Nutrient-specific combinatorial secretion of GLP-1 and neurotransmitters in the enteroendocrine L-cell line GLUTag

Enteroendocrine L cells function as chemosensors in the intestine, secreting glucagon-like peptide-1 (GLP-1) to signal nutrient ingestion to the brain. Since GLP-1 secretion is induced by a variety of nutrients, including sugars, amino acids, and fatty acids, GLP-1 receptor-expressing neurons are also activated regardless of the nutrient identity, making it unclear how the brain discriminates between different nutrients. By examining vesicle dynamics and extracellular signals associated with GLP-1, glutamate, and ATP in GLUTag cells using total internal reflection fluorescence (TIRF) microscopy, a technique that selectively visualizes fluorescent molecules near the plasma membrane, we demonstrated that distinct nutrients evoke specific secretory patterns. D-glucose induced the co-secretion of GLP-1, glutamate, and ATP, whereas L-glutamine induced the secretion of GLP-1 and glutamate but not ATP. Butyrate and sucralose, which are also GLP-1 secretagogues, did not induce the secretion of glutamate or ATP. While Ca²⁺ acts as the trigger for exocytosis, additional regulatory mechanisms are required to generate these distinct secretory patterns. We therefore investigated the cAMP and cGMP pathways and found that neither their activation nor inhibition detectably affected glutamate and ATP secretion, suggesting the involvement of other nutrient-specific signaling pathways. Collectively, our findings suggest that L cells distinguish and encode nutrient identity as combinatorial secretion patterns of GLP-1 and neurotransmitters.