CERT and intracellular trafficking of ceramide

Ceramide and its transport protein (CERT) contribute to deterioration of mitochondrial structure and function in aging oocytes

In women as well as in mice, oocytes exhibit decreased developmental potential (oocyte quality) with advanced age. Our current data implicate alterations in the levels of oocyte ceramide and associated changes in mitochondrial function and structure as being prominent elements contributing to reduced oocyte quality. Both ROS levels and ATP content were significantly reduced in aged oocytes. The decreased in ROS levels are of intrigue because it is contrary to what has been previously reported. Lowered levels of both ROS and ATP indicate diminished mitochondrial function that was accompanied by alterations in mitochondrial structure. Interestingly, developmental potential of old oocytes was improved by microinjection of mitochondria isolated from young oocytes. Co-treatment of aged oocytes with ceramide and a cytoplasmic lipid carrier (l-carnitine) improved both mitochondrial morphology and function, and totally rescued spontaneous in vitro fragmentation. In addition, ceramide localization was altered in old oocytes possibly due to downregulation of the ceramide transport protein (CERT). However, knockdown of CERT alone was not sufficient to increase young oocyte's susceptibility to death, because the sequential manipulation of ceramide levels (its chronic decrease, followed by downregulation of CERT, and finally a ceramide spike) were all necessary to replicate the aging phenotype. These results indicate that oocyte aging is due to a multiplicity of events; and that with increasing biological age, changes in levels of both ceramide and its transport protein contribute to deterioration of oocyte mitochondrial structure and function. Hence, those changes may represent potential targets to manipulate when attempting to ameliorate aging phenotypes in germ cells.

Roles of ceramide and sphingolipids in pancreatic β-cell function and dysfunction

Recent technical advances have re-invigorated the study of sphingolipid metabolism in general, and helped to highlight the varied and important roles that sphingolipids play in pancreatic β-cells. Sphingolipid metabolites such as ceramide, glycosphingolipids, sphingosine 1-phosphate and gangliosides modulate many β-cell signaling pathways and processes implicated in β-cell diabetic disease such as apoptosis, β-cell cytokine secretion, ER-to-golgi vesicular trafficking, islet autoimmunity and insulin gene expression. They are particularly relevant to lipotoxicity. Moreover, the de novo synthesis of sphingolipids occurs on many subcellular membranes, in parallel to secretory vesicle formation, traffic and granule maturation events. Indeed, the composition of the plasma membrane, determined by the activity of neutral sphingomyelinases, affects β-cell excitability and potentially insulin exocytosis while another glycosphingolipid, sulfatide, determines the stability of insulin crystals in granules. Most importantly, sphingolipid metabolism on internal membranes is also strongly implicated in regulating β-cell apoptosis.

PLP-dependent enzymes as entry and exit gates of sphingolipid metabolism

Sphingolipids are membrane constituents as well as signaling molecules involved in many essential cellular processes. Serine palmitoyltransferase (SPT) and sphingosine-1-phosphate lyase (SPL), both PLP (pyridoxal 5'-phosphate)-dependent enzymes, function as entry and exit gates of the sphingolipid metabolism. SPT catalyzes the condensation of serine and a fatty acid into 3-keto-dihydrosphingosine, whereas SPL degrades sphingosine-1-phosphate (S1P) into phosphoethanolamine and a long-chain aldehyde. The recently solved X-ray structures of prokaryotic homologs of SPT and SPL combined with functional studies provide insight into the structure-function relationship of the two enzymes. Despite carrying out different reactions, the two enzymes reveal striking similarities in the overall fold, topology, and residues crucial for activity. Unlike their eukaryotic counterparts, bacterial SPT and SPL lack a transmembrane helix, making them targets of choice for biochemical characterization because the use of detergents can be avoided. Both human enzymes are linked to severe diseases or disorders and might therefore serve as targets for the development of therapeutics aiming at the modulation of their activity. This review gives an overview of the sphingolipid metabolism and of the available biochemical studies of prokaryotic SPT and SPL, and discusses the major similarities and differences to the corresponding eukaryotic enzymes.

Sphingolipid Δ8 unsaturation is important for glucosylceramide biosynthesis and low-temperature performance in Arabidopsis

Plants contain a large diversity of sphingolipid structures, arising in part from C4 hydroxylation and Δ4 and Δ8 desaturation of the component long-chain bases (LCBs). Typically, 85-90% of sphingolipid LCBs in Arabidopsis leaves contain a cis or transΔ8 double bond produced by sphingoid LCB Δ8 desaturase (SLD). To understand the metabolic and physiological significance of Δ8 unsaturation, studies were performed using mutants of the Arabidopsis SLD genes AtSLD1 and AtSLD2. Our studies revealed that both genes are constitutively expressed, the corresponding polypeptides are ER-localized, and expression of these genes in Saccharomyces cerevisiae yields mixtures of cis/transΔ8 desaturation products, predominantly as trans isomers. Consistent in part with the higher expression of AtSLD1 in Arabidopsis plants, AtSLD1 T-DNA mutants showed large reductions in Δ8 unsaturated LCBs in all organs examined, whereas AtSLD2 mutants showed little change in LCB unsaturation. Double mutants of AtSLD1 and AtSLD2 showed no detectable LCB Δ8 unsaturation. Comprehensive analysis of sphingolipids in rosettes of these mutants revealed a 50% reduction in glucosylceramide levels and a corresponding increase in glycosylinositolphosphoceramides that were restored by complementation with a wild-type copy of AtSLD1. Double sld1 sld2 mutants lacked apparent growth phenotypes under optimal conditions, but displayed altered responses to certain stresses, including prolonged exposure to low temperatures. These results are consistent with a role for LCB Δ8 unsaturation in selective channeling of ceramides for the synthesis of complex sphingolipids and the physiological performance of Arabidopsis.

Ceramide and mitochondrial function in aging oocytes: joggling a new hypothesis and old players

Maternal aging adversely affects oocyte quality (function and developmental potential) and consequently lowers pregnancy rates while increasing spontaneous abortions. Substantial evidence, especially from egg donation studies, implicates the decreased quality of an aging oocyte as a major factor in the etiology of female infertility. Nevertheless, the cellular and molecular mechanisms responsible for the decreased oocyte quality with advanced maternal aging are not fully characterized. Herein we present information in the published literature and our own data to support the hypothesis that during aging induced decreases in mitochondrial ceramide levels and associated alterations in mitochondrial structure and function are prominent elements contributing to reduced oocyte quality. Hence, by examining the molecular determinants that underlie impairments in oocyte mitochondria, we expect to sieve to a better understanding of the mechanistic anatomy of oocyte aging.

