Cross-species fertilization: the hamster egg receptor, Juno, binds the human sperm ligand, Izumo1

Abstract

Fertilization is the culminating event in sexual reproduction and requires the recognition and fusion of the haploid sperm and egg to form a new diploid organism. Specificity in these recognition events is one reason why sperm and eggs from different species are not normally compatible. One notable exception is the unusual ability of zona-free eggs from the Syrian golden hamster (Mesocricetus auratus) to recognize and fuse with human sperm, a phenomenon that has been exploited to assess sperm quality in assisted fertility treatments. Following our recent finding that the interaction between the sperm and egg recognition receptors Izumo1 and Juno is essential for fertilization, we now demonstrate concordance between the ability of Izumo1 and Juno from different species to interact, and the ability of their isolated gametes to cross-fertilize each other in vitro. In particular, we show that Juno from the golden hamster can directly interact with human Izumo1. These data suggest that the interaction between Izumo1 and Juno plays an important role in cross-species gamete recognition, and may inform the development of improved prognostic tests that do not require the use of animals to guide the most appropriate fertility treatment for infertile couples.

Keywords: Izumo1; Juno; fertilization; oocyte; sperm; zona-free hamster egg penetration assay.

Figure 1.

The cell biology of mammalian fertilization. (a) Diagram highlighting the main cellular features of both the mammalian sperm and egg important during fertilization. (b) Schematics showing the four main steps in mammalian fertilization. (1) Sperm undergo the acrosome reaction and pass through the egg investments: the cumulus cells and the zona pellucida. (2) The acrosome reaction triggers the re-localization of Izumo1 (red) from the acrosomal membrane to the sperm head plasma membrane. (3) Once within the perivitelline space, the now exposed Izumo1 ligand is able to interact with the egg receptor Juno (green) displayed on the oolemma. (4) Finally, the sperm and egg membranes fuse, fertilizing the egg.

Figure 2.

Alignment of the extracellular amino acid sequences of hamster Juno with the mouse and human orthologues. The amino acid sequence of the hamster (M. auratus) Juno was aligned with the human (Homo sapiens) and mouse (Mus musculus) orthologues and identical amino acids boxed. The signal peptide and the predicted C-terminal propeptide are indicated.

Figure 3.

Human Izumo1 directly interacts with hamster, but not mouse Juno. (a) The hamster Juno biotinylated ‘bait’ protein was immobilized on streptavidin-coated microtitre plates and tested for its ability to bind human, mouse and pig Izumo1 by AVEXIS. Clear binding was observed with Izumo1 orthologues from all species relative to a negative control. (b) Human Izumo1 prey proteins were probed for interactions with either human or mouse Juno proteins presented as baits. Human Izumo1 interacted with human but not mouse Juno. The positive interaction between mouse Juno and mouse Izumo1 demonstrated that the mouse Juno bait protein was functional. Negative controls were the rat CD4 tag alone. Bar graphs represent mean ± s.d., n = 3.