It is well known that limb regeneration in amphibians is nerve-dependent. Several neural-derived factors, including FGFs or substance P, have been suggested to play this role. But it was not until few years ago that the laboratory of Jeremy Brockes identified nAG, an important protein expressed in Schwann cells and with the capacity to rescue, for the first time, regeneration up to the digit stage in a denervated newt limb. A different paradigm in which to study nerve-dependency in amphibian limb regeneration is the so-called accessory limb model (ALM). After a skin wound, amphibians regenerate it normally (without any scar). However, if at the same time, a nerve is deviated to the center of this skin wound a blastema-like outgrowth (bump) is formed. Over time most of these bumps regress, but if a piece from the skin of the opposite site (anterior/posterior) is grafted to the site of the skin wound to which a nerve has been deviated then most of the bumps continue growing and eventually can form an ectopic limb (ALM). In this model, an ALM blastema is considered to display many of the features of a regular limb blastema.
In a recent paper by the laboratory of Akira Satoh the authors continue their research to find which factors can be important to induce blastema formation and regeneration in this context (http://www.ncbi.nlm.nih.gov/pubmed/23769980). To do that they investigated the action of possible factors that can substitute the function of a deviated nerve in inducing a blastema-like outgrowth in a skin wound. From a deep sequencing analysis they decided to investigate the Growth and differentiation factor-5 (Gdf5), a member of the TGF-beta superfamily. After the application of Gdf5 in a skin wound they observed the development of a bump similar to an ALM blastema. Thus, it contained small undifferentiating-looking cells, the expression of tenascin-C and fibronectin was upregulated whereas type I collagen appeared downregulated, resulting in an extracellular matrix typical of a regenerative environment and, finally, the blastema marker Msx2 was also detected. However, the expression of Prrx-1, another marker for an ALM blastema was not observed. Also, when fibroblast dedifferentiate to give rise to blastema cells they acquire the capability to differentiate into cartilage, which can be assessed if those cells are grafted to a bone healing region. Whereas grafted cells from an ALM blastema can end up making cartilage, cells from a Gdf5-induced blastema never did it. Therefore, even Gdf5 was able to induce a blastema-like bump it was not exactly the same as a regeneration blastema.
However, if Gdf5 was applied together with Fgf2 and Fgf8 then the blastema-like bump induced appeared quite similar to a regeneration blastema: Prxx1 was induced and those bump cells could be differentiated into cartilage. Remarkably if a piece of skin from the opposite site was grafted to the original skin wound, the application of Gdf5, Fgf2 and Fgf8 was able to induce the regeneration of a limb-like structure displaying digit-like formation, as it happens in a normal ALM context, but without the requirement of a deviated nerve.
Therefore, it seems that Gdf5, Fgf2 and Fgf8 can cooperate to substitute for the requirement of nerve to induce limb regeneration, at least in the ALM paradigm. In the future it would be interesting to characterize better Gdf5 as it does not seem to be expressed by nerve cells and it is not clear what could be its receptor. Finally, the authors describe how Gdf5 appears to activate phosphorylated ERK (pERK), as it also do Fgf2 and Fgf8, which opens the door to investigate the function of pERK in blastema induction and formation, as it has been described in other models.