The subsynaptic 43-kDa protein is concentrated at developing nerve-muscle synapses in vitro

An Inside Job: Molecular Determinants for Postsynaptic Localization of Nicotinic Acetylcholine Receptors

Nicotinic acetylcholine receptors (nAChRs) mediate fast synaptic transmission at neuromuscular and autonomic ganglionic synapses in the peripheral nervous system. The postsynaptic localization of muscle ((α1)₂β1γδ) and neuronal ((α3β4)₂β4) nicotinic receptors at these synapses is mediated by interactions between the nAChR intracellular domains and cytoplasmic scaffolding proteins. Recent high resolution structures and functional studies provide new insights into the molecular determinants that mediate these interactions. Surprisingly, they reveal that the muscle nAChR binds 1–3 rapsyn scaffolding molecules, which dimerize and thereby form an interconnected lattice between receptors. Moreover, rapsyn binds two distinct sites on the nAChR subunit cytoplasmic loops; the MA-helix on one or more subunits and a motif specific to the β subunit. Binding at the latter site is regulated by agrin-induced phosphorylation of βY390, and increases the stoichiometry of rapsyn/AChR complexes. Similarly, the neuronal nAChR may be localized at ganglionic synapses by phosphorylation-dependent interactions with 14-3-3 adaptor proteins which bind specific motifs in each of the α3 subunit cytoplasmic loops. Thus, postsynaptic localization of nAChRs is mediated by regulated interactions with multiple scaffolding molecules, and the stoichiometry of these complexes likely helps regulate the number, density, and stability of receptors at the synapse.

Laminin is instructive and calmodulin dependent kinase II is non-permissive for the formation of complex aggregates of acetylcholine receptors on myotubes in culture

Previous work has shown that myotubes cultured on laminin-coated substrates form complex aggregates of synaptic proteins that are similar in shape and composition to neuromuscular junctions (NMJs). Here we show that laminin instructs the location of complex aggregates which form only on the lower surface when laminin is coated onto culture dishes but over the entire cell when laminin is added in solution. Silencing of myotubes by agents that block electrical activity (tetrodotoxin, verapamil) or by inhibitors of calmodulin dependent kinase (CaMKII) render the myotube permissive for the formation of complex aggregates. Treatment with laminin alone will facilitate the formation of complex aggregates hours later when myotubes are made permissive by inhibiting CaMKII. The AChR agonist carbachol disperses pre formed aggregates suggesting that non-permissiveness may involve active dispersal of AChRs. The permissive period requires ongoing protein synthesis. The latter may reflect a requirement for rapsyn, which turns over rapidly, and is necessary for aggregation. Consistent with this geldanamycin, an agent that increases rapsyn turnover disrupts complex aggregates. Agrin is well known to induce small clusters of AChRs but does not induce complex aggregates even though aggregate formation requires MuSK, a receptor tyrosine kinase activated by agrin. Dystroglycan (DG) is the major laminin receptor mediating complex aggregate formation with some contribution from β1 integrins. In addition, there is a pool of CaMKII associated with DG. We discuss how these permissive and instructive mechanisms bear on NMJ formation in vivo.

■ REVIEW : Postsynaptic Anchoring of Receptors: A Cellular Approach to Neuronal and Muscular Sensitivity

Significant progress has been made toward the elucidation of the molecular mechanisms underlying the biogenesis and stabilization of postsynaptic membrane specializations at the neuromuscular junction of vertebrate skeletal muscle. The emerging picture reveals a continuous molecular link from the extracellular matrix within the synaptic cleft via integral and peripheral membrane proteins to the subsarcolemmal cytoskeleton. The formation and maintenance of synaptic contacts between neurons in the CNS might follow similar architectural principles but involve different molecules. The biogenesis of glycinergic postsynaptic membrane specializations depends on the widely expressed peripheral membrane protein gephyrin, which anchors the neurotransmitter receptor to underlying cytoskeletal elements in a dynamic manner. This anchoring mechanism could also contribute to the plasticity of glycinergic synapses. Other types of neurotransmitter receptors, like GABAA- and glutamate receptors, may have evolved different molecular mechanisms to ensure their localization in postsynaptic membrane specializations. The Neuroscientist 2:100-108, 1996

Acetylcholine receptors enable the transport of rapsyn from the Golgi complex to the plasma membrane

The accumulation of acetylcholine receptors (AChRs) at nerve terminals is critical for signal transmission at the neuromuscular junction, and rapsyn is essential for this process. Previous studies suggest that AChRs might direct rapsyn self-clusters to the synapse. In vivo experiments with fluorescently tagged AChR or rapsyn in zebrafish larvae revealed that rapsyn self-clusters separate from AChRs did not exist before synapse formation. Examination of rapsyn in the AChR-less mutant sofa potato revealed that rapsyn in the absence of AChR was localized in the Golgi complex. Expression of muscle-type AChR in sofa potato restored synaptic clustering of rapsyn, while neuronal type AChR had no effect. To determine whether this requirement of protein interaction is reciprocal, we examined the mutant twitch once, which has a missense mutation in rapsyn. While the AChRs distributed nonsynaptically on the plasma membrane in twitch once, mutant rapsyn was retained in the Golgi complex. We conclude that AChRs enable the transport of rapsyn from the Golgi complex to the plasma membrane through a molecule-specific interaction.

