The involvement of the T1R3 receptor protein in the control of glucose metabolism in mice at different levels of glycemia

The heterodimeric protein T1R2/T1R3 is a chemoreceptor mediating taste perception of sugars, several amino acids, and non-caloric sweeteners in humans and many other vertebrate species. The T1R2 and T1R3 proteins are expressed not only in the oral cavity, but also in the intestine, pancreas, liver, adipose tissue, and in structures of the central nervous system, which suggests their involvement in functions other than gustatory perception. In this study, we analyzed the role of the T1R3 protein in regulation of glucose metabolism in experiments with the gene-knockout mouse strain C57BL/6J-Tas1r3 ^tm1Rfm (Tas1r3-/-), with a deletion of the Tas1r3 gene encoding T1R3, and the control strain C57BL/6ByJ with the intact gene. Glucose tolerance was measured in euglycemic or food-deprived mice after intraperitoneal or intragastric glucose administration. We have shown that in the Tas1r3-/- strain, in addition to the disappearance of taste preference for sucrose, glucose tolerance is also substantially reduced, and insulin resistance is observed. The effect of the Tas1r3 gene knockout on glucose utilization was more pronounced in the euglycemic state than after food deprivation. The baseline glucose level after food deprivation was lower in the Tas1r3-/- strain than in the control strain, which suggests that T1R3 is involved in regulation of endogenous glucose production. These data suggest that the T1R3-mediated glucoreception interacts with the K_ATP-dependent mechanisms of regulation of the glucose metabolism, and that the main role is likely played by T1R3 expressed in the pancreas and possibly in the central nervous system, but not in the intestinal mucosa, as it was suggested earlier.     

Allelic variation of the Tas1r3 taste receptor gene selectively affects taste responses to sweeteners: evidence from 129.B6-Tas1r3 congenic mice.

The Tas1r3 gene encodes the T1R3 receptor protein, which is involved in sweet taste transduction. To characterize ligand specificity of the T1R3 receptor and the genetic architecture of sweet taste responsiveness, we analyzed taste responses of 129.B6-Tas1r3 congenic mice to a variety of chemically diverse sweeteners and glucose polymers with three different measures: consumption in 48-h two-bottle preference tests, initial licking responses, and responses of the chorda tympani nerve. The results were generally consistent across the three measures. Allelic variation of the Tas1r3 gene influenced taste responsiveness to nonnutritive sweeteners (saccharin, acesulfame-K, sucralose, SC-45647), sugars (sucrose, maltose, glucose, fructose), sugar alcohols (erythritol, sorbitol), and some amino acids (D-tryptophan, D-phenylalanine, L-proline). Tas1r3 genotype did not affect taste responses to several sweet-tasting amino acids (L-glutamine, L-threonine, L-alanine, glycine), glucose polymers (Polycose, maltooligosaccharide), and nonsweet NaCl, HCl, quinine, monosodium glutamate, and inosine 5'-monophosphate. Thus Tas1r3 polymorphisms affect taste responses to many nutritive and nonnutritive sweeteners (all of which must interact with a taste receptor involving T1R3), but not to all carbohydrates and amino acids. In addition, we found that the genetic architecture of sweet taste responsiveness changes depending on the measure of taste response and the intensity of the sweet taste stimulus. Variation in the T1R3 receptor influenced peripheral taste responsiveness over a wide range of sweetener concentrations, but behavioral responses to higher concentrations of some sweeteners increasingly depended on mechanisms that could override input from the peripheral taste system.     

