Cell adhesion molecules regulating neurite growth from amacrine and rod photoreceptor cells

A great deal is now known about the cell adhesion molecules (CAMs) that are responsible for promoting the growth of ganglion cell axons as they project out of the retina through the optic nerve and finally to distant targets in the brain. However, the CAMs important for regulating axon outgrowth from nonprojection neurons, such as amacrine cells and rods, are not known. Such local circuit neurons extend their neurites rather short distances on cellular surfaces not normally encountered by the ganglion cell axons. To study the mechanisms regulating axon or dendrite growth from local circuit neurons, neurite outgrowth from amacrine cells and rod photoreceptor cells derived from the rat was examined in vitro on immunopurified forms of NCAM, L1, and N-cadherin, three well-characterized adhesive molecules found in the developing retina. Either early (P3) or late (P10) postnatal amacrine cells grew neurites on all three CAMs, but there were significant differences in the percentage of the amacrine cells that responded to each CAM. None of the CAMs supported neurite outgrowth from early postnatal rods, but, surprisingly, NCAM stimulated vigorous neurite extension from rods isolated at postnatal day 10. Postnatal ganglion cells were also examined for comparison and were found not to grow neurites on NCAM, but did grow extensive processes on L1 and N-cadherin. These results show that NCAM, L1, and N-cadherin can promote neurite outgrowth from local circuit neurons, but that the effectiveness of any particular CAM is dependent on the cell type and the developmental period.