Ca~(2+)-ATPases in non-failing and failing heart: evidence for a novel cardiac sarco/endoplasmic reticulum Ca~(2+)-ATPase 2 isoform (SERCA2c)

摘要

We recently documented the expression of a novel human mRNA variant encoding a yet uncharacterized SERCA [SR (sarcoplasmic reticulum)/ER (endoplasmic reticulum) Ca~(2+)-ATPase] protein, SERCA2c [Gelebart, Martin, Enouf and Papp (2003) Biochem. Biophys. Res. Commun. 303, 676-684]. In the present study, we have analysed the expression and functional characteristics of SERCA2c relative to SERCA2a and SERCA2b isoforms upon their stable heterologous expression in HEK-293 cells (human embryonic kidney 293cells). All SERCA2 proteins induced an increased Ca~(2+) content in the ER of intact transfected cells. In microsomes prepared from transfected cells, SERCA2c showed a lower apparent affinity for cytosolic Ca~(2+) than SERCA2a and a catalytic turnover rate similar to SERCA2b. We further demonstrated the expression of the endogenous SERCA2c protein in protein lysates isolated from heart left ventricles using a newly generated SERCA2c-specific antibody. Relative to the known uniform distribution of SERCA2a and SERCA2b in cardio-myocytes of the left ventricle tissue, SERCA2c was only detected in a confined area of cardiomyocytes, in close proximity to the sarcolemma. This finding led us to explore the expression of the presently known cardiac Ca~(2+)-ATPase isoforms in heart failure. Comparative expression of SERCAs and PMCAs (plasma-membrane Ca~(2+)-ATPases) was performed in four non-failing hearts and five failing hearts displaying mixed cardio-myopathy and idiopathic dilated cardiomyopathies. Relativeto normal subjects, cardiomyopathic patients express more PMCAs than SERCA2 proteins. Interestingly, SERCA2c expression was significantly increased (166 + 26%) in one patient. Taken together, these results demonstrate the expression of the novel SERCA2cisoform in the heart and may point to a still unrecognized role of PMCAs in cardiomyopathies.