Mammalian cell mutants resistant to a sphingomyelin-directed cytolysin. Genetic and biochemical evidence for complex formation of the LCB1 protein with the LCB2 protein for serine palmitoyltransferase

Non-viral Gene Therapy for Melanoma Using Lysenin from Eisenia Foetida

Earthworms, long utilized in traditional medicine, serve as a source of inspiration for modern therapeutics. Lysenin, a defensive factor in the coelom fluid of the earthworm Eisenia fetida, has multiple bioactivities. However, the inherent toxicity of Lysenin as a pore-forming protein (PFP) restricts its application in therapy. Here, a gene therapy strategy based on Lysenin for cancer treatment is presented. The formulation consists of polymeric nanoparticles complexed with the plasmid encoding Lysenin. After transfection in vitro, melanoma cells can express Lysenin, resulting in necrosis, autophagy, and immunogenic cell death. The secretory signal peptide alters the intracellular distribution of the expressed product of Lysenin, thereby potentiating its anticancer efficacy. The intratumor injection of Lysenin gene formulation can efficiently kill the transfected melanoma cells and activate the antitumor immune response. Notably, no obvious systemic toxicity is observed during the treatment. Non-viral gene therapy based on Lysenin derived from Eisenia foetida exhibits potential in cancer therapy, which can inspire future cancer therapeutics.

Genome deletions to overcome the directed loss of gene function in Leishmania

With the global reach of the Neglected Tropical Disease leishmaniasis increasing, coupled with a tiny armory of therapeutics which all have problems with resistance, cost, toxicity and/or administration, the validation of new drug targets in the causative insect vector borne protozoa Leishmania spp is more important than ever. Before the introduction of CRISPR Cas9 technology in 2015 genetic validation of new targets was carried out largely by targeted gene knockout through homologous recombination, with the majority of genes targeted (~70%) deemed non-essential. In this study we exploit the ready availability of whole genome sequencing technology to reanalyze one of these historic cell lines, a L. major knockout in the catalytic subunit of serine palmitoyltransferase (LCB2), which causes a complete loss of sphingolipid biosynthesis but remains viable and infective. This revealed a number of Single Nucleotide Polymorphisms, but also the complete loss of several coding regions including a gene encoding a putative ABC3A orthologue, a putative sterol transporter. Hypothesizing that the loss of such a transporter may have facilitated the directed knockout of the catalytic subunit of LCB2 and the complete loss of de novo sphingolipid biosynthesis, we re-examined LCB2 in a L. mexicana line engineered for straightforward CRISPR Cas9 directed manipulation. Strikingly, LCB2 could not be knocked out indicating essentiality. However, simultaneous deletion of LCB2 and the putative ABC3A was possible. This indicated that the loss of the putative ABC3A facilitated the loss of sphingolipid biosynthesis in Leishmania, and suggested that we should re-examine the many other Leishmania knockout lines where genes were deemed non-essential.

Natural Ligand-Mimetic and Nonmimetic Inhibitors of the Ceramide Transport Protein CERT

Lipid transfer proteins (LTPs) are recognized as key players in the inter-organelle trafficking of lipids and are rapidly gaining attention as a novel molecular target for medicinal products. In mammalian cells, ceramide is newly synthesized in the endoplasmic reticulum (ER) and converted to sphingomyelin in the trans-Golgi regions. The ceramide transport protein CERT, a typical LTP, mediates the ER-to-Golgi transport of ceramide at an ER-distal Golgi membrane contact zone. About 20 years ago, a potent inhibitor of CERT, named (1R,3S)-HPA-12, was found by coincidence among ceramide analogs. Since then, various ceramide-resembling compounds have been found to act as CERT inhibitors. Nevertheless, the inevitable issue remains that natural ligand-mimetic compounds might directly bind both to the desired target and to various undesired targets that share the same natural ligand. To resolve this issue, a ceramide-unrelated compound named E16A, or (1S,2R)-HPCB-5, that potently inhibits the function of CERT has recently been developed, employing a series of in silico docking simulations, efficient chemical synthesis, quantitative affinity analysis, protein-ligand co-crystallography, and various in vivo assays. (1R,3S)-HPA-12 and E16A together provide a robust tool to discriminate on-target effects on CERT from off-target effects. This short review article will describe the history of the development of (1R,3S)-HPA-12 and E16A, summarize other CERT inhibitors, and discuss their possible applications.

Golgi Apparatus Regulates Plasma Membrane Composition and Function

Golgi apparatus is the central component of the mammalian secretory pathway and it regulates the biosynthesis of the plasma membrane through three distinct but interacting processes: (a) processing of protein and lipid cargoes; (b) creation of a sharp transition in membrane lipid composition by non-vesicular transport of lipids; and (c) vesicular sorting of proteins and lipids at the trans-Golgi network to target them to appropriate compartments. We discuss the molecules involved in these processes and their importance in physiology and development. We also discuss how mutations in these molecules affect plasma membrane composition and signaling leading to genetic diseases and cancer.

Plasma membrane effects of sphingolipid-synthesis inhibition by myriocin in CHO cells: a biophysical and lipidomic study

Suppression of a specific gene effect can be achieved by genetic as well as chemical methods. Each approach may hide unexpected drawbacks, usually in the form of side effects. In the present study, the specific inhibitor myriocin was used to block serine palmitoyltransferase (SPT), the first enzyme in the sphingolipid synthetic pathway, in CHO cells. The subsequent biophysical changes in plasma membranes were measured and compared with results obtained with a genetically modified CHO cell line containing a defective SPT (the LY-B cell line). Similar effects were observed with both approaches: sphingomyelin values were markedly decreased in myriocin-treated CHO cells and, in consequence, their membrane molecular order (measured as laurdan general polarization) and mechanical resistance (AFM-measured breakthrough force values) became lower than in the native, non-treated cells. Cells treated with myriocin reacted homeostatically to maintain membrane order, synthesizing more fully saturated and less polyunsaturated GPL than the non-treated ones, although they achieved it only partially, their plasma membranes remaining slightly more fluid and more penetrable than those from the control cells. The good agreement between results obtained with very different tools, such as genetically modified and chemically treated cells, reinforces the use of both methods and demonstrates that both are adequate for their intended use, i.e. the complete and specific inhibition of sphingolipid synthesis in CHO cells, without apparent unexpected effects.

Intellectual disability-associated mutations in the ceramide transport protein gene CERT1 lead to aberrant function and subcellular distribution

The lipid molecule ceramide is transported from the endoplasmic reticulum to the Golgi apparatus for sphingomyelin production via the ceramide transport protein (CERT), encoded by CERT1. Hyperphosphorylation of CERT’s serine-repeat motif (SRM) decreases its functionality. Some forms of inherited intellectual disability (ID) have been associated with a serine-to-leucine substitution in the SRM (S132L mutation) and a glycine-to-arginine substitution outside the SRM (G243R mutation) in CERT; however, it is unclear if mutations outside the SRM disrupt the control of CERT functionality. In the current investigation, we identified a new CERT1 variant (dupAA) in a patient with mild ID that resulted from a frameshift at the C-terminus of CERT1. However, familial analysis revealed that the dupAA variant was not associated with ID, allowing us to utilize it as a disease-matched negative control for CERT1 variants that are associated with ID. Biochemical analysis showed that G243R and S132L, but not dupAA, impair SRM hyperphosphorylation and render the CERT variants excessively active. Additionally, both S132L and G243R mutations but not dupAA caused the proteins to be distributed in a punctate subcellular manner. On the basis of these findings, we infer that the majority of ID-associated CERT variants may impair SRM phosphorylation-dependent repression, resulting in an increase in sphingomyelin production concurrent with CERT subcellular redistribution.

