Identification of a novel GPI-anchored CRISP glycoprotein, MAK248, located on the posterior head and equatorial segment of cynomolgus macaque sperm

The proteomic landscape of sperm surface deciphers its maturational and functional aspects in buffalo

Buffalo is a dominant dairy animal in many agriculture-based economies. However, the poor reproductive efficiency (low conception rate) of the buffalo bulls constrains the realization of its full production potential. This in turn leads to economic and welfare issues, especially for the marginal farmers in such economies. The mammalian sperm surface proteins have been implicated in the regulation of survival and function of the spermatozoa in the female reproductive tract (FRT). Nonetheless, the lack of specific studies on buffalo sperm surface makes it difficult for researchers to explore and investigate the role of these proteins in the regulation of mechanisms associated with sperm protection, survival, and function. This study aimed to generate a buffalo sperm surface-specific proteomic fingerprint (LC-MS/MS) and to predict the functional roles of the identified proteins. The three treatments used to remove sperm surface protein viz. Elevated salt, phosphoinositide phospholipase C (PI-PLC) and in vitro capacitation led to the identification of N = 1,695 proteins (≥1 high-quality peptide-spectrum matches (PSMs), p < 0.05, and FDR<0.01). Almost half of these proteins (N = 873) were found to be involved in crucial processes relevant in the context of male fertility, e.g., spermatogenesis, sperm maturation and protection in the FRT, and gamete interaction or fertilization, amongst others. The extensive sperm-surface proteomic repertoire discovered in this study is unparalleled vis-à-vis the depth of identification of reproduction-specific cell-surface proteins and can provide a potential framework for further studies on the functional aspects of buffalo spermatozoa.

Extracellular vesicles in mammalian reproduction: a review

Over the last decades, extracellular vesicles (EVs) have been found to be implicated in a complex universal mechanism of communication between different cell types. EVs are nanostructures of lipid nature that have an exosomal or ectosomal biogenesis, responsible for the intercellular transport of proteins, lipids, carbohydrates, nucleic acids, ions, among other molecules. The content of EVs can vary due to various factors such as hormonal stimuli, non-physiological conditions, metabolic state, etc. Once EVs reach their target cell, they can modulate processes such as gene expression, metabolism, response to external factors, and can even be associated with the delivery of molecules involved in epigenetic inheritance processes in germ cells. In mammalian reproduction, EVs have been shown to play an important role, either in vivo or in vitro, modulating a variety of processes in sperm, oocytes and embryos, and in their respective environments. Moreover, EVs represent a biodegradable, harmless and specific vehicle, which makes them attractive allies to consider when improving assisted reproductive technologies (ARTs). Therefore, the present review aims to describe the content of the main EVs involved in mammalian reproduction and how they can vary due to different factors, as well as to detail how EVs modulate, directly or indirectly, different molecular processes in gametes and embryos. In addition, we will highlight the mechanisms that remain to be elucidated. We will also propose new perspectives according to the characteristics of each particular EV to improve the different ARTs.

The functions of CAP superfamily proteins in mammalian fertility and disease

BACKGROUND

Members of the cysteine-rich secretory proteins (CRISPS), antigen 5 (Ag5) and pathogenesis-related 1 (Pr-1) (CAP) superfamily of proteins are found across the bacterial, fungal, plant and animal kingdoms. Although many CAP superfamily proteins remain poorly characterized, over the past decade evidence has accumulated, which provides insights into the functional roles of these proteins in various processes, including fertilization, immune defence and subversion, pathogen virulence, venom toxicology and cancer biology.

OBJECTIVE AND RATIONALE

The aim of this article is to summarize the current state of knowledge on CAP superfamily proteins in mammalian fertility, organismal homeostasis and disease pathogenesis.

SEARCH METHODS

The scientific literature search was undertaken via PubMed database on all articles published prior to November 2019. Search terms were based on following keywords: 'CAP superfamily', 'CRISP', 'Cysteine-rich secretory proteins', 'Antigen 5', 'Pathogenesis-related 1', 'male fertility', 'CAP and CTL domain containing', 'CRISPLD1', 'CRISPLD2', 'bacterial SCP', 'ion channel regulator', 'CatSper', 'PI15', 'PI16', 'CLEC', 'PRY proteins', 'ASP proteins', 'spermatogenesis', 'epididymal maturation', 'capacitation' and 'snake CRISP'. In addition to that, reference lists of primary and review article were reviewed for additional relevant publications.

