A comparison of the molecular specificities of whole cell and endonuclear phosphatidylcholine synthesis

Insight into erythrocyte phospholipid molecular flux in healthy humans and in patients with acute respiratory distress syndrome

Although the distribution of cellular membrane phospholipid composition is well characterised in human erythrocytes, in-vivo turnover and dynamic flux of phospholipids between plasma and erythrocytes in physiological and in particular during disease states are mostly unknown. Erythrocyte mass primarily consisted of lipids and phosphatidylcholine (PC) contributes to the significant proportion of phospholipid membrane composition. Esterified membrane PC can be utilised during pathological processes to generate pro and anti-inflammatory lipid mediators, which can contribute to the pathogenesis of acute respiratory distress syndrome (ARDS). In this study, utilising isotope labelling of choline and analytical methods with electrospray mass spectrometry (ESI-MS/MS), we characterised individual molecular composition and dynamic exchange of PC, sphingomyelins (SM) and lysophosphatidylcholines (LPC) between plasma and erythrocytes. In ARDS patients, there were significant alterations in PC molecular composition, coupled with a continuous loss of arachidonoyl-PC species over time. Infusion of methyl-D9-choline chloride resulted in enrichment of labelled choline into plasma PC and LPC via CDP-choline pathway with subsequent incorporation into erythrocyte PC. As expected, erythrocyte methyl-D9 PC enrichment is much slower than plasma. Patients had much faster and higher fractional enrichment of all PC and LPC molecules suggesting increased flux between plasma and erythrocytes. There was a particular pattern of incorporation, where the arachidonoyl-PC species achieved equilibrium with plasma rapidly and retained highest concentrations of enrichment compared to the other PC species. Increased enrichment of arachidonoyl-PC coupled with virtually no increase or depletion of its concentrations suggests the possibility of substrate donation for other cell types for the participation of eicosanoid biosynthesis during inflammatory conditions like ARDS. In summary, this study revealed an alerted pattern erythrocyte molecular phospholipid composition and flux in patients with acute respiratory distress syndrome and the pathological consequences of these changes needs further exploration.

Formation of inverse topology lyotropic phases in dioleoylphosphatidylcholine/oleic acid and dioleoylphosphatidylethanolamine/oleic acid binary mixtures

The addition of saturated fatty acids (FA) to phosphatidylcholine lipids (PC) that have saturated acyl chains has been shown to promote the formation of lyotropic liquid-crystalline phases with negative mean curvature. PC/FA mixtures may exhibit inverse bicontinuous cubic phases (Im3m, Pn3m) or inverse topology hexagonal phases (HII), depending on the length of the acyl chains/fatty acid. Here we report a detailed study of the phase behavior of binary mixtures of dioleoylphosphatidylcholine (DOPC)/oleic acid (OA) and dioleoylphosphatidylethanolamine (DOPE)/oleic acid at limiting hydration, constructed using small-angle X-ray diffraction (SAXD) data. The phase diagrams of both systems show a succession of phases with increasing negative mean curvature with increasing OA content. At high OA concentrations, we have observed the occurrence of an inverse micellar Fd3m phase in both systems. Hitherto, this phase had not been reported for phosphatidylethanolamine/fatty acid mixtures, and as such it highlights an additional route through which fatty acids may increase the propensity of bilayer lipid membranes to curve. We also propose a method that uses the temperature dependence of the lattice parameters of the HII phases to estimate the spontaneous radii of curvature (R0) of the binary mixtures and of the component lipids. Using this method, we calculated the R0 values of the complexes comprising one phospholipid molecule and two fatty acid molecules, which have been postulated to drive the formation of inverse phases in PL/FA mixtures. These are -1.8 nm (±0.4 nm) for DOPC(OA)2 and -1.1 nm (±0.1 nm) for DOPE(OA)2. R0 values estimated in this way allow the quantification of the contribution that different lipid species make to membrane curvature elastic properties and hence of their effect on the function of membrane-bound proteins.

