Atrial Transcriptional Profiles of Molecular Targets Mediating Electrophysiological Function in Aging and Pgc-1β Deficient Murine Hearts

Abstract

Background Deficiencies in the transcriptional co-activator, peroxisome proliferative activated receptor, gamma, coactivator-1β are implicated in deficient mitochondrial function. The latter accompanies clinical conditions including aging, physical inactivity, obesity, and diabetes. Recent electrophysiological studies reported that Pgc-1 β -/- mice recapitulate clinical age-dependent atrial pro-arrhythmic phenotypes. They implicated impaired chronotropic responses to adrenergic challenge, compromised action potential (AP) generation and conduction despite normal AP recovery timecourses and background resting potentials, altered intracellular Ca 2+ homeostasis, and fibrotic change in the observed arrhythmogenicity. Objective We explored the extent to which these age-dependent physiological changes correlated with alterations in gene transcription in murine Pgc-1 β -/- atria. Methods and Results RNA isolated from murine atrial tissue samples from young (12–16 weeks) and aged (>52 weeks of age), wild type (WT) and Pgc-1β -/- mice were studied by pre-probed quantitative PCR array cards. We examined genes encoding sixty ion channels and other strategic atrial electrophysiological proteins. Pgc-1β -/- genotype independently reduced gene transcription underlying Na + -K + -ATPase, sarcoplasmic reticular Ca 2+ -ATPase, background K + channel and cholinergic receptor function. Age independently decreased Na + -K + -ATPase and fibrotic markers. Both factors interacted to alter Hcn4 channel activity underlying atrial automaticity. However, neither factor, whether independently or interactively, affected transcription of cardiac Na + , voltage-dependent K + channels, surface or intracellular Ca 2+ channels. Nor were gap junction channels, β-adrenergic receptors or transforming growth factor-β affected. Conclusion These findings limit the possible roles of gene transcriptional changes in previously reported age-dependent pro-arrhythmic electrophysiologial changes observed in Pgc-1 β -/- atria to an altered Ca 2+ -ATPase ( Atp2a2 ) expression. This directly parallels previously reported arrhythmic mechanism associated with p21-activated kinase type 1 deficiency. This could add to contributions from the direct physiological outcomes of mitochondrial dysfunction, whether through reactive oxygen species (ROS) production or altered Ca ...

Molecular basis of arrhythmic substrate in ageing murine peroxisome proliferator-activated receptor γ co-activator deficient hearts modelling mitochondrial dysfunction

“…In the atria , neither increased age nor Pgc-1β deficiency affected Na V 1.5 protein levels whether independently or with interaction. This finding is consistent with prior studies of aged and Pgc-1 β deficient mice that have shown no change in RNA levels of Scn5a, the gene coding for Na V 1.5 [70]. However, both factors of age and genotype independently decreased atrial Cx40 and Cx43 expression levels.…”

“…In both atria and ventricles, the unchanged level of NaV1.5 is unexpected even though consistent with the unchanged RNA level in both atrial and ventricular tissues [70,72]. This indicates the presence of other mechanisms than altered sodium current to explain the pro-arrhythmic features, for example Ca 2+ homoeostasis as discussed.…”

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 ].…”

“…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.…”

“…Amongst the above gene expression patterns examined, previous studies had reported situations in which normal Nav1.5 mRNA expression on qPCR accompanied a reduced protein expression on western blotting, suggesting possible specific post-transcriptional regulatory effects. These had both involved atrial [ 27 ] and ventricular Pgc-1β −/− Nav1.5 gene [ 29 ] as well as protein expression [ 44 ], as well as examples in other systems [ 45 , 46 ]. There were similar differences involving gja mRNA markers and Cx40 and Cx43 protein expression in the same reports.…”