, 2006, Stegmüller et al., 2006 and Stegmüller et al., 2008). Expression of SnoN alone can overcome myelin-dependent growth inhibition, suggesting that SnoN drives a genetic program that promotes axon growth under different extrinsic stimuli (Stegmüller et al., 2006). Interestingly, in contrast to the opposing functions of SnoN1 Androgen Receptor signaling Antagonists and SnoN2 in the control of granule neuron migration and positioning, the two isoforms of SnoN collaborate to promote axon growth (Huynh et al., 2011 and Stegmüller et al., 2006). Although SnoN is widely considered to have transcriptional repressive

functions (Luo, 2004), including in the control of neuronal positioning (Huynh et al., 2011), SnoN functions as a transcriptional coactivator in the control of axon growth (Figure 3; Ikeuchi et al., 2009). In particular, SnoN associates with the histone acetyltrasferase p300 and thereby induces the expression of a large set of genes in neurons (Ikeuchi et al., 2009). These findings support

the concept that SnoN acts in a dual transcriptional activating or repressive manner in a cell-or target-specific manner (Pot and Bonni, 2008 and Pot et al., 2010). In promoting axon growth, the cytoskeletal scaffold protein Ccd1 represents a critical downstream target of SnoN (Ikeuchi et al., 2009). Ccd1 localizes to the actin cytoskeleton at growth cones and activates the protein kinase c-Jun kinase (JNK) (Ikeuchi et al., 2009), which has been implicated CHIR 99021 in axon growth (Oliva et al., 2006). Whereas SnoN drives axon growth by triggering the expression of regulators of the actin cytoskeleton, Id2 is thought to promote axon growth by antagonizing the function of the bHLH transcription factor E47, which induces the expression of a number of genes involved in axon repulsion including NogoR, Sema3F, and Unc5A (Lasorella et al., 2006). Thus, Id2 stimulates axon growth by modulating the response of neurons to guidance cues. Interestingly, TGFβ signaling through the mafosfamide protein Smad2 regulates the abundance of SnoN protein and consequently axon growth (Stegmüller et al., 2008), thus highlighting how intrinsic determinants integrate signals

from extrinsic cues for proper development. Although transcriptional regulators such as NFAT, SnoN, and Id2 appear to regulate axon growth in postmitotic neurons, transcription factors that primarily regulate neurogenesis may also coordinate axon growth in differentiated neurons. In studies of retinotectal projection neurons and spinal cord motor neurons, several transcription factors including Vax2, Zic2, Lim1, and Lmx1b have been reported to regulate the timely and cell-specific expression of proteins involved in axon guidance, including Ephrins A and B and their receptors (Barbieri et al., 2002, Dufour et al., 2003, Herrera et al., 2003, Kania and Jessell, 2003, Kania et al., 2000, Mui et al., 2002, Schulte et al., 1999 and Williams et al., 2003).