Imperfectly Regrowing a Frog Limb Using Growth Factors to Change Cell Behavior
Researchers here report on a promising advance in making a non-regenerative species more regenerative. In recent years, research has focused on differences in the behavior of macrophages and injury-induced senescent cells in those species capable of regeneration of organs. It is presently thought likely that the capability for regeneration of organs exists in all higher animals, but it is in some way suppressed after embryonic development. Thus suitable coercion of cell behavior may unlock this ability. In this case, applying a combination of growth factors and other compounds for a short period of time sufficiently changed cell behavior to induce limb regrowth in a frog species not normally capable of this feat. The result was not a fully formed limb, but there was more than enough regrowth of structure to suggest that this approach is worthy of further development.
Limb regeneration is a frontier in biomedical science. Identifying triggers of innate morphogenetic responses in vivo to induce the growth of healthy patterned tissue would address the needs of millions of patients, from diabetics to victims of trauma. Organisms such as the African clawed frog (Xenopus laevis) - whose limited regenerative capacities in adulthood mirror those of humans - are important models with which to test interventions that can restore form and function.
Here, we demonstrate long-term (18 months) regrowth, marked tissue repatterning, and functional restoration of an amputated Xenopus laevis hindlimb following a 24-hour exposure to a multidrug, pro-regenerative treatment delivered by a wearable bioreactor. The treatment used 1,4-dihydrophenonthrolin-4-one-3carboxylic acid (1,4-DPCA), brain-derived neurotrophic factor (BDNF), growth hormone (GH), resolvin D5 (RD5), and retinoic acid (RA).
Regenerated tissues composed of skin, bone, vasculature, and nerves significantly exceeded the complexity and sensorimotor capacities of untreated and control animals' regeneration. RNA sequencing of early tissue buds revealed activation of developmental pathways such as Wnt/β-catenin, TGF-β, hedgehog, and Notch. These data demonstrate the successful "kickstarting" of endogenous regenerative pathways in a vertebrate model.
Competing interests: M.L., D.L.K., and N.J.M. are inventors on a pending patent related to this work filed by Tufts University (PCT/US2020/063665). D.L.K. and M.L. are cofounders of the company Morphoceuticals Inc., which operates in the regenerative medicine space and does research relevant to the subject of this paper.