Journal of Neuroscience Research 90:1–12 (2012)
Review Role of microRNAs in Central Nervous System Development and Pathology Karla F. Meza-Sosa,1 David Valle-Garcı´a,2 Gustavo Pedraza-Alva,1 and Leonor Pe´rez-Martı´nez1* 1 Departamento de Medicina Molecular y Bioprocesos, Instituto de Biotecnologı´a, Universidad Nacional Auto´noma de Me´xico (UNAM), Cuernavaca, Morelos, Me´xico 2 Departamento de Gene´tica Molecular, Instituto de Fisiologı´a Celular, Universidad Nacional Auto´noma de Me´xico, Me´xico, D.F., Me´xico
Gene expression regulation is essential for correct functioning of the cell. Complex processes such as development, apoptosis, cell differentiation, and cell cycling require a fine tuning of gene expression. MicroRNAs (miRNAs) are small RNAs that have been recognized as key components of the gene expression regulatory machinery. By sequence complementarity, miRNAs recognize target mRNAs and inhibit their function through degradation or by repressing their translation. The development of the central nervous system (CNS) requires precise and exquisitely regulated gene expression patterns. It is now widely recognized that miRNAs have the capacity to provide such fine regulation both in time and in space. High-throughput analyses as well as classical molecular biology approaches have allowed the identification of essential miRNAs for CNS development and function. Moreover, recent studies in several model organisms are beginning to show intricate regulatory networks involving miRNAs, transcription factors, and epigenetic regulators during CNS development. Here we review recent findings on the role that miRNAs play in the development of the CNS as well as in neuropathologies such as schizophrenia, Parkinson disease, and Alzheimer’s disease, among others. VC 2011 Wiley Periodicals, Inc. Key words: miRNA; small RNA; neurodevelopment; neuropathology
gene
regulation;
The generation of a particular cell type requires a specific and precise spatiotemporal control of gene expression at different levels. Recently, it has been demonstrated that, in addition to transcriptional regulation, the posttranscriptional level is also crucial for regulating gene expression and that microRNAs (miRNAs) are important players in this process (Fiore et al., 2008; Ivey and Srivastava, 2010). miRNAs are endogenous, small RNA molecules with a length of 19– 21 base pairs (bp). They are present in a variety of organisms, including plants, invertebrates, and vertebrates (Bartel et al., 2004; Du and Zamore, 2005; Cao et al., 2006; JonesRhoades et al., 2006). Several lines of evidence have shown ' 2011 Wiley Periodicals, Inc.
that miRNAs play key regulatory roles not only in normal cellular processes but also in different pathologies. miRNAs represent a precise and efficient manner of posttranscriptional regulation of gene expression because of their tissuespecific distribution (Lagos-Quintana et al., 2001). The generation of these small RNA species begins with the transcription of miRNA genes by RNA polymerase II, generating a 1,000-bp primary transcript (pri-miRNA), which is then cleaved by Drosha together with Dgcr8 to produce an 70–100-bp precursor (pre-miRNA; see Fig. 1; Lee et al., 2002, 2003; Zeng, 2003). Exportin 5 is the protein that mediates the transport of pre-miRNA from the nucleus to the cytoplasm (Yi et al., 2003; Lund et al., 2004). Once at the cytosol, pre-miRNAs are cleaved by Dicer, generating a duplex consisting of the mature miRNA and its corresponding miRNA* (Grishok et al., 2001; Hutva´gner et al., 2001; Ketting et al., 2001; Ambros et al., 2003; Bartel et al., 2004; Du and Zamore, 2005). Most miRNAs* are not functional (Lau et al., 2001); however, recent data have shown specific roles of miRNAs* (discussed below). In animals, miRNAs usually silence their target genes based on the complementarity between their 50 ‘‘seed’’ region and the 30 untranslated region (30 UTR) of the target genes (Lagos-Quintana et al., 2001; Lim et al., 2003). Of particular functional relevance is the ‘‘seed sequence’’ spanning bases 2–8 of the miRNA. This small region is responsible for the miRNA specificity. The complementarity between the miRNA’s seed and its Contract Contract Contract Contract
grant sponsor: DGAPA; Contract grant number: IN227506-3; grant number: IN224909; Contract grant number: 227510; grant sponsor: CONACyT; Contract grant number: 61208; grant number: 51198; Contract grant number: 42605.
*Correspondence to: Leonor Pe´rez-Martı´nez, Departamento de Medicina Molecular y Bioprocesos, Instituto de Biotecnologı´a, UNAM, A.P. 510-3, Cuernavaca, Morelos 62271, Me´xico. E-mail:
[email protected] Received 25 January 2011; Revised 14 April 2011; Accepted 5 May 2011 Published online 15 September 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jnr.22701
2
Meza-Sosa et al.
suggestive of a more general role of miRNAs in cellular events including the appearance and progression of disease (Opin et al., 2009; De Smaele et al., 2010; Sonntag, 2010; Lau and de Strooper, 2010).
Fig. 1. miRNA biogenesis pathway. miRNA genes are transcribed as long transcripts, denominated primary miRNAs (pri-miRNAs), that are then processed to generate a miRNA precursor (pre-miRNA). premiRNAs are exported from nucleus to cytoplasm, where they are cleaved to generate a duplex consisting of the mature miRNA and its complementary strand, called miRNA star (miRNA*). The mature miRNA is capable of recruiting the RNA-induced silencing complex (RISC), and this complex guides the silencing of the target gene(s). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
target mRNA is responsive to mismatches and noncanonical base pairing (Lai, 2002; Lewis et al., 2003). In general, when the complementarity is perfect or almost perfect, the mRNA is silenced by degradation. In contrast, if complementarity is imperfect, the translation of the target mRNA is inhibited (Hammond et al., 2000; Elbashir, 2001; Nyka¨nen et al., 2001; Elbashir et al., 2001; Martinez et al., 2002; Schwarz et al., 2002). Although originally animal miRNAs act through translational repression of their target genes, few studies have focused on understanding how miRNA-mediated translational processes are regulated. RNA-binding proteins (RBPs) constitute a special group of proteins capable of binding to RNA molecules and therefore of regulating their cellular functions. Diverse RBPs have been described as regulate the silencing function of miRNAs in different situations, including cell cycle control (Kedde et al., 2010), stress response (Bhattacharyya et al., 2006), and Parkinson’s disease (Gehrke et al., 2010; see below). Studies in different model organisms have elucidated the role of miRNAs in biological processes that require a very precise switch in the expression of key genes. In this sense, the participation of miRNAs has been demonstrated in the regulation of cellular processes such as development, differentiation pathways (Li and Jin, 2010; Ivey and Srivastava, 2010), proliferation, stress response, metabolism, apoptosis, and heterochromatin formation (Mattick and Makunin, 2005; Giraldez et al., 2005; Jones-Rhoades et al., 2006; Martino et al., 2009; Zhao et al., 2010). In addition, abnormal miRNA expression profiles have been detected in cancer, autoimmune diseases, and some nervous system disorders,
