Commensal bacteria-dependent indole production enhances epithelial barrier function in the colon

Abstract

Microbiota have been shown to have a great influence on functions of intestinal epithelial cells (ECs). The role of indole as a quorum-sensing (QS) molecule mediating intercellular signals in bacteria has been well appreciated. However, it remains unknown whether indole has beneficial effects on maintaining intestinal barriers in vivo. In this study, we analyzed the effect of indole on ECs using a germ free (GF) mouse model. GF mice showed decreased expression of junctional complex molecules in colonic ECs. The feces of specific pathogen-free (SPF) mice contained a high amount of indole; however the amount was significantly decreased in the feces of GF mice by 27-fold. Oral administration of indole-containing capsules resulted in increased expression of both tight junction (TJ)- and adherens junction (AJ)-associated molecules in colonic ECs in GF mice. In accordance with the increased expression of these junctional complex molecules, GF mice given indole-containing capsules showed higher resistance to dextran sodium sulfate (DSS)-induced colitis. A similar protective effect of indole on DSS-induced epithelial damage was also observed in mice bred in SPF conditions. These findings highlight the beneficial role of indole in establishing an epithelial barrier in vivo.

Figure 1. Epithelial barrier functions is impaired in GF mice.

(A) Real-time quantitative RT-PCR analysis of mRNA expression of Cldn7, Ocln, Tjp1, Ctnnb1, Cdh1 in colonic ECs in SPF (n = 4) or GF (n = 4) mice. Values were normalized to that of Gapdh. Data are representative of two independent experiments and show mean values ± S.D. of 4 samples performed in duplicate. *P<0.05. (B) Mouse colonic tissue was stained with anti-occludin antibody. Sections were analyzed using a confocal microscope. Bars, 50 µm. Data are representative of two independent experiments. (C) SPF (n = 8) or GF (n = 8) mice were administered 4% DSS by drinking water for 3 days. Survival rates of the indicated mice are shown. Body weight changes relative to the value prior to colitis induction are shown. Data are mean ± S.E.M of 8 mice at each time point. SPF, specific pathogen free; GF, germ free.

Figure 2. Indole and indole metabolites are absent in GF mice.

(A, B) Feces and serum were collected from either SPF (n = 3) or GF (n = 3) mice. The concentration of indole in the feces was measured by HPLC-FL, and the serum concentration of indoxyl sulfate was measured by LC-MS/MS. Data are representative of two independent experiments and show mean values ± S.D. of 3 mice. *P<0.05. SPF, specific pathogen free; GF, germ free.

Figure 3. Indole, but not indoxyl sulfate, induces the expression of junctional complex molecules in Caco-2 cells.

(A, B) Real-time quantitative RT-PCR analysis of mRNA expression of Cldn7, Ocln, Tjp1, Ctnnb1, Cdh1 in Caco-2 cells cultured with indole or indoxyl sulfate is shown. DMF or PBS was used as a control, respectively. Quadruplicate was used for each condition. Values were normalized to the expression of Gapdh. Data are representative of two independent experiments and show mean values ± S.D. of 4 samples performed in duplicate. *P<0.05. n.s., not significant.

Figure 4. Indole-containing capsules promote epithelial barrier function in GF mice.

(A) Feces were collected from SPF mice, and GF mice treated with indole- or MCT- containing capsules. Three mice was analysed in each group. The concentration of indole in the feces was measured by HPLC-FL. Data show mean values ± S.D. of 3 samples. *P<0.05. n.s., not significant. SPF, specific pathogen free; GF, germ free; MCT, Medium-Chain Triglycerides. (B) Real-time quantitative RT-PCR analysis of mRNA expression of Cldn7, Ocln, Tjp1, Ctnnb1, and Cdh1 in colonic epithelial cells of GF mice treated with indole- (n = 4) or MCT- (n = 4) containing capsules. Values were normalized to the expression of Gapdh. Data are representative of two independent experiments and show mean values ± S.D. of 4 samples performed in duplicate. *P<0.05. (C) Colonic tissues of GF mice treated with indole- or MCT- containing capsules were stained with anti-occludin antibody. Sections were analyzed using a confocal microscope. Bars, 20 µm. Data are representative of two independent experiments. (D) After oral administration with either indole- (n = 6) or MCT- (n = 6) containing capsules for 2 weeks, GF mice were treated by 4% DSS in drinking water for 3 days. Survival rate of the mice in each group is shown. Data are representative of two independent experiments. MCT, Medium-Chain Triglycerides.

Figure 5. Indole-containing capsules show preventative effect on colitis development in SPF mice.

SPF mice were treated with indole- (n = 7) or MCT- (n = 7) containing capsules for 1 week, and then challenged by 5% DSS for 6 days. Body weight changes relative to the value prior to colitis induction are shown. Data are representative of two independent experiments and mean ± S.E.M of 7 mice at each time point is shown. *P<0.05. MCT, Medium-Chain Triglycerides.