Reduced acetylcholine-induced channel activity in dystrophic mouse myotubes

Identification of a determinant of acetylcholine receptor gating kinetics in the extracellular portion of the gamma subunit

A large body of structure-function studies has identified many of the functional motifs underlying ion permeation through acetylcholine receptor (AChR) channels. The structural basis of channel gating kinetics is, however, incompletely understood. We have previously identified a novel shorter form of the AChR gamma subunit, which lacks the 52 amino acids within the extracellular amino-terminal half, encoded by exon 5. To define the contribution of the missing domain to AChR channel function, we have transiently coexpressed the mouse short gamma subunit [gamma(s)] with alpha, beta and delta subunits in human cells and recorded single-channel currents from the resulting AChRs. Our findings show that replacement of the gamma by the gamma(s) subunit confers a long duration characteristic to AChR channel openings without altering unitary conductance sizes or receptor affinity for the transmitter. We also show that alpha beta gamma(s) delta AChR channels exhibit a peculiar voltage sensitivity characterized by a short opening duration when the membrane potential is hyperpolarized. Together, these findings indicate that the domain in the extracellular amino-terminal half of the gamma subunit that encompasses a conserved disulphide loop and a critical tyrosine residue implicated in receptor oligomerization and insertion at the cell surface is a functional motif that also modulates AChR channel gating kinetics. The results also provide a molecular explanation of the functional diversity exhibited by skeletal muscle AChRs during development.

Protein kinase C modulates exogenous acetylcholine current in Xenopus oocytes

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

Amphiphilic properties of molecular forms of acetylcholinesterase in normal and dystrophic muscle

Acetylcholinesterase (AChE) molecular forms were studied in normal (NM) and in dystrophic (DM) 129B6F1/J mouse muscle. Successive extractions of the tissue with saline and saline-Triton X-100 buffers yielded two soluble fractions, S1 and S2. Forty percent of the AChE in NM was measured in S1 and 60% in S2, and 65% and 35%, respectively, in extracts from DM. A12, A8, G4, G2, and G1 forms of AChE were found in S1 and S2 from NM and DM. A similar content of asymmetric molecules was noticed between NM and DM. G4 AChE was a minor species in DM, and G1 and G2 AChE were more abundant in DM than in NM. The amphiphilic properties of the several molecules were assessed by Triton X-114 phase-partitioning and hydrophobic chromatography. Thirty and 70% of the enzyme in a mixture of S1 and S2 partitioned in the detergent-rich and in the detergent-poor phases, respectively, whether the extracts were obtained from NM or DM. Asymmetric and G4 AChE predominated in the aqueous phase and G1 and G2 in the detergent phase. Ten and 25% of the enzyme in S1 from NM or DM, respectively, was adsorbed to the phenyl-agarose. Elution of the retained enzyme followed by sedimentation analysis revealed that a certain amount of asymmetric and most of the G1 and G2 forms were associated with the matrix. The content of amphiphilic asymmetric and light globular forms was notably higher in DM than in NM. The results suggest that dystrophic muscle produces a specific pattern of molecular forms of AChE.

Blockage of nicotinic acetylcholine receptors by 5-hydroxytryptamine

The action of 5-hydroxytryptamine (5HT) on nicotinic acetylcholine receptor (nAChR) channels was investigated in mouse myotubes, human cloned TE671/RD cells, and Xenopus laevis oocytes. The decay of the ACh-activated whole-cell currents was reversibly accelerated in the presence of 5HT (10(-5) to 10(-3) M), in a dose-dependent manner. 5HT also reduced the size and accelerated the decay of currents elicited by ACh in Xenopus oocytes injected with mRNA extracted from C2 myotubes or Torpedo electroplaques, or oocytes injected with cloned mouse muscle AChR subunit mRNAs. The effect of 5HT was promptly reversed after washout, or by depolarizing the oocyte beyond -10 mV. In patch-clamp recordings from myotubes, bath-application of 5HT did not exert an indirect influence on the ACh-activated channels within the patch membrane. In contrast, when the patch membrane was exposed to 5HT (10(-6) M), ACh unit responses appeared as bursts of short pulses. It is concluded that the regulation of ACh responses by 5HT results from a fast noncompetitive blocking action of nAChR-channels. These results show that ligand-gated channels, activated by their specific neurotransmitter, may be regulated by a different neurotransmitter through a direct action on the receptor molecule.

Increased acetylcholine sensitivity in Duchenne muscular dystrophy myotubes

Monolayer cultures were established from explants of muscle obtained from 6 patients with Duchenne muscular dystrophy (DMD) and 9 controls. Electrophysiological studies were made after 3-4 weeks in vitro, when many myotubes had formed. An intracellular electrode was used to record cell membrane potential, and acetylcholine was applied by ionophoresis. The myotubes grown from Duchenne muscle showed greater acetylcholine sensitivity than controls.

Altered protein phosphorylation in murine muscular dystrophy

Protein phosphorylation has been studied in the dydy murine muscular dystrophy, both in intact muscle cells and in various membrane fractions derived from them. The results obtained showed that several polypeptides were more heavily phosphorylated in dystrophic myotubes in culture as well as in dystrophic muscle fibers isolated from tibialis anterior. In vitro phosphorylation studies revealed that a large polypeptide of apparent molecular weight of 170,000-150,000 was phosphorylated under basal conditions (3 mM EGTA) in dydy microsomal membranes. The phosphorylation of this polypeptide was not stimulated further by cAMP, calmodulin, cGMP or 12-O-tetradecanoylphorbol 13-acetate (TPA). Under no condition was the corresponding polypeptide phosphorylated at an appreciable rate in normal microsomal membranes. An antibody raised against the voltage-dependent calcium channel reacted, in an immunoblot assay, with a polypeptide, present in both normal and dydy microsomes, which had migration characteristics identical to the phosphorylated 170-150 kDa polypeptide after one- or two-dimensional gel electrophoresis. Additional differences were identified in the phosphorylation of smaller polypeptides of microsomal membranes. When sarcolemmal membranes of normal and dydy muscle were phosphorylated in vitro, no major differences were observed. These results show the existence of an alteration of protein phosphorylation in dystrophic muscle cells in vitro and in vivo, leading to abnormal phosphorylation of the voltage-dependent calcium channel. The possible causes and consequences of this alteration are discussed.

Properties of acetylcholine-receptor activation in human Duchenne muscular dystrophy myotubes

In human myotubes cultured from biopsies of normal subjects and dystrophic patients we investigated, with the patch-clamp technique, the activation properties of the nicotinic acetylcholine receptor (AChoR) in the presence of acetylcholine and suberyldicholine. The single-channel conductance and the lifetime of the openings were not found to differ. In contrast, the average frequency of openings was about four times higher in Duchenne muscular dystrophy (DMD) myotubes in the presence of equal amounts of acetylcholine, but not of suberyldicholine. The most reasonable conclusion from this observation is that the behaviour of the AChoR is not altered in DMD cells but that there is a greater average concentration of ACho molecules present around AChoRs. This leads to the tentative conclusion that the activity of the enzyme acetylcholinesterase (AChoE) is impaired by some unknown mechanism in the dystrophic myotube.