The sperm agglutination antigen-1 (SAGA-1) glycoforms of CD52 are O-glycosylated

Global view of human protein glycosylation pathways and functions

Glycosylation is the most abundant and diverse form of post-translational modification of proteins that is common to all eukaryotic cells. Enzymatic glycosylation of proteins involves a complex metabolic network and different types of glycosylation pathways that orchestrate enormous amplification of the proteome in producing diversity of proteoforms and its biological functions. The tremendous structural diversity of glycans attached to proteins poses analytical challenges that limit exploration of specific functions of glycosylation. Major advances in quantitative transcriptomics, proteomics and nuclease-based gene editing are now opening new global ways to explore protein glycosylation through analysing and targeting enzymes involved in glycosylation processes. In silico models predicting cellular glycosylation capacities and glycosylation outcomes are emerging, and refined maps of the glycosylation pathways facilitate genetic approaches to address functions of the vast glycoproteome. These approaches apply commonly available cell biology tools, and we predict that use of (single-cell) transcriptomics, genetic screens, genetic engineering of cellular glycosylation capacities and custom design of glycoprotein therapeutics are advancements that will ignite wider integration of glycosylation in general cell biology.

Latheef S, Sharun K, Khurana SK, K. A, Tiwari R, Bhatt P, K. V, Chaicumpa W. Role of Antisperm Antibodies in Infertility, Pregnancy, and Potential forContraceptive and Antifertility Vaccine Designs: Research Progress and Pioneering Vision

Sperm of humans, non-human primates, and other mammalian subjects is considered to be antigenic. The effect of changes in autoimmunity on reproductive cells such as spermatozoa and oocytes play a critical but indistinct role in fertility. Antisperm antibodies (ASAs) are invariably present in both females and males. However, the degree of ASA occurrence may vary according to individual and gender. Although the extent of infertility due to ASAs alone is yet to be determined, it has been found in almost 9-12% of patients who are infertile due to different causes. Postcoital presence of spermatozoa in the reproductive tract of women is not a contributory factor in ASA generation. However, ASA generation may be induced by trauma to the vaginal mucosa, or by anal or oral sex resulting in the deposition of sperm inside the digestive tract. It is strongly believed that, in humans and other species, at least some antibodies may bind to sperm antigens, causing infertility. This form of infertility is termed as immunological infertility, which may be accompanied by impairment of fertility, even in individuals with normozoospermia. Researchers target ASAs for two major reasons: (i) to elucidate the association between ASAs and infertility, the reason ASAs causes infertility, and the mechanism underlying ASA-mediated infertility; and (ii) to assess the potential of ASAs as a contraceptive in humans in case ASAs influences infertility. Therefore, this review explores the potential application of ASAs in the development of anti-spermatozoa vaccines for contraceptive purposes. The usefulness of ASAs for diagnosing obstructive azoospermia, salpingitis, and oligoasthenoteratozoospermia has been reviewed extensively. Important patents pertaining to potential candidates for spermatozoa-derived vaccines that may be utilized as contraceptives are discussed in depth. Antifertility vaccines, as well as treatments for ASA-related infertility, are also highlighted. This review will address many unresolved issues regarding mechanisms involving ASAs in the diagnosis, as well as prognoses, of male infertility. More documented scientific reports are cited to support the mechanisms underlying the potential role of ASA in infertility. The usefulness of sperm antigens or ASAs (recombinant) in human and wild or captive animal contraceptive vaccines has been revealed through research but is yet to be validated via clinical testing.

Specific Sialoforms Required for the Immune Suppressive Activity of Human Soluble CD52

