TRPV4 is endogenously expressed in vertebrate spermatozoa and regulates intracellular calcium in human sperm

Sperm-Specific CatSper is Not Conserved in All Vertebrates and May Not be the Only Progesterone-Responsive Ion Channel Present in Sperm

Progesterone (P4) acts as a key conserved signalling molecule in vertebrate reproduction. P4 is especially important for mature sperm physiology and subsequent reproductive success. "CatSpermasome", a multi-unit molecular complex, has been suggested to be the main if not the only P4-responsive atypical Ca2+-ion channel present in mature sperm. Altogether, here we analyse the protein sequences of CatSper1-4 from more than 500 vertebrates ranging from early fishes to humans. CatSper1 becomes longer in mammals due to sequence gain mainly at the N-terminus. Overall the conservation of full-length CatSper1-4 as well as the individual TM regions remain low. The lipid-water-interface residues (i.e. a 5 amino acid stretch sequence present on both sides of each TM region) also remain highly diverged. No specific patterns of amino acid distributions were observed. The total frequency of positively charged, negatively charged or their ratios do not follow in any specific pattern. Similarly, the frequency of total hydrophobic, total hydrophilic residues or even their ratios remain random and do not follow any specific pattern. We noted that the CatSper1-4 genes are missing in amphibians and the CatSper1 gene is missing in birds. The high variability of CatSper1-4 and gene-loss in certain clades indicate that the "CatSpermasome" is not the only P4-responsive ion channel. Data indicate that the molecular evolution of CatSper is mostly guided by diverse hydrophobic ligands rather than only P4. The comparative data also suggest possibilities of other Ca2+-channel/s in vertebrate sperm that can also respond to P4.

Aqp4a and Trpv4 mediate regulatory cell volume increase for swimming maintenance of marine fish spermatozoa

Volume regulation is essential for cell homeostasis and physiological function. Amongst the sensory molecules that have been associated with volume regulation is the transient receptor potential vanilloid 4 (TRPV4), which is a non-selective cation channel that in conjunction with aquaporins, typically controls regulatory volume decrease (RVD). Here we show that the interaction between orthologous AQP4 (Aqp4a) and TRPV4 (Trpv4) is important for regulatory volume increase (RVI) in post-activated marine fish spermatozoa under high osmotic stress. Based upon electrophysiological, volumetric, and in vivo and ex vivo functional experiments using the pharmacological and immunological inhibition of Aqp4a and Trpv4 our model suggests that upon ejaculation and exposure to the hypertonic seawater, spermatozoon shrinkage is initially mediated by water efflux through Aqp1aa in the flagellar tail. The shrinkage results in an increase in intracellular Ca2+ concentration, and the activation of sperm motility and a Na+/K+/2Cl- (NKCC1) cotransporter. The activity of NKCC1 is required for the initiation of cell swelling, which secondarily activates the Aqp4a-Trpv4 complex to facilitate the influx of water via Aqp4a-M43 and Ca2+ via Trpv4 and L-type channels for the mediation of RVI. The inhibitory experiments show that blocking of each of these events prevents either shrinkage or RVI. Our data thus reveal that post-activated marine fish spermatozoa are capable of initiating RVI under a high hypertonic stress, which is essential for the maintenance of sperm motility.

The multifaceted role of extracellular ATP in sperm function: From spermatogenesis to fertilization

Extracellular adenosine 5'-triphosphate (ATP) is a vital signaling molecule involved in various physiological processes within the body. In recent years, studies have revealed its significant role in male reproduction, particularly in sperm function. This review explores the multifaceted role of extracellular ATP in sperm function, from spermatogenesis to fertilization. We discuss the impact of extracellular ATP on spermatogenesis, sperm maturation and sperm-egg fusion, highlighting the complex regulatory mechanisms and potential clinical applications in the context of male infertility. By examining the latest research, we emphasize the crucial role of extracellular ATP in sperm function and propose future research directions to further.

Effects of cryopreservation in the presence of Natural Deep Eutectic Solvents (NADESs) on sperm parameters

Natural Deep Eutectic Solvents (NADESs) are being considered as a potential alternative to traditional cryoprotective agents (CPAs) in sperm freezing. The study aimed to assess the effects of NADESs as a CPA on human sperm parameters. A total of 32 normozoospermic semen samples were collected from the Alzahra infertility treatment center (Iran) between July 2021 and September 2022. The samples were categorized into eight different groups: 1) a control (nonfrozen), and groups frozen with 2) SpermFreeze Solution, 3) ChX (Choline chloride and Xylitol), 4) ChS (Choline chloride and D-sorbitol), 5) ChG (Choline chloride and Glucose), 6) ChU (Choline chloride and Urea), 7) EtP (Ethylene glycol and l-proline), and 8) GlyP (Glycerol and l-proline). The study also analyzed the quality of sperm parameters, such as chromatin condensation and integrity, acrosome integrity, and survival, along with the expression of some genes that affect sperm fertility (TRPV1, TRPV4, SPACA3, and OGG1). The study found there were notable variations in sperm parameters (such as viability, chromatin condensation and integrity, and acrosome integrity) among frozen groups with some NADESs compared to the SpermFreeze Solution and control groups (P < 0.05). Analysis of gene expression demonstrated that the levels of TRPV1, TRPV4, SPACA3, and OGG1 genes were superior in the GlyP group compared to the other groups (P < 0.05). Additionally, the ChS and ChU groups exhibited preserved expression of these genes compared with the SpermFreeze Solution group. The use of NADESs led to the discovery of a more appropriate CPA that has low toxicity and is highly effective in maintaining the fertility potential of sperm.

