Mechanical hypothesis of sperm penetration.

Sperm have generally been supposed to penetrate the zona pellucida surrounding the mammalian egg with the aid of a protease, acrosin. Difficulties associated with this view are discussed and an alternative, mechanical hypothesis introduced. The calculated force exerted by individual sperm is too small to permit the rupture of any but the weakest of secondary chemical bonds. Mechanical progress through the zona must rely on stress relaxation in a viscoelastic medium. The known properties of the zona appear to be consistent with such a mechanism of penetration.     

Capacitation and the acrosome reaction in equine sperm.

During sexual reproduction, the sperm and oocyte must fuse before the production of a diploid zygote can proceed. In mammals such as equids, fusion depends critically on complex changes in the plasma membrane of the sperm and, not surprisingly, this membrane differs markedly from that of somatic cells. After leaving the testes, sperm cease to synthesize plasma membrane lipids or proteins, and vesicle-mediated transport stops. When the sperm reaches the female reproductive tract, it is activated by so-called capacitation factors that initiate a delicate reorientation and modification of molecules within the plasma membrane. These surface changes enable the sperm to bind to the extracellular matrix of the egg (zona pellucida ZP) and the zona then primes the sperm to initiate the acrosome reaction, an exocytotic event required for the sperm to penetrate the zona. This paper will review the processes that occur at the sperm plasma membrane before and during successful penetration of the equine ZP. It is noted that while several methods have been described for detecting changes that occur during capacitation and the acrosome reaction in bovine and porcine sperm, relatively little has been documented for equine sperm. Special attention will therefore be dedicated to recent attempts to develop and implement new assays for the detection of the capacitation status of live, acrosome-intact and motile equine sperm.     

A model for sperm-egg binding and fusion based on ADAMs and integrins

Once a sperm meets an egg, several events must occur in order for fertilization to proceed. Sperm must bind to the zona pellucida, undergo the acrosome reaction, penetrate the zona pellucida and then bind to and fuse with the egg plasma membrane. Shortly thereafter, the egg must be activated for zygotic development. This review focuses on mammalian sperm-egg plasma membrane binding and fusion, and in particular on the roles of two families of cell-adhesion molecules, ADAMs and integrins, in this important union.     

Mammalian sperm cannot penetrate the zona pellucida solely by force

The zona pellucida of the mammalian egg is a barrier which sperm must penetrate if fertilization is to proceed. The physical properties of zonae remain unknown. The response of intact hamster zonae to defined force and shear stress has been established by using quartz fibres as 'pokers' mounted on tungsten beams of known stiffness. Fibres with cross-sectional areas as small as 2.8 micron 2 fail to penetrate zonae even when subjected to forces of 150 nN, i.e. stresses greater than 50 kN/m2. Moreover, although a force of 150 nN produces extensive strain, zonae recover essentially elastically, and show little or no creep over 1 200 sec. These forces and stresses are in excess of those which are currently thought to be produced by sperm, and it follows, therefore, that sperm cannot penetrate zonae solely by force.     

Sperm penetration through oocyte investments in mammals

Literature on the interactions between eutherian gametes is reviewed. The oocyte cumulus complex of the female is surrounded by a zona pellucida, corona radiata, and cumulus layer. Sperm undergo an acrosome reaction before penetrating the zona pellucida. The morphological consequences of this reaction and its possible role(s) in penetration of the oocyte cumulus complex are considered. The acrosomal enzyme, hyaluronidase, has been thought to aid sperm in penetrating the cumulus layer and corona radiata. Several recent investigations, including one that shows that motile cells lacking hyaluronidase can penetrate to the zona surface, do not support this idea. Other possible roles of this enzyme in fertilization are discussed. The development of in vitro fertilization systems that employ physiological numbers (1-100) of sperm will be valuable in studying the mechanisms used by sperm to penetrate the oocyte cumulus complex.     

