Src, Fyn, and Yes are not required for neuromuscular synapse formation but are necessary for stabilization of agrin-induced clusters of acetylcholine receptors

Accelerated response of the myogenin gene to denervation in mutant mice lacking phosphorylation of myogenin at threonine 87

Gene expression in skeletal muscle is regulated by a family of myogenic basic helix-loop-helix (bHLH) proteins. The binding of these bHLH proteins, notably MyoD and myogenin, to E-boxes in their own regulatory regions is blocked by protein kinase C (PKC)-mediated phosphorylation of a single threonine residue in their basic region. Because electrical stimulation increases PKC activity in skeletal muscle, these data have led to an attractive model suggesting that electrical activity suppresses gene expression by stimulating phosphorylation of this critical threonine residue in myogenic bHLH proteins. We show that electrical activity stimulates phosphorylation of myogenin at threonine 87 (T87) in vivo and that calmodulin-dependent kinase II (CaMKII), as well as PKC, catalyzes this reaction in vitro. We find that phosphorylation of myogenin at T87 is dispensable for skeletal muscle development. We show, however, that the decrease in myogenin (myg) expression following innervation is delayed and that the increase in expression following denervation is accelerated in mutant mice lacking phosphorylation of myogenin at T87. These data indicate that two distinct innervation-dependent mechanisms restrain myogenin activity: an inactivation mechanism mediated by phosphorylation of myogenin at T87, and a second, novel regulatory mechanism that regulates myg gene activity independently of T87 phosphorylation.

Agrin signaling in cortical neurons is mediated by a tyrosine kinase-dependent increase in intracellular Ca2+ that engages both CaMKII and MAPK signal pathways

Agrin has been implicated in multiple aspects of central nervous system (CNS) neuron differentiation and function including neurite formation, synaptogenesis, and synaptic transmission. However, little is known about the signaling mechanisms whereby agrin exerts its effects. We have recently identified a neuronal receptor for agrin, whose activation induces expression of c-fos, and provided evidence that agrin binding to this receptor is associated with a rise in intracellular Ca2+, a ubiquitous second messenger capable of mediating a wide range of effects. To gain further insight into agrin's role in brain, we used Ca2+ imaging to explore agrin signal transduction in cultured cortical neurons. Bath application of either z+ or z-agrin isoforms resulted in marked changes in intracellular Ca2+ concentration specifically in neurons. Propagation of the Ca2+ response was a two-step process characterized by an initial increase in intracellular Ca2+ mediated by ryanodine receptor (RyR) release from intracellular stores, supplemented by influx through voltage-gated calcium channels (VGCCs). Agrin-induced increases in intracellular Ca2+ were blocked by genistein and herbimycin, suggesting that the agrin receptor is a tyrosine kinase. Ca2+ release from intracellular stores activates both calcium/calmodulin-dependent kinase II (CaMKII) and mitogen activated protein kinase (MAPK). Activation of CaMKII is required for propagation of the Ca2+ wave itself, whereas both MAPK and CaMKII play a role in mediating long latency responses such as induction of c-fos. These results suggest that an agrin-dependent tyrosine kinase could play a critical role in modulating levels of intracellular Ca2+ and activity of MAPK and CaMKII in CNS neurons.

A caveolin-3 mutant that causes limb girdle muscular dystrophy type 1C disrupts Src localization and activity and induces apoptosis in skeletal myotubes

