Protein-dependent translocation of aminophospholipids and asymmetric transbilayer distribution of phospholipids in the plasma membrane of ram sperm cells

PUFA-rich phospholipid classes and subclasses of ram spermatozoa are unevenly affected by cryopreservation with a soybean lecithin-based extender

Cryopreservation is known to affect spermatozoa structure and function. Ram sperm are among the most highly sensitive mammalian gametes to freezing, due to their lipid composition, which limit their efficiency in artificial insemination programs. The aim of this study was to investigate the effects of cryopreservation with a chemically defined soybean lecithin-based extender on ram spermatozoa functionality on the one hand, and quantifiable changes in lipid and fatty acid profile on the other. Freeze-thawing decreased sperm quality, as indicated by post-thaw parameters related to membrane integrity, mitochondrial viability and sperm motility. The most relevant lipid change after cryopreservation was a remarkable loss of all glycerophospholipids containing 22:6n-3. Species of sphingomyelin with very long chain polyunsaturated fatty acids (VLC-PUFA), that are exclusively located in the sperm head, where responsible of its reduction after cryostorage. Freezing caused a reduction in mitochondrial function, which was confirmed by significantly decreased of mitochondrial membrane potential and by the generation of 4-HNE. Mitochondria damage was accompanied by a loss in cardiolipin with 18:2n-6 and phosphatidylethanolamine with 20:4n-6, two well-known lipids that are critical components for mitochondrial membrane functionality. Loss of sterols after cryopreservation occurred along with a decrease in the order of sperm membrane lipids. Our research provides new insights on deleterious effects of cryopreservation on PUFA-rich phospholipids of ram sperm and highlight their importance as biomarkers of ultrastructural, biochemical and functional damage that ram spermatozoa undergo after freezing-thawing.

Lipid Signaling During Gamete Maturation

Cell lipids are differentially distributed in distinct organelles and within the leaflets of the bilayer. They can further form laterally defined sub-domains within membranes with important signaling functions. This molecular and spatial complexity offers optimal platforms for signaling with the associated challenge of dissecting these pathways especially that lipid metabolism tends to be highly interconnected. Lipid signaling has historically been implicated in gamete function, however the detailed signaling pathways involved remain obscure. In this review we focus on oocyte and sperm maturation in an effort to consolidate current knowledge of the role of lipid signaling and set the stage for future directions.

Seminal lipid profiling and antioxidant capacity: A species comparison

On their way to the oocyte, sperm cells are subjected to oxidative stress, which may trigger the oxidation of phospholipids (PL). Applying MALDI-TOF MS, HPTLC and ESI-IT MS, we comparatively analyzed the PL compositions of semen and blood of species differing in their reproductive systems and types of nutrition (bull, boar, stallion, lion and man) with regard to the sensitivity to oxidation as well as the accumulation of harmful lyso-PL (LPL), transient products of lipid oxidation. In addition, the protective capacity of seminal fluid (SF) was also examined. The PL composition of erythrocytes and blood plasma is similar across the species, while pronounced differences exist for sperm and SF. Since the blood function is largely conserved across mammalian species, but the reproductive systems may vary in many aspects, the obtained results suggest that the PL composition is not determined by the type of nutrition, but by the relatedness of species and by functional requirements of cell membranes such as fluidity. Sperm motion and fertilization of oocytes require a rather flexible membrane, which is accomplished by significant moieties of unsaturated fatty acyl residues in sperm lipids of most species, but implies a higher risk of oxidation. Due to a high content of plasmalogens (alkenyl ether lipids), bull sperm are most susceptible to oxidation. Our data indicate that bull sperm possess the most effective protective power in SF. Obviously, a co-evolution of PL composition and protective mechanisms has occurred in semen and is related to the reproductive characteristics. Although the protective capacity in human SF seems well developed, we recorded the most pronounced individual contaminations with LPL in human semen. Probably, massive oxidative challenges related to lifestyle factors interfere with natural conditions.

Sperm Lipid Markers of Male Fertility in Mammals

Sperm plasma membrane lipids are essential for the function and integrity of mammalian spermatozoa. Various lipid types are involved in each key step within the fertilization process in their own yet coordinated way. The balance between lipid metabolism is tightly regulated to ensure physiological cellular processes, especially referring to crucial steps such as sperm motility, capacitation, acrosome reaction or fusion. At the same time, it has been shown that male reproductive function depends on the homeostasis of sperm lipids. Here, we review the effects of phospholipid, neutral lipid and glycolipid homeostasis on sperm fertilization function and male fertility in mammals.

Anchoring dipalmitoyl phosphoethanolamine to nanoparticles boosts cellular uptake and fluorine-19 magnetic resonance signal

Magnetic resonance (MR) methods to detect and quantify fluorine (¹⁹F) nuclei provide the opportunity to study the fate of cellular transplants in vivo. Cells are typically labeled with ¹⁹F nanoparticles, introduced into living organisms and tracked by ¹⁹F MR methods. Background-free imaging and quantification of cell numbers are amongst the strengths of ¹⁹F MR-based cell tracking but challenges pertaining to signal sensitivity and cell detection exist. In this study we aimed to overcome these limitations by manipulating the aminophospholipid composition of ¹⁹F nanoparticles in order to promote their uptake by dendritic cells (DCs). As critical components of biological membranes, phosphatidylethanolamines (PE) were studied. Both microscopy and MR spectroscopy methods revealed a striking (at least one order of magnitude) increase in cytoplasmic uptake of ¹⁹F nanoparticles in DCs following enrichment with 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE). The impact of enriching ¹⁹F nanoparticles with PE on DC migration was also investigated. By manipulating the nanoparticle composition and as a result the cellular uptake we provide here one way of boosting ¹⁹F signal per cell in order to overcome some of the limitations related to ¹⁹F MR signal sensitivity. The boost in signal is ultimately necessary to detect and track cells in vivo.

Effects of cationic antimicrobial peptides on liquid-preserved boar spermatozoa

Antibiotics are mandatory additives in semen extenders to control bacterial contamination. The worldwide increase in resistance to conventional antibiotics requires the search for alternatives not only for animal artificial insemination industries, but also for veterinary and human medicine. Cationic antimicrobial peptides are of interest as a novel class of antimicrobial additives for boar semen preservation. The present study investigated effects of two synthetic cyclic hexapeptides (c-WFW, c-WWW) and a synthetic helical magainin II amide derivative (MK5E) on boar sperm during semen storage at 16°C for 4 days. The standard extender, Beltsville Thawing Solution (BTS) containing 250 µg/mL gentamicin (standard), was compared to combinations of BTS with each of the peptides in a split-sample procedure. Examination revealed peptide- and concentration-dependent effects on sperm integrity and motility. Negative effects were more pronounced for MK5E than in hexapeptide-supplemented samples. The cyclic hexapeptides were partly able to stimulate a linear progressive sperm movement. When using low concentrations of cyclic hexapeptides (4 µM c-WFW, 2 µM c-WWW) sperm quality was comparable to the standard extender over the course of preservation. C-WFW-supplemented boar semen resulted in normal fertility rates after AI. In order to investigate the interaction of peptides with the membrane, electron spin resonance spectroscopic measurements were performed using spin-labeled lipids. C-WWW and c-WFW reversibly immobilized an analog of phosphatidylcholine (PC), whereas MK5E caused an irreversible increase of PC mobility. These results suggest testing the antimicrobial efficiency of non-toxic concentrations of selected cyclic hexapeptides as potential candidates to supplement/replace common antibiotics in semen preservation.

