Molecular characterization of Limulus polyphemus C-reactive protein. II. Asparagine-linked oligosaccharides

Identification of the High Mannose N-Glycan Isomers Undescribed by Conventional Multicellular Eukaryotic Biosynthetic Pathways

N-linked glycosylation is one of the most important post-translational modifications of proteins. Current knowledge of multicellular eukaryote N-glycan biosynthesis suggests high mannose N-glycans are generated in the endoplasmic reticulum and Golgi apparatus through conserved biosynthetic pathways. According to conventional biosynthetic pathways, four Man₇GlcNAc₂ isomers, three Man₆GlcNAc₂ isomers, and one Man₅GlcNAc₂ isomer are generated during this process. In this study, we applied our latest mass spectrometry method, logically derived sequence tandem mass spectrometry (LODES/MSⁿ), to re-examine high mannose N-glycans extracted from various multicellular eukaryotes which are not glycosylation mutants. LODES/MSⁿ identified many high mannose N-glycan isomers previously unreported in plantae, animalia, cancer cells, and fungi. A database consisting of retention time and CID MSⁿ mass spectra was constructed for all possible Man_nGlcNAc₂ (n = 5, 6, 7) isomers that include the isomers by removing arbitrary numbers and positions of mannose from canonical N-glycan, Man₉GlcNAc₂. Many N-glycans in this database are not found in current N-glycan mass spectrum libraries. The database is useful for rapid high mannose N-glycan isomeric identification.

Modulation of the Tissue Expression Pattern of Zebrafish CRP-Like Molecules Suggests a Relevant Antiviral Role in Fish Skin

Simple Summary

The clinical use of the human short pentraxin C-reactive protein as a health biomarker is expanded worldwide. The acute increase of the serum levels of short pentraxins in response to bacterial infections is evolutionarily conserved, as are the main functions of pentraxins. Interestingly, fish orthologs have been found to increase similarly after bacterial and viral stimuli, thus becoming promising candidates for health biomarkers of both types of infection in this group of vertebrates. To preliminarily assess their adequacy for this application, zebrafish and a fish rhabdovirus were chosen as infection model systems for the analysis of the levels of gene expression of all short pentraxins in healthy and infected animals in a wide range of tissues. Because some significant increases were found in skin (a very suitable sampling source for testing purposes), further transcript analyses were carried out in this tissue. Due to the functional similarities between pentraxins and antibodies, it was also checked whether short pentraxins can compensate for the deficiencies in adaptive immunity by using mutant zebrafish lacking this system. In conclusion, the obtained results suggest that short pentraxins are highly reactant against viruses in skin and their overexpression seems to reflect a mechanism to compensate for the loss of adaptive immunity.

Abstract

Recent studies suggest that short pentraxins in fish might serve as biomarkers for not only bacterial infections, as in higher vertebrates including humans, but also for viral ones. These fish orthologs of mammalian short pentraxins are currently attracting interest because of their newly discovered antiviral activity. In the present work, the modulation of the gene expression of all zebrafish short pentraxins (CRP-like proteins, CRP1-7) was extensively analyzed by quantitative polymerase chain reaction. Initially, the tissue distribution of crp1-7 transcripts and how the transcripts varied in response to a bath infection with the spring viremia of carp virus, were determined. The expression of crp1-7 was widely distributed and generally increased after infection (mostly at 5 days post infection), except for crp1 (downregulated). Interestingly, several crp transcription levels significantly increased in skin. Further assays in mutant zebrafish of recombinant activation gene 1 (rag1) showed that all crps (except for crp2, downregulated) were already constitutively highly expressed in skin from rag1 knockouts and only increased moderately after viral infection. Similar results were obtained for most mx isoforms (a reporter gene of the interferon response), suggesting a general overcompensation of the innate immunity in the absence of the adaptive one.

The Pentraxins 1975-2018: Serendipity, Diagnostics and Drugs

The phylogenetically ancient, pentraxin family of plasma proteins, comprises C-reactive protein (CRP) and serum amyloid P component (SAP) in humans and the homologous proteins in other species. They are composed of five, identical, non-covalently associated protomers arranged with cyclic pentameric symmetry in a disc-like configuration. Each protomer has a calcium dependent site that mediates the particular specific ligand binding responsible for all the rigorously established functional properties of these proteins. No genetic deficiency of either human CRP or SAP has been reported, nor even any sequence polymorphism in the proteins themselves. Although their actual functions in humans are therefore unknown, gene deletion studies in mice demonstrate that both proteins can contribute to innate immunity. CRP is the classical human acute phase protein, routinely measured in clinical practice worldwide to monitor disease activity. Human SAP, which is not an acute phase protein, is a universal constituent of all human amyloid deposits as a result of its avid specific binding to amyloid fibrils of all types. SAP thereby contributes to amyloid formation and persistence in vivo. Whole body radiolabelled SAP scintigraphy safely and non-invasively localizes and quantifies systemic amyloid deposits, and has transformed understanding of the natural history of amyloidosis and its response to treatment. Human SAP is also a therapeutic target, both in amyloidosis and Alzheimer's disease. Our drug, miridesap, depletes SAP from the blood and the brain and is currently being tested in the DESPIAD clinical trial in Alzheimer's disease. Meanwhile, the obligate therapeutic partnership of miridesap, to deplete circulating SAP, and dezamizumab, a humanized monoclonal anti-SAP antibody that targets residual SAP in amyloid deposits, produces unprecedented removal of amyloid from the tissues and improves organ function. Human CRP binds to dead and damaged cells in vivo and activates complement and this can exacerbate pre-existing tissue damage. The adverse effects of CRP are completely abrogated by compounds that block its binding to autologous ligands and we are developing CRP inhibitor drugs. The present personal and critical perspective on the pentraxins reports, for the first time, the key role of serendipity in our work since 1975. (345 words).

