Hormonal and Neuronal Regulation of Gastrointestinal Physiology

Abstract

Sensory responses to intestinal nutrients, such as lipid or protein, help coordinate the complex process of digestion and maintain metabolic homeostasis. Nutrient ingestion evokes rapid physiological responses, including gut hormone secretion, delayed gastric emptying and gut motility, secretion of bile and digestive enzymes, and changes in feeding behavior. Pathways by which peripheral nutrient detection impact physiology remain poorly understood, including primary chemosensory mechanisms, relevant signaling molecules, and the roles of various peripheral and enteric neurons. Here, I studied (1) gut-innervating vagal afferents, and (2) fat-evoked gallbladder responses to understand signaling pathways involved in intestinal sensation.

First, I studied vagal sensory neurons expressing GPR65, an orphan G-protein-coupled receptor (GPCR). Previous studies using Gpr65-ires-Cre mice, revealed that vagal GPR65 neurons densely innervate intestinal villi, where they function as chemosensors. I performed a high throughput chemical screen to identify GPR65 agonists, and found X to be a potent agonist of GPR65 in cultured cells. Preliminary studies using X and GPR65 knockout mice did not reveal acute modulation of gastric pressure, but the identification of a GPR65 agonist should help pinpoint the role of this receptor in gut-brain signaling in future studies.

Next, I studied sensory pathways involved in fat sensation in the intestine. Several models for intestinal fat detection and signal transmission have been proposed. Here, I monitored gallbladder contraction to measure systemic responses to ingested fat. I developed an in vivo imaging method for observing real-time gallbladder emptying with an automated data analysis pipeline powered by machine learning. Lipid but not amino acid induced gallbladder emptying, and this response was abolished in CCK receptor knockout. These findings raise a conundrum, as dietary amino acid also induced CCK release but does not evoke gallbladder contraction.

Finally, a long-hypothesized role for neuronal regulation was examined in fat-evoked gallbladder contraction. Optogenetic activation, electrical stimulation, or transection of the vagus nerve did not impact gallbladder contraction. Moreover, chemogenetic activation or inhibition of cholinergic neurons (which includes major populations of the enteric nervous system) or cholinergic signaling did not affect gallbladder emptying. Instead, acute injection of CCK in a remote site (tail vein) was sufficient to contract the gallbladder, indicating that CCK likely functions as a humoral factor acting at a distance to mediate signals from the intestine to gallbladder. Together, these studies define several steps in a lipid-sensing pathway that ultimately influences systemic host physiology.