Biochemical evidence for energy-independent flippase activity in bovine epididymal sperm membranes: an insight into membrane biogenesis

During the maturation process spermatozoa undergo a series of changes in their lateral and horizontal lipid profiles. However, lipid metabolism in spermatozoa is not clearly understood for two reasons: i) the mature spermatozoa are devoid of endoplasmic reticulum, which is the major site of phospholipid (PL) synthesis in somatic cells, and ii) studies have been superficial due to the difficulty in culturing spermatozoa. We hypothesize that spermatozoa contain biogenic membrane flippases since immense changes in lipids occur during spermatogenic differentiation. To test this, we isolated spermatozoa from bovine epididymides and reconstituted the detergent extract of sperm membranes into proteoliposomes. In vitro assays showed that proteoliposomes reconstituted with sperm membrane proteins exhibit ATP-independent flip-flop movement of phosphatidylcholine (PC), phosphatidylserine, and phosphatidylglycerol. Half-life time of PC flipping was found to be ∼3.2±1 min for whole sperm membrane, which otherwise would have taken ∼11-12 h in the absence of protein. Further biochemical studies confirm the flip-flop movement to be protein-mediated, based on its sensitivity to protease and protein-modifying reagents. To further determine the cellular localization of flippases, we isolated mitochondria of spermatozoa and checked for ATP-independent flippase activity. Interestingly, mitochondrial membranes showed flip-flop movement but were specific for PC with half-life time of ∼5±2 min. Our results also suggest that spermatozoa have different populations of flippases and that their localization within the cellular compartments depends on the type of PL synthesis.

Effect of different monosaccharides and disaccharides on boar sperm quality after cryopreservation

The aim of the present study was to evaluate the cryoprotectant effect of different non-permeating sugars for boar sperm. Pooled semen from three boars was used for the experiments. In the first experiment, the sperm quality of boar sperm cryopreserved with an egg-yolk based extender supplemented with different monosaccharides (glucose, galactose or fructose) was compared to a control cryopreserved in lactose-egg yolk extender. In the second experiment, the effect of five disaccharides (lactose, sucrose, lactulose, trehalose or melibiose) on boar sperm cryosurvival was studied. Several sperm quality parameters were assessed by flow cytometry in samples incubated for 30 and 150 min at 37°C after thawing: percentages of sperm with intact plasma membrane (SIPM), sperm presenting high plasma membrane fluidity (HPMF), sperm with intracellular reactive oxygen substances production (IROSP) and apoptotic sperm (AS). In addition, the percentages of total motile (TMS) and progressively motile sperm (PMS) were assessed at the same incubation times with a computer-assisted sperm analysis system. Freezing extenders supplemented with each of the monosaccharide presented smaller cryoprotective effect than the control extender supplemented with lactose (P<0.05). However, from the three monosaccharides tested, glucose provided the best sperm quality after freezing-thawing. With respect to the disaccharides studied, samples frozen with the extender supplemented with lactulose exhibited in general the lowest sperm quality, except for the percentage of capacitated sperm, which was highest (P<0.05) in the samples cryopreserved with the trehalose extender. Our results suggest that disaccharides have higher cryoprotective effect than monosaccharides, although the monosaccharide composition of the disaccharides is also important, since the best results were obtained with those disaccharides presenting glucose in their composition.

Phosphatidylserine membrane translocation in human spermatozoa: topography in membrane domains and relation to cell vitality

The complex structure of the human spermatozoa membrane comprises five topographic domains. Transmembrane asymmetry of the distribution of phospholipids including phosphatidylserine (PS) is considered a marker of cell activity. The objective of the study was to determine which cytomembrane domains of human spermatozoa are involved in PS membrane translocation and to identify the possible relationship of PS translocation with spermatozoa morphology and vitality. In normozoospermic semen of 35 donors, annexin-V labeling with fluorescein determined PS translocation. Propidium iodide staining distinguished between vital and dead spermatozoa. Three types of PS membrane translocation have been distinguished: (1) in the midpiece, (2) in the acrosomal part and (3) simultaneously in the midpiece and acrosomal part. In morphologically normal vital spermatozoa, PS translocation occurred in the midpiece but never in the equatorial region. In dead spermatozoa, simultaneous PS translocation in the midpiece and acrosomal part was most often observed. The difference between proportions of, respectively, vital and dead spermatozoa presenting PS translocation located in different domains was significant (P < 0.0001). In vital cells, there was no difference in PS translocation prevalence between morphologically normal and abnormal spermatozoa (P > 0.05). The strict relation of PS translocation to specific membrane domains indicates functional specificity. It seems doubtful to include this phenomenon in physiological mechanisms of elimination of abnormal spermatozoa.

Effects of cryopreservation on mitochondrial function and heterogeneity, lipid raft stability and phosphatidylserine translocation in koala (Phascolarctos cinereus) spermatozoa

Koala sperm mitochondria were examined by cryomicroscopy using the fluorescent probe JC-1, which distinguishes high (red) and low (green) mitochondrial membrane potential (MMP). At normal body temperature, ~70% of live and untreated spermatozoa exhibited high MMP whereas <3% of live untreated spermatozoa exhibited low potential. A third class, in which single midpieces contained mixed mitochondrial populations, was also detected. Heterogeneity was noted in the level of MMP between individual koalas, individual spermatozoa and even between mitochondrial gyres within single midpieces. MMP of the live sperm population was not significantly affected by glycerol but was suppressed by freezing and thawing treatments. After thawing, MMP declined significantly during rewarming, especially as the temperature increased from 5 to 35°C. The distribution of the ganglioside GM1 was examined using fluorescent-labelled cholera toxin B. In fresh, untreated koala spermatozoa GM1 was detected on the head and midpiece, but not on the principal piece. No significant redistribution of GM1 was observed after chilling and cryotreatment. Phosphatidylserine translocation across the plasma membrane was examined using fluorescent-labelled annexin V. Few fresh spermatozoa exhibited phosphatidylserine translocation (~1%); this was not increased by chilling or cryopreservation, thus implying that cryotreatment had little effect on plasma membrane lipid asymmetry.

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.

Localization of phosphatidylserine in boar sperm cell membranes during capacitation and acrosome reaction

One of the essential properties of mammalian, including sperm, plasma membranes is a stable transversal lipid asymmetry with the aminophospholipids, phosphatidylserine (PS) and phosphatidylethanolamine (PE), typically in the inner, cytoplasmic leaflet. The maintenance of this nonrandom lipid distribution is important for the homeostasis of the cell. To clarify the relevance of lipid asymmetry to sperm function, we have studied the localization of PS in boar sperm cell membranes. By using labeled annexin V as a marker for PS and propidium iodide (PI) as a stain for nonviable cells in conjunction with different methods (flow cytometry, fluorescence and electron microscopy), we have assessed the surface exposure of PS in viable cells during sperm genesis, that is, before and during capacitation as well as after acrosome reaction. An approach was set up to address also the presence of PS in the outer acrosome membrane. The results show that PS is localized in the cytoplasmic leaflet of the plasma membrane as well as on the outer acrosome membrane. Our results further indicate the cytoplasmic localization of PS in the postacrosomal region. During capacitation and acrosome reaction of spermatozoa, PS does not become exposed on the outer surface of the viable cells. Only in a subpopulation of PI-positive sperm cells does PS became accessible upon capacitation. The stable cytoplasmic localization of PS in the plasma membrane, as well as in the outer acrosome membrane, is assumed to be essential for a proper genesis of sperm cells during capacitation and acrosome reaction.

