Fluorescently labeled phosphatidylinositol transfer protein isoforms (alpha and beta), microinjected into fetal bovine heart endothelial cells, are targeted to distinct intracellular sites

The expanding roles of PI4P and PI(4,5)P2 at the plasma membrane: Role of phosphatidylinositol transfer proteins

Phosphoinositides are phosphorylated derivatives of phosphatidylinositol, a phospholipid that is synthesised at the endoplasmic reticulum. The plasma membrane contains the enzymes to phosphorylate phosphatidylinositol and is therefore rich in the phosphorylated derivatives, PI4P and PI(4,5)P2. PI(4,5)P2 is a substrate for phospholipase C and during cell signaling, PI(4,5)P2 levels are reduced. Here I discuss a family of proteins, phosphatidylinositol transfer proteins (PITPs) that can restore PI(4,5)P2 levels.

Microcolin H, a novel autophagy inducer, exerts potent antitumour activity by targeting PITPα/β

The identification of effective drug targets and the development of bioactive molecules are areas of high need in cancer therapy. The phosphatidylinositol transfer protein alpha/beta isoform (PITPα/β) has been reported to play an essential role in integrating phosphoinositide trafficking and lipid metabolism in diverse cellular processes but remains unexplored as a potential target for cancer treatment. Herein, data analysis of clinical cancer samples revealed that PITPα/β expression is closely correlated with the poor prognosis. Target identification by chemical proteomic methods revealed that microcolin H, a naturally occurring marine lipopeptide, directly binds PITPα/β and displays antiproliferative activity on different types of tumour cell lines. Furthermore, we identified that microcolin H treatment increased the conversion of LC3I to LC3II, accompanied by a reduction of the level of p62 in cancer cells, leading to autophagic cell death. Moreover, microcolin H showed preeminent antitumour efficacy in nude mouse subcutaneous tumour models with low toxicity. Our discoveries revealed that by targeting PITPα/β, microcolin H induced autophagic cell death in tumours with efficient anti-proliferating activity, which sheds light on PITPα/β as a promising therapeutic target for cancer treatment.

Nuclear Phosphoinositides as Key Determinants of Nuclear Functions

Polyphosphoinositides (PPIns) are signalling messengers representing less than five per cent of the total phospholipid concentration within the cell. Despite their low concentration, these lipids are critical regulators of various cellular processes, including cell cycle, differentiation, gene transcription, apoptosis and motility. PPIns are generated by the phosphorylation of the inositol head group of phosphatidylinositol (PtdIns). Different pools of PPIns are found at distinct subcellular compartments, which are regulated by an array of kinases, phosphatases and phospholipases. Six of the seven PPIns species have been found in the nucleus, including the nuclear envelope, the nucleoplasm and the nucleolus. The identification and characterisation of PPIns interactor and effector proteins in the nucleus have led to increasing interest in the role of PPIns in nuclear signalling. However, the regulation and functions of PPIns in the nucleus are complex and are still being elucidated. This review summarises our current understanding of the localisation, biogenesis and physiological functions of the different PPIns species in the nucleus.

Hippo pathway regulation by phosphatidylinositol transfer protein and phosphoinositides

The Hippo pathway plays a key role in development, organ size control and tissue homeostasis, and its dysregulation contributes to cancer. The LATS tumor suppressor kinases phosphorylate and inhibit the YAP/TAZ transcriptional co-activators to suppress gene expression and cell growth. Through a screen of marine natural products, we identified microcolin B (MCB) as a Hippo activator that preferentially kills YAP-dependent cancer cells. Structure-activity optimization yielded more potent MCB analogs, which led to the identification of phosphatidylinositol transfer proteins α and β (PITPα/β) as the direct molecular targets. We established a critical role of PITPα/β in regulating LATS and YAP. Moreover, we showed that PITPα/β influence the Hippo pathway via plasma membrane phosphatidylinositol-4-phosphate. This study uncovers a previously unrecognized role of PITPα/β in Hippo pathway regulation and as potential cancer therapeutic targets.

Lipid transfer proteins and instructive regulation of lipid kinase activities: Implications for inositol lipid signaling and disease

Cellular membranes are critical platforms for intracellular signaling that involve complex interfaces between lipids and proteins, and a web of interactions between a multitude of lipid metabolic pathways. Membrane lipids impart structural and functional information in this regulatory circuit that encompass biophysical parameters such as membrane thickness and fluidity, as well as chaperoning the interactions of protein binding partners. Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play key roles in intracellular membrane signaling, and these involvements are translated into an impressively diverse set of biological outcomes. The phosphatidylinositol transfer proteins (PITPs) are key regulators of phosphoinositide signaling. Found in a diverse array of organisms from plants, yeast and apicomplexan parasites to mammals, PITPs were initially proposed to be simple transporters of lipids between intracellular membranes. It now appears increasingly unlikely that the soluble versions of these proteins perform such functions within the cell. Rather, these serve to facilitate the activity of intrinsically biologically insufficient inositol lipid kinases and, in so doing, promote diversification of the biological outcomes of phosphoinositide signaling. The central engine for execution of such functions is the lipid exchange cycle that is a fundamental property of PITPs. How PITPs execute lipid exchange remains very poorly understood. Molecular dynamics simulation approaches are now providing the first atomistic insights into how PITPs, and potentially other lipid-exchange/transfer proteins, operate.

Nuclear Phosphoinositides-Versatile Regulators of Genome Functions

The many functions of phosphoinositides in cytosolic signaling were extensively studied; however, their activities in the cell nucleus are much less clear. In this review, we summarize data about their nuclear localization and metabolism, and review the available literature on their involvements in chromatin remodeling, gene transcription, and RNA processing. We discuss the molecular mechanisms via which nuclear phosphoinositides, in particular phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2), modulate nuclear processes. We focus on PI(4,5)P2’s role in the modulation of RNA polymerase I activity, and functions of the nuclear lipid islets—recently described nucleoplasmic PI(4,5)P2-rich compartment involved in RNA polymerase II transcription. In conclusion, the high impact of the phosphoinositide–protein complexes on nuclear organization and genome functions is only now emerging and deserves further thorough studies.