The zona pellucida: a coat of many colors

The zona pellucida is an extracellular coat that surrounds all mammalian eggs. It is a porous matrix of interconnected filaments that are assembled from glycoproteins synthesized and secreted by growing oocytes. The zona pellucida is responsible both for species-specific binding of sperm to unfertilized eggs and inducing bound sperm to undergo the acrosome reaction. The latter enables sperm to penetrate the extracellular coat and fertilize the egg. The zona pellucida also aids in prevention of polyspermy following fertilization and in protection of preimplantation embryos. In mice, several of these important functions can now be ascribed to specific zona pellucida glycoproteins that have been purified and characterized. Furthermore, the enzyme responsible for hatching of embryos from the zona pellucida, just prior to implantation, has been identified and characterized.     

SPERM THRUSTS AND THE PROBLEM OF PENETRATION

An equation of Lighthill's is used to calculate sperm thrusts. They have values in the range 5–350 pN, depending on species. The limitations of this approach are discussed and comparison is made with the measured thrust for human sperm. The effect of sperm thrusts of this magnitude on covalent bonds and reversible bonds is discussed. Sperm cannot break covalent bonds, but can reduce the lifetime of reversible bonds.
The structure and physical properties of the zona pellucida are examined in relation to sperm penetration. The evidence suggests that sperm cannot penetrate it solely by force. A model for sperm penetration is elaborated in which the conjunctive application of thrust and a soluble enzyme leads to strain-induced proteolysis and the formation of the penetration slit. The potential mechanism of the zona block is discussed, as is the site of the acrosome reaction. The effects of other mechanical inputs into fertilization such as stirring and swimming are examined briefly. Evidence suggests that sperm penetration of the cumulus oophorus and cervical mucus is mechanical, but that in the case of cervical mucus, it is affected by changes in the physical properties of the mucus.     

Cell surface beta-1,4-galactosyltransferase-I activates G protein-dependent exocytotic signaling

ZP3 is a protein in the mammalian egg coat (zona pellucida) that binds sperm and stimulates acrosomal exocytosis, enabling sperm to penetrate the zona pellucida. The nature of the ZP3 receptor/s on sperm is a matter of considerable debate, but most evidence suggests that ZP3 binds to beta-1,4-galactosyltransferase-I (GalTase) on the sperm surface. It has been suggested that ZP3 induces the acrosome reaction by crosslinking GalTase, activating a heterotrimeric G protein. In this regard, acrosomal exocytosis is sensitive to pertussis toxin and the GalTase cytoplasmic domain can precipitate G(i) from sperm lysates. Sperm from mice that overexpress GalTase bind more soluble ZP3 and show accelerated G protein activation, whereas sperm from mice with a targeted deletion in GalTase have markedly less ability to bind soluble ZP3, undergo the ZP3-induced acrosome reaction, and penetrate the zona pellucida. We have examined the ability of GalTase to function as a ZP3 receptor and to activate heterotrimeric G proteins using Xenopus laevis oocytes as a heterologous expression system. Oocytes that express GalTase bound ZP3 but did not bind other zona pellucida glycoproteins. After oocyte maturation, ZP3 or GalTase antibodies were able to trigger cortical granule exocytosis and activation of GalTase-expressing eggs. Pertussis toxin inhibited GalTase-induced egg activation. Consistent with G protein activation, both ZP3 and anti-GalTase antibodies increased GTP-gamma[(35)S] binding as well as GTPase activity in membranes from eggs expressing GalTase. Finally, mutagenesis of a putative G protein activation motif within the GalTase cytoplasmic domain eliminated G protein activation in response to ZP3 or anti-GalTase antibodies. These results demonstrate directly that GalTase functions as a ZP3 receptor and following aggregation, is capable of activating pertussis toxin-sensitive G proteins leading to exocytosis.     

Molecular basis of fertilization in the mouse

Recent studies of mouse fertilization have identified two complementary gamete receptors that mediate sperm-egg binding. Sperm surface β1,4-galactosyltransferase (GalTase) binds to specific oligosaccharides of the egg coat (zona pellucida) glycoprotein ZP3. Evidence suggests that these same molecules may stimulate the acrosome reaction in sperm. After the acrosome reaction, it is thought that sperm remain adherent to the zona by binding another glycoprotein, ZP2. The acrosome-reacted sperm releases hydrolytic enzymes, including acrosin and N-acetylglucosaminidase, enabling it to penetrate the zona pellucida. After the penetrating sperm binds to the egg membrane and activates development, N-acetylglucosaminidase is exocytosed from egg cortical granules and, as part of the zona block to polyspermy, globally removes the sperm GalTase binding site from ZP3 oligosaccharides.     

