Production of phosphatidylinositol 5-phosphate via PIKfyve and MTMR3 regulates cell migration

Recent advances in liposome development for studying protein-lipid interactions

Protein-lipid interactions are crucial for various cellular biological processes like intracellular signaling, membrane transport, and cytoskeletal dynamics. Therefore, studying these interactions is essential to understand and unravel their specific functions. Nevertheless, the interacting proteins of many lipids are poorly understood and still require systematic study. Liposomes are the most well-known and familiar biomimetic systems used to study protein-lipid interactions. Although liposomes have been widely used for studying protein-lipid interactions in classical methods such as the co-flotation assay (CFA), co-sedimentation assay (CSA), and flow cytometric assay (FCA), an overview of their current applications and developments in high-throughput methods is not yet available. Here, we summarize the liposome development in low and high-throughput methods to study protein-lipid interactions. Besides, a constructive comment for each platform is presented to stimulate the advancement of these technologies in the future.

Phosphoinositide Signaling in Immune Cell Migration

In response to different immune challenges, immune cells migrate to specific sites in the body, where they perform their functions such as defense against infection, inflammation regulation, antigen recognition, and immune surveillance. Therefore, the migration ability is a fundamental aspect of immune cell function. Phosphoinositide signaling plays critical roles in modulating immune cell migration by controlling cell polarization, cytoskeletal rearrangement, protrusion formation, and uropod contraction. Upon chemoattractant stimulation, specific phosphoinositide kinases and phosphatases control the local phosphoinositide levels to establish polarized phosphoinositide distribution, which recruits phosphoinositide effectors to distinct subcellular locations to facilitate cell migration. In this Special Issue of "Molecular Mechanisms Underlying Cell Adhesion and Migration", we discuss the significance of phosphoinositide production and conversion by phosphoinositide kinases and phosphatases in the migration of different types of immune cells.

PI3P-dependent regulation of cell size and autophagy by phosphatidylinositol 5-phosphate 4-kinase

Phosphatidylinositol 3-phosphate (PI3P) and phosphatidylinositol 5-phosphate (PI5P) are low-abundance phosphoinositides crucial for key cellular events such as endosomal trafficking and autophagy. Phosphatidylinositol 5-phosphate 4-kinase (PIP4K) is an enzyme that regulates PI5P in vivo but can act on both PI5P and PI3P in vitro. In this study, we report a role for PIP4K in regulating PI3P levels in Drosophila Loss-of-function mutants of the only Drosophila PIP4K gene show reduced cell size in salivary glands. PI3P levels are elevated in dPIP4K 29 and reverting PI3P levels back towards WT, without changes in PI5P levels, can rescue the reduced cell size. dPIP4K 29 mutants also show up-regulation in autophagy and the reduced cell size can be reverted by depleting Atg8a that is required for autophagy. Lastly, increasing PI3P levels in WT can phenocopy the reduction in cell size and associated autophagy up-regulation seen in dPIP4K 29 Thus, our study reports a role for a PIP4K-regulated PI3P pool in the control of autophagy and cell size.

Phosphoinositides in New Spaces

Phosphoinositides (PIs) are phospholipids derived from phosphatidylinositol. PIs are regulated via reversible phosphorylation, which is directed by the opposing actions of PI kinases and phosphatases. PIs constitute a minor fraction of the total cellular lipid pool but play pleiotropic roles in multiple aspects of cell biology. Genetic mutations of PI regulatory enzymes have been identified in rare congenital developmental syndromes, including ciliopathies, and in numerous human diseases, such as cancer and metabolic and neurological disorders. Accordingly, PI regulatory enzymes have been targeted in the design of potential therapeutic interventions for human diseases. Recent advances place PIs as central regulators of membrane dynamics within functionally distinct subcellular compartments. This brief review focuses on the emerging role PIs play in regulating cell signaling within the primary cilium and in directing transfer of molecules at interorganelle membrane contact sites and identifies new roles for PIs in subcellular spaces.

RUFY3 regulates endolysosomes perinuclear positioning, antigen presentation and migration in activated phagocytes

Endo-lysosomes transport along microtubules and clustering in the perinuclear area are two necessary steps for microbes to activate specialized phagocyte functions. We report that RUN and FYVE domain-containing protein 3 (RUFY3) exists as two alternative isoforms distinguishable by the presence of a C-terminal FYVE domain and by their affinity for phosphatidylinositol 3-phosphate on endosomal membranes. The FYVE domain-bearing isoform (iRUFY3) is preferentially expressed in primary immune cells and up-regulated upon activation by microbes and Interferons. iRUFY3 is necessary for ARL8b + /LAMP1+ endo-lysosomes positioning in the pericentriolar organelles cloud of LPS-activated macrophages. We show that iRUFY3 controls macrophages migration, MHC II presentation and responses to Interferon-γ, while being important for intracellular Salmonella replication. Specific inactivation of rufy3 in phagocytes leads to aggravated pathologies in mouse upon LPS injection or bacterial pneumonia. This study highlights the role of iRUFY3 in controlling endo-lysosomal dynamics, which contributes to phagocyte activation and immune response regulation.

Beyond PI3Ks: targeting phosphoinositide kinases in disease

Lipid phosphoinositides are master regulators of almost all aspects of a cell's life and death and are generated by the tightly regulated activity of phosphoinositide kinases. Although extensive efforts have focused on drugging class I phosphoinositide 3-kinases (PI3Ks), recent years have revealed opportunities for targeting almost all phosphoinositide kinases in human diseases, including cancer, immunodeficiencies, viral infection and neurodegenerative disease. This has led to widespread efforts in the clinical development of potent and selective inhibitors of phosphoinositide kinases. This Review summarizes our current understanding of the molecular basis for the involvement of phosphoinositide kinases in disease and assesses the preclinical and clinical development of phosphoinositide kinase inhibitors.

miR-100-5p Promotes Epidermal Stem Cell Proliferation through Targeting MTMR3 to Activate PIP3/AKT and ERK Signaling Pathways

