Survival of Drowning Sperm: Do Spermatozoa from External Fertilizers Adapt to Differing Osmotic Environments Through the Use of Aquaporins?

In spawning animals, external fertilization usually requires spermatozoa to become activated (motile) in the environmental medium and then quickly locate and contact the egg. In most teleost fish, sperm are usually quiescent within the testes and seminal fluid but then initiate vigorous motility upon dilution into the external medium in which spawning occurs [1]. In teleost fish sperm, an increase in intracellular calcium occurs upon motility initiation by way of different signaling mechanisms (reviewed by [2]) [3–5]. Following dilution into medium, four different triggers of sperm motility in teleosts have been identified: 1) a reduction in external K þ upon dilution of semen initiates motility in salmonids [6, 7], 2) hypotonic exposure after dilution into freshwater is the trigger in nonsalmonid freshwater fishes [8–11], 3) hypertonic exposure initiates sperm motility in marine teleosts [12, 13], and 4) egg-associated molecules activate sperm motility [14– 17]. While there are many examples in the literature of specific ions and ion channels playing roles in sperm motility initiation and maintenance, it is known that the osmotic pressure of the medium alone can affect both motility initiation (reviewed by [5]) [18] and survival of sperm following dilution. In some of these studies, osmotic changes may stimulate stretch-activated channels that trigger motility initiation [19]. Sperm volume changes are known to occur and are associated with the osmotic change upon dilution in hypotonic medium [20]. These volume changes may, by themselves, activate stretchactivated channels and/or water channel proteins, termed aquaporins, which play a role in the activation of sperm motility in seabream spermatozoa [21]. In this case, the dilution of seabream spermatozoa into seawater results in a hyperosmotic change, and the water efflux through aquaporins that results creates an increase in intracellular ions; this is believed to be responsible for activation of adenylyl cyclase and the cAMP-signaling pathway causing phosphorylation of flagellar proteins [22]. The proposed model for aquaporin-mediated motility initiation is shown in Figure 1 [21]. A new study in this issue of Biology of Reproduction by Chauvigne et al. [23] has now shown that multiple aquaporins are associated with water and fluid homeostasis during spermatogenesis and in the male reproductive tract, as well as in mature motile spermatozoa. This study demonstrates that the water-specific aquaporin Aqp1aa and the glycerol/urea/ water-transporting channel Aqp10b are both present (with others) in the plasma membrane of both the head and flagellum of ejaculated sperm. Inhibitors of aquaporin channels are shown to inhibit motility, indicating a direct role for aquaporins in sperm motility initiation in this marine teleost. The Chauvigne et al. study [23] isolated and functionally characterized five additional members of the aquaporin superfamily using a comprehensive approach of immunolocalization and biochemical fractionation of sperm. They have shown, for the first time, that a nonmammalian vertebrate possesses up to seven classes of aquaporins that are differentially expressed during spermatogenesis and spermiogenesis, with several of these being redistributed in the plasma membrane upon motility initiation. During hyperosmotic shock upon dilution in seawater, Aqp1aa may mediate water efflux, whereas Aqp10b might play a role in maintenance of motility [21, 23]. The data in this new study clearly show the conservation of aquaporin paralogs in the male reproductive tract of a nonmammalian vertebrate and highlight the similarities in the pattern and localization of aquaporins both in the testes and in mature sperm, as has been described in mammals [24, 25, 26]. One of the most intriguing findings of the current Chauvigne et al. study [23] is the change in intensity of immunolabeling of Aqp1ab as motility is initiated, with the head becoming more intense and the midpiece becoming strongly labeled. Aqp8b did not change intensity or labeling pattern in the head after activation, but did relocate to the midpiece. Aqp10b also showed a strong increase in intensity in the head upon motility activation. These changes in intensity and spatial patterns of sperm plasma membrane aquaporins during motility initiation in a hypertonic medium strongly suggest that they are regulating water/fluid efflux and have a direct role in motility initiation and maintenance. Such a direct role for aquaporins in motility regulation in external spawning species needs serious consideration in other species and across phyla. Species that reproduce in environments of varying salinities [3, 27] would be of particular interest to study, as one might hypothesize that at higher salinities aquaporins may play a direct role in motility initiation; however, at lower salinities (iososmotic or hypoosmotic), other mechanisms may be responsible. Now that we know the potential importance of aquaporins in sperm motility initiation in a marine teleost from this study, the functionality and redistribution of these waterregulating permeability membrane proteins would be of great Correspondence: Gary N. Cherr, University of California Davis Bodega Marine Laboratory, P.O. Box 247, Bodega Bay, CA 94923. E-mail: gncherr@ucdavis.edu