TMEM95 is a sperm membrane protein essential for mammalian fertilization

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.

Genome-wide landscape of genetic diversity, runs of homozygosity, and runs of heterozygosity in five Alpine and Mediterranean goat breeds

BACKGROUND

Goat breeds in the Alpine area and Mediterranean basin exhibit a unique genetic heritage shaped by centuries of selection and adaptability to harsh environments. Understanding their adaptive traits can aid breeding programs target enhanced resilience and productivity, especially as we are facing important climate and agriculture challenges. To this aim the genomic architecture of 480 goats belonging to five breeds (i.e., Saanen [SAA], Camosciata delle Alpi [CAM], Murciano-Granadina [MUR], Maltese [MAL], Sarda [SAR]) reared in the Sardinia Island were genotyped and their genomic architecture evaluated to find molecular basis of adaptive traits. Inbreeding, runs of homozygosity (ROH) and runs of heterozygosity (ROHet) were identified. Finally, candidate genes in the ROH and ROHet regions were explored through a pathway analysis to assess their molecular role.

RESULTS

In total, we detected 10,341 ROH in the SAA genome, 11,063 ROH in the CAM genome, 12,250 ROH in the MUR genome, 8,939 ROH in the MAL genome, and 18,441 ROH in the SAR genome. Moreover, we identified 4,087 ROHet for SAA, 3,360 for CAM, 2,927 for MUR, 3,701 for MAL, and 3,576 for SAR, with SAR having the highest heterozygosity coefficient. Interestingly, when computing the inbreeding coefficient using homozygous segment (FROH), SAA showed the lowest value while MAL the highest one, suggesting the need to improve selecting strategies to preserve genetic diversity within the population. Among the most significant candidate genes, we identified several ones linked to different physiological functions, such as milk production (e.g., DGAT1, B4GALT1), immunity (GABARAP, GPS2) and adaptation to environment (e.g., GJA3, GJB2 and GJB6).

CONCLUSIONS

This study highlighted the genetic diversity within and among five goat breeds. The high levels of ROH identified in some breeds might indicate high levels of inbreeding and a lack in genetic variation, which might negatively impact the animal population. Conversely, high levels of ROHet might indicate regions of the genetic diversity, beneficial for breed health and resilience. Therefore, these findings could aid breeding programs in managing inbreeding and preserving genetic diversity.

Paternal Effects in Mammalian Reproduction: Functional, Environmental, and Clinical Relevance of Sperm Components in Early Embryos and Beyond

In addition to widely recognized contributions of the paternal genome, centriole, and oocyte-activation factors, sperm deliver a wide range of macromolecules to the fertilized embryo. The impacts of these factors on the embryo, progeny, and even subsequent generations have become increasingly apparent, along with an understanding of an extensive potential for male health and environmental exposures to exert both immediate and long-term impacts on mammalian reproduction. Available data reveal that sperm factors interact with and regulate the actions of oocyte factors as well as exerting additional direct effects on the early embryo. This review provides a summary of the nature and mechanisms of paternal effects in early mammalian embryos, long-term effects in progeny, susceptibility of sperm components to diverse environmental factors, and potential approaches to mitigate adverse effects of such exposures.

Ribosomal protein phylogeography offers quantitative insights into the efficacy of genome-resolved surveys of microbial communities

The increasing availability of microbial genomes is essential to gain insights into microbial ecology and evolution that can propel biotechnological and biomedical advances. Recent advances in genome recovery have significantly expanded the catalogue of microbial genomes from diverse habitats. However, the ability to explain how well a set of genomes account for the diversity in a given environment remains challenging for individual studies or biome-specific databases. Here we present EcoPhylo, a computational workflow to characterize the phylogeography of any gene family through integrated analyses of genomes and metagenomes, and our application of this approach to ribosomal proteins to quantify phylogeny-aware genome recovery rates across three biomes. Our findings show that genome recovery rates vary widely across taxa and biomes, and that single amplified genomes, metagenome-assembled genomes, and isolate genomes have non-uniform yet quantifiable representation of environmental microbes. EcoPhylo reveals highly resolved, reference-free, multi-domain phylogenies in conjunction with distribution patterns of individual clades across environments, providing a means to assess genome recovery in individual studies and benchmark biome-level genome collections.