A hyaluronidase activity of the sperm plasma membrane protein PH-20 enables sperm to penetrate the cumulus cell layer surrounding the egg

A typical mammalian egg is surrounded by an outer layer of about 3,000 cumulus cells embedded in an extracellular matrix rich in hyaluronic acid. A current, widely proposed model is that the fertilizing sperm, while it is acrosome intact, passes through the cumulus cell layer and binds to the egg zona pellucida. This current model lacks a well- supported explanation for how sperm penetrate the cumulus layer. We report that the sperm protein PH-20 has a hyaluronidase activity and is present on the plasma membrane of mouse and human sperm. Brief treatment with purified, recombinant PH-20 can release all the cumulus cells surrounding mouse eggs. Acrosome intact mouse sperm incubated with anti-PH-20 antibodies can not pass through the cumulus layer and thus can not reach the zona pellucida. These results, indicating that PH-20 enables acrosome intact sperm to penetrate the cumulus barrier, reveal a mechanism for cumulus penetration, and thus provide the missing element in the current model.     

Motile cells lacking hyaluronidase can penetrate the hamster oocyte cumulus complex

Sperm hyaluronidase is thought to assist in penetration of the extracellular matrix (ECM) between the cumulus and corona radiata cells surrounding mammalian oocytes. The question was asked: Can motile cells which lack hyaluronidase penetrate the hamster oocyte cumulus complex (OCC)? Sea urchin (Strongylocentrotus purpuratus) and frog (Rana catesbeiana) sperm and the unicellular, biflagellated, green alga Chlamydomonas reinhardtii were extracted and found to contain no hyaluronidase activity. Moreover, none of these cells was able to disperse the cumulus cells of hamster OCC, nor did they affect the ultrastructure of the ECM between cells. Fresh hamster OCC were challenged with suspensions of each cell type. Frog and sea urchin sperm penetrated to the zona pellucida surface in less than 5 min. A cell wall-less mutant of Chlamydomonas also penetrated to the zona surface but required longer than 5 min. Wild-type Chlamydomonas penetrated only halfway to the zona, perhaps because its cell wall adhered to the ECM between the cumulus cells and retarded its movement. The motility of the frog and sea urchin sperm was not affected by the ECM of the OCC. Frog sperm exhibited slow lethargic motility yet had no difficulty penetrating to the zona; this indicates that hyperactivated motility is not required for penetration of the ECM. None of the challenge cells penetrated the zona pellucida, although the frog sperm did compress the weave of the zona. These data show that motile cells which lack the enzyme hyaluronidase can readily penetrate the ECM of the hamster cumulus and corona radiata and suggest that the significance of hyaluronidase in fertilization should be reevaluated.     

Sperm proteasomes degrade sperm receptor on the egg zona pellucida during mammalian fertilization

