Effect of intracellular pH on depolarization-evoked calcium influx in human sperm

Arresting calcium-regulated sperm metabolic dynamics enables prolonged fertility in poultry liquid semen storage

The preservation of liquid semen is pivotal for both industrial livestock production and genetic management/conservation of species with sperm that are not highly cryo-tolerant. Nevertheless, with regard to poultry semen, even brief in vitro storage periods can lead to a notable decline in fertility, despite the in vivo capacity to maintain fertility for several weeks when within the hen's sperm storage tubules. For fertility in sperm, intracellular calcium ions ([Ca2+]i) play a key role in signaling towards modifying energy metabolism. While reducing [Ca2+]i has been found to enhance the preservation of sperm fertility in some mammals, the connection between semen fertility and calcium availability in avian sperm has received limited attention. In this study, we demonstrate that the use of extracellular and intracellular calcium chelators in liquid semen extenders, specifically EGTA and EGTA-AM, has distinct effects on prolonging the fertility of chicken sperm. These results were validated through in vivo fertility tests. Mechanistically, the effects observed were linked to coordination of mitochondrial metabolism and ATP catabolism. Despite both calcium chelators inducing hypoxia, they differentially regulated mitochondrial respiration and ATP accumulation. This regulation was closely linked to a bimodal control of dynein ATPase activity; a direct initial activation with reduction in [Ca2+]i, and subsequent suppression by cytoplasmic acidification caused by lactic acid. These findings not only contribute to advancing poultry liquid semen preservation techniques, but also elucidates biologically relevant mechanisms that may underlie storage within the female reproductive tract in birds.

SLC4A10 mutation causes a neurological disorder associated with impaired GABAergic transmission

SLC4A10 is a plasma-membrane bound transporter that utilizes the Na+ gradient to drive cellular HCO3- uptake, thus mediating acid extrusion. In the mammalian brain, SLC4A10 is expressed in principal neurons and interneurons, as well as in epithelial cells of the choroid plexus, the organ regulating the production of CSF. Using next generation sequencing on samples from five unrelated families encompassing nine affected individuals, we show that biallelic SLC4A10 loss-of-function variants cause a clinically recognizable neurodevelopmental disorder in humans. The cardinal clinical features of the condition include hypotonia in infancy, delayed psychomotor development across all domains and intellectual impairment. Affected individuals commonly display traits associated with autistic spectrum disorder including anxiety, hyperactivity and stereotyped movements. In two cases isolated episodes of seizures were reported in the first few years of life, and a further affected child displayed bitemporal epileptogenic discharges on EEG without overt clinical seizures. While occipitofrontal circumference was reported to be normal at birth, progressive postnatal microcephaly evolved in 7 out of 10 affected individuals. Neuroradiological features included a relative preservation of brain volume compared to occipitofrontal circumference, characteristic narrow sometimes 'slit-like' lateral ventricles and corpus callosum abnormalities. Slc4a10 -/- mice, deficient for SLC4A10, also display small lateral brain ventricles and mild behavioural abnormalities including delayed habituation and alterations in the two-object novel object recognition task. Collapsed brain ventricles in both Slc4a10-/- mice and affected individuals suggest an important role of SLC4A10 in the production of the CSF. However, it is notable that despite diverse roles of the CSF in the developing and adult brain, the cortex of Slc4a10-/- mice appears grossly intact. Co-staining with synaptic markers revealed that in neurons, SLC4A10 localizes to inhibitory, but not excitatory, presynapses. These findings are supported by our functional studies, which show the release of the inhibitory neurotransmitter GABA is compromised in Slc4a10-/- mice, while the release of the excitatory neurotransmitter glutamate is preserved. Manipulation of intracellular pH partially rescues GABA release. Together our studies define a novel neurodevelopmental disorder associated with biallelic pathogenic variants in SLC4A10 and highlight the importance of further analyses of the consequences of SLC4A10 loss-of-function for brain development, synaptic transmission and network properties.

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

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

[Ca2+]i oscillations in human sperm are triggered in the flagellum by membrane potential-sensitive activity of CatSper

STUDY QUESTION

How are progesterone (P4)-induced repetitive intracellular Ca2+ concentration ([Ca2+]i) signals (oscillations) in human sperm generated?

SUMMARY ANSWER

P4-induced [Ca2+]i oscillations are generated in the flagellum by membrane potential (Vm)-sensitive Ca2+-influx through CatSper channels.

WHAT IS KNOWN ALREADY

A subset of human sperm display [Ca2+]i oscillations that regulate flagellar beating and acrosome reaction. Although pharmacological manipulations indicate involvement of stored Ca2+ in these oscillations, influx of extracellular Ca2+ is also required.

STUDY DESIGN, SIZE, DURATION

This was a laboratory study that used >20 sperm donors and involved more than 100 separate experiments and analysis of more than 1000 individual cells over a period of 2 years.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Semen donors and patients were recruited in accordance with local ethics approval from Birmingham University and Tayside ethics committees. [Ca2+]i responses and Vm of individual cells were examined by fluorescence imaging and whole-cell current clamp.