The Effect of <Emphasis Type="Italic">Tas1r3</Emphasis> Gene Polymorphism on Preference and Consumption of Sucrose and Low-Calorie Sweeteners in Interstrain Hybrid Mice of the First Filial Generation

Inter- and intra-species differences in consumption of sweet tastants formed during the evolution of vertebrates are thought to be due to polymorphism of the Tas1r3 gene encoding T1R3, a sweet taste receptor subunit. The aim of the study was to assess the effect of Tas1r3 polymorphism on nutritional behavior of laboratory mice using the first filial generation (F1) hybrids produced by crossing inbred strains with different sensitivity to sweet: 129P3/J males (129, carriers of a recessive SacD sweet taste receptor allele) and C57BL/6 females (B6, dominant SacB allele) or females of the Tas1r3 gene knockout strain, C57BL/6-Tas1r3KO (B6-Tas1r3KO). SacD/B and SacD/0 hybrids, sharing identical background genotypes, differed only by sets of Sac alleles. In a briefaccess test (BAT) or a 48-h two-bottle free choice test, the presence of the dominant SacD allele in SacD/B hybrids determined increased preference for low sucrose concentrations (1–4%) and higher concentrations of nonmetabolized sweeteners (saccharin Na, sucralose, acesulfame K). A comparison between the 129 parental strain and SacD/0 hybrids or between the B6 parental strain and hybrids from crossing B6 × B6-Tas1r3KO revealed no influence of hemizygosity of SacD or SacB on preference for sweeteners in BAT. A small decrease in sucrose and saccharin preference associated with the lack of the SacB allele was observed during long-term exposure to solutions with low concentrations of these substances. The data obtained indicate the relevance of studying the Tas1r3 polymorphism effects on preference and consumption of sweet tastants using F1 interstrain hybrids and BAT.     

Initial licking responses of mice to sweeteners: effects of tas1r3 polymorphisms

Recent studies have established that the T1R3 receptor plays a central role in the taste-mediated ingestive response to sweeteners by mice. First, transgenic mice lacking the gene for T1R3, Tas1r3, show dramatically reduced lick responsiveness to most sweeteners. Second, strains with the taster allele of Tas1r3 (T strains) are more sensitive to low sweetener concentrations than strains with the nontaster allele (NT strains) and consume greater quantities of low- to midrange concentrations of sweeteners during 24-h tests. We asked how Tas1r3 polymorphisms influence the initial licking responses of four T strains (FVB/NJ, SWR/J, SM/J, and C57BL/6J) and four NT strains (BALB/cJ, 129P3/J, DBA/2J, and C3H/HeJ) to two sweeteners (sucrose and SC-45647, an artificial sweetener). We used the initial licking response as a measure of the taste-mediated ingestive response because its brief duration minimizes the potential contribution of nontaste factors (e.g., negative and positive postingestive feedback). Further, we used two complimentary short-term intake tests (the brief-access taste test and a novel 1-min preference test) to reduce the possibility that our findings were an epiphenomenon of a specific testing procedure. In both tests, the T strains were more responsive than the NT strains to low concentrations of each sweetener. At higher concentrations, however, there was considerable overlap between the T and NT strains. In fact, the initial licking response of several NT strains was more vigorous than (or equivalent to) that of several T strains. There was also considerable variation among strains with the same Tas1r3 allele. We conclude that Tas1r3 polymorphisms contribute to strain differences in initial lick responsiveness to low but not high concentrations of sweeteners.     

Loss of the nutrient sensor TAS1R3 leads to reduced bone resorption

The taste receptor type 1 (TAS1R) family of heterotrimeric G protein-coupled receptors participates in monitoring energy and nutrient status. TAS1R member 3 (TAS1R3) is a bi-functional protein that recognizes amino acids such as L-glycine and L-glutamate or sweet molecules such as sucrose and fructose when dimerized with TAS1R member 1 (TAS1R1) or TAS1R member 2 (TAS1R2), respectively. It was recently reported that deletion of TAS1R3 expression in Tas1R3 mutant mice leads to increased cortical bone mass but the underlying cellular mechanism leading to this phenotype remains unclear. Here, we independently corroborate the increased thickness of cortical bone in femurs of 20-week-old male Tas1R3 mutant mice and confirm that Tas1R3 is expressed in the bone environment. Tas1R3 is expressed in undifferentiated bone marrow stromal cells (BMSCs) in vitro and its expression is maintained during BMP2-induced osteogenic differentiation. However, levels of the bone formation marker procollagen type I N-terminal propeptide (PINP) are unchanged in the serum of 20-week-old Tas1R3 mutant mice as compared to controls. In contrast, levels of the bone resorption marker collagen type I C-telopeptide are reduced greater than 60% in Tas1R3 mutant mice. Consistent with this, Tas1R3 and its putative signaling partner Tas1R2 are expressed in primary osteoclasts and their expression levels positively correlate with differentiation status. Collectively, these findings suggest that high bone mass in Tas1R3 mutant mice is due to uncoupled bone remodeling with reduced osteoclast function and provide rationale for future experiments examining the cell-type-dependent role for TAS1R family members in nutrient sensing in postnatal bone remodeling.     

