University of Cincinnati (UC) researchers have been awarded a five-year, $2.1 million grant from the National Institutes of Health (NIH) to study an isoform that plays a critical role in human resistance to fatigue.
Judith Heiny, PhD, and Jerry Lingrel, PhD, received the award from the National Institute of Arthritis and Musculoskeletal and Skin Diseases, one of 27 units within the NIH. Heiny is an associate professor in the UC College of Medicine's Department of Molecular and Cellular Physiology; Lingrel is a professor in the Department of Molecular Genetics, Biochemistry and Microbiology and acting chair of the Department of Cancer Biology.
Heiny's lab focuses on the molecular mechanisms of muscle fatigue, which occurs when the skeletal muscles (muscle attached to bone, such as the muscles in the arms and legs) are unable to continue contracting.
"Muscle fatigue is a normal property of skeletal muscle," Heiny says. "However, it can become exaggerated in aging or in human heart failure. Skeletal muscle fatigue with exercise intolerance is almost a diagnostic criterion for heart failure."
Isoforms are closely related variants of the same enzyme. Heiny and Lingrel, using genetically modified mice with an isoform of the Na,K-ATPase enzyme known as alpha 2 deleted only in the skeletal muscles (what's known as a "targeted knockout"), observed that the skeletal muscles of these mice fatigued much more quickly than mice without the knockout.
"They didn't run as long as their colleagues, and they had a dramatically reduced exercise capacity," Heiny says.
Heiny likened the action of this isoform to the "turbocharger" on an engine which kicks in when extra work is demanded. She adds: "It's not active in resting muscle. It turns on and becomes activated only when you're using your muscles."
The Na,K-ATPase enzyme pumps sodium out of cells, while pumping potassium into them. Skeletal muscles express very high levels of the alpha 2 isoform of the enzyme, in addition to the more common alpha 1 isoform that is found in all living cells.
Heiny and Lingrel deleted the alpha 2 isoform in their genetically modified mice. They found that this isoform is switched on rapidly at the start of muscle use, providing the "turbocharger" effect.
Working muscles naturally lose some potassium, and the researchers postulate that this isoform provides the extra capacity to pump it back. "The alpha 2 enzyme keeps us moving," Heiny added.
Their new research project will seek a better understanding of the isoform's mechanisms. "The mechanisms of fatigue aren't that well understood," Heiny says. "Some of them are, but it's still an incomplete picture."
"This is a major isoform, and it's sitting there not working until you really need it," says Lingrel. "That happens only in rare cases in other kinds of proteins.
"So that is the major challenge: What turns it on?"
Heiny and Lingrel will focus their research on the regulation of the isoform, with a number of possible translational applications including heart failure and endurance-based sports activities such as swimming, running and bicycling.
"The field of exercise physiology is always looking for something to resist muscle fatigue," says Lingrel. "But we're not at that point yet."
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