Robo and Frazzled/DCC mediate dendritic guidance at the CNS midline

Adjacent Neuronal Fascicle Guides Motoneuron 24 Dendritic Branching and Axonal Routing Decisions through Dscam1 Signaling

The formation and precise positioning of axons and dendrites are crucial for the development of neural circuits. Although juxtracrine signaling via cell-cell contact is known to influence these processes, the specific structures and mechanisms regulating neuronal process positioning within the central nervous system (CNS) remain to be fully identified. Our study investigates motoneuron 24 (MN24) in the Drosophila embryonic CNS, which is characterized by a complex yet stereotyped axon projection pattern, known as 'axonal routing.' In this motoneuron, the primary dendritic branches project laterally toward the midline, specifically emerging at the sites where axons turn. We observed that Scp2-positive neurons contribute to the lateral fascicle structure in the ventral nerve cord (VNC) near MN24 dendrites. Notably, the knockout of the Down syndrome cell adhesion molecule (dscam1) results in the loss of dendrites and disruption of proper axonal routing in MN24, while not affecting the formation of the fascicle structure. Through cell-type specific knockdown and rescue experiments of dscam1, we have determined that the interaction between MN24 and Scp2-positive fascicle, mediated by Dscam1, promotes the development of both dendrites and axonal routing. Our findings demonstrate that the holistic configuration of neuronal structures, such as axons and dendrites, within single motoneurons can be governed by local contact with the adjacent neuron fascicle, a novel reference structure for neural circuitry wiring.

Novel dominant and recessive variants in human ROBO1 cause distinct neurodevelopmental defects through different mechanisms

The Roundabout (Robo) receptors, located on growth cones of neurons, induce axon repulsion in response to the extracellular ligand Slit. The Robo family of proteins controls midline crossing of commissural neurons during development in flies. Mono- and bi-allelic variants in human ROBO1 (HGNC: 10249) have been associated with incomplete penetrance and variable expressivity for a breath of phenotypes, including neurodevelopmental defects such as strabismus, pituitary defects, intellectual impairment, as well as defects in heart and kidney. Here, we report two novel ROBO1 variants associated with very distinct phenotypes. A homozygous missense p.S1522L variant in three affected siblings with nystagmus; and a monoallelic de novo p.D422G variant in a proband who presented with early-onset epileptic encephalopathy. We modeled these variants in Drosophila and first generated a null allele by inserting a CRIMIC T2A-GAL4 in an intron. Flies that lack robo1 exhibit reduced viability but have very severe midline crossing defects in the central nervous system. The fly wild-type cDNA driven by T2A-Gal4 partially rescues both defects. Overexpression of the human reference ROBO1 with T2A-GAL4 is toxic and reduces viability, whereas the recessive p.S1522L variant is less toxic, suggesting that it is a partial loss-of-function allele. In contrast, the dominant variant in fly robo1 (p.D413G) affects protein localization, impairs axonal guidance activity and induces mild phototransduction defects, suggesting that it is a neomorphic allele. In summary, our studies expand the phenotypic spectrum associated with ROBO1 variant alleles.

Novel phospho-switch function of delta-catenin in dendrite development

In neurons, dendrites form the major sites of information receipt and integration. It is thus vital that, during development, the dendritic arbor is adequately formed to enable proper neural circuit formation and function. While several known processes shape the arbor, little is known of those that govern dendrite branching versus extension. Here, we report a new mechanism instructing dendrites to branch versus extend. In it, glutamate signaling activates mGluR5 receptors to promote Ckd5-mediated phosphorylation of the C-terminal PDZ-binding motif of delta-catenin. The phosphorylation state of this motif determines delta-catenin's ability to bind either Pdlim5 or Magi1. Whereas the delta:Pdlim5 complex enhances dendrite branching at the expense of elongation, the delta:Magi1 complex instead promotes lengthening. Our data suggest that these complexes affect dendrite development by differentially regulating the small-GTPase RhoA and actin-associated protein Cortactin. We thus reveal a "phospho-switch" within delta-catenin, subject to a glutamate-mediated signaling pathway, that assists in balancing the branching versus extension of dendrites during neural development.

Deterministic and Stochastic Rules of Branching Govern Dendrite Morphogenesis of Sensory Neurons

Dendrite morphology is necessary for the correct integration of inputs that neurons receive. The branching mechanisms allowing neurons to acquire their type-specific morphology remain unclear. Classically, axon and dendrite patterns were shown to be guided by molecules, providing deterministic cues. However, the extent to which deterministic and stochastic mechanisms, based upon purely statistical bias, contribute to the emergence of dendrite shape is largely unknown. We address this issue using the Drosophila class I vpda multi-dendritic neurons. Detailed quantitative analysis of vpda dendrite morphogenesis indicates that the primary branch grows very robustly in a fixed direction, though secondary branch numbers and lengths showed fluctuations characteristic of stochastic systems. Live-tracking dendrites and computational modeling revealed how neuron shape emerges from few local statistical parameters of branch dynamics. We report key opposing aspects of how tree architecture feedbacks on the local probability of branch shrinkage. Child branches promote stabilization of parent branches, although self-repulsion promotes shrinkage. Finally, we show that self-repulsion, mediated by the adhesion molecule Dscam1, indirectly patterns the growth of secondary branches by spatially restricting their direction of stable growth perpendicular to the primary branch. Thus, the stochastic nature of secondary branch dynamics and the existence of geometric feedback emphasize the importance of self-organization in neuronal dendrite morphogenesis.