miRNAs IN THE DEVELOPING NERVOUS SYSTEM The CNS composed primarily by neurons and accessory-maintenance cells called glial cells, represents one of the most complex structures in an organism. Functions of the CNS include receiving, integrating, conducting, and responding to environmental stimuli, accomplished by the neuronal networks residing in the brain (Kandel et al., 2000; Cao et al., 2006). During development, most of the cells within the CNS are generated from pluripotent stem cells precursors. This process involves the generation of more specialized multipotent cells called neural stem cells (NSCs). According to their brain origin, NSCs are then committed to form neural progenitors, which are lineage-specific cells that, depending on several external and internal fine-tuned signals, will differentiate into particular cell types such as neurons or glial cells (Gage, 2000; Diez del Corral and Storey, 2001). Thus, the proper functioning of the CNS relies on correct cell differentiation, which requires a coordinated program of positive and negative signals to regulate gene expression in an adequate manner through different cellular stages that range from NSCs to terminally differentiated cells (Gage, 2000; Smirnova et al., 2005; Ivey and Srivastava, 2010). Neurogenesis, for example, requires a precise and controlled program of gene expression that depends on regulatory feedback loops to ensure the precise pattern of gene expression to generate a proper cell type in a specific spatiotemporal window (Li and Jin, 2010; Ivey and Srivastava, 2010). As shown in the following examples, miRNAs have an active regulatory role through each stage of neural differentiation. The important role of miRNAs in development of the CNS is further highlighted by the fact that miRNAs have a cross-species conserved function (Zhao and Srivastava, 2007) and show tissue- and cell type-specific expression profiles during CNS development (Coolen and Bally-Cuif, 2009). This makes them attractive regulators of gene expression during the cell differentiation process. Some studies have shown the importance of miRNAs for neural differentiation and CNS development through the deletion of key proteins either for miRNA biogenesis such as Dicer or for miRNA function such as Ago, which is part of the RNA-induced silencing complex (RISC Fiore et al., 2008). Dicer knockout mice die at embryonic stages before neurogenesis occurs, confirming an important role for miRNAs in early development. However, Dicer conditional mutants exhibit defects during CNS development such as size reduction of the forebrain, possibly resulting from apoptosis of differentiating neurons (Makeyev et al., 2007), whereas loss of this protein in the cortex and hippocampus causes microcephaly as well as a decrement in dendritic branching and density (Davis et al., 2008). The role of Dicer in CNS development has also been confirmed in zebrafish, Journal of Neuroscience Research
miRNAs in the CNS
in which Dicer deletion results in abnormal neuronal differentiation and morphological defects in the neural tube. Surprisingly, introduction of miR-430 rescues all the abnormal phenotypes within the nervous system (Giraldez et al., 2005). Interestingly, in mice with mutations in Ago, the closure of the neural tube is prevented (Liu et al., 2004). Together, these findings further support the involvement of miRNAs in the development of the nervous system in different organisms. Several examples of the involvement of particular miRNAs during the establishment of NSCs and neural progenitor cells (NPCs) have also been reported. let-7 and miR-125b were shown to induce neuronal lineage commitment in mouse and human cells, respectively (Rybak et al., 2008; Le et al., 2009). miR-9a is very well conserved among fish, chicken, and mouse, and its expression in these organisms has been detected exclusively in NPCs. In Drosophila melanogaster, miR-9a specifies the correct number of sensory organ precursor cells (SOPs) by inhibiting the expression of the Senseless (sens) gene, which is necessary for the proper generation of SOPs (Li et al., 2006). Moreover, miR-9a plays a key role during the establishment of the midbrain–hindbrain boundary in vertebrates by targeting the antineurogenic genes her5 and her9 (Leucht et al., 2008). Also, suppression of miR-9 expression has been related to enhanced human NPC migration resulting from overexpression of stathmin, a direct target of miR-9 that increases microtubule instability (Delaloy et al., 2010). The existence of a feedback loop between an established signal transduction pathway and miRNAs has been clearly shown in Drosophila melanogaster eye development, in which the expression of miR-7 is activated in retinal progenitor cells as they begin the transition to become photoreceptors. This process depends on epidermal growth factor receptor (Egfr) activation. In the absence of epidermal growth factor (Egf) signaling, the transcription factor Yan represses miR-7 and maintains a progenitor state. When cells are differentiated into photoreceptors, after Egf stimulation, miR-7 binds to the 30 UTR of yan, mediating its degradation. This negative feedback loop ensures a mutually exclusive expression pattern of Yan in progenitor cells and of miR-7 in differentiated cells (Li and Carthew, 2005; Li and Jin, 2010). Furthermore, miRNAs are involved in the establishment of specific neural phenotypes. miR-124 for example is expressed exclusively in neurons, whereas miR-92b has been found only in neuronal progenitors (Liu et al., 2009; Li and Jin, 2010). miR-124 is the most abundant miRNA within the CNS, and it is conserved from nematodes to primates. miR-124 is able to induce a neuronal phenotype when overexpressed in embryonic stem cells (Fiore et al., 2008). miR-124 also regulates the transition from neural progenitor to mature neuron by inhibiting nonneuronal genes (scp1 and sox9; Visvanathan et al., 2007; Cheng et al., 2009) and the polypyrimidine tract-binding protein 1 (Ptbp1 or Ptb), an antineuronal factor that regulates negatively neuron specific alternative splicing (Makeyev et al., 2007; Fiore et al., Journal of Neuroscience Research
3
Fig. 2. Differential miRNA expression generates the chemosensorial ASE left (ASEL) and ASE right (ASER) neurons in C. elegans. To acquire the phenotype of an ASEL neuron, the expression of the die-1 transcription factor is essential to activate the expression of the lsy-6 miRNA, which in turn silences its target, cog-1, resulting in the expression of gcy-7. On the other hand, for the establishment of the ASER phenotype, miR-273 prevents the expression of die-1, so lsy-6 miRNA does not silence cog-1, resulting in the final expression of gcy-5. Black and light gray indicate the genes whose expression is high or low, respectively. Gcy, guanyl cyclase. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
2008). This discovery reveals that the regulation exerted by miR-124 on its different target genes is essential for neuronal differentiation and lineage commitment. Additional examples are miR-129, miR155, and miR122. These miRNAs are able to inhibit bipolar neuron regeneration in the Xenopus retina through the downregulation of otx2 and vsx1, which positively regulate the commitment of progenitor cells to bipolar cells (Li and Jin, 2010). In the nematode Caenorhabditis elegans, the interplay between transcription factors and miRNAs has been described in the specification of the chemosensorial neurons ASE left (ASEL) and ASE right (ASER) in which an asymmetric expression pattern of miRNAs lsy-6 and miR-273 and their targets cog-1 and die-1 is required for the establishment of ASEL and ASER identity respectively (Chang et al., 2004; Fig. 2). miRNAs not only regulate neural functions but also other brain cells. An important signal for the acquisition of a glial cell fate is the phosphorylation of the signal transducer and activator of transcription 3 (Stat3). As mentioned, miR-9 and miR-124 are characterized by their participation in the determination of a neuronal cell fate. By overexpressing these two miRNAs, the levels of phosphorylated Stat3 decrease, so the rate of glial cell fate determination is diminished (Krichevsky et al., 2006). Although the target for these miRNAs has not been identified, one could predict that both might regulate the levels of a specific kinase or a phosphatase inhibitor involved in Stat3 phosphorylation. Thus, it is clear
4
Meza-Sosa et al.
