Acute neurodegeneration and the inflammasome - Semantic Scholar

Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881 & 2008 ISCBFM All rights reserved 0271-678X/08 $30.00 www.jcbfm.com

Review Article

Acute neurodegeneration and the inflammasome: central processor for danger signals and the inflammatory response? George Trendelenburg Experimentelle Neurologie, Charite´-Universita¨tsmedizin Berlin, Berlin, Germany

Activation of the inflammatory response is a crucial event in the adverse outcome of cerebral ischemia, which is promoted by proinflammatory cytokines such as interleukin (IL)-1b. Although caspase-1 is necessary for IL-1b processing, the ‘upstream’ signaling pathways were, until recently, essentially unknown. Fortunately, the inflammasome, a multiprotein complex responsible for activating caspase-1 and caspase-5, has recently been characterized. The activation of the inflammasome can result in one of several consequences such as cytokine secretion, cell death, or the development of a stress-resistant state. The significance of the inflammasome for the initiation of the inflammatory response during systemic diseases has already been shown and members of the inflammasome complex were recently found to be induced in acute brain injury. However, the specific pathophysiologic role of the inflammasome in neurodegenerative disorders still remains to be clarified. The underlying theories (e.g., danger signal theory) along with the signaling pathways that link the inflammasome to acute neurodegeneration will be discussed here. Furthermore, the stimuli that potentially activate the inflammasome in cerebral ischemia will be specified, as well as their relation to well-known pathways activating the innate immune response (e.g., Toll-like receptor signaling) and the consequences that result from their activation (beneficial versus deleterious). Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881; doi:10.1038/sj.jcbfm.9600609; published online 23 January 2008 Keywords: cerebral ischemia; inflammasome; neurodegeneration; Toll-like receptors

Inflammation in Neurodegenerative Disorders Acute brain injury or tissue stress (e.g., ischemic or traumatic) elicits acute inflammation that in turn exacerbates primary brain damage. Activation of the innate immune system (i.e., neutrophils, macrophages, microglia, as well as the noncellular components, such as the complement system) is an important part of the inflammatory response. Accordingly, microglial activation is a hallmark of several central nervous system (CNS) disorders, including multiple sclerosis, Alzheimer’s disease, HIV encephalitis, dementia, and ischemic or traumatic brain injury. These diseases are associated, in Correspondence: Dr G Trendelenburg, Experimentelle Neurologie, Department of Neurology, Charite´—Universita¨tsmedizin Berlin, CCM Charite´platz 1, Berlin D-10117, Germany. E-mail: [email protected] This work was supported by the Deutsche Forschungsgemeinschaft (DFG Grant TR742-1). Received 20 March 2007; revised 14 December 2007; accepted 17 December 2007; published online 23 January 2008

variable degrees, with the neuronal damage that is substantially mediated by the inflammatory reaction (Dirnagl et al, 1999; Block and Hong, 2005; Lucas et al, 2006). The inflammatory reaction is predominantly sustained by microglia, but the astrocytes may also play a major role (Trendelenburg and Dirnagl, 2005; Bramlett and Dietrich, 2004). Products of activated glia have implicated in promoting cell death of neighboring neurons. There is in vivo evidence for a direct link between activation of innate immune response and neuronal injury in the CNS (Block and Hong, 2005, Lucas et al, 2006; Xiong et al, 2003). Toll-like receptors (TLRs) are important mediators of the innate immune response and significantly contribute to neuroinflammation, even in the absence of infectious pathogens (Chen et al, 2007; Babcock et al, 2006) and TLR-deficient mice are protected against ischemic brain damage (Ziegler et al, 2007; Caso et al, 2007; Cao et al, 2007; Tang et al, 2007). Although a substantial amount of data supports the idea of a detrimental role of the inflammatory response in acute brain injury, there is also an increasing body of evidence showing a more

Acute neurodegeneration and the inflammasome G Trendelenburg 868

beneficial role in inflammatory processes. Limited inflammation may promote, repair, and remodel the injured brain tissue, and restrict various toxic substrates released by the damaged cells. Thus, a fine-tuned regulation of the innate immune system in the CNS is crucial for brain homeostasis. Within the following pages, the mechanisms delineated are those that are thought to induce the inflammatory response. The focus is on the regulation of interleukin (IL)-1 processing, the pathways known to regulate caspase-1 activation, as well as the signals that are thought to trigger these events systemically and within the CNS.

K+ [K+] Potential ligands

TLRs NLRs

ATP NALP3 ASC

CARD NF-κB signaling CARD Caspase-1

Transcription of pre-IL-1ß

PYD

NACHT

LRRs

PYD

NALP3 inflammasome CASPASE-1

Secretion of IL-1 and IL-18

Cell death

Cell survival via sterol regulatory binding proteins

Endogenous Activation of the Immune System: the Danger Model Although the immunosurveillance has only restricted access to the brain, the injured CNS requires an innate immune intervention for the purpose of clearing apoptotic cells and toxic debris to limit damage and initiate tissue repair (Elward and Gasque, 2003). In contrast to the ‘foreign’ structures of pathogens (pathogen-associated molecular patterns (PAMPs)) in infectious diseases, the substrates that lead to the activation of the innate immune system in noninfectious disorders such as cerebral ischemia or traumatic brain injury must be endogenous (host-derived). Thus, the activation of the immune system in such conditions cannot simply be explained by discrimination between foreign and host-derived patterns. A better idea is the ‘danger model,’ in which the tissue damage caused by the threat activates the innate immune system by endogenous stress or damage-associated molecular patterns (DAMPs) (Matzinger, 2002, 2007). Host-derived molecules released from injured tissue and cells are detected by receptors (e.g., TLRs), which in turn lead to the production of proinflammatory cytokines such as tumor necrosis factor (TNF), IL-1b, or IL-6. Currently, there is an evolving list of endogenous immunostimulators (‘danger signals’ or DAMPs), such as hyaluronan, heat-shock proteins (HSPs), surfactant protein, interferon-a, uric acid, fibronectin, b-defensin, and cardiolipin. These molecules are thought to use the same receptors as the PAMPs, and many of these may initiate the inflammatory response in neurodegenerative diseases. In addition to the TLRs, which are associated with the plasma membrane or with vesicles, cytosolic proteins have recently been identified, which exhibit feature analogous to the TLRs. These proteins, called nucleotide-binding oligomerization domain (NOD)like receptor proteins (NLRs), also contain leucinerich repeats (LRRs), which may indicate that they are responsible for sensing danger signals in the cytoplasm (Strober et al, 2006; Akira and Takeda, 2004). Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881

Figure 1 Composition of the NALP3 inflammasome. The murine NALP3 inflammasome is composed of NALP3, ASC, and caspase-1 (a second adaptor protein called CARDINAL exists only in humans). ASC interacts with one of the NALP proteins through cognate pyrin domain (PYD) interactions and with pro-caspase-1 through homotypic CARD (caspase recruitment domain) interactions. The human inflammasome complex brings two molecules of pro-caspase-1 (the second via CARDINAL) into close proximity, leading to autocatalysis and the subsequent release of the active catalytic p20 and p10 domains of caspase-1. NALP3 binds ATP via the NACHT domain (nucleoside triphosphatase (NTPase) domain), is an ATPase, and requires ATP binding for inflammasome activation (Duncan et al, 2007). Caspase-1, in turn, cleaves the precursor of IL-1b into its biologically active fragment, a potent mediator of fever and inflammation. There is no CARDINAL homolog in the mouse and, hence, murine NALP3 is thought to recruit only a single caspase-1 molecule. (TLRs, Toll-like receptors; ATP, adenosine triphosphate; NLRs, nucleotide-binding oligomerization domain (NOD)-like receptors; ASC, apoptosis-associated speck-like protein containing a CARD; NALP, NACHT-, LRR-, and pyrin domain-containing protein; LRRs, leucine-rich repeats).

Before specifying the potential danger signals and receptors that are characteristic of acute brain injury, the processing and regulation of proinflammatory cytokines such as IL-1b will be discussed.

Proinflammatory Cytokines such as IL-1 and Their Significance in Acute Brain Injury Interleukin-1, with its subtypes IL-1a and IL-1b, is a potent pyrogen. It is considered to be a master cytokine that mediates both the innate and adaptive immune response either directly or by induction of other cytokines such as IL-6 or TNF-a. There is a plethora of data supporting a central role of the proinflammatory IL-1 in acute brain injury (Allan et al, 2005; Emsley et al, 2005; Dinarello, 2005). Interleukin-1 might have indirect neurotoxic effects by activating glial cells, which in turn release


Tissue stress in neurodegenerative disorders + ATP, uric acid Release of K

Release of ssRNA

(danger signals)

P2X 7 receptors

Stress-associated molecular patterns (e.g. HmgB1, HSP70)

TLRs

Pannexin-1

Interferon

PGE2

(e.g.IFNß)

Assembly of the inflammasome

NLRs Activation of caspase-1

Hypoxia excitotoxins

Hypotonic stress

TLRs

K+

Transcription of pro-IL-1ß

NF-kB MAPK Inflammatory cytokines chemotactic cytokines

Cleavage and release of Interleukin1ß & IL-18 Recruitment of lymphocytes, activation of astrocytes & microglia, etc.

Inflammator y response

Figure 2 The inflammasome and its role in connecting the various local stress signals with the initiation of the inflammatory response (excitotoxins, excitatory amino-acid receptor agonists; HmgB1, high-mobility group box protein 1; HSP, heat-shock protein; IFN-b, interferon-b, a type I interferon; IL, interleukin; MAPK, mitogen-activated protein kinase; NLR, nucleotide-binding oligomerization domain (NOD)-like receptor; PGE2, prostaglandin E2; TLRs, Toll-like receptors).

proinflammatory cytokines. It may also have direct neurotoxic effects by, for example, enhancing the seizure activity or Ca2 + entry. However, there are also data pointing to a neuroprotective role of IL-1 action in neurons, an effect that may be mediated by inhibition of glutamate release, inhibition of Ca2 + entry, or enhancement of synaptic inhibition by g-aminobutyric acid (Allan et al, 2005). Nevertheless, IL-1 has been shown to be a key mediator of experimentally induced neurodegeneration and its inhibition has proven to be neuroprotective in vitro and in vivo (Allan et al, 2005; Friedlander et al, 1997; Betz et al, 1996; Pinteaux et al, 2006; Schielke et al, 1998; Boutin et al, 2001; Hara et al, 1997a, b). Although there is evidence for a significant contribution of IL-1a in injury-induced inflammation in vivo, most of the data concern the subtype IL-1b (Chen et al, 2007). The processing and release of IL-1b depend on caspase-1 activation. The expression of IL-1b is primarily regulated through nuclear factor-kB (NF-kB) signaling, resembling that of other proinflammatory cytokines such as interferon-g or TNF-a (Thornberry et al, 1992; Allan et al, 2005). Various proinflammatory signals can enhance the transcription of IL-1b, including hypoxia, complement components, prostaglandin E2, the b-chain of the S100 calcium-binding protein, excitotoxins (excitatory amino-acid receptor agonists), various TLR agonists, as well as IL-1b itself (Allan et al, 2005). In contrast to NF-kB-regulated pro-IL-1b expression, processing of IL-1b is dependent on a cytosolic

complex that includes NLR proteins (see below). Thus, the IL-1b system with its associated TLR and NLR signaling pathways shows how both extracellular and intracellular innate immune signaling pathways interact (Delbridge and O’riordan, 2007; Mariathasan and Monack, 2007; Allan et al, 2005; Creagh and O’Neill, 2006). However, proinflammatory signaling is not exclusively mediated by IL-1b or other more caspase-1independent cytokines (e.g., TNF-a or IL-6). It may also be determined by the complex time- and sitespecific expression profiles of the corresponding receptors and downstream signaling partners. Thus, the following concept of the inflammasome should only be regarded as a model and may represent a starting point for further hypotheses.

