Acetycholine receptor production and incorporation into membranes of developing muscle fibers

Molecular control of neuromuscular junction development

Skeletal muscle innervation is a multi-step process leading to the neuromuscular junction (NMJ) apparatus formation. The transmission of the signal from nerve to muscle occurs at the NMJ level. The molecular mechanism that orchestrates the organization and functioning of synapses is highly complex, and it has not been completely elucidated so far. Neuromuscular junctions are assembled on the muscle fibers at very precise locations called end plates (EP). Acetylcholine receptor (AChR) clusterization at the end plates is required for an accurate synaptic transmission. This review will focus on some mechanisms responsible for accomplishing the correct distribution of AChRs at the synapses. Recent evidences support the concept that a dual transcriptional control of AChR genes in subsynaptic and extrasynaptic nuclei is crucial for AChR clusterization. Moreover, new players have been discovered in the agrin-MuSK pathway, the master organizer of postsynaptical differentiation. Mutations in this pathway cause neuromuscular congenital disorders. Alterations of the postynaptic apparatus are also present in physiological conditions characterized by skeletal muscle wasting. Indeed, recent evidences demonstrate how NMJ misfunctioning has a crucial role at the onset of age-associated sarcopenia.

Formation of postsynaptic-like membranes during differentiation of embryonic stem cells in vitro

To analyze the formation of neuromuscular junctions, mouse pluripotent embryonic stem (ES) cells were differentiated via embryoid bodies into skeletal muscle and neuronal cells. The developmentally controlled expression of skeletal muscle-specific genes coding for myf5, myogenin, myoD and myf6, alpha 1 subunit of the L-type calcium channel, cell adhesion molecule M-cadherin, and neuron-specific genes encoding the 68-, 160-, and 200-kDa neurofilament proteins, synaptic vesicle protein synaptophysin, brain-specific proteoglycan neurocan, and microtubule-associated protein tau was demonstrated by RT-PCR analysis. In addition, genes specifically expressed at neuromuscular junctions, the gamma- and epsilon-subunits of the nicotinic acetylcholine receptor (AChR) and the extracellular matrix protein S-laminin, were found. At the terminal differentiation stage characterized by the formation of multinucleated spontaneously contracting myotubes, the myogenic regulatory gene myf6 and the AChR epsilon-subunit gene, both specifically expressed in mature adult skeletal muscle, were found to be coexpressed. Only the terminally differentiated myotubes showed a clustering of nicotinic acetylcholine receptors (AChR) and a colocalization with agrin and synaptophysin. The formation of AChRs was also demonstrated on a functional level by using the patch clamp technique. Taken together, our results showed that during ES cell differentiation in vitro neuron- and muscle-specific genes are expressed in a developmentally controlled manner, resulting in the formation of postsynaptic-like membranes. Thus, the embryonic stem cell differentiation model will be helpful for studying cellular interactions at neuromuscular junctions by "loss of function" analysis in vitro.

Localization and developmental expression of the NMDA receptor subunit NR2A in the mammalian retina

The localization of the N-methyl-D-aspartate receptor subunit NR2A was studied, by using light microscopic immunocytochemistry, in the retina of adult rat, rabbit, cat, and monkey. Strong, punctate immunolabeling was observed in the inner plexiform layer indicating a synaptic localization of the NR2A subunit. The punctate labeling was concentrated in two bands corresponding to the on- and off-sublaminae of the inner plexiform layer. The punctate character of immunofluorescence suggested a synaptic localization of the receptor. This was confirmed by electron microscopy of immunostained adult rat retina. The staining was localized postsynaptic to cone bipolar cells, and only one of the two postsynaptic elements of the dyad was labeled. Staining was not observed at extrasynaptic plasma membranes. In situ hybridization of adult rat retina showed expression of the NR2A subunit in virtually all ganglion cells and displaced amacrine cells in the ganglion cell layer and in a subset of amacrine cells in the inner nuclear layer. The postnatal developmental expression of the NR2A subunit was studied in rat retina by light microscopic immunocytochemistry. Punctate immunolabeling appeared prior to eye opening, and the developmental profile of NR2A could be compatible with a role in development of circuitry in the inner plexiform layer.

