Aneural muscle cell cultures make synaptic basal lamina components

Structural and functional organization of synaptic acetylcholinesterase

The expression of the synaptic asymmetric form of the enzyme acetylcholinesterase (AChE) depends of two different genes: the gene that encodes for the catalytic subunit and the gene that encodes for the collagenic tail, ColQ. Asymmetric AChE is specifically localized to the basal lamina at the neuromuscular junction (NMJ). This highly organized distribution pattern suggests the existence of one or more specific binding sites in ColQ required for its anchorage to the synaptic basal lamina. Recent evidence support this notion: first, the presence of two heparin-binding domains in ColQ that interact with heparan sulfate proteoglycans (HSPGs) at the synaptic basal lamina; and second, a knockout mouse for perlecan, a HSPG concentrated in nerve-muscle contact, in which absence of asymmetric AChE at the NMJ is observed. The physiological importance of collagen-tailed AChE form in skeletal muscle has been illustrated by the identification of several mutations in the ColQ gene. These mutations determine end-plate acetylcholinesterase deficiency and induce one type of synaptic functional disorders observed in Congenital Myasthenic Syndromes (CMSs).

Colocalization but differential regulation of neuronal NO synthase and nicotinic acetylcholine receptor in C2C12 myotubes

In mammalian skeletal muscle, neuronal-type nitric oxide synthase (nNOS) is found to be enriched at neuromuscular endplates. Here we demonstrate the colocalization of the nicotinic acetylcholine receptor (nAChR, stained with alpha-bungarotoxin) and nNOS (stained with a specific antibody) in murine C(2)C(12) myotubes. However, coimmunoprecipitation experiments demonstrated no evidence for a direct protein-protein association between the nAChR and nNOS in C(2)C(12) myotubes. An antibody to the alpha(1)-subunit of the nAChR did not coprecipitate nNOS, and an nNOS-specific antibody did not precipitate the alpha(1)-subunit of the nAChR. Treatment of mice with bacterial LPS downregulated the expression of nNOS in skeletal muscle, and treatment of C(2)C(12) cells with bacterial LPS and interferon-gamma markedly decreased nNOS mRNA and protein expression. In contrast, mRNA and protein of the nAChR (alpha-, gamma-, and epsilon-subunits) remained unchanged at the mRNA and protein levels. These data demonstrate that nNOS and the nAChR are colocalized in murine skeletal muscle and C(2)C(12) cells but differ in their expressional regulation.

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

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

Laminins of the neuromuscular system

The mammalian neuromuscular system expresses seven laminin genes (alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 1), produces seven isoforms of the laminin trimer (laminins 1, 2, 4, 8, 9, 10, and 11), and distributes these trimers to at least seven distinct basal laminae (perineurial, endoneurial, terminal Schwann cell, myotendinous junction, synaptic cleft, synaptic fold, and extrajunctional muscle). The patterns of expression, assembly, and distribution are regulated during development, and primary and secondary changes in laminin expression occur in several neuromuscular genetic disorders. Functional studies using knockout and transgenic mice, and purified laminins and cell types, demonstrate that laminins are required components of basal laminae in the neuromuscular system. Collectively, laminins have both structural and signaling functions; individually, laminin isoforms have unique roles in regulating the behavior of nerve, muscle, and Schwann cell. Among them, laminin-2 (alpha 2 beta 1 gamma 1) plays an important structural role in supporting the muscle plasma membrane, laminin-4 regulates adhesion and differentiation of the myotendinous junction, and laminin-11 regulates nerve terminal differentiation and Schwann cell motility. Together, these observations reveal remarkable diversity in the formation and function of laminins and basal laminae, and suggest avenues for addressing some neuromuscular diseases.

Distinct structures and functions of related pre- and postsynaptic carbohydrates at the mammalian neuromuscular junction

Carbohydrates that terminate in beta-linked N-acetylgalactosamine (betaGalNAc) residues are concentrated in the postsynaptic apparatus of the skeletal neuromuscular junction and have been implicated in the differentiation of the postsynaptic membrane. We now report that distinct synapse-specific betaGalNAc-containing carbohydrates are associated with motor nerve terminals. Two monoclonal antibodies that recognize distinct betaGalNAc-containing epitopes, CT1 and CT2, both stain synaptic sites on skeletal muscle fibers. However, CT1 selectively stains nerve terminal, whereas CT2 selectively stains the postsynaptic apparatus. Likewise, CT1 and CT2 selectively stain motoneuron-like and muscle cell lines, respectively. Using the cell lines, we identify distinct CT1- and CT2-reactive glycolipids and glycoproteins. Finally, we show that GalNAc modulates the adhesion of motoneuron-like cells to recombinant fragments of a synaptic cleft component, laminin beta2. Together, these results show that pre- as well as postsynaptic membranes bear and are affected by distinct but related synapse-specific carbohydrates.

Distribution and function of laminins in the neuromuscular system of developing, adult, and mutant mice