MAIN RESULTS AND THE ROLE OF CHANCE

P4-induced [Ca2+]i oscillations originated in the flagellum, spreading to the neck and head (latency of 1-2 s). K+-ionophore valinomycin (1 µM) was used to investigate the role of membrane potential (Vm). Direct assessment by whole-cell current-clamp confirmed that Vm in valinomycin-exposed cells was determined primarily by K+ equilibrium potential (EK) and was rapidly 'reset' upon manipulation of [K+]o. Pre-treatment of sperm with valinomycin ([K+]o = 5.4 mM) had no effect on the P4-induced [Ca2+] transient (P = 0.95; eight experiments), but application of valinomycin to P4-pretreated sperm suppressed activity in 82% of oscillating cells (n = 257; P = 5 × 10-55 compared to control) and significantly reduced both the amplitude and frequency of persisting oscillations (P = 0.0001). Upon valinomycin washout, oscillations re-started in most cells. When valinomycin was applied in saline with elevated [K+], the inhibitory effect of valinomycin was reduced and was dependent on EK (P = 10-25). Amplitude and frequency of [Ca2+]i oscillations that persisted in the presence of valinomycin showed similar sensitivity to EK (P < 0.01). The CatSper inhibitor RU1968 (4.8 and 11 µM) caused immediate and reversible arrest of activity in 36% and 96% of oscillating cells, respectively (P < 10-10). Quinidine (300 µM) which blocks the sperm K+ current (IKsper) completely, inhibited [Ca2+]i oscillations.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This was an in-vitro study and caution must be taken when extrapolating these results to in-vivo regulation of sperm.

WIDER IMPLICATIONS OF THE FINDINGS

[Ca2+]i oscillations in human sperm are functionally important and their absence is associated with failed fertilisation at IVF. The data reported here provide new understanding of the mechanisms that underlie the regulation and generation (or failure) of these oscillations.

STUDY FUNDING/COMPETING INTEREST(S)

E.T.-N. was in receipt of a postgraduate scholarship from the CAPES Foundation (Ministry of Education, Brazil). E.M-M received travel funds from the Programa de Apoyo a los Estudios de Posgrado (Maestria y Doctorado en Ciencias Bioquimicas-Universidad Autonoma de Mexico). SGB and CLRB are recipients of a Chief Scientist Office (NHS Scotland) grant TCS/17/28. The authors have no conflicts of interest.

Machine-learning algorithm incorporating capacitated sperm intracellular pH predicts conventional in vitro fertilization success in normospermic patients

OBJECTIVE

To measure human sperm intracellular pH (pH_i) and develop a machine-learning algorithm to predict successful conventional in vitro fertilization (IVF) in normospermic patients.

DESIGN

Spermatozoa from 76 IVF patients were capacitated in vitro. Flow cytometry was used to measure sperm pH_i, and computer-assisted semen analysis was used to measure hyperactivated motility. A gradient-boosted machine-learning algorithm was trained on clinical data and sperm pH_i and membrane potential from 58 patients to predict successful conventional IVF, defined as a fertilization ratio (number of fertilized oocytes [2 pronuclei]/number of mature oocytes) greater than 0.66. The algorithm was validated on an independent set of data from 18 patients.

SETTING

Academic medical center.

PATIENT(S)

Normospermic men undergoing IVF. Patients were excluded if they used frozen sperm, had known male factor infertility, or used intracytoplasmic sperm injection only.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Successful conventional IVF.

RESULT(S)

Sperm pH_i positively correlated with hyperactivated motility and with conventional IVF ratio (n = 76) but not with intracytoplasmic sperm injection fertilization ratio (n = 38). In receiver operating curve analysis of data from the test set (n = 58), the machine-learning algorithm predicted successful conventional IVF with a mean accuracy of 0.72 (n = 18), a mean area under the curve of 0.81, a mean sensitivity of 0.65, and a mean specificity of 0.80.

CONCLUSION(S)

Sperm pH_i correlates with conventional fertilization outcomes in normospermic patients undergoing IVF. A machine-learning algorithm can use clinical parameters and markers of capacitation to accurately predict successful fertilization in normospermic men undergoing conventional IVF.

Male Decapacitation Factor SPINK3 Blocks Membrane Hyperpolarization and Calcium Entry in Mouse Sperm

Mammalian sperm acquire ability to fertilize through a process called capacitation, occurring after ejaculation and regulated by both female molecules and male decapacitation factors. Bicarbonate and calcium present in the female reproductive tract trigger capacitation in sperm, leading to acrosomal responsiveness and hyperactivated motility. Male decapacitating factors present in the semen avert premature capacitation, until detached from the sperm surface. However, their mechanism of action remains elusive. Here we describe for the first time the molecular basis for the decapacitating action of the seminal protein SPINK3 in mouse sperm. When present in the capacitating medium, SPINK3 inhibited Src kinase, a modulator of the potassium channel responsible for plasma membrane hyperpolarization. Lack of hyperpolarization affected calcium channels activity, impairing the acquisition of acrosomal responsiveness and blocking hyperactivation. Interestingly, SPINK3 acted only on non-capacitated sperm, as it did not bind to capacitated cells. Binding selectivity allows its decapacitating action only in non-capacitated sperm, without affecting capacitated cells.