Compartmentalization of Sphingolipid metabolism: Implications for signaling and therapy

Sphingolipids (SLs) are a family of bioactive lipids implicated in a variety of cellular processes, and whose levels are controlled by an interlinked network of enzymes. While the spatial distribution of SL metabolism throughout the cell has been understood for some time, the implications of this for SL signaling and biological outcomes have only recently begun to be fully explored. In this review, we outline the compartmentalization of SL metabolism and describe advances in tools for investigating and probing compartment-specific SL functions. We also briefly discuss the implications of SL compartmentalization for cell signaling and therapeutic approaches to targeting the SL network.

CHO/LY-B cell growth under limiting sphingolipid supply: Correlation between lipid composition and biophysical properties of sphingolipid-restricted cell membranes

Sphingolipids (SL) are ubiquitous in mammalian cell membranes, yet there is little data on the behavior of cells under SL-restriction conditions. LY-B cells derive from a CHO linein whichserine palmitoyl transferase (SPT), thus de novo SL synthesis, is suppressed, while maintaining the capacity of taking up and metabolizing exogenous sphingoid bases from the culture medium. In this study, LY-B cells were adapted to grow in a fetal bovine serum (FBS)-deficient medium to avoid external uptake of lipids. The lowest FBS concentration that allowed LY-B cell growth, though at a slow rate, under our conditions was 0.04%, that is, 250-fold less than the standard (10%) concentration. Cells grown under limiting SL concentrations remained viable for at least 72 hours. Enriching with sphingomyelin the SL-deficient medium allowed the recovery of growth rates analogous to those of control LY-B cells. Studies including whole cells, plasma membrane preparations, and derived lipid vesicles were carried out. Laurdan fluorescence was recorded to measure membrane molecular order, showing a significant decrease in the rigidity of LY-B cells, not only in plasma membrane but also in whole cell lipid extract, as a result of SL limitation in the growth medium. Plasma membrane preparations and whole cell lipid extracts were also studied using atomic force microscopy in the force spectroscopy mode. Force measurements demonstrated that lower breakthrough forces were required to penetrate samples obtained from SL-poor LY-B cells than those obtained from control cells. Mass-spectroscopic analysis was also a helpful tool to understand the rearrangement undergone by the LY-B cell lipid metabolism. The most abundant SL in LY-B cells, sphingomyelin, decreased by about 85% as a result of SL limitation in the medium, the bioactive lipid ceramide and the ganglioside precursor hexosylceramide decreased similarly, together with cholesterol. Quantitative SL analysis showed that a 250-fold reduction in sphingolipid supply to LY-B cells led only to a sixfold decrease in membrane sphingolipids, underlining the resistance to changes in composition of these cells. Plasma membrane compositions exhibited similar changes, at least qualitatively, as the whole cells with SL restriction. A linear correlation was observed between the sphingomyelin concentration in the membranes, the degree of lipid order as measured by laurdan fluorescence, and membrane breakthrough forces assessed by atomic force microscopy. Smaller, though significant, changes were also detected in glycerophospholipids under SL-restriction conditions.

Enhanced amyloid-β generation by γ-secretase complex in DRM microdomains with reduced cholesterol levels

A neuropathologic hallmark of Alzheimer's disease (AD) is the presence of senile plaques that contain neurotoxic amyloid-β protein (Aβ) species, which are generated by the cleavage of amyloid β-protein precursor by secretases such as the γ-secretase complex, preferentially located in detergent-resistant membrane (DRM) regions and comprising endoproteolysed amino- and carboxy-terminal fragments of presenilin, nicastrin, anterior pharynx defective 1 and presenilin enhancer 2. Whereas some of familial AD patients harbor causative PSEN mutations that lead to more generation of neurotoxic Aβ42, the contribution of Aβ generation to sporadic/late-onset AD remains unclear. We found that the carboxy-terminal fragment of presenilin 1 was redistributed from DRM regions to detergent-soluble membrane (non-DRM) regions in brain tissue samples from individuals with sporadic AD. DRM fractions from AD brain sample had the ability to generate significantly more Aβ and had a lower cholesterol content than DRM fractions from non-demented control subjects. We further demonstrated that lowering the cholesterol content of DRM regions from cultured cells contributed to the redistribution of γ-secretase components and Aβ production. Taken together, the present analyses suggest that the lowered cholesterol content in DRM regions may be a cause of sporadic/late-onset AD by enhancing overall Aβ generation.

Intellectual disability-associated gain-of-function mutations in CERT1 that encodes the ceramide transport protein CERT

Intellectual disability (ID) is a developmental disorder that includes both intellectual and adaptive functioning deficits in conceptual, social, and practical domains. Although evidence-based interventions for patients have long been desired, their progress has been hindered due to various determinants. One of these determinants is the complexity of the origins of ID. The ceramide transport protein (CERT) encoded by CERT1 mediates inter-organelle trafficking of ceramide for the synthesis of intracellular sphingomyelin. Utilizing whole exome sequencing analysis, we identified a novel CERT variant, which substitutes a serine at position 135 (S135) for a proline in a patient with severe ID. Biochemical analysis showed that S135 is essential for hyperphosphorylation of a serine-repeat motif of CERT, which is required for down-regulation of CERT activity. Amino acid replacements of S135 abnormally activated CERT and induced an intracellular punctate distribution pattern of this protein. These results identified specific ID-associated CERT1 mutations that induced gain-of-function effects on CERT activity. These findings provide a possible molecular basis for not only new diagnostics but also a conceivable pharmaceutical intervention for ID disorders caused by gain-of-function mutations in CERT1.

Phase-selective staining of model and cell membranes, lipid droplets and lipoproteins with fluorescent solvatochromic pyrene probes

The push-pull solvatochromic pyrene derivatives PA and PK have been applied to the study of model membrane vesicles, cells and purified human serum lipoproteins, using both confocal fluorescence microscopy and fluorescence spectroscopy. These polarity-sensitive probes provide information similar to that obtained by Laurdan or Prodan, i.e. mainly lipid order in biomembranes, but they have the essential advantage of being excitable by a standard 405 nm laser light, bypassing the use of multiphoton excitation. In addition, they are brighter and much more photostable than those dimethylamino naphthalene derivatives. Our results with model membrane spectroscopy (multilamellar vesicles) and with microscopy (giant unilamellar vesicles) showed the capacity of PA and PK to report differently on liquid-disordered, liquid-ordered and gel phase bilayers. Moreover, a ratiometric parameter, the Red/Blue Intensity Ratio (RBIR) could be used for inter-domain, inter-vesicle and even inter-technique comparison, and the appropriate microscopy-spectroscopy conversion coefficients could be estimated. In studies at the cellular level, PA probe stained almost exclusively the plasma membrane of red blood cells, revealing its high degree of lipid order. Using Chinese Hamster Ovary cells PA was shown to be an excellent probe for the detection of cytoplasmic lipid droplets, superior to Nile Red in that PA provides simultaneously a detailed information of membrane order in the whole cell, in which the lipid droplets appear with a very good contrast. Moreover, spectrofluorometric data of PA-stained serum lipoproteins indicated an essentially identical value of RBIR for lipid droplets and for high-density lipoproteins.

