Researchers led by Tufts University biologists and engineers have found that delivering progesterone to an amputation injury site can induce the regeneration of limbs in otherwise non-regenerative adult frogs - a discovery that furthers understanding of regeneration and could help advance treatment of amputation injuries.

The researchers created a wearable bioreactor attached to the wound site to deliver the progesterone locally for a 24-hour period and observed that it had a lasting beneficial effect on tissue regrowth, allowing the frogs to partially regenerate their hind-limbs.

A mere 24 hour of exposure led to 9 months of changes in gene expression, innervation, and patterned growth. The finding, published in Cell Reports, suggests the drug-device combination could be a new model for systematically testing and deploying therapeutic cocktails that could induce regeneration in non-regenerative species.

Many animals are capable of regeneration - in fact, planarian worms and sea cucumbers can spawn entire individuals from fragments when cut into pieces. Partial regeneration is observed in other species - lizards regrow tails, some crabs regrow claws, and deer regrow antlers each year.

Xenopus laevis, or the African clawed frog examined in this study, can regenerate limbs when in their tadpole and froglet stages, but gradually lose that capability as they develop into adults. Until now, it was not known whether adult frogs were capable of significant regeneration response.

For humans, the ability to regenerate would be a welcome development, especially for millions of people who live with limb amputations, of which there are 2 million in the U.S.

The study authors noted that while restoration of limbs has been an endpoint long sought in biomedical research, very little has been reported of rebuilding or repairing lost limbs in non-regenerative animals.

Starting with the successful result of this study, the researchers are exploring factors and modes of treatment to better understand how to induce regeneration in organisms that have lost, or never had that capability.

"We looked at progesterone because it showed promise for promoting nerve repair and regeneration. It also modulates the immune response to promote healing, and triggers the re-growth of blood vessels and bone," said Celia Herrera-Rincon, Ph.D., a post-doctoral fellow, and lead author of the study.

"Progesterone can also regulate the bioelectric state of cells, caused by cells passing ions across their outer membranes, which is known to drive regeneration and body pattern formation." Examination of the growing limbs in the experiments confirmed these beneficial effects of the drug.

The wearable bioreactor delivering the progesterone was developed in the laboratory of David Kaplan, Ph.D, Stern Family Professor of Engineering and chair of the Department of Biomedical Engineering at Tufts' School of Engineering and director of the Initiative for Neural Science, Disease and Engineering at Tufts.

The device contains a silk protein-based hydrogel which is applied directly to the wound and is capable of delivering small molecule compounds to the site. Future experiments will explore additional factors that can enhance or improve upon the effects of progesterone.

"We'll be using the bioreactor model as a new platform for finding 'master regulator' control points, activated by drugs which, after a very brief treatment, trigger a long program of tissue growth and remodeling - as well as other factors that support the entire process of regeneration," said Michael Levin, Ph.D. Vannevar Bush Professor of Biology in the School of Arts and Sciences and director of the Allen Discovery Center at Tufts, where the regeneration studies were conducted.

"The fact that the model applies treatments locally, which can also be varied over time and location on the wound, makes this a powerful tool for discovering regeneration therapeutics," added Levin, the paper's corresponding author. Studies in mammalian limb models are underway.

Research paper

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