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.

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.

Biomarker-based human and animal sperm phenotyping: the good, the bad and the ugly†

Conventional, brightfield-microscopic semen analysis provides important baseline information about sperm quality of an individual; however, it falls short of identifying subtle subcellular and molecular defects in cohorts of "bad," defective human and animal spermatozoa with seemingly normal phenotypes. To bridge this gap, it is desirable to increase the precision of andrological evaluation in humans and livestock animals by pursuing advanced biomarker-based imaging methods. This review, spiced up with occasional classic movie references but seriously scholastic at the same time, focuses mainly on the biomarkers of altered male germ cell proteostasis resulting in post-testicular carryovers of proteins associated with ubiquitin-proteasome system. Also addressed are sperm redox homeostasis, epididymal sperm maturation, sperm-seminal plasma interactions, and sperm surface glycosylation. Zinc ion homeostasis-associated biomarkers and sperm-borne components, including the elements of neurodegenerative pathways such as Huntington and Alzheimer disease, are discussed. Such spectrum of biomarkers, imaged by highly specific vital fluorescent molecular probes, lectins, and antibodies, reveals both obvious and subtle defects of sperm chromatin, deoxyribonucleic acid, and accessory structures of the sperm head and tail. Introduction of next-generation image-based flow cytometry into research and clinical andrology will soon enable the incorporation of machine and deep learning algorithms with the end point of developing simple, label-free methods for clinical diagnostics and high-throughput phenotyping of spermatozoa in humans and economically important livestock animals.

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.

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.

Molecular quantitative trait loci in reproductive tissues impact male fertility in cattle

Breeding bulls are well suited to investigate inherited variation in male fertility because they are genotyped and their reproductive success is monitored through semen analyses and thousands of artificial inseminations. However, functional data from relevant tissues are lacking in cattle, which prevents fine-mapping fertility-associated genomic regions. Here, we characterize gene expression and splicing variation in testis, epididymis, and vas deferens transcriptomes of 118 mature bulls and conduct association tests between 414,667 molecular phenotypes and 21,501,032 genome-wide variants to identify 41,156 regulatory loci. We show broad consensus in tissue-specific and tissue-enriched gene expression between the three bovine tissues and their human and murine counterparts. Expression- and splicing-mediating variants are more than three times as frequent in testis than epididymis and vas deferens, highlighting the transcriptional complexity of testis. Finally, we identify genes (WDR19, SPATA16, KCTD19, ZDHHC1) and molecular phenotypes that are associated with quantitative variation in male fertility through transcriptome-wide association and colocalization analyses.

Participation of WD repeat-containing protein 54 (WDR54) in rat sperm-oocyte fusion through interaction with both IZUMO1 and JUNO

Fertilization is a complex process that depends on the fusion of the cell membrane of sperm with that of oocyte, and it involves sperm-oocyte recognition, binding, and fusion, which are mediated by multiple proteins. Among those proteins, IZUMO1 and its receptor JUNO have been identified as essential factors for sperm-oocyte recognition and fusion. However, the interaction between IZUMO1 and JUNO alone does not lead to cell membrane fusion, suggesting the involvement of additional proteins in sperm-oocyte membrane fusion. In this study, we have discovered that a protein called WDR54, which consists of WD-repeat modules, is located on the cell membrane of sperm, as well as on the cell membrane and in the cytoplasm of the oocyte. We have found that WDR54 is involved in sperm-oocyte fertilization. When sperm and oocyte were treated with anti-WDR54 ascites, the in vitro fertilization (IVF) rate significantly decreased. Furthermore, our research has shown that WDR54 interacts with both IZUMO1 and JUNO, and it colocalizes with IZUMO1 on the surface of the sperm head and with JUNO on the oocyte surface. Through structural analysis of the putative complexes of WDR54-IZUMO1 and WDR54-JUNO, we infer that these three proteins could form a complex of JUNO-WDR54-IZUMO1-JUNO (referred to as the "JWIJ complex") on the oocyte surface. Our findings suggest that WDR54 is an important factor involved in sperm-oocyte adhesion and fusion. This discovery provides new insight into the mechanisms of mammalian sperm-oocyte adhesion and fusion.