Organelle contacts: Sub-organelle zones to facilitate rapid and accurate inter-organelle trafficking of lipids

When one person wants to communicate securely with another, he/she should contact the other person directly. This rule applies not only to human society, but also to the intracellular micro-society. In the past two decades, it has become increasingly clear that the sub-organelle regions called membrane contact sites (MCSs) are pivotal for inter-organelle transport of lipids in cells, as highlighted in the thematic review series "Interorganelle trafficking of lipids" held in Traffic in 2014-2015. In this commentary, we will describe how the currently prevailing model for lipid trafficking at MCSs was generated, and comment on three important issues that have not been explored: (a1) the principles guiding the generation of an asymmetrical inter-organelle flow of lipids in cells, (b2) the advantages in lipid trafficking at organelle contacts, and (c3) the dynamic network of inter-organelle lipid trafficking.

Transmembrane topology of mammalian ORMDL proteins in the endoplasmic reticulum as revealed by the substituted cysteine accessibility method (SCAM™)

Sphingolipids are diverse lipids with essential, and occasionally opposing, functions in the cell and therefore tight control over biosynthesis is vital. Mechanisms governing this regulation are not understood. Initial steps in sphingolipid biosynthesis take place on the cytosolic face of the endoplasmic reticulum (ER). Serine palmitoyltransferase (SPT) is an ER-resident enzyme catalyzing the first-committed step in sphingolipid biosynthesis. Not surprisingly, SPT activity is tightly regulated. ORMDLs are ER-resident proteins recently identified as regulators of SPT activity. ORMDL proteins interact directly with SPT but the nature of this interaction is unknown. ORMDL protein sequences contain hydrophobic regions, yet algorithm-based predictions of transmembrane segments are highly ambiguous, making topology of this key regulator unclear. Here we report use of substituted cysteine accessibility to analyze topology of mammalian ORMDLs. We constructed multiple mutant ORMDLs, each containing a single cysteine strategically placed along the protein length. Combined use of selective membrane permeabilization with an impermeant cysteine modification reagent allowed us to assign transmembrane and cytosolic segments of ORMDL. We confirmed that mammalian ORMDL proteins transit the membrane four times, with amino- and carboxy termini facing the cytosol along with a large cytosolic loop. This model will allow us to determine details of the ORMDL-SPT interaction and identify regions acting as the "lipid sensor" to detect changes in cellular sphingolipid levels. We also observe that SPT and ORMDL are substantially resistant to extraction from membranes with non-ionic detergent, indirectly suggesting that both proteins reside in a specialized subdomain of the ER.

The ORMs interact with transmembrane domain 1 of Lcb1 and regulate serine palmitoyltransferase oligomerization, activity and localization

Serine palmitoyltransferase (SPT), an endoplasmic reticulum-localized membrane enzymecomposed of acatalytic LCB1/LCB2 heterodimer and a small activating subunit (Tsc3 in yeast; ssSPTs in mammals), is negatively regulated by the evolutionarily conserved family of proteins known as the ORMs. In yeast, SPT, the ORMs, and the PI4P phosphatase Sac1, copurify in the "SPOTs" complex. However, neither the mechanism of ORM inhibition of SPT nor details of the interactions of the ORMs and Sac1 with SPT are known. Here we report that the first transmembrane domain (TMD1) of Lcb1 is required for ORM binding to SPT. Loss of binding is not due to altered membrane topology of Lcb1 since replacing TMD1 with a heterologous TMD restores membrane topology but not ORM binding. TMD1 deletion also eliminates ORM-dependent formation of SPT oligomers as assessed by co-immunoprecipitation assays and in vivo imaging. Expression of ORMs lacking derepressive phosphorylation sites results in constitutive SPT oligomerization, while phosphomimetic ORMs fail to induce oligomerization under any conditions. Significantly, when LCB1-RFP and LCB1ΔTMD1-GFP were coexpressed, more LCB1ΔTMD1-GFP was in the peripheral ER, suggesting ORM regulation is partially accomplished by SPT redistribution. Tsc3 deletion does not abolish ORM inhibition of SPT, indicating the ORMs do not simply prevent activation by Tsc3. Binding of Sac1 to SPT requires Tsc3, but not the ORMs, and Sac1 does not influence ORM-mediated oligomerization of SPT. Finally, yeast mutants lacking ORM regulation of SPT require the LCB-P lyase Dpl1 to maintain long-chain bases at sublethal levels.

Lipid transfer proteins rectify inter-organelle flux and accurately deliver lipids at membrane contact sites

The endoplasmic reticulum (ER) is the main center for the synthesis of various lipid types in cells, and newly synthesized lipids are delivered from the ER to other organelles. In the past decade, various lipid transfer proteins (LTPs) have been recognized as mediators of lipid transport from the ER to other organelles; inter-organelle transport occurs at membrane contact sites (MCSs) and in a nonvesicular manner. Although the intermembrane transfer reaction catalyzed by LTPs is an equilibrium reaction, various types of newly synthesized lipids are transported unidirectionally in cells. This review provides a brief history of the inter-organelle trafficking of lipids and summarizes the structural and biochemical characteristics of the ceramide transport protein (CERT) as a typical LTP acting at MCSs. In addition, this review compares several LTP-mediated inter-organelle lipid trafficking systems and proposes that LTPs generate unidirectional fluxes of specific lipids between different organelles by indirect coupling with the metabolic reactions that occur in specific organelles. Moreover, the available data also suggest that the major advantage of LTP-mediated lipid transport at MCSs may be the accuracy of delivery. Finally, how cholesterol is enriched in the plasma membrane is discussed from a thermodynamic perspective.

Substrate reduction therapy for Krabbe's disease

Krabbe's disease (KD) is a lysosomal storage disorder in which galactosylceramide, a major glycosphingolipid of myelin, and psychosine (galactose-sphingosine) cannot be adequately metabolized because of a deficiency in galactosylceramidase. Substrate reduction therapy (SRT) has been tested in preclinical studies. The premise of SRT is to reduce the synthesis of substrates that are not adequately digested so that the substrate burden is lowered, resulting in less accumulation of unmetabolized material. SRT is used for Gaucher's disease, in which inhibitors of the terminal biosynthetic step are used. Unfortunately, an inhibitor for the final step of galactosylceramide biosynthesis, i.e., UDP glycosyltransferase 8 (a.k.a. UDP-galactose ceramide galactosyltransferase), has not been found. Approaches that inhibit an earlier biosynthetic step or that lessen the substrate burden by other means, such as genetic manipulations, have been tested in the twitcher mouse model of KD. Either as a stand-alone therapy or in combination with other approaches, SRT slowed the disease course, indicating that this approach has potential therapeutic value. For instance, in individuals with adult-onset disease, SRT theoretically could lessen the production of substrates so that residual enzymatic activity could adequately manage the lower substrate burden. In more severe forms of disease, SRT theoretically could be part of a combination therapy. However, SRT has the potential to impair normal function by reducing the synthesis of galactosylceramide to levels that impede myelin function, or SRT could have other deleterious effects. Thus, multiple issues need to be resolved before this approach is ready for testing in humans. © 2016 Wiley Periodicals, Inc.

