Loss of Caveolin-3 Causes Heart Rhythm Abnormalities by Affecting Ca2+-Voltage Coupling in the Mouse Sinoatrial Node

Abstract

Caveolin-3 (Cav3) is a myocyte-specific scaffolding protein within caveolar membranes that compartmentalize a number of ion channels and transporters involved in sinoatrial node (SAN) pacemaking. These channels and transporters are coupled with intracellular Ca2+ cycling (Ca2+-voltage coupling) through rhythmic local Ca2+ releases (LCRs) from ryanodine receptors to initiate an action potential. We hypothesize that disruption of caveolae-associated macromolecular signaling complexes alters SAN automaticity leading to SAN dysfunction. In vivo ECG telemetry, in vitro high-resolution optical mapping, and confocal imaging of intracellular Ca2+ cycling were performed in wild type (WT, n=8) and conditional tamoxifen-induced α-MHC-controlled Cav3 knockout (Cav3-/-, n=8) mice and isolated SAN myocytes. Both in vivo and in vitro,Cav3-/- mice exhibited SAN pacemaking abnormalities characterized by alternating periods of tachycardia-bradycardia rhythm (cycle length varied from 123.6±2.6 ms to 464.4±130 ms), significant beat-to-beat cycle length variability (3.8±1.1 ms in WT vs. 19.5±3.6 ms in Cav3-/-, p<0.01) and shift of the leading pacemaker from the SAN to ectopic foci. In SAN myocytes, Cav3 deficiency led to irregular spontaneous rate with significant cycle length variability (30.9±5.2 ms in WT vs. 644.7±256.4 ms in Cav3-/-, p<0.01). Because LCRs are critically important for the Na+/Ca2+ exchanger mediated component of slow diastolic depolarization of transmembrane potential, we measured the period from LCRs to Ca2+ transient initiation in order to evaluate Ca2+-voltage coupling. Cav3-/- SAN myocytes showed significant prolongation (73.9±6.7 ms in WT vs. 130.6±15.0 ms in Cav3-/-, p<0.01) of this period with large beat-to-beat variability (15.5±2.6 ms in WT vs. 71.6±17.4 ms in Cav3-/-, p<0.01). Our findings demonstrate that Cav3 plays a crucial role in supporting functional integrity of the SAN by synchronizing Ca2+-voltage coupling and regulating rhythm of LCRs.