Semi-automatized segmentation method using image-based flow cytometry to study sperm physiology: the case of capacitation-induced tyrosine phosphorylation

Overexpression of PD-L1 causes germ cell failure and infertility via CRISP1/PD-L1 interaction in mouse epididymis

Spermatogenesis is a highly complex process through which mature sperms are produced, and it requires three important stages; mitosis, meiosis and sperm formation. The expression of genes regulated by transcription factors at specific stages exerts important regulatory effects on the development process of germ cells. Male mice with overexpressed programmed death ligand 1 (PD-L1) (B7 homolog1) in the testis have infertility and abnormal sperm development, thereby exhibiting severe malformation and sloughing throughout spermatid maturation and collapsed and disorganized seminiferous epithelium structure. Furthermore, PD-L1 overexpression causes overexpression of cysteine-rich secretory protein 1 (CRISP1) in the epididymis and adversely affects or precludes sperm energization, sperm-pellucida binding and sperm-oocyte fusion. These findings suggest that CRISP1 and PD-L1 can interact with each other to induce male infertility and germ-cell dissociation.

Clinical Utility of Sperm Function Tests in Predicting Male Fertility: A Systematic Review

Routine semen analysis provides considerable information regarding sperm parameters; however, it is not solely adequate to predict male fertility potential. In the past two decades, several advance sperm function tests have been developed. The present systematic review intends to assess the clinical utility of available advance sperm function tests in predicting the male fertility potential. A systematic literature search was conducted as per PRISMA guidelines using PubMed, MEDLINE, Google Scholar, and Cochrane Library. Different keywords either singly or in combination were used to retrieve the relevant articles related to sperm function tests, male fertility, and pregnancy outcomes. A total of 5169 articles were obtained, out of which 110 meeting the selection criteria were included in this review. The majorly investigated sperm function tests are hypo-osmotic swelling test, acrosome reaction test, sperm capacitation test, hemizona binding assay, sperm DNA fragmentation test, seminal reactive oxygen species test, mitochondrial dysfunction tests, antisperm antibody test, nuclear chromatin de-condensation (NCD) test, etc. The different advance sperm function tests analyse different aspects of sperm function. Hence, any one test may not be helpful to appropriately predict the male fertility potential. Currently, the unavailability of high-quality clinical data, robust thresholds, complex protocols, high cost, etc., are the limiting factors and prohibiting current sperm function tests to reach the clinics. Further multi-centric research efforts are required to fulfil the existing lacunas and pave the way for these tests to be introduced into the clinics.

IZUMO1 Receptor Localization during Hyaluronic Acid Selection in Human Spermatozoa

IZUMO1 is an acrosome transmembrane protein implicated in the adhesion and fusion of gametes. This study aims to describe the distribution of IZUMO1 in human sperm under different physiological conditions: before capacitation (NCS), at one-hour capacitation (CS1), after a hyaluronic acid (HA) selection test (mature, MS1 and immature, IS1), and induced acrosome reaction from one-hour-capacitated sperm (ARS1). The data obtained in NCS, CS1, and MS1 significantly highlight dotted fluorescence in the acrosomal region (P1) as the major staining pattern (~70%). Moreover, we describe a new distribution pattern (P2) with a dotted acrosomal region and a labelled equatorial region that significantly increases in HA-bound spermatozoa, suggesting the onset of the migration of IZUMO1. In contrast, unbound spermatozoa presented an increase in P3 (equatorial region labelled) and P4 (not labelled). Finally, costaining to observe IZUMO1 distribution and acrosome status was performed in ARS1. Interestingly, we reported a variety of combinations between the IZUMO1 staining patterns and the acrosomal stages. In conclusion, these data show as a novelty the diffusion of the IZUMO1 protein during different physiological conditions that could contribute to the improvement in sperm selection techniques.

Analysis of intracellular tyrosine phosphorylation in circulating neutrophils as a rapid assay for the in vivo effect of oral tyrosine kinase inhibitors

