Calmegin is required for fertilin alpha/beta heterodimerization and sperm fertility.

Loss of the endoplasmic reticulum resident chaperone calmegin leads to the production of sterile sperm that do not bind to the egg zona pellucida (M. Ikawa et al., 1997, Nature 387, 607-611). In the present study, we demonstrate that calmegin -/- sperm were defective in migrating into the oviducts and in binding to the egg plasma membrane. Despite the impaired adhesive function, calmegin -/- sperm could fertilize eggs when zonae pellucidae were partially dissected, and eggs fertilized in this manner could develop normally to term. Since these sperm characteristics were similar to those found in fertilin beta -/- sperm, we investigated the interaction of calmegin with fertilin beta. Using immunoprecipitation techniques, calmegin was found to bind to sperm membrane proteins, fertilin alpha and beta, during spermatogenesis. The binding was specific to calmegin: another endoplasmic reticulum chaperone calnexin, a calmegin homologue, was not able to bind to fertilin alpha and beta. In the calmegin -/- mice, a loss of heterodimerization of fertilin alpha and beta was observed and fertilin beta was not detectable in mature sperm. The data not only explain why the calmegin and fertilin beta knockout mouse lines share a common infertile phenotype, but also reveal the importance of the maturation of sperm membrane proteins in the endoplasmic reticulum.     

受精蛋白β在人精子表面的免疫组织化学定位

Fertilin is a kind of sperm plasma membrane protein that mimics snake venom protein. It belongs to the ADAMs family of surface proteins that contain a disintegrin and a metalloprotease domain. Fertilin functions in the sperm-egg binding process by connecting the sperm to the egg plasma membrane via a binding site in the disintegrin domain of fertilin beta (HF93). Its localization on the sperm is in the change. In this study, the monoclonal antibody against human fertilin beta was prepared and used to analyze the localization of fertilin beta on capacitated and acrosome-reacted sperm by immunofluorescence and immunoelectron microscopy techniques. The results were as follows: (1) fertilin beta became restricted to the anterior head during the course of capacitation. (2) During the course of acrosome reaction, the expression and localization of fertilin beta changed immensely on the anterior head and restricted to the lateral of posterior head at last. The restrictions of fertilin beta to the anterior head of capacitated sperm of human beings indicated that fertilin beta may be involved in the binding the sperm to the epithelial cells of the oviduct; the restrictions of fertilin beta to the posterior head domain of acrosome-reacted sperm implied its function in sperm-egg binding and fusion.     

A Role for the Disintegrin Domain of Cyritestin, a Sperm Surface Protein Belonging to the ADAM Family, in Mouse Sperm–Egg Plasma Membrane Adhesion and Fusion

Sperm–egg plasma membrane fusion is preceded by sperm adhesion to the egg plasma membrane. Cell–cell adhesion frequently involves multiple adhesion molecules on the adhering cells. One sperm surface protein with a role in sperm–egg plasma membrane adhesion is fertilin, a transmembrane heterodimer (α and β subunits). Fertilin α and β are the first identified members of a new family of membrane proteins that each has the following domains: pro-, metalloprotease, disintegrin, cysteine-rich, EGF-like, transmembrane, and cytoplasmic domain. This protein family has been named ADAM because all members contain a disintegrin and metalloprotease domain. Previous studies indicate that the disintegrin domain of fertilin β functions in sperm–egg adhesion leading to fusion. Full length cDNA clones have been isolated for five ADAMs expressed in mouse testis: fertilin α, fertilin β, cyritestin, ADAM 4, and ADAM 5. The presence of the disintegrin domain, a known integrin ligand, suggests that like fertilin β, other testis ADAMs could be involved in sperm adhesion to the egg membrane. We tested peptide mimetics from the predicted binding sites in the disintegrin domains of the five testis-expressed ADAMs in a sperm–egg plasma membrane adhesion and fusion assay. The active site peptide from cyritestin strongly inhibited (80–90%) sperm adhesion and fusion and was a more potent inhibitor than the fertilin β active site peptide. Antibodies generated against the active site region of either cyritestin or fertilin β also strongly inhibited (80–90%) both sperm–egg adhesion and fusion. Characterization of these two ADAM family members showed that they are both processed during sperm maturation and present on mature sperm. Indirect immunofluorescence on live, acrosome-reacted sperm using antibodies against either cyritestin or fertilin β showed staining of the equatorial region, a region of the sperm membrane that participates in the early steps of membrane fusion. Collectively, these data indicate that a second ADAM family member, cyritestin, functions with fertilin β in sperm–egg plasma membrane adhesion leading to fusion.     

