Link between Nitric Oxide and Autism and wider implications

Post Reply New Topic

Researchers say high levels of a signaling chemical in the brain called nitric oxide may play a key role in autism spectrum disorder.

Even though autism spectrum disorder (ASD) diagnoses are on the rise, scientists still don't fully understand what mechanisms contribute to its development. Many experts lean towards the idea that autism has a genetic cause, but most also believe there's something else amiss that triggers the disorder in some individuals.

A person with autism may have symptoms such as communication difficulties, sensory challenges, restricted interests, and repetitive behaviors. However, that list doesn't capture the full scope of the disorder. For example, some autistic individuals have difficulty conversing with others yet have an eidetic memory and can remember intricate details other people miss.

Nonetheless, identifying autism's triggers or the mechanisms behind its development could lead to therapies that might help a person with ASD manage the challenges while accentuating the positive attributes of the disorder.

In a new study published on May 22 in Advanced Science, investigators from the Hebrew University of Jerusalem, Israel, have come one step closer to understanding a specific mechanism that might play a role in ASD.

The scientists found that increased nitric oxide (NO) levels in the brains of mice contributed to autism-like symptoms, such as social deficits, repetitive behaviors, and anxiety. And when the team reduced those levels, most of these ASD symptoms reversed.

Investigating nitric oxide’s role in autism
NO is a gaseous molecule produced in organs and tissues, including the central and peripheral nervous systems. When NO levels are within the normal range, the molecules help with normal cell signaling and regulate a host of physiological functions.

However, higher levels can lead to abnormal cell signaling. And this NO dysregulation can result in numerous neurodevelopmental, neuropsychiatric, and neurodegenerative disorders.

Previously, the research team found that mouse models of autism had notably higher levels of NO.

Investigating this further, the team conducted NO experiments in ASD mice, blood samples, human stem cells, and human cell cultures.

First, when the scientists increased NO levels in typical mice, the rodents began to show signs of autism, even at a molecular level. This finding left the team wondering if reducing NO in ASD mice could reverse autism-like symptoms and behaviors.

To test their hypothesis, the scientists injected two types of ASD mice with a compound that inhibited NO production. One type had a mutation in the SHANK3 gene, and the other had a mutation in the CNTNAP2 gene. These gene mutations are associated with autism.

After the injection, the team noticed that as NO levels fell, the mice displayed less autistic-like behaviors and had more normal molecular and biochemical functions.

"The ASD mouse models with high [nitric oxide] levels showed social deficits, reduced novelty seeking, repetitive behavior, and anxiety. When we reduced the NO levels, we found reversal of most of the features," says lead investigator Haitham Amal, an assistant professor at The Hebrew University of Jerusalem.

The team then tested higher and lower NO levels in cultured neuronal cells from typical and ASD mouse models and found similar molecular and biochemical changes as seen in the animal experiments.

Then, to see if these findings could translate to humans, the investigators tested neurons from the stem cells of autistic people with SHANK3 gene mutations. They found that when NO levels fell, so did levels of biomarkers that indicate nitric oxide stress.

Moreover, when they looked at plasma samples from autistic children, they found that these children also had higher levels of biomarkers that indicate NO stress.

The study authors say these findings show that nitric oxide plays a crucial role in ASD.

"Importantly, this was confirmed in cellular, animal models, human iPSC-derived cortical neurons, as well as in clinical samples," the authors note. "Since the molecular mechanisms underlying ASD pathogenesis remain largely unknown, we provided a new mechanism that shows that NO plays a key role in ASD pathology at the molecular, cellular, and behavioral levels."

The authors hope that these findings and future nitric oxide research will lead to the development of therapeutic drugs to help autistic people worldwide.

Abstract

Background: Neuroimmune regulation abnormalities have been implicated in the pathophysiology of autistic disorder. Nitric oxide (NO) is involved in immune reactivity and is known to affect brain neurodevelopmental processes. Recent evidence indicates that NO, and cytokines involved in NO production, may be high in children with autism. The purpose of this study was to verify that plasma NO is high in children with autism and determine whether this elevation is related to plasma levels of cytokines involved in NO production.

Methods: The metabolites of NO, nitrite, and nitrate (NOx), along with the cytokines interferon-gamma (IFN-gamma), tumor necrosis factor-alpha, and interleukin-1beta, were measured in plasma of 29 children with autism (mean age +/- SD = 6.1 +/- 2.8 years) and 27 age- and gender-matched healthy comparison subjects using commercially available assay kits.

Results: Plasma levels of NOx were significantly higher in the autistic subjects (p =.006); plasma levels of the cytokines did not differ between groups. NOx and IFN-gamma levels were positively correlated in the autistic subjects (r =.51; p =.005).

Conclusions: These results confirm that plasma NO is high in some children with autism and suggest that this elevation may be related to IFN-gamma activity.

The researchers injected the mice with an inhibitory chemical that would suppress the production of nitric oxide. As levels of nitric oxide fell, the signs of autism tapered off. Normal levels of neuronal proteins were restored, and dendritic spines returned at typical density. “In an ASD state, the number of spines is reduced, and after treatment we found an almost full recovery,” explained Amal.