Phosphatidylinositol 4-phosphate 5-kinase Research Articles

Phosphatidylinositol 4,5-bisphosphate regulates cilium transition zone maturation in Drosophila melanogaster.

Cilia are cellular antennae that are essential for human development and physiology. A large number of genetic disorders linked to cilium dysfunction are associated with proteins that localize to the ciliary transition zone (TZ), a structure at the base of cilia that regulates trafficking in and out of the cilium. Despite substantial effort to identify TZ proteins and their roles in cilium assembly and function, processes underlying maturation of TZs are not well understood. Here, we report a role for the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2) in TZ maturation in the Drosophila melanogaster male germline. We show that reduction of cellular PIP2 levels through ectopic expression of a phosphoinositide phosphatase or mutation of the type I phosphatidylinositol phosphate kinase Skittles induces formation of longer than normal TZs. These hyperelongated TZs exhibit functional defects, including loss of plasma membrane tethering. We also report that the onion rings (onr) allele of DrosophilaExo84 decouples TZ hyperelongation from loss of cilium-plasma membrane tethering. Our results reveal a requirement for PIP2 in supporting ciliogenesis by promoting proper TZ maturation.

Abstract A078: Towards understanding noncanonical phosphatidylinositol kinases in the maintenance of prostate metabolism

Abstract An estimated 1 in 7 men will develop prostate cancer (PCa) with many progressing to advanced castrate-resistant disease. Unlike other tissue types, normal prostate cell growth and development is heavily dependent on the androgen receptor (AR) signaling pathway. While the introduction of novel AR antagonists for clinical treatment has improved outcomes, most castration-resistant prostate cancer (CRPC) patients ultimately develop resistance to these therapies. A need exists to better understand the mechanisms that control the transition of prostate cells from a hormone-dependent to castrate-resistant state. Androgens strongly influence the metabolic state of PCa cells to favor sustained cellular growth. We hypothesize there are effectors working in conjunction with AR to coordinate alterations to androgen-dependent metabolism that are linchpins in the orchestration of the transition to CRPC. Leading candidates are members of phosphoinositol (PI) pathways, which have a high frequency of alteration in PCa (i.e phosphoinositide 3-kinase (PI3K)). Herein we explore a family of poorly understood lipid kinases called the type II phosphatidylinositol-5-phosphate 4-kinases (PI5P4Ks) and predict them to be critical regulators of cancer cell survival. PI5P4Ks are druggable targets that act by phosphorylating the lipid phosphatidylinositol-5-phosphate (PI 5-P) at the 4 position of the inositol ring to generate phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2; PIP2). We implicate the three PI5P4K isoforms (PI5P4Kα, PI5P4Kβ, and PI5P4Kγ) encoded by the genes PIP4K2A, B, and C, to be important regulators of cancer metabolism that play a role in the maintenance of prostate biology and oncogenesis. Analysis of transcript data revealed expression of PIP4K2A, B, and C in primary PCa patient samples, which was correlated with an AR activation gene signature and hotspot tumor suppressor deletion. As well, isoform expression was assessed for differential expression in relation to an integrated neuroendocrine prostate cancer mRNA score (TCGA; n=333). PI5P4Kα and PI5P4Kβ protein was detected in primary and advanced prostate cancer using optimized antibodies of patient tissue TMAs (n= 72). Using in vitro LNCaP cell models, siRNA knockdown systems were tested to evaluate the molecular consequence of targeting PIP4K2A and PIP4K2B in androgen-dependent systems. Stable knockdown using fluorescently labeled lentiviral shRNA constructs significantly reduced proliferation of shPIP4K2 treated cells. As well, we have produced a prostate-specific PI5P4K knockout mouse model by expressing probasin-driven Cre in a homozygous 129/SvEv Pip4k2aflx/flx murine strain. Finally, implementation of a discovery-based metabolomic platform (Metabolon HD4) was used to profile the overall shift in metabolite species that results from downregulating the expression of PIP4K2A in androgen-dependent cell models. In summary, we have developed novel insights into the role of a family of noncanonical PI kinases in prostate biology. There are a growing number of PI3K/AKT inhibitors being tested in combination with androgen deprivation therapy in clinical trials, but there is still almost nothing known about the potential crosstalk of the greater PI kinase network. These data convincingly implicate a fundamental role for PI5P4Ks in PCa androgen signaling and metabolism, as well as lay the foundation of phenotypic understanding of what PI5P4K is responsible for in the prostate. Citation Format: Joanna Triscott, Matteo Benelli, Verena Sailer, Davide Prandi, Brooke Emerling, Francesca Demichelis, Lewis Cantley, Mark A. Rubin. Towards understanding noncanonical phosphatidylinositol kinases in the maintenance of prostate metabolism [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A078.

