Aquaporins in sperm osmoadaptation: an emerging role for volume regulation

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Acta Pharmacologica Sinica (2011) 32: 721724 2011 CPS and SIMM All rights reserved 1671-4083/11 $32.00 www.nature.com/aps

Review

Aquaporins in sperm osmoadaptation: an emerging role for volume regulation

Qi CHEN1, 2, En-kui DUAN1, *

1State Key Laboratory of Reproductive Biology, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, China; 2Graduate University of Chinese Academy of Sciences, Beijing 100049, China

Upon ejaculation, mammalian sperm experience a natural osmotic decrease during male to female reproductive tract transition. This hypo-osmotic exposure not only activates sperm motility, but also poses potential harm to sperm structure and function by inducing unwanted cell swelling. In this physiological context, regulatory volume decrease (RVD) is the major mechanism that protects cells from detrimental swelling, and is essential to sperm survival and normal function. Aquaporins are selective water channels that enable rapid water transport across cell membranes. Aquaporins have been implicated in sperm osmoregulation. Recent discoveries show that Aquaporin-3 (AQP3), a water channel protein, is localized in sperm tail membranes and that AQP3 mutant sperm show defects in volume regulation and excessive cell swelling upon physiological hypotonic stress in the female reproductive tract, thereby highlighting the importance of AQP3 in the postcopulatory sperm RVD process. In this paper, we discuss current knowledge, remaining questions and hypotheses about the function and mechanismic basis of aquaporins for volume regulation in sperm and other cell types.

Keywords: aquaporins; sperm; water permeability; regulatory volume decrease; osmosensing/mechanosensing; tetrameric structure

Acta Pharmacologica Sinica (2011) 32: 721724; doi: 10.1038/aps.2011.35; published online 9 May 2011

Efficient sperm volume regulation is a prerequisite for normal sperm functionIn most mammalian species studied, the journey of sperm from the male to the female reproductive tract experience a natural osmotic decrease[1], an evolutionary vestige from freshwater sh species[2]. 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]. Mammalian sperm have evolved to effectively reduce the negative impact of hypotonic...