Heads or tails? Structural events and molecular mechanisms that promote mammalian sperm acrosomal exocytosis and motility

Reorganization of the Flagellum Scaffolding Induces a Sperm Standstill During Fertilization

Mammalian sperm delve into the female reproductive tract to fertilize the female gamete. The available information about how sperm regulate their motility during the final journey to the fertilization site is extremely limited. In this work, we investigated the structural and functional changes in the sperm flagellum after acrosomal exocytosis and during the interaction with the eggs. The evidence demonstrates that the double helix actin network surrounding the mitochondrial sheath of the midpiece undergoes structural changes prior to the motility cessation. This structural modification is accompanied by a decrease in diameter of the midpiece and is driven by intracellular calcium changes that occur concomitant with a reorganization of the actin helicoidal cortex. Although midpiece contraction may occur in a subset of cells that undergo acrosomal exocytosis, live-cell imaging during in vitro fertilization showed that the midpiece contraction is required for motility cessation after fusion is initiated. These findings provide the first evidence of the F-actin network's role in regulating sperm motility, adapting its function to meet specific cellular requirements during fertilization, and highlighting the broader significance of understanding sperm motility.

Significant statement

In this work, we demonstrate that the helical structure of polymerized actin in the flagellum undergoes a rearrangement at the time of sperm-egg fusion. This process is driven by intracellular calcium and promotes a decrease in the sperm midpiece diameter as well as the arrest in motility, which is observed after the fusion process is initiated.

α-Synuclein is required for sperm exocytosis at a post-fusion stage

The sperm acrosome is a large dense-core granule whose contents are secreted by regulated exocytosis at fertilization through the opening of numerous fusion pores between the acrosomal and plasma membranes. In other cells, the nascent pore generated when the membrane surrounding a secretory vesicle fuses with the plasma membrane may have different fates. In sperm, pore dilation leads to the vesiculation and release of these membranes, together with the granule contents. α-Synuclein is a small cytosolic protein claimed to exhibit different roles in exocytic pathways in neurons and neuroendocrine cells. Here, we scrutinized its function in human sperm. Western blot revealed the presence of α-synuclein and indirect immunofluorescence its localization to the acrosomal domain of human sperm. Despite its small size, the protein was retained following permeabilization of the plasma membrane with streptolysin O. α-Synuclein was required for acrosomal release, as demonstrated by the inability of an inducer to elicit exocytosis when permeabilized human sperm were loaded with inhibitory antibodies to human α-synuclein. The antibodies halted calcium-induced secretion when introduced after the acrosome docked to the cell membrane. Two functional assays, fluorescence and transmission electron microscopies revealed that the stabilization of open fusion pores was responsible for the secretion blockage. Interestingly, synaptobrevin was insensitive to neurotoxin cleavage at this point, an indication of its engagement in cis SNARE complexes. The very existence of such complexes during AE reflects a new paradigm. Recombinant α-synuclein rescued the inhibitory effects of the anti-α-synuclein antibodies and of a chimeric Rab3A-22A protein that also inhibits AE after fusion pore opening. We applied restrained molecular dynamics simulations to compare the energy cost of expanding a nascent fusion pore between two model membranes and found it higher in the absence than in the presence of α-synuclein. Hence, our results suggest that α-synuclein is essential for expanding fusion pores.

A novel protein encoded by circRsrc1 regulates mitochondrial ribosome assembly and translation during spermatogenesis

BACKGROUND

Circular RNAs (circRNAs) are a large class of mammalian RNAs. Several protein products translated by circRNAs have been reported to be involved in the development of various tissues and systems; however, their physiological functions in male reproduction have yet not been explored.

RESULTS

Here, we report an endogenous circRNA (circRsrc1) that encodes a novel 161-amino-acid protein which we named Rsrc1-161aa through circRNA sequencing coupled with mass spectrometry analysis on mouse testicular tissues. Deletion of Rsrc1-161aa in mice impaired male fertility with a significant decrease in sperm count and motility due to dysfunctions of mitochondrial energy metabolism. A series of in vitro rescue experiments revealed that circRsrc1 regulates mitochondrial functions via its encoded protein Rsrc1-161aa. Mechanistically, Rsrc1-161aa directly interacts with mitochondrial protein C1qbp and enhances its binding activity to mitochondrial mRNAs, thereby regulating the assembly of mitochondrial ribosomes and affecting the translation of oxidative phosphorylation (OXPHOS) proteins and mitochondrial energy metabolism.

CONCLUSIONS

Our studies reveal that Rsrc1-161aa protein encoded by circRsrc1 regulates mitochondrial ribosome assembly and translation during spermatogenesis, thereby affecting male fertility.

The SLC9C2 Gene Product (Na+/H+ Exchanger Isoform 11; NHE11) Is a Testis-Specific Protein Localized to the Head of Mature Mammalian Sperm

Na+/H+ exchangers (NHEs) are a family of ion transporters that regulate the pH of various cell compartments across an array of cell types. In eukaryotes, NHEs are encoded by the SLC9 gene family comprising 13 genes. SLC9C2, which encodes the NHE11 protein, is the only one of the SLC9 genes that is essentially uncharacterized. Here, we show that SLC9C2 exhibits testis/sperm-restricted expression in rats and humans, akin to its paralog SLC9C1 (NHE10). Similar to NHE10, NHE11 is predicted to contain an NHE domain, a voltage sensing domain, and finally an intracellular cyclic nucleotide binding domain. An immunofluorescence analysis of testis sections reveals that NHE11 localizes with developing acrosomal granules in spermiogenic cells in both rat and human testes. Most interestingly, NHE11 localizes to the sperm head, likely the plasma membrane overlaying the acrosome, in mature sperm from rats and humans. Therefore, NHE11 is the only known NHE to localize to the acrosomal region of the head in mature sperm cells. The physiological role of NHE11 has yet to be demonstrated but its predicted functional domains and unique localization suggests that it could modulate intracellular pH of the sperm head in response to changes in membrane potential and cyclic nucleotide concentrations that are a result of sperm capacitation events. If NHE11 is shown to be important for male fertility, it will be an attractive target for male contraceptive drugs due to its exclusive testis/sperm-specific expression.

Diversity of sialic acids and sialoglycoproteins in gametes and at fertilization

Sialic acids are a family of 9-carbon monosaccharides with particular physicochemical properties. They modulate the biological functions of the molecules that carry them and are involved in several steps of the reproductive process. Sialoglycoproteins participate in the balance between species recognition and specificity, and the mechanisms of these aspects remain an issue in gametes formation and binding in metazoan reproduction. Sialoglycoproteins form a specific coat at the gametes surface and specific polysialylated chains are present on marine species oocytes. Spermatozoa are submitted to critical sialic acid changes in the female reproductive tract facilitating their migration, their survival through the modulation of the female innate immune response, and the final oocyte-binding event. To decipher the role of sialic acids in gametes and at fertilization, the dynamical changes of enzymes involved in their synthesis and removal have to be further considered.

