Preferential decarboxylation of hydrophilic phosphatidylserine species in cultured cells. Implications on the mechanism of transport to mitochondria and cellular aminophospholipid species compositions

An organelle-specific photoactivation and dual-isotope labeling strategy reveals phosphatidylethanolamine metabolic flux

Phosphatidylethanolamine metabolism plays essential roles in eukaryotic cells but has not been completely investigated due to its complexity. This is because lipid species, unlike proteins or nucleic acids, cannot be easily manipulated at the single molecule level or controlled with subcellular resolution, two of the key factors toward understanding their functions. Here, we use the organelle-targeting photoactivation method to study PE metabolism in living cells with a high spatiotemporal resolution. Containing predefined PE structures, probes which can be selectively introduced into the ER or mitochondria were designed to compare their metabolic products according to their subcellular localization. We combined photo-uncaging with dual stable isotopic labeling to track PE metabolism in living cells by mass spectrometry analysis. Our results reveal that both mitochondria- and ER-released PE participate in phospholipid remodeling, and that PE methylation can be detected only under particular conditions. Thus, our method provides a framework to study phospholipid metabolism at subcellular resolution.

ORP5/8 and MIB/MICOS link ER-mitochondria and intra-mitochondrial contacts for non-vesicular transport of phosphatidylserine

Mitochondria are dynamic organelles essential for cell survival whose structural and functional integrity rely on selective and regulated transport of lipids from/to the endoplasmic reticulum (ER) and across the mitochondrial intermembrane space. As they are not connected by vesicular transport, the exchange of lipids between ER and mitochondria occurs at membrane contact sites. However, the mechanisms and proteins involved in these processes are only beginning to emerge. Here, we show that the main physiological localization of the lipid transfer proteins ORP5 and ORP8 is at mitochondria-associated ER membrane (MAM) subdomains, physically linked to the mitochondrial intermembrane space bridging (MIB)/mitochondrial contact sites and cristae junction organizing system (MICOS) complexes that bridge the two mitochondrial membranes. We also show that ORP5/ORP8 mediate non-vesicular transport of phosphatidylserine (PS) lipids from the ER to mitochondria by cooperating with the MIB/MICOS complexes. Overall our study reveals a physical and functional link between ER-mitochondria contacts involved in lipid transfer and intra-mitochondrial membrane contacts maintained by the MIB/MICOS complexes.

Molecular species selectivity of lipid transport creates a mitochondrial sink for di-unsaturated phospholipids

Mitochondria depend on the import of phospholipid precursors for the biosynthesis of phosphatidylethanolamine (PE) and cardiolipin, yet the mechanism of their transport remains elusive. A dynamic lipidomics approach revealed that mitochondria preferentially import di-unsaturated phosphatidylserine (PS) for subsequent conversion to PE by the mitochondrial PS decarboxylase Psd1p. Several protein complexes tethering mitochondria to the endomembrane system have been implicated in lipid transport in yeast, including the endoplasmic reticulum (ER)-mitochondrial encounter structure (ERMES), ER-membrane complex (EMC), and the vacuole and mitochondria patch (vCLAMP). By limiting the availability of unsaturated phospholipids, we created conditions to investigate the mechanism of lipid transfer and the contributions of the tethering complexes in vivo. Under these conditions, inactivation of ERMES components or of the vCLAMP component Vps39p exacerbated accumulation of saturated lipid acyl chains, indicating that ERMES and Vps39p contribute to the mitochondrial sink for unsaturated acyl chains by mediating transfer of di-unsaturated phospholipids. These results support the concept that intermembrane lipid flow is rate-limited by molecular species-dependent lipid efflux from the donor membrane and driven by the lipid species' concentration gradient between donor and acceptor membrane.

