Selectins and other mammalian sialic acid-binding lectins

Inhibition of acid sialidase by inorganic sulfate

Sulfated glycosaminoglycans are known to inhibit mammalian acid-active sialidase. Although the inhibition depends clearly on the presence of sulfate groups on these macromolecules, there was no information on the intrinsic inhibitory potency of inorganic sulfate. In this study, we demonstrate that inorganic sulfates inhibit acid-active Mu-Neu5Ac sialidase of U937 cells. This inhibition was found to be reversible and it appeared to be of the mixed competitive type. Sulfate-induced inhibition was also observed in other cells as well as with other substrates such as sialyl lactose and bovine mixed brain gangliosides. We conclude that the intrinsic inhibitory potency of sulfate groups may be significantly involved in the inhibition of acid-active sialidase by sulfated glycosaminoglycans. In addition, inorganic sulfate by its apparent potency to selectively inhibit acid sialidases might constitute an interesting tool for the characterisation of the minor forms of sialidases occurring in mammalian cells.

Zona pellucida glycoprotein mZP3 bioactivity is not dependent on the extent of glycosylation of its polypeptide or on sulfation and sialylation of its oligosaccharides

During fertilization in mice, free-swimming sperm bind to mZP3, one of three egg zona pellucida glycoproteins. Sperm recognize and bind to specific serine/threonine-linked (O-linked) oligosaccharides located at the mZP3 combining site for sperm. Shortly after binding to mZP3, sperm undergo the acrosome reaction, a form of cellular exocytosis. Here, we examined the influence of extent of glycosylation, sulfation, and sialylation of mZP3 (M(r) approximately 65,000-100,000) on its bioactivity; i.e. its ability to inhibit binding of sperm to eggs and to induce the acrosome reaction in vitro. Low (av. M(r) approximately 70,000), medium (av. M(r) approximately 82,000), and high (av. M(r) approximately 94,000) M(r) fractions of mZP3 were purified and shown to vary in extent of asparagine-linked (N-linked) glycosylation. All three size-fractions exhibited bioactivity, suggesting that the ability of mZP3 to inhibit binding of sperm to eggs is not related to the extent of glycosylation of its polypeptide (M(r) approximately 44,000). Digestion of mZP3 by neuraminidase decreased its average M(r) from approximately 83,000 to approximately 77,000 and increased its average pI from approximately 4.7 to approximately 6.0, but did not significantly affect mZP3 bioactivity. Terminal sialic acid largely accounts for the glycoprotein's acidic nature, but is not an essential element of the mZP3 combining site for sperm. Experiments with stably transfected embryonal carcinoma (EC) cells that secrete bioactive EC-mZP3 revealed that, of the sulfate present, approximately 70–75% was located on N-linked and approximately 25–30% on O-linked oligosaccharides. EC-mZP3 devoid of sulfate inhibited binding of sperm to eggs and induced the acrosome reaction to the same extent as sulfated EC-mZP3. These results suggest that sulfation of EC-mZP3 oligosaccharides is not essential for bioactivity. Overall, these findings contrast with those reported for certain other glycoproteins involved in cellular adhesion that require sulfate and/or sialic acid for bioactivity.

The biology of colorectal cancer metastasis

Metastasis is a complex process involving a series of tumor-host interactions that follow tumor initiation and progression. Metastatic cancer cells must be capable of completing all stages of this process to establish secondary foci at distant sites. Most studies infer the functional significance of various observations based on models designed to emphasize the contributions of single molecules or ligand-receptor pairs. While providing useful insights into potential mechanisms of tumorigenesis and metastasis, additional studies are needed to elucidate the complex dynamic interactions involved in these processes. Nonetheless, a better understanding of how tumor cells metastasize is leading to new therapies to prevent and treat micrometastatic disease.

Constitutively hyposialylated human T-lymphocyte clones in the Tn-syndrome: binding characteristics of plant and animal lectins

Previously, beta 1,3-galactosyltransferase-deficient (Tn+) and normal (TF+) T-lymphocyte clones have been established from a patient suffering from Tn-syndrome [Thurnher et al. (1992) Eur J Immunol 22: 1835-42]. Tn+ T lymphocytes express only Tn antigen GalNAc alpha 1-O-R) while other O-glycan structures such as sialosyl-Tn (Neu5Ac alpha 2,6GalNAc alpha 1-O-R) or TF (Gal beta 1-3GalNAc alpha 1-O-R) antigens are absent from these cells as shown by flow cytometry using specific mABs for TF and sialosyl-Tn antigen, respectively. Normal T lymphocytes express the TF antigen and derivatives thereof. The surface glycans of Tn+ and TF+ cells were then analysed by flow cytometry using the following sialic acid-binding lectins: Amaranthus caudatus (ACA), Maackia amurensis (MAA), Limax flavus (LFA), Sambucus nigra (SNA) and Triticum vulgare (WGA). Equal and weak binding of MAA and SNA to both TF+ and Tn+ cells was found. WGA, LFA and ACA bound more strongly to TF+ cells than to Tn+ cells. Binding of ACA to TF+ cells was enhanced after sialidase treatment. To investigate the possible biological consequences of hyposialylation, binding of three sialic acid-dependent adhesion molecules to Tn+ and TF+ cells was estimated using radiolabelled Fc-chimeras of sialoadhesin (Sn), myelin-associated glycoprotein (MAG) and CD22. Equal and strong binding of human CD22 to both TF+ and Tn+ cells was found. Whereas binding of Sn and MAG to TF+ cells was strong (100%), binding to Tn+ cells amounted only to 33% (Sn) and 19% (MAG). These results indicate that the in vivo interactions of T lymphocytes in the Tn syndrome with CD22 are not likely to be affected, whereas adhesion mediated by Sn or MAG could be strongly reduced.

