Yamada, E. et al. Akt2 phosphorylates Synip to regulate docking and fusion of GLUT4-containing vesicles. J. Cell Biol. 168, 921-928

Department of Medicine and Molecular Science, Gunma University Graduate School of Medicine, Maebashi, Gunma, 371-8511, Japan.
The Journal of Cell Biology (Impact Factor: 9.83). 04/2005; 168(6):921-8. DOI: 10.1083/jcb.200408182
Source: PubMed


We have identified an unusual potential dual Akt/protein kinase B consensus phosphorylation motif in the protein Synip (RxKxRS(97)xS(99)). Surprisingly, serine 97 is not appreciably phosphorylated, whereas serine 99 is only a specific substrate for Akt2 but not Akt1 or Akt3. Although wild-type Synip (WT-Synip) undergoes an insulin-stimulated dissociation from Syntaxin4, the Synip serine 99 to phenylalanine mutant (S99F-Synip) is resistant to Akt2 phosphorylation and fails to display insulin-stimulated Syntaxin4 dissociation. Furthermore, overexpression of WT-Synip in 3T3L1 adipocytes had no effect on insulin-stimulated recruitment of glucose transporter 4 (GLUT4) to the plasma membrane, whereas overexpression of S99F-Synip functioned in a dominant-interfering manner by preventing insulin-stimulated GLUT4 recruitment and plasma membrane fusion. These data demonstrate that insulin activation of Akt2 specifically regulates the docking/fusion step of GLUT4-containing vesicles at the plasma membrane through the regulation of Synip phosphorylation and Synip-Syntaxin4 interaction.