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.

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.

Genome-wide landscape of runs of homozygosity and differentiation across Egyptian goat breeds

Understanding the genomic features of livestock is essential for successful breeding programs and conservation. This information is scarce for local goat breeds in Egypt. In the current study, genomic regions with selection signatures were identified as well as runs of homozygosity (ROH), genomic inbreeding coefficients (FROH) and fixation index (FST) were detected in Egyptian Nubian, Damascus, Barki and Boer goat breeds. A total of 46,268 SNP markers and 337 animals were available for the genomic analyses. On average, 145.44, 42.02, 87.90 and 126.95 ROHs were detected per individual in the autosomal genome of the respective breeds. The mean accumulative ROH lengths ranged from 46.5 Mb in Damascus to 360 Mb in Egyptian Nubian. The short ROH segments (< 2 Mb) were most frequent in all breeds, while the longest ROH segments (> 16 Mb) were exclusively found in the Egyptian Nubian. The highest average FROH was observed in Egyptian Nubian (~ 0.12) followed by Boer (~ 0.11), while the lowest FROH was found in Damascus (~ 0.05) and Barki breed (~ 0.03). The estimated mean FST was 0.14 (Egyptian Nubian and Boer), 0.077 (Egyptian Nubian and Barki), 0.075 (Egyptian Nubian and Damascus), 0.071 (Barki and Boer), 0.064 (Damascus and Boer), and 0.015 (Damascus and Barki), for each pair of breeds. Interestingly, multiple SNPs that accounted for high FST values were observed on chromosome 6 in regions harboring ALPK1 and KCNIP4. Genomic regions overlapping both FST and ROH harbor genes related to immunity (IL4R, PHF23, GABARAP, GPS2, and CD68), reproduction (SPATA2L, TNFSF12, TMEM95, and RNF17), embryonic development (TCF25 and SOX15) and adaptation (MC1R, KDR, and KIT), suggesting potential genetic adaptations to local environmental conditions. Our results contribute to the understanding of the genetic architecture of different goat breeds and may provide valuable information for effective preservation and breeding programs of local goat breeds in Egypt.

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.

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.

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.

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.

Study on the expression patterns and function of JUNO and CD9 in bovine oocytes during in vitro maturation

Fertilization proteins JUNO and CD9 play vital roles in sperm-egg fusion, but little is known about their expression patterns during in vitro maturation (IVM) and their function during in vitro fertilization (IVF) of bovine oocytes. In this study, qRT-PCR and immunofluorescence staining were used to detect the mRNA and protein expression levels of JUNO and CD9 genes in bovine oocytes and cumulus cells. Then, fertilization rate of MII oocytes treated with (i) JUNO antibody (1, 5 and 25 μg/ml) or (ii) CD9 antibody (1, 5 and 25 μg/ml) or (iii) CD9 antibody (5 μg/ml) + JUNO antibody (5 μg/ml) were recorded. Our results showed that the mRNA and protein expression levels of JUNO and CD9 genes significantly increased from bovine GV oocytes to MII oocytes, and similar mRNA expression patterns of JUNO and CD9 were also detected in cumulus cells. All groups of oocytes treated with CD9 antibody or JUNO antibody showed significantly decreased fertilization rates (p < .05). Particularly, the fertilization ability of oocytes treated with CD9 antibody (5 μg/ml) + JUNO antibody (5 μg/ml) sharply decreased to 3.48 ± 0.11%. In conclusion, our study revealed the expression levels of JUNO and CD9 genes in oocytes and cumulus cells increased during IVM of bovine oocytes, with JUNO protein playing a major role in the fertilization of bovine oocytes.

