Label-Free Quantitation of Phosphopeptide Changes During Rat Sperm Capacitation

Comparative Proteomic Identification of Ram Sperm before and after In Vitro Capacitation

Ram sperm undergo a sequence of physiological and biochemical changes collectively termed as capacitation to perform oocyte fertilization. However, the protein changes induced by capacitation remain in need of further exploration. Thus, the present study investigated the comparative proteomic profiling in ram spermatozoa under non-capacitating (NC) and capacitating (CAP) conditions in vitro using a liquid chromatography-tandem mass spectrometry combined with tandem mass tag labeling strategy. As a results, 2050 proteins were identified and quantified; 348 of them were differentially abundant, with 280 of the proteins upregulated and 68 of the proteins downregulated between the CAP and NC spermatozoa, respectively. Functional enrichment analysis indicated that the differentially abundant proteins Prune Exopolyphosphatase 1, Galactose-1-Phosphate Uridylyltransferase, and ATP Citrate Lyase were strictly related to energy production and conversion, and Phosphoglycolate phosphatase, Glucosamine-6-Phosphate Deaminase 1 and 2 were related to metabolism, RNA processing, and vesicular transport pathways. Furthermore, the networks of protein-protein interaction indicated a strong interaction among these differential proteins in annotated pathways such as ubiquitin and transport metabolism. Our findings indicate that capacitation progress might be regulated through different pathways, providing insights into mechanisms involved in ram sperm capacitation and fertility.

Quantitative phosphoproteomics analyses reveal the regulatory mechanisms related to frozen-thawed sperm capacitation and acrosome reaction in yak (Bos grunniens)

Mammalian spermatozoa are not mature after ejaculation and must undergo additional functional and structural changes within female reproductive tracts to achieve subsequent fertilization, including both capacitation and acrosome reaction (AR), which are dominated by post-translational modifications (PTMs), especially phosphorylation. However, the mechanism of protein phosphorylation during frozen-thawed sperm capacitation and AR has not been well studied. In this study, the phosphoproteomics approach was employed based on tandem mass tag (TMT) labeling combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS) strategy to analyze frozen-thawed sperm in Ashidan yak under three sequential conditions (density gradient centrifugation-based purification, incubation in the capacitation medium and induction of AR processes by the calcium ionophore A23187 treatment). The identification of 1,377 proteins with 5,509 phosphorylation sites revealed changes in phosphorylation levels of sperm-specific proteins involved in regulation of spermatogenesis, sperm motility, energy metabolism, cilium movement, capacitation and AR. Some phosphorylated proteins, such as AKAP3, AKAP4, SPA17, PDMD11, CABYR, PRKAR1A, and PRKAR2A were found to regulate yak sperm capacitation and AR though the cAMP/PKA signaling pathway cascades. Notably, the phosphorylation level of SPA17 at Y156 increased in capacitated sperm, suggesting that it is also a novel functional protein besides AKAPs during sperm capacitation. Furthermore, the results of this study suggested that the phosphorylation of PRKAR1A and PRKAR2A, and the dephosphorylation of CABYR both play key regulatory role in yak sperm AR process. Protein-protein interaction analysis revealed that differentially phosphorylated proteins (AKAP3, AKAP4, FSIP2, PSMD11, CABYR, and TPPP2) related to capacitation and AR process played a key role in protein kinase A binding, sperm motility, reproductive process, cytoskeleton and sperm flagella function. Taken together, these data provide not only a solid foundation for further exploring phosphoproteome of sperm in yak, but an efficient way to identify sperm fertility-related marker phosphorylated proteins.

Recombinant human OVGP1 increases intracellular calcium and further potentiates the effects of progesterone on human sperm

Purpose

To investigate the effects of recombinant human oviduct–specific glycoprotein (rHuOVGP1) alone and in combination with progesterone (P4) on intracellular Ca²⁺ concentration [Ca²⁺]_i and to investigate if rHuOVGP1 in combination with P4 can further enhance tyrosine phosphorylation (pY) of sperm proteins during human sperm capacitation.

Methods

Fluorometric flow cytometry was performed to examine the effects of rHuOVGP1 on [Ca²⁺]_i in human sperm during capacitation. Confocal microscopy was used in conjunction with live cell imaging to analyze the influence of rHuOVGP1 and P4 on [Ca²⁺]_i in the sperm tail and to examine the involvement of CatSper channels in their effect on [Ca²⁺]_i. Western blot analysis was performed to assess the protein levels of p105, a major tyrosine-phosphorylated sperm protein.

Results

rHuOVGP1 increases [Ca²⁺]_i in human sperm at the beginning of capacitation and further increases and sustains the level of [Ca²⁺]_i in the sperm tail following the addition of P4. Inhibition of CatSper channels impedes the effects of rHuOVGP1 on [Ca²⁺]_i in the sperm tail. P4 alone can increase pY of a major human sperm protein, p105, yet yields a further increase when used in combination with rHuOVGP1.

