EVIDENCE FOR THE INVOLVEMENT OF AQUAPORINS IN SPERM MOTILITY ACTIVATION OF THE TELEOST GILTHEAD SEA BREAM (SPARUS AURATA)1

The expression of aquaporins in the spermatozoa of the marine
teleost gilthead sea bream (Sparus aurata) and their involvement
in the motility activation process were investigated. Sperm
motility was activated by a hyperosmotic shock, but it was
completely inhibited by 10 lM HgCl2, such inhibition being
partially recovered by beta-mercaptoethanol (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 lM 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 analysis 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 hyperosmotic activation, suggesting the
translocation of both aquaporin 1a and aquaglyceroporin into
the plasma membrane after the osmotic shock. This study
therefore provides the first direct demonstration for the presence
of aquaporins in fish sperm. The different sensitivities 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.