Wnt and the tissue-regeneration symphony

Frogs have the enviable ability to regrow lost digits and limbs. Human princes in the prime of life do not, but human infants can spontaneously regrow lost fingertips.

The subsequent loss of this infantile capacity for regeneration suggests the genetic pathways for regenerating tissues are actively repressed beyond infancy. Which genes are involved, and could science remove the shackles?

Randall Moon, Professor of Pharmacology at the University of Washington School of Medicine, and a Howard Hughes Medical Institute investigator, believes it may eventually be possible to re-awaken dormant tissue-regeneration pathways in humans, to regrow damaged or lost digits and limbs, fix broken hearts, and repair paralysing spinal-cord injuries.

Moon is director of the university's Institute for Stem Cell and Regenerative Medicine. His laboratory uses zebrafish, mice, human embryonic stem cells and the African clawed toad, Xenopus, to investigate the role of the Wnt family of cell-signalling proteins and their receptors in embryogenesis and tissue regeneration - and their involvement in the development of primary and metastatic cancers.

"There are a dozen different Wnt receptors, that couple to two or possibly three distinct signalling pathways," Moon says.

"One pathway is involved in cell polarity and cell movement. The second, better-understood pathway operates through the beta-catenin system to influence cell proliferation and differentiation, in the context of both normal development and in tissue regeneration."

Wnt proteins and their receptors are vital to orderly embryonic development and tissue repair.

They provide positional cues that allow undifferentiated cells to migrate to locations where they are required for growth or repair, then orient themselves in relation to their neighbours and undergo final differentiation into tissue-specific forms.

Moon has shown that Wnt signalling through the beta-catenin pathway begins from the moment a sperm fertilise an egg. Beta-catenin molecules accumulate on the hemisphere of the egg opposite to the point of fertilisation, and the daughter cell formed from the beta-catenin enriched region subsequently gives rise to the embryo's dorsal tissues.

Beta catenin pathway

Moon will describe some of his recent, unpublished results on Wnt signalling and tissue regeneration at ComBio in Brisbane this month.

"Wnt signals are clearly produced in response to injury," he says. "If you're going to regenerate tissues at the site of an injury, and get wound healing, you need cells to proliferate then differentiate into missing structures like bone and skin.

"Wnt is involved in the cell proliferation response and in differentiation via the beta-catenin pathway. We've shown that if we block Wnt signalling, we block regeneration.

"Conversely, if we use various tricks to increase Wnt signalling, we can enhance proliferation and actually enhance regeneration, at least in zebrafish.

"It has not escaped our attention that repression of Wnt signalling may in part explain why humans are unable to regenerate lost limbs and digits. It's conceivable that this repression involves too much non-canonical Wnt signalling since in zebrafish this signalling blocks regeneration.

"So if we're interested in enhancing regeneration, we might be able to do it by turning on beta catenin signalling in the region of the injury, or suppressing the antagonising signalling pathway."

Moon says it is important to recognise that while Wnt signalling was essential, complementary pathways were involved.

For example, Dr Mark Keating of the Howard Hughes Medical Institute at the Children's Hospital in Boston, had shown that fibroblast growth factor (FGF) is also essential for tissue regeneration and repair.

"We're looking at a scenario where multiple secreted signals work together to influence cell proliferation and differentiation," Moon says. "If we want to understand regeneration, we need to understand these mechanisms, and to find out what each player is doing in the tissue-regeneration symphony.

"Of course, it may not be important to identify every player. The alternative view is, 'I don't really care how it works, as long as it works'."

CNS response

Other laboratories had shown that if beta-catenin signalling is enhanced in the mouse brain, the animals end up with an over-sized brain. "We have unpublished data showing that Wnt signalling is turned on in the mouse brain in response to traumatic injury, which suggests that Wnt plays a role in the response to injury of the central nervous system.

"One of the nice things about these signalling pathways is that there are a finite number of them, and the same systems are used again and again, in different contexts, with different kinds of cells.

"We've shown, for example, that Wnt signalling also operates in heart muscle.

Working with colleague Dr Charles Murry, Moon has shown that Wnt signals operating through the beta-catenin pathway convert human embryonic stem cells into cardiomyocytes.

"Wnt signalling is important not only in processes such as embryonic development in all species, from flies to hydra to mice , to fish frogs and humans, myriad Wnt pathways that work in a very complex way are operating constantly in the body.

"They are turned on and off in very specific places and times - they direct differentiation and cell polarity. The same processes are employed in the regenerative response.

"Our challenge is to learn enough about how this works during embryonic development, and in species like frogs, that can regenerate organs and limbs."