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Dopamine neurons of the hypothalamic arcuate nucleus (ARC) tonically inhibit the release of the protein hormone prolactin from lactotropic cells in the anterior pituitary gland and thus play a central role in prolactin homeostasis of the body. Prolactin, in turn, orchestrates numerous important biological functions such as maternal behavior, reproduction, and sexual arousal. Here, we identify the canonical transient receptor potential channel Trpc5 as an essential requirement for normal function of dopamine ARC neurons and prolactin homeostasis. By analyzing female mice carrying targeted mutations in the Trpc5 gene including a conditional Trpc5 deletion, we show that Trpc5 is required for maintaining highly stereotyped infraslow membrane potential oscillations of dopamine ARC neurons. Trpc5 is also required for eliciting prolactin-evoked tonic plateau potentials in these neurons that are part of a regulatory feedback circuit. Trpc5 mutant females show severe prolactin deficiency or hypoprolactinemia that is associated with irregular reproductive cyclicity, gonadotropin imbalance, and impaired reproductive capabilities. These results reveal a previously unknown role for the cation channel Trpc5 in prolactin homeostasis of female mice and provide strategies to explore the genetic basis of reproductive disorders and other malfunctions associated with defective prolactin regulation in humans.

Inflammation is a central element of many neurodegenerative diseases. Formyl peptide receptors (FPRs) can trigger several receptor-dependent signal transduction pathways that play a key role in neuroinflammation and neurodegeneration. They are chemotactic receptors that help to regulate pro- and anti-inflammatory responses in most mammals. FPRs are primarily expressed in the immune and nervous systems where they interact with a complex pattern of pathogen-derived and host-endogenous molecules. Mounting evidence points towards a contribution of FPRs – via neuropathological ligands such as Amyloid beta, and neuroprotective ligands such as Humanin, Lipoxin A4, and Annexin A1 – to multiple pathological aspects of neurodegenerative diseases. In this review, we aim to summarize the interplay of FPRs with neuropathological and neuroprotective ligands. Next, we depict their capability to trigger a number of ligand-dependent cell signaling pathways and their potential to interact with additional intracellular cofactors. Moreover, we highlight first studies, demonstrating that a pharmacological inhibition of FPRs helps to ameliorate neuroinflammation, which may pave the way towards novel therapeutic strategies.

The Ca2+-activated monovalent cation channel Trpm5 is a key element in chemotransduction of taste receptor cells of the tongue, but the extent to which Trpm5 channels are expressed in olfactory sensory neurons (OSNs) of the main olfactory epithelium (MOE) of adult mice as part of a specific pheromonal detection system is debated. Here, we used a novel Trpm5-IRES-Cre knockin strain to drive Cre recombinase expression, employed previously validated Trpm5 antibodies, performed in situ hybridization experiments to localize Trpm5 RNA, and searched extensively for Trpm5 splice variants in genetically-labeled, Trpm5-expressing MOE cells. In contrast to previous reports, we find no evidence for the existence in adult mouse OSNs of the classical Trpm5 channel known from taste cells. We show that Trpm5-expressing adult OSNs express a novel Trpm5 splice variant, Trpm5-9, that is unlikely to form a functional cation channel by itself. We also demonstrate that Trpm5 is transiently expressed in a subpopulation of mature OSNs in the embryonic olfactory epithelium, indicating that Trpm5 channels could play a specific role in utero during a narrow developmental time window. Ca2+ imaging with GCaMP3 under the control of the Trpm5-IRES-Cre allele using a newly developed MOE wholemount preparation of the adult olfactory epithelium reveals that Trpm5-GCaMP3 OSNs comprise a heterogeneous group of sensory neurons many of which can detect general odorants. Together, these studies are essential for understanding the role of transient receptor potential channels in mammalian olfaction.