Laminins, heterotrimers of alpha, beta, and gamma chains, are prominent constituents of basal laminae (BLs) throughout the body. Previous studies have shown that laminins affect both myogenesis and synaptogenesis in skeletal muscle. Here we have studied the distribution of the 10 known laminin chains in muscle and peripheral nerve, and assayed the ability of several heterotrimers to affect the outgrowth of motor axons. We show that cultured muscle cells express four different alpha chains (alpha1, alpha2, alpha4, and alpha5), and that developing muscles incorporate all four into BLs. The portion of the muscle's BL that occupies the synaptic cleft contains at least three alpha chains and two beta chains, but each is regulated differently. Initially, the alpha2, alpha4, alpha5, and beta1 chains are present both extrasynaptically and synaptically, whereas beta2 is restricted to synaptic BL from its first appearance. As development proceeds, alpha2 remains broadly distributed, whereas alpha4 and alpha5 are lost from extrasynaptic BL and beta1 from synaptic BL. In adults, alpha4 is restricted to primary synaptic clefts whereas alpha5 is present in both primary and secondary clefts. Thus, adult extrasynaptic BL is rich in laminin 2 (alpha2beta1gamma1), and synaptic BL contains laminins 4 (alpha2beta2gamma1), 9 (alpha4beta2gamma1), and 11 (alpha5beta2gamma1). Likewise, in cultured muscle cells, alpha2 and beta1 are broadly distributed but alpha5 and beta2 are concentrated at acetylcholine receptor-rich "hot spots," even in the absence of nerves. The endoneurial and perineurial BLs of peripheral nerve also contain distinct laminin chains: alpha2, beta1, gamma1, and alpha4, alpha5, beta2, gamma1, respectively. Mutation of the laminin alpha2 or beta2 genes in mice not only leads to loss of the respective chains in both nerve and muscle, but also to coordinate loss and compensatory upregulation of other chains. Notably, loss of beta2 from synaptic BL in beta2(-/-) "knockout" mice is accompanied by loss of alpha5, and decreased levels of alpha2 in dystrophic alpha2(dy/dy) mice are accompanied by compensatory retention of alpha4. Finally, we show that motor axons respond in distinct ways to different laminin heterotrimers: they grow freely between laminin 1 (alpha1beta1gamma1) and laminin 2, fail to cross from laminin 4 to laminin 1, and stop upon contacting laminin 11. The ability of laminin 11 to serve as a stop signal for growing axons explains, in part, axonal behaviors observed at developing and regenerating synapses in vivo.

A synaptic localization domain in the synaptic cleft protein laminin beta 2 (s-laminin)

The basal lamina that ensheaths skeletal muscle fibers traverses the synaptic cleft at the neuromuscular junction. Synaptic and extrasynaptic portions of the basal lamina contain different laminin beta chains: beta 2 (or s) at synapses and beta 1 (or B1) extrasynaptically. Laminin beta 2 is also confined to synapselike patches on myotube surfaces in vitro, whereas beta 1 is present throughout the extracellular matrix. This differential localization of laminin beta chains was analyzed by expression of chimeric beta 1-beta 2 molecules in cultured mouse myotubes. A 16-amino acid carboxyl-terminal sequence in beta 2 was necessary for synaptic localization, and an amino-terminal domain in beta 1 promoted association with extracellular fibrils. The synaptic targeting sequence of beta 2 contains a site previously shown to be adhesive for motor neurons.

Role for a synapse-specific carbohydrate in agrin-induced clustering of acetylcholine receptors

Lectins such as VVA-B4, which bind N-acetylgalactosaminyl (GalNAc)-terminated saccharides, selectively stain the neuromuscular junction, thus defining a synapse-specific carbohydrate. In seeking roles for this carbohydrate, we asked whether VVA-B4 affected the distribution of acetylcholine receptors (AChRs) on cultured muscle cells. We found that incubation of myotubes with VVA-B4 induced formation of AChR clusters and potentiated the effect of a nerve-derived clustering factor, agrin. Additional experiments implicated GalNAc-terminated glycoconjugates as modulators of agrin-induced AChR clustering. Enzymatic removal of GalNAc residues or treatment with a multivalent protein-GalNAc conjugate blocked agrin-induced clustering, whereas enzymatic unmasking of additional GalNAc residues induced clustering in the absence of added agrin. Moreover, incubation with agrin led to redistribution of VVA-B4-binding material on myotubes. Together, these results suggest that agrin-induced clustering of AChRs involves a GalNAc-dependent step.

Sixth Annual Stuart Reiner Memorial Lecture: embryonic development of nerve and muscle

This article provides a basic scheme of sequential anatomic and some physiologic events occurring during the course of embryonic development of motor neurons and muscles, leading to the establishment of mature nerve-muscle relationships. Motor neurons and muscles begin their development independently and during embryogenesis they become dependent on each other for further development and survival. Aspects of development which occur independently and those requiring mutual interactions are identified. The development of motor neurons is discussed with respect to their production, projection, neuromuscular transmission, myelination, sprouting, survival, and death. The development of muscles is discussed with respect to the origin, differentiation, and muscle fiber types. Discussion on the development of neuromuscular junction includes differentiation of presynaptic nerve terminal, postsynaptic components, and elimination of multiple axons.

Dystroglycan-alpha, a dystrophin-associated glycoprotein, is a functional agrin receptor

Aggregation of acetylcholine receptors (AChRs) on skeletal muscle fibers is thought to be mediated by the basal lamina protein agrin. Structural similarities shared by agrin and laminin suggested that the laminin receptor dystroglycan-alpha, part of a dystrophin-receptor complex, might also bind agrin. We show here that dystroglycan-alpha and dystrophin-related protein (DRP/utrophin) are concentrated within AChR aggregates in cultures of C2 myotubes and that agrin binds specifically to dystroglycan-alpha in in vitro assays. This binding is calcium dependent and is inhibited by monoclonal antibody (MAb) IIH6 against dystroglycan-alpha, heparin, and laminin, but not by fibronectin. In S27 cells, which do not aggregate AChRs spontaneously, agrin and laminin binding to dystroglycan-alpha are dramatically decreased. Moreover, MAb IIH6 significantly inhibits agrin-induced AChR aggregation on C2 cells. We conclude that dystroglycan-alpha is an agrin-binding protein and part of a dystrophin-receptor complex involved in AChR aggregation.