Positional cloning of the mouse saccharin preference (Sac) locus

Differences in sweetener intake among inbred strains of mice are partially determined by allelic variation of the saccharin preference (Sac) locus. Genetic and physical mapping limited a critical genomic interval containing Sac to a 194 kb DNA fragment. Sequencing and annotation of this region identified a gene (Tas1r3) encoding the third member of the T1R family of putative taste receptors, T1R3. Introgression by serial backcrossing of the 194 kb chromosomal fragment containing the Tas1r3 allele from the high-sweetener-preferring C57BL/6ByJ strain onto the genetic background of the low-sweetener-preferring 129P3/J strain rescued its low-sweetener-preference phenotype. Polymorphisms of Tas1r3 that are likely to have functional significance were identified using analysis of genomic sequences and sweetener-preference phenotypes of genealogically distant mouse strains. Tas1r3 has two common haplotypes, consisting of six single nucleotide polymorphisms: one haplotype was found in mouse strains with elevated sweetener preference and the other in strains relatively indifferent to sweeteners. This study provides compelling evidence that Tas1r3 is equivalent to the Sac locus and that the T1R3 receptor responds to sweeteners.     

Evolutionary insights into umami,sweet, and bitter taste receptors in amphibians

Umami and sweet sensations provide animals with important dietary information for detecting and consuming nutrients, whereas bitter sensation helps animals avoid potentially toxic or harmful substances. Enormous progress has been made toward animal sweet/umami taste receptor (Tas1r) and bitter taste receptor (Tas2r). However, information about amphibians is mainly scarce. This study attempted to delineate the repertoire of Tas1r/Tas2r genes by searching for currently available genome sequences in 14 amphibian species. This study identified 16 Tas1r1, 9 Tas1r2, and 9 Tas1r3 genes to be intact and another 17 Tas1r genes to be pseudogenes or absent in the 14 amphibians. According to the functional prediction of Tas1r genes, two species have lost sweet sensation and seven species have lost both umami and sweet sensations. Anurans possessed a large number of intact Tas2rs, ranging from 39 to 178. In contrast, caecilians possessed a contractive bitter taste repertoire, ranging from 4 to 19. Phylogenetic and reconciling analysis revealed that the repertoire of amphibian Tas1rs and Tas2rs was shaped by massive gene duplications and losses. No correlation was found between feeding preferences and the evolution of Tas1rs in amphibians. However, the expansion of Tas2rs may help amphibians adapt to both aquatic and terrestrial habitats. Bitter detection may have played an important role in the evolutionary adaptation of vertebrates in the transition from water to land.     

Stimulus processing of glycine is dissociable from that of sucrose and glucose based on behaviorally measured taste signal detection in Sac 'taster' and 'non-taster' mice

