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

In vitro approach points to a chemotactic effect of melatonin on ram spermatozoa

Sperm orientation mechanisms, such as chemotaxis, are essential for the sperm to reach the oocyte and fertilize it. Melatonin is secreted by the cumulus cells and is also present in the follicular fluid in mammals. The presence of membrane receptors for melatonin in ram spermatozoa, and its proven involvement in the sperm functionality, may suggest a possible role in the guided movement towards the oocyte. Hence, the objective of the present work is to study the in vitro potential chemotactic action of melatonin on ram spermatozoa, analysing the influence of the season (breeding and non-breeding) and the sperm capacitation state. The first experimental approach consisted in the inclusion of melatonin in the upper layer of a swim-up selection method. During the non-breeding season, the presence of melatonin at 100 pM and 1 μM concentrations significantly increased the cell recovery rate, and induced changes in the sperm location of the MT₂ melatonin receptor, compared with the standard swim-up. Moreover, the selected sperm population with 100 pM melatonin presented a higher percentage of capacitated spermatozoa. The greater recovery rate obtained with melatonin could be due to the stimulation of sperm movement in random directions, i.e., a chemokinetic effect, or due to a guided movement (chemotaxis) towards the gradient of the melatonin. To elucidate this issue, together with the study of the influence of the sperm capacitation status, we performed a second experimental approach which consisted in the use of chemotaxis chambers and an open-source software (Open-CASA) that analyses the sperm trajectories towards the hormone gradient and calculates a chemotaxis index (SL index). There was a significant difference between the SL index in the presence of 1 μM melatonin and the control without hormone. This effect was only observed in capacitated spermatozoa with cAMP-elevating agents (Cap-CK samples) obtained during the non-breeding season. These results would point to an in vitro chemotactic effect of melatonin on ram spermatozoa, although chemokinesis cannot be ruled out. Nonetheless, the inclusion of this hormone in the swim-up procedure could enhance the sperm recovery rate.

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.

Sperm-Guiding Unconventional Prostaglandins in C. elegans: Synthesis and Signaling

Prostaglandins comprise a family of lipid signaling molecules derived from polyunsaturated fatty acids and are involved in a wide array of biological processes, including fertilization. Prostaglandin-endoperoxide synthase (a.k.a. cyclooxygenase or Cox) initiates prostaglandin synthesis from 20-carbon polyunsaturated fatty acids, such as arachidonic acid. Oocytes of Caenorhabditis elegans (C. elegans) have been shown to secrete sperm-guidance cues prostaglandins, independent of Cox enzymes. Both prostaglandin synthesis and signal transduction in C. elegans are environmentally modulated pathways that regulate sperm guidance to the fertilization site. Environmental factors such as food triggers insulin and TGF-β secretion and their levels regulate tissue-specific prostaglandin synthesis in C. elegans. This novel PG pathway is abundant in mouse and human ovarian follicular fluid, where their functions, mechanism of synthesis and pathways remain to be established. Given the importance of prostaglandins in reproductive processes, a better understanding of how diets and other environmental factors influence their synthesis and function may lead to new strategies towards improving fertility in mammals.

N-formyl-l-aspartate: A novel sperm chemoattractant identified in ovulatory phase oviductal fluid using a microfluidic chip

BACKGROUND

Chemotaxis, as a mechanism for sperm guidance although known, has been difficult to demonstrate in vitro. Consequently, very few chemoattractants have been identified till date.

OBJECTIVES

To investigate sperm motility behavior in response to ovulatory (OV) and preovulatory (preOV) oviductal fluid (OF) and identify potential chemotactic metabolites.

MATERIALS AND METHODS

Intracellular calcium ([Ca²⁺ ]_I ) influx in capacitating sperm was determined by spectrofluorimetry. The chemotactic response of rat caudal sperm to OF from the preOV- and OV- phases of normally cycling female rats was assessed in a microfluidic device developed by us. Hydrophilic metabolites extracted from the OF of both the phases were resolved and identified by LC-MS/MS, followed by data analysis using XCMS and MetaboAnalyst software, and chemotactic potential of the most promising compound was validated using the microfluidic device.

