Phosphorylation of the acetylcholine receptor by protein kinase C and identification of the phosphorylation site within the receptor delta subunit

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.

Modulation of nicotinic receptor channels by adrenergic stimulation in rat pinealocytes

Melatonin secretion from the pineal gland is triggered by norepinephrine released from sympathetic terminals at night. In contrast, cholinergic and parasympathetic inputs, by activating nicotinic cholinergic receptors (nAChR), have been suggested to counterbalance the noradrenergic input. Here we investigated whether adrenergic signaling regulates nAChR channels in rat pinealocytes. Acetylcholine or the selective nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) activated large nAChR currents in whole cell patch-clamp experiments. Norepinephrine (NE) reduced the nAChR currents, an effect partially mimicked by a β-adrenergic receptor agonist, isoproterenol, and blocked by a β-adrenergic receptor antagonist, propranolol. Increasing intracellular cAMP levels using membrane-permeable 8-bromoadenosine (8-Br)-cAMP or 5,6-dichlorobenzimidazole riboside-3',5'-cyclic monophosphorothioate (cBIMPS) also reduced nAChR activity, mimicking the effects of NE and isoproterenol. Further, removal of ATP from the intracellular pipette solution blocked the reduction of nAChR currents, suggesting involvement of protein kinases. Indeed protein kinase A inhibitors, H-89 and Rp-cAMPS, blocked the modulation of nAChR by adrenergic stimulation. After the downmodulation by NE, nAChR channels mediated a smaller Ca(2+) influx and less membrane depolarization from the resting potential. Together these results suggest that NE released from sympathetic terminals at night attenuates nicotinic cholinergic signaling.

Regulation of the ATP-sensitive potassium channel subunit, Kir6.2, by a Ca2+-dependent protein kinase C

The activity of ATP-sensitive potassium (K(ATP)) channels is governed by the concentration of intracellular ATP and ADP and is thus responsive to the metabolic status of the cell. Phosphorylation of K(ATP) channels by protein kinase A (PKA) or protein kinase C (PKC) results in the modulation of channel activity and is particularly important in regulating smooth muscle tone. At the molecular level the smooth muscle channel is composed of a sulfonylurea subunit (SUR2B) and a pore-forming subunit Kir6.1 and/or Kir6.2. Previously, Kir6.1/SUR2B channels have been shown to be inhibited by PKC, and Kir6.2/SUR2B channels have been shown to be activated or have no response to PKC. In this study we have examined the modulation of channel complexes formed of the inward rectifier subunit, Kir6.2, and the sulfonylurea subunit, SUR2B. Using a combination of biochemical and electrophysiological techniques we show that this complex can be inhibited by protein kinase C in a Ca(2+)-dependent manner and that this inhibition is likely to be as a result of internalization. We identify a residue in the distal C terminus of Kir6.2 (Ser-372) whose phosphorylation leads to down-regulation of the channel complex. This inhibitory effect is distinct from activation which is seen with low levels of channel activity.

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.

Addiction research in a simple animal model: the nematode Caenorhabditis elegans

Genetic analysis in the nematode C. elegans has provided important insights into many aspects of neuronal cell biology, including functions related to addiction. Specifically, genetic and molecular screens to have been used to identify molecules involved in long-term responses to drugs of abuse and to analyze the mechanisms underlying their effects on nervous system development, plasticity, and behavior. This review presents a personal view of addiction-related research in C. elegans, and includes a discussion of technical innovations that have facilitated neurobiological analyses in C. elegans and a look at future prospects drug addiction research in simple animal models.

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.

