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 Ca2+ regulates gene expression, but the nature and significance of nuclear Ca2+-changes in AF are largely unknown.

Objective:

To elucidate mechanisms by which AF alters atrial-cardiomyocyte nuclear Ca2+ ([Ca2+](Nuc)) and CaMKII (Ca2+/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 x 1 week]). [Ca2+](Nuc) and cytosolic [Ca2+] ([Ca2+](Cyto)) were recorded via confocal microscopy. Diastolic [Ca2+](Nuc) was greater than [Ca2+](Cyto) under control conditions, while resting [Ca2+](Nuc) was similar to [Ca2+](Cyto); both diastolic and resting [Ca2+](Nuc) increased with AF. IP3R (Inositol-trisphosphate receptor) stimulation produced larger [Ca2+](Nuc) increases in AF versus control cardiomyocytes, and IP3R-blockade suppressed the AF-related [Ca2+](Nuc) differences. AF upregulated nuclear protein expression of IP(3)R1 (IP3R-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 [Ca2+](Nuc) changes and L-type calcium current decreases versus 1-Hz-paced cardiomyocytes; these changes were prevented by IP3R knockdown with short-interfering RNA directed against IP(3)R1. 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 IP3R-knockdown prevented the CaMKII-hyperphosphorylation and nuclear-to-cytosolic HDAC4 shift caused by 3-Hz pacing. In human atrial cardiomyocytes from AF patients, nuclear IP(3)R1-expression was significantly increased, with decreased nuclear/nonnuclear HDAC4 ratio. MicroRNA-26a was predicted to target ITPR1 (confirmed by luciferase assay) and was downregulated in AF atrial cardiomyocytes; microRNA-26a silencing reproduced AF-induced IP(3)R1 upregulation and nuclear diastolic Ca2+-loading.

Conclusions:

AF increases atrial-cardiomyocyte nucleoplasmic [Ca2+] by IP(3)R1-upregulation involving miR-26a, leading to enhanced IP(3)R1-CaMKII-HDAC4 signaling and L-type calcium current downregulation.