Skin epidermal stem cells (EpSCs) play a critical role in wound healing and are ideal seed cells for skin tissue engineering. Exosomes from human adipose-derived stem cells (ADSC-Exos) promote human EpSC proliferation, but the underlying mechanism remains unclear. Here, we investigated the effect of miR-100-5p, one of the most abundant miRNAs in ADSC-Exos, on the proliferation of human EpSCs and explored the mechanisms involved. MTT and BrdU incorporation assays showed that miR-100-5p mimic transfection promoted EpSC proliferation in a time-dependent manner. Cell cycle analysis showed that miR-100-5p mimic transfection significantly decreased the percentage of cells in the G1 phase and increased the percentage of cells in the G2/M phase. Myotubularin-related protein 3 (MTMR3), a lipid phosphatase, was identified as a direct target of miR-100-5p. Knockdown of MTMR3 in EpSCs by RNA interference significantly enhanced cell proliferation, decreased the percentage of cells in the G1 phase and increased the percentage of cells in the S phase. Overexpression of MTMR3 reversed the proproliferative effect of miR-100-5p on EpSCs, indicating that miR-100-5p promoted EpSC proliferation by downregulating MTMR3. Mechanistic studies showed that transfection of EpSCs with miR-100-5p mimics elevated the intracellular PIP3 level, induced AKT and ERK phosphorylation, and upregulated cyclin D1, E1, and A2 expression, which could be attenuated by MTMR3 overexpression. Consistently, intradermal injection of ADSC-Exos or miR-100-5p-enriched ADSC-Exos into cultured human skin tissues significantly reduced MTMR3 expression and increased the thickness of the epidermis and the number of EpSCs in the basal layer of the epidermis. The aforementioned effect of miR-100-5p-enriched ADSC-Exos was stronger than that of ADSC-Exos and was reversed by MTMR3 overexpression. Collectively, our findings indicate that miR-100-5p promotes EpSC proliferation through MTMR3-mediated elevation of PIP3 and activation of AKT and ERK. miR-100-5p-enriched ADSC-Exos can be used to treat skin wound and expand EpSCs for generating epidermal autografts and engineered skin equivalents.

PIKfyve-Dependent Phosphoinositide Dynamics in Megakaryocyte/Platelet Granule Integrity and Platelet Functions

BACKGROUND

Secretory granules are key elements for platelet functions. Their biogenesis and integrity are regulated by fine-tuned mechanisms that need to be fully characterized. Here, we investigated the role of the phosphoinositide 5-kinase PIKfyve and its lipid products, PtdIns5P (phosphatidylinositol 5 monophosphate) and PtdIns(3,5)P₂ (phosphatidylinositol (3,5) bisphosphate) in granule homeostasis in megakaryocytes and platelets.

METHODS

For that, we invalidated PIKfyve by pharmacological inhibition or gene silencing in megakaryocytic cell models (human MEG-01 cell line, human imMKCLs, mouse primary megakaryocytes) and in human platelets.

RESULTS

We unveiled that PIKfyve expression and its lipid product levels increased with megakaryocytic maturation. In megakaryocytes, PtdIns5P and PtdIns(3,5)P₂ were found in alpha and dense granule membranes with higher levels in dense granules. Pharmacological inhibition or knock-down of PIKfyve in megakaryocytes decreased PtdIns5P and PtdIns(3,5)P₂ synthesis and induced a vacuolar phenotype with a loss of alpha and dense granule identity. Permeant PtdIns5P and PtdIns(3,5)P₂ and the cation channel TRPML1 (transient receptor potential mucolipins) and TPC2 activation were able to accelerate alpha and dense granule integrity recovery following release of PIKfyve pharmacological inhibition. In platelets, PIKfyve inhibition specifically impaired the integrity of dense granules culminating in defects in their secretion, platelet aggregation, and thrombus formation.

CONCLUSIONS

These data demonstrated that PIKfyve and its lipid products PtdIns5P and PtdIns(3,5)P₂ control granule integrity both in megakaryocytes and platelets.

Lipid kinases VPS34 and PIKfyve coordinate a phosphoinositide cascade to regulate Retriever-mediated recycling on endosomes

Cell-surface receptors control how cells respond to their environment. Many cell-surface receptors recycle from endosomes to the plasma membrane via a recently discovered pathway, which includes sorting-nexin SNX17, Retriever, WASH and CCC complexes. Here, using mammalian cells, we discover that PIKfyve and its upstream PI3-kinase VPS34 positively regulate this pathway. VPS34 produces PI3P, which is the substrate for PIKfyve to generate PI3,5P2. We show that PIKfyve controls recycling of cargoes including integrins, receptors that control cell migration. Furthermore, endogenous PIKfyve colocalizes with SNX17, Retriever, WASH and CCC complexes on endosomes. Importantly, PIKfyve inhibition results displacement of Retriever and CCC from endosomes. In addition, we show that recruitment of SNX17 is an early step and requires VPS34. These discoveries suggest that VPS34 and PIKfyve coordinate an ordered pathway to regulate recycling from endosomes and suggest how PIKfyve functions in cell migration.

Expanding role of PI5P4Ks in cancer: A promising druggable target

Cancer cells are challenged by a myriad of microenvironmental stresses, and it is their ability to efficiently adapt to the constantly changing nutrient, energy, oxidative, and/or immune landscape that allows them to survive and proliferate. Such adaptations, however, result in distinct vulnerabilities that are attractive therapeutic targets. Phosphatidylinositol 5-phosphate 4-kinases (PI5P4Ks) are a family of druggable stress-regulated phosphoinositide kinases that become conditionally essential as a metabolic adaptation, paving the way to targeting cancer cell dependencies. Further, PI5P4Ks have a synthetic lethal interaction with the tumor suppressor p53, the loss of which is one of the most prevalent genetic drivers of malignant transformation. PI5P4K's emergence as a crucial axis in the expanding landscape of phosphoinositide signaling in cancer has already stimulated the development of specific inhibitors. Thus, a better understanding of the biology of the PI5P4Ks will allow for targeted and effective therapeutic interventions. Here, we attempt to summarize the mounting roles of the PI5P4Ks in cancer, including evidence that targeting them is a therapeutic vulnerability and promising next-in-line treatment for multiple cancer subtypes.

PIP5KIγ90-generated phosphatidylinositol-4,5-bisphosphate promotes the uptake of Staphylococcus aureus by host cells

Staphylococcus aureus, a Gram-positive pathogen, invades cells mainly in an integrin-dependent manner. As the activity or conformation of several integrin-associated proteins can be regulated by phosphatidylinositol-4,5-bisphosphate (PI-4,5-P₂ ), we investigated the roles of PI-4,5-P₂ and PI-4,5-P₂ -producing enzymes in cellular invasion by S. aureus. PI-4,5-P₂ accumulated upon contact of S. aureus with the host cell, and targeting of an active PI-4,5-P₂ phosphatase to the plasma membrane reduced bacterial invasion. Knockdown of individual phosphatidylinositol-4-phosphate 5-kinases revealed that phosphatidylinositol-4-phosphate 5-kinase γ (PIP5KIγ) plays an important role in bacterial internalization. Specific ablation of the talin and FAK-binding motif in PIP5KIγ90 reduced bacterial invasion, which could be rescued by reexpression of an active, but not inactive PIP5KIγ90. Furthermore, PIP5KIγ90-deficient cells showed normal basal PI-4,5-P₂ levels in the plasma membrane but reduced the accumulation of PI-4,5-P₂ and talin at sites of S. aureus attachment and overall lower levels of FAK phosphorylation. These results highlight the importance of local synthesis of PI-4,5-P₂ by a focal adhesion-associated lipid kinase for integrin-mediated internalization of S. aureus.

