Induction of phosphorylation and cell surface redistribution of acetylcholine receptors by phorbol ester and carbamylcholine in cultured chick muscle cells

Regulation of nicotinic acetylcholine receptors by post-translational modifications

Nicotinic acetylcholine receptors (nAChRs) comprise a family of pentameric ligand-gated ion channels widely distributed in the central and peripheric nervous system and in non-neuronal cells. nAChRs are involved in chemical synapses and are key actors in vital physiological processes throughout the animal kingdom. They mediate skeletal muscle contraction, autonomic responses, contribute to cognitive processes, and regulate behaviors. Dysregulation of nAChRs is associated with neurological, neurodegenerative, inflammatory and motor disorders. In spite of the great advances in the elucidation of nAChR structure and function, our knowledge about the impact of post-translational modifications (PTMs) on nAChR functional activity and cholinergic signaling has lagged behind. PTMs occur at different steps of protein life cycle, modulating in time and space protein folding, localization, function, and protein-protein interactions, and allow fine-tuned responses to changes in the environment. A large body of evidence demonstrates that PTMs regulate all levels of nAChR life cycle, with key roles in receptor expression, membrane stability and function. However, our knowledge is still limited, restricted to a few PTMs, and many important aspects remain largely unknown. There is thus a long way to go to decipher the association of aberrant PTMs with disorders of cholinergic signaling and to target PTM regulation for novel therapeutic interventions. In this review we provide a comprehensive overview of what is known about how different PTMs regulate nAChR.

PKC and PKA regulate AChR dynamics at the neuromuscular junction of living mice

The steady state of the acetylcholine receptor (AChR) density at the neuromuscular junction (NMJ) is critical for efficient and reliable synaptic transmission. However, little is known about signaling molecules involved in regulating the equilibrium between the removal and insertion of AChRs that establishes a stable postsynaptic receptor density over time. In this work, we tested the effect of activities of two serine/threonine kinases, PKC and PKA, on the removal rate of AChRs from and the re-insertion rate of internalized recycled AChRs into synaptic sites of innervated and denervated NMJs of living mice. Using an in vivo time-lapse imaging approach and various pharmacological agents, we showed that PKC and PKA activities have antagonistic effects on the removal and recycling of AChRs. Inhibition of PKC activity or activation of PKA largely prevents the removal of pre-existing AChRs and promotes the recycling of internalized AChRs into the postsynaptic membrane. In contrast, stimulation of PKC or inactivation of PKA significantly accelerates the removal of postsynaptic AChRs and depresses AChR recycling. These results indicate that a balance between PKA and PKC activities may be critical for the maintenance of the postsynaptic receptor density.

Protein kinase C isoforms at the neuromuscular junction: localization and specific roles in neurotransmission and development

The protein kinase C family (PKC) regulates a variety of neural functions including neurotransmitter release. The selective activation of a wide range of PKC isoforms in different cells and domains is likely to contribute to the functional diversity of PKC phosphorylating activity. In this review, we describe the isoform localization, phosphorylation function, regulation and signalling of the PKC family at the neuromuscular junction. Data show the involvement of the PKC family in several important functions at the neuromuscular junction and in particular in the maturation of the synapse and the modulation of neurotransmission in the adult.

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

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

Decreased phosphorylation of δ and ε subunits of the acetylcholine receptor coincides with delayed postsynaptic maturation in PKC θ deficient mouse

Protein kinase C (PKC) activity is involved in the nicotinic acetylcholine receptor (nAChR) redistribution at the neuromuscular junction in vivo during postnatal maturation. Here we studied, in PKC theta (PKCtheta) deficient mice (KO), how the theta isoform of PKC is involved in the nAChR cluster maturation that is accompanied by the developmental activity-dependent neuromuscular synapse elimination process. We found that axonal elimination and dispersion of nAChR from the postsynaptic plaques and its redistribution to form the mature postsynaptic apparatus were delayed but not totally suppressed in PKCtheta deficient mice. Moreover, the delay in the maturation of the morphology of the nAChR clusters during the early postnatal synapse elimination period in the PKCtheta deficient mice coincides with a reduction in the PKCtheta-mediated phosphorylation on the delta subunit of the nAChR. In addition, we show evidence for PKCtheta regulation of PKA in normally phosphorylating the epsilon subunit of nAChR. We have also found that the theta isoform of PKC is located on the postsynaptic component of the neuromuscular junction but is also expressed by motoneurons in the spinal cord and in the motor nerve terminals. The results allow us to hypothesize that a spatially specific and opposing action of PKCtheta and PKA may result in activity-dependent alterations to synaptic connectivity at both the nerve inputs and the postsynaptic nAChR clusters.

The function of Shp2 tyrosine phosphatase in the dispersal of acetylcholine receptor clusters

Background

A crucial event in the development of the vertebrate neuromuscular junction (NMJ) is the postsynaptic enrichment of muscle acetylcholine (ACh) receptors (AChRs). This process involves two distinct steps: the local clustering of AChRs at synapses, which depends on the activation of the muscle-specific receptor tyrosine kinase MuSK by neural agrin, and the global dispersal of aneural or "pre-patterned" AChR aggregates, which is triggered by ACh or by synaptogenic stimuli. We and others have previously shown that tyrosine phosphatases, such as the SH2 domain-containing phosphatase Shp2, regulate AChR cluster formation in muscle cells, and that tyrosine phosphatases also mediate the dispersal of pre-patterned AChR clusters by synaptogenic stimuli, although the specific phosphatases involved in this latter step remain unknown.

Results

Using an assay system that allows AChR cluster assembly and disassembly to be studied separately and quantitatively, we describe a previously unrecognized role of the tyrosine phosphatase Shp2 in AChR cluster disassembly. Shp2 was robustly expressed in embryonic Xenopus muscle in vivo and in cultured myotomal muscle cells, and treatment of the muscle cultures with an inhibitor of Shp2 (NSC-87877) blocked the dispersal of pre-patterned AChR clusters by synaptogenic stimuli. In contrast, over-expression in muscle cells of either wild-type or constitutively active Shp2 accelerated cluster dispersal. Significantly, forced expression in muscle of the Shp2-activator SIRPα1 (signal regulatory protein α1) also enhanced the disassembly of AChR clusters, whereas the expression of a truncated SIRPα1 mutant that suppresses Shp2 signaling inhibited cluster disassembly.

Conclusion

Our results suggest that Shp2 activation by synaptogenic stimuli, through signaling intermediates such as SIRPα1, promotes the dispersal of pre-patterned AChR clusters to facilitate the selective accumulation of AChRs at developing NMJs.

