Development of the neuromuscular junction in the chick embryo: the number, distribution, and stability of acetylcholine receptors

The Metabolic Stability of the Nicotinic Acetylcholine Receptor at the Neuromuscular Junction

The clustering and maintenance of nicotinic acetylcholine receptors (AChRs) at high density in the postsynaptic membrane is a hallmark of the mammalian neuromuscular junction (NMJ). The regulation of receptor density/turnover rate at synapses is one of the main thrusts of neurobiology because it plays an important role in synaptic development and synaptic plasticity. The state-of-the-art imaging revealed that AChRs are highly dynamic despite the overall structural stability of the NMJ over the lifetime of the animal. This review highlights the work on the metabolic stability of AChRs at developing and mature NMJs and discusses the role of synaptic activity and the regulatory signaling pathways involved in the dynamics of AChRs.

Motor function recovery: deciphering a regenerative niche at the neuromuscular synapse

The coordinated movement of many organisms relies on efficient nerve–muscle communication at the neuromuscular junction (NMJ), a peripheral synapse composed of a presynaptic motor axon terminal, a postsynaptic muscle specialization, and non‐myelinating terminal Schwann cells. NMJ dysfunctions are caused by traumatic spinal cord or peripheral nerve injuries as well as by severe motor pathologies. Compared to the central nervous system, the peripheral nervous system displays remarkable regenerating abilities; however, this capacity is limited by the denervation time frame and depends on the establishment of permissive regenerative niches. At the injury site, detailed information is available regarding the cells, molecules, and mechanisms involved in nerve regeneration and repair. However, a regenerative niche at the final functional step of peripheral motor innervation, i.e. at the mature neuromuscular synapse, has not been deciphered. In this review, we integrate classic and recent evidence describing the cells and molecules that could orchestrate a dynamic ecosystem to accomplish successful NMJ regeneration. We propose that such a regenerative niche must ensure at least two fundamental steps for successful NMJ regeneration: the proper arrival of incoming regenerating axons to denervated postsynaptic muscle domains, and the resilience of those postsynaptic domains, in morphological and functional terms. We here describe and combine the main cellular and molecular responses involved in each of these steps as potential targets to help successful NMJ regeneration.

Vezatin is required for the maturation of the neuromuscular synapse

Key genes, such as Agrin, Lrp4, and MuSK, are required for the initial formation, subsequent maturation, and long-term stabilization of mammalian neuromuscular synapses. Additional molecules are thought to function selectively during the evolution and stabilization of these synapses, but these molecular players are largely unknown. Here, we used mass spectrometry to identify vezatin, a two-pass transmembrane protein, as an acetylcholine receptor (AChR)–associated protein, and we provide evidence that vezatin binds directly to AChRs. We show that vezatin is dispensable for the formation of synapses but plays a later role in the emergence of a topologically complex and branched shape of the synapse, as well as the stabilization of AChRs. In addition, neuromuscular synapses in vezatin mutant mice display premature signs of deterioration, normally found only during aging. Thus, vezatin has a selective role in the structural elaboration and postnatal maturation of murine neuromuscular synapses.

Neuregulin1 fine-tunes pre-, post-, and perisynaptic neuromuscular junction development

BACKGROUND

Neuromuscular junction (NMJ) development is a multistep process mediated by coordinated interactions between the nerve terminal, target muscle, and perisynaptic Schwann cell that require constant back-and-forth communication. Retrograde and anterograde growth and differentiation factors have been postulated to participate in this communication. While neuregulin1 (NRG1) has been shown to be potent anterograde signal that activates acetylcholine receptor (AChR) transcription and clustering in vitro, its roles in NMJ development in vivo remain elusive.

RESULTS

Using the model of chicken embryo, we measured the effects of NRG1 signaling during NMJ development in ovo using quantitative, sequential measures of AChR cluster size and density, pre- and postsynaptic apposition, and the alignment of perisynaptic Schwann cells. Using in ovo electroporation at early stages and a targeted soluble neuregulin antagonist through all developmental stages, we found soluble NRG1 regulates AChR cluster density and size at the earliest stage prior to nerve-AChR cluster contact. Once the nerve contacts with muscle AChRs, NRG1 has pronounced effects on presynaptic specialization and on the alignment of perisynaptic Schwann cells at endplates.

CONCLUSION

These findings suggest that, while NRG1 may not be critical for overall development, it appears to be important in fine-tuning pre-, post-, and perisynaptic development of the NMJ. Developmental Dynamics 246:368-380, 2017. © 2017 Wiley Periodicals, Inc.

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.

Translational regulation of acetylcholinesterase by the RNA-binding protein Pumilio-2 at the neuromuscular synapse

Acetylcholinesterase (AChE) is highly expressed at sites of nerve-muscle contact where it is regulated at both the transcriptional and post-transcriptional levels. Our understanding of the molecular mechanisms underlying its regulation is incomplete, but they appear to involve both translational and post-translational events as well. Here, we show that Pumilio-2 (PUM2), an RNA binding translational repressor, is highly localized at the neuromuscular junction where AChE mRNA concentrates. Immunoprecipitation of muscle cell extracts with a PUM2 specific antibody precipitated AChE mRNA, suggesting that PUM2 binds to the AChE transcripts in a complex. Gel shift assays using a bacterially expressed PUM2 RNA binding domain showed specific binding using wild type AChE 3'-UTR RNA segment that was abrogated by mutation of the consensus recognition site. Transfecting skeletal muscle cells with shRNAs specific for PUM2 up-regulated AChE expression, whereas overexpression of PUM2 decreased AChE activity. We conclude that PUM2 binds to AChE mRNA and regulates AChE expression translationally at the neuromuscular synapse. Finally, we found that PUM2 is regulated by the motor nerve suggesting a trans-synaptic mechanism for locally regulating translation of specific proteins involved in modulating synaptic transmission, analogous to CNS synapses.

