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

Impaired Glucose Metabolism in Mice Lacking the <i>Tas1r3</i> Taste Receptor Gene

<div><p>The G-protein-coupled sweet taste receptor dimer T1R2/T1R3 is expressed in taste bud cells in the oral cavity. In recent years, its involvement in membrane glucose sensing was discovered in endocrine cells regulating glucose homeostasis. We investigated importance of extraorally expressed T1R3 taste receptor protein in age-dependent control of blood glucose homeostasis <i>in vivo</i>, using nonfasted mice with a targeted mutation of the <i>Tas1r3</i> gene that encodes the T1R3 protein. Glucose and insulin tolerance tests, as well as behavioral tests measuring taste responses to sucrose solutions, were performed with C57BL/6ByJ (<i>Tas1r3</i>+/+) inbred mice bearing the wild-type allele and C57BL/6J-<i>Tas1r3^tm1Rfm</i> mice lacking the entire <i>Tas1r3</i> coding region and devoid of the T1R3 protein (<i>Tas1r3-/-</i>). Compared with <i>Tas1r3</i>+/+ mice, <i>Tas1r3-/-</i> mice lacked attraction to sucrose in brief-access licking tests, had diminished taste preferences for sucrose solutions in the two-bottle tests, and had reduced insulin sensitivity and tolerance to glucose administered intraperitoneally or intragastrically, which suggests that these effects are due to absence of T1R3. Impairment of glucose clearance in <i>Tas1r3-/-</i> mice was exacerbated with age after intraperitoneal but not intragastric administration of glucose, pointing to a compensatory role of extraoral T1R3-dependent mechanisms in offsetting age-dependent decline in regulation of glucose homeostasis. Incretin effects were similar in <i>Tas1r3</i>+/+ and <i>Tas1r3</i>-/- mice, which suggests that control of blood glucose clearance is associated with effects of extraoral T1R3 in tissues other than the gastrointestinal tract. Collectively, the obtained data demonstrate that the T1R3 receptor protein plays an important role in control of glucose homeostasis not only by regulating sugar intake but also via its extraoral function, probably in the pancreas and brain.</p></div

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

Defects in the Peripheral Taste Structure and Function in the MRL/lpr Mouse Model of Autoimmune Disease

<div><p>While our understanding of the molecular and cellular aspects of taste reception and signaling continues to improve, the aberrations in these processes that lead to taste dysfunction remain largely unexplored. Abnormalities in taste can develop in a variety of diseases, including infections and autoimmune disorders. In this study, we used a mouse model of autoimmune disease to investigate the underlying mechanisms of taste disorders. MRL/MpJ-Fas^lpr/J (MRL/lpr) mice develop a systemic autoimmunity with phenotypic similarities to human systemic lupus erythematosus and Sjögren's syndrome. Our results show that the taste tissues of MRL/lpr mice exhibit characteristics of inflammation, including infiltration of T lymphocytes and elevated levels of some inflammatory cytokines. Histological studies reveal that the taste buds of MRL/lpr mice are smaller than those of wild-type congenic control (MRL/+/+) mice. 5-Bromo-2′-deoxyuridine (BrdU) pulse-chase experiments show that fewer BrdU-labeled cells enter the taste buds of MRL/lpr mice, suggesting an inhibition of taste cell renewal. Real-time RT-PCR analyses show that mRNA levels of several type II taste cell markers are lower in MRL/lpr mice. Immunohistochemical analyses confirm a significant reduction in the number of gustducin-positive taste receptor cells in the taste buds of MRL/lpr mice. Furthermore, MRL/lpr mice exhibit reduced gustatory nerve responses to the bitter compound quinine and the sweet compound saccharin and reduced behavioral responses to bitter, sweet, and umami taste substances compared with controls. In contrast, their responses to salty and sour compounds are comparable to those of control mice in both nerve recording and behavioral experiments. Together, our results suggest that type II taste receptor cells, which are essential for bitter, sweet, and umami taste reception and signaling, are selectively affected in MRL/lpr mice, a model for autoimmune disease with chronic inflammation.</p> </div

QTL Analysis of Dietary Obesity in C57BL/6byj X 129P3/J F₂ Mice: Diet- and Sex-Dependent Effects

