TRIM-9 functions in the UNC-6/UNC-40 pathway to regulate ventral guidance

Eight TRIM32 isoforms from oriental river prawn Macrobrachium nipponense are involved in innate immunity during white spot syndrome virus infection

Tripartite motif (TRIM) proteins comprise a large family of RING-type ubiquitin E3 ligases that regulate important biological processes. In this study, full-length MnTRIM32 cDNA was obtained from oriental river prawn Macrobrachium nipponense, and eight MnTRIM32 isoforms generated by alternative splicing were identified. The open reading frames of the eight MnTRIM32 isoforms were predicted to be separately composed of 402, 346, 347, 346, 414, 358, 359, and 358 amino acid residues. Protein structural analysis revealed that all MnTRIM32 isoforms contained a RING domain and a coiled coil region. MnTRIM32 was ubiquitously expressed in all tissues tested, with the highest expression in the hepatopancreas. The mRNA levels of MnTRIM32 in the gills, stomach, and intestine of prawns were found to undergo time-dependent enhancement following white spot syndrome virus (WSSV) stimulation. Double-stranded RNA interference studies revealed that MnTRIM32 silencing significantly downregulated the expression levels of interferon (IFN) regulatory factor MnIRF, IFN-like factor MnVago4, and tumor necrosis factor MnTNF. Furthermore, knockdown of MnTRIM32 in WSSV-challenged prawns increased the expression of VP28 and the number of WSSV copies, suggesting that MnTRIM32 plays a positive role in limiting WSSV infection. These findings provided strong evidence for the important role of MnTRIM32 in the antiviral innate immunity of M. nipponense.

Integration of proteomic and genetic approaches to assess developmental muscle atrophy

Muscle atrophy, or a decline in muscle protein mass, is a significant problem in the aging population and in numerous disease states. Unraveling molecular signals that trigger and promote atrophy may lead to a better understanding of treatment options; however, there is no single cause of atrophy identified to date. To gain insight into this problem, we chose to investigate changes in protein profiles during muscle atrophy in Manduca sexta and Drosophila melanogaster. The use of insect models provides an interesting parallel to probe atrophic mechanisms as these organisms undergo a normal developmental atrophy process during the pupal transition stage. Leveraging the inherent advantages of each model organism, we first defined protein signature changes during M. sexta intersegmental muscle (ISM) atrophy and then used genetic approaches to confirm their functional importance in the D. melanogaster dorsal internal oblique muscles (DIOMs). Our data reveal an upregulation of proteasome and peptidase components and a general downregulation of proteins that regulate actin filament formation. Surprisingly, thick filament proteins that comprise the A-band are increased in abundance, providing support for the ordered destruction of myofibrillar components during developmental atrophy. We also uncovered the actin filament regulator ciboulot (Cib) as a novel regulator of muscle atrophy. These insights provide a framework towards a better understanding of global changes that occur during atrophy and may eventually lead to therapeutic targets.

Trim9 and Klp61F promote polymerization of new dendritic microtubules along parallel microtubules

Axons and dendrites are distinguished by microtubule polarity. In Drosophila, dendrites are dominated by minus-end-out microtubules, whereas axons contain plus-end-out microtubules. Local nucleation in dendrites generates microtubules in both orientations. To understand why dendritic nucleation does not disrupt polarity, we used live imaging to analyze the fate of microtubules generated at branch points. We found that they had different rates of success exiting the branch based on orientation: correctly oriented minus-end-out microtubules succeeded in leaving about twice as often as incorrectly oriented microtubules. Increased success relied on other microtubules in a parallel orientation. From a candidate screen, we identified Trim9 and kinesin-5 (Klp61F) as machinery that promoted growth of new microtubules. In S2 cells, Eb1 recruited Trim9 to microtubules. Klp61F promoted microtubule growth in vitro and in vivo, and could recruit Trim9 in S2 cells. In summary, the data argue that Trim9 and kinesin-5 act together at microtubule plus ends to help polymerizing microtubules parallel to pre-existing ones resist catastrophe.

