By Gabriela Beug
Autism spectrum disorder is a complex condition linked with how the human brain develops. But some scientists think that treating autism might start with looking for therapies directed at the gut, rather than the brain. They are exploring using poop infusions — fecal transplants that aim to change the microbial composition of the gut — to reduce the symptoms of autism.
Chinese researchers recently tested this hypothesis by transplanting feces from children — some of whom had autism spectrum disorder and some of whom were typically developing — into germ-free mice. Their observations suggest that gut microbes have a direct correlation to how certain substances are metabolized in the brain and to autism-like behavior. What’s more, the composition of the microbial population and its plethora of inhabitants was different between both groups of mice. Although these findings are preliminary and only in mice, scientists are one step closer to identifying why fecal transplants may help individuals with autism.
Although autism is on the rise, current treatments are inadequate. According to the Centers for Disease Control and Prevention, while in 2000 only 1 in 1,500 children in the United States experienced autism, this number rose to 1 in 54 children by 2017. The most widely used drug currently approved by the FDA, risperidone, has yet to be widely adopted for long-term use, as the side effects such as anxiety are not ideal for people with a communication disorder.
The thinking behind administering fecal transplants to treat autism has its roots in the current understanding of how two of the body’s nervous systems interact. While the central nervous system consists of the brain and spinal cord, there is a so-called “second brain”, the enteric nervous system, which controls the gastrointestinal system. Researchers think the two systems are joined by physical and biochemical connections, allowing the health of the gut to be communicated to the brain and vice versa. Thus, a microbial imbalance could be communicated to the brain, contributing to developmental disabilities like autism.
For this reason, scientists are exploring how altered gut bacteria could be connected to autism spectrum disorder. Potential treatments include using probiotics and prebiotics to alter microbes in the guts of people with autism — although evidence for the effectiveness of these is limited, a team of scientists led by Andrea Haqq of the University of Alberta in Canada wrote in a review paper in June in Autism Research.
When it comes to fecal transplant treatments, the Chinese researchers wanted to test a hypothesis in mice. They had a special focus on two molecules involved in metabolism: tryptophan and serotonin. Tryptophan is an essential amino acid and a prerequisite to produce serotonin, which is a neurotransmitter responsible for modulating mood and cognition among other functions.
Other studies have established that abnormal tryptophan levels are associated with intestinal inflammatory diseases such as intestinal bowel disease. Altered levels of serotonin have been observed in many different psychiatric disorders such as anxiety and depression.
To explore the role of these molecules in autism spectrum disorder, a team led by Jie Chen of Chongqing Medical University and Hong Wei of Army Medical University, also in Chongqing, transplanted feces from children with autism into mice. They found that these mice had different levels of substances involved in metabolism when compared to mice with feces transplanted from typically developing children; for example, tryptophan and serotonin were present in higher concentrations. Not only that, certain microbiota were different in both groups; for instance, the mice that had received fecal transplants from children with autism had less of a type of bacteria called Verrucomicrobia in the gut. The scientists suggest that gut bacteria may be influencing the ways in which tryptophan and serotonin are metabolized in the body. “This study provides valuable clues for studying the relationship between intestinal microorganisms and [autism spectrum disorder],” the scientists wrote in March in the journal mSystems.
Much more work remains to be done to establish whether fecal transplants are an effective treatment for autism in humans. In 2019, researchers led by Rosa Krajmalnik-Brown of Arizona State University in Tempe found a 50% improvement in autism symptoms in children with autism who had received fecal transplants from a person without autism. She is in the process of running a follow-up study in 84 adults.
These findings might pave the way for unconventional treatments for similar diseases like Parkinson’s or Alzheimer’s, and promote microbial interventions as an alternative approach to medication.
Gabriela Beug is a junior studying biomedicine at the University of Tübingen in Germany. She is a former intern at the National Institutes of Health and the Friedrich Miescher Laboratory of the Max Planck Society. Trading in her pipette for a pen, she now is taking the plunge into the world of science journalism. Currently she is working at the Osteogenesis Imperfecta Foundation as a science communication intern. E-mail her at gabriela.beug@gmail.com.
This story was produced as part of NASW's David Perlman Summer Mentoring Program, which was launched in 2020 by our Education Committee. Beug was mentored by Alexandra Witze.
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