Decrease in acetylcholine-induced current by neomycin in PC12 cells

The effects of neomycin, one of the aminoglycoside antibiotics, on the acetylcholine (ACh)-induced current (I(ACh)) were studied in pheochromocytoma cells by using the whole-cell clamp technique. The I(ACh) proved to be generated through neuronal nicotinic receptor. ACh (30 microM) induced an inward current at a holding potential of -80 mV. When cells were treated with neomycin (0.01-1 mM) and ACh (30 microM) simultaneously, an inhibitory effect of neomycin on the peak of I(ACh) was found. This effect was fast, reversible, and concentration dependent. Pretreatment with neomycin for 3-8 min had no effect on the inhibition of I(ACh) induced by neomycin. External application of 0.1 mM neomycin neither shifted the dose-response curve of the peak I(ACh) to the right (dissociation constant (K(d)) = 16.5 microM) nor affected its coefficient (1.8) but inhibited the curve amplitudes by approximately 33%. Stimulated protein kinase C activation by using an exogenous activator produced inhibition of I(ACh), while using protein kinase C inhibitor (PKCI 19-31) had no effect on the inhibition of I(ACh) induced by neomycin. These results suggest that neomycin has an inhibitory effect on I(ACh) without the involvement of phospholipase C. It indicates that neomycin binds to a specific site on the cell membrane, probably on the neuronal nicotinic receptor-coupled channel, and inhibits the I(ACh) in a noncompetitive manner, thus controlling the immediate catecholamine release from the sympathetic cells.

Cyclic AMP-dependent protein kinase (PKA) and protein kinase C phosphorylate sites in the amino acid sequence corresponding to the M3/M4 cytoplasmic domain of alpha4 neuronal nicotinic receptor subunits

To determine whether alpha4 subunits of alpha4beta2 neuronal nicotinic receptors are phosphorylated within the M3/M4 intracellular region by cyclic AMP-dependent protein kinase A (PKA) or protein kinase C (PKC), immunoprecipitated receptors from Xenopus oocytes and a fusion protein corresponding to the M3/M4 cytoplasmic domain of alpha4 (alpha4(336-597)) were incubated with ATP and either PKA or PKC. Both alpha4 and alpha4(336-597) were phosphorylated by PKA and PKC, providing the first direct biochemical evidence that the M3/M4 cytoplasmic domain of neuronal nicotinic receptor alpha4 subunits is phosphorylated by both kinases. When the immunoprecipitated receptors and the alpha4(336-597) fusion protein were phosphorylated and the labeled proteins subjected to phosphoamino acid analysis, results indicated that alpha4 and alpha4(336-597) were phosphorylated on the same amino acid residues by each kinase. Furthermore, PKA phosphorylated serines exclusively, whereas PKC phosphorylated both serines and threonines. To determine whether Ser(368) was a substrate for both kinases, a peptide corresponding to amino acids 356-371 was synthesized (alpha4(356-371)) and incubated with ATP and the kinases. The phosphorylation of alpha4(356-371) by both PKA and PKC was saturable with K(m)s of 15.3 +/- 3.3 microM and 160.8 +/- 26.8 microM, respectively, suggesting that Ser(368) was a better substrate for PKA than PKC.

Specific phosphorylation of Torpedo 43K rapsyn by endogenous kinase(s) with thiamine triphosphate as the phosphate donor

43K rapsyn is a peripheral protein specifically associated with the nicotinic acetylcholine receptor (nAChR) present in the postsynaptic membrane of the neuromuscular junction and of the electrocyte, and is essential for its clustering. Here, we demonstrate a novel specific phosphorylation of 43K rapsyn by endogenous protein kinase(s) present in Torpedo electrocyte nAChR-rich membranes and identify thiamine triphosphate (TTP) as the phosphate donor. In the presence of Mg(2+) and [gamma-(32)P]-TTP, 43K rapsyn is specifically phosphorylated with a (32)P-half-maximal incorporation at approximately 5-25 microM TTP. The presence of TTP in the cytosol and of 43K rapsyn at the cytoplasmic face of the postsynaptic membrane, together with TTP-dependent phosphorylation of 43K rapsyn without added exokinases, suggests that TTP-dependent-43K-rapsyn phosphorylation may occur in vivo. In addition, phosphoamino acid and chemical stability analysis suggests that the residues phosphorylated are predominantly histidines. Inhibition of phosphorylation by Zn(2+) suggests a possible control of 43K rapsyn phosphorylation state by its zinc finger domain. Endogenous kinase(s) present in rodent brain membranes can also use [gamma-(32)P]-TTP as a phosphodonor. The use of a phosphodonor (TTP) belonging to the thiamine family but not to the classical (ATP, GTP) purine triphosphate family represents a novel phosphorylation pathway possibly important for synaptic proteins.

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

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

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).

