Molecular biology of acetylcholine receptor long-term evolution during motor end-plate morphogenesis

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.

TTX-sensitive and TTX-insensitive sodium channel mRNA transcripts are independently regulated in adult skeletal muscle after denervation

The expression of mRNA encoding the TTX-sensitive (SkM1) and TTX-insensitive (SkM2) voltage-dependent sodium channels in adult skeletal muscle is independently regulated. In normal skeletal muscle, only the SkM1 message is expressed and the level varies with muscle fiber type. After surgical denervation, the steady-state SkM1 mRNA level declines transiently, but returns to control levels within 5 days. Expression of SkM2 transcripts is markedly activated, reaching a peak 3 days after axotomy and then declining to a maintained level at approximately 30% of peak. Chemical denervation with botulinum toxin results in higher levels of SkM2 mRNA, which by 7 days posttreatment are 7-fold greater than levels in paired axotomized muscles. SkM2 expression subsequently declines as functional reinnervation appears. Quantal acetylcholine release appears to play a major role in suppression of SkM2 expression in adult innervated or reinnervated muscle, whereas nonquantal factors in toxin-treated, but not axotomized, muscle may sustain high level SkM2 mRNA expression.