Phosphatidylinositol 3,4-bisphosphate
Phosphatidylinositol -bisphosphate is a minor phospholipid component of cell membranes, yet an important second messenger. The generation of PtdInsP2 at the plasma membrane activates a number of important cell signaling pathways.
Of all the phospholipids found within the membrane, inositol phospholipids make up less than 10%. Phosphoinositides, also known as phosphatidylinositol phosphates, are synthesized in the cell's endoplasmic reticulum by the protein phosphatidylinositol synthase. PIs are highly compartmentalized; their main components include a glycerol backbone, two fatty acid chains enriched with stearic acid and arachidonic acid, and an inositol ring whose phosphate groups' regulation differs between organelles depending on the specific PI and PIP kinases and PIP phosphatases present in the organelle. These kinases and phosphatases conduct phosphorylation and dephosphorylation at the inositol sugar head groups 3’, 4’, and 5’ positions, producing differing phosphoinositides, including PtdInsP2. PI kinases catalyze phosphate group binding while PI phosphatases remove phosphate groups at the three positions on the PI inositol ring, giving seven different combinations of PIs.
PtdInsP2 is dephophosphorylated by the phosphatase INPP4B on the 4' position of the inositol ring and by the TPTE family of phosphatases on the 3 position of the inositol ring.
The PH domain in a number of proteins binds to PtdInsP2 including the PH domain in PKB. The generation of PtdInsP2 at the plasma membrane upon the activation of class I PI 3-kinases and SHIP phosphatases causes these proteins to translocate to the plasma membrane, thereby affecting their activity.
Class I and II phosphoinositide 3-kinases synthesize PtdInsP2 by phosphorylating the phosphoinositide PI4P’s 3' -OH position. Phosphatases SHIP1 and SH2-containing inositol 5’-polyphosphatases produce PtdInsP2 through desphosphorylation of PtdInsP3’s 5’ inositol ring position. In addition to these positive regulators at the plasma membrane, 3-phosphatase tensin homolog acts as a negative regulator of PtdInsP2 production by depleting PtdInsP3 levels at the PM through dephosphorylation of PtdInsP3’s 3’ inositol ring position, giving rise to PtdInsP2. Inositol polyphosphate 4-phosphatase isozymes, INPP4A and INPP4B, also act as negative PtdInsP2 regulators, though through a more direct interaction- by hydrolyzing PtdInsP2’s 4-phosphate, producing PI3P. PtdInsP2 has been indicated to be critical for AKT activation within the PI3K pathway through the PI’s regulation by the SHIP1 and 2 phosphatases. Akt is recruited and subsequently activated through its PH domains interaction with PtdInsP2 and PtdInsP3 both of which have shown to have high affinity with the Akt PH domain. Once bound to the PM through its interaction with PtdInsP2 and PtdInsP3, Akt is activated through release of its auto-inhibitory interaction between the PH and kinase domains. Following this release, T308 in the proteins activation loop and S437 in the proteins hydrophobic domain are phosphorylated by Phosphoinositide-dependent kinase-1 and mechanistic target of Rapamycin Complex 2, respectively. Test tube experiments have shown that the essential recruitment of PDK1 for Akt activation at the PM can be driven through interactions with both PtdInsP2 and PtdInsP3.
It was originally presumed that 5-phosphatases dephosphorylation of PIP3 would be anti-tumoral, similar to tumor suppressor PTEN. Yet the 5-phosphatase SHIP proteins synthesis of PIP2 has been linked to tumor cell survival due to the lipid’s binding and subsequent activation of Akt. Akt activation causes downstream metabolism alterations, apoptosis suppression and a rise in cell proliferation. This pathway and its effects have shown up in 50% of cancers. In conjunction, investigators have shown a rise in PIP2 levels and mutation of 4-phosphatase INPP4B has shown mammary epithelial transformation.
Recently, PtdInsP2 has been shown to play an important role in vesicle maturation during clathrin-mediated endocytosis. PtdInsP synthesizing phosphatases SHIP2 and synaptojanin are recruited to clathrin structures at the beginning of the CME process. This production of PtdInsP subsequently leads to PtdInsP2 synthesis through PI3K-C2α11, and the newly synthesized PtdInsP2 then recruits SNX9 and SNX18 PX-BAR domain proteins which narrow the nascent vesicles neck to eventually be cut and released by dynamin, forming vesicles.
PIP2 plays another possible role at the PM, promoting cytoskeletal rearrangements through actin regulatory proteins like Lamellipodin. Lamellipodin is recruited to the PM where it is believed to interact with PIP2 through its PH domain. Once at the PM, it can regulate lamellipodia actin networks and cell migration by interacting with actin-binding proteins like Ena/VASP.