OUTCOMES

In this review, we discuss the breadth of knowledge on CAP superfamily proteins with regards to their protein structure, biological functions and emerging significance in reproduction, health and disease. We discuss the evolution of CAP superfamily proteins from their otherwise unembellished prokaryotic predecessors into the multi-domain and neofunctionalized members found in eukaryotic organisms today. At least in part because of the rapid evolution of these proteins, many inconsistencies in nomenclature exist within the literature. As such, and in part through the use of a maximum likelihood phylogenetic analysis of the vertebrate CRISP subfamily, we have attempted to clarify this confusion, thus allowing for a comparison of orthologous protein function between species. This framework also allows the prediction of functional relevance between species based on sequence and structural conservation.

WIDER IMPLICATIONS

This review generates a picture of critical roles for CAP proteins in ion channel regulation, sterol and lipid binding and protease inhibition, and as ligands involved in the induction of multiple cellular processes.

GLIPR1L1 is an IZUMO-binding protein required for optimal fertilization in the mouse

Background

The sperm protein IZUMO1 (Izumo sperm-egg fusion 1) and its recently identified binding partner on the oolemma, IZUMO1R, are among the first ligand-receptor pairs shown to be essential for gamete recognition and adhesion. However, the IZUMO1-IZUMO1R interaction does not appear to be directly responsible for promoting the fusion of the gamete membranes, suggesting that this critical phase of the fertilization cascade requires the concerted action of alternative fusogenic machinery. It has therefore been proposed that IZUMO1 may play a secondary role in the organization and/or stabilization of higher-order heteromeric complexes in spermatozoa that are required for membrane fusion.

Results

Here, we show that fertilization-competent (acrosome reacted) mouse spermatozoa harbor several high molecular weight protein complexes, a subset of which are readily able to adhere to solubilized oolemmal proteins. At least two of these complexes contain IZUMO1 in partnership with GLI pathogenesis-related 1 like 1 (GLIPR1L1). This interaction is associated with lipid rafts and is dynamically remodeled upon the induction of acrosomal exocytosis in preparation for sperm adhesion to the oolemma. Accordingly, the selective ablation of GLIPR1L1 leads to compromised sperm function characterized by a reduced ability to undergo the acrosome reaction and a failure of IZUMO1 redistribution.

Conclusions

Collectively, this study characterizes multimeric protein complexes on the sperm surface and identifies GLIPRL1L1 as a physiologically relevant regulator of IZUMO1 function and the fertilization process.

Glioma pathogenesis-related 1-like 1 is testis enriched, dynamically modified, and redistributed during male germ cell maturation and has a potential role in sperm-oocyte binding

The glioma pathogenesis-related 1 (GLIPR1) family consists of three genes [GLIPR1, GLIPR1-like 1 (GLIPR1L1), and GLIPR1-like 2 (GLIPR1L2)] and forms a distinct subgroup within the cysteine-rich secretory protein (CRISP), antigen 5, and pathogenesis-related 1 (CAP) superfamily. CAP superfamily proteins are found in phyla ranging from plants to humans and, based largely on expression and limited functional studies, are hypothesized to have roles in carcinogenesis, immunity, cell adhesion, and male fertility. Specifically data from a number of systems suggests that sequences within the C-terminal CAP domain of CAP proteins have the ability to promote cell-cell adhesion. Herein we cloned mouse Glipr1l1 and have shown it has a testis-enriched expression profile. GLIPR1L1 is posttranslationally modified by N-linked glycosylation during spermatogenesis and ultimately becomes localized to the connecting piece of elongated spermatids and sperm. After sperm capacitation, however, GLIPR1L1 is also localized to the anterior regions of the sperm head. Zona pellucida binding assays indicate that GLIPR1L1 has a role in the binding of sperm to the zona pellucida surrounding the oocyte. These data suggest that, along with other members of the CAP superfamily and several other proteins, GLIPR1L1 is involved in the binding of sperm to the oocyte complex. Collectively these data further strengthen the role of CAP domain-containing proteins in cellular adhesion and propose a mechanism whereby CAP proteins show overlapping functional significance during fertilization.

