Epididymal proteins

One of the first decapacitation factors described is a 40 kDa glycoprotein acquired during mouse sperm epididymal transit known as DF (Fraser et al., Reference Fraser, Harrison and Herod1990). This protein binds to spermatozoa via a glycophosphatidylinositol (GPI)-anchored membrane receptor called the DF receptor, mainly located in the post-acrosomal region (Fraser, Reference Fraser1998). Its elimination from the uncapacitated spermatozoa surface resulted in highly fertile gametes and, when added back to capacitated spermatozoa produced poorly fertile cells in which the acrosome reaction was blocked (Fraser et al., Reference Fraser, Harrison and Herod1990). DF stimulates calmodulin-sensitive calcium ATPase activity, thereby ensuring the maintenance of low intracellular calcium concentrations. As capacitation proceeds, DF is lost and calcium ATPase activity declines, allowing intracellular calcium to rise and hence promoting capacitation-related changes (Adeoya-Osiguwa and Fraser, Reference Adeoya-Osiguwa and Fraser1996). In human, the addition of mouse DF to capacitated sperm suspensions causes a significant reversal in the capacitation state of the cells, suggesting that a similar mechanism could take place in both species (DasGupta et al., Reference DasGupta, Mills and Fraser1994). The DF receptor on sperm is the phosphatidylethanolamine-binding protein 1 (PEBP1), a 23 kDa protein located extracellularly on the acrosome, the post-acrosomal region and the flagellum of mouse and human spermatozoa (Gibbons et al., Reference Gibbons, Adeoya-Osiguwa and Fraser2005). Investigations carried out in the mouse model showed that PEBP1 along with other three proteins, identified as the plasma membrane fatty acid binding protein, the cysteine-rich secretory protein 1 (CRISP1), and a decapacitation factor named DF10, were able to inhibit sperm–zona pellucida (ZP) interaction as well as the sperm ability to acrosome react in response to progesterone (Nixon et al., Reference Nixon, MacIntyre, Mitchell, Gibbs, O’Bryan and Aitken2006). CRISPs are a family of proteins closely involved in the process of fertilization. In human and rat, there are three CRISPs (CRISP1, CRISP2 and CRISP3 for human; CRISP1, CRISP2 and CRISP4 for rat), while mice produce four CRISPs named CRISP1, CRISP2, CRISP3 and CRISP4 (Koppers et al., Reference Koppers, Reddy and O’Bryan2011). CRISP2 (originally known as Tpx-1) is expressed exclusively in the mammalian testicle and is incorporated into the developing sperm acrosome and flagellum (O’Bryan et al., Reference O’Bryan, Sebire, Meinhardt, Edgar, Keah, Hearn and De Kretser2001), while CRISP1 and CRISP4 are secreted by the epididymis and incorporated onto the spermatozoa during epididymal storage (Reddy et al., Reference Reddy, Gibbs, Merriner, Kerr and O’Bryan2008). Experiments performed in the rat model have shown that the addition of CRISP1 to the sperm inhibits tyrosine phosphorylation in a dose-dependent manner, therefore inhibiting capacitation and ultimately the acrosome reaction and that such inhibition occurs upstream the production of cAMP by the sperm (Roberts et al., Reference Roberts, Wamstad, Ensrud and Hamilton2003). However, there are two populations of CRISP1 in spermatozoa: a loosely bound that is released during capacitation and the form strongly associated with the sperm that remains on cells after capacitation (Cohen et al., Reference Cohen, Rochwerger, Ellerman, Morgenfeld, Busso and Cuasnicu2000b). It has been shown that CRISP1 that remain associated with the sperm head after capacitation, migrates to the equatorial segment during the acrosome reaction and is involved in sperm–ZP interaction and subsequent gamete fusion (Rochwerger et al., Reference Rochwerger, Cohen and Cuasnicu1992; Cohen et al., Reference Cohen, Ellerman and Cuasnicu2000a, Reference Cohen, Maldera, Vasen, Ernesto, Munoz, Battistone and Cuasnicu2011). Similarly, human CRISP1 also participates in sperm binding to the ZP and in gamete fusion (Maldera et al., Reference Maldera, Weigel Muñoz, Chirinos, Busso, Raffo, Battistone, Blaquier, Larrea and Cuasnicu2014; Da Ros et al., Reference Da Ros, Munoz, Battistone, Brukman, Carvajal, Curci, Gomez-ElIas, Cohen and Cuasnicu2015). Recent investigations with Crisp1 and Crisp4 knockout mice indicated that these proteins may act redundantly or autonomously on sperm function. CRISP1 is essential for the establishment of normally motile sperm, but CRISP4 enhances capacitation-associated tyrosine phosphorylation, and both are required for normal acrosome function and sperm–egg interaction (Hu et al., Reference Hu, Merriner, O’Connor, Houston, Furic, Hedger and O’Bryan2018). In contrast, CRISP3 shows a wider tissue distribution and is secreted by the prostate and seminal vesicles into the seminal plasma (Koppers et al., Reference Koppers, Reddy and O’Bryan2011), but its participation in sperm function regulation is still unknown.