Quantitative Intracellular pH Determinations in Single Live Mammalian Spermatozoa Using the Ratiometric Dye SNARF-5F

Intracellular pH (pH i ) plays a crucial role in mammalian sperm physiology. However, it is a challenging task to acquire quantitative single sperm pH i images due to their small size and beating flagella. In this study, we established a robust pH i imaging system using the dual-emission ratiometric pH indicator, SNARF-5F. Simultaneous good signal/noise ratio fluorescence signals were obtained exciting with a green high-power LED (532 nm) and acquiring with an EM-CCD camera through an image splitter with two band-pass filters (550-600 nm, channel 1; 630-650 nm, channel 2). After in vivo calibration, we established an imaging system that allows determination of absolute pH i values in spermatozoa, minimizing cell movement artifacts. Using this system, we determined that bicarbonate increases non-capacitated human pH i with slower kinetics than in mouse spermatozoa. This difference suggests that distinct ionic transporters might be involved in the bicarbonate influx into human and mouse spermatozoa. Alternatively, pH i regulation downstream bicarbonate influx into spermatozoa could be different between the two species.

Inhibition of CatSper and Hv1 Channels and NOX5 Enzyme Affect Progesterone-Induced Increase of Intracellular Calcium Concentration and ROS Generation in Human Sperm

BACKGROUND

Normal sperm function depends on appropriate intracellular calcium (Ca_i ²⁺) and reactive oxygen species (ROS) levels. Calcium activates NADPH oxidase-5 (NOX5) that leads to ROS generation. The calcium channel of sperm (CatSper) is activated by progesterone and intracellular alkalization. Herein, the interactive role of CatSper, Hv1 channels, and NOX5 enzyme on Ca_i ²⁺ and ROS generation in human sperm is investigated.

METHODS

The present laboratory in vitro study was carried out in the School of Medicine, Shiraz University of Medical Sciences (Shiraz, Iran) during 2016. Normal semen samples (n=15) were washed and diluted to 20×10⁶ sperm/mL. The diluted samples were divided into 16 groups containing Ham's F-10 (the control group), 2 µM NNC (CatSper inhibitor), 1 mM ZnCl2 (Hv1 inhibitor), 1 µM DPI (NOX5 inhibitor), NNC+Zn, NNC+DPI, and NNC+Zn+DPI. The other 8 groups were the same as the above except that they contained 1 µM progesterone. Cell viability and Ca_i ²⁺ were analyzed by flou-3 AM probe and PI staining, respectively, using flow cytometric method. ROS generation was assessed by chemiluminescence method. Statistical analysis was performed using the one-way ANOVA followed by Tukey's test. P values <0.05 were considered statistically significant.

RESULTS

Progesterone increased Ca_i ²⁺ and ROS generation. The addition of NNC, Zn, or NNC+Zn significantly decreased Ca_i ²⁺ in the control and progesterone containing groups. Progesterone-induced ROS generation was decreased significantly in all groups containing NNC, Zn, or DPI and reached to the control level when DPI was added to NNC or Zn.

CONCLUSION

There is a functional relationship between CatSper and Hv1 channels in increasing Ca_i ²⁺+. The activity of CatSper and Hv1 channels are required for progesterone-induced ROS generation by NOX5 enzyme.

CRISPR/Cas9-mediated genome editing reveals the synergistic effects of β-defensin family members on sperm maturation in rat epididymis

The epididymis is a male reproductive organ involved in posttesticular sperm maturation and storage, but the mechanism underlying sperm maturation remains unclear. β-Defensins (Defbs) belong to a family of small, cysteine-rich, cationic peptides that are antimicrobial and modulate the immune response. A large number of Defb genes are expressed abundantly in the male reproductive tract, especially in the epididymis. We and other groups have shown the involvement of several Defb genes in regulation of sperm function. In this study, we found that Defb23, Defb26, and Defb42 were highly expressed in specific regions of the epididymis. Rats with CRISPR/Cas9-mediated single-gene disruption of Defb23, Defb26, or Defb42 had no obvious fertility phenotypes. Those with the deletion of Defb23/ 26 or Defb23/ 26/ 42 became subfertile, and sperm isolated from the epididymal cauda of multiple-mutant rats were demonstrated decreased motility. Meanwhile, the sperm showed precocious capacitation and increased spontaneous acrosome reaction. Consistent with premature capacitation and acrosome reaction, sperm from multiple-gene-knockout rats had significantly increased intracellular calcium. These results suggest that Defb family members affect sperm maturation by a synergistic pattern in the epididymis.-Zhang, C., Zhou, Y., Xie, S., Yin, Q., Tang, C., Ni, Z., Fei, J., Zhang, Y. CRISPR/Cas9-mediated genome editing reveals the synergistic effects of β-defensin family members on sperm maturation in rat epididymis.

A genetic variant of the sperm-specific SLO3 K+ channel has altered pH and Ca2+ sensitivities

To fertilize an oocyte, sperm must first undergo capacitation in which the sperm plasma membrane becomes hyperpolarized via activation of potassium (K⁺) channels and resultant K⁺ efflux. Sperm-specific SLO3 K⁺ channels are responsible for these membrane potential changes critical for fertilization in mouse sperm, and they are only sensitive to pH _i However, in human sperm, the major K⁺ conductance is both Ca²⁺- and pH _i -sensitive. It has been debated whether Ca²⁺-sensitive SLO1 channels substitute for human SLO3 (hSLO3) in human sperm or whether human SLO3 channels have acquired Ca²⁺ sensitivity. Here we show that hSLO3 is rapidly evolving and reveal a natural structural variant with enhanced apparent Ca²⁺ and pH sensitivities. This variant allele (C382R) alters an amino acid side chain at a principal interface between the intramembrane-gated pore and the cytoplasmic gating ring of the channel. Because the gating ring contains sensors to intracellular factors such as pH and Ca²⁺, the effectiveness of transduction between the gating ring and the pore domain appears to be enhanced. Our results suggest that sperm-specific genes can evolve rapidly and that natural genetic variation may have led to a SLO3 variant that differs from wild type in both pH and intracellular Ca²⁺ sensitivities. Whether this physiological variation confers differences in fertility among males remains to be established.

