Determinants of enteric hyperoxaluria in the SAMP1/YitFc mouse model of spontaneous ileitis

Enteric hyperoxaluria (EH) results from increased oxalate bioavailability in the gastrointestinal (GI) tract, often affecting patients with inflammatory bowel disease (IBD). We investigated the pathophysiology of EH in an ileitis mouse model, hypothesizing that fat malabsorption, increased gut permeability, and microbial shifts collectively contribute to the hyperoxaluric phenotype in the setting of GI tract inflammation. SAMP1/YitFc (SAMP1) mice and their parental AKR controls were fed one of three diets varying in fat content (10%, 45%, or 60% kcal), each supplemented with 1% oxalate, for 6 weeks. Plasma (P), urine (U), oxalate (Ox), and creatinine (Cr) levels were measured, while stool lipid species were analyzed using mass spectrometry. Intestinal permeability was assessed using sucralose and ¹³C₂ oxalate gastric gavage in SAMP1 and AKR mice. Histology, qPCR, and Western blotting were performed on kidney, liver, and GI tissues. Microbial DNA was analyzed at the community, genus, and functional levels. Changes in bacterial metabolic pathways were investigated in the mice fed the highest fat content. The oxalobiome of SAMP1 and AKR mice was characterized using our bioinformatics pipeline. On high-fat diets, SAMP1 mice had higher UOx, POx, and PCr levels than AKR mice. Increased levels of diacylglycerols and free fatty acids in SAMP1 stool samples suggested fat malabsorption. A decrease in ZO1 and occludin intestinal expression, coupled with significantly increased urinary sucralose and oxalate levels, indicated increased intestinal permeability. Microbiome analysis revealed the enrichment of Lactobacilli and Bacteroides in SAMP1 mice, with bacterial pathways favoring lipid synthesis and glyoxylate metabolism. Ileal SLC26A6 protein expression was significantly reduced in SAMP1 mice. SAMP1 mice also developed progressive kidney injury with interstitial inflammation. These findings highlight fat malabsorption as a central pathophysiologic disturbance in EH that reduces luminal calcium availability for oxalate binding, is associated with enhanced intestinal permeability, and accompanies microbiome and enzymatic pathway alterations.