Peptide extraction by alkaline treatment is accompanied by rearrangement of the membrane-bound acetylcholine receptor from Torpedo marmorata

Cell-surface translational dynamics of nicotinic acetylcholine receptors

Synapse efficacy heavily relies on the number of neurotransmitter receptors available at a given time. In addition to the equilibrium between the biosynthetic production, exocytic delivery and recycling of receptors on the one hand, and the endocytic internalization on the other, lateral diffusion and clustering of receptors at the cell membrane play key roles in determining the amount of active receptors at the synapse. Mobile receptors traffic between reservoir compartments and the synapse by thermally driven Brownian motion, and become immobilized at the peri-synaptic region or the synapse by: (a) clustering mediated by homotropic inter-molecular receptor–receptor associations; (b) heterotropic associations with non-receptor scaffolding proteins or the subjacent cytoskeletal meshwork, leading to diffusional “trapping,” and (c) protein-lipid interactions, particularly with the neutral lipid cholesterol. This review assesses the contribution of some of these mechanisms to the supramolecular organization and dynamics of the paradigm neurotransmitter receptor of muscle and neuronal cells -the nicotinic acetylcholine receptor (nAChR). Currently available information stemming from various complementary biophysical techniques commonly used to interrogate the dynamics of cell-surface components is critically discussed. The translational mobility of nAChRs at the cell surface differs between muscle and neuronal receptors in terms of diffusion coefficients and residence intervals at the synapse, which cover an ample range of time regimes. A peculiar feature of brain α7 nAChR is its ability to spend much of its time confined peri-synaptically, vicinal to glutamatergic (excitatory) and GABAergic (inhibitory) synapses. An important function of the α7 nAChR may thus be visiting the territories of other neurotransmitter receptors, differentially regulating the dynamic equilibrium between excitation and inhibition, depending on its residence time in each domain.

Src-class kinases act within the agrin/MuSK pathway to regulate acetylcholine receptor phosphorylation, cytoskeletal anchoring, and clustering

Synaptogenesis at the neuromuscular junction requires agrin-induced stable localization of acetylcholine receptors (AChRs) at the endplate. The effects of agrin are transduced by the muscle-specific receptor tyrosine kinase (MuSK). This study provides evidence that Src-class protein tyrosine kinases mediate the effects of agrin-activated MuSK to regulate clustering and anchoring of AChRs in skeletal muscle. MuSK was complexed with both Src and Fyn in the C2 mouse muscle cell line. These associations were enhanced by agrin and by increasing protein tyrosine phosphorylation with pervanadate. Coupling between MuSK and the Src-class kinases in vivo appeared to be caused by a phosphotyrosine-SH2 domain interaction because binding of MuSK to the SH2 domains of Fyn and Src in vitro was specific, enhanced by phosphorylation, and dependent on MuSK autophosphorylation. In addition, Src and Fyn phosphorylated MuSK. AChR phosphorylation, stimulated by agrin or pervanadate, was inhibited by blocking Src-class kinases with PP1. Furthermore, agrin-induced clustering and cytoskeletal anchoring of AChRs was dependent on Src-family kinases. These data support the conclusion that Fyn and Src act downstream of MuSK to regulate the stable localization of AChRs at the neuromuscular endplate during agrin-induced synaptogenesis.

The torpedo electrocyte: a model system to study membrane-cytoskeleton interactions at the postsynaptic membrane

Many aspects of the organization of the electromotor synapse of electric fish resemble the nerve-muscle junction. In particular, the postsynaptic membrane in both systems share most of their proteins. As a remarquable source of cholinergic synapses, the Torpedo electrocyte model has served to identify the most important components involved in synaptic transmission such as the nicotinic acetylcholine receptor and the enzyme acetylcholinesterase, as well as proteins associated with the subsynaptic cytoskeleton and the extracellular matrix involved in the assembly of the postsynaptic membrane, namely the 43-kDa protein-rapsyn, the dystrophin/utrophin complex, agrin, and others. This review encompasses some representative experiments that helped to clarify essential aspects of the supramolecular organization and assembly of the postsynaptic apparatus of cholinergic synapses.