Purification and multimer formation of allurin, a sperm chemoattractant from Xenopus laevis egg jelly

Allurin, a sperm chemoattractant isolated from Xenopus laevis egg jelly, can be purified in one step from an extract of diffusible jelly proteins ("egg water") using a FPLC or HPLC anion exchange column and a multi-step NaCl gradient. Allurin homomultimers were detected by Western blotting with antibodies prepared against the purified protein or peptides within the protein. Allurin multimers were stable and resisted dissociation by SDS and beta-mercaptoethanol. Alkylation of allurin provided evidence for two free sulfhydryl groups but did not eliminate multimer formation, suggesting that intermolecular disulfide bond formation is not required for allurin aggregation. Concentration of egg water was accompanied by a reduction of chemoattractant activity that could not be fully accounted for by homomultimer formation. Rather, the presence of a multiphasic dose-activity curve upon partial purification and formation of hetero-allurin complexes during concentration suggested that egg water may contain allurin-binding proteins that reduce multimer formation and activity.

The CAP superfamily: cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins--roles in reproduction, cancer, and immune defense

The cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins (CAP) superfamily members are found in a remarkable range of organisms spanning each of the animal kingdoms. Within humans and mice, there are 31 and 33 individual family members, respectively, and although many are poorly characterized, the majority show a notable expression bias to the reproductive tract and immune tissues or are deregulated in cancers. CAP superfamily proteins are most often secreted and have an extracellular endocrine or paracrine function and are involved in processes including the regulation of extracellular matrix and branching morphogenesis, potentially as either proteases or protease inhibitors; in ion channel regulation in fertility; as tumor suppressor or prooncogenic genes in tissues including the prostate; and in cell-cell adhesion during fertilization. This review describes mammalian CAP superfamily gene expression profiles, phylogenetic relationships, protein structural properties, and biological functions, and it draws into focus their potential role in health and disease. The nine subfamilies of the mammalian CAP superfamily include: the human glioma pathogenesis-related 1 (GLIPR1), Golgi associated pathogenesis related-1 (GAPR1) proteins, peptidase inhibitor 15 (PI15), peptidase inhibitor 16 (PI16), cysteine-rich secretory proteins (CRISPs), CRISP LCCL domain containing 1 (CRISPLD1), CRISP LCCL domain containing 2 (CRISPLD2), mannose receptor like and the R3H domain containing like proteins. We conclude that overall protein structural conservation within the CAP superfamily results in fundamentally similar functions for the CAP domain in all members, yet the diversity outside of this core region dramatically alters target specificity and, therefore, the biological consequences.

Beta-defensin 126 on the surface of macaque sperm mediates attachment of sperm to oviductal epithelia

Beta-defensin 126 (DEFB126) coats the entire surface of macaque sperm until sperm become capacitated, and the removal of DEFB126 from over the head of sperm is required for sperm-zona recognition. Viable sperm collected from cervix and the uterine lumen of mated female macaques had DEFB126 coating the entire surface, suggesting that DEFB126 is retained on sperm en route to the oviduct. DEFB126 plays a major role in attachment of sperm to oviductal epithelial cells (OECs). Following treatment to either remove or alter DEFB126, sperm were coincubated with explants of OECs, which were assessed for sperm binding following rinsing to remove superficially attached sperm. Sperm treated with either 1 mM caffeine + 1 mM dibutyryl cyclic adenosine monophosphate (dbcAMP) (induces capacitation and complete release of DEFB126 from sperm), 2 mM caffeine (removes DEFB126 from over the head and midpiece but does not induce capacitation), anti-DEFB126 immunoglobulin, or neuraminidase (cleaves sialic acid from terminal positions on glycosylation sites of DEFB126) resulted in similar and significant levels of inhibition of sperm-OEC binding. Preincubation of OECs with soluble DEFB126 also resulted in significantly reduced sperm-OEC binding. Furthermore, reduced OEC binding capability of sperm lacking DEFB126 could be restored by addition of soluble DEFB126 to the sperm surface prior to incubation with OECs. Finally, purified DEFB126, infused into oviducts in situ, associated primarily with the apical membranes of secretory-type epithelial cells. In summary, treatments of macaque sperm that result in either removal, masking, or alteration of DEFB126 result in loss of sperm-OEC binding that is independent of changes in sperm motility. DEFB126 may be directly involved in the formation of a reservoir of sperm in the oviduct of macaques.

