Sarcolipin a novel regulator of the cardiac sarcoplasmic reticulum calcium ATPase
Sarcolipin (SLN) is a small 31- amino acid protein which co-localizes with the calcium transport ATPase (SERCA2a) in the cardiac sarcoplasmic reticulum (SR) membrane. The main goal of this study was determine the functional relevance of SLN in cardiac physiology. Using a highly specific SLN antibody we showed that SLN levels are altered during muscle development and cardiac pathophysiology. Our protein data indicate that SLN is expressed in cardiac and skeletal muscle tissues. More importantly, in the heart SLN is predominantly expressed in the atria. To determine the role of sarcolipin in SR calcium handling and cardiac contractility, we have generated two mouse models 1) cardiac specific overexpression of SLN and 2) SLN knockout. Overexpression or complete loss of SLN did not affect the cardiac muscle development or induce any cardiac pathology. Overexpression of sarcolipin decreases calcium-dependent calcium uptake, whereas ablation of SLN increases the calcium uptake in both atria and ventricle. Our results showed that loss of SLN significantly increases the rates of contraction and relaxation, whereas an increase in SLN level decreases the contractility. SLN null hearts showed blunted response to isoproterenol treatment. On the other hand, the inhibitory effect of SLN was relieved upon high dose of isoproterenol in SLN overexpressing hearts, which may suggest that SLN also mediates beta-adrenergic response. However, the mechanism of SLN action on SERCA2a is not well understood. Invitro phosphorylation studies revealed that a highly conserved Threonine 5 (T5) residue in SLN can be phosphorylated by CaMKII. To determine the role of T5 phosphorylation in SLN function we mutated T5 to alanine (A) (a non-phosphorylatable mutant), or to glutamic acid (E) (which mimics the phosphorylated form of SLN) and studied their effects on myocyte contractility and calcium transients. Our data indicate that T5 plays a physiological role in affecting SLN function. In conclusion, our data suggest that SLN is an important regulator of SERCA2a activity in the heart and the inhibitory effect is regulated by SLN phosphorylation.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:sarcolipin serca pump cardiac calcium homeostasis
Date of Publication:01/01/2008