Sperm-Specific Factors Implicated in Oolemma Binding and / or Fusion

In mammals, a key event in the process of fertilization occurs when a sperm binds to and fuses with the oocyte plasma membrane (oolemma). However, information regarding the genetic and molecular bases of this complex process of sperm-oolemma penetration (SOP) is limited. Research to elucidate the sperm factors involved in SOP has resulted in the identification of several important candidate molecules, most of which have been recognized using antibodies to sperm surface antigens (immunological approach). Of these, the best characterized are those belonging to the ADAM (A Disintegrin and A Metalloprotease) and CRISP (Cysteine-Rich Secretory Protein) gene families. ADAM family members - Fertilin a, Fertilin b and Cyritestin - are known to be involved in sperm binding to the oolemma. The CRISP family includes DE (also known as acidic epididymal glycoprotein [AEG], CRISP-1) which appears to play a role in sperm-egg fusion, without affecting sperm-oolemma binding. In addition to the immunological approach, studies on mouse t haplotypes have led to the identification of genetic loci important in the process of SOP. Mouse t haplotypes carry mutations that cause defects in several sperm functions. Recent studies have suggested that t haplotypes contain altered alleles of genes that encode sperm-oolemma binding and/or fusion proteins. The molecular identification of these various sperm-specific factors and the elucidation of their roles in SOP will increase our knowledge of the cellular basis of fertilization, and will help in designing therapeutic strategies for contraception and sperm dysfunction.