Faculty Bibliography

A deficit in dietary protein elicits a nutrient-specific appetite, yet the underlying mechanisms remain poorly understood. In this work, we identify coordinated neuronal and systemic mechanisms in Drosophila that drive an essential amino acid (EAA)-specific appetite. EAA deprivation increases neuropeptide CNMamide (CNMa) expression in gut enterocytes, activating enteric neurons and ellipsoid body neurons in the brain to promote EAA intake through two complementary pathways: a rapid neuronal gut-brain axis and a slower hormonal route. CNMa suppresses the activity of sugar-sensing diuretic hormone 44 (DH44) neurons, thereby reducing carbohydrate intake and biasing feeding toward EAAs. Similarly, protein deprivation in mice promotes an EAA-specific appetite independently of fibroblast growth factor 21 (FGF21). Together, these findings reveal multilayered gut-brain mechanisms that regulate nutrient-specific feeding and maintain EAA homeostasis across species.

Animals need daily intakes of three macronutrients: sugar, protein, and fat. Under fasted conditions, however, animals prioritize sugar as a primary source of energy. They must detect ingested sugar-specifically D-glucose-and quickly report its presence to the brain. Hypothalamic neurons that can respond to the caloric content in the gut regardless of the identity of macronutrient have been identified, but until now, the existence of neurons that can encode the specific macronutrients remained unknown. We found that a subset of corticotropin-releasing factor (CRF)-expressing neurons in the hypothalamic paraventricular nucleus (CRF^PVN) respond specifically to D-glucose in the gut, separately from other macronutrients or sugars. CRF^PVN neuronal activity is essential for fasted mice to develop a preference for D-glucose. These responses of CRF^PVN neurons to intestinal D-glucose require a specific spinal gut-brain pathway including the dorsal lateral parabrachial nuclei. These findings reveal the neural circuit that encodes the identity of D-glucose.

A balanced intake of macronutrients-protein, carbohydrate and fat-is essential for the well-being of organisms. An adequate calorific intake but with insufficient protein consumption can lead to several ailments, including kwashiorkor¹. Taste receptors (T1R1-T1R3)² can detect amino acids in the environment, and cellular sensors (Gcn2 and Tor)³ monitor the levels of amino acids in the cell. When deprived of dietary protein, animals select a food source that contains a greater proportion of protein or essential amino acids (EAAs)⁴. This suggests that food selection is geared towards achieving the target amount of a particular macronutrient with assistance of the EAA-specific hunger-driven response, which is poorly understood. Here we show in Drosophila that a microbiome-gut-brain axis detects a deficit of EAAs and stimulates a compensatory appetite for EAAs. We found that the neuropeptide CNMamide (CNMa)⁵ was highly induced in enterocytes of the anterior midgut during protein deprivation. Silencing of the CNMa-CNMa receptor axis blocked the EAA-specific hunger-driven response in deprived flies. Furthermore, gnotobiotic flies bearing an EAA-producing symbiotic microbiome exhibited a reduced appetite for EAAs. By contrast, gnotobiotic flies with a mutant microbiome that did not produce leucine or other EAAs showed higher expression of CNMa and a greater compensatory appetite for EAAs. We propose that gut enterocytes sense the levels of diet- and microbiome-derived EAAs and communicate the EAA-deprived condition to the brain through CNMa.

Nutrient sensors allow animals to identify foods rich in specific nutrients. The Drosophila nutrient sensor, diuretic hormone 44 (DH44) neurons, helps the fly to detect nutritive sugar. This sensor becomes operational during starvation; however, the mechanisms by which DH44 neurons or other nutrient sensors are regulated remain unclear. Here, we identified two satiety signals that inhibit DH44 neurons: (1) Piezo-mediated stomach/crop stretch after food ingestion and (2) Neuromedin/Hugin neurosecretory neurons in the ventral nerve cord (VNC) activated by an increase in the internal glucose level. A subset of Piezo⁺ neurons that express DH44 neuropeptide project to the crop. We found that DH44 neuronal activity and food intake were stimulated following a knockdown of piezo in DH44 neurons or silencing of Hugin neurons in the VNC, even in fed flies. Together, we propose that these two qualitatively distinct peripheral signals work in concert to regulate the DH44 nutrient sensor during the fed state.

