www.elsevier.comrlocateranireprosci
Oocyte–sperm interactions
Edda Topfer-Petersen
¨
), Anna M. Petrounkina,
Mahnaz Ekhlasi-Hundrieser
Institute of ReproductiÕe Medicine, School of Veterinary Medicine, Bunteweg 15,¨
D-30559 HannoÕer, Germany
Abstract
The penetration of the zona pellucida is a crucial step during fertilization. Spermatozoa that are unable to recognize and bind to the zona pellucida glycoproteins or respond to the zona pellucida by undergoing the acrosome reaction fail to fertilize the egg. In most mammalian species, after entering the fallopian tube sperm are stored in the isthmic part of the oviduct under conditions that maintain sperm viability and synchronize both sperm transport and the process of acquisition of fertilizing ability, called capacitation. Only capacitated sperm are enabled to recognize the oocyte and respond to the oocyte signals in an appropriate manner. Close to time of ovulation sperm are released from the oviductal epithelium and swim to site of fertilization. The oviduct and the oocyte itself appear to coordinate sperm function and gamete interaction. The gamete recognition and the next levels of interaction are probably granted by the carbohydrate–protein interactions. Upon binding the signal cascade leading to acrosomal exocytosis is activated, eventually initiated by aggregation of zona pellucida receptor molecules. These signal transducing mechanisms are primed during the capacitation process. Tyrosine phosphorylation, tightly connected to the cholesterol efflux from the plasma membrane, and hyperpolarization seem be involved in this priming by activation of Ca2qpathways. Further preparational steps of the acrosome reaction may
be mediated by osmosensitive signal transducing mechanisms.
The current perspective focuses on the molecules involved in the complex hierarchy of sperm–egg interactions and regulative events priming sperm cell during capacitation for the acrosome reaction.q2000 Elsevier Science B.V. All rights reserved.
Keywords: Sperm; Egg; Gamete recognition; Carbohydrate–protein interaction; Capacitation; Acrosome reaction
)Corresponding author. Tel.:q49-511-9538520; fax:q49-51-9538504.
Ž .
E-mail address: [email protected] E. Topfer-Petersen .¨
0378-4320r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved. Ž .
1. Introduction
The penetration of the oocyte zona pellucida is a crucial step during fertilization. The zona pellucida is the extracellular network-like matrix enveloping the egg that protects the growing egg and the preimplantation embryo against physical damages. Sperm that are unable to recognize and bind to the zona pellucida glycoproteins, as well as sperm, upon binding, that are unable to respond with it by undergoing the acrosome reaction fail to fertilize the egg. After entering the Fallopian tube sperm are stored in the isthmic
Ž part of the oviduct by binding to the ciliated cell lining the oviductal epithelium Suarez,
.
1999 . Sperm are stored under conditions that maintain sperm viability and synchronize sperm transport and the capacitating process. This enables sperm to interact with the oocyte in the appropriate manner.
Reaching the site of fertilization sperm recognize the egg by carbohydrate–protein interactions. It is well accepted that the corresponding carbohydrate-binding proteins of the sperm surface bind defined oligosaccharide ligands of the zona pellucida. Upon
Ž .
binding the signaling cascade leading to the acrosomal exocytosis acrosome reaction is activated. This allows sperm to penetrate the zona pellucida. After penetrating the zona pellucida sperm interact and fuse with egg vitelline membrane and trigger thus the
Ž .
embryonic development program Yanagimachi, 1994 .
The current perspective focuses on the molecules involved in the complex hierarchy of interactions between sperm and egg in pig and other domestic animals.
2. Molecules involved in gamete recognition
Close to the time of ovulation sperm are released from the oviductal epithelium and swim to the site of fertilization to meet the egg. The hyperactivated motility that sperm develop during the capacitating process not only enables the sperm to reach the oocyte but also may be necessary for the collision with the egg assisting the manifestation of
Ž .
the adhesion between sperm and egg Thaler and Cardullo, 1996 . The adhesion between both gametes is a complex sequence of binding events implicating low and high affinity
Ž .
binding sites Thaler and Cardullo, 1996 . The fundamental mechanism of gamete recognition appears to be conserved throughout the evolution from marine invertebrates to eutherian mammals and is based on carbohydrate–protein interactions between the sperm and the oocyte envelope. Oligosaccharides that are presented with a certain arrangement within the supramolecular architecture of the egg envelope are recognized by complementary carbohydrate receptors of the sperm, thereby mediating gamete recognition and coordinating sperm functions to warrant fertilization.
