At the neuromuscular junction, acetylcholinesterase (AChE) is mainly present as asymmetric forms in which tetramers of catalytic subunits are associated to a specific collagen, collagen Q (ColQ). The accumulation of the enzyme in the synaptic basal lamina strictly relies on ColQ. This has been shown to be mediated by interaction between ColQ and perlecan, which itself binds dystroglycan. Here, using transfected mutants of ColQ in a ColQ-deficient muscle cell line or COS-7 cells, we report that ColQ clusterizes through a more complex mechanism. This process requires two heparin-binding sites contained in the collagen domain as well as the COOH terminus of ColQ. Cross-linking and immunoprecipitation experiments in Torpedo postsynaptic membranes together with transfection experiments with muscle-specific kinase (MuSK) constructs in MuSK-deficient myotubes or COS-7 cells provide the first evidence that ColQ binds MuSK. Together, our data suggest that a ternary complex containing ColQ, perlecan, and MuSK is required for AChE clustering and support the notion that MuSK dictates AChE synaptic localization at the neuromuscular junction.

The accumulation of dystrophin and associated proteins at the postsynaptic membrane of the neuromuscular junction and their co-distribution with nicotinic acetylcholine receptor (AChR) clusters in vitro suggested a role for the dystrophin complex in synaptogenesis. Co-transfection experiments in which ␣-and ␤-dystroglycan form a complex with AChR and rapsyn, a peripheral protein required for AChR clustering (Apel, D. A., Roberds, S. L., Campbell, K. P., and Merlie, J. P. (1995) Neuron 15, 115-126), suggested that rapsyn functions as a link between AChR and the dystrophin complex. We have investigated the interaction between rapsyn and ␤-dystroglycan in Torpedo AChR-rich membranes using in situ and in vitro approaches. Cross-linking experiments were carried out to study the topography of postsynaptic membrane polypeptides. A cross-linked product of 90 kDa was labeled by antibodies to rapsyn and ␤-dystroglycan; this demonstrates that these polypeptides are in close proximity to one another. Affinity chromatography experiments and ligand blot assays using rapsyn solubilized from Torpedo AChR-rich membranes and constructs containing ␤-dystroglycan C-terminal fragments show that a rapsyn-binding site is present in the juxtamembranous region of the cytoplasmic tail of ␤-dystroglycan. These data point out that rapsyn and dystroglycan interact in the postsynaptic membrane and thus reinforce the notion that dystroglycan could be involved in synaptogenesis.

Duchenne muscular dystrophy is a prevalent X-linked neuromuscular disease for which there is currently no cure. Recently, it was demonstrated in a transgenic mouse model that utrophin could functionally compensate for the lack of dystrophin and alleviate the muscle pathology (Tinsley, J. M., Potter, A. C., Phelps, S. R., Fisher, R., Trickett, J. I., and Davies, K. E. (1996) Nature 384, 349 -353). In this context, it thus becomes essential to determine the cellular and molecular mechanisms presiding over utrophin expression in attempts to overexpress the endogenous gene product throughout skeletal muscle fibers. In a recent study, we showed that the nerve exerts a profound influence on utrophin gene expression and postulated that nerve-derived trophic factors mediate the local transcriptional activation of the utrophin gene within nuclei located in the postsynaptic sarcoplasm (Gramolini, A. O., Dennis, C. L., Tinsley, J. M., Robertson, G. S., Davies, K. E, Cartaud, J., and Jasmin, B. J. (1997) J. Biol. Chem. 272, 8117-8120). In the present study, we have therefore focused on the effect of agrin on utrophin expression in cultured C2 myotubes. In response to Torpedo-, muscle-, or nerve-derived agrin, we observed a significant 2-fold increase in utrophin mRNAs. By contrast, CGRP treatment failed to affect expression of utrophin transcripts. Western blotting experiments also revealed that the increase in utrophin mRNAs was accompanied by an increase in the levels of utrophin. To determine whether these changes were caused by parallel increases in the transcriptional activity of the utrophin gene, we transfected muscle cells with a 1.3-kilobase pair utrophin promoter-reporter (nlsLacZ) gene construct and treated them with agrin for 24 -48 h. Under these conditions, both muscle-and nerve-derived agrin increased the activity of ␤-galactosidase, indicating that agrin treatment led, directly or indirectly, to the transcriptional activation of the utrophin gene. Furthermore, this increase in transcriptional activity in response to agrin resulted from a greater number of myonuclei expressing the 1.3-kilobase pair utrophin promoter-nlsLacZ construct. Deletion of 800 base pairs 5 from this fragment decreased the basal levels of nlsLacZ expression and abolished the sensitivity of the utrophin promoter to exogenously applied agrin. In addition, site-directed mutagenesis of an N-box motif contained within this 800-base pair fragment demonstrated its essential contribution in this regulatory mechanism. Finally, direct gene transfer studies performed in vivo further revealed the importance of this DNA element for the synapse-specific expression of the utrophin gene along multinucleated muscle fibers. These data show that both muscle and neural isoforms of agrin can regulate expression of the utrophin gene and further indicate that agrin is not only involved in the mechanisms leading to the formation of clusters containing presynthesized synaptic molecules but that it can also participate in the local regulation of genes encoding s...

