Isolation of a Chinese hamster fibroblast variant defective in dihydroxyacetonephosphate acyltransferase activity and plasmalogen biosynthesis: use of a novel two-step selection protocol

Regulation of plasmalogen biosynthesis in mammalian cells and tissues

Plasmalogens are a unique family of cellular glycerophospholipids that contain a vinyl-ether bond. Synthesis of plasmalogens is initiated in peroxisomes and completed in the endoplasmic reticulum. The absence of plasmalogens in several organs of patients with deficiency in peroxisome biogenesis suggests that de novo synthesis of plasmalogens contributes significantly to plasmalogen homeostasis in humans. Plasmalogen biosynthesis is spatiotemporally regulated by a feedback mechanism that senses the amount of plasmalogens in the inner leaflet of the plasma membrane and regulates the stability of fatty acyl-CoA reductase 1 (FAR1), the rate-limiting enzyme for plasmalogen biosynthesis. Dysregulation of plasmalogen synthesis impairs cholesterol synthesis in cells and brain, resulting in the reduced expression of genes such as mRNA encoding myelin basic protein, a phenotype found in the cerebellum of plasmalogen-deficient mice. In this review, we summarize the current knowledge of molecular mechanisms underlying the regulation of plasmalogen biosynthesis and the link between plasmalogen homeostasis and cholesterol biosynthesis, and address the pathogenesis of impaired plasmalogen homeostasis in rodent and humans.

Association of miRNA-145 with the occurrence and prognosis of hydrosalpinx-induced defective endometrial receptivity

MiR-145 is reported to facilitate inflammation and is also associated with unsuccessful embryonic implantation. Whether miR-145 mediates inflammatory response underlying hydrosalpinx-induced defective endometrial receptivity (ER) remains unclear, and this study attempted to clarify this point. Endometrium samples were collected from hydrosalpinx patients (case, n = 243) and patients with tubal patency/obstruction (control, n = 187). The peripheral blood samples of cases and controls were collected to determine the genotypes of miR-145 SNPs. The value of miR-145 expression in the diagnosis and prognostic estimation of hydrosalpinx was assessed using ROC curve and regression analysis, respectively. Lipopolysaccharide (LPS) cell model was established with endometrial cells, and cells were transfected with miR-145 mimic, inhibitor, or negative control. MiR-145 and cytokine levels were quantified by quantitative reverse transcription PCR or western blot. MiR-145 expression was significantly higher in hydrosalpinx compared to control group, and high miR-145 expression was significantly associated with moderate/severe tube lesion, high pulsatility index (>1.06), and high resistance index (>0.61) in hydrosalpinx patients. ROC curve analysis indicated that monitoring miR-145 expression may be useful for the diagnosis of hydrosalpinx (AUC = 0.704). A alleles of rs41291957 (G>A) and rs353292 (G>A) were significantly associated with an increased risk of hydrosalpinx compared to G allele (p < 0.05), yet the mutant allele of rs353291 (A>G) and rs4705343 (T>C) significantly reduced susceptibility to hydrosalpinx compared to the wild type allele. Treatments with miR-145 mimic and LPS in endometrial cells significantly increased the levels of transforming growth factor-β1, tumor necrosis factor -α, interleukin (IL)-6, and IL-8 compared to negative control, while treatment with miR-145 inhibitor decreased the cytokine levels. In conclusion, abnormally expressed miR-145 may be involved in hydrosalpinx-induced ER defects by regulating the inflammatory response.

Plasmalogen mediates integration of adherens junction

Ether glycerolipids, plasmalogens are found in various mammalian cells and tissues. However, physiological role of plasmalogens in epithelial cells remains unknown. We herein show that synthesis of ethanolamine-containing plasmalogens, plasmenylethanolamine (PlsEtn), is deficient in MCF7 cells, an epithelial cell line, with severely reduced expression of alkyl-dihydroxyacetonephosphate synthase (ADAPS), the second enzyme in the PlsEtn biosynthesis. Moreover, expression of ADAPS or supplementation of PlsEtn containing C18-alkenyl residue delays the migration of MCF7 cells as compared to that mock-treated MCF7 and C16-alkenyl-PlsEtn-supplemented MCF7 cells. Localization of E-cadherin to cell-cell junctions is highly augmented in cells containing C18-alkenyl-PlsEtn. Together, these results suggest that PlsEtn containing C18-alkenyl residue plays a distinct role in the integrity of E-cadherin-mediated adherens junction.

