Cadmium exposure persistently modulates the gut-liver axis in an Alzheimer’s disease mouse model

The human Apolipoprotein E4 (ApoE4) variant is the strongest known genetic risk factor for Alzheimer’s disease (AD). Cadmium (Cd) has been shown to impair learning and memory at a greater extent in humanized ApoE4 knock-in (ApoE4-KI) mice as compared to the ApoE3 (common allele)-KI mice. In this study, we determined the extent that cadmium interacts with the ApoE4 gene variants to modify the gut-liver axis, which is important for xenobiotic biotransformation and nutrient homeostasis. Large intestinal content bacterial 16S rDNA sequencing, serum lipid metabolomics, and hepatic transcriptomics were analyzed in ApoE3- and ApoE4-KI mice orally exposed to vehicle, a low dose, or a high dose of Cd in drinking water. Aligning with the previous report showing that the ApoE4-KI males are more susceptible to cadmium-induced memory deficit, ApoE4-KI males had the most prominent changes in gut microbiota, including an up-regulation of A. muciniciphila, which is a biomarker for AD in humans, as well as predicted down-regulation of many essential microbial pathways involved in nutrient and energy homeostasis. Serum lactate was lower only in ApoE4-KI males following Cd exposure. In the host liver, cadmium-exposed ApoE4-KI males had the most differentially regulated pathways; specifically there was an enrichment in several pathways involved in platelet activation, which is known to amplify liver damage and inflammation. These pathways were associated with up-regulated Prevotella and A. muciniphila in intestine at the Cd low dose. Cadmium-exposed ApoE4-KI mice also had the most differentially regulated hepatic drug processing genes, and in particular, the up-regulation of Cyp2 family, Ugts, Gsts, and Slco1b2 in liver associated with the up-regulation of Clostridiaceae in the intestine at the Cd high dose. In conclusion, Cd exposure profoundly modified the gut-liver axis in the most susceptible mouse strain to neurological damage, (ApoE4-KI males) evidenced by up-regulation in microbial AD biomarkers, reduction in energy supply-related pathways in gut and blood, and up-regulation in hepatic pathways involved in inflammation and xenobiotic biotransformation.,Total DNA was isolated from the large intestinal content of male and female ApoE3-KI and ApoE4-KI mice (n=4-5). Briefly, samples were prepared using an E.Z.N.A. DNA Stool Kit (Omega Bio-tek Inc., Norcross, GA) per the manufacturer’s instructions. The V4 hypervariable region of 16S rDNA was amplified and sequenced using a HiSeq 2500 second generation sequencing platform (250-bp paired-end) (Novogene Corporation Inc., Sacramento, CA). Total RNA was isolated from livers of Cd or vehicle exposed ApoE3-KI and ApoE4-KI mice (n=4 per exposure per sex) using RNA zol Bee reagent (Tel-Test Inc., Friendswood, TX). cDNA libraries were prepared using a Clontech cDNA library prep kit (Clontech Laboratories Inc., Mountain View CA), and were sequenced using a NextSeq 500 sequencing platform (75 bp paired end).,