Regrowing limbs a step closer
Ken Muneoka has a history of shaking up the field of regeneration. For instance, in a 2019 groundbreaking story published in Nature, the Texas A&M University College of Veterinary Medicine and Biomedical Sciences (CVMBS) professor proved the possibility of joint regeneration in mammals for the first time.
He and his team are now questioning other long-held notions about the underlying science, this time as to how mammals might regenerate damaged parts of their bodies.
Only some organs, such as the liver, and certain tissues, such as the epidermis — the top layer of the skin — can naturally regenerate in humans. Other species, most notably salamanders, can regenerate complex parts including bones, joints, and even entire limbs.
Consequently, for more than 200 years, researchers have studied these animals in the hope one day we might be able to regenerate limbs in humans. Because of this research, it’s widely accepted that you have to have nerves for limb regeneration.
According to two of Muneoka’s recent studies, although it may be true for salamanders and other species, this isn’t so for mammals. The first research, published in 2021 in the Journal of Bone and Mineral Research, proved that mammals need mechanical loading, or the capacity to exert force on or with the affected part of the body. The second study, published in March 2022 in Developmental Biology, proved that regeneration is not affected by a lack of nerves.
Together, these findings present a sizeable shift in the thinking of how regeneration could work in human medicine. “What these two studies show counteracts the two-century-old dogma that you need nerves to regenerate,” Muneoka says. “What replaces it in mammals is that you need mechanical loading, not nerves.”
Scientists have long held that two things must be present in order to induce regeneration in mammals.
The first is growth factors, which are molecules that can stimulate cells to regrow and reconstruct parts of the body. In natural regeneration, these growth factors are produced by the body; for human-induced regeneration, they must be introduced to the area.
The second factor was nerves. This belief was based on numerous studies on those parts of the body, usually digit tips, without nerves, in which the whole limbs were also no longer usable.
Those studies would have the predicted result: when growth factors were introduced regeneration didn’t take place, leading to the conclusion that, as with other species, nerves were required for regeneration.
But mechanical load was ignored.
In their studies, Muneoka and colleagues decided to take a step back and ask the question: “Is it really the nerves, or is a lack of mechanical load part of the equation as well?”
Connor Dolan, a former graduate student in Muneoka’s lab and first author of both new studies, came up with a way to test the question.
NASA has been using hindlimb suspension for decades to test how mammals react to zero gravity. A similar process is used during medical procedures on the legs of large animals to stop them putting weight on the affected limbs.
“Dolan found that when the limbs were suspended, even though they still had lots of nerves and could move around, they couldn’t actually put pressure on their limbs so the digit tips wouldn’t regenerate. It just completely inhibited regeneration,” Muneoka notes. “Absolutely nothing happens during the suspension. But once the load returns, there will be a couple of weeks of delay, but then they’ll begin to regenerate.”
That first step proved that even though nerves might be required, mechanical loading was critical in regeneration.
Taking the research a step further, Dolan’s second publication showed that nerves weren’t required by demonstrating that if a mouse has no nerves in one of its digits but does in the others — so that it’s still exerting force on the denerved digit — that digit will still regenerate. “He found they regenerate a little bit slower, but they regenerated perfectly normally,” Muneoka adds.
Muneoka is not saying previous research is wrong, just that it doesn’t directly apply to humans.
“There have been a number of studies in salamanders that prove that when you remove the nerves, they do not regenerate,” he says. “Researchers have also been able to put growth factors they know are being produced by nerves into the cells and rescue regeneration.
“So, salamanders probably do need nerves to regenerate,” he notes. “But if we’re going to regenerate limbs in humans, it’s going to be a lot more like what happens in mice.”
Since first beginning to look at regeneration more than 20 years ago, a number of Muneoka’s ideas have pushed back against the generally accepted theories about regeneration. He says getting these two papers published took almost three years because they originally tried to submit them together.
“Many scientists don’t embrace this idea,” he says. “A lot of people’s careers are really dependent on their studies of nerves and how they affect regeneration. For a study to come out and say that for humans it’s unlikely you’ll need the nerves, the whole biomedical application of what people are doing in salamanders and fish kind of goes out the window.”
Nerves not being required for regeneration in mammals may seem like an academic point. After all, what would be the point of regenerating a limb if the person couldn’t feel it or control it because it had no nerves? In that sense, nerves are still going to be important.
From Muneoka’s perspective, the shift is that instead of thinking of nerves as a requirement for regeneration, nerves are a part of what needs to be regenerated.
Larry Suva, head of the CVMBS’ Department of Veterinary Physiology and Pharmacology, says the issue is that nobody was even thinking about the load aspect previously. As Suva puts it, science is full of people looking where the light is best.
“I work on bones, so when I see a problem, I look at the bone problem,” he says. “People who work on nerves, all they look at are nerves. So it’s very rare that someone like Dr Muneoka will take a step back and take a more holistic view. That’s what he brought to this idea, to this 200-year-old data. We now have to look at regeneration through a different lens because now we know the mechanical influences are extremely important.”
One of the results of research focusing on nerves is that scientists have been able to recreate the growth factors that nerves produce, which has allowed them to start regeneration in salamanders, even if the nerves aren’t present. Suva says that with these new findings, scientists will now know they have to do the same with the mechanical load if they want to start regeneration in mammals.
“Scientists already have been able to trick the body into thinking nerves are still present,” he says. “But now they know they’ll also have to trick it into thinking there’s a mechanical load, something that has not been done before.”
Because cells react differently under mechanical load, somehow, that load is being translated biochemically inside the cell.
“There’s a small number of labs looking at the biochemical basis for what mechanical load does to a cell,” Muneoka says. “If we could understand that biochemical signal, then perhaps the physical force of mechanical load can be replaced by some sort of cocktail of molecules that will create the same signals in the cells.”
It may be some time before we see full human regeneration, but Suva says this kind of fundamental shift in thinking is a major milestone.
“Regeneration of a human limb may still be science fiction, but we know some facts about it, and now we know you have to have that mechanical load along with the growth factors. That changes how future scientists and engineers are going to solve this problem,” he concludes.
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