Metabolic stabilization of endplate acetylcholine receptors regulated by Ca2+ influx associated with muscle activity

Effects of delayed repair of peripheral nerve injury on the spatial distribution of motor endplates in target muscle

Motor endplates (MEPs) are important sites of information exchange between motor neurons and skeletal muscle, and are distributed in an organized pattern of lamellae in the muscle. Delayed repair of peripheral nerve injury typically results in unsatisfactory functional recovery because of MEP degeneration. In this study, the mouse tibial nerve was transected and repaired with a biodegradable chitin conduit, immediately following or 1 or 3 months after the injury. Fluorescent α-bungarotoxin was injected to label MEPs. Tissue optical clearing combined with light-sheet microscopy revealed that MEPs were distributed in an organized pattern of lamellae in skeletal muscle after delayed repair for 1 and 3 months. However, the total number of MEPs, the number of MEPs per lamellar cluster, and the maturation of single MEPs in gastrocnemius muscle gradually decreased with increasing denervation time. These findings suggest that delayed repair can restore the spatial distribution of MEPs, but it has an adverse effect on the homogeneity of MEPs in the lamellar clusters and the total number of MEPs in the target muscle. The study procedures were approved by the Animal Ethics Committee of the Peking University People's Hospital (approval No. 2019PHC015) on April 8, 2019.

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.

Bionic ion channel and single-ion conductor design for artificial skin sensors

Ionic skin with a single-ion conductor and a bionic-ordered ion channel could detect human activities due to ion redistribution.

Laminin is instructive and calmodulin dependent kinase II is non-permissive for the formation of complex aggregates of acetylcholine receptors on myotubes in culture

Previous work has shown that myotubes cultured on laminin-coated substrates form complex aggregates of synaptic proteins that are similar in shape and composition to neuromuscular junctions (NMJs). Here we show that laminin instructs the location of complex aggregates which form only on the lower surface when laminin is coated onto culture dishes but over the entire cell when laminin is added in solution. Silencing of myotubes by agents that block electrical activity (tetrodotoxin, verapamil) or by inhibitors of calmodulin dependent kinase (CaMKII) render the myotube permissive for the formation of complex aggregates. Treatment with laminin alone will facilitate the formation of complex aggregates hours later when myotubes are made permissive by inhibiting CaMKII. The AChR agonist carbachol disperses pre formed aggregates suggesting that non-permissiveness may involve active dispersal of AChRs. The permissive period requires ongoing protein synthesis. The latter may reflect a requirement for rapsyn, which turns over rapidly, and is necessary for aggregation. Consistent with this geldanamycin, an agent that increases rapsyn turnover disrupts complex aggregates. Agrin is well known to induce small clusters of AChRs but does not induce complex aggregates even though aggregate formation requires MuSK, a receptor tyrosine kinase activated by agrin. Dystroglycan (DG) is the major laminin receptor mediating complex aggregate formation with some contribution from β1 integrins. In addition, there is a pool of CaMKII associated with DG. We discuss how these permissive and instructive mechanisms bear on NMJ formation in vivo.

Ordered and Active Nanochannel Electrode Design for High-Performance Electrochemical Actuator

A novel electrochemical actuator based on polyaniline@vertically aligned carbon nanotube nanocomposite electrodes, in which the vertically aligned structure is introduced by a facilely electrochemical process. Owing to the vertically aligned nanocomposite electrode providing ordered path channel for faster ion transportation and high electrochemical capacitance for more ion accumulation, our actuator displays intriguing actuation performance including large deformation, fast-actuation speed, and excellent actuation stability.

The craniosacral progression of muscle development influences the emergence of neuromuscular junction alterations in a severe murine model for spinal muscular atrophy

AIMS

As 4-day-old mice of the severe spinal muscular atrophy (SMA) model (dying at 5-8 days) display pronounced neuromuscular changes in the diaphragm but not the soleus muscle, we wanted to gain more insight into the relationship between muscle development and the emergence of pathological changes and additionally to analyse intercostal muscles which are affected in human SMA.

METHODS

Structures of muscle fibres and neuromuscular junctions (NMJs) of the diaphragm, intercostal and calf muscles of prenatal (E21) and postnatal (P0 and P4) healthy and SMA mice were analysed by light and transmission electron microscopy. NMJ innervation was studied by whole mount immunofluorescence in diaphragms of P4 mice.

RESULTS

During this period, the investigated muscles still show a significant neck-to-tail developmental gradient. The diaphragm and calf muscles are most and least advanced, respectively, with respect to muscle fibre fusion and differentiation. The number and depth of subsynaptic folds increases, and perisynaptic Schwann cells (PSCs) acquire a basal lamina on their outer surface. Subsynaptic folds are connected to an extensive network of tubules and beaded caveolae, reminiscent of the T system in adult muscle. Interestingly, intercostal muscles from P4 SMA mice show weaker pathological involvement (that is, vacuolization of PSCs and perineurial cells) than those previously described by us for the diaphragm, whereas calf muscles show no pathological changes.

CONCLUSION

SMA-related alterations appear to occur only when the muscles have reached a certain developmental maturity. Moreover, glial cells, in particular PSCs, play an important role in SMA pathogenesis.

Ca(2+) permeation and/or binding to CaV1.1 fine-tunes skeletal muscle Ca(2+) signaling to sustain muscle function

Background

Ca²⁺ influx through Ca_V1.1 is not required for skeletal muscle excitation-contraction coupling, but whether Ca²⁺ permeation through Ca_V1.1 during sustained muscle activity plays a functional role in mammalian skeletal muscle has not been assessed.

