Eukaryotic fertilization and gamete fusion at a glance

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.

The absence of both RIBC1 and RIBC2 induces decreased sperm motility and litter size in male mice

BACKGROUND

RIBC1 (RIB43A domain with coiled-coils 1) and RIBC2 (RIB43A domain with coiled-coils 2) are homolog proteins of RIB43a which is localized to microtubules in the cilia and flagella of unicellular organisms. Cryo-electron microscopy and artificial intelligence studies showed that RIBC1 and RIBC2 are microtubule inner proteins (MIPs) localized in the inner lumen of the doublet microtubules (DMTs) in mouse sperm flagella. However, the function of RIBC1 and RIBC2 in mammalian reproduction and sperm flagella is still unknown.

OBJECTIVE

To clarify the function of RIBC1 and RIBC2 in mouse spermatozoa.

MATERIALS AND METHODS

We generated Ribc1 knockout (KO), Ribc2 KO, and Ribc1 and Ribc2 double-knockout (Ribc1/2 DKO) mice using the CRISPR/Cas9 system and analyzed their phenotypes.

RESULTS

We revealed that the loss of either RIBC1 or RIBC2 alone did not affect male fertility, but the absence of both caused a decrease in pup numbers. Sperm motility analysis showed that Ribc1 KO spermatozoa had reduced velocity, but Ribc2 KO sperm velocities were comparable with WT mice. However, Ribc1/2 DKO sperm velocities were significantly lower than those from Ribc1 KO mice. No structural abnormalities in the axonemal structure at the transmission electron microscope (TEM) level and no abnormalities in the flagellar waveform pattern were observed in Ribc1/2 DKO spermatozoa.

DISCUSSION AND CONCLUSION

Both RIBC1 and RIBC2 are not significant for maintaining the axonemal structure in mouse spermatozoa, but both proteins function cooperatively in sperm motility. This result may indicate that minor structural changes due to RIBC protein absence are not detected at the TEM level, and RIBC2 function depends on RIBC1 in sperm motility. We think that reduced litter size in Ribc1/2 DKO mice is caused by reduced sperm motility due to minor structural abnormalities caused by the loss of two RIBC proteins.

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.

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.

Cell-cell fusion in cancer: The next cancer hallmark?

In this review, we consider the role of cell-cell fusion in cancer development and progression through an evolutionary lens. We begin by summarizing the origins of fusion proteins (fusogens), of which there are many distinct classes that have evolved through convergent evolution. We then use an evolutionary framework to highlight how the persistence of fusion over generations and across different organisms can be attributed to traits that increase fitness secondary to fusion; these traits map well to the expanded hallmarks of cancer. By studying the tumor microenvironment, we can begin to identify the key selective pressures that may favor higher rates of fusion compared to healthy tissues. The paper concludes by discussing the increasing number of research questions surrounding fusion, recommendations for how to answer them, and the need for a greater interest in exploring cell fusion and evolutionary principles in oncology moving forward.

Sentient cells as basic units of tissues, organs and organismal physiology

Cells evolved some 4 billion years ago, and since then the integrity of the structural and functional continuity of cellular life has been maintained via highly conserved and ancient processes of cell reproduction and division. The plasma membrane as well as all the cytoplasmic structures are reproduced and inherited uninterruptedly by each of the two daughter cells resulting from every cell division. Although our understanding of the evolutionary emergence of the very first cells is obscured by the extremely long timeline since that revolutionary event, the generally accepted position is that the de novo formation of cells is not possible; all present cells are products of other prior cells. This essential biological principle was first discovered by Robert Remak and then effectively coined as Omnis Cellula e Cellula (every cell of the cell) by Rudolf Virchow: all currently living cells have direct structural and functional connections to the very first cells. Based on our previous theoretical analysis, all cells are endowed with individual sentient cognition that guides their individual agency, behaviour and evolution. There is a vital consequence of this new sentient and cognitive view of cells: when cells assemble as functional tissue ecologies and organs within multicellular organisms, including plants, animals and humans, these cellular aggregates display derivative versions of aggregate tissue- and organ-specific sentience and consciousness. This innovative view of the evolution and physiology of all currently living organisms supports a singular principle: all organismal physiology is based on cellular physiology that extends from unicellular roots.

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.

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.

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.

CRISPR/Cas9 Mediated Fluorescent Tagging of Caenorhabditis elegans SPE-38 Reveals a Complete Localization Pattern in Live Spermatozoa

The Caenorhabditis elegans spe-38 gene encodes a four-pass transmembrane molecule that is required in sperm for fertilization. In previous work, the localization of the SPE-38 protein was examined using polyclonal antibodies on spermatids and mature amoeboid spermatozoa. SPE-38 is localized to unfused membranous organelles (MOs) in nonmotile spermatids. Different fixation conditions revealed that SPE-38 either localized to fused MOs and the cell body plasma membrane or the pseudopod plasma membrane of mature sperm. To address this localization paradox in mature sperm, CRISPR/Cas9 genome editing was used to tag endogenous SPE-38 with fluorescent wrmScarlet-I. Homozygous male and hermaphrodite worms encoding SPE-38::wrmScarlet-I were fertile indicating the fluorescent tag does not interfere with SPE-38 function during sperm activation or fertilization. We found that SPE-38::wrmScarlet-I localized to MOs in spermatids consistent with previous antibody localization. In mature and motile spermatozoa we found SPE-38::wrmScarlet-I in fused MOs, the cell body plasma membrane, and the pseudopod plasma membrane. We conclude that the localization pattern observed with SPE-38::wrmScarlet-I represents the complete distribution of SPE-38 in mature spermatozoa and this localization pattern is consistent with a hypothesized role of SPE-38 directly in sperm-egg binding and/or fusion.

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.