Direct activation of the proton channel by albumin leads to human sperm capacitation and sustained release of inflammatory mediators by neutrophils

Bovine serum albumin-induced calcium influx triggers soluble adenylyl cyclase activation and cyclic AMP signalling pathways in mouse sperm capacitation

Sperm capacitation involves a series of biochemical and physiological changes essential for fertilization. A critical regulator of capacitation, the soluble adenylyl cyclase (sAC; ADCY10)-dependent production of the second messenger cyclic AMP (cAMP), drives key downstream events such as protein kinase A (PKA) substrate phosphorylation. sAC activity is directly stimulated by bicarbonate (HCO3 -) and calcium (Ca2+). CatSper, a sperm-specific Ca2+ channel, is considered the primary pathway for Ca2+ influx during capacitation; however, emerging evidence suggests additional pathways exist. This study reveals that bovine serum albumin (BSA) influences the dynamics of intracellular Ca2+ concentration ([Ca2+]i) in CatSper1 knockout (KO) sperm and plays a novel role in sAC activation. Using single-cell live imaging and flow cytometry, we observed a rapid [Ca2+]i rise in the head of CatSper1 KO sperm under capacitating conditions, indicating an alternative Ca2+ entry mechanism. BSA alone, in the absence of HCO3 -, triggered a significant [Ca2+]i rise. Removal of extracellular Ca2+ abolished this [Ca2+]i rise, confirming the necessity of Ca2+ influx. This BSA-induced [Ca2+]i rise was upstream of sAC activation, since it was not affected by sAC inhibitors and led to increased cAMP production and PKA substrate phosphorylation. Our findings provide new insights into the regulatory mechanisms of sAC, highlighting the existence of a CatSper-independent Ca2+ entry pathway activated by BSA during sperm capacitation. This rapid [Ca2+]i rise is initiated in the sperm head and propagates throughout the cell, and is sufficient to activate sAC and stimulate cAMP synthesis independently of HCO3 -. KEY POINTS: Sperm capacitation, essential for fertilization, is regulated by sAC, which produces cAMP in response to HCO3 - and Ca2+, driving key events like protein kinase A substrate phosphorylation. We demonstrate the existence of a CatSper-independent Ca2+ entry pathway that initiates in the sperm head and propagates throughout the cell, occurring rapidly after sperm encounters albumin, a critical component of the capacitation medium used in in vitro fertilization procedures in mammals. This albumin-induced Ca2+ influx is sufficient to activate sAC and stimulate cAMP synthesis independently of HCO3 -. We further reveal a novel role for albumin, beyond its well-established function as a cholesterol acceptor, in triggering this rapid Ca2+ influx and downstream signalling events essential for sperm capacitation. By demonstrating a CatSper-independent regulatory pathway, we expand the current paradigm of Ca2+ signalling in sperm physiology.

The neutrophil percentage-to-albumin ratio is an independent risk factor for poor prognosis in peritoneal dialysis patients

AIM

This study aimed to investigate the predictive ability of the neutrophil percentage-to-albumin Ratio (NPAR) concerning all-cause mortality and cardio-cerebrovascular mortality in patients undergoing peritoneal dialysis (PD).

METHODS

We included a total of 807 PD patients from the Peritoneal Dialysis Center of the Second Affiliated Hospital of Soochow University between January 2009 and December 2019 in this study. Patients were categorized into three groups based on their baseline NPAR. The Kaplan-Meier method, multivariate Cox proportional hazard model, and Fine-Gray competing risk model were employed to examine the relationship between NPAR level and all-cause mortality and cardio-cerebrovascular mortality among PD patients. Furthermore, the ROC curve and calibration plots were utilized to compare the performance between NPAR and other conventional indicators.

RESULTS

The mean follow-up period was 38.2 months. A total of 243 (30.1%) patients passed away, with 128 (52.7%) succumbing to cardio-cerebrovascular diseases. The mortality rates of the Middle and High NPAR groups were significantly greater than that of the Low NPAR group (p < 0.001), and NPAR was independently associated with all-cause mortality and cardio-cerebrovascular mortality. Receiver Operating Characteristic (ROC) analysis indicated that the Area Under the Curve (AUC) of NPAR (0.714) was significantly superior to those of C-reactive protein (CRP) (0.597), neutrophil to lymphocyte ratio (NLR) (0.589), C-reactive protein to albumin ratio (CAR) (0.698) and platelet to lymphocyte ratio (PLR) (0.533).

CONCLUSION

NPAR served as an independent predictive marker for all-cause mortality and cardio-cerebrovascular mortality in PD patients. Moreover, NPAR demonstrated superior predictive potential compared to CRP, CAR, NLR, and PLR.