Sevoflurane inhibits neuronal migration and axon growth in the developing mouse cerebral cortex

The highly organized laminar structure of the mammalian brain is dependent on successful neuronal migration, and migration deficits can cause lissencephaly and behavioral and cognitive defects. Here, we investigated the contribution of neuronal migration dysregulation to anesthesia-induced neurotoxicity in the fetal brain. Pregnant C57BL/6 mice at embryonic day 14.5 received 2.5% sevoflurane daily for two days. Cortical neuron migration and axon lengths were evaluated using GFP immunostaining. Morris water maze tests were performed to assess the effects of sevoflurane exposure on spatial memory in offspring. We found that sevoflurane exposure decreased axon length and caused cognitive defects in young mice. RNA sequencing revealed that these defects were associated with reduced neuro-oncological ventral antigen 2 (Nova2) expression. In utero electroporation experiments using Nova2 shRNA recapitulated this finding. Nova2 shRNA inhibited neuronal migration and decreased axon lengths. Finally, we found that Netrin-1/Deleted in Colorectal Cancer (Dcc) proteins acted downstream of Nova2 to suppresses neuronal migration. These findings describe a novel mechanism by which prenatal anesthesia exposure affects embryonic neural development and postnatal behavior.

Retrograde Tracing of Drosophila Embryonic Motor Neurons Using Lipophilic Fluorescent Dyes

We describe a technique for retrograde labeling of motor neurons in Drosophila. We use an oil-dissolved lipophilic dye and deliver a small droplet to an embryonic fillet preparation by a microinjector. Each motor neuron whose membrane is contacted by the droplet can then be rapidly labeled. Individual motor neurons are continuously labeled, enabling fine structural details to be clearly visualized. Given that lipophilic dyes come in various colors, the technique also provides a means to get adjacent neurons labeled in multicolor. This tracing technique is therefore useful for studying neuronal morphogenesis and synaptic connectivity in the motor neuron system of Drosophila.

De novo assembly of a transcriptome for the cricket Gryllus bimaculatus prothoracic ganglion: An invertebrate model for investigating adult central nervous system compensatory plasticity

The auditory system of the cricket, Gryllus bimaculatus, demonstrates an unusual amount of anatomical plasticity in response to injury, even in adults. Unilateral removal of the ear causes deafferented auditory neurons in the prothoracic ganglion to sprout dendrites across the midline, a boundary they typically respect, and become synaptically connected to the auditory afferents of the contralateral ear. The molecular basis of this sprouting and novel synaptogenesis in the adult is not understood. We hypothesize that well-conserved developmental guidance cues may recapitulate their guidance functions in the adult in order to facilitate this compensatory growth. As a first step in testing this hypothesis, we have generated a de novo assembly of a prothoracic ganglion transcriptome derived from control and deafferented adult individuals. We have mined this transcriptome for orthologues of guidance molecules from four well-conserved signaling families: Slit, Netrin, Ephrin, and Semaphorin. Here we report that transcripts encoding putative orthologues of most of the candidate developmental ligands and receptors from these signaling families were present in the assembly, indicating expression in the adult G. bimaculatus prothoracic ganglion.

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.

NOVA regulates Dcc alternative splicing during neuronal migration and axon guidance in the spinal cord

RNA-binding proteins (RBPs) control multiple aspects of post-transcriptional gene regulation and function during various biological processes in the nervous system. To further reveal the functional significance of RBPs during neural development, we carried out an in vivo RNAi screen in the dorsal spinal cord interneurons, including the commissural neurons. We found that the NOVA family of RBPs play a key role in neuronal migration, axon outgrowth, and axon guidance. Interestingly, Nova mutants display similar defects as the knockout of the Dcc transmembrane receptor. We show here that Nova deficiency disrupts the alternative splicing of Dcc, and that restoring Dcc splicing in Nova knockouts is able to rescue the defects. Together, our results demonstrate that the production of DCC splice variants controlled by NOVA has a crucial function during many stages of commissural neuron development.

Epithelia-derived wingless regulates dendrite directional growth of drosophila ddaE neuron through the Fz-Fmi-Dsh-Rac1 pathway

Background

Proper dendrite patterning is critical for the receiving and processing of information in the nervous system. Cell-autonomous molecules have been extensively studied in dendrite morphogenesis; however, the regulatory mechanisms of environmental factors in dendrite growth remain to be elucidated.

Results

By evaluating the angle between two primary dendrites (PD-Angle), we found that the directional growth of the primary dendrites of a Drosophila periphery sensory neuron ddaE is regulated by the morphogen molecule Wingless (Wg). During the early stage of dendrite growth, Wg is expressed in a group of epithelial cells posteriorly adjacent to ddaE. When Wg expression is reduced or shifted anteriorly, the PD-Angle is markedly decreased. Furthermore, Wg receptor Frizzled functions together with Flamingo and Dishevelled in transducing the Wg signal into ddaE neuron, and the downstream signal is mediated by non-canonical Wnt pathway through Rac1.

Conclusions

In conclusion, we reveal that epithelia-derived Wg plays a repulsive role in regulating the directional growth of dendrites through the non-canonical Wnt pathway. Thus, our findings provide strong in vivo evidence on how environmental signals serve as spatial cues for dendrite patterning.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0228-0) contains supplementary material, which is available to authorized users.

Regulation of dendrite morphogenesis by extrinsic cues

Dendrites play a central role in the integration and flow of information in the nervous system. The morphogenesis and maturation of dendrites is hence an essential step in the establishment of neuronal connectivity. Recent studies have uncovered crucial functions for extrinsic cues in the development of dendrites. We review the contribution of secreted polypeptide growth factors, contact-mediated proteins, and neuronal activity in distinct phases of dendrite development. We also highlight how extrinsic cues influence local and global intracellular mechanisms of dendrite morphogenesis. Finally, we discuss how these studies have advanced our understanding of neuronal connectivity and have shed light on the pathogenesis of neurodevelopmental disorders.

