Glycosylation patterns in cells: an evolutionary marker?

Glycobiology of developing chicken kidney: Profiling the galectin family and selected β-galactosides

The concept of the sugar code interprets the cellular glycophenotype as a rich source of information read by glycan-lectin recognition in situ. This study's aim is the comprehensive characterization of galectin expression by immunohistochemistry during chicken nephrogenesis along with mapping binding sites by (ga)lectin histochemistry. Light and two-color fluorescence microscopy were used. First, six plant/fungal lectins that are specific for galectin-binding parts of N- and O-glycans were applied. The spatiotemporally regulated distributions of these glycans in meso- and metanephros equip cells with potential binding partners for the galectins. Complete galectin profiling from HH Stage 20 (about 70-72 hr) onward revealed cell-, galectin-, and stage-dependent expression patterns. Representatives of all three types of modular architecture of the galectin family are detectable, and overlaps of signal distribution in light and two-color fluorescence microscopy illustrate a possibility for functional cooperation among them. Performing systematic galectin histochemistry facilitated comparisons between staining profiles of plant lectins and galectins. They revealed several cases for differences so that tissue lectins appear to be selective among the β-galactosides. Notably, selectivity is also disclosed in intrafamily comparison. Thus, combining experimental series with plant and tissue lectins is a means to characterize target populations of glycans presented by cellular glycoconjugates for individual galectins. Our results document the presence and sophisticated level of elaboration among β-galactosides and among the members of the family of galectins during organogenesis, using chicken galectins and kidney as model. Thus, they provide a clear guideline for functional assays using supramolecular tools, cells, and organ cultures.

Radioactive fucose as a tool for studying glycoprotein secretion

The efficiency and reliability of radioactive fucose as a specific label for newly synthesized glycoproteins were investigated. Young adult male rabbits were injected intravitreally with [3H]-fucose, [3H]-galactose. [3H]-mannose, N-acetyl-[3H]-glucosamine or N-acetyl-[3H]-mannosamine, and killed 40 h after injection. In another series of experiments rabbits were injected with either [3H]-fucose or several tritiated amino acids and the specific activity of the vitreous proteins was determined. Vitreous samples were also processed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and histological sections of retina, ciliary body and lens (the eye components around the vitreous body) were processed for radioautography. The specific activity (counts per minute per microgram of protein) of the glycoproteins labeled with [3H]-fucose was always much higher than that of the proteins labeled with any of the other monosaccharides or any of the amino acids. There was a good correlation between the specific activity of the proteins labeled by any of the above precursors and the density of the vitreous protein bands detected by fluorography. This was also true for the silver grain density on the radioautographs of the histological sections of retina, ciliary body and lens. The contribution of radioautography (after [3H]-fucose administration) to the elucidation of the biogenesis of lysosomal and membrane glycoproteins and to the determination of the intracellular process of protein secretion was reviewed. Radioactive fucose is the precursor of choice for studying glycoprotein secretion because it is specific, efficient and practical for this purpose.

Active human erythropoietin expressed in insect cells using a baculovirus vector: a role for N-linked oligosaccharide

Biologically active recombinant human erythropoietin has been expressed at high levels in an insect cell background. Expression involved the preparation of a human erythropoietin cDNA, the transfer of this cDNA to the Autographa californica nuclear polyhedrosis virus (AcNPV) genome under the polyhedrin gene promoter, and the subsequent infection of Spodoptera frugiperda cells with recombinant AcNPV. Erythropoietin cDNA was prepared through the expression of the human erythropoietin gene in COS cells using pSV2 and the construction of a COS cell cDNA library in bacteriophage Lambda GT10. Prior to transfer to the AcNPV genome, erythropoietin cDNA isolated from this library was modified at the 3'-terminus in order to replace genomic erythropoietin for SV40 cDNA derived from pSV2. Transfer of this cDNA to AcNPV and the infection of S. frugiperda cells with cloned recombinant virus led to the secretion of erythropoietin: based on bioassay, rates of hormone secretion (over 40 U/ml per h) were 50-fold greater than observed for COS cells. The purified recombinant product possessed full biological activity (at least 200,000 U/mg), but was of lower Mr (23,000) than human erythropoietin produced in COS cells (30,000) or purified from urine (30,000 to 38,000). This difference was attributed to the glycosylation of erythropoietin in S. frugiperda cells with oligosaccharides of only limited size. Further removal of N-linked oligosaccharides from this Mr 23,000 hormone using N-Glycanase yielded an apo-erythropoietin (Mr 18,000) which possessed substantially reduced biological activity. These results indicate that glycosylation, but not the normal processing of oligosaccharides to complex types, is required for the full hormonal activity of human erythropoietin during red cell development.

