Autoradiographic visualization of the mouse egg's sperm receptor bound to sperm

Precocious loss of cortical granules during mouse oocyte meiotic maturation and correlation with an egg-induced modification of the zona pellucida

Fertilization results in cortical granule exocytosis, which is thought to be involved in modifications of the zona pellucida that constitute the zona pellucida block to polyspermy. A previous report demonstrated that a decrease in the number of Lens culinaris agglutinin-staining granules, which are likely to be cortical granules, occurred during in vivo mouse oocyte maturation with arrest at metaphase II, as well as the formation of a cortical granule-free domain in the area of the metaphase II spindle (T. Ducibella, E. Anderson, D.F. Albertini, J. Aalberg, and S. Rangarajan, 1988, Dev. Biol. 130, 184-197). We extend these observations by reporting here that germinal vesicle-intact oocytes matured in vitro to metaphase II in either the absence or the presence of serum develop a cortical granule-free domain and have reduced numbers of cortical granules when compared to germinal vesicle-intact oocytes; these changes are similar to those of oocytes matured in vivo. The reduction in the number of cortical granules requires germinal vesicle breakdown, since it is prevented by dibutyryl cAMP, which inhibits germinal vesicle breakdown in vitro. The ability of oocytes to respond to the calcium ionophore A23187 with a reduction in the number of cortical granules is also associated with meiotic maturation and develops between 7 and 12 hr after initiation of maturation. The maturation-associated reduction in the number of cortical granules is likely to represent cortical granule exocytosis, since this reduction is accompanied by the formation of a cortical granule-free domain and a conversion of ZP2 to ZP2f when the oocytes are matured in vitro in serum-free medium; this zona pellucida modification occurs following fertilization and is thought to be due to cortical granule exocytosis. In contrast, the loss of cortical granules and development of the cortical granule-free domain of oocytes matured in vitro in the presence of serum is not accompanied by the modification of ZP2. The inhibitory effect of serum on the ZP2 modification may afford in vivo a physiological mechanism to prevent a precocious modification of the zona pellucida that could result in a premature block to polyspermy and hence inhibit fertilization.

Endpoint of first stage of zona pellucida-induced acrosome reaction in mouse spermatozoa characterized by acrosomal H+ and Ca2+ permeability: population and single cell kinetics

The acrosome reaction induced by the mouse egg's zona pellucida in mouse sperm has been shown to proceed in two stages as characterized empirically by sequential changes in patterns of chlortetracycline fluorescence on the sperm plasma membrane surfaces. The chlortetracycline fluorescence pattern characteristic of fully intact sperm is designated B; in sperm bound to structurally intact zonae that induce the acrosome reaction, the B pattern changes first to an intermediate pattern S and then to a terminal pattern AR characteristic of the completed acrosome reaction. In the same study, it was shown, using a 9-amino acridine fluorescent pH probe, that completion of the first stage was characterized by increase in H+ permeability such that the H+ gradient between sperm head and medium was dissipated. In this study, we show that the fluorescent pH probe 9-N-dodecylamino acridine and the intracellular Ca2+ fluorescent probe fura-2 are both localized to the anterior part of the sperm head encompassing the acrosomal compartment in intact sperm, and the fluorescence associated with each probe is lost as the first stage of the acrosome reaction is completed. Loss of the pH probe fluorescence, pattern N, corresponds to onset of H+ permeability, and loss of fura-2 fluorescence, pattern F, corresponds to onset of Ca2+ permeability. Localization of intracellular fura-2 fluorescence to the acrosomal compartment required extracellular Mn2+ to quench surface-bound fura-2 AM, the tetra-acetoxymethyl ester of fura-2 used to load the cells. Loss of acrosomal fura-2 fluorescence is due to quenching by tracer Mn2+ accompanying Ca2+. Onset of membrane permeability to both H+ and Ca2+, as seen by loss of patterns N and F, occurred in synchrony in populations of sperm bound to isolated, structurally intact zonae, with an overall time course of 210 min postbinding. The loss of pattern N in individual sperm cells bound to zonae was rapid, with a half time of 2.1 min. Concomitant with this rapid loss of pattern N was a shift in the amplitude of flagellar motion from large to small. The lag times to pattern N loss in 50 individual cells ranged from 30 to 140 min. The variable lag times determine the population kinetics; the rate of the endpoint reaction seen in the individual cells is rapid and constant. Dissipation of the H+ gradient with immediate loss of pattern N was readily achieved by addition of nigericin with no change in the time course of the onset of Ca2+ permeability of the membranes enclosing the acrosome.(ABSTRACT TRUNCATED AT 400 WORDS)

