Phosphatidylinositol 5-phosphate

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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. [1]

Contents

Phosphatidylinositol 5-phosphate is one of the 7 known cellular phosphoinositides with less understood functions. It is phosphorylated on position D-5 of the inositol head group, which is attached via phosphodiester linkage to diacylglycerol (with varying chemical composition of the acyl chains, frequently 1-stearoyl-2-arachidonoyl chain). In quiescent cells, on average, PtdIns5P is of similar or higher abundance as compared to PtdIns3P and ~20-100-fold below the levels of PtdIns4P (Phosphatidylinositol 4-phosphate and PtdIns(4,5)P2 (Phosphatidylinositol 4,5-bisphosphate). [2] Notably, steady-state PtdIns5P levels are more than 5-fold higher than those of PtdIns(3,5)P2. [3] [4]

PtdIns5P was first demonstrated by HPLC (high pressure liquid chromatography) in mouse fibroblasts as a substrate for PtdIns(4,5)P2 synthesis by type II PIP kinases (1-phosphatidylinositol-5-phosphate 4-kinase). [5] In many cell types, however, PtdIns5P is not detected by HPLC due to technical limitations associated with its poor separation from the abundant PtdIns4P. [6] Rather, PtdIns5P is measured by the "mass assay", where PtdIns5P (as a part of the extracted cellular lipids) is converted in vitro by purified PtdIns5P 4-kinase to PtdIns(4,5)P2 that is subsequently quantified. [7]

Based on studies with the mass assay [6] and an improved HPLC technique, [8] PtdIns5P is detected in all studied mammalian cells. Most of the cellular PtdIns5P is found on cytoplasmic membranes whereas a smaller fraction resides in the nucleus. [9] The cytoplasmic and nuclear pools have distinct functions and regulation. [10]

Metabolism

Cellular PtdIns5P could be produced by D-5-phosphorylation of phosphatidylinositol or by dephosphorylation of PtdIns(3,5)P2 or PtdIns(4,5)P2. Each of these possibilities is experimentally supported. PtdIns5P is synthesized in vitro by PIKfyve, an enzyme principally responsible for PtdIns(3,5)P2 production, [11] [12] as well as by [PIP5K]s. [13] A major role for PIKfyve in synthesis of cellular PtdIns5P is suggested by data for reduced PtdIns5P mass levels upon heterologous overexpression of the enzymatically inactive PIKfyve point-mutant (PIKfyveK1831E) [6] [14] and PIKfyve silencing by small interfering RNAs. [15] Such a role is reinforced by data in transgenic fibroblasts with one genetically disrupted PIKfyve allele, demonstrating equal reduction of steady-state levels of PtdIns5P and PtdIns(3,5)P2. [3]

Likewise, similar reduction of PtdIns5P and PtdIns(3,5)P2 is found in fibroblasts with knockout of the PIKfyve activator [16] ArPIKfyve/VAC14. [4] This experimental evidence coupled with the fact that the cellular levels of PtdIns5P exceed more than 5-fold those of PtdIns(3,5)P2 indicate a predominant role of PIKfyve in maintenance of the steady-state PtdIns5P levels via D-5 phosphorylation of phosphatidylinositol.

A role for the myotubularin protein family in PtdIns5P production has been proposed based on dephosphorylation of PtdIns(3,5)P2 by overexpressed myotubularin 1. [17] Concordantly, genetic ablation of the myotubularin-related protein 2 (MTMR2) causes elevation of cellular PtdIns(3,5)P2 and a decrease of PtdIns5P. [18] The low cellular levels of PtdIns(3,5)P2 suggest that myotubularin phosphatase activity plays a minor role in maintaining the steady-state PtdIns5P levels. Importantly, PtdIns(3,5)P2 is synthesized from PtdIns3P by the PIKfyve complex that includes ArPIKfyve and Sac3/Fig4. [19] Noteworthy, the PIKfyve complex underlies both PtdIns(3,5)P2 synthesis from and turnover to PtdIns3P. [20] The relative proportion of PtdIns(3,5)P2 turnover by myotubularin phosphatases versus that by Sac3 is unknown.

PtdIns5P can also be produced by dephosphorylation of PtdIns(4,5)P2. Such phosphatase activity is shown for Shigella flexneri effector IpgD [21] and two mammalian phosphatases – PtdIns(4,5)P2 4-phosphatase type I and type II. [22]

In myoblast, PtdIns5P is rapidly metabolized by the PI5P 4-kinase α into PI(4,5)P2 which accumulates at the plasma membrane thereby facilitating the formation of podosome-like protrusions, playing a crucial role in the spatiotemporal regulation of myoblast fusion. [23]

Currently, there is no known mammalian phosphatase to specifically dephosphorylate PtdIns5P. The pathway for PtdIns5P clearance involves synthesis of PtdIns(4,5)P2. [10]

Functions

The levels of PtdIns5P change significantly in response to physiological and pathological stimuli. Insulin, [8] [24] thrombin, [7] T-cell activation, [25] and cell transformation with nucleophosmin anaplastic lymphoma tyrosine kinase (NPM-ALK), [15] cause elevation of cellular PtdIns5P levels. In contrast, hypoosmotic shock [6] and histamine treatment [26] decrease the levels of PtdIns5P. In T-cells, two “downstream of tyrosine kinase” proteins DOK1 and DOK2 are proposed as PtdIns5P-binding proteins and effectors. [25]

