GLUT4 Storage Vesicles: Specialized Organelles for Regulated Trafficking

Abstract

Fat and muscle cells contain a specialized, intracellular organelle known as the GLUT4 storage vesicle (GSV). Insulin stimulation mobilizes GSVs, so that these vesicles fuse at the cell surface and insert GLUT4 glucose transporters into the plasma membrane. This example is likely one instance of a broader paradigm for regulated, non-secretory exocytosis, in which intracellular vesicles are translocated in response to diverse extracellular stimuli. GSVs have been studied extensively, yet these vesicles remain enigmatic. Data support the view that in unstimulated cells, GSVs are present as a pool of preformed small vesicles, which are distinct from endosomes and other membrane-bound organelles. In adipocytes, GSVs contain specific cargoes including GLUT4, IRAP, LRP1, and sortilin. They are formed by membrane budding, involving sortilin and probably CHC22 clathrin in humans, but the donor compartment from which these vesicles form remains uncertain. In unstimulated cells, GSVs are trapped by TUG proteins near the endoplasmic reticulum - Golgi intermediate compartment (ERGIC). Insulin signals through two main pathways to mobilize these vesicles. Signaling by the Akt kinase modulates Rab GTPases to target the GSVs to the cell surface. Signaling by the Rho-family GTPase TC10α stimulates Usp25m-mediated TUG cleavage to liberate the vesicles from the Golgi. Cleavage produces a ubiquitin-like protein modifier, TUGUL, that links the GSVs to KIF5B kinesin motors to promote their movement to the cell surface. In obesity, attenuation of these processes results in insulin resistance and contributes to type 2 diabetes and may simultaneously contribute to hypertension and dyslipidemia in the metabolic syndrome.

Keywords: GLUT4 storage vesicle; GLUT4 trafficking; Unconventional secretion; Vasopressin inactivation; non-secretory exocytosis.

Figure 1

GLUT4 trafficking pathways. After GLUT4 undergoes endocytosis from the plasma membrane (1), it undergoes retrograde trafficking through recycling endosomes to donor membranes such as trans-Golgi network and the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) (2). GLUT4 storage vesicles (GSVs) then bud from donor membranes (3) and are sequestered by TUG proteins in a static configuration (4). Insulin stimulates TUG cleavage subsequent loading of GSVs onto kinesin motors for transport to the cell surface (5). GSVs are tethered, docked, and fused with the plasma membrane (6) to insert GLUT4. During ongoing stimulation by insulin, GSV components may return to the plasma membrane directly from endosomes, thereby bypassing the GSV compartment. The origin of GSVs remain largely unknown or not fully described.

Figure 2

A model for how insulin-stimulated cleavage of TUG and subsequent trafficking of GSVs coordinately regulates multiple physiologic effects. Insulin signals through at least two pathways. Upstream components in the TC10α pathway remain incompletely understood and are shown in gray. Signaling through TC10α removes an inhibitory effect of PIST to trigger Usp25m-mediated TUG proteolysis. Cleavage releases GSVs from an anchoring site comprising Golgin-160 and ACBD3. Acetylation of TUG mediates its binding to this site and controls the size of a GSV pool and, thus, insulin sensitivity. After cleavage, the TUG N-terminal product (TUGUL) modifies the kinesin motor protein KIF5B and translocates with the GSV to the cell surface. The GSV cargos are inserted into the T-tubule membrane in muscle tissue and the plasma membrane in adipose tissue. The GSV cargos mediate glucose uptake (GLUT4), vasopressin inactivation (IRAP), and possibly effects on lipids (LRP1, sortilin).