Structural insights into phosphatidylethanolamine formation in bacterial membrane biogenesis

Phosphatidylethanolamine (PE), a major component of the cellular membrane across all domains of life, is synthesized exclusively by membrane-anchored phosphatidylserine decarboxylase (PSD) in most bacteria. The enzyme undergoes auto-cleavage for activation and utilizes the pyruvoyl moiety to form a Schiff base intermediate with PS to facilitate decarboxylation. However, the structural basis for self-maturation, PS binding, and decarboxylation processes directed by PSD remain unclear. Here, we present X-ray crystal structures of PSD from Escherichia coli, representing an apo form and a PE-bound complex, in which the phospholipid is chemically conjugated to the essential pyruvoyl residue, mimicking the Schiff base intermediate. The high-resolution structures of PE-complexed PSD clearly illustrate extensive hydrophobic interactions with the fatty acyl chains of the phospholipid, providing insights into the broad specificity of the enzyme over a wide range of cellular PS. Furthermore, these structures strongly advocate the unique topology of the enzyme in a lipid bilayer environment, where the enzyme associates with cell membranes in a monotopic fashion via the N-terminal domain composed of three amphipathic helices. Lastly, mutagenesis analyses reveal that E. coli PSD primarily employs D90/D142-H144-S254 to achieve auto-cleavage for the proenzyme maturation, where D90 and D142 act in complementary to each other.

Is Spontaneous Translocation of Polar Lipids Between Cellular Organelles Negligible?

In most reviews addressing intracellular lipid trafficking, spontaneous diffusion of lipid monomers between the cellular organelles is considered biologically irrelevant because it is thought to be far too slow to significantly contribute to organelle biogenesis. This view is based on intervesicle transfer experiments carried out in vitro with few lipids as well as on the view that lipids are highly hydrophobic and thus cannot undergo spontaneous intermembrane diffusion at a significant rate. However, besides that single-chain lipids can translocate between vesicles in seconds, it has been demonstrated that the rate of spontaneous transfer of two-chain polar lipids can vary even 1000-fold, depending on the number of carbons and double bonds in the acyl chains. In addition, the rate of spontaneous lipid transfer can strongly depend on the experimental conditions such as vesicle composition and concentration. This review examines the studies suggesting that spontaneous lipid transfer is probably more relevant to intracellular trafficking of amphipathic lipids than commonly thought.

Lipid transport between the endoplasmic reticulum and mitochondria

Mitochondria are partially autonomous organelles that depend on the import of certain proteins and lipids to maintain cell survival and membrane formation. Although phosphatidylglycerol, cardiolipin, and phosphatidylethanolamine are synthesized by mitochondrial enzymes, phosphatidylcholine, phosphatidylinositol, phosphatidylserine, and sterols need to be imported from other organelles. The origin of most lipids imported into mitochondria is the endoplasmic reticulum, which requires interaction of these two subcellular compartments. Recently, protein complexes that are involved in membrane contact between endoplasmic reticulum and mitochondria were identified, but their role in lipid transport is still unclear. In the present review, we describe components involved in lipid translocation between the endoplasmic reticulum and mitochondria and discuss functional as well as regulatory aspects that are important for lipid homeostasis.

Checks and balances in membrane phospholipid class and acyl chain homeostasis, the yeast perspective

Glycerophospholipids are the most abundant membrane lipid constituents in most eukaryotic cells. As a consequence, phospholipid class and acyl chain homeostasis are crucial for maintaining optimal physical properties of membranes that in turn are crucial for membrane function. The topic of this review is our current understanding of membrane phospholipid homeostasis in the reference eukaryote Saccharomyces cerevisiae. After introducing the physical parameters of the membrane that are kept in optimal range, the properties of the major membrane phospholipids and their contributions to membrane structure and dynamics are summarized. Phospholipid metabolism and known mechanisms of regulation are discussed, including potential sensors for monitoring membrane physical properties. Special attention is paid to processes that maintain the phospholipid class specific molecular species profiles, and to the interplay between phospholipid class and acyl chain composition when yeast membrane lipid homeostasis is challenged. Based on the reviewed studies, molecular species selectivity of the lipid metabolic enzymes, and mass action in acyl-CoA metabolism are put forward as important intrinsic contributors to membrane lipid homeostasis.

Synthesis and biosynthetic trafficking of membrane lipids

Eukaryotic cells can synthesize thousands of different lipid molecules that are incorporated into their membranes. This involves the activity of hundreds of enzymes with the task of creating lipid diversity. In addition, there are several, typically redundant, mechanisms to transport lipids from their site of synthesis to other cellular membranes. Biosynthetic lipid transport helps to ensure that each cellular compartment will have its characteristic lipid composition that supports the functions of the associated proteins. In this article, we provide an overview of the biosynthesis of the major lipid constituents of cell membranes, that is, glycerophospholipids, sphingolipids, and sterols, and discuss the mechanisms by which these newly synthesized lipids are delivered to their target membranes.