An adhesion-associated agonist from the zona pellucida activates G protein-promoted elevations of internal Ca2+ and pH that mediate mammalian sperm acrosomal exocytosis

Solubilized oocyte zonae pellucidae promoted acrosomal exocytosis in fura-2- or carboxyfluorescein-loaded, mature bovine sperm. Associated elevations of internal [Ca2+] and pH in sperm suspensions were first detectable at 2-5 min, without apparent temporal resolution, and increased monotonically thereafter. Video imaging of fura-2-loaded, single cells identified a responsive subpopulation, destined to undergo exocytosis, that displayed no early transient but manifested lags of 1-7 min then sustained elevations of internal [Ca2+]. Both the zona-induced exocytosis and dye responses were diminished for functionally immature sperm and for mature sperm treated preliminarily with pertussis toxin. Together, these results indicate that a developmentally regulated mechanism of signal transduction employs G protein(s) to couple the physiological (zona) agonist to alterations of the internal ionic mediators of acrosomal exocytosis.

On the number and rate of formation of sperm-zona bonds in the mouse

In mammalian fertilization, sperm bind to the zona pellucida, a glycoprotein matrix forming a shell surrounding the oocyte. Subsequently, one of the bound sperm penetrates the zona and fertilizes the egg. The adhesion between sperm and zona is mediated by complementary receptor-ligand pairs. Recent biochemical evidence has identified likely candidates for these molecules in the mouse. Biophysical studies have predicted that very few (possibly as few as one) bonds are needed to tether a motile sperm to the zona. We have used the data characterizing the putative receptors of the mouse sperm to predict the number of bonds they can form with the zona ligands. Our calculations indicate that few bonds probably form between the sperm and zona during the initial contact when the sperm is captured, supporting the hypothesis that fertilization depends on the action of a very few sperm-zona bonds.

Novel alpha-D-mannosidase of rat sperm plasma membranes: characterization and potential role in sperm-egg interactions

During the course of a study of glycoprotein processing mannosidases in the rat epididymis, we have made an intriguing discovery regarding the presence of a novel alpha-D-mannosidase on the rat sperm plasma membranes. Unlike the sperm acrosomal "acid" mannosidase which has a pH optimum of 4.4, the newly discovered alpha-D-mannosidase has a pH optimum of 6.2, and 6.5 when assayed in sperm plasma membranes and intact spermatozoa, respectively. In addition, the two enzymes show different substrate specificity. The acrosomal alpha-D-mannosidase is active mainly towards synthetic substrate, p-nitrophenyl alpha-D-mannopyranoside, whereas the sperm plasma membrane alpha-D-mannosidase shows activity mainly towards mannose-containing oligosaccharides. Evidence is presented which suggest that the sperm plasma membrane alpha-D-mannosidase is different from several processing mannosidases previously characterized from the rat liver. The newly discovered alpha-D-mannosidase appears to be an intrinsic plasma membrane component, since washing of the purified membranes with buffered 0.4 M NaCl did not release the enzyme in soluble form. The enzyme requires nonionic detergent (Triton X-100) for complete solubilization. The enzyme is activated by Co2+ and Mn2+. However, Cu2+ and Zn2+ are potent inhibitors of the sperm plasma membrane alpha-D-mannosidase. At a concentration of 0.1 mM, these divalent cations caused nearly complete inactivation of the sperm enzyme. In addition methyl-alpha-D-mannoside, methyl-alpha-D-glucoside, mannose, 2-deoxy-D-glucose, and D-mannosamine are inhibitors of the sperm surface alpha-D-mannosidase. The physiological role of the newly discovered enzyme is not yet known. Several published reports in three species, including the rat, suggest that the sperm surface alpha-D-mannosidase may have a role in binding to mannose-containing saccharides presumably present on the zona pellucida.

