Phosphorylation and assembly of nicotinic acetylcholine receptor subunits in cultured chick muscle cells

Ceramides modulate cell-surface acetylcholine receptor levels

The effects of ceramides (Cer) on the trafficking of the nicotinic acetylcholine receptor (AChR) to the plasma membrane were studied in CHO-K1/A5 cells, a clonal cell line that heterologously expresses the adult murine form of the receptor. When cells were incubated with short- (C6-Cer) or long- (brain-Cer) chain Cer at low concentrations, an increase in the number of cell-surface AChRs was observed concomitant with a decrease in intracellular receptor levels. The alteration in AChR distribution by low Cer treatment does not appear to be a general mechanism since the surface expression of the green fluorescent protein derivative of the vesicular stomatitis virus protein (VSVG-GFP) was not affected. High Cer concentrations caused the opposite effects, decreasing the number of cell-surface AChRs, which exhibited higher affinity for [125I]-alpha-bungarotoxin, and increasing the intracellular pool, which colocalized with trans-Golgi/TGN specific markers. The generation of endogenous Cer by sphingomyelinase treatment also decreased cell-surface AChR levels. These effects do not involve protein kinase C zeta or protein phosphatase 2A activation. Taken together, the results indicate that Cer modulate trafficking of AChRs to and stability at the cell surface.

The chaperone protein 14-3-3eta interacts with the nicotinic acetylcholine receptor alpha 4 subunit. Evidence for a dynamic role in subunit stabilization

By using the large cytoplasmic domain of the nicotinic acetylcholine receptor (AChR) alpha4 subunit as a bait in the yeast two-hybrid system, we isolated the first cytosolic protein, 14-3-3eta, known to interact directly with neuronal AChRs. 14-3-3eta is a member of a family of proteins that function as regulatory or chaperone/ scaffolding/adaptor proteins. 14-3-3eta interacted with the recombinant alpha4 subunit alone in tsA 201 cells following activation of cAMP-dependent protein kinase by forskolin. The interaction of 14-3-3eta with recombinant alpha4 subunits was abolished when serine 441 of the alpha4 subunit was mutated to alanine (alpha4(S441A)). The surface levels of recombinant wild-type alpha4beta2 AChRs were approximately 2-fold higher than those of mutant alpha4(S441A)beta2 AChRs. The interaction significantly increased the steady state levels of the alpha4 subunit and alpha4beta2 AChRs but not that of the mutant alpha4(S441A) subunit or mutant alpha4(S441A)beta2 AChRs. The EC50 values for activation by acetylcholine were not significantly different for alpha4beta2 AChRs and alpha4(S441A)beta2 AChRs coexpressed with 14-3-3eta in oocytes following treatment with forskolin. 14-3-3 coimmunopurified with native alpha4 AChRs from brain. These results support a role for 14-3-3 in dynamically regulating the expression levels of alpha4beta2 AChRs through its interaction with the alpha4 subunit.

Biosynthesis and degradation of CFTR

Biosynthesis and Degradation of CFTR. Physiol. Rev. 79, Suppl.: S167-S173, 1999. - Many of the mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that cause cystic fibrosis interfere with the folding and biosynthetic processing of nascent CFTR molecules in the endoplasmic reticulum. Mutations in the cytoplasmic nucleotide binding domains, including the common allele DeltaF508, decrease the efficiency of CFTR folding, reduce the probability of its dissociation from molecular chaperones, and largely prevent its maturation through the secretory pathway to the plasma membrane. These mutant CFTR molecules are rapidly degraded by cytoplasmic proteasomes by a process that requires covalent modification by multiubiquitination. The effects of temperature and chemical chaperones on the intracellular processing of mutant CFTR molecules suggest that strategies aimed at increasing the folding yield of this protein in vivo may eventually lead to the development of novel therapies for cystic fibrosis.

