Soluble CD46 (membrane cofactor protein, MCP) in human reproductive tract fluids

Alternative polyadenylation expands the mRNA isoform repertoire of human CD46

Alternative polyadenylation is a prevalent mechanism regulating mammalian gene expression. While tandem 3'-Untranslated-Region (3'UTR) polyadenylation changes expression levels, Intronic PolyAdenylation generates shorter transcripts encoding truncated proteins. Intronic PolyAdenylation regulates 20% of genes and is especially common in receptor tyrosine-kinase transcripts, generating soluble repressors. Here we report that human CD46, encoding a TransMembrane repressor of complement and T-cell co-stimulator, expresses multiple isoforms by alternative polyadenylation. We provide evidence for polyadenylation at several introns by RT-PCR of 5' intronic fragments, and by increase in such isoforms via functional U1 knockdown. We mapped various Intronic PolyAdenylation Sites by 3' Rapid Amplification of cDNA Ends (3'RACE), which could generate soluble or membrane-bound but tail-less CD46. Intronic PolyAdenylation could add to the source of soluble CD46 isoforms in fluids and tissues, which increase in cancers and autoimmune syndromes. Furthermore, 3'RACE identified three PolyAdenylation Sites within the last intron and exon, whose transcripts with shortened 3'UTRs could support higher CD46 expression. Finally, 3'RACE revealed that the CD46 Pseudogene only expresses short transcripts by early polyadenylation in intron 2. Overall, we report a wide variety of CD46 mRNA isoforms which could generate new protein isoforms, adding to the diverse physiological and pathological roles of CD46.

Soluble forms of CD46 are detected in Bos taurus plasma and neutralize BVDV, the bovine pestivirus

The pestivirus bovine viral diarrhea virus (BVDV) is known to bind to the CD46 molecule, which subsequently promotes entry of the virus. Mapping of the BVD-virion-binding site has shown that two peptides, 66EQIV69 and 82GQVLAL87, located on antiparallel beta sheets in the most distal complement control protein module (CCP1), provide the attachment platform. In the present study, we reveal new CD46-encoding transcripts that are predicted to encode CCP1-containing soluble forms. Further, we show that the serum of most adult cattle contains soluble CD46 (sCD46) and that a recombinant soluble isoform neutralizes BVDV infectivity in an in vitro assay. We have then established an ELISA for determination of plasma sCD46 in a large cohort of animals. Overall, serum sCD46 amounts to 8±18ng/mL (mean±SD, n=440), with a IC [95-105] ranging from 6,4 to 9,8ng/mL and extreme values ​​between 0 and 178ng/mL. We found that sCD46 is not detectable in fetal and neonatal sera and that its plasma concentration increases progressively up to adulthood. We also detected high- and low-sCD46 performers and show that this phenotype does not depend of environment. As modern rearing techniques make it possible to disseminate genetically-determined phenotypes very quickly in a population, a large-scale study examining whether high-sCD46 animals provide epidemiological protection against BVDV infection and transmission should be undertaken.

Factor H in porcine seminal plasma protects sperm against complement attack in genital tracts

We found that factor H (FH) exists in porcine seminal plasma. Purified FH strongly inhibited serum alternative pathway complement activation against lipopolysaccharide. The molecular weight, pI, and heparin-binding activity of the purified protein were different from those of purified FH from porcine serum. The complement regulatory activity of seminal plasma FH was approximately 2-fold stronger than that of serum FH. Treatment of purified serum FH with sialidase and N-glycosidase F gave almost the same results as those of seminal plasma FH. The deletion of sialic acid from the carbohydrate chains of both FHs contributed to heparin-binding and complement regulatory activities. Results of reverse transcriptase-PCR, Western blot analysis, and immunohistochemistry showed that seminal plasma FH is mainly secreted from epithelial cells of the seminal vesicle in male genital tracts. FH was also detected in the outer acrosomal region of ejaculated sperm by immunofluorescence staining, and found that the purified FH from the sperm membrane has the same complement regulatory activity as that of seminal plasma FH. The ejaculated sperm possessing FH in the outer acrosomal region considerably evaded complement attack. We also found that there is strong complement activity in fluids from female genital tract ducts. These findings indicate that FH bound to the outer acrosomal region and soluble FH play important roles in protecting sperm against complement attack in male and female genital tracts.

