Behavioral mechanism during human sperm chemotaxis: involvement of hyperactivation

Microfluidics: The future of sperm selection in assisted reproduction

BACKGROUND

Obtaining functional sperm cells is the first step to treat infertility. With the ever-increasing trend in male infertility, clinicians require access to effective solutions that are able to single out the most viable spermatozoa, which would max out the chance for a successful pregnancy. The new generation techniques for sperm selection involve microfluidics, which offers laminar flow and low Reynolds number within the platforms can provide unprecedented opportunities for sperm selection. Previous studies showed that microfluidic platforms can provide a novel approach to this challenge and since then researchers across the globe have attacked this problem from multiple angles.

OBJECTIVE

In this review, we seek to provide a much-needed bridge between the technical and medical aspects of microfluidic sperm selection. Here, we provide an up-to-date list on microfluidic sperm selection procedures and its application in assisted reproductive technology laboratories.

SEARCH METHOD

A literature search was performed in Web of Science, PubMed, and Scopus to select papers reporting microfluidic sperm selection using the keywords: microfluidic sperm selection, self-motility, non-motile sperm selection, boundary following, rheotaxis, chemotaxis, and thermotaxis. Papers published before March 31, 2023 were selected.

OUTCOMES

Our results show that most studies have used motility-based properties for sperm selection. However, microfluidic platforms are ripe for making use of other properties such as chemotaxis and especially rheotaxis. We have identified that low throughput is one of the major hurdles to current microfluidic sperm selection chips, which can be solved via parallelization.

CONCLUSION

Future work needs to be performed on numerical simulation of the microfluidics chip prior to fabrication as well as relevant clinical assessment after the selection procedure. This would require a close collaboration and understanding among engineers, biologists, and medical professionals. It is interesting that in spite of two decades of microfluidics sperm selection, numerical simulation and clinical studies are lagging behind. It is expected that microfluidic sperm selection platforms will play a major role in the development of fully integrated start-to-finish assisted reproductive technology systems.

Progesterone and estradiol regulate sperm hyperactivation and in vitro fertilization success in mice

Progesterone (P) and 17β-estradiol (Eβ) form the well-known hormone pair that regulates sperm capacitation. Here, we examined the regulatory effects of P and Eβ on sperm hyperactivation in mice and evaluated the in vitro fertilization (IVF) success. Although P enhanced hyperactivation, Eβ dose-dependently suppressed the P-enhanced hyperactivation. Moreover, P increased IVF success, whereas Eβ suppressed the P-induced increase in IVF success in a dose-dependent manner. Thus, P and Eβ competitively regulate hyperactivation and IVF success in mice. Since P and Eβ concentrations generally change during the estrous cycle, sperm are speculated to capacitate in response to the oviductal environment and fertilize the oocyte.

β-Defensin 19/119 mediates sperm chemotaxis and is associated with idiopathic infertility

Sperm chemotaxis is required for guiding sperm toward the egg. However, the molecular identity of physiological chemoattractant and its involvement in infertility remain elusive. Here, we identify DEFB19/119 (mouse/human orthologs) as a physiological sperm chemoattractant. The epithelia of the female reproductive tract and the cumulus-oocyte complex secrete DEFB19/119 that elicits calcium mobilization via the CatSper channel and induces sperm chemotaxis in capacitated sperm. Manipulating the level of DEFB19 in mice determines the number of sperm arriving at the fertilization site. Importantly, we identify exon mutations in the DEFB119 gene in idiopathic infertile women with low level of DEFB119 in the follicular fluid. The level of DEFB119 correlates with the chemotactic potency of follicular fluid and predicts the infertile outcome with positive correlation. This study reveals the pivotal role of DEFB19/119 in sperm chemotaxis and demonstrates its potential application in the diagnosis of idiopathic infertility.

Effect of High Viscosity on Energy Metabolism and Kinematics of Spermatozoa from Three Mouse Species Incubated under Capacitating Conditions

In order to sustain motility and prepare for fertilization, sperm require energy. The characterization of sperm ATP production and usage in mouse species revealed substantial differences in metabolic pathways that can be differentially affected by capacitation. Moreover, spermatozoa encounter different environments with varying viscoelastic properties in the female reproductive tract. Here, we examine whether viscosity affects sperm ATP levels and kinematics during capacitation in vitro. Sperm from three mouse species (Mus musculus, M. spretus, M. spicilegus) were incubated under capacitating conditions in a modified Tyrode's medium containing bicarbonate, glucose, pyruvate, lactate, and bovine serum albumin (mT-BH) or in a bicarbonate-free medium as a non-capacitating control. Viscosity was increased with the inclusion of polyvinylpyrrolidone. ATP was measured with a bioluminescence kit, and kinematics were examined with a computer-aided sperm analysis system. In M. musculus sperm, ATP declined during capacitation, but no differences were found between non-capacitating and capacitating sperm. In contrast, in M. spretus and M. spicilegus, ATP levels decreased in capacitating sperm. Increasing viscosity in the medium did not modify the timing or proportion of cells undergoing capacitation but did result in additional time- and concentration-dependent decreases in ATP in M. spretus and M. spicilegus under capacitating conditions. Additionally, increased viscosity altered both velocity and trajectory descriptors. The limited impact of capacitation and higher viscosity on M. musculus sperm ATP and kinematics could be related to the low intensity of postcopulatory sexual selection in this species. Responses seen in the other two species could be linked to the ability of their sperm to perform better under enhanced selective pressures.

Thermotaxis of mammalian sperm

Sperm are guided through the female reproductive tract. A temperature difference of about 2 °C exists between the storage site and fertilization site of the mammalian oviduct, leading to the hypothesis that sperm can sense and swim towards the oocyte along a rising temperature gradient, known as thermotaxis. Research over the past two decades has reported that sperm feature a sophisticated thermal detection system to detect and track ambient temperature gradients. More recently, thermotaxis is expected to be added to the microfluidic isolation method based on sperm tactic responses for sperm selection. In this paper, mammalian sperm thermotaxis is discussed, explaining the underlying behavioral mechanisms and molecular basis, according to the latest research. Finally, this paper explores the possible application of sperm thermotaxis in assisted reproductive technologies.

Mammalian sperm hyperactivation regulates navigation via physical boundaries and promotes pseudo-chemotaxis

Mammalian sperm migration within the complex and dynamic environment of the female reproductive tract toward the fertilization site requires navigational mechanisms, through which sperm respond to the tract environment and maintain the appropriate swimming behavior. In the oviduct (fallopian tube), sperm undergo a process called "hyperactivation," which involves switching from a nearly symmetrical, low-amplitude, and flagellar beating pattern to an asymmetrical, high-amplitude beating pattern that is required for fertilization in vivo. Here, exploring bovine sperm motion in high-aspect ratio microfluidic reservoirs as well as theoretical and computational modeling, we demonstrate that sperm hyperactivation, in response to pharmacological agonists, modulates sperm-sidewall interactions and thus navigation via physical boundaries. Prior to hyperactivation, sperm remained swimming along the sidewalls of the reservoirs; however, once hyperactivation caused the intrinsic curvature of sperm to exceed a critical value, swimming along the sidewalls was reduced. We further studied the effect of noise in the intrinsic curvature near the critical value and found that these nonthermal fluctuations yielded an interesting "Run-Stop" motion on the sidewall. Finally, we observed that hyperactivation produced a "pseudo-chemotaxis" behavior, in that sperm stayed longer within microfluidic chambers containing higher concentrations of hyperactivation agonists.

