- A new study has highlighted an association between neurodevelopmental abnormalities and autism development.
- Researchers specifically saw that certain issues with neurons could increase the chance of having autism.
- Neurons play a critical role in brain function, sparking connections and sending messages that dictate everyday actions.
- The study findings may help in the future with designing and selecting targeted therapies for those with autism.
Various brain functions are thought to contribute to autism spectrum disorder (ASD) — yet, despite ongoing studies, scientists have not been able to pinpoint its specific roots.
The scientists found that ASD was linked to two neurodevelopmental abnormalities involving excitatory neurons.
Excitatory neurons have a higher likelihood of “firing” or passing an electrical signal in the nervous system.
The researchers also found an association between these abnormalities and the incidence of macrocephaly, which is when the head is larger than average in size.
To assess neural pathway function in ASD, the researchers took skin stem cells from 13 boys diagnosed with the condition — eight of which also had macrocephaly.
These biopsies were used to grow skin fibroblasts (connective tissue cells), which were then reprogrammed into stem cells. Biopsies were also taken from the children’s fathers and underwent the same process so researchers could draw comparisons.
From there, the cells were used to create organoids or “mini-brains” in culture dishes, which are miniature 3D models that resemble the brain. Once these were established, the researchers used single-cell RNA sequencing to study gene patterns in 664,272 brain cells across three stages of brain development.
These results were then compared to the brain development of the children’s fathers.
They found the children with ASD had imbalanced levels of excitatory neurons compared to their fathers. Interestingly, those with macrocephaly had excessive amounts of these neurons, while children without macrocephaly had a deficit of them.
The scientists also learned that these changes occur due to “transcription factors” — proteins that influence gene formation at the early stages of brain development when a baby is still in the womb.
Dr. Flora Vaccarino, the Harris Professor in the Child Study Center at Yale School of Medicine and co-senior author of the paper, said that one finding was particularly unexpected.
“I was not surprised to find different mechanisms of disease in normocephalic [normal head size] and macrocephalic ASD,” she told Healthline.
However, “I was surprised by our discovery that these mechanisms are diametrically opposite — where, for example, some genes/cell types that are increased in autism with macrocephaly are decreased in autistic children that are normocephalic.”
Neurons play a critical role in brain function, sparking connections and sending messages that dictate everyday actions.
“The signals sent between neurons are necessary for activities like eating, talking, breathing, walking, speaking, and thinking,” explained Dr. Peter Chung, medical director at The Center for Autism & Neurodevelopmental Disorders, University of California, Irvine – School of Medicine.
Chung was not involved in the study.
So where do excitatory neurons come into all of this?
“Within those connections, excitatory neurons are responsible for sending or spreading signals. Their counterparts, inhibitory neurons, are responsible for suppressing signals,” Chung told Healthline.
“The balance/number of excitatory and inhibitory neuronal activity is one of the factors essential for neuronal circuit functioning,” he added — along with their location, distribution, and connectivity.
As this research demonstrates, those with autism can display differences in excitatory neuronal connections. But Chung noted that previous studies have highlighted additional changes in neuronal connections among people with ASD.
“For example, some have shown that, when compared with typically developing people, people with
Interestingly, added Dr. Robert Melillo, a brain and autism researcher, such differences in connections aren’t consistent throughout the brain.
“Our research [separate from the new study] shows that [excess excitatory connections and fewer inhibitory connections] is affecting the right hemisphere more than the left, and it mainly affects connectivity between the two hemispheres,” he shared with Healthline.
“The left hemisphere is more excitatory behaviorally and in general, and the right hemisphere is more behaviorally inhibitory. Therefore, an increase in excitatory neurons may increase left hemisphere activity, and the decreased amount of inhibitory neurons may produce a deficit in right hemisphere function and development,” Melillo said.
It’s thought the left brain is more “in charge” of logic and linear thinking. Meanwhile, the right brain controls the visualization of feelings and non-verbal cues — both of which are signs of ASD.
