Tiny worm mazes allow researchers to ID genes linked to spatial perception, risk-taking

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Tiny worm mazes allow researchers to ID genes linked to spatial perception, risk-taking

A chance observation led to a completely new area of behavioral research

March 28, 2017
worms

Could the worms' few hundred neurons really distinguish a physical pattern in their surroundings? That's what Dr. Jihong Bai and his newly hired technician, Bicheng Han, wanted to figure out.

Courtesy of the Bai lab

Dr. Jihong Bai was getting fed up.

The Fred Hutchinson Cancer Research Center neurobiologist had just hired a new technician, Bicheng Han, to work in his research laboratory. Bai had given Han what he thought was a fairly straightforward task: Take some pictures of these roundworms — Bai’s research animal of choice for their simple, well-mapped nervous system — using this microscope. But Han kept coming to his boss with what sounded to Bai like a crackpot theory.

To keep the tiny, squirmy worms still enough to capture a good image, the researchers placed them in fluid-filled chambers small enough to immobilize them. Han was convinced that the worms were acting differently — turning and pausing more or less — depending on the design of the chamber he chose.

“He was saying, ‘The worm is behaving very differently in different chambers,’” Bai said.

Researchers knew that these simple animals, with just 302 neurons, change their behavior based on certain chemical cues in the environment — like the smell of food. But could those few hundred neurons really distinguish a physical pattern in their surroundings? Bai wasn’t buying it. “I was like, ‘Do your work,’” he said.

Eventually Han convinced Bai to watch the worms, formally known as Caenorhabditis elegans, under the microscope for a while. And Bai was pretty sure he could see what the technician saw. The worms moved differently depending on how close together the confines of their chambers were.

And like that, the two scientists decided to embark on what was for them a completely new area of research. Bai’s work up until that point had focused on a specific type of neurochemistry. This was spatial perception and behavioral biology — untested waters for these scientists.

In the course of their research project, described in a study published Tuesday in the journal eLife, Han (who is now enrolled in a doctorate program at Harvard University), Bai and their Fred Hutch colleagues uncovered that the tiny worms are indeed able to sense small differences in their spatial environments; that laboratory worms from Britain prefer tight, dense confines while worms native to Hawaii are more adventurous in their exploration; that starvation causes the animals to take more risks and venture out from their (spatial) comfort zones; and that the brain chemical known as dopamine is responsible for all of this behavior.

Worm mazes

To more precisely test Han’s theories about the worms’ behavior in close confines, the researchers decided to build a series of mazes, to see how the worms handled them.

The mazes the researchers built are really simple: They’re a series of evenly spaced plastic pillars in a shallow dish filled with water. A single worm maze contains four quadrants, each with different spacing between the small pillars. Think the surface of a Lego vs. that of a Nanoblock or a Duplo — submerged under water. The worms can swim between the pillars, but not over them.

When the researchers put the standard C. elegans lab strain — which originally harkens from a patch of mushroom compost in Bristol, U.K. — on the watery mazes, they saw the animals preferred to hang out in quadrant 4, where the pillars were most densely packed. And they noticed another interesting behavior, which Bai dubbed “boundary testing”: the worms would often reach the edge of the tightly spaced pillars, poke their heads out into the neighboring quadrant and quickly pivot back to the more confining part of the maze.

The researchers determined that this close-quarters-seeking behavior relied on dopamine — a neurochemical conserved from worms to humans that regulates the brain’s reward and pleasure centers, among other important roles. When Bai and Han tested worms with a dopamine deficiency, the animals no longer showed the same spatial pattern preference.

Dr. Jihong Bai

At first, Dr. Jihong Bai was skeptical that the worms were behaving differently. Then he looked under the microscope.

Fred Hutch file photo

They then pinpointed a pair of genes responsible — one, known as TRP-4 produces a touch-sensitive protein in neurons; the other, DOP-3, produces a protein on neurons that senses the presence or absence of dopamine. Humans have DOP-3 too, Bai said, although we don’t have the TRP-4 gene. TRP-4 seems to have been lost in evolution at the point animals moved from living in either liquid, like fish or tadpoles, or solid states, like the worms, whose natural home is dirt or rotten fruit, to living above ground in air, like humans.

“Animals interpret their physical environments with different sensations,” Bai said. “Worms sense by touch but humans sense dominantly by vision.”

It turns out our sense of sight is tightly coupled to dopamine, Bai said, implying this conserved neurochemical could be regulating environmental perception from worms to humans. Children with mutations in a gene that affects dopamine sensation have difficulty interpreting visual cues in their environment, a 2007 study found.

Transcontinental differences

It’s at this point in the research that things got even more interesting. The researchers wanted to ask whether worms have a natural level of variation in their spatial perception behavior. So they took 11 different strains of roundworms from around the world and tested their maze-navigating abilities. The laboratory strain most distantly related to the British worms, which was found in a pineapple field in Hawaii, showed no preference for the more constricting maze quadrant, swimming freely throughout all the different fields of pillars.

So not only does this very simple creature change its behavior in response to its physical environment, there’s a level of natural genetic variability that underlies that behavior.

“That’s how diverse a simple animal’s perception is,” Bai said.

Of the 11 strains they tested, six (including the Hawaiian worms) showed this free-ranging behavior. The other five were more like the British in their selectivity for tight spaces. All six of those more adventurous worm strains had small mutations in either TRP-4 or DOP-3, the genes the researchers identified as important for the spatial perception and quadrant preference in the British worms. And subbing in the Bristol version of TRP-4 made the Hawaiian worms more particular in selecting comforting spatial settings, they found.

Their findings don’t name the evolutionary reason for this diversity, Bai said. But he believes that having a natural level of variability in the animals’ behavior could have benefited the population over time. Certain changes in the environment could favor more timid or more adventurous worms, and having both present in the global population could ensure the species’ survival through times of plenty and lean.

As for the specific differences in Hawaiian versus British worms, that requires even more speculation, Bai said. It is possible that the colder environment with fewer food sources in the U.K. favored animals that stay put, while the abundance of different fruits in Hawaii selected for animals willing to venture out from home base, he said. But there’s no scientific evidence to support that hypothesis yet.

It would be even trickier to directly line up these results with human behavior. Our own behavior is “extremely complex,” Bai said. But we do know that dopamine plays an important role in behaviors that could be correlated to the worm experiments, namely, environmental perception and risk taking.

Next up, Bai wants to map the behavior to understand which of the animal’s 302 neurons is responsible. When Bai reflects on the events that took him and his laboratory team down this novel research path, it’s still hard for him to believe. He credits both Han’s ingenuity — “he’s a very creative guy,” Bai said — as well as the freedom the Hutch confers on basic scientists like him to explore new areas of science.

“Once in a while, you find a lab is doing something that is so weird, you don’t expect that they’re working on this topic,” he said.

When asked if his lab is now the “weird lab,” Bai laughed. “At this moment it probably is,” he said. 

Rachel Tompa, a staff writer at Fred Hutchinson Cancer Research Center, joined Fred Hutch in 2009 as an editor working with infectious disease researchers and has since written about topics ranging from nanotechnology to global health. She has a Ph.D. in molecular biology from the University of California, San Francisco and a certificate in science writing from the University of California, Santa Cruz. Reach her at rtompa@fredhutch.org.

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