Fas-, caspase 8-, and caspase 3-dependent signaling regulates the activity of the aminophospholipid translocase and phosphatidylserine externalization in human erythrocytes

Apoptosis and erythrocyte senescence share the common feature of exposure of phosphatidylserine (PS) in the outer leaflet of the cells. Western analysis showed that mature red cells contain Fas, FasL, Fas-associated death domain (FADD), caspase 8, and caspase 3. Circulating, aged cells showed colocalization of Fas with the raft marker proteins Galpha(s) and CD59; the existence of Fas-associated FasL, FADD and caspase 8; and caspase 8 and caspase 3 activity. Aged red cells had significantly lower aminophospholipid translocase activity and higher levels of PS externalization in comparison with young cells. In support of our contention that caspases play a functional role in the mature red cell, the oxidatively stressed red cell recapitulated apoptotic events, including translocation of Fas into rafts, formation of a Fas-associated complex, and activation of caspases 8 and 3. These events were independent of calpain but dependent on reactive oxygen species (ROS) as evident from the effects of the ROS scavenger N-acetylcysteine. Caspase activation was associated with loss of aminophospholipid translocase activity and with PS externalization. ROS was not generated by treatment of cells with t-butyl hydroperoxide at 10 degrees C, and Fas did not translocate into rafts. Concomitantly, neither formation of a Fas-associated signaling complex nor caspase activation could be observed, supporting the view that translocation of Fas into rafts was the trigger for the chain of events leading to caspase 3 activation. Our data demonstrate for the first time the novel involvement of Fas/caspase 8/caspase 3-dependent signaling in an enucleated cell leading to PS externalization, a central feature of erythrophagocytosis and erythrocyte biology.

Bicarbonate-induced membrane processing in sperm capacitation

During capacitation, major changes take place in the sperm plasma membrane so as to render it fusogenic and responsive to zona pellucida glycoproteins. However, the mechanisms involved have not been defined. As bicarbonate is known to be the key component that induces capacitation, we have investigated the bicarbonate-dependent changes in the boar sperm's plasma membrane architecture. We have discovered that bicarbonate induces a rapid collapse of phospholipid transverse asymmetry, exposing phosphatidylethanolamine and phosphatidylserine at the outer surface of the lipid bilayer. The collapse, which is reversible, is brought about as a result of activation of the phospholipid scramblase that exchanges phospholipids in a non-specific fashion between the two leaflets of the lipid bilayer. The activation takes place via a cyclic AMP-protein kinase A-dependent pathway and is initiated via stimulation of the so-called 'soluble' adenylyl cyclase in the sperm cell by bicarbonate. As a result of the collapse and the concurrent increase in phospholipid exchange, removal of cholesterol by albumin is facilitated (perhaps due to increased lipid packing disorder). This finding is in conflict with earlier surmises that cholesterol loss precedes activation of the cyclic AMP-protein kinase A axis. We have noted that not all cells in a given sperm population show rapid changes in response to bicarbonate stimulation; samples from individual boars also differ in their response. Maturation differences between cells have been found to play an important role in such functional heterogeneity.

Activity of phospholipases A and lysophospholipase in turkey semen and oviducal fluid

Changes in lipid composition of turkey semen have previously been reported to occur during in vitro storage and may be mediated by endogenous hydrolysis of phospholipids. To investigate the presence of phospholipases able to initiate such degradation, phospholipaseA2 (PLA2), phospholipase A1 (PLA1), and lysophospholipase (LPLase) activities were measured in turkey spermatozoa and seminal plasma. These enzymes were also measured in the oviductal fluid because they may be involved in the process prior to fertilization in the female. In spermatozoa and seminal plasma, the major PLA2 was a calcium-dependent and sodium deoxycholate (DOC) stimulated enzyme. However, calcium-independent PLA2 activities were also detected with different characteristics in spermatozoa (DOC inhibited enzyme) and seminal plasma (DOC stimulated enzyme). Additionally, PLA1 activity and high LPLase activity were present in spermatozoa and seminal plasma. In vitro storage of semen for 48 h did not affect PLA2 and LPLase activities. By contrast, PLA1 was the major phospholipase activity detected in oviductal fluid. A PLA2 activity stimulated by calcium or DOC and LPLase activity were also detected, but both were low relative to PLA1. These results showed that turkey semen had several enzymatic activities able to hydrolyze phospholipids. In addition, the phospholipase activities described here in the oviductal fluid could be involved in membrane destabilization prior to fertilization.

Presence and significance of somatic cell apoptosis markers in human ejaculated spermatozoa

Ejaculated spermatozoa, particularly in infertile men, have been shown to display morphological and biochemical features that are typical of an apoptotic phenotype in somatic cells. Deregulation of apoptosis is known to play roles in a number of disease processes, but roles for apoptosis in ejaculated spermatozoa and male infertility are poorly defined or have not been studied. Preliminary data demonstrated that populations of ejaculated spermatozoa express: (i) various degrees of plasma membrane translocation of phosphatidylserine and DNA fragmentation; and (ii) active caspase-3, the main executor of apoptosis in somatic cells, with an apparent exclusive cellular location to the mid-piece. Tests are currently being carried out on the effects of well-known apoptosis agonists and caspase inhibitors on such markers using purified populations of leukocyte-free ejaculated human spermatozoa. The main objective is to determine if somatic cell apoptosis markers are relevant indicators and/or causative factors of male infertility.

A novel aminophospholipid transporter exclusively expressed in spermatozoa is required for membrane lipid asymmetry and normal fertilization

Through the use of a functionally unbiased signal peptide trap screen, we have discovered an ATP-dependent aminophospholipid transporter that is exclusively expressed in the acrosomal region of spermatozoa; it is about 62% similar to the flippase, FIC1. We disrupted the transporter gene and found that the size of litters from male null mice was slightly smaller than found with wild-type males. Sperm morphology and motility were the same between null and wild-type littermates, but agents (merocyanine and annexin) that measure phospholipid packing or phosphatidylserine (PS) in the outer membrane leaflet showed that PS already existed in the outer leaflet of null spermatozoa before sperm capacitation. Fertilization rates were normal when null spermatozoa were added to zona pellucida-free eggs, but in the presence of the extracellular matrix, fewer transporter(-/-) spermatozoa bound tightly or penetrated the zona pellucida (ZP), and fewer underwent acrosome reactions. In vitro fertilization was compromised, especially at early time points or at low sperm concentrations after mixing null spermatozoa and eggs. Thus, a new aminophospholipid transporter expressed exclusively in spermatozoa is critical for normal phospholipid distribution in the bilayer, and for normal binding, penetration, and signaling by the zona pellucida.