3. Zona pellucida
Ž .
genes Harris et al., 1994 . In rodents zp glycoproteins are exclusively synthesized by the oocyte whereas, in domestic animals possessing a thick zona pellucida, e.g. in pig and cow, during follicular development the surrounding follicle cells contribute to the
Ž .
formation of the extra cellular matrix for review see Sinowatz et al., 1999 . The three proteins build a typical fibrogranular structure by noncovalent interactions presenting a
Ž .
complex and highly heterogeneous mixture of asparagine N - and serinerthreonine
Ž .O -linked oligosaccharide side chains. It has been proved that in different mammalian species amino acid sequence of the zp glycoproteins is highly conserved between different mammalian species. However, variable posttranslational glycosylation and processing of the polypeptide chains as well as the variable assembling of the
supra-Ž .
molecular structure of the zp matrix possibly due to differing biosynthetic pathways
Ž .
lead to substantial differences of zp structure and function between rodents mouse and
Ž . Ž
domestic animals pig . In mouse the zona pellucida protein 3 mZP3 encoded by the .
ZPC gene carrying the sperm receptor activity and the acrosome reaction-inducing
Ž .
potency forms together with zona pellucida protein 2 mZP2 encoded by the ZPA gene long periodically heterodimeric filaments that are randomly interconnected with the zona
Ž . Ž .
pellucida protein 1 mZP1 encoded by the ZPB gene Wassarman and Mortillo, 1991 .
Ž
In contrast, in pig the biological active zp protein has been found to be ZPB the mZP1
. Ž .
homologue that tends to aggregate with ZPC the mZP3 homologue thereby increasing
Ž .
the sperm binding capacity Yurewicz et al., 1998 . Murine zp proteins can be separated
Ž . Ž .
by SDS-PAGE showing molecular masses of 80 kDa ZP3rZPC , 120 kDa ZP2rZPA
Ž Ž . Ž .
and 220 kDa ZP1 ZPB reviewed by Wassarman, 1999 . In pig the zona pellucida
Ž . Ž .
protein 3 pZP3 has been shown to be a mixture of the so-called pZP3a ZPB and
Ž .
pZP3b ZPB with apparent molecular masses of 55 kDa and in cow the ZP3arb
Ž . Ž
homologues reveal overlapping bands 78–88 kDa after 2-D PAGE Noguchi and .
Nakano, 1992; Topper et al., 1997 . Information on the oligosaccharide structure of zp glycoproteins is almost complete for pig and partially available for mouse and cow
ŽHokke et al., 1994; Nakano et al., 1996; Topfer-Petersen, 1999 . Although the
¨
.structures of the oligosaccharide chains are basically the same in these three species, they differ in the percentage and structure of the neutral N-linked carbohydrates.
Ž Whereas, in mouse the N-glycans are almost acidic, the major neutral N-glycans about
.
25% of the porcine zp glycoproteins belong to the bi-antennary fucosylated complex N-type and is a high mannose-type in cattle. In cattle the major neutral N-glycan has
Ž .
been implicated in sperm–egg recognition Nakano et al., 1996 and may bind to still Ž
poorly described mannose-binding proteins of the capacitated sperm Suarez, personal .
communication . In pig sperm receptor activity has been mapped to O- and N-linked
Ž .
glycans of ZPB Yurewicz et al., 1991; Yonezawa et al., 1995 . The trirtetra-antennary
Ž .
N-glycans localized in the N-terminal region of the mature ZPB pZP3a mediate the
binding of sperm to the zona pellucida whereas, the structural identical trir
tetra-anten-Ž .
nary N-glycans of the ZPC molecule pZP3b appear to play no role in gamete
Ž .
recognition Kudo et al., 1998, Yonezawa et al., 1999 . The only difference between these oligosaccharides is the C-terminally localized glycosylation site within the ZPC molecule, possibly leading to a reduced accessibility of the glycan chains, dependent on
Ž .
