Distinct phenotypes of mutant mice lacking agrin, MuSK, or rapsyn

Differentiation of the postsynaptic membrane at the neuromuscular junction requires agrin, a nerve-derived signal; MuSK, a critical component of the a grin receptor in muscle; and rapsyn, a protein that interacts with acetylch oline receptors (AChRs). We showed previously that nerve-induced AChR aggre gation is dramatically impaired in knockout mice lacking agrin, MuSK, or ra psyn. However, the phenotypes of these mutants differed in several respects , suggesting that the pathway from agrin to MuSK to rapsyn is complex. Here , we compared the effects of these mutations on two aspects of synaptic dif ferentiation: AChR clustering and transcriptional specialization of synapse -associated myonuclei. First, we show that a plant lectin, VVA-B-4, previou sly shown to act downstream of agrin, can induce AChR clusters on MuSK-defi cient but not rapsyn-deficient myotubes in culture. Thus, although both MuS K and rapsyn are required for AChR clustering in vivo, only rapsyn is essen tial for cluster formation per se. Second, we show that neuregulin, a nerve -derived inducer of AChR gene expression, activates AChR gene expression in cultured agrin- and MuSK-deficient myotubes, even though synapse-specific transcriptional specialization is disrupted in agrin and MuSK mutants in vi vo. We propose that agrin works through MuSK to determine a synaptogenic re gion within which synaptic differentiation occurs. (C) 1999 Elsevier Scienc e B.V. All rights reserved.