Localization and regulation of MuSK at the neuromuscular junction

Thrombin reduces MuSK and acetylcholine receptor expression along with neuromuscular contact size in vitro

In the course of studies on thrombin and its inhibitor(s) in synaptic plasticity, we addressed the question of their roles in the formation of neuromuscular junctions (NMJ) and used a model of rat neuron-myotube cocultures. We report that the size of acetylcholinesterase (AChE) patches used as a marker of neuromuscular contacts was decreased in the presence of either thrombin or SFLLRN, the agonist peptide of the thrombin receptor PAR-1, whereas it was increased with hirudin, a specific thrombin inhibitor. In an attempt to relate these neuromuscular contact size variations to molecular changes, we studied muscle-specific tyrosine kinase receptor (MuSK), acetylcholine receptor (AChR) and rapsyn expression in the presence of thrombin. We showed that thrombin did not change rapsyn gene and protein expression. However, the expression of MuSK and surface AChR proteins was diminished in both myotube cultures and neuron-myotube cocultures. These reductions in protein expression were associated with a decrease in MuSK and AChR alpha-subunit gene expression in myotube cultures but not in neuron-myotube cocultures. Moreover, the expression of the AChR epsilon-subunit gene, specifically enhanced by neuron-released factors, was not modified by thrombin in neuron-myotube cocultures. This suggests that thrombin did not affect the expression of synaptic AChRs enhanced by neuron-released factors but rather reduced the level of extrasynaptic AChRs. Taken together, these results indicate that thrombin in balance with its inhibitor(s) could modulate the formation of neuromuscular contacts in vitro by affecting the expression of two essential molecules in NMJ postsynaptic differentiation, MuSK and AChR.

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.

A novel pathway for MuSK to induce key genes in neuromuscular synapse formation

At the developing neuromuscular junction the Agrin receptor MuSK is the central organizer of subsynaptic differentiation induced by Agrin from the nerve. The expression of musk itself is also regulated by the nerve, but the mechanisms involved are not known. Here, we analyzed the activation of a musk promoter reporter construct in muscle fibers in vivo and in cultured myotubes, using transfection of multiple combinations of expression vectors for potential signaling components. We show that neuronal Agrin by activating MuSK regulates the expression of musk via two pathways: the Agrin-induced assembly of muscle-derived neuregulin (NRG)-1/ErbB, the pathway thought to regulate acetylcholine receptor (AChR) expression at the synapse, and via a direct shunt involving Agrin-induced activation of Rac. Both pathways converge onto the same regulatory element in the musk promoter that is also thought to confer synapse-specific expression to AChR subunit genes. In this way, a positive feedback signaling loop is established that maintains musk expression at the synapse when impulse transmission becomes functional. The same pathways are used to regulate synaptic expression of AChR epsilon. We propose that the novel pathway stabilizes the synapse early in development, whereas the NRG/ErbB pathway supports maintenance of the mature synapse.

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.

The agrin/muscle-specific kinase pathway: new targets for autoimmune and genetic disorders at the neuromuscular junction

The increasing understanding of the structural complexity of the neuromuscular junction (NMJ), and the processes that are important in its development, suggests many possible new disease targets. Here, we summarize briefly the genetic and autoimmune disorders that affect neuromuscular transmission, and the identified targets, including new evidence that antibodies to muscle-specific receptor tyrosine kinase (MuSK) are involved in the pathogenesis of acetylcholine receptor (AChR) antibody-negative myasthenia gravis. We then review the development of the NMJ, focusing on the important roles of nerve-derived agrin and MuSK in clustering of AChRs and other essential components of the NMJ.