Occurrence of Calcium Oscillations in Human Spermatozoa Is Based on Spatial Signaling Enzymes Distribution

In human spermatozoa, calcium dynamics control most of fertilization events. Progesterone, present in the female reproductive system, can trigger several types of calcium responses, such as low-frequency oscillations. Here we aimed to identify the mechanisms of progesterone-induced calcium signaling in human spermatozoa. Progesterone-induced activation of fluorophore-loaded spermatozoa was studied by fluorescent microscopy. Two computational models were developed to describe the spermatozoa calcium responses: a homogeneous one based on a system of ordinary differential equations and a three-dimensional one with added space dimensions and diffusion for the cytosolic species. In response to progesterone, three types of calcium responses were observed in human spermatozoa: a single transient rise of calcium concentration in cytosol, a steady elevation, or low-frequency oscillations. The homogenous model provided qualitative description of the oscillatory and the single spike responses, while the three-dimensional model captured the calcium peak shape and the frequency of calcium oscillations. The model analysis demonstrated that an increase in the calcium diffusion coefficient resulted in the disappearance of the calcium oscillations. Additionally, in silico analysis suggested that the spatial distribution of calcium signaling enzymes governs the appearance of calcium oscillations in progesterone-activated human spermatozoa.

Selection of Boar Sperm by Reproductive Biofluids as Chemoattractants

Simple Summary

Both in natural breeding and some assisted reproduction technologies, spermatozoa are deposited into the uterus. The journey the spermatozoa must take from the place of semen deposition to the fertilization site is long, hostile, and selective of the best spermatozoa. For the fertilization to succeed, spermatozoa are guided by chemical stimuli (chemoattractants) to the fertilization site, mainly secreted by the oocyte, cumulus cells, and other substances poured into the oviduct in the periovulatory period. This work studied some sources of chemotactic factors and their action on spermatozoa functionality in vitro, including the fertility. A special chemotactic chamber for spermatozoa selection was designed which consists of two wells communicated by a tube. The spermatozoa are deposited in well A, and the chemoattractants in well B. This study focuses on the use of follicular fluid (FF), periovulatory oviductal fluid (pOF), conditioned medium from the in vitro maturation of oocytes (CM), and progesterone (P4) as chemoattractants to sperm. The chemotactic potential of these substances is also investigated as related to their action on CatSper which is a calcium channel in the spermatozoa known to be sensitive to chemoattractants and essential for motility.

Abstract

Chemotaxis is a spermatozoa guidance mechanism demonstrated in vitro in several mammalian species including porcine. This work focused on follicular fluid (FF), periovulatory oviductal fluid (pOF), the medium surrounding oocytes during in vitro maturation (conditioned medium; CM), progesterone (P4), and the combination of those biofluids (Σ) as chemotactic agents and modulators of spermatozoa fertility in vitro. A chemotaxis chamber was designed consisting of two independent wells, A and B, connected by a tube. The spermatozoa are deposited in well A, and the chemoattractants in well B. The concentrations of biofluids that attracted a higher proportion of spermatozoa to well B were 0.25% FF, 0.25% OF, 0.06% CM, 10 pM P4 and 0.25% of a combination of biofluids (Σ2), which attracted between 3.3 and 12.3% of spermatozoa (p < 0.05). The motility of spermatozoa recovered in well B was determined and the chemotactic potential when the sperm calcium channel CatSper was inhibited, which significantly reduced the % of spermatozoa attracted (p < 0.05). Regarding the in vitro fertility, the spermatozoa attracted by FF produced higher rates of penetration of oocytes and development of expanded blastocysts. In conclusion, porcine reproductive biofluids show an in vitro chemotactic effect on spermatozoa and modulate their fertilizing potential.

Getting to and away from the egg, an interplay between several sperm transport mechanisms and a complex oviduct physiology

In mammals, the architecture and physiology of the oviduct are very complex, and one long-lasting intriguing question is how spermatozoa are transported from the sperm reservoir in the isthmus to the oocyte surface. In recent decades, several studies have improved knowledge of the factors affecting oviduct fluid movement and sperm transport. They report sperm-guiding mechanisms that move the spermatozoa towards (rheotaxis, thermotaxis, and chemotaxis) or away from the egg surface (chemorepulsion), but only a few provide evidence of their occurrence in vivo. This gives rise to several questions: how and when do the sperm transport mechanisms operate inside such an active oviduct? why are there so many sperm guidance processes? is one dominant over the others, or do they cooperate to optimise the success of fertilisation? Assuming that sperm guidance evolved alongside oviduct physiology, in this review we propose a theoretical model that integrates oviduct complexity in space and time with the sperm-orienting mechanisms. In addition, since all of the sperm-guidance processes recruit spermatozoa in a better physiological condition than those not selected, they could potentially be incorporated into assisted reproductive technology (ART) to improve fertility treatment and/or to develop innovative contraceptive methods. All these issues are discussed in this review.

A Higher Abundance of O-Linked Glycans Confers a Selective Advantage to High Fertile Buffalo Spermatozoa for Immune-Evasion From Neutrophils

The glycans on the plasma membrane of cells manifest as the glycocalyx, which serves as an information-rich frontier that is directly in contact with its immediate milieu. The glycoconjugates (GCs) that adorn most of the mammalian cells are also abundant in gametes, especially the spermatozoa where they perform unique reproduction-specific functions e.g., inter-cellular recognition and communication. This study aimed to implicate the sperm glycosylation pattern as one of the factors responsible for low conception rates observed in buffalo bulls. We hypothesized that a differential abundance of glycans exists on the spermatozoa from bulls of contrasting fertilizing abilities endowing them with differential immune evasion abilities. Therefore, we investigated the role of glycan abundance in the phagocytosis and NETosis rates exhibited by female neutrophils (PMNs) upon exposure to such spermatozoa. Our results indicated that the spermatozoa from high fertile (HF) bulls possessed a higher abundance of O-linked glycans e.g., galactosyl (β-1,3)N-acetylgalactosamine and N-linked glycans like [GlcNAc]1-3, N-acetylglucosamine than the low fertile (LF) bull spermatozoa. This differential glycomic endowment appeared to affect the spermiophagy and NETosis rates exhibited by the female neutrophil cells (PMNs). The mean percentage of phagocytizing PMNs was significantly different (P < 0.0001) for HF and LF bulls, 28.44 and 59.59%, respectively. Furthermore, any introduced perturbations in the inherent sperm glycan arrangements promoted phagocytosis by PMNs. For example, after in vitro capacitation the mean phagocytosis rate (MPR) rate in spermatozoa from HF bulls significantly increased to 66.49% (P < 0.01). Likewise, the MPR increased to 70.63% (p < 0.01) after O-glycosidase & α2-3,6,8,9 Neuraminidase A treatment of spermatozoa from HF bulls. Moreover, the percentage of PMNs forming neutrophil extracellular traps (NETs) was significantly higher, 41.47% when exposed to spermatozoa from LF bulls vis-à-vis the spermatozoa from HF bulls, 15.46% (P < 0.0001). This is a pioneer report specifically demonstrating the role of O-linked glycans in the immune responses mounted against spermatozoa. Nevertheless, further studies are warranted to provide the measures to diagnose the sub-fertile phenotype thus preventing the losses incurred by incorrect selection of morphologically normal sperm in the AI/IVF reproduction techniques.