Progesterone interacts with the mutational hot-spot of TRPV4 and acts as a ligand relevant for fast Ca2+-signalling

Steroids are also known to induce immediate physiological and cellular response which occurs within minutes to seconds, or even faster. Such non-genomic actions of steroids are rapid and are proposed to be mediated by different ion channels. Transient receptor potential vanilloid sub-type 4 (TRPV4), is a non-specific polymodal ion channel which is involved in several physiological and cellular processes. In this work, we explored the possibilities of Progesterone (P4) as an endogenous ligand for TRPV4. We demonstrate that P4 docks as well as physically interacts with the TM4-loop-TM5 region of TRPV4, a region which is a mutational hotspot for different diseases. Live cell imaging experiments with a genetically encoded Ca2+-sensor suggests that P4 causes quick influx of Ca2+ specifically in the TRPV4 expressing cells, which can be partially blocked by TRPV4-specific inhibitor, suggesting that P4 can act as a ligand for TRPV4. Such P4-mediated Ca2+-influx is altered in cells expressing disease causing TRPV4 mutants, namely in L596P, R616Q, and also in embryonic lethal mutant L618P. P4 dampens, both in terms of "extent" as well as the "pattern" of the Ca2+-influx by other stimulus too in cells expressing TRPV4-Wt, suggesting that P4 crosstalk with the TRPV4-mediated Ca2+-signalling, both in quick and long-term manner. We propose that P4 crosstalk with TRPV4 might be relevant for both acute and chronic pain as well as for other health-related functions.

Research progress on ion channels and their molecular regulatory mechanisms in the human sperm flagellum

The ion channels in sperm tail play an important role in triggering key physiological reactions, e.g., progressive motility, hyperactivation, required for successful fertilization. Among them, CatSper and KSper have been shown to be important ion channels for the transport of Ca2+ and K+ . Moreover, the voltage-gated proton channel Hv1, the sperm-specific sodium-hydrogen exchanger (sNHE), the epithelial sodium channel (ENaC), members of the temperature-sensitive TRP channel family, and the cystic fibrosis transmembrane regulator (CFTR) are also found in the flagellum. This review focuses on the latest advances in ion channels located at the flagellum, describes how they affect sperm physiological function, and summarizes some primary mutual regulation mechanism between ion channels, including PH, membrane potential, and cAMP. These ion channels may be promising targets for clinical application in infertility.

TRPV4 regulates osteoblast differentiation and mitochondrial function that are relevant for channelopathy

Different ion channels present in the osteoblast regulate the cellular functions including bio-mineralization, a process that is a highly stochastic event. Cellular events and molecular signaling involved in such process is poorly understood. Here we demonstrate that TRPV4, a mechanosensitive ion channel is endogenously present in an osteoblast cell line (MC3T3-E1) and in primary osteoblasts. Pharmacological activation of TRPV4 enhanced intracellular Ca²⁺-level, expression of osteoblast-specific genes and caused increased bio-mineralization. TRPV4 activation also affects mitochondrial Ca²⁺-levels and mitochondrial metabolisms. We further demonstrate that different point mutants of TRPV4 induce different mitochondrial morphology and have different levels of mitochondrial translocation, collectively suggesting that TRPV4-mutation-induced bone disorders and other channelopathies are mostly due to mitochondrial abnormalities. These findings may have broad biomedical implications.

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.

TRPV4 regulates mitochondrial Ca2+-status and physiology in primary murine T cells based on their immunological state

T cell activation process is critically affected by temperature and intracellular Ca²⁺-signalling. Yet, the nature and the key molecules involved in such complex Ca²⁺-signalling is poorly understood. It is mostly assumed that ion channels present in the plasma membrane primarily regulate the cytosolic Ca²⁺-levels exclusively. TRPV4 is a non-selective Ca²⁺ channel which can be activated at physiological temperature. TRPV4 is involved in several physiological, pathophysiological process as well as different forms of pain. Here we demonstrate that TRPV4 is endogenously expressed in T cell and is present in the mitochondria of T cells. TRPV4 activation increases mitochondrial Ca²⁺-levels, and alters mitochondrial temperature as well as specific metabolisms. The TRPV4-dependent increment in the mitochondrial Ca²⁺ is context-dependent and not just passively due to the increment in the cytosolic Ca²⁺. Our work also indicates that mitochondrial Ca²⁺-level correlates positively with a series of essential factors, such as mitochondrial membrane potential, mitochondrial ATP production and negatively correlates with certain factors such as mitochondrial temperature. We propose that TRPV4-mediated mitochondrial Ca²⁺-signalling and other metabolisms has implications in the immune activation process including immune synapse formation. Our data also endorse the re-evaluation of Ca²⁺-signalling in T cell, especially in the light of mitochondrial Ca²⁺-buffering and in higher body temperature, such as in case of fever. Presence of TRPV4 in the mitochondria of T cell is relevant for proper and optimum immune response and may provide evolutionary adaptive benefit. These findings may also have broad implications in different pathophysiological process, neuro-immune cross-talks, and channelopathies involving TRPV4.