Fig. 1. Schematic presentation of the known N-glycosylation sites in porcine zp glycoproteins. Of the six potential N-glycosylation sites of ZPA only Asn268 has been identified carrying biantennary N-glycans
ŽTopfer-Petersen, unpublished . ZPB and ZPC both possess three glycosylation sites Kudo et al., 1998;¨ . Ž
. Ž .
Yonezawa et al., 1999 . The mature ZPB molecule is N-terminally processed Asp 137 and the mature ZPC
Ž .
molecule losses the signal peptide and is N-terminally blocked pyro-Gln23 . Signal peptide is marked in grey.
within the three-dimensional structure are important features to achieve the physio-logical relevant binding between both gametes. In mouse low- and high-affinity binding sites have been identified on the sperm surface implicating a hierarchy of binding events ŽJohnston et al., 1998 . Interestingly, the high-affinity site recognizing fucosylated. oligosaccharides can be occupied with less affinity by other carbohydrate structures. The multivalent presentation of the biological active oligosaccharides may be a necessity to create high-affinity binding. In mouse the relevant O-linked oligosaccharides are
clus-Ž .
tered at the C-terminal sperm-combing sites of mZPC reviewed by Wassarman, 1999 .
In pig the arrangement of the oligosaccharides within the heteromultimeric ZPBrZPC
complex may be responsible for the manifestation of high-affinity binding to the sperm ŽYurewicz et al., 1998 . There is a considerable controversy regarding the structural.
Ž .
entity of the sperm oligosaccharide ligands in both species suggesting that i different Ž
experimental conditions and the possible binding of uncapacitated and capacitated
. Ž .
sperm to the zona; Miranda, 1998 andror ii an allowed structural variety of the
4. Zona pellucida binding proteins
A long list of putative zona pellucida receptors has been described up to date. However, only few are characterized regarding their carbohydrate specificity and structure of the carbohydrate-binding domain, e.g. rabbit sp17, mouse
galactosyltrans-ferase, porcine spermadhesins and proracrosin. Spermadhesins represent a new class of
lectins with specificity to galactose-containing structures, mannose or mannose-6-phos-phate, whereas, proacrosin binds to zp glycoproteins following a sulfate-recognition mechanism. Similarly, sp17 recognizes sulfated carbohydrate as they are presented in the zona pellucida and fucoidan and share consensus sequences with the class of C-type lectins. Porcine zonadhesin and mouse sp56 are proteins containing a molecular
Ž structure with still unknown ligand-binding specificity. Zona receptor kinases mouse
. Ž .
p95 and human hu9 ZRK and human fertility antigen A-1 FA-1 are autophosphory-lated in response to the zona pellucida exhibiting an intrinsic signaling potential. Some proteins have been found by targeted mutagenesis not to be particularly relevant to
Ž .
gamete recognition e.g. mouse galactosyltransferase . They may rather function in
Ž .
earlier events such as capacitating and sperm–oviduct interactions spermadhesins . Others are obviously located in the wrong compartment of the sperm to participate in
Ž .
gamete recognition or primary binding e.g. proracrosin and PH-20 . These may rather
be involved in the transient secondary binding of acrosome-reacted sperm during zona Ž
penetration reviewed by Naz Rajesh, 1996; McLesky et al., 1998; Shur, 1998; Topfer-
¨
. Petersen, 1999 .
5. Receptor aggregation initiates acrosome reaction
The first evidence that the aggregation of zona pellucida receptor molecules within the plane of the sperm membrane triggers the signaling cascade resulting in acrosome
Ž .
reaction was demonstrated by Leyton and Saling 1989 . Zp glycoproteins and
antibod-Ž .
ies directed against zona receptor kinase ZRK , galactosyltransferase and some other sperm surface proteins are able to initiate the acrosome reaction whereas, a zp glycopeptide fraction retaining sperm binding ability and univalent Fab-fragments fail to
Ž .
induce the acrosome reaction McLesky et al., 1998; Shur, 1998 . Putative zona
pellucida binding proteins have been found to traverse the sperm plasma membrane as ZRK, galactosyltransferase, human FA-1 and porcine zonadhesin whereas, porcine p47, spermadhesins and murine proteinase inhibitor-binding protein are peripherally
associ-Ž
ated to the sperm surface Aarons et al., 1991; Naz Rajesh, 1996; McLesky et al., 1998; .
