Propionic Acid Metabolite of Gut Microbiome Implicated in the Pathogenesis of Autism Spectrum Disorder
Introduction. Autism Spectrum Disorder (ASD) is a complex neurodevelopmental disorder characterized by social deficits, repetitive behaviors, and restricted interests. The prevalence of ASD has dramatically increased recently, rising from 4.5 in 10,000 in 1966, to 1 in 68 children aged eight years in 20106. Gastrointestinal comorbidities are common among ASD patients, which has sparked interest in the role of the gut microbiome in ASD6. These studies have focused on short chain fatty acids (SCFAs), namely propionic acid, after propionic acid producing bacteria were found in increased levels within stool samples of ASD patients4. This research primarily examines the effects of injected propionic acid in rodent models and it has found that propionic acid alters gene expression, causes mitochondrial dysfunction, and glutamate excitotoxicity1,2,4. As we learn more, the challenge becomes identifying definitively the causative agent(s) of the very complex profile of behaviors and symptoms in ASD. Methods. Propionic acid was administered to rats. Reverse transcription polymerase chain reaction was used to identify and examine gene expression in the hippocampus1. An acyl-carnitine panel was used to evaluate the function of the fatty-acid oxidation pathway. Glutathione metabolism was measured by taking fasting blood samples and using high-pressure liquid chromatography with electrochemical detection and metabolite quantitation4. Rats were treated with PPA and an assay for GABA by ELISA and Glutamate/Glutamine by HPLC were performed2. The probiotic Protexin was used and fecal samples were analyzed using ELISA and culture plating3. Behavioral models used ethnovision and video recordings to quantify behavioral patterns in response to stimuli5. Results. GFAP and TNF-alpha were significantly increased in expression1. C16OH, C14, and C4OH were increased above normal. Total GSH and free GSH were reduced, while free GSSG was elevated4. Glutamate was elevated, GABA was reduced2. Probiotic treated samples showed reduced Glutamate/GABA ratio3. Increased repetitive behavior, decreased social interaction, and increased object fixation found in behavioral model5. Conclusion. Studies show that propionic acid is a metabolite of species of the gut microbiota, elevated in ASD patients with GI comorbidities. Propionic acid increases an astrocyte marker and pro-inflammatory marker, and results in mitochondrial dysfunction, which together constitute a reduction in neural plasticity. Further, glutamate toxicity helps explain the repetitive and social behaviors in these patients. Behavioral studies confirm that PPA mimics ASD symptoms in rodent models. The elucidated molecular mechanisms, behavioral studies, and probiotic effects together constitute a strong implication of PPA in the pathogenesis of ASD, and demand further study.
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