Peripheral proteins of postsynaptic membranes from Torpedo electric organ identified with monoclonal antibodies

Syntrophin proteins as Santa Claus: role(s) in cell signal transduction

Syntrophins are a family of cytoplasmic membrane-associated adaptor proteins, characterized by the presence of a unique domain organization comprised of a C-terminal syntrophin unique (SU) domain and an N-terminal pleckstrin homology (PH) domain that is split by insertion of a PDZ domain. Syntrophins have been recognized as an important component of many signaling events, and they seem to function more like the cell's own personal 'Santa Claus' that serves to 'gift' various signaling complexes with precise proteins that they 'wish for', and at the same time care enough for the spatial, temporal control of these signaling events, maintaining overall smooth functioning and general happiness of the cell. Syntrophins not only associate various ion channels and signaling proteins to the dystrophin-associated protein complex (DAPC), via a direct interaction with dystrophin protein but also serve as a link between the extracellular matrix and the intracellular downstream targets and cell cytoskeleton by interacting with F-actin. They play an important role in regulating the postsynaptic signal transduction, sarcolemmal localization of nNOS, EphA4 signaling at the neuromuscular junction, and G-protein mediated signaling. In our previous work, we reported a differential expression pattern of alpha-1-syntrophin (SNTA1) protein in esophageal and breast carcinomas. Implicated in several other pathologies, like cardiac dys-functioning, muscular dystrophies, diabetes, etc., these proteins provide a lot of scope for further studies. The present review focuses on the role of syntrophins in membrane targeting and regulation of cellular proteins, while highlighting their relevance in possible development and/or progression of pathologies including cancer which we have recently demonstrated.

Integrated genomics and proteomics of the Torpedo californica electric organ: concordance with the mammalian neuromuscular junction

Background

During development, the branchial mesoderm of Torpedo californica transdifferentiates into an electric organ capable of generating high voltage discharges to stun fish. The organ contains a high density of cholinergic synapses and has served as a biochemical model for the membrane specialization of myofibers, the neuromuscular junction (NMJ). We studied the genome and proteome of the electric organ to gain insight into its composition, to determine if there is concordance with skeletal muscle and the NMJ, and to identify novel synaptic proteins.

Results

Of 435 proteins identified, 300 mapped to Torpedo cDNA sequences with ≥2 peptides. We identified 14 uncharacterized proteins in the electric organ that are known to play a role in acetylcholine receptor clustering or signal transduction. In addition, two human open reading frames, C1orf123 and C6orf130, showed high sequence similarity to electric organ proteins. Our profile lists several proteins that are highly expressed in skeletal muscle or are muscle specific. Synaptic proteins such as acetylcholinesterase, acetylcholine receptor subunits, and rapsyn were present in the electric organ proteome but absent in the skeletal muscle proteome.

Conclusions

Our integrated genomic and proteomic analysis supports research describing a muscle-like profile of the organ. We show that it is a repository of NMJ proteins but we present limitations on its use as a comprehensive model of the NMJ. Finally, we identified several proteins that may become candidates for signaling proteins not previously characterized as components of the NMJ.

Modulation of agrin-induced acetylcholine receptor clustering by extracellular signal-regulated kinases 1 and 2 in cultured myotubes

Agrin released by motoneurons induces and/or maintains acetylcholine receptor (AChR) clustering and other aspects of postsynaptic differentiation at the vertebrate neuromuscular junction. Agrin acts by binding and activating a receptor complex containing LDL receptor protein 4 (Lrp4) and muscle-specific kinase (MuSK). Two critical downstream components of this signaling cascade, Dox-7 and rapsyn, have been identified. However, additional intracellular essential elements remain unknown. Prior observations by others and us suggested antagonistic interactions between agrin and neuregulin-1 (Nrg-1) signaling in cultured myotubes and developing muscle fibers in vivo. A hallmark of Nrg-1 signaling in skeletal muscle cells is the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2). ERK1/2 are also activated in most cells by phorbol 12-myristate 13-acetate, a classical inhibitor of agrin-induced AChR clustering in myotubes. Here, it was investigated whether agrin activates ERK1/2 directly and whether such activation modulates agrin-induced AChR clustering. Agrin induced a rapid but transient activation of ERK1/2 in myotubes that was Lrp4/MuSK-dependent. However, blocking this ERK1/2 activation did not prevent but potentiated AChR clustering induced by agrin. ERK1/2 activation was dispensable for Nrg-1-mediated inhibition of the AChR clustering activity of agrin, but was indispensable for such activity by phorbol 12-myristate 13-acetate. Together, these results suggest agrin-induced activation of ERK1/2 is a negative modulator of agrin signaling in skeletal muscle cells.

Beta1 integrins in muscle, but not in motor neurons, are required for skeletal muscle innervation

In vitro studies have provided evidence that beta1 integrins in motor neurons promote neurite outgrowth, whereas beta1 integrins in myotubes regulate acetylcholine receptor (AChR) clustering. Surprisingly, using genetic studies in mice, we show here that motor axon outgrowth and neuromuscular junction (NMJ) formation in large part are unaffected when the integrin beta1 gene (Itgb1) is inactivated in motor neurons. In the absence of Itgb1 expression in skeletal muscle, interactions between motor neurons and muscle are defective, preventing normal presynaptic differentiation. Motor neurons fail to terminate their growth at the muscle midline, branch excessively, and develop abnormal nerve terminals. These defects resemble the phenotype of agrin-null mice, suggesting that signaling molecules such as agrin, which coordinate presynaptic and postsynaptic differentiation, are not presented properly to nerve terminals. We conclude that Itgb1 expression in muscle, but not in motor neurons, is critical for NMJ development.

