Dendritic targeting in the leg neuropil of Drosophila: the role of midline signalling molecules in generating a myotopic map

scRNA-seq data from the larval Drosophila ventral cord provides a resource for studying motor systems function and development

The Drosophila larval ventral nerve cord (VNC) shares many similarities with the spinal cord of vertebrates and has emerged as a major model for understanding the development and function of motor systems. Here, we use high-quality scRNA-seq, validated by anatomical identification, to create a comprehensive census of larval VNC cell types. We show that the neural lineages that comprise the adult VNC are already defined, but quiescent, at the larval stage. Using fluorescence-activated cell sorting (FACS)-enriched populations, we separate all motor neuron bundles and link individual neuron clusters to morphologically characterized known subtypes. We discovered a glutamate receptor subunit required for basal neurotransmission and homeostasis at the larval neuromuscular junction. We describe larval glia and endorse the general view that glia perform consistent activities throughout development. This census represents an extensive resource and a powerful platform for future discoveries of cellular and molecular mechanisms in repair, regeneration, plasticity, homeostasis, and behavioral coordination.

Neuronal mechanisms regulating locomotion in adult Drosophila

The coordinated action of multiple leg joints and muscles is required even for the simplest movements. Understanding the neuronal circuits and mechanisms that generate precise movements is essential for comprehending the neuronal basis of the locomotion and to infer the neuronal mechanisms underlying several locomotor-related diseases. Drosophila melanogaster provides an excellent model system for investigating the neuronal circuits underlying motor behaviors due to its simple nervous system and genetic accessibility. This review discusses current genetic methods for studying locomotor circuits and their function in adult Drosophila. We highlight recently identified neuronal pathways that modulate distinct forward and backward locomotion and describe the underlying neuronal control of leg swing and stance phases in freely moving flies. We also report various automated leg tracking methods to measure leg motion parameters and define inter-leg coordination, gait and locomotor speed of freely moving adult flies. Finally, we emphasize the role of leg proprioceptive signals to central motor circuits in leg coordination. Together, this review highlights the utility of adult Drosophila as a model to uncover underlying motor circuitry and the functional organization of the leg motor system that governs correct movement.

Characterization of enhancer fragments in Drosophila robo2

Receptor proteins of the Roundabout (Robo) family regulate axon guidance decisions during nervous system development. Among the three Drosophila robo family genes (robo1, robo2 and robo3), robo2 displays a dynamic expression pattern and regulates multiple axon guidance outcomes, including preventing midline crossing in some axons, promoting midline crossing in others, forming lateral longitudinal axon pathways, and regulating motor axon guidance. The identity and location of enhancer elements regulating robo2's complex and dynamic expression pattern in different neural cell types are unknown. Here, we characterize a set of 17 transgenic lines expressing GAL4 under the control of DNA sequences derived from noncoding regions in and around robo2, to identify enhancers controlling specific aspects of robo2 expression in the embryonic ventral nerve cord. We identify individual fragments that confer expression in specific cell types where robo2 is known to function, including early pioneer neurons, midline glia and lateral longitudinal neurons. Our results indicate that robo2's dynamic expression pattern is specified by a combination of enhancer elements that are active in different subsets of cells. We show that robo2's expression in lateral longitudinal axons represents two genetically separable subsets of neurons, and compare their axon projections with each other and with Fasciclin II (FasII), a commonly used marker of longitudinal axon pathways. In addition, we provide a general description of each fragment's expression in embryonic tissues outside of the nervous system, to serve as a resource for other researchers interested in robo2 expression and its functional roles outside the central nervous system.

Post-transcriptional regulation of transcription factor codes in immature neurons drives neuronal diversity

How the vast array of neuronal diversity is generated remains an unsolved problem. Here, we investigate how 29 morphologically distinct leg motoneurons are generated from a single stem cell in Drosophila. We identify 19 transcription factor (TF) codes expressed in immature motoneurons just before their morphological differentiation. Using genetic manipulations and a computational tool, we demonstrate that the TF codes are progressively established in immature motoneurons according to their birth order. Comparing RNA and protein expression patterns of multiple TFs reveals that post-transcriptional regulation plays an essential role in shaping these TF codes. Two RNA-binding proteins, Imp and Syp, expressed in opposing gradients in immature motoneurons, control the translation of multiple TFs. The varying sensitivity of TF mRNAs to the opposing gradients of Imp and Syp in immature motoneurons decrypts these gradients into distinct TF codes, establishing the connectome between motoneuron axons and their target muscles.

Age-dependent degeneration of an identified adult leg motor neuron in a Drosophila SOD1 model of ALS

Mutations in superoxide dismutase 1 (SOD1) cause familial amyotrophic lateral sclerosis (ALS) in humans. ALS is a neurodegenerative disease characterized by progressive motor neuron loss leading to paralysis and inevitable death in affected individuals. Using a gene replacement strategy to introduce disease mutations into the orthologous Drosophila sod1 (dsod1) gene, here, we characterize changes at the neuromuscular junction using longer-lived dsod1 mutant adults. Homozygous dsod1H71Y/H71Y or dsod1null/null flies display progressive walking defects with paralysis of the third metathoracic leg. In dissected legs, we assessed age-dependent changes in a single identified motor neuron (MN-I2) innervating the tibia levitator muscle. At adult eclosion, MN-I2 of dsod1H71Y/H71Y or sod1null/null flies is patterned similar to wild-type flies indicating no readily apparent developmental defects. Over the course of 10 days post-eclosion, MN-I2 shows an overall reduction in arborization with bouton swelling and loss of the post-synaptic marker discs-large (dlg) in mutant dsod1 adults. In addition, increases in polyubiquitinated proteins correlate with the timing and extent of MN-I2 changes. Because similar phenotypes are observed between flies homozygous for either dsod1H71Y or dsod1null alleles, we conclude these NMJ changes are mainly associated with sod loss-of-function. Together these studies characterize age-related morphological and molecular changes associated with axonal retraction in a Drosophila model of ALS that recapitulate an important aspect of the human disease.This article has an associated First Person interview with the first author of the paper.

Genetic Control of Muscle Diversification and Homeostasis: Insights from Drosophila

In the fruit fly, Drosophila melanogaster, the larval somatic muscles or the adult thoracic flight and leg muscles are the major voluntary locomotory organs. They share several developmental and structural similarities with vertebrate skeletal muscles. To ensure appropriate activity levels for their functions such as hatching in the embryo, crawling in the larva, and jumping and flying in adult flies all muscle components need to be maintained in a functionally stable or homeostatic state despite constant strain. This requires that the muscles develop in a coordinated manner with appropriate connections to other cell types they communicate with. Various signaling pathways as well as extrinsic and intrinsic factors are known to play a role during Drosophila muscle development, diversification, and homeostasis. In this review, we discuss genetic control mechanisms of muscle contraction, development, and homeostasis with particular emphasis on the contractile unit of the muscle, the sarcomere.

let-7-Complex MicroRNAs Regulate Broad-Z3, Which Together with Chinmo Maintains Adult Lineage Neurons in an Immature State

During Drosophila melanogaster metamorphosis, arrested immature neurons born during larval development differentiate into their functional adult form. This differentiation coincides with the downregulation of two zinc-finger transcription factors, Chronologically Inappropriate Morphogenesis (Chinmo) and the Z3 isoform of Broad (Br-Z3). Here, we show that br-Z3 is regulated by two microRNAs, let-7 and miR-125, that are activated at the larval-to-pupal transition and are known to also regulate chinmo. The br-Z3 3′UTR contains functional binding sites for both let-7 and miR-125 that confers sensitivity to both of these microRNAs, as determined by deletion analysis in reporter assays. Forced expression of let-7 and miR-125 miRNAs leads to early silencing of Br-Z3 and Chinmo and is associated with inappropriate neuronal sprouting and outgrowth. Similar phenotypes were observed by the combined but not separate depletion of br-Z3 and chinmo. Because persistent Br-Z3 was not detected in let-7-C mutants, this work suggests a model in which let-7 and miR-125 activation at the onset of metamorphosis may act as a failsafe mechanism that ensures the coordinated silencing of both br-Z3 and chinmo needed for the timely outgrowth of neurons arrested during larval development. The let-7 and miR-125 binding site sequences are conserved across Drosophila species and possibly other insects as well, suggesting that this functional relationship is evolutionarily conserved.

