Synthesis of mannan oligosaccharide-sialic acid conjugates and its inhibition on Aβ42 aggregation

In this work, a mannan-oligosaccharide conjugate with sialic acid capable of perturbing Aβ42 aggregation was designed and synthesized. Mannan oligosaccharides with degree polymerization of 3–13 were obtained by stepwise hydrolysis of locust bean gum using β-mannanase and α-galactosidase, named as LBOS. The activated LBOS was further chemically conjugated with sialic acid (Sia, N-acetylneuraminic acid) by fluoro-mercapto chemical coupling to synthesize a conjugate LBOS-Sia, and then phosphorylated to obtain pLBOS-Sia. The successful synthesis of pLBOS-Sia was confirmed by infrared1 chromatography, mass spectrometry, and 1 H NMR. The soluble protein analysis, microscopic observation, thioflavin T-labeling, and circular dichroism spectroscopy revealed that both LBOS-Sia and pLBOS-Sia can inhibit Aβ42 aggregation. MTT assay showed that LBOS-Sia and pLBOS-Sia had no cytotoxicity to BV-2 cells, and could substantially reduce the release of pro-inflammatory factor TNF-α induced by Aβ42 in BV-2 cells, and inhibit the occurrence of neuroinflammation. In future, this novel structure of mannan oligosaccharide-sialic acid conjugate can be potentially used to for the development of glycoconjugates against AD targeting Aβ.