Plasmalogen homeostasis - regulation of plasmalogen biosynthesis and its physiological consequence in mammals

Plasmalogens, mostly ethanolamine-containing alkenyl ether phospholipids, are a major subclass of glycerophospholipids. Plasmalogen synthesis is initiated in peroxisomes and completed in the endoplasmic reticulum. The absence of plasmalogens in several organs of peroxisome biogenesis-defective patients suggests that the de novo synthesis of plasmalogens plays a pivotal role in its homeostasis in tissues. Plasmalogen synthesis is regulated by modulating the stability of fatty acyl-CoA reductase 1 on peroxisomal membranes, a rate-limiting enzyme in plasmalogen synthesis, by sensing plasmalogens in the inner leaflet of plasma membranes. Dysregulation of plasmalogen homeostasis impairs cholesterol biosynthesis by altering the stability of squalene monooxygenase, a key enzyme in cholesterol biosynthesis, implying physiological consequences of plasmalogen homeostasis with respect to cholesterol metabolism in cells, as well as in organs such as the liver.

Regional changes in CNS and retinal glycerophospholipid profiles with age: a molecular blueprint

We present here a quantitative molecular blueprint of the three major glycerophospholipid (GPL) classes, phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylethanolamine (PE), in retina and six regions of the brain in C57Bl6 mice at 2, 10, and 26 months of age. We found an age-related increase in molecular species containing saturated and monoenoic FAs and an overall decrease in the longer-chain PUFA molecular species across brain regions, with loss of DHA-containing molecular species as the most consistent and dramatic finding. Although we found very-long-chain PUFAs (VLC-PUFAs) (C28) in PC in the retina, no detectable levels were found in any brain region at any of the ages examined. All brain regions (except hippocampus and retina) showed a significant increase with age in PE plasmalogens. All three retina GPLs had di-PUFA molecular species (predominantly 44:12), which were most abundant in PS (∼30%). In contrast, low levels of di-PUFA GPL (1-2%) were found in all regions of the brain. This study provides a regional and age-related assessment of the brain's lipidome with a level of detail, inclusion, and quantification that has not heretofore been published.

Biosynthesis of GPI-anchored proteins: special emphasis on GPI lipid remodeling

Glycosylphosphatidylinositols (GPIs) act as membrane anchors of many eukaryotic cell surface proteins. GPIs in various organisms have a common backbone consisting of ethanolamine phosphate (EtNP), three mannoses (Mans), one non-N-acetylated glucosamine, and inositol phospholipid, whose structure is EtNP-6Manα-2Manα-6Manα-4GlNα-6myoinositol-P-lipid. The lipid part is either phosphatidylinositol of diacyl or 1-alkyl-2-acyl form, or inositol phosphoceramide. GPIs are attached to proteins via an amide bond between the C-terminal carboxyl group and an amino group of EtNP. Fatty chains of inositol phospholipids are inserted into the outer leaflet of the plasma membrane. More than 150 different human proteins are GPI anchored, whose functions include enzymes, adhesion molecules, receptors, protease inhibitors, transcytotic transporters, and complement regulators. GPI modification imparts proteins with unique characteristics, such as association with membrane microdomains or rafts, transient homodimerization, release from the membrane by cleavage in the GPI moiety, and apical sorting in polarized cells. GPI anchoring is essential for mammalian embryogenesis, development, neurogenesis, fertilization, and immune system. Mutations in genes involved in remodeling of the GPI lipid moiety cause human diseases characterized by neurological abnormalities. Yeast Saccharomyces cerevisiae has >60 GPI-anchored proteins (GPI-APs). GPI is essential for growth of yeast. In this review, we discuss biosynthesis of GPI-APs in mammalian cells and yeast with emphasis on the lipid moiety.

Homeostasis of phospholipids - The level of phosphatidylethanolamine tightly adapts to changes in ethanolamine plasmalogens

Ethanolamine plasmalogens constitute a group of ether glycerophospholipids that, due to their unique biophysical and biochemical properties, are essential components of mammalian cellular membranes. Their importance is emphasized by the consequences of defects in plasmalogen biosynthesis, which in humans cause the fatal disease rhizomelic chondrodysplasia punctata (RCDP). In the present lipidomic study, we used fibroblasts derived from RCDP patients, as well as brain tissue from plasmalogen-deficient mice, to examine the compensatory mechanisms of lipid homeostasis in response to plasmalogen deficiency. Our results show that phosphatidylethanolamine (PE), a diacyl glycerophospholipid, which like ethanolamine plasmalogens carries the head group ethanolamine, is the main player in the adaptation to plasmalogen insufficiency. PE levels were tightly adjusted to the amount of ethanolamine plasmalogens so that their combined levels were kept constant. Similarly, the total amount of polyunsaturated fatty acids (PUFAs) in ethanolamine phospholipids was maintained upon plasmalogen deficiency. However, we found an increased incorporation of arachidonic acid at the expense of docosahexaenoic acid in the PE fraction of plasmalogen-deficient tissues. These data show that under conditions of reduced plasmalogen levels, the amount of total ethanolamine phospholipids is precisely maintained by a rise in PE. At the same time, a shift in the ratio between ω-6 and ω-3 PUFAs occurs, which might have unfavorable, long-term biological consequences. Therefore, our findings are not only of interest for RCDP but may have more widespread implications also for other disease conditions, as for example Alzheimer's disease, that have been associated with a decline in plasmalogens.

