Efficiently synthesizing lacto-ganglio-series gangliosides by using a glucosyl ceramide cassette approach: the total synthesis of ganglioside X2

Fluorescent GD2 analog for single-molecule imaging

Ganglioside GD2 is associated with the proliferation and migration of breast cancer cells. However, the precise role of GD2 is unclear because its tendency to form dynamic and transient domains in cell plasma membranes (PMs), called lipid rafts, makes it difficult to observe. Previously, we developed fluorescent analogs of gangliosides (e.g., GM3 and GM1), which enabled the observation of lipid raft formation for the first time using single-molecule imaging. In this report, we describe the first chemical synthesis of a fluorescent ganglioside, GD2. A biophysical analysis of the synthesized analog revealed its raft-philic character, suggesting its potential to aid single-molecule imaging-based investigations into raft-associated interactions.

Advanced Chemical Methods for Stereoselective Sialylation and Their Applications in Sialoglycan Syntheses

Sialic acid is an important component of cell surface glycans, which are responsible for many vital body functions and should therefore be thoroughly studied to understand their biological roles and association with disorders. The difficulty of isolating large quantities of homogenous-state sialoglycans from natural sources has inspired the development of the corresponding chemical synthesis methods affording acceptable purities, yields, and amounts. However, the related syntheses are challenging because of the difficulties in α-glycosylation of sialic acid, which arises from its certain structural features such as the absence of a stereodirecting group at the C3 position and presence of carboxyl group at the anomeric position. Moreover, the structural complexities of sialoglycans with diverse numbers and locations of sialic acid on the glycan chains pose additional barriers. Thus, efficient α-stereoselective routes to sialosides remain highly sought after, although various types of sialyl donors/acceptors have been developed for the straightforward synthesis of α-sialosides. Herein, we review the latest progress in the α-stereoselective synthesis of sialosides and their applications in the preparation of gangliosides and other sialoglycans.

A Diversity-Oriented Strategy for Chemical Synthesis of Glycosphingolipids: Synthesis of Glycosphingolipid LcGg4 and Its Analogues and Derivatives

A diversity-oriented strategy was developed for the synthesis of glycosphingolipids (GSLs). This strategy was highlighted by using a simple lactoside containing the core structures of GSL glycan and lipid as the universal starting material to obtain different synthetic targets upon stepwise elongation of the glycan via chemical glycosylations and on-site remodeling of the lipid via chemoselective cross-metathesis and N-acylation. The strategy was verified with the synthesis of a lacto-ganglio GSL, LcGg4, which is a biomarker of undifferentiated malignant myeloid cells, and a series of its analogues or derivatives carrying different sugar chains and unique functionalities or molecular labels. This synthetic strategy should be widely applicable and, therefore, be utilized to rapidly access various GSLs and related derivatives by using different donors for glycosylations and different substrates for lipid remodeling following each glycosylation.

Chemical synthesis of the dimeric repeating unit of type Ia group BStreptococcuscapsular polysaccharide

The first synthesis of the dimeric repeating unit of type Ia GBS CPS containing two sialotrisaccharide side chains and adjacent 3,4-di-branched Gal motifs was achieved.

Highly efficient chemoenzymatic synthesis and facile purification of α-Gal pentasaccharyl ceramide Galα3nLc4βCer

A highly efficient chemoenzymatic method for synthesizing glycosphingolipids using α-Gal pentasaccharyl ceramide as an example is reported here. Enzymatic extension of the chemically synthesized lactosyl sphingosine using efficient sequential one-pot multienzyme (OPME) reactions allowed glycosylation to be carried out in aqueous solutions. Facile C18 cartridge-based quick (<30 minutes) purification protocols were established using minimal amounts of green solvents (CH₃CN and H₂O). Simple acylation in the last step led to the formation of the target glycosyl ceramide in 4 steps with an overall yield of 57%.

Chemical Synthesis of Gangliosides

Various methods for the chemical synthesis of gangliosides have been investigated to date and numerous natural gangliosides and their structural analogues have been synthesized during the past three decades. Key technologies in the synthesis of gangliosides include α-selective sialylation and introduction of the ceramide moiety into the oligosaccharide chain. This chapter introduces two major strategies for ganglioside synthesis-the most commonly used strategy and the recently developed glucosylceramide cassette approach. Synthetic procedures for selected reactions are also presented.

Development of new ganglioside probes and unraveling of raft domain structure by single-molecule imaging

Gangliosides are involved in a variety of biological roles and are a component of lipid rafts found in cell plasma membranes (PMs). Gangliosides are especially abundant in neuronal PMs and are essential to their physiological functions. However, the dynamic behaviors of gangliosides have not been investigated in living cells due to a lack of fluorescent probes that behave like their parental molecules. We have recently developed, using an entirely chemical method, four new ganglioside probes (GM1, GM2, GM3, and GD1b) that act similarly to their parental molecules in terms of raft partitioning and binding affinity. Using single fluorescent-molecule imaging, we have found that ganglioside probes dynamically enter and leave rafts featuring CD59, a GPI-anchored protein. This occurs both before and after stimulation. The residency time of our ganglioside probes in rafts with CD59 oligomers was 48ms, after stimulation. The residency times in CD59 homodimer and monomer rafts were 40ms and 12ms, respectively. In this review, we introduce an entirely chemical-based ganglioside analog synthesis method and describe its application in single-molecule imaging and for the study of the dynamic behavior of gangliosides in cell PMs. Finally, we discuss how raft domains are formed, both before and after receptor engagement. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Syntheses of Fluorescent Gangliosides for the Studies of Raft Domains

