Molecular Interface of Neuronal Innate Immunity, Synaptic Vesicle Stabilization, and Presynaptic Homeostatic Plasticity

Using Electrophysiology to Study Homeostatic Plasticity at the Drosophila Neuromuscular Junction

The Drosophila melanogaster neuromuscular junction (NMJ) is a superb system for studying synapse function. Beyond that, the NMJ is also great for studying forms of synaptic plasticity. Over the last 25 years, Drosophila NMJ neuroscientists have pioneered understanding of a form of plasticity called homeostatic synaptic plasticity, which imparts functional stability on synaptic connections. The reason is straightforward: The NMJ has a robust capacity for stability. Moreover, many strategies that the NMJ uses to maintain appropriate levels of function are mirrored at other metazoan synapses. Here, we introduce core approaches that neurophysiologists use to study homeostatic synaptic plasticity at the peripheral Drosophila NMJ. We focus on methods to study a specific form of homeostatic plasticity termed presynaptic homeostatic potentiation (PHP), which is the most well-characterized one. Other forms such as presynaptic homeostatic depression and developmental forms of homeostasis are briefly discussed. Finally, we share lists of several dozen factors and conditions known to influence the execution of PHP.

An integrative sensor of body states: how the mushroom body modulates behavior depending on physiological context

The brain constantly compares past and present experiences to predict the future, thereby enabling instantaneous and future behavioral adjustments. Integration of external information with the animal's current internal needs and behavioral state represents a key challenge of the nervous system. Recent advancements in dissecting the function of the Drosophila mushroom body (MB) at the single-cell level have uncovered its three-layered logic and parallel systems conveying positive and negative values during associative learning. This review explores a lesser-known role of the MB in detecting and integrating body states such as hunger, thirst, and sleep, ultimately modulating motivation and sensory-driven decisions based on the physiological state of the fly. State-dependent signals predominantly affect the activity of modulatory MB input neurons (dopaminergic, serotoninergic, and octopaminergic), but also induce plastic changes directly at the level of the MB intrinsic and output neurons. Thus, the MB emerges as a tightly regulated relay station in the insect brain, orchestrating neuroadaptations due to current internal and behavioral states leading to short- but also long-lasting changes in behavior. While these adaptations are crucial to ensure fitness and survival, recent findings also underscore how circuit motifs in the MB may reflect fundamental design principles that contribute to maladaptive behaviors such as addiction or depression-like symptoms.

Neuroimmune transcriptome changes in patient brains of psychiatric and neurological disorders

Neuroinflammation has been implicated in multiple brain disorders but the extent and the magnitude of change in immune-related genes (IRGs) across distinct brain disorders has not been directly compared. In this study, 1275 IRGs were curated and their expression changes investigated in 2467 postmortem brains of controls and patients with six major brain disorders, including schizophrenia (SCZ), bipolar disorder (BD), autism spectrum disorder (ASD), major depressive disorder (MDD), Alzheimer's disease (AD), and Parkinson's disease (PD). There were 865 IRGs present across all microarray and RNA-seq datasets. More than 60% of the IRGs had significantly altered expression in at least one of the six disorders. The differentially expressed immune-related genes (dIRGs) shared across disorders were mainly related to innate immunity. Moreover, sex, tissue, and putative cell type were systematically evaluated for immune alterations in different neuropsychiatric disorders. Co-expression networks revealed that transcripts of the neuroimmune systems interacted with neuronal-systems, both of which contribute to the pathology of brain disorders. However, only a few genes with expression changes were also identified as containing risk variants in genome-wide association studies. The transcriptome alterations at gene and network levels may clarify the immune-related pathophysiology and help to better define neuropsychiatric and neurological disorders.

Activation and expansion of presynaptic signaling foci drives presynaptic homeostatic plasticity

Presynaptic homeostatic plasticity (PHP) adaptively regulates synaptic transmission in health and disease. Despite identification of numerous genes that are essential for PHP, we lack a dynamic framework to explain how PHP is initiated, potentiated, and limited to achieve precise control of vesicle fusion. Here, utilizing both mice and Drosophila, we demonstrate that PHP progresses through the assembly and physical expansion of presynaptic signaling foci where activated integrins biochemically converge with trans-synaptic Semaphorin2b/PlexinB signaling. Each component of the identified signaling complexes, including alpha/beta-integrin, Semaphorin2b, PlexinB, talin, and focal adhesion kinase (FAK), and their biochemical interactions, are essential for PHP. Complex integrity requires the Sema2b ligand and complex expansion includes a ∼2.5-fold expansion of active-zone associated puncta composed of the actin-binding protein talin. Finally, complex pre-expansion is sufficient to accelerate the rate and extent of PHP. A working model is proposed incorporating signal convergence with dynamic molecular assemblies that instruct PHP.