Expanding the Limits of Computer-Assisted Sperm Analysis through the Development of Open Software

Computer assisted sperm analysis (CASA) systems can reduce errors occurring in manual analysis. However, commercial CASA systems are frequently not applicable at the forefront of challenging research endeavors. The development of open source software may offer important solutions for researchers working in related areas. Here, we present an example of this, with the development of three new modules for the OpenCASA software (hosted at Github). The first is the Chemotactic Sperm Accumulation Module, a powerful tool for studying sperm chemotactic behavior, analyzing the sperm accumulation in the direct vicinity of the stimuli. This module was validated by comparing fish sperm accumulation, with or without the influence of an attractant. The analysis clearly indicated cell accumulation in the treatment group, while the distribution of sperm was random in the control group. The second is the Sperm Functionality Module, based on the ability to recognize five sperm subpopulations according to their fluorescence patterns associated with the plasma membrane and acrosomal status. The last module is the Sperm Concentration Module, which expands the utilities of OpenCASA. These last two modules were validated, using bull sperm, by comparing them with visual counting by an observer. A high level of correlation was achieved in almost all the data, and a good agreement between both methods was obtained. With these newly developed modules, OpenCASA is consolidated as a powerful free and open-source tool that allows different aspects of sperm quality to be evaluated, with many potential applications for researchers.

Stromal cell-derived factor 1 regulates in vitro sperm migration towards the cumulus-oocyte complex in cattle

Sperm migration towards an oocyte in the female reproductive tract is an important step for successful fertilization. Although several sperm-chemotactic factors have been identified in mammals, it is unclear whether these chemoattractants contribute to sperm migration towards an oocyte that is the final destination for sperm. Furthermore, chemoattractants for bovine sperm are still undiscovered even though the follicular fluid attracts sperm in cattle. Here, we demonstrated that a single bovine cumulus-oocyte complex (COC) had the ability to attract sperm, suggesting that the COC secreted sperm chemoattractants. We identified stromal cell-derived factor 1 (SDF1), which was expressed in COCs, and its receptor CXCR4 in sperm, as a candidate. Our results showed that bovine sperm preferentially migrated to the area with a high SDF1 concentration and occasionally showed turn movements by asymmetric flagellar bends during the migration. We also demonstrated that increasing the intracellular Ca²⁺ concentration via Ca²⁺ channels was related to SDF1-induced sperm chemotaxis. Finally, a CXCR4 inhibitor significantly suppressed the in vitro bovine sperm migration towards a COC. Taken together, we propose that SDF1 is a chemotactic factor for bovine sperm to regulate their migration towards an oocyte via the CXCR4 receptor.

Egg-induced changes to sperm phenotypes shape patterns of multivariate selection on ejaculates

Sperm cells exhibit extraordinary phenotypic diversity and rapid rates of evolution, yet the adaptive value of most sperm traits remains equivocal. Recent findings suggest that to understand how selection targets ejaculates, we must recognize that female-imposed physiological conditions often alter sperm phenotypes. These phenotypic changes may influence the relationships among sperm traits and their association with fitness. Here, we show that chemical substances released by eggs (known to modify sperm physiology and behaviour) alter patterns of selection on a suite of sperm traits in the mussel Mytilus galloprovincialis. We use multivariate selection analyses to characterize linear and nonlinear selection acting on sperm traits in (a) seawater alone and (b) seawater containing egg-derived chemicals (egg water). Our analyses revealed that nonlinear selection on canonical axes of multiple traits (notably sperm velocity, sperm linearity and percentage of motile sperm) was the most important form of selection overall, but importantly these patterns were only evident when sperm phenotypes were measured in egg water. These findings reveal the subtle way that females can alter patterns of selection, with the implication that overlooking environmentally moderated changes to sperm, may result in erroneous interpretations of how selection targets phenotypic (co)variation in sperm traits.

Periovulatory oviductal fluid decreases sperm protein kinase A activity, tyrosine phosphorylation, and in vitro fertilization in pig

BACKGROUND

Molecules from the female reproductive tract modulate capacitation and function of sperm cells in vivo. These molecules vary in a quantitative and qualitative manner throughout the estrous cycle.

OBJECTIVES

This work evaluates the effect of using various female reproductive fluids on capacitation and fertilization of pig spermatozoa in vitro.

MATERIAL AND METHODS

The effects of culturing spermatozoa in different fluids on the levels of sperm protein kinase A (pPKA), tyrosine phosphorylation, acrosome reaction, and in vitro fertilization (IVF) were evaluated. The fluids tested were as follows: oviductal fluid (OF) from five phases of the estrous cycle, namely early and late follicular (OF-EF, OF-LF), early and late luteal (OF-EL, OF-LL) and periovulatory (pOF), follicular fluid from medium-sized follicles, and secretions of cumulus-oocyte complexes (conditioned medium).

RESULTS

The pPKAs and tyrosine phosphorylation were decreased by OF-EF, OF-LF, OF-EL, and pOF but not by follicular fluid and conditioned medium. OF-EF, OF-LF, and pOF also decreased the sperm acrosome reaction. Moreover, the effect of pOF on pPKAs and tyrosine phosphorylation was reversible. In in vitro fertilization, OF-EF, OF-LF, OF-EL, and pOF reduced the percentage of penetrated oocytes, the mean number of spermatozoa per penetrated oocyte, and increased monospermy.

CONCLUSION

OF from follicular, early luteal, and periovulatory phases of the estrous cycle modulates the sperm protein phosphorylation as well as the acrosome reaction involved in capacitation and increases monospermic fertilization in in vitro fertilization. Our findings suggest that fluids from the female reproductive tract could be used as additives in porcine IVF systems to modulate sperm-oocyte interaction.

Novel Techniques of Sperm Selection for Improving IVF and ICSI Outcomes

Almost 50% of the infertility cases are due to male factors. Assisted reproductive technologies (ARTs) allow to overcome the incapacity of these patients' spermatozoa to fertilize the oocyte and produce a viable and healthy offspring, but the efficiency of the different techniques has still the potential to improve. According to the latest reports of the European Society of Human Reproduction and Embryology (ESHRE) and the Centers for Disease Control and Prevention of the United States (CDC), the percentages of deliveries per ART cycle in 2014 and 2016 were 21 and 22%, respectively. Among the reasons for this relatively low efficiency, the quality of the spermatozoa has been pointed out as critical, and the presence of high percentages of DNA-damaged spermatozoa in patients' ejaculates is possibly one of the main factors reducing the ARTs outcomes. Thus, one of the main challenges in reproductive medicine is to ensure the highest quality of the spermatozoa used in ARTs, and specifically, in terms of genetic integrity. The latest techniques for the preparation and selection of human spermatozoa are herein discussed focusing on those proven to improve one or several of the following parameters: sperm genetic integrity, fertilization capacity, embryo production, and in vitro survival, as well as pregnancy and delivery rates following in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI). In addition, we discuss the potential of techniques developed in non-human mammals that could be further transferred to the clinic.