Activation of Caspase-1: the Inflammasome A major portion of the signaling pathways ‘upstream’ of caspase-1 has recently been unraveled with the discovery of the ‘inflammasome.’ Similar to the activation of caspase-9 or caspase-8 by the Apaf-1 (apoptotic protease-activating factor 1)– apoptosome or Fas/CD95-DISC (death-inducing signaling complex), it has been shown that caspase-1 is activated by an B700-kDa multiprotein complex termed the ‘inflammasome’ (Martinon et al, 2002; Ogura et al, 2006). Caspase-activating platforms such as the inflammasome are typically present in the cytoplasm as inactive monomers and oligomerize only upon the reception of a specific signal, such as the binding of a ligand. To process the proinflammatory cytokines IL-1b, IL-33, and IL-18, the assembly of different adaptor proteins activates caspase-1 and caspase-5. The complex is also able to interact with NF-kBdependent pathways. In addition, inflammasome activation can lead to host cell death in certain cell types (Richards et al, 2001; Ohtsuka et al, 2004; Park et al, 2007a) (Figure 1). Types of Inflammasomes

The inflammasome can comprise caspase-1 or caspase-5; members of the NLR family, such as IPAF (ICE-protease-activating factor) or NALPs (NACHT-, LRR-, and pryin domain-containing proteins); and adaptor molecules such as ASC (apoptosis-associated speck-like protein containing a CARD (caspase recruitment domain)) or CARDINAL (CARD inhibitor of NF-kB-activating ligands). Until now, at least three inflammasomes have been described: the IPAF, NALP1, and the NALP2/3 inflammasomes. The IPAF inflammasome is thought to be a homo-oligomer, consisting of IPAF and caspase-1. The NALP1 inflammasome is composed of NALP1, ASC, caspase-1, and caspase-5 (which does not exist in mice). Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881


The NALP2/3 inflammasome contains, in addition to NALP2 or NALP3, CARDINAL (which does not exist in mice), ASC, and caspase-1 (Martinon and Tschopp, 2007; Kummer et al, 2007; Sutterwala et al, 2007; Poyet et al, 2001; Agostini et al, 2004; Lamkanfi et al, 2007; Ogura et al, 2006). Members of the NLR family (e.g., IPAF or NALPs) contain either a CARD or pyrin domain (PYD) caspase interaction motif, a typical nucleotidebinding oligomerization domain (NACHT), and a ligand-sensing site—which is composed of LRRs (also occurring in TLRs) (Martinon and Tschopp, 2004). ASC is an essential component of most NALPs (it connects the NALPs to caspase-1), and it may also play a role in the IPAF inflammasome (Masumoto et al, 1999, 2001a, b, 2003; Miao et al, 2006). Moreover, ASC has recently been shown to be involved in the caspase-1-independent induction of proinflammatory cytokines (e.g., IL-6 or TNF) by TRLs (Taxman et al, 2006). Several inflammasome-associated autoinflammatory disorders have already been identified. Three autosomal dominant diseases are associated with a gain-of-function mutation in the NALP3 gene: familial cold autoinflammatory syndrome, Muckle– Wells syndrome, and chronic infantile neurologic cutaneous and articular syndrome (Agostini et al, 2004; Inohara et al, 2005; Ogura et al, 2006). These autoinflammatory syndromes are characterized by recurrent episodes of fever, skin rashes, and tissue inflammation, and there is a possibility that they might be treated with the IL-1 antagonist anakinra (Fisher et al, 1994; Liao et al, 1984; Dinarello, 2005; Braddock and Quinn, 2004; Goldbach-Mansky et al, 2006; Hawkins et al, 2004; Hoffman et al, 2004). Loss-of-function mutations in another NLR family member, NOD2, have been associated with a susceptibility to autoinflammatory diseases such as Crohn’s disease and Blau’s syndrome (Ogura et al, 2001). Familial Mediterranean fever is caused by a deficiency in an inflammasome inhibitor known as Pyrin (the familial Mediterranean fever gene). Homozygous loss-of-function mutations in Pyrin, thought to negatively regulate NALP3 signaling by disrupting the assembly of the inflammasome, may lead to enhanced inflammation (Inohara et al, 2005; Chae et al, 2006).

Activation of Inflammasomes: TLRs

The TLR family is the best-characterized group of proteins that are known to bind danger signals. Tolllike receptors are single-spanning transmembrane proteins with ectodomains largely composed of LRRs as well as a cytoplasmic domain that consists of a TIR (Toll/IL-1 interacting) domain. In contrast to TLRs such as TLR2 and TLR4, which are located on the cell surface, some TLR family members (TLR3, TLR7, TLR8, TLR9) are located intracellularly on vesicle membranes (Akira and Takeda, 2004). Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881

Toll-like receptors recognize the corresponding PAMPs or DAMPs through the LRRs in their extracellular domains, which have been implicated in ligand binding and autoregulation. Various PAMPs activate individual TLRs: TLR2 is essential for the recognition of lipopeptides, peptidoglycan, or lipoteichoic acid, TLR4 recognizes lipopolysaccharide (LPS), TLR3 is activated by doublestranded viral RNA, TLR5 mediates the recognition of flagellin, TLR9 recognizes unmethylated CpGcontaining DNA, and TLR7 and TLR8 bind singlestranded RNA (Creagh and O’Neill, 2006; Mishra et al, 2006; Mariathasan and Monack, 2007; Akira and Takeda, 2004). Most TLRs are known to be expressed in the CNS, especially in microglial cells, and have been found to be induced during different pathologic conditions (Bsibsi et al, 2002, 2006; Kariko et al, 2004a; Babcock et al, 2006; Nishimura and Naito, 2005; Bowman et al, 2003; Bo¨ttcher et al, 2002; McKimmie et al, 2005; Tang et al, 2007). Various molecules released from damaged cells or the extracellular matrix have been identified as endogenous activators of TLRs (Kariko et al, 2004a, b; Seong and Matzinger, 2004). A key functional outcome of TLR ligation is the production of inflammatory cytokines through transcription factors such as NF-kB. Most TLRs require the adaptor protein MyD88 (myeloid differentiation primary response gene 88) to induce transcription of cytokines such as IL-1b, IL-6, and TNF-a through NF-kB signaling, but can also recruit further adaptor molecules and transcription factors (Beutler et al, 2006; Deane and Bolland, 2006; Creagh and O’Neill, 2006). There has been speculation about the possible superior role of TLR2 and TLR4 signaling in acute brain injury (Babcock et al, 2006; Lehnardt et al, 2003; Coban et al, 2007; Mishra et al, 2006; Maslinska et al, 2004; Tang et al, 2007). Toll-like receptor 2, a receptor with a wide range of ligands, is expressed in several CNS diseases, primarily in microglial and mononuclear cell infiltrates (Mishra et al, 2006; Babcock et al, 2006; Maslinska et al, 2004; Kielian, 2006), but can also be found in neurons (Tang et al, 2007; Ziegler et al, 2007). It has been suggested that oxidant stress is sensed by TLR2-dependent pathways (Mitchell et al, 2007; Zhang et al, 2003). Endogenous TLR2 signaling strongly impacts early glial cytokine and chemokine responses and has been shown to exacerbate postischemic damage in different organs, including the brain (Favre et al, 2007; Ziegler et al, 2007; Leemans et al, 2005; Aliprantis et al, 2000; Hoffmann et al, 2007; Babcock et al, 2006). The LPS-binding molecule CD14 not only signals through TLR4, but also through TLR2 and is expressed by activated microglia in ischemic brain injury (Aliprantis et al, 2000; Beschorner et al, 2002; ¨ zo¨ren et al, 2006; Scott et al, Bsibsi et al, 2007; O 2006). The class B scavenger receptor CD36 also


signals through TLR2. CD36 has recently been shown to mediate free radical production and tissue injury in cerebral ischemia (Cho et al, 2005; Hoebe et al, 2005). Toll-like receptor 4 is also expressed in the CNS. It is induced by oxidative stress, and it exacerbates ischemic injury in vivo (Lehnardt et al, 2003; Caso et al, 2007; Cao et al, 2007; Kielian, 2006; Tang et al, 2007; Powers et al, 2006). Moreover, polymorphisms of the TLR4 gene have been shown to have a significant association with an increased risk for cerebral ischemia (Lin et al, 2005). Monosodium urate, heat-shock proteins (Hsp-60, Hsp-70), heparan sulfate, fibrinogen, and hyaluronan have been identified as endogenous TLR4 ligands. They may be released from the cell surface and are linked to caspase-1 activation and IL-1b processing (Elward and Gasque, 2003; Scott et al, 2006; Stevens and Stenzel-Poore, 2006). For example, high-mobility group box 1 protein (HMGB1), an endogenous ligand potentially triggering the neuroinflammatory response, has been shown, depending on the cell type examined, to differentially use TLR2 or TLR4 signaling for IL release (Yu et al, 2006). It has also been shown to trigger ischemic liver damage through TLR4 signaling (Tsung et al, 2005). The TLR3, TLR7, TLR8, and TLR9 are expressed in the CNS and represent further candidate receptors for the initiation of the innate immune response after acute brain injury. These innate receptors recognize nucleic acid types that normally are found in bacteria and viruses but which can also be found within mammalian cells (Deane and Bolland, 2006; Stevens and Stenzel-Poore, 2006; Bsibsi et al, 2007). Toll-like receptor 9, for example, normally responds to bacterial DNA, viral DNA, and synthetic oligodeoxynucleotides, all of which contain unmethylated CpG motifs. However, some CpG dinucleotides within the mammalian genome are unmethylated and should therefore be recognized by TLR9 (Beutler et al, 2006; Kielian, 2006; Kariko et al, 2005). The microglial-mediated neuronal toxicity as a result of CpG oligodeoxynucleotide treatment is reminiscent of what has been observed after stimulation with the TLR4 agonist LPS or a synthetic TLR2 agonist. This suggests that the microglial response to diverse PAMPs or DAMPs has been conserved and that there may be a link between TLRs and neurodegeneration (Kielian, 2006; Hoffmann et al, 2007; Lehnardt et al, 2003). However, this does not preclude that TLRs might influence neuronal survival or regeneration, or modulate neurogenesis, as has been shown very recently for TLR2 and TLR4 (Rolls et al, 2007). Indeed, TLR ligands exert both beneficial and pathologic effects in the CNS. Accordingly, administration of TLR agonists in the injured CNS has been shown to promote regeneration or myelin clearance, whereas direct injection of TLR ligands into the healthy brain or spinal cord induced robust

inflammation that caused tissue damage (Babcock et al, 2006).

