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

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.Open Acces

Cardiomyocyte Membrane Structure and cAMP Compartmentation Produce Anatomical Variation in beta(2)AR-cAMP Responsiveness in Murine Hearts

Cardiomyocytes from the apex but not the base of the heart increase their contractility in response to β2-adrenoceptor (β2AR) stimulation, which may underlie the development of Takotsubo cardiomyopathy. However, both cell types produce comparable cytosolic amounts of the second messenger cAMP. We investigated this discrepancy using nanoscale imaging techniques and found that, structurally, basal cardiomyocytes have more organized membranes (higher T-tubular and caveolar densities). Local membrane microdomain responses measured in isolated basal cardiomyocytes or in whole hearts revealed significantly smaller and more short-lived β2AR/cAMP signals. Inhibition of PDE4, caveolar disruption by removing cholesterol or genetic deletion of Cav3 eliminated differences in local cAMP production and equilibrated the contractile response to β2AR. We conclude that basal cells possess tighter control of cAMP because of a higher degree of signaling microdomain organization. This provides varying levels of nanostructural control for cAMP-mediated functional effects that orchestrate macroscopic, regional physiological differences within the heart