Expression of beta 1 integrin complexes on the surface of unfertilized mouse oocyte

Activation of embryonic/germ cell-like axis links poor outcomes of gliomas

Background

It is unclear which core events drive the malignant progression of gliomas. Earlier studies have revealed that the embryonic stem (ES) cell/early PGC state is associated with tumourigenicity. This study was designed to investigate the role of ES/PGC state in poor outcomes of gliomas.

Methods

Crispr-Cas9 technology, RT–PCR and animal experiments were used to investigate whether PGC-like cell formation play crucial roles in the tumorigenicity of human glioma cells. Bioinformatic analysis was used to address the link between ES/PGC developmental axis and glioma overall outcomes.

Results

Here, our findings showed that germ cell-like cells were present in human gliomas and cultured glioma cells and that the formation of germ cell-like cells was essential for glioma tumours. Bioinformatic analysis showed that the mRNA levels of genes related to embryonic/germ cell development could be detected in most gliomas. Our findings showed that the activation of genes related to reprogramming or the germ cell-like state alone seemed to be insufficient to lead to a malignant prognosis, whereas increased mRNA levels of genes related to the activation of the embryonic/germ cell-like cycle (somatic PGC-EGC-like cycle and somatic parthenogenetic embryo-like cycle) were positively correlated with malignant prognoses and poor clinical outcomes of gliomas. Genes related to the embryonic/germ cell cycle alone or in combination with the WHO grade or 1p19q codeletion status could be used to subdivide gliomas with distinct clinical behaviours.

Conclusion

Together, our findings indicated that a crucial role of germ cell-like cell formation in glioma initiation as well as activation of genes related with the parthenogenetic embryo-like cycle and PGC-EGC-like cycle link to the malignant prognosis and poor outcomes of gliomas, which might provide a novel way to better understand the nature of and develop targeted therapies for gliomas as well as important markers for predicting clinical outcomes in gliomas.

Integrin β1 mediates vaccinia virus entry through activation of PI3K/Akt signaling

Vaccinia virus has a broad range of infectivity in many cell lines and animals. Although it is known that the vaccinia mature virus binds to cell surface glycosaminoglycans and extracellular matrix proteins, whether additional cellular receptors are required for virus entry remains unclear. Our previous studies showed that the vaccinia mature virus enters through lipid rafts, suggesting the involvement of raft-associated cellular proteins. Here we demonstrate that one lipid raft-associated protein, integrin β1, is important for vaccinia mature virus entry into HeLa cells. Vaccinia virus associates with integrin β1 in lipid rafts on the cell surface, and the knockdown of integrin β1 in HeLa cells reduces vaccinia mature virus entry. Additionally, vaccinia mature virus infection is reduced in a mouse cell line, GD25, that is deficient in integrin β1 expression. Vaccinia mature virus infection triggers the activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling, and the treatment of cells with inhibitors to block P13K activation reduces virus entry in an integrin β1-dependent manner, suggesting that integrin β1-mediates PI3K/Akt activation induced by vaccinia virus and that this signaling pathway is essential for virus endocytosis. The inhibition of integrin β1-mediated cell adhesion results in a reduction of vaccinia virus entry and the disruption of focal adhesion and PI3K/Akt activation. In summary, our results show that the binding of vaccinia mature virus to cells mimics the outside-in activation process of integrin functions to facilitate vaccinia virus entry into HeLa cells.

ADAM2 interactions with mouse eggs and cell lines expressing α4/α9 (ITGA4/ITGA9) integrins: implications for integrin-based adhesion and fertilization

BACKGROUND

Integrins are heterodimeric cell adhesion molecules, with 18 α (ITGA) and eight β (ITGB) subunits forming 24 heterodimers classified into five families. Certain integrins, especially the α(4)/α(9) (ITGA4/ITGA9) family, interact with members of the ADAM (a disintegrin and metalloprotease) family. ADAM2 is among the better characterized and also of interest because of its role in sperm function. Having shown that ITGA9 on mouse eggs participates in mouse sperm-egg interactions, we sought to characterize ITGA4/ITGA9-ADAM2 interactions.

METHODOLOGY/PRINCIPAL FINDINGS

An anti-β(1)/ITGB1 function-blocking antibody that reduces sperm-egg binding significantly inhibited ADAM2 binding to mouse eggs. Analysis of integrin subunit expression indicates that mouse eggs could express at least ten different integrins, five in the RGD-binding family, two in the laminin-binding family, two in the collagen-binding family, and ITGA9-ITGB1. Adhesion assays to characterize ADAM2 interactions with ITGA4/ITGA9 family members produced the surprising result that RPMI 8866 cell adhesion to ADAM2 was inhibited by an anti-ITGA9 antibody, noteworthy because ITGA9 has only been reported to dimerize with ITGB1, and RPMI 8866 cells lack detectable ITGB1. Antibody and siRNA studies demonstrate that ITGB7 is the β subunit contributing to RPMI 8866 adhesion to ADAM2.

CONCLUSIONS/SIGNIFICANCE

These data indicate that a novel integrin α-β combination, ITGA9-ITGB7 (α(9)β(7)), in RPMI 8866 cells functions as a binding partner for ADAM2. ITGA9 had previously only been reported to dimerize with ITGB1. Although ITGA9-ITGB7 is unlikely to be a widely expressed integrin and appears to be the result of "compensatory dimerization" occurring in the context of little/no ITGB1 expression, the data indicate that ITGA9-ITGB7 functions as an ADAM binding partner in certain cellular contexts, with implications for mammalian fertilization and integrin function.

Fusion as the result of sperm–somatic cell interaction

The research has been designed to investigate whether acrosome-reacted spermatozoa can fuse with somatic cells and to check whether this event may involve the molecular machinery implicated in the sperm–egg fusion. Boar spermatozoa were capacitatedin vitroand then treated with A23187 to induce acrosome reaction and activate their fusogenic potential. Reacted spermatozoa, loaded with the membrane-permeant fluorescent dye calcein AM, were incubated with plated granulosa cells or cells derived from stable cell lines: CRFK, VERO, and ESK4. The fusion between spermatozoa and somatic cells was revealed by the diffusion of the fluorescent dye from the sperm to the cell as membrane fusion and cytoplasmic continuity between the two cells were established. The involvement of integrin α6 and tetraspanin CD9 in the process of fusion was assessed by carrying out the experiment in the presence of antibodies against these molecules. Moreover, the incidence of fusion displayed by the different cell types used was analyzed in relation to their content in the above molecules assessed by western blot and immunostaining. The role of CD9 was additionally investigated by using CD9-negative cells. The data presented demonstrate that boar spermatozoa can fuse with different somatic cell types derived from different species and the process requires the combined presence of both integrin and tetraspanin molecules on the cell plasma membrane.

Reduction of mouse egg surface integrin alpha9 subunit (ITGA9) reduces the egg's ability to support sperm-egg binding and fusion

The involvement of egg integrins in mammalian sperm-egg interactions has been controversial, with data from integrin inhibitor studies contrasting with evidence from knockouts showing that specific integrin subunits are not essential for fertility. An alpha(4)/alpha(9) (ITGA4/ITGA9) integrin subfamily member has been implicated in fertilization but not extensively examined, so we tested the following three hypotheses: 1) an ITGA4/ITGA9 integrin participates in sperm-egg interactions, 2) short-term acute knockdown by RNA interference of integrin subunits would result in a fertilization phenotype differing from that of chronic depletion via knockout, and 3) detection of a fertilization phenotype is sensitive to in vitro fertilization (IVF) assay conditions. We show that mouse and human eggs express the alpha(9) integrin subunit (ITGA9). RNA interference-mediated knockdown resulted in reduced levels of Itga9 mRNA and surface protein in mouse eggs. RNA interference attempts to knockdown ITGA9's likely beta partner, beta(1) (ITGB1), resulted in reduced Itgb1 mRNA but no reduction in ITGB1 surface protein. Therefore, studies using a function-blocking anti-ITGB1 antibody tested the hypothesis that ITGB1 participates in gamete interactions. Analyses of sperm-egg interactions with Itga9-knockdown eggs and anti-ITGB1 antibody-treated eggs in IVF assays using specific sperm:egg ratios revealed the following: 1) a reduction, but not complete loss, of sperm-egg binding and fusion was observed and 2) the reduction of sperm-egg binding and fusion was not detected in inseminations with high sperm:egg ratios. These data demonstrate that ITGA9 and ITGB1 participate in sperm-egg interactions but clearly are not the only molecules involved. This also shows that careful design of IVF parameters allows detection of deficiencies in gamete interactions.