Mouse strains have been divided into 'tasters' and 'non-tasters' based on their relatively high and low preference, respectively, for low concentrations of sucrose and saccharin. These phenotypic differences appear to be due to a polymorphism in the gene at the Sac locus encoding for the T1R3 taste receptor selectively affecting the functionality of the T1R2+3 heterodimer. To psychophysically examine whether these phenotypes are due to sensory sensitivity as opposed to hedonic responsiveness, we measured taste signal detection of sucrose, glucose, and glycine by Sac taster (C57BL/6J and SWR/J) and non-taster (129P3/J and DBA/2J) strains in an operant conditioning paradigm using a gustometer. The taster mice had lower detection thresholds for sucrose and glucose compared with the non-taster mice. The detection thresholds corresponded well with reported responsiveness to low concentrations of these sugars in two-bottle intake tests suggesting that the Sac taster phenotype has a sensory basis and is not simply a matter of strain differences in the hedonic evaluation of weak intensities of the stimuli. Taster status did not entirely account for the strain differences in detection thresholds for glycine, a 'sweet' tasting amino acid. Collapsed across strains, detection thresholds for sucrose and glucose were highly correlated with each other (r = 0.81), but only modestly correlated with those for glycine (r < or = 0.43). This suggests that stimulus processing of glycine in the perithreshold intensity domain can be dissociated from that of sucrose and glucose. The mechanism underlying this difference may be related to the ability of glycine to bind with the T1R1+3 heterodimer.     

Loss of NAC1 Expression Is Associated with Defective Bony Patterning in the Murine Vertebral Axis

NAC1 encoded by NACC1 is a member of the BTB/POZ family of proteins and participates in several pathobiological processes. However, its function during tissue development has not been elucidated. In this study, we compared homozygous null mutant Nacc1^-/- and wild type Nacc1^+/+ mice to determine the consequences of diminished NAC1 expression. The most remarkable change in Nacc1^-/- mice was a vertebral patterning defect in which most knockout animals exhibited a morphological transformation of the sixth lumbar vertebra (L6) into a sacral identity; thus, the total number of pre-sacral vertebrae was decreased by one (to 25) in Nacc1^-/- mice. Heterozygous Nacc1^+/- mice had an increased tendency to adopt an intermediate phenotype in which L6 underwent partial sacralization. Nacc1^-/- mice also exhibited non-closure of the dorsal aspects of thoracic vertebrae T10-T12. Chondrocytes from Nacc1^+/+ mice expressed abundant NAC1 while Nacc1^-/- chondrocytes had undetectable levels. Loss of NAC1 in Nacc1^-/- mice was associated with significantly reduced chondrocyte migratory potential as well as decreased expression of matrilin-3 and matrilin-4, two cartilage-associated extracellular matrix proteins with roles in the development and homeostasis of cartilage and bone. These data suggest that NAC1 participates in the motility and differentiation of developing chondrocytes and cartilaginous tissues, and its expression is necessary to maintain normal axial patterning of murine skeleton.     

Is glycine "sweet" to mice? Mouse strain differences in perception of glycine taste

Glycine is an amino acid tasting sweet to humans. In 2-bottle tests, C57BL/6ByJ (B6) mice strongly prefer glycine solutions, whereas 129P3/J (129) mice do not, suggesting that they differ in perception of glycine taste. We examined this question using the conditioned taste aversion (CTA) generalization technique. CTA was achieved by injecting LiCl after drinking glycine, and next its generalization to 10 taste solutions (glycine, sucrose, saccharin, D-tryptophan, L-tryptophan, L-alanine, L-proline, L-glutamine, NaCl, and HCl) was examined by video recording licking behavior. Both B6 and 129 mice generalized the aversion to sucrose, saccharin, L-alanine, and L-proline and did not generalize it to NaCl, HCl, and L-tryptophan. This indicates that both B6 and 129 mice perceive the sweetness (i.e., a sucrose-like taste) of glycine. Thus, the lack of a glycine preference by 129 mice cannot be explained by their inability to perceive its sweetness. Strain differences were observed for CTA generalization to 2 amino acids: 129 mice generalized aversion to L-glutamine but not D-tryptophan, whereas B6 mice generalized it to D-tryptophan but not L-glutamine. 129.B6-Tas1r3 congenic mice with 2 genotypes of the Tas1r3 locus (B6/129 heterozygotes and 129/129 homozygotes) did not differ in aversion generalization, suggesting that the differences between 129 and B6 strains are not attributed to the Tas1r3 allelic variants and that other, yet unknown, genes are involved in taste perception of amino acids.     

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.     