Activation of Inflammasomes: NLRs Whereas extracellular PAMPs and DAMPs are recognized by membranous TLRs, surveillance of the cytoplasm is thought to be the duty of the NLR family of proteins. The NLR family is composed of 23 soluble cytosolic proteins including NALPs, NOD1, NOD2, and IPAF, all of which are expressed primarily in immune cells, although the expression of certain proteins, such as NOD1, is ubiquitous (Zedler and Faist, 2006; Delbridge and O’riordan, 2007; Kanneganti et al, 2006b; Inohara et al, 2005; Swanson and Molofsky, 2005; Ting et al, 2006). The cytosolic NLRs are thought to sense specific danger signals with their LRR domains analogous to the TLRs. Recent studies revealed a specific role of Gram-positive pathogens such as Staphylococcus aureus or Listeria monocytogenes in activating the NALP3 inflammasome, whereas Gram-negative pathogens such as Salmonaella typhimurium, Legionella pneumophila, or Shigella flexneri mediate caspase-1 activation through IPAF (Mariathasan and Monack, 2007). NALPs may also be crucial for detecting tissue injury and they represent key drivers for IL-1b production. NALP3 for example is able to sense monosodium urate, calcium pyrophosphate dehydrate crystals, as well as low intracellular potassium concentration. Thus, factors that induce K + efflux, such as certain toxins, hypotonic stress, and high concentrations of extracellular adenosine triphosphate (ATP), can be sensed by NALP3 (Kanneganti et al, 2006b; Kahlenberg and Dubyak, 2004; Creagh and O’Neill, 2006; Petrilli et al, 2007). The minimal requirements for inflammasome assembly have recently been characterized: in vitro reconstitution of the NALP1 inflammasome revealed that the activation of caspase-1 is a process requiring at least a bacterial ligand such as muramyl dipeptide, pro-caspase-1, NALP1, as well as ATP (Faustin et al, 2007).

Interplay between TLR Signaling and the Inflammasome/NLRs

There are many hints for fine-tuned crosssignaling between caspase-1 activation by the inflammasomes and TLR signaling (Akira and Takeda, 2004; Strober et al, 2006; Zhang et al, 2003; Taxman et al, 2006; Yoo et al, 2002). Toll-like receptor ligands such as flagellin, imiquimod, or single-stranded RNA not only activate TLRs, but also signal through IPAF or NALP inflammasomes (Kanneganti et al, 2006a, b; Prins et al, 2006). Further complexity has recently been noted by the characterization of a third class of PAMP receptors, which are called the retinoic Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881


acid-inducible I-like helicases (including retinoic acid-inducible I and MDA-5 (melanoma differentiation associated gene 5). These helicases have been shown to bind to viral RNA and self-RNA (Malathi et al, 2007; Meylan et al, 2006). Toll-like receptor agonists are known to not only activate IL-1 maturation, but also regulate transcription of proIL-1, NLRs (e.g., NALP3), murine caspase-11 (which corresponds to human caspase-4 and caspase-5, and seems to be required for caspase-1 activation in mice), or caspase-1 through NF-kB (Mariathasan and Monack, 2007; O’Connor et al, 2003; Kanneganti et al, 2006a, b; Martinon and Tschopp, 2004; Kang et al, 2000; Wang et al, 1998). Interleukin-1 and other proinflammatory cytokines are capable of activating NF-kB as well. Furthermore, it was shown that caspase-1 activity cleaves MyD88 adaptor-like (MAL/TIRAP), a protein that has been shown to regulate TLR2-mediated NF-kB and p38 mitogenactivated protein kinase activation. Accordingly, MAL/TIRAP deficiency reduces the functional deficit after ischemia/reperfusion injury in the heart (Miggin et al, 2007; Sakata et al, 2007). There are data arguing for a modulating role of specific NLRs (e.g., NOD1, NOD2) in TLR signaling: for example, the adaptor protein RICK (RIP-like interacting CLARP kinase) is thought to mediate both NLR and TLR signaling (Strober et al, 2006; Watanabe et al, 2004; Kobayashi et al, 2002; Netea et al, 2005; Thome et al, 1998), although this has recently been questioned (Park et al, 2007b). RICK and the NLR family member NOD2 are expressed in the CNS and are induced after exposure to TLR ligands. NOD2 was shown to be a modulator of signals transmitted through TLR2, TLR3, and TLR4, and NOD2 ligands were shown to augment TLR2-mediated immune responses in astrocytes (Sterka et al, 2006; Netea et al, 2005). Moreover, NOD2 was shown to mediate neuronal damage in an early sepsis model, as it is true for TLR2 (Orihuela et al, 2006). There is evidence that the NLR system is largely independent of the TLR system and, as such, might positively or negatively modulate TLR responses (Strober et al, 2006). Interestingly, there is a synergistic interaction between NOD2 and TLR2 signals in mononuclear cells, resulting in a shift toward a Th2-type response, which also occurs in stroke-induced immunodeficiency (Watanabe et al, 2004; Netea et al, 2005; Prass et al, 2003). However, it still remains to be resolved to what extent TLRs contribute to neuroinflammation or activation of the inflammasome, as well as to what extent each TLR contributes to regenerative processes in cerebral ischemia.

Potential Endogenous Danger Signals in Acute Neurodegeneration Because various TLRs, NLRs, and inflammasome components are expressed in the CNS, the Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881

inflammasome is thought to play a significant role in triggering the inflammatory response in acute brain injury (Kinoshita et al, 2005; Kummer et al, 2007; Zhao et al, 2007; Liu et al, 2004). In addition to bacterial PAMPs, such as flagellin, peptidoglycan, or bacterial RNAs, there are several endogenous signals released from dying cells that potentially activate the inflammasome in neurodegenerative diseases (Mariathasan et al, 2005; Kanneganti et al, 2006a, b; Martinon and Tschopp, 2004; Kummer et al, 2007). Substances such as RNA or DNA that are released from necrotic cells can activate the inflammasome or can bind to TLRs (Matzinger, 2007; Martinon and Tschopp, 2007; Ogura et al, 2006; Kanneganti et al, 2006b; Shi et al, 2003; Kariko et al, 2004b) (Figure 2). Although several stimuli, such as bacterial RNA, bacterial toxins, ATP, calcium pyrophosphate dehydrate, and uric acid crystals, have been shown to activate the NALP3 inflammasome, it is unlikely that these various stimuli can all activate NALP3 directly. The NALP3 inflammasome is more likely activated by a common intracellular ‘danger signal’ such as low intracellular potassium concentration or endogenously generated uric acid (Creagh and O’Neill, 2006; Ogura et al, 2006; Seong and Matzinger, 2004; Petrilli et al, 2007). Injured cells rapidly degrade their RNA and DNA and the liberated purines are converted into uric acid, resulting in its accumulation. The production of uric acid does not require protein synthesis, and inhibition of protein synthesis is a critical hallmark of the early ischemic tissue damage (Dirnagl et al, 1999; Hossmann, 2006; Shi et al, 2003; Kanemitsu et al, 1988; Bos et al, 2006). Uric acid forms monosodium urate crystals, which have been shown to activate caspase-1 activation and IL-1b processing by binding to TLR2, TLR4, CD14, or the NALP3 inflammasome (Shi et al, 2003; Scott et al, 2006; Ogura et al, 2006; Martinon et al, 2006; Martinon and Tschopp, 2007; Seong and Matzinger, 2004). Eliminating this endogenous danger signal in vivo was shown to inhibit the immune response to antigens associated with injured cells (Shi et al, 2003). Accordingly, high serum urate levels have been linked to poor outcome and higher vascular event rates in stroke patients (Weir et al, 2003; Hozawa et al, 2006; Bos et al, 2006). With the HSP family, including HSP60, HSP70, and HSP90, another host-derived group of endogenous ligands for TLRs (and very recently also for NLRs) (Mayor et al, 2007) has been identified. Heat-shock proteins are released by necrotic but not apoptotic cells, and have been shown to be induced in neuroinflammation and infection. Although existing data argue for a mainly neuroprotective effect of HSPs, these proteins are also able to induce inflammation (Zedler and Faist, 2006; Dirnagl et al, 1999, 2003; Asea et al, 2002; Vabulas et al, 2002; Basu et al, 2000; Svensson et al, 2006). Interestingly, HSP90, in concert with the ubiquitin ligase-associated protein SGT1


(suppressor of G2 allele of SKP1), has recently been shown to interact with NLRs to activate caspase-1 (Mayor et al, 2007). Extracellular ATP and K + efflux represent further danger signal candidates in cerebral injury. Both ATP and K + were shown to be involved in cerebral injury and both trigger inflammasome assembly in vitro (Gurcel et al, 2006; Cruz et al, 2007; Mariathasan et al, 2006). The best-studied model of caspase-1 activation is the exposure of cells to extracellular ATP. Adenosine triphosphate activates purinergic receptors P2X (P2X7) with its associated hemichannel pannexin-1 and leads to potassium efflux, plasma membrane depolarization, cell swelling, and disaggregation of the cytoskeletal network (Khakh and North, 2006; Kanneganti et al, 2007; Martinon and Tschopp, 2007; Kahlenberg and Dubyak, 2004; Franchi et al, 2007; Solle et al, 2001). The intracellular K + concentration is known to decrease during hypoxic/ischemic cell injury and, interestingly, NALP3 and NALP1 (but not IPAF) activation has recently been shown to depend on low intracellular K + concentration (Petrilli et al, 2007; Somjen, 2001). Accordingly, there is in vitro evidence that extracellular ATP is involved in inflammatory responses and neuronal death induced by ischemic stress. ATP induces the transcription of genes for the oxidative stress response in macrophages and activates caspase-1 as well as the secretion of IL-1b through a reactive oxygen species-dependent phosphoinositide-3kinase pathway (Cruz et al, 2007). However, although ATP levels can increase significantly under inflammatory and ischemic conditions in vivo and in vitro, the high concentrations of ATP that are required for caspase-1 activation are normally not found in the extracellular milieu (Le Feuvre et al, 2003; Trendelenburg and Dirnagl, 2005; Dirnagl et al, 1999; Kanneganti et al, 2007; Ferrari et al, 2006). In vitro results with both P2X7- and K + -loss-induced IL-1b processing support a model in which a putative lipid second messenger is generated by the cytosolic calcium-independent phospholipase A2 and subsequently activates caspase-1 (Kahlenberg and Dubyak, 2004; Mariathasan and Monack, 2007). Phospholipase A2 activity has been found to be induced after cerebral ischemia and phospholipase A2 isoforms have been implicated in cell injury and death by their ability to mediate inflammatory responses (Bonventre et al, 1997; Adibhatla et al, 2006; Cummings et al, 2000). However, the deletion of the P2X7 receptor does not affect neuronal death after cerebral ischemia in vivo, and P2X7 knockout mice can still be protected by the use of an IL-1 antagonist (Le Feuvre et al, 2003). The complement system may also contribute to caspase-1 activation by provoking K + efflux, because disruption of the cell membrane is a consequence of activation of the complement system and formation of the terminal membrane attack complex. The TLRs and complement system are two

