Aldolase-regulated G3BP1/2(+) condensates control insulin mRNA storage in beta cells.

Upregulation of insulin mRNA translation upon hyperglycemia in pancreatic islet β-cells involves several RNA-binding proteins. Here, we found that G3BP1, a stress granule marker downregulated in islets of subjects with type 2 diabetes, binds to insulin mRNA in glucose concentration-dependent manner. We show in mouse insulinoma MIN6-K8 cells exposed to fasting glucose levels that G3BP1 and its paralog G3BP2 colocalize to cytosolic condensates with eIF3b, phospho-AMPKα(Thr172) and Ins1/2 mRNA. Glucose stimulation dissolves G3BP1(+)/2(+) condensates with cytosolic redistribution of their components. The aldolase inhibitor aldometanib prevents the glucose- and pyruvate-induced dissolution of G3BP1(+)/2(+) condensates, increases phospho-AMPKα(Thr172) levels and reduces those of phospho-mTOR(Ser2448). G3BP1 or G3BP2 depletion precludes condensate assembly. KO of G3BP1 decreases Ins1/2 mRNA abundance and translation as well as proinsulin levels, and impaires glucose-stimulated insulin secretion. Further, other insulin secretagogues such as exendin-4 and palmitate, but not high KCl, prompts the dissolution of G3BP1(+)/2(+) condensates. G3BP1(+)/2(+)/Ins mRNA(+) condensates are also found in primary mouse and human β-cells. Hence, G3BP1(+)/2(+) condensates represent a conserved glycolysis/aldolase-regulated compartment for the physiological storage and protection of insulin mRNA in resting β-cells.