Abstract
The histidine-rich Ca(2+)-binding protein (HRC) is located in the lumen of the sarcoplasmic reticulum (SR) and exhibits high-capacity Ca(2+)-binding properties. Overexpression of HRC in the heart resulted in impaired SR Ca(2+) uptake and depressed relaxation through its interaction with SERCA2a. However, the functional significance of HRC in overall regulation of calcium cycling and contractility is not currently well defined. To further elucidate the role of HRC in vivo under physiological and pathophysiological conditions, we generated and characterized HRC-knockout (KO) mice. The KO mice were morphologically and histologically normal compared to wild-type (WT) mice. At the cellular level, ablation of HRC resulted in significantly enhanced contractility, Ca(2+) transients, and maximal SR Ca(2+) uptake rates in the heart. However, after-contractions were developed in 50 % of HRC-KO cardiomyocytes, compared to 11 % in WT mice under stress conditions of high-frequency stimulation (5 Hz) and isoproterenol application. A parallel examination of the electrical activity revealed significant increases in the occurrence of Ca(2+) spontaneous SR Ca(2+) release and delayed afterdepolarizations with ISO in HRC-KO, compared to WT cells. The frequency of Ca(2+) sparks was also significantly higher in HRC-KO cells with ISO, consistent with the elevated SR Ca(2+) load in the KO cells. Furthermore, HRC-KO cardiomyocytes showed significantly deteriorated cell contractility and Ca(2+)-cycling caused possibly by depressed SERCA2a expression after transverse-aortic constriction (TAC). Also HRC-null mice exhibited severe cardiac hypertrophy, fibrosis, pulmonary edema and decreased survival after TAC. Our results indicate that ablation of HRC is associated with poorly regulated SR Ca(2+)-cycling, and severe pathology under pressure-overload stress, suggesting an essential role of HRC in maintaining the integrity of cardiac function.