Asynaptic expression of the adult nicotinic acetylcholine receptor in long-term cultures of mammalian myotubes

To determine whether synaptic contact is required to express adult-type nicotinic acetylcholine receptors (A-AChR) in developing mammalian muscle, we have examined single-channel AChR activity in primary muscle cultures maintained for up to 29 days. A-AChRs were first expressed after day 12 in culture (CD12), during a period characterized by the accumulation of embryonic acetylcholine receptors (E-AChR). The highest rate of A-AChR expression was observed between CD15 and 19, during a period of maximal E-AChR accumulation. Although the level of A-AChR expression between individual patches was quite variable during this period, A-AChRs accounted for up to 40% of the events produced by receptors expressed over a 3-day interval. Between CD19 and 29, the density of E-AChRs diminished while the expression of A-AChRs per patch continued to increase but at a lower rate than that observed between CD15 and 19. In 25-29-day cultures, 70.6% of patches exhibited both E-AChR and A-AChR activity, and the percentage of A-AChR events per patch ranged between 0 and 47% with a mean of 11.7 +/- 3.2%. These results demonstrate that endogenous muscle mechanisms promote developmental increases in the expression of A-AChRs in myotubes that have no history of synaptic contact. This conclusion suggests that synaptic imprinting at developing junctions is mediated in part by endogenous muscle mechanisms, and does not require direct neurotrophic activation of epsilon mRNA transcription.

Mammalian muscle cells bear a cell-autonomous, heritable memory of their rostrocaudal position

We previously documented a greater than 100-fold rostrocaudal gradient of chloramphenicol acetyltransferase (CAT) expression in the muscles of adult mice that bear a myosin light chain-CAT transgene: successively more caudal muscles express successively higher levels of CAT. Here we studied the development and maintenance of this positional information in vitro. CAT levels reflect the rostrocaudal positions of the muscles from which the cells are derived in cultures established from adult muscles, in clones derived from individual adult myogenic (satellite) cells, in cultures prepared from embryonic myoblasts, and in cell lines derived by retrovirus-mediated transfer of an oncogene to satellite cells. Our results suggest that myoblasts bear a positional memory that is established in embryos, retained in adults, cell autonomous, heritable, stable to transformation, and accessible to study in clonal cell lines.

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

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

Activity-independent modulation of acetylcholine receptor levels in rat skeletal muscle following neonatal denervation

Following denervation of adult muscle, levels of acetylcholine receptor (AChR) increase; normal, low levels are restored only after muscle reinnervation. After neonatal denervation, we found a large initial increase in AChR levels during the first days postsurgery, as in adult denervated muscle. However, 1 week after denervation, total AChR levels decreased in the absence of any sign of reinnervation. By 3 weeks after surgery, near-normal levels of AChR were restored and extrajunctional AChR had disappeared. Thus, in sharp contrast to adult muscle, in young denervated muscle a down-regulation of AChR occurs without recovery of innervation and normal muscle contractile activity. These results suggest that different mechanisms regulate the levels of AChR in developing and adult skeletal muscle.

Acetylcholine receptor alpha-subunit mRNA is increased by ascorbic acid in cloned L5 muscle cells: Northern blot analysis and in situ hybridization

Ascorbic acid is the major factor in brain extract responsible for increasing the average acetylcholine receptor (AChR) site density on the cloned muscle cell line L5. In the present study, we show that this effect of ascorbic acid requires mRNA synthesis, and that the mRNA level for the AChR alpha-subunit is increased to about the same level as are the surface receptors. We have found no increase in the mRNA levels of the beta-, gamma-, and delta-subunits, or in the mRNAs of other muscle-specific proteins, such as that of light chain myosin 2, alpha-actin, and creatine kinase. By in situ hybridization, we further show that the increase in alpha-mRNA in response to ascorbic acid is exclusively in myotubes and is located near clusters of nuclei. mRNA levels for the alpha-subunit in mononucleated cells are very low and do not significantly increase in response to ascorbic acid. The mononucleated cells are thus excluded as a possible source for the increase in alpha-subunit mRNA detected by Northern blot analysis. Our results indicate that there is a very specific action of ascorbic acid on the regulation of AChR alpha-mRNA in the L5 muscle cells, and that the expression of surface receptors in these cells is limited by the amount of AChR alpha-subunit mRNA.

