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

Distinctive function of Tetraspanins: Implication in viral infections

Harboring four transmembrane domains in their structural hallmark, Tetraspanins (Tspans) are a family of glycoproteins with pivotal functions in a variety of biological and cellular processes. Through interacting laterally with each other or specific membrane proteins, Tspans organize tetraspanin-enriched microdomains (TEMs), modulating cellular signaling, adhesion, fusion, and proliferation. An abundance of evidence has identified the multiple functions in the progression of cancer as well as the underlying molecular mechanisms. Recently, plenty of studies have focused on the utilities of Tspans by pathogens for infection, especially the infection of viruses. The expression of Tspans correlates with the phase of viral infection, the type of virus, and targeted therapies. In particular, perturbations of Tspans in host cells can affect viral attachment, intracellular trafficking, translation, virus assembly, and release. In this review, we summarize and provide a historical overview of the discovery and characterization of various kinds of virus infection and highlight their diversity and complexity, along with the virus life cycle. Furthermore, we examined the current understanding of how various Tspans are involved in the regulatory mechanisms underlying viral infection. This review aims to offer a comprehensive understanding of the targeting of Tspans for therapeutic intervention in infections caused by diverse pathogens.

Tetraspanin CD9 alters cellular trafficking and endocytosis of tetraspanin CD63, affecting CD63 packaging into small extracellular vesicles

Small extracellular vesicles (sEVs) are particles secreted from cells that play vital roles both in normal physiology and in human disease. sEVs are highly enriched in tetraspanin proteins, such as CD9 and CD63, and contain tetraspanin-enriched membrane microdomains involved in loading of sEVs with macromolecule cargoes and in sEV biogenesis. However, the precise roles of individual tetraspanins in sEV biogenesis and cargo loading remain poorly understood. Here, we report that CD9 negatively regulated CD63 trafficking to tetraspanin-enriched microdomains and its subsequent packaging into sEVs, whereas CD63 had no discernable effect on CD9 localization or packaging. Using super resolution microscopy of individual vesicles, we showed that CD9 governs the fraction of sEVs that are loaded with CD63. Interestingly, CD9-dependent suppression of CD63 packaging was rescued by pharmacological blockade of endocytosis. Together, our data support a model where CD9 contributes to the regulation and secretion of CD63 in an endocytosis-dependent manner to reprogram the contents of sEVs and tetraspanin-enriched microdomains.

Proteomic landscape of tunneling nanotubes reveals CD9 and CD81 tetraspanins as key regulators

Tunneling nanotubes (TNTs) are open actin- and membrane-based channels, connecting remote cells and allowing direct transfer of cellular material (e.g. vesicles, mRNAs, protein aggregates) from the cytoplasm to the cytoplasm. Although they are important especially, in pathological conditions (e.g. cancers, neurodegenerative diseases), their precise composition and their regulation were still poorly described. Here, using a biochemical approach allowing to separate TNTs from cell bodies and from extracellular vesicles and particles (EVPs), we obtained the full composition of TNTs compared to EVPs. We then focused on two major components of our proteomic data, the CD9 and CD81 tetraspanins, and further investigated their specific roles in TNT formation and function. We show that these two tetraspanins have distinct non-redundant functions: CD9 participates in stabilizing TNTs, whereas CD81 expression is required to allow the functional transfer of vesicles in the newly formed TNTs, possibly by regulating docking to or fusion with the opposing cell.

Deep learning insights into the architecture of the mammalian egg-sperm fusion synapse

A crucial event in sexual reproduction is when haploid sperm and egg fuse to form a new diploid organism at fertilization. In mammals, direct interaction between egg JUNO and sperm IZUMO1 mediates gamete membrane adhesion, yet their role in fusion remains enigmatic. We used AlphaFold to predict the structure of other extracellular proteins essential for fertilization to determine if they could form a complex that may mediate fusion. We first identified TMEM81, whose gene is expressed by mouse and human spermatids, as a protein having structural homologies with both IZUMO1 and another sperm molecule essential for gamete fusion, SPACA6. Using a set of proteins known to be important for fertilization and TMEM81, we then systematically searched for predicted binary interactions using an unguided approach and identified a pentameric complex involving sperm IZUMO1, SPACA6, TMEM81 and egg JUNO, CD9. This complex is structurally consistent with both the expected topology on opposing gamete membranes and the location of predicted N-glycans not modeled by AlphaFold-Multimer, suggesting that its components could organize into a synapse-like assembly at the point of fusion. Finally, the structural modeling approach described here could be more generally useful to gain insights into transient protein complexes difficult to detect experimentally.