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.

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.

SPACA6 ectodomain structure reveals a conserved superfamily of gamete fusion-associated proteins

SPACA6 is a sperm-expressed surface protein that is critical for gamete fusion during mammalian sexual reproduction. Despite this fundamental role, little is known about how SPACA6 specifically functions. We elucidated the crystal structure of the SPACA6 ectodomain at 2.2-Å resolution, revealing a two-domain protein containing a four-helix bundle and Ig-like β-sandwich connected via a quasi-flexible linker. This structure is reminiscent of IZUMO1, another gamete fusion-associated protein, making SPACA6 and IZUMO1 founding members of a superfamily of fertilization-associated proteins, herein dubbed the IST superfamily. The IST superfamily is defined structurally by its distorted four-helix bundle and a pair of disulfide-bonded CXXC motifs. A structure-based search of the AlphaFold human proteome identified more protein members to this superfamily; remarkably, many of these proteins are linked to gamete fusion. The SPACA6 structure and its connection to other IST-superfamily members provide a missing link in our knowledge of mammalian gamete fusion.

MAIA, Fc receptor-like 3, supersedes JUNO as IZUMO1 receptor during human fertilization

Gamete fusion is a critical event of mammalian fertilization. A random one-bead one-compound combinatorial peptide library represented synthetic human egg mimics and identified a previously unidentified ligand as Fc receptor-like 3, named MAIA after the mythological goddess intertwined with JUNO. This immunoglobulin super family receptor was expressed on human oolemma and played a major role during sperm-egg adhesion and fusion. MAIA forms a highly stable interaction with the known IZUMO1/JUNO sperm-egg complex, permitting specific gamete fusion. The complexity of the MAIA isotype may offer a cryptic sexual selection mechanism to avoid genetic incompatibility and achieve favorable fitness outcomes.

The vertebrate- and testis- specific transmembrane protein C11ORF94 plays a critical role in sperm-oocyte membrane binding

Sperm-oocyte membrane fusion is necessary for mammalian fertilization. The factors that determine the fusion of sperm with oocytes are largely unknown. So far, spermatozoon factor IZUMO1 and the IZUMO1 counter-receptor JUNO on the oocyte membrane has been identified as a protein requiring fusion. Some sperm membrane proteins such as FIMP, SPACA6 and TEME95, have been proved not to directly regulate fusion, but their knockout will affect the fusion process of sperm and oocytes. Here, we identified a novel gene C11orf94 encoding a testicular-specific small transmembrane protein that emerges in vertebrates likely acquired via horizontal gene transfer from bacteria and plays an indispensable role in sperm-oocyte binding. We demonstrated that the deletion of C11orf94 dramatically decreased male fertility in mice. Sperm from C11orf94-deficient mice could pass through the zona pellucida, but failed to bind to the oocyte membrane, thus accumulating in the perivitelline space. In consistence, when the sperm of C11orf94-deficient mice were microinjected into the oocyte cytoplasm, fertilized oocytes were obtained and developed normally to blastocysts. Proteomics analysis revealed that C11orf94 influenced the expression of multiple gene products known to be indispensable for sperm-oocyte binding and fusion, including IZUMO1, EQTN and CRISP1. Thus, our study indicated that C11ORF94 is a vertebrate- and testis-specific small transmembrane protein that plays a critical role in sperm binding to the oolemma.

C11orf94/Frey is a key regulator for male fertility by controlling Izumo1 complex assembly

Although gamete fusion represents the central event in sexual reproduction, the required protein machinery is poorly defined. In sperm cells, Izumo1 and several Izumo1-associated proteins play an essential role for this process. However, so far, the mechanisms underlying transport and maturation of Izumo1 and its incorporation into high molecular weight complexes are incompletely defined. Here, we provide a detailed characterization of the C11orf94 protein, which we rename Frey, which provides a platform for the assembly of Izumo1 complexes. By retaining Izumo1 in the endoplasmic reticulum, Frey facilitates its incorporation into high molecular weight complexes. To fulfill its function, the unstable Frey protein is stabilized within the catalytic center of an intramembrane protease. Loss of Frey results in reduced assembly of Izumo1 complexes and male infertility due to impaired gamete fusion. Collectively, these findings provide mechanistic insights into the early biogenesis and functional relevance of Izumo1 complexes.