Conclusion

The present study revealed that rHuOVGP1 may work with P4 to upregulate [Ca²⁺]_i at the beginning of capacitation in part through CatSper channels which, in turn, leads to the downstream event of pY of sperm proteins and enhancement of sperm capacitation.

Supplementary information

The online version contains supplementary material available at 10.1007/s10815-022-02591-0.

Sperm Ion Transporters and Channels in Human Asthenozoospermia: Genetic Etiology, Lessons from Animal Models, and Clinical Perspectives

In mammals, sperm fertilization potential relies on efficient progression within the female genital tract to reach and fertilize the oocyte. This fundamental property is supported by the flagellum, an evolutionarily conserved organelle that provides the mechanical force for sperm propulsion and motility. Importantly several functional maturation events that occur during the journey of the sperm cells through the genital tracts are necessary for the activation of flagellar beating and the acquisition of fertilization potential. Ion transporters and channels located at the surface of the sperm cells have been demonstrated to be involved in these processes, in particular, through the activation of downstream signaling pathways and the promotion of novel biochemical and electrophysiological properties in the sperm cells. We performed a systematic literature review to describe the currently known genetic alterations in humans that affect sperm ion transporters and channels and result in asthenozoospermia, a pathophysiological condition defined by reduced or absent sperm motility and observed in nearly 80% of infertile men. We also present the physiological relevance and functional mechanisms of additional ion channels identified in the mouse. Finally, considering the state-of-the art, we discuss future perspectives in terms of therapeutics of asthenozoospermia and male contraception.

Phosphoproteomics and Bioinformatics Analyses Reveal Key Roles of GSK-3 and AKAP4 in Mouse Sperm Capacitation

Protein phosphorylation can induce signal transduction to change sperm motility patterns during sperm capacitation. However, changes in the phosphorylation of sperm proteins in mice are still incompletely understood. Here, capacitation-related phosphorylation in mouse sperms were firstly investigated by label-free quantitative (LFQ) phosphoproteomics coupled with bioinformatics analysis using ingenuity pathway analysis (IPA) methods such as canonical pathway, upstream regulator, and network analysis. Among 1632 phosphopeptides identified at serine, threonine, and tyrosine residues, 1050 novel phosphosites, corresponding to 402 proteins, were reported. Gene heatmaps for IPA canonical pathways showed a novel role for GSK-3 in GP6 signaling pathways associated with capacitation for 60 min. At the same time, the reduction of the abundant isoform-specific GSK-3α expression was shown by western blot (WB) while the LFQ pY of this isoform slightly decreased and then increased. The combined results from WB and LFQ methods explain the less inhibitory phosphorylation of GSK-3α during capacitation and also support the predicted increases in its activity. In addition, pAKAP4 increased at the Y156 site but decreased at the Y811 site in a capacitated state, even though IPA network analysis and WB analysis for overall pAKAP revealed upregulated trends. The potential roles of GSK-3 and AKAP4 in fertility are discussed.

The activation of the chymotrypsin-like activity of the proteasome is regulated by soluble adenyl cyclase/cAMP/protein kinase A pathway and required for human sperm capacitation

One of the first events of mammalian sperm capacitation is the activation of the soluble adenyl cyclase/cAMP/protein kinase A (SACY/cAMP/PKA) pathway. Here, we evaluated whether the increase in PKA activity at the onset of human sperm capacitation is responsible for the activation of the sperm proteasome and whether this activation is required for capacitation progress. Viable human sperm were incubated with inhibitors of the SACY/cAMP/PKA pathway. The chymotrypsin-like activity of the sperm proteasome was evaluated using a fluorogenic substrate. Sperm capacitation status was evaluated using the chlortetracycline assay and tyrosine phosphorylation. To determine whether proteasomal subunits were phosphorylated by PKA, the proteasome was immunoprecipitated and tested on a western blot using an antibody against phosphorylated PKA substrates. Immunofluorescence microscopy analysis and co-immunoprecipitation (IPP) were used to investigate an association between the catalytic subunit alpha of PKA (PKA-Cα) and the proteasome. The chymotrypsin-like activity of the sperm proteasome significantly increased after 5 min of capacitation (P < 0.001) and remained high for the remaining incubation time. Treatment with H89, KT5720 or KH7 significantly decreased the chymotrypsin-like activity of the proteasome (P < 0.001). IPP experiments indicated that PKA inhibition significantly modified phosphorylation of proteasome subunits. In addition, PKA-Cα colocalized with the proteasome in the equatorial segment and in the connecting piece, and co-immunoprecipitated with the proteasome. This is the first demonstration of sperm proteasome activity being directly regulated by SACY/PKA-Cα. This novel discovery extends our current knowledge of sperm physiology and may be used to manage sperm capacitation during assisted reproductive technology procedures.

