Metabolism of pyrenyl fatty acids in baby hamster kidney fibroblasts. Effect of the acyl chain length

A novel assay for the introduction of the vinyl ether double bond into plasmalogens using pyrene-labeled substrates

Plasmanylethanolamine desaturase (PEDS) (EC 1.14.99.19) introduces the 1-prime double bond into plasmalogens, one of the most abundant phospholipids in the human body. This labile membrane enzyme has not been purified and its coding sequence is unknown. Previous assays for this enzyme used radiolabeled substrates followed by multistep processing. We describe here a straight-forward method for the quantification of PEDS in enzyme incubation mixtures using pyrene-labeled substrates and reversed-phase HPLC with fluorescence detection. After stopping the reaction with hydrochloric acid in acetonitrile, the mixture was directly injected into the HPLC system without the need of lipid extraction. The substrate, 1-O-pyrenedecyl-2-acyl-sn-glycero-3-phosphoethanolamine, and the lyso-substrate, 1-O-pyrenedecyl-sn-glycero-3-phosphoethanolamine, were prepared from RAW-12 cells deficient in PEDS activity and were compared for their performance in the assay. Plasmalogen levels in mouse tissues and in cultured cells did not correlate with PEDS levels, indicating that the desaturase might not be the rate limiting step for plasmalogen biosynthesis. Among selected mouse organs, the highest activities were found in kidney and in spleen. Incubation of intact cultivated mammalian cells with 1-O-pyrenedecyl-sn-glycerol, extraction of lipids, and treatment with hydrochloric or acetic acid in acetonitrile allowed sensitive monitoring of PEDS activity in intact cells.

A novel method for determining peroxisomal fatty acid β-oxidation

The purpose of this study is to establish an assay method to screen for chemical compounds that stimulate peroxisomal fatty acid β-oxidation activity in X-linked adrenoleukodystropy (X-ALD) fibroblasts. In this investigation, we used 12-(1-pyrene)dodecanoic acid (pyrene-C12:0), a fluorescent fatty acid analog, as a substrate for fatty acid β-oxidation. When human skin fibroblasts were incubated with pyrene-C12:0, β-oxidation products such as pyrene-C10:0 and pyrene-C8:0 were generated time-dependently. These β-oxidation products were scarcely detected in the fibroblasts from patients with Zellweger syndrome, a peroxisomal biogenesis disorder. In contrast, in fibroblasts with mitochondrial carnitine-acylcarnitine translocase deficiency, the β-oxidation products were detected at a level similar to control fibroblasts. These results indicate that the β-oxidation of pyrene-C12:0 takes place in peroxisomes, but not mitochondria, so pyrene-C12:0 is useful for measuring peroxisomal fatty acid β-oxidation activity. In X-ALD fibroblasts, the β-oxidation activity for pyrene-C12:0 was approximately 40 % of control fibroblasts, which is consistent with previous results using [1-(14)C]lignoceric acid as the substrate. The present study provides a convenient procedure for screening chemical compounds that stimulate the peroxisomal fatty acid β-oxidation in X-ALD fibroblasts.

Fluorescent probes and fluorescence (microscopy) techniques--illuminating biological and biomedical research

Fluorescence, the absorption and re-emission of photons with longer wavelengths, is one of those amazing phenomena of Nature. Its discovery and utilization had, and still has, a major impact on biological and biomedical research, since it enables researchers not just to visualize normal physiological processes with high temporal and spatial resolution, to detect multiple signals concomitantly, to track single molecules in vivo, to replace radioactive assays when possible, but also to shed light on many pathobiological processes underpinning disease states, which would otherwise not be possible. Compounds that exhibit fluorescence are commonly called fluorochromes or fluorophores and one of these fluorescent molecules in particular has significantly enabled life science research to gain new insights in virtually all its sub-disciplines: Green Fluorescent Protein. Because fluorescent proteins are synthesized in vivo, integration of fluorescent detection methods into the biological system via genetic techniques now became feasible. Currently fluorescent proteins are available that virtually span the whole electromagnetic spectrum. Concomitantly, fluorescence imaging techniques were developed, and often progress in one field fueled innovation in the other. Impressively, the properties of fluorescence were utilized to develop new assays and imaging modalities, ranging from energy transfer to image molecular interactions to imaging beyond the diffraction limit with super-resolution microscopy. Here, an overview is provided of recent developments in both fluorescence imaging and fluorochrome engineering, which together constitute the “fluorescence toolbox” in life science research.

