Recombinant hamster sperm receptors that exhibit species-specific binding to sperm

Polypeptide encoded by mouseZP3 exon-7 Is Necessary and Sufficient for binding of mouse sperm in vitro

Fertilization in mice is initiated by species-specific binding of sperm to mZP3, one of three mouse zona pellucida (ZP) glycoproteins. At nanomolar concentrations, purified egg mZP3 binds to acrosome-intact sperm heads and inhibits binding of sperm to eggs in vitro. Although several reports suggest that sperm recognize and bind to a region of mZP3 encoded by mZP3 exon-7 (so-called, sperm combining-site), this issue remains controversial. Here, exon-swapping and an IgG(Fc) fusion construct were used to further evaluate whether mZP3 exon-7 is essential for binding of sperm to mZP3. In one set of experiments, hamster ZP3 (hZP3) exon-6, -7, and -8 were individually replaced with the corresponding exon of mZP3. Stably transfected embryonal carcinoma (EC) cell lines carrying the recombinant genes were produced and secreted recombinant glycoprotein was purified and assayed for the ability to inhibit binding of sperm to eggs. While EC-hZP3, a recombinant form of hZP3 made by EC cells, is unable to inhibit binding of mouse sperm to eggs in vitro, the results suggest that substitution of mZP3 exon-7 for hZP3 exon-7, but not mZP3 exon-6 or -8, can impart inhibitory activity to EC-hZP3. In this context, a fusion construct consisting of human IgG(Fc) and mZP3 exon-7 and -8 was prepared, an EC cell line carrying the recombinant gene was produced, and secreted chimeric glycoprotein, called EC-huIgG(Fc)/mZP3(7), was purified and assayed. It was found that the chimeric glycoprotein binds specifically to plasma membrane overlying sperm heads to a similar extent as egg mZP3 and, at nanomolar concentrations, inhibits binding of mouse sperm to eggs in vitro. Collectively, these observations provide new evidence that sperm recognize and bind to a region of mZP3 polypeptide immediately downstream of its ZP domain that is encoded by mZP3 exon-7. The implications of these findings are discussed.

Species specificity in avian sperm:perivitelline interaction

The interaction of chicken spermatozoa with the inner perivitelline layer from different avian species in vitro during a 5 min co-incubation was measured as the number of points of hydrolysis produced per unit area of inner perivitelline layer. The average degree of interaction, as a proportion of that between chicken spermatozoa and their homologous inner perivitelline layer, was: equal to or greater than 100% within Galliformes (chicken, turkey, quail, pheasant, peafowl and guineafowl); 44% within Anseriformes (goose, duck); and less than 30% in Passeriformes (Zebra Finch) and Columbiformes (collared-dove). The homologue of the putative chicken sperm-binding proteins, chicken ZP1 and ZP3, were identified by Western blotting with anti-chicken ZP1/ZP3 antibody in the perivitelline layers of all species. The functional cross-reactivity between chicken spermatozoa and heterologous inner perivitelline layer appeared to be linked to known phylogenetic distance between the species, although it was not related to the relative affinity of the different ZP3 homologues for anti-chicken ZP3. This work demonstrates that sperm interaction with the egg investment does not represent such a stringent species-specific barrier in birds as it does in mammals and marine invertebrates. This may be a factor in the frequency of hybrid production in birds.

Conversion of Ser to Thr residues at the sperm combining-site of mZP3 does not affect sperm receptor activity

Mammalian eggs are surrounded by a thick extracellular coat, the zona pellucida, that is composed of three glycoproteins, called ZP1-3. Sperm recognize and bind to O-linked oligosaccharides attached to Ser-332 and Ser-334 at the sperm combining-site of mouse ZP3 (mZP3). Mutation of either of these Ser residues to a small aliphatic amino acid results in the loss of sperm binding to mZP3 in vitro. Here, we converted both Ser-332 and Ser-334 to Thr residues by site-directed mutagenesis. Recombinant mutant glycoprotein made by stably transfected EC cells was purified and then assayed for its ability to inhibit binding of sperm to ovulated eggs in vitro. Results of these experiments suggest that Thr residues can replace the two evolutionarily conserved Ser residues as acceptors for essential O-linked oligosaccharides at the sperm combining-site of mZP3 without affecting the glycoprotein's sperm receptor activity.