Ionization properties of monophosphoinositides in mixed model membranes

Phosphoinositides are found in low concentration in cellular membranes but perform numerous functions such as signaling, membrane trafficking, protein recruitment and modulation of protein activity. Spatiotemporal regulation by enzymes that phosphorylate and dephosphorylate the inositol ring results in the production of seven distinct and functionally diverse derivatives. Ionization properties of the phosphorylated headgroups of anionic lipids have been shown to impact how they interact with proteins and lipids in the membrane. While the ionization properties of the three bis and one tris phosphorylated forms have been studied in physiologically relevant model membranes, that of the monophosphorylated forms (i.e., phosphatidylinositol-3-phosphate (PI3P), phosphatidylinositol-4-phosphate (PI4P), phosphatidylinositol-5-phosphate (PI5P)) has received less attention. Here, we used ³¹P MAS NMR to determine the charge of 5 mol% of the monophosphorylated derivatives in pure dioleoylphosphatidylcholine (DOPC) and DOPC/dioleoylphosphatidylethanolamine (DOPE) bilayers as a function of pH. We find that PI3P, PI4P and PI5P each have unique pKa2 values in a DOPC bilayer, and each is reduced in DOPC/DOPE model membranes through the interaction of their headgroups with DOPE according to the electrostatic-hydrogen bond switch model. In this study, using model membranes mimicking the plasma membrane (inner leaflet), Golgi, nuclear membrane, and endosome (outer leaflet), we show that PI3P, PI4P or PI5P maximize their charge at neutral pH. Our results shed light on the electrostatics of the monophosphorylated headgroups of PI3P, PI4P, and PI5P and form the basis of their intracellular functions.

Apilimod alters TGFβ signaling pathway and prevents cardiac fibrotic remodeling

Rationale: TGFβ signaling pathway controls tissue fibrotic remodeling, a hallmark in many diseases leading to organ injury and failure. In this study, we address the role of Apilimod, a pharmacological inhibitor of the lipid kinase PIKfyve, in the regulation of cardiac pathological fibrotic remodeling and TGFβ signaling pathway. Methods: The effects of Apilimod treatment on myocardial fibrosis, hypertrophy and cardiac function were assessed in vivo in a mouse model of pressure overload-induced heart failure. Primary cardiac fibroblasts and HeLa cells treated with Apilimod as well as genetic mutation of PIKfyve in mouse embryonic fibroblasts were used as cell models. Results: When administered in vivo, Apilimod reduced myocardial interstitial fibrosis development and prevented left ventricular dysfunction. In vitro, Apilimod controlled TGFβ-dependent activation of primary murine cardiac fibroblasts. Mechanistically, both Apilimod and genetic mutation of PIKfyve induced TGFβ receptor blockade in intracellular vesicles, negatively modulating its downstream signaling pathway and ultimately dampening TGFβ response. Conclusions: Altogether, our findings propose a novel function for PIKfyve in the control of myocardial fibrotic remodeling and the TGFβ signaling pathway, therefore opening the way to new therapeutic perspectives to prevent adverse fibrotic remodeling using Apilimod treatment.

The effects of PIKfyve inhibitor YM201636 on claudins and malignancy potential of nonsmall cell cancer cells

PIKfyve is an evolutionarily conserved lipid and protein kinase enzyme that has pleiotropic cellular functions. The aim of the present study was to investigate the effects of phosphatidylinositol-3-phosphate 5-kinase (PIKfyve) inhibitor, YM201636, on nonsmall cell lung cancer (NSCLC) cells growth, tumorigenicity, and claudin (CLDN) expressions. Three NSCLC cell lines (Calu-1, H1299 and HCC827) were used to compare the effects of YM201636. Cytotoxic effects of YM201636 were analysed using XTT assay. Malignancy potential of cells assesses with wound healing and soft agar colony-forming assays. mRNA and protein expressions of claudins were analysed by qRT-PCR and immunofluorescence staining. Our results revealed that YM201636 inhibited the proliferation and malignancy potential of Calu-1, H1299, and HCC827 cells in a dose-dependent manner. After YM201636 treatment CLDN1, -3 and -5 expressions increased significantly in HCC827 cells. CLDN3 and -5 expressions also significantly increased in Calu1 cell line. YM201636 treatment significantly reduced the CLDN1 and increased the CLDN5 expression in H1299 cells. Immunofluorescence staining of CLDN1, -3 and -5 proteins showed a significant increase after YM201636 treatment. Besides, YM201636 induced EGFR mRNA expression in all NSCLC cell lines. Our results have shown that YM201636 inhibits tumorigenicity of NSCLC cells. Furthermore, estimated glomerular filtration rate (EGFR) pathway is important signalling involved in the regulation of claudins. Understanding the mechanisms of PIKfyve inhibitors may improve cancer treatment particularly for EGFR overactivated NSCLC.

Monitoring Phosphoinositide Fluxes and Effectors During Leukocyte Chemotaxis and Phagocytosis

The dynamic re-organization of cellular membranes in response to extracellular stimuli is fundamental to the cell physiology of myeloid and lymphoid cells of the immune system. In addition to maintaining cellular homeostatic functions, remodeling of the plasmalemma and endomembranes endow leukocytes with the potential to relay extracellular signals across their biological membranes to promote rolling adhesion and diapedesis, migration into the tissue parenchyma, and to ingest foreign particles and effete cells. Phosphoinositides, signaling lipids that control the interface of biological membranes with the external environment, are pivotal to this wealth of functions. Here, we highlight the complex metabolic transitions that occur to phosphoinositides during several stages of the leukocyte lifecycle, namely diapedesis, migration, and phagocytosis. We describe classical and recently developed tools that have aided our understanding of these complex lipids. Finally, major downstream effectors of inositides are highlighted including the cytoskeleton, emphasizing the importance of these rare lipids in immunity and disease.