Application of stable isotopes to investigate the metabolism of fatty acids, glycerophospholipid and sphingolipid species

Nature provides an enormous diversity of lipid molecules that originate from various pathways. To gain insight into the metabolism and dynamics of lipid species, the application of stable isotope-labeled tracers combined with mass spectrometric analysis represents a perfect tool. This review provides an overview of strategies to track fatty acid, glycerophospholipid, and sphingolipid metabolism. In particular, the selection of stable isotope-labeled precursors and their mass spectrometric analysis is discussed. Furthermore, examples of metabolic studies that were performed in cell culture, animal and clinical experiments are presented.

Structure of SF-1 bound by different phospholipids: evidence for regulatory ligands

Despite the fact that many nuclear receptors are ligand dependent, the existence of obligate regulatory ligands is debated for some receptors, including steroidogenic factor 1 (SF-1). Although fortuitously bound bacterial phospholipids were discovered in the structures of the SF-1 ligand-binding domain (LBD), these lipids might serve merely as structural ligands. Thus, we examined whether exogenously added phospholipids would exchange for these bacterial lipids and bind to SF-1. Here, we report the first crystal structure of the SF-1 LBD bound by the exchanged phosphatidylcholine. Although the bound phosphatidylcholine phospholipid mimics the conformation of bound bacterial phosphoplipids, two surface loops, L2-3 and L11-12, surrounding the entrance to the pocket vary significantly between different SF-1 LBD structures. Based on this observation, we hypothesized that a bound ligand might control the conformations of loops L2-3 and L11-12, and that conserved residues in these dynamic loops could influence ligand binding and the receptor function. Consistent with this hypothesis, impaired phospholipid exchange and diminished transcriptional activity were observed for loop L11-12 SF-1 mutants and for the loop L2-3 human mutant R255L. The endocrine disease associated with this L2-3 mutation coupled with our cellular and biochemical data suggest that critical residues at the mouth of the ligand-binding pocket have evolved for efficient binding of phospholipid ligands and for achieving optimal SF-1 activity.

Developmental changes in rat surfactant lipidomics in the context of species variability

Lung surfactant comprises mainly phosphatidylcholine (PC) species together with phosphatidylglycerols and surfactant proteins (SP) SP-A to -D. Changes in the concentrations of its principal components dipalmitoyl-PC, palmitoylmyristoyl-PC, palmitoylpalmitoleoyl-PC relative to developmental, structural and physiological differences are only partially understood. Particularly, their attribution to differences in air-liquid interface curvature, compared with dynamic parameters, such as respiratory rate, are controversial. We postulated that during alveolarization the changes in these principal PC components of surfactant differ from those in other phospholipid parameters, and that across endothermic vertebrates their concentrations are related to lung physiology rather than structure. We therefore investigated in rats from postnatal day (d)1 to d42 the pattern of surfactant phospholipids relative to alveolarization (d4-d14), and we discuss these changes in terms of molecular adaptation to pulmonary structure or physiology. Contrary to mammals with advanced alveolarization and increased respiratory rate (RR) at term, concentrations of dipalmitoyl-PC (49-52%) and palmitoylmyristoyl-PC (7-9%) in lung lavage fluid were identical at d1 and d42. At d7-d14, when in rats RR is increased, palmitoyl-myristoyl-PC transiently increased by 2.5- to 3.9-fold at the expense of dipalmitoyl-PC (-32% to 34%) and palmitoyl-palmitoleoyl-PC (-16%). Other lipidomic changes followed essentially different patterns of increase or decrease. Palmitoyl-myristoyl-PC was increased in large aggregates suggesting that it is an integral component of active surfactant. In the overall context of vertebrates, irrespective of age and lung structure, fractions of palmitoyl-myristoyl-PC, dipalmitoyl-PC and palmitoyl-palmitoleoyl-PC correlate with differences in RR rather than alveolar curvature. In adult mammals, however, only concentrations of palmitoyl-palmitoleoyl-PC correlate with RR.

Probing phospholipid dynamics by electrospray ionisation mass spectrometry

Recent advances in electrospray ionisation mass spectrometry (ESI-MS) have greatly facilitated the analysis of phospholipid molecular species in a growing diversity of biological and clinical settings. The combination of ESI-MS and metabolic labelling employing substrates labelled with stable isotopes is especially exciting, permitting studies of phospholipid synthesis and turnover in vivo. This review will first describe the methodology involved and will then detail dynamic lipidomic studies that have applied the stable isotope incorporation approach. Finally, it will summarise the increasing number of studies that have used ESI-MS to characterise structural and signalling phospholipid molecular species in development and disease.