Acetylcholine receptor organization in membrane domains in muscle cells: evidence for rapsyn-independent and rapsyn-dependent mechanisms

Nicotinic acetylcholine receptors (nAChR) in muscle fibers are densely packed in the postsynaptic region at the neuromuscular junction. Rapsyn plays a central role in directing and clustering nAChR during cellular differentiation and neuromuscular junction formation; however, it has not been demonstrated whether rapsyn is the only cause of receptor immobilization. Here, we used single-molecule tracking methods to investigate nAChR mobility in plasma membranes of myoblast cells during their differentiation to myotubes in the presence and absence of rapsyn. We found that in myoblasts the majority of nAChR were immobile and that ∼20% of the receptors showed restricted diffusion in small domains of ∼50 nm. In myoblasts devoid of rapsyn, the fraction of mobile nAChR was considerably increased, accompanied by a 3-fold decrease in the immobile population of nAChR with respect to rapsyn-expressing cells. Half of the mobile receptors were confined to domains of ∼120 nm. Measurements performed in heterologously transfected HEK cells confirmed the direct immobilization of nAChR by rapsyn. However, irrespective of the presence of rapsyn, about one-third of nAChR were confined in 300-nm domains. Our results show (i) that rapsyn efficiently immobilizes nAChR independently of other postsynaptic scaffold components; (ii) nAChR is constrained in confined membrane domains independently of rapsyn; and (iii) in the presence of rapsyn, the size of these domains is strongly reduced.

Rapsyn interacts with the muscle acetylcholine receptor via alpha-helical domains in the alpha, beta, and epsilon subunit intracellular loops

At the developing vertebrate neuromuscular junction, the acetylcholine receptor becomes aggregated at high density in the postsynaptic muscle membrane. Receptor localization is regulated by the motoneuron-derived factor, agrin, and requires an intracellular, scaffolding protein called rapsyn. However, it remains unclear where rapsyn binds on the acetylcholine receptor and how their interaction is regulated. In this study, we identified rapsyn's binding site on the acetylcholine receptor using chimeric constructs where the intracellular domain of CD4 was substituted for the major intracellular loop of each mouse acetylcholine receptor subunit. When expressed in heterologous cells, we found that rapsyn clustered and cytoskeletally anchored CD4-alpha, beta and epsilon subunit loops but not CD4-delta loop. Rapsyn-mediated clustering and anchoring was highest for beta loop, followed by epsilon and alpha, suggesting that rapsyn interacts with the loops with different affinities. Moreover, by making deletions within the beta subunit intracellular loop, we show that rapsyn interacts with the alpha-helical region, a secondary structural motif present in the carboxyl terminal portion of the subunit loops. When expressed in muscle cells, rapsyn co-immunoprecipitated together with a CD4-alpha helical region chimera, independent of agrin signaling. Together, these findings demonstrate that rapsyn interacts with the acetylcholine receptor via an alpha-helical structural motif conserved between the alpha, beta and epsilon subunits. Binding at this site likely mediates the critical rapsyn interaction involved in localizing the acetylcholine receptor at the neuromuscular junction.

Identification of a motif in the acetylcholine receptor beta subunit whose phosphorylation regulates rapsyn association and postsynaptic receptor localization

At the neuromuscular junction, the acetylcholine receptor (AChR) is specifically clustered in the postsynaptic membrane via interactions with rapsyn and other scaffolding proteins. However, it remains unclear where these proteins bind on the AChR and how the interactions are regulated. Here, we define a phosphorylation-dependent binding site on the receptor that mediates agrin-induced clustering. Using chimeric proteins in which CD4 is fused to the large intracellular loop of each of the AChR subunits we found that agrin induced clustering of only chimeras containing the beta subunit loop. By making deletions in the beta loop we defined a 20 amino-acid sequence that is sufficient for clustering. The sequence contains a conserved tyrosine (Y390) whose phosphorylation is induced by agrin and whose mutation abolished clustering of beta loop chimeras and their ability to inhibit agrin-induced clustering of the endogenous AChR. Phosphorylation of the AChR beta subunit is correlated with increased rapsyn/AChR binding, suggesting that the effect of betaY390 phosphorylation on clustering is mediated by rapsyn. Indeed, we found that rapsyn associated with CD4-beta loop chimeras in a phosphorylation-dependent manner, and that agrin increased the ratio of rapsyn binding to wild type AChR but not to AChR-beta(3F/3F), which lacks beta loop tyrosine phosphorylation sites. Together, these findings suggest that agrin-induced phosphorylation of the beta subunit motif increases the stoichiometry of rapsyn binding to the AChR, thereby helping to stably cluster the receptor and anchor it at high density in the postsynaptic membrane.

An emerging picture of synapse formation: a balance of two opposing pathways

The neuromuscular junction (NMJ) is a well-studied chemical synapse and has served as a tractable model system to clarify how synapse formation occurs. Proteins on both the presynaptic and postsynaptic sides collaborate to induce the high-density accumulation of acetylcholine receptors (AChRs) at the NMJ. Two opposing pathways work in this process: A dispersing pathway works through acetylcholine and the AChR, and a clustering pathway works through agrin and the transmembrane tyrosine kinase MuSK. The molecular mechanisms underlying these two signaling cascades are beginning to be understood.