Aberrant patterning of neuromuscular synapses in choline acetyltransferase-deficient mice

In this study we examined the developmental roles of acetylcholine (ACh) by establishing and analyzing mice lacking choline acetyltransferase (ChAT), the biosynthetic enzyme for ACh. As predicted, ChAT-deficient embryos lack both spontaneous and nerve-evoked postsynaptic potentials in muscle and die at birth. In mutant embryos, abnormally increased nerve branching occurs on contact with muscle, and hyperinnervation continues throughout subsequent prenatal development. Postsynaptically, ACh receptor clusters are markedly increased in number and occupy a broader muscle territory in the mutants. Concomitantly, the mutants have significantly more motor neurons than normal. At an ultrastructural level, nerve terminals are smaller in mutant neuromuscular junctions, and they make fewer synaptic contacts to the postsynaptic muscle membrane, although all of the typical synaptic components are present in the mutant. These results indicate that ChAT is uniquely essential for the patterning and formation of mammalian neuromuscular synapses.

Neuregulin expression at neuromuscular synapses is modulated by synaptic activity and neurotrophic factors

The proper formation of neuromuscular synapses requires ongoing synaptic activity that is translated into complex structural changes to produce functional synapses. One mechanism by which activity could be converted into these structural changes is through the regulated expression of specific synaptic regulatory factors. Here we demonstrate that blocking synaptic activity with curare reduces synaptic neuregulin expression in a dose-dependent manner yet has little effect on synaptic agrin or a muscle-derived heparan sulfate proteoglycan. These changes are associated with a fourfold increase in number and a twofold reduction in average size of synaptic acetylcholine receptor clusters that appears to be caused by excessive axonal sprouting with the formation of new, smaller acetylcholine receptor clusters. Activity blockade also leads to threefold reductions in brain-derived neurotrophic factor and neurotrophin 3 expression in muscle without appreciably changing the expression of these same factors in spinal cord. Adding back these or other neurotrophic factors restores synaptic neuregulin expression and maintains normal end plate band architecture in the presence of activity blockade. The expression of neuregulin protein at synapses is independent of spinal cord and muscle neuregulin mRNA levels, suggesting that neuregulin accumulation at synapses is independent of transcription. These findings suggest a local, positive feedback loop between synaptic regulatory factors that translates activity into structural changes at neuromuscular synapses.

Sodium channel distribution on uninnervated and innervated embryonic skeletal myotubes

Acetylcholine receptor (AChR) and sodium (Na(+)) channel distributions within the membrane of mature vertebrate skeletal muscle fibers maximize the probability of successful neuromuscular transmission and subsequent action potential propagation. AChRs have been studied intensively as a model for understanding the development and regulation of ion channel distribution within the postsynaptic membrane. Na(+) channel distributions have received less attention, although there is evidence that the temporal accumulation of Na(+) channels at developing neuromuscular junctions (NMJs) may differ between species. Even less is known about the development of extrajunctional Na(+) channel distributions. To further our understanding of Na(+) channel distributions within junctional and extrajunctional membranes, we used a novel voltage-clamp method and fluorescent probes to map Na(+) channels on embryonic chick muscle fibers as they developed in vitro and in vivo. Na(+) current densities on uninnervated myotubes were approximately one-tenth the density found within extrajunctional regions of mature fibers, and showed several-fold variations that could not be explained by a random scattering of single channels. Regions of high current density were not correlated with cellular landmarks such as AChR clusters or myonuclei. Under coculture conditions, AChRs rapidly concentrated at developing synapses, while Na(+) channels did not show a significant increase over the 7 day coculture period. In vivo investigations supported a significant temporal separation between Na(+) channel and AChR aggregation at the developing NMJ. These data suggest that extrajunctional Na(+) channels cluster together in a neuronally independent manner and concentrate at the developing avian NMJ much later than AChRs.

Neural regulation of alpha-dystroglycan biosynthesis and glycosylation in skeletal muscle

Alpha-dystroglycan (alpha-DG) is part of a complex of cell surface proteins linked to dystrophin or utrophin, which is distributed over the myofiber surface and is concentrated at neuromuscular junctions. In laminin overlays of muscle extracts from developing chick hindlimb muscle, alpha-DG first appears at embryonic day (E) 10 with an apparent molecular mass of 120 kDa. By E15 it is replaced by smaller (approximately 100 kDa) and larger (approximately 150 kDa) isoforms. The larger form increases in amount and in molecular mass (>200 kDa) as the muscle is innervated and the postsynaptic membrane differentiates (E10-E20), and then decreases dramatically in amount after hatching. In myoblasts differentiating in culture the molecular mass of alpha-DG is not significantly increased by their replication, fusion, or differentiation into myotubes. Monoclonal antibody IIH6, which recognizes a carbohydrate epitope on alpha-DG, preferentially binds to the larger forms, suggesting that the core protein is differentially glycosylated beginning at E16. Consistent with prior observations implicating the IIH6 epitope in laminin binding, the smaller forms of alpha-DG bind more weakly to laminin affinity columns than the larger ones. In blots of adult rat skeletal muscle probed with radiolabeled laminin or monoclonal antibody IIH6, alpha-DG appears as a >200-kDa band that decreases in molecular mass but increases in intensity following denervation. Northern blots reveal a single mRNA transcript, indicating that the reduction in molecular mass of alpha-DG after denervation is not obviously a result of alternative splicing but is likely due to posttranslational modification of newly synthesized molecules. The regulation of alpha-DG by the nerve and its increased affinity for laminin suggest that glycosylation of this protein may be important in myofiber-basement membrane interactions during development and after denervation.

Development of the vertebrate neuromuscular junction

We describe the formation, maturation, elimination, maintenance, and regeneration of vertebrate neuromuscular junctions (NMJs), the best studied of all synapses. The NMJ forms in a series of steps that involve the exchange of signals among its three cellular components--nerve terminal, muscle fiber, and Schwann cell. Although essentially any motor axon can form NMJs with any muscle fiber, an additional set of cues biases synapse formation in favor of appropriate partners. The NMJ is functional at birth but undergoes numerous alterations postnatally. One step in maturation is the elimination of excess inputs, a competitive process in which the muscle is an intermediary. Once elimination is complete, the NMJ is maintained stably in a dynamic equilibrium that can be perturbed to initiate remodeling. NMJs regenerate following damage to nerve or muscle, but this process differs in fundamental ways from embryonic synaptogenesis. Finally, we consider the extent to which the NMJ is a suitable model for development of neuron-neuron synapses.

