Pseudogenization of a Sweet-Receptor Gene Accounts for Cats' Indifference toward Sugar

Although domestic cats (Felis silvestris catus) possess an otherwise functional sense of taste, they, unlike most mammals, do not prefer and may be unable to detect the sweetness of sugars. One possible explanation for this behavior is that cats lack the sensory system to taste sugars and therefore are indifferent to them. Drawing on work in mice, demonstrating that alleles of sweet-receptor genes predict low sugar intake, we examined the possibility that genes involved in the initial transduction of sweet perception might account for the indifference to sweet-tasting foods by cats. We characterized the sweet-receptor genes of domestic cats as well as those of other members of the Felidae family of obligate carnivores, tiger and cheetah. Because the mammalian sweet-taste receptor is formed by the dimerization of two proteins (T1R2 and T1R3; gene symbols Tas1r2 and Tas1r3), we identified and sequenced both genes in the cat by screening a feline genomic BAC library and by performing PCR with degenerate primers on cat genomic DNA. Gene expression was assessed by RT-PCR of taste tissue, in situ hybridization, and immunohistochemistry. The cat Tas1r3 gene shows high sequence similarity with functional Tas1r3 genes of other species. Message from Tas1r3 was detected by RT-PCR of taste tissue. In situ hybridization and immunohistochemical studies demonstrate that Tas1r3 is expressed, as expected, in taste buds. However, the cat Tas1r2 gene shows a 247-base pair microdeletion in exon 3 and stop codons in exons 4 and 6. There was no evidence of detectable mRNA from cat Tas1r2 by RT-PCR or in situ hybridization, and no evidence of protein expression by immunohistochemistry. Tas1r2 in tiger and cheetah and in six healthy adult domestic cats all show the similar deletion and stop codons. We conclude that cat Tas1r3 is an apparently functional and expressed receptor but that cat Tas1r2 is an unexpressed pseudogene. A functional sweet-taste receptor heteromer cannot form, and thus the cat lacks the receptor likely necessary for detection of sweet stimuli. This molecular change was very likely an important event in the evolution of the cat's carnivorous behavior

Polymorphisms in the Taste Receptor Gene (Tas1r3) Region are Associated with Saccharin Preference in 30 Mouse Strains.

The results of recent studies suggest that the mouse Sac (saccharin preference) locus is identical to the Tas1r3 (taste receptor) gene. The goal of this study was to identify Tas1r3 sequence variants associated with saccharin preference in a large number of inbred mouse strains. Initially, we sequenced approximately 6.7 kb of the Tas1r3 gene and its flanking regions from six inbred mouse strains with high and low saccharin preference, including the strains in which the Sac alleles were described originally (C57BL/6J, Sac(b); DBA/2J, Sac(d)). Of the 89 sequence variants detected among these six strains, eight polymorphic sites were significantly associated with preferences for 1.6 mm saccharin. Next, each of these eight variant sites were genotyped in 24 additional mouse strains. Analysis of the genotype-phenotype associations in all 30 strains showed the strongest association with saccharin preference at three sites: nucleotide (nt) -791 (3 bp insertion/deletion), nt +135 (Ser45Ser), and nt +179 (Ile60Thr). We measured Tas1r3 gene expression, transcript size, and T1R3 immunoreactivity in the taste tissue of two inbred mouse strains with different Tas1r3 haplotypes and saccharin preferences. The results of these experiments suggest that the polymorphisms associated with saccharin preference do not act by blocking gene expression, changing alternative splicing, or interfering with protein translation in taste tissue. The amino acid substitution (Ile60Thr) may influence the ability of the protein to form dimers or bind sweeteners. Here, we present data for future studies directed to experimentally confirm the function of these polymorphisms and highlight some of the difficulties of identifying specific DNA sequence variants that underlie quantitative trait loci

Behavioral genetics and taste

This review focuses on behavioral genetic studies of sweet, umami, bitter and salt taste responses in mammals. Studies involving mouse inbred strain comparisons and genetic analyses, and their impact on elucidation of taste receptors and transduction mechanisms are discussed. Finally, the effect of genetic variation in taste responsiveness on complex traits such as drug intake is considered. Recent advances in development of genomic resources make behavioral genetics a powerful approach for understanding mechanisms of taste

Protein Hydrolysates Are Avoided by Herbivores but Not by Omnivores in Two-Choice Preference Tests

Background: The negative sensory properties of casein hydrolysates (HC) often limit their usage in products intended for human consumption, despite HC being nutritious and having many functional benefits. Recent, but taxonomically limited, evidence suggests that other animals also avoid consuming HC when alternatives exist. Methodology/Principal Findings: We evaluated ingestive responses of five herbivorous species (guinea pig, mountain beaver, gopher, vole, and rabbit) and five omnivorous species (rat, coyote, house mouse, white-footed mouse, and deer mouse; N = 16–18/species) using solid foods containing 20% HC in a series of two-choice preference tests that used a nonprotein, cellulose-based alternative. Individuals were also tested with collagen hydrolysate (gelatin; GE) to determine whether it would induce similar ingestive responses to those induced by HC. Despite HC and GE having very different nutritional and sensory qualities, both hydrolysates produced similar preference score patterns. We found that the herbivores generally avoided the hydrolysates while the omnivores consumed them at similar levels to the cellulose diet or, more rarely, preferred them (HC by the white-footed mouse; GE by the rat). Follow-up preference tests pairing HC and the nutritionally equivalent intact casein (C) were performed on the three mouse species and the guinea pigs. For the mice, mean HC preference scores were lower in the HC v C compared to the HC v Cel tests, indicating that HC’s sensory qualities negatively affected its consumption. However, responses were species-specific. For the guinea pigs, repeated exposure to HC or C (4.7-h sessions; N = 10) were found to increase subsequent HC preference scores in an HC v C preference test, which was interpreted in the light of conservative foraging strategies thought to typify herbivores. Conclusions/Significance: This is the first empirical study of dietary niche-related taxonomic differences in ingestive responses to protein hydrolysates using multiple species under comparable conditions. Our results provide a basis for future work in sensory, physiological, and behavioral mechanisms of hydrolysate avoidance and on the potential use of hydrolysates for pest management

