Diverse roles for the Drosophila fructose sensor Gr43a

The detection of nutrients, both in food and within the body, is crucial for the regulation of feeding behavior, growth, and metabolism. While the molecular basis for sensing food chemicals by the taste system has been firmly linked to specific taste receptors, relatively little is known about the molecular nature of the sensors that monitor nutrients internally. Recent reports of taste receptors expressed in other organ systems, foremost in the gastrointestinal tract of mammals and insects, has led to the proposition that some taste receptors may also be used as sensors of internal nutrients. Indeed, we provided direct evidence that the Drosophila gustatory receptor 43a (Gr43a) plays a critical role in sensing internal fructose levels in the fly brain. In addition to the brain and the taste system, Gr43a is also expressed in neurons of the proventricular ganglion and the uterus. Here, we discuss the multiple potential roles of Gr43a in the fly. We also provide evidence that its activation in the brain is likely mediated by the neuropeptide Corazonin. Finally, we posit that Gr43a may represent only a precedent for other taste receptors that sense internal nutrients, not only in flies but, quite possibly, in other animals, including mammals.     

Spatially restricted expression of candidate taste receptors in the Drosophila gustatory system

BACKGROUND: Taste is an important sensory modality in most animals. In Drosophila, taste is perceived by gustatory neurons located in sensilla distributed on several different appendages throughout the body of the animal. Here we show that the gustatory receptors are encoded by a family of at least 54 genes (Gr genes), most of which are expressed exclusively in a small subset of taste sensilla located in narrowly defined regions of the fly's body. RESULTS: BLAST searches with the predicted amino acid sequences of 6 7-transmembrane-receptor genes of unknown function and 20 previously identified, putative gustatory receptor genes led to the identification of a large gene family comprising at least 54 genes. We investigated the expression of eight genes by using a Gal4 reporter gene assay and found that five of them were expressed in the gustatory system of the fly. Four genes were expressed in 1%-4% of taste sensilla, located in well-defined regions of the proboscis, the legs, or both. The fifth gene was expressed in about 20% of taste sensilla in all major gustatory organs, including the taste bristles on the anterior wing margin. Axon-tracing experiments demonstrated that neurons expressing a given Gr gene project their axons to a spatially restricted domain of the subesophageal ganglion in the fly brain. CONCLUSIONS: Our findings suggest that each taste sensillum represents a discrete, functional unit expressing at least one Gr receptor and that most Gr genes are expressed in spatially restricted domains of the gustatory system. These observations imply the potential for high taste discrimination of the Drosophila brain.     

Sugar receptors in Drosophila

The detection and discrimination of chemical compounds in potential foods are essential sensory processes when animals feed. The fruit fly Drosophila melanogaster employs 68 different gustatory receptors (GRs) for the detection of mostly nonvolatile chemicals that include sugars, a diverse group of toxic compounds present in many inedible plants and spoiled foods, and pheromones [1-6]. With the exception of a trehalose (GR5a) and a caffeine (GR66a) receptor [7-9], the functions of GRs involved in feeding are unknown. Here, we show that the Gr64 genes encode receptors for numerous sugars. We generated a fly strain that contained a deletion for all six Gr64 genes (DeltaGr64) and showed that these flies exhibit no or a significantly diminished proboscis extension reflex (PER) response when stimulated with glucose, maltose, sucrose, and several other sugars. The only considerable response was detected when Gr64 mutant flies were stimulated with fructose. Interestingly, response to trehalose is also abolished in these flies, even though they contain a functional Gr5a gene, which has been previously shown to encode a receptor for this sugar [8, 9]. This observation indicates that two or more Gr genes are necessary for trehalose detection, suggesting that GRs function as multimeric receptor complexes. Finally, we present evidence that some members of the Gr64 gene family are transcribed as a polycistronic mRNA, providing a mechanism for the coexpression of multiple sugar receptors in the same taste neurons.     