Human CD52 is a small glycopeptide (12 amino acid residues) with one N-linked glycosylation site at asparagine 3 (Asn3) and several potential O-glycosylation serine/threonine sites. Soluble CD52 is released from the surface of activated T cells and mediates immune suppression via its glycan moiety. In suppressing activated T cells, it first sequesters the pro-inflammatory high mobility group Box 1 (HMGB1) protein, which facilitates its binding to the inhibitory sialic acid-binding immunoglobulin-like lectin-10 (Siglec-10) receptor. We aimed to identify the features of CD52 glycan that underlie its bioactivity. Analysis of native CD52 purified from human spleen revealed extensive heterogeneity in N-glycosylation and multi-antennary sialylated N-glycans with abundant polyLacNAc extensions, together with mainly di-sialylated O-glycosylation type structures. Glycomic (porous graphitized carbon-ESI-MS/MS) and glycopeptide (C8-LC-ESI-MS) analysis of recombinant soluble human CD52-immunoglobulin Fc fusion proteins revealed that CD52 bioactivity was correlated with a high abundance of tetra-antennary α-2,3/6 sialylated N-glycans. Removal of α-2,3 sialylation abolished bioactivity, which was restored by re-sialylation with α-2,3 sialyltransferases. When glycoforms of CD52-Fc were fractionated by anion exchange MonoQ-GL chromatography, bioactive fractions displayed mainly tetra-antennary, α-2,3 sialylated N-glycan structures and a lower relative abundance of bisecting GlcNAc structures compared to non-bioactive fractions. In addition, O-glycan core type-2 di-sialylated structures at Ser12 were more abundant in bioactive CD52 fractions. Understanding the structural features of CD52 glycan required for its bioactivity will aid its development as an immunotherapeutic agent.

Decoding of O-Linked Glycosylation by Mass Spectrometry

Protein glycosylation (N- and O-linked) plays an important role in many biological processes, including protein structure and function. However, the structural elucidation of glycans, specifically O-linked glycans, remains a major challenge and is often overlooked during protein analysis. Recently, mass spectrometry (MS) has matured as a powerful technology for high-quality analytical characterization of O-linked glycans. This review summarizes the recent developments and insights of MS-based glycomics technologies, with a focus on mucin-type O-glycan analysis. Three main MS-based approaches are outlined: O-glycan profiling (structural analysis of released O-glycan), a "bottom-up" approach (analysis of an O-glycan covalently attached to a glycopeptide), and a "top-down" approach (analysis of a glycan attached to an intact glycoprotein). In addition, the most widely used MS ionization techniques, i.e., matrix-assisted laser desorption ionization and electrospray ionization, as well as ion activation techniques like collision-induced dissociation, electron capture dissociation, and electron transfer dissociation during O-glycan analysis are discussed. The MS technical approaches mentioned above are already major improvements for studying O-linked glycosylation and appear to be valuable for in-depth analysis of the type of O-glycan attached, branching patterns, and the occupancy of O-glycosylation sites.

Eukaryotic protein glycosylation: a primer for histochemists and cell biologists

Proteins undergo co- and posttranslational modifications, and their glycosylation is the most frequent and structurally variegated type. Histochemically, the detection of glycan presence has first been performed by stains. The availability of carbohydrate-specific tools (lectins, monoclonal antibodies) has revolutionized glycophenotyping, allowing monitoring of distinct structures. The different types of protein glycosylation in Eukaryotes are described. Following this educational survey, examples where known biological function is related to the glycan structures carried by proteins are given. In particular, mucins and their glycosylation patterns are considered as instructive proof-of-principle case. The tissue and cellular location of glycoprotein biosynthesis and metabolism is reviewed, with attention to new findings in goblet cells. Finally, protein glycosylation in disease is documented, with selected examples, where aberrant glycan expression impacts on normal function to let disease pathology become manifest. The histological applications adopted in these studies are emphasized throughout the text.

Lectin binding of human sperm associates with DEFB126 mutation and serves as a potential biomarker for subfertility

Coating on the sperm surface, glycocalyx, plays a key role in sperm motility, maturation and fertilization. A comprehensive profile of sperm surface glycans will greatly facilitate both basic researches and clinical studies. Because of the capability of recognizing different glycan moieties, lectins are widely used in glycobiology. However, lacking high-throughput technology, limited lectins have been reported for analyzing the glycan of human sperm. In this study, we employed a lectin microarray for profiling the surface glycans of human sperm, on which 54 out of 91 lectins showed positive binding. Based on this technique, we compared lectin binding profiling of sperm with homozygous DEFB126 mutation (del/del) with that of wild type (wt/wt). DEFB126 was reported to contribute to the sialylation on sperm surface and its homozygous mutation was related to male subfertility. Six lectins (Jacalin/AIA, GHA, ACL, MPL, VVL and ABA) were found to develop lower binding affinity to sperm with del/del. Further validation showed that these lectins, especially ABA and MPL, can be potential biomarkers for clinical diagnosis of subfertility due to the mutation of DEFB126. Our research provides insight into the detection of some unexplained male subfertility, and the lectin microarray is generally applicable for infertility/subfertility sperm biomarker discovery.

