Differences and Similarities: The Richness of Comparative Sperm Physiology

Influence of extracellular ATP on mammalian sperm physiology

In addition to its central role in cellular metabolism, adenosine 5'-triphosphate (ATP) is an important extracellular signalling molecule involved in various physiological processes. In reproduction, extracellular ATP participates in both autocrine and paracrine paths regulating gametogenesis, gamete maturation and fertilisation. This review focusses on how extracellular ATP modulates sperm physiology with emphasis on the mammalian acrosome reaction. The presence of extracellular ATP in the reproductive tract is primarily determined by the ion channels and transporters that influence its movement within the cells comprising the tract. The main targets of extracellular ATP in spermatozoa are its own transporters, particularly species-specific sperm purinergic receptors. We also discuss notable phenotypes from knock-out mouse models and human Mendelian inheritance related to ATP release mechanisms, along with immunological, proteomic, and functional observations regarding sperm purinergic receptors and their involvement in sperm signalling.

Cytosolic and Acrosomal pH Regulation in Mammalian Sperm

As in most cells, intracellular pH regulation is fundamental for sperm physiology. Key sperm functions like swimming, maturation, and a unique exocytotic process, the acrosome reaction, necessary for gamete fusion, are deeply influenced by pH. Sperm pH regulation, both intracellularly and within organelles such as the acrosome, requires a coordinated interplay of various transporters and channels, ensuring that this cell is primed for fertilization. Consistent with the pivotal importance of pH regulation in mammalian sperm physiology, several of its unique transporters are dependent on cytosolic pH. Examples include the Ca2+ channel CatSper and the K+ channel Slo3. The absence of these channels leads to male infertility. This review outlines the main transport elements involved in pH regulation, including cytosolic and acrosomal pH, that participate in these complex functions. We present a glimpse of how these transporters are regulated and how distinct sets of them are orchestrated to allow sperm to fertilize the egg. Much research is needed to begin to envision the complete set of players and the choreography of how cytosolic and organellar pH are regulated in each sperm function.

L-Proline: A Promising Tool for Boosting Cryotolerance and Fertilizing Ability of Cryopreserved Sperm in Animals

Sperm cryopreservation technology significantly contributes to the safeguarding of genetic resources, particularly for endangered species, and supports the use of artificial insemination in domestic animals. Therefore, cryopreservation can negatively affect sperm health and function leading to reduce the freezing ability and fertility potential. Therefore, it is essential to prioritize the improvement of cryotolerance in cryopreserved sperm to enhance reproductive efficiency and ensure sustainability in livestock herds. The main reason for sperm dysfunction after thawing may be related to the excessive amount of oxidative stress (OS) produced during cryopreservation. Scientists have different ways for counteracting this OS including the use of plant extracts, enzymes, minerals, anti-freezing proteins, and amino acids. Recently, one such amino acid is L-proline (LP), which has multiple roles such as osmotic and OS defense, nitrogen, and carbon metabolism, as well as cell survival and signaling. LP has been found in seminal plasma and has recently been added to the freezing extender to improve the various post-thaw parameters of sperm. This improvement is related to the ability of LP to reduce the OS, sustain the plasma membrane and to act as an osmoregulatory agent. Moreover, LP can suppress cell apoptosis by modulating intracellular redox in sperm. This review addresses the ongoing research on the addition of L-proline as an osmoregulatory agent in freezing extenders to increase the cryotolerance of animal spermatozoa to freeze-thaw.

Characterization of a sperm motility signalling pathway in a gonochoric coral suggests conservation across cnidarian sexual systems

Most stony corals liberate their gametes into the water column via broadcast spawning, where fertilization hinges upon the activation of directional sperm motility. Sperm from gonochoric and hermaphroditic corals display distinct morphological and molecular phenotypes, yet it is unknown whether the signalling pathways controlling sperm motility are also distinct between these sexual systems. Here, we addressed this knowledge gap using the gonochoric, broadcast spawning coral Astrangia poculata. We found that cytosolic alkalinization of sperm activates the pH-sensing enzyme soluble adenylyl cyclase (sAC), which is required for motility. Additionally, we demonstrate for the first time in any cnidarian that sAC activity leads to protein kinase A (PKA) activation, and that PKA activity contributes to sperm motility activation. Ultrastructures of A. poculata sperm displayed morphological homology with other gonochoric cnidarians, and sAC exhibited broad structural and functional conservation across this phylum. These results indicate a conserved role for pH-dependent sAC-cAMP-PKA signalling in sperm motility across coral sexual systems, and suggest that the role of this pathway in sperm motility may be ancestral in metazoans. Finally, the dynamics of this pH-sensitive pathway may play a critical role in determining the sensitivity of marine invertebrate reproduction to anthropogenic ocean acidification.

