Synthesis of biotinylated sialoside to probe CD22–ligand interactions

Improved synthesis of CD22-binding sialosides and its application for further development of potent CD22 inhibitors

CD22, one of the sialic acid-binding immunoglobulin-like lectins (Siglecs), regulates B lymphocyte signaling via its interaction with glycan ligands bearing the sequence Neu5Ac/Gcα(2→6)Gal. We have developed the synthetic sialoside GSC-718 as a ligand mimic for CD22 and identified it as a potent CD22 inhibitor. Although the synthesis of CD22-binding sialosides including GSC-718 has been reported by our group, the synthetic route was unfortunately not suitable for large-scale synthesis. In this study, we developed an improved scalable synthetic procedure for sialosides which utilized 1,5-lactam formation as a key step. The improved procedure yielded sialosides incorporating a series of aglycones at the C2 position. Several derivatives with substituted benzyl residues as aglycones were found to bind to mouse CD22 with affinity comparable to that of GSC-718. The new procedure developed in this study affords sialosides in sufficient quantities for cell-based assays, and will facilitate the search for promising CD22 inhibitors that have therapeutic potential.

Design, synthesis and molecular docking study of α-triazolylsialosides as non-hydrolyzable and potent CD22 ligands

Ligand 1 was the first reported example of monomeric high-affinity synthetic CD22 ligand that regulated B cell activation in vitro, augmented antibody production and regulated immune responses in mice. Replacing O-glycoside linkage of 1 by nitrogen of triazole by click reaction afforded compounds which are as potent as the parent compound. The synthesis of the new compounds is straightforward with fewer synthetic steps and higher yield. Such a strategy provided stable ligand that can bind avidly and can be conjugated to drugs for B-cell targeting or multimeric formation. The new compounds were screened for their affinity to CD22, using surface plasmon resonance (SPR). Compound 12 was obtained as a bioisosteric analogue and an anomerically stable imitation of 1. It was, also, screened for MAG to test for selectivity and analyzed by molecular docking and dynamic simulation to explore the potential binding modes and source of selectivity within CD22. Our results could enable the development of small molecule drug capable of modulating the activity of CD22 in autoimmune diseases and malignancies derived from B-cells.

CD22-Binding Synthetic Sialosides Regulate B Lymphocyte Proliferation Through CD22 Ligand-Dependent and Independent Pathways, and Enhance Antibody Production in Mice

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are expressed in various immune cells and most of them carry signaling functions. High-affinity synthetic sialoside ligands have been developed for various Siglecs. Therapeutic potentials of the nanoparticles and compounds that contain multiple numbers of these sialosides and other reagents such as toxins and antigens have been demonstrated. However, whether immune responses can be regulated by monomeric sialoside ligands has not yet been known. CD22 (also known as Siglec-2) is an inhibitory molecule preferentially expressed in B lymphocytes (B cells) and is constitutively bound and functionally regulated by α2,6 sialic acids expressed on the same cell (cis-ligands). Here, we developed synthetic sialosides GSC718 and GSC839 that bind to CD22 with high affinity (IC₅₀ ~100 nM), and inhibit ligand binding of CD22. When B cells are activated by B cell antigen receptor (BCR) ligation, both GSC718 and GSC839 downregulate proliferation of B cells, and this regulation requires both CD22 and α2,6 sialic acids. This result suggests that these sialosides regulate BCR ligation-induced B cell activation by reversing endogenous ligand-mediated regulation of CD22. By contrast, GSC718 and GSC839 augment B cell proliferation induced by TLR ligands or CD40 ligation, and this augmentation requires CD22 but not α2,6 sialic acids. Thus, these sialosides appear to enhance B cell activation by directly suppressing the inhibitory function of CD22 independently of endogenous ligand-mediated regulation. Moreover, GSC839 augments B cell proliferation that depends on both BCR ligation and CD40 ligation as is the case for in vivo B cell responses to antigens, and enhanced antibody production to the extent comparable to CpG oligonuleotides or a small amount of alum. Although these known adjuvants induce production of the inflammatory cytokines or accumulation of inflammatory cells, CD22-binding sialosides do not. Thus, synthetic sialosides that bind to CD22 with high-affinity modulate B cell activation through endogenous ligand-dependent and independent pathways, and carry an adjuvant activity without inducing inflammation.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2009-2010

