External Labeling of Cell Surface Galactose and Galactosamine in Glycolipid and Glycoprotein of Human Erythrocytes

Traceless Protein-Selective Glycan Labeling and Chemical Modification

Executing glycan editing at a molecular level not only is pivotal for the elucidation of complicated mechanisms involved in glycan-relevant biological processes but also provides a promising solution to potentiate disease therapy. However, the precision control of glycan modification or glyco-editing on a selected glycoprotein is by far a grand challenge. Of note is to preserve the intact cellular glycan landscape, which is preserved after editing events are completed. We report herein a versatile, traceless glycan modification methodology for customizing the glycoforms of targeted proteins (subtypes), by orchestrating chemical- and photoregulation in a protein-selective glycoenzymatic system. This method relies on a three-module, ligand-photocleavable linker-glycoenzyme (L-P-G) conjugate. We demonstrated that RGD- or synthetic carbohydrate ligand-containing conjugates (RPG and SPG) would not activate until after the ligand-receptor interaction is accomplished (chemical regulation). RPG and SPG can both release the glycoenzyme upon photoillumination (photoregulation). The adjustable glycoenzyme activity, combined with ligand recognition selectivity, minimizes unnecessary glycan editing perturbation, and photolytic cleavage enables precise temporal control of editing events. An altered target protein turnover and dimerization were observed in our system, emphasizing the significance of preserving the native physiological niche of a particular protein when precise modification on the carbohydrate epitope occurs.

Chemical reporters to study mammalian O-glycosylation

Glycans play essential roles in a range of cellular processes and have been shown to contribute to various pathologies. The diversity and dynamic nature of glycan structures and the complexities of glycan biosynthetic pathways make it challenging to study the roles of specific glycans in normal cellular function and disease. Chemical reporters have emerged as powerful tools to characterise glycan structures and monitor dynamic changes in glycan levels in a native context. A variety of tags can be introduced onto specific monosaccharides via the chemical modification of endogenous glycan structures or by metabolic or enzymatic incorporation of unnatural monosaccharides into cellular glycans. These chemical reporter strategies offer unique opportunities to study and manipulate glycan functions in living cells or whole organisms. In this review, we discuss recent advances in metabolic oligosaccharide engineering and chemoenzymatic glycan labelling, focusing on their application to the study of mammalian O-linked glycans. We describe current barriers to achieving glycan labelling specificity and highlight innovations that have started to pave the way to overcome these challenges.

Identifying the Most Effective Hydatid Cyst Fluid Fraction for Anticancer Vaccination of 4T1 Breast Tumor-Bearing Mice

Background:

The hydatid cyst fluid antigens have high homology with cancer cell antigens and also exhibit considerable immunogenicity. Therefore, their utilization for cancer immunization can cause an effective antitumor immune response. However, the main challenge is identifying the most effective antigens for this purpose.

Methods:

Hydatid cyst fluid fractions including the glycolipid fraction, glycoprotein fraction, 78 kDa fraction, and antigen B fraction were prepared. Then, the BALB/c mice were immunized against different antigens and, subsequently, 4T1 cells were subcutaneously implanted. The tumors' growth, metastasis, and tumor-bearing mice survival were assessed in different immunized groups. In addition, IL-2, IL-4, IFN-γ, and TNF-α serum levels were estimated to evaluate the immune system response.

Results:

BALB/c mice immunization against the complete hydatid cyst fluid antigens exhibited more significant inhibition of the tumors' growth and metastasis and increase of tumor-bearing mice survival in comparison with its derived fractions. However, the 78 kDa fraction exhibited the best results according to the same factors in comparison with all the prepared fractions.

Conclusions:

The 78 kDa fraction of the hydatid cyst fluid was the most effective fraction of hydatid cyst fluid for immunization against 4T1 breast tumors.

Current Technologies for Complex Glycoproteomics and Their Applications to Biology/Disease-Driven Glycoproteomics

Glycoproteomics is an important recent advance in the field of glycoscience. In glycomics, glycan structures are comprehensively analyzed after glycans are released from glycoproteins. However, a major limitation of glycomics is the lack of insight into glycoprotein functions. The Biology/Disease-driven Human Proteome Project has a particular focus on biological and medical applications. Glycoproteomics technologies aimed at obtaining a comprehensive understanding of intact glycoproteins, i.e., the kind of glycan structures that are attached to particular amino acids and proteins, have been developed. This Review focuses on the recent progress of the technologies and their applications. First, the methods for large-scale identification of both N- and O-glycosylated proteins are summarized. Next, the progress of analytical methods for intact glycopeptides is outlined. MS/MS-based methods were developed for improving the sensitivity and speed of the mass spectrometer, in parallel with the software for complex spectrum assignment. In addition, a unique approach to identify intact glycopeptides using MS1-based accurate masses is introduced. Finally, as an advance of glycomics, two approaches to provide the spatial distribution of glycans in cells are described, i.e., MS imaging and lectin microarray. These methods allow rapid glycomic profiling of different types of biological samples and thus facilitate glycoproteomics.

