THE ROLE OF TASTE RECEPTORS IN MALE AND FEMALE FERTILITY

Fertility is decreasing worldwide and male infertility factors contribute to approximately 30% of all infertility cases. The causes of this condition are often unknown, and about 40% of infertile men are oligozoospermic or azoospermic despite normal reproductive hormonal profile, presenting a condition of idiopathic infertility. Several studies have demonstrated the expression of taste receptors and their signaling transduction cascade in the male reproductive system, highlighting the potential role of response to chemical stimuli of taste receptors and their possible involvement in sperm maturation as well as in sperm behavior and fertilization. Considering the strong genetic component in male infertility, and the evidence that polymorphic variant in the gene are often functional, this thesis investigates the possible role of taste receptors in spermatogenesis, as well as the functional effect of selected taste receptor polymorphisms in male fertility. We hypothesized that polymorphisms in taste receptors genes might influence sperm functionality, in term of sperm parameters, as well as sperm behavior and fertilization. To this end we conducted a study to identify possible novel markers of susceptibility in human infertility within the taste receptor clusters. We selected 19 SNPs in 12 taste related genes that have been reported to be expressed in human and or mice testis or sperm and analyzed a possible correlation with different types and degrees of male infertility. For this purpose we enrolled 494 male patients, undergoing spermiogram at the Centre of Couple Sterility, Siena University Hospital. All patients were characterized for main sperm parameters, according to WHO (2010) guidelines, and were genotyped for 19 SNPs in taste receptors genes, using the KASPar SNP genotyping system. To assess the possible functional associations between SNPs and sperm parameters, different bioinformatic tools were used. Among the 19 SNPs investigated in this study for their potential association with male infertility, we found that TAS2R14-rs3741843, TAS1R2-rs4920566 and TAS2R3-rs11763979 showed the strongest significant association with specific sperm parameters. In particular, the homozygous carriers of the minor (G) allele of the TAS2R14-rs3741843 SNP showed a decreased sperm progressive motility compared to common (A) homozygous (coefficient= -0.46; P=0.003). Moreover, GTEx we showed that TAS2R14-rs3741843 has a plethora of expression quantitative trait loci (eQTL) in various tissues, and in the testicular tissue the SNP has one of the biggest effect size, suggesting its importance in this tissue. In addition, we analyzed, in human ejaculated sperm, the gene expression profile of the selected taste receptors, as well as of several genes involved in the signal transduction cascade elicited by these receptors. We partially confirmed the obtained data from the gene expression analysis at the protein level both in human sperm as well as in mouse sperm and testis (this study has been carried out during my training period at the Walther Straub Institute, Ludwig-Maximilians-University Munich). In particular, by using the Western blot and immunohistochemical techniques, we focused on the bitter receptor TAS2R4, TAS2R14, and the umami receptor TAS1R1 and the G proteins α-gustducin and α-transducin. Since we observed the expression of the detected proteins in the male reproductive system, we carried out the same analysis in the female somatic follicular cells, namely cumulus and granulosa cells, that play an important role in the oocyte competence acquiring and fertilization process . Immunofluorescence assay revealed a cytoplasmic localization in cumulus cells and granulose cells, in particular around the nucleus, probably in the transition region between rough endoplasmic reticulum and Golgi apparatus, where the protein concentration inside vesicles prelude to an active transport/release from these cells. In conclusion, our results suggest a role of genetic variability of taste receptors in human male infertility, as indicated by the significant correlation with the main sperm, and the expression of several components of taste receptor transduction cascade in male and female gametes