CD81 and CD9 work independently as extracellular components upon fusion of sperm and oocyte

Abstract

SummaryWhen a sperm and oocyte unite into one cell upon fertilization, membranous fusion between the sperm and oocyte occurs. In mice, Izumo1 and a tetraspanin molecule CD9 are required for sperm-oocyte fusion as one of the oocyte factors, and another tetraspanin molecule CD81 is also thought to involve in this process. Since these two tetraspanins often form a complex upon cell-cell interaction, it is probable that such a complex is also formed in sperm-oocyte interaction; however, this possibility is still under debate among researchers. Here we assessed this problem using mouse oocytes. Immunocytochemical analysis demonstrated that both CD9 and CD81 were widely distributed outside the oocyte cell membrane, but these molecules were separate, forming bilayers, confirmed by immunobiochemical analysis. Electron-microscopic analysis revealed the presence of CD9- or CD81-incorporated extracellular structures in those bilayers. Finally, microinjection of in vitro-synthesized RNA showed that CD9 reversed a fusion defect in CD81-deficient oocytes in addition to CD9-deficient oocytes, but CD81 failed in both oocytes. These results suggest that both CD9 and CD81 independently work upon sperm-oocyte fusion as extracellular components.

Sperm-Egg Fusion: A Molecular Enigma of Mammalian Reproduction

“…It was proposed to play a role in gamete interaction, which was proved by in vitro fertilization assay, where the specific antibody against SLLP1 blocked both fertilization and binding. Receptor sites for this protein are found in the microvillar region of the egg and in the perivitelline space, which is in agreement with localization of CD9 [28]. The binding partner of SLLP1 was however found to be SAS1B (Sperm Acrosomal SLLP1 Binding), a specific oolemal metalloprotease [29].…”

Network Analyses of Sperm–Egg Recognition and Binding: Ready to Rethink Fertility Mechanisms?

“…At present, two different models of interaction with integrins during SERB process have been proposed: in one, fertilin b (ADAM 2) binds to a6b1 that is physically associated with CD9 and tethered to the actin cytoskeleton, whereas in the other model, fertilin b binds to a6b1 that is tethered to the actin cytoskeleton and CD9 indirectly influences the association of a6b1 with the actin cytoskeleton and, therefore, its ability to bind fertilin b (Chen et al, 1999). It has been recently proposed that CD9, together with CD81, could ''play a role as 'extracellular components' in sperm-oocyte membrane fusion'' (Ohnami et al, 2012). Indeed, CD9 and CD81 are present in exosomes released from oocytes: CD9 localizes in the perivitelline space, while CD81 is in zona pellucida (Ohnami et al, 2012).…”

“…It has been recently proposed that CD9, together with CD81, could ''play a role as 'extracellular components' in sperm-oocyte membrane fusion'' (Ohnami et al, 2012). Indeed, CD9 and CD81 are present in exosomes released from oocytes: CD9 localizes in the perivitelline space, while CD81 is in zona pellucida (Ohnami et al, 2012). When the reacted spermatozoa reach the perivitelline space, possibly they acquire CD9 from a transfer of membrane fragments from the oocyte plasma membrane.…”

Extracellular vesicles and reproduction–promotion of successful pregnancy

“…42,43,44 Subcellular expression of CD9 and CD81 in the mouse oocyte demonstrates that these are predominantly localized in the PVS and zona pellucida respectively. It has been proposed that CD9 is primarily produced by the oocyte, whereas CD81 is primarily produced in the surrounding cumulus cells.…”