A Statistically-Oriented Asymmetric Localization (SOAL) Model for Neuronal Outgrowth Patterning by Caenorhabditis elegans UNC-5 (UNC5) and UNC-40 (DCC) Netrin Receptors

Neurons extend processes that vary in number, length, and direction of "outgrowth". Extracellular cues help determine outgrowth patterns. In Caenorhabditis elegans, neurons respond to the extracellular UNC-6 (netrin) cue via UNC-40 (DCC) and UNC-5 (UNC5) receptors. Previously, we presented evidence that UNC-40 asymmetric localization at the plasma membrane is self-organizing, and that UNC-40 can localize and mediate outgrowth at randomly selected sites. Here, we provide further evidence for a statistically-oriented asymmetric localization (SOAL) model in which UNC-5 receptor activity affects patterns of axon outgrowth by regulating UNC-40 asymmetric localization. According to the SOAL model, the direction of outgrowth activity fluctuates across the membrane over time. Random walk modeling predicts that increasing the degree to which the direction of outgrowth fluctuates will decrease the outward displacement of the membrane. By differentially affecting the degree to which the direction of outgrowth activity fluctuates over time, extracellular cues can produce different rates of outgrowth along the surface and create patterns of "extension". Consistent with the SOAL model, we show that unc-5 mutations alter UNC-40 asymmetric localization, increase the degree to which the direction of outgrowth fluctuates, and reduce the extent of outgrowth in multiple directions relative to the source of UNC-6 These results are inconsistent with current models, which predict that UNC-5 mediates a "repulsive" response to UNC-6 Genetic interactions suggest that UNC-5 acts through the UNC-53 (NAV2) cytoplasmic protein to regulate UNC-40 asymmetric localization in response to both the UNC-6 and EGL-20 (Wnt) extracellular cues.

Disruption of Trim9 function abrogates macrophage motility in vivo

The vertebrate immune response comprises multiple molecular and cellular components that interface to provide defense against pathogens. Because of the dynamic complexity of the immune system and its interdependent innate and adaptive functionality, an understanding of the whole-organism response to pathogen exposure remains unresolved. Zebrafish larvae provide a unique model for overcoming this obstacle, because larvae are protected against pathogens while lacking a functional adaptive immune system during the first few weeks of life. Zebrafish larvae were exposed to immune agonists for various lengths of time, and a microarray transcriptome analysis was executed. This strategy identified known immune response genes, as well as genes with unknown immune function, including the E3 ubiquitin ligase tripartite motif-9 (Trim9). Although trim9 expression was originally described as "brain specific," its expression has been reported in stimulated human Mϕs. In this study, we found elevated levels of trim9 transcripts in vivo in zebrafish Mϕs after immune stimulation. Trim9 has been implicated in axonal migration, and we therefore investigated the impact of Trim9 disruption on Mϕ motility and found that Mϕ chemotaxis and cellular architecture are subsequently impaired in vivo. These results demonstrate that Trim9 mediates cellular movement and migration in Mϕs as well as neurons.

Genetics of alcohol consumption in Drosophila melanogaster

Individual variation in alcohol consumption in human populations is determined by genetic, environmental, social and cultural factors. In contrast to humans, genetic contributions to complex behavioral phenotypes can be readily dissected in Drosophila, where both the genetic background and environment can be controlled and behaviors quantified through simple high-throughput assays. Here, we measured voluntary consumption of ethanol in ∼3000 individuals of each sex from an advanced intercross population derived from 37 lines of the Drosophila melanogaster Genetic Reference Panel. Extreme quantitative trait loci mapping identified 385 differentially segregating allelic variants located in or near 291 genes at P < 10-8 . The effects of single nucleotide polymorphisms associated with voluntary ethanol consumption are sex-specific, as found for other alcohol-related phenotypes. To assess causality, we used RNA interference knockdown or P{MiET1} mutants and their corresponding controls and functionally validated 86% of candidate genes in at least one sex. We constructed a genetic network comprised of 23 genes along with a separate trio and a pair of connected genes. Gene ontology analyses showed enrichment of developmental genes, including development of the nervous system. Furthermore, a network of human orthologs showed enrichment for signal transduction processes, protein metabolism and developmental processes, including nervous system development. Our results show that the genetic architecture that underlies variation in voluntary ethanol consumption is sexually dimorphic and partially overlaps with genetic factors that control variation in feeding behavior and alcohol sensitivity. This integrative genetic architecture is rooted in evolutionarily conserved features that can be extrapolated to human genetic interaction networks.

A cell cycle-independent, conditional gene inactivation strategy for differentially tagging wild-type and mutant cells

Here, we describe a novel method based on intronic MiMIC insertions described in Nagarkar-Jaiswal et al. (2015) to perform conditional gene inactivation in Drosophila. Mosaic analysis in Drosophila cannot be easily performed in post-mitotic cells. We therefore, therefore, developed Flip-Flop, a flippase-dependent in vivo cassette-inversion method that marks wild-type cells with the endogenous EGFP-tagged protein, whereas mutant cells are marked with mCherry upon inversion. We document the ease and usefulness of this strategy in differential tagging of wild-type and mutant cells in mosaics. We use this approach to phenotypically characterize the loss of SNF4Aγ, encoding the γ subunit of the AMP Kinase complex. The Flip-Flop method is efficient and reliable, and permits conditional gene inactivation based on both spatial and temporal cues, in a cell cycle-, and developmental stage-independent fashion, creating a platform for systematic screens of gene function in developing and adult flies with unprecedented detail.

