Using a murine monoclonal antibody, Andrews et al. (1981) identified a chromosome 12-determined surface antigen. The antigenic determinant was found to be carried by a cell surface glycoprotein which is reported to be involved in cell adhesion and migration. CD9 is a tetraspan protein that associates with several pi integrins, including alpha 6 beta 1. Alpha 6 beta 1 is present on murine eggs and interacts with the sperm-surface glycoprotein ADAM 2 (fertilin beta). Furthermore, findings of Hirano et al. (1999) indicate that CD9 is a differentiation-related molecule present in the extravillous trophoblasts.
Recently, CD9 was shown to be associated with beta(1)-related integrins.
Ellerman DA,et al 2003 reported the direct binding of the ligand PSG17 to CD9 requires a CD9 site essential for sperm-egg fusion.
The function currently attributed to tetraspanins is to organize molecular complexes in the plasma membrane by using multiple cis-interactions. Additionally, the tetraspanin CD9 may be a receptor that binds the soluble ligand PSG17, a member of the immunoglobulin superfamily (IgSF)/CEA subfamily. However, previous data are also consistent with the PSG17 receptor being a CD9 cis-associated protein. In the current study, CD9 extracellular loop (EC2) specifically bound to PSG17-coated beads, indicating a direct interaction between the two proteins. However, CD9-EC2 did not bind to PSG17-coated beads if the CD9-EC2 had the mutation SFQ (173-175) to AAA, a previously studied mutation in egg CD9 that abolishes sperm-egg fusion. Also, PSG17 bound to 293 T cells transfected with wild-type CD9 but not the mutant CD9. By immunofluorescence, PSG17 bound to wild-type eggs but not to CD9 null eggs. The presence of approximately 2 microM recombinant PSG17 produced a significant and reversible inhibition (60-80%) of sperm-egg fusion. Thus, we conclude that CD9 is a receptor for PSG17 and when the PSG17 binding site is mutated or occupied, sperm-egg fusion is impaired. These findings suggest that egg CD9 may function in gamete fusion by binding to a sperm IgSF/CEA subfamily member and such proteins have previously been identified on sperm.
NCBI Summary:
This gene encodes a member of the transmembrane 4 superfamily, also known as the tetraspanin family. Tetraspanins are cell surface glycoproteins with four transmembrane domains that form multimeric complexes with other cell surface proteins. The encoded protein functions in many cellular processes including differentiation, adhesion, and signal transduction, and expression of this gene plays a critical role in the suppression of cancer cell motility and metastasis. [provided by RefSeq, Jan 2011]
General function
Cell adhesion molecule
Comment
Cellular localization
Plasma membrane
Comment
Ovarian function
Luteinization, Early embryo development
Comment
Unveiling a novel function of CD9 in surface compartmentalization of oocytes. Inoue N et al. (2020) 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 critical role in gamete fusion. Particularly, 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 critical 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 and Mouse Oocyte Microvilli Related to Fertilizing Ability. Benammar A et al. (2016) Our electron microscopy observations demonstrate for the first time that the number of microvilli on the mice oocyte membrane decreases when meiosis progresses from prophase I to metaphase II (MII) stage, and the morphology of the microvilli also changes. Microvilli are significantly shorter and larger on the ovulated oocyte membrane than at the previous stages. Although clathrin vesicles clearly disappear during oocyte maturation, exosome-like vesicles begin to be secreted at the metaphase I stage, more strongly at the MII stage. Multivesicular bodies are visible only at the MII stage. Since several oocyte tetraspanins are involved in the gamete interaction, Cd9 being congregated on the MII oocyte microvilli, we analyzed the effect of tetraspanin deletion on oocyte membrane morphology. The Cd9(-/-) and Cd9(-/-) Cd81(-/-) deletions are associated with a decreased microvilli density on the MII oocyte surface. Microvilli thickness is significantly increased whatever the deleted tetraspanin gene be. Only Cd9 deletion clearly disturbs the vesicular traffic, increasing the number of clathrin and exosome vesicles. Additional investigations are necessary to elucidate how tetraspanins modulate the microvilli morphology, likely in relation with cytoskeleton. The role of oocyte exosomes in gamete adhesion/fusion remains to be further studied.//////////////////
Tetraspanin CD9 in Bovine Oocytes and its Role in Fertilization. Zhou GB et al. 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.
Takao et al. (1999) showed that proteins purified from granulosa cells by immunoaffinity chromatography using anti-integrin alpha(6) monoclonal antibodies contained CD9 as well as integrin beta(1). They suggest that CD9 is a differentiation-related molecule of granulosa and theca cells and that it is associated with integrin alpha(6)beta(1) on the cell surface of GC. Thus, CD9 is implicated in the function of human GC in cooperation with integrin alpha(6)beta(1).