Expression cloning of the Golgi CMP-sialic acid transporter

Translocation of nucleotide sugars across the membrane of the Golgi apparatus is a prerequisite for the synthesis of complex carbohydrate structures. While specific transport systems for different nucleotide sugars have been identified biochemically in isolated microsomes and Golgi vesicles, none of these transport proteins has been characterized at the molecular level. Chinese hamster ovary (CHO) mutants of the complementation group Lec2 exhibit a strong reduction in sialylation of glycoproteins and glycolipids due to a defect in the CMP-sialic acid transport system. By complementation cloning in the mutant 6B2, belonging to the Lec2 complementation group, we were able to isolate a cDNA encoding the putative murine Golgi CMP-sialic acid transporter. The cloned cDNA encodes a highly hydrophobic, multiple membrane spanning protein of 36.4 kDa, with structural similarity to the recently cloned ammonium transporters. Transfection of a hemagglutinin-tagged fusion protein into the mutant 6B2 led to Golgi localization of the hemagglutinin epitope. Our results, together with the observation that the cloned gene shares structural similarities to other recently cloned transporter proteins, strongly suggest that the isolated cDNA encodes the CMP-sialic acid transporter.

CMP-N-acetylneuraminic acid hydroxylase: the first cytosolic Rieske iron-sulphur protein to be described in Eukarya

Electron paramagnetic resonance (EPR) spectroscopy and analysis of the primary structure of the CMP-N-acetylneuraminic acid hydroxylase revealed that this enzyme is the first iron-sulphur protein of the Rieske type to be found in the cytosol of Eukarya. The dithionite-reduced hydroxylase exhibited an EPR signal known to be characteristic for a Rieske iron-sulphur centre (2Fe-2S), the g-values being 1.78, 1.91 and 2.01, respectively. An analysis of the primary structure of the hydroxylase led to the identification of an amino acid sequence, known to be characteristic for Rieske proteins. Furthermore, possible binding sites for cytochrome b5, the substrate CMP-Neu5Ac and a mononuclear iron centre were also identified.

Genomic organization and chromosomal mapping of the Gal beta 1,3GalNAc/Gal beta 1,4GlcNAc alpha 2,3-sialyltransferase

In this report we describe the chromosome mapping and genomic organization of the human Gal beta 1,3GalNAc/Gal beta 1,4GlcNAc alpha 2,3-sialyltransferase gene. The gene is localized to human chromosome 11(q23-q24) by in situ hybridization of metaphase chromosomes. It spans more than 25 kilobases of human genomic DNA and is distributed over 14 exons that range in size from 61 to 679 base pairs. Previous characterization of cDNAs encoding the Gal beta 1,3GalNAc/Gal beta 1,4GlcNAc alpha 2,3-sialyltransferase revealed that the gene produces at least three transcripts in human placenta, which code for identical protein sequences except at the 5' ends (Kitagawa, H., and Paulson, J. C. (1994a) J. Biol. Chem. 269, 1394-1401). Repeated screening for clones that contain the 5' end of the cDNA has identified two additional distinct mRNAs that are expressed in human placenta. Comparison of the genomic DNA sequence with that of the five different mRNAs indicates that these transcripts are produced by a combination of alternative splicing and alternative promoter utilization. Northern analysis indicated that one of them is specifically expressed in placenta, testis, and ovary, indicating that its expression is independently regulated from the others.

Reduction of Myocardial Infarct Size in the Rabbit by a Carbohydrate Analog of Sialyl Lewis(x)

BACKGROUND: Available data suggest that the accumulation of neutrophils within the myocardium following an ischemic event plays an important role in the pathogenesis of myocardial ischemia/reperfusion injury. It is of interest, therefore, to develop pharmacologic agents designed to inhibit neutrophil adhesion to the endothelium. METHODS AND RESULTS: A synthetic carbohydrate analog to the P-selectin ligand sialyl Lewis(x) (sLe(x)) was evaluated for its ability to protect the myocardium from ischemia/reperfusion injury. Open chest anesthetized rabbits were subjected to 30 minutes occlusion of the left circumflex artery followed by 5 hours of reperfusion. Vehicle or sLe(x) analog (10 mg/kg) was administered intravenously before the onset of reperfusion and every hour during the reperfusion period. Myocardial infarct size in rabbits treated with the sLe(x) analog (10 mg/kg) was administered intravenously before the onset of reperfusion and every hour during the reperfusion period. Myocardial infarct size in rabbits treated with the sLe(x) analog was significantly reduced when compared to rabbits treated with vehicle (28 +/- 9% vs 57 +/- 10% of the area at risk, p <.05). The compound did not alter circulating neutrophil counts or myocardial oxygen demand as determined by the rate-pressure product. Furthermore, neutrophil accumulation within the ischemic region was decreased by 44% (P <.05) in the hearts of animals receiving sLe(x) analog as compared to vehicle. CONCLUSIONS: Carbohydrate derivatives of sLe(x) may be effective in reducing the degree of myocardial injury after ischemia/reperfusion.

Enzymatic transfer of sialic acids modified at C-5 employing four different sialyltransferases

We present kinetic studies on the enzymatic transfer of several synthetic sialic acid analogues, modified at C-5, to distinct glycoprotein glycans by sialyltransferases differing in acceptor- and linkage-specificity. Biochemical properties of sialic acids were modified by introducing formyl-, trifluoroacetyl-, benzyloxy-carbonyl-, and aminoacetyl-groups to the amino group at C-5 of neuraminic acid. The latter substitution renders the corresponding alpha-glycoside resistant towards sialidases. The respective CMP-sialic acid analogues were prepared by CMP-sialic acid synthase with a yield of 13-55%. The kinetic parameters of several sialyltransferases for the 5-substituted CMP-glycosides differed significantly. Relative to parent CMP-NeuAc, reaction rates of human- and rat liver Gal beta 1, 4GlcNAc alpha 2,6-sialyltransferases ranged from 50 to 170%, of GalNAc alpha 2,6-sialyltransferases from 40-140%, and of Gal beta 1,3Gal-NAc alpha 2,3-sialyltransferase from 20-50%. Resialylation of asialo-alpha 1-acid glycoprotein by 5-N-formyl- and 5-N-aminoacetyl-neuraminic acid employing rat liver Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase proceeded to about 80% of galactose sites which is identical to the extent achieved with parent NeuAc. According to our data, neosialoglycoconjugates which carry sialic acids modified at the N-acetyl group can be prepared for structure-function analysis, as this position seems crucial for recognition of adhesion proteins and influenza viruses.

