Mammalian chemosensory bile acid detection supports species and gut microbiome evaluation

Abstract

The rodent accessory olfactory system (AOS) detects environmental chemosignals and guides social and survival-oriented behaviors. Fecal bile acids activate neurons in the AOS, potentially serving as mammalian pheromones and kairomones, but few molecules in this large class have been evaluated thus far. We used live volumetric Ca2+ imaging to screen naturally occurring bile acids for their capacity to activate peripheral vomeronasal sensory neurons (VSNs). We found that taurine-conjugated bile acids (tauro-BAs), including taurine-conjugates of cholic acid, deoxycholic acid, lithocholic acid, and chenodeoxycholic acid (TCA, TDCA, TLCA, TCDCA, respectively) activate large populations of VSNs at sub-micromolar concentrations. Tauro-BA-sensitive VSNs rarely responded to unconjugated (CA, CDCA, DCA, LCA), glycine-conjugated (GCA, GDCA, GLCA, GCDCA), or keto-conjugated (7-keto DCA, 12-keto DCA, 7-keto LCA) bile acids. Tauro-BA-sensitive VSNs were also insensitive to well-studied sulfated steroids, suggesting tauro-BAs activate vomeronasal receptors that have not yet been de-orphaned. Among the tauro-BAs, TDCA displayed particularly strong potency, activating many VSNs at sub-micromolar concentrations. Tauro-BAs were not detectable in mouse fecal extracts by mass spectrometry, but were found in reptile fecal extracts and germ-free mouse fecal extracts. VSN responses to germ-free and conventional mouse fecal extracts, conjugated bile acids, and tauro-BAs revealed that non-overlapping populations of VSNs respond to germ-free and conventional mouse feces. A subset of VSNs that were activated by germ-free mouse fecal extracts responded to tauro-BAs, whereas VSNs responsive to conventionally fecal extracts responded to unconjugated bile acids. In vivo exposure to TDCA alone, and to mouse fecal extracts spiked with TDCA, elicited mild aversion and stress-associated behaviors in a non-social context (avoidance, digging, grooming, etc.). These studies establish tauro-BAs as a novel class of aversive vomeronasal ligands that vary in feces across species and gut microbiomes.