Possible Mechanisms of Autism Development Explored

Highlights:

• Autism has been earlier linked to the presence of severe infection and inflammation during pregnancy
• Research on mice evaluated the possible mechanisms for this link
• Reduced number of interneurons in a particular part of the brain and the presence of segmented filamentous bacteria in the maternal digestive tract may play a role

Studies have explored the role of maternal infection and inflammation in the development of autism in the offspring. Autism is a developmental condition characterized by problems with communication and social interaction. The child shows repetitive behavior and appears to be in his/her own world, but usually has normal intelligence. The exact causes of autism are not known, though genetic and environmental factors have been implicated. A better understanding of the causes could help to take steps to prevent the condition, which is important since it does not have a definite cure.

The presence of infection during pregnancy appears to influence the development of autism. It has been earlier suggested that severe infection during pregnancy that requires hospitalization could increase the risk of giving birth to an autistic child. A study from Denmark indicated that viral infections during the first trimester of pregnancy increased the risk of autism three times, while serious bacterial infections during the second trimester increased the risk 1.42 times.

The link between inflammation during pregnancy and the development of brain changes has been suggested by another study in mice. In the study, a type of immune cells called the Th17 cells released a chemical called IL-17 which appeared to affect certain parts of the fetal brain, resulting in ‘patches’.

In one study on mice with behavioral problems, the scientists studied a part of the somatosensory cortex where patches commonly occur called S1DZ. The somatosensory cortex is the part of the brain that is responsible for processing information obtained from our surroundings. They found that:

• In the patches, a type of nerve cells called interneurons are reduced. Interneurons are nerves that connect other nerves, and control their excitation or inhibition.

• The reduced number of interneurons caused an increased excitement in the S1DZ

• When the activity of the brain was normalized, the behavioral changes were reversed

• When the S1DZ was overstimulated in normal mice, the behavioral symptoms appeared, thereby indicating the role of the part of the brain in the development of behavioral symptoms

• When the neurons connecting the S1DZ to two other areas, the temporal association area and the striatum, were inhibited, the problems with sociability and repetitive behaviors in the mice were overcome

Another study in mice demonstrated the role of inflammation during pregnancy in the development of behavioral problems in mice. The scientists found that:

• The presence of segmented filamentous bacteria in the digestive tract, which are actually harmless, was associated with behavioral problems and the patches in the brain in the offspring

• The IL-17 appeared within a day of stimulating the immune system, suggesting that it was produced by pre-existing immune cells

• Treatment with antibiotics acting against the segmented filamentous bacteria, resulted in normal mice

Implication of these studies

• Bacteria in the digestive tract during pregnancy can cause inflammation and result in behavioral problems and brain changes

• It could be possible to reduce the risk of autism if the function of bacteria in the maternal digestive tract that stimulate inflammation is blocked

Further studies in humans are necessary to validate the findings of the two studies in humans. Scientists also warn that though severe maternal infection could be a cause of autism, it is not the only cause, and therefore eliminating bacteria may not be the answer to preventing all cases of autism.

Reference:

1. Nature, September 13,(2017)

Source-Medindia