Tyrosine kinases are crucial signaling components of diverse biological processes and are major therapeutic targets in various malignancies and immune-mediated disorders. A critical step of development of novel tyrosine kinase inhibitors is the transition from the confirmation of the in vitro effects of drug candidates to the analysis of their in vivo efficacy. To facilitate this transition, we have developed a rapid in vivo assay for the analysis of the effect of oral tyrosine kinase inhibitors on basal tyrosine phosphorylation of circulating mouse neutrophils. The assay uses a single drop of peripheral blood without sacrificing the mice. Flow cytometry using intracellular staining by fluorescently labeled anti-phosphotyrosine antibodies revealed robust basal tyrosine phosphorylation in resting circulating neutrophils. This signal was abrogated by the use of isotype control antibodies or by pre-saturation of the anti-phosphotyrosine antibodies with soluble phosphotyrosine amino acids or tyrosine-phosphorylated peptides. Basal tyrosine phosphorylation was dramatically reduced in neutrophils of triple knockout mice lacking the Src-family tyrosine kinases Hck, Fgr, and Lyn. Neutrophil tyrosine phosphorylation was also abrogated by oral administration of the Abl/Src-family inhibitor dasatinib, a clinically used anti-leukemic agent. Detailed dose-response and kinetic studies revealed half-maximal reduction of neutrophil tyrosine phosphorylation by 2.9 mg/kg dasatinib, with maximal reduction observed 2 h after inhibitor administration. Taken together, our assay allows highly efficient analysis of the in vivo effect of orally administered tyrosine kinase inhibitors, and may be used as a suitable alternative to other existing approaches.

Molecular Chaperone HSPA2 Distribution During Hyaluronic Acid Selection in Human Sperm

During fertilization, sperm hyaluronidase activity is essential for spermatozoa to successfully penetrate the hyaluronic acid-enriched extracellular matrix of the cumulus cells. Since molecular chaperones, as the heat shock protein A2, are typically involved in bringing hyaluronic acid receptors to the cell surface, here we evaluated the presence and spatial location of HSPA2 on human spermatozoa based on its hyaluronic acid binding capacity. This study included 16 normozoospermic sperm samples from volunteering donors. The location of HSPA2 was studied in cells before and after 1-h incubation under capacitating conditions, as well as in spermatozoa selected according to their ability of binding to hyaluronic acid. Our results showed no significant differences in HSPA2 immunofluorescent cells before and after 1 h of incubation in capacitating conditions. Nevertheless, after hyaluronic acid selection, the percentage of HSPA2-labelled cells increased significantly, indicating that the interaction with hyaluronic acid may induce the unmasking of HSPA2 epitopes. Furthermore, after swim-up and hyaluronic acid selection, spermatozoa presented a highly immunostained equatorial band with a homogeneous fluorescence throughout the acrosomal region. This distribution has been previously suggested to have important implications in male fertility. Noteworthy, a homogeneous fluorescence among the acrosomal region with a more intense labelling at the apical region was observed only in hyaluronic acid bound sperm cells, which may be associated with primary gamete recognition. Our findings suggest that the hyaluronic acid selection technique and HSPA2 biomarker should be considered candidates to complement the classic seminal analysis before recommending an appropriate assisted reproduction technique.

The stallion sperm acrosome: Considerations from a research and clinical perspective

During the fertilization process, the interaction between the sperm and the oocyte is mediated by a process known as acrosomal exocytosis (AE). Although the role of the sperm acrosome on fertilization has been studied extensively over the last 70 years, little is known about the molecular mechanisms that govern acrosomal function, particularly in species other than mice or humans. Even though subfertility due to acrosomal dysfunction is less common in large animals than in humans, the evaluation of sperm acrosomal function should be considered not only as a complementary but a routine test when individuals are selected for breeding potential. This certainly holds true for stallions, which might display lower levels of fertility in the face of "acceptable" sperm quality parameters determined by conventional sperm assays. Nowadays, the use of high throughput technologies such as flow cytometry or mass spectrometry-based proteomic analysis is commonplace in the research arena. Such techniques can also be implemented in clinical scenarios of males with "idiopathic" subfertility. The current review focuses on the sperm acrosome, with particular emphasis on the stallion. We aim to describe the physiological events that lead to the acrosome formation within the testis, the role of very specific acrosomal proteins during AE, the methods to study the occurrence of AE under in vitro conditions, and the potential use of molecular biology techniques to discover new markers of acrosomal function and subfertility associated with acrosomal dysfunction in stallions.

The sirtuin 1 activator YK 3-237 stimulates capacitation-related events in human spermatozoa

RESEARCH QUESTION

Does sirtuin-1 (SIRT1) have a role in the human spermatozoa capacitation process?

DESIGN

Human spermatozoa were incubated for 6 h in a capacitating medium in presence or absence of the specific SIRT1 activator, YK 3-237. Several sperm parameters were determined by flow cytometry: viability, acrosome reaction and mitochondria membrane status. Sperm motility was determined objectively by computer-assisted semen analysis. Sperm capacitation status was evaluated by the extent of protein tyrosine phosphorylation and by the percentage of spermatozoa with the acrosome reacted by a calcium ionophore challenge.