Hyperglycemia adversely affects critical physiological events related to rat sperm capacitation

Hyperglycemia, the hallmark of diabetes mellitus (DM), is the main cause of DM-related systemic complications, including reproductive issues. Furthermore, the incidence of DM in males of reproductive ages is becoming an increasing concern, as the complexity of sperm capacitation (an essential process for fertilizing the egg) extends beyond conventional sperm parameters such as count, viability, and motility. Capacitation defects cause male infertility, and DM-related hyperglycemia may affect this process. We explore the effects of uncontrolled hyperglycemia on sperm using alloxan-induced hyperglycemic Wistar rats. In addition to assessing conventional sperm parameters, we also evaluated functional indicators, including hyperactivation (HA) with a pharmacological approach and assessed its effects with a computer-assisted sperm analysis (CASA); fluorescence indicators to monitor membrane potential (EmR, DiSC3(5)) and mitochondrial membrane potential (Ψ, JC-1); CatSper activity, using its ability to permeate Na+ ions, and ATP levels with the luciferin-luciferase reaction. We confirmed previous findings with our hyperglycemic model, which replicated the typical reduction on conventional sperm parameters. In sperm from hyperglycemic rats, we observed increased motility and HA levels after pharmacological treatment. Additionally, CatSper activity was unaffected by hyperglycemia, while EmR was hyperpolarized under non-capacitating condition. Finally, we noted a low percentage of hyperpolarized Ψ and reduced ATP content. This study highlights the significance of impact of hyperglycemia on sperm physiology and capacitation. We proposed that low ATP levels perturb energy state, signaling pathways, ion channels activity, motility, and HA. Our findings offer insight into DM-associated infertility and potential treatment strategies.

Albumin reprograms the B cell transcriptional landscape and improves neutrophil antimicrobial function in patients with decompensated cirrhosis

Background & Aims

Patients with acutely decompensated (AD) cirrhosis are immunocompromised and particularly susceptible to infections. This study investigated the immunomodulatory actions of albumin by which this protein may lower the incidence of infections.

Methods

Blood immunophenotyping was performed in 11 patients with AD cirrhosis and 10 healthy volunteers (HV). Bulk and single-cell RNA sequencing (scRNA-seq) and flow cytometry were performed in peripheral blood mononuclear cells (PBMCs) from 20 patients with AD cirrhosis and 34 HV exposed to albumin. Albumin's effects on degranulation, phagocytosis, chemotaxis, and swarming of neutrophils from six patients with AD cirrhosis and nine HV were assessed by measuring myeloperoxidase enzymatic activity, the engulfment of fluorescent-labeled Escherichia coli and zymosan, and interactions of neutrophils with Candida albicans at single-cell resolution in microfluidic chambers, respectively. Whole blood RNA sequencing (RNA-seq) analyses were performed in 49 patients admitted for severe AD cirrhosis, of whom 30 received albumin during hospitalization.

Results

Compared with HV, patients with AD cirrhosis showed severe lymphopenia and defective neutrophil antimicrobial function. Bulk and scRNA-seq analyses revealed significantly (false discovery rate [FDR] <0.05) increased signatures related to B cells, myeloid cells, and CD4+ T cells in PBMCs incubated with albumin. Changes in the B cell population were confirmed by flow cytometry. Neutrophils exposed to albumin also exhibited augmented chemotactic and degranulation responses, enhanced phagocytosis, and increased pathogen-restrictive swarming. RNA-seq data analysis in patients who had received albumin revealed specific upregulation of signatures related to B cells and neutrophils together with transcriptional changes in CD4+ T cells (FDR <0.05).

Conclusions

The finding that albumin promotes the transcriptional reprogramming and expansion of the B cell compartment and improves neutrophil antimicrobial functions indicates mechanisms that may lower the incidence of infections in patients with severe AD cirrhosis receiving albumin therapy.

Impact and implications

Patients with acutely decompensated cirrhosis receiving albumin as treatment have a lower incidence of infections. The reason for this protection is currently unknown, but the present study provides data that support the ability of albumin to boost the antimicrobial functions of immune cells in these patients. Moreover, these findings encourage the design of controlled clinical studies specifically aimed at investigating the effects of albumin administration on the immune system.

Lipid remodeling in acrosome exocytosis: unraveling key players in the human sperm

It has long been thought that exocytosis was driven exclusively by well-studied fusion proteins. Some decades ago, the role of lipids became evident and escalated interest in the field. Our laboratory chose a particular cell to face this issue: the human sperm. What makes this cell special? Sperm, as terminal cells, are characterized by their scarcity of organelles and the complete absence of transcriptional and translational activities. They are specialized for a singular membrane fusion occurrence: the exocytosis of the acrosome. This unique trait makes them invaluable for the study of exocytosis in isolation. We will discuss the lipids' role in human sperm acrosome exocytosis from various perspectives, with a primary emphasis on our contributions to the field. Sperm cells have a unique lipid composition, very rare and not observed in many cell types, comprising a high content of plasmalogens, long-chain, and very-long-chain polyunsaturated fatty acids that are particular constituents of some sphingolipids. This review endeavors to unravel the impact of membrane lipid composition on the proper functioning of the exocytic pathway in human sperm and how this lipid dynamic influences its fertilizing capability. Evidence from our and other laboratories allowed unveiling the role and importance of multiple lipids that drive exocytosis. This review highlights the role of cholesterol, diacylglycerol, and particular phospholipids like phosphatidic acid, phosphatidylinositol 4,5-bisphosphate, and sphingolipids in driving sperm acrosome exocytosis. Furthermore, we provide a comprehensive overview of the factors and enzymes that regulate lipid turnover during the exocytic course. A more thorough grasp of the role played by lipids transferred from sperm can provide insights into certain causes of male infertility. It may lead to enhancements in diagnosing infertility and techniques like assisted reproductive technology (ART).