Cryopreservation of margay (Leopardus wiedii) spermatozoa: Effects of different extenders and frozen protocols

Sperm cryopreservation offers many benefits to wild felids conservation programs. However, the implementation of these programs is limited by the different responses of each species to the cryopreservation protocols and extenders used, requiring the formulation of species-specific protocols. For this purpose, semen samples from 6 margays (Leopardus wiedii) were submitted to 2 cryopreservation protocols: 1) manual freezing (cooling rate of - 0.33 °C/min at 5 °C/180 min and freezing rate with two steps - 9 °C/min for 2 min and -19.1 °C/min for 2 min) and 2) automatic freezing machine (cooling rate of - 0.25 °C/min at 5 °C/120 min and freezing rate with one step -20 °C/min for 8.3 min) using 2 commercial extenders, an egg yolk-based (Test Yolk Buffer; TYB) and an egg yolk-free extender (AndroMed; MED). Post-thawed sperm quality was assessed at 3 time points (immediately after thawing and 1 and 2 h post-thawed) by sperm motility index (SMI), plasma membrane and acrosomal integrity, and mitochondrial membrane potential (MMP). Regarding SMI, TYB yielded superior results (29.4 ± 3.5%) compared to MED (11.2 ± 2.8%; p < 0.002) immediately after thawing until 2 h after thawing (TYB 3.9 ± 1.7% and MED 0.0 ± 0.0%; p < 0.05). Furthermore, the automated freezing method provided higher motility compared to the manual freezing procedure immediately post-thaw (25.08 ± 3.66% and 15.78 ± 3.29%, respectively) and 1 h post-thaw (13.71 ± 2.56% and 6.03 ± 1.97%, respectively; p < 0.05). The percentage of intact acrosomes and plasma membranes and the percentage of sperm with high MMP were superior for TYB when compared to MED regardless of cryopreservation protocol (p < 0.05). Conversely, the interaction between cryopreservation protocols and extenders was observed for MMP where TYB exhibits better results compared to MED (p < 0.05) in both procedures, but it was higher in automated procedures. For MED, no changes were found in MMP between procedures. Considering only TYB, samples showed higher MMP when submitted to an automated procedure (p < 0.05). In conclusion, the slow cooling rates with shorter time of exposure to glycerol contributed to minimize cryodamage in the Margays' sperm. Moreover, results indicated that association between TYB and automatic freezing machine ensured the minimal quality of spermatozoa after thawing required for further use in in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI).

Human sperm ion channel (dys)function: implications for fertilization

BACKGROUND

Intensive research on sperm ion channels has identified members of several ion channel families in both mouse and human sperm. Gene knock-out studies have unequivocally demonstrated the importance of the calcium and potassium conductances in sperm for fertility. In both species, the calcium current is carried by the highly complex cation channel of sperm (CatSper). In mouse sperm, the potassium current has been conclusively shown to be carried by a channel consisting of the pore forming subunit SLO3 and auxiliary subunit leucine-rich repeat-containing 52 (LRRC52). However, in human sperm it is controversial whether the pore forming subunit of the channel is composed of SLO3 and/or SLO1. Deciphering the role of the proton-specific Hv1 channel is more challenging as it is only expressed in human sperm. However, definitive evidence for a role in, and importance for, human fertility can only be determined through studies using clinical samples.

OBJECTIVE AND RATIONALE

This review aims to provide insight into the role of sperm ion channels in human fertilization as evidenced from recent studies of sperm from infertile men. We also summarize the key discoveries from mouse ion channel knock-out models and contrast the properties of mouse and human CatSper and potassium currents. We detail the evidence for, and consequences of, defective ion channels in human sperm and discuss hypotheses to explain how defects arise and why affected sperm have impaired fertilization potential.

SEARCH METHODS

Relevant studies were identified using PubMed and were limited to ion channels that have been characterized in mouse and human sperm. Additional notable examples from other species are included as appropriate.

OUTCOMES

There are now well-documented fundamental differences between the properties of CatSper and potassium channel currents in mouse and human sperm. However, in both species, sperm lacking either channel cannot fertilize in vivo and CatSper-null sperm also fail to fertilize at IVF. Sperm-lacking potassium currents are capable of fertilizing at IVF, albeit at a much lower rate. However, additional complex and heterogeneous ion channel dysfunction has been reported in sperm from infertile men, the causes of which are unknown. Similarly, the nature of the functional impairment of affected patient sperm remains elusive. There are no reports of studies of Hv1 in human sperm from infertile men.

WIDER IMPLICATIONS

Recent studies using sperm from infertile men have given new insight and critical evidence supporting the supposition that calcium and potassium conductances are essential for human fertility. However, it should be highlighted that many fundamental questions remain regarding the nature of molecular and functional defects in sperm with dysfunctional ion channels. The development and application of advanced technologies remains a necessity to progress basic and clinical research in this area, with the aim of providing effective screening methodologies to identify and develop treatments for affected men in order to help prevent failed ART cycles. Conversely, development of drugs that block calcium and/or potassium conductances in sperm is a plausible strategy for producing sperm-specific contraceptives.

S100 A9 is expressed and secreted by the oviduct epithelium, interacts with gametes and affects parameters of human sperm capacitation in vitro

Our previous findings demonstrate that some oviductal secretion proteins bind to gametes and affect sperm physiology and gamete interaction. One of these proteins possesses an estimated molecular weight of 14 kDa. The objective of this study was to isolate and identify this 14 kDa protein, to localize it in the human oviduct, to detect gamete binding sites for the protein, and to evaluate its effects on sperm capacitation parameters and gamete interaction. Explants from the human oviductal tissues of premenopausal women were cultured in the presence of [³⁵ S]-Methionine-proteins ([35S]-Met-proteins). De novo synthesized secreted [³⁵ S]-Met-proteins were isolated from the culture media by affinity chromatography using their sperm membrane binding ability and analysed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using liquid chromatography-tandem mass spectrometry peptide sequencing, human S100 A9 was identified as one of the isolated proteins from the 14 kDa protein band. S100 A9 was detected in oviduct epithelium and oviduct secretion using immunohistochemistry and a Western blot. S100 A9 binding to human oocytes and spermatozoa was assessed by indirect immunofluorescence. The acrosome reaction (AR) affected S100 A9 ability to bind sperm cells. The presence of S100 A9 significantly increased both the induced AR and the sperm protein tyrosine phosphorylation, with respect to controls. However, the protein did not affect sperm-zona pellucida interaction. Results indicate that S100 A9 is present in the human oviduct and that it modulates parameters of sperm capacitation in vitro. Hence, the protein might contribute to the regulation of the reproductive process in the oviductal microenvironment.

OpenCASA: A new open-source and scalable tool for sperm quality analysis

In the field of assisted reproductive techniques (ART), computer-assisted sperm analysis (CASA) systems have proved their utility and potential for assessing sperm quality, improving the prediction of the fertility potential of a seminal dose. Although most laboratories and scientific centers use commercial systems, in the recent years certain free and open-source alternatives have emerged that can reduce the costs that research groups have to face. However, these open-source alternatives cannot analyze sperm kinetic responses to different stimuli, such as chemotaxis, thermotaxis or rheotaxis. In addition, the programs released to date have not usually been designed to encourage the scalability and the continuity of software development. We have developed an open-source CASA software, called OpenCASA, which allows users to study three classical sperm quality parameters: motility, morphometry and membrane integrity (viability) and offers the possibility of analyzing the guided movement response of spermatozoa to different stimuli (useful for chemotaxis, thermotaxis or rheotaxis studies) or different motile cells such as bacteria, using a single software. This software has been released in a Version Control System at Github. This platform will allow researchers not only to download the software but also to be involved in and contribute to further developments. Additionally, a Google group has been created to allow the research community to interact and discuss OpenCASA. For validation of the OpenCASA software, we analysed different simulated sperm populations (for chemotaxis module) and evaluated 36 ejaculates obtained from 12 fertile rams using other sperm analysis systems (for motility, membrane integrity and morphology modules). The results were compared with those obtained by Open-CASA using the Pearson's correlation and Bland-Altman tests, obtaining a high level of correlation in all parameters and a good agreement between the different used methods and the OpenCASA. With this work, we propose an open-source project oriented to the development of a new software application for sperm quality analysis. This proposed software will use a minimally centralized infrastructure to allow the continued development of its modules by the research community.