The ability to detect specific chemical signatures released by bacteria and other microorganisms is a fundamental feature of immune defense against pathogens. There is increasing evidence that chemodetection of such microorganism-associated molecular patterns (MAMPs) occurs at many places in the body including specific sets of chemosensory neurons in the mammalian nose. Formyl peptide receptors (FPRs) are a unique family of G protein-coupled receptors (GPCRs) that can detect the presence of bacteria and function as chemotactic receptors. Here, we highlight the recent discovery of a vast family of natural FPR agonists, the bacterial signal peptides (or signal sequences), thus providing new insight into the molecular mechanisms of bacterial sensing by human and mouse FPRs. Signal peptides in bacteria are formylated, N-terminal protein signatures required for directing the transfer of proteins through the plasma membrane. After their cleavage and release, signal peptides are available for FPR detection and thus provide a previously unrecognized MAMP. With over 170 000 predicted sequences, bacterial signal peptides represent one of the largest families of GPCR ligands and one of the most complex classes of natural activators of the innate immune system. By recognizing a conserved three-dimensional peptide motif, FPRs employ an unusual detection mechanism that combines structural promiscuity with high specificity and sensitivity, thus solving the problem of detecting thousands of distinct sequences yet maintaining selectivity. How signal peptides are released by bacteria and sensed by GPCRs and how these processes shape the responses of other cells and whole organisms represents an important topic for future research.

Formyl peptide receptor 3 (Fpr3, also known as Fpr-rs1) is a G protein-coupled receptor expressed in subsets of sensory neurons of the mouse vomeronasal organ, an olfactory substructure essential for social recognition. Fpr3 has been implicated in the sensing of infection-associated olfactory cues, but its expression pattern and function are incompletely understood. To facilitate visualization of Fpr3-expressing cells, we generated and validated two new anti-Fpr3 antibodies enabling us to analyze acute Fpr3 protein expression. Fpr3 is not only expressed in murine vomeronasal sensory neurons but also in bone marrow cells, the primary source for immune cell renewal, and in mature neutrophils. Consistent with the notion that Fpr3 functions as a pathogen sensor, Fpr3 expression in the immune system is up-regulated after stimulation with a bacterial endotoxin (lipopolysaccharide). These results strongly support a dual role for Fpr3 in both vomeronasal sensory neurons and immune cells. We also identify a large panel of mouse strains with severely altered expression and function of Fpr3, thus establishing the existence of natural Fpr3 knock-out strains. We attribute distinct Fpr3 expression in these strains to the presence or absence of a 12-nucleotide in-frame deletion (Fpr3Δ424–435). In vitro calcium imaging and immunofluorescence analyses demonstrate that the lack of four amino acids leads to an unstable, truncated, and non-functional receptor protein. The genome of at least 19 strains encodes a non-functional Fpr3 variant, whereas at least 13 other strains express an intact receptor. These results provide a foundation for understanding the in vivo function of Fpr3.

Mucociliary clearance through coordinated ciliary beating is a major innate defense removing pathogens from the lower airways, but the pathogen sensing and downstream signaling mechanisms remain unclear. We identified virulence-associated formylated bacterial peptides that potently stimulated ciliary-driven transport in the mouse trachea. This innate response was independent of formyl peptide and taste receptors but depended on key taste transduction genes. Tracheal cholinergic chemosensory cells expressed these genes, and genetic ablation of these cells abrogated peptide-driven stimulation of mucociliary clearance. Trpm5-deficient mice were more susceptible to infection with a natural pathogen, and formylated bacterial peptides were detected in patients with chronic obstructive pulmonary disease. Optogenetics and peptide stimulation revealed that ciliary beating was driven by paracrine cholinergic signaling from chemosensory to ciliated cells operating through muscarinic M3 receptors independently of nerves. We provide a cellular and molecular framework that defines how tracheal chemosensory cells integrate chemosensation with innate defense.

To understand the molecular basis of neuronal excitation in the mammalian olfactory system, we conducted a systematic analysis of the organization of voltage-gated sodium (Nav) channel subunits in the main olfactory epithelium (MOE) and vomeronasal organ (VNO) of adult mice. We also analyzed changes in Nav channel expression during development in these two systems and during regeneration of the MOE. Quantitative PCR shows that Nav1.7 is the predominant isoform in both adult MOE and VNO. We detected pronounced immunoreactivity for Nav1.7 and Nav1.3 in axons of olfactory and vomeronasal sensory neurons. Analysis of Nav1.2 and Nav1.6 revealed an unexpected subsystem-specific distribution. In the MOE, these Nav channels are absent from olfactory sensory neurons but present in non-neuronal olfactory cell types. In the VNO, Nav1.2 and Nav1.6 are confined to vomeronasal sensory neurons, with Nav1.2-immunoreactive somata solely present in the basal layer of the VNO. The subcellular localization of Nav1.3 and Nav1.7 in olfactory sensory neurons can change dramatically during periods of heightened plasticity in the MOE. During the first weeks of development and during regeneration of the olfactory epithelium following chemical lesion, expression of Nav1.3 and Nav1.7 is transiently enhanced in the somata of mature olfactory sensory neurons. Our results demonstrate a highly complex organization of Nav channel expression in the mouse olfactory system, with specific commonalities but also differences between the MOE and the VNO. On the basis of their subcellular localization, Nav1.3 and Nav1.7 should play major roles in action potential propagation in both MOE and VNO, whereas Nav1.2 and Nav1.6 are specific to the function of vomeronasal sensory neurons. The plasticity of Nav channel expression in olfactory sensory neurons during early development and recovery from injury could reflect important physiological requirements in a variety of activity-dependent mechanisms.

