Oocyte CD9 is enriched on the microvillar membrane and required for normal microvillar shape and distribution

Tetraspanin-interacting protein IGSF8 is dispensable for mouse fertility

OBJECTIVE

To determine the physiological role of IGSF8 for fertility.

DESIGN

Experimental prospective study.

SETTING

Academic basic research laboratory.

ANIMAL(S)

C57BL/6J and hybrid B6D2F1 mice, as well as Cd9 and Igsf8 knockout mice (C57BL/6J and 129/SvJ mix background), were used for this study.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

In vitro and in vivo fertility tests of Igsf8 knockout mice.

RESULT(S)

Tetraspanin family member CD9 plays an important role in sperm-egg fusion. Recently, some researchers have reported that CD9 tightly associates with the immunoglobulin superfamily member IGSF8 on the egg surface and that IGSF8 is undetectable on the surface of Cd9-deficient eggs. This led us to hypothesize that IGSF8 participates in sperm-egg fusion together with CD9. To examine the physiological role of IGSF8 in vivo, we generated Igsf8-deficient mice by homologous recombination and examined the fertility of the females.

CONCLUSION(S)

The Igsf8-deficient female mice showed no fertilization defect in vitro or in vivo. We observed that Igsf8-deficient eggs retained the normal level and localization of CD9, resulting in normal microvilli formation, which indicates that IGSF8 is dispensable in fertility.

Membrane transfer from oocyte to sperm occurs in two CD9-independent ways that do not supply the fertilising ability of Cd9-deleted oocytes

Spermatozoa undergo regulation of their functions along their lifespan through exchanges via vesicles or interactions with epithelial cells, in the epididymis, in the seminal fluid and in the female genital tract. Two different ways of oocyte membrane transfer to spermatozoa have been described: trogocytosis and exosomes. We here report an analysis of in vitro exchanges between the membranes of unfertilised oocytes and capacitated spermatozoa. We showed that optimum conditions are fulfilled when unfertilised oocytes interact with acrosome-reacted spermatozoa, a scenario mimicking the events occurring when the fertilising spermatozoon is inside the perivitelline space. Although CD9 tetraspanin is an essential molecule for fertilisation, exosome and trogocytosis transfer persists in Cd9-null oocytes in spite of their dramatic fusion failure. These exchanges are CD9 tetraspanin independent. We also confirm that mice sperm express CD9 tetraspanin and that when Cd9-null oocytes were inseminated with sperm covered with oocyte membrane materials, including CD9 tetraspanin, no rescue of the oocytes' fertilisability could be obtained. Thus, the existence of two ways of exchange between gametes during fertilisation suggests that these events could be of a physiological importance in this process.

Spindle positioning in mammalian oocytes

To preserve the maternal stores accumulated during oogenesis for further embryo development, oocytes divide asymmetrically which minimizes the volume of cytoplasm lost with each set of haploid genome. To ensure asymmetric division to occur, oocytes have to position their division spindle asymmetrically as well as tailor the size of daughter cells to the chromatin mass. In this review, we will discuss the recent advances in the field, with emphasis on the control mechanisms involved in meiotic spindle positioning in mammalian oocytes.

A net-like structure with pores is observed during cell fusion induced by the receptor FGFRL1

FGFRL1 is the fifth member of the fibroblast growth factor receptor (FGFR) family. Similar to the other members, it harbors three Ig loops in its extracellular domain, but in contrast to the other receptors, it lacks the intracellular protein tyrosine kinase domain that would be required for signaling by transphosphorylation. FGFRL1 is mainly found in the musculoskeletal system, where it appears to inhibit cell proliferation but to induce cell adhesion and differentiation. Mice with a targeted disruption of the FGFRL1 gene die during birth due to a malformed diaphragm muscle, which is not strong enough to inflate the lungs after birth. Expression of FGFRL1 is highly upregulated during the differentiation of myoblasts to multinucleated myotubes, suggesting an important role for FGFRL1 in cell-cell fusion. Recently we showed that FGFRL1 does indeed induce fusion of cultured cells into large syncytia. A reporter gene assay demonstrated that the third Ig domain and the transmembrane domain of FGFRL1 are both necessary and sufficient to fuse CHO cells into syncytia comprising several hundred nuclei. At the contact site, the fusing cells reveal a peculiar net-like structure with pores of about 1 µm diameter. It is possible that these structures represent membrane areas with fusion pores that set in motion the cell-cell fusion process. FGFRL1 is the first mammalian protein that is capable of triggering cell-cell fusion in vitro.

Sperm-egg interaction

A crucial step of fertilization is the sperm-egg interaction that allows the two gametes to fuse and create the zygote. In the mouse, CD9 on the egg and IZUMO1 on the sperm stand out as critical players, as Cd9(-/-) and Izumo1(-/-) mice are healthy but infertile or severely subfertile due to defective sperm-egg interaction. Moreover, work on several nonmammalian organisms has identified some of the most intriguing candidates implicated in sperm-egg interaction. Understanding of gamete membrane interactions is advancing through characterization of in vivo and in vitro fertilization phenotypes, including insights from less robust phenotypes that highlight potential supporting (albeit not absolutely essential) players. An emerging theme is that there are varied roles for gamete molecules that participate in sperm-egg interactions. Such roles include not only functioning as fusogens, or as adhesion molecules for the opposite gamete, but also functioning through interactions in cis with other proteins to regulate membrane order and functionality.