The expression of CMP-NeuAc: Gal beta 1,4GlcNAc alpha 2,6 sialyltransferase [EC 2.4.99.1] and glycoproteins bearing alpha 2,6-linked sialic acids in human brain tumours

The expression of CMP-NeuAc: Gal beta 1,4GlcNAc alpha 2,6 sialyltransferase (alpha 2,6-ST) [EC 2.4.99.1] and glycoproteins bearing alpha 2,6-linked sialic acids were examined in primary human brain tumours and cell lines. 79% (19/24) of the meningiomas expressed alpha 2,6-ST mRNA, 42% (10/24) of which showed very high expression. alpha 2,6-ST mRNA expression was undetectable in normal brain tissue. In contrast, only 1/13 of the gliomas examined expressed detectable alpha 2,6-ST mRNA. Metastases to the brain did not express measurable amounts of alpha 2,6-ST mRNA. Less expression was found in malignant (i.e. anaplastic) compared to benign (i.e. meningothelial) meningiomas. Two-dimensional SDS-PAGE of glioma and meningioma proteins, followed by Sambucus nigra lectin staining, revealed the presence of a glycoprotein bearing alpha 2,6-linked sialic acids, M(r) = 53 kDa and a pI = 7.0 (MEN-1) that appeared in all seven of the meningiomas examined, but was expressed at barely detectable levels, if at all, in seven out of the seven glioblastomas examined. Thus, decreased alpha 2,6-ST expression may play a role in the aggressive nature of anaplastic meningiomas, but appears to be virtually absent in all tumours of glial origin.

A human STX cDNA confers polysialic acid expression in mammalian cells

Polysialic acid, or PSA, is a term used to refer to linear homopolymers of alpha(2,8)-sialic acid residues displayed at the surface of some mammalian cells. PSA is typically linked to the neural cell adhesion molecule N-CAM, where it can modulate the homotypic adhesive properties of this polypeptide. PSA expression is developmentally regulated, presumably through mechanisms involving regulated expression of sialyltransferases involved in PSA biosynthesis. Several different sialytransferase sequences have been implicated in PSA expression, although the precise roles of these enzymes in this context remain unclear. One such sequence, termed STX, maintains approximately 59% amino acid sequence identity with another sialyltransferase (PST-1, from hamster; PST, human) that is known to participate in PSA expression. While a murine STX fusion protein can catalyze the synthesis of a single alpha(2,8)-sialic acid linkage in vitro, the ability of STX to participate in PSA expression in vivo has not been demonstrated. We show here that STX transcripts are present in a PSA-positive, N-CAM-positive human small cell carcinoma line (NCI-H69/F3), but are absent in a variant of this line (NCI-H69/E2) selected to be PSA-negative and N-CAM-positive. To functionally confirm this correlation, we have cloned a human cDNA encoding the human STX sequence, and show, by transfection studies, that human STX can restore PSA expression when expressed in the PSA-negative, N-CAM-positive small cell carcinoma variant. We furthermore show that STX can confer PSA expression when expressed in a PSA-negative, N-CAM-positive murine cell line (NIH-3T3 cells), or when expressed in PSA-negative, N-CAM-negative COS-7 cells. These observations imply that STX, like PST-1/PST, can determine PSA expression in vivo. When considered together with the correlation between STX expression and PSA expression in vivo in the brain, these results suggest a regulatory role for STX in PSA expression in the developing central nervous system and small cell lung carcinoma.

Decreased sialidase activity in mononuclear leucocytes of type 1 diabetic subjects: relationship to diabetic complications and glycaemic control

Leucocyte surface sialic acid content influences surface charge, deformability, and leucocyte-endothelial interaction. Abnormal leucocyte structure and function contributes both to microvascular damage and diabetic complications. The aim of this study was to investigate altered leucocyte SA metabolism in diabetic subjects and measure lysosomal sialidase which regulates leucocyte surface sialylation. We examined 26 Type 1 (insulin-dependent) diabetic subjects with retinopathy, 26 Type 1 diabetic subjects without complications, and 38 matched normal control subjects. Sialidase was assayed in freshly prepared sonicates of pure mononuclear leucocytes (MNLs), using the fluorometric substrate 4-methyl-umbelliferyl-N-acetylneuraminic acid. In the subjects with diabetes there was a significant negative correlation between MNL sialidase activity and both HbA1c (rs = 0.37, p = 0.007) and fructosamine (rs = -0.31, p = 0.026). MNL sialidase activity was significantly decreased in diabetic subjects with clinical evidence of complications compared to control subjects. HbA1c was significantly higher (p = 0.036) in diabetic patients with complications compared to those without. The observed decrease in MNL sialidase activity related to diabetic control may be important in the pathogenesis of vascular damage. Diabetes-associated changes in sialylation of functional cell surface glycoconjugates may have important clinical consequences.

Molecular cloning of Sia alpha 2,3Gal beta 1,4GlcNAc alpha 2,8-sialyltransferase from mouse brain

A cDNA encoding a new alpha 2,8-sialyltransferase (ST8Sia III), which exhibits activity toward the Sia alpha 2,3Gal beta 1, 4GlcNAc sequences of N-linked oligosaccharides, was cloned from mouse brain by means of the polymerase chain reaction-based approach. The predicted amino acid sequence of ST8Sia III showed 27.6 and 34.4% identity with those of so far cloned mouse alpha 2,8-sialyltransferases, i.e. GD3 synthase (ST8Sia I) and STX (ST8Sia II), respectively. Transfection of the protein A-fused ST8Sia III gene into COS-7 cells led to alpha 2,8-sialyltransferase activity toward sialylated glycoproteins and alpha 2,3-sialylated glycosphingolipids, such as alpha 2,3-sialylparagloboside and GM3. However, the kinetic properties of ST8Sia III revealed that it is much more specific to N-linked oligosaccharides of glycoproteins than glycosphingolipids. The expression pattern of the ST8Sia III gene was clearly different from those of other alpha 2,8-sialyltransferase genes. The expression of the ST8Sia III gene was tissue and stage specific. The ST8Sia III gene was expressed only in brain and testis, and it appeared first in 20 postcoitum embryonal brain and then decreased. Therefore, the new alpha 2,8-sialyltransferase is closely involved in brain development.