Department of Physiology, Saarland University Faculty of Medicine, Homburg, Germany Key words: CTL; cytotoxicity; killing kinetics; real-time assay; NK cells. Corresponding author: Dr. Bin Qu Institute of Biophysics Saarland University Building 58 66421 Homburg Germany Tel.: +49-6841-162-6458 Fax: +49-6841-162-6060 bin.qu@uks.eu

Humans perceive thousands of compounds as bitter. In sharp contrast, only approximately 25 taste 2 receptors (TAS2R) bitter taste receptors have been identified, raising the question as to how the vast array of bitter compounds can be detected by such a limited number of sensors. To address this issue, we have challenged 25 human taste 2 receptors (hTAS2Rs) with 104 natural or synthetic bitter chemicals in a heterologous expression system. Thirteen cognate bitter compounds for 5 orphan receptors and 64 new compounds for previously identified receptors were discovered. Whereas some receptors recognized only few agonists, others displayed moderate or extreme tuning broadness. Thus, 3 hTAS2Rs together were able to detect approximately 50% of the substances used. Conversely, though 63 bitter substances activated only 1-3 receptors, 19 compounds stimulated up to 15 hTAS2Rs. Our data suggest that the detection of the numerous bitter chemicals is related to the molecular receptive ranges of hTAS2Rs.

The ability to sense bitter taste is vital for detecting toxins in food. Phenylthiocarbamide (PTC) is unusual in that to us it tastes either very bitter, or almost tasteless, depending on an individual's genetics. In a classic Nature paper in 1939, R. A. Foster, E. B. Ford and J. S. Huxley showed that chimpanzees also have variable PTC sensitivity, which was thought to reflect a shared ancient genetic polymorphism maintained by natural selection. Now that the TAS2R38 locus responsible for human PTC sensitivity has been identified, Wooding et al. have revisited the comparison. TAS2R38 is also involved in chimpanzees but, surprisingly, the mutations responsible for the polymorphism differ in the two species. ‘Non-taster’ alleles seem to have evolved at least twice, independently, during hominid evolution. The cover photo by D. J. Cox was taken in Chattanooga Zoo, Tennessee, in March 2003; the chimp is gathering as much fruit as he can carry. It was reported over 65 years ago that chimpanzees, like humans, vary in taste sensitivity to the bitter compound phenylthiocarbamide (PTC)1. This was suggested to be the result of a shared balanced polymorphism, defining the first, and now classic, example of the effects of balancing selection in great apes. In humans, variable PTC sensitivity is largely controlled by the segregation of two common alleles at the TAS2R38 locus, which encode receptor variants with different ligand affinities2,3,4. Here we show that PTC taste sensitivity in chimpanzees is also controlled by two common alleles of TAS2R38; however, neither of these alleles is shared with humans. Instead, a mutation of the initiation codon results in the use of an alternative downstream start codon and production of a truncated receptor variant that fails to respond to PTC in vitro. Association testing of PTC sensitivity in a cohort of captive chimpanzees confirmed that chimpanzee TAS2R38 genotype accurately predicts taster status in vivo. Therefore, although Fisher et al.'s observations1 were accurate, their explanation was wrong. Humans and chimpanzees share variable taste sensitivity to bitter compounds mediated by PTC receptor variants, but the molecular basis of this variation has arisen twice, independently, in the two species.

