Presynaptic activity and CaMKII modulate retrograde semaphorin signaling and synaptic refinement

hkb is required for DIP-α expression and target recognition in the Drosophila neuromuscular circuit

Our nervous system contains billions of neurons that form precise connections with each other through interactions between cell surface proteins. In Drosophila, the Dpr and DIP immunoglobulin protein subfamilies form homophilic or heterophilic interactions to instruct synaptic connectivity, synaptic growth, and cell survival. However, the upstream regulatory mechanisms of Dprs and DIPs are not clear. On the other hand, while transcription factors have been implicated in target recognition, their downstream cell surface proteins remain mostly unknown. We conduct an F1 dominant modifier genetic screen to identify regulators of Dprs and DIPs. We identify huckebein (hkb), a transcription factor previously implicated in target recognition of the dorsal Is motor neuron. We show that hkb genetically interacts with DIP-α and loss of hkb leads to complete removal of DIP-α expression specifically in dorsal Is motor neurons. We then confirm that this specificity is through the dorsal Is motor neuron specific transcription factor, even-skipped (eve), which acts downstream of hkb. Analysis of the genetic interaction between hkb and eve reveals that they act in the same pathway to regulate dorsal Is motor neuron connectivity. Our study provides insight into the transcriptional regulation of DIP-α and suggests that distinct regulatory mechanisms exist for the same CSP in different neurons.

Alterations in Synaptic Connectivity and Synaptic Transmission in Alzheimer's Disease with High Physical Activity

Background

Alzheimer's disease (AD) is a progressive neurodegeneration disease. Physical activity is one of the most promising modifiable lifestyles that can be effective in slowing down the progression of AD at an early stage.

Objective

Explore the molecular processes impaired in AD that were conversely preserved and enhanced by physical activity.

Methods

Integrated transcriptomic analyses were performed in datasets that contain AD patients and elders with different degrees of physical activity. The changes of the hub genes were validated through analyzing another two datasets. The expression of the hub genes was further detected in the hippocampus and cortexes of APP/PS1 transgenic mice with or without physical activity by Quantitative polymerase chain reaction (qPCR).

Results

Cross-comparison highlighted 195 DEGs displaying opposed regulation patterns between AD and high physical activity (HPA). The common DEGs were predominantly involved in synaptic vesicle recycling and synaptic transmission, largely downregulated in AD patients but upregulated in the elders with HPA. Two key modules and four hub genes that were related to synaptic vesicle turnover were obtained from the PPI network. The expression of these hub genes (SYT1, SYT4, SH3GL2, and AP2M1) was significantly decreased in AD transgenic mice and was reversed by HPA training.

Conclusions

HPA may reverse AD pathology by upregulating a range of synaptic vesicle transport related proteins which might improve the efficiency of synaptic vesicle turnover and facilitate inter-neuronal information transfer. The study provides novel insights into the mechanisms underlining the protective effects of HPA on AD.

CaMKII mediates sexually dimorphic synaptic transmission at neuromuscular junctions in C. elegans

Sexually dimorphic behaviors are ubiquitous throughout the animal kingdom. Although both sex-specific and sex-shared neurons have been functionally implicated in these diverse behaviors, less is known about the roles of sex-shared neurons. Here, we discovered sexually dimorphic cholinergic synaptic transmission in C. elegans occurring at neuromuscular junctions (NMJs), with males exhibiting increased release frequencies, which result in sexually dimorphic locomotion behaviors. Scanning electron microscopy revealed that males have significantly more synaptic vesicles (SVs) at their cholinergic synapses than hermaphrodites. Analysis of previously published transcriptome identified the male-enriched transcripts and focused our attention on UNC-43/CaMKII. We ultimately show that differential accumulation of UNC-43 at cholinergic neurons controls axonal SV abundance and synaptic transmission. Finally, we demonstrate that sex reversal of all neurons in hermaphrodites generates male-like cholinergic transmission and locomotion behaviors. Thus, beyond demonstrating UNC-43/CaMKII as an essential mediator of sex-specific synaptic transmission, our study provides molecular and cellular insights into how sex-shared neurons can generate sexually dimorphic locomotion behaviors.

UNC-43/CaMKII-triggered anterograde signals recruit GABAARs to mediate inhibitory synaptic transmission and plasticity at C. elegans NMJs

Disturbed inhibitory synaptic transmission has functional impacts on neurodevelopmental and psychiatric disorders. An essential mechanism for modulating inhibitory synaptic transmission is alteration of the postsynaptic abundance of GABA_ARs, which are stabilized by postsynaptic scaffold proteins and recruited by presynaptic signals. However, how GABAergic neurons trigger signals to transsynaptically recruit GABA_ARs remains elusive. Here, we show that UNC-43/CaMKII functions at GABAergic neurons to recruit GABA_ARs and modulate inhibitory synaptic transmission at C. elegans neuromuscular junctions. We demonstrate that UNC-43 promotes presynaptic MADD-4B/Punctin secretion and NRX-1α/Neurexin surface delivery. Together, MADD-4B and NRX-1α recruit postsynaptic NLG-1/Neuroligin and stabilize GABA_ARs. Further, the excitation of GABAergic neurons potentiates the recruitment of NLG-1-stabilized-GABA_ARs, which depends on UNC-43, MADD-4B, and NRX-1. These data all support that UNC-43 triggers MADD-4B and NRX-1α, which act as anterograde signals to recruit postsynaptic GABA_ARs. Thus, our findings elucidate a mechanism for pre- and postsynaptic communication and inhibitory synaptic transmission and plasticity.

