Expression of Taste Receptor 2 Subtypes in Human Testis and Sperm

Taste receptors (TASRs) are expressed not only in the oral cavity but also throughout the body, thus suggesting that they may play different roles in organ systems beyond the tongue. Recent studies showed the expression of several TASRs in mammalian testis and sperm, indicating an involvement of these receptors in male gametogenesis and fertility. This notion is supported by an impaired reproductive phenotype of mouse carrying targeted deletion of taste receptor genes, as well as by a significant correlation between human semen parameters and specific polymorphisms of taste receptor genes. To better understand the biological and thus clinical significance of these receptors for human reproduction, we analyzed the expression of several members of the TAS2Rs family of bitter receptors in human testis and in ejaculated sperm before and after in vitro selection and capacitation. Our results provide evidence for the expression of TAS2R genes, with TAS2R14 being the most expressed bitter receptor subtype in both testis tissue and sperm cells, respectively. In addition, it was observed that in vitro capacitation significantly affects both the expression and the subcellular localization of these receptors in isolated spermatozoa. Interestingly, α-gustducin and α-transducin, two Gα subunits expressed in taste buds on the tongue, are also expressed in human spermatozoa; moreover, a subcellular redistribution of both G protein α-subunits to different sub-compartments of sperm was registered upon in vitro capacitation. Finally, we shed light on the possible downstream transduction pathway initiated upon taste receptor activation in the male reproductive system. Performing ultrasensitive droplets digital PCR assays to quantify RNA copy numbers of a distinct gene, we found a significant correlation between the expression of TAS2Rs and TRPM5 (r = 0.87), the cation channel involved in bitter but also sweet and umami taste transduction in taste buds on the tongue. Even if further studies are needed to clarify the precise functional role of taste receptors for successful reproduction, the presented findings significantly extend our knowledge of the biological role of TAS2Rs for human male fertility.

Taste receptor polymorphisms and male infertility

STUDY QUESTION

Are polymorphisms of taste receptor genes associated with male infertility?

SUMMARY ANSWER

This study has showed the associations between three single nucleotide polymorphisms (SNPs) in taste receptors genes (TASR) and male infertility.

WHAT IS KNOWN ALREADY

Recent studies showed the expression of taste receptors in the testis and in spermatozoa, suggesting their possible role in infertility. The vast genetic variability in taste genes results in a large degree of diversity in various human phenotypes.

STUDY DESIGN, SIZE, DURATION

In this study, we genotyped 19 SNPs in 12 taste related genes in a total of 494 Caucasian male patients undergoing semen evaluation at the Centre of Couple Sterility of the Siena University Hospital. Consecutive patients were enrolled during infertility investigations from October 2014 to February 2016.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Median age of the patients was 36 years (18-58) and 141 were smokers. Genotyping was performed using the allele-specific PCR. The statistical analysis was carried out using generalized linear model (GLM) to explore the association between age, smoking, the genetic polymorphisms and sperm parameters.

MAIN RESULTS AND THE ROLE OF CHANCE

We observed that the homozygous carriers of the (G) allele of the TAS2R14-rs3741843 polymorphism showed a decreased sperm progressive motility compared to heterozygotes and (A) homozygotes (P = 0.003). Moreover, the homozygous carriers of the (T) allele of the TAS2R3-rs11763979 SNP showed fewer normal acrosome compared with the heterozygous and the homozygous carriers of the (G) allele (P = 0.002). Multiple comparisons correction was applied and the Bonferroni-corrected critical P-value was = 0.003.

LIMITATIONS, REASONS FOR CAUTION

The analysis is restricted to SNPs within genes and to men of Caucasian ancestry.

WIDER IMPLICATIONS OF THE FINDINGS

In silico analyses strongly point towards a functional effect of the two SNPs: TAS2R14-rs3741843 regulates TAS2R43 expression, a gene that is involved in cilia motility and therefore could influences sperm mobility; the (T) allele of TAS2R3-rs11763979 increases the expression of the WEE2 antisense RNA one gene (WEE2-AS1). According to Genotype-Tissue Expression (GTEx) project the WEE2 gene is expressed in the testes where presumably it has the role of down regulating meiotic cell division. It is plausible to hypothesize that the WEE2-AS1 increased expression may down regulate WEE2 which in turn can alter the natural timing of sperm maturation increasing the number of abnormal sperm cells.

STUDY FUNDING/COMPETING INTEREST(S)

None.

