Organelle biogenesis and intracellular lipid transport in eukaryotes

Non-Vesicular Lipid Transport Machinery in Leishmania donovani: Functional Implications in Host-Parasite Interaction

Eukaryotic cells have distinct membrane-enclosed organelles, each with a unique biochemical signature and specialized function. The unique identity of each organelle is greatly governed by the asymmetric distribution and regulated intracellular movement of two important biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid-transfer proteins (LTPs) plays essential roles in intra-cellular lipid trafficking and cellular lipid homeostasis, while vesicular transport regulates protein trafficking. A comparative analysis of non-vesicular lipid transport machinery in protists could enhance our understanding of parasitism and basis of eukaryotic evolution. Leishmania donovani, the trypanosomatid parasite, greatly depends on receptor-ligand mediated signalling pathways for cellular differentiation, nutrient uptake, secretion of virulence factors, and pathogenesis. Lipids, despite being important signalling molecules, have intracellular transport mechanisms that are largely unexplored in L. donovani. We have identified a repertoire of sixteen (16) potential lipid transfer protein (LTP) homologs based on a domain-based search on TriTrypDB coupled with bioinformatics analyses, which signifies the presence of well-organized lipid transport machinery in this parasite. We emphasized here their evolutionary uniqueness and conservation and discussed their potential implications for parasite biology with regards to future therapeutic targets against visceral leishmaniasis.

Saturated fatty acid- and/or monounsaturated fatty acid-containing phosphatidic acids selectively interact with heat shock protein 27

Diacylglycerol kinase (DGK) α, which is a key enzyme in the progression of cancer and, in contrast, in T-cell activity attenuation, preferentially produces saturated fatty acid (SFA)- and/or monounsaturated fatty acid (MUFA)-containing phosphatidic acids (PAs), such as 16:0/16:0-, 16:0/18:0-, and 16:1/16:1-PA, in melanoma cells. In the present study, we searched for the target proteins of 16:0/16:0-PA in melanoma cells and identified heat shock protein (HSP) 27, which acts as a molecular chaperone and contributes to cancer progression. HSP27 more strongly interacted with PA than other phospholipids, including phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, cardiolipin, phosphatidylinositol, phosphatidylinositol 4-monophosphate, and phosphatidylinositol 4,5-bisphosphate. Moreover, HSP27 is more preferentially bound to SFA- and/or MUFA-containing PAs, including 16:0/16:0- and 16:0/18:1-PAs, than PUFA-containing PAs, including 18:0/20:4- and 18:0/22:6-PA. Furthermore, HSP27 and constitutively active DGKα expressed in COS-7 cells colocalized in a DGK activity-dependent manner. Notably, 16:0/16:0-PA, but not phosphatidylcholine or 16:0/16:0-phosphatidylserine, induced oligomer dissociation of HSP27, which enhances its chaperone activity. Intriguingly, HSP27 protein was barely detectable in Jurkat T cells, while the protein band was intensely detected in AKI melanoma cells. Taken together, these results strongly suggest that SFA- and/or MUFA-containing PAs produced by DGKα selectively target HSP27 and regulate its cancer-progressive function in melanoma cells but not in T cells.

Deciphering lipid transfer between and within membranes with time-resolved small-angle neutron scattering

This review focuses on time-resolved neutron scattering, particularly time-resolved small angle neutron scattering (TR-SANS), as a powerful in situ noninvasive technique to investigate intra- and intermembrane transport and distribution of lipids and sterols in lipid membranes. In contrast to using molecular analogues with potentially large chemical tags that can significantly alter transport properties, small angle neutron scattering relies on the relative amounts of the two most abundant isotope forms of hydrogen: protium and deuterium to detect complex membrane architectures and transport processes unambiguously. This review discusses advances in our understanding of the mechanisms that sustain lipid asymmetry in membranes-a key feature of the plasma membrane of cells-as well as the transport of lipids between membranes, which is an essential metabolic process.

Phosphatidic acid-mediated binding and mammalian cell internalization of the Vibrio cholerae cytotoxin MakA

Vibrio cholerae is a noninvasive intestinal pathogen extensively studied as the causative agent of the human disease cholera. Our recent work identified MakA as a potent virulence factor of V. cholerae in both Caenorhabditis elegans and zebrafish, prompting us to investigate the potential contribution of MakA to pathogenesis also in mammalian hosts. In this study, we demonstrate that the MakA protein could induce autophagy and cytotoxicity of target cells. In addition, we observed that phosphatidic acid (PA)-mediated MakA-binding to the host cell plasma membranes promoted macropinocytosis resulting in the formation of an endomembrane-rich aggregate and vacuolation in intoxicated cells that lead to induction of autophagy and dysfunction of intracellular organelles. Moreover, we functionally characterized the molecular basis of the MakA interaction with PA and identified that the N-terminal domain of MakA is required for its binding to PA and thereby for cell toxicity. Furthermore, we observed that the ΔmakA mutant outcompeted the wild-type V. cholerae strain A1552 in the adult mouse infection model. Based on the findings revealing mechanistic insights into the dynamic process of MakA-induced autophagy and cytotoxicity we discuss the potential role played by the MakA protein during late stages of cholera infection as an anti-colonization factor.

Vibrio cholerae is the cause of cholera, an infectious disease causing watery diarrhea that can lead to fatal dehydration. The bacteria can readily adapt to different environments, such as from its natural aquatic habitats to the human digestive system. Recently, we reported a novel V. cholerae cytotoxin, MakA that functions as a potent virulence factor in C. elegans and zebrafish. Here we identified phosphatidic acid as a lipid target for MakA interaction with mammalian cells. This interaction promoted macropinocytosis resulting in the formation of an endomembrane-rich aggregate in intoxicated cells that ultimately lead to activation of autophagy. Importantly, data from bacterial colonization in a mouse infection model suggested that MakA might act as an anti-colonization factor of V. cholerae, presumably expressed during later stage(s) of infection. MakA might be explored as a new target for diagnostics and therapeutic developments against V. cholerae infections. Our findings will contribute to further understanding of the virulence, colonization and post-infection spread of V. cholerae.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Intermembrane transport: Glycerophospholipid homeostasis of the Gram-negative cell envelope

This perspective addresses recent advances in lipid transport across the Gram-negative inner and outer membranes. While we include a summary of previously existing literature regarding this topic, we focus on the maintenance of lipid asymmetry (Mla) pathway. Discovered in 2009 by the Silhavy group [J. C. Malinverni, T. J. Silhavy, Proc. Natl. Acad. Sci. U.S.A. 106, 8009–8014 (2009)], Mla has become increasingly appreciated for its role in bacterial cell envelope physiology. Through the work of many, we have gained an increasingly mechanistic understanding of the function of Mla via genetic, biochemical, and structural methods. Despite this, there is a degree of controversy surrounding the directionality in which Mla transports lipids. While the initial discovery and subsequent studies have posited that it mediated retrograde lipid transport (removing glycerophospholipids from the outer membrane and returning them to the inner membrane), others have asserted the opposite. This Perspective aims to lay out the evidence in an unbiased, yet critical, manner for Mla-mediated transport in addition to postulation of mechanisms for anterograde lipid transport from the inner to outer membranes.