Despite decades of research, the mechanism by which the fertilizing spermatozoon penetrates the mammalian vitelline membrane, the zona pellucida (ZP) remains one of the unexplained fundamental events of human/mammalian development. Evidence has been accumulating in support of the 26S proteasome as a candidate for echinoderm, ascidian and mammalian egg coat lysin. Monitoring ZP protein degradation by sperm during fertilization is nearly impossible because those few spermatozoa that penetrate the ZP leave behind a virtually untraceable residue of degraded proteins. We have overcome this hurdle by designing an experimentally consistent in vitro system in which live boar spermatozoa are co-incubated with ZP-proteins (ZPP) solubilized from porcine oocytes. Using this assay, mimicking sperm-egg interactions, we demonstrate that the sperm-borne proteasomes can degrade the sperm receptor protein ZPC. Upon coincubation with motile spermatozoa, the solubilized ZPP, which appear to be ubiquitinated, adhered to sperm acrosomal caps and induced acrosomal exocytosis/formation of the acrosomal shroud. The degradation of the sperm receptor protein ZPC was assessed by Western blotting band-densitometry and proteomics. A nearly identical pattern of sperm receptor degradation, evident already within the first 5 min of coincubation, was observed when the spermatozoa were replaced with the isolated, enzymatically active, sperm-derived proteasomes. ZPC degradation was blocked by proteasomal inhibitors and accelerated by ubiquitin-aldehyde(UBAL), a modified ubiquitin protein that stimulates proteasomal proteolysis. Such a degradation pattern of ZPC is consistent with in vitro fertilization studies, in which proteasomal inhibitors completely blocked fertilization, and UBAL increased fertilization and polyspermy rates. Preincubation of intact zona-enclosed ova with isolated active sperm proteasomes caused digestion, abrasions and loosening of the exposed zonae, and significantly reduced the fertilization/polyspermy rates after IVF, accompanied by en-mass detachment of zona bound sperm. Thus, the sperm borne 26S proteasome is a candidate zona lysin in mammals. This new paradigm has implications for contraception and assisted reproductive technologies in humans, as well as animals.     

Egg cortical granule N-acetylglucosaminidase is required for the mouse zona block to polyspermy

The mammalian egg must be fertilized by only one sperm to prevent polyploidy. In most mammals studied to date, the primary block to polyspermy occurs at the zona pellucida, the mammalian egg coat, after exocytosis of the contents of the cortical granules into the perivitelline space. The exudate acts on the zona, causing it to lose its ability to bind sperm and to be penetrated by sperm previously bound to the zona. However, the cortical granule components responsible for the zona block have not been identified. Studies described herein demonstrate that N-acetylglucosaminidase is localized in cortical granules and is responsible for the loss in sperm-binding activity leading to the zona block to polyspermy. Before fertilization, sperm initially bind to the zona by an interaction between sperm surface GalTase and terminal N-acetylglucosamine residues on specific oligosaccharides of the zona glycoprotein ZP3 (Miller, D. J., M. B. Macek, and B. D. Shur. 1992. Nature (Lond.). 357:589-593). These GalTase-binding sites are lost from ZP3 after fertilization, an effect that can be duplicated by N-acetylglucosaminidase treatment. Therefore, N-acetylglucosaminidase, or a related glycosidase, may be present in cortical granules and be responsible for ZP3's loss of sperm-binding activity at fertilization. Of eight glycosidases assayed in exudates of ionophore-activated eggs, N-acetylglucosaminidase was 10-fold higher than any other activity. The enzyme was localized to cortical granules using immunoelectron microscopy. Approximately 70 or 90% of the enzyme was released from cortical granules after ionophore activation or in vivo fertilization, respectively. The isoform of N- acetylglucosaminidase found in cortical granules was identified as beta- hexosaminidase B, the beta, beta homodimer. Inhibition of N- acetylglucosaminidase released from activated eggs, with either competitive inhibitors or with specific antibodies, resulted in polyspermic binding to the zona pellucida. Another glycosidase inhibitor or nonimmune antibodies had no effect on sperm binding to activated eggs. Therefore, egg cortical granule N-acetylglucosaminidase is released at fertilization, where it inactivates the sperm GalTase- binding site, accounting for the block in sperm binding to the zona pellucida.     

Identification of a secondary sperm receptor in the mouse egg zona pellucida: role in maintenance of binding of acrosome-reacted sperm to eggs