Sugar-coated sperm: Unraveling the functions of the mammalian sperm glycocalyx

Mammalian spermatozoa are coated with a thick glycocalyx that is assembled during sperm development, maturation, and upon contact with seminal fluid. The sperm glycocalyx is critical for sperm survival in the female reproductive tract and is modified during capacitation. The complex interplay among the various glycoconjugates generates numerous signaling motifs that may regulate sperm function and, as a result, fertility. Nascent spermatozoa assemble their own glycans while the cells still possess a functional endoplasmic reticulum and Golgi in the seminiferous tubule, but once spermatogenesis is complete, they lose the capacity to produce glycoconjugates de novo. Sperm glycans continue to be modified, during epididymal transit by extracellular glycosidases and glycosyltransferases. Furthermore, epididymal cells secrete glycoconjugates (glycophosphatidylinositol-anchored glycoproteins and glycolipids) and glycan-rich microvesicles that can fuse with the maturing sperm membrane. The sperm glycocalyx mediates numerous functions in the female reproductive tract, including the following: inhibition of premature capacitation; passage through the cervical mucus; protection from innate and adaptive female immunity; formation of the sperm reservoir; and masking sperm proteins involved in fertilization. The immense diversity in sperm-associated glycans within and between species forms a remarkable challenge to our understanding of essential sperm glycan functions.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Is there a role for immunocontraception?

The world's population is continuing to grow at an alarming rate and yet no novel methods of contraception have been introduced since 1960s. The paucity of our current contraceptive armoury is indicated by the 46 million abortions that are performed each year, largely in developing countries where population growth is greatest. Thus, whatever new forms of fertility control we develop for the next millennium, the particular needs of developing countries should be borne in mind. Contraceptive vaccines have the potential to provide safe, effective, prolonged, reversible protection against pregnancy in a form that can be easily administered in the Third World. In this review we consider the contraceptive targets that might be pursued, how vaccines might be engineered and the problems generated by inter-individual variations in antibody titre. We conclude that the specifications for a safe, effective, reversible vaccine are more likely to be met in animals than man.

Expeditious chemoenzymatic synthesis of CD52 glycopeptide antigens

CD52 is a glycosylphosphatidylinositol (GPI)-anchored glycopeptide antigen found on sperm cells and human lymphocytes. Recent structural studies indicate that sperm-associated CD52 antigen carries both a complex type N-glycan and an O-glycan on the polypeptide backbone. To facilitate functional and immunological studies of distinct CD52 glycoforms, we report in this paper the first chemoenzymatic synthesis of homogeneous CD52 glycoforms carrying both N- and O-glycans. The synthetic strategy consists of two key steps: monosaccharide primers GlcNAc and GalNAc were first installed at the pre-determined N- and O-glycosylation sites by a facile solid-phase peptide synthesis, and then the N- and O-glycans were extended by respective enzymatic glycosylations. It was found that the endoglycosidase-catalyzed transglycosylation allowed efficient attachment of an intact N-glycan in a single step at the N-glycosylation site, while the recombinant human T-synthase could independently extend the O-linked GalNAc to form the core 1 O-glycan. This chemoenzymatic approach is highly convergent and permits easy construction of various homogeneous CD52 glycoforms from a common polypeptide precursor. In addition, the introduction of a latent thiol group in the form of protected cysteamine at the C-terminus of the CD52 glycoforms will enable site-specific conjugation to a carrier protein to provide immunogens for generating CD52 glycoform-specific antibodies for functional studies.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 5: intercellular junctions and contacts between germs cells and Sertoli cells and their regulatory interactions, testicular cholesterol, and genes/proteins associated with more than one germ cell generation