Netrin-1 directs dendritic growth and connectivity of vertebrate central neurons in vivo

BACKGROUND

Netrins are a family of extracellular proteins that function as chemotropic guidance cues for migrating cells and axons during neural development. In the visual system, netrin-1 has been shown to play a key role in retinal ganglion cell (RGC) axon growth and branching at the target, where presynaptic RGC axons form partnerships with the dendrites of tectal neurons. However, the signals that guide the connections between RGC axons and their postsynaptic partners are yet unknown. Here, we explored dynamic cellular mechanisms by which netrin-1 influences visual circuit formation, particularly those that impact postsynaptic neuronal morphology and connectivity during retinotectal wiring.

RESULTS

Time-lapse in vivo imaging of individual Xenopus laevis optic tectal neurons co-expressing tdTomato and PSD95-GFP revealed rapid remodeling and reorganization of dendritic arbors following acute manipulations in netrin-1 levels. Effects of altered netrin signaling on developing dendritic arbors of tectal neurons were distinct from its effects on presynaptic RGC axons. Within 4 h of treatment, tectal injection of recombinant netrin-1 or sequestration of endogenous netrin with an UNC-5 receptor ectodomain induced significant changes in the directionality and orientation of dendrite growth and in the maintenance of already established dendrites, demonstrating that relative levels of netrin are important for these functions. In contrast, altering DCC-mediated netrin signaling with function-blocking antibodies induced postsynaptic specialization remodeling and changed growth directionality of already established dendrites. Reducing netrin signaling also decreased avoidance behavior in a visually guided task, suggesting that netrin is essential for emergent visual system function.

CONCLUSIONS

These in vivo findings together with the patterns of expression of netrin and its receptors reveal an important role for netrin in the early growth and guidance of vertebrate central neuron dendritic arbors. Collectively, our studies indicate that netrin shapes both pre- and postsynaptic arbor morphology directly and in multiple ways at stages critical for functional visual system development.

Intrinsic and extrinsic mechanisms of dendritic morphogenesis

The complex, branched morphology of dendrites is a cardinal feature of neurons and has been used as a criterion for cell type identification since the beginning of neurobiology. Regulated dendritic outgrowth and branching during development form the basis of receptive fields for neurons and are essential for the wiring of the nervous system. The cellular and molecular mechanisms of dendritic morphogenesis have been an intensely studied area. In this review, we summarize the major experimental systems that have contributed to our understandings of dendritic development as well as the intrinsic and extrinsic mechanisms that instruct the neurons to form cell type-specific dendritic arbors.

The homeodomain transcription factor Hb9 controls axon guidance in Drosophila through the regulation of Robo receptors

Transcription factors establish neural diversity and wiring specificity; however, how they orchestrate changes in cell morphology remains poorly understood. The Drosophila Roundabout (Robo) receptors regulate connectivity in the CNS, but how their precise expression domains are established is unknown. Here, we show that the homeodomain transcription factor Hb9 acts upstream of Robo2 and Robo3 to regulate axon guidance in the Drosophila embryo. In ventrally projecting motor neurons, hb9 is required for robo2 expression, and restoring Robo2 activity in hb9 mutants rescues motor axon defects. Hb9 requires its conserved repressor domain and functions in parallel with Nkx6 to regulate robo2. Moreover, hb9 can regulate the medio-lateral position of axons through robo2 and robo3, and restoring robo3 expression in hb9 mutants rescues the lateral position defects of a subset of neurons. Altogether, these data identify Robo2 and Robo3 as key effectors of Hb9 in regulating nervous system development.

Quantification of dendritic and axonal growth after injury to the auditory system of the adult cricket Gryllus bimaculatus

Dendrite and axon growth and branching during development are regulated by a complex set of intracellular and external signals. However, the cues that maintain or influence adult neuronal morphology are less well understood. Injury and deafferentation tend to have negative effects on adult nervous systems. An interesting example of injury-induced compensatory growth is seen in the cricket, Gryllus bimaculatus. After unilateral loss of an ear in the adult cricket, auditory neurons within the central nervous system (CNS) sprout to compensate for the injury. Specifically, after being deafferented, ascending neurons (AN-1 and AN-2) send dendrites across the midline of the prothoracic ganglion where they receive input from auditory afferents that project through the contralateral auditory nerve (N5). Deafferentation also triggers contralateral N5 axonal growth. In this study, we quantified AN dendritic and N5 axonal growth at 30 h, as well as at 3, 5, 7, 14, and 20 days after deafferentation in adult crickets. Significant differences in the rates of dendritic growth between males and females were noted. In females, dendritic growth rates were non-linear; a rapid burst of dendritic extension in the first few days was followed by a plateau reached at 3 days after deafferentation. In males, however, dendritic growth rates were linear, with dendrites growing steadily over time and reaching lengths, on average, twice as long as in females. On the other hand, rates of N5 axonal growth showed no significant sexual dimorphism and were linear. Within each animal, the growth rates of dendrites and axons were not correlated, indicating that independent factors likely influence dendritic and axonal growth in response to injury in this system. Our findings provide a basis for future study of the cellular features that allow differing dendrite and axon growth patterns as well as sexually dimorphic dendritic growth in response to deafferentation.