Characterization of a phospholipid-regulated β-galactosidase from Akkermansia muciniphila involved in mucin degradation

The gut microbe Akkermansia muciniphila is important for the human health as the occurrence of the organism is inversely correlated with different metabolic disorders. The metabolism of the organism includes the degradation of intestinal mucins. Thus, the gut health‐promoting properties are not immediately obvious and mechanisms of bacteria‐host interactions are mostly unclear. In this study, we characterized a novel extracellular β‐galactosidase (Amuc_1686) with a preference for linkages from the type Galβ1–3GalNAc. Additionally, Amuc_1686 possesses a discoidin‐like domain, which enables the interaction with anionic phospholipids. We detected a strong inhibition by phosphatidylserine, phosphatidylglycerol, phosphatidic acid, and lysophosphatidic acid while phosphatidylcholine and phosphatidylethanolamine had no influence. Amuc_1686 is the first example of a prokaryotic hydrolase that is strongly inhibited by certain phospholipids. These inhibiting phospholipids have important signal functions in immune response and cell clearance processes. Hence, Amuc_1686 might be regulated based on the health status of the large intestine and could therefore contribute to the mutualistic relationship between the microbe and the host on a molecular level. In this sense, Amuc_1686 could act as an altruistic enzyme that does not attack the mucin layer of apoptotic epithelial cells to ensure tissue regeneration, for example, in areas with inflammatory damages.

Non-vesicular Lipid Transport Machinery in Entamoeba histolytica

Eukaryotic cells are organized into separate membrane-bound compartments that have specialized biochemical signature and function. Maintenance and regulation of distinct identity of each compartment is governed by the uneven distribution and intra-cellular movement of two essential biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid transfer proteins plays a pivotal role in intra-cellular lipid trafficking and homeostasis whereas vesicular transport plays a central role in protein trafficking. Comparative study of lipid transport machinery in protist helps to better understand the pathogenesis and parasitism, and provides insight into eukaryotic evolution. Amebiasis, which is caused by Entamoeba histolytica, is one of the major enteric infections in humans, resulting in 40–100 thousand deaths annually. This protist has undergone remarkable alterations in the content and function of its sub-cellular compartments as well represented by its unique diversification of mitochondrion-related organelle, mitosome. We conducted domain-based search on AmoebaDB coupled with bioinformatics analyses and identified 22 potential lipid transfer protein homologs in E. histolytica, which are grouped into several sub-classes. Such in silico analyses have demonstrated the existence of well-organized lipid transport machinery in this parasite. We summarized and discussed the conservation and unique features of the whole repertoire of lipid transport proteins in E. histolytica.

Phospholipase D and phosphatidic acid in the biogenesis and cargo loading of extracellular vesicles

Extracellular vesicles released by viable cells (exosomes and microvesicles) have emerged as important organelles supporting cell-cell communication. Because of their potential therapeutic significance, important efforts are being made toward characterizing the contents of these vesicles and the mechanisms that govern their biogenesis. It has been recently demonstrated that the lipid modifying enzyme, phospholipase D (PLD)2, is involved in exosome production and acts downstream of the small GTPase, ARF6. This review aims to recapitulate our current knowledge of the role of PLD2 and its product, phosphatidic acid, in the biogenesis of exosomes and to propose hypotheses for further investigation of a possible central role of these molecules in the biology of these organelles.

CB1 Receptor Signaling in the Brain: Extracting Specificity from Ubiquity

Endocannabinoids (eCBs) are amongst the most ubiquitous signaling molecules in the nervous system. Over the past few decades, observations based on a large volume of work, first examining the pharmacological effects of exogenous cannabinoids, and then the physiological functions of eCBs, have directly challenged long-held and dogmatic views about communication, plasticity and behavior in the central nervous system (CNS). The eCBs and their cognate cannabinoid receptors exhibit a number of unique properties that distinguish them from the widely studied classical amino-acid transmitters, neuropeptides, and catecholamines. Although we now have a loose set of mechanistic rules based on experimental findings, new studies continue to reveal that our understanding of the eCB system (ECS) is continuously evolving and challenging long-held conventions. Here we will briefly summarize findings on the current canonical view of the 'ECS' and will address novel aspects that reveal how a nearly ubiquitous system can determine highly specific functions in the brain. In particular, we will focus on findings that push for an expansion of our ideas around long-held beliefs about eCB signaling that, while clearly true, may be contributing to an oversimplified perspective on how cannabinoid signaling at the microscopic level impacts behavior at the macroscopic level.

Moderate Expression of SEC16 Increases Protein Secretion by Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is widely used to produce biopharmaceutical proteins. However, the limited capacity of the secretory pathway may reduce its productivity. Here, we increased the secretion of a heterologous α-amylase, a model protein used for studying the protein secretory pathway in yeast, by moderately overexpressing SEC16, which is involved in protein translocation from the endoplasmic reticulum to the Golgi apparatus. The moderate overexpression of SEC16 increased α-amylase secretion by generating more endoplasmic reticulum exit sites. The production of reactive oxygen species resulting from the heterologous α-amylase production was reduced. A genome-wide expression analysis indicated decreased endoplasmic reticulum stress in the strain that moderately overexpressed SEC16, which was consistent with a decreased volume of the endoplasmic reticulum. Additionally, fewer mitochondria were observed. Finally, the moderate overexpression of SEC16 was shown to improve the secretion of two other recombinant proteins, Trichoderma reesei endoglucanase I and Rhizopus oryzae glucan-1,4-α-glucosidase, indicating that this mechanism is of general relevance.IMPORTANCE There is an increasing demand for recombinant proteins to be used as enzymes and pharmaceuticals. The yeast Saccharomyces cerevisiae is a cell factory that is widely used to produce recombinant proteins. Our study revealed that moderate overexpression of SEC16 increased recombinant protein secretion in S. cerevisiae This new strategy can be combined with other targets to engineer cell factories to efficiently produce protein in the future.

