| The pancreatic hormone insulin regulates fat and muscle glucose uptake by controlling the intracellular localization of GLUT4 glucose transporters. GLUT4 is the main glucose transporter expressed in fat and muscle cells, where it cycles to and from the plasma membrane. In cells not stimulated with insulin, GLUT4 is targeted to specialized GLUT4 storage vesicles (GSVs), which sequester it away from the cell surface. Insulin acts within minutes to mobilize these vesicles, translocating GLUT4 to the plasma membrane to enhance glucose uptake. TUG is hypothesized to regulate GSVs as a functional tether, linking GLUT4 to a membrane-associated anchoring site in unstimulated cells. Insulin stimulates rapid dissociation of TUG-GLUT4 complexes. Depletion of TUG, or expression of a dominant negative TUG fragment, disrupts intracellular GLUT4 retention and may mimic insulin action.;Here, I describe data which bolsters the model that insulin signals through a TUG-GLUT4 retention complex to recruit GSVs. First, I present data indicating that TUG depletion in unstimulated adipocytes redistributes GSV cargos VAMP2 and IRAP to the cell surface, in addition to its effect on GLUT4. I also demonstrate the presence of a TUG-IRAP interaction, which could account for how GSVs are regulated independently of GLUT4. Second, I identify two Golgi matrix proteins, Golgin-160 and PIST, which interact with TUG and may be components of the TUG-GLUT4 membrane-associated anchor. Third, in collaborative work I show that in the absence of TUG, GSV cargos recycle in exocytic vesicles with properties of GSVs. Fourth, I report evidence that mutations in GLUT4 that enhance or disrupt its intracellular retention, respectively increase or decrease the glucose transporter's association with TUG. Finally, I provide data to support the idea that insulin signals through TC10a to cleave TUG, and that TUG proteolytic processing is necessary for GLUT4 translocation.;The regulation of GLUT4 trafficking is tissue-specific, but many proteins controlling GSV retention and mobilization, including TUG, are expressed in a wide range of cell types. My data are consistent with the notion that GLUT4 participates in a general mechanism, by which the cell surface delivery of various membrane proteins can be controlled by extracellular stimuli in different tissues. TUG may play a role in this shared pathway. Lastly, it is not known if defects in the formation or intracellular retention of GSVs contribute to human insulin resistance. If so, then the data presented here may have importance for understanding the pathogenesis of type 2 diabetes. |