Subtle membrane changes in cryopreserved bull semen in relation with sperm viability, chromatin structure, and field fertility

This study investigated the use of annexin-V/PI assay to assess sub lethal changes in bull spermatozoa post-thawing, and to further relate these changes to results obtained by fluorometric assessment of sperm viability and sperm chromatin structure assay (SCSA), as well as field fertility (as 56-day non-return rates, 56-day NRR) after AI. Frozen-thawed semen samples were obtained from 18 Swedish Red and White bulls (one to three semen batches/bull) and fertility data was based on 6900 inseminations. The annexin-V/PI assay revealed that post-thaw semen samples contained on average 41.8+/-7.5% annexin-V-positive cells. Most of the annexin-V-positive cells were dying cells, i.e. also PI-positive. The incidence of annexin-V-positive cells was negatively related (r=-0.59, P<0.01) to the percentage of viable cells, as detected by fluorometry. The incidence of annexin-V-positive spermatozoa significantly correlated to the SCSA variable xalphat (r=0.53, P<0.05). The incidence of annexin-V-negative, dead cells was the only annexin-V/PI assay variable that correlated significantly with fertility both at batch (r=-0.40, P<0.05), and bull (r=-0.56, P<0.05) levels. Among sperm viability variables, subjectively assessed sperm motility (r=0.52-0.59, P<0.01), CASA-assessed sperm motility (r=0.43-0.61, P<0.05), and the incidence of live spermatozoa, expressed as total numbers (r=0.39-0.54, P<0.05), or percentage values (r=0.68-0.68, P<0.01), correlated significantly with field fertility both at batch, and bull levels. Among the SCSA variables, only the COMP alphat correlated significantly (r=0.33-0.51, P<0.05) with fertility results. The results indicate a certain proportion of bull spermatozoa express PS on their surface after thawing, e.g. they have altered membrane function, and that the incidence of such cells is inversely correlated to sperm viability, and positively correlated to abnormal sperm chromatin condensation since they eventually undergo necrosis.

Regulation of transbilayer plasma membrane phospholipid asymmetry

Lipids in biological membranes are asymmetrically distributed across the bilayer; the amine-containing phospholipids are enriched on the cytoplasmic surface of the plasma membrane, while the choline-containing and sphingolipids are enriched on the outer surface. The maintenance of transbilayer lipid asymmetry is essential for normal membrane function, and disruption of this asymmetry is associated with cell activation or pathologic conditions. Lipid asymmetry is generated primarily by selective synthesis of lipids on one side of the membrane. Because passive lipid transbilayer diffusion is slow, a number of proteins have evolved to either dissipate or maintain this lipid gradient. These proteins fall into three classes: 1) cytofacially-directed, ATP-dependent transporters ("flippases"); 2) exofacially-directed, ATP-dependent transporters ("floppases"); and 3) bidirectional, ATP-independent transporters ("scramblases"). The flippase is highly selective for phosphatidylserine and functions to keep this lipid sequestered from the cell surface. Floppase activity has been associated with the ABC class of transmembrane transporters. Although they are primarily nonspecific, at least two members of this class display selectivity for their substrate lipid. Scramblases are inherently nonspecific and function to randomize the distribution of newly synthesized lipids in the endoplasmic reticulum or plasma membrane lipids in activated cells. It is the combined action of these proteins and the physical properties of the membrane bilayer that generate and maintain transbilayer lipid asymmetry.

Energy-dependent flip of fluorescence-labeled phospholipids is regulated by nutrient starvation and transcription factors, PDR1 and PDR3

The yeast Saccharomyces cerevisiae readily accumulates short-chain, fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled phosphatidylcholine and phosphatidylethanolamine at the nuclear envelope/endoplasmic reticulum and mitochondria. The net intracellular accumulation reflects the sum of their inwardly and outwardly directed transbilayer translocation across the plasma membrane (flip and flop, respectively). The rate of flop is negligible in energy-depleted cells as well as at low temperature (2 degrees C). Although flip is reduced at 2 degrees C, it can still be measured by flow cytometry, allowing the rate of flip, independent of flop, to be characterized at this temperature. Flip requires the energy of the plasma membrane proton electrochemical gradient and is down-regulated as cells pass through the diauxic shift and enter stationary phase. Furthermore, drug-resistant, gain-of-function mutations in the transcription factors, PDR1 and PDR3, result in a dramatic down-regulation of flip in addition to their already established up-regulation of flop. These results imply that down-regulation of the NBD-phospholipid flip pathway is a physiological response to environmental stress.

NBD-labeled phosphatidylcholine and phosphatidylethanolamine are internalized by transbilayer transport across the yeast plasma membrane

The internalization and distribution of fluorescent analogs of phosphatidylcholine (M-C6-NBD-PC) and phosphatidylethanolamine (M-C6-NBD-PE) were studied in Saccharomyces cerevisiae. At normal growth temperatures, M-C6-NBD-PC was internalized predominantly to the vacuole and degraded. M-C6-NBD-PE was internalized to the nuclear envelope/ER and mitochondria, was not transported to the vacuole, and was not degraded. At 2 degrees C, both were internalized to the nuclear envelope/ER and mitochondria by an energy-dependent, N-ethylmaleimide-sensitive process, and transport of M-C6-NBD-PC to and degradation in the vacuole was blocked. Internalization of neither phospholipid was reduced in the endocytosis-defective mutant, end4-1. However, following pre-incubation at 37 degrees C, internalization of both phospholipids was inhibited at 2 degrees C and 37 degrees C in sec mutants defective in vesicular traffic. The sec18/NSF mutation was unique among the sec mutations in further blocking M-C6-NBD-PC translocation to the vacuole suggesting a dependence on membrane fusion. Based on these and previous observations, we propose that M-C6-NBD-PC and M-C6-NBD-PE are transported across the plasma membrane to the cytosolic leaflet by a protein-mediated, energy-dependent mechanism. From the cytosolic leaflet, both phospholipids are spontaneously distributed to the nuclear envelope/ER and mitochondria. Subsequently, M-C6-NBD-PC, but not M-C6-NBD-PE, is sorted by vesicular transport to the vacuole where it is degraded by lumenal hydrolases.

Dynamics of the mammalian sperm plasma membrane in the process of fertilization

Sexual reproduction requires the fusion of sperm cell and oocyte during fertilization to produce the diploid zygote. In mammals complex changes in the plasma membrane of the sperm cell are involved in this process. Sperm cells have unusual membranes compared to those of somatic cells. After leaving the testes, sperm cells cease plasma membrane lipid and protein synthesis, and vesicle mediated transport. Biophysical studies reveal that lipids and proteins are organized into lateral regions of the sperm head surface. A delicate reorientation and modification of plasma membrane molecules take place in the female tract when sperm cells are activated by so-called capacitation factors. These surface changes enable the sperm cell to bind to the extra cellular matrix of the egg (zona pellucida, ZP). The ZP primes the sperm cell to initiate the acrosome reaction, which is an exocytotic process that makes available the enzymatic machinery required for sperm penetration through the ZP. After complete penetration the sperm cell meets the plasma membrane of the egg cell (oolemma). A specific set of molecules is involved in a disintegrin-integrin type of anchoring of the two gametes which is completed by fusion of the two gamete plasma membranes. The fertilized egg is activated and zygote formation preludes the development of a new living organism. In this review we focus on the involvement of processes that occur at the sperm plasma membrane in the sequence of events that lead to successful fertilization. For this purpose, dynamics in adhesive and fusion properties, molecular composition and architecture of the sperm plasma membrane, as well as membrane derived signalling are reviewed.