Another protein expressed and secreted in the adult rat epididymis that binds to sperm head plasma membrane is HongrES1, a member of the serine proteinase inhibitor (SERPIN) protein family (Hu et al., Reference Hu, Liu, Shang, Zheng, Yang and Zhang2002). The co-culture of caudal spermatozoa with anti-HongrES1 antibodies resulted in an increased proportion of capacitated spermatozoa. Moreover, the percentage of capacitated spermatozoa increased when HongrES1 was downregulated by RNAi in vivo (Zhou et al., Reference Zhou, Zheng, Shi, Zhang, Zhen, Chen and Zhang2008). This protein has also been found in the cauda epididymis of guinea pig, where it is gradually exfoliated during capacitation and redistributed to patches on the sperm head and tail, and disappears after acrosome reaction. Removal of HongrES1 from the sperm surface facilitates extracellular calcium influx, thereby allowing the progress of capacitation and hyperactivation (Ni et al., Reference Ni, Zhou, Chen, Zheng, Yu, Li, Zhang and Shi2009). Moreover, it has been demonstrated that human and equine epididymal secretomes contain lactotransferrin (LTF) (Dacheux et al., Reference Dacheux, Belghazi, Lanson and Dacheux2006, Reference Dacheux, Belleannee, Jones, Labas, Belghazi, Guyonnet, Druart, Gatti and Dacheux2009), a protein that has been shown to decrease sperm ability to interact with ZP (Zumoffen et al., Reference Zumoffen, Gil, Caille, Morente, Munuce and Ghersevich2013) and to have dose-dependent effects on sperm proteins tyrosine phosphorylation (Zumoffen et al., Reference Zumoffen, Massa, Caille, Munuce and Ghersevich2015; Hernández-Silva et al., Reference Hernández-Silva, Durand, Larrea and Chirinos2018).

Seminal vesicle proteins

Seminal vesicles secretions contribute with most of the proteins existing in the seminal fluid and semenogelin-1 (Sg1) is the most abundant protein secreted by seminal vesicles and the main component of semen coagulum. After ejaculation, it is degraded by the serine protease prostate-specific antigen (PSA) to generate peptides of various biological activities and allow coagulum liquefaction (de Lamirande et al., Reference de Lamirande, Yoshida, Yoshiike, Iwamoto and Gagnon2001). Previous studies have shown that Sg1 inhibits human sperm motility (Robert and Gagnon, Reference Robert and Gagnon1996), protein tyrosine phosphorylation and the amount of superoxide anion generated during capacitation (de Lamirande et al., Reference de Lamirande, Yoshida, Yoshiike, Iwamoto and Gagnon2001), which is related to the progress of hyperactivation (de Lamirande and Gagnon, Reference de Lamirande and Gagnon1995). Interestingly, during ejaculation, Sg1 binds to the Eppin protein complex in the surface of human spermatozoa (Wang et al., Reference Wang, Widgren, Sivashanmugam, O’Rand and Richardson2005), a heteromultimer containing the epididymal protease inhibitor (Eppin), LTF and clusterin (Wang et al., Reference Wang, Widgren, Richardson and O’Rand2007), to inhibit the progressive motility of ejaculated spermatozoa (Mitra et al., Reference Mitra, Richardson and O’Rand2010) by promoting the loss of intracellular calcium (O’Rand and Widgren, Reference O’Rand and Widgren2012). However, during Sg1 digestion after ejaculation, Eppin modulates PSA hydrolytic activity on Sg1 resulting in spermatozoa with forward motility (O’Rand et al., Reference O’Rand, Widgren, Wang and Richardson2006).