Systematic Analysis of Phosphatidylinositol-5-phosphate-Interacting Proteins Using Yeast Proteome Microarrays

We used yeast proteome microarrays (∼5800 purified proteins) to conduct a high-throughput and systematic screening of PI5P-interacting proteins with PI5P-tagged fluorescent liposomal nanovesicles. Lissamine rhodamine B-dipalmitoyl phosphatidylethanol was incorporated into the liposome bilayer to provide the nanovesicles with fluorescence without any encapsulants, which not only made the liposome fabrication much easier without the need for purification but also improved the chip-probing quality. A special chip assay was washed very gently without the traditional spin-dry step. Forty-five PI5P-interacting proteins were identified in triplicate with this special chip assay. Subsequently, we used flow cytometry to validate these interactions, and a total of 41 PI5P-interacting proteins were confirmed. Enrichment analysis revealed that these proteins have significant functions associated with ribosome biogenesis, rRNA processing, ribosome binding, GTP binding, and hydrolase activity. Their component enrichment is located in the nucleolus. The InterPro domain analysis indicated that PI5P-interacting proteins are enriched in the P-loop containing nucleoside triphosphate hydrolases domain (P-loop). Additionally, using the MEME program, we identified a consensus motif (IVGPAGTGKSTLF) that contains the Walker A sequence, a well-known nucleotide-binding motif. Furthermore, using a quartz crystal microbalance, both the consensus motif and Walker A motif showed strong affinities to PI5P-containing liposomes but not to PI5P-deprived liposomes or PI-containing liposomes. Additionally, the glycine (G6) and lysine (K7) residues of the Walker A motif (-GPAGTG⁶K⁷S-) were found to be critical to the PI5P-binding ability. This study not only identified an additional set of PI5P-interacting proteins but also revealed the strong PI5P-binding affinity (K_d = 1.81 × 10^-7 M) of the Walker A motif beyond the motif's nucleotide-binding characteristic.

Phosphatidylinositol 4-kinase IIIβ mediates contraction-induced GLUT4 translocation and shows its anti-diabetic action in cardiomyocytes

In the diabetic heart, long-chain fatty acid (LCFA) uptake is increased at the expense of glucose uptake. This metabolic shift ultimately leads to insulin resistance and a reduced cardiac function. Therefore, signaling kinases that mediate glucose uptake without simultaneously stimulating LCFA uptake could be considered attractive anti-diabetic targets. Phosphatidylinositol-4-kinase-IIIβ (PI4KIIIβ) is a lipid kinase downstream of protein kinase D1 (PKD1) that mediates Golgi-to-plasma membrane vesicular trafficking in HeLa-cells. In this study, we evaluated whether PI4KIIIβ is involved in myocellular GLUT4 translocation induced by contraction or oligomycin (an F₁F₀-ATP synthase inhibitor that activates contraction-like signaling). Pharmacological targeting, with compound MI14, or genetic silencing of PI4KIIIβ inhibited contraction/oligomycin-stimulated GLUT4 translocation and glucose uptake in cardiomyocytes but did not affect CD36 translocation nor LCFA uptake. Addition of the PI4KIIIβ enzymatic reaction product phosphatidylinositol-4-phosphate restored oligomycin-stimulated glucose uptake in the presence of MI14. PI4KIIIβ activation by PKD1 involves Ser294 phosphorylation and altered its localization with unchanged enzymatic activity. Adenoviral PI4KIIIβ overexpression stimulated glucose uptake, but did not activate hypertrophic signaling, indicating that unlike PKD1, PI4KIIIβ is selectively involved in GLUT4 translocation. Finally, PI4KIIIβ overexpression prevented insulin resistance and contractile dysfunction in lipid-overexposed cardiomyocytes. Together, our studies identify PI4KIIIβ as positive and selective regulator of GLUT4 translocation in response to contraction-like signaling, suggesting PI4KIIIβ as a promising target to rescue defective glucose uptake in diabetics.

Liposomes Loaded With Phosphatidylinositol 5-Phosphate Improve the Antimicrobial Response to Pseudomonas aeruginosa in Impaired Macrophages From Cystic Fibrosis Patients and Limit Airway Inflammatory Response

Despite intensive antimicrobial and anti-inflammatory therapies, cystic fibrosis (CF) patients are subjected to chronic infections due to opportunistic pathogens, including multidrug resistant (MDR) Pseudomonas aeruginosa. Macrophages from CF patients show many evidences of reduced phagocytosis in terms of internalization capability, phagosome maturation, and intracellular bacterial killing. In this study, we investigated if apoptotic body-like liposomes (ABLs) loaded with phosphatidylinositol 5-phosphate (PI5P), known to regulate actin dynamics and vesicular trafficking, could restore phagocytic machinery while limiting inflammatory response in in vitro and in vivo models of MDR P. aeruginosa infection. Our results show that the in vitro treatment with ABL carrying PI5P (ABL/PI5P) enhances bacterial uptake, ROS production, phagosome acidification, and intracellular bacterial killing in human monocyte-derived macrophages (MDMs) with pharmacologically inhibited cystic fibrosis transmembrane conductance regulator channel (CFTR), and improve uptake and intracellular killing of MDR P. aeruginosa in CF macrophages with impaired bactericidal activity. Moreover, ABL/PI5P stimulation of CFTR-inhibited MDM infected with MDR P. aeruginosa significantly reduces NF-κB activation and the production of TNF-α, IL-1β, and IL-6, while increasing IL-10 and TGF-β levels. The therapeutic efficacy of ABL/PI5P given by pulmonary administration was evaluated in a murine model of chronic infection with MDR P. aeruginosa. The treatment with ABL/PI5P significantly reduces pulmonary neutrophil infiltrate and the levels of KC and MCP-2 cytokines in the lungs, without affecting pulmonary bacterial load. Altogether, these results show that the ABL/PI5P treatment may represent a promising host-directed therapeutic approach to improve the impaired phagocytosis and to limit the potentially tissue-damaging inflammatory response in CF.

Phosphatidylinositol 5 Phosphate (PI5P): From Behind the Scenes to the Front (Nuclear) Stage

Phosphatidylinositol (PI)-related signaling plays a pivotal role in many cellular aspects, including survival, cell proliferation, differentiation, DNA damage, and trafficking. PI is the core of a network of proteins represented by kinases, phosphatases, and lipases which are able to add, remove or hydrolyze PI, leading to different phosphoinositide products. Among the seven known phosphoinositides, phosphatidylinositol 5 phosphate (PI5P) was the last to be discovered. PI5P presence in cells is very low compared to other PIs. However, much evidence collected throughout the years has described the role of this mono-phosphoinositide in cell cycles, stress response, T-cell activation, and chromatin remodeling. Interestingly, PI5P has been found in different cellular compartments, including the nucleus. Here, we will review the nuclear role of PI5P, describing how it is synthesized and regulated, and how changes in the levels of this rare phosphoinositide can lead to different nuclear outputs.

