Society: AGA

Background & Aim:
Phosphatase and tensin homolog (PTEN) is a lipid phosphatase, while its protein phosphatase activity is being suggested. Since the classical role of PTEN as a tumor suppressor is unclear in the gut, we hypothesize PTEN’s alternative function in regulating gut homeostasis.

Methods:
We examined human colonic mucosal biopsies of pediatric Ulcerative Colitis (pUC) patients and control subjects to evaluate PTEN mRNA levels. We generated intestinal epithelial cell (IEC)-Pten-knockout (KO) (Pten-dIEC/dIEC) mice using the Villin-Cre system. We tested in vivo gut permeability with FITC-Dextran, intestinal epithelial cell junction, and autophagy using transmission electron microscopy and fluorescence confocal microscopy. We knocked down (KD) endogenous PTEN gene in human colonic epithelial HT-29 and DLD-1 cells to perform immunoblotting and immunoprecipitation. We used fluorescence in situ hybridization and 16S rRNA gene sequencing. We generated Pten-dIEC/dIEC;Myd88-KO, Pten-dIEC/dIEC;Tlr5-KO, and Pten-dIEC/dIEC;Tlr4-KO mice to study the pathological link of IEC-Pten-KO.

Results:
PTEN mRNA level is 70% reduced (P<0.01, Mann-Whitney U test) in colonic mucosa tissues of pUC (n=9) compared to controls (n=19). IEC-Pten-KO mice exhibit disrupted intestinal epithelial cell junctions and increased in vivo gut permeability. These mice have reduced levels of cell-junction proteins (ZO-1, JAM-C, Claudin-1) in IECs. PTEN-KD decreases cell-junction protein levels, but increases SNAIL/SLUG transcription repressor proteins in HT-29 and DLD-1 cells. PTEN-KD promotes the phosphorylation of Ubiquitin protein and increases the ubiquitination of SNAIL/SLUG in IECs. Loss of PTEN initiates the autophagy pathway in IECs. But, it lowers YKT6 levels to inhibit the fusion between autophagosome and lysosome in IECs. Thereby, PTEN deficiency inhibits the protein degradation machinery of autophagy. We further observed that IEC-Pten-KO mice have increased Bacteroides, but decreased Akkermansia muciniphila in the feces. IEC-Pten-KO mice have increased gut bacterial dissemination to the colon submucosa. Accordingly, Pten-dIEC/dIEC;Myd88-KO, Pten-dIEC/dIEC;Tlr5-KO mice develop massive inflammation in extra-intestinal organs, including the liver and kidney. Pten-dIEC/dIEC;Tlr4-KO mice exhibit mild inflammation in the liver where the abundance of Bacteroides is markedly increased.

Conclusion:
PTEN deficiency promotes the ubiquitination of SNAIL/SLUG transcription repressor protein, while blocking the autophagy flux by inhibiting the autolysosome formation in IECs. Thus, ubiquitinated SNAIL/SLUG cannot be degraded in PTEN-deficient IECs, resulting in augmented SNAIL/SLUG that suppresses the expression of cell-junction proteins and then disrupts the intestinal epithelium. Consequently, PTEN deficiency in IECs increases the gut permeability and risk of gut microbe-induced disorders.

Background and Aims: The Aryl Hydrocarbon Receptor (AhR) plays important functions in intestinal stem cell differentiation, intestinal homeostasis, and immune regulation in the gut, however, its role in regulating intestinal immune tolerance remains poorly understood. In this study, we assessed the role of AhR knockout on the intestinal paracellular tight junction (TJ) barrier, and how it affects the factors influencing intestinal anergy and immune tolerance.
Methods: Ahr^fl/fl, and AhR^Vil-Cre mice were maintained in a specific pathogen-free area until injected with tamoxifen to delete the AhR gene, and then moved to the normal housing. The AhR deletion was maintained by weekly tamoxifen injections for 3 weeks. We performed physiological assessments of the gut epithelial barrier and used flow cytometry, and qRT-PCR analysis to determine the effect of epithelia-specific AhR^-/- on the gut immune system.
Results: Conditional gut epithelium-specific knockout of AhR in AhR^Vil-Cre mice significantly increased the colonic transepithelial resistance (TER) and reduced the paracellular flux of small molecule, urea, and macromolecule inulin, compared to AhR^fl/fl mice. Assessment of transcript levels in the mouse colonic tissues, after 3 weeks of AhR deletion, showed that AhR knockout did not alter the baseline levels of TNF-α, IFN-γ, IL-4, and TGF-β, however, the transcript levels of IL-6, IL-1β, and Il-17A showed marked elevation. Inflammatory stimuli in the form of intraperitoneal LPS administration further amplified the increase in IL-6, IL-1β, and IL-17A mRNA levels in AhR^Vil-Cre mice. Knocking out AhR in the gut epithelial cells also reduced the colonic transcript levels of IL-10, a key anti-inflammatory cytokine, and Programmed death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA4), the two key proteins, involved in limiting the pro-inflammatory responses in the gut mucosa. Flow cytometry analysis using control Ahr^fl/fl and AhR^Vil-Cre mouse colons subjected to the same tamoxifen administration cycles and LPS challenge showed an increased abundance of IL-6^hi inflammatory M1 macrophages and IL-17A^hi RoRγt⁺CD4+ Th17 cells in the AhR^Vil-Cre colonic lamina propria under baseline conditions which were further increased by the administration of LPS.
Conclusion: Our data show that deletion of the AhR gene in the gut epithelial cells reduces the epithelial paracellular TJ permeability. This causes the down-regulation of immunological-anergy-promoting proteins and increases the presence of circulating M1 polarized macrophages and Th17 cells in the mice colon in the event of an inflammatory challenge. Our study also highlights the role of paracellular TJ permeability in intestinal immune homeostasis.

Anirudh Venkatesh

Appearances