Taste perception and coding in Drosophila

BACKGROUND: Discrimination between edible and contaminated foods is crucial for the survival of animals. In Drosophila, a family of gustatory receptors (GRs) expressed in taste neurons is thought to mediate the recognition of sugars and bitter compounds, thereby controlling feeding behavior. RESULTS: We have characterized in detail the expression of eight Gr genes in the labial palps, the fly's main taste organ. These genes fall into two distinct groups: seven of them, including Gr66a, are expressed in 22 or fewer taste neurons in each labial palp. Additional experiments show that many of these genes are coexpressed in partially overlapping sets of neurons. In contrast, Gr5a, which encodes a receptor for trehalose, is expressed in a distinct and larger set of taste neurons associated with most chemosensory sensilla, including taste pegs. Mapping the axonal targets of cells expressing Gr66a and Gr5a reveals distinct projection patterns for these two groups of neurons in the brain. Moreover, tetanus toxin-mediated inactivation of Gr66a- or Gr5a-expressing cells shows that these two sets of neurons mediate distinct taste modalities-the perception of bitter (caffeine) and sweet (trehalose) taste, respectively. CONCLUSION: Discrimination between two taste modalities-sweet and bitter-requires specific sets of gustatory receptor neurons that express different Gr genes. Unlike the Drosophila olfactory system, where each neuron expresses a single olfactory receptor gene, taste neurons can express multiple receptors and do so in a complex Gr gene code that is unique for small sets of neurons.     

Hedonic taste in Drosophila revealed by olfactory receptors expressed in taste neurons

Taste and olfaction are each tuned to a unique set of chemicals in the outside world, and their corresponding sensory spaces are mapped in different areas in the brain. This dichotomy matches categories of receptors detecting molecules either in the gaseous or in the liquid phase in terrestrial animals. However, in Drosophila olfactory and gustatory neurons express receptors which belong to the same family of 7-transmembrane domain proteins. Striking overlaps exist in their sequence structure and in their expression pattern, suggesting that there might be some functional commonalities between them. In this work, we tested the assumption that Drosophila olfactory receptor proteins are compatible with taste neurons by ectopically expressing an olfactory receptor (OR22a and OR83b) for which ligands are known. Using electrophysiological recordings, we show that the transformed taste neurons are excited by odor ligands as by their cognate tastants. The wiring of these neurons to the brain seems unchanged and no additional connections to the antennal lobe were detected. The odor ligands detected by the olfactory receptor acquire a new hedonic value, inducing appetitive or aversive behaviors depending on the categories of taste neurons in which they are expressed i.e. sugar- or bitter-sensing cells expressing either Gr5a or Gr66a receptors. Taste neurons expressing ectopic olfactory receptors can sense odors at close range either in the aerial phase or by contact, in a lipophilic phase. The responses of the transformed taste neurons to the odorant are similar to those obtained with tastants. The hedonic value attributed to tastants is directly linked to the taste neurons in which their receptors are expressed.     

Taste representations in the Drosophila brain

Drosophila taste compounds with gustatory neurons on many parts of the body, suggesting that a fly detects both the location and quality of a food source. For example, activation of taste neurons on the legs causes proboscis extension or retraction, whereas activation of proboscis taste neurons causes food ingestion or rejection. We examined whether the features of taste location and taste quality are mapped in the fly brain using molecular, genetic, and behavioral approaches. We find that projections are segregated by the category of tastes that they recognize: neurons that recognize sugars project to a region different from those recognizing noxious substances. Transgenic axon labeling experiments also demonstrate that gustatory projections are segregated based on their location in the periphery. These studies reveal the gustatory map in the first relay of the fly brain and demonstrate that taste quality and position are represented in anatomical projection patterns.     