Mechanisms of Sperm-Egg Interactions: What Ascidian Fertilization Research Has Taught Us

Fertilization is an essential process in terrestrial organisms for creating a new organism with genetic diversity. Before gamete fusion, several steps are required to achieve successful fertilization. Animal spermatozoa are first activated and attracted to the eggs by egg-derived chemoattractants. During the sperm passage of the egg's extracellular matrix or upon the sperm binding to the proteinaceous egg coat, the sperm undergoes an acrosome reaction, an exocytosis of acrosome. In hermaphrodites such as ascidians, the self/nonself recognition process occurs when the sperm binds to the egg coat. The activated or acrosome-reacted spermatozoa penetrate through the proteinaceous egg coat. The extracellular ubiquitin-proteasome system, the astacin-like metalloproteases, and the trypsin-like proteases play key roles in this process in ascidians. In the present review, we summarize our current understanding and perspectives on gamete recognition and egg coat lysins in ascidians and consider the general mechanisms of fertilization in animals and plants.

Expression of membrane fusion proteins in spermatozoa and total fertilisation failure during in vitro fertilisation

Background

Couples increasingly experience infertility and seek help from assisted reproductive techniques to become pregnant. However, 5%–15% of the couples that are selected for in vitro fertilisation (IVF) experience a total fertilisation failure (TFF), where no zygotes develop despite oocytes and semen parameters appear to be normal. We hypothesise that TFF during IVF could be related to improper membrane fusion of gametes.

Objective

To investigate the membrane integrity and fusion proteins in spermatozoa from men in couples experiencing TFF.

Materials and methods

A total of 33 infertile couples, 17 of which experienced TFF during IVF and 16 matched control couples with normal IVF fertilisation rates, were selected and the men re‐called to deliver an additional semen sample. Proteins involved in gamete membrane fusion on spermatozoa (IZUMO1, SPESP1 and Syncytin‐1) as well as O‐glycosylation patterns (Tn and GALNT3), were investigated by immunofluorescence. The DNA fragmentation index, acrosomal integrity and viability of spermatozoa were determined by flow and image cytometry.

Results

No significant changes in the expression of GALNT3, Tn and Syncytin‐1 were observed between the TFF and control groups. The fraction of spermatozoa expressing SPESP1, the median IZUMO1 staining intensity, and the percentage of viable acrosome‐intact spermatozoa were significantly lower in the TFF group compared to controls. Furthermore, following progesterone‐induced acrosomal exocytosis, a significant difference in the fraction of spermatozoa expressing SPESP1 and the median IZUMO1 staining intensity were observed between the control and TFF group.

Discussion and conclusion

Our results indicate that acrosomal exocytosis, IZUMO1 and SPESP1 expression in spermatozoa could play a crucial role in achieving fertilisation during IVF. However, the size of our cohort was quite small, and our results need to be validated with quantitative methods in larger cohorts.

Targeted Analysis of HSP70 Isoforms in Human Spermatozoa in the Context of Capacitation and Motility

HSP70s constitute a family of chaperones, some isoforms of which appear to play a role in sperm function. Notably, global proteomic studies analyzing proteins deregulated in asthenozoospermia, a main cause of male infertility characterized by low sperm motility, showed the dysregulation of some HSP70 isoforms. However, to date, no clear trend has been established since the variations in the abundance of HSP70 isoforms differed between studies. The HSPA2 isoform has been reported to play a key role in fertilization, but its dysregulation and possible relocation during capacitation, a maturation process making the spermatozoon capable of fertilizing an oocyte, is debated in the literature. The aim of the present study was to investigate the fate of all sperm HSP70 isoforms during capacitation and in relation to sperm motility. Using Multiple-Reaction Monitoring (MRM) mass spectrometry, we showed that the relative abundance of all detected isoforms was stable between non-capacitated and capacitated spermatozoa. Immunofluorescence using two different antibodies also demonstrated the stability of HSP70 isoform localization during capacitation. We also investigated spermatozoa purified from 20 sperm samples displaying various levels of total and progressive sperm motility. We showed that the abundance of HSP70 isoforms is not correlated to sperm total or progressive motility.

