A Comprehensive Review: Sphingolipid Metabolism and Implications of Disruption in Sphingolipid Homeostasis

Sphingolipids are a specialized group of lipids essential to the composition of the plasma membrane of many cell types; however, they are primarily localized within the nervous system. The amphipathic properties of sphingolipids enable their participation in a variety of intricate metabolic pathways. Sphingoid bases are the building blocks for all sphingolipid derivatives, comprising a complex class of lipids. The biosynthesis and catabolism of these lipids play an integral role in small- and large-scale body functions, including participation in membrane domains and signalling; cell proliferation, death, migration, and invasiveness; inflammation; and central nervous system development. Recently, sphingolipids have become the focus of several fields of research in the medical and biological sciences, as these bioactive lipids have been identified as potent signalling and messenger molecules. Sphingolipids are now being exploited as therapeutic targets for several pathologies. Here we present a comprehensive review of the structure and metabolism of sphingolipids and their many functional roles within the cell. In addition, we highlight the role of sphingolipids in several pathologies, including inflammatory disease, cystic fibrosis, cancer, Alzheimer’s and Parkinson’s disease, and lysosomal storage disorders.

One-step synthesis of carbon-13-labeled globotriaosylsphingosine (lyso-Gb3), an internal standard for biomarker analysis of Fabry disease

Globotriaosylsphingosine (lyso-Gb3) is a well-established biomarker for diagnosis and prognosis of Fabry disease. This biomarker is measured in biological samples by liquid chromatography-tandem mass spectrometry using an internal standard. The ideal internal standard is a variant of lyso-Gb3 substituted with heavy isotopes, but the total synthesis of such a compound is very labor intensive. In this report, we describe a simple, one-step synthesis of lyso-Gb3 labeled with carbon-13 in all of the galactosyl carbons.

Versatile synthetic method for sphingolipids and functionalized sphingosine derivatives via olefin cross metathesis

A highly efficient and versatile method for the synthesis of various sphingolipids, such as sphingomyelin, ceramide, sphingosine, sphingosine 1-phosphate, and functionalized sphingosine derivatives, was established by two types of combinations of the olefin cross metathesis reaction. One reaction was between the same olefin part and appropriate amino alcohols, which were prepared starting from N-Boc-L-serine, and the other was between appropriate olefins and the same amino alcohol. [reaction: see text].

Sphingolipid synthesis via olefin cross metathesis: preparation of a differentially protected building block and application to the synthesis of D-erythro-ceramide

The sphingolipid backbone is readily assembled by E-selective olefin cross metathesis of a suitable building block.

The synthesis of internal standards for the quantitative determination of sphingolipids by tandem mass spectrometry

Novel internal standards have been synthesised for the quantitative determination by tandem mass spectrometry (MS/MS) of the sphingolipids that accumulate in lysosomal storage diseases. The [d4]C16- and [d47]C24-isoforms of galactosylceramide (CMH), lactosylceramide (CDH), globotriaosylceramide (CTH), cerebroside sulphate, sphingomyelin and G(M1)-, G(M2)- and G(M3)-gangliosides were synthesised by the reaction of their lyso-forms with the acid chlorides of hexadecanoic 5,5,6,6-d4 acid ([d4]-palmitic acid) and tetracosanoic-d47 acid ([d47]-lignoceric acid), respectively. The acid chlorides were formed using oxalyl chloride. The structures of the internal standards were confirmed by MS/MS. The fragmentation pattern of each novel compound was similar to that of the corresponding natural form of the sphingolipid, making it a good internal standard for the quantitative determination of the natural sphingolipid by ESI-MS/MS. Characteristic product ions were identified for each compound.

