A role for mitsugumin 23 in cardiac sarcoplasmic reticulum calcium leak

Abstract

Carefully controlled intracellular Ca²⁺-release is essential for maintenance of normal cardiac function. In failing hearts, dysregulated Zn²⁺-homeostasis is associated with disrupted intracellular Ca²⁺-homeostasis, however the underlying molecular mechanisms remain elusive. Mitsugumin 23 (MG23) is a newly identified SR Ca²⁺- permeable ion channel found in sarcoplasmic reticulum (SR) membranes, challenging understanding that RyR2 is the only SR Ca²⁺-release channel. The major hypothesis of this thesis is that MG23 is a Zn²⁺-regulated SR Ca²⁺-leak channel, and that this function plays a key role in disease progression mechanisms in heart failure. The aim of this study was to investigate at the molecular level how Zn²⁺ regulates MG23-channel function and how this shapes intracellular Ca²⁺-dynamics in the failing heart.

Using single-channel electrophysiological techniques, this study demonstrated that RyR2 is not the only SR Ca²⁺-channel directly modulated by Zn²⁺. Pathophysiological (≥1 nM) levels of cytosolic Zn²⁺ potentiated MG23-channel activity, with the current amplitude of MG23-channel openings found to be consistent to that previously reported as RyR2 sub-conductance gating. In bilayer experiments using SR vesicles isolated from MG23 knock-out mice, RyR2 sub-conductance gating was never observed. These data reveal that following elevation of Zn²⁺ in heart failure, RyR2 sub-conductance gating does not occur but rather MG23-channel gating becomes exacerbated likely resulting in cardiac dysfunction.

Live-cell Ca²⁺-imaging in isolated mouse cardiomyocytes demonstrated that MG23 function as a Ca²⁺-leak channel is an important determinant of SR Ca²⁺ content. In cardiomyocytes exposed to ischaemia, MG23-mediated Ca²⁺-leak provided cardioprotection against SR Ca²⁺-store overload-induced spontaneous Ca²⁺-release. Increased MG23 protein expression observed following prolonged exposure to hypoxia may contribute to altered Ca²⁺-dynamics associated with cardiac remodelling in chronic heart failure. This study also provided the first demonstration of the Zn²⁺-permeability of MG23, suggesting that MG23 can mediate SR Zn²⁺-flux following redistribution of ionic balance across the SR membrane during EC-coupling or following disruption of homeostatic mechanisms.

Taken together these findings identify a key role for MG23 as a SR Ca²⁺-leak channel in both normal and disrupted cardiac function, highlighting MG23 as a potential therapeutic target in the treatment of the failing heart.