TRPV4 acts as a mitochondrial Ca2+-importer and regulates mitochondrial temperature and metabolism

TRPV4 is associated with the development of neuropathic pain, sensory defects, muscular dystrophies, neurodegenerative disorders, Charcot Marie Tooth and skeletal dysplasia. In all these cases, mitochondrial abnormalities are prominent. Here, we demonstrate that TRPV4, localizes to a subpopulation of mitochondria in various cell lines. Improper expression and/or function of TRPV4 induces several mitochondrial abnormalities. TRPV4 is also involved in the regulation of mitochondrial numbers, Ca²⁺-levels and mitochondrial temperature. Accordingly, several naturally occurring TRPV4 mutations affect mitochondrial morphology and distribution. These findings may help in understanding the significance of mitochondria in TRPV4-mediated channelopathies possibly classifying them as mitochondrial diseases.

TRPV1 channel in spermatozoa is a molecular target for ROS-mediated sperm dysfunction and differentially expressed in both natural and ART pregnancy failure

Bi-directional crosstalk between Ca2+ signaling and ROS modulates physiological processes as a part of a regulatory circuit including sperm function. The role of transient receptor potential vanilloid 1 (TRPV1) in this regard cannot be undermined. This is the first report demonstrating the Ca2+-sensitive TRPV1 channel to be under-expressed in spermatozoa of subfertile men, idiopathic infertile men, and normozoospermic infertile males with high ROS (idiopathic infertility and unilateral varicocele). To study the effect of TRPV1 in determining the fertility outcome, we compared the expression profile of TRPV1 in spermatozoa of male partners who achieved pregnancy by natural conception (NC+, n = 10), IVF (IVF+, n = 23), or ICSI (ICSI +, n = 9) and their respective counterparts with failed pregnancy NC (n = 7), IVF (n = 23), or ICSI (n = 10), by both immunocytochemistry and flow-cytometry. Reduced expression of TRPV1 in sperm of IVF ± and ICSI ± men with respect to that NC+ men imply its role in mediating successful fertilization. Unsuccessful pregnancy outcome with an underexpression of TRPV1 in sperm of NC-/IVF-/ICSI-men suggests its role in conception and maintenance of pregnancy. Since ROS is regarded as one of the major contributors to sperm dysfunction, the effect of H2O2 +/- TRPV1 modulators (RTX/iRTX) on acrosomal reaction and calcium influx was evaluated to confirm TRPV1 as a redox sensor in human sperm. A significant increment in the percentage of acrosome reacted spermatozoa along with augmented Ca2+-influx was observed after H2O2 treatment, both in the presence or absence of TRPV1 agonist resiniferatoxin (RTX). The effect was attenuated by the TRPV1 antagonist iodoresiniferatoxin (iRTX), indicating the involvement of TRPV1 in mediating H2O2 response. Enhancement of motility and triggering of acrosomal reaction post TRPV1 activation suggested that disruption of these signaling cascades in vivo, possibly due to down-regulation of TRPV1 in these subfertile males. Bioinformatic analysis of the crosstalk between TRPV1 with fertility candidate proteins (reported to influence IVF outcome) revealed cell death and survival, cellular compromise, and embryonic development to be the primary networks affected by anomalous TRPV1 expression. We therefore postulate that TRPV1 can act as a redox sensor, and its expression in spermatozoa may serve as a fertility marker.

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.

TRPV1 in male reproductive system: focus on sperm function

The transient receptor potential vanilloid 1 (TRPV1) is a receptor used to perceive external noxious stimuli and participates in the regulation of various pathophysiological mechanisms in vivo by integrating multiple signals. The explosive growth in knowledge of TRPV1 stemmed from research on neuronal pain and heat sensation over the last decades and is being expanded tremendously in peripheral tissue research. The discovery that TRPV1 is functionally active in male animal and human reproductive tissues have attracted increasing attention in recent years. Indeed, many studies have indicated that TRPV1 is an endocannabinoid receptor that mediates Anandamide's regulation of sperm function. Other characteristics of the TRPV1 channel itself, such as calcium penetration and temperature sensitivity, have also been investigated, especially the possibility that TRPV1 could act as a mediator for sperm thermotaxis. In addition, some reproductive diseases appear to be related to the protective effects of TRPV1 on oxidative stress and heat stress. A better understanding of TRPV1 in these areas should provide strategies for tackling male infertility. This paper is the first to review the expression and mechanism of TRPV1 in the male reproductive system from molecular and cellular perspectives. A focus is given on sperm function, including calcium homeostasis, crosstalk with endocannabinoid system, participation in cholesterol-related sperm maturation, and thermotaxis, hoping to capture the current situation of this rapidly developing field.