Regulated and constitutive secretion of distinct molecular forms of acetylcholinesterase from PC12 cells

PC12 cells secrete the enzyme acetylcholinesterase (AChE) while at rest, and increase the overall rate of this secretion 2-fold upon depolarization. This behavior is different from the release of other markers by the constitutive or regulated secretory pathways in PC12 cells. Both the resting and stimulated release of AChE are unchanged after treatment with a membrane-impermeable esterase inhibitor, demonstrating that it represents true secretion and not shedding from the cell surface. The stimulation release of AChE is Ca(2+)-dependent, while the unstimulated release is not. Analysis of the molecular forms of AChE secreted by PC12 cells indicates that the release of AChE actually involves two concurrent but independent secretory processes, and that the G4 form of the enzyme is secreted constitutively, while both the G2 and G4 forms are secreted in a regulated manner, presumably from regulated secretory vesicles. Compared with other regulated secretory proteins, a much smaller fraction of cellular AChE is secreted, and the intracellular localization of this enzyme differs from that of other regulated secretory proteins. The demonstration that a cell line that exhibits regulated secretion of acetylcholine (ACh) is also capable of regulated secretion of AChE provides additional evidence for the existence of multiple regulated secretory pathways within a single cell. Moreover, there appears to be a selective packaging of different molecular forms of AChE into the regulated versus the constitutive secretory pathway. Both the specificity of sorting of AChE and the regulation of its secretion suggest that AChE may play a more dynamic role in synaptic function than has been recognized previously.

Congruity of acetylcholine receptor, acetylcholinesterase, and Dolichos biflorus lectin binding glycoprotein in postsynaptic-like sarcolemmal specializations in noninnervated regenerating rat muscles

Noninnervated regenerating muscles are able to form focal postsynaptic-like sarcolemmal specializations either in places of the former motor endplates ("junctional" specializations) or elsewhere along the muscle fibers (extrajunctional specializations). The triple labeling histochemical method was introduced to analyse the congruity of focalization in such specializations of 3 synaptic components: acetylcholinesterase (AChE), acetylcholine receptor (AChR), and a specific synaptic glycoprotein which binds Dolichos biflorus lectin (DBAR). Noninnervated regenerating soleus and extensor digitorum longus (EDL) muscles of the rat were examined and compared with denervated muscles of neonatal and adult rats. All junctional sarcolemmal specializations in noninnervated regenerating muscles accumulated AChE and AChR. Localization of the 2 components was identical within the limits of resolution of the method. DBAR could not be demonstrated in junctional specializations in 17-day-old regenerating muscles. It seems that an agrin-like inducing substance in the former junctional basal lamina invariably triggers the accumulation of both AChE and AChR in the underlying sarcolemma of the regenerating muscle fiber. However, accumulation of DBAR would probably require the presence of the motor nerve. In most of the extrajunctional sarcolemmal specializations in 8-day-old regenerating soleus and EDL muscles, both AChE and AChR accumulated. However, about 10 percent of AChE accumulations lacked AChR and about 35% of AChR accumulations lacked AChE. Even greater variability was observed in 17-day-old regenerating muscles. The presence of DBAR in the extrajunctional postsynaptic-like sarcolemmal specializations could not be demonstrated. Similar extrajunctional sarcolemmal specializations were observed in denervated postnatal rat muscles. About 70% contained both AChE and AChR, and 30% contained only AChR, but none contained DBAR. In denervated mature muscles, sparse extrajunctional AChR accumulations did not contain detectable amounts of AChE. The ability to form complex postsynaptic-like sarcolemmal specializations in the absence of nerve, which is probably inherent to noninnervated immature muscle fibers, may be reduced with muscle maturation. Variable accumulation of individual components in the postsynaptic-like specializations indicates that different triggering factors may be involved in their accumulation or, at least, the mechanisms of their accumulation can function relatively independently.

Ultroser G and brain extract induce a continuous basement membrane with specific synaptic elements in aneurally cultured human skeletal muscle cells

Basement membrane (BM) components were studied on human muscle and skeletal muscle cells cultured on different media by immunofluorescence and electron microscopy. Their topographical relation with acetylcholine receptors was investigated. Myotubes cultured on a combination of the serum substitute Ultroser G and brain extract show a continuous layer of heparan sulfate proteoglycans (HSPGs), laminin, and type IV collagen. In contrast, myotubes cultured on serum-containing media are associated with granular depositions of HSPG and laminin and only with wisps of type IV collagen. Omission of brain extract or substitution by chicken embryo extract results in an intermediate staining pattern. For all types of cultures, fibronectin is localized in and around mononuclear cells, but hardly associated with myotubes. A codistribution between clusters of acetylcholine receptors and HSPG and laminin and Vicia villosa B4 lectin-positive material exists only in Ultroser G/brain extract-based myotubes like in muscle in vivo. No clustering is observed in serum-based myotubes. Electron microscopy reveals that the former myotubes are surrounded by a continuous BM consisting of a lamina lucida, lamina densa, and lamina fibroreticularis. Proteoglycans are present on the external site of the lamina densa and associated in a regular fashion with collagen fibrils. In conclusion, BMs associated with myotubes cultured on Ultroser G/brain extract resemble in many ways the in vivo situation, including synaptic specializations. Cultured myotubes may serve as a model system for studies on the structure and function of human muscular (synaptic) BM under normal and pathological conditions.

Calcium and ionophore A23187 stimulates deposition of extracellular matrix and acetylcholinesterase release in cultured myotubes

Calcium (Ca2+) and calcium-transporting ionophores stimulate protein secretion in many cellular systems. We demonstrate here than increases in intracellular calcium concentration induce a time- and concentration-dependent deposition of extracellular matrix and an increase in acetylcholinesterase secretion. Scanning and transmission electron-microscopy revealed that treatment with the calcium ionophore A23187, or high extracellular Ca2+ levels (5 mM to 15 mM) produce significant deposits of extracellular matrix around the myotubes, as well as a marked increase in the acetylcholinesterase reaction-product. Blocking muscle contraction was not necessary for the induction of AChE secretory activity. Sucrose density-gradients of media conditioned by muscle cells revealed 3 separate acetylcholinesterase molecular forms. However, incubation with A23187 increased only the 4.5 S and the 7.2 S molecular forms, whereas the 12.0 S form showed no significant differences from controls. Polyacrylamide gel electrophoresis, and autoradiography using [3H]diisopropyl fluorophosphate revealed a broad band at 65,000 daltons. This band was broader than for controls when medium was obtained from A23187-treated cells. Our results show that increasing intracellular Ca2+ concentration induces marked deposition of extracellular matrix and increased acetylcholinesterase secretion, with an apparent selectivity for the monomeric and dimeric acetylcholinesterase molecular forms.