Quantitative insights into the mechanism of proton conduction and selectivity for the human voltage-gated proton channel Hv1

Human voltage-gated proton (hHv1) channels are crucial for regulating essential biological processes such as immune cell respiratory burst, sperm capacitation, and cancer cell migration. Despite the significant concentration difference between protons and other ions in physiological conditions, hHv1 demonstrates remarkable proton selectivity. Our calculations of single-proton, cation, and anion permeation free energy profiles quantitatively demonstrate that the proton selectivity of the wild-type channel originates from its strong proton affinity via the titration of the key residues D112 and D174, although the channel imposes similar kinetic blocking effects for protons compared to other ions. A two-proton knock-on model is proposed to mathematically explain the electrophysiological measurements of the pH-dependent proton conductance in the conductive state. Moreover, it is shown that the anion selectivity of the D112N mutant channel is tied to impaired proton transport and substantial anion leakage.

The sperm-specific K+ channel Slo3 is inhibited by albumin and steroids contained in reproductive fluids

To locate and fertilize the egg, sperm probe the varying microenvironment prevailing at different stages during their journey across the female genital tract. To this end, they are equipped with a unique repertoire of mostly sperm-specific proteins. In particular, the flagellar Ca2+ channel CatSper has come into focus as a polymodal sensor used by human sperm to register ligands released into the female genital tract. Here, we provide the first comprehensive study on the pharmacology of the sperm-specific human Slo3 channel, shedding light on its modulation by reproductive fluids and their constituents. We show that seminal fluid and contained prostaglandins and Zn2+ do not affect the channel, whereas human Slo3 is inhibited in a non-genomic fashion by diverse steroids as well as by albumin, which are released into the oviduct along with the egg. This indicates that not only CatSper but also Slo3 harbours promiscuous ligand-binding sites that can accommodate structurally diverse molecules, suggesting that Slo3 is involved in chemosensory signalling in human sperm.

An essential role for the Hv1 voltage-gated proton channel in Pseudomonas aeruginosa corneal infection

Assembly of NADPH oxidase 2 (NOX2) proteins in neutrophils plays an essential role in controlling microbial infections by producing high levels of reactive oxygen species (ROS). In contrast, the role of the Hv1 voltage-gated proton channel that is required for sustained NOX2 activity is less well characterized. We examined the role of Hv1 in a murine model of blinding Pseudomonas aeruginosa corneal infection and found that in contrast to C57BL/6 mice, Hvcn1 -/- mice exhibit an impaired ability to kill bacteria and regulate disease severity. In vitro, we used a novel Hv1 Inhibitor Flexible (HIF) to block ROS production by human and murine neutrophils and found that HIF inhibits ROS production in a dose-dependent manner following stimulation with PMA or infection with P. aeruginosa. Collectively, these findings demonstrate an important role for Hv1 on controlling bacterial growth in a clinically relevant bacterial infection model.

pH Homeodynamics and Male Fertility: A Coordinated Regulation of Acid-Based Balance during Sperm Journey to Fertilization

pH homeostasis is crucial for spermatogenesis, sperm maturation, sperm physiological function, and fertilization in mammals. HCO3- and H+ are the most significant factors involved in regulating pH homeostasis in the male reproductive system. Multiple pH-regulating transporters and ion channels localize in the testis, epididymis, and spermatozoa, such as HCO3- transporters (solute carrier family 4 and solute carrier family 26 transporters), carbonic anhydrases, and H+-transport channels and enzymes (e.g., Na+-H+ exchangers, monocarboxylate transporters, H+-ATPases, and voltage-gated proton channels). Hormone-mediated signals impose an influence on the production of some HCO3- or H+ transporters, such as NBCe1, SLC4A2, MCT4, etc. Additionally, ion channels including sperm-specific cationic channels for Ca2+ (CatSper) and K+ (SLO3) are directly or indirectly regulated by pH, exerting specific actions on spermatozoa. The slightly alkaline testicular pH is conducive to spermatogenesis, whereas the epididymis's low HCO3- concentration and acidic lumen are favorable for sperm maturation and storage. Spermatozoa pH increases substantially after being fused with seminal fluid to enhance motility. In the female reproductive tract, sperm are subjected to increasing concentrations of HCO3- in the uterine and fallopian tube, causing a rise in the intracellular pH (pHi) of spermatozoa, leading to hyperpolarization of sperm plasma membranes, capacitation, hyperactivation, acrosome reaction, and ultimately fertilization. The physiological regulation initiated by SLC26A3, SLC26A8, NHA1, sNHE, and CFTR localized in sperm is proven for certain to be involved in male fertility. This review intends to present the key factors and characteristics of pHi regulation in the testes, efferent duct, epididymis, seminal fluid, and female reproductive tract, as well as the associated mechanisms during the sperm journey to fertilization, proposing insights into outstanding subjects and future research trends.

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.

High-concentration bovine serum albumin enhances fertilization ability of cold-stored rat sperm

Cold transport of the cauda epididymides is a useful technique for shipping laboratory rat sperm. Cold transport of rat sperm avoids potential risks of microbiological infection, animal escape or death, and animal welfare issues. Previously, we reported that a cold-storage solution containing dimethyl sulfoxide and quercetin maintained the fertility of cold-stored rat sperm. However, cold-stored rat sperm exhibited a decreased fertilization rate after 24-h storage. To recover the fertility of cold-stored sperm, we focused on the effects of bovine serum albumin (BSA), a cholesterol acceptor that induces sperm capacitation. We sought to determine the optimal concentration of BSA in fertilization medium based on the fertility of cold-stored rat sperm. High concentrations of BSA (40 mg/ml) enhanced the fertilization rate of cold-stored rat sperm and maintained sperm fertility for 144 h. Embryos derived from cold-stored and BSA-treated sperm normally developed into pups after embryo transfer. In summary, high BSA concentrations enhanced the fertility of cold-stored rat sperm and prolonged the storage period to 144 h, thereby expanding the transportable region for genetically engineered rats.

