Regeneration of limbs and wounded flesh has long been a part of science fiction and fantasy literature and therefore illicits no surprise with the continued search for such a ability. Many extant species are able to regenerate limbs such as arms, tails, and even spine segments. Some such species include lamprey such as the Lampetra species, the salamander Triturus viridescens, the turtle Trachemys dorbignyi, and of particular note, the axolotl which is known scientifically as Ambystoma mexicanum. The axolotl is an amphibian known commonly, and incorrectly, as the “Mexican walking fish” due to its discovery in Mexico and arms.
While the little guy is so interesting to look at, the question still stands, how is it able to regenerate its tail or legs? Freitas and her research group look to help answer that question and further our understanding of the regenerative process. They state that a common trait to regenerative species is that they have endogenous, or naturally derived, stem cells which are totipotent. This means that given enough instruction, they can be turned into any other type of cell found in that body. This trait is natural to stem cells and is why the axolotl can regenerate tissues which cannot be recovered in humans.
In particular, Freitas et. al. hone in on neuroregulin or Nrg1 to help explain how regeneration occurs in axolotls. Nrg1 is a type of signaling molecule known as a trophic factor ligand which binds to ErbB3 tyrosine receptors and is responsible in many developmental neural processes. These include schwann cell development, axon growth and guidance, neural cell migration, remyelination, and most notably, neural stem cell proliferation. In short it directs the growth of neural cells throughout development and even as an adult when neural cells grow or need protection. It comes as no surpise then that it has must have a hand in regeneration of zebrafish hearts and axolotl limbs and lungs.
Many signaling molecules are responsible for carrying out necessary actions in regeneration but not many pathways can be singled out to be responsible for starting the signal or even regulating the various acting signals. Freitas et. Al. hypothesized that Nrg1 must be influencing the process given its important role in all the aforementioned processes. They used and advanced in situ Hybridization Chain Reaction (HCR) to visually tag and amplify small amounts of present proteins and messenger RNA, responsible for making the proteins, at the same time in an image.
By optimizing an already advanced technique they hoped to find a change in the amount of Nrg1 found in cells. Unfortunately the numbers were not significant which they chalked up to the younger models of axolotls they used. They do however find that the Nrg1 is largely localized in ependymoglial cells found in the center of the spine surrounding the cerebrospinal fluid. This may allude to its importance in regeneration given that the levels of Nrg1 do not change even after the surgical damage to the spine or tail in the axolotls.
To further signal Nrb1 as the molecule of unique regenerative importance, the team set out to inhibit its receptor ErbB2 which binds with ErbB3 using the target specific drug Mubritinib. Then using another visual tag they observed reduced tissue growth and cell proliferation when ErbB3 reaction was inhibited. As such it seems that regeneration is guided by Nrg1 as well as other major components and is then carried out by different pathways to fully regenerate a lost limb. When Nrg1 was unable to signal through the bound ErbB receptors regeneration is hindered. These findings help understand how the regenerative process occurs and might one day help us create treatments to at least help previously unrecoverable wounds recover. While this study does not find the one switch that may allow humans to regrow an arm, it may allow for treatments to allow regeneration of neural cells. In this way the treatments would bring back sensation in limb damaged patients.
In Depth
Freitas et. Al. research an interesting topic of regeneration and make use of prior data on species and understanding of regeneration. While their data is clear, concise, and helpful in identifying role of Nrg1, they require further investigation to truly make a concluding remark. In addition they point out that their younger axolotl models may influence the lack of significant data in proliferating cells which also requires further investigation. Another line of research which was not mentioned includes the receptor involvement in neural regeneration. If ErbB2 and ErbB3 can bind to other ligands and send the same signal, there may be other factors at play. This requires an isolating experiment similar to that of the Mubritinib to ensure the ErbB and Nrg1 binding is specific to the regeneration. Lastly, they do mention that the signaling mechanism is not fully understood. Whether or not Nrg1 works through autocrine of paracrine function is not known and would more than likely be beneficial for creating treatments in the future as it would help dictate dosage concentrations and administration locations. Overall however the research seems to bring up new questions which further progress our understanding of regenerative processes in other species. They conduct thoughtful experiments which highlight the functions of Nrg1.
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