Structural insights into lethal contractural syndrome type 3 (LCCS3) caused by a missense mutation of PIP5Kγ.

Signaling molecule phosphatidylinositol 4,5-bisphosphate is produced primarily by phosphatidylinositol 4-phosphate 5-kinase (PIP5K). PIP5K is essential for the development of the human neuronal system, which has been exemplified by a recessive genetic disorder, lethal congenital contractural syndrome type 3, caused by a single aspartate-to-asparagine mutation in the kinase domain of PIP5Kγ. So far, the exact role of this aspartate residue has yet to be elucidated. In this work, we conducted structural, functional and computational studies on a zebrafish PIP5Kα variant with a mutation at the same site. Compared with the structure of the wild-type (WT) protein in the ATP-bound state, the ATP-associating glycine-rich loop of the mutant protein was severely disordered and the temperature factor of ATP was significantly higher. Both observations suggest a greater degree of disorder of the bound ATP, whereas neither the structure of the catalytic site nor the Km toward ATP was substantially affected by the mutation. Microsecond molecular dynamics simulation revealed that negative charge elimination caused by the mutation destabilized the involved hydrogen bonds and affected key electrostatic interactions in the close proximity of ATP. Taken together, our data indicated that the disease-related aspartate residue is a key node in the interaction network crucial for effective ATP binding. This work provides a paradigm of how a subtle but critical structural perturbation caused by a single mutation at the ATP-binding site abolishes the kinase activity, emphasizing that stabilizing substrate in a productive conformational state is crucial for catalysis.

Cdk5-mediated phosphorylation regulates phosphatidylinositol 4-phosphate 5-kinase type I γ 90 activity and cell invasion.

Phosphatidylinositol 4-phosphate 5-kinase type I γ (PIPKIγ90) regulates cell migration, invasion, and metastasis. However, it is unknown how cellular signals regulate those processes. Here, we show that cyclin-dependent kinase 5 (Cdk5), a protein kinase that regulates cell migration and invasion, phosphorylates PIPKIγ90 at S453, and that Cdk5-mediated PIPKIγ90 phosphorylation is essential for cell invasion. Moreover, Cdk5-mediated phosphorylation down-regulates the activity of PIPKIγ90 and the secretion of fibronectin, an extracellular matrix protein that regulates cell migration and invasion. Furthermore, inhibition of PIPKIγ activity with the chemical inhibitor UNC3230 suppresses fibronectin secretion in a dose-dependent manner, whereas depletion of Cdk5 enhances fibronectin secretion. With total internal reflection fluorescence microscopy, we found that secreted fibronectin appears as round dots, which colocalize with Tks5 and CD9 but not with Zyxin. These data suggest that Cdk5-mediated PIPKIγ90 phosphorylation regulates cell invasion by controlling PIPKIγ90 activity and fibronectin secretion.-Li, L., Kołodziej, T., Jafari, N., Chen, J., Zhu, H., Rajfur, Z., Huang, C. Cdk5-mediated phosphorylation regulates phosphatidylinositol 4-phosphate 5-kinase type I γ 90 activity and cell invasion.