RESULTS

SIRT1 was detected in the connecting piece of human spermatozoa where a lysine acetylation pattern was mainly found along the sperm tail. SIRT1 activation accelerates the occurrence of a phenotype associated with human sperm capacitation, with no differences seen in the lysine acetylation pattern. After 1 h of co-incubation of YK 3-237 with human spermatozoa, tyrosine phosphorylation levels were comparable to control levels after 6 h of incubation in capacitating conditions. In addition, the activator improved sperm responsiveness to a Ca²⁺ ionophore (A23187) challenge determined by an increase in acrosome-reacted spermatozoa (P = 0.025). Importantly, sperm viability and mitochondrial activity-related parameters assessed by flow cytometry were not affected by YK 3-237.

CONCLUSION

YK 3-237 induces capacitation-related events in human spermatozoa such an increase of tyrosine phosphorylation levels and acrosome-reacted spermatozoa after the ionophore challenge. Together, these results show that YK 3-237 affects human spermatozoa capacitation-related events by a mechanism independent of protein lysine acetylation but dependent on bicarbonate and calcium.

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.

Elevated and Sustained Intracellular Calcium Signalling Is Necessary for Efficacious Induction of the Human Sperm Acrosome Reaction

Progesterone and prostaglandin E1 are postulated to trigger the human sperm acrosome reaction (AR). However, their reported efficacy is very variable which likely, in part, reflects the plethora of experimental conditions and methodologies used to detect this physiologically relevant event. The purpose of this study was to develop an assay for the robust induction and objective measurement of the complete AR. Sperm from healthy volunteers or patients undertaking IVF were treated with a variety of ligands (progesterone, prostaglandin E1 or NH4Cl, alone or in combinations). AR, motility and intracellular calcium measurements were measured using flow cytometry, computer-assisted sperm analysis (CASA) and fluorimetry, respectively. The AR was significantly increased by the simultaneous application of progesterone, prostaglandin E1 and NH4Cl, following an elevated and sustained intracellular calcium concentration. However, we observed notable inter- and intra-donor sample heterogeneity of the AR induction. When studying the patient samples, we found no relationship between the IVF fertilization rate and the AR. We conclude that progesterone and prostaglandin E1 alone do not significantly increase the percentage of live acrosome-reacted sperm. This assay has utility for drug discovery and sperm toxicology studies but is not predictive for IVF success.

Bovine ICSI: limiting factors, strategies to improve its efficiency and alternative approaches

Intracytoplasmic sperm injection (ICSI) is an assisted reproductive technique mainly used to overcome severe infertility problems associated with the male factor, but in cattle its efficiency is far from optimal. Artificial activation treatments combining ionomycin (Io) with 6-dimethylaminopurine after piezo-ICSI or anisomycin after conventional ICSI have recently increased the blastocyst rate obtained. Compounds to capacitate bovine spermatozoa, such as heparin and methyl-β-cyclodextrin and compounds to destabilize sperm membranes such as NaOH, lysolecithin and Triton X-100, have been assessed, although they have failed to substantially improve post-ICSI embryonic development. Disulfide bond reducing agents, such as dithiothreitol (DTT), dithiobutylamine and reduced glutathione, have been assessed to decondense the hypercondensed head of bovine spermatozoa, the two latter being more efficient than DTT and less harmful. Although piezo-directed ICSI without external activation has generated high fertilization rates and modest rates of early embryo development, other studies have required exogenous activation to improve the results. This manuscript thoroughly reviews the different strategies used in bovine ICSI to improve its efficiency and proposes some alternative approaches, such as the use of extracellular vesicles (EVs) as 'biological methods of oocyte activation' or the incorporation of EVs in the in vitro maturation and/or culture medium as antioxidant defence agents to improve the competence of the ooplasm, as well as a preincubation of the spermatozoa in estrous oviductal fluid to induce physiological capacitation and acrosome reaction before ICSI, and the use of hyaluronate in the sperm immobilization medium.

Acrosomal alkalinization occurs during human sperm capacitation

Mammalian sperm capacitation is a prerequisite for successful fertilization. Capacitation involves biochemical and physiological modifications of sperm as they travel through the female reproductive tract. These modifications prepare the sperm to undergo the acrosome reaction (AR), an acrosome vesicle exocytosis that is necessary for gamete fusion. Capacitation requires an increase in both intracellular calcium ([Ca2+]i) and pH (pHi). Mouse sperm capacitation is accompanied by acrosomal alkalinization and artificial elevation of the acrosome pH (pHa) is sufficient to trigger the AR in mouse and human sperm, but it is unknown if pHa increases naturally during human sperm capacitation. We used single-cell imaging and image-based flow cytometry to evaluate pHa during capacitation and its regulation. We found that pHa progressively increases during capacitation. The V-ATPase, which immunolocalized to the acrosome and equatorial segment, is mainly responsible for the acidity of the acrosome. It is likely that the regulation of V-ATPase is at least in part responsible for the progressive increase in pHa during capacitation. Acrosome alkalinization was dependent on extracellular HCO3- and Ca2+. Inhibition of the HCO3--dependent adenylyl cyclase and protein kinase A induced significant pHa changes. Overall, alkalinization of the acrosome may be a key step in the path towards the AR.

