Declining FXR expression coordinates neonatal beta cell mass development with microbial bile acid metabolism...

Diet switch during weaning induces gut microbiome maturation, accompanied by the formation of adequate functional beta cell mass. Bile acid (BA), an essential microbial metabolite, regulates host glucose homeostasis by binding to its main receptor, farnesoid X receptor (FXR, encoded by NR1H4). However, the precise roles of microbial BA metabolism and FXR signalling in neonatal beta cell development are still unclear.

Islet FXR levels were determined at different perinatal stages. Postnatal changes in gut microbiome and BA profiles were examined in mice, with changes in germ-free mouse BAs serving as the control. We genetically modified beta cells to sustain FXR expression after birth (using Nr1h4-knockin [βFxrKI] mice) and performed morphological and functional analysis on murine islets. Single-cell RNA-seq and single-cell assay for transposase-accessible chromatin sequencing of islet cells were used to study FXR-mediated downstream regulation in islets. Lineage tracing was performed to evaluate beta cell fate transition. Mendelian randomisation (MR) and human islet proteomics data analysis were applied to study the pathological relevance in human diabetes.

FXR expression in beta cells declined after birth (positive cell proportion, 29.1 ± 3.1% at embryonic day 18.5 vs 4.2 ± 2.4% at 3 weeks postnatal in mice, p<0.001). This physiological change paralleled the ascending of FXR-agonistic BAs derived from gut microbiome maturation (unconjugated BA proportion, 0.9 ± 0.6% at 1 week vs 14.0 ± 5.6% at 3 weeks, p<0.05). βFxrKI mice had limited beta cell mass growth (approximately 70% of the control level at 1 week of age and only 15% of the control level at 8 weeks of age) and developed high blood glucose levels by weaning (random blood glucose, 15.2 ± 1.7 mmol/l in βFxrKI vs 7.7 ± 0.5 mmol/l in control, p<0.001), mainly resulting from elevated cell apoptosis (1.95-, 1.79-, and 3.27-fold increase vs control at 1, 2 and 3 weeks, respectively) and altered beta cell identity. Casp6 was identified as a key downstream target in beta cell FXR. Intervention with antibiotics or a specific caspase-6 (CASP6) inhibitor partially recovered the phenotypes of βFxrKI mice. Further validation in humans showed that islet FXR/CASP6 levels were elevated in individuals with type 2 diabetes (FXR, -0.039 ± 1.257 a.u. in donors without diabetes vs 0.646 ± 1.140 a.u. in donors with diabetes, p=0.0371; CASP6, -1.575 ± 0.307 a.u. in donors without diabetes vs -1.325 ± 0.381 a.u. in donors with diabetes, p=0.011). MR analysis further supported the effect of human islet FXR expression in elevating HbA_1c (β=0.006, p<0.001) with lowering fasting insulin level (β=-0.009, p=0.02) and the effect of CASP6 expression in enhancing 2 h glucose (β=0.039, p=0.01).

The declining FXR-CASP6 signals in neonatal beta cells could serve as a programmed host response to the maturing gut microbial BA metabolism to maintain normal postnatal beta cell mass development and ensure glycaemic homeostasis in adults.

Raw data of scRNA-seq and scATAC-seq are deposited in the Gene Expression Omnibus (GEO) database under the accession number GSE241408. The code used in this Mendelian randomisation study is publicly available at https://github.com/Angela-linyt/Gene_Glu_MR.git .