Zinc Modulates Skeletal Ryanodine Receptor Function Resulting in Altered Sarcoplasmic Reticulum Calcium Release

Impaired Ca2+-homeostasis due to dysfunction in the principal sarcoplasmic reticulum Ca2+-release channel, the type-1 ryanodine receptor (RyR1) contributes to the pathophysiology of muscular dystrophy. Dynamic alterations in intracellular Zn2+ and Ca2+-homeostasis are associated with dystrophinopathies, but the mechanism by which Zn2+ modulates RyR1 is unknown. We have investigated how Zn2+ modulates RyR1-channel gating at the single-channel level. Native RyR1-channels were isolated from guinea-pig skeletal muscle and incorporated into planar phosphatidylethanolamine bilayers under voltage-clamp conditions as described previously (1). We reveal that Zn2+ is a high affinity effector of RyR1. Sequential addition of cytosolic Zn2+ in the range 100 pM to 1 μM significantly increased RyR1-channel open probability (Po) in a dose dependent manner. The increase in channel activity resulted from an increase in the frequency of openings with no significant effect on mean channel open times. These data indicate that Zn2+ increases RyR1 Po by sensitizing the channel to cytosolic Ca2+. In skeletal muscle, Mg2+ is an endogenous inhibitor of RyR1 and helps to prevent inappropriate release of Ca2+. We therefore wanted to investigate the activatory effects of Zn2+ in the presence of physiological levels of Mg2+. Under resting conditions (100 nM cytosolic Ca2+) we observed that the presence of 1 mM cytosolic Mg2+ resulted in complete closure of the channel (Po=0). Unexpectedly, RyR1-channel openings were observed following the addition of pathophysiological levels of Zn2+ (10 nM) where Po increased from 0 to 0.14 (n=2). Dysregulated Zn2+ levels may therefore lead to abnormally active or “leaky” RyR1-channels, suggesting that altered Zn2+-homeostasis could perturb intracellular Ca2+-dynamics and contribute to the progression of muscular dystrophy.(1) Woodier et al.,(2015) JBC;290(28):17599-610.Funded by the BHF