Tetraspanins regulate the protrusive activities of cell membrane

Tetraspanins have gained increased attention due to their functional versatility. But the universal cellular mechanism that governs such versatility remains unknown. Herein we present the evidence that tetraspanins CD81 and CD82 regulate the formation and/or development of cell membrane protrusions. We analyzed the ultrastructure of the cells in which a tetraspanin is either overexpressed or ablated using transmission electron microscopy. The numbers of microvilli on the cell surface were counted, and the radii of microvillar tips and the lengths of microvilli were measured. We found that tetraspanin CD81 promotes the microvillus formation and/or extension while tetraspanin CD82 inhibits these events. In addition, CD81 enhances the outward bending of the plasma membrane while CD82 inhibits it. We also found that CD81 and CD82 proteins are localized at microvilli using immunofluorescence. CD82 regulates microvillus morphogenesis likely by altering the plasma membrane curvature and/or the cortical actin cytoskeletal organization. We predict that membrane protrusions embody a common morphological phenotype and cellular mechanism for, at least some if not all, tetraspanins. The differential effects of tetraspanins on microvilli likely lead to the functional diversification of tetraspanins and appear to correlate with their functional propensity.

β-catenin is a molecular switch that regulates transition of cell-cell adhesion to fusion

When a sperm and an oocyte unite upon fertilization, their cell membranes adhere and fuse, but little is known about the factors regulating sperm-oocyte adhesion. Here we explored the role of β-catenin in sperm-oocyte adhesion. Biochemical analysis revealed that E-cadherin and β-catenin formed a complex in oocytes and also in sperm. Sperm-oocyte adhesion was impaired when β-catenin-deficient oocytes were inseminated with sperm. Furthermore, expression of β-catenin decreased from the sperm head and the site of an oocyte to which a sperm adheres after completion of sperm-oocyte adhesion. UBE1-41, an inhibitor of ubiquitin-activating enzyme 1, inhibited the degradation of β-catenin, and reduced the fusing ability of wild-type (but not β-catenin-deficient) oocytes. These results indicate that β-catenin is not only involved in membrane adhesion, but also in the transition to membrane fusion upon fertilization.

Molecular and cellular mechanisms of mammalian cell fusion

The fusion of one cell with another occurs in development, injury and disease. Despite the diversity of fusion events, five steps in sequence appear common. These steps include programming fusion-competent status, chemotaxis, membrane adhesion, membrane fusion, and post-fusion resetting. Recent advances in the field start to reveal the molecules involved in each step. This review focuses on some key molecules and cellular events of cell fusion in mammals. Increasing evidence demonstrates that membrane lipid rafts, adhesion proteins and actin rearrangement are critical in the final step of membrane fusion. Here we propose a new model for the formation and expansion of membrane fusion pores based on recent observations on myotube formation. In this model, membrane lipid rafts first recruit adhesion molecules and align with opposing membranes, with the help of a cortical actin "wall" as a rigid supportive platform. Second, the membrane adhesion proteins interact with each other and trigger actin rearrangement, which leads to rapid dispersion of lipid rafts and flow of a highly fluidic phospholipid bilayer into the site. Finally, the opposing phospholipid bilayers are then pushed into direct contact leading to the formation of fusion pores by the force generated through actin polymerization. The actin polymerization generated force also drives the expansion of the fusion pores. However, several key questions about the process of cell fusion still remain to be explored. The understanding of the mechanisms of cell fusion may provide new opportunities in correcting development disorders or regenerating damaged tissues by inhibiting or promoting molecular events associated with fusion.

The C-terminal tail of tetraspanin protein CD9 contributes to its function and molecular organization

Tetraspanin protein CD9 supports sperm-egg fusion, and regulates cell adhesion, motility, metastasis, proliferation and signaling. The large extracellular loop and transmembrane domains of CD9 engage in functionally important interactions with partner proteins. However, neither functional nor biochemical roles have been shown for the CD9 C-terminal tail, despite it being highly conserved throughout vertebrate species. To gain new insight into the CD9 tail, three C-terminal amino acids (Glu-Met-Val) were replaced with residues corresponding to C-terminal amino acids from tetraspanin protein CD82 (Pro-Lys-Tyr). Wild-type and mutant CD9 were then stably expressed in MOLT-4, K562, U937, RD and HT1080 cells. Whereas wild-type CD9 inhibited cell adhesion and spreading on fibronectin, mutant CD9 did not. Wild-type CD9 also promoted homotypic cell-cell aggregation and microvilli formation, whereas mutant CD9 did not. Protein interactions of wild-type and mutant CD9 were compared quantitatively using stable isotope labeling with amino acids in cell culture (SILAC) in conjunction with liquid-chromatography-tandem mass spectrometry (LC-MS/MS) technology. SILAC results showed that, despite wild-type and mutant CD9 having identical expression levels, mutant CD9 and its major transmembrane interacting partners were recovered in substantially reduced amounts from 1% Brij 96 lysates. Immunoprecipitation experiments confirmed that mutant CD9 recovery was decreased in Brij 96, but not in more stringent Triton X-100 detergent. Additionally, compared with wild-type CD9 complexes, mutant CD9 complexes were larger and more oligomerized in Brij 96 detergent, consistent with decreased Brij 96 solubility, perhaps due to more membrane domains packing more tightly together. In conclusion, multiple CD9 functions depend on its C-terminal tail, which affects the molecular organization of CD9 complexes, as manifested by their altered solubilization in Brij 96 and organization on the cell surface.

Tetraspanins: Small transmembrane proteins with big impact on membrane microdomain structures

Members of the tetraspanin family of transmembrane proteins including CD9, CD37, CD53, CD63, CD81, CD82, CD151, etc., contribute to the structural organization of the plasma membrane by forming microdomain structures, influencing cell fusion and regulating cell motility. Interestingly, K41, a CD9-specific monoclonal antibody (mAb), inhibits the release of human immunodeficiency virus (HIV-1), and the canine distemper virus (CDV)-, but not measles virus (MV)-induced cell-cell fusion. This mAb, which recognizes a conformational epitope on the large extracellular loop (LEL) of CD9, induced rapid relocation and clustering of CD9 in net-like structures at cell-cell contact areas.1 High-resolution analyses revealed that CD9 clustering is accompanied by the formation of microvilli that protrude from either side of adjacent cell surfaces, thus forming structures like microvilli zippers. While the cellular CD9-associated proteins beta1-integrin and EWI-F were co-clustered with CD9 at cell-cell interfaces, viral proteins in infected cells were differentially affected. MV envelope proteins were detected within, whereas CDV proteins were excluded from CD9 clusters, and thus, the tetraspanin CD9 can regulate cell-cell fusion by controlling the access of the viral fusion machinery to cell contact areas.