Impact of Cryopreservation on Motile Subpopulations and Tyrosine-Phosphorylated Regions of Ram Spermatozoa during Capacitating Conditions

Simple Summary

Spermatozoa go through diverse changes to achieve their fertilizing potential (capacitation) and develop a specific motility pattern (hyperactivation). However, to ensure a greater reproductive success, not all the spermatozoa present in an ejaculate react equally or at the same time. Therefore, a comparative analysis was performed in the present study to improve our current understanding about how cryopreservation may affect the heterogeneous nature of fresh ejaculates during these two events. Among the four motile sperm subpopulations identified in fresh and frozen-thawed ram semen, one of them developed a hyperactivated-like movement and was the main group involve in those changes associated with sperm capacitation based on the marked increase and the positive correlation with mitochondrial activity and tyrosine phosphorylation, two relevant parameters that usually increase during capacitation. In addition, cryopreservation altered the distribution of the motile sperm subpopulations. Although the subpopulation with hyperactivated-like movement increased at the beginning of incubation in frozen-thawed samples, this subpopulation together with the subpopulation of rapid and progressive spermatozoa were replaced after a prolonged incubation by the subpopulation of slow spermatozoa with the lowest mitochondrial activity, which clearly indicate the reduction in sperm quality. These findings will aid to optimize the current cryopreservation and in vitro fertilization protocols.

Abstract

The heterogeneous nature of ejaculates highlights the relevance of studying the behavior of different sperm subpopulations. Changes in sperm motility and the increase in tyrosine phosphorylation are key events that usually occur during capacitation and can be modified by the cryopreservation process. However, the relationship between both events remains poorly defined throughout capacitation in the different sperm subpopulations. Fresh and frozen-thawed spermatozoa were incubated in capacitating (CAP) and non-capacitating (NC) media up to 240 min. Sperm kinematics, tyrosine phosphorylation and mitochondrial activity were measured by the CASA system and imaging flow cytometry. Four motile sperm subpopulations (SP) were identified in fresh and frozen-thawed ram semen after the cluster analysis. Incubation under CAP conditions over time led to greater changes in the percentage of spermatozoa included in each subpopulation compared to NC conditions, being different between fresh and frozen-thawed spermatozoa. The SP1, characterized by slow spermatozoa, progressively increased after 15 min in frozen-thawed samples incubated in both media but not in fresh ones. The SP4, characterized by fast and non-linear spermatozoa, showed a marked increase during CAP, but not under NC conditions, occurring more rapidly in frozen-thawed spermatozoa. This subpopulation (SP4) was also the only one positively and strongly correlated with mitochondrial activity and all phosphorylated sperm regions during capacitation, either in fresh or frozen-thawed samples. Our results indicated that in vitro capacitation induced significant changes in the distribution of motile sperm subpopulations, being affected by cryopreservation. Notwithstanding, the subpopulation which probably represents hyperactivated-like spermatozoa (SP4) also increased in frozen-thawed samples, occurring faster and simultaneously to the increment of mitochondrial activity and tyrosine phosphorylation of different sperm regions.

Comparative Analysis of piRNA Profiles Helps to Elucidate Cryoinjury Between Giant Panda and Boar Sperm During Cryopreservation

Cryopreservation induces sperm cryoinjuries, including physiological and functional changes. However, the molecular mechanisms of sperm cryoinjury and cryoresistance are still unknown. Cryoresistance or the freeze tolerance of sperm varies across species, and boar sperm is more susceptible to cold stress. Contrary to boar sperm, giant panda sperm appears to be strongly freeze-tolerant and is capable of surviving repeated cycles of freeze-thawing. In this study, differentially expressed (DE) PIWI-interacting RNAs (piRNAs) of fresh and frozen-thawed sperm with different freeze tolerance capacity from giant panda and boar were evaluated. The results showed that 1,160 (22 downregulated and 1,138 upregulated) and 384 (110 upregulated and 274 downregulated) DE piRNAs were identified in giant panda and boar sperm, respectively. Gene ontology (GO) enrichment analysis revealed that the target DE messenger RNAs (mRNAs) of DE piRNAs were mainly enriched in biological regulation, cellular, and metabolic processes in giant panda and boar sperm. Moreover, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated that the target DE mRNAs of DE piRNAs were only distributed in DNA replication and the cyclic adenosine monophosphate (cAMP) signaling pathway in giant panda, but the cAMP, cyclic guanosine monophosphate (cGMP), and mitogen-activated protein kinase (MAPK) signaling pathways in boar sperm were considered as part of the olfactory transduction pathway. In conclusion, we speculated that the difference in the piRNA profiles and the DE piRNAs involved in the cAMP signaling pathway in boar and giant panda may have contributed to the different freeze tolerance capacities between giant panda and boar sperm, which helps to elucidate the molecular mechanism behind sperm cryoinjury and cryoresistance.