Phosphorylation and cytoskeletal anchoring of the acetylcholine receptor by Src class protein-tyrosine kinases. Activation by rapsyn

Src class protein-tyrosine kinases bind to and phosphorylate the nicotinic acetylcholine receptor of skeletal muscle. This study provided evidence for the functional importance of Src kinases in regulating the nicotinic acetylcholine receptor at the neuromuscular junction. Three Src class kinases, Fyn, Fyk, and Src, each formed a complex with the endplate-specific cytoskeletal protein rapsyn. In addition, cellular phosphorylation by each kinase was stimulated by rapsyn in heterologous transfected cells. Several lines of evidence supported rapsyn as a substrate for Src kinases. Most importantly, rapsyn regulation of Fyn, Fyk, and Src resulted in phosphorylation of the nicotinic acetylcholine receptor beta and delta subunits and anchoring of the receptor to the cytoskeleton. Both nicotinic acetylcholine receptor phosphorylation and cytoskeletal anchoring were blocked by the Src kinase-selective inhibitor herbimycin A. Rapsyn alone also induced a modest increase in nicotinic acetylcholine receptor phosphorylation and cytoskeletal translocation. However, inhibition by herbimycin A and a catalytically inactive dominant negative Src demonstrated that the effects of rapsyn were mediated by endogenous Src kinases. These data support the importance of Src class kinases for stabilization of the nicotinic acetylcholine receptor at the endplate during synaptic differentiation at the neuromuscular junction.

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

Differentiation of the postsynaptic membrane at the neuromuscular junction requires agrin, a nerve-derived signal; MuSK, a critical component of the agrin receptor in muscle; and rapsyn, a protein that interacts with acetylcholine receptors (AChRs). We showed previously that nerve-induced AChR aggregation is dramatically impaired in knockout mice lacking agrin, MuSK, or rapsyn. However, the phenotypes of these mutants differed in several respects, suggesting that the pathway from agrin to MuSK to rapsyn is complex. Here, we compared the effects of these mutations on two aspects of synaptic differentiation: AChR clustering and transcriptional specialization of synapse-associated myonuclei. First, we show that a plant lectin, VVA-B4, previously shown to act downstream of agrin, can induce AChR clusters on MuSK-deficient but not rapsyn-deficient myotubes in culture. Thus, although both MuSK and rapsyn are required for AChR clustering in vivo, only rapsyn is essential for cluster formation per se. Second, we show that neuregulin, a nerve-derived inducer of AChR gene expression, activates AChR gene expression in cultured agrin- and MuSK-deficient myotubes, even though synapse-specific transcriptional specialization is disrupted in agrin and MuSK mutants in vivo. We propose that agrin works through MuSK to determine a synaptogenic region within which synaptic differentiation occurs.

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.

Evidence for in situ and in vitro association between beta-dystroglycan and the subsynaptic 43K rapsyn protein. Consequence for acetylcholine receptor clustering at the synapse

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

Rapsyn and agrin slow the metabolic degradation of the acetylcholine receptor

Rapsyn is a 43-kDa cytoplasmic protein that clusters nicotinic acetylcholine receptors (AChR) in the postsynaptic membrane. Here we examine the effect of rapsynmediated AChR clustering on the metabolic stability of the AChR. When transfected into QT-6 fibroblasts, cell surface AChRs (alpha, beta, epsilon, and delta subunit combination) pulse labeled with 125I-alpha-bungarotoxin were degraded with a half-life of 16.4 +/- 1.1 h (mean +/- SEM). Cotransfection of rapsyn with AChR caused extensive AChR clustering and increased AChR half-life to 20.5 +/- 1.0 h. Anti-AChR antibodies such as mab 35 cause an increased AChR degradation often associated with myasthenia gravis: 80.8 +/- 2.5% of AChRs labeled at zero time were degraded over a 12-h period. Contransfection of rapsyn reduced this AChR loss to 66.4 +/- 3.8%. Rapsyn also reduced normal AChR degradation, from 53.2 +/- 2.1 to 44.2 +/- 2.2%. Muscle cell lines from wild-type myotubes displayed few AChR clusters, but treatment with neural agrin increased the number of AChR clusters 30-fold. Clustering was accompanied by reductions in AChR degradation (both in the presence and absence of mab 35) similar in magnitude to those produced by overexpression of rapsyn in QT-6 cells. In rapsyn-deficient myotubes, treatment with neural agrin neither caused AChR clustering nor reduced AChR degradation. Thus neural agrin may slow AChR degradation by inducing the rapsyn-dependent clustering of AChRs.

