Infertility of CD9-deficient mouse eggs is reversed by mouse CD9, human CD9, or mouse CD81; polyadenylated mRNA injection developed for molecular analysis of sperm-egg fusion

Epigenetic silencing of the tetraspanin CD9 during disease progression in multiple myeloma cells and correlation with survival

PURPOSE

The purpose of this study was to investigate expression and epigenetic regulation of CD9 in multiple myeloma (MM) cells during disease progression.

EXPERIMENTAL DESIGN

CD9 expression was retrospectively analyzed on bone marrow myeloma samples from 81 patients by immunophenotyping. CD9 expression by murine 5TMM cells was detected by flow cytometric staining and quantitative PCR. The methylation status of the CD9 promoter was determined by bisulfite PCR sequencing.

RESULTS

Primary plasma cells in the majority of MM patients with nonactive disease (n = 28) showed CD9 expression, whereas most cases with active disease (n = 53) were CD9 negative. CD9 expression in diagnostic bone marrow samples (n = 74) correlated with survival. Moreover, CD9 expression on murine 5T33 and 5T2MM cells was significantly down-regulated during disease development. Treatment of CD9-nonexpressing 5T33MMvt cells with the clinically relevant histone deacetylase inhibitor LBH589 resulted in a significant increase in CD9 expression. In contrast, cells treated with the demethylation agent 5-aza-2'deoxycytidine barely showed any increase. A combination study with both compounds resulted in a strong synergistic reactivation of CD9. CD9-expressing 5T33MMvv cells and 5T33MMvt cells stably transduced with a mCD9 lentiviral transferplasmid were shown to be more susceptible to natural killer cell-mediated cytolysis than CD9-negative 5T33MMvt cells.

CONCLUSIONS

CD9 expression correlates with disease status and survival of MM patients. In the murine 5T33MM model, we show that histone modifications, and to a lesser extent CpG methylation, are key epigenetic events in CD9 down-regulation. Furthermore, as CD9 expression becomes down-regulated, 5T33MM cells become less susceptible to natural killer cell-mediated cytolysis.

Molecular control of mammalian myoblast fusion

The fusion of postmitotic mononucleated myoblasts to form syncytial myofibers is a critical step in the formation of skeletal muscle. Myoblast fusion occurs both during development and throughout adulthood, as skeletal muscle growth and regeneration require the accumulation of additional nuclei within myofibers. Myoblasts must undergo a complex series of molecular and morphological changes prior to fusing with one another. Although many molecules regulating myoblast fusion have been identified, the precise mechanism by which these molecules act in concert to control fusion remains to be elucidated. A comprehensive understanding of how myo-blast fusion is controlled may contribute to the treatment of various disorders associated with loss of muscle mass. In this chapter, we examine progress made toward elucidating the cellular and molecular pathways involved in mammalian myoblast fusion. Special emphasis is placed on the molecules that regulate myofiber formation without discernibly affecting biochemical differentiation.

Tetraspanin CD81 is required for the alpha v beta5-integrin-dependent particle-binding step of RPE phagocytosis

Retinal pigment epithelial (RPE) cells are among the most active phagocytes in the body. Every morning, circadian shedding of outer segment fragments by photoreceptor cells activates a synchronized phagocytic response by RPE cells that is critical for vision. RPE cells require alpha v beta5 integrin receptors for particle binding that triggers engulfment. Here, we show that tetraspanins CD81 and CD9 reside in a complex specifically with alpha v beta5 integrin but not the engulfment receptors Mer tyrosine kinase and CD36 at the apical, phagocytic surface of RPE cells. Function blocking and RNA silencing of CD81 but not of CD9 specifically diminish particle binding. CD81 but not CD9 overexpression is sufficient to increase particle binding and surface levels of alpha v beta5 integrin. Wild-type and mutant RPE cells defective in particle engulfment equally reduce and increase particle binding in response to CD81 inhibition and CD81 overexpression, respectively. By striking contrast, neither CD81 inhibition nor CD81 overexpression has any effect on particle binding by RPE lacking alpha v beta5 integrin. These results identify a novel and important role for CD81 in phagocytosis. CD81 does not function as a binding receptor by itself but promotes outer segment particle binding through functional interaction specifically with alpha v beta5 integrin.

