Post-transcriptional regulation of insulin mRNA storage by G3BP1/2+condensates in beta cells

Abstract

AbstractHyperglycemia upregulates insulin translation in pancreatic beta cells. Several RNA- binding proteins involved in this process have been identified, including G3BP1, a stress granule marker downregulated in islets of subjects with type 2 diabetes. We show that in mouse insulinoma MIN6-K8 cells exposed to fasting glucose levels G3BP1 and its paralog G3BP2 colocalize to cytosolic condensates with eIF3b andIns1/2mRNA. Upon glucose stimulation, the condensates dissolve and G3BP1/2, eIF3b, and insulin mRNAs redistribute throughout the cytosol. Intriguingly, G3BP1+condensates in MIN6-K8 cells differ from sodium arsenate-induced stress granules in regards to eIF2α and AMPKα phosphorylation. Knockout ofG3BP1orG3BP2prevented condensate assembly, but onlyG3BP1deletion decreased the levels ofIns1/2mRNA and proinsulin and impaired polysome formation. Like glucose, other insulin secretagogues such as Exendin-4 and palmitate, but not high KCl, prompted the dissolution of G3BP1+condensates. G3BP1+/InsmRNA+condensates were also present in mouse and human beta cells from normoglycemic donors. Hence, G3BP1+condensates represent a glucose-regulated compartment for the physiological storage and protection of insulin mRNA in resting beta cells.