Is direct bodyguard manipulation a parasitoid-induced stress sleep? A new perspective

Bodyguard manipulation is a behavioural manipulation in which the host's behaviour is altered to protect the inducer's offspring from imminent biotic threats. The behaviour of a post-parasitoid-egressed host resembles a quiescence state with a characteristic reduction in motor activities like feeding, locomotion, respiration, and metabolic rate. Yet, they respond aggressively through a defensive response when disturbed, which ensures better fitness for the parasitoid's offspring. The behavioural changes in the parasitized host appear after the parasitoid egression. Several hypotheses have been proposed to elucidate how the parasitized host's behaviour is manipulated for the fitness benefits of the inducers, but the exact mechanism is still unknown. We review evidence to explain the behavioural changes and their mechanism in the parasitized hosts. The evidence suggests that parasitoid pre-pupal egression may drive the host to stress-induced sleep. The elevated octopamine concentration also reflects the stress response in the host. Given the theoretical links between the behavioural and the physiological changes in the post-parasitoid-egressed host and stress-induced sleep of other invertebrates, we suggest that behavioural studies combined with functional genomics, proteomics, and histological analyses might give a better understanding of bodyguard manipulation.

A circular RNA Edis-Relish-castor axis regulates neuronal development in Drosophila

Circular RNAs (circRNAs) are a new group of noncoding/regulatory RNAs that are particularly abundant in the nervous system, however, their physiological functions are underexplored. Here we report that the brain-enriched circular RNA Edis (Ect4-derived immune suppressor) plays an essential role in neuronal development in Drosophila. We show that depletion of Edis in vivo causes defects in axonal projection patterns of mushroom body (MB) neurons in the brain, as well as impaired locomotor activity and shortened lifespan of adult flies. In addition, we find that the castor gene, which encodes a transcription factor involved in neurodevelopment, is upregulated in Edis knockdown neurons. Notably, castor overexpression phenocopies Edis knockdown, and reducing castor levels suppresses the neurodevelopmental phenotypes in Edis-depleted neurons. Furthermore, chromatin immunoprecipitation analysis reveals that the transcription factor Relish, which plays a key role in regulating innate immunity signaling, occupies a pair of sites at the castor promoter, and that both sites are required for optimal castor gene activation by either immune challenge or Edis depletion. Lastly, Relish mutation and/or depletion can rescue both the castor gene hyperactivation phenotype and neuronal defects in Edis knockdown animals. We conclude that the circular RNA Edis acts through Relish and castor to regulate neuronal development.

NMDAR-dependent presynaptic homeostasis in adult hippocampus: Synapse growth and cross-modal inhibitory plasticity

Homeostatic plasticity (HP) encompasses a suite of compensatory physiological processes that counteract neuronal perturbations, enabling brain resilience. Currently, we lack a complete description of the homeostatic processes that operate within the mammalian brain. Here, we demonstrate that acute, partial AMPAR-specific antagonism induces potentiation of presynaptic neurotransmitter release in adult hippocampus, a form of compensatory plasticity that is consistent with the expression of presynaptic homeostatic plasticity (PHP) documented at peripheral synapses. We show that this compensatory plasticity can be induced within minutes, requires postsynaptic NMDARs, and is expressed via correlated increases in dendritic spine volume, active zone area, and docked vesicle number. Further, simultaneous postsynaptic genetic reduction of GluA1, GluA2, and GluA3 in triple heterozygous knockouts induces potentiation of presynaptic release. Finally, induction of compensatory plasticity at excitatory synapses induces a parallel, NMDAR-dependent potentiation of inhibitory transmission, a cross-modal effect consistent with the anti-epileptic activity of AMPAR-specific antagonists used in humans.