An efficient preparation of isosteric phosphonate analogues of sphingolipids by opening of oxirane and cyclic sulfamidate intermediates with alpha-lithiated alkylphosphonic esters

D-erythro-(2S,3R,4E)-Sphingosine-1-phosphonate (1), the isosteric phosphonate analogue of naturally occurring sphingosine 1-phosphate (1a), and D-ribo-phytosphingosine 1-phosphonate (2), the isosteric phosphonate analogue of D-ribo-phytosphingosine-1-phosphate (2a), were synthesized starting with methyl 2,3-O-isopropylidene-d-glycerate (4) and D-ribo-phytosphingosine (3), respectively. Oxirane 12 was formed in eight steps from 4, and cyclic sulfamidate 22 was formed in five steps from 3. The phosphonate group was introduced via regioselective ring-opening reactions of oxirane 12 and cyclic sulfamidate 22 with lithium dialkyl methylphosphonate, affording 13 and 23, respectively. The synthesis of 1 was completed by S(N)2 displacement of chloromesylate intermediate 14b with azide ion, followed by conversion of the resulting azido group to a NHBoc group and deprotection. The synthesis of 2 was completed by cleavage of the acetal, N-benzyl, and alkyl phosphonate ester groups.

Synthetic, non-natural sphingolipid analogs inhibit the biosynthesis of cellular sphingolipids, elevate ceramide and induce apoptotic cell death

Numerous studies have demonstrated the participation of sphingolipids in signal transduction and regulation of cell growth. Several cellular stress agents have been shown to elevate ceramide, the basic precursor of all sphingolipids, initiating a cascade of events leading to arrest of the cell cycle, apoptosis and cell death. Aiming at inhibiting metabolic pathways of sphingolipid metabolism that might lead to an increase of cellular ceramide, we have synthesized non-natural analogs of ceramide, sphingosine and trimethylsphingosine. When the respective analogs were applied to HL60 human myeloid leukemic cells they inhibited the biosynthesis of sphingomyelin (SPM) and glycosphingolipids and induced apoptosis that led to cell death. A fluorescent procedure which has been developed for quantifying the biosynthesis of cellular ceramide indicated an increase in the ceramide content following an incubation with the synthetic analogs. These results suggest that the newly synthesized sphingolipid analogs might be valuable for potential application as a therapeutic modality in leukemia and other malignancies.

Sphingoid bases and de novo ceramide synthesis: enzymes involved, pharmacology and mechanisms of action

The sphingoid base backbones of sphingolipids (sphingosines, sphinganines, 4-hydroxysphinganines and others) are highly bioactive species directly and-in most cases-as their metabolites, the N-acyl-sphingoid bases (ceramides) and sphingoid base 1-phosphates. The complexity of these compounds affords many opportunities to prepare synthetic analogs for studies of sphingolipid metabolism and the functions of the sphingoid bases and metabolites. Described in this review are methods for the preparation of libraries of sphingoid bases, including a series of 1-deoxy-analogs, as well as information about their metabolism and biological activities. Findings with these compounds have uncovered some of the complications of working with compounds that mimic a naturally occurring biomodulator-such as that they are sometimes metabolized by enzymes that handle the endogenous compounds and the products may have potent (and unexpected) biological activities. Through studying such compounds, there is now a greater understanding of the metabolism and mechanism(s) of action of naturally occurring sphingoid bases as well as of these analogs.

[Synthetic study of biologically important nitrogen containing natural products: development of new methodology and design of leading compounds for new pharmaceuticals]

Synthetic study of biologically important nitrogen-containing natural products and development of new methodologies and design of leading compounds for new pharmaceuticals are described. The first total synthesis of eudistomines, manzamine C, martefragin A, cerebroside B1b, and symbioramide was accomplished and the absolute configurations of the stereogenic centers were determined. A novel methodology useful for the synthesis of alkaloids that have perhydroisoquinoline ring system such as manzamine A and B, and related alkaloids, nakadomarin A and dynemicin A, is presented. Sphingolipids, 4-stereoisomers of 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol, were synthesized and antimalaria activity was investigated. Inhibition of DNA primase by sphingosine and its analogues is described. A new synthetic methodology for alkylation and reduction of imines has been developed, and the first example of a reagent-controlled enantioselective Pictet-Spengler reaction is described. Also novel and convenient methods using transition metal and rare earth metals including alkene metathesis, asymmetric Diels-Alder reaction, imino ene reaction, selective allylic halogenation, enantioselective Pictet-Spengler reaction, and enantioselective physostigmine synthesis are described.

