Following myocardial infarction (MI), dead cardiomyocytes are replaced by collagenous scar and result in a heterogenous transition zone between the scar and healthy myocardium. The region of myocardium bordering an infarct (border zone, BZ) undergoes many transcriptional changes, some of which may drive adverse remodelling. Master regulator transcription factor Runx1 is increased in the BZ as early as 1 day post-MI. Previously, we have shown that cardiomyocyte-specific Runx1 deficient (Runx1^Δ/Δ) mice demonstrate remarkably preserved contractility 1 day post-MI and that Runx1 modulates cardiac sarcoplasmic reticulum (SR) calcium uptake and contractile function (McCarroll et al. 2018). Ventricular arrhythmias and sudden cardiac death are major complications over the days and weeks following MI, the BZ region being most vulnerable site for arrhythmogenesis. (Qin et al. 1996). As such, we hypothesised that changes in Runx1 may contribute to arrhythmogenicity in the BZ region post-MI.

It is well understood that both heterogenous tissue electrophysiology and impaired calcium handling can result in an increased propensity for arrhythmia (Smaill et al. 2013). Thus, calcium measurements were performed on BZ and remote zone (RZ) cardiomyocytes loaded with a calcium-sensitive fluorophore (5.0 μmol/L Fura-4F AM, Invitrogen) from Runx1^Δ/Δ and flox control (Runx1^fl/fl) mice.

Overall, we found that at 1-day post-MI, BZ cardiomyocytes from Runx1^fl/fl mice had impaired calcium handling compared to RZ cardiomyocytes, whereas there were no regional differences in Runx1^Δ/Δ mice. Specifically, in BZ cardiomyocytes, calcium transient peak was 64% of the RZ peak, whereas in Runx1^Δ/Δ hearts, calcium transient peak was not different in the BZ compared to the RZ. Calcium transient amplitude in the BZ was 47% of that observed in RZ cardiomyocytes Runx1^fl/fl mice, whereas it was preserved in Runx1^Δ/Δ hearts. This pattern was also consistent for caffeine-induced calcium transient amplitude, representing SR calcium content, was 71% of that observed in RZ cardiomyocytes in Runx1^fl/fl mice but again not different regionally in Runx1^Δ/Δ hearts and SERCA activity, quantified by the rate constant of decay of the caffeine-induced calcium transient, which was 48% of RZ cardiomyocytes in the BZ of Runx1^fl/fl mice but not different between Runx1^Δ/Δ BZ and RZ cardiomyocytes.

Further, in ventricular cardiomyocytes isolated from C57BL/6 mice and incubated with Runx1 small molecule inhibitor Ro5-3335 or with vehicle control DMSO, we utilised a burst pacing protocol followed by a two-minute period of rest and measured spontaneous calcium events as a measure of arrhythmogenicity. We found that cells treated with Ro5-335 had significantly fewer spontaneous events in the rest period compared cells incubated in DMSO (0.5 ± 0.02 vs. 26 ± 8, p = 0.017, n = 9). Overall, these results demonstrate the potential mechanistic contribution of Runx1 to arrhythmogenicity in the BZ post-MI.