Bacteria-Derived Peptidoglycan Triggers a Noncanonical Nuclear Factor-κB-Dependent Response in Drosophila Gustatory Neurons

Probing the external world is essential for eukaryotes to distinguish beneficial from pathogenic micro-organisms. If it is clear that the main part of this task falls to the immune cells, recent work shows that neurons can also detect microbes, although the molecules and mechanisms involved are less characterized. In Drosophila, detection of bacteria-derived peptidoglycan by pattern recognition receptors of the peptidoglycan recognition protein (PGRP) family expressed in immune cells triggers nuclear factor-κB (NF-κB)/immune deficiency (IMD)-dependent signaling. We show here that one PGRP protein, called PGRP-LB, is expressed in bitter gustatory neurons of proboscises. In vivo calcium imaging in female flies reveals that the PGRP/IMD pathway is cell-autonomously required in these neurons to transduce the peptidoglycan signal. We finally show that NF-κB/IMD pathway activation in bitter-sensing gustatory neurons influences fly behavior. This demonstrates that a major immune response elicitor and signaling module are required in the peripheral nervous system to sense the presence of bacteria in the environment.SIGNIFICANCE STATEMENT In addition to the classical immune response, eukaryotes rely on neuronally controlled mechanisms to detect microbes and engage in adapted behaviors. However, the mechanisms of microbe detection by the nervous system are poorly understood. Using genetic analysis and calcium imaging, we demonstrate here that bacteria-derived peptidoglycan can activate bitter gustatory neurons. We further show that this response is mediated by the PGRP-LC membrane receptor and downstream components of a noncanonical NF-κB signaling cascade. Activation of this signaling cascade triggers behavior changes. These data demonstrate that bitter-sensing neurons and immune cells share a common detection and signaling module to either trigger the production of antibacterial effectors or to modulate the behavior of flies that are in contact with bacteria. Because peptidoglycan detection doesn't mobilize the known gustatory receptors, it also demonstrates that taste perception is much more complex than anticipated.

Homeostatic Plasticity of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) is an ideal preparation to study synaptic plasticity. Its simplicity- one input, one postsynaptic target- allows experimental manipulations and mechanistic analyses that are impossible at more complex synapses. Homeostatic synaptic plasticity attempts to maintain normal function in the face of perturbations in activity. At the NMJ, 3 aspects of activity are sensed to trigger 3 distinct mechanisms that contribute to homeostatic plasticity: Block of presynaptic action potentials triggers increased quantal size secondary to increased release of acetylcholine from vesicles. Simultaneous block of pre- and postsynaptic action potentials triggers an increase in the probability of vesicle release. Block of acetylcholine binding to acetylcholine receptors during spontaneous fusion of single vesicles triggers an increase in the number of releasable vesicles as well as increased motoneuron excitability. Understanding how the NMJ responds to perturbations of synaptic activity informs our understanding of its response to diverse neuromuscular diseases.

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.

Insect Behavioral Change and the Potential Contributions of Neuroinflammation-A Call for Future Research

Many organisms are able to elicit behavioral change in other organisms. Examples include different microbes (e.g., viruses and fungi), parasites (e.g., hairworms and trematodes), and parasitoid wasps. In most cases, the mechanisms underlying host behavioral change remain relatively unclear. There is a growing body of literature linking alterations in immune signaling with neuron health, communication, and function; however, there is a paucity of data detailing the effects of altered neuroimmune signaling on insect neuron function and how glial cells may contribute toward neuron dysregulation. It is important to consider the potential impacts of altered neuroimmune communication on host behavior and reflect on its potential role as an important tool in the "neuro-engineer" toolkit. In this review, we examine what is known about the relationships between the insect immune and nervous systems. We highlight organisms that are able to influence insect behavior and discuss possible mechanisms of behavioral manipulation, including potentially dysregulated neuroimmune communication. We close by identifying opportunities for integrating research in insect innate immunity, glial cell physiology, and neurobiology in the investigation of behavioral manipulation.

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.

Homeostatic plasticity fails at the intersection of autism-gene mutations and a novel class of common genetic modifiers

We identify a set of common phenotypic modifiers that interact with five independent autism gene orthologs (RIMS1, CHD8, CHD2, WDFY3, ASH1L) causing a common failure of presynaptic homeostatic plasticity (PHP) in Drosophila. Heterozygous null mutations in each autism gene are demonstrated to have normal baseline neurotransmission and PHP. However, PHP is sensitized and rendered prone to failure. A subsequent electrophysiology-based genetic screen identifies the first known heterozygous mutations that commonly genetically interact with multiple ASD gene orthologs, causing PHP to fail. Two phenotypic modifiers identified in the screen, PDPK1 and PPP2R5D, are characterized. Finally, transcriptomic, ultrastructural and electrophysiological analyses define one mechanism by which PHP fails; an unexpected, maladaptive up-regulation of CREG, a conserved, neuronally expressed, stress response gene and a novel repressor of PHP. Thus, we define a novel genetic landscape by which diverse, unrelated autism risk genes may converge to commonly affect the robustness of synaptic transmission.