Antifungal activity of bifunctional sphingolipids. Intramolecular synergism within long-chain alpha,omega-bis-aminoalcohols

The in vitro antifungal activity of a series of alpha,omega-bifunctionalized aminoalcohols against Candida glabrata was measured. The dimeric bi-functionalized lipids exhibited activity about approximately 10-fold higher higher than D-sphingosine, which is a larger factor than expected from the simple additive effects of vicinal aminoalcohols groups.

A stereocontrolled, efficient synthetic route to bioactive sphingolipids: synthesis of phytosphingosine and phytoceramides from unsaturated ester precursors via cyclic sulfate intermediates

An efficient and highly enantioselective method for the preparation of D-ribo- and L-lyxo-phytosphingosines (1a,b, respectively) and phytoceramides (2a,b) has been developed. The key steps in the syntheses are as follows: (i) osmium-catalyzed asymmetric dihydroxylation of 4-O-protected (E)-alpha,beta-unsaturated ester 5 (generated by dihydroxylation of 1-hexadecene, followed by oxidation to the aldehyde and Horner-Wadsworth-Emmons olefination), (ii) conversion to cyclic sulfate intermediate 7, and (iii) regioselective alpha-azidation of 7. Reduction of 4-O-protected 2-azido ester 8 via alpha-azidolactone 9 afforded phytosphingosine 1a. Staudinger reduction of the azido group of 8, followed by in situ N-acylation in aqueous media and reduction of the ester functionality with NaBH(4)/LiBr, provided phytoceramide 2a. By using a similar approach, phytosphingosine 1b was synthesized. D-erythro-4, 5-Dihydrosphingosine 1c and D-erythro-4,5-dihydroceramide 2c were synthesized in high yield from 1-hexadecanol via cyclic sulfate intermediate 15. The desired configurations at C-2, C-3, and C-4 of the sphingoid chain can be accessed readily by the route described here.

Glucosylceramide synthase: assay and properties

Glucosylceramide synthesis is a key step in the formation of most mammalian glycosphingolipids. The expanding number of cellular functions that may be glycosphinolipid dependent and the identification of this glucosylceramide synthase as a potential therapeutic target for several sphingolipid storage disorders necessitate the availability of a reliable assay for glucosylceramide synthase. Coupled with the recent sequencing of this enzyme, the liposome-based assay utilizing a single extraction step should aid in the understanding of this critical early pathway in glycosphingolipid formation.

Synthesis and evaluation of morpholino- and pyrrolidinosphingolipids as inhibitors of glucosylceramide synthase

This paper describes the new synthesis and evaluation of some morpholino- and pyrrolidinosphingolipids and mimics as inhibitors of glucosylceramide synthase. It was found that the pyrrolidino derivatives are generally more active than the morpholino derivatives and the best one was shown to be a nanomolar inhibitor.

Multivalent ganglioside and sphingosine conjugates modulate myelin protein kinases

