Regular activity not only strengthens muscles but can bolster our bones, blood vessels, and immune system. MIT engineers have now found that exercise can also have benefits at the level of individual neurons. The researchers observed that when muscles contract during exercise they release a “soup” of biochemical signals called myokines. The team’s in vitro, cell-based experiments found that in the presence of these muscle-generated signals, neurons grew four times further, compared with neurons that were not exposed to myokines.
Surprisingly, the neurons appeared to respond not only to the biochemical signals of exercise but also to its physical impacts. The team also observed that when neurons were repeatedly pulled back and forth, similarly to how muscles contract and expand during exercise, the neurons grew just as much as when they were exposed to a muscle’s myokines.
The collective results suggest that exercise can have a significant biochemical effect on nerve growth. And while previous studies have indicated a potential biochemical link between muscle activity and nerve growth, the newly reported work is the first to show that physical effects can be just as important, the researchers say. Shedding light on the connection between muscles and nerves during exercise, the results could inform on exercise-related therapies for repairing damaged and deteriorating nerves.
“Now that we know this muscle-nerve crosstalk exists it can be useful for treating things like nerve injury, where communication between nerve and muscle is cut off,” said Ritu Raman, PhD, the Eugene Bell Career Development Assistant Professor of Mechanical Engineering at MIT. “Maybe if we stimulate the muscle, we could encourage the nerve to heal, and restore mobility to those who have lost it due to traumatic injury or neurodegenerative diseases.”
Raman is the senior author of the team’s paper in Advanced Healthcare Materials, titled “Actuating Extracellular Matrices Decouple the Mechanical and Biochemical Effects of Muscle Contraction on Motor Neurons.” In their paper the researchers concluded, “In the long-term, by leveraging established tools for in vivo biochemical stimulation and mechanotherapy, we hope our in vitro learnings will translate to effective therapeutic strategies that preserve and promote healthy mobility.”
Emerging in vivo evidence suggests that repeated muscle contraction, or exercise, impacts peripheral nerves, the authors wrote. “Several recent studies have highlighted a particular need to study communication between skeletal muscle and motor neurons, since these two cell types work together to coordinate all voluntary movement.”
In 2023, Raman and colleagues reported that they could restore mobility in mice that had experienced a traumatic muscle injury, by first implanting muscle tissue at the site of injury, then exercising the new tissue by stimulating it repeatedly with light. They found that over time the exercised graft helped mice to regain their motor function, reaching activity levels comparable to those of healthy mice. When the researchers analyzed the graft itself it appeared that regular exercise stimulated the grafted muscle to produce certain biochemical signals that are known to promote nerve and blood vessel growth. “Our observation corroborated similar studies by others that indicate exercise may influence innervation, and is correlated with upregulation of circulating neurotrophins, such as ciliary neurotrophic factor and brain-derived neurotrophic factor,” they explained in their newly released paper.
Raman added, “That was interesting because we always think that nerves control muscle, but we don’t think of muscles talking back to nerves. So, we started to think stimulating muscle was encouraging nerve growth. And people replied that maybe that’s the case, but there’s hundreds of other cell types in an animal, and it’s really hard to prove that the nerve is growing more because of the muscle, rather than the immune system or something else playing a role.”
As the team further acknowledged in their newly released paper, while such results indicate a potential role for muscle contraction in mediating motor neuron growth, “the difficulty of deconvolving the muscle-specific role of exercise in vivo motivates investigation in tightly controlled in vitro environments.”
Developing effective in vitro model systems for investigating muscle contraction-mediated intercellular signaling would allow scientists to decouple the biochemical and mechanical impacts of muscle contraction on motor neuron growth and development, they said.
Through their studies reported in Advanced Healthcare Materials the investigators set out to determine whether exercising muscle has any direct effect on how nerves grow by focusing solely on muscle and nerve tissue. “ … we aimed to design an in vitro model system for harvesting myokines from exercised muscle,” they explained.
