The long range goal of this research is to better understand the signal transduction cascades and cytoskeletal rearrangements that are necessary for guided axon extension in vivo. Guidance of growth cones to their targets requires specific and interactions between multiple extracellular ligands with receptors on the surface of growth cones. While large families of diffusible, cell surface and extracellular matrix (ECM) bound ligands and their receptors have been identified, relatively little is known of the intracellular signaling systems and cytoskeletal rearrangements downstream of receptor-ligand interactions. This is especially true within the developing embryo, where growth cones must integrate signals generated by interactions with dynamic combinations of molecular gradients, guideposts and boundaries in the nervous system. Intracellular calcium and cAMP are two key second messenger systems that have been shown to regulate axon growth and guidance. Actin filaments and microtubules are two key cytoskeletal elements that are required for proper growth cone motility and detection of extracellular cues. This study proposes to investigate the role of calcium transients, cAMP signaling and cytoskeletal rearrangements at specific choice points within the developing Xenopus spinal cord and/or at artificial choice points created in vitro using specific fluorescent probes, caged-compounds, pharmacological agents and molecular perturbations. Specifically, we propose to: 1)Determine the role of calcium transients in the divergent choice made by motoneuron (MN) and commissural interneuron (CI) growth cones at the ventral fascicle choice point in the spinal cord. 2) Examine the function of cAMP-dependent protein kinase signaling in the guidance of CI's through the floorplate. 3)Assess the role of local and global calcium transients in growth cone turning at a substrate boundary in vitro. 4)Characterize the effects of spontaneous and imposed calcium transients on growth cone cytoskeletal dynamics. These studies will provide new information about the role of second messenger changes in the guidance of growth cones at choice points in vivo. This is an important and necessary step in our understanding of the mechanisms by which growth cones locate their targets in the complex environment of a developing embryo. In addition, this study will identify how transient global and local elevations of second messengers regulate the dynamic assembly and disassembly of cytoskeletal components that control growth cone motility.
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