Investigating the Role of Gut Microbiota and Bacterial Fermentation Product in Stress-Induced Autistic-Like Behaviors

Autism spectrum disorder (ASD) is a devastating disease with a wide spectrum of manifestations that involve several behavioral deficits, including social communication deficit and stereotypic behaviors. Epidemiology studies suggest that both genetic and environmental factors could contribute to the etiology of ASD. Recently, clinical researches show that a significant proportion of subjects with ASD suffer from gastrointestinal (GI) complications including constipation and abdominal pain, etc. Furthermore, the gut microbiome compositions differ between ASD and healthy subjects. Probiotics and fecal transplants have been proposed for human trials, and have been shown to be therapeutic in ASD animal models. Therefore, we hypothesize that gut commensal microbiota could play a critical role in ASD. Based on our preliminary data, we developed an antibiotic cocktail (ABX) comprised of four different antibiotics to completely deplete gut microbiota in mice. These ABX-treated mice exhibit social deficits similar to the symptom of ASD. Further, social deviation in ABX-treated mice is associated with elevated levels of the stress hormone corticosterone, which is primarily produced via activation of hypothalamus-pituitary-adrenal (HPA) axis. Accordingly, removal of the adrenal gland, antagonism of the glucocorticoid receptor, and pharmacological inhibition of corticosterone synthesis effectively correct social deficits. Chemogenetic inactivation of hypothalamic neurons dramatically increases social behavior. These data strongly suggest that gut microbiota control social behavior via regulating HPA axis. Interestingly, we found that the receptors for bacterial fermentation products, short-chain fatty acids (SCFAs), abundantly express in HPA axis. Administration of SCFAs effectively increases social behavior and suppresses corticosterone. Herein, we aim to identify the specific bacterial strain and SCFA that are able to suppress stress response and promote social behavior. In this three years proposal, we have three specific aims. Specific aim 1: Identify the specific bacterial taxa in mediating stress-induced ASD behaviors. We will screen the gut bacteria that promote social behavior and suppress corticosterone by applying different combinations of antibiotic. The fecal samples from those ABX combinations-treated mice will be analyzed by microbiome sequencing and transplanted to other mice to validate if the behavioural and endocrinal phenotypes can be transferred by fecal materials transplant. The specific bacterial strain will be determined based on the sequencing result. Specific aim 2: Determine the role of SCFAs in stress-induced ASD behaviors. We will establish an in vitro platform to screen the effect of SCFAs on stress response in primary culture cells isolated from the adrenal gland, pituitary gland, and hypothalamus. The SCFA that can potently suppress the stress response in the cells will eventually be validated in microbial-depleted mice by testing their behavioral and hormone changes. Specific aim 3: Test the probiotic and SCFA therapies in mouse models of ASD. To proof our working hypothesis, we will administer the identified bacterial strain and SCFA from previous aims to two different models of ASD. The behaviors, stress hormone, and neuropathologies will be thoroughly examined after the probiotic or SCFA treatment. With the completion of this study, we anticipate developing a safe, non-invasive, and microbiome-based therapy for ASD.