Triangulated mal-signaling in Alzheimer's disease: roles of neurotoxic ceramides, ER stress, and insulin resistance reviewed

Ceramides are lipid signaling molecules that cause cytotoxicity and cell death mediated by insulin resistance, inflammation, and endoplasmic reticulum (ER) stress. However, insulin resistance dysregulates lipid metabolism, which promotes ceramide accumulation with attendant inflammation and ER stress. Herein, we discuss two major pathways, extrinsic and intrinsic, that converge and often overlap in propagating AD-type neurodegeneration via a triangulated mal-signaling network. First, we review evidence that systemic insulin resistance diseases linked to obesity, type 2 diabetes, and non-alcoholic steatohepatitis promote neurodegeneration. Mechanistically, we propose that toxic ceramides generated in extra-CNS tissues (e.g., liver) get released into peripheral blood, and subsequently transit across the blood-brain barrier into the brain where they induce brain insulin resistance, inflammation, and cell death (extrinsic pathway). Then we discuss the role of the intrinsic pathway of neurodegeneration which is mediated by endogenous or primary brain insulin/IGF resistance, and impairs neuronal and oligodendrocyte survival, energy metabolism, membrane integrity, cytoskeletal function, and AβPP-Aβ secretion. The end result is increased ER stress and ceramide generation, which exacerbate brain insulin resistance, cell death, myelin degeneration, and neuroinflammation. Altogether, the data suggest that the triangulated mal-signaling network mediated by toxic ceramides, ER stress, and insulin resistance should be targeted to disrupt positive feedback loops that drive the AD neurodegeneration cascade.

Differential localization of sphingomyelin synthase isoforms in neurons regulates sphingomyelin cluster formation

Sphingomyelin (SM) plays important roles in regulating structure and function of plasma membrane, but how intracellular localization of SM is regulated in neuronal cells is not understood. Here we show that two isoforms of SM synthase (SMS) are differentially expressed in neuronal subtypes and that only SMS2 proteins localize in neurites of hippocampal neurons. Moreover, SMS proteins induce Lysenin-binding SM clusters exclusively in their vicinity although neurons hardly contain such cluster under control condition. These findings indicate three important notions about SM metabolism in neurons. First, the activity of SMS is the rate-limiting step of SM cluster formation. Second, the SM content or clustering can be modulated by SMS activity. Third, SMS1 and SMS2 play distinct roles in regulating local SM clustering. Particularly, SMS2, rather than SMS1, is likely to be the major enzyme that is important for SM synthesis in the long neurites and its tip, the growth cone.

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.

Architecture of the mammalian Golgi

Since its first visualization in 1898, the Golgi has been a topic of intense morphological research. A typical mammalian Golgi consists of a pile of stapled cisternae, the Golgi stack, which is a key station for modification of newly synthesized proteins and lipids. Distinct stacks are interconnected by tubules to form the Golgi ribbon. At the entrance site of the Golgi, the cis-Golgi, vesicular tubular clusters (VTCs) form the intermediate between the endoplasmic reticulum and the Golgi stack. At the exit site of the Golgi, the trans-Golgi, the trans-Golgi network (TGN) is the major site of sorting proteins to distinct cellular locations. Golgi functioning can only be understood in light of its complex architecture, as was revealed by a range of distinct electron microscopy (EM) approaches. In this article, a general concept of mammalian Golgi architecture, including VTCs and the TGN, is described.

Signaling at the Golgi

The protein processing and trafficking function of the Golgi is intimately linked to multiple intracellular signaling pathways. Assembly of Golgi trafficking structures and lipid sorting at the Golgi complex is controlled and coordinated by specific phosphoinositide kinases and phosphatases. The intra-Golgi transport machinery is also regulated by kinases belonging to several functionally distinct families, for example, MAP kinase signaling is required for mitotic disassembly of the Golgi. However, the Golgi plays an additional, prominent role in compartmentalizing other signaling cascades that originate at the plasma membrane or at other organelles. This article summarizes recent advances in our understanding of the signaling network that converges at the Golgi.

Loss of subcellular lipid transport due to ARV1 deficiency disrupts organelle homeostasis and activates the unfolded protein response

The ARV1-encoded protein mediates sterol transport from the endoplasmic reticulum (ER) to the plasma membrane. Yeast ARV1 mutants accumulate multiple lipids in the ER and are sensitive to pharmacological modulators of both sterol and sphingolipid metabolism. Using fluorescent and electron microscopy, we demonstrate sterol accumulation, subcellular membrane expansion, elevated lipid droplet formation, and vacuolar fragmentation in ARV1 mutants. Motif-based regression analysis of ARV1 deletion transcription profiles indicates activation of Hac1p, an integral component of the unfolded protein response (UPR). Accordingly, we show constitutive splicing of HAC1 transcripts, induction of a UPR reporter, and elevated expression of UPR targets in ARV1 mutants. IRE1, encoding the unfolded protein sensor in the ER lumen, exhibits a lethal genetic interaction with ARV1, indicating a viability requirement for the UPR in cells lacking ARV1. Surprisingly, ARV1 mutants expressing a variant of Ire1p defective in sensing unfolded proteins are viable. Moreover, these strains also exhibit constitutive HAC1 splicing that interacts with DTT-mediated perturbation of protein folding. These data suggest that a component of UPR induction in arv1Δ strains is distinct from protein misfolding. Decreased ARV1 expression in murine macrophages also results in UPR induction, particularly up-regulation of activating transcription factor-4, CHOP (C/EBP homologous protein), and apoptosis. Cholesterol loading or inhibition of cholesterol esterification further elevated CHOP expression in ARV1 knockdown cells. Thus, loss or down-regulation of ARV1 disturbs membrane and lipid homeostasis, resulting in a disruption of ER integrity, one consequence of which is induction of the UPR.

