Partial characterization of the N-linked oligosaccharide structures on P-selectin glycoprotein ligand-1 (PSGL-1)

Integration of biomimicry and nanotechnology for significantly improved detection of circulating tumor cells (CTCs)

Circulating tumor cells (CTCs) have received a great deal of scientific and clinical attention as a biomarker for diagnosis and prognosis of many types of cancer. Given their potential significance in clinics, a variety of detection methods, utilizing the recent advances in nanotechnology and microfluidics, have been introduced in an effort of achieving clinically significant detection of CTCs. However, effective detection and isolation of CTCs still remain a tremendous challenge due to their extreme rarity and phenotypic heterogeneity. Among many approaches that are currently under development, this review paper focuses on a unique, promising approach that takes advantages of naturally occurring processes achievable through application of nanotechnology to realize significant improvement in sensitivity and specificity of CTC capture. We provide an overview of successful outcome of this biomimetic CTC capture system in detection of tumor cells from in vitro, in vivo, and clinical pilot studies. We also emphasize the clinical impact of CTCs as biomarkers in cancer diagnosis and predictive prognosis, which provides a cost-effective, minimally invasive method that potentially replaces or supplements existing methods such as imaging technologies and solid tissue biopsy. In addition, their potential prognostic values as treatment guidelines and that ultimately help to realize personalized therapy are discussed.

Molecular cloning and expression analysis of P-selectin glycoprotein ligand-1 from zebrafish (Danio rerio)

To date, the best characterized glycoprotein ligand for P-selectin is P-selectin glycoprotein ligand-1 (PSGL-1). In this study, we cloned the full-length cDNA of PSGL-1 from zebrafish (Danio rerio). Zebrafish PSGl-1 cDNA is 1,594 bp and encodes a putative 284 amino acid protein with a theoretical molecular weight of 30.33 kDa and isoelectric point of 7.96. A signal peptide of 27 amino acids is predicted. The putative protein contains an extracellular mucin-like domain, a transmembrane domain and a cytoplasmic domain, with homology to mammalian PSGL-1. In the putative P-selectin binding region, there are 1 potential tyrosine sulfation site and 12 potential threonine O-glycosylation sites. A single extracellular cysteine, at the junction of the extracellular and transmembrane domains, suggests a disulfide-bonding pattern. The amino acid sequence of zebrafish PSGL-1 is 19-22% identical to that of mammalian PSGL-1. RT-PCR and whole-mount in situ hybridization analysis revealed that zebrafish PSGL-1 was expressed in early embryonic development, and the expression has an increased trend from 0.2 (1-cell stage) to 72 hpf. The results indicate that the general domain structure of PSGL-1 protein is conserved among species, and zebrafish PSGL-1 plays important roles in embryonic development and probably has similar biological function to that of mammalian PSGL-1.

Direct measurements on CD24-mediated rolling of human breast cancer MCF-7 cells on E-selectin

Tumor cell rolling on the endothelium plays a key role in the initial steps of cancer metastasis, i.e., extravasation of circulating tumor cells (CTCs). Identification of the ligands that induce the rolling of cells is thus critical to understanding how cancers metastasize. We have previously demonstrated that MCF-7 cells, human breast cancer cells, exhibit the rolling response selectively on E-selectin-immobilized surfaces. However, the ligand that induces rolling of MCF-7 cells on E-selectin has not yet been identified, as these cells lack commonly known E-selectin ligands. Here we report, for the first time to our knowledge, a set of quantitative and direct evidence demonstrating that CD24 expressed on MCF-7 cell membranes is responsible for rolling of the cells on E-selectin. The binding kinetics between CD24 and E-selectin was directly measured using surface plasmon resonance (SPR), which revealed that CD24 has a binding affinity against E-selectin (K(D) = 3.4 ± 0.7 nM). The involvement of CD24 in MCF-7 cell rolling was confirmed by the rolling behavior that was completely blocked when cells were treated with anti-CD24. A simulated study by flowing microspheres coated with CD24 onto E-selectin-immobilized surfaces further revealed that the binding is Ca(2+)-dependent. Additionally, we have found that actin filaments are involved in the CD24-mediated cell rolling, as observed by the decreased rolling velocities of the MCF-7 cells upon treatment with cytochalasin D (an inhibitor of actin-filament dynamics) and the stationary binding of CD24-coated microspheres (the lack of actins) on the E-selectin-immobilized slides. Given that CD24 is known to be directly related to enhanced invasiveness of cancer cells, our results imply that CD24-based cell rolling on E-selectin mediates, at least partially, cancer cell extravasation, resulting in metastasis.