Analysis of the contribution of phosphoinositides to medial septation in fission yeast highlights the importance of PI(4,5)P2 for medial contractile ring anchoring.

In Schizosaccharomyces pombe, loss of the plasma membrane PI4-kinase scaffold Efr3 leads to sliding of the cytokinetic ring (CR) away from the cell center during anaphase, implicating phosphoinositides (PIPs) in CR anchoring. However, whether other PIP regulators contribute to CR anchoring has not been investigated. Here we report that mutants of other PIP kinases and their regulators divide with off-center septa, similar to efr3∆. Using new biosensors for S. pombe PIPs, we confirm that these mutants have disrupted PIP composition. We extend a previous finding that a mutant known to decrease PI(3,5)P2 levels indirectly affects CR positioning by increasing vacuole size which disrupts nuclear position at the onset of mitosis. Indeed, we found that other mutants with increased vacuole size also disrupt medial division via this mechanism. Although elevated plasma membrane PI(4,5)P2 levels do not affect medial cytokinesis, mutants with decreased levels display CR sliding events indicating a specific role for PI(4,5)P2 in CR anchoring.

NEDD4-induced degradative ubiquitination of phosphatidylinositol 4-phosphate 5-kinase α and its implication in breast cancer cell proliferation.

Phosphatidylinositol 4‐phosphate 5‐kinase (PIP5K) family members generate phosphatidylinositol 4,5‐bisphosphate (PIP2), a critical lipid regulator of diverse physiological processes. The PIP5K‐dependent PIP2 generation can also act upstream of the oncogenic phosphatidylinositol 3‐kinase (PI3K)/Akt pathway. Many studies have demonstrated various mechanisms of spatiotemporal regulation of PIP5K catalytic activity. However, there are few studies on regulation of PIP5K protein stability. Here, we examined potential regulation of PIP5Kα, a PIP5K isoform, via ubiquitin‐proteasome system, and its implication for breast cancer. Our results showed that the ubiquitin ligase NEDD4 (neural precursor cell expressed, developmentally down‐regulated gene 4) mediated ubiquitination and proteasomal degradation of PIP5Kα, consequently reducing plasma membrane PIP2 level. NEDD4 interacted with the C‐terminal region and ubiquitinated the N‐terminal lysine 88 in PIP5Kα. In addition, PIP5Kα gene disruption inhibited epidermal growth factor (EGF)‐induced Akt activation and caused significant proliferation defect in breast cancer cells. Notably, PIP5Kα K88R mutant that was resistant to NEDD4‐mediated ubiquitination and degradation showed more potentiating effects on Akt activation by EGF and cell proliferation than wild‐type PIP5Kα. Collectively, these results suggest that PIP5Kα is a novel degradative substrate of NEDD4 and that the PIP5Kα‐dependent PIP2 pool contributing to breast cancer cell proliferation through PI3K/Akt activation is negatively controlled by NEDD4.

Reproducibility of Phosphoinositide Lipid Extraction & Presentation in Bioanalytical Assays

Phosphatidylinositol 4‐phosphate (PI4P) produced by Phosphatidylinositol 4‐Phosphate kinases is an abundant Golgi phosphoinositide functioning in lipid exchange and vesicle transport within mammalian cells. Researchers wanting to quantify PI4P use the Bligh and Dyer method for extracting and separating phospholipids in cells and biological tissues. This common lipid extraction procedure is known to have inherent variation. Using a human promyelocytic leukemia cell line and an ELISA that is sensitive and specific for PI4P, we examine sources of variation in bioassays including sample (HL‐60 cells), the extraction procedure, and lipid detection. We determined a PI4P Mass ELISA has low coefficient of variation (<10%) if the extracted sample volume is held constant and samples are prepped and lipids presented with standardized procedures. Thus, the organic solvent extraction process is the major source of variation in typical Bligh‐Dyer preparations.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Differential Regulation of Genes Involved in Root Morphogenesis and Cell Wall Modification is Associated with Salinity Tolerance in Chickpea

