Structures of sugar chains of the third component of human complement

High-throughput N-glycan analysis in aging and inflammaging: State of the art and future directions

As the global population ages at an unprecedented rate, the prevalence of age-related diseases is increasing, making inflammaging - a phenomenon characterized by a chronic, low-grade inflammatory state that follows aging - a significant concern. Understanding the mechanisms of inflammaging and its impact on health is critical for developing strategies to improve the quality of life and manage health in the aging population. Despite their crucial roles in various biological processes, including immune response modulation, N-glycans, oligosaccharides covalently attached to many proteins, are often overlooked in clinical and research studies. This repeated oversight is largely due to their inherent complexity and the complexity of the analysis methods. High-throughput N-glycan analysis has emerged as a transformative tool in N-glycosylation research, enabling cost- and time-effective, detailed, and large-scale examination of N-glycan profiles. This paper is the first to explore the application of high-throughput N-glycomics techniques to investigate the complex interplay between N-glycosylation and the immune system in aging. Technological advancements have significantly improved Nglycan detection and characterization, providing insights into age-related changes in Nglycosylation. Key findings highlight consistent shifts in immunoglobulin G (IgG) and plasma/serum glycoprotein glycosylation with age, with a pronounced rise in agalactosylated structures bound to IgG that also affect the composition of the total plasma N-glycome. These N-glycan modifications seem to be strongly associated with inflammaging and have been identified as valuable biomarkers for biological age, predictors of disease risk, and proxy biomarkers for monitoring intervention efficacy at the individual level. Despite current challenges related to data complexity and methodological limitations, ongoing technological innovations and interdisciplinary research are expected tofurther advance our knowledge of glycan biology, improve diagnostic and therapeutic strategies, and promote healthier aging. The integration of glycomics with other omics approaches holds promise for a more comprehensive understanding of the aging immune system, paving the way for personalized medicine and targeted interventions to mitigate inflammaging. In conclusion, this paper underscores the transformative impact of high-throughput Nglycan analysis in aging and inflammaging.

High-Throughput Human Complement C3 N-Glycoprofiling Identifies Markers of Early Onset Type 1 Diabetes Mellitus in Children

Recently, it was shown that children at the onset of type 1 diabetes (T1D) have a higher proportion of oligomannose glycans in their total plasma protein N-glycome compared to their healthy siblings. The most abundant complement component, glycoprotein C3, contains two N-glycosylation sites occupied exclusively by this type of glycans. Furthermore, complement system, as well as C3, was previously associated with T1D. It is also known that changes in glycosylation can modulate inflammatory responses, so our aim was to characterize the glycosylation profile of C3 in T1D. For this purpose, we developed a novel high-throughput workflow for human C3 concanavalin A lectin affinity enrichment and subsequent LC-MS glycopeptide analysis which enables protein-specific N-glycosylation profiling. From the Danish Childhood Diabetes Register, plasma samples of 61 children/adolescents newly diagnosed with T1D and 84 of their unaffected siblings were C3 N-glycoprofiled. Significant changes of C3 N-glycan profiles were found. T1D was associated with an increase in the proportion of unprocessed glycan structures with more mannose units. A regression model including C3 N-glycans showed notable discriminative power between children with early onset T1D and their healthy siblings with area under curve of 0.879. This study confirmed our previous findings of plasma high-mannose glycan changes in a cohort of recent onset T1D cases, suggesting the involvement of C3 N-glycome in T1D development. Our C3 glycan-based discriminative model could be valuable in assessment of T1D risk in children.

Plasma proteome plus site-specific N-glycoprofiling for hepatobiliary carcinomas

Hepatobiliary cancer is the third leading cause of cancer death worldwide. Appropriate markers for early diagnosis, monitoring of disease progression, and prediction of postsurgical outcome are still lacking. As the majority of circulating N‐glycoproteins are originated from the hepatobiliary system, we sought to explore new markers by assessing the dynamics of N‐glycoproteome in plasma samples from patients with hepatocellular carcinoma (HCC), cholangiocarcinoma (CCA), or combined HCC and CCA (cHCC‐CCA). Using a mass spectrometry‐based quantitative proteomic approach, we found that 57 of 5358 identified plasma proteins were differentially expressed in hepatobiliary cancers. The levels of four essential proteins, including complement C3 and apolipoprotein C‐III in HCC, galectin‐3‐binding protein in CCA, and 72 kDa inositol polyphosphate 5‐phosphatase in cHCC‐CCA, were highly correlated with tumor stage, tumor grade, recurrence‐free survival, and overall survival. Postproteomic site‐specific N‐glycan analyses showed that human complement C3 bears high‐mannose and hybrid glycoforms rather than complex glycoforms at Asn85. The abundance of complement C3 with mannose‐5 or mannose‐6 glycoform at Asn85 was associated with HCC tumor grade. Furthermore, stepwise Cox regression analyses revealed that HCC patients with a hybrid glycoform at Asn85 of complement C3 had a lower postsurgery tumor recurrence rate or mortality rate than those with a low amount of complement C3 protein. In conclusion, our data show that particular plasma N‐glycoproteins with specific N‐glycan compositions could be potential noninvasive markers to evaluate oncological status and prognosis of hepatobiliary cancers.

