FGF signaling is required for chemokinesis and ventral migration of trunk neural crest cells.

BACKGROUND: Neural crest cells (NCCs) delaminate from the neural tube (NT) and migrate ventrally to generate the trunk peripheral nervous system (PNS). Although several signaling pathways have been identified that steer NCCs once they are on their ventral trajectory, no molecules have been identified that are required for the initial migration between the NT and the dorsal root ganglion. Given the critical role of fibroblast growth factor (FGF) signaling in embryogenesis, we investigated its function in this initial migration. RESULTS: FGFR1 signaling is required for the migration of newly delaminated NCCs onto the ventral pathway. Live imaging of migrating NCCs revealed that inhibition of FGFR1 signaling caused the dorsally stalled NCCs to lose their dorsal/ventral oriented polarity and instead adopt a rounded morphology while dynamically extending filopodia. FGF8, an FGFR1 ligand, increased motility of NCCs away from the NT by acting chemokinetically. Finally, we provide evidence that inhibition of FGFR1-mediated chemokinesis is partially rescued by increasing Akt signaling, inhibiting RhoA, and activation of N-cadherin signaling. CONCLUSION: These data support a model in which NCCs are stimulated chemokinetically by FGF:FGFR1 signaling, and that this activation positions and orients NCCs on their ventral migratory route-a process that is essential for patterning the trunk PNS.

PACAP promotes sensory neuron differentiation: blockade by neurotrophic factors.

Developing neurons encounter a panoply of extracellular signals as they differentiate. A major goal is to identify these extrinsic cues and define the mechanisms by which neurons simultaneously integrate stimulation by multiple factors yet initiate one specific biological response. Factors that are known to exert potent activities in the developing nervous system include the NGF family of neurotrophic factors, ciliary neurotrophic factor (CNTF), and pituitary adenylate cyclase-activating peptide (PACAP). Here we demonstrate that PACAP promotes the differentiation of nascent dorsal root ganglion (DRG) neurons in that it increases both the number of neural-marker-positive cells and axonogenesis without affecting the proliferation of neural progenitor cells. This response is mediated through the PAC1 receptor and requires MAP kinase activation. Moreover, we find that, in the absence of exogenously added PACAP, blockade of the PAC1 receptor inhibits neuronal differentiation. These data coupled with our finding that both PACAP and the PAC1 receptor are expressed during the peak period of neuronal differentiation in the DRG suggest that PACAP functions in vivo to promote the differentiation of nascent sensory neurons. Interestingly, we also demonstrate that the neurotrophic factors NT-3 and CNTF completely block the PACAP-induced neuronal differentiation. This points to the intricate integration of cellular signals by nascent neurons and, to our knowledge, is the first evidence for neurotrophic factor abrogation of a pathway regulated by G-protein-coupled receptors (GPCRs).