Molecular regulation of postsynaptic differentiation at the neuromuscular junction

The neuromuscular junction (NMJ) is a synapse that develops between a motor neuron and a muscle fiber. A defining feature of NMJ development in vertebrates is the re-distribution of muscle acetylcholine (ACh) receptors (AChRs) following innervation, which generates high-density AChR clusters at the postsynaptic membrane and disperses aneural AChR clusters formed in muscle before innervation. This process in vivo requires MuSK, a muscle-specific receptor tyrosine kinase that triggers AChR re-distribution when activated; rapsyn, a muscle protein that binds and clusters AChRs; agrin, a nerve-secreted heparan-sulfate proteoglycan that activates MuSK; and ACh, a neurotransmitter that stimulates muscle and also disperses aneural AChR clusters. Moreover, in cultured muscle cells, several additional muscle- and nerve-derived molecules induce, mediate or participate in AChR clustering and dispersal. In this review we discuss how regulation of AChR re-distribution by multiple factors ensures aggregation of AChRs exclusively at NMJs.

The actin-driven movement and formation of acetylcholine receptor clusters

A new method was devised to visualize actin polymerization induced by postsynaptic differentiation signals in cultured muscle cells. This entails masking myofibrillar filamentous (F)-actin with jasplakinolide, a cell-permeant F-actin-binding toxin, before synaptogenic stimulation, and then probing new actin assembly with fluorescent phalloidin. With this procedure, actin polymerization associated with newly induced acetylcholine receptor (AChR) clustering by heparin-binding growth-associated molecule-coated beads and by agrin was observed. The beads induced local F-actin assembly that colocalized with AChR clusters at bead-muscle contacts, whereas both the actin cytoskeleton and AChR clusters induced by bath agrin application were diffuse. By expressing a green fluorescent protein-coupled version of cortactin, a protein that binds to active F-actin, the dynamic nature of the actin cytoskeleton associated with new AChR clusters was revealed. In fact, the motive force generated by actin polymerization propelled the entire bead-induced AChR cluster with its attached bead to move in the plane of the membrane. In addition, actin polymerization is also necessary for the formation of both bead and agrin-induced AChR clusters as well as phosphotyrosine accumulation, as shown by their blockage by latrunculin A, a toxin that sequesters globular (G)-actin and prevents F-actin assembly. These results show that actin polymerization induced by synaptogenic signals is necessary for the movement and formation of AChR clusters and implicate a role of F-actin as a postsynaptic scaffold for the assembly of structural and signaling molecules in neuromuscular junction formation.

Metabolic stabilization of muscle nicotinic acetylcholine receptor by rapsyn

Although the metabolic half-life of muscle endplate acetylcholine receptor (AChR) changes during development and after denervation in the adult, little is known about the molecular mechanisms that influence receptor stability. We have investigated the effect on AChR turnover of its interaction with rapsyn, a 43 kDa peripheral membrane protein that is closely associated with the AChR in muscle cells and is required for its clustering at endplates. Both in transfected COS cells and in cultured myotubes from rapsyn-negative and rapsyn-positive mice, we have found that the presence of rapsyn slows the turnover of AChRs by as much as twofold. The effect was similar for both embryonic (alpha2betadeltagamma) and adult (alpha2betadeltaepsilon) AChRs and for AChRs whose beta subunit lacked a putative tyrosine phosphorylation site. Neither colchicine nor cytochalasin D altered AChR turnover or prevented the rapsyn effect. Mutant rapsyn proteins whose N-terminal myristoylation signal was eliminated, or whose C terminus or zinc-finger domains were deleted, failed to change the rate of receptor turnover. Each of these mutations affects the association of the AChR with rapsyn, suggesting that AChR stability is altered by interaction between the two proteins. Our results suggest that, in addition to its role in AChR clustering, rapsyn also functions to metabolically stabilize the AChR.

Metabolic stabilization of muscle nicotinic acetylcholine receptor by rapsyn

Although the metabolic half-life of muscle endplate acetylcholine receptor (AChR) changes during development and after denervation in the adult, little is known about the molecular mechanisms that influence receptor stability. We have investigated the effect on AChR turnover of its interaction with rapsyn, a 43 kDa peripheral membrane protein that is closely associated with the AChR in muscle cells and is required for its clustering at endplates. Both in transfected COS cells and in cultured myotubes from rapsyn-negative and rapsyn-positive mice, we have found that the presence of rapsyn slows the turnover of AChRs by as much as twofold. The effect was similar for both embryonic (alpha2betadeltagamma) and adult (alpha2betadeltaepsilon) AChRs and for AChRs whose beta subunit lacked a putative tyrosine phosphorylation site. Neither colchicine nor cytochalasin D altered AChR turnover or prevented the rapsyn effect. Mutant rapsyn proteins whose N-terminal myristoylation signal was eliminated, or whose C terminus or zinc-finger domains were deleted, failed to change the rate of receptor turnover. Each of these mutations affects the association of the AChR with rapsyn, suggesting that AChR stability is altered by interaction between the two proteins. Our results suggest that, in addition to its role in AChR clustering, rapsyn also functions to metabolically stabilize the AChR.