A coding single-nucleotide polymorphism (cSNP), K172N, in hTAS2R16, a gene encoding a taste receptor for bitter beta -glucopyranosides, shows significant association with alcohol dependence (P = .00018). This gene is located on chromosome 7q in a region reported elsewhere to exhibit linkage with alcohol dependence. The SNP is located in the putative ligand-binding domain and is associated with an increased sensitivity to many bitter beta -glucopyranosides in the presence of the N172 allele. Individuals with the ancestral allele K172 are at increased risk of alcohol dependence, regardless of ethnicity. However, this risk allele is uncommon in European Americans (minor-allele frequency [MAF] 0.6%), whereas 45% of African Americans carry the allele (MAF 26%), which makes it a much more significant risk factor in the African American population.

The closely related neuropeptides orexin A and orexin B mediate their actions, including the regulation of sleep and appetite, by the activation of the orexin 1 and 2 receptors. To elucidate the structural prerequisites for receptor activation and subtype selectivity, we performed multiple amino acid substitutions within the sequence of orexin A and human orexin B-(6-28)-peptide and analyzed their solution structures by CD spectroscopy and their activity at both receptors in Ca(2+) mobilization assays. For orexin A, we showed that the basic amino acids within the segment of residues 6-14 were important for the activation of both receptors. Furthermore, we showed that the restriction via disulfide bonds is not required to maintain the active structure of orexin A. The kink region of h orexin B has been shown to be important for Ox(2)R selectivity, which is not mediated by the restriction of the turn structure. Additionally, we showed that no particular secondary structure is required for receptor subtype selectivity.

Individual differences in perception are ubiquitous within the chemical senses: taste, smell, and chemical somesthesis [1–4]. A hypothesis of this fact states that polymorphisms in human sensory receptor genes could alter perception by coding for functionally distinct receptor types [1, 5–8]. We have previously reported evidence that sequence variants in a presumptive bitter receptor gene (hTAS2R38) correlate with differences in bitterness recognition of phenylthiocarbamide (PTC) [9–11]. Here, we map individual psychogenomic pathways for bitter taste by testing people with a variety of psychophysical tasks and linking their individual perceptions of the compounds PTC and propylthiouracil (PROP) to the in vitro responses of their TAS2R38 receptor variants. Functional expression studies demonstrate that five different haplotypes from the hTAS2R38 gene code for operatively distinct receptors. The responses of the three haplotypes we also tested in vivo correlate strongly with individuals' psychophysical bitter sensitivities to a family of compounds. These data provide a direct molecular link between heritable variability in bitter taste perception to functional variations of a single G protein coupled receptor that responds to compounds such as PTC and PROP that contain the N-C=S moiety. The molecular mechanisms of perceived bitterness variability have therapeutic implications, such as helping patients to consume beneficial bitter-tasting compounds—for example, pharmaceuticals and selected phytochemicals.

Weight-conscious subjects and diabetics use the sulfonyl amide sweeteners saccharin and acesulfame K to reduce their calorie and sugar intake. However, the intrinsic bitter aftertaste, which is caused by unknown mechanisms, limits the use of these sweeteners. Here, we show by functional expression experiments in human embryonic kidney cells that saccharin and acesulfame K activate two members of the human TAS2R family (hTAS2R43 and hTAS2R44) at concentrations known to stimulate bitter taste. These receptors are expressed in tongue taste papillae. Moreover, the sweet inhibitor lactisole did not block the responses of cells transfected with TAS2R43 and TAS2R44, whereas it did block the response of cells expressing the sweet taste receptor heteromer hTAS1R2-hTAS1R3. The two receptors were also activated by nanomolar concentrations of aristolochic acid, a purely bitter-tasting compound. Thus, hTAS2R43 and hTAS2R44 function as cognate bitter taste receptors and do not contribute to the sweet taste of saccharin and acesulfame K. Consistent with thein vitrodata, cross-adaptation studies in human subjects also support the existence of common receptors for both sulfonyl amide sweeteners.

Type II transmembrane serine proteases (TTSPs) are a growing family of multidomain proteins. Among the TTSPs, a new subfamily of HAT/DESC1-like ( human airway trypsin-like protease/ differentially expressed in squamous cell carcinoma gene 1) proteases is emerging consisting so far of four members: DESC1-3 and HAT. The cDNA of a new member of this subfamily, named DESC4, was isolated from rat tongue tissue and characterised. Analysis of selected tissues by RT-PCR demonstrated expression of DESC4 in brain, colon, heart, liver, lung and tongue. At the cellular level, DESC4 expression is confined to epithelial cells within the cleft of the circumvallate papillae extending into the ducts of minor salivary glands, the respiratory epithelium of the nasal cavity and tear gland ducts of the eyes as analysed by in situ hybridisation of sensory organ tissues. In transfected mammalian cells, DESC4 is localised to the plasma membrane as shown by immunocytochemistry and subcellular fractionation experiments. Our results suggest that we have identified a protease that is an important constituent of sensory systems and other organs.