Flagellar pH homeostasis mediated by Na+/H+ exchangers regulates human sperm functions through coupling with CatSper and KSper activation

STUDY QUESTION

Whether and how do Na+/H+ exchangers (NHEs) regulate the physiological functions of human sperm?

SUMMARY ANSWER

NHE-mediated flagellar intracellular pH (pHi) homeostasis facilitates the activation of the pH-sensitive, sperm-specific Ca2+ channel (CatSper) and the sperm-specific K+ channel (KSper), which subsequently modulate sperm motility, hyperactivation, flagellar tyrosine phosphorylation, and the progesterone (P4)-induced acrosome reaction.

WHAT IS KNOWN ALREADY

Sperm pHi alkalization is an essential prerequisite for the acquisition of sperm-fertilizing capacity. Different sperm functions are strictly controlled by particular pHi regulatory mechanisms. NHEs are suggested to modulate sperm H+ efflux.

STUDY DESIGN, SIZE, DURATION

This was a laboratory study that used samples from >50 sperm donors over a period of 1 year. To evaluate NHE action on human sperm function, 5-(N,N-dimethyl)-amiloride (DMA), a highly selective inhibitor of NHEs, was utilized. All experiments were repeated at least five times using different individual sperm samples or cells.

PARTICIPANTS/MATERIALS, SETTING, METHODS

By utilizing the pH fluorescent indicator pHrodo Red-AM, we detected alterations in single-cell pHi value in human sperm. The currents of CatSper and KSper in human sperm were recorded by the whole-cell patch-clamp technique. Changes in population and single-cell Ca2+ concentrations ([Ca2+]i) of human sperm loaded with Fluo 4-AM were measured. Membrane potential (Vm) and population pHi were quantitatively examined by a multimode plate reader after sperm were loaded with 3,3'-dipropylthiadicarbocyanine iodide and 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethyl ester, respectively. Sperm motility parameters were assessed by a computer-assisted semen analysis system. Tyrosine phosphorylation was determined by immunofluorescence, and sperm acrosome reaction was evaluated by Pisum sativum agglutinin-FITC staining.

MAIN RESULTS AND THE ROLE OF CHANCE

DMA-induced NHEs inhibition severely acidified the human sperm flagellar pHi from 7.20 ± 0.04 to 6.38 ± 0.12 (mean ± SEM), while the effect of DMA on acrosomal pHi was less obvious (from 5.90 ± 0.13 to 5.57 ± 0.12, mean ± SEM). The whole-cell patch-clamp recordings revealed that NHE inhibition remarkably suppressed alkalization-induced activation of CatSper and KSper. As a consequence, impairment of [Ca2+]i homeostasis and Vm maintenance were detected in the presence of DMA. During the capacitation process, pre-treatment with DMA for 2 h potently decreased sperm pHi, which in turn decreased sperm motility and kinetic parameters. Sperm capacitation-associated functions, including hyperactivation, tyrosine phosphorylation, and P4-induced acrosome reaction, were also compromised by NHE inhibition.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This was an in vitro study. Caution should be taken when extrapolating these results to in vivo applications.

WIDER IMPLICATIONS OF THE FINDINGS

This study revealed that NHEs are important physiological regulators for human CatSper and KSper, which are indispensable for human sperm fertility, suggesting that malfunction of NHEs could be an underlying mechanism for the pathogenesis of male infertility.

FUNDING/COMPETING INTEREST(S)

This work was supported by the National Natural Science Foundation of China (32271167 and 81871202 to X.Z.), Jiangsu Innovation and Entrepreneurship Talent Plan (JSSCRC20211543 to X.Z.), the Social Development Project of Jiangsu Province (No. BE2022765 to X.Z.), the Society and livelihood Project of Nantong City (No. MS22022087 to X.Z.), and the Natural Science Foundation of Jiangsu Province (BK20220608 to H.K.). The authors have no competing interests to declare.

Anionic omega currents from single countercharge mutants in the voltage-sensing domain of Ci-VSP

The S4 segment of voltage-sensing domains (VSDs) directly responds to voltage changes by reorienting within the electric field as a permion. A narrow hydrophobic "gasket" or charge transfer center at the core of most VSDs focuses the electric field into a narrow region and catalyzes the sequential and reversible translocation of S4 positive gating charge residues across the electric field while preventing the permeation of physiological ions. Mutating specific S4 gating charges can cause ionic leak currents through the VSDs. These gating pores or omega currents play important pathophysiological roles in many diseases of excitability. Here, we show that mutating D129, a key countercharge residue in the Ciona intestinalis voltage-sensing phosphatase (Ci-VSP), leads to the generation of unique anionic omega currents. Neutralizing D129 causes a dramatic positive shift of activation, facilitates the formation of a continuous water path through the VSD, and creates a positive electrostatic potential landscape inside the VSD that contributes to its unique anionic selectivity. Increasing the population or dwell time of the conducting state by a high external pH or an engineered Cd2+ bridge markedly increases the current magnitude. Our findings uncover a new role of countercharge residues in the impermeable VSD of Ci-VSP and offer insights into mechanisms of the conduction of anionic omega currents linked to countercharge residue mutations.