Mechanisms of signal transduction in mouse spermatozoa

Sperm from all species studied thus far contain G-proteins. The presence of such signal-transducing proteins in these cells suggests that the regulation of sperm function might have control elements that are similar to ligand:receptor:G-protein:second messenger systems common to many somatic cells. This hypothesis is supported by experiments that demonstrate a potential intermediary role for the mouse sperm Gi-like protein in the acrosome reaction induced by ZP3. The specific function of this Gi-like protein in this important physiologic event is not known at this time, although possible roles in regulating ionic movements, cyclic nucleotide metabolism, and polyphosphoinositide turnover are possible candidates. Studies directed at the localization and biochemical identity of the mouse sperm Gi-like protein, as well as the nature of the second messenger system(s) modulated by this protein, are in progress and should help to delineate the sequence of events involved in some of the early steps of sperm-ZP interaction.

95 kd sperm proteins bind ZP3 and serve as tyrosine kinase substrates in response to zona binding

In the mouse, the zona pellucida (ZP) glycoprotein ZP3 both binds intact sperm and induces acrosomal exocytosis. The subsequent signaling pathway(s) is still uncertain, but Gi-like proteins have been implicated. By analogy with other signal transduction mechanisms, we examined anti-phosphotyrosine antibody reactivity in mouse sperm. Antibodies reacted with three proteins of 52, 75, and 95 kd. Indirect immunofluorescence localized reactivity to the acrosomal region of the sperm head. The 52 kd and 75 kd phosphoproteins are detected only in capacitated sperm, whereas the 95 kd protein is detected in both fresh and capacitated sperm. For the 95 kd protein, the level of immunoreactivity is not related to sperm motility but is enhanced by both capacitation and sperm interaction with solubilized ZP proteins. In addition, binding of radiolabeled whole ZP or purified ZP3 to blots of separated sperm proteins identified two ZP binding proteins of 95 kd and 42 kd. 95 kd sperm proteins that bind to ZP3 also react with anti-phosphotyrosine antibodies (in a ZP concentration-dependent manner), supporting the idea that the same 95 kd sperm protein serves as a ZP3 receptor and as a tyrosine kinase substrate. These findings and our evidence on acrosome reaction triggering via sperm receptor aggregation suggest that a 95 kd protein in the sperm plasma membrane is aggregated by ZP3, which stimulates tyrosine kinase activity leading to acrosomal exocytosis.

Egg-induced modifications of the zona pellucida of mouse eggs: effects of microinjected inositol 1,4,5-trisphosphate

Mouse eggs microinjected with physiological concentrations of inositol 1,4,5-trisphosphate (IP3) do not emit the second polar body, form a pronucleus, or display a fertilization-associated set of changes in the pattern of protein synthesis. IP3-injected eggs, however, display a conversion of the zona pellucida glycoprotein ZP2 to ZP2f. The effect is concentration-dependent with an EC50 (effective concentration, 50%) of 5-10 nM and also occurs in the presence of reduced levels of extracellular calcium. The egg-induced zona pellucida modification is not elicited by several other inositol phosphates that are not able to release calcium from intracellular stores in other systems. Analysis of individual eggs microinjected with IP3 reveals a strong correlation between a reduced binding of sperm to the zona pellucida and the ZP2 to ZP2f conversion. In addition, solubilized zonae pellucidae isolated from IP3-injected eggs possess reduced levels of acrosome reaction-inducing activity. These egg-induced modifications of the zona pellucida--reduced sperm receptor and acrosome reaction-inducing activities and the ZP2 to ZP2f conversion--elicited by microinjected-IP3 are similar to those that occur following fertilization. Results of these experiments suggest that IP3 generated in response to fertilization may play a role in the egg-induced modifications of the zona pellucida that result in the polyspermy block.

Identification of a deoxyribonucleic acid allelic variant for beta 1-4 galactosyltransferase expression associated with male sperm binding/penetration infertility

Studies on mouse sperm-egg binding and fertilization have been suggested to involve the interaction of sperm-associated beta 1-4 galactosyltransferase with egg zona pellucida glycoproteins. A population of human males, whose sperm demonstrated an inability to penetrate ovulated zona pellucida-free hamster eggs in vitro, were examined for the level of activity of beta 1-4 galactosyltransferase. The level of enzyme activity was found to be reduced in human sperm isolated from this group of individuals compared with a known hamster penetration-positive group. Analysis of the deoxyribonucleic acid from these individuals by Southern hybridization with a putative human complementary deoxyribonucleic acid clone to beta 1-4 galactosyltransferase identified a unique allele lacking 0.8 and 0.4 kb restriction fragments on digestion with the endonuclease Taq I. These results represent the first evidence to suggest that mutations could be associated with the human gene for galactosyltransferase. Our data help to clarify one of the possible molecular mechanisms responsible for sperm-egg binding/penetration interactions.

