Regulation of Elevated Postexercise Insulin-stimulated Glucose Uptake by Skeletal Muscle

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Over 100 million Americans suffer from the devastating consequences of type 2 diabetes (T2D) or prediabetes (a condition associated with elevated risk to develop T2D). Skeletal muscle accounts for up to 85% of insulin- induced blood glucose clearance, and insulin resistance for muscle glucose uptake is an essential, and perhaps primary defect for T2D. It has been known for 40 years that one exercise bout can enhance subsequent insulin- stimulated glucose uptake (ISGU) by muscle, but the mechanisms have remained elusive. The long-range goal is to fully understand the molecular, cellular, and physiological events responsible for this significant health benefit. Recent research using a unique Akt substrate of 160 kDa-knockout (AS160-KO) rat model revealed that expression of AS160 (a key regulator of GLUT4 glucose transporter localization) is essential for the elevated postexercise ISGU. Specific Aim 1 will identify mechanisms whereby AS160 leads to greater postexercise ISGU by muscle. AS160’s canonical Rab-GAP (Rab-GTPase activating protein) domain controls ISGU under sedentary conditions. Experiments using AS160-KO rats with AAV-vector (AAV) to deliver AS160 with a mutation to selectively disable its Rab-GAP domain will test if this domain is required for elevated postexercise ISGU. Although AS160 expression is essential for elevated postexercise ISGU in both sexes, AS160 phosphorylation of key sites is required only for male rats. Experiments will test estrogen’s role in the mechanisms responsible for this important sexual dimorphism. Specific Aim 2 will elucidate the regulation of subcellular GLUT4 localization in skeletal muscle postexercise. Knowledge of AS160’s role in GLUT4 distribution is limited to insulin’s ability to elevate GLUT4 exocytosis to cell surface membranes in unexercised muscle. A powerful new microscopy-based approach (STERM, Sample Thinning Enhanced Resolution Microscopy) will examine GLUT4 distribution in 7 different myocellular compartments. Coupling STERM with the AS160-KO model will be used to test if AS160 is crucial for postexercise regulation of GLUT4 distribution in both cell surface and intracellular membrane compartments. Specific Aim 3 will ascertain the role of postexercise muscle glycogen resynthesis in the reversal of the postexercise increase in ISGU. Because the health benefit of elevated postexercise ISGU could be extended by delaying its reversal, elucidating the mechanism for reversal of elevated ISGU would be valuable. Experiments will test if muscle glycogen resynthesis is crucial for reversal of elevated postexercise ISGU by muscle with carbohydrate refeeding. The innovative approach will be to reduce muscle abundance of glycogen synthase (GS, rate-limiting enzyme for glycogen synthesis) using AAV-vector delivery of shRNA-GS to muscle. Further analysis will seek to identify mechanisms underlying the relationship between muscle glycogen and reversal of elevated postexercise ISGU. The research in this project will use rigorous and innovative methods to enable significant advances in fundamental knowledge and address a critical barrier to progress in the field by elucidating mechanisms for elevated postexercise ISGU, a major health benefit.