Induction of multiple signaling loops by MuSK during neuromuscular synapse formation

At the neuromuscular junction, two motor neuron-derived signals have been implicated in the regulation of synaptogenesis. Neuregulin-1 is thought to induce transcription of acetylcholine receptor (AChR) genes in subsynaptic muscle nuclei by activating ErbB receptors. Neural agrin aggregates AChRs by activating the receptor tyrosine kinase MuSK. Here, we show that these two signals act sequentially. Agrin, by activating MuSK, induces the synthesis and aggregation of both MuSK and ErbB receptors. ErbB acts downstream of MuSK in synapse formation. In this way, MuSK activation leads to the establishment of a neuregulin-1-dependent signaling complex that maintains MuSK, ErbB, and AChR expression at the synapse of electrically active muscle fibers.

Sodium channel mRNAs at the neuromuscular junction: distinct patterns of accumulation and effects of muscle activity

Voltage-gated sodium channels (VGSCs) are highly concentrated at the neuromuscular junction (NMJ) in mammalian skeletal muscle. Here we test the hypothesis that local upregulation of mRNA contributes to this accumulation. We designed radiolabeled antisense RNA probes, specific for the "adult" Na(V)1.4 and "fetal" Na(V)1.5 isoforms of VGSC in mammalian skeletal muscle, and used them in in situ hybridization studies of rat soleus muscles. Na(V)1.4 mRNA is present throughout normal adult muscles but is highly concentrated at the NMJ, in which the amount per myonucleus is more than eightfold greater than away from the NMJ. Na(V)1.5 mRNA is undetectable in innervated muscles but is dramatically upregulated by denervation. In muscles denervated for 1 week, both Na(V)1.4 and Na(V)1.5 mRNAs are present throughout the muscle, and both are concentrated at the NMJ. No Na(V)1.5 mRNA was detectable in denervated muscles stimulated electrically for 1 week in vivo. Neither denervation nor stimulation had any significant effect on the level or distribution of Na(V)1.4 mRNA. We conclude that factors, probably derived from the nerve, lead to the increased concentration of VGSC mRNAs at the NMJ. In addition, the expression of Na(V)1.5 mRNA is downregulated by muscle activity, both at the NMJ and away from it.

Overexpression of muscle specific kinase increases the transcription and aggregation of acetylcholine receptors in Xenopus embryos

Muscle specific kinase (MuSK) mediates agrin-induced acetylcholine receptor (AChR) aggregation on muscle membrane at the neuromuscular junction (NMJ). To examine whether MuSK enhances NMJ formation during embryonic development in vivo, the level of expression of MuSK was manipulated in Xenopus embryos and the functional consequence at the NMJ was assessed. We found that overexpression of MuSK enhanced the formation of NMJ by increasing the aggregation of AChRs at innervated regions in developing embryos. The area of AChR aggregation increased by approximately 2-fold in MuSK injected embryos during the critical stages of NMJ formation. Interestingly, overexpression of MuSK in Xenopus embryos was found to induce the level of AChR transcript. Deletion of the Kringle domain in the MuSK construct did not attenuate the observed induction of AChR transcription and aggregation. Taken together, our findings provide the first demonstration that increased level of MuSK expression in vivo significantly elevate the aggregation and transcription of AChR at the NMJ in developing Xenopus embryos.

Just in time and place: NOS/NO system assembly in neuromuscular junction formation

Recent advances in the molecular, biochemical, and anatomical aspects of postsynaptic membrane components at the neuromuscular junction (NMJ) are briefly reviewed focussing on assembly, architecture, and function of the multi-subunit dystrophin-protein complex (DPC) and its associated nitric oxide (NO)-signaling complex. Elucidation of unique structural binding motifs of NO-synthases (NOS), and microscopical codistribution of neuronal NOS (nNOS), the major isoform of NOS expressed at the NMJ, with known synaptic proteins, i.e., family members of the DPC, nicotinic acetylcholine receptor (AChR), NMDA-receptor, type-1 sodium and Shaker K(+)-channel proteins, and linker proteins (e.g., PSD-95, 43K-rapsyn), suggests targeting and assembly of the NO-signaling pathway at postsynaptic membrane components. NO mediates agrin-induced AChR-aggregation and downstream signal transduction in C2 skeletal myotubes while administration of L-arginine, the limiting substrate for NO-biosynthesis, enhances aggregation of synapse-specific components such as utrophin. At the NMJ, NO appears to be a mediator of (1) early synaptic protein clustering, (2) synaptic receptor activity and transmitter release, or (3) downstream signaling for transcriptional control. Multidisciplinary data obtained from cellular and molecular studies and from immunolocalization investigations have led us to propose a working model for step-by-step binding of nNOS, e.g., to subunit domains of targeted and/or preexisting membrane components. Formation of NOS-membrane complexes appears to be governed by agrin-signaling as well as by NO-signaling, supporting the idea that parallel signaling pathways may account for the spatiotemporally defined postsynaptic assembly thereby linking the NOS/NO-signaling cascade to early membrane aggregations and at the right places nearby preexisting targets (e.g., juxtaposition of NO source and target) in synapse formation.