Calcium signalling during mammalian fertilization

The fertilized mammalian egg is a nice model system for analysing spatiotemporal Ca2+ signalling in the intact cell. Hamster eggs show repetitive Ca2+ transients, associated in the initial response with Ca2+ waves which begin from the site of sperm attachment and are propagated across the deep cytoplasm to the opposite pole. In unfertilized eggs, a regenerative Ca2+ wave is induced by injection of either inositol 1,4,5-trisphosphate (InsP3) or Ca2+, and Ca2+ oscillations are produced by continuous injection of InsP3. These Ca2+ waves and oscillations in both fertilized and unfertilized eggs are inhibited in a dose-dependent manner by a monoclonal antibody to the type 1 InsP3 receptor. Ryanodine receptors (both skeletal and cardiac types) are not detected by physiological or immunoblot analyses. Positive and negative feedback between cytosolic Ca2+ and Ca2+ release from InsP3-sensitive pools accounts for the spatiotemporal Ca2+ signalling. In addition to intracellular Ca2+ release, Ca2+ entry from outside the egg is necessary to refill the Ca2+ pools and maintain Ca2+ oscillations. Evidence suggests that inositol 1,3,4,5-tetrakisphosphate activates the Ca2+ influx. The signal transduction process leading to the production of InsP3 and the mechanism of egg activation following the Ca2+ response still remain to be elucidated.

Influence of arginine-glycine-aspartic acid (RGD), integrins (αV and α5) and osteopontin on bovine sperm–egg binding, and fertilization in vitro

Osteopontin (OPN), a phosphoprotein containing an arginine-glycine-aspartic acid (RGD) sequence, has been identified in cow oviduct epithelium and fluid. To investigate the potential role OPN in fertilization, we evaluated the ability of RGD peptide (arginine-glycine-aspartic), RGE peptide (arginine-glycine-glutamic acid), integrins alphaV and alpha5 antibodies and OPN antibody to influence bovine in vitro sperm-egg binding and fertilization. Treatment of sperm or oocytes with the RGD peptide prior fertilization significantly decreased in vitro sperm-egg binding and fertilization compared to the non-treated controls or those treated with RGE peptide. Binding and fertilization were also significantly decreased when in vitro matured bovine oocytes or sperm were pre-incubated with integrins alphaV and alpha5 antibodies at concentration ranging from 5 to 20 microg/mL. Addition of a rabbit polyclonal IgG antibody against purified bovine milk OPN with sperm or/and oocytes decreased (P<0.05) fertilization compared to the in vitro-fertilized control. These data provided evidence that integrin ligands existed on bovine oocytes and spermatozoa that contained RGD recognition sequences, and that antibody to OPN, a protein that contains that RGD sequence, was capable of reducing sperm-egg binding and fertilization in vitro.

Specific integrin subunits in bovine oocytes, including novel sequences for alpha 6 and beta 3 subunits

Integrins facilitate attachment of cells to the extra-cellular matrix, often binding the arginine-glycine-aspartic acid tri-peptide motif, thus facilitating cell migration, mediating cell-cell adhesion, linking the extracellular matrix (ECM) with cytoskeletal elements, and acting as signaling molecules. Adhesion activates signaling mechanisms that regulate integrin function, cytoskeletal assembly, cell behavior, and protein synthesis. Immunofluorescence was used to determine the presence of integrin alpha and beta subunits on the surface of bovine oocytes using a panel of monoclonal antibodies (mAbs) specific for alphaL, alphaM, alphaX, alphaV, alpha2, alpha4, alpha6, beta1, beta2, and beta3 antigens, with multiple antibodies for each subunit. Confocal microscopy indicated the presence of alphaV, alpha6, alpha4, alpha2, ss1, and ss3 integrin subunits on the plasma membrane of bovine oocytes. The presence of these subunits was verified by RT-PCR analysis using primers designed based on known gene sequences of bovine integrin subunits, or by using sequence information using bovine expressed sequence tags (EST) compared with known human and murine integrin subunit gene sequence information. Previously unpublished sequence information for bovine alpha6 and beta3 integrins was determined. The presence of these integrin subunits on the bovine oocyte vitelline membrane supports the hypothesis that sperm-oocyte interactions in the bovine are mediated by integrins.

Effects of amino acid substitutions in and around the arginine-glycine-aspartic acid (RGD) sequence on fertilization and parthenogenetic development in mature bovine oocytes

Integrins have been shown to be involved in the process of fertilization and many integrin-ligand interactions are mediated through the recognition of an arginine-glycine-aspartic acid (RGD) sequence. Despite the fact the RGD domain is a principal player in determining the functional characteristics of an adhesive protein, increasing evidence has accumulated implicating the amino acids flanking the RGD sequence in determining the functional properties of the RGD-containing protein. A set of linear peptides in which the amino acid sequence in and around the RGD tri-peptide was modified was synthesized to better understand the specificity of the RGD-receptor interaction. Mature oocytes were fertilized in vitro in the presence of RGD-containing and RGD-modified peptides. Both the RGD-containing and RGD-modified peptides impaired the ability of sperm to fertilize bovine oocytes, illustrated by a reduction in cleavage. The linear modified RGD containing peptides were also examined for their ability to induce parthenogenetic development with the objective of providing a linear RGD peptide with greater biological activity than the one (GRGDSPK) used previously (Campbell et al., 2000). The data demonstrate the specificity of the receptor for the RGD sequence, further implicate the involvement of integrins in the process of bovine fertilization, and illustrate the importance of the amino acids surrounding the RGD sequence in determining the binding and functional properties of RGD-containing peptides. The data support the findings that a linear RGD peptide can block fertilization and that amino acids around the RGD sequence have an impact on the biological activity of the receptor.

Effects of arginine-glycine-aspartic acid (RGD) containing snake venom peptides on parthenogenetic development and in vitro fertilization of bovine oocytes

The ability of synthetic arginine-glycine-aspartic acid (RGD)-containing peptides to induce intracellular calcium transients similar to those observed at fertilization by spermatozoa in the bovine has been reported (Campbell et al., 2000: Biol Reprod 62:1702-1709; Sessions et al., 2006. Mol Reprod Dev). These results also indicated the ability of synthetic RGD-containing peptides to induce activation and subsequent parthenogenetic development to the blastocyst stage, although, at numbers lower than observed with control in vitro fertilization (IVF). Evidence has been provided indicating the important effect of surrounding regions on the biological activity of the RGD sequence (Zhu and Evans, 2002; Sessions et al., 2006). The current experiments were designed to use natural RGD-containing sequences (disintegrins) to understand their effects. A total of three RGD-containing snake venom peptides (Kistrin (K), Elegantin (Ele), and Echistatin (Ech)) and one nonRGD-containing venom (Erabutoxin B (EB; control) were used at three concentrations (0.1, 1, and 10 micro g /ml) to induce parthenogenetic development to the blastocyst stage and in conjunction (1.0, 5.0, and 10 micro g/ml) with spermatozoa to evaluate competitive inhibition of fertilization and subsequent development. A (P < 0.01) higher number of bovine oocytes developed to the blastocyst stage after incubation with K, Ele and Ech at 1.0 micro g/ml, and was not different (P > 0.01) from IVF control. Fertilization was significantly reduced (P < 0.01) at all concentrations of K, Ele and Ech as compared to IVF control. No reduction (P > 0.05) was observed in EB (nonRGD) treated oocytes. These results support the involvement of a disintegrin-integrin interaction at fertilization in the bovine resulting in activation and subsequent development.