Peptides corresponding to the epidermal growth factor-like domain of mouse fertilin: synthesis and biological activity

A key step leading to fertilization is the binding of sperm to the egg plasma membrane. When a mammalian sperm reaches the egg plasma membrane, fertilin, an extracellular sperm membrane protein, is believed to bind to an egg plasma membrane receptor mediating fusion. Fertilin is composed of two subunits, and each subunit contains several domains, i.e., metalloprotease, disintegrin, epidermal growth factor (EGF)-like and fusion domains. This investigation examined the role of the EGF-like domains of mouse fertilin alpha and fertilin beta. Peptides corresponding to the N-terminal subdomain, containing four cysteines, and the C-terminal subdomain, containing two cysteines, were synthesized by solid-phase synthesis methods. Disulfide bonds were formed regioselectively according to the canonical EGF-like disulfide pattern. The activity of these peptides and their linear counterparts were tested for activity in a mouse in vitro fertilization assay. One peptide, 4a, corresponding to the cystine-constrained N-terminal subdomain of fertilin beta, had an activating effect on fertilization. The fertilization rate (number of eggs fertilized), fertilization index (number of sperm fused per egg), and level of polyspermy (three or more sperm fused per egg) increased in the presence of 500 microM 4a (12, 56, and 190%, respectively). Its linear counterpart, 4b, had no effect on in vitro fertilization. These data suggest that the EGF-like domain of fertilin beta has a function in sperm-egg binding and fusion. Previously, it has been shown that the fertilin beta disintegrin domain has a role in sperm-egg binding. Considered together, these studies suggest that fertilin is a modular, multidomain protein with more than one mechanism of action. This modularity may be used to design inhibitors of fertilin-receptor interactions that have high specificities for the fertilization process.     

Analysis of fertilin alpha (ADAM1)-mediated sperm-egg cell adhesion during fertilization and identification of an adhesion-mediating sequence in the disintegrin-like domain

Fertilin alpha (also known as ADAM1) is a member of the ADAM (A disintegrin and A metalloprotease domain) family of proteins. In this study, we examine the mechanism of mouse fertilin alpha's in adhesion of sperm to the egg plasma membrane during fertilization. We find that recombinant forms of fertilin alpha corresponding to either the disintegrin-like domain or the cysteine-rich domain and the EGF-like repeat can perturb sperm-egg binding, suggesting that both of these domains can participate in fertilin alpha-mediated adhesion events. In further examination of the fertilin alpha disintegrin-like domain, we find that a subdomain of disintegrin-like domain with the sequence DLEECDCG outside the putative disintegrin loop but with homology to the fertilin beta disintegrin loop can inhibit the binding of both sperm and recombinant fertilin alpha to eggs, suggesting that this is an adhesion-mediating motif of the fertilin alpha disintegrin-like domain. This sequence also inhibits the binding of recombinant fertilin beta to eggs and thus is the first peptide sequence found to block two different sperm ligands. Finally, a monoclonal antibody to the tetraspanin protein CD9, KMC.8, inhibited the binding of recombinant fertilin alpha to eggs in one type of binding assay, suggesting that, under certain conditions, fertilin alpha may interact with a KMC.8-sensitive binding site on the egg plasma membrane.     

Actin-plasma membrane associations in mouse eggs and oocytes

Using rhodamine-phalloidin stained preparations and extracted specimens labeled with heavy meromyosin or run on polyacrylamide gels, actin-plasma membrane associations in mouse mature eggs at the second metaphase of meiosis and oocytes at meiotic prophase have been examined. Cortices of extracted oocytes possessed numerous actin filaments that emanated from the plasma membrane delimiting regions between microvilli and from microvillar apices. The membrane anchorage sites of actin filaments were marked by an electron dense material on the inner leaflet of the plasma membrane. The free ends of filaments emanating from the plasma membrane of oocytes intermeshed to form a dense, cortical layer. With meiotic maturation, changes in the organization of cortical actin were first noted approximately 3 hr after the chromosomes had become localized at the oocyte's periphery. Fewer and shorter actin filaments, which did not form a well-defined layer as in oocytes, were connected with electron-dense material to the inner leaflet of the plasma membrane of extracted egg cortices in regions other than that associated with the meiotic spindle. Cortical actin adjacent to the meiotic spindle, however, was organized into a dense, cresentic aggregation in which clusters of filaments emanated from electron-dense regions associated with both the inner and outer leaflets of the plasma membrane. These observations indicate that mouse oocyte maturation not only involves changes in the distribution of cortical actin but also local alterations in the association of actin with the plasma membrane.     