Developmental changes in the open time and conductance of acetylcholine receptors in aneural cultured Xenopus myocytes treated with cycloheximide or tunicamycin

Aneural cultures of Xenopus myocytes were treated with cycloheximide or tunicamycin in order to determine the influence of continued protein synthesis and insertion of glycosylated acetylcholine (ACh) receptor proteins on developmental changes in ACh-activated single-channel events. The developmental stage of the cultures was estimated from the stage of intact embryos maintained at the same temperature as the cultures. Single-channel recordings (5 x 10(-7) M ACh) from untreated cultures revealed primarily low conductance (27.4 +/- 0.6 pS) events for the first 14 h in culture (temperature = 23-25 degrees C) and a second high conductance class of events (42.5 +/- 0.9 pS) at later developmental stages (after Stage 28). Treatment with cycloheximide (at Stage 28) or tunicamycin (at Stage 22) produced significantly (P less than 0.01) fewer high conductance events at later stages (Stages 31-47), but had no effect on the conductance or open time of the low conductance event. A significant decrease (Chi square, P less than 0.05) in ACh sensitivity was observed at Stage 27 in myocytes treated with tunicamycin at Stage 22. The results strongly suggest that the low and high conductance events represent two distinct receptor molecules synthesized at different developmental stages. The tunicamycin results also suggest that a developmental reduction in the mean open time of low conductance events in untreated cultures does not depend upon the continued insertion of newly synthesized receptors.

Transcriptional and translational requirements for developmental alterations in acetylcholine receptor channel function in Xenopus myotomal muscle

Two functionally distinct types of acetylcholine (ACh) receptor channels are present on embryonic Xenopus myotomal muscle. During differentiation of this muscle, both in vivo and in dissociated cell culture, the occurrence of the high conductance "fast" channel type increases relative to the low conductance "slow" channel type. In order to ascertain whether new receptor synthesis is required for this rapid switch in channel types we examined the effects of inhibitors of transcription (alpha-amanitin and actinomycin D) and translation (cycloheximide) on developing muscle in culture. Inhibition of protein synthesis resulted in greater than 95% reduction in ACh receptor incorporation and also reversibly blocked the developmental appearance of the high conductance channel type. Inhibition of mRNA synthesis only slightly reduced the rate of receptor incorporation into muscle membrane over a 24-hr period but reversibly blocked appearance of the high conductance channel. These findings suggest that the high conductance ACh receptor channel type does not result from post-translational modifications of the low conductance type, but rather from transcription of a different mRNA encoding one or more of the ACh receptor subunits.

Acetylcholine receptor clusters are associated with nuclei in rat myotubes

Clustered and diffuse acetylcholine receptors are present in cultured myotubes. These clustered AChRs represent regions of myotube membrane containing high receptor density. We have studied the distribution of the AChR clusters and nuclei to determine whether there is an association in the distribution of nuclei beneath AChR clusters. AChR clusters were visualized with alpha-bungarotoxin conjugated to tetramethylrhodamine (alpha BTX-TMR) and the nuclei were stained with bisbenzimide which binds specifically to DNA. This double label procedure, and the computerized analysis of the data allowed us to determine the distribution of nuclei and AChR clusters in the same myotube. During early stages of myotube development the nuclei formed aggregates which were comprised of 4 to 10 nuclei in close apposition to one another. This association of AChR clusters with nuclear aggregates was greatest at Day 4 after plating. As the number of nuclear aggregates associated with clusters decreased the number of nuclei in the aggregates also decreased and the AChR clusters decreased in size as well as number. At all time points examined, the concentration of myotube nuclei in the cells was 3 to 12 times higher beneath areas of AChR clusters than away from clusters. Our computerized analysis shows that there is an association of the AChR clusters with the nuclear region during myotube development.

