Greg S. Suh - US grants

DESCRIPTION (provided by applicant): How specific sensory stimuli evoke specific behaviors is a fundamental problem in neurobiology. Most odorants elicit attraction or avoidance depending on their concentrations and identity, as well as the nature of the neural circuits they activate. Such odorants, moreover, typically activate combinations of olfactory sensory neurons (OSNs), complicating the dissection of the circuits translating odor recognition into behavior. Carbon dioxide (CO2), in contrast, elicits avoidance over a wide range of concentrations in the fly, Drosophila melanogaster, and activates only two populations of OSNs when examined by a sensitive, in vivo calcium imaging technique. Previous studies showed that OSNs expressing GR63a &GR21a receptors, the first CO2 olfactory neurons identified, is essential for avoidance to low concentrations of CO2, but it remained unclear the function of the other neurons activated by CO2. Here, we propose to determine that the putative 2nd CO2 OSNs and its cognate receptor that belongs to a member of the recently identified Ionotropic Glutamate Receptors (IRs) family are necessary and sufficient for detection of and avoidance to high CO2 concentrations and similar odorants such as acids. To address these questions, we will perform in vivo calcium imaging and behavioral assays. Determining subcellular localization of the 2nd CO2 receptor will predict whether or not the receptor directly interacts with odorants. To better understand central circuits mediating avoidance behavior, we will trace its projections into higher brain centers and compare them to the 1st CO2 pathway whether these two pathways converge upon a same target neuron in a higher brain center such as the lateral horn. Because CO2 and acids released by warm-blooded hosts are essential olfactory cues for the mosquito, and a homolog of the 2nd CO2 receptor is expressed in the mosquito antenna, we plan to examine whether the mosquito OSNs expressing the homolog are activated by CO2 and acids. PUBLIC HEALTH RELEVANCE: Relevance Insects transmit diseases to humans and animals including livestock, and cause serious threats to health and enormous losses to agricultural output. Many insects respond to their human and animal hosts primarily through carbon dioxide (CO2) and lactic acid, key olfactory cues emanating from mammals. These olfactory cues activate defined populations of olfactory sensory neurons in the mosquito that express the same odorant receptors as in Drosophila. Understanding how the Drosophila sensory receptors are activated by CO2 and acids, and the mechanism by which their neural circuits trigger behavioral responses to these stimuli would help us develop better strategies to prevent transmission of insect-born diseases by mosquitoes, tsetse flies, and other pathogenic insects.

Project Description Feeding behavior is influenced by multiple factors including food palatability and nutritional needs. Peripheral chemosensory taste neurons primarily detect palatable food, but animals lacking these taste neurons can still develop a preference to sugars on the basis of their nutritional value. My laboratory previously determined that dSLC5A11+, EB R4d neurons in the Drosophila brain are required for the selection of nutritive D- glucose over nonnutritive L-glucose after periods of starvation. dSLC5A11 (or cupcake) acts on approximately 12 pairs of EB R4d neurons to trigger the selection of nutritive sugars, but the mechanism underlying this process is not understood. We previously proposed two possible mechanisms by which EB R4 neurons may mediate the selection of nutritive sugars: (1) by detecting the nutritional value of sugar through direct activation or (2) monitoring the internal energy reserves of the fly with a direct nutrient sensor located elsewhere; starved flies lacking functional EB R4d neurons cannot sense the deprived metabolic state and, thus, would not select nutritive sugars. Through calcium imaging and a more-sensitive electrophysiology approach, we tested whether EB R4d neurons respond to nutritive sugars, but failed to observe any responses to glucose or any other sugars. Instead, the activity of EB R4d neurons and the expression of dSLC5A11 transcript increase significantly following periods of starvation. Furthermore, the increased dSLC5A11 suppresses dKCNQ currents, thereby increasing the activity of EB R4d neurons during starvation. Recently, we found that EB R4d neurons are robustly activated by serotonin, which is apparently secreted by the neurons labeled by R50H05-GAL4. These R50H05+ neurons were shown to promote food intake, similar to EB R4d neurons. We also found that another population of the EB Ring neurons, apparently the neighboring EB R4m neurons, suppress food intake. In this renewal application, we propose to study the following questions based on our published and preliminary results. In Aim 1, we will determine the mechanisms by which EB R4d neurons are activated. We will further elucidate the dSLC5A11/dKCNQ- mediated mechanism, but also investigate the dSLC5A11?independent mechanism in which serotonin plays critical roles in stimulating the activity of EB R4d neurons and the expression of dSLC5A11 transcript. In Aim 2, we will determine whether EB R4d neurons function downstream of R50H05+ neurons. In Aim 3, we will validate that the recently identified EB R4m neurons suppress food intake and characterize the interactions between EB R4m and EB R4d neurons, and between EB R4d or EB R4m neurons, and the sleep-promoting EB R2 neurons.