Rapsyn is required for MuSK signaling and recruits synaptic components to a MuSK-containing scaffold

Agrin-induced clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane is a key step in synaptogenesis at the neuromuscular junction. The receptor tyrosine kinase MuSK is a component of the agrin receptor, while the cytoplasmic protein rapsyn is necessary for the clustering of AChRs and all other postsynaptic membrane components studied to date. We show here that MuSK remains concentrated at synaptic sites in rapsyn-deficient mutant mice, suggesting that MuSK forms a primary structural scaffold to which rapsyn attaches other synaptic components. Using nonmuscle cells, we show that rapsyn-MuSK interactions are mediated by the ectodomain of MuSK, suggesting the existence of a transmembrane intermediate. In addition to rapsyn's structural role, we demonstrate that it is required for an early step in MuSK signaling, AChR phosphorylation. This signaling requires the kinase domain of MuSK, but not its ectodomain. Thus, MuSK may interact with rapsyn in multiple ways to play both structural and signaling roles in agrin-induced differentiation.

Rapsyn may function as a link between the acetylcholine receptor and the agrin-binding dystrophin-associated glycoprotein complex

The 43 kDa AChR-associated protein rapsyn is required for the clustering of nicotinic acetylcholine receptors (AChRs) at the developing neuromuscular junction, but the functions of other postsynaptic proteins colocalized with the AChR are less clear. Here we use a fibroblast expression system to investigate the role of the dystrophin-glycoprotein complex (DGC) in AChR clustering. The agrin-binding component of the DGC, dystroglycan, is found evenly distributed across the cell surface when expressed in fibroblasts. However, dystroglycan colocalizes with AChR-rapsyn clusters when these proteins are coexpressed. Furthermore, dystroglycan colocalizes with rapsyn clusters even in the absence of AChR, indicating that rapsyn can cluster dystroglycan and AChR independently. Immunofluorescence staining using a polyclonal antibody to utrophin reveals a lack of staining of clusters, suggesting that the immunoreactive species, like the AChR, does not mediate the observed rapsyndystroglycan interaction. Rapsyn may therefore be a molecular link connecting the AChR to the DGC. At the neuromuscular synapse, rapsyn-mediated linkage of the AChR to the cytoskeleton-anchored DGC may underlie AChR cluster stabilization.

Clustering of the acetylcholine receptor by the 43-kD protein: involvement of the zinc finger domain

A postsynaptic membrane-associated protein of M(r) 43,000 (43-kD protein) is involved in clustering of the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Previous studies have shown that recombinant mouse 43-kD protein forms membrane-associated clusters when expressed in Xenopus oocytes. Coexpression with the AChR results in colocalization of the receptor with the 43-kD protein clusters (Froehner, S. C., C. W. Luetje, P. B. Scotland, and J. Patrick, 1990. Neuron. 5:403-410). To understand the mechanism of this clustering, we have studied the role of the carboxy-terminal region of the 43-kD protein. The amino acid sequence of this region predicts two tandem zinc finger structures followed by a serine phosphorylation site. Both Torpedo 43-kD protein and the carboxy-terminal region of the mouse 43-kD protein bind radioisotopic zinc. Mutation of two histidine residues in this predicted domain greatly attenuates zinc binding, lending support to the proposal that this region forms zinc fingers. When expressed in oocytes, the ability of this mutant 43-kD protein to form clusters is greatly reduced. Its ability to interact with AChR, however, is retained. In contrast, a mutation that eliminates the potential serine phosphorylation site has no effect on clustering of the 43-kD protein or on interaction with the AChR. These findings suggest that protein interactions via the zinc finger domain of the 43-kD protein may be important for AChR clustering at the synapse.

Increased expression of the 43-kD protein disrupts acetylcholine receptor clustering in myotubes

The 43-kD protein is a peripheral membrane protein that is in approximately 1:1 stoichiometry with the acetylcholine receptor (AChR) in vertebrate muscle cells and colocalizes with it in the postsynaptic membrane. To investigate the role of the 43-kD protein in AChR clustering, we have isolated C2 muscle cell lines in which some cells overexpress the 43-kD protein. We find that myotubes with increased levels of the 43-kD protein have small AChR clusters and that those with the highest levels of expression have a drastically reduced number of clusters. Our results suggest that the 1:1 stoichiometry of AChR and 43-kD protein found in muscle cells is important for AChR cluster formation.