Alterations of the Human Skin N- and O-Glycome in Basal Cell Carcinoma and Squamous Cell Carcinoma

The glycome of one of the largest and most exposed human organs, the skin, as well as glycan changes associated with non-melanoma skin cancers have not been studied in detail to date. Skin cancers such as basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) are among the most frequent types of cancers with rising incidence rates in the aging population. We investigated the healthy human skin N- and O-glycome and its changes associated with BCC and SCC. Matched patient samples were obtained from frozen biopsy and formalin-fixed paraffin-embedded tissue samples for glycomics analyses using two complementary glycomics approaches: porous graphitized carbon nano-liquid chromatography electro spray ionization tandem mass spectrometry and capillary gel electrophoresis with laser induced fluorescence detection. The human skin N-glycome is dominated by complex type N-glycans that exhibit almost similar levels of α2-3 and α2-6 sialylation. Fucose is attached exclusively to the N-glycan core. Core 1 and core 2 type O-glycans carried up to three sialic acid residues. An increase of oligomannose type N-glycans and core 2 type O-glycans was observed in BCC and SCC, while α2-3 sialylation levels were decreased in SCC but not in BCC. Furthermore, glycopeptide analyses provided insights into the glycoprotein candidates possibly associated with the observed N-glycan changes, with glycoproteins associated with binding events being the most frequently identified class.

Comparison of RP-HPLC modes to analyse the N-glycome of the free-living nematode Pristionchus pacificus

Pristionchus pacificus is a free-living nematode increasingly used as an organism for comparison to the more familiar model Caenorhabditis elegans. In this study, we examined the N-glycans of this organism isolated after serial release with peptide:N-glycosidases F and A; after fluorescent labelling with 2-aminopyridine, chromatographic fractionation by three types of RP-HPLC (with either classical C18, fused core C18 or alkylamide-bonded phases) followed by mass spectrometric analyses revealed key features of its N-glycome. In addition to paucimannosidic and oligomannosidic glycans typical of invertebrates, N-glycans with two core fucose residues were detected. Furthermore, a range of glycans carrying up to three phosphorylcholine residues was observed whereas, unlike C. elegans, no tetrafucosylated N-glycans were detected. Structures with three fucose residues, unusual methylation of core α1,3-fucose or with galactosylated fucose motifs were found in low amounts; these features may correlate with a different ensemble or expression of glycosyltransferase genes as compared to C. elegans. From an analytical perspective, both the alkylamide RP-amide and fused core C18 columns, as compared to a classical C18 material, offer advantages in terms of resolution and of elution properties, as some minor pyridylamino-labelled glycans (e.g. those carrying phosphorylcholine) appear in earlier fractions and so potential losses of such structures due to insufficient gradient length can be avoided.

A revised mechanism for the activation of complement C3 to C3b: a molecular explanation of a disease-associated polymorphism

The solution structure of complement C3b is crucial for the understanding of complement activation and regulation. C3b is generated by the removal of C3a from C3. Hydrolysis of the C3 thioester produces C3u, an analog of C3b. C3b cleavage results in C3c and C3d (thioester-containing domain; TED). To resolve functional questions in relation to C3b and C3u, analytical ultracentrifugation and x-ray and neutron scattering studies were used with C3, C3b, C3u, C3c, and C3d, using the wild-type allotype with Arg(102). In 50 mm NaCl buffer, atomistic scattering modeling showed that both C3b and C3u adopted a compact structure, similar to the C3b crystal structure in which its TED and macroglobulin 1 (MG1) domains were connected through the Arg(102)-Glu(1032) salt bridge. In physiological 137 mm NaCl, scattering modeling showed that C3b and C3u were both extended in structure, with the TED and MG1 domains now separated by up to 6 nm. The importance of the Arg(102)-Glu(1032) salt bridge was determined using surface plasmon resonance to monitor the binding of wild-type C3d(E1032) and mutant C3d(A1032) to immobilized C3c. The mutant did not bind, whereas the wild-type form did. The high conformational variability of TED in C3b in physiological buffer showed that C3b is more reactive than previously thought. Because the Arg(102)-Glu(1032) salt bridge is essential for the C3b-Factor H complex during the regulatory control of C3b, the known clinical associations of the major C3S (Arg(102)) and disease-linked C3F (Gly(102)) allotypes of C3b were experimentally explained for the first time.

Improved method for drawing of a glycan map, and the first page of glycan atlas, which is a compilation of glycan maps for a whole organism

Glycan Atlas is a set of glycan maps over the whole body of an organism. The glycan map that includes data of glycan structure and quantity displays micro-heterogeneity of the glycans in a tissue, an organ, or cells. The two-dimensional glycan mapping is widely used for structure analysis of N-linked oligosaccharides on glycoproteins. In this study we developed a comprehensive method for the mapping of both N- and O-glycans with and without sialic acid. The mapping data of 150 standard pyridylaminated glycans were collected. The empirical additivity rule which was proposed in former reports was able to adapt for this extended glycan map. The adapted rule is that the elution time of pyridylamino glycans on high performance liquid chromatography (HPLC) is expected to be the simple sum of the partial elution times assigned to each monosaccharide residue. The comprehensive mapping method developed in this study is a powerful tool for describing the micro-heterogeneity of the glycans. Furthermore, we prepared 42 pyridylamino (PA-) glycans from human serum and were able to draw the map of human serum N- and O-glycans as an initial step of Glycan Atlas editing.

Human complement factor 3 polymorphism determination by capillary electrophoresis of serum

Variability of complement factor 3 (C3) mobility in serum protein electrophoresis was investigated. We found that the migration time of C3 can be reproducibly determined (beween-run CV=0.76%) using clinical capillary electrophoresis (CE) equipment (the Capillarys™ 2 system, Sebia). Moreover, we found a significant difference (p<0.001) in migration times of the major C3 phenotypes FF (fast-fast), FS (fast-slow) and SS (slow-slow). Glycosylation did not significantly affect test results. This is the first report on the migration time of C3 phenotypes on a clinical CE instrument. The presented method allows faster data than agarose-electrophoresis or genotyping. Moreover, reference ranges for serum C3 concentration depend on C3 phenotype, which allows a better tailored clinical interpretation of C3 concentrations.