Glycosphingolipids of porcine, bovine, and equine pericardia as potential immune targets in bioprosthetic heart valve grafts

BACKGROUND

Pericardial tissue from various animal species is utilized for the production of the bioprosthetic heart valves (BHV) used clinically. Experimental data show that the eventual breakdown of BHV is partly due to immunological interactions with carbohydrate tissue antigens. To understand these processes, we have examined the glycolipid-based carbohydrate antigens in naïve porcine, bovine, and equine pericardia.

EXPERIMENTAL

Total non-acid and acid glycosphingolipid fractions were isolated from porcine, bovine, and equine pericardia, and individual glycolipid compounds were characterized by thin-layer chromatography, mass spectrometry, and binding of monoclonal antibodies, lectins and bacteria in chromatogram binding assays.

RESULTS

The non-acid glycolipid fractions from all species contained glycosphingolipids based on the globo- and neolacto-series, including pentaglycosylceramides with terminal Galα3 determinants. Terminal blood group A and H (O) structures based on type 2 core chains were present in porcine pericardium, while the Forssman pentaosylceramide was found in equine pericardium. All acid glycolipid fractions contained sulfatide and several gangliosides with both N-acetyl- and N-glycolyl-neuraminic acid as terminal saccharide chain determinants.

CONCLUSION

Several carbohydrate antigens which are potential targets for the human immune system have been identified in the animal pericardial tissues used for the production of BHV. Which of these antigens are left in the tissues after industrial BHV production processes, as well as their potential role in eventual BHV degradation, remains to be elucidated.

Jose G, Shanbhag C, Kalia J. Rapid and reversible hydrazone bioconjugation in cells without the use of extraneous catalysts

Rapid, catalyst-free and reversible bioconjugation in mammalian cells.

Specific Identification of Glycoproteins Bearing the Tn Antigen in Human Cells

Glycoproteins contain a wealth of valuable information regarding the development and disease status of cells. In cancer cells, some glycans (such as the Tn antigen) are highly up-regulated, but this remains largely unknown for glycoproteins with a particular glycan. Herein, an innovative method combining enzymatic and chemical reactions was first designed to enrich glycoproteins with the Tn antigen. Using synthetic glycopeptides with O-GalNAc (the Tn antigen) or O-GlcNAc, we demonstrated that the method is selective for glycopeptides with O-GalNAc and can distinguish between these two modifications. The diagnostic ions from the tagged O-GalNAc further confirmed the effectiveness of the method and confidence in the identification of glycopeptides with the Tn antigen by mass spectrometry. Using this method, we identified 96 glycoproteins with the Tn antigen in Jurkat cells. The method can be extensively applied in biological and biomedical research.

Pneumococcal Neuraminidase Substrates Identified through Comparative Proteomics Enabled by Chemoselective Labeling

Neuraminidases (sialidases) are enzymes that hydrolytically remove sialic acid from sialylated proteins and lipids. Neuraminidases are encoded by a range of human pathogens, including bacteria, viruses, fungi, and protozoa. Many pathogen neuraminidases are virulence factors, indicating that desialylation of host glycoconjugates can be a critical step in infection. Specifically, desialylation of host cell surface glycoproteins can enable these molecules to function as pathogen receptors or can alter signaling through the plasma membrane. Despite these critical effects, no unbiased approaches exist to identify glycoprotein substrates of neuraminidases. Here, we combine previously reported glycoproteomics methods with quantitative proteomics analysis to identify glycoproteins whose sialylation changes in response to neuraminidase treatment. The two glycoproteomics methods-periodate oxidation and aniline-catalyzed oxime ligation (PAL) and galactose oxidase and aniline-catalyzed oxime ligation (GAL)-rely on chemoselective labeling of sialylated and nonsialylated glycoproteins, respectively. We demonstrated the utility of the combined approaches by identifying substrates of two pneumococcal neuraminidases in a human cell line that models the blood-brain barrier. The methods deliver complementary lists of neuraminidase substrates, with GAL identifying a larger number of substrates than PAL (77 versus 17). Putative neuraminidase substrates were confirmed by other methods, establishing the validity of the approach. Among the identified substrates were host glycoproteins known to function in bacteria adherence and infection. Functional assays suggest that multiple desialylated cell surface glycoproteins may act together as pneumococcus receptors. Overall, this method will provide a powerful approach to identify glycoproteins that are desialylated by both purified neuraminidases and intact pathogens.

Advances in mass spectrometry driven O-glycoproteomics

BACKGROUND

Global analyses of proteins and their modifications by mass spectrometry are essential tools in cell biology and biomedical research. Analyses of glycoproteins represent particular challenges and we are only at the beginnings of the glycoproteomic era. Some of the challenges have been overcome with N-glycoproteins and proteome-wide analysis of N-glycosylation sites is accomplishable today but only by sacrificing information of structures at individual glycosites. More recently advances in analysis of O-glycoproteins have been made and proteome-wide analysis of O-glycosylation sites is becoming available as well.

SCOPE OF REVIEW

Here we discuss the challenges of analysis of O-glycans and new O-glycoproteomics strategies focusing on O-GalNAc and O-Man glycoproteomes.

