Biogenesis and regulation of insulin-responsive vesicles containing GLUT4

Abstract

Insulin regulates the trafficking of GLUT4 glucose transporters in fat and muscle cells. In unstimulated cells, GLUT4 is sequestered intracellularly in small, insulin-responsive vesicles. Insulin stimulates the translocation of these vesicles to the cell surface, inserting the transporters into the plasma membrane to enhance glucose uptake. Formation of the insulin-responsive vesicles requires multiple interactions among GLUT4, IRAP, LRP1, and sortilin, as well as recruitment of GGA and ACAP1 adaptors and clathrin. Once formed, the vesicles are retained within unstimulated cells by the action of TUG, Ubc9, and other proteins. In addition to acting at other steps in vesicle recycling, insulin releases this retention mechanism to promote the translocation and fusion of the vesicles at the cell surface.

Figure 1. Intracellular GLUT4 trafficking

In unstimulated cells, GLUT4 is localized predominantly in the perinuclear region in small, insulin-responsive vesicles (IRVs). In addition to GLUT4, these vesicles contain IRAP, LRP1, and VAMP2, as well as sortilin, a cargo adaptor protein. The IRVs exist in a dynamic equilibrium with donor membranes that are likely a sub-domain of the trans-Golgi network (TGN) and/or recycling endosomes. Formation of the IRVs requires GGA and ACAP1 adaptor proteins, clathrin, and probably phosphatidylinositol 4-phosphate (PI4P). Once formed, IRVs are retained intracellularly by TUG, Ubc9, and other proteins, which may constrain an intracellular cycle of budding and fusion. Upon insulin addition, IRVs are mobilized and fuse with the plasma membrane. The IRV component proteins are internalized into sorting endosomes and returned to donor membranes by cellugyrin-containing transport vesicles. As diagrammed, the IRVs are labeled in magenta, proteins and lipids are denoted in blue, donor membranes in red, and retention and mobilization steps are in green.