Activity-Dependent Synaptic Refinement: New Insights from Drosophila

Repetitive and compulsive behavior after Early-Life-Pain associated with reduced long-chain sphingolipid species

BACKGROUND

Pain in early life may impact on development and risk of chronic pain. We developed an optogenetic Cre/loxP mouse model of "early-life-pain" (ELP) using mice with transgenic expression of channelrhodopsin-2 (ChR2) under control of the Advillin (Avil) promoter, which drives expression of transgenes predominantly in isolectin B4 positive non-peptidergic nociceptors in postnatal mice. Avil-ChR2 (Cre +) and ChR2-flfl control mice were exposed to blue light in a chamber once daily from P1-P5 together with their Cre-negative mother.

RESULTS

ELP caused cortical hyperexcitability at P8-9 as assessed via multi-electrode array recordings that coincided with reduced expression of synaptic genes (RNAseq) including Grin2b, neurexins, piccolo and voltage gated calcium and sodium channels. Young adult (8-16 wks) Avil-ChR2 mice presented with nociceptive hypersensitivity upon heat or mechanical stimulation, which did not resolve up until one year of age. The persistent hypersensitivy to nociceptive stimuli was reflected by increased calcium fluxes in primary sensory neurons of aged mice (1 year) upon capsaicin stimulation. Avil-ChR2 mice behaved like controls in maze tests of anxiety, social interaction, and spatial memory but IntelliCage behavioral studies revealed repetitive nosepokes and corner visits and compulsive lickings. Compulsiveness at the behavioral level was associated with a reduction of sphingomyelin species in brain and plasma lipidomic studies. Behavioral studies were done with female mice.

CONCLUSION

The results suggest that ELP may predispose to chronic "pain" and compulsive psychopathology in part mediated by alterations of sphingolipid metabolism, which have been previously described in the context of addiction and psychiatric diseases.

Synaptic and circuit mechanisms prevent detrimentally precise correlation in the developing mammalian visual system

The developing visual thalamus and cortex extract positional information encoded in the correlated activity of retinal ganglion cells by synaptic plasticity, allowing for the refinement of connectivity. Here, we use a biophysical model of the visual thalamus during the initial visual circuit refinement period to explore the role of synaptic and circuit properties in the regulation of such neural correlations. We find that the NMDA receptor dominance, combined with weak recurrent excitation and inhibition characteristic of this age, prevents the emergence of spike-correlations between thalamocortical neurons on the millisecond timescale. Such precise correlations, which would emerge due to the broad, unrefined connections from the retina to the thalamus, reduce the spatial information contained by thalamic spikes, and therefore we term them 'parasitic' correlations. Our results suggest that developing synapses and circuits evolved mechanisms to compensate for such detrimental parasitic correlations arising from the unrefined and immature circuit.

Synaptic Development in Diverse Olfactory Neuron Classes Uses Distinct Temporal and Activity-Related Programs

