e)cycle between various compartments. Insulin triggers downstream cascades that modulate GSV trafficking and boost exocytosis to increase Glut4 presence at the membrane. Phosphatidylinositol 3-kinase-Akt (PI3K-Akt) pathway stimulates insulin dependent Glut4 translocation by impinging on small monomeric G proteins signaling systems, like Rab and Rho. The Rab family of small G-proteins regulates various steps of vesicular trafficking. Like all small G proteins, they cycle between the GTP-bound active, and the GDP-bound inactive,states. The GTP-bound state is promoted by guanine exchange factors (GEF), the GDP-bound state by the guanine activating proteins (GAP). In addition, GDP dissociation inhibitors (GDI) prevent membrane association of Rabs, by masking the lipid moiety with which they are post-translationally modified. There are ~70 members in the Rab family; a subset has been shown to be associated with GSV. Rab4a, 5a, and 11a and Rab2a, 8a, 10, 13 and 14 are associated with GSV and involved in Glut4 trafficking. Rab2a, 8a, 10, 13 and 14 are targets of TBC1D4/AS160 which has GAP activity towards them. Several GEFs for Rab proteins have been identified, such as the Dennd4 proteins and Rab3ip, known as Rabin8 and Dennd1c, for Rab10 and Rab8a, respectively. Rab8a and Rab13 are present in muscle cells while Rab10 is present in adipocytes. TBC1D4/AS160 is an Akt2 substrate; Akt2 phosphorylation of Tbc1d4 has an inhibitory effect upon it, thus relieving the inhibition of Rab.

Downstream of PI3K but independent of Akt2, Glut4 translocation is regulated by another small monomeric G protein family ¿ Rho, involved in the regulation of actin-cytoskeletal rearrangements. The stimulation of Rac1 member, promotes actin remodeling via Arp 2/3 actin branching complex. The seven subunit Arp2/3 complex binds mother filaments to initiate daughter branching at characteristics ~70 degree. The polymerization-depolymerization cycle depends on the phosphorylation state of cofilin, an actin-severing protein, and is necessary for proper Glut4 translocation in response to insulin. Dephosphorylation of cofilin activates its actin-depolymerizing function, eventually inhibited by the action of kinases. Limk1 and Ssh1 have been shown to phosphorylate/dephosphorylate cofilin, respectively, in insulin dependent Glut4 translocation. Downstream Rac1 effectors play a role in cytoskeletal rearrangements via regulation of microtubule and actin reorganization. They include Iqgap1 and Pak1 for microtubules, Pak1, Cyfip1 (Sra-1) and Baiap2 (Irsp53) for actin. Reorganized actin and microtubules act as filaments along which cargo translocation of Glut4 is mediated by motor proteins kinesins (Kif3a and 5b) and myosins Myo1c, 5a and 5b). A few GEFs for Rac1 have been identified (Prex1 and Plkhg4). Rhoq, known as TC10, is another Rho family member. Its activation, in response to insulin stimulation in adipocytes, is mediated by a GEF-containing complex. The complex and the Rho protein are present in lipid rafts and stimulation of Rhoq is necessary for Glut4 tethering and subsequent fusion with the plasma membrane. Likely, the activated Rhoq interacts with the exocyst complex to mediate Glut4 tethering. Rhoq recruits Trp10 (Cip4) which is in complex with Gapvd1 (Gapex5), a GEF for Rab31. Rab31 is a negative regulator of Glut4 and the interaction of Rhoq with the Trip10-Gapvd1 complex precludes its activation, therefore promoting Glut4 translocation.

The interaction of TBC1D4 with Lnpep, known as Irap, may help localize it to GSV. Lnpep and Vamp2 are associated with GSVs. Vamp2 ¿ a v-SNARE, complexes with the t-SNARE Stx4 and Snap23 to promote docking/fusion events; fusion further requiring Stxbp3. In the basal state, Stxbp4 binds Stx4 and inhibits Stx4 interaction with Snap23 and Vamp2. Insulin stimulation leads to Stxbp4 dissociation and is dependent on phosphorylation of Stxbp4 by Akt2. Myo5a is another substrate of Akt2; phosphorylation promotes association with actin and inhibition of Myo5a diminishes insulin dependent Glut4 translocation and glucose uptake. Likely, more molecular details are going to emerge and further add to the complexity of insulin mediated uptake of glucose. In type 2 diabetes (T2D), in addition to an impaired insulin secretion, there is an insufficient, inadequate response to insulin. Several rare mutations have been identified such as rare truncation of TBC1D4 associated with obesity or polymorphism in RapGef1, the Gef within the activating complex of Rhoq, associated with type 2 diabetes, in addition to the Akt2 mutations involved in severe insulin resistance.

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