Calnexin-dependent enhancement of nicotinic acetylcholine receptor assembly and surface expression

The muscle-type nicotinic acetylcholine receptor (AChR)2 is a pentameric membrane ion channel assembled in the endoplasmic reticulum from four homologous subunits by mechanisms that are insufficiently understood. Nascent AChR subunits were recently found to form complexes with the endoplasmic reticulum-resident molecular chaperone calnexin. To determine the contribution of this interaction to AChR assembly and surface expression, we have now used transient transfection of mouse AChR subunits and calnexin into non-muscle cells. Co-transfection of calnexin along with AChR subunits into COS and HEK 293 cells was found to enhance AChR subunit folding and assembly, and to decrease degradation rates of newly synthesized AChR alpha-subunits, resulting in elevated surface expression of assembled AChR. Moreover, inhibition of the interaction between endogenous calnexin and AChR by castanospermine resulted in decreased AChR subunit folding, assembly, and surface expression in muscle and HEK 293 cells. Together, these findings provide evidence that calnexin directly contributes to AChR biogenesis by promoting subunit folding and assembly.

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).

Arrest of subunit folding and assembly of nicotinic acetylcholine receptors in cultured muscle cells by dithiothreitol

In this study we have used cultured muscle cells to investigate the role of disulfide bond formation in the sequence of molecular events leading to nicotinic acetylcholine receptor (AChR) assembly and surface expression. We have observed that disulfide bond formation in newly synthesized AChR alpha-subunits occurs 5-20 min after translation and that this modification can be blocked by dithiothreitol (DTT), a membrane-permeant thiol-reducing agent. DTT treatment was found to arrest AChR alpha-subunit conformational maturation, assembly, and appearance on the cell surface, showing that these events are dependent on prior formation of disulfide bonds. Subunits prevented from maturation by the reducing agent do not irreversibly misfold or aggregate, since upon removal of DTT, AChR alpha-subunits undergo formation of disulfide bonds and resume folding, oligomerization, and surface expression. We have previously found that nascent alpha-subunits form transient complexes with the molecular chaperone calnexin immediately after subunit synthesis (Gelman, M.S., Chang, W., Thomas, D. Y., Bergeron, J. J. M., and Prives, J. M. (1995) J. Biol. Chem. 270, 15085-15092) and have now observed that both the formation and the subsequent dissociation of these complexes are unaffected by DTT treatment. Thus, alpha-subunits appear to dissociate from calnexin independently of their undergoing disulfide bond formation and achieving conformational maturation. This finding together with the absence of irreversible misfolding of DTT-arrested alpha-subunits suggests that calnexin may act to prevent misfolding by aiding in the initial folding events and is not an essential participant in the late stages of alpha-subunit maturation.

Role of the endoplasmic reticulum chaperone calnexin in subunit folding and assembly of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (AChR) is a pentameric complex assembled from four different gene products by mechanisms that are inadequately understood. In this study we investigated the role of the endoplasmic reticulum (ER)-resident molecular chaperone calnexin in AChR subunit folding and assembly. We have shown that calnexin interacts with nascent AChR alpha-subunits (AChR-alpha) in muscle cell cultures and in COS cells transfected with mouse AChR-alpha. In chick muscle cells maximal association of labeled alpha-subunits with calnexin was observed immediately after a 15-min pulse with [35S]methionine/cysteine and subsequently declined with a t1/2 of approximately 20 min. The decrease in association with calnexin was concomitant with the folding of the alpha-subunit to achieve conformational maturation shortly before assembly. Brefeldin A did not inhibit AChR subunit assembly or the dissociation of calnexin from the assembling subunits, confirming that the ER is the site of AChR assembly and that calnexin dissociation is not affected under conditions in which the exit of assembled AChR from the ER is blocked. These results indicate that calnexin participates directly in the molecular events that lead to AChR assembly.

