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[Steven DC]

Muscling in on limb regeneration

Researchers pin down a pathway coming between mammals and the ability to regenerate tissue salamander-style.

Alla Katsnelson

Discovering how salamanders and newts regrow limbs and tails could pave the way to regenerating damaged human tissue. Inaki Relanzon/naturepl.com

Newts and salamanders have brilliant personal-injury insurance: lose a tail or leg, and another grows in its place, good as new. Now, researchers in the United States say that they've found a way to recreate this ability in mice, opening the door to the possibility of regenerating damaged tissue in humans.

Compared with these lower vertebrates' ability to regrow missing limbs, humans and other mammals "are pretty pathetic", says Helen Blau, a regenerative biologist at Stanford University School of Medicine in Stanford, California, and the study's lead author1. "We can regenerate our livers, but not much else," she says.

So the burning question for researchers has been: what do these amphibians have that we lack?

Some have attributed the regenerative potential, in part, to stem cells that remain in adult tissue — but there don't seem to be enough of them in newts to get the job done. Instead, most biologists believe that, in vertebrates endowed with regenerative ability, muscle cells surrounding injured tissue temporarily regress to a more primitive state, re-entering the cell cycle and then proliferating to produce more muscle cells.

Past studies have identified a protein called retinoblastoma protein (Rb) as a key factor in getting muscle cells to differentiate, or specialize. Suppression of the Rb gene in newt muscle cells sends the cells back into the cell cycle, but this doesn't work in mammalian muscle cells.

Blau and her colleagues proposed that, in mammals, an additional mechanism may have evolved atop the Rb pathway to confer tumour suppression. Unlocking regeneration in mammalian tissues may involve interfering with that pathway too.

The researchers homed in on a tumour-suppressor gene called Arf that is present in mammals but not in regenerating vertebrates. Using a gene-silencing technique called RNA interference to temporarily knock down both Arf and Rb in cultured mouse muscle cells, they found that the treated cells re-entered the cell cycle and began proliferating.

When the genes' activities were restored, the cells returned to their differentiated state. Newly generated muscle cells transplanted into living mice were able to integrate into the animals' muscle tissue.
Playing with fire

"There's no question we're playing with fire in knocking down tumour suppression," says co-author Jason Pomerantz, a reconstructive surgeon currently at the University of California, San Francisco. The fear, he says, is that cells not protected by tumour suppressors can start to grow uncontrollably. But follow-up experiments suggested that temporarily suppressing the genes did not lead to tumours.

He and Blau believe that tissue could be treated as an explant — grown outside the body — and then implanted, or that drugs blocking the two genes could be injected directly into a spot at which regeneration is needed. "The heart is really where the bar would be set," says Pomerantz. "There is no regeneration in the mammalian heart, and no bona fide stem cells that have been described that cause regeneration in adults."

Ken Poss, a cell biologist at Duke University Medical Center in Durham, North Carolina, cautions that several unknowns remain in determining whether the technique will aid regeneration.

First, Poss notes, it's still unclear whether de-differentiation is the main trick that animals such as newts rely on for tissue regeneration. Researchers aren't sure how much of the effect can be attributed to muscle stem cells called satellite cells. Furthermore, he says, regenerating large chunks of tissue may involve recreating the connective-tissue scaffolding on which muscle cells grow — a step that's not part of this technique.

The findings are "very suggestive", says David Stocum, a regenerative biologist at Indiana University in Indianapolis, "but there's a lot that needs to happen to nail down the experiment".

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For one thing, he notes, the muscle-cell culture model used in the experiment may not sufficiently resemble living tissue. "I would like to see this kind of thing repeated on fully differentiated muscle explants," Stocum says. Moreover, he says, he would like to be sure that adding mammalian Arf to newt muscle cells prevents the cells from de-differentiating as it does in mice.

So far, the researchers have not demonstrated that the tissue they transplanted is functional — that, says Blau, is a crucial next step.

"It will be exciting to find out whether the capacity or speed of regeneration will be changed" by the strategy the researchers propose, says Poss.

Bron: www.nature.com