Serotonergic Modulation of Walking in Drosophila

To navigate complex environments, animals must generate highly robust, yet flexible, locomotor behaviors. For example, walking speed must be tailored to the needs of a particular environment. Not only must animals choose the correct speed and gait, they must also adapt to changing conditions and quickly respond to sudden and surprising new stimuli. Neuromodulators, particularly the small biogenic amine neurotransmitters, have the ability to rapidly alter the functional outputs of motor circuits. Here, we show that the serotonergic system in the vinegar fly, Drosophila melanogaster, can modulate walking speed in a variety of contexts and also change how flies respond to sudden changes in the environment. These multifaceted roles of serotonin in locomotion are differentially mediated by a family of serotonergic receptors with distinct activities and expression patterns.

Chronology-based architecture of descending circuits that underlie the development of locomotor repertoire after birth

The emergence of new and increasingly sophisticated behaviors after birth is accompanied by dramatic increase of newly established synaptic connections in the nervous system. Little is known, however, of how nascent connections are organized to support such new behaviors alongside existing ones. To understand this, in the larval zebrafish we examined the development of spinal pathways from hindbrain V2a neurons and the role of these pathways in the development of locomotion. We found that new projections are continually layered laterally to existing neuropil, and give rise to distinct pathways that function in parallel to existing pathways. Across these chronologically layered pathways, the connectivity patterns and biophysical properties vary systematically to support a behavioral repertoire with a wide range of kinematics and dynamics. Such layering of new parallel circuits equipped with systematically changing properties may be central to the postnatal diversification and increasing sophistication of an animal’s behavioral repertoire.

Newborn babies have limited abilities. Indeed, most of our actions shortly after birth are the result of reflexes that serve our most basic need: to stay alive. As we get older, however, our behaviour gradually becomes more sophisticated. During this time, the billions of cells in our brain form new connections to build intricate ‘circuits’ of neurons that allow for more complicated thoughts and actions.

It is clear that the brain circuits that support new behaviours must develop in a way that does not interfere with the existing circuits that are vital for survival. However, the challenge has been to find a way to peer into a brain as it develops to see how these new circuits form.

In recent years, zebrafish have revolutionised research into neuronal circuits in animals. Developing over the course of a few days, these small transparent fish provide a window into the brain during the earliest stages of development. Indeed, the circuits of neurons that descend from the brain and connect to the spinal cord have already been mapped in these animals. Now, Pujala and Koyama have begun to follow the careful development of these ‘descending’ neurons, and relate it to the appearance of new behaviours in young zebrafish.

Time-lapse imaging with a fluorescent protein that is active only in specific descending neurons revealed that new circuits are laid down over existing ones, like the growth rings in a tree. Next, at different timepoints in zebrafish development, Pujala and Koyama traced these neurons backwards from the spine to the brain to identify which connections formed first. This showed that the spinal connections develop one after the other, in the same order that the neurons mature.

Next, Pujala and Koyama asked how the activity of neurons that mature early or late in development relates to specific behaviours in young zebrafish. Early-born circuits connect to neurons that produce powerful, reflex-driven, whole-body movements such as an escape response. The later circuits connect to different neurons through slower, less direct pathways; the late-born neurons also generate the refined movements that are acquired later in a zebrafish’s development and help the fish to explore its environment.

These findings show that descending circuits in zebrafish run parallel to each other, but with distinct connections and properties that allow them to control different kinds of movements. While this study was conducted using an animal model, a better understanding of how such circuits develop and the movements they control may one day aid the treatment of patients with neurodegenerative diseases or injuries where connections have been lost.

Reactive oxygen species regulate activity-dependent neuronal plasticity in Drosophila

Reactive oxygen species (ROS) have been extensively studied as damaging agents associated with ageing and neurodegenerative conditions. Their role in the nervous system under non-pathological conditions has remained poorly understood. Working with the Drosophila larval locomotor network, we show that in neurons ROS act as obligate signals required for neuronal activity-dependent structural plasticity, of both pre- and postsynaptic terminals. ROS signaling is also necessary for maintaining evoked synaptic transmission at the neuromuscular junction, and for activity-regulated homeostatic adjustment of motor network output, as measured by larval crawling behavior. We identified the highly conserved Parkinson’s disease-linked protein DJ-1β as a redox sensor in neurons where it regulates structural plasticity, in part via modulation of the PTEN-PI3Kinase pathway. This study provides a new conceptual framework of neuronal ROS as second messengers required for neuronal plasticity and for network tuning, whose dysregulation in the ageing brain and under neurodegenerative conditions may contribute to synaptic dysfunction.

Branching mechanisms shaping dendrite architecture

A neuron's contribution to the information flow within a neural circuit is governed by the structure of its dendritic arbor. The geometry of the dendritic arbor directly determines synaptic density and the size of the receptive field, both of which influence the firing pattern of the neuron. Importantly, the position of individual dendritic branches determines the identity of the neuron's presynaptic partner and thus the nature of the incoming sensory information. To generate the unique stereotypic architecture of a given neuronal subtype, nascent branches must emerge from the dendritic shaft at preprogramed branch points. Subsequently, a complex array of extrinsic factors regulates the degree and orientation of branch expansion to ensure maximum coverage of the receptive field whilst constraining growth within predetermined territories. In this review we focus on studies that best illustrate how environmental cues such as the Wnts and Netrins and their receptors sculpt the dendritic arbor. We emphasize the pivotal role played by the actin cytoskeleton and its upstream regulators in branch initiation, outgrowth and navigation. Finally, we discuss how protocadherin and DSCAM contact-mediated repulsion prevents inappropriate synapse formation between sister dendrites or dendrites and the axon from the same neuron. Together these studies highlight the clever ways evolution has solved the problem of constructing complex branch geometries.

Visualize Drosophila Leg Motor Neuron Axons Through the Adult Cuticle

The majority of work on the neuronal specification has been carried out in genetically and physiologically tractable models such as C. elegans, Drosophila larvae, and fish, which all engage in undulatory movements (like crawling or swimming) as their primary mode of locomotion. However, a more sophisticated understanding of the individual motor neuron (MN) specification-at least in terms of informing disease therapies-demands an equally tractable system that better models the complex appendage-based locomotion schemes of vertebrates. The adult Drosophila locomotor system in charge of walking meets all of these criteria with ease, since in this model it is possible to study the specification of a small number of easily distinguished leg MNs (approximately 50 MNs per leg) both using a vast array of powerful genetic tools, and in the physiological context of an appendage-based locomotion scheme. Here we describe a protocol to visualize the leg muscle innervation in an adult fly.