Developing neurons must meet core molecular, cellular, and temporal requirements to ensure the correct formation of synapses, resulting in functional circuits. However, because of the vast diversity in neuronal class and function, it is unclear whether or not all neurons use the same organizational mechanisms to form synaptic connections and achieve functional and morphologic maturation. Moreover, it remains unknown whether neurons united in a common goal and comprising the same sensory circuit develop on similar timescales and use identical molecular approaches to ensure the formation of the correct number of synapses. To begin to answer these questions, we took advantage of the Drosophila antennal lobe (AL), a model olfactory circuit with remarkable genetic access and synapse-level resolution. Using tissue-specific genetic labeling of active zones, we performed a quantitative analysis of synapse formation in multiple classes of neurons of both sexes throughout development and adulthood. We found that olfactory receptor neurons (ORNs), projection neurons (PNs), and local interneurons (LNs) each have unique time courses of synaptic development, addition, and refinement, demonstrating that each class follows a distinct developmental program. This raised the possibility that these classes may also have distinct molecular requirements for synapse formation. We genetically altered neuronal activity in each neuronal subtype and observed differing effects on synapse number based on the neuronal class examined. Silencing neuronal activity in ORNs, PNs, and LNs impaired synaptic development but only in ORNs did enhancing neuronal activity influence synapse formation. ORNs and LNs demonstrated similar impairment of synaptic development with enhanced activity of a master kinase, GSK-3β, suggesting that neuronal activity and GSK-3β kinase activity function in a common pathway. ORNs also, however, demonstrated impaired synaptic development with GSK-3β loss-of-function, suggesting additional activity-independent roles in development. Ultimately, our results suggest that the requirements for synaptic development are not uniform across all neuronal classes with considerable diversity existing in both their developmental time frames and molecular requirements. These findings provide novel insights into the mechanisms of synaptic development and lay the foundation for future work determining their underlying etiologies.SIGNIFICANCE STATEMENT Distinct olfactory neuron classes in Drosophila develop a mature synaptic complement over unique timelines and using distinct activity-dependent and molecular programs, despite having the same generalized goal of olfactory sensation.

Pruning deep neural networks generates a sparse, bio-inspired nonlinear controller for insect flight

Insect flight is a strongly nonlinear and actuated dynamical system. As such, strategies for understanding its control have typically relied on either model-based methods or linearizations thereof. Here we develop a framework that combines model predictive control on an established flight dynamics model and deep neural networks (DNN) to create an efficient method for solving the inverse problem of flight control. We turn to natural systems for inspiration since they inherently demonstrate network pruning with the consequence of yielding more efficient networks for a specific set of tasks. This bio-inspired approach allows us to leverage network pruning to optimally sparsify a DNN architecture in order to perform flight tasks with as few neural connections as possible, however, there are limits to sparsification. Specifically, as the number of connections falls below a critical threshold, flight performance drops considerably. We develop sparsification paradigms and explore their limits for control tasks. Monte Carlo simulations also quantify the statistical distribution of network weights during pruning given initial random weights of the DNNs. We demonstrate that on average, the network can be pruned to retain a small amount of original network weights and still perform comparably to its fully-connected counterpart. The relative number of remaining weights, however, is highly dependent on the initial architecture and size of the network. Overall, this work shows that sparsely connected DNNs are capable of predicting the forces required to follow flight trajectories. Additionally, sparsification has sharp performance limits.

γ-secretase promotes Drosophila postsynaptic development through the cleavage of a Wnt receptor

Developing synapses mature through the recruitment of specific proteins that stabilize presynaptic and postsynaptic structure and function. Wnt ligands signaling via Frizzled (Fz) receptors play many crucial roles in neuronal and synaptic development, but whether and how Wnt and Fz influence synaptic maturation is incompletely understood. Here, we show that Fz2 receptor cleavage via the γ-secretase complex is required for postsynaptic development and maturation. In the absence of γ-secretase, Drosophila neuromuscular synapses fail to recruit postsynaptic scaffolding and cytoskeletal proteins, leading to behavioral deficits. Introducing presenilin mutations linked to familial early-onset Alzheimer's disease into flies leads to synaptic maturation phenotypes that are identical to those seen in null alleles. This conserved role for γ-secretase in synaptic maturation and postsynaptic development highlights the importance of Fz2 cleavage and suggests that receptor processing by proteins linked to neurodegeneration may be a shared mechanism with aspects of synaptic development.