The complexity of tetraspanins

Tetraspanins compose a family of structurally related molecules with four transmembrane domains. A total of 33 tetraspanins are present in the human genome, and tetraspanins are also found in plants and certain fungi. A well-known property of tetraspanins is their ability to interact with one another and many other surface proteins, which led to the suggestion that they organize a network of molecular interaction referred to as the 'tetraspanin web', and that they play a role in membrane compartmentalization. Recent studies of the dynamics of these molecules provided important new information that helped refining the models of this 'web'. Several genetic studies in mammals and invertebrates have demonstrated key physiological roles for some of the tetraspanins, in particular in immune response, sperm-egg fusion, photoreceptor function and the normal function of certain epitheliums or vascular development. However, in several examples, the phenotypes of tetraspanin-knockout mice are relatively mild or restricted to a particular organ, despite a wide tissue distribution.

The role of tetraspanins in fusion

Membrane fusion underlies such important biological processes as virus entry into host cells, intracellular protein trafficking, fertilization, formation of muscle fibres and bone resorption. In addition, pathologies such as osteoporosis and implant rejection have been attributed to aberrant fusion. Members of the tetraspanin protein superfamily have been ascribed multiple roles in membrane biology, forming extensive lateral associations and regulating the function of effector molecules by clustering them in specific areas of the membrane. The present review aims to summarize the experimental evidence for tetraspanin function in different fusion events and highlight common themes.

CD9 tetraspanin generates fusion competent sites on the egg membrane for mammalian fertilization

CD9 tetraspanin is the only egg membrane protein known to be essential for fertilization. To investigate its role, we have measured, on a unique acrosome reacted sperm brought in contact with an egg, the adhesion probability and strength with a sensitivity of a single molecule attachment. Probing the binding events at different locations of wild-type egg we described different modes of interaction. Here, we show that more gamete adhesion events occur on Cd9 null eggs but that the strongest interaction mode disappears. We propose that sperm-egg fusion is a direct consequence of CD9 controlled sperm-egg adhesion properties. CD9 generates adhesion sites responsible for the strongest of the observed gamete interaction. These strong adhesion sites impose, during the whole interaction lifetime, a tight proximity of the gamete membranes, which is a requirement for fusion to take place. The CD9-induced adhesion sites would be the actual location where fusion occurs.

One-step collagenase method for zona pellucida removal in unfertilized eggs: easy and gentle method for large-scale preparation

Purpose

Zona pellucida (ZP)-free eggs are often used for studies such as evaluating the interaction of sperm-oolemma. To acquire ZP-free eggs, the most commonly used methods employ acidified Tyrode's solution, enzymatic digestion with a trypsin-like enzyme, or mechanical methods using micropipettes. However, acidified Tyrode's solution and trypsin-like enzymes often damage the oolemma, especially when many eggs are treated at once for mass sample analyses. The mechanical method requires skill, and it is time-consuming to prepare many ZP-free eggs. Therefore, in this study, to establish an easy, reliable method for preparing ZP-free eggs, we examined the ZP digestion method originally reported by Zuccotti et al. (J Reprod Fertil 93:515-520, 1991) that uses collagenase.

Methods

Mouse unfertilized eggs were treated with collagenase and acidified Tyrode's solution to compare the ZP-free rates, the effect on the oolemma, and the two-cell development rates of ZP-free eggs by in vitro fertilization. The effects on the oolemma were gauged by observing the polarity of the transmembrane protein localization of enhanced green fluorescence protein tagged CD9 protein (CD9-EGFP) and using differential interference contrast microscopy.

Results

Collagenase removed the ZP and the cumulus cells from the cumulus oocyte complex. The collagenase method had no influence on the localization of CD9-EGFP, resulting in a high two-cell development rate. Additionally, the collagenase method could exclude low quality eggs with hardened ZP, since collagenase could not digest the hardened ZP.

Conclusions

The one-step collagenase method is an easy preparation method for large numbers of high-quality ZP-free eggs.

Ultrastructure of isolated mouse ovarian follicles cultured in vitro

BACKGROUND

In vitro maturation of ovarian follicles, in combination with cryopreservation, might be a valuable method for preserving and/or restoring fertility in mammals with impaired reproductive function. Several culture systems capable of sustaining mammalian follicle growth in vitro have been developed and many studies exist on factors influencing the development of in vitro grown oocytes. However, a very few reports concern the ultrastructural morphology of in vitro grown follicles.

METHODS

The present study was designed to evaluate, by transmission and scanning electron microscopy, the ultrastructural features of isolated mouse preantral follicles cultured in vitro for 6 days in a standard medium containing fetal calf serum (FCS). The culture was supplemented or not with FSH.