Regulatory role of ATP at developing neuromuscular junctions

Neuronal factors co-released with neurotransmitters may play an important role in synaptic development and function. Extracellular application of adenosine 5'-triphosphate (ATP), a substance co-stored and co-released with acetylcholine (ACh) in peripheral nervous systems, potentiated the spontaneous secretion of ACh at developing neuromuscular synapses in Xenopus 1-day-old cell cultures, as shown by a marked increase in the frequency of spontaneous synaptic currents recorded in the post-synaptic muscle cell. ATP also increased the frequency of miniature endplate potentials in the isolated tails of 2-week-old Xenopus tadpoles, with much smaller effect than that observed in cell cultures. The potentiation effect of ATP on ACh release in Xenopus cell cultures was inhibited by L-type Ca2+ channel blockers, suggesting that the L-type Ca2+ channel is responsible for the positive regulation of spontaneous ACh secretion by ATP at the developing neuromuscular synapses. The frequency of spontaneous synaptic events was found to vary greatly from cell to cell in the culture, over two orders of magnitude. Synapses with high frequency events are probably under the influence of endogenously released ATP. In addition, ATP was shown to potentiate the responses of isolated myocytes to iontophoretically-applied ACh. Local application of ATP to one region of the elongated myocyte surface resulted in potentiated ACh responses only at the ATP-treated region. Single channel recording showed that ATP specifically increased the open time and opening frequency of embryonic-type, low conductance ACh channels. Pharmacological experiments suggest that ATP exerted both its pre- and post-synaptic effects by binding to P2-purinoceptors and activating protein kinase C. Moreover, the potentiation effects of ATP were restricted to the early stages of embryos. Taken together, these results suggest that ATP co-released with ACh or released from stimulated myocytes may promote synaptic development by potentiating pre-synaptic ACh release and post-synaptic ACh channel activity during the early phase of synaptogenesis.

Tyrosine phosphorylation and synapse formation at the neuromuscular junction

Protein tyrosine phosphorylation is prevalent throughout the nervous system. It has been implicated to play an important role in the development and maintenance of neuronal functions. In the past few years significant advances have been made in our understanding of the molecular mechanisms of synapse formation and synaptic plasticity. Protein tyrosine phosphorylation appears to be important in the neuron-induced synthesis of the nicotinic acetylcholine receptor and aggregation of synaptic proteins at the neuromuscular junction during development. In addition, protein tyrosine phosphorylation may regulate the ion channel activity of the nicotinic acetylcholine receptor.

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.

Developmental regulation of a protein kinase C isoform localized in the neuromuscular junction

Protein kinase C (PKC) is a family of protein serine/threonine kinases consisting of multiple isoforms whose distinct physiological roles within cells are unknown. The message encoding the nPKC theta isoform, a member of the novel calcium-independent class of PKCs, has recently been shown to be abundant in mouse skeletal muscle. The message for cPKC alpha, a calcium-dependent isoform, was also found to be highly expressed in this tissue. In an effort to distinguish between the physiological roles of these two isoforms of PKC in rat skeletal muscle, we examined their subcellular distribution, developmental expression and intracellular localization. We generated an isotype-specific antiserum directed against a peptide sequence unique to nPKC theta. This antiserum recognized a 79 kDa protein highly enriched in rat skeletal muscle, which is likely to be nPKC theta. cPKC alpha was also readily detectable in skeletal muscle, using another isotype-specific antibody, but it appeared to be ubiquitously expressed in all of the tissues we examined. Together these results suggest that nPKC theta, rather than cPKC alpha, is involved in physiological functions that are specific for skeletal muscle. The immunoreactivity for nPKC theta was highest in the membrane subcellular fraction compared to the cytosolic fraction of skeletal muscle. In contrast, cPKC alpha was found to be predominantly distributed in the cytosolic rather than the membrane fraction. nPKC theta appeared to be developmentally regulated postnatally in rat skeletal muscle, with a 4-fold increase in expression occurring exclusively in the membrane fraction during postnatal days 3 through 21. This time course coincides with the period in rat development associated with maturation of neuromuscular junctions. Expression of nPKC theta in rat spleen, another tissue expressing detectable levels of this isoform, was not found to be developmentally regulated during this time. cPKC alpha expression was found to increase slightly from postnatal days 3 through 11 and no developmental increase in expression of this isoform was observed in skeletal muscle during postnatal days 11 through 21. The intracellular localization of the PKC theta and alpha isoforms in rat skeletal muscle was examined by immunocytochemistry. nPKC theta was detected in association with the sarcolemma of skeletal muscle and was found to be localized in the neuromuscular junction. Enhanced staining for nPKC theta in the neuromuscular junction appeared as early as postnatal day 4 during development. Staining for nPKC theta in the neuromuscular junction persisted after prolonged denervation, suggesting that the enzyme is distributed postsynaptically.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein kinase A-mediated phosphorylation reduces only the fast desensitizing glycine current in acutely dissociated ventromedial hypothalamic neurons