Avoidance of hydrolyzed casein by mice

When casein, a milk protein, is hydrolyzed, it renders human foods that contain it (e.g., hypoallergenic infant formula, cheeses) distasteful to many people. This rejection of hydrolyzed casein (HC)-containing foods has recently been found to also occur in a non-human species (deer, Odocoileus spp.). Identifying other animals that avoid HC would facilitate understanding how and why HC-containing food is often rejected. This study determined whether HC-containing food is avoided by Mus musculus and whether consumption patterns were sensitive to testing conditions, specifically food form (powder, pellet or dough) and food access (ad libitum or 1.5 h/day following 6 h of food deprivation). Diets were offered in two-choice tests that paired an HC-containing food with an intact casein-containing alternative at seven protein concentrations (0%–50% w/w). Five experimental groups were tested under different combinations of food form and food access. Three groups (ad lib/powder, ad lib/pellet, and 1.5 h/pellet) avoided the HC diet starting at the 30% protein level. At the 40% and 50% protein levels, all groups showed strong avoidance of HC. Although testing conditions influenced total caloric intake and body weight gain, avoidance of HC at the highest concentrations was robust to the manipulations in experimental conditions. Our study suggests that mice may be a useful model for understanding the mechanisms of HC rejection

Blood glucose concentration during the intragastric glucose tolerance test (glucose 2 g/kg) in nonfasted <i>Tas1r3+/+</i> and <i>Tas1r3-/-</i> mice.

<p>A) Relationship between glucose AUC and age. Pearson’s coefficient of correlation was calculated. n(<i>Tas1r3+/+</i>) = 23, n(<i>Tas1r3</i>-/-) = 26. B, C) Blood glucose concentration (left) and glucose AUC (right) in 9- to 21-week-old (B) and 22- to 34-week-old (C) mice. B) n(<i>Tas1r3+/+</i>) = 13, n(<i>Tas1r3</i>-/-) = 15; C) n(<i>Tas1r3+/+</i>) = 10; n(<i>Tas1r3</i>-/-) = 11. Post hoc comparisons with Fisher LSD test (<i>Tas1r3</i> +/+ vs. <i>Tas1r3</i>-/-): *—p<0.05, ***—p<0.001.</p

Blood glucose concentration during the intraperitoneal glucose tolerance test (glucose 2 g/kg) in nonfasted <i>Tas1r3+/+</i> and <i>Tas1r3-/-</i> mice.

<p>A) Relationship between glucose AUC and age. Pearson’s coefficient of correlation was calculated; n(<i>Tas1r3</i>+/+) = 29, n(<i>Tas1r3</i>-/-) = 30. B, C) Blood glucose concentration (left) and glucose AUC (right) in 9- to 21-week-old (B) and 22- to 34-week-old (C) mice. B) n(<i>Tas1r3</i>+/+) = 19, n(<i>Tas1r3</i>-/-) = 18; C) n(<i>Tas1r3</i>+/+) = 10; n(<i>Tas1r3</i>-/-) = 12. Post hoc comparisons with Fisher LSD test (<i>Tas1r3</i> +/+ vs. <i>Tas1r3</i>-/-): *—p<0.05, **—p<0.01, ***—p<0.001.</p

Insulin tolerance test in nonfasted Tas1r3+/+ and Tas1r3-/- mice.

<p>A) Relationship between glucose AUC and age. Pearson’s correlation coefficients (r) were calculated. B) Absolute values of blood glucose concentration. C) Percentage relative to baseline level. Insulin (2 U/kg, IP) was injected at zero time point; n(Tas1r3+/+) = 18, n(Tas1r3-/-) = 20. Post hoc comparisons with Fisher LSD test (<i>Tas1r3 +/+</i> vs. <i>Tas1r3-/-</i>): *—p<0.05</p

Taste responses to sucrose solutions in naïve <i>Tas1r3</i> +/+ and <i>Tas1r3</i>-/- mice.

<p>A) Licking ratio (%) as a function of sucrose concentration in the brief-access licking test (mean±SEM); n(<i>Tas1r3</i>+/+) = 15, n(<i>Tas1r3</i>-/-) = 29. B) Sucrose preference scores (%) in the 48-h two bottle test; n(<i>Tas1r3</i>+/+) = 18, n(<i>Tas1r3</i>-/-) = 12. Post hoc comparisons with Fisher LSD test (<i>Tas1r3</i> +/+ vs. <i>Tas1r3</i>-/-): *—p<0.05, ***—p<0.001</p