When Tufts University developmental biologist Michael Levin proposed tweaking cells’ electrical signals to create new patterns of growth, he encountered some resistance. "People thought it was nuts," Levin says.
That's because although all cells have electric potentials (defined as the amount of energy required to move a given electrical unit against an electrical field), and these potentials clearly relate to cellular properties—the assumption in most cases was that the electric potential related primarily to cellular maintenance or “housekeeping.” Disrupting a cell’s electrical potential, the conventional wisdom went, would kill it.
Yet for more than a decade Levin's work has countered this idea. He has manipulated the electrical potentials of cells in various ways to produce a menagerie of strange beasts: tadpoles with eyes on their tails or within their guts and frogs sprouting toes at the site of an amputated leg.
In fact, Levin believes he has found a new role for the bioelectricity of cells. He posits that the pattern of cellular voltages creates a system of electric signals that direct how the body grows. He calls these signals the bioelectric code and believes they are fundamentally as important in understanding growth and development as the genes in the body or the various chemical switches that turn them on and off. Indeed, he thinks that changes in electric potentials across cells can also serve as a so-called epigenetic switch to regulate how genes function.
Although Levin may have coined the phrase “bioelectric code,” the belief that electric signals relate to patterns of growth is not a new concept. "The idea goes back a long way," says regenerative biologist David Stocum of Indiana University–Purdue University Indianapolis. "[Levin]'s taken it to a much higher art,” Stocum says, by actually looking at cellular potentials with specialized dyes.
Some of the earliest investigations go back nearly one hundred years. In the early 20th century Yale University biologist Harold Burr placed various organisms in a voltmeter to study their electric potentials and suggested there was a link between shape and electrical properties. Then in the 1970s Lionel Jaffe, a biologist at the Marine Biological Laboratory in Woods Hole, Mass., used a probe to study electrical currents in and around cells. He noted differences in the electrical properties of creatures that could regenerate, such as salamanders, and those that could not, such as adult frogs. But much of this bioelectric research would be forgotten in subsequent decades in the rush toward molecular biology and genetics.
In recent years, researchers at the University of Aberdeen in Scotland have been examining how electric fields guide the growth of tissues during healing. But Levin's approach is the first to look at electric potentials on the level of individual cells and how they can be incorporated into our knowledge of molecular biology.
All cells have an electric potential that comes from the difference between charged atoms and molecules, or ions, on either side of the cell's membrane. Highly malleable cells, such as stem cells, which have the ability to grow into other cell types as well as tumor cells (which are characterized by abnormal and uncontrolled growth) have low electric potentials whereas mature and stable cells have high potentials.
Levin reasoned that if you could alter a cell's potential you can change how it would grow. And by changing the electric potential of many cells, he hypothesized that he could trigger the growth of a specific structure. Levin sees these patterns of electrical activity as a form of cellular communication, signaling when and how to grow.
To test his hypothesis, Levin has co-opted tools from neuroscience and molecular biology. By inserting new genetic material or compounds into cells, for example, he has found he can manipulate their electric potentials. For example, an injection of the appropriate genetic information leads to the creation of new pumps and channels in the cell membrane that allow ions to cross whereas certain pharmaceutical compounds can facilitate the flow of ions in and out of the cell. More recently his lab has begun publishing work that incorporates opto-genetics, which involves genes that contain the hereditary instructions for making light-sensitive proteins, allowing the researchers to control cellular changes with the flick of a lightswitch.
The approach is simple conceptually—help ions pass in or out by encouraging or constricting flow—but in practice it is more complex, requiring intricate calculations to determine what changes will produce the desired charge within and outside of the cell. "You have to think about the whole mathematics of all the pumps and channels present, and the medium inside and outside of the cell," Levin says.
In the January issue of Development Levin and colleagues describe how they identified the pattern of cellular voltages responsible for growing a frog's eyes. Tweaking those voltages during early development caused them to be malformed. Mimicking this pattern of voltages with other cell clusters in the body induced the growth of eyes in those locations, creating frogs with eyes on their tails or backs.
The work is a proof of concept with implications for regenerative biology. Levin believes one could take any cluster of cells in the body—including mature and fully differentiated cells—and override existing chemical and molecular signals by changing electric potentials. The signals would then direct growth into any shape desired, such as a new nose or eyes as well as manipulate them to repair a lost limb or correct birth defects.
What's most impressive about the study is not the outcome—scientists investigating regenerative medicine have created similar strange creatures for decades with grafts and chemical interventions—but the approach. It's possible that the electric signals serve as a master switch, meaning researchers don't need to know about the subsequent interplay of chemical and molecular signals involved in creating a new structure.
This is not to say genetics and epigenetics are not important. In fact, Levin points out that these signals are all cyclically linked and interdependent. "Genetics determines the cell's channels, for example, which in turn determines the gradient," he says. "And the bioelectric gradient can change gene expression." Precisely how these three sets of signals are intertwined in nature, however, remains unclear.
Levin's lab has also demonstrated how observing patterns of electrical potential can be used to recognize abnormal growths—a finding they believe can have important implications in cancer research. In a study to be published in the May issue of Disease Models & Mechanisms, the team details how it identified electrical signals associated with tumor formation. Levin and graduate student Brook Chernet noticed a bioelectric signature associated with tumor-like structures, which offers a novel approach to spotting cancer. In addition, they even had some success in raising the typically low electrical potentials of these cells to prevent tumor development.
But many challenges remain in understanding the possibilities of Levin's bioelectric code. Levin, for example, believes far more needs to be understood about cellular physiology, and hopes that with more data and tools he and his colleagues can begin creating a more systematic understanding of how sets of bioelectric signals relate to specific growth patterns.
It also remains to be seen how well this work translates to other animals. University of Dayton regenerative biologist Panagiotis Tsonis points out that although mature frogs do not typically regenerate limbs, amphibians generally are more gifted in regeneration than mammals and less prone to cancers. "I would like to see this work extended to animals like mice," Tsonis says. "If that works, that would be fantastic."