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J. Biol. Chem., Vol. 281, Issue 20, 13853-13856, May 19, 2006
Minireview Molecular Models for Murine Sperm-Egg Binding* 1 2
From the
Murine sperm initiate fertilization by binding to the specialized extracellular matrix of mouse eggs, known as the zona pellucida. Over the past decade, powerful genetic, biophysical, and biochemical techniques have been employed to gain new insights into this interaction. Evidence from these studies does not support either of two major models for binding first proposed over two decades ago. Two more recently established models suggest that protein-protein interactions predominate during this initial stage of fertilization. Another model proposes that about 7580% of the murine sperm bound to zona pellucida under well defined in vitro conditions is carbohydrate dependent, with the remaining sperm bound via protein-protein interactions. Mounting evidence suggests that the carbohydrate sequences coating the murine egg could be employed as specific immune recognition markers. Continued investigation of this system may resolve many of these controversial findings and reveal novel functions for murine zona pellucida glycoproteins.
The species-specific binding of sperm to eggs is the initial event in the development of metazoans that propagate solely via sexual reproduction. Considerable evidence indicates that carbohydrate recognition plays a key role in this interaction in lower species (reviewed in Refs. 13), eutherian mammals (4, 5), and marsupials (6). The major animal model for investigating the role of carbohydrate recognition in mammalian fertilization is the mouse. This species is preferred because of its fecundity, short life span, low maintenance costs, and its ability to be genetically manipulated. Murine sperm initiate fertilization by binding to the mZP,3 a matrix composed of three major glycoproteins designated mZP1, mZP2, and mZP3 (7). mZP3 has been implicated as the component that mediates both initial binding and the induction of the acrosome reaction (8). Its genetic deletion leads to the complete loss of a functional mZP and infertility (9). Therefore mZP3 is usually the primary glycoprotein targeted in functional studies.
Two major hypothetical models for murine gamete binding were first presented in 1985. The resistance of mZP3 to denaturing conditions suggested that its protein-linked glycans could be responsible for initial sperm-egg binding (10). Subsequent studies demonstrated that either glycopeptides or O-linked glycans derived from mZP3 inhibit murine sperm-egg binding in a competitive murine sperm-egg binding assay (10, 11). Therefore Wassarman and colleagues suggested that EBPs on the sperm plasma membrane interact with mZP3 associated O-glycans to mediate initial murine sperm-egg binding.
Digestion of the O-glycans derived from mZP3 with
Insertion of the mZP3 gene into the F9 embryonal carcinoma cells yields recombinant mZP3 that is functionally equivalent to native mZP3 (14). Subsequent exon swapping studies suggested that a region from Ser-329
Another carbohydrate-dependent model suggests that a specific
These two models were completely dominant in the murine system for the decade following their proposal. The development of molecular biological approaches (22) and ultrasensitive mass spectrometric methods for sequencing glycans (23) and proteins (24) enabled investigators to directly test these predominant models.
Female transgenic mice lacking Wassarman and co-workers have consistently contested these findings (29, 30). In a recent study, sp56 was again localized to the sperm plasma membrane (29). This group recently suggested that only those mZP3 molecules that actually coat the surface of the egg are modified at Ser-332 and Ser-334 (30). This restricted modification would be extremely difficult to detect because the molar level of mZP3 on the surface of the mZP is quite low.
The model suggesting that the
The two established models for murine gamete binding have been severely compromised by evidence collected over the last decade. This lack of a consensus has led to the development of alternate molecular models for initial sperm-egg binding.
