Sperm proteins SOF1, TMEM95, and SPACA6 are required for sperm-oocyte fusion in mice

Transmembrane protein 95 as a promising molecular marker of ram sperm functionality

The optimization of preservation protocols (refrigeration and freezing) in ovine species is necessary for a wider diffusion of artificial insemination in this species. Besides the ram sperm quality assays, the characterization of novel proteins could be crucial for improving these protocols employing biomarkers. The protein transmembrane 95 (TMEM95) is a sperm membrane protein associated with oocyte-sperm fusion previously described in bull or mouse. However, this protein has not yet been characterized in the ram until now. In this work, different experimental groups based on sperm functionality: capacitated, refrigerated at different times (5 °C 24 h, 5 °C 48 h, and 5 °C 72 h), and frozen-thawed sperm samples were analyzed and compared to initial sperm quality samples (15 °C 3 h) to characterize the expression of this novel protein and its relationship with other sperm quality markers (motility, kinetic parameters, viability, apoptosis-like events, mitochondrial function, acrosome-reacted, zinc content as marker of capacitation). In addition, capacitation status was tested by Fluozin-3, a novel fluorescent probe measuring zinc content used for the first time in ram sperm. After capacitation induction, as expected, acrosome reactive spermatozoa and zinc signature 2 and 3 were significantly increased, while linearity was significantly (P < 0.05) decreased compared to non-capacitated samples. Concerning TMEM95, its profile was significantly (P < 0.05) increased after the capacitation process, confirming its relationship with this spermatozoa status. Attending to preservation processes, as expected, semen quality decreased progressively during liquid storage, and a significant (P < 0.05) decrease was observed at 24 h according to fast progressive motility and linearity. TMEM95 profile showed the same decrease tendency, showing a significant reduction (P < 0.05) at 48 h with respect to the control samples. Finally, after the cryopreservation process, the semen quality of the thawed samples suffered a detrimental effect compared to the initial control sample, concerting all studied parameters accomplished by a significant (P < 0.05) decrease in TMEM95 profile compared to initial control samples. When we analyzed the TMEM95 correlation with other sperm quality markers, the highest positive correlations observed were with low sperm quality parameters in capacitated samples, such as apoptosis-like changes and acrosome-reaction. On the other hand, the highest positive correlations observed between TMEM95 and sperm quality parameters in preservation process samples were observed with suitable sperm quality parameters (motility, viability, and mitochondrial functionality). According to our results, this novel protein could be considered a predictor of early damage in ram sperm preservation protocols (cooling and freezing), considering its relationship with capacitation and membrane integrity status.

The role of sperm protein in mammal fertilization: insights into gamete adhesion, membrane fusion and oocyte activation

Globally, numerous infertile couples have been assisted by extensive research on mammalian fertilization and the rapid development of Assisted Reproductive Technology (ART). However, 5%-15% of the couples that are selected for in vitro fertilization (IVF) experience a total fertilization failure (TFF), where no zygotes develop despite oocytes and semen parameters appear to be normal. Notably, an essential early event in fertilization is the binding of spermatozoa to the oocyte's external envelope, which followed by the spermatozoa-oocyte fusion. Meanwhile, oocyte activation is a crucial cellular process necessary to block polyspermy and start the development of the zygote. Improper membrane fusion of gametes has been demonstrated to be one of the mechanisms of TFF. Moreover, considering the large amount of research on sperm proteins in recent years, thus in this review, we characterize the role and molecular mechanisms of sperm proteins in the three key processes of gamete adhesion and fusion and oocyte activation, which would provide a comprehensive understanding of the role of sperm proteins in fertilization in mammals and a favourable reference for future studies in assisted reproduction due to FF.

Conservation and divergence of the molecular regulators of the vertebrate fertilization synapse

Fertilization - the process during which sperm and egg find each other, bind and eventually fuse - marks the beginning of a new individual. Research over the past years in vertebrates has shed new light on conserved and divergent molecular regulators that mediate the formation of the fertilization synapse, the close apposition of the two plasma membranes before fusion. Here, we review the known proteins that are required for sperm-egg interaction in mammals and fish from a phylogenetic perspective. While some sperm factors are only conserved in vertebrates and share phylogenetic and structural features, others have a longer evolutionary history. In contrast, the egg factors have changed even within vertebrates despite recognizing the preserved sperm machinery. Future functional work on these factors will be essential to understand the fusion mechanism of vertebrate sperm and egg.

Noncanonical phagocytosis-like SEAL establishes mammalian fertilization

In many forms of sexual reproduction, only the most robust spermatozoa, which overcome multiple physiological challenges, reach the oocyte. However, the exact mechanisms of gamete recognition and fusion are unknown. In the present study, we demonstrated that with the onset of gamete recognition, oocyte microvilli form lamellipodium-like structures, activate actin polymerization, and subsequently engulf spermatozoa to initiate gamete fusion. Gamete fusion occurred via a phagocytosis-like process we termed "sperm engulfment activated by IZUMO1-JUNO linkage and gamete fusion-related factors" (SEAL). Gamete adhesion was strictly regulated by binding of sperm IZUMO1 to oocyte JUNO, while SEAL was primarily mediated by sperm DCST1/2, SPACA6, TMEM95, FIMP, and TMEM81, the essential factors for gamete fusion. Interestingly, JUNO was almost depleted from oocyte surfaces in the region where SEAL enveloped spermatozoa by microvilli without actin polymerization. SEAL formation was recapitulated using JUNO-expressing K562 lymphocytic cells rather than oocytes. Together, these findings suggest that dynamic rearrangement of membrane components facilitates SEAL prior to successful fertilization.

Allosteric inhibition of the IZUMO1-JUNO fertilization complex by the naturally occurring antisperm antibody OBF13

Sperm IZUMO1 binds to egg JUNO, and this interaction is essential for mammalian fertilization. Isolated from a female mouse immunized with syngeneic sperm, the antisperm antibody OBF13 recognizes IZUMO1 and inhibits murine fertilization. How OBF13 interferes with sperm-egg interactions was unknown. Here, we present the X-ray crystal structure of IZUMO1 in complex with OBF13. OBF13 binds to the apex of the four-helix domain of IZUMO1, distant from the JUNO-binding site. Our crystal structure of OBF13-bound IZUMO1 resembles apo-IZUMO1 and differs from the structure of IZUMO1 in complex with JUNO. We identify that OBF13 carries a low level of somatic hypermutation, and through deep mutational scanning, we engineer an affinity-enhanced OBF13 variant. This OBF13 variant single-chain fragment variable decreases the apparent affinity of IZUMO1 for membrane-bound murine JUNO and blocks the binding of acrosome-reacted sperm to eggs, thereby preventing fertilization. We propose allostery between the OBF13 epitope and the JUNO-binding site. OBF13 inhibits a conformational change in IZUMO1, preventing fusion-competent sperm from adhering to murine eggs during fertilization. Surprisingly, murine IZUMO1 binds to hamster JUNO with an affinity ~20-fold higher than to murine JUNO. The decreased affinity caused by OBF13 of murine IZUMO1 for hamster JUNO is sufficient for murine sperm to bind to and fuse with hamster eggs. Our studies provide a structural and mechanistic framework for species-specific, allosteric inhibition of IZUMO1 by a naturally occurring antisperm antibody and offer insights into the development of immunocontraceptives.

Fertilization mechanisms in hermaphroditic ascidians and nematodes: Common mechanisms with mammals and plants

Most animals have male and female, whereas flowering plants are hermaphrodites. Exceptionally, a small population of invertebrates, including ascidians and nematodes, has hermaphrodite in reproductive strategies. Several ascidians exhibit strict self-sterility (or self-incompatibility), similar to flowering plants. Such a self-incompatibility mechanism in ascidian has been revealed to be very similar to those of flowering plants. Here, we describe the mechanisms of ascidian fertilization shared with invertebrates and mammals, as well as with plants. In the nematode Caenorhabditis elegans, having self-fertile hermaphrodite and male, several genes responsible for fertilization are homologous to those of mammals. Thus, novel proteins responsible for fertilization will be easily disclosed by the analyses of sterile mutants. In this review, we focus on the same or similar reproductive strategies by shedding lights on the common mechanisms of fertilization, particularly in hermaphrodites.