Self-association and domain rearrangements between complement C3 and C3u provide insight into the activation mechanism of C3

Component C3 is the central protein of the complement system. During complement activation, the thioester group in C3 is slowly hydrolysed to form C3u, then the presence of C3u enables the rapid conversion of C3 into functionally active C3b. C3u shows functional similarities to C3b. To clarify this mechanism, the self-association properties and solution structures of C3 and C3u were determined using analytical ultracentrifugation and X-ray scattering. Sedimentation coefficients identified two different dimerization events in both proteins. A fast dimerization was observed in 50 mM NaCl but not in 137 mM NaCl. Low amounts of a slow dimerization was observed for C3u and C3 in both buffers. The X-ray radius of gyration RG values were unchanged for both C3 and C3u in 137 mM NaCl, but depend on concentration in 50 mM NaCl. The C3 crystal structure gave good X-ray fits for C3 in 137 mM NaCl. By randomization of the TED (thioester-containing domain)/CUB (for complement protein subcomponents C1r/C1s, urchin embryonic growth factor and bone morphogenetic protein 1) domains in the C3b crystal structure, X-ray fits showed that the TED/CUB domains in C3u are extended and differ from the more compact arrangement of C3b. This TED/CUB conformation is intermediate between those of C3 and C3b. The greater exposure of the TED domain in C3u (which possesses the hydrolysed reactive thioester) accounts for the greater self-association of C3u in low-salt conditions. This conformational variability of the TED/CUB domains would facilitate their interactions with a broad range of antigenic surfaces. The second dimerization of C3 and C3u may correspond to a dimer observed in one of the crystal structures of C3b.

Gas-Phase Pyridylamination of Saccharides: Development and Applications

Pyridylamination is a versatile method for fluorescence labeling of oligosaccharides. The technique affords sensitive detection of saccharides with reducing termini and high-resolution separation by high-performance liquid chromatography. The conventional method, based on a liquid-phase reaction, has been extensively used in various aspects of glycobiology and glycotechnology. Unfortunately, the necessity for removing excess 2-aminopyridine makes the technique both laborious and time-consuming. Furthermore, removal of excess reagent can result in a significant loss of short saccharide components. In the present paper, we report an alternative methodology based on a "gas-phase" reaction, in which dried saccharides are reacted with vaporized 2-aminopyridine. The resultant Schiff base was also reduced in the gas phase within the same reaction microtube using a purpose-built device. The newly developed procedure was applied to both monosaccharide (GlcNAc) and oligosaccharides (isomalto-oligosaccharides) at quantitative yields with no requirement to remove excess reagent. The acid-labile sialyl linkages of alpha2-6-disialobiantennary oligosaccharides proved to be fully stable during the procedure. The developed method was also successfully applied to profiling N-linked oligosaccharides liberated from glycoproteins by hydrazinolysis and, thus, should contribute to various fields of glycomics.

Structural characterization of a rhamnose-binding glycoprotein (lectin) from Spanish mackerel (Scomberomorous niphonius) eggs

A rhamnose-binding glycoprotein (lectin), named SML, was isolated from the eggs of Spanish mackerel (Scomberomorous niphonius) by affinity and ion-exchange chromatographies. SML was composed of a non-covalently linked homodimer. The SML subunit was composed of 201 amino acid residues with two tandemly repeated domains, and contained 8 half-Cys residues in each domain, which is highly homologous to the N-terminal lectin domain of calcium-independent alpha-latrotoxin receptor in mammalian brains. Each domain has the same disulfide bonding pattern; Cys10-Cys40, Cys20-Cys99, Cys54-Cys86 and Cys67-Cys73 were located in the N-terminal domain, and Cys108-Cys138, Cys117-Cys195, Cys152-Cys182 and Cys163-Cys169 were in the C-terminal domain. SML was N-glycosylated at Asn168 in the C-terminal domain. The structure of the sugar chain was determined to be NeuAc-Galbeta1-4GlcNAcbeta1-2Manalpha1-6-(NeuAc-Galbeta1-4GlcNAcbeta1-2Manalpha1-3)Manbeta1-4GlcNAcbeta1-4GlcNAc-Asn.

Structure of pentasaccharide of glycopeptide from TIME-EA4, N-glycoprotein in silkworm diapause eggs

The TIME-EA4, from silkworm diapause eggs of pure strain C108, Bombyx mori, has glycosylated chain as tetrasaccharide (Man(2)GlcNAc(2)) attaching to the Asn(22) of T3 peptide from tryptic digests. On the other hand, from Showa silkworm strain we additionally observed a pentasaccharide (Man(3)GlcNAc(2)) on T3 at the same linkage site. The linkage pattern of the 5-sugar chain was studied through Smith degradation combined with LC-MS and MS/MS analyses. These advanced methods led us to conclude that the pentasaccharide was branching as Man 1-->3(Man 1-->6)Man 1-->4GlcNAc 1-->4GlcNAc.

Calnexin is associated with and induced by overexpressed human complement protein C2

C2 is a serum glycoprotein that is essential for activation of the classical and lectin pathways of the complement system. We reported previously that in transiently transfected COS cells, C2 accumulates in the endoplasmic reticulum-Golgi intermediate compartment (ERGIC). Transfection with a cDNA corresponding to a variant C2 mRNA in which exon 17 is spliced out, C2Delta(17), resulted in retention of the mutant polypeptide in the ER. We now show that calnexin, a lectin-like chaperone, colocalizes with wild-type (wt) C2 and C2Delta(17). Biosynthetic labeling and sequential immunoprecipitation experiments indicated that colocalization is due to a physical association between calnexin and C2. Immunofluorescence analysis indicated that calnexin was upregulated in cells transfected with either C2 species. Upregulation of calnexin was not affected by castanospermine, which inhibits glucosidases I and II. However, castanospermine inhibited translocation of calnexin to the ERGIC in wt C2 transfected cells. Upregulation of calnexin was also observed in cells transfected with the complement protein factor B, a glycoprotein with extensive structural and functional similarities to C2, but not in cells transfected with complement proteins C3 or factor D, which have no structural similarity to C2, and low or no glycan content, respectively. Calnexin upregulation by transfection with C2 or factor B, but not factor D, was also demonstrated by quantitative analysis of calnexin immunoprecipitates from biosynthetically labeled cells. Increased calnexin expression by overexpressed C2 and factor B appears to be triggered either by the high glycan content of these proteins or, since it also occurs in the presence of castanospermine, by shared features of the structure of these two proteins.

