Modern medicine clutches at a number of dreams. Some, like developing an AIDS vaccine, can seem tantalisingly close. Others, like curing cancer, have frustrated so many minds for so many years that we’ve learned to temper our expectations.
Then there’s regeneration.
A future in which humans regrow lost or diseased body parts feels like a mirage. But why? After all, many species can accomplish the task with ease. A decapitated flatworm, for example, will grow a new head, replete with a new brain. For the first week of their lives, tadpoles can replace lost tails. And the axolotl has the ability to regenerate everything from its limbs and tail to its spinal cord, all without any evidence of scarring.
Humans have a sliver of regenerative capacity, too. The Greek legend of Prometheus, the god who was cursed to have an eagle peck out his liver each day, only to grow it back overnight, actually contains a grain of physiological truth: if you were to lose part of your liver, it would, in fact, repair itself. With the exception of our skin, it’s the only human organ that can do this.
Regenerating a small body part under special circumstances is one thing, but what if we could regrow entire lost limbs or regenerate a lost eye? Michael Levin doesn’t think this is an outlandish fantasy. In fact, he thinks he may be on the path to figuring out how to do precisely that.
Levin is director of Tufts University’s Centre for Regenerative and Developmental Biology in Medford, near Boston. He’s a 47 year old Russian émigré who looks like a geeked-out Gen-Xer: His smooth hair is parted far to the side; a neat geometric beard frames his face; and he’s most comfortable in a college uniform of T-shirts over long sleeves.
His life, however, is marked by a willingness to embark on undertakings that many would consider arduous in the extreme. He possesses exceptional focus and personal conviction. These traits are common among successful scientists, and essential for any who want to pursue a left-field line of enquiry.
Levin thinks that the key to regeneration – the key to pattern, to shape – may be found in the electrical signals that are transmitted among all our cells. Manipulating these signals has already allowed Levin to produce results more suited to an X-Men comic book than a scientific journal, including the creation of four-headed flatworms and tadpoles with eyes on their bellies.
Levin’s work is little known, perhaps because so many scientists believe that the key to human regeneration – if such a thing exists – lies in studies of genetics and stem cells. Such studies have produced incredible results: a patient’s windpipe repaired in a lab; a segment of functional bladder fashioned on an artificial lattice. These achievements offer the hope that a patient will one day be able to grow a new organ from their own cells, instead of waiting for someone else’s misfortune to be their good luck.
But none of these accomplishments involve recreating the complex organs and limbs that our bodies produce naturally. Scientists can’t command the cells at an amputated elbow to generate a limb replete with muscles, tendons, bone, cartilage and blood vessels. No one has found the signal that gives the command: “Become an arm.”
At least not yet.
One night in the late 1700s, Luigi Galvani, an anatomy professor at the University of Bologna, Italy, stood on his balcony and lined up a string of butchered frog legs. This in itself was not unusual – they were, in all likelihood, awaiting the dinner plate. But on this night the air was crackling with electricity from a storm, and Galvani noticed something odd: when he touched the legs with a pair of scissors, they twitched.
The professor’s curiosity was piqued. Soon after, he laid out some dissected frog legs in his laboratory – where, as it happened, he also kept a newfangled machine that stored static electricity. Any time the machine was on and someone touched the legs with a metal scalpel, they jumped.
Galvani wondered if the limbs contained some sort of charge, an “animal electricity” essential for life. He thought it might come in the form of an undiscovered biological juice, and, while he was wrong about that, he had become perhaps the first person to purposefully stimulate exposed nerve cells with electricity.
In the years that followed, Giovanni Aldini, Galvani’s nephew and former assistant, took the experiments further. In 1802, Aldini connected a primitive battery to a recently severed ox head. It was as if the animal had come back to life: its eyes flew open; its ears wriggled; its tongue jerked. He attempted a similar experiment on the corpse of a murderer who’d been hanged in London’s infamous Newgate Prison. The effects were much the same, Aldini later wrote: “The jaw began to quiver, the adjoining muscles were horrible contorted, and the left eye actually opened.”
These ghoulish experiments were well known in scientific and popular circles – Mary Shelley used the notion that electricity could animate matter as the foundation for Frankenstein – and interest in the effects of electricity on living creatures continued for the next 150 years. Many efforts, including using electricity to treat hysteria and melancholia, amounted to little more than quackery. Fringe thinking may have helped sideline the study of bioelectricity.
In the end though, it was overtaken by a rival branch of science that seemed to hold the key to all biology’s secrets. When the structure of DNA was discovered in the 1950s, the search for the commands that shape our bodies became an investigation into the extraordinary interplay between genes and proteins. The studies that descended from Galvani were all but swept away.
