The Role of MicroRNAs in Neural Stem Cells and Neurogenesis

Abstract

Neural stem cells give rise to neurons through the process of neurogenesis, which includes neural stem cell proliferation, fate determination of new neurons, as well as the new neuron's migration, maturation and integration. Currently, neurogenesis is divided into two phases: embryonic and adult phases. Embryonic neurogenesis occurs at high levels to form the central nervous system. Adult neurogenesis has been consistently identified only in restricted regions and occurs at low levels. As the basic process for embryonic neurodevelopment and adult brain maintenance, neurogenesis is tightly regulated by many factors and pathways. MicroRNA, short non-coding RNA that regulates gene expression at the post-transcriptional level, appears to be involved in multiple steps of neurogenesis. This review summarizes the emerging role of microRNAs in regulating embryonic and adult neurogenesis, with a particular emphasis on the proliferation and differentiation of neural stem cells.

Introduction

Neural stem cells (NSCs) are a class cells capable of self-renewing and giving rise to different neural lineages, such as neurons, astrocytes, and oligodendrocytes (Gage, 2000). To progress from NSC to functional neurons requires the process known as neurogenesis, which includes the proliferation of the neural stem/progenitor cells (NSPCs), the differentiation of neurons, and the integration of new neurons into the existing neural circuitry. As the basic process of neuron generation, neurogenesis plays important roles during the embryonic development and adult nervous system maintenance. In the embryo, the neuroepithelial cells within the neural tube are the common source of new neurons (Kintner, 2002). During the development of the brain, neurogenesis in the embryo occurs at a high level, forming the central nervous system (CNS). In the adult brain, the newborn neuronal cells are derived from adult NSCs. To date, adult NSCs have been persistently found in the subventricular zone (SVZ) and the hippocampal subgranular zone (SGZ). Two types of NSCs have been identified based on their morphology, cellular markers, and proliferative capacity (Zhao et al., 2008). In the SVZ, a population of GFAP⁺ and CD133⁺ radial glia-like progenitors, type-B cells, has been hypothesized to be the primary source of NSCs, which subsequently generate DCX⁺ PSA⁻NCAM⁺ neuroblasts through intermediate progenitors. The neuroblasts then migrate to the olfactory bulb (OB) and differentiate into GABA⁺ dopamine⁺ neurons. In the SGZ, a population of GFAP⁺ Sox2⁺ and Nestin⁺ radial cells was found to be the reservoir of NSCs and may generate the non-radial Sox2⁺ GFAP⁻ cells, which actively self-renew and subsequently differentiate into neurons. Compared with embryonic neurogenesis, adult neurogenesis is largely restricted to niches in which both proliferation and neuronal differentiation are under tight regulation. Moreover, the frequency of neurogenesis in the adult brain occurs at much lower levels. Altogether, NSCs are the cellular basis of neurogenesis, and neurogenesis is a fundamental process for both embryonic neurodevelopment and adult brain maintenance.

The progression from an NSC to a mature neuron is tightly regulated by various signaling pathways and factors. For example, the Wnt signaling pathway regulate NSC proliferation and differentiation both in embryonic development and adult neurogenesis (Lie et al., 2005; Wexler et al., 2009). In addition, the disruption of the Sonic Hedgehog (Shh) signaling pathway decreases the level of NSC proliferation (Lai et al., 2003). TLX (also known as NR2E1, nuclear receptor subfamily 2 group E member 1), an orphan nuclear receptor, plays an essential role in regulating the activity of NSCs (Shi et al., 2004; Liu et al., 2008; Zhang et al., 2008). In addition, some transcription factors, such as AscH, Neurog2, and Tbr2, influence the differentiation of NSCs (Ozen et al., 2007; Jessberger et al., 2008; Brill et al., 2009). Some epigenetic factors, such as MBD1, MeCP2, and Gadd45b, are also closely associated with adult neurogenesis (Zhao et al., 2003; Smrt et al., 2007; Ma et al., 2009).

