The regulation of insulin secretory granule protein production by hnRNP A2/B1 : Final Report DFG Grant SO 818/10-1

This DFG-funded project investigated how pancreatic β-cells regulate insulin biosynthesis in response to glucose fluctuations. β-cells uniquely produce and secrete insulin, the hormone enabling glucose uptake and lowering blood sugar. While insulin synthesis is known to rise rapidly after glucose stimulation, the speed of this response cannot be explained by transcription alone. Our studies uncovered a missing mechanism: insulin mRNA is stored in membrane-less cytosolic condensates that release it when metabolic energy becomes available. These condensates, organised by RNA-binding proteins, keep insulin mRNA protected but untranslated in resting β-cells and dissolve upon stimulation, allowing rapid initiation of insulin translation. In Vasiljević et al., 2021, we showed that mRNAs encoding insulin and other β-cell secretory granule cargoes bind to hnRNP A2/B1 and localise to stress-granule-like particles that disassemble after glucose stimulation. In Quezada et al., EMBO J 2025, we identified G3BP1 and G3BP2 as main structural components of these condensates. Under low glucose, G3BP1/2 cluster with insulin mRNA, translation factors and AMPK; their dissolution by glucose, GLP-1 stimulation or palmitate enables translation. Depolarisation by high KCl, which triggers secretion but not biosynthesis, does not dissolve condensates, uncoupling insulin production from release. Aldolase activity proved essential for condensate dynamics. Its inhibition blocked ATP generation and dissolution even in the presence of pyruvate, revealing glycolysis as a control point for translation upstream of oxidative phosphorylation. Deletion of G3BP1 reduced insulin mRNA and secretion. These findings are intriguing based on our previous evidence indicating that in islets from living donors with type 2 diabetes, aldolase B is increased and G3BP1 decreased (Wigger et al., Nat Metab 2021*), suggesting impaired regulation of insulin translation. Together, these findings define a new principle of β-cell control: insulin mRNA is stored and released for translation through a glycolysis-coupled, energy-sensing mechanism. The dissolution of insulin-mRNA-containing condensates is a physico-chemical process occurring within minutes—much faster than transcription—allowing β-cells to adjust insulin production swiftly to metabolic demand. This is especially relevant in view of the evidence indicating that newly-synthesized insulin is preferentially released.