Polyphosphoinositide-Binding Domains: Insights from Peripheral Membrane and Lipid-Transfer Proteins

Within eukaryotic cells, biochemical reactions need to be organized on the surface of membrane compartments that use distinct lipid constituents to dynamically modulate the functions of integral proteins or influence the selective recruitment of peripheral membrane effectors. As a result of these complex interactions, a variety of human pathologies can be traced back to improper communication between proteins and membrane surfaces; either due to mutations that directly alter protein structure or as a result of changes in membrane lipid composition. Among the known structural lipids found in cellular membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the membrane-anchored precursor of low-abundance regulatory lipids, the polyphosphoinositides (PPIn), which have restricted distributions within specific subcellular compartments. The ability of PPIn lipids to function as signaling platforms relies on both non-specific electrostatic interactions and the selective stereospecific recognition of PPIn headgroups by specialized protein folds. In this chapter, we will attempt to summarize the structural diversity of modular PPIn-interacting domains that facilitate the reversible recruitment and conformational regulation of peripheral membrane proteins. Outside of protein folds capable of capturing PPIn headgroups at the membrane interface, recent studies detailing the selective binding and bilayer extraction of PPIn species by unique functional domains within specific families of lipid-transfer proteins will also be highlighted. Overall, this overview will help to outline the fundamental physiochemical mechanisms that facilitate localized interactions between PPIn lipids and the wide-variety of PPIn-binding proteins that are essential for the coordinate regulation of cellular metabolism and membrane dynamics.

The interface between phosphatidylinositol transfer protein function and phosphoinositide signaling in higher eukaryotes

Phosphoinositides are key regulators of a large number of diverse cellular processes that include membrane trafficking, plasma membrane receptor signaling, cell proliferation, and transcription. How a small number of chemically distinct phosphoinositide signals are functionally amplified to exert specific control over such a diverse set of biological outcomes remains incompletely understood. To this end, a novel mechanism is now taking shape, and it involves phosphatidylinositol (PtdIns) transfer proteins (PITPs). The concept that PITPs exert instructive regulation of PtdIns 4-OH kinase activities and thereby channel phosphoinositide production to specific biological outcomes, identifies PITPs as central factors in the diversification of phosphoinositide signaling. There are two evolutionarily distinct families of PITPs: the Sec14-like and the StAR-related lipid transfer domain (START)-like families. Of these two families, the START-like PITPs are the least understood. Herein, we review recent insights into the biochemical, cellular, and physiological function of both PITP families with greater emphasis on the START-like PITPs, and we discuss the underlying mechanisms through which these proteins regulate phosphoinositide signaling and how these actions translate to human health and disease.

A Golgi Lipid Signaling Pathway Controls Apical Golgi Distribution and Cell Polarity during Neurogenesis

Phosphatidylinositol (PtdIns) transfer proteins (PITPs) stimulate PtdIns-4-P synthesis and signaling in eukaryotic cells, but to what biological outcomes such signaling circuits are coupled remains unclear. Herein, we show that two highly related StART-like PITPs, PITPNA and PITPNB, act in a redundant fashion to support development of the embryonic mammalian neocortex. PITPNA/PITPNB do so by driving PtdIns-4-P-dependent recruitment of GOLPH3, and likely ceramide transfer protein (CERT), to Golgi membranes with GOLPH3 recruitment serving to promote MYO18A- and F-actin-directed loading of the Golgi network to apical processes of neural stem cells (NSCs). We propose the primary role for PITP/PtdIns-4-P/GOLPH3/CERT signaling in NSC Golgi is not in regulating bulk membrane trafficking but in optimizing apically directed membrane trafficking and/or apical membrane signaling during neurogenesis.

Quantitative profiling of the endonuclear glycerophospholipidome of murine embryonic fibroblasts

A reliable method for purifying envelope-stripped nuclei from immortalized murine embryonic fibroblasts (iMEFs) was established. Quantitative profiling of the glycerophospholipids (GPLs) in envelope-free iMEF nuclei yields several conclusions. First, we find the endonuclear glycerophospholipidome differs from that of bulk membranes, and phosphatidylcholine (PtdCho) and phosphatidylethanolamine species are the most abundant endonuclear GPLs by mass. By contrast, phosphatidylinositol (PtdIns) represents a minor species. We also find only a slight enrichment of saturated versus unsaturated GPL species in iMEF endonuclear fractions. Moreover, much lower values for GPL mass were measured in the iMEF nuclear matrix than those reported for envelope-stripped IMF-32 nuclei. The collective results indicate that the nuclear matrix in these cells is a GPL-poor environment where GPL occupies only approximately 0.1% of the total nuclear matrix volume. This value suggests GPL accommodation in this compartment can be satisfied by binding to resident proteins. Finally, we find no significant role for the PtdIns/PtdCho-transfer protein, PITPα, in shuttling PtdIns into the iMEF nuclear matrix.

Zebrafish class 1 phosphatidylinositol transfer proteins: PITPbeta and double cone cell outer segment integrity in retina

Phosphatidylinositol transfer proteins (PITPs) in yeast co-ordinate lipid metabolism with the activities of specific membrane trafficking pathways. The structurally unrelated metazoan PITPs (mPITPs), on the other hand, are an under-investigated class of proteins. It remains unclear what biological activities mPITPs discharge, and the mechanisms by which these proteins function are also not understood. The soluble class 1 mPITPs include the PITPalpha and PITPbeta isoforms. Of these, the beta-isoforms are particularly poorly characterized. Herein, we report the use of zebrafish as a model vertebrate for the study of class 1 mPITP biological function. Zebrafish express PITPalpha and PITPbeta-isoforms (Pitpna and Pitpnb, respectively) and a novel PITPbeta-like isoform (Pitpng). Pitpnb expression is particularly robust in double cone cells of the zebrafish retina. Morpholino-mediated protein knockdown experiments demonstrate Pitpnb activity is primarily required for biogenesis/maintenance of the double cone photoreceptor cell outer segments in the developing retina. By contrast, Pitpna activity is essential for successful navigation of early developmental programs. This study reports the initial description of the zebrafish class 1 mPITP family, and the first analysis of PITPbeta function in a vertebrate.

The Diverse Biological Functions of Phosphatidylinositol Transfer Proteins in Eukaryotes

Phosphatidylinositol/phosphatidylcholine transfer proteins (PITPs) remain largely functionally uncharacterized, despite the fact that they are highly conserved and are found in all eukaryotic cells thus far examined by biochemical or sequence analysis approaches. The available data indicate a role for PITPs in regulating specific interfaces between lipid-signaling and cellular function. In this regard, a role for PITPs in controlling specific membrane trafficking events is emerging as a common functional theme. However, the mechanisms by which PITPs regulate lipid-signaling and membrane-trafficking functions remain unresolved. Specific PITP dysfunctions are now linked to neurodegenerative and intestinal malabsorption diseases in mammals, to stress response and developmental regulation in higher plants, and to previously uncharacterized pathways for regulating membrane trafficking in yeast and higher eukaryotes, making it clear that PITPs are integral parts of a highly conserved signal transduction strategy in eukaryotes. Herein, we review recent progress in deciphering the biological functions of PITPs, and discuss some of the open questions that remain.

