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

在这一资助期间，我们研究了Sema3A在斑马鱼脊髓运动神经元中的作用。在斑马鱼中，Sema3A基因有两个拷贝，Sema3a1和Sema3a2。我们发现这两个基因在脊髓运动神经元发育的不同阶段发挥作用。斑马鱼脊髓运动神经元的发育与邻近的节段性体节相对应。在每个节段，尾侧初级(帽)运动神经元先锋地离开脊髓细胞体的正下方，然后确定所有后续运动轴突的共同出口点。在帽的发育过程中，Sema3a2通过在新分裂的体节的后半部分表达来调节细胞体的位置和共同出口点。事实上，Sema3a2基因敲除胚胎的帽子细胞体和出口点位置不规则，与相应的体节不匹配。随后，Sema3al通过在体节的背侧和腹侧表达而不是在两者之间表达来调节轴突的引导和分支。与Sema3a2不同的是，Sema3a1基因敲除不影响细胞体的位置，但导致帽状轴突的错误走行和异常分支。这些结果表明，斑马鱼Sema3A基因的两个拷贝在帽子发育过程中发挥亚功能，一个用于细胞体定位，另一个用于轴突引导。

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.