Filamentous actin organization in the unfertilized sea urchin egg cortex

We have investigated the organization of filamentous actin in the cortex of unfertilized eggs of the sea urchins Strongylocentrotus purpuratus and Lytechinus variegatus. Rhodamine phalloidin and anti-actin immunofluorescent staining of isolated cortices reveal a punctate pattern of fluorescent sources. Comparison of this pattern with SEM images of microvillar morphology and distribution indicates that filamentous actin in the cortex is predominantly localized in the microvilli. Thin-section TEM and quick-freeze deep-etch ultrastructure of isolated cortices demonstrates that this microvillar-associated actin is in a novel organizational state composed of very short filaments arranged in a tight network and that these filament networks form mounds that extend beyond the plane of the plasma membrane. Actin filaments within the networks do not exhibit free ends and make end-on attachments with the membrane only within the region of the evaginating microvilli. Myosin S-1 dissociable crosslinks, 2-3 nm in diameter, are observed between network filaments and between network filaments and the membrane. A second population of long, individual actin filaments is observed in close lateral association with the plasma membrane and frequently complexes with the microvillar actin networks. The filamentous actin of the unfertilized egg cortex may participate in establishing the mechanical properties of the egg surface and may function in nucleating the assembly of cortical actin following fertilization.     

Sperm from the calmegin-deficient mouse have normal abilities for binding and fusion to the egg plasma membrane

Calmegin is a putative testis-specific molecular chaperone required for the heterodimerization of fertilin alpha/beta and the appearance of fertilin beta on the sperm surface. Calmegin-deficient mice are almost completely sterile. The cause of the sterility initially was considered to be impaired abilities in sperm/zona pellucida (ZP) and sperm/egg plasma membrane (EPM) binding, and in the ascension of sperm to the oviduct, phenotypes similar to those seen in sperm from fertilin beta-deficient animals. We have developed a new method in which eggs were prepared without any detectable ZP3 on their surfaces by using a piezo-driven micromanipulator. Using these eggs and sperm containing the green fluorescent protein in their acrosomes, which can distinguish acrosome-intact from acrosome-reacted sperm, the binding and fusing abilities of calmegin-deficient sperm were reexamined. Under these conditions, acrosome-reacted sperm retained their ability to bind to and fuse with the EPM. The reduction in EPM binding of sperm from the calmegin(-/-) animals was apparently due to the artifactual binding of large numbers of acrosome-intact sperm from calmegin(+/-) mice to ZP remnants remaining on the EPM prepared with acidic Tyrode's solution. Thus, the sperm defect in calmegin-null animals is not at the level of sperm-EPM binding but rather may involve either sperm-ZP binding and/or sperm transit to the oviduct. Because fertilin beta is absent from calmegin-deficient mice, these results also suggest that the role of fertilin beta in sperm-EPM interaction needs to be reevaluated.     

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

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.     

The coelomic envelope of Xenopus laevis eggs: a quick-freeze, deep-etch analysis

The extracellular matrix of Xenopus laevis oocytes was analyzed before and after meiotic maturation using quick-freeze, deep-etch, rotary-shadow electron microscopy. The perivitelline space (PS) of the meiotically immature oocyte contains a filamentous network which connects microvilli (MV) and follicle cell macrovilli to the folded oocyte surface below. The envelope overlying the PS is composed of bundles of large fibers which course between the tips of the MV. Spaces between these bundles contain smaller fibrils which secure the egg envelope to the microvillar tips. Meiotic maturation is accompanied by flattening of the oocyte plasma membrane, formation of an orderly array of MV, and elevation of the egg envelope. In the coelomic eggs, the reorganized envelope is composed of loosely bundled large fibers which course above the microvillar tips rather than between them. The spaces between these bundles contain small fibers similar to those seen in the meiotically immature oocyte. This reorganized envelope, however, will not bind sperm; further modifications must transpire during passage through the oviduct to render it sperm receptive.     