Dynamics of lipid transfer by phosphatidylinositol transfer proteins in cells

Of many lipid transfer proteins identified, all have been implicated in essential cellular processes, but the activity of none has been demonstrated in intact cells. Among these, phosphatidylinositol transfer proteins (PITP) are of particular interest as they can bind to and transfer phosphatidylinositol (PtdIns) – the precursor of important signalling molecules, phosphoinositides – and because they have essential functions in neuronal development (PITPα) and cytokinesis (PITPβ). Structural analysis indicates that, in the cytosol, PITPs are in a ‘closed’ conformation completely shielding the lipid within them. But during lipid exchange at the membrane, they must transiently ‘open’. To study PITP dynamics in intact cells, we chemically targeted their C95 residue that, although non-essential for lipid transfer, is buried within the phospholipid-binding cavity, and so, its chemical modification prevents PtdIns binding because of steric hindrance. This treatment resulted in entrapment of open conformation PITPs at the membrane and inactivation of the cytosolic pool of PITPs within few minutes. PITP isoforms were differentially inactivated with the dynamics of PITPβ faster than PITPα. We identify two tryptophan residues essential for membrane docking of PITPs.

The anti-apoptotic MAP kinase pathway is inhibited in NIH3T3 fibroblasts with increased expression of phosphatidylinositol transfer protein β

Mouse NIH3T3 fibroblast cells overexpressing phosphatidylinositol transfer protein beta (PI-TPbeta, SPIbeta cells) demonstrate a low rate of proliferation and a high sensitivity towards UV-induced apoptosis when compared with wtNIH3T3 cells. In contrast, SPIbetaS262A cells overexpressing a mutant PI-TPbeta that lacks the protein kinase C-dependent phosphorylation site Ser-262, demonstrate a phenotype comparable with wtNIH3T3 cells. This suggests that the phosphorylation of Ser-262 in PI-TPbeta is involved in the regulation of apoptosis. Conditioned medium (CM) from wtNIH3T3 cells contains bioactive factors, presumably arachidonic acid metabolites [H. Bunte, et al., 2006; M. Schenning, et al., 2004] that are able to protect SPIbeta cells against UV-induced apoptosis. CM from SPIbeta cells lacks this protective activity. However, after heat denaturation CM from SPIbeta cells regains a protective activity comparable with that of wtNIH3T3 cells. This indicates that CM from SPIbeta cells contains an antagonistic factor interfering with the anti-apoptotic activity present. SPIbetaS262A cells do not produce the antagonist suggesting that phosphorylation of Ser-262 is required. Moreover, in line with the apparent lack of anti-apoptotic activity, CM from SPIbeta cells does not induce the expression of COX-2 or the activation of p42/p44 MAP kinase in SPIbeta cells. In contrast, CM from wtNIH3T3 and SPIbetaS262A cells or heat-treated CM from SPIbeta cells does induce these anti-apoptotic markers. Since we have previously shown that some of the arachidonic acid metabolites present in CM from wtNIH3T3 cells are prostaglandin (PG) E(2) and PGF(2alpha), we investigated the effect of these PGs on cell survival. Although PGE(2) and PGF(2alpha) were found to protect wtNIH3T3 and SPIbetaS262A cells against UV-induced apoptosis, these PGs failed to rescue SPIbeta cells. The fact that the concentrations of PGE(2) and PGF(2alpha) in the CM from SPIbeta cells and wtNIH3T3 cells were found to be comparable suggests that the failure of these PGs to protect SPIbeta cells could render these cells more apoptosis sensitive. Concomitantly, upon incubation with PGE(2) and PGF(2alpha), an increased expression of COX-2 and activation of p42/p44 MAP kinase were observed in wtNIH3T3 and SPIbetaS262A cells but not in SPIbeta cells. Hence, it appears that specific mechanisms of cell survival are impaired in SPIbeta cells.

The pathologies associated with functional titration of phosphatidylinositol transfer protein α activity in mice

Phosphatidylinositol transfer proteins (PITPs) bind phosphatidylinositol (PtdIns) and phosphatidylcholine and play diverse roles in coordinating lipid metabolism/signaling with intracellular functions. The underlying mechanisms remain unclear. Genetic ablation of PITPalpha in mice results in neonatal lethality characterized by intestinal and hepatic steatosis, spinocerebellar neurodegeneration, and glucose homeostatic defects. We report that mice expressing a PITPalpha selectively ablated for PtdIns binding activity (Pitpalpha(T59D)), as the sole source of PITPalpha, exhibit phenotypes that recapitulate those of authentic PITPalpha nullizygotes. Analyses of mice with graded reductions in PITPalpha activity reveal proportionately graded reductions in lifespan, demonstrate that intestinal steatosis and hypoglycemia are apparent only when PITPalpha protein levels are strongly reduced (>or=90%), and correlate steatotic and glucose homeostatic defects with cerebellar inflammatory disease. Finally, reconstitution of PITPalpha expression in the small intestine substantially corrects the chylomicron retention disease and cerebellar inflammation of Pitpalpha(0/0) neonates, but does not rescue neonatal lethality in these animals. These data demonstrate that PtdIns binding is an essential functional property of PITPalpha in vivo, and suggest a causal linkage between defects in lipid transport and glucose homeostasis and cerebellar inflammatory disease. Finally, the data also demonstrate intrinsic neuronal deficits in PITPalpha-deficient mice that are independent of intestinal lipid transport defects and hypoglycemia.

Biochemical and biological functions of class I phosphatidylinositol transfer proteins

Phosphoinositides function in a diverse array of cellular activities. They include a role as substrate for lipid kinases and phospholipases to generate second messengers, regulators of the cytoskeleton, of enzymes and of ion channels, and docking sites for reversible recruitment of proteins to membranes. Mammalian phosphatidylinositol transfer proteins, PITPalpha and PITPbeta are paralogs that share 77% sequence identity and contain a hydrophobic cavity that can sequester either phosphatidylinositol or phosphatidylcholine. A string of 11 amino acid residues at the C-terminal acts as a "lid" which shields the lipid from the aqueous environment. PITPs in vitro can facilitate inter-membrane lipid transfer and this requires the movement of the "lid" to allow the lipid cargo to be released. Thus PITPs are structurally designed for delivering lipid cargo and could thus participate in cellular events that are dependent on phosphatidylinositol or derivatives of phosphatidylinositol. Phosphatidylinositol, the precursor for all phosphoinositides is synthesised at the endoplasmic reticulum and its distribution to other organelles could be facilitated by PITPs. Here we highlight recent studies that report on the three-dimensional structures of the different PITP forms and suggest how PITPs are likely to dock at the membrane surface for lipid delivery and extraction. Additionally we discuss whether PITPs are important regulators of sphingomyelin metabolism, and finally describe recent studies that link the association of PITPs with diverse functions including membrane traffic at the Golgi, neurite outgrowth, cytokinesis and stem cell growth.