Another protein from seminal vesicles that interacts with sperm plasma membrane is the mouse seminal vesicle autoantigen (SVA), which binds to sperm membrane phospholipids and suppresses sperm motility, BSA-stimulated sperm hyperactivation (Huang et al., Reference Huang, Chu and Chen1999) and capacitation-related protein tyrosine phosphorylation (Huang et al., Reference Huang, Chu and Chen2000). SVA has two orthologues genes in rat (Yoshida et al., Reference Yoshida, Kaneko, Kurachi and Osawa2001) and the human homologue is prolactin induced protein (PIP), a protein highly expressed on the surface of spermatozoa from donors with asthenozoospermia and oligoasthenoteratozoospermia (Capkova et al., Reference Capkova, Elzeinova and Novak2007). Interestingly, PIP interacts with serum albumin, zinc-α-2 glycoprotein and Sg1 (Tomar et al., Reference Tomar, Sooch, Raj, Singh and Yadav2013), suggesting that it may participate in human sperm capacitation regulation. In addition, it seems that the protein family known as seminal vesicle secretory proteins (SVS) may also regulate sperm capacitation. Murine SVS II, the major component of the copulatory plug (Lundwall, Reference Lundwall1996), binds to ganglioside GM1 on the post-acrosomal region of the head sperm (Kawano et al., Reference Kawano, Yoshida, Iwamoto and Yoshida2008) and decreases sperm protein tyrosine phosphorylation and the progesterone-induced acrosome reaction (Kawano and Yoshida, Reference Kawano and Yoshida2007).

Glycodelin (Gd) is a glycoprotein that has four well defined isoforms named S, A, F and C, according to their origin (semen, amniotic fluid, follicular fluid and cumulus oophorus, respectively) (Seppala et al., Reference Seppala, Koistinen, Koistinen, Chiu and Yeung2007). GdS is one of the most abundant glycoproteins in the seminal plasma (Julkunen et al., Reference Julkunen, Wahlstrom, Seppala, Koistinen, Koskimies, Stenman and Bohn1984) and is able to reduce the cyclodextrin-induced cholesterol efflux and down-regulate the adenylyl cyclase/protein kinase A/tyrosine kinase signalling pathway, resulting in suppression of human sperm capacitation (Chiu et al., Reference Chiu, Chung, Tsang, Koistinen, Koistinen, Seppala, Lee and Yeung2005). A different group of seminal vesicle proteins that binds to sperm are spermadhesins, a protein family comprised by five polypeptides called AQN-1, AQN-3, AWN (isoforms 1 and 2), PSP-I, and PSP-II. These proteins have been identified in pig, in which a large subpopulation of spermadhesins is loosely associated with the sperm surface and functions as decapacitation factors (Dostalova et al., Reference Dostalova, Calvete, Sanz and Topfer-Petersen1994). Nonetheless, it seems that each spermadhesin plays specific roles in sperm function regulation, given that a PSP-I/PSP-II heterodimer exerts a decapacitation effect by decreasing sperm intracellular calcium (Caballero et al., Reference Caballero, Vazquez, Mayor, Alminana, Calvete, Sanz, Roca and Martinez2009) while AQN-3 has been described as a ZP-binding protein (Calvete et al., Reference Calvete, Carrera, Sanz and Topfer-Petersen1996).