Epididymal secreted protein Crisp-1 and sperm function

Crisp-1 is a member of the cysteine-rich secretory protein family. This family of proteins is characterized by the presence of 16 conserved cysteine residues, the characteristic from which the family name is derived. Members of the Crisp protein family are found in the secretions of the reproductive tract and salivary glands, including venom toxins from several species of snakes and lizards. The Crisp proteins are modular, each containing an amino terminal pathogenesis-related (PR)-like domain and a carboxyl terminal cysteine-rich domain (CRD) connected by a hinge region. Sequence and structural similarities to proteins with known functions suggest that the Crisp family of proteins may act by regulating cellular ion channels. Rat Crisp-1 is synthesized as two distinct isoforms (referred to as Proteins D and E) by the epididymal epithelium and both are secreted into the luminal fluid where they interact with spermatozoa. Our laboratory has correlated Crisp-1 binding to sperm with inhibiting the signaling cascades that initiate capacitation while others have shown that blocking Crisp-1 binding sites on oocytes interferes with sperm-egg fusion. We hypothesize that the D and E populations of rat Crisp-1 have different interactions with sperm that modulate these distinct biological activities. Through tandem mass spectrometry (MS/MS) and monosaccharide composition analyses, we have identified at least one difference between the D and E forms as an additional single O-linked N-acetyl galactosamine on an amino terminal threonine residue in Protein E. This post-translational modification appears to account for the unique 'E' epitope bound by monoclonal antibody 4E9 developed in our laboratory, and may also lead to differential processing and localization of Protein E on sperm, when compared to Protein D. These findings are the first step in distinguishing the molecular basis of the biological activities of the D and E forms of rat Crisp-1. The epididymal-specific expression of Crisp-1, combined with its role in regulation of sperm capacitation and oocyte interaction, make it an attractive target for post-testicular contraceptive development.

Phospholipase-C sensitive GPI-anchored proteins of goat sperm: possible role in sperm protection

The role of glycosylphosphatidylinositol (GPI)-anchored sperm proteins in reproduction has been investigated. SDS-polyacrylamide gels (PAGE) analysis of goat sperm (Capra indica) indicated that several GPI-anchored proteins were released by phosphatidylinositol-specific phospholipase-C (PI-PLC) treatment. The distribution of this category of PI-PLC-sensitive GPI-anchored proteins on the surface of sperm was examined by indirect immunofluorescence. The fluorescence microscopic study clearly demonstrated that the PI-PLC-sensitive GPI-anchored proteins are confined predominantly to the head region of goat sperm. Further experiments were conducted on intact and PI-PLC treated sperm in order to decipher the function of GPI proteins. Co-incubation of sperm with peritoneal macrophages led to the enhanced phagocytosis of PI-PLC treated sperm by macrophages compared with the untreated intact sperm. Transmission electron micrographs of the macrophages acquired from the phagocytosis assay are provided to corroborate the same. From the results obtained it is inferred that one or more of the PI-PLC-sensitive GPI-anchored proteins on the sperm surface could act as protection factor(s) that shield the sperm from macrophages.

Beta-defensin 126 on the cell surface protects sperm from immunorecognition and binding of anti-sperm antibodies

Beta-defensin 126 (DEFB126), formerly known as epididymal secretory protein 13.2 (ESP13.2), coats the entire primate sperm surface until completion of capacitation, and it is a candidate for providing immune protection in the female reproductive tract. To further examine the potential role of DEFB126 as a means of protection from immune recognition, cynomolgus macaque sperm were exposed to a number of treatments that are known to alter sperm surface coats, including capacitation. We used a novel in vivo assay to determine immune recognition: aldehyde-fixed whole sperm injections into rabbits. Following booster injections, immunoblot analyses of whole sperm prepared in various manners was conducted. On Days 60 and 80 post-initial immunization, the antisera showed a remarkably strong reaction to a single 34-36 kDa protein, which was shown to be DEFB126. Sera from rabbits that were immunized with sperm washed more rigorously using Percoll gradients showed an increase in the number and intensity of proteins recognized on whole sperm Western blots, although DEFB126 was still the major immune response. When capacitated sperm, from which most DEFB126 had been released, were used as the immunogen, there was a dramatic increase in the immune recognition to a variety of protein bands. Sperm treated with neuraminidase to remove sialic acid on DEFB126 before fixation were shown to still possess DEFB126, but lacked the sialic acid component of the glycoprotein. These sperm were as immunogenic as capacitated sperm even though the desialylated DEFB126 still covered the entire cell surface. These sperm lost their highly negative charge (the isoelectric point of DEFB126 shifted from pI 3.0 to pI 6.4). Experiments using different sperm plasma membrane protein-specific Igs showed that recognition did not occur when DEFB126 was present, but following capacitation these Igs readily recognized the exposed sperm membrane. Our data suggest that DEFB126 protects the entire primate sperm surface from immune recognition and that the sialic acid moieties are responsible for the cloaking characteristic of this unique glycoprotein.