Sec14 related proteins in yeast

Lipid transport between membranes of eukaryotic organisms represents an essential aspect of organelle biogenesis. This transport must be strictly selective and directional to assure specific lipid composition of individual membranes. Despite the intensive research effort in the last few years, our understanding of how lipids are sorted and moved within cells is still rather limited. Evidence indicates that at least some of the mechanisms generating and maintaining non-random distribution of lipids in cells are linked to the action of phosphatidylinositol transfer proteins (PITPs). The major PITP in yeast Saccharomyces cerevisiae, Sec14p, is essential in promoting Golgi secretory function by modulating of its membrane lipid composition. This review focuses on a group of five yeast proteins that share significant sequence homology with Sec14p. Based on this sequence identity, they were termed Sfh (Sec fourteen homologue) proteins. It is a diverse group of proteins with distinct subcellular localizations and varied physiological functions related to lipid metabolism, phosphoinositide mediated signaling and membrane trafficking.

Phosphatidylinositol transfer proteins and cellular nanoreactors for lipid signaling

Membrane lipids function as structural molecules, reservoirs for second messengers, membrane platforms that scaffold protein assembly and regulators of enzymes and ion channels. Such diverse lipid functions contribute substantially to cellular mechanisms for fine-tuning membrane-signaling events. Meaningful coordination of these events requires exquisite spatial and temporal control of lipid metabolism and organization, and reliable mechanisms for specifically coupling these parameters to dedicated physiological processes. Recent studies suggest such integration is linked to the action of phosphatidylinositol transfer proteins that operate at the interface of the metabolism, trafficking and organization of specific lipids.

Differential expression of a C-terminal splice variant of phosphatidylinositol transfer protein beta lacking the constitutive-phosphorylated Ser262 that localizes to the Golgi compartment

Mammalian PITPbeta (phosphatidylinositol transfer protein beta) is a 272-amino-acid polypeptide capable of transferring PtdIns, PtdCho and SM (sphingomyelin) between membrane bilayers. It has been reported that Ser262 present in the C-terminus of PITPbeta is constitutively phosphorylated and determines Golgi localization. We provide evidence for the expression of an sp (splice) variant of PITPbeta (PITPbeta-sp2) where the C-terminal 15 amino acids of PITPbeta-sp1 are replaced by an alternative C-terminus of 16 amino acids. PITPbeta-sp1 is the product of the first 11 exons, whereas PITPbeta-sp2 is a product of the first 10 exons followed by the twelfth exon--exon 11 being 'skipped'. Both splice variants are capable of PtdIns and PtdCho transfer, with PITPbeta-sp2 being unable to transport SM. PITPbeta is ubiquitously expressed, with the highest amounts of PITPbeta found in HL60 cells and in rat liver; HL60 cells express only PITPbeta-sp1, whereas rat liver expresses both sp variants in similar amounts. In both cell types, PITPbeta-sp1 is constitutively phosphorylated and both the PtdIns and PtdCho forms of PITPbeta-sp1 are present. In contrast, PITPbeta-sp2 lacks the constitutively phosphorylated Ser262 (replaced with glutamine). Nonetheless, both PITPbeta variants localize to the Golgi and, moreover, dephosphorylation of Ser262 of PITPbeta-sp1 does not affect its Golgi localization. The presence of PITPbeta sp variants adds an extra level of proteome complexity and, in rat liver, the single gene for PITPbeta gives rise to seven distinct protein species that can be resolved on the basis of their charge differences.

Phospholipid transfer proteins in perspective

Since their discovery and subsequent purification from mammalian tissues more than 30 years ago an impressive number of studies have been carried out to characterize and elucidate the biological functions of phosphatidylcholine transfer protein (PC-TP), phosphatidylinositol transfer protein (PI-TP) and non-specific lipid transfer protein, more commonly known as sterol carrier protein 2 (SCP-2). Here I will present information to show that these soluble, low-molecular weight proteins constitute domain structures in StArR-related lipid transfer (START) proteins (i.e. PC-TP), in retinal degeneration protein, type B (RdgB)-related PI-TPs (e.g. Dm RdgB, Nir2, Nir3) and in peroxisomal beta-oxidation enzyme-related SCP-2 (i.e. 3-oxoacyl-CoA thiolase, also denoted as SCP-X and the 80-kDa D-bifunctional protein). Further I will summarize the most recent studies pertaining to the physiological function of these soluble phospholipid transfer proteins in metazoa.

Phosphatidylinositol-transfer protein and its homologues in yeast

Yeast Sec14p acts as a phosphatidylinositol/phosphatidylcholine-transfer protein in vitro. In vivo, it is essential in promoting Golgi secretory function. Products of five genes named SFH1–SFH5 (Sec Fourteen Homologues 1–5) exhibit significant sequence homology to Sec14p and together they form the Sec14p family of lipid-transfer proteins. It is a diverse group of proteins with distinct subcellular localizations and varied physiological functions related to lipid metabolism and membrane trafficking.

Specific and nonspecific membrane-binding determinants cooperate in targeting phosphatidylinositol transfer protein beta-isoform to the mammalian trans-Golgi network

Phosphatidylinositol transfer proteins (PITPs) regulate the interface between lipid metabolism and specific steps in membrane trafficking through the secretory pathway in eukaryotes. Herein, we describe the cis-acting information that controls PITPbeta localization in mammalian cells. We demonstrate PITPbeta localizes predominantly to the trans-Golgi network (TGN) and that this localization is independent of the phospholipid-bound state of PITPbeta. Domain mapping analyses show the targeting information within PITPbeta consists of three short C-terminal specificity elements and a nonspecific membrane-binding element defined by a small motif consisting of adjacent tryptophan residues (the W(202)W(203) motif). Combination of the specificity elements with the W(202)W(203) motif is necessary and sufficient to generate an efficient TGN-targeting module. Finally, we demonstrate that PITPbeta association with the TGN is tolerant to a range of missense mutations at residue serine 262, we describe the TGN localization of a novel PITPbeta isoform with a naturally occurring S262Q polymorphism, and we find no other genetic or pharmacological evidence to support the concept that PITPbeta localization to the TGN is obligately regulated by conventional protein kinase C (PKC) or the Golgi-localized PKC isoforms delta or epsilon. These latter findings are at odds with a previous report that conventional PKC-mediated phosphorylation of residue Ser262 is required for PITPbeta targeting to Golgi membranes.