RESULTS

The follicles cultured in FCS alone, without FSH supplementation (FCS follicles), did not form the antral cavity. They displayed low differentiation (juxta-nuclear aggregates of organelles in the ooplasm, a variable amount of microvilli on the oolemma, numerous granulosa cell-oolemma contacts, signs of degeneration in granulosa cell compartment). Eighty (80)% of FSH-treated follicles formed the antral cavity (FSH antral follicles). These follicles showed various ultrastructural markers of maturity (spreading of organelles in ooplasm, abundant microvilli on the oolemma, scarce granulosa cell-oolemma contacts, granulosa cell proliferation). Areas of detachment of the innermost granulosa cell layer from the oocyte were also found, along with a diffuse granulosa cell loosening compatible with the antral formation. Theca cells showed an immature morphology for the stage reached. Twenty (20)% of FSH-treated follicles did not develop the antral cavity (FSH non-antral follicles) and displayed morphological differentiation features intermediate between those shown by FCS and FSH antral follicles (spreading of organelles in the ooplasm, variable amount of microvilli, scattered granulosa cell-oolemma contacts, signs of degeneration in granulosa cell compartment).

CONCLUSIONS

It is concluded that FSH supports the in vitro growth of follicles, but the presence of a diffuse structural granulosa cell-oocyte uncoupling and the absence of theca development unveil the incomplete efficiency of the system. The present study contributes to explain, from a morphological point of view, the effects of culture conditions on the development of mouse in vitro grown follicles and to highlight the necessity of maintaining efficient intercellular communications to obtain large numbers of fully-grown mature germ cells.

Lipid rafts: keys to sperm maturation, fertilization, and early embryogenesis

Cell membranes are composed of many different lipids and protein receptors, which are important for regulating intracellular functions and cell signaling. To orchestrate these activities, the cell membrane is compartmentalized into microdomains that are stably or transiently formed. These compartments are called "lipid rafts". In gamete cells that lack gene transcription, distribution of lipids and proteins on these lipid rafts is focused during changes in their structure and functions such as starting flagella movement and membrane fusion. In this paper, we describe the role of lipid rafts in gamete maturation, fertilization, and early embryogenesis.

Dynamics of the mammalian sperm membrane modification leading to fertilization: a cytological study

Fertilization occurs when the sperm penetrates the egg, resulting in the combination of paternal and maternal genomes for the propagation of generations. To perform the task, the mammalian sperm membrane system, constructed during spermatogenesis, undergoes biochemical and cytological modifications. In this review, the following three points are discussed: (i) the nature of the acrosomal membrane disclosed by various types of microscopy, including transmission electron microscopy and the recently developed high-resolution fluorescence microscopy, (ii) the nascent acrosomal membrane dysfunction during acrosome biogenesis and (iii) the modification of the sperm membrane during sperm-egg interaction.

Functional and biochemical studies of CD9 in fibrosarcoma cell line

CD9, a member of the tetraspanin family, plays important roles in a variety of cell activities. Fibrosarcoma is a malignant tumor that arises from fibroblasts. Low CD9 expression is found in fibrosarcoma tumor, but function of CD9 in fibrosarcoma has been rarely studied. In this study, stable cell lines for CD9 overexpression and vector were generated in HT1080, a human fibroscarcoma cell line, and cellular functions were widely investigated. In CD9-HT1080 cells, CD9 mainly localized in the membrane and co-localized with F-actin in the filopodia of cell surface. In functional assays, we demonstrated that CD9 could up-regulate total and active caspase-3 expression and induce cell apoptosis, but cell proliferation remained unchanged. CD9 overexpression inhibited HT1080 cell adhesion to FN but promoted cell spreading on FN. We also observed CD9 reduced cell migration using FN a chemoattractant and inhibited cell colony formation in soft agar medium. To explore the biochemical mechanism for functional changes, we investigated the effects of CD9 overexpression on cellular pathways and protein association. CD9 overexpression induced Akt phosphorylation on FN but did not change total Akt expression. Phosphorylation of p38 but not ERK was increased by CD9 overexpression, total p38 and ERK were not affected. CD9 overexpression did not affect the expression of TGFα, EGFR, β1, and EWI-2, but EWI-F expression was up-regulated. Moreover, CD9 could associate with TGFα, EGFR, β1, EWI-2, and EWI-F in HT1080 cell line. Take together, CD9 overexpression had promoting effects on cell apoptosis and cell spreading, but had inhibitory effects on cell adhesion, migration, and cell colony formation. These effects might be ascribed to CD9 associations with EWI-2/EWI-F/β1 complex and EGFR pathway, and the activation of Akt and p38 signalings as well.

The mechanism of sperm-egg interaction and the involvement of IZUMO1 in fusion

An average human ejaculate contains over 100 million sperm, but only a few succeed in accomplishing the journey to an egg by migration through the female reproductive tract. Among these few sperm, only one participates in fertilization. There might be an ingenious molecular mechanism to ensure that the very best sperm fertilize an egg. However, recent gene disruption experiments in mice have revealed that many factors previously described as important for fertilization are largely dispensable. One could argue that the fertilization mechanism is made robust against gene disruptions. However, this is not likely, as there are already six different gene-disrupted mouse lines (Calmegin, Adam1a, Adam2, Adam3, Ace and Pgap1), all of which result in male sterility. The sperm from these animals are known to have defective zona-binding ability and at the same time lose oviduct-migrating ability. Concerning sperm-zona binding, the widely accepted involvement of sugar moiety on zona pellucida 3 (ZP3) is indicated to be dispensable by gene disruption experiments. Thus, the landscape of the mechanism of fertilization is revolving considerably. In the sperm-egg fusion process, CD9 on egg and IZUMO1 on sperm have emerged as essential factors. This review focuses on the mechanism of fertilization elucidated by gene-manipulated animals.