Fig. 3. miRNAs involved in CNS development. Different miRNAs participate during the differentiation process of neuronal and glial phenotypes in a variety of organisms. Cell type-specific miRNA expression (miRNAs below/beside each cell type) is necessary to maintain exquisite control of gene expression within specific spatiotemporal windows. NSC, neural stem cell; NPC, neural progenitor cell. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
that the interaction between different miRNAs and signaling pathways contributes to determine specific cell fates. miR-219 and miR-338 have been identified as regulators of the oligodendrocyte differentiation pathway. Oligodendrocytes represent one type of CNS glial cells whose proper differentiation is crucial for brain function, insofar as they produce myelin, a protein that sheaths the axons of neurons thus allowing neural transmission (Dugas et al., 2010; Zhao et al., 2010). Different in vivo and in vitro assays performed in zebrafish, chicken, rat, and mouse have shown that miR-219 and miR-338 negatively regulate the expression of sox6 and hes5, two known oligodendrocyte differentiation repressors. Therefore, correct expression of both miRNAs is needed for a proper generation of functional oligodendrocytes (Zhao et al., 2010). In addition to sox6 and hes5, other oligodendrocyte lineage repressors may be targeted by these miRNAs or other, unidentified miRNAs. It is also possible that the expression of proneuronal genes is negatively regulated by miRNAs in order to promote an oligodendrocytic differentiation. Astrocytes represent another class of glial cells within the CNS that is important in providing glucose to neurons, in neurotransmitter recycling, and in buffering extracellular potassium (Kandel et al., 2000). In this sense, miR-23 and miR-29 have been demonstrated as astrocyte-specific expressed miRNAs (Smirnova et al., 2005); however, their role during astrocytic differentiation and/or functioning has to be elucidated. Taken together, all the examples given so far show a general picture in which miRNA-mediated regulation
is involved in every step during CNS differentiation and development (Fig. 3, Table I). Although miRNAs are negative regulators, they can positively promote NSC renewal as well as differentiation depending on cell and time-specific signals. This is achieved through complex regulatory interactions among miRNAs, their targets, and miRNA regulatory proteins, which often can form both positive and negative regulatory feedback loops. This kind of feedback loop is crucial during NSC development; it allows the establishment of time- and tissuespecific transcriptional patterns. Given their ubiquitous functions, it is not surprising that miRNAs are involved in all stages during CNS development. Although far from being extensive, the examples given above may help us to generate a speculative overview of the miRNAs function during CNS differentiation process: in renewing NSC, miRNAs will tend to silence prodifferentiation genes as well as to repress negative regulators of proliferative genes. As differentiation goes on, the miRNA expression profile may change, and miRNAs that target renewal and proliferative genes may be expressed. During terminal phases of differentiation, miRNAs should silence several nonneuronal or nonglial genes and repress genes involved in cell cycle progression (in the particular case of neurons) while allowing the expression of particular neuron or glial specific markers, giving the cell a defined identity. All these processes should regulate and constantly be regulated by diverse signaling pathways as well as epigenetic mechanisms, both fundamental processes during differentiation. Journal of Neuroscience Research
miRNAs in the CNS
5
TABLE I. miRNAs involved during Central Nervous System development* miRNA(s)
Target(s)
Function
Cell type
References
let-7
Lin28 (mouse)
NSC
Rybak et al., 2008
miR-7
Yan (fruit fly)
Photoreceptor cells
Li and Carthew, 2005
miR-9 miR-9
Her5 and Her9 (zebrafish) Stat3 (mouse)
NSC to NPC ES cells
Leucht, et al., 2008 Krichevsky, et al., 2006
miR-9a miR-23
Sens (fruit fly) Unknown
Epithelial cells ES cells and astrocytes
Li, et al., 2006 Smirnova et al., 2005
Astrocytes NPC NPC
Smirnova et al., 2005 Liu, et al., 2009 Li and Jin, 2010
REST-Scp1, Sox9, and Ptbp1 (mouse)
Promotes neuronal lineage commitment Promotes neuronal lineage commitment Inhibits proliferation Promotes neuronal lineage commitment Inhibits neuronal fate Promotes glial lineage commitment Unknown Unknown Promotes neuronal lineage commitment Promotes neuronal lineage commitment
NPC to neuron
Tbc1d1, Dgat1, and Sgpl1 (human) Sox6 and Hes5 (vertebrates)
Promotes neuronal lineage commitment Promotes glial lineage commitment
SH-SY5Y
Visvanathan et al., 2007; Cheng, et al., 2009 and Makeyev et al., 2007 Le et al., 2009
NPC to oligodendrocyte
Zhao et al., 2010
miR-29 miR-92b miR-122, miR-129, and miR-155 miR-124
miR-125b miR-219 and miR-338
Unknown Unknown Otx2 and Vsx1 (frog)
*Lin28, Lin-28 homolog A; Yan, AOP or anterior open; Her, Hairy/E(spl) transcription factor; Stat3, signal transducer and activator of transcription 3; Sens, senseless; Otx2, orthodenticle homeobox 2; Vsx1, visual system homeobox 1; REST, RE1-silencing transcription factor; Scp1, CTD (carboxy-terminal domain RNA polymerase II polypeptide A) small phosphatase 1; Sox, SRY (sex determining region Y)-box; Ptbp1, polypyrimidine tract binding protein 1; Tbc1d1, TBC1 (tre-2/USP6, BUB2, cdc16) domain family, member 1; Dgat1, diacylglycerol O-acyltransferase 1; Sgpl1, sphingosine-1-phosphate lyase 1; Hes5, hairy and enhancer of split 5; NSC, neural stem cell; NPC, neural precursor cell; ES, embryonic stem cell.
Finally, it is clear that the current regulatory networks composed mainly by transcription factors have to be redesigned by including posttranscriptional regulators such as miRNAs. Thus, it is necessary to think about gene expression regulation as a comprehensive and designed program with specific players acting in the correct space and time within the different levels by which regulation is achieved. miRNAs AND NEURODEGENERATIVE DISEASES Neurodegenerative diseases are those in which neurons are lost progressively with age, resulting in the loss of multiple functions such as speech and memory, among others. In this section, we discuss recent findings indicating a possible link between miRNAs and neurodegenerative disorders. Alzheimer’s Disease Alzheimer’s disease (AD) is the most common type of dementia. It is characterized by the accumulation of amyloid b (Ab) peptides. Bace1 is a secretase involved in the generation of the Ab peptides. Different studies have shown that Bace1 protein expression is higher in AD patients than in healthy individuals, suggesting that, in AD, the overexpression of Bace1 results either from overproduction or from increased stability of this protein (Provost, 2010). Although, little research has been performed regarding the participation of miRNAs in the development of this neurodegenerative disorder, some Journal of Neuroscience Research
miRNAs have been shown to target the 30 UTR of bace1 (Martino et al., 2009). A decrease in the expression of miR-107 has been detected in cortices of AD patients that correlates with a higher production of Bace1 (Wang et al., 2008b). In addition, the bace1 30 UTR contains putative binding sites for miR-298 and miR-328. Interestingly, these miRNAs decrease with age (Boissonneault et al., 2009). Additionally, the miR-29a/b-1 cluster has been reported as another regulator of Bace1 expression (He´bert et al., 2008). Thus, Base1 overexpression observed in AD patients may result from defects in miRNA function. Recently, it has been shown that an important component of AD is the overproduction of inflammatory cytokines resulted from the accumulation of Ab peptides (Martino et al., 2009). Proinflammatory cytokines induce the activation of the transcription factor nuclear factor-jB (NF-jB), which in turns activates miR-146 expression, resulting in chronic inflammation by targeting complement factor H (CFH) that acts as a repressor of brain inflammatory processes (Lukiw et al., 2008; Fig. 4). On the other hand, it is not surprising that apoptotic pathways are also involved in AD etiology, insofar as its final consequence is the progressive loss of neurons. Using the AD mouse model, it has been proved that miR-34a is overexpressed in these animals compared with control mice. miR-34a targets the antiapoptotic gene bcl2; accordingly, the inhibition of this miRNA results in an increase of the Bcl2 protein, which correlates with a decrease in the caspase 3 activity (Wang
6
Meza-Sosa et al.