Multivalent adhesion molecule 7 clusters act as signaling platform for host cellular GTPase activation and facilitate epithelial barrier dysfunction

Vibrio parahaemolyticus is an emerging bacterial pathogen which colonizes the gastrointestinal tract and can cause severe enteritis and bacteraemia. During infection, V. parahaemolyticus primarily attaches to the small intestine, where it causes extensive tissue damage and compromises epithelial barrier integrity. We have previously described that Multivalent Adhesion Molecule (MAM) 7 contributes to initial attachment of V. parahaemolyticus to epithelial cells. Here we show that the bacterial adhesin, through multivalent interactions between surface-induced adhesin clusters and phosphatidic acid lipids in the host cell membrane, induces activation of the small GTPase RhoA and actin rearrangements in host cells. In infection studies with V. parahaemolyticus we further demonstrate that adhesin-triggered activation of the ROCK/LIMK signaling axis is sufficient to redistribute tight junction proteins, leading to a loss of epithelial barrier function. Taken together, these findings show an unprecedented mechanism by which an adhesin acts as assembly platform for a host cellular signaling pathway, which ultimately facilitates breaching of the epithelial barrier by a bacterial pathogen.

Vibrio parahaemolyticus is a bacterial pathogen which occurs in marine and estuarine environments. It is a main cause of gastrointestinal illness following the consumption of raw or undercooked seafood. In immunocompromised people, the bacteria can sometimes enter the bloodstream and cause septicemia, a serious and often fatal condition. V. parahaemolyticus attaches to host tissues using adhesive proteins. Multivalent Adhesion Molecule (MAM) 7 is an adhesin which helps the bacteria to hold onto the host cells early on during infection. It does so by binding two different molecules on the host, a protein (fibronectin) and phospholipids called phosphatidic acids. We show that MAM7 does not only play a role in sticking to host cells. By forming adhesin clusters on the host surface and binding to host lipids, it triggers signaling processes in the host. These include activation of RhoA, an important mediator of cytoskeletal dynamics. By doing so, MAM7 perturbs proteins at cellular junctions, which normally maintain the cells in the gut as a tightly sealed layer protective of environmental influences. When bacteria use MAM7 to attach to the intestine, the seals between cells break, permitting bacteria to cross the barrier and cause infection of underlying tissues.

Lipid interactions during virus entry and infection

For entry and infection viruses have developed numerous strategies to subjugate indispensable cellular factors and functions. Host cell lipids and cellular lipid synthesis machinery are no exception. Not only do viruses exploit existing lipid signalling and modifications for virus entry and trafficking, they also reprogram lipid synthesis, metabolism, and compartmentalization for assembly and egress. Here we review these various concepts and highlight recent progress in understanding viral interactions with host cell lipids during entry and assembly.

Lipid transport in the lactating mammary gland

Mammalian cells depend on phospholipid (PL) and fatty acid (FA) transport to maintain membrane structure and organization, and to fuel and regulate cellular functions. In mammary glands of lactating animals, copious milk secretion, including large quantities of lipid in some species, requires adaptation and integration of PL and FA synthesis and transport processes to meet secretion demands. At present few details exist about how these processes are regulated within the mammary gland. However, recent advances in our understanding of the structural and molecular biology of membrane systems and cellular lipid trafficking provide insights into the mechanisms underlying the regulation and integration of PL and FA transport processes the lactating mammary gland. This review discusses the PL and FA transport processes required to maintain the structural integrity and organization of the mammary gland and support its secretory functions within the context of current molecular and cellular models of their regulation.

Monitoring phosphatidic acid formation in intact phosphatidylcholine bilayers upon phospholipase D catalysis

We have monitored the production of the negatively charged lipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidic acid acid (POPA), in supported lipid bilayers via the enzymatic hydrolysis of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC), a zwitterionic lipid. Experiments were performed with phospholipase D (PLD) in a Ca(2+) dependent fashion. The strategy for doing this involved using membrane-bound streptavidin as a biomarker for the charge on the membrane. The focusing position of streptavidin in electrophoretic-electroosmotic focusing (EEF) experiments was monitored via a fluorescent tag on this protein. The negative charge increased during these experiments due to the formation of POPA lipids. This caused the focusing position of streptavidin to migrate toward the negatively charged electrode. With the use of a calibration curve, the amount of POPA generated during this assay could be read out from the intact membrane, an objective that has been otherwise difficult to achieve because of the lack of unique chromophores on PA lipids. On the basis of these results, other enzymatic reactions involving the change in membrane charge could also be monitored in a similar way. This would include phosphorylation, dephosphorylation, lipid biosynthesis, and additional phospholipase reactions.

The mitochondrial fatty acid synthesis (mtFASII) pathway is capable of mediating nuclear-mitochondrial cross talk through the PPAR system of transcriptional activation

Mammalian cells contain two fatty acid synthesis pathways, the cytosolic FASI pathway, and the mitochondrial FASII pathway. The selection behind the conservation of the mitochondrial pathway is not completely understood, given the presence of the cytosolic FAS pathway. In this study, we show through heterologous gene reporter systems and PCR-based arrays that overexpression of MECR, the last step in the mtFASII pathway, causes modulation of gene expression through the PPAR pathway. Electromobility shift assays (EMSAs) demonstrate that overexpression of MECR causes increased binding of PPARs to DNA, while cell fractionation and imaging studies show that MECR remains localized to the mitochondria. Interestingly, knock down of the mtFASII pathway lessens the effect of MECR on this transcriptional modulation. Our data are most consistent with MECR-mediated transcriptional activation through products of the mtFASII pathway, although we cannot rule out MECR acting as a coactivator. Further investigation into the physiological relevance of this communication will be necessary to better understand some of the phenotypic consequences of deficits in this pathway observed in animal models and human disease.

Regulation of glucose and protein metabolism in growing steers by long-chain n-3 fatty acids in muscle membrane phospholipids is dose-dependent

A previous study showed that long-chain n-3 polyunsaturated fatty acids (LCn-3PUFA; >18 carbons n-3) exert an anabolic effect on protein metabolism through the upregulation of insulin sensitivity and activation of the insulin signaling pathway. This study further delineates for the first time whether the anabolic effect of LCn-3PUFA on metabolism is dose responsive. Six steers were used to test three graded amounts of menhaden oil rich in LCn-3PUFA (0%, 2% and 4%; enteral infusions) according to a double 3 × 3 Latin square design. Treatment comparisons were made using iso-energetic substitutions of control oil for menhaden oil and using 6-week experimental periods. The LCn-3PUFA in muscle total membrane phospholipids increased from 8%, 14% to 20% as dietary menhaden oil increased. Feeding graded amounts of menhaden oil linearly decreased plasma insulin concentration (49, 35 and 25 μU/ml, P = 0.01). The insulin-stimulated amino acid disposal rates as assessed using hyperinsulinemic-euglycemic-euaminoacidemic clamps (20, 40 and 80 mU/kg per h) were linearly increased by the incremental administrations of menhaden oil from 169, 238 to 375 μmol/kg per h (P = 0.005) during the 40 mU/kg per h clamp, and from 295, 360 and 590 μmol/kg per h (P = 0.02) during the 80 mU/kg per h clamp. Glucose disposal rate responded according to a quadratic relationship with the incremental menhaden oil amounts (P < 0.05). A regression analysis showed that 47% of the amino acid disposal rates elicited during the hyperinsulinemic clamp was related to muscle membrane LCn-3PUFA content (P = 0.003). These results show for the first time that both protein and glucose metabolism respond in a dose-dependent manner to menhaden oil and to muscle membrane LCn-3PUFA.