Relationship between the M42 antigen of mouse sperm and the acrosome reaction induced by ZP3

A murine monoclonal antibody, M42 mAb, directed against 200/220 Kd protein of mouse sperm, has been employed to study the molecular events of gamete interaction. We have reported previously that M42 mAb blocks mouse fertilization in a zona-dependent manner; the reagent specifically inhibits physiologically induced (zonae), but not pharmacologically induced (A23187), acrosome reactions in mouse sperm. Using solubilized mouse zonae pellucidae and purified ZP3, we demonstrate that M42 mAb inhibits acrosome reactions (ARs) induced by ZP3 to the same extent as those induced by total zonae. We have also studied AR inhibition using the fluorescent antibiotic chlortetracycline (CTC), which permits visualization of three different acrosomal patterns during the AR. In the presence of M42 IgG, greater than 70% of capacitated sperm treated with zonae are arrested in the acrosome-intact state (B-pattern), in contrast to the majority of sperm (60-70%) in the absence of M42 IgG, which progress through the intermediate phase (S-pattern) to the fully acrosome-reacted (AR-pattern) state. Incubation of sperm with zona proteins modified by incubating eggs with phorbol esters arrests sperm in the S-pattern (Y. Endo, R.M. Schultz, and G.S. Kopf, 1987, Dev. Biol. 119, 199-209). We show that once sperm have reached such a state, M42 mAb no longer exerts an inhibitory effect. The addition of unmodified ZP to S-pattern sperm permits the completion of the acrosome reaction. These results indicate that M42 mAb blocks an early step in the AR cascade and that M42 mAb is unable to prevent subsequent events of this cascade once it has been initiated.

Characterization of a proteinase that cleaves zona pellucida glycoprotein ZP2 following activation of mouse eggs

Here, we describe an in vitro assay that has permitted further characterization of a proteinase (called "ZP2-proteinase") that is released upon activation of ovulated mouse eggs and cleaves ZP2, one of three glycoproteins present in mouse zonae pellucidae. Results presented suggest that ZP2-proteinase readily diffuses through the zona pellucida within 5 min of activation of eggs by ionophore A23187 and carries out limited proteolysis of ZP2. Appearance of ZP2-proteinase is completely dependent upon activation of eggs, consistent with it being present in cortical granule exudate. The proteinase is insensitive to a wide variety of proteinase inhibitors, but is inhibited when either an anti-ZP2 monoclonal antibody or an Fab fragment of the antibody is bound to ZP2. Proteolysis occurs near the amino- or carboxy-terminus of ZP2, producing a 23,000 Mr glycopeptide(s) that remains attached to ZP2 by intramolecular disulfide bonds. HPLC fractionation of activated egg exudate suggests that ZP2-proteinase has an apparent Mr between 21,000 and 34,000. Proteolysis of ZP2 correlates with "hardening" of the zona pellucida following egg activation and, thus, may be responsible for one aspect of the zona reaction.

Effects of in vitro incubation on a zona binding site found on murine spermatozoa

Murine cauda epididymal sperm possess a site, the acceptor, on the plasma membrane over the apical cap region of the acrosome which recognizes both a proteinase inhibitor of seminal vesicle origin and homologous zonae. The acceptor site may participate in both capacitation and zona binding. This presentation explores the effect of in vitro incubation in a medium known to induce capacitation on the binding capabilities of this site. Approximately 80% of fresh cauda epididymal sperm will bind the seminal inhibitor in vitro. Incubating sperm, pretreated with inhibitor for 2 hr in a medium (M199-M) known to support capacitation, reduces by 60% the number of sperm showing evidence of the inhibitor. No such decrease is seen when sperm are incubated in a medium (M199) that does not support capacitation. During the 2-hr incubation in either medium, 60-70% of the sperm retain two diverse components on the plasma membrane over the acrosome: a receptor for the Fc portion of IgG and an epitope recognized by a monoclonal antibody to the acceptor site. These observations suggest that the plasma membrane in the acrosome region of the cell remains structurally intact during incubation. Furthermore, sperm retain the ability to bind the seminal inhibitor during incubation. After a 2-hr incubation in M199-M, sperm pretreated with heat-solubilized zonae no longer bind the inhibitor. These sperm, however, retain the plasma membrane over the acrosomal cap region. When the sperm are incubated in M199, no decrease in inhibitor binding due to zona treatment is noted.(ABSTRACT TRUNCATED AT 250 WORDS)