Identification and purification of aminophospholipid flippases

Transbilayer phospholipid asymmetry is a common structural feature of most biological membranes. This organization of lipids is generated and maintained by a number of phospholipid transporters that vary in lipid specificity, energy requirements and direction of transport. These transporters can be divided into three classes: (1) bidirectional, non-energy dependent 'scramblases', and energy-dependent transporters that move lipids (2) toward ('flippases') or (3) away from ('floppases') the cytofacial surface of the membrane. One of the more elusive members of this family is the plasma membrane aminophospholipid flippase, which selectively transports phosphatidylserine from the external to the cytofacial monolayer of the plasma membrane. This review summarizes the characteristics of aminophospholipid flippase activity in intact cells and describes current strategies to identify and isolate this protein. The biochemical characteristics of candidate flippases are critically compared and their potential role in flippase activity is evaluated.

The capacitating agent bicarbonate induces protein kinase A-dependent changes in phospholipid transbilayer behavior in the sperm plasma membrane

A flow cytometric procedure was used to follow the effect of bicarbonate, a key inducer of sperm capacitation in vitro, on the transbilayer behavior of C6NBD-phospholipids in the plasma membrane of living acrosome-intact boar spermatozoa under physiological conditions. In the absence of bicarbonate, 97% of C6NBD-phosphatidylserine and 78% of C6NBD-phosphatidylethanolamine was rapidly translocated from the outer leaflet to the inner, whereas relatively little C6NBD-phosphatidylcholine and C6NBD-sphingomyelin was translocated (15% and 5%, respectively). Inclusion of 15 mM bicarbonate/5%CO(2) markedly slowed down the rates of translocation of the aminophospholipids without altering their final distribution, whereas it increased the proportions of C6NBD-phosphatidylcholine and C6NBD-sphingomyelin translocated (30% and 20%, respectively). Bicarbonate activated very markedly the outward translocation of all four phospholipid classes. The changes in C6NBD-phospholipid behavior were accompanied by increased membrane lipid disorder as detected by merocyanine 540, and also by increased potential for phospholipase catabolism of the C6NBD-phospholipid probes. All three changes were mediated via a cAMP-dependent protein phosphorylation pathway. We suspect that the changes result from an activation of the non- specific bidirectional translocase ('scramblase'). They have important implications with respect to sperm fertilizing function.

P-glycoprotein plays a drug-efflux–independent role in augmenting cell survival in acute myeloblastic leukemia and is associated with modulation of a sphingomyelin-ceramide apoptotic pathway

P-glycoprotein (pgp), which is the product of the MDR1(multidrug resistance-1) gene, has an established role as a mediator of cytotoxic drug resistance in acute myeloid leukemia (AML). To study the role of pgp in mediating apoptosis resistance in AML cells deprived of serum and growth factors, apoptosis was quantified by flow cytometry using uptake of the dye 7-amino-actinomycin D (7-AAD) alongside low forward scatter. In pgp+ve primary AML samples, there was a significant increase in apoptosis in the presence of the pgp-specific antibody UIC2 (mean increase: 58%; range: 11%-95%; P &lt; .05). Likewise, apoptosis in growth factor–deprived TF1 cells cultured for 30 hours increased 2.5-fold in the presence of 25 μg/mL UIC2. The pgp reversal agent PSC-833 (1 μmol/L) augmented in vitro apoptosis by a median of 52% in pgp+ve patient samples and to a comparable degree in 6 pgp−ve samples. To determine whether the sphingomyelin-ceramide (SM-ceramide) pathway of apoptosis occurs in AML blasts in response to cytotoxic drugs, cells were incubated with daunorubicin at the patient-specific IC30 (the concentration of daunorubicin that caused apoptotic cell death in 30% of cells) in the presence of the ceramide synthase inhibitor fumonisin B1, which inhibited apoptosis by 18%-81% (median: 40%). Exogenous SM failed to augment apoptosis induced by growth factor withdrawal in pgp+ve TF1 cells and was significantly more effective at augmenting apoptosis in pgp−ve patient blasts (median increase in cell death: 33%; range: 19%-88%) than in pgp+ve samples (median: 7%; range: 0%-27%;P = .028). Cellular accumulation of exogenous SM was associated with apoptosis and also occurred in nonapoptotic patient cells treated with PSC-833. However, this effect was not seen following treatment with the UIC2 antibody. These results indicate that pgp is able to exert a protective effect on AML cell viability and that this is associated with a reduced effect of exogenous SM on apoptosis. The pgp reversal agent PSC-833 acts, at least in part, by a pgp independent mechanism to alter SM distribution and to augment apoptosis induced in AML cells by serum and growth factor withdrawal.

The transbilayer distribution of phospholipids in disc membranes is a dynamic equilibrium evidence for rapid flip and flop movement

We studied the transbilayer redistribution of phospholipids in bovine rod outer segment membranes on thoroughly washed, Ficoll-floated osmotically intact disc vesicles; freshly prepared membranes separated from the disc stack by osmotic shock; and intact disc stacks with a permeabilized plasma membrane (A-discs, B-discs C-discs, respectively). In all cases, spin-labelled phospholipid analogues (SL-PL) with choline, serine and ethanolamine head groups (PtdCho, PtdSer and PtdEtn, respectively) were taken up into the outer leaflet of the membranes by > 90% and within less than 30 s after SL-PL addition, as deduced from the disappearance of spin-label from the suspension medium and from the specific ESR spectrum of membrane-associated spin-label. Using BSA extraction, the amount of SL-PL in the outer leaflet of the bilayer was determined. It decreased with a mean half-time of < 5 min at 25 degrees C, indicating rapid redistribution of all spin-labelled phospholipids into the inner leaflet of the disc membranes. After 1 h, PtdCho and PtdEtn were distributed almost symmetrically, whereas PtdSer was 35 : 65% (in/out). Using subsequent incubation with BSA, the outward movement (flop) of the analogues was observed directly, demonstrating that inward and outward movements proceed in thermodynamic equilibrium. No effect of N-ethylmaleimide or ATP on the redistribution could be measured, which makes it unlikely that energy-consuming translocase or flippase processes are involved in the redistribution in the dark. We reason that the solubilization zone around the photoreceptor rhodopsin may be the locus of rapid redistribution of the highly unsaturated disc phospholipid.

Fatty acid composition of spermatozoa and immature germ cells

A great deal of attention has recently been given to the essential role of polyunsaturated fatty acids (PUFA) of sperm membranes. We studied the fatty acid composition of the immature germ cells (IGC) and of the sperm populations separated by Percoll gradient in the ejaculate of normozoospermic patients. Fatty acid pattern was analysed by combined gas chromatography-mass spectrometry on a capillary column. In IGC, differences were found compared with mature spermatozoa, with a higher percentage of saturated fatty acids and of essential fatty acids. On the contrary, the long-chain PUFA were significantly lower in IGC. The highest concentration of n3 PUFA docohexaenoic acid (DHA) was detected in the spermatozoa deriving from 70-100% Percoll layers and a direct linear correlation was found between the increase of DHA and increased percentage of Percoll gradient. An inverse relationship between the percentage of atypical sperm forms in each layer and the percentage of DHA was also observed. This study demonstrates that the human germ cell line can elongate and desaturate essential fatty acids and that the percentage of long-chain PUFA is correlated with the normal morphology of sperm cells.

