Because of the large size and modest selectivity of the connexin hemichannel aqueous pore, hemichannel opening must be highly regulated to maintain cell viability. At normal resting potentials, this regulation is achieved predominantly by the physiological extracellular Ca2+ concentration, which drastically reduces hemichannel activity. Here, we characterize the Ca2+ regulation of channels formed by wild-type human connexin26 (hCx26) and its human mutations, D50N/Y, that cause aberrant hemichannel opening and result in deafness and skin disorders. We found that in hCx26 wild-type channels, deactivation kinetics are accelerated as a function of Ca2+ concentration, indicating that Ca2+ facilitates transition to, and stabilizes, the closed state of the hemichannels. The D50N/Y mutant hemichannels show lower apparent affinities for Ca2+-induced closing than wild-type channels and have more rapid deactivation kinetics, which are Ca2+ insensitive. These results suggest that D50 plays a role in (a) stabilizing the open state in the absence of Ca2+, and (b) facilitating closing and stabilization of the closed state in the presence of Ca2+. To explore the role of a negatively charged residue at position 50 in regulation by Ca2+, this position was substituted with a cysteine residue, which was then modified with a negatively charged methanethiosulfonate reagent, sodium (2-sulfanoethyl) methanethiosulfonate (MTSES)−. D50C mutant hemichannels display properties similar to those of D50N/Y mutants. Recovery of the negative charge with chemical modification by MTSES− restores the wild-type Ca2+ regulation of the channels. These results confirm the essential role of a negative charge at position 50 for Ca2+ regulation. Additionally, charge-swapping mutagenesis studies suggest involvement of a salt bridge interaction between D50 and K61 in the adjacent connexin subunit in stabilizing the open state in low extracellular Ca2+. Mutant cycle analysis supports a Ca2+-sensitive interaction between these two residues in the open state of the channel. We propose that disruption of this interaction by extracellular Ca2+ destabilizes the open state and facilitates hemichannel closing. Our data provide a mechanistic understanding of how mutations at position 50 that cause human diseases are linked to dysfunction of hemichannel gating by external Ca2+.

Abnormal intracellular Ca(2+) handling is an important factor in the progressive functional decline of dystrophic muscle. In the present study, we investigated the function of sarco(endo)plasmic reticulum (SR) Ca(2+) ATPase (SERCA) in various dystrophic muscles of mouse models of Duchenne muscular dystrophy. Our studies show that the protein expression of sarcolipin, a key regulator of the SERCA pump is abnormally high and correlates with decreased maximum velocity of SR Ca(2+) uptake in the soleus, diaphragm and quadriceps of mild (mdx) and severe (mdx:utr-/-) dystrophic mice. These changes are more pronounced in the muscles of mdx:utr-/- mice. We also found increased expression of SERCA2a and calsequestrin specifically in the dystrophic quadriceps. Immunostaining analysis further showed that SERCA2a expression is associated both with fibers expressing slow-type myosin and regenerating fibers expressing embryonic myosin. Together, our data suggest that sarcolipin upregulation is a common secondary alteration in all dystrophic muscles and contributes to the abnormal elevation of intracellular Ca(2+) concentration via SERCA inhibition.

Duchenne muscular dystrophy (DMD) is caused by an X-linked mutation that leads to the absence of dystrophin, resulting in life-threatening arrhythmogenesis and associated heart failure. We targeted the gap junction protein connexin43 (Cx43) responsible for maintaining cardiac conduction. In mild mdx and severe mdx:utr mouse models of DMD, and human DMD tissues, Cx43 was found to be pathologically mislocalized to lateral sides of cardiomyocytes. In addition, overall Cx43 protein levels were markedly increased in mouse and human DMD heart tissues examined. Electrocardiography on isoproterenol challenged mice showed that both models developed arrhythmias and died within 24 hours, while wild-type mice were free of pathology. Administering peptide mimetics to inhibit lateralized Cx43 function prior to challenge protected mdx mice from arrhythmogenesis and death, while mdx:utr mice displayed markedly improved ECG scores. These findings suggest that Cx43 lateralization contributes significantly to DMD arrhythmogenesis and that selective inhibition may provide substantial benefit.

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SYNOPSIS Duchenne muscular dystrophy (DMD) is an incurable, rapidly-worsening neuromuscular degenerative disease caused by the absence of dystrophin. In skeletal muscle, lack of dystrophin disrupts the recruitment of neuronal nitric oxide synthase (nNOS) to the sarcolemma thus affecting nitric oxide (NO) production. Utrophin is a dystrophin homolog which expression is greatly upregulated in the sarcolemma of dystrophin-negative fibers from mdx mice, a mouse model of DMD. Although cardiomyopathy is an important cause of death, little is known about the NO signaling pathway in cardiac muscle of DMD patients. Thus, we used cardiomyocytes and hearts from two month-old mdx and mdx:utrophin (−/−) double knockout mice (mdx:utr) to study key steps in NO signaling: L-arginine transporters, NOS, and soluble guanylyl cyclase (sGC). nNOS did not co-localize with dystrophin or utrophin to the cardiomyocyte membrane. Despite this, nNOS activity was markedly decreased in both mdx and mdx:utr mice while nNOS expression was only decreased in mdx:utr hearts, suggesting that utrophin upregulation in cardiomyocytes maintains nNOS levels but not function. sGC protein levels and activity remained at control levels. Unexpectedly, L-arginine transporter expression and function were significantly increased, suggesting a novel biochemical compensatory mechanism of the NO pathway and a potential entry site for therapeutics.