Health Science Reviews

by Chiara Foret

What do starfish, chestnut trees, and Wolverine have in common? Stumped? While humans’ regenerative capacity is largely limited to liver and skin cells, they all have the capability to re-grow entire functional limbs. This regenerative potential is one that is highly sought after, as worldwide more than 57 million people live with the loss of a limb. Although prosthetic technologies have advanced, the mechanism behind limb regrowth remains frustratingly elusive, and as of yet doctors have been unable to induce the generation of human limbs from previously amputated sites. This is partially because our wounds tend to scar over, which is great for preventing blood loss and infection, but not ideal for regenerative growth.

Luckily for us, scientists are closer than ever to conquering this biomedical frontier. A study published earlier this year offers new hope in the form of glassy-eyed frogs, which face the same limitations we do. Using a wearable hydrogel cap – the BioDome – infused with a potent cocktail of drugs, Harvard researchers were able to successfully stimulate the regrowth of surgically amputated legs in adult African clawed frogs. This distinction is important, as unlike tadpoles, adult frogs are normally unable to muster a regenerative response.

In the experiment, the adult frogs’ right hindlimbs were amputated before exposure to one of three treatments: the BioDome alone, the Biodome with the cocktail, or a control treatment with neither. The cocktail consisted of five pro-regenerative drugs designed to facilitate regrowth processes, such as the suppression of collagen production (which causes scarring), the regulation of inflammation, and the promotion of neuromuscular repair, blood vessel integration, and tissue outgrowth. In all cases the treatments were applied within 24 hours of amputation, and then removed 24 hours later. Over the next 18 months, the amputation site were regularly assessed for soft tissue repatterning, bone regrowth, and sensory-motor reflexes.

Incredibly, the drug-infused BioDome was able to restore what was lost, regrowing hindlimbs containing a rich complement of nerves, tissue, muscle and bone similar to its natural state. Although the stubby toes lacked webbing or a supportive bone structure – and it certainly wouldn’t win any beauty pageants – the regrown limbs were sensitive enough to respond to external stimuli and robust enough to allow the frogs to carry on with their usual amphibious activities. The hindlimbs treated with the BioDome only also exhibited a regenerative response, albeit not as strongly as the drug enhanced version. It would appear that although the BioDome created the nurturing micro-environment necessary for scarless wound healing, the brief exposure to the drug-cocktail provided the molecular triggers essential for the regrowth of the limb. Their data would concur, as exploration into the mechanisms responsible for this process revealed that the drug-cocktail stimulated molecular pathways normally active in developing embryos as they take shape.

While of course we are not frogs, this milestone result has encouraged the researchers, who plan to optimise the treatment for use in mice, with hopes of producing more structurally and functionally complete limbs. Being mammals, mice models are more applicable to the human condition, and so success could potentially lead to human trials. While limbs that have already been lost sadly could not be regrown, use of the treatment in hospital settings could allow the regeneration of human limbs lost to trauma or illness provided it is applied soon after the injury occurs. This might offer an alternative to prosthetics, which in their current state provide minimal limb function restoration. Furthermore, effective regenerative treatments developed for amputated human limbs could potentially be co-opted for the regrowth of other human organs in the future, which is currently in scarce supply.

It’s the stuff of science fiction, but one that is edging closer and closer to a reality.

References
• McDonald CL, Westcott-McCoy S, Weaver MR, Haagsma J, Kartin D. Global prevalence of traumatic non-fatal limb amputation. Prosthetics and Orthotics International. 4 December 2020. doi: 10.1177/0309364620972258.
• Nirosha J. Murugan, Hannah J. Vigran, Kelsie A. Miller, Annie Golding, Quang L. Pham, Megan M. Sperry, Cody Rasmussen-Ivey, Anna W. Kane, David L. Kaplan, Michael Levin. Acute multidrug delivery via a wearable bioreactor facilitates long-term limb regeneration and functional recovery in adult Xenopus laevis. Science Advances, 2022; 8 (4) DOI: 10.1126/sciadv.abj2164