Blastocysts don't go it alone. Extrinsic signals fine-tune the intrinsic developmental program of trophoblast cells

The preimplantation embryo floats freely within the oviduct and is capable of developing into a blastocyst independently of the maternal reproductive tract. While establishment of the trophoblast lineage is dependent on expression of developmental regulatory genes, further differentiation leading to blastocyst implantation in the uterus requires external cues emanating from the microenvironment. Recent studies suggest that trophoblast differentiation requires intracellular signaling initiated by uterine-derived growth factors and integrin-binding components of the extracellular matrix. The progression of trophoblast development from the early blastocyst stage through the onset of implantation appears to be largely independent of new gene expression. Instead, extrinsic signals direct the sequential trafficking of cell surface receptors to orchestrate the developmental program that initiates blastocyst implantation. The dependence on external cues could coordinate embryonic activities with the developing uterine endometrium. Biochemical events that regulate trophoblast adhesion to fibronectin are presented to illustrate a developmental strategy employed by the peri-implantation blastocyst.

The ADAM-integrin-tetraspanin complex in fetal and postnatal testicular cords

New insights have emerged about the expression, during testicular cord formation, of the ADAM (a disintegrin and metalloprotease) domain family of proteins that combines both cell surface adhesion and proteolytic activity; this family includes integrins alpha3beta1 and alpha6beta1 and tetraspanins, a distinct family of proteins containing four transmembrane domains, a small and a large extracellular loop, and short cytoplasmic tails. ADAM3 (cyritestin), ADAM5, ADAM6, and ADAM15 are expressed in fetal rat testes. In contrast, the expression of the ADAM1/ADAM2 pair (fertilin alpha/fertilin beta, respectively) is not detected in fetal testis. Yet the expression of ADAM1 starts immediately after birth, and is followed within 24 hr by the expression of ADAM2. Therefore, the ADAM1/ADAM2 heterodimer is visualized far in advance of the meiotic and spermiogenic phase of spermatogenesis. A similar expression pattern was observed for integrin subunits alpha3, alpha6, and beta1, as well as for tetraspanins CD9, CD81, and CD98; the latter is a single-pass integrin subunit beta1-binding protein. ADAM2, integrin subunits alpha3, alpha6, and beta1, and tetraspanin CD9 and CD81 immunoreactive sites are observed in prespermatogonia (also known as primordial germ cells or gonocytes). A model is proposed in which the ADAM-integrin-tetraspanin complex, known to constitute a network of membrane microdomains called the tetraspanin web, may be involved in the migration of prespermatogonia from the center to the periphery of the testicular cords and in the reinitiation of mitotic activity during the initial wave of spermatogenesis. A complementary model consists in the rearrangement of the tetraspanin web in prespermatogonia/spermatogonia undergoing spontaneous or Fas-induced apoptosis upon coculturing with Sertoli cells. In this model, the cellular site involved in the formation of preapoptotic bodies is devoid of tetraspanin-integrin clusters, in contrast with nonapoptotic cells, which display a diffuse circumferential distribution. In apoptotic prespermatogonia, immunoreactive clusters are restricted to sites where the attachment of prespermatogonia/spermatogonia to Sertoli cell surfaces is still preserved.

Proteolytic cleavage of the cell surface protein p160 is required for detachment of the fertilization envelope in the sea urchin

Sea urchin eggs secrete a serine protease activity, CGSP1, at fertilization that is essential for the block to polyspermy. Several targets of this proteolytic activity on the plasma membrane were identified here using a cell surface biotinylation approach. Amino acid microsequencing of one of these proteins led to the identification of a 4.75-kb cDNA clone from a Strongylocentrotus purpuratus ovary cDNA library that encodes a 160-kDa protein called p160. This protein contains five CUB domains and a putative transmembrane domain suggesting that p160 is an integral membrane protein with protein-protein interaction motifs facing the extracellular matrix of the egg. Whole-mount immunolocalization studies demonstrate that p160 is on the surface of the egg, enriched at the tips of microvilli. The protein is removed at fertilization in a protease-dependent manner, and functional assays suggest that p160 serves to link the plasma membrane to the vitelline layer until fertilization. Thus, p160 is a key candidate for a vitelline-layer linker protein, the selective proteolysis of which functions in the block to polyspermy in the sea urchin egg.

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.

None of the integrins known to be present on the mouse egg or to be ADAM receptors are essential for sperm-egg binding and fusion

Antibody inhibition and alpha6beta1 ligand binding experiments indicate that the egg integrin alpha6beta1 functions as a receptor for sperm during gamete fusion; yet, eggs null for the alpha6 integrin exhibit normal fertilization. Alternative integrins may be involved in sperm-egg binding and fusion and could compensate for the absence of alpha6beta1. Various beta1 integrins and alphav integrins are present on mouse eggs. Some of these integrins are also reported to be receptors for ADAMs, which are expressed on sperm. Using alpha3 integrin null eggs, we found that the alpha3beta1 integrin was not essential for sperm-egg binding and fusion. Oocyte-specific, beta1 integrin conditional knockout mice allowed us to obtain mature eggs lacking all beta1 integrins. We found that the beta1 integrin null eggs were fully functional in fertilization both in vivo and in vitro. Furthermore, neither anti-mouse beta3 integrin function-blocking monoclonal antibody (mAb) nor alphav integrin function-blocking mAb inhibited sperm binding to or fusion with beta1 integrin null eggs. Thus, function of beta3 or alphav integrins does not seem to be involved in compensating for the absence of beta1 integrins. These results indicate that none of the integrins known to be present on mouse eggs or to be ADAM receptors are essential for sperm-egg binding/fusion, and thus, egg integrins may not play the role in gamete fusion previously attributed to them.

Analysis of the roles of RGD-binding integrins, alpha(4)/alpha(9) integrins, alpha(6) integrins, and CD9 in the interaction of the fertilin beta (ADAM2) disintegrin domain with the mouse egg membrane

Fertilin beta (also known as ADAM2), a mammalian sperm protein that mediates gamete cell adhesion during fertilization, is a member of the ADAM protein family whose members have disintegrin domains with homology to integrin ligands found in snake venoms. Fertilin beta utilizes an ECD sequence within its disintegrin domain to interact with the egg plasma membrane; the Asp is especially critical. Based on what is known about different integrin subfamilies and their ligands, we sought to characterize fertilin beta binding sites on mouse eggs, focusing on integrin subfamilies that recognize short peptide sequences that include an Asp residue: the alpha(5)/alpha(8)/alpha(v)/alpha(IIb) or RGD-binding subfamily (alpha(5)beta(1), alpha(8)beta(1), alpha(V)beta(1), alpha(V)beta(3), alpha(V)beta(5), alpha(V)beta(6), alpha(V)beta(8), and alpha(IIb)beta(3)) and the alpha(4)/alpha(9) subfamily (alpha(4)beta(1), alpha(9)beta(1), and alpha(4)beta(7)). We tested peptide sequences known to perturb interactions mediated by these integrins in two different assays for fertilin beta binding. Peptides with the sequence MLDG, which perturb alpha(4)/alpha(9) integrin-mediated interactions, significantly inhibit fertilin beta binding to eggs, which suggests a role for a member of this integrin subfamily as a fertilin beta receptor. RGD peptides, which perturb alpha(5)/alpha(8)/alpha(v)/alpha(IIb) integrin-mediated interactions, have partial inhibitory activity. The anti-alpha(6) antibody GoH3 has little or no inhibitory activity. An antibody to the integrin-associated tetraspanin protein CD9 inhibits the binding of a multivalent presentation of fertilin beta (immobilized on beads) but not soluble fertilin beta, which we speculate has implications for the role of CD9 in the strengthening of fertilin beta-mediated cell adhesion but not in initial ligand binding.

Structural analysis of fertilin(beta) cyclic peptide mimics that are ligands for alpha6beta1 integrin

The NMR structural analysis of two fertilin(beta) mimics cyclo(EC2DC1)YNH2, 1, and cyclo(D2EC2D1C1)YNH2, 2 is described. Both of these mimics are moderate inhibitors of sperm-egg binding with IC50 values of 500 microm in a mouse in vitro fertilization assay. For peptide 1, the optimized conformations that best match the NMR data have a pseudo-type II' beta-turn with the linker and Glu at the i+1 and i+2 positions, respectively. The EC2D1C1 sequence is in a nonclassical (type IV) beta-turn. For peptide 2, the conformation that best matches the NMR data has two turns: a pseudo-type II' beta-turn in the D2EC2D1 sequence followed by a nonclassical beta-turn in the EC2D1C1 sequence. The Cbeta-Cbeta distance between E and D1 in peptide 1 is 9.1 A, in peptide 2, it is 7.7 A. Thus, one possibility for the high IC50 values of these cyclic peptides is that the acidic residues are not constrained to a sufficiently tight turn, and thus much entropy must still be lost upon binding to the alpha6beta1 integrin. This explains why the cyclic peptides are the same as linear peptides at inhibiting sperm-egg binding.

