Salamanders’ ‘Fight or flight’ Response Holds Key to Limb Regeneration, Harvard Study Finds
A groundbreaking new study reveals that the same adrenaline-fueled “fight or flight” response that prepares animals for immediate danger also plays a critical role in triggering limb regeneration in salamanders, offering potential insights into future regenerative medicine for humans.
For decades, biologists have been captivated by the remarkable ability of salamanders to completely regrow lost limbs. Now, researchers at Harvard University have pinpointed a key mechanism: the activation of stem cells throughout the entire body, not just at the site of injury. The findings, published in the journal cell on October 24th, demonstrate that this systemic response is orchestrated by the sympathetic nervous system.
“We’ve shown the importance of the adrenaline stress signaling hormone in regeneration,” said Dr.Payzin-Dogru, lead author of the study. “It’s not just about what’s happening at the amputation site, but what’s happening to the rest of the body.” Salamanders, unlike most other vertebrates, possess an unusual capacity for regeneration. While some invertebrates, like planarian flatworms, can regenerate entire bodies from fragments, salamanders are unique among vertebrates in their ability to fully regrow limbs.
When a salamander loses a limb, a blastema – a mass of precursor cells – forms at the amputation site. These cells then specialize to rebuild a new arm, leg, or tail. This process has long held promise for regenerative medicine,wiht some scientists theorizing that the common ancestor of all four-limbed vertebrates possessed this ability,which was subsequently lost in most species – except salamanders.
The Harvard team’s inquiry began in 2018 with the observation that limb amputation triggered cell proliferation throughout the salamander’s body, even in undamaged tissues. Over six years, and with the collaboration of 38 researchers, they sought to understand the underlying mechanisms. their work revealed that the adrenergic signaling network, a component of the sympathetic nervous system responsible for involuntary responses like heart rate and breathing during stress, coordinates this systemic response. This system is famously linked to the “fight or flight” response, first described over a century ago by Harvard physiologist Walter Bradford Cannon.
Adrenergic signaling involves the hormones noradrenaline and adrenaline, which also function as neurotransmitters. The study found that activating these pathways primes uninjured limbs for faster regeneration, perhaps aiding salamanders in surviving encounters with predators or cannibalism. Researchers discovered that activated cells altered their DNA architecture, making certain genes more accessible for activation – essentially preparing them for future regenerative events.
“The animal seems to form a short-term memory of the injury, bodywide,” Payzin-Dogru noted. “There is something that senses the injury and kind of goes into ‘getting ready’ mode for a subsequent injury so it can respond faster.”
However, this priming effect is transient. Systemic activation lasts only a few cell cycles, likely due to the high metabolic demands. After four weeks, no difference was observed in the speed of limb regeneration. The research also delineated the specific roles of different adrenergic signaling pathways: alpha-adrenergic signaling is crucial for preparing distant cells, while beta-adrenergic signaling drives regrowth at the amputation site. Furthermore, the adrenergic signaling triggered activation of the mTOR signaling pathway, which promotes cell growth and division.
For centuries, scientists have understood that nerve supply is essential for limb regeneration, but the focus was typically on sensory or motor nerves. “I heard very few people talking about sympathetic nerves,” Whited remarked.
The study challenges the conventional view of limb regeneration as a localized phenomenon.Whited believes growing evidence suggests it should be considered a whole-body event. “I think it’s paradigm-shifting,” she said.”I think it’s going to inspire a lot of future work to try to figure out not just how this works in an axolotl but also how it works in other systems.”
This research opens new avenues for exploring regenerative therapies in humans, potentially unlocking the ability to repair damaged tissues and organs by harnessing the power of the body’s own stress response system.