The future of assessing bull fertility: Can the 'omics fields identify usable biomarkers?

Breeding soundness examinations for bulls rely heavily on the subjective, visual assessment of sperm motility and morphology. Although these criteria have the potential to identify infertile males, they cannot be used to guarantee fertility or provide information about varying degrees of bull fertility. Male factor fertility is complex, and the success of the male gamete is not necessarily realized until well after the spermatozoon enters the oocyte. This paper reviews our existing knowledge of the bull’s contribution from a standpoint of the sperm’s cargo and the impact that this can have on fertilization and the development of the embryo. There has been a plethora of recent research characterizing the many molecular attributes that can affect the functional competence of a spermatozoon. A better understanding of the molecular factors influencing fertilization and embryo development in cattle will lead to the identification of biomarkers for the selection of bulls of superior fertility, which will have major implications for livestock production. To see this improvement in reproductive performance, we believe incorporation of modern technology into breeding soundness examinations will be necessary—although many of the discussed technologies are not ready for large-scale field application. Each of the ‘omics fields discussed in this review have shown promise for the identification of biomarkers of fertility, with certain families of biomarkers appearing to be better suited to different evaluations throughout a bull’s lifetime. Further research is needed for the proposed biomarkers to be of diagnostic or predictive value.

SLC22A14 is a mitochondrial riboflavin transporter required for sperm oxidative phosphorylation and male fertility

Ablation of Slc22a14 causes male infertility in mice, but the underlying mechanisms remain unknown. Here, we show that SLC22A14 is a riboflavin transporter localized at the inner mitochondrial membrane of the spermatozoa mid-piece and show by genetic, biochemical, multi-omic, and nutritional evidence that riboflavin transport deficiency suppresses the oxidative phosphorylation and reprograms spermatozoa energy metabolism by disrupting flavoenzyme functions. Specifically, we find that fatty acid β-oxidation (FAO) is defective with significantly reduced levels of acyl-carnitines and metabolites from the TCA cycle (the citric acid cycle) but accumulated triglycerides and free fatty acids in Slc22a14 knockout spermatozoa. We demonstrate that Slc22a14-mediated FAO is essential for spermatozoa energy generation and motility. Furthermore, sperm from wild-type mice treated with a riboflavin-deficient diet mimics those in Slc22a14 knockout mice, confirming that an altered riboflavin level causes spermatozoa morphological and bioenergetic defects. Beyond substantially advancing our understanding of spermatozoa energy metabolism, our study provides an attractive target for the development of male contraceptives.

The protein phosphatase with EF-hand domain 1 is a calmodulin-binding protein that interacts with proteins involved in sperm capacitation, binding to the zona pellucida, and motility

Spermatozoa are highly specialized cells whose fertilizing and motility functions highly depend on intracellular Ca²⁺ -mediated events and protein posttranslational modifications like phosphorylation. Our group previously identified PPEF1, the Ser/Thr phosphatase with EF-hand domain 1, among calmodulin-affinity pulled down sperm proteins. As the mammalian ortholog of the Drosophila phosphatase rdgC that dephosphorylates rhodopsin, PPEF1 has been studied mostly in the retina. The presence and importance of this Ca²⁺ /calmodulin-binding protein phosphatase has not been studied in sperm or testicular functions despite its high expression level. In this study, we show that PPEF1 is present in testicular germ cells, and in mouse, human and bull spermatozoa where it is localized predominantly in the neck and acrosome areas. Different transcript variants encoding four predicted isoforms were detected by reverse transcription polymerase chain reaction in bull testis, spermatocytes and spermatids. Phosphatase activity of immunoprecipitated sperm PPEF1 was detected using the substrate pNPP and analysis of the coimmunoprecipitated proteins reveal an enrichment in the biological processes of sperm capacitation, binding to the zona pellucida and motility. Although this is the first demonstration of the presence of PPEF1 in sperm and testicular germ cells, its involvement in sperm fertilizing ability and motility, and the mechanisms regulating its activity remain to be further investigated.

Molecular mechanisms and evolution of fertilization proteins

Sexual reproduction involves a cascade of molecular interactions between the sperm and the egg culminating in cell-cell fusion. Vital steps mediating fertilization include chemoattraction of the sperm to the egg, induction of the sperm acrosome reaction, dissolution of the egg coat, and sperm-egg plasma membrane binding and fusion. Despite decades of research, only a handful of interacting gamete recognition proteins (GRPs) have been identified across taxa mediating each of these steps, most notably in abalone, sea urchins, and mammals. This review outlines and compares notable GRP pairs mediating sperm-egg recognition in these three significant model systems and discusses the molecular basis of species-specific fertilization driven by GRP function. In addition, we explore the evolutionary theory behind the rapid diversification of GRPs between species. In particular, we focus on how the coevolution between interacting sperm and egg proteins may contribute to the formation of boundaries to hybridization. Finally, we discuss how pairing structural information with evolutionary insights can improve our understanding of mechanisms of fertilization and their origins.

Male reproductive health and intergenerational metabolic responses from a small RNA perspective

The world has recently experienced a decline in male reproductive (e.g. sperm counts and motility) and metabolic (e.g. obesity and diabetes) health. Accumulated evidence from animal models also shows that the metabolic health of the father may influence the metabolic health in his offspring. Vectors for such paternal intergenerational metabolic responses (IGMRs) involve small noncoding RNAs (sncRNAs) that often increase in spermatozoa during the last days of maturation in the epididymis. We and others have shown that the metabolic state - depending on factors such as diet, obesity and physical exercise - may affect sperm quality and sperm sncRNA. Together, this suggests that there are overlapping aetiologies between the male metabolic syndrome, male factor infertility and intergenerational responses. In this review, we present a theoretical framework for an overlap of these aetiologies by exploring the advances in our understanding of the roles of sncRNA in spermatogenesis and offspring development. A special focus will lie on novel findings about tRNA-derived small RNA (tsRNA), rRNA-derived small RNA (rsRNA) and small mitochondrial RNA (mitoRNA), and their emerging roles in intergenerational metabolic and reproductive health.