Direct exposure of mouse ovaries and oocytes to high doses of an adenovirus gene therapy vector fails to lead to germ cell transduction

The risk of insertion of adenovirus gene therapy DNA into female germ cells during the course of somatic gene therapy was stringently tested in the mouse by injecting up to 10(10) infectious particles directly into the ovary and by incubating naked oocytes in a solution of 2 x 10(8) particles/ml for 1 h prior to in vitro fertilization (IVF). The vector used was a recombinant adenovirus carrying the bacterial lacZ gene driven by the cytomegalovirus promoter (Adbeta-gal). Ovaries were stained for LacZ activity, or immunochemically for LacZ, 5-7 days after injection. Although very large amounts of LacZ activity and protein were detected, all positive staining was in the thecal portion of the ovary, with no staining seen in oocytes. In another series of experiments, mice with injected ovaries were mated, and preimplantation embryos or fetuses were analyzed either for LacZ expression or by PCR for lacZ DNA. None of 202 preimplantation embryos stained positively for LacZ and none of 58 fetuses were positive for DNA by PCR analysis. Finally, more than 1400 eggs were fertilized after exposure to the vector prior to IVF and stained as morulae for LacZ activity. Fewer than 2% of the embryos stained positively for LacZ, and experiments indicated that the staining was due to incomplete washing of the eggs prior to IVF. These data provide strong evidence that adenoviruses cannot infect oocytes and that the risk of female germ-line transduction with such vectors is very low.

Effects of human hydrosalpinx fluid on in-vitro murine fertilization

Patients with hydrosalpinges show a decrease of both fertility and clinical outcome of IVF and embryo transfer treatment. Several reports have demonstrated the negative effects of hydrosalpinx fluid (HSF) on embryo development and implantation. The aim of this study was to determine whether human HSF, collected from infertile patients, might exhibit a deleterious effect on gametes and fertilization using a murine IVF system. Murine gametes were co-incubated during IVF until first cleavage with human HSF diluted to 50% from four patients (HSF1-4). It was demonstrated that HSF affected fertilization, as determined by the count of the 2-cell embryos. Pre-incubation of spermatozoa with HSF during capacitation significantly lowered the percentage of 2-cell embryos (P < 0.05). While HSF1-3 had no significant effect on motility and viability of spermatozoa, HSF4 almost completely affected their survival. In contrast, pre-incubation of ovulated oocytes surrounded by their cumulus cells with HSF before IVF did not impede first cleavage. Taken together, these results suggest that HSF has a cytotoxic effect on spermatozoa and/or impairs the fertilization process, probably by altering capacitation/acrosome reaction and/or ligand(s)-receptor(s) interactions. Hydrosalpinges may be partly associated with sterility through HSF inhibitory effects on fertilization.

Analysis of the role of egg integrins in sperm-egg binding and fusion

Sperm-egg fusion is believed to be mediated via specific molecular interactions. Integrin alpha6beta1 is a strong candidate for a sperm receptor on the egg plasma membrane. However, the ability of the egg integrin alpha6beta1 to interact with molecules on intact sperm has not yet been proven. In this report, possible involvement of integrin alpha6beta1 in sperm-egg interactions was examined by biochemical and immunocytochemical analyses. To identify egg molecules that specifically interact with sperm, we first incubated sperm with biotin-labeled egg surface proteins. Under this condition, solubilized proteins from eggs inhibited sperm-egg fusion. Western blot analysis under reducing conditions indicated that a major-labeled band of 135 kDa bound to sperm. An immunodepletion experiment using the anti-integrin alpha6 antibody GoH3 indicated that the 135 kDa egg surface molecule that bound to sperm was the integrin alpha6 subunit. To investigate the potential involvement of integrin alpha6beta1 in sperm-egg fusion, we next examined the localization of integrin alpha6 and beta1 subunits before and after fertilization by confocal laser microscopy. At an early stage of sperm-egg fusion, the integrin alpha6 and beta1 subunits were accumulated at the sperm binding site. The frequency of cluster formation was closely related to that of sperm-egg fusion, indicating that integrin receptors are accumulated by sperm destined for fusion. Taken together, these results strongly suggest that the integrin alpha6beta1 is involved in sperm-egg binding leading to fusion via direct association of the integrin alpha6 with sperm.

Integrin trafficking regulates adhesion to fibronectin during differentiation of mouse peri-implantation blastocysts

Trophoblast cells of the peri-implantation blastocyst differentiate from a polarized epithelium, the trophectoderm, into invasive cells having an apical surface occupied by integrins that mediate adhesion to the extracellular matrix. Blastocyst differentiation was assessed during serum-free culture using a fibronectin binding assay with intact mouse blastocysts. Fibronectin binding activity became elevated during a 24-h "window" after approximately 72 h of culture. Blastocyst differentiation was unaffected by transcriptional inhibition with alpha-amanitin, however, exposure of cavitating morulae to the drug significantly delayed the onset of maximal fibronectin-binding activity. Inhibition of de novo protein synthesis with cycloheximide delayed development only when added during the first 24 h of blastocyst culture, indicating that proteins required for adhesion to fibronectin were synthesized at least 24 h before blastocyst differentiation was completed. Since blastocyst differentiation did not appear to be regulated temporally by gene expression, the possible role of protein trafficking was investigated using the inhibitor, brefeldin A. Brefeldin A caused a reversible, dose-dependent decrease in fibronectin-binding activity when added to the culture medium between 48 and 72 h of culture. During the period of brefeldin A sensitivity, alpha 5 beta 1 integrin, a major fibronectin receptor, translocated to the apical surface of trophoblast cells, as determined by immunohistochemistry and confocal microscopy. Mouse blastocysts expressed other integrins that recognize the central cell-binding domain of fibronectin, including the alpha v integrins and alpha llb beta 3, but not alpha4 which recognizes the lllCS site. Trafficking of alpha 5 beta 1, and possibly other integrins, to the apical surface of trophoblast cells appears to be a critical step in the differentiation of the mouse blastocyst to an invasive phenotype.

Normal fertilization occurs with eggs lacking the integrin alpha6beta1 and is CD9-dependent

Previous results, based on inhibition of fertilization by an anti-alpha6 integrin mAb (GoH3), suggest that the alpha6beta1 integrin on mouse eggs functions as the receptor for sperm (Almeida, E.A., A.P. Huovila, A.E. Sutherland, L.E. Stephens, P.G. Calarco, L. M. Shaw, A.M. Mercurio, A. Sonnenberg, P. Primakoff, D.G. Myles, and J.M. White. 1995. Cell. 81:1095-1104). Because the egg surface tetraspanin CD9 is essential for gamete fusion (Kaji, K., S. Oda, T. Shikano, T. Ohnuki, Y. Uematsu, J. Sakagami, N. Tada, S. Miyazaki, and A. Kudo. 2000. Nat. Genet. 24:279-282; Le Naour, F., E. Rubinstein, C. Jasmin, M. Prenant, and C. Boucheix. 2000. Science. 287:319-321; Miyado, K., G. Yamada, S. Yamada, H. Hasuwa, Y. Nakamura, F. Ryu, K. Suzuki, K. Kosai, K. Inoue, A. Ogura, M. Okabe, and E. Mekada. 2000. Science. 287:321-324) and CD9 is known to associate with integrins, recent models of gamete fusion have posited that egg CD9 acts in association with alpha6beta1 in fusion (Chen, M.S., K.S. Tung, S.A. Coonrod, Y. Takahashi, D. Bigler, A. Chang, Y. Yamashita, P.W. Kincade, J.C. Herr, and J.M. White. 1999. Proc. Natl. Acad. Sci. USA. 96:11830-11835; Kaji, K., S. Oda, T. Shikano, T. Ohnuki, Y. Uematsu, J. Sakagami, N. Tada, S. Miyazaki, and A. Kudo. 2000. Nat. Genet. 24:279-282; Le Naour, F., E. Rubinstein, C. Jasmin, M. Prenant, and C. Boucheix. 2000. Science. 287:319-321; Miyado, K., G. Yamada, S. Yamada, H. Hasuwa, Y. Nakamura, F. Ryu, K. Su- zuki, K. Kosai, K. Inoue, A. Ogura, M. Okabe, and E. Mekada. 2000. Science. 287:321-324). Using eggs from cultured ovaries of mice lacking the alpha6 integrin subunit, we found that the fertilization rate, fertilization index, and sperm binding were not impaired compared with wild-type or heterozygous controls. Furthermore, a reexamination of antibody inhibition, using an assay that better simulates in vivo fertilization conditions, revealed no inhibition of fusion by the GoH3 mAb. We also found that an anti-CD9 mAb completely blocks sperm fusion with either wild-type eggs or eggs lacking alpha6beta1. Based on these results, we conclude that the alpha6beta1 integrin is not essential for sperm-egg fusion, and we suggest a new model in which CD9 acts by itself, or interacts with egg protein(s) other than alpha6beta1, to function in sperm-egg fusion.