DOI:http://dx.doi.org/10.7554/eLife.26420.001

The instructions needed to build and maintain cells in an organism are encoded in their DNA. There are many different cell types, and each type only needs a small portion of the information found in the DNA to do its job. Hence, only some of the instructions, in the form of genes, need to be active or ‘expressed’ in any given cell type.

To understand how a gene works, it is necessary to know in which cell the gene is expressed and where in the cell the gene product – normally a protein – is located. Researchers may study a gene by deleting it, which prevents the protein from being made, or by attaching a new instruction into the gene, which generates a fluorescent tag on the protein to determine where and when it is expressed. Until now, it was not possible to selectively inactivate a gene and simultaneously mark both normal cells containing the protein and mutant cells lacking the protein.

Based on an existing tagging approach, Nagarkar-Jaiswal et al. have now developed a method in which normal and mutant cells of fruit flies are marked differently. A gene of interest is tagged with a fluorescent marker called green fluorescent protein (or GFP). The same gene is then inactivated in some of the cells, which are tagged with a red marker called mCherry. Nagarkar-Jaiswal et al. compared normal and mutant cells, and were able to determine how long it takes before the mutant cells become abnormal.

With this new method, the role of numerous genes in any tissue of adult flies can be reassessed. This will allow to investigate what happens when a protein is removed in specific cells in adult flies. A future goal will be to apply this method to other animals that are more closely related to humans, such as mice, to gain a clearer picture of the role of genes in different cell types and how faulty genes may cause disease.

DOI:http://dx.doi.org/10.7554/eLife.26420.002

Frazzled promotes growth cone attachment at the source of a Netrin gradient in the Drosophila visual system

Axon guidance is proposed to act through a combination of long- and short-range attractive and repulsive cues. The ligand-receptor pair, Netrin (Net) and Frazzled (Fra) (DCC, Deleted in Colorectal Cancer, in vertebrates), is recognized as the prototypical effector of chemoattraction, with roles in both long- and short-range guidance. In the Drosophila visual system, R8 photoreceptor growth cones were shown to require Net-Fra to reach their target, the peak of a Net gradient. Using live imaging, we show, however, that R8 growth cones reach and recognize their target without Net, Fra, or Trim9, a conserved binding partner of Fra, but do not remain attached to it. Thus, despite the graded ligand distribution along the guidance path, Net-Fra is not used for chemoattraction. Based on findings in other systems, we propose that adhesion to substrate-bound Net underlies both long- and short-range Net-Fra-dependent guidance in vivo, thereby eroding the distinction between them.

DOI:http://dx.doi.org/10.7554/eLife.20762.001

The brain of the fruit fly contains hundreds of thousands of neurons, while the human brain contains more than 80 billion. Each of these consists of a cell body that bears an array of branches called dendrites, plus a single cable-like axon. During development, the neurons organize themselves into complex networks by forming connections with one another via their axons and dendrites. But it is not clear exactly how the correct connections form in the correct places.

As they grow out, axons rely on specialized moving structures at their tips – known as growth cones – to probe their environment in search of attractive and repulsive chemical signals released by other cells. When sensors on the surface of growth cones detect a target signal, they initiate processes that cause the growth cone to expand or collapse. This enables the axons to move towards or away from the signal, as appropriate. In all animals studied, proteins called DCC and Netrin form one of the best-known sensor-signal pairs. Growth cones bearing DCC sensors are thought to detect ‘wafting plumes’ or gradients of Netrin and then grow towards the Netrin source.

However, nobody had directly watched neurons respond to Netrin in a living intact animal. Using a type of microscope that can look deep into the developing fly brain, Akin and Zipursky have now followed the movement of growth cones on cells called R8 neurons in fruit fly pupae. Unexpectedly, Akin and Zipursky found that the growth cones of mutant flies that lack Netrin or Frazzled (the fruit fly version of DCC) navigate successfully to their intended destinations. Once there, however, the mutant growth cones were unable to attach to their targets.

Akin and Zipursky’s work is consistent with other observations in a number of animal and insect systems that suggest that Netrin may not attract growth cones via wafting plumes of signal. Instead, Netrin may form a sticky trail that helps growth cones to gain traction as they crawl towards or stick to their destinations. Further experiments are now needed to test whether other neurons in fruit flies and in different animals use Netrin in this way.