As the other phosphoinositides, PtdIns5P is also present in the nucleus of mammalian cells. [27] The nuclear PtdIns5P pool is controlled by the nuclear type I PtdIns(4,5)P2 4-phosphatase that, in conjunction with the PIPKIIbeta kinase, plays a role in UV stress, apoptosis and cell cycle progression. [9] [28] [29]

The function of PtdIns5P in nuclear signaling likely involves ING2, a member of the ING family. The proteins of this family associate with and modulate the activity of histone acetylases and deacetylases as well as induce apoptosis through p53 acetylation. The ING2 interacts with PtdIns5P via its plant homeodomain (PHD) finger motif. [30]

In summary, the available evidence indicates that PIKfyve activity is the major source of steady-state cellular PtdIns5P. Under certain conditions, PtdIns5P is produced by dephosphorylation of bis-phosphoinositides. PtdIns5P is involved in regulation of both basic cellular functions and responses to a multitude of physiological and pathological stimuli by yet- to- be specified molecular mechanisms.

Related Research Articles

<span class="mw-page-title-main">Phosphatidylinositol</span> Signaling molecule

Phosphatidylinositol or inositol phospholipid is a biomolecule. It was initially called "inosite" when it was discovered by Léon Maquenne and Johann Joseph von Scherer in the late 19th century. It was discovered in bacteria but later also found in eukaryotes, and was found to be a signaling molecule.

<span class="mw-page-title-main">Phosphatidylinositol (3,4,5)-trisphosphate</span> Chemical compound

Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3), abbreviated PIP3, is the product of the class I phosphoinositide 3-kinases' (PI 3-kinases) phosphorylation of phosphatidylinositol (4,5)-bisphosphate (PIP2). It is a phospholipid that resides on the plasma membrane.

<span class="mw-page-title-main">Phosphoinositide 3-kinase</span> Class of enzymes

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.

<span class="mw-page-title-main">Phosphatidylinositol 4,5-bisphosphate</span> Chemical compound

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.

<span class="mw-page-title-main">Phosphatidylinositol 3-phosphate</span> Chemical compound

Phosphatidylinositol 3-phosphate (PI3P) 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.

<span class="mw-page-title-main">Phosphatidylinositol 3,4-bisphosphate</span>

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.

<span class="mw-page-title-main">Phosphatidylinositol 3,5-bisphosphate</span> Chemical compound

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.

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.

<span class="mw-page-title-main">PX domain</span>

The PX domain is a phosphoinositide-binding structural domain involved in targeting of proteins to cell membranes.

<span class="mw-page-title-main">Yunis–Varon syndrome</span> Medical condition

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.

The enzyme phosphoinositide 5-phosphatase (EC 3.1.3.36) catalyzes the reaction

<span class="mw-page-title-main">PIKFYVE</span> Protein-coding gene in the species Homo sapiens

PIKfyve, a FYVE finger-containing phosphoinositide kinase, is an enzyme that in humans is encoded by the PIKFYVE gene.

<span class="mw-page-title-main">PIP4K2A</span> Kinase enzyme

Phosphatidylinositol-5-phosphate 4-kinase type-2 alpha is an enzyme that in humans is encoded by the PIP4K2A gene.

<span class="mw-page-title-main">PIP4K2B</span> Protein-coding gene in the species Homo sapiens

Phosphatidylinositol-5-phosphate 4-kinase type-2 beta is an enzyme that in humans is encoded by the PIP4K2B gene.

Bisphosphate may refer to:

<span class="mw-page-title-main">Fig4</span> Protein-coding gene in the species Homo sapiens

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.

<span class="mw-page-title-main">VAC14</span> Protein-coding gene in the species Homo sapiens

Protein VAC14 homolog, also known as ArPIKfyve, is a protein that in humans is encoded by the VAC14 gene.

Phosphatidylinositol-4,5-bisphosphate 4-phosphatase (EC 3.1.3.78, phosphatidylinositol-4,5-bisphosphate 4-phosphatase I, phosphatidylinositol-4,5-bisphosphate 4-phosphatase II, type I PtdIns-4,5-P2 4-Ptase, type II PtdIns-4,5-P2 4-Ptase, IpgD, PtdIns-4,5-P2 4-phosphatase type I, PtdIns-4,5-P2 4-phosphatase type II, type I phosphatidylinositol-4,5-bisphosphate 4-phosphatase, type 1 4-phosphatase) is an enzyme with systematic name 1-phosphatidyl-1D-myo-inositol-4,5-bisphosphate 4-phosphohydrolase. This enzyme catalyses the following chemical reaction

Phosphatidylinositol-4-phosphate 5-kinases are a class of enzymes that phosphorylate phosphatidylinositol 4-phosphate. They perform this reaction on the fifth hydroxyl of the myo-inositol ring to form phosphatidylinositol 4,5-bisphosphate.

<span class="mw-page-title-main">Apilimod</span> Chemical compound

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.

References

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