Synaptic transmission is impaired at neuronal autonomic synapses in agrin-null mice

Neuronal synapse formation is a multistep process regulated by several pre- and postsynaptic adhesion and signaling proteins. Recently, we found that agrin acts as one such synaptogenic factor at neuronal synapses in the PNS by demonstrating that structural synapse formation is impaired in the superior cervical ganglia (SCG) of z+ agrin-deficient mice and in SCG cultures derived from those animals. Here, we tested whether synaptic function is defective in agrin-null (AGD-/-) ganglia and began to define agrin's mechanism of action. Our electrophysiological recordings of compound action potentials showed that presynaptic stimulation evoked action potentials in approximately 40% of AGD-/- ganglionic neurons compared to 90% of wild-type neurons; moreover, transmission could not be potentiated as in wild-type or z+ agrin-deficient ganglia. Intracellular recordings also showed that nerve-evoked excitatory postsynaptic potentials in AGD-/- neurons were only 1/3 the size of those in wild-type neurons and mostly subthreshold. Consistent with these defects in transmission, we found an approximately 40-50% decrease in synapse number in AGD-/- ganglia and cultures, and decreased levels of differentiation at the residual synapses in culture. Furthermore, surface levels of acetylcholine receptors (AChRs) were equivalent in cultured AGD-/- and wild-type neurons, and depolarization reduced the synaptic localization of AChRs in AGD-/- but not wild-type neurons. These findings provide the first direct demonstration that agrin is required for proper structural and functional development of an interneuronal synapse in vivo. Moreover, they suggest a novel role for agrin, in stabilizing the postsynaptic density of nAChR at nascent neuronal synapses.

Phosphorylation of the nicotinic acetylcholine receptor in myotube-cholinergic neuron cocultures

Acetylcholine receptor (AChR) stability in the postsynaptic membrane is affected by serine kinases. AChR are phosphorylated by protein kinase C (PKC) and PKA, and we have shown that activation of PKA and PKC have opposite effects on AChR stability and that this may play some role in the selective, activity-dependent synapse loss that occurs during development of the neuromuscular junction. Myotube cultures with and without added spinal motor neurons were probed with immunoaffinity-purified antibodies prepared against phosphorylated peptides with amino acid sequences from different AChR subunits. Different treatments activating PKC (phorbol 12-myristate 13-acetate; PMA) or PKA (dibutyryl cyclic adenosine monophosphate; cAMP) or blocking electrical activity (tetrodotoxin; TTX) of the cocultures were chosen because of their known effects, direct or indirect, on receptor stability. We asked whether the phospho-specific antibody staining in conjunction with alpha-bungarotoxin (BTX) identification of AChR aggregates could provide a direct demonstration of changes in receptor phosphorylation produced by the treatments. We found that PMA treatment did increase phosphorylation of the delta subunit and cAMP increased phosphorylation of the epsilon subunit relative to total BTX labeling in muscle-nerve cocultures, but not in muscle-only cultures. Blockade of electrical activity with TTX increased the incidence of aggregates that showed no phospho-epsilon staining. Myotube cultures grown in the absence of neurons did not show the responses of myotubes in cocultures. The results show that manipulations that alter receptor stability also produce changes in receptor phosphorylation. We suggest that phosphorylation may be a mechanism mediating the changes in receptor stability.

Molecular regulation of postsynaptic differentiation at the neuromuscular junction

The neuromuscular junction (NMJ) is a synapse that develops between a motor neuron and a muscle fiber. A defining feature of NMJ development in vertebrates is the re-distribution of muscle acetylcholine (ACh) receptors (AChRs) following innervation, which generates high-density AChR clusters at the postsynaptic membrane and disperses aneural AChR clusters formed in muscle before innervation. This process in vivo requires MuSK, a muscle-specific receptor tyrosine kinase that triggers AChR re-distribution when activated; rapsyn, a muscle protein that binds and clusters AChRs; agrin, a nerve-secreted heparan-sulfate proteoglycan that activates MuSK; and ACh, a neurotransmitter that stimulates muscle and also disperses aneural AChR clusters. Moreover, in cultured muscle cells, several additional muscle- and nerve-derived molecules induce, mediate or participate in AChR clustering and dispersal. In this review we discuss how regulation of AChR re-distribution by multiple factors ensures aggregation of AChRs exclusively at NMJs.

Cooperative regulation by Rac and Rho of agrin-induced acetylcholine receptor clustering in muscle cells

A key aspect of neuromuscular synapse formation is the clustering of muscle acetylcholine receptors (AChR) at synaptic sites in response to neurally secreted agrin. Agrin-induced AChR clustering in cultured myotubes proceeds via the initial formation of small microclusters, which then aggregate to form AChR clusters. Here we show that the coupling of agrin signaling to AChR clustering is dependent on the coordinated activities of Rac and Rho GTPases. The addition of agrin induces the sequential activation of Rac and Rho in C2 muscle cells. The activation of Rac is rapid and transient and constitutes a prerequisite for the subsequent activation of Rho. This temporal pattern of agrin-induced Rac and Rho activation reflects their respective roles in AChR cluster formation. Whereas agrin-induced activation of Rac is necessary for the initial phase of AChR cluster formation, which involves the aggregation of diffuse AChR into microclusters, Rho activation is crucial for the subsequent condensation of these microclusters into full-size AChR clusters. Co-expression of constitutively active forms of Rac and Rho is sufficient to induce the formation of mature AChR clusters in the absence of agrin. These results establish that Rac and Rho play distinct but complementary roles in the mechanism of agrin-induced AChR clustering.

Overexpression of nPKC theta is inhibitory for agrin-induced nicotinic acetylcholine receptor clustering in C2C12 myotubes

Protein kinase C (PKC) activity has been implicated in nicotinic acetylcholine receptor (nAChR) cluster disruption but the specific PKC isoforms involved have not been identified. We first tested whether phorbol esters, which activate PKCs, regulate agrin-induced nAChR clustering in C(2)C(12) cells. We found that extended phorbol ester treatment (6 hr) increased nAChR clustering by two-fold. This increase correlated in time with downregulation of PKCs, as indicated by the disappearance of cPKC alpha, suggesting that the presence of PKCs is inhibitory for maximal nAChR clustering. To address the question whether nPKC theta, a specific PKC isoform restricted in expression to skeletal muscle and localized to neuromuscular junctions, regulates agrin-induced nAChR cluster formation we overexpressed an nPKC theta -green fluorescent protein (GFP) fusion protein in C(2)C(12) myotubes. The number of nAChR clusters was significantly reduced in nPKC theta-GFP compared to GFP overexpressing myotubes at less-than-maximal clustering concentrations of agrin. These data indicate that nPKC theta activity inhibits nAChR cluster formation. To examine whether nPKC theta activation by phorbol esters regulates agrin-induced nAChR clustering, we treated overexpressing myotubes overnight with maximal agrin concentrations followed by phorbol esters for 1 hr. Phorbol ester treatment reduced preexisting nAChR cluster numbers in nPKC theta-GFP compared to GFP-overexpressing myotubes, suggesting that stimulating nPKC theta activity disrupts nAChR clusters in the presence of maximal clustering concentrations of agrin. Together these findings, that nPKC theta activity inhibits agrin-induced nAChR cluster formation and disrupts preexisting agrin-induced nAChR clusters, suggest that nPKC theta activity is inhibitory for agrin function.