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    • "Synip was reported to bind to syntaxin to inhibit ternary SNARE complex, furthermore overexpression of synip in β-cell derived clonal cells resulted in the inhibition of glucose-induced insulin secretion [51]. Phosphorylation of Synip by Akt promotes the dissociation of synip from syntaxin, which allows vesicles to dock and fuse with the plasma membrane [52]. Thus, synip may link the inhibition of Akt activity with the suppression of fusions from previously docked granules. "
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    ABSTRACT: In glucose-induced insulin secretion from pancreatic β-cells, a population of insulin granules fuses with the plasma membrane without the typical docking process (newcomer granule fusions), however, its mechanism is unclear. In this study, we investigated the PI3K signaling pathways involved in the upregulation of newcomer granule fusions. Acute treatment with the class IA-selective PI3K inhibitors, PIK-75 and PI-103, enhanced the glucose-induced insulin secretion. Total internal reflection fluorescent microscopy revealed that the PI3K inhibitors increased the fusion events from newcomer granules. We developed a new system for transfection into pancreatic islets and demonstrated the usefulness of this system in order for evaluating the effect of transfected genes on the glucose-induced secretion in primary cultured pancreatic islets. Using this transfection system together with a series of constitutive active mutants, we showed that the PI3K-3-phosphoinositide dependent kinase-1 (PDK1)-Akt pathway mediated the potentiation of insulin secretion. The Akt inhibitor also enhanced the glucose-induced insulin secretion in parallel with the upregulation of newcomer granule fusions, probably via increased motility of intracellular insulin granules. These data suggest that the PI3K-PDK1-Akt pathway plays a significant role in newcomer granule fusions, probably through an alteration of the dynamics of the intracellular insulin granules.
    PLoS ONE 10/2012; 7(10):e47381. DOI:10.1371/journal.pone.0047381 · 3.23 Impact Factor
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    • "Synip binds syntaxin 4 in the basal state and inhibits the interaction between syntaxin 4 and the V-SNARE vesicle associated protein-2 (Vamp2) that is present in Glut4 containing vesicles [9]. Insulin stimulation results in the Akt2-mediated phosphorylation of Synip and its dissociation from syntaxin 4 [9], [10]. These events are thought to allow Glut4 vesicle Vamp2 access to syntaxin 4 and the binding of Vamp2 to syntaxin 4 generates a fusion competent complex [12]. "
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    ABSTRACT: The insulin responsive Glut4 transport vesicles contain the v-SNARE protein Vamp2 that associate with the plasma membrane t-SNARE protein Syntaxin 4 to drive insulin-stimulated Glut4 translocation in skeletal muscle and adipocytes. The syntaxin 4 interacting protein (Synip) binds to syntaxin 4 in the basal state and dissociates in the insulin-stimulated state allowing for the subsequent binding of Vamp2 containing Glut4 vesicles and fusion with the plasma membrane. In this study, we have found that Synip binds phosphatidylinositol 3,4,5-triphosphate (PIP3), but not phosphatidylinositol 3 phosphate (PIP) or phosphatidylinositol 3,4-biphosphate (PIP2) through the Synip WW domain as deletion of this domain (Synip ΔWW) failed to bind PIP3. Over-expressed Synip ΔWW in 3T3L1 adipocytes reduced the basal levels of Glut4 at the plasma membrane with no effect on the binding to syntaxin 4 in vitro. Subcellular fractionation demonstrated that the amount of Synip ΔWW at the PM was decreased in response to insulin in 3T3L1 adipocytes whereas the amount of Synip WT increased. These data suggest that in the presence of insulin, the dissociated Synip remains anchored to the plasma membrane by binding to PIP3.
    PLoS ONE 08/2012; 7(8):e42782. DOI:10.1371/journal.pone.0042782 · 3.23 Impact Factor
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    ABSTRACT: Because of their great abundance, high specific surface area, layer charge, laminar morphology, and chemical reactivity towards both neutral and charged species, clay minerals are of great importance to agriculture, industry, and the environment. The presence of iron in the structures of clay minerals infuses an additional facet into their importance. This is because the oxidation state of iron can be rather easily modified in situ and such a change can evoke profound differences in the surface-chemical and structural behaviour of the mineral. Examples of clay mineral properties that are greatly affected by changes in iron oxidation state are swelling in water, CEC, cation fixation capacity, surface area, clay mineral-organic interactions, surface pH, reduction potential, ability to transform chlorinated organic compounds, and ability to degrade pesticides and thereby alter their toxicity to mammals. Reduction of structural iron from Fe3+ to Fe2+ in smectites has been observed both in the laboratory and in situ in the field. Bacteria are second only to dithionite in their effectiveness to reduce structural iron in clay mineral and are the most important agent responsible for this phenomenon in natural soils and sediments. Because the manipulation of the iron oxidation state causes such large changes in chemical and physical behaviour and because such changes can be invoked under field conditions, a great opportunity exists to exploit this phenomenon for a myriad of purposes beneficial to mankind. Although such exploitation has yet to occur to any large extent, it has found application in the remediation of subsurface soils contaminated with radioactive and other harmful metals. Studies are also beginning to emerge that recognise this as an important factor in sustaining the fertility and use of flooded soils. Clearly, other opportunities will arise for its use in creating designer minerals for industrial uses. Challenges and many unanswered questions still face those who study redox processes of iron in clay minerals, especially with respect to the mechanisms governing the electron transfer and the linkages between Fe2+ and surface behaviour. How is the electron passed from the outer surfaces of the clay mineral layers into the octahedral sheet? What are the precise energies associated with this process? The exact surface forces altered by the redox process appear to be both coulombic and non-coulombic, but the precise nature of the latter is not well understood. What is the mechanism for electron transfer from bacteria to clay mineral layers-is it done through a direct membrane contact or are electron shuttles or mediators utilised? Is the mechanism the same for all bacteria? Even though the phenomenon of iron redox in clay minerals has been studied for several decades, the number of scientists participating in such studies is still rather small. Interest in this field of inquiry is beginning to grow, however, and answers to these and other questions are anticipated to be forthcoming. On a personal note, this author has been the beneficiary of many intriguing twists and turns along the path in the realm of iron redox chemistry, which has provided a most interesting, challenging, and rewarding perspective to his study of clay science. He has been awed by the intricacies of Nature as seen at such a seemingly insignificant level in the grand overall scheme of things, but which reveal such majestic order and complexity at the same time. His feelings about this are well captured in the words of the poet Elizabeth Barrett Browning (1937), who declared, "Earth's crammed with heaven, and every common bush afire with God; only he who sees takes off his shoes." My shoes are off!
    Developments in Clay Science 01/2006; 1. DOI:10.1016/S1572-4352(05)01013-5
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