Cholecystokinin receptors regulate sperm protein tyrosine phosphorylation via uptake of HCO3-

Cholecystokinin (CCK), a peptide hormone and a neurotransmitter, was detected in mature sperm two decades ago. However, the exact role of CCK and the types of CCK receptors (now termed CCK1 and CCK2) in sperm have not been identified. Here, we find that CCK1 and CCK2 receptors are immunolocalized to the acrosomal region of mature sperm. The antagonist of CCK1 or CCK2 receptor strongly activated the soluble adenylyl cyclase/cAMP/protein kinase A signaling pathway that drives sperm capacitation-associated protein tyrosine phosphorylation in dose- and time-dependent manners. But these actions of stimulation were abolished when sperm were incubated in the medium in the absence of HCO3-. Further investigation demonstrated that the inhibitor of CCK1 or CCK2 receptor could accelerate the uptake of HCO3- and significantly elevate the intracellular pH of sperm. Interestingly, the synthetic octapeptide of CCK (CCK8) showed the same action and mechanism as antagonists of CCK receptors. Moreover, CCK8 and the antagonist of CCK1 or CCK2 receptor were also able to accelerate human sperm capacitation-associated protein tyrosine phosphorylation by stimulating the influx of HCO3-. Thus, the present results suggest that CCK and its receptors may regulate sperm capacitation-associated protein tyrosine phosphorylation by modulating the uptake of HCO3-.

Odyssey of the spermatozoon

This Opinion piece is offered as a cursory overview of sperm development, function, and transport through the eyes of an equine veterinarian. My professional background is predominantly clinical in nature, but my fascination with sperm function and preservation has led to a fairly sizeable review of the scientific literature over the years in hopes of extracting laboratory findings that have application to my daily activities in the clinical arena. Spermatozoa are quite unique among cellular types with regard to both form and function, and represent the only endogenously derived cell type that exerts its action in a separate being. This paper takes the reader on a voyage with a mammalian spermatozoon, from its formative stages through its transport in the male and female reproductive tracts, and culminating with its interaction with an ovulated oocyte at the time of fertilization. Specific emphasis is placed on equine spermatozoa when notable research findings have been unveiled.

Inhibiting sperm pyruvate dehydrogenase complex and its E3 subunit, dihydrolipoamide dehydrogenase affects fertilization in Syrian hamsters

Background/Aims

The importance of sperm capacitation for mammalian fertilization has been confirmed in the present study via sperm metabolism. Involvement of the metabolic enzymes pyruvate dehydrogenase complex (PDHc) and its E3 subunit, dihydrolipoamide dehydrogenase (DLD) in hamster in vitro fertilization (IVF) via in vitro sperm capacitation is being proposed through regulation of sperm intracellular lactate, pH and calcium.

Methodology and Principal Findings

Capacitated hamster spermatozoa were allowed to fertilize hamster oocytes in vitro which were then assessed for fertilization, microscopically. PDHc/DLD was inhibited by the use of the specific DLD-inhibitor, MICA (5-methoxyindole-2-carboxylic acid). Oocytes fertilized with MICA-treated (MT) [and thus PDHc/DLD-inhibited] spermatozoa showed defective fertilization where 2^nd polar body release and pronuclei formation were not observed. Defective fertilization was attributable to capacitation failure owing to high lactate and low intracellular pH and calcium in MT-spermatozoa during capacitation. Moreover, this defect could be overcome by alkalinizing spermatozoa, before fertilization. Increasing intracellular calcium in spermatozoa pre-IVF and in defectively-fertilized oocytes, post-fertilization rescued the arrest seen, suggesting the role of intracellular calcium from either of the gametes in fertilization. Parallel experiments carried out with control spermatozoa capacitated in medium with low extracellular pH or high lactate substantiated the necessity of optimal sperm intracellular lactate levels, intracellular pH and calcium during sperm capacitation, for proper fertilization.

Conclusions

This study confirms the importance of pyruvate/lactate metabolism in capacitating spermatozoa for successful fertilization, besides revealing for the first time the importance of sperm PDHc/ DLD in fertilization, via the modulation of sperm intracellular lactate, pH and calcium during capacitation. In addition, the observations made in the IVF studies in hamsters suggest that capacitation failures could be a plausible cause of unsuccessful fertilization encountered during human assisted reproductive technologies, like IVF and ICSI. Our studies indicate a role of sperm capacitation in the post-penetration events during fertilization.