Egg envelope conversion following fertilization in Bufo japonicus

The envelope of the Bufo japonicus egg becomes impenetrable to sperm following fertilization. Electrophoretic analysis of envelopes showed that two glycoprotein components with apparent molecular weights of 65,000 and 61,000 were hydrolyzed during fertilization to 62,000 and 58,000, respectively. These two envelope components were structurally related as shown by peptide mapping and deglycosylation studies. Hardening of the envelope following egg activation was also observed, as detected by an increase in the envelope melting temperature. The involvement of proteolytic activities in the envelope hydrolysis and hardening reactions was demonstrated using protease inhibitors, and was verified for the hydrolysis reaction by observing a loss of mass in deglycosylated envelope components obtained before and after fertilization. A low ionic strength medium (less than 50 mM) was required for both the hardening and hydrolysis reactions. Envelopes from eggs activated in a high ionic strength medium were resistant to lysin from sperm, indicating that neither hydrolysis nor hardening was necessary to block lysin activity on the envelope. Both envelope hydrolysis and hardening could be effected in the absence of sperm (i.e., when eggs were activated by electric shock) and after egg jelly had been removed, indicating that neither sperm nor jelly factors were required for the envelope modifications. In addition, when eggs were activated in the presence of NH4Cl to suppress cortical granule exocytosis, envelope hardening and hydrolysis were still observed, indicating that a cortical granule-derived factor may not be involved.

Galactose at the nonreducing terminus of O-linked oligosaccharides of mouse egg zona pellucida glycoprotein ZP3 is essential for the glycoprotein's sperm receptor activity

During fertilization in mice, zona pellucida glycoprotein ZP3 mediates initial sperm-egg interactions by serving as receptor for sperm. Purified egg ZP3, as well as ZP3-derived O-linked oligosaccharides, exhibit sperm receptor activity in vitro. We report that treatment of purified egg ZP3 and ZP3-derived O-linked oligosaccharides with either alpha-galactosidase or galactose oxidase results in loss of sperm receptor activity. In the latter case, sperm receptor activity can be restored to the oxidized glycoprotein and O-linked oligosaccharides by treatment with sodium borohydride. We conclude that galactose, located in alpha-linkage at the nonreducing terminus of O-linked oligosaccharides, is at least one of the sugar determinants on ZP3 responsible for binding of sperm to the zona pellucida.

Primary structure of the mouse sperm receptor polypeptide determined by genomic cloning

The mouse sperm receptor, a glycoprotein called ZP3, is synthesized and secreted by growing oocytes. It is present in more than a billion copies in the unfertilized egg's extracellular coat, or zona pellucida. We have cloned and characterized a region of the mouse (CD-1) genome that spans 10 kilobases of the ZP3 locus. The genomic clones described encompass the entire ZP3 coding region, which contains eight exons. The exons were identified, mapped, and sequenced, yielding the entire primary structure of the ZP3 polypeptide chain (424 amino acids; Mr, 46,300), which includes a 22-amino acid signal sequence. In addition, sequencing of genomic clones has revealed some unusual features of ZP3 mRNA and a region just downstream of the ZP3 gene.

The regulation of acrosomal exocytosis. I. Sperm capacitation is required for the induction of acrosome reactions by the bovine zona pellucida in vitro

The regulation of acrosomal exocytosis in capacitated bovine spermatozoa by soluble extracts of zonae pellucidae was examined. Kinetic studies demonstrated that zonae pellucidae stimulated synchronous acrosome reactions. The t1/2 of this process was 5-10 min and response was maximal at 20 min. The apparent initial rate of exocytosis in sperm populations was dependent upon the concentration of zona pellucida protein, with an ED50 and a maximally effective dosage of 20 and 50 ng protein/microliter, respectively. Zonae pellucidae caused up to a 48-fold increase in the apparent initial rate and a 3- to 4-fold stimulation in the net occurrence of exocytosis. In contrast, solubilized zonae pellucidae did not induce acrosome reactions in uncapacitated sperm. The development of a capacitated state, as assayed by the ability of sperm to fertilize eggs in vitro, was compared to the expression of zona pellucida-regulated acrosome reactions in a series of kinetic experiments. Both activities were manifest with similar kinetics and displayed identical dependencies toward stimulatory and inhibitory agents in vitro. It is concluded that capacitation is an essential prerequisite for the induction of acrosomal exocytosis in bovine sperm by 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.

