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Wriggling tadpoles may hold clue to how autism develops

You could say Hollis Cline’s lab at The Scripps Research Institute is building better tadpoles.

To better understand how humans learn, Cline, PhD, Hahn Professor of Neuroscience and co-chair of the Department of Neuroscience, leads experiments designed to spark learning in tadpole brains. Over the years, her lab’s work with tadpoles has shed light on neuroplasticity — how new experiences flood brain cells with proteins that fuel brain development and learning.

Now the lab’s latest study, published in eLife, suggests a key to neuroplasticity is not just the presence of new proteins, but how the brain makes proteins in the first place. The research also points to a possible new role for proteins in sensory processing in some people with autism spectrum disorder.

“The idea that visual experience can influence how we make proteins is something brand new,” Cline says. “This is interesting to think about because we live in a very busy sensory world.”

The researchers used tadpoles that naturally have translucent skin — which makes them an excellent model for peering into the wirings of a living brain. The tadpoles were kept in the dark and then exposed to either ambient light (for the control group) or a screen with moving bars (simulating normal visual experience) for four hours.

Working closely with Professor John Yates, PhD, of the Scripps Research Department of Molecular Medicine, Cline’s team measured changes in protein expression — the production of proteins in cells — before and after each experiment. They found that the expression of 83 proteins shifted either up or down in the experimental group.

Many of these were effector proteins — the proteins doing specific jobs in cells. But the team also spotted three outliers: proteins eIF3A, FUS and RPS17. These three are regulatory proteins, meaning they construct the machinery that makes the effector proteins further down the line. Cline was surprised. She and her colleagues always thought regulatory protein expression would hold steady even when visual experience varied.

“We just thought the regulatory machine would be just humming along,” Cline says. “So, we were surprised to see them on our list. We thought, ‘Is this accurate? Is this true?'”

It turned out that these regulatory proteins are essential for learning from visual experience. Cells are better at building connections and reinforcing learning when they synthesize these proteins at a certain rate during visual experience.

In fact, researchers could tag neurons with fluorescent proteins to see the physical signature that visual experience left in the brain. Thanks to eIF3A, FUS and RPS17, tadpoles had significant neuronal growth — seen in how their neurons sent out branch-like tendrils — after just four hours of visual experience.

Next, the scientists investigated whether changes in protein expression affected tadpole behavior. How important were these proteins for teaching tadpoles?

To find out, the researchers took advantage of a natural tadpole behavior: the instinct to avoid any large shape that may be a looming predator. The researchers had tadpoles swim above a screen that projected large, predator-like spots. Then they tracked whether a tadpole would turn to avoid the dark spots.

The tadpoles with exposure to visual experience did significantly better on the avoidance test than tadpoles in the control group. This suggests they had formed the neural circuits to better process visual information. Interestingly, tadpoles did not do as well on the test — even after exposure to visual experience — when they could not express all three key proteins (eIF3A, FUS and RPS17). This finding further confirmed the importance of the regulatory proteins in neuronal plasticity.

Finally, the researchers were curious whether the 83 total proteins they identified were expressed differently in human brain disorders, so they cross referenced their list with two databases — one of people with risk factors for autism spectrum disorders, and one with people with fragile X syndrome, which has similar characteristics as autism.

The results came as a surprise. Twenty-five percent of the proteins on the Scripps Research list overlapped with the database lists of genes thought to cause autism spectrum disorder and fragile X syndrome. That was a much bigger number than Cline expected, and it prompts new questions about what makes an autism “risk factor” actually risky.

Cline thinks mutations in regulatory proteins might keep some people from expressing the other proteins needed for processing sights, smells, textures, tastes and sounds. “This brings to mind a new dimension for understanding autism,” Cline says.

Cline says future studies could focus on understanding all 83 synthesized proteins. She says the work has also made her consider the visual experience humans take in every day.

“It’s fascinating to think about how sensory experience affects the brains of our children,” says Cline. “We may wittingly or unwittingly affect how their brains develop.”

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