Membrane traffic in polarized neurons

The plasma membrane of neurons can be divided into two domains, the soma-dendritic and the axonal. These domains perform different functions: the dendritic surface receives and processes information while the axonal surface is specialized for the rapid transmission of electrical impulses. This functional specialization is generated by sorting and anchoring mechanisms that guarantee the correct delivery and retention of specific membrane proteins. Our understanding of neuronal membrane protein sorting is primarily based on studies of protein overexpression in cultured neurons. These studies revealed that newly synthesized membrane proteins are segregated in the Golgi apparatus in the cell body from where they are transported to the axonal or dendritic surface. Such segregation presumably depends on sorting motifs in the proteins' primary structure. They appear to be located in the cytoplasmic tail for dendritic proteins and in the transmembrane-ectodomain for axonal proteins. Recent studies on neurotransmitter segregation suggest that anchoring in the correct subdomain of the plasma membrane also requires cytoplasmic tail information for binding to the cytoskeleton either directly or by linker proteins. Both mechanisms, sorting and retention, gradually mature during neural development. Young neurons appear to develop initial polarity by other mechanisms, presumably analogous to the mechanisms used by migrating cells.

Recombinant neuromuscular synapses

The developing neuromuscular junction has provided an important paradigm for studying synapse formation. An outstanding feature of neuromuscular differentiation is the aggregation of acetylcholine receptors (AChRs) at high density in the postsynaptic membrane. While AChR aggregation is generally believed to be induced by the nerve, the mechanisms underlying aggregation remain to be clarified. A 43-kD protein (43k) normally associated with the cytoplasmic aspect of AChR clusters has long been suspected of immobilizing AChRs by linking them to the cytoskeleton. In recent studies, the AChR clustering activity of 43k has, at last, been demonstrated by expressing recombinant AChR and 43k in non-muscle cells. Mutagenesis of 43k has revealed distinct domains within the primary structure which may be responsible for plasma membrane targeting and AChR binding. Other lines of study have provided clues as to how nerve-derived (extracellular) AChR-cluster inducing factors such as agrin might activate 43k-driven postsynaptic membrane specialization.

Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane

An extrinsic membrane protein of apparent molecular mass 43 kDa is specifically localized in postsynaptic membranes closely associated with the nicotinic acetylcholine receptor (AChR). Since its discovery in 1977, biochemical and morphological studies have combined to provide relatively clear pictures of 43K protein structure and subcellular compartmentalization. Nevertheless, despite these advances, the precise function of this synapse-specific protein remains unclear. Data gathered in recent years indicate that the postsynaptic apparatus develops through the incremental agglomeration of receptor microaggregates; evidence derived from a number of sources points to a role for 43K protein in certain underlying reactions. In this paper, I review 43K protein structural and anatomical data and analyze evidence for its role in the organization and maintenance of the postsynaptic membrane. Finally, I offer a model presenting a view of the role of 43K protein in the ontogeny of the motor endplate.

Mutagenesis of the 43-kD postsynaptic protein defines domains involved in plasma membrane targeting and AChR clustering

The postsynaptic membrane of the neuromuscular junction contains a myristoylated 43-kD protein (43k) that is closely associated with the cytoplasmic face of the nicotinic acetylcholine receptor (AChR)-rich plasma membrane. Previously, we described fibroblast cell lines expressing recombinant AChRs. Transfection of these cell lines with 43k was necessary and sufficient for reorganization of AChR into discrete 43k-rich plasma membrane domains (Phillips, W. D., C. Kopta, P. Blount, P. D. Gardner, J. H. Steinbach, and J. P. Merlie. 1991. Science (Wash. DC). 251:568-570). Here we demonstrate the utility of this expression system for the study of 43k function by site-directed mutagenesis. Substitution of a termination codon for Asp254 produced a truncated (28-kD) protein that associated poorly with the cell membrane. The conversion of Gly2 to Ala2, to preclude NH2-terminal myristoylation, reduced the frequency with which 43k formed plasma membrane domains by threefold, but did not eliminate the aggregation of AChRs at these domains. Since both NH2 and COOH-termini seemed important for association of 43k with the plasma membrane, a deletion mutant was constructed in which the codon Gln15 was fused in-frame to Ile255 to create a 19-kD protein. This mutated protein formed 43k-rich plasma membrane domains at wild-type frequency, but the domains failed to aggregate AChRs, suggesting that the central part of the 43k polypeptide may be involved in AChR aggregation. Our results suggest that membrane association and AChR interactions are separable functions of the 43k molecule.

Dystrophin is a component of the subsynaptic membrane

A subsynaptic protein of Mr approximately 300 kD is a major component of Torpedo electric organ postsynaptic membranes and copurifies with the AChR and the 43-kD subsynaptic protein. mAbs against this protein react with neuromuscular synapses in higher vertebrates, but not at synapses in dystrophic muscle. The Torpedo 300-kD protein comigrates in SDS-PAGE with murine dystrophin and reacts with antibodies against murine dystrophin. The sequence of a partial cDNA isolated by screening an expression library with mAbs against the Torpedo 300-kD protein shows striking homology to mammalian dystrophin, and in particular to the b isoform of dystrophin. These results indicate that dystrophin is a component of the postsynaptic membrane at neuromuscular synapses and raise the possibility that loss of dystrophin from synapses in dystrophic muscle may have consequences that contribute to muscular dystrophy.