The mammalian egg's zona pellucida, fertilization, and fertility

The zona pellucida (ZP) is a relatively thick extracellular matrix (ECM) that surrounds all mammalian eggs and plays vital roles during oogenesis, fertilization, and preimplantation development. The ZP is a semi-permeable, viscous ECM that consists of three or four glycosylated proteins, called ZP1-4, that differ from proteoglycans and proteins of somatic cell ECM. Mammalian ZP proteins are encoded by single-copy genes on different chromosomes and synthesized and secreted by growing oocytes arrested in meiosis. Secreted ZP proteins assemble in the extracellular space into long fibrils that are crosslinked polymers of ZP proteins and exhibit a structural repeat. Several regions of nascent ZP proteins, the signal-sequence, ZP domain, internal and external hydrophobic patches, transmembrane domain, and consensus furin cleavage-site regulate secretion and assembly of the proteins. The ZP domain is required for assembly of ZP fibrils, as well as for assembly of other kinds of ZP domain-containing proteins. ZP proteins adopt immunoglobulin (Ig)-like folds that resemble C- and V-type Ig-like domains, but represent new immunoglobulin-superfamily subtype structures. Interference with synthesis, processing, or secretion of ZP proteins by either gene-targeting in mice or mutations in human ZP genes can result in failure to assemble a ZP and female infertility. ZP2 and ZP3 must be present to assemble a ZP during oocyte growth and both serve as receptors for binding of free-swimming sperm to ovulated eggs. Acrosome-reacted sperm bind to ZP2 polypeptide by inner-acrosomal membrane and acrosome-intact sperm bind to ZP3 oligosaccharides by plasma membrane overlying the sperm head. Binding of acrosome-intact sperm to ZP3 induces them to undergo cellular exocytosis, the acrosome reaction. Only acrosome-reacted sperm can penetrate the ZP, bind to, and then fuse with the egg's plasma membrane to produce a zygote. Following sperm-egg fusion (fertilization) the ZP undergoes structural and functional changes (zona reaction) induced by cortical granule components (cortical reaction) deposited into the ZP. The latter include zinc and ovastacin, a metalloendoprotease that cleaves ZP2 near its amino-terminus and hardens the egg's ZP. The changes prevent penetration of bound sperm through and binding of supernumerary sperm to the ZP of fertilized eggs as part of a secondary or slow block to polyspermy. Therefore, ZP proteins act as structural proteins and sperm receptors, and help to prevent fertilization by more than one sperm. Here we review some of this information and provide details about several key features of ZP proteins, ZP matrix, and mammalian fertilization.

Rat copper transport protein 2 (CTR2) is involved in fertilization through interaction with IZUMO1 and JUNO

In mammalian reproduction, testis-specific protein IZUMO1 and its receptor JUNO on the oocyte surface are essential for sperm-oocyte recognition, binding, and membrane fusion. However, these factors alone are insufficient to accomplish cytoplasmic membrane fusion. It is believed that other gametic proteins interact with them to facilitate sperm-oocyte interaction on the head and mid-tail of rat spermatozoa as well as on the surface of oocytes. In this study, Copper Transport Protein 2 (CTR2) has been identified on the head and mid-tail of rat spermatozoa as well as on the surface of oocytes. CTR2 directly interacts with both IZUMO1 and JUNO, colocalizing with IZUMO1 on the sperm head and with JUNO on the oocyte membrane. Treatment of the capacitated sperm and zona pellucida-free oocytes with anti-CTR2 antibody resulted in a significant decrease in fertilization rates in IVF experiments. These findings suggest that CTR2 plays an important role in mammalian fertilization by interacting with IZUMO1 and JUNO, providing new insights into the molecular mechanisms of mammalian sperm-oocyte adhesion and fusion.

Decoding the Genes Orchestrating Egg and Sperm Fusion Reactions and Their Roles in Fertility

Mammalian fertilization is a complex and highly regulated process that has garnered significant attention, particularly with advancements in assisted reproductive technologies such as in vitro fertilization (IVF). The fusion of egg and sperm involves a sequence of molecular and cellular events, including capacitation, the acrosome reaction, adhesion, and membrane fusion. Critical genetic factors, such as IZUMO1, JUNO (also known as FOLR4), CD9, and several others, have been identified as essential mediators in sperm-egg recognition and membrane fusion. Additionally, glycoproteins such as ZP3 within the zona pellucida are crucial for sperm binding and triggering the acrosome reaction. Recent gene-editing technologies, such as CRISPR/Cas9 and conditional knockout models, have facilitated the functional annotation of genes such as SPAM1 and ADAM family members, further elucidating their roles in capacitation and adhesion. Furthermore, the integration of CRISPR-Cas9 with omics technologies, including transcriptomics, proteomics, and lipidomics, has unlocked new avenues for identifying previously unknown genetic players and pathways involved in fertilization. For instance, transcriptomics can uncover gene expression profiles during gamete maturation, while proteomics identifies key protein interactions critical for processes such as capacitation and the acrosome reaction. Lipidomics adds another dimension by revealing how membrane composition influences gamete fusion. Together, these tools enable the discovery of novel genes, pathways, and molecular mechanisms involved in fertility, providing insights that were previously unattainable. These approaches not only deepen our molecular understanding of fertility mechanisms but also hold promise for refining diagnostic tools and therapeutic interventions for infertility. This review summarizes the current molecular insights into genes orchestrating fertilization and highlights cutting-edge methodologies that propel the field toward novel discoveries. By integrating these findings, this review aims to provide valuable knowledge for clinicians, researchers, and technologists in the field of reproductive biology and assisted reproductive technologies.

A conserved fertilization complex bridges sperm and egg in vertebrates

Fertilization, the basis for sexual reproduction, culminates in the binding and fusion of sperm and egg. Although several proteins are known to be crucial for this process in vertebrates, the molecular mechanisms remain poorly understood. Using an AlphaFold-Multimer screen, we identified the protein Tmem81 as part of a conserved trimeric sperm complex with the essential fertilization factors Izumo1 and Spaca6. We demonstrate that Tmem81 is essential for male fertility in zebrafish and mice. In line with trimer formation, we show that Izumo1, Spaca6, and Tmem81 interact in zebrafish sperm and that the human orthologs interact in vitro. Notably, complex formation creates the binding site for the egg fertilization factor Bouncer in zebrafish. Together, our work presents a comprehensive model for fertilization across vertebrates, where a conserved sperm complex binds to divergent egg proteins-Bouncer in fish and JUNO in mammals-to mediate sperm-egg interaction.

Reorganization of the flagellum scaffolding induces a sperm standstill during fertilization

Mammalian sperm delve into the female reproductive tract to fertilize the female gamete. The available information about how sperm regulate their motility during the final journey to the fertilization site is extremely limited. In this work, we investigated the structural and functional changes in the sperm flagellum after acrosomal exocytosis (AE) and during the interaction with the eggs. The evidence demonstrates that the double helix actin network surrounding the mitochondrial sheath of the midpiece undergoes structural changes prior to the motility cessation. This structural modification is accompanied by a decrease in diameter of the midpiece and is driven by intracellular calcium changes that occur concomitant with a reorganization of the actin helicoidal cortex. Midpiece contraction occurs in a subset of cells that undergo AE, and live-cell imaging during in vitro fertilization showed that the midpiece contraction is required for motility cessation after fusion is initiated. These findings provide the first evidence of the F-actin network's role in regulating sperm motility, adapting its function to meet specific cellular requirements during fertilization, and highlighting the broader significance of understanding sperm motility.

Eighteen genes primarily expressed in the testis are not required for male fertility in mice†

There are approximately 20 000 protein-coding genes in humans and mice. More than 1000 of these genes are predominantly expressed in the testis or are testis-specific and thought to play an important role in male reproduction. Through the production of gene knockout mouse models and phenotypic evaluations, many genes essential for spermatogenesis, sperm maturation, and fertilization have been discovered, greatly contributing to the elucidation of their molecular mechanisms. On the other hand, there are many cases in which single-gene knockout models do not affect fertility, indicating that tissue-specific genes are not always critical. Here, we selected 18 genes whose mRNA expression is restricted to the testis or higher than in other tissues, but whose function in male reproduction is unknown. We then created single-gene KO mouse models using the CRISPR/Cas9 system. The established KO males were subjected to mating tests and screened for effects on fecundity, revealing that these genes were not essential for spermatogenesis and male fertility. This knowledge will contribute to understanding the functions of genes characteristic of the testis and identify the cause of male infertility.