Crystal structures of Limulus SAP-like pentraxin reveal two molecular aggregations

The serum-amyloid-P-component-like pentraxin from Limulus polyphemus, a recently discovered pentraxin species and important effector protein of the hemolymph immune system, displays two distinct doubly stacked cyclic molecular aggregations, heptameric and octameric. The refined three-dimensional structures determined by X-ray crystallography, both based on the same cDNA sequence, show that each aggregate is constructed from a similar dimer of protomers, which is repeated to make up the ring structure. The native octameric form has been refined at a resolution of 3 A, the native heptameric form at 2.3 A, and the phosphoethanolamine (PE)-bound octameric form at 2.7 A. The existence of the hitherto undescribed heptameric form was confirmed by single-particle analysis using cryo-electron microscopy. In the native structures, the calcium-binding site is similar to that in human pentraxins, with two calcium ions bound in each subunit. Upon binding PE, however, each subunit binds a third calcium ion, with all three calcium ions contributing to the binding and orientation of the bound phosphate group within the ligand-binding pocket. While the phosphate is well-defined in the electron density, the ethanolamine group is poorly defined, suggesting structural and binding variabilities of this group. Although sequence homology with human serum amyloid P component is relatively low, structural homology is high, with very similar overall folds and a common affinity for PE. This is due, in part, to a "topological" equivalence of side-chain position. Identical side chains that are important in both function and fold, from different regions of the sequence in human and Limulus structures, occupy similar space within the overall subunit fold. Sequence and structure alignment, based on the refined three-dimensional structures presented here and the known horseshoe crab pentraxin sequences, suggest that adaptation and refinement of C-reactive-protein-mediated immune responses in these ancient creatures lacking antibody-based immunity are based on adaptation by gene duplication.

Human C-reactive protein does not protect against acute lipopolysaccharide challenge in mice

The physiological and pathophysiological functions of C-reactive protein (CRP), the classical acute-phase protein, are not well established, despite many reports of biological effects of CRP in vitro and in model systems in vivo. Limited, small scale experiments have suggested that rabbit and human CRP may both protect mice against lethal toxicity of Gram-negative bacterial LPS. However, in substantial well-controlled studies in C57BL/6 mice challenged with Escherichia coli O111:B4 LPS, we show in this work that significant protection against lethality was conferred neither by an autologous acute-phase response to sterile inflammatory stimuli given to wild-type mice 24 h before LPS challenge, nor by human CRP, whether passively administered or expressed transgenically. Male mice transgenic for human CRP, which mount a major acute-phase response of human CRP after LPS injection, were also not protected against the lethality of LPS from either E. coli O55:B5 or Salmonella typhimurium. Even when the acute-phase human CRP response was actively stimulated in transgenic mice before LPS challenge, no protection against LPS toxicity was observed. Indeed, male mice transgenic for human CRP that were pretreated with casein to stimulate an acute-phase response 24 h before LPS challenge suffered significantly greater mortality than unstimulated human CRP transgenic controls. Rather than being protective in this situation, human CRP may thus have pathogenic proinflammatory effects in vivo.

Molecular characterization of Limulus polyphemus C-reactive protein. I. Subunit composition

The molecular mass of whole native C-reactive protein (CRP) from Limulus polyphemus, the horseshoe crab, was precisely determined for the first time, by analytical ultracentrifugation after measurement of its absorption coefficient, A1cm, 1% at 280 nm (15.49) and its partial specific volume (0.72 +/- 0.003 ml/g). The apparent weight-average molecular mass, in the range over which it was independent of the concentration, was 300 kDa. Limulus CRP, isolated from an individual animal, was also analyzed by electrospray mass spectrometry after dissociation into its subunits, and nine components were detected in the mass range 24935-25617 Da. Four of these components corresponded precisely to one of the previously reported protein sequences of Limulus CRP subunits which is glycosylated by a single oligosaccharide chain of composition, Man2GlcNAc2, Man3GlcNAc2, Man4GlcNAc2 or Man5GlcNAc2, the structures of which are elucidated in the accompanying paper [Amatayakul-Chantler, S., Dwek, R. A., Tennent, G. A., Pepys, M. B. & Rademacher, T. W. Molecular characterization of Limulus polyphemus C-reactive protein. II. Asparagine-linked oligosaccharides (1993) Eur. J. Biochem. 214, 99-110]. All the remaining components, except one minor component, corresponded to the analogous glycoforms of peptides of molecular mass 24281 +/- 2 Da and 24523 +/- 1 Da, neither of which have previously been characterized. These results are consistent with the presence of 12 subunits in the Limulus CRP molecule, in contrast to the number of subunits in vertebrate members of the CRP family. The CRP family members in vertebrates are known as pentraxins because of the symmetrical pentameric arrangement of their subunits. The concentration of Limulus CRP in hemolymph from 20 individual animals, measured precisely by specific electroimmunoassay, covered the wide range 0.275-6.64 mg/ml, mean (SD) = 1.83 (2.06) mg/ml. It remains to be determined whether this protein is an acute-phase reactant, like its human and some other mammalian homologues.