Na+/H+ Exchangers (NHEs) in Mammalian Sperm: Essential Contributors to Male Fertility

Na+/H+ exchangers (NHEs) are known to be important regulators of pH in multiple intracellular compartments of eukaryotic cells. Sperm function is especially dependent on changes in pH and thus it has been postulated that NHEs play important roles in regulating the intracellular pH of these cells. For example, in order to achieve fertilization, mature sperm must maintain a basal pH in the male reproductive tract and then alkalize in response to specific signals in the female reproductive tract during the capacitation process. Eight NHE isoforms are expressed in mammalian testis/sperm: NHE1, NHE3, NHE5, NHE8, NHA1, NHA2, NHE10, and NHE11. These NHE isoforms are expressed at varying times during spermatogenesis and localize to different subcellular structures in developing and mature sperm where they contribute to multiple aspects of sperm physiology and male fertility including proper sperm development/morphogenesis, motility, capacitation, and the acrosome reaction. Previous work has provided evidence for NHE3, NHE8, NHA1, NHA2, and NHE10 being critical for male fertility in mice and NHE10 has recently been shown to be essential for male fertility in humans. In this article we review what is known about each NHE isoform expressed in mammalian sperm and discuss the physiological significance of each NHE isoform with respect to male fertility.

Control of intracellular pH and bicarbonate by CO2 diffusion into human sperm

The reaction of CO2 with H2O to form bicarbonate (HCO3-) and H+ controls sperm motility and fertilization via HCO3--stimulated cAMP synthesis. A complex network of signaling proteins participates in this reaction. Here, we identify key players that regulate intracellular pH (pHi) and HCO3- in human sperm by quantitative mass spectrometry (MS) and kinetic patch-clamp fluorometry. The resting pHi is set by amiloride-sensitive Na+/H+ exchange. The sperm-specific putative Na+/H+ exchanger SLC9C1, unlike its sea urchin homologue, is not gated by voltage or cAMP. Transporters and channels implied in HCO3- transport are not detected, and may be present at copy numbers < 10 molecules/sperm cell. Instead, HCO3- is produced by diffusion of CO2 into cells and readjustment of the CO2/HCO3-/H+ equilibrium. The proton channel Hv1 may serve as a unidirectional valve that blunts the acidification ensuing from HCO3- synthesis. This work provides a new framework for the study of male infertility.

Role of the Voltage-Gated Proton Channel Hv1 in Nervous Systems

Hv1 is the only voltage-gated proton-selective channel in mammalian cells. It contains a conserved voltage-sensor domain, shared by a large class of voltage-gated ion channels, but lacks a pore domain. Its primary role is to extrude protons from the cytoplasm upon pH reduction and membrane depolarization. The best-known function of Hv1 is the regulation of cytosolic pH and the nicotinamide adenine dinucleotide phosphate oxidase-dependent production of reactive oxygen species. Accumulating evidence indicates that Hv1 is expressed in nervous systems, in addition to immune cells and others. Here, we summarize the molecular properties, distribution, and physiological functions of Hv1 in the peripheral and central nervous systems. We describe the recently discovered functions of Hv1 in various neurological diseases, including brain or spinal cord injury, ischemic stroke, demyelinating diseases, and pain. We also summarize the current advances in the discovery and application of Hv1-targeted small molecules in neurological diseases. Finally, we discuss the current limitations of our understanding of Hv1 and suggest future research directions.

Voltage-Gated Proton Channels in the Tree of Life

With a single gene encoding H_V1 channel, proton channel diversity is particularly low in mammals compared to other members of the superfamily of voltage-gated ion channels. Nonetheless, mammalian H_V1 channels are expressed in many different tissues and cell types where they exert various functions. In the first part of this review, we regard novel aspects of the functional expression of H_V1 channels in mammals by differentially comparing their involvement in (1) close conjunction with the NADPH oxidase complex responsible for the respiratory burst of phagocytes, and (2) in respiratory burst independent functions such as pH homeostasis or acid extrusion. In the second part, we dissect expression of H_V channels within the eukaryotic tree of life, revealing the immense diversity of the channel in other phylae, such as mollusks or dinoflagellates, where several genes encoding H_V channels can be found within a single species. In the last part, a comprehensive overview of the biophysical properties of a set of twenty different H_V channels characterized electrophysiologically, from Mammalia to unicellular protists, is given.

Arachidonic acid reverses cholesterol and zinc inhibition of human voltage-gated proton channels

Unlike other members of the voltage-gated ion channel superfamily, voltage-gated proton (Hv) channels are solely composed of voltage sensor domains without separate ion-conducting pores. Due to their unique dependence on both voltage and transmembrane pH gradients, Hv channels normally open to mediate proton efflux. Multiple cellular ligands were also found to regulate the function of Hv channels, including Zn²⁺, cholesterol, polyunsaturated arachidonic acid, and albumin. Our previous work showed that Zn²⁺ and cholesterol inhibit the human voltage-gated proton channel hHv1 by stabilizing its S4 segment at resting state conformations. Released from phospholipids by phospholipase A2 in cells upon infection or injury, arachidonic acid regulates the function of many ion channels, including hHv1. In the present work, we examined the effects of arachidonic acid on purified hHv1 channels using liposome flux assays and revealed underlying structural mechanisms using single-molecule Fluorescence Resonance Energy Transfer (smFRET). Our data indicated that arachidonic acid strongly activates hHv1 channels by promoting transitions of the S4 segment towards opening or 'pre-opening' conformations. Moreover, we found that arachidonic acid even activates hHv1 channels inhibited by Zn²⁺ and cholesterol, providing a biophysical mechanism to activate hHv1 channels in non-excitable cells upon infection or injury.