Introduction of phospholipids to cultured cells with cyclodextrin

Previous studies indicate that methyl-β-cyclodextrin (meβ-CD) can greatly enhance translocation of long-chain phospholipids from vesicles to cells in culture, which is very useful when studying, e.g., phospholipid metabolism and trafficking. However, the parameters affecting the transfer have not been systematically studied. Therefore, we studied the relevant parameters including meβ-CD and vesicle concentration, incubation time, phospholipid structure, and cell type. Because meβ-CD can extract cholesterol and other lipids from cells, thereby potentially altering cell growth or viability, these issues were studied as well. The results show that efficient incorporation of phospholipid species with hydrophobicity similar to that of natural species can be obtained without significantly compromising cell growth or viability. Cellular content of phosphatidyl-serine, -ethanolamine, and -choline could be increased dramatically, i.e., 400, 125, and 25%, respectively. Depletion of cellular cholesterol could be prevented or alleviated by inclusion of the proper amount of cholesterol in the donor vesicles. In summary, meβ-CD mediates efficient transfer of long-chain (phospho) lipids from vesicles to cells without significantly compromising their growth or viability. This lays a basis for detailed studies of phospholipid metabolism and trafficking as well as enables extensive manipulation of cellular phospholipid composition, which is particularly useful when investigating mechanisms underlying phospholipid homeostasis.

Phosphatidylserine decarboxylases, key enzymes of lipid metabolism

Phosphatidylserine decarboxylases (PSDs) (E.C. 4.1.1.65) are enzymes which catalyze the formation of phosphatidylethanolamine (PtdEtn) by decarboxylation of phosphatidylserine (PtdSer). This enzymatic activity has been identified in both prokaryotic and eukaryotic organisms. PSDs occur as two types of proteins depending on their localization and the sequence of a conserved motif. Type I PSDs include enzymes of eukaryotic mitochondria and bacterial origin which contain the amino acid sequence LGST as a characteristic motif. Type II PSDs are found in the endomembrane system of eukaryotes and contain a typical GGST motif. These characteristic motifs are considered as autocatalytic cleavage sites where proenzymes are split into alpha- and beta-subunits. The S-residue set free by this cleavage serves as an attachment site of a pyruvoyl group which is required for the activity of the enzymes. Moreover, PSDs harbor characteristic binding sites for the substrate PtdSer. Substrate supply to eukaryotic PSDs requires lipid transport because PtdSer synthesis and decarboxylation are spatially separated. Targeting of PSDs to their proper locations requires additional intramolecular domains. Mitochondrially localized type I PSDs are directed to the inner mitochondrial membrane by N-terminal targeting sequences. Type II PSDs also contain sequences in their N-terminal extensions which might be required for subcellular targeting. Lack of PSDs causes various defects in different cell types. The physiological relevance of these findings and the central role of PSDs in lipid metabolism will be discussed in this review.

The superlattice model of lateral organization of membranes and its implications on membrane lipid homeostasis

Most biological membranes are extremely complex structures consisting of hundreds of different lipid and protein molecules. According to the famous fluid-mosaic model lipids and many proteins are free to diffuse very rapidly in the plane of the membrane. While such fast diffusion implies that different membrane lipids would be laterally randomly distributed, accumulating evidence indicates that in model and natural membranes the lipid components tend to adopt regular (superlattice-like) distributions. The superlattice model, put forward based on such evidence, is intriguing because it predicts that 1) there is a limited number of allowed compositions representing local minima in membrane free energy and 2) those energy minima could provide set-points for enzymes regulating membrane lipid compositions. Furthermore, the existence of a discrete number of allowed compositions could help to maintain organelle identity in the face of rapid inter-organelle membrane traffic.

Mitochondrial phospholipids of rat skeletal muscle are less polyunsaturated than whole tissue phospholipids: implications for protection against oxidative stress

