Dual Function of PI(4,5)P in Insulin-Regulated Exocytic Trafficking of GLUT4 in Adipocytes.

Phosphoinositides are important signaling molecules involved in the regulation of vesicular trafficking. It has been implicated that phosphatidylinositol 4,5-bisphosphate [PI(4,5)P] is involved in insulin-regulated GLUT4 translocation in adipocytes. However, it remains unclear where and how PI(4,5)P regulates discrete steps of GLUT4 vesicle translocation in adipocytes, especially on the exocytic arm of regulation. Here, we employed optogenetic tools to acutely control the PI(4,5)P metabolism in living cells. By combination of TIRFM imaging, we were able to monitor the temporal-spatial-dependent PI(4,5)P regulation on discrete steps of GLUT4 translocation in adipocytes. We found that the plasma membrane localized PI(4,5)P is crucial for proper insulin signaling propagation and for insulin-stimulated GLUT4 vesicle translocation in 3T3-L1 adipocytes. Global depletion of PI(4,5)P on the cell surface blunted insulin-stimulated Akt phosphorylation and abolished insulin effects in promotion of the docking and fusion of GLUT4 vesicle with the plasma membrane. Furthermore, by development of a novel optogenetic module to selectively modulate PI(4,5)P levels on the GLUT4 vesicle docking site, we identified an important regulatory role of PI(4,5)P in controlling of vesicle docking process. Local depletion of PI(4,5)P at the vesicle docking site promoted GLUT4 vesicle undocking, diminished insulin-stimulated GLUT4 vesicle docking and fusion, but without perturbation of insulin signaling propagation in adipocytes. Our results provide strong evidence that cell surface PI(4,5)P plays two distinct functions on regulation of the exocytic trafficking of GLUT4 in adipocytes. PI(4,5)P not only regulates the proper activation of insulin signaling in general but also controls GLUT4 vesicle docking process at the vesicle-membrane contact sites.