PIKfyve, a FYVE finger-containing phosphoinositide kinase, is an enzyme that in humans is encoded by the PIKFYVE gene. [5] [6]
PIKfyve, a FYVE finger-containing phosphoinositide kinase, is an enzyme that in humans is encoded by the PIKFYVE gene. [5] [6]
The principal enzymatic activity of PIKfyve is to phosphorylate PtdIns3P to PtdIns(3,5)P2. PIKfyve activity is responsible for the production of both PtdIns(3,5)P2 and phosphatidylinositol 5-phosphate (PtdIns5P). [7] [8] [9] [10] PIKfyve is a large protein, containing a number of functional domains and expressed in several spliced forms. The reported full-length mouse and human cDNA clones encode proteins of 2052 and 2098 amino acid residues, respectively. [6] [11] [8] [12] By directly binding membrane PtdIns(3)P, [13] the FYVE finger domain of PIKfyve is essential in localizing the protein to the cytosolic leaflet of endosomes. [6] [13] Impaired PIKfyve enzymatic activity by dominant-interfering mutants, siRNA- mediated ablation or pharmacological inhibition causes lysosome enlargement and cytoplasmic vacuolation due to impaired PtdIns(3,5)P2 synthesis and impaired lysosome fission process and homeostasis. [14] Thus, via PtdIns(3,5)P2 production, PIKfyve participates in several aspects of vesicular dynamics, [15] [16] thereby affecting a number of trafficking pathways that emanate from or traverse the endosomal system en route to the trans-Golgi network or later compartments along the endocytic pathway. [17] [18] [19] [20] [21] [22]
PIKfyve mutations affecting one of the two PIKFYVE alleles are found in 8 out of 10 families with Francois-Neetens corneal fleck dystrophy. [23] Disruption of both PIKFYVE alleles in the mouse is lethal at the stage of pre-implantation embryo. [24] PIKfyve’s role in pathogen invasion is deduced by evidence from cell studies implicating PIKfyve activity in HIV and Salmonella replication. [20] [25] [26] A link of PIKfyve with type 2 diabetes is inferred by the observations that PIKfyve perturbation inhibits insulin-regulated glucose uptake. [27] [28] Concordantly, mice with selective Pikfyve gene disruption in skeletal muscle, the tissue mainly responsible for the decrease of postprandial blood sugar, exhibit systemic insulin resistance; glucose intolerance; hyperinsulinemia; and increased adiposity, i.e. symptoms, typical for human prediabetes. [29]
Several small molecule PIKfyve inhibitors have shown promise as cancer therapeutics in preclinical studies due to selective toxicity in non-Hodgkin lymphoma B cells [30] or in U-251 glioblastoma cells. [31] PIKfyve inhibitors cause cell death also in A-375 melanoma cells, which depend on autophagy for growth and proliferation, due to impaired lysosome homeostasis. [32] The potential therapeutic use of PIKfyve inhibitors awaits clinical trials.
PIKfyve physically associates with its regulator ArPIKfyve, a protein encoded by the human gene VAC14, and the Sac1 domain-containing PtdIns(3,5)P2 5-phosphatase Sac3, encoded by FIG4, to form a stable ternary heterooligomeric complex that is scaffolded by ArPIKfyve homooligomeric interactions. The presence of two enzymes with opposing activities for PtdIns(3,5)P2 synthesis and turnover in a single complex indicates the requirement for a tight control of PtdIns(3,5)P2 levels. [16] [33] [34] PIKfyve also interacts with the Rab9 effector RABEPK and the kinesin adaptor JLP, encoded by SPAG9. [18] [22] These interactions link PIKfyve to microtubule-based endosome to trans-Golgi network traffic. Under sustained activation of glutamate receptors PIKfyve binds to and facilitates the lysosomal degradation of Cav1.2, voltage-dependent calcium channel type 1.2, thereby protecting the neurons from excitotoxicity. [35] PIKfyve negatively regulates Ca2+-dependent exocytosis in neuroendocrine cells without affecting voltage-gated calcium channels. [36]
PIKFYVE belongs to a large family of evolutionarily-conserved lipid kinases. Single copy genes, encoding similarly-structured FYVE-domain–containing phosphoinositide kinases exist in most genomes from yeast to man. The plant A. thaliana has several copies of the enzyme. Higher eukaryotes (after D. melanogaster ), acquire an additional DEP domain. The S. cerevisiae enzyme Fab1p is required for PtdIns(3,5)P2 synthesis under basal conditions and in response to hyperosmotic shock. PtdIns5P, made by PIKfyve kinase activity in mammalian cells, is not detected in budding yeast. [37] Yeast Fab1p associates with Vac14p (the ortholog of human ArPIKfyve) and Fig4p (the ortholog of Sac3). [38] The yeast Fab1 complex also includes Vac7p and probably Atg18p, proteins that are not detected in the mammalian PIKfyve complex. [39] S. cerevisiae could survive without Fab1. [40] In contrast, the knockout of the FYVE domain-containing enzymes in A. thaliana, D. melanogaster, C. elegans and M. musculus leads to embryonic lethality indicating that the FYVE-domain–containing phosphoinositide kinases have become essential in embryonic development of multicellular organisms. [24] [41] [42] [43] Thus, in evolution, the FYVE-domain-containing phosphoinositide kinases retain several aspects of the structural organization, enzyme activity and protein interactions from budding yeast. In higher eukaryotes, the enzymes acquire one additional domain, a role in the production of PtdIns5P, a new set of interacting proteins and become essential in embryonic development.