The bovine uterine fluid proteome is more impacted by the stage of the estrous cycle than the proximity of the ovulating ovary in the periconception period

Uterine secretions provide a suitable environment for sperm selective migration during a couple of days preceding ovulation and for early embryo development before implantation. Our goal was to identify and quantify proteins in the bovine uterine fluid during the periovulatory period of the estrous cycle. Genital tracts with normal morphology were collected from adult cyclic Bos taurus females in a local slaughterhouse and classified into pre-ovulatory or post-ovulatory stages of cycle (around days 19-21 and 0-5 of cycle, respectively; n = 8 cows per stage) based on ovarian morphology. Proteins from uterine fluid collected from the utero-tubal junction to the base of each horn (four pools of two cows per condition) were analyzed by nanoLiquid Chromatography coupled with tandem Mass Spectrometry (nanoLC-MS/MS). A total of 1214 proteins were identified, of which 91% were shared between all conditions. Overall, 57% of proteins were predicted to be secreted and 17% were previously reported in uterine extracellular vesicles. Paired comparisons between uterine horns ipsilateral and contralateral to ovulation evidenced 12 differentially abundant proteins, including five at pre-ovulatory stage. Furthermore, 35 proteins differed in abundance between pre- and post-ovulatory stages, including 21 in the ipsilateral side of ovulation. Functional analysis of identified proteins demonstrated roles in binding, metabolism, cellular detoxification and the immune response. This study provides a valuable database of uterine proteins for functional studies on sperm physiology and early embryo development.

Network pharmacology- and molecular simulation-based exploration of therapeutic targets and mechanisms of heparin for the treatment of sepsis/COVID-19

Critically infected patients with COVID-19 (coronavirus disease 2019) are prone to develop sepsis-related coagulopathy as a result of a robust immune response. The mechanism underlying the relationship between sepsis and COVID-19 is largely unknown. LMWH (low molecular weight heparin) exhibits both anti-inflammatory and anti-coagulating properties that result in a better prognosis of severely ill patients with COVID-19 co-associated with sepsis-induced coagulopathy or with a higher D-dimer value. Heparin-associated molecular targets and their mechanism of action in sepsis/COVID-19 are not well understood. In this work, we characterize the pharmacological targets, biological functions and therapeutic actions of heparin in sepsis/COVID-19 from the perspective of network pharmacology. A total of 38 potential targets for heparin action against sepsis/COVID-19 and 8 core pharmacological targets were identified, including IL6, KNG1, CXCL8, ALB, VEGFA, F2, IL10 and TNF. Moreover, enrichment analysis showed that heparin could help in treating sepsis/COVID-19 through immunomodulation, inhibition of the inflammatory response, regulation of angiogenesis and antiviral activity. The pharmacological effects of heparin against these targets were further confirmed by molecular docking and simulation analysis, suggesting that heparin exerts effective binding capacity by targeting the essential residues in sepsis/COVID-19. Prospective clinical practice evaluations may consider the use of these key prognostic indicators for the treatment of sepsis/COVID-19.Communicated by Ramaswamy H. Sarma.

Protection from acute lung injury by a peptide designed to inhibit the voltage-gated proton channel

There are no targeted medical therapies for Acute Lung Injury (ALI) or its most severe form acute respiratory distress syndrome (ARDS). Infections are the most common cause of ALI/ARDS and these disorders present clinically with alveolar inflammation and barrier dysfunction due to the influx of neutrophils and inflammatory mediator secretion. We designed the C6 peptide to inhibit voltage-gated proton channels (Hv1) and demonstrated that it suppressed the release of reactive oxygen species (ROS) and proteases from neutrophils in vitro. We now show that intravenous C6 counteracts bacterial lipopolysaccharide (LPS)-induced ALI in mice, and suppresses the accumulation of neutrophils, ROS, and proinflammatory cytokines in bronchoalveolar lavage fluid. Confirming the salutary effects of C6 are via Hv1, genetic deletion of the channel similarly protects mice from LPS-induced ALI. This report reveals that Hv1 is a key regulator of ALI, that Hv1 is a druggable target, and that C6 is a viable agent to treat ALI/ARDS.

The role of Phe150 in human voltage-gated proton channel

The voltage-gated proton channel H_v1 is a member of voltage-gated ion channels containing voltage-sensing domains (VSDs). The VSDs are made of four membrane-spanning segments (S1 through S4), and their function is to detect changes in membrane potential in the cells. A highly conserved phenylalanine 150 (F150) is located in the S2 segment of human voltage-gated proton channels. We previously discovered that the F150 is a binding site for the open channel blocker 2GBI. Here, we show that the H_v1 VSD voltage-dependent activation requires a hydrophobic group at position F150. We perform double-mutant cycle analysis to probe interactions between F150 and positively charged arginines in the S4 segment of the channel. Our results indicate that F150 interacts with two arginines (R2 and R3) in the S4 segment and catalyzes the transfer of the S4 arginines in the process of voltage-dependent activation.