well-characterized arms of the immune system, which are known to play a key role in priming the adaptive immune system. It has recently been shown that both systems interact at the molecular level in vivo (Zhang et al, 2007). However, the exact contribution of the complement system in neurodegeneration still remains controversial (Stahel et al, 1998; Trendelenburg and Dirnagl, 2005; Xiong et al, 2003; Huang et al, 1999, Del Zoppo, 1999; Elward and Gasque, 2003; van Beek et al, 2003; D’Ambrosio et al, 2001). Heparan sulfate, a biologically active saccharide released from cell surfaces during almost every type of inflammation, activates dendritic cells as fully as LPS and may further represent a danger signal during brain injury. Both heparan sulfate and fragments of hyaluronic acid, a related saccharide, act through TLR4 (Zedler and Faist, 2006; Leadbeater et al, 2006; Gomez-Pinilla et al, 1995). Myeloidrelated protein-8, an agonist for TLR4 signaling that has recently been identified, represents a further endogenous danger signal (Vogl et al, 2007). The list of endogenous molecules that are induced after brain injury and which activate cells through TLRdependent pathways also includes fibronectin and saturated fatty acids (Kielian, 2006; Tate et al, 2007; Yanqing et al, 2006). Another group of structurally diverse multifunctional host proteins that are rapidly released after pathogen challenge or cell stress is comprised of defensins, cathelicidins, eosinophil-derived neurotoxins, and HMGB1. Cathelicidin, defensins, or its rodent homologs are antimicrobial peptides that have also been shown to be expressed in the CNS. These peptides, as well as eosinophil-derived neurotoxin and HMGB1, are able to both recruit and activate antigen-presenting cells (Bergman et al, 2005; Hao et al, 2001; Zedler and Faist, 2006). The nuclear protein HMBG1 is involved in transcriptional activation and DNA folding. In addition to its nuclear role, extracellular HMGB1 has been shown to be a critical mediator of the innate immune response to infection and also ischemic brain injury (Tsung et al, 2005; Qiu et al, 2007). HMGB1 is released by necrotic cells and activated macrophages; however, it is not released by apoptotic cells. HMGB1 interacts with TLRs and the receptor for advanced glycation end products, resulting in the activation of NF-kB and the release of proinflammatory cytokines such as TNF-a, IL-6, or IL-1b (Orlova et al, 2007; Scaffidi et al, 2002; Lotze and Tracey, 2005; Park et al, 2004; Qiu et al, 2007). Besides its proinflammatory role, HMGB1 has also been identified as an active player during tissue repair, stimulating homing of endothelial progenitor cells to ischemic tissues (Chavakis et al, 2007). Whereas some stimuli mentioned above may result in the assembly of the inflammasome through TLR signaling, several of them may also be detected directly by the LRRs of cytosolic NLR proteins, or by a hypothetical common ‘danger signal’ mediator Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881


such as low intracellular potassium or uric acid. Furthermore, genomic responses under neuronal injury (e.g., stroke) may alter and induce specific components of TLR or NLR pathways, as shown, for example, for NOD1, NOD2, NALP1, TLR2, TLR4, or RICK/RIP2, thus fine-tuning the innate immune response (Sterka et al, 2006; Tang et al, 2007; Ziegler et al, 2007; Liu et al, 2004).

Inflammasome Activation and Its Potential Role in Neuroprotection Besides the essential role of caspase-1 in the generation of the proinflammatory cytokine IL-1b, its activation has also been shown to promote cell survival by activating protective pathways. Toxininduced K + efflux in human fibroblasts leads to improved cell survival by the inflammasomemediated induction of lipogenic genes (Gurcel et al, 2006; Saleh, 2006). Thus, pathophysiologic conditions associated with lowering of cytoplasmic K + concentration may trigger specific signaling pathways, some of which are aimed at triggering the immune system (e.g., by the induction of IL-1b processing), whereas others promote death or survival of the attacked cell (Petrilli et al, 2007; Mariathasan and Monack, 2007; Gurcel et al, 2006). These data support the hypothetical neuroprotective abilities of inflammasome activation, for example, for ischemic preconditioning phenomena, and are consistent with the observation that proinflammatory substances such as LPS or IL-1b are able to induce ischemic tolerance (Dirnagl et al, 2003; Shi et al, 2003; Akahoshi et al, 2006; Glantz et al, 2005; Romanos et al, 2007; Stenzel-Poore et al, 2007). It is well known that small doses of an otherwise harmful (or proinflammatory) stimulus can induce protection against a subsequent injurious challenge, thereby causing a ‘preconditioned’ state with activated antiinflammatory and neuroprotective pathways (Stenzel-Poore et al, 2007; Izuishi et al, 2006; Dirnagl et al, 2003; Hossmann, 2006).

Inflammasome Modulation Although a lot of data already exist regarding the regulation of TLR signaling in the brain, there is only a small quantity of information available regarding the regulation of the cytosolic surveillance system (Prins et al, 2006; Kariko et al, 2004a; Kielian, 2006; Stevens and Stenzel-Poore, 2006; Hoffmann et al, 2007; Kinoshita et al, 2005). Toll-like-receptor-mediated signaling is regulated at several levels and at least five levels of negative regulation have so far been uncovered (Liew et al, 2005). These include extracellular decoy receptors, intracellular inhibitors, membrane-bound suppressors, degradation of TLRs, and TLR-induced Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881

apoptosis. Soluble splicing variants of TLR2 and TLR4 were identified and have been shown to attenuate TLR-mediated NF-kB activation. The most likely mechanism through which these TLR variants exert this effect is by blocking the interaction between TLRs and other co-receptor complexes, such as CD14. Moreover, a short form of MyD88— the most crucial adaptor in TLR signaling—has been shown to be expressed in the brain; it has also been shown to inhibit IL-1- and LPS-mediated NF-kB activation. Further negative regulators of TLRs have been shown to interfere with the signaling pathways of transmembrane protein regulators or various other levels further downstream: for example, SIGIRR (single immunoglobulin interleukin-1related receptor), TRAILR (tumor necrosis factorrelated apoptosis-inducing ligand receptor), RP105 (which negatively regulates TLR4 signaling), IRAKM (interleukin-1 receptor-associated kinase-M; the expression of which is restricted to macrophages and monocytes), SOCS1 (suppressor of cytokine signaling 1), NOD2 (nucleotide-binding oligomerization domain protein 2), phosphoinositide3-kinase, TOLLIP (Toll-interacting protein), Bcl-3 (B-cell leukemia-3), or A20. Moreover, TLR signaling can be controlled by TLR-induced apoptosis or the reduced expression of TLRs, the complement system, or downstream signaling molecules (e.g., ASC) (Kariko et al, 2004a; Naiki et al, 2005; Liew et al, 2005; Wald et al, 2003; Zhang et al, 2007; Miggin et al, 2007; Akira and Takeda, 2004; Divanovic et al, 2005; Taxman et al, 2006; Carmody et al, 2007). Although little is known about the regulation of the cytosolic immune surveillance system at the physiologic level, several proteins have been proposed to interfere with inflammasome assembly and caspase activation (Shiohara et al, 2002; Martinon and Tschopp, 2007; Park et al, 2007a). Two types of inflammasome regulators can be distinguished based on their modular structure. The first type is characterized by the presence of a CARD (a group including decoy caspase-1 genes such as iceberg, INCA (inhibitory caspase recruitment domain protein), or human caspase-12) and inhibits caspase-1 activation by interacting with caspase-1, hypoxiainducible RICK/RIP2, or ASC CARDs. The second type is characterized by the presence of a PYD and is thought to interfere with interactions between ASC and NALPs (such as pyrin, POP (pyrin only protein), PYNOD (protein containing a PYD and a NOD domain), and viral PYDs) (Martinon and Tschopp, 2007; Chae et al, 2006; Wang et al, 2004; Kinoshita et al, 2005; Stehlik et al, 2002, 2003; Dorfleutner et al, 2007; Bedoya et al, 2007; Zhang et al, 2003). However, human COPs (CARD-only proteins) and POPs lack mouse orthologs and are therefore not part of murine inflammasome regulation (Stehlik and Dorfleutner, 2007). The clinical significance of such interactions has been documented by the mutations in the Pyrin


gene, which cause the familial Mediterranean fever. NLR inhibitors also include the ErbB2-interacting protein; NOD2-S, a short isoform of NOD2; caspase1 inhibitors, such as COP and ICEBERG; proteinase inhibitor 9 (PI-9); a decoy ASC molecule; and caspase-12 (Creagh and O’Neill, 2006; Saleh et al, 2006; Rosenstiel et al, 2006). Furthermore, viral negative regulators of caspase-1 activation were identified, such as the myxoma virus encoded M13L, which interacts with ASC to inhibit caspase-1 activation (Johnston et al, 2005; Mariathasan and Monack, 2007). Beyond the manifold interactions mentioned above, one must also be aware of additional dimensions of complexity: not only the specific cellular expression pattern, but also the time-wise expression patterns of various inflammatory mediators. The regulation of the inflammatory response is highly dependent on the temporal nature of the inflammatory mediators, which rise as waves of gene expression and gene products and then shortly dissipate (Wang et al, 1995; Dirnagl et al, 1999). Accordingly, corresponding proteins have recently been detected in different cells in the brain, some of which (e.g., caspase-11; RICK/RIP2) are highly inducible under ischemic conditions (Kummer et al, 2007; Zhang et al, 2003; Kang et al, 2000), whereas others, such as TLR4, might alter the cellular distribution under ischemic conditions (Powers et al, 2006). Whereas specific drugs interfering with inflammasome assembly are still under development, caspase-1 inhibitors (e.g., pralnacasan) and IL-1 antagonizing drugs (e.g., IL-1Ra or anakinra) have already been successfully introduced in clinical treatments for autoinflammatory diseases such as rheumatoid arthritis or gout (Dinarello, 2005; Fisher et al, 1994; Liao et al, 1984; So et al, 2007; Braddock and Quinn, 2004; Hauff et al, 2005) and recent results of a phase II study of IL-1Ra treatment in stroke patients add much promise (Emsley et al, 2005). Nevertheless, it still remains to be proven whether the described signaling pathways (e.g., TLR signaling) identified in in vitro and in vivo models can be reproduced in the clinical setting. Whether the high expectations can be fulfilled in the treatment of human diseases still remains to be confirmed and one has to be aware of the fact that most of the antagonists or inhibitors still remain to be tested in humans. It represents an enormous challenge to unravel the interactions that exist between membrane receptors for danger signals (e.g., TLRs), cytoplasmic receptors (e.g., NLRs), the inflammasomes, and their negative regulators. Because there is such a large impact of the innate immune response on the outcome of various neurodegenerative diseases, drugs interfering with the activation of the inflammatory response in the CNS are eagerly being awaited.

Summary The prominent role of caspase-1 activation and IL-1b production in acute brain injury has been known for many years, but it is only recently that the multiprotein complex—the inflammasome—has been characterized. The inflammasome activates pro-caspase-1, which in turn processes IL-1b; however, it can also under specific circumstances lead to apoptotic cell death. It has been shown that specific pathogens and danger signals activate different inflammasomes and yet the exact contribution of each inflammasome in neurodegenerative disorders remains to be resolved. Activation of the inflammasome in neurons may lead directly to neuronal cell death (Liu et al, 2004), at the same time activation within microglial cells may induce inflammatory responses or repair mechanisms. In conclusion, future research should not only focus on cell type-specific and time-wise activation patterns, but should examine the differential activation patterns of the various inflammasome types responsible for regulating tissue damage during brain injury.

Acknowledgements I am grateful to Martin Holtkamp, Ronald Bottlender, and Ryan Cordell for critical reading of the manuscript.

