Regulation of lung surfactant phospholipid synthesis and metabolism

The alveolar type II epithelial (ATII) cell is highly specialised for the synthesis and storage, in intracellular lamellar bodies, of phospholipid destined for secretion as pulmonary surfactant into the alveolus. Regulation of the enzymology of surfactant phospholipid synthesis and metabolism has been extensively characterised at both molecular and functional levels, but understanding of surfactant phospholipid metabolism in vivo in either healthy or, especially, diseased lungs is still relatively poorly understood. This review will integrate recent advances in the enzymology of surfactant phospholipid metabolism with metabolic studies in vivo in both experimental animals and human subjects. It will highlight developments in the application of stable isotope-labelled precursor substrates and mass spectrometry to probe lung phospholipid metabolism in terms of individual molecular lipid species and identify areas where a more comprehensive metabolic model would have considerable potential for direct application to disease states.     

Quantitative determination of phospholipid compositions by ESI-MS: effects of acyl chain length, unsaturation, and lipid concentration on instrument response

Electrospray ionization-mass spectrometry (ESI-MS) is a very promising tool for the analysis of phospholipid compositions, but is hampered by the fact that not all molecular species are detected with equal efficiency. We studied this and other issues that need to be taken into account to obtain truly quantitative compositional data. The key findings were as follows: First, the instrument response for both saturated and unsaturated phospholipid species decreased with increasing acyl chain length. This effect became increasingly prominent with increasing overall lipid concentration. Second, the degree of acyl chain unsaturation also had a significant effect on instrument response. At the highest concentration studied (10 pmol/microl), polyunsaturated species gave 40% higher intensity than the fully saturated ones. The effect of unsaturation diminished and nearly disappeared with progressive dilution. Third, the instrument response for the different head group classes varied markedly depending on the infusion solvent used. Notably, inclusion of ammonia in the infusion solvent eliminated sodium adduct formation in the positive ion mode, thus greatly simplifying the interpretation of the spectra. The fact that instrument response is dependent on many structural features, overall lipid concentration, solvent composition, and instrument settings makes it necessary to include several internal standards for each phospholipid class to obtain accurate data. Preferably, both unsaturated and saturated standards should be used. Finally, we quantified the major phospholipid classes of BHK cells using ESI-MS. The data agreed closely with those obtained with thin-layer chromatography and phosphorus analysis. This study indicates that quantitative compositional data can be obtained with ESI-MS, provided that proper attention is paid to experimental details, particularly the choice of internal standards.     

Effects of small organic molecules on phospholipid phase transitions

Small organic molecules are known to exhibit a wide spectrum of physiological or pharmacological effects and many of them are thought to be membrane associated. Therefore a great number of studies is devoted to the interaction between these molecules and phospholipid model membranes. Results obtained for molecular species of varying hydrophobic/hydrophilic balances will be described. It will be shown that, in general, these different molecules induce similar effects on phospholipid phase transitions, although they are located differently in the membrane. Detailed studies of these interactions will help to understand these processes on a molecular level.     

Molecular species composition of rat liver phospholipids by ESI-MS/MS: the effect of chromatography.

Using electrospray ionization tandem mass spectrometry (ESI-MS/MS) this study shows that the loss of glycerophospholipid (GPL) after chromatography was unevenly distributed across the GPL molecular species. Both TLC and HPLC caused a preferential loss of GPL with 0 to 3 double bonds: 20% and 7.2% for choline glycerophosphates (PC) and 19.7% and 7.5% for ethanolamine glycerophosphates (PE), respectively. A consequence of these losses was that GPLs containing fatty acids with four or more double bonds had a greater contribution to the total after chromatography. ESI-MS/MS analysis also showed that PC molecular species with four or more double bonds migrated at the front of the TLC band of PCs. GPLs extracted from TLC plates occasionally contained PCs that were smaller than those in the original extract. These low molecular mass PCs were easily reduced to alcohols and formed derivatives with 2,4-dinitrophenylhydrazine, suggesting that aldehydes were generated by the oxidation of unsaturated fatty acids. Directly analyzing lipid extracts by ESI-MS/MS without preliminary chromatographic separation gives an accurate distribution of GPL molecular species in lipid mixtures. However, the ionization of the phospholipids in the electrospray jet maximized at relatively low concentrations of GPL. There was a linear response between phospholipid mass and ion intensity for concentrations around 1-2 nmol/ml for both PC and PE. The total ion intensity continued to increase with concentrations above 1-2 nmol/ml, but the response was non-linear.     