The glt operon of Escherichia coli comprises the structural genes for the glutamate synthase subunits (gltB and gltD) and gltF, whose product was previously suggested to have regulatory functions. The A/T-rich region between gltD and gltF contains a weak promoter and a translation initiation site for gltF. The GltF protein is preceded by a signal peptide, which is cleaved off during export into the periplasmic space. A gltF::Km(R) insertion mutant was constructed and shown here to have no detectable phenotype with respect to amino acid utilization or ammonium transport. Thus, GltF is apparently not involved in regulation of nitrogen catabolism.

The capacity of the mammalian olfactory system to detect an enormous collection of different chemical compounds is based on a large repertoire of odorant receptors (ORs). A small group of these ORs, the OR37 family, is unique due to a variety of special features. Members of this subfamily are exclusively found in mammals, they share a high degree of sequence homology and are highly conserved during evolution. It is still elusive which odorants may activate these atypical receptors. We have reasoned that compounds from skin, hairs, or skin glands might be potential candidates. We have exposed mice to such compounds and monitored activation of glomeruli through the expression of the activity marker c-fos in juxtaglomerular cells surrounding ventrally positioned glomeruli in the olfactory bulb (OB). Employing this methodology it was found that stimulation with long-chain alkanes elicits activation in the ventral part of the OB, however, none of the OR37 glomeruli. Analyses of long-chain hydrocarbon compounds with different functional groups revealed that long-chain aliphatic aldehydes elicited an activation of defined OR37 glomeruli, each of them responding preferentially to an aldehyde with different chain lengths. These results indicate that OR37 receptors may be tuned to distinct fatty aldehydes with a significant degree of ligand specificity.

Bitter taste perception is initiated by TAS2R receptors, which respond to agonists by triggering depolarization of taste bud cells. Mutations in TAS2Rs are known to affect taste phenotypes by altering receptor function. Evidence that TAS2Rs overlap in ligand specificity suggests that they may also contribute joint effects. To explore this aspect of gustation, we examined bitter perception of saccharin and acesulfame K, widely used artificial sweeteners with aversive aftertastes. Both substances are agonists of TAS2R31 and -43, which belong to a five-member subfamily (TAS2R30-46) responsive to a diverse constellation of compounds. We analyzed sequence variation and linkage structure in the ∼140 kb genomic region encoding TAS2R30-46, taste responses to the two sweeteners in subjects, and functional characteristics of receptor alleles. Whole-gene sequences from TAS2R30-46 in 60 Caucasian subjects revealed extensive diversity including 34 missense mutations, two nonsense mutations and high-frequency copy-number variants. Thirty markers, including non-synonymous variants in all five genes, were associated (P0.001) with responses to saccharin and acesulfame K. However, linkage disequilibrium (LD) in the region was high (D', r(2)0.95). Haplotype analyses revealed that most associations were spurious, arising from LD with variants in TAS2R31. In vitro assays confirmed the functional importance of four TAS2R31 mutations, which had independent effects on receptor response. The existence of high LD spanning functionally distinct TAS2R loci predicts that bitter taste responses to many compounds will be strongly correlated even when they are mediated by different genes. Integrative approaches combining phenotypic, genetic and functional analysis will be essential in dissecting these complex relationships.

Loss of function of the gene SCN9A, encoding the voltage-gated sodium channel Na(v)1.7, causes a congenital inability to experience pain in humans. Here we show that Na(v)1.7 is not only necessary for pain sensation but is also an essential requirement for odour perception in both mice and humans. We examined human patients with loss-of-function mutations in SCN9A and show that they are unable to sense odours. To establish the essential role of Na(v)1.7 in odour perception, we generated conditional null mice in which Na(v)1.7 was removed from all olfactory sensory neurons. In the absence of Na(v)1.7, these neurons still produce odour-evoked action potentials but fail to initiate synaptic signalling from their axon terminals at the first synapse in the olfactory system. The mutant mice no longer display vital, odour-guided behaviours such as innate odour recognition and avoidance, short-term odour learning, and maternal pup retrieval. Our study creates a mouse model of congenital general anosmia and provides new strategies to explore the genetic basis of the human sense of smell.

