Lectin-based three-color flow cytometric approach for studying cell surface glycosylation changes that occur during apoptosis

Glycan Metabolic Fluorine Labeling for In Vivo Visualization of Tumor Cells and In Situ Assessment of Glycosylation Variations

The abnormality in the glycosylation of surface proteins is critical for the growth and metastasis of tumors and their capacity for immunosuppression and drug resistance. This anomaly offers an entry point for real-time analysis on glycosylation fluctuations. In this study, we report a strategy, glycan metabolic fluorine labeling (MEFLA), for selectively tagging glycans of tumor cells. As a proof of concept, we synthesized two fluorinated unnatural monosaccharides with distinctive 19 F chemical shifts (Ac4 ManNTfe and Ac4 GalNTfa). These two probes could undergo selective uptake by tumor cells and subsequent incorporation into surface glycans. This approach enables efficient and specific 19 F labeling of tumor cells, which permits in vivo tracking of tumor cells and in situ assessment of glycosylation changes by 19 F MRI. The efficiency and specificity of our probes for labeling tumor cells were verified in vitro with A549 cells. The feasibility of our method was further validated with in vivo experiments on A549 tumor-bearing mice. Moreover, the capacity of our approach for assessing glycosylation changes of tumor cells was illustrated both in vitro and in vivo. Our studies provide a promising means for visualizing tumor cells in vivo and assessing their glycosylation variations in situ through targeted multiplexed 19 F MRI.

Effect of Dexamethasone on the Expression of the α2,3 and α2,6 Sialic Acids in Epithelial Cell Lines

N-acetylneuraminic acid linked to galactose by α2,6 and α2,3 linkages (Siaα2,6 and Siaα2,3) is expressed on glycoconjugates of animal tissues, where it performs multiple biological functions. In addition, these types of sialic acid residues are the main targets for the binding and entry of influenza viruses. Here we used fluorochrome-conjugated Sambuccus nigra, Maackia amurensis, and peanut lectins for the simultaneous detection of Siaα2,3 and Siaα2,6 and galactosyl residues by two-color flow cytometry on A549 cells, a human pneumocyte cell line used for in vitro studies of the infection by influenza viruses, as well as on Vero and MDCK cell lines. The dexamethasone (DEX) glucocorticoid (GC), a widely used anti-inflammatory compound, completely abrogated the expression of Siaα2,3 in A549 cells and decreased its expression in Vero and MDCK cells; in contrast, the expression of Siaα2,6 was increased in the three cell lines. These observations indicate that DEX can be used for the study of the mechanism of sialylation of cell membrane molecules. Importantly, DEX may change the tropism of avian and human/pig influenza viruses and other infectious agents to animal and human epithelial cells.

O-GlcNAcylation in early stages of chronic lymphocytic leukemia; protocol development for flow cytometry

BACKGROUND

Recent studies proved that metabolic changes in malignant disorders have an impact on protein glycosylation, however, only a few attempts have been made so far to use O-GlcNAc analysis as a prognostic tool. Glucose metabolism is reported to be altered in hematological malignancies thus, we hypothesized that monitoring intracellular O-GlcNAc levels in Rai stage 0-I (Binet A) CLL patients could give deeper insights regarding subtle metabolic changes of progression which are not completely detected by the routine follow-up procedures.

OBJECTIVE

In this proof of concept study we established a flow cytometric detection method for the assessment of O-GlcNAcylation as a possible prognostic marker in CLL malignancy which was supported by fluorescence microscopy.

METHODS

Healthy volunteers and CLL patients were recruited for this study. Lymphocytes were isolated, fixed and permeabilised by various methods to find the optimal experimental condition for O-GlcNAc detection by flow cytometry. O-GlcNAc levels were measured and compared to lymphocyte count and various blood parameters including plasma glucose level.

RESULTS

The protocol we developed includes red blood cell lysis, formalin fixation, 0.1% Tween 20 permeabilisation and employs standardized cell number per sample and unstained controls. We have found significant correlation between O-GlcNAc levels and WBC (R2= 0.8535, p< 0.0029) and lymphocyte count (R2= 0.9225, p< 0.0006) in CLL patients. Interestingly, there was no such correlation in healthy individuals (R2= 0.05664 for O-GlcNAc vs WBC and R2= 0.04379 for O-GlcNAc vs lymphocytes).

CONCLUSION

Analyzing O-GlcNAc changes in malignant disorders, specifically in malignant hematologic diseases such as CLL, could be a useful tool to monitor the progression of the disease.