Influence of innervation on molecular forms of acetylcholinesterase in regenerating fast and slow skeletal muscles

Nerve-intact muscle regenerates were prepared by ischemic-toxic injury of slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles of the rat. Rapid innervation of regenerating myotubes modified intrinsic patterns of AChE molecular forms, revealed by velocity sedimentation in linear sucrose gradients. Regarding their onset, the effects of innervation can be classified as early and late. The earliest changes in the SOL regenerates appeared a few days after innervation by their motoneurons: the activity of the 13 S AChE form (A 8) increased significantly in comparison to non-innervated regenerates. The pattern of AChE molecular forms became similar to that in the normal SOL muscle during the 2nd week after injury. In contrast, no major differences were observed between 8 day-old innervated and non-innervated EDL regenerates. Their patterns of AChE molecular forms resembled that in the normal EDL. However, the predominance of the 10 S AChE form (G 4) characteristic for the 2-week old non-innervated regenerates was prevented by innervation. Early effect of innervation observed in the SOL regenerates but not in the EDL may be due to intrinsically different response of the regenerating SOL myotubes to innervation. Rather high extrajunctional activity of the asymmetric 16 S (A 12) molecular form of AChE in early regenerates was reduced to adult level in about 3 weeks in the SOL, and nearly completely suppressed in 5 weeks after innervation in the EDL regenerates. This reduction is assumed to be a late effect of innervation, as well as a decrease of the activity of the 4 S AChE form (G 1) in the SOL regenerates. A suppressive mechanism is activated in the extra-junctional regions of the innervated muscle regenerates during their maturation.(ABSTRACT TRUNCATED AT 250 WORDS)

Translocation of the brain-type glucose transporter largely accounts for insulin stimulation of glucose transport in BC3H-1 myocytes

Insulin-stimulated glucose transport was examined in BC3H-1 myocytes. Insulin treatment lead to a 2.7 +/- 0.3-fold increase in the rate of deoxyglucose transport and, under the same conditions, a 2.1 +/- 0.1-fold increase in the amount of the brain-type glucose transporter (GLUT 1) at the cell surface. It has been shown that some insulin-responsive tissues express a second, immunologically distinct, transporter, namely GLUT 4. We report here that BC3H-1 myocytes and C2 and G8 myotubes express only GLUT 1; in contrast, rat soleus muscle and heart express 3-4 times higher levels of GLUT 4 than GLUT 1. Thus translocation of GLUT 1 can account for most, if not all, of the insulin stimulation of glucose transport in BC3H-1 myocytes. On the other, hand, neither BC3H-1 myocytes nor the other muscle-cell lines are adequate as models for the study of insulin regulation of glucose transport in muscle tissue.

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.

Two types of focal accumulations of acetylcholinesterase appear in noninnervated regenerating skeletal muscles of the rat

Muscle fibers in the soleus muscle of the rat, injured by bupivacaine and free autografting, were allowed to regenerate within their old basal laminae. Histochemical and cytochemical analysis of newly synthesized acetylcholinesterase (AChE) revealed that two kinds of focal accumulations of AChE appeared in regenerating myotubes. First, AChE gets concentrated at the sites of the former motor endplates. Accumulation of AChE starts in places where a tight contact between the remnants of the old junctional basal lamina and the budding surface of the myotube engulf the extracellular material. Appearance of these AChE accumulations can be prevented by papain treatment of the soleus muscle before autografting but not by predenervating it for 1 month. Focalization of AChE is probably induced by a component of the junctional basal lamina, possibly a protein, the existence of which is not dependent upon continuous presence of the motor nerve and may be produced by the muscle. This view is corroborated by the fact that an additional kind of AChE accumulation appeared in regenerating muscles in regions remote from the sites where motor endplates were located in the muscles of origin. Although differing in localization, size, and appearance, both kinds of AChE accumulations ultrastructurally resemble the postsynaptic specialization of the motor endplate: they consist of tubelike sarcolemmal invaginations containing AChE. The extrajunctional AChE accumulations seem to arise spontaneously and are usually located more than 750 micron away from the junctional ones as if some local inhibitory mechanism prevents their formation in the immediate vicinity.

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

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

Development of postsynaptic-like specializations of the neuromuscular synapse in the absence of motor nerve

It was previously reported that the acetylcholine receptor clusters and acetylcholinesterase appear on embryonic superior oblique muscle cells developing in vivo without motor nerve contacts. The objective of this study was to examine whether some other components of neuromuscular junction also form on muscle cells developing in vivo in the absence of motor neurons. In the present study, postsynaptic specializations such as junctional folds, postsynaptic density and basal lamina were studied in normal and aneural muscles. The superior oblique muscle of duck embryos was made aneural by permanent destruction of trochlear motor neurons by cauterizing midbrain on embryonic day 7; 3 days before the motor neurons normally project their axons into the muscle. Normal and aneural muscles from embryonic days 10 to 25 were processed for electron microscopy. The results indicate that morphological specializations such as junction-like folds, postsynaptic-like density, and basal lamina also develop in the absence of motor neuron contacts. Whether the differentiation of specialized synaptic basal lamina is dependent on the presence of motor neurons was examined by utilizing a monoclonal antibody against heparan sulfate proteoglycan. Immunohistochemical studies indicate that specialized synaptic basal lamina differentiates in the absence of motor neurons. Thus, the mechanism of development of postsynaptic components of neuromuscular junction in this muscle is not dependent on motor neuron contacts. These results also suggest that the postsynaptic cell plays a more active role in synapse formation than previously realized. The results are discussed in relation to the control of synapse numbers by the postsynaptic cell.