CALR3 defects disrupt sperm-zona pellucida binding in humans: new insights into male factor fertilization failure and relevant clinical therapeutic approaches

STUDY QUESTION

Do biallelic deleterious variants of Calreticulin 3 (CALR3) cause fertilization failure (FF), resulting in male infertility in humans?

SUMMARY ANSWER

Biallelic mutations in CALR3 were identified in two infertile men from unrelated families and were shown to cause FF associated with failed sperm-zona pellucida (ZP) binding.

WHAT IS KNOWN ALREADY

In male mice, the Calr3-knockout has been reported to cause male infertility and FF. However, the mechanism behind this remains unclear in humans.

STUDY DESIGN, SIZE, DURATION

Sequencing studies were conducted in a research hospital on samples from Han Chinese families with primary infertility and sperm head deformations to identify the underlying genetic causes.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Data from two infertile probands characterized by sperm head deformation were collected through in silico analysis. Sperm cells from the probands were characterized using light and electron microscopy and used to verify the pathogenicity of genetic factors through functional assays. Subzonal insemination (SUZI) and IVF assays were performed to determine the exact pathogenesis of FF. ICSI were administered to overcome CALR3-affected male infertility.

MAIN RESULTS AND THE ROLE OF CHANCE

Novel biallelic deleterious mutations in CALR3 were identified in two infertile men from unrelated families. We found one homozygous frameshift CALR3 mutation (M1: c.17_27del, p.V6Gfs*34) and one compound heterozygous CALR3 mutation (M2: c.943A>G, p.N315D; M3: c.544T>C, p.Y182H). These mutations are rare in the general population and cause acrosomal ultrastructural defects in affected sperm. Furthermore, spermatozoa from patients harbouring the CALR3 mutations were unable to bind to the sperm-ZP or they disrupted gamete fusion or prevented oocyte activation. Molecular assays have revealed that CALR3 is crucial for the maturation of the ZP binding protein in humans. Notably, the successful fertilization via SUZI and ICSI attempts for two patients, as well as the normal expression of PLCζ in the mutant sperm, suggests that ICSI is an optimal treatment for CALR3-deficient FF.

LIMITATIONS, REASONS FOR CAUTION

The results are based on sperm-related findings from two patients. Further studies are required to gain insight into the developmental stage and function of CALR3 in human testis.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings highlight the underlying risk of FF associated with sperm defects and provide a valuable reference for personalized genetic counselling and clinical treatment of these patients.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the National Key R&D Program of China (2021YFC2700901), Hefei Comprehensive National Science Center Medical-Industrial Integration Medical Equipment Innovation Research Platform Project (4801001202), the National Natural Science Foundation of China (82201803, 82371621, 82271639), Foundation of the Education Department of Anhui Province (gxgwfx2022007), Key Project of Natural Science Research of Anhui Educational Committee (2023AH053287), and the Clinical Medical Research Transformation Project of Anhui Province (202204295107020037). The authors declare no competing interests.

TRIAL REGISTRATION NUMBER

N/A.

Selective regulation of aspartyl intramembrane protease activity by calnexin

Signal peptide peptidase-like 2c (SPPL2c) is a testis-specific aspartyl intramembrane protease that contributes to male gamete function both by catalytic and non-proteolytic mechanisms. Here, we provide an unbiased characterisation of the in vivo interactome of SPPL2c identifying the ER chaperone calnexin as novel binding partner of this enzyme. Recruitment of calnexin specifically required the N-glycosylation within the N-terminal protease-associated domain of SPPL2c. Importantly, mutation of the single glycosylation site of SPPL2c or loss of calnexin expression completely prevented SPPL2c-mediated intramembrane proteolysis of all tested substrates. By contrast and despite rather promiscuous binding of calnexin to other SPP/SPPL proteases, expression of the chaperone was exclusively required for SPPL2c-mediated proteolysis. Despite some impact on the stability of SPPL2c most presumably due to assistance in folding of the luminal domain of the protease, calnexin appeared to be recruited rather constitutively to the protease thereby boosting its catalytic activity. In summary, we describe a novel, highly specific mode of intramembrane protease regulation, highlighting the need to systematically approach control mechanisms governing the proteolytic activity of other members of the aspartyl intramembrane protease family.

Update on the research on the antigens of anti-sperm antibodies over the last decade

This review summarizes the advancements over a decade of research on antigens of anti-sperm antibodies (ASAs), which are key to male immune infertility. Despite the progress in assisted reproductive technologies, understanding the roles and mechanisms of ASAs and their antigens remains vital for immune infertility management. We conducted a comprehensive literature search on PubMed from January 2013 to December 2023 using the following keywords: "anti-sperm antibody," "sperm antigen," and "immune infertility." In this review, we focus on the discoveries in sperm antigen identification and characterization through proteomics, gene disruption technology, and immunoinformatics, along with the development of fertility biomarkers. Here, we discuss the clinical applications of improved ASA detection methods and the progress in the development of immunocontraceptive vaccines. The intersection of advanced diagnostic techniques and vaccine development represents a promising frontier in reproductive health. The findings also highlight the need for standardized ASA detection methods and a comprehensive molecular-level approach to understanding ASA-related infertility. These insights underscore the significance of ongoing reproductive immunology research in enhancing clinical fertility outcomes and contraceptive vaccine development.

Induction of acrosome reaction by 4-Br-A23187 alters the glycoproteomic profile of boar spermatozoa

Protein glycosylation is a post-translational modification involved in wide range of biological processes. In mammalian spermatozoa this modification has been identified in numerous proteins, and membrane glycoproteins are involved in the fertilization process. The objective of the present study was to identify changes in protein glycosylation after acrosome reaction (AR) induction using the 4-Br-A23187 ionophore. Our results showed that treatment with 10 μM of 4-Br-A23187 for 20 min significantly increased the percentage of live acrosome-reacted spermatozoa compared to the control (69.8 ± 0.8 vs. 6.4 ± 0.5; mean % ± SEM, respectively). Also, we observed an increase in 32 kDa tyrosine-phosphorylated protein (p32) and a decrease in serine/threonine phosphorylation of the protein kinase A substrates (phospho-PKA-substrates) after ionophore treatment. Furthermore, changes in glycosylated proteins following AR induction were analyzed using different HRP-conjugated lectins (GNA, DSA, and SNA), revealing changes in mannose and sialic acid residues. Proteomic analysis of isolated proteins using GNA lectin revealed that 50 proteins exhibited significantly different abundance (q-value < 0.01). Subsequent analysis using Uniprot database identified 39 downregulated and 11 upregulated proteins in the presence of 4-Br-A23187. Notably, six of these proteins were classified as transmembrane proteins, namely LRRC37A/B like protein 1 C-terminal domain-containing protein, Membrane metalloendopeptidase like 1, VWFA domain-containing protein, Syndecan, Membrane spanning 4-domains A14 and Serine protease 54. This study shows a novel protocol to induce acrosome reaction in boar spermatozoa and identifies new transmembrane proteins containing mannose residues. Further work is needed to elucidate the role of these proteins in sperm-oocyte fusion.

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.