References Adibhatla RM, Hatcher JF, Larsen EC, Chen X, Sun D, Tsao FH (2006) CDP-choline significantly restores phosphatidylcholine levels by differentially affecting phospholipase A2 and CTP: phosphocholine cytidylyltransferase after stroke. J Biol Chem 281:6718–25 Agostini L, Martinon F, Burns K, McDermott MF, Hawkins PN, Tschopp J (2004) NALP3 forms an IL-1betaprocessing inflammasome with increased activity in Muckle–Wells autoinflammatory disorder. Immunity 20:319–25 Akahoshi T, Murakami Y, Kitasato H (2006) Recent advances in crystal-induced acute inflammation. Curr Opin Rheumatol 19:146–50 Akira S, Takeda K (2004) Toll-like receptor signaling. Nat Rev Immunol 4:499–511 Aliprantis AO, Yang RB, Weiss DS, Godowski P, Zychlinsky A (2000) The apoptotic signaling pathway activated by Toll-like receptor-2. EMBO J 19:3325–36 Allan SM, Tyrrell PJ, Rothwell NJ (2005) Interleukin-1 and neuronal injury. Nat Rev Immunol 5:629–40 Asea A, Rehli M, Kabingu E, Boch JA, Bare O, Auron PE, Stevenson MA, Calderwood SK (2002) Novel signal transduction pathway utilized by extracellular HSP70: role of toll-like receptor (TLR) 2 and TLR4. J Biol Chem 277:15028–34 Babcock AA, Wirenfeldt M, Holm T, Nielsen HH, DissingOlesen L, Toft-Hansen H, Millward JM, Landmann R, Rivest S, Finsen B, Owens T (2006) Toll-like receptor 2 Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881


signaling in response to brain injury: an innate bridge to neuroinflammation. J Neurosci 26:12826–37 Basu S, Binder RJ, Suto R, Anderson KM, Srivastava PK (2000) Necrotic but not apoptotic cell death releases heat shock proteins, whichdeliver a partial maturation signal to dendritic cells and activate the NF-kappa B pathway. Int Immunol 12:1539–46 Bedoya F, Sandler LL, Harton JA (2007) Pyrin-only protein 2 modulates NF-kB and disrupts ASC:CLR interactions. J Immunol 178:3837–45 Bergman P, Termen S, Johansson L, Nystrom L, Arenas E, Jonsson AB, Hokfelt T, Gudmundsson GH, Agerberth B (2005) The antimicrobial peptide rCRAMP is present in the central nervous system of the rat. J Neurochem 93:1132–40 Beschorner R, Schluesener HJ, Gozalan F, Meyermann R, Schwab JM (2002) Infiltrating CD14+ monocytes and expression of CD14 by activated parenchymal microglia/macrophages contribute to the pool of CD14+ cells in ischemic brain lesions. J Neuroimmunol 126:107–15 Betz AL, Schielke GP, Yang GY (1996) Interleukin-1 in cerebral ischemia. Keio J Med 45:230–7 Beutler B, Jiang Z, Georgel P, Crozat K, Croker B, Rutschmann S, Du X, Hoebe K (2006) Genetic analysis of host resistance: Toll-like receptor signaling and immunity at large. Annu Rev Immunol 24:353–89 Block ML, Hong JS (2005) Microglia and inflammationmediated neurodegeneration: multiple triggers with a common mechanism. Prog Neurobiol 76:77–98 Bonventre JV, Huang Z, Taheri MR, O’Leary E, Li E, Moskowitz MA, Sapirstein A (1997) Reduced fertility and postischaemic brain injury in mice deficient in cytosolic phospholipase A2. Nature 390:622–5 Bos MJ, Koudstaal PJ, Hofman A, Witteman JC, Breteler MM (2006) Uric acid is a risk factor for myocardial infarction and stroke: the Rotterdam study. Stroke 37:1503–7 Bo¨ttcher T, von Mering M, Ebert S, Meyding-Lamade U, Kuhnt U, Gerber J, Nau R (2002) Differential regulation of Toll-like receptor mRNAs in experimental murine central nervous system infections. Neurosci Lett 344: 17–20 Boutin H, LeFeuvre RA, Horai R, Asano M, Iwakura Y, Rothwell NJ (2001) Role of IL-1alpha and IL-1beta in ischemic brain damage. J Neurosci 21:5528–34 Bowman CC, Rasley A, Tranguch SL, Marriott I (2003) Cultured astrocytes express toll-like receptors for bacterial products. Glia 43:281–91 Braddock M, Quinn A (2004) Targeting IL-1 in inflammatory disease: new opportunities for therapeutic intervention. Nat Rev Drug Discov 3:330–9 Bramlett HM, Dietrich WD (2004) Pathophysiology of cerebral ischemia and brain trauma: similarities and differences. J Cereb Blood Flow Metab 24:133–50 Bsibsi M, Bajramovic JJ, Van Duijvenvoorden E, Persoon C, Ravid R, Van Noort JM, Vogt MH (2007) Identification of soluble CD14 as an endogenous agonist for Toll-like receptor 2 on human astrocytes by genome-scale functional screening of glial cell derived proteins. Glia 55:473–82 Bsibsi M, Persoon-Deen C, Verwer RW, Meeuwsen S, Ravid R, Van Noort JM (2006) Toll-like receptor 3 on adult human astrocytes triggers production of neuroprotective mediators. Glia 53:688–95 Bsibsi M, Ravid R, Gveric D, van Noort JM (2002) Broad expression of Toll-like receptors in the human central nervous system. J Neuropathol Exp Neurol 61:1013–21

Journal of Cerebral Blood Flow & Metabolism (2008) 28, 867–881

Cao CX, Yang QW, Lv FL, Cui J, Fu HB, Wang JZ (2007) Reduced cerebral ischemia–reperfusion injury in Tolllike receptor 4 deficient mice. Biochem Biophys Res Commun 353:509–14 Carmody RJ, Ruan Q, Palmer S, Hilliard B, Chen YH (2007) Negative regulation of toll-like receptor signaling by NF-kappaB p50 ubiquitination blockade. Science 317:675–8 Caso JR, Pradillo JM, Hurtado O, Lorenzo P, Moro MA, Lizasoain I (2007) Toll-like receptor 4 is involved in brain damage and inflammation after experimental stroke. Circulation 115:1599–608 Chae JJ, Wood G, Masters SL, Richard K, Park G, Smith BJ, Kastner DL (2006) The B30.2 domain of pyrin, the familial Mediterranean fever protein, interacts directly with caspase-1 to modulate IL-1beta production. Proc Natl Acad Sci USA 103:9982–7 Chavakis E, Hain A, Vinci M, Carmona G, Bianchi ME, Vajkoczy P, Zeiher AM, Chavakis T, Dimmeler S (2007) High-mobility group box 1 activates integrin-dependent homing of endothelial progenitor cells. Circ Res 100:204–12 Chen CJ, Kono H, Golenbock D, Reed G, Akira S, Rock KL (2007) Identification of a key pathway required for the sterile inflammatory response triggered by dying cells. Nat Med 13:851–6 Cho S, Park EM, Febbraio M, Anrather J, Park L, Racchumi G, Silverstein RL, Iadecola C (2005) The class B scavenger receptor CD36 mediates free radical production and tissue injury in cerebral ischemia. J Neurosci 25:2504–12 Coban C, Ishii KJ, Uematsu S, Arisue N, Sato S, Yamamoto M, Kawai T, Takeuchi O, Hisaeda H, Horii T, Akira S (2007) Pathological role of Toll-like receptor signaling in cerebral malaria. Int Immunol 19:67–79 Creagh EM, O’Neill LA (2006) TLRs, NLRs and RLRs: a trinity of pathogen sensors that co-operate in innate immunity. Trends Immunol 27:352–7 Cruz CM, Rinna A, Forman HJ, Ventura AL, Persechini PM, Ojcius DM (2007) ATP activates an ros-dependent oxidative stress response and secretion of pro-inflammatory cytokines in macrophages. J Biol Chem 282:2871–9 Cummings BS, McHowat J, Schnellmann RG (2000) Phospholipase A(2)s in cell injury and death. J Pharmacol Exp Ther 294:793–9 D’Ambrosio AL, Pinsky DJ, Connolly ES (2001) The role of the complement cascade in ischemia/reperfusion injury: implications for neuroprotection. Mol Med 7:367–82 Deane JA, Bolland S (2006) Nucleic acid-sensing TLRs as modifiers of autoimmunity. J Immunol 177:6573–8 del Zoppo GJ (1999) In stroke, complement will get you nowhere. Nat Med 5:995–6 Delbridge LM, O’riordan MX (2007) Innate recognition of intracellular bacteria. Curr Opin Immunol 19:10–6 Dinarello CA (2005) Blocking IL-1 in systemic inflammation. J Exp Med 201:1355–9 Dirnagl U, Iadecola C, Moskowitz MA (1999) Pathobiology of ischaemic stroke: an intergrated view. Trends Neurosci 22:391–7 Dirnagl U, Simon RP, Hallenbeck JM (2003) Ischemic tolerance and endogenous neuroprotection. Trends Neurosci 26:248–54 Divanovic S, Trompette A, Atabani SF, Madan R, Golenbock DT, Visintin A, Finberg RW, Tarakhovsky A, Vogel SN, Belkaid Y, Kurt-Jones EA, Karp CL (2005) Negative