GABAergic inhibition of leg motoneurons is required for normal walking behavior in freely moving Drosophila

Inhibition is an important feature of the neuronal circuit, and in walking, it aids in controlling coordinated movement of legs, leg segments, and joints. Recent studies in Drosophila report the role of premotor inhibitory interneurons in regulation of larval locomotion. However, in adult walking, the identity and function of premotor interneurons are poorly understood. Here, we use genetic methods for targeted knockdown of inhibitory neurotransmitter receptors in leg motoneurons, combined with automated video recording methods we have developed for quantitative analysis of fly leg movements and walking parameters, to reveal the resulting slower walking speed and defects in walking parameters. Our results indicate that GABAergic premotor inhibition to leg motoneurons is required to control the normal walking behavior in adult Drosophila.

Walking is a complex rhythmic locomotor behavior generated by sequential and periodical contraction of muscles essential for coordinated control of movements of legs and leg joints. Studies of walking in vertebrates and invertebrates have revealed that premotor neural circuitry generates a basic rhythmic pattern that is sculpted by sensory feedback and ultimately controls the amplitude and phase of the motor output to leg muscles. However, the identity and functional roles of the premotor interneurons that directly control leg motoneuron activity are poorly understood. Here we take advantage of the powerful genetic methodology available in Drosophila to investigate the role of premotor inhibition in walking by genetically suppressing inhibitory input to leg motoneurons. For this, we have developed an algorithm for automated analysis of leg motion to characterize the walking parameters of wild-type flies from high-speed video recordings. Further, we use genetic reagents for targeted RNAi knockdown of inhibitory neurotransmitter receptors in leg motoneurons together with quantitative analysis of resulting changes in leg movement parameters in freely walking Drosophila. Our findings indicate that targeted down-regulation of the GABA_A receptor Rdl (Resistance to Dieldrin) in leg motoneurons results in a dramatic reduction of walking speed and step length without the loss of general leg coordination during locomotion. Genetically restricting the knockdown to the adult stage and subsets of motoneurons yields qualitatively identical results. Taken together, these findings identify GABAergic premotor inhibition of motoneurons as an important determinant of correctly coordinated leg movements and speed of walking in freely behaving Drosophila.

Islet Coordinately Regulates Motor Axon Guidance and Dendrite Targeting through the Frazzled/DCC Receptor

Motor neuron axon targeting in the periphery is correlated with the positions of motor neuron inputs in the CNS, but how these processes are coordinated to form a myotopic map remains poorly understood. We show that the LIM homeodomain factor Islet (Isl) controls targeting of both axons and dendrites in Drosophila motor neurons through regulation of the Frazzled (Fra)/DCC receptor. Isl is required for fra expression in ventrally projecting motor neurons, and isl and fra mutants have similar axon guidance defects. Single-cell labeling indicates that isl and fra are also required for dendrite targeting in a subset of motor neurons. Finally, overexpression of Fra rescues axon and dendrite targeting defects in isl mutants. These results indicate that Fra acts downstream of Isl in both the periphery and the CNS, demonstrating how a single regulatory relationship is used in multiple cellular compartments to coordinate neural circuit wiring.

Spatial patterning of excitatory and inhibitory neuropil territories during spinal circuit development

To generate rhythmic motor behaviors, both single neurons and neural circuits require a balance between excitatory inputs that trigger action potentials and inhibitory inputs that promote a stable resting potential (E/I balance). Previous studies have focused on individual neurons and have shown that, over a short spatial scale, excitatory and inhibitory (E/I) synapses tend to form structured territories with inhibitory inputs enriched on cell bodies and proximal dendrites and excitatory inputs on distal dendrites. However, systems-level E/I patterns, at spatial scales larger than single neurons, are largely uncharted. We used immunostaining for PSD-95 and gephyrin postsynaptic scaffolding proteins as proxies for excitatory and inhibitory synapses, respectively, to quantify the numbers and map the distributions of E/I synapses in zebrafish spinal cord at both an embryonic stage and a larval stage. At the embryonic stage, we found that PSD-95 puncta outnumber gephyrin puncta, with the number of gephyrin puncta increasing to match that of PSD-95 puncta at the larval stage. At both stages, PSD-95 puncta are enriched in the most lateral neuropil corresponding to distal dendrites while gephyrin puncta are enriched on neuronal somata and in the medial neuropil. Significantly, similar to synaptic puncta, neuronal processes also exhibit medial-lateral territories at both developmental stages with enrichment of glutamatergic (excitatory) processes laterally and glycinergic (inhibitory) processes medially. This establishment of neuropil excitatory-inhibitory structure largely precedes dendritic arborization of primary motor neurons, suggesting that the structured neuropil could provide a framework for the development of E/I balance at the cellular level. J. Comp. Neurol. 525:1649-1667, 2017. © 2016 Wiley Periodicals, Inc.

A multi-protein receptor-ligand complex underlies combinatorial dendrite guidance choices in C. elegans

Ligand receptor interactions instruct axon guidance during development. How dendrites are guided to specific targets is less understood. The C. elegans PVD sensory neuron innervates muscle-skin interface with its elaborate dendritic branches. Here, we found that LECT-2, the ortholog of leukocyte cell-derived chemotaxin-2 (LECT2), is secreted from the muscles and required for muscle innervation by PVD. Mosaic analyses showed that LECT-2 acted locally to guide the growth of terminal branches. Ectopic expression of LECT-2 from seam cells is sufficient to redirect the PVD dendrites onto seam cells. LECT-2 functions in a multi-protein receptor-ligand complex that also contains two transmembrane ligands on the skin, SAX-7/L1CAM and MNR-1, and the neuronal transmembrane receptor DMA-1. LECT-2 greatly enhances the binding between SAX-7, MNR-1 and DMA-1. The activation of DMA-1 strictly requires all three ligands, which establishes a combinatorial code to precisely target and pattern dendritic arbors.

DOI:http://dx.doi.org/10.7554/eLife.18345.001

In vivo functional analysis of Drosophila Robo1 immunoglobulin-like domains

BACKGROUND

In animals with bilateral symmetry, midline crossing of axons in the developing central nervous system is regulated by Slit ligands and their neuronal Roundabout (Robo) receptors. Multiple structural domains are present in an evolutionarily conserved arrangement in Robo family proteins, but our understanding of the functional importance of individual domains for midline repulsive signaling is limited.

METHODS

We have examined the functional importance of each of the five conserved immunoglobulin-like (Ig) domains within the Drosophila Robo1 receptor. We generated a series of Robo1 variants, each lacking one of the five Ig domains (Ig1-5), and tested each for their ability to bind Slit when expressed in cultured Drosophila cells. We used a transgenic approach to express each variant in robo1's normal expression pattern in wild-type and robo1 mutant embryos, and examined the effects of deleting each domain on receptor expression, axonal localization, regulation, and midline repulsive signaling in vivo.

