Galectinas: estructura, especificidad glicídica y ligandos específicos / Galectins: structure, sugar specificity and specific ligands

Las galectinas se definen por dos propiedades: secuencias de aminoácidos características compartidas y afinidad por azúcares ß-galactosídicos. Numerosas galactinas de mamíferos fueron secuenciadas y bien caracterizadas en diferentes especies, siendo clasificadas como galectina-1 a galectina-10, según sus homologías de secuencia. La identidad entre dominios que ligan carbohidratos de distintas galectinas de una especie de mamífero oscila entre 20-40 por ciento, mientras que la identidad de galectina-1, por ejemplo, entre distintas especies es de 80-90 por ciento. En la presente revisión, se describen las principales propiedades distintivas de las galectinas de mamífero en cuanto a estructura proteica, estructura cristalina, especificidad glicídica y ligandos específicos

Mouse mammary tumor virus (MMTV), a retrovirus that exploits the immune system. Genetics of susceptibility to MMTV infection

All animals, including humans, show differential susceptibility to infection with viruses. Study of the genetics of susceptibility or resistance to specific pathogens is most easily studied in inbred mice. We have been using mouse mammary tumor virus (MMTV), a retrovirus that causes mammary tumors in mice, to study virus/host interactions. These studies have focused on understanding the mechanisms that determine genetic susceptibility to MMTV-induced mammary tumors, the regulation of virus gene expression in vivo and how the virus is transmitted between different cell types. We have found that some endogenous MMTVs are only expressed in lymphoid tissue and that a single base pair change in the long terminal repeat of MMTV determines whether the virus is expressed in mammary gland. This expression in lymphoid cells is necessary for the infectious cycle of MMTV, and both T and B cells express and shed MMTV. Infected lymphocytes are required not only for the initial introduction of MMTV to the mammary gland, but also for virus spread at later times. Without this virus spread, mammary tumorigenesis is dramatically reduced. Mammary tumor incidence is also affected by the genetic background of the mouse and at least one gene that affects infection of both lymphocytes and mammary cells has not yet been identified. The results obtained from these studies will greatly increase our understanding of the genetic mechanisms that viruses use to infect their hosts and how genetic resistance to such viruses in the hosts occurs.

Mouse mammary tumor virus (MMTV), a retrovirus that exploits the immune system. Genetics of susceptibility to MMTV infection

All animals, including humans, show differential susceptibility to infection with viruses. Study of the genetics of susceptibility or resistance to specific pathogens is most easily studied in inbred mice. We have been using mouse mammary tumor virus (MMTV), a retrovirus that causes mammary tumors in mice, to study virus/host interactions. These studies have focused on understanding the mechanisms that determine genetic susceptibility to MMTV-induced mammary tumors, the regulation of virus gene expression in vivo and how the virus is transmitted between different cell types. We have found that some endogenous MMTVs are only expressed in lymphoid tissue and that a single base pair change in the long terminal repeat of MMTV determines whether the virus is expressed in mammary gland. This expression in lymphoid cells is necessary for the infectious cycle of MMTV, and both T and B cells express and shed MMTV. Infected lymphocytes are required not only for the initial introduction of MMTV to the mammary gland, but also for virus spread at later times. Without this virus spread, mammary tumorigenesis is dramatically reduced. Mammary tumor incidence is also affected by the genetic background of the mouse and at least one gene that affects infection of both lymphocytes and mammary cells has not yet been identified. The results obtained from these studies will greatly increase our understanding of the genetic mechanisms that viruses use to infect their hosts and how genetic resistance to such viruses in the hosts occurs.

Sequencing of heparan sulfate proteoglycans: identification of variable and constant oligosaccharide regions in eight heparan sulfate proteoglycans of different origins

The sequence of the disacharide units of eight heparan sulfate proteoglycans of different origins is described. All heparan sulfates contain 5 variable regions made of oligosaccharide blocks of disaccharides, namely GlcUA(1-4) GlcNAc, GlcUA(1-4)GlcNS, IdoUA (104)GlcNS) and monosaccharides (GlcNS, and GlcNS,65) at the non-reducing terminal. The N-acetylated region of the heparan sulfates is linked to the serine of the protein core through a trisaccharide of Xyl-Gal-Gal. Heparan sulfates differ from one another in terms of the number of disaccharides that compose each block