The formation of the neuromuscular junction is characterized by the progressive accumulation of nicotinic acetylcholine receptors (AChRs) in the postsynaptic membrane facing the nerve terminal, induced predominantly through the agrin/muscle-specific kinase (MuSK) signaling cascade. However, the cellular mechanisms linking MuSK activation to AChR clustering are still poorly understood. Here, we investigate whether lipid rafts are involved in agrinelicited AChR clustering in a mouse C2C12 cell line. We observed that in C2C12 myotubes, both AChR clustering and cluster stability were dependent on cholesterol, because depletion by methyl-b-cyclodextrin inhibited cluster formation or dispersed established clusters. Importantly, AChR clusters resided in ordered membrane domains, a biophysical property of rafts, as probed by Laurdan two-photon fluorescence microscopy. We isolated detergent-resistant membranes (DRMs) by three different biochemical procedures, all of which generate membranes with similar cholesterol/ GM1 ganglioside contents, and these were enriched in several postsynaptic components, notably AChR, syntrophin, and raft markers flotillin-2 and caveolin-3. Agrin did not recruit AChRs into DRMs, suggesting that they are present in rafts independently of agrin activation. Consequently, in C2C12 myotubes, agrin likely triggers AChR clustering or maintains clusters through the coalescence of lipid rafts. These data led us to propose a model in which lipid rafts play a pivotal role in the assembly of the postsynaptic membrane at the neuromuscular junction upon agrin signaling.-StetzkowskiMarden, F., K. Gaus, M. Recouvreur, A. Cartaud, and J. Cartaud. Agrin elicits membrane lipid condensation at sites of acetylcholine receptor clusters in C2C12 myotubes.

The muscle-specific kinase MuSK is part of an agrin receptor complex that stimulates tyrosine phosphorylation and drives clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane at the vertebrate neuromuscular junction (NMJ). MuSK also regulates synaptic gene transcription in subsynaptic nuclei. Over the past few years, decisive progress has been made in the identification of MuSK effectors, helping to understand its function in the formation of the NMJ. Similarly to AChR, MuSK and several of its partners are the target of mutations responsible for diseases of the NMJ, such as congenital myasthenic syndromes. This minireview will focus on the multiple MuSK effectors so far identified that place MuSK at the center of a multifunctional signaling complex involved in the organization of the NMJ and associated disorders.

The muscle-specific receptor tyrosine kinase (MuSK) is part of a receptor complex, activated by neural agrin, that orchestrates the differentiation of the neuromuscular junction (NMJ). To gain insight into the function of the MuSK complex, we have developed a proteomic approach to identify new MuSK partners. MS analysis of MuSK crosslink products from postsynaptic membranes of the Torpedo electrocytes identified the adaptor protein 14-3-3 ␥. The 14-3-3 ␥ protein was found localized at the adult rat NMJ. Cotransfection experiments in COS-7 cells showed that MuSK codistributed with the 14-3-3 ␥ protein at the plasma membrane. Furthermore, 14-3-3 ␥ was copurified by affinity chromatography with MuSK from transfected COS-7 cells and myotubes. The 14-3-3 ␥ protein did not colocalize with agrin-elicited acetylcholine receptor (AChR) aggregates in cultured myotubes, suggesting that it is not involved in AChR clustering. Expression of 14-3-3 ␥ specifically repressed the transcription of several synaptic reporter genes in cultured myotubes. This repression was potentiated by MuSK expression. Moreover, the expression of 14-3-3 ␥ in muscle fibers in vivo caused both the repression of synaptic genes transcription and morphological perturbations of the NMJ. Our data extend the notion that, apart from its well documented role in AChR clustering, the MuSK complex might also be involved in the regulation of synaptic gene expression at the NMJ. neuromuscular junction ͉ synaptogenesis

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Dystrophin has been shown to occur in Torpedo electrocyte [Chang, H. W., Bock, E. & Bonilla, E. (1989) J. Biol. Chem. 264, 20831-208341, a highly polarized syncytium that is embryologically derived from skeletal muscle and displays functionally distinct plasma membrane domains on its innervated and noninnervated faces. In the present study, we investigated the subcellular distribution of dystrophin in the adult electrocyte from Torpedo marmorata and the evolution of its distribution during embryogenesis. Immunofluorescence experiments performed on adult electrocytes with a polyclonal antibody directed against chicken dystrophin revealed that dystrophin immunoreactivity codistributed exclusively with the acetylcholine receptor along the innervated membrane. At the ultrastructural level, dystrophin immunoreactivity appears confined to the face of the subsynaptic membrane exposed to the cytoplasm. In developing electrocytes (45-mm embryo), dystrophin is already detectable at the acetylcholine receptorrich ventral pole ofthe cells before the entry of the electromotor axons. Furthermore, we show that dystrophin represents a major component of purified membrane fractions rich in acetylcholine receptor. A putative role of dystrophin in the organization and stabilization of the subsynaptic membrane domain of the electrocyte is discussed.The Duchenne/Becker muscular dystrophy gene has recently been identified by reversed genetics and localized on human chromosome X (reviewed in refs.