Gene and protein expression profiles of selected molecular targets mediating electrophysiological function in pgc-1-alpha deficient murine ventricles

Background

The risk of cardiac arrhythmias increases significantly in patients with metabolic disorders such as obesity and diabetes mellitus. The mechanisms linking metabolic conditions and electrophysiological changes underlying cardiac arrhythmias remain poorly understood. Central to the energetic abnormalities characterising these conditions is mitochondrial dysfunction. Peroxisome proliferator activated receptor-γ (PPARγ) coactivator-1 (Pgc-1) regulate mitochondrial biogenesis and function. Their expression is impaired in metabolic disorders. Murine Pgc-1α−/− hearts replicate disrupted mitochondrial function and model the associated pro-arrhythmic electrophysiological abnormalities.

Purpose

To explore the molecular mechanisms underlying the pro-arrhythmic electrophysiological changes in the Pgc-1α−/− murine model of mitochondrial dysfunction.

Methods

Ventricular tissue samples were obtained from aged (>12 months) wild-type (WT) and homozygous Pgc-1α−/− mice. Quantitative PCR was used to examine transcription of 60 genes underlying cardiac tissue excitability, western blotting was used to examine expression of proteins relating to cardiac conduction velocity, and histological analysis was used to examine cardiac tissue fibrotic change.

Results

qPCR analysis implicated downregulation of genes related to Na+-K+ ATPase activity (Atp1b1), surface Ca2+ entry (Cacna1c), action potential repolarisation (Kcnn1), autonomic function (Adra1d, Adcy4, Pde4d, Prkar2a), and morphological properties (Myh6, Tbx3) in murine Pgc-1α−/− ventricles. Western blotting revealed reduced NaV1.5 but normal Cx43 expression. Histological analysis revealed increased tissue fibrosis in the Pgc-1α−/− ventricles.

Conclusions

These results identified molecular mechanisms underlying previously reported electrophysiological abnormalities such as impaired ventricular activation and reduced conduction velocity in arrhythmic substrate associated with Pgc-1 deficiencies. Results also correlated with earlier findings including functional effects of elevated Ca2+ concentrations. The present findings clarify possible mechanisms by which mitochondrial dysfunction affects electrophysiological function and identify potential pharmacological targets for anti-arrhythmic therapy.

Funding Acknowledgement

Type of funding sources: Public Institution(s). Main funding source(s): Medical Research Council; the Wellcome Trust; British Heart Foundation

Apolipoprotein B messenger RNA editing in rat liver: developmental and hormonal modulation is divergent from apolipoprotein A-IV gene expression despite increased hepatic lipogenesis

Rat hepatic apolipoprotein B (apoB) mRNA editing is regulated developmentally as well as by hormonal and nutritional modulation of hepatic lipogenesis, changes previously associated with coordinate modulation of hepatic apoA-IV gene expression. We have examined the effects of dexamethasone administration on apoB mRNA editing and the expression of other apolipoprotein genes in both neonatal and adult rats. Administration of dexamethasone increased hepatic triglyceride content in neonatal rats and increased hepatic but not intestinal apoA-IV mRNA abundance. However, neither the developmental profile nor the extent of hepatic apoB mRNA editing was changed after hormone administration. Dexamethasone produced a dose-dependent increase in adult hepatic triglyceride content and a coordinate fourfold increase in hepatic but not intestinal apoA-IV mRNA abundance, and hepatic and serum apoA-IV protein concentrations. Immunocytochemical localization revealed apoA-IV to be expressed in hepatocytes around the central vein while dexamethasone treatment produced a dose-dependent appearance of fat-filled hepatocytes throughout the lobule that were immunoreactive for apoA-IV. Despite these changes in hepatic triglyceride accumulation there was no change in the extent of hepatic apoB mRNA editing at any dose of dexamethasone. The data suggest that hormonal and metabolic modulation of hepatic apoB mRNA editing may be independent of factors that modulate apoA-IV gene expression despite alterations in hepatic triglyceride content.