Characterization of phosphoinositol oligosaccharides from parasitic protozoa by fast atom bombardment and collisional activation mass spectrometry

Fast atom bombardment mass spectrometry (FAB MS) enables the rapid, accurate and sensitive determination of the molecular masses of glycosylphosphatidylinositol(GPI)-derived oligosaccharides, from which the composition in terms of monosaccharide residues and non-carbohydrate substituents can be determined. Interpretation of fragment ions in collisional activation mass spectra further enables the determination of residue sequence, the positions of branch points, and the location of non-carbohydrate substituents. We have applied these techniques to the characterization of phosphoinositol oligosaccharides from Leptomonas samueli, Endotrypanum schaudinni and Leishmania adleri. The mass spectral data permit the postulation of candidate structures for the oligosaccharides, which provide a set of constraints that can assist the interpretation of results from other techniques such as NMR

Structures of four oligosaccharides derived from the glycoinositolphospholipid of Leishmania adleri

Glycoinositolphospholipids (GIPLs) were extracted from the trypanosomatid Leishmania adleri by hot phenol extraction and the carbohydrate moieties isolated after base cleavage. Purification of the crude oligosaccharides by high performance anion exchange (HPAE) chromatography yielded four fractions whose structures were determined by a combination of methylation analysis, fast atom bombardment (FAB) mass spectrometry and two-dimensional nuclear magnetic resonance (NMR) spectroscopy

The use of NMR spectroscopy in the structure determination of a Leptomonas samueli glycosylphosphosphingolipid-derived oligosaccharide

Nuclear magnetic resonance (NMR) spectroscopy provides an extremely powerful technique to determine the structure of oligosaccharides, particularly when used in conjuction with other physical techniques such as methylation analysis and fast atom bombardment mass spectroscopy (FAB-MS). This brief review describes the application of NMR to the determination of the structure of an oligosaccharide isolated from the glycophosphosphingolipid (GPS) from the monogenetic trypanosomatid Leptomonas samueli. Where ambiguities arise in the NMR interpretation, the use of other data will be discussed

Structural features of LPPG and related compounds isolated from Trypanosoma cruzi trypomastigotes

A hydrophobic fraction isolated from trypomastigotes of Trypanosoma cruzi is being characterized using immunological and chemical techniques. The lipopeptidophosphoglycan (LPPG) was identified in this fraction since it gave a positive reaction with anti-LPPG rabbit serum and had similar structural features such as the presence of ceramide as the lipid moiety, furanoic galactose, and a glycan moiety consistent with that obtained from an authentic sample of epimastigote LPPG, as judged by thin-layer chromatography. Furthermore, the hydrophobic fraction contained other glycolipids with different structural features. The lipid moiety of these compounds is alkylglycerol rather than a ceramide, the carbohydrate chain appears to be less complex than that in LPPG and no reactivity was observed towards an anti-LPPG serum

Characterization of a phospholipase C-resistant inositol containing glycolipid from Trypanosoma cruzi

Since glycosylphosphatidylinositol is the most common form of attachment of proteins to membranes in T. cruzi, and that this parasite depends on surface-mediated interactions for survival within the vector and mammalian host, it is probable that a drug which interfers with the metabolism of glycosylphosphatidylinositol (GPI) could be successfully employed in chemotherapy. Over the last few years several groups have been characterizing this mode of attachment in T. cruzi and more recently we have been concentrating our efforts on the identification of candidate precursors for protein anchors in metacyclic trypomastigotes. Previously detected GPI heterogeneity regarding solubilization of a major stage-specific antigen (1G7-Ag) by phospholipase C led us to investigate whether biosynthetic precursors with similar properties could also be identified. Two glycolipid species whose migration properties resemble glycolipids A and C of T. brucei were amenable to biosynthetic radiolabelling with palmitic acid, inositol, ethanolamine, glucosamine and mannose. Following purification, these species were submitted to classical GPI diagnostic treatments. In both cases digestion with GPI-specific phospholipase D (GPIPLD) produced phosphatidic acid and treatment with either mild base or phospholipase A2 (PLA2) produced free fatty acid, indicating an acylation at least at position 2 of the glycerol. The glycolipid A-like species proved to be susceptible to solubilization by PIPLC of B. thuriengiensis and by GPIPLC of T. brucei and the glycolipid C-like material proved to be fully resistant to both lipases. Although the glycolipid A-like species indeed presents these and other properties compatible with a precursor for the chemically characterized 1G7-Ag anchor, the PLC-resistant species which is completely insensitive to nitrous acid deamination might be an exception to the general finding of a non-acetylated glucosamine in the GPI moieties so far described

