Recent research published in cell death and disease Suggests that certain cellular abnormalities in the brain associated with autism spectrum disorders may be reversible. By artificially activating targeted neural pathways in mouse models, scientists were able to restore the structure of key neuronal components and improve social and repetitive behaviors. This provides evidence that some of the core symptoms of the disorder are due to adaptive brain changes rather than permanent damage.
Autism spectrum disorder, commonly known as ASD, is a complex developmental disorder characterized by difficulties in social communication and the presence of restricted or repetitive behaviors. Genetic factors play an important role in the development of ASD. One known genetic risk factor is duplication of a specific chromosomal region called 15q11-13. Mouse models with this gene duplication tend to exhibit behavioral symptoms that mirror ASD in humans.
To better understand the biological causes of these symptoms, scientists are focusing on the microstructure of the brain. Neurons, or nerve cells, communicate by sending electrical signals called action potentials. These signals are generated at specific locations on the neuron called the axon initial segment. The first part of the axon is highly adaptable and can change its length and position to control how easily the neuron emits electrical signals. This is a type of brain plasticity.
The collaborative research group aimed to determine whether the structural changes seen in axon initial segments in an ASD mouse model represent permanent structural damage or a reversible condition. The team was led by Masashi Fujitani, a professor in the Department of Anatomical Neuroscience at Shimane University School of Medicine, and colleagues from Kobe University and Hyogo University of Medicine.
“This research was motivated by my interest in identifying abnormal neural circuits in the brain,” Fujitani said. “Since the axon initial segment (AIS) is known to vary in length depending on activity, we hypothesized that its properties may differ between different neural circuits depending on the projection target.”
The authors analyzed a total of 214 mice and compared normal healthy mice and an ASD mouse model with a gene duplication. They used high-resolution microscopy and fluorescent markers to measure the length of the first segments of axons in different brain regions. They focused on pyramidal neurons, the main type of excitatory neurons in the cortex. The researchers focused specifically on the medial prefrontal cortex, a region of the brain known to control social behavior, decision-making, and emotional responses.
The researchers used 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 initial segments of axons were significantly shortened in specific lower layers of the medial prefrontal cortex. This shortened structure reduced the excitability of the neuron. This means it has become much harder for neurons to fire electrical signals. This shortening acts as a homeostatic adaptation and is the brain’s way of trying to balance overall electrical activity.
Interestingly, this structural abnormality was not widespread in all brain cells. The researchers used a method called retrograde tracking to map where the abnormal neurons were sending 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 observed projection-specific changes in AIS structure,” Fujitani said. One of the most affected pathways was the connection with the dorsal raphe nucleus, which is the main source of serotonin in the brain and is deeply involved in social engagement.
To test whether this structural abnormality could be repaired, scientists employed an advanced technique called chemical genetics. This method uses engineered viral vectors to deliver specialized receptors to specific groups of neurons. These newly introduced receptors remain inactive until researchers administer a specific designer drug.
The researchers targeted precise neural circuits that project from the medial prefrontal cortex to the dorsal raphe nucleus. After introducing the receptors, they waited four weeks before giving the mice a single injection of the activating drug.
Follow-up analysis of brain tissue revealed that this single, targeted activation was sufficient to re-lengthen the initial segment of the shortened axon. The structure of neurons in the ASD mouse model returned to a length comparable to that of neurons in healthy control mice. The researchers confirmed that the length of related sodium channel proteins was also normalized. This confirmed that the shortened state of the initial segment of the axon is not a permanent defect but a reversible adaptation.
“One of the surprising findings was that the abnormalities we observed were not necessarily permanent,” Fujitani said. “In some ways, it appeared as though the system had simply been ‘switched on’ rather than being fundamentally broken. These changes were found to be reversible, suggesting that the underlying neural circuit dysfunction is not irreversible. This gives us hope that we may be able to restore normal function and develop treatments in the future.”
The researchers also wanted to see if restoring this cellular structure would improve behavioral symptoms in the animals. They conducted two standard behavioral assessments using a sample of 11 mice per group. The three-chamber test was used to measure social preference and social novelty. In this test, the researchers tracked the amount of time the mice spent interacting with new and unfamiliar mouse and inanimate wooden blocks.
The second behavioral measure was the marble embedding test. This test is used to assess repetitive and anxiety-like behaviors in rodents. The researchers placed a mouse in a cage and placed 20 glass beads evenly on top of it. They then counted the number of marbles the mice were forced to bury in a 30-minute period.
Before treatment, the ASD mouse model showed significant social deficits in the three-chamber test, burying large numbers of marbles. An hour after the researchers used the synthetic drug to activate specific neural circuits, the mice were again subjected to behavioral tests. Treated mice showed dramatic improvements in social interactions and spent normal time with unfamiliar mice. The number of buried marbles was also significantly reduced, completely consistent with the normal behavior of healthy control mice.
“One important takeaway from our study is that changes in the axon initial segment (AIS) may serve as a biomarker for detecting abnormal neural circuits in the brain,” Fujitani told PsyPost. “Furthermore, we found that these AIS abnormalities are reversible, suggesting that the underlying dysfunction is not necessarily permanent, and under certain conditions increases the likelihood that neural circuit abnormalities can recover.”
Although these findings are promising, several limitations and potential misinterpretations should be considered. Rather than observing living cells over time, the researchers used fixed, archived tissue samples to assess the plasticity of brain cells. In the future, live-cell imaging techniques will provide a more accurate, real-time picture of how the initial segment shape of an axon changes. The authors also note that while they measured electrical excitability across brain regions, they did not directly record the electrical activity of individual repaired neurons.
“One important limitation of our study is that it was performed in mice, so further research is needed to determine whether the findings are directly applicable to humans,” Professor Fujitani cautioned. “Furthermore, we only investigated a single model of autism spectrum disorder. It will be important to test whether similar results are observed in different models and conditions.”
The findings suggest that future therapeutic strategies for neurodevelopmental conditions may benefit from targeting specific neural pathways. Subsequent studies utilizing targeted recording methods are required to directly demonstrate the functional consequences of these structural changes at the single-cell level.
“Autism spectrum disorders are relatively common, and our findings suggest that the underlying neural circuits are not permanently fixed and may retain the ability to change,” Fujitani said. “Rather than simply trying to broadly modulate brain activity with drug therapy, 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, 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 could include combining techniques such as focused ultrasound and viral vectors to selectively modulate neural circuits. However, this is still a distant goal and requires further significant research.”
The study, “Restoration of axonal early segment plasticity through chemogenetic activation rescues autism-related behaviors,” was authored by Yoshinori Otani, Xiaowei Zhu, Xinrou Liu, Kohei Koga, Ryo Kawabata, Nao Miyajima, Toru Takami, and Masashi Fujitani.