Sperm Ion Transporters and Channels in Human Asthenozoospermia: Genetic Etiology, Lessons from Animal Models, and Clinical Perspectives

In mammals, sperm fertilization potential relies on efficient progression within the female genital tract to reach and fertilize the oocyte. This fundamental property is supported by the flagellum, an evolutionarily conserved organelle that provides the mechanical force for sperm propulsion and motility. Importantly several functional maturation events that occur during the journey of the sperm cells through the genital tracts are necessary for the activation of flagellar beating and the acquisition of fertilization potential. Ion transporters and channels located at the surface of the sperm cells have been demonstrated to be involved in these processes, in particular, through the activation of downstream signaling pathways and the promotion of novel biochemical and electrophysiological properties in the sperm cells. We performed a systematic literature review to describe the currently known genetic alterations in humans that affect sperm ion transporters and channels and result in asthenozoospermia, a pathophysiological condition defined by reduced or absent sperm motility and observed in nearly 80% of infertile men. We also present the physiological relevance and functional mechanisms of additional ion channels identified in the mouse. Finally, considering the state-of-the art, we discuss future perspectives in terms of therapeutics of asthenozoospermia and male contraception.

Perspectives on Potential Fatty Acid Modulations of Motility Associated Human Sperm Ion Channels

Human spermatozoan ion channels are specifically distributed in the spermatozoan membrane, contribute to sperm motility, and are associated with male reproductive abnormalities. Calcium, potassium, protons, sodium, and chloride are the main ions that are regulated across this membrane, and their intracellular concentrations are crucial for sperm motility. Fatty acids (FAs) affect sperm quality parameters, reproductive pathologies, male fertility, and regulate ion channel functions in other cells. However, to date the literature is insufficient to draw any conclusions regarding the effects of FAs on human spermatozoan ion channels. Here, we aimed to discern the possible effects of FAs on spermatozoan ion channels and direct guidance for future research. After investigating the effects of FAs on characteristics related to human spermatozoan motility, reproductive pathologies, and the modulation of similar ion channels in other cells by FAs, we extrapolated polyunsaturated FAs (PUFAs) to have the highest potency in modulating sperm ion channels to increase sperm motility. Of the PUFAs, the ω-3 unsaturated fatty acids have the greatest effect. We speculate that saturated and monounsaturated FAs will have little to no effect on sperm ion channel activity, though the possible effects could be opposite to those of the PUFAs, considering the differences between FA structure and behavior.

Activation of TRPV4 stimulates transepithelial K+ secretion in rat epididymal epithelium

The maturation of sperms is dependent on the coordinated interactions between sperm and the unique epididymal luminal milieu, which is characterized by high K+ content. This study investigated the involvement of transient receptor potential vanilloid 4 (TRPV4) in the K+ secretion of epididymal epithelium. The expression level and cellular localization of TRPV4 and Ca2+- activated K+ channels (KCa) were analyzed via RT-PCR, real-time quantitative PCR, western blot, and immunofluorescence. The functional role of TRPV4 was investigated using short circuit current (ISC) and intracellular Ca2+ imaging techniques. We found a predominant expression of TRPV4 in the corpus and cauda epididymal epithelium. Activation of TRPV4 with a selective agonist, GSK1016790A, stimulated a transient decrease in the ISC of the epididymal epithelium. The ISC response was abolished by either the TRPV4 antagonists, HC067047 and RN-1734, or the removal of basolateral K+. Simultaneously, the application of GSK1016790A triggered Ca2+ influx in epididymal epithelial cells. Our data also indicated that the big conductance KCa (BK), small conductance KCa (SK), and intermediate conductance KCa (IK) were all expressed in rat epididymis. Pharmacological studies revealed that BK, but not SK and IK, mediated TRPV4-elicited transepithelial K+ secretion. Finally, we demonstrated that TRPV4 and BK were localized in the epididymal epithelium, which showed an increased expression level from caput to cauda regions of rat epididymis. This study implicates that TRPV4 plays an important role in the formation of high K+ concentration in epididymal intraluminal fluid via promoting transepithelial K+ secretion mediated by BK.