MAJOR CONCLUSIONS

A variety of strategies are now available for proteome-wide analysis of O-glycosylation sites enabling functional studies. However, further developments are still needed for complete analysis of glycan structures at individual sites for both N- and O-glycoproteomics strategies.

GENERAL SIGNIFICANCE

The advances in O-glycoproteomics have led to identification of new biological functions of O-glycosylation and a new understanding of the importance of where O-glycans are positioned on proteins.

Glycoproteomics enabled by tagging sialic acid- or galactose-terminated glycans

In this paper, we present two complementary strategies for enrichment of glycoproteins on living cells that combine the desirable attributes of "robust enrichment" afforded by covalent-labeling techniques and "specificity for glycoproteins" typically provided by lectin or antibody affinity reagents. Our strategy involves the selective introduction of aldehydes either into sialic acids by periodate oxidation (periodate oxidation and aniline-catalyzed oxime ligation (PAL)) or into terminal galactose and N-acetylgalactosamine residues by galactose oxidase (galactose oxidase and aniline-catalyzed oxime ligation (GAL)), followed by aniline-catalyzed oxime ligation with aminooxy-biotin to biotinylate the glycans of glycoprotein subpopulations with high efficiency and cell viability. As expected, the two methods exhibit reciprocal tagging efficiencies when applied to fully sialylated cells compared with sialic acid-deficient cells. To assess the utility of these labeling methods for glycoproteomics, we enriched the PAL- and GAL-labeled (biotinylated) glycoproteome by adsorption onto immobilized streptavidin. Glycoprotein identities (IDs) and N-glycosylation site information were then obtained by liquid chromatography-tandem mass spectrometry on total tryptic peptides and on peptides subsequently released from N-glycans still bound to the beads using peptide N-glycosidase F. A total of 175 unique N-glycosylation sites were identified, belonging to 108 nonredundant glycoproteins. Of the 108 glycoproteins, 48 were identified by both methods of labeling and the remainder was identified using PAL on sialylated cells (40) or GAL on sialic acid-deficient cells (20). Our results demonstrate that PAL and GAL can be employed as complementary methods of chemical tagging for targeted proteomics of glycoprotein subpopulations and identification of glycosylation sites of proteins on cells with an altered sialylation status.

Chemo-enzymatic modification of poly-N-acetyllactosamine (LacNAc) oligomers and N,N-diacetyllactosamine (LacDiNAc) based on galactose oxidase treatment

The importance of glycans in biological systems is highlighted by their various functions in physiological and pathological processes. Many glycan epitopes on glycoproteins and glycolipids are based on N-acetyllactosamine units (LacNAc; Galβ1,4GlcNAc) and often present on extended poly-LacNAc glycans ([Galβ1,4GlcNAc](n)). Poly-LacNAc itself has been identified as a binding motif of galectins, an important class of lectins with functions in immune response and tumorigenesis. Therefore, the synthesis of natural and modified poly-LacNAc glycans is of specific interest for binding studies with galectins as well as for studies of their possible therapeutic applications. We present the oxidation by galactose oxidase and subsequent chemical or enzymatic modification of terminal galactose and N-acetylgalactosamine residues of poly-N-acetyllactosamine (poly-LacNAc) oligomers and N,N-diacetyllactosamine (LacDiNAc) by galactose oxidase. Product formation starting from different poly-LacNAc oligomers was characterised and optimised regarding formation of the C6-aldo product. Further modification of the aldehyde containing glycans, either by chemical conversion or enzymatic elongation, was established. Base-catalysed β-elimination, coupling of biotin-hydrazide with subsequent reduction to the corresponding hydrazine linkage, and coupling by reductive amination to an amino-functionalised poly-LacNAc oligomer were performed and the products characterised by LC-MS and NMR analysis. Remarkably, elongation of terminally oxidised poly-LacNAc glycans by β3GlcNAc- and β4Gal-transferase was also successful. In this way, a set of novel, modified poly-LacNAc oligomers containing terminally and/or internally modified galactose residues were obtained, which can be used for binding studies and various other applications.

Innate Immunity in Lobsters: Partial Purification and Characterization of a Panulirus cygnus Anti-A Lectin

A lectin detected in haemolymph from the Australian spiny lobster Panulirus cygnus agglutinated human ABO Group A cells to a higher titre than Group O or B. The lectin also agglutinated rat and sheep erythrocytes, with reactivity with rat erythrocytes strongly enhanced by treatment with the proteolytic enzyme papain, an observation consistent with reactivity via a glycolipid. The lectin, purified by affinity chromatography on fixed rat-erythrocyte stroma, was inhibited equally by N-acetylglucosamine and N-acetylgalactosamine. Comparison of data from gel filtration of haemolymph (behaving as a 1,800,000 Da macromolecule), and polyacrylamide gel electrophoresis of purified lectin (a single 67,000 Da band), suggested that in haemolymph the lecin was a multimer. The purified anti-A lectin autoprecipitated unless the storage solution contained chaotropic inhibitors (125 mmol/L sucrose: 500 mmol/L urea). The properties of this anti-A lectin and other similar lectins are consistent with a role in innate immunity in these invertebrates.