Modulation of cellular cholesterol transport and homeostasis by Rab11

To analyze the contribution of vesicular trafficking pathways in cellular cholesterol transport we examined the effects of selected endosomal Rab proteins on cholesterol distribution by filipin staining. Transient overexpression of Rab11 resulted in prominent accumulation of free cholesterol in Rab11-positive organelles that sequestered transferrin receptors and internalized transferrin. Sphingolipids were selectively redistributed as pyrene-sphingomyelin and sulfatide cosequestered with Rab11-positive endosomes, whereas globotriaosyl ceramide and GM2 ganglioside did not. Rab11 overexpression did not perturb the transport of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine-perchlorate-labeled low-density lipoprotein (LDL) to late endosomes or the Niemann-Pick type C1 (NPC1)-induced late endosomal cholesterol clearance in NPC patient cells. However, Rab11 overexpression inhibited cellular cholesterol esterification in an LDL-independent manner. This effect could be overcome by introducing cholesterol to the plasma membrane by using cyclodextrin as a carrier. These results suggest that in Rab11-overexpressing cells, deposition of cholesterol in recycling endosomes results in its impaired esterification, presumably due to defective recycling of cholesterol to the plasma membrane. The findings point to the importance of the recycling endosomes in regulating cholesterol and sphingolipid trafficking and cellular cholesterol homeostasis.

Translocation of pyrene-labeled phosphatidylserine from the plasma membrane to mitochondria diminishes systematically with molecular hydrophobicity: implications on the maintenance of high phosphatidylserine content in the inner leaflet of the plasma membrane

To study the translocation of phosphatidylserine (PS) from plasma membrane to mitochondria, dipyrene PS molecules (diPyr(n)PS; n=acyl chain length) were introduced to the plasma membrane of baby hamster kidney cells (BHK cells) using either cyclodextrin-mediated monomer transfer or fusion of cationic vesicles. Translocation of diPyr(n)PS to mitochondria was assessed based on decarboxylation by mitochondrial PS decarboxylase (PSD). It was found that the rate of translocation diminishes systematically with acyl chain length (molecular hydrophobicity) of diPyr(n)PS. Using an in vitro assay, it was shown that the spontaneous translocation rates of long-chain diPyr(n)PS species are similar to those of common natural PS species, thus supporting the biological relevance of the data. These results, and other data arguing against the involvement of vesicular traffic and lipid transfer proteins, imply that spontaneous monomeric diffusion via the cytoplasm is the main mechanism of PS movement from the plasma membrane to mitochondria. This finding could explain why a major fraction of PS synthesized by BHK cells consists of hydrophobic species: such species have little tendency to efflux from the plasma membrane to mitochondria where they would be decarboxylated. Thus, adequate molecular hydrophobicity seems to be crucial for the maintenance of high PS content in the inner leaflet of the plasma membrane.

Pyrene-labeled lipids as tools in membrane biophysics and cell biology

Pyrene is one of the most frequently used lipid-linked fluorophores. Its most characteristic features are a long excited state lifetime and (local) concentration-dependent formation of excimers. Pyrene is also hydrophobic and thus does not significantly distort the conformation of the labeled lipid molecule. These characteristics make pyrene lipids well-suited for studies on a variety of biophysical phenomena like lateral diffusion, inter- or transbilayer movement of lipids and lateral organization of membranes. Pyrene lipids have also been widely employed to determine protein binding to membranes, lipid conformation and the activity of lipolytic enzymes. In cell biology, pyrene lipids are promising tools for studies on lipid trafficking and metabolism, as well as for microscopic mapping of membrane properties. The main disadvantage of pyrene lipids is the relatively large size of the fluorophore. Another disadvantage is that they require UV-excitation, which is not feasible with all microscopes.

Fluorescence imaging of pyrene-labeled lipids in living cells

Microscopic imaging of fluorescent lipid derivatives is a powerful tool to study membrane organization and lipid trafficking but it is complicated by cellular autofluorescence background and photobleaching of the fluorophore as well as by the difficulty to selectively image membranes stacked on top of each other. Here we describe protocols that strongly alleviate such problems when pyrene-labeled lipids are being used. First, photobleaching of these lipids is virtually eliminated when oxygen is depleted from the medium by using a gentle and simple enzymatic method. Second, an image practically free of cellular autofluorescence contribution can be obtained simply by subtracting from the pyrene image the background image obtained at a slightly different excitation wavelength. This type of background subtraction more properly accounts for the typically uneven distribution of cellular background fluorescence than other, commonly used methods. Third, it is possible to selectively image the pyrene lipids in the plasma membrane by using plasma membrane-specific quencher trinitrophenyl lysophosphatidylethanolamine and image subtraction. Importantly, either the outer or the inner leaflet can be selectively imaged by labeling the cells with pyrene phosphatidylcholine or phosphatidylserine, respectively. These protocols should be of considerable help when studying organization of the plasma membrane or intracellular lipid trafficking.