A comparison of three different methods of eliciting rapid activity-dependent synaptic plasticity at the Drosophila NMJ

The Drosophila NMJ is a system of choice for investigating the mechanisms underlying the structural and functional modifications evoked during activity-dependent synaptic plasticity. Because fly genetics allows considerable versatility, many strategies can be employed to elicit this activity. Here, we compare three different stimulation methods for eliciting activity-dependent changes in structure and function at the Drosophila NMJ. We find that the method using patterned stimulations driven by a K+-rich solution creates robust structural modifications but reduces muscle viability, as assessed by resting potential and membrane resistance. We argue that, using this method, electrophysiological studies that consider the frequency of events, rather than their amplitude, are the only reliable studies. We contrast these results with the expression of CsChrimson channels and red-light stimulation at the NMJ, as well as with the expression of TRPA channels and temperature stimulation. With both these methods we observed reliable modifications of synaptic structures and consistent changes in electrophysiological properties. Indeed, we observed a rapid appearance of immature boutons that lack postsynaptic differentiation, and a potentiation of spontaneous neurotransmission frequency. Surprisingly, a patterned application of temperature changes alone is sufficient to provoke both structural and functional plasticity. In this context, temperature-dependent TRPA channel activation induces additional structural plasticity but no further increase in the frequency of spontaneous neurotransmission, suggesting an uncoupling of these mechanisms.

Modelling and Refining Neuronal Circuits with Guidance Cues: Involvement of Semaphorins

The establishment of neuronal circuits requires neurons to develop and maintain appropriate connections with cellular partners in and out the central nervous system. These phenomena include elaboration of dendritic arborization and formation of synaptic contacts, initially made in excess. Subsequently, refinement occurs, and pruning takes places both at axonal and synaptic level, defining a homeostatic balance maintained throughout the lifespan. All these events require genetic regulations which happens cell-autonomously and are strongly influenced by environmental factors. This review aims to discuss the involvement of guidance cues from the Semaphorin family.

Tyramine induces dynamic RNP granule remodeling and translation activation in the Drosophila brain

Ribonucleoprotein (RNP) granules are dynamic condensates enriched in regulatory RNA binding proteins (RBPs) and RNAs under tight spatiotemporal control. Extensive recent work has investigated the molecular principles underlying RNP granule assembly, unraveling that they form through the self-association of RNP components into dynamic networks of interactions. How endogenous RNP granules respond to external stimuli to regulate RNA fate is still largely unknown. Here, we demonstrate through high-resolution imaging of intact Drosophila brains that Tyramine induces a reversible remodeling of somatic RNP granules characterized by the decondensation of granule-enriched RBPs (e.g. Imp/ZBP1/IGF2BP) and helicases (e.g. Me31B/DDX-6/Rck). Furthermore, our functional analysis reveals that Tyramine signals both through its receptor TyrR and through the calcium-activated kinase CamkII to trigger RNP component decondensation. Finally, we uncover that RNP granule remodeling is accompanied by the rapid and specific translational activation of associated mRNAs. Thus, this work sheds new light on the mechanisms controlling cue-induced rearrangement of physiological RNP condensates.

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.

Synaptic Properties and Plasticity Mechanisms of Invertebrate Tonic and Phasic Neurons

Defining neuronal cell types and their associated biophysical and synaptic diversity has become an important goal in neuroscience as a mechanism to create comprehensive brain cell atlases in the post-genomic age. Beyond broad classification such as neurotransmitter expression, interneuron vs. pyramidal, sensory or motor, the field is still in the early stages of understanding closely related cell types. In both vertebrate and invertebrate nervous systems, one well-described distinction related to firing characteristics and synaptic release properties are tonic and phasic neuronal subtypes. In vertebrates, these classes were defined based on sustained firing responses during stimulation (tonic) vs. transient responses that rapidly adapt (phasic). In crustaceans, the distinction expanded to include synaptic release properties, with tonic motoneurons displaying sustained firing and weaker synapses that undergo short-term facilitation to maintain muscle contraction and posture. In contrast, phasic motoneurons with stronger synapses showed rapid depression and were recruited for short bursts during fast locomotion. Tonic and phasic motoneurons with similarities to those in crustaceans have been characterized in Drosophila, allowing the genetic toolkit associated with this model to be used for dissecting the unique properties and plasticity mechanisms for these neuronal subtypes. This review outlines general properties of invertebrate tonic and phasic motoneurons and highlights recent advances that characterize distinct synaptic and plasticity pathways associated with two closely related glutamatergic neuronal cell types that drive invertebrate locomotion.

Synaptic Plasticity Induced by Differential Manipulation of Tonic and Phasic Motoneurons in Drosophila

Structural and functional plasticity induced by neuronal competition is a common feature of developing nervous systems. However, the rules governing how postsynaptic cells differentiate between presynaptic inputs are unclear. In this study, we characterized synaptic interactions following manipulations of tonic Ib or phasic Is glutamatergic motoneurons that coinnervate postsynaptic muscles of male or female Drosophila melanogaster larvae. After identifying drivers for each neuronal subtype, we performed ablation or genetic manipulations to alter neuronal activity and examined the effects on synaptic innervation and function at neuromuscular junctions. Ablation of either Ib or Is resulted in decreased muscle response, with some functional compensation occurring in the Ib input when Is was missing. In contrast, the Is terminal failed to show functional or structural changes following loss of the coinnervating Ib input. Decreasing the activity of the Ib or Is neuron with tetanus toxin light chain resulted in structural changes in muscle innervation. Decreased Ib activity resulted in reduced active zone (AZ) number and decreased postsynaptic subsynaptic reticulum volume, with the emergence of filopodial-like protrusions from synaptic boutons of the Ib input. Decreased Is activity did not induce structural changes at its own synapses, but the coinnervating Ib motoneuron increased the number of synaptic boutons and AZs it formed. These findings indicate that tonic Ib and phasic Is motoneurons respond independently to changes in activity, with either functional or structural alterations in the Ib neuron occurring following ablation or reduced activity of the coinnervating Is input, respectively.SIGNIFICANCE STATEMENT Both invertebrate and vertebrate nervous systems display synaptic plasticity in response to behavioral experiences, indicating that underlying mechanisms emerged early in evolution. How specific neuronal classes innervating the same postsynaptic target display distinct types of plasticity is unclear. Here, we examined whether Drosophila tonic Ib and phasic Is motoneurons display competitive or cooperative interactions during innervation of the same muscle, or compensatory changes when the output of one motoneuron is altered. We established a system to differentially manipulate the motoneurons and examined the effects of cell type-specific changes to one of the inputs. Our findings indicate Ib and Is motoneurons respond differently to activity mismatch or loss of the coinnervating input, with the Ib subclass responding robustly compared with Is motoneurons.