Expression of IZUMO2 in colorectal cancer in association with clinicopathological features

IZUMO2 belongs to the testis-expressed IZUMO family of proteins, which are characterized by an N-terminal IZUMO domain. Based on integrated analysis of expression profiles and matched DNA methylation data from a public database, IZUMO2 represents a prognosis-related methylation-driven gene in colorectal cancer. However, it remains unclear whether IZUMO2 protein expression is suppressed or overexpressed in colorectal cancer cells. In this study, we aimed to elucidate the expression of the IZUMO2 protein in colorectal cancer, with a focus on the clinicopathological features. Sixty-four colorectal cancer tissue specimens were immunohistochemically stained using specific antibodies against IZUMO2. IZUMO2 immunoreactivity was detected at the invasion front in 30 of the 64 colorectal cancer samples. Kaplan-Meier analysis demonstrated that patients with IZUMO2 immunoreactivity had a relatively shorter overall and progression-free survival (log-rank test, P = 0.046 and 0.019, respectively). IZUMO2 immunoreactivity served as an independent factor predictive of poor progression-free survival in colorectal cancer (P = 0.025) as determined via the Cox proportional hazard regression model. Moreover, IZUMO2 immunoreactivity represented an independent factor for poor overall survival (P = 0.035) and progression-free survival (P = 0.013) in patients with colon cancer. The present findings suggest that IZUMO2 is expressed in many colorectal cancers, especially at the cancer invasion front, and may represent an indicator of poor prognosis in colorectal cancer.

ZP2 cleavage blocks polyspermy by modulating the architecture of the egg coat

Following the fertilization of an egg by a single sperm, the egg coat or zona pellucida (ZP) hardens and polyspermy is irreversibly blocked. These events are associated with the cleavage of the N-terminal region (NTR) of glycoprotein ZP2, a major subunit of ZP filaments. ZP2 processing is thought to inactivate sperm binding to the ZP, but its molecular consequences and connection with ZP hardening are unknown. Biochemical and structural studies show that cleavage of ZP2 triggers its oligomerization. Moreover, the structure of a native vertebrate egg coat filament, combined with AlphaFold predictions of human ZP polymers, reveals that two protofilaments consisting of type I (ZP3) and type II (ZP1/ZP2/ZP4) components interlock into a left-handed double helix from which the NTRs of type II subunits protrude. Together, these data suggest that oligomerization of cleaved ZP2 NTRs extensively cross-links ZP filaments, rigidifying the egg coat and making it physically impenetrable to sperm.

Prothioconazole exposure disrupts oocyte maturation and fertilization by inducing mitochondrial dysfunction and apoptosis in mice

Prothioconazole (PTC), a novel broad-spectrum triazole fungicide, has attracted widespread concern due to its wide use and toxicological effects on non-target organisms. However, little is known about the impact of PTC on oocyte quality and female fertility, especially on oocyte maturation and fertilization. In the present study, we reported that PTC exposure affects the oocyte developmental competence and oocyte fertilization ability to weaken female fertility. Firstly, PTC compromises oocyte development ability by disrupting spindle morphology and chromosome alignment, as well as decreasing acetylation level of α-tubulin and disrupting kinetochore-microtubule attachments. In addition, PTC compromises oocyte fertilization ability by weakening the sperm binding ability and impairing the dynamics of Juno, Cortical granule and Ovastacin. Finally, single-cell transcriptome analysis revealed that PTC exposure has potentially toxic effects on oocyte development and fertilization, which is caused by the mitochondrial dysfunction and the occurrence of oxidative stress and apoptosis. In summary, our results indicated that PTC exposure had potentially toxic effects on female fertility and led to poor oocyte quality in female mice.

No evidence for a direct extracellular interaction between human Fc receptor-like 3 (MAIA) and the sperm ligand IZUMO1

Fertilization involves the recognition and fusion of sperm and egg to form a previously unidentified organism. In mammals, surface molecules on the sperm and egg have central roles, and while adhesion is mediated by the IZUMO1-JUNO sperm-egg ligand-receptor pair, the molecule/s responsible for membrane fusion remain mysterious. Recently, MAIA/FCRL3 was identified as a mammalian egg receptor, which bound IZUMO1 and JUNO and might therefore have a bridging role in gamete recognition and fusion. Here, we use sensitive assays designed to detect extracellular protein binding to investigate the interactions between MAIA and both IZUMO1 and JUNO. Despite using reagents with demonstrable biochemical activity, we did not identify any direct binding between MAIA/FCRL3 and either IZUMO1 or JUNO. We also observed no fusogenic activity of MAIA/FCRL3 in a cell-based membrane fusion assay. Our findings encourage caution in further investigations on the role played by MAIA/FCRL3 in fertilization.

Sperm induction of somatic cell-cell fusion as a novel functional test

The fusion of mammalian gametes requires the interaction between IZUMO1 on the sperm and JUNO on the oocyte. We have recently shown that ectopic expression of mouse IZUMO1 induces cell-cell fusion and that sperm can fuse to fibroblasts expressing JUNO. Here, we found that the incubation of mouse sperm with hamster fibroblasts or human epithelial cells in culture induces the fusion between these somatic cells and the formation of syncytia, a pattern previously observed with some animal viruses. This sperm-induced cell-cell fusion requires a species-matching JUNO on both fusing cells, can be blocked by an antibody against IZUMO1, and does not rely on the synthesis of new proteins. The fusion is dependent on the sperm's fusogenic capacity, making this a reliable, fast, and simple method for predicting sperm function during the diagnosis of male infertility.

CRISPR/Cas9-mediated genome editing reveals seven testis-enriched transmembrane glycoproteins dispensable for male fertility in mice

BACKGROUND

Mammalian fertilization is mediated by multiple sperm acrosomal proteins, many of which are testis-enriched transmembrane glycoproteins expressed during spermiogenesis (e.g., Izumo sperm-egg fusion 1, Sperm acrosome associated 6, and Transmembrane protein 95).

METHODS

We hypothesized that proteins with these features might have a role in sperm-egg interaction and thus carried out an in-silico screen based on multiple public databases. We generated knockout mouse lines lacking seven candidate proteins by the CRISPR/Cas9 system and conducted detailed analyses on the fecundity of the knockout males, as well as their testis appearance and weight, testis and epididymis histology, and sperm motility and morphology.

RESULTS

Through the in-silico screen, we identified 4932438H23Rik, A disintegrin and metalloproteinase domain-containing protein 29, SAYSvFN domain-containing protein 1, Sel-1 suppressor of lin-12-like 2 (C. elegans), Testis-expressed protein 2, Transmembrane and immunoglobulin domain-containing 3, and Zinc and ring finger 4. Phenotypic analyses unveiled that the knockout males showed normal testis gross appearance, normal testis and epididymis histology, and normal sperm morphology and motility. Fertility tests further indicated that the knockout male mice could sire pups with normal litter sizes when paired with wild-type females.

DISCUSSION AND CONCLUSION

These findings suggest that these seven proteins are individually dispensable for male reproduction and fertilization. Future studies are warranted to devise advanced in-silico screening approaches that permit effective identification of gamete fusion-required sperm proteins.

Molecular dynamics of JUNO-IZUMO1 complexation suggests biologically relevant mechanisms in fertilization

JUNO-IZUMO1 binding is the first known physical link created between the sperm and egg membranes in fertilization, however, how this initiates sperm-egg fusion remains elusive. As advanced structural insights will help to combat the infertility crisis, or advance fertility control, we employed all-atom Molecular Dynamics (MD) to derive dynamic structural insights that are difficult to obtain experimentally. We found that the hydrated JUNO-IZUMO1 interface is composed of a large set of short-lived non-covalent interactions. The contact interface is destabilized by strategically located point mutations, as well as by Zn2+ ions, which shift IZUMO1 into the non-binding "boomerang" conformation. We hypothesize that the latter might explain how the transient zinc spark, as released after sperm entry into the oocyte, might contribute to block polyspermy. To address a second mystery, we performed another set of simulations, as it was previously suggested that JUNO in solution is unable to bind to folate despite it belonging to the folate receptor family. MD now suggests that JUNO complexation with IZUMO1 opens up the binding pocket thereby enabling folate insertion. Our MD simulations thus provide crucial new hypotheses how the dynamics of the JUNO-IZUMO1 complex upon solvation might regulate fertility.