Protease inhibitors are widely distributed in nearly all species and are crucial in balancing protease activities. In mouse, the serine protease inhibitor Kazal-type 3 (SPINK3), also called P12, has been found in the seminal vesicle, coagulating gland and prostate of adults. The P12-binding sites are on the anterior region of the acrosome and the protein inhibits sperm binding to the ZP and the ZP-induced acrosome reaction (Boettger-Tong et al., Reference Boettger-Tong, Aarons, Biegler, Lee and Poirier1992). Furthermore, the protein suppresses calcium uptake by spermatozoa during capacitation (Chen et al., Reference Chen, Lin, Lai and Chen1998). Additionally, a related serine protease inhibitor known as Kazal-type-like (SPINKL) is secreted by mouse seminal vesicle and has been found to bind to sperm, enhance sperm progressive motility, suppress BSA-stimulated sperm capacitation and block sperm–oocyte interactions in vitro (Lin et al., Reference Lin, Lee, Hwu, Lu, Chu, Chen, Chang and Li2008).

Binder of sperm proteins (BSP, previously known as bovine seminal plasma proteins) are part of a family of structurally related proteins characterized by the presence of tandem fibronectin type II domains. They are highly expressed by seminal vesicles, but BSP-related genes have been found to be also expressed in the epididymis (Fan et al., Reference Fan, Lefebvre and Manjunath2006; Plante and Manjunath, Reference Plante and Manjunath2015a). BSPs are involved in sperm binding to oviductal epithelium assisting in the formation of the sperm storage reservoir (Gwathmey et al., Reference Gwathmey, Ignotz and Suarez2003). Bovine seminal vesicles secrete three BSP (BSP1, BSP3 and BSP5) that are adsorbed onto sperm (Manjunath et al., Reference Manjunath, Chandonnet, Leblond and Desnoyers1994), but after in vitro capacitation BSP5 is almost completely released from sperm surface while BSP1 and a small molecular mass isoform of BSP3 remain associated to sperm, suggesting that BSP3 undergoes proteolytic modifications on the sperm surface that may regulate sperm release from the storage reservoir (Hung and Suarez, Reference Hung and Suarez2012). Moreover, when BSPs interact with the sperm surface remove cholesterol from the plasma membrane (Therien et al., Reference Therien, Moreau and Manjunath1999) and bind to choline phospholipids (Desnoyers and Manjunath, Reference Desnoyers and Manjunath1992), preventing free movement of phospholipids and therefore stabilizing sperm membrane. However, they have also been found to promote sperm capacitation in bulls, boars, humans and mice and such effects on capacitation are mediated by stimulation of sperm membrane cholesterol and phospholipid efflux in the presence of heparin or HDL, both components of follicular and oviductal fluids (for review see Manjunath and Thérien, Reference Manjunath and Thérien2002). Recently, it has been shown that antibodies against murine BSPH1 could block capacitation induced by HDLs, suggesting a specific interaction between HDL and BSPH1 (Plante and Manjunath, Reference Plante and Manjunath2015b). Nevertheless, as the capacitation promoting effect is observed under conditions resembling the milieu of the female reproductive tract while in seminal fluid BSPs stabilize sperm membrane and prevent premature capacitation, it has been proposed that they are multifunctional proteins that employ different mechanisms to exert such effects (Plante et al., Reference Plante, Prud’homme, Fan, Lafleur and Manjunath2016).