Fate of glycosylphosphatidylinositol (GPI)-less procyclin and characterization of sialylated non-GPI-anchored surface coat molecules of procyclic-form Trypanosoma brucei

A Trypanosoma brucei TbGPI12 null mutant that is unable to express cell surface procyclins and free glycosylphosphatidylinositols (GPI) revealed that these are not the only surface coat molecules of the procyclic life cycle stage. Here, we show that non-GPI-anchored procyclins are N-glycosylated, accumulate in the lysosome, and appear as proteolytic fragments in the medium. We also show, using lectin agglutination and galactose oxidase-NaB(3)H(4) labeling, that the cell surface of the TbGPI12 null parasites contains glycoconjugates that terminate in sialic acid linked to galactose. Following desialylation, a high-apparent-molecular-weight glycoconjugate fraction was purified by ricin affinity chromatography and gel filtration and shown to contain mannose, galactose, N-acetylglucosamine, and fucose. The latter has not been previously reported in T. brucei glycoproteins. A proteomic analysis of this fraction revealed a mixture of polytopic transmembrane proteins, including P-type ATPase and vacuolar proton-translocating pyrophosphatase. Immunolocalization studies showed that both could be labeled on the surfaces of wild-type and TbGPI12 null cells. Neither galactose oxidase-NaB(3)H(4) labeling of the non-GPI-anchored surface glycoconjugates nor immunogold labeling of the P-type ATPase was affected by the presence of procyclins in the wild-type cells, suggesting that the procyclins do not, by themselves, form a macromolecular barrier.

The relevance of the aldehyde bisulfite toluidine blue reaction and its variants in the submicroscopic carbohydrate research

Carbohydrates are chemical compounds that contain only oxygen, hydrogen and carbon. They are classified by their number of sugar units: monosaccharides (such as glucose and fructose), and disaccharides (such as sucrose and lactose) are simple carbohydrates; oligosaccharides and polysaccharides (such as starch, glycogen and cellulose) are complex carbohydrates. Carbohydrates play a crucial role in diverse biological systems [Hricovín M. Structural aspects of carbohydrates and the relation with their biological properties. Curr Med Chem 2004;11:2565-83]. According to Roseman [Sugars of the cell membrane. In: Weissmann G, Clairborn E, editors. Cell membranes. Biochemistry, Cell Biology, Pathology. New York: H. P. Publ. Co; 1975. p. 55-64], two classes of glycoproteins are described. Free glycoproteins are localised in the surface coat of the membranes and form a thick mobile layer, without any association to the membrane itself. Functionally, however, they are located in a close association with the membrane (e.g. in the duodenal mucosa). The other group consists of the membrane glycoproteins, which are integral to the membranes and are located in the outer layer. The oligosaccharide chains are bound to the N-terminal part of proteins, and are situated in the hydrophilic zone. Glycoproteins have diverse functions. They are important in specific receptor functions, in immunological cell destruction and play a significant role in reactions with lectins, antibodies, as well as in cell association and mutual recognition of the cells. This paper focuses on aspects of a summary of polarisation optical investigations and biological functions of the following three groups of carbohydrates: oligosaccharides, glycoproteins and glycosaminoglycans.

ICAM-5--a novel two-facetted adhesion molecule in the mammalian brain

Cell adhesion is of utmost importance for normal development and cellular functions. ICAM-5 (intercellular adhesion molecule-5, telencephalin) is a member of the ICAM-family of adhesion proteins. These proteins bind to leukocyte beta(2)-integrins (CD11/CD18), but ICAM-5 is exceptional in several ways. It is solely expressed in the mammalian forebrain, appears at the time of birth, and is located in the soma and dendrites of neurons. It is structurally more complex than the others, and also shows homophilic adhesion. Recent studies show that it is important for the regulation of immunological activity in the brain and for the development of neuronal synapses and signal transmission.

Endo-beta-galactosidases and keratanase

This overview covers the endo-beta-galactosidases; enzyme is capable of hydrolyzing a wide range of glycoconjugates. Endo-beta-galactosidases from numerous sources are discussed in terms of their substrate specificities and substrates, as well as their practical research applications.

Importance of molecular studies on major blood groups--intercellular adhesion molecule-4, a blood group antigen involved in multiple cellular interactions

Several blood groups, including the LW-blood group were discovered in the first part of last century, but their biochemical characteristics and cellular functions have only more recently been elucidated. The LW-blood group, renamed ICAM-4 (CD242), is red cell specific and belongs to the intercellular adhesion molecule family. ICAM-4 binds to several integrin receptors on blood and endothelial cells and is thus able to form large cellular complexes containing red cells. Its physiological function(s) has remained incompletely understood, but recent work shows that macrophage integrins can bind red cells through this ligand. In this article we discuss molecular properties of major blood group antigens, describe ICAM-4 in more detail, and show that phagocytosis of senescent red cells is in part ICAM-4/beta(2)-integrin dependent.