Insertion and internalization of acetylcholine receptors at clustered and diffuse domains on cultured myotubes

Two populations of acetylcholine receptors (AChRs) are present in cultured myotubes. One forms large aggregates or clusters and the other has a much lower density of AChRs, which are diffusely distributed. Both clustered and diffuse AChRs are inserted and removed (internalized) from the sarcolemma. To determine the insertion and removal rates of AChRs in these two plasma membrane domains, we used a double label technique to distinguish and quantitate newly inserted and "old" AChRs. Application of our method revealed that the rate of AChR internalization is the same at the clustered and diffuse regions of the plasma membrane, whereas the rate of insertion is threefold greater at the clusters than elsewhere in the plasma membrane. Thus, the increase in AChR number at the clusters is not due to an increase in their half-life, but to an increase in their rate of insertion.

Some properties of acetylcholine receptors in human cultured myotubes

The distribution and single channel properties of acetylcholine (ACh) receptors in human myotubes grown in tissue culture have been examined. Radioautography of myotubes labelled with [125I]alpha-bungarotoxin showed that ACh receptors are distributed uniformly over the myotube surface at a density of 3.9 +/- 0.5 receptors per square micrometre. Accumulations of ACh receptors (hot spots) were found rarely. The conductance and kinetics of ACh-activated channels were investigated with the patch-clamp technique. Cell-attached membrane patches were used in all experiments. A single channel conductance in the range 40-45 pS was calculated. No sublevels of conductance (substates) of the activated channel were observed. The distribution of channel open-times varied with ACh concentration. With 100 nM ACh, the distribution was best fitted by the sum of two exponentials, whereas with 1 microM ACh a single exponential could be fitted. The mean channel open-time at the myotube resting potential (ca. -70 mV, 22 degrees C) was 8.2 ms. The distribution of channel closed-times was complex at all concentrations of ACh studied (100 nM to 10 microM). With desensitizing doses of ACh (10 microM), channel openings occurred in obvious bursts; each burst usually appeared as part of a 'cluster' of bursts. Both burst duration and mean interval between bursts increased with membrane hyperpolarization. Individual channel open-times and burst durations showed similar voltage dependence (e-fold increase per 80 mV hyperpolarization), whereas both the channel closed-times within a burst and the number of openings per burst were independent of membrane potential.

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.

Freeze-fracture and electrophysiological studies of newly developed acetylcholine receptors in Xenopus embryonic muscle cells

The development of acetylcholine receptors on Xenopus embryonic muscle cells both in culture and in situ was studied using electrophysiology and freeze-fracture electron microscopy. Acetylcholine sensitivity first appeared at developmental stage 20 and gradually increased up to about stage 31. Freeze-fracture of muscle cells that were nonsensitive to acetylcholine revealed diffusely distributed small P-face intramembraneous particles. When cells acquired sensitivity to acetylcholine, a different group of diffusely distributed large P-face particles began to appear. This group of particles was analyzed by subtracting the size distribution found on nonsensitive cells from that found on sensitive cells. We call this group of particles difference particles. The sizes of difference particles were large (peak diameter 11 nm). The density of difference particles gradually increased with development. The density of small particles (less than 9 nm) did not change with development. At later stages (32-36) aggregates of large particles appeared, which probably represent acetylcholine receptor clusters. The size distribution of difference particles was close to that of the aggregated particles, suggesting that at least part of difference particles represent diffusely distributed acetylcholine receptors. Difference particles exist mostly in solitary form (occasionally double), indicating that an acetylcholine receptor can be functional in solitary form. This result also shows that diffuse acetylcholine receptors that have previously been observed with 125I-alpha-bungarotoxin autoradiography do indeed exist in solitary forms not as microaggregates.

Differential response of satellite cells and embryonic myoblasts to a tumor promoter

Treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) reversibly suppressed myotube formation and expression of acetylcholine receptors in cultures of Day 15 mouse embryo presumptive myoblasts, but was totally ineffective in cultures of adult mouse satellite cells. A subpopulation of TPA-resistant myogenic cells became apparent in cultures prepared from older embryos or newborn mice. Thus, limb presumptive myoblasts are a heterogeneous population, and part of the distinct TPA-resistant subpopulation may represent satellite cell precursors.