Expression of adult fast pattern of acetylcholinesterase molecular forms by mouse satellite cells in culture

The pattern of acetylcholinesterase (AChE) molecular forms, obtained by sucrose gradient sedimentation, was studied at different in vitro developmental stages of myogenic cells isolated from adult mouse skeletal muscle. Only the globular forms were present in rapidly dividing satellite cells during the first days in culture. After myotube formation, a pattern similar to that described in mammalian fast-twitch skeletal muscle was observed. This pattern did not change during the following period in culture (up to 1 month) nor could it be modified by co-culturing with spinal cord motoneurons or by addition of brain-derived extracts. The internal-external localization of AChE molecular forms has been determined by the use of echothiophate iodide, a membrane-impermeant irreversible inhibitor of AChE. Echothiophate-treated cultures showed about 40% of both asymmetric and globular forms localized on the sarcolemma, with their active sites oriented outward. Analysis of culture medium from untreated cultures revealed the presence of both asymmetric and globular forms. When the same analysis was repeated on cultures of myoblasts derived from 16-day-old mouse embryos, the pattern of AChE forms was different. The myotubes derived from these cells exhibit a very small proportion of asymmetric form, which was not released into the medium. This pattern was not further modified during the following days of culture, nor by co-cultures with spinal cord motoneurons or by incubations with brain-derived extracts. Thus, the myotubes derived from myoblasts express in culture a clear phenotypic difference when compared to the corresponding myotubes from satellite cells, supporting the view that these two myogenic cells are endowed with different developmental programs.

De novo neuromuscular junction formation on human muscle fibres cultured in monolayer and innervated by foetal rat spinal cord: ultrastructural and ultrastructural--cytochemical studies

Ultrastructural features of neuromuscular junction formation and transverse tubule development were studied utilizing a newly developed model in which human muscle fibres cultured in monolayer are innervated by foetal rat spinal cord with dorsal root ganglia attached. At early innervation (7-10 days), when distinct 'boutons' are contacting muscle fibres, the contacts of nerve terminals with the muscle fibres are, ultrastructurally, superficial and unorganized, and there is no basal lamina-like material between nerve terminals and muscle fibres. A bouton consists, ultrastructurally, of a cluster of small nerve terminals contacting the muscle fibre. At 2-3 weeks of innervation, shallow 'beds' are formed on the muscle fibre just beneath nerve terminals, and occasionally there are irregular and miniscule fragments of basal lamina-like material in the cleft. There is no Schwann cell apposing the nerve terminal at this stage of innervation. After 4-5 weeks of innervation there is more definite basal lamina material in the cleft and suggestive postsynaptic plasmalemmal densities and invaginations. However, there is no Schwann cell apposing the nerve terminal at this stage. At 6-8 weeks of innervation, deep postsynaptic folds are present, a Schwann cell apposes the nerve terminal, and basal lamina surrounds the entire muscle fibre. At all four stages of innervation examined, ultrastructural cytochemistry of alpha-bungarotoxin binding reveals that nicotinic ACh receptors are located exclusively at the neuromuscular junctions. After 1-2 weeks of innervation, very few lanthanum-positive transverse tubules are observed and only in close proximity to the surface membrane. After 3 weeks of innervation, more lanthanum-positive tubules are present, and they are located deeper within the muscle fibre. Five weeks after innervation, somewhat more elaborated tubules (but no lateral sacs) appear, and honeycomb structures are often present. After 6-7 weeks of innervation the tubular system is very elaborate and lateral sacs are present. Hence, this study describes consecutive stages of the formation of neuromuscular junctions and transverse tubules in innervated cultured human muscle, and provides an important basis to which similar studies related to the diseased human muscle can be compared.

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.

Receptors to agglutinin from Dolichus biflorus (DBA) at the synaptic basal lamina of rat neuromuscular junction. A histochemical study during development and denervation

The binding of agglutinin from Dolichus biflorus (DBA) and other lectins (Concanavalin A, agglutinin from wheat germ and lectin from Bandeiraea simplicifolia) to synaptic and extrasynaptic portions of the basal lamina of muscle fibers, was studied with histochemical methods. In rat muscle, DBA-binding is specifically detected at the basal lamina of neuromuscular junction. However, long-term (6 months) denervated end-plate in adult rat muscle failed to bind DBA. During normal development, synaptic DBA receptors appear later than acetylcholine receptors or acetylcholinesterase at the rat neuromuscular junction. Generalized DBA-binding to motor end-plates is first visualized in 3-day-old rats, but section of sciatic nerve in 1-day-old rats prevents the appearance of synaptic DBA-binding on the leg end-plates. It is suggested, therefore, that the synaptic DBA receptors could be related to the postnatal stabilization of rat neuromuscular synapses.

Kinetics of activation of acetylcholine receptors in a mouse muscle cell line under a range of acetylcholine concentrations

We studied, using the patch-clamp technique, the kinetics of single acetylcholine (ACh)-activated channels in a mouse muscle cell line. In the presence of high ACh concentrations we estimated the rate of channel isomerization into the open state (beta) from the dwell time between openings. Also, we obtained estimates for beta under low agonist concentrations by assuming a linear sequential model of channel activation and applying burst analysis. If the linear model is correct, then the two estimates of beta should agree since beta should be independent of ACh concentration. However, the estimates of beta obtained under low ACh concentrations were slower than those obtained independently under high ACh concentrations. The discrepancy in the estimates of beta suggests that the linear model is inadequate, but the discrepancy can be explained if open channels can close through two separate pathways. Two alternative kinetic models that can account for our data are discussed.