Hyperactivated stallion spermatozoa fail to exhibit a rheotaxis-like behaviour, unlike other species

The journey of spermatozoa through the female genital tract is facilitated by rheotaxis, or the cell's preference to swim against a flow, as well as thigmotaxis, the wall tracking behaviour, which guides them to the site of fertilisation. The aim of this study was to characterise the rheotactic and thigmotactic response of stallion sperm within a microfluidic channel. Stallion sperm rheotaxis was assessed within the microfluidic channel with regard to: (i) A range of flow velocities, (ii) Varying media viscosity and (iii) Sperm hyperactivation. Sperm distribution across the microfluidic channel was also studied and compared to human and ram sperm. Stallion sperm progressed furthest at a velocity range of 10-30 µm/s, with an optimum velocity of 20 µm/s. A flow viscosity of 2.5cP or greater reduced sperm rheotaxis (P < 0.05). Stallion sperm that were hyperactivated were unable to exhibit rheotaxis within the microfluidic channel, whereas, both hyperactivated human and ram sperm did exhibit positive rheotaxis under the same conditions. The number of sperm swimming near the microfluidic channel walls was higher than in the microfluidic channel centre (P < 0.05). This is the first study to illustrate that stallion sperm are rheotactically responsive and increasing viscosity reduces this response. We also demonstrated that sperm are predominantly inclined to swim along a surface and uniquely, hyperactivated stallion sperm are non-progressive and do not exhibit a rheotactic response unlike other species.

Introduction: A Brief Guide to the Periconception Environment

Definition of the periconception period is not an exact science and is probably somewhat arbitrary. One can define it as spanning the period from the final stages of gamete maturation until formation of the embryo and the stages of embryonic development and implantation. Hence, the periconception period includes periods when spermatozoa are in the female reproductive tract, oocytes are matured and ovulated into the oviduct, fertilization occurs and the embryo undergoes development. By definition the implantation process and the early stages of placenta formation are also regarded as a part of the periconception period. In this article we highlight a few of the major advances which have transformed this topic over the last two decades. It is now clear that the fitness and wellbeing of developing mammalian embryos, including the human, are highly dependent on the health status, diet and habits of both parents especially in the months and weeks that precede the formation of oocytes and spermatozoa.

Migrating Myeloid Cells Sense Temporal Dynamics of Chemoattractant Concentrations

Chemoattractant-mediated recruitment of hematopoietic cells to sites of pathogen growth or tissue damage is critical to host defense and organ homeostasis. Chemotaxis is typically considered to rely on spatial sensing, with cells following concentration gradients as long as these are present. Utilizing a microfluidic approach, we found that stable gradients of intermediate chemokines (CCL19 and CXCL12) failed to promote persistent directional migration of dendritic cells or neutrophils. Instead, rising chemokine concentrations were needed, implying that temporal sensing mechanisms controlled prolonged responses to these ligands. This behavior was found to depend on G-coupled receptor kinase-mediated negative regulation of receptor signaling and contrasted with responses to an end agonist chemoattractant (C5a), for which a stable gradient led to persistent migration. These findings identify temporal sensing as a key requirement for long-range myeloid cell migration to intermediate chemokines and provide insights into the mechanisms controlling immune cell motility in complex tissue environments.

Human sperm steer with second harmonics of the flagellar beat

Sperm are propelled by bending waves traveling along their flagellum. For steering in gradients of sensory cues, sperm adjust the flagellar waveform. Symmetric and asymmetric waveforms result in straight and curved swimming paths, respectively. Two mechanisms causing spatially asymmetric waveforms have been proposed: an average flagellar curvature and buckling. We image flagella of human sperm tethered with the head to a surface. The waveform is characterized by a fundamental beat frequency and its second harmonic. The superposition of harmonics breaks the beat symmetry temporally rather than spatially. As a result, sperm rotate around the tethering point. The rotation velocity is determined by the second-harmonic amplitude and phase. Stimulation with the female sex hormone progesterone enhances the second-harmonic contribution and, thereby, modulates sperm rotation. Higher beat frequency components exist in other flagellated cells; therefore, this steering mechanism might be widespread and could inspire the design of synthetic microswimmers.

Possible ability of bovine follicular fluid to attract migrating bull spermatozoa

Aim

To examine the potential of bovine follicular fluid (BFF) to attract bull spermatozoa.

Methods

The ability of the BFF to attract bull sperm was evaluated by observing changes in sperm migration after being placed in a cross‐column chamber. The movement parameters of the heads and flagella of the sperm that were attracted to the BFF were analyzed by using the Computer Assisted Sperm Analysis system.

Results

It was observed that 61.6% of the bull sperm migrated toward the BFF when the BFF was used at a concentration of 0.1%, but 67.2% of the sperm did not migrate toward the BFF at a concentration of 10%. Relatively larger numbers of both precapacitated and postcapacitated bull sperm migrated toward the BFF (0.1%). The ability of the 0.1% BFF to attract sperm probably affected both the normal artificial insemination (AI) fertility sperm and the poor AI fertility spermatozoa. The flagellar curvilinear ratio of the sperm winding to the 0.1% BFF was significantly higher than that of the prewinding sperm.

Conclusion

These results could suggest that BFF potentially attracts bull sperm at a certain concentration, irrespective of the capacitation status of the sperm. Although the mechanism by which this attraction occurs remains unclear, these data imply that it could be related to BFF‐dependent changes in the sperm flagellar curvilinear ratio.

Improving porcine in vitro fertilization output by simulating the oviductal environment

Differences between the in vitro and in vivo environment in which fertilization occurs seem to play a key role in the low efficiency of porcine in vitro fertilization (IVF). This work proposes an IVF system based on the in vivo oviductal periovulatory environment. The combined use of an IVF medium at the pH found in the oviduct in the periovulatory stage (pH_e 8.0), a mixture of oviductal components (cumulus-oocyte complex secretions, follicular fluid and oviductal periovulatory fluid, OFCM) and a device that interposes a physical barrier between gametes (an inverted screw cap of a Falcon tube, S) was compared with the classical system at pH_e 7.4, in a 4-well multidish (W) lacking oviduct biological components. The results showed that the new IVF system reduced polyspermy and increased the final efficiency by more than 48%. This higher efficiency seems to be a direct consequence of a reduced sperm motility and lower capacitating status and it could be related to the action of OFCM components over gametes and to the increase in the sperm intracellular pH (pH_i) caused by the higher pH_e used. In conclusion, a medium at pH 8.0 supplemented with OFCM reduces polyspermy and improves porcine IVF output.