Lipidomic analysis of signaling pathways

This chapter outlines methods that can be applied to determine the levels of lipids in cells and tissues. In particular, the methods focus upon the extraction and analysis of those lipids critical for monitoring signal transduction pathways. The methods address the analysis of the phosphoinositides, the lipid agonists lysophosphatidic acid and sphingosine 1-phosphate, and the neutral lipid messengers diacylglycerol and ceramide. Additionally, because of the increasing need to determine the dynamics of signaling, the analysis of phospholipids synthesis using stable isotope methods is described. The use of these methods as described or adaptation to permit both approaches should allow investigators to determine changes in signaling lipids and to better understand such processes in most cell types. The increasing appreciation of the central roles played by lipid signaling pathways has dispelled the misconception that lipids are inert structural components that are involved solely in keeping a cell intact. Advances in our understanding of cell-signaling pathways have identified particular lipids that act to regulate the functions of a number of proteins either by controlling enzyme activity directly, or by localizing proteins to particular intracellular compartments where they perform a specialized role. These lipid-binding domains (e.g., PH, PX, FYVE) have been found in many proteins, and considerable detail is recorded of the structural basis of lipid protein interaction. Additional lipid-binding domains exist, which remain less well characterized (e.g., those that bind phosphatidic acid [PA] or ceramide); however, the important regulatory roles that these lipids play and the pathways involving these messengers are increasingly appreciated. While the downstream targets are thus being defined, the actual changes in lipid concentration in a stimulated cell or membrane are less characterized. The primary reason for this lack has been a deficiency in methodology. Much of the reported studies of lipid messengers in stimulated cells have depended upon monitoring changes in radio-labeled cells. Many well-documented problems are associated with this type of methodology, including lack of isotopic equilibrium, distinct pools with different turnover rates, and inadequate separation of radio-labeled metabolites; however, much important information has been generated. The second approach has been to make use of the lipid-binding properties of the target protein domains and to generate a tagged fusion protein, generally GFP, which permits identification of a region rich in a signaling lipid (Guillou et al., 2007). This has proved useful in monitoring PI-3-kinase activation in stimulated cells; however, considerable caveats must be raised, not least the problems associated with lipid specificity and the fact that many of these domains have associated protein-binding regions that can compromise the findings. A further problem associated with these two methodologies is that they tend to group lipids together and take no account of the multiple acyl chain structures that occur in all lipids. These concerns point to the need to determine actual changes in lipid compositions. Until relatively recently, such an analysis was unachievable; however, advances in both chromatographic separation and mass spectrometry (MS) have permitted the development of lipidomic analysis. This chapter outlines a number of methods that allow determination of changes in signaling lipids. Adaptation of the methods here for the analysis of other molecules should be relatively straightforward in the future. Much of the lipidomic research in the United Kingdom is focused upon signaling lipidomics, with particular foci upon phosphoinositide-related signaling in Birmingham and Cambridge (Wakelam) and London (Larijani), upon eicosanoids in Cardiff (O'Donnell), and steroids in London (Griffiths). Meanwhile, the use of stable isotopes has been particularly developed in Southampton (Postle).

Endonuclear lipids in liver cells

Lipids are not only components of cell nucleus membranes, but are also found in the membrane-depleted nuclei where they fulfill special functions. We have investigated the lipid composition of membrane-depleted rat liver nuclei obtained by incubation with low Triton X-100 concentrations of 0.04% and 0.08%, which rendered them unaltered or hardly altered. Under these conditions, 26% of proteins and 22% of phospholipids were recovered. The main phospholipids were phosphatidylcholine > phosphatidylethanolamine > phosphatidylinositol = or > phosphatidylserine and sphingomyelin (in decreasing concentrations). The fatty acid components of total lipids and phosphatidylcholine were mainly unsaturated. Over 40% belonged to the n-6 series (arachidonic > or = 25% and linoleic 15%); approximately 40% corresponded to saturated acids and <10% were monoenoic. Endonuclear phosphatidylcholine was built up by 16 molecular species, the most abundant being 18:0-20:4 (32%), 16:0-20:4 (19%), 16:0-18:2 (13%), and 18:0-18:2 (11%). The fatty acid composition and phosphatidylcholine molecular species distribution in the membrane-depleted nucleus of rat liver showed patterns similar to the whole nucleus, mitochondria, microsomes, and homogenate of the parent liver cells, suggesting that endonuclear lipid pool composition is mainly determined by a liver organ profile.