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.

De Novo Germline Mutations in SEMA5A Associated With Infantile Spasms

Infantile spasm (IS) is an early-onset epileptic encephalopathy that usually presents with hypsarrhythmia on an electroencephalogram with developmental impairment or regression. In this study, whole-exome sequencing was performed to detect potential pathogenic de novo mutations, and finally we identified a novel damaging de novo mutation in SEMA5A and a compound heterozygous mutation in CLTCL1 in three sporadic trios with IS. The expression profiling of SEMA5A in the human brain showed that it was mainly highly expressed in the cerebral cortex, during the early brain development stage (8 to 9 post-conception weeks and 0 to 5 months after birth). In addition, we identified a close protein-protein interaction network between SEMA5A and candidate genes associated with epilepsy, autism spectrum disorder (ASD) or intellectual disability. Gene enrichment and function analysis demonstrated that genes interacting with SEMA5A were significantly enriched in several brain regions across early fetal development, including the cortex, cerebellum, striatum and thalamus (q < 0.05), and were involved in axonal, neuronal and synapse-associated processes. Furthermore, SEMA5A and its interacting genes were associated with ASD, epilepsy syndrome and developmental disorders of mental health. Our results provide insightful information indicating that SEMA5A may contribute to the development of the brain and is associated with IS. However, further genetic studies are still needed to evaluate the role of SEMA5A in IS to definitively establish the role of SEMA5A in this disorder.

Cell-type-Specific Patterned Stimulus-Independent Neuronal Activity in the Drosophila Visual System during Synapse Formation

Stereotyped synaptic connections define the neural circuits of the brain. In vertebrates, stimulus-independent activity contributes to neural circuit formation. It is unknown whether this type of activity is a general feature of nervous system development. Here, we report patterned, stimulus-independent neural activity in the Drosophila visual system during synaptogenesis. Using in vivo calcium, voltage, and glutamate imaging, we found that all neurons participate in this spontaneous activity, which is characterized by brain-wide periodic active and silent phases. Glia are active in a complementary pattern. Each of the 15 of over 100 specific neuron types in the fly visual system examined exhibited a unique activity signature. The activity of neurons that are synaptic partners in the adult was highly correlated during development. We propose that this cell-type-specific activity coordinates the development of the functional circuitry of the adult brain.

Plexin-Semaphorin Signaling Modifies Neuromuscular Defects in a Drosophila Model of Peripheral Neuropathy

Dominant mutations in GARS, encoding the ubiquitous enzyme glycyl-tRNA synthetase (GlyRS), cause peripheral nerve degeneration and Charcot-Marie-Tooth disease type 2D (CMT2D). This genetic disorder exemplifies a recurring paradigm in neurodegeneration, in which mutations in essential genes cause selective degeneration of the nervous system. Recent evidence suggests that the mechanism underlying CMT2D involves extracellular neomorphic binding of mutant GlyRS to neuronally-expressed proteins. Consistent with this, our previous studies indicate a non-cell autonomous mechanism, whereby mutant GlyRS is secreted and interacts with the neuromuscular junction (NMJ). In this Drosophila model for CMT2D, we have previously shown that mutant gars expression decreases viability and larval motor function, and causes a concurrent build-up of mutant GlyRS at the larval neuromuscular presynapse. Here, we report additional phenotypes that closely mimic the axonal branching defects of Drosophila plexin transmembrane receptor mutants, implying interference of plexin signaling in gars mutants. Individual dosage reduction of two Drosophila Plexins, plexin A (plexA) and B (plexB) enhances and represses the viability and larval motor defects caused by mutant GlyRS, respectively. However, we find plexB levels, but not plexA levels, modify mutant GlyRS association with the presynaptic membrane. Furthermore, increasing availability of the plexB ligand, Semaphorin-2a (Sema2a), alleviates the pathology and the build-up of mutant GlyRS, suggesting competition for plexB binding may be occurring between these two ligands. This toxic gain-of-function and subversion of neurodevelopmental processes indicate that signaling pathways governing axonal guidance could be integral to neuropathology and may underlie the non-cell autonomous CMT2D mechanism.

Neuropilin-2/PlexinA3 Receptors Associate with GluA1 and Mediate Sema3F-Dependent Homeostatic Scaling in Cortical Neurons

Regulation of AMPA-type glutamate receptor (AMPAR) number at synapses is a major mechanism for controlling synaptic strength during homeostatic scaling in response to global changes in neural activity. We show that the secreted guidance cue semaphorin 3F (Sema3F) and its neuropilin-2 (Npn-2)/plexinA3 (PlexA3) holoreceptor mediate homeostatic plasticity in cortical neurons. Sema3F-Npn-2/PlexA3 signaling is essential for cell surface AMPAR homeostatic downscaling in response to an increase in neuronal activity, Npn-2 associates with AMPARs, and Sema3F regulates this interaction. Therefore, Sema3F-Npn-2/PlexA3 signaling controls both synapse development and synaptic plasticity.

Retrograde semaphorin-plexin signalling drives homeostatic synaptic plasticity

Homeostatic signaling systems ensure stable, yet flexible neural activity and animal behavior^1–4. Defining the underlying molecular mechanisms of neuronal homeostatic signaling will be essential in order to establish clear connections to the causes and progression of neurological disease. Presynaptic homeostatic plasticity (PHP) is a conserved form of neuronal homeostatic signaling, observed in organisms ranging from Drosophila to human^1,5. Here, we demonstrate that Semaphorin2b (Sema2b) is target-derived signal that acts upon presynaptic PlexinB (PlexB) receptors to mediate the retrograde, homeostatic control of presynaptic neurotransmitter release at the Drosophila neuromuscular junction. Sema2b-PlexB signaling regulates the expression of PHP via the cytoplasmic protein Mical and the oxoreductase-dependent control of presynaptic actin^6,7. During neural development, Semaphorin-Plexin signaling instructs axon guidance and neuronal morphogenesis^8–10. Yet, Semaphorins and Plexins are also expressed in the adult brain^11–16. Here we demonstrate that Semaphorin-Plexin signaling controls presynaptic neurotransmitter release. We propose that Sema2b-PlexB signaling is an essential platform for the stabilization of synaptic transmission throughout life.

