In the summer of 1986, futuristic magnetic trains and life-size robots drew a teenaged Michael Levin to the Vancouver World’s Expo. But what changed his life was an obscure used book he found on the trip—one that described how animals regenerated lost limbs using bioelectric signals.
Levin’s fascination with bioelectricity has turned into a quest to answer one of life’s longstanding mysteries: How do animals know what shape to take when they grow a head or repair their bodies? Although the genome is thought to be life’s instruction manual, those assembly instructions aren’t in our DNA.
It’s currently impossible to look solely at an animal’s genome and predict what shape or size it’s going to be. “We can say this gene makes that protein, but to understand the shape of a head or an animal’s size…there’s very little understanding of how large-scale shape emerges or is controlled,” says Levin, who is now a biology professor at Tufts University.
To him, the answers might lie in bioelectric signals. An animal cell hoards sodium and chloride ions inside its body and actively ushers potassium ions out, creating a negatively charged interior and a voltage of -40 to -80 mV across its membranes—about 20 times less than a watch battery. Electric currents are created when ions shuttle from one side of the membrane to the other. Our hearts wouldn’t beat without that flow, and our nerves wouldn’t relay signals from brain to limb without it. That transmembrane potential is what keeps us alive.
Researchers overwhelmingly agree that bioelectric currents are essential to nerve and muscle function. But there’s less certainty of what they do in other tissues. Some of the earliest evidence stemmed from studies of growing embryos. In 2002, Levin’s lab demonstrated that in a developing tadpole, a flow of hydrogen and potassium ions helped to establish left and right sides—and the animal’s line of symmetry—as well as correctly position the head and tail. Disrupting this flow randomized the body’s axis, and flipped the locations of the heart, stomach, and other organs.
The notion that electric currents within an embryo could influence maturing cells “caught many people by surprise,” says Alejandro Sánchez Alvarado, a regeneration researcher at the Stowers Institute for Medical Research. “That’s the seed that launched the idea that bioelectricity plays a much broader role in development.”
Researchers now know that bioelectric signals are important to wound healing, spinal cord development, and cell differentiation in a menagerie that includes bacteria, tadpoles, salamanders, and human stem cells.
“The concept that bioelectric signals are important is not so outlandish,” says Jochen Rink, planarian researcher at the Max Planck Institute of Molecular Cell Biology and Genetics. “Collectively, there’s quite a bit of evidence that they may be at work outside the brain as well. The question now is, where do these signals fit in with other cellular mechanisms?”
Preliminary data hint at bioelectricity’s potential. By tweaking cellular electric signals with the right tools, scientists may one day be able to heal wounds or spur lost tissues to regenerate. But the quest to find such breathtaking bioelectricity-based cures is, in part, what led to the field’s historic decline.
A Complicated History
In the mid-19 th century, German physiologist Emil Reymond cut his own finger, then used nearly two miles of wire to record a current of one micro-ampere flowing from the wound. Other scientists reported that electric signals were essential for severed newt limbs to regenerate. These tiny currents are about 10,000 times less than what’s needed to power the blinking LED light on a TV remote. They stem from a voltage that exists across intact skin. Skin cells are constantly shuttling ions in and out of themselves, helping to maintain an electric potential across their membranes. That charge—much like a battery—can be short-circuited by a cut or wound that disrupts the movement of ions.
At about the same time that researchers were discovering cellular electricity, Alessandro Volta invented the first battery. Inventors were quick to couple the two advances together, leading to a flurry of electricity-based therapies. One example was an electric air bath, where a patient was charged with static electricity, then hooked up to a grounded electrode to draw sparks away from specific body parts. A “positive breeze” treated kidney disease, while a negative one was purported to cure insomnia and baldness.
Thanks to such quackery, “this area of study that was initially very promising found itself in decline,” Alvarado says. Unlike other research into electricity—the sort that used metal wires—researchers at the time found it difficult to explain how electricity flowed in animal tissues.
So attempts to understand electric fields in animal bodies dwindled and were forgotten about for a long time. Biochemistry, molecular biology, and genetics flourished as the tools to probe these fields grew more refined. While the study of electrical phenomena in nerves is still booming, electric fields in regeneration or wound healing are now considered a somewhat fringe field.
Spotting bioelectric signals in tissues isn’t easy. They have to be pinpointed in living cells, not preserved samples or cell fractions. “If you freeze a cell, or extract proteins or RNA to examine them, the signal disappears,” Levin says. “That makes it very tough.”
Another issue is finding the right animal model. Many labs use flatworms known as planaria, whose powers of regeneration are almost legendary: a minuscule snip of tissue from the middle of a worm can remake a head, tail, gut, and other organs. But genetic manipulations in some species of planaria are tricky, and—because of their already-immense regenerative abilities—it’s difficult to know if tweaking an electric field can enhance that ability. So Levin’s lab has turned to other model systems such as clawed frogs, chicken embryos, and human stem cells to resolve that question. But do the data extend across the animal world? “I don’t think any of this is species-specific,” Levin says. “These are general biophysical principles that transcend whatever species they’re found in.”
And lastly, bioelectricity research needs to show how these signals sync up with what else we know about biology, especially genetics. In regenerating flatworms, for example, disrupting the way certain genes are expressed can also produce two-headed worms similar to those formed after bioelectric tweaks. Integrating these data requires that bioelectricity first be tested with modern-day tools. “One of my missions was to bring studies of bioelectricity into the molecular age,” Levin says.