Phosphoinositide 3-kinases (PI3Ks), also called phosphatidylinositol 3-kinases, are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which in turn are involved in cancer.
The phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit alpha, also called p110α protein, is a class I PI 3-kinase catalytic subunit. The human p110α protein is encoded by the PIK3CA gene.
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 also forms lipid clusters that sort proteins.
Phosphatidylinositol 3-phosphate (PtdIns3P) is a phospholipid found in cell membranes that helps to recruit a range of proteins, many of which are involved in protein trafficking, to the membranes. It is the product of both the class II and III phosphoinositide 3-kinases activity on phosphatidylinositol.
Phosphatidylinositol (3,4)-bisphosphate is a minor phospholipid component of cell membranes, yet an important second messenger. The generation of PtdIns(3,4)P2 at the plasma membrane activates a number of important cell signaling pathways.
Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit delta isoform also known as phosphoinositide 3-kinase (PI3K) delta isoform or p110δ is an enzyme that in humans is encoded by the PIK3CD gene.
Phosphatidylinositol 3,5-bisphosphate is one of the seven phosphoinositides found in eukaryotic cell membranes. In quiescent cells, the PtdIns(3,5)P2 levels, typically quantified by HPLC, are the lowest amongst the constitutively present phosphoinositides. They are approximately 3 to 5-fold lower as compared to PtdIns3P and PtdIns5P levels, and more than 100-fold lower than the abundant PtdIns4P and PtdIns(4,5)P2. PtdIns(3,5)P2 was first reported to occur in mouse fibroblasts and budding yeast S. cerevisiae in 1997. In S. cerevisiae PtdIns(3,5)P2 levels increase dramatically during hyperosmotic shock. The response to hyperosmotic challenge is not conserved in most tested mammalian cells except for differentiated 3T3L1 adipocytes.
In molecular biology the FYVE zinc finger domain is named after the four cysteine-rich proteins: Fab 1, YOTB, Vac 1, and EEA1, in which it has been found. FYVE domains bind phosphatidylinositol 3-phosphate, in a way dependent on its metal ion coordination and basic amino acids. The FYVE domain inserts into cell membranes in a pH-dependent manner. The FYVE domain has been connected to vacuolar protein sorting and endosome function.
Phosphatidylinositol phosphate kinases (PIPK) are kinases that phosphorylate the phosphoinositides PtdInsP and PtdInsP2 that are derivatives of phosphatidylinositol (PtdIns). It has been found that PtdIns is only phosphorylated on three (3,4,5) of its five hydroxyl groups, possibly because D-2 and D-6 hydroxyl groups cannot be phosphorylated because of steric hindrance. All 7 combinations of phosphorylated PtdIns have been found in animals, all except PtdIns(3,4,5)P3 have been found in plants.
The PX domain is a phosphoinositide-binding structural domain involved in targeting of proteins to cell membranes.
Yunis–Varon syndrome (YVS), also called cleidocranial dysplasia with micrognathia or absent thumbs and distal aphalangia, is an extremely rare autosomal recessive multisystem congenital disorder which affects the skeletal system, ectodermal tissue, heart and respiratory system. It was first described by Emilio Yunis and Humberto Váron from the National University of Colombia.
Phosphatidylinositol 3-kinase catalytic subunit type 3 is an enzyme subunit that in humans is encoded by the PIK3C3 gene. It's a class III phosphoinositide 3-kinase.
Phosphatidylinositol-5-phosphate 4-kinase type-2 alpha is an enzyme that in humans is encoded by the PIP4K2A gene.
Phosphatidylinositol 4-kinase 2-alpha is an enzyme that in humans is encoded by the PI4K2A gene.
Rab9 effector protein with Kelch motifs also known as p40 is a protein that in humans is encoded by the RABEPK gene.
Phosphatidylinositol 5-phosphate (PtdIns5P) is a phosphoinositide, one of the phosphorylated derivatives of phosphatidylinositol (PtdIns), that are well-established membrane-anchored regulatory molecules. Phosphoinositides participate in signaling events that control cytoskeletal dynamics, intracellular membrane trafficking, cell proliferation and many other cellular functions. Generally, phosphoinositides transduce signals by recruiting specific phosphoinositide-binding proteins to intracellular membranes.
Polyphosphoinositide phosphatase also known as phosphatidylinositol 3,5-bisphosphate 5-phosphatase or SAC domain-containing protein 3 (Sac3) is an enzyme that in humans is encoded by the FIG4 gene. Fig4 is an abbreviation for Factor-Induced Gene.
Protein VAC14 homolog, also known as ArPIKfyve, is a protein that in humans is encoded by the VAC14 gene.
Vacuolar segregation protein 7 is a protein that in yeast is encoded by the VAC7 gene. VAC7 is a component of the PI(3,5)P2 regulatory complex, composed of ATG18, FIG4, FAB1, VAC14 and VAC7.
Apilimod (STA-5326) is a drug that was initially identified as an inhibitor of production of the interleukins IL-12 and IL-23, and developed for the oral treatment of autoimmune conditions such as Crohn's disease and rheumatoid arthritis, though clinical trial results were disappointing and development for these applications was not continued.