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Regulating the Motor for GLUT4 Vesicle Traffic

2008; Cell Press; Volume: 8; Issue: 5 Linguagem: Inglês

10.1016/j.cmet.2008.10.006

1932-7420

Geoffrey D. Holman, Kei Sakamoto,

Pancreatic function and diabetes

Insulin-triggered trafficking of GLUT4 glucose transporter-loaded vesicles and their fusion with the plasma membrane are mechanical processes involving multiprotein complexes that coordinate and facilitate vesicle movement. Now, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar link myosin-1c to insulin signaling by demonstrating direct CaMKII-driven phosphorylation of this critical motor protein. Insulin-triggered trafficking of GLUT4 glucose transporter-loaded vesicles and their fusion with the plasma membrane are mechanical processes involving multiprotein complexes that coordinate and facilitate vesicle movement. Now, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar link myosin-1c to insulin signaling by demonstrating direct CaMKII-driven phosphorylation of this critical motor protein. Insulin signaling in its target tissues of fat and muscle is known to lead to the exocytosis of vesicles loaded with the glucose transporter GLUT4 from an intracellular reservoir compartment to the plasma membrane. Now, the challenge for researchers is to link early signaling to the vesicle-trafficking machinery. Early events in insulin signaling include the coupling of insulin-receptor tyrosine-kinase activity to phosphorylation of the insulin-receptor-substrate molecules IRS1 and IRS2. These steps lead to activation of PDK1 and PDK2 (mTOR/Rictor), which in turn lead to the phosphorylation of Akt/PKB at dual sites. Recent studies have explored the roles of the Akt substrates TBC1D1 and TBC1D4 as these proteins are Rab-GAPs and can therefore potentially link signaling to Rab-facilitated vesicle traffic of GLUT4, reviewed in Sakamoto and Holman, 2008Sakamoto K. Holman G.D. Am. J. Physiol. Endocrinol. Metab. 2008; 295: E29-E37Crossref PubMed Scopus (318) Google Scholar. Now, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar have discovered a new cascade that links insulin signaling to GLUT4 vesicle traffic; they report that insulin action in fat cells leads to CaM-KinaseII-mediated phosphorylation of the motor protein Myo1c. Myo1c is a class 1 unconventional myosin, present in many tissues, which has been implicated in vesicle translocation. Like other unconventional myosins, it is likely to primarily interact with actin (Bose et al., 2002Bose A. Guilherme A. Robida S.I. Nicoloro S.M. Zhou Q.L. Jiang Z.Y. Pomerleau D.P. Czech M.P. Nature. 2002; 420: 821-824Crossref PubMed Scopus (210) Google Scholar), but it has been recently found to occur in complexes with calmodulin and the G protein RalA (Chen et al., 2007Chen X.W. Leto D. Chiang S.H. Wang Q. Saltiel A.R. Dev. Cell. 2007; 13: 391-404Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar) and with Rictor (Hagan et al., 2008Hagan G.N. Lin Y. Magnuson M.A. Avruch J. Czech M.P. Mol. Cell. Biol. 2008; 28: 4215-4226Crossref PubMed Scopus (50) Google Scholar). Rictor was discovered as an essential component of the separate mTORC2 complex that leads to Akt phosphorylation. However, the association of Rictor with Myo1c potentially links insulin signaling with a regulated Myo1c and actin interaction (Hagan et al., 2008Hagan G.N. Lin Y. Magnuson M.A. Avruch J. Czech M.P. Mol. Cell. Biol. 2008; 28: 4215-4226Crossref PubMed Scopus (50) Google Scholar). Myo1c has also been found to be associated with GLUT4 vesicles (Bose et al., 2002Bose A. Guilherme A. Robida S.I. Nicoloro S.M. Zhou Q.L. Jiang Z.Y. Pomerleau D.P. Czech M.P. Nature. 2002; 420: 821-824Crossref PubMed Scopus (210) Google Scholar). Some Myo1c has been found to move to the vicinity of the plasma membrane in response to insulin treatment of target cells (Bose et al., 2002Bose A. Guilherme A. Robida S.I. Nicoloro S.M. Zhou Q.L. Jiang Z.Y. Pomerleau D.P. Czech M.P. Nature. 