Role of MAP kinase and myosin light chain kinase in chromosome-induced development of mouse egg polarity

During maturation, the mouse oocyte is transformed into a highly polarized egg, characterized by an actin cap and cortical granule-free domain (CGFD) overlying the meiotic spindle that is in close proximity to the cortex. The presence of spindle/chromosomes or microinjected sperm chromatin in the cortical region initiates this cortical reorganization, but the pathway is unknown. We report that cortical reorganization induced by microinjected sperm chromatin is blocked by inhibitors of microfilament assembly or disassembly. Active mitogen-activated protein kinase (MAPK), which becomes enriched in the region of sperm chromatin, is required for cortical reorganization, because microinjected sperm chromatin fails to induce cortical reorganization in Mos-/- eggs, which lack MAPK activity. Last, myosin light chain kinase (MLCK), which can be directly phosphorylated and activated by MAPK, appears involved, because the MLCK inhibitors ML-7 and Peptide 18 prevent sperm chromatin-induced cortical reorganization. These results provide new insights into how cortical reorganization occurs independently of extracellular signals to generate egg polarity.     

Guinea pig fertilin exhibits restricted lateral mobility in epididymal sperm and becomes freely diffusing during capacitation

The guinea pig sperm protein fertilin functions in sperm-egg plasma membrane binding. Fertilin is initially present in the plasma membrane of the whole head in testicular sperm, then becomes concentrated into the posterior head domain during epididymal passage. Fertilin remains localized to the posterior head plasma membrane following the acrosome reaction, when it functions in sperm-egg interaction. Fluorescence redistribution after photobleaching was used to examine the lateral mobility of fertilin in both acrosome-intact and acrosome-reacted sperm. Fertilin exhibited highly restricted lateral mobility in both testicular and epididymal sperm (D < 10(-10) cm(2)/s). However, fertilin in acrosome-reacted sperm was highly mobile within the membrane bilayer (D = 1.8 x 10(-9) cm(2)/s and %R = 84). Measurement of the lateral mobility of fertilin in capacitated, acrosome-intact sperm revealed two populations of cells. In approximately one-half of the cells, lateral mobility of fertilin was similar to sperm freshly isolated from the cauda epididymis; while in the other half fertilin was highly mobile. The release of fertilin from interactions that restrict its lateral mobility may regulate its function in sperm-egg interaction.     

Analysis of mouse fertilin in wild-type and fertilin beta(-/-) sperm: evidence for C-terminal modification, alpha/beta dimerization, and lack of essential role of fertilin alpha in sperm-egg fusion

The sperm surface protein fertilin functions in sperm-egg interaction. On guinea pig and bovine sperm, fertilin is a heterodimer of alpha and beta subunits. Both subunits are initially synthesized as precursors and then proteolytically processed by removing N-terminal domains. Since the mouse is currently the main mammalian species in which fertilization is studied, in the present report, we analyzed the structure, processing, and expression of fertilin in mouse. We found that the processing of mouse fertilin beta occurs during epididymal maturation and involves changes in the cytoplasmic tail domain as well as the N-terminal domains. Although we (R. Yuan et al., 1997, J. Cell Biol. 137, 105-112) and others (M. S. Chen et al., 1999, J. Cell Biol. 144, 549-561) have previously reported that mature fertilin beta is 55-57 kDa, here we show that 55 kDa is an unrelated protein in the sperm extract which cross-reacts with an antibody that recognizes precursor, but not mature, fertilin beta. Comparison of Western blots of wild-type and fertilin beta knockout sperm revealed that authentic, mature fertilin beta is 45 kDa. We also obtained direct evidence that mouse fertilin alpha and beta exist as a heterodimer. In addition, we found that in mice lacking the fertilin beta subunit, fertilin alpha is absent from mature sperm. A widely proposed model for sperm-egg fusion suggests that fertilin alpha is the sperm component that promotes membrane fusion by undergoing a conformational change that exposes a virus-like, hydrophobic fusion peptide. Because sperm lacking fertilin alpha and fertilin beta can fuse with eggs at 50% the wild-type rate, this model is called into question. The results suggest instead that other gamete surface molecules act to promote membrane fusion and that fertilin's role in gamete fusion is in sperm-egg plasma membrane adhesion.     