RESULTS

We show that individual deletion of Ig domains 2-5 does not interfere with Robo1's ability to bind Slit, while deletion of Ig1 strongly disrupts Slit binding. None of the five Ig domains (Ig1-5) are individually required for proper expression of Robo1 in embryonic neurons, for exclusion from commissural axon segments in wild-type embryos, or for downregulation by Commissureless (Comm), a negative regulator of Slit-Robo repulsion in Drosophila. Each of the Robo1 Ig deletion variants (with the exception of Robo1∆Ig1) were able to restore midline crossing in robo1 mutant embryos to nearly the same extent as full-length Robo1, indicating that Ig domains 2-5 are individually dispensable for midline repulsive signaling in vivo.

CONCLUSIONS

Our findings indicate that four of the five Ig domains within Drosophila Robo1 are dispensable for its role in midline repulsion, despite their strong evolutionary conservation, and highlight a unique requirement for the Slit-binding Ig1 domain in the regulation of midline crossing.

Lineage mapping identifies molecular and architectural similarities between the larval and adult Drosophila central nervous system

Neurogenesis in Drosophila occurs in two phases, embryonic and post-embryonic, in which the same set of neuroblasts give rise to the distinct larval and adult nervous systems, respectively. Here, we identified the embryonic neuroblast origin of the adult neuronal lineages in the ventral nervous system via lineage-specific GAL4 lines and molecular markers. Our lineage mapping revealed that neurons born late in the embryonic phase show axonal morphology and transcription factor profiles that are similar to the neurons born post-embryonically from the same neuroblast. Moreover, we identified three thorax-specific neuroblasts not previously characterized and show that HOX genes confine them to the thoracic segments. Two of these, NB2-3 and NB3-4, generate leg motor neurons. The other neuroblast is novel and appears to have arisen recently during insect evolution. Our findings provide a comprehensive view of neurogenesis and show how proliferation of individual neuroblasts is dictated by temporal and spatial cues.

DOI:http://dx.doi.org/10.7554/eLife.13399.001

Fruit flies undergo a process called metamorphosis in which they change from a maggot or larva into an adult fly. These two life stages look and behave differently and appear to have strikingly different nervous systems. The relationship between the two nervous systems has been most extensively studied in the ventral nerve cord (which is the equivalent to the spinal cord in humans). Although the ventral nerve cords of a larva and an adult fly look quite different, they are generated by the same set of stem cells known as neuroblasts. This is made possible because the neuroblasts proliferate in two separate phases: the first phase occurs in the embryo to generate the neurons of the larval nervous system, and the second phase occurs in the larva to generate neurons for the adult’s nervous system.

Now, Lacin and Truman have paired each of the neurons in the adult fruit fly’s nerve cord with their corresponding neurons in the nerve cords of fruit fly larvae. This involved identifying the original neuroblasts that gave rise to each of the neurons in both larval and adult fruit flies. The results suggest that most neurons that arise from a given neuroblast produce a similar set of molecules and extend similar nerve fibers, even though they work in two different nervous systems. Since neuroblasts in non-metamorphosing insects proliferate continuously, these findings also suggest that, when metamorphosis evolved, a pause was introduced to create the two separate phases of proliferation without a big effect on the types of neurons generated.

Lacin and Truman then went on to discover three neuroblasts that appear to be unique to the middle (or thoracic) segments of a fruit fly. The experiments reveal that the presence of these neuroblasts depended on specific genes that control the development of animal body plans. Two of these neuroblasts generate the so-called motor neurons that control the movement of a fly’s legs. Flies only have legs on their thoracic segments, so this indicates that the development of new neurons is coordinated with the development of the body plan at the stem cell level. The third neuroblast generates neurons that connect with the leg motor neurons, and Lacin and Truman propose that this neuroblast arose from a copy of a neighboring stem cell. The resulting extra neurons may have enabled finer control over the leg movements required for activities such as walking and grooming.

Following on from this work, it is now possible to investigate how molecular events that occur from the embryonic to the adult stages of a fruit fly’s life control the formation and function of its nervous system.

DOI:http://dx.doi.org/10.7554/eLife.13399.002

Glial and neuronal Semaphorin signaling instruct the development of a functional myotopic map for Drosophila walking

Motoneurons developmentally acquire appropriate cellular architectures that ensure connections with postsynaptic muscles and presynaptic neurons. In Drosophila, leg motoneurons are organized as a myotopic map, where their dendritic domains represent the muscle field. Here, we investigate mechanisms underlying development of aspects of this myotopic map, required for walking. A behavioral screen identified roles for Semaphorins (Sema) and Plexins (Plex) in walking behavior. Deciphering this phenotype, we show that PlexA/Sema1a mediates motoneuron axon branching in ways that differ in the proximal femur and distal tibia, based on motoneuronal birth order. Importantly, we show a novel role for glia in positioning dendrites of specific motoneurons; PlexB/Sema2a is required for dendritic positioning of late-born motoneurons but not early-born motoneurons. These findings indicate that communication within motoneurons and between glia and motoneurons, mediated by the combined action of different Plexin/Semaphorin signaling systems, are required for the formation of a functional myotopic map.

DOI:http://dx.doi.org/10.7554/eLife.11572.001

Nerve cells enable us to both sense the world around us and to move about it. The nerves responsible for movement are called motor neurons. While one end of a motor neuron stimulates the muscle it is connected to, the other end receives signals from nerves in the spinal cord that relay messages about movement from the brain. Motor neuron connections in the spinal cord, or its equivalent in insects, the ventral nerve cord, are organized into an arrangement known as a myotopic map, which reflects the anatomical arrangement of the muscles in the body. Much remains to be learnt about how these maps form.

Syed et al. have investigated how the myotopic map develops for motor neurons in the legs of fruit flies by reducing the function of chosen genes in the ventral nerve cord and asking how this affects the myotopic map. The experiments disrupted a signaling system called the Semaphorin signaling pathway that guides motor neurons to the right target muscle and consists of different receptor-signaling molecule pairs.

By looking for flies with an abnormal walk and with disrupted motor neuron organization, Syed et al. identified receptor-signal pairs that guide motor neurons to different leg muscles. Specific receptor-signal pairs also guide the organisation of motor neurons in the ventral nerve cord. This guidance depends on when neurons are ‘born’. While a receptor-signal pair targets early born neurons to one leg muscle, the same receptor-signal pair regulates a different aspect of guidance in late-born neurons. Cells called glia, which are related to neurons, also help to position the connections of late-born motor neurons in the ventral nerve cord.

Overall, the Semaphorin signaling system assists communication both within motor neurons and between glia cells and motor neurons during the formation of the myotopic map for leg motor neurons. These discoveries open new avenues of investigation into how else these cells communicate with each other to aid the development and organization of motor neurons.

DOI:http://dx.doi.org/10.7554/eLife.11572.002

Lamination Speeds the Functional Development of Visual Circuits

A common feature of the brain is the arrangement of synapses in layers. To examine the significance of this organizational feature, we studied the functional development of direction-selective (DS) circuits in the tectum of astray mutant zebrafish in which lamination of retinal ganglion cell (RGC) axons is lost. We show that although never laminar, the tuning of DS-RGC axons targeting the mutant tectum is normal. Analysis of mutant tectal neurons at late developmental stages reveals that directional tuning is indistinguishable from wild-type larvae. Furthermore, we show that structural plasticity of tectal dendrites and RGC axons compensates for the loss of lamination, establishing connectivity between DS-RGCs and their normal tectal targets. However, tectal direction selectivity is severely perturbed at earlier developmental stages. Thus, the formation of synaptic laminae is ultimately dispensable for the correct wiring of direction-selective tectal circuits, but it is crucial for the rapid assembly of these networks.