Role of calcium oscillations in sperm physiology

Intracellular Ca²⁺ is a key regulator of cell signaling and sperm are not the exception. Cells often use cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) oscillations as a means to decodify external and internal information. [Ca²⁺]_i oscillations faster than those usually found in other cells and correlated with flagellar beat were the first to be described in sperm in 1993 by Susan Suarez, in the boar. More than 20 years passed before similar [Ca²⁺]_i oscillations were documented in human sperm, simultaneously examining their flagellar beat in three dimensions by Corkidi et al. 2017. On the other hand, 10 years after the discovery of the fast boar [Ca²⁺]_i oscillations, slower ones triggered by compounds from the egg external envelope were found to regulate cell motility and chemotaxis in sperm from marine organisms. Today it is known that sperm display fast and slow spontaneous and agonist triggered [Ca²⁺]_i oscillations. In mammalian sperm these Ca²⁺ transients may act like a multifaceted tool that regulates fundamental functions such as motility and acrosome reaction. This review covers the main sperm species and experimental conditions where [Ca²⁺]_i oscillations have been described and discusses what is known about the transporters involved, their regulation and the physiological purpose of these oscillations. There is a lot to be learned regarding the origin, regulation and physiological relevance of these Ca²⁺ oscillations.

Role and Modulation of TRPV1 in Mammalian Spermatozoa: An Updated Review

Based on the abundance of scientific publications, the polymodal sensor TRPV1 is known as one of the most studied proteins within the TRP channel family. This receptor has been found in numerous cell types from different species as well as in spermatozoa. The present review is focused on analyzing the role played by this important channel in the post-ejaculatory life of spermatozoa, where it has been described to be involved in events such as capacitation, acrosome reaction, calcium trafficking, sperm migration, and fertilization. By performing an exhaustive bibliographic search, this review gathers, for the first time, all the modulators of the TRPV1 function that, to our knowledge, were described to date in different species and cell types. Moreover, all those modulators with a relationship with the reproductive process, either found in the female tract, seminal plasma, or spermatozoa, are presented here. Since the sperm migration through the female reproductive tract is one of the most intriguing and less understood events of the fertilization process, in the present work, chemotaxis, thermotaxis, and rheotaxis guiding mechanisms and their relationship with TRPV1 receptor are deeply analyzed, hypothesizing its (in)direct participation during the sperm migration. Last, TRPV1 is presented as a pharmacological target, with a special focus on humans and some pathologies in mammals strictly related to the male reproductive system.

Structure and Function of Ion Channels Regulating Sperm Motility-An Overview

Sperm motility is linked to the activation of signaling pathways that trigger movement. These pathways are mainly dependent on Ca²⁺, which acts as a secondary messenger. The maintenance of adequate Ca²⁺ concentrations is possible thanks to proper concentrations of other ions, such as K⁺ and Na⁺, among others, that modulate plasma membrane potential and the intracellular pH. Like in every cell, ion homeostasis in spermatozoa is ensured by a vast spectrum of ion channels supported by the work of ion pumps and transporters. To achieve success in fertilization, sperm ion channels have to be sensitive to various external and internal factors. This sensitivity is provided by specific channel structures. In addition, novel sperm-specific channels or isoforms have been found with compositions that increase the chance of fertilization. Notably, the most significant sperm ion channel is the cation channel of sperm (CatSper), which is a sperm-specific Ca²⁺ channel required for the hyperactivation of sperm motility. The role of other ion channels in the spermatozoa, such as voltage-gated Ca²⁺ channels (VGCCs), Ca²⁺-activated Cl-channels (CaCCs), SLO K⁺ channels or voltage-gated H⁺ channels (VGHCs), is to ensure the activation and modulation of CatSper. As the activation of sperm motility differs among metazoa, different ion channels may participate; however, knowledge regarding these channels is still scarce. In the present review, the roles and structures of the most important known ion channels are described in regard to regulation of sperm motility in animals.

TRPM8 channel inhibitor-encapsulated hydrogel as a tunable surface for bone tissue engineering

A major limitation in the bio-medical sector is the availability of materials suitable for bone tissue engineering using stem cells and methodology converting the stochastic biological events towards definitive as well as efficient bio-mineralization. We show that osteoblasts and Bone Marrow-derived Mesenchymal Stem Cell Pools (BM-MSCP) express TRPM8, a Ca²⁺-ion channel critical for bone-mineralization. TRPM8 inhibition triggers up-regulation of key osteogenesis factors; and increases mineralization by osteoblasts. We utilized CMT:HEMA, a carbohydrate polymer-based hydrogel that has nanofiber-like structure suitable for optimum delivery of TRPM8-specific activators or inhibitors. This hydrogel is ideal for proper adhesion, growth, and differentiation of osteoblast cell lines, primary osteoblasts, and BM-MSCP. CMT:HEMA coated with AMTB (TRPM8 inhibitor) induces differentiation of BM-MSCP into osteoblasts and subsequent mineralization in a dose-dependent manner. Prolonged and optimum inhibition of TRPM8 by AMTB released from the gels results in upregulation of osteogenic markers. We propose that AMTB-coated CMT:HEMA can be used as a tunable surface for bone tissue engineering. These findings may have broad implications in different bio-medical sectors.

