Sperm SPACA6 protein is required for mammalian Sperm-Egg Adhesion/Fusion

Sandrine Barbaux,Côme Ialy-Radio,Méline Homps-Legrand,Jean-Philippe Wolf,Ahmed Ziyyat,Marcio Do Cruzeiro,Daniel Vaiman,Elisa Dybal,Myriam Chalbi

doi:10.1038/s41598-020-62091-y

Abstract

Three genes are known to be essential for gamete adhesion/fusion (Cd9, Izumo1 and Juno). Here, we confirmed that Spaca6 null males are infertile and showed that their sperm accumulate in the perivitelline space but are unable to fuse with oocyte. Like IZUMO1, SPACA6 which is expressed by human sperm, is remained on the equatorial segment after acrosomal reaction and is involved in human fertilization since an anti-SPACA6 antibody inhibited it. Despite the similarity of the phenotypes caused by Spaca6 and Izumo1 knockouts, these are not redundant and the essential relocation of IZUMO1 is not affected by the lack of SPACA6. We propose a model in which IZUMO1 and SPACA6 would be part of a molecular complex necessary for gamete fusion and that their concomitant presence would be required for the recruitment of another essential molecular actor, such as a fusogen, for the fusion to take place.

Highlights

Among the membrane molecular players involved in mammalian gamete adhesion/fusion, only three, the spermatic immunoglobulin IZUMO11, the glycosylphosphatidylinositol-anchored protein (GPI-AP) JUNO, its oocyte receptor[2] and the oocyte CD9 tetraspanin are described as essential[3,4,5]
In addition to the three molecular actors described as essential for the gamete adhesion/fusion step during www.nature.com/scientificreports mammalian fertilization, which are the oocyte CD9, organizer of molecular complexes, sperm IZUMO1 and oocyte JUNO, a ligand-receptor pair, a fourth sperm actor has been suggested playing an important role during these steps: SPACA6, sperm acrosome associated 6
The one directed against the murine form of the protein proved to be totally non-specific. This is probably due to the polyclonal nature of our antibodies and to the fact that the protein belongs to a superfamily, that of immunoglobulins, which are known to be involved in cell adhesion

Summary

Introduction

Among the membrane molecular players involved in mammalian gamete adhesion/fusion, only three, the spermatic immunoglobulin IZUMO11, the glycosylphosphatidylinositol-anchored protein (GPI-AP) JUNO, its oocyte receptor[2] and the oocyte CD9 tetraspanin are described as essential[3,4,5]. Www.nature.com/scientificreports have reported that these depletions take place after the fusion stage[13] This result was made possible by the use of a microfluidics approach which, combined with relevant fluorescent probe, guarantiees the best frontal view of the sperm/egg interaction area and the timeline of the fertilization events. Thanks to this approach, these authors have added a new element in the understanding of the stages of gamete adhesion/fusion. It is known that this gene encodes an immunoglobulin-like protein, a single-pass type I membrane protein containing a signal peptide (aa 1 to 26), a large extracellular domain (aa 27 to 295), a transmembrane helical domain (aa 296 to 316) and small cytoplasmic domain (aa 317 to 324)

Methods

Results

Conclusion