In vitro Induction and Detection of Acrosomal Exocytosis in Human Spermatozoa

The acrosome reaction is a highly regulated exocytotic event that primes spermatozoa for successful fertilization. Upon induction, acrosomal exocytosis proceeds via a wave of vesiculation that radiates across the sperm head, destabilizing the acrosomal vesicle and resulting in the release of the acrosomal contents. Having shed their acrosome, spermatozoa are then capable of penetrating the outer vestments of the oocyte and initiating fertilization. Accordingly, the failure of spermatozoa to complete an acrosome reaction represents a relatively common etiology in male infertility patients, and the ability to induce acrosomal exocytosis has found clinical utility in the evaluation of sperm fertilizing capacity. Here, we firstly describe protocols for driving the capacitation of human spermatozoa in vitro using chemically defined media in order to prime the cells for completion of acrosomal exocytosis. We then describe methodology routinely used for the induction of acrosomal exocytosis incorporating either a physiological agonist (i.e., the steroidal hormone, progesterone) or pharmacological reagent (i.e., the divalent cation ionophore, A23187). Finally, we describe the application of histochemical and immunofluorescence techniques that can be applied to study the completion of the acrosome reaction. Such protocols have important diagnostic utility for sperm function testing in both clinical and andrological research laboratories.

Recent introgression between Taiga Bean Goose and Tundra Bean Goose results in a largely homogeneous landscape of genetic differentiation

Several studies have uncovered a highly heterogeneous landscape of genetic differentiation across the genomes of closely related species. Specifically, genetic differentiation is often concentrated in particular genomic regions (“islands of differentiation”) that might contain barrier loci contributing to reproductive isolation, whereas the rest of the genome is homogenized by introgression. Alternatively, linked selection can produce differentiation islands in allopatry without introgression. We explored the influence of introgression on the landscape of genetic differentiation in two hybridizing goose taxa: the Taiga Bean Goose (Anser fabalis) and the Tundra Bean Goose (A. serrirostris). We re-sequenced the whole genomes of 18 individuals (9 of each taxon) and, using a combination of population genomic summary statistics and demographic modeling, we reconstructed the evolutionary history of these birds. Next, we quantified the impact of introgression on the build-up and maintenance of genetic differentiation. We found evidence for a scenario of allopatric divergence (about 2.5 million years ago) followed by recent secondary contact (about 60,000 years ago). Subsequent introgression events led to high levels of gene flow, mainly from the Tundra Bean Goose into the Taiga Bean Goose. This scenario resulted in a largely undifferentiated genomic landscape (genome-wide F_ST = 0.033) with a few notable differentiation peaks that were scattered across chromosomes. The summary statistics indicated that some peaks might contain barrier loci while others arose in allopatry through linked selection. Finally, based on the low genetic differentiation, considerable morphological variation and incomplete reproductive isolation, we argue that the Taiga and the Tundra Bean Goose should be treated as subspecies.

The aryl hydrocarbon receptor mediates sex ratio distortion in the embryos sired by TCDD-exposed male mice

Many reports describe an association between preconceptional paternal exposure to environmental chemicals, including the persistent organic pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) with an increased number of female offspring. We chronically treated wild-type C57BL/6 male mice with TCDD to investigate a role for the aryl hydrocarbon receptor (AHR) transcription factor. These mice had a 14 % lower male:female sex ratio than control mice, which was not observed in TCDD-treated Ahr knock out mice. AHR target genes Cyp1a1 and Ahrr were upregulated in the liver and testis of WT mice and Ahr expression was higher in the epididymis (2-fold) and liver (18-fold) than in whole testis tissue. The AHR protein was localized to round spermatids, elongating spermatids, and Leydig cells in the testis of WT mice. These studies demonstrate AHR involvement in the sex ratio distortion of TCDD-exposed males and the need for evaluating the molecular and genetic mechanism of this process.

Evaluation of cannabinoid receptors Type 1 and Type 2 mRNA expression in mature versus immature spermatozoa from fertile and infertile males

The expression of the main cannabinoid receptors (CBR1 and CBR2) was investigated to evaluate the possible association with the sperm maturation from fertile and infertile individuals. One hundred subjects were classified into fertile (n = 50) and infertile groups (n = 50). Fresh semen samples were collected. Computer-assisted semen analysis and acrosin activity test were done. RNA was extracted from mature and immature sperm pellets. Reverse transcriptase reaction and real-time PCR were done to assess the levels of both CBR1 and CBR2 genes expression in all samples. Mature spermatozoa from both groups showed significantly higher levels of both CBR1 and CBR2 compared with the immature spermatozoa (p < .05). This increment was significantly more important in the fertile group (p < .05). In mature spermatozoa, CBR1 expression was significantly related to variation in sperm morphology, and CBR2 was significantly related to both sperm morphology and linearity index. In conclusion, CBR1 and CBR2 mRNA expression may closely direct the sperm maturation at different steps of the reproductive process.

Capacitation-associated alkalization in human sperm is differentially controlled at the subcellular level

Capacitation in mammalian sperm involves the accurate balance of intracellular pH (pHi), but the mechanisms controlling this process are not fully understood, particularly regarding the spatiotemporal regulation of the proteins involved in pHi modulation. Here, we employed an image-based flow cytometry technique combined with pharmacological approaches to study pHi dynamics at the subcellular level during capacitation. We found that, upon capacitation induction, sperm cells undergo intracellular alkalization in the head and principal piece regions. The observed localized pHi increases require the initial uptake of HCO3-, which is mediated by several proteins acting consistently with their subcellular localization. Hv1 proton channel (also known as HVCN1) and cAMP-activated protein kinase (protein kinase A, PKA) antagonists impair alkalization mainly in the principal piece. Na+/HCO3- cotransporter (NBC) and cystic fibrosis transmembrane regulator (CFTR) antagonists impair alkalization only mildly, predominantly in the head. Motility measurements indicate that inhibition of alkalization in the principal piece prevents the development of hyperactivated motility. Altogether, our findings shed light on the complex control mechanisms of pHi and underscore their importance during human sperm capacitation.This article has an associated First Person interview with the first author of the paper.

Proteomic Changes in Human Sperm During Sequential in vitro Capacitation and Acrosome Reaction

The male gamete is not completely mature after ejaculation and requires further events in the female genital tract to acquire fertilizing ability, including the processes of capacitation and acrosome reaction. In order to shed light on protein changes experienced by the sperm cell in preparation for fertilization, a comprehensive quantitative proteomic profiling based on isotopic peptide labeling and liquid chromatography followed by tandem mass spectrometry was performed on spermatozoa from three donors of proven fertility under three sequential conditions: purification with density gradient centrifugation, incubation with capacitation medium, and induction of acrosome reaction by exposure to the calcium ionophore A23187. After applying strict selection criteria for peptide quantification and for statistical analyses, 36 proteins with significant changes in their relative abundance within sperm protein extracts were detected. Moreover, the presence of peptide residues potentially harboring sites for post-translational modification was revealed, suggesting that protein modification may be an important mechanism in sperm maturation. In this regard, increased levels of proteins mainly involved in motility and signaling, both regulated by protein modifiers, were detected in sperm lysates following incubation with capacitation medium. In contrast, less abundant proteins in acrosome-reacted cell lysates did not contain potentially modifiable residues, suggesting the possibility that all those proteins might be relocated or released during the process. Protein-protein interaction analysis revealed a subset of proteins potentially involved in sperm maturation, including the proteins Erlin-2 (ERLIN2), Gamma-glutamyl hydrolase (GGH) and Transmembrane emp24 domain-containing protein 10 (TMED10). These results contribute to the current knowledge of the molecular basis of human fertilization. It should now be possible to further validate the potential role of the detected altered proteins as modulators of male infertility.