ATP increases head volume in capacitated human sperm via a purinergic channel

Scanning ion-conductance microscopy allowed us to document an external Ca²⁺ dependent ATP driven volume increase (ATPVI) in capacitated human sperm heads. We examined the involvement of purinergic receptors (PRs) P2X2R and P2X4R in ATPVI using their co-agonists progesterone and Ivermectin (Iver), and Cu²⁺, which co-activates P2X2Rs and inhibits P2X4Rs. Iver enhanced ATPVI and Cu²⁺ and 5BDBD inhibited it, indicating P2X4Rs contributed to this response. Moreover, Cu²⁺ and 5BDBD inhibited the ATP-induced acrosome reaction (AR) which was enhanced by Iver. ATP increased the concentration of intracellular Ca²⁺ ([Ca²⁺]_i) in >45% of individual sperm, most of which underwent AR monitored using FM4-64. Our findings suggest that human sperm P2X4R activation by ATP increases [Ca²⁺]_i mainly due to Ca²⁺ influx which leads to a sperm head volume increase, likely involving acrosomal swelling, and resulting in AR.

Hyperpolarization induces cytosolic alkalization of mouse sperm flagellum probably through sperm Na+/H+ exchanger

The sperm-specific sodium/proton exchanger (sNHE) is an indispensable protein for male fertility in mammals. Nevertheless, it is still unknown how mammalian sNHE is regulated. Evidence obtained from sea urchin sNHE indicates that hyperpolarization of plasma membrane potential (Vm), which is a hallmark of mammalian capacitation, positively regulates the sNHE. Therefore, we explored the activity of sNHE in mouse and human sperm by fluorescence imaging of intracellular pH (pHi) with a ratiometric dye, SNARF-5F. A valinomycin-induced Vm hyperpolarization elevated sperm flagellar pHi of wild-type mouse, but not in sNHE-KO mouse. Moreover, this pHi increase was inhibited in a high K+ (40 mM) medium. These results support the idea that mouse sNHE is activated by Vm hyperpolarization. Interestingly, we observed different types of kinetics derived from valinomycin-induced alkalization, including some (30 %) without any pHi changes. Our quantitative pHi determinations revealed that unresponsive cells had a high resting pHi (> 7.5), suggesting that the activity of mouse sNHE is regulated by the resting pHi. On the other hand, valinomycin did not increase the pHi of human sperm in the head or the flagellum, regardless of their resting pHi values. Our findings suggest that the regulatory mechanisms of mammalian sNHEs are probably distinct depending on the species.

Simultaneous recording of multiple cellular signaling events by frequency- and spectrally-tuned multiplexing of fluorescent probes

Fluorescent probes that change their spectral properties upon binding to small biomolecules, ions, or changes in the membrane potential (Vm) are invaluable tools to study cellular signaling pathways. Here, we introduce a novel technique for simultaneous recording of multiple probes at millisecond time resolution: frequency- and spectrally-tuned multiplexing (FASTM). Different from present multiplexing approaches, FASTM uses phase-sensitive signal detection, which renders various combinations of common probes for Vm and ions accessible for multiplexing. Using kinetic stopped-flow fluorimetry, we show that FASTM allows simultaneous recording of rapid changes in Ca2+, pH, Na+, and Vm with high sensitivity and minimal crosstalk. FASTM is also suited for multiplexing using single-cell microscopy and genetically encoded FRET biosensors. Moreover, FASTM is compatible with optochemical tools to study signaling using light. Finally, we show that the exceptional time resolution of FASTM also allows resolving rapid chemical reactions. Altogether, FASTM opens new opportunities for interrogating cellular signaling.