<div><p>Obesity is a heritable trait caused by complex interactions between genes and environment, including diet. Gene-by-diet interactions are difficult to study in humans because the human diet is hard to control. Here, we used mice to study dietary obesity genes, by four methods. First, we bred 213 F₂ mice from strains that are susceptible [C57BL/6ByJ (B6)] or resistant [129P3/J (129)] to dietary obesity. Percent body fat was assessed after mice ate low-energy diet and again after the same mice ate high-energy diet for 8 weeks. Linkage analyses identified QTLs associated with dietary obesity. Three methods were used to filter candidate genes within the QTL regions: (a) association mapping was conducted using >40 strains; (b) differential gene expression and (c) comparison of genomic DNA sequence, using two strains closely related to the progenitor strains from Experiment 1. The QTL effects depended on whether the mice were male or female or which diet they were recently fed. After feeding a low-energy diet, percent body fat was linked to chr 7 (LOD = 3.42). After feeding a high-energy diet, percent body fat was linked to chr 9 (<i>Obq5</i>; LOD = 3.88), chr 12 (<i>Obq34</i>; LOD = 3.88), and chr 17 (LOD = 4.56). The Chr 7 and 12 QTLs were sex dependent and all QTL were diet-dependent. The combination of filtering methods highlighted seven candidate genes within the QTL locus boundaries: <i>Crx</i>, <i>Dmpk</i>, <i>Ahr</i>, <i>Mrpl28</i>, <i>Glo1</i>, <i>Tubb5</i>, and <i>Mut</i>. However, these filtering methods have limitations so gene identification will require alternative strategies, such as the construction of congenics with very small donor regions.</p></div

Number of gustducin-positive taste receptor cells is reduced in circumvallate taste buds of MRL/lpr mice.

<p>(<b>A</b>) Confocal images of immunofluorescent staining with antibodies against gustducin and NCAM. Circumvallate sections from MRL/+/+ and MRL/lpr mice were processed for immunostaining with anti-gustducin or anti-NCAM antibody as indicated. Scale bars, 40 µm. (<b>B</b> and <b>C</b>) Quantitative analyses of the average number of gustducin-positive (<b>B</b>) or NCAM-positive (<b>C</b>) cells per taste bud profile based on immunostaining data. Five mice per group were included in the experiment. Four circumvallate tissue sections per animal were included. Student's <i>t</i> tests were used. Data are means ± SEM. ** <i>p</i><0.01.</p

MRL/lpr mice show decreased CT nerve responses to quinine and saccharin.

<p>(<b>A</b>) Representative CT nerve responses to taste compounds. CT nerve responses to 0.1 M NH₄Cl are shown as reference. (<b>B</b>) CT nerve responses (mean ± SEM) to several concentrations of QHCl, saccharin, sucrose, MSG, NaCl, citric acid, and HCl. Nerve responses to taste compounds were normalized against responses to 0.1 M NH₄Cl. Nerve recordings were done with 8–12 MRL/+/+ mice and 7–10 MRL/lpr mice. Data were analyzed with two-way ANOVA with <i>post hoc</i> Fisher LSD tests. Responses to QHCl were significantly affected by strain (F_1,20 = 4.7, <i>p</i> = 0.04) and strain×concentration interaction (F_3,60 = 4.6, <i>p</i> = 0.006). Responses to saccharin were significantly affected by strain×concentration interaction (F_4,52 = 2.7, <i>p</i> = 0.04). * <i>p</i><0.05.</p

MRL/lpr mice exhibit reduced responses to bitter, sweet, and umami taste compounds in brief-access tests.

<p>Five-second brief-access tests were performed with MRL/lpr and MRL/+/+ mice to QHCl (<b>A</b>), sucrose (<b>B</b>), IMP (<b>C</b>), NaCl (<b>D</b>), and citric acid (<b>E</b>). Taste compounds and the tested concentrations are as indicated in the graphs. Lick ratios (mean ± SEM) were calculated as the number of licks of taste stimuli divided by the number of licks of water in each test session. N = 8 for MRL/+/+ mice; N = 7 for MRL/lpr mice. Data were analyzed with two-way ANOVA with <i>post hoc t</i> tests. * <i>p</i><0.05.</p