Alzheimer's APP mangles mitochondria - Nature

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NEWS AND VIEWS PGC-1α opposite in the two striatal cell types, neurons and interneurons? Will PGC-1α also be implicated in the effects of mutant huntingtin in other neurons in the brain? Finally, the key question is: to what extent will these observations from disease models reflect the pathophysiology of human Huntington disease? Patients with Huntington disease have lower body weight than expected13, and have been reported to have increased prevalence of diabetes, though the prevalence, specificity and potential mechanisms are unknown. The new findings might suggest an involvement of PGC-1α in these symptoms. The current results may also have implications for developing approaches to treat Huntington disease, which remains without effective treatments. Several compounds already in clinical trials act directly on mitochondria, including coenzyme Q10 and creatine. In addition, several agents that may act at the level of transcription are in clinical trials or under study for clinical trials. These

include phenylbutyrate and suberoylanilide hydroxamic acid (SAHA), which act by inhibiting histone deactylases—enzymes that counteract the effect of histone acetylases such as CBP and P300, and can thus restore decreased transcription. Other therapeutic approaches may be suggested by the biology of PGC-1α, which interacts with a variety of other proteins including sirtuins (NAD+dependent histone deacetylases that are the target of extensive drug development). It may also be possible to target PPAR, which is a transcription factor that interacts with PGC-1α, and is activated by intracellular metabolic changes. Eicosapentanoic acid is a natural ligand for PPAR and ethyl eicosapentanoic acid (ethyl EPA) is already in therapeutic trials for symptomatic treatment of Huntington disease—but the compound might now also be tested for neuroprotective effects. Furthermore, many drugs used to treat diabetes and other medical conditions target isoforms of PPAR. It is conceivable that some of these may be of benefit in Huntington disease.

The current findings may have implications for therapeutics as well as for unifying our postulated mechanisms of pathogenesis. In addition they reinforce the central place of mitochondria and oxidative stress in neurodegenerative disease in general, now with a molecular basis involving transcription and intracellular communication. 1. Cui, L. et al. Cell 127, 59–69 (2006). 2. Weydt, P. et al. Cell Metab. published online 19 October 2006 (doi: 10.1016/jcmet.2006.10.004). 3. St-Pierre, J. et al. Cell 127, 397–408 (2006). 4. Beal, M.F. Ann. Neurol. 58, 495–505 (2005). 5. Ross, C.A. Neuron 35, 819–822 (2002). 6. Sugars, K.L. & Rubinsztein, D.C. Trends Genet. 19, 233–238 (2003). 7. Handschin, C. & Spiegelman, B.M. Endocr. Rev., published online 3 October 2006 (doi:10.1210/ er.2006-0037). 8. Lin, J. et al. Cell 119, 121–135 (2004). 9. Mantamadiotis, T. et al. Nat. Genet. 31, 47–54 (2002). 10. Bae, B.I. et al. Neuron 47, 29–41 (2005). 11. Cattaneo, E., Zuccato, C. & Tartari, M. Nat. Rev. Neurosci. 6, 919–930 (2005). 12. Nonomura, T. et al. Int. J. Exp. Diabetes Res. 2, 201–209 (2001). 13. Djousse, L. et al. Neurology 59, 1325–1330 (2002).

Alzheimer’s APP mangles mitochondria Michael T Lin & M Flint Beal New findings in humans examine how mitochondrial function declines during Alzheimer disease. Although the amyloid plaques characteristic of Alzheimer disease consist of extracellular aggregates of the toxic amyloid-β peptide, researchers are increasingly recognizing that amyloid species may exert toxicity from within the cell1. Well before plaques are observed, intracellular aggregates of amyloid-β form early in mice overexpressing amyloid precursor protein (APP), which gives rise to amyloid-β. These aggregates seem to congregate in synaptic compartments and are the best pathologic correlate of cognitive impairment2. How intracellular amyloid might cause cellular dysfunction remains unclear, though some sites of action have been identified. For example, toxic amyloid-β oligomers accumulate within late endosomal multivesicular bodies and can be linked with alterations in endocytic function, ubiquitin-proteasome activity and synaptic receptor levels3. The authors are in the Department of Neurology and Neuroscience, Weill Medical College of Cornell University, 525 East 68th Street, F-6, New York, New York 10021, USA. E-mail: [email protected]