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PE accurately compensates for the lack of plasmalogens in vitro and in vivo.

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PE levels decrease to adapt to excess of ethanolamine plasmalogens (PlsEtn).

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Plasmalogen deficiency favors incorporation of arachidonic acid into PE.

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Docosahexaenoic acid in ethanolamine phospholipids decreases upon PlsEtn depletion.

Lipidomics identifies a requirement for peroxisomal function during influenza virus replication

Influenza virus acquires a host-derived lipid envelope during budding, yet a convergent view on the role of host lipid metabolism during infection is lacking. Using a mass spectrometry-based lipidomics approach, we provide a systems-scale perspective on membrane lipid dynamics of infected human lung epithelial cells and purified influenza virions. We reveal enrichment of the minor peroxisome-derived ether-linked phosphatidylcholines relative to bulk ester-linked phosphatidylcholines in virions as a unique pathogenicity-dependent signature for influenza not found in other enveloped viruses. Strikingly, pharmacological and genetic interference with peroxisomal and ether lipid metabolism impaired influenza virus production. Further integration of our lipidomics results with published genomics and proteomics data corroborated altered peroxisomal lipid metabolism as a hallmark of influenza virus infection in vitro and in vivo. Influenza virus may therefore tailor peroxisomal and particularly ether lipid metabolism for efficient replication.

Mild reduction of plasmalogens causes rhizomelic chondrodysplasia punctata: functional characterization of a novel mutation

Rhizomelic chondrodysplasia punctata (RCDP) is an autosomal recessive disorder due to the deficiency in ether lipid synthesis. RCDP type 1, the most prominent type, is caused by the dysfunction of the receptor of peroxisome targeting signal type 2, Pex7 (peroxisomal biogenesis factor 7), and the rest of the patients, RCDP types 2 and 3, have defects in peroxisomal enzymes catalyzing the initial two steps of alkyl-phospholipid synthesis, glyceronephosphate O-acyltransferase and alkylglycerone phosphate synthase (Agps). We herein investigated defects of two patients with RCDP type 3. Patient 1 had a novel missense mutation, T1533G, resulting in the I511M substitution in Agps. The plasmalogen level was mildly reduced, whereas the protein level and peroxisomal localization of Agps-I511M in fibroblasts were normal as in the control fibroblasts. Structure prediction analysis suggested that the mutated residue was located in the helix α15 on the surface of V-shaped active site tunnel in Agps, likely accounting for the mild defects of plasmalogen synthesis. These results strongly suggest that an individual with mildly affected level of plasmalogen synthesis develops RCDP. In fibroblasts from patient 2, the expression of AGPS mRNA and Agps protein was severely affected, thereby giving rise to the strong reduction of plasmalogen synthesis.

Topogenesis and homeostasis of fatty acyl-CoA reductase 1

Peroxisomal fatty acyl-CoA reductase 1 (Far1) is essential for supplying fatty alcohols required for ether bond formation in ether glycerophospholipid synthesis. The stability of Far1 is regulated by a mechanism that is dependent on cellular plasmalogen levels. However, the membrane topology of Far1 and how Far1 is targeted to membranes remain largely unknown. Here, Far1 is shown to be a peroxisomal tail-anchored protein. The hydrophobic C terminus of Far1 binds to Pex19p, a cytosolic receptor harboring a C-terminal CAAX motif, which is responsible for the targeting of Far1 to peroxisomes. Far1, but not Far2, was preferentially degraded in response to the cellular level of plasmalogens. Experiments in which regions of Far1 or Far2 were replaced with the corresponding region of the other protein showed that the region flanking the transmembrane domain of Far1 is required for plasmalogen-dependent modulation of Far1 stability. Expression of Far1 increased plasmalogen synthesis in wild-type Chinese hamster ovary cells, strongly suggesting that Far1 is a rate-limiting enzyme for plasmalogen synthesis.