The fatty acid composition of phospholipids is an important determinant of membrane function. Although the mitochondria play a pivotal role in skeletal muscle function, the fatty acid composition of their individual phospholipids has not been examined. The purpose of this study was to determine the fatty acid profile of each phospholipid in rat skeletal muscle mitochondria and compare it with that of the whole muscle. Lipids were extracted from the gastrocnemius muscles of 10 Wistar rats, and phospholipids were separated by thin-layer chromatography. The fatty acid composition of each phospholipid was then determined by gas chromatography. The same procedure was applied to a mitochondrial preparation from these muscles. We found that the fatty acid composition of the individual mitochondrial phospholipids (phosphatidyl choline, phosphatidyl ethanolamine, cardiolipin, phosphatidyl inositol, phosphatidyl serine, sphingomyelin, and lysophosphatidyl choline) and of the total mitochondrial phospholipids differed markedly (P < 0.05) from the fatty acid composition of the corresponding whole muscle phospholipids. Notably, the mitochondrial phospholipids had higher percentages of MUFA [13.9 (2.1) vs. 10.3 (0.9)] and lower percentages of PUFA [34.8 (4.3) vs. 39.5 (5.2)] and n6 fatty acids [25.0 (2.5) vs. 27.6 (2.5)]. Overall, the mitochondrial phospholipids had a lower unsaturation index than whole muscle phospholipids [135 (20) vs. 161 (26)]. Because PUFA are susceptible to peroxidation, unlike saturated fatty acids and MUFA, we propose that the low polyunsaturation of mitochondrial phospholipids is the result of selective pressure toward membranes that are more resistant to oxidative damage by reactive oxygen species produced in their vicinity. The negative effect of the low polyunsaturation on membrane fluidity may be counterbalanced by the higher percentage of MUFA and the known low cholesterol content of mitochondrial membranes.

Automated quantitative analysis of complex lipidomes by liquid chromatography/mass spectrometry

Recent advances in mass spectrometry have revolutionized the analysis of lipid compositions of cells and other biomaterials by simplifying the analytical protocol dramatically and by increasing the sensitivity of detection by several orders of magnitude. However, the throughput of the published mass spectrometric methods is severely limited by data analysis, which requires extensive operator involvement. Consequently, we have developed an automated method that allows unattended identification and quantification of lipid molecular species of all the major lipid classes from a two-dimensional chromatographic/mass spectrometric data set. More than 100 polar lipid species could be automatically quantified from different biological samples with good accuracy and reproducibility. The response was linear over approximately 3 orders of magnitude with the equipment used, and approximately 35 samples could be analyzed in a day. This method makes high-throughput lipidomics feasible in biology, biotechnology, and medicine.

The bovine seminal plasma protein PDC-109 extracts phosphorylcholine-containing lipids from the outer membrane leaflet

The bovine seminal plasma protein PDC-109 modulates the maturation of bull sperm cells by removing lipids, mainly phosphatidylcholine and cholesterol, from their cellular membrane. Here, we have characterized the process of extraction of endogenous phospholipids and of their respective analogues. By measuring the PDC-109-mediated release of fluorescent phospholipid analogues from lipid vesicles and from biological membranes (human erythrocytes, bovine epididymal sperm cells), we showed that PDC-109 extracts phospholipids with a phosphorylcholine headgroup mainly from the outer leaflet of these membranes. The ability of PDC-109 to extract endogenous phospholipids from epididymal sperm cells was followed by mass spectrometry, which allowed us to characterize the fatty acid pattern of the released lipids. From these cells, PDC-109 extracted phosphatidylcholine and sphingomyelin that contained an enrichment of mono- and di-unsaturated fatty acids as well as short-chain and lyso-phosphatidylcholine species. Based on the results, a model explaining the phospholipid specificity of PDC-109-mediated lipid release is presented.

Contribution of different pathways to the supply of phosphatidylethanolamine and phosphatidylcholine to mitochondrial membranes of the yeast Saccharomyces cerevisiae

In the yeast, three biosynthetic pathways lead to the formation of phosphatidylethanolamine (PtdEtn): (i) decarboxylation of phosphatidylserine (PtdSer) by phosphatidylserine decarboxylase 1 (Psd1p) in mitochondria; (ii) decarboxylation of PtdSer by Psd2p in a Golgi/vacuolar compartment; and (iii) the CDP-ethanolamine (CDP-Etn) branch of the Kennedy pathway. The major phospholipid of the yeast, phosphatidylcholine (PtdCho), is formed either by methylation of PtdEtn or via the CDP-choline branch of the Kennedy pathway. To study the contribution of these pathways to the supply of PtdEtn and PtdCho to mitochondrial membranes, labeling experiments in vivo with [(3)H]serine and [(14)C]ethanolamine, or with [(3)H]serine and [(14)C]choline, respectively, and subsequent cell fractionation were performed with psd1Delta and psd2Delta mutants. As shown by comparison of the labeling patterns of the different strains, the major source of cellular and mitochondrial PtdEtn is Psd1p. PtdEtn formed by Psd2p or the CDP-Etn pathway, however, can be imported into mitochondria, although with moderate efficiency. In contrast to mitochondria, microsomal PtdEtn is mainly derived from the CDP-Etn pathway. PtdEtn formed by Psd2p is the preferred substrate for PtdCho synthesis. PtdCho derived from the different pathways appears to be supplied to subcellular membranes from a single PtdCho pool. Thus, the different pathways of PtdEtn biosynthesis play different roles in the assembly of PtdEtn into cellular membranes.

