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Septin 7 forms a complex with CD2AP and nephrin and regulates glucose transporter trafficking.

ABSTRACT: Podocytes are insulin-sensitive and take up glucose in response to insulin. This requires nephrin, which interacts with vesicle-associated membrane protein 2 (VAMP2) on GLUT4 storage vesicles (GSVs) and facilitates their fusion with the plasma membrane. In this paper, we show that the filament-forming GTPase septin 7 is expressed in podocytes and associates with CD2-associated protein (CD2AP) and nephrin, both essential for glomerular ultrafiltration. In addition, septin 7 coimmunoprecipitates with VAMP2. Subcellular fractionation of cultured podocytes revealed that septin 7 is found in both cytoplasmic and membrane fractions, and immunofluorescence microscopy showed that septin 7 is expressed in a filamentous pattern and is also found on vesicles and the plasma membrane. The filamentous localization of septin 7 depends on CD2AP and intact actin organization. A 2-deoxy-d-glucose uptake assay indicates that depletion of septin 7 by small interfering RNA or alteration of septin assembly by forchlorfenuron facilitates glucose uptake into cells and further, knockdown of septin 7 increased the interaction of VAMP2 with nephrin and syntaxin 4. The data indicate that septin 7 hinders GSV trafficking and further, the interaction of septin 7 with nephrin in glomeruli suggests that septin 7 may participate in the regulation of glucose transport in podocytes.

SUBMITTER: Wasik AA

PROVIDER: S-EPMC3431928 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The effect of culture conditions simulating hypo- and hyper-glycaemia on glucose transport and on the subcellular localization of the glucose transporter GLUT-1 was studied in L8 myocytes. Incubation of the cells with 20 mM-glucose for 25 h decreased the rate of 2-deoxy-D-[3H]glucose (dGlc) uptake to 0.106 +/- 0.016 nmol/min per 10(6) cells compared with 0.212 +/- 0.025 in cells maintained at 2 mM-glucose (final glucose concentrations at the end of the incubation period were 16-17 mM and 0.7-1.0 mM respectively). An additional 5 h incubation of these cells with medium containing the opposite glucose concentration (i.e. change from 17 mM to 1 mM and from 1 mM to 17 mM) increased the transport rate to 0.172 +/- 0.033 nmol/min per 10(6) cells in cultures initially conditioned at high glucose, and decreased the transport to 0.125 +/- 0.029 in those conditioned at low glucose. Plasma-membrane- and microsomal-membrane-enriched fractions were prepared from these cells for [3H]cytochalasin B (CB) binding and Western-blot analysis with antibodies against GLUT-1 and GLUT-4. A decrease in glucose concentration increased the number of D-glucose-displaceable CB-binding sites and GLUT-1 protein in the plasma-membrane fraction to the same extent as the increase in dGlc transport. Under downregulatory conditions, the lower dGlc-transport capacity could be accounted for by a decreased number of transporters in the plasma membrane of the cells. No apparent modification of the intrinsic activity of the glucose transporters was observed in up- or down-regulated cells. Under downregulatory conditions, the CB-binding data indicated a large increase in the number of transporters in the intracellular membranes of the myocytes. Western blots of the same membranes also indicated an increase in GLUT-1 content. However, the interaction of the intracellular GLUT-1 protein with the polyclonal antibodies was much weaker than that of the plasma-membrane-associated GLUT-1. The GLUT-4 concentration was too low to permit quantification in membrane fractions. Our findings suggest that autoregulation of glucose transport in L8 myocytes is accompanied by parallel changes in the number of GLUT-1 transporters in the plasma membrane, and that the rate of transporter degradation may be augmented in the upregulated myocytes. These glucose-induced changes are fully reversible.

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Septin 7 forms a complex with CD2AP and nephrin and regulates glucose transporter trafficking.

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