An alkaline follicular fluid fraction induces capacitation and limited release of oviduct epithelium-bound stallion sperm

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.

A stallion spermatozoon's journey through the mare's genital tract: In vivo and in vitro aspects of sperm capacitation

Conventional in vitro fertilization is not efficacious when working with equine gametes. Although stallion spermatozoa bind to the zona pellucida in vitro, these gametes fail to initiate the acrosome reaction in the vicinity of the oocyte and cannot, therefore, penetrate into the perivitelline space. Failure of sperm penetration most likely relates to the absence of optimized in vitro fertilization media containing molecules essential to support stallion sperm capacitation. In vivo, the female reproductive tract, especially the oviductal lumen, provides an environmental milieu that appropriately regulates interactions between the gametes and promotes fertilization. Identifying these 'fertilization supporting factors' would be a great contribution for development of equine in vitro fertilization media. In this review, a description of the current understanding of the interactions stallion spermatozoa undergo during passage through the female genital tract, and related specific molecular changes that occur at the sperm plasma membrane is provided. Understanding these molecular changes may hold essential clues to achieving successful in vitro fertilization with equine gametes.

Developing a reproducible protocol for culturing functional confluent monolayers of differentiated equine oviduct epithelial cells

We describe the development of two methods for obtaining confluent monolayers of polarized, differentiated equine oviduct epithelial cells (EOEC) in Transwell inserts and microfluidic chips. EOECs from the ampulla were isolated post-mortem and seeded either (1) directly onto a microporous membrane as differentiated EOECs (direct seeding protocol) or (2) first cultured to a confluent de-differentiated monolayer in conventional wells, then trypsinized and seeded onto a microporous membrane (re-differentiation protocol). Maintenance or induction of EOEC differentiation in these systems was achieved by air-liquid interface introduction. Monolayers cultured via both protocols were characterized by columnar, cytokeratin 19-positive EOECs in Transwell inserts. However, only the re-differentiation protocol could be transferred successfully to the microfluidic chips. Integrity of the monolayers was confirmed by transepithelial resistance measurements, tracer flux and the demonstration of an intimate network of tight junctions. Using the direct protocol, 28% of EOECs showed secondary cilia at the apical surface in a diffuse pattern. In contrast, re-differentiated polarized EOECs rarely showed secondary cilia in either culture system (>90% of the monolayers showed <1% ciliated EOECs). Occasionally (5-10%), re-differentiated monolayers with 11-27% EOECs with secondary cilia in a diffuse pattern were obtained. Additionally, nuclear progesterone receptor expression was found to be inhibited by simulated luteal phase hormone concentrations, and sperm binding to cilia was higher for re-differentiated EOEC monolayers exposed to estrogen-progesterone concentrations mimicking the follicular rather than luteal phase. Overall, a functional equine oviduct model was established with close morphological resemblance to in vivo oviduct epithelium.

The Presence of D-Penicillamine during the In Vitro Capacitation of Stallion Spermatozoa Prolongs Hyperactive-Like Motility and Allows for Sperm Selection by Thermotaxis

Assisted reproductive technologies (ARTs) in the horse still yield suboptimal results in terms of pregnancy rates. One of the reasons for this is the lack of optimal conditions for the sperm capacitation in vitro. This study assesses the use of synthetic human tubal fluid (HTF) supplemented with D-penicillamine (HTF + PEN) for the in vitro capacitation of frozen/thawed stallion spermatozoa by examining capacitation-related events over 180 min of incubation. Besides these events, we explored the in vitro capacity of the spermatozoa to migrate by thermotaxis and give rise to a population of high-quality spermatozoa. We found that HTF induced higher levels of hyperactive-like motility and protein tyrosine phosphorylation (PTP) compared to the use of a medium commonly used in this species (Whitten's). Also, HTF + PEN was able to maintain this hyperactive-like motility, otherwise lost in the absence of PEN, for 180 min, and also allowed for sperm selection by thermotaxis in vitro. Remarkably, the selected fraction was enriched in spermatozoa showing PTP along the whole flagellum and lower levels of DNA fragmentation when compared to the unselected fraction (38% ± 11% vs 4.4% ± 1.1% and 4.2% ± 0.4% vs 11% ± 2% respectively, t-test p < 0.003, n = 6). This procedure of in vitro capacitation of frozen/thawed stallion spermatozoa in HTF + PEN followed by in vitro sperm selection by thermotaxis represents a promising sperm preparation strategy for in vitro fertilization and intracytoplasmic sperm injection in this species.

