The goal of my research is to understand how axons grow along the correct pathways during development. To address this issue, we study sensory axons projecting into the chick hindlimb. In contrast to motoneuron axons, which do not require the presence of muscle per se to form muscle nerves, the skin is essential for the normal development of cutaneous projections. Our surgical manipulations have shown that the initial formation of a major cutaneous nerve in the embryonic chick limb, the lateral femoral cutaneous (LFCt) nerve requires the presence of the target ectoderm during a critical time period, when those axons are about to diverge from the hindlimb plexus. To elucidate the underlying mechanisms, we characterized the effects of ectoderm removal on a variety of molecules expressed in the limb. Importantly, Bmp4 expression was downregulated and BMP4-soaked beads placed on the denuded limb surface rescued LFCt nerve formation. In the proposed studies, we will continue our ongoing efforts to elucidate the mechanisms responsible for cutaneous nerve formation, taking advantage of the accessibility of chick embryos for carrying out surgical and molecular manipulations. 1 major goal will be to determine whether BMP4 exerts its effects by directly affecting the outgrowth of cutaneous axons or by signaling the limb mesenchyme in a way that it in turn elicits LFCt nerve formation. To distinguish between these possibilities and elucidate the molecular mechanisms underlying BMP4 action, the proposed experiments will characterize the expression of BMP receptors in the DRGs and in the limb; perturb BMP4 signaling, using various loss-of-function approaches, targeted by means of electroporation, to either the limb bud or to the neural crest (to transfect DRG neurons); and use complementary tissue culture assays to examine the cellular mechanisms by which BMP4 rescues LFCt nerve formation. Other studies will determine how cues from the skin contribute to LFCt nerve formation. These will focus on NGF and NTS, characterizing their expression in the limb, the expression of the relevant receptors on the first cutaneous axons to project to the skin, and examining whether exogenous NGF and/or NT3 rescues LFCt nerve formation after ectoderm removal. The proposed studies will help us understand how discrete regions of the skin bring about the formation of the cutaneous nerve projecting to it, and thereby also provide insight into ways to improve regeneration following peripheral nerve injury. ? ?
