Agrin is synthesized by retinal cells and colocalizes with gephyrin [corrected]

At the neuromuscular junction, motoneuron-derived agrin is concentrated in the basal lamina of the synaptic cleft where it is responsible for the formation of the postsynaptic apparatus. As a step toward understanding agrin's role in the developing central nervous system, we have characterized agrin in retinal cell cultures as well as in the intact retina. Agrin was synthesized and secreted by cultured retinal cells and at least part of this agrin was active in acetylcholine receptor aggregation. Punctate anti-agrin immunoreactivity was associated with the cell bodies and processes of retinal neurons. Double labeling with antibodies against agrin and against the synapse-associated molecule gephyrin demonstrated that approximately half of the gephyrin clusters colocalized with the punctate agrin immunoreactivity in vitro. In the in vivo retina, more than 80% of the gephyrin clusters colocalized with extracellular agrin. These data indicate that agrin is concentrated at synapses in the retina as it is at the neuromuscular junction.