Ability of integrins to mediate fertilization, intracellular calcium release, and parthenogenetic development in bovine oocytes

The ability of arginine-glycine-aspartic acid (RGD; a sequence recognized by integrins) or non-RGD-containing peptides to block fertilization, induce intracellular Ca(2+) oscillations, and initiate parthenogenetic development in bovine oocytes was investigated. Addition of a soluble RGD peptide during fertilization at concentrations ranging from 10 to 1000 microg/ml significantly decreased (P<0.05) fertilization as compared to the in vitro-fertilized controls. The addition of non-RGD peptide had no effect on fertilization. Two intracellular Ca(2+) transients 21.5+/- 1.9 min apart were observed in 56 of 60 oocytes incubated in RGD peptide concentrations ranging from 20 to 1000 microg/ml. No intracellular Ca(2+) transients were observed in medium alone, non-RGD treatment groups or in the RGD peptide at 10 microg/ml. The percentage of oocytes activated with ionomycin and 6-dimethylaminopurine (63% cleavage and 34% blastocyst development) was significantly higher (P<0.05) than those activated with the RGD peptide and 6-dimethylaminopurine (35% cleavage and 19% blastocyst development). These groups were significantly higher (P<0.05) than either peptide alone, 6-dimethylaminopurine alone, or the non-RGD peptide and 6-dimethylaminopurine treatment groups. These data provide evidence that ligation of an integrin on bovine oocytes with a soluble RGD peptide is capable of blocking fertilization, inducing intracellular Ca(2+) transients, and initiating parthenogenetic development.

A monoclonal antibody to human SP-10 inhibits in vitro the binding of human sperm to hamster oolemma but not to human Zona pellucida

SP-10 is a sperm intra-acrosomal protein, specific to the testis, that is believed to play an important role in egg-sperm binding. While the molecular characterization of the SP-10 protein has been clarified, little is yet known of its functional role in fertilization. We therefore established a monoclonal antibody (mAb pep-SP10) against a peptide (pep-SP10) that included the most hydrophilic portion of human SP-10 between the 135th and 149th amino acids. Human SP-10 was found to be localized in the equatorial region of acrosome-reacted sperm by immunofluorescent staining using our mAb pep-SP10. Monoclonal Ab pep-SP10 inhibited sperm-oolemma binding in the zona-free hamster egg penetration test, but it did not inhibit sperm-zona binding in the hemizona assay. Furthermore, we demonstrated that the oolemmal ligands of human SP-10 did not include beta(1) integrins, the most promising candidates for oocyte ligands involved in sperm-oolemma binding, based on the findings of a human sperm-cultured cell binding assay using F9 mouse embryonal carcinoma cells and F9-transformed cells lacking beta(1) integrins. In conclusion, our present data suggest that human SP-10, expressed on the equatorial region of acrosome-reacted sperm, indeed mediates sperm-oolemma binding in a beta(1) integrin-independent manner, but not sperm-zona binding.

Characterization of fertilin beta-disintegrin binding specificity in sperm-egg adhesion

An essential step leading to fertilization is the binding of a sperm to the egg plasma membrane. Fertilin beta, a membrane bound protein on the extracellular surface of sperm, partially mediates this binding via the alpha6beta1 integrin. Fertilin beta is a member of the still expanding family of ADAM proteins (a disintegrin and metalloprotease) that are implicated in many cellular functions ranging from neurogenesis to myoblast fusion and cytokine processing. Fertilin beta contains a highly conserved motif (D/E)ECD in the disintegrin domain. This suggests that (D/E)ECD could be the consensus sequence for recognition of disintegrins by alpha6beta1 integrins. Previously, it has been demonstrated that small peptides containing different moieties of this consensus sequence are inhibitors of in vitro fertilization. In the present study, we sought to determine whether a four amino acid peptide sequence with two adjacent acidic residues improved inhibition, and investigated the importance for inhibition of a cysteine versus a cystine. A series of linear and cyclic peptides were synthesized, in which either one or both adjacent acidic residues in the sequence DECD were mutated to their corresponding amides (N or Q). To explore the required oxidation state of the cysteine in the (D/E)ECD sequence, it was protected as a mixed disulfide. Our results indicate that only one acidic residue is required for inhibition of fertilization and a reduced C is required.

The role of integrins in reproduction

Fertilization, implantation, and placentation are dynamic cellular events that require not only synchrony between the maternal environment and the embryo, but also complex cell-to-cell communication. This communication involves integrins, a large family of proteins involved in the attachment, migration, invasion, and control of cellular function. Over the past decade, investigators have learned that integrins participate in multiple reproductive events including fertilization, implantation, and placentation in many species. This review will describe: (i) the expression of integrins on gametes and during the establishment and development of the placenta; (ii) regulatory pathways for controlling expression of integrins in the uterus and developing placenta; (iii) the function of integrins as determined by null-mutations; and (iv) reproductive dysfunction in women related to inappropriate integrin expression in the uterus and/or placenta.

Severely reduced female fertility in CD9-deficient mice

CD9 is a widely expressed cell surface molecule that belongs to the tetraspanin superfamily of proteins. The tetraspanins CD9, KAI-1/CD82, and CD63 are involved in metastasis suppression, an effect that may be related to their association with beta1 integrins. Knockout mice lacking CD9 were created to evaluate the physiological importance of CD9. CD9-/- females displayed a severe reduction of fertility. Oocytes were ovulated but were not successfully fertilized because sperm did not fuse with the oocytes from CD9-/- females. Thus, CD9 appears to be essential for sperm-egg fusion, a process involving the CD9-associated integrin alpha6beta1.

Molecular phenotype of the human oocyte by PCR-SAGE

Consecutive application of PCR and serial analysis of gene expression (SAGE) was used to generate a catalog of approximately 50, 000 SAGEtags from nine human oocytes. Matches for known genes were identified using the National Institutes of Health SAGEtag database. This database links directly to the UniGene database, providing rapid discrimination between SAGEtags that match known genes and expressed sequence tags and those that currently have no match. Matches in the oocyte SAGE catalog were found for surface receptors, second-messenger systems, and cytoskeletal, apoptotic, and secreted proteins. Many of these proteins were not previously known to be expressed in mammalian oocytes. The relative abundances of transcripts for cytoskeletal proteins and proteins known to be in oocytes are consistent with their documented expression, suggesting an absence of representational distortion by the PCR step. The expression profile of the human oocyte may help identify factors that reprogram somatic cell nuclei to totipotency.

Effects of perturbation of cell polarity on molecular markers of sperm-egg binding sites on mouse eggs

The mouse germinal vesicle (GV)-intact oocyte is a symmetric cell, with the GV centrally localized and with components of the plasma membrane and cortex symmetrically distributed around the periphery of the oocyte. During oocyte maturation, two distinct regions of the egg plasma membrane and cortex develop: the amicrovillar region overlying the meiotic spindle and the microvillar region. The development of this polarity is significant, since sperm bind to and fuse with the microvillar region. We are interested in the development of egg polarity and have characterized the localizations of several markers for egg polarity in normal metaphase II eggs and GV-intact oocytes. The asymmetric distributions of these markers (including actin, cortical granules, binding sites for the sperm proteins fertilin alpha and fertilin beta, and two different beta(1) integrin epitopes) develop during oocyte maturation in vitro, and this polarity can be perturbed by treatments that disrupt the actin microfilaments or microtubules. In addition, immunoelectron microscopy reveals that binding sites for recombinant fertilin beta are specifically localized to the microvillar region, suggesting that the binding sites for this sperm ligand are either specifically localized or activated in this region. These results indicate that structural remodeling of the mouse egg plasma membrane is accompanied by molecular remodeling, resulting in the localization or activation of specific molecules in subdomains of the plasma membrane.