Two Gr genes underlie sugar reception in Drosophila

We have analyzed the molecular basis of sugar reception in Drosophila. We define the response spectrum, concentration dependence, and temporal dynamics of sugar-sensing neurons. Using in situ hybridization and reporter gene expression, we identify members of the Gr5a-related taste receptor subfamily that are coexpressed in sugar neurons. Neurons expressing reporters of different Gr5a-related genes send overlapping but distinct projections to the brain and thoracic ganglia. Genetic analysis of receptor genes shows that Gr5a is required for response to one subset of sugars and Gr64a for response to a complementary subset. A Gr5a;Gr64a double mutant shows no physiological or behavioral responses to any tested sugar. The simplest interpretation of our results is that Gr5a and Gr64a are each capable of functioning independently of each other within individual sugar neurons and that they are the primary receptors used in the labellum to detect sugars.     

Review: Chemosensing of nutrients and non-nutrients in the human and porcine gastrointestinal tract

The gastrointestinal tract (GIT) is an interface between the external and internal milieus that requires continuous monitoring for nutrients or pathogens and toxic chemicals. The study of the physiological/molecular mechanisms, mediating the responses to the monitoring of the GIT contents, has been referred to as chemosensory science. While most of the progress in this area of research has been obtained in laboratory rodents and humans, significant steps forward have also been reported in pigs. The objective of this review was to update the current knowledge on nutrient chemosensing in pigs in light of recent advances in humans and laboratory rodents. A second objective relates to informing the existence of nutrient sensors with their functionality, particularly linked to the gut peptides relevant to the onset/offset of appetite. Several cell types of the intestinal epithelium such as Paneth, goblet, tuft and enteroendocrine cells (EECs) contain subsets of chemosensory receptors also found on the tongue as part of the taste system. In particular, EECs show specific co-expression patterns between nutrient sensors and/or transceptors (transport proteins with sensing functions) and anorexigenic hormones such as cholecystokinin (CCK), peptide tyrosine tyrosine (PYY) or glucagon-like peptide-1 (GLP-1), amongst others. In addition, the administration of bitter compounds has an inhibitory effect on GIT motility and on appetite through GLP-1-, CCK-, ghrelin- and PYY-labelled EECs in the human small intestine and colon. Furthermore, the mammalian chemosensory system is the target of some bacterial metabolites. Recent studies on the human microbiome have discovered that commensal bacteria have developed strategies to stimulate chemosensory receptors and trigger host cellular functions. Finally, the study of gene polymorphisms related to nutrient sensors explains differences in food choices, food intake and appetite between individuals.     

Gastrointestinal chemosensation: chemosensory cells in the alimentary tract

Sensing potentially beneficial or harmful constituents in the luminal content by specialized cells in the gastrointestinal mucosa is an essential prerequisite for governing digestive processes, initiating protective responses and regulating food intake. Until recently, it was poorly understood how the gastrointestinal tract senses and responds to nutrients and non-nutrients in the diet; however, the enormous progress in unraveling the molecular machinery underlying the responsiveness of gustatory cells in the lingual taste buds to these compounds has been an important starting point for studying intestinal chemosensation. Currently, the field of nutrient sensing in the gastrointestinal tract is evolving rapidly and is benefiting from the deorphanization of previously unliganded G-protein-coupled receptors which respond to important nutrients, such as protein degradation products and free fatty acids as well as from the FACS-assisted isolation of distinct cell populations. This review focuses on mechanisms and principles underlying the chemosensory responsiveness of the alimentary tract. It describes the cell types which might potentially contribute to chemosensation within the gut: cells that can operate as specialized sensors and transducers for luminal factors and which communicate information from the gut lumen by releasing paracrine or endocrine acting messenger molecules. Furthermore, it addresses the current knowledge regarding the expression and localization of molecular elements that may be part of the chemosensory machinery which render some of the mucosal cells responsive to constituents of the luminal content, concentrating on candidate receptors and transporters for sensing nutrients.     