Expression patterns of transmembrane and released forms of neuregulin during spinal cord and neuromuscular synapse development

We mapped the distribution of neuregulin and its transmembrane precursor in developing, embryonic chick and mouse spinal cord. Neuregulin mRNA and protein were expressed in motor and sensory neurons shortly after their birth and levels steadily increased during development. Expression of the neuregulin precursor was highest in motor and sensory neuron cell bodies and axons, while soluble, released neuregulin accumulated along early motor and sensory axons, radial glia, spinal axonal tracts and neuroepithelial cells through associations with heparan sulfate proteoglycans. Neuregulin accumulation in the synaptic basal lamina of neuromuscular junctions occurred significantly later, coincident with a reorganization of muscle extracellular matrix resulting in a relative concentration of heparan sulfate proteoglycans at endplates. These results demonstrate an early axonal presence of neuregulin and its transmembrane precursor at developing synapses and a role for heparan sulfate proteoglycans in regulating the temporal and spatial sites of soluble neuregulin accumulation during development.

Evidence for a neural inhibitory factor which downregulates fetal-type acetylcholine receptor expression in skeletal muscle cell lines

Nerve-evoked muscle depolarisation plays an important role in the downregulation of extrasynaptic AChRs which accompanies the increase in synaptic AChR expression at the neuromuscular junction during embryonic development. However, additional mechanisms may be involved in the AChR downregulation. This study provides evidence for a neurotrophic factor present in adult and embryonic chick neural extracts which downregulates fetal-type AChR density independently of depolarisation. Treatment of skeletal muscle cell lines with crude neural extracts decreased AChR density up to 50%, as measured by changes in 125I-alpha-bungarotoxin binding levels. Decreases in membrane-bound AChR density were accompanied by a decrease in the size of the intracellular AChR pool; RT-PCR analysis demonstrated that extract treatment also induced a decrease in gamma-subunit mRNA expression. These studies demonstrate that crude neural extracts contain a factor which may account for the activity-independent regulatory mechanism previously proposed to operate in concert with activity-dependent mechanisms to downregulate fetal-type AChR expression.

Neurotrophins regulate agrin-induced postsynaptic differentiation

The precise orchestration of synaptic differentiation is critical for efficient information exchange in the nervous system. The nerve-muscle synapse forms in response to agrin, which is secreted from the motor nerve terminal and induces the clustering of acetylcholine receptors (AChRs) and other elements of the postsynaptic apparatus on the subjacent muscle cell surface. In view of the highly restricted spatial localization and the plasticity of neuromuscular junctions, it seems likely that synapse formation and maintenance are regulated by additional, as-yet-unidentified factors. Here, we tested whether neurotrophins modulate the agrin-induced differentiation of postsynaptic specializations. We show that both brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4) inhibit agrin-induced AChR clustering on cultured myotubes. Nerve growth factor and NT-3 are without effect. Muscle cells express full-length TrkB, the cognate receptor for BDNF and NT-4. Direct activation of this receptor by anti-TrkB antibodies mimicked the BDNF/NT-4 inhibition of agrin-induced AChR clustering. This BDNF/NT-4 inhibition is likely to be an intrinsic mechanism for regulating AChR clustering, because neutralization of endogenous TrkB ligands resulted in elevated levels of AChR clustering even in the absence of added agrin. Finally, high concentrations of agrin can occlude the BDNF/NT-4 inhibition of AChR clustering. These results indicate that an interplay between agrin and neurotrophins can regulate the formation of postsynaptic specializations. They also suggest a mechanism for the suppression of postsynaptic specializations at nonjunctional regions.

Temporal correlations between functional and molecular changes in NMDA receptors and GABA neurotransmission in the superior colliculus

Activation of the NMDA subtype of glutamate receptor is required for activity-dependent structural plasticity in many areas of the young brain. Previous work has shown that NMDA receptor currents decline approximately at the time that developmental synaptic plasticity ends, and in situ hybridization studies have suggested that receptor subunit changes may be occurring during the same developmental interval. To establish a system in which the relationship between these properties of developing synapses can be explored, we have combined patch-clamp recordings with mRNA- and protein-level biochemical analyses to study the developmental regulation of NMDA receptors in the superficial layers of the rat superior colliculus. These experiments document an abrupt decrease in the NMDA receptor contribution to synaptic currents that occurs before eye opening and is closely associated with changes in NR1 protein, rapidly rising levels of the NMDA receptor subunit NR2A, and decreasing levels of NR2B. The functional and molecular changes also are correlated with the developmental decline in structural plasticity in these layers. In addition, both physiological and biochemical methods show evidence of GABA-mediated inhibition in the superficial collicular layers beginning after eye opening. This may provide an additional heterosynaptic mechanism for controlling excitation and plasticity in this neuropil by pattern vision. Thus our findings lend support to the idea that high levels of NMDA receptor function are associated with the potential for structural rearrangement in CNS neuropil and that the functional downregulation of this molecule results, at least partially, from changes in its subunit composition.

Stabilization of acetylcholine receptors by exogenous ATP and its reversal by cAMP and calcium

Innervation of the neuromuscular junction (nmj) affects the stability of acetylcholine receptors (AChRs). A neural factor that could affect AChR stabilization was studied using cultured muscle cells since they express two distinct populations of AChRs similar to those seen at the nmjs of denervated muscle. These two AChR populations are (in a ratio of 9 to 1) a rapidly degrading population (Rr) with a degradation half-life of approximately 1 d and a slowly degrading population (Rs) that can alternate between an accelerated form (half-life approximately 3-5 d) and a stabilized form (half-life approximately 10 d), depending upon the state of innervation of the muscle. Previous studies have shown that elevation of intracellular cAMP can stabilize the Rs, but not the Rr. We report here that in cultured rat muscle cells, exogenous ATP stabilized the degradation half-life of Rr and possibly also the Rs. Furthermore, pretreatment with ATP caused more stable AChRs to be inserted into the muscle membrane. Thus, in the presence of ATP, the degradation rates of the Rr and Rs overlap. This suggests that ATP released from the nerve may play an important role in the regulation of AChR degradation. Treatment with either the cAMP analogue dibutyryl-cAMP (dB-cAMP) or the calcium mobilizer ryanodine caused the ATP-stabilized Rr to accelerate back to a half-life of 1 d. Thus, at least three signaling systems (intracellular cAMP, Ca2+, and extracellular ATP) have the potential to interact with each other in the building of an adult neuromuscular junction.

