Phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate or PtdIns(4,5)P2, also known simply as PIP2 or PI(4,5)P2, is a minor phospholipid component of cell membranes. PtdIns(4,5)P2 is enriched at the plasma membrane where it is a substrate for a number of important signaling proteins. Phosphatidylinositol 4,5-bisphosphate or PtdIns(4,5)P2, also known simply as PIP2 or PI(4,5)P2, is a minor phospholipid component of cell membranes. PtdIns(4,5)P2 is enriched at the plasma membrane where it is a substrate for a number of important signaling proteins. PIP2 is formed primarily by the type I phosphatidylinositol 4-phosphate 5-kinases from PI(4)P. In metazoans, PIP2 can also be formed by type II phosphatidylinositol 5-phosphate 4-kinases from PI(5)P. The fatty acids of PIP2 are variable in different species and tissues, but the most common fatty acids are stearic in position 1 and arachidonic in 2. PIP2 is a part of many cellular signaling pathways notably PIP2 cycle, PI3K signalling and PI5P metabolism. Recently it has been found in the nucleus with unknown function. PIP2 regulates the organization, polymerization, and branching of filamentous actin (F-actin) via direct binding to F-actin regulatory proteins. The first evidence that indicated phosphoinositides(PIs) (especially PI(4,5)P2) are important during the exocytosis process was in 1990. Emberhard et al. found that the application of PI-specific phospholipase C into digitonin-permeablized chromaffin cells decreased PI levels, and inhibited calcium-triggered exocytosis. This exocytosis inhibition was preferential for an ATP-dependent stage, indicating PI function was required for secretion. Later studies identified associated proteins necessary during this stage, such as phosphatidylinositol transfer protein, and phosphoinositol-4-monophosphatase 5 kinase type Iγ (PIPKγ), which mediates PI(4,5)P2 restoration in permeable cell incubation in an ATP-dependent way. In these later studies, PI(4,5)P2 specific antibodies strongly inhibited exocytosis, thus providing direct evidence that PI(4,5)P2 plays a pivotal role during the LDCV (Large dense core vesicle) exocytosis process. Through the use of PI-specific kinase/phosphatase identification and PI antibody/drug/blocker discovery, the role of PI (especially PI(4,5)P2) in secretion regulation was extensively investigated. Studies utilizing PHPLCδ1 domain over-expression (acting as PI(4,5)P2 buffer or blocker), PIPKIγ knockout in chromaffin cell and in central nerve system, PIPKIγ knockdown in beta cell lines, and over-expression of membrane-tethered inositol 5-phosphatase domain of synaptojanin 1, all suggested vesicle (synaptic vesicle and LDCV) secretion were severely impaired after PI(4,5)P2 depletion or blockage. Moreover, some studies showed an impaired/reduced RRP of those vesicles, though the docked vesicle number were not altered after PI(4,5)P2 depletion, indicating a defect at a pre-fusion stage (priming stage). Follow-up studies indicated that PI(4,5)P2 interactions with CAPS, Munc13 and synaptotagmin1 are likely to play a role in this PI(4,5)P2 dependent priming defect. PIP2 functions as an intermediate in the , which is initiated by ligands binding to G protein-coupled receptors activating the Gq alpha subunit. PtdIns(4,5)P2 is a substrate for hydrolysis by phospholipase C (PLC), a membrane-bound enzyme activated through protein receptors such as α1 adrenergic receptors. PIP2 regulates the function of many membrane proteins and ion channels, such as the M-channel. The products of the PLC catalyzation of PIP2 are inositol 1,4,5-trisphosphate (InsP3; IP3) and diacylglycerol (DAG), both of which function as second messengers. In this cascade, DAG remains on the cell membrane and activates the signal cascade by activating protein kinase C (PKC). PKC in turn activates other cytosolic proteins by phosphorylating them. The effect of PKC could be reversed by phosphatases. IP3 enters the cytoplasm and activates IP3 receptors on the smooth endoplasmic reticulum (ER), which opens calcium channels on the smooth ER, allowing mobilization of calcium ions through specific Ca2+ channels into the cytosol. Calcium participates in the cascade by activating other proteins. Class I PI 3-kinases phosphorylate PtdIns(4,5)P2 forming phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) and PtdIns(4,5)P2 can be converted from PtdIns4P. PtdIns4P, PtdIns(3,4,5)P3 and PtdIns(4,5)P2 not only act as substrates for enzymes but also serve as docking phospholipids that bind specific domains that promote the recruitment of proteins to the plasma membrane and subsequent activation of signaling cascades.