Correction: Boronate affinity-based photoactivatable magnetic nanoparticles for the oriented and irreversible conjugation of Fc-fused lectins and antibodies.

[This corrects the article DOI: 10.1039/C9SC01613A.].

Preparation of Recombinant Siglecs and Identification of Their Ligands.

Siglecs are transmembrane receptor-like vertebrate lectins that recognize glycans containing sialic acid. Most Siglecs also interact with intracellular signal transduction molecules, and modulate immune responses. Recombinant soluble Siglecs fused with the fragment crystallizable (Fc) region of immunoglobulin G (Siglec-Fc) are a versatile tool for the investigation of Siglec functions. We describe protocols for the production of recombinant Siglec-Fc, the analysis of expression of Siglec ligands by flow cytometry, and the identification of the Siglec ligand candidates based on proximity labeling.

Boronate affinity-based photoactivatable magnetic nanoparticles for the oriented and irreversible conjugation of Fc-fused lectins and antibodies.

The utilization of immuno-magnetic nanoparticles (MNPs) for the selective capture, enrichment, and separation of specific glycoproteins from complicated biological samples is appealing for the discovery of disease biomarkers. Herein, MNPs were designed and anchored with abundant boronic acid (BA) and photoreactive alkyl diazirine (Diaz) functional groups to obtain permanently tethered Fc-fused Siglec-2 and antiserum amyloid A (SAA) mAb with the assistance of reversible boronate affinity and UV light activation in an orientation-controlled manner. The Siglec-2-Fc-functionalized MNPs showed excellent stability in fetal bovine serum (FBS) and excellent efficiency in the extraction of cell membrane glycoproteins. The anti-SAA mAb-functionalized MNPs maintained active Ab orientation and preserved antigen recognition capability in biological samples. Thus, the BA-Diaz-based strategy holds promise for the immobilization of glycoproteins, such as antibodies, with the original protein binding activity maintained, which can provide better enrichment for the sensitive detection of target proteins.

Expedient assembly of Oligo-LacNAcs by a sugar nucleotide regeneration system: Finding the role of tandem LacNAc and sialic acid position towards siglec binding.

Sialosides containing (oligo-)N-acetyllactosamine (LacNAc, Galß(1,4)GlcNAc) as core structure are known to serve as ligands for Siglecs. However, the role of tandem inner epitope for Siglec interaction has never been reported. Herein, we report the effect of internal glycan (by length and type) on the binding affinity and describe a simple and efficient chemo-enzymatic sugar nucleotide regeneration protocol for the preparative-scale synthesis of oligo-LacNAcs by the sequential use of ß1,4-galactosyltransferase (ß4GalT) and ß1,3-N-acetylglucosyl transferase (ß3GlcNAcT). Further modification of these oligo-LacNAcs was performed in one-pot enzymatic synthesis to yield sialylated and/or fucosylated analogs. A glycan library of 23 different sialosides containing various LacNAc lengths or Lac core with natural/unnatural sialylation and/or fucosylation was synthesized. These glycans were used to fabricate a glycan microarray that was utilized to screen glycan binding preferences against five different Siglecs (2, 7, 9, 14 and 15).

Soluble Siglec-14 glycan-recognition protein is generated by alternative splicing and suppresses myeloid inflammatory responses.

Human sialic acid-binding immunoglobulin-like lectin 14 (Siglec-14) is a glycan-recognition protein that is expressed on myeloid cells, recognizes bacterial pathogens, and elicits pro-inflammatory responses. Although Siglec-14 is a transmembrane protein, a soluble form of Siglec-14 is also present in human blood. However, the mechanism that generates soluble Siglec-14 and what role this protein form may play remain unknown. Here, investigating the generation and function of soluble Siglec-14, we found that soluble Siglec-14 is derived from an alternatively spliced mRNA that retains intron 5, containing a termination codon and thus preventing the translation of exon 6, which encodes Siglec-14's transmembrane domain. We also note that the translated segment in intron 5 encodes a unique C-terminal 7-amino acid extension, which allowed the specific antibody-mediated detection of this isoform in human blood. Moreover, soluble Siglec-14 dose-dependently suppressed pro-inflammatory responses of myeloid cells that expressed membrane-bound Siglec-14, likely by interfering with the interaction between membrane-bound Siglec-14 and Toll-like receptor 2 on the cell surface. We also found that intron 5 contains a G-rich segment that assumes an RNA tertiary structure called a G-quadruplex, which may regulate the efficiency of intron 5 splicing. Taken together, we propose that soluble Siglec-14 suppresses pro-inflammatory responses triggered by membrane-bound Siglec-14.

Identification of Siglec Ligands Using a Proximity Labeling Method.

Siglecs are a family of receptor-type glycan recognition proteins (lectins) involved in self-nonself discrimination by the immune system. Identification of Siglec ligands is necessary to understand how Siglec-ligand interaction translates into biological outcomes. However, this is challenging because the interaction is weak. To facilitate identification of Siglec ligands, we adopted a proximity labeling method based on the tyramide radicalization principle. Cells that express Siglec ligands were labeled with Siglec-peroxidase complexes and incubated with biotin tyramide and hydrogen peroxide to generate short-lived tyramide radicals that covalently label the proteins near the Siglec-peroxidase complex. A proof-of-principle experiment using CD22 (Siglec-2) probe identified its known ligands on B cells, including CD22 itself, CD45, and IgM, among others, demonstrating the validity of this method. The specificity of labeling was confirmed by sialidase treatment of target cells and using glycan recognition-deficient mutant CD22 probes. Moreover, possible interactions between biotin-labeled proteins were revealed by literature-based protein-protein interaction network analysis, implying the presence of a molecular cluster comprising CD22 ligands. Further application of this method identified CD44 as a hitherto unknown Siglec-15 ligand on RAW264.7-derived osteoclasts. These results demonstrated the utility of proximity labeling for the identification of Siglec ligands, which may extend to other lectins.