Myotubes were transfected with either scrambled (scr) or nexilin specific siRNA (si-nex) oligos. Serum depleted cells were stimulated with 100 nM insulin A) or 10 nM B) for the Methionine enkephalin web indicated times. IRS1 was immunoprecipitated from cell lysates and complexes probed with either 4G10, nexilin or p85a PI3K abs as indicated. doi:10.1371/journal.pone.0055634.gNexilin Binds and Regulates IRSFigure 5. Silencing of nexilin enhances insulin-stimulated PIP3 production. A) L6 myoblasts were transfected with either scr or si-nex oligos together with GRP1-PH-GFP (GRP1PH) cDNA. Serum-starved cells were stimulated for 5 min with 10 nM insulin, fixed, permeabilized and incubated with anti-nexilin abs and Cy3-conjugated secondary abs (red). GFP was visualized using the appropriate filter. Arrows indicate regions of focal GRP1PH protein localization. B) L6 cells were transfected with either scr or si-nex oligos and left unstimulated or treated with 10 nM inulin for the indicated times. Cells were stained with rhodamine-phalloidin. Images were obtained on a Zeiss LSM510 laser scanning confocal microscope and manipulated using Canvas 9.04 (ACD Systems). doi:10.1371/journal.pone.0055634.gassociated with changes in insulin-induced formation of cortical actin bundles (Fig. 6C). Importantly, pre-treatment of L6 cells with the PI3K inhibitor LY294002 abolished the insulin-stimulated gain in GRP1-PH-GFP detection along the plasma membrane, confirming that mobilization of this reporter was dependent on PIP3 production (Fig. 6B). Given that Akt is a key mediator in the insulin-signaling pathway linking IRS1/PI3K activity to glucose uptake, we next tested the 
effect of nexilin knockdown on insulin-stimulated Akt phosphorylation. siRNA-treated L6 myotubes were incubated with a range of insulin concentrations for 5 min, and levels of Akt phosphorylation at LED 209 serine 473 (S473) and threonine 308 (T308) were determined through immunoblot analysis. As shown in Figure 7A, siRNA-mediated depletion of nexilin in L6 myotubes led to sensitization of insulin-stimulated Akt S473 phosphorylation. Furthermore, analysis of T308 pAkT levels revealed that nexilin knockdown enhanced the robustness of the Akt response especially noticeable at 10 nM and 100 nM insulin doses (Fig. 7B).From these experiments it appears that nexilin might influence the quantitative characteristics of signals broadcast from the IRS/ PI3K signalling node. Akt activation leads to the translocation of GLUT4 containing vesicles to the cell surface promoting the uptake of glucose into the cell. To determine the role of nexilin in GLUT4 transport, we measured glucose uptake in nexilindepleted L6 myotubes. Consistent with our observation on Akt activation, nexilin knockdown significantly augmented insulinstimulated 2-deoxyglucose uptake into siRNA-nexilin treated myotubes compared to control scr cells 
(Fig. 7C). Given the abundance of nexilin in L6 cells, we chose to use 3T3-L1 adipocytes (3T3-L1) as a model system to investigate the effect of nexilin overexpression on insulin/IRS1 signaling as these cells express very low levels of nexilin. To this end, we generated adenoviruses expressing Flag-tagged nexilin (Ad-Nex) that efficiently transduced differentiated 3T3-L1s (Fig. 8A). Once infected with control Ad-GFP or Ad-Nex adenoviruses, 3T3-L1s were serum starved for at least 2 hours prior to treatment with a rangeNexilin Binds and Regulates IRSFigure 6. Overexpression of Flag-nexilin inhibits localized PI3K activation.Myotubes were transfected with either scrambled (scr) or nexilin specific siRNA (si-nex) oligos. Serum depleted cells were stimulated with 100 nM insulin A) or 10 nM B) for the indicated times. IRS1 was immunoprecipitated from cell lysates and complexes probed with either 4G10, nexilin or p85a PI3K abs as indicated. doi:10.1371/journal.pone.0055634.gNexilin Binds and Regulates IRSFigure 5. Silencing of nexilin enhances insulin-stimulated PIP3 production. A) L6 myoblasts were transfected with either scr or si-nex oligos together with GRP1-PH-GFP (GRP1PH) cDNA. Serum-starved cells were stimulated for 5 min with 10 nM insulin, fixed, permeabilized and incubated with anti-nexilin abs and Cy3-conjugated secondary abs (red). GFP was visualized using the appropriate filter. Arrows indicate regions of focal GRP1PH protein localization. B) L6 cells were transfected with either scr or si-nex oligos and left unstimulated or treated with 10 nM inulin for the indicated times. Cells were stained with rhodamine-phalloidin. Images were obtained on a Zeiss LSM510 laser scanning confocal microscope and manipulated using Canvas 9.04 (ACD Systems). doi:10.1371/journal.pone.0055634.gassociated with changes in insulin-induced formation of cortical actin bundles (Fig. 6C). Importantly, pre-treatment of L6 cells with the PI3K inhibitor LY294002 abolished the insulin-stimulated gain in GRP1-PH-GFP detection along the plasma membrane, confirming that mobilization of this reporter was dependent on PIP3 production (Fig. 6B). Given that Akt is a key mediator in the insulin-signaling pathway linking IRS1/PI3K activity to glucose uptake, we next tested the effect of nexilin knockdown on insulin-stimulated Akt phosphorylation. siRNA-treated L6 myotubes were incubated with a range of insulin concentrations for 5 min, and levels of Akt phosphorylation at serine 473 (S473) and threonine 308 (T308) were determined through immunoblot analysis. As shown in Figure 7A, siRNA-mediated depletion of nexilin in L6 myotubes led to sensitization of insulin-stimulated Akt S473 phosphorylation. Furthermore, analysis of T308 pAkT levels revealed that nexilin knockdown enhanced the robustness of the Akt response especially noticeable at 10 nM and 100 nM insulin doses (Fig. 7B).From these experiments it appears that nexilin might influence the quantitative characteristics of signals broadcast from the IRS/ PI3K signalling node. Akt activation leads to the translocation of GLUT4 containing vesicles to the cell surface promoting the uptake of glucose into the cell. To determine the role of nexilin in GLUT4 transport, we measured glucose uptake in nexilindepleted L6 myotubes. Consistent with our observation on Akt activation, nexilin knockdown significantly augmented insulinstimulated 2-deoxyglucose uptake into siRNA-nexilin treated myotubes compared to control scr cells (Fig. 7C). Given the abundance of nexilin in L6 cells, we chose to use 3T3-L1 adipocytes (3T3-L1) as a model system to investigate the effect of nexilin overexpression on insulin/IRS1 signaling as these cells express very low levels of nexilin. To this end, we generated adenoviruses expressing Flag-tagged nexilin (Ad-Nex) that efficiently transduced differentiated 3T3-L1s (Fig. 8A). Once infected with control Ad-GFP or Ad-Nex adenoviruses, 3T3-L1s were serum starved for at least 2 hours prior to treatment with a rangeNexilin Binds and Regulates IRSFigure 6. Overexpression of Flag-nexilin inhibits localized PI3K activation.