Altered membrane lipid composition and functional parameters of circulating cells in cockles (Cerastoderma edule) affected by disseminated neoplasia

Membrane lipid composition and morpho-functional parameters were investigated in circulating cells of the edible cockle (Cerastoderma edule) affected by disseminated neoplasia (neoplastic cells) and compared to those from healthy cockles (hemocytes). Membrane sterol levels, phospholipid (PL) class and subclass proportions and their respective fatty acid (FA) compositions were determined. Morpho-functional parameters were evaluated through total hemocyte count (THC), mortality rate, phagocytosis ability and reactive oxygen species (ROS) production. Both morpho-functional parameters and lipid composition were profoundly affected in neoplastic cells. These dedifferentiated cells displayed higher THC (5×), mortality rate (3×) and ROS production with addition of carbonyl cyanide m-chloro phenylhydrazone (1.7×) but lower phagocytosis ability (½×), than unaffected hemocytes. Total PL amounts were higher in neoplastic cells than in hemocytes (12.3 and 5.1 nmol×10(-6) cells, respectively). However, sterols and a particular subclass of PL (plasmalogens; 1-alkenyl-2-acyl PL) were present in similar amounts in both cell type membranes. This led to a two times lower proportion of these membrane lipid constituents in neoplastic cells when compared to hemocytes (20.5% vs. 42.1% of sterols in total membrane lipids and 21.7% vs. 44.2% of plasmalogens among total PL, respectively). Proportions of non-methylene interrupted FA- and 20:1n-11-plasmalogen molecular species were the most impacted in neoplastic cells when compared to hemocytes (⅓× and ¼×, respectively). These changes in response to this leukemia-like disease in bivalves highlight the specific imbalance of plasmalogens and sterols in neoplastic cells, in comparison to the greater stability of other membrane lipid components.

Plasmalogens inhibit APP processing by directly affecting γ-secretase activity in Alzheimer's disease

Lipids play an important role as risk or protective factors in Alzheimer's disease (AD). Previously it has been shown that plasmalogens, the major brain phospholipids, are altered in AD. However, it remained unclear whether plasmalogens themselves are able to modulate amyloid precursor protein (APP) processing or if the reduced plasmalogen level is a consequence of AD. Here we identify the plasmalogens which are altered in human AD postmortem brains and investigate their impact on APP processing resulting in Aβ production. All tested plasmalogen species showed a reduction in γ-secretase activity whereas β- and α-secretase activity mainly remained unchanged. Plasmalogens directly affected γ-secretase activity, protein and RNA level of the secretases were unaffected, pointing towards a direct influence of plasmalogens on γ-secretase activity. Plasmalogens were also able to decrease γ-secretase activity in human postmortem AD brains emphasizing the impact of plasmalogens in AD. In summary our findings show that decreased plasmalogen levels are not only a consequence of AD but that plasmalogens also decrease APP processing by directly affecting γ-secretase activity, resulting in a vicious cycle: Aβ reduces plasmalogen levels and reduced plasmalogen levels directly increase γ-secretase activity leading to an even stronger production of Aβ peptides.

Defective lipid remodeling of GPI anchors in peroxisomal disorders, Zellweger syndrome, and rhizomelic chondrodysplasia punctata

Many cell surface proteins in mammalian cells are anchored to the plasma membrane via glycosylphosphatidylinositol (GPI). The predominant form of mammalian GPI contains 1-alkyl-2-acyl phosphatidylinositol (PI), which is generated by lipid remodeling from diacyl PI. The conversion of diacyl PI to 1-alkyl-2-acyl PI occurs in the ER at the third intermediate in the GPI biosynthetic pathway. This lipid remodeling requires the alkyl-phospholipid biosynthetic pathway in peroxisome. Indeed, cells defective in dihydroxyacetone phosphate acyltransferase (DHAP-AT) or alkyl-DHAP synthase express only the diacyl form of GPI-anchored proteins. A defect in the alkyl-phospholipid biosynthetic pathway causes a peroxisomal disorder, rhizomelic chondrodysplasia punctata (RCDP), and defective biogenesis of peroxisomes causes Zellweger syndrome, both of which are lethal genetic diseases with multiple clinical phenotypes such as psychomotor defects, mental retardation, and skeletal abnormalities. Here, we report that GPI lipid remodeling is defective in cells from patients with Zellweger syndrome having mutations in the peroxisomal biogenesis factors PEX5, PEX16, and PEX19 and in cells from patients with RCDP types 1, 2, and 3 caused by mutations in PEX7, DHAP-AT, and alkyl-DHAP synthase, respectively. Absence of the 1-alkyl-2-acyl form of GPI-anchored proteins might account for some of the complex phenotypes of these two major peroxisomal disorders.