Methods

We generated a mouse with a Ca²⁺ binding and/or permeation defect in the voltage-dependent Ca²⁺ channel, Ca_V1.1, and used Ca²⁺ imaging, western blotting, immunohistochemistry, proximity ligation assays, SUnSET analysis of protein synthesis, and Ca²⁺ imaging techniques to define pathways modulated by Ca²⁺ binding and/or permeation of Ca_V1.1. We also assessed fiber type distributions, cross-sectional area, and force frequency and fatigue in isolated muscles.

Results

Using mice with a pore mutation in Ca_V1.1 required for Ca²⁺ binding and/or permeation (E1014K, EK), we demonstrate that Ca_V1.1 opening is coupled to CaMKII activation and refilling of sarcoplasmic reticulum Ca²⁺ stores during sustained activity. Decreases in these Ca²⁺-dependent enzyme activities alter downstream signaling pathways (Ras/Erk/mTORC1) that lead to decreased muscle protein synthesis. The physiological consequences of the permeation and/or Ca²⁺ binding defect in Ca_V1.1 are increased fatigue, decreased fiber size, and increased Type IIb fibers.

Conclusions

While not essential for excitation-contraction coupling, Ca²⁺ binding and/or permeation via the Ca_V1.1 pore plays an important modulatory role in muscle performance.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-014-0027-1) contains supplementary material, which is available to authorized users.

Recycling of acetylcholine receptors at ectopic postsynaptic clusters induced by exogenous agrin in living rats

During the development of the neuromuscular junction, motor axons induce the clustering of acetylcholine receptors (AChRs) and increase their metabolic stability in the muscle membrane. Here, we asked whether the synaptic organizer agrin might regulate the metabolic stability and density of AChRs by promoting the recycling of internalized AChRs, which would otherwise be destined for degradation, into synaptic sites. We show that at nerve-free AChR clusters induced by agrin in extrasynaptic membrane, internalized AChRs are driven back into the ectopic synaptic clusters where they intermingle with pre-existing and new receptors. The extent of AChR recycling depended on the strength of the agrin stimulus, but not on the development of junctional folds, another hallmark of mature postsynaptic membranes. In chronically denervated muscles, in which both AChR stability and recycling are significantly decreased by muscle inactivity, agrin maintained the amount of recycled AChRs at agrin-induced clusters at a level similar to that at denervated original endplates. In contrast, AChRs did not recycle at agrin-induced clusters in C2C12 or primary myotubes. Thus, in muscles in vivo, but not in cultured myotubes, neural agrin promotes the recycling of AChRs and thereby increases their metabolic stability.

Regulation of muscle acetylcholine receptor turnover by β subunit tyrosine phosphorylation

At the neuromuscular junction (NMJ), the postsynaptic localization of muscle acetylcholine receptor (AChR) is regulated by neural signals and occurs via several processes including metabolic stabilization of the receptor. However, the molecular mechanisms that influence receptor stability remain poorly defined. Here, we show that neural agrin and the tyrosine phosphatase inhibitor, pervanadate slow the degradation of surface receptor in cultured muscle cells. Their action is mediated by tyrosine phosphorylation of the AChR β subunit, as agrin and pervandate had no effect on receptor half-life in AChR-β(3F/3F) muscle cells, which have targeted mutations of the β subunit cytoplasmic tyrosines. Moreover, in wild type AChR-β(3Y) muscle cells, we found a linear relationship between average receptor half-life and the percentage of AChR with phosphorylated β subunit, with half-lives of 12.7 and 23 h for nonphosphorylated and phosphorylated receptor, respectively. Surprisingly, pervanadate increased receptor half-life in AChR-β(3Y) myotubes in the absence of clustering, and agrin failed to increase receptor half-life in AChR-β(3F/3F) myotubes even in the presence of clustering. The metabolic stabilization of the AChR was mediated specifically by phosphorylation of βY390 as mutation of this residue abolished β subunit phosphorylation but did not affect δ subunit phosphorylation. Receptor stabilization also led to higher receptor levels, as agrin increased surface AChR by 30% in AChR-β(3Y) but not AChR-β(3F/3F) myotubes. Together, these findings identify an unexpected role for agrin-induced phosphorylation of β(Y390) in downregulating AChR turnover. This likely stabilizes AChR at developing synapses, and contributes to the extended half-life of AChR at adult NMJs.

Ca(V)1.1: The atypical prototypical voltage-gated Ca²⁺ channel

Ca(V)1.1 is the prototype for the other nine known Ca(V) channel isoforms, yet it has functional properties that make it truly atypical of this group. Specifically, Ca(V)1.1 is expressed solely in skeletal muscle where it serves multiple purposes; it is the voltage sensor for excitation-contraction coupling and it is an L-type Ca²⁺ channel which contributes to a form of activity-dependent Ca²⁺ entry that has been termed Excitation-coupled Ca²⁺ entry. The ability of Ca(V)1.1 to serve as voltage-sensor for excitation-contraction coupling appears to be unique among Ca(V) channels, whereas the physiological role of its more conventional function as a Ca²⁺ channel has been a matter of uncertainty for nearly 50 years. In this chapter, we discuss how Ca(V)1.1 supports excitation-contraction coupling, the possible relevance of Ca²⁺ entry through Ca(V)1.1 and how alterations of Ca(V)1.1 function can have pathophysiological consequences. This article is part of a Special Issue entitled: Calcium channels.

