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.[1] PIP2 also forms
lipid clusters[2] that sort proteins.[3][4][5]
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.[6]
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.[7]
Signaling pathways
PIP2 is a part of many cellular signaling pathways, including
PIP2 cycle,
PI3K signalling, and PI5P metabolism.[8] Recently, it has been found in the
nucleus[9] with unknown function.
Functions
Cytoskeleton dynamics near membranes
PIP2 regulates the organization, polymerization, and branching of filamentous actin (F-actin) via direct binding to F-actin regulatory proteins.[10]
Endocytosis and exocytosis
The first evidence that indicated phosphoinositides(PIs) (especially PI(4,5)P2) are important during the exocytosis process was in 1990. Emberhard et al.
[11]
found that the application of PI-specific
phospholipase C into digitonin-permeabilized 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
,[12] and phosphoinositol-4-monophosphatase 5 kinase type Iγ (PIPKγ)
,[13]
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.[citation needed]
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)
,[14] PIPKIγ knockout in chromaffin cell
[15] and in central nerve system,[16] PIPKIγ knockdown in beta cell lines
,[17] and over-expression of membrane-tethered inositol 5-phosphatase domain of synaptojanin 1
,[18] all suggested vesicle (synaptic vesicle and LDCV) secretion were severely impaired after PI(4,5)P2 depletion or blockage. Moreover, some studies[18][16][15] showed an impaired/reduced RRP of those vesicles, though the docked vesicle number were not altered[15] 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,[19] Munc13[20] and synaptotagmin1[21] are likely to play a role in this PI(4,5)P2 dependent priming defect.
IP3/DAG pathway
PIP2 functions as an intermediate in the
IP3/DAG pathway, 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.[22]
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.[23][24]
Examples of proteins activated by PtdIns(3,4,5)P3 are
Akt,
PDPK1,
Btk1.
PtdIns(4,5)P2 has been shown to stabilize the active states of Class A
G protein-coupled receptors (GPCRs) via direct binding, and enhance their selectivity toward certain G proteins.[28]
PIP2 is regulated by many different components. One emerging hypothesis is that PIP2 concentration is maintained locally. Some of the factors involved in PIP2 regulation are:[30]
^
Hay, Jesse C, Thomas M (1993). "Phosphatidylinositol transfer protein required for ATP-dependent priming of Ca2+-activated secretion". Nature. 366 (6455): 572–575.
doi:
10.1038/366572a0.
PMID8255295.
S2CID4348488.