At the vertebrate neuromuscular junction the extracellular matrix molecule
agrin is responsible for the formation, maintenance and regeneration of most if not all postsynaptic specializations. Several
agrin isoforms are generated by alternative splicing which differ in their function and which are all expressed in the CNS. To analyse the role of
agrin in the CNS, we investigated the expression and ultrastructural localization of
agrin in the posthatched chick retina. In situ hybridization revealed the presence of
agrin mRNA in all cellular layers of the mature retina, indicating that most if not all major
retinal cell types synthesize
agrin. Pan-specific as well as
isoform-specific antiagrin
antisera stained the optic fibre layer and the outer 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 the photoreceptor terminals and the invaginating horizontal and bipolar cell dendrites. In the synapse-containing inner plexiform layer both
antisera revealed punctate immunoreactivity. This staining corresponded to
agrin concentrated in the synaptic cleft of conventional synapses as determined by preembedding immunoelectron microscopy.
Agrin is thus concentrated at mature interneuronal synapses as it is at the neuromuscular junction, consistent with a role of
agrin during formation and/or maintenance of synapses in the CNS.