Fig. 4. miR-146 promotes chronic inflammation in AD. In normal individuals, the activity of NF-jB is normally regulated and thus the expression of miR-146 is moderated and maintains normal levels of complement factor (CFH). However, in AD, proinflammatory cytokines promote the activation of the NF-jB transcription factor, which in turn activates the miR-146, resulting in permanent inhibition of CFH and leading to chronic inflammation. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
et al., 2009; Provost, 2010) and probably apoptosis. Thus, abnormal expression of miRNAs might contribute to the etiology of AD at least by modulating Bcl2 expression and consequently altering the apoptotic process during AD. Therefore, the above-described data support the idea that miRNAs deregulation plays a key role during AD pathogenesis by affecting different cellular processes that result in increased Ab peptides production, which in turns promotes inflammatory and apoptotic processes causing neuronal loss. Parkinson’s Disease The second most common neurodegenerative disorder is Parkinson’s disease (PD). PD is characterized by bradykinesia, resting tremor, muscular rigidity, disrupted coordination and balance, and loss of dopaminergic neurons in the substantia nigra (Martino et al., 2009). Different analyses have revealed the possible participation of miRNAs in the onset and progression of this disorder. Accordingly, Dicer conditional mutant mice present abnormal locomotor activity that correlates with that observed in PD (Martino et al., 2009). Additionally, it has been demonstrated that the transcription factor Pitx3, which is essential for the establishment and maintenance of the dopaminergic neuronal population in the CNS, is targeted by miR-133b, whose expression is higher in PD patients brains compared with healthy individuals (Fuchs et al., 2009). Previous studies have identified a-synuclein as target of miR-7. During oxidative stress, miR-7 function is blocked, so expression of
a-synuclein increases, causing disruption of the proteasome system as has been observed in PD (Junn et al., 2009). miR-433 represents another putative miRNA participating in PD. It has been shown that mutations in the 30 UTR of fibroblast growth factor 20 (fgf20) disrupt the miR-433 binding site, so more Fgf20 protein is generated in the brain of PD patients (Wang et al., 2008a). Accordingly, it has been shown that a particular polymorphism in the fgf20 gene correlates with increased expression of a-synuclein, which suggests that Fgf20 may constitute a risk factor for PD. On the other hand, mutations in the leucine-rich repeat kinase 2 (lrrk2) gene have been described as a common cause of PD (Santosh et al., 2009). A protein– protein interaction between pathogenic Lrrk2 and Ago has been demonstrated. This interaction inactivates let-7 and miR-184*, causing an overproduction of their respective targets, E2f1 and Dp, that results in increased neuronal cell death (Gehrke et al., 2010). Therefore, these data indicate a link between the miRNA silencing pathway and Lrrk2 pathogenesis, which may be useful in future therapeutic strategies. Thus, alterations in miRNA expression and function in PD result in disrupted proteasome activity and cell cycle control, leading to neuronal death, pointing out that miRNA deregulation is an important component of PD etiology Huntington’s Disease Most Huntington’s disease (HD) cases are explained by an increase in the number of tandem repeats of the codon ‘‘CAG’’ in the htt gene, which generates a glutamine-enriched huntingtin protein (Johnson et al., 2008). However, recent studies have suggested that some miRNAs might contribute to the development of HD. miRNA expression pattern analysis in brains of HD patients revealed that miR-9/9*, miR29b, and miR-124 are downregulated, whereas miR-29a and miR-132 are upregulated (Johnson et al., 2008; Packer et al., 2008). A more detailed analysis demonstrated that miR-9/9* inhibits the expression of REST and co-REST transcription factors. In normal individuals, REST is a repressor of neuronal genes in nonneuronal cells, and it is usually sequestered in the cytoplasm by interacting with huntingtin. As mentioned, in HD, the huntingtin protein is characterized by a longer tract of polyglutamine, which prevents its interaction with REST, causing REST translocation to the nucleus and thus the inhibition of its target genes not only in nonneuronal but also in neuronal cells. The existence of a double-negative feedback loop involving miR-9/9*, REST, and co-REST has been suggested, because miR9/9* is downregulated by REST (Packer et al., 2008). It will be interesting to elucidate whether miRNA deregulation is a cause or consequence of the disease. Furthermore, the identification of new miRNAs that target genes showing deregulated expression in HD may contribute to a better understanding of the molecular mechanisms underlying the etiology of this disease. Journal of Neuroscience Research
miRNAs in the CNS
Spinocerebellar Ataxia Spinocerebellar ataxia type 1 is caused by multiple repetitions of the CAG codon in the atxn1 gene, generating a polyglutamine expanded ataxin-1 (Atxn1) protein. The main etiology of this disorder is the loss of cerebellar Purkinje cells (Martino et al., 2009). In 2008, Lee and colleagues demonstrated that the inhibition of miR-19, miR-101, and miR-130 enhanced the cytotoxicity caused by the Atxn1 protein. Consistently, Dicer mutation also promotes the cytotoxicity caused by the Atxn1 protein (Lee et al., 2008). Thus, it is possible that these miRNAs regulate proapoptotic genes; however, further experiments are required to test this hypothesis. miRNA correct expression and activity are essential for an adequate functioning of the CNS, especially as age increases. Evidence suggests that neurodegenerative diseases present miRNA deregulation as a common feature. Thus, it is possible that the counteraction of these abnormal events with miRNA gain- or loss-of-function therapeutic tools could help to control these characteristic aging syndromes. miRNAs AND DEVELOPMENTAL CNS DISORDERS As mentioned above, CNS development requires activation and repression of specific genes. In this sense, the role of several miRNAs repressing their target genes in a time- and region-specific manner has been proved to be essential for the proper functioning of the CNS (Li and Jin, 2010; Ivey and Srivastava, 2010). Evidently, when the inhibitory action of miRNAs is compromised, different pathologies arise from abnormal CNS development caused by aberrant gene expression patterns. Fragile X Mental Retardation This is the most common X-linked inherited type of mental retardation, in which patients have IQ values between 20 and 70. The development to this disorder has been associated with decreased expression of the fragile mental retardation 1 (fmr1) gene, which encodes the Fmr protein (Fmrp), an RNA-binding protein involved in the transport and translation of different mRNAs. Fmrp is also one of the proteins forming the RISC complex (Bicker and Schratt, 2008). Alterations in the Fmrp function have been proposed as the main cause for this disease, and it has been linked to the miRNA biogenesis pathway because of the interaction between Dicer, Ago, and miRNAs with the human Fmrp (Jin et al., 2004) and also with the Drosophila melanogaster Fmrp homolog (Caudy et al., 2002; Ishizuka et al., 2002). These molecular interactions suggest that Fmrp could act as an miRNA acceptor from Dicer, so fmr1 mutations might cause abnormalities not only in miRNA function but also in its target mRNAs (Provost, 2010). Because downregulation of Fmr1 expression leads to the fragile X mental retardation disorder, a recent study sought to determine whether the 30 UTR of the fmr1 gene is a target of miRNAs. Accordingly, the Journal of Neuroscience Research
7
miR-19b, miR-302b*, and miR-323-3p were identified as repressors of fmr1 gene expression (Yi et al., 2010). However, future studies are required to determine whether these miRNAs are involved in the pathway affected in this disease and to identify the signals controlling their expression. Rett Syndrome Rett syndrome is an X-linked neurodevelopmental disorder that has been related to the autism-spectrum disorder. This disease is typically caused by mutations in the gene that encodes the methyl CpG binding protein 2 (MeCP2), a protein that binds to methylated DNA and that is involved in epigenetic silencing. miR-184 expression is regulated by MeCP2. The expression of this miRNA is restricted to the brain and it is subjected to an imprinting phenomenon, implying that only the paternal allele of miR-184 is expressed. In vitro experiments show that, after neuron depolarization, MeCP2 paternal allele expression is lost, so the expression of its target genes is increased (miR-184). Strikingly, in MeCP2-deficient mice, miR-184 expression is lost in both alleles (Nomura et al., 2008). These results indicate that MeCP2 and miR-184 may have an intricate regulation; however, the role of miR-184 in Rett syndrome etiology is still unknown. MeCP2 plays a key role during neuronal development. Consistently, MeCP2 has been implicated in gene expression control of brain-derived neurotrophic factor (Bdnf), an important neurotrophin for neural differentiation, survival, and synapse formation. miR-132 targets the long 30 UTR of mecp2 that is present exclusively in brain transcripts. Interestingly, a negative regulatory loop among miR-132, Bdnf, and MeCP2 that finely regulates MeCP2 levels has been proposed. MeCP2 overexpression increases Bdnf transcript levels, which in turn results in the activation of miR-132, leading to a decrease in the MeCP2 levels. Experimental data have shown that blocking miR-132 leads to increased levels of MeCP2 and Bdnf, which may cause abnormal neural development (Klein et al., 2007). Even though miRNAs are not directly involved in epigenetic processes, the examples given above reveal a clear connection between miRNAs and MeCP2, both in normal and in disease conditions. An intriguing possibility is that, in some Rett syndrome patients, miR-132 may be abnormally upregulated, leading to low levels of MeCP2 and the development of the disease, this in the absence of mecp2 gene mutations. Alternatively, MeCP2 may promote expression of neuronal protein-coding genes indirectly by repressing specific miRNAs. In support to this notion, a recent study showed decreased levels of Bdnf resulting from aberrant overexpression of several miRNAs caused by the loss of MeCP2 (Wu et al., 2010b). MeCP2-regulated miRNAs in neurons, therefore, may serve as another control point in the course of Rett disease. However, further experiments are required to understand the physiological relevance of the MeCP2-regulated miRNAs and their
8
Meza-Sosa et al.