Lipid composition of cell membranes and its relevance in type 2 diabetes mellitus

Identifying the causative relationship between the fatty acid composition of cell membranes and type 2 diabetes mellitus fundamentally contributes to the understanding of the basic pathophysiological mechanisms of the disease. Important outcomes of the reviewed studies appear to support the hypotheses that the flexibility of a membrane determined by the ratio of (poly)unsaturated to saturated fatty acyl chains of its phospholipids influences the effectiveness of glucose transport by insulin-independent glucose transporters (GLUTs) and the insulin-dependent GLUT4, and from the prediabetic stage on a shift from unsaturated towards saturated fatty acyl chains of membrane phospholipids directly induces a decrease in glucose effectiveness and insulin sensitivity. In addition, it has become evident that a concomitant increase in stiffness of both plasma and erythrocyte membranes may decrease the microcirculatory flow, leading ultimately to tissue hypoxia, insufficient tissue nutrition, and diabetes-specific microvascular pathology. As to the etiology of type 2 diabetes mellitus, a revised hypothesis that attempts to accommodate the reviewed findings is presented.

Enzymatic measurement of phosphatidylserine in cultured cells

Phosphatidylserine (PS) is a quantitatively minor membrane phospholipid involved in diverse cellular functions. In this study, we developed a new fluorometric method for measuring PS using combinations of specific enzymes and Amplex Red. The calibration curve for PS measurement was linear and hyperbolic at low (0-50 µM) and high (50-1000 µM) concentrations, respectively, and the detection limit was 5 µM (50 pmol in the reaction mixture). This assay quantified PS regardless of the chain length and the number of double bonds. We applied this new method to the determination of PS content in HEK293 cells, which was validated by a recovery study and comparison with TLC-phosphorus assay. We showed that the PS content was high in sparse cells. The overexpression of PS synthase 1 elevated not only the cellular PS content but also the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) contents, suggesting the conversion of PS into PE and the enhancement of PC production. This new assay for PS measurement is simple, specific, sensitive, and high throughput, and it will be useful to clarify the metabolism and biological functions of PS.

Functional characterization of the interaction between bacterial adhesin multivalent adhesion molecule 7 (MAM7) protein and its host cell ligands

The ability of a pathogen to rapidly form a stable interaction with the host cell surface is key to its success. Bacterial pathogens use a repertoire of virulence factors, but their efficient use relies on close contact between the host and the pathogen. We have recently identified a constitutively expressed MAM7 (multivalent adhesion molecule 7), which is widely distributed in gram-negative pathogens and enables them to establish initial contact with the host cell. Here, we describe the dissection of the MAM7 interaction with the host cell surface into two distinct binding events, involving the host protein fibronectin and the membrane phospholipid phosphatidic acid. We analyzed which domains within MAM7 and fibronectin are necessary for complex formation. We further studied phosphatidic acid binding by MAM7 using site-directed mutagenesis and liposome association assays and demonstrated that a specific distribution of basic charge on MAM7 is required for high affinity binding. Finally, we showed that fibronectin and phosphatidic acid binding to MAM7 are not mutually exclusive and that the three molecules likely assemble into a tripartite complex on the host cell surface.

Genetic analysis of Arabidopsis mutants impaired in plastid lipid import reveals a role of membrane lipids in chloroplast division

The biogenesis of photosynthetic membranes in plants relies largely on lipid import from the endoplasmic reticulum (ER) and this lipid transport process is mediated by TGD proteins in Arabidopsis. Such a dependency of chloroplast biogenesis on ER-to-plastid lipid transport was recently exemplified by analyzing double mutants between tgd1-1 or tgd4-3 and fad6 mutants. The fad6 mutants are defective in the desaturation of membrane lipids in chloroplasts and therefore dependent on import of polyunsaturated lipid precursors from the ER for constructing a competent thylakoid membrane system. In support of a critical role of TGD proteins in ER-to-plastid lipid trafficking, we showed that the introduction of the tgd mutations into fad6 mutant backgrounds led to drastic reductions in relative amounts of thylakoid lipids. Moreover, the tgd1-1 fad6 and tgd4-3 fad6 double mutants were deficient in polyunsaturated fatty acids in chloroplast membrane lipids, and severely compromised in the biogenesis of photosynthetic membrane systems. Here we report that these double mutants are severely impaired in chloroplast division. The possible role of membrane lipids in chloroplast division is discussed. :

Functional analysis of two isoforms of phosphatidylethanolamine N-methyltransferase

The enzyme catalysing the conversion of PE (phosphatidylethanolamine) into PC (phosphatidylcholine), PEMT (PE N-methyltransferase), exists as two isoforms, PEMT-L (longer isoform of PEMT) and PEMT-S (shorter isoform of PEMT). In the present study, to compare the functions of the two isoforms of PEMT, we established HEK (human embryonic kidney)-293 cell lines stably expressing PEMT-L and PEMT-S. Both PEMT-L and PEMT-S were localized in the ER (endoplasmic reticulum). PEMT-L, but not PEMT-S, was N-glycosylated with high-mannose oligosaccharides. The enzymatic activity of PEMT-S was much higher than that of PEMT-L. By using novel enzymatic assays for measuring PC and PE, we showed that PEMT-L and PEMT-S expression remarkably increased the cellular PC content, whereas the PE content was decreased by PEMT-S expression, but was hardly affected by PEMT-L expression. The cellular content of phosphatidylserine was also reduced by the expression of PEMT-L or PEMT-S. MS analyses demonstrated that the expression of PEMT-S led to more increases in the molecular species of PC and PC-O (ether-linked PC) with longer polyunsaturated chains than that of PEMT-L, whereas the PC-O species with shorter chains were increased more by PEMT-L expression than by PEMT-S expression, suggesting a difference in the substrate specificity of PEMT-L and PEMT-S. On the other hand, various PE and PE-O species were decreased by PEMT-S expression. In addition, PEMT-L and PEMT-S expression promoted the proliferation of HEK-293 cells. Based upon these findings, we propose a model in which the enzymatic activity and substrate specificity are regulated by the glycosylated N-terminal region of PEMT-L localized in the ER lumen.

