A new sialic acid (Sia)‐containing glycopolymer—a fluorescent probe with high‐density disialic acid (diSia) on the surface of polysaccharide dextran (diSia‐Dex)—was synthesized as a key molecule to regulate the Sia recognition lectins, Siglecs, that are involved in the immune system. According to our original methods, diSia was synthesized by α‐selective sialylation, and a dextran template possessing terminal acetylenes and amino groups was prepared. A diSia and a fluorescent molecule were subsequently introduced to surface‐modified dextran by Hüisgen reaction and amidation, respectively. The modulatory activity of Siglec7 was evaluated by using synthetic probes. DiSia‐Dex showed high binding avidity toward Siglec7, with a KD value of 5.87×10−10 m, and a high inhibitory activity for the interaction between Siglec7 and a ligand (GD3), with a IC50 value of 1.0 nm. Notably, diSia‐Dex was able to release Siglec7 from the pre‐existing Siglec7–GD3 complex, possibly due to its unique properties of a slow dissociation rate and a high association rate. Together, these data show that diSia‐Dex can be widely applicable as a modulator of Siglec7 functions.

Siglec-7 is a human CD33-like siglec, and is localised predominantly on human natural killer (NK) cells and monocytes. Siglec-7 is considered to function as an immunoreceptor in a sialic acid-dependent manner. However, the underlying mechanisms linking sialic acid-binding and function remain unknown. Here, to gain new insights into the ligand-binding properties of Siglec-7, we carried out in silico analysis and site-directed mutagenesis, and found a new sialic acid-binding region (site 2 containing R67) in addition to the well-known primary ligand-binding region (site 1 containing R124). This was supported by equilibrium dialysis, STD-NMR experiments, and inhibition analysis of GD3-binding toward Siglec-7 using synthetic sialoglycoconjugates and a comprehensive set of ganglioside-based glycoconjugates. Our results suggest that the two ligand-binding sites are potentially controlled by each other due to the flexible conformation of the C-C′ loop of Siglec-7. Siglecs are sialic acid (Sia)-binding immunoglobulin-like lectins and the majority of them function as transmembrane receptors in the immune system via recognition of Sia residues 1. The expression pattern of siglecs is often cell type-specific. Through cytosolic regions or adaptor proteins, they are able to regulate inhibitory or activating signals. Siglecs contain a Sia-binding N-terminal V-set domain followed by 1 to 16 C2-set Ig-domains that are considered to act as spacers or regulators of oligomerisation. Siglecs differ from most other mammalian Sia-binding lectins, such as selectins, with respect to the absolute requirement for Sia, but the details of their molecular interactions, including specific natural ligands and ligand-binding mechanisms, remain largely unclear. These are thought to be important for immune cell functions via Sia-binding because disrupting the interactions between Sias and siglecs can alter responses to infection, inflammation, autoimmunity, and cancer 1. Therefore, understanding the role of ligand-binding in biological functions of siglecs is important. The binding affinity between siglecs and naturally occurring sialylated glycoconjugates is known to be relatively weak (0.1-3 mM) 2 , and therefore biological interactions between siglecs and their ligands depend on clustering and multivalent binding. For example, mucins can display multiple ligands, and gangliosides can be clustered within lipid microdomains. To conquer the low affinity of Sia ligands for siglecs, recent research in Sia chemistry has focussed on the development of biochemical probes toward siglecs with high affinity and specificity 2-6. Attempts to increase the affinity were successful, and in some cases materials were used for clinical therapy 7,8. However, the precise details of how siglecs engage their ligands remain unclear. Siglec-7 (CDw328) is the major siglec expressed by NK cells where it can function as an inhibitory receptor 4,9,10. Siglec-7 consists of an extracellular N-terminal V-set Ig domain that is responsible for a Sia-binding, two

An allene monomer containing an α(2,8) disialic acid was polymerized by a π-allyl nickel complex with an azido group to produce end-functional glycopolymers with an excellent polydispersity index. The polymers allowed terminal modification with a fluorescent dye by click chemistry. The glycopolymers can dissociate Siglec-7-GD3 interactions at low concentrations.

The inside cover picture shows a new synthetic inhibitor, named diSia‐Dex, for binding the sialic‐acid‐binding lectin Siglec‐7 to its typical ligand, GD3. DiSia‐Dex is a dextran polymer conjugated with multiple disialic acid (diSia) units that are, in silico, predicted to bind to the sialic‐acid‐binding site of Siglec‐7 with a high affinity. DiSia‐Dex has a strong affinity for Siglec‐7 with a very slow dissociation rate, and shows an inhibitory activity (IC50=1 nm) strong enough to release the Siglec‐7 from its binding state with GD3. DiSia‐Dex would be a key molecule to regulate the ligand binding of Siglec‐7. More information can be found in the full paper by C. Sato et al. on page 1194 in Issue 13, 2017 (DOI: 10.1002/cbic.201600694).