Stereoselective Synthesis of Novel Sphingoid Bases Utilized for Exploring the Secrets of Sphinx.

Sphingolipids are ubiquitous in eukaryotic plasma membranes and play major roles in human and animal physiology and disease. This class of lipids is usually defined as being derivatives of sphingosine, a long-chain 1,3-dihydroxy-2-amino alcohol. Various pathological conditions such as diabetes or neuropathy have been associated with changes in the sphingolipidome and an increased biosynthesis of structurally altered non-canonical sphingolipid derivatives. These unusual or non-canonical sphingolipids hold great promise as potential diagnostic markers. However, due to their low concentrations and the unavailability of suitable standards, the research to explore the secret of this class of 'Sphinx' lipids is ultimately hampered. Therefore, the development of efficient and facile syntheses of standard compounds is a key endeavor. Here, we present various chemical approaches for stereoselective synthesis and in-depth chemical characterization of a set of novel sphingoid bases which were recently utilized as valuable tools to explore the metabolism and biophysical properties of sphingolipids, but also to develop efficient analytical methods for their detection and quantification.

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.

Total Synthesis of Sialyl Inositol Phosphosphingolipids CJP-2, CJP-3, and CJP-4 Isolated from Feather Star Comanthus japonica.

The first total synthesis of three echinodermatous sialyl inositol phosphosphingolipids, which exhibit unusual neuritogenic activity in the absence of nerve growth factor, are reported. Highlights of the syntheses include 9- O-methylation on sialic acid, inter-residual amide bond formation between sialic acid residues, and highly stereo- and regioselective sialylation of inositol. A key phosphodiester linkage between the mono-, di-, and trisialyl inositols and ceramide was formed at a late state employing the phosphoramidite method.

Stereoselective synthesis of unnatural (2S,3S)-6-hydroxy-4-sphingenine-containing sphingolipids.

6-Hydroxy-(4E)-sphingenine-containing sphingolipids are found in mammalian and bacterial membranes and have multiple intra- and intercellular functions. Most sphingolipids contain a (2S,3R)-2-amino-1,3-diol core structure, but only limited examples of unnatural (2S,3S)-2-amino-1,3-diol derivates have so far been reported. Using an underexplored hydrozirconation-transmetalation reaction and an unusual three-step-one-pot deprotection sequence, we were able to synthesize several unnatural (2S,3S)-6-hydroxy-(4E)-sphingenine-containing sphingolipids in only three (protected) or four (deprotected) consecutive steps, respectively, including a fluoresence-labeled derivative suitable for future biological studies.

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.

Click reactions with functional sphingolipids.

Sphingolipids and glycosphingolipids can regulate cell recognition and signalling. Ceramide and sphingosine-1-phosphate are major players in the sphingolipid pathways and are involved in the initiation and regulation of signalling, apoptosis, stress responses and infection. Specific chemically synthesised sphingolipid derivatives containing small functionalities like azide or alkyne can mimic the biological properties of natural lipid species, which turns them into useful tools for the investigation of the highly complex sphingolipid metabolism by rapid and selective 'click chemistry' using sensitive tags like fluorophores. Subsequent analysis by various fluorescence microscopy techniques or mass spectrometry allows the identification and quantification of the corresponding sphingolipid metabolites as well as the research of associated enzymes. Here we present an overview of recent advances in the synthesis of ceramide and sphingosine analogues for bioorthogonal click reactions to study biosynthetic pathways and localization of sphingolipids for the development of novel therapeutics against lipid-dependent diseases.

Localization of 1-deoxysphingolipids to mitochondria induces mitochondrial dysfunction.