Physics Clues on the Mind Substrate and Attributes

The last decade has witnessed a remarkable progress in our understanding of the brain. This has mainly been based on the scrutiny and modeling of the transmission of activity among neurons across lively synapses. A main conclusion, thus far, is that essential features of the mind rely on collective phenomena that emerge from a willful interaction of many neurons that, mediating other cells, form a complex network whose details keep constantly adapting to their activity and surroundings. In parallel, theoretical and computational studies developed to understand many natural and artificial complex systems, which have truthfully explained their amazing emergent features and precise the role of the interaction dynamics and other conditions behind the different collective phenomena they happen to display. Focusing on promising ideas that arise when comparing these neurobiology and physics studies, the present perspective article shortly reviews such fascinating scenarios looking for clues about how high-level cognitive processes such as consciousness, intelligence, and identity can emerge. We, thus, show that basic concepts of physics, such as dynamical phases and non-equilibrium phase transitions, become quite relevant to the brain activity while determined by factors at the subcellular, cellular, and network levels. We also show how these transitions depend on details of the processing mechanism of stimuli in a noisy background and, most important, that one may detect them in familiar electroencephalogram (EEG) recordings. Thus, we associate the existence of such phases, which reveal a brain operating at (non-equilibrium) criticality, with the emergence of most interesting phenomena during memory tasks.

Drosophila Corazonin Neurons as a Hub for Regulating Growth, Stress Responses, Ethanol-Related Behaviors, Copulation Persistence and Sexually Dimorphic Reward Pathways

The neuronal mechanisms by which complex behaviors are coordinated and timed often involve neuropeptidergic regulation of stress and reward pathways. Recent studies of the neuropeptide Corazonin (Crz), a homolog of the mammalian Gonadotrophin Releasing Hormone (GnRH), have suggested its crucial role in the regulation of growth, internal states and behavioral decision making. We focus this review on Crz neurons with the goal to (1) highlight the diverse roles of Crz neuron function, including mechanisms that may be independent of the Crz peptide, (2) emphasize current gaps in knowledge about Crz neuron functions, and (3) propose exciting ideas of novel research directions involving the use of Crz neurons. We describe the different developmental fates of distinct subsets of Crz neurons, including recent findings elucidating the molecular regulation of apoptosis. Crz regulates systemic growth, food intake, stress responses and homeostasis by interacting with the short Neuropeptide F (sNPF) and the steroid hormone ecdysone. Additionally, activation of Crz neurons is shown to be pleasurable by interacting with the Neuropeptide F (NPF) and regulates reward processes such as ejaculation and ethanol-related behaviors in a sexually dimorphic manner. Crz neurons are proposed to be a motivational switch regulating copulation duration using a CaMKII-dependent mechanism described as the first neuronal interval timer lasting longer than a few seconds. Lastly, we propose ideas to use Crz neuron-induced ejaculation to study the effects of fictive mating and sex addiction in flies, as well as to elucidate dimorphic molecular mechanisms underlying reward behaviors and feeding disorders.

Nutraceutical and Probiotic Approaches to Examine Molecular Interactions of the Amyloid Precursor Protein APP in Drosophila Models of Alzheimer's Disease

Studies using animal models have shed light into the molecular and cellular basis for the neuropathology observed in patients with Alzheimer’s disease (AD). In particular, the role of the amyloid precursor protein (APP) plays a crucial role in the formation of senile plaques and aging-dependent degeneration. Here, we focus our review on recent findings using the Drosophila AD model to expand our understanding of APP molecular function and interactions, including insights gained from the fly homolog APP-like (APPL). Finally, as there is still no cure for AD, we review some approaches that have shown promising results in ameliorating AD-associated phenotypes, with special attention on the use of nutraceuticals and their molecular effects, as well as interactions with the gut microbiome. Overall, the phenomena described here are of fundamental significance for understanding network development and degeneration. Given the highly conserved nature of fundamental signaling pathways, the insight gained from animal models such as Drosophila melanogaster will likely advance the understanding of the mammalian brain, and thus be relevant to human health.

Building a circuit through correlated spontaneous neuronal activity in the developing vertebrate and invertebrate visual systems

During the development of the vertebrate nervous systems, genetic programs assemble an immature circuit that is subsequently refined by neuronal activity evoked by external stimuli. However, prior to sensory experience, the intrinsic property of the developing nervous system also triggers correlated network-level neuronal activity, with retinal waves in the developing vertebrate retina being the best documented example. Spontaneous activity has also been found in the visual system of Drosophila Here, we compare the spontaneous activity of the developing visual system between mammalian and Drosophila and suggest that Drosophila is an emerging model for mechanistic and functional studies of correlated spontaneous activity.

