Abstracts 1 / 18 / 16 A . Development of the Nervous System Class-6 Semaphorins have receptor function during neural circuit formationA

Class-6 Semaphorins form a transmembrane subgroup of the Semaphorin family. They were implicated in various processes, such as differentiation, cell migration, and angiogenesis. However, in most of these processes they were shown to act as ligands of Plexin receptors. Evidence for a receptor functions was suggested based on our studies on Sema6A's role in the formation of boundary cap cell clusters, a gate keeper structure between the peripheral and the central nervous system during development (Mauti et al., Neural Development, 2(2007)28). Loss of Sema6A in boundary cap cell precursors resulted in motoneurons exiting the spinal cord along the ventral roots, and in the failure of dorsal roots to form properly. Loss of Sema6D had an effect only at the dorsal root entry site but did not result in the loss of motoneurons from the spinal cord. More recently, we demonstrated a cell-autonomous role of Sema6B as a receptor in commissural axon guidance (Andermatt et al., Development 141(2014)3709-3720). In the absence of Sema6B, post-crossing commissural axons failed to turn rostrally along the longitudinal axis of the spinal cord. A cis-versus trans-interaction between Sema6B and its binding partner PlexinA2 was shown to regulate responsiveness of commissural axons at the midline. We have now extended our analysis of class-6 Semphorins' function during neural development to the peripheral nervous system. We used the chicken embryo to individually downregulate each member of the family (Sema6A, Sema6B and Sema6D) and study aberrations in axon guidance using whole-mount staining. Disruption of Sema6 signaling resulted in abnormalities in the crural, sciatic and epaxial nerves at E5 (HH25). Together, these findings suggest an important, previously unexplored role for class 6 Semaphorins in PNS neural circuit formation. For the formation of neural circuits, axons have to be guided to their appropriate target cells by a combination of attractive and repulsive cues. Recently, we have shown that Wnt5a and Wnt7a act as attractants for post-crossing commissural axons in the spinal cord of chicken embryos. However, the molecular signaling pathways activated by Wnts during axon guidance are not completely understood. In addition to atypical protein kinase C, members of the Wnt planar cell polarity (PCP) pathway were implicated in commissural axon guidance in the mouse. We confirmed that PCP components are also required for post-crossing commissural axon guidance in the chicken spinal cord. However, taking advantage of the precise temporal control of gene silencing provided by in ovo RNAi, we demonstrate that …