An FXR-Casp6 axis-mediated bile acid/gut microbiota sensing modulates neonatal ß-cell mass expansion [scATAC-Seq]
Weaning diet switch brings gut microbiome maturation along with postnatal formation of sufficient matured ß-cell mass. The matured gut microbiota elevated agonistic components of bile acid (BA) pool towards farnesoid X receptor (FXR) that was paralleling with the declined ß-cell FXR expression. To investigate whether BA/FXR could link postnatal ß-cell development and gut microbiota maturation, we forced persistent FXR expression in ß cells (ßFxrKI) and found decreased neonatal ß-cell mass growth and increased glycemia in weaned ßFxrKI mice, which could be partially recovered by ablating gut microbiota before weaning. scRNA and scATAC seq analysis showed different ß cell growth trajectories with suppressed intrinsic cell proliferation and elevated cell apoptosis in ßFxrKI. Caspase-6 was then identified as a dominant ß-cell FXR downstream effector to mediate its regulation. The negative regulation of the FXR-Casp6 axis on postnatal ß-cell mass expansion reflected a programmed cellular response to gut microbiota maturation in neonatal mice. Overall design: We constructed a mouse model of ß-cell-specific FXR overexpression (ßFxrKI). The overall goal of this study was to define the transcriptome alterations for pancreatic islet cells in control and ßFxrKI mice from postnatal 1 week to 3 weeks, and we further identified dominant FXR targets in ß cells. For the construction of  ßFxrKI mice, a cassette of Fxr (Nr1h4) cDNA sequence with a 3xflag in N terminal followed a sequence containing the CAG promoter and loxp-PGK-Neo-polyA-loxp and was knocked in the first intron of the Rosa26 site. Fragment of loxp-PGK-Neo-polyA-loxp prevents the transcription of Nr1h4 cDNA as a “STOP” sequence. When mice with this knock-in fragment were mating with Ins2-Cre mice, the floxed sequence was cut off, Nr1h4 can be driven to express, and the mice with specifically forced Fxr expression in ß cells were generated.