Importance of SLC26 Transmembrane Anion Exchangers in Sperm Post-testicular Maturation and Fertilization Potential

In mammals, sperm cells produced within the testis are structurally differentiated but remain immotile and are unable to fertilize the oocyte unless they undergo a series of maturation events during their transit in the male and female genital tracts. This post-testicular functional maturation is known to rely on the micro-environment of both male and female genital tracts, and is tightly controlled by the pH of their luminal milieus. In particular, within the epididymis, the establishment of a low bicarbonate (HCO₃^–) concentration contributes to luminal acidification, which is necessary for sperm maturation and subsequent storage in a quiescent state. Following ejaculation, sperm is exposed to the basic pH of the female genital tract and bicarbonate (HCO₃^–), calcium (Ca²⁺), and chloride (Cl^–) influxes induce biochemical and electrophysiological changes to the sperm cells (cytoplasmic alkalinization, increased cAMP concentration, and protein phosphorylation cascades), which are indispensable for the acquisition of fertilization potential, a process called capacitation. Solute carrier 26 (SLC26) members are conserved membranous proteins that mediate the transport of various anions across the plasma membrane of epithelial cells and constitute important regulators of pH and HCO₃^– concentration. Most SLC26 members were shown to physically interact and cooperate with the cystic fibrosis transmembrane conductance regulator channel (CFTR) in various epithelia, mainly by stimulating its Cl^– channel activity. Among SLC26 members, the function of SLC26A3, A6, and A8 were particularly investigated in the male genital tract and the sperm cells. In this review, we will focus on SLC26s contributions to ionic- and pH-dependent processes during sperm post-testicular maturation. We will specify the current knowledge regarding their functions, based on data from the literature generated by means of in vitro and in vivo studies in knock-out mouse models together with genetic studies of infertile patients. We will also discuss the limits of those studies, the current research gaps and identify some key points for potential developments in this field.

Human OVGP1 enhances tyrosine phosphorylation of proteins in the fibrous sheath involving AKAP3 and increases sperm-zona binding

Purpose

To investigate if the recombinant human oviduct-specific glycoprotein (rHuOVGP1)–enhanced tyrosine-phosphorylated (pY) proteins are components of specific structure(s) of the sperm tail and if rHuOVGP1 binds to the oocyte and enhances sperm-egg binding.

Methods

Immunofluorescent staining and confocal microscopy were performed to examine the localization of pY proteins, outer dense fiber (ODF), and A-Kinase Associated Protein 3 (AKAP3) in human sperm during capacitation. Western blot and immunoprecipitation were employed to analyze protein levels of pY proteins and AKAP3. Immunofluorescent staining was performed to examine the binding of rHuOVGP1 to human oocytes. The effect of rHuOVGP1 on enhancing sperm-zona binding was examined using hemizona assay.

Results

pY proteins were detected mainly in the fibrous sheath (FS) surrounding the ODF with a relatively weak immunoreaction in the neck and mid-piece. Western blot analysis revealed co-migration of the pY 105 kDa protein with AKAP3, which was further confirmed by immunoprecipitation correlating immunofluorescent results of co-localization of pY proteins with AKAP3 in the sperm tail. rHuOVGP1 binds specifically to the zona pellucida (ZP) of human oocytes. Prior incubation of sperm and/or ZP with rHuOVGP1 increased sperm-egg binding.

Conclusions

The present study revealed that one of the major rHuOVGP1-enhanced pY proteins could be AKAP3 of the FS and that rHuOVGP1 is capable of binding to human ZP and its presence in the medium results in an increase in sperm-zona binding. Supplement of rHuOVGP1 in in vitro fertilization media could be beneficial for enhancement of the fertilizing ability of human sperm.

Comparative proteomic identification buffalo spermatozoa during in vitro capacitation

To investigate the proteomic profiling in buffalo spermatozoa before and after capacitation, a liquid chromatography-tandem mass spectrometry (LC-MS/MS) combined with Tandem Mass Tag (TMT) labeling strategy was applied. As a result, 1461 proteins were identified, 93 of them were found to be differentially expressed (>1.5-fold), including 52 up-regulated proteins and 41 down-regulated proteins during sperm capacitation. 88 out of 93 proteins were annotated and classified. Gene ontology (GO) analysis revealed that most of the differently expressed proteins (DEPs) were involved in the Biological Process of transport, cytoskeleton organization, sexual reproduction, and spermatogenesis. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated that DEPs were mainly involved in the pathways of metabolic pathways, PPAR signaling pathway, and oxidative phosphorylation. Western blot (WB) assay confirmed the expressional variation of VAMP4 and APOC3 proteins. Our date provided a foundation for studying the changes in protein expression during sperm capacitation, which contributing to identifying marker proteins that may be associated with sperm capacitation.

Revolutionizing male fertility factor research in mice by using the genome editing tool CRISPR/Cas9

Background

Reproductive research is quintessential in understanding not only the cause of infertility, but also for creating family planning tools. The knockout (KO) system approach is conducive to discovering genes that are essential for fertility in mice. However, in vivo research has been limited due to its high cost and length of time needed to establish KO mice.

Methods

The mechanisms behind the CRISPR/Cas9 system and its application in investigating male fertility in mice are described by using original and review articles.

Results

The CRISPR/CAS9 SYSTEM has enabled researchers to rapidly, efficiently, and inexpensively produce genetically modified mice to study male fertility. Several genes have been highlighted that were found to be indispensable for male fertility by using the CRISPR/Cas9 system, as well as more complicated gene manipulation techniques, such as point mutations, tag insertions, and double knockouts, which have become easier with this new technology.