Butylparaben weakens female fertility via causing oocyte meiotic arrest and fertilization failure in mice

Butylparaben is an ubiquitous environmental endocrine disruptor, that is commonly used in cosmetics and personal care product due to its anti-microbial properties. Butylparaben has been shown to cause developmental toxicity, endocrine and metabolic disorders and immune diseases. However, little is known about the impact on female fertility, especially oocyte quality. In the present study, we reported that butylparaben influenced female fertility by showing the disturbed oocyte meiotic capacity and fertilization potential. Specifically, butylparaben results in the oocyte maturation arrest by impairing spindle/chromosome structure and microtubule stability. Besides, butylparaben results in fertilization failure by impairing the dynamics of Juno and ovastacin and the sperm binding ability. Last, single-cell transcriptome analysis showed that butylparaben-induced oocyte deterioration was caused by mitochondrial dysfunction, which led to the accumulation of ROS and occurrence of apoptosis. Collectively, our study indicates that mitochondrial dysfunction and redox perturbation is the major cause of the weakened female fertility expoesd to butylparaben.

Adenylate kinase 9 is essential for sperm function and male fertility in mammals

Despite passing routine laboratory tests for semen quality, bulls used in artificial insemination exhibit significant variation in fertility. Routine analysis of fertility data identified a dairy bull with extreme subfertility (10% pregnancy rate). To characterize the subfertility phenotype, a range of in vitro, in vivo, and molecular assays were carried out. Sperm from the subfertile bull exhibited reduced motility and severely reduced caffeine-induced hyperactivation compared to controls. Ability to penetrate the zona pellucida, cleavage rate, cleavage kinetics, and blastocyst yield after IVF or AI were significantly lower than in control bulls. Whole-genome sequencing from semen and RNA sequencing of testis tissue revealed a critical mutation in adenylate kinase 9 (AK9) that impaired splicing, leading to a premature termination codon and a severely truncated protein. Mice deficient in AK9 were generated to further investigate the function of the gene; knockout males were phenotypically indistinguishable from their wild-type littermates but produced immotile sperm that were incapable of normal fertilization. These sperm exhibited numerous abnormalities, including a low ATP concentration and reduced motility. RNA-seq analysis of their testis revealed differential gene expression of components of the axoneme and sperm flagellum as well as steroid metabolic processes. Sperm ultrastructural analysis showed a high percentage of sperm with abnormal flagella. Combined bovine and murine data indicate the essential metabolic role of AK9 in sperm motility and/or hyperactivation, which in turn affects sperm binding and penetration of the zona pellucida. Thus, AK9 has been found to be directly implicated in impaired male fertility in mammals.

SPE-51, a sperm-secreted protein with an immunoglobulin-like domain, is required for fertilization in C. elegans

Fertilization is a fundamental process in sexual reproduction during which gametes fuse to combine their genetic material and start the next generation in their life cycle. Fertilization involves species-specific recognition, adhesion, and fusion between the gametes.1,2 In mammals and other model species, some proteins are known to be required for gamete interactions and have been validated with loss-of-function fertility phenotypes.3,4 Yet, the molecular basis of sperm-egg interaction is not well understood. In a forward genetic screen for fertility mutants in Caenorhabditis elegans, we identified spe-51. Mutant worms make sperm that are unable to fertilize the oocyte but otherwise normal by all available measurements. The spe-51 gene encodes a secreted protein that includes an immunoglobulin (Ig)-like domain and a hydrophobic sequence of amino acids. The SPE-51 protein acts cell autonomously and localizes to the surface of the spermatozoa. We further show that the gene product of the mammalian sperm function gene Sof1 is likewise secreted. This is the first example of a secreted protein required for the interactions between the sperm and egg with genetic validation for a specific function in fertilization in C. elegans (also see spe-365). This is also the first experimental evidence that mammalian SOF1 is secreted. Our analyses of these genes begin to build a paradigm for sperm-secreted or reproductive-tract-secreted proteins that coat the sperm surface and influence their survival, motility, and/or the ability to fertilize the egg.

The EGF-motif-containing protein SPE-36 is a secreted sperm protein required for fertilization in C. elegans

Identified through forward genetics, spe-9 was the first gene to be identified in C. elegans as necessary for fertilization.1 Since then, genetic screens in C. elegans have led to the identification of nine additional sperm genes necessary for fertilization (including spe-51 reported by Mei et al.2 and the spe-36 gene reported here).3,4,5,6,7,8,9 This includes spe-45, which encodes an immunoglobulin-containing protein similar to the mammalian protein IZUMO1, and spe-42 and spe-49, which are homologous to vertebrate DCST2 and DCST1, respectively.4,7,8,10,11,12,13 Mutations in any one of these genes result in healthy adult animals that are sterile. Sperm from these mutants have normal morphology, migrate to and maintain their position at the site of fertilization in the reproductive tract, and make contact with eggs but fail to fertilize the eggs. This same phenotype is observed in mammals lacking Izumo1, Spaca6, Tmem95, Sof1, FIMP, or Dcst1 and Dcst2.10,14,15,16,17,18,19 Here we report the discovery of SPE-36 as a sperm-derived secreted protein that is necessary for fertilization. Mutations in the Caenorhabditis elegans spe-36 gene result in a sperm-specific fertilization defect. Sperm from spe-36 mutants look phenotypically normal, are motile, and can migrate to the site of fertilization. However, sperm that do not produce SPE-36 protein cannot fertilize. Surprisingly, spe-36 encodes a secreted EGF-motif-containing protein that functions cell autonomously. The genetic requirement for secreted sperm-derived proteins for fertilization sheds new light on the complex nature of fertilization and represents a paradigm-shifting discovery in the molecular understanding of fertilization.

A Frame-by-Frame Glance at Membrane Fusion Mechanisms: From Viral Infections to Fertilization

Viral entry and fertilization are distinct biological processes that share a common mechanism: membrane fusion. In viral entry, enveloped viruses attach to the host cell membrane, triggering a series of conformational changes in the viral fusion proteins. This results in the exposure of a hydrophobic fusion peptide, which inserts into the host membrane and brings the viral and host membranes into close proximity. Subsequent structural rearrangements in opposing membranes lead to their fusion. Similarly, membrane fusion occurs when gametes merge during the fertilization process, though the exact mechanism remains unclear. Structural biology has played a pivotal role in elucidating the molecular mechanisms underlying membrane fusion. High-resolution structures of the viral and fertilization fusion-related proteins have provided valuable insights into the conformational changes that occur during this process. Understanding these mechanisms at a molecular level is essential for the development of antiviral therapeutics and tools to influence fertility. In this review, we will highlight the biological importance of membrane fusion and how protein structures have helped visualize both common elements and subtle divergences in the mechanisms behind fusion; in addition, we will examine the new tools that recent advances in structural biology provide researchers interested in a frame-by-frame understanding of membrane fusion.

Analyzing the effect of heparin on in vitro capacitation and spermatozoal RNA population in goats

Heparin is a glycosaminoglycan polymer that is commonly used as an anticoagulant. Heparin also induces in vitro capacitation in spermatozoa, although its molecular mechanism is elusive. This study investigated the effect of heparin on in vitro capacitation and spermatozoal RNA (spRNA) population in goats. Goat spermatozoa were treated with 20 μM heparin for 0-6 h and evaluated for motility, capacitation, acrosome reaction, and spRNA population by RNA sequencing (RNA-seq). It was observed that heparin enhanced sperm motility up to 6 h of incubation (p < 0.05). Heparin also induced capacitation and acrosome reaction within 4 h. RNA-seq identified 1254 differentially expressed genes (DEGs) between heparin-treated and control spermatozoa. Most DEGs (1251 nos.) were upregulated and included 1090 protein-coding genes. A few genes (PRND, ITPR1, LLCFC1, and CHRM2) showed >5-fold increased expression in heparin-treated spermatozoa compared to the control. The upregulated genes were found to be involved in cAMP-PKA, PI3-Akt, calcium, MAPK signaling, and oxidative stress pathways. DCFDA staining confirmed the increased oxidative stress in heparin-treated spermatozoa compared to the control (p < 0.05). In conclusion, the results of the present study suggest that heparin enhances sperm motility and induces capacitation by upregulation of the spRNA population and oxidative stress pathway.