Apoptotic cells selectively uptake minor glycoforms of vitronectin from serum

Apoptosis profoundly alters the carbohydrate layer coating the membrane of eukaryotic cells. Previously we showed that apoptotic cells became reactive with the α2,6-sialyl-specific lectin from Sambucus nigra agglutinin (SNA), regardless of their histological origin and the nature of the apoptotic stimulus. Here we reveal the basis of the phenomenon by showing that in apoptotic cancer cell lines SNA reactivity was mainly associated with a 67 kDa glycoprotein which we identified by MALDI-TOF/TOF and immunoblot analysis as bovine vitronectin (bVN). bVN was neither present in non-apoptotic cells, nor in cells induced to apoptosis in serum-free medium, indicating that its uptake from the cell culture serum occurred only during apoptosis. The bVN molecules associated with apoptotic cancer cell lines represented minor isoforms, lacking the carboxyterminal sequence and paradoxically containing a few α2,6-linked sialic acid residues. Despite their poor α2,6-sialylation, these bVN molecules were sufficient to turn apoptotic cells to SNA reactivity, which is a late apoptotic event occurring in cells positive to both annexin-V and propidium iodide. Unlike in cancer cell lines, the major bVN form taken up by apoptotic neutrophils and mononuclear cells was a 80 kDa form. In apoptotic SW948 cells we also detected the α2,6-sialylated forms of the stress-70 mitochondrial precursor (mortalin) and of tubulin-β2C. These data indicate that the acquisition of vitronectin isoforms from the environment is a general, although cell specific phenomenon, potentially playing an important role in post-apoptotic events and that the α2,6-sialylation of intracellular proteins is a new kind of posttranslational modification associated with apoptosis.

Targeting aberrant sialylation in cancer cells using a fluorinated sialic acid analog impairs adhesion, migration, and in vivo tumor growth

Cancer cells decorate their surface with a dense layer of sialylated glycans by upregulating the expression of sialyltransferases and other glycogenes. Although sialic acids play a vital role in many biologic processes, hypersialylation in particular has been shown to contribute to cancer cell progression and metastasis. Accordingly, selective strategies to interfere with sialic acid synthesis might offer a powerful approach in cancer therapy. In the present study, we assessed the potential of a recently developed fluorinated sialic acid analogue (P-3F(ax)-Neu5Ac) to block the synthesis of sialoglycans in murine melanoma cells and the consequences on cell adhesion, migration, and in vivo growth. The results showed that P-3F(ax)-Neu5Ac readily caused depletion of α2,3-/α2,6-linked sialic acids in B16F10 cells for several days. Long-term inhibition of sialylation for 28 days was feasible without affecting cell viability or proliferation. Moreover, P-3F(ax)-Neu5Ac proved to be a highly potent inhibitor of sialylation even at high concentrations of competing sialyltransferase substrates. P-3F(ax)-Neu5Ac-treated cancer cells exhibited impaired binding to poly-l-lysine, type I collagen, and fibronectin and diminished migratory capacity. Finally, blocking sialylation of B16F10 tumor cells with this novel sialic acid analogue reduced their growth in vivo. These results indicate that P-3F(ax)-Neu5Ac is a powerful glycomimetic capable of inhibiting aberrant sialylation that can potentially be used for anticancer therapy.

High-throughput lectin microarray-based analysis of live cell surface glycosylation

Lectins, plant-derived glycan-binding proteins, have long been used to detect glycans on cell surfaces. However, the techniques used to characterize serum or cells have largely been limited to mass spectrometry, blots, flow cytometry, and immunohistochemistry. While these lectin-based approaches are well established and they can discriminate a limited number of sugar isomers by concurrently using a limited number of lectins, they are not amenable for adaptation to a high-throughput platform. Fortunately, given the commercial availability of lectins with a variety of glycan specificities, lectins can be printed on a glass substrate in a microarray format to profile accessible cell-surface glycans. This method is an inviting alternative for analysis of a broad range of glycans in a high-throughput fashion and has been demonstrated to be a feasible method of identifying binding-accessible cell surface glycosylation on living cells. The current unit presents a lectin-based microarray approach for analyzing cell surface glycosylation in a high-throughput fashion.

Incorporation of podoplanin into HIV released from HEK-293T cells, but not PBMC, is required for efficient binding to the attachment factor CLEC-2

BACKGROUND

Platelets are associated with HIV in the blood of infected individuals and might modulate viral dissemination, particularly if the virus is directly transmitted into the bloodstream. The C-type lectin DC-SIGN and the novel HIV attachment factor CLEC-2 are expressed by platelets and facilitate HIV transmission from platelets to T-cells. Here, we studied the molecular mechanisms behind CLEC-2-mediated HIV-1 transmission.

RESULTS

Binding studies with soluble proteins indicated that CLEC-2, in contrast to DC-SIGN, does not recognize the viral envelope protein, but a cellular factor expressed on kidney-derived 293T cells. Subsequent analyses revealed that the cellular mucin-like membranous glycoprotein podoplanin, a CLEC-2 ligand, was expressed on 293T cells and incorporated into virions released from these cells. Knock-down of podoplanin in 293T cells by shRNA showed that virion incorporation of podoplanin was required for efficient CLEC-2-dependent HIV-1 interactions with cell lines and platelets. Flow cytometry revealed no evidence for podoplanin expression on viable T-cells and peripheral blood mononuclear cells (PBMC). Podoplanin was also not detected on HIV-1 infected T-cells. However, apoptotic bystander cells in HIV-1 infected cultures reacted with anti-podoplanin antibodies, and similar results were obtained upon induction of apoptosis in a cell line and in PBMCs suggesting an unexpected link between apoptosis and podoplanin expression. Despite the absence of detectable podoplanin expression, HIV-1 produced in PBMC was transmitted to T-cells in a CLEC-2-dependent manner, indicating that T-cells might express an as yet unidentified CLEC-2 ligand.