Interaction of the 43 kd postsynaptic protein with all subunits of the muscle nicotinic acetylcholine receptor

The 43 kd postsynaptic protein (43K) plays a key role in the aggregation of muscle nicotinic acetylcholine receptors (AChRs) in the postsynaptic membrane of the neuromuscular junction. By transiently coexpressing 43K and a single AChR subunit (alpha, beta, gamma, or delta) in the quail fibroblast cell line, QT-6, we show that 43K interacts with each subunit to form cell surface clusters in which 43K and receptor subunit are precisely colocalized. Although the level of cell surface expression of single subunits is much lower than that of fully assembled receptor, the clustering of both single subunits and fully assembled AChR occurs efficiently. In addition, 43K-induced clustering is specific for AChR subunits. From these results, we conclude that each pentameric AChR has five potential sites for interacting with 43K during cluster formation.

The 87K postsynaptic membrane protein from Torpedo is a protein-tyrosine kinase substrate homologous to dystrophin

Postsynaptic peripheral membrane proteins at the neuromuscular junction have been proposed to participate in the immobilization of the nicotinic acetylcholine receptor at the synapse. An 87 kd cytoplasmic peripheral membrane protein has been demonstrated to colocalize with the nicotinic acetylcholine receptor in the Torpedo electric organ and at the mammalian neuromuscular junction. We have cloned the cDNA encoding the 87K protein from Torpedo electric organ, and the predicted protein sequence is homologous to the C-terminal domains of dystrophin, the protein product of the Duchenne muscular dystrophy gene. The 87K protein displays a restricted pattern of expression detected only in electric organ, brain, and skeletal muscle. Analysis of the in vitro and in vivo phosphorylation of the 87K protein indicates that it is multiply phosphorylated on serine, threonine, and tyrosine residues. The 87K protein is in a complex with other proteins associated with the postsynaptic membrane, including dystrophin and a 58 kd protein. These results suggest that the 87K protein is involved in the formation and stability of synapses and is regulated by protein phosphorylation.

Molecular properties of 5-hydroxytryptamine3 receptor-type binding sites purified from NG108-15 cells

5-Hydroxytryptamine3 (5-HT3) receptor-type binding sites were solubilised from NG108-15 mouse neuroblastoma x rat glioma hybrid cells using five different detergents [n-octyl-beta-D-glucoside, Triton X-100, 3-[3-(cholamidopropyl)dimethylammonio]-1-propanesulphonate (CHAPS), sodium cholate, and deoxycholate] and the solubilisation efficiencies compared. The equilibrium binding, kinetic properties, and pharmacological profile of solubilised binding sites were similar to those of 5-HT3 receptor-type binding sites (5-HT3R) in membrane preparations determined using [3H]GR65630. The solubilised binding sites were purified using an affinity column constructed by coupling the high-affinity antagonist GR119566X to an Affi-Gel 15 resin. The affinity of purified 5-HT3R for [3H]-GR65630 was reduced threefold compared to the crude soluble preparation, but the pharmacological profile was similar. The sedimentation coefficient of the purified protein (11S, detergent: CHAPS) was determined by sucrose density gradient centrifugation. The apparent molecular mass of the detergent/binding site complex (370 kDa) was determined by size exclusion chromatography in the presence of n-dodecyl-beta-D-maltoside. Gel electrophoresis of the purified protein revealed bands at apparent molecular masses of 36, 40, 50, and 76 kDa. Electron microscopy of the negatively stained purified protein showed the presence of round particles of 8-9 nm diameter with a 2-nm stained pit in the centre, closely resembling the doughnut shapes described for nicotinic acetylcholine receptors.

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.

Functional properties of acetylcholine receptors coexpressed with the 43K protein in heterologous cell systems

The nicotinic acetylcholine (ACh) receptor is an integral membrane protein which mediates synaptic transmission at the skeletal neuromuscular junction. A key event in the development of the neuromuscular junction is the formation of high density aggregates of ACh receptors in the postsynaptic membrane. Receptor clustering has been attributed, in part, to their association with a peripheral membrane protein of Mr 43,000 (43K protein). We have addressed whether the association of the 43K protein can alter the single channel properties of the ACh receptor, and thus influence neuromuscular transmission at developing synapses, by expressing ACh receptors with and without the 43K protein in heterologous expression systems. We found that coexpression of the 43K protein with the receptor did not significantly alter either its single channel conductance or its mean channel open time. This was true in oocytes and also in COS cells where it was possible to localize 43K-induced clusters by fluorescence microscopy and to record from those clustered receptors. These data are in agreement with previous single channel studies which have shown that the properties of diffusely distributed and clustered receptors in native muscle cells from both mice and Xenopus do not differ.

