O-284 Evidence for gamete recognition mechanisms in humans
Authors: Bianchi E , Wright G  J 
Published: July 2024

Abstract
Egg-sperm binding and fusion are the central events of fertilization and are essential for the formation of a new individual. While decades of research have provided a thorough understanding of the cellular processes, the identity of molecules orchestrating these events, along with their mechanism of action, remain elusive. Often the interactions established by cell surface proteins are weak and transient, making them challenging to deal with using standard biochemical methods, like purifications that involve stringent washing and denaturing conditions. Therefore, our knowledge of membrane proteins required for the sperm-egg interaction remains scarce and the research is further complicated by the limited amount of biological material, especially the eggs. Thanks to experimental approaches specifically developed to investigate cell surface interactions, we identified the first binding pair essential for fertilization in mammals 1. The pair is made of the egg protein JUNO and the sperm protein IZUMO1. Recently another membrane protein, the Fc receptor-like 3 (MAIA), has been indicated as a binding partner of IZUMO1 and suggested to be required for human fertilization2, but the data needs to be further investigated. Indeed, the same biochemical methods used to identify the JUNO-IZUMO1 binding pair, found no direct evidence of binding between MAIA and IZUMO13.
In the last decade more sperm proteins have been shown to be essential for sperm-egg interaction other than IZUMO1, and all findings have been validated using transgenic mouse models. Among these sperm proteins, Tmem95 was firstly discovered in bulls, it was later confirmed to be required for fertilization in mice, and new evidence suggests that its role is conserved in humans4,5.
Notably, the role played by each of the novel sperm candidates remains elusive and little is known on whether and how these cell surface molecules interact with one another.
Insights into the architecture of the mammalian egg-sperm fusion synapse are now emerging with the use of AlphaFold2, an artificial intelligence-based program capable of predicting proteins’ 3D structure. Remarkably, AlphaFold2 not only predicts the structure of extracellular proteins essential for fertilization, also suggests the formation of a pentameric complex composed of three sperm (IZUMO1, SPACA6 and TMEM81) and two egg (JUNO and CD9) proteins which is likely to be important for gamete recognition and fusion6,7.
In a high throughput screening we have recently identified a novel binding partner of IZUMO1, expressed in human leukocytes, potentially linking the reproductive and immune system (unpublished).
References
1. Bianchi, E., Doe, B., Goulding, D. & Wright, G. J. Juno is the egg Izumo receptor and is essential for mammalian fertilization. Nature 508, 483–487 (2014).
2. Vondrakova, J. et al. MAIA, Fc receptor-like 3, supersedes JUNO as IZUMO1 receptor during human fertilization. Sci Adv 8, eabn0047 (2022).
3. Bianchi, E. et al. No evidence for a direct extracellular interaction between human Fc receptor–like 3 (MAIA) and the sperm ligand IZUMO1. Science Advances 10, eadk6352 (2024).
4. Lamas-Toranzo, I. et al. TMEM95 is a sperm membrane protein essential for mammalian fertilization. eLife 9, e53913 (2020).
5. Tang, S. et al. Human sperm TMEM95 binds eggs and facilitates membrane fusion. Proceedings of the National Academy of Sciences 119, e2207805119 (2022).
6. Elofsson, A., Han, L., Bianchi, E., Wright, G. J. & Jovine, L. Deep learning insights into the architecture of the mammalian egg-sperm fusion synapse. eLife 13, (2024).
7. Deneke, V. E. et al. A conserved fertilization complex of Izumo1, Spaca6, and Tmem81 mediates sperm-egg interaction in vertebrates. 2023.07.27.550750 Preprint at https://doi.org/10.1101/2023.07.27.550750 (2023).