Caveolins are membrane proteins that are the major coat proteins of caveolae, specialized lipid rafts in the plasma membrane that serve as scaffolding sites for many signaling complexes. Among the many signaling molecules associated with caveolins are the Src tyrosine kinases, whose activation regulates numerous cellular functions including the balance between cell survival and cell death. Several mutations in the muscle-specific caveolin, caveolin-3, lead to a form of autosomal dominant muscular dystrophy referred to as limb girdle muscular dystrophy type 1C (LGMD-1C). One of these mutations (here termed the `TFT mutation') results in a deletion of a tripeptide (ΔTFT(63-65)) that affects the scaffolding and oligomerization domains of caveolin-3. This mutation causes a 90-95% loss of caveolin-3 protein levels and reduced formation of caveolae in skeletal muscle fibers. However, the effects of this mutation on the specific biochemical processes and cellular functions associated with caveolae have not been elucidated. We demonstrate that the TFT caveolin-3 mutation in post-mitotic skeletal myotubes causes severely reduced localization of caveolin-3 to the plasma membrane and to lipid rafts, and significantly inhibits caveolar function. The TFT mutation reduced the binding of Src to caveolin-3, diminished targeting of Src to lipid rafts, and caused abnormal perinuclear accumulation of Src. Along with these alterations of Src localization and targeting, there was elevated Src activation in myotubes expressing the TFT mutation and an increased incidence of apoptosis in those cells compared with control myotubes. The results of this study demonstrate that caveolin-3 mutations associated with LGMD-1C disrupt normal cellular signal transduction pathways associated with caveolae and cause apoptosis in muscle cells, all of which may reflect pathogenetic pathways that lead to muscle degeneration in these disorders.

Postsynaptic requirement for Abl kinases in assembly of the neuromuscular junction

Agrin signals through the muscle-specific receptor tyrosine kinase (MuSK) to cluster acetylcholine receptors (AChRs) on the postsynaptic membrane of the neuromuscular junction (NMJ). This stands as the prevailing model of synapse induction by a presynaptic factor, yet the agrin-dependent MuSK signaling cascade is largely undefined. Abl1 (previously known as Abl) and the Abl1-related gene product Abl2 (previously known as Arg) define a family of tyrosine kinases that regulate actin structure and presynaptic axon guidance. Here we show that the Abl kinases are critical mediators of postsynaptic assembly downstream of agrin and MuSK. In mouse muscle, Abl kinases were localized to the postsynaptic membrane of the developing NMJ. In cultured myotubes, Abl kinase activity was required for agrin-induced AChR clustering and enhancement of MuSK tyrosine phosphorylation. Moreover, MuSK and Abl kinases effected reciprocal tyrosine phosphorylation and formed a complex after agrin engagement. Our findings suggest that Abl kinases provide the developing synapse with the kinase activity required for signal amplification and the intrinsic cytoskeletal regulatory capacity required for assembly and remodeling.

New insights into the roles of agrin

The heparan sulphate proteoglycan agrin is expressed as several isoforms in various tissues. Agrin is best known as a crucial organizer of postsynaptic differentiation at the neuromuscular junction, but it has recently also been implicated in the formation of the immunological synapse, the organization of the cytoskeleton and the amelioration of function in diseased muscle. So the activities of agrin might be of broader significance than previously anticipated.

Tyrosine-phosphorylated and nonphosphorylated isoforms of alpha-dystrobrevin: roles in skeletal muscle and its neuromuscular and myotendinous junctions

alpha-Dystrobrevin (DB), a cytoplasmic component of the dystrophin-glycoprotein complex, is found throughout the sarcolemma of muscle cells. Mice lacking alphaDB exhibit muscular dystrophy, defects in maturation of neuromuscular junctions (NMJs) and, as shown here, abnormal myotendinous junctions (MTJs). In normal muscle, alternative splicing produces two main alphaDB isoforms, alphaDB1 and alphaDB2, with common NH2-terminal but distinct COOH-terminal domains. alphaDB1, whose COOH-terminal extension can be tyrosine phosphorylated, is concentrated at the NMJs and MTJs. alphaDB2, which is not tyrosine phosphorylated, is the predominant isoform in extrajunctional regions, and is also present at NMJs and MTJs. Transgenic expression of either isoform in alphaDB-/- mice prevented muscle fiber degeneration; however, only alphaDB1 completely corrected defects at the NMJs (abnormal acetylcholine receptor patterning, rapid turnover, and low density) and MTJs (shortened junctional folds). Site-directed mutagenesis revealed that the effectiveness of alphaDB1 in stabilizing the NMJ depends in part on its ability to serve as a tyrosine kinase substrate. Thus, alphaDB1 phosphorylation may be a key regulatory point for synaptic remodeling. More generally, alphaDB may play multiple roles in muscle by means of differential distribution of isoforms with distinct signaling or structural properties.