Expression of the integrin subunits alpha 5, alpha 6 and beta 1 in the testes of the common marmoset

Integrin subunits alpha 5, alpha 6 and beta 1 were localized in the testis of pre-pubertal or adult non-human primates (Callithrix jacchus) by immunofluorescence staining and in situ hybridization. In animals of all ages subunits alpha 5 and beta 1 were localized in cells of the lamina propria of the seminiferous epithelium. In prepubertal animals, the integrin subunits alpha 5, alpha 6, as well as beta 1, were distributed all over the plasma membrane of Sertoli cells. In adult animals the integrin subunits were confined to those plasma membrane regions of Sertoli cells which are assigned to the basal compartment, including the basement membrane of the seminiferous tubules. Protein expression of integrin subunits alpha 6 and beta 1 was most pronounced in tubular stages in which elongated spermatids were not yet present in the adluminal compartment of the epithelium, suggesting that these integrin subunits are particularly essential at certain developmental stages of spermatogenesis. Non-radioactive in situ hybridization revealed that the mRNA for integrin subunits alpha 5, alpha 6 and beta 1 was expressed by Sertoli cells. In situ hybridization, together with immunofluorescence data, shows that these integrin subunits were exclusively synthesized in Sertoli cells. As to functional aspects, it is concluded that during primate spermatogenesis. Sertoli cell integrins may be involved in both cell matrix as well as cell-cell interactions, particularly during early spermatogenesis.

Anchorage-dependent regulation of the mitogen-activated protein kinase cascade by growth factors is supported by a variety of integrin alpha chains

Integrin cooperation with growth factor receptors to enable permissive signaling to the mitogen-activated protein (MAP) kinase pathway has important implications for cell proliferation, differentiation, and survival. Here we have sought to determine whether anchorage regulation of the MAP kinase pathway is specific to the alpha chain subunit of the integrins employed during adhesion. Human umbilical vein endothelial cells (HUVECs) anchored via endogenous alpha(2), alpha(3), or alpha(5) integrin subunits or NIH3T3 fibroblast cells lines anchored via ectopically expressed human integrin alpha(2) or alpha(5) subunits displayed comparable MAP kinase activation upon growth factor stimulation, regardless of the integrin alpha chain employed. In contrast, when either cell type was maintained in suspension, growth factor treatment inefficiently activated the MAP kinase pathway. The integrin-mediated enhancement of MAP kinase activation by growth factor correlated with the tyrosine phosphorylation of focal adhesion kinase but was independent of Shc. These data indicate that integrin modulation of the MAP kinase pathway is supported by a variety of integrin complexes and imply that other pathways may be required for the previously reported alpha chain-specific effects on cell cycle regulation and cell differentiation.

The potential use of sperm antigens as targets for immunocontraception; past, present and future

Immunocontraception, and in particular the targeting of antibodies to gamete-specific antigens implicated in sperm egg binding and fertilisation, offers an attractive approach to the growing global problem of overpopulation. Such an idea is not new; indeed several immunocontraception trials, using animal model systems, have been reported in recent years and a number are reviewed here. However, the results of these studies have been largely disappointing. We believe that two fundamental flaws attribute to the poor success of most of these preliminary immunocontraceptive trials. Firstly, loss of fertility has invariably been used as the assay. This presupposes that immuno-neutralisation of a single, gamete-specific antigen will be sufficient to cause a significant reduction in fertility; however, recent data suggests that such a premise may not be well-founded for a number of reasons. Secondly, and arguably the most important flaw, is the almost universal, but largely inappropriate, use of systemic immunisation as the sole route of antigen delivery. Whilst systemic immunisation regimes may lead to high serum IgG levels, these levels do not correlate with specific antibody levels in the reproductive tract or with contraceptive efficacy. Hence, an alternative antigen delivery approach is required which will induce an effective local immune response in the reproductive tract. Here we discuss the ways in which this might be achieved.

Mouse primordial germ cells lacking beta1 integrins enter the germline but fail to migrate normally to the gonads

Primordial germ cells are the founder cells of the gametes. They are set aside at the initial stages of gastrulation in mammals, become embedded in the hind-gut endoderm, then actively migrate to the sites of gonad formation. The molecular basis of this migration is poorly understood. Here we sought to determine if members of the integrin family of cell surface receptors are required for primordial germ cell migration, as integrins have been implicated in the migration of several other motile cell types. We have established a line of mice which express green fluorescent protein in germline cells that has enabled us to efficiently purify primordial germ cells at different stages by flow cytometry. We have catalogued the spectrum of integrin subunit expression by primordial germ cells during and after migration, using flow cytometry, immunocytochemistry and RT-PCR. Through analysis of integrin beta1(−/−)--&gt;wild-type chimeras, we show that embryonic cells lacking beta1 integrins can enter the germline. However, integrin beta1(−/−) primordial germ cells do not colonize the gonad efficiently. Embryos with targeted deletion of integrin subunit alpha3, alpha6, or alphaV show no major defects in primordial germ cell migration. These results demonstrate a role for beta1-containing integrins in the development of the germline, although an equivalent role for * integrin subunit(s) has yet to be established.

Evidence that distinct states of the integrin alpha6beta1 interact with laminin and an ADAM

Integrins can exist in different functional states with low or high binding capacity for particular ligands. We previously provided evidence that the integrin alpha6beta1, on mouse eggs and on alpha6-transfected cells, interacted with the disintegrin domain of the sperm surface protein ADAM 2 (fertilin beta). In the present study we tested the hypothesis that different states of alpha6beta1 interact with fertilin and laminin, an extracellular matrix ligand for alpha6beta1. Using alpha6-transfected cells we found that treatments (e.g., with phorbol myristate acetate or MnCl2) that increased adhesion to laminin inhibited sperm binding. Conversely, treatments that inhibited laminin adhesion increased sperm binding. Next, we compared the ability of fluorescent beads coated with either fertilin beta or with the laminin E8 fragment to bind to eggs. In Ca2+-containing media, fertilin beta beads bound to eggs via an interaction mediated by the disintegrin loop of fertilin beta and by the alpha6 integrin subunit. In Ca2+-containing media, laminin E8 beads did not bind to eggs. Treatment of eggs with phorbol myristate acetate or with the actin disrupting agent, latrunculin A, inhibited fertilin bead binding, but did not induce laminin E8 bead binding. Treatment of eggs with Mn2+ dramatically increased laminin E8 bead binding, and inhibited fertilin bead binding. Our results provide the first evidence that different states of an integrin (alpha6beta1) can interact with an extracellular matrix ligand (laminin) or a membrane-anchored cell surface ligand (ADAM 2).

Mediation of sperm-egg fusion: evidence that mouse egg alpha6beta1 integrin is the receptor for sperm fertilinbeta

BACKGROUND

INTRODUCTION

A key step leading to fertilization is the binding of sperm to the egg plasma membrane. When a mammalian sperm reaches the egg plasma membrane, fertilinbeta, an extracellular sperm membrane protein, is believed to bind to an egg plasma membrane receptor triggering fusion. We set out to identify the fertilinbeta binding partner on the egg plasma membrane.

RESULTS

We synthesized an 125-labeled peptide with the consensus Asp-Glu-Cys-Asp (DECD) sequence of fertilinbeta's disintegrin domain. This peptide contains a benzophenone photoaffinity probe and inhibits sperm-egg fusion. Upon photoactivation in the presence of whole mouse eggs, a single polypeptide was covalently labeled. This polypeptide has been identified by immunoprecipitation as an alpha6 integrin complexed with beta1 integrin.

CONCLUSIONS

Our experiments establish that small peptides containing the consensus DECD sequence of sperm fertilinbeta bind specifically to an alpha6beta1 integrin receptor on the egg membrane. We conclude that fertilinbeta binds directly to the alpha6beta1 integrin on the egg surface and this partnership mediates sperm-egg fusion.