Neuromuscular synaptic patterning requires the function of skeletal muscle dihydropyridine receptors

Developing skeletal myofibers in vertebrates are intrinsically “pre-patterned” for motor nerve innervation. However, the intrinsic factors that regulate muscle pre-patterning remain unknown. Here we show that a functional skeletal muscle dihydropyridine receptor (DHPR, the L-type Ca²⁺ channel in muscle) is required for muscle pre-patterning during the development of the neuromuscular junction (NMJ). Targeted deletion of the β1 subunit of DHPR (Cacnb1) in mice leads to muscle pre-patterning defects, aberrant innervation and precocious maturation of the NMJ. Reintroducing the Cacnb1 gene into Cacnb1^−/− muscles reverses the pre-patterning defects and restores normal development of the NMJ. The mechanism by which DHPRs govern muscle pre-patterning is independent of their role in excitation-contraction coupling (E-C coupling), but requires Ca²⁺ influx through the L-type Ca²⁺ channel. Our findings demonstrate that the skeletal muscle DHPR retrogradely regulates the patterning and formation of the NMJ.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Myosin Va cooperates with PKA RIalpha to mediate maintenance of the endplate in vivo

Myosin V motor proteins facilitate recycling of synaptic receptors, including AMPA and acetylcholine receptors, in central and peripheral synapses, respectively. To shed light on the regulation of receptor recycling, we employed in vivo imaging of mouse neuromuscular synapses. We found that myosin Va cooperates with PKA on the postsynapse to maintain size and integrity of the synapse; this cooperation also regulated the lifetime of acetylcholine receptors. Myosin Va and PKA colocalized in subsynaptic enrichments. These accumulations were crucial for synaptic integrity and proper cAMP signaling, and were dependent on AKAP function, myosin Va, and an intact actin cytoskeleton. The neuropeptide and cAMP agonist, calcitonin-gene related peptide, rescued fragmentation of synapses upon denervation. We hypothesize that neuronal ligands trigger local activation of PKA, which in turn controls synaptic integrity and turnover of receptors. To this end, myosin Va mediates correct positioning of PKA in a postsynaptic microdomain, presumably by tethering PKA to the actin cytoskeleton.

Neural agrin increases postsynaptic ACh receptor packing by elevating rapsyn protein at the mouse neuromuscular synapse

Fluorescence resonance energy transfer (FRET) experiments at neuromuscular junctions in the mouse tibialis anterior muscle show that postsynaptic acetylcholine receptors (AChRs) become more tightly packed during the first month of postnatal development. Here, we report that the packing of AChRs into postsynaptic aggregates was reduced in 4-week postnatal mice that had reduced amounts of the AChR-associated protein, rapsyn, in the postsynaptic membrane (rapsyn(+/-) mice). We hypothesize that nerve-derived agrin increases postsynaptic expression and targeting of rapsyn, which then drives the developmental increase in AChR packing. Neural agrin treatment elevated the expression of rapsyn in C2 myotubes by a mechanism that involved slowing of rapsyn protein degradation. Similarly, exposure of synapses in postnatal muscle to exogenous agrin increased rapsyn protein levels and elevated the intensity of anti-rapsyn immunofluorescence, relative to AChR, in the postsynaptic membrane. This increase in the rapsyn-to-AChR immunofluorescence ratio was associated with tighter postsynaptic AChR packing and slowed AChR turnover. Acute blockade of synaptic AChRs with alpha-bungarotoxin lowered the rapsyn-to-AChR immunofluorescence ratio, suggesting that AChR signaling also helps regulate the assembly of extra rapsyn in the postsynaptic membrane. The results suggest that at the postnatal neuromuscular synapse agrin signaling elevates the expression and targeting of rapsyn to the postsynaptic membrane, thereby packing more AChRs into stable, functionally-important AChR aggregates.

Running to stand still: ionotropic receptor dynamics at central and peripheral synapses

For synapses to form and function, neurotransmitter receptors must be recruited to a location on the postsynaptic cell in direct apposition to presynaptic neurotransmitter release. However, once receptors are inserted into the postsynaptic membrane, they are not fixed in place but are continually exchanged between synaptic and extrasynaptic regions, and they cycle between the surface and intracellular compartments. This article highlights and compares the current knowledge about the dynamics of acetylcholine receptors at the vertebrate peripheral neuromuscular junction and AMPA, N-methyl-D-aspartate, and gamma-aminobutyric acid receptors in central synapses.

Developmental increase in the amount of rapsyn per acetylcholine receptor promotes postsynaptic receptor packing and stability

Neuromuscular synaptic transmission depends upon tight packing of acetylcholine receptors (AChRs) into postsynaptic AChR aggregates, but not all postsynaptic AChRs are aggregated. Here we describe a new confocal Fluorescence Resonance Energy Transfer (FRET) assay for semi-quantitative comparison of the degree to which AChRs are aggregated at synapses. During the first month of postnatal life the mouse tibialis anterior muscle showed increases both in the number of postsynaptic AChRs and the efficiency with which AChR was aggregated (by FRET). There was a concurrent two-fold increase in immunofluorescent labeling for the AChR-associated cytoplasmic protein, rapsyn. When 1-month old muscle was denervated, postsynaptic rapsyn immunostaining was reduced, as was the efficiency of AChR aggregation. In vivo electroporation of rapsyn-EGFP into muscle fibers increased postsynaptic rapsyn levels. Those synapses with higher ratios of rapsyn-EGFP to AChR displayed a slower metabolic turnover of AChR. Conversely, the reduction of postsynaptic rapsyn after denervation was accompanied by an acceleration of AChR turnover. Thus, a developmental increase in the amount of rapsyn targeted to the postsynaptic membrane may drive enhanced postsynaptic AChRs aggregation and AChR stability within the postsynaptic membrane.