Gangliosides, glycosphingolipids containing one or more sialic acids in the glycan chain, are involved in various important biological processes in cell plasma membranes (PMs). However, the behaviors and functions of gangliosides are poorly understood, primarily because of the lack of fluorescent analogs that are equivalent to native gangliosides that can be used as chemical and physical probes. In this study, we developed entirely chemical methods to synthesize fluorescent gangliosides (GM3, GM2, GM1, and GD1b) in which the glycan components are site-specifically labeled with various fluorescent dyes. The functional evaluations of the synthesized fluorescent gangliosides demonstrated the great influence of fluorescent dye on the physical properties of gangliosides in PMs and revealed the fluorescent ganglioside analogs which show similar behaviors to the native gangliosides.

A General Chemoenzymatic Strategy for the Synthesis of Glycosphingolipids

A concise, prototypical, and stereoselective strategy for the synthesis of therapeutically and immunologically significant glycosphingolipids has been developed. This strategy provides a universal platform for glycosphingolipid synthesis by block coupling of enzymatically prepared free oligosaccharideglycans to lipids using glycosyl N-phenyltrifluoroacetimidates as efficient activated intermediates. As demonstrated here, two different types of glycosphingolipids were obtained in excellent yields using the method.

Raft-based interactions of gangliosides with a GPI-anchored receptor

Gangliosides, glycosphingolipids containing one or more sialic acid(s) in the glyco-chain, are involved in various important physiological and pathological processes in the plasma membrane. However, their exact functions are poorly understood, primarily because of the scarcity of suitable fluorescent ganglioside analogs. Here, we developed methods for systematically synthesizing analogs that behave like their native counterparts in regard to partitioning into raft-related membrane domains or preparations. Single-fluorescent-molecule imaging in the live-cell plasma membrane revealed the clear but transient colocalization and codiffusion of fluorescent ganglioside analogs with a fluorescently labeled glycosylphosphatidylinisotol (GPI)-anchored protein, human CD59, with lifetimes of 12 ms for CD59 monomers, 40 ms for CD59's transient homodimer rafts in quiescent cells, and 48 ms for engaged-CD59-cluster rafts, in cholesterol- and GPI-anchoring-dependent manners. The ganglioside molecules were always mobile in quiescent cells. These results show that gangliosides continually and dynamically exchange between raft domains and the bulk domain, indicating that raft domains are dynamic entities.

Total syntheses of disulphated glycosphingolipid SB1a and the related monosulphated SM1a

Total syntheses of two natural sulphoglycolipids, disulphated glycosphingolipid SB1a and the structurally related monosulphated SM1a, are described. They have common glycan sequences and ceramide moieties and are associated with human epithelial carcinomas. The syntheses featured efficient glycan assembly and the glucosyl ceramide cassette as a versatile building block. The binding of the synthetic sulphoglycolipids by the carcinoma-specific monoclonal antibody AE3 was investigated using carbohydrate microarray technology.

Lower motor neuron syndrome associated with IgG anti-GM1 antibodies revisited

A patient, who developed an amyotrophic lateral sclerosis-like disorder subsequent to ganglioside treatment, had IgM antibodies to GM2 as well as to minor gangliosides X1 and X2 containing GM2 epitope. These gangliosides as well as GM1 were tested in 655 sera obtained from patients who were suspected of having amyotrophic lateral sclerosis or motor neuron disease to find a treatable condition. Three patients had high titers of IgG anti-GM1 antibodies, but no IgM anti-GM1 antibodies. One of the patients also had IgG anti-X2 antibodies. The patients, being diagnosed with having lower motor neuron syndrome, had neither upper motor neuron signs nor multifocal conduction block. Both IgM and IgG anti-GM1 antibodies should be tested in patients who have lower motor neuron syndrome.

Extending the glucosyl ceramide cassette approach: application in the total synthesis of ganglioside GalNAc-GM1b

The development of a novel cyclic glucosyl ceramide cassette acceptor for efficient glycolipid syntheses was investigated. p-Methoxybenzyl (PMB) groups were selected as protecting groups at C2 and C3 of the glucose residue with the aim of improving the functionality of the cassette acceptor. The choice of the PMB group resulted in a loss of β-selectivity, which was corrected by using an appropriate tether to control the spatial arrangement and the nitrile solvent effect. To investigate the effect of linker structure on the β-selectivity of intramolecular glycosylation, several linkers for tethering the glucose and ceramide moiety were designed and prepared, namely, succinyl, glutaryl, dimethylmalonyl, and phthaloyl esters. The succinyl ester linker was the best for accessing the cassette form. The newly designed glucosyl ceramide cassette acceptor was then applied in the total synthesis of ganglioside GalNAc-GM1b.

Design and synthesis of a novel ganglioside ligand for influenza A viruses

A novel ganglioside bearing Neuα2-3Gal and Neuα2-6Gal structures as distal sequences was designed as a ligand for influenza A viruses. The efficient synthesis of the designed ganglioside was accomplished by employing the cassette coupling approach as a key reaction, which was executed between the non-reducing end of the oligosaccharide and the cyclic glucosylceramide moiety. Examination of its binding activity to influenza A viruses revealed that the new ligand is recognized by Neuα2-3 and 2-6 type viruses.