Scooped from a fountain in Spain, a colony of planarians help scientists unravel the rules of regeneration
In a Petri dish filled with water, about ten juvenile planaria float. The tiny flatworms are decidedly underwhelming. Their tiny gray forms morph from stationary circular blobs to gliding oval blobs depending on how active they are. They’re not particularly hardy either. “They’re like the consistency of wet tissue paper,” says Katherine Browder, a graduate student who manages operations in the lab run by Phil Newmark and Melanie Issigonis.
Even their mating is understated. In a humming room in the basement, Browder scans one of the many tanks teeming with planarians. This one contains adults, the largest about the length of a macaroni noodle. She is hoping to catch a pair in the act. “Oh, there they are,” she exclaims. “They’re just high fiving each other from the back.”
But the drab appearance of these worms belies what’s inside: a treasure trove of amazing biological capabilities. They can regenerate their tails, but also their heads. They can repopulate their germ cells. Some can switch from asexual reproduction to sexual reproduction, or vice versa. The list of fascinating oddities goes on and on and on.
It was this long list of unique capabilities that drew Phil Newmark to the flatworms when he was in graduate school in the early 1990s. He spent the next two decades working tirelessly to push planarian research into the age of molecular biology, overcoming failed experiments, colony collapses, and the skepticism of many of his colleagues. The plan worked. When Newmark began, just a handful of labs studied planarians (and none in the U.S.). Today, there are dozens. “It’s become a pretty popular model for trying to understand stem cell-based regeneration,” he says.
Now Newmark, part of the regenerative biology research focus at the Morgridge Institute for Research, uses these tools he helped develop to study what first interested him: planarian stem cells and their amazing ability to regenerate. But his planarian research has also opened a new avenue of exploration into parasitic flatworms, which share many similarities with their benign, free-swimming cousins. A better understanding of how parasitic flatworms infect their hosts and reproduce inside could help them develop new strategies to control the parasites.
“I just thought, I can totally see studying planaria for the rest of my life.”
Phil Newmark
Warming to worms
Newmark first became interested in planarians as a graduate student at the University of Colorado. At the time, he was studying fruit flies. But he was also a member of the developmental biology journal club, which had a rule that each member had to present a paper outside of their area of research. Newmark stumbled across a paper on planarian regeneration. A team of researchers from Barcelona had taken planarians, irradiated them to eliminate their ability to regenerate, and then transplanted stem cells, called neoblasts, from healthy planarians to restore their regenerative capabilities. But here is what blew Newmark’s mind: They demonstrated that they could introduce neoblasts from a planarian that reproduces sexually into a planarian that reproduces asexually and convert it to sexual reproduction. “I thought that was probably one of the coolest things I’ve ever seen,” he says.
He presented the paper and then dove into the literature. One paper led to another and Newmark became more and more fascinated by planarian biology and the possibility of applying new molecular biology techniques to studying these worms. “I can totally see studying this for the rest of my life,” he remembers thinking.
At the time there were no planarian labs in North America. So Newmark joined the lab of Jaume Baguñà at the University of Barcelona to study them. When he returned to the U.S., he packed the worms in thermoses and brought them with him to Alejandro Sánchez Alvarado’s lab at the Carnegie Institution for Science in Baltimore, Md. There, he painstakingly built up a colony. When that colony collapsed because of a mishap with a water filter, he and Sánchez Alvarado flew to Barcelona, collected more from the fountain where he had found the original planarians, and flew them back to Baltimore again.
Together, Newmark and Sánchez Alvarado developed the tools they needed to study planarian biology. “Back then it was not even an emerging model,” Newmark says. “We were just getting everything going.” Today, he is using those tools to investigate some of the most perplexing questions related to planarian biology.
Fateful decision
Melanie Issigonis, who joined the lab in 2011 when Newmark was still at the University of Illinois in Urbana-Champaign, has been focused on a single overarching question: how do cells decide whether to become reproductive cells or the building blocks for the rest of the body? “It’s just really cool fundamental biology,” she says.
All species that reproduce sexually have essentially two main cell types. Germ cells give rise to sperm and eggs. Somatic cells form everything else. This process typically happens during embryogenesis, but in planarians, it also happens during regeneration. “If you chop the head right above where any of the reproductive tissues are, you get these little head fragments that don’t have any germ cells at all,” Issigonis says. But over time the worm will regenerate everything, including germ cells. “It will have all of its reproductive organs again,” she says. “So we know that it’s going from having only somatic tissues to being able to regenerate de novo germ cells.” That’s a rare capability in the animal kingdom.
But how the new cells decide whether to become somatic cells or germ cells is still mostly a mystery. “That’s a big fate choice,” Issigonis says.
Planarians have a number of other reproduction-related quirks as well. The worms are hermaphrodites, so each organism contains both eggs and sperm. In addition, they lay unusual eggs. Most eggs have a yolk on the inside, but planarians have specialized organs that produce yolks on the outside of their eggs.
The team recently reported that the worms’ yolk cells and germ cells share a gene that seems to play a key role in the function of both. When they knocked down expression of the gene, germ cells lost their ability to make sperm and eggs and yolk cell production plummeted. As a result, the planarians couldn’t reproduce.
