The authors discussed the basic strategies for uniting the membranes and cytoplasms …

• Summary
• Introduction
• Attachment and fusion proteins in virus-cell and cell-cell fusion
• The process of fertilization
• Sperm-oocyte attachment and fusion
• The egg
• The sperm
• Perspectives
• References

 
ADAMs
ADAM-family proteins are ideal candidates for attachment proteins on sperm owing to the presence of a transmembrane domain and two cell-cell adhesion domains – the disintegrin and cysteine-rich domains (Iba et al., 2000; Primakoff and Myles, 2000). The founding ADAM-family members, subunits of the heterodimer fertilin (ADAM1b-ADAM2), were originally identified in a monoclonal antibody screen for guinea pig sperm proteins with a role in gamete fusion (Primakoff et al., 1987). To date, 39 ADAM-family members have been identified. About half of these are testis-specific or testis-enriched proteins, which indicates that this family of proteins might have important roles in sperm function.

The best-studied sperm ADAMs are fertilin ß (ADAM2) and cyritestin (ADAM3). Initial experiments in the mouse system showed that peptides from the disintegrin loop of these two ADAMs potently inhibit sperm-oocyte fusion in vitro, whereas the same regions of ADAM1, ADAM4 and ADAM5 are ineffective (Yuan et al., 1997). Targeted deletion of Adam2 or Adam3 yields infertile male mice (Cho et al., 1998; Nishimura et al., 2001; Shamsadin et al., 1999). However, both mouse lines show defects in the fertilization process upstream of gamete membrane interactions. Adam2^–/– sperm exhibit defective transit through the female reproductive tract (Cho et al., 1998). Adam2^–/– and Adam3^–/– mice exhibit defective ZP binding (Cho et al., 1998; Nishimura et al., 2001; Shamsadin et al., 1999). In vitro tests of ZP-free oocytes found that Adam3^–/– sperm bind to the oocyte plasma membrane at very low levels (9% compared with wild type), but fuse at a normal rate (Nishimura et al., 2001). This finding eliminates a required role for cyritestin in physiologically relevant plasma membrane binding or fusion. Such a conclusion is supported by recent data showing that ADAM3 is released from the cell during the acrosome reaction and thus may not be present on the sperm surface as an integral protein at the time of plasma membrane binding (Kim et al., 2004). Adam2^–/– sperm show a similar reduction in plasma membrane binding, and a 50% decrease in fusion under these in vitro conditions (Cho et al., 1998). The Adam2^–/–Adam3^–/– double-knockout mice mimic Adam2^–/– sperm (Nishimura et al., 2001). ADAM2 is not essential for plasma membrane binding or fusion, but could have a contributing role in this process. A molecular dissection of this protein might provide a better understanding of its direct role, if any, in fusion.

Finally, one other factor complicates the interpretation of these data. Adam2^–/– and Adam3^–/– sperm do not lack only the genetically targeted protein. Through an as-yet-undetermined mechanism, the levels of at least two other ADAMs (ADAM3, ADAM1b) are reduced on Adam2^–/– sperm, and it is possible that the loss extends to other classes of proteins (Nishimura et al., 2001). Adam3^–/– sperm show a similar, although less-severe, phenotype. An essential role for an ADAM protein in fusion is still possible, because several other ADAMs are expressed on sperm (Brachvogel et al., 2002; Choi et al., 2003; Wolfsberg et al., 1995; Zhu et al., 2001). Recombinant protein and disintegrin peptide loop inhibition studies show a somewhat specific and potent inhibition of fusion (Chen and Sampson, 1999; Yuan et al., 1997). These peptides may inhibit the interaction of some other ADAM with the oocyte.