Selective Translation of Maternal mRNA by eIF4E1B Controls Oocyte to Embryo Transition

Maternal messenger ribonucleic acids (mRNAs) are driven by a highly orchestrated scheme of recruitment to polysomes and translational activation. However, selecting and regulating individual mRNAs for the translation from a competitive pool of mRNAs are little-known processes. This research shows that the maternal eukaryotic translation initiation factor 4e1b (Eif4e1b) expresses during the oocyte-to-embryo transition (OET), and maternal deletion of Eif4e1b leads to multiple defects concerning oogenesis and embryonic developmental competence during OET. The linear amplification of complementary deoxyribonucleic acid (cDNA) ends, and sequencing (LACE-seq) is used to identify the distinct subset of mRNA and its CG-rich binding sites within the 5' untranslated region (UTR) targeted by eIF4E1B. The proteomics analyses indicate that eIF4E1B-specific bound genes show stronger downregulation at the protein level, which further verify a group of proteins that plays a crucial role in oocyte maturation and embryonic developmental competence is insufficiently synthesized in Eif4e1b-cKO oocytes during OET. Moreover, the biochemical results in vitro are combined to further confirm the maternal-specific translation activation model assembled by eIF4E1B and 3'UTR-associated mRNA binding proteins. The findings demonstrate the indispensability of eIF4E1B for selective translation activation in mammalian oocytes and provide a potential network regulated by eIF4E1B in OET.

Significance of Complement Regulatory Protein Tetraspanins in the Male Reproductive System and Fertilization

Fertilization is a very sophisticated and unique process involving several key steps resulting in a zygote's formation. Recent research has indicated that some immune system-related cell surface molecules (CD molecules from the tetraspanin superfamily) may have a role in fertilization. Extracellular vesicles are undeniably involved in a variety of cellular functions, including reproduction. Tetraspanin proteins identified in extracellular vesicles are now used mostly as markers; mounting evidence indicates that they also participate in cell targeting, cargo selection, and extracellular vesicle formation. Their significance and potential in mammalian reproduction are currently being studied extensively. Despite the fact that the current data did not establish any theory, the crucial function of tetraspanins in the fertilization process was not ruled out, and the specific role of tetraspanins is still unknown. In this review, we bring insight into the existing knowledge regarding the expression of tetraspanins in spermatozoa and seminal fluid and their role in gamete binding and fusion.

Liposomes and Extracellular Vesicles as Drug Delivery Systems: A Comparison of Composition, Pharmacokinetics, and Functionalization

Over the past decades, lipid-based nanoparticle drug delivery systems (DDS) have caught the attention of researchers worldwide, encouraging the field to rapidly develop improved ways for effective drug delivery. One of the most prominent examples is liposomes, which are spherical shaped artificial vesicles composed of lipid bilayers and able to encapsulate both hydrophilic and hydrophobic materials. At the same time, biological nanoparticles naturally secreted by cells, called extracellular vesicles (EVs), have emerged as promising more complex biocompatible DDS. In this review paper, the differences and similarities in the composition of both vesicles are evaluated, and critical mediators that affect their pharmacokinetics are elucidate. Different strategies that have been assessed to tweak the pharmacokinetics of both liposomes and EVs are explored, detailing the effects on circulation time, targeting capacity, and cytoplasmic delivery of therapeutic cargo. Finally, whether a hybrid system, consisting of a combination of only the critical constituents of both vesicles, could offer the best of both worlds is discussed. Through these topics, novel leads for further research are provided and, more importantly, gain insight in what the liposome field and the EV field can learn from each other.

Exploring interactions between extracellular vesicles and cells for innovative drug delivery system design

Extracellular vesicles (EVs) are submicron cell-secreted structures containing proteins, nucleic acids and lipids. EVs can functionally transfer these cargoes from one cell to another to modulate physiological and pathological processes. Due to their presumed biocompatibility and capacity to circumvent canonical delivery barriers encountered by synthetic drug delivery systems, EVs have attracted considerable interest as drug delivery vehicles. However, it is unclear which mechanisms and molecules orchestrate EV-mediated cargo delivery to recipient cells. Here, we review how EV properties have been exploited to improve the efficacy of small molecule drugs. Furthermore, we explore which EV surface molecules could be directly or indirectly involved in EV-mediated cargo transfer to recipient cells and discuss the cellular reporter systems with which such transfer can be studied. Finally, we elaborate on currently identified cellular processes involved in EV cargo delivery. Through these topics, we provide insights in critical effectors in the EV-cell interface which may be exploited in nature-inspired drug delivery strategies.

Tetraspanin Cd9b and Cxcl12a/Cxcr4b have a synergistic effect on the control of collective cell migration

Collective cell migration is essential for embryonic development and homeostatic processes. During zebrafish development, the posterior lateral line primordium (pLLP) navigates along the embryo flank by collective cell migration. The chemokine receptors, Cxcr4b and Cxcr7b, as well as their cognate ligand, Cxcl12a, are essential for this process. We corroborate that knockdown of the zebrafish cd9 tetraspanin orthologue, cd9b, results in mild pLL abnormalities. Through generation of CRISPR and TALEN mutants, we show that cd9a and cd9b function partially redundantly in pLLP migration, which is delayed in the cd9b single and cd9a; cd9b double mutants. This delay led to a transient reduction in neuromast numbers. Loss of both Cd9a and Cd9b sensitized embryos to reduced Cxcr4b and Cxcl12a levels. Together these results provide evidence that Cd9 modulates collective cell migration of the pLLP during zebrafish development. One interpretation of these observations is that Cd9 contributes to more effective chemokine signalling.