Non-vesicular lipid transport by lipid-transfer proteins and beyond

The movement of lipids within and between intracellular membranes is mediated by different lipid transport mechanisms and is crucial for maintaining the identities of different cellular organelles. Non-vesicular lipid transport has a crucial role in intracellular lipid trafficking and distribution, but its underlying mechanisms remain unclear. Lipid-transfer proteins (LTPs), which regulate diverse lipid-mediated cellular processes and accelerate vectorial transport of lipid monomers between membranes in vitro, could potentially mediate non-vesicular intracellular lipid trafficking. Understanding the mechanisms by which lipids are transported and distributed between cellular membranes, and elucidating the role of LTPs in intracellular lipid transport and homeostasis, are currently subjects of intensive study.

Intersection of the unfolded protein response and hepatic lipid metabolism

The liver plays a central role in whole-body lipid metabolism by governing the synthesis, oxidization, transport and excretion of lipids. The unfolded protein response (UPR) was identified as a signal transduction system that is activated by ER stress. Recent studies revealed a critical role of the UPR in hepatic lipid metabolism. The IRE1/XBP1 branch of the UPR is activated by high dietary carbohydrates and controls the expression of genes involved in fatty acid and cholesterol biosynthesis. PERK mediated eIF2alpha phosphorylation is also required for the expression of lipogenic genes and the development of hepatic steatosis, likely by activating C/EBP and PPARgamma transcription factors. Further studies to define the molecular pathways that lead to the activation of the UPR by nutritional cues in the liver, and their contribution to human metabolic disorders such as hepatic steatosis, atherosclerosis and type 2 diabetes that are associated with dysregulation of lipid homeostasis, are warranted.

Biophysical characterization of a new phospholipid analogue with a spin-labeled unsaturated fatty acyl chain

Spin-labeled analogs of phospholipids have been used widely to characterize the biophysical properties of membranes. We describe synthesis and application of a new spin-labeled phospholipid analog, SL-POPC. The advantage of this molecule is that the EPR active doxyl group is linked to an unsaturated fatty acyl chain different to saturated phospholipid analogs used so far. The need for those analogs arises from the fact that biological membranes contain unsaturated phospholipids to a large extent. The biophysical properties of SL-POPC in membranes were characterized using EPR and NMR spectroscopy and compared with those of the saturated spin-labeled phospholipid, SL-PSPC. To this end, POPC membranes were labeled with either analog to assess whether the spin-labeled counterpart SL-POPC mimics the membrane properties better than the often used SL-PSPC. The results show that SL-POPC and SL-PSPC explore different molecular environments of the bilayer, and that the type and degree of perturbation of bilayer caused by the label moiety also differs between both analogs. We found that SL-POPC is more appropriate to assess the versatile dynamics of POPC membranes than SL-PSPC.

Long-chain n-3 fatty acids enhance neonatal insulin-regulated protein metabolism in piglets by differentially altering muscle lipid composition

This study investigated the role of long-chain n-3 polyunsaturated fatty acids (LCn-3PUFAs) of muscle phospholipids in the regulation of neonatal metabolism. Twenty-eight piglets were weaned at 2 days of age and raised on one of two milk formulas that consisted of either a control formula supplying 0% or a formula containing 3.5% LCn-3PUFAs until 10 or 28 days of age. There was a developmental decline in the insulin sensitivity of amino acid disposal in control pigs during the first month of life, with a slope of -2.24 micromol.kg(-1).h(-1) (P = 0.01) per unit of insulin increment, as assessed using hyperinsulinemic-euglycemic-euaminoacidemic clamps. LCn-3PUFA feeding blunted this developmental decline, resulting in differing insulin sensitivities (P < 0.001). When protein metabolism was assessed under parenteral feeding-induced hyperinsulinemia, LCn-3PUFAs reduced by 16% whole body oxidative losses of amino acids (from 238 to 231 micromol.kg(-1).h(-1); P = 0.06), allowing 41% more amino acids to accrete into body proteins (from 90 to 127 micromol.kg(-1).h(-1); P = 0.06). The fractional synthetic rate of muscle mixed proteins remained unaltered by the LCn-3PUFA feeding. However, LCn-3PUFAs retarded a developmental increase in the essential-to-nonessential amino acid ratio of the muscle intracellular free pool (P = 0.05). Overall, alterations in metabolism were concomitant with a preferential incorporation of LCn-3PUFAs into muscle total membrane phospholipids (P < 0.001), in contrast to intramuscular triglycerides. These results underscore the potential role of LCn-3PUFAs as regulators of different aspects of protein metabolism in the neonate.

Long-chain omega-3 fatty acids regulate bovine whole-body protein metabolism by promoting muscle insulin signalling to the Akt-mTOR-S6K1 pathway and insulin sensitivity

The ability of the skeletal musculature to use amino acids to build or renew constitutive proteins is gradually lost with age and this is partly due to a decline in skeletal muscle insulin sensitivity. Since long-chain omega-3 polyunsaturated fatty acids (LCn-3PUFA) from fish oil are known to improve insulin-mediated glucose metabolism in insulin-resistant states, their potential role in regulating insulin-mediated protein metabolism was investigated in this study. Experimental data are based on a switchback design composed of three 5 week experimental periods using six growing steers to compare the effect of a continuous abomasal infusion of LCn-3PUFA-rich menhaden oil with an iso-energetic control oil mixture. Clamp and insulin signalling observations were combined with additional data from a second cohort of six steers. We found that enteral LCn-3PUFA potentiate insulin action by increasing the insulin-stimulated whole-body disposal of amino acids from 152 to 308 micromol kg(-1) h(-1) (P=0.006). The study further showed that in the fed steady-state, chronic adaptation to LCn-3PUFA induces greater activation (P<0.05) of the Akt-mTOR-S6K1 signalling pathway. Simultaneously, whole-body total flux of phenylalanine was reduced from 87 to 67 micromol kg(-1) h(-1) (P=0.04) and oxidative metabolism was decreased (P=0.05). We conclude that chronic feeding of menhaden oil provides a novel nutritional mean to enhance insulin-sensitive aspects of protein metabolism.

Subcellular localization of phosphatidylinositol synthesis

It is well-established that the endoplasmic reticulum is the major site of phosphatidylinositol (PtdIns) synthesis. The PtdIns synthetic ability of other organelles, such as plasma membrane and nucleus, remains controversial. In the present study, we re-examine this question by comparing PtdIns synthesis in isolated cytoplasts (enucleated cells) with that in corresponding karyoplasts (nuclei surrounded by plasma membrane but lacking most cytoplasmic components). We report that cytoplasts are competent to carry out both basal and stimulated PtdIns synthesis as well as polyphosphoinositide hydrolysis, while karyoplasts can neither synthesize PtdIns nor hydrolyze phosphoinositides in response to agonists. The karyoplasts are, however, capable of synthesizing phosphatidylcholine (PtdCho), as previously reported. From these data, we conclude that PtdIns synthesis is limited to cytoplasmic components, and cannot be sustained by either plasma membrane or nucleus under conditions that permit robust PtdCho synthesis.