Ca2+ signals generated by CatSper and Ca2+ stores regulate different behaviors in human sperm

[Ca²⁺]_i signaling regulates sperm motility, enabling switching between functionally different behaviors that the sperm must employ as it ascends the female tract and fertilizes the oocyte. We report that different behaviors in human sperm are recruited according to the Ca²⁺ signaling pathway used. Activation of CatSper (by raising pH_i or stimulating with progesterone) caused sustained [Ca²⁺]_i elevation but did not induce hyperactivation, the whiplash-like behavior required for progression along the oviduct and penetration of the zona pellucida. In contrast, penetration into methylcellulose (mimicking penetration into cervical mucus or cumulus matrix) was enhanced by activation of CatSper. NNC55-0396, which abolishes CatSper currents in human sperm, inhibited this effect. Treatment with 5 μm thimerosal to mobilize stored Ca²⁺ caused sustained [Ca²⁺]_i elevation and induced strong, sustained hyperactivation that was completely insensitive to NNC55-0396. Thimerosal had no effect on penetration into methylcellulose. 4-Aminopyridine, a powerful modulator of sperm motility, both raised pH_i and mobilized Ca²⁺ stored in sperm (and from microsomal membrane preparations). 4-Aminopyridine-induced hyperactivation even in cells suspended in Ca²⁺-depleted medium and also potentiated penetration into methylcellulose. The latter effect was sensitive to NNC55-039, but induction of hyperactivation was not. We conclude that these two components of the [Ca²⁺]_i signaling apparatus have strikingly different effects on sperm motility. Furthermore, since stored Ca²⁺ at the sperm neck can be mobilized by Ca²⁺-induced Ca²⁺ release, we propose that CatSper activation can elicit functionally different behaviors according to the sensitivity of the Ca²⁺ store, which may be regulated by capacitation and NO from the cumulus.

2-APB-potentiated channels amplify CatSper-induced Ca(2+) signals in human sperm

Ca²⁺_i signalling is pivotal to sperm function. Progesterone, the best-characterized agonist of human sperm Ca²⁺_i signalling, stimulates a biphasic [Ca²⁺]_i rise, comprising a transient and subsequent sustained phase. In accordance with recent reports that progesterone directly activates CatSper, the [Ca²⁺]_i transient was detectable in the anterior flagellum (where CatSper is expressed) 1–2 s before responses in the head and neck. Pre-treatment with 5 μM 2-APB (2-aminoethoxydiphenyl borate), which enhances activity of store-operated channel proteins (Orai) by facilitating interaction with their activator [STIM (stromal interaction molecule)] ‘amplified’ progesterone-induced [Ca²⁺]_i transients at the sperm neck/midpiece without modifying kinetics. The flagellar [Ca²⁺]_i response was unchanged. 2-APB (5 μM) also enhanced the sustained response in the midpiece, possibly reflecting mitochondrial Ca²⁺ accumulation downstream of the potentiated [Ca²⁺]_i transient. Pre-treatment with 50–100 μM 2-APB failed to potentiate the transient and suppressed sustained [Ca²⁺]_i elevation. When applied during the [Ca²⁺]_i plateau, 50–100 μM 2-APB caused a transient fall in [Ca²⁺]_i, which then recovered despite the continued presence of 2-APB. Loperamide (a chemically different store-operated channel agonist) enhanced the progesterone-induced [Ca²⁺]_i signal and potentiated progesterone-induced hyperactivated motility. Neither 2-APB nor loperamide raised pH_i (which would activate CatSper) and both compounds inhibited CatSper currents. STIM and Orai were detected and localized primarily to the neck/midpiece and acrosome where Ca²⁺ stores are present and the effects of 2-APB are focussed, but store-operated currents could not be detected in human sperm. We propose that 2-APB-sensitive channels amplify [Ca²⁺]_i elevation induced by progesterone (and other CatSper agonists), amplifying, propagating and providing spatio-temporal complexity in [Ca²⁺]_i signals of human sperm.

Participation of the Cl-/HCO(3)- exchangers SLC26A3 and SLC26A6, the Cl- channel CFTR, and the regulatory factor SLC9A3R1 in mouse sperm capacitation

Sperm capacitation is required for fertilization and involves several ion permeability changes. Although Cl(-) and HCO(3)(-) are essential for capacitation, the molecular entities responsible for their transport are not fully known. During mouse sperm capacitation, the intracellular concentration of Cl(-) ([Cl(-)](i)) increases and membrane potential (Em) hyperpolarizes. As in noncapacitated sperm, the Cl(-) equilibrium potential appears to be close to the cell resting Em, opening of Cl(-) channels could not support the [Cl(-)](i) increase observed during capacitation. Alternatively, the [Cl(-)](i) increase might be mediated by anion exchangers. Among them, SLC26A3 and SLC26A6 are good candidates, since, in several cell types, they increase [Cl(-)](i) and interact with cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) channel present in mouse and human sperm. This interaction is known to be mediated and probably regulated by the Na(+)/H(+) regulatory factor-1 (official symbol, SLC9A3R1). Our RT-PCR, immunocytochemistry, Western blot, and immunoprecipitation data indicate that SLC26A3, SLC26A6, and SLC9A3R1 are expressed in mouse sperm, localize to the midpiece, and interact between each other and with CFTR. Moreover, we present evidence indicating that CFTR and SLC26A3 are involved in the [Cl(-)](i) increase induced by db-cAMP in noncapacitated sperm. Furthermore, we found that inhibitors of SLC26A3 (Tenidap and 5099) interfere with the Em changes that accompany capacitation. Together, these findings indicate that a CFTR/SLC26A3 functional interaction is important for mouse sperm capacitation.