Pre- and postsynaptic mechanisms in Hebbian activity-dependent synapse modification

We have used a three compartment tissue culture system that involved two separate populations of cholinergic neurons in the side compartments that converged on a common target population of myotubes in the center compartment. Activation of the axons from one population of neurons produced selective down-regulation of the synaptic inputs from the other neuronal population (when the two inputs innervated the same myotubes). The decrease in heterosynaptic inputs was mediated by protein kinase C (PKC). An activity-dependent action of protein kinase A (PKA) was associated with the stimulated input and this served to selectively stabilize this input. These changes associated with PKA and PKC activation were mediated by alterations in the number of acetylcholine receptors at the neuromuscular junction. These results suggest that neuromuscular electrical activity produces postsynaptic activation of both PKA and PKC, with the latter producing generalized synapse weakening and the former a selective synapse stabilization. Treatment of the neuronal cell body and axon to increase PKC activity by putting phorbal ester (PMA) in the side chamber did not affect synaptic transmission (with or without stimulation). By contrast, PKA blockade in the side compartment did produce an activity-dependent decrease in synaptic efficacy, which was due to a decrease in quantal release of neurotransmitter. Thus, when the synapse is activated, it appears that presynaptic PKA action is necessary to maintain transmitter output.

Activation of protein kinase C isozymes in primary mouse myotubes by carbachol

The activation of muscle PKC isozymes following treatment with carbachol, an acetylcholine receptor agonist, has been investigated. Primary mouse myotubes were treated with carbachol, and protein extracts from the cytosol and membrane fractions of the myotubes were subjected to Western blot analyses. Carbachol treatment resulted in a rapid translocation of PKC-theta; to the membrane. This effect was dependent on both carbachol concentration and incubation time. The treatment also resulted in a drastic increase of PKC-alpha in the cytosol followed by an increase of PKC-alpha in the membrane. The regulation of PKC-alpha in response to carbachol was quite distinct from that produced by the PKC activator, PMA, which rapidly translocated PKC-alpha from the cytosol to the membrane without any increases in PKC-alpha in the cytosol. Confocal microscopy demonstrated an enhanced membrane localization of PKC-theta; and overall increased intensity of PKC-alpha staining in the cytosol accompanied by a characteristic membrane staining of PKC-alpha in the myotubes treated with carbachol. Taken together, the results suggested that the activation of PKC isozymes in response to the receptor agonist is quite distinct, which indicates their diverse role in the muscle upon the release of neurotransmitter at the neuromuscular junction.

Pre- and postsynaptic maturation of the neuromuscular junction during neonatal synapse elimination depends on protein kinase C

The distribution of acetylcholine receptors (AChRs) within and around the neuromuscular junction changes dramatically during the first postnatal weeks, a period during which polyneuronal innervation is eliminated. We reported previously that protein kinase C (PKC) activation accelerates postnatal synapse loss. Because of the close relationship between axonal retraction and AChR cluster dispersal, we hypothesize that PKC can modulate morphological maturation changes of the AChR clusters in the postsynaptic membrane during neonatal axonal reduction. We applied substances affecting PKC activity to the neonatal rat levator auris longus muscle in vivo. Muscles were then stained immunohistochemically to detect both AChRs and axons. We found that, during the first postnatal days of normal development, substantial axonal loss preceded the formation of areas in synaptic sites that were free of AChRs, implying that axonal loss could occur independently of changes in AChR cluster organization. Nevertheless, there was a close relationship between axonal loss and AChR organization; PKC modulates both, although differently. Block of PKC activity with calphostin C prevented both AChR loss and axonal loss between postnatal days 4 and 6. PKC may act primarily to influence AChR clusters and not axons, insofar as phorbol ester activation of PKC accelerated changes in receptor aggregates but produced relatively little axon loss.

Pertussis toxin-sensitive G-protein and protein kinase C activity are involved in normal synapse elimination in the neonatal rat muscle

Individual skeletal muscle fibers in most new-born rodents are innervated at a single endplate by several motor axons. During the first postnatal weeks, the polyneuronal innervation decreases in a process of synaptic elimination. Previous studies showed that the naturally occurring serine-protease thrombin mediates the activity-dependent synapse reduction at the neuromuscular junction (NMJ) in vitro and that thrombin-receptor activation may modulate nerve terminal consolidation through a protein kinase mechanism. To test whether these mechanisms may be operating in vivo, we applied external thrombin and its inhibitor hirudin, and several substances affecting the G protein-protein kinase C system (GP-PKC) directly over the external surface of the neonatal rat Levator auris longus muscle. Muscles were processed for immunocytochemistry to simultaneously detect acetylcholine receptors (AChRs) and axons for counting the percentage of polyinnervated NMJ. We found that exogenous thrombin accelerated synapse loss and hirudin blocked axonal removal. Phorbol-12-myristate-13-acetate, a potent PKC activator, had a similar effect as thrombin, whereas the PKC inhibitors, calphostin C and staurosporine, prevented axonal removal. Pertussis toxin, an effective blocker of GP function, blocked synapse elimination. These findings suggest that the normal synapse elimination in the neonatal rat muscle may be modulated, at least in part, by the pertussis-sensitive G-protein and PKC activity and that thrombin could play a role in the postnatal synaptic maturation in vivo.

Protein kinase C-mediated changes in synaptic efficacy at the neuromuscular junction in vitro: the role of postsynaptic acetylcholine receptors

Activation of a mouse in vitro neuromuscular synapse produces a reduction in synaptic efficacy which is greater for nonactivated than for activated inputs to the myotubes. This has been shown to require thrombin and thrombin receptor activation and to involve a protein kinase C (PKC)-mediated step. We show in the present work that phorbol ester activation of PKC produces physiological loss of synapses in a time- and dose-related manner. We observe, using quantitative imaging methods, a parallel loss of acetylcholine receptors (AChR) from synaptically functional neurite-associated receptor aggregates in nerve-muscle cocultures. Biochemical measurements of total AChR show that PKC activation reduces both AChR stability (increases receptor loss) and receptor insertion into the surface membrane. Taken together, the data suggest that PKC activation decreases the stability of AChR aggregates in the muscle surface membrane. We conclude that PKC plays a crucial role in activity-dependent synapse reduction and does so, at least in part, by altering AChR stability.