Structure and function of glycosphingolipids and sphingolipids: recollections and future trends

Based on development of various methodologies for isolation and characterization of glycosphingolipids (GSLs), we have identified a number of GSLs with globo-series or lacto-series structure. Many of them are tumor-associated or developmentally regulated antigens. The major question arose, what are their functions in cells and tissues? Various approaches to answer this question were undertaken. While the method is different for each approach, we have continuously studied GSL or glycosyl epitope interaction with functional membrane components, which include tetraspanins, growth factor receptors, integrins, and signal transducer molecules. Often, GSLs were found to interact with other carbohydrates within a specific membrane microdomain termed "glycosynapse", which mediates cell adhesion with concurrent signal transduction. Future trends in GSL and glycosyl epitope research are considered, including stem cell biology and epithelial-mesenchymal transition (EMT) process.

Characterization of a sialate pyruvate-lyase in the cytosol of human erythrocytes

Sialate pyruvate-lyases, also known as sialate aldolases (EC 4.1.3.3), reversibly catalyse the cleavage of free N-acetylneuraminic acids to form pyruvate and N-acetylmannosamine. These enzymes are widely distributed and are present in numerous pro- and eukaryotic cells, in which they are localized only in the cytosol. They play an important role in the regulation of sialic acid metabolism by controlling the intracellular concentration of sialic acids of biosynthetic or exogenous origin, thus preventing the accumulation of toxic levels of this sugar. Application of an original colorimetric micromethod for N-acetylmannosamine determination, as well as the use of [4,5,6,7,8,9-14C]N-acetylneuraminic acid, led us to evidence a cytosolic neuraminate aldolase activity in human red blood cells (RBCs) and then to define the main characteristics of this enzyme: Michaelis-Menten type, K(m:) 1.4 +/- 0.05 mM, optimal pH: 7.6 +/- 0.2, optimal temperature: 70 +/- 2 degrees C, inhibition by heavy metals: Ag(+) and Hg(++). These enzyme parameters are close to those of the bacterial and mammalian aldolases described up to now. At the moment, the presence of sialate pyruvate-lyase in the cytosol of red blood cells remains an enigma.

Uptake and metabolism of exogenous glycosphingolipids by cultured cells

Exogenous glycosphingolipids, especially gangliosides, are used to study transport and metabolism of their endogenous counterparts as well as their role in cell adhesion, cell recognition and signal transduction. Unlike monodispersed solutes, in aqueous media ganglioside molecules aggregate into micelles (or bilayer structures) with a very low critical micellar concentration. Upon addition to cells in culture, exogenous gangliosides bind to the cell surface in three operationally defined modes: loosely associated micelles removable by serum; tightly attached micelles removable by proteases such as trypsin; and ganglioside molecules inserted into the outer leaflet of the plasma membrane. As shown by a biotin-labeled derivative of the ganglioside GM1 these inserted molecules are endocytosed and transported to intralysosomal membranes for catabolism. The benefit from using (partially) nondegradable as well as semi-truncated glycosphingolipids in transport studies is discussed.

An interaction between ricin and calreticulin that may have implications for toxin trafficking

Here we demonstrate that ricin is able to interact with the molecular chaperone calreticulin both in vitro and in vivo. The interaction occurred with ricin holotoxin, but not with free ricin A chain; and it was prevented in the presence of lactose, suggesting that it was mediated by the lectin activity of the ricin B chain. This lectin is galactose-specific, and metabolic labeling with [(3)H]galactose or treating galactose oxidase-modified calreticulin with sodium [(3)H]borohydride indicated that Vero cell calreticulin possesses a terminally galactosylated oligosaccharide. Brefeldin A treatment indicated that the intracellular interaction occurred initially in a post-Golgi stack compartment, possibly the trans-Golgi network, whereas the reductive separation of ricin subunits occurred in an earlier part of the secretory pathway, most probably the endoplasmic reticulum (ER). Intoxicating Vero cells with ricin whose A chain had been modified to include either a tyrosine sulfation site or the sulfation site plus available N-glycosylation sites, in the presence of Na(2)35SO(4), confirmed that calreticulin interacted with endocytosed ricin that had already undergone retrograde transport to both the Golgi and the ER. Although we cannot exclude the possibility that the interaction between ricin and calreticulin is an indirect one, the data presented are consistent with the idea that calreticulin may function as a recycling carrier for retrograde transport of ricin from the Golgi to the ER.

Fluorescence-based evaluation of the partitioning of lipids and lipidated peptides into liquid-ordered lipid microdomains: a model for molecular partitioning into "lipid rafts"

A fluorescence-quenching assay is described that can directly monitor the relative extents of partitioning of different but structurally homologous fluorescent molecules into liquid-ordered (l(o)) domains in lipid vesicles exhibiting liquid-ordered/liquid-disordered (l(o)/l(d)) phase coexistence. Applying this assay to a series of bimane-labeled diacyl phospholipid probes in cholesterol-containing ternary lipid mixtures exhibiting l(o)/l(d) phase separation, we demonstrate that partitioning into l(o)-phase domains is negligible for diunsaturated species and greatest for long-chain disaturated species. These conclusions agree well with those derived from previous studies of the association of lipids and lipid-anchored molecules with l(o)-phase domains, using methods based on the isolation of a detergent-insoluble fraction from model or biological membranes at low temperatures. However, we also find that monounsaturated and shorter-chain saturated species partition into l(o) phases with significant, albeit modest affinities, and that the level of partitioning of these latter species into l(o)-phase domains is significantly underestimated (relative to that of their long-chain saturated counterparts) by the criterion of low-temperature detergent insolubility. Finally, applying the fluorescence-quenching method to a family of lipid-modified peptides, we demonstrate that the S-palmitoyl/S-isoprenyl dual-lipidation motif found in proteins such as H- and N-ras and yeast Ste18p does not promote significant association with l(o) domains in l(o)/l(d)-phase-separated bilayers.