Secretion and assembly of zona pellucida glycoproteins by growing mouse oocytes microinjected with epitope-tagged cDNAs for mZP2 and mZP3

The zona pellucida (ZP) is a highly organized extracellular coat that surrounds all mammalian eggs. The mouse egg ZP is composed of three glycoproteins, called mZP1-3, that are synthesized, secreted, and assembled into a ZP exclusively by growing oocytes. Here, we microinjected epitope-tagged (Myc and Flag) cDNAs for mZP2 and mZP3 into the germinal vesicle (nucleus) of growing oocytes isolated from juvenile mice. Specific antibodies and laser scanning confocal microscopy were used to follow nascent, recombinant ZP glycoproteins in both permeabilized and nonpermeabilized oocytes. When such cDNAs were injected, epitope-tagged mZP2 (Myc-mZP2) and mZP3 (Flag-mZP3) were synthesized, packaged into large intracellular vesicles, and secreted by the vast majority of oocytes. Secreted glycoproteins were incorporated into only the innermost layer of the thickening ZP, and the amount of nascent glycoprotein in this region increased with increasing time of oocyte culture. Consistent with prior observations, the putative transmembrane domain at the C terminus of mZP2 and mZP3 was missing from nascent glycoprotein incorporated into the ZP. When the consensus furin cleavage site near the C terminus of mZP3 was mutated, such that it should not be cleaved by furin, secretion and assembly of mZP3 was reduced. On the other hand, mZP3 incorporated into the ZP lacked the transmembrane domain downstream of the mutated furin cleavage site, suggesting that some other protease(s) excised the domain. These results strongly suggest that nascent mZP2 and mZP3 are incorporated into only the innermost layer of the ZP and that excision of the C-terminal region of the glycoproteins is required for assembly into the oocyte ZP.

Zona pellucida glycoprotein mZP3 produced in milk of transgenic mice is active as a sperm receptor, but can be lethal to newborns

Mouse egg zona pellucida glycoprotein mZP3 (approximately 83 kDa M(r)) serves as a species-specific sperm receptor and acrosome reaction-inducer during fertilization in mice. These biological activities are dependent on certain mZP3 serine/threonine- (O-) linked oligosaccharides present at the combining-site for sperm. In an attempt to produce large amounts of biologically active mZP3, we generated several transgenic mouse lines carrying the full-length mZP3 gene fused to the beta-casein gene promoter and transcription termination sequence. We found that different transgenic mouse lines have different amounts of recombinant mZP3 (approximately 63 kDa M(r)) in milk of lactating females, from approximately 0.3 to 3.5 micrograms/microliter of milk. In all cases, purified milk-mZP3 is active as a sperm receptor and acrosome reaction-inducer in vitro. Unexpectedly, we also found that development of litters from these transgenic mice is related to the amount of mZP3 in the mother's milk. In the most extreme case, litters from the highest expressers fail to live beyond about day-7 post partum unless placed immediately after birth with surrogate wild-type mothers. Litters from lower expressers initially display a complex phenotype that includes effects on hair and body growth, but some of the mice survive and, in time, are restored to a wild-type phenotype. These results demonstrate that relatively large amounts of biologically active mZP3 can be produced in transgenic mouse milk for structural and other studies, but that the presence of mZP3 in milk has dramatic developmental effects on litters, ranging from retarded hair and body growth to death of newborn pups.

Isolation and characterisation of a cDNA encoding a zona pellucida protein (ZPB) from the marsupial Trichosurus vulpecula (brushtail possum)

We have cloned a cDNA containing the entire coding sequence of a marsupial (the brushtail possum, Trichosurus vulpecula) zona pellucida protein (ZPB). The open reading frame of 1,581 nt is predicted to encode a ZPB polypeptide of 527 amino acids which contains 20 cysteine residues, 7 potential N-linked glycosylation sites, a potential N-terminal signal peptide and a potential C-terminal trans-membrane domain, preceded by a furin proteolytic processing signal. Sequence comparisons between possum ZPB and orthologous polypeptides from 7 eutherian species and from Xenopus laevis, reveal the existence of a high degree of sequence similarity, particularly in the central portion of the molecule. Cysteine residues are highly conserved, and all nine species possess potential N-terminal signal peptide sequences and C-terminal trans-membrane domains of approximately the same length. In situ hybridisation revealed that expression of ZPB was restricted to oocytes of primordial and primary follicles of adult possums; no expression was detected in the surrounding granulosa cells. The broad conservation of ZPB sequence, structure and expression over a wide range of mammalian species, revealed by our studies, makes it unlikely that these features account for the different properties of the marsupial and eutherian zona pellucidae.