Increased ratio of rapsyn to ACh receptor stabilizes postsynaptic receptors at the mouse neuromuscular synapse

The metabolic turnover of nicotinic ACh receptors (AChR) at the neuromuscular synapse is regulated over a tenfold range by innervation status, muscle electrical activity and neural agrin, but the downstream effector of such changes has not been defined. The AChR-associated protein rapsyn is essential for forming AChR clusters during development. Here, rapsyn was tagged with enhanced green fluorescent protein (EGFP) to begin to probe its influence at the adult synapse. In C2 myotubes, rapsyn-EGFP participated with AChR in agrin-induced AChR cluster formation. When electroporated into the tibialis anterior muscle of young adult mice, rapsyn-EGFP accumulated in discrete subcellular structures, many of which colocalized with Golgi markers, consistent with the idea that rapsyn assembles with AChR in the exocytic pathway. Rapsyn-EGFP also targeted directly to the postsynaptic membrane where it occupied previously vacant rapsyn binding sites, thereby increasing the rapsyn to AChR ratio. At endplates displaying rapsyn-EGFP, the metabolic turnover of AChR (labelled with rhodamine-alpha-bungarotoxin) was slowed. Thus, the metabolic half-life of receptors at the synapse may be modulated by local changes in the subsynaptic ratio of rapsyn to AChR.

Laminin-alpha1 globular domains 3 and 4 induce heterotrimeric G protein binding to alpha-syntrophin's PDZ domain and alter intracellular Ca2+ in muscle

Alpha-syntrophin is a component of the dystrophin glycoprotein complex (DGC). It is firmly attached to the dystrophin cytoskeleton via a unique COOH-terminal domain and is associated indirectly with alpha-dystroglycan, which binds to extracellular matrix laminin. Syntrophin contains two pleckstrin homology (PH) domains and one PDZ domain. Because PH domains of other proteins are known to bind the betagamma-subunits of the heterotrimeric G proteins, whether this is also a property of syntrophin was investigated. Isolated syntrophin from rabbit skeletal muscle binds bovine brain Gbetagamma-subunits in gel blot overlay experiments. Laminin-1-Sepharose or specific antibodies against syntrophin, alpha- and beta-dystroglycan, or dystrophin precipitate a complex with Gbetagamma from crude skeletal muscle microsomes. Bacterially expressed syntrophin fusion proteins and truncation mutants allowed mapping of Gbetagamma binding to syntrophin's PDZ domain; this is a novel function for PDZ domains. When laminin-1 is bound, maximal binding of Gsalpha and Gbetagamma occurs and active Gsalpha, measured as GTP-gamma35S bound, decreases. Because intracellular Ca2+ is elevated in Duchenne muscular dystrophy and Gsalpha is known to activate the dihydropyridine receptor Ca2+ channel, whether laminin also altered intracellular Ca2+ was investigated. Laminin-1 decreases active (GTP-gammaS-bound) Gsalpha, and the Ca2+ channel is inhibited by laminin-1. The laminin alpha1-chain globular domains 4 and 5 region, the region bound by DGC alpha-dystroglycan, is sufficient to cause an effect, and an antibody that specifically blocks laminin binding to alpha-dystroglycan inhibits Gbeta binding by syntrophin in C2C12 myotubes. These observations suggest that DGC is a matrix laminin, G protein-coupled receptor.

Functional reassembly of a split PH domain

The pleckstrin homology (PH) domain forms a structurally conserved protein module of approximately 120 amino acid residues. Several proteins involved in cellular signaling and cytoskeletal organization possess split PH domains while their biological roles and ligand binding activity remain to be clarified. We have designed a split PH domain from a structurally well-characterized PH domain of phospholipase Cdelta(1) by dissecting the PH domain and tethering a coiled coil module to each subunit to ask a question of whether the coiled coil could mediate a functional reassembly of the split PH domain. Isothermal titration microcalorimetry measurements indicated a formation of a thermodynamically stable 1:1 complex of the N-terminal and C-terminal halves of the split PH domain by the coiled coil formation. The reassembled split PH domain binds to IP(3), a target molecule of the parent PLCdelta(1) PH domain, but not to L-IP(3), indicating that the split PH domain maintains a binding selectivity similar to the native PLCdelta(1) PH domain. These results demonstrate that the split PH domain folds into a functional structure when the split halves are brought to close proximity, and suggest that the native split PH domains, such as found in PLCgamma(1), have distinctive functions upon the reassembly.

Duchenne muscular dystrophy: current knowledge, treatment, and future prospects

The cloning of the dystrophin gene has led to major advances in the understanding of the molecular genetic basis of Duchenne, Becker, and other muscular dystrophies associated with mutations in genes encoding members of the dystrophin-associated glycoprotein complex. The recent introduction of pharmaceutical agents such as prednisone has shown great promise in delaying the progression of Duchenne muscular dystrophy but there remains a need to develop more long-term therapeutic interventions. Knowledge of the nature of the dystrophin gene and the glycoprotein complex has led many researchers to think that somatic gene replacement represents the most promising approach to treatment. The potential use of this strategy has been shown in the mdx mouse model of Duchenne muscular dystrophy, where germ line gene transfer of either a full-length or a smaller Becker-type dystrophin minigene prevents necrosis and restores normal muscle function.

Proteomics of multiprotein complexes: answering fundamental questions in neuroscience

Proteomics tools offer new ways to analyse networks of proteins that control important neurobiological phenomena such as learning and memory. In this review, we discuss how a combined proteomic, pharmacological and genetic approach reveals that multiprotein complexes process neural information and encode memories. Simultaneous analysis of multiple proteins enables the development of new concepts and approaches for neuroscience research.