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.

ACh receptor-rich membrane domains organized in fibroblasts by recombinant 43-kildalton protein

Neurotransmitter receptors are generally clustered in the postsynaptic membrane. The mechanism of clustering was analyzed with fibroblast cell lines that were stably transfected with the four subunits for fetal (alpha, beta, gamma, delta) or adult (alpha, beta, epsilon, delta) type mouse muscle nicotinic acetylcholine receptors (AChRs). Immunofluorescent staining indicated that AChRs were dispersed on the surface of these cells. When transiently transfected with an expression construct encoding a 43-kilodalton protein that is normally concentrated under the postsynaptic membrane, AChRs expressed in these cells became aggregated in large cell-surface clusters, colocalized with the 43-kilodalton protein. This suggests that 43-kilodalton protein can induce AChR clustering and that cluster induction involves direct contact between AChR and 43-kilodalton protein.

The postsynaptic 43K protein clusters muscle nicotinic acetylcholine receptors in Xenopus oocytes

Nicotinic acetylcholine receptors (AChRs) are localized at high concentrations in the postsynaptic membrane of the neuromuscular junction. A peripheral membrane protein of Mr 43,000 (43K protein) is closely associated with AChRs and has been proposed to anchor receptors at postsynaptic sites. We have used the Xenopus oocyte expression system to test the idea that the 43K protein clusters AChRs. Mouse muscle AChRs expressed in oocytes after injection of RNA encoding receptor subunits are uniformly distributed in the surface membrane. Coinjection of AChR RNA and RNA encoding the mouse muscle 43K protein causes AChRs to form clusters of 0.5-1.5 microns diameter. AChR clustering is not a consequence of increased receptor expression in the surface membrane or nonspecific clustering of all membrane proteins. The 43K protein is colocalized with AChRs in clusters when the two proteins are expressed together and forms clusters of similar size even in the absence of AChRs. These results provide direct evidence that the 43K protein causes clustering of AChRs and suggest that regulation of 43K protein clustering may be a key step in neuromuscular synaptogenesis.

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.

Presence of a protein immunologically related to lamin B in the postsynaptic membrane of Torpedo marmorata electrocyte

The Torpedo electrocyte is a flattened syncytium derived from skeletal muscle, characterized by two functionally distinct plasma membrane domains. The electrocyte is filled up with a transversal network of intermediate filaments (IF) of desmin which contact in an end-on fashion both sides of the cell. In this work, we show that polyclonal antibodies specific for lamin B recognizes a component of the plasma membrane of Torpedo electrocyte. This protein which thus shares epitopes with lamin B has a relative molecular mass of 54 kD, an acidic IP of 5.4. It is localized exclusively on the cytoplasmic side of the innervated membrane of the electrocyte at sites of IF-membrane contacts. Since our previous work showed that the noninnervated membrane contains ankyrin (Kordeli, E., J. Cartaud, H. O. Nghiêm, L. A. Pradel, C. Dubreuil, D. Paulin, and J.-P. Changeux. 1986. J. Cell Biol. 102:748-761), the present results suggest that desmin filaments may be anchored via the 54-kD protein to the innervated membrane and via ankyrin to the noninnervated membrane. These findings would represent an extension of the model proposed by Georgatos and Blobel (Georgatos, S. D., and G. Blobel. 1987a. J. Cell Biol. 105:105-115) in which type III intermediate size filaments are vectorially inserted to plasma and nuclear membranes by ankyrin and lamin B, respectively.

The intactness and orientation of acetylcholine receptor-rich membrane from Torpedo californica electric tissue

By a mild and highly reproducible fractionation of Torpedo californica electric tissue, we prepared membrane which was 30 times enriched in nicotinic acetylcholine receptor (AChR). This preparation was neither alkali-stripped nor reconstituted and consequently contained nu (43-kDa protein), which is associated with the cytoplasmic aspect of the receptor. We tested this membrane for the presence of sealed vesicles and determined the orientation of these vesicles by combining three methods. Two of these methods were based on the accessibilities, in the presence and absence of detergent, of the extracellular acetylcholine binding site to alpha-bungarotoxin and of the intracellular nu to trypsin. These two methods are specific for AChR-containing membrane. The third method was morphometry of electron micrographs, by which we estimated the proportion of sequestered membrane. These methods taken together indicated that approximately 45% of the AChR-containing membrane was in the form of leaky vesicles or sheets, 33% was sealed right-side-out vesicles, 11% was sealed inside-out vesicles, and 11% was sequestered within multilamellar or multivesicular vesicles. The complexity of this membrane needs to be taken into account in sidedness studies of the AChR.