Issigonis is still unraveling the secrets of planarian reproduction, but it’s clear that what she finds will have implications beyond planarians. For example, all animals specify which cells will become germ cells. “That’s a fundamental thing that every animal does,” Newmark says. “It’s relevant to how animals make germ cells across evolution.”
A Lucky Fluke
As Newmark and his colleagues began sequencing planarian genes, they found many that were shared with the worms’ parasitic cousins, the blood flukes. Blood flukes, or schistosomes, are a massive public health issue, affecting more than 200 million people worldwide. “We get infected by this thing that swims out of the snail,” Newmark says. They burrow through the skin within minutes, and then enter the bloodstream, causing, in some case, massive damage.
In 2009, one of Newmark’s postdoctoral fellows, Jim Collins, started characterizing some of the chemical messengers in the brain that are in involved in reproduction. One of these molecules, called neuropeptides, seemed to regulate the maturation of the reproductive system. When the team blocked expression of that gene, called npy8, in adult planarians, the worms’ reproductive systems regressed to a juvenile state. “Jim had identified this peptide hormone that basically, when you knocked it down, made a flatworm reproductive system go away,” Newmark says.
Collins realized the implications right away. Blood flukes are flatworms too. He found the same neuropeptide gene in the schistosome genome. If he could inhibit that neuropeptide, he might be able to stop the flukes from producing eggs and prevent the worst effects of blood fluke infection. “He came into my office and said, I think we should look at schistosomes,” Newmark recalls.
Newmark always hoped that a parasitologist would see his lab’s work, notice the similarities between planarians and parasitic flatworms, and then reach out to collaborate. But that never happened. So in 2009, he and Collins went to visit the main resource center for schistosomes in Rockville, Md. to learn how to culture the animals and maintain their wildly complex lifecycle. Schistosomes need freshwater snails as hosts in order to produce millions of cercariae, tiny fork-tailed creatures that swim in the water until they find a mammal to serve as a host.
While the pair visited the lab, then-director Fred Lewis cautioned them to make sure that the water used to house the infected snails was free of rotifers, tiny organisms that sometimes live on the snails’ shells. He and his colleagues had discovered in the 1980s that rotifers can release a chemical into the water the paralyzes the infectious larvae.
This caught Newark’s attention. Could he and his colleagues identify this chemical and leverage it to prevent schistosomiasis?
Stopping schistosomiasis
Cercariae, the free-swimming form of schistosomes, aren’t visible to the naked eye, but under the microscope they thrash wildly. “Their one goal in life is to seek out a host,” says Ian Donovan, a research specialist in Newmark’s lab. “They have about 24 hours of energy reserves before they perish.” The thrashing serves two purposes. It propels them through the water in search of a host, but it also helps them wiggle their way through the skin once they find a suitable mammal. “So in the lab, that would be a mouse. In real life, it could be a human walking through, say, a rice paddy that might be flooded,” Donovan says.
In the 1980s, Lewis and his collaborator Peg Stirewalt noticed that rotifers produced a factor that paralyzed cercariae. But the pair ran out of funding before they could uncover the identity of the mysterious substance. Lewis shared his unpublished observations with Newmark and his colleagues, and once he had established schistosomes in the lab, the team picked up the quest.
Newmark grew rotifers and confirmed what Lewis and Stirewalt had observed: the rotifers were producing something that stopped the wiggling. They then collaborated with chemist Jonathan Sweedler’s lab at University of Illinois Urbana-Champaign to isolate the compound. They named it schistosome paralysis factor or SPF for short. “It’s a pretty odd-looking molecule for something that’s secreted by an animal. The closest known analogs are actually in bacteria and plants and fungi,” Donovan says.
The team also showed that SPF could prevent infection in mice. “We’d like to come up with ways to prevent an infection in the first place,” Newmark says.
The next step will require a bit more work though. In order to make SPF a useful preventative, they need to have enough of it to run tests. But so far, no one has had any luck synthesizing the compound. “We actually have a company that’s still working on it. They’ve been working on it for years,” Newmark says. “We’re their best customer,” Issigonis adds.
In the lab, Donovan is focused on uncovering how the rotifers make the compound. If he could identify the enzymes responsible, the team might be able to engineer yeast or bacteria to produce it.
In the meantime, there are enough questions about flatworm biology to keep the lab busy for decades. But Newmark finds the many questions exciting rather than daunting. Now he and his colleagues have cutting-edge tools to answer them. “Back when the only tool was a razor blade, you do this experiment, and you throw your hands up in the air,” he says. But now his team can dive deep into planarian biology and investigate the mechanisms that underlie their amazing capabilities.
On the wall behind Newmark’s desk, there’s an image of the planarian nervous system in all its vast complexity. The team has identified a few of the neuropeptides they produce, but they’ve just scratched the surface. There are so many types of neurons, each producing a different suite of neuropeptides, Issigonis muses. Planarians might seem like a “humble unassuming looking critter on the outside,” she says. But just look at that brain. “They’re quite complex,” she says.
Fearless Science Magazine
This story was featured in Fearless Science Magazine. The inaugural issue focuses on regenerative biology. How do some of the world’s most clever and fascinating organisms redevelop critical body parts after injury? And what might it mean for future advances in human health, from heart repair to infectious disease?