Live imaging of multicolor-labeled cells in Drosophila

We describe LOLLIbow, a Brainbow-based live imaging system with applications in developmental biology and neurobiology. The development of an animal, including the environmentally sensitive adaptation of its brain, is thought to proceed through continual orchestration among diverse cell types as they divide, migrate, transform and interact with one another within the body. To facilitate direct visualization of such dynamic morphogenesis by individual cells in vivo, we have modified the original Brainbow for Drosophila in which live imaging is practical during much of its development. Our system offers permanent fluorescent labels that reveal fine morphological details of individual cells without requiring dissection or fixation of the samples. It also features a non-invasive means to control the timing of stochastic tricolor cell labeling with a light pulse. We demonstrate applicability of the new system in a variety of settings that could benefit from direct imaging of the developing multicellular organism with single-cell resolution.

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.

Juvenile exposure to methylphenidate reduces cocaine reward and alters netrin-1 receptor expression in adulthood

The guidance cue netrin-1 acting on mesocorticolimbic dopamine (DA) neurons through its receptor DCC (deleted in colorectal cancer) has been implicated in the neuronal plasticity induced by psychostimulant drugs. We examined in C57/BL6 mice the effects of repeated juvenile methylphenidate (MPH) exposure on cocaine-reward sensitivity in adulthood and determined whether early MPH treatment alters adult expression of DCC in mesocorticolimbic DA regions. By using place conditioning, we show that adult mice exposed to MPH during the juvenile period are less sensitive to cocaine-reward compared to saline-controls. Furthermore, by means of immunoblotting, we demonstrate that early MPH treatment attenuates adult DCC expression in the ventral tegmental area (VTA) selectively. These results support previous evidence that developmental MPH treatment diminishes cocaine-reward in adulthood and are the first to suggest that DCC in the VTA may participate in this enduring effect.

Role of netrin-1 in the organization and function of the mesocorticolimbic dopamine system

Changes in mesocorticolimbic dopamine (DA) neurons and their target cells can be induced throughout life and are important determinants of individual differences in susceptibility to psychopathology. The goal of my research is to gain insight into the nature of the cellularand molecular mechanism underlying the selective plasticity of mesocorticolimbic DA neurons. Here, I review work showing that the guidance cue netrin-1 is implicated in the organization, plasticity and function of mesocorticolimbic DA neurons in rodents. Developmental variations in netrin-1 receptor function result in selective reorganization of medial prefrontal DA circuitry during adolescence and in an adult phenotype protected against schizophrenia-like dopaminergic and behavioural abnormalities. Furthermore, in adulthood, expression of netrin-1 receptors is upregulated by repeated exposure to stimulant drugs of abuse in DA somatodendritic regions and is necessary for drug-induced behavioural plasticity. I propose that risk factors associated with DA-related adult psychiatric disorders alter netrin-1 function.

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.

Bilateral consequences of chronic unilateral deafferentation in the auditory system of the cricket Gryllus bimaculatus

The auditory system of the cricket has the unusual ability to respond to deafferentation by compensatory growth and synapse formation. Auditory interneurons such as ascending neuron 2 (AN-2) in the cricket Gryllus bimaculatus possess a dendritic arbor that normally grows up to, but not over, the midline of the prothoracic ganglion. After chronic deafferentation throughout larval development, however, the AN-2 dendritic arbor changes dramatically, and medial dendrites sprout across the midline where they form compensatory synapses with the auditory afferents from the contralateral ear. We quantified the extent of the effects of chronic, unilateral deafferentation by measuring several cellular parameters of 3 different neuronal components of the auditory system: the deafferented AN-2, the contralateral (or nondeafferented) AN-2 and the contralateral auditory afferents. Neuronal tracers and confocal microscopy were used to visualize neurons, and double-label experiments were performed to examine the cellular relationship between pairs of cells. Dendritic complexity was quantified using a modified Sholl analysis, and the length and volume of processes and presynaptic varicosities were assessed under control and deafferented conditions. Chronic deafferentation significantly influenced the morphology of all 3 neuronal components examined. The overall dendritic complexity of the deafferented AN-2 dendritic arbor was reduced, while both the contralateral AN-2 dendritic arbor and the remaining, intact, auditory afferents grew longer. We found no significant changes in the volume or density of varicosities after deafferentation. These complex cellular changes after deafferentation are interpreted in the light of the reported differential regulation of vesicle-associated membrane protein and semaphorin 2a.

UNC-6 and UNC-40 promote dendritic growth through PAR-4 in Caenorhabditis elegans neurons

Axons navigating through the developing nervous system are instructed by external attractive and repulsive cues. Emerging evidence suggests the same cues control dendrite development, but it is not understood how they differentially instruct axons and dendrites. We studied a C. elegans motor neuron whose axon and dendrite adopt different trajectories and lengths. We found that the guidance cue UNC-6 (Netrin) is required for both axon and dendrite development. Its repulsive receptor UNC-5 repelled the axon from the ventral cell body, whereas the attractive receptor UNC-40 (DCC) was dendritically enriched and promotes antero-posterior dendritic growth. Although the endogenous ventrally secreted UNC-6 instructs axon guidance, dorsal or even membrane-tethered UNC-6 could support dendrite development. Unexpectedly, the serine-threonine kinase PAR-4 (LKB1) was selectively required for the activity of the UNC-40 pathway in dendrite outgrowth. These data suggest that axon and dendrite of one neuron interpret common environmental cues with different receptors and downstream signaling pathways.