[Localization of Arylsulfatase A during in vitro incubation of human spermatozoa in capacitation media]

OBJECTIVE

To determine the localization and distribution of the ArylsulfataseA receptor (ARSA) in human spermatozoa before and after their incubation in capacitation medium for 1 and 4hours.

MATERIAL AND METHODS

Semen samples were obtained from five normozoospermic donors. Capacitation was by swim-up technique using capacitation medium for 1 and 4hours. Localization of the ARSA receptor was assessed by indirect immunofluorescence using confocal microscopy. A minimum of 200cells were observed in each physiological condition.

RESULTS

Before incubation, no representative pattern was observed among the cells positive for this biomarker (8.61%). This percentage increased significantly after incubation in the capacitation medium for 1 and 4hours (61.86% and 63.38% respectively). A majority pattern was observed among the capacitated cells, with intense labelling in the acrosomal region (27.11% and 28.20% after 1 and 4hours respectively). It should be noted that the pattern corresponding to fluorescence at the level of the periacrosomal region was not observed in the spermatozoa prior to incubation. Only after incubation in capacitation medium for 1 and 4hours, 9.13% and 12.78% of cells with such distribution were detected.

CONCLUSIONS

In vitro capacitation, regardless of time, favours the immunolocalization of ARSA in the cephalic region of the spermatozoa. The most representative subpopulation after this process was the one in which ARSA was intensely and homogeneously distributed in the acrosome region, involved in primary gamete recognition.

Sperm ion channels and transporters in male fertility and infertility

Mammalian sperm cells must respond to cues originating from along the female reproductive tract and from the layers of the egg in order to complete their fertilization journey. Dynamic regulation of ion signalling is, therefore, essential for sperm cells to adapt to their constantly changing environment. Over the past 15 years, direct electrophysiological recordings together with genetically modified mouse models and human genetics have confirmed the importance of ion channels, including the principal Ca²⁺-selective plasma membrane ion channel CatSper, for sperm activity. Sperm ion channels and membrane receptors are attractive targets for both the development of contraceptives and infertility treatment drugs. Furthermore, in this era of assisted reproductive technologies, understanding the signalling processes implicated in defective sperm function, particularly those arising from genetic abnormalities, is of the utmost importance not only for the development of infertility treatments but also to assess the overall health of a patient and his children. Future studies to improve reproductive health care and overall health care as a function of the ability to reproduce should include identification and analyses of gene variants that underlie human infertility and research into fertility-related molecules.

Human sperm chaperone HSPA2 distribution during in vitro capacitation

Human fertilization success depends on the ability of the spermatozoa to undergo capacitation. Even though this process can be conducted in vitro, the optimal time for a sperm cell to complete capacitation in vitro is still under discussion due to the lack of proper capacitation biomarkers. Here, we evaluated the influence of in vitro capacitation time on HSPA2 distribution over human sperm head testing this chaperone as a potential capacitation biomarker. The chaperone was assessed in human spermatozoa from 16 normozoospermic donors using indirect immunofluorescence in uncapacitated, one and four-hour capacitated spermatozoa. The percentage of HSPA2 immunofluorescent cells before and after one hour of capacitation did not differ significantly. However, after four hours of capacitation, we observed a significantly higher percentage of HSPA2 labelled cells. In fluorescent cells analysed before capacitation, we could not identify a predominant distribution pattern. Meanwhile, after capacitation, most sperm showed a highly labelled equatorial band accompanied by a homogeneous fluorescence throughout the acrosomal region. Our findings suggest that HSPA2 needs more than one hour of in vitro capacitation for being correctly distributed in the anterior region of the sperm head. In conclusion, the present study provides solid evidences for the utility of HSPA2 as a biomarker of human sperm in vitro capacitation. Due to its importance during egg-sperm recognition, the use of HSPA2 as a biomarker before an artificial reproduction technique may be suggested, in addition to a longer capacitation time during sperm preparation.