RESULTS

Spectrofluorimetric analysis depicts a significant increase in sperm [Ca²⁺ ]_I in response to OV-OF. With the microfluidic chemotaxis assay, sperm population shows a significantly increased directionality and velocity to an ascending gradient of 0.06 µg/µl OV-OF compared to preOV-OF. LC-MS/MS of the OFs demonstrates five and four metabolites to be exclusive to the OV-OF and preOV-OF, respectively, and 25 metabolites common to both, of which 14 metabolites, including N-formyl-l-aspartate (NFA), are increased in OV-OF; NFA was tested for its ability to influence sperm movement, and shows chemotaxis potential.

DISCUSSION AND CONCLUSION(S)

This is the first study that has systematically demonstrated sperm chemotaxis with OV phase rat OF, identified NFA present in this fluid as a novel chemoattractant to sperm, and proven the utility of the device to test putative chemoattractants. It remains to be seen whether NFA is present in the follicular fluid (FF) of infertile women, and whether it may likely be a reason for the failure of natural conception in idiopathic infertile women.

Analysis of sperm chemotaxis

In many species, sperm must locate the female gamete to achieve fertilization. Molecules diffusing from the egg envelope, or the female genital tract, guide the sperm toward the oocyte through a process called chemotaxis. Sperm chemotaxis has been studied for more than 100 years being a widespread phenomenon present from lower plants to mammals. This process has been mostly studied in external fertilizers where gametes undergo a significant dilution, as compared to internal fertilizers where the encounter is more defined by the topology of the female tract and only a small fraction of sperm appear to chemotactically respond. Here, we summarize the main methods to measure sperm swimming responses to a chemoattractant, both in populations and in individual sperm. We discuss a novel chemotactic index (CI) to score sperm chemotaxis in external fertilizers having circular trajectories. This CI is based on the sperm progressive displacement and its orientation angle to the chemoattractant source.

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.

Human sperm rheotaxis: a passive physical process

A long-standing question in natural reproduction is how mammalian sperm navigate inside female reproductive tract and finally reach the egg cell, or oocyte. Recently, fluid flow was proposed as a long–range guidance cue for sperm navigation. Coitus induces fluid flow from oviduct to uterus, and sperm align themselves against the flow direction and swim upstream, a phenomenon termed rheotaxis. Whether sperm rheotaxis is a passive process dominated by fluid mechanics, or sperm actively sense and adapt to fluid flow remains controversial. Here we report the first quantitative study of sperm flagellar motion during human sperm rheotaxis and provide direct evidence indicating that sperm rheotaxis is a passive process. Experimental results show that there is no significant difference in flagellar beating amplitude and asymmetry between rheotaxis-turning sperm and those sperm swimming freely in the absence of fluid flow. Additionally, fluorescence image tracking shows no Ca²⁺ influx during sperm rheotaxis turning, further suggesting there is no active signal transduction during human sperm rheotaxis.

Cross-talk between free and bound spermatozoa to modulate initial sperm:egg ratios at the site of fertilization in the mammalian oviduct

This essay proposes that highly localized communication between free and bound spermatozoa in the caudal portion of the oviduct acts to regulate the numbers detaching from the epithelium and progressing to the site of fertilization close to the time of ovulation. Low initial sperm:egg ratios are essential for monospermic fertilization. Liberation of surface macromolecules and metabolic prompting from activated spermatozoa, together with altered patterns of sperm movement and dynamic differences in intracellular Ca(2+) ion status between neighboring sperm cells, would influence the progressive release of spermatozoa from the reservoir in the oviduct isthmus. Different intensities of preovulatory epithelial binding, reflecting a range of states in the sperm surface membranes and associated proteins, would provide a further explanation for a chronologically staggered periovulatory detachment of spermatozoa. Intimate sperm-sperm interactions within the confines of the oviduct isthmus offer a sensitive means of fine-tuning the vanguard of competent male gametes reaching the isthmo-ampullary junction.

The computational sperm cell

Sperm are guided to the egg by a gradient of chemical attractants - a process called chemotaxis. The binding of the chemoattractant to receptors on the surface of the flagellum triggers a cascade of signaling events that eventually lead to an influx of Ca(2+) ions. Based on these Ca(2+) surges, which control the waveform of the flagellar beat, sperm adjust their swimming path toward the egg. In past years, many components of chemotactic signaling have been identified. Moreover, kinetic spectroscopy and imaging techniques unraveled the sequence of cellular events controlling swimming behavior. During navigation in a chemical gradient, sperm perform a surprising variety of computational operations. Here we discuss theoretical concepts of navigation strategies and the cellular underpinnings.