Phosphoinositides as regulators of membrane trafficking in health and disease.

Membrane trafficking is crucial in the homeostasis of the highly compartmentalized eukaryotic cells. This compartmentalization occurs both at the organelle level, with distinct organelles maintaining their identities while also intensely interchanging components, and at a sub-organelle level, with adjacent subdomains of the same organelle containing different sets of lipids and proteins. A central question in the field is thus how this compartmentalization is established and maintained despite the intense exchange of components and even physical continuities within the same organelle. The phosphorylated derivatives of phosphatidylinositol, known as the phosphoinositides, have emerged as key components in this context, both as regulators of membrane trafficking and as finely tuned spatial and temporal landmarks for organelle and sub-organelle domains. The central role of the phosphoinositides in cell homeostasis is highlighted by the severe consequences of the derangement of their metabolism caused by genetic deficiencies of the enzymes involved, and from the systematic hijacking of phosphoinositide metabolism that pathogens operate to promote their entry and/or survival in host cells.

Morphological changes in the Golgi complex correlate with actin cytoskeleton rearrangements.

In this report we have studied the morphological changes of the Golgi complex (GC) that specifically accompany F-actin reorganizations. In starved rat RBL-2H3 tumor mast cells, the GC, that was visualized at immunofluorescence level with antibodies raised against the Golgi-resident proteins giantin, mannosidase II, or TGN-38, showed a compacted morphology with a supranuclear positioning. Concomitant to membrane ruffle formation induced by epidermal growth factor (EGF) or phorbol 12-myristate 13-acetate (PMA), and stress fiber formation induced by lysophosphatidic acid (LPA), specific GC morphological changes were observed. When cells were stimulated with EGF or PMA, the compacted GC morphology was transformed into a reticular network that was extended towards the cell periphery. When cells were incubated with LPA, the GC acquired a characteristic ring-shaped morphology. Brefeldin A (BFA) did not affect the PMA- or LPA-induced membrane ruffling and stress fiber formation, respectively, indicating that actin rearrangements occurred independent of the presence of the GC. Upon BFA removal, the presence of PMA or LPA during the recovery process induced the GC to acquire the morphological appearance described above for each agent. Moreover, the PMA- but not the LPA-induced GC rearrangements were sensitive to the actin perturbing agents cytochalasin D and jasplakinolide. When cells were preincubated with the phosphatidylinositide 3-kinase (PI3K) inhibitors wortmannin or LY294002, the PMA-induced GC morphological changes were inhibited but not membrane ruffles. Finally, the PMA-induced increase in the post-Golgi transport of glycosaminoglycans to the cell surface was not altered by cytochalasin D or jasplakinolide. Altogether, these data suggest that: (1) the shape of the GC is influenced by the 3D arrangement of actin microfilaments; (2) PI3K regulates the association of the GC with actin microfilaments; and (3) actin microfilaments are not essential for the post-Golgi transport to the plasma membrane.