L-type calcium channels mediate acetylcholine receptor aggregation on cultured muscle

Agrin activation of muscle specific kinase (MuSK) initiates postsynaptic development on skeletal muscle that includes the aggregation of acetylcholine receptors (AChRs; Glass et al. [1996]: Cell 85: 513-523; Gautam et al. [1996]: Cell 85: 525-535). Although the agrin/MuSK signaling pathway remains largely unknown, changes in intracellular calcium levels are required for agrin-induced AChR aggregation (Megeath and Fallon [1998]: J Neurosci 18: 672-678). Here, we show that L-type calcium channels (L-CaChs) are required for full agrin-induced aggregation of AChRs and sufficient to induce agrin-independent AChR aggregation. Blockade of L-CaChs in muscle cultures inhibited agrin-induced AChR aggregation but not tyrosine phosphorylation of MuSK or AChR beta subunits. Activation of L-CaChs in the absence of agrin induced AChR aggregation but not tyrosine phosphorylation of MuSK or AChR beta subunits. Agrin responsiveness was significantly reduced in primary muscle cultures from the muscular dysgenesis mouse, a natural mutant, which does not express the L-CaCh. Our results establish a novel role for L-CaChs as important sources of the intracellular calcium necessary for the aggregation of AChRs.

Activity-dependent neurotransmitter-receptor matching at the neuromuscular junction

Signaling in the nervous system requires matching of neurotransmitter receptors with cognate neurotransmitters at synapses. The vertebrate neuromuscular junction is the best studied cholinergic synapse, but the mechanisms by which acetylcholine is matched with acetylcholine receptors are not fully understood. Because alterations in neuronal calcium spike activity alter transmitter specification in embryonic spinal neurons, we hypothesized that receptor expression in postsynaptic cells follows changes in transmitter expression to achieve this specific match. We find that embryonic vertebrate striated muscle cells normally express receptors for glutamate, GABA, and glycine as well as for acetylcholine. As maturation progresses, acetylcholine receptor expression prevails. Receptor selection is altered when early neuronal calcium-dependent activity is perturbed, and remaining receptor populations parallel changes in transmitter phenotype. In these cases, glutamatergic, GABAergic, and glycinergic synaptic currents are recorded from muscle cells, demonstrating that activity regulates matching of transmitters and their receptors in the assembly of functional synapses.

The dynamics of recycled acetylcholine receptors at the neuromuscular junction in vivo

At the peripheral neuromuscular junction (NMJ), a significant number of nicotinic acetylcholine receptors (AChRs) recycle back into the postsynaptic membrane after internalization to intermingle with not-yet-internalized ;pre-existing' AChRs. However, the way in which these receptor pools are maintained and regulated at the NMJ in living animals remains unknown. Here, we demonstrate that recycled receptors in functional synapses are removed approximately four times faster than pre-existing receptors, and that most removed recycled receptors are replaced by new recycled ones. In denervated NMJs, the recycling of AChRs is significantly depressed and their removal rate increased, whereas direct muscle stimulation prevents their loss. Furthermore, we show that protein tyrosine phosphatase inhibitors cause the selective accumulation of recycled AChRs in the peri-synaptic membrane without affecting the pre-existing AChR pool. The inhibition of serine/threonine phosphatases, however, has no effect on AChR recycling. These data show that recycled receptors are remarkably dynamic, and suggest a potential role for tyrosine dephosphorylation in the insertion and maintenance of recycled AChRs at the postsynaptic membrane. These findings may provide insights into long-term recycling processes at less accessible synapses in the central nervous system in vivo.

In vivo regulation of acetylcholinesterase insertion at the neuromuscular junction

The efficiency of synaptic transmission between nerve and muscle depends on the number and density of acetylcholinesterase molecules (AChE) at the neuromuscular junction. However, little is known about the way this density is maintained and regulated in vivo. By using time lapse and quantitative fluorescence imaging assays in living mice, we demonstrated that insertion of new AChEs occurs within hours of saturating pre-existing AChEs with fasciculin2, a snake toxin that selectively labels AChE. In the absence of muscle postsynaptic activity or evoked nerve presynaptic neurotransmitter release, AChE insertion was decreased significantly, whereas direct stimulation of the muscle completely restored AChE insertion to control levels. This activity-dependent AChE insertion is mediated by intracellular calcium. In muscle stimulated in the presence of a Ca2+ channel blocker or calcium-permeable Ca2+ chelator, AChE insertion into synapses was significantly decreased, whereas ryanodine or ionophore A12387 treatment of blocked and unstimulated synapses significantly increased AChE insertion. These results demonstrated that synaptic activity is critical for AChE insertion and indicated that a rise in intracellular calcium either through voltage-gated calcium channels or from intracellular stores is critical for proper AChE insertion into the adult synapse.

Dual role for calcium in agrin signaling and acetylcholine receptor clustering

Agrin is a motoneuron-derived factor that initiates neuromuscular synapse formation; however, the signaling pathway underlying postsynaptic differentiation is not yet understood. We have investigated the role of calcium in agrin signaling through the MuSK receptor tyrosine kinase and in the intracellular signaling cascade that leads to AChR phosphorylation and clustering. We find that agrin- and neuramindase-induced MuSK activation in cultured myotubes is completely blocked by removal of extracellular calcium, but only slightly reduced by clamping of intracellular calcium transients with BAPTA. Following agrin's activation of MuSK, we find that the downstream tyrosine phosphorylation of the AChR beta-subunit was inhibited by BAPTA but not by a slower acting chelator, EGTA. Similarly, agrin-induced clustering of the AChR was blocked by BAPTA but not EGTA. These findings indicate that extracellular calcium is required for the formation of a MuSK signaling complex, and that intracellular calcium regulates phosphorylation and clustering of the AChR in the postsynaptic membrane.

