The cell adhesion molecule, connectin, and the development of the Drosophila neuromuscular system.

The connectin gene of Drosophila has been identified as a candidate direct target of homeotic gene control and has also been implicated in the formation of specific neuromuscular connections. The gene product, connectin, is a member of the leucine-rich repeat protein family and we show that it is attached to the cell surface via a glycosylphosphatidylinositol linkage and that it can mediate homotypic cell-cell adhesion in vitro. The expression of connectin protein during Drosophila embryogenesis provides support for a role in adhesion in vivo. In the central nervous system, it is initially expressed on longitudinal glia and on a few identified neurons. These cells extend processes and connect up to form a continuous scaffold of connectin-expressing cells, presaging the development of axonal pathways. Later, connectin is expressed on specific axons as they track along the connectin scaffold. Glial expression then declines and connectin appears on axons that fasciculate with pre-existing connectin-positive bundles. Thus scaffold formation, axon pathfinding and fasciculation involve specific contacts between connectin-positive cells. The timing and pattern of connectin expression suggest that it may play an important role in mediating specific interactions through homotypic cell adhesion.