Disparate dysfunction of skeletal muscles located near and distant from burn site in the rat

This study tested the hypotheses that burn-induced change in muscle function varies at sites local and distant from burn and is related to changes in expression of acetylcholine receptors (AChRs) and muscle mass. In anesthetized rats, approximately 4% burn was inflicted over the tibialis anterior muscle of one limb. The contralateral leg served as control. In another study, a approximately 45% body surface area burn was produced on the trunk; controls were body sham-burned rats. The evoked twitch tensions of tibialis anterior muscles in both legs were measured together with AChR proteins and their transcripts. Compared with the contralateral leg, absolute tensions in the burned leg declined at days 1, 4, and 7 without loss of muscle mass so that tension per unit wet muscle mass (specific tension) decreased; at day 14, the tension decreased with muscle atrophy so that specific tension was unchanged. Membrane AChRs and/or the immature subunit transcript, AChRgamma messenger ribonucleic acid (mRNA) increased at days 4, 7, and 14, and both were inversely related to evoked tension (r =.43, P <.01 and r =.61, P <.0001, respectively). There was a direct correlation between AChR and AChRgamma mRNA (r =.82, P <.001), suggesting that the upregulated AChRs may contain the immature gamma-subunit isoform. After approximately 45% body burn, AChRs and mRNA did not change and the evoked tensions did not decline, but there was relative loss of muscle mass at days 7 and 14 so that specific tension increased. Burn trauma initially causes weakness of muscles directly under the burn, and this weakness may be partially related to increased expression of immature AChRs and later to muscle atrophy.

Aplysia ror forms clusters on the surface of identified neuroendocrine cells

The ror receptors are a highly conserved family of receptor tyrosine kinases genetically implicated in the establishment of cellular polarity. We have cloned a ror receptor from the marine mollusk Aplysia californica. Aplysia ror (Apror) is expressed in most developing neurons and some adult neuronal populations, including the neuroendocrine bag-cell neurons. The Apror protein is present in peripheral neuronal processes and ganglionic neuropil, implicating the kinase in the regulation of growth and/or synaptic events. In cultured bag-cell neurons, the majority of the protein is stored in intracellular organelles, whereas only a small fraction is expressed on the surface. When expressed on the cell surface, the protein is clustered on neurites, suggesting that Apror is involved in the organization of functional domains within neurons. Apror immunoreactivity partially colocalizes with the P-type calcium channel BC-alpha1A at bag-cell neuron varicosities, suggesting a role for Apror in organizing neuropeptide release sites.