1-Deoxysphingolipids (deoxySLs) are atypical sphingolipids that are elevated in the plasma of patients with type 2 diabetes and hereditary sensory and autonomic neuropathy type 1 (HSAN1). Clinically, diabetic neuropathy and HSAN1 are very similar, suggesting the involvement of deoxySLs in the pathology of both diseases. However, very little is known about the biology of these lipids and the underlying pathomechanism. We synthesized an alkyne analog of 1-deoxysphinganine (doxSA), the metabolic precursor of all deoxySLs, to trace the metabolism and localization of deoxySLs. Our results indicate that the metabolism of these lipids is restricted to only some lipid species and that they are not converted to canonical sphingolipids or fatty acids. Furthermore, exogenously added alkyne-doxSA [(2S,3R)-2-aminooctadec-17-yn-3-ol] localized to mitochondria, causing mitochondrial fragmentation and dysfunction. The induced mitochondrial toxicity was also shown for natural doxSA, but not for sphinganine, and was rescued by inhibition of ceramide synthase activity. Our findings therefore indicate that mitochondrial enrichment of an N-acylated doxSA metabolite may contribute to the neurotoxicity seen in diabetic neuropathy and HSAN1. Hence, we provide a potential explanation for the characteristic vulnerability of peripheral nerves to elevated levels of deoxySLs.

Highly efficient preparation of sphingoid bases from glucosylceramides by chemoenzymatic method.

Sphingoid base derivatives have attracted increasing attention as promising chemotherapeutic candidates against lifestyle diseases such as diabetes and cancer. Natural sphingoid bases can be a potential resource instead of those derived by time-consuming total organic synthesis. In particular, glucosylceramides (GlcCers) in food plants are enriched sources of sphingoid bases, differing from those of animals. Several chemical methodologies to transform GlcCers to sphingoid bases have already investigated; however, these conventional methods using acid or alkaline hydrolysis are not efficient due to poor reaction yield, producing complex by-products and resulting in separation problems. In this study, an extremely efficient and practical chemoenzymatic transformation method has been developed using microwave-enhanced butanolysis of GlcCers and a large amount of readily available almond ß-glucosidase for its deglycosylation reaction of lysoGlcCers. The method is superior to conventional acid/base hydrolysis methods in its rapidity and its reaction cleanness (no isomerization, no rearrangement) with excellent overall yield.

A reflection of the lasting contributions from Dr. Robert Bittman to sterol trafficking, sphingolipid and phospholipid research.

With the passing of Dr. Robert Bittman from pancreatic cancer on the 1st October 2014, the lipid research field lost one of the most influential and significant personalities. Robert Bittman's genius was in chemical design and his contribution to the lipid research field was truly immense. The reagents and chemicals he designed and synthesised allowed interrogation of the role of lipids in constituting complex biophysical membranes, sterol transfer and in cellular communication networks. Here we provide a review of these works which serve as a lasting memory to his life.

Aziridine ring opening for the synthesis of sphingolipid analogues: inhibitors of sphingolipid-metabolizing enzymes.

A library of sphingolipid analogues is designed and tested as inhibitors against mammalian and fungal sphingolipid enzymes. The synthesis of sphingolipid analogues is based on the nucleophilic ring-opening reactions of N-activated aziridine derivatives with thiols, ß-thioglycosyl thiols, phosphorothioates, phosphates, and amines to afford compounds having different lipid backbones and substituents representative of the naturally occurring sphingolipid families. The screening on mammalian sphingomyelin synthase (SMS) and glucosylceramide synthase (GCS) and yeast inositol phosphorylceramide synthase (IPCS) enzymes identified several inhibitors of GCS and IPCS, but no inhibition of SMS was observed among the tested compounds.

Synthesis of fungal glycolipid asperamide B and investigation of its ability to stimulate natural killer T cells.

The relationship between mold and asthma has been recognized for decades, but the molecular triggers of asthma generated by molds have not been fully elucidated. A glycolipid generated by Aspergillus species has recently been identified that triggers airway hyperreactivity via natural killer T cell activation. The synthesis of this glycolipid and structural variants designed to allow identification of the features of this glycolipid required for recognition by natural killer T cells is described.

