Development of inhibition by Ephrin-A5 on outgrowth of embryonic spinal motor neurites

The spinal motor pool maps systematically onto the surface of muscles. This map is detectable in rat embryonic muscles, and is partially restored afte r reinnervation, Recent evidence shows that either overexpression or deleti on of the ephrin-A5 gene significantly disrupts the map, suggesting that ep hrin-A5 plays a critical role in the formation of this topography. Several studies have demonstrated that ephrin-A5 is a repulsive molecule in the ner vous system, including the neuromuscular system. To examine the development of sensitivity of ventral spinal axons to this inhibitory ligand, slices o f Ell to E15 embryonic rat spinal cords were cocultured with membranes deri ved from ephrin-A5-expressing cell lines. We detected a progressive express ion of inhibition by ephrin-A5 between Ell and E15, By E15, rostral and cau dal spinal neurites showed clear differences in responsiveness to the ephri n-A5 ligand, Further, we found that at this age caudal neurites are more se nsitive to changes of ephrin-A5 concentration along a gradient. In addition , growth cones of caudal, more than rostral, neurites tended to assume a co llapsed shape in the presence of the ligand, These results demonstrate a pr ogressive development of sensitivity to ephrin-A5, and suggest a divergence in this sensitivity between rostral and caudal spinal cord neurites. These results provide further insight into how subtle rostrocaudal differences i n the development of sensitivity to ephrin-A5 may explain, in part, neuromu scular topography. (C) 2001 John Wiley & Sons. Inc.