The functional anatomy of the human spermatozoon: relating ultrastructure and function

The Internet, magazine articles, and even biomedical journal articles, are full of cartoons of spermatozoa that bear minimal resemblance to real spermatozoa, especially human spermatozoa, and this had led to many misconceptions about what spermatozoa look like and how they are constituted. This review summarizes the historical and current state of knowledge of mammalian sperm ultrastructure, with particular emphasis on and relevance to human spermatozoa, combining information obtained from a variety of electron microscopic (EM) techniques. Available information on the composition and configuration of the various ultrastructural components of the spermatozoon has been related to their mechanistic purpose and roles in the primary aspects of sperm function and fertilization: motility, hyperactivation, capacitation, the acrosome reaction and sperm-oocyte fusion.

C-type natriuretic peptide regulates sperm capacitation by the cGMP/PKG signalling pathway via Ca2+ influx and tyrosine phosphorylation

RESEARCH QUESTION

What is the effect of C-type natriuretic peptide (CNP) on human sperm capacitation in vitro and what is the mechanism of this effect?

DESIGN

CNP/NPR-B expression in the female rat genital tract was examined by immunohistochemistry and western blot assay, and then the role of CNP in human sperm capacitation was determined. The signal transduction pathway of CNP in the process was determined to elucidate the regulation mechanism of CNP by enzyme-linked immunosorbent assay and flow cytometry.

RESULTS

Both CNP and NPR-B were expressed in the genital tract of female rats, especially in the mucosa epithelium cell of the oviduct; the CNP level in the rat oviduct was higher than that in the cervix. Both CNP and NPR-B level in the rat oviduct varied during the oestrus cycle, maximal expression being observed at proestrus. Furthermore, intracellular cGMP level in spermatozoa was significantly enhanced by CNP (P < 0.01). PKG activity was detected in the spermatozoa, and it can be activated by the CNP and 8-Br-cGMP (cGMP analogue). The PKG inhibitor KT5823 inhibited the effect of CNP on sperm hyperactivation and the acrosome reaction. Finally, Ca²⁺ and tyrosine phosphorylation levels in spermatozoa were markedly improved by CNP and 8-Br-cGMP but significantly inhibited by the addition of KT5823 (P < 0.05).

CONCLUSIONS

CNP secreted by the female genital tract might bind to NPR-B on the spermatozoa. It successively stimulated intracellular cGMP/PKG signalling, increased Ca²⁺ and tyrosine-phosphorylated proteins, promoted hyperactivation and induced the acrosome reaction, which ultimately facilitated sperm capacitation.

Role of the Na+/K+-ATPase ion pump in male reproduction and embryo development

Na⁺/K⁺-ATPase was one of the first ion pumps studied because of its importance in maintaining osmotic and ionic balances between intracellular and extracellular environments, through the exchange of three Na⁺ ions out and two K⁺ ions into a cell. This enzyme, which comprises two main subunits (α and β), with or without an auxiliary polypeptide (γ), can have specific biochemical properties depending on the expression of associated isoforms (α1β1 and/or α2β1) in the cell. In addition to the importance of Na⁺/K⁺-ATPase in ensuring the function of many tissues (e.g. brain, heart and kidney), in the reproductive tract this protein is essential for embryo development because of its roles in blastocoel formation and embryo hatching. In the context of male reproduction, the discovery of a very specific subunit (α4), apparently restricted to male germ cells, only expressed after puberty and able to influence sperm function (e.g. motility and capacitation), opened a remarkable field for further investigations regarding sperm biology. Therefore, the present review focuses on the importance of Na⁺/K⁺-ATPase on male reproduction and embryo development.

Removal of sialic acid from bull sperm decreases motility and mucus penetration ability but increases zona pellucida binding and polyspermic penetration in vitro

This study tested the hypothesis that sperm sialic acid (Sia) is required to reach the site of fertilization, and that successful fertilization requires recognition of Sia from both the sperm and oocyte to occur. In addition, it has recently been reported that Siglecs (Sia-binding-immunoglobulin-like lectins) are present on the sperm surface. Thus, the possibility that the recognition of oocyte Sia was sperm-Siglec-mediated was also addressed. Sperm exposed to neuraminidase (NMase) exhibited lower overall and progressive motility, which translated to a decreased ability to swim through cervical mucus from cows in oestrus. In addition, when either sperm or cumulus-oocyte complexes (COCs) were treated with NMase, a decrease in cleavage and blastocyst rate was observed. However, incubation of sperm with increasing concentrations of anti-Siglec-2, -5, -6 and -10 antibodies prior to fertilization had no effect on their fertilizing ability. Interestingly, treatment with NMase increased the number of sperm bound to the ZP but also the rate of polyspermic fertilization. Flow cytometry analysis revealed no differences in the percentage of capacitated or acrosome-reacted sperm. These results suggest that Sia are required to reach the site of fertilization but need to be removed for sperm-oocyte interaction. However, fine regulation is needed to avoid abnormal fertilization which can lead to impaired embryo development.

Design, Synthesis, and in Vitro and in Vivo Evaluation of Ouabain Analogues as Potent and Selective Na,K-ATPase α4 Isoform Inhibitors for Male Contraception

Na,K-ATPase α4 is a testis-specific plasma membrane Na⁺ and K⁺ transporter expressed in sperm flagellum. Deletion of Na,K-ATPase α4 in male mice results in complete infertility, making it an attractive target for male contraception. Na,K-ATPase α4 is characterized by a high affinity for the cardiac glycoside ouabain. With the goal of discovering selective inhibitors of the Na,K-ATPase α4 and of sperm function, ouabain derivatives were modified at the glycone (C3) and the lactone (C17) domains. Ouabagenin analogue 25, carrying a benzyltriazole moiety at C17, is a picomolar inhibitor of Na,K-ATPase α4, with an outstanding α4 isoform selectivity profile. Moreover, compound 25 decreased sperm motility in vitro and in vivo and affected sperm membrane potential, intracellular Ca²⁺, pH, and hypermotility. These results proved that the new ouabagenin triazole analogue is an effective and selective inhibitor of Na,K-ATPase α4 and sperm function.