A family of 25 G protein-coupled receptors, TAS2Rs, mediates bitter taste in humans. Many of the members of this family are coexpressed in a subpopulation of taste receptor cells on the tongue, thereby allowing the possibility of receptor-receptor interactions with potential influences on their function. In this study, we used several experimental approaches to investigate whether TAS2Rs can form oligomers and if this has an effect on receptor function. Coimmunoprecipitations clearly demonstrated that TAS2Rs can physically interact in HEK293T cells. Further bioluminescence resonance energy transfer analysis of all 325 possible binary combinations of TAS2Rs established that the vast majority of TAS2R pairs form oligomers, both homomers and heteromers. Subsequent screenings of coexpressed bitter receptors with 104 different tastants did not reveal any heteromer-specific agonists. Additional studies also showed no obvious influence of TAS2R hetero-oligomerization on plasma membrane localization or pharmacological properties of the receptors. Thus, our results show that receptor oligomerization occurs between TAS2R bitter taste receptors; however, functional consequences of hetero-oligomerization were not obvious.

The mammalian sense of taste is crucial for evaluating food palatability and nutritional quality. To achieve the detection of relevant chemicals, evolution has shaped a set of receptor molecules that allow the detection of five basic taste qualities: sweet, umami, bitter, salty, and sour. Each taste modality has unique characteristics and serves a distinct function for an animal's nutrition. Therefore, we address the characteristics, the recent advances, the persisting difficulties, and the future perspectives of the basic taste qualities in separate paragraphs. Enormous progress has been made in the identification and the characterization of taste receptor molecules, and in the growing number of animal genomes accessible from databases, which has inspired us to devote a section of this review to discuss a series of sophisticated evolutionary studies on taste receptor molecules. Finally, evidence is accumulating to show that taste receptor and signal transduction molecules have nongustatory functions as well. The extragustatory expression of such genes and the resulting implications are summarized in the final section.

Background Differences in sweet taste perception among species depend on structural variations of the sweet taste receptor. The commercially used isovanillyl sweetener neohesperidin dihydrochalcone activates the human but not the rat sweet receptor TAS1R2+TAS1R3. Analysis of interspecies combinations and chimeras of rat and human TAS1R2+TAS1R3 suggested that the heptahelical domain of human TAS1R3 is crucial for the activation of the sweet receptor by neohesperidin dihydrochalcone. Results By mutational analysis combined with functional studies and molecular modeling we identified a set of different amino acid residues within the heptahelical domain of human TAS1R3 that forms the neohesperidin dihydrochalcone binding pocket. Sixteen amino acid residues in the transmembrane domains 2 to 7 and one in the extracellular loop 2 of hTAS1R3 influenced the receptor's response to neohesperidin dihydrochalcone. Some of these seventeen residues are also part of the binding sites for the sweetener cyclamate or the sweet taste inhibitor lactisole. In line with this observation, lactisole inhibited activation of the sweet receptor by neohesperidin dihydrochalcone and cyclamate competitively, whereas receptor activation by aspartame, a sweetener known to bind to the N-terminal domain of TAS1R2, was allosterically inhibited. Seven of the amino acid positions crucial for activation of hTAS1R2+hTAS1R3 by neohesperidin dihydrochalcone are thought to play a role in the binding of allosteric modulators of other class C GPCRs, further supporting our model of the neohesperidin dihydrochalcone pharmacophore. Conclusion From our data we conclude that we identified the neohesperidin dihydrochalcone binding site at the human sweet taste receptor, which overlaps with those for the sweetener cyclamate and the sweet taste inhibitor lactisole. This readily delivers a molecular explanation of our finding that lactisole is a competitive inhibitor of the receptor activation by neohesperidin dihydrochalcone and cyclamate. Some of the amino acid positions crucial for activation of hTAS1R2+hTAS1R3 by neohesperidin dihydrochalcone are involved in the binding of allosteric modulators in other class C GPCRs, suggesting a general role of these amino acid positions in allosterism and pointing to a common architecture of the heptahelical domains of class C GPCRs.

