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‘Exercise Hormone’ Tied to Bone-Strengthening Benefits

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Exercise
Credit: gettyimages/kali9

There’s no doubt that exercise is good for us—strengthening our muscles, helping us maintain a healthy weight, maybe even boosting our moods and memories. There’s also been intriguing evidence that exercise may help build strong bones.

Now, an NIH-funded study is shedding light on the mechanism behind exercise’s bone-strengthening benefits [1]. The new work—which may lead to new approaches for treating osteoporosis, a disease that increases the risk of bone fracture—centers on a hormone called irisin that is secreted by muscles during exercise.

In a series of mouse experiments, the researchers found that irisin works directly on a common type of bone cell, stimulating the cells to produce a protein that encourages bones to thin. However, this chain of molecular events ultimately takes a turn for the better and reverses bone loss.

Bruce Spiegelman’s lab at the Dana-Farber Cancer Institute and Harvard University Medical School, Boston, first discovered the irisin hormone in 2012 [2]. In the years since, evidence has accumulated suggesting a connection between irisin and many of the benefits that come with regular workouts. For example, delivering low doses of irisin—sometimes called “the exercise hormone”—increase bone density and strength in mice.

But how does irisin act on bones? The answer hasn’t been at all clear. A major reason is the protein receptor on our cells that binds and responds to irisin wasn’t known.

In the new study reported in the journal Cell, Spiegelman’s team has now identified irisin’s protein receptor, called αVβ5 integrin. Those receptors are present on the surface of osteocytes, the most common cell type found in mature bone tissue.

The researchers went on to show that irisin helps osteocytes to live longer. It also leads the bone cells to begin secreting a protein called sclerostin, known for its role in preparing bones for remodeling and rebuilding by first breaking them down. Interestingly, previous studies also showed sclerostin levels increase in response to the mechanical stresses that come with exercise.

To further explore the role of irisin in mouse studies, the researchers gave the animals the hormone for six days. And indeed, after the treatment, the animals showed higher levels of sclerostin in their blood.

The findings suggest that irisin could form the basis of a new treatment for osteoporosis, a condition responsible for almost nine million fractures around the world each year. While it might seem strange that a treatment intended to strengthen bone would first encourage them to break down, this may be similar to the steps you have to follow when fixing up a house that has weakened timbers. And Spiegelman notes that there’s precedent for such a phenomenon in bone remodeling—treatment for osteoporosis, parathyroid hormone, also works by thinning bones before they are rebuilt.

That said, it’s not yet clear how best to target irisin for strengthening bone. In fact, locking in on the target could be a little complicated. The Speigelman lab found, for example, that mice prone to osteoporosis following the removal of their ovaries were paradoxically protected from weakening bones by the inability to produce irisin.

This new study fits right in with other promising NIH-funded efforts to explore the benefits of exercise. One that I’m particularly excited about is the Molecular Transducers of Physical Activity Consortium (MoTrPAC), which aims to develop a comprehensive map of the molecular changes that arise with physical activity, leading to a range of benefits for body and mind.

Indeed, the therapeutic potential for irisin doesn’t end with bone. In healthy people, irisin circulates throughout the body. In addition to being produced in muscle, its protein precursor is produced in the heart and brain.

The hormone also has been shown to transform energy-storing white fat into calorie-burning brown fat. In the new study, Spiegelman’s team confirms that this effect on fat also depends on the very same integrin receptors present in bone. So, these new findings will no doubt accelerate additional study in Speigelman’s lab and others to explore the many other benefits of irisin—and of exercise—including its potential to improve our moods, memory, and metabolism.

References:

[1] Irisin Mediates Effects on Bone and Fat via αV Integrin Receptors. Kim H, Wrann CD, Jedrychowski M, Vidoni S, Kitase Y, Nagano K, Zhou C, Chou J, Parkman VA, Novick SJ, Strutzenberg TS, Pascal BD, Le PT, Brooks DJ, Roche AM, Gerber KK, Mattheis L, Chen W, Tu H, Bouxsein ML, Griffin PR, Baron R, Rosen CJ, Bonewald LF, Spiegelman BM. Cell. 2018 Dec 13;175(7):1756-1768. 

[2] A PGC1-α-dependent myokine that drives brown-fat-like development of white fat and thermogenesis. Boström P, Wu J, Jedrychowski MP, Korde A, Ye L, Lo JC, Rasbach KA, Boström EA, Choi JH, Long JZ, Kajimura S, Zingaretti MC, Vind BF, Tu H, Cinti S, Højlund K, Gygi SP, Spiegelman BM. Nature. 2012 Jan 11;481(7382):463-8.