Modulation of glycine receptor-ionophore complex by internally perfused cyclic AMP was investigated and compared to that of GABA in the acutely dissociated ventromedial hypothalamic neurons using whole-cell and outside-out patch-clamp techniques. Cyclic AMP significantly reduced both GABA- and glycine-gated chloride currents. The reduction in glycine-induced chloride current was specific in that only the fast-desensitizing one gated by high concentrations of glycine (30-100 microM) was affected. Cyclic AMP did not modulate the non-desensitizing current induced by lower concentrations (6-10 microM). Addition of N-[-2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, a protein kinase A inhibitor, did not have a significant effect on its own but prevented the attenuation of fast desensitizing glycine current induced by cyclic AMP. Both the reversal potential and inactivation kinetics of glycine current were not affected by the activation of protein kinase A, suggesting that cyclic AMP-mediated attenuation is not due to an enhancement of desensitization. In outside-out patch studies intracellular perfusion of cyclic AMP reduced the open probability of the 100 microM glycine-activated channels without affecting that of the 6 microM glycine-activated channels. In conclusion, cyclic AMP selectively modulates the channel open frequency of the glycine receptor when activated at higher concentrations through a protein kinase A-mediated phosphorylation.

The functional architecture of the acetylcholine nicotinic receptor explored by affinity labelling and site-directed mutagenesis

The scientific community will remember Peter Läuger as an exceptional man combining a generous personality and a sharp and skilful mind. He was able to attract by his views the interest of a large spectrum of biologists concerned by the mechanism of ion translocation through membranes. Yet, he was not a man with a single technique or theory. Using an authentically multidisciplinary approach, his ambition was to ‘understand transmembrane transport at the microscopic level, to capture its dynamics in the course of defined physiological processes’ (1987). According to him, ‘new concepts in the molecular physics of proteins’ had to be imagined, and ‘the traditional static picture of proteins has been replaced by the notions that proteins represent dynamic structures, subjected to conformational fluctuations covering a very wide time-range’ (1987).

Modulation of anti-acetylcholine receptor antibody specificities and of experimental autoimmune myasthenia gravis by synthetic peptides

Synthetic peptides corresponding to selected sequences from the nicotinic acetylcholine receptor (AChR) were employed to identify possible antigenic determinants within the receptor which can modulate the anti-AChR response and experimental autoimmune myasthenia gravis (EAMG). Immunization of rabbits with peptides T alpha 73-89, T alpha 351-368, T delta 354-367 and H alpha 351-368, prior to AChR inoculation, affected the course of EAMG in six out of eight rabbits. These six protected rabbits survived three inoculations of AChR and survived for at least five months after the third injection with AChR, whereas control rabbits died following one or two injections of AChR. The survival of peptide-preimmunized rabbits injected with AChR seemed to correlate with the antibody specificities in immunoblots. Following AChR inoculation there was a shift in reactivity, from a subunit-restricted response, to reactivity with all subunits of the receptor. This shift was delayed in protected rabbits. This may indicate that the reactivity with the entire Torpedo receptor molecule represents a loss of tolerance to AChR which culminates in the autoimmune disease, EAMG.

