Mitofusin 2 Is Essential for IP-Mediated SR/Mitochondria Metabolic Feedback in Ventricular Myocytes.

Endothelin-1 (ET-1) and angiotensin II (Ang II) are multifunctional peptide hormones that regulate the function of the cardiovascular and renal systems. Both hormones increase the intracellular production of inositol-1,4,5-trisphosphate (IP) by activating their membrane-bound receptors. We have previously demonstrated that IP-mediated sarcoplasmic reticulum (SR) Ca release results in mitochondrial Ca uptake and activation of ATP production. In this study, we tested the hypothesis that intact SR/mitochondria microdomains are required for metabolic IP-mediated SR/mitochondrial feedback in ventricular myocytes. As a model for disrupted mitochondrial/SR microdomains, cardio-specific tamoxifen-inducible mitofusin 2 (Mfn2) knock out (KO) mice were used. Mitochondrial Ca uptake, membrane potential, redox state, and ATP generation were monitored in freshly isolated ventricular myocytes from Mfn2 KO mice and their control wild-type (WT) littermates. Stimulation of ET-1 receptors in healthy control myocytes increases mitochondrial Ca uptake, maintains mitochondrial membrane potential and redox balance leading to the enhanced ATP generation. Mitochondrial Ca uptake upon ET-1 stimulation was significantly higher in interfibrillar (IFM) and perinuclear (PNM) mitochondria compared to subsarcolemmal mitochondria (SSM) in WT myocytes. Mfn2 KO completely abolished mitochondrial Ca uptake in IFM and PNM mitochondria but not in SSM. However, mitochondrial Ca uptake induced by beta-adrenergic receptors activation with isoproterenol (ISO) was highest in SSM, intermediate in IFM, and smallest in PNM regions. Furthermore, Mfn2 KO did not affect ISO-induced mitochondrial Ca uptake in SSM and IFM mitochondria; however, enhanced mitochondrial Ca uptake in PNM. In contrast to ET-1, ISO induced a decrease in ATP levels in WT myocytes. Mfn2 KO abolished ATP generation upon ET-1 stimulation but increased ATP levels upon ISO application with highest levels observed in PNM regions. When the physical link between SR and mitochondria by Mfn2 was disrupted, the SR/mitochondrial metabolic feedback mechanism was impaired resulting in the inability of the IP-mediated SR Ca release to induce ATP production in ventricular myocytes from Mfn2 KO mice. Furthermore, we revealed the difference in Mfn2-mediated SR-mitochondrial communication depending on mitochondrial location and type of communication (IPR-mRyR1 ryanodine receptor type 2-mitochondrial calcium uniporter).