Cardiac microdomains in cyclic nucleotide signalling in mouse atrial cardiomyocytes: role of the caveolar compartments and popeye proteins

Abstract

Transverse axial tubules (TAT) and caveolae are essential structural microdomains in cardiomyocytes that recruit components of various signalling pathways. Of particular interest, β- adrenergic receptors (β-ARs) are localised to these structures and in response to catecholamines elicit compartmentalised cyclic adenosine 3′,5′-monophosphate (cAMP) signals. Both Popeye domain-containing (POPDC) and Caveolin-3 (CAV3) proteins are localised to these membrane compartments. POPDC1 is a CAV3-interacting protein, and the Popeye domain acts as a highaffinity cAMP binding site. The function of these proteins in atrial myocytes (AMs) remains elusive. This study has investigated whether AMs isolated from CAV3 and POPDC null mutants display an altered TAT structure and aberrant cAMP response. The TAT structure was investigated in AMs isolated from the left and right atria of CAV3-/- and Popdc1-/- mice. In both mutants, the TAT structure of AMs originating from the right atria were stronger affected than from the left. cAMP compartmentation was studied with the help of a transgenic FRET sensor. In response to β2-AR-stimulation, phosphodiesterase (PDE) 4 is critical compared to PDE3 for cAMP compartmentation. To understand how changes in TAT structure and cAMP signalling might alter atrial function, sinoatrial pacemaking and atrial conduction were studied in Popdc1-/- and Popdc2-/- isolated atrial tissues. Both mutants demonstrated abnormal pacemaker activity, associated with depressed sinoatrial pacemaking, enhanced ectopy and tachycardia-bradycardia arrhythmias increasing HR lability in response to β-AR stimulation. Additionally, Popdc1-/- displayed slower atrial conduction, increased fibrosis and downregulated connexin-43 expression. Along with the altered fast sodium current and the elevated late sodium current, these facilitated the development of atrial tachyarrhythmias in Popdc1-/- mutants. These data suggest that Cav3-/- and Popdc1-/- mutants are associated with structural changes resulting in aberrant cAMP compartmentation, which may result in an increased risk of developing atrial arrhythmogenesis.