Ceramide Transport from the Endoplasmic Reticulum to the Trans Golgi Region at Organelle Membrane Contact Sites

Lipids are the major constituents of all cell membranes and play dynamic roles in organelle structure and function. Although the spontaneous transfer of lipids between different membranes rarely occurs, lipids are appropriately transported between different organelles for their metabolism and to exert their functions in living cells. Proteins that have the biochemical capability to catalyze the intermembrane transfer of lipids are called lipid transfer proteins (LTPs). All organisms possess many types of LTPs. Recent studies revealed that LTPs are key players in the interorganelle transport of lipids at organelle membrane contact sites (MCSs). This chapter depicts how LTPs rationally operate at MCSs by using the ceramide transport protein CERT as a typical model for the LTP-mediated interorganelle transport of lipids.

Sphingolipids modulate the function of human serotonin1A receptors: Insights from sphingolipid-deficient cells

Sphingolipids are essential components of eukaryotic cell membranes and are known to modulate a variety of cellular functions. It is becoming increasingly clear that membrane lipids play a crucial role in modulating the function of integral membrane proteins such as G protein-coupled receptors (GPCRs). In this work, we utilized LY-B cells, that are sphingolipid-auxotrophic mutants defective in sphingolipid biosynthesis, to monitor the role of cellular sphingolipids in the function of an important neurotransmitter receptor, the serotonin_1A receptor. Serotonin_1A receptors belong to the family of GPCRs and are implicated in behavior, development and cognition. Our results show that specific ligand binding and G-protein coupling of the serotonin_1A receptor exhibit significant enhancement under sphingolipid-depleted conditions, which reversed to control levels upon replenishment of cellular sphingolipids. In view of the reported role of sphingolipids in neuronal metabolism and pathogenesis of several neuropsychiatric disorders, exploring the role of serotonin_1A receptors under conditions of defective sphingolipid metabolism assumes relevance, and could contribute to our overall understanding of such neuropsychiatric disorders. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Dynamics of sphingomyelin- and cholesterol-enriched lipid domains during cytokinesis

Sphingomyelin (SM) and cholesterol (Chol) are the major lipids in the mammalian cells, which are mainly localized to the plasma membrane. Multiple lines of evidence suggest that these lipids form local lipid domains in the plasma membrane, playing functional roles in the cell. Several observations have suggested that these lipid domains are required for cytokinesis. In this chapter, we show the methods for visualizing SM-rich and/or Chol-rich membrane domains at cytokinesis by using specific lipid-binding proteins. Lysenin, equinatoxin II, perfringolysin O, and pleurotolysin A2 bind specifically to clustered SM-rich domain, dispersed SM-rich domain, Chol-rich domain, and SM/Chol mixtures, respectively. Nontoxic forms of these lipid-binding proteins fused to fluorescent proteins are used for imaging lipid domains in biological membranes at cytokinesis. The image analysis reveals the structures and functions of SM-rich and/or Chol-rich domains at the time of cytokinesis.

HSAN1 mutations in serine palmitoyltransferase reveal a close structure-function-phenotype relationship

Hereditary sensory and autonomic neuropathy type 1 (HSAN1) is a rare autosomal dominant inherited peripheral neuropathy caused by mutations in the SPTLC1 and SPTLC2 subunits of serine palmitoyltransferase (SPT). The mutations induce a permanent shift in the substrate preference from L-serine to L-alanine, which results in the pathological formation of atypical and neurotoxic 1-deoxy-sphingolipids (1-deoxySL). Here we compared the enzymatic properties of 11 SPTLC1 and six SPTLC2 mutants using a uniform isotope labelling approach. In total, eight SPT mutants (STPLC1p.C133W, p.C133Y, p.S331F, p.S331Y and SPTLC2p.A182P, p.G382V, p.S384F, p.I504F) were associated with increased 1-deoxySL synthesis. Despite earlier reports, canonical activity with l-serine was not reduced in any of the investigated SPT mutants. Three variants (SPTLC1p.S331F/Y and SPTLC2p.I505Y) showed an increased canonical activity and increased formation of C20 sphingoid bases. These three mutations are associated with an exceptionally severe HSAN1 phenotype, and increased C20 sphingosine levels were also confirmed in plasma of patients. A principal component analysis of the analysed sphingoid bases clustered the mutations into three separate entities. Each cluster was related to a distinct clinical outcome (no, mild and severe HSAN1 phenotype). A homology model based on the protein structure of the prokaryotic SPT recapitulated the same grouping on a structural level. Mutations associated with the mild form clustered around the active site, whereas mutations associated with the severe form were located on the surface of the protein. In conclusion, we showed that HSAN1 mutations in SPT have distinct biochemical properties, which allowed for the prediction of the clinical symptoms on the basis of the plasma sphingoid base profile.

Pore-forming toxins: Properties, diversity, and uses as tools to image sphingomyelin and ceramide phosphoethanolamine

Pore-forming toxins (PFTs) represent a unique class of highly specific lipid-binding proteins. The cytotoxicity of these compounds has been overcome through crystallographic structure and mutation studies, facilitating the development of non-toxic lipid probes. As a consequence, non-toxic PFTs have been utilized as highly specific probes to visualize the diversity and dynamics of lipid nanostructures in living and fixed cells. This review is focused on the application of PFTs and their non-toxic analogs as tools to visualize sphingomyelin and ceramide phosphoethanolamine, two major phosphosphingolipids in mammalian and insect cells, respectively. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

ABCB4 exports phosphatidylcholine in a sphingomyelin-dependent manner

ABCB4, which is specifically expressed on the canalicular membrane of hepatocytes, exports phosphatidylcholine (PC) into bile. Because SM depletion increases cellular PC content and stimulates PC and cholesterol efflux by ABCA1, a key transporter involved in generation of HDL, we predicted that SM depletion also stimulates PC efflux through ABCB4. To test this prediction, we compared the lipid efflux activity of ABCB4 and ABCA1 under SM depletion induced by two different types of inhibitors for SM synthesis, myriocin and (1R,3S)-N-(3-hydroxy-1-hydroxymethyl-3-phenylpropyl)dodecanamide, in human embryonic kidney 293 and baby hamster kidney cells. Unexpectedly, SM depletion exerted opposite effects on ABCB4 and ABCA1, suppressing PC efflux through ABCB4 while stimulating efflux through ABCA1. Both ABCB4 and ABCA1 were recovered from Triton-X-100-soluble membranes, but ABCB4 was mainly recovered from CHAPS-insoluble SM-rich membranes, whereas ABCA1 was recovered from CHAPS-soluble membranes. These results suggest that a SM-rich membrane environment is required for ABCB4 to function. ABCB4 must have evolved to exert its maximum activity in the SM-rich membrane environment of the canalicular membrane, where it transports PC as the physiological substrate.