Effect of aracnotoxin from Latrodectus mactans on bovine sperm function: modulatory action of bovine oviduct cells and their secretions

Latrodectus mactans' aracnotoxin (Atx) induces changes in sperm function that could be used as a co-adjuvant in male contraceptive barrier methods. This effect includes the suppression of intracellular reactive oxygen species (ROS), an event necessary for capacitation, chemotaxis and acrosome reaction (AR). The sperm that are not trapped by the barrier method can reach the oviduct before fertilisation and be exposed to the secretions of the oviducts. This study evaluated the effect of bovine tubal explants (TU) and conditioned media (CM) from the ampullar and isthmal regions on spermatozoa exposed to Atx. Thawed bovine sperm were incubated with Atx, TU and CM from the ampullar and isthmal regions for 4 h and then DNA integrity, intracellular ROS and lysophosphatidylcholine-induced AR were determined. Spermatozoa exposed to Atx and co-incubated with TU and CM for 4 h produced an increase in sperm DNA damage, a decrease in ROS production and a decrease in %AR, compared with the control. A similar result was obtained from the co-incubation of spermatozoa with Atx. In conclusion, the effect of Atx is not modified by tubal cells or their secretions and this opens the door to future studies to evaluate the application of synthetic peptides obtained from Atx as a co-adjuvant of contraceptive barrier methods.

Differential pH in embryo culture

OBJECTIVE

To determine the optimum pH in sequential media for embryo culture around the fertilization-zygote stage and cleavage stage, with use of a mouse embryo assay.

DESIGN

Experimental laboratory study.

SETTING

University Hospital and University Research Unit.

ANIMAL(S)

F1 hybrids between CD1 female and BDF male mice.

INTERVENTION(S)

Fertilized, one-cell mouse embryos were cultured 5 days in test media where pH was changed at defined time intervals.

MAIN OUTCOME MEASURE(S)

Percentage of good-quality embryos, defined by strict morphology.

RESULT(S)

A significantly improved development was observed when pH was as high as 7.30 before the pronuclear stage and lowered to pH 7.15 during the cleavage period.

CONCLUSION(S)

Good embryo development is consistent with two different pH values in sequential culture media. This could have important implications for embryo culture in human IVF.

The BSA-induced Ca2+ influx during sperm capacitation is CATSPER channel-dependent

BACKGROUND

Serum albumin is a key component in mammalian sperm capacitation, a functional maturation process by which sperm become competent to fertilize oocytes. Capacitation is accompanied by several cellular and molecular changes including an increased tyrosine phosphorylation of sperm proteins and a development of hyperactivated sperm motility. Both of these processes require extracellular calcium, but how calcium enters sperm during capacitation is not well understood.

METHODS

BSA-induced changes in intracellular calcium concentration were studied using Fluo-4 and Fura-2 calcium imaging with wild-type and Catsper1 knockout mouse sperm.

RESULTS

We found that the fast phase of the BSA-induced rises in intracellular calcium concentration was absent in the Catsper1 knockout sperm and could be restored by an EGFP-CATSPER1 fusion protein. The calcium concentration increases were independent of G-proteins and phospholipase C but could be partially inhibited when intracellular pH was clamped. The changes started in the principal piece and propagated toward the sperm head.

CONCLUSION

We conclude that the initial phase of the increases in intracellular calcium concentration induced by BSA requires the CATSPER channel, but not the voltage-gated calcium channel. Our findings identify the molecular conduit responsible for the calcium entry required for the sperm motility changes that occur during capacitation.

Cl- is required for HCO3- entry necessary for sperm capacitation in guinea pig: involvement of a Cl-/HCO3- exchanger (SLC26A3) and CFTR

Our previous study demonstrated the involvement of cystic fibrosis transmembrane conductance regulator (CFTR) in transporting bicarbonate that is necessary for sperm capacitation; however, whether its involvement is direct or indirect remains unclear. The present study investigated the possibility of a Cl-/HCO3- exchanger (solute carrier family 26, number 3 [SLC26A3]) operating with CFTR during guinea pig sperm capacitation. Incubating sperm in media with various concentrations of Cl- resulted in varied percentages of capacitated sperm in a concentration-dependent manner. Depletion of Cl-, even in the presence of HCO3-, abolished sperm capacitation and vice versa, indicating the involvement of both anions in the process. Capacitation-associated HCO3--dependent events, including increased intracellular pH, cAMP production, and protein tyrosine phosphorylation, also depend on Cl- concentrations. Similar Cl- dependence and inhibitor sensitivity were observed for sperm-hyperactivated motility and for sperm-egg fusion. The expression and localization of CFTR and SLC26A3 were demonstrated using immunostaining and Western blot analysis. Taken together, our results indicate that Cl- is required for the entry of HCO3- that is necessary for sperm capacitation, implicating the involvement of SLC26A3 in transporting HCO3-, with CFTR providing the recycling pathway for Cl-.