cAMP-dependent protein kinase control of plasma membrane lipid architecture in boar sperm

Bicarbonate/CO(2), a physiological effector of sperm capacitation, has been shown to induce a rapid and reversible change in the lipid architecture of the plasma membrane of live boar sperm: the change is detectable as an increase in the cells' ability to bind the fluorescent dye merocyanine, a characteristic which implied an increase in lipid packing disorder (Harrison et al. 1996. Mol Reprod Dev 45:378-391). Evidence suggested that cAMP may act as a second messenger in the system, and we have therefore investigated this cAMP-dependency in more detail. Bicarbonate stimulates cAMP levels within 1 min in a dose-dependent fashion, prior to parallel increases in merocyanine binding. Although the potent somatic cell adenylyl cyclase activator forskolin is unable to induce significant increases in cAMP or merocyanine binding, increases in merocyanine binding are inducible in a dose-dependent fashion by 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole 3',5'-cyclic monophosphothioate, a cAMP analogue highly specific in its ability to stimulate protein kinase A; moreover, the bicarbonate-induced membrane change is inhibited by H89, a specific protein kinase A inhibitor. Neither bisindolylmaleimide I (protein kinase C inhibitor) nor lavendustin A (protein tyrosine kinase inhibitor) are inhibitory. In the presence of low levels of the potent phosphodiesterase inhibitor papaverine, increases in merocyanine binding are enhanced by okadaic acid and (more effectively) by calyculin (both protein phosphatase inhibitors). We conclude that boar sperm plasma membrane lipid architecture is controlled via a target protein that is dynamically phosphorylated by cAMP-dependent protein kinase and dephosphorylated by protein phosphatase type 1. Mol. Reprod. Dev. 55:220-228, 2000.

Rapid transbilayer movement of fluorescent phospholipid analogues in the plasma membrane of endocytosis-deficient yeast cells does not require the Drs2 protein

Evidence is presented that endocytosis-deficient Saccharomyces cerevisiae end4 yeast cells rapidly internalize the fluorescent phospholipid analogues 1-palmitoyl-2-{6-[7-nitro-2,1, 3-benzoxadiazol-4-yl(NBD)amino] caproyl}phosphatidylcholine (P-C6-NBD-PtdCho) and P-C6-NBD-phosphatidylserine (P-C6-NBD-PtdSer). Both analogues redistributed between the exoplasmic and cytoplasmic leaflet with a half-time of < 15 min at 0 degrees C. The plateau of internalized analogues was about 70%. Transbilayer movement is probably protein-mediated, as the flip-flop of both analogues was very slow in liposomes composed of plasma-membrane lipids. Rapid analogue internalization was not abolished on depletion of intracellular ATP by about 90%. For P-C6-NBD-PtdCho only was a moderate decrease in the plateau of internalized analogues of about 20% observed, while that of P-C6-NBD-PtdSer was not affected. The Drs2 protein plays only a minor role, if any, in the rapid transbilayer movement of analogues in S. cerevisiae end4 cells. In S. cerevisiae end4 Deltadrs2 cells harbouring both an end4 allele and a drs2 null allele, about 60% and 50% of P-C6-NBD-PtdCho and P-C6-NBD-PtdSer, respectively, became internalized within 15 min at 0 degrees C. The preferential orientation of P-C6-NBD-PtdSer to the cytoplasmic leaflet is in qualitative agreement with the sequestering of endogenous phosphatidylserine to the cytoplasmic leaflet, as assessed by binding of annexin V. Virtually no binding of annexin V to spheroplasts of the parent wild-type strain or the mutant strains was observed. Likewise, no difference in the exposure of endogenous aminophospholipids to the exoplasmic leaflet between these strains was found by labelling with trinitrobenzenesulfonic acid. Thus, lipid asymmetry, at least of aminophospholipids, was preserved in S. cerevisiae end4 cells independently of the presence of the Drs2 protein.

Flow cytometric detection of transbilayer movement of fluorescent phospholipid analogues across the boar sperm plasma membrane: elimination of labeling artifacts

Reliable protocols were established for investigating asymmetric distributions of 6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino-caproyl (C6NBD) phospholipids in the plasma membrane of boar sperm cells under physiological conditions. A method based on fluorescence resonance energy transfer was used to ensure that incorporation of the fluorescent phospholipids into the sperm proceeded via monomeric transfer. The total amount of incorporated phospholipid fluorescence and the proportion of translocated phospholipid fluorescence were determined by flow cytometric analysis before, and after, dithionite destruction of outer leaflet fluorescence. Catabolism of incorporated fluorescent phospholipids was blocked with phenylmethylsulfonyl fluoride. Membrane-damaged cells were detected with impermeant DNA stains, thereby enabling their exclusion from subsequent analyses of the flow cytometric data, whence it could be demonstrated that the labeled phospholipids were incorporated only via the outer plasma membrane leaflet in living sperm cells. Phospholipid uptake and internalization was followed at 38 degrees C. After 1 hr of labeling, about 96% of the incorporated C6NBD-phosphatidylserine, 80% of C6NBD-phosphatidylethanolamine, 18% of C6NBD-phosphatidylcholine, and 4% of C6NBD-sphingomyelin were found to have moved across the plasma membrane bilayer to the interior of the spermatozoa. These inward movements of fluorescent phospholipids were ATP-dependent and could be blocked with sulfhydryl reagents. Movements from the inner to the outer leaflet of the sperm plasma membrane were minimal for intact fluorescent phospholipids, but were rapid and ATP-independent for fluorescent lipid metabolites. The described method enables, for the first time, assessment of changes in lipid asymmetry under fertilizing conditions.

Internalization of phospholipids from the plasma membrane of human osteoblasts depends on the lipid head group

The redistribution of spin- or fluorescence-labeled phospholipid analogs across the plasma membrane of human osteoblast cells, either in suspension or grown as monolayers, was investigated. After incorporation into the outer membrane leaflet, analogs of the aminophospholipids phosphatidylserine and phosphatidylethanolamine moved rapidly to the inner monolayer, whereas the choline-containing analogs of phosphatidylcholine and sphingomyelin disappeared more slowly from the outer leaflet. The fast inward movement of the aminophospholipids became reduced after lowering the intracellular ATP, suggesting the presence of an aminophospholipid translocase activity in the plasma membrane of these cells. From these data, a transverse phospholipid asymmetry in osteoblasts can be inferred with the aminophospholipids mainly concentrated in the inner monolayer and the choline-containing phospholipids in the outer leaflet. A similar pattern of phospholipid internalization was inferred for osteoblasts from human osteoporotic bones and for a human osteosarcoma cell line. The relevance of the enrichment of phosphatidylserine in the cytoplasmic membrane leaflet for calcification in skeletal tissues is emphasized.