CFAP157 is a murine downstream effector of FOXJ1 that is specifically required for flagellum morphogenesis and sperm motility

Motile cilia move extracellular fluids or mediate cellular motility. Their function is essential for embryonic development, adult tissue homeostasis and reproduction throughout vertebrates. FOXJ1 is a key transcription factor for the formation of motile cilia but its downstream genetic programme is only partially understood. Here, we characterise a novel FOXJ1 target, Cfap157, that is specifically expressed in motile ciliated tissues in mouse and Xenopus in a FOXJ1-dependent manner. CFAP157 protein localises to basal bodies and interacts with tubulin and the centrosomal protein CEP350. Cfap157 knockout mice appear normal but homozygous males are infertile. Spermatozoa display impaired motility and a novel phenotype: Cfap157-deficient sperm exhibit axonemal loops, supernumerary axonemal profiles with ectopic accessory structures, excess cytoplasm and clustered mitochondria in the midpiece regions, and defective axonemes along the flagella. Our study thus demonstrates an essential sperm-specific function for CFAP157 and suggests that this novel FOXJ1 effector is part of a mechanism that acts during spermiogenesis to suppress the formation of supernumerary axonemes and ensures a correct ultrastructure.

Autophagy-related proteins are functionally active in human spermatozoa and may be involved in the regulation of cell survival and motility

Macroautophagy (hereafter autophagy) is an evolutionarily highly conserved cellular process that participates in the maintenance of intracellular homeostasis through the degradation of most long-lived proteins and entire organelles. Autophagy participates in some reproductive events; however, there are not reports regarding the role of autophagy in the regulation of sperm physiology. Hence, the aim of this study was to investigate whether autophagy-related proteins are present and functionally active in human spermatozoa. Proteins related to autophagy/mitophagy process (LC3, Atg5, Atg16, Beclin 1, p62, m-TOR, AMPKα 1/2, and PINK1) were present in human spermatozoa. LC3 colocalized with p62 in the middle piece of the spermatozoa. Autophagy activation induced a significant increase in motility and a decrease in PINK1, TOM20 expression and caspase 3/7 activation. In contrast, autophagy inhibition resulted in decreased motility, viability, ATP and intracellular calcium concentration whereas PINK1, TOM20 expression, AMPK phosphorylation and caspase 3/7 activation were significantly increased. In conclusion our results show that autophagy related proteins and upstream regulators are present and functional in human spermatozoa. Modification of mitochondrial proteins expression after autophagy activation/inhibition may be indicating that a specialized form of autophagy named mitophagy may be regulating sperm function such as motility and viability and may be cooperating with apoptosis.

The Rab3A-22A Chimera Prevents Sperm Exocytosis by Stabilizing Open Fusion Pores

At the final stage of exocytotis, a fusion pore opens between the plasma and a secretory vesicle membranes; typically, when the pore dilates the vesicle releases its cargo. Sperm contain a large dense-core secretory granule (the acrosome) whose contents are secreted by regulated exocytosis at fertilization. Minutes after the arrival of the triggering signal, the acrosomal and plasma membranes dock at multiple sites and fusion pores open at the contact points. It is believed that immediately afterward, fusion pores dilate spontaneously. Rab3A is an essential component of human sperm exocytotic machinery. Yet, recombinant, persistently active Rab3A halts calcium-triggered secretion when introduced after docking into streptolysin O-permeabilized cells; so does a Rab3A-22A chimera. Here, we applied functional assays, electron and confocal microscopy to show that the secretion blockage is due to the stabilization of open fusion pores. Other novel findings are that sperm SNAREs engage in α-SNAP/NSF-sensitive complexes at a post-fusion stage. Complexes are disentangled by these chaperons to achieve vesiculation and acrosomal contents release. Thus, post-fusion regulation of the pores determines their expansion and the success of the acrosome reaction.

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.

Small GTPases in acrosomal exocytosis

Regulated exocytosis employs a conserved molecular machinery in all secretory cells. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) and Rab superfamilies are members of this machinery. Rab proteins are small GTPases that organize membrane microdomains on organelles by recruiting specific effectors that strongly influence the movement, fusion and fission dynamics of intracellular compartments. Rab3 and Rab27 are the prevalent exocytotic isoforms. Many events occur in mammalian spermatozoa before they can fertilize the egg, one of them is the acrosome reaction (AR), a type of regulated exocytosis. The AR relies on the same fusion machinery as all other cell types, which includes members of the exocytotic SNARE and Rab superfamilies. Here, we describe in depth two protocols designed to determine the activation status of small G proteins. One of them also serves to determine the subcellular localization of active Rabs, something not achievable with other methods. By means of these techniques, we have reported that Rab27 and Rab3 act sequentially and are organized in a RabGEF cascade during the AR. Although we developed them to scrutinize the exocytosis of the acrosome in human sperm, the protocols can potentially be extended to study other Ras-related proteins in virtually any cellular model.

Coiled-coil domain containing 42 (Ccdc42) is necessary for proper sperm development and male fertility in the mouse

Spermiogenesis is the differentiation of spermatids into motile sperm consisting of a head and a tail. The head harbors a condensed elongated nucleus partially covered by the acrosome-acroplaxome complex. Defects in the acrosome-acroplaxome complex are associated with abnormalities in sperm head shaping. The head-tail coupling apparatus (HTCA), a complex structure consisting of two cylindrical microtubule-based centrioles and associated components, connects the tail or flagellum to the sperm head. Defects in the development of the HTCA cause sperm decapitation and disrupt sperm motility, two major contributors to male infertility. Here, we provide data indicating that mutations in the gene Coiled-coil domain containing 42 (Ccdc42) is associated with malformation of the mouse sperm flagella. In contrast to many other flagella and motile cilia genes, Ccdc42 expression is only observed in the brain and developing sperm. Male mice homozygous for a loss-of-function Ccdc42 allele (Ccdc42(KO)) display defects in the number and location of the HTCA, lack flagellated sperm, and are sterile. The testes enriched expression of Ccdc42 and lack of other phenotypes in mutant mice make it an ideal candidate for screening cases of azoospermia in humans.

A Specific Transitory Increase in Intracellular Calcium Induced by Progesterone Promotes Acrosomal Exocytosis in Mouse Sperm

During capacitation, sperm acquire the ability to undergo the acrosome reaction (AR), an essential step in fertilization. Progesterone produced by cumulus cells has been associated with various physiological processes in sperm, including stimulation of AR. An increase in intracellular Ca²⁺ ([Ca²⁺]i) is necessary for AR to occur. In this study, we investigated the spatiotemporal correlation between the changes in [Ca²⁺]i and AR in single mouse spermatozoa in response to progesterone. We found that progesterone stimulates an [Ca²⁺]i increase in five different patterns: gradual increase, oscillatory, late transitory, immediate transitory, and sustained. We also observed that the [Ca²⁺]i increase promoted by progesterone starts at either the flagellum or the head. We validated the use of FM4-64 as an indicator for the occurrence of the AR by simultaneously detecting its fluorescence increase and the loss of EGFP in transgenic EGFPAcr sperm. For the first time, we have simultaneously visualized the rise in [Ca²⁺]i and the process of exocytosis in response to progesterone and found that only a specific transitory increase in [Ca²⁺]i originating in the sperm head promotes the initiation of AR.