Inactivation of the mouse sperm receptor, mZP3, by site-directed mutagenesis of individual serine residues located at the combining site for sperm

To initiate fertilization, mouse sperm bind to Ser- (O-) linked oligosaccharides located at the sperm combining site of zona pellucida glycoprotein mZP3. Apparently, the oligosaccharides are present on one or more of five Ser residues clustered in the carboxyl-terminal region of the mZP3 polypeptide. Here, each of the Ser residues, as well as an intervening Asn residue, was converted to a small, nonhydroxy amino acid by site-directed mutagenesis. Mouse embryonal carcinoma (EC) cells were then stably transfected with the wild-type and mutated mZP3 genes. In each case, transfected cells synthesized and secreted recombinant EC-mZP3 into the culture medium. The glycoproteins were partially purified and assayed for their ability to inhibit binding of sperm to ovulated eggs in vitro. As compared with wild-type EC-mZP3, mutations of Ser-329, Ser-331, or Ser-333 had no effect on sperm receptor activity. Mutation of Asn-330, a potential N-linked glycosylation site, also had no effect on sperm receptor activity. On the other hand, mutation of either Ser-332 or Ser-334, or mutation of Ser-332, Ser-333, and Ser-334, resulted in complete inactivation of EC-mZP3 as a sperm receptor. These results suggest that Ser-332 and Ser-334, residues conserved in mouse, hamster, and human ZP3, are essential for sperm receptor activity.

The abalone egg vitelline envelope receptor for sperm lysin is a giant multivalent molecule

Abalone sperm lysin is a 16-kDa acrosomal protein, which nonenzymatically and species selectively creates a hole in the egg vitelline envelope (VE) through which the sperm passes to reach the egg cell membrane. The crystal structures of both monomeric and dimeric lysins have been solved and the sequences of lysins from 20 abalone species have been determined. As a first step in understanding the molecular mechanism by which lysin creates a hole in the VE, its VE receptor was isolated. The VE receptor for lysin (VERL) is an unbranched, rod-like molecule with an approximate relative molecular mass of 2 million; half the mass being carbohydrate. Fluorescence polarization studies showed positive cooperativity in the binding of lysin to VERL (EC50 approximately 9 nM) and were consistent with the species selectivity of lysin in dissolving VEs. Each molecule of VERL bound between 126 and 142 molecules of monomeric lysin (two independent assays), showing that VERL possesses repetitive lysin-binding motifs.

Zona pellucida glycoprotein mZP3 bioactivity is not dependent on the extent of glycosylation of its polypeptide or on sulfation and sialylation of its oligosaccharides

During fertilization in mice, free-swimming sperm bind to mZP3, one of three egg zona pellucida glycoproteins. Sperm recognize and bind to specific serine/threonine-linked (O-linked) oligosaccharides located at the mZP3 combining site for sperm. Shortly after binding to mZP3, sperm undergo the acrosome reaction, a form of cellular exocytosis. Here, we examined the influence of extent of glycosylation, sulfation, and sialylation of mZP3 (M(r) approximately 65,000-100,000) on its bioactivity; i.e. its ability to inhibit binding of sperm to eggs and to induce the acrosome reaction in vitro. Low (av. M(r) approximately 70,000), medium (av. M(r) approximately 82,000), and high (av. M(r) approximately 94,000) M(r) fractions of mZP3 were purified and shown to vary in extent of asparagine-linked (N-linked) glycosylation. All three size-fractions exhibited bioactivity, suggesting that the ability of mZP3 to inhibit binding of sperm to eggs is not related to the extent of glycosylation of its polypeptide (M(r) approximately 44,000). Digestion of mZP3 by neuraminidase decreased its average M(r) from approximately 83,000 to approximately 77,000 and increased its average pI from approximately 4.7 to approximately 6.0, but did not significantly affect mZP3 bioactivity. Terminal sialic acid largely accounts for the glycoprotein's acidic nature, but is not an essential element of the mZP3 combining site for sperm. Experiments with stably transfected embryonal carcinoma (EC) cells that secrete bioactive EC-mZP3 revealed that, of the sulfate present, approximately 70–75% was located on N-linked and approximately 25–30% on O-linked oligosaccharides. EC-mZP3 devoid of sulfate inhibited binding of sperm to eggs and induced the acrosome reaction to the same extent as sulfated EC-mZP3. These results suggest that sulfation of EC-mZP3 oligosaccharides is not essential for bioactivity. Overall, these findings contrast with those reported for certain other glycoproteins involved in cellular adhesion that require sulfate and/or sialic acid for bioactivity.