The Heart and Heredity; Unromantic as It May Sound, Genetics Rules the Normal, Everyday Function of the Heart

By Silberner, Joanne | Science News, February 22, 1986 | Go to article overview

The Heart and Heredity; Unromantic as It May Sound, Genetics Rules the Normal, Everyday Function of the Heart


Silberner, Joanne, Science News


THE HEART AND HEREDITY Unromatic as it may sound, genetics rules the normal, everyday function of the heart. And in that control lie clues to a basic mechanism of genetic expression.

Heart research spans everthing from finding a replacement for the ailing organ to figuring out on a molecular level exactly what goes on in a normal heart. Working at the basic-research end of the scale, Bernardo Nadal-Ginard and co-workers at his Harvard University laboratory are delving into the way proteins in heart muscle cells respond to changes in the environment. While the work does not have any direct application to cures or treatments, it has enabled these researchers to delineate how a single gene could produce several dozen forms of a single type of molecule, and it is helping to clarify exactly how genes work.

Scientists once believed that an individual gene produced an individual protein. But with a finite gene bank, sometimes more diversity is needed, and researchers now know that certain cell types can generate more than one protein from a single gene. White blood cells called lymphocytes, for example, can produce billions of different immunoglobulins through rearrangement of their DNA.

There is a second way to generate diversity, called alternative RNA splicing. The Harvard group and others have found evidence of it in muscle proteins, and researchers at other laboratories have seen it in immune system proteins, some nervous system proteins and blood proteins.

Nadal-Ginard found the alternative RNA splicing in a muscle protein serendipitously, he says. HE had originally set out to study how different proteins respond to calcium ions.

The scene for Nadal-Ginard's study is the contraction of muscle cells, a process that begins when an electrical change releases calcium stored within the cell. The contraction is effected by four key proteins in the cell--actin, myosin, tropomyosin and troponin.

The myosin molecules are aligned in thick parallel filaments; the other three molecules are intertwined in parallel thin filaments. Within each cell, rows of thin filaments jut out toward each other, like combs aligned teeth-to-teeth. The thick filaments lie between the teeth, bridging the gap between each set of "combs."

When the fiber receives a signal to contract, the thick filaments pull the combs together by "rowing" against the thin filaments. Globular ends of the myosin protein stick out toward the actin and act as little paddles. They grab hold of actin at an angle and pull the molecule slightly before the bond breaks. Then another bond is made a little farther down the actin filament and the process is repeated. Each cycle shortens the muscle by about 1 percent.

Troponin--which has subunits labeled I, T and C -- and tropomyosin control binding between actin and myosin. When the fiber is at rest, the troponin sits on the tropomyosin, which in turn sits on the actin molecule in such a way that it blocks its binding site. But when the contraction-initiating calcium ions bind to troponin C, the system is displaced so that the tropomyosin is pulled off actin's binding site.

It's complicated enough system. But as Nadal-Ginard, Roger E. Breitbart and their co-workers at Harvard have found, unraveling the genetics is even trickier. They are working specifically on the genetics of troponin T.

Troponin T comes in many different forms. The Harvard group has identified 10; in theory the gene has the capacity to encode an additional 54. "As far as we know, all can fulfill the same physical function," says Nadal-Ginard. But some may be "faster" than others -- more responsive to the presence of calcium. In that case, a quicker move off the tropomyosin would result in a speedier contraction. …

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