Tetraspanins are involved in Burkholderia pseudomallei-induced cell-to-cell fusion of phagocytic and non-phagocytic cells

Tetraspanins are four-span transmembrane proteins of host cells that facilitate infections by many pathogens. Burkholderia pseudomallei is an intracellular bacterium and the causative agent of melioidosis, a severe disease in tropical regions. This study investigated the role of tetraspanins in B. pseudomallei infection. We used flow cytometry to determine tetraspanins CD9, CD63, and CD81 expression on A549 and J774A.1 cells. Their roles in B. pseudomallei infection were investigated in vitro using monoclonal antibodies (MAbs) and recombinant large extracellular loop (EC2) proteins to pretreat cells before infection. Knockout of CD9 and CD81 in cells was performed using CRISPR Cas9 to confirm the role of tetraspanins. Pretreatment of A549 cells with MAb against CD9 and CD9-EC2 significantly enhanced B. pseudomallei internalization, but MAb against CD81 and CD81-EC2 inhibited MNGC formation. Reduction of MNGC formation was consistently observed in J774.A1 cells pretreated with MAbs specific to CD9 and CD81 and with CD9-EC2 and CD81-EC2. Data from knockout experiments confirmed that CD9 enhanced bacterial internalization and that CD81 inhibited MNGC formation. Our data indicate that tetraspanins are host cellular factors that mediated internalization and membrane fusion during B. pseudomallei infection. Tetraspanins may be the potential therapeutic targets for melioidosis.

Long-Term Low-Dose Delta-9-Tetrahydrocannbinol (THC) Administration to Simian Immunodeficiency Virus (SIV) Infected Rhesus Macaques Stimulates the Release of Bioactive Blood Extracellular Vesicles (EVs) that Induce Divergent Structural Adaptations and Signaling Cues

Blood extracellular vesicles (BEVs) carry bioactive cargo (proteins, genetic materials, lipids, licit, and illicit drugs) that regulate diverse functions in target cells. The cannabinoid drug delta-9-tetrahydrocannabinol (THC) is FDA approved for the treatment of anorexia and weight loss in people living with HIV. However, the effect of THC on BEV characteristics in the setting of HIV/SIV infection needs to be determined. Here, we used the SIV-infected rhesus macaque model of AIDS to evaluate the longitudinal effects of THC (THC/SIV) or vehicle (VEH/SIV) treatment in HIV/SIV infection on the properties of BEVs. While BEV concentrations increased longitudinally (pre-SIV (0), 30, and 150 days post-SIV infection (DPI)) in VEH/SIV macaques, the opposite trend was observed with THC/SIV macaques. SIV infection altered BEV membrane properties and cargo composition late in infection, since i) the electrostatic surface properties (zeta potential, ζ potential) showed that RM BEVs carried negative surface charge, but at 150 DPI, SIV infection significantly changed BEV ζ potential; ii) BEVs from the VEH/SIV group altered tetraspanin CD9 and CD81 levels compared to the THC/SIV group. Furthermore, VEH/SIV and THC/SIV BEVs mediated divergent changes in monocyte gene expression, morphometrics, signaling, and function. These include altered tetraspanin and integrin β1 expression; altered levels and distribution of polymerized actin, FAK/pY397 FAK, pERK1/2, cleaved caspase 3, proapoptotic Bid and truncated tBid; and altered adhesion of monocytes to collagen I. These data indicate that HIV/SIV infection and THC treatment result in the release of bioactive BEVs with potential to induce distinct structural adaptations and signaling cues to instruct divergent cellular responses to infection.

Unveiling a novel function of CD9 in surface compartmentalization of oocytes

Gamete fusion is an indispensable process for bearing offspring. In mammals, sperm IZUMO1-oocyte JUNO recognition essentially carries out the primary step of this process. In oocytes, CD9 is also known to play a crucial role in gamete fusion. In particular, microvilli biogenesis through CD9 involvement appears to be a key event for successful gamete fusion, because CD9-disrupted oocytes produce short and sparse microvillous structures, resulting in almost no fusion ability with spermatozoa. In order to determine how CD9 and JUNO cooperate in gamete fusion, we analyzed the molecular profiles of each molecule in CD9- and JUNO-disrupted oocytes. Consequently, we found that CD9 is crucial for the exclusion of GPI-anchored proteins, such as JUNO and CD55, from the cortical actin cap region, suggesting strict molecular organization of the unique surface of this region. Through distinct surface compartmentalization due to CD9 governing, GPI-anchored proteins are confined to the appropriate fusion site of the oocyte.

