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MicroRNA-124: Micromanager of Neurogenesis

2009; Elsevier BV; Volume: 4; Issue: 5 Linguagem: Inglês

10.1016/j.stem.2009.04.007

1934-5909

Thales Papagiannakopoulos, Kenneth S. Kosik,

RNA Research and Splicing

In a recent issue of Nature Neuroscience, Cheng et al., 2009Cheng L.-C. Pastrana E. Tavazoie M. Doetsch F. Nat. Neurosci. 2009; 12: 399-408Crossref PubMed Scopus (760) Google Scholar demonstrate that miR-124, the most abundant of the microRNAs in the adult brain, positively modulates the transitory progression of adult subventricular zone (SVZ) neurogenesis. In a recent issue of Nature Neuroscience, Cheng et al., 2009Cheng L.-C. Pastrana E. Tavazoie M. Doetsch F. Nat. Neurosci. 2009; 12: 399-408Crossref PubMed Scopus (760) Google Scholar demonstrate that miR-124, the most abundant of the microRNAs in the adult brain, positively modulates the transitory progression of adult subventricular zone (SVZ) neurogenesis. In settings as diverse as malignant transformation to stem cell differentiation, when cells change their identities, microRNAs are likely to be involved. Undergoing an identity change must be a precarious moment for a cell, which has to compress into a very brief interval a complete sweep of its former lifestyle while simultaneously making the transition to its new lifestyle. Maintaining a selection of key messages poised and ready for expression (rather than reaching back into the nucleus in response to any differentiation cue) appears to be a fundamental element of developmental transitions. This "stand-by" task belongs to the microRNAs. With over 700 miRNA genes identified in the human genome, and a plethora of computationally predicted mRNA targets, these small noncoding RNAs (21–22 nt) bind 3′ untranslated regions (3′UTRs) of target mRNAs and repress their translation and/or lead to degradation of the target mRNAs. The partial complementarity of the miRNA/mRNA duplex allows for miRNAs to target multiple 3′UTRs of genes that may share in the regulation of common cellular processes (Gangaraju and Lin, 2009Gangaraju V.K. Lin H. Nat. Rev. Mol. Cell Biol. 2009; 10: 116-125Crossref PubMed Scopus (563) Google Scholar). One brain-enriched miRNA, miR-124, first rose to fame when it was transfected into nonneuronal HeLa cells and resulted in a shift in the expression profile toward brain genes (Lim et al., 2005Lim L.P. Lau N.C. Garrett-Engele P. Grimson A. Schelter J.M. Castle J. Bartel D.P. Linsley P.S. Johnson J.M. Nature. 2005; 433: 769-773Crossref PubMed Scopus (3785) Google Scholar). More than 100 messages were downregulated, and most of these "targets" were nonneural genes in need of suppression to achieve a neural identity. However, it is now apparent that the in vitro experiments conducted in this study did not reveal the subtly of miR-124 regulation. Cheng et al., 2009Cheng L.-C. Pastrana E. Tavazoie M. Doetsch F. Nat. Neurosci. 2009; 12: 399-408Crossref PubMed Scopus (760) Google Scholar have now performed elegant analyses of the role of miR-124 in the adult SVZ, a brain region of keen interest due to its ongoing neurogenesis into adulthood. SVZ neurogenesis is regulated by the olfactory experience and learning of animals (Lledo et al., 2006Lledo P.-M. Alonso M. Grubb M.S. Nat. Rev. Neurosci. 2006; 7: 179-193Crossref PubMed Scopus (1075) Google Scholar), which induces resident cells to migrate outward along a path called the rostral migratory stream (RMS) and ultimately differentiate into olfactory bulb (OB) neurons that express NeuN (Lledo et al., 2006Lledo P.-M. Alonso M. Grubb M.S. Nat. Rev. Neurosci. 2006; 7: 179-193Crossref PubMed Scopus (1075) Google Scholar) (Figure 1). The call for new olfactory neurons is answered by cells in the SVZ, by way of a sequence of epigenetic changes in cell identity starting from type B glial fibrillary acidic protein (GFAP)-positive progenitors that give rise to type C transit-amplifying cells, which in turn expand the progenitor pool and produce type A migrating neuroblasts, which express doublecortin (Dcx) and Tuj1. After traversing the RMS, type A cells populate the OB as granule and periglomerular neurons. Multiple cell-autonomous and nonautonomous signals regulate the migration and differentiation of these cells. Extrinsic factors such as cell-cell interactions and signals from the extracellular matrix control the migration and differentiaion of SVZ neuroblasts. These intracellular and extracellular events orchestrate transcriptional and posttranscriptional gene regulation, controlling cellular differentiation through precise transitions between cell types in a migratory path. The question posed by Cheng et al., 2009Cheng L.-C. Pastrana E. Tavazoie M. Doetsch F. Nat. Neurosci. 2009; 12: 399-408Crossref PubMed Scopus (760) Google Scholar related to how miRNA-124 might integrate these various signals within the differentiated progenitors. miR-124 expression increases during CNS development (Krichevsky et al., 2006Krichevsky A.M. Sonntag K.-C. Isacson O. Kosik K.S. Stem Cells. 2006; 24: 857-864Crossref PubMed Scopus (566) Google Scholar). Furthermore, miR-124 expression is negatively regulated by RE-1-silencing transcription repressor (REST) binding to its promoter (Conaco et al., 2006Conaco C. Otto S. Han J.-J. Mandel G. Proc. Natl. Acad. Sci. USA. 2006; 103: 2422-2427Crossref PubMed Scopus (575) Google Scholar). Loss of REST during neuronal differentiation is thought to lead to derepression of miR-124. Blocking miR-124 activity in mature neurons leads selectively to increased levels of nonneuronal transcripts (Conaco et al., 2006Conaco C. Otto S. Han J.