Gene and Protein Expression Profile of Selected Molecular Targets Mediating Electrophysiological Function in Pgc-1α Deficient Murine Atria

Abstract

Increases in the prevalence of obesity, insulin resistance, and metabolic syndrome has led to the increase of atrial fibrillation (AF) cases in the developed world. These AF risk factors are associated with mitochondrial dysfunction, previously modelled using peroxisome proliferator activated receptor-γ (PPARγ) coactivator-1 (Pgc-1)-deficient murine cardiac models. We explored gene and protein expression profiles of selected molecular targets related to electrophysiological function in murine Pgc-1α−/− atria. qPCR analysis surveyed genes related to Na+-K+-ATPase, K+ conductance, hyperpolarisation-activated cyclic nucleotide-gated (Hcn), Na+ channels, Ca2+ channels, and indicators for adrenergic and cholinergic receptor modulation. Western blot analysis for molecular targets specific to conduction velocity (Nav1.5 channel and gap junctions) was performed. Transcription profiles revealed downregulation of molecules related to Na+-K+-ATPase transport, Hcn-dependent pacemaker function, Na+ channel-dependent action potential activation and propagation, Ca2+ current generation, calsequestrin-2 dependent Ca2+ homeostasis, and adrenergic α1D dependent protection from hypertrophic change. Nav1.5 channel protein expression but not gap junction expression was reduced in Pgc-1α−/− atria compared to WT. Nav1.5 reduction reflects corresponding reduction in its gene expression profile. These changes, as well as the underlying Pgc-1α−/− alteration, suggest potential pharmacological targets directed towards either upstream PGC-1 signalling mechanisms or downstream ion channel changes.

Molecular basis of ventricular arrhythmogenicity in a Pgc-1α deficient murine model

“…Yet Section 2.1.9 reported qPCR results which revealed no change in transcription of molecular markers of fibrosis. However, the absence of increased transcription in cardiac fibrosis related genes, particularly TGF-β1, parallels previous reported histological incidences of increased tissue fibrosis without increased transcription of corresponding fibrosis-related genes [ [27] , [28] , [29] , 43 ]. Thus, murine Pgc-1β deficient atria and ventricles similarly showed unaltered transcription of cardiac fibrosis genes including Tgfb1 and Col3a1 despite histological evidence of fibrotic change [ 23 , 26 , 27 , 29 ].…”

“…Of biomolecules related to the background ionic gradients supporting excitable activity, Pgc-1α −/− ventricles demonstrated reduced regulatory Atp1b1 , but normal catalytic Atp1a1 or Atp1a2 , subunit transcription required for Na + -K + ATPase activity, compared to WT. This complements previous reports that Pgc-1α −/− atria showed reduced transcription of all three catalytic and regulatory subunits [ 28 ].…”

“…The latter included molecular qPCR investigations of gene transcription, western blotting for protein expression, and histological analysis for fibrotic change. The present study complete this sequence of three previous reports bearing on molecular qPCR investigations of gene transcription in Pgc-1β atria and ventricles and Pgc-1α atria [ [27] , [28] , [29] ], investigating genes related to electrophysiological function in Pgc-1α ventricles.…”

Isolated downregulation of HCN2 in ventricles of rats with streptozotocin-induced diabetic cardiomyopathy

“…HCN downregulation in sinus node is reported in aged heart [ 36 ] and heart failure and relates to sinus node dysfunction [ 37 ] and even atrial tachyarrhythmia [ 38 ]. Similarly, a decrease of HCN was found in the atria of metabolic murine model of mitochondrial dysfunction [ 19 ] and in sinoatrial node of Goto-Kakizaki type 2 diabetic rats [ 39 ]. Importantly, HCN downregulation was reported also in pacemaker cells [ 20 ], in sinoatrial node [ 20 ] and whole cardiac conduction system [ 40 ] of rats with type 1 diabetes mellitus induced by STZ manifesting in lower intrinsic heart rate, a lengthened sinoatrial conduction time and rate-corrected maximal sinoatrial node recovery time in vivo as well as a longer cycle length in vitro [ 20 ].…”

“…Furthermore, I f augmentation induces a diastolic influx of Na + cations leading to increased intracellular Ca 2+ due to the Na + /Ca 2+ exchanger shift towards 'reverse mode' resulting in increased arrhythmogenicity [ 16 ]. The overexpression of myocardial HCN occurs in various conditions, including cardiac hypertrophy [ 17 ], acute myocardial infarction [ 18 ] and heart failure [ 15 ] while the reduction of myocardial HCN is rather related to atria and to impaired sinus rhythm [ 19 , 20 ].…”