'Water-tastes' are gustatory after-impressions elicited by water following the removal of a chemical solution from the mouth, akin to colour after-images appearing on 'white' paper after fixation on coloured images. Unlike colour after-images, gustatory after-effects are poorly understood. One theory posits that 'water-tastes' are adaptation phenomena, in which adaptation to one taste solution causes the water presented subsequently to act as a taste stimulus. An alternative hypothesis is that removal of the stimulus upon rinsing generates a receptor-based, positive, off-response in taste-receptor cells, ultimately inducing a gustatory perception. Here we show that a sweet 'water-taste' is elicited when sweet-taste inhibitors are rinsed away. Responses of cultured cells expressing the human sweetener receptor directly parallel the psychophysical responses-water rinses remove the inhibitor from the heteromeric sweetener receptor TAS1R2-TAS1R3, which activates cells and results in the perception of strong sweetness from pure water. This 'rebound' activity occurs when equilibrium forces on the two-state allosteric sweet receptors result in their coordinated shift to the activated state upon being released from inhibition by rinsing.

Background The sweet taste inhibitor lactisole acts on the human sweet taste receptor heteromer TAS1R2-TAS1R3 but not on its rodent counterpart. Recently, it was shown that the lactisole sensitivity of the human sweet taste receptor involves the part of TAS1R3 encompassing the seven transmembrane regions but not the huge N-terminal domain. Using mutational analysis we investigated which amino acid residues distinguish lactisole insensitive rat from sensitive human T1R3 receptors. Results The functional analysis of specific receptor mutants in HEK293T cells revealed that the exchange of valine 738 in the fifth transmembrane domain of rTas1r3 by an alanine is sufficient to confer lactisole sensitivity to the rat sweet taste receptor. The sensitivity of this receptor mutant is ~2 fold lower than the sensitivity of the human sweet taste receptor. Additional substitution of lysine 735 by phenylalanine in rTas1r3 results in a rat sweet taste receptor that is as sensitive to lactisole as its human counterpart. The exchange of valine 738 to alanine was accompanied by a ~50% reduction in receptor efficacy. This effect was seen with all six different sweet compounds examined. Conclusion The lactisole insensitivity of rat sweet taste receptor is caused by only two amino acids in transmembrane region five, which is critical for the interaction of lactisole with the sweet taste receptor. The observation that the mutant receptor simultaneously displays a generally reduced sensitivity towards all agonists suggests that the lactisole insensitivity of the rodent receptor might be more likely caused by the inaccessibility of the lactisole binding site rather then by its direct disruption.

The recent advances in the functional expression of TAS2Rs in heterologous systems resulted in the identification of bitter tastants that specifically activate receptors of this family. All bitter taste receptors reported to date exhibit a pronounced selectivity for single substances or structurally related bitter compounds. In the present study we demonstrate the expression of the hTAS2R14 gene by RT-PCR analyses and in situ hybridisation in human circumvallate papillae. By functional expression in HEK-293T cells we show that hTAS2R14 displays a, so far, unique broad tuning towards a variety of structurally diverse bitter compounds, including the potent neurotoxins, (-)-alpha-thujone, the pharmacologically active component of absinthe, and picrotoxinin, a poisonous substance of fishberries. The observed activation of heterologously expressed hTAS2R14 by low concentrations of (-)-alpha-thujone and picrotoxinin suggests that the receptor is sufficiently sensitive to caution us against the ingestion of toxic amounts of these substances.

Bitter taste generally causes aversion, which protects humans from ingesting toxic substances. But bitter flavors also contribute to the palatability of food and beverages, thereby influencing nutritional habits in humans. Although many studies have examined bitter taste, the underlying receptor mechanisms remain poorly understood. Anatomical, functional and genetic data from rodents suggest the existence of a family of receptors that are responsive to bitter compounds. Here we report that a human member of this family, TAS2R16, is present in taste receptor cells on the tongue and is activated by bitter beta-glucopyranosides. Responses to these phytonutrients show a similar concentration dependence and desensitization in transfected cells and in experiments assessing taste perception in humans. Bitter compounds consisting of a hydrophobic residue attached to glucose by a beta-glycosidic bond activate TAS2R16. Thus, TAS2R16 links the recognition of a specific chemical structure to the perception of bitter taste. If the ability of TAS2R16 to detect substances with common molecular properties is typical of the bitter receptor family, it may explain how a few receptors permit the perception of numerous bitter substances.

