Protein kinase C modulates exogenous acetylcholine current in Xenopus oocytes

Selective effects of a 4-oxystilbene derivative on wild and mutant neuronal chick alpha7 nicotinic receptor

1. We assessed the pharmacological activity of triethyl-(beta-4-stilbenoxy-ethyl) ammonium (MG624), a drug that is active on neuronal nicotinic receptors (nicotinic AChR). Experiments on the major nicotinic AChR subtypes present in chick brain, showed that it inhibits the binding of [125I]-alphaBungarotoxin (alphaBgtx) to the alpha7 subtype, and that of [3H]-epibatidine (Epi) to the alpha4beta2 subtype, with Ki values of respectively 106 nM and 84 microM. 2. MG624 also inhibited ACh elicited currents (I(ACh)) in the oocyte-expressed alpha7 and alpha4beta2 chick subtypes with half-inhibitory concentrations (IC50) of respectively 109 nM and 3.2 microM. 3. When tested on muscle-type AChR, it inhibited [125I]-alphaBgtx binding with a Ki of 32 microM and ACh elicited currents (I(ACh)) in the oocyte-expressed alpha1beta1gammadelta chick subtype with an IC50 of 2.9 microM. 4. The interaction of MG624 with the alpha7 subtype was investigated using an alpha7 homomeric mutant receptor with a threonine-for-leucine 247 substitution (L247T alpha7). MG624 did not induce any current in oocytes expressing the wild type alpha7 receptor, but did induce large currents in the oocyte-expressed L247T alpha7 receptor. The MG624 elicited current (I(MG62)) has an EC50 of 0.2 nM and a Hill coefficient nH of 1.9, and is blocked by the nicotinic receptor antagonist methyllycaconitine (MLA). 5. These binding and electrophysiological studies show that MG624 is a potent antagonist of neuronal chick alpha7 nicotinic AChR, and becomes a competitive agonist following the mutation of the highly conserved leucine residue 247 located in the M2 channel domain.

Neuronal nicotinic threonine-for-leucine 247 alpha7 mutant receptors show different gating kinetics when activated by acetylcholine or by the noncompetitive agonist 5-hydroxytryptamine

Mutation of the highly conserved leucine residue (Leu-247) converts 5-hydroxytryptamine (5HT) from an antagonist into an agonist of neuronal homomeric alpha7 nicotinic acetylcholine receptor expressed in Xenopus oocytes. We show here that acetylcholine (AcCho) activates two classes of single channels with conductances of 44 pS and 58 pS, similar to those activated by 5HT. However, the mean open time of AcCho-gated ion channels (11 ms) is briefer than that of 5HT-gated ion channels (18 ms). Furthermore, whereas the open time of AcCho channels lengthens with hyperpolarization, that of 5HT channels is decreased. In voltage-clamped oocytes, the apparent affinity of the alpha7 mutant receptor for 5HT is not modified by the presence of dihydro-beta-erythroidine, which acts on the AcCho binding site in a competitive manner. This indicates a noncompetitive action of 5HT on nicotinic acetylcholine receptors. Considered together, our findings show that AcCho gates alpha7 mutant channels with similar conductance but with different kinetic profile than the channels gated by 5HT, suggesting that the two agonists act on different docking sites. These results will help to understand the crosstalk between cholinergic and serotonergic systems in the central nervous system.

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).

Serotonin 5-HT2 receptor activation potentiates N-methyl-D-aspartate receptor-mediated ion currents by a protein kinase C-dependent mechanism

Modulation of N-methyl-D-aspartate (NMDA) receptor-mediated ion currents by serotonin was investigated with a two-electrode voltage clamp technique in Xenopus oocytes injected with rat brain RNA. After a 1-min application of 200 nM serotonin a transient potentiation of the NMDA receptor-mediated ion currents was observed. The serotonin-induced enhancement was mimicked by the protein kinase C activators 1-oleoyl-2-acetyl-sn-glycerol (100 microM) and phorbol 12-myristate 13-acetate (10 nM), whereas the inactive phorbol ester 4-alpha-phorbol 12-myristate 13-acetate (10 nM) had no effect. From these observations it was concluded that protein kinase C was involved in the enhancement of NMDA-induced currents. In agreement with this conclusion, it was found that the serotonin effect was inhibited by the protein kinase C inhibitors sphingosine (1 microM) or staurosporine (1 microM) added 20 min before NMDA application and by oocyte injection of protein kinase C (PKC)-inhibitor peptide (500 ng/oocyte) 1 hr prior to recordings. The serotonin receptor involved was identified as a 5-HT2 receptor subtype by the finding that 200 nM of the selective 5-HT2 receptor agonist alpha-methyl-5-hydroxytryptamine mimicked the potentiation of NMDA-induced ion currents by serotonin. Furthermore, the observed potentiation was significantly reduced by co-application of serotonin with 100 microM of the selective 5-HT2 receptor antagonist ketanserin. These results indicate that 5-HT2 receptors enhance NMDA receptor function via phosphoinositol hydrolysis and subsequent stimulation of PKC.

The desensitization of the embryonic mouse muscle acetylcholine receptor depends on the cellular environment

The rate of desensitization of nicotinic acetylcholine (ACh) receptor (nAChR), an important characteristic of nAChR function, was studied in myotubes of the mouse C2C12 cell line at different times after fusion, by measuring the decay of ACh-evoked currents (IACh) under various patch-clamp configurations. We observed a progressive slowing of IACh decay rate (half-decay time rose from about 0.5 s to over 5 s) in myotubes of increasing size (i.e. age) under all experimental conditions, except in outside-out patches, when IACh decayed as fast as in the smallest myotubes. Single-channel conductance (about 35 pS) and open time (about 3.5 ms), measured in outside-out and cell-attached patches, were independent of myotube size. In Xenopus oocytes injected with poly(A+)RNA extracted from C2C12 myoblasts or mature myotubes, IACh decay was about 50 times slower than in myotubes. Neither cAMP-dependent nor diacylglycerol-dependent protein kinases, actin nor microtubule polymerization state influenced IACh decay. Our data indicate that the cellular environment, but not readily dialysable cytosolic factors, markedly influences the functional behaviour of nAChR.