Differential localization of conventional protein kinase C isoforms during mouse oocyte development

Protein kinase C (PKC), the major cell target for tumor-promoting phorbol esters, plays a central role in signal transduction pathways. In many biological systems where Ca(2+) serves as a second messenger, regulatory control is mediated by PKC. The activation of PKC depends on its binding to RACK1 receptor, which is an intracellular protein anchor for activated PKC. We demonstrate that the conventional PKC (cPKC) isoforms, PKC-alpha, PKC-betaI, and PKC-betaII, as well as RACK1, are expressed in mouse oocytes (germinal vesicle [GV]) and mature eggs (metaphase II [MII]). In GV oocytes, PKC-alpha, PKC-betaII, and RACK1 were uniformly distributed in the cytoplasm, while PKC-betaI was localized in the cytoplasm and in the plasma membrane as well. Treatment of GV oocytes with the biologically active phorbol ester, 12-o-tetradecanoyl phorbol-13-acetate (TPA), resulted in a rapid translocation of the cytosolic PKC-alpha, but not PKC-betaI, PKC-betaII, or RACK1, to the plasma membrane. This was associated with inhibition of GV breakdown. In MII eggs (17 h post-hCG), PKC-alpha was uniformly distributed in the cytoplasm while PKC-betaI and -betaII were distributed in the cytoplasm and in the plasma membrane as well. Treatment with TPA resulted in a rapid translocation of PKC-alpha from the cytoplasm to the plasma membrane and a significant decrease of PKC-betaI throughout the cytoplasm, while it also remained in the cell periphery. No change in the distribution of PKC-betaII or RACK1 was observed. TPA also induced pronucleus formation. Physiological activation of MII eggs by sperm induced cortical granule exocytosis associated with significant translocation of PKC-alpha and -betaI, but not -betaII, to the plasma membrane. Overall, these results suggest a possible involvement of cPKC isoforms in the mechanism of mouse oocyte maturation and egg activation.     

绵羊fertilin β基因编码区的钓取与结构分析

Fertilin β与精卵的结合和融合有密切关系。为探讨fertilin β蛋白在绵羊受精过程中的作用机理, 采用RACE技术, 首次钓取了该基因的编码区。结果绵羊fertilin β基因的编码区cDNA全长为2,217 bp。同源性分析显示, 绵羊的fertilin β氨基酸序列与牛、猪和人的fertilin β具有79.4%、66.7%和58.1%的同源性。系统发育分析表明, 绵羊fertilin β与牛属于同一分支, 并且也显示了绵羊和牛分类地位最近, 这和传统的分类一致。Fertilin β蛋白结构域分析显示, 绵羊fertilin β去整合素识别序列为TDE, 与牛的序列相同。除了上述三肽序列外, 紧随X-D/E-E的ECD保守序列, 从而形成了X-D/E-ECD五肽保守序列, 在绵羊fertilin β中该五肽序列为TDECE。     

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

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

Positive selection within sperm-egg adhesion domains of fertilin: an ADAM gene with a potential role in fertilization

Genes with a role in fertilization show a common pattern of rapid evolution. The role played by positive selection versus lack of selective constraints has been more difficult to establish. One problem arises from attempts to detect selection in an overall gene sequence analysis. I have analyzed the pattern of molecular evolution of fertilin, a gene coding for a heterodimeric sperm protein belonging to the ADAM (A disintegrin and A metalloprotease) gene family. A nonsynonymous to synonymous rate ratio (d(N)/d(S)) analysis for different protein domains of fertilin alpha and fertilin beta showed d(N)/d(S) < 1, suggesting that purifying selection has shaped fertilin's evolution. However, an analysis of the distribution of single positively selected codon sites using phylogentic analysis by maximum likelihood (PAML) showed sites within adhesion domains (disintegrin and cysteine-rich) of fertilin beta evolving under positive selection. The region 3' to the EGF-like domain of fertilin alpha, where the transmembrane and cytoplasmic tail regions are supposed to be localized, showed higher d(N) and d(S) than any other fertilin alpha region. However, it was not possible to identify positively selected codon sites due to ambiguous alignments of the carboxy-end region (ClustalX vs. DiAlign2). When this region was excluded from the PAML analysis, most single positively selected codon sites were concentrated within adhesion domains (cysteine-rich and EGF-like). The use of an ancestral sequence prior to a recent duplication event of fertilin alpha among non-Hominidae primates (Macaca, Papio, and Saguinus) revealed that the duplication is partially responsible for masking the detection of positively selected sites within the disintegrin domain. Finally, most ADAM genes with a potential role in sperm maturation and/or fertilization showed significantly higher d(N) estimates than other ADAM genes.     