Synthesis and biological evaluation of a polyyne-containing sphingoid base probe as a chemical tool.

The sphingolipid metabolites have emerged as a starting point for the development of novel therapeutics for many diseases. However, details of the functions and mechanisms of sphingolipids remain unknown. To better understand the roles of sphingolipids, chemical tools with unique biological and physicochemical properties are needed. In this regard, we previously reported the synthesis of sphingoid base analogues in which the carbon chains are restricted by triple bonds. Here, we have conjugated a fluorescent dye to the polyyne analogues of the sphingoid bases to generate optical probes. Like the parent polyyne-containing sphingoid base, the 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled triyne-sphingosine inhibited cancer cell growth far more effectively than did the corresponding sphingosine. NBD-triyne-sphingosine was rapidly incorporated into the cells and displayed broad cytoplasmic distribution. According to the results of a flow cytometric analysis, cancer cells fed with NBD-triyne-sphingosine showed significantly increased fluorescence intensity compared with the NBD-sphingosine treated cells. The metabolism of NBD-triyne-sphingosine was somewhat different from that of NBD-sphingosine. These results indicated that the incorporated rigid polyyne moiety in the sphingoid base altered the physicochemical properties of the sphingolipid, thereby affecting its biological behavior. The higher antiproliferative activity in the SRB assay and the significantly higher fluorescence intensity observed in the flow cytometric analysis are some of the interesting and distinct aspects of NBD-triyne-sphingosine compared to standard NBD-sphingosine probes. Thus, it is believed that the fluorescently labeled polyyne-containing sphingoid base developed in this study will be a useful chemical tool in sphingolipid research.

Practical multigram-scale synthesis of 4,6- and 4,8-sphingadienes, chemopreventive sphingoid bases.

Sphingadienes are chemopreventive agents that act by blocking signaling pathways that are activated in cancer. A practical synthesis of 4,6- and 4,8-sphingadienes on a scale of gram quantities is reported here in order to allow evaluation of the biological properties of these sphingolipids. The key steps in the preparation of 4,6-sphingadiene (1a) are an intramolecular cyclization of N-Boc derivative 5a to oxazolidinone derivative 6a, followed by conversion to carbamate intermediate 7a and base-mediated hydrolysis to afford the product without further purification. 4,8-Sphingadiene (1b) was prepared in a similar fashion; the requisite trans-γ,δ-unsaturated aldehyde 15 was prepared by an ester enolate Ireland-Claisen rearrangement.

Design and synthesis of pyrrolidine-containing sphingomimetics.

Based on the structures of natural sphingolipids, we designed heterocyclic sphingoid base mimetics in which the conformational restriction is introduced by incorporation of a pyrrolidine moiety between the 2-amino group and the C-4 carbon atom of the sphingoid base. Our synthesis features a regioselective nucleophilic ring opening of a cyclic sulfate with cyanide and subsequent manipulation of the cyanide group. During the course of synthesis, Staudinger-type reductive cyclization of 1,3-azido carboxylic acid and 1,4-azido alcohol offers a direct route to the five-membered pyrrolidone and pyrrolidine products. The preliminary biological evaluation indicates that the designed pyrrolidine analog is biologically active and its cytotoxic effect is associated with the induction of apoptosis.

Sphingo-guanidines and their use as inhibitors of sphingosine kinase (WO2010078247).

The sphingolipid metabolic pathway controls the balance of bioactive lipids including apoptotic ceramide and proliferative sphingosine 1-phosphate and, therefore, represents a potential source of new therapeutic targets for numerous diseases. Targets, such as sphingosine kinases (SphK), have been extensively studied, and numerous strategies have been used to develop inhibitors against these enzymes. The WO2010078247 patent application shows the development of some novel sphingo-guanidines, including their water soluble salts, as inhibitors of SphK, and for use in treating and/or preventing diseases and disorders related to undesirable ceramidase, ceramidase-related or SphK activity, including cancer and other proliferative diseases. The synthesis of two agents D-erythro-2-N-(1'-carboxamidino)sphingosine hydrochloride (LCL146) and L-erythro-2-N-(1'-carboxamidino)sphingosine hydrochloride (LCL351) is described and they have been shown to inhibit SphK activity, be cytotoxic to cancer cells and decrease the migration rate of human prostate (DU145) cells.

