There has been a high level of evolutionary conservation in the function that several signaling pathways play during regeneration. Thus, it is well known the role that pathways such us BMP/TGF-b, Wnt/b-catenin, notch, Hedgehog and RTKs have during the regeneration of different structures in a variety of animals from axolotls and zebrafish to Hydra and planarians. There are some cases in which the ability to regenerate in different species depends on different tissue dependencies. An example is the regeneration of the tail in axolotls and Xenopus tadpoles. In axolotls the spinal cord is necessary for tail regeneration, whereas in Xenopus tadpoles is the notochord and not the spinal cord what is required. In axolotls this dependency seems to be determined by Hedgehog signaling as shh (sonic hedgehog) is exclusively expressed in the spinal cord. Now, a recent paper from the laboratory of Yuka Taniguchi and Makoto Mochii reports that in Xenopus tadpoles shh is expressed in the notochord and required for tail regeneration (http://www.ncbi.nlm.nih.gov/pubmed/24941877).
Whereas in axolotls shh is expressed in the spinal cord during tail regeneration, the authors show here that shh was exclusively expressed in the notochord in the entire regeneration region in Xenopus tadpoles. On the other side, shh receptors patched 1 and 2 (ptc-1 and ptc-2) were expressed in the spinal cord in them. In order to determine the function of Hh signaling on tail regeneration the authors used cyclopamine a widely used inhibitor for Hh signaling. Upon cyclopamine treatment tail regeneration was severely impaired as well as the length of the regenerating notochord. These effects were rescued by the treatment of pumorphamine, an agonist for the Hh pathway. Cyclopamine treatment did not result in an increase in apoptotic cells, but a significant down-regulation of genes related to the Hh pathway such as ptc-1, ptc-2, gli-1 and smo was observed in those treated animals.
During normal tail regeneration, undifferentiated notochord cells accumulate at the distal edge of the amputated notochord by day 2. Then these cells align perpendicular respect to the AP axis and differentiate into cells containing large vacuoles after day 3. In contrast, upon cyclopamine treatment, undifferentiated notochord cells normally accumulated at the edge of the amputated structure and proliferated; however, their posterior alignment and final differentiation was inhibited. Interestingly, cyclopamine treatment impaired the growth of the regenerating spinal cord as well as the formation of myofibers. In fact, the expression of myoD, a well-known myogenic transcription factor, was suppressed upon treatment. Also, the authors observed a strong reduction in the number of cells positive for Pax-7, another myogenic marker. These results suggest a strong dependence of muscle regeneration on Hh signaling, being this in agreement with previous results in mouse and chicken in which shh has a positive effect on the proliferation and differentiation of the satellite cells (muscle stem cells).
Overall, the results presented here uncover a pivotal role of shh in tail regeneration in Xenopus tadpoles as its inhibition impairs regeneration by affecting several processes such as the final differentiation of new notochord cells as well as the proliferation of progenitors for the spinal cord and muscle fibers. Future experiments should determine whether the defects observed here in the differentiation of the notochord are due to a direct autocrine action of shh (as it is expressed in the notochord) or an indirect effect of shh function on spinal cord regeneration.
In summary, this work describes how the differential expression of shh either in the notochord or the spinal cord could account for the different tissue dependency of tail regeneration in Xenopus tadpoles and axolotls, respectively.