Regulation of acrosomal exocytosis. II. The zona pellucida-induced acrosome reaction of bovine spermatozoa is controlled by extrinsic positive regulatory elements

The effects of accessory sex gland secretions on the zona pellucida-induced acrosome reaction of bovine spermatozoa were investigated. Soluble extracts of zonae pellucidae initiated exocytosis in ejaculated spermatozoa. This process had an ED50 of 20 ng/microliter zona pellucida protein and saturated at 50 ng/microliter (Florman and First, 1988. Dev. Biol. 128, 453-463). In epididymal sperm this dose-response relationship was shifted toward greater agonist concentrations by at least a factor of 10(3). Reconstitution of high potency agonist response was achieved in vitro by incubation of epididymal sperm with bovine seminal plasma. Reconstitution was dependent on the seminal plasma protein concentration. The ED50 of this process was 62 micrograms protein/10(8) sperm and saturation was observed with 124 micrograms protein/10(8) sperm. Agonist responses in reconstituted epididymal sperm and in ejaculated sperm were indistinguishable with regard to dependence on the zona pellucida protein concentration and the kinetics of induced acrosome reactions. Kinetic studies suggest that reconstitution is due to adsorption of regulatory factors from seminal plasma. In addition to the positive regulatory elements responsible for reconstituting activity, seminal plasma also contains negative regulatory elements which inhibit agonist response. These negative factors are inactivated during sperm capacitation, permitting the expression of positive regulators. Acting together, these regulatory elements could coordinate high affinity agonist response with the availability of eggs in vivo.

Protein-carbohydrate complementarity in mammalian gamete recognition

Recent studies suggest that gamete recognition in a number of species is mediated by complementary proteins and carbohydrates on opposing gamete surfaces. Studies in invertebrates and vertebrates have shown that carbohydrate-binding proteins on the sperm surface recognize and bind to complementary glycoconjugates on the egg's extracellular coat. This chapter reviews our current knowledge of gamete recognition in the mouse. The complementary receptors for both mouse sperm and egg have been identified, purified, and characterized. Their synthesis during gametogenesis has been defined, as have the effects of sperm capacitation and of the acrosome reaction on their expression and distribution. Their relationship to gamete receptors that function in other species is discussed. Finally, evidence is presented that suggests that one of the receptors that mediate mouse gamete recognition belongs to a family of cell surface receptors that function during multiple cellular interactions in development.

A computer-assisted assay for mouse sperm hyperactivation demonstrates that bicarbonate but not bovine serum albumin is required

Mammalian sperm hyperactivation (HA) is a change in motility that accompanies capacitation (CAP) and is dependent on calcium (Ca) (Yanagimachi and Usui, Exp Cell Res 89:161, 1974). HA may be important for transport through the female tract and/or for fertilization. To develop an objective and quantitative assay for HA in individual mouse sperm, a computer-assisted motion-analysis system was used to describe sperm translational movements. To determine which movements were characteristic of HA, Ca-dependent motility was identified. This was done by incubating sperm with or without calcium (Ca+ or Ca- sperm, respectively), and determining the range of values for each motility parameter that was present only among Ca+ sperm. To do this, we compared frequency distributions of motility parameter values at the time of maximal CAP (90 min). CAP was monitored by measuring the level of in vitro fertilization and by evaluating the pattern of chlortetracycline binding to individual sperm heads [Ward and Storey, Dev Biol 104:287, 1984]. Two Ca-dependent motility subgroups were apparent: 1) a "slow-speed" subgroup with a curvilinear velocity (Vc) less than 169 microns/sec that had none of the characteristics expected of HA sperm; and 2) a subgroup with higher speeds (Vc greater than 169 microns/sec) and wider-amplitude head movements as measured by curvilinear progressiveness ratio (PRc less than 0.56). The latter subgroup was selected as HA, since the frequencies and time course were similar to those for CAP in the same population. Two media components known to be important for CAP, bicarbonate and bovine serum albumin (BSA) were then tested to determine whether they were necessary for HA. Incubation of sperm without bicarbonate prevented HA, but omitting BSA did not affect HA during the first 3 hrs. These data suggest that HA is not tightly coupled with CAP.