Video Abstract

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Robo2 directs lamination of direction-selective retinal axons and tectal dendrites

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Tectal lamination is required for rapid assembly of direction-selective circuits

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Functional direction-selective circuits eventually form when lamination is lost

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Structural plasticity compensates for the loss of tectal lamination

Development of dendritic form and function

The nervous system is populated by numerous types of neurons, each bearing a dendritic arbor with a characteristic morphology. These type-specific features influence many aspects of a neuron's function, including the number and identity of presynaptic inputs and how inputs are integrated to determine firing properties. Here, we review the mechanisms that regulate the construction of cell type-specific dendrite patterns during development. We focus on four aspects of dendrite patterning that are particularly important in determining the function of the mature neuron: (a) dendrite shape, including branching pattern and geometry of the arbor; (b) dendritic arbor size;

Importance of gene dosage in controlling dendritic arbor formation during development

Proper dendrite morphology is crucial for normal nervous system functioning. While a number of genes have been implicated in dendrite morphogenesis in both invertebrates and mammals, it remains unclear how developing dendrites respond to changes in gene dosage and what type of patterns their responses may follow. To understand this, I review here evidence from the recent literature, focusing on the genetic studies performed in the Drosophila larval dendritic arborization class IV neuron, an excellent cell type to understand dendrite morphogenesis. I summarize how class IV arbors change morphology in response to developmental fluctuations in the expression levels of 47 genes, studied by means of genetic manipulations such as loss-of-function and gain-of-function, and for which sufficient information is available. I find that arbors can respond to changing gene dosage in several distinct ways, each characterized by a singular dose-response curve. Interestingly, in 72% of cases arbors are sensitive, and thus adjust their morphology, in response to both decreases and increases in the expression of a given gene, indicating that dendrite morphogenesis is a process particularly sensitive to gene dosage. By summarizing the parallels between Drosophila and mammals, I show that many Drosophila dendrite morphogenesis genes have orthologs in mammals, and that some of these are associated with mammalian dendrite outgrowth and human neurodevelopmental disorders. One notable disease-related molecule is kinase Dyrk1A, thought to be a causative factor in Down syndrome. Both increases and decreases in Dyrk1A gene dosage lead to impaired dendrite morphogenesis, which may contribute to Down syndrome pathoetiology.

Specification of individual adult motor neuron morphologies by combinatorial transcription factor codes

How the highly stereotyped morphologies of individual neurons are genetically specified is not well understood. We identify six transcription factors (TFs) expressed in a combinatorial manner in seven post-mitotic adult leg motor neurons (MNs) that are derived from a single neuroblast in Drosophila. Unlike TFs expressed in mitotically active neuroblasts, these TFs do not regulate each other's expression. Removing the activity of a single TF resulted in specific morphological defects, including muscle targeting and dendritic arborization, and in a highly specific walking defect in adult flies. In contrast, when the expression of multiple TFs was modified, nearly complete transformations in MN morphologies were generated. These results show that the morphological characteristics of a single neuron are dictated by a combinatorial code of morphology TFs (mTFs). mTFs function at a previously unidentified regulatory tier downstream of factors acting in the NB but independently of factors that act in terminally differentiated neurons.

Development of connectivity in a motoneuronal network in Drosophila larvae

Background

Much of our understanding of how neural networks develop is based on studies of sensory systems, revealing often highly stereotyped patterns of connections, particularly as these diverge from the presynaptic terminals of sensory neurons. We know considerably less about the wiring strategies of motor networks, where connections converge onto the dendrites of motoneurons. Here, we investigated patterns of synaptic connections between identified motoneurons with sensory neurons and interneurons in the motor network of the Drosophila larva and how these change as it develops.

Results

We find that as animals grow, motoneurons increase the number of synapses with existing presynaptic partners. Different motoneurons form characteristic cell-type-specific patterns of connections. At the same time, there is considerable variability in the number of synapses formed on motoneuron dendrites, which contrasts with the stereotypy reported for presynaptic terminals of sensory neurons. Where two motoneurons of the same cell type contact a common interneuron partner, each postsynaptic cell can arrive at a different connectivity outcome. Experimentally changing the positioning of motoneuron dendrites shows that the geography of dendritic arbors in relation to presynaptic partner terminals is an important determinant in shaping patterns of connectivity.

Conclusions

In the Drosophila larval motor network, the sets of connections that form between identified neurons manifest an unexpected level of variability. Synapse number and the likelihood of forming connections appear to be regulated on a cell-by-cell basis, determined primarily by the postsynaptic dendrites of motoneuron terminals.

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Growing motoneurons consolidate synapses with existing presynaptic partners

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Motoneuron dendritic arbors are active parties in setting connectivity patterns

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Cell-type-specific features coexist with variations at the individual cell level

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Motoneuron wiring strategies may contrast with those of sensory neurons

Neural control of wing coordination in flies

At the onset of each flight bout in flies, neural circuits in the CNS must rapidly integrate multimodal sensory stimuli and synchronously engage hinges of the left and right wings for coordinated wing movements. Whereas anatomical and physiological investigations of flight have been conducted on larger flies, molecular genetic studies in Drosophila have helped identify neurons that mediate various levels of flight control. However, neurons that might mediate bilateral coordination of wing movements to precisely synchronize left and right wing engagement at flight onset and maintain their movement in perfect coordination at rapid frequencies during flight maneuvers remain largely unexplored. Wing coordination could be directly modulated via bilateral sensory inputs to motoneurons of steering muscles and/or through central interneurons. Using a Ca(2+)-activity-based GFP reporter, we identified three flight-activated central dopaminergic interneurons in the ventral ganglion, which connect to and activate motoneurons that innervate a pair of direct-steering flight muscles. The activation of these newly identified dopaminergic interneurons is context specific. Whereas bilateral wing engagement for flight requires these neurons, they do not control unilateral wing extension during courtship. Thus, independent central circuits function in the context of different natural behaviors to control the motor circuit for Drosophila wing movement.

A synaptic mechanism for network synchrony

Within neural networks, synchronization of activity is dependent upon the synaptic connectivity of embedded microcircuits and the intrinsic membrane properties of their constituent neurons. Synaptic integration, dendritic Ca²⁺ signaling, and non-linear interactions are crucial cellular attributes that dictate single neuron computation, but their roles promoting synchrony and the generation of network oscillations are not well understood, especially within the context of a defined behavior. In this regard, the lamprey spinal central pattern generator (CPG) stands out as a well-characterized, conserved vertebrate model of a neural network (Smith et al., 2013a), which produces synchronized oscillations in which neural elements from the systems to cellular level that control rhythmic locomotion have been determined. We review the current evidence for the synaptic basis of oscillation generation with a particular emphasis on the linkage between synaptic communication and its cellular coupling to membrane processes that control oscillatory behavior of neurons within the locomotor network. We seek to relate dendritic function found in many vertebrate systems to the accessible lamprey central nervous system in which the relationship between neural network activity and behavior is well understood. This enables us to address how Ca²⁺ signaling in spinal neuron dendrites orchestrate oscillations that drive network behavior.

