Neuronal primary cilia are not required for hippocampal circuit function or behavior in adult mice
Using genetic and behavioral studies in adult mice, this paper demonstrates that the deletion of primary cilia from hippocampal neurons does not impair neuronal excitability, synaptic plasticity, or hippocampal-dependent behaviors, challenging the prevailing view that these structures are essential signaling hubs in mature brain circuits.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Inside nearly every cell of the human body, and in the brains of mice, there exists a tiny, hair-like projection called a primary cilium. For a long time, scientists thought these structures were useless leftovers from evolution, like the appendix in the gut. However, recent discoveries have revealed that during the early formation of a brain, these cilia act as critical command centers. They receive chemical signals that tell growing cells where to go and what to become. When these structures fail to form correctly in a developing brain, the result can be severe, leading to malformations and intellectual disabilities. This has led many researchers to believe that these tiny antennae must continue to play a vital role in the mature brain, acting as essential hubs that keep adult neurons communicating and behaving correctly.
The question that remained unanswered was whether these structures are still necessary once the brain is fully grown. Do adult neurons rely on their primary cilia to fire electrical signals, learn new things, or remember the past? To find out, a team of researchers at St. Jude Children's Research Hospital decided to remove these structures from the brains of adult mice and watch what happened. They focused on the hippocampus, a seahorse-shaped region deep in the brain that is famous for its role in forming memories and navigating space. By using precise genetic tools, they were able to delete the genes required to build primary cilia, but only in the mature neurons of adult mice, leaving the rest of the body and the developing brain untouched.
The researchers used two different methods to ensure their results were solid. First, they bred mice with a genetic switch that turned off the ability to make cilia in specific brain regions as the animals matured. This allowed them to study mice that had lived their entire lives without these structures in their hippocampal neurons. Second, to rule out the possibility that the brain had simply adapted over time to compensate for the missing parts, they used a high-powered laser to surgically remove the cilia from individual neurons in adult mice just minutes before testing them. They also used a virus to delete the cilia in a different group of adult mice. In every case, the goal was the same: to see if the brain stopped working without these tiny antennae.
What they found was surprising. Despite the complete absence of primary cilia in the mature neurons, the brain continued to function exactly as it should. The electrical activity of the neurons remained normal. The cells fired their signals with the same strength and timing as healthy cells. When the researchers tested how these neurons communicated with one another, the connections were just as strong, and the ability to strengthen those connections over time—a process essential for learning—was completely unaffected. The neurons did not become confused, sluggish, or overactive. They behaved as if the cilia had never been there in the first place.
The researchers then moved from the microscope to the real world to see if the mice could still learn and remember. They placed the mice in a series of tests designed to measure memory and behavior. In one test, the mice learned to associate a specific sound or a specific room with a mild, harmless shock. In another, they had to remember the location of a hidden platform in a pool of water. They also tested the mice for anxiety, social interaction, and motor skills. In every single instance, the mice without cilia performed just as well as the mice with cilia. They learned the tasks quickly, remembered them days later, and showed no signs of anxiety or social withdrawal. Their ability to navigate and interact with the world was indistinguishable from that of a normal mouse.
These results challenge a long-held assumption in neuroscience. While it is clear that primary cilia are indispensable for building a brain, this study suggests they are not required to keep it running once it is finished. The findings indicate that in the adult hippocampus, these structures are not the major signaling hubs that many scientists believed them to be. The brain appears to have other ways of managing the complex chemistry of learning and memory that do not depend on these tiny projections. This does not mean cilia are useless in the adult brain; they may still play a role during times of stress, disease, or aging. However, under normal, healthy conditions, the adult brain can function perfectly well without them, suggesting that the rules for how a brain works change significantly from childhood to adulthood.
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