CYLD-associated K63-linked ubiquitin dysregulation contributes to synaptic and autism-related behavioral abnormalities after maternal thyroid hormone deficiency

The developing brain is a construction site that never truly closes, but it has a few critical windows where the foundation must be poured perfectly. One of the most important foremen on this site is a hormone called thyroid hormone. Before a baby's own thyroid gland begins to function, the entire brain relies on the mother's supply to guide the growth of neurons, the migration of cells to their proper neighborhoods, and the formation of connections between them. When this supply is short, the blueprint gets scrambled. This is a known risk factor for neurodevelopmental issues, including conditions that affect how a person interacts with the world and processes information. For decades, scientists have known that a lack of thyroid hormone during pregnancy leads to lasting changes in the brain, but the specific molecular machinery that locks these changes in place has remained a mystery. It was unclear how a temporary shortage of a hormone could cause permanent structural problems in the brain's wiring long after the hormone levels were corrected.

A team of researchers at China Medical University has now traced a specific chain of events that links a mother's thyroid hormone deficiency to these lasting brain changes. They focused on a tiny, invisible process inside brain cells called ubiquitination. Imagine the brain as a bustling city where proteins are the buildings and machines. To keep the city running, the brain needs to constantly build new structures and tear down old or broken ones. A system of molecular tags, known as ubiquitin chains, acts like a postal service that marks proteins for delivery, repair, or demolition. There are different types of these tags; some mark a protein for the trash, while others, specifically those linked in a K63 pattern, act more like a construction manager's clipboard, organizing how proteins assemble and move without destroying them. The researchers suspected that when thyroid hormone is missing, this construction management system goes haywire, leading to a buildup of misplaced proteins and a failure to build the necessary connections for learning and social behavior.

To test this, the scientists created a model using rats. They surgically removed the thyroid glands from pregnant female rats and gave them a very small, controlled dose of thyroid hormone replacement. This ensured the mothers remained in a state of mild deficiency throughout the pregnancy and early nursing period, mimicking the human condition of insufficient thyroid hormone. They then waited until the male offspring grew into adolescence, an age roughly equivalent to human teenagers, to see how the early hormone shortage had affected them. The results were striking. The young rats from the hormone-deficient mothers struggled with social interactions, showing little interest in meeting new rats. They spent more time grooming themselves in repetitive loops, a sign of anxiety and stereotyped behavior, and they had trouble navigating mazes that required them to remember where they had been. These behaviors mirrored the core symptoms seen in autism spectrum disorder and other neurodevelopmental conditions in humans.

When the researchers looked inside the brains of these adolescent rats, they found the physical evidence of this struggle. In the hippocampus, a region of the brain essential for memory and learning, the neurons were disorganized and damaged. More importantly, they found a specific molecular imbalance. The rats had significantly lower levels of a protein called CYLD. CYLD acts as a specialized tool that removes the K63 ubiquitin tags when they are no longer needed, keeping the construction site tidy. Because CYLD was missing, the K63 tags piled up in high-molecular-weight chains, creating a molecular traffic jam. This accumulation was directly linked to a shortage of critical synaptic proteins—the very materials needed to build and maintain the connections between neurons. The rats' brains were missing key components like PSD95, Synapsin I, and GluA1, which are essential for transmitting signals and forming memories. The researchers also found that the signaling pathway driven by BDNF, a growth factor that helps neurons survive and connect, was severely dampened, further stalling the brain's ability to repair and mature.

The most compelling part of the study came when the researchers tried to fix the problem after the rats were already born. They used a harmless virus to deliver extra copies of the CYLD gene directly into the hippocampus of the adolescent rats, effectively giving the brain the missing tool it needed to clean up the ubiquitin tags. This intervention worked. The excess K63 tags disappeared, and the levels of the missing synaptic proteins began to rise. The behavioral changes were equally dramatic. The rats that received the CYLD treatment started to interact socially again, showed less repetitive grooming, and were able to navigate the maze with much greater success. They did not become perfect, but they recovered a significant portion of the abilities they had lost.

This study suggests that the damage caused by a lack of thyroid hormone during pregnancy is not just a static injury but a dynamic process driven by a failure in protein management. The absence of thyroid hormone leads to a drop in CYLD, which causes a buildup of K63 ubiquitin chains. This buildup prevents the brain from assembling the synaptic proteins necessary for healthy communication between neurons. The fact that restoring CYLD in adolescence could reverse many of these effects indicates that the brain retains a degree of plasticity even after the initial insult. While the researchers note that their work was done in male rats and focused on the hippocampus, the findings offer a clear, testable pathway for understanding how early endocrine disruptions can lead to autism-related behaviors. It moves the conversation from a vague idea of "brain damage" to a specific molecular mechanism that could one day be targeted for intervention, suggesting that the key to fixing the wiring might lie in managing the tags that tell the brain's proteins where to go.