2002; 420: 821-824Crossref PubMed Scopus (210) Google Scholar) and has been directly implicated in the mechanics of fusion of GLUT4 vesicles with the plasma membrane (Bose et al., 2004Bose A. Robida S. Furcinitti P.S. Chawla A. Fogarty K. Corvera S. Czech M.P. Mol. Cell. Biol. 2004; 24: 5447-5458Crossref PubMed Scopus (126) Google Scholar). Importantly, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar now find that phosphorylation of Myo1c is regulated. They first identified Myo1c as an interacting partner for the phosphoprotein regulator 14-3-3. Among the Myo1c phosphorylation sites that could potentially bind 14-3-3, only serine 701 showed reduced 14-3-3 interaction when mutated to alanine. By use of specific inhibitors of CaMKII, and by knockdown of the major CaMKII isoform, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar demonstrate that this kinase is directly required for phosphorylation of Myo1c on Ser701 in adipocytes. To link the CaMKII-mediated phosphorylation site at Myo1c Ser701 to insulin-regulated GLUT4 translocation, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar carried out a series of siRNA experiments. Consistent with a previous report (Bose et al., 2002Bose A. Guilherme A. Robida S.I. Nicoloro S.M. Zhou Q.L. Jiang Z.Y. Pomerleau D.P. Czech M.P. Nature. 2002; 420: 821-824Crossref PubMed Scopus (210) Google Scholar), they report that knockdown of Myo1c reduced insulin-stimulated GLUT4 translocation; additionally, this effect is reversed by reintroduction of wild-type Myo1c but not by Ser701-to-alanine-mutated Myo1c. The requirement for Myo1c interaction with calmodulin has been investigated by Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar, and these studies have led them to propose the mechanism illustrated in Figure 1. Insulin is thought to stimulate calcium influx in the area just below the plasma membrane, which leads to activation of CaMKII, which then phosphorylates Myo1c. These steps are proposed to be associated with or lead to dissociation of calmodulin from Myo1c. In support of this hypothesis, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar demonstrate that 14-3-3 binding to phosphorylated Ser701 on Myo1c, and calmodulin interaction with Myo1c are mutually exclusive. This phosphorylation-driven activation of Myo1c is associated with increased ATPase activity, but the 14-3-3 interaction at the phosphorylated site does not alter its ATPase activity. To complete the experimental support for the proposed activation cascade, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar demonstrate that the ATPase activity of phosphorylated Myo1c is required for insulin-stimulated GLUT4 translocation. The studies by Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar have shown that pharmacological inhibition of PI3-kinase activity with wortmannin leads to reduced Myo1c phosphorylation and 14-3-3 binding to phosphorylated Myo1c. However, wortmannin inhibition of PI3-kinase does not directly lead to reduction of insulin-activated CaMKII catalytic activity. Instead, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar have suggested the interesting possibility that PI 3-kinase activity may be required for juxtaposing Myo1c and CaMKII. Some of the effects of Myo1c in the GLUT4 translocation process are PI 3-kinase independent. Bose et al., 2002Bose A. Guilherme A. Robida S.I. Nicoloro S.M. Zhou Q.L. Jiang Z.Y. Pomerleau D.P. Czech M.P. Nature. 2002; 420: 821-824Crossref PubMed Scopus (210) Google Scholar, Bose et al., 2004Bose A. Robida S. Furcinitti P.S. Chawla A. Fogarty K. Corvera S. Czech M.P. Mol. Cell. Biol. 2004; 24: 5447-5458Crossref PubMed Scopus (126) Google Scholar found that insulin-activated Myo1c translocation to the plasma membrane was PI3-kinase independent and that high expression of Myo1c could overcome a wortmannin (PI3-kinase inhibition) -induced block in plasma membrane fusion of GLUT4 vesicles. The separate PI3-kinase-dependent and -independent actions of Myo1c may perhaps be mediated by distinct Myo1c complexes. It will be important in future to further resolve whether PI 3-kinase-dependent or PI 3-kinase-independent actions of Myo1c are essential to insulin action on GLUT4 translocation, particularly at the GLUT4 vesicle membrane fusion step. As Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar suggest, the relationship of the newly discovered CaMKII and Myo1c insulin-activated cascade to the more established activation of GLUT4 translocation via an Akt cascade is not fully resolved. A recent report indicates that, in the absence of insulin, Akt activation alone (using a rapid, chemically activated form of Akt, the rapalog system) is sufficient to mimic the effects of insulin on GLUT4 translocation (Ng et al., 2008Ng Y. Ramm G. Lopez J.A. James D.E. Cell Metab. 