Update on mammalian sperm capacitation: how much does the horse differ from other species?

In contrast to various other mammalian species, conventional in vitro fertilization (IVF) with horse gametes is not reliably successful. In particular, stallion spermatozoa fails to penetrate the zona pellucida, most likely due to incomplete activation of stallion spermatozoa (capacitation) under in vitro conditions. In other mammalian species, specific capacitation triggers have been described; unfortunately, none of these is able to induce full capacitation in stallion spermatozoa. Nevertheless, knowledge of capacitation pathways and their molecular triggers might improve our understanding of capacitation-related events observed in stallion sperm. When sperm cells are exposed to appropriate capacitation triggers, several molecular and biochemical changes should be induced in the sperm plasma membrane and cytoplasm. At the level of the sperm plasma membrane, (1) an increase in membrane fluidity, (2) cholesterol depletion and (3) lipid raft aggregation should occur consecutively; the cytoplasmic changes consist of protein tyrosine phosphorylation and elevated pH, cAMP and Ca2+ concentrations. These capacitation-related events enable the switch from progressive to hyperactivated motility of the sperm cells, and the induction of the acrosome reaction. These final capacitation triggers are indispensable for sperm cells to migrate through the viscous oviductal environment, penetrate the cumulus cells and zona pellucida and, finally, fuse with the oolemma. This review will focus on molecular aspects of sperm capacitation and known triggers in various mammalian species. Similarities and differences with the horse will be highlighted to improve our understanding of equine sperm capacitation/fertilizing events.

Stage-specific metabolomic changes in equine oviductal fluid: New insights into the equine fertilization environment

A repeatable protocol for equine in vitro fertilization (IVF) has remained elusive. This is likely, in part, due to suboptimal composition of capacitation or IVF media that are currently in use. Hence, we aimed to analyse the metabolome of equine oviductal fluid (OF) at the pre- (PRE) and immediate post-ovulatory (PST) stages using proton magnetic resonance spectroscopy (¹H NMR). Oviductal fluid from eight PRE and six PST mares were used to prepare a total of five samples per group. A total of 18 metabolites were identified. The five metabolites with the highest concentrations in the OF samples were lactate, myoinositol, creatine, alanine and carnitine. Only fumarate and glycine showed significant differences in their concentrations between PRE and PST OF samples, with higher concentrations in the PST samples. In a preliminary study, stallion spermatozoa (n = 3 ejaculates) were incubated with different concentrations of PST OF from one mare (0, 0.0625, 0.125, 0.25, 0.5 or 1%; v:v). After 4 h of sperm incubation, protein tyrosine phosphorylation (PY) by western blotting, sperm motility, and acrosomal status were evaluated. An increase of PY was observed in sperm from two stallions when treated with 0.0625% and 0.125% of OF; however no change in PY was noted in the other stallion. There were no effects of OF on spermatozoa motility or acrosome status. These results provide the first information on the metabolomics of equine OF at different stages of the estrus cycle, and present the possibility that OF may affect PY in stallion spermatozoa.

pH-dependent effects of procaine on equine gamete activation†

Procaine directly triggers pH-dependent cytokinesis in equine oocytes and induces hypermotility in stallion spermatozoa, an important event during capacitation. However, procaine-induced hyperactivated motility is abolished when sperm is washed to remove the procaine prior to sperm-oocyte co-incubation. To understand how procaine exerts its effects, the external Ca2+ and Na+ and weak base activity dependency of procaine-induced hyperactivation in stallion spermatozoa was assessed using computer-assisted sperm analysis. Percoll-washed stallion spermatozoa exposed to Ca2+-depleted (+2 mM EGTA) procaine-supplemented capacitating medium (CM) still demonstrated hyperactivated motility, whereas CM without NaCl or Na+ did not. Both procaine and NH4Cl, another weak base, were shown to trigger a cytoplasmic pH increase (BCECF-acetoxymethyl (AM)), which is primarily induced by a pH rise in acidic cell organelles (Lysosensor green dnd-189), accompanied by hypermotility in stallion sperm. As for procaine, 25 mM NH4Cl also induced oocyte cytokinesis. Interestingly, hyperactivated motility was reliably induced by 2.5-10 mM procaine, whereas a significant cytoplasmic cAMP increase and tail-associated protein tyrosine phosphorylation were only observed at 10 mM. Moreover, 25 mM NH4Cl did not support the latter capacitation characteristics. Additionally, cAMP levels were more than 10× higher in boar than stallion sperm incubated under similar capacitating conditions. Finally, stallion sperm preincubated with 10 mM procaine did not fertilize equine oocytes. In conclusion, 10 mM procaine causes a cytoplasmic and acidic sperm cell organelle pH rise that simultaneously induces hyperactivated motility, increased levels of cAMP and tail-associated protein tyrosine phosphorylation in stallion spermatozoa. However, procaine-induced hypermotility is independent of the cAMP/protein tyrosine phosphorylation pathway.