Contribution of different biosynthetic pathways to species selectivity of aminoglycerophospholipids assembled into mitochondrial membranes of the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, three pathways lead to the formation of cellular phosphatidylethanolamine (PtdEtn), namely the mitochondrial conversion of phosphatidylserine (PtdSer) to PtdEtn catalyzed by phosphatidylserine decarboxylase 1 (Psd1p), the equivalent reaction catalyzed by phosphatidylserine decarboxylase 2 (Psd2p) in the Golgi, and the CDP-ethanolamine branch of the so-called Kennedy pathway which is located to the microsomal fraction. To investigate the contributions of these three pathways to the cellular pattern of PtdEtn species (fatty acid composition) we subjected lipids of wild-type and yeast mutant strains with distinct defects in the respective pathways to mass spectrometric analysis. We also analyzed species of PtdSer and phosphatidylcholine (PtdCho) of these strains because formation of the three aminoglycerophospholipids is linked through their biosynthetic route. We demonstrate that all three pathways involved in PtdEtn synthesis exhibit a preference for the formation of C34:2 and C32:2 species resulting in a high degree of unsaturation in total cellular PtdEtn. In PtdSer, the ratio of unsaturated to saturated fatty acids is much lower than in PtdEtn, suggesting a high species selectivity of PtdSer decarboxylases. Finally, PtdCho is characterized by its higher ratio of C16 to C18 fatty acids compared to PtdSer and PtdEtn. In contrast to biosynthetic steps, import of all three aminoglycerophospholipids into mitochondria of wild-type and mutant cells is not highly specific with respect to species transported. Thus, the species pattern of aminoglycerophospholipids in mitochondria is mainly the result of enzyme specificities, but not of translocation processes involved. Our results support a model that suggests equilibrium transport of aminoglycerophospholipids between mitochondria and microsomes based on membrane contact between the two compartments.

The Yeast Phospholipid N-Methyltransferases Catalyzing the Synthesis of Phosphatidylcholine Preferentially Convert Di-C16:1 Substrates Both in Vivo and in Vitro

Phosphatidylcholine (PC) is an important and abundant structural component of the membranes of eukaryotic cells. In the yeast Saccharomyces cerevisiae, the primary route for the biosynthesis of PC consists of three consecutive methylation steps of phosphatidylethanolamine (PE) catalyzed by the phospholipid N-methyltransferases Cho2p and Opi3p. To investigate how these biosynthetic enzymes contribute to the composition of the PC species profile, the precursor-product relationships between PE and newly synthesized PC were determined at the level of the molecular species by using electrospray ionization tandem mass spectrometry and stable isotope labeling. In vivo labeling of yeast cells for 10 min with [methyl-D3]methionine revealed the preferential methylation of di-C16:1 PE over a range of PE species compositions. A similar preferential conversion of di-C16:1 PE to PC was found in vitro upon incubating isolated microsomes with S-adenosyl[methyl-D3]methionine. Yeast opi3 and cho2 deletion strains were used to distinguish between the substrate selectivities of Cho2p and Opi3p, respectively. Both biosynthetic enzymes were found to participate in the speciesselective methylation with Cho2p contributing the most. The combined results indicate that the selective methylation of PE species by the methyltransferases plays an important role in shaping the steady-state profile of PC molecular species in yeast.

