Molecular Basis and Role of Siglec-7 Ligand Expression on Chronic Lymphocytic Leukemia B Cells.

Siglec-7 (sialic acid-binding immunoglobulin-like lectin 7) is an immune checkpoint-like glycan recognition protein on natural killer (NK) cells. Cancer cells often upregulate Siglec ligands to subvert immunosurveillance, but the molecular basis of Siglec ligands has been elusive. In this study, we investigated Siglec-7 ligands on chronic lymphocytic leukemia (CLL) B cells. CLL B cells express higher levels of Siglec-7 ligands compared with healthy donor B cells, and enzymatic removal of sialic acids or sialomucins makes them more sensitive to NK cell cytotoxicity. Gene knockout experiments have revealed that the sialyltransferase ST6GalNAc-IV is responsible for the biosynthesis of disialyl-T (Neu5Acα2-3Galß1-3[Neu5Acα2-6]GalNAcα1-), which is the glycotope recognized by Siglec-7, and that CD162 and CD45 are the major carriers of this glycotope on CLL B cells. Analysis of public transcriptomic datasets indicated that the low expression of GCNT1 (encoding core 2 GlcNAc transferase, an enzyme that competes against ST6GalNAc-IV) and high expression of ST6GALNAC4 (encoding ST6GalNAc-IV) in CLL B cells, together enhancing the expression of the disialyl-T glycotope, are associated with poor patient prognosis. Taken together, our results determined the molecular basis of Siglec-7 ligand overexpression that protects CLL B cells from NK cell cytotoxicity and identified disialyl-T as a potential prognostic marker of CLL.

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.

The Potential Role of Krüppel-Like Zinc-Finger Protein Glis3 in Genetic Diseases and Cancers.

Gli-similar 3 (Glis3) belongs to a Glis subfamily of Krüppel-like zinc-finger transcription factors characterized to regulate a set of downstream targets essential for cellular functions, including pancreatic development, ß-cell maturation and maintenance, and insulin production. Examination of the DNA-binding domain of Glis3 reveals that this domain contains a repeated cysteine 2/histidine 2 (Cys2/His2) zinc-finger motif in the central region where the recognized DNA sequence binds. The loss of the production of pancreatic hormones, such as insulin 1 and 2, is linked to the down-regulation of ß cells-related genes and promotes the apoptotic death of ß cells found in mutant Glis3. Although accumulating studies converge on the Glis3 functioning in ß cells, recently, there have been developments in the field of Glis3 using knockdown/mutant mice to better understand the role of Glis3 in diseases. The Glis3 mutant mice have been characterized for their propensity to develop congenital hypothyroidism, polycystic kidney disease, and some types of cancer. In this review, we attempt to comprehensively summarize the knowledge of Glis3, including its structure and general function in cells. We also collected and organized the academic achievements related to the possible mechanisms of Glis3-related diseases.

Correction: Chon-Kit Chou, et al. The Regulations of Deubiquitinase USP15 and Its Pathophysiological Mechanisms in Diseases. Int. J. Mol. Sci. 2017, 18, 483.

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The Regulations of Deubiquitinase USP15 and Its Pathophysiological Mechanisms in Diseases.

Deubiquitinases (DUBs) play a critical role in ubiquitin-directed signaling by catalytically removing the ubiquitin from substrate proteins. Ubiquitin-specific protease 15 (USP15), a member of the largest subfamily of cysteine protease DUBs, contains two conservative cysteine (Cys) and histidine (His) boxes. USP15 harbors two zinc-binding motifs that are essential for recognition of poly-ubiquitin chains. USP15 is grouped into the same category with USP4 and USP11 due to high degree of homology in an N-terminal region consisting of domains present in ubiquitin-specific proteases (DUSP) domain and ubiquitin-like (UBL) domain. USP15 cooperates with COP9 signalosome complex (CSN) to maintain the stability of cullin-ring ligase (CRL) adaptor proteins by removing the conjugated ubiquitin chains from RBX1 subunit of CRL. USP15 is also implicated in the stabilization of the human papillomavirus type 16 E6 oncoprotein, adenomatous polyposis coli, and IκBα. Recently, reports have suggested that USP15 acts as a key regulator of TGF-ß receptor-signaling pathways by deubiquitinating the TGF-ß receptor itself and its downstream transducers receptor-regulated SMADs (R-SMADs), including SMAD1, SMAD2, and SMAD3, thus activating the TGF-ß target genes. Although the importance of USP15 in pathologic processes remains ambiguous so far, in this review, we endeavor to summarize the literature regarding the relationship of the deubiquitinating action of USP15 with the proteins involved in the regulation of Parkinson's disease, virus infection, and cancer-related signaling networks.