ApoA-I induces a preferential efflux of monounsaturated phosphatidylcholine and medium chain sphingomyelin species from a cellular pool distinct from HDL(3) mediated phospholipid efflux

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used for a detailed analysis of cellular phospholipid and cholesterol efflux in free cholesterol (FC) loaded human primary fibroblasts and human monocyte-derived macrophages (HMDM) loaded with enzymatically modified LDL (E-LDL). Although both cell models differed significantly in their cellular lipid composition, a higher apoA-I specific efflux was found for monounsaturated phosphatidylcholine (PC) species together with a decreased contribution of polyunsaturated PC species in both cell types. Moreover, medium chain sphingomyelin (SPM) species SPM 14:0 and SPM 16:1 were translocated preferentially to apoA-I in both cell types. In contrast to fibroblasts, HMDM displayed a considerable proportion of cholesteryl esters (CE) in basal and apoA-I specific efflux media, most likely due to secretion of CE associated to apoE. Analysis of HDL(3) mediated lipid efflux from HMDM using D(9)-choline and (13)C(3)-FC stable isotope labeling revealed significantly different D(9)-PC and D(9)-SPM species pattern for apoA-I and HDL(3) specific efflux media, which indicates a contribution of distinct cellular phospholipid pools to apoA-I and HDL(3) mediated efflux. Together with a partial loading of fibroblasts and HMDM with HDL(3)-derived CE species, these data add further evidence for retroendocytosis of HDL. In summary, analysis of apoA-I/ABCA1 and HDL(3) mediated lipid efflux by ESI-MS/MS demonstrated a preferential efflux of monounsaturated PC and medium chain SPM to apoA-I. Moreover, this is the first study, which provides evidence for distinct cellular phospholipid pools used for lipid transfer to apoA-I and HDL(3) from the analysis of phospholipid species pattern in HMDM.     

Metabolomic identification of potential phospholipid biomarkers for chronic glomerulonephritis by using high performance liquid chromatography-mass spectrometry

Plasma phospholipids metabolic profile of chronic glomerulonephritis was investigated using high performance liquid chromatography/mass spectrometry (LC/MS) and principal component analysis. The plasma samples of 18 patients with chronic glomerulonephritis, 17 patients with chronic renal failure (CRF) without renal replacement therapy and 18 healthy persons were collected and analyzed. It was found that combination of the LC/MS technique with PCA can be successfully applied to phospholipid profile analysis. The results showed that primary chronic glomerulonephritis and CRF had phospholipids metabolic abnormality. Nineteen phospholipid species were identified as possible biomarkers in plasma samples of chronic glomerulonephritis and chronic renal failure. Moreover, the identification of the molecular structure of the potential phospholipid markers was obtained by ESI-MS/MS experiment. It suggests that phospholipids can be used as potential biomarkers on the progress of primary chronic glomerulonephritis.     

The ethanolamine branch of the Kennedy pathway is essential in the bloodstream form of Trypanosoma brucei

Phosphatidylethanolamine (GPEtn), a major phospholipid component of trypanosome membranes, is synthesized de novo from ethanolamine through the Kennedy pathway. Here the composition of the GPEtn molecular species in the bloodstream form of Trypanosoma brucei is determined, along with new insights into phospholipid metabolism, by in vitro and in vivo characterization of a key enzyme of the Kennedy pathway, the cytosolic ethanolamine-phosphate cytidylyltransferase ( Tb ECT) . Gene knockout indicates that Tb ECT is essential for growth and survival, thus highlighting the importance of the Kennedy pathway for the pathogenic stage of the African trypanosome. Phosphatiylserine decarboxylation, a potential salvage pathway, does not appear to be active in cultured bloodstream form T. brucei , and it is not upregulated even when the Kennedy pathway is disrupted. In vivo metabolic labelling and phospholipid composition analysis by ESI-MS/MS of the knockout cells confirmed a significant decrease in GPEtn species, as well as changes in the relative abundance of other phospholipid species. Reduction in GPEtn levels had a profound influence on the morphology of the mutants and it compromised mitochondrial structure and function, as well as glycosylphosphatidylinositol anchor biosynthesis. Tb ECT is therefore genetically validated as a potential drug target against the African trypanosome.     