Heat Sensing Receptor TRPV1 Is a Mediator of Thermotaxis in Human Spermatozoa

The molecular bases of sperm thermotaxis, the temperature-oriented cell motility, are currently under investigation. Thermal perception relies on a subclass of the transient receptor potential [TRP] channels, whose member TRPV1 is acknowledged as the heat sensing receptor. Here we investigated the involvement of TRPV1 in human sperm thermotaxis. We obtained semen samples from 16 normozoospermic subjects attending an infertility survey programme, testis biopsies from 6 patients with testicular germ cell cancer and testis fine needle aspirates from 6 patients with obstructive azoospermia undergoing assisted reproductive technologies. Expression of TRPV1 mRNA was assessed by RT-PCR. Protein expression of TRPV1 was determined by western blot, flow cytometry and immunofluorescence. Sperm motility was assessed by Sperm Class Analyser. Acrosome reaction, apoptosis and intracellular-Ca²⁺ content were assessed by flow cytometry. We found that TRPV1 mRNA and protein were highly expressed in the testis, in both Sertoli cells and germ-line cells. Moreover, compared to no-gradient controls at 31°C or 37°C (Ctrl 31°C and Ctrl 37°C respectively), sperm migration towards a temperature gradient of 31–37°C (T gradient) in non-capacitated conditions selected a higher number of cells (14,9 ± 4,2×10⁶ cells T gradient vs 5,1± 0,3×10⁶ cells Ctrl 31°C and 5,71±0,74×10⁶ cells Ctrl 37°C; P = 0,039). Capacitation amplified the migrating capability towards the T gradient. Sperms migrated towards the T gradient showed enriched levels of both TRPV1 protein and mRNA. In addition, sperm cells were able to migrate toward a gradient of capsaicin, a specific agonist of TRPV1, whilst capsazepine, a specific agonist of TRPV1, blocked this effect. Finally, capsazepine severely blunted migration towards T gradient without abolishing. These results suggest that TRPV1 may represent a facilitating mediator of sperm thermotaxis.

Gamete activation: basic knowledge and clinical applications

Background

The first clues to the process of gamete activation date back to nearly 60 years ago. The mutual activation of gametes is a crucial event during fertilization. In the testis and ovaries, spermatozoa and oocytes are in a state of meiotic and metabolic quiescence and require reciprocal signals in order to undergo functional changes that lead to competence for fertilization. First, the oocyte activates sperm by triggering motility, chemoattraction, binding and the acrosome reaction, culminating with the fusion of the two plasma membranes. At the end of this cascade of events, collectively known as sperm capacitation, sperm-induced oocyte activation occurs, generating electrical, morphological and metabolic modifications in the oocyte.

Objective and rationale

The aim of this review is to provide the current state of knowledge regarding the entire process of gamete activation in selected specific animal models that have contributed to our understanding of fertilization in mammals, including humans. Here we describe in detail the reciprocal induction of the two activation processes, the molecules involved and the mechanisms of cell interaction and signal transduction that ultimately result in successful embryo development and creation of a new individual.

Search methods

We carried out a literature survey with no restrictions on publication date (from the early 1950s to March 2016) using PubMed/Medline, Google Scholar and Web of Knowledge by utilizing common keywords applied in the field of fertilization and embryo development. We also screened the complete list of references published in the most recent research articles and relevant reviews published in English (both animal and human studies) on the topics investigated.

Outcomes

Literature on the principal animal models demonstrates that gamete activation is a pre-requisite for successful fertilization, and is a process common to all species studied to date. We provide a detailed description of the dramatic changes in gamete morphology and behavior, the regulatory molecules triggering gamete activation and the intracellular ions and second messengers involved in active metabolic pathways in different species. Recent scientific advances suggest that artificial gamete activation may represent a novel technique to improve human IVF outcomes, but this approach requires caution.

Wider implications

Although controversial, manipulation of gamete activation represents a promising tool for ameliorating the fertilization rate in assisted reproductive technologies. A better knowledge of mechanisms that transform the quiescent oocyte into a pluripotent cell may also provide new insights for the clinical use of stem cells.

Regulation of hamster sperm hyperactivation by extracellular Na+

Mammalian sperm motility has to be hyperactivated to be fertilization-competent. Hyperactivation is regulated by extracellular environment. Osmolality of mammalian semen is higher than that in female reproductive tract; however, the effect of them on hyperactivation has not been investigated. So we investigated the effect of osmotic environment on hyperactivation using hamster spermatozoa at first. Increase in the osmolality of the media (∼370 mOsm) by increasing the concentration of NaCl (∼150 mmol/L) caused the delay of the expression of hyperactivation. When NaCl concentration varied in the same range (75–150 mmol/L) whereas the osmolality was fixed at 370 mOsm by adding mannitol, the delay of hyperactivation occurred dependent on NaCl concentration. Increase in NaCl concentration also caused suppression of curvilinear velocity, bend angle, and sliding velocity of the flagellum at the onset of incubation, suggesting that NaCl concentration affect both activation and hyperactivation in hamster spermatozoa. Hamster sperm intracellular Ca2+ concentration decreased as extracellular NaCl concentration increased, whereas membrane potential and intracellular pH were unaffected by extracellular NaCl concentration. SN-6 and SEA0400, inhibitors of Na+-Ca2+ exchanger (NCX), increased intracellular Ca2+ and accelerated hyperactivation in the presence of 150 mmol/L NaCl. Tyrosine phosphorylation on fibrous sheath proteins was unaffected by extracellular NaCl concentration. These results suggest that extracellular Na+ suppresses hamster sperm hyperactivation by reducing intracellular Ca2+ via an action of NCX in a tyrosine phosphorylation-independent manner. It seems that the removal of suppression by extracellular Na+ leads to the expression of hyperactivated motility.

Progesterone effects on mouse sperm kinetics in conditions of viscosity

The spermatozoa delivered to the female genital tract need to swim towards the oocyte through viscous secretions. Once close to the oocyte, the spermatozoa are guided by a gradient of progesterone (P4) and other unknown chemoattractants via a process known as chemotaxis. Using polyvinylpyrrolidone to establish the conditions of viscosity, we examined the response of mouse spermatozoa to P4. Herein, we show that in low-viscous media, P4 induces hyperactive-like motility whereby sperm show erratic trajectories and non-progressive movement. However, an opposite response is produced in viscous medium in that trajectories are linear and motility is more progressive and less erratic. Our observations provide a behavioural explanation for the chemotaxis of spermatozoa swimming under viscous conditions in a spatial gradient of the chemoattractant P4. They also highlight the importance of using viscous solutions to mimic in vivo conditions when analysing sperm behaviour in response to any stimulus.

Reproduction (2016) 151 501–507

Behavioral mechanisms of mammalian sperm guidance

In mammals, sperm guidance in the oviduct appears essential for successful sperm arrival at the oocyte. Hitherto, three different potential sperm guidance mechanisms have been recognized: thermotaxis, rheotaxis, and chemotaxis, each of them using specific stimuli – a temperature gradient, fluid flow, and a chemoattractant gradient, respectively. Here, we review sperm behavioral in these mechanisms and indicate commonalities and differences between them.

Search for a small egg by spermatozoa in restricted geometries

The search by swimmers for a small target in a bounded domain is ubiquitous in cellular biology, where a prominent case is that of the search by spermatozoa for an egg in the uterus. This is one of the severest selection processes in animal reproduction. We present here a mathematical model of the search, its analysis, and numerical simulations. In the proposed model the swimmers' trajectories are rectilinear and the speed is constant. When a trajectory hits an obstacle or the boundary, it is reflected at a random angle and continues the search with the same speed. Because hitting a small target by a trajectory is a rare event, asymptotic approximations and stochastic simulations are needed to estimate the mean search time in various geometries. We consider searches in a disk, in convex planar domains, and in domains with cusps. The exploration of the parameter space for spermatozoa motion in different uterus geometries leads to scaling laws for the search process.