Bacterial Peptidoglycan as a Driver of Chronic Brain Inflammation

Peptidoglycan (PGN) is a cell wall component of both Gram-positive and Gram-negative bacteria. Signature fragments of PGN are proinflammatory through engagement of pattern recognition receptors (PRR) on resident tissue cells and circulating leukocytes. Despite its abundance in the gut microbiota, there is limited recognition that PGN could contribute to chronic neuroinflammation. This review highlights current insights into the roles of PGN as a determinant of brain inflammation, notably in multiple sclerosis (MS) and its experimental autoimmune encephalomyelitis (EAE) models. Recent studies demonstrate PGN in blood of healthy adult humans. PGN amplifies autoimmune pathology via activation of innate immune cells. Novel uptake routes through (altered) gut mucosa by myeloid leukocyte subsets promote PGN transport to the brain.

Target-wide Induction and Synapse Type-Specific Robustness of Presynaptic Homeostasis

Presynaptic homeostatic plasticity (PHP) is an evolutionarily conserved form of adaptive neuromodulation and is observed at both central and peripheral synapses. In this work, we make several fundamental advances by interrogating the synapse specificity of PHP. We define how PHP remains robust to acute versus long-term neurotransmitter receptor perturbation. We describe a general PHP property that includes global induction and synapse-specific expression mechanisms. Finally, we detail a novel synapse-specific PHP expression mechanism that enables the conversion from short- to long-term PHP expression. If our data can be extended to other systems, including the mammalian central nervous system, they suggest that PHP can be broadly induced and expressed to sustain the function of complex neural circuitry.

Peptidoglycan-dependent NF-κB activation in a small subset of brain octopaminergic neurons controls female oviposition

When facing microbes, animals engage in behaviors that lower the impact of the infection. We previously demonstrated that internal sensing of bacterial peptidoglycan reduces Drosophila female oviposition via NF-κB pathway activation in some neurons (Kurz et al., 2017). Although we showed that the neuromodulator octopamine is implicated, the identity of the involved neurons, as well as the physiological mechanism blocking egg-laying, remained unknown. In this study, we identified few ventral nerve cord and brain octopaminergic neurons expressing an NF-κB pathway component. We functionally demonstrated that NF-κB pathway activation in the brain, but not in the ventral nerve cord octopaminergic neurons, triggers an egg-laying drop in response to infection. Furthermore, we demonstrated via calcium imaging that the activity of these neurons can be directly modulated by peptidoglycan and that these cells do not control other octopamine-dependent behaviors such as female receptivity. This study shows that by sensing peptidoglycan and hence activating NF-κB cascade, a couple of brain neurons modulate a specific octopamine-dependent behavior to adapt female physiology status to their infectious state.

Homeostatic control of Drosophila neuromuscular junction function

The ability to adapt to changing internal and external conditions is a key feature of biological systems. Homeostasis refers to a regulatory process that stabilizes dynamic systems to counteract perturbations. In the nervous system, homeostatic mechanisms control neuronal excitability, neurotransmitter release, neurotransmitter receptors, and neural circuit function. The neuromuscular junction (NMJ) of Drosophila melanogaster has provided a wealth of molecular information about how synapses implement homeostatic forms of synaptic plasticity, with a focus on the transsynaptic, homeostatic modulation of neurotransmitter release. This review examines some of the recent findings from the Drosophila NMJ and highlights questions the field will ponder in coming years.

A Path(way) to Keeping Your Synapses on an Even Keel

In this issue of Neuron, Harris et al. (2018) show that a signal transduction pathway normally exploited by the innate immune system in recognizing foreign agents plays a critical role in controlling a synapse's ability to maintain stability in the efficacy of synaptic transmission over both rapid and prolonged timescales.