The immune status of the bovine uterus during the peripartum period

The post-partum period in cattle is characterised by an increased risk of infection of the uterus, as the anatomical barriers are broached during parturition and remain open for several days. Infection of the uterus is largely influenced by the balance between bacterial contamination and the local and systemic immune status during pregnancy and around parturition. Infectious diseases are more prevalent during this period, because of an impaired immune status before and immediately after parturition. Neutrophils play a primary role in the defence of the uterus against infection. Influx of neutrophils into the uterus is thought to be mediated by chemoattractants, chemokines and adhesion molecules, such as beta2-integrin (complement receptor 3) and L-selectin (CD62L). Other cellular components activated in the uterus during this period include lymphocytes, eosinophils, mast cells and macrophages. The major classes of immunoglobulins (IgM, IgA and IgG), either by passive diffusion or local production, play an important protective role in the uterus by acting as opsonins to enhance phagocytosis, stimulating the complement pathways or blocking pathogens from adhering to mucosal surfaces. Endometrial cells express toll-like receptor 4 (TLR4), which recognises lipopolysaccharides of Escherichia coli and other Gram negative bacteria, the most common causes of bovine endometritis. Activation of TLR4 triggers the production of tumour necrosis factor alpha and other pro-inflammatory cytokines. The periparturient period is also characterised by an increased secretion of prostaglandin F(2alpha), which enhances uterine immune defences.

Complement and complement regulators in the male reproductive system

Spermatozoa are almost unique among cells in that they must survive transplantation into a foreign host in order to perform their physiological role. The biggest hurdle to overcome is innate immune defence that will target the invaders in the female genital tract. Complement is a major player in innate immunity and is present in the female genital tract. Spermatozoa must therefore evade complement attack if they are to reach their goal. Complement evasion is achieved by the presence of complement regulators both in seminal plasma and on the spermatozoa. Here we discuss the parts played by complement and complement regulators in permitting spermatozoa to survive long enough to reach the oocyte, in clearance of the excess spermatozoa that have outlived their usefulness and in aiding activation of spermatozoa to engage the oocyte. In particular, we focus on the unique distribution patterns of complement regulators on spermatozoa, patterns that strongly suggest roles in spermatozoal development and oocyte binding. An understanding of these roles will inform studies of their contribution to fertility and infertility in man.

Co-purification of soluble membrane cofactor protein (CD46) and human herpesvirus 6 variant A genome in serum from multiple sclerosis patients

The association of human herpesvirus 6 (HHV-6) and multiple sclerosis (MS) has been supported by several immunological and molecular studies. Recently, membrane cofactor protein (CD46) has been identified as the cellular receptor for the A and B variants of HHV-6. Elevated levels of soluble CD46 (sCD46) have been reported in the serum and CSF of MS patients. The aim of this study was to investigate a possible correlation between elevated levels of soluble CD46 and the presence of serum HHV-6 DNA in MS patients. An immunoaffinity column comprised of immobilized monoclonal antibodies to CD46 was developed to isolate sCD46 from cell free body fluids of MS patients and controls. After immunoaffinity purification, DNA was extracted from anti-CD46 column eluates and subjected to PCR amplification. Of the 42 MS samples tested, 4 serum samples were HHV-6 positive, 3 of which were typed as HHV-6A. The co-purification of sCD46 and HHV-6 DNA from MS sera indicates that HHV-6 is tightly connected to its receptor, CD46, in the serum of MS patients.