Specific glycoconjugates are present at the oolemma of the fertilization site in the egg of Discoglossus pictus (Anurans) and bind spermatozoa in an in vitro assay

In the egg of the anuran Discoglossus pictus, the site of fertilization is restricted to the central portion of an animal hemisphere indentation (the dimple). Previous studies showed that the acrosome reaction of D. pictus sperm is triggered in the jelly, and yet sperm arrive at the dimple surface with the plasma membrane at an early stage of vesiculation. Reactivity of the dimple surface with specific lectins suggests that fucose might be utilized as a marker of glycoproteins located at the dimple surface. In this paper, proteins of the egg surface were labeled with the membrane impermeable sulfo-NHS-biotin. Four main bands of 200, 230, 260, and 270 kDa labeled only at the dimple surface, although they were detected in the cortex of the whole egg. The 270-kDa band reacted with Galanthus nivalis agglutinin only in the cortex of the dimple, suggesting that this band is differently glycosylated according to its localization. The alpha-l-fucose-specific lectin Ulex europaeus agglutinin I was utilized both in lectin blotting and in affinity chromatography and cross-reacted with the 200- and 270/260-kDa bands. Furthermore, two polypeptides were obtained by exposure of intact eggs to lysylendoproteinase C. They were also reactive to Ulex europaeus agglutinin I. The 200- and 270/260-kDa bands were eluted from the acrylamide gels and adsorbed to polystyrene beads. An assay for sperm binding to 200-kDa glycoprotein-bound beads was developed. Sperm stuck to the beads before but not after Ca-ionophore treatment. When the beads were coated with the 270/260-kDa glycoproteins, binding occurred after ionophore treatment. In these assays, the 200- and 270/260-kDa glycoproteins competitively inhibited sperm binding to the beads coated with the corresponding glycoprotein. These results indicate that the assayed glycoproteins, located either in the glycocalyx or in the plasma membrane of the fertilization site, are involved in sperm binding.

Voltage-dependent activation of frog eggs by a sperm surface disintegrin peptide

Fertilin, a sperm protein of the metalloprotease/disintegrin/cysteine-rich (MDC) family, plays a critical role in sperm-egg binding in mammals. Peptides corresponding to the disintegrin domain of fertilin and antibodies against fertilin have been shown to inhibit mammalian sperm-egg binding and fusion. A protein from the same family, xMDC16, was recently cloned from frog (Xenopus laevis) testis and was found to be involved in frog sperm-egg binding. Here we report that xMDC16 is localized predominantly on the posterior surface of egg jelly-activated sperm, and peptides from the disintegrin domain of this protein activate eggs when applied near the egg surface. Egg activation was dependent on (1) specific amino acid residues (KTX); (2) the presence of divalent cations, but not external Ca2+ alone; and (3) voltage across the egg plasma membrane. This is the first demonstration of egg activation in vertebrates by the surface application of a peptide derived from a sperm surface protein, supporting a model for egg activation that involves a signal transducing receptor for sperm in the egg's plasma membrane.

Molecular mechanisms of sperm-egg interactions and egg activation

The binding of acrosome reacted mammalian sperm to the egg plasma membrane initiates a series of signaling events in the egg, termed "egg activation", which lead to the completion of meiosis II and the initiation of a mitotic cell cycle. Many of these signaling events have characteristics of classical signal transduction events in somatic cells. Currently, there are two hypotheses for how sperm-induced egg activation is initiated. In the "receptor" hypothesis, the fertilizing sperm interacts with a specific egg surface receptor, and this interaction leads to signal transduction and effector activation. In the "fusion" hypothesis it is postulated that following fusion of the sperm and egg plasma membranes a soluble sperm-derived factor enters the egg's cytoplasm and activates pathways leading to egg activation. This chapter will provide an overview of the processes of cell-cell interaction and signal transduction leading to mammalian egg activation. It will concentrate on specific molecules proposed to be involved in sperm-egg interaction, signal transduction and effector mechanisms involved in egg activation, and a discussion of sperm-associated factors that have been implicated in egg activation.

Timing of gene expression and oolemma localization of mouse alpha6 and beta1 integrin subunits during oogenesis

The sperm antigen fertilin alpha/beta and the integrin complex alpha6beta1 present on the oolemma are two of the most promising candidates to mediate gamete interaction. During growth, the plasma membrane of both hamster and mouse zona-free oocytes acquires the capacity to fuse with acrosome-reacted sperm when oocytes reach the size of 25-30 microm in diameter, suggesting changes in the membrane molecular composition. The present study has two aims: to determine the timing of (1) gene expression of alpha6 and beta1 integrins and (2) localization of these integrin subunits on the plasma membrane in primordial germ cells and in oocytes during oogenesis. We found that both alpha6 and beta1 genes are expressed in female germ cells during all the stages of development analyzed, from 10.5 to 18.5 d.p. c., during oocyte growth, and in ovulated eggs. The alternatively spliced isoform alpha6B is expressed from 10.5 d.p.c., whereas alpha6A begins to be expressed at 12.5 d.p.c., suggesting a different role for the two variants. In situ immunodetection of alpha6 or beta1 shows a ring of fluorescence on the female germ cell plasma membrane for both integrins at 10.5 d.p.c., then the fluorescent signal becomes undetectable at 12.5 d.p.c. to reappear again, this time with a patchy distribution, at 18.5 d.p.c. This pattern of localization is maintained in oocytes isolated from newborn individuals and only when oocytes during growth reach the size of about 25-30 microm in diameter does the fluorescence become homogenous all around the whole oocyte surface. These data, although not conclusive, support the hypothesis of an involvement of alpha6 and beta1 integrins in sperm-egg fusion.

The stage-specific expression of TEC-1, -2, -3, and -4 antigens on bovine preimplantation embryos

The preimplantation developmental period is associated with constant changes within the embryo, and some of these changes are apparent on the embryo cell surface. For example, during transition from maternal to embryonic genome control and the compaction and differentiation of embryonic cells, the cell surface undergoes morphologic alterations that reflect changes in gene control. In order to gain insight into the events occurring during embryonic development and cellular differentiation, monoclonal antibodies specific for cell surface antigens (TEC antigens) of embryonic cells have been generated previously and shown to recognise either the carbohydrate moiety of embryoglycan or a developmentally regulated protein epitope. The TEC antigens have been identified on mouse preimplantation embryos, and their expression is specific to particular developmental stages. To determine whether these antigens are conserved in higher mammals, we examined the expression of four TEC antigens (TEC-1 to TEC-4) on in vitro-derived bovine and murine embryos during the preimplantation stage of development. It was found that bovine oocytes and embryos derived from in vitro maturation (IVM) and in vitro fertilisation (IVF) showed stage-specific expression of each of the TEC antigens investigated, with the pattern of expression overlapping but not identical to that seen in the mouse. Immunoprecipitation together with Western blot analysis showed that the TEC monoclonal antibodies recognised a single glycoprotein band with an apparent molecular weight of 70 kDa. Confocal microscopy of immunofluorescence staining of the bovine cells showed this protein to be located on the cell surface. The apparent negative expression of these TEC antigens by immunohistochemistry and immunoprecipitation at particular stages of development appears to be due to the epitopes being inaccessible to the TEC antibodies, since Western blotting revealed the TEC antigens to be present at all stages of development examined. Antibodies identifying stage-specific antigens will provide useful markers to characterise early embryonic cells, monitor normal embryonic development in vitro, and identify cell surface structures having a function in cell-cell interactions during embryogenesis and differentiation.

Measurement of vitronectin content of human spermatozoa and vitronectin concentration within seminal fluid

OBJECTIVE

Vitronectin previously has been extracted from human spermatozoa and messenger RNA (mRNA) encoding vitronectin localized by reverse transcriptase in situ polymerase chain reaction (PCR) to spermatocytes of human testis. In the present experiments, we have established ranges for the content of vitronectin in living human spermatozoa and vitronectin concentration within seminal fluid of human ejaculates.

DESIGN

Seminal fluid was obtained by centrifugation and motile sperm selected by swim-up from men with normal and abnormal ejaculates, according to World Health Organization criteria, for vitronectin determinations.

SETTING

Academic research environment.

MAIN OUTCOME MEASURE(S)

Seminal fluid vitronectin concentrations were measured by ELISA and sperm vitronectin content by polyacrylamide gel electrophoresis and semiquantitative Western blots.