Sphingolipid transport in eukaryotic cells

Sphingolipids constitute a sizeable fraction of the membrane lipids in all eukaryotes and are indispensable for eukaryotic life. First of all, the involvement of sphingolipids in organizing the lateral domain structure of membranes appears essential for processes like protein sorting and membrane signaling. In addition, recognition events between complex glycosphingolipids and glycoproteins are thought to be required for tissue differentiation in higher eukaryotes and for other specific cell interactions. Finally, upon certain stimuli like stress or receptor activation, sphingolipids give rise to a variety of second messengers with effects on cellular homeostasis. All sphingolipid actions are governed by their local concentration. The intricate control of their intracellular topology by the proteins responsible for their synthesis, hydrolysis and intracellular transport is the topic of this review.

Characterization of protein-glycolipid recognition at the membrane bilayer

A growing number of important molecular recognition events are being shown to involve the interactions between proteins and glycolipids. Glycolipids are molecules in which one or more monosaccharides are glycosidically linked to a lipid moiety. The lipid moiety is generally buried in the cell membrane or other bilayer, leaving the oligosaccharide moiety exposed but in close proximity to the bilayer surface. This presents a unique environment for protein-carbohydrate interactions, and studies to determine the influence of the bilayer on these phenomena are in their infancy. One important property of the bilayer is the ability to orient and cluster glycolipid species, as strong interactions in biological systems are often achieved through multivalency arising from the simultaneous association of two or more proteins and receptors. This is especially true of protein-carbohydrate binding because of the unusually low affinities that characterize the monovalent interactions. More recent studies have also shown that the composition of the lipid bilayer is a critical parameter in protein-glycolipid recognition. The fluidity of the bilayer allows for correct geometric positioning of the oligosaccharide head group relative to the binding sites on the protein. In addition, there are activity-based and structural data demonstrating the impact of the bilayer microenvironment on the modulation of oligosaccharide presentation. The use of model membranes in biosensor-based methods has supplied decisive evidence of the importance of the membrane in receptor presentation. These data can be correlated with three-dimensional structural information from X-ray crystallography, NMR, and molecular mechanics to provide insight into specific protein-carbohydrate inter--actions at the bilayer.

Leishmania donovani has distinct mannosylphosphoryltransferases for the initiation and elongation phases of lipophosphoglycan repeating unit biosynthesis

Lipophosphoglycan (LPG) is the predominant surface glycoconjugate of Leishmania promastigotes and plays several roles in the infectious cycle of this protozoan parasite. The salient feature of LPG is the presence of 15-30 copies of a disaccharide-phosphate repeating unit Gal(beta1,4)Man(alpha1-PO4), which is also found on many other secreted molecules (secretory acid phosphatase, phosphoglycan, proteophosphoglycan). This structural diversity suggests that a multiplicity of enzymes mediating repeating unit addition may exist, especially for the mannosylphosphoryltransferases (MPTs), which initiate repeating unit synthesis. This work has taken a combined biochemical-genetic approach to resolve this issue. An lpg- mutant of Leishmania donovani, JEDI, was obtained by antibody selection against cells expressing a repeating unit epitope of LPG. Metabolic and surface labeling experiments revealed that JEDI cells accumulated a truncated form of LPG bearing only a single repeating unit: [Gal(beta 1,4)Man(alpha1-PO4)][Gal(alpha1,6)Gal(alpha1,3)Gal(f)(beta1,3)[Glc(alpha 1-PO4)]Man(alpha1,3)Man(alpha1,4)GlcN(alpha1,6)]-PI. Enzymatic assays of microsomal preparations showed that JEDI lacked MPT activity when tested with a repeating unit acceptor but retained wild-type levels of the MPT activity with an LPG glycan core acceptor. These data indicate that at least two distinct MPT activities are required for LPG repeating unit synthesis: one involved in the 'initiation' of repeating unit synthesis on the LPG core (iMPT), and a second (lacking in JEDI) participating in the 'elongation' phase of repeating unit addition (eMPT), leading to the mature full-length LPG.