impact on the development of the disease and also whether altered miRNA expression may indeed contribute to Rett syndrome pathoetiology. miRNAs AND PSYCHIATRIC DISORDERS Psychiatric disorders are characterized by a global deregulation of multiple signaling pathways. Although these disorders might have a genetic component, the variability between patients has made difficult the identification of key genes that participate in the development of psychiatric disorders (Miller and Wahlestedt, 2010). Accordingly, the efficiency of different drug-based therapies is not the same between patients. These observations suggest the participation of additional regulatory molecules as yet uncharacterized. Studies involving miRNAs have been focused mainly on schizophrenia and to a lesser extent on bipolar disorder. The mechanism of action of this class of small RNAs makes them suitable for exploration as putative therapeutic tools by their silencing of specific target genes in patients suffering of such disorders. Schizophrenia Schizophrenia is a psychiatric disorder present in approximately 1% of the general population. It is a complex disease characterized by a disintegration of the processes of thinking and of emotional responsiveness. Some common symptoms of the disease are visual and acoustic hallucinations, paranoid delusions, social dysfunction, and disorganized speech and thinking (Feng et al., 2009). The causes of the disease are intricate and poorly understood. Abnormal levels of some miRNAs have been detected in brains of schizophrenic patients compared with healthy brains. A link between miRNA biogenesis and schizophrenia has been suggested. Dgcr8, a key protein for miRNA biogenesis (Fig. 1), is encoded within a fragment of chromosome 22, which is commonly deleted in schizophrenic patients. Thus, a disrupted miRNA biogenesis might contribute to the appearance and development of schizophrenia. Recently, the genes of miRNAs encoded in the X chromosome have been sequenced in order to identify alterations that might be related to the appearance of X-linked schizophrenia (Bicker and Schratt, 2008). Results confirmed that ultrarare variants in the sequence of mature premiRNAs (miR-18b, miR-502, and miR-505) or mature miRNAs (let-7f-2, miR-188, miR-325, miR-509-3, miR-510, and miR-660) are frequent in men affected with this disorder compared with healthy individuals (Feng et al., 2009). This shows that altered function of miRNAs might also contribute to the development of this disorder; however, further studies are required to prove this association formally. The identification of mRNAs targeted by those miRNAs is a key step in forming a comprehensive idea of the pathways affected by abnormal expression of specific miRNAs resulting in disease.
Tourette’s Syndrome This syndrome is a neuropsychiatric disorder characterized by the presence of motor and vocal tics. The Slit and Trk-like family member 1 (slitrk1) gene participates in growth, guidance, and arborization of neurons. Mutations within the slitrk1 gene coding sequence have been proposed as one of the genetic factors responsible for this syndrome (Abelson et al., 2005; Bicker and Schratt, 2008). However, it has been found that, in some cases, an abnormal posttranscriptional regulation of slitrk1 might be responsible for the disease. In particular, it has been proposed that miR-189 targets the slitrk1 mRNA. Mutations in the 30 UTR of the slitrk1 gene enhance the binding of miR-189 to the slitrk1 mRNA (Abelson et al., 2005; Bicker and Schratt, 2008; Martino et al., 2009). Given that Slitrk1 shares homology with the TRK intracellular domain and that TRK signaling induced by Bdnf binding is required for neurite outgrowth and synaptic function, it is possible that Slitrk1, trough its interaction with an unknown ligand, promotes neurite outgrowth and synaptic function by activating a similar transduction pathway (Proenca et al., 2011). Therefore, it is possible that the altered synaptic functions observed in Tourette’s syndrome patients may be a consequence of reduced Slitrk1 signaling resulting from decreased Slitrk1 protein levels imposed by the sequence variations found in the Slitrk1 30 UTR that favor its interaction with miR-189. Bipolar Disorder Analyses of differentially expressed miRNAs between healthy individuals and bipolar disorder patients revealed a significant reduction in the expression of miR-132 in the prefrontal cortex of the patients (Miller and Wahlestedt, 2010). Although a direct target for this miRNA has not yet been identified in the context of this psychiatric disorder, the fact that miR-132 regulates Gap250 levels and that this protein negatively regulates Bdnf-induced neurite outgrowth and synaptic formation by inactivating the small GTPase Rac1, it is possible that, in patients with bipolar disorder, the reduced levels of miR-132 might result in increased levels of the Gap protein, leading to reduced Rac1 signaling and thus altered neurite growth and synaptic function (Vo et al., 2006). This idea is supported by the fact that lithium chloride improves bipolar disorder symptoms and increases neurogenesis and synaptogenesis. Furthermore, lithium chloride and valproate, another mood stabilizer used to treat bipolar disorder, alter the expression of 31– 37 miRNAs. Interestingly, the targets of the miRNAs downregulated by lithium chloride and valproate are involved in neurite outgrowth and neurogenesis (Zhou et al., 2009; Miller and Wahlestedt, 2010). Together these result indicate that alteration in miRNA expression contributes to bipolar disorder by targeting genes involved in neurogenesis and neurite outgrowth. However, additional miRNAs may also be involved, because lithium chloride also affected the expression of miRNAs Journal of Neuroscience Research
miRNAs in the CNS
9
TABLE II. miRNAs involved in Central Nervous System pathologies* Disease Huntington’s disease Huntington’s disease Huntington’s disease Alzheimer’s disease Alzheimer’s disease Alzheimer’s disease Parkinson’s disease Parkinson’s disease Parkinson’s disease Spinocerebellar ataxia Rett syndrome Rett syndrome Schizophrenia Tourette’s syndrome Bipolar disorder
miRNA(s) miR-9/9* miR-29b and miR-124 miR-29a and miR-132 miR-107, miR-29a/b-1, miR-298, and miR-328 miR-146 miR-34a miR-7 miR-133b miR-433 miR-19, miR-101 and miR-130 miR-184 miR-132 let-7f-2, miR-188, miR-325, miR-509-3, miR-510, and miR-660 miR-189 miR-132
Target(s)
References
REST/co-REST Unknown Unknown Bace1
Packer et al., 2008 Johnson et al., 2008 Johnson et al., 2008 Martino, 2009; Boissonneault, 2010
CFH Bcl2 a-Synuclein Pitx3 Fgf20 Unknown Unknown MeCP2 Unknown
Lukiw, 2008 Provost, 2010 Junn et al., 2009 Fuchs, 2009 Wang et al., 2008a Lee et al., 2008 Nomura et al., 2008 Klein et al., 2007 Feng et al., 2009
Slitrk1 Unknown
Bicker and Schratt, 2008; Martino et al., 2009 Miller and Wahlestedt, 2010
*REST, RE1-silencing transcription factor; Co-REST, REST corepressor 1; Bace1, beta-site APP-cleaving enzyme 1; CFH, complement factor H; Bcl2, B-cell CLL/lymphoma 2; Pitx3, paired-like homeodomain 3; Fgf20, fibroblast growth factor 20; MeCP2, methyl CpG binding protein 2; Slitrk1, SLIT and NTRK-like family, member 1.
whose predicted targets are genes such as the thyroid hormone receptor b (thrb), the dipeptidyl-peptidase 10 (dpp10), and the metabotropic glutamate receptor 7 (grm7) previously identified as risk factors for bipolar disorder (Zhou et al., 2009; Miller and Wahlestedt, 2010); nevertheless, experimental data are required to validate these predictions. OTHER NEUROLOGICAL DISEASES Epilepsy Temporal lobe epilepsy (TLE) is a neurological disorder characterized by a damaged hippocampus as a consequence of specific neuronal loss, gliosis, and reorganization of synapses. An inflammatory component is also present in this disease, resulting from the upregulation of proinflammatory genes. Accordingly, in a rat model of TLE, miR-146a was upregulated compared with normal rats (Aronica et al., 2010). As mentioned previously, miR-146a targets the brain inflammatory repressor CFH (Lukiw et al., 2008), so an increase in miR-146a levels would result in constitutive proinflammatory cytokine production and inflammation. So far, no mutations in the miR-146a gene have been found that could explain its increased levels in epilepsy, suggesting that the cause is not genetically associated. Therefore, these data indicate a potential epigenetic role in miRNA regulation leading to TLE as well as in other neurological disorders in which an inflammatory process has been implicated. However, experimental evidence demonstrating that upregulation of miR-146a leads to chronic inflammation and that this promotes TLE remains to be provided. Overall these observations suggest that miRNA deregulation is an important characteristic of several neuropathologies (Table II). Given the wide roles of miRNAs during CNS differentiation, it is quite feasible that miRNA deregulation may be one of the driving steps in Journal of Neuroscience Research
the generation of developmental disorders, although more research is needed to address this issue. Moreover, the presence of miRNA deregulation in neurodegenerative diseases as well as in psychiatric disorders highlights the importance of miRNAs during normal CNS function. Further studies will be needed to determine the causes of miRNA deregulation, in order to understand the etiology of several neuropathologies. Finally, some miRNAs, their regulators, or their targets may serve as promising therapeutic targets. CONCLUSIONS Development of the CNS is one of the most complex and fascinating processes in nature. Recently, the role of miRNAs as essential regulators of gene expression has been documented. Here, we discussed a few representative cases that highlight the importance of miRNAs during normal CNS development as well as under pathological circumstances. As described, miRNAs are involved in every step of CNS development, from the maintenance of the pluripotent state of emryonic stem cells to the establishment of neural phenotypes. Nevertheless, many issues must be addressed to elucidate the specific role of miRNAs during normal development as well as in the pathological CNS. Although considerable efforts have been made in finding and validating specific targets for miRNAs, we still do not understand the fact that a typical miRNA has several targets, nor the reasons why single mRNA can be targeted by multiple miRNAs. There is little information concerning the mechanisms that may control miRNAs specificity from one target to another. This is particularly relevant during development because miRNAs function may be modulated depending on the cell and the developmental stage. Even though it is known that a canonical 30 UTR contains several miRNA response elements, information
10
Meza-Sosa et al.