Progesterone, spermatozoa and reproduction: An updated review

The rapid effects of steroids on spermatozoa have been demonstrated for the first time more than three decades ago. Progesterone (P), which is present throughout the female genital tract with peaks of levels in the cumulus matrix surrounding the oocyte, has been shown to stimulate several sperm functions in vitro, including capacitation, hyperactivation, chemotaxis and acrosome reaction (AR). Besides an increase of intracellular calcium, P has been shown to activate other sperm signalling pathways including tyrosine phosphorylation of several sperm proteins. All these effects are mediated by extra-nuclear pathways likely involving interaction with molecules present on the sperm surface. In particular, the increase in intracellular calcium ([Ca²⁺]_i) in spermatozoa from human and several other mammalian species is mediated by the sperm specific calcium channel CatSper, whose expression and function are required for sperm hyperactive motility. P-mediated CatSper activation is indeed involved in promoting sperm hyperactivation, but the involvement of this channel in other P-stimulated sperm functions, such as AR and chemotaxis, is less clear and further studies are required to disclose all the involved pathways. In human spermatozoa, responsiveness to P in terms of [Ca²⁺]_i increase and AR is highly related to sperm fertilizing ability in vitro, suggesting that the steroid is a physiological inducer of AR during in vitro fertilization. In view of their physiological relevance, P-stimulated sperm functions are currently investigated to develop new tools to select highly performant spermatozoa for assisted reproduction.

Does the Pre-Ovulatory Pig Oviduct Rule Sperm Capacitation In Vivo Mediating Transcriptomics of Catsper Channels?

Spermatozoa need to conduct a series of biochemical changes termed capacitation in order to fertilize. In vivo, capacitation is sequentially achieved during sperm transport and interaction with the female genital tract, by mechanisms yet undisclosed in detail. However, when boar spermatozoa are stored in the tubal reservoir pre-ovulation, most appear to be in a non-capacitated state. This study aimed at deciphering the transcriptomics of capacitation-related genes in the pig pre-ovulatory oviduct, following the entry of semen or of sperm-free seminal plasma (SP). Ex-vivo samples of the utero-tubal junction (UTJ) and isthmus were examined with a microarray chip (GeneChip^® Porcine Gene 1.0 ST Array, Thermo Fisher Scientific) followed by bioinformatics for enriched analysis of functional categories (GO terms) and restrictive statistics. The results confirmed that entry of semen or of relative amounts of sperm-free SP modifies gene expression of these segments, pre-ovulation. It further shows that enriched genes are differentially associated with pathways relating to sperm motility, acrosome reaction, single fertilization, and the regulation of signal transduction GO terms. In particular, the pre-ovulation oviduct stimulates the Catsper channels for sperm Ca²⁺ influx, with AKAPs, CATSPERs, and CABYR genes being positive regulators while PKIs and CRISP1 genes appear to be inhibitors of the process. We postulate that the stimulation of PKIs and CRISP1 genes in the pre-ovulation sperm reservoir/adjacent isthmus, mediated by SP, act to prevent premature massive capacitation prior to ovulation.

Assessments of sperm quality integrating morphology, swimming patterns, bioenergetics and cell signalling

Spermatozoa are diverse in form and function and these differences impact on their fertilizing capacity. Because of considerable inter-male and inter-species differences in sperm traits, assessments of sperm quality demand that we consider variations at different levels. We should thus pay attention not only to average values but also intra- and inter-sperm population variations and subpopulation structure. Sperm shape and size evolve in reponse to postcopulatory sexual selection. Assessments of morphological variation, with conventional microscopy or with computer-assisted systems, should bear this in mind. In rodents sperm head shape is asymmetric so it requires more complex tools, such as geometric morphometrics. Sperm function also evolves under postcopulatory sexual selection and this could be used as a basis to assess sperm performance. Sperm cells swim actively to overcome barriers in the female tract and develop a peculiar motility pattern in the final stages prior to and during fertilization. Both types of movement can be analyzed by computer-assisted microscopy systems. Sperm have high energetic demands for cell homeostasis, motility, and signalling. Bioenergetics can be analyzed by various means, including extracellular flux analyses to characterize glycolysis and mitochondrial respiration. Finally, cell signalling during capacitation has received much attention and can be assessed by microscopy (conventional or computer-assisted) or flow cytometry. Recent advances in image-flow cytometry affords analyses of high cell numbers with spatial localization of subcellular changes, which will have a big impact in the development of functional tests for the andrology clinic and in sperm preservation and use in artificial insemination.

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.

