Calcium regulatory proteins and their alteration by transgenic approaches

Abstract

Abnormalities in calcium flux have been linked to abnormal contractile behavior of the heart in patients with congestive heart failure as well as in animal models. Decreased activity or levels of the calcium adenosine triphosphatase of the sarco(endo)plasmic reticulum (SERCA2) particularly have been known to cause a delay in calcium transients. The SERCA2 protein pumps 2 moles of calcium per mole of adenosine triphosphate (ATP) split from the cytoplasm into the sarcoplasmic reticulum, thus lowering the free cytoplasmic calcium concentration. It therefore is of interest to identify mechanisms by which SERCA activity could be increased in the heart. To determine influences of increased expression of the SERCA2 gene on calcium transient and contractile behavior, we constructed transgenic mice and rats expressing a SERCA2 transgene in their heart. In these animals, a 20% increase in SERCA levels occurs due to additional expression of the SERCA transgene. This leads to a corresponding increase in contractile activity as determined by the increase in left ventricular pressure measured as dP/dt(max) and decrease in diastolic ventricular pressure determined as dP/dt(min). Similarly, isolated cardiac myocytes obtained from the heart of transgenic mice showed an accelerated calcium transient and increased speed of shortening and relengthening as determined by edge detection. To determine if SERCA2 transgene expression could have a compensatory effect on the contractile behavior of the heart in transgenic mice expressing SERCA2, these mice were made hypothyroid, and papillary muscle function was determined. Contractile behavior of the papillary muscle of wild-type hypothyroid mice showed a significant increase in muscle relaxation time (RT50). In contrast, SERCA2 transgenic hypothyroid mice showed normal contractile behavior of papillary muscle. A compensatory effect of SERCA transgene expression was therefore demonstrated. In addition, we constructed transgenic rats expressing a SERCA2 transgene in which constriction of the ascending aorta induced cardiac hypertrophy and a delayed contraction of papillary muscle. In preliminary results, we found that SERCA2 transgenic rats submitted to ascending aortic constriction did not show the delayed relaxation of papillary muscle as was found in wild-type rats submitted to aortic constriction. In addition, adenoviral vectors expressing transgenes for calcium-handling proteins can be used to improve cardiac myocyte contraction. Adenoviruses expressing a SERCA transgene or a mutant phospholamban transgene exhibiting dominant negative action were used to infect isolated myocytes treated with a phorbol ester (phorbol 12-myristate 13-acetate), which delays the calcium transients. The calcium transients and contractile behavior of the isolated myocytes indicated that increased SERCA expression or increased expression of mutant phospholamban transgene led to increased SERCA2 activity, resulting in an increased contractile phenotype. Recent findings by other investigators also indicate that decreased SERCA2 activity can be increased under in vivo conditions using adenoviral vector-based SERCA2 expression. A gene therapy type of approach delivering increased amounts of SERCA or phospholamban mutants leading to increased SERCA activity should therefore be considered in the future.