Rat membrane cofactor protein (MCP; CD46) is expressed only in the acrosome of developing and mature spermatozoa and mediates binding to immobilized activated C3

The rat analogue of the complement regulator membrane cofactor protein (MCP; CD46) was recently cloned and analysis at the mRNA level suggested that expression was restricted to testis. In light of the proposed roles of human MCP in sperm-egg interaction, we undertook to analyze rat MCP expression at the protein level in order better to address its putative role in fertilization. Recombinant fusion proteins comprising antibody Fc and specific domains of rat MCP were generated and used to develop a monoclonal antibody, MM.1, specific for rat MCP. Immunohistochemistry using these reagents confirmed the reported testis-specific expression of MCP in sexually mature rats and demonstrated that MCP was expressed only by spermatozoa and their immediate precursors in spermiogenesis, spermatids. Prepubertal male rats did not express MCP, and there was no evidence of MCP expression at any site in the embryo. Spermatozoal MCP expression was restricted to the inner acrosomal membrane, exposed only after fixation or induction of the acrosome reaction. Acrosome-reacted but not unreacted spermatozoa bound methylamine-activated C3 immobilized on plastic. The retention of MCP at this subcellular site, which is probably crucial to sperm-egg interaction, and the functional demonstration of binding to activated C3 strengthen suggestions from human studies that MCP may play an important role in fertilization. The reagents and results described here will enable studies of the role of spermatozoal MCP in sperm-egg interaction using a relevant animal model system.

Expression of complement regulatory proteins [membrane cofactor protein (CD46), decay accelerating factor (CD55), and protectin (CD59)] in endometrial stressed cells

In the female reproductive tract, the complement system represents a defense mechanism that can act directly against pathogens and cells, and mediates inflammatory response. Endometrial cells are protected from autologous complement attack by membrane-bound complement regulatory proteins (CRPs) that prevent complement activation: membrane cofactor protein (CD46), decay accelerating factor (CD55), and protectin (CD59). In this work we show that all CRPs were overexpressed after LPS exposure. Maximal stimulatory effect was detected after 6h, and was declining after 12h, reaching control levels in 24h. CD59 was the protein showing the more prominent effect. There seems to be a slight increase of CRP expression in the endometrium of sterile patients that have anti-endometrial antibodies (AEA) in their serum. Our results suggest that under stress, the high expression of CRPs (CD46, CD55, and CD59) could protect endometrial injured cells against complement mediated lysis. The survival of these cells with some biochemical modifications would enable autoimmune response.

Induction of monocyte chemotactic protein-1 in peritoneal mesothelial and endometrial cells by oxidized low-density lipoprotein and peritoneal fluid from women with endometriosis

OBJECTIVE

To elucidate the effect of oxidized low-density lipoprotein (LDL) and peritoneal fluid of women with endometriosis on monocyte chemotactic protein-1 (MCP-1) production by peritoneal mesothelial cells and endometrial cells.

DESIGN

In vitro study.

SETTING

University medical center.

PATIENT(S)

Five women undergoing surgery for pelvic pain, infertility, or endometriosis; five women without endometriosis who were undergoing tubal ligation were the controls.

INTERVENTION(S)

Mesothelial cells and endometrial cells in culture were treated with oxidized LDL and peritoneal fluid from control and endometriosis patients, then MCP-1 levels were measured.

MAIN OUTCOME MEASURE(S)

ELISA was used to measure MCP-1 in the culture supernatants exposed to oxidized LDL and peritoneal fluid from control and endometriosis patients. Cellular MCP-1 messenger RNA expression was evaluated by reverse transcription-polymerase chain reaction (RT-PCR) assay.