PtdIns3P phosphatases MTMR3 and MTMR4 negatively regulate innate immune responses to DNA through modulating STING trafficking

The innate immune system plays an essential role in initial recognition of pathogen infection by producing inflammatory cytokines and type I interferons. cGAS is a cytoplasmic sensor for DNA derived from DNA viruses. cGAS binding with DNA induces the production of cGAMP, a second messenger that associates with STING in endoplasmic reticulum (ER). STING changes its cellular distribution from ER to perinuclear Golgi, where it activates the protein kinase TBK1 that catalyzes the phosphorylation of IRF3. Here we found that STING trafficking is regulated by myotubularin-related protein (MTMR) 3 and MTMR4, members of protein tyrosine phosphatases that dephosphorylate 3' position in phosphatidylinositol (PtdIns) and generate PtdIns5P from PtdIns3,5P₂ and PtdIns from PtdIns3P. We established MTMR3 and MTMR4 double knockout (DKO) RAW264.7 macrophage cells and found that they exhibited increased type I interferon production after interferon-stimulatory DNA (ISD) stimulation and herpes simplex virus 1 infection concomitant with enhanced IRF3 phosphorylation. In DKO cells, STING rapidly trafficked from ER to Golgi after ISD stimulation. Notably, DKO cells exhibited enlarged cytosolic puncta positive for PtdIns3P and STING was aberrantly accumulated in this puncta. Taken together, these results suggest that MTMR3 and MTMR4 regulate the production of PtdIns3P, which plays a critical role in suppressing DNA-mediated innate immune responses via modulating STING trafficking.

Phosphoinositide phosphatases in cancer cell dynamics-Beyond PI3K and PTEN

Phosphoinositides are a group of lipids that regulate intracellular signaling and subcellular biological events. The signaling by phosphatidylinositol-3,4,5-trisphosphate and Akt mediates the action of growth factors that are essential for cell proliferation, gene transcription, cell migration, and polarity. The hyperactivation of this signaling has been identified in different cancer cells; and, it has been implicated in oncogenic transformation and cancer cell malignancy. Recent studies have argued the role of phosphoinositides in cancer cell dynamics, including actin cytoskeletal rearrangement at the plasma membrane and the organization of intracellular compartments. The focus of this review is to summarize the impact of the activities of phosphoinositide phosphatases on intracellular signaling related to cancer cell dynamics and to discuss how the abnormalities in the activities of the enzymes alter the levels of phosphoinositides in cancer cells.

Phosphoinositides: multipurpose cellular lipids with emerging roles in cell death

Phosphorylated phosphatidylinositol lipids, or phosphoinositides, critically regulate diverse cellular processes, including signalling transduction, cytoskeletal reorganisation, membrane dynamics and cellular trafficking. However, phosphoinositides have been inadequately investigated in the context of cell death, where they are mainly regarded as signalling secondary messengers. However, recent studies have begun to highlight the importance of phosphoinositides in facilitating cell death execution. Here, we cover the latest phosphoinositide research with a particular focus on phosphoinositides in the mechanisms of cell death. This progress article also raises key questions regarding the poorly defined role of phosphoinositides, particularly during membrane-associated events in cell death such as apoptosis and secondary necrosis. The review then further discusses important future directions for the phosphoinositide field, including therapeutically targeting phosphoinositides to modulate cell death.

Development of Three Orthogonal Assays Suitable for the Identification and Qualification of PIKfyve Inhibitors

FYVE-type zinc finger-containing phosphoinositide kinase (PIKfyve) catalyzes the formation of phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) from phosphatidylinositol 3-phosphate (PI(3)P). PIKfyve has been implicated in multiple cellular processes, and its role in the regulation of toll-like receptor (TLR) pathways and the production of proinflammatory cytokines has sparked interest in developing small-molecule PIKfyve inhibitors as potential therapeutics to treat autoimmune and inflammatory diseases. We developed three orthogonal assays to identify and qualify small-molecule inhibitors of PIKfyve: (1) a purified component microfluidic enzyme assay that measures the conversion of fluorescently labeled PI(3)P to PI(3,5)P₂ by purified recombinant full-length human 6His-PIKfyve (rPIKfyve); (2) an intracellular protein stabilization assay using the kinase domain of PIKfyve expressed in HEK293 cells; and (3) a cell-based functional assay that measures the production of interleukin (IL)-12p70 in human peripheral blood mononuclear cells stimulated with TLR agonists lipopolysaccharide and R848. We determined apparent K_m values for both ATP and labeled PI(3)P in the rPIKfyve enzyme assay and evaluated the enzyme's ability to use phosphatidylinositol as a substrate. We also tested four reference compounds in the three assays and showed that together these assays provide a platform that is suitable to select promising inhibitors having appropriate functional activity and confirmed cellular target engagement to advance into preclinical models of inflammation.

Interaction analysis between germline susceptibility loci and somatic alterations in lung cancer

Emerging evidence indicates that germline variations may interact with somatic events in carcinogenesis. However, the germline-somatic interaction in lung cancer remains largely unknown. We investigated whether lung cancer driver genes (CDGs) were more likely to locate within cancer susceptibility regions. Pathway analysis was performed to identify common pathways underlying CDGs and cancer susceptibility genes (CSGs). Next, we analyzed the associations between lung cancer risk SNPs and somatic alterations, including mutations and copy number alterations, in the level of genes, pathways, and overall burden of alterations. Enrichment analysis showed that lung CDGs are more likely to locate within cancer susceptibility regions (p = 8.40 × 10^-3 ). Both of lung CSGs and CDGs showed significant enrichment in pathways such as cell cycle and p53 signaling pathway. Gene-based analysis showed that rs36600 (22q12.2) was associated with somatic mutations within ARID1A (OR = 2.45, 95%CI: 1.47-4.08, p = 5.78 × 10^-4 ). Pathway-based analysis of somatic truncation mutations identified rs2395185 and rs3817963 at 6p22.1 was associated with cell cycle pathway (OR = 1.56, p = 3.61 × 10^-4 for rs2395185; OR = 1.58, p = 4.15 × 10^-4 for rs3817963), and rs3817963 was also associated with MAPK signaling pathway (OR = 1.54, p = 8.58 × 10^-4 ). Further analysis associated rs2395185 at 6p22.1 (HLA class II genes) with increased APOBEC3A expression (p = 9.50 × 10^-3 ) and elevated APOBEC mutagenesis (p = 3.58 × 10^-3 ). These results indicate germline-somatic interactions in lung tumorigenesis, and help to uncover the molecular mechanisms underlying lung cancer risk SNPs.

Phosphoinositides, Major Actors in Membrane Trafficking and Lipid Signaling Pathways

Phosphoinositides are lipids involved in the vesicular transport of proteins and lipids between the different compartments of eukaryotic cells. They act by recruiting and/or activating effector proteins and thus are involved in regulating various cellular functions, such as vesicular budding, membrane fusion and cytoskeleton dynamics. Although detected in small concentrations in membranes, their role is essential to cell function, since imbalance in their concentrations is a hallmark of many cancers. Their synthesis involves phosphorylating/dephosphorylating positions D3, D4 and/or D5 of their inositol ring by specific lipid kinases and phosphatases. This process is tightly regulated and specific to the different intracellular membranes. Most enzymes involved in phosphoinositide synthesis are conserved between yeast and human, and their loss of function leads to severe diseases (cancer, myopathy, neuropathy and ciliopathy).