Cooperative regulation by Rac and Rho of agrin-induced acetylcholine receptor clustering in muscle cells

A key aspect of neuromuscular synapse formation is the clustering of muscle acetylcholine receptors (AChR) at synaptic sites in response to neurally secreted agrin. Agrin-induced AChR clustering in cultured myotubes proceeds via the initial formation of small microclusters, which then aggregate to form AChR clusters. Here we show that the coupling of agrin signaling to AChR clustering is dependent on the coordinated activities of Rac and Rho GTPases. The addition of agrin induces the sequential activation of Rac and Rho in C2 muscle cells. The activation of Rac is rapid and transient and constitutes a prerequisite for the subsequent activation of Rho. This temporal pattern of agrin-induced Rac and Rho activation reflects their respective roles in AChR cluster formation. Whereas agrin-induced activation of Rac is necessary for the initial phase of AChR cluster formation, which involves the aggregation of diffuse AChR into microclusters, Rho activation is crucial for the subsequent condensation of these microclusters into full-size AChR clusters. Co-expression of constitutively active forms of Rac and Rho is sufficient to induce the formation of mature AChR clusters in the absence of agrin. These results establish that Rac and Rho play distinct but complementary roles in the mechanism of agrin-induced AChR clustering.

Lithium inhibits a late step in agrin-induced AChR aggregation

Agrin activates an intracellular signaling pathway to induce the formation of postsynaptic specializations on muscle fibers. In myotubes in culture, this pathway has been shown to include autophosphorylation of the muscle-specific kinase MuSK, activation of Src-family kinases, tyrosine phosphorylation of the acetylcholine receptor (AChR) beta subunit, a decrease in receptor detergent extractability, and the accumulation of AChRs into high-density aggregates. Here we report that treating chick myotubes with lithium prevented any detectable agrin-induced change in AChR distribution without affecting the number of AChRs or the agrin-induced change in AChR tyrosine phosphorylation and detergent extractability. Lithium treatment also increased the rate at which AChR aggregates disappeared when agrin was removed. The effects of lithium developed slowly over the course of approximately 12 h. Thus, sensitivity to lithium identifies a late step in the agrin signaling pathway, after agrin-induced MuSK and AChR phosphorylation, that is necessary for the recruitment of AChRs into visible aggregates.

Overexpression of nPKC theta is inhibitory for agrin-induced nicotinic acetylcholine receptor clustering in C2C12 myotubes

Protein kinase C (PKC) activity has been implicated in nicotinic acetylcholine receptor (nAChR) cluster disruption but the specific PKC isoforms involved have not been identified. We first tested whether phorbol esters, which activate PKCs, regulate agrin-induced nAChR clustering in C(2)C(12) cells. We found that extended phorbol ester treatment (6 hr) increased nAChR clustering by two-fold. This increase correlated in time with downregulation of PKCs, as indicated by the disappearance of cPKC alpha, suggesting that the presence of PKCs is inhibitory for maximal nAChR clustering. To address the question whether nPKC theta, a specific PKC isoform restricted in expression to skeletal muscle and localized to neuromuscular junctions, regulates agrin-induced nAChR cluster formation we overexpressed an nPKC theta -green fluorescent protein (GFP) fusion protein in C(2)C(12) myotubes. The number of nAChR clusters was significantly reduced in nPKC theta-GFP compared to GFP overexpressing myotubes at less-than-maximal clustering concentrations of agrin. These data indicate that nPKC theta activity inhibits nAChR cluster formation. To examine whether nPKC theta activation by phorbol esters regulates agrin-induced nAChR clustering, we treated overexpressing myotubes overnight with maximal agrin concentrations followed by phorbol esters for 1 hr. Phorbol ester treatment reduced preexisting nAChR cluster numbers in nPKC theta-GFP compared to GFP-overexpressing myotubes, suggesting that stimulating nPKC theta activity disrupts nAChR clusters in the presence of maximal clustering concentrations of agrin. Together these findings, that nPKC theta activity inhibits agrin-induced nAChR cluster formation and disrupts preexisting agrin-induced nAChR clusters, suggest that nPKC theta activity is inhibitory for agrin function.