Expression of CD9 in frozen–thawed mouse oocytes: preliminary experience

CD9 mRNA and protein expression levels in mouse slow frozen-rapid thawed oocytes were compared with those in fresh oocytes by using comparative quantitative real time reverse transcription-PCR and semiquantitative Western blot, respectively. The expression levels of both CD9 mRNA and protein in the frozen oocytes were significantly lower than those found in the fresh oocytes.

Characterization of Xenopus egg membrane microdomains containing uroplakin Ib/III complex: roles of their molecular interactions for subcellular localization and signal transduction

A single-transmembrane protein uroplakin III (UPIII) and its tetraspanin binding-partner uroplakin Ib (UPIb) are members of the UP proteins that were originally identified in mammalian urothelium. In Xenopus laevis eggs, these proteins: xUPIII and xUPIb, are components of the cholesterol-enriched membrane microdomains or "rafts" and involved in the sperm-egg membrane interaction and subsequent egg activation signaling via Src tyrosine kinase at fertilization. Here, we investigate whether the xUPIII-xUPIb complex is in close proximity to CD9, a tetraspanin that has been implicated in the sperm-egg fusion in the mouse and GM1, a ganglioside typically enriched in egg rafts. Preparation of the egg membrane microdomains using different non-ionic detergents (Brij 98 and Triton X-100), chemical cross-linking, co-immunoprecipitation, in vitro kinase assay and in vitro fertilization experiments demonstrated that GM1, but not CD9, is in association with the xUPIII-xUPIb complex and contributes to the sperm-dependent egg activation. Transfection experiments using HEK293 cells demonstrated that xUPIII and xUPIb localized efficiently to the cholesterol-dependent membrane microdomains when they were co-expressed, whereas co-expression of xUPIII and CD9, instead of xUPIb, did not show this effect. Furthermore, xUPIII and xUPIb were shown to suppress kinase activity of the wild type, but not a constitutively active form of, Xenopus Src protein co-expressed in HEK293 cells. These results provide novel insight into the molecular architecture of the egg membrane microdomains containing xUPIII, xUPIb and Src, which may contribute to the understanding of sperm-egg interaction and signaling during Xenopus fertilization.

Cell-cell membrane fusion during mammalian fertilization

The mechanism of sperm-egg fusion in mammals is a research area that has greatly benefited from the use of gene deletion technology. Because fertilization is internal in mammals and the gametes (particularly the eggs) are sparse in number, in vitro studies have considerable limitations. Using gene deletions, a few cell surface proteins in both gametes have been identified as essential for gamete fusion. Ongoing studies are directed at analysis of the function of these proteins and the search for additional proteins that may be involved in this process. So far, no mammalian proteins have been found that also function in sperm-egg fusion of non-mammalian species or in other types of cell-cell fusion.

Possible involvement of CD81 in acrosome reaction of sperm in mice

Tetraspanin CD81 is closely homologous in amino acid sequence with CD9. CD9 is well known to be involved in sperm-egg fusion, and CD81 has also been reported to be involved in membrane fusion events. However, the function of CD81 as well as that of CD9 in membrane fusion remains unclear. Here, we report that disruption of the mouse CD81 gene led to a reduction in the fecundity of female mice, and CD81-/- eggs had impaired ability to fuse with sperm. Furthermore, we demonstrated that when CD81-/- eggs were incubated with sperm, some of the sperm that penetrated into the perivitelline space of CD81-/- eggs had not yet undergone the acrosome reaction, indicating that the impaired fusibility of CD81-/- eggs may be in part caused by failure of the acrosome reaction of sperm. In addition, we showed that CD81 was highly expressed in granulosa cells, somatic cells that surround oocytes. Our observations suggest that there is an interaction between sperm and CD81 on somatic cells surrounding eggs before the direct interaction of sperm and eggs. Our results may provide new clues for clarifying the cellular mechanism of the acrosome reaction, which is required for sperm-egg fusion.