In vivo development of dendritic orientation in wild-type and mislocalized retinal ganglion cells

Background

Many neurons in the central nervous system, including retinal ganglion cells (RGCs), possess asymmetric dendritic arbors oriented toward their presynaptic partners. How such dendritic arbors become biased during development in vivo is not well understood. Dendritic arbors may become oriented by directed outgrowth or by reorganization of an initially unbiased arbor. To distinguish between these possibilities, we imaged the dynamic behavior of zebrafish RGC dendrites during development in vivo. We then addressed how cell positioning within the retina, altered in heart-and-soul (has) mutants, affects RGC dendritic orientation.

Results

In vivo multiphoton time-lapse analysis revealed that RGC dendrites initially exhibit exploratory behavior in multiple directions but progressively become apically oriented. The lifetimes of basal and apical dendrites were generally comparable before and during the period when arbors became biased. However, with maturation, the addition and extension rates of basal dendrites were slower than those of the apical dendrites. Oriented dendritic arbors were also found in misplaced RGCs of the has retina but there was no preferred orientation amongst the population. However, has RGCs always projected dendrites toward nearby neuropil where amacrine and bipolar cell neurites also terminated. Chimera analysis showed that the abnormal dendritic organization of RGCs in the mutant was non-cell autonomous.

Conclusions

Our observations show that RGC dendritic arbors acquire an apical orientation by selective and gradual restriction of dendrite addition to the apical side of the cell body, rather than by preferential dendrite stabilization or elimination. A biased arbor emerges at a stage when many of the dendritic processes still appear exploratory. The generation of an oriented RGC dendritic arbor is likely to be determined by cell-extrinsic cues. Such cues are unlikely to be localized to the basal lamina of the inner retina, but rather may be provided by cells presynaptic to the RGCs.

Assisted morphogenesis: glial control of dendrite shapes

Neurons display a myriad of dendritic architectures, reflecting their diverse roles in information processing and transduction in the nervous system. Recent findings suggest that neuronal signals may not account for all aspects of dendrite morphogenesis. Observations from C. elegans and other organisms suggest that glial cells can affect dendrite length and guidance, as well as localization and shapes of dendritic receptive structures, such as dendritic spines and sensory cilia. Thus, besides direct roles in controlling neuronal activity, glia contribute to neuron function by ensuring that neurons attain their proper shapes.

Dendrite reshaping of adult Drosophila sensory neurons requires matrix metalloproteinase-mediated modification of the basement membranes

In response to changes in the environment, dendrites from certain neurons change their shape, yet the mechanism remains largely unknown. Here we show that dendritic arbors of adult Drosophila sensory neurons are rapidly reshaped from a radial shape to a lattice-like shape within 24 hr after eclosion. This radial-to-lattice reshaping arises from rearrangement of the existing radial branches into the lattice-like pattern, rather than extensive dendrite pruning followed by regrowth of the lattice-shaped arbors over the period. We also find that the dendrite reshaping is completely blocked in mutants for the matrix metalloproteinase (Mmp) 2. Further genetic analysis indicates that Mmp2 promotes the dendrite reshaping through local degradation of the basement membrane upon which dendrites of the sensory neurons innervate. These findings suggest that regulated proteolytic alteration of the extracellular matrix microenvironment might be a fundamental mechanism to drive a large-scale change of dendritic structures during reorganization of neuronal circuits.

Distinct roles for Robo2 in the regulation of axon and dendrite growth by retinal ganglion cells

Guidance factors act on the tip of a growing axon to direct it to its target. What role these molecules play, however, in the control of the dendrites that extend from that axon's cell body is poorly understood. Slits, through their Robo receptors, guide many types of axons, including those of retinal ganglion cells (RGCs). Here we assess and contrast the role of Slit/Robo signalling in the growth and guidance of the axon and dendrites extended by RGCs in Xenopus laevis. As Xenopus RGCs extend dendrites, they express robo2 and robo3, while slit1 and slit2 are expressed in RGCs and in the adjacent inner nuclear layer. Interestingly, our functional data with antisense knockdown and dominant negative forms of Robo2 (dnRobo2) and Robo3 (dnRobo3) indicate that Slit/Robo signalling has no role in RGC dendrite guidance, and instead is necessary to stimulate dendrite branching, primarily via Robo2. Our in vitro culture data argue that Slits are the ligands involved. In contrast, both dnRobo2 and dnRobo3 inhibited the extension of axons and caused the misrouting of some axons. Based on these data, we propose that Robo signalling can have distinct functions in the axon and dendrites of the same cell, and that the specific combinations of Robo receptors could underlie these differences. Slit acts via Robo2 in dendrites as a branching/growth factor but not in guidance, while Robo2 and Robo3 function in concert in axons to mediate axonal interactions and respond to Slits as guidance factors. These data underscore the likelihood that a limited number of extrinsic factors regulate the distinct morphologies of axons and dendrites.

Establishment of axon-dendrite polarity in developing neurons

Neurons are among the most highly polarized cell types in the body, and the polarization of axon and dendrites underlies the ability of neurons to integrate and transmit information in the brain. Significant progress has been made in the identification of the cellular and molecular mechanisms underlying the establishment of neuronal polarity using primarily in vitro approaches such as dissociated culture of rodent hippocampal and cortical neurons. This model has led to the predominant view suggesting that neuronal polarization is specified largely by stochastic, asymmetric activation of intracellular signaling pathways. Recent evidence shows that extracellular cues can play an instructive role during neuronal polarization in vitro and in vivo. In this review, we synthesize the recent data supporting an integrative model whereby extracellular cues orchestrate the intracellular signaling underlying the initial break of neuronal symmetry leading to axon-dendrite polarization.