Effect of cycloheximide on ionic channels in neuroblastoma cell membrane

The effect of cycloheximide (an inhibitor of protein synthesis) on the ionic currents through sodium and potassium channels was investigated in dialysed voltage-clamped N18 A-1 neuroblastoma cells. The cycloheximide concentration needed for half-inhibition of sodium peak conductance was about 0.5 micrograms/ml for 24 h of incubation. Half-inhibition time of the sodium peak conductance in cells incubated with 15.0 micrograms/ml of cycloheximide was about 9 h. Sodium against potassium ion selectivity, the activation and inactivation parameters were shown to be not affected by cycloheximide. Potassium conductance in similarly treated cells exhibited no consistent changes. The main conclusion is that the decay in peak sodium conductance is caused by diminishing the sodium channel density in the membrane (from 25 to 2.2 channels per micron2). The inhibition effect was evidently mediated by block of protein synthesis and was not the result of direct drug-channel interaction. The half-decay time of sodium peak conductance is interpreted as a possible life-time characteristic of sodium channels in the neuroblastoma cell membrane.

Acetylcholine receptors and cholinesterase in developing chick skeletal muscle fibers

The developmental changes in the distribution of acetylcholine receptors (AChRs) and cholinesterase (ChE) were investigated in the posterior latissimus dorsi (PLD) muscle of chick embryos by double staining with rhodamine-labeled erabutoxin b (TMR-Eb) for AChRs and Karnovsky's method for ChE. During the development, the TMR-Eb and ChE positive areas changed in their shapes and sizes. In early stages, the TMR-Eb positive areas appeared as fine fluorescent dots of about 0.3 micrometer in diameter or small aggregates of such dots. These areas then became enlarged and exhibited the following sequential changes in their configuration: spindle-, round-, ring-, C- and finally tree-shaped. The transformation in the configuration of the areas appeared to be caused by the changes of the fluorescent dots in their number and distribution. Simultaneous staining with ChE revealed that at early stages the TMR-Eb positive areas were not stained with ChE, but then they were stained later. The ChE deposits usually accumulated at the borders of the TMR-Eb positive areas and thereby outlined them. Electron microscopy using horseradish peroxidase-labeled erabutoxin b revealed that the fluorescent dots represent the discrete regions of high AChR concentration at the muscle membrane.

Interaction of the cytoskeletal framework with acetylcholine receptor on th surface of embryonic muscle cells in culture

To monitor the interaction of cell surface acetylcholine (AcCho) receptors with the cytoskeleton, cultured muscle cells were labeled with radioactive or fluorescent alpha-bungarotoxin and extracted with Triton X-100, using conditions that preserve internal structure. A significant population of the AcCho receptors is retained on the skeletal framework remaining after detergent extraction. The skeleton organization responsible for restricting AcCho receptors to a patched region may also result in their retention after detergent extraction.

In vitro synthesis, glycosylation, and membrane insertion of the four subunits of Torpedo acetylcholine receptor

We have characterized the early biosynthetic forms of the Torpedo electroplax acetylcholine receptor by using a cell-free protein synthesizing system. We obtained primary translation products of approximately 38, 50, 49, and 60 kilodaltons for the alpha, beta, gamma, and delta polypeptides, respectively, by using immunoprecipitation with subunit-specific antisera. These chains could each be labeled by the formylated initiator [35S]Met-tRNA. On cotranslational incubation with pancreatic rough microsomes, glycosylated forms of each subunit were obtained that had molecular weights close to those of their mature authentic counterparts. Extensive trypsinization reduced the glycosylated forms of the receptor subunits to glycosylated membrane-protected fragments of approximately 35 (alpha), 37 (beta), 45 (gamma), and 44 (delta) kilodaltons. In this system, then, each receptor chain spans the membrane at least once. This in vitro-synthesized material apparently exhibited neither oligomeric assembly nor alpha-bungarotoxin binding.