Photoreceptor renewal: a role for peripherin/rds

Visual transduction begins with the detection of light within the photoreceptor cell layer of the retina. Within this layer, specialized cells, termed rods and cones, contain the proteins responsible for light capture and its transduction to nerve impulses. The phototransductive proteins reside within an outer segment region that is connected to an inner segment by a thin stalk rich in cytoskeletal elements. A unique property of the outer segments is the presence of an elaborate intracellular membrane system that holds the phototransduction proteins and provides the requisite lipid environment. The maintenance of normal physiological function requires that these postmitotic cells retain the unique structure of the outer segment regions--stacks of membrane saccules in the case of rods and a continuous infolding of membrane in the case of cones. Both photoreceptor rod and cone cells achieve this through a series of coordinated steps. As new membranous material is synthesized, transported, and incorporated into newly forming outer segment membranes, a compensatory shedding of older membranous material occurs, thereby maintaining the segment at a constant length. These processes are collectively referred to as ROS (rod outer segment) or COS (cone outer segment) renewal. We review the cellular and molecular events responsible for these renewal processes and present the recent but compelling evidence, drawn from molecular genetic, biochemical, and biophysical approaches, pointing to an essential role for a unique tetraspanning membrane protein, called peripherin/rds, in the processes of disk morphogenesis.

Unique aspects of mitochondrial biogenesis in trypanosomatids

Mitochondrial biogenesis consists of the sum of all processes required for the formation of the mitochondrial membranes as well as the soluble compartments they contain. Furthermore, it includes the replication of the mitochondrial genome and correct segregation of the organelles during cell division. Mitochondrial proteins come from two sources, a limited but essential set of inner membrane proteins is encoded by the mitochondrial genome, whereas the large majority (90-95%) is derived from nucleus-encoded genes and are posttranslationally imported into the organelle. Trypanosomatids belong to the earliest diverging branches of the eukaryotic evolutionary tree which have mitochondria. This is reflected in the organisation of their mitochondrial DNA that consists of a network of two classes of topologically interlocked circular DNA molecules as well as many unique features in their mitochondrial biogenesis. The proteins encoded on the mitochondrial genome are conventional for a mitochondrial genome, their expression, however, involves a complex series of processes. Many genes represent incomplete open reading frames and their primary transcripts have to remodelled by RNA editing to convert them into translatable mRNAs. RNA editing is mediated by small mitochondria-encoded transcripts, the guide RNAs, and is in that form specific for trypanosomatids and closely related organisms. Mitochondrial translation is also unconventional. No tRNA genes are encoded on the mitochondrial genome. Instead, mitochondrial protein synthesis functions exclusively with imported cytosolic, eukaryotic-type tRNAs. The composition of mitochondrial ribosomes is also unusual in that they contain the smallest known rRNAs. They are about 30% shorter than the already much reduced rRNAs in human mitochondria. Furthermore, the topological organisation of the mitochondrial genome requires an elaborate replication machinery involving topoisomerases. Finally, some trypanosomatids have life cycle stages exhibiting very different mitochondrial activities and can therefore serve as a model system for the regulation of mitochondrial biogenesis.

Biogenesis of respiratory complex I

Proteins specifically involved in the biogenesis of respiratory complex I in eukaryotes have been characterized. The complex I intermediate associated proteins CIA30 and CIA84 are tightly bound to an assembly intermediate of the membrane arm. Like chaperones, they are involved in multiple rounds of membrane arm assembly without being part of the mature structure. Two biosynthetic subunits of eukaryotic complex I have been characterized. The acyl carrier subunit is needed for proper assembly of the peripheral arm as well as the membrane arm of complex I. It may interact with enzymes of a mitochondrial fatty acid synthetase. The 39/40-kDa subunit appears to be an isomerase with a tightly bound NADPH. It is related to a protein family of reductases/isomerases. Both subunits have been discussed to be involved in the synthesis of a postulated, novel, high-potential redox group.

Reconstitution of yeast microsomal lipid flip-flop using endogenous aminophospholipids

The molecular basis of transbilayer movement or flipping of phospholipids in the endoplasmic reticulum is largely unknown. To circumvent the problems inherent to studies with artificial phospholipid analogs, we studied microsomal flip-flop of endogenous phosphatidylethanolamine in yeast. The transbilayer transport of phosphatidylethanolamine was measured in reconstituted proteoliposomes derived from microsomal detergent extracts. Our results demonstrate that flipping is protease sensitive but does not require metabolic energy. Our assay is the first to use the endogenous substrate of the so-called 'flippase' to study phospholipid translocation in endomembranes and may therefore be crucial for the understanding of the catalytic properties of this elusive enzyme.

Phosphatidylinositol/phosphatidylcholine transfer proteins in yeast

Phosphatidylinositol transfer proteins (PITPs) are now becoming widely recognized as intriguing proteins that participate in the coordination and coupling of phospholipid metabolism with vesicle trafficking, and in the regulation of important signaling cascades. Yet, only in one case is there a large body of evidence that speaks to the precise identities of PITP-dependent cellular reactions, and to the mechanisms by which PITPs execute function in eukaryotic cells. At present, yeast provide the most powerful system for analysis of the physiology of PITP function in vivo, and the mechanism by which this function is carried out. Here, we review the recent progress and remaining questions in the area of PITP function in yeast.

Charged membrane surfaces impede the protein-mediated transfer of glycosphingolipids between phospholipid bilayers

A lipid transfer protein that facilitates the transfer of glycolipids between donor and acceptor membranes has been investigated using a fluorescence resonance energy transfer assay. The glycolipid transfer protein (23-24 kDa, pI 9.0) catalyzes the high specificity transfer of lipids that have sugars beta-linked to either a ceramide or a diacylglycerol backbone, such as simple glycolipids and gangliosides, but not the transfer of phospholipids, cholesterol, or cholesterol esters. In this study, we examined the effect of different charged lipids on the rate of transfer of anthrylvinyl-labeled galactosylceramide (1 mol %) from a donor to acceptor vesicle population at neutral pH. Compared to neutral donor vesicle membranes, introduction of negatively charged lipid at 5 or 10 mol % into the donor vesicles significantly decreased the transfer rate. Introduction of the same amount of negative charge into the acceptor vesicle membrane did not impede the transfer rate as effectively. Also, positive charge in the donor vesicle membrane was not as effective at slowing the transfer rate as was negative charge in the donor vesicle. Increasing the ionic strength of the buffer with NaCl significantly reversed the charge effects. At neutral pH, the transfer protein (pI congruent with 9.0) is expected to be positively charged, which may promote association with the negatively charged donor membrane. Based on these and other experiments, we conclude that the transfer process follows first-order kinetics and that the off-rate of the transfer protein from the donor vesicle surface is the rate-limiting step in the transfer process.