Shur, 1998; Topfer-Petersen, 1999 . Those surface-associated and transmembrane pro-
¨
teins may form the multimeric receptor upon binding to the zona pellucida thus initiating Ž
the aggregation of the signaling molecules of the receptor complex ZRK, galactosyl-.
transferase, zonadhesin or other still unknown components that then trigger the different
Ž .
pathways of acrosome reaction Florman et al., 1998 . The postulated multimeric sperm
Ž .
The following model of the ionic events in ZP signal transduction is largely based on
Ž .
the information reviewed by Florman et al. 1998 . According to this model the
activation of the sperm surface receptor by the association with mouse ZP3 initiates two separate pathways. One signaling sequence leads to the activation of a cation channel C producing inward currents depolarizing sperm membrane potential and opening low voltage-activated T-type Ca2q channel. The other pathway initiates internal
alkaliniza-tion by mechanisms likely reflecting the mediaalkaliniza-tion of G proteins. This pH increase and
transient Ca2q current in response to ZP or membrane depolarization promote a
sustained Ca2q increase. The elevation of Ca2q level, a rise of intracellular pH and the
enhancement of membrane fusogenity are postulated to be the driving forces triggering Ž
the cascade of acrosomal expcytosis Harrison and Roldan, 1990; Roldan and Harrison, .
1990; Kopf and Gerton, 1991; Aitken, 1997 .
6. Priming of signal transducing mechanisms during capacitation
Ejaculated mammalian sperm need a period of incubation in the female reproductive
Ž .
tract in order to acquire the capacity to fertilize an egg Yanagimachi, 1994 . This period
Ž .
of attaining functional competence, referred to as capacitation Austin, 1951 , is required for undergoing the acrosome reaction induced by physiological stimuli such as ZP ŽFlorman and First, 1988 . Since the response to the oocyte signals can be initiated only. in capacitated sperm, the signal-transducing mechanisms appear to be primed during capacitation.
Capacitation is accompanied by an increase of membrane fluidity and remodeling of
the sperm surface, protein phosphorylation, an increase of internal Ca2q and pH and
Ž .
membrane hyperpolarization Storey, 1995 . Generally accepted is the concept of
Ž .
capacitation as series of positive destabilizing events Harrison, 1996 . The initial event
is the BSArLDH-mediated cholesterol efflux resulting in an increase of plasma
Ž
membrane fluidity thus supporting membrane remodeling Davis et al., 1979; Visconti et .
al., 1999 . Kinase-mediated tyrosine posphorylations are tightly connected with this
Ž .
event Visconti et al., 1994a,b .
Tyrosine phosphorylation appears to be an essential process of capacitation: if tyrosine kinase inhibitors block it the spermatozoa lose the ability to respond to
Ž .
physiological agonists Aitken et al., 1996 . Since cross talk with cAMP is involved in the regulation of protein tyrosine phosphorylation, it is tempting to speculate that the loss of cholesterol may be involved in the regulation of cAMP pathway. The monotonic
elevating of Ca2q with the rate about 0.5 nm
rmin seems to be insufficient to initiate
acrosome reaction and might occur due to the modulation of Ca2q-ATPase activity.
Ž
The mechanism related to voltage-dependent T-channel described above Florman et .
al., 1998 and references therein appears to be responsible for the explosive increase of
Ca2q in response to physiological agonists leading to the acrosomal exocytosis. The
sperm T-channel may be primed and held in steady state during capacitating by
Ž q .
hyperpolarization probably due to K -permeability contribution at the level of an upstream action channel and by tyrosine phosphorylation. The tyrosine phosphorylation
prevent-ing the acrosome reaction, whereas, the dephosphorylation may stimulate the activation
Ž .
of the channel Arnoult et al., 1997 .
However, it appears that further signal transduction mechanisms may be involved in
the activation of Ca2q pathways. A mild hypoosmotic shock has been shown to be a
Ž .
potent stimulus of the acrosome reaction. Rossato et al. 1996 demonstrated that human
spermatozoa possess Ca2qinflux pathways activated by plasma membrane stretching. A
2q Ž q.
blocker of mechano-sensitive Ca channels Gd diminished osmotically sensitive
Ca2q-rises in a dose-dependent manner and completely blocked the osmotically sensitive
acrosome reaction.