Regulation of peroxisomal lipid metabolism by catalytic activity of tumor suppressor H-rev107

H-rev107 is a mammalian protein belonging to the HRAS-like suppressor family. Although the protein was originally found as a tumor suppressor, currently it is receiving considerable attention as a regulator of adipocyte lipolysis. We recently revealed that purified recombinant H-rev107 has phospholipase A(1/2) activity, releasing free fatty acids from glycerophospholipids with a preference for esterolysis at the sn-1 position. In the present study, we constitutively expressed H-rev107 in cloned HEK293 cells to examine its biological function in living cells. Initially, the cells accumulated free fatty acids. We also found a remarkable decrease in the levels of ether-type lipids, including plasmalogen and ether-type triglyceride, with a concomitant increase in fatty alcohols, substrates for the biosynthesis of ether-type lipids. Considering that peroxisomes are involved in the ether-type lipid biosynthesis, we next focused on peroxisomes and found that the peroxisomal markers 70-kDa peroxisomal membrane protein and catalase were abnormally distributed in the transfected cells. These biochemical and morphological abnormalities were not seen in HEK293 cells stably expressing a catalytically inactive mutant of H-rev107. When H-rev107 or its fusion protein with enhanced green fluorescence protein was transiently expressed in mammalian cells, both proteins were associated with peroxisomes in some of the observed cells. These results suggest that H-rev107 interferes with the biosynthesis of ether-type lipids and is responsible for the dysfunction of peroxisomes in H-rev107-expressing cells.

The Cubic "Faces" of Biomembranes

Biomembranes are traditionally viewed as flat phospholipid-bilayer sheets delineating the cell boundaries and dividing the cell into multiple subcellular organelles with specialized functions. However, biological membranes may also fold up into three-dimensional nanoperiodic arrangements, termed cubic membranes. This type of geometry is mathematically well described and extensively studied in lipidic cubic phase systems. This chapter will (1) summarize similarities and dissimilarities between cubic membranes and cubic phases; (2) provide an update on the experimental data describing the role of lipids, proteins and electrostatic charges on the biogenesis of cubic membranes; and (3) discuss their potential function in intracellular macromolecular transport and as optical filters, as well as potential practical applications such as gene delivery vehicles.

Membrane plasmalogen composition and cellular cholesterol regulation: a structure activity study

Background

Disrupted cholesterol regulation leading to increased circulating and membrane cholesterol levels is implicated in many age-related chronic diseases such as cardiovascular disease (CVD), Alzheimer's disease (AD), and cancer. In vitro and ex vivo cellular plasmalogen deficiency models have been shown to exhibit impaired intra- and extra-cellular processing of cholesterol. Furthermore, depleted brain plasmalogens have been implicated in AD and serum plasmalogen deficiencies have been linked to AD, CVD, and cancer.

Results

Using plasmalogen deficient (NRel-4) and plasmalogen sufficient (HEK293) cells we investigated the effect of species-dependent plasmalogen restoration/augmentation on membrane cholesterol processing. The results of these studies indicate that the esterification of cholesterol is dependent upon the amount of polyunsaturated fatty acid (PUFA)-containing ethanolamine plasmalogen (PlsEtn) present in the membrane. We further elucidate that the concentration-dependent increase in esterified cholesterol observed with PUFA-PlsEtn was due to a concentration-dependent increase in sterol-O-acyltransferase-1 (SOAT1) levels, an observation not reproduced by 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase inhibition.

Conclusion

The present study describes a novel mechanism of cholesterol regulation that is consistent with clinical and epidemiological studies of cholesterol, aging and disease. Specifically, the present study describes how selective membrane PUFA-PlsEtn enhancement can be achieved using 1-alkyl-2-PUFA glycerols and through this action reduce levels of total and free cholesterol in cells.