Genetic diversity of bitter taste receptor gene family in Sichuan domestic and Tibetan chicken populations

The sense of bitter taste plays a critical role in animals as it can help them to avoid intake of toxic and harmful substances. Previous research had revealed that chicken has only three bitter taste receptor genes (Tas2r1, Tas2r2 and Tas2r7). To better understand the genetic polymorphisms and importance of bitter taste receptor genes (Tas2rs) in chicken, here, we sequenced Tas2rs of 30 Sichuan domestic chickens and 30 Tibetan chickens. Thirteen single-nucleotide polymorphisms (SNPs) including three nonsynonymous mutations (m.359G >C, m.503C >A and m.583A >G) were detected in Tas2r1 (m. is the abbreviation for mutation); three SNPs were detected in Tas2r2, but none of them were missense mutation; eight SNPs were detected in Tas2r7 including six nonsynonymous substitutions (m.178G >A, m.421A >C, m.787C >T, m.832G >T, m.907A >T and m.943G >A). Tajima’s D neutral test indicates that there is no population expansion in both populations, and the size of the population is relatively stable. All the three networks indicate that red jungle fowls share haplotypes with domestic chickens. In addition, we found that haplotypes H1 and HE1 were positively associated with high-altitude adaptation, whereas haplotypes H4 and HE4 showed a negative correlation with high-altitude adaptation in Tas2rs. Although, chicken has only three Tas2rs, our results showed that both Sichuan domestic chickens and Tibetan chickens have abundant haplotypes in Tas2rs, especially in Tas2r7, which might help chickens to recognize a wide variety of bitter-tasting compounds.     

Mice Abundant in Muricholic Bile Acids Show Resistance to Dietary Induced Steatosis,Weight Gain,and to Impaired Glucose Metabolism

High endogenous production of, or treatment with muricholic bile acids, strongly reduces the absorption of cholesterol. Mice abundant in muricholic bile acids may therefore display an increased resistance against dietary induced weight gain, steatosis, and glucose intolerance due to an anticipated general reduction in lipid absorption. To test this hypothesis, mice deficient in steroid 12-alpha hydroxylase (Cyp8b1^-/-) and therefore abundant in muricholic acids were monitored for 11 weeks while fed a high fat diet. Food intake and body and liver weights were determined, and lipids in liver, serum and feces were measured. Further, responses during oral glucose and intraperitoneal insulin tolerance tests were evaluated.On the high fat diet, Cyp8b1^-/- mice displayed less weight gain compared to wildtype littermates (Cyp8b1^+/+). In addition, liver enlargement with steatosis and increases in serum LDL-cholesterol were strongly attenuated in Cyp8b1^-/- mice on high fat diet. Fecal excretion of cholesterol was increased and there was a strong trend for doubled fecal excretion of free fatty acids, while excretion of triglycerides was unaltered, indicating dampened lipid absorption. On high fat diet, Cyp8b1^-/- mice also presented lower serum glucose levels in response to oral glucose gavage or to intraperitoneal insulin injection compared to Cyp8b1^+/+.In conclusion, following exposure to a high fat diet, Cyp8b1^-/- mice are more resistant against weight gain, steatosis, and to glucose intolerance than Cyp8b1^+/+ mice. Reduced lipid absorption may in part explain these findings. Overall, the results suggest that muricholic bile acids may be beneficial against the metabolic syndrome.     

No relationship between sequence variation in protein coding regions of the Tas1r3 gene and saccharin preference in rats