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.

Activity-Dependent Synaptic Refinement: New Insights from Drosophila

During development, neurons establish inappropriate connections as they seek out their synaptic partners, resulting in supernumerary synapses that must be pruned away. The removal of miswired synapses usually involves electrical activity, often through a Hebbian spike-timing mechanism. A novel form of activity-dependent refinement is used by Drosophila that may be non-Hebbian, and is critical for generating the precise connectivity observed in that system. In Drosophila, motoneurons use both glutamate and the biogenic amine octopamine for neurotransmission, and the muscle fibers receive multiple synaptic inputs. Motoneuron growth cones respond in a time-regulated fashion to multiple chemotropic signals arising from their postsynaptic partners. Central to this mechanism is a very low frequency (<0.03 Hz) oscillation of presynaptic cytoplasmic calcium, that regulates and coordinates the action of multiple downstream effectors involved in the withdrawal from off-target contacts. Low frequency calcium oscillations are widely observed in developing neural circuits in mammals, and have been shown to be critical for normal connectivity in a variety of neural systems. In Drosophila these mechanisms allow the growth cone to sample widely among possible synaptic partners, evaluate opponent chemotropic signals, and withdraw from off-target contacts. It is possible that the underlying molecular mechanisms are conserved widely among invertebrates and vertebrates.

Low dose extended exposure to saxitoxin and its potential neurodevelopmental effects: A review

Saxitoxin (STX) and its analogs, the paralytic shellfish toxins (PSTs), are a group of potent neurotoxins well known for their role in acute paralytic poisoning by preventing the generation of action potentials in neuronal cells. They are found in both marine and freshwater environments globally and although acute exposure from the former has previously received more attention, low dose extended exposure from both sources is possible and to date has not been investigated. Given the known role of cellular electrical activity in neurodevelopment this pattern of exposure may be a significant public health concern. Additionally, the presence of PSTs is likely to be an ongoing and possibly increasing problem in the future. This review examines the neurodevelopmental toxicity of STX, the risk of extended or repeated exposure to doses with neurodevelopmental effects, the potential implications of this exposure and briefly, the steps taken and difficulties faced in preventing exposure.

A fly's view of neuronal remodeling

Developmental neuronal remodeling is a crucial step in sculpting the final and mature brain connectivity in both vertebrates and invertebrates. Remodeling includes degenerative events, such as neurite pruning, that may be followed by regeneration to form novel connections during normal development. Drosophila provides an excellent model to study both steps of remodeling since its nervous system undergoes massive and stereotypic remodeling during metamorphosis. Although pruning has been widely studied, our knowledge of the molecular and cellular mechanisms is far from complete. Our understanding of the processes underlying regrowth is even more fragmentary. In this review, we discuss recent progress by focusing on three groups of neurons that undergo stereotypic pruning and regrowth during metamorphosis, the mushroom body γ neurons, the dendritic arborization neurons and the crustacean cardioactive peptide peptidergic neurons. By comparing and contrasting the mechanisms involved in remodeling of these three neuronal types, we highlight the common themes and differences as well as raise key questions for future investigation in the field. WIREs Dev Biol 2016, 5:618–635. doi: 10.1002/wdev.241

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Cyclic nucleotide signaling is required during synaptic refinement at the Drosophila neuromuscular junction

The removal of miswired synapses is a fundamental prerequisite for normal circuit development, leading to clinical problems when aberrant. However, the underlying activity-dependent molecular mechanisms involved in synaptic pruning remain incompletely resolved. Here the dynamic properties of intracellular calcium oscillations and a role for cAMP signaling during synaptic refinement in intact Drosophila embryos were examined using optogenetic tools. We provide In vivo evidence at the single gene level that the calcium-dependent adenylyl cyclase rutabaga, the phosphodiesterase dunce, the kinase PKA, and Protein Phosphatase 1 (PP1) all operate within a functional signaling pathway to modulate Sema2a-dependent chemorepulsion. It was found that presynaptic cAMP levels were required to be dynamically maintained at an optimal level to suppress connectivity defects. It was also proposed that PP1 may serve as a molecular link between cAMP signaling and CaMKII in the pathway underlying refinement. The results introduced an in vivo model where presynaptic cAMP levels, downstream of electrical activity and calcium influx, act via PKA and PP1 to modulate the neuron's response to chemorepulsion involved in the withdrawal of off-target synaptic contacts. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 39-60, 2017.

Control of Synaptic Connectivity by a Network of Drosophila IgSF Cell Surface Proteins

We have defined a network of interacting Drosophila cell surface proteins in which a 21-member IgSF subfamily, the Dprs, binds to a nine-member subfamily, the DIPs. The structural basis of the Dpr-DIP interaction code appears to be dictated by shape complementarity within the Dpr-DIP binding interface. Each of the six dpr and DIP genes examined here is expressed by a unique subset of larval and pupal neurons. In the neuromuscular system, interactions between Dpr11 and DIP-γ affect presynaptic terminal development, trophic factor responses, and neurotransmission. In the visual system, dpr11 is selectively expressed by R7 photoreceptors that use Rh4 opsin (yR7s). Their primary synaptic targets, Dm8 amacrine neurons, express DIP-γ. In dpr11 or DIP-γ mutants, yR7 terminals extend beyond their normal termination zones in layer M6 of the medulla. DIP-γ is also required for Dm8 survival or differentiation. Our findings suggest that Dpr-DIP interactions are important determinants of synaptic connectivity.