Substrate preference in phosphatidylserine biosynthesis for docosahexaenoic acid containing species

Neuronal membranes contain high levels of phosphatidylserine (PS) and docosahexaenoic acid (22:6n-3, DHA). In this study, substrate preference in PS synthesis was determined to gain insight on the biochemical basis for concentrating PS in neuronal membranes where 22:6n-3 is highly enriched. We first established an in vitro assay method using unilamellar vesicles (LUV) of deuterium-labeled substrates and reversed-phase HPLC/electrospray ionization (ESI) mass spectrometry. The PS production by the incubation of deuterium-labeled substrate and microsomal fractions was monitored. We found that tissue-specific substrate preference exists in PS synthesis. Microsomes from the cerebral cortex synthesized PS from 18:0,22:6-PC most favorably among the PC substrates tested, followed by 18:0,22:5-PC, resulting in the PC substrate preference in the order of 18:0,22:6 > 18:0,22:5 > 18:0,20:4 = 18:0,18:1. Liver microsomes also preferred 18:0,22:6-PC as the substrate in PS synthesis but did not use 18:0,22:5-PC favorably. The 18:0,22:5-PC species was converted to PS at the similar extent as 18:0,20:4- or 18:0,18:1-PC species in the liver. Both brain and liver microsomes showed a preference for 18:0 over 16:0 as the sn-1 fatty acid. From these data it was deduced that preferential conversion of 18:0,22:6-PC to the corresponding PS species is at least partly responsible for concentrating PS in neuronal tissues where 22:6n-3 is particularly abundant. The distinctive preference for 18:0,22:5-PS observed with brain microsomes may help to maintain PS at a high level in the brain when 22:6n-3 is replaced by 22:5n-3 as in the case of n-3 fatty acid deficiency.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

The two biosynthetic routes leading to phosphatidylcholine in yeast produce different sets of molecular species. Evidence for lipid remodeling

Phosphatidylcholine (PC), a major lipid class in the membranes of eukaryotes, is synthesized either via the triple methylation of phosphatidylethanolamine (PE) or via the CDP-choline route. To investigate whether the two biosynthetic routes contribute differently to the steady-state profile of PC species, i.e., PC molecules with specific acyl chain compositions, the pools of newly synthesized PC species were monitored by labeling Saccharomyces cerevisiae with deuterated precursors of the two routes, (methyl-D3)-methionine and (D13)-choline, respectively. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) revealed that the two PC biosynthetic pathways yield different sets of PC species, with the CDP-choline route contributing most to the molecular diversity. Moreover, yeast was shown to be capable of remodeling PC by acyl chain exchange at the sn-1 position of the glycerol backbone. Remodeling was found to be required to generate the steady-state species distribution of PC. This is the first study demonstrating a functional difference between the two biosynthetic routes in yeast.

Age-related changes in central nervous system phosphatidylserine decarboxylase activity

A significant portion of brain phosphatidylethanolamine (PE) is synthesized by a pathway involving the mitochondrial enzyme phosphatidylserine decarboxylase (PSDC), in a process by which phosphatidylserine (PS) is transferred from the endoplasmic reticulum to mitochondria. Aging changes the fatty acid composition of brain phospholipids, PS and PE being the most affected. The present study was carried out to determine PSDC activity in cerebral cortex (CC) and cerebellum (CRBL) mitochondrial fraction from adult (4-month-old) and aged (30-month-old) rats and to compare these activities with that found in liver. To study the effect of 22:6n-3 content on the PSDC activity, PSs from different sources were prepared: rPS (from bovine retina, containing 36 mol % of 22:6n-3); adPS (from adult rat CC microsomal membranes, with 25 mole % 22:6n-3 content) and agPS (from aged rat CC microsomal membranes, with 21 mole % 22:6n-3 content). For aged CC PSDC, the preferred substrate was agPS (the physiological substrate for aged animals), whereas in adult CC PSDC the substrate preference was inverse (rPS > adPS > agPS). Furthermore, CRBL PSDC does not show any substrate preference based on 22:6n-3 content. CRBL PSDC activity in aged membranes using agPS as substrate is lower than PSDC activity in adult membranes in the presence of adPS. These results indicate that under physiological conditions, cerebellar PSDC is inhibited during aging. Liver PSDC activity showed the same substrate preference in adult and aged rats as adult CC PSDC. These findings lead us to conclude that PSDC activity has a differential tissue-dependent substrate preference characteristic of the aging process.

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.

