Colonic gene expression in conventional and germ-free mice with a focus on the butyrate receptor GPR109A and the butyrate transporter SLC5A8

Introduction: Butyrate is a bacterial fermentation product that produces its beneficial effects on colon through GPR109A, a butyrate receptor, and SLC5A8, a butyrate transporter. In this study, we compared the expression of GPR109A and SLC5A8 between conventional mice and germ-free mice to test the hypothesis that the expression of these two proteins will be decreased in germ-free mice compared to conventional mice because of the absence of bacterial fermentation products and that colonization of germ-free mouse colon with conventional bacteria will reverse these changes. Methods: RNA was prepared from the ileum and colon of conventional mice and germ-free mice and used for RT-PCR to determine mRNA levels. Tissue sections were used for immunohistochemical analysis to monitor the expression of GPR109A and SLC5A8 at the protein level. cDNA microarray was used to determine the differential expression of the genes in the colon between conventional mice and germ-free mice. Results: In conventional mice with normal bacterial colonization of the intestinal tract, GPR109A and SLC5A8 are expressed on the apical membrane of epithelial cells lining the ileum and colon. In germ-free mice, the expression of GPR109A and SLC5A8 is reduced markedly in the ileum and colon. The expression returns to normal levels when the intestinal tract of germ-free mice is colonized with bacteria. The expression of the Na⁺-coupled glucose transporter, SGLT1, follows a similar pattern. Microarray analysis identifies ~700 genes whose expression is altered more than twofold in germ-free mice compared to conventional mice. Among these genes are the chloride/bicarbonate exchanger SLC26A3 and the water channel aquaporin 4. The expression of SLC26A3 and AQP4 in ileum and colon is reduced in germ-free mice, but the levels return to normal upon bacterial colonization. Conclusion: Gut bacteria play an active role in the control of gene expression in the host intestinal tract, promoting the expression of the genes that are obligatory for the biological actions of the bacterial fermentation product butyrate and also the genes that are related to electrolyte and water absorption.