In a recent study4, Anandatheerthavarada et al. have carried the idea of intracellular amyloid toxicity further, and with two twists. They link amyloid to another intracellular organelle—the mitochondrion—which has not yet been widely recognized as a site of amyloid accumulation or toxicity. Moreover, they focus on APP as the primary culprit, even though its amyloid-β cleavage product is much better known to exert toxicity. Previous work by this group5 had shown that APP carries a dual leader sequence, permitting targeting to the endoplasmic reticulum (ER) or to mitochondria (Fig. 1). When overexpressed in cultured cells, APP was found in mitochondrially enriched cell fractions, and APP immunoreactivity was seen in mitochondria by immunoelectron microscopy. This mitochondrial association was not seen if the leader sequence was mutated. Using chemical cross-linkers, they showed that APP was in contact with the mitochondrial protein importation machinery. However, a large acidic domain spanning APP residues 220–290 caused APP to become ‘stuck’ during importation, with

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the amino terminus inside and the carboxy terminus outside. Accumulation of this transmembrane-arrested APP was associated with reduced cytochrome oxidase activity, decreased ATP synthesis and loss of the mitochondrial membrane potential. When the acidic APP220–290 domain was deleted, transmembrane arrest did not occur and mitochondrial function was not impaired. These findings were intriguing but it remained possible that they were an artifact of overexpressing APP and not necessarily applicable to human disease. In the current work, the authors extend their results to post-mortem brain samples from human Alzheimer disease and control subjects. They found that nonglycosylated full-length and C-terminally-truncated APP was associated with mitochondria in samples from the brains of individuals with Alzheimer disease, but not with mitochondria in samples from subjects without the disease. Moreover, within Alzheimer disease brain samples, levels of mitochondrial APP were higher in more affected brain areas and in subjects with more advanced disease.

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Without resorting to chemical cross-linking, the authors showed, using both blue native gels and immunoelectron microscopy, that APP was stably associated with two components of the mitochondrial protein translocation machinery, TOM40 and TIM23 (translocases of the outer and inner membranes), again suggesting that APP ‘clogs’ this machinery. Consistent with clogging the protein importation machinery, higher mitochondrial APP levels were associated with decreased importation of respiratory chain subunits in vitro, decreased cytochrome oxidase activity, increased H2O2 generation and impaired mitochondrial reducing capacity. The data from Anandatheerthavarada and colleagues4,5 provide a potential explanation for the well-established observation that mitochondrial function and energy metabolism are impaired early in Alzheimer disease6,7. Their data on mitochondrial APP also complement a small but growing literature that mitochondria may interact with factors involved in amyloid-β metabolism. Several groups have found that amyloid-β interacts with mitochondria, impairing mitochondrial function and increasing free radical generation8–11. Functional complexes with γ-secretase activity, which is essential to cleave APP and create amyloid-β, have been found in mitochondria12. Insulin degrading enzyme (IDE), which is important for amyloid-β removal, can be targeted to mitochondria by alternative translation initiation13. The presequence peptidase PreP, which is localized to the mitochondrial matrix and is responsible for degrading presequences and other short peptides, can also degrade amyloid-β (ref. 14). At the same time, the new data raise questions. First, what is the relationship between mitochondrial involvement in amyloid metabolism and the other (nonmitochondrial) subcellular abnormalities well documented in Alzheimer disease? Is the mitochondrial involvement primary or secondary to these other processes? In particular, what process causes APP to get imported (and stuck) in mitochondria in Alzheimer disease, or prevents its importation in controls? Phosphorylation of the leader sequence has been reported to affect targeting in other examples of proteins with dual ER-mitochondria leader sequences15, but this effect has not been reported for APP. Second, what, if any, are the normal functions of APP within mitochondria? In particular, how is it that APP should have both a mitochondrial targeting sequence as well as a large domain that prevents its importation?