Regulation of ligand-gated ion channels by protein phosphorylation

The studies discussed in this review demonstrate that phosphorylation is an important mechanism for the regulation of ligand-gated ion channels. Structurally, ligand-gated ion channels are heteromeric proteins comprised of homologous subunits. For both the AChR and the GABA(A) receptor, each subunit has a large extracellular N-terminal domain, four transmembrane domains, a large intracellular loop between transmembrane domains M3 and M4, and an extracellular C-terminal domain (Fig. 1B). All the phosphorylation sites on these receptors have been mapped to the major intracellular loop between M3 and M4 (Table 1). In contrast, glutamate receptors appear to have a very large extracellular N-terminal domain, one membrane hairpin loop, three transmembrane domains, a large extracellular loop between transmembrane domains M3 and M4, and an intracellular C-terminal domain (Fig. 1C). Most phosphorylation sites on glutamate receptors have been shown to be on the intracellular C-terminal domain, although some have been suggested to be on the putative extracellular loop between M3 and M4 (Table 1). A variety of extracellular factors and intracellular signal transduction cascades are involved in regulating phosphorylation of these ligand-gated ion channels (Fig. 2). Once again, the AChR at the neuromuscular junction is the most fully understood system. Phosphorylation of the AChR by PKA is stimulated synaptically by the neuropeptide CGRP and in an autocrine fashion by adenosine released from the muscle in response to acetylcholine. In addition, acetylcholine, via calcium influx through the AChR, appears to activate calcium-dependent kinases including PKC to stimulate serine phosphorylation of the receptor. Presently, agrin is the only extracellular factor known to stimulate phosphorylation of the AChR on tyrosine residues. For glutamate receptors, non-NMDA receptor phosphorylation by PKA is stimulated by dopamine, while NMDA receptor phosphorylation by PKA and PKC can be induced via the activation of beta-adrenergic receptors, and metabotropic glutamate or opioid receptors, respectively. In addition, Ca2+ influx through the NMDA receptor has been shown to activate PKC. CaMKII, and calcineurin, resulting in phosphorylation of AMPA receptors (by CaMKII) and inactivation of NMDA receptors (at least in part through calcineurin). In contrast to the AChR and glutamate receptors, no information is presently available regarding the identities of the extracellular factors and intracellular signal transduction cascades that regulate phosphorylation of the GABA(A) receptor. Surely, future studies will be aimed at further clarifying the molecular mechanisms by which the central receptors are regulated. The presently understood functional effects of ligand-gated ion channel phosphorylation are diverse. At the neuromuscular junction, a regulation of the AChR desensitization rate by both serine and tyrosine phosphorylation has been demonstrated. In addition, tyrosine phosphorylation of the AChR or other synaptic components appears to play a role in AChR clustering during synaptogenesis. For the GABA(A) receptor, the data are complex. Both activation and inhibition of GABA(A) receptor currents as a result of PKA and PKC phosphorylation have been reported, while phosphorylation by PTK enhances function. The predominant effect of glutamate receptor phosphorylation by a variety of kinases is a potentiation of the peak current response. However, PKC also modulates clustering of NMDA receptors. This complexity in the regulation of ligand-gated ion channels by phosphorylation provides diverse mechanisms for mediating synaptic plasticity. In fact, accumulating evidence supports the involvement of protein phosphorylation and dephosphorylation of AMPA receptors in LTP and LTD respectively. There has been a dramatic increase in our understanding of the nature by which phosphorylation regulates ligand-gated ion channels. However, many questions remain unanswered. (AB

Identification of phosphorylation sites on AChR delta-subunit associated with dispersal of AChR clusters on the surface of muscle cells

The innervation of embryonic skeletal muscle cells is marked by the redistribution of nicotinic acetylcholine receptors (AChRs) on muscle surface membranes into high-density patches at nerve-muscle contacts. To investigate the role of protein phosphorylation pathways in the regulation of AChR surface distribution, we have identified the sites on AChR delta-subunits that undergo phosphorylation associated with AChR cluster dispersal in cultured myotubes. We found that PKC-catalyzed AChR phosphorylation is targeted to Ser378, Ser393, and Ser450, all located in the major intracellular domain of the AChR delta-subunit. Adjacent to one of these sites is a PKA consensus target site (Ser377) that was efficiently phosphorylated by purified PKA in vitro. The PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA) and the phosphoprotein phosphatase inhibitor okadaic acid (OA) produced increased phosphorylation of AChR delta-subunits on the three serine residues that were phosphorylated by purified PKC in vitro. In contrast, treatment of these cells with the PKA activator forskolin, or with the cell-permeable cAMP analogue 8-bromo-cAMP, did not alter the phosphorylation state of surface AChR, suggesting that PKA does not actively phosphorylate the delta-subunit in intact chick myotubes. The effects of TPA and OA included an increase in the proportion of surface AChR that is extracted in Triton X-100, as well as the spreading of AChR from cluster regions to adjacent areas of the muscle cell surface. These findings suggest that PKC-catalyzed phosphorylation on the identified serine residues of AChR delta-subunits may play a role in the surface distribution of these receptors.

Muscle-specific agrin isoforms reduce phosphorylation of AChR gamma and delta subunits in cultured muscle cells

The accumulation of nicotinic acetylcholine receptors (AChRs) at neuromuscular synapses is triggered by agrin, a protein that is synthesized by both nerve and muscle. Nerve-derived agrin, which contains an amino acid insert at a conserved splice site in the carboxy-terminal part of the protein, induces AChR aggregation and causes tyrosine phosphorylation of the AChR beta subunit. In contrast, agrin isoforms synthesized by muscle cells lack such an insert and have no effect on AChR distribution. In order to identify possible functional roles of muscle-derived agrin we have analyzed further the effect of various fragments of recombinant agrin on AChR phosphorylation. A carboxy-terminal fragment of muscle agrin, c95A0B0, reduced AChR gamma and delta subunit phosphorylation when added to C2C12 myotubes in culture. Although c95A0B0 had no effect on AChR beta subunit phosphorylation when added alone, it inhibited AChR beta subunit phosphorylation and AChR aggregation by the nerve-specific agrin isoform c95A4B8. We conclude that muscle-derived agrin can influence, both directly and indirectly, AChR phosphorylation. Such changes may play a role in the formation, maintenance, or function of the neuromuscular junction.