Sterol carrier protein-2 directly interacts with caveolin-1 in vitro and in vivo

HDL-mediated reverse-cholesterol transport as well as phosphoinositide signaling are mediated through plasma membrane microdomains termed caveolae/lipid rafts. However, relatively little is known regarding mechanism(s) whereby these lipids traffic to or are targeted to caveolae/lipid rafts. Since sterol carrier protein-2 (SCP-2) binds both cholesterol and phosphatidylinositol, the possibility that SCP-2 might interact with caveolin-1 and caveolae was examined. Double immunolabeling and laser scanning fluorescence microscopy showed that a small but significant portion of SCP-2 colocalized with caveolin-1 primarily at the plasma membrane of L-cells and more so within intracellular punctuate structures in hepatoma cells. In SCP-2 overexpressing L-cells, SCP-2 was detected in close proximity to caveolin, 48 +/- 4 A, as determined by fluorescence resonance energy transfer (FRET) and immunogold electron microscopy. Cell fractionation of SCP-2 overexpressing L-cells and Western blotting detected SCP-2 in purified plasma membranes, especially in caveolae/ lipid rafts as compared to the nonraft fraction. SCP-2 and caveolin-1 were coimmunoprecipitated from cell lysates by anti-caveolin-1 and anti-SCP-2. Finally, a yeast two-hybrid assay demonstrated that SCP-2 directly interacts with caveolin-1 in vivo. These interactions of SCP-2 with caveolin-1 were specific since a functionally related protein, phosphatidyinositol transfer protein (PITP), colocalized much less well with caveolin-1, was not in close proximity to caveolin-1 (i.e., >120 A), and was not coimmunoprecipitated by anti-caveolin-1 from cell lysates. In summary, it was shown for the first time that SCP-2 (but not PITP) selectively interacted with caveolin-1, both within the cytoplasm and at the plasma membrane. These data contribute significantly to our understanding of the role of SCP-2 in cholesterol and phosphatidylinositol targeted from intracellular sites of synthesis in the endoplasmic reticulum to caveolae/lipid rafts at the cell surface plasma membrane.

Phosphorylation of a distinct structural form of phosphatidylinositol transfer protein alpha at Ser166 by protein kinase C disrupts receptor-mediated phospholipase C signaling by inhibiting delivery of phosphatidylinositol to membranes

Phosphatidylinositol transfer protein alpha (PITPalpha) participates in the supply of phosphatidylinositol (PI) required for many cellular events including phospholipase C (PLC) beta and gamma signaling by G-protein-coupled receptors and receptor-tyrosine kinases, respectively. Protein kinase C has been known to modulate PLC signaling by G-protein-coupled receptors and receptor-tyrosine kinases, although the molecular target has not been identified in most instances. In each case phorbol myristate acetate pretreatment of HL60, HeLa, and COS-7 cells abrogated PLC stimulation by the agonists formyl-Met-Leu-Phe, ATP, and epidermal growth factor, respectively. Here we show that phosphorylation of PITPalpha at Ser166 resulted in inhibition of receptor-stimulated PLC activity. Ser166 is localized in a small pocket between the 165-172 loop and the rest of the protein and was not solvent-accessible in either the PI- or phosphatidylcholine-loaded structures of PITPalpha. To allow phosphorylation at Ser166, a distinct structural form is postulated, and mutation of Thr59 to alanine shifted the equilibrium to this form, which could be resolved on native PAGE. The elution profile observed by size exclusion chromatography of phosphorylated PITPalpha from rat brain or in vitro phosphorylated PITPalpha demonstrated that phosphorylated PITPalpha is structurally distinct from the non-phosphorylated form. Phosphorylated PITPalpha was unable to deliver its PI cargo, although it could deliver phosphatidylcholine. We conclude that the PITPalpha structure has to relax to allow access to the Ser166 site, and this may occur at the membrane surface where PI delivery is required for receptor-mediated PLC signaling.

Phosphatidylinositol transfer protein α regulates growth and apoptosis of NIH3T3 cells

Mouse fibroblast cells overexpressing phosphatidylinositol transfer protein alpha [PI-TPalpha; sense PI-TPalpha (SPIalpha) cells] show a significantly increased rate of proliferation and an extreme resistance toward ultraviolet- or tumor necrosis factor-alpha-induced apoptosis. The conditioned medium (CM) from SPIalpha cells or the neutral lipid extract from CM stimulated the proliferation of quiescent wild-type NIH3T3 cells. CM was also highly effective in increasing resistance toward induced apoptosis in both wild-type cells and the highly apoptosis-sensitive SPIbeta cells (i.e., wild-type cells overexpressing PI-TPbeta). CM from SPIalpha cells grown in the presence of NS398, a specific cyclooxygenase-2 (COX-2) inhibitor, expressed a diminished mitogenic and antiapoptotic activity. This strongly suggests that at least one of the bioactive factor(s) is an eicosanoid. In accordance, SPIalpha cells express enhanced levels of COX-1 and COX-2. The antiapoptotic activity of CM from SPIalpha cells tested on SPIbeta cells was inhibited by approximately 50% by pertussis toxin and suramin as well as by SR141716A, a specific antagonist of the cannabinoid 1 receptor. These inhibitors had virtually no effect on the COX-2-independent antiapoptotic activity of CM from SPIalpha cells. The latter results imply that PI-TPalpha mediates the production of a COX-2-dependent eicosanoid that activates a G-protein-coupled receptor, most probably a cannabinoid 1-like receptor.