Sour taste is initiated by protons acting at receptor proteins or channels. In vertebrates, transduction of this taste quality involves several parallel pathways1,2,3,4,5. Here we examine the effects of sour stimuli on taste cells in slices of vallate papilla from rat. From a subset of cells, we identified a hyperpolarization-activated current that was enhanced by sour stimulation at the taste pore. This current resembled Ih found in neurons and cardio-myocytes6,7, a current carried by members of the family of hyperpolarization-activated and cyclic-nucleotide-gated (HCN) channels8,9,10,11,12,13. We show by in situ hybridization and immunohistochemistry that HCN1 and HCN4 are expressed in a subset of taste cells. By contrast, gustducin, the G-protein involved in bitter and sweet taste14, is not expressed in these cells. Lowering extracellular pH causes a dose-dependent flattening of the activation curve of HCN channels and a shift in the voltage of half-maximal activation to more positive voltages. Our results indicate that HCN channels are gated by extracellular protons and may act as receptors for sour taste.

Bitter taste perception is innate and induces aversive reactions. Since numerous harmful compounds, including secondary plant metabolites, synthetic chemicals, inorganic ions and rancid fats, do taste bitter, this basic taste modality may be considered as a defence mechanism against the ingestion of potential poisons. For a complete understanding of this defence mechanism it is obligatory to identify and characterize the chemical detectors of the bitter compounds, which display the remarkable ability to recognize thousands of different chemicals. Screening of the genome data bases ultimately led to the discovery of a novel gene family of ∼40 members in mice and ∼30 in humans. Some of the genes identified by this approach are located within chromosomal loci associated with tasting various distinct bitter compounds (Adler et al., 2000; Matsunami et al., 2000; Bufe et al., 2002). Therefore, these genes encoding G-proteincoupled receptors, TAS2Rs (previously referred to as T2Rs or TRBs), have been suggested to represent bitter taste receptors. Several lines of independent evidence further support this assumption (Adler et al., 2000; Chandrashekar et al., 2000; Matsunami et al., 2000). First, the expression pattern of these genes on the rodent tongue was consistent with that of bitter taste receptors. Secondly, functional expression studies identified the bitter compound cycloheximide as an agonist for mTAS2R105. Thirdly, mice strains with impaired cycloheximide tasting have variant mTAS2R105 genes encoding receptors that are less responsive to cycloheximide. Fourthly, mTAS2R105 couples in vitro to α-gustducin (Chandrashekar et al., 2000), a G-protein α-subunit expressed in taste tissue that has amply been shown before to play a role in bitter taste transduction (Margolskee, 2002). Although this question has not been directly addressed, these investigations led to the impression of a narrow tuning of bitter taste receptors and raised the intriguing problem of how organisms that are equipped with a limited number of TAS2R genes are able to perceive numerous chemicals bitter. In the present report we address two questions that are important for the understanding of bitter taste in general and human bitter taste in particular. First, given the largely uncharacterized receptor repertoire, are all TAS2Rs true bitter taste receptors and, secondly, can their broad tuning explain how humans equipped with a fairly small number of TAS2R genes are able to perceive thousands of bitter compounds.

Cytotoxicity is an essential parameter to examine killer cell activity, to screen hits for new drugs, and to proof safety evaluation. Cytotoxicity assays are routinely used in basic research, clinical studies, and industrial applications. However, the most commonly applied standard end-point cytotoxicity assays largely suffer from the lack of kinetic information, which is indispensable to assess killing competence. Here we report a quantitative and highly sensitive calcein-based real-time high-throughput cytotoxicity assay on a single cell level and for population use. By determining fluorescence loss as a function of the fraction of lysed cells, this assay measures the kinetics of cytotoxicity over a broad dynamic range. The high time resolution of the assay also provides kinetic information with the potential to reveal neglected/immeasurable properties in the process of cytotoxicity of both purified human cytotoxic T lymphocytes and natural killer cells. To test its clinical potential, we have also applied this assay to screen the cytotoxic potential of (easy to prepare) peripheral blood mononuclear cells in healthy blood donors. In summary, this assay should substantially increase the screening efficiency for clinical and industrial purposes and offer quantitative kinetic data with a high time resolution.