Expression of Eph receptors in skeletal muscle and their localization at the neuromuscular junction

The participation of ephrins and Eph receptors in guiding motor axons during muscle innervation has been well documented, but little is known about their expression and functional significance in muscle at later developmental stages. Our present study investigates the expression and localization of Eph receptors and ephrins in skeletal muscle. Prominent expression of EphA4, EphA7, and ephrin-A ligands was detected in muscle during embryonic development. More importantly, both EphA4 and EphA7, as well as ephrin-A2, were localized at the neuromuscular junction (NMJ) of adult muscle. Despite their relative abundance, they were not localized at the synapses during embryonic stages. The concentration of EphA4, EphA7, and ephrin-A2 at the NMJ was observed at postnatal stages and the synaptic localization became prominent at later developmental stages. In addition, expression of Eph receptors was increased by neuregulin and after nerve injury. Furthermore, we demonstrated that overexpression of EphA4 led to tyrosine phosphorylation of the actin-binding protein cortactin and that EphA4 was coimmunoprecipitated with cortactin in muscle. Taken together, our findings indicate that EphA4 is associated with the actin cytoskeleton. Since actin cytoskeleton is critical to the formation and stability of NMJ, the present findings raise the intriguing possibility that Eph receptors may have a novel role in NMJ formation and/or maintenance.

Agrin-induced phosphorylation of the acetylcholine receptor regulates cytoskeletal anchoring and clustering

At the developing neuromuscular junction, a motoneuron-derived factor called agrin signals through the muscle-specific kinase receptor to induce postsynaptic aggregation of the acetylcholine receptor (AChR). The agrin signaling pathway involves tyrosine phosphorylation of the AChR beta subunit, and we have tested its role in receptor localization by expressing tagged, tyrosine-minus forms of the beta subunit in mouse Sol8 myotubes. We find that agrin-induced phosphorylation of the beta subunit occurs only on cell surface AChR, and that AChR-containing tyrosine-minus beta subunit is targeted normally to the plasma membrane. Surface AChR that is tyrosine phosphorylated is less detergent extractable than nonphosphorylated AChR, indicating that it is preferentially linked to the cytoskeleton. Consistent with this, we find that agrin treatment reduces the detergent extractability of AChR that contains tagged wild-type beta subunit but not tyrosine-minus beta subunit. In addition, agrin-induced clustering of AChR containing tyrosine-minus beta subunit is reduced in comparison to wild-type receptor. Thus, we find that agrin-induced phosphorylation of AChR beta subunit regulates cytoskeletal anchoring and contributes to the clustering of the AChR, and this is likely to play an important role in the postsynaptic localization of the receptor at the developing synapse.

Electrical activity and postsynapse formation in adult muscle: gamma-AChRs are not required

Skeletal muscle fibers will not accept hyperinnervation by foreign motor axons unless they are paralyzed, suggesting that paralysis makes them receptive to innervation, e.g., by upregulating extrasynaptic expression of gamma-AChRs and/or of the agrin receptor MuSK. To examine the involvement of these parameters in paralysis-mediated synapse induction, ectopic expression of agrin, a factor from motor neurons controlling neuromuscular synapse formation, was made dependent on the administration of doxycycline in innervated adult muscle fibers. In response to doxycycline-induced agrin secretion, adult fibers did form ectopic postsynaptic specializations, even when they were electrically active, lacked fetal AChRs, and expressed normal low levels of MuSK. These data demonstrate that paralysis and changes associated with it are not required for agrin-induced postsynapse formation. They suggest that paralyzed muscle induces synapse formation via the release of factors that make motor neurites contact muscle fibers and secrete agrin.

Cloning and characterization of muscle-specific kinase in chicken

Muscle-specific kinase (MuSK) is part of the receptor complex that is involved in the agrin-induced formation of the neuromuscular junction. In the rodent, prominent mRNA expression of MuSK is restricted to skeletal muscle while the expression of agrin can also be detected in brain and certain nonneuronal tissues. The recent identification of Xenopus MuSK reveals that MuSK can be detected in tissues other than skeletal muscle, such as the neural tube, eye vesicles, and spleen. In this study, we describe the cloning and characterization of the chicken ortholog of MuSK and demonstrate that the regulation of MuSK expression in muscle is conserved from avian to rodent. Abundant mRNA expression of MuSK can be detected in early embryonic chick muscle and is up-regulated after nerve injury. More importantly, we also demonstrate that, in the chicken, MuSK mRNA is expressed during development in brain and liver, suggesting possible novel functions for MuSK. Furthermore, the regulatory profile of MuSK expression in chick muscle closely parallels that observed for acetylcholine receptor, in terms of both mRNA expression and protein localization. Finally, studies with paralyzed chicken muscle as well as with cultured chick myotubes demonstrate the dependence of MuSK on both electrical activity and trophic factors.