gamma-cyclodextrins greatly enhance translocation of hydrophobic fluorescent phospholipids from vesicles to cells in culture. Importance of molecular hydrophobicity in phospholipid trafficking studies

Short-chain, fluorescent derivatives are commonly used to investigate intracellular phospholipid trafficking. However, their use can yield misleading results because they, unlike the native species, can rapidly distribute between organelles due to their low hydrophobicity. On the other hand, hydrophobic derivatives are very difficult to introduce to cells and thus have hardly been used. Here we show that carboxyethylated gamma-cyclodextrin (CE-gamma-CD) greatly enhances transfer of a variety of hydrophobic fluorescent phospholipid derivatives from vesicles to cultured cells. Several lines of evidence indicate that CE-gamma-CD enhances transfer of lipid molecules by increasing their effective concentration in the aqueous phase, rather than by inducing membrane fusion or hemifusion. Incubation with CE-gamma-CD and donor lipid vesicles does not extract cholesterol or phospholipids from the cells or compromise plasma membrane intactness or long term cell viability. Using CE-gamma-CD-mediated transfer, we introduced hydrophobic pyrene-labeled phosphatidylserine to the plasma membrane of fibroblast cells and followed their distribution with time. In contrast to what has been previously observed for other, less hydrophobic species, transport of this lipid to the Golgi apparatus or mitochondria was not detected. Rather, much of this fluorescent PS remained in the plasma membrane or was incorporated to various endocytotic compartments. These findings indicate that the native, typically hydrophobic phosphatidylserine molecules efflux only very slowly via the cytoplasm to intracellular organelles. This helps to explain how cells can maintain a very high concentration of phosphatidylserine in the inner leaflet of their plasma membrane. Furthermore, the present results underline the importance of using hydrophobic analogues when studying intracellular trafficking of many phospholipid classes.

The turnover of cytoplasmic triacylglycerols in human fibroblasts involves two separate acyl chain length-dependent degradation pathways

Cultured fibroblasts from patients affected with the genetic metabolic disorder named neutral lipid storage disease (NLSD) exhibit a dramatic accumulation of cytoplasmic triacylglycerols (Radom, J., Salvayre, R., Nègre, A., Maret, A., and Douste-Blazy, L. (1987) Eur. J. Biochem. 164, 703-708). We compared here the metabolism of radiolabeled short-, medium- and long-chain fatty acids in these cells. Short/medium-chain fatty acids (C4-C10) were incorporated into polar lipids (60-80%) and triacylglycerols (20-40%) at a lower rate (5-10 times lower) than long-chain fatty acids. Pulse-chase experiments allowed to evaluate the degradation rate of cytoplasmic triacylglycerols in normal and NLSD fibroblasts and to discriminate between two catabolic pathways of cytoplasmic triacylglycerols. Short/medium-chain (C4-C10) triacylglycerols were degraded at a normal rate in NLSD fibroblasts, whereas long-chain (C12 and longer) triacylglycerols remained undegraded. These data are confirmed by mass analysis. The use of diethylparanitrophenyl phosphate (E600) and parachloromercuribenzoate (PCMB) inhibitors allows to discriminate between the two triacylglycerol degradation pathways. E600 inhibited selectively the in situ degradation of short/medium-chain triacylglycerols without inhibition of the degradation of long-chain triacylglycerols, whereas PCMB inhibited selectively the in situ hydrolysis of long-chain triacylglycerols without affecting the degradation of long-chain triacylglycerols. This was correlated with the in vitro properties of cellular triacylglycerol-hydrolyzing enzymes characterized by their substrate specificity and their susceptibility to inhibitors; the neutral lipase specific to long-chain triacylglycerols is inhibited by PCMB, but not by E600, in contrast to short/medium-chain lipase, which is inhibited by E600 but not by PCMB. The data of in vitro and in situ experiments suggest the existence in fibroblasts of two separate acyl chain length-dependent pathways involved in the degradation of cytoplasmic triacylglycerols, one mediated by a neutral long-chain lipase and another one mediated by a short/medium-chain lipase.