Acute neurodegeneration and the inflammasome G Trendelenburg

regulation of Toll-like receptor 4 signaling by the Toll-like receptor homolog RP105. Nat Immunol 6: 571–8 Dorfleutner A, Bryan NB, Talbott SJ, Funya KN, Rellick SL, Reed JC, Shi X, Rojanasakul Y, Flynn DC, Stehlik C (2007) Cellular pyrin domain-only protein 2 is a candidate regulator of inflammasome activation. Infect Immun 75:1484–92 Duncan JA, Bergstralh DT, Wang Y, Willingham SB, Ye Z, Zimmermann AG, Ting JP (2007) Cryopyrin/NALP3 binds ATP/dATP, is an ATPase, and requires ATP binding to mediate inflammatory signaling. Proc Natl Acad Sci USA 104:8041–6 Elward K, Gasque P (2003) ‘Eat me’ and ‘don’t eat me’ signals govern the innate immune response and tissue repair in the CNS: emphasis on the critical role of the complement system. Mol Immunol 40:85–94 Emsley HC, Smith CJ, Georgiou RF, Vail A, Hopkins SJ, Rothwell NJ, Tyrrell PJ, Acute Stroke Investigators (2005) A randomised phase II study of interleukin-1 receptor antagonist in acute stroke patients. J Neurol Neurosurg Psychiatry 76:1366–72 Faustin B, Lartigue L, Bruey JM, Luciano F, Sergienko E, Bailly-Maitre B, Volkmann N, Hanein D, Rouiller I, Reed JC (2007) Reconstituted NALP1 inflammasome reveals two-step mechanism of caspase-1 activation. Mol Cell 25:713–24 Favre J, Musette P, Douin-Echinard V, Laude K, Henry JP, Arnal JF, Thuillez C, Richard V (2007) Toll-like receptors 2-deficient mice are protected against postischemic coronary endothelial dysfunction. Arterioscler Thromb Vasc Biol 27:1064–71 Ferrari D, Pizzirani C, Adinolfi E, Lemoli RM, Curti A, Idzko M, Panther E, Di Virgilio F (2006) The P2X7 receptor: a key player in IL-1 processing and release. J Immunol 176:3877–83 Fisher CJ, Jr, Dhainaut JF, Opal SM, Pribble JP, Balk RA, Slotman GJ, Iberti TJ, Rackow EC, Shapiro MJ, Greenman RL et al (1994) Recombinant human interleukin 1 receptor antagonist in the treatment of patients with sepsis syndrome. Results from a randomized, doubleblind, placebo-controlled trial. Phase III rhIL-1Ra Sepsis Syndrome Study Group. JAMA 271:1836–43 Franchi L, Kanneganti TD, Dubyak GR, Nunez G (2007) Differential requirement of P2X7 receptor and intracellular K+ for caspase-1 activation induced by intracellular and extracellular bacteria. J Biol Chem 282: 18810–8 Friedlander RM, Gagliardini V, Hara H, Fink KB, Li W, MacDonald G, Fishman MC, Greenberg AH, Moskowitz MA, Yuan J (1997) Expression of a dominant negative mutant of interleukin-1 beta converting enzyme in transgenic mice prevents neuronal cell death induced by trophic factor withdrawal and ischemic brain injury. J Exp Med 185:933–40 Glantz L, Avramovich A, Trembovler V, Gurvitz V, Kohen R, Eidelman LA, Shohami E (2005) Ischemic preconditioning increases antioxidants in the brain and peripheral organs after cerebral ischemia. Exp Neurol 192: 117–24 Goldbach-Mansky R, Dailey NJ, Canna SW, Gelabert A, Jones J, Rubin BI et al (2006) Neonatal-onset multisystem inflammatory disease responsive to interleukin1beta inhibition. N Engl J Med 355:581–92 Gomez-Pinilla F, Vu L, Cotman CW (1995) Regulation of astrocyte proliferation by FGF-2 and heparan sulfate in vivo. J Neurosci 15:2021–9

Gurcel L, Abrami L, Girardin S, Tschopp, van der Goot (2006) Caspase-1 activation of lipid metabolic pathways in response to bacterial pore-forming toxins promotes cell survival. Cell 126:1135–45 Hao HN, Zhao J, Lotoczky G, Grever WE, Lyman WD (2001) Induction of human beta-defensin-2 expression in human astrocytes by lipopolysaccharide and cytokines. J Neurochem 77:1027–35 Hara H, Fink K, Endres M, Friedlander RM, Gagliardini V, Yuan J, Moskowitz MA (1997b) Attenuation of transient focal cerebral ischemic injury in transgenic mice expressing a mutant ICE inhibitory protein. J Cereb Blood Flow Metab 17:370–5 Hara H, Friedlander RM, Gagliardini V, Ayata C, Fink K, Huang Z, Shimizu-Sasamata M, Yuan J, Moskowitz MA (1997a) Inhibition of interleukin 1beta converting enzyme family proteases reduces ischemic and excitotoxic neuronal damage. Proc Natl Acad Sci USA 94: 2007–12 Hauff K, Zamzow C, Law WJ, De Melo J, Kennedy K, Los M (2005) Peptide-based approaches to treat asthma, arthritis, other autoimmune diseases and pathologies of the central nervous system. Arch Immunol Ther Exp (Warsz) 53:308–20 Hawkins PN, Lachmann HJ, Aganna E, McDermott MF (2004) Spectrum of clinical features in Muckle–Wells syndrome and response to anakinra. Arthritis Rheum 50:607–12 Hoebe K, Georgel P, Rutschmann S, Du X, Mudd S, Crozat K, Sovath S, Shamel L, Hartung T, Zahringer U, Beutler B (2005) CD36 is a sensor of diacylglycerides. Nature 433:523–7 Hoffman HM, Rosengren S, Boyle DL, Cho JY, Nayar J, Mueller JL, Anderson JP, Wanderer AA, Firestein GS (2004) Prevention of cold-associated acute inflammation in familial cold autoinflammatory syndrome by interleukin-1 receptor antagonist. Lancet 364:1779–85 Hoffmann O, Braun JS, Becker D, Halle A, Freyer D, Dagand E, Lehnardt S, Weber JR (2007) Toll-like receptor 2 mediates neuroinflammation and neuronal damage. J Immunol 178:6476–81 Hossmann K-A (2006) Pathophysiology and therapy of experimental stroke. Cell Mol Neurobiol 26:1055–81 Hozawa A, Folsom AR, Ibrahim H, Javier Nieto F, Rosamond WD, Shahar E (2006) Serum uric acid and risk of ischemic stroke: the ARIC Study. Atherosclerosis 187:401–7 Huang J, Kim LJ, Mealey R, Marsh HC, Jr, Zhang Y, Tenner AJ, Connolly ES, Jr, Pinsky DJ (1999) Neuronal protection in stroke by an sLex-glycosylated complement inhibitory protein. Science 285:595–9 Inohara N, Chamaillard M, McDonald C, Nunez G (2005) NOD-LRR proteins: role in host–microbial interactions and inflammatory disease. Annu Rev Biochem 74:355–84 Izuishi K, Tsung A, Jeyabalan G, Critchlow ND, Li J, Tracey KJ, Demarco RA, Lotze MT, Fink MP, Geller DA, Billiar TR (2006) Cutting edge: high-mobility group box 1 preconditioning protects against liver ischemia–reperfusion injury. J Immunol 176:7154–8 Johnston JB, Barrett JW, Nazarian SH, Goodwin M, Ricuttio D, Wang G, McFadden G (2005) A poxvirusencoded pyrin domain protein interacts with ASC-1 to inhibit host inflammatory and apoptotic responses to infection. Immunity 23:587–98 Kahlenberg JM, Dubyak GR (2004) Mechanisms of caspase-1 activation by P2X7 receptor-mediated K+ release. Am J Physiol Cell Physiol 286:C1100–8

877



Kanemitsu H, Tamura A, Kirino T, Karasawa S, Sano K, Iwamoto T, Yoshiura M, Iriyama K (1988) Xanthine and uric acid levels in rat brain following focal ischemia. J Neurochem 51:1882–5 Kang SJ, Wang S, Hara H, Peterson EP, Namura S, AminHanjani S, Huang Z, Srinivasan A, Tomaselli KJ, Thornberry NA, Moskowitz MA, Yuan J (2000) Dual role of caspase-11 in mediating activation of caspase-1 and caspase-3 under pathological conditions. J Cell Biol 149:613–22 Kanneganti TD, Body-Malapel M, Amer A, Park JH, Whitfield J, Taraporewala ZF, Miller D, Patton JT, Inohara N, Nunez G (2006b) Critical role for cryopyrin/Nalp3 in activation of caspase-1 in response to viral infection and double-stranded RNA. J Biol Chem 281:36560–8 Kanneganti TD, Lamkanfi M, Kim YG, Chen G, Park JH, Franchi L, Vandenabeele P, Nunez G (2007) Pannexin1-mediated recognition of bacterial molecules activates the cryopyrin inflammasome independent of Toll-like receptor signaling. Immunity 26:433–43 Kanneganti TD, Ozoren N, Body-Malapel M, Amer A, Park JH, Franchi L, Whitfield J, Barchet W, Colonna M, Vandenabeele P, Bertin J, Coyle A, Grant EP, Akira S, Nunez G (2006a) Bacterial RNA and small antiviral compounds activate caspase-1 through cryopyrin/ Nalp3. Nature 440:233–6 Kariko K, Buckstein M, Ni H, Weissman D (2005) Suppression of RNA recognition by Toll-like receptors: the impact of nucleoside modification and the evolutionary origin of RNA. Immunity 23:165–75 Kariko K, Ni H, Capodici J, Lamphier M, Weissman D (2004b) mRNA is an endogenous ligand for Toll-like receptor 3. J Biol Chem 279:12542–50 Kariko K, Weissman D, Welsh FA (2004a) Inhibition of toll-like receptor and cytokine signaling—a unifying theme in ischemic tolerance. J Cereb Blood Flow Metab 24:1288–304 Khakh BS, North RA (2006) P2X receptors as cellsurface ATP sensors in health and disease. Nature 442: 527–32 Kielian T (2006) Toll-like receptors in central nervous system glial inflammation and homeostasis. J Neurosci Res 83:711–30 Kinoshita T, Wang Y, Hasegawa M, Imamura R, Suda T (2005) PYPAF3, a PYRIN-containing APAF-1-like protein, is a feedback regulator of caspase-1-dependent interleukin-1beta secretion. J Biol Chem 280:21720–5 Kobayashi K, Inohara N, Hernandez LD, Galan JE, Nunez G, Janeway CA, Medzhitov R, Flavell RA (2002) RICK/ Rip2/CARDIAK mediates signaling for receptors of the innate and adaptive immune systems. Nature 416:194–9 Kummer JA, Broekhuizen R, Everett H, Agostini L, Kuijk L, Martinon F, van Bruggen R, Tschopp J (2007) Inflammasome components NALP 1 and 3 show distinct but separate expression profiles in human tissues, suggesting a site-specific role in the inflammatory response. J Histochem Cytochem 55:443–52 Lamkanfi M, Kanneganti TD, Franchi L, Nunez G (2007) Caspase-1 inflammasomes in infection and inflammation. J Leukoc Biol 82:220–5 Le Feuvre RA, Brough D, Touzani O, Rothwell NJ (2003) Role of P2X7 receptors in ischemic and excitotoxic brain injury in vivo. J Cereb Blood Flow Metab 23:381–4 Leadbeater WE, Gonzalez AM, Logaras N, Berry M, Turnbull JE, Logan A (2006) Intracellular trafficking


