Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Phosphoinositides: Tiny Lipids With Giant Impact on Cell Regulation

Capacitative Ca(2+) entry involves the regulation of plasma membrane Ca(2+) channels by the filling state of intracellular Ca(2+) stores in the endoplasmic reticulum (ER). Several theories have been advanced regarding the mechanism by which the stores communicate with the plasma membrane. One such mechanism, supported by recent findings, is conformational coupling: inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) receptors in the ER may sense the fall in Ca(2+) levels through Ca(2+)-binding sites on their lumenal domains, and convey this conformational information directly by physically interacting with Ca(2+) channels in the plasma membrane. In support of this idea, in some cell types, store-operated channels in excised membrane patches appear to depend on the presence of both Ins(1,4,5)P(3) and Ins(1,4,5)P(3) receptors for activity; in addition, inhibitors of Ins(1,4,5)P(3) production that either block phospholipase C or inhibit phosphatidylinositol 4-kinase can block capacitative Ca(2+) entry. However, the electrophysiological current underlying capacitative Ca(2+) entry is not blocked by an Ins(1,4,5)P(3) receptor antagonist, and the blocking effects of a phospholipase C inhibitor are not reversed by the intracellular application of Ins(1,4,5)P(3). Furthermore, cells whose Ins(1,4,5)P(3) receptor genes have been disrupted can nevertheless maintain their capability to activate capacitative Ca(2+) entry channels in response to store depletion. A tentative conclusion is that multiple mechanisms for signaling capacitative Ca(2+) entry may exist, and involve conformational coupling in some cell types and perhaps a diffusible signal in others.

Mechanisms of capacitative calcium entry

Inositol 1,4,5-trisphosphate (IP3) is a second messenger that elicits complex spatiotemporal patterns of calcium ion (Ca2+) mobilization and has essential roles in the regulation of many cellular functions. In Madin-Darby canine kidney epithelial cells, green fluorescent protein-tagged pleckstrin homology domain translocated from the plasma membrane to the cytoplasm in response to increased concentration of IP3. The detection of translocation enabled monitoring of IP3 concentration changes within single cells and revealed spatiotemporal dynamics in the concentration of IP3 synchronous with Ca2+ oscillations and intracellular and intercellular IP3 waves that accompanied Ca2+ waves. Such changes in IP3 concentration may be fundamental to Ca2+ signaling.

https://science.sciencemag.org/content/sci/284/5419/1527.full.pdf

Spatiotemporal dynamics of inositol 1,4,5-trisphosphate that underlies complex Ca2+ mobilization patterns.

Vascular Endothelial Growth Factor Signals Endothelial Cell Production of Nitric Oxide and Prostacyclin Through flk-1/KDR Activation of c-Src

The aminosteroid U-73122 inhibits muscarinic receptor sequestration and phosphoinositide hydrolysis in SK-N-SH neuroblastoma cells. A role for Gp in receptor compartmentation.

1-[6-[[17 beta-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]- 1H-pyrrole-2,5-dione (U-73122), an inhibitor of phospholipase C (PLC)-dependent processes in human platelets, was found to be a potent inhibitor of human polymorphonuclear neutrophil (PMN) activation by structurally unrelated receptor-specific agonists. U-73122 caused a time- and concentration-dependent (0.1-1 microM) inhibition of myeloperoxidase and vitamin B12-binding protein release from PMNs exposed to N-formyl-methionyl-leucyl-phenylalanine, recombinant human C5a, leukotriene B4 and platelet-activating factor. Activation of the respiratory burst, as measured by superoxide anion production, in PMNs stimulated with these agonists was also suppressed by U-73122. These data suggested that U-73122 inhibited a component of signal transduction that was common to the mechanisms of action of these stimuli. Production of inositol 1,4,5-trisphosphate and 1,2-diacylglycerol and the rise in the cytosolic free calcium concentration, which are early postreceptor events in PMN activation, were all suppressed in U-73122-treated PMNs stimulated with the agonists. These signal transduction events require activation of PLC. Receptor-coupled activation of PLC in membranes isolated from PMNs was potently inhibited by U-73122. U-73122, however, had no direct effect on PMN protein kinase C activity. 1-[6-[[17 beta-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl] -2,5- pyrrolidine-dione (U-73343), a close analog of U-73122 that does not suppress PLC activity, did not inhibit receptor-specific agonist-induced PMN responsiveness. U-73122, therefore, is a novel reagent that is useful in investigating PLC function in receptor-mediated PMN activation.

Receptor-coupled signal transduction in human polymorphonuclear neutrophils: effects of a novel inhibitor of phospholipase C-dependent processes on cell responsiveness.

L M Sam

Papers

Receptor-coupled signal transduction in human polymorphonuclear neutrophils: effects of a novel inhibitor of phospholipase C-dependent processes on cell responsiveness.