Co-Adaptation of Physical Attributes of the Mammalian Female Reproductive Tract and Sperm to Facilitate Fertilization

The functions of the female reproductive tract not only encompass sperm migration, storage, and fertilization, but also support the transport and development of the fertilized egg through to the birth of offspring. Further, because the tract is open to the external environment, it must also provide protection against invasive pathogens. In biophysics, sperm are considered “pusher microswimmers”, because they are propelled by pushing fluid behind them. This type of swimming by motile microorganisms promotes the tendency to swim along walls and upstream in gentle fluid flows. Thus, the architecture of the walls of the female tract, and the gentle flows created by cilia, can guide sperm migration. The viscoelasticity of the fluids in the tract, such as mucus secretions, also promotes the cooperative swimming of sperm that can improve fertilization success; at the same time, the mucus can also impede the invasion of pathogens. This review is focused on how the mammalian female reproductive tract and sperm interact physically to facilitate the movement of sperm to the site of fertilization. Knowledge of female/sperm interactions can not only explain how the female tract can physically guide sperm to the fertilization site, but can also be applied for the improvement of in vitro fertilization devices.

Activation of parasperm and eusperm upon ejaculation in Lepidoptera

Almost all Lepidoptera produce two kinds of spermatozoa, a typical nucleated spermatozoön and an anucleate one. Inactive in the male prior to ejaculation, both of these spermatozoa become motile upon ejaculation and move to the female's sperm storage organ. This study shows that in several phylogenetically and morphologically diverse species of Lepidoptera, the anucleate spermatozoa, or parasperm (also known as apyrene spermatozoa), and the nucleated spermatozoa, or eusperm (also known as eupyrene spermatozoa), are activated by a protein of approximately 37.7 kDa added by a secretion from the male. Although proteases have been shown to activate these parasperm, inhibitors of proteases did not prevent activation of the tobacco hornworm moth, Manduca sexta, parasperm, even at well over normal working concentrations of the inhibitors. Parasperm could also be activated by an ionophore, indicating that a trans-membrane ionic event is involved. In contrast to parasperm, eusperm are first ejaculated as bundles of 256 spermatozoa. This study identified a male protein of similar molecular weight that dissociates the eusperm from the bundles, but that is sensitive to proteases. Based on these characteristics, the activators of both types of spermatozoa appear to be different from the initiatorin imputed to be the activator of commercial silkmoth, Bombyx mori, spermatozoa. The role of these proteins in these unique modes of lepidopterous sperm activation may have been adapted from other roles in other kinds of insects, and indicates especially an important function of parasperm in the reproductive physiology and/or behavior of female lepidopterans.

Odorant and Taste Receptors in Sperm Chemotaxis and Cryopreservation: Roles and Implications in Sperm Capacitation, Motility and Fertility

Sperm chemotaxis, which guide sperm toward oocyte, is tightly associated with sperm capacitation, motility, and fertility. However, the molecular mechanism of sperm chemotaxis is not known. Reproductive odorant and taste receptors, belong to G-protein-coupled receptors (GPCR) super-family, cause an increase in intracellular Ca²⁺ concentration which is pre-requisite for sperm capacitation and acrosomal reaction, and result in sperm hyperpolarization and increase motility through activation of Ca²⁺-dependent Cl^¯ channels. Recently, odorant receptors (ORs) in olfactory transduction pathway were thought to be associated with post-thaw sperm motility, freeze tolerance or freezability and cryo-capacitation-like change during cryopreservation. Investigation of the roles of odorant and taste receptors (TRs) is important for our understanding of the freeze tolerance or freezability mechanism and improve the motility and fertility of post-thaw sperm. Here, we reviewed the roles, mode of action, impact of odorant and taste receptors on sperm chemotaxis and post-thaw sperm quality.