Translocation of phosphatidylthreonine and -serine to mitochondria diminishes exponentially with increasing molecular hydrophobicity

Some cultured cells contain significant amounts of a rarely recognized phospholipid, phosphatidylthreonine. Since phosphatidylthreonine is a structural analog of phosphatidylserine, the question rises whether it is transported to mitochondria and decarboxylated to phosphatidylisopropanolamine therein. We studied this issue with hamster kidney cell-line using a novel approach, i.e. electrospray mass-spectrometry and stable isotope-labeled precursors. Scanning for a neutral loss of 155, which is characteristic for phosphatidylisopropanolamine, indicated that this lipid is indeed present. The identity of phosphatidylisopropanolamine was supported by the following: (i) it co-chromatographed with phosphatidylethanolamine; (ii) its molecular species profile was similar to that of phosphatidylethanolamine; (iii) its head group was labeled from 13C-threonine; and (iv) its concentration increased in parallel with phosphatidylthreonine. Tests with solubilized decarboxylase and subcellular fractionation studies indicated that the low cellular content of phosphatidylisopropanolamine is due to inefficient decarboxylation, rather than poor translocation of phosphatidylthreonine to mitochondria. Importantly, the average hydrophobicity of phosphatidylisopropanolamine molecular species was significantly less than that of phosphatidylthreonine species, indicating that hydrophilic phosphatidylthreonine species translocate to mitochondria far more rapidly than hydrophobic ones. Parallel results were obtained for phosphatidylserine. These findings imply that efflux from the ER membrane could be the rate-limiting step in the phosphatidylthreonine and -serine translocation to mitochondria.

Characteristics of pyrene phospholipid/gamma-cyclodextrin complex

Recently, it was demonstrated that gamma-cyclodextrins (gamma-CDs) greatly accelerates transfer of hydrophobic pyrene-labeled and other fluorescent phospholipid derivatives from vesicles to cells in culture (). To understand better the characteristics of this process, we studied the interaction of gamma-CD with pyrene-labeled phosphatidylcholines (PyrPCs) using a variety of physical methods. Either one or both of the acyl chains of PC was labeled with a pyrene moiety (monoPyrPCs and diPyrPCs, respectively), and the length of the labeled chain(s) varied from 4 to 14 carbons. Fluorescent binding assays showed that the association constant decreases strongly with increasing acyl chain length. PyrPC/gamma-CD stoichiometry was 1:2 for the shorter chain species, but changed to 1:3 when the acyl chain length exceeded 8 (diPyrPCs) or 10 (monoPyrPCs) carbons. The activation energy for the formation of diPyr(10)PC/gamma-CD complex was high, i.e., +92 kJ/mol, indicating that the phospholipid molecule has to fully emerge from the bilayer before complex formation can take place. The free energy, enthalpy, and entropy of transfer of monoPyrPC from bilayer to gamma-CD complex were close to zero. The absorption, Fourier transform infrared, and fluorescence spectral measurements and lifetime analysis indicated that the pyrene moiety lies inside the CD cavity and is conformationally restricted, particularly when the labeled chain is short. The acyl chains of a PyrPC molecule seem to share a CD cavity rather than occupy different ones. The present data provide strong evidence that the ability of gamma-CD to enhance intermembrane transfer of pyrene-labeled phospholipids is based on the formation of stoichiometric complexes in the aqueous phase. This information should help in designing CD derivatives that are more efficient lipid carriers then those available at present.

Determination of substrate preference in phosphatidylserine decarboxylation by liquid chromatography-electrospray ionization mass spectrometry

A method has been developed to determine the substrate preference in phosphatidylserine decarboxylation (PSD), the process by which phosphatidylserine is converted to phosphatidylethanolamine (PE) in the mitochondria. The in vitro assay utilized liposomes containing deuterium-labeled PS molecular species incubated with liver and brain cortex mitochondria, and the conversion of PS to the corresponding PE species was monitored by electrospray ionization mass spectrometry in conjunction with reversed-phase liquid chromatography. Employing this approach we were able to establish for the first time that there exists a substrate preference in PSD in liver (18:0,18:1 > or = 18:0,22:6 > 18:0,20:4-PS) and brain cortex (18:0,22:6 > 18:0,18:1 > 18:0,20:4-PS). The observed PSD molecular species preference, however, did not reflect the mitochondrial PE profile, suggesting that selectivity in other processes such as de novo PE synthesis, intracellular transport of phospholipid molecules, or remodeling by deacylation-reacylation may be important contributors in maintaining a specific lipid profile in mitochondria.

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.

