miRNAs are essential for survival and differentiation of newborn neurons but not for expansion of neural progenitors

INTRODUCTION The cerebral cortex, the site of higher brain function, has undergone dramatic expansion during mammalian, and notably primate, evolution (Abdel-Mannan et al., 2008; Caviness et al., 1995; Molnar et al., 2006; Rakic, 1995; Rakic, 2007). The concomitant increase in neuron number is, in essence, due to an increase in neural progenitors that undergo neurogenic divisions (Götz and Huttner, 2005; Kriegstein et al., 2006). There are two principal classes of neural progenitors that generate the neurons of the mammalian cerebral cortex: (1) the progenitors dividing at the ventricular (apical) surface of the ventricular zone (VZ) (neuroepithelial cells, radial glia and short neural precursors, collectively referred to as apical progenitors); and (2) the progenitors that divide in the basal region of the VZ and in the subventricular zone (SVZ) (referred to as basal progenitors, also called intermediate, non-surface or SVZ progenitors) (Götz and Huttner, 2005; Kriegstein et al., 2006). These distinct neural progenitors can divide to generate either progenitors, neurons, or both. The molecular machinery that regulates the balance between apical and basal progenitors, and between their neurogenic and non-neurogenic divisions, is largely unknown. MicroRNAs (miRNAs) are a class of small RNAs that bind to specific mRNA targets, directing their degradation and/or repressing their translation (Hannon et al., 2006; Stefani and Slack, 2008). Approximately 70% of known miRNAs are expressed in the mammalian brain (Cao et al., 2006), and the level of many miRNAs changes dramatically during brain development (Krichevsky et al., 2003; Miska et al., 2004; Sempere et al., 2004). Indeed, based on observations obtained with cell culture models in vitro, miRNAs have been implicated in the control of neuronal differentiation (Conaco et al., 2006; Krichevsky et al., 2006; Makeyev et al., 2007; Smirnova et al., 2005; Wu and Belasco, 2005). Many of these investigations have focused on the in vitro role of miR-124, one of the most abundant miRNAs in the brain, which is highly enriched in neurons (De Pietri Tonelli et al., 2006; Hohjoh and Fukushima, 2007; Lagos-Quintana et al., 2002). These studies have revealed an important role of miRNAs in the differentiation of postmitotic neurons in vitro. To explore a possible role of miRNAs in neuronal differentiation during the development of the mammalian nervous system in vivo, recent studies have investigated the consequences of the genetic ablation of Dicer (Dicer1 – Mouse Genome Informatics), one of the essential enzymes for the production of endogenous small interfering RNAs (siRNAs) (Watanabe et al., 2008) and for miRNA maturation (Bernstein et al., 2001; Hutvagner et al., 2001). Dicer ablation in various specific subpopulations of neurons has been found to impair neuronal differentiation and cause neurodegeneration and neuronal cell death (Cuellar et al., 2008; Davis et al., 2008; Kim et al., 2007; Schaefer et al., 2007). Although the most recent of these reports [Cuellar et al., 2008; Davis et al., 2008 (which appeared while the present study was being prepared miRNAs are essential for survival and differentiation of newborn neurons but not for expansion of neural progenitors during early neurogenesis in the mouse embryonic neocortex