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Hydrophobicity of F150 is crucial for human H_v1 channel voltage-dependent activation

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F150 interacts with R2 to stabilize the closed state of the H_v1 channel

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When depolarized, R3 moves upward to interact with F150 stabilizing the open state of H_v1

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F150 is essential for the transfer of the H_v1 arginines in the process of voltage sensing

Multiple mechanisms contribute to fluorometry signals from the voltage-gated proton channel

Voltage-clamp fluorometry (VCF) supplies information about the conformational changes of voltage-gated proteins. Changes in the fluorescence intensity of the dye attached to a part of the protein that undergoes a conformational rearrangement upon the alteration of the membrane potential by electrodes constitute the signal. The VCF signal is generated by quenching and dequenching of the fluorescence as the dye traverses various local environments. Here we studied the VCF signal generation, using the Hv1 voltage-gated proton channel as a tool, which shares a similar voltage-sensor structure with voltage-gated ion channels but lacks an ion-conducting pore. Using mutagenesis and lipids added to the extracellular solution we found that the signal is generated by the combined effects of lipids during movement of the dye relative to the plane of the membrane and by quenching amino acids. Our 3-state model recapitulates the VCF signals of the various mutants and is compatible with the accepted model of two major voltage-sensor movements.

Fluorometry signals indicating conformational change in an ion channel are generated by quenching amino acids and lipid effects during movement of the dye relative to the plane of the membrane.

Cholesterol inhibits human voltage-gated proton channel hHv1

Although human sperm is morphologically mature in the epididymis, it cannot fertilize eggs before capacitation. Cholesterol efflux from the sperm plasma membrane is a key molecular event essential for cytoplasmic alkalinization and hyperactivation, but the underlying mechanism remains unclear. The human voltage-gated proton (hHv1) channel functions as an acid extruder to regulate intracellular pHs of many cell types, including sperm. Aside from voltage and pH, Hv channels are also regulated by distinct ligands, such as Zn²⁺ and albumin. In the present work, we identified cholesterol as an inhibitory ligand of the hHv1 channel and further investigated the underlying mechanism using the single-molecule fluorescence resonance energy transfer (smFRET) approach. Our results indicated that cholesterol inhibits the hHv1 channel by stabilizing the voltage-sensing S4 segment at resting conformations, a similar mechanism also utilized by Zn²⁺. Our results suggested that the S4 segment is the central gating machinery in the hHv1 channel, on which voltage and distinct ligands are converged to regulate channel function. Identification of membrane cholesterol as an inhibitory ligand provides a mechanism by which the hHv1 channel regulates fertilization by linking the cholesterol efflux with cytoplasmic alkalinization, a change that triggers calcium influx through the CatSper channel. These events finally lead to hyperactivation, a remarkable change in the mobility pattern indicating fertilization competence of human sperm.

The Aging Features of Thyrotoxicosis Mice: Malnutrition, Immunosenescence and Lipotoxicity

The problem of aging is mainly the increase of age-related diseases, and elderly patients have longer hospitalization and worse prognosis. Poorer nutritional status and immunosenescence may be predisposing and severe factors. The mechanism of the high incidence of diseases and poor prognosis behind aging is complex. Finding suitable aging models is of great significance to find strategies to prevent aging related events. In this study, the relationship between thyrotoxicosis and aging was investigated in mice. The results of routine blood tests and flow cytometry showed that immunosenescence occurred in thyrotoxicosis mice, which was characterized by a significant decrease in neutrophils, lymphocytes, CD4+/CD8+ and CD4+IFN-γ+ lymphocytes. Biochemical examination results showed that there were hypocholesterolemia, hypolipoproteinemia, and hyperlipidemia in thyrotoxicosis mice. Serum proteomics analysis showed that the downregulation of complement and coagulation proteins was another manifestation of declined immunity. Moreover, proteomics analysis showed that many downregulated proteins were related to homeostasis, mainly transport proteins. Their downregulation led to the disturbance of osmotic pressure, ion homeostasis, vitamin utilization, lipid transport, hyaluronic acid processing, and pH maintenance. Serum metabolomics analysis provided more detailed evidence of homeostasis disturbance, especially lipid metabolism disorder, including the downregulation of cholesterol, vitamin D, bile acids, docosanoids, and the upregulation of glucocorticoids, triglycerides, sphingolipids, and free fatty acids. The upregulated lipid metabolites were related to lipotoxicity, which might be one cause of immunosenescence and many aging related syndromes. This study provides evidence for the aging model of thyrotoxicosis mice, which can be used for exploring anti-aging drugs and strategies.

Molecular determinants of inhibition of the human proton channel hHv1 by the designer peptide C6 and a bivalent derivative

We designed C6 peptide to address the absence of specific inhibitors of human voltage-gated proton channels (hHv1). Two C6 bind to the two hHv1 voltage sensors at the resting state, inhibiting activation on depolarization. Here, we identify the C6–hHv1 binding interface using tethered-toxin variants and channel mutants, unveil an important role for negatively charged lipids, and present a model of the C6–hHv1 complex. Inspired by nature, we create a peptide with two C6 epitopes (C6₂) that binds to both channel subunits simultaneously, yielding picomolar affinity and significantly improved inhibition at high potentials. C6 and C6₂ are peptides designed to regulate hHv1, a channel involved in innate immune-system inflammatory pathophysiology, sperm capacitation, cancer-cell proliferation, and tissue damage in ischemic stroke.