Live imaging-based assay for visualising species-specific interactions in gamete adhesion molecules

Successful gamete fusion requires species-specific membrane adhesion. However, the interaction of adhesion molecules in gametes is difficult to study in real time through low-throughput microscopic observation. Therefore, we developed a live imaging-based adhesion molecule (LIAM) assay to study gamete adhesion molecule interactions in cultured cells. First, we modified a fusion assay previously established for fusogens introduced into cultured cells, and confirmed that our live imaging technique could visualise cell-cell fusion in the modified fusion assay. Next, instead of fusogen, we introduced adhesion molecules including a mammalian gamete adhesion molecule pair, IZUMO1 and JUNO, and detected their temporal accumulation at the contact interfaces of adjacent cells. Accumulated IZUMO1 or JUNO was partly translocated to the opposite cells as discrete spots; the mutation in amino acids required for their interaction impaired accumulation and translocation. By using the LIAM assay, we investigated the species specificity of IZUMO1 and JUNO of mouse, human, hamster, and pig in all combinations. IZUMO1 and JUNO accumulation and translocation were observed in conspecific, and some interspecific, combinations, suggesting potentially interchangeable combinations of IZUMO1 and JUNO from different species.

The Sperm Olfactory Receptor OLFR601 is Dispensable for Mouse Fertilization

Fertilization involves the fusion of two gametes by means of yet unknown membrane binding and fusion events. Over the last years, many sperm proteins have been uncovered to play essential roles in sperm-egg fusion in mammals, but their precise role in fertilization remains unknown, being unclear how these proteins interact with each other or with other yet unknown sperm proteins. The aim of this study has been to identify possible sperm proteins interacting with TMEM95, a protein essential for fertilization located in the sperm membrane. A list of 41 sperm proteins that were pulled down with TMEM95 and identified by mass spectrometry did not include other sperm proteins known to play a role in fertilization, suggesting an independent role of TMEM95 in fertilization. Between these lists, OLFR601 is allocated to the acrosomal region and may mediate affinity for an odorant involved in fertilization. However, Olfr601 disruption did not impair the sperm fertilization ability, suggesting that its function may be redundant with that of other sperm proteins.

Sperm membrane proteins DCST1 and DCST2 are required for sperm-egg interaction in mice and fish

The process of sperm-egg fusion is critical for successful fertilization, yet the underlying mechanisms that regulate these steps have remained unclear in vertebrates. Here, we show that both mouse and zebrafish DCST1 and DCST2 are necessary in sperm to fertilize the egg, similar to their orthologs SPE-42 and SPE-49 in C. elegans and Sneaky in D. melanogaster. Mouse Dcst1 and Dcst2 single knockout (KO) sperm are able to undergo the acrosome reaction and show normal relocalization of IZUMO1, an essential factor for sperm-egg fusion, to the equatorial segment. While both single KO sperm can bind to the oolemma, they show the fusion defect, resulting that Dcst1 KO males become almost sterile and Dcst2 KO males become sterile. Similar to mice, zebrafish dcst1 KO males are subfertile and dcst2 and dcst1/2 double KO males are sterile. Zebrafish dcst1/2 KO sperm are motile and can approach the egg, but are defective in binding to the oolemma. Furthermore, we find that DCST1 and DCST2 interact with each other and are interdependent. These data demonstrate that DCST1/2 are essential for male fertility in two vertebrate species, highlighting their crucial role as conserved factors in fertilization.