Structural characterisation of neutrophil glycans by ultra sensitive mass spectrometric glycomics methodology

Neutrophils are the most abundant white blood cells in humans and play a vital role in several aspects of the immune response. Numerous reports have implicated neutrophil glycosylation as an important factor in mediating these interactions. We report here the application of high sensitivity glycomics methodologies, including matrix assisted laser desorption ionisation (MALDI-TOF) and MALDI-TOF/TOF analyses, to the structural analysis of N- and O-linked carbohydrates released from two samples of neutrophils, prepared by two separate and geographically remote laboratories. The data produced demonstrates that the cells display a diverse range of sialylated and fucosylated complex glycans, with a high level of similarity between the two preparations.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

PSGL-1 from the murine leukocytic cell line WEHI-3 is enriched for core 2-based O-glycans with sialyl Lewis x antigen

Leukocyte trafficking involves specific recognition between P-selectin and L-selectin and PSGL-1 containing core 2-based O-glycans expressing sialyl Lewis x (SLe(x)) antigen. However, the structural identity of the glycan component(s) displayed by murine neutrophil PSGL-1 that contributes to its P-selectin counter-receptor activity has been uncertain, since these cells express little if any SLe(x) antigen, and because there have been no direct studies to examine murine PSGL-1 glycosylation. To address this uncertainty, we studied PSGL-1 glycosylation in the murine cell line WEHI-3 using metabolic-radiolabeling with (3)H-monosaccharide precursors to detect low-abundance O-glycan structures. We report that PSGL-1 from WEHI-3 cells expresses a di-sialylated core 2 O-glycan containing the SLe(x) antigen. This fucosylated O-glycan is scarce on PSGL-1 and essentially undetectable in total leukocyte glycoproteins from WEHI-3 cells. These results demonstrate that WEHI-3 cells selectively fucosylate PSGL-1 to generate functionally important core 2-based O-glycans containing the SLe(x) antigen.

Distinct kinetic and mechanical properties govern selectin-leukocyte interactions

Leukocytes are recruited from the bloodstream to sites of inflammation by the selectin family of adhesion receptors. In vivo and in vitro studies reveal distinctive rolling velocities of polymorphonuclear leukocytes over E-, P- and L-selectin substrates. The kinetic and mechanical properties of the selectin-ligand bonds responsible for these differences at the single-molecule level are not well understood. Using single-molecule force spectroscopy, we probe in situ the rupture force, unstressed off-rate and reactive compliance of single selectin receptors to single ligands on whole human polymorphonuclear leukocytes (PMNs) under conditions that preserve the proper orientation and post-translational modifications of the selectin ligands. Single L-selectin bonds to PMNs were more labile than either E- or P-selectin in the presence of an applied force. This outcome, along with a higher unstressed off-rate and a higher reactive compliance, explain the faster L-selectin-mediated rolling. By quantifying binding frequency in the presence of a specific blocking monoclonal antibody or following enzyme treatment, we determined that P-selectin glycoprotein ligand-1 is a high-affinity ligand for E-selectin on PMNs under force. The rupture force spectra and corresponding unstressed off-rate and reactive compliance of selectin-ligand bonds provide mechanistic insights that might help to explain the variable rolling of leukocytes over different selectin substrates.