The translational regulator Cup controls NMJ presynaptic terminal morphology

During oogenesis and early embryonic development in Drosophila, translation of proteins from maternally deposited mRNAs is tightly controlled. We and others have previously shown that translational regulatory proteins that function during oogenesis also have essential roles in the nervous system. Here we examine the role of Cup in neuromuscular system development. Maternal Cup controls translation of localized mRNAs encoding the Oskar and Nanos proteins and binds to the general translation initiation factor eIF4E. In this paper, we show that zygotic Cup protein is localized to presynaptic terminals at larval neuromuscular junctions (NMJs). cup mutant NMJs have strong phenotypes characterized by the presence of small clustered boutons called satellite boutons. They also exhibit an increase in the frequency of spontaneous glutamate release events (mEPSPs). Reduction of eIF4E expression synergizes with partial loss of Cup expression to produce satellite bouton phenotypes. The presence of satellite boutons is often associated with increases in retrograde bone morphogenetic protein (BMP) signaling, and we show that synaptic BMP signaling is elevated in cup mutants. cup genetically interacts with two genes, EndoA and Dap160, that encode proteins involved in endocytosis that are also neuronal modulators of the BMP pathway. Endophilin protein, encoded by the EndoA gene, is downregulated in a cup mutant. Our results are consistent with a model in which Cup and eIF4E work together to ensure efficient localization and translation of endocytosis proteins in motor neurons and control the strength of the retrograde BMP signal.

Functional loss of semaphorin 3C and/or semaphorin 3D and their epistatic interaction with ret are critical to Hirschsprung disease liability

Innervation of the gut is segmentally lost in Hirschsprung disease (HSCR), a consequence of cell-autonomous and non-autonomous defects in enteric neuronal cell differentiation, proliferation, migration, or survival. Rare, high-penetrance coding variants and common, low-penetrance non-coding variants in 13 genes are known to underlie HSCR risk, with the most frequent variants in the ret proto-oncogene (RET). We used a genome-wide association (220 trios) and replication (429 trios) study to reveal a second non-coding variant distal to RET and a non-coding allele on chromosome 7 within the class 3 Semaphorin gene cluster. Analysis in Ret wild-type and Ret-null mice demonstrates specific expression of Sema3a, Sema3c, and Sema3d in the enteric nervous system (ENS). In zebrafish embryos, sema3 knockdowns show reduction of migratory ENS precursors with complete ablation under conjoint ret loss of function. Seven candidate receptors of Sema3 proteins are also expressed within the mouse ENS and their expression is also lost in the ENS of Ret-null embryos. Sequencing of SEMA3A, SEMA3C, and SEMA3D in 254 HSCR-affected subjects followed by in silico protein structure modeling and functional analyses identified five disease-associated alleles with loss-of-function defects in semaphorin dimerization and binding to their cognate neuropilin and plexin receptors. Thus, semaphorin 3C/3D signaling is an evolutionarily conserved regulator of ENS development whose dys-regulation is a cause of enteric aganglionosis.

Microtubule dynamics in axon guidance

Precise modulation of the cytoskeleton is involved in a variety of cellular processes including cell division, migration, polarity, and adhesion. In developing post-mitotic neurons, extracellular guidance cues not only trigger signaling cascades that act at a distance to indirectly regulate microtubule distribution, and assembly and disassembly in the growth cone, but also directly modulate microtubule stability and dynamics through coupling of guidance receptors with microtubules to control growth-cone turning. Microtubule-associated proteins including classical microtubule-associated proteins and microtubule plus-end tracking proteins are required for modulating microtubule dynamics to influence growth-cone steering. Multiple key signaling components, such as calcium, small GTPases, glycogen synthase kinase-3β, and c-Jun N-terminal kinase, link upstream signal cascades to microtubule stability and dynamics in the growth cone to control axon outgrowth and projection. Understanding the functions and regulation of microtubule dynamics in the growth cone provides new insights into the molecular mechanisms of axon guidance.

Semaphorins and the dynamic regulation of synapse assembly, refinement, and function

Semaphorins are phylogenetically conserved proteins expressed in most organ systems, including the nervous system. Following their description as axon guidance cues, semaphorins have been implicated in multiple aspects of nervous system development. Semaphorins are key regulators of neural circuit assembly, neuronal morphogenesis, assembly of excitatory and inhibitory synapses, and synaptic refinement. Semaphorins contribute to the balance between excitatory and inhibitory synaptic transmission, and electrical activity can modulate semaphorin signaling in neurons. This interplay between guidance cue signaling and electrical activity has the potential to sculpt the wiring of neural circuits and to modulate their function.

Optogenetic-mediated increases in in vivo spontaneous activity disrupt pool-specific but not dorsal-ventral motoneuron pathfinding

Rhythmic waves of spontaneous electrical activity are widespread in the developing nervous systems of birds and mammals, and although many aspects of neural development are activity-dependent, it has been unclear if rhythmic waves are required for in vivo motor circuit development, including the proper targeting of motoneurons to muscles. We show here that electroporated channelrhodopsin-2 can be activated in ovo with light flashes to drive waves at precise intervals of approximately twice the control frequency in intact chicken embryos. Optical monitoring of associated axial movements ensured that the altered frequency was maintained. In embryos thus stimulated, motor axons correctly executed the binary dorsal-ventral pathfinding decision but failed to make the subsequent pool-specific decision to target to appropriate muscles. This observation, together with the previous demonstration that slowing the frequency by half perturbed dorsal-ventral but not pool-specific pathfinding, shows that modest changes in frequency differentially disrupt these two major pathfinding decisions. Thus, many drugs known to alter early rhythmic activity have the potential to impair normal motor circuit development, and given the conservation between mouse and avian spinal cords, our observations are likely relevant to mammals, where such studies would be difficult to carry out.

Plum, an immunoglobulin superfamily protein, regulates axon pruning by facilitating TGF-β signaling

Axon pruning during development is essential for proper wiring of the mature nervous system, but its regulation remains poorly understood. We have identified an immunoglobulin superfamily (IgSF) transmembrane protein, Plum, that is cell autonomously required for axon pruning of mushroom body (MB) γ neurons and for ectopic synapse refinement at the developing neuromuscular junction in Drosophila. Plum promotes MB γ neuron axon pruning by regulating the expression of Ecdysone Receptor-B1, a key initiator of axon pruning. Genetic analyses indicate that Plum acts to facilitate signaling of Myoglianin, a glial-derived TGF-β, on MB γ neurons upstream of the type-I TGF-β receptor Baboon. Myoglianin, Baboon, and Ecdysone Receptor-B1 are also required for neuromuscular junction ectopic synapse refinement. Our study highlights both IgSF proteins and TGF-β facilitation as key promoters of developmental axon elimination and demonstrates a mechanistic conservation between MB axon pruning during metamorphosis and the refinement of ectopic larval neuromuscular connections.