In the testis, cell adhesion and junctional molecules permit specific interactions and intracellular communication between germ and Sertoli cells and apposed Sertoli cells. Among the many adhesion family of proteins, NCAM, nectin and nectin-like, catenins, and cadherens will be discussed, along with gap junctions between germ and Sertoli cells and the many members of the connexin family. The blood-testis barrier separates the haploid spermatids from blood borne elements. In the barrier, the intercellular junctions consist of many proteins such as occludin, tricellulin, and claudins. Changes in the expression of cell adhesion molecules are also an essential part of the mechanism that allows germ cells to move from the basal compartment of the seminiferous tubule to the adluminal compartment thus crossing the blood-testis barrier and well-defined proteins have been shown to assist in this process. Several structural components show interactions between germ cells to Sertoli cells such as the ectoplasmic specialization which are more closely related to Sertoli cells and tubulobulbar complexes that are processes of elongating spermatids embedded into Sertoli cells. Germ cells also modify several Sertoli functions and this also appears to be the case for residual bodies. Cholesterol plays a significant role during spermatogenesis and is essential for germ cell development. Lastly, we list genes/proteins that are expressed not only in any one specific generation of germ cells but across more than one generation.

Mucin-type O-glycosylation--putting the pieces together

The O-glycosylation of Ser and Thr by N-acetylgalactosamine-linked (mucin-type) oligosaccharides is often overlooked in protein analysis. Three characteristics make O-linked glycosylation more difficult to analyse than N-linked glycosylation, namely: (a) no amino acid consensus sequence is known; (b) there is no universal enzyme for the release of O-glycans from the protein backbone; and (c) the density and number of occupied sites may be very high. For significant biological conclusions to be drawn, the complete picture of O-linked glycosylation on a protein needs to be determined. This review specifically addresses the analytical approaches that have been used, and the challenges remaining, in the characterization of both the composition and structure of mucin-type O-glycans, and the determination of the occupancy and heterogeneity at each amino acid attachment site.

7th HUPO World Congress: the human disease glycomics/proteomics initiative (HGPI) session 17 August 2008, Amsterdam, The Netherlands

The Human Disease Glycomics/Proteomics Initiative (HGPI) Session was held on August 17, 2008, at the HUPO World Congress in Amsterdam. Reports were made on the progress of the first and second analytical pilot studies to profile N- and O-linked glycan structures of standard glycoproteins utilizing laboratories from around the world. In addition, recent advances in glycan structural analyses were presented, including the use of O-linked glycan libraries of standards, use of negative mode nano-LC-MS for O-linked glycan analysis, and identification of aberrant O-glycosylation of IgA1 as a cause of IgA nephropathy. A report was made of a newly discovered lectin, malectin, which appears to function in the folding/quality control of glycoproteins with N-linked glycans and may regulate several human disorders whose etiology involves protein quality control in the ER. The major glycan ligand for malectin was identified using a novel printed glycan microarray. Advances in the analysis of the genes that are associated with glycan expression and recognition--the glycotranscriptome--were described, as well as technologies to determine the relative quantitation of N- and O-linked glycans from as few as 2 x 10(6) cells. These technologies are being applied to identify potential biomarkers of stem and cancer cells.

Expression of CD52 mRNA in the rat embryo

CD52 is a leukocyte differentiation antigen first discovered in humans as expressed on the surface of lymphocytes, monocytes and eosinophils. The human CD52 is found on chromosome 1, and two alleles are both known to be reasonably common. A closely homologous gene has been identified in the cynomologous monkey and related genes have been found in mouse, rat and dog. The role of CD52 in lymphocyte is still unclear but the anti-CD52 antibodies named CAMPATH-1 antibodies are largely used for therapy where depletion of lymphocytes is required. In the past expression of the antigen on progenitors of leukocytes in bone marrow had been excluded, but recent work indicates CD52 is highly expressed on cells with colony-forming and NOD/SCID (non-obese diabetic-severe combined immunodeficiency)-engrafting capacities, both at the mRNA and membrane protein level. We have investigated CD52 expression during development in rat embryos by in situ hybridization. We report here that the antigen is highly expressed in the liver that is the major organ where multipotent hematopietic stem cells differentiate but also in the splancnopleuric mesoderm, at early stages of embryo differentiation, where hematopietic stem cells are suggested to arise. CD52+ cells were found in areas active in vasculogenesis at early embryo stages and in the walls of the vessels in the liver at mid gestation. CD52+ cells were also found to emerge among c-Kit positive cells.