DOI:http://dx.doi.org/10.7554/eLife.20762.002

Functional Conservation of the Glide/Gcm Regulatory Network Controlling Glia, Hemocyte, and Tendon Cell Differentiation in Drosophila

High-throughput screens allow us to understand how transcription factors trigger developmental processes, including cell specification. A major challenge is identification of their binding sites because feedback loops and homeostatic interactions may mask the direct impact of those factors in transcriptome analyses. Moreover, this approach dissects the downstream signaling cascades and facilitates identification of conserved transcriptional programs. Here we show the results and the validation of a DNA adenine methyltransferase identification (DamID) genome-wide screen that identifies the direct targets of Glide/Gcm, a potent transcription factor that controls glia, hemocyte, and tendon cell differentiation in Drosophila. The screen identifies many genes that had not been previously associated with Glide/Gcm and highlights three major signaling pathways interacting with Glide/Gcm: Notch, Hedgehog, and JAK/STAT, which all involve feedback loops. Furthermore, the screen identifies effector molecules that are necessary for cell-cell interactions during late developmental processes and/or in ontogeny. Typically, immunoglobulin (Ig) domain-containing proteins control cell adhesion and axonal navigation. This shows that early and transiently expressed fate determinants not only control other transcription factors that, in turn, implement a specific developmental program but also directly affect late developmental events and cell function. Finally, while the mammalian genome contains two orthologous Gcm genes, their function has been demonstrated in vertebrate-specific tissues, placenta, and parathyroid glands, begging questions on the evolutionary conservation of the Gcm cascade in higher organisms. Here we provide the first evidence for the conservation of Gcm direct targets in humans. In sum, this work uncovers novel aspects of cell specification and sets the basis for further understanding of the role of conserved Gcm gene regulatory cascades.

Anti-inflammatory and antiapoptotic effects of mesenchymal stem cells transplantation in rat brain with cerebral ischemia

BACKGROUND

Excessive inflammation and apoptosis contribute to the pathogenesis of ischemic brain damage. Nuclear factor-kappa B (NF-κB) is considered to be a key protein complex involved in this cascade of events. The aim of the present study was to clarify the protection mechanism of the mesenchymal stem cells (MSCs).

METHODS

Lewis rats (N = 90) were randomly assigned to three groups: (1) the sham-operated group; (2) the saline group, in which the animals underwent rat transient middle cerebral artery occlusion (tMCAO, for 2 hours) and were treated with saline through the tail vein; and (3) the MSCs group, in which the animals underwent tMCAO (for 2 hours) and were infused with cultured human MSCs (4 × 10(6)/0.4 ml PBS) through the tail vein. At days 1 and 3 post-MSCs infusion, real-time PCR, and Western blot, immunohistochemical analyses were applied for tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and P-IKKβ, p53, and B-cell lymphoma 2 (Bcl-2) expression levels.

RESULTS

TNF-α, IL-1β messenger RNA (mRNA) and P-IκB-α, P-IKKβ, p53 protein expression levels were significantly increased in the saline group compared with the sham group. However, IκB-α and Bcl-2 protein expression levels were markedly decreased in the saline group. After injection of BrdU(+) MSCs, the expression levels of TNF-α, IL-1β mRNA and P-IκB-α, P-IKKβ, p53 protein were significantly decreased. Contrary to these findings, IκB-α, Bcl-2 protein expression levels were markedly increased. In addition, we found that infarct area was significantly reduced in MSCs group.

CONCLUSIONS

These results suggest that MSCs' neuroprotection is attributable to its anti-inflammatory and antiapoptotic effect through inhibition of NF-κB.

Negative regulation of NF-κB activity by brain-specific TRIpartite Motif protein 9

The TRIpartite Motif (TRIM) family of RING-domain-containing proteins participate in a variety of cellular functions. The β-transducin repeat-containing protein (β-TrCP), a component of the Skp-Cullin-F-box-containing (SCF) E3 ubiquitin ligase complex, recognizes the NF-κB inhibitor IκBα and precursor p100 for proteasomal degradation and processing, respectively. β-TrCP thus plays a critical role in both canonical and non-canonical NF-κB activation. Here we report that TRIM9 is a negative regulator of NF-κB activation. Interaction between the phosphorylated degron motif of TRIM9 and the WD40 repeat region of β-TrCP prevented β-TrCP from binding its substrates, stabilizing IκBα and p100 and thereby blocking NF-κB activation. Consequently, expression or depletion of the TRIM9 gene significantly affected NF-κB-induced inflammatory cytokine production. This study not only elucidates a mechanism for TRIM9-mediated regulation of the β-TrCP SCF complex activity but also identifies TRIM9 as a brain-specific negative regulator of the NF-κB pro-inflammatory signalling pathway.