Development of γ-aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on the rat genioglossal motoneurons

Detailed information about the development of excitatory and inhibitory synapses on the genioglossal (GG) motoneuron may help to understand the mechanism of fine control of GG motoneuron firing and the coordinated tongue movement during postnatal development. For this, we investigated the development of γ-aminobutyric acid (GABA)-immunopositive (GABA +), glycine + (Gly +), and glutamate + (Glut +) axon terminals (boutons) on the somata of rat GG motoneurons at a postnatal day 2 (P2), P6 and P18 by retrograde labeling of GG motoneurons with horseradish peroxidase, electron microscopic postembedding immunogold staining with GABA, Gly, and Glut antisera, and quantitative analysis. The number of boutons per GG motoneuron somata and the mean length of bouton apposition, measures of bouton size and synaptic covering percentage, were significantly increased from P2/P6 to P18. The number and fraction of GABA + only boutons of all boutons decreased significantly, whereas those of Gly + only boutons increased significantly from P2/P6 to P18, suggesting developmental switch from GABAergic to glycinergic synaptic transmission. The fraction of mixed GABA +/Gly + boutons of all boutons was the highest among inhibitory bouton types throughout the postnatal development. The fractions of excitatory and inhibitory boutons of all boutons remained unchanged during postnatal development. These findings reveal a distinct developmental pattern of inhibitory synapses on the GG motoneurons different from that on spinal or trigeminal motoneurons, which may have an important role in the regulation of the precise and coordinated movements of the tongue during the maturation of the oral motor system.

Structural and Functional Synaptic Plasticity Induced by Convergent Synapse Loss in the Drosophila Neuromuscular Circuit

Throughout the nervous system, the convergence of two or more presynaptic inputs on a target cell is commonly observed. The question we ask here is to what extent converging inputs influence each other's structural and functional synaptic plasticity. In complex circuits, isolating individual inputs is difficult because postsynaptic cells can receive thousands of inputs. An ideal model to address this question is the Drosophila larval neuromuscular junction (NMJ) where each postsynaptic muscle cell receives inputs from two glutamatergic types of motor neurons (MNs), known as 1b and 1s MNs. Notably, each muscle is unique and receives input from a different combination of 1b and 1s MNs; we surveyed multiple muscles for this reason. Here, we identified a cell-specific promoter that allows ablation of 1s MNs postinnervation and measured structural and functional responses of convergent 1b NMJs using microscopy and electrophysiology. For all muscles examined in both sexes, ablation of 1s MNs resulted in NMJ expansion and increased spontaneous neurotransmitter release at corresponding 1b NMJs. This demonstrates that 1b NMJs can compensate for the loss of convergent 1s MNs. However, only a subset of 1b NMJs showed compensatory evoked neurotransmission, suggesting target-specific plasticity. Silencing 1s MNs led to similar plasticity at 1b NMJs, suggesting that evoked neurotransmission from 1s MNs contributes to 1b synaptic plasticity. Finally, we genetically blocked 1s innervation in male larvae and robust 1b synaptic plasticity was eliminated, raising the possibility that 1s NMJ formation is required to set up a reference for subsequent synaptic perturbations.SIGNIFICANCE STATEMENT In complex neural circuits, multiple convergent inputs contribute to the activity of the target cell, but whether synaptic plasticity exists among these inputs has not been thoroughly explored. In this study, we examined synaptic plasticity in the structurally and functionally tractable Drosophila larval neuromuscular system. In this convergent circuit, each muscle is innervated by a unique pair of motor neurons. Removal of one neuron after innervation causes the adjacent neuron to increase neuromuscular junction outgrowth and functional output. However, this is not a general feature as each motor neuron differentially compensates. Further, robust compensation requires initial coinnervation by both neurons. Understanding how neurons respond to perturbations in adjacent neurons will provide insight into nervous system plasticity in both healthy and disease states.