Human testicular protein TPX1/CRISP-2: localization in spermatozoa, fate after capacitation and relevance for gamete interaction

Testicular protein Tpx-1, also known as CRISP-2, is a cysteine-rich secretory protein specifically expressed in the male reproductive tract. Since the information available on the human protein is limited to the identification and expression of its gene, in this work we have studied the presence and localization of human Tpx-1 (TPX1) in sperm, its fate after capacitation and acrosome reaction (AR), and its possible involvement in gamete interaction. Indirect immunofluorescence studies revealed the absence of significant staining in live or fixed non-permeabilized sperm, in contrast to a clear labelling in the acrosomal region of permeabilized sperm. These results, together with complementary evidence from protein extraction procedures strongly support that TPX1 would be mainly an intra-acrosomal protein in fresh sperm. After in vitro capacitation and ionophore-induced AR, TPX1 remained associated with the equatorial segment of the acrosome. The lack of differences in the electrophoretic mobility of TPX1 before and after capacitation and AR indicates that the protein would not undergo proteolytical modifications during these processes. The possible involvement of TPX1 in gamete interaction was evaluated by the hamster oocyte penetration test. The presence of anti-TPX1 during gamete co-incubation produced a significant and dose-dependent inhibition in the percentage of penetrated zona-free hamster oocytes without affecting sperm motility, the AR or sperm binding to the oolema. Together, these results indicate that human TPX1 would be a component of the sperm acrosome that remains associated with sperm after capacitation and AR, and is relevant for sperm-oocyte interaction.

Post-testicular sperm environment and fertility

When mammalian spermatozoa exit the testis, they show a highly specialized morphology; however, they are not yet able to carry out their task: to fertilize an oocyte. This property, that includes the acquisition of motility and the ability to recognize and to fuse with the oocyte investments, is gained only after a transit through the epididymis during which the spermatozoa from the testis travel to the vas deferens. The exact molecular mechanisms that turn these cells into fertile gametes still remain mysterious, but surface-modifying events occurring in response to the external media are key steps in this process. Our laboratory has established cartographies of secreted (secretomes) and present proteins (proteomes) in the epididymal fluid of different mammals and have shown the regionalized variations in these fluid proteins along the epididymis. We have found that the main secreted proteins are common in different species and that enzymatic activities, capable of controlling the sperm surface changes, are present in the fluid. Our studies also indicate that the epididymal fluid is more complex than previously thought; it contains both soluble and particulate compartments such as exosome-like vesicles (epididymosomes) and certainly specific glycolipid-protein micelles. Understanding how these different compartments interplay to modify sperm components during their transit will be a necessary step if one wants to control and to ameliorate sperm quality and to obtain valuable fertility markers helpful to establish a male fertility based genetic selection.

ESP13.2, a member of the beta-defensin family, is a macaque sperm surface-coating protein involved in the capacitation process

Female macaques produced isoantibodies to a limited number of sperm surface proteins following immunization with sperm components released by phosphatidylinositol-specific phospholipase C (PI-PLC). Washed, acrosome-intact, fixed sperm injected into rabbits elicited a major immune response to one of the same PI-PLC-released proteins, which was shown to be a sperm surface-coating protein. After purification and digestion of the glycoprotein, four peptides were analyzed for amino acid sequence, and all had 100% homology with an epididymal secretory protein, ESP13.2, reported previously to be a small, cationic-rich peptide and a member of the beta-defensin family. Antibodies to purified ESP13.2 recognized a number of protein bands on Western blots of nonreduced PI-PLC-released sperm components and nonreduced whole-sperm extracts. After chemical disulfide reduction, only a single, broad band from 31 to 35 kDa was recognized by anti-ESP13.2 antibodies. Indirect immunofluorescence showed ESP13.2 over the entire surface of ejaculated macaque sperm. Fluorescence was only slightly reduced after sperm were washed through 80% Percoll. A 24-h incubation in capacitating medium significantly reduced the amount of ESP13.2 over the head and midpiece, whereas exposure of the incubated sperm to dbcAMP and caffeine (capacitation activators) resulted in almost complete loss of ESP13.2 from the sperm surface. After activation, ESP13.2 was the primary component released into the medium as judged electrophoretically. Lignosulfonic acid, a potent inhibitor of macaque fertilization in vitro, completely blocked release of ESP13.2 from the sperm surface, even following treatment with activators. These findings suggest that the beta-defensin, ESP13.2, has a function in the capacitation of macaque spermatozoa and may modulate sperm surface-receptor presentation at the time of fertilization.