The ceramide-enriched trans-Golgi compartments reorganize together with other parts of the Golgi apparatus in response to ATP-depletion

In this study, the ceramide-enriched trans-Golgi compartments representing sites of synthesis of sphingomyelin and higher organized lipids were visualized in control and ATP-depleted hepatoma and endothelial cells using internalization of BODIPY-ceramide and the diaminobenzidine photooxidation method for combined light-electron microscopical exploration. Metabolic stress induced by lowering the cellular ATP-levels leads to reorganizations of the Golgi apparatus and the appearance of tubulo-glomerular bodies and networks. The results obtained with three different protocols, in which BODIPY-ceramide either was applied prior to, concomitantly with, or after ATP-depletion, revealed that the ceramide-enriched compartments reorganize together with other parts of the Golgi apparatus under these conditions. They were found closely associated with and integrated in the tubulo-glomerular bodies formed in response to ATP-depletion. This is in line with the changes of the staining patterns obtained with the Helix pomatia lectin and the GM130 and TGN46 immuno-reactions occurring in response to ATP-depletion and is confirmed by 3D electron tomography. The 3D reconstructions underlined the glomerular character of the reorganized Golgi apparatus and demonstrated continuities of ceramide positive and negative parts. Most interestingly, BODIPY-ceramide becomes concentrated in compartments of the tubulo-glomerular Golgi bodies, even though the reorganization took place before BODIPY-ceramide administration. This indicates maintained functionalities although the regular Golgi stack organization is abolished; the results provide novel insights into Golgi structure-function relationships, which might be relevant for cells affected by metabolic stress.

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.

High-throughput computational structure-based characterization of protein families: START domains and implications for structural genomics

SkyLine, a high-throughput homology modeling pipeline tool, detects and models true sequence homologs to a given protein structure. Structures and models are stored in SkyBase with links to computational function annotation, as calculated by MarkUs. The SkyLine/SkyBase/MarkUs technology represents a novel structure-based approach that is more objective and versatile than other protein classification resources. This structure-centric strategy provides a multi-dimensional organization and coverage of protein space at the levels of family, function, and genome. The concept of "modelability", the ability to model sequences on related structures, provides a reliable criterion for membership in a protein family ("leverage") and underlies the unique success of this approach. The overall procedure is illustrated by its application to START domains, which comprise a Biomedical Theme for the Northeast Structural Genomics Consortium as part of the Protein Structure Initiative. START domains are typically involved in the non-vesicular transport of lipids. While 19 experimentally determined structures are available, the family, whose evolutionary hierarchy is not well determined, is highly sequence diverse, and the ligand-binding potential of many family members is unknown. The SkyLine/SkyBase/MarkUs approach provides significant insights and predicts: (1) many more family members (approximately 4,000) than any other resource; (2) the function for a large number of unannotated proteins; (3) instances of START domains in genomes from which they were thought to be absent; and (4) the existence of two types of novel proteins, those containing dual START domain and those containing N-terminal START domains.

The ceramide transporter and the Goodpasture antigen binding protein: one protein--one function?

The Goodpasture antigen-binding protein (GPBP) and its splice variant the ceramide transporter (CERT) are multifunctional proteins that have been found to play important roles in brain development and biology. However, the function of GPBP and CERT is controversial because of their involvement in two apparently unrelated research fields: GPBP was initially isolated as a protein associated with collagen IV in patients with the autoimmune disease Goodpasture syndrome. Subsequently, a splice variant lacking a serine-rich domain of 26 amino acids (GPBPDelta26) was found to mediate the cytosolic transport of ceramide and was therefore (re)named CERT. The two splice forms likely carry out different functions in specific sub-cellular localizations. Selective GPBP knockdown induces extensive apoptosis and tissue loss in the brain of zebrafish. GPBP/GPBPDelta26 knock-out mice die as a result of structural and functional defects in endoplasmic reticulum and mitochondria. Because both mitochondria and ceramide play an important role in many biological events that regulate neuronal differentiation, cellular senescence, proliferation and cell death, we propose that GPBP and CERT are pivotal in neurodegenerative processes. In this review, we discuss the current state of knowledge on GPBP and CERT, including the molecular and biochemical characterization of GPBP in the field of autoimmunity as well as the fundamental research on CERT in ceramide transport, biosynthesis, localization, metabolism and cell homeostasis.

Glycosphingolipids--nature, function, and pharmacological modulation

The discovery of the glycosphingolipids is generally attributed to Johan L. W. Thudichum, who in 1884 published on the chemical composition of the brain. In his studies he isolated several compounds from ethanolic brain extracts which he coined cerebrosides. He subjected one of these, phrenosin (now known as galactosylceramide), to acid hydrolysis, and this produced three distinct components. One he identified as a fatty acid and another proved to be an isomer of D-glucose, which is now known as D-galactose. The third component, with an "alkaloidal nature", presented "many enigmas" to Thudichum, and therefore he named it sphingosine, after the mythological riddle of the Sphinx. Today, sphingolipids and their glycosidated derivatives are the subjects of intense study aimed at elucidating their role in the structural integrity of the cell membrane, their participation in recognition and signaling events, and in particular their involvement in pathological processes that are at the basis of human disease (for example, sphingolipidoses and diabetes type 2). This Review details some of the recent findings on the biosynthesis, function, and degradation of glycosphingolipids in man, with a focus on the glycosphingolipid glucosylceramide. Special attention is paid to the clinical relevance of compounds directed at interfering with the factors responsible for glycosphingolipid metabolism.

Lipid-regulated sterol transfer between closely apposed membranes by oxysterol-binding protein homologues

The ORP lipid-binding domain can contact two membranes simultaneously to facilitate sterol extraction or delivery at one membrane in response to the lipid composition of the other.

Sterols are transferred between cellular membranes by vesicular and poorly understood nonvesicular pathways. Oxysterol-binding protein–related proteins (ORPs) have been implicated in sterol sensing and nonvesicular transport. In this study, we show that yeast ORPs use a novel mechanism that allows regulated sterol transfer between closely apposed membranes, such as organelle contact sites. We find that the core lipid-binding domain found in all ORPs can simultaneously bind two membranes. Using Osh4p/Kes1p as a representative ORP, we show that ORPs have at least two membrane-binding surfaces; one near the mouth of the sterol-binding pocket and a distal site that can bind a second membrane. The distal site is required for the protein to function in cells and, remarkably, regulates the rate at which Osh4p extracts and delivers sterols in a phosphoinositide-dependent manner. Together, these findings suggest a new model of how ORPs could sense and regulate the lipid composition of adjacent membranes.

Membrane traffic within the Golgi apparatus

Newly synthesized secretory cargo molecules pass through the Golgi apparatus while resident Golgi proteins remain in the organelle. However, the pathways of membrane traffic within the Golgi are still uncertain. Most of the available data can be accommodated by the cisternal maturation model, which postulates that Golgi cisternae form de novo, carry secretory cargoes forward and ultimately disappear. The entry face of the Golgi receives material that has been exported from transitional endoplasmic reticulum sites, and the exit face of the Golgi is intimately connected with endocytic compartments. These conserved features are enhanced by cell-type-specific elaborations such as tubular connections between mammalian Golgi cisternae. Key mechanistic questions remain about the formation and maturation of Golgi cisternae, the recycling of resident Golgi proteins, the origins of Golgi compartmental identity, the establishment of Golgi architecture, and the roles of Golgi structural elements in membrane traffic.

