The Role of Inflammation in Age-Related Sinus Node Dysfunction

Abstract

The sinus node (SAN) is the natural pacemaker of the heart, and it generates the cardiac action potential. In mammals the function of the SAN declines with advancing age and can result in SAN dysfunction (SND). The incidence of SND increases exponentially with age and has been attributed to electrophysiological remodelling. The underlying mechanisms are poorly understood but there is evidence that upregulation in immune signalling and expression of pro-inflammatory cytokines, notably TNFα, seen in aged hearts, can trigger electrical remodelling. During my PhD studies, I aimed to define the precise immune cell populations present in the mouse and human SAN, and to simulate local inflammation by inducing cardiac conduction system (CCS)-specific TNFα overexpression with a view to assess the role of inflammation in age-related SND in mice. Experiments were performed in young (2-3 months) and aged (22 months) C57BL/6JRccHsd mice to confirm the presence of an SND phenotype. I observed a significant resting bradycardia, slowed intrinsic rate, increased SAN recovery time, conduction slowing, an inferior shift in the leading pacemaker site, and an increased incidence of complex fractionated electrograms in aged mice – characteristic of the SND phenotype in aged humans. To assess age-related changes to immune cell populations, a spatial proteomics approach using imaging mass cytometry (IMC) was applied for the first time in the SAN. While the novelty of this technique posed challenges for in-depth characterisation of the immune environment, complementary imaging methods helped reveal an age-related increase in macrophages and TNFα in the mouse SAN, as well as increased TNFα in the human SAN. To my knowledge, this represents the first registration of the immune populations in both the young and aged SAN in mice and humans. To directly assess how TNFα impacts SAN function, I generated a transgenic mouse model enabling tamoxifen-inducible, CCS-specific TNFα overexpression by crossing an Hcn4-Cre line with mice harbouring a loxP-flanked EGFP cassette that prevents transcription of CAG promoter-driven Tnf cDNA. SAN-specific TNFα induction followed Cre-mediated recombination in HCN4/TNFαglo mice. Six weeks following Cre-mediated recombination, an increased TNFα protein and mRNA signal was observed in the CCS of HCN4/TNFαglo mice vs. tamoxifen-treated controls with no associated structural cardiac remodelling. This was accompanied by a reduction in SAN conduction velocity, an increased incidence of atrial ectopic beats and sinus pauses, and a trend towards a prolonged SAN recovery time, which could indicate reduced SAN automaticity as a result of enhanced TNFα expression. These data provide the first evidence directly linking an inflammatory microenvironment within the SAN with electrical remodelling, and offer further mechanistic insight into the underlying processes leading to age-related SND.

Date of Award	9 Jul 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Delvac Oceandy (Main Supervisor), Alicia D'Souza (Co Supervisor) & Tara Sutherland (Co Supervisor)