Stability of transbilayer phospholipid asymmetry in viable ram sperm cells after cryotreatment

The transbilayer dynamics of lipids in the plasma membrane of mammalian sperm cells is crucial for the fertilization process. Here, the transbilayer movement and distribution of phospholipids in the plasma membrane of fresh, ejaculated and cryopreserved ram spermatozoa was studied by labeling cells with fluorescent analogues of phosphatidylserine and phosphatidylcholine. By co-labeling cells with the DNA-binding dye propidiumiodide as well as by employing fluorescence microscopy and flow cytometry we were able to determine the transbilayer redistribution of fluorescent phospholipid analogues in intact (propidiumiodide-negative) and in impaired (propidiumiodide-positive) spermatozoa. The transbilayer distribution of the fluorescent phosphatidylserine and phosphatidylcholine analogues was not perturbed in intact sperm cells after cryopreservation. In those cells, the phosphatidylserine analogue became rapidly enriched on the cytoplasmic leaflet by the activity of a putative aminophospholipid translocase similar to intact cells of fresh, ejaculated samples. However, upon cryopreservation the activity of the putative aminophospholipid translocase was significantly reduced in intact cells. Employing annexin V-FITC, we found that even after cryopreservation the sequestering of endogenous phosphatidylserine to the cytoplasmic leaflet is maintained in intact cells, but not in impaired cells. The phosphatidylcholine analogue redistributed very slowly remaining essentially confined to the exoplasmic leaflet of the plasma membrane of intact cells from both fresh, ejaculated and cryopreserved samples. The physiological consequences of a perturbed transbilayer asymmetry in sperm plasma membranes is discussed.

Fatty acid pattern of human prostasome lipid

Prostasomes are organelles of prostatic origin found in human semen. Their average diameter is about 150 nm and they appear as a lipoprotein membrane surrounding less organized material. Their lipid composition is peculiar, having much cholesterol and sphingomyelin. On the other hand, many of their proteins possess catalytic activity and are involved in the immune response. In previous work, we have shown that prostasomes may fuse to sperm at slightly acidic pH values, thereby modifying the composition of the sperm plasma membrane. In this paper, we examine the fatty acid pattern of prostasome lipid and find that it is completely different from that of sperm membrane lipid. Polyunsaturated phosphatidylcholines, common in sperm membrane, are rare in prostasome. Therefore, the fusion between prostasomes and sperm should stabilize sperm plasma membrane by enriching it in cholesterol, sphingomyelin, and saturated glycerophospholipid. This would prevent the untimely occurrence of the acrosome reaction.

Compensating lipid fluxes generated by the aminophospholipid translocase

By a combined kinetic and thermodynamic model on the transbilayer dynamics and asymmetric distribution of lipids in the red blood cell, compensating lipid fluxes to the exoplasmic leaflet have been analysed, counterbalancing the active transport of aminophospholipids to the cytoplasmic monolayer by the aminophospholipid translocase. The compensating fluxes are assumed to be of passive nature generated by forces of lateral mechanical stress and of lipid concentration differences between the two monolayers. These forces are shown to be caused and maintained by the operation of the aminophospholipid translocase. Simulations reveal that a reduction of the compensating fluxes upon ATP-depletion can be attributed to the inhibition of the aminophospholipid translocase. Thus, a Mg(2+)- and ATP-dependence of the outward movement of phospholipid analogues in the plasma membrane of red blood cells can be expected independent of the existence and operation of an ATP-dependent 'floppase' activity.

Rapid kinetics of insertion and accessibility of spin-labeled phospholipid analogs in lipid membranes: a stopped-flow electron paramagnetic resonance approach

Spin-labeled phospholipid analogs have been employed to probe the transbilayer distribution of endogenous phospholipids in various membrane systems. To determine the transmembrane distribution of the spin-labeled analogs, the analogs are usually inserted into the membrane of interest and subsequently the amount of analog in the outer membrane leaflet is determined either by chemical reduction with ascorbate or by back-exchange to bovine serum albumin (BSA). For accurate determination of the transbilayer distribution of analogs, both the kinetics of incorporation and those of accessibility of analogs to ascorbate or BSA have to be fast in comparison to their transbilayer movement. By means of stopped-flow electron paramagnetic resonance (EPR) spectroscopy, we have studied the kinetics of incorporation of the spin-labeled phosphatidylcholine (PC) analog 1-palmitoyl-2-(4-doxylpentanoyl)-sn-glycero-3-phosphocholine (SL-PC) and of its accessibility to chemical reduction and to back-exchange at room temperature. Incorporation of SL-PC into the outer leaflet of egg phosphatidylcholine (EPC) and red cell ghost membranes was essentially completed within 5 s. Ninety percent of the SL-PC molecules located in the outer membrane leaflet of those membranes were extracted by BSA within 15 s. All exterior-facing SL-PC molecules were reduced by ascorbate in a pseudo-first-order reaction within 60 s in EPC membranes and within 90 s in red cell ghost membranes. The rate of the reduction process could be enhanced by approximately 30-fold when 6-O-phenyl-ascorbic acid was used instead of ascorbate as the reducing agent. The results are discussed in light of assaying rapid transbilayer movement of spin-labeled analogs in biological membranes.

Influence of pH on Phospholipid Redistribution in Human Erythrocyte Membrane

The influence of the suspension pH (pHo ) on the transmembrane mobility of spin-labeled phospholipid analogues in the human red blood cell was investigated. The passive transverse diffusion of spin-labeled phospholipid analogues was independent of pHo in the investigated range (5.8 to 8.5). However, upon acidification to pHo 5.8, a significant decrease of the rapid adenosine triphosphate (ATP)-dependent inward movement of aminophospholipids was found at physiologic ionic concentration, whereas a change of pH from 7.4 to 8.5 did not affect this transport. Evidence is given that the intracellular pH affects the active transport of aminophospholipids but not the extracellular pH. Suppression of the ATP-dependent outside-inside redistribution of aminophospholipid analogues by low pH was reversible because original transport activity was re-established upon reneutralization. pH dependence of the active phospholipid transport was not caused by the spin-labeled reporter group or by depletion of intracellular ATP. Because the same influence of pH on aminophospholipid movement could be observed for resealed ghosts, constituents of the red blood cell cytoplasm do not mediate the influence of pH on the ATP-dependent inward movement of aminophospholipids.

Influence of pH on Phospholipid Redistribution in Human Erythrocyte Membrane

The influence of the suspension pH (pHo ) on the transmembrane mobility of spin-labeled phospholipid analogues in the human red blood cell was investigated. The passive transverse diffusion of spin-labeled phospholipid analogues was independent of pHo in the investigated range (5.8 to 8.5). However, upon acidification to pHo 5.8, a significant decrease of the rapid adenosine triphosphate (ATP)-dependent inward movement of aminophospholipids was found at physiologic ionic concentration, whereas a change of pH from 7.4 to 8.5 did not affect this transport. Evidence is given that the intracellular pH affects the active transport of aminophospholipids but not the extracellular pH. Suppression of the ATP-dependent outside-inside redistribution of aminophospholipid analogues by low pH was reversible because original transport activity was re-established upon reneutralization. pH dependence of the active phospholipid transport was not caused by the spin-labeled reporter group or by depletion of intracellular ATP. Because the same influence of pH on aminophospholipid movement could be observed for resealed ghosts, constituents of the red blood cell cytoplasm do not mediate the influence of pH on the ATP-dependent inward movement of aminophospholipids.

