Oviductal high concentration of K+ suppresses hyperpolarization but does not prevent hyperactivation, acrosome reaction and in vitro fertilization in hamsters

Hamster Sperm Possess Functional Na+/Ca2+-Exchanger 1: Its Implication in Hyperactivation

Previous studies demonstrated that hamster sperm hyperactivation is suppressed by extracellular Na⁺ by lowering intracellular Ca²⁺ levels, and Na⁺/Ca²⁺-exchanger (NCX) specific inhibitors canceled the suppressive effects of extracellular Na⁺. These results suggest the involvement of NCX in the regulation of hyperactivation. However, direct evidence of the presence and functionality of NCX in hamster spermatozoa is still lacking. This study aimed to reveal that NCX is present and is functional in hamster spermatozoa. First, NCX1 and NCX2 transcripts were detected via RNA-seq analyses of hamster testis mRNAs, but only the NCX1 protein was detected. Next, NCX activity was determined by measuring the Na⁺-dependent Ca²⁺ influx using the Ca²⁺ indicator Fura-2. The Na⁺-dependent Ca²⁺ influx was detected in hamster spermatozoa, notably in the tail region. The Na⁺-dependent Ca²⁺ influx was inhibited by the NCX inhibitor SEA0400 at NCX1-specific concentrations. NCX1 activity was reduced after 3 h of incubation in capacitating conditions. These results, together with authors' previous study, showed that hamster spermatozoa possesses functional NCX1 and that its activity was downregulated upon capacitation to trigger hyperactivation. This is the first study to successfully reveal the presence of NCX1 and its physiological function as a hyperactivation brake.

Sperm calcium flux and membrane potential hyperpolarization observed in the Mexican big-eared bat Corynorhinus mexicanus

Mammalian sperm capacitation involves biochemical and physiological changes, such as an increase in intracellular calcium ion concentration ([Ca2+]i), hyperpolarization of the plasma membrane potential and sperm hyperactivation, among others. These changes provide sperm with the ability to fertilize. In the bat Corynorhinus mexicanus, there is an asynchrony between spermatogenesis and sperm storage in the male with the receptivity of the female. For instance, in C. mexicanus, spermatogenesis occurs before the reproductive season. During the reproductive period, sperm are stored in the epididymis for a few months and the testis undergoes a regression, indicating low or almost null sperm production. Therefore, it is unclear whether the elements necessary for sperm fertilization success undergo maturation or preparation during epididymis storage. Here, we characterized pH-sensitive motility hyperactivation and Ca2+ influx in sperm, regulated by alkalinization and progesterone. In addition, by electrophysiological recordings, we registered currents that were stimulated by alkalinization and inhibited by RU1968 (a CatSper-specific inhibitor), strongly suggesting that these currents were evoked via CatSper, a sperm Ca2+-specific channel indispensable for mammalian fertilization. We also found hyperpolarization of the membrane potential, such as in other mammalian species, which increased according to the month of capture, reaching the biggest hyperpolarization during the mating season. In conclusion, our results suggest that C. mexicanus sperm have functional CatSper and undergo a capacitation-like process such as in other mammals, particularly Ca2+ influx and membrane potential hyperpolarization.

[Regulation of sperm hyperactivation by transporters involved in Na+ homeostasis]

Mammalian sperm have to undergo several physiological and biochemical changes to be fertilization-competent. These changes are collectively called "capacitation". Capacitated sperm show frantic whiplash-like flagellar movement called "hyperactivation". Recently, it has been reported that treatments of sperm to enhance hyperactivation improve in vitro fertilization (IVF) and development of the fertilized embryos. This fact indicates that hyperactivation is an attractive target to improve assisted reproductive technologies. However, the detailed mechanisms which regulate hyperactivation have not been fully elucidated. Recently, it was found out that Na⁺ and K⁺ concentration of the oviductal fluid considerably differs from those in the medium used for IVF. Thus, the effect of the Na⁺ and K⁺ concentration on sperm hyperactivation was investigated using hamsters as a model. The results revealed that hamster sperm hyperactivation was suppressed by extracellular Na⁺ via an action of Na⁺/Ca²⁺ exchanger. Moreover, it was shown that the activity of testis specific isoform of Na⁺/K⁺ ATPase (NKA) α subunit, α4, is necessary for the hyperactivation-associated change of the flagellar movement. By contrast, there was no significant effect of raising the K⁺ concentration up to 20 mM on sperm hyperactivation although it significantly depolarized the membrane potential. These results suggests that transporters involved in primary and secondary active transport which regulates cellular Na⁺ homeostasis has a crucial role in the regulation of hyperactivation, and these transporters can be an attractive target for the improvement of assisted reproductive technologies.