Rapid and reversible effects of activity on acetylcholine receptor density at the neuromuscular junction in vivo

Quantitative fluorescence imaging was used to study the regulation of acetylcholine receptor (AChR) number and density at neuromuscular junctions in living adult mice. At fully functional synapses, AChRs have a half-life of about 14 days. However, 2 hours after neurotransmission was blocked, the half-life of the AChRs was now less than a day; the rate was 25 times faster than before. Most of the lost receptors were not quickly replaced. Direct muscle stimulation or restoration of synaptic transmission inhibited this process. AChRs that were removed from nonfunctional synapses resided for hours in the perijunctional membrane before being locally internalized. Dispersed AChRs could also reaggregate at the junction once neurotransmission was restored. The rapid and reversible alterations in AChR density at the neuromuscular junction in vivo parallel changes thought to occur in the central nervous system at synapses undergoing potentiation and depression.

Role of calcium permeation in dihydropyridine receptor function. Insights into channel gating and excitation-contraction coupling

The skeletal and cardiac muscle dihydropyridine receptors (DHPRs) differ with respect to their rates of channel activation and in the means by which they control Ca2+ release from the sarcoplasmic reticulum (Adams, B.A., and K.G. Beam. 1990. FASEB J. 4:2809-2816). We have examined the functional properties of skeletal (SkEIIIK) and cardiac (CEIIIK) DHPRs in which a highly conserved glutamate residue in the pore region of repeat III was mutated to a positively charged lysine residue. Using expression in dysgenic myotubes, we have characterized macroscopic ionic currents, intramembrane gating currents, and intracellular Ca2+ transients attributable to these two mutant DHPRs. CEIIIK supported very small inward Ca2+ currents at a few potentials (from -20 to +20 mV) and large outward cesium currents at potentials greater than +20 mV. SkEIIIK failed to support inward Ca2+ flux at any potential. However, large, slowly activating outward cesium currents were observed at all potentials greater than + 20 mV. The difference in skeletal and cardiac Ca2+ channel activation kinetics was conserved for outward currents through CEIIIK and SkEIIIK, even at very depolarized potentials (at +100 mV; SkEIIIK: tau(act) = 30.7 +/- 1.9 ms, n = 11; CEIIIK: tau(act) = 2.9 +/- 0.5 ms, n = 7). Expression of SkEIIIK in dysgenic myotubes restored both evoked contractions and depolarization-dependent intracellular Ca(2+) transients with parameters of voltage dependence (V(0.5) = 6.5 +/- 3.2 mV and k = 9.3 +/- 0.7 mV, n = 5) similar to those for the wild-type DHPR (Garcia, J., T. Tanabe, and K.G. Beam. 1994. J. Gen. Physiol. 103:125-147). However, CEIIIK-expressing myotubes never contracted and failed to exhibit depolarization-dependent intracellular Ca2+ transients at any potential. Thus, high Ca2+ permeation is required for cardiac-type excitation-contraction coupling reconstituted in dysgenic myotubes, but not skeletal-type. The strong rectification of the EIIIK channels made it possible to obtain measurements of gating currents upon repolarization to -50 mV (Qoff) following either brief (20 ms) or long (200 ms) depolarizing pulses to various test potentials. For SkEIIIK, and not CEIIK, Qoff was significantly (P < 0.001) larger after longer depolarizations to +60 mV (121.4 +/- 2.0%, n = 6). The increase in Qoff for long depolarizations exhibited a voltage dependence similar to that of channel activation. Thus, the increase in Q(off) may reflect a voltage sensor movement required for activation of L-type Ca2+ current and suggests that most DHPRs in skeletal muscle undergo this voltage-dependent transition.

Regulation of the size and distribution of ectopic neuromuscular junctions in adult skeletal muscle by nerve-derived trophic factor and electrical muscle activity

Transplanted axons induced multiple, irregularly distributed acetylcholine receptor (AChR) aggregates on muscle fibers at early stages of ectopic neuromuscular junction formation in denervated adult rat soleus muscles. Subsequently, most AChR aggregates disappeared (the losers). A few aggregates survived (the winners) and, as part of the surviving junctions, reached a certain size and spatial separation along the fibers. This elimination of losers and development of winners occurred only in electrically active muscles whether the activity was elicited by intact axons or by electrical muscle stimulation after the axons had been cut early. We conclude that electrical muscle activity regulates the size and distribution of ectopic neuromuscular junctions by acting in conjunction with a nerve-derived priming influence that does not require the continued presence of nerve terminals.

Synaptic segregation at the developing neuromuscular junction

Throughout the developing nervous system, competition between axons causes the permanent removal of some synaptic connections. In mouse neuromuscular junctions at birth, terminal branches of different axons are intermingled. However, during the several weeks after birth, these branches progressively segregated into nonoverlapping compartments before the complete withdrawal of all but one axon. Segregation was caused by selective branch atrophy, detachment, and withdrawal; the axon branches that were nearest to the competitor's branches were removed before the more distant branches were removed. This progression suggests that the signals that mediate the competitive removal of synapses must decrease in potency over short distances.

Rapsyn and agrin slow the metabolic degradation of the acetylcholine receptor

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

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.