Posttranslational regulation of fatty acyl-CoA reductase 1, Far1, controls ether glycerophospholipid synthesis

Plasmalogens are a major subclass of ethanolamine and choline glycerophospholipids in which a long chain fatty alcohol is attached at the sn-1 position through a vinyl ether bond. This ether-linked alkyl bond is formed in peroxisomes by replacement of a fatty acyl chain in the intermediate 1-acyl-dihydroxyacetone phosphate with a fatty alcohol in a reaction catalyzed by alkyl dihydroxyacetone phosphate synthase. Here, we demonstrate that the enzyme fatty acyl-CoA reductase 1 (Far1) supplies the fatty alcohols used in the formation of ether-linked alkyl bonds. Far1 activity is elevated in plasmalogen-deficient cells, and conversely, the levels of this enzyme are restored to normal upon plasmalogen supplementation. Down-regulation of Far1 activity in response to plasmalogens is achieved by increasing the rate of degradation of peroxisomal Far1 protein. Supplementation of normal cells with ethanolamine and 1-O-hexadecylglycerol, which are intermediates in plasmalogen biosynthesis, accelerates degradation of Far1. Taken together, our results indicate that ether lipid biosynthesis in mammalian cells is regulated by a negative feedback mechanism that senses cellular plasmalogen levels and appropriately increases or decreases Far1.

Circulating plasmalogen levels and Alzheimer Disease Assessment Scale-Cognitive scores in Alzheimer patients

BACKGROUND

Plasmalogens, which are key structural phospholipids in brain membranes, are decreased in the brain and serum of patients with Alzheimer disease (AD). We performed this pilot study to evaluate the relation between the levels of circulating plasmalogens and Alzheimer Disease Assessment Scale-Cognitive (ADAS-Cog) scores in patients with AD.

METHODS

We evaluated participants' ADAS-Cog scores and serum plasmalogen levels. For the 40 included AD patients with an ADAS-Cog score between 20 and 46, were tested their ADAS-Cog score 1 year later. The levels of docosahexaenoic acid plasmalogen were measured by use of liquid chromatography-tandem mass spectrometry.

RESULTS

We found that the ADAS-Cog score increased significantly in AD patients with circulating plasmalogen levels that were <or= 75% of that of age-matched controls at entry into the study. There was no change in score among participants with normal serum plasmalogen levels at baseline (> 75%).

LIMITATIONS

This was a pilot study with 40 patients, and the results require validation in a larger population.

CONCLUSION

Our study demonstrates that decreased levels of plasmalogen precursors in the central nervous system correlate with functional decline (as measured by ADAS-Cog scores) in AD patients. The use of both ADAS-Cog and serum plasmalogen data may be a more accurate way of predicting cognitive decline in AD patients, and may be used to decrease the risk of including patients with no cognitive decline in the placebo arm of a drug trial.

Peroxisome dependency of alkyl-containing GPI-anchor biosynthesis in the endoplasmic reticulum

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) play various roles in cell-cell and cell-environment interactions. GPI is synthesized in the endoplasmic reticulum (ER) from phosphatidylinositol (PI) through step-wise reactions including transfers of monosaccharides and preassembled GPI is transferred en bloc to proteins. Cellular PI contains mostly diacyl glycerol and unsaturated fatty acid in the sn-2 position, whereas mammalian GPI-APs have mainly 1-alkyl-2-acyl PI and almost exclusively stearic acid, a saturated chain, at the sn-2 position. The latter characteristic is the result of fatty acid remodeling occurring in the Golgi, generating GPI-anchors compatible with raft membrane. The former characteristic is the result of diacyl to alkyl-acyl change occurring in the third GPI intermediate, glucosaminyl-inositolacylated-PI (GlcN-acyl-PI). Here we investigated the origin of the sn-1 alkyl-chain in GPI-APs. Using cell lines defective in the peroxisomal alkyl-phospholipid biosynthetic pathway, we demonstrated that generation of alkyl-containing GPI is dependent upon the peroxisomal pathway. We further demonstrated that in cells defective in the peroxisome pathway, the chain composition of the diacyl glycerol moiety in GlcN-acyl-PI is different from those in the first intermediate N-acetylglucosaminyl-PI and cellular PI, indicating that not only diacyl to alkyl-acyl change but also diacyl to diacyl change occurs in GlcN-acyl-PI. We therefore propose a biosynthetic step within GlcN-acyl-PI in which the diacyl glycerol (or diacyl phosphatidic acid) part is replaced by diradyl glycerol (or diradyl phosphatidic acid). These results highlight cooperation of three organelles, the ER, the Golgi, and the peroxisome, in the generation of the lipid portion of GPI-APs.

