Emerging evidence of signalling roles for PI(3,4)P₂ in Class I and II PI3K-regulated pathways

There are eight members of the phosphoinositide family of phospholipids in eukaryotes; PI, PI3P, PI4P, PI5P, PI(4,5)P₂, PI(3,4)P₂, PI(3,5)P₂ and PI(3,4,5)P₃. Receptor activation of Class I PI3Ks stimulates the phosphorylation of PI(4,5)P₂ to form PI(3,4,5)P₃. PI(3,4,5)P₃ is an important messenger molecule that is part of a complex signalling network controlling cell growth and division. PI(3,4,5)P₃ can be dephosphorylated by both 3- and 5-phosphatases, producing PI(4,5)P₂ and PI(3,4)P₂, respectively. There is now strong evidence that PI(3,4)P₂ generated by this route does not merely represent another pathway for removal of PI(3,4,5)P₃, but can act as a signalling molecule in its own right, regulating macropinocytosis, fast endophilin-mediated endocytosis (FEME), membrane ruffling, lamellipodia and invadopodia. PI(3,4)P₂ can also be synthesized directly from PI4P by Class II PI3Ks and this is important for the maturation of clathrin-coated pits [clathrin-mediated endocytosis (CME)] and signalling in early endosomes. Thus PI(3,4)P₂ is emerging as an important signalling molecule involved in the coordination of several specific membrane and cytoskeletal responses. Further, its inappropriate accumulation contributes to pathology caused by mutations in genes encoding enzymes responsible for its degradation, e.g. Inpp4B.