Abstract
Aim: 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 (IP3) by activating their membrane-bound receptors. We have previously demonstrated that IP3-mediated sarcoplasmic reticulum (SR) Ca2+ release results in mitochondrial Ca2+ uptake and activation of ATP production. In this study, we tested the hypothesis that intact SR/mitochondria microdomains are required for metabolic IP3-mediated SR/mitochondrial feedback in ventricular myocytes.Methods: As a model for disrupted mitochondrial/SR microdomains, cardio-specific tamoxifen-inducible mitofusin 2 (Mfn2) knock out (KO) mice were used. Mitochondrial Ca2+ 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.Results: Stimulation of ET-1 receptors in healthy control myocytes increases mitochondrial Ca2+ uptake, maintains mitochondrial membrane potential and redox balance leading to the enhanced ATP generation. Mitochondrial Ca2+ 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 Ca2+ uptake in IFM and PNM mitochondria but not in SSM. However, mitochondrial Ca2+ 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 Ca2+ uptake in SSM and IFM mitochondria; however, enhanced mitochondrial Ca2+ 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.Conclusion: 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 IP3-mediated SR Ca2+ 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 (IP3R-mRyR1 vs. ryanodine receptor type 2-mitochondrial calcium uniporter).
Highlights
Heart failure (HF) is one of the most common chronic conditions associated with aging
We found that when the physical link between sarcoplasmic reticulum (SR) and mitochondria by mitofusin 2 (Mfn2) was disrupted, the SR/mitochondrial metabolic feedback mechanism was impaired resulting in the inability of the Inositol 1 (IP3)-mediated SR Ca2+ release to induce ATP production in ventricular myocytes from Mfn2 knockout (Mfn2 KO) mice
When cardiac myocytes from these mice were stimulated with the Inositol 1 (IP3R) agonist ET-1, no mitochondrial Ca2+ uptake could be detected. In agreement with this finding, IP3-mediated SR Ca2+ release in Mfn2-deficient myocytes was not able to increase cellular ATP content. These results clearly show that intact tethering of SR and mitochondria via Mfn2 is crucial for the channeling of intracellular Ca2+ signals from IP3R to Mitochondrial ryanodine receptor type 1 (mRyR1) located in mitochondria
Summary
Heart failure (HF) is one of the most common chronic conditions associated with aging. Despite advances in HF treatment, it has a poor prognosis with a steady increase in mortality and incidence in the aging population (Lloyd-Jones et al, 2002; Nieminen et al, 2006). Current treatments of HF rely almost entirely on altering the neurohormonal milieu, using agents such as angiotensin converting enzyme (ACE) inhibitors, angiotensin receptor blockers, and beta-adrenergic blockers to improve cardiac function and reduce mortality. Depending on the type of cardiomyopathy, both decreased respiratory chain activity due to impaired mitochondrial complexes I and III (Ventura-Clapier et al, 2011; Thai et al, 2018) or increased respiratory chain activity due to increased expression of uncoupling proteins (Sebastiani et al, 2007), could lead to increased proton leak and decline in ATP generation. The heart has the highest energy demand by weight of any organ in the body, and its function fails within minutes if mitochondrial ATP production is interrupted. Addressing mitochondrial dysfunction and decrease in mitochondrial ATP generation may represent an effective
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