Histidine-Rich Ca Binding Protein and Cardiac Functions
The Ca uptake and release properties of sarcoplasmic reticulum (SR) play a key role in regulating myocardial Ca homeostasis, and aberrant Ca cycling properties may lead to elevated ventricular automaticity and cardiac arrhythmias. The SR histidine-rich Ca binding protein (HRC) regulates SR Ca cycling by binding to both triadin and SERCA2a. Overexpression of HRC in heart resulted in impaired SR Ca uptake and cardiac relaxation. To further elucidate the functional role of HRC in SR Ca cycling, HRC-knockout (KO) mice were generated and characterized.
The results demonstrated that the inhibitory effect of HRC on SR Ca uptake was eliminated in HRC-KO mouse myocytes, which exhibited enhanced contractile and Ca-cycling properties under basal conditions. The enhanced contractility was not associated with any alterations in Na/Ca exchange or L-type Ca-channel activities. However, under stress conditions (1µM isoproterenol at 5 Hz), the KO cells were overloaded with Ca in the SR. As a consequence, we observed elevated frequencies of SR Ca sparks and Ca after-transients in the isolated HRC-KO cardiomyocytes using confocal microscopy, demonstrating an enhanced Ca release from the SR. Correspondingly, this aberrant RyR Ca release in the HRC-deficient cardiomyocytes induced a higher frequency of after-contractions and delayed after-depolarizations, compared to the WT cells, under the same stress conditions. Thus, maintenance of HRC expression level appears necessary for proper SR Ca handling in the heart, and complete ablation of HRC increases the susceptibility of cardiomyocytes to cardiac arrhythmias at the cellular level.
Interestingly, a human S96A HRC mutation was recently identified, which is associated with cardiac arrhythmias in dilated cardiomyopathy patients. In order to determine the functional significance of this mutation, we have generated adenoviruses and transgenic mouse lines expressing the human wild-type HRC and HRCS96A. Characterization of the infected adult rat cardiomyocytes and the transgenic mouse models expressing the mutant protein will help us to understand the mechanisms by which HRC regulates myocardial SR Ca cycling, and overall cardiac function in health and disease.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:calcium cycling cardiac arrhythmias delayed afterdepolarization sarcoplasmic reticulum histidine rich ca binding protein
Date of Publication:07/17/2009