Conclusion

In order to increase efficiency and usage, new methods of CRISPR/Cas9 integration are being developed, such as electroporation and applying the system to embryonic stem cells. The hidden mysteries of male fertility will be unraveled with the help of this new technology.

Phosphorylation of Izumo1 and Its Role in Male Infertility

Izumo1 is a testis-specific gene product, whose function is essential for sperm-egg fusion. Throughout its lifespan, Izumo1 is posttranslationally modified, being both N-linked glycosylated on its extracellular domain and phosphorylated on the intracellular C-terminal tail. Within the caput regions of the rat epididymis, two phosphorylation events have been documented. However, as sperm pass through the epididymis, this cytoplasmic portion of Izumo1 has been shown to contain up to seven phosphorylation sites. Remarkably, in the rat, in correlation with these events, Izumo1 undergoes sub-cellular re-location, moving from the head/tail regions of the spermatozoa, to a predominantly equatorial segment location once they have reached the caudal end of the epididymis.

Regulation of Sperm Capacitation by the 26S Proteasome: An Emerging New Paradigm in Spermatology

The ubiquitin proteasome system (UPS) participates in many biological processes ranging from cell cycle and antigen processing to cellular defense and signaling. Work of the last decade has made it evident that the UPS is involved in many sperm-related processes leading up to and as part of fertilization. The current knowledge of UPS involvement and changes during sperm capacitation are reviewed together with a list of known proteasome-associated sperm proteins and a discussion of the relationships between these proteins and the proteasome. Proteasomal inhibitors such as MG-132 and epoxomicin significantly alter capacitation and prevent acrosome reaction. The 26S proteasome degrades AKAP3, an A-kinase anchoring protein, partially regulating the release of protein-kinase A (PKA), a vital component necessary for the steps leading up to capacitation. Further, changes occur in 20S core subunit localization and abundance throughout capacitation. Proteasome-interacting valosine-containing protein (VCP) undergoes tyrosine phosphorylation; however, its physiological roles in capacitation and fertilization remain unknown. The E1-type ubiquitin-activating enzyme (UBA1) inhibitor PYR-41 also alters acrosomal membrane remodeling during capacitation. Furthermore, after capacitation, the acrosomal proteasomes facilitate the degradation of zona pellucida glycoproteins leading up to fertilization. Methods to modulate the sperm proteasome activity during sperm storage and capacitation may translate to increased reproductive efficiency in livestock animals. Human male infertility diagnostics may benefit from incorporation of research outcomes built upon relationships between UPS and capacitation. Altogether, the studies reviewed here support the involvement of UPS in sperm capacitation and present opportunities for new discoveries.

Identification of bovine sperm acrosomal proteins that interact with a 32-kDa acrosomal matrix protein

Mammalian fertilization is accomplished by the interaction between sperm and egg. Previous studies from this laboratory have identified a stable acrosomal matrix assembly from the bovine sperm acrosome termed the outer acrosomal membrane-matrix complex (OMC). This stable matrix assembly exhibits precise binding activity for acrosin and N-acetylglucosaminidase. A highly purified OMC fraction comprises three major (54, 50, and 45 kDa) and several minor (38-19 kDa) polypeptides. The set of minor polypeptides (38-19 kDa) termed "OMCrpf polypeptides" is selectively solubilized by high-pH extraction (pH 10.5), while the three major polypeptides (55, 50, and 45 kDa) remain insoluble. Proteomic identification of the OMC32 polypeptide (32 kDa polypeptide isolated from high-pH soluble fraction of OMC) yielded two peptides that matched the NCBI database sequence of acrosin-binding protein. Anti-OMC32 recognized an antigenically related family of polypeptides (OMCrpf polypeptides) in the 38-19-kDa range with isoelectric points ranging between 4.0 and 5.1. Other than glycohydrolases, OMC32 may also be complexed to other acrosomal proteins. The present study was undertaken to identify and localize the OMC32 binding polypeptides and to elucidate the potential role of the acrosomal protein complex in sperm function. OMC32 affinity chromatography of a detergent-soluble fraction of bovine cauda sperm acrosome followed by mass spectrometry-based identification of bound proteins identified acrosin, lactadherin, SPACA3, and IZUMO1. Co-immunoprecipitation analysis also demonstrated the interaction of OMC32 with acrosin, lactadherin, SPACA3, and IZUMO1. Our immunofluorescence studies revealed the presence of SPACA3 and lactadherin over the apical segment, whereas IZUMO1 is localized over the equatorial segment of Triton X-100 permeabilized cauda sperm. Immunoblot analysis showed that a significant portion of SPACA3 was released after the lysophosphatidylcholine (LPC)-induced acrosome reaction, whereas the IZUMO1 and lactadherin polypeptides remain associated to the particulate fraction. Almost entire population of bovine sperm IZUMO1 relocates to the equatorial segment during the LPC-induced acrosome reaction. We propose that the interaction of OMC32 matrix polypeptide with detergent-soluble acrosomal proteins regulates the release of hydrolases/other acrosomal protein(s) during the acrosome reaction.