Mechanisms of Carboxylic Acid Attraction in Drosophila melanogaster

Sour is one of the fundamental taste modalities that enable taste perception in animals. Chemoreceptors embedded in taste organs are pivotal to discriminate between different chemicals to ensure survival. Animals generally prefer slightly acidic food and avoid highly acidic alternatives. We recently proposed that all acids are aversive at high concentrations, a response that is mediated by low pH as well as specific anions in Drosophila melanogaster. Particularly, some carboxylic acids such as glycolic acid, citric acid, and lactic acid are highly attractive to Drosophila compared with acetic acid. The present study determined that attractive carboxylic acids were mediated by broadly expressed Ir25a and Ir76b, as demonstrated by a candidate mutant library screen. The mutant deficits were completely recovered via wild-type cDNA expression in sweet-sensing gustatory receptor neurons. Furthermore, sweet gustatory receptors such as Gr5a, Gr61a, and Gr64a-f modulate attractive responses. These genetic defects were confirmed using binary food choice assays as well as electrophysiology in the labellum. Taken together, our findings demonstrate that at least two different kinds of receptors are required to discriminate attractive carboxylic acids from other acids.     

Drosophila gustatory receptors: from gene identification to functional expression

Recent years have seen long-awaited progress in understanding of the molecular mechanisms of taste perception in insects. The breakthrough came in the early 2000 with the identification of a novel family of candidate gustatory receptor (Gr) genes in the first release of the Drosophila melanogaster genome sequence. The 60 Gr genes are expressed in the subsets of gustatory neurons in the fly's taste organs and, without exception, encode heptahelical G protein-coupled receptors (GPCRs). Here I review our current knowledge about Gr genes and their products focusing on the newly emerging information regarding the function of the Gr-encoded proteins.     

The Gr64 cluster of gustatory receptors promotes survival and proteostasis of epithelial cells in Drosophila

Gustatory Receptor 64 (Gr64) genes are a cluster of 6 neuronally expressed receptors involved in sweet taste sensation in Drosophila melanogaster. Gr64s modulate calcium signalling and excitatory responses to several different sugars. Here, we discover an unexpected nonneuronal function of Gr64 receptors and show that they promote proteostasis in epithelial cells affected by proteotoxic stress. Using heterozygous mutations in ribosome proteins (Rp), which have recently been shown to induce proteotoxic stress and protein aggregates in cells, we show that Rp/+ cells in Drosophila imaginal discs up-regulate expression of the entire Gr64 cluster and depend on these receptors for survival. We further show that loss of Gr64 in Rp/+ cells exacerbates stress pathway activation and proteotoxic stress by negatively affecting autophagy and proteasome function. This work identifies a noncanonical role in proteostasis maintenance for a family of gustatory receptors known for their function in neuronal sensation.

GR64 genes are a cluster of neuronally expressed gustatory receptors normally involved in taste sensation in Drosophila melanogaster. This study reveals a surprising role for these receptors in regulating proteostasis and cell survival in epithelial cells exposed to proteotoxic stress.     

Molecular receptors of taste agents

All representatives of higher eukaryotes can probably differentially perceive nutrients and poisonous substances. Molecular mechanisms of transduction of taste information have been best studied for mammals and for the fruit fly Drosophila. Here, we consider receptor mechanisms and conjugated primary signal processes of stimulation of taste receptor cells by stimuli of various taste modalities.     

Imaging taste responses in the fly brain reveals a functional map of taste category and behavior

The sense of taste allows animals to distinguish nutritious and toxic substances and elicits food acceptance or avoidance behaviors. In Drosophila, taste cells that contain the Gr5a receptor are necessary for acceptance behavior, and cells with the Gr66a receptor are necessary for avoidance. To determine the cellular substrates of taste behaviors, we monitored taste cell activity in vivo with the genetically encoded calcium indicator G-CaMP. These studies reveal that Gr5a cells selectively respond to sugars and Gr66a cells to bitter compounds. Flies are attracted to sugars and avoid bitter substances, suggesting that Gr5a cell activity is sufficient to mediate acceptance behavior and that Gr66a cell activation mediates avoidance. As a direct test of this hypothesis, we inducibly activated different taste neurons by expression of an exogenous ligand-gated ion channel and found that cellular activity is sufficient to drive taste behaviors. These studies demonstrate that taste cells are tuned by taste category and are hardwired to taste behaviors.     

