A chemical in acne medicine can help regenerate limbs

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If an axolotl loses a leg, it gets a new one–complete with a functional foot and all four toes. Over just a few weeks or months, bone, muscle, skin, and nerves grow back in exactly the same formation as the lost limb. The endangered, aquatic, Mexican salamander are masters of regeneration, showcasing the best of an ability shared by many other amphibians, reptiles, and fish species. But how do these cold-blooded creatures do it?

That some species can regrow limbs while others can’t is one of the oldest mysteries in biology, says James Monaghan, a developmental biologist at Northeastern University. Aristotle noted that lizards can regenerate their tails more than 2,400 years ago, in one of the earliest known written observations of the phenomenon. And since the 18th century, a subset of biologists studying regeneration have been working to find a solution to the puzzle, in the hopes it will enable medical treatments that help human bodies behave more like axolotls. It may sound sci-fi, but Monaghan and others in his field firmly believe people might one day be able to grow back full arms and legs post-amputation. After all that time, the scientists are getting closer.

New findings in an old biological mystery

Monaghan and a team of regeneration researchers have identified a critical molecular pathway that aids in limb mapping during regrowth, ensuring that axolotls’ cells know how to piece themselves together in the same arrangement as before. Using gene-edited, glow-in-the-dark salamanders, the scientists parsed out the important role of a chemical called retinoic acid, a form of vitamin A and also the active ingredient in the acne medicine isotretinoin (commonly known as Accutane). The concentration of retinoic acid along the gradient of a developing replacement limb dictates where an axolotl’s foot, joint, and leg segments go, according to the study published June 10 in the journal Nature Communications. Those concentrations are tightly controlled by just one protein also identified in the new work and, in turn, have a domino effect on a suite of other genes.

12/11/24 - BOSTON, MA. - Northeastern chair and professor of biology James Monaghan works with axolotls in his lab in the Mugar Life Sciences on Wednesday, Dec. 11, 2024. Photo by Alyssa Stone/Northeastern University — Northeastern chair and professor of biology James Monaghan works with axolotls in his lab. *Image: Alyssa Stone/Northeastern University* Stone, Alyssa

“This is really a question that has been fascinating developmental and regenerative biologists forever: How does the regenerating tissue know and make the blueprint of exactly what’s missing?,” Catherine McCusker, a developmental biologist at the University of Massachusetts Boston who was uninvolved in the new research, tells Popular Science.

The findings are “exciting,” she says, because they show how even the low levels of retinoic acid naturally present in salamander tissues can have a major impact on limb formation. Previous work has examined the role of the vitamin A-adjacent molecule, but generally at artificially high dosages. The new study proves retinoic acid’s relevance at normal concentrations. And, by identifying how retinoic acid is regulated as well as the subsequent effects of the compound in the molecular cascade, Monaghan and his colleagues have “figured out something that’s pretty far upstream” in the process of limb regeneration, says McCusker.

Understanding these initial steps is a big part of decoding the rest of the process, she says. Once we know the complete chemical and genetic sequence that triggers regeneration, biomedical applications become more feasible.

“I really think that we’ll be able to figure out how to regenerate human limbs,” McCusker says. “I think it’s a matter of time.” On the way there, she notes that findings could boost our ability to treat cancer, which can behave in similar ways to regenerating tissues, or enhance wound and burn healing.

Mutant and glow-in-the-dark salamanders

Monaghan and his colleagues started on their path to discovery by first assessing patterns of protein expression and retinoic acid concentration in salamander limbs. They used genetically modified axolotls that express proteins which fluoresce in the presence of the target compounds, so they could easily visualize where those molecules were present in the tissue under microscopes. Then, they used a drug to tamp down naturally occurring retinoic acid levels, and observed the effects on regenerating limbs. Finally, they produced a line of mutant salamanders lacking one of the genes in the chain, to pinpoint what alterations lead to which limb deformities.

They found that higher concentrations of retinoic acid tell an Axolotl’s body to keep growing leg length, while lower concentrations signal it’s time to sprout a foot, according to the new research. Too much retinoic acid, and a limb can grow back deformed and extra-long, with segments and joints not present in a well-formed leg, hampering an axolotl’s ability to easily move. One protein, in particular, is most important for setting the proper retinoic acid concentration.

“We discovered it’s essentially a single enzyme called CYP26b1, that regulates the amount of tissue that regenerates,” Monaghan says. CYP26b1 breaks down retinoic acid, so when the gene that makes the protein is activated, retinoic acid concentrations drop, allowing the conditions for foot and digit formations.

At least three additional genes vital to limb mapping and bone formation seem to be directly controlled by concentrations of retinoic acid. So, when retinoic acid concentrations are off, expression of these genes is also abnormal. Resulting limbs have shortened segments, repeat sections, limited bone development, and other deformations.

The promising future of limb regeneration

Based on their observations, Monaghan posits that retinoic acid could be a tool for “inducing regeneration.” There’s “probably not a silver bullet for regeneration,” he says, but adds that many pieces of the puzzle do seem to be wrapped up in the presence or absence of retinoic acid. “It’s shown promise before in the central nervous system and the spinal cord to induce regeneration. It’s not out of the question to also [use it] to induce regeneration of a limb tissue.”

Retinoic acid isn’t just produced inside axolotls. It’s a common biological compound made across animal species that plays many roles in the body. In human embryo development, retinoic acid pathways are what help map our bodily orientation, prompting a head to grow atop our shoulders instead of a tail. That’s a big part of why isotretinoin can cause major birth defects if taken during pregnancy–because all that extra retinoic acid disrupts the normal developmental blueprint.

Yet retinoic acid isn’t the only notable factor shared by humans and amphibians alike. In fact, most of the genes identified as part of the axolotl limb regrowth process are also present in our own DNA. What’s different seems to be how easily accessed those genetic mechanisms are after maturity. Axolotls, says Monaghan, have an uncanny ability to activate these developmental genes as needed.

Much more research is needed to understand exactly how and why that is, and to get to the very root of regeneration ability, but the implication is that inducing human limbs to regrow could be easier than it sounds.

“We might not need to turn on thousands of genes or turn off thousands of genes or knock out genes. It might just be triggering the reprogramming of a cell into the proper state where it thinks it’s an embryo,” he says.

And lots of research is already underway. Other scientists, McCusker included, have also made big recent strides in attempting to unlock limb regeneration. Her lab published a study in April finding key mechanisms in the lateral mapping of limbs–how the top and bottom of a leg differentiate and grow. Another major study from scientists in Austria came out last month pinpointed genetic feedback loops involved in positional memory, which help axolotl tissues keep tabs on where lost limbs once were and how they should be structured.

Still, it’s likely to be decades more before human amputees can regain their limbs. Right now, the major findings fall in the realm of foundational science, says McCusker. Getting to the eventual goal of boosting human regenerative abilities will continue to take “a huge investment and bit of trust.” But every medical treatment we have today was similarly built off of those fundamental building blocks, she says.

“We need to remember to continue to invest in these basic biology studies.” Otherwise, the vision of a more resilient future, where peoples’ extremities can come back from severe injury, will remain out of reach.