PI5P and PI(3,5)P2: Minor, but Essential Phosphoinositides

In most eukaryotes, phosphoinositides (PIs) have crucial roles in multiple cellular functions. Although the cellular levels of phosphatidylinositol 5-phosphate (PI5P) and phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) are extremely low relative to some other PIs, emerging evidence demonstrates that both lipids are crucial for the endocytic pathway, intracellular signaling, and adaptation to stress. Mutations that causes defects in the biosynthesis of PI5P and PI(3,5)P₂ are linked to human diseases including neurodegenerative disorders. Here, we review recent findings on cellular roles of PI5P and PI(3,5)P₂, as well as the pathophysiological importance of these lipids.Key words: Phosphoinositides, Membrane trafficking, Endocytosis, Vacuoles/Lysosomes, Fab1/PIKfyve.

Phosphatidylinositol 3,5-bisphosphate: regulation of cellular events in space and time

Phosphorylated phosphatidylinositol lipids are crucial for most eukaryotes and have diverse cellular functions. The low-abundance signalling lipid phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2] is critical for cellular homoeostasis and adaptation to stimuli. A large complex of proteins that includes the lipid kinase Fab1-PIKfyve, dynamically regulates the levels of PI(3,5)P2. Deficiencies in PI(3,5)P2 are linked to some human diseases, especially those of the nervous system. Future studies will probably determine new, undiscovered regulatory roles of PI(3,5)P2, as well as uncover mechanistic insights into how PI(3,5)P2 contributes to normal human physiology.

The amyloid precursor protein (APP) binds the PIKfyve complex and modulates its function

Phosphoinositides are important components of eukaryotic membranes that are required for multiple forms of membrane dynamics. Phosphoinositides are involved in defining membrane identity, mediate cell signalling and control membrane trafficking events. Due to their pivotal role in membrane dynamics, phosphoinositide de-regulation contributes to various human diseases. In this review, we will focus on the newly emerging regulation of the PIKfyve complex, a phosphoinositide kinase that converts the endosomal phosphatidylinositol-3-phosphate [PI(3)P] to phosphatidylinositol-3,5-bisphosphate [PI(3,5)P2)], a low abundance phosphoinositide of outstanding importance for neuronal integrity and function. Loss of PIKfyve function is well known to result in neurodegeneration in both mouse models and human patients. Our recent work has surprisingly identified the amyloid precursor protein (APP), the central molecule in Alzheimer's disease aetiology, as a novel interaction partner of a subunit of the PIKfyve complex, Vac14. Furthermore, it has been shown that APP modulates PIKfyve function and PI(3,5)P2 dynamics, suggesting that the APP gene family functions as regulator of PI(3,5)P2 metabolism. The recent advances discussed in this review suggest a novel, unexpected, β-amyloid-independent mechanism for neurodegeneration in Alzheimer's disease.

Phosphoinositides in Control of Membrane Dynamics

Most functions of eukaryotic cells are controlled by cellular membranes, which are not static entities but undergo frequent budding, fission, fusion, and sculpting reactions collectively referred to as membrane dynamics. Consequently, regulation of membrane dynamics is crucial for cellular functions. A key mechanism in such regulation is the reversible recruitment of cytosolic proteins or protein complexes to specific membranes at specific time points. To a large extent this recruitment is orchestrated by phosphorylated derivatives of the membrane lipid phosphatidylinositol, known as phosphoinositides. The seven phosphoinositides found in nature localize to distinct membrane domains and recruit distinct effectors, thereby contributing strongly to the maintenance of membrane identity. Many of the phosphoinositide effectors are proteins that control membrane dynamics, and in this review we discuss the functions of phosphoinositides in membrane dynamics during exocytosis, endocytosis, autophagy, cell division, cell migration, and epithelial cell polarity, with emphasis on protein effectors that are recruited by specific phosphoinositides during these processes.

The genetic architecture of type 2 diabetes

The genetic architecture of common traits, including the number, frequency, and effect sizes of inherited variants that contribute to individual risk, has been long debated. Genome-wide association studies have identified scores of common variants associated with type 2 diabetes, but in aggregate, these explain only a fraction of the heritability of this disease. Here, to test the hypothesis that lower-frequency variants explain much of the remainder, the GoT2D and T2D-GENES consortia performed whole-genome sequencing in 2,657 European individuals with and without diabetes, and exome sequencing in 12,940 individuals from five ancestry groups. To increase statistical power, we expanded the sample size via genotyping and imputation in a further 111,548 subjects. Variants associated with type 2 diabetes after sequencing were overwhelmingly common and most fell within regions previously identified by genome-wide association studies. Comprehensive enumeration of sequence variation is necessary to identify functional alleles that provide important clues to disease pathophysiology, but large-scale sequencing does not support the idea that lower-frequency variants have a major role in predisposition to type 2 diabetes.

Phosphatidylinositol 5-phosphate 4-kinase γ (PI5P4Kγ), a lipid signalling enigma

The phosphatidylinositol 5-phosphate 4-kinases (PI5P4Ks) are an important family of enzymes, whose physiological roles are being teased out by a variety of means. Phosphatidylinositol-5-phosphate 4-kinase γ (PI5P4Kγ) is especially intriguing as its in vitro activity is very low. Here we review what is known about this enzyme and discuss some recent advances towards an understanding of its physiology. Additionally, the effects of the ATP-competitive inhibitor I-OMe Tyrphostin AG-538 on all three mammalian PI5P4Ks was explored, including two PI5P4Kγ mutants with altered ATP- or PI5P-binding sites. The results suggest a strategy for targeting non-ATP binding sites on inositol lipid kinases.

PIP4K and the role of nuclear phosphoinositides in tumour suppression

Phosphatidylinositol-5-phosphate (PtdIns5P)-4-kinases (PIP4Ks) are stress-regulated lipid kinases that phosphorylate PtdIns5P to generate PtdIns(4,5)P₂. There are three isoforms of PIP4Ks: PIP4K2A, 2B and 2C, which localise to different subcellular compartments with the PIP4K2B isoform being localised predominantly in the nucleus. Suppression of PIP4K expression selectively prevents tumour cell growth in vitro and prevents tumour development in mice that have lost the tumour suppressor p53. p53 is lost or mutated in over 70% of all human tumours. These studies suggest that inhibition of PIP4K signalling constitutes a novel anti-cancer therapeutic target. In this review we will discuss the role of PIP4K in tumour suppression and speculate on how PIP4K modulates nuclear phosphoinositides (PPIns) and how this might impact on nuclear functions to regulate cell growth. This article is part of a Special Issue entitled Phosphoinositides.