Substrate specificity and inhibition of UDP-GlcNAc:GlcNAc beta 1-2Man alpha 1-6R beta 1,6-N-acetylglucosaminyltransferase V using synthetic substrate analogues

UDP-GlcNAc:GlcNAc beta 1-2Man alpha 1-6R (GlcNAc to Man) beta 1,6- N-acetylglucosaminyltransferase V (GlcNAc-T V) adds a GlcNAc beta 1-6 branch to bi- and triantennary N-glycans. An increase in this activity has been associated with cellular transformation, metastasis and differentiation. We have used synthetic substrate analogues to study the substrate specificity and inhibition of the partially purified enzyme from hamster kidney and of extracts from hen oviduct membranes and acute myeloid leukaemia leukocytes. All compounds with the minimum structure GlcNAc beta 1-2Man alpha 1-6Glc/Man beta-R were good substrates for GlcNAc-T V. The presence of structural elements other than the minimum trisaccharide structure affected GlcNAc-T V activity without being an absolute requirement for activity. Substrates with a biantennary structure were preferred over linear fragments of biantennary structures. Kinetic analysis showed that the 3-hydroxyl of the Man alpha 1-3 residue and the 4-hydroxyl of the Man beta- residue of the Man alpha 1-6(Man alpha 1-3)Man beta-R N-glycan core are not essential for catalysis but influence substrate binding. GlcNAc beta 1-2(4,6-di-O-methyl-)Man alpha 1-6Glc beta-pnp was found to be an inhibitor of GlcNAc-T V from hamster kidney, hen oviduct microsomes and acute and chronic myeloid leukaemia leukocytes.

Chicken B cells undergo discrete developmental changes in surface carbohydrate structure that appear to play a role in directing lymphocyte migration during embryogenesis

The migration of progenitor cells to specific microenvironments is essential for the development of complex organisms. Avian species possess a unique primary lymphoid organ, the bursa of Fabricius, that plays a central role in the development of B cells. B cell progenitors, however, arise outside the bursa of Fabricius and, during embryonic development, must migrate through the vasculature to the bursa of Fabricius. In this report, we demonstrate that these progenitor B cells express the sialyl Lewis × carbohydrate structure previously shown to be a ligand for the selectin family of vascular adhesion receptors. Soon after migration to the bursa of Fabricius, B cell progenitors are induced to undergo a developmental switch and terminate the expression of sialyl Lewis × in a temporal pattern that correlates with the developmental decline in the ability of these cells to home to the bursa of Fabricius upon transplantation. The induction of the developmental switch in the glycosylation pattern of developing B cells requires the bursal environment. In addition, sialyl Lewis × carbohydrate determinants or structurally similar determinants on the surface of immortalized bursal lymphoid stem cells participate in the adherence of these cells to the vascular regions of the bursal microenvironment. These data demonstrate that the carbohydrate structure sialyl Lewis × is developmentally regulated during chicken B cell development and may facilitate the migration of B cell progenitors to the bursal microenvironment by serving as a ligand for a lectin-like adhesion receptor.

Cyclohexylamine inhibits the adhesion of lymphocytic cells to human syncytiotrophoblast

We have previously shown that lymphocytic cells adhere to cultured syncytiotrophoblast and that this may be important in the lymphocyte-mediated infection of trophoblast with the human immunodeficiency virus (HIV). During the course of studies aimed at investigating the role of cell surface carbohydrates in adhesion, it was discovered that a contaminant of commercial fucose-1-phosphate, dicyclohexylamine, inhibited MOLT-trophoblast adhesion. Dicyclohexylamine and the related compounds, cyclohexylamine and hexylamine, inhibited adhesion in a dose-responsive manner with half-maximal inhibition seen at about 4 mM. While the pressor effects of cyclohexylamine, the principal metabolite of cyclamate, are well known, this is the first report of an effect of this and related compounds on cell adhesion activity. The inhibitory effect was reversible and, at concentrations less than 25 mM, did not result in loss of cell viability. Several possible mechanisms of action of cyclohexylamine were examined in an attempt to explain the effect on adhesion. No evidence was found to suggest that the effects of cyclohexylamine were due to inhibition of polyamine synthesis, increase in intracellular Ca2+ concentration or to a lysosomotropic effect. The concentrations of cyclohexylamine used are within the range of plasma concentrations attainable in humans, raising the possibility that the in vitro effects described here may also occur in vivo. The results also suggest that caution should be used in the interpretation of results obtained from experiments where cell adhesion is blocked using exogenous monosaccharides that are in the form of dicyclohexylammonium salts. Appropriate controls must be included or, if possible, sodium, potassium or barium salts should be chosen.

