Dysregulation of RyR Calcium Channel and Impaired Mitochondrial Calcium Uptake Cause the Onset of Mitochondrial Retrograde Signaling

Abstract

Mitochondrial dysfunction, such as reduction in mitochondrial DNA (mtDNA) content induce mitochondria-to-nucleus retrograde signaling (MtRs) which involves elevated cytoplasmic Ca2+ ([Ca2+]c) followed by activation of calcineurin (Cn) as the initiating events of MtRs. However, the mechanism of elevated [Ca2+]c in response to mitochondrial dysfunction remains unclear. In this study, we show that multiple modes of mitochondrial stress generated by partial mtDNA depletion or cytochrome c oxidase disruption cause ryanodine receptor channel (RyR) dysregulation which instigates the release of Ca2+ upon the addition of caffeine in C2C12 myoblast and HCT116 carcinoma cells. We also observed a reciprocal downregulation of IP3R channel activity and reduced mitochondrial uptake of Ca2+. Ryanodine, a RyR antagonist, abrogated the mitochondrial stress-mediated increase in [Ca2+]c and the entire downstream signaling cascades of MtRs. Interestingly, Ryanodine also inhibited mitochondrial stress-induced invasive behavior in mtDNA depleted C2C12 cells and HCT116 carcinoma cells. Additionally, co-immunoprecipitation shows that RyR protein interaction with FKBP12 proteins is affected suggesting altered function of the Ca2+ channel. These results document how the endoplasmic reticulum-associated RyR channels in combination with inhibition of mitochondrial uniporter system modulate cellular Ca2+ homeostasis and signaling under mitochondrial stress conditions.