During fertilization in mice, acrosome-intact sperm bind via plasma membrane overlying their head to a glycoprotein, called ZP3, present in the egg extracellular coat or zona pellucida. Bound sperm then undergo the acrosome reaction, which results in exposure of inner acrosomal membrane, penetrate through the zona pellucida, and fuse with egg plasma membrane. Thus, in the normal course of events, acrosome-reacted sperm must remain bound to eggs, despite loss of plasma membrane from the anterior region of the head and exposure of inner acrosomal membrane. Here, we examined maintenance of binding of sperm to the zona pellucida following the acrosome reaction. We found that polyclonal antisera and monoclonal antibodies directed against ZP2, another zona pellucida glycoprotein, did not affect initial binding of sperm to eggs, but inhibited maintenance of binding of sperm that had undergone the acrosome reaction on the zona pellucida. On the other hand, polyclonal antisera and monoclonal antibodies directed against ZP3 did not affect either initial binding of acrosome-intact sperm to eggs or maintenance of binding following the acrosome reaction. We also found that soybean trypsin inhibitor, a protein reported to prevent binding of mouse sperm to eggs, did not affect initial binding of sperm to eggs, but, like antibodies directed against ZP2, inhibited maintenance of binding of sperm that had undergone the acrosome reaction on the zona pellucida. These and other observations suggest that ZP2 serves as a secondary receptor for sperm during the fertilization process in mice and that maintenance of binding of acrosome-reacted sperm to eggs may involve a sperm, trypsin-like proteinase.     

Sperm penetration through cumulus mass and zona pellucida

Mammalian fertilization requires sperm to penetrate the cumulus mass and egg zona pellucida prior to fusion with the egg. Although sperm penetration through these physical barriers is essential, the molecular mechanism has not yet been completely elucidated. In addition to sperm motility, hyaluronan-hydrolyzing and proteolytic enzymes of sperm have been suggested to participate in the penetration events. Here we focus on the functional roles of hyaluronidase and protease in sperm passage through the cumulus mass and zona pellucida.     

Signal transduction pathways that regulate sperm capacitation and the acrosome reaction

Ejaculated spermatozoa must undergo physiological priming as they traverse the female reproductive tract before they can bind to the egg’s extracellular coat, the zona pellucida (ZP), undergo the acrosome reaction, and fertilize the egg. The preparatory changes are the net result of a series of biochemical and functional modifications collectively referred to as capacitation. Accumulated evidence suggests that the event that initiates capacitation is the efflux of cholesterol from the sperm plasma membrane (PM). The efflux increases permeability and fluidity of the sperm PM and causes influx of Ca²⁺ ions that starts a signaling cascade and result in sperm capacitation. The binding of capacitated spermatozoa to ZP further elevates intrasperm Ca²⁺ and starts a new signaling cascade which open up Ca²⁺ channels in the sperm PM and outer acrosomal membrane (OAM) and cause the sperm to undergo acrosomal exocytosis. The hydrolytic action of the acrosomal enzymes released at the site of sperm-egg (zona) binding, along with the hyperactivated beat pattern of the bound spermatozoon, are important factors in directing the sperm to penetrate the ZP and fertilize the egg. The role of Ca²⁺-signaling in sperm capacitation and induction of the acrosome reaction (acrosomal exocytosis) has been of wide interest. However, the precise mechanism(s) of its action remains elusive. In this article, we intend to highlight data from this and other laboratories on Ca²⁺ signaling cascades that regulate sperm functions.     

A structural view of egg coat architecture and function in fertilization

Species-restricted interaction between gametes at the beginning of fertilization is mediated by the extracellular coat of the egg, a matrix of cross-linked glycoprotein filaments called the zona pellucida (ZP) in mammals and the vitelline envelope in nonmammals. All egg coat subunits contain a conserved protein-protein interaction module-the "ZP domain"-that allows them to polymerize upon dissociation of a C-terminal propeptide containing an external hydrophobic patch (EHP). Recently, the first crystal structures of a ZP domain protein, sperm receptor ZP subunit zona pellucida glycoprotein 3 (ZP3), have been reported, giving a glimpse of the structural organization of the ZP at the atomic level and the molecular basis of gamete recognition in vertebrates. The ZP module is divided in two related immunoglobulin-like domains, ZP-N and ZP-C, that contain characteristic disulfide bond patterns and, in the case of ZP-C, also incorporate the EHP. This segment lies at the interface between the two domains, which are connected by a long loop carrying a conserved O-glycan important for binding to sperm in vitro. The structures explain several apparently contradictory observations by reconciling the variable disulfide bond patterns found in different homologues of ZP3 as well as the multiple ZP3 determinants alternatively involved in gamete interaction. These findings have implications for our understanding of ZP subunit biogenesis; egg coat assembly, architecture, and interaction with sperm; structural rearrangements leading to postfertilization hardening of the ZP and the block to sperm binding; and the evolutionary origin of egg coats.     

