The axolotl, a unique salamander native to the murky lakes around Mexico City, has long fascinated scientists with its remarkable ability to regrow lost limbs. Living among aggressive and cannibalistic neighbors, these creatures are at constant risk of losing a limb to a neighbor's nibble. Yet, they can regrow these limbs, fully functional, in as few as eight weeks.
Now, researchers at the Institute of Molecular Biotechnology of the Austrian Academy of Sciences (IMBA) have cracked the code behind this extraordinary regenerative capability. Elly Tanaka and her team have discovered how axolotl cells "remember" their position within the body, allowing them to regenerate the correct structures after an injury. Their groundbreaking study was published in the journal *Nature* on May 21.
Upon injury, axolotl cells activate signals that instruct them to regenerate structures that match their location. Once regeneration begins, stem cells on the anterior (thumb) side of the limb express the signaling factor FGF8, while those on the posterior (pinky) side express the signaling factor Shh. These two signals reinforce each other, guiding the cells to grow and pattern the regenerating arm correctly.
"What we did not know was which cues ensure that FGF8 and Shh are turned on at the two sides of the limb during regeneration, the master mechanism that underlies positional information," explains Leo Otsuki, the first author of the study.
Through recent advances in molecular tools, the scientists systematically searched for the signaling cues at the root of axolotl regeneration. They found that the gene Hand2 is only expressed on the posterior side of the limb and plays an important role in switching on Shh after injury. Hand2 acts as a positional cue, maintaining the cells' stable memory of "being in the pinky zone."
Experiments confirmed that upon injury, posterior-side cells increase Hand2 expression, which then switches on the Shh signal in a subset of these cells. Cells near the Shh source regenerate as posterior-part cells, while cells farther away regenerate as anterior-part cells. Once the limb is fully regenerated, cells return to expressing Hand2 at a low level, ready for the next cycle of injury and regeneration.
The researchers liken this process to a radio broadcast. The Shh signal acts as a broadcast that instructs cells within its range to adopt a posterior identity. "We uncovered a more flexible model of regeneration than we had expected, and this is really exciting," says Otsuki. "Our model predicted that we should be able to switch cells from an anterior identity to a posterior identity by taking advantage of the Shh broadcast."
To test this, the team placed cells from the axolotl arm's thumb side into the pinky side. Remarkably, these thumb cells regenerated and behaved like pinky cells. "We were able to reprogram cells from the anterior and change their identity," Otsuki notes.
This ability to alter cell identities holds immense potential for tissue engineering and regenerative therapies. "Being able to convert cells remaining after an injury and change their function is critically important for applications in regenerative therapies," Otsuki points out. "It also enhances our ability to work with organoids and engineer tissues: We now know signals that can transform cell identity and change their regenerative outputs. Harnessing such signals might allow us to push cells beyond their normal biological limits."
The discovery that the axolotl relies on the Hand2-Shh signaling circuit for limb regeneration is particularly promising. "These same genes are also present in humans, and the fact that the axolotl reuses this circuit during adult life to regenerate a limb is exciting," says Tanaka. "It suggests that, if similar memory exists in human limbs, scientists may one day be able to target them to unlock new regenerative capabilities."
By expressing the Hand2 gene in areas where it is not typically active, it could potentially direct cells to initiate limb formation from scratch. "This finding fuels optimism that, by using Hand2 expression along with other insights from the axolotl model, we may eventually be able to regrow limbs in mammals," Tanaka adds. "Such advances hold promise for the field of regenerative medicine."
The team's breakthrough was made possible by genetic manipulation and cell tracing tools developed in the Tanaka lab, overcoming the challenges posed by the axolotl's very large and complex genome. Genetic tools readily available for other model organisms often do not exist for axolotls, but recent technological advancements allowed the scientists to delve deeper into the mechanisms of regeneration.
The preparation of this article relied on a news-analysis system.