The Effects of Age, APOE Genotype and Sex on the Gut Microbiome of EFAD Transgenic Mice

Currently, there are 5.7 million Americans diagnosed with Alzheimer’s disease (AD), a fatal neurodegenerative disease with no cure and few palliative treatments. Emerging as an important component of AD pathology, the gut microbiome (GM) consists of a vast population of gastrointestinal bacteria involved in the digestion of carbohydrates, lipids, and proteins, providing critical metabolites for the host. AD patients exhibit dysbiosis, a significant decrease in diversity and in putative beneficial bacteria, disrupting many metabolic processes. Thus, the GM would be potential AD therapeutic target, as the reestablishment of a healthy GM has reportedly improved cognition in humans. However, the development of effective GM-targeted therapeutics is stalled, likely by the unclear effects of the universal biological variables (UBVs) of AD risk (age, APOE genotype, and sex) on the bacterial composition and metabolic function of the GM. While age is the greatest risk factor for AD, APOE4 is the greatest genetic risk factor compared to the common APOE3, and females (♀) exhibit a 2-fold greater risk than males (♂). To study the interactions among UBVs, we utilize the EFAD transgenic mice (5xFAD+/-/APOE+/+), a mouse model that exhibits an age-dependent AD-like phenotype with increased behavioral deficits, Abeta deposition and neuroinflammation in the following hierarchy: ♀E4FAD > ♂E4FAD = ♀E3FAD > ♂E3FAD. The timepoints chosen for this study are 4M, which represented very early pathology though significant in ♀E4FAD, and 8M, where pathology reached significance for all four cohorts. Bacteria from 8M EFAD fecal samples were isolated and identified using high throughput 16S rRNA gene amplicon sequencing and compared against the 4M dataset. Shotgun metagenomic sequencing (SMS) was also performed to identify biological pathways associated with the 8M EFAD GM across APOE genotype and sex. Our results demonstrated that the UBVs interact to shape the compositional profile and metabolic function of the GM, corresponding with the age-associated increase in AD-like pathology, resulting in dysbiosis greatest in ♀E4FAD > ♂E4FAD = ♀E3FAD > ♂E3FAD. Therefore, the characterization of the AD GM and further development of GM-targeted AD therapeutics will require further examination of the interactive effects among the UBVs.