Mouse alpha1-syntrophin binding to Grb2: further evidence of a role for syntrophin in cell signaling

Syntrophins have been proposed to serve as adapter proteins. Syntrophins are found in the dystrophin glycoprotein complex (DGC); defects in the constituents of this complex are linked to various muscular dystrophies. Blot overlay experiments demonstrate that alpha-dystroglycan, beta-dystroglycan, and syntrophins all bind Grb2, the growth factor receptor bound adapter protein. Mouse alpha1-syntrophin sequences were produced as chimeric fusion proteins in bacteria and found to also bind Grb2 in a Ca2+-independent manner. This binding was localized to the proline rich sequences adjacent to and overlapping with the N-terminal pleckstrin homology domain (PH1). Grb2 bound syntrophin with an apparent KD of 563 +/- 15 nM. Grb2-C-SH3 domain bound syntrophin with slightly higher affinity than Grb2-N-SH3 domain. Crk-L, an SH2/SH3 protein of similar domain structure but different specificity, does not bind these syntrophin sequences.

Oligomerization of mouse alpha 1-syntrophin and self-association of its pleckstrin homology domain 1 containing sequences

Syntrophins are known to self-associate to form oligomers. Mouse alpha 1-syntrophin sequences were produced as chimeric fusion proteins in bacteria and were found to also oligomerize and in a micromolar Ca(2+)-dependent manner. The oligomerization was localized to the N-terminal pleckstrin homology domain (PH1) or adjacent sequences; the second, C-terminal PH2 domain did not show oligomerization. PH1 was found to self-associate, and calmodulin or Ca(2+)-chelating agents such as ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) could effectively prevent this oligomerization. A single calmodulin bound per syntrophin to cause inhibition of the precipitation. Since calmodulin inhibited syntrophin oligomerization in the presence or absence of Ca(2+), Ca(2+) binding to syntrophin is responsible for the inhibition by EGTA of syntrophin oligomerization.

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.

The presynaptic calcium channel is part of a transmembrane complex linking a synaptic laminin (alpha4beta2gamma1) with non-erythroid spectrin

Nerve regeneration studies at the neuromuscular junction (NMJ) suggest that synaptic basal lamina components tell the returning axon where to locate neurotransmitter release machinery, including synaptic vesicle clusters and active zones. Good candidates for these components are the synaptic laminins (LNs) containing alpha4, alpha5, or beta2 chains. Results from a beta2 laminin knockout mouse have suggested a linkage of this extracellular laminin to cytosolic synaptic vesicle clusters. Here we report such a transmembrane link at the electric organ synapse, which is homologous to the NMJ. We immunopurified electric organ synaptosomes and found on their surface two laminins of 740 and 900 kDa. The 740 kDa laminin has a composition of alpha4beta2gamma1 (laminin-9). Immunostaining reveals that as in the NMJ, alpha4 and beta2 chains are concentrated at the electric organ synapse. Using detergent-solubilized synaptosomes, we immunoprecipitated a complex containing alpha4beta2gamma1 laminin, the voltage-gated calcium channel, and the cytoskeletal protein spectrin. Other presynaptic proteins such as 900 kDa laminin are not found in this complex. We hypothesize that alpha4beta2gamma1 laminin in the synaptic basal lamina attaches to calcium channel, which in turn is attached to cytosolic spectrin. Spectrin could then organize synaptic vesicle clusters by binding vesicle-associated proteins.

Plasma membrane cytoskeleton of muscle: a fine structural analysis

The discovery of dystrophin and its definition as the causative molecule in Duchenne Muscular Dystrophy has led to a renewed interest in the molecular structure of the muscle fiber plasma membrane and its association with the extracellular basal lamina. The original identification of dystrophin gave credence to the possibility that the plasma membrane of the muscle fiber may be highly organized and involved in maintaining appropriate homeostasis in this actively contracting cellular system. In this review, we examine the currently known members of the muscle fiber plasma membrane cytoskeleton and the interactions that occur between the different members of this complex using histological, electron microscopic, and confocal methods. From our studies and others cited in this review, it is clear that the dystrophin cytoskeletal complex is not completely understood and component molecules continue to be discovered. Perhaps equally importantly, currently defined molecules (such as alpha-actinin or neuronal nitric oxide synthase) are being recognized as being specifically associated with the complex. What is striking from all of the studies, to date, is that while we are able to identify members of the dystrophin cytoskeletal complex and while we are able to associate mutations of individual molecules with disease(s), we are still unable to truly define the roles of each of the molecules in maintaining the normal physiology of the muscle fiber.

The presynaptic calcium channel is part of a transmembrane complex linking a synaptic laminin (alpha4beta2gamma1) with non-erythroid spectrin

Nerve regeneration studies at the neuromuscular junction (NMJ) suggest that synaptic basal lamina components tell the returning axon where to locate neurotransmitter release machinery, including synaptic vesicle clusters and active zones. Good candidates for these components are the synaptic laminins (LNs) containing alpha4, alpha5, or beta2 chains. Results from a beta2 laminin knockout mouse have suggested a linkage of this extracellular laminin to cytosolic synaptic vesicle clusters. Here we report such a transmembrane link at the electric organ synapse, which is homologous to the NMJ. We immunopurified electric organ synaptosomes and found on their surface two laminins of 740 and 900 kDa. The 740 kDa laminin has a composition of alpha4beta2gamma1 (laminin-9). Immunostaining reveals that as in the NMJ, alpha4 and beta2 chains are concentrated at the electric organ synapse. Using detergent-solubilized synaptosomes, we immunoprecipitated a complex containing alpha4beta2gamma1 laminin, the voltage-gated calcium channel, and the cytoskeletal protein spectrin. Other presynaptic proteins such as 900 kDa laminin are not found in this complex. We hypothesize that alpha4beta2gamma1 laminin in the synaptic basal lamina attaches to calcium channel, which in turn is attached to cytosolic spectrin. Spectrin could then organize synaptic vesicle clusters by binding vesicle-associated proteins.