This review is the sixth update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2010. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, arrays and fragmentation are covered in the first part of the review and applications to various structural typed constitutes the remainder. The main groups of compound that are discussed in this section are oligo and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals. Many of these applications are presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions and applications to chemical synthesis.

Toward libraries of biotinylated chondroitin sulfate analogues: from synthesis to in vivo studies

Chondroitin sulfate-E (CS-E) oligosaccharidic analogues (di to hexa) were prepared from lactose. In these compounds, the 2-acetamido group was replaced by a hydroxyl group. This modification speeded up the synthesis, and large oligosaccharides were constructed in a few steps from a lactose-originated block. The protecting groups used were as follows; Fmoc for hydroxyl groups to be glycosylated, allyl group for anomeric position protection, and trichoroacetimidate leaving groups were used to prepare up to octasaccharides. We took advantage of the presence of allyl group to develop a click biotinylation, through its transformation into a 3-azido-2-hydroxyl propyl group in two steps (epoxidation and sodium azide epoxide opening). The biotinylating agent was a water-soluble propargylated and biotinylated triethylene glycol (PEG). By using surface plasmon resonance (SPR), it was shown that the di-, tetra-, and hexasaccharides display a binding affinity and selectivity toward HSF/GSF and CXCL12 similar to that of CS-E. A parallel study confirmed their mimicry of natural compounds, based on the hexasaccharide interaction with Otx2, a homeodomain protein involved in brain maturation, thus validating our simplification approach to synthesize bioactive GAG.

CD22-antagonists with nanomolar potency: the synergistic effect of hydrophobic groups at C-2 and C-9 of sialic acid scaffold

In earlier studies, we identified the C-9 amido derivative 1 (9-(4'-hydroxy-4-biphenyl)acetamido-9-deoxy-Neu5Gcα2-6GalOMP) and the C-9 amino derivative 2 (9-(4'-hydroxy-4-biphenyl)methylamino-9-deoxy-Neu5Gcα2-6GalOMP) have the most promising affinity for mouse CD22 and human CD22, respectively. Replacing the subterminal galactose residue (2-6Gal-OMP) of 1 with benzyl (5) or biphenylmethyl (6) as aglycone led to even higher potency for mCD22. In this study, both compounds showed improved potency and selectivity for CD22 (IC(50) 70 nM) and 712-fold more selective for CD22 than for MAG. The corresponding derivatives of 2, compounds 8 and 9, showed comparable activity to 2 but lower potency and selectivity than 5 and 6. Although compounds 5-9 are simple and small molecular weight antagonists, they showed much high potency and selectivity than the corresponding compounds having α 2-6Gal linkage. Both biological and computational docking simulation studies suggest that the 2-6Gal-OMP residues of 1 and 2 are not critical for binding process and could be replaced with hydrophobic non-carbohydrate moieties. The data presented herein has significant implications for the design and discovery of next-generation CD22-antagonists.

Potent small molecule mouse CD22-inhibitors: exploring the interaction of the residue at C-2 of sialic acid scaffold

Our previous study revealed that compound 1 (9-(4'-hydroxy-4-biphenyl)acetamido-9-deoxy-Neu5Gcalpha2-6GalOMP) has the most promising affinity for mCD22. Replacing the subterminal galactose residue of 1 with benzyl or biphenylmethyl as aglycone led to 38- and 20-fold higher potency, respectively. This discovery represents a new direction in inhibitor design suitable for pharmaceutical development.