Aquaporins in spermatozoa and testicular germ cells: identification and potential role

Abstract

Mammalian spermatozoa have relatively high water permeability and swell readily, as in the hypo-osmotic swelling test used in the andrology clinic. Physiologically, spermatozoa experience changes in the osmolality of the surrounding fluids in both the male and the female tracts on their journey from the testis to the ovum. Sperm volume regulation in response to such osmotic challenges is important to maintain a stable cell size for the normal shape and function of the sperm tail. Alongside ion channels for the fluxes of osmolytes, water channels would be crucial for sperm volume regulation. In contrast to the deep knowledge and numerous studies on somatic cell aquaporins (AQPs), the understanding of sperm AQPs is limited. Among the 13 AQPs, convincing evidence for their presence in spermatozoa has been confined to AQP7, AQP8 and AQP11. Overall, current findings indicate a major role of AQP8 in water influx and efflux for sperm volume regulation, which is required for natural fertilization. The preliminary data suggestive of a role for AQP7 in sperm glycerol metabolism needs further substantiation. The association of AQP11 with the residual cytoplasm of elongated spermatids and the distal tail of spermatozoa supports the hypothesis of more than just a role in conferring water permeability and also in the turnover and recycling of surplus cellular components made redundant during spermiogenesis and spermiation. This would be crucial for the maintenance of a germinal epithelium functioning efficiently in the production of spermatozoa.

Figure 1

Localization of AQP7 (A, E), AQP8 (B, F) and AQP11 (C, D, G) in germ cells and spermatozoa. (A): Human testis with AQP7 absent from spermatogonia (1) and spermatocytes (2), but present in round spermatids (3). (B): Human testis with AQP8 localized on the plasma membrane of all germ cells (1–3) and elongated spermatids (4), and possibly cytoplasm of Sertoli cells. (C), (D): Rat testis with AQP11 localized in caudal cytoplasm of elongated spermatids (arrow, D) later in the proximal residual cytoplasm (arrow, C) and distal tail of testicular spermatozoa (arrow head, C). (E): AQP7 along the whole tail of human ejaculated spermatozoa except the end piece (arrow heads). (F): AQP8 in cytoplasmic droplets (arrow heads) and tail of human ejaculated spermatozoa. (G): AQP11 in end piece (arrow head) of rat epididymal spermatozoa. Bars = 10 μm; A–D have the same magnification. Corresponding negative controls can be seen in previous reports ^{, ,} .