CD22-mediated cell adhesion to cytokine-activated human endothelial cells. Positive and negative regulation by alpha 2-6-sialylation of cellular glycoproteins

We previously showed that cultured human umbilical vein endothelial cells (HEC) exposed to the inflammatory cytokines tumor necrosis factor-alpha or interleukin-1 display increased activity of beta-galactoside alpha 2,6-sialyltransferase. This is associated with enhanced expression of ligands for the B cell receptor CD22 beta, which recognizes alpha 2-6-linked sialic acids (Hanasaki, K., Varki, A., Stamenkovic, I., and Bevilacqua, M. P. (1994) J. Biol. Chem. 269, 10637-10643). Here we report that increased expression of CD22 ligands is a feature of dermal microvascular endothelial cells as well, and is also observed in response to the cytokine interleukin-4. Tumor necrosis factor-alpha stimulation of HEC causes no change in the profile of endothelial glycoproteins recognized by CD22, but doubles the proportion of total cellular N-linked oligosaccharides capable of binding tightly to CD22. This modest change is sufficient to cause a marked increase in alpha 2-6-linked sialic acid-dependent binding of Chinese hamster ovary (CHO) cells expressing recombinant human CD22. In contrast, B lymphoma cell lines expressing higher levels of cell surface CD22 do not show such sialic acid-dependent binding to activated HEC. Since B lymphoma cells themselves also express high levels of alpha 2-6-linked sialic acids, their CD22 molecules might be rendered nonfunctional by endogenous ligands. In support of this, the lectin function of CD22 can be directly detected on transfected CHO cells, but not on B lymphoma cells. Furthermore, coexpression of beta-galactoside alpha 2,6-sialyltransferase with CD22 in the CHO cells abrogates sialic acid-dependent binding to cytokine-activated HEC. However, such co-transfected cells can bind to B lymphoma cells in a manner apparently less dependent upon alpha 2-6-linked sialic acid, suggesting CD22-mediated interactions that may not be directly dependent on its lectin function. Thus, CD22-mediated interactions between B cells and activated vascular endothelium may be positively regulated by induction of alpha 2-6-linked sialic acid-bearing endothelial cell ligands, but negatively regulated by such ligands on the B cells expressing CD22. Since expression of both CD22 and beta-galactoside alpha 2,6-sialyltransferase are regulated during B cell ontogeny, these findings could be of importance in B cell function and/or trafficking.

Enzymatic activity of a developmentally regulated member of the sialyltransferase family (STX): evidence for alpha 2,8-sialyltransferase activity toward N-linked oligosaccharides

We have detected sialyltransferase activity of recombinant mouse STX, which was cloned from rat brain as a new member of the sialyltransferase family, but sialyltransferase activity of which had not been detected previously [Livingston and Paulson, J. Biol. Chem. (1993) 268, 11504-11507]. The activity of mouse STX was specific toward sialylated glycoproteins. N-Glycanase treatment and linkage-specific sialidase treatment of glycoproteins revealed that STX transfers sialic acids through alpha 2,8-linkages to only N-linked oligosaccharides of glycoproteins. However, polymerase activity for polysialic acid synthesis was not detected for this sialyltransferase. Since this alpha 2,8-sialyltransferase gene is highly restricted in fetal and newborn brain, it may be involved in the polysialylation of glycoproteins, especially of N-CAM.

P-selectin glycoprotein ligand-1 mediates rolling of human neutrophils on P-selectin

Neutrophils roll on P-selectin expressed by activated platelets or endothelial cells under the shear stresses in the microcirculation. P-selectin glycoprotein ligand-1 (PSGL-1) is a high affinity ligand for P-selectin on myeloid cells. However, it has not been demonstrated that PSGL-1 contributes to the rolling of neutrophils on P-selectin. We developed two IgG mAbs, PL1 and PL2, that appear to recognize protein-dependent epitopes on human PSGL-1. The mAbs bound to PSGL-1 on all leukocytes as well as on heterologous cells transfected with PSGL-1 cDNA. PL1, but not PL2, blocked binding of 125-I-PSGL-1 to immobilized P-selectin, binding of fluid-phase P-selectin to myeloid and lymphoid leukocytes, adhesion of neutrophils to immobilized P-selectin under static conditions, and rolling of neutrophils on P-selectin-expressing CHO cells under a range of shear stresses. PSGL-1 was localized to microvilli on neutrophils, a topography that may facilitate its adhesive function. These data indicate that (a) PSGL-1 accounts for the high affinity binding sites for P-selectin on leukocytes, and (b) PSGL-1 must interact with P-selectin in order for neutrophils to roll on P-selectin at physiological shear stresses.

Sialoadhesin binds preferentially to cells of the granulocytic lineage

Sialoadhesin is a macrophage-restricted, sialic acid-dependent receptor of 185 kD that binds to the oligosaccharide sequence NeuAc alpha 2,3Gal on cell surface glycoconjugates. Recent cDNA cloning has shown that sialoadhesin is a new member of the immunoglobulin superfamily with sequence similarity to CD22, a sialic acid-dependent receptor of B lymphocytes. Sialoadhesin has been implicated in cellular interactions of stromal macrophages with developing myeloid cells. In this study, direct evidence for this interaction was obtained in cell-cell binding assays using both native and recombinant forms of the protein. In all assays, sialoadhesin exhibited specific, differential binding to various murine cell populations of hemopoietic origin. In rank order, sialoadhesin bound neutrophils > bone marrow cells = blood leukocytes > lymphocytes > thymocytes. Single-cell analyses confirmed that sialoadhesin selectively bound myeloid cells in complex cell mixtures obtained from the bone marrow and blood. In comparison, a recombinant Fc-chimeric form of murine CD22 showed high binding to B and T lymphocytes, but very low binding to immature and mature myeloid cells. These results are consistent with the notion that sialoadhesin in involved in interactions with granulocytes at different stages of their life histories.

