THE DEVELOPING AVIAN RETINA EXPRESSES AGRIN ISOFORMS DURING SYNAPTOGENESIS

The localization, isoform pattern, and mRNA distribution of the synaps e-organizing molecule agrin was investigated in the developing avian r etina. Injection of anti-agrin Fab fragments into the vitreous humor o f chick eyes of embryonic days 3 to 20, a procedure that labels only e xtracellular agrin, reveals staining in the inner and outer plexiform layers before, during, and after the period of synapse formation. The labelling in these layers changes from a diffuse to a punctate pattern at the time when synapses form. At all stages investigated, the inner limiting membrane (a basal lamina that separates vitreons from neural retina) is intensely labeled, as are the axonal processes of retinal ganglion cells in the optic fiber layer and in the optic nerve, althou gh the staining intensity declines after embryonic day 10 in both reti na and optic nerve. In culture, axons of retinal ganglion cells also e xpress agrin-like immunoreactivity on their surfaces. Polymerase chain reaction analysis reveals that several different agrin isoforms are e xpressed in the developing neural retina. In situ hybridization studie s show that agrin isoforms are expressed in the ganglion cell and inne r nuclear layers, correlating well with the staining for agrin protein in the optic fiber and plexiform layers. The expression of mRNA codin g for several agrin isoforms and the presence of extracellular agrin i n the synapse-containing layers during the period of synapse formation is consistent with the idea that agrin isoforms might play a role dur ing synapse formation in the central nervous system. (C) 1996 John Wil ey & Sons, Inc.