Abstract
Contraction of the heart relies on the precisely timed contraction of individual cardiomyocytes. This is achieved by the highly regulated and synchronous movement of calcium from the sarcoplasmic reticulum into the cardiomyocyte cytosol. Specialised signalling nanodomains called dyads play a crucial role in orchestrating this calcium movement. Key dyadic proteins include junctophilin-2 (JPH2) which structurally anchors dyads, and the ryanodine receptor II (RyR2) as the principal intracellular calcium release channel. Disruption in the expression and organisation of these proteins is a hallmark of cardiovascular disease, where structural remodelling processes such as fibrosis and cardiomyocyte hypertrophy can further distort dyadic architecture and impair calcium handling. Aberrant calcium homeostasis promotes arrhythmias – conditions characterised by mistimed contraction of the heart. Atrial fibrillation (AF) is the most common form of arrhythmia, with patients being at an increased risk of developing heart failure. A major risk factor for the development of AF is diabetes mellitus (DM), a globally prevalent metabolic disorder that, alongside AF, is associated with aberrant calcium signalling in the heart. While there is a strong clinical association between these two pathologies, whether this is attributed to a shared underlying mechanism, such as dyadic disorganisation, remains poorly understood.
This project aimed to examine myocardial structural changes at the tissue, cellular, and protein levels in DM and AF. Right atrial appendages (RAA) were collected from control, DM, AF, and DM-AF patients undergoing coronary artery bypass graft surgery at Dunedin Hospital. Using immunofluorescence labelling and Airyscan confocal microscopy, we assessed cardiomyocyte hypertrophy and fibrosis, with no significant differences observed between patient groups. Furthermore, protein co-localisation of RyR2 and JPH2 revealed no significant differences across patient groups, and did not correlate with echocardiographic parameters. This suggests that structural changes in the localisation of these two proteins are unlikely to underlie DM and AF pathophysiology. Interestingly, preliminary western blot data suggest that JPH2 expression may be upregulated in patients with DM and DM-AF. We hypothesise that this may be indicative of adaptive remodelling in these patients.
This project offers novel insights into the complex interplay of factors that link DM and AF, and establishes a basis for future investigation into the temporal progression and potential therapeutic interventions aimed at preserving contractile function.