Adhesion of carcinoma cells to rat hepatocytes and rat fibronectin is inhibited by the OPAR monoclonal antibody, which is directed against a rat liver-specific carbohydrate epitope

The OPAR mouse monoclonal antibody (mAb) directed against rat hepatocytes was previously shown to inhibit adhesion of TA3/Ha mammary carcinoma cells to hepatocytes. The antigen is abundantly present at the surface of hepatocytes beneath the endothelium of liver capillaries where we have observed invasion of carcinoma cells to occur. The OPAR mAb reacted with three major bands on a Western blot of liver plasma membrane proteins. The same proteins were also seen upon immunoprecipitation from iodinated liver plasma membrane proteins. We have isolated OPAR antigens by lectin wheat germ agglutinin (WGA) and OPAR affinity chromatography. Amino acid sequence analysis revealed that two of the bands were alpha 1-macroglobulin and C4-binding protein, which are serum components produced by hepatocytes. The presence of the epitope on distinct proteins and our previous observation that it can be detected in the Golgi apparatus but not in the endoplasmic reticulum, suggested that OPAR reacts with a liver-specific glycoconjugate. Loss of OPAR reactivity after neuraminidase and N-glycosidase F treatment showed that the epitope contains sialic acid residues on N-linked sugar moieties. OPAR also reacted with rat fibronectin, and inhibited adhesion of TA3/St cells to fibronectin. This explains the inhibition by the OPAR mAb of TA3/St cell adhesion to hepatocytes, which we have shown to be due mainly to interaction with hepatocyte surface-associated fibronectin. However, adhesion of the related TA3/Ha cells to hepatocytes, which is mediated by the alpha 6 beta 4 integrin, and does not involve binding to fibronectin, is also inhibited. This suggests that alpha 6 beta 4 on liver-metastasizing carcinoma cells binds to an OPAR epitope-carrying glycoprotein produced by hepatocytes.

Adhesion molecule expression on B-cells from acute and chronic lymphoid leukemias

Adhesion molecule expression on acute and chronic lymphoid leukemia cells of B lineage (B-ALL and B-CLL) may subserve several functions. Adhesion of leukemic cells to endothelial cells and to extracellular matrix components is relevant to homing, trafficking and spread of the malignant cells, and thus to clinical presentation, course and disease prognosis. Adhesive interactions between malignant cells and accessory cells, particularly stromal cells in the bone marrow environment, may support growth of the malignant cells via cytokine-delivered messages. They may also deliver signals that prevent or trigger programmed cell death of tumor cells. Here we review data on the adhesive phenotype of leukemic blasts from pro-B (CALLA +) ALL and of cells from B-CLL cases. We show that expression of certain adhesion molecules may help define disease subsets with distinctive clinical and prognostic features. One adhesion molecule, the lymphocyte homing receptor CD44, allows definition of two groups of B-CLL patients with significantly different survival.

Modifications of cell surface sialic acids modulate cell adhesion mediated by sialoadhesin and CD22

An increasing number of mammalian cell adhesion molecules, including sialoadhesion, CD22 and the family of selectins, have been found to bind cell surface glycoconjugates containing sialic acids. Here we describe how the structural diversity of this sugar influences cell adhesion mediated by the related molecules sialoadhesin and CD22 in murine macrophages and B-cells respectively. We show that the 9-O-acetyl group of Neu5,9Ac2 and the N-glycoloyl residue of Neu5Gc interfere with sialoadhesin binding. In contrast, CD22 binds more strongly to Neu5Gc compared to Neu5Ac. Of two synthetic sialic acids tested, only CD22 bound the N-formyl derivative, whereas a N-trifluoroacetyl residue was accepted by sialoadhesin. The potential significance for the regulation of sialic acid dependent cell adhesion phenomena is discussed.

Carbohydrate induced modulation of cell membrane. I. Interaction of sialic acid with peripheral blood lymphocytes: a spin label study

Sialic acid is now known to serve as ligand for lymphocyte lectins known as selectins. Although its role as a ligand for adhesion molecules has been studied extensively, no studies have been performed to determine the physiological changes in lymphocytes after binding of sialic acid to lymphocyte lectin. We report for the first time that interaction of lymphocytes with sialic acid severely restricts the rotational mobility of the cell surface proteins as well as membrane lipids, as studied by EPR spectroscopy using spin probes. Binding of mucin totally immobilizes the lymphocyte membrane. Surprisingly the binding of sialic acid or mucin also immobilized the aqueous probe TEMPO, indicating an appreciable increase in cytoplasmic viscosity.

Lymphocytic traffic and homing into target tissue and the generation of endocrine autoimmunity

Endocrine autoimmunity is known to be characterized by the presence of specific autoantibodies and from the histopathological point of view by lymphocytic infiltration in the target tissue. The presence of mononuclear cell infiltrates is the pathological hallmark of most endocrine diseases characterized by an autoimmune process directed against antigens expressed on endocrine cells. Infiltrating cells can usually be detected by biopsy or by using other, non-invasive, techniques. However, in endocrine tissue such as the islets of Langerhans and the adrenal glands it is difficult to perform biopsies and diagnosis of the autoimmune process is dependent mainly upon detection of specific autoantibodies. A crucial aspect of endocrine autoimmunity and of all processes of organ specific autoimmunity is why and how lymphocytes migrate from primary lymphoid tissue to their specific targets. This occurs mainly through contact with specific adhesion molecules which enable lymphocytes to adhere to the endothelial vessels in close proximity to the target tissue. In this review we discuss the homing of peripheral mononuclear cells into target endocrine tissues and the mediating role of adhesion molecules.

Transient inducible events in different tissues: in situ studies in the context of the development and expression of the immune responses to intracellular pathogens

Intracellular pathogens whether facultative like Mycobacterium sp., e.g. Bacillus Calmette Guérin, Listeria monocytogenes or strictly intracellular like Leishmania sp. initiate either asymptomatic infectious processes or disease depending both on factors of the host (genetic as well as environmental ones) and the infectious/pathogenic agents. In this contribution, we first summarized informations which justify to develop in situ analysis to decipher the sequential events that result in different modes/classes of immune responses. How the mode of the immune response is determined remains a main question to address. Although it has recently become clear, in vitro, that immunocompetent cells and their cytokines are critical to set on a stable mode of immune response, acting on naive T cells, this area deserves more in vivo studies. Indeed, peripheral T cells, at different stages of differentiation, may exist in vivo (a) naive/virgin, (b) experienced, (c) effector T cells, depending on the level of stimulation of the immune system by either endogenous or exogenous (e.g. gut flora) signals. The three chosen examples illustrate our contributions in this field focusing on three different non-lymphoid tissues which may become infected: bone marrow (Bacille de Calmette Guérin), liver (Listeria monocytogenes), skin (Leishmania major). These three illustrations also allow to attract attention on the interest of using mice of genetically different strains the immune response of which is set up under different modes.