Phosphorylation of dystrophin and alpha-syntrophin by Ca(2+)-calmodulin dependent protein kinase II

A Ca(2+)-calmodulin dependent protein kinase activity (DGC-PK) was previously shown to associate with skeletal muscle dystrophin glycoprotein complex (DGC) preparations, and phosphorylate dystrophin and a protein with the same electrophoretic mobility as alpha-syntrophin (R. Madhavan, H.W. Jarrett, Biochemistry 33 (1994) 5797-5804). Here, we show that DGC-PK and Ca(2+)-calmodulin dependent protein kinase II (CaM kinase II) phosphorylate a common site (RSDS(3616)) within the dystrophin C terminal domain that fits the consensus CaM kinase II phosphorylation motif (R/KXXS/T). Furthermore, both kinase activities phosphorylate exactly the same three fusion proteins (dystrophin fusions DysS7 and DysS9, and the syntrophin fusion) out of a panel of eight fusion proteins (representing nearly 100% of syntrophin and 80% of dystrophin protein sequences), demonstrating that DGC-PK and CaM kinase II have the same substrate specificity. Complementing these results, anti-CaM kinase II antibodies specifically stained purified DGC immobilized on nitrocellulose membranes. Renaturation of electrophoretically resolved DGC proteins revealed a single protein kinase band (M(r) approximately 60,000) that, like CaM kinase II, underwent Ca(2+)-calmodulin dependent autophosphorylation. Based on these observations, we conclude DGC-PK represents a dystrophin-/syntrophin-phosphorylating skeletal muscle isoform of CaM kinase II. We also show that phosphorylation of the dystrophin C terminal domain sequences inhibits their syntrophin binding in vitro, suggesting a regulatory role for phosphorylation.

Apocalmodulin

Intracellular Ca2+ is normally maintained at submicromolar levels but increases during many forms of cellular stimulation. This increased Ca2+ binds to receptor proteins such as calmodulin (CaM) and alters the cell's metabolism and physiology. Calcium-CaM binds to target proteins and alters their function in such a way as to transduce the Ca2+ signal. Calcium-free or apocalmodulin (ApoCaM) binds to other proteins and has other specific effects. Apocalmodulin has roles in the cell that apparently do not require the ability to bind Ca2+ at all, and these roles appear to be essential for life. Apocalmodulin differs from Ca2+-CaM in its tertiary structure. It binds target proteins differently, utilizing different binding motifs such as the IQ motif and noncontiguous binding sites. Other kinds of binding potentially await discovery. The ApoCaM-binding proteins are a diverse group of at least 15 proteins including enzymes, actin-binding proteins, as well as cytoskeletal and other membrane proteins, including receptors and ion channels. Much of the cellular CaM is bound in a Ca2+-independent manner to membrane structures within the cell, and the proportion bound changes with cell growth and density, suggesting it may be a storage form. Apocalmodulin remains tightly bound to other proteins as subunits and probably hastens the response of these proteins to Ca2+. The overall picture that emerges is that CaM cycles between its Ca2+-bound and Ca2+-free states and in each state binds to different proteins and performs essential functions. Although much of the research focus has been on the roles of Ca2+-CaM, the roles of ApoCaM are equally vital but less well understood.

Pleckstrin homology domain 1 of mouse alpha 1-syntrophin binds phosphatidylinositol 4,5-bisphosphate

Mouse alpha 1-syntrophin sequences were produced as chimeric fusion proteins in bacteria and found to bind phosphatidylinositol 4, 5-bisphosphate (PtdIns4,5P2). Half-maximal binding occurred at 1.9 microM PtdIns4,5P2 and when 1.2 PtdIns4,5P2 were added per syntrophin. Binding was specific for PtdIns4,5P2 and did not occur with six other tested lipids including the similar phosphatidylinositol 4-phosphate. Binding was localized to the N-terminal pleckstrin homology domain (PH1); the second, C-terminal PH2 domain did not bind lipids. Key residues in PtdIns4,5P2 binding to a PH domain were found to be conserved in alpha-syntrophins' PH1 domains and absent in PH2 domains, suggesting a molecular basis for binding.

A minigene of neural agrin encoding the laminin-binding and acetylcholine receptor-aggregating domains is sufficient to induce postsynaptic differentiation in muscle fibres

The extracellular matrix molecule agrin is both necessary and sufficient for inducing the formation of postsynaptic specializations at the neuromuscular junction (NMJ). At the mature NMJ, agrin is stably incorporated in synaptic basal lamina. The postsynapse-inducing activity of chick agrin, as assayed by its capability of causing aggregation of acetylcholine receptors (AChRs) on cultured muscle cells, maps to a 21 kDa, C-terminal domain. Binding of chick agrin to muscle basal lamina is mediated by the laminins and maps to a 25 kDa, N-terminal fragment of agrin. Here we show that an expression construct encoding a 'mini'-agrin, in which the laminin-binding fragment was fused to the AChR-clustering domain, is sufficient to induce postsynaptic differentiation in vivo when injected into non-synaptic sites of rat soleus muscle. As shown for ectopic postsynaptic differentiation induced by full-length neural agrin, myonuclei underneath the ectopic sites expressed the gene for the AChR epsilon-subunit. Altogether, our data show that a 'mini'-agrin construct encoding only a small fraction of the entire agrin protein is sufficient to induce postsynapse-like structures that are reminiscent of those induced by full-length neural agrin or innervation by motor neurons.

Role of the target organ in determining susceptibility to experimental autoimmune myasthenia gravis

Injection of anti-AChR antibodies in passive transfer experimental autoimmune myasthenia gravis (EAMG) results in increased degradation of acetylcholine receptor (AChR) and increased synthesis of AChR alpha-subunit mRNA. Passive transfer of anti-Main Immunogenic Region (MIR) mAb 35 in aged rats does not induce clinical signs of disease nor AChR loss. The expression of the AChR subunit genes was analyzed in susceptible and resistant rats. In aged EAMG resistant rats, no increase in the amount of AChR alpha-subunit mRNA was measured. In vivo AChR degradation experiments did not show any increase in AChR degradation rates in aged resistant rats, in contrast to young susceptible rats. Taken together, these data demonstrate that resistance of the AChR protein to antibody-mediated degradation is the primary mechanism that accounts for the resistance to passive transfer EAMG in aged rats.