Slit/Robo signaling regulates cell fate decisions in the intestinal stem cell lineage of Drosophila

In order to maintain tissue homeostasis, cell fate decisions within stem cell lineages have to respond to the needs of the tissue. This coordination of lineage choices with regenerative demand remains poorly characterized. Here, we identify a signal from enteroendocrine cells (EEs) that controls lineage specification in the Drosophila intestine. We find that EEs secrete Slit, a ligand for the Robo2 receptor in intestinal stem cells (ISCs) that limits ISC commitment to the endocrine lineage, establishing negative feedback control of EE regeneration. Furthermore, we show that this lineage decision is made within ISCs and requires induction of the transcription factor Prospero in ISCs. Our work identifies a function for the conserved Slit/Robo pathway in the regulation of adult stem cells, establishing negative feedback control of ISC lineage specification as a critical strategy to preserve tissue homeostasis. Our results further amend the current understanding of cell fate commitment within the Drosophila ISC lineage.

Dual role for Hox genes and Hox co-factors in conferring leg motoneuron survival and identity in Drosophila

Adult Drosophila walk using six multi-jointed legs, each controlled by ∼50 leg motoneurons (MNs). Although MNs have stereotyped morphologies, little is known about how they are specified. Here, we describe the function of Hox genes and homothorax (hth), which encodes a Hox co-factor, in Drosophila leg MN development. Removing either Hox or Hth function from a single neuroblast (NB) lineage results in MN apoptosis. A single Hox gene, Antennapedia (Antp), is primarily responsible for MN survival in all three thoracic segments. When cell death is blocked, partially penetrant axon branching errors are observed in Hox mutant MNs. When single MNs are mutant, errors in both dendritic and axon arborizations are observed. Our data also suggest that Antp levels in post-mitotic MNs are important for specifying their identities. Thus, in addition to being essential for survival, Hox and hth are required to specify accurate MN morphologies in a level-dependent manner.

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.

Cooperative slit and netrin signaling in contralateralization of the mouse trigeminothalamic pathway

Ascending somatosensory pathways are crossed pathways representing each side of the body in the contralateral neocortex. The principal sensory nucleus of the trigeminal nerve (PrV) relays the facial sensations to the contralateral somatosensory cortex via the ventrobasal thalamus. In the companion article (Kivrak and Erzurumlu [2012] J. Comp. Neurol. 12-0013) we described the normal development of the trigeminal lemniscal pathway in the mouse. In this study we investigated the role of midline axon navigation signals, the netrin and slit proteins. In situ hybridization assays revealed that both netrin and slit mRNAs are expressed along the midline facing the PrV axons and their receptors are expressed in developing PrV neurons. In wild-type mouse embryos, PrV axons cross the midline and take a sharp rostral turn heading toward the contralateral thalamus. Examination of trigeminal lemniscal axons in dcc knockout mice revealed absence of midline crossing between E11 and E15. However, a few axons crossed the midline at E17 and reached the contralateral thalamus, resulting in a bilateral PrV lemniscal pathway at P0. We also found that slit1, -2 or -3 single or double knockout mice have impaired development of the trigeminal-lemniscal pathway. These include axon stalling along the midline, running within the midline, and recrossing of axons back to the site of origin. Collectively, our studies indicate a cooperative role for netrin and slit proteins in midline attraction and crossing behavior of the ascending facial somatosensory projections during development.

Neuroblast lineage-specific origin of the neurons of the Drosophila larval olfactory system

The complete neuronal repertoire of the central brain of Drosophila originates from only approximately 100 pairs of neural stem cells, or neuroblasts. Each neuroblast produces a highly stereotyped lineage of neurons which innervate specific compartments of the brain. Neuroblasts undergo two rounds of mitotic activity: embryonic divisions produce lineages of primary neurons that build the larval nervous system; after a brief quiescence, the neuroblasts go through a second round of divisions in larval stage to produce secondary neurons which are integrated into the adult nervous system. Here we investigate the lineages that are associated with the larval antennal lobe, one of the most widely studied neuronal systems in fly. We find that the same five neuroblasts responsible for the adult antennal lobe also produce the antennal lobe of the larval brain. However, there are notable differences in the composition of larval (primary) lineages and their adult (secondary) counterparts. Significantly, in the adult, two lineages (lNB/BAlc and adNB/BAmv3) produce uniglomerular projection neurons connecting the antennal lobe with the mushroom body and lateral horn; another lineage, vNB/BAla1, generates multiglomerular neurons reaching the lateral horn directly. lNB/BAlc, as well as a fourth lineage, vlNB/BAla2, generate a diversity of local interneurons. We describe a fifth, previously unknown lineage, BAlp4, which connects the posterior part of the antennal lobe and the neighboring tritocerebrum (gustatory center) with a higher brain center located adjacent to the mushroom body. In the larva, only one of these lineages, adNB/BAmv3, generates all uniglomerular projection neurons. Also as in the adult, lNB/BAlc and vlNB/BAla2 produce local interneurons which, in terms of diversity in architecture and transmitter expression, resemble their adult counterparts. In addition, lineages lNB/BAlc and vNB/BAla1, as well as the newly described BAlp4, form numerous types of projection neurons which along the same major axon pathways (antennal tracts) used by the antennal projection neurons, but which form connections that include regions outside the "classical" olfactory circuit triad antennal lobe-mushroom body-lateral horn. Our work will benefit functional studies of the larval olfactory circuit, and shed light on the relationship between larval and adult neurons.

Localized netrins act as positional cues to control layer-specific targeting of photoreceptor axons in Drosophila

A shared feature of many neural circuits is their organization into synaptic layers. However, the mechanisms that direct neurites to distinct layers remain poorly understood. We identified a central role for Netrins and their receptor Frazzled in mediating layer-specific axon targeting in the Drosophila visual system. Frazzled is expressed and cell autonomously required in R8 photoreceptors for directing their axons to the medulla-neuropil layer M3. Netrin-B is specifically localized in this layer owing to axonal release by lamina neurons L3 and capture by target neuron-associated Frazzled. Ligand expression in L3 is sufficient to rescue R8 axon-targeting defects of Netrin mutants. R8 axons target normally despite replacement of diffusible Netrin-B by membrane-tethered ligands. Finally, Netrin localization is instructive because expression in ectopic layers can retarget R8 axons. We propose that provision of localized chemoattractants by intermediate target neurons represents a highly precise strategy to direct axons to a positionally defined layer.

Developmental origins and architecture of Drosophila leg motoneurons

Motoneurons are key points of convergence within motor networks, acting as the "output channels" that directly control sets of muscles to maintain posture and generate movement. Here we use genetic mosaic techniques to reveal the origins and architecture of the leg motoneurons of Drosophila. We show that a small number of leg motoneurons are born in the embryo but most are generated during larval life. These postembryonic leg motoneurons are produced by five neuroblasts per hemineuromere, and each lineage generates stereotyped lineage-specific projection patterns. Two of these postembryonic neuroblasts generate solely motoneurons that are the bulk of the leg motoneurons. Within the largest lineage, lineage 15, we see distinct birth-order differences in projection patterns. A comparison of the central projections of leg motoneurons and the muscles they innervate reveals a stereotyped architecture and the existence of a myotopic map. Timeline analysis of axonal outgrowth reveals that leg motoneurons reach their sites of terminal arborization in the leg at the time when their dendrites are elaborating their subtype-specific shapes. Our findings provide a comprehensive description of the origin, development, and architecture of leg motoneurons that will aid future studies exploring the link between the assembly and organization of connectivity within the leg motor system of Drosophila.

