Scientists at Duke University have supported naval oceanographer Carina Blocks' claim that the dust pollution she and her fellow female sailors were regularly exposed to, combined with unavoidable job stress, were causing adverse health outcomes for their children. Air pollution, alongside housing insecurity during pregnancy, contribute to autism-like social behavior and altered wiring in the brains in male, but not female, pups.
Block recalls: "I was pregnant, stressed, and worked near airplanes." Hydrocephalus was eventually diagnosed in Block's daughter, but the new research by Block and colleagues, published in Cell Reports, suggests that if Block had been expecting a son, he might have been diagnosed with autism."
The authors note that increased incidences of neurodevelopmental disorders (NDDs) are linked to air pollution, such as diesel engine exhaust in trucks, as well as a strong male bias.
Despite the fact that 99% of people across the world live in dangerous environments, only one out of every 44 children develops autism, and four times more boys than girls, Bilbo believes the cause is maternal stress, which is associated with poverty and housing instability.
While there is evidence to support Bilbos' claim, it is impossible and illegal to directly test these ideas in pregnant women, with the intention to discover any biological mechanisms by which air pollution and stress might combine to rewire the brains of developing children. Instead, Block and her team exposed pregnant mice to the deplorable housing and air quality conditions that many people encounter every day.
Diesel exhaust particles were found in mouse mothers' urine as a proxy for air pollution. At the conclusion of their pregnancy, the female mice were given less building materials than usual for constructing nests for their pups, effectively implying inadequate housing.
The results of the research demonstrated that stressed mothers remained excellent parents, in that they nursed and groomed their pups just as much as mouse mothers who were protected from prenatal stressors. However, male pups misread social cues throughout their adult lives.
Block and her colleagues then investigated if the male pups' brains had been rewired early on, leading to shyer male teens. Specifically, the research team examined if the male brains had not received the necessary training early in development.
All animals are born with an overabundance of brain cell synapses, which are reduced as the animal grows. The synapses leading to successful activities, such as picking up a glass, are maintained and strengthened, whereas the connections leading to failed attempts are removed. In early postnatal brain development, an exuberant period of synaptogenesis is closely followed by and overlaps with a period of synaptic pruning, where weak or unnecessary synapses are eliminated.
Stressed mothers who had inhaled diesel fumes gave birth to male pups who, as toddlers, appeared to miss this synapse organization in the anterior cingulate cortex (ACC), a brain area crucial for perceiving and producing social cues. However, this later left open the question of how pregnancy induced typical brain sculpting.
Block and his team examined the immune system more closely, and specifically the brain's microglia, which are keen observers for damaged or dead synapses, which they will readily remove to help tidy up the brain. Microglia are the primary immune competent cells of the brain and may therefore be poised to act as immediate responders to environmental disturbances, according to the study.
Block argued that if there were more synapses than usual in male pups born to stressed and pollution-exposed mothers, there might also not be as many microglia in affected males' brains, as previously reported, including an increased relative expression of microglial genes in male brains, as well as females.
Despite smog and housing stress, Block and his colleagues found that adolescent males from stressed mothers had just as many microglia in the ACC as did their peers from non-stressed mothers. However, microglia have less of the protein that stimulates their appetite for synapses, and this might explain the abundance of synapses.
The effect on male mice born to smog- and stress-exposed mothers was strikingly reversed by adulthood. Kafui Dzirasa, MD, PhD, a Duke neurobiology professor and psychiatrist, studied mice with autism-linked genes and observed their behavior and behavior in recent experiments.
People with autism are often portrayed as less social, but Block acknowledged that if you have met one person with autism, you have also encountered one person with autism. These tests, essentially pigeon-holes rodents as autism-like if they have less desire to socialize.
Block and Bilbo claim that their new investigation reveals a clear mechanism in mice that might explain why high levels of air pollution increase the likelihood that a child will develop autism only if they are born in a poor neighborhood. It might also lead to the development of medications that help to prevent microglia from being manipulated by environmental stressors, since diesel exhaust and housing stress trigger a similar immune response when pregnant women get the flu.
Bilbo and her colleagues hope that their recent findings on the effect of stress and air pollution during pregnancy will inspire policymakers to support clean air initiatives and social services, such as improved and expanded public housing.
Our findings reveal a mechanism by which environmental pollutants can synergize with psychosocial stress in pregnant mothers and induce MIA, which has specific long-term effects on male brain development and function, according to the authors, in recent years. Increased economic activity and lower environmental protection measures have increased the likelihood that increased air pollution will worsen the health of future generations.