Stable isotope analysis of dynamic lipidomics

Metabolic pathway flux is a fundamental element of biological activity, which can be quantified using a variety of mass spectrometric techniques to monitor incorporation of stable isotope-labelled substrates into metabolic products. This article contrasts developments in electrospray ionisation mass spectrometry (ESI-MS) for the measurement of lipid metabolism with more established gas chromatography mass spectrometry and isotope ratio mass spectrometry methodologies. ESI-MS combined with diagnostic tandem MS/MS scans permits the sensitive and specific analysis of stable isotope-labelled substrates into intact lipid molecular species without the requirement for lipid hydrolysis and derivatisation. Such dynamic lipidomic methodologies using non-toxic stable isotopes can be readily applied to quantify lipid metabolic fluxes in clinical and metabolic studies in vivo. However, a significant current limitation is the absence of appropriate software to generate kinetic models of substrate incorporation into multiple products in the time domain. Finally, we discuss the future potential of stable isotope-mass spectrometry imaging to quantify the location as well as the extent of lipid synthesis. This article is part of a Special Issue entitled: BBALIP_Lipidomics Opinion Articles edited by Sepp Kohlwein.     

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.     

Lipidomic approaches to the study of phospholipase A2-regulated phospholipid fatty acid incorporation and remodeling

The distribution of fatty acids among cellular glycerophospholipids is finely regulated by the CoA-dependent acylation of lysophospholipids followed by transacylation reactions. Arachidonic acid is the fatty acid precursor of a wide family of bioactive compounds called the eicosanoids, with key roles in innate immunity and inflammation. Because availability of free AA constitutes a rate-limiting step in the generation of eicosanoids by mammalian cells, many studies have been devoted to characterize the processes of arachidonate liberation from phospholipids by phospholipase A₂s and its re-incorporation and further remodeling back into phospholipids by acyltransferases and transacylases. These studies have traditionally been conducted by using radioactive precursors which do not allow the identification of the phospholipid molecular species involved in these processes. Nowadays, lipidomic approaches utilizing mass spectrometry provide a new frame for the analysis of unique phospholipid species involved in fatty acid release and phospholipid incorporation and remodeling. This review focuses on the mass spectrometry techniques applied to the study of phospholipid fatty acid trafficking and the recent advances that have been achieved by the use of this technique.     

Evidence for the release of arachidonic acid through the selective action of phospholipase A2 in thrombin-stimulated human platelets

The release of arachidonic acid from thrombin-stimulated platelets can be attributed to the action of phospholipase A2 on membrane phospholipid. Previously, analysis of individual subclasses of phospholipid demonstrated that 1-acyl-2-[3H]arachidonoyl-sn-glycerophosphocholine and to a lesser degree 1-acyl-2-[3H]arachidonoyl-sn-glycerophosphoethanolamine were the main source of [3H]arachidonic acid in thrombin-stimulated cells. In the present work, 1,2-diacyl phospholipid subclasses were analyzed as 1,2-diacylglycerobenzoates by high-pressure liquid chromatography in order to analyze arachidonate release as mass changes in individual molecular species of phospholipid. Following thrombin stimulation (5 U/ml, 5 min, 37 degrees C) all arachidonoyl-containing molecular species of 1,2-diacyl-sn-glycerophosphocholine decreased in mass and [3H]arachidonate content by almost 50%, while those of 1,2-diacyl-sn-glycerophosphoethanolamine decreased by 20%. The mass change was substantial and indicated that these phospholipids are a major source of arachidonate in stimulated cells. No variation was seen in the other non-arachidonate-containing molecular species of either subclass. Thus, deacylation of membrane 1,2-diacylglycerophosphocholine and 1,2-diacylglycerophosphoethanolamine by phospholipase A2 is selective for those molecular species of phospholipid containing arachidonic acid, suggesting that a certain proportion of arachidonoyl-containing molecular species of phospholipid are compartmentalized with the platelet membrane proximal to the site of action of this enzyme. These studies demonstrate that the human platelet is a cell poised and specialized to release rapidly substantial amounts of arachidonic acid upon stimulation.     