Tetraspanins in mammalian reproduction: spermatozoa, oocytes and embryos

It is known that tetraspanin proteins are involved in many physiological somatic cell mechanisms. Additionally, research has indicated they also have a role in various infectious diseases and cancers. This review focuses on the molecular interactions underlying the tetraspanin web formation in gametes. Primarily, tetraspanins act in the reproductive tract as organizers of membrane complexes, which include the proteins involved in the contact and association of sperm and oocyte membranes. In addition, recent data shows that tetraspanins are likely to be involved in these processes in a complex way. In mammalian fertilization, an important role is attributed to CD molecules belonging to the tetraspanin superfamily, particularly CD9, CD81, CD151, and also CD63; mostly as part of extracellular vesicles, the significance of which and their potential in reproduction is being intensively investigated. In this article, we reviewed the existing knowledge regarding the expression of tetraspanins CD9, CD81, CD151, and CD63 in mammalian spermatozoa, oocytes, and embryos and their involvement in reproductive processes, including pathological events.

Exosomes and Female Infertility

BACKGROUND

Exosomes are small Extracellular Vesicles (EVs) (40-100 nm) secreted by living cells and mediate the transmission of information between cells. The number and contents of exosomes are associated with diseases such as inflammatory diseases, cancer, metabolic diseases and what we are focusing in this passage-female infertility.

OBJECTIVE

This review focused on the role of exosomes in oocyte development, declined ovarian function, PCOS, uterine diseases, endometrial receptivity and fallopian tube dysfunction in the female.

METHODS

We conducted an extensive search for research articles involving relationships between exosomes and female infertility on the bibliographic database.

RESULTS

It has been reported that exosomes can act as a potential therapeutic device to carry cargoes to treat female infertility. However, the pathophysiological mechanisms of exosomes in female infertility have not been entirely elucidated. Further researches are needed to explore the etiology and provide evidence for potential clinical treatment.

CONCLUSIONS

This review systematically summarized the role exosomes play in female infertility and its potential as drug delivery.

Minocycline impairs TNF-α-induced cell fusion of M13SV1-Cre cells with MDA-MB-435-pFDR1 cells by suppressing NF-κB transcriptional activity and its induction of target-gene expression of fusion-relevant factors

Background

To date, several studies have confirmed that driving forces of the inflammatory tumour microenvironment trigger spontaneous cancer cell fusion. However, less is known about the underlying factors and mechanisms that facilitate inflammation-induced cell fusion of a cancer cell with a normal cell. Recently, we demonstrated that minocycline, a tetracycline antibiotic, successfully inhibited the TNF-α-induced fusion of MDA-MB-435 cancer cells with M13SV1 breast epithelial cells. Here, we investigated how minocycline interferes with the TNF-α induced signal transduction pathway.

Methods

A Cre-LoxP recombination system was used to quantify the fusion of MDA-MB-435-pFDR1 cancer cells and M13SV1-Cre breast epithelial cells. The impact of minocycline on the TNF-α signalling pathway was determined by western blotting. The transcriptional activity of NF-κB was characterised by immunocytochemistry, western blot and ChIP analyses. An NF-κB-luciferase reporter assay was indicative of NF-κB activity.

Results

Minocycline treatment successfully inhibited the TNFR1-TRAF2 interaction in both cell types, while minocycline abrogated the phosphorylation of IκBα and NF-κB-p65 to suppress nuclear NF-κB and its promotor activity only in M13SV1-Cre cells, which attenuated the expression of MMP9 and ICAM1. In MDA-MB-435-pFDR1 cells, minocycline increased the activity of NF-κB, leading to greater nuclear accumulation of NF-κB-p65, thus increasing promoter activity to stimulate the expression of ICAM1. Even though TNF-α also activated all MAPKs (ERK1/2, p38 and JNK), minocycline differentially affected these kinases to either inhibit or stimulate their activation. Moreover, SRC activation was analysed as an upstream activator of MAPKs, but no activation by TNF-α was revealed. The addition of several specific inhibitors that block the activation of SRC, MAPKs, AP-1 and NF-κB confirmed that only NF-κB inhibition was successful in inhibiting the TNF-α-induced cell fusion process.

Conclusion

Minocycline is a potent inhibitor in the TNF-α-induced cell fusion process by targeting the NF-κB pathway. Thus, minocycline prevented NF-κB activation and nuclear translocation to abolish the target-gene expression of MMP9 and ICAM1 in M13SV1-Cre cells, resulting in reduced cell fusion frequency.

Can cytoplasmic donation rescue aged oocytes?

BACKGROUND

The pregnancy and delivery rates following assisted reproductive technology (ART) start to decrease and that the miscarriage rate increases rapidly from 35 years old. The miscarriage rate exceeds 50% at 43 years old. The number of aneuploid fetuses in miscarriages increases according to female age, reaching more than 90% when women are over 40 years old.