Sphingolipid metabolism and interorganellar transport: localization of sphingolipid enzymes and lipid transfer proteins

In recent decades, many sphingolipid enzymes, sphingolipid-metabolism regulators and sphingolipid transfer proteins have been isolated and characterized. This review will provide an overview of the intracellular localization and topology of sphingolipid enzymes in mammalian cells to highlight the locations where respective sphingolipid species are produced. Interestingly, three sphingolipids that reside or are synthesized in cytosolic leaflets of membranes (ceramide, glucosylceramide and ceramide-1-phosphate) all have cytosolic lipid transfer proteins (LTPs). These LTPs consist of ceramide transfer protein (CERT), four-phosphate adaptor protein 2 (FAPP2) and ceramide-1-phosphate transfer protein (CPTP), respectively. These LTPs execute functions that affect both the location and metabolism of the lipids they bind. Molecular details describing the mechanisms of regulation of LTPs continue to emerge and reveal a number of critical processes, including competing phosphorylation and dephosphorylation reactions and binding interactions with regulatory proteins and lipids that influence the transport, organelle distribution and metabolism of sphingolipids.

The Human ABCG1 Transporter Mobilizes Plasma Membrane and Late Endosomal Non-Sphingomyelin-Associated-Cholesterol for Efflux and Esterification

We have previously shown that GFP-tagged human ABCG1 on the plasma membrane (PM) and in late endosomes (LE) mobilizes sterol on both sides of the membrane lipid bilayer, thereby increasing cellular cholesterol efflux to lipid surfaces. In the present study, we examined ABCG1-induced changes in membrane cholesterol distribution, organization, and mobility. ABCG1-GFP expression increased the amount of mobile, non-sphingomyelin(SM)-associated cholesterol at the PM and LE, but not the amount of SM-associated-cholesterol or SM. ABCG1-mobilized non-SM-associated-cholesterol rapidly cycled between the PM and LE and effluxed from the PM to extracellular acceptors, or, relocated to intracellular sites of esterification. ABCG1 increased detergent-soluble pools of PM and LE cholesterol, generated detergent-resistant, non-SM-associated PM cholesterol, and increased resistance to both amphotericin B-induced (cholesterol-mediated) and lysenin-induced (SM-mediated) cytolysis, consistent with altered organization of both PM cholesterol and SM. ABCG1 itself resided in detergent-soluble membrane domains. We propose that PM and LE ABCG1 residing at the phase boundary between ordered (L_o) and disordered (L_d) membrane lipid domains alters SM and cholesterol organization thereby increasing cholesterol flux between L_o and L_d, and hence, the amount of cholesterol available for removal by acceptors on either side of the membrane bilayer for either efflux or esterification.

Expression of the ORMDLS, modulators of serine palmitoyltransferase, is regulated by sphingolipids in mammalian cells

The relationship between serine palmitoyltransferase (SPT) activity and ORMDL regulation of sphingolipid biosynthesis was investigated in mammalian HEK293 cells. Each of the three human ORMDLs reduced the increase in long-chain base synthesis seen after overexpression of wild-type SPT or SPT containing the C133W mutation in hLCB1, which produces the non-catabolizable sphingoid base, 1-deoxySa. ORMDL-dependent repression of sphingoid base synthesis occurred whether SPT was expressed as individual subunits or as a heterotrimeric single-chain SPT fusion protein. Overexpression of the single-chain SPT fusion protein under the control of a tetracycline-inducible promoter in stably transfected cells resulted in increased endogenous ORMDL expression. This increase was not transcriptional; there was no significant increase in any of the ORMDL mRNAs. Increased ORMDL protein expression required SPT activity since overexpression of a catalytically inactive SPT with a mutation in hLCB2a had little effect. Significantly, increased ORMDL expression was also blocked by myriocin inhibition of SPT as well as fumonisin inhibition of the ceramide synthases, suggesting that increased expression is a response to a metabolic signal. Moreover, blocking ORMDL induction with fumonisin treatment resulted in significantly greater increases in in vivo SPT activity than was seen when ORMDLs were allowed to increase, demonstrating the physiological significance of this response.

ABCA1, ABCG1, and ABCG4 are distributed to distinct membrane meso-domains and disturb detergent-resistant domains on the plasma membrane

ATP-binding cassette A1 (ABCA1), ABCG1, and ABCG4 are lipid transporters that mediate the efflux of cholesterol from cells. To analyze the characteristics of these lipid transporters, we examined and compared their distributions and lipid efflux activity on the plasma membrane. The efflux of cholesterol mediated by ABCA1 and ABCG1, but not ABCG4, was affected by a reduction of cellular sphingomyelin levels. Detergent solubility and gradient density ultracentrifugation assays indicated that ABCA1, ABCG1, and ABCG4 were distributed to domains that were solubilized by Triton X-100 and Brij 96, resistant to Triton X-100 and Brij 96, and solubilized by Triton X-100 but resistant to Brij 96, respectively. Furthermore, ABCG1, but not ABCG4, was colocalized with flotillin-1 on the plasma membrane. The amounts of cholesterol extracted by methyl-β-cyclodextrin were increased by ABCA1, ABCG1, or ABCG4, suggesting that cholesterol in non-raft domains was increased. Furthermore, ABCG1 and ABCG4 disturbed the localization of caveolin-1 to the detergent-resistant domains and the binding of cholera toxin subunit B to the plasma membrane. These results suggest that ABCA1, ABCG1, and ABCG4 are localized to distinct membrane meso-domains and disturb the meso-domain structures by reorganizing lipids on the plasma membrane; collectively, these observations may explain the different substrate profiles and lipid efflux roles of these transporters.

Stress-induced ceramide generation and apoptosis via the phosphorylation and activation of nSMase1 by JNK signaling

Neutral sphingomyelinase (nSMase) activation in response to environmental stress or inflammatory cytokine stimuli generates the second messenger ceramide, which mediates the stress-induced apoptosis. However, the signaling pathways and activation mechanism underlying this process have yet to be elucidated. Here we show that the phosphorylation of nSMase1 (sphingomyelin phosphodiesterase 2, SMPD2) by c-Jun N-terminal kinase (JNK) signaling stimulates ceramide generation and apoptosis and provide evidence for a signaling mechanism that integrates stress- and cytokine-activated apoptosis in vertebrate cells. An nSMase1 was identified as a JNK substrate, and the phosphorylation site responsible for its effects on stress and cytokine induction was Ser-270. In zebrafish cells, the substitution of Ser-270 for alanine blocked the phosphorylation and activation of nSMase1, whereas the substitution of Ser-270 for negatively charged glutamic acid mimicked the effect of phosphorylation. The JNK inhibitor SP600125 blocked the phosphorylation and activation of nSMase1, which in turn blocked ceramide signaling and apoptosis. A variety of stress conditions, including heat shock, UV exposure, hydrogen peroxide treatment, and anti-Fas antibody stimulation, led to the phosphorylation of nSMase1, activated nSMase1, and induced ceramide generation and apoptosis in zebrafish embryonic ZE and human Jurkat T cells. In addition, the depletion of MAPK8/9 or SMPD2 by RNAi knockdown decreased ceramide generation and stress- and cytokine-induced apoptosis in Jurkat cells. Therefore the phosphorylation of nSMase1 is a pivotal step in JNK signaling, which leads to ceramide generation and apoptosis under stress conditions and in response to cytokine stimulation. nSMase1 has a common central role in ceramide signaling during the stress and cytokine responses and apoptosis.