Changes in the surface membrane during myoblast fusion

1. During fusion of chick-embryo myoblasts in culture, the surface membrane is affected as follows. Uptake of 2-aminoisobutyrate and 2-deoxyglucose, each of which is concentrated 20-fold relative to its concentration in the medium, is unaltered; uptake of alpha-methyl glucoside and choline (15 mM), each of which equilibrates relative to its concentration in the medium, approximately doubles. An approximate doubling also occurs in iodinatable surface protein (and in total protein) and in cell surface area as judged by light-microscopy. Adenylate cyclase (in the absence or the presence of fluoride) increases by more than 2-fold. 2. It is concluded that, during myoblast fusion cells increase in size, and this is reflected in an increased rate of simple diffusion; the rate of facilitated processes such as the uptake of amino acids and sugars, on the other hand, remains unaltered, though the activity of certain enzymes is increased. These results indicate that specific changes in the function of surface membrane occur during myoblast fusion in vitro.

Acetylcholine receptors in cultured human muscle cells

Primary cultures prepared from human muscle biopsies were examined for the presence and distribution of acetylcholine receptors, as measured by the binding of 125iodine-labelled alpha-bungarotoxin. The toxin bound to the human muscle cultures with a similar time dependence and specificity as found in muscle cultures from other species. The amount of toxin bound was lower than that obtained for neonatal mouse muscle under similar conditions. The distribution of receptors was similar in cultures derived from the muscles of patients with a variety of neuromuscular disorders. The toxin was located along the myotubes in a fairly even distribution; however, variations in labelling along a single myotube were observed, as well as variations between different myotubes in the same culture. Occasionally, the toxin also bound to other cells, which may have been mononucleated. Cultures prepared from patients with Duchenne muscular dystrophy produced multi-layered cell clusters, instead of the usual monolayer of cells. Within these clusters, only the myotubes bound alpha-bungarotoxin.

Development of synaptic ultrastructure at neuromuscular contacts in an amphibian cell culture system

Cultures of dissociated myotomal muscle and spinal cord derived from embryos of Xenopus laevis were grown in the presence of curare in order to abolish neuromuscular activity and were examined by electron microscopy. In one-day-old cultures a few of the neuromuscular contacts already displayed several synaptic specializations including 500 A vesicles clustered against the axolemma, increased axolemmal densities, basal lamina in the cleft, an increased sarcolemmal density and subsarcolemmal filamentous material. Contacts with these specializations were observed more frequently in two and three-day-old cultures. Throughout the three-day culture period nerve fibres and neuromuscular contacts were devoid of Schwann cells. Isolated patches of basal lamina were relatively scarce and were usually accompanied by an increase in sarcolemmal density and subsarcolemmal filamentous material even in cultures in which spinal cord cells were not included. These observations indicate that the myotomal neuromuscular synapse differentiates in culture in much the same way as it does in vivo, that muscle contractions are not required for its differentiation, and that apparent postsynaptic specializations can develop in the absence of innervation.

Appearance of new alpha-bungarotoxin-acetylcholine receptors in cultured sympathetic ganglia of chick embryos

alpha-Bungarotoxin (alpha-BuTX) has been used as a marker for studying the production of alpha-bungarotoxin-acetylcholine receptors (alpha-BuTX-AChRs) in explants of chick embryo sympathetic ganglia cultured in vitro. New alpha-BuTX-AChRs appear rapidly in the explants after blocking of the pre-existent ones with the toxin (40% of the total receptors at 3 h). There is a portion of alpha-BuTX-AChRs in the explants which for a short time is not accessible to the toxin. This portion constitutes the precursor pool of receptors and represents 18% of the total. The precursor pool of receptors supplies the neurons with new receptors for 1-2 h in the absence of protein synthesis. The appearance of new receptors from the precursor pool is an energy-dependent process.

A possible modulation of acetylcholine receptors of embryonic chick muscle cells by alpha-bungarotoxin

Acetylcholine receptors were assayed with alpha-bugarotoxin on embryonic chick skeletal muscle growing in primary cell culture. Toxin was bound specifically to muscle cells and could be competed with D-tubocurarine. Two dissociation constants were obtained by equilibrium binding: 7.2 x 10(-9)M and 2.7 x 10(-7)M at 25 degrees C. Two sets of rate constants were also obtained from dissociation kinetics. There are five times more low affinity sites on cells than high affinity sites. The average density of high-affinity receptors is about 200/micrometers2. A time course of toxin binding to receptors at 37 degrees C in growth medium revealed that under conditions permitting growth and metabolism, toxin bound to cells was lost. The possibility that the growth medium was inactivating toxin molecules was ruled out by showing that unbound toxin molecules in the medium were fully capable of binding to fresh cultures.

