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Frog tadpoles have a remarkable ability to regenerate their tissues—a process that requires a lot of energy, fast. In other animals, rapidly growing tissues, such as tumors and embryos, are thought to get this energy from glycolysis—the products of which can be used to build biomass more easily than other forms of ATP production.

So when Andrea Wills, a biologist at the University of Washington, and her team started looking into what fuels tail regeneration in tadpoles, they expected to find that tadpoles shuttle glucose into this metabolic pathway too, Wills says.

Their first experiment seemed to bear this out. After the team snipped the tails off tadpoles of the western clawed frog (Xenopus tropicalis), they observed increased glucose levels in the regenerating cells. “But then we ran into a problem,” says Wills. The researchers’ RNA-seq experiments didn’t show increased gene expression of any of the enzymes involved in glycolysis, except hexokinase, which performs the first step. Levels of lactate, a product of glycolysis, were also normal, and blocking glycolysis had no effect on tail regrowth. “We just kept putting on more of these inhibitors and they didn’t do anything,” she says.

So the researchers started searching for alternative metabolic pathways that consume glucose. They looked for increased expression of other genes and found higher levels of all but one enzyme used in the pentose phosphate pathway (PPP). And blocking the PPP prevented tail regrowth, suggesting that it was this pathway, not glycolysis, fueling regeneration.

     Infographic showing the process of tail regeneration in tadpoles
Tadpole tails regenerate when lost (1). In this study, researchers found that to do so, tadpoles increase the production of genes for proteins that shuttle glucose into the cell (2). Glucose feeds into the pentose phosphate pathway (PPP) (3). The PPP produces two molecules, NADPH and ribonucleotide-5-phosphate (R5P), which are precursors for fatty acids and nucleotides, respectively (4). The cells use fatty acids to build more cell membrane, and nucleotides to build more DNA as cells rapidly divide and increase their numbers. This leads to tissue growth, and eventually, the tadpole has its tail back. WEB | PDF

The PPP makes NADPH, a molecule used to build fatty acids, and ribose-5-phosphate, a nucleotide precursor. Wills says it makes sense that the pathway would be upregulated because “if you’re going to be a proliferative tissue, you will need to make lots of new membrane out of fatty acids, [and] you’re going to need a lot of nucleotides” for DNA.

Caroline Beck, a developmental biologist at the University of Otago in New Zealand who was not involved in the work, says that the authors “have done a good job” presenting a compelling story, but cautions that “these pathways cross-talk with each other all the time, so it’s quite difficult to rule [a pathway] in or out completely.”

“One of the long-term goals,” Wills says, “is to be able to use this knowledge to enhance regenerative outcomes in people,” but she adds that by itself, switching on the PPP probably wouldn’t give cells regenerative capability. “You have to have growth . . . and patterning, and you also have to have inhibition of the scarring response. . . . One of the elements that we’ve been missing is how to enable growth.” The PPP may be critical to enabling that growth.