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Figure 1 Mitochondria and Alzheimer disease. Amyloid precursor protein (APP) has a dual leader sequence, permitting targeting to the endoplasmic reticulum or to mitochondria. The targeting of APP to mitochondria may be an Alzheimer disease–specific process: mitochondrial localization of APP occurs only in disease subjects and only in affected brain areas, and mitochondrial APP levels increase with disease severity. Mitochondrial APP forms complexes with the protein importation translocases of the outer and inner membranes (TOM and TIM). However, a stretch of acidic residues prevents the importation of APP, which is arrested in an amino-terminus-in/carboxy-terminus-out position. Importation of respiratory chain subunits and other mitochondrial proteins is reduced, presumably because the importation machinery is clogged by the transmembrane-arrested APP. This reduced importation is associated with decreased activity of respiratory chain enzymes, increased free radical generation and impaired reducing capacity. Other aspects of amyloid metabolism may also involve mitochondria. Active γ-secretase complexes, which are involved in cleaving APP to produce amyloid-β, have been found in mitochondria, though it is not clear that this can generate intramitochondrial amyloid-β, given the C-terminus-out orientation of APP. Nonetheless, amyloid-β has been found in mitochondria, interacting with amyloid-β–binding alcohol dehydrogenase (ABAD) and producing reactive oxygen species (ROS). Amyloid-β also inhibits cytochrome oxidase and α-ketoglutarate dehydrogenase activities (not shown), enzymes known to have decreased activity in the brains of subjects with Alzheimer disease. Amyloid-β degrading enzymes, such as insulin degrading enzyme (IDE) and presequence peptidase (PreP), are also found in mitochondria. C-IV, complex IV.

The authors speculate that APP might be imported normally, without transmembrane arrest, depending on whether metal ions complex with the acidic stretch. However, no APP was found in mitochondria from normal control brains. Third, what exactly is the relationship between APP and amyloid-β with respect to toxicity in mitochondria? Does APP actually undergo processing within mitochondria? Cleavage of mitochondrial APP to form amyloid-β could provide an explanation for the reports that amyloid-β is found in mitochondria, and would be consistent

with the presence of amyloid-β–producing (γ-secretase) and amyloid-β–degrading (IDE and PreP) enzymes within mitochondria. However, the orientation of amyloid-β in the membrane, with the N terminus in and the C terminus out, suggests that amyloid-β formed by such cleavage would not be intramitochondrial (Fig. 1). A great deal remains to be unraveled, but it is clear that in addition to forming extracellular aggregates, amyloid-β, or its precursor APP, has complicated intracellular effects involving a variety of subcellular organelles, including mitochondria.

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Ann Thomson


NEWS AND VIEWS

NEWS AND VIEWS 6. Parker, W.D. Jr., Parks, J., Filley, C.M. & KleinschmidtDeMasters, B.K. Neurology 44, 1090–1096 (1994). 7. Hirai, K. et al. J. Neurosci. 21, 3017–3023 (2001). 8. Casley, C.S., Canevari, L., Land, J.M., Clark, J.B. & Sharpe, M.A. J. Neurochem. 80, 91–100 (2002). 9. Lustbader, J.W. et al. Science 304, 448–452 (2004). 10. Crouch, P.J. et al. J. Neurosci. 25, 672–679 (2005).

11. Manczak, M. et al. Hum. Mol. Genet. 15, 1437–1449 (2006). 12. Hansson, C.A. et al. J. Biol. Chem. 279, 51654– 51660 (2004). 13. Leissring, M.A. et al. Biochem. J. 383, 439–446 (2004). 14. Falkevall, A. et al. J. Biol. Chem. 281, 29096–29104 (2006). 15. Robin, M.A. et al. J. Biol. Chem. 277, 40583–40593 (2002).

