Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCA041

Poster Communications

Molecular basis of arrhythmic substrate in ageing, Pgc-1β-deficient murine hearts modelling mitochondrial dysfunction

I. T. Fazmin1,2, C. E. Edling2, K. R. Chadda1,2, S. Ahmad1, H. Valli1, C. L. Huang1,3, K. Jeevaratnam1,2

1. Physiological Laboratory, University of Cambridge, Cambridge, United Kingdom. 2. Faculty of Health and Medical Sciences, University of Surrey, Guildford, United Kingdom. 3. Division of Cardiovascular Biology, Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom.

Background: Ageing and chronic metabolic disorders are associated with mitochondrial dysfunction, in turn associated with increased incidences of arrhythmia. An ageing Pgc-1β-deficient mouse model of mitochondrial dysfunction demonstrates atrial and ventricular arrhythmias and reduced myocardial action potential (AP) conduction velocity, θ (1, 2). However, the molecular changes underlying these electrophysiological findings are currently unknown. We investigated expression levels of voltage-gated NaV1.5 channels and gap junction connexin (Cx) proteins, both known to affect AP conduction. Methods: Generation, breeding and sacrifice (cervical dislocation) of mice and tissue retrieval by cardiectomy are described in (1, 2). The four experimental groups consisted of young and old, and wild-type and Pgc-1β-/- C57/B6 mice (min. n=4/group). Western blotting (WB) was performed on cardiac tissue lysates. Expression levels of NaV1.5 and the Cx isoforms Cx40 and Cx43 were obtained through densitometry. Immunofluorescence (IF) was also used to quantify Cx40 and Cx43 expression in paraffin-embedded tissue sections. Two-way ANOVA with Tukey's post-hoc test was used to investigate for independent or interacting effects of age or Pgc-1β deletion. Results: In the atria, WB demonstrated that increased age and genotype showed no significant independent or interacting effects on NaV1.5 expression levels. Significant independent effects of increased age (Cx40: P<0.0001; Cx43: P=0.0018) and Pgc-1β deletion (Cx40: P=0.0098; Cx43: P=0.0286) decreased atrial Cx40 and Cx43 expression levels, without interactions between factors. IF experiments demonstrated increased age had no effect on atrial Cx40 expression, but Pgc-1β deletion reduced Cx40 expression (P=0.0027). Increased age (P<0.0001) and Pgc-1β deletion (P<0.0001) independently reduced Cx43 expression. In the ventricles, WB experiments demonstrated interacting effects of age and genotype on NaV1.5 expression levels (P=0.0418). Young Pgc-1β-/- ventricles showed higher NaV1.5 expression than young WT ventricles (P=0.0253). Both WB and IF experiments did not show either interacting or independent effects of age or Pgc-1β deletion on ventricular Cx43 expression. Discussion and conclusion: The present results demonstrate functional rather than expressional changes in atrial and ventricular NaV1.5, and ventricular Cx43. In contrast, downregulated Cx40/Cx43 expression can be implicated in reduced θ following increased age or Pgc-1β deletion in the atria. These results suggest differing atrial and ventricular susceptibilities in protein expression patterns nevertheless culminating in proarrhythmic phenotype. Additionally, altered Ca2+ homeostasis, oxidative stress or altered cellular redox potentials may contribute to the altered electrophysiological abnormalities seen in response to mitochondrial dysfunction.

Where applicable, experiments conform with Society ethical requirements