Sperm ion channels and transporters in male fertility and infertility

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

Differential expression and localization of TRPV channels in the mature sperm of Anas platyrhynchos

Sperm cells perform precise chemotactic and thermotactic movement which is crucial for fertilization. However, the key molecules involved in detection of different chemical and physical stimuli which guide the sperm during navigation are not well understood. Ca²⁺ -signalling mediated by ion channels seem to play important role in motility and other fertility parameters. In this work, we explored the endogenous localization pattern of TRPV channels in the mature spermatozoa of avian species. Using sperm from white pekin duck (Anas platyrhynchos) as the representative avian model, we demonstrate that duck sperm endogenously express the thermosensitive channels TRPV1, TRPV2, TRPV3, TRPV4, and highly Ca²⁺ -selective channels TRPV5 and TRPV6 in specific yet differential locations. All of these TRPV channels are enriched in the sperm tail, indicating their relevance in sperm motility. Interestingly, the TRPV3 and TRPV4 channels are present in the mitochondrial region. Calcium selective TRPV5 channel is exclusively present in sperm tail and is most abundant among the TRPV channels. This is the first report describing the endogenous presence of TRPV2 and TRPV3 channels in the sperm of any species. Using confocal imaging and super-resolution imaging, we demonstrate that though the TRPV channels are evolutionarily closely related, they have distinct localization pattern in the duck sperm, which could impact their role in fertilization.

Ca2+ signaling in mammalian spermatozoa

Calcium is an essential ion which regulates sperm motility, capacitation and the acrosome reaction (AR), three processes necessary for successful fertilization. The AR enables the spermatozoon to penetrate into the egg. In order to undergo the AR, the spermatozoon must reside in the female reproductive tract for several hours, during which a series of biochemical transformations takes place, collectively called capacitation. An early event in capacitation is relatively small elevation of intracellular Ca²⁺ (in the nM range) and bicarbonate, which collectively activate the soluble adenylyl cyclase to produce cyclic-AMP; c-AMP activates protein kinase A (PKA), leading to indirect tyrosine phosphorylation of proteins. During capacitation, there is an increase in the membrane-bound phospholipase C (PLC) which is activated prior to the AR by relatively high increase in intracellular Ca²⁺ (in the μM range). PLC catalyzes the hydrolysis of phosphatidyl-inositol-4,5-bisphosphate (PIP₂) to diacylglycerol and inositol-trisphosphate (IP₃), leading to activation of protein kinase C (PKC) and the IP₃-receptor. PKC activates a Ca²⁺- channel in the plasma membrane, and IP₃ activates the Ca²⁺- channel in the outer acrosomal membrane, leading to Ca²⁺ depletion from the acrosome. As a result, the plasma-membrane store-operated Ca²⁺ channel (SOCC) is activated to increase cytosolic Ca²⁺ concentration, enabling completion of the acrosome reaction. The hydrolysis of PIP₂ by PLC results in the release and activation of PIP₂-bound gelsolin, leading to F-actin dispersion, an essential step prior to the AR. Ca²⁺ is also involved in the regulation of sperm motility. During capacitation, the sperm develops a unique motility pattern called hyper-activated motility (HAM) which is essential for successful fertilization. The main Ca²⁺-channel that mediates HAM is the sperm-specific CatSper located in the sperm tail.

Novel pharmacological actions of trequinsin hydrochloride improve human sperm cell motility and function

BACKGROUND AND PURPOSE

Asthenozoospermia is a leading cause of male infertility, but development of pharmacological agents to improve sperm motility is hindered by the lack of effective screening platforms and knowledge of suitable molecular targets. We have demonstrated that a high-throughput screening (HTS) strategy and established in vitro tests can identify and characterise compounds that improve sperm motility. Here, we applied HTS to identify new compounds from a novel small molecule library that increase intracellular calcium ([Ca²⁺ ]_i ), promote human sperm cell motility, and systematically determine the mechanism of action.

EXPERIMENTAL APPROACH

A validated HTS fluorometric [Ca²⁺ ]_i assay was used to screen an in-house library of compounds. Trequinsin hydrochloride (a PDE3 inhibitor) was selected for detailed molecular (plate reader assays, electrophysiology, and cyclic nucleotide measurement) and functional (motility and acrosome reaction) testing in sperm from healthy volunteer donors and, where possible, patients.

KEY RESULTS

Fluorometric assays identified trequinsin as an efficacious agonist of [Ca²⁺ ]_i , although less potent than progesterone. Functionally, trequinsin significantly increased cell hyperactivation and penetration into viscous medium in all donor sperm samples and cell hyperactivation in 22/25 (88%) patient sperm samples. Trequinsin-induced [Ca²⁺ ]_i responses were cross-desensitised consistently by PGE₁ but not progesterone. Whole-cell patch clamp electrophysiology confirmed that trequinsin activated CatSper and partly inhibited potassium channel activity. Trequinsin also increased intracellular cGMP.