Insights into soy lecithin and egg yolk-based extenders for chilling canine spermatozoa

SummaryThe aim of this study was to compare different concentrations of soy lecithin (LEC0.01%, LEC0.05% and LEC0.1%) with egg yolk (Control) in cooling extenders during the storage of semen at 5ºC for 5 days. Twelve dogs (n = 12) were selected, and semen was cooled and assessed after 2, 24, 48, 72, 96 or 120 h. At each time point, sperm were analyzed for kinetic patterns (using computer-assisted sperm analysis), mitochondrial activity (3'3- diaminobenzidine assay), lipid peroxidation (TBARS assay), DNA fragmentation (SCSA®) and plasma and acrosome membrane integrity (eosin/nigrosin and fast green/rose Bengal stains, respectively). The Control group (1814.4 ± 197.2) presented the highest rates of lipid peroxidation at 120 h. Conversely, progressive motility (42.8 ± 4%), linearity (45.4 ± 1%), and VAP (88 ± 3%) were higher in the Control group. In addition, there was lower mitochondrial activity in the Control group at 72 h. Therefore, our data show that lecithin used at these concentrations was not able to maintain sperm viability at as high qualities as would egg yolk. Moreover, the decrease in high mitochondrial activity and the persistence of sperm motility may indicate a compensatory mechanism in canine spermatozoa (i.e., glycolytic pathway). Furthermore, these higher lipid peroxidation indexes could indicate the necessity for future therapy using extenders and antioxidants over a long cooling time for dog sperm.

Zinc ion flux during mammalian sperm capacitation

Sperm capacitation, the ultimate maturation event preparing mammalian spermatozoa for fertilization, was first described in 1951, yet its regulatory mechanisms remain poorly understood. The capacitation process encompasses an influx of bicarbonate and calcium ions, removal of decapacitating factors, changes of pH and sperm proteasomal activities, and the increased protein tyrosine phosphorylation. Here, we document a novel biological phenomenon of a unique zinc (Zn²⁺) ion redistribution associated with mammalian sperm in vitro capacitation (IVC). Using image-based flow cytometry (IBFC), we identified four distinct types of sperm zinc ion distribution patterns (further zinc signature) and their changes during IVC. The zinc signature was altered after sperm capacitation, reduced by proteasomal inhibitors, removed by zinc chelators, and maintained with addition of external ZnCl₂. These findings represent a fundamental shift in the understanding of mammalian fertilization, paving the way for improved semen analysis, in vitro fertilization (IVF), and artificial insemination (AI).

During sperm capacitation physiological changes occur that are required for fertilization. Here, the authors describe four sperm zinc signatures during invitro capacitation that are indicative of sperm quality and capacity to fertilize.

Follicular fluid stimulates capacitation and acrosome reaction in alpaca sperm (Vicugna pacos)