METHODS

Different cytoplasmic donation technologies used to rescue aged oocytes with high percentage of aneuploidy were analyzed, and their efficacy compared.

MAIN FINDINGS RESULTS

Germinal vesicle transfer (GVT) might be superior to spindle chromosome transfer (ST) theoretically from the point of higher capability of rescuing the disjunction at meiosis I which cannot be helped by ST. However, actually, in vitro maturation (IVM) of oocyte after GVT has not yet been totally completed. ST among other nuclear donations showed the higher possibility to rescue them, due to the fact it does not require in vitro maturation and it has an ethical advantage over pronuclear transfer (PNT) which requires the destruction of an embryo.

CONCLUSION

Spindle chromosome transfer has the potential to rescue aged oocytes to some extent, but we have to continue the basic study further to establish the clinical application of cytoplasmic donation to rescue aged oocytes.

OC-STAMP promotes osteoclast fusion for pathogenic bone resorption in periodontitis via up-regulation of permissive fusogen CD9

Cell fusion-mediated formation of multinuclear osteoclasts (OCs) plays a key role in bone resorption. It is reported that 2 unique OC-specific fusogens [ i.e., OC-stimulatory transmembrane protein (OC-STAMP) and dendritic cell-specific transmembrane protein (DC-STAMP)], and permissive fusogen CD9, are involved in OC fusion. In contrast to DC-STAMP-knockout (KO) mice, which show the osteopetrotic phenotype, OC-STAMP-KO mice show no difference in systemic bone mineral density. Nonetheless, according to the ligature-induced periodontitis model, significantly lower level of bone resorption was found in OC-STAMP-KO mice compared to WT mice. Anti-OC-STAMP-neutralizing mAb down-modulated in vitro: 1) the emergence of large multinuclear tartrate-resistant acid phosphatase-positive cells, 2) pit formation, and 3) mRNA and protein expression of CD9, but not DC-STAMP, in receptor activator of NF-κB ligand (RANKL)-stimulated OC precursor cells (OCps). While anti-DC-STAMP-mAb also down-regulated RANKL-induced osteoclastogenesis in vitro, it had no effect on CD9 expression. In our mouse model, systemic administration of anti-OC-STAMP-mAb suppressed the expression of CD9 mRNA, but not DC-STAMP mRNA, in periodontal tissue, along with diminished alveolar bone loss and reduced emergence of CD9⁺ OCps and tartrate-resistant acid phosphatase-positive multinuclear OCs. The present study demonstrated that OC-STAMP partners CD9 to promote periodontal bone destruction by up-regulation of fusion during osteoclastogenesis, suggesting that anti-OC-STAMP-mAb may lead to the development of a novel therapeutic regimen for periodontitis.-Ishii, T., Ruiz-Torruella, M., Ikeda, A., Shindo, S., Movila, A., Mawardi, H., Albassam, A., Kayal, R. A., Al-Dharrab, A. A., Egashira, K., Wisitrasameewong, W., Yamamoto, K., Mira, A. I., Sueishi, K., Han, X., Taubman, M. A., Miyamoto, T., Kawai, T. OC-STAMP promotes osteoclast fusion for pathogenic bone resorption in periodontitis via up-regulation of permissive fusogen CD9.

Genes of cellular components of morphogenesis in porcine oocytes before and after IVM

Proper oocyte maturation in mammals produces an oocyte capable of monospermic fertilization and embryo preimplantation. The cumulus-oocyte complexes (COCs), surrounding an oocyte, play a significant role in oocyte maturation. During this process, when the COCs undergo cumulus expansion wherein tightly compact cumulus cells (CCs) form a dispersed structure, permanent biochemical and molecular modifications occur in the maturing oocytes, indicating that the gene expression between immature and mature oocytes differs significantly. This study focuses on the genes responsible for the cellular components of morphogenesis within the developing oocyte. Brilliant cresyl blue (BCB) was used to determine the developmental capability of porcine oocytes. The immature oocytes (GV stage) were compared with matured oocytes (MII stage), using microarray and qRT-PCR analysis to track changes in the genetic expression profile of transcriptome genes. The data showed substantial upregulation of genes influencing oocyte's morphology, cellular migration and adhesion, intracellular communication, as well as plasticity of nervous system. Conversely, downregulation involved genes related to microtubule reorganization, regulation of adhesion, proliferation, migration and cell differentiation processes in oocytes. This suggests that most genes recruited in morphogenesis in porcine oocyte in vitro, may have cellular maturational capability, since they have a higher level of expression before the oocyte's matured form. It shows the process of oocyte maturation and developmental capacity is orchestrated by significant cellular modifications during morphogenesis.

