GLUT4 distribution in the plasma membrane: from single molecules to whole muscles

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Insulin promotes glucose transport by moving GLUT4 from insulin-sensitive intracellular stores to the plasma membrane (PM). Recent work has revealed that in addition to promotingGLUT4 traffic to the PM, insulin also stimulates the dispersal of GLUT4 within the PM. In the absence of insulin, clusters of GLUT4 are retained at the site of delivery and are rapidly re-internalised. Insulin promotes a 60-fold increase in the rate of dispersal of GLUT4 away from the fusion site; this is thought to be an essential step in the activation of the glucose transport function and thus represents a new paradigm for insulin action. The mechanism(s) which underpin this dispersal remain unresolved. At present it is unclear whether insulin stimulates dispersal in either cardiomyocytes or human skeletal muscle cells, how exercise impacts on this phenomenon, or whether this dispersal is modulated in diabetes. In the transgenic model GLUT4 is tagged with GFP and can be imaged in intact tissues using microscopy. We shall marry this opportunity with the imaging capability of the mesolens which offers a level of unparalleled structural analysis of tissues over a significant scale. We will use the power of this system to probe the distribution of GLUT4 in whole tissues, notably heart and muscle. GLUT4 translocation in muscle is poorly understood in part because of difficulties in isolating muscle cells and by difficulties in studying behaviour of tagged-GLUT4 molecules in muscle cells which may cover several mm in length - way beyond conventional microscopy. We will use this system to study GLUT4 distribution in intact hearts and muscle cells and compare the actions of insulin and exercise on GLUT4 distribution. Does GLUT4 traffic to the sarcolemma in the same way in response to different stimuli? Are they additive? Does each myofibril look the same? These are examples of questions which cannot be answered using existing technology.The key unanswered questions which this studentship will address are:(i) Does insulin promote GLUT4 dispersal in cardiomyocytes or muscle cells?(ii) Is this dispersal impaired in T2DM or obesity?(iii) How is the distribution of GLUT4 modulated by insulin in intact tissues - do all cells behave similarly? If not, why not? and(iv) Can we study changes in this during disease progression?