Agrin-induced postsynaptic-like apparatus in skeletal muscle fibers in vivo

We find that when extrajunctional regions of denervated soleus muscles in adult rats are transfected with cDNA encoding rat agrin isoform Y4Z8, which is normally secreted by motor neurons at adult neuromuscular junctions, the myofibers express and secrete the neural agrin. Muscle fibers in the vicinity of transfection form at their surface specialized areas having extracellular, plasma membrane, and cytoplasmic protein aggregates, narrow and deep plasma membrane infoldings, and an accumulation of myonuclei, all of which are characteristic of the postsynaptic apparatus at neuromuscular junctions. We conclude that at ectopic neuromuscular junctions that form in the extrajunctional region of denervated adult soleus muscles after implantation of a foreign nerve, a single neural-derived factor, agrin, is sufficient not only to cause protein aggregation in the early stages of postsynaptic apparatus formation, as predicted by the agrin hypothesis, but also to bring about changes in conformation of the muscle fiber surface and distribution of organelles which appear as the apparatus reaches maturity.

Intracellular Ca2+ changes and Ca2+-activated K+ channel activation induced by acetylcholine at the endplate of mouse skeletal muscle fibres

1. Enzymatically isolated skeletal muscle fibres were used to investigate the effects of applying acetylcholine (ACh) onto the endplate area on intracellular free calcium concentration ([Ca2+]i) measured using the indicator indo-1 and single channel activity using the patch clamp technique. 2. Using a Tyrode solution containing 5 microM tetrodotoxin (TTX) as extracellular solution, ACh applications (at 0.1 or 1 mM) onto the endplate induced intracellular free calcium transients the mean maximal amplitude of which was 360 +/- 30 nM from a mean resting value of 72 +/- 7 nM (n = 13). In cells bathed with a K(+)-rich solution (145 mM K+), applications of ACh (0.1 mM) induced transient rises in [Ca2+]i from a mean resting value of 53 +/- 7 nM to a maximum of 222 +/- 24 nM (n = 33). 3. In cell-attached membrane patches at the endplate membrane of muscle fibres bathed in a K(+)-rich external solution, using a pipette filled with Tyrode solution, external application of 0.1 mM ACh could induce a transient burst opening of channels carrying an outward current of an average amplitude of 4.6 +/- 0.2 pA at 0 mV (n = 8). 4. These channels were characterized as Ca2(+)-activated K+ channels. At 0 mV, in inside-out patches excised from the endplate membrane area, they displayed a conductance of 60 and 224 pS in the presence of Tyrode and K(+)-rich solution in the pipette, respectively. Half-maximum activation was found for a [Ca2+]i close to 4 microM. The channels showed a typical voltage dependence. In outside-out patches these channels were shown to be blocked by 100 nM charybdotoxin (CTX). 5. In fibres bathed in a Tyrode solution containing TTX (5 microM), CTX had no clear effect on the change in membrane voltage, recorded near the endplate with a single intracellular microelectrode, in response to the application of ACh. 6. Although the physiological relevance of this ACh-induced K+ channel activation remains unclear, results suggest that, in the presence of a physiological extracellular [Ca2+], Ca2+ entry through the endplate nicotinic receptors can produce a local increase in [Ca2+]i, sufficient to trigger the opening of Ca2+-activated K+ channels in the adjacent surface membrane.

Activation of nicotinic acetylcholine receptors expressed in quail fibroblasts: effects on intracellular calcium

1. The aim of these experiments was to determine the ability of the muscle-type nicotinic acetylcholine receptor (AChR) stably expressed in quail fibroblasts (QF18 cells) to elevate intracellular calcium ([Ca2+]i) upon activation. Ratiometric confocal microscopy, with the calcium-sensitive fluorescent dye Indo-1 was used. 2. Application of the nicotine agonist, suberyldicholine (SDC), to the transfected QF18 cells caused an increase in [Ca2+]i. Control [Ca2+]i levels in QF18 cells were found to be 164 +/- 22 nM (mean +/- s.e. mean; n = 40 cells) rising to 600 +/- 81 nM on addition of SDC (10 microM; n = 15 cells), whereas no increase in [Ca2+]i was seen in non-transfected control QT6 fibroblasts (before: 128 +/- 9 nM, n = 40; after; 113 +/- 13 nM, n = 15). 3. The increase in [Ca2+]i caused by application of SDC was dose-dependent, with an EC50 value of 12.7 +/- 5.9 microM (n = 14). 4. The responses to SDC in QF18 cells were blocked by prior application of alpha-bungarotoxin (200 nM), by the addition of Ca2+ (100 microM), by removal of Na+ ions from the extracellular solution, or by the voltage-sensitive calcium channel blockers nifedipine and omega-conotoxin, which act with IC50 values of 100 nM and 100 pM respectively. 5. We conclude that activation of the nicotinic AChRs leads to a Na(+)-dependent depolarization and hence activation of endogenous voltage-sensitive Ca2+ channels in the plasma membrane and an increase in [Ca2+]i. There is no significant entry of Ca2+ through the nicotinic receptor itself.

Protein kinase A regulates the degradation rate of Rs acetylcholine receptors

Acetylcholine receptors at the neuromuscular junction of innervated vertebrate muscle (called Rs AChRs) have a stable degradation rate (t1/2 approximately 8-12 days) which accelerates after denervation to a half-life of approximately 3 days, but can be restabilized by reinnervation or by cAMP. We examined the mechanism by which cAMP regulates the Rs degradation rate. When dibutyryl cAMP (DB-cAMP) was applied to denervated mouse diaphragms in organ culture, it stabilized the accelerated degradation rate of the Rs. We found that this stabilization is reversible upon removal of the DB-cAMP, is cAMP specific and is mediated by intracellular cAMP. A major observation of this study is that the cAMP-induced stabilization of Rs AChRs is via protein kinase A (PKA), since H89, a PKA inhibitor, blocked the DB-cAMP induced stabilization of Rs, and H85, an analog of H89, which does not inhibit PKA but does inhibit other kinases as efficiently as H89, did not prevent the DB-cAMP-induced stabilization of Rs degradation. These results suggest that the cAMP messenger system via a PKA-dependent pathway could be among the mechanisms whereby the nerve regulates AChR degradation.