Tetraspanins Function as Regulators of Cellular Signaling

Tetraspanins are molecular scaffolds that distribute proteins into highly organized microdomains consisting of adhesion, signaling, and adaptor proteins. Many reports have identified interactions between tetraspanins and signaling molecules, finding unique downstream cellular consequences. In this review, we will explore these interactions as well as the specific cellular responses to signal activation, focusing on tetraspanin regulation of adhesion-mediated (integrins/FAK), receptor-mediated (EGFR, TNF-α, c-Met, c-Kit), and intracellular signaling (PKC, PI4K, β-catenin). Additionally, we will summarize our current understanding for how tetraspanin post-translational modifications (palmitoylation, N-linked glycosylation, and ubiquitination) can regulate signal propagation. Many of the studies outlined in this review suggest that tetraspanins offer a potential therapeutic target to modulate aberrant signal transduction pathways that directly impact a host of cellular behaviors and disease states.

The Dark Side of Cell Fusion

Cell fusion is a physiological cellular process essential for fertilization, viral entry, muscle differentiation and placental development, among others. In this review, we will highlight the different cancer cell-cell fusions and the advantages obtained by these fusions. We will specially focus on the acquisition of metastatic features by cancer cells after fusion with bone marrow-derived cells. The mechanism by which cancer cells fuse with other cells has been poorly studied thus far, but the presence in several cancer cells of syncytin, a trophoblastic fusogen, leads us to a cancer cell fusion mechanism similar to the one used by the trophoblasts. The mechanism by which cancer cells perform the cell fusion could be an interesting target for cancer therapy.

Tetraspanins displayed in retrovirus-derived virus-like particles and their immunogenicity

Virus-like particles (VLPs) are a particular subset of subunit vaccines which are currently explored as safer alternatives to live attenuated or inactivated vaccines. VLPs derived from retrovirus (retroVLPs) are commonly used as scaffolds for vaccine candidates due to their ability to incorporate heterologous envelope proteins. Pseudotyping retroVLPs is however not a selective process therefore, host cellular proteins such as tetraspanins are also included in the membrane. The contribution of these host-proteins to retrovirus immunogenicity remains unclear. In this work, human cells silenced and not silenced for tetraspanin CD81 were used to produce CD81(-) or CD81(+) retroVLPs. We first analyzed mice immune response against human CD81. Despite effective silencing of CD81 in retroVLP producing cells, both humoral and cellular immune responses showed persistent anti-CD81 immunogenicity, suggesting cross reactivity to related antigens. We thus compared the incorporation of related tetraspanins in retroVLPs and showed that decreased CD81 incorporation in CD81(-) retro-VLPs is compensated by an increased incorporation of CD9 and CD63 tetraspanins. These results highlight the dynamic nature of host-derived proteins incorporation in retroVLPs membrane, which should be considered when retrovirus-based biopharmaceuticals are produced in xenogeneic cells.

Apoptosis-induced cancer cell fusion: a mechanism of breast cancer metastasis

Although cancer cell fusion has been suggested as a mechanism of cancer metastasis, the underlying mechanisms defining this process are poorly understood. In a recent study, apoptotic cells were newly identified as a type of cue that induces signaling via phosphatidylserine receptors to promote fusion of myoblasts. The microenvironment of breast tumors is often hypoxic, and because apoptosis is greatly increased in hypoxic conditions, we decided to investigate whether the mechanism of breast cancer cell fusion with mesenchymal stem/multipotent stromal cells (MSCs) involves apoptosis. We used a powerful tool for identification and tracking of hybrids based on bimolecular fluorescence complementation (BiFC) and found that breast cancer cells fused spontaneously with MSCs. This fusion was significantly enhanced with hypoxia and signaling associated with apoptotic cells, especially between nonmetastatic breast cancer cells and MSCs. In addition, the hybrids showed a significantly higher migratory capacity than did the parent cells. Taken together, these findings describe a mechanism by which hypoxia-induced apoptosis stimulates fusion between MSCs and breast tumor cells resulting in hybrids with an enhanced migratory capacity that may enable their dissemination to distant sites or metastases. In the long run, this study may provide new strategies for developing novel drugs for preventing cancer metastasis.

Tetraspanins at a glance

Tetraspanins are a family of proteins with four transmembrane domains that play a role in many aspects of cell biology and physiology; they are also used by several pathogens for infection and regulate cancer progression. Many tetraspanins associate specifically and directly with a limited number of proteins, and also with other tetraspanins, thereby generating a hierarchical network of interactions. Through these interactions, tetraspanins are believed to have a role in cell and membrane compartmentalization. In this Cell Science at a Glance article and the accompanying poster, we describe the basic principles underlying tetraspanin-based assemblies and highlight examples of how tetraspanins regulate the trafficking and function of their partner proteins that are required for the normal development and function of several organs, including, in humans, the eye, the kidney and the immune system.

