Intestinal Transporters and Gut Function

Nutrient Detection in the Gastrointestinal Tract
Helen E Raybould, Shuzhen Hao, Samara Freeman, Alice Liou, Gabriel Paulino, James Sharp, Michael Donovan, Diana Chavez, Nicolas Darcel, Lauraine Rivier and Kellie Whited
Dept of Anatomy, Physiology and Cell Biology, UC Davis School of Veterinary Medicine, UC Davis, CA 95616

The presence of nutrients in the gut is important in coordination of GI function in the postprandial period, including control of short term food intake.  Each macronutrient group elicits separate and distinct GI responses, evidence for specific pathways for detection of each macronutrient group. The primary sensory mechanism for nutrient detection in the gut is likely the entero-endocrine (EC) cells.  These secretory cells represent less than 1% of the total population of epithelial cells in the gut. EC cells store and release regulatory peptides, such as cholecystokinin (CCK), glucagon-like peptides (GLP-1 and GLP-2) and peptide YY (PYY), in response to nutrients.  These regulatory peptides activate the gut-brain axis via interaction with specific receptors expressed by vagal afferent fibers terminating in close apposition to EC cells, which drives parasympathetic outflow to the viscera and input to the hindbrain and hypothalamus for control of food intake.  EC cells respond to nutrients using a number of different cell surface proteins, including G protein-coupled receptors (sweet and bitter taste receptors, fatty acid receptors), nutrients transporters, such as sodium-glucose co-transporter (SGLT – detection of glucose) and PepT1 (detection of protein hydrolysates), and proteins expressed by other epithelial cells, such as apolipoprotein A-IV (detection of lipids).  There is evidence for considerable plasticity in these pathways, in particular those induced by long term changes in diet; an understanding of the mechanisms and pathways underlying nutrients detection in the GI tract has important implications in metabolic disease, such as insulin resistance and obesity.