Aminophospholipid translocase activity in JEG-3; a choriocarcinoma model of cytotrophoblast differentiation

The plasma membrane is characterized by a non-symmetrical distribution of phospholipids; the outer monolayer of the plasma membrane consists primarily of phosphatidylcholine (PC), and the aminophospholipids, phosphatidylserine (PS) and phosphatidylethanolamine (PE), preferentially reside in the inner monolayer. Asymmetry is maintained by a membrane associated ATP-dependent aminophospholipid translocase that preferentially relocates PS and PE from the outer to the inner monolayer. Although in most cells the translocase minimizes expression of PS on the outer surface, differentiating trophoblasts express increasing levels of surface PS. One possible explanation of prolonged PS externalization is that trophoblasts lack an effective aminophospholipid translocase. To test this hypothesis, fluorescent PC and PS analogues, NBD-PC and NBD-PS, were introduced into the plasma membrane of a choriocarcinoma model of trophoblast, JEG-3 cells. After incubation, the fluorescent lipid remaining on the outer monolayer was removed by incubation with fetal bovine serum. JEG-3 cells selectively translocated 80 per cent of the NBD-PS without significant translocation of NBD-PC. The process was significantly inhibited by N-ethylmaleimide (NEM) and vanadate. It is concluded that this model of trophoblast contains an active aminophospholipid translocase.

Altered membrane phospholipid organization and erythrophagocytosis in E beta-thalassemia

The underlying cause behind the accelerated destruction of erythrocytes in the bone marrow and in the peripheral circulation, accompanying the beta-thalassemic syndromes is still not clearly understood. The present investigation demonstrates that increased phagocytosis of erythrocytes in E beta-thalassemia is inhibited by the presence of phosphatidylserine (PS) vesicles, suggesting a PS-'PS-receptor' type of interaction in premature recognition of these erythrocytes by macrophages. Increased exposure of both aminophospholipids phosphatidylethanolamine (PE) and PS was demonstrated by fluorescamine labeling and annexin binding, respectively. The slower rate of translocation of PS across the bilayer suggested that this contributed towards the increased exposure of PS in E beta-thalassemic erythrocytes.

Bicarbonate/CO2, an effector of capacitation, induces a rapid and reversible change in the lipid architecture of boar sperm plasma membranes

Bicarbonate/CO2 is believed to be the key in vitro effector of sperm capacitation, a process which induces major changes in the sperm plasma membrane in preparation for fertilization. In a flow cytometric study, we examined the effect of bicarbonate on boar spermatozoa using merocyanine, an impermeant lipophilic probe which binds to plasma membranes with increasing affinity as their lipid components become more disordered. We found that bicarbonate causes a rapid increase in the ability of live boar spermatozoa to bind merocyanine. First detected about 100 sec after exposure to bicarbonate and largely complete by 300 sec, this increase appears to result from individual cells within the sperm population switching from a low merocyanine-binding state to a high binding state. The majority of live spermatozoa are capable of responding in this way, and do so in proportion to bicarbonate concentration, half-maximal response being induced by about 3 mM bicarbonate; however, overall population response varies greatly between ejaculates. Increased merocyanine stainability is observed over the whole surface area of the cell, and is reversible both with respect to temperature (it is only manifested above 30 degrees C) and with respect to presence of bicarbonate. A similar effect can be induced by phosphodiesterase inhibitors such as isobutylmethylxanthine, and enhanced by a permeant cyclic nucleotide analogue. We conclude that bicarbonate causes a major alteration in sperm plasma membrane lipid architecture, apparently by perturbing enzymic control processes. This novel action of bicarbonate may represent an initial permissive event in the capacitation sequence.

Transverse mobility of aminophospholipids in the plasma membrane of ram sperm cells

The transbilayer movement of aminophospholipids in the plasma membrane of ram sperm cells was investigated using spin-labeled lipid analogues. After incorporation, spinlabeled phosphatidylserine (SL-PS) and phosphatidyl-ethanolamine (SL-PE) rapidly disappeared from the exoplasmic monolayer. Even at lower temperatures (10 degrees C) the inward movement of SL-PE is fast. The initial velocities of the internalization of SL-PS and SL-PE were compared with those of ram and human erythrocytes.

Transbilayer movement of fluorescent and spin-labeled phospholipids in the plasma membrane of human fibroblasts: a quantitative approach

All phospholipids in the plasma membrane of eukaryotic cells are subject to a slow passive transbilayer movement. In addition, aminophospholipids are recognized by the so-called aminophospholipid translocase, and are rapidly moved from the exoplasmic to the cytoplasmic leaflet of the plasma membrane at the expense of ATP hydrolysis. Though these principal pathways of transbilayer movement of phospholipids probably apply to all eukaryotic plasma membranes, studies of the actual kinetics of phospholipid redistribution have been largely confined to non-nucleated cells (erythrocytes). Experiments on nucleated cells are complicated by endocytosis and metabolism of the lipid probes inserted into the plasma membrane. Taking these complicating factors into account, we performed a detailed kinetic study of the transbilayer movement of short-chain fluorescent (N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl); NBD) and, for the first time, spin-labeled analogues of phosphatidylcholine (PC), -ethanolamine (PE), -serine (PS), and sphingomyelin (SM) in the plasma membrane of cultured human gingival fibroblasts. At 20 degrees C, the passive transbilayer diffusion of NBD analogues was very slow, and the choline-containing NBD analogues were internalized predominantly by endocytosis. Spin-labeled analogues of PC and SM showed higher passive transbilayer diffusion rates, and probably entered the cell by both passive transbilayer movement and endocytosis. In contrast, the rapid uptake of NBD- and spin-labeled aminophospholipid analogues could be mainly ascribed to the action of the aminophospholipid translocase, since it was inhibited by ATP depletion and N-ethylmaleimide pretreatment. The initial velocity of NBD-aminophospholipid translocation was eight to ten times slower than that of the corresponding spin-labeled lipid, and the half-times of redistribution of NBD-PS and spin-labeled PS were 7.2 and 3.6 minutes, respectively. Our data indicate that in human fibroblasts the initial velocity of aminophospholipid translocation is at least one order of magnitude higher than that in human erythrocytes, which should be sufficient to maintain the phospholipid asymmetry in the plasma membrane.

Flow cytometric analysis of transmembrane phospholipid movement in bull sperm

Fluorescent phospholipids are useful to investigate phospholipid dynamics in biological membranes. We used flow cytometry to investigate transbilayer phospholipid movement in live sperm cells. Acyl-labeled N-4-nitrobenzo-2-oxa-1,3-diazole (NBD) -phosphatidylcholine (-PC), -phosphatidylethanolamine (-PE), or -phosphatidylserine (-PS) were incorporated into sperm cells, and the transbilayer location was determined by extraction of probe from cell with excess bovine serum albumin (BSA) or by chemical destruction of probe by sodium dithionite. Using these methods, we have measured the head group specific outer leaflet to inner leaflet movement (flip) of the aminophospholipids NBD-PS and NBD-PE. The fluorescent phospholipids moved inward across the plasma membrane with half-times of 1.8, 2.5, and 11.2 min, for NBD-PS, NBD-PE, and NBD-PC and reached apparent equilibrium levels of 88%, 94%, and 32% inside, respectively. The inward movement of NBD-PE was inhibited by sulfhydryl reagents, elevated intracellular Ca2+, and depletion of cellular ATP. Analysis of the kinetics of NBD-PE and -PS extraction by BSA allows determination of the rates for outward movement (flop) across the plasma membrane. Half-times for flop were 4.7 and 4.5 min for NBD-PS and -PE, respectively. Based on these measurements, a simple model of NBD-phospholipid equilibria was developed and fit to the kinetic data. Computer-generated fits reflected major features of the experimental data and provide a potential tool for predicting the dynamics of endogenous lipids.