Background & Aims: The gall bladder stores bile between meals and empties into the duodenum upon demand, thereby being exposed to the intestinal microbiome. This raises the need for antimicrobial factors, among them mucins produced by gall bladder epithelial cells. The role of the much less frequent biliary tuft cells in this scenario is still unknown.
Methods: Gall bladder contraction and mucin granule exocytosis were measured by force recording and electron microscopy, respectively, in wildtype and genetically modified mice. Stimuli were blue light in an appropriate optogenetic model, expressing channelrhodopsin-2 selectively in tuft cells, and short chain fatty acids. Acetylcholine, prostanoids and cysteinyl leukotrienes were directly assayed in supernatants of stimulated, explanted gall bladders. Reporter mice, in situ-hybridization and immunolabeling localized mediator synthesizing enzymes and receptors.
Results: Selective optogenetic stimulation of gall bladder tuft cells revealed corelease of acetylcholine and cysteinyl leukotrienes. Acetylcholine triggers exocytosis of mucin granules from cholangiocytes through the muscarinic receptor M3, and cysteinyl leukotrienes cause bladder contraction through the receptor CysLTR1. We identify propionate, a major metabolite of intestinal bacteria, as a naturally occurring stimulus activating tuft cells via the short chain free fatty acid receptor 2 and downstream signalling involving the cation channel TRPM5.
Conclusions: Our results establish gall bladder tuft cells as sensors of a microbial product, initiating two independent innate defence mechanisms through cotransmission. Acetylcholine, best characterized as a neurotransmitter, serves here as a paracrine factor triggering epithelial defence, and cysteinyl leukotrienes, known from immune effector cells, target the muscular component, emptying and closing the bladder.