The remarkable axolotl is helping to advance cancer research

Abstract

In 1864, naturalists shipped a group of 34 axolotl salamanders from their native Mexico to Paris, where the strange amphibians became attractions in two Parisian zoos.¹ The axolotls thrived, multiplied, and soon populated laboratories, zoos, and home aquariums throughout the world—a sharp contrast to their decline in the wild. Unlike many other salamanders, the axolotl proved easy to house and breed: It stays in its larval stage throughout its life, remains fully aquatic with six feathery gills fanning out behind its head, and mates throughout the year.

More than 160 years later, the “Peter Pan of salamanders” is critically endangered in Mexico, a popular laboratory animal around the globe, and a growing source of tantalizing insights in cancer research. Although it has only partly divulged its many secrets, the axolotl has revealed new clues about its remarkable ability to regrow entire limbs, organs, and even parts of its brain and heart while supplying an expanding list of anticancer peptides and extracts.

The axolotl is normally dark brown in the wild but exists in a wide array of color variants in captivity, including an intentional cross-mating with an albino tiger salamander in 1964 that yielded a nearly translucent (and easier to study) albino axolotl. Joshua Currie, PhD, assistant professor of biology at Wake Forest University in Winston-Salem, North Carolina, says that the unusual salamander species has been a compelling model for studying tissue regeneration. “As a cell biologist, I was really fascinated with this idea that cells sometimes work together in collectives to do pretty extraordinary things, and I think regeneration is one of the most extraordinary things that cells do as a cohort.”

In a recent study of axolotls, zebrafish, and mice, Dr Currie and his colleagues discovered that two highly conserved gene-activating transcription factors appear to play a critical role in regenerating the tips of digits in mice and of entire limbs in axolotls.² The SP6 and SP8 factors, members of the SP family, were previously known to be involved in embryonic development. Researchers found that they were also highly active in the animals’ skin during tissue regeneration. “That was really intriguing: that this core set of transcription factors is in the regenerating skin of all three organisms,” Dr Currie says. “So that was quite cool, and then we went about trying to figure out, are they essential for this process?”

When the team specifically knocked out the SP8 gene in axolotls, they found that its deletion had no apparent impact on the animals’ embryonic development. Conversely, the gene knockout led to significant disruptions in tissue regeneration after an amputation. “That was very cool for us, to find a regeneration-specific role for SP8,” Dr Currie says. Knockouts of the Sp6 or Sp8 genes similarly disrupted digit regrowth in mice, whereas the researchers were able to partially restore that regeneration ability by replacing the lost genes through a gene therapy approach coupled with a regulatory element called an enhancer.

Dr Currie likens the mass of disorganized cells involved in embryogenesis and regeneration to those involved in cancer. The difference, he says, is the orchestration that resolves the cells’ fate—perhaps the role played by SP genes. In fact, multiple studies in axolotls and mice suggest that the morphogenesis accompanying development and regeneration can actually supersede tumorigenesis. “There are differentiation therapies around this idea that cancer is a reapplication of developmental programs, and that potentially you can have some overlying signals that supersede that and tell cancer cells what to do,” he says.

From a clinical application standpoint, Dr Currie says that continued research might reveal how to restart what could be a similar regeneration program in humans. “So that’s being able to tell our own tissues and cells to kickstart regeneration,” he says. After an injury, cells and tissues can work toward a resolution; in humans, that more often takes the form of scar tissue. “But what happens in the axolotl is that there’s some kind of reacquisition of developmental-like programs and fates and that may be why these SP genes, which are involved in development, are part of this orchestration.” Understanding how they also orchestrate regeneration as the wound-resolution program of choice could yield new advances toward future tissue-repair therapies.

Other lines of evidence have linked axolotls to antimicrobial and anticancer compounds. Sarah Strauss, PhD, head of the Kerstin Reimers Laboratory for Regenerative Biology at the Clinic for Plastic, Aesthetic, Hand and Reconstructive Surgery at Hannover Medical School in Germany, has had pet axolotls for more than 30 years. She began her research career by studying human antimicrobial peptides in the gut and their role in wound healing and inflammation before switching directions to study axolotls. “I knew from my pet axolotl that they have a very weak adaptive immune system, and so my thought always was that their innate immunity must be very strong and effective,” she says.

Three of those four peptides also triggered apoptosis, or programmed cell death, in laboratory-grown breast cancer cell lines. The peptides, the researchers found, altered the activity of multiple cancer-linked genes and specifically targeted cancer cells while sparing healthy breast tissue cells; these results point to their early promise as therapeutic candidates. With new funding in hand, Dr Strauss hopes to conduct more detailed testing of the axolotl peptides against other infection-causing bacteria such as Acinetobacter and with minimal inhibitory growth assays that incorporate different temperature and pH conditions. The team will repeat its tests on breast cancer cell lines as well to solidify the initial findings and delve more deeply into the effects on tumor markers and the potential mechanism of action.

Other studies have likewise suggested that axolotl skin and other tissues contain anticancer compounds. In one report, researchers described how a crude extract derived from axolotl limb tissue halted the proliferation of acute myeloid leukemia cells by arresting the cell cycle replication process and prompting the cells to begin differentiating.⁴ Like Dr Currie, the authors suggested a close linkage among the mechanisms governing regeneration, wound healing, development, and cancer formation. “There is increasing evidence highlighting the common signaling pathways which link cancer growth and regeneration whereby dysregulation of the well-balanced and coordinated process of regeneration leads to cancer,” the authors stated.

Despite the new revelations, the question of whether axolotls themselves derive any cancer protection from their own peptides and regeneration-specific cells remains unresolved. Dr Currie occasionally sees melanoma in his colony of roughly 200 adults but has not observed any metastatic cancer. Other researchers have proposed that the animals’ unique molecular and physiological characteristics indeed help them to ward off tumors, whereas Dr Strauss has seen little evidence of such resistance in her own colony, which now consists of roughly 40 adults. In collaboration with an anatomy and pathology laboratory, she has performed micro-computed tomography scans on deceased axolotls and observed a wide range of malignancies, especially in older animals (the oldest in her laboratory lived for 25 years).

In fact, Dr Strauss and a colleague have advised veterinarians on how to perform compassionate-use surgery on axolotl pets with malignant growths. Amputating an affected limb or tail, she says, often resolves the cancer: The regenerated bone and tissue grow back cancer-free. One axolotl with cancer of the spleen even had a successful spleen-removal surgery and lived for a few more years, Dr Strauss recalls. When it died, she and a colleague discovered that the cancer had spread to the animal’s liver. Its fully regenerated spleen, however, remained cancer-free—the result of an extraordinary process that is only now coming to light.