Wnt signalling plays an important role in the development and maintenance of many organs and tissues, and appears to be especially important in regulating bone mass. Enhanced Wnt signalling has the potential to speed up healing and mice genetically modified to have prolonged Wnt signalling heal more quickly than control mice. So far it hasn’t been possible to directly test the effect of administering Wnt ligands because of difficulties in purifying and formulating the proteins, but researchers at Stanford University School of Medicine have now solved the problem by packaging the proteins in liposomal vesicles.
When Wnt3a-loaded liposomes were administered to mice with bone injury, within 3 days the animals had 3.5 times more new bone growth than animals that received no treatment or animals that received Wnt protein without the carrier liposomes. After 4 weeks, the bone had completely healed in the animals treated with Wnt-carrying liposomes whereas untreated animals took another 2 weeks. Wnt was shown to act by increasing the proliferation of bone progenitor cells – Wnt-responsive cells are found on the inner surface of the bone and participate in normal bone maintenance and in bone growth in response to injury. A liposomal formulation would also be suitable for use in people and could, after much further evaluation, be used to speed bone healing. Members of the bone morphogenetic protein family are currently used in spinal fusions and to treat some fractures but can cause bone to grow in the wrong place.
In some animals, such as flatworms and zebrafish, the Wnt pathway allows tissue regeneration without scarring and the authors hope that their study might also provide the basis for new treatments for stroke and heart attack where scar tissue formed as part of the normal healing process impairs later function.
Although some species of amphibians – such as salamanders and newts – are able to regenerate lost or damaged tissue, the capacity for regeneration diminished as vertebrates evolved and adult mammals generally have limited regenerative capacity. Over a decade ago, it was discovered that the Murphy Roths Large (MRL) mouse strain was able to regenerate new epidermis, new hair follicles, and new cartilage to repair punch holes in the ears, unlike other strains of mice that close the holes with scar tissue. The MRL mice were later shown to have the ability to repair damaged heart muscle and spinal cord with restoration of normal structure and function and to have some limited capacity for digit re-growth. Scientists have been trying to identify the gene or genes that are responsible for the increased capacity for regeneration in MRL mice and researchers at the Wistar Institute and Washington University have now shown that the p21 gene is involved in regulating the regeneration process.
The team showed that p21, a cell cycle regulator, was found to be consistently inactive in cells from MRL mice. When they looked at p21 knockout mice, they found that, unlike normal mice which heal wounds by forming a scar, mice that lack p21 begin by forming a blastema, a mass of cells capable of rapid growth and de-differentiation which behave more like embryonic stem cells than adult mammalian cells. The p21 knockout mice were able to replace missing or damaged tissue with healthy tissue that showed no signs of scarring.
Since the cyclin-dependent kinase p21 is one of the best characterized downstream targets of the tumour suppressor p53, knockout of p21 might be expected to increase the incidence of cancer and other studies have suggested that p21-deficient mice develop tumours at an earlier age than their wild-type counterparts and are more susceptible to the effects of some carcinogens. Although increased DNA damage was observed in the present study, there was also an increased incidence of apoptosis and no net increase in the incidence of cancer. If MRL mice and p21 knockout mice are a good model for tissue regeneration in humans, temporary inactivation of the p21 gene could eventually be used to speed up wound healing in people.
Although cutaneous wounds are known to heal more slowly in elderly men than in elderly women –and this observation had been linked to androgens – the exact role of androgens in the wound healing process is not clear. Researchers at the University of Rochester have now shown that, in general androgen receptor (AR) knockout mice, wound healing was faster than in wild type litter mates: re-epithelialisation and collagen deposition were also found to be enhanced in knockout versus wild type animals. Because serum testosterone levels are significantly reduced in the knockout animals and androgens are known to regulate cellular activities through AR-independent pathways, the team subcutaneously implanted dihydrotestosterone (DHT) pellets into the knockout mice to restore their serum androgen levels. The observation that DHT implantation did not reverse the acceleration of wound healing in the knockout animals suggests that AR-dependent pathways are critical for the accelerated wound-healing phenotype.
Using cell-specific AR knockout mice, the team went on to show that it is AR on infiltrating macrophages, rather than on resident keratinocytes or dermal fibroblasts, that plays a critical role in delaying wound healing and collagen deposition but that AR on keratinocytes and fibroblasts plays an opposing role in the regulation of re-epithelialization. Examination of inflammatory mediators showed that increased local TNF-α production following AR activation on macrophages plays a critical role in suppressing wound healing.
Topical application of ASC-J9, an anti-AR compound that causes increased AR degradation and reduced AR transactivation, was shown to accelerate wound healing. Earlier in vitro and in vivo studies have shown that ASC-J9 reduces AR-promoted tumour growth in liver and bladder cancer as well as AR-mediated spinal and bulbar muscular atrophy, with little influence on serum testosterone concentrations. The present study, which is published in the Journal of Clinical Investigation, suggests that topical treatment with ASC-J9 also has the potential to be effective in promoting wound healing.