Hepatitis C virus entry: potential receptors and their biological functions

Several cellular molecules have been identified as putative receptors forHepatitis C virus(HCV): CD81 tetraspanin, scavenger receptor class B type I (SR-BI), mannose-binding lectins DC-SIGN and L-SIGN, low-density lipoprotein receptor, heparan sulphate proteoglycans and the asialoglycoprotein receptor. Due to difficulties in propagating HCV in cell culture, most of these molecules have been identified by analysing their interaction with a soluble, truncated form of HCV glycoprotein E2. A recent major step in investigating HCV entry was the development of pseudoparticles (HCVpp), consisting of unmodified HCV envelope glycoproteins assembled onto retroviral core particles. This system has allowed the investigation of the role of candidate receptors in the early steps of the HCV life cycle and the data obtained can now be confirmed with the help of a newly developed cell-culture system that allows efficient amplification of HCV (HCVcc). Interestingly, CD81 and SR-BI have been shown to play direct roles in HCVpp and/or HCVcc entry. However, co-expression of CD81 and SR-BI in non-hepatic cell lines does not lead to HCVpp entry, indicating that other molecule(s), expressed only in hepatic cells, are necessary for HCV entry. In this review, the molecules that have been proposed as potential HCV receptors are described and the experimental data indicating that CD81 and SR-BI are potentially involved in HCV entry are presented.

Reduced fertility of female mice lacking CD81

In somatic cells, the tetraspanins CD81 and CD9 associate with each other, with additional tetraspanins and with non-tetraspanin molecules to form proteolipidic complexes. Here we show that CD81 is expressed on the surface of oocytes where it associates with tetraspanin-enriched membrane structures. A major CD9 and CD81 partner, CD9P-1, is also expressed by oocytes. Deletion of CD81 gene in mice results in a 40% reduction of female fertility. In vitro insemination indicated that this infertility is due to a deficiency of oocytes to fuse with sperm. While the fertility of CD9-/- mice is severely but not completely impaired, double knock-out CD9-/- CD81-/- mice were completely infertile indicating that CD9 and CD81 play complementary roles in sperm-egg fusion. Finally, a fraction of CD9 was transferred from CD81-/- oocytes to sperm present in the perivitelline space indicating that the defect of fusion of CD81-/- oocytes does not result from an impaired initial gamete interaction.

CD9 controls the formation of clusters that contain tetraspanins and the integrin α6β1, which are involved in human and mouse gamete fusion

The process of gamete fusion has been largely studied in the mouse and has revealed the crucial role of the tetraspanin CD9. By contrast, human gamete fusion remains largely unknown. We now show that an anti-α6 integrin mAb (GoH3) strongly inhibited human sperm-egg fusion in human zona-free eggs. Furthermore, a mAb directed against CD151, a tetraspanin known to associate with α6β1, partially inhibited sperm-egg fusion. By contrast, the addition of an anti-CD9 mAb to zona free eggs had no effect. The integrin α6β1, CD151 and CD9 tetraspanins were evenly distributed on human zona-intact oocytes. On zona-free eggs, the integrin α6β1 and tetraspanin CD151 patched and co-localized but the tetraspanin CD9 remained unchanged. CD9 mAb prevented α6β1 integrin clustering and gamete fusion when added prior to, but not after, zona removal. Antibody-mediated aggregation of integrin α6β1 yielded patches that were bigger and more heterogeneous in mouse oocytes lacking CD9. Moreover, a strong labelling of α6β1 could be observed at the sperm entry point. Altogether, these data show that CD9 controls the redistribution of some membrane proteins including the α6β1 integrin into clusters that may be necessary for gamete fusion.