Long-term segmentation-free assessment of head-flagellum movement and intracellular calcium in swimming human sperm

Human spermatozoa are the archetype of long-term self-organizing transport in nature and are critical for reproductive success. They utilize coordinated head and flagellar movements to swim long distances within the female reproductive tract in order to find and fertilize the egg. However, to date, long-term analysis of the sperm head-flagellar movements, or indeed those of other flagellated microorganisms, remains elusive due to limitations in microscopy and flagellar-tracking techniques. Here, we present a novel methodology based on local orientation and isotropy of bio-images to obtain long-term kinematic and physiological parameters of individual free-swimming spermatozoa without requiring image segmentation (thresholding). This computer-assisted segmentation-free method evaluates, for the first time, characteristics of the head movement and flagellar beating for up to 9.2 min. We demonstrate its powerful use by showing how releasing Ca²⁺ from internal stores significantly alters long-term sperm behavior. The method allows for straightforward generalization to other bio-imaging applications, such as studies of bull sperm and Trypanosoma, or indeed of other flagellated microorganisms - appealing to communities other than those investigating sperm biology.

Time-Lapse Flow Cytometry: A Robust Tool to Assess Physiological Parameters Related to the Fertilizing Capability of Human Sperm

Plasma membrane (PM) hyperpolarization, increased intracellular pH (pH_i), and changes in intracellular calcium concentration ([Ca²⁺]_i) are physiological events that occur during human sperm capacitation. These parameters are potential predictors of successful outcomes for men undergoing artificial reproduction techniques (ARTs), but methods currently available for their determination pose various technical challenges and limitations. Here, we developed a novel strategy employing time-lapse flow cytometry (TLFC) to determine capacitation-related membrane potential (E_m) and pH_i changes, and progesterone-induced [Ca²⁺]_i increases. Our results show that TLFC is a robust method to measure absolute E_m and pH_i values and to qualitatively evaluate [Ca²⁺]_i changes. To support the usefulness of our methodology, we used sperm from two types of normozoospermic donors: known paternity (subjects with self-reported paternity) and no-known paternity (subjects without self-reported paternity and no known fertility problems). We found relevant differences between them. The incidences of membrane hyperpolarization, pH_i alkalinization, and increased [Ca²⁺]_i were consistently high among known paternity samples (100%, 100%, and 86%, respectively), while they varied widely among no-known paternity samples (44%, 17%, and 45%, respectively). Our results indicate that TLFC is a powerful tool to analyze key physiological parameters of human sperm, which pending clinical validation, could potentially be employed as fertility predictors.

Understanding sperm physiology: Proximate and evolutionary explanations of sperm diversity

Much can be gained from the comprehensive study of a biological system. Based on what is known as Mayr's proximate-ultimate causation and the subsequent expansion to Tinbergen's four questions, biological traits can be understood by taking into account different approximations that try to explain mechanisms, development, adaptive significance or phylogeny. These, in principle, separate areas, can be integrated crossing boundaries, but bearing in mind that answers to one question would not explain a different query. Studies of sperm biology have, until now, not benefited much from this framework and potential integration. Proximate causes (particularly mechanisms) have been the subject of interest for reproductive biologists, and evolutionary explanations have been the domain of behavioural ecologists with interest in adaptive significance of traits in the context of post-copulatory sexual selection. This review will summarize opportunities for research in the different areas, focusing on sperm preparation for fertilization and suggesting possible integration within and between proximate and evolutionary studies.

The role of oxidative stress in postcopulatory selection

Two decades ago, von Schantz et al. (von Schantz T, Bensch S, Grahn M, Hasselquist D, Wittzell H. 1999 Good genes, oxidative stress and condition-dependent sexual signals. Proc. R. Soc. B 266, 1-12. (doi:10.1098/rspb.1999.0597)) united oxidative stress (OS) biology with sexual selection and life-history theory. This set the scene for analysis of how evolutionary trade-offs may be mediated by the increase in reactive molecules resulting from metabolic processes at reproduction. Despite 30 years of research on OS effects on infertility in humans, one research area that has been left behind in this integration of evolution and OS biology is postcopulatory sexual selection-this integration is long overdue. We review the basic mechanisms in OS biology, why mitochondria are the primary source of ROS and ATP production during oxidative metabolism, and why sperm, and its performance, is uniquely susceptible to OS. We also review how postcopulatory processes select for antioxidation in seminal fluids to counter OS and the implications of the net outcome of these processes on sperm damage, sperm storage, and female and oocyte manipulation of sperm metabolism and repair of DNA to enhance offspring fitness. This article is part of the theme issue 'Fifty years of sperm competition'.