Developmental progression of myosin gene expression in cultured muscle cells

Myosin heavy chains are encoded by distinct members of a multigene family at different stages of muscle development. Study of the underlying regulatory mechanisms has been hindered because transitions in myosin expression have not been readily attained in tissue culture. Here we show a transition from early (fetal) to late (perinatal/adult) myosins defined by two monoclonal antibodies, F1.652 and N3.36, in the myotubes of mouse C2C12 cells. On day 1 of differentiation, essentially all myosin was early myosin. By day 8, early myosin dropped to 25% of its day 1 value and was replaced by late myosin. The transition occurred without neural contact, connective tissue components, or complex substrates, suggesting that its regulation may be intrinsic to the muscle cell. Our results demonstrate that a developmental progression in myosin gene expression, which occurs rapidly, with high frequency, and under relatively simple conditions, is now amenable to molecular analysis in cultured muscle cells.

A nerve terminal anchorage protein from electric organ

The nerve terminal and the postsynaptic receptor-containing membranes of the electric organ are both linked to the basal lamina that runs between them. We have identified an extracellular matrix protein whose physical properties suggest it anchors the nerve terminal to the basal lamina. The protein was identified because it shares an epitope with a proteoglycan component of electric organ synaptic vesicles. It too behaves like a proteoglycan. It is solubilized with difficulty from extracellular matrix fractions, elutes from DEAE Sephacel at pH 4.9 only at high ionic strength, and binds to a laminin affinity column from which it can be eluted with heparin. Under denaturing conditions it sediments rapidly and has a large excluded volume although it can be included in Sephacryl S-1000 columns. This large, highly charged extracellular matrix molecule can be readily reconstituted into liposomes consistent with the presence of a hydrophobic tail. By immunoelectron microscopy the antigen is found both in synaptic vesicles and on the plasma membrane of the nerve terminal. Since this is the first protein described that links the nerve terminal membrane to the extracellular matrix, we propose calling it terminal anchorage protein one (TAP-1).

Novel antigens at the neuromuscular junction

Three novel components of neuromuscular junctions have been identified by use of monoclonal antibodies (McAb) against glycoproteins obtained from a mouse neuroblastoma X human dorsal root ganglion cell hybrid line. Antigen distribution was assessed by fluorescent immunohistochemistry on frozen sections of human intercostal muscle counterstained with labeled alpha-bungarotoxin to identify neuromuscular junctions. Antigen SOS 6 stained exclusively in the neuromuscular junction, whereas antigens SOS 5 and SOS 13 were highly enriched in the junction but also stained extrasynaptic regions. These antigens can be distinguished from previously described components of the neuromuscular junction by their molecular weights, insensitivity to collagenase treatment, and solubility in 0.1% Triton X-100. Indirect evidence suggests that these species-specific antigens are located in the postsynaptic muscle membrane, but location in the junctional basal lamina or subsarcolemmal region cannot be excluded.

Developmental regulation of laminin accumulation in the extracellular matrix of a mouse muscle cell line

We have investigated the synthesis, accumulation, and secretion of laminin, an extracellular matrix glycoprotein, during differentiation of the C2 mouse skeletal muscle cell line in culture. Myoblasts actively synthesized laminin, as measured by incorporation of [35S]methionine and by a dot-immunobinding assay. In myoblast cultures laminin accumulated in an intracellular compartment and could be extracted with a physiological salt solution containing the detergent Triton X-100. After the culture medium was replaced to promote differentiation of myoblasts to myotubes, laminin synthesis was increased, and laminin began to accumulate in the medium in soluble form. During differentiation, laminin also accumulated in an insoluble cell-associated fraction that required guanidinium chloride for extraction. Indirect immunofluorescence and immunobinding assays showed that myotubes but not myoblasts contained laminin on their external surface. The time course of increase in surface laminin paralleled that of the accumulation of insoluble laminin. These results suggest that the insoluble fraction represents laminin bound to the extracellular matrix at the cell surface. Our experiments demonstrate, contrary to previous observations, that myotube cultures synthesize and accumulate laminin, and further, that the differentiation of proliferating myoblasts to multinucleated myotubes is accompanied by increased laminin synthesis, by secretion of laminin into the medium, and by the deposition of laminin into an extracellular matrix on the myotube surface.

Cellular and secreted forms of acetylcholinesterase in mouse muscle cultures

When grown in primary cell culture in the absence of neurons, muscle cells from a variety of species synthesize several forms of acetylcholinesterase (AChE), including the collagen-tailed A12 form. A12 AChE has been the subject of much study because it is thought to be a major functional enzyme form normally found in the basal lamina at the neuromuscular junction. In this paper, we show that muscle fibers derived from mouse embryos and neonates are also able to synthesize substantial percentages of their AChE as the A12 form when grown in vitro. This synthesis is modulated by a process associated with spontaneous muscle contractile activity since both total enzyme levels and the proportion of A12 AChE expressed on the cell surface are decreased when the cells are grown in the sodium channel blocker tetrodotoxin, which blocks muscle contraction. On the other hand, when the cells are treated with veratridine, which opens sodium channels, thereby mimicking one aspect of muscle contraction, their AChE levels are comparable to those of untreated cells. Although smaller in magnitude, these changes are similar to those seen in rat muscle cultures. A novel feature of mouse muscle cultures, not seen in those from rat and chick, is the presence of a secreted enzyme form that sediments in the same position as the cellular A12 form (when separated on sucrose density gradients containing high salt) and is also collagenase sensitive.

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

A 43-kDa peripheral membrane protein is known to copurify with acetylcholine receptor (AcChoR)-rich membranes isolated from the electric organ of Torpedo californica. Immunoelectron microscopy and crosslinking studies have demonstrated that this 43-kDa protein is closely associated with the cytoplasmic domain(s) of the AcChoR and suggest that the 43-kDa protein could regulate the distribution of the AcChoR in the postsynaptic membrane. This paper demonstrates that this postsynaptic protein appears at developing neuromuscular synapses in Xenopus nerve/muscle cocultures as early as AcChoRs become clustered at synaptic sites. Moreover, this protein is concentrated at AcChoR clusters that occur on noninnervated muscle cells. The close spatial and temporal relationship of this subsynaptic protein and AcChoR clusters is consistent with a role for the 43-kDa protein in the formation and/or stabilization of AcChoR clusters.