Metabolism of acetylcholine receptor in chick embryo muscle cells: effects of RSV and PMA

We have investigated some aspects of the metabolism of the integral membrane protein acetylcholine receptor (AChR) in normal and transformed cultures of chick embryo muscle cells. Turnover of AChR in control muscle cell cultures was compared with turnover in cultures infected and transformed by a temperature-sensitive mutant of Rous sarcoma virus (RSV) and with cultures treated with the tumor promoter phorbol myristate acetate (PMA). The parameters of AChR metabolism were estimated using 125I-alpha-bungarotoxin as a stoichiometric high affinity ligand for the AChR. We found that both RSV transformation and PMA increased the rate of degradation and decreased the rate of synthesis of AChR. The consequent reduction in steady state receptor levels suggests that oncogenic transformation and tumor promoter significantly alter the metabolism of cell surface membranes. We also observed that parameters of AChR metabolism in control cultures changed systematically in a pattern which depended upon the age of the culture as well as the use of embryo extract or fetal bovine serum as medium supplements. The muscle cell system allows quantitative measurement of an integral membrane protein and its metabolism, and may serve as a more general model for alterations in membrane and surface receptor metabolism associated with the transformed state.

Time course of appearance of alpha-bungarotoxin binding sites during development of chick ciliary ganglion and iris

The binding of [125I]alpha-bungarotoxin (ABTX) to homogenates of ciliary ganglia and irises from embryonic and posthatching chickens has been examined. Specific, high-affinity binding was found in both tissues [KD (iris) equals 2.5 nM; KD (ganglion) equals 2.7 nM]. Binding is saturated above 10 nM toxin concentration and is inhibited by low concentrations of the nicotinic antagonist d-tubocurarine. The binding may be associated with a nicotinic cholinergic receptor in both tissues. The amount of binding in the iris begins to increase soon after functional innervation is first observed, at 12 days of incubation (d.i.), and continues to increase up to four months after hatching (a.h.), the oldest age tested. In contrast, ABTX binding in the ciliary ganglion increases fourfold between 7 and 11 d.i., after which the amount of binding remains unchanged up to four months a.h. When compared to the development of choline acetyltransferase (ChAc) and acetylcholinesterase (AChE) activities in the ganglion and iris, ABTX binding follows a pattern similar to that of AChE activity. The largest increases in ChAc activity occur later than those of the postsynaptic markers. After 16 d.i. there are approximately 3 x 10(6) toxin molecules bound per neuron in the ciliary ganglion.

Interrelated lipid alterations and their influence on the proliferation and fusion of cultured myogenic cells

We have cultured myogenic cells derived from primary explants and a cell line (L6) in a lipid-depleted medium (LDM) and produced large alterations of the fatty acyl and polar headgroup composition and of the cellular sterol levels. These alterations were produced by altering the composition of the media as follows: removing biotin and providing exogenous fatty acid; removing choline and providing exogenous ethanolamine or choline analogues; and by adding 25-OH cholesterol, an inhibitor of 3-hydroxy-3-methylglutarate (HMG)-CoA reductase. Relatively small, secondary alterations of other lipid classes accompany the large primary alteration. In general, they are not obviously compensatory for the primary alteration by retaining some physical property. We have explored the influence of these lipid alterations on myoblast proliferation and fusion into myotubes. In general, considerable variability appears tolerated, but there also appear to be limits. Long-term cultures grown in media containing a single fatty acid do not proliferate indefinitely, and the fatty acid does not become the sole fatty acyl component of the phospholipids. This phenomenon is also observed for cultures enriched in phosphatidylethanolamine (PE) or phosphatidyldimethylethanolamine (PDME). The influence of the lipid alterations on fusion is particularly interesting. The inclusion of 25-OH cholesterol inhibits fusion. Enrichment of the fatty acyl chains with elaidate or the polar headgroups with PE also inhibits fusion, but in contrast to that by 25-OH cholesterol, a significant fraction of the myoblasts are aligned and interacting with each other. Oleate enrichment enhances the rate of fusion.