Nicotine-related brain disorders: the neurobiological basis of nicotine dependence

1. This paper was written at a moment when the dependence liability of nicotine, the psychoactive component from tobacco, was the center of a dispute between the tobacco manufacturing companies and the scientific community (Nowak, 1994a-c). Without being comprehensive, it tries to summarize evidence compiled from several disciplines within neuroscience demonstrating that nicotine produces a true psychiatric disease, behaviorally expressed as dependence to the drug (American Psychiatric Association, 1994). Nicotine dependence has a biological substratum defined as "neuroadaptation to nicotine." 2. The first part of the article defines terms such as "abuse," "tolerance," "dependence," and "withdrawal." It discusses clinical and experimental facts at the whole-organism level, showing that animals and humans will seek and self-administer nicotine because of its rewarding properties. 3. The second part discusses the neurobiological basis of neuroadaptation to nicotine. It presents information on neuroanatomical circuits which may be involved in nicotine-related brain disorders, such as the mesocorticolimbic pathway and the basal forebrain-frontal cortex pathway. It also discusses work from several laboratories, including our own, that support the notion of a molecular basis for neuroadaptative changes induced by nicotine in the brain of a chronic smoker. 4. Although still under experimental scrutiny, the hallmark of neuroadaptation to nicotine is up-regulation of nicotinic receptors, possibly due to nicotine-induced desensitization of their function (Marks et al., 1983; Schwartz and Kellar, 1985). A correlation between these plastic changes and the behavioral data obtained from animal and human experiments is still needed to understand dependence to nicotine fully.

Cyclic AMP-regulated AChR assembly is independent of AChR subunit phosphorylation by PKA

Forskolin treatment of cells expressing Torpedo acetylcholine receptors leads to enhanced assembly efficiency of subunits, which correlates with increased phosphorylation of the gamma subunit. To determine the role of the two potential protein kinase A sites of the gamma subunit in receptor assembly, cell lines expressing different mutant receptors were established. Mouse fibroblast cell lines stably expressing wild-type Torpedo acetylcholine receptor alpha, beta, delta subunits plus one of three gamma subunit mutations (S353A, S354A, or S353,354A) were established to identify the protein kinase A phosphorylation sites of gamma in vivo, and to determine if increased phosphorylation of the gamma subunit leads to enhanced expression of receptors. We found that both serines (353, 354) in gamma are phosphorylated in vivo by protein kinase A, however, phosphorylation of either or both of these sites does not lead to increased assembly efficiency. We established a cell line expressing alpha, beta, and gamma(S353,354A) subunits only (no delta), and found that the presence of delta (or its phosphorylation) is also not necessary for the observed stimulation by forskolin. alpha beta gamma, alpha gamma, and beta gamma associations were stimulated by forskolin but alpha beta and alpha delta interactions were not. These data imply that the presence of gamma is necessary for forskolin action. We postulate that forskolin may stimulate acetylcholine receptor expression through a cellular protein that is involved in the folding and/or assembly of protein complexes, and that forskolin may regulate the action of such a protein through phosphorylation.

Phosphorylation of rat brain nicotinic acetylcholine receptor by cAMP-dependent protein kinase in vitro

The participation of protein kinases in phosphorylation of nicotinic acetylcholine receptor (nAChR) in electric organ and muscle has been precisely investigated in vitro and in vivo whereas phosphorylation of neuronal nAChR is not yet fully characterized. Here, we first report the in vitro phosphorylation of brain nAChR. nAChR purified from rat brains was phosphorylated in vitro by cAMP-dependent protein kinase (PKA), immunoprecipitated with monoclonal antibody against the receptor, and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by autoradiography. PKA specifically phosphorylated nAChR on the alpha 4 subunits, and H8, an inhibitor of PKA, inhibited completely the phosphorylation. Under the conditions used, a maximal stoichiometry of the phosphorylation by PKA was near to 1 mol of phosphate/mol of the alpha 4 subunits. The 32P-labeled subunits were digested with S. aureas V8 protease followed by SDS-PAGE autoradiography and the resultant phosphopeptide maps revealed three distinct phosphopeptide bands, one major band and two minor bands. Phosphoamino acid analysis of the 32P-labeled alpha 4 subunits showed that serine residues were exclusively phosphorylated. Based on these results, participation of PKA in the regulation of neuronal nAChR is discussed.

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.

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.