The researchers grew mouse muscle cells into long fibers that then fused to form a small sheet of mature muscle tissue about the size of a quarter. They genetically modified the muscle to contract in response to light. With this modification the team could flash a light repeatedly, causing the muscle to squeeze in response, in a way that mimicked the act of exercise. Raman had previously developed a novel gel mat on which to grow and exercise muscle tissue. The gel’s properties are such that it can support muscle tissue and prevent it from peeling away as the researchers stimulated the muscle to exercise.
The team then collected samples of the surrounding solution in which the muscle tissue was exercised, reasoning that the solution may hold myokines, including growth factors, RNA, and a mix of other proteins. “A growing body of literature, largely in animal models, has provided compelling evidence that repeated muscle contraction upregulates secretion of a range of biochemical factors, termed “myokines,” which are released into the circulatory system and can modulate cell signaling throughout the body,” they noted. Raman added, “I would think of myokines as a biochemical soup of things that muscles secrete, some of which could be good for nerves and others that might have nothing to do with nerves. Muscles are pretty much always secreting myokines, but when you exercise them, they make more.”
The scientists transferred the myokine solution to a separate dish containing stem cell-derived mouse motor neurons—nerves found in the spinal cord that control muscles involved in voluntary movement. As with the muscle tissue, the neurons were grown on an appropriate gel mat. The team observed that after exposure to the myokine mixture the neurons quickly began to grow, four times faster than neurons that did not receive the biochemical solution. “Motor neurons stimulated with exercised muscle-secreted factors significantly upregulate neurite outgrowth and migration, with an effect size dependent on muscle contraction intensity,” they noted. Raman continued, “They grow much farther and faster, and the effect is pretty immediate.”
For a closer look at how neurons changed in response to the exercise-induced myokines, the team ran a genetic analysis, extracting RNA from the neurons to see whether the myokines induced any change in the expression of certain neuronal genes. “We saw that many of the genes up-regulated in the exercise-stimulated neurons was not only related to neuron growth, but also neuron maturation, how well they talk to muscles and other nerves, and how mature the axons are,” Raman noted. “Exercise seems to impact not just neuron growth but also how mature and well-functioning they are.”
The results suggested that biochemical effects of exercise can promote neuron growth. The group then wondered whether the purely physical impacts of exercise have a similar benefit. “Neurons are physically attached to muscles, so they are also stretching and moving with the muscle,” Raman said. “We also wanted to see, even in the absence of biochemical cues from muscle, could we stretch the neurons back and forth, mimicking the mechanical forces (of exercise), and could that have an impact on growth as well?”
To answer this, the researchers grew a different set of motor neurons on a gel mat that they embedded with tiny magnets. They then used an external magnet to jiggle the mat—and the neurons—back and forth. In this way, they “exercised” the neurons, for 30 minutes a day. To their surprise, they found that this mechanical exercise stimulated the neurons to grow just as much as the myokine-induced neurons, growing significantly farther than neurons that received no form of exercise. “Interestingly, we observed that dynamic mechanical stimulation of motor neurons increased neurite length and migration area by a similar amount as biochemical stimulation over 5 days,” they wrote. “Despite the fact that the mechanical impact of exercise on motor
neurons has rarely been investigated, to our knowledge, our findings indicate that dynamic forces have a significant impact on neuron growth and migration.” Raman pointed out, “That’s a good sign because it tells us both biochemical and physical effects of exercise are equally important.”
Now that the researchers have shown that exercising muscle can promote nerve growth at the cellular level they plan to study how targeted muscle stimulation can be used to grow and heal damaged nerves, and restore mobility for people who are living with a neurodegenerative disease such as ALS. “This is just our first step toward understanding and controlling exercise as medicine,” Raman stated.
In their report the authors concluded “Our study is a first step toward unraveling how muscle contraction regulates motor neuron growth and maturation from the bottom-up through both biochemical and mechanical modes of signaling.”