Caveolar Compartmentalization of Pacemaker Signaling Ensures Stable Sinoatrial Rhythmicity Which Is Disrupted in Heart Failure.

BACKGROUND

Caveolae are nanoscale, plasma membrane invaginations that compartmentalize ion channels and transporters, including those involved in sinoatrial node (SAN) activity. However, role of caveolae in cardiac pacemaking remains unknown.

OBJECTIVES

This study sought to determine the role of caveolae in SAN pacemaking and sinus node dysfunction (SND).

METHODS

In vivo electrocardiography, ex vivo optical mapping, in vitro Ca2+ imaging, immunofluorescent and electron microscopy were performed in wild-type, cardiac-specific Cav3 knockout and 8-week post-myocardial infarction heart failure mice. Mouse and human donor SAN tissues were used for biochemical protein copurification studies. A novel 3-dimensional single SAN cell mathematical model was used to determine the impact of protein localization on SAN pacemaking.

RESULTS

In both mouse and human SANs, caveolae compartmentalized HCN4, Cav1.2, Cav1.3, Cav3.1, and Na+-Ca2+ exchanger (NCX1) proteins within discrete pacemaker signalosomes via direct association with Cav3. This compartmentalization positioned electrogenic sarcolemmal proteins near the subsarcolemmal sarcoplasmic reticulum membrane and ensured fast and robust activation of NCX1 by subsarcolemmal local sarcoplasmic reticulum Ca2+ release events, which diffuse across ∼15-nm subsarcolemmal cleft. Disruption of caveolae led to the development of SND via suppression of pacemaker automaticity through a 50% decrease of the L-type Ca2+ current, a negative shift of the HCN current (If) activation curve, and a 40% reduction of NCX1 function, along with ∼2.3-times widening of the sarcolemma-sarcoplasmic reticulum distance. These changes significantly decreased the SAN depolarizing force, both during diastolic depolarization and upstroke phase, leading to bradycardia, sinus pauses, recurrent development of SAN quiescence, and significant increase in heart rate lability. Computational modeling, supported by biochemical studies, identified NCX1 redistribution to extracaveolar membrane as the primary mechanism of SAN pauses and quiescence due to the impaired ability of NCX1 to be effectively activated by local sarcoplasmic reticulum Ca2+ release events and trigger action potentials. Heart failure remodeling mirrored caveolae disruption leading to NCX1-local sarcoplasmic reticulum Ca2+ release event uncoupling and SND.

CONCLUSIONS

SAN pacemaking is driven by complex protein interactions within a nanoscale caveolar pacemaker signalosome. Disruption of caveolae leads to SND, demonstrating a new dimension of SAN remodeling and revealing a novel therapeutic target.