Role of forward-motility-stimulating factor as an extracellular activator of soluble adenylyl cyclase

Forward-motility-stimulating factor (FMSF) is a protein, originally purified from bubaline serum, that promotes progressive motility of mature spermatozoa. FMSF binds to sperm surface receptors and activates transmembrane adenylyl cyclase (tmAC), causing a rise in intracellular cyclic AMP level ([cAMP]i) and subsequent activation of a protein kinase A/tyrosine kinase-mediated pathway that enhances forward motility. This article further evaluates how FMSF works in the caprine system, particularly identifying the stimulatory effect of this glycoprotein on soluble adenylyl cyclase (sAC). Elevated [cAMP]i, initially resulting from FMSF-dependent activation of tmAC, was associated with the release of Ca(2+) from an intracellular calcium store in the sperm head, likely via an inositol triphosphate-sensitive calcium ion channel. This peak Ca(2+) concentration of ∼125-175 nM was capable of stimulating sAC in vitro in a calmodulin-independent manner, thereby triggering more cAMP production. Our model proposes that a positive-feedback loop mediated by cAMP and Ca(2+) is established in FMSF-stimulated sperm, with cAMP playing the role of a chemical messenger at multiple steps, resulting in the observed progressive motility. Thus, FSMF stimulates a novel signaling cascade that synergistically activate both tmAC and sAC to achieve forward sperm motility.

The IP3 R Binding Protein Released With Inositol 1,4,5-Trisphosphate Is Expressed in Rodent Reproductive Tissue and Spermatozoa

Besides its capacity to inhibit the 1,4,5-trisphosphate (IP3) receptor, the regulatory protein IRBIT (IP3 receptor binding protein released with IP3) is also able to control the activity of numerous ion channels and electrolyte transporters and thereby creates an optimal electrolyte composition of various biological fluids. Since a reliable execution of spermatogenesis and sperm maturation critically depends on the establishment of an adequate microenvironment, the expression of IRBIT in male reproductive tissue was examined using immunohistochemical approaches combined with biochemical fractionation methods. The present study documents that IRBIT is expressed in Leydig and Sertoli cells. In addition, pronounced IRBIT expression was detected in sperm precursors during early stages of spermatogenesis as well as in spermatozoa. Analyzing tissue sections of rodent epididymides, IRBIT was found to co-localize with the proton pumping V-ATPase and the cystic fibrosis transmembrane conductance regulator (CFTR) at the apical surface of narrow and clear cells. A similar co-localization of IRBIT with CFTR was also observed for Sertoli cells and developing germ cells. Remarkably, assaying caudal sperm in immunogold electron microscopy, IRBIT was found to localize to the acrosomal cap and the flagellum as well as to the sperm nucleus; moreover, a prominent oligomerization was observed for spermatozoa. The pronounced occurrence of IRBIT in the male reproductive system and mature spermatozoa indicates a potential role for IRBIT in establishing the essential luminal environment for a faithful execution of spermatogenesis and epididymal sperm maturation, and suggest a participation of IRBIT during maturation steps after ejaculation and/or the final fertilization process.

The Histone Methyltransferase Gene Absent, Small, or Homeotic Discs-1 Like Is Required for Normal Hox Gene Expression and Fertility in Mice

Chromatin remodeling influences gene expression in developing and adult organisms. Active and repressive marks of histone methylation dictate the embryonic expression boundaries of developmentally regulated genes, including the Hox gene cluster. Drosophila ash1 (absent, small or homeotic discs 1) gene encodes a histone methyltransferase essential for regulation of Hox gene expression that interacts genetically with other members of the trithorax group (TrxG). While mammalian members of the mixed lineage leukemia (Mll) family of TrxG genes have roles in regulation of Hox gene expression, little is known about the expression and function of the mammalian ortholog of the Drosophila ash1 gene, Ash1-like (Ash1l). Here we report the expression of mouse Ash1l gene in specific structures within various organs and provide evidence that reduced Ash1l expression has tissue-specific effects on mammalian development and adult homeostasis. Mutants exhibit partially penetrant postnatal lethality and failure to thrive. Surviving mutants have growth insufficiency, skeletal transformations, and infertility associated with developmental defects in both male and female reproductive organs. Specifically, expression of Hoxa11 and Hoxd10 are altered in the epididymis of Ash1l mutant males and Hoxa10 is reduced in the uterus of Ash1l mutant females. In summary, we show that the histone methyltransferase Ash1l is important for the development and function of several tissues and for proper expression of homeotic genes in mammals.

PKA-dependent phosphorylation of LIMK1 and Cofilin is essential for mouse sperm acrosomal exocytosis

Mammalian sperm must acquire their fertilizing ability after a series of biochemical modifications in the female reproductive tract collectively called capacitation to undergo acrosomal exocytosis, a process that is essential for fertilization. Actin dynamics play a central role in controlling the process of exocytosis in somatic cells as well as in sperm from several mammalian species. In somatic cells, small GTPases of the Rho family are widely known as master regulators of actin dynamics. However, the role of these proteins in sperm has not been studied in detail. In the present work we characterized the participation of small GTPases of the Rho family in the signaling pathway that leads to actin polymerization during mouse sperm capacitation. We observed that most of the proteins of this signaling cascade and their effector proteins are expressed in mouse sperm. The activation of the signaling pathways of cAMP/PKA, RhoA/C and Rac1 is essential for LIMK1 activation by phosphorylation on Threonine 508. Serine 3 of Cofilin is phosphorylated by LIMK1 during capacitation in a transiently manner. Inhibition of LIMK1 by specific inhibitors (BMS-3) resulted in lower levels of actin polymerization during capacitation and a dramatic decrease in the percentage of sperm that undergo acrosomal exocytosis. Thus, we demonstrated for the first time that the master regulators of actin dynamics in somatic cells are present and active in mouse sperm. Combining the results of our present study with other results from the literature, we have proposed a working model regarding how LIMK1 and Cofilin control acrosomal exocytosis in mouse sperm.

