Expression, distribution and ultrastructural localization of the synapse-organizing molecule agrin in the mature avian retina

At the vertebrate neuromuscular junction the extracellular matrix molecule agrin is responsible for the formation, maintenance and regeneration of mos t if not all postsynaptic specializations. Several agrin isoforms are gener ated by alternative splicing which differ in their function and which are a ll expressed in the CNS. To analyse the role of agrin in the CNS, we invest igated the expression and ultrastructural localization of agrin in the post hatched chick retina. In situ hybridization revealed the presence of agrin mRNA in all cellular layers of the mature retina, indicating that most if n ot all major retinal cell types synthesize agrin. Pan-specific as well as i soform-specific antiagrin antisera stained the optic fibre layer and the ou ter plexiform layer. However, only the pan-specific antiserum additionally stained the inner limiting membrane. Immunoelectron microscopy showed that in the optic fibre layer agrin was associated with ganglion cell axons and that at least part of this agrin corresponds to a neuronal isoform of agrin . In the outer plexiform layer, agrin was localized in the cleft between th e photoreceptor terminals and the invaginating horizontal and bipolar cell dendrites. In the synapse-containing inner plexiform layer both antisera re vealed punctate immunoreactivity. This staining corresponded to agrin conce ntrated in the synaptic cleft of conventional synapses as determined by pre embedding immunoelectron microscopy. Agrin is thus concentrated at mature i nterneuronal synapses as it is at the neuromuscular junction, consistent wi th a role of agrin during formation and/or maintenance of synapses in the C NS.