Three-dimensional structure of the nicotinic acetylcholine receptor and location of the major associated 43-kD cytoskeletal protein, determined at 22 A by low dose electron microscopy and x-ray diffraction to 12.5 A

The three-dimensional structure of the nicotinic acetylcholine receptor (AChR) from Torpedo californica, crystallized both before and after removal of associated proteins, most notably the main 43-kD cytoskeletal protein that interacts both with AChR and actin, is determined to a resolution of 22 A. This is the first structural analysis where the 43-kD protein has been removed from the sample before crystallization. Thus, it provides the most reliable assessment of what constitutes the structure of the minimal five subunit AChR complex, and, by comparison with the native membrane, of the location of the 43-kD cytoskeletal protein. Image reconstruction of two-dimensional crystals includes information from electron images of up to +/- 52 degrees tilted specimens of latticed AChR. Hybrid density maps that include x-ray diffraction perpendicular to the membrane to 12.5 A resolution were used and eliminate some of the distortions introduced in maps based only on electron microscopic analyses. Comparison of the difference Fourier density maps between AChR with its normal complement of associated proteins, and without them shows that the main density, assigned to the actin-binding 43-kD component is closely associated with the lipid bilayer as well as with the cytoplasmic domain of the AChR. It binds beside the AChR, not beneath it as suggested by others (C. Toyoshima and N. Unwin 1988. Nature [Lond.]. 336:237-240). There is good agreement between the volumes of density for structural components and expected volumes based on their molecular weight. Acetylcholine receptors aggregate in the absence of any cytoskeletal proteins, suggesting that the AChR alone is sufficient to encode and stabilize clustering, and perhaps to do so during synaptogenesis. The main 43-kD component may play a role in location and rate of association of AChR. We show that the disulfide bond that cross-links delta-delta chains of adjacent pentamers in about 80% of AChR, is not required to stabilize the lattice of AChR. Latticed tube structures are stable indefinitely. The lattices described here have 20% less volume of lipid than those originally obtained and characterized by J. Kistler and R. M. Stroud (1981. Proc. Natl. Acad. Sci. USA. 78:3678-3682), or those subsequently characterized by A. Brisson and P. N. T. Unwin (1984. J. Cell Biol. 99:1202-1211) and A. Brisson and P. N. T. Unwin (1985. Nature (Lond.). 315:474-477).

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.

Clusters of 43-kDa protein are absent from genetic variants of C2 muscle cells with reduced acetylcholine receptor expression

Genetic variants of the C2 muscle cell line were used to investigate the relation between acetylcholine receptor (AChR) clustering and clustering of the 43-kDa protein. Two variants that express severely reduced amounts of the alpha subunit of the AChR and consequently lack AChR clusters were found also to lack clusters of the 43-kDa protein. The amount of 43-kDa protein in the variants measured by immunoassay was reduced to about one-third the levels found in wild-type cells. The beta subunit of the AChR was reduced to a similar extent. Northern blot analysis showed that neither the 43-kDa protein mRNA nor the beta subunit mRNA was reduced in the variants. Taken together, these results suggest that the amounts of beta subunit and 43-kDa protein may be regulated coordinately by a post-transcriptional mechanism.

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.

The lipid environment of the nicotinic acetylcholine receptor in native and reconstituted membranes

Detailed knowledge of the membrane framework surrounding the nicotinic acetylcholine receptor (AChR) is key to an understanding of its structure, dynamics, and function. Recent theoretical models discuss the structural relationship between the AChR and the lipid bilayer. Independent experimental data on the composition, metabolism, and dynamics of the AChR lipid environment are analyzed in the first part of the review. The composition of the lipids in which the transmembrane AChR chains are inserted bears considerable resemblance among species, perhaps providing this evolutionarily conserved protein with an adequate milieu for its optimal functioning. The effects of lipids on the latter are discussed in the second part of the review. The third part focuses on the information gained on the dynamics of AChR and lipids in the membrane, a section that also covers the physical properties and interactions between the protein, its immediate annulus, and the bulk lipid bilayer.

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.