Subcellular localization of phosphatidylinositol synthesis

It is well-established that the endoplasmic reticulum is the major site of phosphatidylinositol (PtdIns) synthesis. The PtdIns synthetic ability of other organelles, such as plasma membrane and nucleus, remains controversial. In the present study, we re-examine this question by comparing PtdIns synthesis in isolated cytoplasts (enucleated cells) with that in corresponding karyoplasts (nuclei surrounded by plasma membrane but lacking most cytoplasmic components). We report that cytoplasts are competent to carry out both basal and stimulated PtdIns synthesis as well as polyphosphoinositide hydrolysis, while karyoplasts can neither synthesize PtdIns nor hydrolyze phosphoinositides in response to agonists. The karyoplasts are, however, capable of synthesizing phosphatidylcholine (PtdCho), as previously reported. From these data, we conclude that PtdIns synthesis is limited to cytoplasmic components, and cannot be sustained by either plasma membrane or nucleus under conditions that permit robust PtdCho synthesis.

Mass spectrometry determination of endonuclear phospholipid composition and dynamics

Mammalian cell lipid analyses using tandem electrospray ionization mass spectrometry, in conjunction with stable isotope labeling, permit unparalleled access to membrane phospholipid molecular species compositions and turnover. Lipidomic data from isolable compartments of lipid second messenger generation, such as membrane-free nuclei, can provide dynamic insights into the topology of phospholipid turnover. For example, ESI-MS/MS precursor scans of characteristic phosphocholine m/z 184(+) fragments reveal a highly saturated endonuclear phosphatidylcholine pool with homeostatic maintenance properties. A spatially distinct CDPcholine pathway yields, within minutes of choline-d(9) labeling, unsaturated endonuclear phosphatidylcholines progressively remodeled to more saturated species evidenced by tracking the deuteriated headgroup through precursor scans of phosphocholine-d(9) (m/z 193(+) fragment). Among the other endonuclear phospholipids, diacyl phosphatidylethanolamines (neutral loss of m/z 141(+)) are also highly saturated compared with those of whole cell whereas, phophatidylinositols (precursor scans of m/z 241(-) fragment) are essentially identical in nuclei and whole cells. Moreover, the pattern of myo-inositol-d(6) acquisition into endonuclear phosphatidylinositol (precursor scans of m/z 247(-) fragment) is inconsistent with compartment-specific synthesis. Endonuclear sphingomyelins (seen in precursor scans of m/z 184(+) and confirmed from precursor scans of m/z 168(-) fragments) are enriched but similar in composition to whole cell species whereas endonuclear phosphatidylserines (neutral loss of m/z 87(-)) are more saturated than their whole cell counterparts. The focus of described methodologies emphasize their value in probing the compositions and dynamics of endonuclear phospholipids, but in principle may be extended to exploration of other isolable compartments including ER or plasma membranes.

Identification of mammalian cell lines using MALDI-TOF and LC-ESI-MS/MS mass spectrometry

Direct mass spectrometric analysis of complex biological samples is becoming an increasingly useful technique in the field of proteomics. Matrix-assisted laser desorption/ionization mass spectroscopy (MALDI-MS) is a rapid and sensitive analytical tool well suited for obtaining molecular weights of peptides and proteins from complex samples. Here, a fast and simple approach to cellular protein profiling is described in which mammalian cells are lysed directly in the MALDI matrix 2,5-dihydroxybenzoic acid (DHB) and mass analyzed using MALDI-time of flight (TOF). Using the unique MALDI mass spectral "fingerprint" generated in these analyses, it is possible to differentiate among several different mammalian cell lines. A number of techniques, including MALDI-post source decay (PSD), MALDI tandem time-of-flight (TOF-TOF), MALDI-Fourier transform ion cyclotron resonance (FTICR), and nanoflow liquid chromatography followed by electrospray ionization and tandem mass spectrometry (LC-ESI-MS/MS) were employed to attempt to identify the proteins represented in the MALDI spectra. Performing a tryptic digestion of the supernatant of the cells lysed in DHB with subsequent LC-ESI-MS/MS analysis was by far the most successful method to identify proteins.