CRISP1 as a novel CatSper regulator that modulates sperm motility and orientation during fertilization

Ca(2+)-dependent mechanisms are critical for successful completion of fertilization. Here, we demonstrate that CRISP1, a sperm protein involved in mammalian fertilization, is also present in the female gamete and capable of modulating key sperm Ca(2+) channels. Specifically, we show that CRISP1 is expressed by the cumulus cells that surround the egg and that fertilization of cumulus-oocyte complexes from CRISP1 knockout females is impaired because of a failure of sperm to penetrate the cumulus. We provide evidence that CRISP1 stimulates sperm orientation by modulating sperm hyperactivation, a vigorous motility required for penetration of the egg vestments. Moreover, patch clamping of sperm revealed that CRISP1 has the ability to regulate CatSper, the principal sperm Ca(2+) channel involved in hyperactivation and essential for fertility. Given the critical role of Ca(2+) for sperm motility, we propose a novel CRISP1-mediated fine-tuning mechanism to regulate sperm hyperactivation and orientation for successful penetration of the cumulus during fertilization.

Sperm navigation along helical paths in 3D chemoattractant landscapes

Sperm require a sense of direction to locate the egg for fertilization. They follow gradients of chemical and physical cues provided by the egg or the oviduct. However, the principles underlying three-dimensional (3D) navigation in chemical landscapes are unknown. Here using holographic microscopy and optochemical techniques, we track sea urchin sperm navigating in 3D chemoattractant gradients. Sperm sense gradients on two timescales, which produces two different steering responses. A periodic component, resulting from the helical swimming, gradually aligns the helix towards the gradient. When incremental path corrections fail and sperm get off course, a sharp turning manoeuvre puts sperm back on track. Turning results from an ‘off' Ca²⁺ response signifying a chemoattractant stimulation decrease and, thereby, a drop in cyclic GMP concentration and membrane voltage. These findings highlight the computational sophistication by which sperm sample gradients for deterministic klinotaxis. We provide a conceptual and technical framework for studying microswimmers in 3D chemical landscapes.

Sperm use external cues to find the egg using ill-defined principles. Here the authors use holographic microscopy and optochemical tools to study sperm swimming in light-sculpted chemical 3D landscapes; they show that sperm translate the temporal stimulation pattern into multiple swimming behaviours to orient deterministically in a gradient.

The galectin-1-glycan axis controls sperm fertilizing capacity by regulating sperm motility and membrane hyperpolarization

Lectin-glycan recognition systems play central roles in many physiologic and pathologic processes. We identified a role for galectin-1 (Gal-1), a highly conserved glycan-binding protein, in the control of sperm function. We found that Gal-1 is expressed in the epididymis and associates with sperm during epididymal maturation. Exposure of sperm to Gal-1 resulted in glycan-dependent modulation of the acrosome reaction (AR), a key event in the fertilization process. Gal-1-deficient (Lgals1(-/-)) mice revealed the essential contribution of this lectin for full sperm fertilizing ability both in vitro and in vivo. Mechanistically, Lgals1(-/-) sperm exhibited defects in their ability to develop hyperactivation, a vigorous motility required for penetration of the egg vestments. Moreover, Lgals1(-/-) sperm showed a decreased ability to control cell volume and to undergo progesterone-induced AR, phenotypes that were rescued by exposure of the cells to recombinant Gal-1. Interestingly, the AR defect was associated with a deficiency in sperm membrane potential hyperpolarization. Our study highlights the relevance of the Gal-1-glycan axis in sperm function with critical implications in mammalian reproductive biology.

Three-dimensional numerical model of cell morphology during migration in multi-signaling substrates

Cell Migration associated with cell shape changes are of central importance in many biological processes ranging from morphogenesis to metastatic cancer cells. Cell movement is a result of cyclic changes of cell morphology due to effective forces on cell body, leading to periodic fluctuations of the cell length and cell membrane area. It is well-known that the cell can be guided by different effective stimuli such as mechanotaxis, thermotaxis, chemotaxis and/or electrotaxis. Regulation of intracellular mechanics and cell's physical interaction with its substrate rely on control of cell shape during cell migration. In this notion, it is essential to understand how each natural or external stimulus may affect the cell behavior. Therefore, a three-dimensional (3D) computational model is here developed to analyze a free mode of cell shape changes during migration in a multi-signaling micro-environment. This model is based on previous models that are presented by the same authors to study cell migration with a constant spherical cell shape in a multi-signaling substrates and mechanotaxis effect on cell morphology. Using the finite element discrete methodology, the cell is represented by a group of finite elements. The cell motion is modeled by equilibrium of effective forces on cell body such as traction, protrusion, electrostatic and drag forces, where the cell traction force is a function of the cell internal deformations. To study cell behavior in the presence of different stimuli, the model has been employed in different numerical cases. Our findings, which are qualitatively consistent with well-known related experimental observations, indicate that adding a new stimulus to the cell substrate pushes the cell to migrate more directionally in more elongated form towards the more effective stimuli. For instance, the presence of thermotaxis, chemotaxis and electrotaxis can further move the cell centroid towards the corresponding stimulus, respectively, diminishing the mechanotaxis effect. Besides, the stronger stimulus imposes a greater cell elongation and more cell membrane area. The present model not only provides new insights into cell morphology in a multi-signaling micro-environment but also enables us to investigate in more precise way the cell migration in the presence of different stimuli.

Sperm guidance to the egg finds calcium at the helm

Sperm respond to multiple cues during guidance to the egg including chemical attractants, temperature, and fluid flow. Of these, sperm chemotaxis has been studied most extensively-over 100 years-but only recently has it started to be understood at the molecular level. The long gestation in this understanding has largely been due to technical limitations that include the detection of calcium signal dynamics in a relatively small structure-the flagellum, measurement of actual chemoattractant gradients, the fact that only subpopulations of sperm respond at any given time, and the diversity in swimming behaviors that sperm exhibit from different species. Today, measurements of flagellar calcium signals on a fast time scale, discovery of the ion channels and organelles that may regulate these signals, and better understanding and quantitation of sperm swimming behaviors involved have given more certainty to our understanding of sperm directional swimming and its control by characteristic, calcium-directed asymmetric flagellar bends. Future research will need to apply these technical advances to other forms of sperm guidance such as thermotaxis and rheotaxis as well as gaining an understanding of how the flagellar apparatus is controlled by calcium.