The gastrointestinal tract in the adult Drosophila serves as a model system for exploring the mechanisms underlying digestion, absorption and excretion, stem cell plasticity, and inter-organ communication, particularly through the gut-brain axis. It is also useful for studying the cellular and adaptive responses to dietary changes, alterations in microbiota and immunity, and systematic and endocrine signals. Despite the various cell types and distinct regions in the gastrointestinal tract, few tools are available to target and manipulate the activity of each cell type and region, and their gene expression. Here, we report 353 GAL4 lines and several split-GAL4 lines that are expressed in enteric neurons (ENs), progenitors (ISCs and EBs), enterocytes (ECs), enteroendocrine cells (EEs), or/and other cell types that are yet to be identified in distinct regions of the gut. We had initially collected approximately 600 GAL4 lines that may be expressed in the gut based on RNA sequencing data, and then crossed them to UAS-GFP to perform immunohistochemistry to identify those that are expressed selectively in the gut. The cell types and regional expression patterns that are associated with the entire set of GAL4 drivers and split-GAL4 combinations are annotated online at http://kdrc.kr/index.php (K-Gut Project). This GAL4 resource can be used to target specific populations of distinct cell types in the fly gut, and therefore, should permit a more precise investigation of gut cells that regulate important biological processes.

Olfactory receptor neurons (ORNs) transform scant chemical inputs into significant neural signals. This transformation requires signal amplification. In this issue of Neuron, Ng et al. (2019) identified a mechanism by which the signals evoked by pheromones are amplified in the ORNs that selectively promote courtship behavior in Drosophila.

Although glucose-sensing neurons were identified more than 50 years ago, the physiological role of glucose sensing in metazoans remains unclear. Here we identify a pair of glucose-sensing neurons with bifurcated axons in the brain of Drosophila. One axon branch projects to insulin-producing cells to trigger the release of Drosophila insulin-like peptide 2 (dilp2) and the other extends to adipokinetic hormone (AKH)-producing cells to inhibit secretion of AKH, the fly analogue of glucagon. These axonal branches undergo synaptic remodelling in response to changes in their internal energy status. Silencing of these glucose-sensing neurons largely disabled the response of insulin-producing cells to glucose and dilp2 secretion, disinhibited AKH secretion in corpora cardiaca and caused hyperglycaemia, a hallmark feature of diabetes mellitus. We propose that these glucose-sensing neurons maintain glucose homeostasis by promoting the secretion of dilp2 and suppressing the release of AKH when haemolymph glucose levels are high.

Corticotropin-releasing factor (CRF) that is released from the paraventricular nucleus (PVN) of the hypothalamus is essential for mediating stress response by activating the hypothalamic-pituitary-adrenal axis. CRF-releasing PVN neurons receive inputs from multiple brain regions that convey stressful events, but their neuronal dynamics on the timescale of behavior remain unknown. Here, our recordings of PVN CRF neuronal activity in freely behaving mice revealed that CRF neurons are activated immediately by a range of aversive stimuli. By contrast, CRF neuronal activity starts to drop within a second of exposure to appetitive stimuli. Optogenetic activation or inhibition of PVN CRF neurons was sufficient to induce a conditioned place aversion or preference, respectively. Furthermore, conditioned place aversion or preference induced by natural stimuli was significantly decreased by manipulating PVN CRF neuronal activity. Together, these findings suggest that the rapid, biphasic responses of PVN CRF neurons encode the positive and negative valences of stimuli.

Sweet-insensitiveDrosophilamutants are unable to readily identify sugar. In presence of wild-type (WT) flies, however, these mutant flies demonstrated a marked increase in their preference for nutritive sugar. Real-time recordings of starved WT flies revealed that these flies discharge a drop from their gut end after consuming nutritive sugars, but not nonnutritive sugars. We proposed that the drop may contain a molecule(s) named calorie-induced secreted factor (CIF), which serves as a signal to inform other flies about its nutritional value. Consistent with this, we observed a robust preference of flies for nutritive sugar containing CIF over nutritive sugar without CIF. Feeding appears to be a prerequisite for the release of CIF, given that fed flies did not produce it. Additionally, correlation analyses and pharmacological approaches suggest that the nutritional value, rather than the taste, of the consumed sugar correlates strongly with the amount (or intensity) of the released CIF. We observed that the release of this attractant signal requires the consumption of macronutrients, specifically nutritive sugars and l-enantiomer essential amino acids (l-eAAs), but it is negligibly released when flies are fed nonnutritive sugars, unnatural d-enantiomer essential amino acids (d-eAAs), fatty acids, alcohol, or salts. Finally, CIF (i) is not detected by the olfactory system, (ii) is not influenced by the sex of the fly, and (iii) is not limited to one species ofDrosophila.