A sphingomyelin-transferring protein from chicken liver. Use of pyrene-labeled phospholipid

A phospholipid transfer protein was purified from chicken liver which, in addition to phosphatidylinositol (PI) and phosphatidylcholine (PC), carries sphingomyelin (SM) between membranes. For comparison, the PI-transfer protein from chicken liver only carries PI and PC. Specificity was established by use of phospholipids that carry a pyrene-labeled acyl chain. Based on the N-terminal sequence and Western blot analysis we conclude that this protein is an isoform of the PI-transfer protein. At increasing length of the pyrene-labeled acyl chain, the isoform expresses a high activity toward SM, a low activity toward PI, and virtually no activity toward PC.

Intramitochondrial distribution and transport of phosphatidylserine and its decarboxylation product, phosphatidylethanolamine. Application of pyrene-labeled species

To investigate the mechanism of intramitochondrial translocation of phosphatidylserine and its decarboxylation product, phosphatidylethanolamine, the distribution of these lipids between the outer (OM) and inner (IM) mitochondrial membranes, as well as their transversal and lateral distribution in OM were studied. Fluorescent, pyrenyl derivatives of phosphatidylserine (PyrxPS) and phosphatidylethanolamine (PyrxPE) species were employed because they allow: (i), direct monitoring of PS (and PE) loading to the mitochondria; (ii) assay of PS decarboxylation by high-performance liquid chromatography with fluorescence detection and (iii), determination of the lateral distributions of PS and PE within the mitochondrial membranes. All PyrxPS species tested were efficiently decarboxylated by the solubilized decarboxylase and thus the distribution of the endogenous PE could be also studied. When the PyrxPS species were loaded to isolated mitochondria very little, if any, of the loaded PyrxPS or of the PyrxPE product was found in IM independent of the time and temperature of incubation, strongly suggesting that these lipids either never enter IM or their residence there is only transient. When mitochondria preloaded with Pyr4PS were incubated with an excess of acceptor vesicles in the presence of the lipid transfer protein, 80% of Pyr4PS and 30-40% of the Pyr4PE product were transported to the acceptor vesicles, indicating that at least corresponding fractions of these lipid were located in, or were in rapid equilibrium with the outer leaflet of OM. Since the decarboxylase is located in the inner membrane, these results signify that both PS and PE must be able to move readily across OM. Determination of the excimer to monomer ratio as the function of pyrenyl lipid concentration in mitochondria (i.e., OM) gave parallel results for PyrxPS and -PE species suggesting the lateral distribution of PS and PE in OM is similar and thus there is no specific enrichment of PS to the contact sites. To investigate the mechanism of PS transport from the outer leaflet to the decarboxylation site, the influence of PyrxPS hydrophobicity, i.e., pyrenylacyl chain length, on the rate of decarboxylation was determined. The variation of the length of the pyrenyl acyl chain from 4 to 12 carbons did not significantly affect the rate of PyrxPS decarboxylation in intact mitochondria, indicating that the transport of PS from the outer leaflet of OM to the site of decarboxylation takes place by lateral diffusion rather than by spontaneous or protein-mediated transport. The implications of these findings on the mechanism of intramitochondrial transport of PS and PE are discussed in terms of alternative models.

A novel fluorescent fatty acid, 5-methyl-BDY-3-dodecanoic acid, is a potential probe in lipid transport studies by incorporating selectively to lipid classes of BHK cells

The 5-methyl-BDY-3-dodecanoic acid (B12FA) labelling of BHK cell lipids was analyzed by thin layer and reverse phase column chromatography. Incorporation to phospholipids was selective: over 90% of B12FA label was enriched in phosphatidylcholine. The major molecular species of PC was that containing palmitate as the unlabelled fatty acid. Small amounts of label was also found in other phosphoglycerides, but not in sphingomyelin. Triglycerides and diglycerides constituted the main B12FA-labelled neutral lipid classes; however, no label was found in cholesterol esters. B12FA was degraded to shorter homologues, which had significantly slower lipid incorporation rates. B12FA-labelled cells displayed in a microscope initially green reticular type fluorescence, but later red spherical structures, representing neutral lipid droplets, could also be seen. It is concluded that B12FA does not incorporate indiscriminately to all lipid classes of BHK cells, but is enriched to PC, diglycerides and triglycerides, which could be utilized in studies on lipid transport as well as metabolism.