Binding of both acrosome-intact and acrosome-reacted guinea pig sperm to the zona pellucida during in vitro fertilization

Mammalian sperm-egg adhesion occurs when sperm bind to the zona pellucida of the egg. In this study with guinea pig gametes, we have asked if sperm can initiate binding to the zona before and after the acrosome reaction and if the sperm surface protein PH-20 is involved in the binding at these two stages. Sperm binding to the zona was examined under a variety of conditions. Sperm were suspended in 0.9% NaCl or capacitated by two different methods. Eggs were immobilized on lectin-coated coverslips, compressed between a coverslip and a glass slide, or free in tissue culture dishes. The sperm-egg interaction was recorded on videotape or assessed after fixation of the eggs with bound sperm. Under all conditions studied, both acrosome-intact and acrosome-reacted sperm initiated binding to the zona. The binding was persistent and not transitory. In particular, acrosome-intact sperm that bound the zona were observed to remain bound for up to 80 min. One acrosome-intact sperm, bound to the zona, was videotaped while it acrosome-reacted. When mixed sperm populations (on the average 24% acrosome-intact and 76% acrosome-reacted) were incubated with eggs for 30 min, an average of 10% of the bound sperm were acrosome-intact. The PH-20 monoclonal antibody has previously been shown to inhibit zona binding by guinea pig sperm of undetermined acrosomal status (P. Primakoff, H. Hyatt, and D. G. Myles (1985), J. Cell Biol. 101, 2239-2244). In this study, when the two populations of sperm were counted separately, PH-20 inhibited the binding of acrosome-reacted but not acrosome-intact sperm. Our results show that both acrosome-intact and acrosome-reacted guinea pig sperm can initiate binding to the zona; however, the binding in the two cases may not occur by the same mechanism.

Lymphocyte major histocompatibility complex-encoded class II structures may act as sperm receptors

Human sperm and blood cells were cocultured in vitro to determine whether specific interactions occur between gametes and blood cells. Evidence for cell type-specific sperm binding and penetration of lymphocytes is presented together with findings that suggest that either or both events involve major histocompatibility complex-encoded class II molecules on lymphocytes and a sperm ligand that is immunoreactive with antibodies to T-cell surface antigen T4. Involvement of HLA-DR is suggested by the pattern of sperm interactions with HLA-DR-positive and -negative cells and by inhibition of sperm binding to HLA-DR-positive cells by a monoclonal antibody that identifies a nonpolymorphic determinant on the HLA-DR molecule. That the complementary sperm ligand may be a T4-like structure is suggested by specific inhibition of sperm-lymphocyte binding with monoclonal antibodies OKT4 and OKT4A. The results are discussed in terms of possible roles for immunoglobulin-related structures in human fertilization and in the sexual transmission of the acquired immunodeficiency syndrome.

Localization of zona pellucida binding sites on rabbit spermatozoa and induction of the acrosome reaction by solubilized zonae

The binding of mammalian spermatozoa to the egg's extracellular coat, the zona pellucida, is a complex process which culminates in species-specific penetration of the sperm to the egg plasma membrane. To investigate where on the spermatozoon's surface the zona binding sites are located, whole rabbit zonae were labeled with FITC, heat solubilized and used to observe the surface binding patterns on live spermatozoa. Before the acrosome reaction the zona binding sites are located either over the entire head as well as the middle piece or alternatively in patches along the apical ridge of the head. After the acrosome reaction there is a 29% loss of fluorescence and the zona binding sites are present in the posterior aspect of the acrosomal region, the anterior postacrosomal region and the middle piece. These results demonstrate the presence of zona binding sites after the acrosome reaction which would account for the sperm's ability to remain bound to the zona after the acrosome reaction. Further, we report for the first time that solubilized rabbit zonae pellucidae will induce the acrosome reaction in in vitro capacitated rabbit sperm whereas solubilized pig zonae pellucidae will not. Since rabbit sperm bind pig zonae, the induction and specificity of the physiological acrosome reaction must reside in the affinity of the binding rather than the binding itself.

The biology and chemistry of fertilization

Fertilization of eggs by sperm, the means by which sexual reproduction takes place in nearly all multicellular organisms, is fundamental to the maintenance of life. In both mammals and nonmammals, the pathway that leads to fusion of an egg with a single sperm consists of many steps that occur in a compulsory order. These steps include species-specific cellular recognition, intracellular and intercellular membrane fusions, and enzyme-catalyzed modifications of cellular investments. In several instances, the molecular mechanisms that underlie these events during mammalian fertilization are beginning to be revealed.