Prostate proteins

The major protein secreted by the prostate is the serine protease known as PSA (or human kallikrein) that plays a well known role in seminal coagulum liquefaction that allows the release of motile spermatozoa (Malm et al., Reference Malm, Hellman, Hogg and Lilja2000), but there is no evidence that it may directly interact with the sperm membrane. Instead, the boar prostate-derived seminal plasma protein identified as WGA16 (wheat germ agglutinin) has been proven to bind to the sperm surface during ejaculation and is later removed during capacitation. It has two functional sites that allow sperm surface association through interaction between its N-glycans and the surface galactosyltransferase, whereas the heparin-binding domain may be involved in binding to sulfated glycosaminoglycans from the female tract, enabling removal of WGA16 from the sperm surface during capacitation (Garenaux et al., Reference Garenaux, Kanagawa, Tsuchiyama, Hori, Kanazawa, Goshima, Chiba, Yasue, Ikeda, Yamaguchi, Sato and Kitajima2015), but its involvement in sperm function regulation has not been examined.

Prostasomes are small vesicles secreted by prostatic epithelial cells that fuse with sperm plasma membranes and are the main source of cholesterol in seminal fluid. Previous studies have shown that incubation of sperm with prostasomes decreases the tyrosine phosphorylation intensity and induces motility changes (Bechoua et al., Reference Bechoua, Rieu, Sion and Grizard2011; Pons-Rejraji et al., Reference Pons-Rejraji, Artonne, Sion, Brugnon, Canis, Janny and Grizard2011). The prostasome molecular entities responsible for those effects are unknown, but among other proteins human prostasomes enclose LTF (Utleg et al., Reference Utleg, Yi, Xie, Shannon, White, Goodlett, Hood and Lin2003), supporting the idea that proteins transferred by prostasomes to sperm may regulate its capability to fertilize.

Uterine proteins

Although GdA is abundant in the amniotic fluid, it is also present in the endometrial secretions of non-pregnant females. It has immunosuppressive properties that protect the spermatozoa from immune attack in the female reproductive tract (Bolton et al., Reference Bolton, Pockley, Clough, Mowles, Stoker, Westwood and Chapman1987), but is also able to inhibit sperm binding to the ZP (Oehninger et al., Reference Oehninger, Coddington, Hodgen and Seppala1995). However, as it is a progesterone-dependent protein, it is mainly secreted during the luteal phase of the menstrual cycle and therefore spermatozoa are not exposed to significant concentrations of GdA during the fertile window (Durand et al., Reference Durand, Koistinen, Chirinos, Rodriguez, Zambrano, Seppala and Larrea2010). Another progesterone-dependent protein expressed and secreted in the human uterus and cervix is the cysteine protease inhibitor cystatin C (CST3), which binds to sperm enhancing motility but inhibits cholesterol efflux and sperm protein tyrosine phosphorylation, indicating CST3’s ability to inhibit sperm capacitation (Lee et al., Reference Lee, Tseng, Hwu, Fan, Lin, Yu, Yeh and Li2018). Interestingly, this protein is also prominently expressed in the male reproductive tract and has been associated with human prostasomes (Carlsson et al., Reference Carlsson, Ronquist, Eliasson, Egberg and Larsson2011), but its potential role as a decapacitation factor has not been investigated. Other proteins of uterine origin that have been shown to inhibit sperm–ZP interaction are LTF (Teng et al., Reference Teng, Gladwell, Beard, Walmer, Teng and Brenner2002; Zumoffen et al., Reference Zumoffen, Gil, Caille, Morente, Munuce and Ghersevich2013) and glucose-regulated protein 78 (Grp78) (Marin-Briggiler et al., Reference Marin-Briggiler, Gonzalez-Echeverria, Munuce, Ghersevich, Caille, Hellman, Corrigall and Vazquez-Levin2010), and it has been proposed that such inhibition may contribute to regulate the number of sperm with the ability to interact and fertilize the oocyte (Zumoffen et al., Reference Zumoffen, Gil, Caille, Morente, Munuce and Ghersevich2013). Conversely, heat shock protein 60 (Hsp60), a chaperone protein secreted by uterus and oviduct epithelial cells from human and bovine (Boilard et al., Reference Boilard, Reyes-Moreno, Lachance, Massicotte, Bailey, Sirard and Leclerc2004; Lachance et al., Reference Lachance, Bailey and Leclerc2007), binds to spermatozoa and increases the intracellular calcium concentration with no apparent effects on sperm motility or acrosome reaction (Lachance et al., Reference Lachance, Bailey and Leclerc2007).