The formal evidence is missing that Ca2q rise occurs via Ca2q channels, but this
hypothesis seems likely due to fast kinetics, sensitivity to membrane depolarization and Gdq treatment.
Osmosensitive mechanisms may also play a role during preparation steps of the
Ž .
acrosome reaction capacitation . The regulation mediated by osmotic events may be reflected in cell volume behavior during capacitation. Boar sperm cell volume tends to show cyclical variations during incubation under capacitating conditions with the frequency varying dependent on the incubation conditions. The expression of different levels of swelling at fixed osmotic conditions is coupled to the change in cell osmole content. The osmosensing regulatory performance, such as swelling and subsequent regulatory volume decrease, could be modulated during capacitation and may reflect the ongoing destabilization of the membrane on earlier stages of capacitation treatment ŽPetrounkina et al., 2000 . Preliminary studies on boar sperm let suggest that i changes. Ž . in cell volume are closely related to the expression of tyrosine phosphorylation as shown by indirect immunfluorescence on the earlier stages of incubation under capacitation
Ž .
conditions, and ii cell volume regulation is mediated in the first line by
quinine-sensi-Ž .
tive channels Petrounkina et al., 2000 . The participation of quinine-sensitive channels Ž . on cell volume regulation of bull sperm was reported by Kulkarni et al. 1997 . For
Ž .
other cell types e.g. in erythrocytes protein tyrosine phosphorylation has already been
Ž .
found to be osmo- and volume-sensitive Mush et al., 1994, 1998 . Hypoosmotic shock triggered a rapid increase in tyrosine phosphorylation; furthermore, the volume regula-tion of swollen cells was sensitive to tyrosine phosphatase inhibiregula-tion. Phosphorylaregula-tion of a band 3 protein in human erythrocytes, observed by ionophore treatment or
shrinking-induced volume regulation, was suppressed by quinine, an inhibitor of Ca2q-activated
q Ž .
K channel Minetti et al., 1996 . Moving into field of speculation, the similar
modulation of osmosensing signal transduction by tyrosine phosphorylation may be expected also in sperm cells while priming for activation in response to ZP and for zona penetration. Further studies are in progress to investigate this hypothetical regulatory mechanism of signal transduction during capacitation.
7. Conclusion
activated stage and be able to fertilize the egg. In response to the zona pellucida
phosphorylationrdephosphorylation reactions may switch the sperm from the steady to
the active state. By interacting with the capacitated sperm the clustered presentation of the oligosaccharide chains within the three-dimensional structure of the zona pellucida may initiate the formation of a multimeric receptor complex within the sperm mem-brane, thereby aggregating the signaling molecules of the complex that then trigger the cascade leading to acrosomal exocytosis, zona penetration and fusion with the egg.
References
Aarons, D., Boettger-Tong, H., Holt, G., Poirer, G.R., 1991. Acrosome reaction induced by immunoaggrega-tion of a proteinase inhibitor bound to the murine sperm head. Mol. Reprod. Dev. 30, 258–264. Arnoult, C., Lemos, J.R., Florman, H.M., 1997. Voltage-dependent modulation of T-type Ca2q-channels by
protein tyrosine phosphorylation. EMBO J. 16, 1593–1599.
Ž .
Aitken, R.J., 1997. The cell biology of fertilization. In: Ivell, Holstein Eds. , The Fate of the Male Germ Cell. pp. 291–299.
Aitken, R.J., Buckingham, D.W., Harkiss, D., Fisher, H., Paterson, M., Irvine, D.S., 1996. The extragenomic action of progesterone on human spermatozoa is influenced by redox regulated changes in tyrosine phosphorylation during capacitation. Mol. Cell. Endocrinol. 117, 83–93.
Austin, C.R., 1951. Observation of the penetration of sperm into the mammalian egg. Aust. J. Sci. Res. 4, 581–596.
Davis, B.K., Byrne, R., Hungund, B., 1979. Studies on the mechanism of capacitation: II. Evidence for lipide transfer between plasma membrane of rat sperm and serum albumin during capacitaton in vitro. Biochim. Biophys. Acta 558, 257–266.
