Abstract
The sequential events of excitation-contraction coupling (ECC) are dependent on the controlled cycling of calcium ions (Ca2+) through the microdomains of a cardiomyocyte. Any alterations from homeostatic Ca2+ levels can increase the susceptibility for acute and chronic contractile dysfunction of the heart, known as systolic heart failure (HF). Ca2+ mishandling is, in part, stimulated by a depleted Ca2+ store of the sarcoplasmic reticulum (SR), reduced systolic transient amplitudes, and diastolic leak. All of these mechanisms are associated with increased luminal sensitisation of the ryanodine receptor type II (RyR2) and myocardial hypocontractility, which are observed phenotypes in both HF and its precursor, diabetes mellitus (DM). Calsequestrin-2 (CSQ2) is an essential luminal protein that prevents Ca2+ mishandling by dynamically polymerising to bind Ca2+, relative to the fluctuating SR store. The organisation of CSQ2 and its localisation to RyR2, is hypothesised to be central to its function, with alterations expected to underlie the deterioration of cardiac structure and performance in DM and HF.
This project aimed to determine the chamber, cellular, and protein remodelling in diabetic and failing human cardiac tissue. Right atrial appendage (RAA) tissue was collected from male and female patients at Dunedin Hospital undergoing coronary artery bypass surgery (CABG) and analysed with confocal microscopy and western blotting. No significant differences were observed between patient groups for the structural cellular remodelling of cardiomyocyte hypertrophy or fibrosis. Interestingly, cardiomyocyte hypertrophy increased linearly with chamber dilatation, whereas increased myocardial fibrosis was associated with female patients and altered with age. Furthermore, co-localisation of CSQ2 with RyR2 remained unchanged between groups and did not significantly correlate with any patient characteristics, suggesting that altered subcellular distribution of CSQ2 is unlikely to underlie RyR2 Ca2+ mishandling in the failing or diabetic heart RAA. Therefore, we hypothesise that in DM and HF, CSQ2 localises to RyR2, but is unable to buffer Ca2+ due to defective polymerisation. Our preliminary western blot analysis further reveals that an altered ratio of CSQ2 to RyR2 may also contribute to Ca2+ mishandling, adding complexity to the poorly understood role of CSQ2 in HF-related pathologies. These findings provide new insights into the underlying mechanisms shared between DM and HF, highlighting the therapeutic potential of restoring functional CSQ2 to treat cardiac dysfunction.