A pyridylamination method aimed at automatic oligosaccharide analysis of N-linked sugar chains

The procedure for preparation of pyridylaminated sugar chains from glycoproteins was improved with a view to its eventual automation. Following on the coupling reaction improvement already reported [N. Kuraya and S. Hase (1992) J. Biochem. 112, 122-126], two further aspects were improved in this study. Instead of sodium bicarbonate-acetic anhydride, volatile reagents were adopted for the re-N-acetylation of hexosamine residues after hydrazinolysis to give rapid removal of excess reagents. Subsequent to the pyridylamination reaction, excess reagents were removed by cation-exchange to isolate the pyridylaminated oligosaccharides in place of gel filtration. These alterations rendered a one-pot reaction possible and resulted in a large reduction in the amount of time needed compared with other methods so far reported. The procedure was successfully applied to the detection of sugar chains from Taka-amylase A and human erythrocyte membranes.

Cloning and expression of a novel galactoside beta1, 3-glucuronyltransferase involved in the biosynthesis of HNK-1 epitope

We isolated a cDNA encoding a novel glucuronyltransferase, designated GlcAT-D, involved in the biosynthesis of the HNK-1 carbohydrate epitope from rat embryo cDNA by the degenerate polymerase chain reaction method. The new cDNA sequence revealed an open reading frame coding for a protein of 324 amino acids with type II transmembrane protein topology. The amino acid sequence of GlcAT-D displayed 50.0% identity to rat GlcAT-P, which is involved in the biosynthesis of the HNK-1 epitope on glycoproteins. Expression of GlcAT-D in COS-7 cells resulted in the formation of the HNK-1 epitope on the cell surface. The enzyme expressed in COS-7 cells transferred a glucuronic acid (GlcA) not only to asialo-orosomucoid, a glycoprotein bearing terminal N-acetyllactosamine structure, but also to paragloboside (lacto-N-neotetraosylceramide), a precursor of the HNK-1 epitope on glycolipids. Furthermore, substrate specificity analysis using a soluble chimeric form of GlcAT-D revealed that GlcAT-D transfers a GlcA not only to Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc-pyridylamine++ + but also to Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc-pyridylamine++ +. Enzymatic hydrolysis and Smith degradation of the reaction product indicated that GlcAT-D transfers a GlcA through a beta1,3-linkage to a terminal galactose. The GlcAT-D transcripts were detected in embryonic, postnatal, and adult rat brain. In situ hybridization analysis revealed that the expression pattern of GlcAT-D transcript in embryo is similar to that of GlcAT-P, but distinct expression of GlcAT-D was observed in the embryonic pallidum and retina. Regions that expressed GlcAT-D and/or GlcAT-P were always HNK-1-positive, indicating that both GlcATs are involved in the synthesis of the HNK-1 epitope in vivo.

Distinction between processing of normal and mutant complement C3 within human skin fibroblasts

Inherited C3 deficiency may result from mutations in the C3 gene affecting transcription or translation (type I deficiency). We described a type II C3 deficiency caused by a mutation yielding an abnormal non-secreted C3. The post-translational processing of mutant and normal C3 was analyzed in fibroblasts grown from skin biopsies. Mutant C3 is located mainly in the endoplasmic reticulum (ER), whereas normal C3 is seen evenly distributed throughout the cytoplasm. Most of the mutant C3 is degraded within the cell, and only a small fraction (around 8%) is secreted after 20 h chase. Processing of C3 at 19 degrees C was reduced in normal fibroblasts but completely blocked in mutant fibroblasts. ATP depletion blocked processing of normal proC3 to C3. In contrast, the mutant proC3 was partly degraded in ATP-depleted cells, yet its complete degradation and secretion were blocked. Intracellular degradation of the mutant C3 was not inhibited by NH4Cl, thus excluding cleavage within lysosomes. These results demonstrate that the type II mutant C3 studied here is retained in the ER probably by a quality contol machinery that identifies abnormal protein folding. Consequently, it is destined to undergo a two-step intracellular degradation; an initial ATP-independent step followed by an ATP-dependent step.

Substrate specificities of recombinant murine Golgi alpha1, 2-mannosidases IA and IB and comparison with endoplasmic reticulum and Golgi processing alpha1,2-mannosidases