Changes in the stainability and sulfhydryl level in the sperm nucleus during sperm-oocyte interaction in mice

During maturation in the epididymis, mouse sperm nuclei become difficult to stain with Giemsa and its component basic dyes. Mature sperm from the cauda epididymis can be stained only after DTT treatment. Stainable sperm such as those from the testis accumulate ³H NEM when examined by autoradiography, while unstainable sperm do not, indicating a close correlation between the basic dye binding capacity and SH levels in the sperm nuclei. During insemination of zonafree ovarian oocytes with a germinal vesicle (GV), mature sperm nuclei become stainable and capable of binding with ³H NEM. At the same time, sperm have established pronase-resistant contact with the oocyte. Similarly, sperm nuclei become stainable during fertilization when the sperm attachment to the egg becomes pronase resistant. However, these changes occur before sperm chromatin decondensation begins. Therefore, it is suggested that S-S bonds in sperm nucleoproteins are reduced when the sperm establish a stable contact with the egg plasma membrane, thus reversing sperm maturational changes. The reduction of S-S bonds may be a prerequisite for sperm chromatin decondensation.     

Possible function of the ADAM1a/ADAM2 Fertilin complex in the appearance of ADAM3 on the sperm surface

In mouse, two different isoforms of ADAM1 (fertilin alpha), ADAM1a and ADAM1b, are produced in the testis. ADAM1a is localized within the endoplasmic reticulum of testicular germ cells, whereas epididymal sperm contain only ADAM1b on the plasma membrane. In this study, we show that the loss of ADAM1a results in the male infertility because of the severely impaired ability of sperm to migrate from the uterus into the oviduct through the uterotubal junction. However, epididymal sperm of ADAM1a-deficient mice were capable of fertilizing cumulus-intact, zona pellucida-intact eggs in vitro despite the delayed dispersal of cumulus cells and the reduced adhesion/binding to the zona pellucida. Among testis (sperm)-specific proteins examined, only the level of ADAM3 (cyritestin) was strongly reduced in ADAM1a-deficient mouse sperm. Moreover, the appearance of ADAM3 on the sperm surface was dependent on the formation of a fertilin protein complex between ADAM1a and ADAM2 (fertilin beta) in testicular germ cells, although no direct interaction between the fertilin complex and ADAM3 was found. These results suggest that ADAM1a/ADAM2 fertilin may be implicated in the selective transport of specific sperm proteins including ADAM3 from the endoplasmic reticulum of testicular germ cells onto the cell surface. These proteins then can participate in sperm migration into the oviduct, the dispersal of cumulus cells, and sperm binding to the zona pellucida.     

Fertilin beta and other ADAMs as integrin ligands: insights into cell adhesion and fertilization.

One of the most important cell-cell interactions is that of the sperm with the egg. This interaction, which begins with cell adhesion and culminates with membrane fusion, is mediated by multiple molecules on the gametes. One of the best-characterized of these molecules is fertilin beta, a ligand on mammalian sperm and one of the first ADAMs (A Disintegrin and A Metalloprotease domain) to be identified. Fertilin beta (also known as ADAM2) participates in sperm-egg membrane binding, and it has long been hypothesized that this function is achieved through the interaction of the disintegrin domain of fertilin beta with an integrin on the egg surface. There are now approximately 30 members of the ADAM family and, to date, five different ADAMs (fertilin beta, ADAM9, ADAM12, ADAM15, ADAM23) have been described to interact with integrins (specifically alpha(6)beta(1), alpha(v)beta(3), alpha(9)beta(1), alpha(v)beta(5), and/or alpha(5)beta(1)). This field will be discussed with respect to what is known about specific ADAMs and the integrins with which they interact, and what the implications are for sperm-egg interactions and for integrin function. These data will also be discussed in the context of recent knockout studies, which show that eggs lacking the alpha(6) integrin subunit can be fertilized, and eggs lacking the integrin-associated tetraspanin protein CD9 fail to fertilize. Key issues in cell adhesion that pertain to gametes and fertilization will also be highlighted.