A connection between reversible tyrosine phosphorylation and SNARE complex disassembly activity of N-ethylmaleimide-sensitive factor unveiled by the phosphomimetic mutant N-ethylmaleimide-sensitive factor-Y83E

N-ethylmaleimide-sensitive factor (NSF) disassembles fusion-incompetent cis soluble-NSF attachment protein receptor (SNARE) complexes making monomeric SNAREs available for subsequent trans pairing and fusion. In most cells the activity of NSF is constitutive, but in Jurkat cells and sperm it is repressed by tyrosine phosphorylation; the phosphomimetic mutant NSF–Y83E inhibits secretion in the former. The questions addressed here are if and how the NSF mutant influences the configuration of the SNARE complex. Our model is human sperm, where the initiation of exocytosis (acrosome reaction (AR)) de-represses the activity of NSF through protein tyrosine phosphatase 1B (PTP1B)-mediated dephosphorylation. We developed a fluorescence microscopy-based method to show that capacitation increased, and challenging with an AR inducer decreased, the number of cells with tyrosine-phosphorylated PTP1B substrates in the acrosomal domain. Results from bioinformatic and biochemical approaches using purified recombinant proteins revealed that NSF–Y83E bound PTP1B and thereupon inhibited its catalytic activity. Mutant NSF introduced into streptolysin O-permeabilized sperm impaired cis SNARE complex disassembly, blocking the AR; subsequent addition of PTP1B rescued exocytosis. We propose that NSF–Y83E prevents endogenous PTP1B from dephosphorylating sperm NSF, thus maintaining NSF’s activity in a repressed mode and the SNARE complex unable to dissociate. The contribution of this paper to the sperm biology field is the detection of PTP1B substrates, one of them likely being NSF, whose tyrosine phosphorylation status varies during capacitation and the AR. The contribution of this paper to the membrane traffic field is to have generated direct evidence that explains the dominant-negative role of the phosphomimetic mutant NSF–Y83E.

Boar semen improvement through sperm capacitation management, with emphasis on zinc ion homeostasis

Critical to fertilization success, sperm capacitation within the female oviductal sperm reservoir endows mammalian spermatozoa with hyperactivated motility and capacity to fertilize. An elaborate cascade of signaling events during capacitation guides the redistribution of sperm plasma membrane seminolipid and cholesterol, Ca-influx and increases tyrosine phosphorylation to promote hyperactivated motility. Such events result in the remodeling of the sperm acrosome, increased fluidity and fusability of the plasma membrane, shedding of surface-adsorbed seminal plasma proteins that glue sperm heads to the oviductal epithelium and ultimately the release of hyperactivated spermatozoa from the oviductal sperm reservoir. Discovered recently, the capacitation-induced sperm zinc ion efflux and resultant zinc signatures are reflective of sperm capacitation status and fertilizing ability, inspiring the retrospection of zinc ion functions in the physiology and fertility of boar sperm and that of other species. This review also highlights the merit of the domestic boar as a biomedical model for spermatology and fertilization research. Relevant to the quest for better fertility management in the livestock industries, the benefits of zinc ion supplementation through nutrition and direct addition to extended semen are discussed in the context of artificial insemination (AI). Ideas are shared on future technologies for zinc management in AI doses and research on the sperm zinc-interacting proteome.

Sperm physiology varies according to ultradian and infradian rhythms

The spermatozoon must be physiologically prepared to fertilize the egg, process called capacitation. Human sperm samples are heterogeneous in their ability to capacitate themselves, which leads to variability between samples from the same or different donors, and even along the seasons. Here we studied sperm variation in the capacitation state according to the ability of capacitated spermatozoa to acrosome react upon stimulation (% ARi) and to be recruited by chemotaxis (% Chex). Both indirect indicators of sperm capacitation increased along the incubation time with fluctuations. Those capacitated sperm recruited by chemotaxis showed an ultradian rhythm with a cycle every 2 h, which might be influenced by unknown intrinsic sperm factors. Two infradian rhythms of 12 months for the % ARi and of 6 months for % Chex were observed, which are associated with the joint action of temperature and photoperiod. Thus, to avoid false negative results, human sperm samples are recommended to be incubated for a long period (e.g. 18 h) preferably in spring time. This innovative point of view would lead to better comprehend human reproductive biology and to think experimental designs in the light of sperm cyclicity or to improve sperm aptitude for clinical purposes.

Tyrosine phosphorylation signaling regulates Ca2+ entry by affecting intracellular pH during human sperm capacitation