Dendrite branching and self-avoidance are controlled by Turtle, a conserved IgSF protein in Drosophila

The dendritic trees of neurons result from specific patterns of growth and branching, and dendrite branches of the same neuron avoid one another to spread over a particular receptive field. Recognition molecules on the surfaces of dendrites influence these patterning and avoidance processes by promoting attractive, repulsive or adhesive responses to specific cues. The Drosophila transmembrane protein Turtle (Tutl) and its orthologs in other species are conserved members of the immunoglobulin superfamily, the in vivo functions of which are unknown. In Drosophila sensory neurons, we show that the tutl gene is required to restrain dendrite branch formation in neurons with simple arbors, and to promote dendrite self-avoidance in neurons with complex arbors. The cytoplasmic tail of Tutl is dispensable for control of dendrite branching, suggesting that Tutl acts as a ligand or co-receptor for an unidentified recognition molecule to influence the architecture of dendrites and their coverage of receptive territories.

Patterning and organization of motor neuron dendrites in the Drosophila larva

Precise patterns of motor neuron connectivity depend on the proper establishment and positioning of the dendritic arbor. However, how different motor neurons orient their dendrites to selectively establish synaptic connectivity is not well understood. The Drosophila neuromuscular system provides a simple model to investigate the underlying organizational principles by which distinct subclasses of motor neurons orient their dendrites within the central neuropil. Here we used genetic mosaic techniques to characterize the diverse dendritic morphologies of individual motor neurons from five main nerve branches (ISN, ISNb, ISNd, SNa, and SNc) in the Drosophila larva. We found that motor neurons from different nerve branches project their dendrites to largely stereotyped mediolateral domains in the dorsal region of the neuropil providing full coverage of the receptive territory. Furthermore, dendrites from different motor neurons overlap extensively, regardless of subclass, suggesting that repulsive dendrite-dendrite interactions between motor neurons do not influence the mediolateral positioning of dendritic fields. The anatomical data in this study provide important information regarding how different subclasses of motor neurons organize their dendrites and establishes a foundation for the investigation of the mechanisms that control synaptic connectivity in the Drosophila motor circuit.

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

Neural maps are emergent, highly ordered structures that are essential for organizing and presenting synaptic information. Within the embryonic nervous system of Drosophila motoneuron dendrites are organized topographically as a myotopic map that reflects their pattern of innervation in the muscle field. Here we reveal that this fundamental organizational principle exists in adult Drosophila, where the dendrites of leg motoneurons also generate a myotopic map. A single postembryonic neuroblast sequentially generates different leg motoneuron subtypes, starting with those innervating proximal targets and medial neuropil regions and producing progeny that innervate distal muscle targets and lateral neuropil later in the lineage. Thus the cellular distinctions in peripheral targets and central dendritic domains, which make up the myotopic map, are linked to the birth-order of these motoneurons. Our developmental analysis of dendrite growth reveals that this myotopic map is generated by targeting. We demonstrate that the medio-lateral positioning of motoneuron dendrites in the leg neuropil is controlled by the midline signalling systems Slit-Robo and Netrin-Fra. These results reveal that dendritic targeting plays a major role in the formation of myotopic maps and suggests that the coordinate spatial control of both pre- and postsynaptic elements by global neuropilar signals may be an important mechanism for establishing the specificity of synaptic connections.

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."

Differential gene expression during compensatory sprouting of dendrites in the auditory system of the cricket Gryllus bimaculatus

Neurones that lose their presynaptic partners because of injury usually retract or die. However, when the auditory interneurones of the cricket Gryllus bimaculatus are denervated, dendrites respond by growing across the midline and forming novel synapses with the opposite auditory afferents. Suppression subtractive hybridization was used to detect transcriptional changes 3 days after denervation. This is a stage at which we demonstrate robust compensatory dendritic sprouting. Whereas 49 unique candidates were down-regulated, no sufficiently up-regulated candidates were identified at this time point. Several candidates identified in this study are known to influence the translation and degradation of proteins in other systems. The potential role of these factors in the compensatory sprouting of cricket auditory interneurones in response to denervation is discussed.

Molecules and mechanisms of dendrite development in Drosophila

Neurons are one of the most morphologically diverse cell types, in large part owing to their intricate dendrite branching patterns. Dendrites are structures that are specialized to receive and process inputs in neurons, thus their specific morphologies reflect neural connectivity and influence information flow through circuits. Recent studies in Drosophila on the molecular basis of dendrite diversity, dendritic guidance, the cell biology of dendritic branch patterning and territory formation have identified numerous intrinsic and extrinsic cues that shape diverse features of dendrites. As we discuss in this review, many of the mechanisms that are being elucidated show conservation in diverse systems.

Positional cues in the Drosophila nerve cord: semaphorins pattern the dorso-ventral axis

During the development of neural circuitry, neurons of different kinds establish specific synaptic connections by selecting appropriate targets from large numbers of alternatives. The range of alternative targets is reduced by well organised patterns of growth, termination, and branching that deliver the terminals of appropriate pre- and postsynaptic partners to restricted volumes of the developing nervous system. We use the axons of embryonic Drosophila sensory neurons as a model system in which to study the way in which growing neurons are guided to terminate in specific volumes of the developing nervous system. The mediolateral positions of sensory arbors are controlled by the response of Robo receptors to a Slit gradient. Here we make a genetic analysis of factors regulating position in the dorso-ventral axis. We find that dorso-ventral layers of neuropile contain different levels and combinations of Semaphorins. We demonstrate the existence of a central to dorsal and central to ventral gradient of Sema 2a, perpendicular to the Slit gradient. We show that a combination of Plexin A (Plex A) and Plexin B (Plex B) receptors specifies the ventral projection of sensory neurons by responding to high concentrations of Semaphorin 1a (Sema 1a) and Semaphorin 2a (Sema 2a). Together our findings support the idea that axons are delivered to particular regions of the neuropile by their responses to systems of positional cues in each dimension.

