Elevated subsarcolemmal Ca ²⁺ in mdx mouse skeletal muscle fibers detected with Ca ²⁺ -activated K ⁺ channels

Abstract

Duchenne muscular dystrophy results from the lack of dystrophin, a cytoskeletal protein associated with the inner surface membrane, in skeletal muscle. The cellular mechanisms responsible for the progressive skeletal muscle degeneration that characterizes the disease are still debated. One hypothesis suggests that the resting sarcolemmal permeability for Ca 2؉ is increased in dystrophic muscle, leading to Ca 2؉ accumulation in the cytosol and eventually to protein degradation. However, more recently, this hypothesis was challenged seriously by several groups that did not find any significant increase in the global intracellular Ca 2؉ in muscle from mdx mice, an animal model of the human disease. In the present study, using plasma membrane Ca 2؉ -activated K ؉ channels as subsarcolemmal Ca 2؉ probe, we tested the possibility of a Ca 2؉ accumulation at the restricted subsarcolemmal level in mdx skeletal muscle fibers. Using the cell-attached configuration of the patch-clamp technique, we demonstrated that the voltage threshold for activation of high conductance Ca 2؉ -activated K ؉ channels is significantly lower in mdx than in control muscle, suggesting a higher subsarcolemmal [Ca 2؉ ]. In inside-out patches, we showed that this shift in the voltage threshold for high conductance Ca 2؉ -activated K ؉ channel activation could correspond to a Ϸ3-fold increase in the subsarcolemmal Ca 2؉ concentration in mdx muscle. These data favor the hypothesis according to which an increased calcium entry is associated with the absence of dystrophin in mdx skeletal muscle, leading to Ca 2؉ overload at the subsarcolemmal level.

Tuning the mechanosensitivity of a BK channel by changing the linker length

“…The channel activity was enhanced by membrane stretch, resulting in a parallel left-shift of the Ca 2+ -P o curve without affecting the slope, suggesting that Ca 2+ sensitivity is not altered by membrane stretch. This is in agreement with the results previously described for the BK channels in smooth muscle cells [19] and skeletal muscle cells [32], whose activation in response to membrane stretch is not due to enhancement of Ca 2+ sensitivity. Although activation of SAKCaC by membrane stretch is independent of the Ca 2+ or voltage sensitivity, the spring-based gating model suggests that the effect of membrane stretch on the SAKCaC gating is similar to that produced by increasing intracellular Ca 2+ or membrane depolarization, as illustrated in Figure 7.…”

Proteome analysis of the dystrophin-deficient MDX diaphragm reveals a drastic increase in the heat shock protein cvHSP

“…The loss of a proper linkage between the actin membrane cytoskeleton and the extracellular matrix in the dystrophic plasmalemma seems to result in major disturbances in fibre homeostasis and muscle metabolism, which encompasses both the compensatory up-regulation of key enzymes and the loss of other metabolic regulators. Microrupturing of the dystrophic surface membrane has previously been implicated in causing disturbed ion fluxes [37,38]. Besides abnormal ion cycling in dystrophic fibres, a reduction in overall cellular integrity may also cause disturbed fluxes of metabolic substrates and protein factors [39] and thereby negatively influence the regulation of important pathways.…”

Proteomic profiling of animal models mimicking skeletal muscle disorders

“…The lack of dystrophin and resulting loss in the dystrophin-associated glycoprotein complex appears to damage the integrity of the skeletal muscle surface to such an extent that increased rupturing of the sarcolemma initiates a natural membrane resealing process [131]. However, the introduction of sarcolemmal Ca 21 -leak channels triggers a drastic elevation of cytosolic Ca 21 -levels [132], which in turn causes proteolysis and thereby muscle degeneration [131]. A complicating factor is the decreased Ca 21 -buffering capacity of the SR due to low levels of the Ca 21 -binding proteins calsequestrin (CSQ) [60], CSQ-like proteins [95] and sarcalumenin (SAR) [96], and impaired cystosolic Ca 21 -binding due to a reduction in regucalcin [41,61].…”