CRISPR/Cas9 Mediated Disruption of Seminal Fluid Protein Sfp62 Induces Male Sterility in Bombyx mori

Simple Summary

In gamogenetic animals, seminal fluid proteins are essential for male fertility. In this study, we investigated the function of the seminal fluid protein Sfp62 by using the CRISPR/Cas9 system in lepidopteran model insect Bombyx mori. Sfp62 mutation led to male sterility and can be inherited stably. The mutation did not affect growth and developmental nor female fertility. These data indicate that Sfp62 is an ideal target for sterile insect technology (SIT), in which genetically modified insects are released on a large scale to mate with wild-type insects in order to reduce or even eradicate the target pests. The determining factors for the effective implementation of SIT include the strong competitiveness of the modified individuals and multi-generational effects resulting from the mutation. Sfp62 meets these criteria and is therefore a promising target for biological pest control.

Abstract

Seminal fluid proteins provide factors necessary for development, storage, and activation of sperm. Altered expression of seminal fluid proteins can lead to defect in male infertility. We investigated the function of seminal fluid protein Sfp62 in the model lepidopteran insect Bombyx mori using CRISPR/Cas9 mediated mutagenesis. The knockout of BmSfp62 gene led to male sterility but has no effect on female fertility. The mutation did not affect growth and development of the silkworm of both sexes. Motility of sperm in male mutants was decreased and the mRNA expression levels of other genes encoding seminal fluid proteins were altered in BmSfp62 mutants compared to the wild-type controls. The male sterility caused by mutation of BmSfp62 was stably inherited. As the proteins encoded by Sfp62 genes are conserved among lepidopteran species, Sfp62 is a potential target for the biological management of lepidopteran pests.

The Role of Sperm Proteins IZUMO1 and TMEM95 in Mammalian Fertilization: A Systematic Review

Gamete membrane fusion is a critical cellular event in sexual reproduction. In addition, the generation of knockout models has provided a powerful tool for testing the functional relevance of proteins thought to be involved in mammalian fertilization, suggesting IZUMO1 and TMEM95 (transmembrane protein 95) as essential proteins. However, the molecular mechanisms underlying the process remain largely unknown. Therefore, the aim of this study was to summarize the current knowledge about IZUMO1 and TMEM95 during mammalian fertilization. Hence, three distinct databases were consulted—PubMed, Scopus and Web of Science—using single keywords. As a result, a total of 429 articles were identified. Based on both inclusion and exclusion criteria, the final number of articles included in this study was 103. The results showed that IZUMO1 is mostly studied in rodents whereas TMEM95 is studied primarily in bovines. Despite the research, the topological localization of IZUMO1 remains controversial. IZUMO1 may be involved in organizing or stabilizing a multiprotein complex essential for the membrane fusion in which TMEM95 could act as a fusogen due to its possible interaction with IZUMO1. Overall, the expression of these two proteins is not sufficient for sperm–oocyte fusion; therefore, other molecules must be involved in the membrane fusion process.

Involvement of cellular protrusions in gamete interactions

Gamete fusion is of considerable importance in reproductive events, as it determines the gamete pairs or chromosomes that the next generation will inherit. To preserve species specificity with an appropriate karyotype, the fusion between gametes requires regulatory mechanisms to ensure limited fusion competency. In many organisms, gamete surfaces are not smooth, but present constitutive or transient cellular protrusions suggested to be involved in gamete fusion. However, the molecular mechanisms and the factors essential for the membrane-membrane fusion process and cellular protrusion involvement have remained unclear. Recent advances in the identification and functional analysis of the essential factors for gamete interaction have revealed the molecular mechanisms underlying their activity regulation and dynamics. In homogametic fertilization, dynamic regulation of the fusion core machinery on cellular protrusions was precisely uncovered. In heterogametic fertilization, oocyte fusion competency was suggested to correlate with the compartmentalization of the fusion essential factor and protrusion formation. These findings shed light on the significance of cellular protrusions in gamete fusion as a physically and functionally specialized site for cellular fusion. In this review, we consider the developments in gamete interaction research in various species with different fertilization modes, highlighting the commonalities in the relationship between gamete fusion and cellular protrusions.