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

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

Sphingomyelin-cholesterol interactions in biological and model membranes

Cholesterol and sphingomyelin are both important plasma membrane constituents in cells. It is now becoming evident that these two lipid classes affect each other's metabolism in the cell to an extent that was not previously appreciated. It is the aim of this review to present recent data in the literature concerning both molecular and membrane properties of the two lipid classes, how they interact in membranes (both biological and model), and the consequences their mutual interaction have on different functional and metabolic processes in cells and lipoproteins.

Overlapping distribution of the 130- and 110-kDa myosin I isoforms on rat liver membranes

The biochemical and mechanochemical properties and localization of myosin I suggest the involvement of these small members of the myosin superfamily in some aspects of intracellular motility in higher cells. We have determined by quantitative immunoblotting with isoform-specific antibodies that the 130-kDa myosin I (myr 1 gene product) and 110-kDa myosin I (myr 2 gene product) account for 0.5 and 0.4%, respectively, of total rat liver protein. Immunoblot analyses reveal that the 130- and 110-kDa myosins I are found in several purified subcellular fractions from rat liver. The membrane-associated 130-kDa myosin I is found at the highest concentration in the plasma membrane (28 ng/microg plasma membrane protein) followed by the endoplasmic reticulum-like mitochondria-associated membrane fraction (MAM; 10 ng/microg MAM protein), whereas the 110-kDa myosin I is found at the highest concentration in Golgi (50 ng/¿g Golgi protein) followed by plasma membrane (20 ng/microg) and MAM (7 ng/microg). Our analyses indicate that myosin I is peripherally associated with Golgi and MAM and its presence in these fractions is not a consequence of myosin I bound to contaminating actin filaments. Although found in relatively low concentrations in microsomes, because of the abundance of microsomes, in liver most of the membrane-associated myosin I is associated with microsomes. Neither myosin I isoform is detected in purified mitochondria. This is the first quantitative analysis addressing the cellular distribution of these mammalian class I myosins.

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

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

Toxoplasma gondii resides in a vacuole that avoids fusion with host cell endocytic and exocytic vesicular trafficking pathways

Toxoplasma gondii actively penetrates its vertebrate host cell to establish a nonfusigenic compartment called the parasitophorous vacuole (PV) that has previously been characterized primarily in phagocytic cells. To determine the fate of this unique compartment in nonphagocytic cells, we examined the trafficking of host cell proteins and lipids in Toxoplasma-infected fibroblasts using quantitative immunofluorescence and immunoelectron microscopy. Toxoplasma-containing vacuoles remained segregated from all levels of the endocytic pathway, as shown by the absence of delivery of transferrin receptors, mannose phosphate receptors, and the lysosomal-associated protein LAMP1 to the vacuole. The PV was also inaccessible to lipids (DiIC16, and GM1) that were internalized from the plasma membrane via the endocytic system. In contrast, vacuoles containing dead parasites or zymosan sequentially acquired both endosomal and lysosomal protein markers and host lipids, reflecting the competency of fibroblasts to process phagocytic vacuoles. The mature PV often lies adjacent to the host cell Golgi, suggesting that it may intersect with vesicles from the exocytic pathway. Despite this proximity, the PV was inaccessible to nitrobenzadiazole-labeled sphingolipids exported from the Golgi and did not contain the host protein markers AP1 or beta-COP. Our results demonstrate that Toxoplasma resides in a compartment that excludes delivery of protein and lipid components from the host cell endocytic and exocytic pathways.

Identification and characterization of a mammalian mitochondrial ATP-binding cassette membrane protein

Membrane proteins of the ATP-binding cassette (ABC) superfamily are involved in the transport of diverse substrates across organellar and plasma membranes of the mammalian cell. Most human ABC proteins identified to date are associated with genetically linked diseases or clinically relevant phenotypes. We describe a new human half-molecule ABC protein, designated M-ABC1, that contains a predicted single membrane and ATP-binding cassette domain. M-ABC1 is localized to membranes of the mitochondria and its transcript is expressed in all tissues. The N-terminal region of the M-ABC1 protein was shown to function independently as a mitochondrial signal sequence by its ability to target the green fluorescent protein to the mitochondria. The monomeric 60 kDa M-ABC1 protein was chemically crosslinked in vivo into a major protein species of 120-130 kDa, thereby confirming that M-ABC1 exists within a higher ordered ABC protein complex. A dominant negative repression approach using M-ABC1 protein with site-directed mutations in its Walker A motif revealed that the mutant protein was rapidly degraded and indicated that the intact Walker A motif of M-ABC1 was required for its stability. The identification of M-ABC1 extends the known distribution of members of the ABC protein family into the mammalian mitochondrion.

The rate of sphingomyelin synthesis de novo is influenced by the level of cholesterol in cultured human skin fibroblasts

Plasma membrane sphingomyelin (SM) is known to affect the cellular distribution of cholesterol. The aim of this work was to examine how SM homoeostasis in human skin fibroblasts is affected by alterations in the level of cholesterol in the cell. The cellular cholesterol level was decreased by exposing cells to 2-hydroxypropyl-beta-cyclodextrin, and increased by exposing cells to cholesterol-methyl-beta-cyclodextrin inclusion complexes. A lowering of the cellular unesterified cholesterol content by 20% was shown to increase the incorporation of [14C]palmitic acid into SM by 70%. Subsequently, the cellular SM mass was shown to be increased (24% increase after a 24 h period). Since l-cycloserine completely abolished the increased incorporation of [14C]palmitic acid into SM in cholesterol-depleted cells, we concluded that the de novo synthesis of the sphingosine backbone of SM was activated in cholesterol-depleted cells. This conclusion was further verified by performing a cell-free assay of serine C-palmitoyltransferase (SPT) in cholesterol-depleted cells, which showed that the activity of the enzyme was increased by 30% after cholesterol depletion. Most of the newly synthesized SM in cholesterol-depleted cells was susceptible to degradation by sphingomyelinase, indicating that it was transported efficiently to the cell surface. Loading of fibroblasts with cholesterol had essentially the opposite effects on SM homoeostasis to those of cholesterol depletion, i.e. 20-30% decreased incorporation of [14C]palmitic acid into SM and decreased activity of SPT. The results of this study show that cellular cholesterol levels have marked effects on the homoeostasis of SM.

The contribution of lipids and lipid metabolism to cellular functions of the Golgi complex

The history of the Golgi complex now reaches its 100 year anniversary. Over the past several decades, tremendous effort has gone into cataloguing Golgi resident proteins, measuring the lipid compositions of Golgi membranes, and in elucidating the pathways by which proteins and lipids traffic through this unique organelle. Only in the past 8 years or so has experimental scrutiny extended to the investigation of how lipids and proteins cooperate to endow the Golgi with its various capabilities regarding protein/lipid transport and sorting. In this chapter we review some of the most recent advances in deciphering the functional interfaces between lipids and proteins of the Golgi complex.