Evolution of programmed cell fusion: common mechanisms and distinct functions

Eukaryotic cells have evolved diverged mechanisms to merge cells. Here, we discuss three types of cell fusion: (1) Non-self-fusion, cells with different genetic contents fuse to start a new organism and fusion between enveloped viruses and host cells; (2) Self-fusion, genetically identical cells fuse to form a multinucleated cell; and (3) Auto-fusion, a single cell fuses with itself by bringing specialized cell membrane domains into contact and transforming itself into a ring-shaped cell. This is a new type of selfish fusion discovered in C. elegans. We divide cell fusion into three stages: (1) Specification of the cell-fusion fate; (2) Cell attraction, attachment, and recognition; (3) Execution of plasma membrane fusion, cytoplasmic mixing and cytoskeletal rearrangements. We analyze cell fusion in diverse biological systems in development and disease emphasizing the mechanistic contributions of C. elegans to the understanding of programmed cell fusion, a genetically encoded pathway to merge specific cells.

Tetraspanin family protein CD9 in the mouse sperm: unique localization, appearance, behavior and fate during fertilization

A tetraspanin family protein, CD9, has not previously been identified in sperm cells. Here, we characterize sperm CD9 in the mouse, including its unique localization in sperm, appearance during spermatogenesis, and behavior and fate during mouse fertilization. In sperm, CD9 is an inner acrosomal membrane-associated protein, not a plasma membrane-associated protein. Its molecular weight is approximately 24 kDa throughout its processing, from testicular germ cells to acrosome-reacted sperm. A temporal difference was found between mRNA and protein expression; CD9 mRNA was detected in the stages from spermatogonia through round spermatids showing the strongest levels in midpachytene spermatocytes. CD9 protein was detected in the cytoplasm throughout the stages from spermatogonia to spermatocytes. While CD9 was weakly expressed in the spermatids from step 1 through step 14, the signals became clearly positive at the marginal region of the anterior acrosome in elongated spermatids. After the acrosome reaction, the majority of sperm CD9 was retained in the inner acrosomal membrane, but some quantity of CD9 was found on the plasma membrane covering the equatorial segment as detected by immunogold electron microscopy using anti-CD9 antibody. CD9 was maintained on the sperm head after reaching the perivitelline space of CD9-deficient eggs that were recovered after natural mating with wild males. Thus, this study characterizes CD9 in sperm development and fertilization.

The roles of tetraspanins in HIV-1 replication

Tetraspanins are small integral membrane proteins that are known to control a variety of cellular processes, including signaling, migration and cell-cell fusion. Research over the past few years established that they are also regulators of various steps in the HIV-1 replication cycle, but the mechanisms through which these proteins either enhance or repress virus spread remain largely unknown.

Decrease in CD9 content and reorganization of microvilli may contribute to the oolemma block to sperm penetration during fertilization of mouse oocyte

Tetraspanin CD9 is the only protein of the oocyte membrane (oolemma) known to be required for the fusion of gametes during fertilization in the mouse. Using electron microscopy and immunostaining we examined the differences in localization of CD9 between ovulated oocytes, zygotes and parthenogenetically activated eggs (parthenogenotes). Changes in ultrastructure of oolemma, which take place in oocytes after fertilization or artificial activation, were also assessed. We demonstrated that after fertilization the level of CD9 present on microvilli of zygote was two times lower than its level on the oolemma of the oocyte. In addition, we showed that the distribution of microvilli is less uniform in the zygotes than in the unfertilized oocytes. We propose that the changes of microvilli distribution and their CD9 content are responsible for the development of the oocyte membrane block to sperm penetration.

Fyn kinase activity is required for normal organization and functional polarity of the mouse oocyte cortex

The objective of the present study was to determine whether Fyn kinase participated in signaling events during sperm-egg interactions, sperm incorporation, and meiosis II. The functional requirement of Fyn kinase activity in these events was tested through the use of the protein kinase inhibitor SKI-606 (Bosutinib) and by analysis of Fyn-null oocytes. Suppression of Fyn kinase signaling prior to fertilization caused disruption of the functional polarity of the oocyte with the result that sperm were able to fuse with the oocyte in the immediate vicinity of the meiotic spindle, a region that normally does not allow sperm fusion. The loss of functional polarity was accompanied by disruption of the microvilli and cortical granule-free zone that normally overlie the meiotic spindle. Changes in the distribution of cortical granules and filamentous actin provided further evidence of disorganization of the oocyte cortex. Rho B, a molecular marker for oocyte polarity, was unaffected by suppression of Fyn activity; however, the polarized association of Par-3 with the cortex overlying the meiotic spindle was completely disrupted. The defects in oocyte polarity in Fyn-null oocytes correlated with a failure of the MII chromosomes to maintain a position close to the oocyte cortex which seemed to underlie the above defects in oocyte polarity. This was associated with a delay in completion of meiosis II. Pronuclei, however, eventually formed and subsequent mitotic cleavages and blastocyst formation occurred normally.

Lateral organization of membrane proteins: tetraspanins spin their web

Despite high expression levels at the plasma membrane or in intracellular vesicles, tetraspanins remain among the most mysterious transmembrane molecules 20 years after their discovery. Several genetic studies in mammals and invertebrates have demonstrated key physiological roles for some of these tetraspanins, in particular in the immune response, sperm-egg fusion, photoreceptor function and the normal function of certain epithelia. Other studies have highlighted their ability to modulate cell migration and metastasis formation. Their role in the propagation of infectious agents has drawn recent attention, with evidence for HIV budding in tetraspanin-enriched plasma membrane domains. Infection of hepatocytic cells by two major pathogens, the hepatitis C virus and the malaria parasite, also requires the tetraspanin CD81. The function of tetraspanins is thought to be linked to their ability to associate with one another and a wealth of other integral proteins, thereby building up an interacting network or 'tetraspanin web'. On the basis of the biochemical dissection of the tetraspanin web and recent analysis of the dynamics of some of its constituents, we propose that tetraspanins tightly regulate transient interactions between a variety of molecules and as such favour the efficient assembly of specialized structures upon proper stimulation.