Connecting the Dots
Levin’s lab began their studies by using voltage-sensitive fluorescent dyes to see how electrical gradients controlled development in frog and chicken embryos. Researchers typically relied on electrodes for such experiments, either through a hole poked through a cell’s membrane or by measuring around it. But those tests are more invasive, and they can only plumb a few cells at a time—not provide a map of the entire animal. Using the dyes “was incredible, because we could see for the first time in a living animal how voltage gradients were determining which side was left or right, where the head goes, where the tail goes,” Levin recalls.
Then, Levin’s team reached out to other researchers to request plasmids, which are pieces of DNA that can be used to add or remove genes. The team wanted to deploy plasmids bearing genes for ion channel proteins to control how cells shuttled ions. People told him that the cells would die if he messed with the voltage across cell membranes, Levin recalls.
But the cells did more than just survive. The genetic tweaks revealed that the left and right halves of a developing body “talked” to each other via electric gradients transmitted through ion channels known as gap junctions. “Much like how neurons in the brain are connected, there’s a very specific set of electrical connections that occurs in order for the left and right sides of the body to communicate during development,” Levin says. Without those cues, organs such as the heart and stomach get flipped around to the wrong side.
Now, the team uses a suite of methods to spot and measure electric signals, including many that are used in genetics and molecular biology. When used to tweak bioelectric cues, these treatments can cause frogs to grow eyes in their gut tissue, change facial patterns, and drive cells to repair damage to the spinal cord. In planaria, they control what organ forms when a worm is cut in two—a head, tail, or the head of a different species of flatworm.
And other researchers have found electric signals can regulate how human stem cells migrate to repair wounds. Min Zhao, a dermatologist at the University of California, Davis, studies how electrical gradients at wounds—the same currents Reymond detected in his cut finger—direct the process of healing. These gradients guide cells toward injuries that need repair; upending the field makes them migrate away instead. Zhao’s team found that cells sense and respond to these electric fields via genetic signals. Last year, they reported that in regenerating tadpole tails, oxygen-containing chemicals that guide cells to repair wounds also help generate electricity, and electric cues trigger the production of these chemicals. “This links these two separate mechanisms to show how they work together,” Zhao says. “If you lose either one, regeneration slows down.”
A wound-induced electric current—triggered when cells are damaged and ions pour out—starts to flow the instant skin is cut. “It happens faster than any chemical changes,” Zhao points out. “When I first started working on this, I thought the electric field would be just another of hundreds of physical and chemical factors. But when we put all these chemical signals together and compared them, we found that the electric field’s physiological strength overrides the rest.”
Mind the Gap
The possibility that bioelectric signals might act as a sort of master regulator led Levin to test a provocative new idea: that electric pattern memories—not just genes—can encode and permanently alter the shape and size of an adult animal. In a 2017 publication, his team cut flatworms into small fragments and dipped them briefly in a chemical that blocks intercellular ionic currents. Some worms regenerated into two-headed animals. The fleeting exposure made a permanent change; the animals consistently produced double-headed offspring across generations. Even worms that seemed unaffected—with normal shapes, gene expression, tissues, and stem cell distribution—recalled that quick dip. When cut again, these worms generated two-headed offspring. The experiments suggest that the pattern to make a two-headed worm isn’t stored in cells or genes, but in body-wide electrical circuits.
“Electrical tissue memories could be storing all kinds of records of physiological experiences that could show up in future situations,” Levin says. “This is the first time where we actually show what this memory looks like.”
Other researchers remain polarized in their views of these data, in part because it’s still unclear how a temporary tweak to electric signals might affect offspring several generations down the line. Many researchers remain skeptical, including Carrie Adler, who studies planarian regeneration at Cornell University. She says that our current understanding of how currents are transmitted between cells “doesn’t explain the finding that a brief perturbation makes a lasting change.”
But on the other hand, even inhibiting chemical signals for brief windows of time can cause long-lasting changes in tissue healing, says Anna Huttenlocher of the University of Wisconsin-Madison. “So it’s convincing that there are key moments where if you alter bioelectric signaling, you can see a long term effect on regeneration or development because some key switch has occurred,” she says.
Those who remain unconvinced also point out that two-headed flatworms can be formed by disrupting the expression of the genes that help determine body shapes. For double-headedness to persist from generation to generation, there needs to be some altering of the organism’s genetics—or how those genes are expressed. “Electric signaling has not been clearly connected to that pathway yet,” Adler says.
For now, the link between bioelectricity and animal forms remains an intriguing correlation—not a definitive means to heal injuries or lost limbs. The evidence to place bioelectric signals on par with enzymes, genes, or metabolites is still missing. Researchers know how a mutation can alter a gene’s function, or how an enzyme’s botched folds or molecular typos can instigate disease. Although studies suggest that ion channel proteins can transmit bioelectric signals—and potentially carry a bioelectric code—how and where this code is stored and inherited isn’t obvious like it is with proteins and DNA. “At the end of the day, irrespective of any properties that proteins may produce, they’re still codified at some point by the genome,” Alvarado says.
To find the missing links, researchers need to piece together information from studies of genetics, electric signals, embryonic development, and other areas, Alvarado says.
But even what’s currently known about bioelectric signals might be enough. Wound healing is a localized, superficial process, Huttenlocher says, so “you could apply topical drugs to a wound to enhance ion transport and see if this affects wound closure by strengthening electric fields.”
In the long run, learning precisely when and how an electric jolt can alter development or regeneration could spark waves of new therapies—and also establish a definitive role for bioelectricity in the grand scheme of life’s devices.
Image credits: Tufts University, Wendy Beane, Nestor Oviedo, and Junji Morokuma/Levin Lab, Dany S. Adams, AiSun Tseng/Levin Lab, Douglas J. Blackiston/Levin Lab, Rawpixel/iStockphoto