Decoded: How muscles build bones during exercise

Although exercise is a well-known stimulus for new bone formation, it has remained unclear how muscle "talks" to bone despite their close proximity. An Indian-origin scientist and his team have now figured out how this process takes shape.

Dr Mone Zaidi from Mount Sinai Hospital in New York and researchers from Universities of Ancona and Bari in Italy discovered that a newly-found molecule produced by skeletal muscle in response to exercise has been shown to increase bone mass.

"It establishes for the first time a molecule released from muscle during exercise can act directly on long bones to increase their strength. These are the bones utilised during exercise and also the ones most likely to break," explained Zaidi, professor of medicine at the Icahn School of Medicine at Mount Sinai.

In the experiment, young male mice were injected with irisin, a recently identified signalling protein molecule.

In the injected mice, researchers saw significant increases in bone mass and strength, specifically cortical bone, which is a dense and compact type of bone tissue that constitutes about 80 percent of skeletal weight.

The study suggests irisin is fundamental to muscle-bone communication and likely translates the well-known skeletal anabolic action of exercise by directly stimulating new bone synthesis by osteoblasts.

Decreases in the level of physical activity, for example in former athletes, can lead to progressive loss of bone and increase fracture risk.

Disuse, or the weightlessness of space, can cause acute, rapid, and severe bone loss.

For example, it is known astronauts lose bone mass 10 times faster than women in early menopause and patients in a vegetative state or with spinal cord injuries display a high risk of fragility fractures.

"Identifying irisin could lead to the development of future therapies for sarcopenia, the gradual loss of muscle mass seen as one gets older, and osteoporosis," the authors emphasised.

"Understanding this molecular connection between muscle and bone gives us hope for treating age-related bone and muscle loss at the same time, with the same agent," Dr Zaidi pointed out.

Dr Zaidi did his graduation from the King George's Medical College (KGMC) in Lucknow in 1983.

Currently, he is affiliated with Mount Sinai Beth Israel, Mount Sinai St Lukes Hospital and New York Eye and Ear Infirmary of Mount Sinai.

The paper was published in the journal Proceedings of the National Academy of Sciences (PNAS).