Cytoplasmic components of acetylcholine receptor clusters of cultured rat myotubes: the 58-kD protein

A 58-kD protein, identified in extracts of postsynaptic membrane from Torpedo electric organ, is enriched at sites where acetylcholine receptors (AChR) are concentrated in vertebrate muscle (Froehner, S. C., A. A. Murnane, M. Tobler, H. B. Peng, and R. Sealock. 1987. J. Cell Biol. 104:1633-1646). We have studied the 58-kD protein in AChR clusters isolated from cultured rat myotubes. Using immunofluorescence microscopy we show that the 58-kD protein is highly enriched at AChR clusters, but is also present in regions of the myotube membrane lacking AChR. Within clusters, the 58-kD protein codistributes with AChR, and is absent from adjacent membrane domains involved in myotube-substrate contact. Semiquantitative fluorescence measurements suggest that molecules of the 58-kD protein and AChR are present in approximately equal numbers. Differential extraction of peripheral membrane proteins from isolated AChR clusters suggests that the 58-kD protein is more tightly bound to cluster membrane than is actin or spectrin, but less tightly bound than the receptor-associated 43-kD protein. When AChR clusters are disrupted either in intact cells or after isolation, the 58-kD protein still codistributes with AChR. Clusters visualized by electron microscopy after immunogold labeling and quick-freeze, deep-etch replication show that, within AChR clusters, the 58-kD protein is sharply confined to AChR-rich domains, where it is present in a network of filaments lying on the cytoplasmic surface of the membrane. Additional actin filaments overlie, and are attached to, this network. Our results suggest that within AChR domains of clusters, the 58-kD protein lies between AChR and the receptor-associated 43-kD protein, and the membrane-skeletal proteins, beta-spectrin, and actin.

Extracellular synaptic factors induce clustering of acetylcholine receptors stably expressed in fibroblasts

The clustering of nicotinic acetylcholine receptors (AChRs) is one of the first events observed during formation of the neuromuscular junction. To determine the mechanism involved in AChR clustering, we established a nonmuscle cell line (mouse fibroblast L cells) that stably expresses just one muscle-specific gene product, the AChR. We have shown that when Torpedo californica AChRs are expressed in fibroblasts, their immunological, biochemical, and electrophysiological properties all indicate that fully functional cell surface AChRs are produced. In the present study, the cell surface distribution and stability of Torpedo AChRs expressed in fibroblasts (AChR-fibroblasts) were analyzed and shown to be similar to nonclustered AChRs expressed in muscle cells. AChR-fibroblasts incubated with antibodies directed against the AChR induced the formation of small AChR microclusters (less than 0.5 micron 2) and caused an increase in the internalization rate and degradation of surface AChRs (antigenic modulation) in a manner similar to that observed in muscle cells. Two disparate sources of AChR clustering factors, extracellular matrix isolated from Torpedo electric organ and conditioned media from a rodent neuroblastoma-glioma hybrid cell line, each induced large (1-3 microns 2), stable AChR clusters with no change in the level of surface AChR expression. By exploiting the temperature-sensitive nature of Torpedo AChR assembly, we were able to demonstrate that factor-induced clusters were produced by mobilization of preexisting surface AChRs, not by directed insertion of newly synthesized AChRs. AChR clusters were never observed in the absence of extracellular synaptic factors. Our results suggest that these factors can interact directly with the AChR.

A protein homologous to the Torpedo postsynaptic 58K protein is present at the myotendinous junction

The 58K protein is a peripheral membrane protein enriched in the acetylcholine receptor (AChR)-rich postsynaptic membrane of Torpedo electric organ. Because of its coexistence with AChRs in the postsynaptic membrane in both electrocytes and skeletal muscle, it is thought to be involved in the formation and maintenance of AChR clusters. Using an mAb against the 58K protein of Torpedo electric organ, we have identified a single protein band in SDS-PAGE analysis of Xenopus myotomal muscle with an apparent molecular mass of 48 kD. With this antibody, the distribution of this protein was examined in the myotomal muscle fibers with immunofluorescence techniques. We found that the 48K protein is concentrated at the myotendinous junctions (MTJs) of these muscle fibers. The MTJ is also enriched in talin and vinculin. By double labeling muscle fibers with antibodies against talin and the 48K protein, these two proteins were found to colocalize at the membrane invaginations of the MTJ. In cultured myotomal muscle cells, the 48K protein and talin are also colocalized at sites of membrane-myofibril interaction. The 48K protein is, however, not found at focal adhesion sites in nonmuscle cells, which are enriched in talin. These data suggest that the 48K protein is specifically involved in the interaction of myofibrillar actin filaments with the plasma membrane at the MTJ. In addition to the MTJ localization, 48K protein is also present at AChR clusters both in vivo and in vitro. Thus, this protein is shared by both the MTJ and the neuromuscular junction.

A novel 87,000-Mr protein associated with acetylcholine receptors in Torpedo electric organ and vertebrate skeletal muscle

To identify proteins associated with nicotinic postsynaptic membranes, mAbs have been prepared to proteins extracted by alkaline pH or lithium diiodosalicylate from acetylcholine receptor-rich (AChR) membranes of Torpedo electric organ. Antibodies were obtained that recognized two novel proteins of 87,000 Mr and a 210,000:220,000 doublet as well as previously described proteins of 43,000 Mr, 58,000 (51,000 in our gel system), 270,000, and 37,000 (calelectrin). The 87-kD protein copurified with acetylcholine receptors and with 43- and 51-kD proteins during equilibrium centrifugation on continuous sucrose gradients, whereas a large fraction of the 210/220-kD protein was separated from AChRs. The 87-kD protein remained associated with receptors and 43-kD protein during velocity sedimentation through shallow sucrose gradients, a procedure that separated a significant amount of 51-kD protein from AChRs. The 87- and 270-kD proteins were cleaved by Ca++-activated proteases present in crude preparations and also in highly purified postsynaptic membranes. With the exception of anti-37-kD antibodies, some of the monoclonals raised against Torpedo proteins also recognized determinants in frozen sections of chick and/or rat skeletal muscle fibers and in permeabilized chick myotubes grown in vitro. Anti-87-kD sites were concentrated at chick and rat endplates, but the antibodies also recognized determinants present at lower site density in the extrasynaptic membrane. Anti-210:220-kD labeled chick endplates, but studies of neuron-myotube cocultures showed that this antigen was located on neurites rather than the postsynaptic membrane. As reported in other species, 43-kD determinants were restricted to chick endplates and anti-51-kD and anti-270-kD labeled extrasynaptic as well as synaptic membranes. None of the cross reacting antibodies recognized determinants on intact (unpermeabilized) myotubes, so the antigens must be located on the cytoplasmic aspect of the surface membrane. The role that each intracellular determinant plays in AChR immobilization at developing and mature endplates remains to be investigated.

