The sperm acrosome reaction (AR), an essential event for mammalian fertilization, involves Ca permeability changes leading to exocytosis of the acrosomal vesicle. The acrosome, an intracellular Ca store whose luminal pH is acidic, contains hydrolytic enzymes. It is known that acrosomal pH (pH ) increases during capacitation and this correlates with spontaneous AR. Some AR inducers increase intracellular Ca concentration ([Ca ] ) through Ca release from internal stores, mainly the acrosome. Catsper, a sperm specific Ca channel, has been suggested to participate in the AR. Curiously, Mibefradil and NNC55-0396, two CatSper blockers, themselves elevate [Ca ] by unknown mechanisms. Here we show that these compounds, as other weak bases, can elevate pH , trigger Ca release from the acrosome, and induce the AR in both mouse and human sperm. To our surprise, μM concentrations of NNC55-0396 induced AR even in nominally Ca free media. Our findings suggest that alkalization of the acrosome is critical step for Ca release from the acrosome that leads to the acrosome reaction.
SUMMARYThe rate of motile sperm recovery after cryopreservation is very variable and difficult to predict. Anti-M€ ullerian hormone (AMH) and inhibin B are produced by Sertoli cells and released into the seminal plasma, where they could be functional markers of spermatogenesis and sperm resistance to thermal stress. The aim of this study was to evaluate whether seminal plasma levels of AMH and inhibin B predict sperm recovery after cryopreservation. The study included 153 men enrolled prospectively during a semen analysis. The cohort was stratified by the fresh semen characteristics into: normal (n = 52), high sperm count (n = 55), asthenozoospermia (n = 23), and oligozoospermia (n = 23). The main outcome measure was motile sperm recovery rate, defined as post-thaw total motile sperm count 9 100/pre-freezing total motile sperm count. In men with asthenozoospermia there was a significant correlation between motile sperm recovery rate and the pre-freezing concentrations of AMH (r = 0.522, p < 0.05) and inhibin B (0.471, p < 0.05). In this group, the areas under the receiver operating characteristic curves of AMH and inhibin B for prediction of ≥50% motile sperm recovery after cryopreservation were, respectively, 0.808 and 0.638. AMH was particularly useful, with sensitivity of 0.85, specificity of 0.80, positive predictive value of 0.84 and negative predictive value of 0.80. The sensitivity, specificity, positive, and negative predictive values of inhibin B for the same outcome were, respectively, 0.62, 0.60, 0.67, and 0.55. The median motile sperm recovery rate was 83% when seminal plasma AMH concentration was ≥0.84 ng/mL, vs. 27% when AMH concentration was <0.84 ng/ mL (p < 0.05). In other patient groups, there was no correlation between the two hormone levels in seminal plasma and the motile sperm recovery rate. In conclusion, seminal plasma AMH and inhibin B concentrations correlate with and can be used to predict motile sperm recovery after semen cryopreservation in asthenozoospermic men.
In brief
Hyperpolarization of the membrane potential is a crucial step for mammalian sperm maturation. This work demonstrates that this membrane potential change likely activates a sperm-specific sodium/proton exchanger to induce alkalization in mouse sperm flagellum.
Abstract
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 WT 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.
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