Aggregates of acetylcholine receptors are not observed under anti-agrin staining Schwann cell processes at the frog neuromuscular junction

The frog neuromuscular junction offers a unique structure in which to obser ve fine details in the relationship between the motor neuron, muscle, and S chwann cell, which together comprise the neuromuscular junction. Schwann ce ll processes that extended from the synapse stained positively with anti-ag rin antibodies. Immunocytochemistry revealed strong anti-agrin staining of the extracellular matrix surrounding the entire Schwann cell and the Schwan n cell processes come in close contact with the muscle fiber. Dual-labeling experiments revealed a lack of acetylcholine receptor (AChR) aggregates on the surface of the muscle fiber directly under these anti-agrin-staining S chwann cell processes. The cDNA that codes for the C-terminal portion of ag rin in frog (Rana pipiens) was cloned and sequenced. Polymerase chain react ion of frog brain, nerve, and muscle cDNA revealed that like other species, agrin transcripts that contain the B-8, B-11, or B-19 inserts were observe d only in brain tissue. Unlike other species, the exact site of the B inser ts in frog was slightly altered, with the B insertion site occurring within a codon sequence. Our data are consistent with the hypothesis that Schwann cells produce agrin which lacks inserts at the B region, and that agrin la cking B inserts does not direct the aggregation of AChRs in vivo under phys iological conditions and concentrations. (C) 1999 John Wiley & Sons, Inc.