Muscle Clocks Play a Role in Regulating Metabolism

Just 20 years ago, scientists didn’t even realize muscles had their own circadian clocks. Now they are beginning to appreciate their importance in health.

In the early 2000s, Stefano Schiaffino, a muscle physiologist at the University of Padova in Italy, was faced with puzzling results: two seemingly identical experiments involving hind leg muscles in rats had yielded different findings.

Schiaffino and his team were investigating the nuclear factor of activated T cells (NFAT), a transcription factor that responds to the level of muscle activity.  Despite using similar procedures, the researchers found that in the tissues from one set of animals, NFAT had moved from the cytoplasm into the nucleus in a large proportion of cells, while in tissues from another experiment, this change had not occurred.

The explanation for this difference turned out to be simple: timing. The researcher responsible for one trial had experimented on the nocturnal animals in the evening, while another had conducted the same procedure for the second trial in the morning. This meant that the first group of animals was more active at the time of measurement than the second. When the scientists repeated the second experiment late in the day, when the animals were more likely to be awake, they observed high levels of NFAT in the nuclei of the muscle cells, essentially replicating the first experiment. “At that time, I’d been working for many years on muscle, but had never thought about the circadian rhythms,” recalls Schiaffino, whose research now focuses on this aspect of muscle biology.

Around the same time, on other side of the globe, muscle physiologist
Karyn Esser, then at the University of Illinois at Chicago, also
stumbled upon a surprising discovery: that genes encoded essential elements of
biological clocks which were being expressed in rat muscle tissue.

A growing body of evidence now points to these cyclical dynamics as mediators of metabolism. Disrupting them may have consequences for health, predisposing individuals to conditions such as diabetes. Research also indicates that these clocks cycles that regulate tissue and cell function, may influence muscle strength and structure, and may even regulate neurological processes such as sleep.

Managing metabolism
One of the most clearly defined roles of a muscle’s clock is maintaining the tissue’s ability to take up glucose, a process that occurs in response to insulin levels and muscle contractions. During an animal’s waking hours, feeding releases insulin from the pancreas and physical activity induces the movement of the glucose transporter called GLUT4 to the cell membrane.

Studies show that disrupting clock genes in the muscle impairs the transcription of GLUT4 and other key genes involved in this process. Proteins required to metabolize sugars and lipids are also produced in a circadian manner. Researchers have found that genes that regulate the storage of these fuels reach peak expression levels when animals are preparing for rest, while those involved in breaking them down for energy production peak just before the active phase begins.

These findings implied that “the intrinsic muscle clock is an important controller of glucose metabolism,” Schiaffino says. This makes sense, he adds, because a muscle can become a “sponge for glucose” when insulin is released in a healthy animal—after a meal, for example. In fact, skeletal muscle is the body’s major glucose storage unit, responsible for around 70 percent of the body’s uptake of the sugar.

This daily cycle helps muscles prepare to transition from rest (and fasting), when the cells tend to store fuels, to a wakeful period, when the animal is eating and its cells burn fuels to generate energy for activity.

Earlier this year, Charna Dibner of the University of Geneva and her colleagues reported similar outcomes in human muscle cells: disrupting the clock in vitro altered the expression of a number of genes, including those encoding proteins involved in glucose transport and lipid metabolism, and impaired the muscle cells’ ability to take up glucose in response to insulin.5 

The muscle clock also appears to regulate the type of fuel that the cell burns.  Although active tissues require more energy, cells still need some fuel during sleep, but rather than rely predominately on glucose, which powers contractile activity during waking hours, they burn lipids and amino acids while at rest.

Those who have flown overseas or large distances by air will understand what it feels like when the body’s rhythms are out of sync. Traveling long distances across multiple time zones throws off the usual clock-setting cues, or zeitgebers, such as the daily light-dark cycle. Jet lag can cause a variety of temporary symptoms, including dizziness, irritability, and indigestion.

Longer-term perturbations of these rhythms can have lasting effects on the body. Researchers have also found that, in rodents, mutations in circadian clock genes can cause obesity, metabolic syndrome (a cluster of conditions that includes high sugar and low insulin levels in the blood), and diabetes. A number of epidemiological studies have shown that people who work night shifts are at a higher risk for these conditions as well.

Muscle clocks may affect aspects of health other than metabolism as well. Esser and her colleagues have found that knocking out tissue-specific timekeepers leads to weaker muscles in mice. 

Many questions remain about the way muscle keeps time, such as how external signals are incorporated. Studies, primarily in rodents, suggest that feeding and exercise may serve as primary triggers. Oxygen levels may also play a role. And when it comes to uncovering the molecular pathways that keep time in muscles and control that clock’s effects on the body, Esser says, researchers have only just begun. “There’s a lot still to learn.”