Nuclear calcium transients: Hermes Propylaios in the heart

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 × 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 IP₃R1 (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 IP₃R1. 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 IP₃R1-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 IP₃R1 upregulation and nuclear diastolic Ca²⁺-loading.

CONCLUSIONS

AF increases atrial-cardiomyocyte nucleoplasmic [Ca²⁺] by IP₃R1-upregulation involving miR-26a, leading to enhanced IP₃R1-CaMKII-HDAC4 signaling and L-type calcium current downregulation. Graphic Abstract: A graphic abstract is available for this article.