Expression of RNA transcripts for the postsynaptic 43 kDa protein in innervated and denervated rat skeletal muscle

A cDNA clone encoding the mouse muscle postsynaptic 43 kDa protein was isolated and sequenced. The amino acid sequence of this protein, which is closely associated with nicotinic acetylcholine receptors at Torpedo electrocyte and vertebrate skeletal muscle synapses, is very similar in different species. A cysteine-rich region homologous to part of the regulatory domain of protein kinase C may be important in interactions of this protein with the lipid bilayer. RNA transcripts for the 43 kDa protein increase only 2-3 fold after denervation of rat skeletal muscle, in sharp contrast to the alpha-subunit of the muscle nicotinic receptor which increases more than 30-fold. Thus, the expression of these two proteins is regulated by different mechanisms.

The mammalian 43-kD acetylcholine receptor-associated protein (RAPsyn) is expressed in some nonmuscle cells

Torpedo electric organ and vertebrate neuromuscular junctions contain the receptor-associated protein of the synapse (RAPsyn) (previously referred to as the 43K protein), a nonactin, 43,000-Mr peripheral membrane protein associated with the cytoplasmic face of postsynaptic membranes at areas of high nicotinic acetylcholine receptor (AChR) density. Although not directly demonstrated, several lines of evidence suggest that RAPsyn is involved in the synthesis and/or maintenance of such AChR clusters. Microscopic and biochemical studies had previously indicated that RAPsyn expression is restricted to differentiated, AChR-synthesizing cells. Our recent finding that RAPsyn is also produced in undifferentiated myocytes (Frail, D.E., L.S. Musil, a. Bonanno, and J.P. Merlie, 1989. Neuron. 2:1077-1086) led to to examine whether RAPsyn is synthesized in cell types that never express AChR (i.e., cells of other than skeletal muscle origin). Various primary and established rodent cell lines were metabolically labeled with [35S]methionine, and extracts were immunoprecipitated with a monospecific anti-RAPsyn serum. Analysis of these immunoprecipitates by SDS-PAGE revealed detectable RAPsyn synthesis in some (notably fibroblast and Leydig tumor cell lines and primary cardiac cells) but not all (hepatocyte- and lymphocyte-derived) cell types. These results were further substantiated by peptide mapping studies of RAPsyn immunoprecipitated from different cells and quantitation of RAPsyn-encoding mRNA levels in mouse tissues. RAPsyn synthesized in both muscle and nonmuscle cells was shown to be tightly associated with membranes. These findings demonstrate that RAPsyn is not specific to skeletal muscle-derived cells and imply that it may function in a capacity either in addition to or instead of AChR clustering.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

An unusual beta-spectrin associated with clustered acetylcholine receptors

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane is an early event in the formation of the neuromuscular junction. The mechanism of clustering is still unknown, but is generally believed to be mediated by the postsynaptic cytoskeleton. We have identified an unusual isoform of beta-spectrin which colocalizes with AChR in AChR clusters isolated from rat myotubes in vitro. A related antigen is present postsynaptically at the neuromuscular junction of the rat. Immunoprecipitation, peptide mapping and immunofluorescence show that the beta-spectrin in AChR clusters resembles but is distinct from the beta-spectrin of human erythrocytes. alpha-Spectrin appears to be absent from AChR clusters. Semiquantitative immunofluorescence techniques indicate that there are from two to seven beta-spectrin molecules present for every clustered AChR, the higher values being obtained from rapidly prepared clusters, the lower values from clusters that require several minutes or more for isolation. Upon incubation of isolated AChR clusters for 1 h at room temperature, beta-spectrin is slowly depleted and the AChR redistribute into microaggregates. The beta-spectrin that remains associated with the myotube membrane is concentrated at these microaggregates. beta-Spectrin is quantitatively lost from clusters upon digestion with chymotrypsin, which causes AChR to redistribute in the plane of the membrane. These results suggest that AChR in clusters is closely linked to an unusual isoform of beta-spectrin.

Myristic acid is the NH2-terminal blocking group of the 43-kDa protein of Torpedo nicotinic post-synaptic membranes

The NH2-terminal blocking group of the 43-kDa peripheral membrane protein (43-kDa protein) of Torpedo post-synaptic membranes has been identified as myristic acid. To identify that blocking group pure 43-kDa protein was digested with trypsin and the blocked tryptic peptide was isolated by reverse phase HPLC. That peptide coeluted with and had the same amino acid composition as a synthetic peptide, myristoyl-Gly-Gln-Asp-Gln-Thr-Lys, the structure of the amino terminus predicted from the protein sequence deduced from a cDNA clone. The presence of myristate was confirmed by the precise molecular mass of the peptide, 886.5266, determined by fast atom bombardment mass spectroscopy.