Salinity is a major constraint for intrinsically salt sensitive grain legume chickpea. Chickpea exhibits large genetic variation amongst cultivars, which show better yields in saline conditions but still need to be improved further for sustainable crop production. Based on previous multi-location physiological screening, JG 11 (salt tolerant) and ICCV 2 (salt sensitive) were subjected to salt stress to evaluate their physiological and transcriptional responses. A total of ~480 million RNA-Seq reads were sequenced from root tissues which resulted in identification of 3,053 differentially expressed genes (DEGs) in response to salt stress. Reproductive stage shows high number of DEGs suggesting major transcriptional reorganization in response to salt to enable tolerance. Importantly, cationic peroxidase, Aspartic ase, NRT1/PTR, phosphatidylinositol phosphate kinase, DREB1E and ERF genes were significantly up-regulated in tolerant genotype. In addition, we identified a suite of important genes involved in cell wall modification and root morphogenesis such as dirigent proteins, expansin and casparian strip membrane proteins that could potentially confer salt tolerance. Further, phytohormonal cross-talk between ERF and PIN-FORMED genes which modulate the root growth was observed. The gene set enrichment analysis and functional annotation of these genes suggests they may be utilised as potential candidates for improving chickpea salt tolerance.

Identification of human Phosphatidyl Inositol 5-Phosphate 4-Kinase as an RNA binding protein that is imported into Plasmodium falciparum

Plasmodium falciparum is a causative agent for malaria and has a complex life cycle in human and mosquito hosts. Translation repression of specific set of mRNA has been reported in gametocyte stages of this parasite. A conserved element present in the 3′UTR of some of these transcripts was identified. Biochemical studies have identified components of the RNA storage and/or translation inhibitor complex but it is not yet clear how the complex is specifically recruited on the RNA targeted for translation regulation. We used the 3′UTR region of translationally regulated transcripts to identify Phosphatidyl-inositol 5-phosphate 4-kinase (PIP4K2A) as the protein that associates with these RNAs. We further show that recombinant PIP4K2A has the RNA binding activity and can associate specifically with Plasmodium 3′UTR RNAs. Immunostainings show that hPIP4K2A is imported into the Plasmodium parasite from RBC. These results identify a novel RNA binding role for PIP4K2A that may play a role in Plasmodium propagation.

Nuclear Phosphatidylinositol-Phosphate Type I Kinase α-Coupled Star-PAP Polyadenylation Regulates Cell Invasion.

ABSTRACTStar-PAP, a nuclear phosphatidylinositol (PI) signal-regulated poly(A) polymerase (PAP), couples with type I PI phosphate kinase α (PIPKIα) and controls gene expression. We show that Star-PAP and PIPKIα together regulate 3′-end processing and expression of pre-mRNAs encoding key anti-invasive factors (KISS1R, CDH1, NME1, CDH13, FEZ1, and WIF1) in breast cancer. Consistently, the endogenous Star-PAP level is negatively correlated with the cellular invasiveness of breast cancer cells. While silencing Star-PAP or PIPKIα increases cellular invasiveness in low-invasiveness MCF7 cells, Star-PAP overexpression decreases invasiveness in highly invasive MDA-MB-231 cells in a cellular Star-PAP level-dependent manner. However, expression of the PIPKIα-noninteracting Star-PAP mutant or the phosphodeficient Star-PAP (S6A mutant) has no effect on cellular invasiveness. These results strongly indicate that PIPKIα interaction and Star-PAP S6 phosphorylation are required for Star-PAP-mediated regulation of cancer cell invasion and give specificity to target anti-invasive gene expression. Our study establishes Star-PAP–PIPKIα-mediated 3′-end processing as a key anti-invasive mechanism in breast cancer.