Oligosaccharides and discordant xenotransplantation

The initiating factor in the hyperacute rejection of pig organs by human or non-human primates is believed to be related to the presence of preformed "natural" antibodies in the host. In 1991, we demonstrated that human anti-pig antibodies were IgG, IgM and IgA and bound most strongly to oligosaccharides with an alpha galactose (alpha Gal) terminal residue. These included (i) alpha Gal-R (alpha galactose), (ii) alpha Gall-3 beta Gal-R (B disaccharide), (iii) alpha Gall-3 beta Gall-4 beta GlcNAc-R (linear B type 2 trisaccharide) and (iv) alpha Gall-3 beta Gall-4 beta Glc-R (linear B type 6 trisaccharide) where R is (CH2) 8COOCH3. In vitro studies using both the chromium release assay and a live/dead staining technique demonstrated that the cytotoxicity of human sera towards pig cells can be significantly reduced or abolished by immunoadsorption of the serum with immunoaffinity columns of an alpha Gal structure, particularly those with an alpha 1-3 linkage, and not by a large selection of other carbohydrates. Similarly, human anti-pig antibodies can be largely inhibited or "neutralized" by the addition of an alpha 1-3Gal di- or trisaccharide to the serum. Staining of pig vascular endothelium utilizing a panel of carbohydrate-specific lectins and immunoaffinity antibodies demonstrated the presence of three different carbohydrate epitopes, namely (i) alpha Gall-3 beta Gall-4 beta GlcNAc-R (linear B type 2 trisaccharide (ii) alpha NeuAc2-3 beta Gall-4 beta GlcNAc-R (sialyl-N-acetyllactosamine), and (iii) beta Gall-4 beta GlcNAc-R (N-acetyllactosamine). We have investigated organs from several breeds of pig and have concluded that the alpha Gal epitope is either monomorphic or at least has a high incidence in porcine species, since we have not found any pig negative for this antigen. Human vascular endothelial cells have at their surface the same lactosamine-ended precursor and sialylated chains as pigs, but instead of terminal alpha Gal they express the fucosylated polymorphic ABH histo-blood group epitopes. As we have found no evidence that human or baboon plasma contain antibodies directed against sialic acid or lactosamine, and as human tissues contain both of these carbohydrates, it seems unlikely that either of these epitopes plays a role in the vascular rejection that takes place when pig organs are transplanted into primates. Unfortunately, the alpha Gal disaccharide and trisaccharides were not available to us in the large quantities required for extracorporeal immunoadsorption or continuous intravenous infusion in adult baboons.(ABSTRACT TRUNCATED AT 400 WORDS)

Polymorphonuclear leukocyte-induced vasocontraction and endothelial dysfunction. Role of selectins

The roles of selectin adhesion molecules (P- and L-selectin) and their counterreceptor sialyl Lewisx were investigated in polymorphonuclear leukocyte (PMN)-induced cat coronary vasocontraction and endothelial dysfunction. Unstimulated autologous PMNs (10(6) cells/mL) were added to organ chambers containing cat coronary artery rings stimulated with either thrombin (2 U/mL) or hydrogen peroxide (100 mumol/L). PMNs elicited a significant vasocontraction in thrombin- (119 +/- 14 mg) and hydrogen peroxide- (132 +/- 15 mg) stimulated coronary rings. This PMN-induced vasocontraction was significantly attenuated by pretreatment with either an anti-P-selectin, an anti-L-selectin monoclonal antibody (ie, MAb PB 1.3 and MAb DREG-200), or a sialyl Lewis(x)-containing oligosaccharide (SLe(x)-OS). Endothelial function as assessed by endothelium-dependent vasorelaxation to acetylcholine was also significantly attenuated after PMN-induced vasocontraction in stimulated coronary rings. This endothelial dysfunction was significantly prevented by either PB 1.3, DREG-200, or SLe(x)-OS. In contrast, endothelium-independent relaxation to acidified sodium nitrite was not altered by PMN incubation, indicating that vascular smooth muscle function was unaffected. Adherence of PMNs to coronary endothelium also significantly increased following stimulation of endothelium with either thrombin or hydrogen peroxide, but this was significantly attenuated by PB 1.3, DREG-200, or SLe(x)-OS. Thus, PMN-endothelial interaction mediated by either selectin adhesion molecules (ie, P-selectin and L-selectin) or sialyl Lewis(x) may play an important role in PMN-induced vasocontraction and endothelial dysfunction. This mechanism may be important in the early endothelial dysfunction observed following reperfusion of an ischemic coronary vasculature.

Regulation of lymphocyte activation by the cell-surface molecule CD22

Accessory molecules play an important role in the regulation of lymphocyte activation mediated by the B-cell antigen receptor (BCR). CD22 is one such accessory molecule expressed on B-lineage cells. Here, Che-Leung Law and colleagues review current knowledge on the structure-function relationship between CD22 and the BCR, discuss the role of CD22 as a cell-adhesion molecule and suggest models for potential in vivo functions of CD22.

The necrotic venom of the brown recluse spider induces dysregulated endothelial cell-dependent neutrophil activation. Differential induction of GM-CSF, IL-8, and E-selectin expression

Brown recluse spider (Loxosceles reclusa) venom induces severe dermonecrotic lesions. The mechanism for this is unknown but presents an interesting paradox: necrosis is completely dependent on the victim's neutrophils, yet neutrophils are not activated by the venom. We show Loxosceles venom is a potent, but disjointed, endothelial cell agonist. It weakly induced E-selectin expression, but not intercellular adhesion molecule-1 or IL-6 expression, yet significantly stimulated release of IL-8 and large amounts of GM-CSF by 4 h. In contrast, TNF strongly induced all of these, except for GM-CSF. PMN bound to E-selectin on venom-activated endothelial cells, apparently via counterreceptors different from those that bind E-selectin on TNF alpha-activated monolayers. Notably, PMN bound venom-activated monolayers only at intercellular junctions, did not polarize, and completely failed to migrate beneath the monolayer. Despite this, bound PMN demonstrated increased intracellular Ca2+ levels and secreted primary and secondary granule markers. The latter event was suppressed by sulfones used to treat envenomation. We have defined a new endothelial cell agonist, Loxosceles venom, that differentially stimulates the inflammatory response of endothelial cells. This, in turn, leads to a dysregulated PMN response where adhesion and degranulation are completely dissociated from shape change and transmigration.