Today, we understand the importance of electricity when it comes to, for instance, the pulses of the nervous system and the beating of the heart. In many ways, though, we remain stuck in an 18th‑century mind frame, aware of the electric signals that course through us but oblivious to the ways in which they could play a subtler and more profound role in our development.
Reviving bioelectricity would require someone who knew little about the frivolous side of bioelectrical research and who wasn’t concerned about how his interests would appear to colleagues.
In high school, Levin came across a book called The Body Electric, which, while including a number of eccentricities such as linking electromagnetic fields to conditions such as AIDS and Lyme disease, described genuine scientific research from the 1900s on the role electricity plays in sparking the regrowth of animal limbs. At Tufts University, Levin chased down all the studies referenced in The Body Electric, and then every other paper referenced in those studies.
In 1992, Levin graduated from Tufts with a dual degree in computer science and biology, and enrolled in a PhD in genetics at Harvard Medical School. There he’d eventually work with Cliff Tabin, who was known for his work on embryonic development. In Tabin’s lab, Levin identified the genetic pathway that determines how cells in a chick embryo know to put the heart on the left side of the body. Later, he discovered a chemical that enables the two sides of the body to communicate and determine which side will develop into which. (Nature would call this work a “key breakthrough” and a “milestone” in the past century of developmental biology.)
As it happened, the movement of that chemical reminded Levin of the movement of a charge in a battery-driven circuit, which prompted him to wonder whether there was a bioelectric signal that set the whole process in motion. It turned out there was. Tabin says that, to this day, he doesn’t believe anyone other than Levin would have thought to look for this connection.
Levin noticed that the Forsyth Institute, an independent organisation affiliated with the Harvard School of Dental Medicine, had an opening for a developmental biologist. He was not only offered the job, but told he could pursue any line of investigation he wanted, so long as he could drum up funding.
By the time Levin was 31 years old, he’d formulated the questions that would shape his life’s work. What is the role of the body’s intrinsic electric signals? How do they interact with genes and proteins? How do they affect the development of an embryo? To find the answers, Levin picked up a thread that led back to the work of the bioelectricity pioneers he’d been reading about from 60 years earlier: he set about trying to control regeneration in animals that already had some regenerative abilities.
He began with the African clawed frog, or Xenopus laevis. A Xenopus tadpole can grow back its tail, provided it’s lost during the first seven days of its life. But from day eight – right around the time the tadpole begins to metamorphose into a frog – it begins to lose that capacity, and at 10 days the ability has gone completely. Levin thought that the regeneration signal – the thing that instructs a tadpole’s cells to form a replacement tail – was what stopped functioning during the transition period. If that signal was an electric one, as he suspected, and if he could figure out what it looked like, perhaps he could work out how to turn it back on in tadpoles that had entered the non-regenerating stage.
Levin’s intuition told him to start studying the electrical properties of cells. Cell walls are dotted with pumps and channels that pull charged atoms – calcium, potassium, sodium – into the cell and spit them back out, creating a voltage change across the cell. He thought controlling regeneration might depend on controlling the flow of charge across the cell walls.
Levin decided to work backwards, to figure out if he could stop regeneration in tadpoles by switching off a particular channel. It was not an easy task. First he had to compile a list of the hundreds of drugs that act on channels, a painstaking project that took months to complete – there are drugs that block a single channel, and drugs that take out an entire family of channels. There was also the risk of side effects. When Levin started experimenting with drugs from his list, colleagues told him he was wasting his time: surely closing off an ion channel in a days-old tadpole – a channel that is present in cells throughout the body – would kill the tiny creature.
The tadpoles didn’t die, and within a few months Levin had narrowed in on concanamycin, a drug that disables a pump that propels hydrogen ions across cell walls. Dosed with the drug, tadpoles that should have been able to grow back their tails were no longer able to.
In the autumn of 2005, Levin took a tadpole that was too old to regenerate a tail, amputated its tail, and added a cell wall pump that he hoped would signal to the tadpole that it was still young enough to grow a new tail.
It worked. The tadpole grew its entire tail back.
Levin does not lack self-confidence. Even so, he was astounded at his experiment’s success. “Boom!” he says, describing the perfect tail that was produced. “That was the most amazing thing. Without tweaking the specifics of it, you get a perfectly normal tail that knows when to stop.”
