Aquaporin3 is a sperm water channel essential for postcopulatory sperm osmoadaptation and migration

Abstract

In the journey from the male to female reproductive tract, mammalian sperm experience a natural osmotic decrease (e.g., in mouse, from ~415 mOsm in the cauda epididymis to ~310 mOsm in the uterine cavity). Sperm have evolved to utilize this hypotonic exposure for motility activation, meanwhile efficiently silence the negative impact of hypotonic cell swelling. Previous physiological and pharmacological studies have shown that ion channel-controlled water influx/efflux is actively involved in the process of sperm volume regulation; however, no specific sperm proteins have been found responsible for this rapid osmoadaptation. Here, we report that aquaporin3 (AQP3) is a sperm water channel in mice and humans. Aqp3-deficient sperm show normal motility activation in response to hypotonicity but display increased vulnerability to hypotonic cell swelling, characterized by increased tail bending after entering uterus. The sperm defect is a result of impaired sperm volume regulation and progressive cell swelling in response to physiological hypotonic stress during male-female reproductive tract transition. Time-lapse imaging revealed that the cell volume expansion begins at cytoplasmic droplet, forcing the tail to angulate and form a hairpin-like structure due to mechanical membrane stretch. The tail deformation hampered sperm migration into oviduct, resulting in impaired fertilization and reduced male fertility. These data suggest AQP3 as an essential membrane pathway for sperm regulatory volume decrease (RVD) that balances the "trade-off" between sperm motility and cell swelling upon physiological hypotonicity, thereby optimizing postcopulatory sperm behavior.

Flagellar ion channels of sperm: similarities and differences between species

“…Human spermatozoa, however, do not show the same I ATP which indicates that P2X2 ion channels might not be expressed in human sperm cells ( [153] and unpublished data from our lab). Calciumactivated chloride channels (anoctamins) have been recently found in human sperm [154], but not in mouse [30], as well as aquaporins, water channels that are required for sperm osmoadaptation [155]. Several members of the transient receptor potential (TRP) ion channel family [156][157][158] were thought to function in mammalian sperm cells.…”

Aquaporins in sperm osmoadaptation: an emerging role for volume regulation

“…However, genetic deletion of AQP7 and 8 in mice did not result in obvious abnormalities in sperm morphology and function [11,16,17] , Review www.nature.com/aps Chen Q et al Acta Pharmacologica Sinica npg suggesting that these AQPs are not essential for mouse sperm function or that they can be functionally substituted by other aquaporin members. After a previous study identified Aquaporin-3 (AQP3) expression in mouse testes [18] , we demonstrated that AQP3 was present in both mouse and human sperm and was located in the plasma membrane of the principle piece of fl agellum [3] ( Figure 1A). Further functional studies using AQP3 knockout mice have shown that upon exposure to physiological hypotonic stress, AQP3 mutant sperm show normal initial motility but display increased vulnerability to hypotonic cell swelling characterized by increased tail bending after entering the uterus [3] .…”

“…After a previous study identified Aquaporin-3 (AQP3) expression in mouse testes [18] , we demonstrated that AQP3 was present in both mouse and human sperm and was located in the plasma membrane of the principle piece of fl agellum [3] ( Figure 1A). Further functional studies using AQP3 knockout mice have shown that upon exposure to physiological hypotonic stress, AQP3 mutant sperm show normal initial motility but display increased vulnerability to hypotonic cell swelling characterized by increased tail bending after entering the uterus [3] . The observed sperm defect was due to impaired cell volume regulation and progressive cell swelling in response to physiological hypotonic stress, as revealed by sperm volume detection using flow cytometry in a population-based manner and by time-lapse imaging of individual sperm [3] .…”

“…Further functional studies using AQP3 knockout mice have shown that upon exposure to physiological hypotonic stress, AQP3 mutant sperm show normal initial motility but display increased vulnerability to hypotonic cell swelling characterized by increased tail bending after entering the uterus [3] . The observed sperm defect was due to impaired cell volume regulation and progressive cell swelling in response to physiological hypotonic stress, as revealed by sperm volume detection using flow cytometry in a population-based manner and by time-lapse imaging of individual sperm [3] . The tail deformation hampers normal sperm migration into the oviduct, resulting in impaired fertilization and reduced male fertility [3] .…”

“…Before ejaculation, the mammalian sperm are quiescent in the relatively hypertonic male reproductive tract with no or very low motility. Upon copulation, the sperm enter into the relatively hypotonic female reproductive tract and quickly show motility activation [3,4] , indicating that osmotic changes are beneficial for initial sperm motility activation. However, postcopulatory hypotonic stress also has a negative effects as it induces osmotic cell swelling, which if uncontrolled, can be detrimental to sperm function and survival [5] .…”