-J. Mandel G. Proc. Natl. Acad. Sci. USA. 2006; 103: 2422-2427Crossref PubMed Scopus (575) Google Scholar), consistent with the pattern observed in the original HeLa experiment described above (Lim et al., 2005Lim L.P. Lau N.C. Garrett-Engele P. Grimson A. Schelter J.M. Castle J. Bartel D.P. Linsley P.S. Johnson J.M. Nature. 2005; 433: 769-773Crossref PubMed Scopus (3785) Google Scholar). Building on these in vitro results, Cheng et al. have now demonstrated an active role of miR-124 in adult mouse SVZ neurogenesis (Cheng et al., 2009Cheng L.-C. Pastrana E. Tavazoie M. Doetsch F. Nat. Neurosci. 2009; 12: 399-408Crossref PubMed Scopus (760) Google Scholar). The authors first assessed the levels of miR-124 present in precursor subpopulations as they migrate through different physical regions of the RMS. By in situ hybridization of tissue sections and by qRT-PCR of fluorescence-activated cell sorting (FACS)-isolated cells, they showed that an increasing gradient of miR-124 expression is present throughout the RMS starting from the SVZ and ending at the OB (Figure 1). miR-124 was most significantly elevated in type A neuroblasts after their transition from transit-amplifying type C cells. Interestingly, within the population of neuroblasts, miR-124 was highly expressed in cells at G0/G1. These data correlate well with previous studies demonstrating that miR-124 represses Cdk6, a protein mediating cell-cycle progression from G0/G1 (Silber et al., 2008Silber J. Lim D. Petritsch C. Persson A. Maunakea A. Yu M. Vandenberg S. Ginzinger D. James C.D. Costello J. et al.BMC Med. 2008; 6: 14Crossref PubMed Scopus (744) Google Scholar). To test experimentally the role of miR-124 in neurogenesis, Cheng et al., 2009Cheng L.-C. Pastrana E. Tavazoie M. Doetsch F. Nat. Neurosci. 2009; 12: 399-408Crossref PubMed Scopus (760) Google Scholar performed miR-124 gain-of-function (GOF) and loss-of-function (LOF) experiments using retrovirus (RV-124) and modified antisense miR-124 oligos (AS-124), respectively. These experiments were performed in FACS-isolated cells as well as in vivo. Knockdown of miR-124 (AS-124) led to an increase in type C and dividing type A cells. This expansion corresponds with an elevation in the neurosphere-forming capacity of type C transit-amplifying cells, but not neuroblasts. On the other hand, overexpression of miR-124 (RV-124) led to cell-cycle exit and promoted neuronal differentiation, as observed by an increase in neuronal markers and a decrease in astrocyte markers. Next, to determine the role of miR-124 in regeneration, the authors performed antimitotic treatment, which leads to regeneration of the adult SVZ by nondividing type B stem cell astrocytes. Knockdown of miR-124 upon antimitotic treatment (Ara-C) led to an increase in proliferation markers and the appearance of hyperplasias. Overall, there was a delay in regeneration, but neuronal differentiation was not permanently blocked. These results indicate that miR-124 is necessary, but not sufficient, to drive the differentiation of stem cells/progenitors to neurons. Both LOF and GOF experiments support the hypothesis that miR-124 is a regulator of the SVZ lineage progression primarily at the transition of the type C transit-amplifying cells to the type A neuroblasts, both in homeostasis and regeneration. How is miR-124 exerting its effect? To address this question, the authors identified three targets of miR-124—jag1, Dlx2, and Sox9. By LOF and GOF of sox9, they demonstrate that miR-124's induction of neurogenesis is mediated in part by repressing sox9, whose role in SVZ neurogenesis was previously unknown. Fine control of Sox9 by miR-124 is critical in the glial-to-neuron transition along the SVZ stem cell lineage. In addition to sox9, jag1 and dlx2, previously identified miR-124 targets, which are known to regulate processes such as proliferation, differentiation, and cell adhesion/migration, are likely to be involved in this migratory neurogenic cascade (Cao et al., 2007Cao X. Pfaff S.L. Gage F.H. Genes Dev. 2007; 21: 531-536Crossref PubMed Scopus (289) Google Scholar, Makeyev and Maniatis, 2008Makeyev E.V. Maniatis T. Science. 2008; 319: 1789-1790Crossref PubMed Scopus (214) Google Scholar, Visvanathan et al., 2007Visvanathan J. Lee S. Lee B. Lee J.W. Lee S.-K. Genes Dev. 2007; 21: 744-749Crossref PubMed Scopus (544) Google Scholar). The exquisite control over levels of miR-124 suggests that this specific miRNA is critical for the homeostasis of differentiation versus proliferation. This role is supported by a report of miR-124 upregulation in brain-tumor-derived stem cells that leads to differentiation and loss of stem cell identity (Silber et al., 2008Silber J. Lim D. Petritsch C. Persson A. Maunakea A. Yu M. Vandenberg S. Ginzinger D. James C.D. Costello J. et al.BMC Med. 2008; 6: 14Crossref PubMed Scopus (744) Google Scholar). However, a complete understanding of such a highly coordinated phenomenon as neurogenesis will require the precise quantification of miR-124 copy numbers concomitantly with copy numbers of its target set. The findings of Cheng et al., 2009Cheng L.-C. Pastrana E. Tavazoie M. Doetsch F. Nat. Neurosci. 2009; 12: 399-408Crossref PubMed Scopus (760) Google Scholar, in conjunction with previous reports, support the hypothesis that miRNAs exercise considerable control over cellular identity and differentiation, and highlight that specific miRNAs likely mediate, alone or in combination, particular developmental transitions.