Asynchronous assembly of the acetylcholine receptor and of the 43-kD nu1 protein in the postsynaptic membrane of developing Torpedo marmorata electrocyte

The assembly of the nicotinic acetylcholine receptor (AchR) and the 43-kD protein (v1), the two major components of the post synaptic membrane of the electromotor synapse, was followed in Torpedo marmorata electrocyte during embryonic development by immunocytochemical methods. At the first developmental stage investigated (45-mm embryos), accumulation of AchR at the ventral pole of the newly formed electrocyte was observed within columns before innervation could be detected. No concomitant accumulation of 43-kD immunoreactivity in AchR-rich membrane domains was observed at this stage, but a transient asymmetric distribution of the extracellular protein, laminin, which paralleled that of the AchR, was noticed. At the subsequent stage studied (80-mm embryos), codistribution of the two proteins was noticed on the ventral face of the cell. Intracellular pools of AchR and 43-kD protein were followed at the EM level in 80-mm electrocytes. AchR immunoreactivity was detected within membrane compartments, which include the perinuclear cisternae of the endoplasmic reticulum and the plasma membrane. On the other hand, 43-kD immunoreactivity was not found associated with the AchR in the intracellular compartments of the cell, but codistributed with the AchR at the level of the plasma membrane. The data reported in this study suggest that AchR clustering in vivo is not initially determined by the association of the AchR with the 43-kD protein, but rather relies on AchR interaction with extracellular components, for instance from the basement membrane, laid down in the tissue before the entry of the electromotor nerve endings.

Acetylcholine receptor-associated 43K protein contains covalently bound myristate

Torpedo electroplaque and vertebrate neuromuscular junctions contain high levels of a nonactin, 43,000-Mr peripheral membrane protein referred to as the 43K protein. 43K protein is associated with the cytoplasmic face of postsynaptic membranes at areas of high acetylcholine receptor density and has been implicated in the establishment and/or maintenance of these receptor clusters. Cloning of cDNAs encoding Torpedo 43K protein revealed that its amino terminus contains a consensus sequence sufficient for the covalent attachment of the rare fatty acid myristate. To examine whether 43K protein is, in fact, myristoylated, mouse muscle BC3H1 cells were metabolically labeled with either [35S]cysteine or [3H]myristate and immunoprecipitated with a monospecific antiserum raised against isolated Torpedo 43K protein. In cells incubated with either precursor, a single labeled species was specifically recovered that comigrated on SDS-PAGE with 43K protein purified from Torpedo electric organ. Approximately 95% of the 3H labeled material released from [3H]myristate-43K protein by acid methanolysis was extractable in organic solvents and eluted from a C18 reverse-phase HPLC column exclusively at the position of the methyl myristate internal standard. Thus, 43K protein contains authentic myristic acid rather than an amino or fatty acid metabolite of [3H]myristate. Myristate appears to be added to 43K protein cotranslationally and cannot be released from it by prolonged incubation in SDS, 2-mercaptoethanol, or hydroxylamine (pH 7.0 or 10.0), characteristics consistent with amino terminal myristoylation. Covalently linked myristate may be responsible for the high affinity of purified 43K protein for lipid bilayers despite the absence of a notably hydrophobic amino acid sequence.

Molecular events in synaptogenesis: nerve-muscle adhesion and postsynaptic differentiation

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane of newly innervated muscle fibers is one of the earliest events in the development of the vertebrate neuromuscular junction. Here, we describe two hypotheses that can account for AChR clustering in response to innervation. The "trophic factor" hypothesis proposes that the neuron releases a soluble factor that interacts with the muscle cell in a specific manner and that this interaction results in the local accumulation of AChR. The "contact and adhesion" hypothesis proposes that the binding of the nerve to the muscle cell surface is itself sufficient to induce AChR clustering, without the participation of soluble factors. We present a model for the molecular assembly of AChR clusters based on the contact and adhesion hypothesis. The model involves the sequential assembly of three distinct membrane domains. The first domain to form serves to attach microfilaments to the cytoplasmic surface of the muscle cell membrane at sites of muscle-nerve adhesion. The second domain to form is clathrin-coated membrane; it serves as a site of insertion of additional membrane elements, including AChR. Upon insertion of AChR into the cell surface, a membrane skeleton assembles by anchoring itself to the AChR. The skeleton, composed in part of actin and spectrin, binds and immobilizes significant numbers of AChR, thereby forming the third membrane domain of the AChR cluster. We make several predictions that should distinguish this model of AChR clustering from one that invokes soluble, trophic factors.

cDNAs for the postsynaptic 43-kDa protein of Torpedo electric organ encode two proteins with different carboxyl termini

Postsynaptic membranes isolated from Torpedo electric organ are highly enriched in the nicotinic acetylcholine receptor and a nonreceptor protein of 43 kDa; the distribution of the 43-kDa protein and the receptor is coextensive in the electrical membrane. As a first step in understanding the regulation of 43-kDa protein expression, we have isolated and characterized 43-kDa protein cDNAs. A lambda gt11 cDNA library was constructed from Torpedo californica electric organ mRNA and screened with a pool of 26-mer oligonucleotides encoding a short tryptic fragment of the 43-kDa synaptic protein. Positive clones were purified and sequenced; the amino acid sequences were deduced, and they matched chemically determined protein sequences of the 43-kDa protein. Two distinct classes of cDNAs were obtained; one class encoded a 43-kDa protein of 389 amino acids with a calculated molecular mass of 43,988 daltons, and another class encoded a second 43-kDa protein containing 23 additional amino acids at the C terminus. Therefore, it appears that two 43-kDa proteins with different carboxyl termini are encoded by separate mRNAs. Consistent with this idea, blot hybridization analysis revealed multiple polyadenylylated 43-kDa mRNAs in electric organ. One polyadenylylated mRNA of approximately equal to 2.0 kilobases in length was apparent in both embryonic day-11 chick muscle and the mouse muscle cell line BC3H1.