The zinc finger domain of the 43-kDa receptor-associated protein, rapsyn: role in acetylcholine receptor clustering

We injected rat myotubes with proteins and antibodies to assess the importance of the zinc finger (ZnF) domain of the 43-kDa receptor-associated protein, rapsyn, in clustering acetylcholine receptors (AChR). Injection of rat myotubes with a fusion protein containing the ZnF domain of rapsyn disrupted AChR clusters. Clusters were unaffected by a fusion protein containing a double mutant that does not bind zinc. Similar results were obtained with the purified wild type and mutant ZnF domains. The ZnF of HIV-1 nucleocapsid protein had no effect. AChR clusters were also disrupted in myotubes injected with antibodies to the ZnF domain, followed by injection of anti-antibodies. Injection of antibodies directed against a different rapsyn epitope or against the cytoplasmic domain of the AChR had no effect. In transfection experiments with HEK 293 cells, the ZnF domain failed to associate with membrane aggregates containing full-length rapsyn, AChR, or rapsyn and AChR together. We conclude that the ZnF domain of rapsyn provides a binding site essential for AChR clustering, but that this site is unlikely to be involved in high affinity binding of rapsyn to itself or to AChR.

Localization and regulation of MuSK at the neuromuscular junction

The receptor tyrosine kinase, MuSK, is required for the formation of the neuromuscular junction (NMJ) where MuSK becomes phosphorylated when exposed to neuronally synthesized isoforms of agrin. To understand better the mechanisms by which MuSK mediates the formation of the NMJ, we have examined how MuSK expression is regulated during development in the embryo, by neuromuscular injury in the adult and by agrin in vitro. Here we show that MuSK is associated with the earliest observable AChR clusters at the developing motor endplate and that MuSK and AChRs codistribute throughout the development of the NMJ. These two proteins are also coordinately regulated on the surfaces of cultured myotubes where MuSK and AChRs colocalize both in spontaneous and agrin-induced clusters. While MuSK is normally restricted to the motor endplate in adult muscle, denervation results in its extrajunctional expression, although a discernible concentration of MuSK remains localized to the motor endplate even 14 days after denervation. Extrajunctional MuSK is first apparent 3 days after denervation and is sharply reduced upon reinnervation. Muscle paralysis also markedly alters the expression of MuSK in adult muscle and results in increased expression of MuSK as well as increased transcription of MuSK mRNA by extrasynaptic myonuclei. Together, these findings demonstrate that MuSK expression is highly regulated by innervation, muscle activity, and agrin, while the distribution of MuSK is precisely coordinated with that of the AChR.

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.

Alpha1-syntrophin has distinct binding sites for actin and calmodulin

Overlay and co-sedimentation assays using recombinant alpha1-syntrophin proteins revealed that two regions of alpha1-syntrophin, i.e. aa 274-315 and 449-505, contain high-affinity binding sites for F-actin (Kd 0.16-0.45 microM), although only a single high-affinity site (Kd 0.35 microM) was detected in the recombinant full-length syntrophin. We also found that actomyosin fractions prepared from both cardiac and skeletal muscle contain proteins recognized by anti-syntrophin antibody. These data suggest a novel role for syntrophin as an actin binding protein, which may be important for the function of the dystrophin-glycoprotein complex or for other cell functions. We also found that alpha1-syntrophin binds calmodulin at two distinct sites with high (Kd 15 nM) and low (Kd 0.3 microM) affinity.

Nerve-induced disruption and reformation of beta1-integrin aggregates during development of the neuromuscular junction

The earliest biochemical change detected during synaptogenesis is a local elimination of muscle basal lamina proteins. To explore whether this provides signal(s) that regulate postsynaptic differentiation, we examined the effects of innervation on the distribution of beta1-integrins, which were initially present in scattered aggregates complexed with basal lamina ligands. These beta1-integrin aggregates disappear along paths of nerve contact as their basal lamina ligands are eliminated. New accumulations of these proteins then form during assembly of the postsynaptic apparatus. The new beta1-integrin aggregates at developing synapses form partly via a redistribution of mobile molecules on muscle surface. We thus consider whether (a) the removal of integrins' basal lamina ligands alters their cytoplasmic ligand-interactions, causing the dissociation of integrin clusters, and (b) this receptor modulation helps to transduce local changes in pericellular protease activity into cytoplasmic signals that control postsynaptic differentiation.

Neural agrin induces ectopic postsynaptic specializations in innervated muscle fibers

Neural agrin, in the absence of a nerve terminal, can induce the activity-resistant expression of acetylcholine receptor (AChR) subunit genes and the clustering of synapse-specific adult-type AChR channels in nonsynaptic regions of adult skeletal muscle fibers. Here we show that, when expression plasmids for neural agrin are injected into the extrasynaptic region of innervated muscle fibers, the following components of the postsynaptic apparatus are aggregated and colocalized with ectopic agrin-induced AChR clusters: laminin-beta2, MuSK, phosphotyrosine-containing proteins, beta-dystroglycan, utrophin, and rapsyn. These components have been implicated to play a role in the differentiation of neuromuscular junctions. Furthermore, ErbB2 and ErbB3, which are thought to be involved in the regulation of neurally induced AChR subunit gene expression, were colocalized with agrin-induced AChR aggregates at ectopic nerve-free sites. The postsynaptic muscle membrane also contained a high concentration of voltage-gated Na+ channels as well as deep, basal lamina-containing invaginations comparable to the secondary synaptic folds of normal endplates. The ability to induce AChR aggregation in vivo was not observed in experiments with a muscle-specific agrin isoform. Thus, a motor neuron-specific agrin isoform is sufficient to induce a full ectopic postsynaptic apparatus in muscle fibers kept electrically active at their original endplate sites.