Effect of sphingomyelin and its metabolites on the activity of human recombinant PLC delta 1

In an attempt to obtain sufficient quantities of pure phospholipase C delta 1 (PLC delta 1) necessary for structural and kinetic studies, human fibroblast PLC delta 1 was cloned in the pPROEX-1 vector, expressed in E. coli cells as a (6xHis) fusion protein and purified to homogeneity. From 11 of E. coli culture 21 mg of pure PLC delta 1 was obtained by a two-step purification procedure, which includes Ni(2+)-NAT agarose and Mono S cation exchange chromatography. Catalytic properties of recombinant PLC delta 1 with respect to activation by spermine and calcium ions and inhibition by sphingomyelin were similar to or identical to PLC delta 1 purified from rat liver. Calcium activation of PLC delta 1 was dependent on the presence of spermine. Half-maximal activity was attained at 250 and 170 nM of free Ca2+ in the presence and absence of spermine, respectively. Sphingomyelin and lysosphingomyelin were mixed type inhibitors with respect to PIP2. Ceramide inhibits PLC delta 1 very weakly. GM1, which is a ceramide bound glucosidically to the oligosaccharide moiety, was a strong non-competitive inhibitor of PLC delta 1. In the absence of spermine, sphingosine and phytosphingosine weakly activated PLC delta 1. The results indicate that the effect of sphingomyelin and its metabolites on PLC delta 1 activity depends on the presence of spermine. It is postulated that, among other factors, in vivo, activity of PLC delta 1 may depend on the turnover of sphingomyelin.

[125I]IgM (KAU) human monoclonal cold agglutinin: labelling and studies on its biological activity

In order to study the interaction between an IgM cold agglutinin and the erythrocyte I antigen, the former antibody was labelled with 125I using the Chloramine-T, IODOGEN and Bolton-Hunter methods. High incorporation and adequate stability of the labelled IgM were obtained with all procedures. However, suitable biological activity was maintained only with the Bolton-Hunter method. Further studies suggest that tyrosine iodination affects antigen recognition by this IgM, whereas iodination of amino groups does not. The reagent thus prepared allowed the determination of the number of I sites per erythrocyte as well as the antibody affinity constant.

Immunohistological analyses of neutral glycosphingolipids and gangliosides in normal mouse skeletal muscle and in mice with neuromuscular diseases

The expression of neutral glycosphingolipids (GSLs) and gangliosides was investigated in cryosections of normal mouse skeletal muscle and in muscle of mice with neuromuscular diseases using indirect immunofluorescence microscopy. Transversal and longitudinal sections were immunostained with specific polyclonal antibodies against lactosylceramide, lacto-N-neotetraosylceramide, globoside, GM3(Neu5Ac), GM3(Neu5Gc) and Gm1(Neu5Ac) as well as monoclonal anti-Forssman GSL antibody. In normal CBA/J mouse muscle (control) the main immunohistochemically detected ganglioside was GM3(Neu5Ac) followed by moderately expressed GM3(Neu5Gc) and GM1. The neutral GSLs lactosylceramide and globoside were stained with almost identical, high fluorescence intensity. Low amounts of lacto-N-neotetraosylceramide and trace quantities of Forssman GSL were immunostained. All GSLs were detected in the sarcolemma, but also in considerable amounts at the intracellular level. Mice with neuromuscular diseases were the A2G-adr mouse mutant (a model for human recessive myotonia of Becker type), the BL6-wr mutant (a model for motor neuron disease) and the BL10-mdx mouse mutant (a model for human Duchenne muscular dystrophy). No changes in GSL expression were found in the A2G-adr mouse, while muscle of the BL6-wr mouse showed increased intensity of immunofluorescence in stainings with anti-lactosylceramide and anti-GM3(Neu5Ac) antibodies. Muscle of BL10-mdx mice showed the most prominent changes in GSL expression with reduced fluorescence intensity for all antibodies. Major differences were not observed in the intensities of GSLs, but there were significant differences in the patterns of distribution on plasma membrane and at the subcellular level. The exact nature and pathogenesis of these changes should be elucidated since such investigations could furnish advances in understanding the functional role of neutral GSLs and gangliosides in normal as well as in diseased muscle.

Epitope masking of rat esophageal carcinoma tumor-associated antigen by certain coexisting glycolipid and phospholipid molecules: a potential mechanism for tumor cell escape from the host immune responses

A monoclonal antibody (mAb-5G) produced against a tumorigenic rat esophageal cell line, B2T, was shown to react specifically with a unique glycolipid antigen expressed on the cell surface of tumorigenic and certain non-tumorigenic, immortalized rat esophageal cell lines [Cancer Immunol Immunother 36: 94 (1993)]. In enzyme-linked immunosorbent assay experiments, mAb-5G reacted with crude lipid extracts prepared from B2T cells cultured in vitro, but showed very little reactivity with crude lipid extracts prepared from the same cell line passaged once in vivo, unless the antigen was separated from other lipid components by column or thin-layer chromatography (TLC). When a secondary tissue-culture cell line was established from the above B2T tumor tissues and serially subcultured in vitro, the percentage of positively stained cells was increased significantly in immunofluorescence assay. It was also demonstrated that the amount of extractable antigen was increased as the cells were subcultured in vitro up to passage 15, and stabilized thereafter. These results indicate the presence of certain lipid components in crude lipid extracts from B2T cells grown in vivo that are capable of interfering with antigen-antibody binding. On TLC plates, these interfering lipids were identified as phosphatidylcholine, phosphatidylserine, sphingomyelin and gangliosides. The interfering lipids did not bind the antibody, rather they appeared to interfere with antigen accessibility. These lipid substances may modify tumor cell surface antigen(s), thus protecting the tumor cells from host immune destruction.