Calcium-dependent maintenance of agrin-induced postsynaptic specializations

Although much progress has been made in understanding synapse formation, little is known about the mechanisms underlying synaptic maintenance and loss. The formation of agrin-induced AChR clusters on cultured myotubes requires both activation of the receptor tyrosine kinase MuSK and intracellular calcium fluxes. Here, we provide evidence that such AChR clusters are maintained by agrin/MuSK-induced intracellular calcium fluxes. Clamping intracellular calcium fluxes after AChR clusters have formed leads to rapid MuSK and AChR tyrosine dephosphorylation and cluster dispersal, even in the continued presence of agrin. Both the dephosphorylation and the dispersal are inhibited by the tyrosine phosphatase inhibitor pervanadate. In contrast, clamping intracellular calcium at the time of initial agrin stimulation has no effect on agrin-induced MuSK or AChR phosphorylation, but blocks AChR cluster formation. These findings suggest an avenue by which postsynaptic stability can be regulated by modification of intracellular signaling pathways that are distinct from those used during synapse formation.

Restoration of synapse formation in Musk mutant mice expressing a Musk/Trk chimeric receptor

Mice lacking Musk, a muscle-specific receptor tyrosine kinase that is activated by agrin, fail to form neuromuscular synapses and consequently die at birth because of their failure to move or breathe. We produced mice that express a chimeric receptor, containing the juxtamembrane region of Musk and the kinase domain of TrkA, selectively in muscle, and we crossed this transgene into Musk mutant mice. Expression of this chimeric receptor restores presynaptic and postsynaptic differentiation, including the formation of nerve terminal arbors, synapse-specific transcription, and clustering of postsynaptic proteins, allowing Musk mutant mice to move, breathe and survive as adults. These results show that the juxtamembrane region of Musk, including a single phosphotyrosine docking site, even in the context of a different kinase domain, is sufficient to activate the multiple pathways leading to presynaptic and postsynaptic differentiation in vivo. In addition, we find that Musk protein can be clustered at synaptic sites, even if Musk mRNA is expressed uniformly in muscle. Moreover, acetylcholine receptor clustering and motor terminal branching are restored in parallel, indicating that the extent of presynaptic differentiation is matched to the extent of postsynaptic differentiation.

Dystrobrevin dynamics in muscle-cell signalling: a possible target for therapeutic intervention in Duchenne muscular dystrophy?

The dystrophin-protein complex forms one of the connections between the extracellular matrix and the cytoskeleton of muscle. This link is disrupted in patients with Duchenne and Becker muscular dystrophies. Dystrobrevin is a component of the dystrophin-protein complex that binds to the C-terminus of dystrophin and also to syntrophin. As its name suggests, dystrobrevin is a relative of dystrophin participating in similar intermolecular interactions. Dystrobrevin-deficient mice have a form of muscular dystrophy that leaves the sarcolemma and dystrophin-protein complex intact but affects an as yet unidentified signalling pathway in muscle. Given that the up-regulation of several genes has a beneficial effect on the muscle in some dystrophic mouse models, alpha-dystrobrevin has a number of properties that might be protective in muscular dystrophy. This article discusses the function of dystrobrevin in muscle and reviews its suitability as a therapeutic target for treating patients with Duchenne and Becker muscular dystrophies.

Regulation of AChR clustering by Dishevelled interacting with MuSK and PAK1

An important aspect of synapse development is the clustering of neurotransmitter receptors in the postsynaptic membrane. Although MuSK is required for acetylcholine receptor (AChR) clustering at the neuromuscular junction (NMJ), the underlying molecular mechanisms remain unclear. We report here that in muscle cells, MuSK interacts with Dishevelled (Dvl), a signaling molecule important for planar cell polarity. Disruption of the MuSK-Dvl interaction inhibits Agrin- and neuron-induced AChR clustering. Expression of dominant-negative Dvl1 in postsynaptic muscle cells reduces the amplitude of spontaneous synaptic currents at the NMJ. Moreover, Dvl1 interacts with downstream kinase PAK1. Agrin activates PAK, and this activation requires Dvl. Inhibition of PAK1 activity attenuates AChR clustering. These results demonstrate important roles of Dvl and PAK in Agrin/MuSK-induced AChR clustering and reveal a novel function of Dvl in synapse development.