PIP4K2A regulates intracellular cholesterol transport through modulating PI(4,5)P2 homeostasis

The transport of LDL-derived cholesterol from lysosomes to peroxisomes is facilitated by membrane contacts formed between the lysosomal protein synaptotagmin VII and the peroxisomal lipid phosphatidylinositol 4, 5-bisphosphate [PI(4,5)P2]. Here, we used RNA interference to search for regulators of PI(4,5)P2 and to study the effects of altered PI(4,5)P2 homeostasis on cholesterol transport. We found that knockdown of phosphatidylinositol 5-phosphate 4-kinase type-2 α (PIP4K2A) reduced peroxisomal PI(4,5)P2 levels, decreased lysosome-peroxisome membrane contacts, and increased accumulation of lysosomal cholesterol in human SV-589 fibroblasts. Forced expression of peroxisome-localized, kinase-active PIP4K2A in the knockdown cells reduced cholesterol accumulation, and in vitro addition of recombinant PIP4K2A restored membrane contacts. These results suggest that PIP4K2A plays a critical role in intracellular cholesterol transport by upregulating PI(4,5)P2 levels in the peroxisomal membrane. Further research into PIP4K2A activity may inform future therapeutic interventions for managing lysosomal storage disorders.

PIP5K1\u03b1 promotes myogenic differentiation via AKT activation and calcium release

BackgroundSkeletal muscle satellite cell-derived myoblasts are mainly responsible for postnatal muscle growth and injury-induced regeneration. Many intracellular signaling pathways are essential for myogenic differentiation, while a number of kinases are involved in this modulation process. Type I phosphatidylinositol 4-phosphate 5-kinase (PIP5KI) was identified as one of the key kinases involved in myogenic differentiation, but the underlying molecular mechanism is still unclear.MethodsPIP5K1α was quantified by quantitative reverse transcriptase PCR and western blot assay. Expression levels of myogenin and myosin heavy chain, which showed significant downregulation in PIP5K1α siRNA-mediated knockdown cells in western blot analysis, were confirmed by immunostaining. Phosphatidylinositol 4,5-bisphosphate in PIP5K1α siRNA-mediated knockdown cells was also measured by the PI(4,5)P2 Mass ELISA Kit. C2C12 cells were overexpressed with different forms of AKT, followed by western blot analysis on myogenin and myosin heavy chain, which reveals their function in myogenic differentiation. FLIPR assays are used to test the release of calcium in PIP5K1α siRNA-mediated knockdown cells after histamine or bradykinin treatment. Statistical significances between groups were determined by two-tailed Student’s t test.ResultsSince PIP5K1α was the major form in skeletal muscle, knockdown of PIP5K1α consistently inhibited myogenic differentiation while overexpression of PIP5K1α promoted differentiation and rescued the inhibitory effect of the siRNA. PIP5K1α was found to be required for AKT activation and calcium release, both of which were important for skeletal muscle differentiation.ConclusionsTaken together, these results suggest that PIP5K1α is an important regulator in myoblast differentiation.

Phosphoinositide 3-kinase pathways and autophagy require phosphatidylinositol phosphate kinases.

Phosphatidylinositol phosphate kinases (PIPKs) generate a lipid messenger phosphatidylinositol 4,5-bisphosphate (PI4,5P2) that controls essentially all aspects of cellular functions. PI4,5P2 rapidly diffuses in the membrane of the lipid bilayer and does not greatly change in membrane or cellular content, and thus PI4,5P2 generation by PIPKs is tightly linked to its usage in subcellular compartments. Based on this verity, recent study of PI4,5P2 signal transduction has been focused on investigations of individual PIPKs and their underlying molecular regulation of cellular processes. Here, we will discuss recent advances in the study of how PIPKs control specific cellular events through assembly and regulation of PI4,5P2 effectors that mediate specific cellular processes. A focus will be on the roles of PIPKs in control of the phosphoinositide 3-kinase pathway and autophagy.