The phylogenetic analysis of tetraspanins projects the evolution of cell–cell interactions from unicellular to multicellular organisms

In animals, the tetraspanins are a large superfamily of membrane proteins that play important roles in organizing various cell-cell and matrix-cell interactions and signal pathways based on such interactions. However, their origin and evolution largely remain elusive and most of the family's members are functionally unknown or less known due to difficulties of study, such as functional redundancy. In this study, we rebuilt the family's phylogeny with sequences retrieved from online databases and our cDNA library of amphioxus. We reveal that, in addition to in metazoans, various tetraspanins are extensively expressed in protozoan amoebae, fungi, and plants. We also discuss the structural evolution of tetraspanin's major extracellular domain and the relation between tetraspanin's duplication and functional redundancy. Finally, we elucidate the coevolution of tetraspanins and eukaryotes and suggest that tetraspanins play important roles in the unicell-to-multicell transition. In short, the study of tetraspanin in a phylogenetic context helps us understand the evolution of intercellular interactions.

Paternal contribution: new insights and future challenges

It has been widely held that all that fathers essentially contribute to the next generation is half their genome. However, recent progress towards understanding biological processes such as sperm maturation and fertilization now indicates that the paternal contribution has been underestimated. To tackle some of the misconceptions surrounding the paternal contribution, the factors that are actually delivered by the sperm at fertilization and their potential developmental functions will be discussed using data from humans and animal models. Although still in their infancy, the practical applications of using sperm RNAs have already emerged in reproductive medicine as markers that are indicative of successful vasectomy. They are also beginning to appear in the forensic sciences and, within the next decade, might appear in the environmental sciences.

Protein-protein interactions in the tetraspanin web

Tetraspanins are evolutionarily conserved membrane proteins that tend to associate laterally with one another and to cluster dynamically with numerous partner proteins in membrane microdomains. Consequently, members of this family are involved in the coordination of intracellular and intercellular processes, including signal transduction; cell proliferation, adhesion, and migration; cell fusion; and host-parasite interactions.

TheCaenorhabditis elegans spe-38gene encodes a novel four-pass integral membrane protein required for sperm function at fertilization

A mutation in the Caenorhabditis elegans spe-38 gene results in a sperm-specific fertility defect. spe-38 sperm are indistinguishable from wild-type sperm with regards to their morphology, motility and migratory behavior. spe-38 sperm make close contact with oocytes but fail to fertilize them. spe-38 sperm can also stimulate ovulation and engage in sperm competition. The spe-38 gene is predicted to encode a novel four-pass (tetraspan) integral membrane protein. Structurally similar tetraspan molecules have been implicated in processes such as gamete adhesion/fusion in mammals, membrane adhesion/fusion during yeast mating, and the formation/function of tight-junctions in metazoa. In antibody localization experiments, SPE-38 was found to concentrate on the pseudopod of mature sperm,consistent with it playing a direct role in gamete interactions.

Unveiling the mechanisms of cell-cell fusion

Cell-cell fusion is fundamental to the development and physiology of multicellular organisms, but little is known of its mechanistic underpinnings. Recent studies have revealed that many proteins involved in cell-cell fusion are also required for seemingly unrelated cellular processes such as phagocytosis, cell migration, axon growth, and synaptogenesis. We review advances in understanding cell-cell fusion by contrasting it with virus-cell and intracellular vesicle fusion. We also consider how proteins involved in general aspects of membrane dynamics have been co-opted to control fusion of diverse cell types by coupling with specialized proteins involved in cell-cell recognition, adhesion, and signaling.

The tetraspanin web modulates immune-signalling complexes

The tetraspanin web represents a new concept of molecular interactions in the immune system. Whereas most surface immune-modulating molecules involve receptor-ligand interactions, tetraspanins associate with partner proteins and facilitate their lateral positioning in the membrane. Moreover, the same tetraspanin molecule can associate with different proteins depending on the cell type. Most importantly, members of this family tend to associate with each other, together with their partners, in membrane microdomains that provide a scaffold for the transmission of external stimuli to intracellular-signalling components.

