Mexican Axolotl Provides Insights into Potential of Human Regenerative Medicine

According to a team of researchers at the Salk Institute for Biological Studies in La Jolla, California, mechanisms learned from the early phase of limb regeneration in a lizard-like amphibian known as the Mexican axolotl – wound healing, cellular dedifferentiation and blastemal formation – will reveal therapeutic approaches for tissue regeneration in humans.

In two genetic studies, the Salk team has explored why the Mexican axolotl, Ambystoma mexicanum, can regenerate complex tissues such as entire limbs, while others including humans cannot.

“What our work suggests is that jumping genes would be an issue in any situation where you wanted to turn on regeneration,” says Prof Tony Hunter, director of the Salk Institute Cancer Center and senior author of both studies.

The scientists have found that two proteins, called piwi-like 1 (PL1) and piwi-like 2 (PL2), perform the job of quieting down jumping genes in this weird creature whose name means water monster and who can regenerate everything from parts of its brain to eyes, spinal cord, and tail. They say that in the axolotl, jumping genes have to be shackled or they might move around in the genomes of cells in the tissue destined to become a new limb, and disrupt the process of regeneration.

“As complex as it already seems, it might seem a hopeless task to try to regenerate a limb or body part in humans, especially since we don’t know if humans even have all the genes necessary for regeneration,” Prof Hunter says.

“For this reason, it is important to understand how regeneration works at a molecular level in a vertebrate that can regenerate as a first step. What we learn may eventually lead to new methods for treating human conditions, such as wound healing and regeneration of simple tissues.”

The team sought to characterize the transcriptional fingerprint emerging from the early phase of axolotl regeneration. They specifically looked at the blastema, a structure that forms at a limb’s stump. There the scientists found transcriptional activation of some genes, usually found only in germline cells, which indicated cellular reprogramming of differentiated cells into a germline state.

In the first study, published in the journal Development, Growth & Differentiation, the team focused on one of these genes, the long interspersed nucleotide element-1 (LINE-1) retrotransposon. LINE-1 elements are jumping genes that arose early in vertebrate evolution. They are pieces of DNA that copy themselves in two stages – first from DNA to RNA by transcription, and then from RNA to DNA by reverse transcription. These DNA copies can then insert themselves into the cell’s genome at new positions.

A few years ago, Prof Fred Gage of the Laboratory of Genetics at the Salk Institute, discovered that LINE-1 elements move around during neuronal development, and may program the identities of individual neurons. “Most of these copies appear to be ‘junk’ DNA, because they are defective and can never jump again,” Prof Hunter explains. “But all mammals, including humans, still have active LINE-1 genes, and the salamander, whose genome is 10 times larger than a human’s, contains many more.”

Active LINE-1 retrotransposons can keep jumping, and that was true in the developing blastema where LINE-1 jumping was dramatically switched on.

The second study, published in the journal Developmental Biology, shows that PL1 and PL2 switch off transcription of repeat elements, such as LINE-1.

“The idea is that in the development of germ cells, you definitely don’t want these things hopping around,” Prof Hunter says. “The mobilization of these jumping genes can introduce harmful genomic rearrangements or even abort the regeneration process.” In fact, when the researchers inhibited PL1 and PL2 activity in the axolotl’s limb blastema, regeneration was significantly slowed down.

“The need to switch on one set of genes to stop other genes from jumping just illustrates how amazingly difficult it would be to regenerate something as complex as a limb in humans,” Prof Hunter concludes. “But that doesn’t mean we won’t learn valuable lessons about how to treat degenerative diseases.”

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Bibliographic information: Wei Zhu et al. 2012. Retrotransposon long interspersed nucleotide element-1 (LINE-1) is activated during salamander limb regeneration. Development, Growth & Differentiation, volume 54, issue 7, pages 673–685; doi: 10.1111/j.1440-169X.2012.01368.x

Wei Zhu et al. 2012. Activation of germline-specific genes is required for limb regeneration in the Mexican axolotl. Developmental Biology, volume 370, issue 1, pages 42–51; doi: 10.1016/j.ydbio.2012.07.021