Mouse gamete adhesion molecules.

Complementary adhesion molecules are located on the surface of mouse eggs and sperm. These molecules support species-specific interactions between sperm and eggs that lead to gamete fusion (fertilization). Modification of these molecules shortly after gamete fusion assists in prevention of polyspermic fertilization. mZP3, an 83,000-Mr glycoprotein located in the egg extracellular coat, or zona pellucida, serves as primary sperm receptor. Gamete adhesion in mice is carbohydrate-mediated, since sperm recognize and bind to certain mZP3 serine/threonine- (O-) linked oligosaccharides. As a consequence of binding to mZP3, sperm undergo the acrosome reaction, which enables them to penetrate the zona pellucida and fertilize the egg. A 56,000-Mr protein called sp56, which is located in plasma membrane surrounding acrosome-intact mouse sperm heads, is a putative primary egg-binding protein. It is suggested that sp56 recognizes and binds to certain mZP3 O-linked oligosaccharides. Acrosome-reacted sperm remain bound to eggs by interacting with mZP2, a 120,000-Mr zona pellicida glycoprotein. Thus, mZP2 serves as secondary sperm receptor. Perhaps a sperm protease associated with inner acrosomal membrane, possibly (pro)acrosin, serves as secondary egg-binding protein. These and, perhaps, other egg and sperm surface molecules regulate fertilization in mice. Homologous molecules apparently regulate fertilization in other mammals.     

ELECTRON MICROSCOPIC OBSERVATIONS OF GUINEA PIG SPERMATOZOA PENETRATING EGGS IN VITRO*

Guinea pig ovarian oocytes matured in vitro were inseminated in vitro with capacitated, acrosome-reacted spermatozoa and sperm penetration through the zona pellucida and into the egg cytoplasm were examined. Sperm heads passing through the zona pellucida had already lost all their acrosomal elements except for the inner acrosomal membrane and the equatorial segment. It was often observed that the texture of the zona material around the sperm head was distorted, giving the impression that the zona pellucida was parted, at least partially, by a shearing force produced by the sperm head advancing through the zona. When eggs were freed from their zonae pellucidae and inseminated, the acrosome-reacted spermatozoa immediately bound to the egg surfaces and began to fuse with the eggs; whereas the spermatozoa with intact acrosomes failed to do so. Fusion began between the egg plasma membrane and the sperm plasma membrane at the central region of the sperm head. The anterior half of the sperm head was engulfed by the egg in a phagocytic fashion, while its posterior half was incorporated into the egg by a fussion between egg and sperm plasma membranes. Incorporation of the sperm tail into the egg was achieved by fusion between the sperm and egg plasma membranes.     

Swimming of Xenopus laevis sperm exhibits multiple gears and its duration is extended by egg jelly constituents

The motility of Xenopus sperm is initiated by the osmotic shock experienced when these cells are ejaculated into low-salinity pond water. Motility is brief and is required for the sperm to penetrate the jelly layers and fertilize the egg. In this study we demonstrate that extracts of egg jelly contain factors that extend the period of sperm motility as well as providing a chemoattractant activity as previously reported. Both activities are partially dependent on extracellular calcium. Time-lapse and video microscopy show that after activation of motility the number of motile sperm decreases rapidly, with a half-time of about 2 min. Addition of 10% v/v egg jelly extract ("egg water") increased the number of motile sperm 2-fold over controls at 20 s and about 4- to 10-fold over controls at 10 min after initiation of motility. Extension of motility lifetime was not mediated by a nonspecific protein or by allurin, the egg-water protein that has chemoattractant activity. The helical path of Xenopus sperm exhibited tight coupling between rotational and forward velocities in egg jelly, but coupling changed rapidly from moment to moment in low-salinity buffer. Our observations suggest that jelly-derived factors regulate both the longevity and directionality of sperm propulsion.