1,3-Propanediol binds inside the water-conducting pore of aquaporin 4: Does this efficacious inhibitor have sufficient potency?

Relevance of Aquaporins for Gamete Function and Cryopreservation

Simple Summary

The interaction between cells and the extracellular medium is of great importance; changes in medium composition can drive water movement across plasma membranes. Aquaporins (AQPs) are membrane channels involved in the transport of water and some solutes across membranes. When sperm enter the female reproductive tract after ejaculation, they encounter a drastic change in extracellular composition, which leads to water flowing across the plasma membrane. This triggers a series of events that are crucial to allowing fertilization to take place, such as regulation of sperm motility. In the context of assisted reproduction techniques (ART), long-term storage of gametes is sometimes required, and, during cryopreservation, these cells undergo drastic changes in extracellular medium composition. As a result, AQPs are crucial in both sperm and oocytes during this process. Cryopreservation is of considerable importance for fertility preservation in livestock, endangered species and for individuals undergoing certain medical treatments that compromise their fertility. Further research to fully elucidate the roles and underlying mechanisms of AQPs in mammalian sperm is therefore warranted.

Abstract

The interaction between cells and the extracellular medium is of great importance, and drastic changes in extracellular solute concentrations drive water movement across the plasma membrane. Aquaporins (AQPs) are a family of transmembrane channels that allow the transport of water and small solutes across cell membranes. Different members of this family have been identified in gametes. In sperm, they are relevant to osmoadaptation after entering the female reproductive tract, which is crucial for sperm motility activation and capacitation and, thus, for their fertilizing ability. In addition, they are relevant during the cryopreservation process, since some members of this family are also permeable to glycerol, one of the most frequently used cryoprotective agents in livestock. Regarding oocytes, AQPs are very important in their maturation but also during cryopreservation. Further research to define the exact sets of AQPs that are present in oocytes from different species is needed, since the available literature envisages certain AQPs and their roles but does not provide complete information on the whole set of AQPs. This is of considerable importance because, in sperm, specific AQPs are known to compensate the role of non-functional members.

Molecular Dynamics Simulations of Mite Aquaporin DerfAQP1 from the Dust Mite Dermatophagoides farinae (Acariformes: Pyroglyphidae)

Aquaporins are a large family of transmembrane channel proteins that facilitate the passive but highly selective transport of water and other small solutes across biological membranes. House dust mite (Dermatophagoides farinae) is the major source of household immunogens, and we have recently reported six cDNA sequence encoding aquaporins from this mite species. To better understand the structure and role of mite aquaporin, we constructed a tertiary structure for DerfAQP1 by homology modeling from the X-ray structure of malaria aquaporin PfAQP (Protein Data Bank code No. 3C02) and conducted molecular dynamics simulation. The simulation arranged seven water molecules in a single file through the pores of the DerfAQP1. Further, two conserved Asn-Pro-Ala motifs were located on Asn203 and Asn77; residues Arg206, Trp57, Met190, Gly200, and Asp207 constituted an extracellular vestibule of the pore; and residues His75, Val80, Ile65, and Ile182 constituted the cytoplasmic portions. The overall free energy profile for water transport through DerfAQP1 revealed an energy barrier of ~2.5 kcal/mol. These results contribute to the understanding of mite physiology and pathology.

Effect of AQP Inhibition on Boar Sperm Cryotolerance Depends on the Intrinsic Freezability of the Ejaculate

Aquaporins (AQPs) are transmembrane channels with permeability to water and small solutes that can be classified according to their structure and permeability into orthodox AQPs, aquaglyceroporins (GLPs), and superAQPs. In boar spermatozoa, AQPs are related to osmoregulation and play a critical role in maturation and motility activation. In addition, their levels differ between ejaculates with good and poor cryotolerance (GFE and PFE, respectively). The aim of this work was to elucidate whether the involvement of AQPs in the sperm response to cryopreservation relies on the intrinsic freezability of the ejaculate. With this purpose, two different molecules: phloretin (PHL) and 1,3-propanediol (PDO), were used to inhibit sperm AQPs in GFE and PFE. Boar sperm samples were treated with three different concentrations of each inhibitor prior to cryopreservation, and sperm quality and functionality parameters were evaluated in fresh samples and after 30 and 240 min of thawing. Ejaculates were classified as GFE or PFE, according to their post-thaw sperm viability and motility. While the presence of PHL caused a decrease in sperm quality and function compared to the control, samples treated with PDO exhibited better quality and function parameters than the control. In addition, the effects of both inhibitors were more apparent in GFE than in PFE. In conclusion, AQP inhibition has more notable consequences in GFE than in PFE, which can be related to the difference in relative levels of AQPs between these two groups of samples.