Capacitation is a mandatory process for the acquisition of mammalian sperm fertilization competence and involves the activation of a complex and still not fully understood system of signaling pathways. Under in vitro conditions, there is an increase in both protein tyrosine phosphorylation (pTyr) and intracellular Ca²⁺ levels in several species. In human sperm, results from our group revealed that pTyr signaling can be blocked by inhibiting proline-rich tyrosine kinase 2 (PYK2). Based on the role of PYK2 in other cell types, we investigated whether the PYK2-dependent pTyr cascade serves as a sensor for Ca ²⁺ signaling during human sperm capacitation. Flow cytometry studies showed that exposure of sperm to the PYK2 inhibitor N-[2-[[[2-[(2,3-dihydro-2-oxo-1 H-indol-5-yl)amino]-5-(trifluoromethyl)-4-pyrimidinyl]amino]methyl]phenyl]- N-methyl-methanesulfonamide hydrate (PF431396) produced a significant and concentration-dependent reduction in intracellular Ca ²⁺ levels during capacitation. Further studies revealed that PF431396-treated sperm exhibited a decrease in the activity of CatSper, a key sperm Ca ²⁺ channel. In addition, time course studies during capacitation in the presence of PF431396 showed a significant and sustained decrease in both intracellular Ca ²⁺ and pH levels after 2 hr of incubation, temporarily coincident with the activation of PYK2 during capacitation. Interestingly, decreases in Ca ²⁺ levels and progressive motility caused by PF431396 were reverted by inducing intracellular alkalinization with NH ₄ Cl, without affecting the pTyr blockage. Altogether, these observations support pTyr as an intracellular sensor for Ca ²⁺ entry in human sperm through regulation of cytoplasmic pH. These results contribute to a better understanding of the modulation of the polymodal CatSper and signaling pathways involved in human sperm capacitation.

Molecular Basis of Human Sperm Capacitation

In the early 1950s, Austin and Chang independently described the changes that are required for the sperm to fertilize oocytes in vivo. These changes were originally grouped under name of “capacitation” and were the first step in the development of in vitro fertilization (IVF) in humans. Following these initial and fundamental findings, a remarkable number of observations led to characterization of the molecular steps behind this process. The discovery of certain sperm-specific molecules and the possibility to record ion currents through patch-clamp approaches helped to integrate the initial biochemical observation with the activity of ion channels. This is of particular importance in the male gamete due to the fact that sperm are transcriptionally inactive. Therefore, sperm must control all these changes that occur during their transit through the male and female reproductive tracts by complex signaling cascades that include post-translational modifications. This review is focused on the principal molecular mechanisms that govern human sperm capacitation with particular emphasis on comparing all the reported pieces of evidence with the mouse model.

Only a subpopulation of mouse sperm displays a rapid increase in intracellular calcium during capacitation

Mammalian sperm must undergo a functionally defined process called capacitation to be able to fertilize oocytes. They become capacitated in vivo by interacting with the female reproductive tract or in vitro in a defined capacitation medium that contains bovine serum albumin, calcium (Ca²⁺ ), and bicarbonate (HCO₃ ^- ). In this work, sperm were double stained with propidium iodide and the Ca²⁺ dye Fluo-4 AM and analyzed by flow cytometry to determine changes in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in individual live sperm. An increase in [Ca²⁺ ]_i was observed in a subpopulation of capacitated live sperm when compared with noncapacitated ones. Sperm exposed to the capacitating medium displayed a rapid increase in [Ca²⁺ ]_i within 1 min of incubation, which remained sustained for 90 min. These rise in [Ca²⁺ ]_i after 90 min of incubation in the capacitating medium was evidenced by an increase in the normalized median fluorescence intensity. This increase was dependent on the presence of extracellular Ca²⁺ and, at least in part, reflected the contribution of a new subpopulation of sperm with higher [Ca²⁺ ]_i . In addition, it was determined that the capacitation-associated [Ca²⁺ ]_i increase was dependent of CatSper channels, as sperm derived from CatSper knockout (CatSper KO) or incubated in the presence of CatSper inhibitors failed to increase [Ca²⁺ ]_i . Surprisingly, a minimum increase in [Ca²⁺ ]_i was also observed in CatSper KO sperm suggesting the existence of other Ca²⁺ transport systems. Altogether, these results indicate that a subpopulation of sperm increases [Ca²⁺ ]_i very rapidly during capacitation mainly due to a CatSper-mediated influx of extracellular Ca²⁺ .

Regulation mechanisms and implications of sperm membrane hyperpolarization

Mammalian sperm are unable to fertilize the egg immediately after ejaculation. In order to gain fertilization competence, they need to undergo a series of biochemical and physiological modifications inside the female reproductive tract, known as capacitation. Capacitation correlates with two essential events for fertilization: hyperactivation, an asymmetric and vigorous flagellar motility, and the ability to undergo the acrosome reaction. At a molecular level, capacitation is associated to: phosphorylation cascades, modification of membrane lipids, alkalinization of the intracellular pH, increase in the intracellular Ca²⁺ concentration and hyperpolarization of the sperm plasma membrane potential. Hyperpolarization is a crucial event in capacitation since it primes the sperm to undergo the exocytosis of the acrosome content, essential to achieve fertilization of the oocyte.