Humans do not have the ability to regenerate lost limbs, but there are particular vertebrate animals that possess this remarkable ability. Amphibians, for example, can regenerate lost appendages and this ability has made these remarkable creatures the focus of a good deal of research in order to understand how to translate this regenerative ability into other mammals, such as ourselves.
Mammals, unfortunately, have no such regenerative ability and we also tend to form scars over injury sites as a result of wound repair. Fortunately, there are some experimental mammalian models that do display enhanced wound regeneration and these organisms permit the study of the underlying processes at play during regeneration. Many of the processes which mediate wound regeneration are controlled through regulatory mechanisms stimulated by a protein called the “hypoxia-inducible factor-1a” or HIF-1α transcription factor. The laboratory of Ellen Heber-Katz at the Wistar Institute in Philadelphia, PA, has worked hard to characterize the function and specific biological activity of HIF-1α in MRL (Murphy Roths Large) mice. MRL mice show spontaneous regenerative healing. In a new study, Heber-Katz and her colleagues confirmed the importance of HIF-1α in regeneration and extended their understanding of this protein by establishing that small molecule-mediated stabilization of HIF-1α protein in wounded animals promotes regenerative wound healing after injury in mouse strains that do not possess the ability to spontaneously regenerate.
Initial studies wounding healing in mice that had been subjected to ear-hole punch injuries. Such injuries induced high HIF-1α levels only in MRL mice but not in control animals (C57BL/6 mice). Also, if small interfering RNAs were used to knock down the levels of HIF-1α in MRL mice, wound closure was delayed and inefficient. These data suggested that HIF-1α was essential for spontaneous healing, and implied that artificial induction of HIF-1α at injury sites in strains of mice that do not show spontaneous healing could induce regenerative healing.
Heber-Katz and her co-workers subjected Swiss Webster mice (a mouse strain that does not show spontaneous regeneration) to ear-punch injuries, but also treated their ear injuries with a hydrogel laced with a drug called 1,4-DPCA (1,4-dihydrophenonthrolin-4-one-3-carboxylic acid). 1,4-DPCA is an inhibitor of the prolyl hydroxylases (PHDs) enzymes that degrade HIF-1α. This did not work because the mice kept grooming their ears and wiping off the hydrogel. So, an undaunted Heber-Katz team implanted the hydrogel underneath the skin of the animals’ necks. They observed HIF-1α expression increase on day 1 and reached maximal expression on days 3 to 4 after injection. If they repeated injection of 1,4-DPCA every 5 days in the neck, the full wound closed on day 35, with no harmful long-term effects observed at 3 months post-injection.
How does HIF-1α enable regenerative wound healing in these mice? Interestingly, 1,4-DPCA treatment induced the expression of stem-cell marker genes such as Nanog and Sox2. These induced cellular de-differentiation in the cells and stimulated the very early and rapid re-casting of the cell layers over the wound that are so characteristic of regeneration, but not wound repair. Additionally, 1,4-DPCA treatment reduced tissue remodeling, inflammatory responses, and scar formation. It also stimulated promoted events associated with the latter stages of the regenerative processes; such as the growth of new cells and the growth and redifferentiation of those cells into things like cartilage, and hair.
These data suggest that induction of the transcription factor HIF-1α promotes tissue regeneration over scar formation in mammals. Thus this is a potential strategy to stimulate the regeneration of lost or damaged tissues. The non-toxic and specific nature of the HIF-1α inhibitor used in these experiments should lead to fruitful studies in appropriate human systems (cell culture) but although mouse models and maybe larger animal models as well will assess the ability of particular drugs to induce repair in other parts of the body and should prove equally as useful.
This new study was published in Science Translational Medicine.