CRISPR/Cas9-Mediated Rapid Generation of Multiple Mouse Lines Identified Ccdc63 as Essential for Spermiogenesis

Spermatozoa are flagellated cells whose role in fertilization is dependent on their ability to move towards an oocyte. The structure of the sperm flagella is highly conserved across species, and much of what is known about this structure is derived from studies utilizing animal models. One group of proteins essential for the movement of the flagella are the dyneins. Using the advanced technology of CRISPR/Cas9 we have targeted three dynein group members; Dnaic1, Wdr63 and Ccdc63 in mice. All three of these genes are expressed strongly in the testis. We generated mice with amino acid substitutions in Dnaic1 to analyze two specific phosphorylation events at S124 and S127, and generated simple knockouts of Wdr63 and Ccdc63. We found that the targeted phosphorylation sites in Dnaic1 were not essential for male fertility. Similarly, Wdr63 was not essential for male fertility; however, Ccdc63 removal resulted in sterile male mice due to shortened flagella. This study demonstrates the versatility of the CRISPR/Cas9 system to generate animal models of a highly complex system by introducing point mutations and simple knockouts in a fast and efficient manner.

Advances in sperm proteomics: best-practise methodology and clinical potential

The recent application of mass spectrometry to the study of the sperm cell has led to an unprecedented capacity for identification of sperm proteins in a variety of species. Knowledge of the proteins that make up the sperm cell represents the first step towards understanding its normal function and the molecular anomalies associated with male infertility. The present review starts with an introduction of the sperm cell biology and is followed by the consideration of the methodological key aspects to be aware of during sample sourcing and preparation, including data interpretation. It then overviews the initiatives developed so far towards the completion of the sperm proteome, with a particular focus in human but with the inclusion of some comments on different model species. Finally, all studies performing differential proteomics in infertile patients are reviewed, pointing to future potential applications.

Quantitative phosphoproteomics analysis reveals a key role of insulin growth factor 1 receptor (IGF1R) tyrosine kinase in human sperm capacitation

One of the most important changes during sperm capacitation is the enhancement of tyrosine phosphorylation. However, the mechanisms of protein tyrosine phosphorylation during sperm capacitation are not well studied. We used label-free quantitative phosphoproteomics to investigate the overall phosphorylation events during sperm capacitation in humans and identified 231 sites with increased phosphorylation levels. Motif analysis using the NetworKIN algorithm revealed that the activity of tyrosine phosphorylation kinases insulin growth factor 1 receptor (IGF1R)/insulin receptor is significantly enriched among the up-regulated phosphorylation substrates during capacitation. Western blotting further confirmed inhibition of IGF1R with inhibitors GSK1904529A and NVP-AEW541, which inhibited the increase in tyrosine phosphorylation levels during sperm capacitation. Additionally, sperm hyperactivated motility was also inhibited by GSK1904529A and NVP-AEW541 but could be up-regulated by insulin growth factor 1, the ligand of IGF1R. Thus, the IGF1R-mediated tyrosine phosphorylation pathway may play important roles in the regulation of sperm capacitation in humans and could be a target for improvement in sperm functions in infertile men.

Systematic analysis of the phosphoproteome and kinase-substrate networks in the mouse testis

Spermatogenesis is a complex process closely associated with the phosphorylation-orchestrated cell cycle. Elucidating the phosphorylation-based regulations should advance our understanding of the underlying molecular mechanisms. Here we present an integrative study of phosphorylation events in the testis. Large-scale phosphoproteome profiling in the adult mouse testis identified 17,829 phosphorylation sites in 3955 phosphoproteins. Although only approximately half of the phosphorylation sites enriched by IMAC were also captured by TiO2, both the phosphoprotein data sets identified by the two methods significantly enriched the functional annotation of spermatogenesis. Thus, the phosphoproteome profiled in this study is a highly useful snapshot of the phosphorylation events in spermatogenesis. To further understand phosphoregulation in the testis, the site-specific kinase-substrate relations were computationally predicted for reconstructing kinase-substrate phosphorylation networks. A core sub-kinase-substrate phosphorylation networks among the spermatogenesis-related proteins was retrieved and analyzed to explore the phosphoregulation during spermatogenesis. Moreover, network-based analyses demonstrated that a number of protein kinases such as MAPKs, CDK2, and CDC2 with statistically more site-specific kinase-substrate relations might have significantly higher activities and play an essential role in spermatogenesis, and the predictions were consistent with previous studies on the regulatory roles of these kinases. In particular, the analyses proposed that the activities of POLO-like kinases (PLKs) might be dramatically higher, while the prediction was experimentally validated by detecting and comparing the phosphorylation levels of pT210, an indicator of PLK1 activation, in testis and other tissues. Further experiments showed that the inhibition of POLO-like kinases decreases cell proliferation by inducing G2/M cell cycle arrest. Taken together, this systematic study provides a global landscape of phosphoregulation in the testis, and should prove to be of value in future studies of spermatogenesis.

