A Northeastern University researcher has identified a molecular pathway that moves the possibility of human limb regeneration forward. The research team in the university’s biology department and Institute for Chemical Imaging of Living Systems discovered how axolotls regrow the right limb in the right place, which is the next step toward reproducing the salamander’s regenerative abilities in human medical treatment.
Axolotls are famous for their remarkable regenerative abilities that allow them to regrow entire limbs and organs and uncovering how it is done could be used to advance human regenerative medicine.
“It could help with scar-free wound healing but also something even more ambitious, like growing back an entire finger,” said James Monaghan, PhD, biology chair and professor at Northeastern University, who coauthored a paper about the work. “It’s not out of the realm [of possibility] to think that something larger could grow back like a hand.”
Monaghan traces this ability, positional memory, back to a molecule known as retinoic acid, which is responsible for telling an axolotl’s cells what body part to grow back. Importantly, retinoic acid is not an axolotl-specific molecule—humans also have it, although we mostly get it from our diet and in skin medication like retinol.
By examining axolotls, Monaghan discovered the animals have a gradient of retinoic acid signaling. In the arm, for example, this means axolotls have more retinoic acid in their shoulders—and less of the enzyme CYP26B1 that breaks down the molecule—and less retinoic acid in their hands. The retinoic acid acts as a cue to the regenerative cells, called fibroblasts, telling them what to grow back and how much to grow back.
“The cells can interpret this cue to say, ‘I’m at the elbow, and then I’m going to grow back the hand,’ or ‘I’m at the shoulder. I have high levels of retinoic acid, so I’m going to then enable those cells to grow back the entire limb,’” Monaghan said.
Once he understood how key retinoic acid was to the body’s signaling, Monaghan started testing the limits of this system in ways that were “pretty Frankensteiny,” he said. By adding extra retinoic acid in an axolotl’s hand, the salamander grew a duplicated limb instead of just a hand.
When humans have an arm injury, our fibroblasts lay down collagen and start making a scar. In axolotls, the fibroblasts listen to retinoic acid and grow a new skeleton.
“If we can find ways of making our fibroblasts listen to these regenerative cues, then they’ll do the rest,” Monaghan said. “They know how to make a limb already because, just like the salamander, they made it during development.”
Monaghan says that understanding the signals in an axolotl’s regenerative system is only part of the key before humans can start regrowing limbs. The next step is to understand the mechanics of the cells themselves and what retinoic acid is targeting inside the cells.
Monaghan has already figured out one target: the short homeobox gene, or shox. When retinoic acid signaling increased, shox activated, indicating that the gene is extremely important for regeneration. Removing shox from the axolotl’s genome altogether with a gene editing technique called CRISPR-Cas9, Monaghan found that axolotls would grow very short arms with normal sized hands.
Notably, this is exactly what happens when humans have a shox mutation, Monaghan adds.
“In order for regenerative biology or regenerative medicine to move forward, we need to understand where positional memory lies and how to manipulate it and engineer it,” Monaghan said. “How do you make a cell move where you want? Changing its positional memory is critical for this.”
Editor’s note: This story was adapted from materials provided by Northeastern University.
The open-access study, “Retinoic acid breakdown is required for proximodistal positional identity during axolotl limb regeneration,” was published in Nature Communications.
To learn more about Monaghan’s work on limb regeneration, visit Northeastern University’s YouTube channel.