Expression of exogenous genes transferred into the avian limb in ovo

We report on a simple method of direct gene transfer which allows the ectopic expression of proteins and the study of mesoderm-specific genes in the chick embryo. We microinjected into the avian embryonic limb several plasmids containing reporter genes under the control of various promoter sequences, including a minimal chicken muscle acetylcholine receptor alpha-subunit promoter [Klarsfeld, A., Daubas, A., Bourachot, B. and Changeux, J.P., Mol. Cell. Biol., 7 (1987) 951-955]. Gene expression is detectable for 3 days, is reproducible, is restricted to the site of injection, and correlates with the amount of DNA injected. Our observations indicate that it is possible to transfer and express genes in ectodermal and mesodermal cells of the chick limb by direct DNA injection and that the method can be used to analyze promoter sequences in vivo during specific windows of development.

Neuronal bungarotoxin displaces (125I) alpha-bungarotoxin binding at the neuromuscular junction as well as to the spinal cord during embryogenesis

Alpha-Bungarotoxin (alpha BTX) administration in ovo prevents motoneuron apoptosis during development. This process may be mediated by alpha BTX-sensitive nicotinic cholinoceptors in the spinal cord, at the neuromuscular junction or at both sites. In order to differentiate between these possibilities, neuronal bungarotoxin binding (NBTX) binding to embryonic muscle and spinal cord was investigated in the chick.

Activity-dependent interactions between motoneurones and muscles: their role in the development of the motor unit

In this review article we have attempted to provide an overview of the various forms of activity-dependent interactions between motoneurones and muscles and its consequences for the development of the motor unit. During early development the components of the motor unit undergo profound changes. Initially the two cell types develop independently of each other. The mechanisms that regulate their characteristic properties and prepare them for their encounter are poorly understood. However, when motor axons reach their target muscles the interaction between these cells profoundly affects their survival and further development. The earliest interactions between motoneurones and muscle fibres generate a form of activity which is in many ways different from that seen at later stages. This difference may be due to the immature types of ion channels and neurotransmitter receptors present in the membranes of both motoneurones and muscle fibres. For example, spontaneous release of acetylcholine may influence the myotube even before any synaptic specialization appears. This initial form of activity-dependent interaction does not necessarily depend on the generation of action potentials in either the motoneurone or the muscle fibre. Nevertheless, the ionic fluxes and electric fields produced by such interactions are likely to activate second messenger systems and influence the cells. An important step for the development of the motor unit in its final form is the initial distribution of synaptic contacts to primary and secondary myotubes and their later reorganization. Mechanisms that determine these events are proposed. It is argued that the initial layout of the motor unit territory depends on the matching of immature muscle fibres (possibly secondary myotubes) to terminals with relatively weak synaptic strength. Such matching can be the consequence of the properties of the muscle fibre at a particular stage of maturation which will accept only nerve terminals that match their developmental stage. Refinements of the motor unit territory follows later. It is achieved by activity-dependent elimination of nerve terminals from endplates that are innervated by more than one motoneurone. In this way the territory of the motor unit is established, but not necessarily the homogeneity of the physiological and biochemical properties of its muscle fibres. These properties develop gradually, largely as a consequence of the activity pattern that is imposed upon the muscle fibres supplied by a given motoneurone. This occurs when the motor system in the CNS completes its development so that specialized activity patterns are transmitted by particular motoneurones to the muscle fibres they supply.(ABSTRACT TRUNCATED AT 400 WORDS)

Stabilization of acetylcholine receptors at the neuromuscular synapse: the role of the nerve

The majority of acetylcholine receptors (AChRs) at innervated neuromuscular junctions (NMJs) are stable, with half-lives averaging about 11 days in rodent muscles. In addition to the stable AChRs, approximately 18% of AChRs at these innervated junctions are rapidly turned over (RTOs), with half lives of less than 24 h. We have postulated that RTOs may be precursors of stable AChRs, and that the motor nerve may influence their stabilization. This hypothesis was tested by: (i) labeling AChRs in mouse sternomastoid (SM) muscles with 125I-alpha-BuTx; (ii) denervating one SM muscle in each mouse, and (iii) following the fate of the labeled AChRs through a 5-day period when RTOs were either stabilized or degraded. The hypothesis predicts that denervation should preclude stabilization of RTOs, resulting in a deficit of stable AChRs in denervated muscles. The results showed a highly significant (P less than 0.002) deficit of stable AChRs in denervated as compared with innervated muscles. Control experiments excluded the possibility that this deficit could be attributed to independent accelerated degradation of either RTOs or pre-existing stable AChRs. The observed deficit was quantitatively consistent with the deficit predicted by a mathematical model based on interruption of stabilization following denervation. We conclude that: (i) the observed deficit after denervation of NMJs is due to failure of stabilization of pre-existing RTOs; (ii) RTOs at normally innervated NMJs are precursors of stable AChRs; (iii) stabilization occurs after the insertion of AChRs at NMJs, and (iv) motor nerves play a key role in stabilization of RTOs. The concept of receptor stabilization has important implications for understanding the biology of the neuromuscular junction and post-synaptic plasticity.

Neuromuscular development following tetrodotoxin-induced inactivity in mouse embryos

Developmental aspects of the neuromuscular system in mouse embryos chronically paralyzed in utero with tetrodotoxin (TTX) between embryonic days 14 and 18 were studied using biochemical and histological methods. The number of lumbar spinal motoneurons (MNs) was higher in inactive embryos than in controls suggesting a decreased motoneuron cell death. In association with the increase in MN number, choline acetyltransferase activity was significantly increased in both spinal cord and peripheral synaptic sites. Paralyzed muscles exhibited a decreased number of mature myofibers and the nuclei were centrally located. Creatine kinase activity was greatly decreased and total acetylcholine receptor and receptor cluster numbers per myofiber were significantly increased in paralyzed muscles. A similar pattern of changes occurs in the neuromuscular system of the mutant mouse muscular dysgenesis (mdg). However, in contrast to the mdg mutant, tetrodotoxin-treated muscles were similar to controls in their innervation pattern, in the ultrastructural aspects of the excitation-contraction coupling system (i.e., dyads and triads) and in the extent of dihydropyridine binding. Thus, neuromuscular inactivity is not sufficient to impair the pattern of muscle innervation or the appearance of either the triadic junctions or dihydropyridine receptors. These results indicate that alterations of dihydropyridine binding sites and triads in muscular dysgenesis cannot be accounted for by inactivity but rather must reflect a more primary defect involving the structural gene(s) regulating the development of one or more aspects of muscle differentiation.