Nearly all mammalian species like sweet-tasting foods and drinks, but there are differences in the degree of 'sweet tooth' both between species and among individuals of the same species. Some individual differences can be explained by genetic variability. Polymorphisms in a sweet taste receptor (Tas1r3) account for a large fraction of the differences in consumption of sweet solutions among inbred mouse strains. We wondered whether mice and rats share the same Tas1r3 alleles, and whether this gene might explain the large difference in saccharin preference among rats. We conducted three experiments to test this. We examined DNA sequence differences in the Tas1r3 gene among rats that differed in their consumption of saccharin in two-bottle choice tests. The animals tested were from an outbred strain (Sprague-Dawley; experiment 1), selectively bred to be high- or low-saccharin consumers (HiS and LoS; experiment 2), or from inbred strains with established differences in saccharin preference (FH/Wjd and ACI; experiment 3). Although there was considerable variation in saccharin preference among the rats there was no variation in the protein-coding regions of the Tas1r3 gene. DNA variants in intronic regions were detected in 1 (of 12) outbred rat with lower-than-average saccharin preference and in the ACI inbred strain, which also has a lower saccharin preference than the FH/Wjd inbred partner strain. Possible effects of these intronic nucleotide variants on Tas1r3 gene expression or the presence of T1R3 protein in taste papillae were evaluated in the ACI and FH/Wjd strains. Based upon the results of these studies, we conclude that polymorphisms in the protein-coding regions of the sweet receptor gene Tas1r3 are uncommon and do not account for individual differences in saccharin preference for these strains of rats. DNA variants in intron 4 and 5 are more common but appear to be innocuous.     

Expression of taste receptors in Solitary Chemosensory Cells of rodent airways

Background

Chemical irritation of airway mucosa elicits a variety of reflex responses such as coughing, apnea, and laryngeal closure. Inhaled irritants can activate either chemosensitive free nerve endings, laryngeal taste buds or solitary chemosensory cells (SCCs). The SCC population lies in the nasal respiratory epithelium, vomeronasal organ, and larynx, as well as deeper in the airway. The objective of this study is to map the distribution of SCCs within the airways and to determine the elements of the chemosensory transduction cascade expressed in these SCCs.

Methods

We utilized a combination of immunohistochemistry and molecular techniques (rtPCR and in situ hybridization) on rats and transgenic mice where the Tas1R3 or TRPM5 promoter drives expression of green fluorescent protein (GFP).

Results

Epithelial SCCs specialized for chemoreception are distributed throughout much of the respiratory tree of rodents. These cells express elements of the taste transduction cascade, including Tas1R and Tas2R receptor molecules, α-gustducin, PLCβ2 and TrpM5. The Tas2R bitter taste receptors are present throughout the entire respiratory tract. In contrast, the Tas1R sweet/umami taste receptors are expressed by numerous SCCs in the nasal cavity, but decrease in prevalence in the trachea, and are absent in the lower airways.

Conclusions

Elements of the taste transduction cascade including taste receptors are expressed by SCCs distributed throughout the airways. In the nasal cavity, SCCs, expressing Tas1R and Tas2R taste receptors, mediate detection of irritants and foreign substances which trigger trigeminally-mediated protective airway reflexes. Lower in the respiratory tract, similar chemosensory cells are not related to the trigeminal nerve but may still trigger local epithelial responses to irritants. In total, SCCs should be considered chemoreceptor cells that help in preventing damage to the respiratory tract caused by inhaled irritants and pathogens.     

TLR2 mediates immunity to experimental cysticercosis

Information concerning TLR-mediated antigen recognition and regulation of immune responses during helminth infections is scarce. TLR2 is a key molecule required for innate immunity and is involved in the recognition of a wide range of viruses, bacteria, fungi and parasites. Here, we evaluated the role of TLR2 in a Taenia crassiceps cysticercosis model. We compared the course of T. crassiceps infection in C57BL/6 TLR2 knockout mice (TLR2^-/-) with that in wild type C57BL/6 (TLR2^+/+) mice. In addition, we assessed serum antibody and cytokine profiles, splenic cellular responses and cytokine profiles and the recruitment of alternatively activated macrophages (AAMφs) to the site of the infection. Unlike wild type mice, TLR2^-/- mice failed to produce significant levels of inflammatory cytokines in either the serum or the spleen during the first two weeks of Taenia infection. TLR2^-/- mice developed a Th2-dominant immune response, whereas TLR2^+/+ mice developed a Th1-dominant immune response after Taenia infection. The insufficient production of inflammatory cytokines at early time points and the lack of Th1-dominant adaptive immunity in TLR2^-/- mice were associated with significantly elevated parasite burdens; in contrast, TLR2^+/+ mice were resistant to infection. Furthermore, increased recruitment of AAMφs expressing PD-L1, PD-L2, OX40L and mannose receptor was observed in TLR2^-/- mice. Collectively, these findings indicate that TLR2-dependent signaling pathways are involved in the recognition of T. crassiceps and in the subsequent activation of the innate immune system and production of inflammatory cytokines, which appear to be essential to limit infection during experimental cysticercosis.     