Immunoglobulin superfamily member IgSF8 (EWI-2) and CD9 in fertilisation: evidence of distinct functions for CD9 and a CD9-associated protein in mammalian sperm-egg interaction

On the mouse egg, the tetraspanin CD9 is nearly essential for sperm-egg fusion, with another tetraspanin, CD81, playing a complementary role. Based on what is known about these proteins, egg tetraspanins are likely to be involved in regulation of membrane order through associations with other egg membrane proteins. Here, we identify a first-level interaction (stable in 1% Triton X-100) between CD9 and the immunoglobulin superfamily member IgSF8 (also known as EWI-2), the first evidence in eggs of such an interaction of CD9 with another protein. We also compared the effects of antibody-mediated perturbation of IgSF8 and CD9, evaluating the robustness of these perturbations in IVF conditions that heavily favour fertilisation and those in which fertilisation occurs less frequently. These studies demonstrate that IgSF8 participates in mouse gamete interactions and identify discrete effects of antibody-mediated perturbation of CD9 and IgSF8. An anti-IgSF8 antibody had moderate inhibitory effects on sperm-egg binding, whereas an anti-CD9 antibody significantly inhibited sperm-egg fusion and, in certain assays, had an inhibitory effect on binding as well. The present study highlights the critical importance of design of IVF experiments for the detection of different effects of experimental manipulations on gamete interactions.

Sperm–egg adhesion and fusion in mammals

Fertilisation is an orchestrated, stepwise process during which the participating male and female gametes undergo irreversible changes, losing some of their structural components while contributing others to the resultant zygote. Following sperm penetration through the egg coat, the sperm plasma membrane fuses with its oocyte counterpart, the oolemma. At least two plasma membrane proteins essential for sperm–oolemma fusion – IZUMO and CD9 on the male and female gametes, respectively – have been identified recently by classical cell biology approaches and confirmed by gene deletion. Oolemma-associated tetraspanin CD81, closely related to CD9, also appears to have an essential role in fusion. Additional proteins that may have nonessential yet still facilitating roles in sperm–oolemma adhesion and fusion include oolemma-anchored integrins and oocyte-expressed retroviral envelope proteins, sperm disintegrins, and sperm-borne proteins of epididymal origin such as CRISP1 and CRISP2. This review discusses these components of the gamete fusion mechanism within the framework of gamete structure, membrane biology, cell signalling and cytoskeletal dynamics, and revisits the topic of antipolyspermy defence at the oolemma level. Harnessing the mechanisms of sperm–egg fusion is of importance to animal biotechnology and to human assisted fertilisation, wherein male patients with reduced sperm fusibility have been identified.

Tetraspanin CD9 in bovine oocytes and its role in fertilization

This study was conducted in bovine to investigate whether CD9 (a member of the tetraspanin superfamily of proteins) is present on oocytes and whether it functions in sperm-oocyte binding and fusion. First, the presence of CD9 in bovine matured oocytes was examined by immunofluorescence with the anti-CD9 monoclonal antibody (mAb) and fluorescein isothiocyanate-conjugated goat anti-mouse antibody, and the results showed that CD9 was expressed on the plasma membrane of matured oocytes. Sperm binding and fusion with oocytes was then examined by in vitro fertilization. When the zona pellucida-free matured oocytes were fertilized, both sperm binding to ooplasma and sperm penetrating into oocytes were significantly (P<0.01) reduced in anti-CD9 antibody-treated oocytes (6.3 +/- 0.7 per oocyte and 41.6%, respectively) compared with untreated control oocytes (19.0 +/- 0.7 per oocyte and 81.3%, respectively), indicating that the anti-CD9 mAb potentially inhibits sperm-oocyte binding and fusion. These results demonstrated that the CD9 present on bovine matured oocytes is involved in sperm-oocyte interaction during fertilization.

Glioblastoma inhibition by cell surface immunoglobulin protein EWI-2, in vitro and in vivo

EWI-2, a cell surface IgSF protein, is highly expressed in normal human brain but is considerably diminished in glioblastoma tumors and cell lines. Moreover, loss of EWI-2 expression correlated with a shorter survival time in human glioma patients, suggesting that EWI-2 might be a natural inhibitor of glioblastoma. In support of this idea, EWI-2 expression significantly impaired both ectopic and orthotopic tumor growth in nude mice in vivo. In vitro assays provided clues regarding EWI-2 functions. Expression of EWI-2 in T98G and/or U87-MG malignant glioblastoma cell lines failed to alter two-dimensional cell proliferation but inhibited glioblastoma colony formation in soft agar and caused diminished cell motility and invasion. At the biochemical level, EWI-2 markedly affects the organization of four molecules (tetraspanin proteins CD9 and CD81 and matrix metalloproteinases MMP-2 and MT1-MMP), which play key roles in the biology of astrocytes and gliomas. EWI-2 causes CD9 and CD81 to become more associated with each other, whereas CD81 and other tetraspanins become less associated with MMP-2 and MT1-MMP. We propose that EWI-2 inhibition of glioblastoma growth in vivo is at least partly explained by the capability of EWI-2 to inhibit growth and/or invasion in vitro. Underlying these functional effects, EWI-2 causes a substantial molecular reorganization of multiple molecules (CD81, CD9, MMP-2, and MT1-MMP) known to affect proliferation and/or invasion of astrocytes and/or glioblastomas.

Targeting of tetraspanin proteins--potential benefits and strategies

The tetraspanin transmembrane proteins have emerged as key players in malignancy, the immune system, during fertilization and infectious disease processes. Tetraspanins engage in a wide range of specific molecular interactions, occurring through the formation of tetraspanin-enriched microdomains (TEMs). TEMs therefore serve as a starting point for understanding how tetraspanins affect cell signalling, adhesion, morphology, motility, fusion and virus infection. An abundance of recent evidence suggests that targeting tetraspanins, for example, by monoclonal antibodies, soluble large-loop proteins or RNAi technology, should be therapeutically beneficial.