Binding of pregnancy-specific glycoprotein 17 to CD9 on macrophages induces secretion of IL-10, IL-6, PGE2, and TGF-beta1

Pregnancy-specific glycoproteins (PSGs) are a family of secreted proteins produced by the placenta, which are believed to have a critical role in pregnancy success. Treatment of monocytes with three members of the human PSGs induces interleukin (IL)-10, IL-6, and transforming growth factor-beta(1) (TGF-beta(1)) secretion. To determine whether human and murine PSGs have similar functions and use the same receptor, we treated wild-type and CD9-deficient macrophages with murine PSG17N and human PSG1 and -11. Our data show that murine PSG17N induced secretion of IL-10, IL-6, prostaglandin E(2), and TGF-beta(1) and that CD9 expression is required for the observed induction of cytokines. Therefore, the ability of PSG17 to induce anti-inflammatory cytokines parallels that of members of the human PSG family, albeit human and murine PSGs use different receptors, as CD9-deficient and wild-type macrophages responded equally to human PSGs. We then proceeded to examine the signaling mechanisms responsible for the CD9-mediated response to PSG17. Inhibition of cyclooxygenase 2 significantly reduced the PSG17N-mediated increase in IL-10 and IL-6. Further characterization of the response to PSG17 indicated that cyclic adenosine monophosphate-dependent protein kinase A (PKA) is involved in the up-regulation of IL-10 and IL-6, and it is not required for the induction of TGF-beta(1). Conversely, treatment of macrophages with a PKC inhibitor reduced the PSG17-mediated induction of TGF-beta(1), IL-6, and IL-10 significantly. The induction of anti-inflammatory cytokines by various PSGs supports the hypothesis that these glycoproteins have an essential role in the regulation of the maternal immune response in species with hemochorial placentation.

Microarray analysis of fiber cell maturation in the lens

The mammalian lens consists of an aged core of quiescent cells enveloped by layers of mature fully elongated cells and younger, continuously elongating transcriptionally active cells. The fiber cell maturation is initiated when fiber cells cease to elongate. The process of maturation represents a radical switch from active elongation to a life-long quiescence and has not been studied previously. It may also include critical stages of preparation for the organelle removal and denucleation. In the present study, we used laser capture microdisection (LCM) microdissection and RNA amplification to compare global gene expression profiles of young elongating and mature, non-elongating fiber cells. Analysis of microarray data from three independent dye-swap experiments identified 65 differentially expressed genes (FDR<0.1) with greater than 2-fold change in expression levels. Microarray array results for a group of randomly selected genes were confirmed by quantitative RT-PCR. These microarray results provide clues to understanding the molecular pathways underlying lens development. The identified changes in the profile of gene expression reflected a shift in cell physiology characterizing the lens fiber maturation.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Sperm-egg fusion: events at the plasma membrane

Sperm-egg fusion is a cell-cell membrane fusion event essential for the propagation of sexually reproducing organisms. In gamete fusion, as in other fusion events, such as virus-cell and intracellular vesicle fusion, membrane fusion is a two-step process. Attachment of two membranes through cell-surface molecules is followed by the physical merger of the plasma membrane lipids. Recent progress has demonstrated an essential role for an oocyte tetraspanin, CD9, in mouse sperm-egg fusion, and a specific molecular site crucial for CD9 function has been identified. Absence of glycosylphosphatidylinositol-anchored proteins on the oocyte surface also results in loss of oocyte fusion competence in this gamete. These discoveries provide a strong starting point for the identification of additional proteins that have roles in sperm-egg fusion.