Diagnosis and prognosis of male infertility in mammal: the focusing of tyrosine phosphorylation and phosphotyrosine proteins

Male infertility refers to the inability of a man to achieve a pregnancy in a fertile female. In more than one-third of cases, infertility arises due to the male factor. Therefore, developing strategies for the diagnosis and prognosis of male infertility is critical. Simultaneously, a satisfactory model for the cellular mechanisms that regulate normal sperm function must be established. In this regard, tyrosine phosphorylation is one of the most common mechanisms through which several signal transduction pathways are adjusted in spermatozoa. It regulates the various aspects of sperm function, for example, motility, hyperactivation, capacitation, the acrosome reaction, fertilization, and beyond. Several recent large-scale studies have identified the proteins that are phosphorylated in spermatozoa to acquire fertilization competence. However, most of these studies are basal and have not presented an overall mechanism through which tyrosine phosphorylation regulates male infertility. In this review, we focus of this mechanism, discussing most of the tyrosine-phosphorylated proteins in spermatozoa that have been identified to date. We categorized tyrosine-phosphorylated proteins in spermatozoa that regulate male infertility using MedScan Reader (v5.0) and Pathway Studio (v9.0).

Evolutionary conservation of mammalian sperm proteins associates with overall, not tyrosine, phosphorylation in human spermatozoa

We investigated possible associations between sequence evolution of mammalian sperm proteins and their phosphorylation status in humans. As a reference, spermatozoa from three normozoospermic men were analyzed combining two-dimensional gel electrophoresis, immunoblotting, and mass spectrometry. We identified 99 sperm proteins (thereof 42 newly described) and determined the phosphorylation status for most of them. Sequence evolution was studied across six mammalian species using nonsynonymous/synonymous rate ratios (dN/dS) and amino acid distances. Site-specific purifying selection was assessed employing average ratios of evolutionary rates at phosphorylated versus nonphosphorylated amino acids (α). According to our data, mammalian sperm proteins do not show statistically significant sequence conservation difference, no matter if the human ortholog is a phosphoprotein with or without tyrosine (Y) phosphorylation. In contrast, overall phosphorylation of human sperm proteins, i.e., phosphorylation at serine (S), threonine (T), and/or Y residues, associates with above-average conservation of sequences. Complementary investigations suggest that numerous protein-protein interactants constrain sequence evolution of sperm phosphoproteins. Although our findings reject a special relevance of Y phosphorylation for sperm functioning, they still indicate that overall phosphorylation substantially contributes to proper functioning of sperm proteins. Hence, phosphorylated sperm proteins might be considered as prime candidates for diagnosis and treatment of reduced male fertility.

Head and flagella subcompartmental proteomic analysis of human spermatozoa

Subcellular proteomics not only deepens our knowledge of what proteins are present within cells, but also opens our understanding as to where those proteins reside. Given the highly differentiated, cross-linked state of spermatozoa, such studies have proven difficult to perform. In this study we have fractionated spermatozoa into two components, consisting of either the head or flagellar region. Following SDS-PAGE, 1 mm slices were digested and used for LC-MS/MS analysis. In total, 1429 proteins were identified with 721 proteins being exclusively found in the tail and 521 exclusively in the head. Not only is this the largest reported proteomic analysis of human spermatozoa, but also it has provided novel insights into the compartmentalization of proteins, particularly receptors, never previously reported to be present in this cell type.

Sperm phosphoproteomics: historical perspectives and current methodologies

Mammalian sperm are differentiated germ cells that transfer genetic material from the male to the female. Owing to this essential role in the reproductive process, an understanding of the complex mechanisms that underlie sperm function has implications ranging from the development of novel contraceptives to the treatment of male infertility. While the importance of phosphorylation in sperm differentiation, maturation and fertilization has been well established, the ability to directly determine the sites of phosphorylation within sperm proteins and to quantitate the extent of phosphorylation at these sites is a recent development that has relied almost exclusively on advances in the field of proteomics. This review will summarize the work that has been carried out to date on sperm phosphoproteomics and discuss how the resulting qualitative and quantitative information has been used to provide insight into the manner in which protein phosphorylation events modulate sperm function. The authors also present the proteomics process as it is most often utilized for the elucidation of protein expression, with a particular emphasis on the way in which the process has been modified for the analysis of protein phosphorylation in sperm.

Protein-tyrosine kinase signaling in the biological functions associated with sperm

In sexual reproduction, two gamete cells (i.e., egg and sperm) fuse (fertilization) to create a newborn with a genetic identity distinct from those of the parents. In the course of these developmental processes, a variety of signal transduction events occur simultaneously in each of the two gametes, as well as in the fertilized egg/zygote/early embryo. In particular, a growing body of knowledge suggests that the tyrosine kinase Src and/or other protein-tyrosine kinases are important elements that facilitate successful implementation of the aforementioned processes in many animal species. In this paper, we summarize recent findings on the roles of protein-tyrosine phosphorylation in many sperm-related processes (from spermatogenesis to epididymal maturation, capacitation, acrosomal exocytosis, and fertilization).