Different functions of fetal and adult AChR subtypes for the formation and maintenance of neuromuscular synapses revealed in epsilon-subunit-deficient mice

Mice deficient in epsilon-subunits of the acetylcholine receptor (AChR) channel die prematurely due to severe AChR deficiency that leads to the progressive reduction in AChR density at the neuromuscular endplate [Witzemann, V., Schwarz, H., Koenen, M., Berberich, C., Villarroel, A., Wernig, A., Brenner, H.R. & Sakmann, B. (1996) Proc. Natl Acad. Sci. USA, 93, 13286-13291]. The mice may serve as a model for studying AChR-related myasthenic diseases. The postnatal development of the subsynaptic apparatus takes place in the absence of the adult type, epsilon-subunit-containing receptors which normally replace the fetal gamma-subunit-containing receptors. During later development the secondary folds of the postsynaptic membrane disappear concomitant with the decrease in AChR density, so that the flattened-out membrane with its remaining nicotinic receptors is in close proximity to the subsynaptic cytoplasmatic compartment and the subsynaptic myonuclei. The decrease in AChR concentration is correlated with a decrease of postsynaptic rapsyn, but has less effect on agrin, a neuronally released aggregating factor for AChRs. Thus, despite the presence of agrin at the synapse, AChR expression is not maintained at the level required to stabilize normal synaptic structure comprising secondary postsynaptic membrane folds. Collectively the results suggest that the postnatal switch from the global, activity-sensitive gamma-subunit gene transcription to the synapse-specific, activity-independent epsilon-subunit gene transcription is not required for the formation and differentiation of synapses but is essential for the maintenance of the highly organized structure of the neuromuscular endplate.

The Ets transcription factor GABP is required for postsynaptic differentiation in vivo

At chemical synapses, neurotransmitter receptors are concentrated in the postsynaptic membrane. During the development of the neuromuscular junction, motor neurons induce aggregation of acetylcholine receptors (AChRs) underneath the nerve terminal by the redistribution of existing AChRs and preferential transcription of the AChR subunit genes in subsynaptic myonuclei. Neural agrin, when expressed in nonsynaptic regions of muscle fibers in vivo, activates both mechanisms resulting in the assembly of a fully functional postsynaptic apparatus. Several lines of evidence indicate that synaptic transcription of AChR genes is primarily dependent on a promoter element called N-box. The Ets-related transcription factor growth-associated binding protein (GABP) binds to this motif and has thus been suggested to regulate synaptic gene expression. Here, we assessed the role of GABP in synaptic gene expression and in the formation of postsynaptic specializations in vivo by perturbing its function during postsynaptic differentiation induced by neural agrin. We find that neural agrin-mediated activation of the AChR epsilon subunit promoter is abolished by the inhibition of GABP function. Importantly, the number of AChR aggregates formed in response to neural agrin was strongly reduced. Moreover, aggregates of acetylcholine esterase and utrophin, two additional components of the postsynaptic apparatus, were also reduced. Together, these results are the first direct in vivo evidence that GABP regulates synapse-specific gene expression at the neuromuscular junction and that GABP is required for the formation of a functional postsynaptic apparatus.