Multiscale Simulations of Membrane Recognition by Lipid Kinases

Peripheral membrane proteins associate with lipids in the plasma membrane in many important cell signalling processes. Type I phosphatidylinositol phosphate kinase A (PIP5K1A) is one such example, which phosphorylates the head group of the lipid phosphatidylinositol 4-phosphate (PI4P) to generate phosphatidylinositol (4,5)-bisphosphate (PIP2). The dynamic interactions of peripheral proteins with their target membranes is difficult to capture on experimentally. A recent crystal structure at 3.3 Å resolution enables the further investigation of the structure and dynamics of PIP5K1A and its interactions with lipids in the plasma membrane. Here we use coarse-grained (CG) and atomistic (AT) MD simulations of the PIP5K1A protein kinase to investigate the nature of monomer and dimer binding to the plasma membrane and their effects on lipid clustering. The AT simulations suggest that whilst dimerisation of the kinase results in an increase in the area of the binding surface, the monomers may not bind concurrently. This in turn suggests that an allosteric mechanism for stabilisation of the productive binding conformation is more likely. We have also used simulations to investigate the relationship between conformational dynamics of the activation loop and interactions with lipids in the cell membrane. Furthermore, large-scale CG simulations (1 million particles) reveal how lipid composition and membrane curvature relate to the interactions of an ensemble of 25 kinases with a model membrane. PIP kinases are implicated in many cancers as a result of their influence on cell processes. Studies in mice and human tissue samples suggest that inhibition of PIP5K1A is a potential target for treatment. Simulations of the interactions of PIP5K1A with realistic models of cell membranes will aid design of drugs targeted at these and other related lipid kinases.

Muscarinic Receptor Neuromodulation of KCNQ M-Type K+, and Other, Channels in Hippocampal Principal Neurons Involves Striking Cell-Specific Regulation Controlling Excitability

Voltage-gated “M-type” channels are important for neuronal discharge properties and require interactions with phosphatidylinositol 4,5-bisphosphate(PIP2). In sympathetic ganglia, M-current(IM) is strongly depressed by Gq-coupled-M1AChR stimulation via PIP2 depletion by phospholipase C(PLC), and other GqPCRs by Ca2+ signals. Ca2+ rises are suggested to stimulate synthesis of PIP2. In hippocampus, M/KCNQ-channels and M1AChRs are widely expressed, and cholinergic innervation strongly affects circuit excitability. We investigated mAChR neuromodulation of hippocampal neurons using brain-slice electrophysiology and Ca2+ imaging. Using whole-cell voltage-clamp, we observed fraction of deactivating relaxation at −60 mV due to IM/ERG current by kinetics and highly-specific blockers (XE991,E-4031). In dentate gyrus granule cells(DGGCs), >95% of current was from M-channels, whereas in CA1 neurons, M/ERG current ratio was 60/40. We stimulated mAChRs with oxotremorine-M(1-10μM) or M1R-selective agonist 77LH281(3-10μM). Unexpectedly, both agonists enhanced IM in DGGCs (maximum 2.4-fold), whereas IM was suppressed in CA1 neurons (78% block). M1R-induced enhancement of IM was abolished by edelfosine blockade of PLC. Testing M1R-induced increase of PIP2 abundance in DGGCs as underlying increases in IM, we quantified commensurate increase of GIRK current during muscarinic stimulation, thus acting as a PIP2 biosensor. Blockade of PI(4)P5-kinase by UNC3230(3μM) caused muscarinic stimulation to instead suppress IM in DGGCs by ∼60%. Ca2+ imaging revealed robust [Ca2+]i increase in DGGCs by muscarinic stimulation. Despite pronounced increase in IM by mAChR stimulation in DGGCs, it was accompanied by robust increase in excitability (action potential frequency), leading us to hypothesize PLC-induced activation of other channels. When the TRPC-blocker, ML204 (10μM), was applied concurrently with muscarinic agonist, neither excitability nor [Ca2+]i were significantly changed. Thus, DGGC excitability is regulated by muscarinic signals in concert with M-type/TRPC channels, whereas CA1 neurons are regulated in a distinct fashion.