Flormann, H.M., First, N.L., 1988. The regulation of the acrosomal exocytosis: I. Sperm capacitation is required for the induction of the acrosome reactions by the bovine zona pellucida in vitro. Dev. Biol. 128, 453–463.
Florman, H.M., Arnoult, C., Kasam, I.G., Chongqing, L., O’Toole, C.M.B., 1998. A perspective on the control of mammalian fertilization by egg-activated ion channels in sperm: a tale of two channels. Biol. Reprod. 59, 12–16.
Harris, J.D., Hibler, D.W., Fontenot, G.K., Hsu, K.T., Yurewicz, E.C., Sacco, A.G., 1994. Cloning and characterisation of zona pellucida genes and cDNAs from a variety of mammalian species: the ZPA, ZPB and ZPC gene families. DNA Sequence 4, 361–393.
Harrison, R.A.P., 1996. Capacitation mechanisms, and the role of capacitation as seen in eutherian mammals. Reprod. Fertil. Dev. 8, 581–594.
Harrison, R.A.P., Roldan, E.R.S., 1990. Phosphoinositides and their products in the mammalian sperm
Ž .
acrosome reaction. J. Reprod. Fertil. 42 Suppl. , 51–67.
Hokke, C.H., Damm, J.B., Penninkhof, B., Aitken, R.J., Kamerling, J.P., Vliegenhardt, J.F.G., 1994. Structure of the O-linked carbohydrate chains of porcine zona pellucida glycoproteins. J. Biochem. 221, 491–512. Johnston, D.S., Wright, W.V., Shaper, J.H., Hokke, C.H., Vanden Eijinden, D.H., Joziasse, D.H., 1998. Murine sperm–zona binding, a fucosyl residue is required for a high affinity sperm-binding ligand. J. Biol. Chem. 273, 1888–1895.
Kopf, G.S., Gerton, G.L., 1991. The mammalian sperm acrosome and the acrosome reaction. In: Wassarman,
Ž .
P.M. Ed. , Elements of Mammalian Fertilization Vol. 9 CRC Press, Boca Raton, FL, pp. 153–203. Kudo, K., Yonezawa, N., Katsumata, T., Aoki, H., Nakano, M., 1998. Localization of carbohydrate chains of
pig sperm ligand in the glycoprotein ZPB of egg zona pellucida. Eur. J. Biochem. 252, 492–499. Kulkarni, S.B., Sauna, Z.E., Somlata, V., Sitaramam, V., 1997. Volume regulation of spermatozoa by
quinine-sensitive channels. Mol. Reprod. Dev. 46, 535–550.
McLesky, S.B., Dowds, C., Carbadalla, R., White, R.R., Saling, P.M., 1998. Molecules involved in mammalian sperm–egg interaction. Int. Rev. Cytol. 177, 57–113.
Minetti, G., Piccini, G., Balduini, C., Seppi, C., Brovelli, A., 1996. Tyrosine phosphorylation of band 3 protein in Ca2qrA23187-treated human erythrocytes. Biochem. J. 320, 445–450.
Miranda, P.V., 1998. Human sperm–zona interaction: while trying to reproduce. Mol. Hum. Reprod. 4, 523–525.
Mush, M.W., Leffingwell, T.R., Goldstein, L., 1994. Band 3 modulation and hypotonic-stimulated taurine efflux in skate erythrocytes. Am. J. Physiol. 266, R65–R74.
Mush, M.W., Davis-Amaral, E.M., Leibowitz, K.L., Goldstein, L., 1998. Hypotonic stimulated taurine efflux in skate erythrocytes: regulation by tyrosine phosphatase activity. Am. J. Physiol. 274, R1677–R1686. Nakano, M., Yonezawa, N., Hatanaka, Y., Noguchi, S., 1996. Structure and function of the N-linked
carbohydrate chains of pig zona pellucida glycoproteins. J.Reprod. Fertil. 50, 25–34.
Naz Rajesh, K., 1996. Involvement of protein tyrosine phosphorylation of human sperm in capacitationracrosome reaction and zona pellucida binding. Front. Biosci. 1, d206–213.
Noguchi, S., Nakano, M., 1992. Structure of the acidic N-linked carbohydrate chains of the 55-kDa
Ž .
glycoprotein family PZP3 from porcine zona pellucida. Eur. J. Biochem. 209, 883–894.
