Jean-Pierre Montmayeur

A family of candidate taste receptors in human and mouse.

The gustatory system of mammals can sense four basic taste qualities, bitter, sweet, salty and sour, as well as umami, the taste of glutamate. Previous studies suggested that the detection of bitter and sweet tastants by taste receptor cells in the mouth is likely to involve G-protein-coupled receptors. Although two putative G-protein-coupled bitter/sweet taste receptors have been identified, the chemical diversity of bitter and sweet compounds leads one to expect that there is a larger number of different receptors. Here we report the identification of a family of candidate taste receptors (the TRBs) that are members of the G-protein-coupled receptor superfamily and that are specifically expressed by taste receptor cells. A cluster of genes encoding human TRBs is located adjacent to a Prp gene locus, which in mouse is tightly linked to the SOA genetic locus that is involved in detecting the bitter compound sucrose octaacetate. Another TRB gene is found on a human contig assigned to chromosome 5p15, the location of a genetic locus (PROP) that controls the detection of the bitter compound 6-n-propyl-2-thiouracil in humans.

CD36 involvement in orosensory detection of dietary lipids, spontaneous fat preference, and digestive secretions

Rats and mice exhibit a spontaneous attraction for lipids. Such a behavior raises the possibility that an orosensory system is responsible for the detection of dietary lipids. The fatty acid transporter CD36 appears to be a plausible candidate for this function since it has a high affinity for long-chain fatty acids (LCFAs) and is found in lingual papillae in the rat. To explore this hypothesis further, experiments were conducted in rats and in wild-type and CD36-null mice. In mice, RT-PCR experiments with primers specific for candidate lipid-binding proteins revealed that only CD36 expression was restricted to lingual papillae although absent from the palatal papillae. Immunostaining studies showed a distribution of CD36 along the apical side of circumvallate taste bud cells. CD36 gene inactivation fully abolished the preference for LCFA-enriched solutions and solid diet observed in wild-type mice. Furthermore, in rats and wild-type mice with an esophageal ligation, deposition of unsaturated LCFAs onto the tongue led to a rapid and sustained rise in flux and protein content of pancreatobiliary secretions. These findings demonstrate that CD36 is involved in oral LCFA detection and raise the possibility that an alteration in the lingual fat perception may be linked to feeding dysregulation.

Genetic tracing reveals a stereotyped sensory map in the olfactory cortex

This corrects the article DOI: 10.1038/35102506

The gustatory pathway is involved in CD36-mediated orosensory perception of long-chain fatty acids in the mouse

The sense of taste informs the body about the quality of ingested foods. Tastant‐mediated signals are generated by a rise in free intracellular calcium levels ([Ca2+]i) in the taste bud cells and then are transferred to the gustatory area of brain via connections between the gustatory nerves (chorda tym‐ pani and glossopharyngeal nerves) and the nucleus of solitary tract in the brain stem. We have recently shown that lingual CD36 contributes to fat preference and early digestive secretions in the mouse. We show here that 1) the induction of an increase in [Ca2+]i by linoleic acid is CD36‐dependent in taste receptor cells, 2) the spontaneous preference for or conversely con ditioned aversion to linoleic acid requires intact gusta tory nerves, and 3) the activation of gustatory neurons in the nucleus of the solitary tract elicited by a linoleic acid deposition on the tongue in wild‐type mice cannot be reproduced in CD36‐null animals. We conclude that the CD36‐mediated perception of long‐chain fatty acids involves the gustatory pathway, suggesting that the mouse may have a “taste“ for fatty foods. This system would constitute a potential physiological advantage under conditions of food scarcity by leading the mouse to select and absorb fatty foods. However, it might also lead to a risk of obesity and associated diseases in a context of constantly abundant food.—Gaillard, D., Laugerette, F., Darcel, N., El‐Yassimi, A., Passilly‐De grace, P., Hichami, A., Khan, N. A., Montmayeur, J.‐P., Besnard, P. The gustatory pathway is involved in CD36‐ mediated orosensory perception of long‐chain fatty acids in the mouse. FASEB J. 22, 1458–1468 (2008)

ENDOTHELIN RECEPTORS AND PAIN

UNLABELLED The endogenous endothelin (ET) peptides participate in a remarkable variety of pain-relatedprocesses. Pain that is elevated by inflammation, by skin incision, by cancer, during a Sickle Cell Disease crisis and by treatments that mimic neuropathic and inflammatory pain and are all reduced by local administration of antagonists of endothelin receptors. Many effects of endogenously released endothelin are simulated by acute, local subcutaneous administration of endothelin, which at very high concentrations causes pain and at lower concentrations sensitizes the nocifensive reactions to mechanical, thermal and chemical stimuli. PERSPECTIVE In this paper we review the biochemistry, second messenger pathways and hetero-receptor coupling that are activated by ET receptors, the cellular physiological responses to ET receptor activation, and the contribution to pain of such mechanisms occurring in the periphery and the CNS. Our goal is to frame the subject of endothelin and pain for a broad readership, and to present the generally accepted as well as the disputed concepts, including important unanswered questions.