CONCLUSIONS

Virion incorporation of podoplanin mediates CLEC-2 interactions of HIV-1 derived from 293T cells, while incorporation of a different cellular factor seems to be responsible for CLEC-2-dependent capture of PBMC-derived viruses. Furthermore, evidence was obtained that podoplanin expression is connected to apoptosis, a finding that deserves further investigation.

Regressing amphibian tail as a model for the cadherin/beta-catenin complex disruption and glycosylation alteration during epithelial apoptosis

Epidermis is one of the many tissues that are resorbed during metamorphosis in the regressing tail of amphibian tadpoles. Apoptotic mechanisms play an important role in this process. In this study, loss of intercellular contacts and alterations in plasma membrane glycosylation were observed during apoptosis. The cadherin/beta-catenin complex represents one of the major adhesive systems in multiple epithelial tissues. Here, we analysed the fate of cadherin/beta-catenin complex and alterations of plasma membrane glycoconjugate compositions in apoptotic epithelial cells. Our results showed that the cadherin molecules were cleaved into extracellular and beta-catenin associated cytosolic domains by an intracellular mechanism. However, the extracellular domains were probably removed completely by matrix metalloproteinases. Lectin histochemistry studies suggested that mannose and alpha(2-->6) linked (but not alpha(2-->3) linked) sialic acids were major sugar motifs in plasma membranes of apoptotic tadpole epithelial cells. Although previous studies indicated reduced levels of sialic acid residues during apoptosis, elevated Sambucus nigra agglutinin (SNA) reactivity might be due to the degradation of high molecular weight glycoproteins (probably including cadherin) that masked the SNA-binding residues of the plasma membrane prior to apoptosis.

Lectin microarrays identify cell-specific and functionally significant cell surface glycan markers

Glycosylation is among the most complex posttranslational modifications with an extremely high level of diversity that has made it refractory to high-throughput analyses. Despite its resistance to high-throughput techniques, glycosylation is important in many critical cellular processes that necessitate a productive approach to their analysis. To facilitate studies in glycosylation, we developed a high-throughput lectin microarray for defining mammalian cell surface glycan signatures. Using the lectin microarray we established a binary analysis of cell binding and hierarchical organization of 24 mammalian cell lines. The array was also used to document changes in cell surface glycosylation during cell development and differentiation of primary murine immune system cells. To establish the biological and clinical importance of glycan signatures, the lectin microarray was applied in two systems. First, we analyzed the cell surface glycan signatures and were able to predict mannose-dependent tropism using a model pathogen. Second, we used the glycan signatures to identify novel lectin biomarkers for cancer stem-like cells in a murine model. Thus, lectin microarrays are an effective tool for analyzing diverse cell processes including cell development and differentiation, cell-cell communication, pathogen-host recognition, and cell surface biomarker identification.

Human galectins as sensors for apoptosis/necrosis-associated surface changes of granulocytes and lymphocytes

Changes in the glycomic profile can significantly affect the cells' communication with the environment. Plant lectins have so far been used to address the issue as to whether the courses of apoptosis or necrosis are associated with such alterations. We, here, initiate the study of members of the family of functionally pleiotropic human galectins in this respect. Established protocols for the induction of apoptosis/necrosis of blood cells and for flow cytometry using annexin V/propidium iodide were combined with cell surface staining using biotinylated galectins at a nontoxic concentration. The galectin panel covered members from all three subfamilies. Flow cytometry revealed specific binding of galectins to viable control cells and conspicuous staining differences when testing apoptotic or necrotic cells. Onset and especially progression of cell death led to pronounced reactivity with the proto-type galectins-1, -2, and -7 and tandem-repeat-type galectin-4. Extent of staining depended on the nature and stage of cell death, type of dying cell, and type of galectin. Galectins act as sensors for cell-death-associated surface changes. Staining of late-apoptotic polymorphonuclear cells was particularly strong. Examining the functional significance of this result may reveal a new aspect within the surveillance system to protect against autoinflammation.

Search for novel cell surface markers of apoptotic cells

Surface markers of apoptotic cells are of great interest as potential targets for non-destructive detection and study of these cells. They are also important for apoptotic cell recognition and subsequent clearance by cells of the immune system. Recently, it was found that apoptosis is accompanied by not only the loss of plasma membrane asymmetry detected by Annexin V, but also by changes in cell surface glycoconjugates. These novel markers of apoptosis are alpha-D-mannose and beta-D-galactose-specific plasma membrane glycoproteins whose expression is substantially increased after induction of apoptosis. The glyconeoepitopes described in this article are proposed to be useful for both, the detection of apoptotic cells and the isolation of the latter, from mixed populations.