Structural and functional characterization of the cardiac mitochondrial-associated reticulum membranes in the obesogenic and diabetic ob/ob mouse model

Abstract

Diabetic cardiomyopathy (DCM) strongly leads to metabolic heart failure with preserved ejection fraction (HFpEF). The involvement of mitochondria-associated reticular membranes (MAMs) in T2D-related metabolic disorders starts to be demonstrated. We recently discovered a reticulum-mitochondria Ca2 + uncoupling in a diet-induced mouse model of DCM with metabolic HFpEF. However, whether cardiac MAMs are affected by T2D and obesity specifically or by DCM with metabolic HFpEF remains unknown. We aimed to study the cardiac phenotype of another obesogenic T2D mouse model (leptin-deficient) together with the proteomic composition and function of their cardiac MAMs. Cardiac contractile function and structure were evaluated by echocardiography, electron microscopy and histology in 12 weeks old male WT and ob/ob mice. MAMs protein composition was assessed by mass spectrometry and by Uniprot and Panther softwares. Cell death in cardiomyocytes (CM) after hypoxia/reoxygenation stress was assessed using flow cytometry. Reticulum-mitochondria Ca2+ fluxes were assessed on CM using a FRET-based mitochondrial Ca2+ sensor expressed by adenoviral infection. Echocardiography analysis revealed strain rate dysfunction and a concentric hypertrophy remodeling while no change was observed in fractional shortening or diastolic function in ob/ob mice. Histological assessment showed an increased lipid deposition but similar fibrosis in the ob/ob heart compared to WT ones. Cardiac MAM length and width were similar between both groups. However, a trend towards an increased MAM protein content was measured in the ob/ob heart. Further MAM proteome analyses showed mainly increased processes in ob/ob hearts, notably the cellular response to stress, lipid metabolism, ion transport and membrane organization. Indeed, functionally, hypoxic stress induces a cell death increase in the ob/ob CM, while MAM-driven Ca2+ fluxes were unchanged. Mitochondrial respiration, CM shortening, ATP and ROS content were also similar between both groups. The T2D ob/ob mouse model does not recapitulate the main hallmarks of metabolic HFpEF and does not display any cardiac MAM-driven Ca2+ uncoupling, contrary to the high-fat-high-sucrose diet-induced obesogenic mouse model. Therefore, the alteration of the cardiac MAM-driven Ca2+ coupling seems specific to the development of DCM with metabolic HFpEF.