In addition to the above-mentioned signaling pathways and factors, neurogenesis is regulated by a novel class of modulators named microRNAs (miRNAs). MiRNAs are endogenously encoded single-stranded RNAs that can post-transcriptionally regulate gene expression by targeting mRNAs for cleavage or translational repression (Bartel, 2004). Thus far, hundreds of miRNAs have been identified in mammals (miRBase, http://www.mirbase.org/). These molecules are derived from independent transcription units or the introns of protein-coding genes with single or clustered distributions in the genome. A miRNA gene is transcribed into a primary transcript called the pri-miRNA. The pri-miRNA is then cleaved in the nucleus by the RNAase III endonuclease Drosha to produce a 60-70 nt stem-loop intermediate known as the pre-miRNA. The pre-miRNA is subsequently exported to the cytoplasm, where it is processed by Dicer to produce a shorter, double-stranded RNA duplex. The Dicer cleavage process is coupled with the binding of the mature miRNA to the RNA-induced silencing complex (RISC) (Rische and Klug, 2012). The miRNA then directs the RISC to its target mRNA by a perfect match in the so-called seed region of 6-8 nt at the 5′ end (Bartel, 2009).

The interaction between the miRNA and mRNA leads to the downregulation of protein expression by translational repression, mRNA degradation, or the promotion of mRNA decay. More than 60% of all mammalian mRNAs are under the control of miRNAs. Because the size and binding specificity of miRNAs are limited, a single miRNA always targets hundreds of mRNAs, and one mRNA can be targeted by multiple miRNAs. Therefore, miRNAs often act as fine-tuning devices rather than as primary gene regulators. Nevertheless, miRNAs may impact physiological processes by regulating the key cellular proteins in a single or related pathway, because many signaling pathways have been identified in the proliferation and differentiation of NSCs.

In the mouse brain, approximately 70% of the miRNAs are detectable. In mice, Dicer deficiency results in the abnormal development of the CNS and the failure to form appropriate cellular and tissue morphogenesis in the cortex and hippocampus; it also affects neurogenesis and gliogenesis (Davis et al., 2008; Kawase-Koga et al., 2009; Huang et al., 2010). The deficiency of Ago2, a component of the RISC, results in defects in neural tube closure and mis-patterning of the forebrain (Liu et al., 2004). Together, these observations indicate that miRNAs biogenesis pathway plays important roles in the development of the central nervous system.

To date, many miRNAs that are specifically or richly expressed in mammalian brain have been identified. For example, miR-124, which accounts for 25%–48% of all brain miRNAs, is the best studied (Lagos-Quintana et al., 2002). miR-9 is a brain-specific miRNA, abundantly expressed in neurogenic regions in embryos and adults (Deo et al., 2006). Let-7 and miR-137 are richly expressed in both embryonic and adult brains (Miska et al., 2004). miR-124a, miR-125b, miR-128, miR-132, and miR-219 are abundantly detected in the fetal hippocampus (Lukiw, 2007). During the differentiation of embryonic stem cells, the expression dynamics of a set of highly expressed neural miRNAs were also evaluated (Smirnova et al., 2005). For instance, miR-124 and miR-128 tend to be expressed in neurons, whereas miR-23 expression is mostly restricted to astrocytes. The expression of miR-26 and miR-29 is higher in astrocytes than in neurons. Indeed, the spatial and temporal expression characteristics of miRNAs indicate their important roles in NSC proliferation and differentiation.

Many miRNAs are associated with the pathogenesis of neurological diseases. For example, miR-19b, miR-302* and miR-323-3p contribute to the pathology of Fragile X syndrome by repressing the expression of fragile X mental retardation protein (FMRP) (Yi et al., 2010). miR-181b plays a role in schizophrenia by targeting visinin-like 1 (VSNL1) and glutamate receptor subunit (GRIA2) (Beveridge et al., 2008). In patients with Alzheimer's disease, the expression of miR-9 and miR-128 is upregulated, while miR-15a and miR-107 are downregulated (Lukiw, 2007; Wang et al., 2008). These studies strongly support the important roles of miRNAs in neurogenesis. In the present review, we summarize the effects of miRNAs on NSC proliferation and differentiation at both the embryonic and adult levels (Fig. 1).