Mitochondrial cardiomyopathies feature increased uptake and diminished efflux of mitochondrial calcium

Abstract

Calcium (Ca²⁺) influx into the mitochondrial matrix stimulates ATP synthesis. Here, we investigate whether mitochondrial Ca²⁺ transport pathways are altered in the setting of deficient mitochondrial energy synthesis, as increased matrix Ca²⁺ may provide a stimulatory boost. We focused on mitochondrial cardiomyopathies, which feature such dysfunction of oxidative phosphorylation. We study a mouse model where the main transcription factor for mitochondrial DNA (transcription factor A, mitochondrial, Tfam) has been disrupted selectively in cardiomyocytes. By the second postnatal week (10-15day old mice), these mice have developed a dilated cardiomyopathy associated with impaired oxidative phosphorylation. We find evidence of increased mitochondrial Ca²⁺ during this period using imaging, electrophysiology, and biochemistry. The mitochondrial Ca²⁺ uniporter, the main portal for Ca²⁺ entry, displays enhanced activity, whereas the mitochondrial sodium-calcium (Na⁺-Ca²⁺) exchanger, the main portal for Ca²⁺ efflux, is inhibited. These changes in activity reflect changes in protein expression of the corresponding transporter subunits. While decreased transcription of Nclx, the gene encoding the Na⁺-Ca²⁺ exchanger, explains diminished Na⁺-Ca²⁺ exchange, the mechanism for enhanced uniporter expression appears to be post-transcriptional. Notably, such changes allow cardiac mitochondria from Tfam knockout animals to be far more sensitive to Ca²⁺-induced increases in respiration. In the absence of Ca²⁺, oxygen consumption declines to less than half of control values in these animals, but rebounds to control levels when incubated with Ca²⁺. Thus, we demonstrate a phenotype of enhanced mitochondrial Ca²⁺ in a mitochondrial cardiomyopathy model, and show that such Ca²⁺ accumulation is capable of rescuing deficits in energy synthesis capacity in vitro.

Keywords: Cardiac metabolism; Mitochondrial calcium uniporter; Mitochondrial sodium-calcium exchanger; OXPHOS deficient cardiomyopathy; Respiratory-chain deficient cardiomyopathy; Whole-mitoplast electrophysiology.