Gangliosides, added exogenously at concentrations of 10-100 microM, inhibit intrinsic protein kinase activities in purified rat brain myelin. Multivalent neoganglioproteins--gangliosides covalently attached, via their lipid moieties, to bovine serum albumin--were much more potent, inhibiting myelin protein phosphorylation half-maximally at a concentration of 100 nM. Different ganglioside conjugates varied 10-fold in inhibitory potency; GT1b-conjugates being the most potent and GM3-conjugates being the least. Conjugates of ganglioside oligosaccharides, lacking the lipid moiety, did not inhibit myelin protein phosphorylation, whereas conjugates of sphingosine inhibited nearly as potently as GT1b conjugates. Conjugate-mediated inhibition of myelin protein phosphorylation was due to inhibition of a protein serine kinase activity rather than activation of a phosphatase activity. We conclude that (i) clustered gangliosides or sphingosine are potent myelin protein kinase inhibitors, and (ii) sphingolipid metabolism is not required for myelin protein kinase inhibition. In contrast to their effects on myelin protein phosphorylation, ganglioside conjugates stimulated phosphorylation of a presumptive axon membrane protein. The data support the conclusion that gangliosides and other sphingolipids, when appropriately clustered, are potent modulators of central nervous system protein phosphorylation.

Formation of high-axial-ratio-microstructures from natural and synthetic sphingolipids

Amphiphiles that form high-axial-ratio-microstructures (HARMs) are being considered as novel materials for controlled release of drugs and other biologically functional molecules. HARMs consisting of tubules, ribbons, solid rods and helices are formed from sphingolipids by addition of water to a solution of amphiphile in DMF. Single molecular species of galactocerebroside (GalCer) containing long unsaturated fatty acid chains or natural GalCer containing mixed-length, non-hydroxy fatty acids (NFA-GalCer) or alpha-hydroxy fatty acids (HFA-GalCer) form cylindrical structures. In contrast, single molecular species of GalCer containing long saturated fatty acids form ribbons and helices. GalCer HARMs are typically under 100 nm in diameter and have lengths of several microns. The importance of the amide of GalCer for HARM formation was evaluated using psychosine, which forms solid fibers, whereas sphingosine and an analog of GalCer in which the amide is reduced to a secondary amine form amorphous aggregates. Single molecular species of ceramide containing long unsaturated fatty acid chains form cylindrical structures, whereas those with long saturated fatty acids form ribbons and helices. Short chain saturated ceramide also forms cylindrical structures. GalCer analogs with N-acetyl-glycine in place of the galactose form fibers whereas those with N-acetyl-proline yield amorphous material. The N-acetyl-proline-containing amphiphile can de doped into pure GalCer or NFA-GalCer without perturbing tubule formation.

Sphingolipid-derived signalling modulators: interaction with phosphatidylserine

We previously described the synthesis of two deuterium-labelled sphingoid bases, which made it possible to perform NMR spectroscopy on this family of signalling modulators in membranes (Rigby, A.C, Barber, K.R and Grant, C.W.M. (1995) Biochim. Biophys. Acta 1240, 75-82). In the present work we sought to test the concept that such mediators may display altered physical behaviour through association with anionic lipids - as a possible mechanism of involvement in signal transduction. Lyso-dihydrogalactosylceramide with deuterium nuclei at C4 and C5 of the sphingosine backbone and at C'3 and C'4 of the galactose ring ([2H4]lyso-GalCer), and N,N-dimethylsphingosine with deuterated amino-methyl groups ([2H6]dimethylsphingosine), were assembled as minor components into unsonicated fluid bilayer membranes of 1-palmitoyl-2-oleoylphosphatidylcholine/cholesterol. The effect of (anionic) phosphatidylserine was considered in this zwitterionic host matrix. The results present a picture of rapidly reversible interaction. The (-) charged phosphatidylserine exerted readily-measurable control over the orientation of the carbohydrate residue of [2H4]lyso-GalCer. In contrast, surrounding (-) charges exerted little spectral influence at the level of C4 and C5 of the lyso-GalCer, membrane-inserted, backbone; or at the level of the amino group of dimethylsphingosine. It would appear that packing alterations induced by the phosphatidylserine/sphingoid base association can translate into sizeable spatial constraints in the neighbouring aqueous domain.