Endometrial cells also secrete interleukin-6 (IL-6), a mediator of the inflammatory response that increases during the periovulatory phase (Tabibzadeh and Sun, Reference Tabibzadeh and Sun1992). IL-6 induces sperm capacitation by increasing protein tyrosine phosphorylation and enhances spontaneous as well as calcium ionophore-induced acrosome reaction (Naz and Kaplan, Reference Naz and Kaplan1994; Laflamme et al., Reference Laflamme, Akoum and Leclerc2005). Moreover, uterine secretions contain the sialic acid binding protein (SABP), a protein that binds to spermatozoa head plasma membrane (Banerjee and Chowdhury, Reference Banerjee and Chowdhury1994) and stimulates in vitro sperm capacitation, increases the percentage of motile cells and the subsequent acrosome reaction (Banerjee and Chowdhury, Reference Banerjee and Chowdhury1995). Furthermore, SABP induces the exposure of mannose ligand receptors on the sperm surface and increases the production of the superoxide anion (Banerjee and Chowdhury, Reference Banerjee and Chowdhury1997). Furthermore, recent investigations indicate a widespread presence of fibroblast growth factor 2 (FGF2) in the mouse uterus and oviduct, and that this protein is able to increase sperm motility, intracellular calcium levels and acrosomal loss in vitro (Saucedo et al., Reference Saucedo, Sobarzo, Brukman, Guidobaldi, Lustig, Giojalas, Buffone, Vazquez-Levin and Marin-Briggiler2018).

In addition to uterine secreted proteins, uterosomes may also contribute to sperm function regulation, as they carry proteins that may promote sperm capacitation (Martin-DeLeon, Reference Martin-DeLeon2016). Moreover, it has been shown that uterosome-like vesicles secreted by endometrial epithelial cells are able to fuse with human spermatozoa, prompting their fertilizing capacity by increasing protein tyrosine phosphorylation and the acrosome reaction (Franchi et al., Reference Franchi, Cubilla, Guidobaldi, Bravo and Giojalas2016). In mouse, uterosomes contain sperm adhesion molecule 1 (SPAM1 or PH-20) (Griffiths et al., Reference Griffiths, Galileo, Reese and Martin-Deleon2008a), a protein that binds to spermatozoa on the acrosome and the midpiece of the flagella (Griffiths et al., Reference Griffiths, Miller, Galileo and Martin-DeLeon2008b). SPAM1 increases sperm intracellular calcium in macaque (Cherr et al., Reference Cherr, Yudin, Li, Vines and Overstreet1999) and enables sperm penetration of the cumulus oophorus surrounding the oocyte in mouse (Lin et al., Reference Lin, Mahan, Lathrop, Myles and Primakoff1994). Moreover, SPAM1 is required for hyaluronic acid enhancement of progesterone-induced acrosome reaction in human sperm (Sabeur et al., Reference Sabeur, Cherr, Yudin and Overstreet1998).

Oviduct proteins

When spermatozoa reach the oviduct, they either migrate to the ampulla region to meet the oocyte or are retained in the isthmus region by oviduct epithelial cells to give rise to the sperm reservoir (Jeulin et al., Reference Jeulin, Soumah and Jouannet1985). Sperm interactions with the oviductal epithelial cells increase the intracellular superoxide dismutase and glutathione peroxidase activities as protective mechanisms against reactive oxygen species (Huang et al., Reference Huang, Zhao, Lee, Lee, Lam, Ko, Yeung, Ho and Chiu2013). It has been shown that spermatozoa bound to oviductal epithelial cells show normal morphology and lower DNA fragmentation (Ellington et al., Reference Ellington, Evenson, Wright, Jones, Schneider, Hiss and Brisbois1999), but the role of the oviductal epithelium on sperm capacitation has been a controversial issue. It has been shown that incubation of sperm with oviductal cells enhances sperm viability and hyperactivated motility (Kervancioglu et al., Reference Kervancioglu, Djahanbakhch and Aitken1994), but other investigations have indicated that, although there is enhancement in sperm motility, there is a delay in capacitation (Murray and Smith, Reference Murray and Smith1997) by reducing protein tyrosine phosphorylation and avoiding follicular fluid-induced acrosome reaction (Morales et al., Reference Morales, Palma, Salgado and Villalon1996; Zumoffen et al., Reference Zumoffen, Caille, Munuce, Cabada and Ghersevich2010). These observations are in agreement with oviduct cells’ role as a sperm reservoir, where oviduct cells must stimulate some early capacitation-associated features but delay capacitation late events that are expected to happen in the oocyte vicinity.

