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Introduction Type 2 diabetic cardiomyopathy has been linked to Ca2+ signaling alterations, notably a decreased mitochondrial Ca2+ uptake. Recent discovery of Ca2+ microdomains between cardiac mitochondria and reticulum launched a new investigation avenue for cardiometabolic diseases. Objective Hereby, we aimed to investigate if the mitochondrial Ca2+ mishandling is due to a dysregulation of the reticulum-mitochondria interactions or of the mitochondrial Ca2+ uniporter in the diabetic mouse heart. Methods and results Mice fed for 16 weeks with a high fat high sucrose diet (HFHSD) displayed a cardiac insulin resistance combined to cardiac fibrosis, hypertrophy and contractile dysfunction (decreased strain rate) versus standard diet-fed mice (SD). Fractionation of HFHSD mice hearts revealed a decreased protein content of their mitochondria associated-reticulum membranes, together with a lower expression of tethering proteins quantified by mass spectrometry. On functional terms, reduced IP3R-VDAC proximity and co-immunoprecipitation were associated with a reduced IP3-stimulated Ca2+ transfer to mitochondria in isolated HFHSD cardiomyocytes, yet no changes in mitochondrial calcium uniporter protein expression and function were detected. Moreover, HFHSD cardiomyocytes displayed no significant changes in the reticular Ca2+ release level and the amplitude of cytosolic Ca2+ transients. Eight weeks of diet reversal restored the cardiac insulin signaling, MAM content, Ca2+ transfer and cardiac function in HFHSD mice. Conclusion Therefore, our data indicate that disruption of the cardiac reticulum-mitochondria interactions is an early and reversible event impairing mitochondrial Ca2+ handling and contributing to the cardiac contractile dysfunction in the diabetic mouse heart, which could be counteracted by diet reversal.

Cardiovascular diseases associated with metabolic syndrome remain frequent and still disabling for patients. The uncovering of Ca2+ microdomains between mitochondria and reticulum (SR) in the heart has launched a new area of investigation for cardiometabolic diseases. Here, we sought to determine the structural and functional role of the SR-mitochondria interactions in the mice heart during diabetes. We hypothesized that an alteration of the SR-mitochondria interactions would impair the Ca2+ signaling between the two organelles, thus leading to contractile dysfunction. Mice were either fed with a standard diet (SD: 16.9% proteins, 4.3% lipids) or a high-fat-high-sucrose diet (HFHSD:20% proteins, 36% lipids) for 16 weeks. In the HFHSD group, half of the mice were subjected to metformin gavage, an antidiabetic agent, for the last 4 weeks. Cardiac mitochondria associated membranes (MAM) isolation showed an increased protein amount of MAM/pure mitochondria in the HFHSD group vs. SD mice, which was prevented by metformin treatment (SD: 2.0 ± 0.4, HFHSD: 5.5 ± 0.9, HFHSD + MET: 2.8 ± 0.4, n = 5). After 70 min hypoxia-2 hrs reoxygenation, HFHSD isolated cardiomyocytes displayed higher propidium-iodide positive cells compared to SD and HFHSD + MET groups (60 ± 1 vs. SD: 46 ± 3 and HFHSD + MET: 50 ± 2%, n = 5). Interestingly, histamine-stimulated Ca2+ transfer to mitochondria was reduced by 50% in HFHSD ± MET cardiomyocytes. Concomitantly, IP3R-VDAC interactions assessed by proximity ligation assay were 30–40% reduced in HFHSD ± MET cardiomyocytes. Hence, our data indicate a role for the SR-mitochondria interactions during diabetes in the heart in hypoxia-reoxygenation-induced cell death and in the Ca2+ signaling dysfunction. Indeed, reduced formation of the IP3R-VDAC Ca2+ channeling complex and of the IP3R-stimulated Ca2+ transfer to mitochondria in the HFHSD ± MET cardiomyocytes suggest an inappropriate thickness of the extended MAM (too close or too far) for the IP3R complex formation.

Introduction Diabetic cardiomyopathy has been linked to Ca2 + signaling alterations, notably a decreased mitochondrial Ca2 + uptake. Uncovering the changes occurring at Ca2 + microdomains between reticulum and mitochondria in the heart has launched a new area of investigation for cardiometabolic diseases. Objective We here aimed to study if the impairment of mitochondrial Ca2 + handling could be due to a dysregulation of the reticulum-mitochondria interactions or of the mitochondrial Ca2 + uniporter in the diabetic mice heart. Methods Mice were either fed with a standard diet (SD: 16.9% proteins, 4.3% lipids) or a high-fat high sucrose diet (HFHSD: 20% proteins, 36% lipids) for 16 weeks. Cardiac mitochondria associated membranes (MAM) composition was analyzed by proteomics and immunoblotting. Proximity ligation assay, calcium imaging and hypoxia/reoxygenation were performed on isolated cardiomyocytes. Results Our HFHSD mice displayed a cardiac insulin resistance and hypertrophy. Decreased MAM/pure mitochondria content in the HFHSD vs SD heart was observed, with an elevated level of proteins involved in lipid metabolism and a decrease in tethering proteins. Decreased IP3R-VDAC proximity upon HFHSD was concomitant to a reduced IP3R-stimulated Ca2 + transfer to mitochondria, with no changes in mitochondrial calcium uniporter protein expression and function. Additionally, decreased amplitude of cytosolic Ca2 + transients was seen in the HFHSD cardiomyocytes, with no significant changes in the reticular Ca2 + release level. Besides, increased cell death susceptibility was measured after both in vitro hypoxia/reoxygenation and in vivo ischemia/reperfusion under HFHSD. Conclusion Our data, therefore, indicate that decreased reticulum-mitochondria interactions trigger an impaired mitochondrial Ca2 + handling in the diabetic mice heart, with no alteration of the mitochondrial Ca2 + uniporter, while the enhanced susceptibility to cell death could be referred to lipid toxicity.