Intestinal colonization of germ-free mice with indole-producing E. coli modulates the central and peripheral endocannabinoidome

Colonization of germ-free (GF) mice with indole-producing E . coli exacerbates anxiety-like and stress-induced depression-like behaviours. Lack and subsequent reintroduction of gut microbiota in mice alter in opposing ways the brain and intestinal expression and levels of receptors and lipid mediators of the expanded endocannabinoid system, or endocannabinoidome (eCBome), which controls, among others, affective behaviour. By using LC/MS-MS-based targeted lipidomics and qPCR, we investigated the effect of GF mouse colonization with E. coli strains capable (wild-type, I+) or not (knock-out, I-) of producing indole, on the brain and intestinal eCBome. Unsurprisingly, indole was only detected in the feces of I+ mice (20 nmol/g). Compared to untreated GF mice, and unlike the I- strain, the I+ strain reduced the levels of 2-monoacylglycerols (2-MAGs), and 2-arachidonoyl-glycerol in particular, in the amygdala; and of all 2-MAGs except 2-palmitoyl-glycerol, and of all NAEs in the hippocampus. Moreover, it elevated 2-MAGs in the ileum, and reduced NAEs in the cecum. The mRNA expression of some eCBome receptors and metabolic enzymes was also selectively altered by the I+ strain, including Gpr55 (upregulated in the amygdala), Gpr18 (downregulated in the ileum), Cnr2 (upregulated in the colon), Gpr119 (downregulated in the cecum) and Mgll and Gde1 (upregulated in the cecum). LC/MS-MS-based targeted metabolomics revealed the presence of the indole metabolite indoxyl-3-sulfate, in the plasma and liver, but not brain, of I+ colonized mice, suggesting that indole modulates the central eCBome via indirect mechanisms. Our data pinpoint the eCBome as a potential mediator of peripheral and central effects of indole.