Essential roles of the unfolded protein response in intestinal physiology

The intestinal epithelium serves as an essential interface between the host and microbiota, regulating innate and adaptive immunity, absorption of nutrients and systemic metabolism, and mediating bidirectional communication with the nervous system. The intestinal epithelium suffers constant challenges to the proteostasis machinery due to its exposure to the dynamically changing and microbial laden lumenal gut environment and to the high secretory demand placed on multiple epithelial cell types to accommodate gut and systemic physiology-especially goblet, enteroendocrine and Paneth cells. In all cases, intestinal cells require an active unfolded protein response (UPR) to sustain their physiological function, the main pathway that monitors and adjusts secretory function changes in the environment. A specialised endoplasmic reticulum (ER) stress sensor uniquely expressed in epithelial cells lining mucosal surfaces, termed inositol-requiring transmembrane kinase/endoribonuclease β, has specific roles in intestinal epithelial homeostasis, regulating mucus production and communication with microbiota. Chronic ER stress or genetic mutations affecting key UPR mediators contribute to the occurrence of inflammatory bowel disease and ulcerative colitis, in addition to colon cancer. Here, we review recent advances linking the UPR and ER stress with gut physiology and intestinal disease. Therapeutic strategies to alleviate ER stress or enforce UPR function to improve intestinal function in ageing and in bowel diseases are also discussed.