Basal lamina directs acetylcholinesterase accumulation at synaptic sites in regenerating muscle

In skeletal muscles that have been damaged in ways which spare the basal lamina sheaths of the muscle fibers, new myofibers develop within the sheaths and neuromuscular junctions form at the original synaptic sites on them. At the regenerated neuromuscular junctions, as at the original ones, the muscle fibers are characterized by junctional folds and accumulations of acetylcholine receptors and acetylcholinesterase (AChE). The formation of junctional folds and the accumulation of acetylcholine receptors is known to be directed by components of the synaptic portion of the myofiber basal lamina. The aim of this study was to determine whether or not the synaptic basal lamina contains molecules that direct the accumulation of AChE. We crushed frog muscles in a way that caused disintegration and phagocytosis of all cells at the neuromuscular junction, and at the same time, we irreversibly blocked AChE activity. New muscle fibers were allowed to regenerate within the basal lamina sheaths of the original muscle fibers but reinnervation of the muscles was deliberately prevented. We then stained for AChE activity and searched the surface of the new muscle fibers for deposits of enzyme they had produced. Despite the absence of innervation, AChE preferentially accumulated at points where the plasma membrane of the new muscle fibers was apposed to the regions of the basal lamina that had occupied the synaptic cleft at the neuromuscular junctions. We therefore conclude that molecules stably attached to the synaptic portion of myofiber basal lamina direct the accumulation of AChE at the original synaptic sites in regenerating muscle. Additional studies revealed that the AChE was solubilized by collagenase and that it remained adherent to basal lamina sheaths after degeneration of the new myofibers, indicating that it had become incorporated into the basal lamina, as at normal neuromuscular junctions.

Molecular forms of acetylcholinesterase in Xenopus muscle

Xenopus adult muscle, whole Xenopus embryos, and cultured embryonic myocytes together contain five acetylcholinesterase forms which can be resolved by sucrose density gradient centrifugation. These are identified as the collagenase-sensitive asymmetric forms A12 and A8, and the globular forms G4, G2, and G1. Asymmetric forms rise in whole embryos during the period of neuromuscular synapse formation, but their rise is not prevented by tricaine methanesulfonate, which abolishes motor activity. Aneural myocyte cultures synthesize primarily asymmetric acetylcholinesterase, much of which is extracellular. Prior nerve contact is not required for its expression. The proportion of asymmetric forms is neither decreased by tetrodotoxin, nor enhanced by veratridine and aconitine. We conclude that muscle activity does not modulate the expression of asymmetric acetylcholinesterase in Xenopus.

Rabies virus binding to cellular membranes measured by enzyme immunoassay

The binding of rabies virus to cellular membranes was measured using an enzyme-linked immunosorbent assay (ELISA). Virus binding to membranes adsorbed to the wells of microtiter plates was detected with rabies virus antibody and alkaline phosphatase-linked second antibody. The greatest degree of binding was to myotube, neuroblastoma, and salivary gland membranes; intermediate levels occurred in striated muscle and nerve membranes; and low levels of binding were found in other membranes, including those of most parenchymal organs. Binding of rabies virus to myotube membranes was saturable, dependent on pH (with an optimum of pH 6.0), facilitated by the divalent cations Ca++, Mn++, and Mg++, and was temperature dependent. Binding was greatly reduced by inactivation of virus with beta-propiolactone or treatment of virus with trypsin. In embryonic chick myotubes, total acetylcholine receptor content and acetylcholinesterase activity undergo marked changes during development, first increasing and then decreasing at the time of hatching. Binding of rabies virus followed a similar pattern, indicating that the virus may interact with the acetylcholine receptor or other surface molecules undergoing similar developmental changes.

Asymmetric molecular forms of acetylcholinesterase in mammalian skeletal muscles

Velocity sedimentation analysis of acetylcholinesterase (AChE) molecular forms was performed separately in endplate-rich and endplate-free regions of the diaphragm muscle of the rat, guinea pig, rabbit, dog, and pig, and in mm. erectores trunci and m. vastus lateralis in man. Several high-ionic-strength media were first tested to achieve better solubilization of AChE from rat muscles than by the usual 1 M NaCl-Triton X-100 medium. Ninety-five percent of the AChE from the motor endplate region of the rat diaphragm was solubilized in a single extraction step by medium containing 1 M lithium chloride instead of NaCl. Homologous molecular forms of AChE were found in all species. The asymmetric forms were invariably present in the endplate regions of muscles but their activity in endplate-free regions was much lower than in endplate regions in all investigated mammals except in man. Essentially the same pattern of AChE molecular forms was present in both regions in human muscles. High extrajunctional activity of the asymmetric forms makes human muscles similar to immature rodent muscles in vivo and in culture. The pattern of AChE molecular forms in the endplate region of the diaphragm in senile 24-month-old rats was not significantly different from that in 3-month-old animals. The persistence of the asymmetric AChE forms in the diaphragm of senile rats suggests that neuromuscular interactions do not become deficient with age in this muscle.

Membrane-related specializations associated with acetylcholine receptor aggregates induced by electric fields

The localization of membrane-associated specializations (basal lamina and cytoplasmic density) at sites of acetylcholine receptor (AChR) aggregation is consistent with an involvement of these structures in receptor stabilization. We investigated the occurrence of these specializations in association with AChR aggregates that develop at the cathode-facing edge of Xenopus muscle cells during exposure to a DC electric field. The cultures were labeled with a fluorescent conjugate of alpha-bungarotoxin and the receptor distribution on selected cells was determined before and after exposure to the field. In thin sections taken from the same cells, the cathode-facing edge was characterized by plaques of basal lamina and cytoplasmic density co-extensive with sarcolemma of increased density. In sections cut in a plane similar to the fluorescence image, it was possible to demonstrate that the specializations were concentrated at areas of field-induced AChR aggregation, and at receptor clusters existing on control cells. This finding further indicates that these structures participate in AChR stabilization, and that the mechanisms involved in AChR aggregation that result from field exposure and nerve contact may be similar.