Oral administration of semisynthetic sphingolipids promotes recovery of striatal dopamine concentrations in a murine model of parkinsonism

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administered to young C57/B16J black mice caused a striatal dopamine depletion that could be at least partially reversed by chronic intraperitoneal administration of GM1 ganglioside. The present study shows that the semisynthetic sphingolipids LIGA 4 (II3Neu-5-AcGgOse4-2-d-erythro-1,3-dihydroxy-2-acetamide-4-t rans-octadacene) and LIGA 20 (II3Neu-5-AcGgOse4-2-d-erythro-1,3-dihydroxy-2-chloro-acetam ide-4- trans-octadacene) are also effective in at least partially reversing MPTP-induced striatal dopamine depletions in mice after oral administration. These results suggest that semisynthetic ganglioside derivatives may be superior to the parent GM1 ganglioside for human clinical use.

Semisynthetic sphingolipids prevent protein kinase C translocation and neuronal damage in the perifocal area following a photochemically induced thrombotic brain cortical lesion

A vascular thrombotic lesion localized to the rat sensorimotor cortex was produced following intravenous injection of the photosensitive dye rose bengal, and its activation with a small beam of high-intensity white light focused to the skull overlaying the sensorimotor cortex. In the sensorimotor cortex at various times after the triggering event, two contiguous brain regions with different degree(s) of neuronal damage can be distinguished: (1) a primary thrombotic ischemic core where the majority of cells are dead and (2) a penumbra region surrounding the core lesion in which a slower progressive neuronal degeneration is occurring. Importantly, in both brain regions the neuronal degeneration is associated with the activation and persistent translocation of protein kinase C (PKC) as indicated by an increase in 4-beta-3H-phorbol-12,13-dibutyrate (3H-PDBu) binding. Moreover, the demonstration that in the area penumbra the neuronal degeneration and the persistent translocation of PKC can be inhibited by a pretreatment with dizocilpine (i.e., MK-801) indicates that the dynamics of the progression of the neuronal degeneration are maintained by glutamate accumulating in the extraneuronal fluids. MK-801 additionally prevents the transcriptional activation of several immediate-early genes (IEGs) (e.g., c-fos) and their cognate third nuclear messenger (i.e., c-Fos) expression present in the hemisphere ipsilateral to the lesion. On the other hand, LIGA4 and LIGA20 derivatives of GM1 lysoganglioside reduce the membrane translocation of PKC and the neuronal damage in the penumbra area, but fail to change the increase of IEG expression in the cortex ipsilateral to the lesion.

Use of N-([1-14C]hexanoyl)-D-erythro-sphingolipids to assay sphingolipid metabolism

An advantage of using N-([1-14C]hexanoyl)sphingolipids to assay sphingolipid metabolism is their ability to rapidly and spontaneously transfer into biological membranes without destroying membrane integrity. This property allows analysis of the activity of enzymes of sphingolipid metabolism under conditions in which the rate of product formation is not limited by availability of substrate, as is often the case with naturally occurring lipids whose rates of spontaneous transfer are extremely slow. Thus, the use of N-([1-14C]hexanoyl)sphingolipids provides an alternative means for studying sphingolipid metabolism in vitro.

Total synthesis of globotriaosylceramide (Gb3) and lysoglobotriaosylceramide (lysoGb3)

We have recently reported a highly efficient and stereocontrolled synthesis of globotriaosylceramide (Gb3, 1) in optically pure form. Key to our synthetic strategy was the implementation of the two-stage activation of thioglycosides for formation of the glycosidic bonds and the utilization of (2S, 3S, 4E)-2-azido-3-O-(tert-butyldimethylsilyl)-4-octadecen-1,3-di ol (9) as a sphingosine equivalent. The syntheses of Gb3 (1) and lysoGb3 (2) were achieved by stereocontrolled coupling of 2,3,4,6-tetra-O-benzyl-alpha-D-galactosyl fluoride (15) with phenyl O-(6-O-benzoyl-2,3-di-O-pivaloyl-beta-D-galactopyranosyl)- (1----4)-2,3,6-tri-O-pivaloyl-1-thio-beta-D-glucopyranoside (14) to form the P kappa antigen trisaccharide masked as a phenyl 1-thioglycoside at the reducing end. Thioglycoside 16 was converted into glycosyl fluoride 19, which was coupled to 9 in high yield. The coupled product 20 was converted into the title compounds 1 and 2 in four and three steps, respectively. This article presents the total synthesis of 1 and 2 in full experimental detail.