The ATP-binding cassette transporter-2 (ABCA2) regulates esterification of plasma membrane cholesterol by modulation of sphingolipid metabolism

The ATP-binding cassette transporters are a large family (~48 genes divided into seven families A-G) of proteins that utilize the energy of ATP-hydrolysis to pump substrates across lipid bilayers against a concentration gradient. The ABC "A" subfamily is comprised of 13 members and transport sterols, phospholipids and bile acids. ABCA2 is the most abundant ABC transporter in human and rodent brain with highest expression in oligodendrocytes, although it is also expressed in neurons. Several groups have studied a possible connection between ABCA2 and Alzheimer's disease as well as early atherosclerosis. ABCA2 expression levels have been associated with changes in cholesterol and sphingolipid metabolism. In this paper, we hypothesized that ABCA2 expression level may regulate esterification of plasma membrane-derived cholesterol by modulation of sphingolipid metabolism. ABCA2 overexpression in N2a neuroblastoma cells was associated with an altered bilayer distribution of the sphingolipid ceramide that inhibited acylCoA:cholesterol acyltransferase (ACAT) activity and cholesterol esterification. In contrast, depletion of endogenous ABCA2 in the rat schwannoma cell line D6P2T increased esterification of plasma membrane cholesterol following treatment with exogenous bacterial sphingomyelinase. These findings suggest that control of ABCA2 expression level may be a key locus of regulation for esterification of plasma membrane-derived cholesterol through modulation of sphingolipid metabolism.

Earthworm-derived pore-forming toxin lysenin and screening of its inhibitors

Lysenin is a pore-forming toxin from the coelomic fluid of earthworm Eisenia foetida. This protein specifically binds to sphingomyelin and induces erythrocyte lysis. Lysenin consists of 297 amino acids with a molecular weight of 41 kDa. We screened for cellular signal transduction inhibitors of low molecular weight from microorganisms and plants. The purpose of the screening was to study the mechanism of diseases using the obtained inhibitors and to develop new chemotherapeutic agents acting in the new mechanism. Therefore, our aim was to screen for inhibitors of Lysenin-induced hemolysis from plant extracts and microbial culture filtrates. As a result, we isolated all-E-lutein from an extract of Dalbergia latifolia leaves. All-E-lutein is likely to inhibit the process of Lysenin-membrane binding and/or oligomer formation rather than pore formation. Additionally, we isolated tyrosylproline anhydride from the culture filtrate of Streptomyces as an inhibitor of Lysenin-induced hemolysis.

Imaging local sphingomyelin-rich domains in the plasma membrane using specific probes and advanced microscopy

Sphingomyelin (SM) is one of the major lipids in the mammalian plasma membrane. Multiple lines of evidence suggest that SM plays at least two functional roles in the cell, as a reservoir of lipid second messengers and as a platform for signaling molecules. To understand the molecular organization and dynamics of the SM-rich membrane domains, new approaches have been developed utilizing newly characterized specific SM-binding probes and state-of-the-art microscopy techniques. The toxic protein from the sea anemone, equinatoxin II, has been characterized as a specific probe for SM. The cytolytic protein from the earthworm, lysenin, has also been used as a SM-specific probe for the analysis of the heterogeneity of SM-rich membrane domains. Recently, using a non-toxic form of lysenin, we showed the spatial and temporal localization of SM in the plasma membrane by confocal and super-resolution microscopy. New microscopy techniques have also been introduced by other groups to help visualize membrane lipid domains. Here we review the most recent studies on imaging the SM-rich domains in biological membranes. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Co-evolution of sphingomyelin and the ceramide transport protein CERT

Life creates many varieties of lipids. The choline-containing sphingophospholipid sphingomyelin (SM) exists ubiquitously or widely in vertebrates and lower animals, but is absent or rare in bacteria, fungi, protists, and plants. In the biosynthesis of SM, ceramide, which is synthesized in the endoplasmic reticulum, is transported to the Golgi region by the ceramide transport protein CERT, probably in a non-vesicular manner, and is then converted to SM by SM synthase, which catalyzes the reaction of phosphocholine transfer from phosphatidylcholine (PtdCho) to ceramide. Recent advances in genomics and lipidomics indicate that the phylogenetic occurrence of CERT and its orthologs is nearly parallel to that of SM. Based on the chemistry of lipids together with evolutionary aspects of SM and CERT, several concepts are here proposed. SM may serve as a chemically inert and robust, but non-covalently interactive lipid class at the outer leaflet of the plasma membrane. The functional domains and peptidic motifs of CERT are separated by exon units, suggesting an exon-shuffling mechanism for the generation of an ancestral CERT gene. CERT may have co-evolved with SM to bypass a competing metabolic reaction at the bifurcated point in the anabolism of ceramide. Human CERT is identical to the splicing variant of human Goodpasture antigen-binding protein (GPBP) annotated as an extracellular non-canonical serine/threonine protein kinase. The relationship between CERT and GPBP has also been discussed from an evolutionary aspect. Moreover, using an analogy of "compatible (or osmoprotective) solutes" that can accumulate to very high concentrations in the cytosol without denaturing proteins, choline phospholipids such as PtdCho and SM may act as compatible phospholipids in biomembranes. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Toxoplasma gondii salvages sphingolipids from the host Golgi through the rerouting of selected Rab vesicles to the parasitophorous vacuole

The obligate intracellular protozoan Toxoplasma gondii actively invades mammalian cells and, upon entry, forms its own membrane-bound compartment, named the parasitophorous vacuole (PV). Within the PV, the parasite replicates and scavenges nutrients, including lipids, from host organelles. Although T. gondii can synthesize sphingolipids de novo, it also scavenges these lipids from the host Golgi. How the parasite obtains sphingolipids from the Golgi remains unclear, as the PV avoids fusion with host organelles. In this study, we explore the host Golgi-PV interaction and evaluate the importance of host-derived sphingolipids for parasite growth. We demonstrate that the PV preferentially localizes near the host Golgi early during infection and remains closely associated with this organelle throughout infection. The parasite subverts the structure of the host Golgi, resulting in its fragmentation into numerous ministacks, which surround the PV, and hijacks host Golgi-derived vesicles within the PV. These vesicles, marked with Rab14, Rab30, or Rab43, colocalize with host-derived sphingolipids in the vacuolar space. Scavenged sphingolipids contribute to parasite replication since alterations in host sphingolipid metabolism are detrimental for the parasite's growth. Thus our results reveal that T. gondii relies on host-derived sphingolipids for its development and scavenges these lipids via Golgi-derived vesicles.

Sphingolipid homeostasis in the endoplasmic reticulum and beyond

Sphingolipids are a diverse group of lipids that have essential cellular roles as structural components of membranes and as potent signaling molecules. In recent years, a detailed picture has emerged of the basic biochemistry of sphingolipids-from their initial synthesis in the endoplasmic reticulum (ER), to their elaboration into complex glycosphingolipids, to their turnover and degradation. However, our understanding of how sphingolipid metabolism is regulated in response to metabolic demand and physiologic cues remains incomplete. Here I discuss new insights into the mechanisms that ensure sphingolipid homeostasis, with an emphasis on the ER as a critical regulatory site in sphingolipid metabolism. In particular, Orm family proteins have recently emerged as key ER-localized mediators of sphingolipid homeostasis. A detailed understanding of how cells sense and control sphingolipid production promises to provide key insights into membrane function in health and disease.

The essential structures of ISP-I that influence serine palmitoyltransferase inhibition in Chinese hamster ovary cells

We investigated the structure-activity relationship between various ISP-I (myriocin, thermozymocidin) analogous which has sphingosine-like structure and serine palmitoyltransferase (SPT) in Chinese hamster ovary (CHO) cells utilizing sphingolipid production as a marker. Our data suggest that the double bond and/or ketone group within the alkyl chain as well as the alkyl chain are necessary for ISP-I to inhibit SPT. In addition, a serine structure is necessary for SPT inhibitory activity, which confirms previous findings.