Freeze-stable sialidase activity in human leucocytes: substrate specificity, inhibitor susceptibility, detergent requirements and subcellular localization

Human leucocytes contain a freeze-stable sialidase (neuraminidase; EC 3.2.1.18) activity in addition to the better-characterized lysosomal freeze-labile enzyme. In order to discriminate between the sialidase activities detected with the synthetic fluorimetric substrate 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (MU-Neu5Ac), different tritiated sialoglycoconjugate substrates were prepared. Using this sensitive radioactive assay system, leucocyte sialidase activity towards glycoproteins was shown to be labile to repeated freeze-thawing, but a Triton-stimulated activity towards gangliosides was entirely freeze-stable. Assay conditions were optimized for this freeze-stable ganglioside sialidase activity. Subcellular fractionation of mononuclear leucocytes (MNLs) on Percoll-density gradients showed that this ganglioside sialidase activity was entirely associated with the plasma membrane. Study of the detergent requirements showed that MNLs also demonstrated ganglioside sialidase activity when sodium cholate was present in place of Triton. Cholate-stimulated ganglioside sialidase activity was found to be entirely freeze-stable and localized at the plasma membrane. Studies on whole homogenates of MNLs demonstrated that the Triton-stimulated and cholate-stimulated activities showed similar acidic pH optima at < or = 3.9 and were both strongly inhibited by 2-deoxy-2,3-didehydro-N-acetylneuraminic acid and Cu2+, but not by free N-acetylneuraminic acid, N-(4-nitrophenyl)oxamic acid or heparan sulphate. These results suggest that human MNLs contain, in addition to the lysosomal freeze-labile sialidase, a single sialidase activity which is freeze-stable, ganglioside-specific, plasma membrane-associated and stimulated both by Triton and by cholate.

Non-carbohydrate binding partners/domains of animal lectins

1. Protein-carbohydrate interactions are involved in a large number of biologically important recognition processes. 2. Among the participating classes of proteins lectins are defined as carbohydrate-binding proteins other than an antibody or an enzyme. 3. In addition to the essential carbohydrate-binding domain other functionally and/or structurally important sites, defined by sequence comparison or by experimental demonstration of protein-protein interactions, can be present within the lectin molecule and may be relevant for its physiological significance. 4. Sequence motifs of lectins for protein-protein interactions include amino acid structures designed for cell adhesion, growth regulatory biosignalling, intracellular routing and enzymatic activity. 5. Elucidation of the complete functional role(s) of a lectin requires accurate delineation of its carbohydrate and, if present, of its protein ligands. 6. Presence of more than one carbohydrate-binding domain in a single lectin, potential ligand properties of the glycopart of a lectin, regulatory interplay between different sites and possible interaction of complementarily shaped peptide sequences to the sugar-recognizing site should all be assessed in the quest to comprehensively explain the physiological role(s) of a lectin.

Sialyl Lewisx-containing oligosaccharide attenuates myocardial reperfusion injury in cats

Neutrophil (PMN) adhesion to the vascular endothelium is an important mechanism of myocardial reperfusion injury. The adhesion process is initially mediated by selectins (e.g., P- and L-selectin), and monoclonal antibodies directed against these adhesion molecules exert cardioprotective activity in ischemia/reperfusion models. The counterreceptors for these selectins are thought to be carbohydrate-containing moieties. In this connection, we studied the effect of a soluble sialyl Lewisx-containing oligosaccharide (SLex-OS) on PMN-endothelial interactions in a feline model of myocardial ischemia/reperfusion (MI/R). SLex-OS (10 mg/kg), administered 10 min before R, significantly reduced myocardial necrosis compared with its vehicle 270 min after reperfusion (6 +/- 1% vs. 35 +/- 4% of area at risk, P < 0.01). The cardioprotection was confirmed by significantly lower plasma creatine kinase activities in SLex-OS vs. vehicle-treated cats (P < 0.01). Cardiac contractility (dP/dt max) of cats receiving SLex-OS was significantly preserved after 270 min of R (97 +/- 2% vs. 78 +/- 5% of initial, P < 0.01). Furthermore, endothelium-dependent relaxation to acetylcholine in coronary artery rings isolated from MI/R cats treated with SLex-OS was significantly preserved (73 +/- 7% vs. 22 +/- 6% vasorelaxation, P < 0.01). In vitro PMN adherence to coronary vascular endothelium after 270 min of R was significantly attenuated in the SLex-OS-treated group compared with the vehicle group (14 +/- 5 vs. 91 +/- 12 PMN/mm2, P < 0.01). Our results indicate that a SLex-OS is cardioprotective and preserves coronary endothelial function after MI/R, indicating an important role of sialyl Lewisx in PMN accumulation, endothelial dysfunction, and myocardial injury in myocardial ischemia/reperfusion.

Selectins

The selectins are three related receptors that initiate rolling of leukocytes on activated platelets or endothelium through Ca(2+)-dependent recognition of cell surface carbohydrates. Cell adhesion may be enhanced by a limited number of membrane glycoproteins that present high affinity carbohydrate ligands to specific selectins. The synthesis and surface display of the selectins is normally tightly controlled, but inappropriate expression may contribute to inflammatory disorders. Recent in vivo studies confirm the importance of the selectins in both physiological and pathological leukocyte recruitment.