CONCLUSION AND IMPLICATIONS

Trequinsin exhibits a novel pharmacological profile in human sperm and may be a suitable lead compound for the development of new agents to improve patient sperm function and fertilisation potential.

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.

NMDA-type glutamate receptors mediate the acrosome reaction and motility initiation in newt sperm

The N-methyl d-aspartate type glutamate receptor (NMDAR) is a ligand-gated cation channel that causes Ca²⁺ influx in nerve cells. An NMDAR agonist is effective to the sperm motility in fowls, although the actual role of NMDAR in sperm function is unknown. In the present study, RNA-seq of the spermatogenic testes suggested the presence of NMDAR in the sperm of the newt Cynops pyrrhogaster. Glutamate of at least 0.7 ± 0.5 mM was detected in the egg-jelly substances along with acrosome reaction-inducing substance (ARIS) and sperm motility-initiating substance (SMIS). In the egg-jelly extract (JE) that included the ARIS and SMIS, the acrosome reaction was inhibited by a NMDAR antagonists, memantine and MK801. MK801 also inhibited the spontaneous acrosome reaction in Steinberg's salt solution (ST). Furthermore, memantine and MK801 suppressed the progressive motility of the sperm in JE and spontaneous waving of the undulating membrane, which is the tail structure giving thrust for forward motility, in ST. The spontaneous waving of the undulating membrane was promoted when Mg²⁺ , which blocks Ca²⁺ influx through gated NMDARs, was removed from the ST. In addition, the ARIS-induced acrosome reaction was inhibited by a selective antagonist of the transient receptor potential vanilloid 4, whose activation might result in the membrane depolarization to release Mg²⁺ from the NMDAR. These results suggest that NMDAR acts together with other cation channels in the induction of the acrosome reaction and motility of the sperm during the fertilization process of C. pyrrhogaster.

TRPA1 is selected as a semi-conserved channel during vertebrate evolution due to its involvement in spermatogenesis

TRPA1 is a non-selective cation channel originated in invertebrates. The genomic locus containing TRPA1 gene remains highly conserved and retained in all vertebrates. TRPA1 gene is evolutionarily selected, yet maintained as a highly diverged protein. Throughout the vertebrate evolution, the extracellular loops of TRPA1 become most diverged indicating that TRPA1 may be involved in detecting large spectrum and uncertain stimulus which is critical for adaptive benefit. We tested the expression of TRPA1 in mature sperm from different vertebrates. This is the first report demonstrating that TRPA1 is expressed endogenously in mature spermatozoa of multiple species representing entire vertebrate phyla. However, its specific localization within sperm remains species-specific. Accordingly, we report that in rodents TRPA1 expression correlates with different stages of spermatogenesis. We propose that presence of endogenous TRPA1 in testes and in mature sperm provides reproductive benefit.

Genetic mechanisms underlying spermatic and testicular traits within and among cattle breeds: systematic review and prioritization of GWAS results

Reduced bull fertility imposes economic losses in bovine herds. Specifically, testicular and spermatic traits are important indicators of reproductive efficiency. Several genome-wide association studies (GWAS) have identified genomic regions associated with these fertility traits. The aims of this study were as follows: 1) to perform a systematic review of GWAS results for spermatic and testicular traits in cattle and 2) to identify key functional candidate genes for these traits. The identification of functional candidate genes was performed using a systems biology approach, where genes shared between traits and studies were evaluated by a guilt by association gene prioritization (GUILDify and ToppGene software) in order to identify the best functional candidates. These candidate genes were integrated and analyzed in order to identify overlapping patterns among traits and breeds. Results showed that GWAS for testicular-related traits have been developed for beef breeds only, whereas the majority of GWAS for spermatic-related traits were conducted using dairy breeds. When comparing traits measured within the same study, the highest number of genes shared between different traits was observed, indicating a high impact of the population genetic structure and environmental effects. Several chromosomal regions were enriched for functional candidate genes associated with fertility traits. Moreover, multiple functional candidate genes were enriched for markers in a species-specific basis, taurine (Bos taurus) or indicine (Bos indicus). For the different candidate regions identified in the GWAS in the literature, functional candidate genes were detected as follows: B. Taurus chromosome X (BTX) (TEX11, IRAK, CDK16, ATP7A, ATRX, HDAC6, FMR1, L1CAM, MECP2, etc.), BTA17 (TRPV4 and DYNLL1), and BTA14 (MOS, FABP5, ZFPM2). These genes are responsible for regulating important metabolic pathways or biological processes associated with fertility, such as progression of spermatogenesis, control of ciliary activity, development of Sertoli cells, DNA integrity in spermatozoa, and homeostasis of testicular cells. This study represents the first systematic review on male fertility traits in cattle using a system biology approach to identify key candidate genes for these traits.