The catalytic domains of murine Golgi alpha1,2-mannosidases IA and IB that are involved in N-glycan processing were expressed as secreted proteins in P.pastoris . Recombinant mannosidases IA and IB both required divalent cations for activity, were inhibited by deoxymannojirimycin and kifunensine, and exhibited similar catalytic constants using Manalpha1,2Manalpha-O-CH3as substrate. Mannosidase IA was purified as a 50 kDa catalytically active soluble fragment and shown to be an inverting glycosidase. Recombinant mannosidases IA and IB were used to cleave Man9GlcNAc and the isomers produced were identified by high performance liquid chromatography and proton-nuclear magnetic resonance spectroscopy. Man9GlcNAc was rapidly cleaved by both enzymes to Man6GlcNAc, followed by a much slower conversion to Man5GlcNAc. The same isomers of Man7GlcNAc and Man6GlcNAc were produced by both enzymes but different isomers of Man8GlcNAc were formed. When Man8GlcNAc (Man8B isomer) was used as substrate, rapid conversion to Man5GlcNAc was observed, and the same oligosaccharide isomer intermediates were formed by both enzymes. These results combined with proton-nuclear magnetic resonance spectroscopy data demonstrate that it is the terminal alpha1, 2-mannose residue missing in the Man8B isomer that is cleaved from Man9GlcNAc at a much slower rate. When rat liver endoplasmic reticulum membrane extracts were incubated with Man9GlcNAc2, Man8GlcNAc2was the major product and Man8B was the major isomer. In contrast, rat liver Golgi membranes rapidly cleaved Man9GlcNAc2to Man6GlcNAc2and more slowly to Man5GlcNAc2. In this case all three isomers of Man8GlcNAc2were formed as intermediates, but a distinctive isomer, Man8A, was predominant. Antiserum to recombinant mannosidase IA immunoprecipitated an enzyme from Golgi extracts with the same specificity as recombinant mannosidase IA. These immunodepleted membranes were enriched in a Man9GlcNAc2to Man8GlcNAc2-cleaving activity forming predominantly the Man8B isomer. These results suggest that mannosidases IA and IB in Golgi membranes prefer the Man8B isomer generated by a complementary mannosidase that removes a single mannose from Man9GlcNAc2.

Overexpression, purification, and characterization of third component of complement

The third component of complement (C3) plays a critical role in both pathways of complement activation by interacting with numerous other complement proteins. To elucidate the molecular features of C3 that relate to the functional activities of the molecule, we expressed the cDNA of human complement component C3 in cultured insect cells using a baculovirus expression vector system derived from the baculovirus Autographa california nuclear polyhedrosis virus (AcNPV). The expression of C3 was controlled by the promoter of the polyhedrin gene and, when recombinant baculovirus infected insect cells were cultured in serum-free medium, C3 was detected at a level of 10 micrograms/ml of culture medium. Characterization of the recombinant C3 (rC3) by SDS-PAGE revealed that the C3 gene product was translated as a 188 kDa protein comprised of two chains of 115 kDa and 73 kDa analogous to the alpha and beta chains of serum-derived human C3 (sC3). An analysis of the glycosylation pattern of purified rC3 revealed that, whereas both the alpha and beta chains were glycosylated as in sC3, the proC3 moiety of rC3 also was glycosylated. When rC3 was produced in the High Five cell line of insect cells and evaluated for reactivity with a panel of anti-C3 monoclonal antibodies (MoAb), the results suggested that the conformation of the baculovirus expressed C3 was similar to that of native C3. When the rC3 was purified by anion exchange column chromatography, it was able to react with several C3-binding proteins (CR1, P and H), reconstitute C3-deficient serum and support the activation of both complement pathways thus demonstrating that a baculovirus-expressed C3 can participate in the formation of and can be cleaved by both the classical and alternative pathway convertases. Incubation of rC3 with factor I and H revealed that both C3 and proC3 are susceptible to cleavage by factor I.

Structural characterization of the N-linked carbohydrate chains from mouse zona pellucida glycoproteins ZP2 and ZP3

N-linked oligosaccharides of mouse zona pellucida glycoproteins ZP2 and ZP3 were prepared as pyridylaminated derivatives. Anion-exchange HPLC revealed that 95% or more of them are acidic. About 80% of these acidic chains were neutralized by digestion with Arthrobacter ureafaciens sialidase and a small amount of sulfate was found in the residual acidic chains. The neutralized fractions (SN fractions) from ZP2 and ZP3 were similarly fractionated by size-fractionation HPLC into many peaks having different numbers of LacNAc in the non-reducing regions. The SN fractions from ZP2 were cleaved by endo-beta-galactosidase into the core and non-reducing regions. The tri- and tetra-antennary complex-type chains with a Fuc residue were predominant in the core region. On the other hand, four kinds of fragment containing the LacNAc sequence were obtained from the non-reducing region. Sialic acids were shown to be linked to the core fragments, as well as the non-reducing fragments. The N-linked oligosaccharides of ZP3 were suggested to have essentially the same structures as those of ZP2.

The cDNA cloning of conglutinin and identification of liver as a primary site of synthesis of conglutinin in members of the Bovidae

Bovine conglutinin is a collagen-like, C-type, plasma lectin which belongs to the group of proteins called 'collectins'. Two inosine-containing oligonucleotides were synthesized, based on the published protein sequence for bovine conglutinin [Lee, Leiby, Allar, Paris, Lerch and Okarma (1991) J. Biol. Chem. 266, 2715-2723], and PCR on target DNA from a bovine liver lambda gt 11 cDNA library yielded a product of the expected size of 210 bp. Screening of the library with this cDNA fragment identified a single positive clone, with an insert of 0.9 kb, coding for bovine conglutinin from residue 70 to the C-terminus. The 5' cDNA sequence, encompassing 150 bp of the 5' non-translated sequence plus the sequence encoding the leader peptide and the N-terminal residues 1-70, was completed by the use of PCR techniques. The cDNA sequence of bovine conglutinin showed 86% identity with that of bovine lung surfactant protein D (SP-D), and the derived amino acid sequence of bovine conglutinin showed 78% identity with that of bovine SP-D, which included complete identity of the leader-peptide sequences. The amino acid sequence derived from the cDNA sequence differs from the published protein sequence at four positions. Northern-blot analysis on total RNA, purified from various tissues from cattle, sheep, humans, rats and mice, showed that a strong signal of approx. 1.8 kb is present in bovine liver RNA. A weak signal of similar size was also observed in sheep liver, but not in human, rat and mouse livers. A weak signal, also of 1.8 kb, is present in the lung RNAs of all the species tested. The signals from the lung tissues are likely to be due to the cross-hybridization of the bovine conglutinin cDNA to the SP-D mRNAs of the respective species. The finding of significant signals in only the bovine and sheep liver RNA samples is indicative that serum conglutinin may be present in significant amounts only in members of the Bovidae (the family encompassing cattle, antelopes, sheep and goats) and closely related species.