The proteins of exocytosis: lessons from the sperm model

Exocytosis is a highly regulated process that consists of multiple functionally, kinetically and/or morphologically definable stages such as recruitment, targeting, tethering and docking of secretory vesicles with the plasma membrane, priming of the fusion machinery and calcium-triggered membrane fusion. After fusion, the membrane around the secretory vesicle is incorporated into the plasma membrane and the granule releases its contents. The proteins involved in these processes belong to several highly conserved families: Rab GTPases, SNAREs (soluble NSF-attachment protein receptors), α-SNAP (α-NSF attachment protein), NSF (N-ethylmaleimide-sensitive factor), Munc13 and -18, complexins and synaptotagmins. In the present article, the molecules of exocytosis are reviewed, using human sperm as a model system. Sperm exocytosis is driven by isoforms of the same proteinaceous fusion machinery mentioned above, with their functions orchestrated in a hierarchically organized and unidirectional signalling cascade. In addition to the universal exocytosis regulator calcium, this cascade includes other second messengers such as diacylglycerol, inositol 1,4,5-trisphosphate and cAMP, as well as the enzymes that synthesize them and their target proteins. Of special interest is the cAMP-binding protein Epac (exchange protein directly activated by cAMP) due in part to its enzymatic activity towards Rap. The activation of Epac and Rap leads to a highly localized calcium signal which, together with assembly of the SNARE complex, governs the final stages of exocytosis. The source of this releasable calcium is the secretory granule itself.

The protein tyrosine phosphorylation during in vitro capacitation and cryopreservation of mammalian spermatozoa

Before the process of fertilization, spermatozoa necessitate a period of residence in the female reproductive environment, and undergo a sequence of physiological and biochemical changes collectively referred to as capacitation. Accumulated evidences from several laboratories indicated that the protein tyrosine phosphorylation (PTP) is one of the most important intracellular signaling events regulating sperm function, and is a meaningful indicator of capacitation. Different factors that affect PTP are cholesterol efflux, influx of HCO3(-), increased intracellular Ca(2+), cAMP and reactive oxygen species (ROS). cAMP/PKA and extracellular signal regulated kinases (ERKs) are the known important signaling pathways primarily involved in PTP. Advanced proteomics approaches have revealed several proteins that undergo tyrosine phosphorylation during capacitation. Semen cryopreservation subjects spermatozoa to frequent stressors, which result in capacitation like changes (cryo-capacitation). The cryo-capacitated spermatozoa usually show different patterns of PTP than the normal in vitro capacitated spermatozoa. In the current manuscript, we have summarized some information about the proteins undergoing tyrosine phosphorylation during capacitation and the effect of cryopreservation on PTP as well as the possibilities to reduce the changes associated with cryopreservation process.

Metabolic syndrome-associated sperm alterations in an experimental rabbit model: relation with metabolic profile, testis and epididymis gene expression and effect of tamoxifen treatment

The influence of metabolic syndrome (MetS) on sperm quality and function is debated. Using a well-established high fat diet (HFD) rabbit model resembling human MetS, including development of hypogonadism, we demonstrate that HFD decreased sperm motility, morphology and acrosome reaction in response to progesterone and increased sperm cholesterol content. All the above parameters were associated with most MetS features, its severity and plasma testosterone (T) at univariate analysis. After T adjustment, sperm morphology and motility retained a significant association, respectively, with mean arterial pressure and circulating cholesterol levels. MetS modified the expression of inflammatory and tissue remodelling genes in the testis and of aquaporins in the epididymis. In a multivariate analysis, sperm morphology resulted associated with testis expression of fibronectin and collagen type 1 genes, whereas motility with epididymis aquaporin 1 gene. Administration of tamoxifen, used in the treatment of idiopathic male infertility, to HFD rabbits partially restored motility, but further decreased morphology and increased spontaneous acrosome reaction, without restoring responsiveness to progesterone. Overall our results indicate that development of MetS produces detrimental effects on sperm quality and functionality by inducing metabolic disorders leading to alterations in testis and epididymis functions and evidence a role of hypertension as a new determinant of abnormal sperm morphology, in line with a previous human study from our group.

Electroacupuncture enhances spermatogenesis in rats after scrotal heat treatment

Spermatogenesis is regulated by a cascade of steroid regulated genes in the testis. Recent studies suggested that acupuncture may improve fertility in men with abnormal semen parameters. Yet, the underlying mechanisms in which acupuncture enhances spermatogenesis remain largely unknown. Here we used a scrotal heat-treated rat model to study the effect of electroacupuncture (EA) on recovery of spermatogenesis. In this model, spermatogenesis was disrupted by 30 min scrotal heat treatment at 43°C. Ten sessions of EA were given at Baihui (GV20), Guanyuan (CV4), Zusanli (ST36) and Sanyinjiao (SP6) from day 9 to day 36 post-treatment. Sperm motility and production, morphology of the germinal epithelium by Johnsen’s scoring, germ cell apoptosis by TUNEL staining, proliferation by proliferating cell nuclear antigen (PCNA) staining, as well as serum testosterone and inhibin B levels by immunoassays were evaluated on day 0, 1, 9, 25, 37, 46, 56 and 79. When compared with the heat-treated (H) group, the heat-treated plus EA (H⁺EA) group showed a significant increase (p < 0.05) in PCNA-positive cells and inhibin B levels on days 37 and 46, and a higher Johnsen’s score till day 56. On day 79, motile spermatozoa could be found in the vas deferens of H⁺EA group only. Consistently, there was a trend of improved motility and increased number of motile epididymal spermatozoa in the H⁺EA group than the H group; while apoptosis of germ cells and serum testosterone levels were similar between the two groups. Taken together, EA enhanced germ cell proliferation through improvement of Sertoli cell functions. This may facilitate the recovery of spermatogenesis and may restore normal semen parameters in subfertile patients.

Central role of soluble adenylyl cyclase and cAMP in sperm physiology

Cyclic adenosine 3',5'-monophosphate (cAMP), the first second messenger to be described, plays a central role in cell signaling in a wide variety of cell types. Over the last decades, a wide body of literature addressed the different roles of cAMP in cell physiology, mainly in response to neurotransmitters and hormones. cAMP is synthesized by a wide variety of adenylyl cyclases that can generally be grouped in two types: transmembrane adenylyl cyclase and soluble adenylyl cyclases. In particular, several aspects of sperm physiology are regulated by cAMP produced by a single atypical adenylyl cyclase (Adcy10, aka sAC, SACY). The signature that identifies sAC among other ACs, is their direct stimulation by bicarbonate. The essential nature of cAMP in sperm function has been demonstrated using gain of function as well as loss of function approaches. This review unifies state of the art knowledge of the role of cAMP and those enzymes involved in cAMP signaling pathways required for the acquisition of fertilizing capacity of mammalian sperm. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Roles of intracellular cyclic AMP signal transduction in the capacitation and subsequent hyperactivation of mouse and boar spermatozoa

It is not until accomplishment of a variety of molecular changes during the transit through the female reproductive tract that mammalian spermatozoa are capable of exhibiting highly activated motility with asymmetric whiplash beating of the flagella (hyperactivation) and undergoing acrosomal exocytosis in the head (acrosome reaction). These molecular changes of the spermatozoa are collectively termed capacitation and promoted by bicarbonate, calcium and cholesterol acceptors. Such capacitation-promoting factors can stimulate intracellular cyclic AMP (cAMP) signal transduction in the spermatozoa. Meanwhile, hyperactivation and the acrosome reaction are essential to sperm fertilization with oocytes and are apparently triggered by a sufficient increase of intracellular Ca²⁺ in the sperm flagellum and head, respectively. Thus, it is necessary to investigate the relationship between cAMP signal transduction and calcium signaling cascades in the spermatozoa for the purpose of understanding the molecular basis of capacitation. In this review, I cover updated insights regarding intracellular cAMP signal transduction, the acrosome reaction and flagellar motility in mammalian spermatozoa and then account for possible roles of intracellular cAMP signal transduction in the capacitation and subsequent hyperactivation of mouse and boar spermatozoa.