The mechanism of sperm-oocyte fusion in mammals

Sperm-oocyte fusion is one of the most impressive events in sexual reproduction, and the elucidation of its molecular mechanism has fascinated researchers for a long time. Because of the limitation of materials and difficulties in analyzing membrane protein-protein interactions, many attempts have failed to reach this goal. Recent studies involving gene targeting have clearly demonstrated the various molecules that are involved in sperm-oocyte binding and fusion. Sperm ADAMs (family of proteins with a disintegrin and metalloprotease domain), including fertilin alpha, fertilin beta and cyritestin, have been investigated and found to be important for binding rather than for fusion and painstaking studies have raised suspicions that their putative receptors, oocyte integrins, are necessary for the sperm-oocyte interaction. Recently, several studies have focused the spotlight on CD9 and glycosylphosphatidylinositol (GPI)-anchored proteins on oocytes, and epididymal protein DE on sperm, as candidate molecules involved in sperm-oocyte fusion. Lack of, or interference with the function of, these proteins can disrupt the sperm-oocyte fusion without changing the binding. In this review we highlight the candidate molecules involved in the sperm-oocyte interaction suggested from the recent progress in this research field.

Tetraspanin proteins mediate cellular penetration, invasion, and fusion events and define a novel type of membrane microdomain

This review summarizes key aspects of tetraspanin proteins, with a focus on the functional relevance and structural features of these proteins and how they are organized into a novel type of membrane microdomain. Despite the size of the tetraspanin family and their abundance and wide distribution over many cell types, most have not been studied. However, from studies of prototype tetraspanins, information regarding functions, cell biology, and structural organization has begun to emerge. Genetic evidence points to critical roles for tetraspanins on oocytes during fertilization, in fungi during leaf invasion, in Drosophila embryos during neuromuscular synapse formation, during T and B lymphocyte activation, in brain function, and in retinal degeneration. From structure and mutagenesis studies, we are beginning to understand functional subregions within tetraspanins, as well as the levels of connections among tetraspanins and their many associated proteins. Tetraspanin-enriched microdomains (TEMs) are emerging as entities physically and functionally distinct from lipid rafts. These microdomains now provide a context in which to evaluate tetraspanins in the regulation of growth factor signaling and in the modulation of integrin-mediated post-cell adhesion events. Finally, the enrichment of tetraspanins within secreted vesicles called exosomes, coupled with hints that tetraspanins may regulate vesicle fusion and/or fission, suggests exciting new directions for future research.

Structural requirements for the inhibitory action of the CD9 large extracellular domain in sperm/oocyte binding and fusion

CD9 has been shown to be essential for sperm/oocyte fusion in mice, the only non-redundant role found for a member of the tetraspanin family. CD9 can act in cis, reconstituting sperm/oocyte fusion when ectopically expressed in oocytes from CD9 null mice, or in trans, inhibiting sperm fusion when the large extracellular domain (LED) is added to CD9-positive oocytes as a soluble protein. In contrast to cis inhibition, the structural requirements of the trans inhibition by soluble CD9 LED are unknown. Here we show that human CD9 LED is as potent an inhibitor as mouse CD9 LED in mouse sperm/oocyte fusion assays and that CD9 LED can also inhibit sperm/oocyte binding. The two disulphide bridges that define membership of the tetraspanin family are critical for structure and function of human CD9 LED and mutation of a pentapeptide sequence in the hypervariable region further defines the critical region for trans inhibition.

Endosomal compartmentalization in three dimensions: implications for membrane fusion

