Developmental regulation of motor neurons: Growth cone guidance, axonal branching, and synapse formation

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In this grant period, we investigated roles of Sema3A in zebrafish spinal motor neurons. In zebrafish, there are two copies of sema3A genes, sema3al and sema3a2. We found these two genes functions in different steps of spinal motor neuron development. Zebrafish spinal motor neurons develop in iterative pattern that correspond to neighboring segmented somites. In each segment, caudal primary (CaP) motor neuron pioneers to exit the spinal cord just beneath its cell body, then determines the common exit points for all following motor axons. Upon the development of CaP, Sema3a2 regulates the position of its cell body and the common exit point by expressing at posterior half of newly segmented somite. Indeed, Sema3a2 knockdown embryos exhibits irregular position of CaP cell bodies and exit points that do not match with corresponding somites. Subsequently, Sema3al regulates axon guidance and branching by expressing at dorsal and ventral portion of the somite but not in between. In contrast to Sema3a2, Sema3al knockdown does not affect the position of cell body, but induce misrouting and abnormal branches on the CaP axon. These results indicate two copies of zebrafish sema3A genes subfunctions their roles during CaP development, one for the cell body positioning, and the other for the axon guidance.