The acetylcholine receptor: its molecular biology and biotechnological prospects

Myoblast and myotube nuclei display similar patterns of heterogeneous acetylcholine receptor subunit mRNA expression

Muscle progenitor cells differentiate to myoblasts, and subsequently myotubes, upon expression of muscle specific genes. We and others have previously shown that myotube nuclei, even in the absence of nerve, express AChR alpha subunit RNA at varying levels, with a small subset (about ten percent) of the nuclei expressing at high levels. These findings raised two important questions: 1) is the observed heterogeneity a unique property of the alpha subunits, and 2) when does the heterogeneity begin? In particular, is it induced only at or after the time of fusion, or does it exist at the myoblast stage? We have, therefore, extended our observations to the gamma and delta subunits and we also have examined the distributions of AChR alpha, gamma, and delta subunit RNAs in both myoblasts and myotubes. We used intron and intron-exon probes to detect prespliced transcripts or mature mRNAs in the cells. Because intron-containing transcripts are not transported out of the nuclei, the distributions of these transcripts can indicate their expression patterns among nuclei in the same myotubes. Our results show that both myotubes and myoblasts have distributions of the AChR alpha, gamma, and delta subunit RNAs which differ sharply from that of the U1 RNA or Myo D. Thus, the heterogeneous expression of AChR genes is not only an intrinsic property of muscle cell nuclei (in the sense that it does not require the presence of nerves), but it also exists prior to fusion. Our results suggest that muscle nuclei attain individualized capacities for AChR subunit mRNA production early in their development. Conceptual models consistent with such individuality imply an additional level of regulation beyond the known diffusible transcriptional factors.

Time-lapse total internal reflection fluorescence video of acetylcholine receptor cluster formation on myotubes

To study when and where acetylcholine receptor (AChR) clusters appear on developing rat myotubes in primary culture, we have made time-lapse movies of total internal reflection fluorescence (TIRF) overlaid with schlieren transmitted light images. The receptors, including the ones newly incorporated into the membrane, were labeled with rhodamine alpha-bungarotoxin (R-BT) continuously present in the medium. Since TIRF illuminates only cell-substrate contact regions where almost all of the AChR clusters are located, background fluorescence from fluorophores either in the bulk solution or inside the cells can be suppressed. Also, because TIRF minimizes the exposure of the cell interior to light, the healthy survival of the culture during imaging procedures is much enhanced relative to standard epi- (or trans-) illumination. During the experiment, cells were kept alive on the microscope stage at 37 degrees C in an atmosphere of 10% CO2. Two digital images were recorded by a CCD camera every 20 min: the schlieren image of the cells and the TIRF image of the clusters. After background subtraction, the cluster image was displayed in pseudocolors, overlaid onto the cell images, and recorded as 3 frames on a videotape. The final movies are thus able to summarize a week-long experiment in less than a minute. These movies and images show that clusters form often shortly after the myoblast fusion but sometimes much later, and the formation takes place very rapidly (a few hours). The clusters have an average lifetime of around a day, much shorter than the lifetime of a typical myotube. The brightest and largest clusters tend to be the longest-lived. The cluster formation seems to be associated with the contacts of myotubes at the glass substrate, but not with cell-cell contacts or myoblast fusion into myotubes. New AChR continuously appear in preexisting clusters: after photobleaching, the fluorescence of some clusters recovers within an hour.

Interactions between spinal cord stimulation and activity blockade in the regulation of synaptogenesis and motoneuron survival in the chick embryo

The present study investigated the effects of spinal cord stimulation, neuromuscular blockade, or a combination of the two on neuromuscular development both during and after the period of naturally occurring motoneuron death in the chick embryo. Electrical stimulation of the spinal cord was without effect on motoneuron survival, synaptogenesis, or muscle properties. By contrast, activity blockade rescued motoneurons from cell death and altered synaptogenesis. A combination of spinal cord stimulation and activity blockade resulted in a marked increase in motoneuron death, and also altered synaptogenesis similar to that seen with activity blockade alone. Perturbation of normal nerve-muscle interactions by activity blockade may increase the vulnerability of developing motoneurons to excessive excitatory afferent input (spinal cord stimulation) resulting in excitotoxic-induced cell death.

The distribution of intracellular acetylcholine receptors and nuclei in avian slow muscle fibres during establishment of distributed synapses

The distribution of intracellular acetylcholine receptor was studied by 125I-alpha-bungarotoxin autoradiography as a measure of the local acetylcholine receptor synthesis at junctional and extrajunctional sites in single fibres of the developing anterior latissimus dorsi muscle of the chicken. Large (longer than 2 microns) acetylcholine receptor clusters characteristic of synaptic contacts were localized by immunofluorescence with anti-acetylcholine receptor antibodies. The distance between acetylcholine receptor clusters at embryonic day 11 was 166 +/- 10.5 microns and this distance did not increase despite growth until after 4 days posthatch. The distance between acetylcholine receptor clusters subsequently increased proportionately with the increase in the length of fibres. Intracellular acetylcholine receptors were labelled with 125I-alpha-BGT after first blocking cell-surface acetylcholine receptor with unlabelled alpha-BGT, and treatment with saponin. Intracellular acetylcholine receptor represented about 5-15% of total cellular acetylcholine receptor. Cycloheximide experiments indicated that 80-90% of intracellular acetylcholine receptor examined represented newly synthesized acetylcholine receptor. The spatial distribution of this pool, studied by autoradiography, was determined in relation to the acetylcholine receptor clusters labelled with anti-acetylcholine receptor antibody. Between embryonic day 11 and posthatch day 14 there was a continual increase in intracellular acetylcholine receptor at both junctional and extrajunctional parts of the fibres, but with the greater increases occurring at the junctional regions. Peaks of intracellular acetylcholine receptor became associated with an increasing number of acetylcholine receptor clusters so that by posthatch day 14 there was an 80% correspondence. The accumulation of newly synthesized intracellular acetylcholine receptor under acetylcholine receptor clusters was not the result of the aggregation of nuclei at these sites, suggesting that a higher rate of acetylcholine receptor synthesis per nucleus develops at distributed synaptic sites on anterior latissimus dorsi fibres.

Nicotinic acetylcholine receptors in porcine hypophyseal intermediate lobe cells

1. Acetylcholine (ACh) was found to depolarize isolated porcine intermediate lobe cells maintained in primary cells culture. We investigated the ACh-induced responses in both whole-cell and cell-attached configurations of the patch-clamp technique. 2. From noise analysis of ACh-evoked whole-cell currents, we estimated an elementary conductance of 20 pS and a channel open duration of about 1.7 ms at -60 mV. From single-channel recordings, we obtained a slope conductance of 26 pS and a mean open time of 1.8 ms at membrane potentials between -60 and -80 mV. 3. ACh-evoked responses were blocked by d-tubocurarine (d-TC), hexamethonium and mecamylamine, but were insensitive to alpha-bungarotoxin. These characteristics define a neuronal type of nicotinic receptors. 4. The whole-cell current induced by ACh showed a strong inward rectification with no outward current being obtained. This phenomenon was observed when the intracellular ion is either sodium or caesium, and even when Ca2+ and Mg2+ were totally removed from the intracellular medium. 5. ACh-gated channels in intermediate lobe cells were cation selective and were permeable to Na+ and Cs+. In Ca2(+)-free extracellular solution, single-channel conductances were much larger (46 pS) than in the presence of 2 mM-Ca2+ (26 pS). 6. The possibility of an excitatory cholinergic control of intermediate lobe cells is discussed.