CEP128 is involved in spermatogenesis in humans and mice

Centrosomal proteins are necessary components of the centrosome, a conserved eukaryotic organelle essential to the reproductive process. However, few centrosomal proteins have been genetically linked to fertility. Herein we identify a homozygous missense variant of CEP128 (c.665 G > A [p.R222Q]) in two infertile males. Remarkably, male homozygous knock-in mice harboring the orthologous CEP128^R222Q variant show anomalies in sperm morphology, count, and motility. Moreover, Cep128 knock-out mice manifest male infertility associated with disrupted sperm quality. We observe defective sperm flagella in both homozygous Cep128 KO and KI mice; the cilia development in other organs is normal—suggesting that CEP128 variants predominantly affected the ciliogenesis in the testes. Mechanistically, CEP128 is involved in male reproduction via regulating the expression of genes and/or the phosphorylation of TGF-β/BMP-signalling members during spermatogenesis. Altogether, our findings unveil a crucial role for CEP128 in male fertility and provide important insights into the functions of centrosomal proteins in reproductive biology.

CEP128 is a centrosomal protein important for the organization of centriolar microtubules. Here, the authors show that a CEP128 variant observed in human male siblings causes reduced sperm counts and morphologically abnormal sperm when modeled in mice, suggesting a role for CEP128 in male fertility.

The future of assessing bull fertility: Can the 'omics fields identify usable biomarkers?

Breeding soundness examinations for bulls rely heavily on the subjective, visual assessment of sperm motility and morphology. Although these criteria have the potential to identify infertile males, they cannot be used to guarantee fertility or provide information about varying degrees of bull fertility. Male factor fertility is complex, and the success of the male gamete is not necessarily realized until well after the spermatozoon enters the oocyte. This paper reviews our existing knowledge of the bull’s contribution from a standpoint of the sperm’s cargo and the impact that this can have on fertilization and the development of the embryo. There has been a plethora of recent research characterizing the many molecular attributes that can affect the functional competence of a spermatozoon. A better understanding of the molecular factors influencing fertilization and embryo development in cattle will lead to the identification of biomarkers for the selection of bulls of superior fertility, which will have major implications for livestock production. To see this improvement in reproductive performance, we believe incorporation of modern technology into breeding soundness examinations will be necessary—although many of the discussed technologies are not ready for large-scale field application. Each of the ‘omics fields discussed in this review have shown promise for the identification of biomarkers of fertility, with certain families of biomarkers appearing to be better suited to different evaluations throughout a bull’s lifetime. Further research is needed for the proposed biomarkers to be of diagnostic or predictive value.

Sperm IZUMO1 Is Required for Binding Preceding Fusion With Oolemma in Mice and Rats

Fertilization occurs as the culmination of multi-step complex processes. First, mammalian spermatozoa undergo the acrosome reaction to become fusion-competent. Then, the acrosome-reacted spermatozoa penetrate the zona pellucida and adhere to and finally fuse with the egg plasma membrane. IZUMO1 is the first sperm protein proven to be essential for sperm-egg fusion in mammals, as Izumo1 knockout mouse spermatozoa adhere to but fail to fuse with the oolemma. However, the IZUMO1 function in other species remains largely unknown. Here, we generated Izumo1 knockout rats by CRISPR/Cas9 and found the male rats were infertile. Unlike in mice, Izumo1 knockout rat spermatozoa failed to bind to the oolemma. Further investigation revealed that the acrosome-intact sperm binding conceals a decreased number of the acrosome-reacted sperm bound to the oolemma in Izumo1 knockout mice. Of note, we could not see any apparent defects in the binding of the acrosome-reacted sperm to the oolemma in the mice lacking recently found fusion-indispensable genes, Fimp, Sof1, Spaca6, or Tmem95. Collectively, our data suggest that IZUMO1 is required for the sperm-oolemma binding prior to fusion at least in rat.