This study confirms the validity of the Eyes test. Both internal consistency and test-retest stability were good for the Italian version of the Eyes test.
Taste sensitivity to PROP varies greatly among individuals and is associated with polymorphisms in the bitter receptor gene TAS2R38, and with differences in fungiform papilla density on the anterior tongue surface. Recently we showed that the PROP non-taster phenotype is strongly associated with the G variant of polymorphism rs2274333 (A/G) of the gene that controls the salivary trophic factor, gustin. The aims of this study were 1) to investigate the role of gustin gene polymorphism rs2274333 (A/G), in PROP sensitivity and fungiform papilla density and morphology, and 2) to investigate the effect of this gustin gene polymorphism on cell proliferation and metabolic activity. Sixty-four subjects were genotyped for both genes by PCR techniques, their PROP sensitivity was assessed by scaling and threshold methods, and their fungiform papilla density, diameter and morphology were determined. In vitro experiments examined cell proliferation and metabolic activity, following treatment with saliva of individuals with and without the gustin gene mutation, and with isolated protein, in the two iso-forms. Gustin and TAS2R38 genotypes were associated with PROP threshold (p=0.0001 and p=0.0042), but bitterness intensity was mostly determined by TAS2R38 genotypes (p<0.000001). Fungiform papillae densities were associated with both genotypes (p<0.014) (with a stronger effect for gustin; p=0.0006), but papilla morphology was a function of gustin alone (p<0.0012). Treatment of isolated cells with saliva from individuals with the AA form of gustin or direct application of the active iso-form of gustin protein increased cell proliferation and metabolic activity (p<0.0135). These novel findings suggest that the rs2274333 polymorphism of the gustin gene affects PROP sensitivity by acting on fungiform papilla development and maintenance, and could provide the first mechanistic explanation for why PROP super-tasters are more responsive to a broad range of oral stimuli.
Taste sensitivity to the bitter compound 6-n-propylthiouracil (PROP) is considered a marker for individual differences in taste perception that may influence food preferences and eating behavior, and thereby energy metabolism. This review describes genetic factors that may contribute to PROP sensitivity including: (1) the variants of the TAS2R38 bitter receptor with their different affinities for the stimulus; (2) the gene that controls the gustin protein that acts as a salivary trophic factor for fungiform taste papillae; and (3) other specific salivary proteins that could be involved in facilitating the binding of the PROP molecule with its receptor. In addition, we speculate on the influence of taste sensitivity on energy metabolism, possibly via modulation of the endocannabinoid system, and its possible role in regulating body composition homeostasis.
Thiourea tasting can be predictive of individual differences in bitter taste responses, general food preferences and eating behavior, and could be correlated with saliva chemical composition. We investigated the possible relationship between PROP bitter taste responsiveness and the salivary proteome in subjects genotyped for TAS2R38 and gustin gene polymorphisms. Taste perception intensity evoked by PROP and NaCl solutions was measured in sixty-three volunteers (21 males, 42 females, age 25±3 y) to establish their PROP taster status, and 24 PROP super-tasters and 21 nontasters were selected to participate in the study. TAS2R38 and gustin gene molecular analysis were performed using PCR techniques. Qualitative and quantitative determination of salivary proteins was performed by HPLC-ESI-MS before and after PROP taste stimulation. PROP super-tastings was strongly associated with the ‘taster’ variant (PAV haplotype) of TAS2R38 and the A allele of rs2274333 polymorphism in the gustin gene and nontasting was associated with the minor alleles at both loci. ANOVA revealed that basal levels of II-2 and Ps-1 proteins, belonging to the basic proline-rich protein (bPRPs) family, were significantly higher in PROP super-taster than in nontaster un-stimulated saliva, and that PROP stimulation elicited a rapid increase in the levels of these same proteins only in PROP super-taster saliva. These data show for the first time that responsiveness to PROP is associated with salivary levels of II-2 peptide and Ps-1 protein, which are products of the PRB1 gene. These findings suggest that PRB1, in addition to TAS2R38 and gustin, could contribute to individual differences in thiourea sensitivity, and the expression of the PROP phenotype as a complex genetic trait.
Orosensory perception of dietary fat varies in individuals, thus influencing nutritional status. Several studies associated fat detection and preference with CD36 or 6-n-propylthiouracil (PROP) sensitivity. Other studies have not confirmed the latter association. We analyzed the relationship between orosensory perception of oleic acid, two CD36 variants, and PROP tasting. Thresholds of oleic acid perception were assessed in 64 subjects using a modification of the three-alternative forced-choice procedure. Subjects were classified for PROP taster status and genotyped for TAS2R38 and CD36 (SNPs: rs1761667 and rs1527483). Subjects homozygous for GG of the rs1761667 polymorphism showed higher sensitivity to oleic acid than AA subjects. The capability to detect oleic acid was directly associated with TAS2R38 or PROP responsiveness. PROP non-tasters had a lower papilla density than tasters, and those with genotype GG of the rs1761667 polymorphism had lower oleic acid thresholds than PROP non-tasters with genotype AA. In conclusion, results showed a direct association between orosensory perception of oleic acid and PROP tasting or rs1761667 polymorphism of CD36, which play a significant role in PROP non-tasters, given their low number of taste papillae. Characterization of individual capability to detect fatty acids may have important nutritional implications by explaining variations in human fat preferences.
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