High-yielding synthesis of sphingoid-type bases

An efficient methodology for the synthesis of sphingoid-type bases is reported. It involves the stereoselective addition of a racemic 3-alkoxy allenylzinc to enantiopure N-tert-butylsulfinyl imines and a cross-metathesis reaction as the key steps. It has been successfully applied to the syntheses of sphinganine and naturally occurring bioactive related compounds, among which the hydrolysis product of clavaminol H and two spisulosines. All of these compounds have been prepared in six steps from N-tert-butylsulfinyl imines in high overall yields (>56%).

An overview of sphingolipid metabolism: from synthesis to breakdown

Sphingolipids constitute a class of lipids defined by their eighteen carbon amino-alcohol backbones which are synthesized in the ER from nonsphingolipid precursors. Modification of this basic structure is what gives rise to the vast family of sphingolipids that play significant roles in membrane biology and provide many bioactive metabolites that regulate cell function. Despite the diversity of structure and function of sphingolipids, their creation and destruction are governed by common synthetic and catabolic pathways. In this regard, sphingolipid metabolism can be imagined as an array of interconnected networks that diverge from a single common entry point and converge into a single common breakdown pathway. In their simplest forms, sphingosine, phytosphingosine and dihydrosphingosine serve as the backbones upon which further complexity is achieved. For example, phosphorylation of the C1 hydroxyl group yields the final breakdown products and/or the important signaling molecules sphingosine-1-phosphate, phytosphingosine-1-phosphate and dihydrosphingosine-1-phosphate, respectively. On the other hand, acylation of sphingosine, phytosphingosine, or dihydrosphingosine with one of several possible acyl CoA molecules through the action of distinct ceramide synthases produces the molecules defined as ceramide, phytoceramide, or dihydroceramide. Ceramide, due to the differing acyl CoAs that can be used to produce it, is technically a class of molecules rather than a single molecule and therefore may have different biological functions depending on the acyl chain it is composed of. At the apex of complexity is the group of lipids known as glycosphingolipids (GSL) which contain dozens of different sphingolipid species differing by both the order and type of sugar residues attached to their headgroups. Since these molecules are produced from ceramide precursors, they too may have differences in their acyl chain composition, revealing an additional layer of variation. The glycosphingolipids are divided broadly into two categories: glucosphingolipids and galactosphingolipids. The glucosphingolipids depend initially on the enzyme glucosylceramide synthase (GCS) which attaches glucose as the first residue to the C1 hydroxyl position. Galactosphingolipids, on the other hand, are generated from galactosylceramide synthase (GalCerS), an evolutionarily dissimilar enzyme from GCS. Glycosphingolipids are further divided based upon further modification by various glycosyltransferases which increases the potential variation in lipid species by several fold. Far more abundant are the sphingomyelin species which are produced in parallel with glycosphingolipids, however they are defined by a phosphocholine headgroup rather than the addition of sugar residues. Although sphingomyelin species all share a common headgroup, they too are produced from a variety of ceramide species and therefore can have differing acyl chains attached to their C-2 amino groups. Whether or not the differing acyl chain lengths in SMs dictate unique functions or important biophysical distinctions has not yet been established. Understanding the function of all the existing glycosphingolipids and sphingomyelin species will be a major undertaking in the future since the tools to study and measure these species are only beginning to be developed (see Fig 1 for an illustrated depiction of the various sphingolipid structures). The simple sphingolipids serve both as the precursors and the breakdown products of the more complex ones. Importantly, in recent decades, these simple sphingolipids have gained attention for having significant signaling and regulatory roles within cells. In addition, many tools have emerged to measure the levels of simple sphingolipids and therefore have become the focus of even more intense study in recent years. With this thought in mind, this chapter will pay tribute to the complex sphingolipids, but focus on the regulation of simple sphingolipid metabolism.

Sphingolipid transport

Sphingolipids are a family of ubiquitous membrane components that exhibit multiple functional properties fundamental to cell properties. Sphingolipid transport represents a crucial aspect in the metabolism, signaling and biological role of sphingolipids. Different mechanisms of sphingolipid movements contribute to their selective localization in different membranes but also in different portions and sides of the same membrane, thus ensuring and regulating their interaction with different enzymes and target molecules. In this chapter we will describe the knowledge of the different mechanisms ofsphingolipid movements within and between membranes, focusing on the recent advances in this field and considering the role played by selective sphingolipid molecules in the regulation of these mechanisms.

Crystal structures of the CERT START domain with inhibitors provide insights into the mechanism of ceramide transfer

The cytosolic protein CERT transfers ceramide from the endoplasmic reticulum to the Golgi apparatus where ceramide is converted to SM. The C-terminal START (steroidogenic acute regulatory protein-related lipid transfer) domain of CERT binds one ceramide molecule in its central amphiphilic cavity. (1R,3R)-N-(3-Hydroxy-1-hydroxymethyl-3-phenylpropyl)alkanamide (HPA), a synthesized analogue of ceramide, inhibits ceramide transfer by CERT. Here we report crystal structures of the CERT START domain in complex with HPAs of varying acyl chain lengths. In these structures, one HPA molecule is buried in the amphiphilic cavity where the amide and hydroxyl groups of HPA form a hydrogen-bond network with specific amino acid residues. The Omega1 loop, which has been suggested to function as a gate of the cavity, adopts a different conformation when bound to HPA than when bound to ceramide. In the Omega1 loop region, Trp473 shows the largest difference between these two structures. This residue exists inside of the cavity in HPA-bound structures, while it is exposed to the outside of the protein in the apo-form and ceramide-bound complex structures. Surface plasmon resonance experiments confirmed that Trp473 is important for interaction with membranes. These results provide insights into not only the molecular mechanism of inhibition by HPAs but also possible mechanisms by which CERT interacts with ceramide.

The Sec14 superfamily and mechanisms for crosstalk between lipid metabolism and lipid signaling

Lipid signaling pathways define central mechanisms for cellular regulation. Productive lipid signaling requires an orchestrated coupling between lipid metabolism, lipid organization and the action of protein machines that execute appropriate downstream reactions. Using membrane trafficking control as primary context, we explore the idea that the Sec14-protein superfamily defines a set of modules engineered for the sensing of specific aspects of lipid metabolism and subsequent transduction of 'sensing' information to a phosphoinositide-driven 'execution phase'. In this manner, the Sec14 superfamily connects diverse territories of the lipid metabolome with phosphoinositide signaling in a productive 'crosstalk' between these two systems. Mechanisms of crosstalk, by which non-enzymatic proteins integrate metabolic cues with the action of interfacial enzymes, represent unappreciated regulatory themes in lipid signaling.