Analysis of binding and activating functions of the chick muscle acetylcholine receptor gamma-subunit upstream sequence

1. The skeletal muscle acetylcholine receptor comprises several subunits whose coordinated expression during myogenesis is probably controlled by cis elements in the individual subunit genes. We have previously analyzed promoter regions of the alpha and delta genes (Wang et al., 1988, 1990); to gain further insight into receptor regulation, we have now studied the promoter of the chick muscle gamma-subunit gene. 2. This analysis was faciliated by the close upstream proximity of the coding region of the delta-subunit gene and the consequent brevity (740 bp) of the untranslated linker connecting the two genes (Nef et al., 1984). Nuclease protection and primer extension analysis revealed that transcription of the gamma-subunit gene starts at position 56 upstream of the translational initiation site. 3. Nested deletions of the promoter region were employed to identify functionally important elements. A 360-bp sequence (-324 to +36) was found to activate transcription, in a position- and orientation-independent manner, during myotube formation. This sequence comprises 5 M-CAT (Nikovits et al., 1986) similarities and contains, at positions -52/-47 and -33/-28, two CANNTG (Lassar et al., 1989) motifs. 4. Binding experiments were performed by means of gel retardation, gel shift competition, and footprint analysis. The CANNTG motifs were found to bind MyoD and myogenin fusion proteins and to interact with proteins in nuclear extracts from cultured myotubes. 5. Point mutations in the CANNTG motifs revealed that these elements are crucial for full promoter activity in myotubes and essential in fibroblasts cotransfected with a myogenin expression vector. 6. We conclude that the activity of the gamma-subunit gene is determined largely by E boxes, which in vivo are likely to be activated by MyoD family proteins; in addition, other transactivators such as the M-CAT binding protein presumably play a role. Both CANNTG elements and M-CAT motifs are also present in the alpha- and delta-subunit enhancer and may therefore account for the coordinate expression of the three subunits during muscle differentiation.

Two isoforms of the muscle acetylcholine receptor alpha-subunit are translated in the human cell line TE671

We have previously reported the existence of 2 forms of mRNA for the human muscle acetylcholine receptor (AChR) alpha-subunit, thought to be generated by alternate splicing of a primary transcript and to encode 2 alpha-subunit protein isoforms. The 2 predicted alpha-subunit isoforms, differing by the insertion of 25 amino acids at position 58/59, have been synthesized from cRNA transcripts using rabbit reticulocyte lysates; these protein isoforms could be differentiated by immunoprecipitation using antibodies raised against synthetic peptides. The antibodies were used to demonstrate translation of both AChR alpha-subunit isoforms in the rhabdomyosarcoma (muscle) cell line TE671, in an approximate 1:1 ratio.

Acetylcholine receptor assembly is stimulated by phosphorylation of its gamma subunit

Different combinations of Torpedo acetylcholine receptor (AChR) subunits stably expressed in mouse fibroblasts were used to establish a role for phosphorylation in AChR biogenesis. When cell lines expressing fully functional AChR complexes (alpha 2 beta gamma delta) were labeled with 32P, only gamma and delta subunits were phosphorylated. Forskolin, which causes a 2- to 3-fold increase in AChR expression by stimulating subunit assembly, increased unassembled gamma phosphorylation, but had little effect on unassembled delta. The forskolin effect on subunit phosphorylation was rapid, significantly preceding its effect on expression. The pivotal role of the gamma subunit was established by treating alpha beta gamma and alpha beta delta cell lines with forskolin and observing increased expression of only alpha beta gamma complexes. This effect was also observed in alpha gamma, but not alpha delta cells. We conclude that the cAMP-induced increase in expression of cell surface AChRs is due to phosphorylation of unassembled gamma subunits, which leads to increased efficiency of assembly of all four subunits.

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.