Neural influence on protein kinase C isoform expression in skeletal muscle

Protein kinase C (PKC) is a family of enzymes involved in synapse formation and signal transduction at the neuromuscular junction. Two PKC isoforms, classical PKC alpha and novel PKC theta, have been shown to be enriched in skeletal muscle or localized to the endplate. We examined the role of nerve in regulating the expression of these PKC isoforms in rat skeletal muscle by denervating diaphragm muscle and measuring PKC protein expression at various postoperative times. nPKC theta protein levels decreased 65% after denervation, whereas cPKC alpha levels increased 80% compared with control hemidiaphragms. These results suggest that innervation regulates PKC theta and alpha isoform expression in skeletal muscle. To explore further how nerve regulates PKC expression, we characterized PKC isoform expression in rat myotubes deprived of neural input. Myoblast expression of nPKC theta was low, and the increase in nPKC theta expression that occurred during differentiation into myotubes resulted in levels of nPKC theta significantly below adult skeletal muscle. cPKC alpha expression in myoblastic increased during differentiation to levels that exceeded expression in adult skeletal muscle. Coculturing myotubes within neuroblastoma X glioma hybrid clonal cell line (NG108-15) increased nPKC theta expression, but not cPKC alpha, suggesting that nPKC theta in skeletal muscle and myotubes is regulated by nerve contact or by a factor(s) provided by nerve. Treating myotubes with tetrodotoxin did not affect either basal- or NG108-15 cell-stimulated nPKC theta expression. Together these results suggest that expression of nPKC theta in skeletal muscle is regulated by a transynaptic interaction with nerve that specifically influences nPKC theta expression.

Phorbol ester modulation of both delta-mutant and subunit-omitted nicotinic receptors expressed in Xenopus oocytes

The action of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), the potent stimulator of protein kinase C (PKC), on acetylcholine-activated currents (I(Ach)) was investigated in voltage clamped Xenopus laevis oocytes injected with RNAs encoding murine embryonic nicotinic acetylcholine receptor (AChR) subunits. Comparable potentiation and acceleration of decay of I(ACh) were observed within minutes of phorbol ester application in oocytes injected with various RNA subunit combinations: (i) alpha beta gamma delta; (ii) alpha beta gamma; (iii) alpha beta delta; and (iv) alpha beta gamma delta(AAA), a mutant of the delta subunit with serine residues 360-361-362 mutated to alanine. Our findings indicate that the effects on I(ACh) induced by PKC stimulation are independent of both gamma and delta subunits and, accordingly, of the presence of PKC phosphorylation sites on delta subunit. It is here suggested a novel PKC-dependent modulatory mechanism of cholinergic receptor which does not involve direct phosphorylation of the AChR and requires phosphorylation of intermediate regulatory protein(s).

Neural influence on protein kinase C isoform expression in skeletal muscle

Protein kinase C (PKC) is a family of enzymes involved in synapse formation and signal transduction at the neuromuscular junction. Two PKC isoforms, classical PKC alpha and novel PKC theta, have been shown to be enriched in skeletal muscle or localized to the endplate. We examined the role of nerve in regulating the expression of these PKC isoforms in rat skeletal muscle by denervating diaphragm muscle and measuring PKC protein expression at various postoperative times. nPKC theta protein levels decreased 65% after denervation, whereas cPKC alpha levels increased 80% compared with control hemidiaphragms. These results suggest that innervation regulates PKC theta and alpha isoform expression in skeletal muscle. To explore further how nerve regulates PKC expression, we characterized PKC isoform expression in rat myotubes deprived of neural input. Myoblast expression of nPKC theta was low, and the increase in nPKC theta expression that occurred during differentiation into myotubes resulted in levels of nPKC theta significantly below adult skeletal muscle. cPKC alpha expression in myoblastic increased during differentiation to levels that exceeded expression in adult skeletal muscle. Coculturing myotubes within neuroblastoma X glioma hybrid clonal cell line (NG108-15) increased nPKC theta expression, but not cPKC alpha, suggesting that nPKC theta in skeletal muscle and myotubes is regulated by nerve contact or by a factor(s) provided by nerve. Treating myotubes with tetrodotoxin did not affect either basal- or NG108-15 cell-stimulated nPKC theta expression. Together these results suggest that expression of nPKC theta in skeletal muscle is regulated by a transynaptic interaction with nerve that specifically influences nPKC theta expression.

Acetylcholine-activated inward current induces cytosolic Ca2+ mobilization in mouse C2C12 myotubes

We examined the spatiotemporal pattern of intracellular Ca2+ liberation in mouse myotubes by means of fluorescence imaging of cytosolic free Ca2+ together with the simultaneous recording of membrane whole-cell currents. Acetylcholine (ACh) applications to C2C12 myotubes equilibrated in Ca(2+)-free medium and voltage clamped at -50 mV evoked localized fluorescence transients of variable amplitude with less than 0.5 s delay. Under the same experimental conditions, fluorescence transients were elicited by ACh also in mouse primary myotubes. Ca2+ transients were inhibited in myotubes clamped at depolarized potentials (-10 mV to +50 mV), or equilibrated in a Na+,Ca(2+)-free medium as well as in cells loaded with heparin, or with inositol (1,4,5) trisphosphate (InsP3). To investigate whether InsP3 could induce Ca2+ mobilization, [Ca2+]i determinations were carried out in myotubes loaded with InsP3 through the whole-cell patch-clamp recording pipette or by extracellular application in permeabilized cells. InsP3 diffusion into the myoplasm caused Ca2+ spikes with 5 +/- 1 s (mean +/- SEM) delay from the rupture of the membrane patch. Spikes were followed by sustained increases in fluorescence or by damped oscillations. In permeabilized myotubes, InsP3 induced the release of sequestered 45Ca2+ with a half-maximally effective concentration (EC50) of 0.28 +/- 0.05 microM, and Hill coefficient of 0.79 +/- 0.09. It is concluded that the ACh-activated inward current in mouse myotubes is coupled to cytosolic Ca2+ mobilization from internal InsP3-sensitive pools.