Current concepts of molecular events during bovine and porcine spermatozoa capacitation

Spermatozoa are required to undergo the processes of capacitation before they obtain fertilizing ability. The molecular changes of capacitation are still not fully understood. However, it is accepted that capacitation is a sequential process involving numerous physiological changes including destabilization of the plasma membrane, alterations of intracellular ion concentrations and membrane potential, and protein phosphorylation. There are no known morphological changes that occur to the spermatozoon during capacitation. The purpose of this review is to summarize current evidence on the molecular aspects of capacitation both in vivo and in vitro in bovine and porcine spermatozoa. For the purpose of this review, the process of sperm capacitation will encompass maturational events that occur following ejaculation up to binding to the zona pellucida, that triggers acrosomal exocytosis and initiates fertilization.

Effect of pH on the development of acrosomal responsiveness of human sperm

Human sperm incubated in vitro gradually become responsive to inducers of the acrosome reaction. The roles of constituents of the incubation medium are not well understood. These experiments tested the effect of the extracellular pH on sperm acrosomal responsiveness. Sperm were incubated 24 h in media with pH varying between 6.7 and 7.6 and then exposed to progesterone to determine the number of sperm that had become acrosomally responsive. The number of responsive sperm was very low following incubation at pH 6.7-7.0, and increased with the pH over the range 7.0-7.6. Sperm incubated at low pH were not damaged as assessed by motility or viability, and if they were transferred to medium of high pH for 8 h, the inhibition of acrosomal responsiveness was reversed. Inhibition of acrosomal responsiveness at low pH was not due to impaired loss of sperm cholesterol, but was correlated with a reduced intracellular pH. The inhibition of acrosomal responsiveness by media of low pH may result from preventing the normal capacitation-related rise in intracellular pH.

Bovine Sperm Hyperactivation Is Promoted by Alkaline-Stimulated Ca2+ Influx1

Sperm hyperactivated motility is characterized by high flagellar bend amplitude and asymmetrical beating, which are detected by computer-assisted sperm motility analysis as increased curvilinear velocity and lateral head movement. It is required for sperm penetration of the oocyte zona pellucida during fertilization and is induced by an increase in flagellar Ca(2+). Our objective was to determine whether pH plays a role in promoting Ca signaling of hyperactivated motility. The cell-permeant weak base NH(4)Cl increased curvilinear velocity and amplitude of lateral head movement of bovine sperm, indicative of hyperactivation. Fluorometric recordings of sperm loaded with BCECF-AM or fluo3-AM, revealed that NH(4)Cl evoked elevations of intracellular pH and Ca(2+), respectively, with the rise in pH occurring more rapidly than that of Ca(2+). Single-cell image analysis showed increased Ca(2+) levels in the flagellum in response to NH(4)Cl. When extracellular Ca(2+) was lowered with BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) prior to treatment with NH(4)Cl, intracellular pH was increased, but elevation of Ca(2+) and hyperactivation were diminished. This suggests that the rise in intracellular pH precedes an influx of Ca(2+). The Ca(2+) channel blocker Ni(2+) also diminished NH(4)Cl stimulation of hyperactivation, demonstrating that Ca(2+) entry is required for maximal expression of hyperactivation. Ca(2+) ionophore produced an increase in Ca(2+) that was 3-fold greater than that produced by NH(4)Cl; however, it produced a weaker hyperactivation response. These results indicate that a rise in pH increases intracellular Ca(2+)and promotes hyperactivation primarily by stimulating Ca(2+) influx, but also by other mechanisms.

The non-genomic effects on Na+/H+-exchange 1 by progesterone and 20α-hydroxyprogesterone in human T cells

Progesterone is an endogenous immunomodulator and can suppress T-cell activation during pregnancy. We have previously shown that the non-genomic effects of progesterone, especially acidification, are exerted via plasma membrane sites and suppress cellular genomic responses to mitogens. This study aimed to show that acidification is due to a non-genomic inhibition of Na(+)/H(+)-exchange 1 (NHE1) by progesterone and correlate this with immunosuppressive phytohemagglutinin (PHA)-induced T-cell proliferation. The presence of amiloride-sensitive NHE 1 was identified in T cells. The activity of NHE1 was inhibited by progesterone but not by 20alpha-hydroxyprogesterone (20alpha-OHP). Furthermore, 20alpha-OHP was able to compete with progesterone and release the inhibitory effect on the NHE1. The inhibition of NHE1 activity by progesterone-BSA demonstrated non-genomic action via plasma membrane sites. Finally, co-stimulation with PHA and progesterone or amiloride, (5-(N, N-dimethyl)-amiloride, DMA), inhibited PHA-induced T-cell proliferation, but this inhibition did not occur with 20alpha-OHP and PHA co-stimulation. However, when DMA was applied 72 h after PHA stimulation, it was able to suppress PHA-induced T-cell proliferation. This is the first study to show that progesterone causes a rapid non-genomic inhibition of plasma membrane NHE1 activity in T cells within minutes which is released by 20alpha-OHP. The inhibition of NHE1 leads to immunosuppressive T-cell proliferation and suggests that progesterone might exert a major rapid non-genomic suppressive effect on NHE1 activity at the maternal-fetal interface in vivo and that 20alpha-OHP may possibly be able to quickly release the suppression when T cells circulated away from the interface.