2008; 7: 348-356Abstract Full Text Full Text PDF PubMed Scopus (125) Google Scholar). The full activation of GLUT4 translocation that results from use of this approach (Ng et al., 2008Ng Y. Ramm G. Lopez J.A. James D.E. Cell Metab. 2008; 7: 348-356Abstract Full Text Full Text PDF PubMed Scopus (125) Google Scholar) suggests that activation of the Myo1c cascade must in some way be associated with the activation of Akt cascade, as the insulin signaling link to Myo1c is absent. However, data from Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar indicates that activation of Akt is not directly required for phosphorylation of Myo1c at Ser701. Establishing an alternative means by which Akt and Myo1c phosphorylation and activation are mutually dependent will require further investigation. GLUT4 vesicle fusion is now known to be an important regulated step in the exocytosis pathway (Bose et al., 2004Bose A. Robida S. Furcinitti P.S. Chawla A. Fogarty K. Corvera S. Czech M.P. Mol. Cell. Biol. 2004; 24: 5447-5458Crossref PubMed Scopus (126) Google Scholar, Lizunov et al., 2005Lizunov V.A. Matsumoto H. Zimmerberg J. Cushman S.W. Frolov V.A. J. Cell Biol. 2005; 169: 481-489Crossref PubMed Scopus (138) Google Scholar, Koumanov et al., 2005Koumanov F. Jin B. Yang J. Holman G.D. Cell Metab. 2005; 2: 179-189Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar, Bai et al., 2007Bai L. Wang Y. Fan J. Chen Y. Ji W. Qu A. Xu P. James D.E. Xu T. Cell Metab. 2007; 5: 47-57Abstract Full Text Full Text PDF PubMed Scopus (170) Google Scholar). The Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar study adds important new data that strengthen the idea that the mechanical properties of the Myo1c motor are essential for the terminal events in GLUT4 vesicle exocytosis. This also appears to be the case for exocytosis of other channel proteins (for example, ADH-regulated translocation of sodium channels [Wagner et al., 2005Wagner M.C. Blazer-Yost B.L. Boyd-White J. Srirangam A. Pennington J. Bennett S. Am. J. Physiol. Cell Physiol. 2005; 289: C120-C129Crossref PubMed Scopus (17) Google Scholar]). The Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar study strengthens the notion that control of exocytosis may occur in a specialized fusion zone that occurs, or is generated in response to insulin, just underneath the plasma membrane. Although CaMKII is activated through insulin action in muscle, Yip et al., 2008Yip M.F. Ramm G. Larance M. Hoehn K.L. Wagner M.C. Guilhaus M. James D.E. Cell Metab. 2008; 8 (this issue): 384-398Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar were not able to detect Myo1c phosphorylation in this tissue. An alternative isoform of Myo1c is therefore likely to be present. Identifying this motor protein and its mode of regulation will be important in the future. Collectively, further studies on the insulin-regulated motor proteins in fat and muscle may lead to identification of novel drug targets for the prevention of insulin resistance and type 2 diabetes. CaMKII-Mediated Phosphorylation of the Myosin Motor Myo1c Is Required for Insulin-Stimulated GLUT4 Translocation in AdipocytesYip et al.Cell MetabolismNovember 05, 2008In BriefThe unconventional myosin Myo1c has been implicated in insulin-regulated GLUT4 translocation to the plasma membrane in adipocytes. We show that Myo1c undergoes insulin-dependent phosphorylation at S701. Phosphorylation was accompanied by enhanced 14-3-3 binding and reduced calmodulin binding. Recombinant CaMKII phosphorylated Myo1c in vitro and siRNA knockdown of CaMKIIδ abolished insulin-dependent Myo1c phosphorylation in vivo. CaMKII activity was increased upon insulin treatment and the CaMKII inhibitors CN21 and KN-62 or the Ca2+ chelator BAPTA-AM blocked insulin-dependent Myo1c phosphorylation and insulin-stimulated glucose transport in adipocytes. Full-Text PDF Open Archive