Regulation of a Golgi flippase by phosphoinositides and an ArfGEF

The essential role for phosphatidylinositol 4-phosphate (PtdIns[4]P) in vesicle-mediated protein transport from the trans-Golgi network (TGN) was first described in budding yeast1–3. However, the identity of downstream effectors of PtdIns[4]P in this system has been elusive. Here, we show that Drs2p, a type IV P-type ATPase required for phospholipid translocase (flippase) activity and transport vesicle budding from the TGN4–8, is an effector of PtdIns[4]P. Drs2p-dependent flip of a fluorescent phosphatidylserine analogue across purified TGN membranes requires synthesis of PtdIns[4]P by the PtdIns 4-kinase Pik1p. PtdIns[4]P binds to a regulatory domain in the C-terminal tail of Drs2p that has homology to a split PH domain and is required for Drs2p activity. In addition, basic residues required for phosphoinositide binding overlap a binding site for the ArfGEF Gea2p that was previously mapped9. ArfGEF binding to this C-terminal domain also stimulates flippase activity in TGN membrane preparations. These interactions imply the presence of a novel coincidence detection system used to activate phospholipid translocation at sites of vesicle formation.

Molecular facets of sphingolipids: mediators of diseases

Sphingolipids constitute a biologically active lipid class that is significantly important from both structural and regulatory aspects. The manipulation of sphingolipid metabolism is currently being studied as a novel strategy for cancer therapy. The basics of this therapeutic approach lie in the regulation property of sphingolipids on cellular processes, which are important in a cell's fate, such as cell proliferation, apoptosis, cell cycle arrest, senescence, and inflammation. Furthermore, the mutations in the enzymes catalyzing some specific reactions in the sphingolipid metabolism cause mortal lysosomal storage diseases like Fabry, Gaucher, Niemann-Pick, Farber, Krabbe, and Metachromatic Leukodystrophy. Therefore, the alteration of the sphingolipid metabolic pathway determines the choice between life and death. Understanding the sphingolipid metabolism and regulation is significant for the development of new therapeutic approaches for all sphingolipid-related diseases, as well as for cancer. An important feature of the sphingolipid metabolic pathway is the compartmentalization into endoplasmic reticulum, the Golgi apparatus, lysosome and plasma membrane, and this compartmentalization makes the transport of sphingolipids critical for proper functioning. This paper focuses on the structures, metabolic pathways, localization, transport mechanisms, and diseases of sphingolipids in Saccharomyces cerevisiae and humans, and provides the latest comprehensive information on sphingolipid research.

De novo ceramide synthesis is required for N-linked glycosylation in plasma cells

Plasma cells (PCs) are terminally differentiated B lymphocytes responsible for the synthesis and secretion of Igs. The differentiation of B cells into PCs involves a remarkable expansion of both lipid and protein components of the endoplasmic reticulum. Despite their importance in many signal transduction pathways, the role of ceramides, and of complex sphingolipids that are derived from ceramide, in PC differentiation has never been directly studied. To assess their putative role in PC differentiation, we blocked ceramide synthesis with fumonisin B1, a specific inhibitor of ceramide synthase. Under fumonisin B1 treatment, N-linked glycosylation was severely impaired in LPS-activated, but not in naive, B cells. We also show that ceramide synthesis is strongly induced by XBP-1 (X box-binding protein-1). In the absence of ceramide synthesis, ER expansion was dramatically diminished. Our results underscore ceramide biosynthesis as a key metabolic pathway in the process of PC differentiation and reveal a previously unknown functional link between sphingolipids and N-linked glycosylation in PCs.

Interactions between the endoplasmic reticulum, mitochondria, plasma membrane and other subcellular organelles

Several recent works show structurally and functionally dynamic contacts between mitochondria, the plasma membrane, the endoplasmic reticulum, and other subcellular organelles. Many cellular processes require proper cooperation between the plasma membrane, the nucleus and subcellular vesicular/tubular networks such as mitochondria and the endoplasmic reticulum. It has been suggested that such contacts are crucial for the synthesis and intracellular transport of phospholipids as well as for intracellular Ca(2+) homeostasis, controlling fundamental processes like motility and contraction, secretion, cell growth, proliferation and apoptosis. Close contacts between smooth sub-domains of the endoplasmic reticulum and mitochondria have been shown to be required also for maintaining mitochondrial structure. The overall distance between the associating organelle membranes as quantified by electron microscopy is small enough to allow contact formation by proteins present on their surfaces, allowing and regulating their interactions. In this review we give a historical overview of studies on organelle interactions, and summarize the present knowledge and hypotheses concerning their regulation and (patho)physiological consequences.

Par j 1 and Par j 2, the two major allergens in Parietaria judaica, bind preferentially to monoacylated negative lipids

Par j 1 and Par j 2 proteins are the two major allergens in Parietaria judaica pollen, one of the main causes of allergic diseases in the Mediterranean area. Each of them contains eight cysteine residues organized in a pattern identical to that found in plant nonspecific lipid transfer proteins. The 139- and 102-residue recombinant allergens, corresponding respectively to Par j 1 and Par j 2, refold properly to fully functional forms, whose immunological properties resemble those of the molecules purified from the natural source. Molecular modeling shows that, despite the lack of extensive primary structure homology with nonspecific lipid transfer proteins, both allergens contain a hydrophobic cavity suited to accommodate a lipid ligand. In the present study, we present novel evidence for the formation of complexes of these natural and recombinant proteins from Parietaria pollen with lipidic molecules. The dissociation constant of oleyl-lyso-phosphatidylcholine is 9.1 +/- 1.2 microm for recombinant Par j 1, whereas pyrenedodecanoic acid shows a much higher affinity, with a dissociation constant of approximately 1 microm for both recombinant proteins, as well as for the natural mixture. Lipid binding does not alter the secondary structure content of the protein but is very efficient in protecting disulfide bonds from reduction by dithiothreitol. We show that Par j 1 and Par j 2 not only bind lipids from micellar dispersions, but also are able to extract and transfer negative phospholipids from bilayers.