Chapter 6: cubic membranes the missing dimension of cell membrane organization

Biological membranes are among the most fascinating assemblies of biomolecules: a bilayer less than 10 nm thick, composed of rather small lipid molecules that are held together simply by noncovalent forces, defines the cell and discriminates between “inside” and “outside”, survival, and death. Intracellular compartmentalization—governed by biomembranes as well—is a characteristic feature of eukaryotic cells, which allows them to fulfill multiple and highly specialized anabolic and catabolic functions in strictly controlled environments. Although cellular membranes are generally visualized as flat sheets or closely folded isolated objects, multiple observations also demonstrate that membranes may fold into “unusual”, highly organized structures with 2D or 3D periodicity. The obvious correlation of highly convoluted membrane organizations with pathological cellular states, for example, as a consequence of viral infection, deserves close consideration. However, knowledge about formation and function of these highly organized 3D periodic membrane structures is scarce, primarily due to the lack of appropriate techniques for their analysis in vivo. Currently, the only direct way to characterize cellular membrane architecture is by transmission electron microscopy (TEM). However, deciphering the spatial architecture solely based on two-dimensionally projected TEM images is a challenging task and prone to artifacts. In this review, we will provide an update on the current progress in identifying and analyzing 3D membrane architectures in biological systems, with a special focus on membranes with cubic symmetry, and their potential role in physiological and pathophysiological conditions. Proteomics and lipidomics approaches in defined experimental cell systems may prove instrumental to understand formation and function of 3D membrane morphologies.

Magainin 2 in action: distinct modes of membrane permeabilization in living bacterial and mammalian cells

Interactions of cationic antimicrobial peptides with living bacterial and mammalian cells are little understood, although model membranes have been used extensively to elucidate how peptides permeabilize membranes. In this study, the interaction of F5W-magainin 2 (GIGKWLHSAKKFGKAFVGEIMNS), an equipotent analogue of magainin 2 isolated from the African clawed frog Xenopus laevis, with unfixed Bacillus megaterium and Chinese hamster ovary (CHO)-K1 cells was investigated, using confocal laser scanning microscopy. A small amount of tetramethylrhodamine-labeled F5W-magainin 2 was incorporated into the unlabeled peptide for imaging. The influx of fluorescent markers of various sizes into the cytosol revealed that magainin 2 permeabilized bacterial and mammalian membranes in significantly different ways. The peptide formed pores with a diameter of approximately 2.8 nm (< 6.6 nm) in B. megaterium, and translocated into the cytosol. In contrast, the peptide significantly perturbed the membrane of CHO-K1 cells, permitting the entry of a large molecule (diameter, >23 nm) into the cytosol, accompanied by membrane budding and lipid flip-flop, mainly accumulating in mitochondria and nuclei. Adenosine triphosphate and negatively charged glycosaminoglycans were little involved in the magainin-induced permeabilization of membranes in CHO-K1 cells. Furthermore, the susceptibility of CHO-K1 cells to magainin was found to be similar to that of erythrocytes. Thus, the distinct membrane-permeabilizing processes of magainin 2 in bacterial and mammalian cells were, to the best of our knowledge, visualized and characterized in detail for the first time.

Lessons from peroxisome-deficient Chinese hamster ovary (CHO) cell mutants

Cells with a genetic defect affecting a biological activity and/or a cell phenotype are generally called "cell mutants" and are a highly useful tool in genetic, biochemical, as well as cell biological research. To investigate peroxisome biogenesis and human peroxisome biogenesis disorders, more than a dozen complementation groups of Chinese hamster ovary (CHO) cell mutants defective in peroxisome assembly have been successfully isolated and established as a model system. Moreover, successful PEX gene cloning studies by taking advantage of rapid functional complementation assay of CHO cell mutants invaluably contributed to the accomplishment of isolation of pathogenic genes responsible for peroxisome biogenesis diseases. Molecular mechanisms of peroxisome assembly are currently investigated by making use of such mammalian cell mutants.

Synthesis and biological properties of the fluorescent ether lipid precursor 1-O-[9'-(1''-pyrenyl)]nonyl-sn-glycerol

The synthesis of an omega-pyrene-labeled 1-O-alkyl-sn-glycerol was performed using a chirospecific method starting from R-(-)-2,3-O-isopropylidene-sn-glycerol. The product, 1-O-[9'-(1''-pyrenyl)]nonyl-sn-glycerol (pAG), is a fluorescent ether lipid that has a pyrene moiety covalently attached at the alkyl chain terminus. pAG was taken into CHO-K1 cells and a plasmalogen-deficient variant of CHO-K1, NRel-4. This variant is defective in dihydroxyacetonephosphate acyltransferase, which catalyzes the first step in plasmenylethanolamine (PlsEtn) biosynthesis. pAG was incorporated primarily into ethanolamine and choline phospholipids as well as a neutral lipid fraction tentatively identified as alkyldiacylglycerol. NRel-4 accumulated more fluorescence in the phospholipid fraction than CHO-K1, specifically in the ethanolamine phospholipids. Analysis of the fluorescent lipids showed that 93% of the pAG was incorporated into glycerolipids with the ether bond intact. Although the addition of 20 microM 1-O-hexadecyl-sn-glycerol to the medium fully restored PlsEtn biosynthesis in NRel-4 cells, pAG only partially restored PlsEtn synthesis. Incubation of cells with pAG followed by irradiation with long-wavelength (>300 nm) ultraviolet light resulted in cytotoxicity. NRel-4 cells displayed an increased sensitivity to this treatment compared with CHO-K1 cells. This photodynamic cytotoxicity approach could be used to select for mutants that are defective in downstream steps in ether lipid biosynthesis.