As the time of ovulation approaches, the sperm in the reservoir must reassume capacitation. In mouse, hyperactivation assists the sperm to pull off the epithelium and escape out of mucosal pockets (Demott and Suarez, Reference Demott and Suarez1992). At this stage of the journey, oviductosomes transfer proteins to the head and midpiece of spermatozoa (Al-Dossary et al., Reference Al-Dossary, Bathala, Caplan and Martin-DeLeon2015), delivering calcium ATPase 4 (PMCA4), a membrane protein whose deletion leads to severe loss of hyperactivated motility and male infertility (Al-Dossary et al., Reference Al-Dossary, Strehler and Martin-Deleon2013).

Oviduct also secretes oviductin, a high-molecular-weight protein that binds to the head and middle piece of the sperm (King and Killian, Reference King and Killian1994; Lyng and Shur, Reference Lyng and Shur2009). Bovine oviductin promotes in vitro sperm viability and total motility (Abe et al., Reference Abe, Sendai, Satoh and Hoshi1995). Moreover, this protein stimulates sperm–ZP interaction in bovine (Martus et al., Reference Martus, Verhage, Mavrogianis and Thibodeaux1998) and mice (Ensslin et al., Reference Ensslin, Lyng, Raymond, Copland and Shur2007), but has an inhibiting effect in pig (Kouba et al., Reference Kouba, Abeydeera, Alvarez, Day and Buhi2000) and hamster (Kimura et al., Reference Kimura, Matsuda, Ogura, Asano and Naiki1994), indicating that its function may not be conserved among mammalian species. In human, the expression of oviductin at the time of ovulation has been demonstrated (Briton-Jones et al., Reference Briton-Jones, Lok, Yuen, Chiu, Cheung and Haines2001; Lok et al., Reference Lok, Briton-Jones, Yuen and Haines2002). Recently, a secretory form of human oviductin expressed in HEK293 cells has been shown to enhance tyrosine phosphorylation and the incidence of acrosome-reacted sperm induced by calcium ionophore (Zhao et al., Reference Zhao, Yang, Jia, Reid, Leclerc and Kan2016). Finally, sperm binding glycoprotein (SBG) that is synthesized at the apical surface of pig isthmic and ampullar epithelial cells (Perez et al., Reference Perez, Roma, Cabada and Marini2006), binds to carbohydrates on the sperm plasma membrane (Marini and Cabada, Reference Marini and Cabada2003) to increase tyrosine phosphorylation of a polypeptide with an apparent molecular mass of 97 kDa, although suppressing motility (Teijeiro et al., Reference Teijeiro, Cabada and Marini2008).

Follicular fluid proteins

In the ampulla of the oviduct, sperm should meet the cumulus–oocyte complex containing the oocyte surrounded by the hyaluronic acid matrix, cumulus cells and follicular fluid. The hyaluronic acid matrix works as a mechanical filter for the elimination of sperm peripheral proteins that are no longer necessary for ZP penetration and fusion with the oocyte. In addition, follicular fluid stimulates the acrosome reaction rate (Calvo et al., Reference Calvo, Vantman, Banks, Tezon, Koukoulis, Dennison and Sherins1989), as a consequence of its high concentrations of progesterone (De Jonge, Reference De Jonge2005) that stimulates a biphasic calcium influx (Kirkman-Brown et al., Reference Kirkman-Brown, Bray, Stewart, Barratt and Publicover2000) and acrosome reaction in capacitated sperm (Blackmore, Reference Blackmore1993).