CERT-mediated trafficking of ceramide

The transport and sorting of lipids from the sites of their synthesis to their appropriate destinations are fundamental for membrane biogenesis. In the synthesis of sphingolipids in mammalian cells, ceramide is newly produced at the endoplasmic reticulum (ER), and transported from the ER to the trans Golgi regions, where it is converted to sphingomyelin. CERT mediates the ER-to-Golgi trafficking of ceramide. CERT contains several functional domains and motifs including i) a START domain capable of catalyzing inter-membrane transfer of ceramide, ii) a pleckstrin homology domain, which serves to target the Golgi apparatus, iii) a FFAT motif which interacts with the ER-resident membrane protein VAP, and iv) a serine-repeat motif, of which hyperphosphorylation down-regulates CERT activity. It has been suggested that CERT extracts ceramide from the ER and carries it to the Golgi apparatus in a non-vesicular manner and that efficient CERT-mediated trafficking of ceramide occurs at membrane contact sites between the ER and the Golgi apparatus.

Tritium suicide selection identifies proteins involved in the uptake and intracellular transport of sterols in Saccharomyces cerevisiae

Sterol transport between the plasma membrane (PM) and the endoplasmic reticulum (ER) occurs by a nonvesicular mechanism that is poorly understood. To identify proteins required for this process, we isolated Saccharomyces cerevisiae mutants with defects in sterol transport. We used Upc2-1 cells that have the ability to take up sterols under aerobic conditions and exploited the observation that intracellular accumulation of exogenously supplied [(3)H]cholesterol in the form of [(3)H]cholesteryl ester requires an intact PM-ER sterol transport pathway. Upc2-1 cells were mutagenized using a transposon library, incubated with [(3)H]cholesterol, and subjected to tritium suicide selection to isolate mutants with a decreased ability to accumulate [(3)H]cholesterol. Many of the mutants had defects in the expression and trafficking of Aus1 and Pdr11, PM-localized ABC transporters that are required for sterol uptake. Through characterization of one of the mutants, a new role was uncovered for the transcription factor Mot3 in controlling expression of Aus1 and Pdr11. A number of mutants had transposon insertions in the uncharacterized Ydr051c gene, which we now refer to as DET1 (decreased ergosterol transport). These mutants expressed Aus1 and Pdr11 normally but were severely defective in the ability to accumulate exogenously supplied cholesterol. The transport of newly synthesized sterols from the ER to the PM was also defective in det1Delta cells. These data indicate that the cytoplasmic protein encoded by DET1 is involved in intracellular sterol transport.

Casein kinase I{gamma}2 down-regulates trafficking of ceramide in the synthesis of sphingomyelin

Intracellullar trafficking of lipids is fundamental to membrane biogenesis. For the synthesis of sphingomyelin, ceramide is transported from the endoplasmic reticulum to the Golgi apparatus by the ceramide transfer protein CERT. CERT is phosphorylated by protein kinase D at S132 and subsequently multiple times in a serine-repeat motif, resulting in its inactivation. However, the kinase involved in the multiple phosphorylation remains unclear. Here, we identify the gamma2 isoform of casein kinase I (CKIgamma2) as a kinase whose overexpression confers sphingomyelin-directed toxin-resistance to Chinese hamster ovary cells. In a transformant stably expressing CKIgamma2, CERT was hyperphosphorylated, and the intracellular trafficking of ceramide was retarded, thereby reducing de novo sphingomyelin synthesis. The reduction in the synthesis of sphingomyelin caused by CKIgamma2 was reversed by the expression of CERT mutants that are not hyperphosphorylated. Furthermore, CKIgamma2 directly phosphorylated CERT in vitro. Among three gamma isoforms, only knockdown of gamma2 isoform caused drastic changes in the ratio of hypo- to hyperphosphorylated form of CERT in HeLa cells. These results indicate that CKIgamma2 hyperphosphorylates the serine-repeat motif of CERT, thereby inactivating CERT and down-regulating the synthesis of sphingomyelin.

The life of lipid droplets

Lipid droplets are the least characterized of cellular organelles. Long considered simple lipid storage depots, these dynamic and remarkable organelles have recently been implicated in many biological processes, and we are only now beginning to gain insights into their fascinating lives in cells. Here we examine what we know of the life of lipid droplets. We review emerging data concerning their cellular biology and present our thoughts on some of the most salient questions for investigation.

Acute perturbations in Golgi organization impact de novo sphingomyelin synthesis

The mammalian Golgi apparatus is composed of multiple stacks of cisternal membranes organized laterally into a ribbon-like structure, with close apposition of trans Golgi regions with specialized endoplasmic reticulum (ER) membranes. These contacts may be the site of ceramide transfer from its site of synthesis (ER) to sphingomyelin (SM) synthase through ceramide transfer protein (CERT). CERT extracts ceramide from the ER and transfers it to Golgi membranes but the role of overall Golgi structure in this process is unknown. We show here that localization of CERT in puncta around the Golgi complex requires both ER- and Golgi-binding domains of CERT. To examine how Golgi structure contributes to SM synthesis, we treated cells with Golgi-perturbing drugs and measured newly synthesized SM. Interestingly, disruption of Golgi morphology with nocodazole, but not ilimaquinone inhibited SM synthesis. Decreased localization of CERT with a Golgi marker correlated with decreased SM synthesis. We propose that some Golgi structural perturbations interfere with efficient ceramide trafficking through CERT, and thus SM synthesis. The organization of the mammalian Golgi ribbon together with CERT may promote specific ER-Golgi interactions for efficient delivery of ceramide for SM synthesis.

The Bet v 1 fold: an ancient, versatile scaffold for binding of large, hydrophobic ligands

Background

The major birch pollen allergen, Bet v 1, is a member of the ubiquitous PR-10 family of plant pathogenesis-related proteins. In recent years, a number of diverse plant proteins with low sequence similarity to Bet v 1 was identified. In addition, determination of the Bet v 1 structure revealed the existence of a large superfamily of structurally related proteins. In this study, we aimed to identify and classify all Bet v 1-related structures from the Protein Data Bank and all Bet v 1-related sequences from the Uniprot database.

Results

Structural comparisons of representative members of already known protein families structurally related to Bet v 1 with all entries of the Protein Data Bank yielded 47 structures with non-identical sequences. They were classified into eleven families, five of which were newly identified and not included in the Structural Classification of Proteins database release 1.71. The taxonomic distribution of these families extracted from the Pfam protein family database showed that members of the polyketide cyclase family and the activator of Hsp90 ATPase homologue 1 family were distributed among all three superkingdoms, while members of some bacterial families were confined to a small number of species. Comparison of ligand binding activities of Bet v 1-like superfamily members revealed that their functions were related to binding and metabolism of large, hydrophobic compounds such as lipids, hormones, and antibiotics. Phylogenetic relationships within the Bet v 1 family, defined as the group of proteins with significant sequence similarity to Bet v 1, were determined by aligning 264 Bet v 1-related sequences. A distance-based phylogenetic tree yielded a classification into 11 subfamilies, nine exclusively containing plant sequences and two subfamilies of bacterial proteins. Plant sequences included the pathogenesis-related proteins 10, the major latex proteins/ripening-related proteins subfamily, and polyketide cyclase-like sequences.