Glycosylphosphatidylinositol anchors regulate glycosphingolipid levels

Glycosylphosphatidylinositol (GPI) anchor biosynthesis takes place in the endoplasmic reticulum (ER). After protein attachment, the GPI anchor is transported to the Golgi where it undergoes fatty acid remodeling. The ER exit of GPI-anchored proteins is controlled by glycan remodeling and p24 complexes act as cargo receptors for GPI anchor sorting into COPII vesicles. In this study, we have characterized the lipid profile of mammalian cell lines that have a defect in GPI anchor biosynthesis. Depending on which step of GPI anchor biosynthesis the cells were defective, we observed sphingolipid changes predominantly for very long chain monoglycosylated ceramides (HexCer). We found that the structure of the GPI anchor plays an important role in the control of HexCer levels. GPI anchor-deficient cells that generate short truncated GPI anchor intermediates showed a decrease in very long chain HexCer levels. Cells that synthesize GPI anchors but have a defect in GPI anchor remodeling in the ER have a general increase in HexCer levels. GPI-transamidase-deficient cells that produce no GPI-anchored proteins but generate complete free GPI anchors had unchanged levels of HexCer. In contrast, sphingomyelin levels were mostly unaffected. We therefore propose a model in which the transport of very long chain ceramide from the ER to Golgi is regulated by the transport of GPI anchor molecules.

Ceramide transfer protein and cancer

Sphingolipids are important structural components of membranes, and play an equally important role in basic cellular processes as second messengers. Recently, sphingolipids are receiving increasing attention in cancer research. Ceramide is the central molecule that regulates sphingolipid metabolism forming the basic structural backbone of sphingolipids and the precursor of all complex sphingolipids. It is been proposed to be an important regulator of tumor cell death following exposure to stress stimuli. The increase or decrease of ceramide levels leading to change in sensitivity of cancer cells to stress stimuli provides support for a central role of ceramide signaling in cell death. In this review, we have focused on ceramide transfer protein (CERT) as a major regulator of ceramide flux in the cell.

Reminiscence of our research on membrane phospholipids in mammalian cells by using the novel technology

By using "our devised up-to-the-second technique" over 30 years ago, we succeeded in the first isolation in the world of the three different kinds of mammalian cell mutants defective in the biosynthesis on each of phosphatidylserine (PS), cardiolipin (CL) and sphingomyelin (SM) from the parental CHO cells. As the results, we found that during the biosyntheses of PS and SM, the biosynthetic precursor or the final lipids are transported from their synthesized intracellular organelles to the plasma membranes via the other intracellular organelles. We further clarified the presence of the reversed routes for PS and SM from the plasma membranes to their synthesized organelles too. Our first epoch-making finding is not only the cycling inter-conversion reactions between PS and PE catalyzed by PSS-II and PSD but also their simultaneous transferring between MAM and Mit (found by O. Kuge). Our second finding is "the ceramide-trafficking protein (CERT)" working as the specific transfer protein of ceramide from the ER to the Golgi apparatus, during the SM biosynthesis (by K. Hanada). As for their new biological roles, we clarified possible contribution of PS and/or PE to the fusion process between viral envelope and endosomal membrane, releasing the genetic information of the virus to the host cytoplasm. CL is contributing to the functional NADH-ubiquinone reductase activity by keeping the right structure of Coenzyme Q9 for its functioning. SM and cholesterol form the microdomain within the plasma membrane, so-called "the raft structure" where the GPI-anchored proteins are specifically located for their functioning.

Reconstitution assay system for ceramide transport with semi-intact cells

The intracellular transport of lipids from the sites of their synthesis to their appropriate destination is a critical step for lipid metabolism. One well-defined inter-organelle lipid movement is the transport of ceramide by ceramide transport protein (CERT). Ceramide, a key intermediate for both sphingomyelin and glycosphingolipids, is synthesized at the endoplasmic reticulum and delivered to the Golgi apparatus to be converted to sphingomyelin. CERT delivers ceramide from the ER to the Golgi apparatus in a non-vesicular and ATP-dependent manner. This chapter describes a reconstitution assay system for ceramide transport with semi-intact cells, which is useful for the study of the CERT-mediated inter-organelle transport of ceramide.

Chlamydia trachomatis co-opts GBF1 and CERT to acquire host sphingomyelin for distinct roles during intracellular development

The obligate intracellular pathogen Chlamydia trachomatis replicates within a membrane-bound inclusion that acquires host sphingomyelin (SM), a process that is essential for replication as well as inclusion biogenesis. Previous studies demonstrate that SM is acquired by a Brefeldin A (BFA)-sensitive vesicular trafficking pathway, although paradoxically, this pathway is dispensable for bacterial replication. This finding suggests that other lipid transport mechanisms are involved in the acquisition of host SM. In this work, we interrogated the role of specific components of BFA-sensitive and BFA-insensitive lipid trafficking pathways to define their contribution in SM acquisition during infection. We found that C. trachomatis hijacks components of both vesicular and non-vesicular lipid trafficking pathways for SM acquisition but that the SM obtained from these separate pathways is being utilized by the pathogen in different ways. We show that C. trachomatis selectively co-opts only one of the three known BFA targets, GBF1, a regulator of Arf1-dependent vesicular trafficking within the early secretory pathway for vesicle-mediated SM acquisition. The Arf1/GBF1-dependent pathway of SM acquisition is essential for inclusion membrane growth and stability but is not required for bacterial replication. In contrast, we show that C. trachomatis co-opts CERT, a lipid transfer protein that is a key component in non-vesicular ER to trans-Golgi trafficking of ceramide (the precursor for SM), for C. trachomatis replication. We demonstrate that C. trachomatis recruits CERT, its ER binding partner, VAP-A, and SM synthases, SMS1 and SMS2, to the inclusion and propose that these proteins establish an on-site SM biosynthetic factory at or near the inclusion. We hypothesize that SM acquired by CERT-dependent transport of ceramide and subsequent conversion to SM is necessary for C. trachomatis replication whereas SM acquired by the GBF1-dependent pathway is essential for inclusion growth and stability. Our results reveal a novel mechanism by which an intracellular pathogen redirects SM biosynthesis to its replicative niche.

C. trachomatis is the leading cause of non-congenital blindness in developing countries and is the number one cause of sexually transmitted disease and non-congenital infertility in Western countries. The capacity of Chlamydia infections to lead to infertility and blindness, their association with chronic diseases, and the extraordinary prevalence and array of these infections make them public concerns of primary importance. This pathogen must establish a protective membrane-bound niche and acquire essential lipids from the host cell during infection in order to survive and replicate. This study identifies novel mechanisms by which C. trachomatis hijacks various lipid trafficking proteins for distinct roles during intracellular development. Disruption of these lipid trafficking pathways results in alterations in the growth and stability of its protective niche as well as a defect in replication. Understanding the molecular mechanisms of these host-pathogen interactions will lead to rational approaches for the development of novel therapeutics, diagnostics, and preventative strategies.

