Mammalian spermatozoa and cumulus cells bind to a 3D model generated by recombinant zona pellucida protein-coated beads

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.

Expression Analysis of ZPB2a and Its Regulatory Role in Sperm-Binding in Viviparous Teleost Black Rockfish

Black rockfish is a viviparous teleost whose sperm could be stored in the female ovary for five months. We previously proposed that zona pellucida (ZP) proteins of black rockfish play a similar sperm-binding role as in mammals. In this study, SsZPB2a and SsZPB2c were identified as the most similar genes with human ZPA, ZPB1 and ZPB2 by Blastp method. Immunohistochemistry showed that ovary-specific SsZPB2a was initially expressed in the cytoplasm of oocytes at stage III. Then it gradually transferred to the region close to the cell membrane and zona pellucida of oocytes at stage IV. The most obvious protein signal was observed at the zona pellucida region of oocytes at stage V. Furthermore, we found that the recombinant prokaryotic proteins rSsZPB2a and rSsZPB2c could bind with the posterior end of sperm head and rSsZPB2a was able to facilitate the sperm survival in vitro. After knocking down Sszpb2a in ovarian tissues cultivated in vitro, the expressions of sperm-specific genes were down-regulated (p < 0.05). These results illustrated the regulatory role of ZP protein to the sperm in viviparous teleost for the first time, which could advance our understanding about the biological function of ZP proteins in the teleost.

Sperm-Binding Assay Using an In Vitro 3D Model of the Mammalian Cumulus-Oocyte Complex

We have recently described a new model to study gamete interaction in mammalian species. The model recreates the spherical surface of the oocyte by using magnetic Sepharose beads coated with a layer of a recombinant protein involved in gamete interaction (such as ZP2, or the IZUMO1 receptor JUNO) and an external layer of cumulus oophorus cells, thus mimicking, to some extent, a native cumulus-oocyte complex. Once generated, this 3D model can be used in a sperm-binding assay to obtain valuable information about the molecular basis of gamete interaction, since different recombinant proteins can be used to coat the bead surface, thus generating a variety of models to be used for several species. Furthermore, thanks to the ability of the model to decoy sperm, the physiological status of the bound sperm can be studied, making this a powerful tool to select sperm with high fertilizing capacity, to unmask subfertile animals in livestock breeding centers, or for toxicological studies. Here, we describe how to generate and use this model for sperm-binding assays, using porcine sperm as an example, and ZP2, a protein from zona pellucida, as the recombinant protein of interest. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Generation of the in vitro 3D model Alternate Protocol 1: Binding cumulus oophorus cells to the model Basic Protocol 2: Quality control of the model by SDS-PAGE electrophoresis and western blot Support Protocol 1: Immunochemistry to confirm proper protein distribution on surface of beads Support Protocol 2: Elution of recombinant conjugated proteins Basic Protocol 3: Sperm-binding assay Alternate Protocol 2: Sperm preparation by the swim-up method.

Oolemma Receptors in Mammalian Molecular Fertilization: Function and New Methods of Study

Fertilization is a key process in biology to the extent that a new individual will be born from the fusion of two cells, one of which leaves the organism in which it was produced to exert its function within a different organism. The structure and function of gametes, and main aspects of fertilization are well known. However, we have limited knowledge about the specific molecules participating in each of the steps of the fertilization process due to the transient nature of gamete interaction. Moreover, if we specifically focus in the fusion of both gametes’ membrane, we might say our molecular knowledge is practically null, despite that molecular mechanisms of cell-to-cell adhesion are well studied in somatic cells. Moreover, between both gametes, the molecular knowledge in the egg is even scarcer than in the spermatozoon for different reasons addressed in this review. Sperm-specific protein IZUMO1 and its oocyte partner, JUNO, are the first cell surface receptor pair essential for sperm–egg plasma membrane binding. Recently, thanks to gene editing tools and the development and validation of in vitro models, new oocyte molecules are being suggested in gamete fusion such as phosphatidylserine recognition receptors. Undoubtedly, we are in a new era for widening our comprehension on molecular fertilization. In this work, we comprehensively address the proposed molecules involved in gamete binding and fusion, from the oocyte perspective, and the new methods that are providing a better understanding of these crucial molecules.