A postsynaptic Mr 58,000 (58K) protein concentrated at acetylcholine receptor-rich sites in Torpedo electroplaques and skeletal muscle

In the study of proteins that may participate in the events responsible for organization of macromolecules in the postsynaptic membrane, we have used a mAb to an Mr 58,000 protein (58K protein) found in purified acetylcholine receptor (AChR)-enriched membranes from Torpedo electrocytes. Immunogold labeling with the mAb shows that the 58K protein is located on the cytoplasmic side of Torpedo postsynaptic membranes and is most concentrated near the crests of the postjunctional folds, i.e., at sites of high AChR concentration. The mAb also recognizes a skeletal muscle protein with biochemical characteristics very similar to the electrocyte 58K protein. In immunofluorescence experiments on adult mammalian skeletal muscle, the 58K protein mAb labels endplates very intensely, but staining of extrasynaptic membrane is also seen. Endplate staining is not due entirely to membrane infoldings since a similar pattern is seen in neonatal rat diaphragm in which postjunctional folds are shallow and rudimentary, and in chicken muscle, which lacks folds entirely. Furthermore, clusters of AChR that occur spontaneously on cultured Xenopus myotomal cells and mouse muscle cells of the C2 line are also stained more intensely than the surrounding membrane with the 58K mAb. Denervation of adult rat diaphragm muscle for relatively long times causes a dramatic decrease in the endplate staining intensity. Thus, the concentration of this evolutionarily conserved protein at postsynaptic sites may be regulated by innervation or by muscle activity.

300-kD subsynaptic protein copurifies with acetylcholine receptor-rich membranes and is concentrated at neuromuscular synapses

Acetylcholine receptor-rich membranes from the electric organ of Torpedo californica are enriched in the four different subunits of the acetylcholine receptor and in two peripheral membrane proteins at 43 and 300 kD. We produced monoclonal antibodies against the 300-kD protein and have used these antibodies to determine the location of the protein, both in the electric organ and in skeletal muscle. Antibodies to the 300-kD protein were characterized by Western blots, binding assays to isolated membranes, and immunofluorescence on tissue. In Torpedo electric organ, antibodies to the 300-kD protein stain only the innervated face of the electrocytes. The 300-kD protein is on the intracellular surface of the postsynaptic membrane, since antibodies to the 300-kD protein bind more efficiently to saponin-permeabilized, right side out membranes than to intact membranes. Some antibodies against the Torpedo 300-kD protein cross-react with amphibian and mammalian neuromuscular synapses, and the cross-reacting protein is also highly concentrated on the intracellular surface of the post-synaptic membrane.

The relationship of the postsynaptic 43K protein to acetylcholine receptors in receptor clusters isolated from cultured rat myotubes

We have examined the relationship of acetylcholine receptors (AChR) to the Mr 43,000 receptor-associated protein (43K) in the AChR clusters of cultured rat myotubes. Indirect immunofluorescence revealed that the 43K protein was concentrated at the AChR domains of the receptor clusters in intact rat myotubes, in myotube fragments, and in clusters that had been purified approximately 100-fold by extraction with saponin. The association of the 43K protein with clustered AChR was not affected by buffers of high or low ionic strength, by alkaline pHs up to 10, or by chymotrypsin at 10 micrograms/ml. However, the 43K protein was removed from clusters with lithium diiodosalicylate or at alkaline pH (greater than 10). Upon extraction of 43K, several changes were observed in the AChR population. Receptors redistributed in the plane of the muscle membrane in alkali-extracted samples. The number of binding sites accessible to an anti-AChR monoclonal antibody directed against cytoplasmic epitopes (88B) doubled. Receptors became more susceptible to digestion by chymotrypsin, which destroyed the binding sites for the 88B antibody only after 43K was extracted. These results suggest that in isolated AChR clusters the 43K protein covers part of the cytoplasmic domain of AChR and may contribute to the unique distribution of this membrane protein.

nu 1, a Mr 43,000 component of postsynaptic membranes, is a protein kinase

Acetylcholine receptor-enriched membranes from the electric organ of Torpedo californica show a major band at Mr 43,000 on NaDodSO4/polyacrylamide gels. This band is composed of three polypeptides: nu 1, nu 2, and nu 3. Polypeptide nu 1 has been found to be localized exclusively at the innervated face of the electrocyte and at the neuromuscular junction in rat muscle. We show here that monoclonal antibody to nu 1 precipitates a radioactive Mr 43,000 polypeptide from detergent-solubilized extracts of Torpedo membranes covalently labeled with periodate-oxidized [alpha-32P]ATP. The monoclonal antibody also precipitates protein kinase activity from neutralized pH 11 extracts of the acetylcholine receptor-rich membranes. These data suggest that nu 1 is a postsynaptic membrane protein kinase.