Drosophila Aversive Behavior toward Erwinia carotovora carotovora Is Mediated by Bitter Neurons and Leukokinin

Evolution of resistance and disease tolerance mechanisms to oral bacterial infection in Drosophila melanogaster

Pathogens exert strong selection on hosts that evolve and deploy different defensive strategies, namely minimizing pathogen exposure (avoidance), directly promoting pathogen elimination (resistance) and/or managing the deleterious effects of illness (disease tolerance). However, how the host response partitions across these processes has not been directly tested in a single host-pathogen system, let alone in the context of known adaptive trajectories resulting from experimental evolution. Here, we compare a Drosophila melanogaster population adapted to oral infection with its natural pathogen Pseudomonas entomophila (BactOral), to its control population to find no evidence for behavioural changes but measurable differences in both resistance and disease tolerance. In BactOral, no differences were detected in bacterial intake or defecation, nor gut cell renewal. However, a measurable relative decrease in bacterial loads correlates with an increase in gut-specific anti-microbial peptide production, pointing to a strengthening in resistance. Additionally, we posit that disease tolerance also contributes to the response of BactOral through a tighter control of self- and pathogen-derived damage caused by bacteria exposure. This study reveals a genetically complex and mechanistically multi-layered response, possibly reflecting the structure of adaptation to infection in natural populations.

Tastant-receptor interactions: insights from the fruit fly

Across species, taste provides important chemical information about potential food sources and the surrounding environment. As details about the chemicals and receptors responsible for gustation are discovered, a complex view of the taste system is emerging with significant contributions from research using the fruit fly, Drosophila melanogaster, as a model organism. In this brief review, we summarize recent advances in Drosophila gustation and their relevance to taste research more broadly. Our goal is to highlight the molecular mechanisms underlying the first step of gustatory circuits: ligand-receptor interactions in primary taste cells. After an introduction to the Drosophila taste system and how it encodes the canonical taste modalities sweet, bitter, and salty, we describe recent insights into the complex nature of carboxylic acid and amino acid detection in the context of sour and umami taste, respectively. Our analysis extends to non-canonical taste modalities including metals, fatty acids, and bacterial components, and highlights unexpected receptors and signaling pathways that have recently been identified in Drosophila taste cells. Comparing the intricate molecular and cellular underpinnings of how ligands are detected in vivo in fruit flies reveals both specific and promiscuous receptor selectivity for taste encoding. Throughout this review, we compare and contextualize these Drosophila findings with mammalian research to not only emphasize the conservation of these chemosensory systems, but to demonstrate the power of this model organism in elucidating the neurobiology of taste and feeding.

Larval microbiota primes the Drosophila adult gustatory response

The survival of animals depends, among other things, on their ability to identify threats in their surrounding environment. Senses such as olfaction, vision and taste play an essential role in sampling their living environment, including microorganisms, some of which are potentially pathogenic. This study focuses on the mechanisms of detection of bacteria by the Drosophila gustatory system. We demonstrate that the peptidoglycan (PGN) that forms the cell wall of bacteria triggers an immediate feeding aversive response when detected by the gustatory system of adult flies. Although we identify ppk23+ and Gr66a+ gustatory neurons as necessary to transduce fly response to PGN, we demonstrate that they play very different roles in the process. Time-controlled functional inactivation and in vivo calcium imaging demonstrate that while ppk23+ neurons are required in the adult flies to directly transduce PGN signal, Gr66a+ neurons must be functional in larvae to allow future adults to become PGN sensitive. Furthermore, the ability of adult flies to respond to bacterial PGN is lost when they hatch from larvae reared under axenic conditions. Recolonization of germ-free larvae, but not adults, with a single bacterial species, Lactobacillus brevis, is sufficient to restore the ability of adults to respond to PGN. Our data demonstrate that the genetic and environmental characteristics of the larvae are essential to make the future adults competent to respond to certain sensory stimuli such as PGN.

