Aquaporin biology during spermatogenesis and sperm physiology in the marine teleost gilthead seabream (Sparus aurata).Biología de las acuaporinas durante la espermatogénesis y la fisiología espermática en el teleósteo marino (Sparus aurata)

Résumé

Water transport during gametogenesis is an essential physiological process associated with the successful reproduction of vertebrates. In the mammalian male reproductive tract, several studies have highlighted the key role that aquaporins could play during the differentiation of spermatozoa in the germinal epithelium, in the concentration and maturation of sperm in the testicular ducts, and in the regulation of osmotically induced volume changes in ejaculated spermatozoa. In teleosts, several studies have reported the expression of aquaporin mRNAs in the testis, but their role is completely unknown. In the marine teleost gilthead seabream (Sparus aurata), aquaporin-1aa and -10b (Aqp1aa and -10b) have been localized in ejaculated spermatozoa, and it has been hypothesized that Aqp1aa possibly mediates the water efflux during the hyperosmotic shock in seawater, which is required for the activation of sperm motility. Using a combination of molecular and biochemical approaches, the main objective of this thesis was to identify the aquaporin paralogs present in the testis and spermatozoa of the gilthead seabream, and investigate the potential physiological role of these channels during sperm motility. The following objectives have been addressed: 1. To isolate and functionally characterize the repertoire of the aquaporins present in testis, and to identify the cell types, including spermatozoa, in which they are expressed. 2. To study the in vivo developmental expression of testicular aquaporins, and to examine the in vitro regulatory effects of recombinant piscine gonadotropins, follicle-stimulating (rFsh) and luteinizing (rLh) hormones, and sex steroids, on aquaporin expression during the spermatogenic cycle. 3. To investigate the potential physiological role of sperm aquaporins controlling volume changes and/or solute transport occurring during sperm motility activation. The present thesis reports for the first time the presence of a complex aquaporin network in the testis of the marine teleost gilthead seabream and uncovers previously unknown mechanisms in sperm physiology and motility. The results indicate that the expression of the different aquaporin paralogs in testicular somatic and germ cells during spermatogenesis is differentially regulated by the pituitary gonadotropins, Fsh and Lh, through androgen-dependent and independent mechanisms, providing the first model of aquaporin regulation during the spermatogenesis of a non mammalian vertebrate as a step forward towards the elucidation of the physiological roles of these important channels in the reproductive systems. The findings also provide evidence for the specialized and coordinated role of aquaporins during sperm motility initiation and maintenance in a marine teleost. The data show that Aqp1aa facilitates the intracellular Ca2+ wave in spermatozoa during the hypertonic shock, which is needed for the initiation of flagellar movement and the trafficking of Aqp8b to the mitochondrion, where facilitates H2O2 efflux during ATP production.