The effect of oviductal deleted in malignant brain tumor 1 over porcine sperm is mediated by a signal transduction pathway that involves pro-AKAP4 phosphorylation

The interaction between sperm and oviduct results in the selection of sperm with certain qualities. Porcine oviductal deleted in malignant brain tumor 1, DMBT1 (previously called sperm-binding glycoprotein, SBG), has been proposed to be implicated in sperm selection through acrosome alteration and suppression of motility of a subpopulation of sperm that have begun capacitation prematurely. It producesin vitroacrosome alteration and decrease of motility of boar sperm, concomitant with tyrosine phosphorylation of a 97 kDa sperm protein (p97). We hypothesized that the phosphorylation of p97 may be a link between DMBT1 sensing by a subpopulation of boar sperm and its biological effect. In this work, p97 was identified by mass spectrometry and immunoprecipitation as a porcine homologue of AKAP4. Pro-AKAP4 was localized by immunofluorescence and subcellular fractionation to the periacrosomal membranes and was shown to be tyrosine phosphorylated by DMBT1 regardless of the presence of calcium or bicarbonate, and of cAMP analogs, protein kinase A inhibitors, or a protein kinase C inductor. A processed ∼80 kDa form of AKAP4 was also detected at the tail of boar sperm, which was not tyrosine phosphorylated by DMBT1 under the conditions tested. Immunohistochemistry of testis showed presence of AKAP4 in boar sperm precursor cells. The evidence presented here supports the involvement of AKAP4 in the formation of the fibrous sheath on boar precursor sperm cells and implicates the phosphorylation of pro-AKAP4 as an early step in the signal transduction pathway gated by DMBT1 that leads to sperm selection through acrosome alteration.

Ubiquitin-activating enzyme (UBA1) is required for sperm capacitation, acrosomal exocytosis and sperm-egg coat penetration during porcine fertilization

Protein ubiquitination is a stable, covalent post-translational modification that alters protein activity and/or targets proteins for proteolysis by the 26S proteasome. The E1-type ubiquitin-activating enzyme (UBA1) is responsible for ubiquitin activation, the initial step of ubiquitin-protein ligation. Proteasomal proteolysis of ubiquitinated spermatozoa and oocyte proteins occurs during mammalian fertilization, particularly at the site of sperm acrosome contact with oocyte zona pellucida. However, it is not clear whether the substrates are solely proteins ubiquitinated during gametogenesis or if de novo ubiquitination also occurs during fertilization supported by ubiquitin-activating and -conjugating enzymes present in the sperm acrosome. Along this line of inquiry, UBA1 was detected in boar sperm-acrosomal extracts by Western blotting (WB). Immunofluorescence revealed accumulation of UBA1 in the nuclei of spermatogonia, spermatocytes and spermatids, and in the acrosomal caps of round and elongating spermatids. Thiol ester assays utilizing biotinylated ubiquitin and isolated sperm acrosomes confirmed the enzymatic activity of the resident UBA1. A specific UBA1 inhibitor, PYR-41, altered the remodelling of the outer acrosomal membrane (OAM) during sperm capacitation, monitored using flow cytometry of fluorescein isothiocyanate-conjugated peanut agglutinin (FITC-PNA). Although viable and motile, the spermatozoa capacitated in the presence of PYR-41, showed significantly reduced fertilization rates during in vitro fertilization (IVF; p < 0.05). Similarly, the fertilization rate was lowered by the addition of PYR-41 directly into fertilization medium during IVF. In WB, high Mr bands, suggestive of protein ubiquitination, were detected in non-capacitated spermatozoa by antibodies against ubiquitin; WB with anti-phosphotyrosine antibodies and antibodies against acrosomal proteins SPINK2 (acrosin inhibitor) and AQN1 (spermadhesin) revealed that the capacitation-induced modification of those proteins was altered by PYR-41. In summary, it appears that de novo protein ubiquitination involving UBA1 contributes to sperm capacitation and acrosomal function during fertilization.

The chaperonin containing TCP1 complex (CCT/TRiC) is involved in mediating sperm-oocyte interaction

Sperm-oocyte interactions are among the most remarkable processes in cell biology. These cellular recognition events are initiated by an exquisitely specific adhesion of free-swimming spermatozoa to the zona pellucida, an acellular matrix that surrounds the ovulated oocyte. Decades of research focusing on this interaction have led to the establishment of a widely held paradigm that the zona pellucida receptor is a single molecular entity that is constitutively expressed on the sperm cell surface. In contrast, we have employed the techniques of blue native-polyacrylamide gel electrophoresis, far Western blotting, and proximity ligation to secure the first direct evidence in support of a novel hypothesis that zona binding is mediated by multimeric sperm receptor complex(es). Furthermore, we show that one such multimeric association, comprising the chaperonin-containing TCP1 complex (CCT/TRiC) and a zona-binding protein, zona pellucida-binding protein 2, is present on the surface of capacitated spermatozoa and could account for the zona binding activity of these cells. Collectively, these data provide an important biochemical insight into the molecular basis of sperm-zona pellucida interaction and a plausible explanation for how spermatozoa gain their ability to fertilize.

