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 (CM) nuclear Ca2+ ([Ca2+]Nuc) and Ca2+/calmodulin-dependent protein kinase-II (CaMKII)-related signaling.Methods and Results: Atrial CMs 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. Inositol-trisphosphate-receptor (IP3R) stimulation produced larger [Ca2+]Nuc increases in AF versus control CMs, and IP3R-blockade suppressed the AF-related [Ca2+]Nuc-differences. AF upregulated nuclear protein-expression of IP3R-type 1 (IP3R1) and of phosphorylated CaMKII (immunohistochemistry and immunoblot), while decreasing the nuclear/cytosolic expression-ratio for histone deacetylase type-4 (HDAC4). Isolated atrial CMs tachypaced at 3 Hz for 24 hours mimicked AF-type [Ca2+]Nuc changes and L-type calcium current (ICaL) decreases versus 1-Hz-paced CMs; these changes were prevented by IP3R knockdown with short-interfering RNA directed against IP3R1. Nuclear/cytosolic HDAC4 expression-ratio was decreased by 3-Hz pacing, while nuclear CaMKII and HDAC4 phosphorylation were 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 CMs from AF patients, nuclear IP3R1-expression was significantly increased, with decreased nuclear/non-nuclear HDAC4 ratio. MicroRNA-26a was predicted to target ITPR1 (confirmed by Luciferase assay) and was downregulated in AF atrial CMs; microRNA-26a silencing reproduced AF-induced IP3R1 upregulation and nuclear diastolic Ca2+-loading. Conclusions: AF increases atrial CM nucleoplasmic Ca2+-handling by IP3R1-upregulation involving miR-26a, leading to enhanced IP3R1-CaMKII-HDAC4 signaling and ICaL-downregulation.