Evidence for the role of a guanine nucleotide-binding regulatory protein in the zona pellucida-induced mouse sperm acrosome reaction

Recently, it has been demonstrated that mouse sperm contain a protein with properties similar to the inhibitory guanine nucleotide-binding regulatory protein, Gi (Kopf, G. S., Woolkalis, M. J., and Gerton, G. L. 1986. J. Biol. Chem. 261, 7327-7331). Since sperm-zona pellucida interaction represents a specialized form of intercellular communication and signal transduction we examined the role of the mouse sperm Gi-like protein in the zona pellucida-induced acrosome reaction using mechanically isolated, structurally intact zonae pellucidae. Sperm capacitated for 90 min in the presence of increasing concentrations of islet-activating protein (IAP) bind to the zona pellucida to a similar extent as control sperm incubated in the absence of this toxin. The zona pellucida-induced acrosome reaction, however, is inhibited in a concentration dependent manner by IAP, with half-maximal effects at 0.1-1.0 ng/ml IAP. IAP does not affect the ability of the sperm to become capacitated, but inhibits the cells from progressing into an intermediate stage prior to the completion of the acrosome reaction. When sperm are capacitated in the presence of 100 microM guanosine-5'-O-(3-thiotriphosphate) for 60 min prior to the addition of IAP during the final 30 min, the IAP-induced inhibition of the zona pellucida-induced acrosome reaction is abolished; capacitation in the presence of 100 microM guanosine-5'-O-(2-thiodiphosphate) does not abolish the inhibitory effects of IAP. The target of the IAP effect on intact sperm appears to be at the level of the Gi-like protein since IAP-catalyzed 32P-ADP-ribosylation of the Mr = 41,000 substrate in detergent extracts of sperm is reduced when intact sperm are preincubated with IAP during capacitation. These data suggest that the mouse sperm Gi-like protein plays an intermediary role in the zona pellucida-induced acrosome reaction.

Mouse sperm antigens that participate in fertilization. IV. A monoclonal antibody prevents zona penetration by inhibition of the acrosome reaction

To investigate the molecular basis of gamete interaction in mammals, monoclonal antibodies (mAbs) have been generated by syngeneic immunization with mouse testis. Previous work has described two particular mAbs, M41 and M42, which localize indistinguishably to the plasma membrane overlying a restricted portion of the acrosome, but recognize different antigens. One of the mAbs, M42, inhibits mouse fertilization in vitro significantly, but only in the presence of the zona pellucida, whereas M41 has no apparent effect upon any assayable event in the fertilization process. The experiments described here were performed to identify the precise event of sperm-zona interaction (sperm-zona binding, induction of the acrosome reaction, or penetration through the zona) that is affected by M42 mAb. Capacitated mouse sperm binding to the zona pellucida was undiminished following pretreatment with M42 mAb, when compared to levels achieved using either no mAb- or to M41 mAb-treated control sperm. When the effect of mAbs on the zona-induced AR was examined, the percentage of acrosome reacted (AR) sperm at the zona surface increased with time, plateauing at approximately 90 min post-insemination, with 78% of the bound cells AR in the control and the M41 mAb-treated groups. M42-treated sperm never achieved greater than 23% AR cells over the 120-min interval assayed. To quantitate this effect, capacitated sperm were exposed to increasing concentrations of acid-solubilized zonae. Increased proportions of AR sperm were found in the control and M41 mAb-treated groups, up to a maximum of 70-76% AR cells with 8 or 12 zonae/microliter. In contrast, M42-treated sperm displayed only 21-28% AR cells over the entire range of zonae concentrations tested. An entirely different result emerged when acrosome reactions were induced with A23187: M42 was no longer able to prevent the AR. This ability of A23187 to override M42 mAb's inhibitory effect on the AR permitted specific examination of the possible effect of M42 mAb on sperm penetration through the zona pellucida. In the presence of A23187, zona penetration levels for M42 mAb-treated sperm were equivalent, both qualitatively and quantitatively, to control and to M41 mAb-treated sperm under the same conditions. It appears, therefore, that M42 mAb identifies a high molecular weight doublet (220-240 kDa) of mouse sperm that participates specifically in the induction of the sperm's acrosome reaction as it occurs under physiological conditions.