in neurones and glia of fibroblast growth factor-2, fibroblast growth factor receptor 1 and heparan sulphate proteoglycans in the injured adult rat cerebral cortex. J Neurochem 96:1189–200 Leemans JC, Stokman G, Claessen N, Rouschop KM, Teske GJ, Kirschning CJ, Akira S, van der Poll T, Weening JJ, Florquin S (2005) Renal-associated TLR2 mediates ischemia/reperfusion injury in the kidney. J Clin Invest 115:2894–903 Lehnardt S, Massillon L, Follett P, Jensen FE, Ratan R, Rosenberg PA, Volpe JJ, Vartanian T (2003) Activation of innate immunity in the CNS triggers neurodegeneration through a Toll-like receptor 4-dependent pathway. Proc Natl Acad Sci USA 100:8514–9 Liao Z, Grimshaw RS, Rosenstreich DL (1984) Identification of a specific interleukin 1 inhibitor in the urine of febrile patients. J Exp Med 159:126–36 Liew FY, Xu D, Brint EK, O’Neill LA (2005) Negative regulation of toll-like receptor-mediated immune responses. Nat Rev Immunol 5:446–58 Lin YC, Chang YM, Yu JM, Yen JH, Chang JG, Hu CJ (2005) Toll-like receptor 4 gene C119A but not Asp299Gly polymorphism is associated with ischemic stroke among ethnic Chinese in Taiwan. Atherosclerosis 180:305–9 Liu F, Lo CF, Ning X, Kajkowski EM, Jin M, Chiriac C, Gonzales C, Naureckiene S, Lock YW, Pong K, Zaleska MM, Jacobsen JS, Silverman S, Ozenberger BA (2004) Expression of NALP1 in cerebellar granule neurons stimulates apoptosis. Cell Signal 16:1013–21 Lotze MT, Tracey KJ (2005) High-mobility group box 1 protein (HMGB1): nuclear weapon in the immune arsenal. Nat Rev Immunol 5:331–42 Lucas SM, Rothwell NJ, Gibson RM (2006) The role of inflammation in CNS injury and disease. Br J Pharmacol 147:S232–40 Malathi K, Dong B, Gale M, Jr, Silverman RH (2007) Small self-RNA generated by RNase L amplifies antiviral innate immunity. Nature 448:816–9 Mariathasan S, Monack DM (2007) Inflammasome adaptors and sensors: intracellular regulators of infection and inflammation. Nat Rev Immunol 7:31–40 Mariathasan S, Weiss DS, Dixit VM, Monack DM (2005) Innate immunity against Francisella tularensis is dependent on the ASC/caspase-1 axis. J Exp Med 202:1043–9 Mariathasan S, Weiss DS, Newton K, McBride J, O’rourke K, Roose-Girma M, Lee WP, Weinrauch Y, Monack DM, Dixit VM (2006) Cryopyrin activates the inflammasome in response to toxins and ATP. Nature 440:228–32 Martinon F, Burns K, Tschopp J (2002) The inflammasome: a molecular platform triggering activation of inflammatory caspases and processing of proIL-beta. Mol Cell 10:417–26 Martinon F, Petrilli V, Mayor A, Tardivel A, Tschopp J (2006) Gout-associated uric acid crystals activate the NALP3 inflammasome. Nature 440:237–41 Martinon F, Tschopp J (2004) Inflammatory caspases: linking an intracellular innate immune system to autoinflammatory diseases. Cell 117:561–74 Martinon F, Tschopp J (2007) Inflammatory caspases and inflammasomes: master switches of inflammation. Cell Death Differ 14:10–22 Maslinska D, Laure-Kamionowska M, Maslinski S (2004) Toll-like receptors in rat brains injured by hypoxicischaemia or exposed to staphylococcal alpha-toxin. Folia Neuropathol 42:125–32


Masumoto J, Dowds TA, Schaner P, Chen FF, Ogura Y, Li M, Zhu L, Katsuyama T, Sagara J, Taniguchi S, Gumucio DL, Nunez G, Inohara N (2003) ASC is an activating adaptor for NF-kappa B and caspase-8dependent apoptosis. Biochem Biophys Res Commun 303:69–73 Masumoto J, Taniguchi S, Ayukawa K, Sarvotham H, Kishino T, Niikawa N, Hidaka E, Katsuyama T, Higuchi T, Sagara J (1999) ASC, a novel 22-kDa protein, aggregates during apoptosis of human promyelocytic leukemia HL-60 cells. J Biol Chem 274:33835–8 Masumoto J, Taniguchi S, Nakayama K, Ayukawa K, Sagara J (2001a) Murine ortholog of ASC, a CARDcontaining protein, self-associates and exhibits restricted distribution in developing mouse embryos. Exp Cell Res 262:128–33 Masumoto J, Taniguchi S, Nakayama J, Shiohara M, Hidaka E, Katsuyama T, Murase S, Sagara J (2001b) Expression of apoptosis-associated speck-like protein containing a caspase recruitment domain, a pyrin N-terminal homology domain-containing protein, in normal human tissues. J Histochem Cytochem 49: 1269–75 Matzinger P (2002) The danger model: a renewed sense of self. Science 296:301–5 Matzinger P (2007) Friendly and dangerous signals: is the tissue in control? Nat Immunol 8:11–3 Mayor A, Martinon F, De Smedt T, Petrilli V, Tschopp J (2007) A crucial function of SGT1 and HSP90 in inflammasome activity links mammalian and plant innate immune responses. Nat Immunol 8:497–503 McKimmie CS, Johnson N, Fooks AR, Fazakerley JK (2005) Viruses selectively upregulate Toll-like receptors in the central nervous system. Biochem Biophys Res Commun 336:925–33 Meylan E, Tschopp J, Karin M (2006) Intracellular pattern recognition receptors in the host response. Nature 442:39–44 Miao EA, Alpuche-Aranda CM, Dors M, Clark AE, Bader MW, Miller SI, Aderem A (2006) Cytoplasmic flagellin activates caspase-1 and secretion of interleukin 1beta via Ipaf. Nat Immunol 7:569–75 Miggin SM, Palsson-McDermott E, Dunne A, Jefferies C, Pinteaux E, Banahan K, Murphy C, Moynagh P, Yamamoto M, Akira S, Rothwell N, Golenbock D, Fitzgerald KA, O’Neill LA (2007) NF-kappaB activation by the Toll-IL-1 receptor domain protein MyD88 adapter-like is regulated by caspase-1. Proc Natl Acad Sci USA 104:3372–7 Mishra BB, Mishra PK, Teale JM (2006) Expression and distribution of Toll-like receptors in the brain during murine neurocysticercosis. J Neuroimmunol 181:46–56 Mitchell JA, Paul-Clark MJ, Clarke GW, McMaster SK, Cartwright N (2007) Critical role of toll-like receptors and nucleotide oligomerisation domain in the regulation of health and disease. J Endocrinol 193:323–30 Naiki Y, Michelsen KS, Zhang W, Chen S, Doherty TM, Arditi M (2005) Transforming growth factor-beta differentially inhibits MyD88-dependent, but not TRAMand TRIF-dependent, lipopolysaccharide-induced TLR4 signaling. J Biol Chem 280:5491–5 Netea MG, Ferwerda G, de Jong DJ, Jansen T, Jacobs L, Kramer M, Naber TH, Drenth JP, Girardin SE, Kullberg BJ, Adema GJ, Van der Meer JW (2005) Nucleotide-binding oligomerization domain-2 modulates specific TLR pathways for the induction of cytokine release. J Immunol 174:6518–23

Nishimura M, Naito S (2005) Tissue-specific mRNA expression profiles of human toll-like receptors and related genes. Biol Pharm Bull 28:886–92 O’Connor W, Jr, Harton JA, Zhu X, Linhoff MW, Ting JP (2003) Cutting edge: CIAS1/cryopyrin/PYPAF1/ NALP3/CATERPILLER 1.1 is an inducible inflammatory mediator with NF-kappa B suppressive properties. J Immunol 171:6329–33 Ogura Y, Bonen DK, Inohara N, Nicolae DL, Chen FF, Ramos R, Britton H, Moran T, Karaliuskas R, Duerr RH, Achkar JP, Brant SR, Bayless TM, Kirschner BS, Hanauer SB, Nunez G, Cho JH (2001) A frameshift mutation in NOD2 associated with susceptibility to Crohn’s disease. Nature 411:603–6 Ogura Y, Sutterwala FS, Flavell RA (2006) The inflammasome: first line of the immune response to cell stress. Cell 126:659–62 Ohtsuka T, Ryu H, Minamishima YA, Macip S, Sagara J, Nakayama KI, Aaronson SA, Lee SW (2004) ASC is a Bax adaptor and regulates the p53–Bax mitochondrial apoptosis pathway. Nat Cell Biol 6:121–8 Orihuela CJ, Fillon S, Smith-Sielicki SH, El Kasmi KC, Gao G, Soulis K, Patil A, Murray PJ, Tuomanen EI (2006) Cell wall-mediated neuronal damage in early sepsis. Infect Immun 74:3783–9 Orlova VV, Choi EY, Xie C, Chavakis E, Bierhaus A, Ihanus E, Ballantyne CM, Gahmberg CG, Bianchi ME, Nawroth PP, Chavakis T (2007) A novel pathway of HMGB1-mediated inflammatory cell recruitment that requires Mac-1integrin. EMBO J 26:1129–39 ¨ zo¨ren N, Masumoto J, Franchi L, Kanneganti TD, BodyO Malapel M, Erturk I, Jagirdar R, Zhu L, Inohara N, Bertin J, Coyle A, Grant EP, Nunez G (2006) Distinct roles of TLR2 and the adaptor ASC in IL-1beta/IL-18 secretion in response to Listeria monocytogenes. J Immunol 176:4337–42 Park HH, Lo YC, Lin SC, Wang L, Yang JK, Wu H (2007a) The death domain superfamily in intracellular signaling of apoptosis and inflammation. Annu Rev Immunol 25:561–86 Park JH, Kim YG, McDonald C, Kanneganti TD, Hasegawa M, Body-Malapel M, Inohara N, Nunez G (2007b) RICK/ RIP2 mediates innate immune responses induced through Nod1 and Nod2 but not TLRs. J Immunol 178: 2380–6 Park JS, Svetkauskaite D, He Q, Kim JY, Strassheim D, Ishizaka A, Abraham E (2004) Involvement of toll-like receptors 2 and 4 in cellular activation by high mobility group box 1 protein. J Biol Chem 279:7370–7 Petrilli V, Papin S, Dostert C, Mayor A, Martinon F, Tschopp J (2007) Activation of the NALP3 inflammasome is triggered by low intracellular potassium concentration. Cell Death Differ 14:1583–9 Pinteaux E, Rothwell NJ, Boutin H (2006) Neuroprotective actions of endogenous interleukin-1 receptor antagonist (IL-1Ra) are mediated by glia. Glia 53:551–6 Powers KA, Szaszi K, Khadaroo RG, Tawadros PS, Marshall JC, Kapus A, Rotstein OD (2006) Oxidative stress generated by hemorrhagic shock recruits Toll-like receptor 4 to the plasma membrane in macrophages. J Exp Med 203:1951–61 Poyet JL, Srinivasula SM, Tnani M, Razmara M, Fernandes-Alnemri T, Alnemri ES (2001) Identification of Ipaf, a human caspase-1-activating protein related to Apaf-1. J Biol Chem 276:28309–13 Prass K, Meisel C, Hoflich C, Braun J, Halle E, Wolf T, Ruscher K, Victorov IV, Priller J, Dirnagl U, Volk HD,

