In those models, such as freshwater planarians, in which regeneration depends upon the presence of pluripotent stem cells, it is easy to imagine that epigenetic marks and genes involved in chromatin remodeling will play an important role in regulating this process. Both in maintaining pluripotency and regulating cell differentiation. Thus, a recent paper from the laboratory of Ricardo Zayas has identified several homologues of the SET1/MLL family of histone methyltransferases and reported an important role of some of these genes during planarian adult pluripotent stem cell maintenance, proliferation and regeneration (http://www.ncbi.nlm.nih.gov/pubmed/23235145).
But what happens in those other systems in which regeneration does not depend upon pluripotent stem cells? In amphibians, amputation triggers dedifferentiation of postmitotic cells that re-enter the cell cycle to provide new cells for regeneration. We know now that in axolotls (as well as for zebrafish cardiomyocites) cells do not appear to go through an extensive dedifferentiation process and do not really achieve a “pluripotent-like” state. In fact, there seems to be an important lineage restriction in terms of the final fate that dedifferentiated cells will reach once differentiate again (for most lineages cells redifferentiate into their original type). Still, certain level of re-programming must occur in those cells.
Following the results reported by the laboratory of Michael Levin (http://www.ncbi.nlm.nih.gov/pubmed/22022609), the laboratory of Carol Beck published recently a paper that confirms and expands on the function of histone deacetylases (HDAC) in amphibian appendage regeneration (http://www.ncbi.nlm.nih.gov/pubmed/22947425). The authors use two different inhibitors of HDAC activity: valproic acid (VPA) and sodium butyrate (NaBu). VPA is able to inhibit tail regeneration in Xenopus tadpoles at stages 40 and 48 of development. Moreover, VPA treatment significantly reduced the level of deacetylated lysine in those animals with inhibited regeneration. Xenopus tadpoles go through a so-called “refractory period” (stage 46/47) in which tail regeneration is significantly reduced in wildtype conditions. VPA treatment reduces even more the rate of regenerative success at this refractory period. Whereas Xenopus frogs lose their regenerative capabilities in adulthood, other amphibians such as axolotls maintain a high power if regeneration when adults. As showed in this paper by Taylor and Beck, VPA also inhibits tail regeneration in axolotls.
In addition, VPA is able to inhibit also limb regeneration in Xenopus. Interestingly, when tadpoles are treated with VPA for only 24h after amputation of their limbs, these regenerate normally. However, when VPA is maintained for the first 48h of regeneration, this process is inhibited. This suggests that HDAC activity is dispensable during the first 24h of limb regeneration but it becomes necessary between 24 and 48h post-amputation. In the context of tail regeneration, on the other side, HDAC activity appears to be required very early during the first 6 hours of regeneration. This is based on the fact that when VPA is added 6 hours post-amputation, the percentage of successful cases of tail regeneration is significantly increased in comparison to those cases in which VPA is added 1, 2 or 4 hours post-amputation.
Even though VPA is able to inhibit tail regeneration when added for just 1 hour after amputation, VPA treatment before amputation does not have any negative effect on regeneration. This points out towards a rapid induction of HDAC activity triggered by wounding and indispensable for regeneration. As the authors suggest, it will be important to determine whether this rapid burst of HDAC activity after amputation induces the formation of a regeneration-competent wound epithelium.
Finally, the authors show that VPA treatment does not inhibit normal limb development or the autonomous development of extirpated tail buds. This together with the rapid requirement of HDAC activity after amputation allow the authors to suggest that it might be the amputation itself that would induce HDAC activity which, in turn, would re-activate developmental patterning genes and pathways needed for a successful regeneration.