Characterization of a cell surface glycoprotein associated with maturation of rat spermatozoa

The principal galactose oxidase/NaB[3H]4-labeled membrane protein of rat caudal epididymal spermatozoa was isolated by hydrophobic interaction chromatography. The protein is released from the membrane by the action of phosphatidylinositol specific phospholipase C, and thereby its properties are transformed from those of a protein anchored to the hydrophobic membrane to those of a hydrophilic solution protein. Because it is the only membrane-associated protein released by the enzyme which did not absorb to a propylaspartate resin, a simple, single step purification procedure was devised. Although the amino terminus of the protein is blocked to Edman degradation, the majority of the protein structure was determined from a series of tryptic peptides and from limited acid hydrolysis. Approximately 65% of the protein mass is carbohydrate which is primarily attached through O-glycosidic bonds to the 18 threonines. The molecular weight of the glycoprotein was estimated to be 16,600, considerably smaller than the M(r) = 26,000 to 37,000 previously determined by gel electrophoresis. The anomalous electrophoretic behavior is undoubtedly due to the large percentage of carbohydrate. The distribution of carbohydrate on the protein side chains suggests the protein may form a positively charged, specialized scaffolding for the presentation of the carbohydrate moieties. Because the appearance of the ability to label the protein with galactose oxidase is correlated with sperm maturation in the epididymis, the glycoprotein structures may be an important component in the fertilization process. The combination of linkage by glycosylphosphatidylinositol and low molecular weight mucin-like structure indicates this may be a member of a new class of membrane proteins.

Characterization of the high-affinity oligosaccharide-binding site of the 205-kDa porcine large granular lymphocyte lectin, a member of the leukocyte common antigen family

Membrane lectins of mammalian large granular lymphocytes are thought to be important receptors in their non-major-histocompatibility complex-restricted activation. A triantennary desialylated oligosaccharide has been reported as the most effective triggering structure [Pospísil M., Kubrycht J., Bezouska K., Táborský O., Novák M. & Kocourek J. (1986) Immunol. Lett. 12, 83-90] while its cell surface receptor has recently been identified in pig natural killer cells as a 205-kDa membrane lectin resembling the proteins of the leukocyte common antigen family (LCA). In this study we have prepared 4-azidophenyl (photoactivatable) and 4-hydroxyphenyl (radio-iodinatable) derivatives of triantennary oligosaccharides by a new procedure which allows the natural conformation of the N-glycosidic linkage between the oligosaccharide and the respective labeling group to be retained. We used these high-affinity ligands to investigate the oligosaccharide-combining site of the 205-kDa lectin. Photoaffinity labeling of the whole cells and solubilized proteins confirmed that a 205-kDa polypeptide constitutes the major cell-surface calcium-independent receptor for triantennary oligosaccharides in pig lymphocytes. Isolation and manual sequencing of two ligand-labeled and eleven other peptides proved that the 205-kDa lectin represents a member of the LCA family expressing exons 4 and 6 during alternative splicing and that the high-affinity binding site is localized in the N-terminal 70-kDa extracellular domain. Binding studies with radiolabeled oligosaccharides and the above carbohydrate-recognition domain subjected to various chemical and enzymatic treatments indicated that the binding of oligosaccharides might be significantly modulated by sialylated O-glycosidically linked lineage-specific carbohydrate epitopes localized within this domain. Affinity chromatography of LCA isolated by conventional methods on immobilized oligosaccharides revealed that only a fraction of these cell-surface glycoproteins expressed high-affinity binding sites for the oligosaccharide ligands. Thus, N-linked oligosaccharide moieties of cell-surface glycoproteins seem to represent possible ligands of LCA that may be important in intercellular adhesion and oligosaccharide-mediated activation of lymphocytes.

Spectroscopical characteristics of galactosylceramide-pyrene and ceramide-pyrene incorporated in model and in clathrin coated vesicles

In order to control the ability of two pyrene-sphingolipids (ceramide-pyrene (Cpyr) and galactosylceramide-pyrene (GCpyr)) to monitor the changes in the lipid bilayer dynamics of cellular membranes, their incorporation in three populations of clathrin coated vesicles which differ in their structural characteristics (Bomsel et al. (1988) Biochemistry 27, 6808-6812) was studied by both absorbance and fluorescence spectroscopy. The method of injection of an ethanolic solution of probe was used. The analysis of the spectra recorded after injection into a free-membrane buffer allowed to discriminate two dispersion states (micellar or aggregated) of the probes. The micellar state was identified as the one suitable for the incorporation within the bilayer. Rising the temperature up to 18 degrees C for a membrane labeling with GCpyr and to 37 degrees C for a membrane labeling with Cpyr was found to be necessary because it allowed to slow down the aggregation process which inhibited the incorporation within the lipid bilayer. The excimer/monomer (E/M) fluorescence intensities ratio of GCpyr was found to be characteristic of each population of coated vesicles. Cpyr could not be used as a diffusion probe because it partly aggregated during the cooling step necessary to establish the E/M versus temperature plot in the heating mode. An important point which arises from these data is that the use of absorbance spectroscopy can avoid misinterpretation of the pyrene derivatives fluorescence spectra in terms of diffusion.