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Researchers identify protein responsible for keeping the heart beating on time.

The average heart beats 35 million times a year, 2.5 billion times over a lifetime. Those beats must be precisely calibrated; even a small divergence from the metronomic rhythm can cause sudden death. For decades, scientists have wondered exactly how the heart stays so precisely on rhythm even though it contains so many moving parts.

Now, researchers at the University of Maryland, the University of Pennsylvania, the University of Massachusetts, Cincinnati Children’s Hospital, Eulji University and the University of Vermont have helped identify how a particular protein plays a central role in this astonishing consistency. This is the first time the mechanism has been described; the discovery could eventually help scientists treat heart problems that kill millions of people every year.

The team describe how myosin-binding protein C (‘C protein’) allows the muscle fibers in the heart to work in perfect synchrony. The opensource study is published in the journal Science Advances.

For years, researchers have known that calcium acts as a trigger for the heartbeat, activating proteins that cause the sarcomeres, the fibrous proteins that make up heart muscle cells, to contract. The team found that the calcium molecules are not distributed evenly across the length of each sarcomere; the molecules are released from the ends. Despite this, the sarcomeres contract uniformly. But exactly how has remained a thorny mystery.

The researchers found the answer, C protein. This protein was known to exist in all heart muscle cells, but until now, its function was unknown. Using an animal model, the researchers studied the physiology of sarcomeres, measuring calcium release and the muscle fibers’ mechanical reaction. It turns out that C protein sensitizes certain parts of the sarcomere to calcium. As a result, the middle of the sarcomere contracts just as much as the ends, despite having much less calcium. In other words, C protein enables the sarcomeres to contract synchronously.

The team explain that calcium is like the sparkplugs in an automobile engine and C protein acts like the rings that increase the efficiency of the movement of the pistons.

Previous studies state that C protein appears to play a large part in many forms of heart disease. In the most severe cases, defects in C-protein lead to extremely serious arrhythmias, which cause sudden death when the heart loses the ability to pump blood. In the U.S., arrhythmias contribute to about 300,000 deaths a year according to the American Heart Association, with not all arrhythmias being fatal, being controlled with medicines and electrical stimulation.

The team theorise that it may be possible to affect arrhythmias by modifying the activity of C protein through drugs adding that the protein is definitely a drug target.

Beyond the findings of this work the team summise that there remain many challenges in unravelling how C protein mutations produce contractile and arrhythmic dysfunction in disease.

Source:  The University of Maryland School of Medicine 

Sarcomeric organization and MyBP-C.   Cardiac muscle sarcomere with interdigitating thick and thin filaments. MyBP-C localized to the C-zone, whereas the ryanodine receptors are localized in puncta (CRUs) along the Z-lines, forming the boundaries of each sarcomere.  Myosin-binding protein C corrects an intrinsic inhomogeneity in cardiac excitation-contraction coupling.  Warshaw et al 2015.
Sarcomeric organization and MyBP-C. Cardiac muscle sarcomere with interdigitating thick and thin filaments. MyBP-C localized to the C-zone, whereas the ryanodine receptors are localized in puncta (CRUs) along the Z-lines, forming the boundaries of each sarcomere. Myosin-binding protein C corrects an intrinsic inhomogeneity in cardiac excitation-contraction coupling. Warshaw et al 2015.

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