Xanthine derivatives IBMX and S-9977-2 potentiate transmission at an Aplysia central cholinergic synapse

In an attempt to investigate the role of cAMP-dependent phosphorylations on synaptic transmission at an Aplysia cholinergic buccal ganglion synapse, the effects of xanthine derivatives such as 3-isobutyl-1-methylxanthine (IBMX), which is well known to inhibit phosphodiesterase activity thereby promoting cAMP accumulation, and a novel xanthine derivative, S-9977-2 were evaluated. They were found to potentiate cholinergic transmission by significantly increasing the time constant of decay (Tc) of inhibitory postsynaptic currents (IPSCs). The postsynaptic origin of the phenomenon was supported by the observation that responses to the ionophoretic application of acetylcholine (ACh) were also potentiated in duration as well as in amplitude. No effects of S-9977-2 on the ACh-gated Cl- channel conductance or mean open time were observed. The finding that responses to the hydrolysis-resistant cholinergic analogue carbachol were unaffected by the two xanthines suggested that the observed effects were at least partly caused by an inhibition of acetylcholinesterase (AChE) activity. That these substances inhibit AChE activity was confirmed in vitro. Phosphorylation processes nonetheless appear to be partly involved in the synaptic effect of the xanthines as the kinase blocker H-8 blocked part of the IPSC Tc lengthening. Possible mechanisms are discussed.

Cytoskeleton and molecular mechanisms in neurotransmitter release by neurosecretory cells

The process of exocytosis is a fascinating interplay between secretory vesicles and cellular components. Secretory vesicles are true organelles which not only store and protect neurotransmitters from inactivation but also provide the cell with efficient carriers of material for export. Different types of secretory vesicles are described and their membrane components compared. Associations of several cytoplasmic proteins and cytoskeletal components with secretory vesicles and the importance of such associations in the mechanism of secretion are discussed. A description of possible sites of action for Ca2+ as well as possible roles for calmodulin, G-proteins and protein kinase C in secretion are also presented. Important aspects of the cytoskeleton of neurosecretory cells are discussed. The cytoskeleton undergoes dynamic changes as a result of cell stimulation. These changes (i.e. actin filament disassembly) which are a prelude to exocytosis, play a central role in secretion. Moreover, advanced electrophysiological techniques which allow the study of secretory vesicle-plasma membrane fusion in real-time resolution and at the level of the single secretory vesicle, have also provided a better understanding of the secretory process.

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.

Kinetics of phosphorylation of Na+/K(+)-ATPase by protein kinase C

The kinetics of phosphorylation of an integral membrane enzyme, Na+/K(+)-ATPase, by calcium- and phospholipid-dependent protein kinase C (PKC) were characterized in vitro. The phosphorylation by PKC occurred on the catalytic alpha-subunit of Na+/K(+)-ATPase in preparations of purified enzyme from dog kidney and duck salt-gland and in preparations of duck salt-gland microsomes. The phosphorylation required calcium (Ka approximately 1.0 microM) and was stimulated by tumor-promoting phorbol ester (12-O-tetradecanoylphorbol 13-acetate) in the presence of a low concentration of calcium (0.1 microM). PKC phosphorylation of Na+/K(+)-ATPase was rapid and plateaued within 30 min. The apparent Km of PKC for Na+/K(+)-ATPase as a substrate was 0.5 microM for dog kidney enzyme and 0.3 microM for duck salt-gland enzyme. Apparent substrate inhibition of PKC activity was observed at concentrations of purified salt-gland Na+/K(+)-ATPase greater than 1.0 microM. Phosphorylation of purified kidney and salt-gland Na+/K+ ATPases occurred at both serine and threonine residues. The 32P-phosphopeptide pattern on 15% sodium dodecyl sulfate-polyacrylamide gel electrophoresis after hydroxylamine cleavage of pure 32P-phosphorylated alpha subunit was the same for the two sources of enzyme, which suggests that the phosphorylation sites are similar. The results indicate that Na+/K(+)-ATPase may serve as a substrate for PKC phosphorylation in intact cells and that the Na+/K(+)-ATPase could be a useful in vitro model substrate for PKC interaction with integral membrane proteins.