Regulation of locomotion and motoneuron trajectory selection and targeting by the Drosophila homolog of Olig family transcription factors

During the development of locomotion circuits it is essential that motoneurons with distinct subtype identities select the correct trajectories and target muscles. In vertebrates, the generation of motoneurons and myelinating glia depends on Olig2, one of the five Olig family bHLH transcription factors. We investigated the so far unknown function of the single Drosophila homolog Oli. Combining behavioral and genetic approaches, we demonstrate that oli is not required for gliogenesis, but plays pivotal roles in regulating larval and adult locomotion, and axon pathfinding and targeting of embryonic motoneurons. In the embryonic nervous system, Oli is primarily expressed in postmitotic progeny, and in particular, in distinct ventral motoneuron subtypes. oli mediates axonal trajectory selection of these motoneurons within the ventral nerve cord and targeting to specific muscles. Genetic interaction assays suggest that oli acts as part of a conserved transcription factor ensemble including Lim3, Islet and Hb9. Moreover, oli is expressed in postembryonic leg-innervating motoneuron lineages and required in glutamatergic neurons for walking. Finally, over-expression of vertebrate Olig2 partially rescues the walking defects of oli-deficient flies. Thus, our findings reveal a remarkably conserved role of Drosophila Oli and vertebrate family members in regulating motoneuron development, while the steps that require their function differ in detail.

Dscam1-mediated self-avoidance counters netrin-dependent targeting of dendrites in Drosophila

Dendrites and axons show precise targeting and spacing patterns for proper reception and transmission of information in the nervous system. Self-avoidance promotes complete territory coverage and nonoverlapping spacing between processes from the same cell [1, 2]. Neurons that lack Drosophila Down syndrome cell adhesion molecule 1 (Dscam1) show aberrant overlap, fasciculation, and accumulation of dendrites and axons, demonstrating a role in self-recognition and repulsion leading to self-avoidance [3-11]. Fasciculation and accumulation of processes suggested that Dscam1 might promote process spacing by counterbalancing developmental signals that otherwise promote self-association [9, 12]. Here we show that Dscam1 functions to counter Drosophila sensory neuron dendritic targeting signals provided by secreted Netrin-B and Frazzled, a netrin receptor. Loss of Dscam1 function resulted in aberrant dendrite accumulation at a Netrin-B-expressing target, whereas concomitant loss of Frazzled prevented accumulation and caused severe deficits in dendritic territory coverage. Netrin misexpression was sufficient to induce ectopic dendritic targeting in a Frazzled-dependent manner, whereas Dscam1 was required to prevent ectopic accumulation, consistent with separable roles for these receptors. Our results suggest that Dscam1-mediated self-avoidance counters extrinsic signals that are required for normal dendritic patterning, but whose action would otherwise favor neurite accumulation. Counterbalancing roles for Dscam1 may be deployed in diverse contexts during neural circuit formation.

Turtle functions downstream of Cut in differentially regulating class specific dendrite morphogenesis in Drosophila

Background

Dendritic morphology largely determines patterns of synaptic connectivity and electrochemical properties of a neuron. Neurons display a myriad diversity of dendritic geometries which serve as a basis for functional classification. Several types of molecules have recently been identified which regulate dendrite morphology by acting at the levels of transcriptional regulation, direct interactions with the cytoskeleton and organelles, and cell surface interactions. Although there has been substantial progress in understanding the molecular mechanisms of dendrite morphogenesis, the specification of class-specific dendritic arbors remains largely unexplained. Furthermore, the presence of numerous regulators suggests that they must work in concert. However, presently, few genetic pathways regulating dendrite development have been defined.

Methodology/Principal Findings

The Drosophila gene turtle belongs to an evolutionarily conserved class of immunoglobulin superfamily members found in the nervous systems of diverse organisms. We demonstrate that Turtle is differentially expressed in Drosophila da neurons. Moreover, MARCM analyses reveal Turtle acts cell autonomously to exert class specific effects on dendritic growth and/or branching in da neuron subclasses. Using transgenic overexpression of different Turtle isoforms, we find context-dependent, isoform-specific effects on mediating dendritic branching in class II, III and IV da neurons. Finally, we demonstrate via chromatin immunoprecipitation, qPCR, and immunohistochemistry analyses that Turtle expression is positively regulated by the Cut homeodomain transcription factor and via genetic interaction studies that Turtle is downstream effector of Cut-mediated regulation of da neuron dendrite morphology.

Conclusions/Significance

Our findings reveal that Turtle proteins differentially regulate the acquisition of class-specific dendrite morphologies. In addition, we have established a transcriptional regulatory interaction between Cut and Turtle, representing a novel pathway for mediating class specific dendrite development.

Role of leucine-rich repeat proteins in the development and function of neural circuits

The nervous system consists of an ensemble of billions of neurons interconnected in a highly specific pattern that allows proper propagation and integration of neural activities. The organization of these specific connections emerges from sequential developmental events including axon guidance, target selection, and synapse formation. These events critically rely on cell-cell recognition and communication mediated by cell-surface ligands and receptors. Recent studies have uncovered central roles for leucine-rich repeat (LRR) domain-containing proteins, not only in organizing neural connectivity from axon guidance to target selection to synapse formation, but also in various nervous system disorders. Their versatile LRR domains, in particular, serve as key sites for interactions with a wide diversity of binding partners. Here, we focus on a few exquisite examples of secreted or membrane-associated LRR proteins in Drosophila and mammals and review the mechanisms by which they regulate diverse aspects of nervous system development and function.

Transcriptome analysis of the desert locust central nervous system: production and annotation of a Schistocerca gregaria EST database

BACKGROUND

The desert locust (Schistocerca gregaria) displays a fascinating type of phenotypic plasticity, designated as 'phase polyphenism'. Depending on environmental conditions, one genome can be translated into two highly divergent phenotypes, termed the solitarious and gregarious (swarming) phase. Although many of the underlying molecular events remain elusive, the central nervous system (CNS) is expected to play a crucial role in the phase transition process. Locusts have also proven to be interesting model organisms in a physiological and neurobiological research context. However, molecular studies in locusts are hampered by the fact that genome/transcriptome sequence information available for this branch of insects is still limited.

METHODOLOGY

We have generated 34,672 raw expressed sequence tags (EST) from the CNS of desert locusts in both phases. These ESTs were assembled in 12,709 unique transcript sequences and nearly 4,000 sequences were functionally annotated. Moreover, the obtained S. gregaria EST information is highly complementary to the existing orthopteran transcriptomic data. Since many novel transcripts encode neuronal signaling and signal transduction components, this paper includes an overview of these sequences. Furthermore, several transcripts being differentially represented in solitarious and gregarious locusts were retrieved from this EST database. The findings highlight the involvement of the CNS in the phase transition process and indicate that this novel annotated database may also add to the emerging knowledge of concomitant neuronal signaling and neuroplasticity events.

CONCLUSIONS

In summary, we met the need for novel sequence data from desert locust CNS. To our knowledge, we hereby also present the first insect EST database that is derived from the complete CNS. The obtained S. gregaria EST data constitute an important new source of information that will be instrumental in further unraveling the molecular principles of phase polyphenism, in further establishing locusts as valuable research model organisms and in molecular evolutionary and comparative entomology.