Nucleic acid degradation as barrier to gene delivery: a guide to understand and overcome nuclease activity

Gene therapy is on its way to revolutionize the treatment of both inherited and acquired diseases, by transferring nucleic acids to correct a disease-causing gene in the target cells of patients. In the fight against infectious diseases, mRNA-based therapeutics have proven to be a viable strategy in the recent Covid-19 pandemic. Although a growing number of gene therapies have been approved, the success rate is limited when compared to the large number of preclinical and clinical trials that have been/are being performed. In this review, we highlight some of the hurdles which gene therapies encounter after administration into the human body, with a focus on nucleic acid degradation by nucleases that are extremely abundant in mammalian organs, biological fluids as well as in subcellular compartments. We overview the available strategies to reduce the biodegradation of gene therapeutics after administration, including chemical modifications of the nucleic acids, encapsulation into vectors and co-administration with nuclease inhibitors and discuss which strategies are applied for clinically approved nucleic acid therapeutics. In the final part, we discuss the currently available methods and techniques to qualify and quantify the integrity of nucleic acids, with their own strengths and limitations.

Metal-Organic Framework Optical Thermometer Based on Cr3+ Ion Luminescence

Metal-organic frameworks with perovskite structures have recently attracted increasing attention due to their structural, optical, and phonon properties. Herein, we report the structural and luminescence studies of a series of six heterometallic perovskite-type metal-organic frameworks with the general formula [EA]₂NaCr_xAl_1-x(HCOO)₆, where x = 1, 0.78, 0.57, 0.30, 0.21, and 0. The diffuse reflectance spectral analysis provided valuable information, particularly on crystal field strength (Dq/B) and energy band gap (E_g). We showed that the Dq/B varies in the 2.33-2.76 range depending on the composition of the sample. Performed Raman, XRD, and lifetime decay analyses provided information on the relationship between those parameters and the chemical composition. We also performed the temperature-dependent luminescence studies within the 80-400 K range, which was the first attempt to use an organic-inorganic framework luminescence thermometer based solely on the luminescence of Cr³⁺ ions. The results showed a strong correlation between the surrounding temperature, composition, and spectroscopic properties, allowing one to design a temperature sensing model. The temperature-dependent luminescence of the Cr³⁺ ions makes the investigated materials promising candidates for noncontact thermometers.

Transcriptional Genetically Encoded Calcium Indicators in Drosophila

Knowing which neurons are active during behavior is a crucial step toward understanding how nervous systems work. Neuronal activation is generally accompanied by an increase in intracellular calcium levels. Therefore, intracellular calcium levels are widely used as a proxy for neuronal activity. Many types of synthetic components and bioluminescent or fluorescent proteins that report transient and long-term changes in intracellular calcium levels have been developed over the past 60 years. Calcium indicators that enable imaging of the dynamic activity of a large ensemble of neurons in behaving animals have revolutionized the field of neuroscience. Among these, transcription-based genetically encoded calcium indicators (transcriptional GECIs) have proven easy to use and do not depend on sophisticated imaging systems, offering unique advantages over other types of calcium indicators. Here, we describe the two currently available fly transcriptional GECIs-calcium-dependent nuclear import of LexA (CaLexA) and transcriptional reporter of intracellular calcium (TRIC)-and review studies that have used them. In the accompanying protocol, we present step-by-step details for generating CaLexA- and TRIC-ready flies and for imaging CaLexA and TRIC signals in dissected brains after experimental manipulations of intact free-moving flies.

Enteric bacterial infection in Drosophila induces whole-body alterations in metabolic gene expression independently of the immune deficiency signaling pathway

When infected by intestinal pathogenic bacteria, animals initiate both local and systemic defence responses. These responses are required to reduce pathogen burden and also to alter host physiology and behavior to promote infection tolerance, and they are often mediated through alterations in host gene expression. Here, we have used transcriptome profiling to examine gene expression changes induced by enteric infection with the Gram-negative bacteria Pseudomonas entomophila in adult female Drosophila. We find that infection induces a strong upregulation of metabolic gene expression, including gut and fat body-enriched genes involved in lipid transport, lipolysis, and beta-oxidation, as well as glucose and amino acid metabolism genes. Furthermore, we find that the classic innate immune deficiency (Imd)/Relish/NF-KappaB pathway is not required for, and in some cases limits, these infection-mediated increases in metabolic gene expression. We also see that enteric infection with Pseudomonas entomophila downregulates the expression of many transcription factors and cell–cell signaling molecules, particularly those previously shown to be involved in gut-to-brain and neuronal signaling. Moreover, as with the metabolic genes, these changes occurred largely independent of the Imd pathway. Together, our study identifies many metabolic, signaling, and transcription factor gene expression changes that may contribute to organismal physiological and behavioral responses to enteric pathogen infection.

