Abstract
Aquaporins are a family of ubiquitous transmembrane proteins that allow the transport of water and small molecules across the cell plasma membrane. The different members of this family present a characteristic distribution across different cell types, which is species-specific. In mammalian sperm, different AQPs, including AQP3, AQP7, and AQP11, have been identified; their main roles are related to osmoadaptation and sperm motility activation after ejaculation. Capacitation, which is a post-ejaculatory process that sperm must undergo to achieve fertilizing ability, is triggered by pH changes and different extracellular ions that are present in the female reproductive tract. Considering the function of AQPs and their influence on pH through the regulation of water flow, this study aimed to elucidate the potential role of different AQPs during in vitro sperm capacitation using three different transition metal compounds as AQP inhibitors. Cooper sulfate, a specific inhibitor of AQP3, caused a drastic increase in peroxide intracellular levels compared to the control. Mercury chloride, an unspecific inhibitor of all AQPs except AQP7 produced an increase in membrane lipid disorder and led to a decrease in sperm motility and kinetics parameters. Finally, the addition of silver sulfadiazine, an unspecific inhibitor of all AQPs, generated the same effects than mercury chloride, decreased the intracellular pH and altered tyrosine phosphorylation levels after the induction of the acrosome reaction. In the light of the aforementioned, (a) the permeability of AQP3 to peroxides does not seem to be crucial for sperm capacitation and acrosome reaction; (b) AQPs have a key role in preserving sperm motility during that process; and (c) AQPs as a whole seem to contribute to the maintenance of lipid membrane architecture during capacitation and may be related to the intracellular signaling pathways involved in the acrosome reaction. Hence, further research aimed to elucidate the mechanisms underlying the involvement of AQPs in mammalian sperm capacitation and acrosome reaction is warranted.
Highlights
Cell homeostasis relies on both the integrity and permeability of plasma membrane to water and electrolytes
Several studies to address the function of ion channels during sperm capacitation were conducted
It is well known that soluble adenylate cyclase, which is regulated by pH, is crucial for the cAMP-PKA pathway triggered during capacitation (Chang and Oude-Elferink, 2014)
Summary
Cell homeostasis relies on both the integrity and permeability of plasma membrane to water and electrolytes. Aquaporins (AQPs) are ubiquitous transmembrane water proteins that allow the transport of water and small solutes across cell membranes (reviewed by Yeste et al, 2017). In mammalian cells, this family of channel proteins includes different members that are classified according to sequence similarity and solute permeability into: orthodox AQPs, aquaglyceroporins (GLPs) and superAQPs. The group of orthodox AQPs includes AQP0, AQP1, AQP2, AQP4, AQP5, AQP6, and AQP8, which are exclusively permeable to water. Aquaglyceroporins, which are permeable to water, glycerol, urea and other small electrolytes, include AQP3, AQP7, AQP9, and AQP10. The group of superAQPs comprises AQP11 and AQP12, which are localized in the membranes of intracellular organelles, present sequence variations to the other groups of AQPs and are involved in both water and glycerol transport
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