A recent paper published in PLoS ONE imaginatively titled “Transcriptomic Analysis of Tail Regeneration in the Lizard Anolis carolinensis Reveals Activation of Conserved Vertebrate Developmental and Repair Mechanisms” has genuinely excited the imaginations of many people interested in regenerative medicine, but how? Well it all boils down to the process of tail regeneration in lizards: by identifying the biochemical pathways that enable lizards to regrow their limbs, scientists at 4 different universities realised that many of the proteins required for this process exist in many other mammals, including humans.
In fact, humans can already to some extent undergo regeneration. Anyone with an adventurous baby will attest to the fact that they can re-grow the tips of their fingers if they are hurt. It appears that it is this property of youth that lizards, salamanders and zebrafish take advantage of throughout their life, including adulthood, and understanding what enables the reactivation of this process could path the way to autologous transplantation (growing a fully structured organ from someone’s own tissues for re-transplantation) – the holy grail of regenerative medicine.
What is likely the cause for this regenerative capacity in young babies is the high rate of cell proliferation required for development. This is mediated by totipotent and pluripotent stem cell lineages, that is, cells that remain “unspecialised” and can differentiate into any one of a sub-set of other specialised cells. In adulthood the pool of totipotent cells disappears, and although the hunt for adult stem cells is ongoing, it is now possible to induce normal skin cells to become pluripotent and divide into different tissues by exposing these cells to specific proteins that induce cellular de-differentiation.
It is how the two principles of artificial pluripotency and tail regeneration combine that is of interest to the medical field, however far down the line this may be.
During the study over 326 different genes and their corresponding RNA were found to be either up or down-regulated along the length of the regenerating tail, pointing to a highly complex concerted biological response for tail regrowth. So complex in fact that in a creature only 4-8cm long this process takes 60 days from start to finish. To determine which genes where required for this process, the relative levels of all cellular RNA in 5 segments of the tail (tip to base) where analysed. To understand this assay you must know that every cell in your body contains identical DNA, known in its entirety as your genome. However, how each gene is used to produce the similar molecule called RNA differs greatly between cells and tissues, as – for instance – the liver will require different genes to be expressed than the brain. Therefore the level of all RNAs in each cell indicates both what genes are required for tail re-growth and how much each gene is used in each tail segment, collectively known as the transcriptome.
Unsurprisingly the genes found to be enriched promoted cell proliferation, but were surprisingly analogous to the human pathways in which they were employed. Scientists among you may well recognise the canonical MAP Kinase / fibroblast growth factor pathway and the Wnt pathway involved in cell division. Additional pathways involved include the control of skeletal muscle expansion, the innervation of this muscle and the formation of blood vessels inside newly formed tissue, forming a highly structured limb.
How the lizards manage this though is unlike their salamander and zebrafish counterparts, in which the process has been more closely studied. In these animals the regenerating limb arises from a single location of highly proliferative cells called the blastema, as the cells continue to grow and become further from the blastema they begin to differentiate into specified cells required.
In A. carolinensis the outgrowth has no blastema and simply progresses outwards from the wounded area. It appears that distributed locations of stem cell provide the expanding tissue with a supply of undifferentiated precursors, although the mechanism of how these stem cells arise is contentious among scientists, being the product of either de-differentiation, or trans differentiation to new cell types. It was observed that the Wnt pathway, responsible for cellular differentiation, was rendered inactive along the leading half of the tail, enabling cells to maintain their undifferentiated state.
Since the mechanism is unlike salamanders and zebrafish and contains homologs of many human proteins it is likely that the process could proceed similarly in higher vertebrates and mammals, although this is untested. What remains a mystery is how lizards “kick-start” this process of regeneration as to do so in human cells requires the artificial addition of factors only expressed during embryonic development. Scientists have actually been successful in inducing normal human skin cells called fibroblasts to become stem cells, and then from these produce liver precursor cells that when implanted in mice with liver disease, underwent a full recovery with functional livers!
This study highlighted that for successful autologous transplantation the induced stem cells need an environment similar to the cells they are intended to become, to grow in. However, insights from in situ tissues regeneration in lizards could lead to a better adaptation of existing stem cell technologies for the re-growth of human tissues in situ, although it might take a while.
Finally I leave you with this thought: while the ability to regenerate a limb using similar biology to our own is extremely exciting, the pathways that are activated in order to do so are frequently found to also be activated in numerous cancers, making not only understanding the extreme complexity of this system a challenge, but also recreating the extreme care nature takes to control its power.
Hutchins ED, Markov GJ, Eckalbar WL, George RM, King JM, Tokuyama MA, Geiger LA, Emmert N, Ammar MJ, Allen AN, Siniard AL, Corneveaux JJ, Fisher RE, Wade J, DeNardo DF, Rawls JA, Huentelman MJ, Wilson-Rawls J, & Kusumi K (2014). Transcriptomic Analysis of Tail Regeneration in the Lizard Anolis carolinensis Reveals Activation of Conserved Vertebrate Developmental and Repair Mechanisms. PloS one, 9 (8) PMID: 25140675
Zhu, S., Rezvani, M., Harbell, J., Mattis, A., Wolfe, A., Benet, L., Willenbring, H., & Ding, S. (2014). Mouse liver repopulation with hepatocytes generated from human fibroblasts Nature, 508 (7494), 93-97 DOI: 10.1038/nature13020