Phosphorylation sites of the nicotinic acetylcholine receptor. A novel site detected in position delta S362

The delta-subunit of the nicotinic acetylcholine receptor from Torpedo californica electric tissue isolated form receptor purified in the absence of protein phosphatase inhibitors contains a total of four phosphate groups. Three of these are shown to represent phosphoserine groups. The fourth possible represents phosphotyrosine. The phosphate groups are localized within the primary structure: We found phosphoserine in positions delta S361 and delta S377, the predicted sites phosphorylated by PKA and PKC, respectively. In addition, we found that position delta S362 is also phosphorylated. Phosphorylation experiments with the synthetic peptide delta L357-delta K368 show that phosphorylation of this novel site can be catalyzed by PKA and by PKC. It is concluded that the delat-subunit of the acetylcholine receptor is stably and not transiently phosphorylated. Implications for the physiological functions of receptor phosphorylation are discussed.

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.

Identification of phosphorylation sites for adenosine 3',5'-cyclic phosphate dependent protein kinase on the voltage-sensitive sodium channel from Electrophorus electricus

The voltage-sensitive sodium channel from the electroplax of Electrophorus electricus is selectively phosphorylated by the catalytic subunit of cyclic-AMP-dependent protein kinase (protein kinase A) but not by protein kinase C. Under identical limiting conditions, the protein was phosphorylated 20% as rapidly as the synthetic model substrate kemptamide. A maximum of 1.7 +/- 0.6 equiv of phosphate is incorporated per mole. Phosphoamino acid analysis revealed labeled phosphoserine and phosphothreonine at a constant ratio of 3.3:1. Seven distinct phosphopeptides were identified among tryptic fragments prepared from radiolabeled, affinity-purified protein and resolved by HPLC. The three most rapidly labeled fragments were further purified and sequenced. Four phosphorylated amino acids were identified deriving from three consensus phosphorylation sites. These were serine 6, serine 7, and threonine 17 from the amino terminus and a residue within 47 amino acids of the carboxyl terminus, apparently serine 1776. The alpha-subunits of brain sodium channels, like the electroplax protein, are readily phosphorylated by protein kinase A. However, these are also phosphorylated by protein kinase C and exhibit a markedly different pattern of incorporation. Each of three brain alpha-subunits displays an approximately 200 amino acid segment between homologous repeat domains I and II, which is missing from the electroplax and skeletal muscle proteins [Noda et al. (1986) Nature (London) 320, 188; Kayano et al. (1988) FEBS Lett. 228, 1878; Trimmer et al. (1989) Neuron 3, 33]. Most of the phosphorylation of the brain proteins occurs on a cluster of consensus phosphorylation sites located in this segment. This contrasts with the pattern of highly active sites on the amino and carboxyl termini of the electroplax protein. The detection of seven labeled tryptic phosphopeptides compared to the maximal labeling stoichiometry of approximately 2 suggests that many of the acceptor sites on the protein may be blocked by endogenous phosphorylation.

Calcitonin gene-related peptide regulates phosphorylation of the nicotinic acetylcholine receptor in rat myotubes

The nicotinic acetylcholine receptor (AChR) is a substrate for at least three different protein kinases, and phosphorylation of the receptor has been shown to increase its rate of desensitization. However, the first messengers that regulate AChR phosphorylation have not yet been identified. This study demonstrates that calcitonin gene-related peptide (CGRP), a neuropeptide present in the axon terminals of the neuromuscular junction, regulates phosphorylation of the AChR in primary rat myotube cultures. CGRP, in the presence of the phosphodiesterase inhibitor Ro 20-1724, increased phosphorylation of the alpha and delta subunits of the AChR. CGRP-induced phosphorylation of the AChR had the same subunit specificity and temporal sequence as previously observed using forskolin or cAMP analogs. Phosphorylation of the AChR in the presence of CGRP appears to be mediated by CGRP-stimulated increases in cAMP levels leading to activation of cAMP-dependent protein kinase. The present results, taken together with the recent demonstration that CGRP increases the rate of AChR desensitization in mouse myotubes, suggest that CGRP may play a physiological role as a regulator of AChR desensitization by modulating AChR phosphorylation at the neuromuscular junction.