Presynaptic CaMKIIα modulates dopamine D3 receptor activation in striatonigral terminals of the rat brain in a Ca²⁺ dependent manner

CaMKIIα is expressed at high density in the nucleus accumbens where it binds to postsynaptic D3 receptors inhibiting their effects. In striatonigral projections, activation of presynaptic D3 receptors potentiates D1 receptor-induced stimulation of cAMP production and GABA release. In this study we examined whether the presynaptic effects of D3 receptor stimulation in the substantia nigra reticulata (SNr) are modulated by Ca²⁺ activation of CaMKIIα. In SNr synaptosomes two procedures that increase cytoplasmic Ca²⁺, ionomycin and K⁺-depolarization, blocked the additional stimulation of cAMP accumulation produced by coactivating D3 and D1 dopamine receptors. The selective CaMKIIα inhibitor KN-62 reversed the blockade produced by ionomycin and K⁺-depolarization. Incubation in either Ca²-free solutions or with the selective Ca²⁺ blocker nifedipine, also reversed the blocking effects of K⁺-depolarization. Immunoblot studies showed that K⁺-depolarization increased CaMKIIα phosphorylation in a KN-62 sensitive manner and promoted CaMKIIα binding to D3 receptors. In K⁺-depolarized tissues, D3 receptors potentiated D1 receptor-induced stimulation of [³H]GABA release only when CaMKIIα was blocked with KN-62. In the presence of this inhibitor, the selective D3 agonist PD 128,907 reduced the ED₅₀ for the D1 agonist SKF 38393 from 56 to 4 nM. KN-62 also enhanced the effects of dopamine on depolarization induced [³H]GABA release. KN-62 changed ED₅₀ for dopamine from 584 to 56 nM. KN-62 did not affect D1 and D4 receptor responses. These experiments show that in striatonigral projections, CaMKIIα inhibits the action of D3 receptors in a Ca²⁺ dependent manner blocking their modulatory effects on GABA release. These findings suggest a mechanism through which the frequency of action potential discharge in presynaptic terminals regulates dopamine effects.

Temporal coherency between receptor expression, neural activity and AP-1-dependent transcription regulates Drosophila motoneuron dendrite development

Neural activity has profound effects on the development of dendritic structure. Mechanisms that link neural activity to nuclear gene expression include activity-regulated factors, such as CREB, Crest or Mef2, as well as activity-regulated immediate-early genes, such as fos and jun. This study investigates the role of the transcriptional regulator AP-1, a Fos-Jun heterodimer, in activity-dependent dendritic structure development. We combine genetic manipulation, imaging and quantitative dendritic architecture analysis in a Drosophila single neuron model, the individually identified motoneuron MN5. First, Dα7 nicotinic acetylcholine receptors (nAChRs) and AP-1 are required for normal MN5 dendritic growth. Second, AP-1 functions downstream of activity during MN5 dendritic growth. Third, using a newly engineered AP-1 reporter we demonstrate that AP-1 transcriptional activity is downstream of Dα7 nAChRs and Calcium/calmodulin-dependent protein kinase II (CaMKII) signaling. Fourth, AP-1 can have opposite effects on dendritic development, depending on the timing of activation. Enhancing excitability or AP-1 activity after MN5 cholinergic synapses and primary dendrites have formed causes dendritic branching, whereas premature AP-1 expression or induced activity prior to excitatory synapse formation disrupts dendritic growth. Finally, AP-1 transcriptional activity and dendritic growth are affected by MN5 firing only during development but not in the adult. Our results highlight the importance of timing in the growth and plasticity of neuronal dendrites by defining a developmental period of activity-dependent AP-1 induction that is temporally locked to cholinergic synapse formation and dendritic refinement, thus significantly refining prior models derived from chronic expression studies.

Development and plasticity of the Drosophila larval neuromuscular junction

The Drosophila larval neuromuscular system is relatively simple, containing only 32 motor neurons in each abdominal hemisegment, and its neuromuscular junctions (NMJs) have been studied extensively. NMJ synapses exhibit developmental and functional plasticity while displaying stereotyped connectivity. Drosophila Type I NMJ synapses are glutamatergic, while the vertebrate NMJ uses acetylcholine as its primary neurotransmitter. The larval NMJ synapses use ionotropic glutamate receptors (GluRs) that are homologous to AMPA-type GluRs in the mammalian brain, and they have postsynaptic scaffolds that resemble those found in mammalian postsynaptic densities. These features make the Drosophila neuromuscular system an excellent genetic model for the study of excitatory synapses in the mammalian central nervous system. The first section of the review presents an overview of NMJ development. The second section describes genes that regulate NMJ development, including: (1) genes that positively and negatively regulate growth of the NMJ, (2) genes required for maintenance of NMJ bouton structure, (3) genes that modulate neuronal activity and alter NMJ growth, (4) genes involved in transsynaptic signaling at the NMJ. The third section describes genes that regulate acute plasticity, focusing on translational regulatory mechanisms. As this review is intended for a developmental biology audience, it does not cover NMJ electrophysiology in detail, and does not review genes for which mutations produce only electrophysiological but no structural phenotypes.

Activity-dependent competition regulates motor neuron axon pathfinding via PlexinA3

The role of electrical activity in axon guidance has been extensively studied in vitro. To better understand its role in the intact nervous system, we imaged intracellular Ca(2+) in zebrafish primary motor neurons (PMN) during axon pathfinding in vivo. We found that PMN generate specific patterns of Ca(2+) spikes at different developmental stages. Spikes arose in the distal axon of PMN and were propagated to the cell body. Suppression of Ca(2+) spiking activity in single PMN led to stereotyped errors, but silencing all electrical activity had no effect on axon guidance, indicating that an activity-based competition rule regulates this process. This competition was not mediated by synaptic transmission. Combination of PlexinA3 knockdown with suppression of Ca(2+) activity in single PMN produced a synergistic increase in the incidence of pathfinding errors. However, expression of PlexinA3 transcripts was not regulated by activity. Our results provide an in vivo demonstration of the intersection of spontaneous electrical activity with the PlexinA3 guidance molecule receptor in regulation of axon pathfinding.