The mammalian ABC transporter ABCA1 induces lipid-dependent drug sensitivity in yeast

ABCA1 belongs to the A class of ABC transporter, which is absent in yeast. ABCA1 elicits lipid translocation at the plasma membrane through yet elusive processes. We successfully expressed the mouse Abca1 gene in Saccharomyces cerevisiae. The cloned ABCA1 distributed at the yeast plasma membrane in stable discrete domains that we name MCA (membrane cluster containing ABCA1) and that do not overlap with the previously identified punctate structures MCC (membrane cluster containing Can1p) and MCP (membrane cluster containing Pma1p). By comparison with a nonfunctional mutant, we demonstrated that ABCA1 elicits specific phenotypes in response to compounds known to interact with membrane lipids, such as papuamide B, amphotericin B and pimaricin. The sensitivity of these novel phenotypes to the genetic modification of the membrane lipid composition was studied by the introduction of the cho1 and lcb1-100 mutations involved respectively in phosphatidylserine or sphingolipid biosynthesis in yeast cells. The results, corroborated by the analysis of equivalent mammalian mutant cell lines, demonstrate that membrane composition, in particular its phosphatidylserine content, influences the function of the transporter. We thus have reconstituted in yeast the essential functions associated to the expression of ABCA1 in mammals and characterized new physiological phenotypes prone to genetic analysis. This article is a part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Segregation of fluorescent membrane lipids into distinct micrometric domains: evidence for phase compartmentation of natural lipids?

Background

We recently reported that sphingomyelin (SM) analogs substituted on the alkyl chain by various fluorophores (e.g. BODIPY) readily inserted at trace levels into the plasma membrane of living erythrocytes or CHO cells and spontaneously concentrated into micrometric domains. Despite sharing the same fluorescent ceramide backbone, BODIPY-SM domains segregated from similar domains labelled by BODIPY-D-e-lactosylceramide (D-e-LacCer) and depended on endogenous SM.

Methodology/Principal Findings

We show here that BODIPY-SM further differed from BODIPY-D-e-LacCer or -glucosylceramide (GlcCer) domains in temperature dependence, propensity to excimer formation, association with a glycosylphosphatidylinositol (GPI)-anchored fluorescent protein reporter, and lateral diffusion by FRAP, thus demonstrating different lipid phases and boundaries. Whereas BODIPY-D-e-LacCer behaved like BODIPY-GlcCer, its artificial stereoisomer, BODIPY-L-t-LacCer, behaved like BODIPY- and NBD-phosphatidylcholine (PC). Surprisingly, these two PC analogs also formed micrometric patches yet preferably at low temperature, did not show excimer, never associated with the GPI reporter and showed major restriction to lateral diffusion when photobleached in large fields. This functional comparison supported a three-phase micrometric compartmentation, of decreasing order: BODIPY-GSLs > -SM > -PC (or artificial L-t-LacCer). Co-existence of three segregated compartments was further supported by double labelling experiments and was confirmed by additive occupancy, up to ∼70% cell surface coverage. Specific alterations of BODIPY-analogs domains by manipulation of corresponding endogenous sphingolipids suggested that distinct fluorescent lipid partition might reflect differential intrinsic propensity of endogenous membrane lipids to form large assemblies.

Conclusions/Significance

We conclude that fluorescent membrane lipids spontaneously concentrate into distinct micrometric assemblies. We hypothesize that these might reflect preexisting compartmentation of endogenous PM lipids into non-overlapping domains of differential order: GSLs > SM > PC, resulting into differential self-adhesion of the two former, with exclusion of the latter.

Diverse endogenous antigens for mouse NKT cells: self-antigens that are not glycosphingolipids

NKT cells with an invariant Ag receptor (iNKT cells) represent a highly conserved and unique subset of T lymphocytes having properties of innate and adaptive immune cells. They have been reported to regulate a variety of immune responses, including the response to cancers and the development of autoimmunity. The development and activation of iNKT cells is dependent on self-Ags presented by the CD1d Ag-presenting molecule. It is widely believed that these self-Ags are glycosphingolipids (GSLs), molecules that contain ceramide as the lipid backbone. In this study, we used a variety of methods to show that mammalian Ags for mouse iNKT cells need not be GSLs, including the use of cell lines deficient in GSL biosynthesis and an inhibitor of GSL biosynthesis. Presentation of these Ags required the expression of CD1d molecules that could traffic to late endosomes, the site where self-Ag is acquired. Extracts of APCs contain a self-Ag that could stimulate iNKT cells when added to plates coated with soluble, rCD1d molecules. The Ag(s) in these extracts are resistant to sphingolipid-specific hydrolase digestion, consistent with the results using live APCs. Lyosphosphatidylcholine, a potential self-Ag that activated human iNKT cell lines, did not activate mouse iNKT cell hybridomas. Our data indicate that there may be more than one type of self-Ag for iNKT cells, that the self-Ags comparing mouse and human may not be conserved, and that the search to identify these molecules should not be confined to GSLs.

Transmembrane BAX inhibitor motif containing (TMBIM) family proteins perturbs a trans-Golgi network enzyme, Gb3 synthase, and reduces Gb3 biosynthesis

Globotriaosylceramide (Gb3) is a well known receptor for Shiga toxin (Stx), produced by enterohemorrhagic Escherichia coli and Shigella dysenteriae. The expression of Gb3 also affects several diseases, including cancer metastasis and Fabry disease, which prompted us to look for factors involved in its metabolism. In the present study, we isolated two cDNAs that conferred resistance to Stx-induced cell death in HeLa cells by expression cloning: ganglioside GM3 synthase and the COOH terminus region of glutamate receptor, ionotropic, N-methyl-D-asparate-associated protein 1 (GRINA), a member of the transmembrane BAX inhibitor motif containing (TMBIM) family. Overexpression of the truncated form, named GRINA-C, and some members of the full-length TMBIM family, including FAS inhibitory molecule 2 (FAIM2), reduced Gb3, and lactosylceramide was accumulated instead. The change of glycolipid composition was restored by overexpression of Gb3 synthase, suggesting that the synthase is affected by GRINA-C and FAIM2. Interestingly, the mRNA level of Gb3 synthase was unchanged. Rather, localization of the synthase as well as TGN46, a trans-Golgi network marker, was perturbed to form punctate structures, and degradation of the synthase in lysosomes was enhanced. Furthermore, GRINA-C was associated with Gb3 synthase. These observations may demonstrate a new type of posttranscriptional regulation of glycosyltransferases.

The serine palmitoyltransferase from Sphingomonas wittichii RW1: An interesting link to an unusual acyl carrier protein

Serine palmitoyltransferase (SPT) catalyses the first step in the de novo biosynthesis of sphingolipids (SLs). It uses a decarboxylative Claisen-like condensation reaction to couple L-serine with palmitoyl-CoA to generate a long-chain base product, 3-ketodihydrosphingosine. SLs are produced by mammals, plants, yeast, and some bacteria, and we have exploited the complete genome sequence of Sphingomonas wittichii to begin a complete analysis of bacterial sphingolipid biosynthesis. Here, we describe the enzymatic characterization of the SPT from this organism and present its high-resolution x-ray structure. Moreover, we identified an open reading frame with high sequence homology to acyl carrier proteins (ACPs) that are common to fatty acid biosynthetic pathways. This small protein was co-expressed with the SPT and we isolated and characterised the apo- and holo-forms of the ACP. Our studies suggest a link between fatty acid and sphingolipid metabolism.