Host cell phospholipids are trafficked to and then modified by Chlamydia trachomatis

There is little information on the trafficking of eukaryotic lipids from a host cell to either the cytoplasmic membrane of or the vacuolar membrane surrounding intracellular pathogens. Purified Chlamydia trachomatis, an obligate intracellular bacterial parasite, contains several eukaryotic glycerophospholipids, yet attempts to demonstrate transfer of these lipids to the chlamydial cell membrane have not been successful. In this report, we demonstrate that eukaryotic glycerophospholipids are trafficked from the host cell to C. trachomatis. Phospholipid trafficking was assessed by monitoring the incorporation of radiolabelled isoleucine, a precursor of C. trachomatis specific branched-chain fatty acids, into host-derived glycerophospholipids and by monitoring the transfer of host phosphatidylserine to chlamydiae and its subsequent decarboxylation to form phosphatidylethanolamine. Phospholipid trafficking to chlamydiae was unaffected by brefeldin A, an inhibitor of Golgi function. Furthermore, no changes in trafficking were observed when C. trachomatis was grown in a mutant cell line with a nonfunctional, nonspecific phospholipid transfer protein. Host glycerophospholipids are modified by C. trachomatis, such that a host-synthesized straight-chain fatty acid is replaced with a chlamydia-synthesized branched-chain fatty acid. We also demonstrate that despite the acquisition of host-derived phospholipids, C. trachomatis is capable of de novo synthesis of phospholipids typically synthesized by prokaryotic cells. Our results provide novel information on chlamydial phospholipid metabolism and eukaryotic cell lipid trafficking, and they increase our understanding of the evolutionary steps leading to the establishment of an intimate metabolic association between an obligate intracellular bacterial parasite and a eukaryotic host cell.

Phosphatidylserine translocation into brain mitochondria: involvement of a fusogenic protein associated with mitochondrial membranes

Data reported in the literature indicate that lipid movement between intracellular organelles can occur through contacts and close physical association of membranes (Vance, J.E. 1990. J Biol Chem 265: 7248-7256). The advantage of this mechanism is that the direct interaction of membranes provides the translocation event without the involvement of lipid-transport systems. However, pre-requisite for the functioning of this machinery is the presence of protein factors controlling membrane association and fusion. In the present work we have found that liposomes fuse to mitochondria at acidic pH and that the pre-treatment of mitochondria with pronase inhibits the fusogenic activity. Mixing of 14C-phosphatidylserine (PS) labeled liposomes with mitochondria at pH 6.0 results in the translocation of 14C-PS into mitochondria and in its decarboxylation to 14C-phosphatidylethanolamine through the PS decarboxylase activity localized on the outer surface of the inner mitochondrial membrane. Incorporation of 14C-PS is inhibited by the pre-treatment of mitochondria with pronase or with EEDQ, a reagent for the derivatization of the protonated form of carboxylic groups. These results indicate the presence of a protein associated with mitochondria which is able to trigger the fusion of liposomes to the mitochondrial membrane. A partial purification of a mitochondrial fusogenic glycoprotein is described in this work. The activity of the fusogenic protein appears to be dependent on the extent of protonation of the residual carboxylic groups and is influenced by the glucidic moiety, as demonstrated by its interaction with Concanavalin A. The purified protein is able to promote the recover of the 14C-PS import from liposomes to pronase-treated mitochondria. Therefore, the protein is candidate to be an essential component in the machinery for the mitochondrial import of PS.

Mucolipidosis type IV: abnormal transport of lipids to lysosomes

Mucolipidosis type IV (MLIV) is a lysosomal storage disorder in which various phospholipids and gangliosides accumulate. The cause of this storage has not yet been identified. Loading experiments in cultured fibroblasts with radioactive phosphatidylcholine ([14C]PC) labelled either in the acyl groups or in the choline residue, indicated increased retention of this lipid in the lysosomes of these patients. Chase experiments in intact fibroblasts, on the other hand, indicated normal degradation and discharge of the radioactive PC in MLIV lysosomes. This was further supported by measurements of the degradation of [14C]PC by isolated lysosomes which indicated normal breakdown of PC in MLIV. Cultured fibroblasts from Hunter (MPSII) patients, which contain enlarged and numerous lysosomes, did not store [14C]PC when compared to normal controls, indicating that the storage of this lipid in MLIV is not a secondary phenomenon caused by the presence of enlarged and numerous lysosomes in these cells. Incubation of [14C]PC at 18 degrees C limits the endocytosis process only up to early endosomes. This temperature did not yield increased retention of [14C]PC in MLIV, indicating that accumulation occurs only after the PC reached late endosomes or the lysosomes. The data indicate that PC as well as other phospholipids and gangliosides accumulate in MLIV apparently owing to a defect in the endocytosis process of membranous components. This defect apparently leads to excessive transportation of these macromolecules into lysosomes rather than their recycling to the plasma membrane. The endocytosis of membrane components is different from receptor-mediated endocytosis which is not affected in MLIV. Once the membrane macromolecules reach the lysosomes in MLIV they are normally catabolized and normally discharged. This may explain the heterogeneity of the stored materials in MLIV. The normal catabolism of macromolecules in the lysosomes is reflected in the minor deterioration in the clinical manifestations of these patients.

Uptake and cellular localization of exogenous lipids by Giardia lamblia, a primitive eukaryote

Giardia lamblia trophozoites are unable to carry out de novo lipid synthesis. It is therefore likely that lipids are acquired from the small intestine of the host, in which the trophozoites are exposed to free and conjugated fatty acids, various sterols, phospholipids, bile acids, and bile-lipid mixed micelles. Here we show that G. lamblia is capable of taking up exogenous phosphatidylcholine (PC), phosphatidylinositol (PI), sphingomyelin (SM), cholesterol, ceramide (Cer), and fatty acids. Results from epifluorescence and high-resolution confocal microscopy suggest that fluorescent analogs of SM and PC were accumulated in the plasma membranes, whereas palmitic acid and Cer were localized intracellularly. Interestingly, many of these analogs were also concentrated in perinuclear regions. Similar labeling patterns were observed when the fluorescent analogs were delivered to the parasite via liposomes. To test whether G. lamblia was capable of esterifying exogenous fatty acids into membrane or cellular phospholipids, trophozoites were pulse-labeled with 3H-labeled palmitic or myristic acids and the phospholipids analyzed by thin-layer chromatography. Results document that G. lamblia was able to incorporate exogenous fatty acids into various phospholipids, i.e., PI, PC, PE, and PG. Interestingly, a major portion of radiolabeled fatty acids was incorporated into PG, a phospholipid characteristic of prokaryotic membranes.