Overexpression of phosphatidylinositol transfer protein β in NIH3T3 cells has a stimulatory effect on sphingomyelin synthesis and apoptosis

Phosphatidylinositol transfer proteins (PI-TPs) consist of two isoforms (PI-TPalpha and PI-TPbeta), which differ in phospholipid transfer properties and intracellular localization. Both PI-TP isoforms are substrates for protein kinase C and contain a minor phosphorylation site (Ser166 in PI-TPalpha; Ser165 in PI-TPbeta). Only PI-TPbeta contains a major phosphorylation site at Ser262, which must be phosphorylated for PI-TPbeta to be associated with the Golgi. The PI-TP isoforms are completely conserved between mammals. Although their function is still not clear, their importance follows from knock-out studies, showing that mice lacking PI-TPalpha die soon after birth and that embryonic stems cells lacking PI-TPbeta cannot be generated [Mol. Biol. Cell 13 (2002) 739]. We determined the levels of the PI-TP isoforms in various mouse tissues by immunoblotting. PI-TPalpha is present in all tissues investigated, with highest levels in brain (167 ng/100 microg total protein). The levels of PI-TPbeta are 50-100 times lower than those of PI-TPalpha, with relatively high levels found in liver and brain (1.2 and 1.8 ng/100 microg of total protein, respectively). In contrast to NIH3T3 cells overexpressing PI-TPalpha, cells overexpressing PI-TPbeta (SPIbeta cells) were able to maintain steady-state levels of sphingomyelin in plasma membrane under conditions where this lipid is degraded by exogenous sphingomyelinase. This process of rapid sphingomyelin replenishment is dependent on PI-TPbeta being associated with the Golgi as cells overexpressing a mutant PI-TPbeta in which the major phosphorylation site is replaced (PI-TPbeta(S262A) behave as wild-type NIH3T3 cells. Since the SPIbeta cells display a decreased growth rate (35 h as compared to 21 h for wtNIH3T3 cells), we have investigated the sensitivity of these cells towards UV-induced apoptosis. We have found that the SPIbeta cells, but not the cells overexpressing PI-TPbeta(S262A), are very sensitive. We are currently investigating whether a relationship exists between PI-TPbeta being involved in maintaining plasma membrane sphingomyelin levels and the enhanced sensitivity towards apoptosis.

Structure-Function Analysis of Phosphatidylinositol Transfer Protein Alpha Bound to Human Phosphatidylinositol

Phosphatidylinositol transfer protein alpha (PITPalpha) selectively transports and promotes exchange of phosphatidylinositol (PI) and phosphatidylcholine (PC) between lipid bilayers. In higher eukaryotes PITPalpha is required for cellular functions such as phospholipase C-mediated signaling, regulated exocytosis, and secretory vesicle formation. We have determined the crystal structure of human PITPalpha bound to its physiological ligand, PI, at 2.95 A resolution. The structure identifies the critical side chains within the lipid-headgroup binding pocket that define the exquisite specificity for PI. Mutational analysis of the PI binding pocket is in good agreement with the structural data and allows manipulation of functional properties of PITPalpha. Surprisingly, there are no major conformational differences between PI- and PC-loaded PITPalpha, despite previous predictions. In the crystal, PITPalpha-PI is dimeric, with two identical dimers in the asymmetric unit. The dimer interface masks precisely the sequence we identify as contributing to PITPalpha membrane interaction. Our structure represents a soluble, transport-competent form of PI-loaded PITPalpha.

Nuclear phosphoinositides and their functions

Phosphoinositides are minor components of biological membranes, which have emerged as essential regulators of a variety of cellular processes, both on the plasma membrane and on several intracellular organelles. The versatility of these lipids stems from their ability to function either as substrates for the generation of second messengers, as membrane-anchoring sites for cytosolic proteins or as regulators of the actin cytoskeleton. Despite a vast literature demonstrating the presence of phosphoinositides in the nucleus, only recently has the function(s) of the nuclear pool of these lipids and their soluble analogues, inositol polyphosphates, started to emerge. These compounds have been shown to serve as essential co-factors for several nuclear processes, including DNA repair, transcription regulation and RNA dynamics. In this light, phosphoinositides and inositol polyphosphates might represent high turnover activity switches for nuclear complexes responsible for these processes. The regulation of these large machineries would be linked to the phosphorylation state of the inositol ring and limited temporally and spatially based on the synthesis and degradation of these molecules.

Exposure of Phosphatidylinositol Transfer Proteins to Sphingomyelin−Cholesterol Membranes Suggests Transient but Productive Interactions with Raft-Like, Liquid-Ordered Domains

Both isoforms of rat phosphatidylinositol transfer protein (PITP) mediate the intermembrane transfer of sphingomyelin (CerPCho). In the plasma membrane, CerPCho often segregates with cholesterol into microdomains such as lipid rafts and caveolae. To test the hypothesis that PITP exhibits a preference for CerPCho- and cholesterol-rich membranes, we prepared unilamellar vesicles containing variable amounts of these two lipids. We also used CerPCho species with different acyl composition and treated vesicles with agents known to sequester and remove cholesterol. We observed that the beta isoform of rat PITP was more sensitive to membrane cholesterol than was the alpha isoform, as shown by increases in specific activities of lipid transfer of 2-6-fold. A relatively high membrane content of cholesterol (mole fraction > 0.4) was required to elicit such enhancements. Treatment of cholesterol-rich membranes with a series of beta cyclodextrins demonstrated that, upon depletion of cholesterol from participating membranes, the PITPbeta activity changes were fully reversible. We finally noted that the mechanism by which cholesterol enhances the activity of PITPbeta appeared to involve a decreased affinity of the protein for the membrane surface, in a manner that was independent of vesicle size and membrane microviscosity. We conclude that PITPbeta interacts transiently but productively with the liquid-ordered phase formed by CerPCho and cholesterol and discuss the possibility of PITP interactions in vivo with sphingolipid- and cholesterol-rich membrane microdomains.

Subcellular localization of yeast Sec14 homologues and their involvement in regulation of phospholipid turnover

Sec14p of the yeast Saccharomyces cerevisiae is involved in protein secretion and regulation of lipid synthesis and turnover in vivo, but acts as a phosphatidylinositol-phosphatidylcholine transfer protein in vitro. In this work, the five homologues of Sec14p, Sfh1p-Sfh5p, were subjected to biochemical and cell biological analysis to get a better view of their physiological role. We show that overexpression of SFH2 and SFH4 suppressed the sec14 growth defect in a more and SFH1 in a less efficient way, whereas overexpression of SFH3 and SFH5 did not complement sec14. Using C-terminal yEGFP fusions, Sfh2p, Sfh4p and Sfh5p are mainly localized to the cytosol and microsomes similar to Sec14p. Sfh1p was detected in the nucleus and Sfh3p in lipid particles and in microsomes. In contrast to Sec14p, which inhibits phospholipase D1 (Pld1p), overproduction of Sfh2p and Sfh4p resulted in the activation of Pld1p-mediated phosphatidylcholine turnover. Interestingly, Sec14p and the two homologues Sfh2p and Sfh4p downregulate phospholipase B1 (Plb1p)-mediated turnover of phosphatidylcholine in vivo. In summary, Sfh2p and Sfh4p are the Sec14p homologues with the most pronounced functional similarity to Sec14p, whereas the other Sfh proteins appear to be functionally less related to Sec14p.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Sterol carrier protein-2 functions in phosphatidylinositol transfer and signaling

