Several growth factor signalling pathways (i.e. Wnt, FGFs, TGFβ/BMP…) play conserved key functions during embryonic development. Similarly, these pathways have been shown to be conserved in several models of regeneration in which they appear also to have conserved roles in processes such as cell proliferation and differentiation, blastema growth or the establishment of axial polarity, among others. In many cases these different signalling pathways are being characterized in depth to determine their exact roles during regeneration. On the other side, when considering regeneration it is also very important to study what happens at those very early stages just after amputation. Thus, for example, wound healing appears as an important process that can either prevent regeneration (in a non-regenerating context) or promote a successful regeneration (in a regeneration-capable context). The same could be said for the inflammatory response after amputation in regenerating vs non-regenerating animals. Therefore, and going back to the role of those different signalling pathways mentioned above, we may wonder what are the upstream signals that activate them.
A recent and beautiful paper from the laboratory of Enrique Amaya shows that reactive oxygen species (ROS) induced very rapidly after amputation are indispensable for the proper tail regeneration in Xenopus tadpoles (http://www.ncbi.nlm.nih.gov/pubmed/23314862). The authors used different approaches to determine the function of ROS in this process. In order to visualize ROS in vivo the authors used a fluorophore HyperYFP reporter that possesses an oxidative sensitive domain, particularly sensitive to H2O2 over other ROS. They first made several transgenic lines of animals expressing this HyPerYFP reporter ubiquitously. Upon tail amputation in those animals they observed a significant and rapid increase in intracellular H2O2, that was maintained high from 6 hours to about 4 days. Then, and in order to determine the function of such increase in ROS they used two chemicals (diphenyleneiodonium, DPI and apocynin, APO) that inhibit the NADPH oxidase (NOX) complex, required for ROS production. Amazingly, these two inhibitors were able to block tail regeneration. A similar result was obtained when the authors decrease ROS production not by using specific drugs but by silencing a gene called cyba (cytochrome b-245 a polypeptide) that codes for a subunit of the NOX complex.
Next they wanted to check which specific processes could be affected after ROS decrease and that could explain the inhibition of a proper regenerative response. Based on previous data on NOX they showed that proliferation was indeed decrease in their treated animals. Finally, and again bearing in mind previous results on the role of Wnt/βcatenin and FGF signalling pathways on proliferation during regeneration, they showed that after ROS production inhibition the Wnt/βcatenin signalling was significantly reduced. This decrease in Wnt/βcatenin signalling explains also the decrease in the expression of fgf20 detected, as fgf20 is a known target of this signalling pathway. In fact fgf20 has been previously shown to play a key role in blastema formation during zebrafish tail regeneration (http://www.ncbi.nlm.nih.gov/pubmed/16373575). Remarkably, fgf20 morphants showed an impaired regenerative response in Xenopus tadpoles.
In summary, I think that these are very interesting and attractive results as they report on signals that act at the very early stages of regeneration and function as activators of signalling pathways that we know are essential for regeneration. As Wnt/βcatenin plays conserved functions in different regeneration models it will be very interesting to analyse whether an initial increase in ROS production has been equally conserved in those other models previous to the activation of downstream signalling pathways.