Divergent molecular signatures in fish Bouncer proteins define cross-fertilization boundaries

Molecular compatibility between gametes is a prerequisite for successful fertilization. As long as a sperm and egg can recognize and bind each other via their surface proteins, gamete fusion may occur even between members of separate species, resulting in hybrids that can impact speciation. The egg membrane protein Bouncer confers species specificity to gamete interactions between medaka and zebrafish, preventing their cross-fertilization. Here, we leverage this specificity to uncover distinct amino acid residues and N-glycosylation patterns that differentially influence the function of medaka and zebrafish Bouncer and contribute to cross-species incompatibility. Curiously, in contrast to the specificity observed for medaka and zebrafish Bouncer, seahorse and fugu Bouncer are compatible with both zebrafish and medaka sperm, in line with the pervasive purifying selection that dominates Bouncer's evolution. The Bouncer-sperm interaction is therefore the product of seemingly opposing evolutionary forces that, for some species, restrict fertilization to closely related fish, and for others, allow broad gamete compatibility that enables hybridization.

Deficiency of the Tmem232 Gene Causes Male Infertility with Morphological Abnormalities of the Sperm Flagellum in Mice

The axoneme and accessory structures of flagella are critical for sperm motility and male fertilization. Sperm production needs precise and highly ordered gene expression to initiate and sustain the many cellular processes that result in mature spermatozoa. Here, we identified a testis enriched gene transmembrane protein 232 (Tmem232), which is essential for the structural integrity of the spermatozoa flagella axoneme. Tmem232 knockout mice were generated for in vivo analyses of its functions in spermatogenesis. Phenotypic analysis showed that deletion of Tmem232 in mice causes male-specific infertility. Transmission electron microscopy together with scanning electron microscopy were applied to analyze the spermatozoa flagella and it was observed that the lack of TMEM232 caused failure of the cytoplasm removal and the absence of the 7th outer microtubule doublet with its corresponding outer dense fiber (ODF). Co-IP assays further identified that TMEM232 interacts with ODF family protein ODF1, which is essential to maintain sperm motility. In conclusion, our findings indicate that TMEM232 is a critical protein for male fertility and sperm motility by regulating sperm cytoplasm removal and maintaining axoneme integrity.

Spermatogenic cell-specific SPACA4 is essential for efficient sperm-zona pellucida binding in vitro

Fertilization is a complex and highly regulated process that involves a series of molecular interactions between sperm and oocytes. However, the mechanisms of proteins involved in human fertilization, such as that of testis-specific SPACA4, remain poorly understood. Here we demonstrated that SPACA4 is a spermatogenic cell-specific protein. SPACA4 is expressed during spermatogenesis, upregulated in early-stage spermatids, and downregulated in elongating spermatids. SPACA4 is an intracellular protein that locates in the acrosome and is lost during the acrosome reaction. Incubation with antibodies against SPACA4 inhibited the binding of spermatozoa to zona pellucida. SPACA4 protein expression levels across different semen parameters were similar but varied significantly among patients. A prospective clinical study found no association between SPACA4 protein levels and fertilization or cleavage rates. Thus, the study suggests a novel function for SPACA4 in human fertilization in a non-dose-dependent manner. However, a larger clinical trial is required to evaluate the potential use of sperm SPACA4 protein levels to predict fertilization potential.

Review: Genetic mutations affecting bull fertility

Cattle are a well-suited "model organism" to study the genetic underpinnings of variation in male reproductive performance. The adoption of artificial insemination and genomic prediction in many cattle breeds provide access to microarray-derived genotypes and repeated measurements for semen quality and insemination success in several thousand bulls. Similar-sized mapping cohorts with phenotypes for male fertility are not available for most other species precluding powerful association testing. The repeated measurements of the artificial insemination bulls' semen quality enable the differentiation between transient and biologically relevant trait fluctuations, and thus, are an ideal source of phenotypes for variance components estimation and genome-wide association testing. Genome-wide case-control association testing involving bulls with either aberrant sperm quality or low insemination success revealed several causal recessive loss-of-function alleles underpinning monogenic reproductive disorders. These variants are routinely monitored with customised genotyping arrays in the male selection candidates to avoid the use of subfertile or infertile bulls for artificial insemination and natural service. Genome-wide association studies with quantitative measurements of semen quality and insemination success revealed quantitative trait loci for male fertility, but the underlying causal variants remain largely unknown. Moreover, these loci explain only a small part of the heritability of male fertility. Integrating genome-wide association studies with gene expression and other omics data from male reproductive tissues is required for the fine-mapping of candidate causal variants underlying variation in male reproductive performance in cattle.

ADAD2 functions in spermiogenesis and piRNA biogenesis in mice

BACKGROUND

Adenosine deaminase domain containing 2 (ADAD2) is a testis-specific protein composed of a double-stranded RNA binding domain and a non-catalytic adenosine deaminase domain. A recent study showed that ADAD2 is indispensable for the male reproduction in mice. However, the detailed functions of ADAD2 remain elusive.

OBJECTIVES

This study aimed to investigate the cause of male sterility in Adad2 mutant mice and to understand the molecular functions of ADAD2.

MATERIALS AND METHODS

Adad2 homozygous mutant mouse lines, Adad2-/- and Adad2Δ/Δ , were generated by CRISPR/Cas9. Western blotting and immunohistochemistry were used to reveal the expression and subcellular localization of ADAD2. Co-immunoprecipitation tandem mass spectrometry was employed to determine the ADAD2-interacting proteins in mouse testes. RNA-sequencing analyses were carried out to analyze the transcriptome and PIWI-interacting RNA (piRNA) populations in wildtype and Adad2 mutant testes.

RESULTS

Adad2-/- and Adad2Δ/Δ mice exhibit male-specific sterility because of abnormal spermiogenesis. ADAD2 interacts with multiple RNA-binding proteins involved in piRNA biogenesis, including MILI, MIWI, RNF17, and YTHDC2. ADAD2 co-localizes and forms novel granules with RNF17 in spermatocytes. Ablation of ADAD2 impairs the formation of RNF17 granules, decreases the number of cluster-derived pachytene piRNAs, and increases expression of ping-pong-derived piRNAs.

DISCUSSION AND CONCLUSION

In collaboration with RNF17 and other RNA-binding proteins in spermatocytes, ADAD2 directly or indirectly functions in piRNA biogenesis.

Characterization of SPINK2, SPACA7 and PDCL2: Effect of immunization on fecundity, sperm function and testicular transcriptome

Testicular factors play a vital role in spermatogenesis. We characterized the functional role of rat Spink2, Spaca7 and Pdcl2 genes. Their primary, secondary and tertiary structure were deduced in silico. The genes of rat Spink2, Spaca7 and Pdcl2 mRNA were predominantly expressed in the testis. SPINK2, SPACA7 and PDCL2 protein expression was evident in all the cell types of testis and on spermatozoa. Ablation of each of these proteins by active immunization resulted in reduced fecundity and sperm count. Damage to the anatomical architecture of testis and epididymis was evident. In SPINK2 immunized rats, 283 genes were differentially regulated while it was 434 and 872 genes for SPACA7 and PDCL2 respectively. Genes that were differentially regulated in the testis of SPINK2 immunized rats primarily belonged to extracellular exosome formation, extracellular space and response to drugs. SPACA7 ablation affected genes related to extracellular space, oxidation-reduction processes, endoplasmic reticulum membrane and response to drugs. Differential gene expression was observed for nuclear function, protein binding and positive regulation of transcription from RNA polymerase II promoter in testis of PDCL2 immunized rats. Results of our study demonstrate the role of SPINK2, SPACA7 and PDCL2 in spermatogenesis and in important molecular processes that may dictate testicular function and other physiological responses as well.

1700029I15Rik orchestrates the biosynthesis of acrosomal membrane proteins required for sperm-egg interaction

Sperm acrosomal membrane proteins, such as Izumo sperm-egg fusion 1 (IZUMO1) and sperm acrosome-associated 6 (SPACA6), play essential roles in mammalian gamete binding or fusion. How their biosynthesis is regulated during spermiogenesis has largely remained elusive. Here, we show that 1700029I15Rik knockout male mice are severely subfertile and their spermatozoa do not fuse with eggs. 1700029I15Rik is a type-II transmembrane protein expressed in early round spermatids but not in mature spermatozoa. It interacts with proteins involved in N-linked glycosylation, disulfide isomerization, and endoplasmic reticulum (ER)-Golgi trafficking, suggesting a potential role in nascent protein processing. The ablation of 1700029I15Rik destabilizes non-catalytic subunits of the oligosaccharyltransferase (OST) complex that are pivotal for N-glycosylation. The knockout testes exhibit normal expression of sperm plasma membrane proteins, but decreased abundance of multiple acrosomal membrane proteins involved in fertilization. The knockout sperm show upregulated chaperones related to ER-associated degradation (ERAD) and elevated protein ubiquitination; strikingly, SPACA6 becomes undetectable. Our results support for a specific, 1700029I15Rik-mediated pathway underpinning the biosynthesis of acrosomal membrane proteins during spermiogenesis.