RESULT(S)

Vitronectin seminal fluid concentration was 1.35 +/- 1.0 mg/mL (mean +/- SD) for normospermic samples (n = 26) and 0.78 +/- 0.4 mg/mL for azoospermic specimens (n = 6). Vitronectin sperm content ranged from 1 to 15 ng/10(6) motile cells (n = 20). Both high- and low-molecular-weight material was observed. Sperm content of vitronectin did not vary with sperm morphology.

CONCLUSION(S)

These results suggest that spermatozoa represent a major source of seminal fluid vitronectin, but that a secondary source exists, perhaps through transudation from serum.

Laminin chain-specific gene expression during mouse oocyte maturation

Expression patterns of laminin chain mRNAs (A, B1, and B2) during mouse oocyte maturation were examined using the competitive reverse transcription-polymerase chain reaction (RT-PCR) method. Total and poly (A)-rich mRNAs isolated from various stages of maturing oocytes in vitro were subjected to RT-PCR and the precise amount of laminin chain-specific mRNA transcripts was estimated by adding externally known amounts of in vitro transcribed mutant cRNA transcripts as an internal control. The estimated copy numbers for A, B1, and B2 chain mRNAs in a single germinal vesicle-stage oocyte were 1.34 +/- 0.19 x 10(5), 6.95 +/- 0.32 x 10(6), and 2.0 +/- 0.56 x 10(5), respectively. Although notable changes of all laminin chain mRNA levels were not observed at any stage of meiotic maturation in total mRNA preparation, chain- and meiotic stage-dependent alterations of poly (A)-tailed mRNA quantities were observed in poly (A)-rich mRNA preparation. A potent RNA synthesis blocker, alpha-amanitin did not influence the changes of mRNA levels, implying the presence of posttranscriptional regulation mechanism in the expression of laminin chains during mouse oocyte maturation. Discrete and time-dependent deadenylation of A and B1 chain, but not B2 chain mRNA, was observed during oocyte maturation by a rapid amplification of cDNA ends (RACE)-PCR. In germinal vesicle (GV)-stage oocytes, only B1 chain was found to be present in a highly polyadenylated state and subsequent deadenylation was observed as meiosis progressed. The poly (A) tail modification was dependent on the initiation of meiotic resumption. Although all laminin chain mRNAs were found in fully grown and meiotically competent mouse oocytes, Western blot analysis detected the B1 chain polypeptide only in GV- and polar body (PB)-stage eggs. These results suggest that the expression of laminin B1 chain in mouse oocytes may be due to its large amount of mRNA transcripts and/or high level of polyadenylation state that is efficient for translational activation.

Characterization of the binding of recombinant mouse sperm fertilin beta subunit to mouse eggs: evidence for adhesive activity via an egg beta1 integrin-mediated interaction

The sperm protein fertilin (also known as PH-30) is a candidate for mediating the interactions between sperm and egg plasma membranes. Fertilin is a heterodimer. The beta subunit, which has a region with homology to the family of integrin ligands known as disintegrins, has been hypothesized to be involved in the binding of sperm to the egg surface. To investigate this hypothesis and determine what role fertilin beta plays in fertilization, we have expressed the putative extracellular domain of mouse fertilin beta in bacteria as a fusion protein with maltose-binding protein (hereafter referred to as recombinant fertilin beta-EC) and used two assays to characterize its binding to mouse eggs. Immunocytochemistry was used to examine the localization of recombinant fertilin beta-EC binding. A luminometric assay was also developed to quantify levels of binding of recombinant fertilin beta-EC to single eggs. We find that recombinant fertilin beta-EC binds to the region of the plasma membrane of the egg to which sperm bind, thus providing the first direct evidence that fertilin beta has adhesive properties. Peptides corresponding to the disintegrin domain of fertilin beta reduce its binding to eggs, suggesting that this domain is at least partially involved in the recognition of fertilin beta by binding sites on the egg. Treatment of zona pellucida-free eggs with chymotrypsin reduces the ability of the eggs to support the binding of recombinant fertilin beta-EC, implicating an egg surface protein as a binding site for recombinant fertilin beta-EC. Binding of recombinant fertilin beta-EC to eggs is also reduced in the absence of divalent cations and is supported by 2.0 mM Ca2+, Mg2+, or Mn2+. Furthermore, eggs incubated in recombinant fertilin beta-EC prior to in vitro fertilization show reduced levels of sperm binding. Finally, we have examined the possible role of integrins on eggs as receptors for fertilin beta, since an anti-alpha6 integrin subunit monoclonal antibody, GoH3, has been shown to inhibit sperm binding (E. A. C. Almeida et al. (1995) Cell 81, 1095-1104). We find that: (a) an increased amount of GoH3 epitope on the egg surface does not correlate with an increased ability of the eggs to bind sperm or recombinant fertilin beta-EC; (b) the GoH3 antibody has virtually no inhibitory effect on recombinant fertilin beta-EC binding; and (c) recombinant fertilin beta-EC binding is reduced in the presence of anti-beta1 integrin antibodies. These results suggest that a beta1-containing integrin participates in the binding of recombinant fertilin beta-EC to mouse eggs.

Sperm-egg interaction in the painted frog (Discoglossus pictus): an ultrastructural study

The ultrastructure of sperm changes and penetration in the egg was studied in the anuran Discoglossus pictus, whose sperm have an acrosome cap with a typical tip, the apical rod. The first stage of the sperm apical rod and acrosome reaction (AR) consists in vesiculation between the plasma membrane and the outer acrosome membrane. The two components of the acrosome cap are released in sequence. The innermost component (component B) is dispersed first. The next acrosome change is the dispersal of the outermost acrosome content (component A). At 30 sec postinsemination, when the loss of component B is first observed, holes are seen in the innermost jelly coat (J1), surrounding the penetrating sperm. Therefore, this acrosome constituent might be related to penetration through the innermost egg investments. At 1 min postinsemination, during sperm penetration into the egg, a halo of finely granular material is observed around the inner acrosome membrane of the spermatozoon, suggesting a role for component A at this stage of penetration. Gamete-binding and fusion take place between D1 (the egg-specific site for sperm interaction) and the perpendicularly oriented sperm. Spermatozoa visualized at their initial interaction (15 sec postinsemination) with the oolemma are undergoing vesiculation. The first interaction is likely to occur between the D1 glycocalyx and the plasma membrane of the hybrid vesicles surrounding the apical rod. As fusion is observed between the internal acrosome membrane and the oolemma, it can be postulated that gametic interaction might be followed by fusion of the latter with the apical rod internal membrane that extends posteriorly into the inner acrosome membrane. Insemination of the outermost jelly layer (J3) dissected out of the egg, and observations of the ultrastructural changes of spermatozoa in this coat, indicate that J3 rather than the vitelline coat (VC) induces the AR. Interestingly, at the late postinsemination stage, VC fibrils are seen crosslinking the inner acrosome membrane. The role of this binding is here discussed.

Integrins and reproductive physiology: expression and modulation in fertilization, embryogenesis, and implantation

OBJECTIVE

To review the available information regarding the role of integrins in reproductive physiology and to discuss their potential clinical implications.

DESIGN

Studies that specifically relate to the expression and modulation of integrins in fertilization, embryogenesis, and implantation were identified through the literature and Medline searches.

RESULT(S)

Integrins are a class of adhesion molecules that participate in cell-to-cell and cell-to-substratum interactions and are present on essentially all human cells. All mammalian eggs express integrins at their surface, and the integrin alpha 6 beta 1 serves as a sperm receptor that mediates sperm-egg binding. In addition, certain integrin moieties appear to be regulated within the cycling endometrium. Specifically, the expression of beta 1 integrins in the early proliferative phase is restricted to the glandular epithelium, whereas stromal cells also express beta 1 integrins in the midsecretory phase. The expression of beta 1 integrins increases at the time of implantation and remains elevated in the decidua during early pregnancy. A disruption of integrin expression is associated with certain types of infertility in women. The apical surface of the mural trophectoderm does indeed possess functional integrins, and trophoblast interactions with extracellular matrix proteins largely depend on the integrin family of adhesion receptors.

CONCLUSION(S)

Integrins play particularly important roles in both fertilization and embryogenesis, including the process of implantation.