Macrocephaly — when the size of a child’s head is in the 98th percentile or greater at birth — occurs in around
So how is it related to ASD? “We believe that the excess of excitatory neurons and the macrocephaly are two different aspects of the same mechanism of autism pathogenesis,” said Vaccarino.
“During development, in macrocephalic ASD, there is an increase in proliferation of progenitor cells of the dorsal cortical plate [a part of the brain linked to neural function], leading to an increased production of excitatory cortical neurons,” she continued.
“The combined effect of increased number of neurons and their interconnections would result in increased brain size (macrocephaly).”
Chung noted that, in the first few years of life, those with macrocephaly and ASD may have larger brain volumes. However, brain growth slows down by the time they start school — meaning that the final brain sizes are equivalent between ASD patients with macrocephaly and those without.
“Some researchers have suggested that the larger brains of children with ASD is related to the lack of ‘pruning’,” he said.
“[This] is a normal process in early childhood where some unused connections between neurons are removed to increase the efficiency of the more frequently used connections.”
It’s important to note that macrocephaly isn’t exclusive to ASD: it can occur due to genetics, fluid on the brain, or as a result of health concerns, such as tumors and infections.
Furthermore, it “is not universally linked to excitatory neurons,” stated Chung. That said, “In cases of ASD, previous research studies have shown certain genetic changes can lead to both macrocephaly and imbalance of excitatory and inhibitory neurons.”
The new research findings support this thinking, Chung said, and highlight that “More research needs to be done to better characterize and understand the potential relationship(s) between these two factors.”
ASD affects around
“The symptoms of ASD typically emerge during early childhood, and in many cases, parents and caregivers begin to notice differences in behavior and development around 18 to 24 months of age,” said Dr. Sanam Hafeez, a neuropsychologist in New York and director of Comprehend the Mind.
“However, the severity and combination of symptoms can vary widely, leading to challenges in diagnosis and identification,” she told Healthline. “Some individuals with milder forms of autism may not receive a formal diagnosis until later in childhood or even adolescence.”
The main signs of ASD fall into three categories, explained Hafeez: impaired social interaction, communication difficulties, and repetitive behaviors and restricted interests.
Within these, symptoms include:
- Difficulty interpreting and understanding social cues, such as facial expressions and body language.
- Delayed or absent speech development in some cases.
- Challenges forming and maintaining relationships, including engaging in reciprocal conversations.
- Difficulty understanding and using abstract language, humor, and sarcasm.
- Strong attachment to specific objects or topics of interest, often to the exclusion of other activities.
- Resistance to changes in routines and a need for predictability.
While some medications are prescribed to treat co-occurring conditions, such as depression and anxiety, none are currently used to target the neurotransmitters involved in excitation and inhibition, said Chung.
“Existing medications directly impacting neuronal excitation (i.e. anti-seizure medications) are not routinely prescribed for seizure-free individuals with ASD,” he revealed. However, “the findings of this study suggest that those with ASD and macrocephaly could uniquely benefit from this approach.”
Vaccarino said the new research data “may be important to properly design clinical trials and choose/design appropriate therapeutics.”
For example, she continued, “Trying to compensate for increased excitatory neuron function will require different drugs than compensating for decreased functioning of the same cells.”
Melillo added that brain-stimulating treatments, such as transcranial magnetic stimulation (TMS) therapy, when used in conjunction with other therapies, hold “great promise” — especially in influencing excitatory neurons.
“Many of these brain stimulation tools can be used to inhibit excitatory neurons and excite inhibitory neurons,” he said. [This] can reestablish a proper balance of excitation and inhibition in the brain and help promote proper brain development.”
Ultimately, said Chung, “More research needs to be done to investigate the specific neurobiological makeup of patients and whether targeted (existing or novel) treatments will lead to improved outcomes.”
New research highlights a potential mechanism in ASD onset: abnormalities in excitatory neurons, which occur early in brain development.
The research also observed a link between ASD, excitatory neurons, and macrocephaly.
While further studies are required to understand the association(s) between these factors, the study authors noted that the ability to track specific neuron growth could aid doctors in diagnosing ASD and identifying existing drugs that may support those with the condition.