Endogenous activation patterns of Cdc42 GTPase within Drosophila embryos

Knowing when and where a given protein is activated within intact animals assists in elucidating its in vivo function. With the use of a genetically encoded A-probe (activation bioprobe), we revealed that Cdc42 guanosine triphosphatase (GTPase) remains inactive within Drosophila embryos during the first two-thirds of embryogenesis. Within the central nervous system where Cdc42 activity first becomes up-regulated, individual neurons display patterns restricted to specific subcellular compartments. At both organismal and cellular levels, Cdc42's endogenous activation patterns in the wild type allow predictions of where loss-of-function phenotypes will emerge in cdc42/cdc42 mutants. Genetic tests support the importance of suppressing endogenous Cdc42 activities until needed. Thus, bioprobe-assisted analysis uncovers how ubiquitously expressed signaling proteins control cellular events through continual regulation of their activities within animals.

Slits and their receptors

Slit was identified in Drosophila embryo as a gene involved in the patterning of larval cuticle. It was later shown that Slit is synthesized in the fly central nervous system by midline glia cells. Slit homologues have since been found in C. elegans and many vertebrate species, from amphibians, fishes, birds to mammals. A single slit was isolated in invertebrates, whereas there are three slit genes (slit1-slit3) in mammals, that have around 60% homology. All encodes large ECM glycoproteins of about 200 kDa (Fig. 1A), comprising, from their N terminus to their C terminus, a long stretch of four leucine rich repeats (LRR) connected by disulphide bonds, seven to nine EGF repeats, a domain, named ALPS (Agrin, Perlecan, Laminin, Slit) or laminin G-like module (see ref 17), and a cystein knot (Fig. 1A). Alternative spliced transcripts have been reported for Drosophila Slit2, human Slit2 and Slit3, and Slit1. Moreover, two Slit1 isoforms exist in zebrafish as a consequence of gene duplication. Last, in mammals, two Slit2 isoforms can be purified from brain extracts, a long 200 kDa one and a shorter 150 kDa form (Slit2-N) that was shown to result from the proteolytic processing of full-length Slit2. Human Slit and Slit3 and Drosophila Slit are also cleaved by an unknown protease in a large N-terminal fragment and a shorter C-terminal fragment, suggesting conserved mechanisms for Slit cleavage across species. Moreover, Slit fragments have different cell association characteristics in cell culture suggesting that they may also have different extents of diffusion, different binding properties, and, hence, different functional activities in vivo. This conclusion is supported by in vitro data showing that full-length Slit2 functions as an antagonist of Slit2-N in the DRG branching assay, and that Slit2-N, not full-length Slit2, causes collapse of OB growth cones. In addition, Slit1-N and full-length Slit1 can induce branching of cortical neurons (see below), but only full-length Slit1 repels cortical axons. Structure-function analysis in vertebrates and Drosophila demonstrated that the LRRs of Slits are required and sufficient to mediate their repulsive activities in neurons. More recent detailed structure function analysis of the LRR domains of Drosophila Slit, revealed that the active site of Slit (at least regarding its pro-angiogenic activity) is located on the second of the fourth LRR (LRR2), which is highly conserved between Slits. Slit can also dimerize through the LRR4 domain and the cystein knot.However, a Slit1 spliced-variant that lacks the cysteine knot and does not dimerize is still able to repel OB axons.

Slit and Robo regulate dendrite branching and elongation of space-filling neurons in Drosophila

Space-filling neurons extensively sample their receptive fields with fine dendritic branches. In this study we show that a member of the conserved Robo receptor family, Robo, and its ligand Slit regulate the dendritic differentiation of space-filling neurons. Loss of Robo or Slit function leads to faster elongating and less branched dendrites of the complex and space-filling class IV multi-dendritic dendrite-arborization (md-da) neurons in the Drosophila embryonic peripheral nervous system, but not of the simpler class I neurons. The total dendrite length of Class IV neurons is not modified in robo or slit mutant embryos. Robo mediates this process cell-autonomously. Upon Robo over-expression in md-da neurons the dendritic tree is simplified and time-lapse analysis during larval stages indicates that this is due to reduction in the number of newly formed branches. We propose that Slit, through Robo, provides an extrinsic signal to coordinate the growth rate and the branching level of space-filling neurons, thus allowing them to appropriately cover their target field.

Identification of genes influencing dendrite morphogenesis in developing peripheral sensory and central motor neurons

Background

Developing neurons form dendritic trees with cell type-specific patterns of growth, branching and targeting. Dendrites of Drosophila peripheral sensory neurons have emerged as a premier genetic model, though the molecular mechanisms that underlie and regulate their morphogenesis remain incompletely understood. Still less is known about this process in central neurons and the extent to which central and peripheral dendrites share common organisational principles and molecular features. To address these issues, we have carried out two comparable gain-of-function screens for genes that influence dendrite morphologies in peripheral dendritic arborisation (da) neurons and central RP2 motor neurons.

