Biologists manipulate electrical fields to regenerate tadpole tails at an unusual stage of development
By Stuart Blackman | February 28, 2007
Electricity can provide the initial spark for the regeneration of damaged animal tissues, according to research reported today (February 28) in the journal Development. Biologists have succeeded in manipulating the electrical fields present in tissues to regenerate the amputated tails of frog tadpoles at a stage of development where such regeneration does not occur naturally.
"This gives us a whole new set of control knobs on cells," said Michael Levin of the Forsyth Institute in Boston, Mass., who led the research. Electrical fields help "control cell identity, cell number, position and movement, which is relevant to everything from embryonic development to regeneration to cancer and almost any biomedical phenomenon you could imagine."
Electrical currents applied to wounds have long been known to enhance regeneration of lost limbs and severed spinal cords in a variety of species from fish to mammals. As part of the current study, Levin's team screened Xenopus laevis tadpoles with various ion-transporter blockers to identify transporters important for tail regeneration. The experiment pinpointed the V-ATPase H+ pump.
Levin and his team found that V-ATPase was expressed in the tail stump six hours after amputation -- much earlier than established markers of regeneration, which, said Levin, do not appear for 24 hours.
The researchers mapped the electrical properties of the wound before and after amputation using voltage-sensitive dyes, and found that the V-ATPase H+ pump polarizes the cells of the regeneration bud after amputation, and creates a long-range electrical field across the bud by pumping protons out of the wound site.
In the absence of these fields, cells necessary for regeneration failed to both proliferate and express downstream genetic markers of regeneration. And neuronal growth -- long held to be an essential precursor to the generation of other tissues, said Levin -- was disrupted.
"What we have here is a master regulator," Levin told The Scientist. "It's a different master regulator to normal tail development, but it activates the same components. And by turning on this one signal, we get the whole program of tail growth."
To demonstrate that it is the electrical signal rather than the V-ATPase gene itself that induces regeneration, the researchers inserted an unrelated yeast hydrogen pump into the damaged tails of tadpoles that were at a stage of development when they are typically unable to regenerate. This resulted in full tail regeneration.
However, in an Email to The Scientist, HHMI investigator Alejandro Sánchez Alvarado, who works on regeneration at the University of Utah Medical School in Salt Lake City, pointed out that there are also considerable limitations of the frog model. Tadpoles are not adult organisms, so can't represent vertebrate regeneration, he said. "And the tails are, in any case, fated to be resorbed during metamorphosis."
Neither is Sánchez Alvarado convinced that the electric fields generated by V-ATPase deserve to be regarded as a master regulator. "Levin has found evidence that bioelectricity plays a significant role in a special case of regeneration," he said. "That is all we can reasonably conclude from this study at this juncture."
Should the findings prove to be more general than the Xenopus system, electrical induction of tissue regeneration might have significant applications, according to Levin. "In a therapeutic context, this really gives the hope that we can activate very complex regeneration programs without having to go in and micro-manage every step," said Levin. Furthermore, in contrast to regenerative techniques involving stem cells or growth factors, for example, where there is a danger of excessive cell proliferation resulting in tumors, manipulating endogenous growth programs using electric fields may induce a natural, self-limiting process. "When regeneration is complete, the process stops," said Levin, who also has an appointment at Harvard University.
Indeed, Richard Borgens, a regenerative biophysicist at Purdue University, called the experiments "very elegant," and suggested researchers may one day control stem cell proliferation using ion pumps to manipulate stem cell polarity. "It could be a very interesting way to restrict their potency," he told The Scientist.
Links within this article
D.S. Adams et al, "H+pump-dependent changes in membrane voltage are an early mechanism necessary and sufficient to induce Xenopus tail regeneration," Development, published online, February 28, 2007.
C.D. McCaig et al, Controlling cell behavior electrically: current views and future potential, Physiol Rev, 85(3):943-78, 2005.
Alejandro Sánchez Alvarado
S. Rothman, "Planarians enter the genomic era," The Scientist, May 2, 2005.
B. Maher, "Peering into Carnegie," The Scientist, February 1, 2007.
I. Weissman and M. Clarke, "Leukemia and cancer stem cells," The Scientist, April 1, 2006.
Clarification (posted March 2): When originally posted, the article said the study authors were based at Harvard University. Two have appointments at Harvard, but all are employees of the Forsyth Institute in Boston, Mass.