PI(5)P regulates autophagosome biogenesis

Phosphatidylinositol 3-phosphate (PI(3)P), the product of class III PI3K VPS34, recruits specific autophagic effectors, like WIPI2, during the initial steps of autophagosome biogenesis and thereby regulates canonical autophagy. However, mammalian cells can produce autophagosomes through enigmatic noncanonical VPS34-independent pathways. Here we show that PI(5)P can regulate autophagy via PI(3)P effectors and thereby identify a mechanistic explanation for forms of noncanonical autophagy. PI(5)P synthesis by the phosphatidylinositol 5-kinase PIKfyve was required for autophagosome biogenesis, and it increased levels of PI(5)P, stimulated autophagy, and reduced the levels of autophagic substrates. Inactivation of VPS34 impaired recruitment of WIPI2 and DFCP1 to autophagic precursors, reduced ATG5-ATG12 conjugation, and compromised autophagosome formation. However, these phenotypes were rescued by PI(5)P in VPS34-inactivated cells. These findings provide a mechanistic framework for alternative VPS34-independent autophagy-initiating pathways, like glucose starvation, and unravel a cytoplasmic function for PI(5)P, which previously has been linked predominantly to nuclear roles.

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PI(5)P positively regulates autophagy

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PI(5)P is associated with autophagy effectors that bind PI(3)P

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PI(5)P sustains noncanonical autophagy in PI(3)P-depleted cells

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PI(5)P is essential for VPS34-independent, glucose-starvation-induced autophagy

Plentiful PtdIns5P from scanty PtdIns(3,5)P2 or from ample PtdIns? PIKfyve-dependent models: Evidence and speculation (response to: DOI 10.1002/bies.201300012)

Recently, we have presented data supporting the notion that PIKfyve not only produces the majority of constitutive phosphatidylinositol 5-phosphate (PtdIns5P) in mammalian cells but that it does so through direct synthesis from PtdIns. Another group, albeit obtaining similar data, suggests an alternative pathway whereby the low-abundance PtdIns(3,5)P2 undergoes hydrolysis by unidentified 3-phosphatases, thereby serving as a precursor for most of PtdIns5P. Here, we review the experimental evidence supporting constitutive synthesis of PtdIns5P from PtdIns by PIKfyve. We further emphasize that the experiments presented in support of the alternative pathway are also compatible with a direct mechanism for PIKfyve-catalyzed synthesis of PtdIns5P. While agreeing with the authors that constitutive PtdIns5P could theoretically be produced from PtdIns(3,5)P2 by 3-dephosphorylation, we argue that until direct evidence for such an alternative pathway is obtained, we should adhere to the existing experimental evidence and quantitative considerations, which favor direct PIKfyve-catalyzed synthesis for most constitutive PtdIns5P.

Exploring phosphatidylinositol 5-phosphate 4-kinase function

The family of phosphatidylinositol 5-phosphate 4-kinases (PI5P4Ks) is emerging from a comparative backwater in inositide signalling into the mainstream, as is their substrate, phosphatidylinositol 5-phosphate (PI5P). Here we review some of the key questions about the PI5P4Ks, their localisation, interaction, and regulation and also we summarise our current understanding of how PI5P is synthesised and what its cellular functions might be. Finally, some of the evidence for the involvement of PI5P4Ks in pathology is discussed.

Expression, purification, crystallization and preliminary crystallographic analysis of human myotubularin-related protein 3

Myotubularin-related proteins are a large family of phosphatases that have the catalytic activity of dephosphorylating the phospholipid molecules phosphatidylinositol 3-phosphate and phosphatidylinositol 3,5-bisphosphate. Each of the 14 family members contains a phosphatase catalytic domain, which is inactive in six family members owing to amino-acid changes in a key motif for the activity. All of the members also bear PH-GRAM domains, which have low homologies between them and have roles that are not yet clear. Here, the cloning, expression, purification and crystallization of human myotubularin-related protein 3 encompassing the PH-GRAM and the phosphatase catalytic domain are reported. Preliminary X-ray crystallographic analysis shows that the crystals diffracted to 3.30 Å resolution at a synchrotron X-ray source. The crystals belonged to space group C2, with unit-cell parameters a = 323.3, b = 263.3, c = 149.4 Å, β = 109.7°.

PIKfyve, MTMR3 and their product PtdIns5P regulate cancer cell migration and invasion through activation of Rac1

We have found that the activity of Rac1 is regulated by the lipid PtdIns5P produced by PIKfyve and MTMR3 and that the activities of these druggable enzymes are important for cancer cell migration and invasion.

Phosphatidylinositol 5-phosphate regulates invasion through binding and activation of Tiam1

PtdIns5P is a lipid messenger acting as a stress-response mediator in the nucleus, and known to maintain cell activation through traffic alterations upon bacterial infection. Here, we show that PtdIns5P regulates actin dynamics and invasion via recruitment and activation of the exchange factor Tiam1 and Rac1. Restricted Rac1 activation results from the binding of Tiam1 DH-PH domains to PtdIns5P. Using an assay that mimics Rac1 membrane anchoring by using Rac1-His and liposomes containing Ni(2+)-NTA modified lipids, we demonstrate that intrinsic Tiam1 DH-PH activity increases when Rac1 is anchored in a PtdIns5P-enriched environment. This pathway appears to be general since it is valid in different pathophysiological models: receptor tyrosine kinase activation, bacterial phosphatase IpgD expression and the invasive NPM-ALK(+) lymphomas. The discovery that PtdIns5P could be a keystone of GTPases and cytoskeleton spatiotemporal regulation opens important research avenues towards unravelling new strategies counteracting cell invasion.

Association of polymorphisms at HORMAD2 and prognosis in advanced non-small-cell lung cancer patients

BACKGROUND

Cancer-testis (CT) genes are predominantly expressed in the testis and are ectopically activated in a wide range of cancers. The expression of CT antigens has been shown to significantly affect the survival of patients with non-small-cell lung cancer (NSCLC). Recently, a genome-wide association study (GWAS) and expression analysis have identified a novel CT gene (HORMAD2) associated with lung cancer risk in Han Chinese people. Thus, the aim of this study is to evaluate the potential prognostic value of HORMAD2 polymorphisms in Han Chinese patients with advanced NSCLC and undergoing first-line platinum-based chemotherapy.

MATERIALS AND METHODS

We selected eight single-nucleotide polymorphisms (SNPs) of HORMAD2 with the potential function of affecting the binding of transcription factors, and we genotyped these SNPs in 303 patients with advanced NSCLC using the MassARRAY platform. All patients were treated with first-line platinum-based chemotherapy but without surgery. Log-rank test and Cox proportional hazard models were used for the survival analyses.