The human voltage-gated proton channel (hHv1) is important for control of intracellular pH. We designed C6, a specific peptide inhibitor of hHv1, to evaluate the roles of the channel in sperm capacitation and in the inflammatory immune response of neutrophils [R. Zhao et al., Proc. Natl. Acad. Sci. U.S.A. 115, E11847–E11856 (2018)]. One C6 binds with nanomolar affinity to each of the two S3–S4 voltage-sensor loops in hHv1 in cooperative fashion so that C6-bound channels require greater depolarization to open and do so more slowly. As depolarization drives hHv1 sensors outwardly, C6 affinity decreases, and inhibition is partial. Here, we identified residues essential to C6–hHv1 binding by scanning mutagenesis, five in the hHv1 S3–S4 loops and seven on C6. A structural model of the C6–hHv1 complex was then generated by molecular dynamics simulations and validated by mutant-cycle analysis. Guided by this model, we created a bivalent C6 peptide (C6₂) that binds simultaneously to both hHv1 subunits and fully inhibits current with picomolar affinity. The results help delineate the structural basis for C6 state-dependent inhibition, support an anionic lipid-mediated binding mechanism, and offer molecular insight into the effectiveness of engineered C6 as a therapeutic agent or lead.

A Review on the Role of Bicarbonate and Proton Transporters during Sperm Capacitation in Mammals

Alkalinization of sperm cytosol is essential for plasma membrane hyperpolarization, hyperactivation of motility, and acrosomal exocytosis during sperm capacitation in mammals. The plasma membrane of sperm cells contains different ion channels implicated in the increase of internal pH (pH_i) by favoring either bicarbonate entrance or proton efflux. Bicarbonate transporters belong to the solute carrier families 4 (SLC4) and 26 (SLC26) and are currently grouped into Na⁺/HCO₃⁻ transporters and Cl⁻/HCO₃⁻ exchangers. Na⁺/HCO₃⁻ transporters are reported to be essential for the initial and fast entrance of HCO₃⁻ that triggers sperm capacitation, whereas Cl⁻/HCO₃⁻ exchangers are responsible for the sustained HCO₃⁻ entrance which orchestrates the sequence of changes associated with sperm capacitation. Proton efflux is required for the fast alkalinization of capacitated sperm cells and the activation of pH-dependent proteins; according to the species, this transport can be mediated by Na⁺/H⁺ exchangers (NHE) belonging to the SLC9 family and/or voltage-gated proton channels (HVCN1). Herein, we discuss the involvement of each of these channels in sperm capacitation and the acrosome reaction.

In vitro assessment of Prunus japonica seed extract on human spermatozoa hypermotility and intracellular alkalization

Prunus japonica var. nakaii is used in traditional Korean medicine to treat various conditions; however, it has not been investigated for treating male infertility. In this study, we investigated the in vitro effects of the ethanolic extract of P. japonica seeds on human sperm motility and identified its mechanism of action. Eleven male volunteers were selected, and the effects of the extract on human spermatozoa were assessed through a computer-assisted semen analysis. The P. japonica seed extract increased the percentage of total and progressive motility of spermatozoa. To understand the mechanism of action, we monitored intracellular alkalization using flow cytometry and obtained electrophysiological recordings of human voltage-gated proton channels hHv1 that were overexpressed in HEK-293 cells. The extract shifted the activation curves in a concentration-dependent manner. Two major constituents of the extract, linoleic acid and oleic acid, exhibited proton channel activity. Our in vitro experiments suggested that P. japonica seed extract could be potentially used to rescue sperm motility in idiopathic infertility patients via pharmacological modulation of the proton channels during capacitation. Therefore, our results indicate the therapeutic potential of P. japonica seed extract for treating male infertility.

Neutrophil Percentage-to-Albumin Ratio: A Good Parameter for the Evaluation of the Severity of Anti-NMDAR Encephalitis at Admission and Prediction of Short-Term Prognosis

Objectives

The purpose of this study was to investigate the association of neutrophil percentage-to-albumin ratio (NPAR) with the severity at admission and discharge (short-term prognosis) in patients with anti-N-methyl-D-aspartic acid receptor (NMDAR) encephalitis.

Methods

Multivariable logistic regression models such as NPAR were constructed based on univariable regression results. Receiver operating characteristic (ROC) curves, nomograms, and concordance index (c-index) were used to evaluate the efficacy of the models in assessing disease severity at admission and predicting short-term prognosis, validated by bootstrap, Hosmer–Lemeshow goodness-of-fit test, calibration curves, and decision curve analysis.

Results

A total of 181 patients with anti-NMDAR encephalitis diagnosed at the First Affiliated Hospital of Zhengzhou University were included. The results showed that NPAR had good sensitivity and specificity in assessing disease severity at admission and predicting short-term prognosis. The multivariable logistic regression models based on NPAR and other influencing factors had good discrimination, consistency, accuracy, calibration ability, applicability, and validity in assessing the severity at admission and predicting short-term prognosis.

Conclusion

NPAR has good clinical value in assessing disease severity at admission and predicting short-term prognosis of patients with anti-NMDAR encephalitis.

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.