Shur and colleagues recently proposed that the
Powerful molecular biological approaches have been employed to investigate the specificity of murine sperm-egg binding in the context of the human interaction (22). Human sperm do not bind to murine eggs (37). Dean and his colleagues hypothesized that female transgenic mice expressing huZP3 instead of mZP3 ("huZP3 rescue mice") would produce eggs that could bind human sperm (38). Eggs from the huZP3 rescue mice bind murine but not human sperm. These investigators proposed that the supramolecular complex of all huZP glycoproteins is required for human sperm binding. To test this hypothesis, mice expressing huZP2 (huZP2 rescue mice) and both huZP2 and huZP3 (huZP2/huZP3 rescue mice) were created (39). However, eggs from both mouse lines continue to bind murine but not human sperm. These findings are more consistent with the model suggesting that glycosylation confers the taxon binding specificity of human sperm (40). A recent study suggests that the huZP is composed of four glycoproteins (41). Therefore Dean and his co-workers (42) have recently suggested that a complex of four rather than three huZP glycoproteins may be required for human sperm binding.
In 1983, Yamagata and colleagues (43) reported that murine sperm bind to rabbit erythrocytes but did not characterize this interaction. A later study revealed very robust binding between these cell types, leading to the formation of aggregates (Fig. 1, panel A). Only acrosome-intact sperm bind, indicating that this interaction is between their plasma membranes (44) (Fig. 1, panel B). However, the molecular basis of this somatic cell adhesion was not determined. Rabbit erythrocytes express very large glycosphingolipids known as polyglycosylceramides (45). The oligosaccharide component of these polyglycosylceramides is composed of between one and seven branches (Gal 13Gal 14GlcNAc 16) linked to a linear polylactosamine backbone. The largest derivative in this family (PG-7) is shown in Fig. 2. This result suggested that sperm-erythrocyte binding could be mediated solely via terminal Gal 13Gal sequences, based on the evidence available at that time (12). However, exhaustive treatment of rabbit erythrocytes with -galactosidase reduced but did not eliminate sperm binding (46), indicating that glycans terminated with 14-linked Gal could mediate sperm binding to this cell type and by analogy the mZP (46).
Evidence that polyglycosylceramides could mediate this binding by interacting with sperm EBPs was provided in another study. Artificial oligosaccharide Constructs expressing the same terminal branched sequence as the rabbit erythrocyte polyglycosylceramides (Fig. 2, dashed boxes in PG-7, S1, and S4) were tested in the competitive murine sperm-egg binding assay at a final concentration of 4 µM (13). Construct S1 blocks murine sperm-egg binding by 7075%, whereas Construct S4 is poorly inhibitory (13). Construct S2 is equally as active as Construct S1 in this assay system, indicating that terminal
Data obtained in other laboratories were consistent with this specificity. Digestion of murine eggs with a highly specific
There was one major problem with this potential specificity, however. Evidence available at that time indicated that linear, but not branched, polylactosamine type sequences are linked to mZP3 glycans (20, 48). There was evidence confirming that mZP3 expresses tri- and tetraantennary N-linked oligosaccharides bearing the
This sequence is primarily expressed on Core 2 O-glycans, complex type tri- and tetraantennary N-glycans, and branched polylactosamine sequences in mice. Ultrasensitive biophysical analyses confirmed that the majority of the mZP O-glycans are Core 2 type sequences linked almost exclusively to mZP3 (32, 33). Complex type tri- and tetraantennary type N-glycans bearing Another distinct possibility was that rabbit erythrocytes could express protein-linked oligosaccharides that resemble those found on the mZP. However, a recent preliminary analysis indicates that the majority of the N-glycans from this cell type are biantennary complex type sequences with one antennae consisting of a highly branched polylactosamine type chain exactly like those present in polyglycosylceramides from this cell type (49). These data suggest that murine sperm bind to both N-glycans and polyglycosylceramides via the recognition of the terminal branched polylactosamine sequence present in Constructs S1 and S4 (Fig. 2). However, adhesion is likely achieved via multivalent interactions with monovalent branched polylactosamine sequences expressed on this cell type, rather than with divalent presentations like those involving Construct S1. Constructs S1 and S2 have never been found in the natural setting.
The binding of murine sperm to mouse eggs is blocked by the exoglycosidase digestion of the mZP (34) or the presence of specific carbohydrate inhibitors (13, 47). However, the maximal inhibitory effect obtained in these studies ranges from 7078% (13, 34, 47). Mild periodate oxidation of human ZP glycans also reduces human sperm binding by 79% in the human hemizona assay system (50). These results suggest that perhaps 2025% of the sperm binding sites on mZP in these assay systems are carbohydrate-independent.
