Fertility and Taxon-Specific Sperm Binding Persist after Replacement of Mouse Sperm Receptors with Human Homologs

Abstract

The zona pellucida surrounding ovulated mouse eggs contains three glycoproteins, two of which (ZP2 and ZP3) are reported sperm receptors. After fertilization, the zona pellucida is modified ad minimus by cleavage of ZP2, and sperm no longer bind. Crosstaxa sperm binding is limited among mammals, and human sperm do not bind to mouse eggs. Using transgenesis to replace mouse ZP2 and/or ZP3 with human homologs, mouse lines with human-mouse chimeric zonae pellucidae have been established. Unexpectedly, mouse, but not human, sperm bind to huZP2 and huZP2/huZP3 rescue eggs, eggs fertilized in vitro with mouse sperm progress to two-cell embryos, and rescue mice are fertile. Also unanticipated, human ZP2 remains uncleaved after fertilization, and mouse sperm continue to bind early rescue embryos. These observations are consistent with a model in which the supramolecular structure of the zona pellucida necessary for sperm binding is modulated by the cleavage status of ZP2.

Speciation and the evolution of gamete recognition genes: pattern and process

“…Interpretation of the ZP protein results is complicated by elegant experiments in which the human versions of ZP2 and ZP3 have been inserted into mouse lines in which mouse ZP2 and ZP3 have been removed. Inserting the human ZP2 or ZP3 genes restores fertility with mouse but not human sperm (Rankin et al, 1998(Rankin et al, , 2003. These results suggest that positive selection does not produce species-specific ZP3 or ZP2 sperm binding.…”

Protease activity involvement in the passage of mammalian sperm through the zona pellucida

“…This model comes from results where mouse ZP1 and ZP3 has been replaced by their human homologus in transgenic mice. Nonetheless, the human proteins were able to form a stable and well-organized zona pellucida at the macroscopic level; the zona is not functionally active, as with the mouse proteins (Gahlay et al, 2010;Rankin et al, 2003). Thus, the simplest explanation for our results is that acrosin treatment disrupts and/or disorganizes the supramolecular structure of hamster zona pellucida, (mainly the carbohydrates moieties), making the binding sites for spermatozoa unavailable.…”

“…However, the fertilization rate decreased under this condition. Recently it has been proposed that a specifi c supramolecular structure of mammalian zona pellucida is important to achieve fertilization and polyspermy block in mammals (Gahlay et al, 2010;Rankin et al, 2003). This model comes from results where mouse ZP1 and ZP3 has been replaced by their human homologus in transgenic mice.…”

“…In order to penetrate the zona pellucida, as one hypothesis states, acrosome-intact sperm recognizes and binds with a single O-linked oligosaccharides chain, which is probably ZP3 (Florman and Wassarman, 1985;Wassarman et al, 2001). On the other hand, a ZP2 cleavage model has recently been proposed that argues that rather than sperm binding to a single ligand, sperm binding is supported by a three-dimensional zona structure (Gahlay et al, 2010;Rankin et al, 2003). This model is predicated on the cleavage status of ZP2, rendering the zona pellucida either permissive (uncleaved ZP2) or non-permissive (cleaved ZP2) to account for sperm binding to the zona pellucida surrounding eggs, but not to those surrounding two-cell embryos (Gahlay et al, 2010;Rankin et al, 2003).…”

“…On the other hand, a ZP2 cleavage model has recently been proposed that argues that rather than sperm binding to a single ligand, sperm binding is supported by a three-dimensional zona structure (Gahlay et al, 2010;Rankin et al, 2003). This model is predicated on the cleavage status of ZP2, rendering the zona pellucida either permissive (uncleaved ZP2) or non-permissive (cleaved ZP2) to account for sperm binding to the zona pellucida surrounding eggs, but not to those surrounding two-cell embryos (Gahlay et al, 2010;Rankin et al, 2003). ZP2 has also been proposed as a binding partner of acrosome-reacted sperm (Bleil et al, 1988;Howes and Jones, 2002).…”