Dexamethasone-induced muscular atrophy is mediated by functional expression of connexin-based hemichannels

Long-term treatment with high glucocorticoid doses induces skeletal muscle atrophy. However, the molecular mechanism of such atrophy remains unclear. We evaluated the possible involvement of connexin-based hemichannels (Cx HCs) in muscle atrophy induced by dexamethasone (DEX), a synthetic glucocorticoid, on control (Cx43^fl/flCx45^fl/fl) and Cx43/Cx45 expression-deficient (Cx43^fl/flCx45^fl/fl:Myo-Cre) skeletal myofibers. Myofibers of Cx43^fl/flCx45^fl/fl mice treated with DEX (5 h) expressed several proteins that form non-selective membrane channels (Cx39, Cx43, Cx45, Panx1, P2X₇ receptor and TRPV2). After 5 h DEX treatment in vivo, myofibers of Cx43^fl/flCx45^fl/fl mice showed Evans blue uptake, which was absent in myofibers of Cx43^fl/flCx45^fl/fl:Myo-Cre mice. Similar results were obtained in vitro using ethidium as an HC permeability probe, and DEX-induced dye uptake in control myofibers was blocked by P2X₇ receptor inhibitors. DEX also induced a significant increase in basal intracellular Ca^2 + signal and a reduction in resting membrane potential in Cx43^fl/flCx45^fl/fl myofibers, changes that were not elicited by myofibers deficient in Cx43/Cx45 expression. Moreover, treatment with DEX induced NFκB activation and increased mRNA levels of TNF-α in control but not in Cx43/Cx45 expression-deficient myofibers. Finally, a prolonged DEX treatment (7 days) increased atrogin-1 and Murf-1 and reduced the cross sectional area of Cx43^fl/flCx45^fl/fl myofibers, but these parameters remained unaffected in Cx43^fl/flCx45^fl/fl:Myo-Cre myofibers. Therefore, DEX-induced expression of Cx43 and Cx45 plays a critical role in early sarcolemma changes that lead to atrophy. Consequently, this side effect of chronic glucocorticoid treatment might be avoided by co-administration with a Cx HC blocker.