This redundant binding is supported by the inactivation of glycosyltransferase genes. The obligatory enzyme required for the synthesis of Core 2 O-glycans is the
The molecular basis underlying this carbohydrate independent interaction is currently unknown. The initial events of murine fertilization involve both sperm binding and the induction of the acrosome reaction. The binding of murine sperm to rabbit erythrocytes clearly does not induce the acrosome reaction (44), suggesting that the signal transduction event that triggers this reaction is not carbohydrate-mediated. Thus the binding of a currently unidentified signal transduction molecule or complex on murine sperm to mZP3 may be essential for inducing the acrosome reaction. This interaction could also be responsible for this residual carbohydrate-independent murine sperm-egg binding, although that hypothesis has yet to be rigorously tested.
It is very easy to see how polar oligosaccharides with very complex sequences could act as very useful "species recognition markers" to ensure rapid and highly species-specific binding in an aquatic environment (reviewed in Refs. 13). However, mammals employ rather sophisticated physical stimuli and behaviors to ensure that mating occurs only between members of the same species. Gene deletion studies indicate that murine sperm continue to bind to mouse eggs in the absence of putative carbohydrate ligands. Thus two major questions arise: (i) why would mice continue to employ carbohydrate dependent gamete binding in the uterus; and (ii) why would initial murine sperm-egg binding need to be species-specific under such circumstances? There exists abundant evidence that glycosylation is essential for many housekeeping functions such as protein folding, stabilization against proteolysis, and specific targeting to intracellular and extracellular compartments (reviewed in Ref. 54). However, in some cases only glycosylation site occupancy is required to fulfill such housekeeping functions. It is only logical that further modification of oligosaccharides at such sites could expand the biological roles of a specific glycoprotein. These added functionalities would be positively selected and could conceivably explain the enormous structural diversity of the oligosaccharides linked to glycoproteins like mZP3. This type of operating logic underlies all studies in the area now known as "functional glycomics." Such carbohydrate-mediated activities would not be immediately obvious from protein structure but could only be exposed by precise carbohydrate sequencing methods in combination with functional analyses. Therefore the existing precedent in lower species combined with the selection pressure to optimize for sperm binding could explain why carbohydrate recognition remains a major component of mammalian sperm-egg binding. The analysis of the protein-linked mZP glycans has recently revealed another possible function associated with mZP glycans. Eggs from any female member of a mammalian species must be highly protected from potential immune responses to ensure the transmission of that individual's genetic legacy. Murine eggs lack histocompatibility markers for "self" and thus can avoid certain types of deleterious cell mediated responses (reviewed in Ref. 55). However, the egg may require recognition signals for other immune effector cells, especially natural killer cells that target cells lacking histocompatibility markers, a concept known as "missing self" (56). The murine egg could also become susceptible to immune destruction after fertilization when it begins to express foreign paternal histocompatibility markers. It is therefore quite significant that the majority of the carbohydrate sequences expressed on the mZP are also highly up-regulated on murine cytotoxic T lymphocytes and T helper cells during their activation (reviewed in Ref. 55). These carbohydrate sequences could be employed in the immune system as protective markers to prevent inadvertent destruction of lymphocytes during their activation. It is only logical that eggs would express such sequences to provide "fail-safe" protection against unintended cytolysis. These egg glycans as a set could therefore be considered markers for the immune recognition of "species" rather than self (reviewed in Ref. 55). Species recognition very likely preceded the recognition of self during the evolution of the metazoan immune system. Therefore carbohydrate recognition markers could have been easily integrated into the matrix coating the eggs of the earliest sexually reproducing metazoans to mediate recognition by immune effector cells. This type of "handshaking" could be one of the key components that protects histoincompatible mammalian blastocysts prior to their hatching.