Links:

Osteoporosis (NIH)

Guide to Physical Activity (National Heart, Lung, and Blood Institute/NIH)

Spiegelman Lab (Dana-Farber Cancer Institute, Boston)

Molecular Transducers of Physical Activity in Humans (Common Fund/NIH)

Video: MoTrPAC (Common Fund)

NIH Support: National Institute of Diabetes and Digestive and Kidney Diseases; National Heart, Lung, and Blood Institute; National Institute on Aging; National Institute of Neurological Disorders and Stroke


Study Suggests Light Exercise Helps Memory

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Fitness group doing tai chi in park

Credit: iStock/Wavebreakmedia

How much exercise does it take to boost your memory skills? Possibly a lot less than you’d think, according to the results of a new study that examined the impact of light exercise on memory.

In their study of 36 healthy young adults, researchers found surprisingly immediate improvements in memory after just 10 minutes of low-intensity pedaling on a stationary bike [1]. Further testing by the international research team reported that the quick, light workout—which they liken in intensity to a short yoga or tai chi session—was associated with heightened activity in the brain’s hippocampus. That’s noteworthy because the hippocampus is known for its involvement in remembering facts and events.


Snapshots of Life: Building Muscle in a Dish

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Fibers from cultured muscle stem cells

Credit: Kevin Murach, Charlotte Peterson, and John McCarthy, University of Kentucky, Lexington

As many of us know from hard experience, tearing a muscle while exercising can be a real pain. The good news is that injured muscle will usually heal quickly for many of us with the help of satellite cells. Never heard of them? They are the adult stem cells in our skeletal muscles long recognized for their capacity to make new muscle fibers called myotubes.

This striking image shows what happens when satellite cells from mice are cultured in a lab dish. With small adjustments to the lab dish’s growth media, those cells fuse to form myotubes. Here, you see the striated myotubes (red) with multiple cell nuclei (blue) characteristic of mature muscle fibers. The researchers also used a virus to genetically engineer some of the muscle to express a fluorescent protein (green).


Creative Minds: Designing Personalized Clinical Trials

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Karina Davidson

Karina Davidson/Jörg Meyer

It might have been 25 years ago, but Karina Davidson remembers that day like yesterday. She was an intern in clinical psychology, and two concerned parents walked into the hospital with their troubled, seven-year-old son. The boy was severely underweight at just 37 pounds and had been acting out violently toward himself and others. It seemed as though Ritalin, a drug commonly prescribed for Attention Deficit Disorder, might help. But would it?

To find out, the clinical team did something unconventional: they designed for the boy a clinical trial to test the benefit of Ritalin versus a placebo. The boy was randomly assigned to take either the drug or placebo each day for four weeks. As a controlled study, neither clinical staff nor the family knew whether he was taking the drug or placebo at any given time. The result: Ritalin wasn’t the answer. The boy was spared any side effects from long term administration of a medication that wouldn’t help him, and his doctors could turn to other potentially more beneficial approaches to his treatment.

Davidson, now an established clinical psychologist at the Columbia University Irving Medical Center, New York, wants to take the unconventional approach that helped this boy and make it more of the norm in medicine. With support from a 2017 NIH Director’s Transformative Research Award, she and her colleagues will develop three pilot computer applications—or digital platforms—to help doctors conduct one-person studies in their offices.


NIH Family Members Giving Back: Toben Nelson

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Roseville Raiders

Caption: Toben Nelson (back row, far left) celebrates with his Roseville Raiders after winning Gopher State Tournament of Champions.
Caption: Heather Hammond Nelson

What was Toben Nelson, a University of Minnesota epidemiologist who studies the health risks of alcohol abuse and obesity, doing this summer lugging around a heavy equipment bag after work? Giving back to his community. Nelson volunteered as a coach for the Roseville Raiders, a 13-year-old-and-under traveling baseball team that just wrapped up its season by winning the prestigious Gopher State Tournament of Champions in their age group.

In the fall, Nelson will gear up for hoops as the volunteer president of the Roseville Youth Basketball Association, which provides an opportunity for kids in this Minneapolis-St. Paul suburb to take part in organized sports. Nelson says volunteering grounds him as a scientist. It reminds him every single day that his NIH-supported research back at the office affects real lives and benefits real communities like his own.


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