Use of a replica technique to isolate muscle cell lines defective in expressing the acetylcholine receptor

We have isolated genetic variants of the C2 muscle cell line that are defective in expressing the acetylcholine receptor (AcChoR). Because the AcChoR is expressed only after C2 myoblasts have fused to form myotubes, we employed a replica technique to detect the variants. This technique yields two copies of each clone, one of which can be used for screening and the other, as a source of dividing cells. In a screening of about 10,000 clones derived from mutagenized cells, we found 2 that fused normally and expressed normal levels of acetylcholinesterase but had reduced amounts of AcChoR on their surface. One of these also had a reduced level of intracellular AcChoR, but, in the other, the amount of intracellular AcChoR was 5-fold higher than normal. Several variants were found that failed to fuse and had reduced levels of both AcChoR and acetylcholinesterase. Though we relied on 125I-labeled alpha-bungarotoxin to distinguish wild-type from deficient clones, we found that an antiserum to the AcChoR, followed by a biotinylated second antibody and a horseradish peroxidase-avidin complex, could also be used. Therefore, it should be possible to obtain muscle cell variants defective in the expression of a variety of proteins for which specific antibodies are available.

Extracellular matrix organization in developing muscle: correlation with acetylcholine receptor aggregates

Monoclonal antibodies recognizing laminin, heparan sulfate proteoglycan, fibronectin, and two apparently novel connective tissue components have been used to examine the organization of extracellular matrix of skeletal muscle in vivo and in vitro. Four of the five monoclonal antibodies are described for the first time here. Immunocytochemical experiments with frozen-sectioned muscle demonstrated that both the heparan sulfate proteoglycan and laminin exhibited staining patterns identical to that expected for components of the basal lamina. In contrast, the remaining matrix constituents were detected in all regions of muscle connective tissue: the endomysium, perimysium, and epimysium. Embryonic muscle cells developing in culture elaborated an extracellular matrix, each antigen exhibiting a unique distribution. Of particular interest was the organization of extracellular matrix on myotubes: the build-up of matrix components was most apparent in plaques overlying clusters of an integral membrane protein, the acetylcholine receptor (AChR). The heparan sulfate proteoglycan was concentrated at virtually all AChR clusters and showed a remarkable level of congruence with receptor organization; laminin was detected at 70-95% of AChR clusters but often was not completely co-distributed with AChR within the cluster; fibronectin and the two other extracellular matrix antigens occurred at approximately 20, 8, and 2% of the AChR clusters, respectively, and showed little or no congruence with AChR. From observations on the distribution of extracellular matrix components in tissue cultured fibroblasts and myogenic cells, several ideas about the organization of extracellular matrix are suggested. (a) Congruence between AChR clusters and heparan sulfate proteoglycan suggests the existence of some linkage between the two molecules, possibly important for regulation of AChR distribution within the muscle membrane. (b) The qualitatively different patterns of extracellular matrix organization over myotubes and fibroblasts suggest that each of these cell types uses somewhat different means to regulate the assembly of extracellular matrix components within its domain. (c) The limited co-distribution of different components within the extracellular matrix in vitro and the selective immune precipitation of each antigen from conditioned medium suggest that each extracellular matrix component is secreted in a form that is not complexed with other matrix constituents.

Development of basal lamina in synaptic and extrasynaptic portions of embryonic rat muscle

Each vertebrate skeletal muscle fiber is ensheathed by a basal lamina (BL) which passes through the synaptic cleft of the neuromuscular junction. In the adult, the synaptic portion of the BL is both functionally and chemically specialized. We have used an immunofluorescence method to compare the development of synaptic and extrasynaptic portions of BL in embryonic rat intercostal muscles. Immunohistochemical staining of adult muscle fibers with monoclonal and serum antibodies defines "synaptic" antigens (including acetylcholinesterase) that are concentrated in synaptic BL, "extrasynaptic" antigens that are concentrated in extrasynaptic regions, and "shared" antigens (including collagen IV, fibronectin, laminin, and a heparan sulfate proteoglycan) that are present in both synaptic and extrasynaptic BL ( Sanes and Chiu , 1983). Synapses appear on newly formed myotubes on embryonic Day 14 (E14; birth is on E22 ). Patches of BL that contain shared and extrasynaptic antigens are present on myotube surfaces by E15, and BL forms a continuous sheath by E17. Shared antigens are present at but not confined to synaptic areas by E15. Two synaptic antigens appear in synaptic areas a day later, and are not detectable extrasynaptically . At least one extrasynaptic antigen is present at immature synapses, and lost or masked by E19 . Thus synaptic BL is not assembled as a unit; rather, components are added, lost, or modified as synaptogenesis proceeds.

The location of neuromuscular junctions on regenerating adult mouse muscle in culture

The location of neuromuscular junctions which form in vitro between regenerating adult mouse muscle fibres and sections of embryonic mouse spinal cord was examined. The position of the original motor end-plates on the explanted muscle fibres was determined by using either rhodamine-labelled alpha-bungarotoxin (R alpha BT) binding to the acetylcholine receptors, or by stains to demonstrate acetylcholinesterase (AChE) also located at the end-plate. In this culture system, the explanted muscle fibres degenerate and regenerate to form new myotubes which develop cross-striations and contractions. The location of the newly-formed neuromuscular junctions in these mature cultures was then demonstrated using R alpha BT-binding to acetylcholine receptors, silver impregnation and cholinesterase techniques. Less than half the new neuromuscular junctions were at the original end-plate areas indicating that, at least in this system, junctions can form at sites other than those of the original end-plate.