Role of adenylate cyclase 1 in retinofugal map development

The development of topographic maps of the sensory periphery is sensitive to the disruption of adenylate cyclase 1 (AC1) signaling. AC1 catalyzes the production of cAMP in a Ca2+/calmodulin-dependent manner, and AC1 mutant mice (AC1−/−) have disordered visual and somatotopic maps. However, the broad expression of AC1 in the brain and the promiscuous nature of cAMP signaling have frustrated attempts to determine the underlying mechanism of AC1-dependent map development. In the mammalian visual system, the initial coarse targeting of retinal ganglion cell (RGC) projections to the superior colliculus (SC) and lateral geniculate nucleus (LGN) is guided by molecular cues, and the subsequent refinement of these crude projections occurs via an activity-dependent process that depends on spontaneous retinal waves. Here, we show that AC1−/− mice have normal retinal waves but disrupted map refinement. We demonstrate that AC1 is required for the emergence of dense and focused termination zones and elimination of inaccurately targeted collaterals at the level of individual retinofugal arbors. Conditional deletion of AC1 in the retina recapitulates map defects, indicating that the locus of map disruptions in the SC and dorsal LGN of AC1−/− mice is presynaptic. Finally, map defects in mice without AC1 and disrupted retinal waves (AC1−/−;β2−/− double KO mice) are no worse than those in mice lacking only β2−/−, but loss of AC1 occludes map recovery in β2−/− mice during the second postnatal week. These results suggest that AC1 in RGC axons mediates the development of retinotopy and eye-specific segregation in the SC and dorsal LGN.

Getting neural circuits into shape with semaphorins

Semaphorins are key players in the control of neural circuit development. Recent studies have uncovered several exciting and novel aspects of neuronal semaphorin signalling in various cellular processes--including neuronal polarization, topographical mapping and axon sorting--that are crucial for the assembly of functional neuronal connections. This progress is important for further understanding the many neuronal and non-neuronal functions of semaphorins and for gaining insight into their emerging roles in the perturbed neural connectivity that is observed in some diseases. This Review discusses recent advances in semaphorin research, focusing on novel aspects of neuronal semaphorin receptor regulation and previously unexplored cellular functions of semaphorins in the nervous system.

EGFR signaling modulates synaptic connectivity via Gurken

Synaptic target selection is critical for establishing functional neuronal circuits. The mechanisms regulating target selection remain incompletely understood. We describe a role for the EGF receptor and its ligand Gurken in target selection of octopaminergic Type II neurons in the Drosophila neuromuscular system. Mutants in happyhour, a regulator of EGFR signaling, form ectopic Type II neuromuscular junctions. These ectopic innervations are due to inappropriate target selection. We demonstrate that EGFR signaling is necessary and sufficient to inhibit synaptic target selection by these octopaminergic Type II neurons, and that the EGFR ligand Gurken is the postsynaptic, muscle-derived repulsive cue. These results identify a new pathway mediating cell-type and branch-specific synaptic repulsion, a novel role for EGFR signaling in synaptic target selection, and an unexpected role for Gurken as a muscle-secreted repulsive ligand.

Generation of neuromuscular specificity in Drosophila: novel mechanisms revealed by new technologies

The Drosophila larval neuromuscular system is one of the best-characterized model systems for axon targeting. In each abdominal hemisegment, only 36 identified motor neurons form synaptic connections with just 30 target muscles in a highly specific and stereotypic manner. Studies in the 1990s identified several cell-surface and secreted proteins that are expressed in specific muscles and contribute to target specificity. Emerging evidence suggests that target selection is determined not only by attraction to the target cells but also by exclusion from non-target cells. Proteins with leucine-rich repeats (LRR proteins) appear to be a major molecular family of proteins responsible for the targeting. While the demonstrated roles of the target-derived cues point to active recognition by presynaptic motor neurons, postsynaptic muscles also reach out and recognize specific motor neurons by sending out cellular protrusions called myopodia. Simultaneous live imaging of myopodia and growth cones has revealed that local and mutual recognition at the tip of myopodia is critical for selective synapse formation. A large number of candidate target cues have been identified on a single muscle, suggesting that target specificity is determined by the partially redundant and combinatorial function of multiple cues. Analyses of the seemingly simple neuromuscular system in Drosophila have revealed an unexpected complexity in the mechanisms of axon targeting.

Lola regulates glutamate receptor expression at the Drosophila neuromuscular junction

Communication between pre- and post-synaptic cells is a key process in the development and modulation of synapses. Reciprocal induction between pre- and postsynaptic cells involves regulation of gene transcription, yet the underlying genetic program remains largely unknown. To investigate how innervation-dependent gene expression in postsynaptic cells supports synaptic differentiation, we performed comparative microarray analysis of Drosophila muscles before and after innervation, and of prospero mutants, which show a delay in motor axon outgrowth. We identified 84 candidate genes that are potentially up- or downregulated in response to innervation. By systematic functional analysis, we found that one of the downregulated genes, longitudinals lacking (lola), which encodes a BTB-Zn-finger transcription factor, is required for proper expression of glutamate receptors. When the function of lola was knocked down in muscles by RNAi, the abundance of glutamate receptors (GluRs), GluRIIA, GluRIIB and GluRIII, as well as that of p-21 activated kinase (PAK), was greatly reduced at the neuromuscular junctions (NMJs). Recordings of the synaptic response revealed a decrease in postsynaptic quantal size, consistent with the reduction in GluR levels. Lola appears to regulate the expression of GluRs and PAK at the level of transcription, because the amount of mRNAs encoding these molecules was also reduced in the mutants. The transcriptional level of lola, in turn, is downregulated by increased neural activity. We propose that Lola coordinates expression of multiple postsynaptic components by transcriptional regulation.