? DESCRIPTION (provided by applicant): Sugar in the natural environment can be detected through taste-independent and taste-dependent modalities. Taste-independent modalities consist mainly of peripheral chemosensory neurons such as sweet taste receptors, which primarily detect the orosensory value of sugar (i.e. sweetness). My laboratory and others have shown that there exist taste-independent, internal sensors that detect the nutritional value of sugar in Drosophila and rodents. In this proposal, we will test a hypothesis that six neurons in the fly brain that produce diuretic hormone (Dh44), a homologue of the mammalian corticotropin-releasing hormone (CRH), directly detects the nutritional content of sugar in a fast time scale. Our preliminary results indicate that DH44 neurons are required for the selection of nutritive sugars in the two-choice assay and are activated by nutritive D-glucose, but not by nonnutritive L-glucose in calcium imaging experiment. Furthermore, we made a surprising observation that artificial activation of DH44 pathway resulted in rapid extensions of the mouthpart, and frequent episodes of excretion. These actions would facilitate the ingestion and digestion of nutritive foods. In the proposal, we will also determine the mechanism by which the activation of DH44 pathway leads to a rapid increase of ingestion and digestion through a positive feedback loop to continue consumption of nutritive foods. Identification and characterization of the taste-independent sugar sensor in Drosophila would provide a framework to understand how appetite is regulated by energy need in normal and eating disorder patients. Given its strong sequence homology, CRH and its neurons in the hypothalamus may offer similar functions in mammals. RELEVANCE: The proposed study to investigate the function of six neurons in the Drosophila brain that produce diuretic hormone (Dh44), a homologue of the mammalian corticotropin-releasing hormone (CRH), suggests a hypothesis that CRH neurons in the hypothalamus function as internal sensors that detect the nutritional value of sugar. Indeed, CRH was shown to play a significant role in the regulation of feeding and food intake, but the exact nature of it role is controversial. The homology between Drosophila DH44 and mammalian CRH is approximately 30% and between Drosophila and mammalian receptors is approximately 40%. Similar to Drosophila DH44, mammalian CRH regulates gastric and colonic movements, and stimulates defecation in rodents. CRH also mediates glucose homeostasis by regulating hypoglycemia-induced counterregulation. It was suggested that the function of glucose-sensing neurons is to generate neuroendocrine stress responses to the hypoglycemic challenge, but the identity of these neurons is unknown. It is possible that CRH neurons in the hypothalamus are glucose-sensing neurons as in the Drosophila brain and are capable of mediating starvation-induced behavioral responses to the nutritional value of sugar in mammals. Mice with compromised CRH function are indeed obese or anorexic. It is intriguing to speculate that obese individuals may be insensitive to the heightened circulating sugar levels after meals and therefore, continues to eat. By contrast, anorexic individuals may be too sensitive to circulating sugar levels and therefore, feel satiated and are reluctant to eat. The proposed research using the Drosophila model would provide a foundation for understanding the mechanisms by which internal sensors respond to the nutritional value of sugar in normal and obese individuals.