The Ancient Link between G-Protein-Coupled Receptors and C-Terminal Phospholipid Kinase Domains.

ABSTRACTSensing external signals and transducing these into intracellular responses requires a molecular signaling system that is crucial for every living organism. Two important eukaryotic signal transduction pathways that are often interlinked are G-protein signaling and phospholipid signaling. Heterotrimeric G-protein subunits activated by G-protein-coupled receptors (GPCRs) are typical stimulators of phospholipid signaling enzymes such as phosphatidylinositol phosphate kinases (PIPKs) or phospholipase C (PLC). However, a direct connection between the two pathways likely exists in oomycetes and slime molds, as they possess a unique class of GPCRs that have a PIPK as an accessory domain. In principle, these so-called GPCR-PIPKs have the capacity of perceiving an external signal (via the GPCR domain) that, via PIPK, directly activates downstream phospholipid signaling. Here we reveal the sporadic occurrence of GPCR-PIPKs in all eukaryotic supergroups, except for plants. Notably, all species having GPCR-PIPKs are unicellular microorganisms that favor aquatic environments. Phylogenetic analysis revealed that GPCR-PIPKs are likely ancestral to eukaryotes and significantly expanded in the last common ancestor of oomycetes. In addition to GPCR-PIPKs, we identified five hitherto-unknown classes of GPCRs with accessory domains, four of which are universal players in signal transduction. Similarly to GPCR-PIPKs, this enables a direct coupling between extracellular sensing and downstream signaling. Overall, our findings point to an ancestral signaling system in eukaryotes where GPCR-mediated sensing is directly linked to downstream responses.

MAPKs Influence Pollen Tube Growth by Controlling the Formation of Phosphatidylinositol 4,5-Bisphosphate in an Apical Plasma Membrane Domain.

An apical plasma membrane domain enriched in the regulatory phospholipid phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] is critical for polar tip growth of pollen tubes. How the biosynthesis of PtdIns(4,5)P2 by phosphatidylinositol 4-phosphate 5-kinases (PI4P 5-kinases) is controlled by upstream signaling is currently unknown. The pollen-expressed PI4P 5-kinase PIP5K6 is required for clathrin-mediated endocytosis and polar tip growth in pollen tubes. Here, we identify PIP5K6 as a target of the pollen-expressed mitogen-activated protein kinase MPK6 and characterize the regulatory effects. Based on an untargeted mass spectrometry approach, phosphorylation of purified recombinant PIP5K6 by pollen tube extracts could be attributed to MPK6. Recombinant MPK6 phosphorylated residues T590 and T597 in the variable insert of the catalytic domain of PIP5K6, and this modification inhibited PIP5K6 activity in vitro. PIP5K6 interacted with MPK6 in yeast two-hybrid tests, immuno-pull-down assays, and by bimolecular fluorescence complementation at the apical plasma membrane of pollen tubes. In vivo, MPK6 expression resulted in reduced plasma membrane association of a fluorescent PtdIns(4,5)P2 reporter and decreased endocytosis without impairing membrane association of PIP5K6. Effects of PIP5K6 expression on pollen tube growth and cell morphology were attenuated by coexpression of MPK6 in a phosphosite-dependent manner. Our data indicate that MPK6 controls PtdIns(4,5)P2 production and membrane trafficking in pollen tubes, possibly contributing to directional growth.

A PIP2-derived amplification loop fuels the sustained initiation of B cell activation.

