Isoform pattern and AChR aggregation activity of agrin expressed by embryonic chick retinal ganglion neurons

Several isoforms of chick agrin, which differ in their activity to aggregate AChRs at the neuromuscular junction, are generated by alternative splicing at splice site B. We analyzed the isoform pattern and the functional properties of agrin in a defined population of CNS neurons. At all developmental stages retinal ganglion cells purified by immunopanning expressed the agrin B0, B11, and B19 isoforms. Single-cell RT-PCR of individual retinal ganglion cells revealed simultaneous expression of B0 and B11 isoforms in about half of the neurons analyzed. Despite the expression of agrin isoforms active in AChR aggregation, ganglion cells did not aggregate AChRs when cocultured with myotubes. Addition of exogenous agrin to myotube-ganglion cell cocultures indicated that AChR aggregation is inhibited. These results demonstrate that a defined population of CNS neurons can simultaneously express several agrin isoforms and that the AChR aggregation activity of agrin might be regulated not only by alternative splicing but also on the protein level.