Atrial Transcriptional Profiles of Molecular Targets Mediating Electrophysiological Function in Aging and Pgc-1 β 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-1β^-/- 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 Ca²⁺ 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-1β^-/- 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 Ca²⁺-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 Ca²⁺ 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-1β^-/- atria to an altered Ca²⁺-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 Ca²⁺ homeostasis.

Keywords: arrhythmias; coactivator-1 transcriptional coactivator (Pgc-1); ion channels; mitochondria; peroxisome proliferator activated receptor-γ (PPARγ); quantitative PCR.

FIGURE 1

Heatmap visualizing differential expression between the sample groups grouped by electrophysiological function at the individual gene level combined with two-way ANOVA results examining for independent and interacting effects of Pgc-1β^-/- genotype and age. All fold changes normalized to the mean of the expression in samples from young WT hearts. The heatmap is coded yellow for no change, red for increased and green for decreased in fold gene transcription.

FIGURE 2

Volcano plot of differentially expressed genes comparing young Pgc-1β^-/- with young WT. Plots of stratified P-values against expression fold changes for results expressing young Pgc-1β^-/- normalized to young-WT.

FIGURE 3

Effect of aging in the WT: volcano plot of differentially expressed genes comparing aged WT with young WT. Plots of stratified P-values against expression fold changes for results expressing aged WT normalized to young WT.

FIGURE 4

Effect of aging in the Pgc-1β^-/-: volcano plot of differentially expressed genes comparing aged Pgc-1β^-/- with young Pgc-1β^-/-. Plots of stratified P-values against expression fold changes for results expressing aged Pgc-1β^-/- normalized to young Pgc-1β^-/-.

FIGURE 5

Aged Pgc-1β^-/- compared with aged WT: volcano plot of differentially expressed genes comparing young Pgc-1β^-/- with young WT. Plots of stratified P-values against expression fold changes for results expressing Pgc-1β^-/- normalized to young WT.