Lymphocytes have evolved sophisticated signaling amplification mechanisms to efficiently activate downstream signaling after detection of rare ligands in their microenvironment. B cell receptor microscopic clusters (BCR microclusters) are assembled on the plasma membrane and recruit signaling molecules for the initiation of lymphocyte signaling after antigen binding. We identified a signaling amplification loop derived from phosphatidylinositol 4,5-biphosphate (PIP2) for the sustained B cell activation. Upon antigen recognition, PIP2 was depleted by phospholipase C-γ2 (PLC-γ2) within the BCR microclusters and was regenerated by phosphatidic acid-dependent type I phosphatidylinositol 4-phosphate 5-kinase outside the BCR microclusters. The hydrolysis of PIP2 inside the BCR microclusters induced a positive feedback mechanism for its synthesis outside the BCR microclusters. The falling gradient of PIP2 across the boundary of BCR microclusters was important for the efficient formation of BCR microclusters. Our results identified a PIP2-derived amplification loop that fuels the sustained initiation of B cell activation.

ROCK1 is a novel Rac1 effector to regulate tubular endocytic membrane formation during clathrin-independent endocytosis

Clathrin-dependent and -independent pathways contribute for β1-integrin endocytosis. This study defines a tubular membrane clathrin-independent endocytic network, induced with the calmodulin inhibitor W13, for β1-integrin internalization. This pathway is dependent on increased phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) levels and dynamin activity at the plasma membrane. Exogenous addition of PI(4,5)P2 or phosphatidylinositol-4-phosphate 5-kinase (PIP5K) expression mimicked W13-generated-tubules which are inhibited by active Rac1. Therefore, the molecular mechanisms downstream of Rac1, that controls this plasma membrane tubulation, were analyzed biochemically and by the expression of different Rac1 mutants. The results indicate that phospholipase C and ROCK1 are the main Rac1 effectors that impair plasma membrane invagination and tubule formation, essentially by decreasing PI(4,5)P2 levels and promoting cortical actomyosin assembly respectively. Interestingly, among the plethora of proteins that participate in membrane remodeling, this study revealed that ROCK1, the well-known downstream RhoA effector, has an important role in Rac1 regulation of actomyosin at the cell cortex. This study provides new insights into Rac1 functioning on plasma membrane dynamics combining phosphatidylinositides and cytoskeleton regulation.

Phosphatidylinositol 4-phosphate 5-kinase α contributes to Toll-like receptor 2-mediated immune responses in microglial cells stimulated with lipoteichoic acid

Phosphatidylinositol 4,5-bisphosphate (PIP2) is an important lipid regulator of membrane signaling and remodeling processes. Accumulating evidence indicates a link between PIP2 metabolism and Toll-like receptor (TLR) signaling, a key transducer of immune responses such as inflammation, phagocytosis, and autophagy. Microglia are immune effector cells that serve as macrophages in the brain. Here, we examined the potential role of phosphatidylinositol 4-phosphate 5-kinase α (PIP5Kα), a PIP2-producing enzyme, in TLR2 signaling in microglial cells. Treatment of BV2 microglial cells with lipoteichoic acid (LTA), a TLR2 agonist, increased PIP5Kα expression in BV2 and primary microglial cells, but not in primary cultures from TLR2-deficient mice. PIP5Kα knockdown of BV2 cells with shRNA significantly suppressed LTA-induced activation of TLR2 downstream signaling, including the production of proinflammatory cytokines and phosphorylation of NF-κB, JNK, and p38 MAP kinase. Such suppression was reversed by complementation of PIP5Kα. PIP5Kα knockdown lowered PIP2 levels and impaired LTA-induced plasma membrane targeting of TIRAP, a PIP2-dependent adaptor required for TLR2 activation. Besides, PIP5Kα knockdown inhibited phagocytic uptake of E. coli particles and autophagy-related vesicle formation triggered by LTA. Taken together, these results support that PIP5Kα can positively mediate TLR2-associated immune responses through PIP2 production in microglial cells.