Maintenance of homeostatic plasticity at the Drosophila neuromuscular synapse requires continuous IP3-directed signaling

Synapses and circuits rely on neuroplasticity to adjust output and meet physiological needs. Forms of homeostatic synaptic plasticity impart stability at synapses by countering destabilizing perturbations. The Drosophila melanogaster larval neuromuscular junction (NMJ) is a model synapse with robust expression of homeostatic plasticity. At the NMJ, a homeostatic system detects impaired postsynaptic sensitivity to neurotransmitter and activates a retrograde signal that restores synaptic function by adjusting neurotransmitter release. This process has been separated into temporally distinct phases, induction and maintenance. One prevailing hypothesis is that a shared mechanism governs both phases. Here, we show the two phases are separable. Combining genetics, pharmacology, and electrophysiology, we find that a signaling system consisting of PLCβ, inositol triphosphate (IP₃), IP₃ receptors, and Ryanodine receptors is required only for the maintenance of homeostatic plasticity. We also find that the NMJ is capable of inducing homeostatic signaling even when its sustained maintenance process is absent.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

The Effects of a Bacterial Endotoxin on Behavior and Sensory-CNS-Motor Circuits in Drosophila melanogaster

The effect of bacterial sepsis on animal behavior and physiology is complex due to direct and indirect actions. The most common form of bacterial sepsis in humans is from gram-negative bacterial strains. The endotoxin (lipopolysaccharide, LPS) and/or associated peptidoglycans from the bacteria are the key agents to induce an immune response, which then produces a cascade of immunological consequences. However, there are direct actions of LPS and associated peptidoglycans on cells which are commonly overlooked. This study showed behavioral and neural changes in larval Drosophila fed commercially obtained LPS from Serratia marcescens. Locomotor behavior was not altered, but feeding behavior increased and responses to sensory tactile stimuli were decreased. In driving a sensory-central nervous system (CNS)-motor neural circuit in in-situ preparations, direct application of commercially obtained LPS initially increased evoked activity and then decreased and even stopped evoked responses in a dose-dependent manner. With acute LPS and associated peptidoglycans exposure (10 min), the depressed neural responses recovered within a few minutes after removal of LPS. Commercially obtained LPS induces a transitory hyperpolarization of the body wall muscles within seconds of exposure and alters activity within the CNS circuit. Thus, LPS and/or associated peptidoglycans have direct effects on body wall muscle without a secondary immune response.

The effects of bacterial endotoxin LPS on synaptic transmission at the neuromuscular junction

The direct action of bacterial lipopolysaccharides (LPS) endotoxin was shown to enhance synaptic transmission and hyperpolarize the membrane potential at low doses, but block glutamatergic receptors and decrease observable spontaneous events at a high dosage. The dosage effects are LPS type specific. The hyperpolarization is not due to voltage-gated potassium channels or to activation of nitric oxide synthase (NOS). The effects are induced directly by LPS, independent of an immune response.

The effects of bacterial endotoxin (LPS) on cardiac function in a medicinal blow fly (Phaenicia sericata) and a fruit fly (Drosophila melanogaster)

The bacterial endotoxins, lipopolysaccharides (LPS), are known to have direct effects on mammalian heart cells; thus, LPS is likely to have some effects in other cardiac models. Drosophila melanogaster was used since it serves as a model for cardiac physiology. Larvae of blow flies (Phaenicia sericata) commonly used as therapy for debriding dead tissue, are exposed to high levels of bacterial endotoxins, but their mechanisms of LPS resistance are not entirely understood. Comparative effects of LPS on heart rate (HR) were examined for both Drosophila and blowfly larvae. Acute 10-min direct exposure of in situ heart tubes with saline containing 1, 100, and 500 μg/ml LPS from two common bacterial stains (Pseudomonas aeruginosa and Serratia marcescens) revealed a dose-dependent effect. The effects differed between the two fly models. Larval hearts of Drosophila stopped rapidly in low Ca²⁺ containing saline, but the hearts of blow flies appear unaffected for >30 min. S. marcescens increased HR initially in Drosophila followed by a reduction for low and high doses, but no change was observed in larvae of blow flies. Whereas P. aeruginosa at a high dose decreased HR in larvae of Drosophila but increased HR in larvae of blow flies. The goal of this study is to better the understanding in the direct action of LPS on HR. Knowing the acute and direct actions of LPS exposure on HR in different species of larvae may aid in understanding the underlying mechanisms in other animals during septicemia.