1700029I15Rik orchestrates the biosynthesis of acrosomal membrane proteins required for sperm-egg interaction

Sperm acrosomal membrane proteins, such as Izumo sperm-egg fusion 1 (IZUMO1) and sperm acrosome-associated 6 (SPACA6), play essential roles in mammalian gamete binding or fusion. How their biosynthesis is regulated during spermiogenesis has largely remained elusive. Here, we show that 1700029I15Rik knockout male mice are severely subfertile and their spermatozoa do not fuse with eggs. 1700029I15Rik is a type-II transmembrane protein expressed in early round spermatids but not in mature spermatozoa. It interacts with proteins involved in N-linked glycosylation, disulfide isomerization, and endoplasmic reticulum (ER)-Golgi trafficking, suggesting a potential role in nascent protein processing. The ablation of 1700029I15Rik destabilizes non-catalytic subunits of the oligosaccharyltransferase (OST) complex that are pivotal for N-glycosylation. The knockout testes exhibit normal expression of sperm plasma membrane proteins, but decreased abundance of multiple acrosomal membrane proteins involved in fertilization. The knockout sperm show upregulated chaperones related to ER-associated degradation (ERAD) and elevated protein ubiquitination; strikingly, SPACA6 becomes undetectable. Our results support for a specific, 1700029I15Rik-mediated pathway underpinning the biosynthesis of acrosomal membrane proteins during spermiogenesis.

Reproductive genomics of the mouse: implications for human fertility and infertility

Genetic analyses of mammalian gametogenesis and fertility have the potential to inform about two important and interrelated clinical areas: infertility and contraception. Here, we address the genetics and genomics underlying gamete formation, productivity and function in the context of reproductive success in mammalian systems, primarily mouse and human. Although much is known about the specific genes and proteins required for meiotic processes and sperm function, we know relatively little about other gametic determinants of overall fertility, such as regulation of gamete numbers, duration of gamete production, and gamete selection and function in fertilization. As fertility is not a binary trait, attention is now appropriately focused on the oligogenic, quantitative aspects of reproduction. Multiparent mouse populations, created by complex crossing strategies, exhibit genetic diversity similar to human populations and will be valuable resources for genetic discovery, helping to overcome current limitations to our knowledge of mammalian reproductive genetics. Finally, we discuss how what we know about the genomics of reproduction can ultimately be brought to the clinic, informing our concepts of human fertility and infertility, and improving assisted reproductive technologies.

Dynamic cellular and molecular characteristics of spermatogenesis in the viviparous marine teleost Sebastes schlegelii†

Spermatogenesis is a dynamic cell developmental process that is essential for reproductive success. Vertebrates utilize a variety of reproductive strategies, including sperm diversity, and internal and external fertilization. Research on the cellular and molecular dynamic changes involved in viviparous teleost spermatogenesis, however, is currently lacking. Here, we combined cytohistology, 10 × genomic single-cell RNA-seq, and transcriptome technology to determine the dynamic development characteristics of the spermatogenesis of Sebastes schlegelii. The expressions of lhcgr (Luteinizing hormone/Choriogonadotropin receptor), fshr (follicle-stimulating hormone receptor), ar (androgen receptor), pgr (progesterone receptor), and cox (cyclo-oxygen-ase), as well as the prostaglandin E and F levels peaked during the maturation period, indicating that they were important for sperm maturation and mating. Fifteen clusters were identified based on the 10 × genomic single-cell results. The cell markers of the sub-cluster were identified by their upregulation; piwil, dazl, and dmrt1 were upregulated and identified as spermatogonium markers, and sycp1/3 and spo11 were identified as spermatocyte markers. For S. schlegelii, the sperm head nucleus was elongated (spherical to streamlined in shape), which is a typical characteristic for sperm involved in internal fertilization. We also identified a series of crucial genes associated with spermiogenesis, such as spata6, spag16, kif20a, trip10, and klf10, while kif2c, kifap3, fez2, and spaca6 were found to be involved in nucleus elongation. The results of this study will enrich our cellular and molecular knowledge of spermatogenesis and spermiogenesis in fish that undergo internal fertilization.

Mammalian fertilization: Does sperm IZUMO1 mediate fusion as well as adhesion?

The molecular mechanism of sperm-egg fusion is a long-standing mystery in reproduction. Brukman and colleagues (2022. J. Cell Biol.https://doi.org/10.1083/jcb.202207147) now provide evidence that the sperm surface protein IZUMO1, which is essential for mammalian fertilization, can induce membrane fusion in cultured cells.

Testis-enriched ferlin, FER1L5, is required for Ca2+-activated acrosome reaction and male fertility

Spermatozoa need to undergo an exocytotic event called the acrosome reaction before fusing with eggs. Although calcium ion (Ca2+) is essential for the acrosome reaction, its molecular mechanism remains unknown. Ferlin is a single transmembrane protein with multiple Ca2+-binding C2 domains, and there are six ferlins, dysferlin (DYSF), otoferlin (OTOF), myoferlin (MYOF), fer-1-like 4 (FER1L4), FER1L5, and FER1L6, in mammals. Dysf, Otof, and Myof knockout mice have been generated, and each knockout mouse line exhibited membrane fusion disorders such as muscular dystrophy in Dysf, deafness in Otof, and abnormal myogenesis in Myof. Here, by generating mutant mice of Fer1l4, Fer1l5, and Fer1l6, we found that only Fer1l5 is required for male fertility. Fer1l5 mutant spermatozoa could migrate in the female reproductive tract and reach eggs, but no acrosome reaction took place. Even a Ca2+ ionophore cannot induce the acrosome reaction in Fer1l5 mutant spermatozoa. These results suggest that FER1L5 is the missing link between Ca2+ and the acrosome reaction.

A comprehensive investigation of human endogenous retroviral syncytin proteins and their receptors in men with normozoospermia and impaired semen quality

PURPOSE

The study aims to investigate first the presence of Syncytin 2 and its receptor, MFSD2, in human sperm, and second whether the expressions of Syncytin 1, Syncytin 2, and their receptors, SLC1A5 and MFSD2, differ between normozoospermic, asthenozoospermic, oligozoospermic, and oligoasthenozoospermic human sperm samples.

METHODS

The localization patterns and expression levels of syncytins and their receptors were evaluated in normozoospermic (concentration = 88.9 ± 5.5 × 10⁶, motility = 79.2 ± 3.15%, n = 30), asthenozoospermic (concentration = 51.7 ± 7.18 × 10⁶, motility = 24.0 ± 3.12%, n = 15), mild oligozoospermic (concentration = 13.5 ± 2.17 × 10⁶, motility = 72.1 ± 6.5%, n = 15), moderate oligozoospermic (concentration = 8.4 ± 3.21 × 10⁶, motility = 65.1 ± 8.9%, n = 15), severe oligozoospermic (concentration = 2.1 ± 1.01 × 10⁶, motility = 67.5 ± 3.2%, n = 15), and oligoasthenozoospermic (concentration = 5.5 ± 3.21 × 10⁶, motility = 18.5 ± 1.2%, n = 15) samples by immunofluorescence staining and western blot.

RESULTS

Syncytins and their receptors visualized by immunofluorescence showed similar staining patterns with slight staining of the tail in all spermatozoa regardless of normozoospermia, asthenozoospermia, oligozoospermia, or oligoasthenozoospermia. The localization patterns were categorized as equatorial segment, midpiece region, acrosome, and post-acrosomal areas. The combined staining patterns were also detected as acrosomal cap plus post acrosomal region, the midpiece plus equatorial segment, and midpiece plus acrosomal region. However, some sperm cells were categorized as non-stained. Both syncytin proteins were most intensely localized in the midpiece region, while their receptors were predominantly present in the midpiece plus acrosomal region. Conspicuously, syncytins and their receptors showed decreased expression in asthenozospermic, oligozoospermic, and oligoasthenozoospermic samples compared to normozoospermic samples.