Rapid induction of acetylcholine receptor aggregates by a neural factor and extracellular Ca2+

A soluble fetal brain extract (EBX) induces acetylcholine receptor (AChR) aggregation in cultured rat myotubes within 4 hr at 36 degrees C in a defined medium containing 1.8 mM (normal) extracellular Ca2+ (Olek et al., 1983). The activity of EBX was Ca2+ dependent; reducing extracellular Ca2+ significantly inhibited EBX-induced AChR aggregation and a 15-50% increase in extracellular Ca2+ synergistically enhanced the activity of EBX. Synergism was specific for Ca2+ as increases in other divalent cations (Ba2+, Co2+, Mg2+, Mn2+, Sr2+) had no effect. A large increase (300-500%) in extracellular Ca2+ alone also induced AChR aggregation within 4 hr at 36 degrees C. An equivalent increase in other cations (Ba2+, Co2+, Mg2+, Mn2+, Sr2+) did not promote AChR aggregation. An initial 15-min pulse of increased extracellular Ca2+ alone or with EBX was adequate to induce AChR aggregation. Aggregates induced by EBX, Ca2+ alone, or EBX/Ca2+ were found predominantly on the top surface of the myotube. These treatments did not detectably alter preexisting aggregates present at substrate contact sites on the bottom surface of myotubes. AChR aggregation induced by any treatment was not inhibited by cycloheximide, Ca2+ channel blockers, or protease inhibitors but was blocked by Co2+ and sodium azide.

Developmental changes in the half-life of acetylcholine receptors in the myotomal muscle of Xenopus laevis

1. Tail preparations, containing myotomal muscle and associated spinal cord, were isolated from embryos and tadpoles of Xenopus laevis between stages 25 and 49 (1.15-12 days) and were pulse-labelled with 125I-alpha-bungarotoxin (125I alpha BT) so that the half-life (T1/2) of their acetylcholine receptors (AChRs) could be estimated in organ culture. 2. For the entire population of AChRs, estimates of T1/2 based on a single exponential decline in radioactivity (but see item 4 below) increased from 53-55 h at stages 25-31 (1.15-1.56 days) to approximately 135 h at stage 47 (5.5 days). Beyond stage 47 T1/2 increased only slightly. 3. Radioautographic estimates of the T1/2 of extrajunctional AChRs at stages 47-48 (5.5-7.5 days) were 41-50 h. It follows that the developmental change in the T1/2 of the entire population of AChRs was due to the junctional AChRs. 4. At stages 47-49 (5.5-12 days) the decline in radioactivity for the entire population of AChRs was fitted well by a double exponential. Assuming a T1/2 of 50 h for the extrajunctional AChRs and 210 h for the junctional AChRs, the correlation coefficient (r) was 0.9947 +/- 0.0014 (mean +/- S.E.M.; n = 14) and junctional AChRs were estimated to comprise 80 +/- 3% of the entire population. Similar analysis, as well as experiments in which the degradation of junctional AChRs was assessed by pulse-labelling with fluorescent alpha-bungarotoxin, suggested that at earlier stages of development the junctional AChRs have a shorter T1/2 and comprise a smaller fraction of the entire population. 5. The developmental increase in T1/2 occurred even when animals were raised in the anaesthetic tricaine or in tetrodotoxin, conditions which abolished all motor activity. 6. Developmental increases in T1/2 also occurred in culture but were smaller than those in vivo. The increases in culture did not occur amongst those AChRs which were pre-labelled with 125I alpha BT. 7. It is concluded that in Xenopus myotomal muscle the T1/2 of junctional AChRs begins to increase within a day after the onset of innervation and that the increase does not require nerve or muscle impulse activity. We suggest, among other possibilities, that it may depend upon incorporation of a different molecular species of AChR into the postsynaptic membrane.

The distribution of intracellular acetylcholine receptors and nuclei in avian slow muscle fibres during establishment of distributed synapses

The distribution of intracellular acetylcholine receptor was studied by 125I-alpha-bungarotoxin autoradiography as a measure of the local acetylcholine receptor synthesis at junctional and extrajunctional sites in single fibres of the developing anterior latissimus dorsi muscle of the chicken. Large (longer than 2 microns) acetylcholine receptor clusters characteristic of synaptic contacts were localized by immunofluorescence with anti-acetylcholine receptor antibodies. The distance between acetylcholine receptor clusters at embryonic day 11 was 166 +/- 10.5 microns and this distance did not increase despite growth until after 4 days posthatch. The distance between acetylcholine receptor clusters subsequently increased proportionately with the increase in the length of fibres. Intracellular acetylcholine receptors were labelled with 125I-alpha-BGT after first blocking cell-surface acetylcholine receptor with unlabelled alpha-BGT, and treatment with saponin. Intracellular acetylcholine receptor represented about 5-15% of total cellular acetylcholine receptor. Cycloheximide experiments indicated that 80-90% of intracellular acetylcholine receptor examined represented newly synthesized acetylcholine receptor. The spatial distribution of this pool, studied by autoradiography, was determined in relation to the acetylcholine receptor clusters labelled with anti-acetylcholine receptor antibody. Between embryonic day 11 and posthatch day 14 there was a continual increase in intracellular acetylcholine receptor at both junctional and extrajunctional parts of the fibres, but with the greater increases occurring at the junctional regions. Peaks of intracellular acetylcholine receptor became associated with an increasing number of acetylcholine receptor clusters so that by posthatch day 14 there was an 80% correspondence. The accumulation of newly synthesized intracellular acetylcholine receptor under acetylcholine receptor clusters was not the result of the aggregation of nuclei at these sites, suggesting that a higher rate of acetylcholine receptor synthesis per nucleus develops at distributed synaptic sites on anterior latissimus dorsi fibres.