Association between the endoplasmic reticulum and mitochondria of yeast facilitates interorganelle transport of phospholipids through membrane contact

Membrane association between mitochondria and the endoplasmic reticulum of the yeast Saccharomyces cerevisiae is probably a prerequisite for phospholipid translocation between these two organelles. This association was visualized by fluorescence microscopy and computer-aided three-dimensional reconstruction of electron micrographs from serial ultrathin sections of yeast cells. A mitochondria-associated membrane (MAM), which is a subfraction of the endoplasmic reticulum, was isolated and re-associated with mitochondria in vitro. In the reconstituted system, phosphatidylserine synthesized in MAM was imported into mitochondria independently of cytosolic factors, bivalent cations, ATP, and ongoing synthesis of phosphatidylserine. Proteolysis of mitochondrial surface proteins by treatment with proteinase K reduced the capacity to import phosphatidylserine. Phosphatidylethanolamine formed in mitochondria by decarboxylation of phosphatidylserine is exported to the endoplasmic reticulum where part of it is converted into phosphatidylcholine. In contrast with previous observations with permeabilized yeast cells [Achleitner, G., Zweytick, D., Trotter, P., Voelker, D. & Daum, G. (1995) J. Biol. Chem. 270, 29836-29842], export of phosphatidylethanolamine from mitochondria to the endoplasmic reticulum was shown to be energy-independent in the reconstituted yeast system.

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis of the lipid molecular species composition of yeast subcellular membranes reveals acyl chain-based sorting/remodeling of distinct molecular species en route to the plasma membrane

Nano-electrospray ionization tandem mass spectrometry (nano-ESI-MS/MS) was employed to determine qualitative differences in the lipid molecular species composition of a comprehensive set of organellar membranes, isolated from a single culture of Saccharomyces cerevisiae cells. Remarkable differences in the acyl chain composition of biosynthetically related phospholipid classes were observed. Acyl chain saturation was lowest in phosphatidylcholine (15.4%) and phosphatidylethanolamine (PE; 16.2%), followed by phosphatidylserine (PS; 29.4%), and highest in phosphatidylinositol (53.1%). The lipid molecular species profiles of the various membranes were generally similar, with a deviation from a calculated average profile of approximately +/- 20%. Nevertheless, clear distinctions between the molecular species profiles of different membranes were observed, suggesting that lipid sorting mechanisms are operating at the level of individual molecular species to maintain the specific lipid composition of a given membrane. Most notably, the plasma membrane is enriched in saturated species of PS and PE. The nature of the sorting mechanism that determines the lipid composition of the plasma membrane was investigated further. The accumulation of monounsaturated species of PS at the expense of diunsaturated species in the plasma membrane of wild-type cells was reversed in elo3Delta mutant cells, which synthesize C24 fatty acid-substituted sphingolipids instead of the normal C26 fatty acid-substituted species. This observation suggests that acyl chain-based sorting and/or remodeling mechanisms are operating to maintain the specific lipid molecular species composition of the yeast plasma membrane.

Release of phospholipids from erythrocyte membranes by taurocholate is determined by their transbilayer orientation and hydrophobic backbone

Bile salts mediate a specific release of phosphatidylcholine (PC) from the canalicular membrane into the bile fluid. We utilized human red blood cells (RBC) as a model system to study the release of endogenous phospholipids as well as phospholipid analogues from plasma membranes in the presence of the bile salt taurocholate (TC). Short- and long-chain fluorescent as well as spin-labeled analogues with various headgroups were chosen. RBC were labeled either on the exoplasmic or on the cytoplasmic leaflet with the analogues and incubated with various concentrations of TC. Analogues on the exoplasmic layer could be readily released by TC. Release was most efficient above the critical micellar concentration (CMC) of TC. Release was independent of the headgroup, but depended on the fatty acid chain length of the analogues; i.e., it was lower for long-chain than for short-chain labeled phospholipids. Analogues on the cytoplasmic leaflet were efficiently shielded from TC-mediated release. The preferential release of endogenous PC and sphingomyelin (SM) from the erythrocyte membrane above the CMC supports the conclusion that TC-mediated release of phospholipids occurs preferentially from the exoplasmic leaflet independent of their headgroup. However, the extent of release of endogenous phospholipids was significantly lower in comparison to that of analogues, endorsing the relevance of the hydrophobic backbone for bile salt mediated release of phospholipids. Implications for the mechanism of the release of PC from the canalicular membrane into the bile fluid are discussed.