Disulfide bridges in human complement component C3b

The disulfide bridges of human complement component C3b, derived from C3 by removal of the 77-residue C3a, have been determined. The 10 bridges are Cys537-Cys794, Cys605-Cys640, Cys851-Cys1491, Cys1079-Cys1136, Cys1336-Cys1467, Cys1367-Cys1436, Cys1484-Cys1489, Cys1496-Cys1568, Cys1515-Cys1639, and Cys1615-Cys1624. Including the 3 bridges in C3a (Cys670-Cys698, Cys672-Cys705, and Cys685-Cys706) previously determined by high-resolution X-ray crystallography [Hoppe-Seyler's Z. Physiol. Chem. 361 (1980) 1389-1399] all disulfide bridges of C3 are localized. C3 and the strongly related C4 and C5 are members of the alpha 2-macroglobulin superfamily. The predicted bridge patterns of C4 and C5 are discussed and compared with that of alpha 2-macroglobulin.

Hereditary deficiency of C3 in animals and humans

Inherited deficiency of complement C3 has been described in guinea pigs, dogs and 20 humans. Homozygous deficiency of C3 is associated with recurrent pyogenic infections by encapsulated bacteria, especially H. influenzae, S. pneumoniae and N. meningitidis. In dogs and humans there is also an association with development of glomerulonephritis of the mesangiocapillary type. Some patients also develop transient erythematous rashes in association with pyogenic infections, with histology showing predominantly neutrophil infiltration and small vessel vasculitis. Studies of antibody responses, mainly in experimental animals have shown impaired primary and secondary responses to both thymus-dependent and -independent antigens at low immunizing doses, with a reduced switch from IgM to IgG production. The molecular basis of C3 deficiency has been established in two humans with C3 deficiency. In one it was due to a splice junction mutation and in another, to a partial gene deletion. These mutations are not compatible with the production of functional C3 in any tissue. Such patients with absolute C3 deficiency are a valid model for understanding the physiological role of C3 in vivo.

Structure of the acidic N-linked carbohydrate chains of the 55-kDa glycoprotein family (PZP3) from porcine zona pellucida

N-linked carbohydrate chains of the major 55-kDa family, PZP3, of porcine zona pellucida glycoproteins are composed of neutral (28%) and acidic (72%) complex-type chains. The structures of the main components of the neutral chain have been established [Noguchi, S., Hatanaka, Y., Tobita, T. & Nakano, M. (1992) Eur. J. Biochem. 204, 1089-1100]. Here we report the structures of the acidic chains. Only two kinds of acidic fragments were released from PZP3 by endo-beta-galactosidase digestion following beta-elimination of O-linked chains. 500-MHz one-dimensional and two-dimensional 1H-NMR spectroscopy revealed their structures to be Sia alpha(2-3)Gal beta(1-4) [HSO3-6]GlcNAc beta(1-3)Gal and HSO3-6GlcNAc beta(1-3)Gal, showing that the sulfate-containing acidic chains are constructed with non-branched N-acetyllactosamine repeats which have sialic acid(s) at the non-reducing end(s) and sulfate at the C-6 position of GlcNAc residues. The acidic N-linked chains obtained from PZP3 by hydrazinolysis were separated into diantennary chains (34%) and tri- and tetra-antennary chains (66%) by concanavalin-A--agarose gel chromatography. The diantennary chains and their sialidase digests were fractionated by DEAE-HPLC. From the analyses of the endo-beta-galactosidase digests of each fraction, structures of the diantennary acidic chains were determined. They are classified into four groups. The first group is the sialylated chains without the sulfated N-acetyllactosamine repeating unit. The other three groups have the chains of various lengths differing in the number of monosulfated N-acetyllactosamine unit. These chains are extended from the Man alpha(1-3) branch of the trimannosyl core in the second group, from the Man alpha(1-6) branch in the third group, and from both branches in the fourth group. The structural features of the tri- and tetra-antennary acidic chains are also presented.

Cis- and trans-acting elements required for constitutive and cytokine-regulated expression of the mouse complement C3 gene

The third component of complement (C3) is an important mediator of inflammation. Murine and human genomic cosmid clones were isolated, characterized and sequenced 5' to the complement C3 gene transcriptional initiation sites to determine cis elements that participate in constitutive and regulated C3 gene expression. The murine and human 5' flanking regions are 51% identical overall, with positions -36 to -1 and -146 to -68 showing 80% identity. Four TATA boxes were identified upstream of the murine transcriptional initiation site, but deletion and transfection analysis using reporter gene constructs in HepG2 cells indicated that only the TATA element at position -30, together with sequences -395 to -111, are essential for constitutive expression of murine C3 in hepatocytes. Deletion analysis also suggested that sequences between -1457 and -800 contain regulatory elements that are involved in suppressing basal expression. Sequences between -90 to -41 confer both enhancer activity and interleukin-1/-6 (IL-1/IL-6)-responsiveness. Mutation analyses showed that both sequences between -88 and -83 and -77 to -72 are essential for enhancer activity and responsiveness to IL-1, but only sequences between -88 and -83 are necessary for IL-6-responsiveness. A gel-retardation assay showed that several nucleoproteins, perhaps of the C/EBP family, from HepG2 cells bound to sequences between -88 to -83. Collectively, these results localize cis-acting elements involved in constitutive and IL-1/IL-6-regulated murine C3 gene expression and provide evidence for specific transacting factors.

