Evidence for the involvement of aquaporins in sperm motility activation of the teleost gilthead sea bream (Sparus aurata)

The expression of aquaporins in the spermatozoa of the marine teleost gilthead sea bream (S. aurata) and their involvement in the motility activation process was investigated. Sperm motility was activated by a hyper-osmotic shock but it was completely inhibited by 10 μM HgCl2, such inhibition being partially recovered by β-mercaptoehanol (ME). Conventional RT-PCR using primers specific for S. aurata aquaglyceroporin (glp) and aquaporin-1a (aqp1a) demonstrated the presence of both mRNAs in spermatozoa. Heterologous expression in Xenopus laevis oocytes showed that 10 and 100 μM HgCl2 equally inhibited water and solute transport through S. aurata aquaporin-1a and S. aurata aquaglyceroporin, but treatment with ME only recovered aquaporin-1a-mediated water permeability. Western blot using isoform-specific antisera on protein extracts from spermatozoa revealed bands that corresponded to the predicted molecular mass of S. aurata aquaglyceroporin (31 kDa) and S. aurata aquaporin-1a (28 kDa). The antisera also demonstrated that both aquaporins were localized in the head and flagellum of the spermatozoa. However, the immunoreaction at the plasma membrane of the spermatozoa head was more intense after the hyper-osmotic activation, suggesting the translocation of both aquaporin-1a and aquaglyceroporin into the plasma membrane following the osmotic shock. This study therefore provides the first direct demonstration for the presence of aquaporins in fish sperm. The different sensitivity of S. aurata aquaporin-1a and S. aurata aquaglyceroporin to ME may explain the failure of this reducing agent to fully recover the mercurial inhibition of sperm motility, suggesting that these aquaporins may play different physiological roles during the activation and maintenance of sperm motility in sea bream.