Regulation of tyrosine phosphorylation of the nicotinic acetylcholine receptor at the rat neuromuscular junction

The nicotinic acetylcholine receptor (AChR) from the electric organ of T. californica is highly phosphorylated on tyrosine residues in vivo. In contrast, tyrosine phosphorylation of the AChR in rat myotube cultures is barely detectable. To determine whether this low level of tyrosine phosphorylation of the AChR in muscle cell cultures is due to a lack of neuronal innervation, we examined tyrosine phosphorylation of the AChR in rat diaphragm in vivo. Immunofluorescent double labeling of cryostat sections of rat diaphragm using antibodies specific for phosphotyrosine or the AChR showed a direct colocalization of phosphotyrosine with the AChR at the neuromuscular junction. Using anti-phosphotyrosine antibodies, immunoblots of AChR partially purified from rat diaphragm demonstrated that the rat AChR contains high levels of phosphotyrosine. Denervation of rat diaphragm induced a time-dependent decrease in tyrosine phosphorylation of the AChR, as measured by immunocytochemical and immunoblot techniques. Tyrosine phosphorylation of the AChR occurred late in the development of the neuromuscular junction, between postnatal days 7 and 14. These studies suggest that muscle innervation regulates tyrosine phosphorylation of the AChR and that tyrosine phosphorylation may play an important role in the developmental regulation of the AChR.

Acetylcholine-activated currents in quail myotubes expressing viral oncogenes

Acetylcholine-activated currents were recorded in cultured myotubes arising from embryonic quail myoblasts transformed by the v-src and v-ras oncogenes. In src-myotubes, the whole cell inward current decayed more slowly than in non-transformed controls. In ras-myotubes, the current had a faster decay and smaller amplitude than in the controls. The single-channel conductance and mean open times recorded from cell-attached patches were similar in transformed and control cells. However, in ras-myotubes the frequency of channel openings strongly decreased with time. It is concluded that oncogenic tyrosine-specific protein kinase (v-src product) and G-like p21 protein (v-ras product) can induce differential changes in the function of nicotinic ACh receptor, perhaps related to specific biochemical events elicited in the establishment of the transformed state.

Phorbol esters and d-tubocurarine up-regulate alpha-bungarotoxin sites in chromaffin cells in culture via distinct mechanisms

Previous work had shown that nicotinic antagonists resulted in a marked up-regulation of alpha-bungarotoxin sites in chromaffin cells in culture. The present experiments were done to determine the intracellular mechanism(s) whereby nicotinic antagonists might mediate their effects on these receptors. Chromaffin cells were cultured for three days with various concentrations of 4 beta-phorbol 12-myristate 13-acetate, an agent which affects protein kinase C by mimicking the actions of diacylglycerol. The phorbol ester resulted in a dose-dependent increase in alpha-bungarotoxin binding which was maximal with 100 nM 4 beta-phorbol 12-myristate 13-acetate. This increase in binding appeared to be due to an increase in the maximal number of alpha-bungarotoxin sites. Time dependence studies showed that the effect of the phorbol was undetectable with incubations of 24 h or less and appeared to plateau by 72-96 h. A similar increase in toxin binding was also observed with 4 beta-phorbol 12,13-dibutyrate. On the other hand, an inactive analog of 4 beta-phorbol 12-myristate 13-acetate had no significant effect on binding. D-Sphingosine, an inhibitor of protein kinase C, was able to partially block the phorbol ester-induced increase in toxin binding while polymyxin B, another protein kinase C inhibitor, completely prevented the up-regulation of the alpha-bungarotoxin sites. Carbachol and nicotine prevented this enhancement of toxin binding in the presence of 4 beta-phorbol 12-myristate 13-acetate. Although the phorbol ester resulted in an increase in toxin binding, acetylcholine-evoked catecholamine secretion from chromaffin cells in culture was decreased, indicating a dissociation between the functional nicotinic acetylcholine receptor population and the alpha-bungarotoxin sites. To determine whether agents which affect protein kinase C can alter the up-regulation of alpha-bungarotoxin sites by d-tubocurarine, 4 beta-phorbol 12-myristate 13-acetate was added to the cells in combination with the nicotinic antagonist. The up-regulation of toxin binding sites induced by d-tubocurarine was additive with that induced by the phorbol and was not affected by polymyxin B. Thus, the results would suggest that there are at least two mechanisms by which alpha-bungarotoxin binding sites can be regulated. One is mediated via an interaction at nicotinic receptors, while the other occurs in response to phorbol esters and thus may be mediated by protein kinase C. Interestingly, although the molecular mechanisms resulting in alpha-bungarotoxin receptor up-regulation differ, both the d-tubocurarine- and the phorbol ester-induced increases were prevented by nicotinic receptor ligands.