Structure of the major oligosaccharide of cobra venom factor

Cobra venom factor (CVF), the complement-activating glycoprotein in cobra venom, contains three or possibly four N-linked oligosaccharide chains per molecule and is devoid of O-linked saccharides. Analysis by lectin-affinity staining revealed the presence of complex-type oligosaccharides containing non-reducing terminal alpha-galactosyl residues and fucose residues linked to the proximal N-acetylglucosamine. Sialic acid residues could not be detected. For their structural analysis, the oligosaccharides were released by hydrazinolysis and fractionated on Bio-Gel P-4. Approximately 80% of the eluted oligosaccharides have a size equivalent of 17 +/- 2 glucose units. The major oligosaccharide representing about 45% of the total carbohydrate present in CVF was purified to homogeneity by MicroPak AX-5 HPLC and its structure was analyzed by sequential exoglycosidase digestion. The positions of the glycosidic linkages of the sugar residues were established by methylation analysis of CVF-derived glycopeptides. The data of these analyses indicated that the major oligosaccharide has a symmetrical fucosylated biantennary complex-type structure terminating with unusual alpha-galactosyl residues.

The substrate-specificity of human lysosomal alpha-D-mannosidase in relation to genetic alpha-mannosidosis

The specificity of human liver lysosomal alpha-mannosidase (EC 3.2.1.24) towards a series of oligosaccharide substrates derived from high-mannose, complex and hybrid asparagine-linked glycans and from the storage products in alpha-mannosidosis was investigated. The enzyme hydrolyses all alpha(1-2)-, alpha(1-3)- and alpha(1-6)-mannosidic linkages in these glycans without a requirement for added Zn2+, albeit at different rates. A major finding of this study is that all the substrates are hydrolysed by non-random pathways. These pathways were established by determining the structures of intermediates in the digestion mixtures by a combination of h.p.t.l.c. and h.p.l.c. before and after acetolysis. The catabolic pathway for a particular substrate appears to be determined by its structure, raising the possibility that degradation occurs by an uninterrupted sequence of steps within one active site. The structures of the digestion intermediates are compared with the published structures of the storage products in mannosidosis and of intact asparagine-linked glycans. Most but not all of the digestion intermediates derived from high-mannose glycans have structures found in intact asparagine-linked glycans of human glycoproteins or among the storage products in the urine of patients with mannosidosis. However, the relative abundances of these structures suggests that the catabolic pathway is not the same as the processing pathway. In contrast, the intermediates formed from the digestion of oligosaccharides derived from hybrid and complex N-glycans are completely different from any processing intermediates and also from the oligosaccharides of composition Man2-4GlcNAc that account for 80-90% of the storage products in alpha-mannosidosis. It is postulated that the structures of these major storage products arise from the action of an exo/endo-alpha(1-6)-mannosidase on the partially catabolized oligomannosides that accumulate in the absence of the main lysosomal alpha-mannosidase.

Isolation and characterization of the third complement component of axolotl (Ambystoma mexicanum)

1. Using a monoclonal anti-human C3 antibody and a polyclonal anti-cobra venom factor antibody as probes, a protein homologous to the mammalian third complement component (C3) was purified from axolotl plasma and found to be axolotl C3. 2. Axolotl C3 consists of two polypeptide chains (Mr = 110,000 and 73,000) linked by disulfide bonds. An internal thiolester bond in the alpha chain was identified by the incorporation of [14C]methylamine and NH2-terminal sequence from the C3d fragment of C3. 3. Digestion of C3 by trypsin resulted in the cleavage of both the alpha and beta chains, generating fragments with a cleavage pattern similar to that of human C3. 4. The amino acid composition of axolotl C3 and the amino acid sequences of the thiolester site (and the surrounding amino acids), the cleavage site for the C3-convertase, and one of the factor I cleavage sites are similar to C3 from other vertebrates. 5. In contrast to human C3, which has concanavalin A binding carbohydrates on both the alpha and beta chains, only the beta chain of axolotl C3 contains such carbohydrates.

Bovine serum conglutinin is a lectin which binds non-reducing terminal N-acetylglucosamine, mannose and fucose residues

Carbohydrate recognition by bovine serum conglutinin has been investigated by inhibition and direct binding assays using glycoproteins and polysaccharides from Saccharomyces cerevisiae (baker's yeast), and neoglycolipids derived from N-acetylglucosamine oligomers, mannobiose and human milk oligosaccharides. The results clearly show that conglutinin is a lectin which binds terminal N-acetylglucosamine, mannose and fucose residues as found in chitobiose (GlcNAc beta 1-4GlcNAc), mannobiose (Man alpha 1-3Man) and lacto-N-fucopentaose II [Fuc alpha 1-4(Gal beta 1-3)GlcNAc beta 1-3Gal beta 1-4Glc] respectively.

Effect of lentinan and mannan on phagocytosis of fluorescent latex microbeads by mouse peritoneal macrophages: a flow cytometric study

Lentinan, an immunopotentiating beta-1,3-glucan polysaccharide stimulated the in vitro phagocytosis of BSA-coated, C3b- or monoclonal immunoglobulin (IgG2b)-coated fluorescent microspheres by resident or thioglycollate-elicited mouse macrophages in a dose-dependent manner. Analysis of flow cytometric data has shown that microbead phagocytosis of resident macrophages, which exhibit a lower basic phagocytic activity than the thioglycollate elicited ones, has been augmented by up to 900% due to lentinan. The percent ratio of phagocytes among peritoneal exudate cells, however, remained unchanged after short-term lentinan stimulation. Preincubation of the cells with lentinan resulted in increased ingestion of the microbeads. Activation of phagocytosis by lentinan is therefore due in part to the direct stimulation of the cells, however, lentinan also serves as supplementary opsonin for C3b-coated beads. Mannan inhibited the ingestion of C3b-coated microspheres by 75%, which was abolished in part when lentinan was also added to the cells. Mannan did not influence the phagocytosis of BSA-coated or IgG-coated beads. Our data, based solely on in vitro studies, suggest a beta-glucan receptor mediated activation of phagocytes by lentinan. These receptors are different from the C3b, Fc or mannose receptors. It is very likely that stimulation of phagocytic activity of macrophages by lentinan may contribute to the antitumor action of this immunopotentiating polysaccharide.

