The correct wiring together of the developing nervous system requires that growing neuronal axons navigate precisely over long distances to connect with their appropriate synaptic targets. The long term objective of this grant has been to identify and characterize some of the guidance cues that direct axonal elongation. This grant supported the discovery of sema3A (Collapsin-1) and some of the other class III semaphorin relatives; the characterization of their biological activities as well as some of the receptor components with which they interact, and studies of the roles that these molecules play in axonal guidance in vivo. In the coming funding period we plan to shift our focus to signaling molecules that modulate axonal responses to known guidance cues. All known guidance cues are thought to act as either attractants or repellents. We have discovered a new kind of guidance cue without attractive or repellent properties of its own that modulates the effectiveness of other traditional guidance cues. We have discovered that the chemokine SDF-1 acts through its receptor CXCR4 to elevate neuronal cAMP levels and thereby makes a variety of different embryonic neurons less responsive to the known axonal repellents sema3A, sema3C, and slit-2. SDF-1/CXCR4 signaling is required for normal axon pathfinding in the spinal cord. We hypothesize that SDF-1/CXCR4 mediated signaling pathways will help to balance repellents and effectively act as attractants in the developing embryo. We will test this hypothesis by determining if a reduction in SDF-1/CXCR4 function can rescue in vivo axonal phenotypes that arise from loss of repellent function. Similarly, we have found that the neurotransmitter glutamate acts through class I metabotropic receptors to reduce the effectiveness of the axonal repellents sema3A, sema3C, and slit-2. We hypothesize that class I metabotropic glutamate receptor activitation will promote cell survival and synapse formation, and we will test these ideas in in vitro assays.
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