Ubiquitin-dependent regulation of a conserved DMRT protein controls sexually dimorphic synaptic connectivity and behavior

Sex-specific synaptic connectivity is beginning to emerge as a remarkable, but little explored feature of animal brains. We describe here a novel mechanism that promotes sexually dimorphic neuronal function and synaptic connectivity in the nervous system of the nematode Caenorhabditis elegans. We demonstrate that a phylogenetically conserved, but previously uncharacterized Doublesex/Mab-3 related transcription factor (DMRT), dmd-4, is expressed in two classes of sex-shared phasmid neurons specifically in hermaphrodites but not in males. We find dmd-4 to promote hermaphrodite-specific synaptic connectivity and neuronal function of phasmid sensory neurons. Sex-specificity of DMD-4 function is conferred by a novel mode of posttranslational regulation that involves sex-specific protein stabilization through ubiquitin binding to a phylogenetically conserved but previously unstudied protein domain, the DMA domain. A human DMRT homolog of DMD-4 is controlled in a similar manner, indicating that our findings may have implications for the control of sexual differentiation in other animals as well.

Activity-Dependent Synaptic Plasticity in Drosophila melanogaster

The Drosophila nervous system is a valuable model to examine the mechanisms of activity-dependent synaptic modification (plasticity) owing to its relatively simple organization and the availability of powerful genetic tools. The larval neuromuscular junction (NMJ) in particular is an accessible model for the study of synaptic development and plasticity. In addition to the NMJ, huge strides have also been made on understanding activity-dependent synaptic plasticity in the Drosophila olfactory and visual systems. In this review, we focus mainly on the underlying processes of activity-dependent synaptic plasticity at both pre-synaptic and post-synaptic terminals, and summarize current knowledge on activity-dependent synaptic plasticity in different parts of the Drosophila melanogaster nervous system (larval NMJ, olfactory system, larval visual system, and adult visual system). We also examine links between synaptic development and activity-dependent synaptic plasticity, and the relationships between morphological and physiological plasticity. We provide a point of view from which we discern that the underlying mechanism of activity-dependent plasticity may be common throughout the nervous systems in Drosophila melanogaster.

Visual Experience-Dependent Expression of Fn14 Is Required for Retinogeniculate Refinement

Sensory experience influences the establishment of neural connectivity through molecular mechanisms that remain unclear. Here, we employ single-nucleus RNA sequencing to investigate the contribution of sensory-driven gene expression to synaptic refinement in the dorsal lateral geniculate nucleus of the thalamus, a region of the brain that processes visual information. We find that visual experience induces the expression of the cytokine receptor Fn14 in excitatory thalamocortical neurons. By combining electrophysiological and structural techniques, we show that Fn14 is dispensable for early phases of refinement mediated by spontaneous activity but that Fn14 is essential for refinement during a later, experience-dependent period of development. Refinement deficits in mice lacking Fn14 are associated with functionally weaker and structurally smaller retinogeniculate inputs, indicating that Fn14 mediates both functional and anatomical rearrangements in response to sensory experience. These findings identify Fn14 as a molecular link between sensory-driven gene expression and vision-sensitive refinement in the brain.

The interplay between electrical and chemical synaptogenesis

Neurons communicate with each other via electrical or chemical synaptic connections. The pattern and strength of connections between neurons are critical for generating appropriate output. What mechanisms govern the formation of electrical and/or chemical synapses between two neurons? Recent studies indicate that common molecular players could regulate the formation of both of these classes of synapses. In addition, electrical and chemical synapses can mutually coregulate each other’s formation. Electrical activity, generated spontaneously by the nervous system or initiated from sensory experience, plays an important role in this process, leading to the selection of appropriate connections and the elimination of inappropriate ones. In this review, we discuss recent studies that shed light on the formation and developmental interactions of chemical and electrical synapses.