The boxy four-storey building that houses the Tufts Centre for Regenerative and Developmental Biology is tucked behind a parking lot in Medford, Massachusetts. Levin’s lab has been housed on the top floor here ever since his alma mater lured him from Forsyth in 2008. Large posters of scientific journal covers line a hallway; one of them features a crimson-hued four-headed flatworm. Levin’s team has figured out how to manipulate these centimetre-long creatures so that they grow in a variety of ways – a head at both ends, or no heads at all. The “monsters”, as one of Levin’s post-docs affectionately calls them, live in plastic containers filled with Poland Spring water from Maine – the only water in which the worms thrive and regenerate properly, and the same water that’s available in the lab’s snack room – and are fed 100% organic beef liver. Examined under a microscope, the mottled beige and brown worms twist and twirl their cartoonish bodies into knots, with triangular heads peeking out every which way. They recall the world of Doctor Doolittle, these skinny “pushmi-pullyus” with a head at either end of their bodies.
To get a tadpole to regrow a tail when it shouldn’t have been able to, Levin had to unlock one part of the regeneration code through a laborious process of trial and error, testing various drugs until he identified one he believed might work. But if animals do, in fact, store information about their shape in electrical signals, Levin will need to figure out how to actually read those signals – and that will require deciphering a whole new language.
A clue to how this might work came from his colleague and collaborator Dany Adams, an associate professor in his lab. By filming a growing embryo that had been treated with voltage‑ sensitive dyes, she discovered that electric patterns appeared before the corresponding facial features did. Adams believes that the flashes in her light show represent a series of switches being flicked on at different voltages. When a cell reaches a certain voltage, she says, the activity of the cell’s genes changes, triggering a chain of events that lead to the formation of an eye or a nose or a mouth. The signal is transmitted from cell to cell and may also shape the process as it unfolds: the voltage that means “eye” might tell cells to start differentiating into a lens, a cornea, a retina, all while simultaneously shaping the eye’s overall organisation. By revealing that specific voltages trigger the growth of specific organs, Adams’ video allowed Levin’s lab to decipher a few more grooves on the Rosetta Stone of electrical signalling.
Levin’s team soon made use of this discovery. Against a wall of the room where Galvani’s drawings hang there are nearly a dozen rectangular Tupperware containers, stacked on metal shelving, each housing tadpoles of various ages. The tadpoles inside stretch to an inch long, their tails ending in transparent wisps that twitch to and fro. Buds that will eventually become legs push out of their abdomen. And, in one container, there are tadpoles that have a tiny round black stain where body melts into tail.
The tiny black dot, almost a perfectly round birthmark, is what Levin refers to as an ectopic eye – literally, one occurring outside of its normal place. This eye is growing on the tadpole’s abdomen. It functions, albeit not fully, and is connected to the brain by a thin highway of nerves. It was prompted to form there, and on other parts of the animal’s body, by changing the voltage on nearby cells to one that signifies “eye”.
These tiny, mutant tadpoles represent the first time anyone has succeeded in prompting a working eye to grow anywhere but the head.
Levin’s tadpole work had produced stunning results, but tadpoles can already regenerate tails; he was simply able to endow older tadpoles with the ability they’d lost. Would the same drug produce the identical result for frogs’ legs? It was a slow process. Legs, unlike tails, can take half a year to regrow. Eventually, that, too, worked: in a photo, tiny digits – frog toes – push out of the skin, the final stage of frog limb regeneration. It was another significant step in decrypting the electrical code of regeneration.
Levin’s word is lauded by many, but a surprising number of regenerative medicine researchers remain unfamiliar with his efforts. When peers do voice scepticism, they tend to focus on the question of whether he’ll be able to replicate achievements such as leg regeneration in humans. “Mike’s work is fascinating, and potentially very important,” says David Mooney, a Harvard bioengineer. “How much of his data and approaches apply to mammals is an open question.”
Levin’s lab has continued to achieve surprising results. In 2016, he and his colleagues demonstrated that electric information can override the genome. Normally, when a flatworm loses a head, the exact same head regrows, with the appropriate structure, brain size, and stem cell distribution. But when Levin tweaked the bioelectric network of a decapitated flatworm, the head that grew back had a different size, brain shape, and stem cell distribution. It resembled that of a different species of flatworm entirely. “This is a kind of stunning thing,” said Levin. “Big-picture wise, it tells us that, if we are to understand evolution, where does the body plan actually come from, what determines that – it’s not just genetics.”
As a result of Levin’s work, his lab received US$10 million from Microsoft co-founder Paul Allen through his fund for out-of-the-box, high‑risk research to solve some of the biggest, unsolved questions in bioscience. Another $10 million is on offer in four years’ time if they can find other funders to match it. Levin and his colleagues know that they face significant hurdles in their research into regeneration, just as they know that even if they eventually find success with mammals such as mice, there’s no guarantee that they’ll be able to replicate the feat in humans.
But Levin insists he is undaunted. “I read tons of science fiction as a kid,” he says. “But I think the reality is, I don’t know too many works of science fiction that are as wild and out there as things that have been discovered by real science.”