Evaluation of synthetic sphingosine, lysosphingolipids and glycosphingolipids as inhibitors of functional responses of human neutrophils

Human neutrophils, when exposed to soluble stimuli, aggregate, release oxygenated products of arachidonic acid and generate active oxygen species. Sphingolipid-derived products such as sphingosine and lysosphingolipids have been shown to exert selective actions on a variety of cell types, including neutrophils. Therefore, to determine the structural basis for selective inhibition of neutrophil responses by naturally occurring sphingolipids, seven compounds were prepared by total organic synthesis, and their impact on neutrophils in suspension has been studied. The compounds synthesized included sphingosine, psychosine, lactosyl lysosphingolipid, globotriaosyl (Gb3) lysosphingolipid, galactosyl cerebroside, lactosyl ceramide and Gb3 ceramide. The neutrophil responses studied were aggregation, leukotriene generation and superoxide anion production. When exposed to non-cytotoxic levels of the synthetic compounds, as monitored by exclusion of Trypan Blue, none of the synthetic sphingolipids inhibited A23187-induced aggregation of neutrophils. Only lactosyl lysosphingolipid, at a concentration of 1 microM, significantly inhibited aggregation induced by fMetLeuPhe; the other compounds in this series including sphingosine were without effect at equal molar concentrations (1 microM). Aggregation induced by phorbol 12-myristate 13-acetate (PMA) (0.1 microM) was significantly blocked by only two of the synthetic sphingolipids (1 microM). At concentrations below 1 microM, these inhibitory actions were not evident, nor was it possible to assign a structure-activity relationship for this series of compounds. None of the synthetic sphingolipids effectively inhibited the generation of superoxide anions induced by PMA. In addition, neither synthetic sphingosine nor psychosine affected either the formation or metabolism of leukotriene B4. Taken together, the results provide further evidence that sphingolipids, when added to intact cells, are not potent selective inhibitors of functional responses of human neutrophils.

Partitioning of exchangeable fluorescent phospholipids and sphingolipids between different lipid bilayer environments

Exchangeable phospho- and sphingolipid probes (phosphatidylcholine, -ethanolamine, -serine, and -glycerol, phosphatidic acid, sphingomyelin, cerebroside, and sulfatide) have been synthesized in which one acyl chain is substituted with a fluorescent bimanyl, 7-(dimethylamino)coumarin-3-yl, or diphenyl-hexatrienyl group. The distribution of these probes between two different populations of lipid vesicles can be readily monitored by fluorescence intensity measurements, as described by Nichols and Pagano [Nichols, J. W., & Pagano, R. E. (1982) Biochemistry 21, 1720-1726], when one of the vesicle populations contains a low mole fraction of a nonexchangeable quencher, (12-DABS)-18-PC. The probes examined in this study exchange between phospholipid vesicles on a time scale of minutes, with kinetics indicating that the transfer process takes place by diffusion of probe monomers through the aqueous phase. As expected, lipid probes with different charges differ markedly in their equilibrium distributions between neutral and charged lipid vesicles. However, probes with different polar headgroups differ only modestly in their relative affinities for vesicles composed of "hydrogen-bonding" lipids (PE and PS) vs "non-hydrogen-bonding" lipids (PC and PG or O-methyl-PA). Probes with different headgroups also show modest, albeit reproducible, differences in their relative affinities for cholesterol-containing vs cholesterol-free PC/PG vesicles. Our results suggest that lipids with different headgroup structures may mix more nearly ideally in liquid-crystalline lipid bilayers than would be predicted from previous analyses of the phase diagrams for binary lipid mixtures.