Coupled Sensing of Hunger and Thirst Signals Balances Sugar and Water Consumption.

Hunger and thirst are ancient homeostatic drives for food and water consumption. Although molecular and neural mechanisms underlying these drives are currently being uncovered, less is known about how hunger and thirst interact. Here, we use molecular genetic, behavioral, and anatomical studies in Drosophila to identify four neurons that modulate food and water consumption. Activation of these neurons promotes sugar consumption and restricts water consumption, whereas inactivation promotes water consumption and restricts sugar consumption. By calcium imaging studies, we show that these neurons are directly regulated by a hormone signal of nutrient levels and by osmolality. Finally, we identify a hormone receptor and an osmolality-sensitive ion channel that underlie this regulation. Thus, a small population of neurons senses internal signals of nutrient and water availability to balance sugar and water consumption. Our results suggest an elegant mechanism by which interoceptive neurons oppositely regulate homeostatic drives to eat and drink.

C. elegans fat storage and metabolic regulation.

C. elegans has long been used as an experimentally tractable organism for discovery of fundamental mechanisms that underlie metazoan cellular function, development, neurobiology, and behavior. C. elegans has more recently been exploited to study the interplay of environment and genetics on lipid storage pathways. As an experimental platform, C. elegans is amenable to an extensive array of forward and reverse genetic, a variety of "omics" and anatomical approaches that together allow dissection of complex physiological pathways. This is particularly relevant to the study of fat biology, as energy balance is ultimately an organismal process that involves behavior, nutrient digestion, uptake and transport, as well as a variety of cellular activities that determine the balance between lipid storage and utilization. C. elegans offers the opportunity to dissect these pathways and various cellular and organismal homeostatic mechanisms in the context of a genetically tractable, intact organism.

Representations of Taste Modality in the Drosophila Brain.

Gustatory receptors and peripheral taste cells have been identified in flies and mammals, revealing that sensory cells are tuned to taste modality across species. How taste modalities are processed in higher brain centers to guide feeding decisions is unresolved. Here, we developed a large-scale calcium-imaging approach coupled with cell labeling to examine how different taste modalities are processed in the fly brain. These studies reveal that sweet, bitter, and water sensory cells activate different cell populations throughout the subesophageal zone, with most cells responding to a single taste modality. Pathways for sweet and bitter tastes are segregated from sensory input to motor output, and this segregation is maintained in higher brain areas, including regions implicated in learning and neuromodulation. Our work reveals independent processing of appetitive and aversive tastes, suggesting that flies and mammals use a similar coding strategy to ensure innate responses to salient compounds.

Analyses of C. elegans fat metabolic pathways.

In Caenorhabdatis elegans as in other animals, fat regulation reflects the outcome of behavioral, physiological, and metabolic processes. The amenability of C. elegans to experimentation has led to utilization of this organism for elucidating the complex homeostatic mechanisms that underlie energy balance in intact organisms. The optical advantages of C. elegans further offer the possibility of studying cell biological mechanisms of fat uptake, transport, storage, and utilization, perhaps in real time. Here, we discuss the rationale as well as advantages and potential pitfalls of methods used thus far to study metabolism and fat regulation, specifically triglyceride metabolism, in C. elegans. We provide detailed methods for visualization of fat depots in fixed animals using histochemical stains and in live animals by vital dyes. Protocols are provided and discussed for chloroform-based extraction of total lipids from C. elegans homogenates used to assess total triglyceride or phospholipid content by methods such as thin-layer chromatography or used to obtain fatty acid profiles by methods such as gas chromatography/mass spectrometry. Additionally, protocols are provided for the determination of rates of intestinal fatty acid uptake and fatty acid breakdown by ß-oxidation. Finally, we discuss methods for determining rates of de novo fat synthesis and Raman scattering approaches that have recently been employed to investigate C. elegans lipids without reliance on invasive techniques. As the C. elegans fat field is relatively new, we anticipate that the indicated methods will likely be improved upon and expanded as additional researchers enter this field.

Regulation of C. elegans fat uptake and storage by acyl-CoA synthase-3 is dependent on NR5A family nuclear hormone receptor nhr-25.

Acyl-CoA synthases are important for lipid synthesis and breakdown, generation of signaling molecules, and lipid modification of proteins, highlighting the challenge of understanding metabolic pathways within intact organisms. From a C. elegans mutagenesis screen, we found that loss of ACS-3, a long-chain acyl-CoA synthase, causes enhanced intestinal lipid uptake, de novo fat synthesis, and accumulation of enlarged, neutral lipid-rich intestinal depots. Here, we show that ACS-3 functions in seam cells, epidermal cells anatomically distinct from sites of fat uptake and storage, and that acs-3 mutant phenotypes require the nuclear hormone receptor NHR-25, a key regulator of C. elegans molting. Our findings suggest that ACS-3-derived long-chain fatty acyl-CoAs, perhaps incorporated into complex ligands such as phosphoinositides, modulate NHR-25 function, which in turn regulates an endocrine program of lipid uptake and synthesis. These results reveal a link between acyl-CoA synthase function and an NR5A family nuclear receptor in C. elegans.