Cryotolerance of Stallion Spermatozoa Relies on Aquaglyceroporins rather than Orthodox Aquaporins

Aquaporins (AQPs), a family of ubiquitous water channels divided into orthodox AQPs, aquaglyceroporins (GLPs), and superAQPs, are present in stallion spermatozoa. The aim of this study was to elucidate the functional relevance of each group of AQPs during stallion sperm cryopreservation through the use of three different inhibitors: acetazolamide (AC), phloretin (PHL) and propanediol (PDO). Sperm quality and function parameters were evaluated in the presence or absence of each inhibitor in fresh and frozen–thawed samples. In the presence of AC, different parameters were altered (p < 0.05), but not in a concentration- or time-depending manner. PHL was found to decrease sperm motility, viability, acrosome integrity, and the percentages of spermatozoa with low membrane lipid disorder, high mitochondrial membrane potential (MMP) and high intracellular levels of calcium and superoxides (p < 0.05). Finally, the sperm motility, viability, acrosome integrity, the percentages of spermatozoa with low membrane lipid disorder, high MMP and high intracellular calcium levels were higher (p < 0.05) in PDO treatments than in the control. The sperm response to AC, PHL and PDO indicates that GLPs, rather than orthodox AQPs, play a crucial role during stallion sperm cryopreservation. Furthermore, post-thaw sperm quality was higher in PDO treatments than in the control, suggesting that this molecule is a potential permeable cryoprotectant.

Aquaglyceroporins but not orthodox aquaporins are involved in the cryotolerance of pig spermatozoa

Background

Aquaporins (AQPs) are a family of transmembrane water channels that includes orthodox AQPs, aquaglyceroporins (GLPs) and superAQPs. AQP3, AQP7, AQP9 and AQP11 have been identified in boar sperm, and they are crucial for sperm maturation and osmoregulation. Water exchange is an important event in cryopreservation, which is the most efficient method for long-term storage of sperm. However, the freeze-thaw process leads to sperm damage and a loss of fertilizing potential. Assuming that the quality of frozen-thawed sperm partially depends on the regulation of osmolality variations during this process, AQPs might play a crucial role in boar semen freezability. In this context, the aim of this study was to unravel the functional relevance of the different groups of AQPs for boar sperm cryotolerance through three different inhibitors.

Results

Inhibition of different groups of AQPs was found to have different effects on boar sperm cryotolerance. Whereas the use of 1,3-propanediol (PDO), an inhibitor of orthodox AQPs and GLPs, decreased total motility (P < 0.05), it increased post-thaw sperm viability, lowered membrane lipid disorder and increased mitochondrial membrane potential (MMP) (P < 0.05). When acetazolamide (AC) was used as an inhibitor of orthodox AQPs, the effects on post-thaw sperm quality were restricted to a mild increase in MMP in the presence of the intermediate concentration at 30 min post-thaw and an increase in superoxide levels (P < 0.05). Finally, the addition of phloretin (PHL), a GLP inhibitor, had detrimental effects on post-thaw total and progressive sperm motilities, viability and lipid membrane disorder (P < 0.05).

Conclusions

The effects of the different inhibitors suggest that GLPs rather than orthodox AQPs are relevant for boar sperm freezability. Moreover, the positive effect of PDO on sperm quality suggests a cryoprotective role for this molecule.

Computing osmotic permeabilities of aquaporins AQP4, AQP5, and GlpF from near-equilibrium simulations

Measuring or computing the single-channel permeability of aquaporins/aquaglyceroporins (AQPs) has long been a challenge. The measured values scatter over an order of magnitude but the corresponding Arrhenius activation energies converge in the current literature. Osmotic flux through an AQP was simulated as water current forced through the channel by kilobar hydraulic pressure or theoretically approximated as single-file diffusion. In this paper, we report large scale simulations of osmotic current under sub M gradient through three AQPs (water channels AQP4 and AQP5 and glycerol-water channel GlpF) using the mature particle mesh Ewald technique (PME) for which the established force fields have been optimized with known accuracy. These simulations were implemented with hybrid periodic boundary conditions devised to avoid the artifactitious mixing across the membrane in a regular PME simulation. The computed single-channel permeabilities at 5°C and 25°C are in agreement with recently refined experiments on GlpF. The Arrhenius activation energies extracted from our simulations for all the three AQPs agree with the in vitro measurements. The single-file diffusion approximations from our large-scale simulations are consistent with the current literature on smaller systems. From these unambiguous agreements among the in vitro and in silico studies, we observe the quantitative accuracy of the all-atom force fields of the current literature for water-channel biology. We also observe that AQP4, that is particularly rich in the central nervous system, is more efficient in water conduction and more temperature-sensitive than other water-only channels (excluding glycerol channels that also conduct water when not inhibited by glycerol).