Extraction of peripheral proteins is accompanied by selective depletion of certain glycerophospholipid classes and changes in the phosphorylation pattern of acetylcholine-receptor-rich-membrane proteins

The widely used alkaline treatment of acetylcholine-receptor (AChR)-rich membranes from Torpedo marmorata (electric fish) and Discopyge tschudii (a marine ray) results not only in the extraction of non-receptor peripheral proteins but also in that of glycerophospholipids (approximately 13%). Minor acidic phospholipids, notably phosphatidic acid and polyphosphoinositides, are particularly enriched in the NaOH extracts. When electrocytes or receptor-rich membranes are incubated with [32P]Pi or [gamma-32P]ATP, polyphosphoinositides accumulate most of the label (approximately 45% in D. tschudii; 96% in T. marmorata) and exhibit the highest specific radioactivity. Furthermore, more than 50% of these phosphorylated lipids are extracted by NaOH together with the peripheral membrane proteins. NaOH treatment also results in modification of the phosphorylation pattern of AChR membrane proteins. Phosphorylation decreases in the Mr-43,000 group of peripheral proteins and in the gamma-subunit of the receptor. The results indicate that polyphosphoinositides constitute a metabolically very active lipid pool in the postsynaptic membrane, and that a substantial proportion of these phospholipids are preferentially released from the membrane together with other acidic phospholipids upon peripheral-protein extraction. The conclusion is drawn that membranes submitted to the above treatments can no longer be considered equivalent to native ones in terms of their phospholipid composition and phosphorylation characteristics.

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.

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.

The Torpedo electrocyte: a model system for the study of receptor-cytoskeleton interactions

We have used the electrocyte of Torpedo electric organ as a model system for the study of AchR stabilization in the postsynaptic membrane. Attention was focused on membrane cytoskeleton interactions in particular on a peripheral protein of 43 KD that is believed to participate in AchR immobilization. Using immunocytochemical methods, we have shown that the cortical skeleton in Torpedo electrocyte displays a local differentiation proper for each specialized domain of the plasma membrane. In the postsynaptic membrane, characterized by an accumulation and a geometrical organization of the receptors in the plane of the membrane, the 43 KD protein participates in a submembraneous coating or "postsynaptic densities" that strictly codistribute with the AchR. The 43 KD protein might also account for the anchoring of intermediate-sized filaments. The organization of the postsynaptic domain appears readily different from that of the non-innervated one where the membrane folds are maintained by a cortical meshwork of cytoskeletal proteins such as ankyrin, spectrin and oligomeric actin. In conclusion, the asymmetrical organization of the cortical skeleton in the electrocyte offers a unique opportunity for the study of the specific aspects of membrane-skeleton interactions that take place in the postsynaptic domain.

Creatine kinase isoenzymes in Torpedo californica: absence of the major brain isoenzyme from nicotinic acetylcholine receptor membranes

Creatine kinase, actin, and nu 1 are three proteins of Mr 43 000 associated with membranes from electric organ highly enriched in nicotinic acetylcholine receptor. High levels of creatine kinase are required to maintain adequate ATP levels, while actin may play a role in maintaining the synaptic cytoskeleton. Previous investigations have prompted the conclusion that postsynaptic specializations at the receptor-enriched membrane domains in electroplax contain the brain form of creatine kinase rather than the form of creatine kinase predominantly found in muscle. We have examined this conclusion by purifying Torpedo brain creatine kinase to virtual homogeneity in order to examine its immunochemical, molecular, and electrophoretic properties. On the basis of immunological cross-reactivity and isozyme analysis, the receptor-associated creatine kinase is identified to be of the muscle type. When the molecular characteristics of Torpedo brain and muscle creatine kinase are compared, the brain enzyme is positioned at a more basic pH during chromatofocusing and on two-dimensional gel electrophoresis (pI = 7.5-7.9). Furthermore, electrophoretic mobilities of the brain and muscle forms of creatine kinase differ in sodium dodecyl sulfate electrophoresis: the brain isozyme of creatine kinase has lower apparent molecular weight (Mr 41 000) when compared with the muscle enzyme (Mr 43 000). On the basis of the results of our current investigations, the hypothesis that the brain isozyme of creatine kinase is a component of the postsynaptic specializations of the Torpedo californica electroplax must be abandoned. Recent sequence data have established close homology between Torpedo and mammalian muscle creatine kinases. On the basis of electrophoretic criteria, our results indicate that a lower degree of homology exists between the brain isozymes.