concerning the silencing of a single gene by different miRNAs is still scarce. An additional area that has just begun to be explored is the regulation of miRNAs expression. This is particularly interesting, insofar as miRNA synthesis is a multistep biogenesis process (Fig. 1). It has been demonstrated that alternative splicing (Wu et al., 2010a) and the availability and adequate functioning of the proteins involved in miRNA biogenesis are crucial for the generation of mature and functional miRNAs (Wu et al., 2010a; Larsson et al., 2010). Other studies have focused on the regulation of miRNAs at the transcriptional and posttranscriptional levels. Some of these studies have revealed that different transcription factors are able to bind to the miRNA promoter regions, regulating miRNA expression in distinct cellular events such as stress response, development, cell cycling, cancer, and apoptosis (Liang et al., 2009). Moreover, methylation of CpG islands present in the promoter region of miRNA genes has also been reported; deregulation of this process has been identified as a main characteristic of cancer (Liang et al., 2009). Additionally, the participation of miRNAs within regulatory networks involving transcription factors has been reported as a common feature in different biological events (Liang et al., 2009; Li et al., 2010). This is in accordance with the suggested role for miRNAs in providing robustness to programs of gene expression by buffering the effects of environmental and intracellular variations. One example is the case of miR-7, which provides robustness against temperature changes during the differentiation of photoreceptor cells in D. melanogaster (Li et al., 2010). Likewise, the dynamic kinetic properties of miRNAs make them suitable to take part in regulatory network motifs as feedback and feedforward loops that are known to be important during development (Li et al., 2010). In terms of time, the biogenesis of miRNAs is more suitable than the production of transcription factors for processes that require rapid tuning of gene expression (Li et al., 2010). Despite this knowledge, the specific molecular mechanisms governing the regulatory networks among miRNAs, transcription factors, and their target genes are just beginning to be discovered and understood. Development involves not only the establishment of specific expression patterns but also the generation of epigenetic landscapes that allow the inheritance of such patterns. These processes are mutually interdependent, and both are crucial for correct development. Here, we have presented a few examples in which both epigenetic regulators and miRNAs are related. This cross-talk between miRNAs and epigenetic regulators may be common and essential during CNS differentiation and maturation; however, further studies involving miRNAs and epigenetic changes are needed. miRNAs are not the only class of regulatory noncoding RNA in the cell. Long noncoding RNAs (lncRNAs) represent another particular important class of RNAs that has also been involved in neural development (Mehler and Mattick, 2007). A current field of research consists of determining whether miRNAs regulate lncRNAs function.
As pointed out in this review, miRNAs are important not only in the normal developing CNS but also in several neuropathologies. Understanding the role of miRNAs in disease is a growing area of study. Most of the analyses performed so far have shown that miRNAs are associated with a vast variety of neuropathologies, but it is not yet clear whether the aberrant miRNA expression profiles are cause or consequence of the disease. Additionally, the molecular mechanisms that might be affected by anomalous miRNA expression are poorly understood. It is quite feasible that in the next few years more and more examples of miRNAs’ functions during CNS development will be revealed. Furthermore, as more comprehensive analyses of miRNA functions are performed, the contribution of other research areas such as epigenetics and systems biology will be necessary to gain a better understanding of miRNAs and the regulatory networks in which they are involved. Therapeutic applications using miRNA overexpression or downregulation will be of key importance for the development of treatment for several diseases. However, it is necessary to consider that some miRNAs exhibit a tissue- and time-specific expression pattern as well as the role of stabilizing and destabilizing factors on the miRNA–target interaction (Kai and Pasquinelli, 2010; Larsson et al., 2010). Therefore, innovative tools must be developed to modulate properly the transcription of certain genes in a restricted spatiotemporal window, avoiding pleiotropic and nonspecific effects. Finally, the combination of new high-throughput technologies with classical molecular biology techniques will be crucial for the study and understanding of the huge world that these tiny regulators conform. ACKNOWLEDGMENTS We are grateful to Oswaldo Lo´pez and Virginia Barajas for technical support. REFERENCES Abelson JF, Kwan KY, O’Roak BJ, Baek DY, Stillman AA, Morgan TM, Mathews CA, Pauls DL, Rasin MR, Gunel M, Davis NR, ErcanSenicek AG, Guez DH, Spertus JA, Leckman JF, Dure LS IV, Kurlan R, Singer HS, Gilbert DL, Farhi A, Louvi A, Lifton RP, Sestan N, State MW. 2005. Sequence variants in SLITRK1 are associated with Tourette’s syndrome. Science 310:317–320. Ambros V, Bartel B, Bartel DP, Burge CB, Carrington JC, Chen X, Dreyfuss G, Eddy SR, Griffiths-Jones S, Marshall M, Matzke M, Ruvkun G, Tuschl T. 2003. A uniform system for microRNA annotation. RNA 9:277–279. Aronica E, Fluiter K, Iyer A, Zurolo E, Vreijling J, Van Vliet EA, Baayen JC, Gorter JA. 2010. Expression pattern of miR-146a, an inflammation-associated microRNA, in experimental and human temporal lobe epilepsy. Eur J Neurosci 31:1100–1107. Bartel DP, Lee R, Feinbaum R. 2004. MicroRNAs: genomics, biogenesis, mechanism and function genomics: the miRNA genes. Cell 116:281–297. Bhattacharyya SN, Habermacher R, Martine U, Closs EI, Filipowicz W. 2006. Relief of microRNA-mediated translational repression in human cells subjected to stress. Cell 125:1111–1124. Bicker S, Schratt G. 2008. microRNAs: tiny regulators of synapse function in development and disease. J Cell Mol Med 12:1466–1476. Journal of Neuroscience Research
