ECTOPIC EXPRESSION OF CONNECTIN REVEALS A REPULSIVE FUNCTION DURING GROWTH CONE GUIDANCE AND SYNAPSE FORMATION

Connectin, a cell surface protein of the leucine-rich repeat family in Drosophila, is expressed on the surface of a subset of embryonic musc les (primarily lateral muscles), on the growth cones and axons of the motoneurons that innervate these muscles (primarily SNa motoneurons), and on several associated glial cells. When coupled with its ability t o mediate hemophilic cell adhesion, these results led to the suggestio n that Connectin functions as an attractive signal for SNa pathfinding and targeting. In the present study, we ectopically expressed Connect in on ventral muscles normally innervated by SNb motoneurons. The SNb growth cones change both their morphology and their trajectory when th ey encounter ectopic Connectin positive ventral muscles, displaying '' bypass,'' ''detour,'' and ''stall'' phenotypes. Moreover, SNb synapse formation is prevented by Connectin expression on ventral muscles. The se results reveal a repulsive function for Connectin during motoneuron growth cone guidance and synapse formation.