Effect of oxamic analogues on functional mice sperm parameters

The present study evaluates the effect of oxamate derivatives (N-ethyl, N-propyl, N-butyl oxamates) on functional murine sperm parameters, towards a new male non-hormonal contraceptive. These derivatives are selective inhibitors of lactate dehydrogenase-C4 (LDH-C4). LDH-C4 is a sperm-specific enzyme that plays an important role in ATP production for maintaining progressive motility as well as to induce capacitation and hyperactivation. The results demonstrate that all oxamate derivatives selectively inhibited LDH-C4 in mouse sperm extracts. The IC(50) values for hexokinase and glyceraldehyde-3-phosphate dehydrogenase were at least an order of magnitude greater than LDH-C4 IC(50) values. Prodrugs of oxamate derivatives assayed on sperm cells diminished normal sperm motility parameters, acrosome reaction, and cell viability in a concentration dependent manner. Also, we performed in vivo studies to determine the potential toxicity and possible contraceptive ability of these inhibitors. Mouse sperm were more sensitive to the N-butyl oxamate ethyl ester (NBOXet). Furthermore, results showed that NBOXet was of a low toxicity substance that diminished the total and progressive motility as well as the kinematic parameters of sperm cells. Data from in vitro and in vivo studies showed that N-butyl oxamate and its prodrug, are selective inhibitors of sperm LDH-C4, has low toxicity, and inhibits sperm progressive motility, offering some of the desirable characteristics of a male contraceptive: effect, low toxicity, and selectivity.

Fucose, mannose, and β-N-acetylglucosamine glycopolymers initiate the mouse sperm acrosome reaction through convergent signaling pathways

The sperm acrosome reaction (AR), an essential exocytosis step in mammalian fertilization, is mediated by a species-specific interaction of sperm surface molecules with glycans on the egg. Previous studies indicate that a subset of terminal carbohydrates on the mouse egg zona pellucida (ZP) trigger the AR by cross-linking or aggregating receptors on the sperm membrane. However, the exact role of those carbohydrates in AR has not been identified and the mechanism underlying the AR still needs further investigation. To study this process, a series of glycopolymers was synthesized. The glycopolymers are composed of a multivalent scaffold (norbornene), a functional ligand (previously identified ZP terminal monosaccharides), and a linker connecting the ligand and the scaffold. The polymers were tested for their ability to initiate AR and through which signaling pathways AR induction occurred. Our data demonstrate that mannose, fucose, and β-N-acetylglucosamine 10-mers and 100-mers initiate AR in a dose-dependent manner, and the 100-mers are more potent on a per monomer basis than the 10-mers. Although nearly equipotent in inducing the AR at the optimal concentrations, their AR activation kinetics are not identical. Similar to mouse ZP3, all 100-mer-activated AR are sensitive to guanine-binding regulatory proteins (G-proteins), tyrosine kinase, protein kinase A, protein kinase C, and Ca²⁺-related antagonists. Thus, the chemotypes of synthetic glycopolymers imitate the physiologic AR-activation agents and provide evidence that occupation of one of at least three different receptor binding sites is sufficient to initiate the AR.

Odorant receptor-mediated sperm activation in disease vector mosquitoes

Insects, such as the malaria vector mosquito, Anopheles gambiae, depend upon chemoreceptors to respond to volatiles emitted from a range of environmental sources, most notably blood meal hosts and oviposition sites. A subset of peripheral signaling pathways involved in these insect chemosensory-dependent behaviors requires the activity of heteromeric odorant receptor (OR) ion channel complexes and ligands for numerous A. gambiae ORs (AgOrs) have been identified. Although AgOrs are expressed in nonhead appendages, studies characterizing potential AgOr function in nonolfactory tissues have not been conducted. In the present study, we explore the possibility that AgOrs mediate responses of spermatozoa to endogenous signaling molecules in A. gambiae. In addition to finding AgOr transcript expression in testes, we show that the OR coreceptor, AgOrco, is localized to the flagella of A. gambiae spermatozoa where Orco-specific agonists, antagonists, and other odorant ligands robustly activate flagella beating in an Orco-dependent process. We also demonstrate Orco expression and Orco-mediated activation of spermatozoa in the yellow fever mosquito, Aedes aegypti. Moreover, we find Orco localization in testes across distinct insect taxa and posit that OR-mediated responses in spermatozoa may represent a general characteristic of insect reproduction and an example of convergent evolution.

Endocannabinoids as biomarkers of human reproduction

BACKGROUND

Infertility is a condition of the reproductive system that affects ∼10-15% of couples attempting to conceive a baby. More than half of all cases of infertility are a result of female conditions, while the remaining cases can be attributed to male factors, or to a combination of both. The search for suitable biomarkers of pregnancy outcome is a challenging issue in human reproduction, aimed at identifying molecules with predictive significance of the reproductive potential of male and female gametes. Among the various candidates, endocannabinoids (eCBs), and in particular anandamide (AEA), represent potential biomarkers of human fertility disturbances. Any perturbation of the balance between synthesis and degradation of eCBs will result in local changes of their tone in human female and male reproductive tracts, which in turn regulates various pathophysiological processes, oocyte and sperm maturation included.

METHODS

PubMed and Web of Science databases were searched for papers using relevant keywords like 'biomarker', 'endocannabinoid', 'infertility', 'pregnancy' and 'reproduction'.

RESULTS

In this review, we discuss different studies on the measurements of AEA and related eCBs in human reproductive cells, tissues and fluids, where the local contribution of these bioactive lipids could be critical in ensuring normal sperm fertilizing ability and pregnancy.

CONCLUSION

Based on the available data, we suggest that the AEA tone has the potential to be exploited as a novel diagnostic biomarker of infertility, to be used in association with assays of conventional hormones (e.g. progesterone, β-chorionic gonadotrophin) and semen analysis. However further quantitative research of its predictive capacity is required.