Laminin-1 redistributes postsynaptic proteins and requires rapsyn, tyrosine phosphorylation, and Src and Fyn to stably cluster acetylcholine receptors

Clustering of acetylcholine receptors (AChRs) is a critical step in neuromuscular synaptogenesis, and is induced by agrin and laminin which are thought to act through different signaling mechanisms. We addressed whether laminin redistributes postsynaptic proteins and requires key elements of the agrin signaling pathway to cause AChR aggregation. In myotubes, laminin-1 rearranged dystroglycans and syntrophins into a laminin-like network, whereas inducing AChR-containing clusters of dystrobrevin, utrophin, and, to a marginal degree, MuSK. Laminin-1 also caused extensive coclustering of rapsyn and phosphotyrosine with AChRs, but none of these clusters were observed in rapsyn -/- myotubes. In parallel with clustering, laminin-1 induced tyrosine phosphorylation of AChR beta and delta subunits. Staurosporine and herbimycin, inhibitors of tyrosine kinases, prevented laminin-induced AChR phosphorylation and AChR and phosphotyrosine clustering, and caused rapid dispersal of clusters previously induced by laminin-1. Finally, laminin-1 caused normal aggregation of AChRs and phosphotyrosine in myotubes lacking both Src and Fyn kinases, but these clusters dispersed rapidly after laminin withdrawal. Thus, laminin-1 redistributes postsynaptic proteins and, like agrin, requires tyrosine kinases for AChR phosphorylation and clustering, and rapsyn for AChR cluster formation, whereas cluster stabilization depends on Src and Fyn. Therefore, the laminin and agrin signaling pathways overlap intracellularly, which may be important for neuromuscular synapse formation.

Rapsyn mutations in humans cause endplate acetylcholine-receptor deficiency and myasthenic syndrome

Congenital myasthenic syndromes (CMSs) stem from genetic defects in endplate (EP)-specific presynaptic, synaptic, and postsynaptic proteins. The postsynaptic CMSs identified to date stem from a deficiency or kinetic abnormality of the acetylcholine receptor (AChR). All CMSs with a kinetic abnormality of AChR, as well as many CMSs with a deficiency of AChR, have been traced to mutations in AChR-subunit genes. However, in a subset of patients with EP AChR deficiency, the genetic defect has remained elusive. Rapsyn, a 43-kDa postsynaptic protein, plays an essential role in the clustering of AChR at the EP. Seven tetratricopeptide repeats (TPRs) of rapsyn subserve self-association, a coiled-coil domain binds to AChR, and a RING-H2 domain associates with beta-dystroglycan and links rapsyn to the subsynaptic cytoskeleton. Rapsyn self-association precedes recruitment of AChR to rapsyn clusters. In four patients with EP AChR deficiency but with no mutations in AChR subunits, we identify three recessive rapsyn mutations: one patient carries L14P in TPR1 and N88K in TPR3; two are homozygous for N88K; and one carries N88K and 553ins5, which frameshifts in TPR5. EP studies in each case show decreased staining for rapsyn and AChR, as well as impaired postsynaptic morphological development. Expression studies in HEK cells indicate that none of the mutations hinders rapsyn self-association but that all three diminish coclustering of AChR with rapsyn.

Dual role for calcium in agrin signaling and acetylcholine receptor clustering

Agrin is a motoneuron-derived factor that initiates neuromuscular synapse formation; however, the signaling pathway underlying postsynaptic differentiation is not yet understood. We have investigated the role of calcium in agrin signaling through the MuSK receptor tyrosine kinase and in the intracellular signaling cascade that leads to AChR phosphorylation and clustering. We find that agrin- and neuramindase-induced MuSK activation in cultured myotubes is completely blocked by removal of extracellular calcium, but only slightly reduced by clamping of intracellular calcium transients with BAPTA. Following agrin's activation of MuSK, we find that the downstream tyrosine phosphorylation of the AChR beta-subunit was inhibited by BAPTA but not by a slower acting chelator, EGTA. Similarly, agrin-induced clustering of the AChR was blocked by BAPTA but not EGTA. These findings indicate that extracellular calcium is required for the formation of a MuSK signaling complex, and that intracellular calcium regulates phosphorylation and clustering of the AChR in the postsynaptic membrane.