Tiling among stereotyped dendritic branches in an identified Drosophila motoneuron

Different types of neurons can be distinguished by the specific targeting locations and branching patterns of their dendrites, which form the blueprint for wiring the brain. Unraveling which specific signals control different aspects of dendritic architecture, such as branching and elongation, pruning and cessation of growth, territory formation, tiling, and self-avoidance requires a quantitative comparison in control and genetically manipulated neurons. The highly conserved shapes of individually identified Drosophila neurons make them well suited for the analysis of dendritic architecture principles. However, to date it remains unclear how tightly dendritic architecture principles of identified central neurons are regulated. This study uses quantitative reconstructions of dendritic architecture of an identified Drosophila flight motoneuron (MN5) with a complex dendritic tree, comprising more than 4,000 dendritic branches and 6 mm total length. MN5 contains a fixed number of 23 dendritic subtrees, which tile into distinct, nonoverlapping volumes of the diffuse motor neuropil. Across-animal comparison and quantitative analysis suggest that tiling of the different dendritic subtrees of the same neuron is caused by competitive and repulsive interactions among subtrees, perhaps allowing different dendritic compartments to be connected to different circuit elements. We also show that dendritic architecture is similar among different wildtype and GAL4 driver fly lines. Metric and topological dendritic architecture features are sufficiently constant to allow for studies of the underlying control mechanisms by genetic manipulations. Dendritic territory and certain topological measures, such as tree compactness, are most constant, suggesting that these reflect the intrinsic molecular identity of the neuron. J. Comp. Neurol. 518:2169-2185, 2010. (c) 2010 Wiley-Liss, Inc.

Moving away from the midline: new developments for Slit and Robo

In most tissues, the precise control of cell migration and cell-cell interaction is of paramount importance to the development of a functional structure. Several families of secreted molecules have been implicated in regulating these aspects of development, including the Slits and their Robo receptors. These proteins have well described roles in axon guidance but by influencing cell polarity and adhesion, they participate in many developmental processes in diverse cell types. We review recent progress in understanding both the molecular mechanisms that modulate Slit/Robo expression and their functions in neural and non-neural tissue.

Branching out: mechanisms of dendritic arborization

Type-specific dendrite morphology is a hallmark of the neuron and has important functional implications in determining what signals a neuron receives and how these signals are integrated. During the past two decades, studies on dendritic arborization neurons in Drosophila melanogaster have started to identify mechanisms of dendrite morphogenesis that may have broad applicability to vertebrate species. Transcription factors, receptor-ligand interactions, various signalling pathways, local translational machinery, cytoskeletal elements, Golgi outposts and endosomes have been identified as contributors to the organization of dendrites of individual neurons and the placement of these dendrites in the neuronal circuitry. Further insight into these mechanisms will improve our understanding of how the nervous system functions and might help to identify the underlying causes of some neurological and neurodevelopmental disorders.

Role of Notch signaling in establishing the hemilineages of secondary neurons in Drosophila melanogaster

The secondary neurons generated in the thoracic central nervous system of Drosophila arise from a hemisegmental set of 25 neuronal stem cells, the neuroblasts (NBs). Each NB undergoes repeated asymmetric divisions to produce a series of smaller ganglion mother cells (GMCs), which typically divide once to form two daughter neurons. We find that the two daughters of the GMC consistently have distinct fates. Using both loss-of-function and gain-of-function approaches, we examined the role of Notch signaling in establishing neuronal fates within all of the thoracic secondary lineages. In all cases, the 'A' (Notch(ON)) sibling assumes one fate and the 'B' (Notch(OFF)) sibling assumes another, and this relationship holds throughout the neurogenic period, resulting in two major neuronal classes: the A and B hemilineages. Apparent monotypic lineages typically result from the death of one sibling throughout the lineage, resulting in a single, surviving hemilineage. Projection neurons are predominantly from the B hemilineages, whereas local interneurons are typically from A hemilineages. Although sibling fate is dependent on Notch signaling, it is not necessarily dependent on numb, a gene classically involved in biasing Notch activation. When Numb was removed at the start of larval neurogenesis, both A and B hemilineages were still generated, but by the start of the third larval instar, the removal of Numb resulted in all neurons assuming the A fate. The need for Numb to direct Notch signaling correlated with a decrease in NB cell cycle time and may be a means for coping with multiple sibling pairs simultaneously undergoing fate decisions.

Midline crossing by gustatory receptor neuron axons is regulated by fruitless, doublesex and the Roundabout receptors

Although nervous system sexual dimorphisms are known in many species, relatively little is understood about the molecular mechanisms generating these dimorphisms. Recent findings in Drosophila provide the tools for dissecting how neurogenesis and neuronal differentiation are modulated by the Drosophila sex-determination regulatory genes to produce nervous system sexual dimorphisms. Here we report studies aimed at illuminating the basis of the sexual dimorphic axonal projection patterns of foreleg gustatory receptor neurons (GRNs): only in males do GRN axons project across the midline of the ventral nerve cord. We show that the sex determination genes fruitless (fru) and doublesex (dsx) both contribute to establishing this sexual dimorphism. Male-specific Fru (Fru(M)) acts in foreleg GRNs to promote midline crossing by their axons, whereas midline crossing is repressed in females by female-specific Dsx (Dsx(F)). In addition, midline crossing by these neurons might be promoted in males by male-specific Dsx (Dsx(M)). Finally, we (1) demonstrate that the roundabout (robo) paralogs also regulate midline crossing by these neurons, and (2) provide evidence that Fru(M) exerts its effect on midline crossing by directly or indirectly regulating Robo signaling.

Midline signalling systems direct the formation of a neural map by dendritic targeting in the Drosophila motor system

A fundamental strategy for organising connections in the nervous system is the formation of neural maps. Map formation has been most intensively studied in sensory systems where the central arrangement of axon terminals reflects the distribution of sensory neuron cell bodies in the periphery or the sensory modality. This straightforward link between anatomy and function has facilitated tremendous progress in identifying cellular and molecular mechanisms that underpin map development. Much less is known about the way in which networks that underlie locomotion are organised. We recently showed that in the Drosophila embryo, dendrites of motorneurons form a neural map, being arranged topographically in the antero-posterior axis to represent the distribution of their target muscles in the periphery. However, the way in which a dendritic myotopic map forms has not been resolved and whether postsynaptic dendrites are involved in establishing sets of connections has been relatively little explored. In this study, we show that motorneurons also form a myotopic map in a second neuropile axis, with respect to the ventral midline, and they achieve this by targeting their dendrites to distinct medio-lateral territories. We demonstrate that this map is "hard-wired"; that is, it forms in the absence of excitatory synaptic inputs or when presynaptic terminals have been displaced. We show that the midline signalling systems Slit/Robo and Netrin/Frazzled are the main molecular mechanisms that underlie dendritic targeting with respect to the midline. Robo and Frazzled are required cell-autonomously in motorneurons and the balance of their opposite actions determines the dendritic target territory. A quantitative analysis shows that dendritic morphology emerges as guidance cue receptors determine the distribution of the available dendrites, whose total length and branching frequency are specified by other cell intrinsic programmes. Our results suggest that the formation of dendritic myotopic maps in response to midline guidance cues may be a conserved strategy for organising connections in motor systems. We further propose that sets of connections may be specified, at least to a degree, by global patterning systems that deliver pre- and postsynaptic partner terminals to common "meeting regions."