Sperm channel diversity and functional multiplicity

Ion channels are extraordinarily efficient machines that move ions in diversely controlled manners, allowing cells to rapidly exchange information with the outside world and with other cells. Communication is the currency of fertilization, as it is of most fundamental cell signaling events. Ion channels are deeply involved in the dialogue between sperm, its surroundings, and the egg. How sperm swim, find the egg and fertilize it depend on ion permeability changes modulated by environmental cues and components of the egg outer layer. Different ion channels distinctly localized in these tiny, amazing cells perform specific decoding functions that shape the sophisticated behavior of sperm. It is not surprising that certain sperm ion channels are turning out to be unique. New strategies to characterize sperm ion transport have opened exciting possibilities to dissect sperm-egg signaling and unveil novel contraception targets.

A remarkable increase in the pHi sensitivity of voltage-dependent calcium channels occurs in human sperm incubated in capacitating conditions

Human sperm are endowed with voltage-dependent calcium channels (VDCC) that produce increases in [Ca2+]i in response to depolarization with KCl. These channels are stimulated during "capacitation", a complex biochemical process, accompanied by a slight pHi alkalization, that sperm must accomplish to acquire the ability to fertilize the egg. The stimulation can be explained in part by the fact that in non-capacitated sperm, calcium influx through VDCC is stimulated by pHi alkalization in the range of pHi observed during capacitation. In this work, we explored the effect of pHi on VDCC in capacitated sperm loaded with fura ff. Strikingly, the pHi sensitivity of VDCC increased approximately 7-fold when sperm was capacitated, as compared with non-capacitated sperm. This finding suggests that the pHi sensitivity of VDCC can be modulated during capacitation so that a combined effect of pHi alkalization and biochemical regulation enhances calcium influx through these channels.

Whole-cell patch-clamp measurements of spermatozoa reveal an alkaline-activated Ca2+ channel

In mammals, sperm cells become motile during ejaculation and swim up the female reproductive tract. Before fertilization and to overcome various barriers, their motility must be hyperactivated, a motion that is characterized by vigorous asymmetric tail beating. Hyperactivation requires an increase in calcium in the flagella, a process that probably involves plasmalemmal ion channels. Numerous attempts in the past two decades to understand sperm cell channels have been frustrated by the difficulty of measuring spermatozoan transmembrane ion currents. Here, by using a simple approach to patch-clamp spermatozoa and to characterize whole-spermatozoan currents, we describe a constitutively active flagellar calcium channel that is strongly potentiated by intracellular alkalinization. This current is not present in spermatozoa lacking the sperm-specific putative ion channel protein, CatSper1. This plasma membrane protein of the six transmembrane-spanning ion channel superfamily is specifically localized to the principal piece of the sperm tail and is required for sperm cell hyperactivation and male fertility. Our results identify CatSper1 as a component of the key flagellar calcium channel, and suggest that intracellular alkalinization potentiates CatSper current to increase intraflagellar calcium and induce sperm hyperactivation.

Calcium signalling in human spermatozoa: a specialized 'toolkit' of channels, transporters and stores

Ca(2+) is a ubiquitous intracellular messenger which encodes information by temporal and spatial patterns of concentration. In spermatozoa, several key functions, including acrosome reaction and motility, are regulated by cytoplasmic Ca(2+) concentration. Despite the very small size and apparent structural simplicity of spermatozoa, evidence is accumulating that they possess sophisticated mechanisms for regulation of cytoplasmic Ca(2+) concentration and generation of complex Ca(2+) signals. In this review, we consider the various components of the Ca(2+)-signalling 'toolkit' that have been characterized in somatic cells and summarize the evidence for their presence and activity in spermatozoa. In particular, data accumulated over the last few years show that spermatozoa possess one (and probably two) Ca(2+) stores as well as a range of plasma membrane pumps and channels. Selective regulation of the various components of the 'toolkit' by agonists probably allows spermatozoa to generate localized Ca(2+) signals despite their very small cytoplasmic volume, permitting the discrete and selective activation of cell functions.

Reassessing the role of progesterone in fertilization—compartmentalized calcium signalling in human spermatozoa?

Progesterone is present at micromolar concentrations in the vicinity of the oocyte. Human spermatozoa generate a biphasic rise in intracellular calcium concentration ([Ca(2+)](i)) and undergo the acrosome reaction upon progesterone stimulation, suggesting that the hormone acts as a secondary inducer or 'primer' of the acrosome reaction in association with the zona pellucida. However, the sensitivity of human spermatozoa to progesterone is such that many cells may undergo the acrosome reaction prematurely, compromising their ability to fertilize. We have shown that exposing human spermatozoa to a progesterone gradient, simulating the stimulus encountered as sperm approach the oocyte, results in a novel response. A slow rise in [Ca(2+)](i) occurs, upon which, in many cells, [Ca(2+)](i) oscillations are superimposed. Cells showing this pattern of response do not undergo the acrosome reaction, but instead show an alternating pattern of flagellar activity associated with peaks and troughs of [Ca(2+)](i). A Ca(2+) store in the rear of the sperm head apparently generates this complex signal, functioning as an '[Ca(2+)](i) oscillator'. We propose that: (i) the acrosome reaction and flagellar beat are regulated by separate Ca(2+) stores; (ii) these stores are mobilized through different mechanisms by different agonists; and (iii) progesterone in vivo acts as a switch for the oscillator which regulates the flagellar beat mode.