Structural and Molecular Properties of Insect Type II Motor Axon Terminals

A comparison between the axon terminals of octopaminergic efferent dorsal or ventral unpaired median neurons in either desert locusts (Schistocerca gregaria) or fruit flies (Drosophila melanogaster) across skeletal muscles reveals many similarities. In both species the octopaminergic axon forms beaded fibers where the boutons or varicosities form type II terminals in contrast to the neuromuscular junction (NMJ) or type I terminals. These type II terminals are immunopositive for both tyramine and octopamine and, in contrast to the type I terminals, which possess clear synaptic vesicles, only contain dense core vesicles. These dense core vesicles contain octopamine as shown by immunogold methods. With respect to the cytomatrix and active zone peptides the type II terminals exhibit active zone-like accumulations of the scaffold protein Bruchpilot (BRP) only sparsely in contrast to the many accumulations of BRP identifying active zones of NMJ type I terminals. In the fruit fly larva marked dynamic changes of octopaminergic fibers have been reported after short starvation which not only affects the formation of new branches (“synaptopods”) but also affects the type I terminals or NMJs via octopamine-signaling (Koon et al., 2011). Our starvation experiments of Drosophila-larvae revealed a time-dependency of the formation of additional branches. Whereas after 2 h of starvation we find a decrease in “synaptopods”, the increase is significant after 6 h of starvation. In addition, we provide evidence that the release of octopamine from dendritic and/or axonal type II terminals uses a similar synaptic machinery to glutamate release from type I terminals of excitatory motor neurons. Indeed, blocking this canonical synaptic release machinery via RNAi induced downregulation of BRP in neurons with type II terminals leads to flight performance deficits similar to those observed for octopamine mutants or flies lacking this class of neurons (Brembs et al., 2007).

In Vivo Calcium Signaling during Synaptic Refinement at the Drosophila Neuromuscular Junction

Neural activity plays a key role in pruning aberrant synapses in various neural systems, including the mammalian cortex, where low-frequency (0.01 Hz) calcium oscillations refine topographic maps. However, the activity-dependent molecular mechanisms remain incompletely understood. Activity-dependent pruning also occurs at embryonic Drosophila neuromuscular junctions (NMJs), where low-frequency Ca²⁺ oscillations are required for synaptic refinement and the response to the muscle-derived chemorepellant Sema2a. We examined embryonic growth cone filopodia in vivo to directly observe their exploration and to analyze the episodic Ca²⁺ oscillations involved in refinement. Motoneuron filopodia repeatedly contacted off-target muscle fibers over several hours during late embryogenesis, with episodic Ca²⁺ signals present in both motile filopodia as well as in later-stabilized synaptic boutons. The Ca²⁺ transients matured over several hours into regular low-frequency (0.03 Hz) oscillations. In vivo imaging of intact embryos of both sexes revealed that the formation of ectopic filopodia is increased in Sema2a heterozygotes. We provide genetic evidence suggesting a complex presynaptic Ca²⁺-dependent signaling network underlying refinement that involves the phosphatases calcineurin and protein phosphatase-1, as well the serine/threonine kinases CaMKII and PKA. Significantly, this network influenced the neuron's response to the muscle's Sema2a chemorepellant, critical for the removal of off-target contacts.SIGNIFICANCE STATEMENT To address the question of how synaptic connectivity is established during development, we examined the behavior of growth cone filopodia during the exploration of both correct and off-target muscle fibers in Drosophila embryos. We demonstrate that filopodia repeatedly contact off-target muscles over several hours, until they ultimately retract. We show that intracellular signals are observed in motile and stabilized "ectopic" contacts. Several genetic experiments provide insight in the molecular pathway underlying network refinement, which includes oscillatory calcium signals via voltage-gated calcium channels as a key component. Calcium orchestrates the activity of several kinases and phosphatases, which interact in a coordinated fashion to regulate chemorepulsion exerted by the muscle.