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

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

Protein phosphorylation of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAcChR) is a ligand-gated ion channel found in the postsynaptic membranes of electric organs, at the neuromuscular junction, and at nicotinic cholinergic synapses of the mammalian central and peripheral nervous system. The nAcChR from Torpedo electric organ and mammalian muscle is the most well-characterized neurotransmitter receptor in biology. It has been shown to be comprised of five homologous (two identicle) protein subunits (alpha 2 beta gamma delta) that form both the ion channel and the neurotransmitter receptor. The nAcChR has been purified and reconstituted into lipid vesicles with retention of ion channel function and the primary structure of all four protein subunits has been determined. Protein phosphorylation is a major posttranslational modification known to regulate protein function. The Torpedo nAcChR was first shown to be regulated by phosphorylation by the discovery that postsynaptic membranes contain protein kinases that phosphorylate the nAcChR. Phosphorylation of the nAcChR has since been shown to be regulated by the cAMP-dependent protein kinase, protein kinase C, and a tyrosine-specific protein kinase. Phosphorylation of the nAcChR by cAMP-dependent protein kinase has been shown to increase the rate of nAcChR desensitization, the process by which the nAcChR becomes inactivated in the continued presence of agonist. In cultured muscle cells, phosphorylation of the nAcChR has been shown to be regulated by cAMP-dependent protein kinase, a Ca2+-sensitive protein kinase, and a tyrosine-specific protein kinase. Stimulation of the cAMP-dependent protein kinase in muscle also increases the rate of nAcChR desensitization and correlates well with the increase in nAcChR phosphorylation. The AcChR represents a model system for how receptors and ion channels are regulated by second messengers and protein phosphorylation.

Acetylcholine receptor expression in primary cultures of embryonic chick myotubes--I. Discoordinate regulation of alpha-, gamma- and delta-subunit gene expression by calcitonin gene-related peptide and by muscle electrical activity

The neuropeptide calcitonin gene-related peptide calcitonin gene-related peptide, and muscle electrical activity regulate in opposite directions the content of nicotinic acetylcholine receptor alpha-subunit mRNA in primary cultures of chick embryonic myotubes. Indeed, treating the cells with calcitonin gene-related peptide or blocking the spontaneous activity of muscle cells by tetrodotoxin (an inhibitor of sodium channels) increases, although to different levels, the content of acetylcholine receptor alpha-subunit mRNA [Fontaine B., Klarsfeld A. and Changeux J. P. (1987) J. Cell Biol. 105, 1337-1342; Klarsfeld A. and Changeux J. P. (1985) Proc. natn. Acad. Sci. U.S.A. 82, 4558-4562]. In this paper, we demonstrate that, under these in vitro culture conditions, calcitonin gene-related peptide (0.1 microM) and tetrodotoxin (0.5 microM) regulate to a smaller extent (no more than 2.5-fold above control) the levels of acetylcholine receptor gamma- and delta-subunit mRNAs. No effect of either compound on acetylcholine receptor biosynthesis was observed during the initial three days of culture. The response to calcitonin gene-related peptide was already maximal when the cells were treated between days three and four after plating (about 3-fold increase of the alpha-subunit mRNA level). The effect of tetrodotoxin resulted in a six-fold increase of the acetylcholine receptor alpha-subunit mRNA level in cells treated between days three and four, and still increased when the cells were exposed to tetrodotoxin through days six and eight (up to a maximum of 20-fold).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of nicotinic acetylcholine receptors by protein phosphorylation

Neurotransmitter receptors and ion channels play a critical role in the transduction of signals at chemical synapses. The modulation of neurotransmitter receptor and ion channel function by protein phosphorylation is one of the major regulatory mechanisms in the control of synaptic transmission. The nicotinic acetylcholine receptor (nAcChR) has provided an excellent model system in which to study the modulation of neurotransmitter receptors and ion channels by protein phosphorylation since the structure and function of this receptor have been so extensively characterized. In this article, the structure of the nAcChR from the electric organ of electric fish, skeletal muscle, and the central and peripheral nervous system will be briefly reviewed. Emphasis will be placed on the regulation of the phosphorylation of nAcChR by second messengers and by neurotransmitters and hormones. In addition, recent studies on the functional modulation of nicotinic receptors by protein phosphorylation will be reviewed.