Role of dihydroxyacetonephosphate acyltransferase in the biosynthesis of plasmalogens and nonether glycerolipids

The variant CHO-K1 cell line, NRel-4, is unable to synthesize plasmalogens because of a severe reduction in dihydroxyacetonephosphate acyltransferase (DHAPAT) activity (Nagan, N., A. K. Hajra, L. K. Larkins, P. Lazarow, P. E. Purdue, W. B. Rizzo, and R. A. Zoeller. 1998. Isolation of a Chinese hamster fibroblast variant defective in dihydroxyacetonephosphate acyltransferase activity and plasmalogen biosynthesis: use of a novel two-step selection protocol. Biochem. J. 332: 273-279). Northern analysis demonstrated that the loss of this activity was attributable to a severe reduction in mRNA levels for DHAPAT. Transfection of NRel-4 cells with a plasmid bearing the human DHAPAT cDNA recovered DHAPAT activity and plasmalogen biosynthesis. Examination of clonal isolates from the transfected population showed that recovery of as little as 10% of wild-type DHAPAT activity restored plasmalogen levels to 55% of normal, whereas in one isolate, NRel-4.15, which overexpressed DHAPAT activity by 6-fold over wild-type cells, plasmalogen levels were returned only to wild-type values. Although the rate of plasmenylethanolamine biosynthesis was restored in NRel-4.15, the biosynthesis of nonether glycerolipids was either decreased or unaffected, suggesting that peroxisomal DHAPAT does not normally contribute to nonether glycerolipid biosynthesis. These data demonstrate that a defect in the gene that codes for peroxisomal DHAPAT is the primary lesion in the NRel-4 cell line and that the peroxisomal DHAPAT is essential for the biosynthesis of plasmalogens in animal cells.

Deficiency in ethanolamine plasmalogen leads to altered cholesterol transport

Plasmalogens are a major sub-class of ethanolamine and choline phospholipids in which the sn-1 position has a long chain fatty alcohol attached through a vinyl ether bond. These phospholipids are proposed to play a role in membrane fusion-mediated events. In this study, we investigated the role of the ethanolamine plasmalogen plasmenylethanolamine (PlsEtn) in intracellular cholesterol transport in Chinese hamster ovary cell mutants NRel-4 and NZel-1, which have single gene defects in PlsEtn biosynthesis. We found that PlsEtn was essential for specific cholesterol transport pathways, those from the cell surface or endocytic compartments to acyl-CoA/cholesterol acyltransferase in the endoplasmic reticulum. The movement of cholesterol from the endoplasmic reticulum or endocytic compartments to the cell surface was normal in PlsEtn-deficient cells. Also, vesicle trafficking was normal in PlsEtn-deficient cells, as measured by fluid phase endocytosis and exocytosis, as was the movement of newly-synthesized proteins to the cell surface. The mutant cholesterol transport phenotype was due to the lack of PlsEtn, since it was corrected when NRel-4 cells were transfected with a cDNA encoding the missing enzyme or supplied with a metabolic intermediate that enters the PlsEtn biosynthetic pathway downstream of the defect. Future work must determine the precise role that plasmalogens have on cholesterol transport to the endoplasmic reticulum.

Ethanolamine plasmalogens protect cholesterol-rich liposomal membranes from oxidation caused by free radicals

The aim of the present study is to investigate the effect of ethanolamine plasmalogens on the oxidative stability of cholesterol-rich membranes by comparing it with that of diacyl glycerophosphoethanolamine, using bovine brain ethanolamine plasmalogen (BBEP) or egg yolk phosphatidylethanolamine (EYPE)-containing large unilamellar vesicles (LUVs) and the water-soluble radical initiator AAPH. Electron microscopic observation and particle size measurement visually demonstrated that ethanolamine plasmalogens protect cholesterol-rich phospholipid bilayers from oxidative collapse. Lipid analyses suggested that the effect of ethanolamine plasmalogens in stabilizing membranes against oxidation is partly due to the antioxidative action of plasmalogens involved in scavenging radicals at vinyl ether linkage.