P2Y2R Deficiency Attenuates Experimental Autoimmune Uveitis Development

We aimed to study the role of the nucleotide receptor P2Y₂R in the development of experimental autoimmune uveitis (EAU). EAU was induced in P2Y₂^+/+ and P2Y₂^-/- mice by immunization with IRBP peptide or by adoptive transfer of in vitro restimulated semi-purified IRBP-specific enriched T lymphocytes from spleens and lymph nodes isolated from native C57Bl/6 or P2Y₂^+/+ and P2Y₂^-/- immunized mice. Clinical and histological scores were used to grade disease severity. Splenocytes and lymph node cell phenotypes were analyzed using flow cytometry. Semi-purified lymphocytes and MACS-purified CD4⁺ T lymphocytes from P2Y₂^+/+ and P2Y₂^-/- immunized mice were tested for proliferation and cytokine secretion. Our data show that clinical and histological scores were significantly decreased in IRBP-immunized P2Y₂^-/- mice as in P2Y₂^-/- mice adoptively transfered with enriched T lymphocytes from C57Bl/6 IRBP-immunized mice. In parallel, naïve C57Bl/6 mice adoptively transferred with T lymphocytes from P2Y₂^-/- IRBP-immunized mice also showed significantly less disease. No differences in term of spleen and lymph node cell recruitment or phenotype appeared between P2Y₂^-/- and P2Y₂^+/+ immunized mice. However, once restimulated in vitro with IRBP, P2Y₂^-/- T cells proliferate less and secrete less cytokines than the P2Y₂^+/+ one. We further found that antigen-presenting cells of P2Y₂^-/- immunized mice were responsible for this proliferation defect. Together our data show that P2Y₂^-/- mice are less susceptible to mount an autoimmune response against IRBP. Those results are in accordance with the danger model, which makes a link between autoreactive lymphocyte activation, cell migration and the release of danger signals such as extracellular nucleotides.     

Transformation of postingestive glucose responses after deletion of sweet taste receptor subunits or gastric bypass surgery

The glucose-dependent secretion of the insulinotropic hormone glucagon-like peptide-1 (GLP-1) is a critical step in the regulation of glucose homeostasis. Two molecular mechanisms have separately been suggested as the primary mediator of intestinal glucose-stimulated GLP-1 secretion (GSGS): one is a metabotropic mechanism requiring the sweet taste receptor type 2 (T1R2) + type 3 (T1R3) while the second is a metabolic mechanism requiring ATP-sensitive K(+) (K(ATP)) channels. By quantifying sugar-stimulated hormone secretion in receptor knockout mice and in rats receiving Roux-en-Y gastric bypass (RYGB), we found that both of these mechanisms contribute to GSGS; however, the mechanisms exhibit different selectivity, regulation, and localization. T1R3(-/-) mice showed impaired glucose and insulin homeostasis during an oral glucose challenge as well as slowed insulin granule exocytosis from isolated pancreatic islets. Glucose, fructose, and sucralose evoked GLP-1 secretion from T1R3(+/+), but not T1R3(-/-), ileum explants; this secretion was not mimicked by the K(ATP) channel blocker glibenclamide. T1R2(-/-) mice showed normal glycemic control and partial small intestine GSGS, suggesting that T1R3 can mediate GSGS without T1R2. Robust GSGS that was K(ATP) channel-dependent and glucose-specific emerged in the large intestine of T1R3(-/-) mice and RYGB rats in association with elevated fecal carbohydrate throughout the distal gut. Our results demonstrate that the small and large intestines utilize distinct mechanisms for GSGS and suggest novel large intestine targets that could mimic the improved glycemic control seen after RYGB.