A single protein buried in a tiny organelle inside heart cells could have a major impact on heart function, and might even play a role in heart failure. Despite its importance, researchers know little about this protein called calsequestrin. Steven Cala, PhD, an assistant professor in internal medicine/cardiology, plans to change that.

Through his research, Dr. Cala hopes to learn more about the role of calsequestrin in excitation-contraction (EC) coupling, which is the relay between the electrical signal from moving across the cell surface and the subsequent contraction of the heart. EC coupling, he contends, depends upon proper functioning of calsequestrin.

 “It's a little misleading to say only that calsequestrin is the major calcium-binding protein in the heart. Actually, its location makes it more important than simply its calcium-binding properties,” Dr. Cala said, noting that scientists currently aren’t sure how its ability to bind calcium is significant in its function.

Calsequestrin is located in an area of the heart often described as “where all the action is,” Dr. Cala said. “The ‘action’ is excitation-contraction coupling.” He explained that the cellular electrical signal, or event, couples to a series of steps that eventually trigger the intracellular release of calcium inside the heart. The site of that release is the junctional sarcoplasmic reticulum (SR), part of the organelle that serves as the inner meshwork of the cytoplasm. Calsequestrin is located there, along with about a half dozen critical proteins needed for EC coupling.

 “Because the calsequestrin is tucked into this tiny locale inside the cell, it has basically escaped investigation. Up until now, we haven't figured out exactly how to get at and observe experimentally that mechanism,” he said. Through a $1.2 million, four-year grant from the National Institutes of Health, he and his research team are beginning studies that will shed light on calsequestrin’s role in EC coupling.

 “One of the ways that we are going about it is to use recombinant adenoviruses, which are viruses that are designed to increase the levels of calsequestrin in heart cells,” he said. “We are investigating two types of calsequestrin: normal calsequestrin and also a mutant that cannot be phosphorylated.” He believes the phosphorylation plays a part in maintaining adequate levels of calsequestrin.

 “The reaction that leads to the phosphorylation of calsequestrin is brought on by a particular enzyme called protein kinase CK2. This kinase continues to be enigmatic in biology – nobody can quite get a handle on this very active enzyme, but recent research has shown that it is involved in localizing proteins to their appropriate places inside the cell,” Dr. Cala said. “As it turns out, calsequestrin is one of the best substrates in the body for this enzyme. My hypothesis is that the phosphorylation event guarantees that adequate amounts of calsequestrin end up where they're supposed to be.”

By comparing heart cells to which each calsequestrin virus has been added, Dr. Cala and his research group will be able to compare the two protein forms. “We may find, for example, that the mutant is found in a different part of the cell, perhaps on its way to being secreted or degraded, which in turn would preclude calsequestrin from localizing to junctional SR.”

The results of this study will provide clues to overall heart function, he added. “For some reason in heart failure, the ability of the heart to utilize the calcium in the heart is impaired. Why is not clear, but plenty of people suspect that there is a breakdown in some part of the EC coupling.” Since calsequestrin is critical to EC coupling, Dr. Cala’s research may provide a key piece to the puzzle.