Myosin heavy chain isoform expression following reduced neuromuscular activity: potential regulatory mechanisms

In this review, the adaptations in myosin heavy chain (MHC) isoform expression induced by chronic reductions in neuromuscular activity (including electrical activation and load bearing) of the intact neuromuscular unit are summarized and evaluated. Several different animal models and human clinical conditions of reduced neuromuscular activity are categorized based on the manner and extent to which they alter the levels of electrical activation and load bearing, resulting in three main categories of reduced activity. These are: 1) reduced activation and load bearing (including spinal cord injury, spinal cord transection, and limb immobilization with the muscle in a shortened position); 2) reduced loading (including spaceflight, hindlimb unloading, bed rest, and unilateral limb unloading); and 3) inactivity (including spinal cord isolation and blockage of motoneuron action potential conduction by tetrodotoxin). All of the models discussed resulted in increased expression of fast MHC isoforms at the protein and/or mRNA levels in slow and fast muscles (with the possible exception of unilateral limb unloading in humans). However, the specific fast MHC isoforms that are induced (usually the MHC-IIx isoform in slow muscle and the MHC-IIb isoform in fast muscle) and the degree and rate of adaptation are dependent upon the animal species and the specific model or condition that is being studied. Recent studies designed to elucidate the mechanisms by which electrical activation and load bearing alter expression of MHC isoforms at the cellular and genetic levels are also reviewed. Two main mechanisms have been proposed, the myogenin:MyoD and calcineurin:NF-AT pathways. Collectively, the data suggest that the regulation of MHC isoform expression involves a complex interaction of multiple control mechanisms including the myogenin:MyoD and calcineurin:NF-AT pathways; however, other intracellular signaling pathways are likely to contribute.

Dystrophin and utrophin: genetic analyses of their role in skeletal muscle

Since the identification of dystrophin as the causitive factor in Duchenne muscular dystrophy, there has been substantial progress in understanding the functions and interactions of this protein. Dystrophin has been shown to interact with a group of peripheral- and trans-membrane proteins known as the dystrophin-associated protein complex (DAPC) and mutations in some of the members of this complex have been shown to account for other forms of muscular dystrophy. This review summarizes the experiments using transgenic and knockout mouse models that have defined the roles of dystrophin, and the dystrophin-related protein utrophin at the skeletal muscle membrane and at the neuromuscular junction. These studies are presented in the context of other known interactions at the muscle membrane. Studies of the dystrophin-deficient mdx mouse have lead to a greater understanding of the human disease. Knockouts and transgenics of utrophin have shown this protein to be sufficient to functionally compensate for dystrophin. Dystrophin transgenic mice combined with the mdx mouse have been used to study the function of specific domains of the dystrophin protein. Together these animal models have led to a delineation of protein functions and localization patterns that will be useful for the generation of potential therapies for DMD.

Acetylcholine receptors are required for postsynaptic aggregation driven by the agrin signalling pathway

To investigate the role of acetylcholine receptors (AChRs) in the aggregation of postsynaptic molecules on muscle cells, we utilized the 1R- genetic variant of C2 muscle cells which has very little expression of AChRs in its cell membrane. On C2 myotubes, AChRs cluster spontaneously, with the frequency of clustering greatly enhanced by motor neuron-derived agrin. Signal transduction events driven by agrin, including the tyrosine phosphorylation of muscle-specific kinase (MuSK) and the AChR beta subunit, have been implicated as requirements of postsynaptic scaffold assembly. We show here that some molecules of the postsynaptic scaffold spontaneously aggregate and colocalize on 1R- myotubes at very low frequency, including an as yet unidentified agrin binding molecule, beta-dystroglycan and MuSK. Agrin is unable to increase the frequency of these aggregations, but does cause tyrosine phosphorylation of MuSK. We conclude that free molecules can associate into aggregates independently of AChRs, but AChRs are required for high-frequency molecular aggregation driven by the agrin signalling pathway. MuSK tyrosine phosphorylation appears to precede a requisite event involving AChRs that aggregates postsynaptic molecules.