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.

Tetraspanins: push and pull in suppressing and promoting metastasis

Tumours progress through a cascade of events that enable the formation of metastases. Some of the components that are required for this fatal process are well established. Tetraspanins, however, have only recently received attention as both metastasis suppressors and metastasis promoters. This late appreciation is probably due to their capacity to associate with various molecules, which they recruit into special membrane microdomains, and their abundant presence in tumour-derived small vesicles that aid intercellular communication. It is reasonable to assume that differences in the membrane and vesicular web components that associate with individual tetraspanins account for their differing abilities to promote and suppress metastasis.

Alteration of the cortical actin cytoskeleton deregulates Ca2+ signaling, monospermic fertilization, and sperm entry

BACKGROUND

When preparing for fertilization, oocytes undergo meiotic maturation during which structural changes occur in the endoplasmic reticulum (ER) that lead to a more efficient calcium response. During meiotic maturation and subsequent fertilization, the actin cytoskeleton also undergoes dramatic restructuring. We have recently observed that rearrangements of the actin cytoskeleton induced by actin-depolymerizing agents, or by actin-binding proteins, strongly modulate intracellular calcium (Ca2+) signals during the maturation process. However, the significance of the dynamic changes in F-actin within the fertilized egg has been largely unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We have measured changes in intracellular Ca2+ signals and F-actin structures during fertilization. We also report the unexpected observation that the conventional antagonist of the InsP(3) receptor, heparin, hyperpolymerizes the cortical actin cytoskeleton in postmeiotic eggs. Using heparin and other pharmacological agents that either hypo- or hyperpolymerize the cortical actin, we demonstrate that nearly all aspects of the fertilization process are profoundly affected by the dynamic restructuring of the egg cortical actin cytoskeleton.

CONCLUSIONS/SIGNIFICANCE

Our findings identify important roles for subplasmalemmal actin fibers in the process of sperm-egg interaction and in the subsequent events related to fertilization: the generation of Ca2+ signals, sperm penetration, cortical granule exocytosis, and the block to polyspermy.

Adaptive evolution of gamete-recognition proteins in birds

Gamete-recognition proteins have been shown to evolve by positive selection in diverse organism groups, such as marine invertebrates and mammals, although underlying evolutionary mechanisms driving this rapid divergence are poorly understood. However, several hypotheses have been put forward to explain the observed pattern, including different forms of sexual conflict and sperm competition. Because female gametes require more energy to produce than male gametes, female organisms suffer more when fertilisation goes wrong. One process that results in a failed mammalian fertilisation is polyspermy, when >1 sperm fertilises the egg. However in birds, there is no such sexual conflict because multiple sperm typically bind and fuse with the egg. If sexual conflict driven by polyspermy avoidance is important for the evolution of gamete-recognition proteins in vertebrates, we expect to find positive selection in the genes to be less pronounced in birds. We therefore sequenced six genes (ZP1, ZP2, ZP4, ZPAX, CD9, and Acrosin) encoding gamete-recognition proteins in several bird species to test for positive selection. For comparison, we also analysed ortologous sequences in a set of mammalian species. We found no major differences in the occurrence of adaptive evolution and the strength of selection between bird and mammal orthologs. From this we conclude that polyspermy avoidance does not act as the main underlying evolutionary force shaping the rate of evolution in these genes. We discuss other possible processes that could explain positive selection of gamete-recognition proteins in birds and mammals, such as hybridisation avoidance, cryptic female choice, and postcopulatory sperm competition.

The fusing ability of sperm is bestowed by CD9-containing vesicles released from eggs in mice

Membrane fusion is an essential step in the encounter of two nuclei from sex cells-sperm and egg-in fertilization. However, aside from the involvement of two molecules, CD9 and Izumo, the mechanism of fusion remains unclear. Here, we show that sperm-egg fusion is mediated by vesicles containing CD9 that are released from the egg and interact with sperm. We demonstrate that the CD9(-/-) eggs, which have a defective sperm-fusing ability, have impaired release of CD9-containing vesicles. We investigate the fusion-facilitating activity of CD9-containing vesicles by examining the fusion of sperm to CD9(-/-) eggs with the aid of exogenous CD9-containing vesicles. Moreover, we show, by examining the fusion of sperm to CD9(-/-) eggs, that hamster eggs have a similar fusing ability as mouse eggs. The CD9-containing vesicle release from unfertilized eggs provides insight into the mechanism required for fusion with sperm.

CD9 clustering and formation of microvilli zippers between contacting cells regulates virus-induced cell fusion

Members of the tetraspanin family including CD9 contribute to the structural organization and plasticity of the plasma membrane. K41, a CD9-specific monoclonal antibody, inhibits the release of HIV-1 and canine distemper virus (CDV)- but not measles virus (MV)-induced cell-cell fusion. We now report that K41, which recognizes a conformational epitope on the large extracellular loop of CD9, induces rapid relocation and clustering of CD9 in net-like structures at cell-cell contact areas. High-resolution analyses revealed that CD9 clustering is accompanied by the formation of microvilli that protrude from either side of adjacent cell surfaces, thus forming structures like microvilli zippers. While the cellular CD9-associated proteins beta(1)-integrin and EWI-F were co-clustered with CD9 at cell-cell interfaces, viral proteins in infected cells were differentially affected. MV envelope proteins were detected within CD9 clusters, whereas CDV proteins were excluded from CD9 clusters. Thus, the tetraspanin CD9 can regulate cell-cell fusion by controlling the access of the fusion machinery to cell contact areas.