Conclusion

The ubiquitous distribution of Bet v 1-related proteins among all superkingdoms suggests that a Bet v 1-like protein was already present in the last universal common ancestor. During evolution, this protein diversified into numerous families with low sequence similarity but with a common fold that succeeded as a versatile scaffold for binding of bulky ligands.

Sphingolipids in the lungs

Sphingolipids such as sphingosine-1-phosphate (S1P), ceramide, or sphingomyelin are essential constituents of plasma membranes and regulate many (patho)physiological cellular responses inducing apoptosis and cell survival, vascular permeability, mast cell activation, and airway smooth muscle functions. The complexity of sphingolipid biology is generated by a great variety of compounds, diverse receptors, and often antagonistic functions of different sphingolipids. For instance, apoptosis is promoted by ceramide and prevented by S1P, and pulmonary vascular permeability is increased by S1P2/3 receptors and by ceramide, whereas S1P1 receptors stabilize barrier integrity. Several enzymes of the sphingolipid metabolism respond to external stimuli such as sphingomyelinase isoenzymes that are activated by many stress stimuli and the sphingosine kinase isoenzymes that are activated by allergens. The past years have provided increasing evidence that these processes contribute to pulmonary disorders including asthma, chronic obstructive pulmonary disease, acute lung injury, and cystic fibrosis. Sphingolipid metabolism offers several novel therapeutic targets for the treatment of lung diseases such as emphysema, asthma, cystic fibrosis, respiratory tract infection, sepsis, and acute lung injury.

Coordinated lipid transfer between the endoplasmic reticulum and the Golgi complex requires the VAP proteins and is essential for Golgi-mediated transport

Lipid transport between intracellular organelles is mediated by vesicular and nonvesicular transport mechanisms and is critical for maintaining the identities of different cellular membranes. Nonvesicular lipid transport between the endoplasmic reticulum (ER) and the Golgi complex has been proposed to affect the lipid composition of the Golgi membranes. Here, we show that the integral ER-membrane proteins VAP-A and VAP-B affect the structural and functional integrity of the Golgi complex. Depletion of VAPs by RNA interference reduces the levels of phosphatidylinositol-4-phosphate (PI4P), diacylglycerol, and sphingomyelin in the Golgi membranes, and it leads to substantial inhibition of Golgi-mediated transport events. These effects are coordinately mediated by the lipid-transfer/binding proteins Nir2, oxysterol-binding protein (OSBP), and ceramide-transfer protein (CERT), which interact with VAPs via their FFAT motif. The effect of VAPs on PI4P levels is mediated by the phosphatidylinositol/phosphatidylcholine transfer protein Nir2, which is required for Golgi targeting of OSBP and CERT and the subsequent production of diacylglycerol and sphingomyelin. We propose that Nir2, OSBP, and CERT function coordinately at the ER-Golgi membrane contact sites, thereby affecting the lipid composition of the Golgi membranes and consequently their structural and functional identities.

From ER to Eph receptors: new roles for VAP fragments

Dominantly inherited mutations in an endoplasmic reticulum protein called VAPB have been found in a subset of patients with a rare familial form of amyotrophic lateral sclerosis (ALS). In this issue, Tsuda et al. (2008) identify a secreted form of VAPB that binds directly to Eph receptors inducing their activation and signaling, providing fresh insights into ALS pathogenesis, including non-neuronal aspects of this disorder.

The VAP protein family: from cellular functions to motor neuron disease

The VAMP-associated proteins (VAPs) are highly conserved integral endoplasmic reticulum membrane proteins implicated in diverse cellular functions, including the regulation of lipid transport and homeostasis, membrane trafficking, neurotransmitter release, stabilization of presynaptic microtubules, and the unfolded protein response. Recently, a single missense mutation within the human VAP-B gene was identified in three forms of familial motor neuron disease. In this review, we integrate results from studies of yeast, fly and mammalian VAPs that provide insight into the structural features of these proteins, the network of VAP-interacting proteins, their possible physiological functions, and their involvement in motor neuron disease.

Retrograde traffic in the biosynthetic-secretory route

In the biosynthetic-secretory route from the rough endoplasmic reticulum, across the pre-Golgi intermediate compartments, the Golgi apparatus stacks, trans Golgi network, and post-Golgi organelles, anterograde transport is accompanied and counterbalanced by retrograde traffic of both membranes and contents. In the physiologic dynamics of cells, retrograde flow is necessary for retrieval of molecules that escaped from their compartments of function, for keeping the compartments' balances, and maintenance of the functional integrities of organelles and compartments along the secretory route, for repeated use of molecules, and molecule repair. Internalized molecules may be transported in retrograde direction along certain sections of the secretory route, and compartments and machineries of the secretory pathway may be misused by toxins. An important example is the toxin of Shigella dysenteriae, which has been shown to travel from the cell surface across endosomes, and the Golgi apparatus en route to the endoplasmic reticulum, and the cytosol, where it exerts its deleterious effects. Most importantly in medical research, knowledge about the retrograde cellular pathways is increasingly being utilized for the development of strategies for targeted delivery of drugs to the interior of cells. Multiple details about the molecular transport machineries involved in retrograde traffic are known; a high number of the molecular constituents have been characterized, and the complicated fine structural architectures of the compartments involved become more and more visible. However, multiple contradictions exist, and already established traffic models again are in question by contradictory results obtained with diverse cell systems, and/or different techniques. Additional problems arise by the fact that the conditions used in the experimental protocols frequently do not reflect the physiologic situations of the cells. Regular and pathologic situations often are intermingled, and experimental treatments by themselves change cell organizations. This review addresses physiologic and pathologic situations, tries to correlate results obtained by different cell biologic techniques, and asks questions, which may be the basis and starting point for further investigations.