Sperm proteasome and fertilization

The omnipresent ubiquitin–proteasome system (UPS) is an ATP-dependent enzymatic machinery that targets substrate proteins for degradation by the 26S proteasome by tagging them with an isopeptide chain composed of covalently linked molecules of ubiquitin, a small chaperone protein. The current knowledge of UPS involvement in the process of sperm penetration through vitelline coat (VC) during human and animal fertilization is reviewed in this study, with attention also being given to sperm capacitation and acrosome reaction/exocytosis. In ascidians, spermatozoa release ubiquitin-activating and conjugating enzymes, proteasomes, and unconjugated ubiquitin to first ubiquitinate and then degrade the sperm receptor on the VC; in echinoderms and mammals, the VC (zona pellucida/ZP in mammals) is ubiquitinated during oogenesis and the sperm receptor degraded during fertilization. Various proteasomal subunits and associated enzymes have been detected in spermatozoa and localized to sperm acrosome and other sperm structures. By using specific fluorometric substrates, proteasome-specific proteolytic and deubiquitinating activities can be measured in live, intact spermatozoa and in sperm protein extracts. The requirement of proteasomal proteolysis during fertilization has been documented by the application of various proteasome-specific inhibitors and antibodies. A similar effect was achieved by depletion of sperm-surface ATP. Degradation of VC/ZP-associated sperm receptor proteins by sperm-borne proteasomes has been demonstrated in ascidians and sea urchins. On the applied side, polyspermy has been ameliorated by modulating sperm-associated deubiquitinating enzymes. Diagnostic and therapeutic applications could emerge in human reproductive medicine. Altogether, the studies on sperm proteasome indicate that animal fertilization is controlled in part by a unique, gamete associated, extracellular UPS.

Identification of the human testis protein phosphatase 1 interactome

Protein phosphorylation is a critical regulatory mechanism in cellular signalling. To this end, PP1 is a major eukaryotic serine/threonine-specific phosphatase whose cellular functions, in turn, depend on complexes it forms with PP1 interacting proteins-PIPs. The importance of the testis/sperm-enriched variant, PP1γ2, in sperm motility and spermatogenesis has previously been shown. Given the key role of PIPs, it is imperative to identify the physiologically relevant PIPs in testis and sperm. Hence, we performed Yeast Two-Hybrid screens of a human testis cDNA library using as baits the different PP1 isoforms and also a proteomic approach aimed at identifying PP1γ2 binding proteins. To the best of our knowledge this is the largest data set of the human testis PP1 interactome. We report the identification of 77 proteins in human testis and 7 proteins in human sperm that bind PP1. The data obtained increased the known PP1 interactome by reporting 72 novel interactions. Confirmation of the interaction of PP1 with 5 different proteins was also further validated by co-immunoprecipitation or protein overlays. The data here presented provides important insights towards the function of these proteins and opens new possibilities for future research. In fact, such diversity in PP1 regulators makes them excellent targets for pharmacological intervention.

Catch me if you can: mass spectrometry-based phosphoproteomics and quantification strategies

Phosphorylation of proteins is one of the most prominent PTMs and for instance a key regulator of signal transduction. In order to improve our understanding of cellular phosphorylation events, considerable effort has been devoted to improving the analysis of phosphorylation by MS-based proteomics. Different enrichment strategies for phosphorylated peptides/proteins, such as immunoaffinity chromatography (IMAC) or titanium dioxide, have been established and constantly optimized for subsequent MS analysis. Concurrently, specific MS techniques were developed for more confident identification and phosphorylation site localization. In addition, more attention is paid to the LC-MS instrumentation to avoid premature loss of phosphorylated peptides within the analytical system. Despite major advances in all of these fields, the analysis of phosphopeptides still remains far from being routine in proteomics. However, to reveal cellular regulation by phosphorylation events, not only qualitative information about the phosphorylation status of proteins but also, in particular, quantitative information about distinct changes in phosphorylation patterns upon specific stimulation is mandatory. Thus, yielded insights are of outstanding importance for the emerging field of systems biology. In this review, we will give an insight into the historical development of phosphoproteome analysis and discuss its recent progress particularly regarding phosphopeptide quantification and assessment of phosphorylation stoichiometry.

Proteomics and the genetics of sperm chromatin condensation

Spermatogenesis involves extremely marked cellular, genetic and chromatin changes resulting in the generation of the highly specialized sperm cell. Proteomics allows the identification of the proteins that compose the spermatogenic cells and the study of their function. The recent developments in mass spectrometry (MS) have markedly increased the throughput to identify and to study the sperm proteins. Catalogs of thousands of testis and spermatozoan proteins in human and different model species are becoming available, setting up the basis for subsequent research, diagnostic applications and possibly the future development of specific treatments. The present review intends to summarize the key genetic and chromatin changes at the different stages of spermatogenesis and in the mature sperm cell and to comment on the presently available proteomic studies.