Agrin-related molecules are concentrated at acetylcholine receptor clusters in normal and aneural developing muscle

Agrin induces the clustering of acetylcholine receptors (AchRs) and other postsynaptic components on the surface of cultured muscle cells. Molecules closely related if not identical to agrin are highly concentrated in the synaptic basal lamina, a structure known to play a key part in orchestrating synapse regeneration. Agrin or agrin-related molecules are thus likely to play a role in directing the differentiation of the postsynaptic apparatus at the regenerating neuromuscular junction. The present studies are aimed at understanding the role of agrin at developing synapses. We have used anti-agrin monoclonal antibodies combined with alpha-bungarotoxin labeling to establish the localization and time of appearance of agrin-related molecules in muscles of the chick hindlimb. Agrinlike immunoreactivity was observed in premuscle masses from as early as stage 23. AchR clusters were first detected late in stage 25, coincident with the entry of axons into the limb. At this and all subsequent stages examined, greater than 95% of the AchR clusters colocalized with agrin-related molecules. This colocalization was also observed in unpermeabilized whole mount preparations, indicating that the agrin-related molecules were disposed on the external surface of the cells. Agrin-related molecules were also detected in regions of low AchR density on the muscle cell surface. To examine the role of innervation in the expression of agrin-related molecules, aneural limbs were generated by two methods. Examination of these limbs revealed that agrin-related molecules were expressed in the aneural muscle and they colocalized with AchR clusters. Thus, in developing muscle, agrin or a closely related molecule (a) is expressed before AchR clusters are detected; (b) is colocalized with the earliest AchR clusters formed; and (c) can be expressed in muscle and at sites of high AchR density independently of innervation. These results indicate that agrin or a related molecule is likely to play a role in synapse development and suggest that the muscle cell may be at least one source of this molecule.

The distribution of intracellular acetylcholine receptors and nuclei in developing avian fast-twitch muscle fibres during synapse elimination

The spatial distribution of intracellular acetylcholine receptors along the length of fibres from the avian posterior latissimus dorsi muscle has been investigated during embryonic development, when distributed synaptic sites are eliminated from the muscle fibres. Cell surface AChR were irreversibly blocked with unlabelled alpha-bungarotoxin (alpha-BGT). Muscles were then fixed and ultrasonically dissociated into fibre fragments, treated with 0.5% saponin and stained with 125I-alpha-BGT. This revealed an intracellular pool of curare sensitive binding sites equivalent to about 10% of total cell AChR. The spatial distribution of this pool was studied by autoradiography. Large (longer than 2 microns) AChR-clusters (AChR-C) characteristic of neuromuscular contacts were localized on the same fibres by immunofluorescence with an anti-AChR antibody. At E11, relatively high levels of intracellular AChR were observed throughout the length of fibres. Between E11 and E18 intracellular AChR declined (19 fold) in extrajunctional parts of fibres but remained high in segments of fibre corresponding to AChR-clusters. Treatment of E14 embryos with an inhibitor of protein synthesis (cycloheximide) reduced intracellular AChR to 22 +/- 6% (mean +/- SE) of control levels, suggesting that most of the intracellular binding represented newly-synthesized AChR. Between E11 and E18 cell nuclei were found to accumulate beneath AChR-C. The mean density of nuclei in segments of fibre corresponding to AChR-C increased 5 fold between E11 and E18, but remained unchanged in extrajunctional segments. It is suggested that the elimination of excess distributed AChR-C may be due to the preferential accumulation of nuclei at a single AChR-C on each fibre accompanied by the down regulation of AChR synthesis associated with nuclei at the remaining AChR-C.

Neural regulation of gene expression by an acetylcholine receptor promoter in muscle of transgenic mice

Motor neurons regulate the quantity and distribution of acetylcholine receptors (AChR) in the muscles they innervate. Here, we report that an AChR alpha subunit gene fragment contains cis-acting regulatory sequences that confer neural regulation as well as tissue-specific regulation of transcription. An 850 bp fragment from the 5' end of the chicken AChR alpha gene fused to the reporter gene, chloramphenicol acetyltransferase (CAT), has been introduced into the genomes of several lines of transgenic mice. Expression of CAT enzyme activity in these mice is tissue-specific; the onset of expression in embryonic muscle correlates well with that of many other muscle-specific proteins. Most importantly, CAT enzyme is down-regulated 100-fold soon after birth, an effect that can be completely reversed by denervation.

Localization of nicotinic acetylcholine receptor alpha-subunit transcripts during myogenesis and motor endplate development in the chick

In 15-d-old chick latissimi dorsi muscles, the nicotinic acetylcholine receptor (AChR) alpha-subunit mRNA is densely accumulated at the level of subsynaptic nuclei of the motor endplate (Fontaine et al., 1988). In this paper, using in situ hybridization with genomic probes, we further show that the expression of the AChR alpha-subunit gene in the embryo, revealed by the accumulation of mature mRNAs, starts in myotomal cells and persists during the first stages of muscle development in a majority of muscle nuclei. Subsequently, the distribution of AChR alpha-subunit mRNAs becomes restricted to the newly formed motor endplates as neuromuscular junctions develop. To assess the transcriptional activity of individual nuclei in developing muscles, a strictly intronic fragment of the AChR alpha-subunit gene was used to probe in situ the level of unspliced transcripts. AChR alpha-subunit unspliced transcripts accumulate around a large number of sarcoplasmic nuclei at embryonic day 11, but can no longer be detected at their level after embryonic day 16 in the embryo. A similar decrease in the accumulation of AChR alpha-subunit transcripts is observed between day 4 and day 6 in primary cultures of muscle cells. On the other hand, in vivo denervation and in vitro blocking of muscle electrical activity by the sodium channel blocker tetrodotoxin results in an increase in the labeling of muscle nuclei. Yet, only 6% of the muscle nuclei appear labeled by the strictly intronic probes after denervation. The possible significance of such heterogeneity of muscle nuclei during motor endplate formation in AChR gene expression is discussed.

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

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

Neuronal acetylcholine receptors in Drosophila: mature and immature transcripts of the ard gene in the developing central nervous system

The ARD protein is a Drosophila homolog of vertebrate nicotinic acetylcholine receptor (AChR) polypeptides. Here, an analysis of transcripts of the corresponding ard gene is presented. In situ hybridization experiments revealed ard gene expression in nervous tissue only. During development, ard transcripts are prevalent in late embryos, pupae, and newly eclosed flies. Both the spatial and the temporal pattern of ard gene expression is consistent with the ARD protein being part of a neuronal AChR that is produced in large amounts during major periods of neuronal differentiation. In situ hybridization with an intron-specific probe indicated codistribution of immature and mature ard RNAs in pupae and adult flies. In addition to the mature 3.2 kb RNA species, two large immature transcripts are found in newly eclosed flies but not in embryos, suggesting a developmentally regulated processing of ard RNA.