Completing the cycles; the dynamics of endonuclear lipidomics

Signal transductions via periodic generation and mobilisation of lipid second messengers within the nuclear matrix of eukaryotic cells have focused renewed attention on their precursor phospholipids' location, structure, form and function. The nuclear matrix contains and supports dynamic pools of phosphatidylcholine and phosphatidylinositol which serve as parent molecules of lipid second messengers but also of other phospholipids requiring cyclical replacement as cells proliferate. Applications of new, highly sensitive and specific analytical methodologies based on tandem electrospray ionisation mass spectrometry and the use of stable isotopes have allowed both static and dynamic lipidomic profiling of these endonuclear phospholipid pools. Together with more conventional enzymatic analyses and evaluation of the effect of specific "knock-out" of phospholipid transfer capacity, a number of important principles have been established. Specifically, a compartmental capacity to synthesise and remodel highly saturated phosphatidylcholine exists alongside transport mechanisms that facilitate the nuclear import of phosphatidylinositol and other phospholipids synthesised elsewhere within the cell. Subnuclear fractionation and the use of newly emerging techniques for sensitive lipidomic profiling of polyphosphoinositides, diacylglycerols and phosphatidate molecular species offer the potential for further significant advances in the near future.

Dynamic lipidomics of the nucleus

Once nuclear envelope membranes have been removed from isolated nuclei, around 6% of mammalian cell phospholipid is retained within the nuclear matrix, which calculations suggest may occupy 10% of the volume of this subcellular compartment. It is now acknowledged that endonuclear phospholipid, largely ignored for the past 40 years, provides substrate for lipid-mediated signaling events. However, given its abundance, it likely also has other as yet incompletely defined roles. Endonuclear phosphatidylcholine is the predominant phospholipid comprising distinct and highly saturated molecular species compared with that of the whole cell. Moreover, this unusual composition is subject to tight homeostatic maintenance even under conditions of extreme dietary manipulation, presumably reflecting a functional requirement for highly saturated endonuclear phosphatidylcholine. Recent application of new lipidomic technologies exploiting tandem electrospray ionization mass spectrometry in conjunction with deuterium stable isotope labeling have permitted us to probe not just molecular species compositions but endonuclear phospholipid acquisition and turnover with unparalleled sensitivity and specificity. What emerges is a picture of a dynamic pool of endonuclear phospholipid subject to autonomous regulation with respect to bulk cellular phospholipid metabolism. Compartmental biosynthesis de novo of endonuclear phosphatidylcholine contrasts with import of phosphatidylinositol synthesized elsewhere. However, irrespective of the precise temporal-spatial-dynamic relationships underpinning phospholipid acquisition, derangement of endonuclear lipid-mediated signaling from these parental phospholipids halts cell growth and division indicating a pivotal control point. This in turn defines the manipulation of compartmentalized endonuclear phospholipid acquisition and metabolism as intriguing potential targets for the development of future antiproliferative strategies.

Use of mass spectrometry-based lipidomics to probe PITPalpha (phosphatidylinositol transfer protein alpha) function inside the nuclei of PITPalpha+/+ and PITPalpha-/- cells

The mammalian phospholipid exchange protein PITPalpha (phosphatidylinositol transfer protein alpha), found in both extranuclear and endonuclear compartments, is thought in part to facilitate nuclear import of the PtdIns (phosphatidylinositol) consumed in the generation of proliferation-associated endonuclear diacylglycerol accumulations. Unlike phosphatidylcholine, endonuclear PtdIns is not synthesized in situ. However, despite progressive postnatal lethality of PITPalpha ablation in mice, PITPalpha(-/-) MEF (mouse embryonic fibroblasts) lack an obviously impaired proliferative capacity. We used ESI-MS (tandem electrospray ionization-MS) to monitor incorporation of the deuterated phospholipid precursors, choline-d(9) and inositol-d(6), into molecular species of whole cell and endonuclear phosphatidylcholine and PtdIns over 24 h to assess the contribution of PITPalpha to the nuclear import of PtdIns into MEF cells. In cells labelled for 1, 3, 6, 12 and 24 h fractional inositol-d(6) incorporation into whole-cell PtdIns species was consistently higher in PITPalpha(-/-) MEF implying greater flux through its biosynthetic pathway. Moreover, endonuclear accumulation of PtdIns-d(6) was apparent in the PITPalpha(-/-) cells and mirrored that in PITPalpha(+/+) cells. Together, these results suggest that the essential endonuclear PtdIns import via PITPalpha can be accommodated by other mechanisms.