Receptors for bitter and sweet taste

The identification of two families of receptors, T1Rs and T2Rs, for sweet and bitter taste stimuli has opened the door to understanding some of the basic mechanisms underlying taste transduction in mammals. Studies of the functions of these receptors and their patterns of expression provide important information regarding the detection of structurally diverse taste compounds and the manner in which different taste qualities are encoded in the mouth.

Differential expression of the mouse D2 dopamine receptor isoforms.

We have identified and characterized the cDNAs corresponding to the mouse D2 dopamine receptors. We show that in the mouse the D2 dopamine receptor is found in two forms, generated by alternative splicing of the same gene, mRNA distribution analysis of areas expressing the D2 receptors shows that the larger form is the most abundant, except in the brain stem where the shorter form is predominant. Membranes of mammalian cells translently transfected with both forms of D2 receptor bind [1H]spiperone with a high affinity.

Nonsynonymous single nucleotide polymorphisms in human tas1r1, tas1r3, and mGluR1 and individual taste sensitivity to glutamate

Several studies indicate an essential role of the heterodimer Tas1R1-Tas1R3 for monosodium l-glutamate (MSG) detection, although others suggest alternative receptors. Human subjects show different taste sensitivities to MSG, and some are unable to detect the presence of glutamate. Our objective was to study possible relations between phenotype (sensitivity to glutamate) and genotype (polymorphisms in candidate glutamate taste receptors tas1r1, tas1r3, mGluR4, and mGluR1) at the individual level. The sensitivity was measured with a battery of tests to distinguish the effect of sodium ions from the effect of glutamate ions in MSG. A total of 142 genetically unrelated white French subjects were categorized into 27 nontasters (specific ageusia), 21 hypotasters, and 94 tasters. Reverse transcriptase polymerase chain reaction and immunohistochemistry showed expression of tas1r1, tas1r3, and alpha-gustducin in fungiform papillae in all 12 subjects tested, including subjects who presented specific ageusia for glutamate. Amplification and sequencing of cDNA and genomic DNA allowed the identification of 10 nonsynonymous single nucleotide polymorphisms (nsSNPs) in tas1r1 (n = 3), tas1r3 (n = 3), and mGluR1 (n = 4). In our sample of subjects, the frequencies of 2 nsSNPs, C329T in tas1r1 and C2269T in tas1r3, were significantly higher in nontasters than expected, whereas G1114A in tas1r1 was more frequent in tasters. These nsSNPs along with minor variants and other nsSNPs in mGluR1, including T2977C, account for only part of the interindividual variance, which indicates that other factors, possibly including additional receptors, contribute to glutamate sensitivity.

Human genetic polymorphisms in T1R1 and T1R3 taste receptor subunits affect their function.

Umami is the typical taste induced by monosodium glutamate (MSG), which is thought to be detected by the heterodimeric G protein-coupled receptor, T1R1 and T1R3. Previously, we showed that MSG detection thresholds differ substantially between individuals and we further showed that nontaster and hypotaster subjects are associated with nonsynonymous single polymorphisms occurring in the T1R1 and T1R3 genes. Here, we show using functional expression that both amino acid substitutions (A110V and R507Q) in the N-terminal ligand-binding domain of T1R1 and the 2 other ones (F749S and R757C), located in the transmembrane domain of T1R3, severely impair in vitro T1R1/T1R3 response to MSG. A molecular model of the ligand-binding region of T1R1/T1R3 provides a mechanistic explanation supporting functional expression data. The data presented here support causal relations between the genotype and previous in vivo psychophysical studies in human evaluating sensitivity to MSG.

The platelet-derived growth factor beta receptor triggers multiple cytoplasmic signaling cascades that arrive at the nucleus as distinguishable inputs

Stimulation of the platelet-derived growth factor β receptor (βPDGFR) activates enzymes such as phosphatidylinositol 3-kinase (PI3K) and phospholipase Cγ1 (PLCγ), which ultimately initiate nuclear responses such as enhanced expression of immediate early genes. In an attempt to compare the signaling cascades initiated by PI3K and PLCγ, we examined the activation of a panel of immediate early genes by βPDGFR mutants, which preferentially engage PI3K or PLCγ. When expressed in A431 cells, the wild type receptor and to a lesser extent the mutant receptor that associates with PLCγ (Y1021) was able to up-regulate c-fos, junB, andKC mRNA expression. In contrast, the receptor mutant that engages PI3K (Y740/51) poorly stimulated c-fosmRNA expression and did not significantly stimulate expression of either JunB or KC. Receptor mutants that did not associate with either PI3K or PLCγ were dramatically compromised or unable to increase expression of any of these immediate early genes. The differential ability of the Y1021 and Y740/51 receptors to activate c-fos correlated well with an apparent difference in their ability to engage distinct protein kinase C family members. However there did appear to be a degree of redundancy in the cytoplasmic signaling pathways initiated by PI3K and PLCγ, since both the Y1021 and Y740/51 receptors were able to activate an AP-1-responsive element. We conclude that recruitment of signal relay enzymes to the βPDGFR is necessary for PDGF-dependent activation of at least some immediate early genes. In addition, whereas the βPDGFR activates multiple signaling enzymes capable of activating the same nuclear response (activation of c-fos), these signaling cascades do not appear to converge in the cytoplasm but arrive at the nucleus as distinguishable inputs.