Inositol Trisphosphate Receptors and Nuclear Calcium in Atrial Fibrillation.

Rationale: The mechanisms underlying atrial fibrillation (AF), the most common clinical arrhythmia, are poorly understood. Nucleoplasmic Ca regulates gene expression, but the nature and significance of nuclear Ca-changes in AF are largely unknown.

Objective: To elucidate mechanisms by which AF alters atrial-cardiomyocyte nuclear Ca ([Ca]) and CaMKII (Ca/calmodulin-dependent protein kinase-II)-related signaling.

Methods And Results: Atrial cardiomyocytes were isolated from control and AF dogs (kept in AF by atrial tachypacing [600 bpm × 1 week]). [Ca] and cytosolic [Ca] ([Ca]) were recorded via confocal microscopy. Diastolic [Ca] was greater than [Ca] under control conditions, while resting [Ca] was similar to [Ca]; both diastolic and resting [Ca] increased with AF. IPR (Inositol-trisphosphate receptor) stimulation produced larger [Ca] increases in AF versus control cardiomyocytes, and IPR-blockade suppressed the AF-related [Ca] differences. AF upregulated nuclear protein expression of IPR1 (IPR-type 1) and of phosphorylated CaMKII (immunohistochemistry and immunoblot) while decreasing the nuclear/cytosolic expression ratio for HDAC4 (histone deacetylase type-4). Isolated atrial cardiomyocytes tachypaced at 3 Hz for 24 hours mimicked AF-type [Ca] changes and L-type calcium current decreases versus 1-Hz-paced cardiomyocytes; these changes were prevented by IPR knockdown with short-interfering RNA directed against IPR1. Nuclear/cytosolic HDAC4 expression ratio was decreased by 3-Hz pacing, while nuclear CaMKII phosphorylation was increased. Either CaMKII-inhibition (by autocamtide-2-related peptide) or IPR-knockdown prevented the CaMKII-hyperphosphorylation and nuclear-to-cytosolic HDAC4 shift caused by 3-Hz pacing. In human atrial cardiomyocytes from AF patients, nuclear IPR1-expression was significantly increased, with decreased nuclear/nonnuclear HDAC4 ratio. MicroRNA-26a was predicted to target (confirmed by luciferase assay) and was downregulated in AF atrial cardiomyocytes; microRNA-26a silencing reproduced AF-induced IPR1 upregulation and nuclear diastolic Ca-loading.

Conclusions: AF increases atrial-cardiomyocyte nucleoplasmic [Ca] by IPR1-upregulation involving miR-26a, leading to enhanced IPR1-CaMKII-HDAC4 signaling and L-type calcium current downregulation. Graphic Abstract: A graphic abstract is available for this article.