Hydrocortisone influences voltage-dependent L-type calcium channels in cultured human skeletal muscle

The glucocorticoid hydrocortisone (HC), applied for up to 2 weeks to either aneurally or innervated cultured human muscle, produced 2-fold increase of the number of dihydropyridine ([3H]PN200-110) binding sites. The K(+)-induced, nifedipine-inhibited Ca2+ uptake was increased 40%. The effect of HC was concentration- and time-dependent. [3H]PN200-110 affinity for its receptor was not affected by HC treatment. HC did not exert significant influence on the total amount of protein, CK activity, and the number of myotubes. These results indicate that voltage-dependent L-type Ca2+ channel expression in human muscle is regulated by glucocorticoid.

Local neurotrophic repression of gene transcripts encoding fetal AChRs at rat neuromuscular synapses

The spatio-temporal expression patterns of mRNA transcripts coding for acetylcholine receptor (AChR) subunits and myogenic factors were measured in denervated rat soleus muscle and in soleus muscle chronically paralyzed for up to 12 d by conduction block of the sciatic nerve by tetrodotoxin (TTX). In denervated muscle the AChR alpha-, beta-, gamma-, and delta-subunit mRNAs were elevated with highest expression levels in the former synaptic and the perisynaptic region and with lower levels in the extrasynaptic fiber segments. In muscle paralyzed by nerve conduction block the alpha-, beta-, gamma-, and delta-subunit mRNA levels increased only in extrasynaptic fiber segments. Surprisingly, in the synaptic region the gamma-subunit mRNA that specifies the fetal-type AChR, and alpha-, beta-, delta-subunit mRNAs were not elevated. The expression of the gene encoding the epsilon-subunit, which specifies the adult-type AChR, was always restricted to synaptic nuclei. The mRNA for the regulatory factor myogenin showed after denervation similar changes as the subunit transcripts of the fetal AChR. When the muscle was paralyzed by nerve conduction block the increase of myogenin transcripts was also less pronounced in synaptic regions compared to extrasynaptic fiber segments. The results suggest that in normal soleus muscle a neurotrophic signal from the nerve locally down-regulates the expression of fetal-type AChR channel in the synaptic and perisynaptic muscle membrane by inhibiting the expression of the gamma-subunit gene and that inhibition of the myogenin gene expression may contribute to this down-regulation.

Neurotoxins in the study of neural regulation of membrane proteins in skeletal muscle

The discovery and purification of several neurotoxins, including alpha-bungarotoxin and tetrodotoxin has provided very high-affinity ligands which have proved to be central to the elucidation of the neural control of skeletal muscle membrane proteins and to the purification and characterization of the nicotinic acetylcholine receptor (AChR) and the Na+ channel, respectively. This review describes the use of neurotoxins for quantification and localization of receptors and ion channels in normal and denervated skeletal muscles with particular emphasis on the appropriateness of the muscle preparation and ligand used in the studies. It is now clear that the nerve controls the synthesis and spatial distribution of AChRs and Na+ channels by regulating gene expression in extrajunctional and subjunctional nuclei. The down-regulation of extrajunctional AChRs is primarily mediated by neuromuscular activity and the concentration of AChRs and Na+ channels in specific membrane domains at the neuromuscular junction is controlled by a number of neurotrophic substances at the neuromuscular junction. These include agrin, ARIA, and CGRP.

Ca2+ signalling pathways activated by acetylcholine in mouse C2C12 myotubes

In mouse C2C12 myotubes acetylcholine (ACh) elevates the concentration of myoplasmic Ca2+ ([Ca2+]i) by inducing Ca2+ influx through transmitter-gated and voltage-gated channels, and by mobilizing Ca2+ from internal stores. The relative contribution of each of these ACh-activated sources to the global [Ca2+]i elevation was estimated. We found that Ca2+ entry through voltage- and ACh-gated channels accounts for roughly 80% of the total [Ca2+]i increment, while mobilization from internal caffeine-sensitive and inositoltrisphosphate- (InsP3-) sensitive stores contributes the remaining 20% to the maximal [Ca2+]i increment. Furthermore, we found that ACh-induced mobilization from InsP3-sensitive stores also develops in embryonic chick myotubes. The differential importance of the Ca2+ signalling pathways activated by ACh during myogenesis is discussed.

Depolarization-transcription signals in skeletal muscle use calcium flux through L channels, but bypass the sarcoplasmic reticulum

Membrane depolarization inactivates acetylcholine receptor (AChR) genes in skeletal muscle. We have studied this process in C2C12 cells, focusing on the role of calcium. Cytoplasmic calcium was monitored with fluo-3, and the activity of receptor genes was measured with a sensitive transcript elongation assay. Removal of extracellular calcium or blockage of L-type calcium channels disrupts signaling, even when release of calcium from the sarcoplasmic reticulum (SR) is not impeded, whereas L channel agonists induce signaling without membrane depolarization or release of calcium from intracellular stores. Activators of calcium release from the SR do not inhibit AChR genes, either in C2C12 or in chicken skeletal muscle in vivo. It appears that calcium ions do not act as messengers between sarcolemma and nucleus but target a sensor near their port of entry where they initiate a signal that bypasses the SR.