Nuclear envelope assembly is promoted by phosphoinositide-specific phospholipase C with selective recruitment of phosphatidylinositol-enriched membranes

Nuclear envelope (NE) formation in a cell-free egg extract proceeds by precursor membrane vesicle binding to chromatin in an ATP-dependent manner, followed by a GTP-induced NE assembly step. The requirement for GTP in the latter step of this process can be mimicked by addition of bacterial PI-PLC [phosphoinositide (PtdIns)-specific phospholipase C]. The NE assembly process is here dissected in relation to the requirement for endogenous phosphoinositide metabolism, employing recombinant eukaryotic PI-PLC, inhibitors and direct phospholipid analysis using ESI-MS (electrospray ionization mass spectrometry). PtdIns (phosphatidylinositol) species analysis by ESI-MS indicates that the chromatin-bound NE precursor vesicles are enriched for specific PtdIns species. Moreover, during GTP-induced precursor vesicle fusion, the membrane vesicles become partially depleted of the PtdIns 18:0/20:4 species. These data indicate that eukaryotic PI-PLC can support NE formation, and the sensitivity to exogenous recombinant PtdIns-5-phosphatases shows that the endogenous PLC hydrolyses a 5-phosphorylated species. It is shown further that the downstream target of this DAG (diacylglycerol) pathway does not involve PKC (protein kinase C) catalytic function, but is mimicked by phorbol esters, indicating a possible engagement of one of the non-PKC phorbol ester receptors. The results show that ESI-MS can be used as a sensitive means to measure the lipid composition of biological membranes and their changes during, for example, membrane fusogenic events. We have exploited this and the intervention studies to illustrate a pivotal role for PI-PLC and its product DAG in the formation of NEs.

Acyl chain-based molecular selectivity for HL60 cellular phosphatidylinositol and of phosphatidylcholine by phosphatidylinositol transfer protein α

Mammalian phosphatidylinositol transfer protein alpha (PITP) is an intracellular lipid transporter with a binding site that can accommodate a single molecule of phosphatidylinositol (PI) or phosphatidylcholine (PC). Phospholipids are a heterogeneous population of molecular species that can be distinguished by their characteristic headgroups as well as their acyl chains at the sn-1 and sn-2 position. In this study, we have defined the acyl chain preference for PITPalpha when presented with a total population of cellular lipids. Recombinant PITPalpha loaded with bacterial lipid, phosphatidylglycerol (PG), was incubated with permeabilised HL60 cells, followed by recovery of PITPalpha by affinity chromatography. Lipids extracted from the PITPalpha were analysed by tandem electrospray ionisation mass spectrometry (ESI-MS) and showed total exchange of acquired bacterial lipids for HL60 cellular PI and PC. Detailed comparison of the molecular species composition of bound phospholipids with those in whole cells permitted the assessment of selectivity of acyl chain binding. For both phospholipid classes, progressive fractional enrichments in bound species possessing shorter acyl chains were apparent with a preference order: 16:1>16:0>18:1>18:0>20:4. A recapitulation of this specificity order was also seen from a dramatically altered range of molecular species present in HL60 cells enriched with arachidonate over many weeks of culture. We speculate that short-chain, saturate-binding preferences under both conditions may reflect properties in vivo. This is consistent with target cell membranes actively remodelling newly delivered phospholipids after transport rather than relying on the transport of the specific molecular species conventionally found in mammalian membranes.

