In striking contrast to ERK1/2 regulation of the Schwann cell lineage, Erk1/2 deleted oligodendrocytes were capable of myelination. The early lethality of Erk1/2CKO(Olig2) mice limited our analysis to

only the initial stages of myelination, however, a clear increase in MBP labeling is apparent in P1 Erk1/2CKO(Olig2) ventral spinal cords ( Figures 8E and 8F). S100β labeled oligodendrocytes in the white matter of mutant embryos exhibited a more ramified, complex morphology than controls, further suggesting that loss of Erk1/2 triggered premature differentiation ( Figures 8A and 8B, data not shown). Coimmunostaining of MBP positive cells with an ERK2 antibody confirmed that myelinating oligodendrocytes were truly ERK1/2 deficient in mutants ( Figures S8C and S8D). These Selleckchem ATR inhibitor data show that, in contrast to Schwann cells, myelination by oligodendrocytes can proceed in the absence of Erk1/2. We have assessed the functions of ERK1/2 and ERK5 in distinct cell types during PNS development in vivo. Our data lead to several clear conclusions. First, many aspects of embryonic sensory and motor neuron development occur normally in the setting of Erk1/2 deletion, although sensory axons do not invade NGF-expressing target fields. Second, ERK5 does not appear to strongly regulate embryonic PNS development.

Third, ERK1/2 is critical for fundamental aspects of Schwann cell development. Erk1/2 deletion phenotypes resemble those of Nrg-1 and ErbB mutants, and Erk1/2 deleted Schwann cell progenitors do not respond to neuregulin-1. Finally, the requirement of ERK1/2 for myelination is specific PD0325901 in vitro to Schwann cells, as myelination

by oligodendrocytes can proceed in the absence of Erk1/2. Overall, our findings tightly link in vivo functions of ERK/MAPK signaling to biological actions of specific RTK activating factors. Gene targeting studies have defined roles for numerous trophic factors, ECM molecules, and axon guidance cues in directing PNS neuron development (Marmigere and Ernfors, 2007). However, the signaling pathways mediating these effects in vivo old have not been defined. Many growth promoting cues appear to converge upon ERK1/2, and combinations of trophic stimuli, such as integrins and growth factors, trigger synergistic ERK1/2 activation (Perron and Bixby, 1999). Overall, these data predict that ERK1/2 is a central regulator of neuronal morphology and development in vivo. In spite of a wealth of in vitro data, our in vivo findings provide surprisingly little support for a broad and essential role for ERK1/2 for the acquisition of neuronal phenotypes, survival, or initial axon outgrowth. Our results instead show that ERK1/2 signaling is required for NGF-mediated cutaneous sensory neuron innervation at late embryonic and early postnatal stages. These results are generally consistent with previous findings in B-Raf/C-Raf and SRF conditional knockout mice ( Wickramasinghe et al., 2008 and Zhong et al.