Sperm-egg fusion: a molecular enigma of mammalian reproduction

The mechanism of gamete fusion remains largely unknown on a molecular level despite its indisputable significance. Only a few of the molecules required for membrane interaction are known, among them IZUMO1, which is present on sperm, tetraspanin CD9, which is present on the egg, and the newly found oolema protein named Juno. A concept of a large multiprotein complex on both membranes forming fusion machinery has recently emerged. The Juno and IZUMO1, up to present, is the only known extracellular receptor pair in the process of fertilization, thus, facilitating the essential binding of gametes. However, neither IZUMO1 nor Juno appears to be the fusogenic protein. At the same time, the tetraspanin is expected to play a role in organizing the egg membrane order and to interact laterally with other factors. This review summarizes, to present, the known molecules involved in the process of sperm-egg fusion. The complexity and expected redundancy of the involved factors makes the process an intricate and still poorly understood mechanism, which is difficult to comprehend in its full distinction.

Critical role of exosomes in sperm-egg fusion and virus-induced cell-cell fusion

In mammals, two integral membrane proteins, sperm IZUMO1 and egg CD9, regulate sperm-egg fusion, and their roles are critical, but yet unclear. Recent studies, however, indicate interesting connections between the sperm-egg fusion and virus-induced cell-cell fusion. First, CD9-containing exosome-like vesicles, which are released from wild-type eggs, can induce the fusion between sperm and CD9-deficient egg, even though CD9-deficient eggs are highly refractory to the fusion with sperm. This finding provides strong evidence for the involvement of CD9-containing, fusion-facilitating vesicles in the sperm-egg fusion. Secondly, there are similarities between the generation of retroviruses in the host cells and the formation of small cellular vesicles, termed exosomes, in mammalian cells. The exosomes are involved in intercellular communication through transfer of proteins and ribonucleic acids (RNAs) including mRNAs and microRNAs. These collective studies provide an insight into the molecular mechanism of membrane fusion events.

Normal viability of Kai1/Cd82 deficient mice

The KAI1/CD82 tetraspanin is a widely expressed cell surface molecule thought to organize diverse cellular signaling processes. KAI1/CD82 suppresses metastasis but not tumorigenicity, establishing it as one of a class of metastasis suppressor genes. In order to further assess its functions, we have characterized the phenotypic properties of Kai1/Cd82 deleted mice, including viability, fertility, lymphocyte composition, blood chemistry and tissue histopathology, and of their wild-type and heterozygote littermates. Interestingly, Kai1/Cd82(-/-) showed no obvious genotype associated defects in any of these processes and displayed no genotype associated histopathologic abnormalities after 12 or 18 months of life. Expression profiles of non-immortal, wild-type and Kai1/Cd82(-/-) mouse embryo fibroblast (MEFs) indicated distinct sex-specific and genotype-specific profiles. These data identify 191 and 1,271 differentially expressed transcripts (by twofold at P < 0.01) based on Kai1/CD82 genotype status in female and male MEFs, respectively. Differentially expressed genes in male MEFs were surprisingly enriched for cell division related processes, suggesting that Kai1/Cd82 may functionally affect these processes. This suggests that Kai/Cd82 has an unappreciated role in the early establishment of proliferation and division when challenged with a new environment that might play a role in adaptability to new metastatic sites.

Tsp66E, the Drosophila KAI1 homologue, and Tsp74F function to regulate ovarian follicle cell and wing development by stabilizing integrin localization

The metastasis suppressor KAI1/CD82 has been implicated in various cellular processes; however, its function in development is not fully understood. Here, we generated and characterized mutants of Tsp66E and Tsp74F, which are Drosophila homologues of KAI1/CD82 and Tspan11, respectively. These mutants exhibited egg elongation defects along with disturbed integrin localization and actin polarity. Moreover, the defects were enhanced by mutation of inflated, an αPS2 integrin gene. Mutant ovaries had elevated αPS2 integrin levels and reduced endocytic trafficking. These results suggest that Drosophila KAI1/CD82 affects the polarized localization and the level of integrin, which may contribute to epithelial cell polarity.

Tetraspanin genes in plants

Tetraspanins represent a four-transmembrane protein superfamily with a conserved structure and amino acid residues that are present in mammals, insects, fungi and plants. Tetraspanins interact with each other or with other membrane proteins to form tetraspanin-enriched microdomains that play important roles in development, pathogenesis and immune responses via facilitating cell-cell adhesion and fusion, ligand binding and intracellular trafficking. Here, we emphasize evolutionary aspects within the plant kingdom based on genomic sequence information. A phylogenetic tree based on 155 tetraspanin genes of 11 plant species revealed ancient and fast evolving clades. Tetraspanins were only present in multicellular plants, were often duplicated in the plant genomes and predicted by the electronic Fluorescent Pictograph for gene expression analysis to be either functionally redundant or divergent. Tetraspanins contain a large extracellular loop with conserved cysteines that provide the binding sites for the interactions. The Arabidopsis thaliana TETRASPANIN1/TORNADO2/EKEKO has a function in leaf and root patterning and TETRASPANIN3 was identified in the plasmodesmatal proteome, suggesting a role in cell-cell communication during plant development.

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.