Expression regulated by
LH
Comment
Gene expression increased. Luteinization of porcine preovulatory follicles leads to systematic changes in follicular gene expression. Agca C et al. The LH surge initiates the luteinization of preovulatory follicles and causes hormonal and structural changes that ultimately lead to ovulation and the formation of corpora lutea. The objective of the study was to examine gene expression in ovarian follicles (n = 11) collected from pigs (Sus scrofa domestica) approaching estrus (estrogenic preovulatory follicle; n = 6 follicles from two sows) and in ovarian follicles collected from pigs on the second day of estrus (preovulatory follicles that were luteinized but had not ovulated; n = 5 follicles from two sows). The follicular status within each follicle was confirmed by follicular fluid analyses of estradiol and progesterone ratios. Microarrays were made from expressed sequence tags that were isolated from cDNA libraries of porcine ovary. Gene expression was measured by hybridization of fluorescently labeled cDNA (preovulatory estrogenic or -luteinized) to the microarray. Microarray analyses detected 107 and 43 genes whose expression was decreased or increased (respectively) during the transition from preovulatory estrogenic to -luteinized (P<0.01). Cells within preovulatory estrogenic follicles had a gene-expression profile of proliferative and metabolically active cells that were responding to oxidative stress. Cells within preovulatory luteinized follicles had a gene-expression profile of nonproliferative and migratory cells with angiogenic properties. Approximately, 40% of the discovered genes had unknown function.
Ovarian localization
Oocyte, Granulosa, Theca, Luteal cells
Comment
CD9 Expression by Human Granulosa Cells and Platelets as a Predictor of Fertilization Success during IVF. Jaslow CR et al. 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.
Chen et al. (1999) reported that CD9 is present on the plasma membrane of oocytes in the ovary as well as on eggs isolated from the oviduct. The anti-CD9 mAb, JF9, potently inhibits sperm-egg binding and fusion in vitro. Alpha 6A and CD8 both are expressed on the apical epithelial surface at the uterine-oviduct junction. These findings correlate
with the observation that fertilin beta ''knockout'' sperm traverse the uterus but do not progress into the oviduct, contributing to the infertility of
fertilin beta(-/-) male mice.
Maintenance of pluripotency and self-renewal ability of mouse embryonic stem cells in the absence of tetraspanin CD9. Akutsu H et al. We have previously demonstrated that the tetraspanin CD9 is necessary for membrane fusion between sperm and oocyte during fertilization. While knockout mice for CD9 are viable, CD9(-/-) females are sterile due to the inability of their oocytes to fuse with sperm. While CD9 is not essential for subsequent development, a role in embryonic stem (ES) cell self-renewal was hypothesised on the basis of two observations: CD9 is highly expressed in murine and human ES cells and the CD9-blocking antibody inhibits mouse ES cell colony formation and survival. To investigate whether CD9 has a direct effect on ES cells, we generated and characterised several CD9 knockout murine ES cell lines. These CD9(-/-) ES cell lines exhibited equivalent morphology and growth properties to wild-type ES cells. Furthermore, the CD9(-/-) ES cell lines also displayed similar expression of pluripotency factors Oct3/4, Sox2 and Nanog. CD9(-/-) ES cells were found to be pluripotent in vivo, as their cells injected into immunocompromised mice gave rise to teratomas consisting of tissues representative of all three germ layers. Additionally several high contribution mouse chimeras were generated by blastocyst injection with several CD9(-/-) ES cell lines. Taken together, our results reveal that CD9 is dispensable for mouse ES cell self-renewal and pluripotency. The generation of CD9(-/-) ES cells should prove to be a useful tool with which to study the function of this protein and a range of other associated cellular processes.
Oocyte CD9 is enriched on the microvillar membrane and required for normal microvillar shape and distribution. Runge KE et al. Microvilli are found on the surface of many cell types, including the mammalian oocyte, where they are thought to act in initial contact of sperm and oocyte plasma membranes. CD9 is currently the only oocyte protein known to be required for sperm-oocyte fusion. We found CD9 is localized to the oocyte microvillar membrane using transmission electron microscopy (TEM). Scanning electron microscopy (SEM) showed that CD9 null oocytes, which are unable to fuse with sperm, have an altered length, thickness and density of their microvilli. One aspect of this change in morphology was quantified using TEM by measuring the radius of curvature at the microvillar tips. A small radius of curvature is thought to promote fusibility and the radius of curvature of microvillar tips on CD9 wild-type oocytes was found to be half that of the CD9 null oocytes. We found that oocyte CD9 co-immunoprecipitates with two Ig superfamily cis partners, EWI-2 and EWI-F, which could have a role in linking CD9 to the oocyte microvillar actin core. We also examined latrunculin B-treated oocytes, which are known to have reduced fusion ability, and found altered microvillar morphology by SEM and TEM. Our data suggest that microvilli may participate in sperm-oocyte fusion. Microvilli could act as a platform to concentrate adhesion/fusion proteins and/or provide a membrane protrusion with a low radius of curvature. They may also have a dynamic interaction with the sperm that serves to capture the sperm cell and bring it into close contact with the oocyte plasma membrane.