Distinct domains of MuSK mediate its abilities to induce and to associate with postsynaptic specializations

Agrin released from motor nerve terminals activates a muscle-specific receptor tyrosine kinase (MuSK) in muscle cells to trigger formation of the skeletal neuromuscular junction. A key step in synaptogenesis is the aggregation of acetylcholine receptors (AChRs) in the postsynaptic membrane, a process that requires the AChR-associated protein, rapsyn. Here, we mapped domains on MuSK necessary for its interactions with agrin and rapsyn. Myotubes from MuSK(-/)- mutant mice form no AChR clusters in response to agrin, but agrin-responsiveness is restored by the introduction of rat MuSK or a Torpedo orthologue. Thus, MuSK(-/)- myotubes provide an assay system for the structure-function analysis of MuSK. Using this system, we found that sequences in or near the first of four extracellular immunoglobulin-like domains in MuSK are required for agrin responsiveness, whereas sequences in or near the fourth immunoglobulin-like domain are required for interaction with rapsyn. Analysis of the cytoplasmic domain revealed that a recognition site for the phosphotyrosine binding domain-containing proteins is essential for MuSK activity, whereas consensus binding sites for the PSD-95/Dlg/ZO-1-like domain-containing proteins and phosphatidylinositol-3-kinase are dispensable. Together, our results indicate that the ectodomain of MuSK mediates both agrin- dependent activation of a complex signal transduction pathway and agrin-independent association of the kinase with other postsynaptic components. These interactions allow MuSK not only to induce a multimolecular AChR-containing complex, but also to localize that complex to a primary scaffold in the postsynaptic membrane.

Constitutively active MuSK is clustered in the absence of agrin and induces ectopic postsynaptic-like membranes in skeletal muscle fibers

In skeletal muscle fibers, neural agrin can direct the accumulation of acetylcholine receptors (AChR) and transcription of AChR subunit genes from the subsynaptic nuclei. Although the receptor tyrosine kinase MuSK is required for AChR clustering, it is less clear whether MuSK regulates gene transcription. To elucidate the role of MuSK in these processes, we constructed a constitutively active MuSK receptor, MuSKneuTMuSK, taking advantage of the spontaneous homodimerization of the transmembrane domain of neuT, an oncogenic variant of the neu/erbB2 receptor. In the extrasynaptic region of innervated muscle fibers, MuSKneuTMuSK formed highly concentrated aggregates that colocalized with AChR clusters. Associated with MuSK-induced AChR clusters was a normal complement of synaptic proteins. Moreover, transcription of the AChR-epsilon subunit gene was increased, albeit via an indirect mechanism by MuSK-induced aggregation of erbB receptors and neuregulin. Although neural agrin was not required, the activity of MuSKneuTMuSK was nevertheless potentiated by ectopic expression of a muscle agrin isoform inactive in AChR clustering. To define the role of the kinase domain in the formation of a postsynaptic-like membrane, a second fusion receptor, neuneuTMuSK, which included the MuSK kinase but not the MuSK extracellular domain, was expressed. Significantly, neuneuTMuSK induced AChR clusters that colocalized with aggregates of endogenous MuSK. Taken together, it was concluded that the MuSK kinase domain is sufficient to initiate the recruitment of additional MuSK receptors, which then develop into highly concentrated aggregates by means of a positive feedback loop to induce a postsynaptic membrane in the absence of neural agrin.

Identification and characterization of a novel splice variant of MuSK

MuSK is a receptor tyrosine kinase that initiates the formation of neuromuscular junctions in response to agrin. Little is known about the ligand-induced activation and kinase-dependent signalling that leads to the clustering of acetylcholine receptors. The ectodomain of these molecule is composed of four Ig-like domains. We describe here the isolation of a novel MuSK splice variant that lacks the third Ig-like domain in its ectodomain. The corresponding RNA is the result of alternative splicing which eliminates two exons. There is 10 times less mRNA for this shorter form than for the long form of MuSK and both forms are regulated coordinately. They decrease strongly after birth and are elevated in denervated muscle. Gene transfer by muscle injection of MuSK DNA into individual muscle fibers demonstrates that kinase-induced acetylcholine receptor clustering caused by overexpression of the two kinases does not depend on the presence of the third Ig-like domain.