Sheryl A. Scott - US grants

The proposed experiments investigate the development of the segmental pattern of cutaneous sensory innervation in skin of the hindlimb of the chick. Cutaneous axons from each dorsal root ganglion (DRG) innervate a characteristic region of skin, referred to as a dermatome. The objective of the proposed experiments is to determine whether DRG neurons are specified to innervate their dermatome prior to axonal outgrowth, and whether these neurons are directed to this skin regions by cues in the surrounding environment. As a first approach to this problem, the dermatome of each DRG supplying the hindlimb will be mapped at selected embryonic stages to determine the sequence of events during the normal development of dermatomes. The mapping will involve extracellular electrophysiological recording from each dorsal root, and labeling cutaneous sensory nerves with the marker horseradish peroxidase. In later investigations surgical manipulations involving skin transplantations, DRG deletions, and limb bud and spinal cord rotations will be made in embryos, and the resulting dermatomes mapped. The establishment of correct innervation by the DRGs in operated embryos will be taken as evidence that the cutaneous afferents are prespecified, and that the specification plays a role in the normal development of dermatomes. The failure of DRG neurons to innervate their correct skin region would suggest that during normal development the axons are directed to their characteristic region of skin by cues in the limb environment.

In adults there is a striking concordance in the trajectories and connectivity of sensory and motor neurons and their target muscles in the limb. Whereas much is known about cellular processes responsible for the development of this pattern, identification of the molecular signals is in its infancy. The goal of this proposal is to characterize the role of selected members of two molecular families -- Eph receptor tyrosine kinases and ETS transcription factors -- in developmental events that lead to the exquisite matching of sensory and motor pathways and connections in a classical model system, the chick hindlimb, Eph receptors and ephrins show complex patterns of expression in limb and in sensory and motor neurons. Aim I is to assess the contribution of Eph receptors, in particular EphA7, and ephrins in patterning axon growth in the limb. Expression or function of EphA receptors or ephrins will be manipulated, and the resulting innervation patterns will be assessed. Alterations in innervation patterns would provide evidence that Eph receptors or ephrins are involved in patterning axon growth in the limb.The ETS transcription factors ER81 and PEA3 are expressed in monosynaptically-connected sensory and motor neurons, and are required for formation of these connections. Aim II is to determine the nature of signals from the limb that regulate ETS expression, and whether these factors determine the specificity of sensory-motor connections. ETS expression will be assayed with antibody staining or in situ hybridization after surgical manipulations in ovo. CNS connectivity will be assayed electrophysiologically. Alterations in ETS expression would show that signals from the limb determine this pattern. Concordance of ETS expression in monosynaptically-connected sensory and motor neurons would suggest that ETS proteins confer specificity to these connections. These studies will elucidate molecular mechanisms involved in the normal development of limb innervation patterns. This information should be applicable more complex neural circuits involved in behavior and higher order functions. Moreover, knowledge gained here will provide insight in the design and implementation of therapeutic strategies to promote axon growth and to restore innervation and function following nerve injury or disease.