RESULTS

Four SNPs at HORMAD2 (rs9620953, rs8135823, rs5753025 and rs9625921) were significantly associated with the survival of advanced NSCLC patients. Among these, patients with the rs9620953 T allele had a significantly reduced risk of death compared to those with the C allele (additive model: HR, 0.53, 95%CI, 0.32-0.89, P=0.016; dominant model: HR, 0.50, 95%CI, 0.29-0.84, P=0.010). Similarly, the G allele at rs8135823 could decrease the death risk of NSCLC patients compared to the T allele (additive model: HR, 0.63, 95%CI, 0.41-0.95, P=0.028; dominant model: HR, 0.60, 95%CI, 0.39-0.93, P=0.022). Furthermore, both the rs5753025 C allele and the rs9625921 G allele also decreased the death risk in NSCLC in different genetic models (additive model for rs5753025: HR, 0.80, 95%CI, 0.65-0.98, P=0.032; heterozygote model for rs9625921: HR, 0.71, 95%CI, 0.51-0.99, P=0.040). In the joint effect analyses, we found that patients with one, two, and three to eight favorable alleles had a better survival compared with patients carrying no alleles.

CONCLUSIONS

These findings indicate that polymorphisms at the CT gene HORMAD2 might be involved in the prognosis of advanced NSCLC in Han Chinese. Further larger and functional studies are needed to confirm the results.

Phosphatidylinositol 3,5-bisphosphate: low abundance, high significance

Recent studies of the low abundant signaling lipid, phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2 ), reveal an intriguingly diverse list of downstream pathways, the intertwined relationship between PI(3,5)P2 and PI5P, as well as links to neurodegenerative diseases. Derived from the structural lipid phosphatidylinositol, PI(3,5)P2 is dynamically generated on multiple cellular compartments where interactions with an increasing list of effectors regulate many cellular pathways. A complex of proteins that includes Fab1/PIKfyve, Vac14, and Fig4/Sac3 mediates the biosynthesis of PI(3,5)P2 , and mutations that disrupt complex function and/or formation cause profound consequences in cells. Surprisingly, mutations in this pathway are linked with neurological diseases, including Charcot-Marie-Tooth syndrome and amyotrophic lateral sclerosis. Future studies of PI(3,5)P2 and PI5P are likely to expand the roles of these lipids in regulation of cellular functions, as well as provide new approaches for treatment of some neurological diseases.

Reactivation of fetal splicing programs in diabetic hearts is mediated by protein kinase C signaling

Diabetic cardiomyopathy is one of the complications of diabetes that eventually leads to heart failure and death. Aberrant activation of PKC signaling contributes to diabetic cardiomyopathy by mechanisms that are poorly understood. Previous reports indicate that PKC is implicated in alternative splicing regulation. Therefore, we wanted to test whether PKC activation in diabetic hearts induces alternative splicing abnormalities. Here, using RNA sequencing we identified a set of 22 alternative splicing events that undergo a developmental switch in splicing, and we confirmed that splicing reverts to an embryonic pattern in adult diabetic hearts. This network of genes has important functions in RNA metabolism and in developmental processes such as differentiation. Importantly, PKC isozymes α/β control alternative splicing of these genes via phosphorylation and up-regulation of the RNA-binding proteins CELF1 and Rbfox2. Using a mutant of CELF1, we show that phosphorylation of CELF1 by PKC is necessary for regulation of splicing events altered in diabetes. In summary, our studies indicate that activation of PKCα/β in diabetic hearts contributes to the genome-wide splicing changes through phosphorylation and up-regulation of CELF1/Rbfox2 proteins. These findings provide a basis for PKC-mediated cardiac pathogenesis under diabetic conditions.

Evolutionarily conserved structural changes in phosphatidylinositol 5-phosphate 4-kinase (PI5P4K) isoforms are responsible for differences in enzyme activity and localization

Mammals have genes coding for three PI5P4Ks (PtdIns5P 4-kinases), and these have different cellular localizations, tissue distributions and lipid kinase activities. We describe in the present paper a detailed molecular exploration of human PI5P4Ks α, β and γ, as well as their fly and worm homologues, to understand how and why these differences came to be. The intrinsic ATPase activities of the three isoforms are very similar, and we show that differences in their G-loop regions can account for much of their wide differences in lipid kinase activity. We have also undertaken an extensive in silico evolutionary study of the PI5P4K family, and show experimentally that the single PI5P4K homologues from Caenorhabditis elegans and Drosophila melanogaster are as widely different in activity as the most divergent mammalian isoforms. Finally we show that the close association of PI5P4Ks α and γ is a true heterodimerization, and not a higher oligomer association of homodimers. We reveal that structural modelling is consistent with this and with the apparently random heterodimerization that we had earlier observed between PI5P4Kα and PI5P4Kβ [Wang, Bond, Letcher, Richardson, Lilley, Irvine and Clarke (2010), Biochem. J. 430, 215–221]. Overall the molecular diversity of mammalian PI5P4Ks explains much of their properties and behaviour, but their physiological functionality remains elusive.

Lipid sorting and multivesicular endosome biogenesis

Intracellular organelles, including endosomes, show differences not only in protein but also in lipid composition. It is becoming clear from the work of many laboratories that the mechanisms necessary to achieve such lipid segregation can operate at very different levels, including the membrane biophysical properties, the interactions with other lipids and proteins, and the turnover rates or distribution of metabolic enzymes. In turn, lipids can directly influence the organelle membrane properties by changing biophysical parameters and by recruiting partner effector proteins involved in protein sorting and membrane dynamics. In this review, we will discuss how lipids are sorted in endosomal membranes and how they impact on endosome functions.

Inositol lipids: from an archaeal origin to phosphatidylinositol 3,5-bisphosphate faults in human disease

The last couple of decades have seen an extraordinary transformation in our knowledge and understanding of the multifarious biological roles of inositol phospholipids. Herein, I briefly consider two topics. The first is the role that recently acquired biochemical and genomic information - especially from archaeons - has played in illuminating the possible evolutionary origins of the biological employment of inositol in lipids, and some questions that these studies raise about the 'classical' biosynthetic route to phosphatidylinositol. The second is the growing recognition of the importance in eukaryotic cells of phosphatidylinositol 3,5-bisphosphate. Phosphatidylinositol 3,5-bisphosphate only entered our phosphoinositide consciousness quite recently, but it is speedily gathering a plethora of roles in diverse cellular processes and diseases thereof. These include: control of endolysosomal vesicular trafficking and of the activity of ion channels and pumps in the endolysosomal compartment; control of constitutive and stimulated protein traffic to and from plasma membrane subdomains; control of the nutrient and stress-sensing target of rapamycin complex 1 pathway (TORC1); and regulation of key genes in some central metabolic pathways.