The establishment of neuronal properties is controlled by Sox4 and Sox11

Abstract

The progression of vertebrate neurogenesis relies on mechanisms that in an orderly fashion direct precursor cells to exit the cell cycle, down-regulate progenitor cell identities, and to subsequently initiate the expression of neuronal properties. Despite advances in defining mechanisms that control the initiation of neurogenesis, the genetic program that drives the acquisition of the neuronal phenotype of post-mitotic neurons remains to be characterized.

Insights into the mechanisms that regulate pan-neuronal gene expression have been derived from studies of the zinc finger repressor protein REST (RE1 silencing transcription factor, also known as NRSF) (Chong et al. 1995; Schoenherr and Anderson 1995), which is ubiquitously expressed in neural precursors and has the capacity to bind and repress a large number of genes encoding neuronal proteins. In contrast, the proneural basic helix–loop–helix (bHLH) transcription factors, including Ngn1, Ngn2, and Mash1, function in neural stem cells to initiate the progression of neurogenesis (Bertrand et al. 2002). While proneural bHLH proteins mediate this function by committing stem cells to the neuronal linage and by inducing cell cycle exit, their expression is generally suppressed before progenitor cells exit the proliferative zone and begin to express neuronal properties (Gradwohl et al. 1996; Fode et al. 2000). Thus, the ability of proneural proteins to promote the terminal steps of neurogenesis must rely on downstream regulatory programs that subsequently establish the expression of neuronal properties in post-mitotic neural cells.

The bHLH genes Math3 and NeuroD (Lee et al. 1995; Perron et al. 1999) and the non-basic HLH gene Ebf1 (Garcia-Dominguez et al. 2003) are examples of transcription factors that have been suggested to function downstream from proneural bHLH proteins during the maturation steps of neurogenesis. Despite the fact that these proteins can induce ectopic formation of neurons in Xenopus (Lee et al. 1995; Perron et al. 1999; Garcia-Dominguez et al. 2003), mice deficient for NeuroD, Math3, or Ebf1 display only minor neurogenic defects (Naya et al. 1997; Garel et al. 1999; Tomita et al. 2000), and their role during neurogenesis remains unclear. Furthermore, a substantial number of neurons are generated prior to the induction of NeuroD expression (Lee et al. 1995; Roztocil et al. 1997). Thus, the molecular mechanism that controls the terminal steps of neurogenesis and the expression of neuronal properties has not yet been identified.

The HMG-box transcription factors of the Sox gene family have diverse regulatory functions during the formation of the vertebrate CNS (Pevny and Placzek 2005). Sox1, Sox2, and Sox3, which are expressed by most precursor cells, act to maintain the expression of progenitor identities and thus preserve cells in an undifferentiated state (Bylund et al. 2003; Graham et al. 2003), whereas another HMG-box protein, Sox21, has the opposite activity and allows cells to initiate a differentiation program (Sandberg et al. 2005). Hence, B-group Sox proteins appear to have key regulatory roles in the commitment of progenitors to neurogenesis. In contrast to Sox1–3 and Sox21, Sox4 and Sox11, which constitute the C-group of the Sox gene family (Kamachi et al. 2000), are mainly expressed in neural cells that have already been committed to neuronal differentiation (Uwanogho et al. 1995; Cheung et al. 2000), raising the possibility that these proteins control later aspects of neurogenesis. Mice, in which either the function of Sox4 or Sox11 has been inactivated, do not reveal any significant role of C-group Sox proteins during neurogenesis (Cheung et al. 2000; Sock et al. 2004), but structural similarities and the conserved expression patterns among these proteins indicate that functional redundancy may compensate for the loss of an individual Sox4 or Sox11 gene.

In this study, we have examined the role of Sox4 and Sox11 in the formation of neurons in the vertebrate CNS. We report that Sox4 and Sox11 operate downstream from proneural bHLH proteins and are vital for the establishment of pan-neuronal protein expression. Interestingly, misexpression of Sox4 and Sox11 does not cause progenitor cells to exit the division cycle or commit to a neuronal differentiation program. Instead, Sox4 and Sox11 can induce precocious expression of neuronal markers in self-renewing precursors. Examination of a neuronal gene promoter indicates that Sox4 and Sox11 can mediate their functions as transcriptional activators. Collectively, these findings establish an essential role of Sox4 and Sox11 in neuronal maturation and separate mechanistically cell cycle exit and the induction of pan-neuronal protein expression.

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