Wound healing is a universal response to injury conserved in all animals. However, not in all cases wound healing is followed by a successful functional regeneration. Thus, for example, skin injuries in adult mammals are usually solved through a so-called scarring wound healing that does not allow a functional recovery of the damaged skin. On the other side, in regeneration-competent species wound healing does not a have a negative effect but on the contrary is a key first step to trigger a regenerative response. Thus, in those cases, impairing wound healing results in the inhibition of regeneration. A deep characterization of the cellular and molecular events that result in scarring or regenerative wound healing may be very important to try to develop strategies and therapies to enhance the poor regenerative abilities shown by many animals. Now, a recent paper from the laboratory of Mark Martindale has characterized the regenerative wound healing in the cnidarian Nematostella vectensis (http://www.ncbi.nlm.nih.gov/pubmed/24670243).
In a first set of experiments the authors characterized the cellular and molecular events that occurred after injuring the animals by making punctures in their bodies with a glass needle. Two hours after injury an enrichment of actin was seen around the injury site and the wounds were healed after 6 hours. In another cnidarian, Hydra, and some vertebrates, apoptosis is required to trigger a proliferative response that leads to a successful regeneration. Similarly, upon injury along the ectodermal surface of Nematostella, apoptosis was significantly upregulated. Next, the authors decided to conduct a pharmacological screen to see which signaling pathways could have a role in wound healing and regeneration in these animals. Inhibition of the Notch pathway blocked head regeneration without affecting wound healing. On the other side, and unexpectedly, they did not found any defect after blocking the TGFB signaling. Finally, they inhibited ERK signaling and found a strong impairment of wound healing and regeneration. The MAPK signaling pathway plays many functions including immune response, cell proliferation, apoptosis and cell movement. In Drosophila, ERK (through MAPK) regulates actin dynamics at the injury site a the early stages of wound healing. Using their puncture assay they found that inhibiting ERK signaling with the drug U0126 caused wound to remain open after six hours and also eliminated the local phosphorylation of ERK at one hour after injury, compared to the wound response of untreated animals. U0216 did not blocked the initial apoptotic response to injury indicating that apoptosis by itself is not sufficient to trigger a regenerative response. Also, the animals treated with U0216 did not show much actin concentration around the injury suggesting that ERK could be targeting cell movement and adhesion.
Then, the authors used Nematostella genome-wide microarrays to identify genes involved in wound healing. They analyzed the gene profiles from samples taken 1 hour and 4 hours after injury in untreated and U0216 treated animals, which allowed them to do many comparisons. Thus, they generated a profile of genes not only up- or down-regulated at early (1h) and late (4h) stages of normal wound healing, but also how the expression of those genes was affected after inhibiting the ERK pathway. After injury and wound healing genes upregulated included genes with peptidase activity, modulators of MAPK signaling, Sox E1 and runt transcription factors, growth factor-related genes as well as genes related to mucus proteins. Some of these genes were validated by qPCR and/or in situ hybridizations. The authors focused then in several genes: uromodulin, soxE, thiamine enzyme, a matrix metalloproteinase (MMP) inhibitor and a maltase-like gene. In all cases these genes were upregulated upon injury and this upregulation appeared dependent of ERK signaling, as it was not observed after treatment with U0216. Remarkably, all these genes were upregulated during regeneration after amputation through the oral-aboral axis. Again, the expression of these genes during regeneration was dependent of ERK signaling.
To conclude, the authors propose that ERK signaling would be necessary for the initiation of the early wound healing response in Nematostella, agreeing with the important functions of the ERK signaling during regeneration reported in other systems. Future functional analyses on the genes identified here should help to confirm this hypothesis and to better characterize wound healing at the gene expression level. In summary, this is the first report of genes involved in wound healing in Nematostella. Comparisons of the cellular and molecular events that characterize Nematostella wound healing with those found in other regenerative models as well as in regeneration-incompetent animals could help to understand better this key initial process that takes place after any injury.