An unusual beta-spectrin associated with clustered acetylcholine receptors

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

Acetylcholine receptor expression in primary cultures of embryonic chick myotubes--II. Comparison between the effects of spinal cord cells and calcitonin gene-related peptide

Spinal cord cells co-cultured with primary chick myotubes caused a 1.5-3-fold increase in the number of muscle surface acetylcholine receptors assayed with [125I]alpha-bungarotoxin. This increase did not result from the metabolic stabilization of the acetylcholine receptor protein and was at least partially due to a stimulation of acetylcholine receptor biosynthesis up to the level of the accumulation of alpha-subunit mature and partially spliced precursor mRNAs. A medium conditioned by spinal cord cells also caused a rise in acetylcholine receptor number. This increase did not coincide with an augmentation of the intracellular cyclic AMP level as reported for the neuropeptide calcitonin gene-related peptide. In contrast, spinal cord cells and the medium conditioned by them potentiated the effect of calcitonin gene-related peptide on acetylcholine receptor number. Stimulation of acetylcholine receptor synthesis by the conditioned medium was blocked by the protein kinase C activator 12-O-tetradecanoyl phorbol-13-acetate and by the calcium ionophore A23187. These two compounds have already been reported to block the increase of acetylcholine receptor number produced by the voltage sensitive sodium channel antagonist tetrodotoxin which stimulates acetylcholine receptor biosynthesis by blocking spontaneous electrical activity of the cultured muscle cells. The possibility that different neural factors and second messenger systems are involved in the regulation of acetylcholine receptor biosynthesis during the development of the neuromuscular junction is discussed.

Isolation and characterization of the mouse acetylcholine receptor delta subunit gene: identification of a 148-bp cis-acting region that confers myotube-specific expression

We have isolated the gene encoding the delta subunit of the mouse skeletal muscle acetylcholine receptor (AChR) and have identified a 148-bp cis-acting region that controls cell type-specific and differentiation-dependent gene expression. The 5' flanking region of the delta subunit gene was fused to the protein-coding region of the chloramphenicol acetyltransferase (CAT) gene and gene fusions were transfected into C2 mouse skeletal muscle cells. Both transiently and stably transfected cells were assayed for CAT gene expression. Deletions from the 5' end of the mouse delta gene demonstrate that 148 bp of 5' flanking DNA is sufficient to confer cell type-specific and differentiation-dependent expression: CAT activity is present in transfected myotubes, but not in transfected 3T3 cells or 10T1/2 cells. Moreover, the level of CAT expression in myotubes transfected with constructs containing 148 bp of 5' flanking DNA from the delta subunit gene is identical to that in myotubes transfected with constructs containing 3.2 kb of 5' flanking DNA and similar to expression from the SV-40 early promoter. Increased CAT activity in myotubes is a result of an increased rate of transcription from the delta subunit promoter, since CAT RNA levels are also 35-fold more abundant in myotubes than myoblasts. In contrast, the SV-40 early promoter is similarly active in all cell types. Thus, 148 bp of 5' flanking DNA from the delta subunit gene contains all the information required for cell type-specific and differentiation-dependent expression of the AChR delta subunit.

The development of cholinergic neurons

Motoneuron precursors acquire some principles of their spatial organization early in their cell lineage, probably at the blastula stage. A predisposition to the cholinergic phenotype in motoneurons and some neural crest cells is detectable at the gastrula to neurula stages. Cholinergic expression is evident upon cessation of cell division. Cholinergic neurons can synthesize ACh during their migration and release ACh from their growth cones prior to target contact or synapse formation. Neurons of different cell lineages can express the cholinergic phenotype, suggesting the importance of secondary induction. Early cholinergic commitment can be modified or reversed until later in development when it is amplified during interaction with target. Motoneurons extend their axons and actively sort out in response to local environmental cues to make highly specific connections with appropriate muscles. The essential elements of the matching mechanism are not species-specific. A certain degree of topographic matching is present throughout the nervous system. In dissociated cell culture, most topographic specificity is lost due to disruption of local environmental cues. Functional cholinergic transmission occurs within minutes of contact between the growth cone and a receptive target. These early contacts contain a few clear vesicles but lack typical ultrastructural specializations and are physiologically immature. An initial stabilization of the nerve terminal with a postsynaptic AChR cluster is not prevented by blocking ACh synthesis, electrical activity, or ACh receptors, but AChR clusters are not induced by non-cholinergic neurons. After initial synaptic contact, there is increasing deposition of presynaptic active zones and synaptic vesicles, extracellular basal lamina and AChE, and postjunctional ridges over a period of days to weeks. There is a concomitant increase in m.e.p.p. frequency, mean quantal content, metabolic stabilization of AChRs, and maturation of single channel properties. At the onset of synaptic transmission, cell death begins to reduce the innervating population of neurons by about half over a period of several days. If target tissue is removed, almost all neurons die. If competing neurons are removed or additional target is provided, cell death is reduced in the remaining population. Pre- or postsynaptic blockade of neuromuscular transmission postpones cell death until function returns.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of acetylcholine receptor channel function during development of skeletal muscle

The nicotinic acetylcholine (ACh) receptor channel mediates synaptic transmission at the neuromuscular junction. During the development of skeletal muscle, ACh receptors undergo changes in distribution, antigenic determinants, degradation rate, and function. Now that these developmental hallmarks have been identified, attention has turned toward understanding both the structural bases for such changes and the role of nerve in triggering these changes. Recently, a much clearer understanding of one of these developmental processes, namely, the alterations in channel function, has emerged through both sensitive patch-clamp measurements and the application of recombinant DNA technology. In light of these new advances, we now reevaluate the processes governing the developmental changes in the functional properties of the ACh receptor.

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.