A Drosophila Gustatory Receptor Essential for Aversive Taste and Inhibiting Male-to-Male Courtship

Contact chemosensation is required for several behaviors that promote insect survival. These include evasive behaviors such as suppression of feeding on repellent compounds, known as antifeedants, and inhibition of male-to-male courtship. However, the gustatory receptors (GRs) required for responding to nonvolatile avoidance chemicals are largely unknown. Exceptions include Drosophila GR66a and GR93a, which are required to prevent ingestion of caffeine [1] and [2], and GR32a, which is necessary for inhibiting male-to-male courtship [3]. However, GR32a is dispensable for normal taste. Thus, distinct GRs may function in sensing avoidance pheromones and antifeedants. Here, we describe the requirements for GR33a, which is expressed widely in gustatory receptor neurons (GRNs) that respond to aversive chemicals. Gr33a mutant flies were impaired in avoiding all nonvolatile repellents tested, ranging from quinine to denatonium, lobeline, and caffeine. Gr33a mutant males also displayed increased male-to-male courtship, implying that it functioned in the detection of a repulsive male pheromone. In contrast to the broadly required olfactory receptor (OR) OR83b, which is essential for trafficking other ORs [4], GR66a and GR93a are localized normally in Gr33a mutant GRNs. Thus, rather than regulating GR trafficking, GR33a may be a coreceptor required for sensing all nonvolatile repulsive chemicals, including tastants and pheromones.     

Drosophila Fatty Acid Taste Signals through the PLC Pathway in Sugar-Sensing Neurons

Taste is the primary sensory system for detecting food quality and palatability. Drosophila detects five distinct taste modalities that include sweet, bitter, salt, water, and the taste of carbonation. Of these, sweet-sensing neurons appear to have utility for the detection of nutritionally rich food while bitter-sensing neurons signal toxicity and confer repulsion. Growing evidence in mammals suggests that taste for fatty acids (FAs) signals the presence of dietary lipids and promotes feeding. While flies appear to be attracted to fatty acids, the neural basis for fatty acid detection and attraction are unclear. Here, we demonstrate that a range of FAs are detected by the fly gustatory system and elicit a robust feeding response. Flies lacking olfactory organs respond robustly to FAs, confirming that FA attraction is mediated through the gustatory system. Furthermore, flies detect FAs independent of pH, suggesting the molecular basis for FA taste is not due to acidity. We show that low and medium concentrations of FAs serve as an appetitive signal and they are detected exclusively through the same subset of neurons that sense appetitive sweet substances, including most sugars. In mammals, taste perception of sweet and bitter substances is dependent on phospholipase C (PLC) signaling in specialized taste buds. We find that flies mutant for norpA, a Drosophila ortholog of PLC, fail to respond to FAs. Intriguingly, norpA mutants respond normally to other tastants, including sucrose and yeast. The defect of norpA mutants can be rescued by selectively restoring norpA expression in sweet-sensing neurons, corroborating that FAs signal through sweet-sensing neurons, and suggesting PLC signaling in the gustatory system is specifically involved in FA taste. Taken together, these findings reveal that PLC function in Drosophila sweet-sensing neurons is a conserved molecular signaling pathway that confers attraction to fatty acids.     

The calcium-sensing receptor in taste tissue

Calcium is an essential nutrient that induces a distinctive taste quality, but the sensing mechanism of calcium in the tongue is poorly understood. A recent study linked calcium to T1R3 receptor. Here, we propose another system for calcium taste involving the extracellular calcium-sensing receptor (CaSR). This G protein-coupled receptor that responds to calcium and magnesium cations is involved in calcium homeostasis regulating parathyroid and kidney functions. In this study, CaSR was found in isolated taste buds from rats and mice. It was expressed in a subset of cells in circumvallate and foliate papillae, with fewer cells in the fungiform papillae. This is the first evidence in mammals that locates CaSR in gustatory tissue and provides the basis for better understanding not only calcium taste but also the taste of multiple CaSR agonists.