miRNAs in the CNS Boissonneault V, Plante I, Rivest S, Provost P. 2009. MicroRNA-298 and microRNA-328 regulate expression of mouse b-amyloid precursor protein converting enzyme 1. J Biol Chem 284:1971–1981. Cao X, Yeo G, Muotri AR, Kuwabara T, Gage FH. 2006. Noncoding RNAs in the mammalian central nervous system. Annu Rev Neurosci 29:77–103. Caudy AA, Myers M, Hannon GJ, Hammond SM. 2002. Fragile Xrelated protein and VIG associate with the RNA interference machinery. Genes Dev 16:2491–2496. Chang S, Johnson RJ, Frøkjær-Jensen C, Lockery S, Hobert O. 2004. MicroRNAs act sequentially and asymmetrically to control chemosensory laterality in the nematode. Nature 430:785–789. Cheng LC, Pastrana E, Tavazoie M, Doetsch F. 2009. miR-124 regulates adult neurogenesis in the subventricular zone stem cell niche. Nat Neurosci 12:399–408. Coolen M, Bally-Cuif L. 2009. MicroRNAs in brain development and physiology. Curr Opinion Neurobiol 19:461–470. Davis TH, Cuellar TL, Koch SM, Barker AJ, Harfe BD, McManus MT, Ullian EM. 2008. Conditional loss of Dicer disrupts cellular and tissue morphogenesis in the cortex and hippocampus. J Neurosci 28:4322–4330. De Smaele E, Ferretti E, Gulino A. 2010. miRNAs as biomarkers for CNS cancer and other disorders. Brain Res 1338:100–111. Delaloy C, Liu L, Lee JA, Su H, Shen F, Yang GY, Young WL, Ivey KN, Gao FB. 2010. MicroRNA-9 coordinates proliferation and migration of human embryonic stem cell-derived neural progenitors. Cell Stem Cell 6:323–335. Diez del Corral R, Storey KG. 2001. Markers in vertebrate neurogenesis. Nat Rev Neurosci 2:835–839. Du T, Zamore PD. 2005. MicroPrimer: the biogenesis and function of microRNA. Development 132:4645–4652. Dugas JC, Cuellar TL, Scholze A, Ason B, Ibrahim A, Emery B, Zamanian JL, Foo LC, McManus MT, Barres BA. 2010. Dicer1 and miR219 are required for normal oligodendrocyte differentiation and myelination. Neuron 65:597–611. Elbashir SM. 2001. RNA interference is mediated by 21- and 22-nucleotide RNAs. Genes Dev 15:188–200. Elbashir SM, Martinez J, Patkaniowska A, Lendeckel W, Tuschl T. 2001. Functional anatomy of siRNAs for mediating efficient RNAi in Drosophila melanogaster embryo lysate. EMBO J 20:6877–6888. Feng J, Sun G, Yan J, Noltner K, Li W, Buzin CH, Longmate J, Heston LL, Rossi J, Sommer SS. 2009. Evidence for X-chromosomal schizophrenia associated with microRNA alterations. PloS One 4:1–11. Fiore R, Siegel G, Schratt G. 2008. MicroRNA function in neuronal development, plasticity and disease. Biochim Biophys Acta 1779:471–478. Fuchs J, Mueller JC, Lichtnerbe P, Schulte C, Munz M, Berga D, Wu¨llnerce U, Illigde T, Sharma M, Gasser T. 2009. The transcription factor PITX3 is associated with sporadic Parkinson’s disease. Neurobiol Aging 30:731–738. Gage FH. 2000. Mammalian neural stem cells. Science 287:1433–1438. Gehrke S, Imai Y, Sokol N, Lu B. 2010. Pathogenic LRRK2 negatively regulates microRNA-mediated translational repression. Nature 466:637–641. Giraldez A, Cinalli R, Glasner M, Enright AJ, Thomson M, Baskerville S, Hammond SM, Bartel DP, Schier ,AF. 2005. MicroRNAs regulate brain morphogenesis in zebrafish. Science 308:833–838. Grishok A, Pasquinelli AE, Conte D, Li N, Parrish S, Ha I, Baillie DL, Fire A, Ruvkun G, Mello CC. 2001. Genes and mechanisms related to RNA interference regulate expression of the small temporal RNAs that control C. elegans developmental timing. Cell 106:23–34. Hammond SM, Bernstein E, Beach D, Hannon GJ. 2000. An RNAdirected nuclease mediates post-transcriptional gene silencing in Drosophila cells. Nature 404:293–296. He´bert S, Horre K, Nicolaı¨ L, Papadopoulou AS, Mandemakers W, Strooper BD. 2008. Loss of microRNA cluster miR-29a/b-1 in sporadic Alzheimer’s disease correlates with increased BACE1/b-secretase expression. Proc Natl Acad Sci U S A 105:6415–6420. Journal of Neuroscience Research
11
Hutva´gner G, McLachlan J, Pasquinelli AE, Ba´lint E, Tuschl T, Zamore PD. 2001. A cellular function for the RNA-interference enzyme Dicer in the maturation of the let-7 small temporal RNA. Science 293:834– 838. Ishizuka A, Siomi MC, Siomi H. 2002. A Drosophila fragile X protein interacts with components of RNAi and ribosomal proteins. Genes Dev 16:2497–2508. Ivey KN, Srivastava D. 2010. MicroRNAs as regulators of differentiation and cell fate decisions. Cell Stem Cell 7:36–41. Jin P, Zarnescu DC, Ceman S, Nakamoto M, Mowrey J, Jongens TA, Nelson DL, Moses K, Warren ST. 2004. Biochemical and genetic interaction between the fragile X mental retardation protein and the microRNA pathway. Nat Neurosci 7:113–117. Johnson R, Zuccato C, Belyaev ND, Guest DJ, Cattaneo E, Buckley NJ. 2008. A microRNA-based gene dysregulation pathway in Huntington’s disease. Neurobiol Dis 29:438–445. Jones-Rhoades MW, Bartel DP, Bartel B. 2006. MicroRNAS and their regulatory roles in plants. Annu Rev Plant Biol 57:19–53. Junn E, Lee KW, Jeong BS, Chan TW, Im JY, Mouradian MM. 2009. Repression of alpha-synuclein expression and toxicity by microRNA-7. Proc Nat Acad Sci U S A 106:13052–13057. Kai ZS, Pasquinelli AE. 2010. MicroRNA assassins: factors that regulate the disappearance of miRNAs. Nat Struct Mol Biol 17:5–10. Kandel ER, Schwartz JH, Jessell TM. 2000. Principles of neural science. New York: McGraw-Hill. Kedde M, Kouwenhove M van, Zwart W, Oude Vrielink JAF, Elkon R, Agami R. 2010. A Pumilio-induced RNA structure switch in p27– 30 UTR controls miR-221 and miR-222 accessibility. Nat Cell Biol 12:1014–1020. Ketting RF, Fischer SE, Bernstein E, Sijen T, Hannon GJ, Plasterk RH. 2001. Dicer functions in RNA interference and in synthesis of small RNA involved in developmental timing in C. elegans. Genes Dev 15:2654–2659. Klein ME, Lioy DT, Ma L, Impey S, Mandel G, Goodman RH. 2007. Homeostatic regulation of MeCP2 expression by a CREB-induced microRNA. Nat Neurosci 10:1513–1514. Krichevsky AM, Sonntag KC, Isacson O, Kosik KS. 2006. Specific microRNAs modulate embryonic stem cell-derived neurogenesis. Stem Cells 24:857–864. Lagos-Quintana M, Rauhut R, Lendeckel W, Tuschl T. 2001. Identification of novel genes coding for small expressed RNAs. Science 294:853–858. Lai EC. 2002. Micro RNAs are complementary to 30 UTR sequence motifs that mediate negative posttranscriptional regulation. Nat Genet 30:363–364. Larsson E, Sander C, Marks D. 2010. mRNA turnover rate limits siRNA and microRNA efficacy. Mol Syst Biol 6:1–9. Lau NC, Lim LP, Weinstein EG, Bartel DP. 2001. An abundant class of tiny RNAs with probable regulatory roles in Caenorhabditis elegans. Science 294:858–862. Lau P, De Strooper B. 2010. Dysregulated microRNAs in neurodegenerative disorders. Semin Cell Dev Biol 21:768–773. Le MTN, Xie H, Zhou B, Chia PH, Rizk P, Um M, Udolph G, Yang H, Lim B, Lodish HF. 2009. MicroRNA-125b promotes neuronal differentiation in human cells by repressing multiple targets. Mol Cell Biol 29:5290–5305. Lee Y, Jeon K, Lee JT, Kim S, Kim VN. 2002. MicroRNA maturation: stepwise processing and subcellular localization. EMBO J 21:4663– 4670. Lee Y, Ahn C, Han J, Choi H, Kim J, Yim J, Lee J, Provost P, Ra˚dmark O, Kim S, Kim VN. 2003. The nuclear RNase III Drosha initiates microRNA processing. Nature 425:415–419.
12
Meza-Sosa et al.
