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.
Highlights
In sexually reproducing species, the spermatozoon and the egg are terminally differentiated haploid cells that are responsible for the propagation of genetic information from one generation to the
The discovery that zona-free hamster eggs could fuse with acrosome-reacted human sperm pro- 4 vided an experimentally amenable model system which played an important role in the development of in vitro fertilization (IVF) and the functional screening of human sperm
Our recent discovery of Juno as an egg receptor for the sperm Izumo1 ligand and the demonstration that the interaction between these molecules was essential for fertilization suggested that they might provide a molecular explanation for the remarkable ability of hamster eggs to fuse with sperm from other species
Summary
The spermatozoon (sperm) and the egg are terminally differentiated haploid cells that are responsible for the propagation of genetic information from one generation to the next. We show using recombinant proteins that hamster Juno can directly interact with mouse, pig and human Izumo, matching the ability of zona-free hamster eggs to fuse with human, mouse and pig sperm. These results provide a molecular explanation for the ability of gametes from different species to recognize and fuse with each other. The regions encoding the ectodomains of Juno and Izumo were flanked by unique NotI and AscI sites and subcloned into a derivative of the pTT3 expression vector [12] that contains a rat CD4 (Ig-like domains 3 and 4) tag for quantitation, and either an enzymatically biotinylatable peptide tag (‘bait’ vector), or a pentamerization domain from the rat cartilage oligomeric matrix protein (COMP) and b-lactamase (‘prey’ vector) Both bait and prey proteins contained a C-terminal 6-His tag for purification [13]. The assays were repeated three times using independent protein preparations
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