Acetylcholine receptor synthesis rate and levels of receptor subunit messenger RNAs in chick muscle

Levels of mRNAs specific for the alpha-, gamma- and delta-subunit of the nicotinic acetylcholine receptor were measured in chick skeletal muscle by solution hybridization, using a genomic DNA probe containing the intramembrane segments M2 and M3 of the alpha-subunit and probes comprising exons 2-6 and exons 4-8, respectively, of the gamma- and delta-subunit. In the innervated calf musculature of adult chickens, receptor-specific messages were detected in approx. 100-fold excess over the amount required to account for the observed synthesis rate. Within 1 week after section of the sciatic nerve, alpha-, gamma- and delta-subunit message levels rose 112-, 42- and 24-fold, respectively, while receptor expression rate increased about 150-fold. The rise in message levels preceded the denervation-induced increase in receptor concentration. In differentiating myogenic cells all three messages were found in excess over the amounts required for the observed rate of receptor synthesis. Treatment of differentiated myotubes with drugs that change receptor synthesis rate selectively affects alpha-subunit mRNA. In all situations in vitro and in vivo the alpha-subunit mRNA was found to reach final levels faster, and to be from 3 to over 30 times more abundant, than the other messages. These observations corroborate earlier evidence for a regulatory mechanism in which the supply of mRNA determines acetylcholine receptor synthesis rate. They also suggest that receptor expression is not simply proportional to acetylcholine receptor subunit mRNA concentrations, but rather is controlled, to a considerable extent, by the efficiency with which the receptor-specific mRNAs and/or the subunits they code for are subsequently utilized.

Elimination of distributed synaptic acetylcholine receptor clusters on developing avian fast-twitch muscle fibres accompanies loss of polyneuronal innervation

Changes in the distribution of large acetylcholine receptor clusters (AChR-Cs) on developing fast-twitch fibres of the chicken posterior latissimus dorsi (PLD) muscle have been studied during the period of loss of polyneuronal innervation using fluorescein-conjugated alpha-bungarotoxin. Embryonic muscles were ultrasonically dissociated into single fibre fragments and presumptive fast-twitch fibres were distinguished from the minority of slow-type fibres in the PLD by immunofluorescence using an antibody against slow-type myosin. Whereas mature PLD muscle fibres are focally innervated, at embryonic day 11 (E11) many fibre fragments from the PLD displayed two or more large (longer than 2 micron) AChR-Cs. Double labelling with anti-neurofilament antibody suggested that most of these AChR-Cs (82 +/- 2%) were associated with neuromuscular contacts. There was a progressive decline in the number of large (synaptic) AChR-Cs per 1000 micron of fibre, from 3.2 +/- 0.5 at E11 to 0.4 +/- 0.1 at E18. No further decline occurred between E18 and one week post-hatch. Primary generation muscle cells identified at E11 and E16 by tritiated thymidine labelling showed a decline in the number of large AChR-Cs per 1000 micron proportional to that seen in the fibre population as a whole, suggesting that distributed synaptic AChR-Cs are eliminated from individual fibres as they mature. When embryos were treated with d-tubocurarine starting at E6 the loss of distributed AChR-Cs from fast-type PLD fibres between E11 and E14 did not occur, suggesting that neuromuscular activity may play an important role in establishing the focal synaptic site AChR-C.

Elimination of distributed acetylcholine receptor clusters from developing fast-twitch fibres in an avian muscle

The development of the focal localization of large acetylcholine receptor clusters (AChR-Cs) on avian fast muscle fibres has been investigated in the triceps brachii pars humeralis (TH) muscle of the chick embryo. The mature TH muscle consists of both fast fibres, which usually receive a focal innervation at single synaptic sites, and slow fibres which receive a distributed innervation at multiple synaptic sites. Single fibre fragments dissociated from the embryonic muscle were typed using anti-myosin antibodies; fluorescently labelled alpha-bungarotoxin was used to identify large AChR-Cs which serve as synaptic markers. In contrast to the mature focal innervation, at embryonic day 11 (E11), many fast-type fibres in the TH muscle displayed large, distributed AChR-Cs (3.7 +/- 0.7 per 1000 microns fibre length; n = 6 embryos) like neighbouring slow-type fibres. By E16 distributed AChR-Cs were rare on fast type fibres (0.9 +/- 0.2 per 1000 microns fibre length). As it was possible that the frequency of fast fibres with distributed AChR-Cs declined simply as a consequence of the increase in number of secondary generation fibres, tritiated thymidine was injected at E7 in order to identify the primary generation fibres at E14. The great majority of fast fibres that were heavily labelled with thymidine at E14 appeared to possess a focal AChR-C. The results suggest that at E11 fast-type primary fibres in the TH muscle receive a distributed innervation very similar to neighbouring slow-type fibres; this subsequently evolves into the mature focal innervation following the elimination of synaptic sites between E11 and E14.

Disruption and reformation of the acetylcholine receptor clusters of cultured rat myotubes occur in two distinct stages

We have examined the redistribution of acetylcholine receptor (AChR) intramembrane particles (IMPs) when AChR clusters of cultured rat myotubes are experimentally disrupted and allowed to reform. In control myotubes, the AChR IMPs are evenly distributed within the AChR domains of cluster membrane. Shortly after addition of azide to disrupt clusters, IMPs become unevenly scattered, with some microaggregation. After longer treatment, IMPs are depleted from AChR domains with no further change in IMP distribution. Contact domains of clusters are relatively poor in IMPs both before and after cluster dispersal. Upon visualization with fluorescent alpha-bungarotoxin, some AChR in azide-treated samples appear as small, bright spots. These spots do not correspond to microaggregates seen in freeze-fracture replicas, and probably represent receptors that have been internalized. The internalization rate is insufficient to account completely for the loss of IMPs from clusters, however. During reformation of AChR clusters upon removal of azide, IMP concentration in receptor domains increases. At early stages of reformation, IMPs appear in small groups containing compact microaggregates. At later times, AChR domains enlarge and IMPs within them assume the evenly spaced distribution characteristic of control clusters. These observations suggest that the disruption of clusters is accompanied by mobilization of AChR from a fixed array, allowing AChR IMPs to diffuse away from the clusters, to form microaggregates, and to become internalized. Cluster reformation appears to be the reverse of this process. Our results are thus consistent with a two-step model for AChR clustering, in which the concentration of IMPs into a small membrane region precedes their rearrangement into evenly spaced sites.