Suppression of galactocerebrosidase premature termination codon and rescue of galactocerebrosidase activity in twitcher cells

Krabbe's disease (KD) is a degenerative lysosomal storage disease resulting from deficiency of β-galactocerebrosidase activity. Over 100 mutations are known to cause the disease, and these usually occur in compound heterozygote patterns. In affected patients, nonsense mutations leading to a nonfunctional enzyme are often found associated with other mutations. The twitcher mouse is a naturally occurring model of KD, containing in β-galactocerebrosidase a premature stop codon, W339X. Recent studies have shown that selected compounds may induce the ribosomal bypass of premature stop codons without affecting the normal termination codons. The rescue of β-galactocerebrosidase activity induced by treatment with premature termination codon (PTC) 124, a well-characterized compound known to induce ribosomal read-through, was investigated on oligodendrocytes prepared from twitcher mice and on human fibroblasts from patients bearing nonsense mutations. The effectiveness of the nonsense-mediated mRNA decay (NMD) inhibitor 1 (NMDI1), a newly identified inhibitor of NMD, was also tested. Incubation of these cell lines with PTC124 and NMDI1 increased the levels of mRNA and rescued galactocerebrosidase enzymatic activity in a dose-dependent manner. The low but sustained expression of β-galactocerebrosidase in oligodendrocytes was sufficient to improve the morphology of the differentiated cells. Our in vitro approach provides the basis for further investigation of ribosomal read-through as an alternative therapeutic strategy to ameliorate the quality of life in selected KD patients. © 2016 Wiley Periodicals, Inc.

Impaired spermatogenesis in the twitcher mouse: A morphological evaluation from the seminiferous tubules to epididymal transit

Spermatogenesis is a complex process of proliferation and differentiation during male germ cell development whereby undifferentiated spermatogonial germ cells evolve into maturing spermatozoa. In this developmental process the interactions between different cell types are finely regulated, hence any disruption in these relationships leads to male infertility. The twitcher mouse, the murine model of Krabbe disease, is characterized by deficiency of galactosylceramidase, an enzyme also involved in the metabolism of the galactosyl-alkyl-acyl-glycerol, the precursor of sulfogalactosyl-alkyl-acyl-glycerol, the most abundant glycolipid in spermatozoa. Twitcher mice are sterile due to alterations of spermatogenesis resulting in the production of spermatozoa with abnormally swollen acrosomes and bent flagella, mainly at the midpiece-principal piece junction. The current study employs light, fluorescence, and electron microscopy to examine the defective spermiogenesis leading to the morphological abnormalities of mature sperm. This study reveals that alterations in germ cell development can be initially detected at the stage VIII and IX of spermatogenesis. The disrupted spermatogenetic process leads to a reduced number of elongating spermatids and spermatozoa in these mutant animals. Electron microscopy analysis demonstrates major acrosomal and chromatin condensation defects in the mutants. In addition, in twitcher mice, the epididymal architecture is impaired, with stereocilia of caput and corpus broken, detached and completely spread out into the lumen. These findings indicate that seminolipid expression is crucial for proper development of spermatocytes and spermatids and for their normal differentiation into mature spermatozoa.

ABBREVIATIONS

GALC: galactosylceramidase; GalAAG: galactosyl-alkyl-acyl-glycerol; SGalAAG: sulfogalactosylalkylacylglycerol; PND: postnatal day; PAS: periodic acid-Schiff stain; TEM: transmission electron microscopy; SEM: scanning electron microscopy; PFA: paraformaldheyde.

Single nucleotide polymorphisms of USP26 in azoospermic men

Some studies have focused on the association between male infertility and single nucleotide polymorphisms (SNPs) in the ubiquitin-specific protease 26 (USP26) gene, but the results are controversial. In this case-control study including both normozoospermic men and patients with nonobstructive azoospermia, we analyzed both the entire coding region and 5' and 3' untranslated regions of USP26 in order to identify genetic variants in this gene to investigate the role of USP26 on spermatogenesis. We reported variations in the USP26 gene sequence in 82% of azoospermic and in 50% normospermic men. The synonymous variation c.576G>A has a frequency significantly different in the azoospermic (60.2%) and normozoospermic (23.6%) groups, while the frequencies in the two groups of both c.1090C>T and c.1737G>A missense mutations did not reach statistical significance. A cluster mutation (c.371insACA, c.494T>C) was detected in 2 normozoospermic men (2.7%). In the 5'UTR we identified the -33C>T variation both in azoospermic (3.8%) and in normozoospermic (2.7%) men. In a normozoospermic man we detected the nonsense mutation c.882C>A, never reported to date. According to our results, we suggest that only the variation c.576G>A has a frequency significantly different in azoospermic compared to normozoospermic men. Moreover, the identification in a normozoospermic man of a nonsense mutation (c.882C>A) which causes the production of a truncated protein, suggests a marginal role of USP26 in male spermatogenesis. Additional studies may be useful as we cannot exclude that the other SNPs may represent risk factors for male fertility acting by an oligogenic/polygenic mechanism.