RESULT(S)

Treatment with oxidized LDL caused an increase in accumulation of immunoreactive MCP-1 in the medium of cultured mesothelial and endometrial cells (primary endometrial stromal cells and endometrial cell line EM42). The mesothelial cells secreted more MCP-1 than did endometrial cells under the culture condition. The EM42 cells cultured in the presence of peritoneal fluid from endometriosis patients secreted more MCP-1 than those cultured with peritoneal fluid from normal women. However, no differences were found in MCP-1 levels in the supernatant of endometrial stromal cells cultured with peritoneal fluid.

CONCLUSION(S)

This is the first report of MCP-1 expression in mesothelial cells induced by oxidized LDL, and provides direct evidence of inflammatory action of peritoneal fluid of women with endometriosis.

Elevated serum and cerebrospinal fluid levels of soluble human herpesvirus type 6 cellular receptor, membrane cofactor protein, in patients with multiple sclerosis

Membrane cofactor protein (CD46) is a member of a family of glycoproteins that are regulators of complement and prevent activation of complement on autologous cells. Recently, CD46 has been identified as the cellular receptor for human herpesvirus Type 6 (HHV-6). Elevated levels of soluble CD46 have been described in several autoimmune disorders, and may be implicated in the pathogenesis of these diseases. As several reports have supported an association of HHV-6 and multiple sclerosis, it was of interest to compare levels of soluble CD46 in the sera of multiple sclerosis patients to that of healthy controls, other neurological disease controls, and other inflammatory disease controls. Using an immunoaffinity column comprised of immobilized monoclonal antibodies to CD46, serum levels of soluble CD46 were found to be significantly elevated in multiple sclerosis patients compared with healthy and other neurological disease controls. Moreover, multiple sclerosis patients who tested positive for HHV-6 DNA in serum had significantly elevated levels of soluble CD46 in their serum compared with those who were negative for HHV-6 DNA. A significant increase in soluble CD46 was also found in the serum of other inflammatory disease controls tested compared to healthy controls. Additionally, a significant correlation was demonstrated between levels of soluble CD46 in the serum and cerebrospinal fluid of multiple sclerosis patients. Collectively, these data suggest that elevated levels of soluble CD46 may contribute to the pathogenesis of inflammatory diseases, including multiple sclerosis.

Urine levels of CD46 (membrane cofactor protein) are increased in patients with glomerular diseases

Soluble membrane cofactor protein (MCP, CD46) has not been detected by conventional ELISA in human urine. Here, we established a highly sensitive assay method for determination of urinary MCP (uMCP) using monoclonal antibody-coated paramagnetic beads. This method enabled us to detect less than 0.05 ng/ml of purified membrane and recombinant soluble MCP, a sensitivity 10-fold higher than that of conventional ELISA. In normal subjects, the levels of uMCP were <0. 05 ng/ml. The levels of uMCP were elevated in patients with IgA nephropathy and more prominently in patients with rapidly progressive glomerulonephritis. The levels of uMCP were correlated significantly with those of serum MCP (sMCP) and N-acetyl-beta-glucosaminidase and nonsignificantly with those of beta(2)-microglobulin, total urine protein, or serum creatinine. The properties of uMCP were inconsistent with those of the reported sMCP, since uMCP showed three bands on SDS-PAGE/immunoblotting with molecular mass profiles different from those of sMCP. uMCP exhibited factor I cofactor activity for cleavage of C3b comparable to that of sMCP. The origin of uMCP, however, remains to be determined. These results, taken together with the parameter correlation profiles, suggested that uMCP is secreted or produced secondary to tubular or glomerular damage. The physiological role and clinical significance of uMCP are now within the scope of our investigation by establishment of this assay.