Neural agrin induces ectopic postsynaptic specializations in innervated muscle fibers

Neural agrin, in the absence of a nerve terminal, can induce the activity-resistant expression of acetylcholine receptor (AChR) subunit genes and the clustering of synapse-specific adult-type AChR channels in nonsynaptic regions of adult skeletal muscle fibers. Here we show that, when expression plasmids for neural agrin are injected into the extrasynaptic region of innervated muscle fibers, the following components of the postsynaptic apparatus are aggregated and colocalized with ectopic agrin-induced AChR clusters: laminin-beta2, MuSK, phosphotyrosine-containing proteins, beta-dystroglycan, utrophin, and rapsyn. These components have been implicated to play a role in the differentiation of neuromuscular junctions. Furthermore, ErbB2 and ErbB3, which are thought to be involved in the regulation of neurally induced AChR subunit gene expression, were colocalized with agrin-induced AChR aggregates at ectopic nerve-free sites. The postsynaptic muscle membrane also contained a high concentration of voltage-gated Na+ channels as well as deep, basal lamina-containing invaginations comparable to the secondary synaptic folds of normal endplates. The ability to induce AChR aggregation in vivo was not observed in experiments with a muscle-specific agrin isoform. Thus, a motor neuron-specific agrin isoform is sufficient to induce a full ectopic postsynaptic apparatus in muscle fibers kept electrically active at their original endplate sites.

Ca2+-calmodulin binding to mouse alpha1 syntrophin: syntrophin is also a Ca2+-binding protein

Syntrophins are peripheral membrane proteins which have been found associated with dystrophin, the protein product of the Duchenne muscular dystrophy gene locus. Mouse alpha1 syntrophin binds the COOH-terminal domain of dystrophin, and calmodulin inhibits this interaction in a Ca2+-dependent fashion. Where calmodulin binds to syntrophin was investigated by constructing fusion proteins containing different regions of syntrophin's sequence. Syntrophin contains at least two regions which bind calmodulin in different ways. The COOH-terminal 24 residues contain a Ca2+-calmodulin binding site, named CBS-C, which binds calmodulin with an apparent affinity of 18 nM and which is highly conserved in all syntrophins. The amino-terminal 174 residue section of syntrophin contains other calmodulin binding, and binding occurs in either the presence or absence of Ca2+ with an apparent affinity of 100 nM. Syntrophin was shown to bind Ca2+ at two or more sites residing in the amino-terminal 274 residues, and Ca2+ binding to syntrophin affects calmodulin binding at high concentrations of syntrophin. Syntrophin A (residues 4-274) is predominantly a dimer in EGTA. A model of syntrophin's complex interactions with itself (i.e., oligomerization), calmodulin, and Ca2+ is presented.

Co-distribution of tropomyosin and alpha-actinin with actin in Psammobatis extenta electrocytes brings out their similarity with muscle fiber cytoplasm

Electric organs of Psammobatis extenta (Rajiformes) electric fish derive from myoblasts of the caudal region (16). Here we study the presence of muscle proteins, actin and the actin-binding proteins, alpha-actinin and tropomyosin, in the electrocytes by means of biochemical approaches, scanning electron microscopy and immunocytochemical methods. NBD-phallacidin is employed to detect the filamentous form of actin (F-actin). Immunoblots of actin and alpha-actinin from P. extenta skeletal and smooth muscle show that the electric organ forms of actin and alpha-actinin correspond to muscle types. Scanning electron microscopy shows that P. extenta electrocytes are highly polarized cells, semicircular in shape, with an anterior, concave innervated face and a posterior, convex, non-innervated face. The immunofluorescence patterns of alpha-actinin and tropomyosin distribution are similar to those of actin, in that these epitopes appear to occur throughout the entire electrocyte cytoplasm. F-actin, as revealed by NBD-phallacidin fluorescence, was also found throughout the cytoplasm. This is the first time that evidence is presented to demonstrate the existence of muscle actin in this weak electric fish species electrocyte. The close evolutionary connection to that of muscle cells is discussed.

Inhibition of agrin-mediated acetylcholine receptor clustering by utrophin C-terminal peptides

BACKGROUND

Agrin is an extracellular matrix protein that is required for neuromuscular synaptogenesis and is particularly important in the clustering of acetylcholine receptors at post-synaptic sites. Little is known about the signal transduction pathway of agrin-mediated receptor clustering, although cytoskeletal elements and a dystrophin associated glycoprotein complex (DGC) have been implicated. Because agrin binds to alpha-dystroglycan, a member of the DGC, and the DGC is linked to actin through utrophin at postsynaptic sites, it has been suggested that binding of utrophin to the DGC plays a central role in agrin mediated receptor clustering.

RESULTS

To test this hypothesis, we expressed at high levels the DGC binding domains of utrophin in cultured myotubes using recombinant Semliki Forest Virus. Myotubes expressing the utrophin and dystrophin DGC binding domain formed significantly fewer acetylcholine receptor clusters in response to agrin than myotubes expressing other proteins.

CONCLUSIONS

These results suggest involvement of the DGC and utrophin in the signal transduction pathway of agrin-mediated acetylcholine receptor cluster formation or stabilization.