Feeling pain? Who’s your daddy… Gavril W Pasternak & Charles E Inturrisi A new modulator of pain comes to light in studies of rats and people (pages 1269–1277).

The most feared consequence of pain from disease or trauma is the possibility that it will become chronic, persisting long beyond the normal recovery time. However, not every herniated disc leads to chronic back pain and not every patient with herpes zoster infection develops postherpetic neuralgia. Recent animal and human studies indicate that genetic factors may predispose subsets of individuals to a greater likelihood of developing chronic pain—a difficult area to study owing to the wide range of potential mechanisms involved. In this issue, Woolf et al.1 home in on a factor that may be common to multiple modes of pain transmission—tetrahydrobiopterin (BH4). They found that BH4 has a role in neuropathic and inflammatory pain in animal models, and provide intriguing evidence that it may also operate in people to help define sensitivity to pain. The observations are quite unexpected as BH4 is not a neurotransmitter but a cofactor important for a number of enzymes, including aromatic acid hydrolase, which is involved in the synthesis of catecholamines and serotonin, and nitric oxide (NO) synthase. The identification of BH4 therefore provides a common factor influencing neurotransmitters involved in increasing pain

Gavril W. Pasternak is in the Department of Neurology and the Molecular Pharmacology and Chemistry Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, New York 10021, USA. Charles E. Inturrisi is in the Departments of Pharmacology and Neurology & Neuroscience, Weill College of Medicine of Cornell University, 1300 York Avenue, New York, New York 10021, USA. E-mail: [email protected]

transmission in the central nervous system (CNS) (Fig. 1). Although a common sensation, pain is complex, and of several different types. Among them, neuropathic pain is unique2–4; patients find it hard to describe and clinicians find it hard to treat. Neuropathic pain is typically seen following nerve injury as in neuropathies, plexopathies and trauma, and following injury to selected sites within the CNS (for example, thalamic pain). Three hallmarks of neuropathic pain are spontaneous pain, allodynia and hypersen-

sitivity. Spontaneous pain is unprovoked and has a variety of descriptors, including lancinating, stabbing or electric shock–like. With allodynia, patients feel pain from sensory stimuli that do not normally cause pain. Hyperalgesia, on the other hand, denotes an exaggerated perception of pain to a noxious stimulus. Each of these pains can persist long after the injury. These aspects illustrate the complexity of the syndrome and the role of neuronal plasticity in its initiation and propagation. Neuropathic pain is only modestly sensitive to opioids and is typically

Injury

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Kim Caesar


1. Gouras, G.K. et al. Am. J. Pathol. 156, 15–20 (2000). 2. Oddo, S. et al. Neuron 39, 409–421 (2003). 3. Almeida, C.G., Takahashi, R.H. & Gouras, G.K. J. Neurosci. 26, 4277–4288 (2006). 4. Devi, L., Prabhu, B.M., Galati, D.F., Avadhani, N.G. & Anandatheerthavarada, H.K. J. Neurosci. 26, 9057– 9068 (2006). 5. Anandatheerthavarada, H.K., Biswas, G., Robin, M.A. & Avadhani, N.G. J. Cell Biol. 161, 41–54 (2003).

Figure 1 Role of BH4 in modulating pain systems. Pain induces the upregulation of GTP cyclohydrolase (GCH1), an early rate-limiting step in the synthesis of tetrahydrobiopterin (BH4). BH4 modulates several enzymes, including neuronal nitric oxide synthase (nNOS), which generates nitric oxide (NO), and aromatic amino acid hydrolase, which is involved in the synthesis of the catecholamines and serotonin. nNOS is also under the control of NMDA (N-methyl-D-aspartate) receptors through their regulation of intracellular calcium ions.

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