Establishing life is a calcium-dependent TRiP: Transient receptor potential channels in reproduction

Calcium plays a key role in many different steps of the reproduction process, from germ cell maturation to placental development. However, the exact function and regulation of calcium throughout subsequent reproductive events remains rather enigmatic. Successful pregnancy requires the establishment of a complex dialogue between the implanting embryo and the endometrium. On the one hand, endometrial cell will undergo massive changes to support an implanting embryo, including stromal cell decidualization. On the other hand, trophoblast cells from the trophectoderm surrounding the inner cell mass will differentiate and acquire new functions such as hormone secretion, invasion and migration. The need for calcium in the different gestational processes implicates the presence of specialized ion channels to regulate calcium homeostasis. The superfamily of transient receptor potential (TRP) channels is a class of calcium permeable ion channels that is involved in the transformation of extracellular stimuli into the influx of calcium, inducing and coordinating underlying signaling pathways. Although the necessity of calcium throughout reproduction cannot be negated, the expression and functionality of TRP channels throughout gestation remains elusive. This review provides an overview of the current evidence regarding the expression and function of TRP channels in reproduction.

TRPV4 is the temperature-sensitive ion channel of human sperm

Ion channels control the ability of human sperm to fertilize the egg by triggering hyperactivated motility, which is regulated by membrane potential, intracellular pH, and cytosolic calcium. Previous studies unraveled three essential ion channels that regulate these parameters: (1) the Ca²⁺ channel CatSper, (2) the K⁺ channel KSper, and (3) the H⁺ channel Hv1. However, the molecular identity of the sperm Na⁺ conductance that mediates initial membrane depolarization and, thus, triggers downstream signaling events is yet to be defined. Here, we functionally characterize DSper, the Depolarizing Channel of Sperm, as the temperature-activated channel TRPV4. It is functionally expressed at both mRNA and protein levels, while other temperature-sensitive TRPV channels are not functional in human sperm. DSper currents are activated by warm temperatures and mediate cation conductance, that shares a pharmacological profile reminiscent of TRPV4. Together, these results suggest that TRPV4 activation triggers initial membrane depolarization, facilitating both CatSper and Hv1 gating and, consequently, sperm hyperactivation.

TMEM16A inhibition impedes capacitation and acquisition of hyperactivated motility in guinea pig sperm

Ca²⁺ -activated Cl^- channels (CaCCs) are anionic channels that regulate many important physiological functions associated with chloride and calcium flux in some somatic cells. The molecular identity of CaCCs was revealed to be TMEM16A and TMEM16B (also known as Anoctamin or ANO1 and ANO2, respectively) in all eukaryotes. A recent study suggests the presence of TMEM16A in human sperm and a relationship with the rhZP-induced acrosome reaction. However, to the best of our knowledge, little is known about the role of TMEM16A in other spermatic processes such as capacitation or motility. In this study, we evaluated the effects of two TMEM16A antagonists on capacitation, acrosome reaction, and motility in guinea pig sperm; these antagonists were T16Ainh-A01, belonging to a second generation of potent antagonists of TMEM16A, and niflumic acid (NFA), a well-known antagonist of TMEM16A (CaCCs). First of all, we confirmed that the absence of Cl^- in the capacitation medium changes motility parameters, capacitation, and the progesterone-induced acrosome reaction. Using a specific antibody, TMEM16A was found as a protein band of ∼120 kDa, which localization was in the apical crest of the acrosome and the middle piece of the flagellum. Inhibition of TMEM16A by T16Ainh-A01 affected sperm physiology by reducing capacitation, blocking the progesterone-induced acrosome reaction under optimal capacitation conditions, inhibiting progressive motility, and the acquisition of hyperactivated motility, diminishing [Ca²⁺ ]i, and increasing [Cl^- ]i. These changes in sperm kinematic parameters provide new evidence of the important role played by TMEM16A in the production of sperm capable of fertilizing oocytes.

Involvement of calcium channels and intracellular calcium in bull sperm thermotaxis

Thermotaxis that sperm migrate to higher temperature area has been confirmed in rabbit and human. In this study, we examined the migration ability of bull sperm in a temperature gradient to confirm thermotaxis and elucidate the involvement of calcium in such thermotaxis, as well as the relation between sperm capacitation and bull fertility. Thermotaxis was evaluated in a temperature gradient of 34–42ºC using a cross-type column 22-mm long, 40-mm wide, and 100-μm deep. Significantly more sperm migrated to the high-temperature area of 39ºC in a 2ºC temperature gradient, and to 40ºC in a 1ºC temperature gradient. In calcium-free, BAPTA containing medium, and EGTA containing medium, the migrated sperm ratio in the two temperature areas was almost the same. In media containing lanthanum, ruthenium red, and 2APB, we could not confirm thermotaxis. Pre- and post-capacitated sperm migrated to the high-temperature area, expressing thermotaxis. The sperm from high-fertility bulls showed clear thermotaxis. Based on these results, thermotaxis of bull sperm was confirmed and the involvement of both calcium channels and intracellular stored calcium in thermotaxis was suggested. Although the sample size of bulls was quite small, the difference in thermotaxis may have been associated with bull fertility. Sperm thermotaxis evaluation has potential as a predictor of bull fertility.