Identification method for twelve oligomannose-type sugar chains thought to be processing intermediates of glycoproteins

A mixture of the pyridylamino (PA) derivatives of 12 oligomannose-type sugar chains was fractionated into five fractions (mannose5N-acetylglucosamine2-PA approximately mannose9N-acetylglucosamine2-PA) by size-fractionation HPLC with a MicroPak AX-5 column. Each fraction thus obtained was then analyzed by reversed-phase HPLC with a Cosmosil 5C18-P column. In this way, the 12 PA-oligomannose-type sugar chains were completely separated from each other. The method was used to identify the structures of oligomannose-type sugar chains of human C3, the third component of human complement.

Fluorescence method for the structural analysis of oligomannose-type sugar chains by partial acetolysis

Fluorescence labeling was used in the analysis of partial acetolysis products of oligomannose-type sugar chains with five to nine mannose residues. The principle of the method was the pyridylamination of fragments obtained by the partial acetolysis of pyridylamino sugar chains and the identification of the fragments with an HPLC apparatus equipped with a fluorescence spectrophotometer. The method was tested by analysis of eight oligomannose-type sugar chains with known chemical structures and was found to be effective for analysis of branching structures with samples of 0.5 nmol.

The carbohydrate specificity of conglutinin and its homology to proteins in the hepatic lectin family

Inhibition experiments with D-mannose oligosaccharides establish that conglutinin recognises terminal alpha 1----2 mannobiosyl units present in the glycopeptide of the alpha-chain of the complement component C3b. On the basis of its three domain structure and the homology of its N-terminal amino-acid sequence to that of the dog pulmonary surfactant protein, it is proposed that conglutinin is a member of the hepatic lectin family.

Carbohydrate structures of the third component of rat complement. Presence of both high-mannose and complex type oligosaccharide chains

We investigated the carbohydrate structure of the third component of complement (C3) newly synthesized by cultured rat hepatocytes. When the cells were incubated with [3H]mannose, [3H]galactose or [3H]glucosamine, these radioactive precursors were incorporated only into the alpha subunit of C3, demonstrating that only the alpha subunit contains oligosaccharide chains. [3H]Mannose-labelled C3 was purified from the culture medium by immunoaffinity chromatography. Oligosaccharides prepared by Pronase digestion and strong alkaline hydrolysis were separated into two fractions by Bio-Gel P-2 chromatography (Fractions I and II). The two fractions were analysed by concanavalin A-Sepharose chromatography, ion-exchange high-performance liquid chromatography, and Bio-Gel P-4 gel filtration before and after sequential exoglycosidase digestions. It was found that Fraction I contained two complex type oligosaccharide chains, (NeuAc)2(Gal)2(GlcNAc)2(Man)3(GlcNAc)2 and (NeuAc)3(Gal)3(GlcNAc)3(Man)3(GlcNAc)2, and Fraction II contained the high-mannose type, consisting mainly of (Man)8(GlcNAc)2. Taken together with the carbohydrate composition of rat serum C3, the results suggest that rat C3 has one high-mannose type oligosaccharide chain and two complex type chains in the alpha subunit, which is different from the proposal for human C3.

Amino acid sequence analysis of the glycopeptides from human complement component C3

Human complement component C3 has been cleaved completely by trypsin in the presence of 2-propanol. The hydrolysate was fully solubilized and fractionated by reversed-phase HPLC. Two peptides only contained glucosamine, Unambiguous sequence analyses identified Asn-63 of the beta-chain and Asn-268 of the alpha-chain as the sites of carbohydrate attachment. A third potential Asn-Xaa-Thr/Ser glycosylation site, Asn-946 of the alpha-chain, is not modified. The different states of glycosylation of the sites cannot be explained by differences in exposure or secondary structure. All three are predicted reverse turn.

Molecular modelling of human complement component C3 and its fragments by solution scattering

Solution scattering experiments using both X-rays and neutrons are reported for human complement component C3 and up to six other glycoprotein fragments that are derived from C3. The X-ray and neutron molecular masses and neutron matchpoints are in agreement with the known primary sequence of C3. The X-ray radius of gyration RG of C3 is 5.2 nm and is similar for the related forms C3u, C3(a + b) and C3b. The X-ray cross-sectional radius of gyration RXS of C3b is however less than that of C3, C3u and C3(a + b). The major fragments of C3b, namely C3c and C3dg, were studied. The RG of C3c is 4.7 nm and for C3dg is 2.9 nm. C3c and C3dg do not interact when they coexist in solution in equimolar amounts. When C3u is cleaved into iC3u, the RG of iC3u increases to 5.9 nm and its RXS decreases, showing that C3c and C3dg behave as independent entities within the parent glycoprotein. Analyses of the neutron RG and RXS values by contrast variation techniques confirm the X-ray analyses, and show no evidence for significant hydrophobic or hydrophilic domains within C3 or any of its fragments. Shape analyses show that C3, C3c and C3dg are elongated particles. Debye models were developed using the scattering curve out to Q = 1.6 nm-1. These show that C3 and C3c resemble oblate ellipsoids while C3dg resembles a prolate ellipsoid. C3dg lies on the long edge of C3c within C3. The dimensions of the models are 18 nm X 2 nm X 10 nm for C3, 18 nm X 2 nm X 7 nm for C3c and 10 nm X 2 nm X 3 nm for C3dg. These models are compatible with analyses of the scattering curve RG and RXS values, data from sedimentation coefficients, and images of C3 and C3c seen by electron microscopy.