Glow-in-the-dark axolotls reveal a clue in the mystery of limb regeneration

A tiny creature with frilly gills, a polite smile and glowing green skin just gave scientists a major clue to solve one of biology’s biggest mysteries: limb regeneration. Aquatic salamanders called axolotls are known for their unusual ability to regrow limbs lost to injury or amputation. Now, researchers have uncovered more about the complex process behind this superpower in a new study published Tuesday in Nature Communications. “A longstanding question in the field has been, what are the cues that tell cells at the injury site to grow back just the hand, for example, or to grow back an entire arm,” said senior study author James Monaghan, a professor of biology and director of the Institute for Chemical Imaging of Living Systems at Northeastern University. It turns out a substance called retinoic acid that’s commonly found in retinol acne treatments is responsible for signaling what body parts an axolotl’s injured cells should regenerate — and how, the study found. Retinoic acid is important in the development of human embryos too, telling the cells where to grow a head, heads and feet, Monaghan explained. But for an unknown reason, most of our cells lose the ability to “listen” to the molecule’s regenerative cues while in utero. And though regrowing entire human limbs still seems like the distant stuff of science fiction, Monaghan said studying the signaling function of retinoic acid in these amphibians could help develop new human healing methods and gene therapies. Studying retinoic acid in axolotls Axolotls don’t naturally glow in the dark. To observe the signaling cues of retinoic acid, Monaghan’s team used genetically modified axolotls that gleam fluorescent green wherever the molecule was activating injured cells. At first, the research team took a more “Frankenstein” approach by injecting excessive amounts of retinoic acid into the salamanders’ systems and observing the effect. At the site of amputations, the axolotls would grow more than what they needed — replacing a hand with an entire arm. “If you throw a ton of retinoic acid into (an injury site), all of these different genes that probably have nothing to do with the necessary blueprint are going to be activated,” said Catherine McCusker, an associate professor of biology at University of Massachusetts Boston who was not involved in the study but also conducts research on salamander limb regeneration. To better understand how axolotls used their natural levels of retinoic acid for limb regeneration, Monaghan and his team shifted their approach. “We discovered that a single enzyme is responsible for breaking down retinoic acid in (axolotls’) bodies,” Monaghan said. When his team blocked this enzyme, the same Frankenstein effects happened again. “This is really exciting and blew us away, as it shows that the levels of (natural) retinoic acid are controlled by their breakdown.” In other words, an injured axolotl hand knows not to grow into an arm partly because the enzyme, called CYP26B1, blocks the regeneration process from going further, McCusker explained. So far, understanding this relationship in an axolotl’s regenerative system is only one piece of the puzzle, Monaghan said. The next step will be to identify exactly what genes retinoic acid is targeting inside cells during regeneration to further uncover the “blueprint” those cells follow. What humans can learn from axolotls When an axolotl’s cells are injured, they go through a process called dedifferentiation, in which they lose their “memory” and revert to an embryonic state, Monaghan said. In this embryonic state, the cells become focused on generating new limbs, and they can once again listen to the retinoic acid signals to build and grow. Human cells, however, don’t dedifferentiate when injured, so they can’t respond to the retinoic acid signals. Instead, our tissues react to injury by scarring, laying down heaps of collagen and calling it a day, Monaghan said. But what if there was a way human cells could take these orders to build limbs once again? “This question is super interesting when it comes to gene therapy because maybe we don’t need to add genes or remove genes to induce regeneration in humans — we can just turn on the appropriate genes at the right time or turn off the appropriate genes at the right time,” Monaghan said, referencing technology like CRISPR that allows scientists to make changes to DNA to prevent and treat disease. Human limb regeneration is likely far off in the future, but once scientists understand more about retinoic acid signaling, technology could help return this regenerative ability to human cells to heal wounds and prevent scarring, McCusker said. Part of McCusker’s research focuses on how to speed up the process of limb regeneration. For axolotls, it may take only a couple of days to regrow their tiny hands, but in a fully grown human, that process could take years, McCusker said. “It’s important that we continue to do this basic biology research,” McCusker said. “We’re finding super novel ways of doing things that we don’t think are possible right now with current human medicine.”