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.

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.

Protein kinase C-mediated phosphorylation of retinal rod outer segment membrane proteins

We have previously reported that the purified GDP-bound alpha-subunit of the GTP-binding protein transducin (TD), present in outer segments of retinal rod cells (ROS), serves as a high affinity substrate (Km = 1 microM) for protein kinase C (PKC) [Zick et al. (1986) Proc. natn. Acad. Sci., U.S.A. 83, 9294-9297]. In the present study we demonstrate that TD-alpha undergoes phosphorylation by PKC when present in its native form in intact ROS membranes. This phosphorylation is inhibited by GTP-gamma-S which activates TD, suggesting that it is only the inactive conformation of TD-alpha that serves as a substrate for PKC. Indeed, both vanadate and AlF4, that confer an active conformation on TD-alpha-GDP, inhibit PKC-mediated phosphorylation of purified TD-alpha-GDP. We demonstrate that the purified beta subunit of TD also serves as an in vitro substrate for PKC. Moreover, following their phosphorylation, both TD-alpha and beta form high affinity complexes with PKC. This is evident from the findings that PKC coprecipitates with both the alpha and beta subunits of TD when the latter are immunoprecipitated by their respective antibodies. PKC phosphorylates additional ROS proteins of 36, 48 and 92 kDa, tentatively identified as rhodopsin, arrestin and the cGMP-phosphodiesterase. Taken together our results strongly suggest that phosphorylation of TD is of physiological relevance and that through phosphorylation of endogenous ROS proteins, PKC could play a key role in regulating phototransduction.

Functional modulation of the nicotinic acetylcholine receptor by tyrosine phosphorylation

Tyrosine-specific protein phosphorylation has been implicated in the regulation of cell transformation and proliferation. However, recent studies have shown that the expression of protein tyrosine kinases in adult brain is very high, suggesting that tyrosine-specific protein phosphorylation may also have a role in the regulation of neuronal function. Although a number of substrate proteins are phosphorylated on tyrosine residues, the functional alteration of proteins by tyrosine phosphorylation has previously been convincingly demonstrated only for protein tyrosine kinases. The nicotinic acetylcholine receptor, a neurotransmitter-gated ion channel, is phosphorylated by a protein tyrosine kinase in post-synaptic membranes in vitro and in vivo. We demonstrate here that this tyrosine phosphorylation increases the rate of the rapid phase of desensitization of the nicotinic receptor, as measured by single channel recording of purified nicotinic acetylcholine receptor, when reconstituted in lipid vesicles. These data provide direct evidence for the regulation of ion channel properties by tyrosine phosphorylation. The results, which demonstrate a functional role of tyrosine phosphorylation in the nervous system, suggest a widespread role for tyrosine phosphorylation in neuronal signal transduction.

Modulation of high-affinity interleukin 2 receptors on activated human T lymphocytes by activators of protein kinase C

Phorbol myristate acetate (PMA) and 1-oleoyl-2-acetyl-rac-glycerol (OAG) are shown to induce a rapid (within 30 min) down-regulation of the capacity of activated human T lymphocytes to bind interleukin 2. This was associated with a manifold increase in membrane-associated protein kinase C, whereas no change in free cytoplasmic calcium was observed. In contrast, a 10-fold increase in free cytoplasmic calcium by ionomycin had no effect on interleukin 2 binding or subcellular distribution of protein kinase C. The reduction of interleukin 2 binding was caused by a decreased number of high-affinity interleukin 2 receptors, whereas the affinity of the remaining receptors was unchanged. However, PMA and OAG had no effect on the rate of internalization of the interleukin receptor. These data suggest that activation of protein kinase C, but not an increase in free cytoplasmic calcium, leads to a rapid decrease in the number of high-affinity interleukin 2 receptors on activated human T lymphocytes. However, the mechanism and biological importance of this phenomenon have to be further elucidated.