Characterization of a novel ZP3-independent sperm-binding ligand that facilitates sperm adhesion to the egg coat

Abstract

During mammalian fertilization, sperm adhere to the extracellular coat of the egg, or zona pellucida, in a species-specific manner. In mouse, evidence suggests that sperm recognize and bind to specific oligosaccharide ligands within the zona pellucida glycoprotein, ZP3, viaβ1,4-galactosyltransferase I (GalT I), a lectin-like receptor on the sperm surface. Although in vitro experiments using isolated gametes lend support to this model, recent in vivo studies of genetically altered mice question whether ZP3 and/or GalT I are solely responsible for sperm-egg binding. In this regard, sperm from GalT I-null mice bind poorly to ZP3 and fail to undergo a zona-induced acrosome reaction; however, they still bind to the ovulated egg coat in vitro. In this report, we characterize a novel ZP3- and GalT I-independent mechanism for sperm adhesion to the egg coat. Results show that the ovulated zona pellucida contains at least two distinct ligands for sperm binding: a ZP3-independent ligand that is peripherally associated with the egg coat and facilitates gamete adhesion; and a ZP3-dependent ligand that is present in the insoluble zona matrix and is recognized by sperm GalT I to facilitate acrosomal exocytosis. The ZP3-independent ligand is not a result of contamination by egg cortical granules, nor is it the mouse homolog of oviduct-specific glycoprotein. It behaves as a 250 kDa, WGA-reactive glycoprotein with a basic isoelectric point, distinguishing it from the acidic glycoproteins that form the insoluble matrix of the egg coat. When eluted from isoelectric focusing gels, the acidic matrix glycoproteins possess sperm-binding activity for wild-type sperm, but not for GalT I-null sperm,whereas the basic glycoprotein retains sperm-binding activity for both wild-type and GalT I-null sperm. Thus, GalT I-null sperm are able to resolve gamete recognition into at least two distinct binding events, leading to the characterization of a novel, peripherally associated, sperm-binding ligand on the ovulated zona pellucida.

Oviduct-specific glycoprotein and heparin modulate sperm–zona pellucida interaction during fertilization and contribute to the control of polyspermy

“…Moreover, we have shown that mouse eggs exposed to bOF do not acquire resistance to proteinase and, intriguingly, mouse is an exception for the OGP association with the ZP, despite its evidence in the perivitelline space (21). These two issues suggest that in the mouse OGP is unrelated to ZP resistance to proteinases or to sperm-ZP binding, as has been recently described (15).…”

“…However, it cannot be ruled out that oviductal modifications of the ZP could also involve the coating of ZP sperm-binding sites, representing a different mechanism to select the fertilizing spermatozoon. In fact, previous observations have revealed differences between ovulated and antral follicular mouse oocytes in ZP sites for sperm (15). Whether an equivalent situation occurs in ungulates is unknown.…”

Molecular cloning and characterization of a C-type lectin from Ancylostoma ceylanicum: Evidence for a role in hookworm reproductive physiology

“…In mammals, CTLs have been shown to affect a variety of important functions mediated by cell-cell interactions, including inflammation, thrombosis, development, and reproductive physiology [6,27,28]. There are more than 120 C-type lectin domain containing proteins predicted from the genome sequence of the non-parasitic nematode C. elegans, some of which have been characterized for their physiologic roles [4,29].…”

“…Although little is known about molecular mechanisms of fertilization in hookworms, the in vitro binding specificity of rAceCTL-1 provided the first evidence of its potential role in reproductive physiology, as GlcNAC is a likely to be a major component of the nematode egg membrane [6,28,41].…”

“…In mammalian reproductive physiology, CTLs are involved in the molecular mechanisms underlying successful fertilization of embryos, a process through which sperm lectins recognize specific carbohydrate moieties on egg surface glycoproteins [6]. In the free living and hermaphroditic nematode Caenorhabditis elegans, some of the genes involved in fertilization (fer) and spermatogenesis (spe) have been characterized, and their role in progeny development defined [7,8].…”