Functional interplay between tetraspanins and proteases

Several recent publications have described examples of physical and functional interations between tetraspanins and specific membrane proteases belonging to the TM-MMP and α-(ADAMs) and γ-secretases families. Collectively, these examples constitute an emerging body of evidence supporting the notion that tetraspanin-enriched microdomains (TEMs) represent functional platforms for the regulation of key cellular processes including the release of surface protein ectodomains ("shedding"), regulated intramembrane proteolysis ("RIPing") and matrix degradation and assembly. These cellular processes in turn play a crucial role in an array of physiological and pathological phenomena. Thus, TEMs may represent new therapeutical targets that may simultaneously affect the proteolytic activity of different enzymes and their substrates. Agonistic or antagonistic antibodies and blocking soluble peptides corresponding to tetraspanin functional regions may offer new opportunities in the treatment of pathologies such as chronic inflammation, cancer, or Alzheimer's disease. In this review article, we will discuss all these aspects of functional regulation of protease activities by tetraspanins.

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 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 Expression by Human Granulosa Cells and Platelets as a Predictor of Fertilization Success during IVF

Objective. To determine whether CD9 expression on human granulosa cells (GCs) and platelets could predict the success of conventional fertilization of human oocytes during in vitro fertilization (IVF). Methods. Thirty women undergoing IVF for nonmale factor infertility participated. Platelets from venous blood and GCs separated from retrieved oocytes were prepared for immunofluorescence. Flow cytometry quantified the percent of GCs expressing CD9, and CD9 surface density on GCs and platelets. Fertilization rate was determined for the total number of oocytes, and the number of mature oocytes per patient. Correlations tested for significant relationships (P < .05) between fertilization rates and CD9 expression. Results. CD9 surface density on human GCs is inversely correlated with fertilization rate of oocytes (P = .04), but the relationship was weak. Conclusion. More studies are needed to determine if CD9 expression on GCs would be useful for predicting conventional fertilization success during IVF.

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.

Novel partners of SPAG11B isoform D in the human male reproductive tract

Human sperm-associated antigen 11 (SPAG11) is closely related to beta-defensins in structure, expression, and function. Like the beta-defensins, SPAG11 proteins are predominantly expressed in the male reproductive tract, where their best-known major roles are in innate host defense and reproduction. Although several hypotheses have emerged to describe the evolution of beta-defensin and SPAG11 multifunctionality, few describe these multiple functions in terms of defensin interactions with specific proteins. To gain insight into the protein interaction potentials of SPAG11 and the signaling pathways that SPAG11 may influence, we used a yeast two-hybrid screening of a human testis-epididymis library. The results reveal human SPAG11B isoform D (SPAG11B/D) interactions with tryptase alpha/beta 1 (TPSAB1), tetraspanin 7 (TSPAN7), and attractin (ATRN). These interactions were confirmed by coimmunoprecipitation and glutathione S-transferase affinity matrix binding. SPAG11B/D and the three interacting proteins are expressed in the proximal epididymis, and all function in immunity and fertility pathways. We analyzed the functional consequences of SPAG11B/D interaction with TPSAB1 and showed that SPAG11B/D is both a substrate and a potent inhibitor of TPSAB1 activity. Furthermore, we show that (like SPAG11B/D) TSPAN7 and ATRN are associated with spermatozoa.

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.

Tetraspanins and vascular functions

Tetraspanins are multiple membrane-spanning proteins that likely function as the organizers of membrane microdomains. Tetraspanins associate with other membrane-bound molecules such as cell-adhesion proteins, growth factor receptors, and Ig superfamily members and regulate key cellular processes such as adhesion, migration, and fusion. Tetraspanins are widely expressed in vascular and haematopoietic cells and are involved in both physiological and pathological processes related to angiogenesis, vascular injury, thrombosis, and haemostasis. A wide body of evidence suggests that tetraspanins directly regulate the development and functions of the vascular system and the pathogenesis of vascular diseases. This article reviews current understanding of the roles of tetraspanins in vascular functions.

Tetraspanin proteins regulate membrane type-1 matrix metalloproteinase-dependent pericellular proteolysis

Membrane type-1 matrix metalloproteinase (MT1-MMP) supports tumor cell invasion through extracellular matrix barriers containing fibrin, collagen, fibronectin, and other proteins. Here, we show that simultaneous knockdown of two or three members of the tetraspanin family (CD9, CD81, and TSPAN12) markedly decreases MT1-MMP proteolytic functions in cancer cells. Affected functions include fibronectin proteolysis, invasion and growth in three-dimensional fibrin and collagen gels, and MMP-2 activation. Tetraspanin proteins (CD9, CD81, and TSPAN2) selectively coimmunoprecipitate and colocalize with MT1-MMP. Although tetraspanins do not affect the initial biosynthesis of MT1-MMP, they do protect the newly synthesized protein from lysosomal degradation and support its delivery to the cell surface. Interfering with MT1-MMP-tetraspanin collaboration may be a useful therapeutic approach to limit cancer cell invasion and metastasis.

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.