SERP1: a new GLP-1 receptor interactor
Gluco-incretins are intestinal hormones released by endocrine cells of the intestinal epithelium during nutrient digestion. They play an important role in potentiating the effect of glucose on pancreatic β cells, mainly GLP-1 (glucagon like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). GLP-1 is a peptide produced in jejunum and ileum by L cells through a mechanism of proteolytic cleavage of the pre-pro-glucagon molecule. It acts by binding to its receptor (GLP-1R), expressed in the pancreas. Unlike GIP, insulinotropic glucose-dependent activity of GLP-1 is preserved in type 2 diabetes mellitus. Significant effort has been undertaken by researchers to find GLP-1 like molecules, or GLP-1R agonists, considered as potential drugs for treating this type of diabetes as well as NAFLD.
GLP-1R is a G-protein coupled receptor that transduces many signaling pathways upon GLP-1 binding. Like all other GPCRs, its expressions at the cell surface are tightly controlled by N-glycosylation mechanisms, ER retention, mediated by complex mechanisms involving protein partners.
In this article, Yuanyuan Xiao developed an effective split-ubiquitin based membrane yeast two-hybrid (MYTH) system to identify new GLP-1R interactors. Among all the proteins identified through screening, she identified SERP1 (stress-associated endoplasmic reticulum protein 1) to be a promising interactor candidate for GLP1-R. This was confirmed by using co-immunoprecipitation experiments from HEK cells co-transfected with both partners. The article also includes a full description of the role of SERP1 on GLP-1R N-glycosylation. The role of SERP1 in regulating GLP-1R signal transduction is assessed by measuring cAMP production.
Moving from MYTH to reality, identifying a new GLP1R protein interactor and understanding its impact on receptor function. Read about this new step forward in GLP-1 research and for the discovery of new treatments for metabolic diseases such as diabetes and NAFLD.
Glucagon-like peptide 1 (GLP-1) exerts multiple effects on metabolism through its receptor, GLP-1R, in the liver. Activation and transduction of GLP-1R require complex interactions of largely unknown accessory proteins, and these processes are crucial to the response to endoplasmic reticulum (ER) stress. Using the membrane-based split ubiquitin yeast two-hybrid system (MYTH) and a human liver cDNA library, we obtained the human GLP-1R interactome and identified SERP1 as a potential interacting protein based on its ability to stabilize membrane proteins and facilitate N-linked glycosylation. GLP-1R and SERP1 were co-expressed in HEK-293 cells, and their interaction was confirmed by co-immunoprecipitation. We then found that overexpression of SERP1 could rescue GLP-1R glycosylation after application of tunicamycin to block N-linked glycosylation. SERP1 overexpression also attenuated exendin-4-stimulated cAMP accumulation and AMPK activation. However, the glycosylation and function of mutant GLP-1R, in which all three sites for N-linked glycosylation were mutated, were not increased with overexpression of SERP1. Moreover, as a GLP-1R interactor, SERP1 could also partly reverse the accumulation of tunicamycin-induced ER stress. Taken together, our findings identify a group of proteins that interact with GLP-1R and show that one specific interacting protein, SERP1, has an important role in facilitating the glycosylation of GLP-1R and rescuing its activities after ER stress induced by tunicamycin
Journal of Cellular Biochemistry. 2017 Nov;118(11):3616-3626.