Exploring the Structural and Functional Aspects of Sialic acid Mimetics

Abstract

Sialic acids (Sia) are nine-carbon backbone monosaccharides, typically found at the terminal position of the glycoproteins and glycolipids. Over the decades, sias have been reported in various pathological and physiological processes. The invasion of virus, bacteria also depends on specific sia patterns and aberrant sialylation is the feature of numerous of neurological, immune, cancer and congenital disorder of glycosylation. Despite its widespread medical applications, the structure-function relation of the defined sialic acid sequence is still poorly understood. In my thesis, we describe the three distinct sialic acid modifications that independently generate crucial structural-functional relation of sialic acid glycans. The first target was to decipher the role of galactose as a penultimate sugar in sialic acid glycans. We have synthesized Neu5Acα(2-6)Gal structural analogs and studies their binding to a series of Siglec. The results showed distinct binding patterns with conserved Siglec (hCD22 and mCD22) compared to rapid evolving Siglec (Siglec -3 & -10) and constitute a valuable tool for designing Siglec specific molecules for therapeutic applications. Furthermore, these isostructural glycans were conjugated to KLH proteins and immunize the mice. The immune response showed that mannose modification at the penultimate position of the sialic acid glycan induced high titer IgG antibody responses and these IgG antibodies showed efficient cross-reactivity with native glycan. Overall, our results highlight a new potential approach to synthesize antigenic sugars to modulate immune responses. Next, we have synthesized sialic acid hydroxamate to understand the structural-functional relation of the acidic group. We found that biomimetic analogs possessed three distinct properties: antioxidant nature, metals chelating ability and Sia backbone to target AD. Focusing on cytotoxicity caused by radicals and A-metal complexes, we demonstrated that hydroxamate ligand on sialic acid backbone protects the neural cells more efficiently in comparisons to sialic acid alone. Finally, we locked the sialic acid in two opposite orientations on micelles to effectively tune sia dependent ligand-receptor interactions in their native context. To obtain sialo-micelles that could serve as the multivalent probe, we conjugated amphiphilic group at C-2 and C-9 position of sia respectively. Upon dissolution of amphiphiles in water, self-assembled highly regular micelles were obtained. The bioavailability of resultant sialo-micelles with plant and human sialic acid binding protein (SBP) was evaluated by surface plasma resonance (SPR) and in-vitro assays. Sambucusnigra agglutinin (SNA), Limaxflavus agglutinin (LFA),30 P, E-selectin and CD22 (Siglec-2) were selected as sialic acid binding proteins (SBP). Where SNA and LFA binds to all common sialic acid residues,31 human CD22-Fc only recognizes sia(2-6)-linked sias, while E and P-selectin, which belong to the subgroup of the C-type lectins that mediate leukocyte trafficking, are specific to sialyl Lewisx and sialyl Lewisa glycans respectively. Animal models to test the biodistribution and sequestration of specific biomarkers vary from laboratory to laboratory, resulting variable results. Thus, a head-to-head comparison of the different models at same experimental conditions will undoubtedly assist to identify the better model for the therapeutic purposes. Herein, we performed in vivo head-to-head comparison of (2-6) and (2-3) sialic acid glycan conjugated quantum dots in zebrafish (Daniorerio) and mouse model (intraperitoneal injection) to advocate fish as an ideal living simple system for the preliminary screening of pharmacokinetics of sugar conjugated-QDs. The results suggest that (2-6) sialylated-QDs preferentially has prolonged circulation half-life and uptake by the liver, kidney and spleen, while (2-3), sialic acid and galactose conjugated-QDs accumulated in the liver and cleared after several hours in both models. These results suggested that zebrafish is a competitive model for sialic acid-mediated pharmacokinetics studies.