Expression of CD9 in frozen-thawed mouse oocytes: preliminary experience. Wen Y et al. CD9 mRNA and protein expression levels in mouse slow frozen-rapid thawed oocytes were compared with those in fresh oocytes by using comparative quantitative real time reverse transcription-PCR and semiquantitative Western blot, respectively. The expression levels of both CD9 mRNA and protein in the frozen oocytes were significantly lower than those found in the fresh oocytes.
Follicle stages
Antral, Preovulatory, Corpus luteum
Comment
Molecular profiling of polycystic ovaries for markers of cell invasion and matrix turnover Oksjoki S, et al .
OBJECTIVE: To study gene expression profiles of connective tissue components in polycystic ovaries using complementary deoxyribonucleic acid (cDNA) array technology. DESIGN: Descriptive study of normal and polycystic human ovarian biopsy samples analyzed by cDNA array hybridizations. SETTING: Experimental laboratory research. PATIENT(S): Eight women with polycystic ovary syndrome (PCOS) and two normally cycling women treated with electrocauterization and hysterectomy, respectively. INTERVENTIONS: Ovarian biopsy samples. MAIN OUTCOME MEASURE(S): Expression levels of 588 genes involved in cellular invasion, extracellular matrix (ECM) turnover, and cell-ECM interactions in polycystic ovaries. RESULT(S): A majority of the 30 genes down-regulated in PCOS ovaries represented those related to cell adhesion and motility, as well as angiogenesis, followed by regulators of cell cycle and growth. The 14 up-regulated genes represented those regulating cell fate and development, growth factors, cytokines, chemokines, and cell-cell interactions. Of the 44 transcripts exhibiting marked changes in the cDNA array analysis, only one - proliferating cell nuclear antigen messenger ribonucleic acid (PCNA mRNA) - was systematically down-regulated; 2 transcripts, for CDC27HS protein and CD9 antigen, were down-regulated in 7 out of 8 PCOS samples.
Phenotypes
Mutations
2 mutations
Species: mouse
Mutation name: None
type: null mutation fertility: subfertile Comment: Mice lacking CD9 were produced by homologous recombination by Miyado et al . Both male
and female CD9 null mice were born healthy and grew normally. However, the litter size
from CD9 null females was less than 2% of that of the wild type. In vitro fertilization
experiments indicated that the cause of this infertility was due to the failure of sperm-egg fusion. When sperm were injected
into oocytes with assisted microfertilization techniques, however, the fertilized eggs developed to term. These results
indicate that CD9 has a crucial role in sperm-egg fusion.
Le Naour et al reported that knockout mice lacking CD9 were created to evaluate the physiological importance of CD9. CD9 null females displayed a severe reduction of fertility. Oocytes were ovulated but were not
successfully fertilized because sperm did not fuse with the oocytes from CD9/ females. Thus, CD9 appears to be essential
for sperm-egg fusion, a process involving the CD9-associated integrin alpha 6 beta 1.
Species: mouse
Mutation name: type: null mutation fertility: subfertile Comment: Role of tetraspanin CD9 molecule in fertilization of mammals. Jankovičová J et al. (2014) Fertilization process is a very clever and unique process comprising some essential steps resulting in formation of zygote. Tetraspanin CD9 is considered to be a serious candidate molecule participating in these events. The importance of CD9 has been discussed in relation to acrosome reaction, sperm-binding, sperm-penetration, sperm-egg fusion and eventually, egg activation. The abundant expression of CD9 oocyte plasma membrane and the presence of CD9-containing vesicles in the perivitelline space of intact oocytes have been confirmed. Despite the fact that majority of authors analyzed CD9 expressed on oocytes, several studies considered the function of sperm CD9, too. To understand CD9 involvement, various conditions of in vitro fertilization (IVF) assays using polyclonal as well as monoclonal antibodies or knockout mice were carried out. However, ambiguous data have been obtained about the importance of CD9 in sperm-egg binding or fusion. Although the current findings did not prove any hypothesis, the indispensable role of CD9 in fertilization process was not excluded and the precise role of CD9 remains unexplained.//////////////////