Protein kinase C modulates exogenous acetylcholine current in Xenopus oocytes

The modulation of acetylcholine-activated current (IACh) by protein kinase C (PKC) was studied in Xenopus laevis oocytes microinjected with either mRNA extracted from C2C12 myotubes (C2C12 mRNA) or RNAs encoding murine alpha beta gamma delta subunits of the nicotinic ACh receptor (nAChR). Voltage-clamped oocytes were treated for 90 sec with 12-O-tetradecanoylphorbol-13-acetate (TPA, 300 nM), a potent PKC activator. Transient increase in the amplitude and acceleration in the decay of IACh were invariably observed within minutes of TPA application, and were independent of extracellular Ca2+ concentration. Both parameters recovered to control within 20-30 min; then a slight depression of IACh developed. By this time, an initial PKC down regulation was observed. At the peak of TPA-induced potentiation, dose-response relations suggested an increased binding affinity of nAChR for the neurotransmitter. 4 alpha-phorbol 12,13-didecanoate (300 nM), a biologically inactive analogue of TPA, did not affect IACh, while staurosporine (5-10 microM), a potent inhibitor of PKC activity, suppressed the action of TPA on IACh. In oocytes co-injected with C2C12 mRNA and with rat brain mRNA, IACh was potentiated by 5-hydroxy-tryptamine (10 microM), whose receptors are coupled to phosphoinositide hydrolysis. The nAChR-channel activity in cell-attached patches increased when TPA was applied to the oocytes. In 50% of the oocytes examined, a sustained depression of the single channel activity followed. We conclude that in Xenopus oocytes an endogenous PKC system regulates the function of embryonic-type muscle nAChRs.

Additive effect of ADP and CGRP in modulation of the acetylcholine receptor channel in Xenopus embryonic myocytes

1. We have previously shown that the activation of either protein kinase A (PKA) or protein kinase C (PKC) enhanced the responses of muscle membrane to acetylcholine (ACh) by increasing the mean open time of embryonic-type ACh channels in Xenopus cultured myocytes. In the present study, we further investigated the interaction between these two kinases in the modulation of ACh channels by using the receptor ligands, adenosine diphosphate (ADP) and calcitonin gene-related peptide (CGRP) which selectively activate PKC and PKA, respectively. 2. ADP concentration-dependently increased the mean open time of embryonic-type ACh channels and 0.3 mM ADP is sufficient to achieve the maximal potentiating effect. alpha, beta-Methylene ATP and PMA (phorbol 12-myristate 13-acetate) but not adenosine, AMP, dibutyryl cyclic GMP have similar potentiating action. 3. Suramin (0.3 mM) pretreatment abolished the potentiating effect of ADP but left that of PMA unchanged. 4. CGRP increased the mean open time of embryonic-type ACh channels in a concentration-dependent manner and 1 microM CGRP produced the maximal effect. 5. The maximal effects of both ADP (0.3 mM) and CGRP (1 microM) in the prolongation of mean open time of ACh channels were additive. 6. These results suggest that the modulation of embryonic-type ACh channels by the endogenously released ligands via the activation of PKA and PKC is additive and possibly different sites of ACh channels may be involved in the potentiation effect of either PKC or PKA.

Phosphorylation of the nicotinic acetylcholine receptor by protein tyrosine kinases

Most neurotransmitter receptors examined to date are either regulated by phosphorylation or contain consensus sequences for phosphorylation by protein kinases. The nicotinic acetylcholine receptor (AChR), which mediates depolarization at the neuromuscular junction, has served as a model for the study of the structure, function, and regulation of ligand-gated ion channels. The AChR is phosphorylated by protein kinase A, protein kinase C, and an unidentified protein tyrosine kinase. Tyrosine phosphorylation of the AChR is correlated with a modulation of the rate of receptor desensitization and is associated with AChR clustering. We showed that agrin, a neuronally derived extracellular matrix protein, induces AChR clustering and tyrosine phosphorylation. In addition, we identified two protein tyrosine kinases, Fyn and Fyk, that appear to be involved in the regulation of synaptic transmission at the neuromuscular junction by phosphorylating the AChR. The two kinases are highly expressed in Torpedo electric organ, a tissue enriched in synaptic components including the AChR. As demonstrated by coimmunoprecipitation, Fyn and Fyk associate with the AChR. Furthermore, the AChR is phosphorylated in Fyn and Fyk immunoprecipitates. We investigated the molecular basis for the association of the AChR with Fyn and Fyk using fusion proteins derived from the kinases. The AChR bound specifically to the SH2 domain fusion proteins of Fyn and Fyk. The association of the AChR with the SH2 domains is dependent on the state of AChR tyrosine phosphorylation and is mediated by the delta subunit of the receptor. These data provide evidence that the protein tyrosine kinases Fyn and Fyk may act to phosphorylate the AChR in vivo.

Requirement of a colchicine-sensitive component of the cytoskeleton for acetylcholine receptor recovery

1. The effect of colchicine treatment on acetylcholine receptor function was examined in potassium depolarized, voltage-clamped snake twitch fibre endplates. Receptor function was assessed by analysis of miniature endplate currents (m.e.p.c.) as well as acetylcholine (ACh)-induced single channel currents. 2. Pretreatment of snake muscle fibres with colchicine (10 microM to 100 microM) for 16-18 h had no effect on m.e.p.c. amplitude or decay rates. At higher concentrations (1 mM), there was a slight decrease in the average m.e.p.c. amplitude. 3. Colchicine produced a concentration-dependent decrease in the extent of m.e.p.c. amplitude recovery following a 10 min exposure to 540 microM carbachol. Exposure of 100 microM colchicine-treated preparations to 0.5 microM staurosporine further reduced the extent of m.e.p.c. amplitude recovery following carbachol exposure. 4. The decrease in m.e.p.c. amplitude following carbachol exposure was not due to a shift in the m.e.p.c. reversal potential. In addition, the distribution of m.e.p.c. amplitudes remained unimodal in both control and colchicine (100 microM)-treated preparations following carbachol exposure. 5. In addition to the normal, large conductance (approximately 48 pS) ACh-activated channels, a population of small conductance (approximately 29 pS) channels was observed in colchicine-treated preparations following exposure to carbachol. In preparations treated with both colchicine and staurosporine and then exposed to carbachol, the conductance of these small channels was identical to that of colchicine or staurosporine alone. 6. We suggest that prolonged exposure of snake twitch fibre endplates to agonist results in the activation and desensitization of ACh receptors. Furthermore, we propose that for a subpopulation of the inactivated receptors, restoration of function requires both the integrity of a subsynaptic cytoskeletal component and phosphorylation by a staurosporine-sensitive protein kinase. One plausible mechanism is that some receptors become destabilized in the membrane and phosphorylation of a cytoskeletal component, whose distribution may depend on an intact microtubular system, is required to re-anchor these receptors. If this anchoring process is inhibited either by disruption of the cytoskeleton with colchicine, or inhibition of the kinase by staurosporine, these receptors remain activatable, but have a reduced conductance.