Gut-derived peptidoglycan remotely inhibits bacteria dependent activation of SREBP by Drosophila adipocytes

Bacteria that colonize eukaryotic gut have profound influences on the physiology of their host. In Drosophila, many of these effects are mediated by adipocytes that combine immune and metabolic functions. We show here that enteric infection with some bacteria species triggers the activation of the SREBP lipogenic protein in surrounding enterocytes but also in remote fat body cells and in ovaries, an effect that requires insulin signaling. We demonstrate that by activating the NF-κB pathway, the cell wall peptidoglycan produced by the same gut bacteria remotely, and cell-autonomously, represses SREBP activation in adipocytes. We finally show that by reducing the level of peptidoglycan, the gut born PGRP-LB amidase balances host immune and metabolic responses of the fat body to gut-associated bacteria. In the absence of such modulation, uncontrolled immune pathway activation prevents SREBP activation and lipid production by the fat body.

An increasing body of evidence indicates that microbes, which live closely associated with animals, significantly influence their development, physiology and even their behavior. The mechanisms that underly these mutual interactions are not yet completely understood. Using Drosophila as a model system, we study the impact of gut bacteria on the host physiology. We present here data showing that some bacteria present in the fly gut can stimulate the production of lipids in the remote fat body tissue via gut autophagy and insulin signaling. However, these bacteria produce many compounds and metabolites such as the cell wall peptidoglycan. Our data show that by cell-autonomously activating the NF-κB signaling pathway in the remote fat body, cell wall peptidoglycan antagonizes bacteria-triggered lipogenesis. We finally show that to prevent this antagonistic effect, flies produce an enzyme, called PGRP-LB, that cleaves the peptidoglycan into its inactive form. Our data highlight the multiple layers of interactions that take place between gut-associated bacteria and a eukaryotic host.

Impact of Microorganisms and Parasites on Neuronally Controlled Drosophila Behaviours

Like all invertebrates, flies such as Drosophila lack an adaptive immune system and depend on their innate immune system to protect them against pathogenic microorganisms and parasites. In recent years, it appears that the nervous systems of eucaryotes not only control animal behavior but also cooperate and synergize very strongly with the animals’ immune systems to detect and fight potential pathogenic threats, and allow them to adapt their behavior to the presence of microorganisms and parasites that coexist with them. This review puts into perspective the latest progress made using the Drosophila model system, in this field of research, which remains in its infancy.

Gut cytokines modulate olfaction through metabolic reprogramming of glia

Infection-induced aversion against enteropathogens is a conserved sickness behaviour that can promote host survival^1,2. The aetiology of this behaviour remains poorly understood, but studies in Drosophila have linked olfactory and gustatory perception to avoidance behaviours against toxic microorganisms^3-5. Whether and how enteric infections directly influence sensory perception to induce or modulate such behaviours remains unknown. Here we show that enteropathogen infection in Drosophila can modulate olfaction through metabolic reprogramming of ensheathing glia of the antennal lobe. Infection-induced unpaired cytokine expression in the intestine activates JAK-STAT signalling in ensheathing glia, inducing the expression of glial monocarboxylate transporters and the apolipoprotein glial lazarillo (GLaz), and affecting metabolic coupling of glia and neurons at the antennal lobe. This modulates olfactory discrimination, promotes the avoidance of bacteria-laced food and increases fly survival. Although transient in young flies, gut-induced metabolic reprogramming of ensheathing glia becomes constitutive in old flies owing to age-related intestinal inflammation, which contributes to an age-related decline in olfactory discrimination. Our findings identify adaptive glial metabolic reprogramming by gut-derived cytokines as a mechanism that causes lasting changes in a sensory system in ageing flies.