Formation of molecular species of mitochondrial cardiolipin: 2. A mathematical model of pattern formation by phospholipid transacylation

Formation of the unique molecular species of mitochondrial cardiolipin requires tafazzin, a transacylase that exchanges acyl groups between phospholipid molecular species without strict specificity for acyl groups, head groups, or carbon positions. However, it is not known whether phospholipid transacylations can cause the accumulation of specific fatty acids in cardiolipin. Here, a model is shown in linear algebra representation, in which acyl specificity emerges from the transacylation equilibrium of multiple molecular species, provided that different species have different free energies. The model defines the conditions and energy terms, under which transacylations may generate the characteristic composition of mitochondrial cardiolipin. It is concluded that acyl-specific cardiolipin patterns could arise from phospholipid transacylations in the tafazzin domain, even if tafazzin itself does not have substrate specificity.     

Distinctive structure and function of human apolipoprotein variant ApoA-IV-2

We have investigated the molecular structure, phospholipid binding, and lecithin-cholesterol acyltransferase catalytic activity of pure apoA-IV-2, a basic variant isoform of apoA-IV which is inherited as a classical Mendelian allele with a gene frequency of 0.09. Circular dichroism spectroscopy established that the alpha-helical content of apoA-IV-2 was 75% in the native state (versus 56% for apoA-IV-1), and increased to 88% in the presence of phospholipid. Fluorescence titration established that apoA-IV-2 bound to egg phospholipid vesicles with a Ka of 3.3 x 10(6) liter/mol, 2.4-fold greater than the affinity of apoA-IV-1. Fluorescence quenching studies revealed that, unlike apoA-IV-1, binding of apoA-IV-2 to phospholipid vesicles induced strong shielding of the amino-terminal tryptophan against iodide quenching. Enzyme kinetic studies using both saturated and unsaturated phospholipid substrates demonstrated that apoA-IV-2 was 36-71% more efficient in activating lecithin-cholesterol acyltransferase than apoA-IV-1. We conclude that apoA-IV-2 has more alpha-helical structure, is more stable in solution, and is more hydrophobic than apoA-IV-1, and that these distinctive structural features are associated with a higher affinity for phospholipid surfaces and an increased catalytic efficiency of lecithin:cholesterol acyltransferase activation. The biophysical basis for this latter characteristic may be the ability of apoA-IV-2 to penetrate phospholipid surfaces to a greater depth than apoA-IV-1. These molecular properties may be responsible for the increased levels of high density lipoproteins which have been observed in apoA-IV-2 heterozygotes.     

Activation of D-beta-hydroxybutyrate apodehydrogenase using molecular species of mixed fatty acyl phospholipids

D-beta-Hydroxybutyrate apodehydrogenase is a lipid-requiring enzyme with a specific requirement of lecithin for enzymatic function. The purified enzyme which is devoid of lipid can be reactivated with lecithin or mixtures of natural phospholipid-containing lecithin. However, it is mitochondrial phospholipid which activates the enzyme optimally and with kinetic parameters similar to that of the native membrane-bound enzyme. Mitochondrial phospholipid consists of three classes of phospholipid (lecithin:phosphatidylethanolamine:diphosphatidylglycerol in a ratio of approximately 2:2:1 by phosphorus); each class consists of a multiplicity of different molecular species due to diversity in the fatty acyl substituents. In this study, we have synthesized defined molecular species of mixed fatty acyl phospholipids to evaluate whether multiplicity of phospholipid molecular species are essential for optimal reactivation. We find that: 1) ternary mixtures of single molecular species of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylpropan-1,3-diol in the liquid crystalline state mimic the optimal reactivation of the enzyme obtained with mitochondrial phospholipids; 2) although some negatively charged phospholipid appears necessary for optimizing the efficiency of activation, diphosphatidylglycerol can be replaced by phosphatidylpropan-1,3-diol, another negatively charged phospholipid; and 3) biphasic Arrhenius plots can be correlated with the liquid crystalline and gel states of the phospholipid.     