Computational modelling of multi-cell migration in a multi-signalling substrate

Cell migration is a vital process in many biological phenomena ranging from wound healing to tissue regeneration. Over the past few years, it has been proven that in addition to cell-cell and cell-substrate mechanical interactions (mechanotaxis), cells can be driven by thermal, chemical and/or electrical stimuli. A numerical model was recently presented by the authors to analyse single cell migration in a multi-signalling substrate. That work is here extended to include multi-cell migration due to cell-cell interaction in a multi-signalling substrate under different conditions. This model is based on balancing the forces that act on the cell population in the presence of different guiding cues. Several numerical experiments are presented to illustrate the effect of different stimuli on the trajectory and final location of the cell population within a 3D heterogeneous multi-signalling substrate. Our findings indicate that although multi-cell migration is relatively similar to single cell migration in some aspects, the associated behaviour is very different. For instance, cell-cell interaction may delay single cell migration towards effective cues while increasing the magnitude of the average net cell traction force as well as the local velocity. Besides, the random movement of a cell within a cell population is slightly greater than that of single cell migration. Moreover, higher electrical field strength causes the cell slug to flatten near the cathode. On the other hand, as with single cell migration, the existence of electrotaxis dominates mechanotaxis, moving the cells to the cathode or anode pole located at the free surface. The numerical results here obtained are qualitatively consistent with related experimental works.

Suppression of progesterone-enhanced hyperactivation in hamster spermatozoa by γ-aminobutyric acid

It has been recently shown that mammalian spermatozoa were hyperactivated by steroids, amines and amino acids. In the present study, we investigated whether hyperactivation of hamster sperm is regulated by progesterone (P) and γ-aminobutyric acid (GABA). Although sperm hyperactivation was enhanced by P, GABA significantly suppressed P-enhanced hyperactivation in a dose-dependent manner. Suppression of P-enhanced hyperactivation by GABA was significantly inhibited by an antagonist of the GABAA receptor (bicuculline). Moreover, P bound to the sperm head, and this binding was decreased by GABA. Because the concentrations of GABA and P change in association with the estrous cycle, these results suggest that GABA and P competitively regulate the enhancement of hyperactivation through the GABAA receptor.

Regulation and disruption of hamster sperm hyperactivation by progesterone, 17β-estradiol and diethylstilbestrol

Purpose

Hyperactivation of hamster sperm is dose-dependently enhanced by progesterone (P) and 17β-estradiol (E). In the first part of the present study, enhancement of hyperactivation in response to the concentrations of P and E was examined in detail and in the second part, it was examined whether enhancement of hyperactivation by P and E was disrupted by diethylstilbestrol (DES).

Methods

Hamster spermatozoa were hyperactivated by incubation in modified Tyrode's albumin lactate pyruvate medium with P, E and/or DES. After spermatozoa were recorded using a video-microscope, observations were quantified by manually counting the numbers of total, motile and hyperactivated spermatozoa.

Results

Hyperactivation was enhanced in response to the concentrations of P and E. When spermatozoa were exposed to DES with E, moreover, DES significantly and strongly suppressed P-enhanced hyperactivation by accelerating the effect of E, but DES itself only weakly suppressed P-enhanced hyperactivation.

Conclusions

Enhancement of hyperactivation was regulated by the concentrations of P and E, suggesting that in vivo hamster spermatozoa are hyperactivated through "monitoring" these concentrations in the oviduct. DES in combination with E suppressed P-enhanced hyperactivation, suggesting that DES significantly disrupts hyperactivation by acting as an accelerator of the effect of E.

A role for the chemokine receptor CCR6 in mammalian sperm motility and chemotaxis

Although recent evidence indicates that several chemokines and defensins, well-known as inflammatory mediators, are expressed in the male and female reproductive tracts, the location and functional significance of chemokine networks in sperm physiology and sperm reproductive tract interactions are poorly understood. To address this deficiency in our knowledge, we examined the expression and function in sperm of CCR6, a receptor common to several chemoattractant peptides, and screened several reproductive tract fluids for the presence of specific ligands. CCR6 protein is present in mouse and human sperm and mainly localized in the sperm tail with other minor patterns in sperm from mice (neck and acrosomal region) and men (neck and midpiece regions). As expected from the protein immunoblotting and immunofluorescence results, mouse Ccr6 mRNA is expressed in the testis. Furthermore, the Defb29 mRNA encoding the CCR6 ligand, β-defensin DEFB29, is expressed at high levels in the epididymis. As determined by protein chip analysis, several chemokines (including some that act through CCR6, such as CCL20/MIP-3α (formerly macrophage inflammatory protein 3α) and protein hormones were present in human follicular fluid, endometrial secretions, and seminal plasma. In functional chemotaxis assays, capacitated human sperm exhibited a directional movement towards CCL20, and displayed modifications in motility parameters. Our data indicate that chemokine ligand/receptor interactions in the male and female genital tracts promote sperm motility and chemotaxis under non-inflammatory conditions. Therefore, some of the physiological reactions mediated by CCR6 ligands in male reproduction extend beyond a pro-inflammatory response and might find application in clinical reproduction and/or contraception.

Different migration patterns of sea urchin and mouse sperm revealed by a microfluidic chemotaxis device

Chemotaxis refers to a process whereby cells move up or down a chemical gradient. Sperm chemotaxis is known to be a strategy exploited by marine invertebrates such as sea urchins to reach eggs efficiently in moving water. Less is understood about how or whether chemotaxis is used by mammalian sperm to reach eggs, where fertilization takes place within the confinement of a reproductive tract. In this report, we quantitatively assessed sea urchin and mouse sperm chemotaxis using a recently developed microfluidic model and high-speed imaging. Results demonstrated that sea urchin Arbacia punctulata sperm were chemotactic toward the peptide resact with high chemotactic sensitivity, with an average velocity V_x up the chemical gradient as high as 20% of its average speed (238 μm/s), while mouse sperm displayed no statistically significant chemotactic behavior in progesterone gradients, which had been proposed to guide mammalian sperm toward eggs. This work demonstrates the validity of a microfluidic model for quantitative sperm chemotaxis studies, and reveals a biological insight that chemotaxis up a progesterone gradient may not be a universal strategy for mammalian sperm to reach eggs.

Thermotaxis of human sperm cells in extraordinarily shallow temperature gradients over a wide range

On the basis of the finding that capacitated (ready to fertilize) rabbit and human spermatozoa swim towards warmer temperatures by directing their movement along a temperature gradient, sperm thermotaxis has been proposed to be one of the processes guiding these spermatozoa to the fertilization site. Although the molecular mechanism underlying sperm thermotaxis is gradually being revealed, basic questions related to this process are still open. Here, employing human spermatozoa, we addressed the questions of how wide the temperature range of thermotaxis is, whether this range includes an optimal temperature or whether spermatozoa generally prefer swimming towards warmer temperatures, whether or not they can sense and respond to descending temperature gradients, and what the minimal temperature gradient is to which they can thermotactically respond. We found that human spermatozoa can respond thermotactically within a wide temperature range (at least 29–41°C), that within this range they preferentially accumulate in warmer temperatures rather than at a single specific, preferred temperature, that they can respond to both ascending and descending temperature gradients, and that they can sense and thermotactically respond to temperature gradients as low as <0.014°C/mm. This temperature gradient is astonishingly low because it means that as a spermatozoon swims through its entire body length (46 µm) it can sense and respond to a temperature difference of <0.0006°C. The significance of this surprisingly high temperature sensitivity is discussed.

Testing human sperm chemotaxis: how to detect biased motion in population assays

Biased motion of motile cells in a concentration gradient of a chemoattractant is frequently studied on the population level. This approach has been particularly employed in human sperm chemotactic assays, where the fraction of responsive cells is low and detection of biased motion depends on subtle differences. In these assays, statistical measures such as population odds ratios of swimming directions can be employed to infer chemotactic performance. Here, we report on an improved method to assess statistical significance of experimentally determined odds ratios and discuss the strong impact of data correlations that arise from the directional persistence of sperm swimming.