, 2008) also contributes to the impaired polarization phenotype, because defective migration may prevent

proper reception of other polarizing factors along their migratory route. Furthermore, downregulation of Sema3A signaling in these cortical VEGFR inhibitor progenitors resulted in significant reduction of the growth of the leading process in cells located at the IZ and CP (Figure 6). Together with our findings on the effect of Sema3A on the selective promotion of dendrite growth and suppression of axon growth in cultured hippocampal neurons (Figure 5), these results would support the notion that Sema3A might regulate neuronal polarization and dendrite development by acting directly to promote dendrite growth. We note that the identity of the AZD2281 specific Plexin coreceptors that mediate the Sema3A effects on neuronal polarization together with NP1 remain to be determined.

Intracellular signaling pathways involved in neuronal polarization have been extensively examined in cultured neurons, but the extracellular polarizing factors that activate these signaling pathways in vivo remain largely unknown. Secreted molecules such as BDNF (Yoshimura et al., 2005 and Shelly et al., 2007), NGF (Da Silva et al., 2005), Insulin-like growth factor-1 (IGF-1) (Sosa et al., 2006), netrin-1 (Mai et al., 2009), and transforming growth factor beta (TGF-β) (Yi et al., 2010) were shown to promote axon initiation and growth in cultured hippocampal neurons, although our stripe assay failed to show the polarizing effect of NGF and netrin-1 (Figure 1). CYTH4 The latter discrepancy may be caused by the differences in the culture conditions or the sensitivity of the

methods for assaying polarization. However, no axon formation defect was detected in mice with targeted deletion of genes for NGF (Crowley et al., 1994) and BDNF (Jones et al., 1994), or for their respective receptors TrkA (Smeyne et al., 1994) and TrkB (Klein et al., 1993). A notable exception is TGF-β, whose receptors are essential for axon formation in embryonic cortical neurons in vivo (Yi et al., 2010). We note that in utero electroporation was used in the latter study to perturb the signaling of TGF-β receptors in a subpopulation of cortical neurons, unlike the earlier studies with genetic deletion over entire population of neurons throughout prolonged developmental period. Differences between the methods used to assay effects of gene downregulation may account for the lack of apparent effects in some of these studies. In this study, we provided evidence that Sema3A may also regulate neuronal polarization in vivo. However, neuronal polarization in the developing brain is likely to depend on the coordinated actions of many extracellular factors. In addition to Sema3A (Polleux et al., 2000 and Chen et al., 2008), the spatially regulated expression of other secreted molecules in the developing cortex has been reported.