[New 4,4-difluoro-3alpha,4alpha-diaza-s-indacene (BODIPY) labeled sphingolipids for membrane studies].

The synthesis of a series of new fluorescence labeled sphingolipids containing 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3alpha,4alpha-diaza-s-indacene-8-yl (Me4-BODIPY-8) group at omega-position of a fatty acyl residue is described. The obtained probes were used in studies of biological and model membrane systems.

Evaluation of synthetic sphingolipid analogs as ligands for peroxisome proliferator-activated receptors.

In this Letter, we assessed newly synthesized sphingolipid analogs as ligands for peroxisome proliferator-activated receptor (PPAR)alpha, PPARbeta or PPARgamma, using a dual-luciferase reporter system. We tested 640 sphingolipid analogs for ligand activity. As a result, seven types: A9, B9, C9, C50, F66, G66 and H66, were found to show agonistic activities for PPARs.

Leucettamol A: a new inhibitor of Ubc13-Uev1A interaction isolated from a marine sponge, Leucetta aff. microrhaphis.

A compound that inhibits the formation of a complex composed of the ubiquitin E2 enzyme Ubc13 and Uev1A was isolated from the marine sponge Leucetta aff. microrhaphis. The compound was identified as leucettamol A (1) by spectroscopic analysis. Its inhibition of Ubc13-Uev1A interaction was tested by the ELISA method, revealing an IC(50) value of 50 microg/mL. The compound is the first inhibitor of Ubc13-Uev1A interaction, that is, that of the E2 activity of Ubc13. Such inhibitors are presumed to be leads for anti-cancer agents that upregulate activity of the tumor suppressor p53 protein. Interestingly, hydrogenation of 1 increased its inhibitory activity with an IC(50) value of 4 microg/mL, while its tetraacetate derivative was inactive, indicating that the hydroxy and/or amino groups of 1 are required for the inhibition.

Chemical synthesis of fluorescent glycero- and sphingolipids.

Glycero- and sphingolipids have been shown to be building blocks of membranes and lipoproteins, metabolites and important intermediaries in the signalling cascades involved in stress responses, proliferation of cells and also apoptosis. Investigations into the exact functions of these lipids have found that they are fundamentally more important than previously thought and that they are intricately involved in the processes of many significant metabolic pathways and diseases. Investigation of these functions requires the detection of the lipids in their natural environment within membranes. To this end, fluorescent labelling has become one of the preferred means in which to study these essential components due to the relative ease of detection. This review will look at the novel compounds that have been synthesised recently through various methodologies including classical lipid synthesis as well as the innovative application of organometallic chemistry. This field has expanded with the advancements in fluorescence detection and these lipids are being used as specific probes for an extensive range of applications in order to ascertain the mechanisms and signalling capabilities of this very important class of biological compounds.

Biochemistry of inositol lipids.

Nature has created an immense combinatorial and structural heterogeneity among lipids. It is becoming increasingly accepted that the vast range of unique chemical entities encodes for distinct functions within biological systems. A unique group of lipids which stands out in terms of diversity as well as biological activity are inositol-containing lipids. The most well characterized inositol lipids are the phosphoinositides, phosphorylated derivatives of glycerophosphoinositol, which play a wide variety of cellular roles in many eukaryotic cells. Less well understood are ceramides containing inositol in fungi, and inositol glycolipids in pathogens. Here we review biochemical aspects of inositol-containing lipids with a focus on novel analytical procedures for their characterization.