A cAMP-dependent Ca2+ signalling pathway at the endplate provided by the gamma to epsilon subunit switch in ACh receptors

During development of neuromuscular junctions there is a switch in the expression of acetylcholine receptors (AChR) from an embryonic form (gamma-AChR) to an adult form (epsilon-AChR). Studies with gamma- and epsilon-AChRs expressed in Xenopus oocytes showed that the gamma to epsilon subunit switch accelerates rates of desensitization and increases Ca2+ permeability. Site-directed mutagenesis of the gamma and epsilon subunits suggests that these changes are regulated by cAMP-dependent phosphorylation on the epsilon subunit. These results suggest that the gamma to epsilon subunit switch could provide for a cAMP-dependent Ca2+ signalling pathway at the endplate.

The neuromuscular junction. Muscle fibre type differences, plasticity and adaptability to increased and decreased activity

The neuromuscular junction (NMJ) of adult mammalian muscle is the site of the transduction of electrical stimuli, generated by the nervous system, to the underlying muscle fibres, resulting in muscle action. It has been demonstrated that, in some ways, the morphology of the NMJ is specific to muscle fibre type. It is also known that while the structure of the NMJ generally remains stable in young, healthy adults, a subtle form of remodelling continuously occurs at this synapse. The morphology and physiology of the NMJ have been shown to adapt to both increased, and decreased use. Indeed, morphological changes of the NMJ are associated with functional alterations in neuromuscular transmission. Increased activity of the myoneural synapse results in adaptations that enhance neuromuscular transmission and, thus, muscle performance. Similarly to increased usage, decreased neuromuscular activity results in structural alterations of the NMJ. However, unlike those responses observed with enhanced activity, decreased recruitment of the myoneural synapse can impair neuromuscular transmission and muscle performance. Thus, the NMJ demonstrates both anatomical and physiological adaptations following substantial changes in its pattern of activity. These NMJ adaptations can affect the functional capacity of skeletal muscle in vivo.

Co-localization of 1,4-dihydropyridine receptor alpha 2/delta subunit and N-CAM during early myogenesis in vitro

The surface distribution of the alpha 2/delta subunit of the 1,4-dihydropyridine receptor and its topographical relationship with the neural cell adhesion molecule (N-CAM) were investigated during early myogenesis in vitro, by double immunocytochemical labeling with the monoclonal antibody 3007 and an anti-N-CAM polyclonal antiserum. The monoclonal antibody 3007 has been previously shown to immunoprecipitate dihydropyridine receptor from skeletal muscle T-tubules. In further immunoprecipitation experiments on such preparations and muscle cell cultures, it was demonstrated here that the monoclonal antibody 3007 exclusively recognizes the alpha 2/delta subunit of the 1,4-dihydropyridine receptor. In rabbit muscle cell cultures, the labeling for both alpha 2/delta and N-CAM was first detected on myoblasts, in the form of spots on the membrane and perinuclear patches. Spots of various sizes organized in aggregates were then found on the membrane of myotubes. At fusion (T0), aggregates of N-CAM spots alone were found at the junction between fusing cells. At T6 and later stages, all alpha 2/delta aggregates present on myotubes co-localized with N-CAM, while less than 3% of N-CAM aggregates did not co-localize with alpha 2/delta. A uniform N-CAM staining also made its appearance. At T12, when myotubes showed prominent contractility, alpha 2/delta-N-CAM aggregates diminished in size. Dispersed alpha 2/delta spots of a small regular size spread over the whole surface of the myotubes and alignments of these spots became visible. Corresponding N-CAM spots were now occasionally seen, and uniform N-CAM staining was prominent. These results show that alpha 2/delta and N-CAM are co-localized and that their distributions undergo concomitant changes during early myogenesis until the T-tubule network starts to be organized. This suggest that these two proteins might jointly participate in morphogenetic events preceding the formation of T-tubules.

Long-term treatment with glucocorticoids increases synthesis and stability of junctional acetylcholine receptors on innervated cultured human muscle

We have studied the effect of long-term treatment with hydrocortisone on the expression of acetylcholine receptors (AChRs) at the neuromuscular junctions of human muscle cultured in monolayer and innervated de novo by fetal rat spinal cord motoneurons. Hydrocortisone increased accumulation of junctional AChRs in a dose- and time-dependent fashion. This increase was due to both decreased degradation and increased synthesis of AChRs. Other glucocorticoids, dexamethasone and prednisolone, exerted similar effects. Our study demonstrates a novel action of glucocorticoids on human junctional AChRs.

Measures of skeletal muscle calcium channels and acetylcholine receptors in thyroidectomized rats

Hypothyroidism frequently presents with muscle complaints. No consistent histopathology nor electrophysiology explains these symptoms and signs. As well, no previous study shows specific changes in components of the nerve-muscle synapse nor excitation-contraction coupling in adult muscles, but changes are seen in hormone-treated embryonic myoblasts. In this study, adult male Holtzman rats underwent thyroidectomy and their age-matched euthyroid controls were simultaneously subjected to sham operation. Thirty days post-operative, animals were sacrificed for anterior tibialis muscles harvest. Muscle dihydropyridine type calcium channel (isradipine) and acetylcholine receptor (alpha-bungarotoxin) binding were measured and compared between experimental treatment groups. There were no significant differences in either the affinity or density of isradipine binding. However, hypothyroid muscles showed a nearly 50% reduction in acetylcholine receptor density when compared to control muscles. Thyroidectomy is associated with specific effects on components of neuromuscular transmission in adult fast twitch muscle.

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.