Free and protein-linked glycoinositolphospholipids in Trypanosoma cruzi

Two glycoinositol phospholipids (GIPL A and GIPL B) have been purified from epimastigotes of Trypanosoma cruzi at the logarithmic phase of growth (2 days). The GIPLs differ mainly in the lipid moiety and are similar to the lipopeptidophosphoglycan (LPPG) previously isolated from epimastigotes at the stationary phase (4-5 days). [3H]-palmitic acid was incorporated into 1-O-hexadecyl-2-O-palmitoylglycerol in GIPL A and into a sphinganine ceramide with palmitic acid and lignoceric acid as the fatty acids in GIPL B. The lipids could be released by incubation with phosphatidylinositol-specific phospholipase C (PI-PLC) or glycosylphosphatidylinositol phospholipase D (GPI-PLD) from rat serum. The oligosaccharides share the common core structure of the glycosylphosphatidilinositol (GPI) membrane anchors. Microheterogeneity was demonstrated, as well as substitution by galactose, which is mainly in the furanose configuration as was previously described for the LPPG. However, methylation analysis indicated that 20 percent of the galactose is present as terminal pyranose units. In infective trypomastigotes, [3H]-palmitic acid was incorporated into the anchor of the Tc-85 glycoprotein. The lipid cleaved by phospholipase C digestion was identified as 1-O-hexadecylglycerol and the main oligosaccharide has the structure of the conserved core of all GPI anchors. [3H]-palmitic acid-labelled Tc-85 released into the culture medium as membrane vesicles showed 80 percent resistance to the action of PI-PLC. However, after mild alkaline hydrolysis, part of the radioactivity was released by the enzyme

Molecular mimicry between Trypanosoma cruzi and host nervous tissues

Increasing evidence suggests that in Chagas' disease chronic-phase pathology is autoimmune in nature. There are at least two nonexclusive explanations for the generation of autoimmunity in Chagas disease: a) infection with the parasite perturbs immunoregulation, leading to loss of tolerance for self-antigens; b) immune recognition of T. cruzi antigens is crossreactive with selected mammalian antigens, leading to autoimmunity (molecular mimicry). Through this latter mechanism, T. cruzi antigens that share epitopes with mammalian nervous tissue may drive autoreactive B- or T-cell clones to expand and cause autoimmune lesions in chronic chagasic patients. Several different antigens sharing this characteristic have been studied, as for example the 160-kDa flagellum-associated surface protein (Fl-160), which has a nervous tissue crossreactive epitope composed by twelve amino acids. Additionally, it has been demonstrated that a trypomastigote stage-specific 85kDa surface glycoprotein (Gp85) has terminal galactosyl(alpha 1-3)galactose terminal residues, which are reactive with chronic chagasic sera. Common glycolipid antigens have also been reported, as for example galactocerebroside, sulfogalactocerebroside and sulfoglucuronylcerebroside, all of them specifically present at high concentrations in mammalian nervous system and in T. cruzi trypomastigotes. Chronic chagasic patients produce elevated levels of antibodies against these three glycolipid antigens. They also do against terminal galactosyl(alpha 1-3)galactose residues present on several acid and neutral glycolipids common either to nervous system or parasite. These antibodies are powerful lytic for circulating T. cruzi trypomastigotes. Another common strongly immunogenic residues are galactosyl(alpha 1-2)galactose, galactosyl(alpha 1-6)galactose and galactofuranosyl(beta 1-3)mannose residues present on several glycoinositolphospholipids (GIPL), against which chronic chagasic patients have elevated levels of specific antibodies. In brief, very specific host-parasite relationships existing only in Chagas' disease may explain the particular peripheral nervous tissue damage seen in acute or chronic stages of this disease. This specificity could depend either on invasion of autonomic ganglia by T. cruzi trypomastigotes and modification of nervous cell surface structures by some of the several mechanisms of acquired molecular mimicry