Signal transduction of fertilization in frog eggs and anti-apoptotic mechanism in human cancer cells: common and specific functions of membrane microdomains

Membrane microdomains or lipid/membrane rafts are distinct areas on the plasma membranes, where a specific subset of lipids (e.g. cholesterol, sphingolipids) and proteins (e.g. glycosylphosphatidylinositol-anchored proteins, growth factor receptor/kinases) are getting together and functioning for several aspects of cellular functions. Our recent investigation has revealed that fertilization of African clawed frog, Xenopus laevis, requires cholesterol-dependent nature of egg membrane microdomains. Moreover, fertilization of Xenopus eggs involves proteolytic cleavage of the extracellular part and subsequent phosphorylation of a cytoplasmic tyrosine residue of uroplakin III, an egg membrane microdomain-associated protein. Protease activity toward uroplakin III seems to be derived from fertilizing sperm, while phosphorylation of uroplakin III seems to be catalyzed by the egg tyrosine kinase Src, whose activation is required for cytoplasmic rearrangement of fertilized eggs; so-called 'egg activation'. Therefore, it is assumed that uroplakin III serves an integral part of signal transduction in fertilization of Xenopus. Our more recent study on human cancer cells has revealed that a similar but distinct scheme of signal transduction operates in anti-apoptotic growth of cells. Namely, in human bladder carcinoma cells, cooperation of uroplakin III and Src, both of which localize to the membrane microdomains, allows cells to escape from apoptotic cell death and proliferate under culture conditions deprived of serum. In this review, I briefly introduce about biology of fertilization and cancer, and then present and discuss our experimental data on general importance and specific features of membrane microdomains in Xenopus fertilization and anti-apoptosis in human bladder carcinoma cells.

Proteome analysis of matrix vesicles isolated from femurs of chicken embryo

Matrix vesicles (MVs) are extracellular organelles that initiate mineral formation, accumulating inorganic phosphate (P(i)) and calcium leading to the formation of hydroxyapatite (HA) crystals, the main mineral component of bones. MVs are produced during bone formation, as well as during the endochondral calcification of cartilage. MVs are released into the extracellular matrix from osseous cells such as osteoblasts and hypertrophic chondrocytes. In this report, using 1-D SDS-PAGE, in-gel tryptic digestion and an LC-MS-MS/MS protein identification protocol, we characterized the proteome of MVs isolated from chicken embryo (Gallus gallus) bones and cartilage. We identified 126 gene products, including proteins related to the extracellular matrix and ion transport, as well as enzymes, cytoskeletal, and regulatory proteins. Among the proteins recognized for the first time in MVs were aquaporin 1, annexin A1 (AnxA1), AnxA11, glycoprotein HT7, G(i) protein alpha2, and scavenger receptor type B. The pathways for targeting the identified proteins into MVs and their particular functions in the biomineralization process are discussed. Obtaining a knowledge of the functions and roles of these proteins during embryonic mineralization is a prerequisite for the overall understanding of the initial mineral formation mechanisms.

Mapping mouse gamete interaction forces reveal several oocyte membrane regions with different mechanical and adhesive properties

This study focuses on the interaction involved in the adhesion of mouse gametes and on the mechanical properties of the oocyte membrane. The oocyte has an asymmetrical shape, and its membrane is composed of two distinct areas. One is rich in microvilli, and the other is smoother and without microvilli. With a biomembrane force probe (BFP) adapted to cell-cell measurements, we have quantified the separation forces between a spermatozoon and an oocyte. Microvillar and amicrovillar areas of the oocyte surface have been systematically probed and compared. In addition to a substantial difference in the elastic stiffness of these two regions, the experiments have revealed the presence of two types of membrane domains with different mechanical and adhesive properties, both distributed over the entire oocyte surface (i.e., in both microvillar and amicrovillar regions). If gamete contact occurs in the first type of domain, then the oocyte membrane deforms only elastically under traction. The pull-off forces in these domains are higher in the amicrovillar region. For a spermatozoon contact with the other type of domain, there can be a transition from the elastic to viscoelastic regime, and then tethers are extruded from the oocyte membrane.

Cell fusion during development

Most readers of this review originated from a sperm-egg fusion event. Cell fusion is a process that is crucial at many intersections later during development. However, we do not know which molecules (fusogens) fuse the membranes of gametes to form zygotes, myoblasts to form myotubes in muscles, macrophages to form osteoclasts in bones, or cytotrophoblasts to form syncytiotrophoblasts in placentas. There are five gold standards that can be applied for the identification of genuine fusogens. Based on these criteria, a numerical score can be used to assess the likelihood of protein fusogenicity. We compare distinct families of candidate developmental, viral and intracellular fusogens and analyze current models of membrane fusion.

Phase transitions of the coupled membrane-cytoskeleton modify cellular shape

Formation of protrusions and protein segregation on the membrane is of a great importance for the functioning of the living cell. This is most evident in recent experiments that show the effects of the mechanical properties of the surrounding substrate on cell morphology. We propose a mechanism for the formation of membrane protrusions and protein phase separation, which may lay behind this effect. In our model, the fluid cell membrane has a mobile but constant population of proteins with a convex spontaneous curvature. Our basic assumption is that these membrane proteins represent small adhesion complexes, and also include proteins that activate actin polymerization. Such a continuum model couples the membrane and protein dynamics, including cell-substrate adhesion and protrusive actin force. Linear stability analysis shows that sufficiently strong adhesion energy and actin polymerization force can bring about phase separation of the membrane protein and the appearance of protrusions. Specifically, this occurs when the spontaneous curvature and aggregation potential alone (passive system) do not cause phase separation. Finite-size patterns may appear in the regime where the spontaneous curvature energy is a strong factor. Different instability characteristics are calculated for the various regimes, and are compared to various types of observed protrusions and phase separations, both in living cells and in artificial model systems. A number of testable predictions are proposed.