CONCLUSION

The expression patterns of HERV-derived syncytins and their receptors were identical regardless of the spermatozoa in men with normozoospermia versus impaired semen quality. Further, asthenozoospermia, oligozoospermia, and oligoasthenozoospermia as male fertility issues are associated with decreased expression of both syncytins and their receptors.

Various gene modification techniques to discover molecular targets for nonhormonal male contraceptives: A review

The identification and characterization of relevant targets are necessary for developing nonhormonal male contraceptives. The molecules must demonstrate that they are necessary for reproduction. As a result, a sophisticated technique is required to identify the molecular targets for nonhormonal male contraceptives. Genetic modification (GM) techniques are one method that can be applied. This technique has been widely used to study gene function that effected male fertility and has resulted in the discovery of numerous nonhormonal male contraceptive target molecules. We examined GM techniques and approaches used to investigate genes involved in male fertility as potential targets for nonhormonal contraceptives. The discovery of nonhormonal contraceptive candidate molecules was increased by using GM techniques, especially the Clustered Regularly Interspaced Short Palindromic Repeats/Cas9 method. The discovery of candidate nonhormonal contraceptive molecules can be a wide-open research for the development of nonhormonal male contraceptives. Therefore, we are believing that one day nonhormonal male contraceptives will be released.

Impaired fertility in 4930590J08Rik mutant male mice is associated with defective sperm energy metabolism

Testis-specifically expressed genes are important for male reproduction according to their unique expression patterns. However, the functions of most of these genes in reproduction are unclear. Here, we showed that mouse 4930590J08Rik was a testis-specifically expressed gene. 4930590J08Rik knockout mice exhibited a delay in the first wave of spermatogenesis and a reduction of cauda epididymal sperm. Furthermore, knockout spermatozoa exhibited defective acrosome reactions and decreased progressive motility, which led to impaired in vivo fertilization. Transcriptome analysis of testes revealed that most of the differentially expressed genes in knockout testes were associated with metabolic processes. 4930590J08Rik knockout sperm exhibited oxidative phosphorylation deficiency and were highly dependent on increased anaerobic glycolysis to compensate for ATP demands. Taken together, the 4930590J08Rik-disrupted mouse partially mimics the phenotypes of human asthenospermia and oligozoospermia, which provides a new model for further understanding the pathogenesis of idiopathic male infertility.

A novel function for the sperm adhesion protein IZUMO1 in cell-cell fusion

Mammalian sperm-egg adhesion depends on the trans-interaction between the sperm-specific type I glycoprotein IZUMO1 and its oocyte-specific GPI-anchored receptor JUNO. However, the mechanisms and proteins (fusogens) that mediate the following step of gamete fusion remain unknown. Using live imaging and content mixing assays in a heterologous system and structure-guided mutagenesis, we unveil an unexpected function for IZUMO1 in cell-to-cell fusion. We show that IZUMO1 alone is sufficient to induce fusion, and that this ability is retained in a mutant unable to bind JUNO. On the other hand, a triple mutation in exposed aromatic residues prevents this fusogenic activity without impairing JUNO interaction. Our findings suggest a second function for IZUMO1 as a unilateral mouse gamete fusogen.

Eukaryotic fertilization and gamete fusion at a glance

In sexually reproducing organisms, the genetic information is transmitted from one generation to the next via the merger of male and female gametes. Gamete fusion is a two-step process involving membrane recognition and apposition through ligand-receptor interactions and lipid mixing mediated by fusion proteins. HAP2 (also known as GCS1) is a bona fide gamete fusogen in flowering plants and protists. In vertebrates, a multitude of surface proteins have been demonstrated to be pivotal for sperm-egg fusion, yet none of them exhibit typical fusogenic features. In this Cell Science at a Glance article and the accompanying poster, we summarize recent advances in the mechanistic understanding of gamete fusion in eukaryotes, with a particular focus on mammalian species.

Fertilization, but Not Post-Implantation Development, Can Occur in the Absence of Sperm and Oocyte Beta1 Integrin in Mice

Fertilization is a complex process that requires successive stages and culminates in the adhesion/fusion of gamete membranes. If the question of the involvement of oocyte integrins has been swept away by deletion experiments, that of the involvement of sperm integrins remains to be further characterized. In the present study, we addressed the question of the feasibility of sperm-oocyte adhesion/fusion and early implantation in the absence of sperm β1 integrin. Males and females with β1 integrin-depleted sperm and oocytes were mated, and fertilization outcome was monitored by a gestational ultrasound analysis. Results suggest that although the sperm β1 integrin participates in gamete adhesion/fusion, it is dispensable for fertilization in mice. However, sperm- and/or oocyte-originated integrin β1 is essential for post-implantation development. Redundancy phenomena could be at the origin of a compensatory expression or alternative dimerization pattern.

DMP8 and 9 regulate HAP2/GCS1 trafficking for the timely acquisition of sperm fusion competence

Sexual reproduction involves the fusion of two gametes of opposite sex. Although the sperm-expressed fusogen HAPLESS 2 (HAP2) or GENERATIVE CELL SPECIFIC 1 (GCS1) plays a vital role in this process in many eukaryotic organisms and an understanding of its regulation is emerging in unicellular systems [J. Zhang et al., Nat. Commun. 12, 4380 (2021); J. F. Pinello et al. Dev. Cell 56, 3380-3392.e9 (2021)], neither HAP2/GCS1 interactors nor mechanisms for delivery and activation at the fusion site are known in multicellular plants. Here, we show that Arabidopsis thaliana HAP2/GCS1 interacts with two sperm DUF679 membrane proteins (DMP8 and DMP9), which are required for the EGG CELL 1 (EC1)-induced translocation of HAP2/GCS1 from internal storage vesicle to the sperm plasma membrane to ensure successful fertilization. Our studies in Arabidopsis and tobacco provide evidence for a conserved function of DMP8/9-like proteins as HAP2/GCS1 partner in seed plants. Our data suggest that seed plants evolved a DMP8/9-dependent fusogen translocation process to achieve timely acquisition of sperm fusion competence in response to egg cell-derived signals, revealing a previously unknown critical step for successful fertilization.

Human sperm TMEM95 binds eggs and facilitates membrane fusion

Membrane fusion of sperm and eggs is pivotal in sexual reproduction. Tmem95 knockout mice produce sperm that can bind to, but do not fuse with, eggs. How TMEM95 facilitates membrane fusion was unknown. We show here that human TMEM95 binds eggs. Our crystal structure of TMEM95 suggests a region where this binding may occur. We develop monoclonal antibodies against TMEM95 that impair sperm-egg fusion but do not block sperm-egg binding. Thus, we propose that there is a receptor-mediated interaction of sperm TMEM95 with eggs, and that this interaction may have a direct role in membrane fusion. Our work suggests avenues for the identification of the TMEM95 egg receptor and the development of infertility treatments and contraceptives for humans.

Tmem95 encodes a sperm acrosomal membrane protein, whose knockout has a male-specific sterility phenotype in mice. Tmem95 knockout murine sperm can bind to, but do not fuse with, eggs. How TMEM95 plays a role in membrane fusion of sperm and eggs has remained elusive. Here, we utilize a sperm penetration assay as a model system to investigate the function of human TMEM95. We show that human TMEM95 binds to hamster egg membranes, providing evidence for a TMEM95 receptor on eggs. Using X-ray crystallography, we reveal an evolutionarily conserved, positively charged region of TMEM95 as a putative receptor-binding surface. Amino acid substitutions within this region of TMEM95 ablate egg-binding activity. We identify monoclonal antibodies against TMEM95 that reduce the number of human sperm fused with hamster eggs in sperm penetration assays. Strikingly, these antibodies do not block binding of sperm to eggs. Taken together, these results provide strong evidence for a specific, receptor-mediated interaction of sperm TMEM95 with eggs and suggest that this interaction may have a role in facilitating membrane fusion during fertilization.