Mouse alpha 1- and beta 2-syntrophin gene structure, chromosome localization, and homology with a discs large domain

The syntrophin family of dystrophin-associated proteins consists of three isoforms, alpha 1, beta 1, and beta 2, each encoded by a distinct gene. We have cloned and characterized the mouse alpha 1- and beta 2-syntrophin genes. The mouse alpha 1-syntrophin gene ( > 24 kilobases) is comprised of eight exons. The mouse beta 2-syntrophin gene ( > 33 kilobases) contains seven exons, all of which have homologues at the corresponding position in the alpha 1-syntrophin gene. Primer extension analysis reveals two transcription initiation sites in the alpha 1-syntrophin gene and a single site in the beta 2-syntrophin gene. The sequence immediately 5' of the transcription start sites of both genes lacks a TATA box but is GC-rich and has multiple putative SP1 binding sites. The alpha 1-syntrophin gene is located on human chromosome 20 and mouse chromosome 2, while the beta 2-syntrophin gene is on human chromosome 16 and mouse chromosome 8. Analysis of the amino acid sequence of the syntrophins reveals the presence of four conserved domains. The carboxyl-terminal 56 amino acids are highly conserved and constitute a syntrophin unique domain. Two pleckstrin homology domains are located at the amino-terminal end of the protein. The first pleckstrin homology domain is interrupted by a domain homologous to repeated sequences originally found in the Drosophila discs-large protein.

Mammalian alpha 1- and beta 1-syntrophin bind to the alternative splice-prone region of the dystrophin COOH terminus

The carboxy-terminal region of dystrophin has been suggested to be crucially important for its function to prevent muscle degeneration. We have previously shown that this region is the locus that interacts with the sarcolemmal glycoprotein complex, which mediates membrane anchoring of dystrophin, as well as with the cytoplasmic peripheral membrane protein, A0 and beta 1-syntrophin (Suzuki, A., M. Yoshida, K. Hayashi, Y. Mizuno, Y. Hagiwara, and E. Ozawa. 1994. Eur. J. Biochem. 220:283-292). In this work, by using the overlay assay technique developed previously, we further analyzed the dystrophin-syntrophin/A0 interaction. Two forms of mammalian syntrophin, alpha 1- and beta 1-syntrophin, were found to bind to very close but discrete regions on the dystrophin molecule. Their binding sites are located at the vicinity of the glycoprotein-binding site, and correspond to the amino acid residues encoded by exons 73-74 which are alternatively spliced out in some isoforms. This suggests that the function of syntrophin is tightly linked to the functional diversity among dystrophin isoforms. Pathologically, it is important that the binding site for alpha 1-syntrophin, which is predominantly expressed in skeletal muscle, coincides with the region whose deletion was suggested to result in a severe phenotype. In addition, A0, a minor component of dystrophin-associated proteins with a molecular mass of 94 kD which is immunochemically related to syntrophin, binds to the same site as beta 1-syntrophin. Finally, based on our accumulated evidence, we propose a revised model of the domain organization of dystrophin from the view point of protein-protein interactions.

Differential neural regulation of a neuromuscular junction-associated antigen in muscle fibers and Schwann cells

Monoclonal antibodies 3G2 and 4E2 recognize a postsynaptic component of rat neuromuscular junctions. In contrast to many other postsynaptic junctional antigens, expression of this antigen is nerve-dependent: immunoreactivity disappears from junctions following denervation and returns upon reinnervation (Astrow et al., 1992 J. Neurosci. 12:1602-1615). Here we show that the epitope is also expressed by Schwann cells and that this expression is also neurally regulated. Weak mAb 3G2/4E2 immunoreactivity was found in myelinating Schwann cells but was not detected in either nonmyelinating Schwann cells or in terminal Schwann cells at the neuromuscular junction. Following axotomy, immunoreactivity increased in myelinating Schwann cells, and nonmyelinating and terminal Schwann cells became immunopositive. Moreover, the immunoreactivity in terminal Schwann cells revealed their extensive sprouting in response to denervation (Reynolds and Woolf, 1992, J. Neurocytol. 21: 50-66). After nerve regeneration, mAb 3G2/4E2 immunoreactivity in all Schwann cells returned towards normal: it disappeared from terminal Schwann cells, returned to low levels in myelinating Schwann cells, and decreased in nonmyelinating Schwann cells. Immunoblots of axotomized nerve and cultured muscle fibers revealed the same set of immunoreactive bands. Therefore, Schwann cells and muscle fibers share the expression of an epitope that is under neural control, but is regulated differently at each site. In Schwann cells, the presence of the nerve suppresses expression of the epitope, whereas in muscle fibers, the nerve terminal promotes this expression. The differential regulation of mAb 3G2/4E2 immunoreactivity in terminal Schwann cells and muscle fibers suggests that the epitope may be involved in interactions between nerve terminals and these cells.

Association of acetylcholine receptors with peripheral membrane proteins: evidence from antibody-induced coaggregation

Acetylcholine receptors (AChR) are associated with several peripheral membrane proteins that are concentrated on the cytoplasmic face of the plasma membrane at the neuromuscular junction, and at aggregates of AChR that form in vitro. We tested the linkage among these proteins by inducing microaggregation of AChR, then determining if a given peripheral membrane protein accumulated with the receptors in microaggregates. In most experiments, we used isolated membrane fragments that are rich in AChR and accessible to antibodies against intracellular antigens. We showed that the 43 kD receptor-associated protein always aggregated with AChR, whether microaggregation was driven by antibodies to the 43 kD protein, or to the receptor itself. Antibodies to the 58 kD receptor-associated protein also always aggregated the 58 kD protein with the receptor. Our results are consistent with a model for AChR-rich membrane in which the 43 kD and 58 kD proteins are both closely associated with the AChR. When we induced microaggregation in intact muscle cells with anti-AChR antibodies, our results were less definitive. The 43 kD receptor-associated protein microaggregated with AChR, but the 58 kD protein was not especially enriched at AChR microaggregates. We discuss the advantages of using isolated AChR-rich membrane fragments to study the association of AChR with peripheral membrane proteins.