Staurosporine inhibits agrin-induced acetylcholine receptor phosphorylation and aggregation

Agrin, a protein that mediates nerve-induced acetylcholine receptor (AChR) aggregation at developing neuromuscular junctions, has been shown to cause an increase in phosphorylation of the beta, gamma, and delta subunits of AChRs in cultured myotubes. As a step toward understanding the mechanism of agrin-induced AChR aggregation, we examined the effects of inhibitors of protein kinases on AChR aggregation and phosphorylation in chick myotubes in culture. Staurosporine, an antagonist of both protein serine and tyrosine kinases, blocked agrin-induced AChR aggregation in a dose-dependent manner; 50% inhibition occurred at approximately 2 nM. The extent of inhibition was independent of agrin concentration, suggesting an effect downstream of the interaction of agrin with its receptor. Staurosporine blocked agrin-induced phosphorylation of the AChR beta subunit, which occurs at least in part on tyrosine residues, but did not reduce phosphorylation of the gamma and delta subunits, which occurs on serine/threonine residues. Staurosporine also prevented the agrin-induced decrease in the rate at which AChRs are extracted from intact myotubes by mild detergents. H-7, an antagonist of protein serine kinases, inhibited agrin-induced phosphorylation of the gamma and delta subunits but did not block agrin-induced phosphorylation of the AChR beta subunit, AChR aggregation, or the decrease in AChR extractability. The results provide support for the hypothesis that tyrosine phosphorylation of the beta subunit plays a role in agrin-induced AChR aggregation.

Clustering of the acetylcholine receptor by the 43-kD protein: involvement of the zinc finger domain

A postsynaptic membrane-associated protein of M(r) 43,000 (43-kD protein) is involved in clustering of the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Previous studies have shown that recombinant mouse 43-kD protein forms membrane-associated clusters when expressed in Xenopus oocytes. Coexpression with the AChR results in colocalization of the receptor with the 43-kD protein clusters (Froehner, S. C., C. W. Luetje, P. B. Scotland, and J. Patrick, 1990. Neuron. 5:403-410). To understand the mechanism of this clustering, we have studied the role of the carboxy-terminal region of the 43-kD protein. The amino acid sequence of this region predicts two tandem zinc finger structures followed by a serine phosphorylation site. Both Torpedo 43-kD protein and the carboxy-terminal region of the mouse 43-kD protein bind radioisotopic zinc. Mutation of two histidine residues in this predicted domain greatly attenuates zinc binding, lending support to the proposal that this region forms zinc fingers. When expressed in oocytes, the ability of this mutant 43-kD protein to form clusters is greatly reduced. Its ability to interact with AChR, however, is retained. In contrast, a mutation that eliminates the potential serine phosphorylation site has no effect on clustering of the 43-kD protein or on interaction with the AChR. These findings suggest that protein interactions via the zinc finger domain of the 43-kD protein may be important for AChR clustering at the synapse.

Receptor-receptor interactions as an integrative mechanism in nerve cells

Several lines of evidence indicate that interactions among transmission lines can take place at the level of the cell membrane via interactions among macromolecules, integral or associated to the cell membrane, involved in signal recognition and transduction. The present view will focus on this last subject, i.e., on the interactions between receptors for chemical signals at the level of the neuronal membrane (receptor-receptor interaction). By receptor-receptor interaction we mean that a neurotransmitter or modulator, by binding to its receptor, modifies the characteristics of the receptor for another transmitter or modulator. Four types of interactions among transmission lines may be considered, but mainly intramembrane receptor-receptor interactions have been dealt with in this article, exemplified by the heteroregulation of D2 receptors via neuropeptide receptors and A2 receptors. The role of receptor-receptor interactions in the integration of signals is discussed, especially in terms of filtration of incoming signals, of integration of coincident signals, and of neuronal plasticity.

Activation of the nicotinic acetylcholine receptor mobilizes calcium from caffeine-insensitive stores in C2C12 mouse myotubes

In cultured mouse C2C12 myotubes, digital Ca2+ imaging fluorescence microscopy using the acetoxymethyl ester of Fura-2, Fura-2-AM, showed that, in the absence of extracellular Ca2+, acetylcholine (ACh) and nicotine, but not muscarine, raised the intracellular concentration of Ca2+ ([Ca2+]i) by about tenfold. ACh-induced Ca2+ mobilization was prevented by thapsigargin, a drug known to deplete inositol 1,4,5-trisphosphate (InsP3)-sensitive stores, and was concomitant with InsP3 accumulation. Caffeine, which releases Ca2+ from the ryanodine-sensitive stores of the sarcoplasmic reticulum, did not interfere with the ACh-induced [Ca2+]i increase. Ca2+ mobilization was also inhibited when myotubes were depolarized by high K+, or when extracellular Na+ was omitted. Nicotinic ACh receptor (nAChR) stimulation lowered intracellular pH with a time course slower than the [Ca2+]i increase. Possible mechanisms linking the current flowing through the nAChR pore to [Ca2+]i increase are discussed.

Mechanism of agrin-induced acetylcholine receptor aggregation

Agrin induces the formation of specializations on chick myotubes in culture at which several components of the postsynaptic apparatus accumulate, including acetylcholine receptors (AChRs). Agrin also induces AChR phosphorylation. Several lines of evidence suggest that agrin-induced phosphorylation of tyrosine residues in the beta subunit of the AChR is an early step in receptor aggregation: agrin-induced phosphorylation and aggregation have the same dose dependence; treatments that prevent aggregation block phosphorylation; phosphorylation begins before any detectable change in receptor distribution, reaches a maximum hours before aggregation is complete, and declines slowly together with the disappearance of aggregates after agrin is withdrawn; agrin slows the rate at which receptors are solubilized from intact myotubes by detergent extraction; and the change in receptor extractability parallels the change in phosphorylation. A model for agrin-induced AChR aggregation is presented in which phosphorylation of AChRs by an agrin-activated protein tyrosine kinase causes receptors to become attached to the cytoskeleton, which reduces their mobility and detergent extractability, and leads to the accumulation of receptors in the vicinity of the activated kinase, forming an aggregate.

Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane

An extrinsic membrane protein of apparent molecular mass 43 kDa is specifically localized in postsynaptic membranes closely associated with the nicotinic acetylcholine receptor (AChR). Since its discovery in 1977, biochemical and morphological studies have combined to provide relatively clear pictures of 43K protein structure and subcellular compartmentalization. Nevertheless, despite these advances, the precise function of this synapse-specific protein remains unclear. Data gathered in recent years indicate that the postsynaptic apparatus develops through the incremental agglomeration of receptor microaggregates; evidence derived from a number of sources points to a role for 43K protein in certain underlying reactions. In this paper, I review 43K protein structural and anatomical data and analyze evidence for its role in the organization and maintenance of the postsynaptic membrane. Finally, I offer a model presenting a view of the role of 43K protein in the ontogeny of the motor endplate.