Lipidomic analysis of the molecular specificity of a cholinephosphotransferase in situ

Dynamic lipidomics using ESI–MS (tandem electrospray ionization mass spectrometry) of 9-deuterated choline (choline-d9) incorporation into mammalian cell PtdCho (phosphatidylcholine) permits assessment of the molecular specificity of synthesis. Bulk cell PtdCho synthesis occurs in spatially distinct locations, using separate CPTs (1,2 diacylglycerol CDP:choline cholinephosphotransferases). We assessed whether in vitro molecular selectivity of DAG (diacylglycerol) incorporation between CPTs is manifest in situ, by monitoring choline-d9 incorporation into PtdCho and lyso-PtdCho molecular species over 3 h in control cells and in CHO-K1 cells overexpressing hCEPT1. Compared with controls, the basal molecular species composition of hCEPT1 overexpressors was significantly enriched in arachidonate. This was not due to net accretion of cellular PtdCho arguing against effects of inadequate unsaturated PtdCho degradation or remodelling. Rather, time-course analyses of PtdCho and lyso-PtdCho pools showed that both arachidonate-containing DAG incorporation and turnover of PtdCho is increased in hCEPT1 overexpressors. Increased choline-d9 incorporation into arachidonyl lyso-PtdCho shows that both phospholipase A1- and A2-mediated turnover is involved. Spatially distinct molecular specificity of DAG incorporation into cellular PtdCho at the level of hCEPT1 exists in situ.

Mass Spectrometric Analysis of Surfactant Metabolism in Human Volunteers Using Deuteriated Choline

Surfactant reduces surface tension at pulmonary air-liquid interfaces. Although its major component is dipalmitoyl-phosphatidylcholine (PC16:0/16:0), other PC species, principally palmitoylmyristoyl-PC, palmitoylpalmitoleoyl-PC, and palmitoyloleoyl-PC, are integral components of surfactant. The composition and metabolism of PC species depend on pulmonary development, respiratory rate, and pathologic alterations, which have largely been investigated in animals using radiolabeled precursors. Recent advances in mass spectrometry and availability of precursors carrying stable isotopes make metabolic experiments in human subjects ethically feasible. We introduce a technique to quantify surfactant PC synthesis in vivo using deuteriated choline coupled with electrospray ionization tandem mass spectrometry. Endogenous PC from induced sputa of healthy volunteers comprised 54.0 +/- 1.5% PC16:0/16:0, 9.7 +/- 0.7% palmitoylmyristoyl-PC, 10.0 +/- 1.0% palmitoylpalmitoleoyl-PC, and 13.1 +/- 0.3% palmitoyloleoyl-PC. Infusion of deuteriated choline chloride (3.6 mg/kg body weight) over 3 hours resulted in linear incorporation into PC over 30 hours. After a plateau of 0.61 +/- 0.04% labeled PC between 30 and 48 hours, incorporation decreased to 0.30 +/- 0.02% within 7 days. Compared with native PC, fractional label was initially lower for PC16:0/16:0 (31.9 +/- 8.3%) but was higher for palmitoyloleoyl-PC (21.0 +/- 1.2%), and equilibrium was achieved after only 48 hours. We conclude that infusion of deuteriated choline and electrospray ionization tandem mass spectrometry is useful to investigate surfactant metabolism in humans in vivo.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

Nuclear lipids: key signaling effectors in the nervous system and other tissues

Lipids have long been recognized as quantitatively minor components of the nucleus, where they were initially thought to have little functional importance; but they now command growing interest, with recognition of their diverse signaling and modulating properties in that organelle. This applies to the lipid-poor compartments of the nucleoplasm as well as the relatively lipid-rich nuclear envelope. Phosphoglycerides and sphingomyelin, as the predominant lipids, have attracted the most interest among researchers, but some of the less-abundant lipids such as gangliosides, sphingosine, and sphingosine phosphate are now becoming recognized as functionally important nuclear constituents. Among recent advances in this emerging field are detailed findings on the metabolic enzymes that synthesize and catabolize nuclear lipids; the fact that these are localized primarily within the nucleus itself indicates considerable autonomy with respect to lipid metabolism. Current studies suggest several key processes involving RNA and DNA reactivity that are dependent on these lipid-initiated events. Neural cell nuclei have been the subject of such investigations, with results that closely parallel the more numerous studies on nuclei of extraneural cells. This review attempts to outline some of the major findings on nuclear lipids of diverse cell types; results with nonneural nuclei will hopefully provide useful guideposts to further studies of neural systems.