Endosomes are major sorting stations in the endocytic route that send proteins and lipids to multiple destinations in the cell, including the cell surface, Golgi complex, and lysosomes. They have an intricate architecture of internal membrane structures enclosed by an outer membrane. Recycling proteins remain on the outer membrane, whereas proteins that are destined for degradation in the lysosome are sorted to the interior. Recently, a retrograde pathway was discovered whereby molecules, like MHC class II of the immune system, return from the internal structures to the outer membrane, allowing their further transport to the cell surface for T cell activation. Whether this return involves back fusion of free vesicles with the outer membrane, or occurs via the continuity of the two membrane domains, is an unanswered question. By electron tomography of cryo-immobilized cells we now demonstrate that, in multivesicular endosomes of B-lymphocytes and dendritic cells, the inner membranes are free vesicles. Hence, protein transport from inner to outer membranes cannot occur laterally in the plane of the membrane, but requires fusion between the two membrane domains. This implies the existence of an intracellular machinery that mediates fusion between the exoplasmic leaflets of the membranes involved, which is opposite to regular intracellular fusion between cytoplasmic leaflets. In addition, our 3D reconstructions reveal the presence of clathrin-coated areas at the cytoplasmic face of the outer membrane, known to participate in protein sorting to the endosomal interior. Interestingly, profiles reminiscent of inward budding vesicles were often in close proximity to the coats.

Role of multiple beta1 integrins in cell adhesion to the disintegrin domains of ADAMs 2 and 3

ADAM disintegrin domains can support integrin-mediated cell adhesion. However, the profile of which integrins are employed for adhesion to a given disintegrin domain remains unclear. For example, we suggested that the disintegrin domains of mouse sperm ADAMs 2 and 3 can interact with the alpha6beta1 integrin on mouse eggs. Others concluded that these disintegrin domains interact instead with the alpha9beta1 integrin. To address these differing results, we first studied adhesion of mouse F9 embryonal carcinoma cells and human G361 melanoma cells to the disintegrin domains of mouse ADAMs 2 and 3. Both cell lines express alpha6beta1 and alpha9beta1 integrins at their surfaces. Antibodies to the alpha6 integrin subunit inhibited adhesion of both cell lines. An antibody that recognizes human alpha9 integrin inhibited adhesion of G361 cells. VLO5, a snake disintegrin that antagonizes alpha4beta1 and alpha9beta1 integrins, potently inhibited adhesion of both cell lines. We next explored expression of the alpha9 integrin subunit in mouse eggs. In contrast to our ability to detect alpha6beta1, we were unable to convincingly detect alpha9beta1 integrin on the surface of mouse eggs. Moreover, treatment of mouse eggs with 250 nm VLO5, which is 250 fold over its approximately IC(50) for inhibition of somatic cell adhesion, had minimal effect on sperm-egg binding or fusion. We did detect alpha9 integrin protein on epithelial cells of the oviduct. Additional studies showed that antibodies to the alpha6 and alpha7 integrins additively inhibited adhesion of mouse trophoblast stem cells and that an antibody to the alpha4 integrin inhibited adhesion of MOLT-3 cells to these disintegrin domains: Our data suggest that multiple integrins (on the same cell) can participate in adhesion to a given ADAM disintegrin domain and that interactions between ADAMs and integrins may be important for sperm transit through the oviduct.

Tetraspanins CD9 and CD81 function to prevent the fusion of mononuclear phagocytes

Tetraspanins CD9 and CD81 facilitate the fusion between gametes, myoblasts, or virus-infected cells. Here, we investigated the role of these tetraspanins in the fusion of mononuclear phagocytes. Expression of CD9 and CD81 and their complex formation with integrins were up-regulated when blood monocytes were cultured under normal conditions. Under fusogenic conditions in the presence of Con A, CD9 and CD81 up-regulation was inhibited, and their complex formation with integrins was down-regulated. Anti-CD9 and -CD81 antibodies, which were previously shown to inhibit the fusion of gametes, myoblasts, and virus-infected cells, unexpectedly promoted the fusion of monocytes and alveolar macrophages. However, these effects were not due to altered cell adhesion, aggregation, or cytokine production. When stimulated in vitro or in vivo, alveolar macrophages and bone marrow cells of CD9- and CD81-null mice formed larger numbers of multinucleated cells than those of wild-type mice. Finally, CD9/CD81 double-null mice spontaneously developed multinucleated giant cells in the lung and showed enhanced osteoclastogenesis in the bone. These results suggest that CD9 and CD81 coordinately prevent the fusion of mononuclear phagocytes.