Dimeric galectin-1 binds with high affinity to alpha2,3-sialylated and non-sialylated terminal N-acetyllactosamine units on surface-bound extended glycans

Galectin-1 is a member of the galectin family of glycan-binding proteins and occurs as an approximately 29.5-kDa noncovalent homodimer (dGal-1) that is widely expressed in many tissues. Here, we report that human recombinant dGal-1 bound preferentially and with high affinity (apparent K(d) approximately 2-4 microM) to immobilized extended glycans containing terminal N-acetyllactosamine (LN; Galbeta1-4GlcNAc) sequences on poly-N-acetyllactosamine (PL; (-3Galbeta1-4GlcNAcbeta1-)(n)) sequences, complex-type biantennary N-glycans, or novel chitin-derived glycans modified to contain terminal LN. Although terminal Gal residues are important for dGal-1 recognition, dGal-1 bound similarly to alpha3-sialylated and alpha2-fucosylated terminal LN, but not to alpha6-sialylated and alpha3-fucosylated terminal LN. The binding specificity of human recombinant dGal-1 was similar to that observed with purified bovine heart-derived dGal-1. Unexpectedly, dGal-1 bound free ligands in solution with relatively low affinity and displayed no preference for extended glycans, indicating that dGal-1 preferentially recognizes extended glycans only when they are surface-bound, such as found on cell surfaces. Human dGal-1 also bound to both native and desialylated human promyelocytic HL-60 cells with similar affinity as observed for immobilized long chain PL. Binding to these cells was reduced upon treatment with endo-beta-galactosidase, which cleaves PL sequences, indicating that cell-surface PLs are ligands. To test the role of dimerization in dGal-1 binding, we examined the binding of a mutated form of dGal-1 that weakly dimerizes (monomeric Gal-1 (mGal-1)) and a covalently dimerized (chemically cross-linked) form of mGal-1 (cd-mGal-1). dGal-1 and cd-mGal-1 had similar affinities that were both approximately 3.5-fold higher for immobilized PL than observed for mGal-1, suggesting that dGal-1 acts as a dimer to cross-link terminal LN units on immobilized PL. These results indicate that dGal-1 functions as a dimer to recognize LN units on extended PLs on cell surfaces.

Absorption of anti-blood group A antibodies on P-selectin glycoprotein ligand-1/immunoglobulin chimeras carrying blood group A determinants: core saccharide chain specificity of the Se and H gene encoded alpha1,2 fucosyltransferases in different host cells

To specifically eliminate recipient anti-blood group ABO antibodies prior to ABO-incompatible organ or bone marrow transplantation, an efficient absorber of ABO antibodies has been developed in which blood group determinants may be carried at high density and by different core saccharide chains on a mucin-type protein backbone. The absorber was made by transfecting different host cells with cDNAs encoding a P-selectin glycoprotein ligand-1/mouse immunoglobulin G(2b) chimera (PSGL-1/mIgG(2b)), the H- or Se-gene encoded alpha1,2-fucosyltransferases (FUT1 or FUT2) and the blood group A gene encoded alpha1,3 N-acetylgalactosaminyltransferase (alpha1,3 GalNAcT). Western blot analysis of affinity-purified recombinant PSGL-1/mIgG(2b) revealed that different precursor chains were produced in 293T, COS-7m6, and Chinese hamster ovary (CHO)-K1 host cells coexpressing FUT1 or FUT2. FUT1 directed expression of H type 2 structures mainly, whereas FUT2 preferentially made H type 3 structures. None of the host cells expressing either FUT1 or FUT2 supported expression of H type 1 structures. Furthermore, the highest A epitope density was on PSGL-1/mIgG2(2b) made in CHO-K1 cells coexpressing FUT2 and the alpha1,3 GalNAcT. This PSGL-1/mIgG(2b) was used for absorption of anti-blood group A antibodies in human blood group O serum. At least 80 times less A trisaccharides on PSGL-1/mIgG(2b) in comparison to A trisaccharides covalently linked to macroporous glass beads were needed for the same level of antibody absorption. In conclusion, PSGL-1/mIgG(2b), if substituted with A epitopes, was shown to be an efficient absorber of anti-blood group A antibodies and a suitable model protein for studies on protein glycosylation.