Two forms of mouse syntrophin, a 58 kd dystrophin-associated protein, differ in primary structure and tissue distribution

Syntrophin, a 58 kd extrinsic membrane protein, is concentrated at postsynaptic sites at the neuromuscular junction and may be involved in clustering acetylcholine receptors. In muscle and nonmuscle tissues, syntrophin is associated with dystrophin, utrophin, and two homologs of the dystrophin carboxy-terminal region. We have isolated three cDNAs encoding Torpedo and mouse syntrophins. The Torpedo cDNA encodes a full-length protein, and on Northern blots recognizes a 3.5 kb mRNA. The two mouse syntrophin cDNAs are products of separate genes but encode proteins that share 50% identity. Syntrophin-1 mRNA (2.2 kb) is expressed at highest levels in skeletal muscle. Syntrophin-2 mRNAs (2.2, 5.0, and 10 kb) are expressed in all mouse tissues examined. These patterns of expression suggest that syntrophin-1 and syntrophin-2 may associate with different members of the dystrophin family.

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.

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.

Localization of dystrophin relative to acetylcholine receptor domains in electric tissue and adult and cultured skeletal muscle

Two high-affinity mAbs were prepared against Torpedo dystrophin, an electric organ protein that is closely similar to human dystrophin, the gene product of the Duchenne muscular dystrophy locus. The antibodies were used to localize dystrophin relative to acetylcholine receptors (AChR) in electric organ and in skeletal muscle, and to show identity between Torpedo dystrophin and the previously described 270/300-kD Torpedo postsynaptic protein. Dystrophin was found in both AChR-rich and AChR-poor regions of the innervated face of the electroplaque. Immunogold experiments showed that AChR and dystrophin were closely intermingled in the AChR domains. In contrast, dystrophin appeared to be absent from many or all AChR-rich domains of the rat neuromuscular junction and of AChR clusters in cultured muscle (Xenopus laevis). It was present, however, in the immediately surrounding membrane (deep regions of the junctional folds, membrane domains interdigitating with and surrounding AChR domains within clusters). These results suggest that dystrophin may have a role in organization of AChR in electric tissue. Dystrophin is not, however, an obligatory component of AChR domains in muscle and, at the neuromuscular junction, its roles may be more related to organization of the junctional folds.

Molecular alterations in the perijunctional region of frog skeletal muscle fibres following denervation

The anatomical distribution of a frog skeletal muscle antigen was studied using immunofluorescence microscopy and a monoclonal antibody 3B6 that was produced against denervated skeletal muscle. In innervated muscles, the monoclonal antibody 3B6 stain was associated with the inner surface of the muscle plasma membrane at the endplate and myotendinous junction. After denervation, the monoclonal antibody 3B6 stain extended from the endplate laterally around the perimeter of muscle fibres and longitudinally well beyond the endplate for a total length of 600-1000 microns. The monoclonal antibody 3B6 stain thus forms a cylindrical structure centred on the endplate. This observation shows that denervation produces a non-homogeneous molecular change in skeletal muscle fibres: an antigen that is present in high concentrations at innervated endplates appears in restricted perijunctional regions of denervated muscle fibres. It further suggests that perijunctional regions of denervated muscle fibres differ from the remaining non-endplate regions in molecular composition and possibly also in function.

Organization and dynamics of microtubules in Torpedo marmorata electrocyte: selective association with specialized domains of the postsynaptic membrane

The distribution and subcellular organization of two components of the secretory pathway, the Golgi apparatus and microtubules, have been investigated in Torpedo marmorata electrocyte. This highly polarized syncytium, embryologically derived from skeletal muscle cells, displays distinct plasma membrane domains on its innervated and non-innervated faces, and it played a critical role in the identification of the acetylcholine receptor. By immunocytochemical analysis, we show that in the electrocyte, numerous focal Golgi bodies are dispersed throughout the cytoplasm in frequent association with nuclei. Under experimental conditions known to stabilize microtubules, we reveal an elaborate network composed of two populations of microtubules exhibiting different dynamic properties as evaluated by cold-stability, resistance to nocodazole and post-translational modification. This network appears organized from several nucleating centers located in the medial plane of the cell that are devoided of centrioles. The network displays an asymmetric distribution with individual microtubules converging towards the troughs of the postsynaptic membrane folds. In these particular regions, we consistently observed clusters of non-coated vesicles in association with the microtubules. The organization of the microtubules in the electrocyte may thus result in a functional polarization of the cytoplasm. In other polarized cells, the particular organization of the secretory pathway accounts for the intracellular routing of membrane proteins. The organization that we have observed in the electrocyte may thus lead to the vectorial delivery of synaptic proteins to the innervated plasma membrane. Furthermore, the abundance of synaptic proteins makes the electrocyte a unique model with which to decipher the mechanisms involved in the sorting and targeting of these glycoproteins.

Gap junctional communication during neuromuscular junction formation

We have tested whether gap junctions form between nerve and muscle during their initial contact, before establishing the chemical synapse. Embryonic Xenopus stage 18-20 myotomes and neural tubes were permeabilized with DMSO to load appropriate reagents, dissociated, and cocultured. When myotomes, loaded with Lucifer yellow, were cocultured with unlabeled neural tube cells, 23% of the neurons contained dye after 24 hr. Affinity-purified gap junction antibodies loaded into myocytes or neurons reduced neuronal labeling significantly to 5%. [3H]uridine nucleotide transfer was observed in both directions between myocytes and neurons. Again gap junction antibodies substantially reduced recipient label. In all cases preimmune IgGs did not reduce transfer. When acetylcholine receptor clustering was examined in cultures containing gap junction antibodies, no difference in the number of neuronally induced AChR clusters was observed. This suggests that the cluster-inducing signal between nerve and muscle does not pass through gap junctions.