Limb regeneration: a lesson from salamanders

As much as we would love to enlist limb regeneration among modern science's best accomplishments, so far it is still very much confined to science fiction. That doesn't mean it won't happen, though. Key to limb regeneration is cellular reprogramming that allows differentiated cells to return to a germline-like (undifferentiated) state. Genes involved in embryonic development need to be reactivated in order to restart the same process that created the limb during the growth of the embryo.

The vertebrates with the best ability to regenerate limbs are salamanders, making these little critters the most studied in the field. When a salamander loses a limb, a layer of epidermis grows to cover the wound, and beneath this layer new, undifferentiated cells start proliferating, forming a mass called blastema. Recent research shows that this first wave of cell dedifferentiation may recapitulate events occurring during embryogenesis.

"The cells in the limb blastema are believed to be a heterogeneous collection of dedifferentiated cells that have been reprogrammed to achieve varying levels of developmental potential exhibited by the cells involved in embryogenesis [1]."

Germline stem cells are cells that give rise to gametes (the reproductive cells) and have the ability to divide into another stem cell as wells as a more differentiated cell. This mechanism, called asymmetric division, is controlled by a protein called PIWI through small, non-coding RNAs called piRNAs. In [1] Zhu et al. showed that when salamanders regenerate a limb, a germline-like state is established in the growing tissue. In particular, they found that germline-specific genes were expressed in the regenerated limb. In order to show this, they looked specifically at the PIWI proteins.

Zhu and colleagues found a significant amount of upregulated transposable elements in the regenerated limbs. If you remember, transposable elements is a segment of DNA that can move from one locus to another within the genome of the same cell. During the limb regeneration process, transposable elements can impart a deleterious amount of instability, which is counteracted by a corresponding upregulation of the PIWI genes. Conversely, when the PIWI genes were knocked down (i.e. their expression was reduced) in the blastema, limb growth following the amputation was significantly reduced compared to controls.

Zhu et al. conclude

"In the future, further characterization of the subpopulations of these reprogrammed cells with additional germline-specific markers might provide more insight into exactly how far cellular dedifferentiation can proceed and whether there are indeed a small number of cells that could be isolated before a certain developmental threshold and exhibit true pluripotency when isolated from the influence of the partially programmed blastemal cells in the proximity."

[1] Wei Zhu, Gerald M. Pao, Akira Satoh, Gillian Cummings, James R. Monaghan, Timothy T. Harkins, Susan V. Bryant, S. Randal Voss, David M. Gardiner, & Tony Hunter (2012). Activation of germline-specific genes is required for limb regeneration in the Mexican axolotl Developmental Biology DOI: 10.1016/j.ydbio.2012.07.021