Petrounkina, A.M., Harrison, R.A.P., Petzoldt, R., Weitze, K.F., Topfer-Petersen, E., 2000. Cyclical changes¨
in sperm volume during in-vitro incubation under capacitation conditions: a novel boar sperm characteris-tic. J. Reprod. Fertil. 118, 283–293.
Roldan, E.R.S., Harrison, R.A.P., 1990. Molecular mechanisms leading to exocytosis during the sperm
Ž .
acrosome reaction. In: Bavister, B.D., Cummins, J., Roldan, E.R.S. Eds. , Fertilization in Mammals. Serono Symposia, Norwell, Massachusetts, USA. pp. 179–196.
Rossato, M., DiVirgilio, F., Foresta, C., 1996. Involvement of osmo-sensitive calcium influx in human sperm activation. Mol. Hum. Reprod. 2, 903–909.
Shur, D.B., 1998. Is sperm galactosyltransferase a signaling subunit of a multimeric gamete receptor?. Biochem. Biophys. Res. Commun. 250, 537–543.
Sinowatz, F., Topfer-Petersen, E., Kolle, S., Plendl, J., 1999. Structure and function of the zona pellucida in¨ ¨
domestic animals. Anat., Histol., Embryol., in press.
Ž .
Suarez, S.S., 1999. Regulation of sperm transport in the mammalian oviduct. In: Gabno, C. Ed. , Spermatol-ogy. Cache Review Progress, Vienna, in press.
Storey, B.T., 1995. Interaction between gametes leading to fertilization: the sperm’s eye view. Reprod. Fertil. Dev. 7, 927–942.
Thaler, C.D., Cardullo, R.A., 1996. The initial molecular interaction between mouse sperm and the zona pellucida is a complex binding event. J. Biol. Chem. 271, 23289–23297.
Topper, E.K., Kruijt, L., Calvete, J.J., 1997. Identification of bovine zona pellucida glycoproteins. Mol. Reprod. Dev. 46, 344–350.
Topfer-Petersen, E., 1999. Carbohydrate-based interactions on the route of a spermatozoon to fertilization.¨
Hum. Reprod. Update 5, 314–329.
Wassarmann, P.M., 1999. Mammalian fertilization: molecular aspects of gamete adhesion, exocytosis and
Ž .
fusion. Cell 96 2 , 175–183.
Wassarman, P.M., Mortillo, S., 1991. Structure of the mouse egg extracellular coat, the zona pellucida. Int. Rev. Cytol. 130, 85–110.
Visconti, P.E., Bailey, J.L., Moore, G.D., Pan, D., Olds-Clarke, P., Kopf, G., 1994a. Capacitation of mouse spermatozoa: I. Correlation between the capacitation state and protein tyrosine phosphorylation. Develop-ment 121, 1129–1137.
Visconti, P.E., Moore, G.D., Bailey, J.L., Leclerc, P., Connors, S.A., Pan, D., Olds-Clarke, P., Kopf, G., 1994b. Capacitation of mouse spermatozoa: II. Protein tyrosine phosphorylation and capacitation are regulated by a cAMP-dependent pathway. Development 121, 1139–1150.
Visconti, P., Galantino-Homer, E.H., Ning, X.P., Moore, G.D., Valenzuela, J.P., Jorgez, C.J., Alvarez, J.G., Kopf, G.S., 1999. Cholesterol efflux mediated signal transduction in mammalian sperm. J. Biol. Chem. 274, 3235–3240.
Ž .
Yonezawa, N., Aoki, H., Hatanaka, Y., Nakano, M., 1995. Involvement of N-linked carbohydrate chains of pig zona pellucida in sperm–egg binding. Eur. J. Biochem. 233, 35–41.
Yonezawa, N., Fukui, N., Kudo, K., Nakano, M., 1999. Localization of neutral N-linked carbohydrate chains in pig zona pellucida glycoprotein ZPC. Eur. J. Biochem. 260, 57–63.
Yurewicz, E.C., Pack, B.A., Sacco, A.G., 1991. Isolation, composition and biological activity of sugar chains of porcine zona pellucida 55K glycoproteins. Mol. Reprod. Dev. 30, 126–134.
