Structural Insights to Insulin Receptor Ligand Interactions

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ABSTRACT Insulin treatment dramatically improves the health of people with diabetes, and is usually administered as a daily long-acting insulin and a prandial fast-acting insulin. Despite considerable success, a number of important challenges remain, including three major limitations that are addressed in this proposal. First, even the best clinically-available fast-acting insulins are too slow and last too long to provide tight control of serum glucose within the physiological range, resulting in substantial excursions outside of this range and chronic hyperglycemia or acute hypoglycemic complications. Second, currently available insulins require continual refrigeration to avoid aggregation, whereas therapeutic insulins that do not require cold-chain delivery would offer considerable advantages, especially for use in long-term insulin pumps and in circumstances where electrical power is unreliable. Third, because insulin stimulates two signals, one therapeutically advantageous for metabolic control (Akt pathway) and one therapeutically concerning for mitogenic growth (Erk pathway), there is interest in developing analogs that preferentially stimulate the Akt pathway. This proposal takes a biochemical and structure-based approach to gain mechanistic insight to each of these concerns, including cryo-EM structure determination of receptor-ligand complexes complemented by a variety of approaches, including cell signaling and mouse glucose-responsiveness studies. Aim 1 focusses on two humanized variants of cone snail venom insulins, which lack residues that make native insulin dimeric and inherently slow acting upon subcutaneous injection, and have been engineered to provide fast response, short duration of action, and high potency. Aim 2 focusses on approaches to render insulins resistant to aggregation/fibrillation, including following up on the surprising finding that one of the humanized venom insulins is highly resistant to aggregation. Aim 3 explores the remarkable property of some receptor ligands to elicit biased signaling that emphasizes either the Akt or the Erk pathways, and offers potential to understand the structural basis for these effects. Completion of these aims will provide fundamental mechanistic insights and inform efforts to develop improved therapeutics.