Over 20 years ago, it was reported that liver cytosol contains at least two distinct proteins that transfer phosphatidylinositol in vitro, phosphatidylinositol transfer protein (PITP) and a pH 5.1 supernatant fraction containing sterol carrier protein-2 (SCP-2). In contrast to PITP, there has been minimal progress on the structural and functional significance of SCP-2 in phosphatidylinositol transport. As shown herein, highly purified, recombinant SCP-2 stimulated up to 13-fold the rapid (s) transfer of radiolabeled phosphatidylinositol (PI) from microsomal donor membranes to highly curved acceptor membranes. SCP-2 bound to microsomes in vitro and overexpression of SCP-2 in transfected L-cells resulted in the following: (i) redistribution of phosphatidylinositols from intracellular membranes (mitochondria and microsomes) to the plasma membrane; (ii) enhancement of insulin-mediated inositol-triphosphate production; and (iii) 5.5-fold down regulation of PITP. Like PITP, SCP-2 binds two ligands required for vesicle budding from the Golgi, PI, and fatty acyl CoA. Double immunolabeling confocal microscopy showed SCP-2 significantly colocalized with caveolin-1 in the cytoplasm (punctate) and plasma membrane of SCP-2 overexpressing hepatoma cells (72%), HT-29 cells (58%), and SCP-2 overexpressing L-cells (37%). Taken together, these data show for the first time that SCP-2 plays a hitherto unrecognized role in intracellular phosphatidylinositol transfer, distribution, and signaling.

Cloning and characterization of a novel variant (mM-rdgBbeta1) of mouse M-rdgBs, mammalian homologs of Drosophila retinal degeneration B gene proteins, and its mRNA localization in mouse brain in comparison with other M-rdgBs

We report the cloning, characterization and localization in the brain of a novel isoform termed mM-rdgBbeta1 (mouse type of mammalian retinal degeneration Bbeta1 protein) in comparison with the localization of three known mammalian homologs (M-rdgBbeta, M-rdgB1, M-rdgB2). mM-rdgBbeta1 cDNA contains a sequence of 119 bp as a form of insertion in the open reading frame of the known mM-rdgBbeta, and encodes a protein of 269 amino acids with a calculated molecular mass of 31.7 kDa, different from the molecular mass of 38.3 kDa of mM-rdgBbeta. It also contains a phosphatidylinositol transfer protein (PITP)-like domain similar to the known three homologs, as well as D-rdgB. The recombinant mM-rdgBbeta1 protein shows the specific binding activity to phosphatidylinositol but not to other phospholipids. This novel molecule is localized not only in the cytoplasm but also in the nucleus, different from the cytoplasmic localization of mM-rdgBbeta. In in situ hybridization analysis, the gene expression for mM-rdgBbeta1 in the brain, though weak, is rather confined to the embryonic stage, different from wider expression of mM-rdgBbeta in the gray matters of pre- and post-natal brains. Taken together, mM-rdgBbeta1 is suggested to play a role in the phosphoinositide-mediated signaling in the neural development.

EGF regulation of PITP dynamics is blocked by inhibitors of phospholipase C and of the Ras-MAP kinase pathway

Phosphatidylinositol transfer proteins (PITP) function in signal transduction and in membrane traffic. Studies aimed at elucidating the mechanism of action of PITP have yielded a singular theme; the activity of PITP stems from its ability to transfer phosphatidylinositol (PI) from its site of synthesis to sites of cellular activity and to stimulate the local synthesis of phosphorylated forms of PI. The participation of various phosphoinositides in EGF signal transduction and in the trafficking of the EGF receptors is well documented. Using fluorescence lifetime imaging microscopy (FLIM) to measure fluorescence resonance energy transfer (FRET) between EGFP-PITP proteins and fluorescently labeled phospholipids, we report that PITPalpha and PITPbeta can dynamically interact with PI or PC at the plasma membrane when stimulated with EGF. Additionally, PITPbeta is localized at the Golgi, and EGF stimulation resulted in enhanced FRET. Inhibitors of the PLC and the Ras/MAP kinase pathway were both able to inhibit the EGF-stimulated interaction of PITPalpha with PI at the plasma membrane. The mobility of PITP proteins was determined by using fluorescence recovery after photobleaching (FRAP), and EGF stimulation reduced the mobility at the plasma membrane. We conclude that the dynamic behavior of PITPalpha and PITPbeta in vivo is a regulated process involving multiple mechanisms.

The structure of phosphatidylinositol transfer protein alpha reveals sites for phospholipid binding and membrane association with major implications for its function

Elucidation of the three-dimensional structure of phosphatidylinositol transfer protein alpha (PI-TPalpha) void of phospholipid revealed a site of membrane association connected to a channel for phospholipid binding. Near the top of the channel specific binding sites for the phosphorylcholine and phosphorylinositol head groups were identified. The structure of this open form suggests a mechanism by which PI-TPalpha preferentially binds PI from a membrane interface. Modeling predicts that upon association of PI-TPalpha with the membrane the inositol moiety of bound PI is accessible from the medium. Upon release from the membrane PI-TPalpha adopts a closed structure with the phospholipid bound fully encapsulated. This structure provides new insights as to how PI-TPalpha may play a role in PI metabolism.

Nuclear lipid signaling

Abundant evidence now supports the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope as part of a bilayer membrane, but are actually within the nucleus in the form of proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol (DAG) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet entirely clear. Generation of DAG within the nucleus is believed to recruit protein kinase C (PKC) to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P3 may mobilize Ca2+ from the space between the nuclear membranes and thus increase nucleoplasmic Ca2+. Less well understood are the increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus, (i) two routes of synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]; (ii) two sources of DAG, one from the PI-PLC pathway and the other probably from phosphatidylcholine; (iii) several isoforms of PKC translocating to nuclei; (iv) increases in activity of the PI-PLC pathway at two points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P3, which may have several functions, including a role in the transfer of mRNA out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and, in particular, 3-phosphorylated inositol lipids, which are now emerging as possible major players in nuclear signaling.