Results

We found 35 unique loci that influenced da neuron dendrites, including five previously shown as required for da dendrite patterning. Several phenotypes were class-specific and many resembled those of known mutants, suggesting that genes identified in this study may converge with and extend known molecular pathways for dendrite development in da neurons. The second screen used a novel technique for cell-autonomous gene misexpression in RP2 motor neurons. We found 51 unique loci affecting RP2 dendrite morphology, 84% expressed in the central nervous system. The phenotypic classes from both screens demonstrate that gene misexpression can affect specific aspects of dendritic development, such as growth, branching and targeting. We demonstrate that these processes are genetically separable. Targeting phenotypes were specific to the RP2 screen, and we propose that dendrites in the central nervous system are targeted to territories defined by Cartesian co-ordinates along the antero-posterior and the medio-lateral axes of the central neuropile. Comparisons between the screens suggest that the dendrites of peripheral da and central RP2 neurons are shaped by regulatory programs that only partially overlap. We focused on one common candidate pathway controlled by the ecdysone receptor, and found that it promotes branching and growth of developing da neuron dendrites, but a role in RP2 dendrite development during embryonic and early larval stages was not apparent.

Conclusion

We identified commonalities (for example, growth and branching) and distinctions (for example, targeting and ecdysone response) in the molecular and organizational framework that underlies dendrite development of peripheral and central neurons.

Molecular mechanisms controlling midline crossing by precerebellar neurons

Precerebellar neurons of the inferior olive (IO) and lateral reticular nucleus (LRN) migrate tangentially from the rhombic lip toward the floor plate following parallel pathways. This process is thought to involve netrin-1 attraction. However, whereas the cell bodies of LRN neurons cross the midline, IO neurons are unable to do so. In many systems and species, axon guidance and cell migration at the midline are controlled by Slits and their receptor Robos. We showed previously that precerebellar axons and neurons do not cross the midline in the absence of the Robo3 receptor. To determine whether this signaling by Slits and the two other Robo receptors, Robo1 and Robo2, also regulates precerebellar neuron behavior at the floor plate, we studied the phenotype of Slit1/2 and Robo1/2/3 compound mutants. Our results showed that many IO neurons can cross the midline in absence of Slit1/2 or Robo1/2, supporting a role for midline repellents in guiding precerebellar neurons. We also show that these molecules control the development of the lamellation of the inferior olivary complex. Last, the analysis of Robo1/2/3 triple mutants suggests that Robo3 inhibits Robo1/2 repulsion in precrossing LRN axons but not in IO axons in which it has a dominant and distinct function.

Genetic modifier screens reveal new components that interact with the Drosophila dystroglycan-dystrophin complex

The Dystroglycan-Dystrophin (Dg-Dys) complex has a capacity to transmit information from the extracellular matrix to the cytoskeleton inside the cell. It is proposed that this interaction is under tight regulation; however the signaling/regulatory components of Dg-Dys complex remain elusive. Understanding the regulation of the complex is critical since defects in this complex cause muscular dystrophy in humans. To reveal new regulators of the Dg-Dys complex, we used a model organism Drosophila melanogaster and performed genetic interaction screens to identify modifiers of Dg and Dys mutants in Drosophila wing veins. These mutant screens revealed that the Dg-Dys complex interacts with genes involved in muscle function and components of Notch, TGF-β and EGFR signaling pathways. In addition, components of pathways that are required for cellular and/or axonal migration through cytoskeletal regulation, such as Semaphorin-Plexin, Frazzled-Netrin and Slit-Robo pathways show interactions with Dys and/or Dg. These data suggest that the Dg-Dys complex and the other pathways regulating extracellular information transfer to the cytoskeletal dynamics are more intercalated than previously thought.

Postnatal development of the cholinergic innervation in the dorsal hippocampus of rat: Quantitative light and electron microscopic immunocytochemical study

Choline acetyltransferase (ChAT) immunocytochemistry was used to examine the distribution and ultrastructural features of the acetylcholine (ACh) innervation in the dorsal hippocampus of postnatal rat. The length of ChAT-immunostained axons was measured and the number of ChAT-immunostained varicosities counted, in each layer of CA1, CA3, and dentate gyrus, at postnatal ages P8, P16, and P32. At P8, an elaborate network of varicose ChAT-immunostained axons was already visible. At P16, the laminar distribution of this network resembled that in the adult, but adult densities were reached only by P32. Between P8 and P32, the mean densities for the three regions increased from 8.4 to 14 meters of axons and 2.3 to 5.7 million varicosities per cubic millimeter of tissue. At the three postnatal ages, the ultrastructural features of ChAT-immunostained axon varicosities from the strata pyramidale and radiatum of CA1 were similar between layers and comparable to those in adult, except for an increasing frequency of mitochondria (up to 41% at P32). The proportion of these profiles displaying a synaptic junction was equally low at all ages, indicating an average synaptic incidence of 7% for whole varicosities, as previously found in adult. The observed junctions were small, usually symmetrical, and made mostly with dendritic branches. These results demonstrate the precocious and rapid maturation of the hippocampal cholinergic innervation and reveal its largely asynaptic nature as soon as it is formed. They emphasize the remarkable growth capacities of individual ACh neurons and substantiate a role for diffuse transmission by ACh during hippocampal development.

Slit and Robo control the development of dendrites in Drosophila CNS

The molecular mechanisms that generate dendrites in the CNS are poorly understood. The diffusible signal molecule Slit and the neuronally expressed receptor Robo mediate growth cone collapse in vivo. However, in cultured neurons, these molecules promote dendritic development. Here we examine the aCC motoneuron, one of the first CNS neurons to generate dendrites in Drosophila. Slit displays a dynamic concentration topography that prefigures aCC dendrogenesis. Genetic deletion of Slit leads to complete loss of aCC dendrites. Robo is cell-autonomously required in aCC motoneurons to develop dendrites. Our results demonstrate that Slit and Robo control the development of dendrites in the embryonic CNS.