Leucokinin and Associated Neuropeptides Regulate Multiple Aspects of Physiology and Behavior in Drosophila

Leucokinins (LKs) constitute a family of neuropeptides identified in numerous insects and many other invertebrates. LKs act on G-protein-coupled receptors that display only distant relations to other known receptors. In adult Drosophila, 26 neurons/neurosecretory cells of three main types express LK. The four brain interneurons are of two types, and these are implicated in several important functions in the fly's behavior and physiology, including feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. The 22 neurosecretory cells (abdominal LK neurons, ABLKs) of the abdominal neuromeres co-express LK and a diuretic hormone (DH44), and together, these regulate water and ion homeostasis and associated stress as well as food intake. In Drosophila larvae, LK neurons modulate locomotion, escape responses and aspects of ecdysis behavior. A set of lateral neurosecretory cells, ALKs (anterior LK neurons), in the brain express LK in larvae, but inconsistently so in adults. These ALKs co-express three other neuropeptides and regulate water and ion homeostasis, feeding, and drinking, but the specific role of LK is not yet known. This review summarizes Drosophila data on embryonic lineages of LK neurons, functional roles of individual LK neuron types, interactions with other peptidergic systems, and orchestrating functions of LK.

Leucokinins: Multifunctional Neuropeptides and Hormones in Insects and Other Invertebrates

Leucokinins (LKs) constitute a neuropeptide family first discovered in a cockroach and later identified in numerous insects and several other invertebrates. The LK receptors are only distantly related to other known receptors. Among insects, there are many examples of species where genes encoding LKs and their receptors are absent. Furthermore, genomics has revealed that LK signaling is lacking in several of the invertebrate phyla and in vertebrates. In insects, the number and complexity of LK-expressing neurons vary, from the simple pattern in the Drosophila larva where the entire CNS has 20 neurons of 3 main types, to cockroaches with about 250 neurons of many different types. Common to all studied insects is the presence or 1-3 pairs of LK-expressing neurosecretory cells in each abdominal neuromere of the ventral nerve cord, that, at least in some insects, regulate secretion in Malpighian tubules. This review summarizes the diverse functional roles of LK signaling in insects, as well as other arthropods and mollusks. These functions include regulation of ion and water homeostasis, feeding, sleep-metabolism interactions, state-dependent memory formation, as well as modulation of gustatory sensitivity and nociception. Other functions are implied by the neuronal distribution of LK, but remain to be investigated.

Protocol for a Binary Choice Feeding Assay Using Adult, Axenic Drosophila

This protocol is designed to prepare adult axenic Drosophila before monitoring their behavior in a two-choice feeding assay, where flies are confronted with an axenic versus a dead or alive bacteria-contaminated feeding solution. Several aspects of the procedure, including raising and aging flies in axenic conditions, starving adult flies, and composing feeding solutions, are detailed. The bacterium used in this protocol, Erwiniacarotovora carotovora-15²¹⁴¹ (Ecc-15²¹⁴¹), is commonly used to decipher the mechanisms controlling host-pathogen interactions in the Drosophila model.

For complete details on the use and execution of this protocol, please refer to Charroux et al. (2020).

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Antibiotics are used to raise flies in axenic conditions

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Flies are starved with a straightforward procedure

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Easy preparation of bacteria-contaminated feeding solutions

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The procedure is suitable for any behavioral assay using axenic flies

How Bacteria Impact Host Nervous System and Behaviors: Lessons from Flies and Worms

Behavior is the neuronally controlled, voluntary or involuntary response of an organism to its environment. An increasing body of evidence indicates that microbes, which live closely associated with animals or in their immediate surroundings, significantly influence animals' behavior. The extreme complexity of the nervous system of animals, combined with the extraordinary microbial diversity, are two major obstacles to understand, at the molecular level, how microbes modulate animal behavior. In this review, we discuss recent advances in dissecting the impact that bacteria have on the nervous system of two genetically tractable invertebrate models, Drosophila melanogaster and Caenorhabditis elegans.