LIPID arrays: new tools in the understanding of membrane dynamics and lipid signaling

Phospholipids are the structural building blocks of the membrane bilayer, which retains and regulates intra-cellular content. In addition to creating a protective barrier around the cell, lipids modulate membrane trafficking and are themselves precursors of important intracellular signaling molecules. Identification and quantification of these molecular species is essential for a more complete understanding of cell signaling pathways, and more reliable and sensitive methods are needed for determining membrane phospholipid content. Recent improvements in electrospray ionization mass spectrometry have made possible the direct identification of more than 400 phospholipid species from biological extracts of a single cell type. Changes in the cellular concentration of diverse lipids can be determined by analysis of the mass spectra by statistical algorithms. In the future, lipid arrays will be integrated with other high-throughput profiling technologies, and computational lipidomics will expand our understanding of the molecular basis of cellular processes and diseases.     

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

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.     

An approach to the identification of molecular fractions of human erythrocyte phospholipids using HPLC with mass-spectrometric detection

A modified method for a single run separation and identification of the molecular species of different phospholipid classes in a complex extract has been proposed. It includes reverse phase HPLC with a mass spectrometric detection. This approach has been employed for the analysis of glycerophospholipids and sphingolipids of human erythrocytes and several ceramide fractions have been identified[L2], that were missed in previous studies employing similar methods. The proposed scheme of experiment decreased the number of procedures needed for a complete phospholipid profiling of the sample.     

Acyl chain and head group regulation of phospholipid catabolism in senescing carnation flowers

Microsomal membranes from the petals of senescing carnation (Dianthus caryophyllus L.) flowers contain phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, and phosphatidylinositol. These phospholipid classes decline essentially in parallel during natural senescence of the flower and when microsomal membranes isolated from young flowers are aged in vitro. However, measurements of changes in the endogenous molecular species composition of microsomal phospholipids during natural senescence of the flower petals and during in vitro aging of isolated membranes have indicated that the various molecular species of phospholipids have quite different susceptibilities to catabolism. Acyl chain composition and the nature of the head group are both determinants of their susceptibility to catabolism. As well, a comparison of the phospholipid catabolism data for naturally senesced membranes and for membranes aged in vitro suggests that the phospholipid composition of membranes is continuously altered during senescence by acyl chain desaturation and possibly retailoring so as to generate molecular species that are more prone to catabolism. The results collectively indicate that provision of particular molecular species of phospholipids with increased susceptibility to degradation contributes to enhanced phospholipid catabolism in the senescing carnation petal.     

Electrospray ionization mass spectrometry in the study of biomolecular non-covalent interactions

In the past mass spectrometry has been limited to the study of small, stable molecules, however, with the emergence of electrospray ionization mass spectrometry (ESI-MS) large biomolecules as well as non-covalent biomolecular complexes can be studied. ESI-MS has been used to study non-covalent interactions involving proteins with metals, ligands, peptides, oligonucleotides, as well as other proteins. Although complementary to other well-established techniques such as circular dichroism and fluorescence spectroscopy, ESI-MS offers some advantages in speed, sensitivity, and directness particularly in the determination of the stoichiometry of the complex. One major advantage is the ability of ESI-MS to provide multiple signals each arising from a distinct population within the sample. In this review I will discuss some of the different types of non-covalent biomolecular interactions that have been studied using ESI-MS, highlighting examples which show the efficacy of using ESI-MS to probe the structure of biomolecular complexes.     

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.