Clusters of neural cell adhesion molecule at sites of cell-cell contact

I have examined the distribution of neural cell adhesion molecule (N-CAM) in cultured C2 myogenic cells and other cell lines to determine if N-CAM accumulates at sites of cell-cell contact. C2 cells growing in log phase display large clusters of neural cell adhesion molecule where they contact each other. These clusters are remarkably stable, do not form at cell-substrate contacts, and appear not to be enriched in a number of other cytoskeletal, membrane, or extracellular proteins. Thus, N-CAM clusters form preferentially in response to cell-cell contact and are specifically enriched in N-CAM. As C2 cultures mature and differentiate, clusters persist at contacts between aligning myoblasts and between myotubes, consistent with a role in myogenesis. N-CAM is also enriched at cell-cell contacts in cultures of PC12, NRK, and CHO cells. These cells have significant amounts of N-CAM as detected on immunoblots. Clusters are not seen in L929 cells, which do not have detectable amounts of N-CAM. Coculture of these cells with C2 cells results in the clustering of N-CAM at heterologous contacts between C2 cells and NRK, CHO, or PC12 cells, but not between C2 cells and L929 cells. These results suggest that N-CAM specifically accumulates where N-CAM-bearing cells contact one another. Clustering of N-CAM may be an important step in strengthening intercellular adhesion.

Agrin induces phosphorylation of the nicotinic acetylcholine receptor

Agrin causes acetylcholine receptors (AChRs) on chick myotubes in culture to aggregate, forming specializations that resemble the postsynaptic apparatus at the vertebrate skeletal neuromuscular junction. Here we report that treating chick myotubes with agrin caused an increase in phosphorylation of the AChR beta, gamma, and delta subunits. H-7, a potent inhibitor of several protein serine kinases, blocked agrin-induced phosphorylation of the gamma and delta subunits, but did not prevent either agrin-induced AChR aggregation or phosphorylation of the beta subunit. Experiments with anti-phosphotyrosine antibodies demonstrated that agrin caused an increase in tyrosine phosphorylation of the beta subunit that began within 30 min of adding agrin to the myotube cultures, reached a plateau by 3 hr, and was blocked by treatments known to block agrin-induced AChR aggregation. Anti-phosphotyrosine antibodies labeled agrin-induced specializations as they do the postsynaptic apparatus. These results suggest that agrin-induced tyrosine phosphorylation of the beta subunit may play a role in regulating AChR distribution.

The mechanism of agrin-induced acetylcholine receptor aggregation

Agrin, a protein isolated from the synapse-rich electric organ of Torpedo californica, induces the formation of specializations on myotubes in culture which resemble the post-synaptic apparatus at the vertebrate skeletal neuromuscular junction. For example, the specializations contain aggregates of acetylcholine receptors and acetylcholinesterase. This report summarizes the evidence that the formation of the post-synaptic apparatus at developing and regenerating neuromuscular junctions is triggered by the release of agrin from motor axon terminals and describes results of recent experiments which suggest that agrin-induced tyrosine phosphorylation of the beta subunit of the acetylcholine receptor may play a role in receptor aggregation.

Rotational diffusion of acetylcholine receptors on cultured rat myotubes

The rotational mobility of acetylcholine receptors (AChR) in the plasma membrane of living rat myotubes in culture is measured in this study by polarized fluorescence recovery after photobleaching (PFRAP). These AChR are known to exist in two distinct classes, evident by labeling with rhodamine alpha-bungarotoxin; clustered AChR that are aggregated in a pattern of highly concentrated speckles and streaks, with each cluster occupying an area of approximately 1,000 microns 2; and nonclustered AChR that appear as diffuse labeling. PFRAP results reported here show that: (a) most clustered AChR (approximately 86%) are rotationally immobile within a time scale of at least several seconds; and (b) most nonclustered AChR (approximately 76%) are rotationally mobile with characteristic times ranging from less than 50 ms to 0.1 s. External cross-linking with the tetravalent lectin concanavalin A immobilizes many nonclustered AChR. PFRAP experiments in the presence of carbachol or cytochalasin D show that the restraints to rotational motion in clusters are remarkably immune to treatments that disperse clusters or disrupt cytoplasmic actin. The experiments also demonstrate the feasibility of using PFRAP to measure rotational diffusion on selected microscopic areas of living nondeoxygenated cells labeled with standard fluorescence probes over a very wide range of time scales, and they also indicate what technical improvements would make PFRAP even more practicable.

Desensitization of central cholinergic mechanisms and neuroadaptation to nicotine

This review focuses on neuroadaptation to nicotine. The first part of the paper delineates some possible general mechanisms subserving neuroadaptation to commonly abused drugs. The postulated role of the mesocorticolimbic neuroanatomical pathway and drug-receptor desensitization mechanisms in the establishment of tolerance to, dependence on, and withdrawal from psychoactive drugs are discussed. The second part of the review deals with the pharmacological effects of nicotine at both pre- and postsynaptic locations within the central nervous system, and the still-perplexing upregulation of brain nicotine-binding sites seen after chronic nicotine administration. A special emphasis has been put on desensitization of presynaptic cholinergic mechanisms, and postsynaptic neuronal nicotinic-receptor function and its modulation by endogenous substances. A comparison with the inactivation process occurring at peripheral nicotinic receptors is also included. Finally, a hypothesis on the possible connections between desensitization of central cholinergic mechanisms and neuroadaptation to nicotine is advanced. A brief comment on the necessity of fully understanding the effects of nicotine on the developing nervous system closes this work.

Localization of beta-adrenergic receptors in A431 cells in situ. Effect of chronic exposure to agonist

The status of beta-adrenergic receptors was investigated in A431 cells exposed to chronic stimulation by the beta-adrenergic agonist, (-)-isoproterenol. Specific binding of beta-adrenergic antagonist (-)-[125I]iodocyanopindolol declined to 60-80% below control values within 12 h of agonist treatment. This decline in ligand binding was also observed in high-speed membrane fractions prepared from agonist-treated cells. Immunoblots probed with anti-receptor antibodies revealed both that beta-adrenergic receptors from untreated and treated cells migrated as 65,000-Mr peptides and that the cellular complement of receptor was unchanged. Indirect immunofluorescence localization of beta-adrenergic receptors was comparable in control (untreated) cells and cells challenged with (-)-isoproterenol for 1, 12, or 24 h. Thus receptor complement, migration on SDS/polyacrylamide-gel electrophoresis, and localization in situ are largely unaffected by agonist stimulation. Receptor binding of antagonist radioligands, in contrast, is markedly down-regulated in cells stimulated chronically with beta-adrenergic agonists. These data argue in favour of agonist-induced alteration(s) in the conformation of the receptor that preclude radioligand binding rather than agonist-induced receptor sequestration and/or degradation.