There is evidence for this type of system on the murine egg. The There is also evidence for this type of protective effect in the human system. Based on lectin binding studies, human sperm, huZP, and egg cell are coated with specific carbohydrate sequences (bisecting type N-glycans) that have been implicated in the suppression of natural killer cells (57). As noted earlier, germ cells should be susceptible to this immune cell type based on the missing self model (56). Recent data suggest that bisecting type glycans are also expressed on persistent pathogens like HIV-1 and aggressive tumor cells (reviewed in Ref. 55). The coupling of gamete protection to the survival of persistent pathogens and tumor cells makes considerable pathophysiological sense. Thus the carbohydrate sequences that coat any metazoan egg need to be carefully investigated for their potential immunomodulatory effects in both natural and pathological states. As for the issue of species-specific binding, Bedford (37) reported that murine sperm bind to eggs from many different mammalian species. These results are consistent with the concept that no selection pressure for species-specific binding exists in the murine reproductive system. In contrast to the avid binding to heterologous eggs shown by the epididymal sperm of mice and some other mammals, human sperm do not bind to foreign eggs except those of some other hominoids like the gibbon (37) and the lowland gorilla (58). The selection pressure that created this taxon-specific binding of human sperm is currently unknown. Do these findings indicate that xenogenic sperm recognize exactly the same carbohydrate sequence as murine sperm do during their binding to the murine egg? The answer to this question is not necessarily. Xenogenic sperm could recognize and bind to a specific carbohydrate sequence presented on the mZP that is not employed for murine gamete binding. Such sperm could also recognize the same carbohydrate sequence as murine eggs but as a non-physiological ligand (i.e. not expressed on the ZP of eggs from that xenogenic species). There is data indicating that the binding of murine sperm to rabbit erythrocytes involves a nonphysiological ligand. Another possibility is that xenogenic sperm could bind to murine eggs in whole or in part via protein-protein interactions. Thus the very strong temptation to draw any conclusions about the binding specificity of sperm-egg interactions from such cross-species binding studies clearly should be avoided. Only a combination of structural and functional analyses has the potential to unambiguously define this precise specificity in any mammalian species.
Our understanding of the initial murine sperm-egg binding interaction has progressed substantially over the past decade, due to the development of powerful genetic, biophysical, and biochemical tools. This area of investigation remains controversial because of the conflicting results and multiple models. As outlined here, two models propose that protein-protein interactions predominate. Another model suggests that murine gamete binding involves both protein-carbohydrate and protein-protein interactions. This latter model is attractive because of the redundancy it offers and the evidence obtained from several laboratories that support it. Continued investigation of the murine sperm-egg binding interaction will eventually lead to a level of understanding that will be useful for practical application in many mammals including humans. This information may also provide insights into other immunological and pathophysiological processes that are only now being appreciated.
* This minireview will be reprinted in the 2006 Minireview Compendium, which will be available in January, 2007. Studies cited in this review that were performed by G. F. C. and collaborators were supported by National Institutes of Health Grants HD35652 and AI60397, the Breeden-Adams Foundation, the Elsa U. Pardee Foundation, and the Jeffress Trust. Studies performed by A. D. were supported by the Biotechnology and Biological Sciences Research Council (BBSRC) and the Wellcome Trust. 1 Member of the Consortium for Functional Glycomics supported by the NIGMS of the NIH. To whom correspondence may be addressed. Tel.: 573-882-1725; Fax: 757-624-2269; E-mail: clarkgf{at}health.missouri.edu. 2 BBSRC Professorial Fellow. Member of the Consortium for Functional Glycomics supported by the NIGMS of NIH. To whom correspondence may be addressed. Tel.: 44-207-594-5219; Fax: 44-207-225-0458; E-mail: a.dell{at}imperial.ac.uk.
3 The abbreviations used are: ZP, zona pellucida; mZP, murine zona pellucida; huZP, human zona pellucida;
4 J. Dennis, personal communication.
We thank Drs. Kathy Sharpe-Timms, J. Michael Bedford, and R. Michael Roberts for their comments and advice during the writing of this review.
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