Processing of pro-vitellogenin in insect fat body: a role for high-mannose oligosaccharide

Several discrete events were resolved in the processing of vitellogenin in Blattella germanica. Using tunicamycin to inhibit the synthesis of high-mannose oligosaccharide, a high molecular weight pro-vitellogenin peptide (apo-proVG, Mr 215,000) was identified in fat body. Dosages of tunicamycin which inhibited glycosylation of vitellogenin by 98% inhibited its synthesis by as much as 59%, yet led to an intracellular accumulation of apo-proVG. Reversibility and dose dependency of these effects on vitellogenin synthesis, glycosylation, proteolytic processing, and secretion were demonstrated. In control insects, glycosylation of apo-proVG yielded a Mr 240,000 pro-vitellogenin peptide (proVG). FITC-Concanavalin A bound to purified proVG but not to apo-proVG, thus confirming an absence of high-mannose oligosaccharide in the apo-protein. Following its glycosylation, proVG was processed rapidly in fat body to Mr 160,000 (VG160) and Mr 102,000 (VG102) peptides which subsequently were secreted into hemolymph. After uptake into developing oocytes, the VG160 peptide was processed further prior to chorionation, yielding subunits of Mr 95,000 and 50,000. Uniqueness of the peptides of mature vitellin (Mr 102,000, 95,000, and 50,000) was indicated by comparison of the CNBr fragments of each purified subunit. Staining of CNBr fragments with FITC-Concanavalin A also indicated that high-mannose oligosaccharides are attached at one or more sites within each vitellin subunit. Resolution of the substructure of this insect vitellin and identification of events involved in the processing and secretion of its fat body apo-protein provide a basis for further study of the assembly and transport of vitellogenin, its packaging in eggs, and utilization during embryogenesis.

A comparative study of the size-heterogeneous high mannose oligosaccharides of some insect vitellins

Comparative studies of the carbohydrate component from vitellins of the cockroaches Blattella germanica, Blaberus discoidalis, Periplaneta americana and Simploce capitata and the locust Locusta migratoria have been conducted. Chemical, enzymatic and chromatographic analyses show that each vitellin contains variably processed high mannose type oligosaccharides. While all have a common size range they occur as two distinct classes based on the proportion of individual saccharides present. Oligosaccharide size distribution is not a characteristic of an individual animal but of the species. Because oligosaccharide heterogeneity also occurs in B. germanica vitellogenin (the hemolymph precursor of vitellin), it does not result from structural changes during or after its uptake by the egg.

Swainsonine affects the processing of glycoproteins in vivo

Rats, sheep and guinea pigs treated with swainsonine excrete 'high mannose' oligosaccharides in urine. The major rat and guinea pig oligosaccharide is (Man)5GlcNAc, whereas sheep excrete a mixture of oligosaccharides of composition (Man)2-5GlcNAc2 and (Man)3-5GlcNAc. The presence of these oligosaccharides suggests that Golgi alpha-D-mannosidase II as well as lysosomal alpha-D-mannosidase is inhibited by swainsonine resulting in storage of abnormally processed asparagine-linked glycans from glycoproteins. Altered glycoprotein processing appears to have little effect on the health of the intoxicated animal, but the accompanying lysosomal storage produces a disease state.