Phosphatidylinositol transfer protein beta displays minimal sphingomyelin transfer activity and is not required for biosynthesis and trafficking of sphingomyelin

Mammalian phosphatidylinositol transfer proteins (PITPs) alpha and beta, which share 77% identity, have been shown to exhibit distinct lipid-transfer activities. In addition to transferring phosphatidylinositol (PI) and phosphatidylcholine (PC), PITPbeta has been shown to transfer sphingomyelin (SM), and this has led to the suggestion that PITPbeta is important for the regulation of SM metabolism. In the present study, we have analysed the ability of human PITPbeta to transfer and regulate the metabolism of cellular SM. We report that, in vitro, the two PITP isoforms were comparable in mediating PI, PC or SM transfer. Using permeabilized HL-60 cells as the donor compartment, both PITP isoforms efficiently transferred PI and PC, and were slightly active towards SM, with the activity of PITPbeta being slightly greater. To identify which cellular lipids were selected by PITPs, PITPalpha and PITPbeta were exposed to permeabilized HL-60 cells, and subsequently repurified and analysed for their bound lipids. Both PITPs were able to select only PI and PC, but not SM. SM synthesis takes place at the Golgi, and PITPbeta was shown to localize in that compartment. To examine the role of PITPbeta in SM biosynthesis, Golgi membranes were used. Purified Golgi membranes had lost their endogenous PITPbeta, but were able to recruit PITPbeta when added exogenously. However, PITPbeta did not enhance the activities of either SM synthase or glucosylceramide synthase. Further analysis in COS-7 cells overexpressing PITPbeta showed no effects on (a) SM and glucosylceramide biosynthesis, (b) diacylglycerol or ceramide levels, (c) SM transport from the Golgi to the plasma membrane, or (d) resynthesis of SM after exogenous sphingomyelinase treatment. Altogether, these observations do not support the suggestion that PITPbeta participates in the transfer of SM, the regulation of SM biosynthesis or its intracellular trafficking.

The Golgi localization of phosphatidylinositol transfer protein beta requires the protein kinase C-dependent phosphorylation of serine 262 and is essential for maintaining plasma membrane sphingomyelin levels

Recombinant mouse phosphatidylinositol transfer protein (PI-TP)beta is a substrate for protein kinase C (PKC)-dependent phosphorylation in vitro. Based on site-directed mutagenesis and two-dimensional tryptic peptide mapping, Ser(262) was identified as the major site of phosphorylation and Ser(165) as a minor phosphorylation site. The phospholipid transfer activities of wild-type PI-TP beta and PI-TP beta(S262A) were identical, whereas PI-TP beta(S165A) was completely inactive. PKC-dependent phosphorylation of Ser(262) also had no effect on the transfer activity of PI-TP beta. To investigate the role of Ser(262) in the functioning of PI-TP beta, wtPI-TP beta and PI-TP beta(S262A) were overexpressed in NIH3T3 fibroblast cells. Two-dimensional PAGE analysis of cell lysates was used to separate PI-TP beta from its phosphorylated form. After Western blotting, wtPI-TP beta was found to be 85% phosphorylated, whereas PI-TP beta(S262A) was not phosphorylated. In the presence of the PKC inhibitor GF 109203X, the phosphorylated form of wtPI-TP beta was strongly reduced. Immunolocalization showed that wtPI-TP beta was predominantly associated with the Golgi membranes. In the presence of the PKC inhibitor, wtPI-TP beta was distributed throughout the cell similar to what was observed for PI-TP beta(S262A). In contrast to wtPI-TP beta overexpressors, cells overexpressing PI-TP beta(S262A) were unable to rapidly replenish sphingomyelin in the plasma membrane upon degradation by sphingomyelinase. This implies that PKC-dependent association with the Golgi complex is a prerequisite for PI-TP beta to express its effect on sphingomyelin metabolism.

Genetic ablation of phosphatidylinositol transfer protein function in murine embryonic stem cells

Phosphatidylinositol transfer proteins (PITPs) regulate the interface between signal transduction, membrane-trafficking, and lipid metabolic pathways in eukaryotic cells. The best characterized mammalian PITPs are PITP alpha and PITP beta, two highly homologous proteins that are encoded by distinct genes. Insights into PITP alpha and PITP beta function in mammalian systems have been gleaned exclusively from cell-free or permeabilized cell reconstitution and resolution studies. Herein, we report for the first time the use of genetic approaches to directly address the physiological functions of PITP alpha and PITP beta in murine cells. Contrary to expectations, we find that ablation of PITP alpha function in murine cells fails to compromise growth and has no significant consequence for bulk phospholipid metabolism. Moreover, the data show that PITP alpha does not play an obvious role in any of the cellular activities where it has been reconstituted as an essential stimulatory factor. These activities include protein trafficking through the constitutive secretory pathway, endocytic pathway function, biogenesis of mast cell dense core secretory granules, and the agonist-induced fusion of dense core secretory granules to the mast cell plasma membrane. Finally, the data demonstrate that PITP alpha-deficient cells not only retain their responsiveness to bulk growth factor stimulation but also retain their pluripotency. In contrast, we were unable to evict both PITP beta alleles from murine cells and show that PITP beta deficiency results in catastrophic failure early in murine embryonic development. We suggest that PITP beta is an essential housekeeping PITP in murine cells, whereas PITP alpha plays a far more specialized function in mammals than that indicated by in vitro systems that show PITP dependence.

Both isoforms of mammalian phosphatidylinositol transfer protein are capable of binding and transporting sphingomyelin

The structurally related mammalian alpha and beta isoforms of phosphatidylinositol (PtdIns) transfer protein (PITP) bind reversibly a single phospholipid molecule, preferably PtdIns or phosphatidylcholine (PtdCho), and transport that lipid between membrane surfaces. PITPbeta, but not PITPalpha, is reported extensively in the scientific literature to exhibit the additional capacity to bind and transport sphingomyelin (CerPCho). We undertook a detailed investigation of the lipid binding and transfer specificity of the soluble mammalian PITP isoforms. We employed a variety of donor and acceptor membrane lipid compositions to determine the sensitivity of recombinant rat PITPalpha and PITPbeta isoforms toward PtdIns, PtdCho, CerPCho, and phosphatidate (PtdOH). Results indicated often striking differences in protein-phospholipid and protein-membrane interactions. We demonstrated unequivocally that both isoforms were capable of binding and transferring CerPCho; we confirmed that the beta isoform was the more active. The order of transfer specific activity was similar for both isoforms: PtdIns>PtdCho>CerPCho>>PtdOH. Independently, we verified the binding of CerPCho to both isoforms by showing an increase in holoprotein isoelectric point following the exchange of protein-bound phosphatidylglycerol for membrane-associated CerPCho. We conclude that PITPalpha and PITPbeta are able to bind and transport glycero- and sphingophospholipids.