ARTICLE IN BRIEF
Investigators describe work in animal models with a “master molecule” that appears to bulk up muscle and improve function. They say further work may lead not only to new treatments of muscle disease, but also help reverse the inevitable frailty associated with aging.
Anew study shows that a “master molecule” can make a muscle burn fat instead of sugar, and that doing so has real benefits: its mitochondria proliferate and become more active, the muscle increases in bulk, and the organism can run further and longer.
Proponents think that exploiting this switch may lead not only to new treatments of muscle disease, but also help reverse the inevitable frailty associated with aging. But, says one expert who was not involved with the study, while the finding is an important advance in the basic biology of muscle, the therapeutic applications are far less certain.
The switch from sugar to fat occurs through the action of a complex set of signaling pathways under the control of transcription factors, proteins that sit on DNA and promote gene expression. But other proteins, called corepressors, can bind to the transcription factors, attenuating their action, “acting like a dimmer switch,” according to Johan Auwerx, PhD, lead author of the study in the Nov. 11, 2011, edition of Cell, and professor of life sciences at the Swiss Federal Institutes of Technology in Lausanne, Switzerland.
One such corepressor is nuclear corepressor 1 (NCoR1), but, because deleting it in the embryo is lethal, its function in muscle has remained largely mysterious. To overcome that hurdle and study the effect of its deletion in adult mice, Dr. Auwerx generated mice whose NCoR1 gene was flanked by muscle-specific deletion sites, which were driven by a promoter that was triggered by an exclusively adult form of actin. This allowed him to turn the gene off in adulthood, and only in muscle. “During development, you don't want to mess with NCoR1, because you need it,” he said. But during adulthood, at least in muscle, not only can mice live without it, but “you can get powerful muscles without it.”
Mice without NCoR1 production in their muscles had higher levels of locomotor activity, running further and for longer amounts of time than untreated littermates, and they were more cold tolerant. Oxygen consumption increased, as did the ability to utilize oxygen during exercise, a critical determinant of endurance performance. Meanwhile, heart muscle performance and morphology were not affected, despite a slight reduction of NCoR1 in cardiac muscle.
On a cellular level, reduction of NCoR1 increased the diameter of single muscle fibers, and increased both mitochondrial number and their activity. Electron microscopy revealed the mitochondria were larger and more abundant, but were normal in structure. Markers of glycolytic muscle fibers were decreased, while markers for oxidative fibers were increased.
The switch to more oxidative and less glycolytic fibers, Dr. Auwerx said, is in keeping with the increased endurance the mice displayed. “When you run only 100 meters, you don't need oxidative metabolism,” because the muscle can derive quick bursts of energy from non-oxygen-using glycolysis. “But if you run for a long time, you need oxidative metabolism,” to continue to provide a rich source of energy.
Active NCoR1, he concluded, kept the muscle cell in primarily a glycolytic mode. As evidence, his study showed that a loss of NCoR1 expression led to an increase in activity of several transcription factors known to promote oxidative function, and others that promote muscle mass, including myocyte enhancer factor 2 (MEF2). When NCoR1 was in place, MEF2 activity was low. But without NCoR1, MEF2 was exposed to acetylation enzymes, which acetylated and therefore activated MEF2, likely contributing to the increase in muscle bulk observed in the mice. “So if you inhibit NCoR1, you are not only going to stimulate more oxidative muscle, but you are also going to get more muscle,” Dr. Auwerx said.
Taken together, the results suggest that NCoR1 “acts as a master modulator of mitochondrial metabolism in muscle,” he said. NCoR1 “may have evolved to facilitate metabolic adaptation of the mitochondria to energy availability.”
This fundamental discovery in muscle biology may have clinical implications. Treatment of muscle diseases may be the most obvious target, but Dr. Auwerx has his sights set, at least speculatively, on larger targets. “I see the major impact on getting old,” he said, specifically addressing the loss of muscle mass that is currently an inevitable aspect of aging. Muscle building slows down with age, but by suppressing NCoR1, “you are going to get your foot off the brake.”
Safety, of course, is the first concern for any strategy that contemplates affecting a protein with such profound and system-wide effects. The role of NCoR1 in adult tissues such as brain and liver is currently unknown, and is the focus of part of the lab's research program. “We need to make sure we can inhibit it in other tissues, without negative effects. That is ongoing, and in a year or two we will know what it does in these tissues. Our preliminary data suggest it will be very good.”
Returning to the dimmer switch analogy, Dr. Auwerx said that removing NCoR1 is not like turning off a light completely. “You are not going to completely alter gene expression, just fine tune it. The body can live with that, while it may not be able to go along with the sledgehammer effect of entirely knocking out a transcription factor.”
As in the mice, in which gene deletion was delayed until adolescence, NCoR1 suppression would not be appropriate in the very young, out of concern for interfering with developmental processes. For any other drug, he said, “It is going to be slow,” with perhaps five to ten years needed to fully work out safety concerns, targeting, and delivery issues. While systemic inhibition of NCoR1 function would be the easiest therapy to administer, muscle-targeted gene therapy with an NCoR1 inhibitor might be possible as well, he suggested.
“This is an important advance in understanding muscle biology,” said Eric Hoffman, PhD, chairman of the department of integrative systems biology at George Washington University School of Medicine and Health Sciences, and director of the Center for Genetic Medicine Research at Children's National Medical Center in Washington, DC. But is it a discovery with therapeutic observations? “That's two different questions right from the beginning.” Dr. Hoffman said.
A key question, he said, is whether increasing muscle mass, by itself, is likely to be therapeutic in disorders such as the muscular dystrophies. “There is a lot of enthusiasm for the idea that bigger is better,” and it continues to be an active area of drug development. But it may be focusing on the wrong endpoint.
In Duchenne muscular dystrophy, for instance, “the dystrophin deficiency is stimulating the hypertrophy pathways,” and the muscle increases in bulk early in the disease course. (The old idea that the muscle was only “pseudohypertrophic” due to fibrosis is incorrect, Dr. Hoffman noted, at least until much later in the disease.) Therapies such as myostatin and activin 2B antibodies, which are aimed at bulking up muscle through related pathways, have either been halted because of side effects, or have not shown efficacy in clinical trials.
There may still be value to pursuing this strategy further, he said, “but so far, the jury is out,” Dr. Hoffman said.