879



Meisel A (2003) Stroke-induced immunodeficiency promotes spontaneous bacterial infections and is mediated by sympathetic activation reversal by poststroke T helper cell type 1-like immunostimulation. J Exp Med 198:725–36 Prins RM, Craft N, Bruhn KW, Khan-Farooqi H, Koya RC, Stripecke R, Miller JF, Liau LM (2006) The TLR-7 agonist, imiquimod, enhances dendritic cell survival and promotes tumor antigen-specific T cell priming: relation to central nervous system antitumor immunity. J Immunol 176:157–64 Qiu J, Nishimura M, Wang Y, Sims JR, Qiu S, Savitz SI, Salomone S, Moskowitz MA (2007) Early release of HMGB-1 from neurons after the onset of brain ischemia. J Cereb Blood Flow Metab (in press) Richards N, Schaner P, Diaz A, Stuckey J, Shelden E, Wadhwa A, Gumucio DL (2001) Interaction between pyrin and the apoptotic speck protein (ASC) modulates ASC-induced apoptosis. J Biol Chem 276:39320–9 Rolls A, Shechter R, London A, Ziv Y, Ronen A, Levy R, Schwartz M (2007) Toll-like receptors modulate adult hippocampal neurogenesis. Nat Cell Biol 9:1081–9 Romanos E, Planas AM, Amaro S, Chamorro A (2007) Uric acid reduces brain damage and improves the benefits of rt-PA in a rat model of thromboembolic stroke. J Cereb Blood Flow Metab 27:14–20 Rosenstiel P, Huse K, Till A, Hampe J, Hellmig S, Sina C, Billmann S, von Kampen O, Waetzig GH, Platzer M, Seegert D, Schreiber S (2006) A short isoform of NOD2/ CARD15, NOD2-S, is an endogenous inhibitor of NOD2/receptor-interacting protein kinase 2-induced signaling pathways. Proc Natl Acad Sci USA 103: 3280–5 Sakata Y, Dong JW, Vallejo JG, Huang CH, Baker JS, Tracey KJ, Tacheuchi O, Akira S, Mann DL (2007) Toll-like receptor 2 modulates left ventricular function following ischemia– reperfusion injury. Am J Physiol Heart Circ Physiol 292:H503–9 Saleh M (2006) Caspase-1 builds a new barrier to infection. Cell 126:1028–30 Saleh M, Mathison JC, Wolinski MK, Bensinger SJ, Fitzgerald P, Droin N, Ulevitch RJ, Green DR, Nicholson DW (2006) Enhanced bacterial clearance and sepsis resistance in caspase-12-deficient mice. Nature 440: 1064–8 Scaffidi P, Misteli T, Bianchi ME (2002) Release of chromatin protein HMGB1 by necrotic cells triggers inflammation. Nature 418:191–5 Schielke GP, Yang GY, Shivers BD, Betz AL (1998) Reduced ischemic brain injury in interleukin-1 beta converting enzyme-deficient mice. J Cereb Blood Flow Metab 18:180–5 Scott P, Ma H, Viriyakosol S, Terkeltaub R, Liu-Bryan R (2006) Engagement of CD14 mediates the inflammatory potential of monosodium urate crystals. J Immunol 177: 6370–8 Seong SY, Matzinger P (2004) Hydrophobicity: an ancient damage-associated molecular pattern that initiates innate immune responses. Nat Rev Immunol 4:469–78 Shi Y, Evans JE, Rock KL (2003) Molecular identification of a danger signal that alerts the immune system to dying cells. Nature 425:516–21 Shiohara M, Taniguchi S, Masumoto J, Yasui K, Koike K, Komiyama A, Sagara J (2002) ASC, which is composed of a PYD and a CARD, is up-regulated by inflammation and apoptosis in human neutrophils. Biochem Biophys Res Commun 293:1314–8


So A, De Smedt T, Revaz S, Tschopp J (2007) A pilot study of IL-1 inhibition by anakinra in acute gout. Arthritis Res Ther 9:R28 Solle M, Labasi J, Perregaux DG, Stam E, Petrushova N, Koller BH, Griffiths RJ, Gabel CA (2001) Altered cytokine production in mice lacking P2X(7) receptors. J Biol Chem 276:125–32 Somjen GG (2001) Mechanisms of spreading depression and hypoxic spreading depression-like depolarization. Physiol Rev 81:1065–96 Stahel PF, Morganti-Kossmann MC, Kossmann T (1998) The role of the complement system in traumatic brain injury. Brain Res Rev 27:243–56 Stehlik C, Dorfleutner A (2007) COPs and POPs: modulators of inflammasome activity. J Immunol 179:7993–8 Stehlik C, Fiorentino L, Dorfleutner A, Bruey JM, Ariza EM, Sagara J, Reed JC (2002) The PAAD/PYRIN-family protein ASC is a dual regulator of a conserved step in nuclear factor kappaB activation pathways. J Exp Med 196:1605–15 Stehlik C, Krajewska M, Welsh K, Krajewski S, Godzik A, Reed JC (2003) The PAAD/PYRIN-only protein POP1/ ASC2 is a modulator of ASC-mediated nuclear-factorkappa B and pro-caspase-1 regulation. Biochem J 373:101–13 Stenzel-Poore MP, Stevens SL, King JS, Simon RP (2007) Preconditioning reprograms the response to ischemic injury and primes the emergence of unique endogenous neuroprotective phenotypes: a speculative synthesis. Stroke 38:680–5 Sterka D, Jr, Rati DM, Marriott I (2006) Functional expression of NOD2, a novel pattern recognition receptor for bacterial motifs, in primary murine astrocytes. Glia 53:322–30 Stevens SL, Stenzel-Poore MP (2006) Toll-like receptors and tolerance to ischaemic injury in the brain. Biochem Soc Trans 34:1352–5 Strober W, Murray PJ, Kitani A, Watanabe T (2006) Signaling pathways and molecular interactions of NOD1 and NOD2. Nat Rev Immunol 6:9–20 Sutterwala FS, Ogura Y, Flavell RA (2007) The inflammasome in pathogen recognition and inflammation. J Leukoc Biol 82:259–64 Svensson PA, Asea A, Englund MC, Bausero MA, Jernas M, Wiklund O, Ohlsson BG, Carlsson LM, Carlsson B (2006) Major role of HSP70 as a paracrine inducer of cytokine production in human oxidized LDL treated macrophages. Atherosclerosis 185:32–8 Swanson MS, Molofsky AB (2005) Autophagy and inflammatory cell death, partners of innate immunity. Autophagy 1:174–6 Tang SC, Arumugam TV, Xu X, Cheng A, Mughal MR, Jo DG, Lathia JD, Siler DA, Chigurupati S, Ouyang X, Magnus T, Camandola S, Mattson MP (2007) Pivotal role for neuronal Toll-like receptors in ischemic brain injury and functional deficits. Proc Natl Acad Sci USA 104:13798–803 Tate CC, Tate MC, LaPlaca MC (2007) Fibronectin and laminin increase in the mouse brain after controlled cortical impact injury. J Neurotrauma 24:226–30 Taxman DJ, Zhang J, Champagne C, Bergstralh DT, Iocca HA, Lich JD, Ting JP (2006) Cutting edge: ASC mediates the induction of multiple cytokines by Porphyromonas gingivalis via caspase-1-dependent and -independent pathways. J Immunol 177:4252–6 Thome M, Hofmann K, Burns K, Martinon F, Bodmer JL, Mattmann C, Tschopp J (1998) Identification of


CARDIAK, a RIP-like kinase that associates with caspase-1. Curr Biol 8:885–8 Thornberry NA, Bull HG, Calaycay JR, Chapman KT, Howard AD, Kostura MJ, Miller DK, Molineaux SM, Weidner JR, Aunins J et al (1992) A novel heterodimeric cysteine protease is required for interleukin-1 beta processing in monocytes. Nature 356:768–74 Ting JP, Kastner DL, Hoffman HM (2006) CATERPILLERs, pyrin and hereditary immunological disorders. Nat Rev Immunol 6:183–95 Trendelenburg G, Dirnagl U (2005) Neuroprotective role of astrocytes in cerebral ischemia: focus on ischemic preconditioning. Glia 50:307–20 Tsung A, Sahai R, Tanaka H, Nakao A, Fink MP, Lotze MT, Yang H, Li J, Tracey KJ, Geller DA, Billiar TR (2005) The nuclear factor HMGB1 mediates hepatic injury after murine liver ischemia–reperfusion. J Exp Med 201: 1135–43 Vabulas RM, Ahmad-Nejad P, Ghose S, Kirschning CJ, Issels RD, Wagner H (2002) HSP70 as endogenous stimulus of the Toll/interleukin-1 receptor signal pathway. J Biol Chem 277:15107–12 Van Beek J, Elward K, Gasque P (2003) Activation of complement in the central nervous system. Ann NY Acad Sci 992:56–71 Vogl T, Tenbrock K, Ludwig S, Leukert N, Ehrhardt C, van Zoelen MA, Nacken W, Foell D, van der Poll T, Sorg C, Roth J (2007) Mrp8 and Mrp14 are endogenous activators of Toll-like receptor 4, promoting lethal, endotoxin-induced shock. Nat Med 13:1042–9 Wald D, Qin J, Zhao Z, Qian Y, Naramura M, Tian L, Towne J, Sims JE, Stark GR, Li X (2003) SIGIRR, a negative regulator of Toll-like receptor–interleukin 1 receptor signaling. Nat Immunol 4:920–7 Wang S, Miura M, Jung YK, Zhu H, Li E, Yuan J (1998) Murine caspase-11, an ICE-interacting protease, is essential for the activation of ICE. Cell 92:501–9 Wang X, Yue TL, Young PR, Barone FC, Feuerstein GZ (1995) Expression of interleukin-6, c-fos, and zif268 mRNAs in rat ischemic cortex. J Cereb Blood Flow Metab 15:166–71 Wang Y, Hasegawa M, Imamura R, Kinoshita T, Kondo C, Konaka K, Suda T (2004) PYNOD, a novel Apaf-1/ CED4-like protein is an inhibitor of ASC and caspase-1. Int Immunol 16:777–86

Watanabe T, Kitani A, Murray PJ, Strober W (2004) NOD2 is a negative regulator of Toll-like receptor 2-mediated T helper type 1 responses. Nat Immunol 5:800–8 Weir CJ, Muir SW, Walters MR, Lees KR (2003) Serum urate as an independent predictor of poor outcome and future vascular events after acute stroke. Stroke 34: 1951–6 Xiong ZQ, Qian W, Suzuki K, McNamara JO (2003) Formation of complement membrane attack complex in mammalian cerebral cortex evokes seizures and neurodegeneration. J Neurosci 23:955–60 Yanqing Z, Yu-Min L, Jian Q, Bao-Guo X, Chuan-Zhen L (2006) Fibronectin and neuroprotective effect of granulocyte colony-stimulating factor in focal cerebral ischemia. Brain Res 1098:161–9 Yoo NJ, Park WS, Kim SY, Reed JC, Son SG, Lee JY, Lee SH (2002) Nod1, a CARD protein, enhances pro-interleukin-1beta processing through the interaction with pro-caspase-1. Biochem Biophys Res Commun 299: 652–8 Yu M, Wang H, Ding A, Golenbock DT, Latz E, Czura CJ, Fenton MJ, Tracey KJ, Yang H (2006) HMGB1 signals through toll-like receptor (TLR) 4 and TLR2. Shock 26:174–9 Zedler S, Faist E (2006) The impact of endogenous triggers on trauma-associated inflammation. Curr Opin Crit Care 12:595–601 Zhang WH, Wang X, Narayanan M, Zhang Y, Huo C, Reed JC, Friedlander RM (2003) Fundamental role of the Rip2/caspase-1 pathway in hypoxia and ischemiainduced neuronal cell death. Proc Natl Acad Sci USA 100:16012–7 Zhang X, Kimura Y, Fang C, Zhou L, Sfyroera G, Lambris JD, Wetsel RA, Miwa T, Song WC (2007) Regulation of Toll-like receptor-mediated inflammatory response by complement in vivo. Blood 110:228–36 Zhao TY, Zou SP, Knapp PE (2007) Exposure to cell phone radiation up-regulates apoptosis genes in primary cultures of neurons and astrocytes. Neurosci Lett 412:34–8 Ziegler G, Harhausen D, Prinz V, Ko¨nig J, Schepers C, Ro¨hr C, Hoffmann O, Lehrach H, Nietfeld W, Trendelenburg G (2007) TLR has a detrimental role in mouse transient focal cerebral ischemia. Biochem Biophys Res Commun 359: 574–9

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