Roles of l-serine and sphingolipid synthesis in brain development and neuronal survival

Sphingolipids represent a class of membrane lipids that contain a hydrophobic ceramide chain as its common backbone structure. Sphingolipid synthesis requires two simple components: l-serine and palmitoyl CoA. Although l-serine is classified as a non-essential amino acid, an external supply of l-serine is essential for the synthesis of sphingolipids and phosphatidylserine (PS) in particular types of central nervous system (CNS) neurons. l-Serine is also essential for these neurons to undergo neuritogenesis and to survive. Biochemical analysis has shown that l-serine is synthesized from glucose and released by astrocytes but not by neurons, which is the major reason why this amino acid is an essential amino acid for neurons. Biosynthesis of membrane lipids, such as sphingolipids, PS, and phosphatidylethanolamine (PE), in neurons is completely dependent on this astrocytic factor. Recent advances in lipid biology research using transgenic mice have demonstrated that synthesis of endogenous l-serine and neuronal sphingolipids is essential for brain development. In this review, we discuss the metabolic system that coordinates sphingolipid synthesis with the l-serine synthetic pathway between neurons and glia. We also discuss the crucial roles of the metabolic conversion of l-serine to sphingolipids in neuronal development and survival. Human diseases associated with serine and sphingolipid biosynthesis are also discussed.

Structural basis for specific lipid recognition by CERT responsible for nonvesicular trafficking of ceramide

In mammalian cells, ceramide is synthesized in the endoplasmic reticulum and transferred to the Golgi apparatus for conversion to sphingomyelin. Ceramide transport occurs in a nonvesicular manner and is mediated by CERT, a cytosolic 68-kDa protein with a C-terminal steroidogenic acute regulatory protein-related lipid transfer (START) domain. The CERT START domain efficiently transfers natural D-erythro-C16-ceramide, but not lipids with longer (C20) amide-acyl chains. The molecular mechanisms of ceramide specificity, both stereo-specific recognition and length limit, are not well understood. Here we report the crystal structures of the CERT START domain in its apo-form and in complex with ceramides having different acyl chain lengths. In these complex structures, one ceramide molecule is buried in a long amphiphilic cavity. At the far end of the cavity, the amide and hydroxyl groups of ceramide form a hydrogen bond network with specific amino acid residues that play key roles in stereo-specific ceramide recognition. At the head of the ceramide molecule, there is no extra space to accommodate additional bulky groups. The two aliphatic chains of ceramide are surrounded by the hydrophobic wall of the cavity, whose size and shape dictate the length limit for cognate ceramides. Furthermore, local high-crystallographic B-factors suggest that the alpha-3 and the Omega1 loop might work as a gate to incorporate the ceramide into the cavity. Thus, the structures demonstrate the structural basis for the mechanism by which CERT can distinguish ceramide from other lipid types yet still recognize multiple species of ceramides.

A lipid transfer protein that transfers lipid

Very few lipid transfer proteins (LTPs) have been caught in the act of transferring lipids in vivo from a donor membrane to an acceptor membrane. Now, two studies (Halter, D., S. Neumann, S.M. van Dijk, J. Wolthoorn, A.M. de Maziere, O.V. Vieira, P. Mattjus, J. Klumperman, G. van Meer, and H. Sprong. 2007. J. Cell Biol. 179:101-115; D'Angelo, G., E. Polishchuk, G.D. Tullio, M. Santoro, A.D. Campli, A. Godi, G. West, J. Bielawski, C.C. Chuang, A.C. van der Spoel, et al. 2007. Nature. 449:62-67) agree that four-phosphate adaptor protein 2 (FAPP2) transfers glucosylceramide (GlcCer), a lipid that takes an unexpectedly circuitous route.

Protein phosphatase 2Cepsilon is an endoplasmic reticulum integral membrane protein that dephosphorylates the ceramide transport protein CERT to enhance its association with organelle membranes

Protein phosphatase 2Cepsilon (PP2Cepsilon), a mammalian PP2C family member, is expressed in various tissues and is implicated in the negative regulation of stress-activated protein kinase pathways. We show that PP2Cepsilon is an endoplasmic reticulum (ER) transmembrane protein with a transmembrane domain at the amino terminus and the catalytic domain facing the cytoplasm. Yeast two-hybrid screening of a human brain library using PP2Cepsilon as bait resulted in the isolation of a cDNA that encoded vesicle-associated membrane protein-associated protein A (VAPA). VAPA is an ER resident integral membrane protein involved in recruiting lipid-binding proteins such as the ceramide transport protein CERT to the ER membrane. Expression of PP2Cepsilon resulted in dephosphorylation of CERT in a VAPA expression-dependent manner, which was accompanied by redistribution of CERT from the cytoplasm to the Golgi apparatus. The expression of PP2Cepsilon also enhanced the association between CERT and VAPA. In addition, knockdown of PP2Cepsilon expression by short interference RNA attenuated the interaction between CERT and VAPA and the sphingomyelin synthesis. These results suggest that CERT is a physiological substrate of PP2Cepsilon and that dephosphorylation of CERT by PP2Cepsilon may play an important role in the regulation of ceramide trafficking from the ER to the Golgi apparatus.

Regulation and traffic of ceramide 1-phosphate produced by ceramide kinase: comparative analysis to glucosylceramide and sphingomyelin

Ceramide 1-phosphate (C1P) has been characterized as a sphingolipid that participates in cell signaling. Although C1P synthesis is thought to occur via phosphorylation of ceramide by ceramide kinase (CerK), the processes that regulate C1P formation and fate remain largely unknown. In this study we analyzed bone marrow-derived macrophages (BMDM) from CerK-null mice (Cerk(-/-)) and found significant levels of C1P, suggesting that previously unrecognized pathways may also lead to C1P formation. After these experiments we used an overexpression system, BMDM from Cerk(-/-) mice, and short-chain fluorescent ceramides to trace CerK-dependent formation of C1P. Because the ceramide analogs can also be converted to glucosylceramide (GlcCer) and sphingomyelin (SM), they allowed us to directly compare all three metabolites. We found that C1P produced by CerK is turned over rapidly when serum is removed or upon calcium chelation, whereas GlcCer and SM are stable under these conditions. We further demonstrated that ceramide must be transported to the Golgi complex to be phosphorylated by CerK. Inhibition of the ceramide transfer protein slowed down SM formation without decreasing C1P, suggesting an alternate route of ceramide transport. Other experiments indicated that, like GlcCer and SM, C1P traffics along the secretory pathway to reach the plasma membrane. Furthermore, in BMDM C1P was secreted more readily than was GlcCer or SM. Altogether, our results indicate that CerK is essential to C1P formation via phosphorylation of Cer, providing the first insights into mechanisms underlying ceramide access to CerK and C1P trafficking as well as clarifying C1P as a signaling entity.