There are also secreted proteins in the follicular fluid content that interact with the sperm. Atrial natriuretic peptide (ANP) has been identified in human follicular fluid (Sundsfjord et al., Reference Sundsfjord, Forsdahl and Thibault1989) and pig oviductal fluid in which it increases the acrosome reaction, the oocyte penetration rate and decreases polyspermy (Zhang et al., Reference Zhang, Hong, Zhou, Jin, Wang, Fu, Wang and Xia2006). Another follicular fluid protein that interacts with the sperm is Gd, existing as two isoforms: GdF that prevents premature acrosomal reaction and interferes with sperm–ZP interaction (Chiu et al., Reference Chiu, Koistinen, Koistinen, Seppala, Lee and Yeung2003), and GdC that promotes sperm–ZP interaction (Chiu et al., Reference Chiu, Chung, Koistinen, Koistinen, Seppala, Ho, Ng, Lee and Yeung2007). It has been proposed that cumulus cells may use GdF as a substrate in the production of GdC, which is then released during cumulus expansion and displaces sperm previously bound Gd isoforms (GdS and/or GdA) to promote sperm–ZP interaction (Yeung et al., Reference Yeung, Lee, Koistinen, Koistinen, Seppala and Chiu2009).

Glycoprotein fibronectin (Fn) is secreted during cumulus expansion (Sutovsky et al., Reference Sutovsky, Flechon and Pavlok1995) and in the fallopian tube (Makrigiannakis et al., Reference Makrigiannakis, Karamouti, Petsas, Makris, Nikas and Antsaklis2009). Its receptor, the integrin α₅β₁, has been detected on both male and female human (Fusi et al., Reference Fusi, Vignali, Gailit and Bronson1993, Reference Fusi, Tamburini, Mangili, Montesano, Ferrari and Bronson1996) and bovine gametes (Thys et al., Reference Thys, Nauwynck, Maes, Hoogewijs, Vercauteren, Rijsselaere, Favoreel and Van Soom2009). By contrast, it has been demonstrated that Fn strongly inhibits sperm penetration during bovine IVF by diminishing sperm binding to ZP and oolemma as well as sperm–oocyte fusion (Tanghe et al., Reference Tanghe, Van Soom, Duchateau, Nauwynck and de Kruif2004; Thys et al., Reference Thys, Nauwynck, Maes, Hoogewijs, Vercauteren, Rijsselaere, Favoreel and Van Soom2009), although its mechanism of action under physiological conditions remains unknown. Recent investigations have indicated that Fn is present in the oviductal fluid as well as in the extracellular matrix of oviductal epithelial cells and that it is a key regulator of sperm binding to the epithelium when forming the sperm reservoir (Osycka-Salut et al., Reference Osycka-Salut, Castellano, Fornes, Beltrame, Alonso, Jawerbaum, Franchi, Diaz and Perez Martinez2017).

Interestingly, despite CRISP1 being considered as a male reproductive tract protein, recent studies have shown that is also expressed by mouse cumulus cells and that fertilization of CRISP1 knockout females is impaired because of sperm failure to penetrate the cumulus. Apparently, CRISP1 produced in the cumulus cells may regulate sperm orientation by modulating sperm hyperactivation through a mechanism that involves inhibition of CatSper and TRPM8 calcium channels, indicating a novel role for this protein in mammalian fertilization (Ernesto et al., Reference Ernesto, Weigel Munoz, Battistone, Vasen, Martinez-Lopez, Orta, Figueiras-Fierro, De la Vega-Beltran, Moreno, Guidobaldi, Giojalas, Darszon, Cohen and Cuasnicu2015).