Ca2+-ATPases in non-failing and failing heart: evidence for a novel cardiac sarco/endoplasmic reticulum Ca2+-ATPase 2 isoform (SERCA2c)

Abstract

We recently documented the expression of a novel human mRNA variant encoding a yet uncharacterized SERCA [SR (sarcoplasmic reticulum)/ER (endoplasmic reticulum) Ca2+-ATPase] protein, SERCA2c [Gélébart, 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 293 cells). All SERCA2 proteins induced an increased Ca2+ content in the ER of intact transfected cells. In microsomes prepared from transfected cells, SERCA2c showed a lower apparent affinity for cytosolic Ca2+ 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 cardiomyocytes 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 Ca2+-ATPase isoforms in heart failure. Comparative expression of SERCAs and PMCAs (plasma-membrane Ca2+-ATPases) was performed in four nonfailing hearts and five failing hearts displaying mixed cardiomyopathy and idiopathic dilated cardiomyopathies. Relative to 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 SERCA2c isoform in the heart and may point to a still unrecognized role of PMCAs in cardiomyopathies.

Calcium Cycling in Synthetic and Contractile Phasic or Tonic Vascular Smooth Muscle Cells

“…SERCA2b differs from SERCA2a by an extension of 46 amino acids that forms an additional trans-membrane domain setting the SERCA2b C terminus in the SR lumen [37]. Functional characterization of SERCA2 isoforms performed in transfected HEK-298 cells clearly indicated that the SERCA2a isoform displays a lower affinity for Ca 2+ (K0.5 = 0.985 µM) but has a higher turnover rate (ATP hydrolysis 70 s -1 ) compared to SERCA2b (K0.5 = 0.508 µM; 35 s -1 ) [38]. Diversity of SERCA isoforms in the same cell suggests that each of them could be responsible for controlling unique cell functions.…”

Determination of apparent calcium affinity for endogenously expressed human sarco(endo)plasmic reticulum calcium-ATPase isoform SERCA3

“…The ATP2A2 gene encoding the SERCA2 protein is alternatively spliced and processed in a tissue-specific manner, giving rise to three isoforms: SERCA2a, which is expressed predominantly in cardiac/slow-twitch skeletal muscle; SERCA2b, which is expressed ubiquitously; and SERCA2c, which has been reported to have a broader expression pattern than that typically attributed to SERCA2a. The level of SERCA2c protein relative to coexpressed SERCA2a or SERCA2b is not known (13,19,35). SERCA2b has a higher apparent affinity for Ca 2ϩ and turns over at a rate twofold slower than SERCA2a.…”

“…SERCA2c has a lower apparent Ca 2ϩ affinity and a slower turnover rate than SERCA2a. The functional differences are due to replacement of the COOH-terminal sequence AILE (amino acids 994 -997) in SERCA2a, by a 49-to 50-amino acid hydrophobic tail in SERCA2b (32,34), and by the 6 amino acids (i.e., VLSSEL) in SERCA2c (13). SERCA1 and SERCA2a are subject to regulation by phospholamban and sarcolipin, which modulate their activity by decreasing their apparent affinity for Ca 2ϩ (36), so that the values for this parameter determined in the recombinant system do not match those observed in the endogenous setting.…”

Deletion of one SERCA2 allele confers protection against bladder wall hypertrophy in a murine model of partial bladder outlet obstruction

“…SERCA1 is expressed in fast-twitch skeletal muscle (26). SERCA2-ATPases exist in three different isoforms: SERCA2a and SERCA2c are the Ca 2ϩ -ATPases of cardiac and slowtwitch skeletal muscle (13,21), whereas SERCA2b is expressed in all tissues including heart and smooth muscle (17). SERCA3 can be found in different nonmuscle tissues as vascular and tracheal epithelium and pancreatic ␤-cells (1,2).…”