Investigation of the mechanisms underlying the effects of hyperglycaemia on cardiac structural and electrical remodelling

Background: Diabetes mellitus with uncontrolled hyperglycaemia is a major cause of cardiovascular complications and mortality. The developing foetal heart in-utero is particularly susceptible to hyperglycaemia through pathological remodelling, which results in life-long structural abnormalities such as cardiomyopathy and electrical defects like arrhythmias. However, the underlying mechanisms and potential therapeutic drug targets remain unclear. In this study, a cardiac developmental cellular model was used to study hyperglycaemia-induced remodelling. Methods: Mouse embryonic stem cells (mESCs) were differentiated into pulsatile, cardiac-like cells via embryoid body (EB) formation and cultured under baseline- or high glucose conditions. A Ca2+ -sensitive fluorescent dye Fluo-4 was used to measure calcium transients and a voltage-sensitive dye di-4-ANEPPS was used to record action potentials. Cellular biomarkers were detected using immunocytochemistry, confocal microscopy, and Western blotting as well as terminal deoxynucleotidyl transferase dUTP nick-end labelling (TUNEL) and 5-ethynyl-2-deoxyuridine (EdU) assay. Results: Undifferentiated mESCs were positive for pluripotent transcription factors Nanog and Oct3/4, whereas the cardiac differentiated mESCs were positive for cardiac proteins troponin T, α-actinin 2, connexin 43, sarco-endoplasmic reticulum calcium ATPase 2 (SERCA 2) and α- and β-myosin heavy chain. Hyperglycaemia decreased the number of beating EBs, their beating rate, and their amplitude of contraction. It also decreased the calcium transient amplitude and the contractile response to ryanodine receptor stimulation by caffeine but did not alter the SERCA 2 expression. The amplitude and duration of action potentials in beating EBs were not altered by hyperglycaemia. However, structural changes included a decrease in EB size and expression of myofilament proteins, α-actinin and α- and β-myosin heavy chain and a disruption of the striated organization of the myofilaments. Hyperglycaemia increased the proportion of TUNEL-positive cells and the expression of the pro-apoptotic marker cytochrome c and decreased the anti-apoptotic protein Bcell lymphoma 2 but did not alter the mitochondrial staining with Mitotracker. It also increased the oxidative stress marker nitrotyrosine but did not alter the extent of EdU nuclear staining nor the expression of the receptor of advanced glycation end-product. The antioxidant n-acetyl cysteine decreased the fraction of hyperglycaemia-induced TUNEL-positive cells and improved the α-actinin striated pattern. Conclusion: Hyperglycaemia suppressed the cardiac differentiation and contractile activity of mESCs as well as disrupted the cardiac myofilament organisation and expression. These effects of hyperglycaemia were likely mediated by mitochondrial-dependent apoptosis triggered by oxidative stress as well as by the abnormalities in calcium signalling. These results have potential clinical implications in foetal diabetic cardiac disease and add novel insights into the mechanistic factors that represent new therapeutic drug targets in the developing foetal heart