How exercise protects cartilage: a microRNA-29 signal

How exercise protects cartilage is not as straightforward as it might seem. Articular cartilage has no blood vessels of its own, yet physical activity can influence a tissue whose cells are physically isolated inside a dense extracellular matrix. A study in Nature Aging points to a possible molecular courier: extracellular vesicles circulating in the blood.

Extracellular vesicles are tiny membrane-bound packages released by cells. They can carry proteins, lipids and RNA molecules, and their cargo changes in response to physiological conditions. The new work follows one potential signal from aerobic exercise to aging cartilage and identifies microRNA-29 as a major part of that route.

Exercise changes the message inside vesicles

The researchers began with previously collected small-RNA sequencing data from circulating extracellular vesicles. In that study, ten men with a mean age of about 69 years completed 40 minutes of aerobic cycling. Reanalysis identified 16 vesicular microRNAs that changed substantially after exercise, 15 of which increased. Two belonged to the microRNA-29 family.

The investigators then examined samples from their own three-month aerobic exercise intervention. Four adults aged 65–85 years, half of them women, followed a moderate-intensity program that gradually progressed from roughly 70 to 150 minutes of activity per week. After training, their circulating extracellular vesicles contained higher levels of microRNA-29 than at baseline.

That finding established a response in blood, but it did not show what the vesicles could do to cartilage. The researchers therefore moved to a model of aging human chondrocytes.

Chondrocytes maintain the extracellular matrix of articular cartilage. As cartilage ages, that matrix becomes mechanically stiffer, and the change in stiffness can itself push the cells toward an older, less healthy state. The team cultured human chondrocytes on stiff, aged-like substrates and exposed them to vesicles collected from participants either before or after the exercise program.

Vesicles obtained after training shifted the cells toward a phenotype resembling chondrocytes in a softer, more youthful mechanical environment. They also increased type II collagen and SOX9, two markers associated with healthy cartilage-producing cells.

An epigenetic link to Klotho

A particularly interesting part of the mechanism involved KL, the gene encoding α-Klotho, a protein widely studied in aging biology.

Under stiff, aged-like conditions, the promoter region controlling KL became more heavily methylated. DNA methylation can reduce gene activity without changing the DNA sequence itself. Treatment with post-exercise vesicles reduced methylation at the KL promoter and increased KL expression.

The result suggested that the circulating signal generated by exercise could influence the epigenetic state of a recipient cartilage cell.

Network analysis then pointed to microRNA-29 as a major regulator of this response. When the researchers computationally removed microRNA-29 from the regulatory network, much of the predicted health-promoting effect was attenuated.

They next engineered extracellular vesicles to contain additional microRNA-29. These vesicles reproduced several of the anabolic effects seen with naturally exercise-primed vesicles in aged human chondrocytes. Disrupting α-Klotho reduced the response, showing that the effect was at least partly dependent on the KL/α-Klotho pathway rather than being a nonspecific consequence of vesicle exposure.

The study did not stop at cultured cells. Exercise-primed extracellular vesicles were injected directly into the knee joints of aged mice, where they improved measures of cartilage health and reduced features associated with age-related degeneration.

That in vivo experiment strengthens the biological case for the pathway, but it does not demonstrate cartilage regeneration in exercising humans.

The human training component was very small: the three-month intervention used samples from only four older adults. Direct effects on cartilage were examined in cultured human cells and aged mice rather than in the joints of those participants. The study also focused on aerobic exercise, so it cannot establish whether resistance training or other forms of activity generate the same vesicular signal.

There are additional open pieces in the pathway. The tissue sources of the circulating vesicles remain uncertain, and the investigators focused primarily on microRNAs. Proteins, lipids or other vesicle cargo could contribute to the biological effects as well. Even within the pathway identified here, α-Klotho explains only part of the response to microRNA-29-enriched vesicles.

What the study provides is a relatively coherent molecular chain connecting several levels of biology: aerobic exercise changes circulating extracellular-vesicle cargo; microRNA-29 emerges as a key signal; the vesicles alter epigenetic regulation of KL in aging chondrocytes; and experimentally enriching vesicles with the same microRNA reproduces part of the effect.

Exercise, in other words, may influence an aging joint through more than forces generated by muscles and movement. Some of the message may travel through the bloodstream, sealed inside microscopic biological packages.