The follicular fluid exerts an effect on the sperm capacitation of several species; however, these effects vary according to species, both in the sperm motility and in the subsequent acrosome reaction. In this study, the effect of alpaca follicular fluid (aFF) on the motility and acrosome reaction of alpaca spermatozoa was observed, using follicular fluid of three follicle sizes: small (<3 mm), medium (3-6 mm) and large (>6 mm), in a concentration of 30%. Sperm motility at the first hour of incubation with aFF of small follicles was 48.0%, with aFF of medium follicles it was 43.33% and with aFF of large follicles, it was 34.53%, while control averaged 26.00%. At the second hour, control achieved an average of 28.13%, treatment with aFF from small follicles showed an average of 46.53%, with aFF from medium follicles it was 40.00% and with aFF from large follicles it was 35.60%. The acrosome reaction after 4 hours of incubation was 30.06% for control, whereas for aFF of small follicles it was 66.3%, with aFF of medium follicles it was 58.86% and for aFF of large follicles, it was 67.63%. In the case of sperm motility, a significant difference is demonstrated for all treatments in relation to the control at the first hour, whereas only the treatments with aFF of small and medium follicles show a significant difference with respect to the control at the second hour. In the case of the acrosome reaction, all treatments with follicular fluid show a significant difference with respect to the control. It was concluded that alpaca follicular fluid favours sperm capacitation and the acrosome reaction in alpaca spermatozoa.

Why doesn't conventional IVF work in the horse? The equine oviduct as a microenvironment for capacitation/fertilization

In contrast to man and many other mammalian species, conventional in vitro fertilization (IVF) with horse gametes is not reliably successful. The apparent inability of stallion spermatozoa to penetrate the zona pellucida in vitro is most likely due to incomplete activation of spermatozoa (capacitation) because of inadequate capacitating or fertilizing media. In vivo, the oviduct and its secretions provide a microenvironment that does reliably support and regulate interaction between the gametes. This review focuses on equine sperm-oviduct interaction. Equine sperm-oviduct binding appears to be more complex than the presumed species-specific calcium-dependent lectin binding phenomenon; unfortunately, the nature of the interaction is not understood. Various capacitation-related events are induced to regulate sperm release from the oviduct epithelium and most data suggest that exposure to oviduct secretions triggers sperm capacitation in vivo However, only limited information is available about equine oviduct secreted factors, and few have been identified. Another aspect of equine oviduct physiology relevant to capacitation is acid-base balance. In vitro, it has been demonstrated that stallion spermatozoa show tail-associated protein tyrosine phosphorylation after binding to oviduct epithelial cells containing alkaline secretory granules. In response to alkaline follicular fluid preparations (pH 7.9), stallion spermatozoa also show tail-associated protein tyrosine phosphorylation, hyperactivated motility and (limited) release from oviduct epithelial binding. However, these 'capacitating conditions' are not able to induce the acrosome reaction and fertilization. In conclusion, developing a defined capacitating medium to support successful equine IVF will depend on identifying as yet uncharacterized capacitation triggers present in the oviduct.

Combined albumin and bicarbonate induces head-to-head sperm agglutination which physically prevents equine sperm–oviduct binding

In many species, sperm binding to oviduct epithelium is believed to be an essential step in generating a highly fertile capacitated sperm population primed for fertilization. In several mammalian species, this interaction is based on carbohydrate-lectin recognition.d-galactose has previously been characterized as a key molecule that facilitates sperm–oviduct binding in the horse. We used oviduct explant and oviduct apical plasma membrane (APM) assays to investigate the effects of various carbohydrates; glycosaminoglycans; lectins; S-S reductants; and the capacitating factors albumin, Ca2+and HCO3−on sperm–oviduct binding in the horse. Carbohydrate-specific lectin staining indicated thatN-acetylgalactosamine,N-acetylneuraminic acid (sialic acid) andd-mannose ord-glucose were the most abundant carbohydrates on equine oviduct epithelia, whereasd-galactose moieties were not detected. However, in a competitive binding assay, sperm–oviduct binding density was not influenced by any tested carbohydrates, glycosaminoglycans, lectins ord-penicillamine, nor did the glycosaminoglycans induce sperm tail-associated protein tyrosine phosphorylation. Furthermore,N-glycosidase F (PNGase) pretreatment of oviduct explants and APM did not alter sperm–oviduct binding density. By contrast, a combination of the sperm-capacitating factors albumin and HCO3−severely reduced (>10-fold) equine sperm–oviduct binding density by inducing rapid head-to-head agglutination, both of which events were independent of Ca2+and an elevated pH (7.9). Conversely, neither albumin and HCO3−nor any other capacitating factor could induce release of oviduct-bound sperm. In conclusion, a combination of albumin and HCO3−markedly induced sperm head-to-head agglutination which physically prevented stallion sperm to bind to oviduct epithelium.