Crystal structure of two CD46 domains reveals an extended measles virus-binding surface

Measles virus is a paramyxovirus which, like other members of the family such as respiratory syncytial virus, is a major cause of morbidity and mortality worldwide. The cell surface receptor for measles virus in humans is CD46, a complement cofactor. We report here the crystal structure at 3.1 A resolution of the measles virus-binding fragment of CD46. The structure reveals the architecture and spatial arrangement of two glycosylated short consensus repeats with a pronounced interdomain bend and some flexibility at the domain interface. Amino acids involved in measles virus binding define a large, glycan-free surface that extends from the top of the first to the bottom of the second repeat. The extended virus-binding surface of CD46 differs strikingly from those reported for the human virus receptor proteins CD4 and intercellular cell adhesion molecule-1 (ICAM-1), suggesting that the CD46 structure utilizes a novel mode of virus recognition. A highly hydrophobic and protruding loop at the base of the first repeat bears a critical virus-binding residue, thereby defining an important recognition epitope. Molecules that mimic the conformation of this loop potentially could be effective anti-viral agents by preventing binding of measles virus to CD46.

The complement regulatory proteins CD46 and CD59, but not CD55, are highly expressed by glandular epithelium of human breast and colorectal tumour tissues

Three of the proteins protecting cells from autologous lysis by complement are: membrane cofactor protein (MCP; CD46), an inhibitor of the membrane attack complex formation (CD59), and decay accelerating factor (DAF; CD55). We have investigated the expression of these proteins in breast and colorectal carcinoma by immunohistochemistry and immunoblotting of breast tissue for CD46. CD46 was consistently and strongly expressed in the epithelial compartment in 26/28 ductal carcinomas of the breast, 9/9 fibroadenomas, and 9/11 cases of control non-neoplastic breast tissue. CD59 showed a similar degree of expression in the fibroadenomas (9/9), but was less strongly expressed in carcinomatous (22/28) and control (5/11) tissues. In marked contrast, no CD55 expression was detected in tissue from 15 ductal carcinomas. Immunoblotting of breast tissue for CD46 showed the same size of the molecule as for lymphocytes. It had however considerably stronger expression in tumour tissue than in non-neoplastic tissue. CD46 and CD59 were either lacking or only weakly expressed in the epithelial component of control colorectal mucosa: 2/15 and 5/15, respectively. In contrast, tissue samples from colorectal adenocarcinomas showed clear staining for both CD59 (10/18) and, more markedly, CD46 (15/18). There was no association between the pattern or intensity of CD46 and CD59 expression and tumour differentiation. As the complement regulatory proteins CD46 and CD59 are also strongly expressed by trophoblast at the feto-maternal tissue interface, these results support the concept that similar mechanisms are employed both by the genetically dissimilar fetus and certain tumours to evade immune attack by their host.

A novel isoform of human membrane cofactor protein (CD46) mRNA generated by intron retention

The reverse transcription polymerase chain reaction (RT-PCR) with primers specific for each of the 14 exons of the human complement regulatory protein membrane cofactor protein (MCP;CD46) has been utilized to determine MCP mRNA transcript expression in peripheral blood mononuclear cells (PBMC). An additional transcript of a larger size than predicted was consistently detected in reactions with a sense primer for exon 7, that encodes the first alternatively spliced serine-threonine-rich region (ST-A), together with an antisense exon 12 primer, RT-PCR with primers for other exons both 5' and 3' of exon 7 further showed that these MCP transcripts contain additional sequences immediately both 5' and 3' to the exon 7-encoded sequence. Comparison of genomic DNA with cDNA by PCR, in combination with sequence analysis, demonstrated the presence of the complete invariant sequences of both introns adjacent to exon 7, i.e. intron 6 (411 bp) and intron 7 (127 bp). RT-PCR using primers specific for the intron 6 sequence, together with Southern and Northern blotting using an intron 6-specific probe, confirmed retention of this intron within a novel 4.8-kb mRNA transcript in human PBMC. Due to the presence of a stop codon within intron 6, translation would result in a novel truncated MCP isoform (MCPi) containing the four invariant short consensus repeat (SCR) regions and a unique C-terminal 39 amino acid transmembrane and cytoplasmic tail region that may promote endoplasmic reticulum retention.