Lee Y, Samaco RC, Gatchel JR, Thaller C, Orr HT, Zoghbi HY. 2008. miR-19, miR-101 and miR-130 coregulate ATXN1 levels to potentially modulate SCA1 pathogenesis. Nat Neurosci 11:1137–1139. Leucht C, Stigloher C, Wizenmann A, Klafke R, Folchert A, Bally-Cuif L. 2008. MicroRNA-9 directs late organizer activity of the midbrain– hindbrain boundary. Nat Neurosci 11:641–648. Lewis BP, Shih I-Hung, Jones-Rhoades MW, Bartel DP, Burge CB. 2003. Prediction of mammalian microRNA targets. Cell 115:787–798. Li X, Carthew RW. 2005. A microRNA mediates EGF receptor signaling and promotes photoreceptor differentiation in the Drosophila eye. Cell 123:1267–1277. Li X, Jin P. 2010. Roles of small regulatory RNAs in determining neuronal identity. Nat Rev Neurosci 11:329–338. Li X, Cassidy JJ, Reinke CA, Fischboeck S, Carthew RW. 2010. A microRNA imparts robustness against environmental fluctuation during development. Cell 137:273–282. Li Y, Wang F, Lee JA, Gao FB. 2006. MicroRNA-9a ensures the precise specification of sensory organ precursors in Drosophila. Genes Dev 20:2793–2805. Liang R, Bates DJ, Wang E. 2009. Epigenetic control of microRNA expression and aging. Curr Genomics 10:184–193. Lim LP, Lau NC, Weinstein EG, Abdelhakim A, Yekta S, Rhoades MW, Burge CB, Bartel DP. 2003. The microRNAs of Caenorhabditis elegans. Genes Dev 17:991–1008. Liu J, Carmell MA, Rivas FV, Marsden CG, Thomson JM, Song JJ, Hammond SM, Joshua-Tor L, Hannon GJ. 2004. Argonaute 2 is the catalytic engine of mammalian RNAi. Science 305:1437–1441. Liu S-P, Fu R-H, Yu H-H, Li K-W, Tsai C-H, Shyu W-C, Lin S-Z. 2009. MicroRNAs regulation modulated self-renewal and lineage differentiation of stem cells. Cell Transplant 18:1039–1045. Lukiw WJ, Zhao Y, Cui JG. 2008. An NF-kappaB-sensitive micro RNA-146a-mediated inflammatory circuit in Alzheimer disease and in stressed human brain cells. J Biol Chem 283:31315–31322. Lund E, Gu¨ttinger S, Calado A, Dahlberg JE, Kutay U. 2004. Nuclear export of microRNA precursors. Science 303:95–98. Makeyev EV, Zhang J, Carrasco MA, Maniatis T. 2007. The microRNA miR-124 promotes neuronal differentiation by triggering brain-specific alternative pre-mRNA splicing. Mol Cell 27:435–448. Martinez J, Patkaniowska A, Urlaub H, Lu¨hrmann R, Tuschl T. 2002. Single-stranded antisense siRNAs guide target RNA cleavage in RNAi. Cell 110:563–574. Martino S, Di Girolamo I, Orlacchio A, Datti A, Orlacchio A. 2009. MicroRNA implications across neurodevelopment and neuropathology. J Biomed Biotechnol 2009:1–13. Mattick JS, Makunin IV. 2005. Small regulatory RNAs in mammals. Hum Mol Genet 14:R121–R132. Mehler M, Mattick J. 2007. Noncoding RNAs and RNA editing in brain development, functional diversification, and neurological disease. Physiol Rev 87:799–823. Miller BH, Wahlestedt C. 2010. MicroRNA dysregulation in psychiatric disease. Brain Res 1338:89–99. Nomura T, Kimura M, Horii T, Morita S, Soejima H, Kudo S, Hatada I. 2008. MeCP2-dependent repression of an imprinted miR-184 released by depolarization. Hum Mol Genet 17:1192–1199. Nyka¨nen A, Haley B, Zamore PD. 2001. ATP requirements and small interfering RNA structure in the RNA interference pathway. Cell 107:309–321. Opin C, August P, Tsitsiou E, Lindsay MA. 2009. MicroRNAs and the immune response. Curr Opin Pharmacol 9:514–520. Packer AN, Xing Y, Harper SQ, Jones L, Davidson BL. 2008. The bifunctional microRNA miR-9/miR-9* regulates REST and CoREST and is downregulated in Huntington’s disease. J Neurosci 28:14341–14346. Proenca CC, Gao KP, Shmelkov SV, Raffi S, Lee FS. 2011. Slitrks as emerging candidate genes involved in neuropsychiatric disorders. Trends Neurosci 34:143–153.
Provost P. 2010. MicroRNAs as a molecular basis for mental retardation, Alzheimer’s and prion diseases. Brain Res 1338:58–66. Rybak A, Fuchs H, Smirnova L, Brandt C, Pohl EE, Nitsch R, Wulczyn FG. 2008. A feedback loop comprising lin-28 and let-7 controls prelet-7 maturation during neural stem-cell commitment. Nat Cell Biol 10:987–993. Santosh PS, Arora N, Sarma P, Pal-Bhadra M, Bhadra U. 2009. Interaction map and selection of microRNA targets in Parkinson’s diseaserelated genes. J Biomed Biotechnol 363145:1–11. Schwarz DS, Hutva´gner G, Haley B, Zamore PD. 2002. Evidence that siRNAs function as guides, not primers, in the Drosophila and human RNAi pathways. Mol Cell 10:537–548. Smirnova L, Gra¨fe A, Seiler A, Schumacher S, Nitsch R, Wulczyn FG. 2005. Regulation of miRNA expression during neural cell specification. Eur J Neurosci 21:1469–1477. Sonntag KC. 2010. miRNAs and deregulated gene expression networks in neurodegeneration. Brain Res 1338:48–57. Visvanathan J, Lee S, Lee B, Lee JW, Lee SK. 2007. The microRNA miR-124 antagonizes the anti-neural REST/SCP1 pathway during embryonic CNS development. Genes Dev 21:744–749. Vo N, Klein ME, Varlamova O, Keller DM, Yamamoto T, Goodman RH, Impey S. 2006. A cAMP-response element binding proteininduced microRNA regulates neuronal morphogenesis. Proc Natl Acad Sci U S A 102:16426–16431. Wang G, Van der Walt JM, Mayhew G, Li YJ, Zu¨chner S, Scott WK, Martin ER, Vance JM. 2008a. Variation in the miRNA-433 binding site of FGF20 confers risk for Parkinson disease by overexpression of alpha-synuclein. Am J Hum Genet 82:283–289. Wang WX, Rajeev BW, Stromberg AJ, Ren N, Tang G, Huang Q, Rigoutsos I, Nelson PT. 2008b. The expression of microRNA miR107 decreases early in Alzheimer’s disease and may accelerate disease progression through regulation of beta-site amyloid precursor proteincleaving enzyme 1. J Neurosci 28:1213–1223. Wang X, Liu P, Zhu H, Xu Y, Ma C, Dai X, Huang L, Liu Y, Zhang L, Qin C. 2009. miR-34a, a microRNA up-regulated in a double transgenic mouse model of Alzheimer’s disease, inhibits bcl2 translation. Brain Res 80:268–273. Wu H, Sun S, Tu K, Gao Y, Xie B, Krainer AR, Zhu J. 2010a. A splicing-independent function of SF2/ASF in microRNA processing. Mol Cell 38:67–77. Wu H, Tao J, Chen PJ, Shahab A, Ge W, Hart RP, Ruan X, Ruan Y, Sun YE. 2010b. Genome-wide analysis reveals methyl-CpG-binding protein 2-dependent regulation of microRNAs in a mouse model of Rett syndrome. Proc Natl Acad Sci U S A 107:18161–18166. Yi R, Qin Y, Macara IG, Cullen BR. 2003. Exportin-5 mediates the nuclear export of pre-microRNAs and short hairpin RNAs. Genes Dev 17:3011–3016. Yi YH, Sun XS, Qin JM, Zhao QH, Liao WP, Long YS. 2010. Experimental identification of microRNA targets on the 30 untranslated region of human FMR1 gene. J Neurosci Methods 190:34–38. Zeng Y. 2003. Sequence requirements for micro RNA processing and function in human cells. RNA 9:112–123. Zhao X, He X, Han X, Yu Y, Ye F, Chen Y, Hoang T, Xu X, Mi QS, Xin M, Wang F, Appel B, Lu QR. 2010. MicroRNA-mediated control of oligodendrocyte differentiation. Neuron 65:612–626. Zhao Y, Srivastava D. 2007. A developmental view of microRNA function. Trends Biochem Sci 32:189–197. Zhou R, Yuan P, Wang Y, Hunsberger JG, Elkahloun A, Wei Y, Damschroder-Williams P, Du J, Chen G, Manji HK. 2009. Evidence for selective microRNAs and their effectors as common long-term targets for the actions of mood stabilizers. Neuropsychopharmacology 34: 1395–1405.
Journal of Neuroscience Research
