The Germ Theory of Regeneration
Amphibians have remarkable regenerative capabilities, but the mechanisms they use to regenerate are largely unknown. Identifying these mechanism would be of great interest for applications in medicine. Reactive oxygen species (ROS) are continually produced and required during tadpole tail regeneration (1). Production of ROS and tadpole tail regeneration are prevented by NADPH oxidase (Nox) inhibitors, suggesting Nox complexes as the source of ROS. However, the role of ROS and the mechanism of their sustained production throughout regeneration, were not known. NF-{kappa}B is a rapid-acting transcription factor with the potential to dramatically alter the activity and function of a cell (2). NF-{kappa}B is necessary for maintaining the undifferentiated state of human embryonic stem cells (3), human induced pluripotent stem cells (4) and mesenchymal stem cells (5), so may similarly be involved in maintaining the de-differentiated state of regeneration blastema cells. In the absence of an activating signal, NF-{kappa}B is sequestered in the cytoplasm by IkB (inhibitor of NF-{kappa}B), preventing its nuclear localisation and activity. The IkB kinase (IKK) complex inhibits IkB in response to multiple extracellular stimuli, but ROS can also inhibit IkB (6). Nuclear NF-{kappa}B directly activates transcription of several genes encoding Nox proteins (7, 8), so could thereby facilitate ROS production. A positive-feedback loop was hypothesised where ROS inhibit IkB to help maintain continual NF-{kappa}B activity and, in turn, facilitate the continual production of ROS by activating the transcription of Nox-encoding genes. Here we demonstrate the involvement of microorganisms in the initiation of tadpole tail regeneration. Microorganisms offer sources of ligands for toll-like receptor (TLR) pathway activation and consequently, IKK complex activity. It was also suggested that sustained NF-{kappa}B activity allows the continual expression of the genes encoding Nox4 in blastema cells and Nox2 in professional phagocytes. These findings provide potential targets for the activation of regeneration in non-regenerative animals.