Optogenetics and thermogenetics: technologies for controlling the activity of targeted cells within intact neural circuits

In recent years, interest has grown in the ability to manipulate, in a temporally precise fashion, the electrical activity of specific neurons embedded within densely wired brain circuits, in order to reveal how specific neurons subserve behaviors and neural computations, and to open up new horizons on the clinical treatment of brain disorders. Technologies that enable temporally precise control of electrical activity of specific neurons, and not these neurons' neighbors-whose cell bodies or processes might be just tens to hundreds of nanometers away-must involve two components. First, they require as a trigger a transient pulse of energy that supports the temporal precision of the control. Second, they require a molecular sensitizer that can be expressed in specific neurons and which renders those neurons specifically responsive to the triggering energy delivered. Optogenetic tools, such as microbial opsins, can be used to activate or silence neural activity with brief pulses of light. Thermogenetic tools, such as thermosensitive TRP channels, can be used to drive neural activity downstream of increases or decreases in temperature. We here discuss the principles underlying the operation of these two recently developed, but widely used, toolboxes, as well as the directions being taken in the use and improvement of these toolboxes.

The genetics of calcium signaling in Drosophila melanogaster

BACKGROUND

Genetic screens for behavioral and physiological defects in Drosophila melanogaster, helped identify several components of calcium signaling of which some, like the Trps, were novel. For genes initially identified in vertebrates, reverse genetic methods have allowed functional studies at the cellular and systemic levels.

SCOPE OF REVIEW

The aim of this review is to explain how various genetic methods available in Drosophila have been used to place different arms of Ca2+ signaling in the context of organismal development, physiology and behavior.

MAJOR CONCLUSION

Mutants generated in genes encoding a range of Ca2+ transport systems, binding proteins and enzymes affect multiple aspects of neuronal and muscle physiology. Some also affect the maintenance of ionic balance and excretion from malpighian tubules and innate immune responses in macrophages. Aspects of neuronal physiology affected include synaptic growth and plasticity, sensory transduction, flight circuit development and function. Genetic interaction screens have shown that mechanisms of maintaining Ca2+ homeostasis in Drosophila are cell specific and require a synergistic interplay between different intracellular and plasma membrane Ca2+ signaling molecules.

GENERAL SIGNIFICANCE

Insights gained through genetic studies of conserved Ca2+ signaling pathways have helped understand multiple aspects of fly physiology. The similarities between mutant phenotypes of Ca2+ signaling genes in Drosophila with certain human disease conditions, especially where homologous genes are causative factors, are likely to aid in the discovery of underlying disease mechanisms and help develop novel therapeutic strategies. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Differential control of presynaptic CaMKII activation and translocation to active zones

The release of neurotransmitters, neurotrophins, and neuropeptides is modulated by Ca(2+) mobilization from the endoplasmic reticulum (ER) and activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Furthermore, when neuronal cultures are subjected to prolonged depolarization, presynaptic CaMKII redistributes from the cytoplasm to accumulate near active zones (AZs), a process that is reminiscent of CaMKII translocation to the postsynaptic side of the synapse. However, it is not known how presynaptic CaMKII activation and translocation depend on neuronal activity and ER Ca(2+) release. Here these issues are addressed in Drosophila motoneuron terminals by imaging a fluorescent reporter of CaMKII activity and subcellular distribution. We report that neuronal excitation acts with ER Ca(2+) stores to induce CaMKII activation and translocation to a subset of AZs. Surprisingly, activation is slow, reflecting T286 autophosphorylation and the function of presynaptic ER ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP3Rs). Furthermore, translocation is not simply proportional to CaMKII activity, as T286 autophosphorylation promotes activation, but does not affect translocation. In contrast, RNA interference-induced knockdown of the AZ scaffold protein Bruchpilot disrupts CaMKII translocation without affecting activation. Finally, RyRs comparably stimulate both activation and translocation, but IP3Rs preferentially promote translocation. Thus, Ca(2+) provided by different presynaptic ER Ca(2+) release channels is not equivalent. These results suggest that presynaptic CaMKII activation depends on autophosphorylation and global Ca(2+) in the terminal, while translocation to AZs requires Ca(2+) microdomains generated by IP3Rs.

An instructive role for patterned spontaneous retinal activity in mouse visual map development

Complex neural circuits in the mammalian brain develop through a combination of genetic instruction and activity-dependent refinement. The relative role of these factors and the form of neuronal activity responsible for circuit development is a matter of significant debate. In the mammalian visual system, retinal ganglion cell projections to the brain are mapped with respect to retinotopic location and eye of origin. We manipulated the pattern of spontaneous retinal waves present during development without changing overall activity levels through the transgenic expression of β2-nicotinic acetylcholine receptors in retinal ganglion cells of mice. We used this manipulation to demonstrate that spontaneous retinal activity is not just permissive, but instructive in the emergence of eye-specific segregation and retinotopic refinement in the mouse visual system. This suggests that specific patterns of spontaneous activity throughout the developing brain are essential in the emergence of specific and distinct patterns of neuronal connectivity.

Cracking the combinatorial semaphorin code

In this issue of Neuron, Wu et al. describe a combinatorial code of repulsive Sema-2a and attractive Sema-2b signaling that mediates mechanosensory axonal guidance, fasciculation, and synaptic target selection within the CNS of Drosophila. Their work exemplifies how a detailed, multilevel molecular-genetic analysis (from molecules to behavior) provides fundamental insights into neural circuit development.

A combinatorial semaphorin code instructs the initial steps of sensory circuit assembly in the Drosophila CNS

Longitudinal axon fascicles within the Drosophila embryonic CNS provide connections between body segments and are required for coordinated neural signaling along the anterior-posterior axis. We show here that establishment of select CNS longitudinal tracts and formation of precise mechanosensory afferent innervation to the same CNS region are coordinately regulated by the secreted semaphorins Sema-2a and Sema-2b. Both Sema-2a and Sema-2b utilize the same neuronal receptor, plexin B (PlexB), but serve distinct guidance functions. Localized Sema-2b attraction promotes the initial assembly of a subset of CNS longitudinal projections and subsequent targeting of chordotonal sensory afferent axons to these same longitudinal connectives, whereas broader Sema-2a repulsion serves to prevent aberrant innervation. In the absence of Sema-2b or PlexB, chordotonal afferent connectivity within the CNS is severely disrupted, resulting in specific larval behavioral deficits. These results reveal that distinct semaphorin-mediated guidance functions converge at PlexB and are critical for functional neural circuit assembly.