A recent study published in Cell Death & Disease suggests that specific cellular abnormalities in the brain associated with autism spectrum disorder might be reversible. By artificially activating a targeted neural pathway in a mouse model, scientists successfully restored the structure of a key neuron component and improved social and repetitive behaviors. This provides evidence that some core symptoms of the disorder stem from adaptable brain changes rather than permanent damage.
Autism spectrum disorder, commonly known as ASD, is a complex developmental condition characterized by challenges with social communication and the presence of restricted or repetitive behaviors. Genetic factors play a significant role in the development of ASD. One known genetic risk factor is the duplication of a specific chromosomal region, referred to as 15q11-13. Mouse models carrying this genetic duplication tend to exhibit behavioral symptoms that mirror human ASD.
To better understand the biological roots of these symptoms, scientists focus on the microscopic structures of the brain. Neurons, or nerve cells, communicate by sending electrical signals called action potentials. These signals are generated at a specific site on the neuron called the axon initial segment. The axon initial segment is highly adaptable, meaning it can change its length and position to regulate how easily a neuron fires an electrical signal, which is a form of brain plasticity.
A collaborative research group aimed to determine if the structural changes seen in the axon initial segment of ASD mouse models represent permanent structural damage or a reversible state. The team was led by Masashi Fujitani, a professor in the Department of Anatomy and Neuroscience at Shimane University’s Faculty of Medicine, alongside colleagues from Kobe University and Hyogo Medical University.
“This research was motivated by my interest in identifying abnormal neural circuits in the brain,” Fujitani said. “Because the axon initial segment (AIS) is known to change its length in an activity-dependent manner, we hypothesized that its properties might vary across different neural circuits depending on their projection targets.”
The authors analyzed a total of 214 mice, comparing normal healthy mice with the ASD mouse model that carries the genetic duplication. They used high-resolution microscopes and fluorescent markers to measure the length of the axon initial segment in various brain regions. They focused on pyramidal neurons, which are a primary type of excitatory nerve cell in the cortex. The researchers specifically looked at the medial prefrontal cortex, an area of the brain known to regulate social behavior, decision-making, and emotional responses.
The researchers utilized a technique called whole-cell patch-clamp recording to measure the electrical properties of the brain slices. The results showed that in the ASD mouse model, the axon initial segment was significantly shortened in a specific sub-layer of the medial prefrontal cortex. This shortened structure resulted in reduced neuronal excitability, meaning the neurons had a much harder time firing electrical signals. This shortening acts as a homeostatic adaptation, which is the brain’s way of trying to balance its overall electrical activity.
Interestingly, this structural abnormality was not widespread across all brain cells. The researchers used a method called retrograde tracing to map where the abnormal neurons were sending their signals. They found that the shortened cellular structures were highly specific to neurons that connect the medial prefrontal cortex to other distant brain regions.
“Consistent with this idea, we found projection-specific changes in AIS structure,” Fujitani noted. One of the most affected pathways was the connection to the dorsal raphe nucleus, which is a major source of serotonin in the brain and is heavily involved in social engagement.
To test if this structural abnormality could be fixed, the scientists employed a sophisticated technique called chemogenetics. This method involves using engineered viral vectors to deliver specialized receptors into specific groups of neurons. These newly introduced receptors remain inactive until the researchers administer a specific designer drug.
The team targeted the precise neural circuit projecting from the medial prefrontal cortex to the dorsal raphe nucleus. After introducing the receptors, they waited four weeks and then gave the mice a single injection of the activating drug.
A follow-up analysis of the brain tissue revealed that this single targeted activation was sufficient to re-elongate the shortened axon initial segments. The structure of the nerve cells in the ASD mouse model returned to a length comparable to that of the healthy control mice. The researchers verified that the lengths of the associated sodium channel proteins also normalized. This confirmed that the shortened state of the axon initial segment was a reversible adaptation rather than a permanent defect.
“One surprising finding was that the abnormalities we observed were not necessarily permanent,” Fujitani said. “In a way, it seemed as if the system was simply ‘switched too far’ rather than fundamentally broken. We found that these changes were reversible, suggesting that the underlying neural circuit dysfunction is not irreversible. This gives us hope that it may be possible to restore normal function and develop therapeutic approaches in the future.”
The researchers also wanted to see if fixing this cellular structure would improve the animals’ behavioral symptoms. They used a sample of 11 mice per group to conduct two standard behavioral assessments. The three-chamber test was used to measure social preference and social novelty. In this test, researchers tracked how much time a mouse spent interacting with a new, unfamiliar mouse versus an inanimate wooden block.
The second behavioral measure was the marble-burying test. This test is used to assess repetitive and anxiety-like behaviors in rodents. The researchers placed the mice in a cage with 20 glass marbles evenly distributed on top of the bedding. They then counted how many marbles the mice compulsively buried over a 30-minute period.
Before the treatment, the ASD mouse models exhibited significant social deficits in the three-chamber test and buried a high number of marbles. One hour after the researchers activated the specific neural circuit using the chemogenetic drug, the mice underwent the behavioral tests again. The treated mice showed a dramatic improvement in social interactions, spending a normal amount of time engaging with unfamiliar mice. They also buried significantly fewer marbles, entirely matching the normal behavior of the healthy control mice.
“One important takeaway from our study is that changes in the axon initial segment (AIS) could potentially serve as a biomarker for detecting abnormal neural circuits in the brain,” Fujitani told PsyPost. “In addition, we found that these AIS abnormalities are reversible. This suggests that the underlying dysfunction is not necessarily permanent, which raises the possibility that neural circuit abnormalities could be restored under certain conditions.”
While these findings are promising, there are several limitations and potential misinterpretations to consider. The team evaluated the brain’s cellular plasticity using preserved, fixed tissue samples rather than observing live cells over time. Utilizing live-cell imaging techniques in the future would provide a more accurate, real-time picture of how the axon initial segment changes shape. The authors also note that they measured the overall electrical excitability of the brain region, but they did not directly record the electrical activity of the individual neurons they repaired.
“One important limitation of our study is that it was conducted in mice, so further research is needed to determine whether the findings can be directly applied to humans,” Fujitani cautioned. “In addition, we examined only a single model of autism spectrum disorder. It will be important to test whether similar results are observed across different models and conditions.”
The findings suggest that future therapeutic strategies for neurodevelopmental conditions might benefit from targeting specific neural pathways. Subsequent studies utilizing targeted recording methods are needed to directly demonstrate the functional impact of these structural changes at the single-cell level.
“Autism spectrum disorder is relatively common, and our findings suggest that the underlying neural circuits may not be permanently fixed, but instead retain the ability to change,” Fujitani said. “Rather than simply trying to broadly adjust brain activity with medication, it may be more important to think in terms of specific ‘switches’ within neural circuits. We believe that identifying and appropriately controlling these switches could be an important step toward more precise and effective treatments.”
Looking ahead, the researchers aim to refine these targeted approaches for potential clinical applications.
“In the long term, we hope to develop treatments that can target specific neural circuits in the human brain,” Fujitani added. “One potential approach might involve combining techniques such as focused ultrasound and viral vectors to selectively modulate neural circuits. However, this is still a distant goal, and significant further research will be required.”
The study, “Restoration of axon initial segment plasticity via chemogenetic activation rescues autism-related behaviors,” was authored by Yoshinori Otani, Xiaowei Zhu, Xinlang Liu, Kohei Koga, Ryo Kawabata, Hisao Miyajima, Toru Takumi, and Masashi Fujitani.
