Prebiotics Intervention to Reduce Alzheimer's Disease Risk via Brain-Gut Axis in an APOE4 Mouse Model

Apolipoprotein å4 (APOE4) allele is the strongest genetic risk factor for Alzheimer’s disease [1]. APOE4 carriers accumulate beta-amyloid (Aâ) and neurofibrillary tau tangles earlier and with more extensive pathology compared to non-carriers. However, decades before the aggregation of Aâ and tau, cognitively normal APOE4 carriers have developed neurometabolic and neurovascular deficits, including reduced glucose oxidative metabolism, cerebral blood flow (CBF), and blood-brain barrier (BBB) function [2-6]. Emerging evidence shows that gut microbiota plays a critical role in determining brain metabolic and vascular integrity. Brain metabolite scyllo-inositol produced by microbiota Bacillus subtilis is able to reduce Aâ burden in AD patients [7, 8]. Brainiste et al. recently showed that BBB permeability is increased in germ free mice due to lack of butyrate, a short chain fatty acid (SCFA) produced by gut microbiota such as Clostridium tyrobutyricum [9]. BBB dysfunction further leads to impaired clearance of Aâ, increased inflammation, reduced CBF, and increased anxiety [10, 11]. In addition, Escherichia coli and Salmonella enterica are among the many bacterial strains that express and secrete amyloid proteins, suggesting Aâ deposits in the brain may also be influenced by the microbiota [12-14]. Moreover, amyloid and lipopolysaccharides secreted by gut microbiota activate microglia, which results in neuroinflammation [15, 16]. Collectively, changes in gut microbiome balance have the potential to contribute greatly to AD pathogenesis. Consistent findings were observed in our pilot study using a mouse model that overexpresses human Aâ via 5 familial-AD (5xFAD) mutations, and expresses human APOE4 (E4FAD). We found that E4FAD mice had significantly different microbiome diversity, increased inflammation-associated microbiota, impaired BBB transporter activity, reduced CBF, and increased neuroinflammation, compared to their APOE3 (E3FAD) littermates (Fig. 1 and 2). Our preliminary findings suggest that modifying the gut microbiome of E4FAD mice may be critical to preserve brain functions and potentially prevent the development of AD-related neuropathology for the APOE4 carriers. As diet is an important modulator of the gut microbiome, our goal is to determine if we can use dietary intervention to restore the gut microbiome composition, which might protect brain vascular and metabolic functions, and reduce neuroinflammation and AD-like pathology of the asymptomatic E4FAD mice compared to their E3FAD littermates. To achieve the goal, we have formulated a dietary supplement containing prebiotic Inulin, a non-digestible fiber of carbohydrates, known to be fermented in the gastrointestinal tract and increase beneficial microbiota and SCFAs [17, 18]. We will feed 2-month-old cognitively intact E4FAD and E3FAD mice (before they show Aâ in brain [19]) with either Inulin or vehicle control diet for 16 weeks. The central hypothesis is that Inulin is protective to brain functions via the modulation of gut microbiome. We will use multidisciplinary methods to test the hypothesis by pursuing the following three Specific Aims: Aim 1. Use “omics” approach to identify Inulin effects on gut microbiome and associated metabolism. Aim 1.1: Measure the effect of inulin diet on the gastrointestinal microbial community taxonomic structure and diversity, and on the functional gene content. Aim 1.2: Determine SCFAs and microbiota-associated metabolites using metabolomics. Aim 2. Use neuroimaging to assess inulin effects on brain vascular and metabolic functions. Aim 2.1: Identify CBF and scyllo-inositol changes in hippocampus using MRI. Aim 2.2: Determine oxidative metabolism using near-infrared diffusion correlation spectroscopy (DSC). Aim 3. Use molecular assays to determine Inulin effects on neuroinflammation. Aim 3.1: Aâ staining, neuroinflammatory gene expression and glial activation Aim 3.2: Inflammation-associated BBB leakage and Aâ transporter activity