A simple high throughput assay to evaluate water consumption in the fruit fly

Future avenues in Drosophila mushroom body research

How does the brain translate sensory information into complex behaviors? With relatively small neuronal numbers, readable behavioral outputs, and an unparalleled genetic toolkit, the Drosophila mushroom body (MB) offers an excellent model to address this question in the context of associative learning and memory. Recent technological breakthroughs, such as the freshly completed full-brain connectome, multiomics approaches, CRISPR-mediated gene editing, and machine learning techniques, led to major advancements in our understanding of the MB circuit at the molecular, structural, physiological, and functional levels. Despite significant progress in individual MB areas, the field still faces the fundamental challenge of resolving how these different levels combine and interact to ultimately control the behavior of an individual fly. In this review, we discuss various aspects of MB research, with a focus on the current knowledge gaps, and an outlook on the future methodological developments required to reach an overall view of the neurobiological basis of learning and memory.

Dehydrated Drosophila melanogaster track a water plume in tethered flight

Perception of sensory stimuli can be modulated by changes in internal state to drive contextually appropriate behavior. For example, dehydration is a threat to terrestrial animals, especially to Drosophila melanogaster due to their large surface area to volume ratio, particularly under the energy demands of flight. While hydrated D. melanogaster avoid water cues, while walking, dehydration leads to water-seeking behavior. We show that in tethered flight, hydrated flies ignore a water stimulus, whereas dehydrated flies track a water plume. Antennal occlusions eliminate odor and water plume tracking, whereas inactivation of moist sensing neurons in the antennae disrupts water tracking only upon starvation and dehydration. Elimination of the olfactory coreceptor eradicates odor tracking while leaving water-seeking behavior intact in dehydrated flies. Our results suggest that while similar hygrosensory receptors may be used for walking and in-flight hygrotaxis, the temporal dynamics of modulating the perception of water vary with behavioral state.

Transcriptional and toxic responses to saxitoxin exposure in the marine copepod Tigriopus japonicus

Saxitoxin (STX) is a highly toxic marine neurotoxin produced by phytoplankton and a growing threat to ecosystems worldwide due to the spread of toxic algae. Although STX is an established sodium channel blocker, the overall profile of transcriptional levels in STX-exposed organisms has yet to be described. Here, we describe a toxicity assay and transcriptome analysis of the copepod Tigriopus japonicus exposed to STX. The half-maximal lethal concentration of STX was 12.35 μM, and a rapid mortality slope was evident at concentrations between 12 and 13 μM. STX induced changes in swimming behavior among the copepods after 10 min of exposure. In transcriptome analysis, gene ontology revealed that the genes involved in nervous system and gene expression were highly enriched. In addition, the congenital neurological disorder and nuclear factor erythroid 2-related factor 2-mediated oxidative stress pathways were identified to be the most significant in network analysis and toxicity pathway analysis, respectively. This study provides valuable information about the effects of STX and related transcriptional responses in T. japonicus.

Thirst interneurons that promote water seeking and limit feeding behavior in Drosophila

Thirst is a motivational state that drives behaviors to obtain water for fluid homeostasis. We identified two types of central brain interneurons that regulate thirsty water seeking in Drosophila, that we term the Janu neurons. Janu-GABA, a local interneuron in the subesophageal zone, is activated by water deprivation and is specific to thirsty seeking. Janu-AstA projects from the subesophageal zone to the superior medial protocerebrum, a higher order processing area. Janu-AstA signals with the neuropeptide Allatostatin A to promote water seeking and to inhibit feeding behavior. NPF (Drosophila NPY) neurons are postsynaptic to Janu-AstA for water seeking and feeding through the AstA-R2 galanin-like receptor. NPF neurons use NPF to regulate thirst and hunger behaviors. Flies choose Janu neuron activation, suggesting that thirsty seeking up a humidity gradient is rewarding. These findings identify novel central brain circuit elements that coordinate internal state drives to selectively control motivated seeking behavior.

Dopamine modulation of sensory processing and adaptive behavior in flies

Behavioral flexibility for appropriate action selection is an advantage when animals are faced with decisions that will determine their survival or death. In order to arrive at the right decision, animals evaluate information from their external environment, internal state, and past experiences. How these different signals are integrated and modulated in the brain, and how context- and state-dependent behavioral decisions are controlled are poorly understood questions. Studying the molecules that help convey and integrate such information in neural circuits is an important way to approach these questions. Many years of work in different model organisms have shown that dopamine is a critical neuromodulator for (reward based) associative learning. However, recent findings in vertebrates and invertebrates have demonstrated the complexity and heterogeneity of dopaminergic neuron populations and their functional implications in many adaptive behaviors important for survival. For example, dopaminergic neurons can integrate external sensory information, internal and behavioral states, and learned experience in the decision making circuitry. Several recent advances in methodologies and the availability of a synaptic level connectome of the whole-brain circuitry of Drosophila melanogaster make the fly an attractive system to study the roles of dopamine in decision making and state-dependent behavior. In particular, a learning and memory center-the mushroom body-is richly innervated by dopaminergic neurons that enable it to integrate multi-modal information according to state and context, and to modulate decision-making and behavior.

Endocrine regulation of MFS2 by branchless controls phosphate excretion and stone formation in Drosophila renal tubules

How inorganic phosphate (Pi) homeostasis is regulated in Drosophila is currently unknown. We here identify MFS2 as a key Pi transporter in fly renal (Malpighian) tubules. Consistent with its role in Pi excretion, we found that dietary Pi induces MFS2 expression. This results in the formation of Malpighian calcium-Pi stones, while RNAi-mediated knockdown of MFS2 increases blood (hemolymph) Pi and decreases formation of Malpighian tubule stones in flies cultured on high Pi medium. Conversely, microinjection of adults with the phosphaturic human hormone fibroblast growth factor 23 (FGF23) induces tubule expression of MFS2 and decreases blood Pi. This action of FGF23 is blocked by genetic ablation of MFS2. Furthermore, genetic overexpression of the fly FGF branchless (bnl) in the tubules induces expression of MFS2 and increases Malpighian tubule stones suggesting that bnl is the endogenous phosphaturic hormone in adult flies. Finally, genetic ablation of MFS2 increased fly life span, suggesting that Malpighian tubule stones are a key element whereby high Pi diet reduces fly longevity previously reported by us. In conclusion, MFS2 mediates excretion of Pi in Drosophila, which is as in higher species under the hormonal control of FGF-signaling.

The thirsty fly: Ion transport peptide (ITP) is a novel endocrine regulator of water homeostasis in Drosophila

Animals need to continuously adjust their water metabolism to the internal and external conditions. Homeostasis of body fluids thus requires tight regulation of water intake and excretion, and a balance between ingestion of water and solid food. Here, we investigated how these processes are coordinated in Drosophila melanogaster. We identified the first thirst-promoting and anti-diuretic hormone of Drosophila, encoded by the gene Ion transport peptide (ITP). This endocrine regulator belongs to the CHH (crustacean hyperglycemic hormone) family of peptide hormones. Using genetic gain- and loss-of-function experiments, we show that ITP signaling acts analogous to the human vasopressin and renin-angiotensin systems; expression of ITP is elevated by dehydration of the fly, and the peptide increases thirst while repressing excretion, promoting thus conservation of water resources. ITP responds to both osmotic and desiccation stress, and dysregulation of ITP signaling compromises the fly's ability to cope with these stressors. In addition to the regulation of thirst and excretion, ITP also suppresses food intake. Altogether, our work identifies ITP as an important endocrine regulator of thirst and excretion, which integrates water homeostasis with feeding of Drosophila.