Matrix metalloproteinase-3 removes agrin from synaptic basal lamina

Agrin, a heparin sulfate proteoglycan. is an integral member of the synapti c basal lamina and plays a critical role in the formation and maintenance o f the neuromuscular junction, The N-terminal region of agrin binds tightly to basal lamina, while the C-terminal region interacts with a muscle-specif ic tyrosine kinase (MuSK) to induce the formation of the postsynaptic appar atus. Although the binding of agrin to basal lamina is tight, the binding o f agrin to MuSK has yet to be shown; therefore, basal lamina binding is cri tical for maintaining the presentation of agrin to MuSK, Here we report evi dence that supports our hypothesis that matrix metalloproteinase-3 (MMP-3) is responsible for the removal of agrin from synaptic basal lamina. Antibod ies to the hinge region of human MMP-3 recognize molecules concentrated at the frog neuromuscular junction in both cross sections and whole mounts, El ectron microscopy of neuromuscular junctions stained with antibodies to MMP -3 reveals that staining is found in the extracellular matrix surrounding t he Schwann cell. Treatment of sections from frog anterior tibialis muscle w ith MMP-3 results in a clear and reproducible removal of agrin immunoreacti vity from synaptic basal lamina. The same MMP-3 treatment does not alter an tilaminin staining. These results support our hypothesis that synaptic acti vity results in the activation of MMP-3 at the neuromuscular junction and t hat MMP-3 specifically removes agrin from synaptic basal lamina. (C) 2000 J ohn Wiley & Sons, Inc. J Neurobiol 43: 140-149. 2000.