UNC-6 (netrin) stabilizes oscillatory clustering of the UNC-40 (DCC) receptor to orient polarity

The receptor deleted in colorectal cancer (DCC) directs dynamic polarizing activities in animals toward its extracellular ligand netrin. How DCC polarizes toward netrin is poorly understood. By performing live-cell imaging of the DCC orthologue UNC-40 during anchor cell invasion in Caenorhabditis elegans, we have found that UNC-40 clusters, recruits F-actin effectors, and generates F-actin in the absence of UNC-6 (netrin). Time-lapse analyses revealed that UNC-40 clusters assemble, disassemble, and reform at periodic intervals in different regions of the cell membrane. This oscillatory behavior indicates that UNC-40 clusters through a mechanism involving interlinked positive (formation) and negative (disassembly) feedback. We show that endogenous UNC-6 and ectopically provided UNC-6 orient and stabilize UNC-40 clustering. Furthermore, the UNC-40-binding protein MADD-2 (a TRIM family protein) promotes ligand-independent clustering and robust UNC-40 polarization toward UNC-6. Together, our data suggest that UNC-6 (netrin) directs polarized responses by stabilizing UNC-40 clustering. We propose that ligand-independent UNC-40 clustering provides a robust and adaptable mechanism to polarize toward netrin.

EVA-1 functions as an UNC-40 Co-receptor to enhance attraction to the MADD-4 guidance cue in Caenorhabditis elegans

We recently discovered a secreted and diffusible midline cue called MADD-4 (an ADAMTSL) that guides migrations along the dorsoventral axis of the nematode Caenorhabditis elegans. We showed that the transmembrane receptor, UNC-40 (DCC), whose canonical ligand is the UNC-6 (netrin) guidance cue, is required for extension towards MADD-4. Here, we demonstrate that MADD-4 interacts with an EVA-1/UNC-40 co-receptor complex to attract cell extensions. EVA-1 is a conserved transmembrane protein with predicted galactose-binding lectin domains. EVA-1 functions in the same pathway as MADD-4, physically interacts with both MADD-4 and UNC-40, and enhances UNC-40's sensitivity to the MADD-4 cue. This enhancement is especially important in the presence of UNC-6. In EVA-1's absence, UNC-6 interferes with UNC-40's responsiveness to MADD-4; in UNC-6's absence, UNC-40's responsiveness to MADD-4 is less dependent on EVA-1. By enabling UNC-40 to respond to MADD-4 in the presence of UNC-6, EVA-1 may increase the precision by which UNC-40-directed processes can reach their MADD-4-expressing targets within a field of the UNC-6 guidance cue.

Experimental evidence for UNC-6 (netrin) axon guidance by stochastic fluctuations of intracellular UNC-40 (DCC) outgrowth activity

How the direction of axon guidance is determined is not understood. In Caenorhabditis elegans the UNC-40 (DCC) receptor mediates a response to the UNC-6 (netrin) guidance cue that directs HSN axon development. UNC-40 becomes asymmetrically localized within the HSN neuron to the site of axon outgrowth. Here we provide experimental evidence that the direction of guidance can be explained by the stochastic fluctuations of UNC-40 asymmetric outgrowth activity. We find that the UNC-5 (UNC5) receptor and the cytoskeletal binding protein UNC-53 (NAV2) regulate the induction of UNC-40 localization by UNC-6. If UNC-40 localization is induced without UNC-6 by using an unc-53 mutation, the direction of UNC-40 localization undergoes random fluctuations. Random walk models describe the path made by a succession of randomly directed movement. This model was experimentally tested using mutations that affect Wnt/PCP signaling. These mutations inhibit UNC-40 localization in the anterior and posterior directions. As the axon forms in Wnt/PCP mutants, the direction of UNC-40 localization randomly fluctuates; it can localize in either the anterior, posterior, or ventral direction. Consistent with a biased random walk, over time the axon will develop ventrally in response to UNC-6, even though at a discrete time UNC-40 localization and outgrowth can be observed anterior or posterior. Also, axon formation is slower in the mutants than in wild-type animals. This is also consistent with a random walk since this model predicts that the mean square displacement (msd) will increase only linearly with time, whereas the msd increases quadratically with time for straight-line motion.

TRIM67 protein negatively regulates Ras activity through degradation of 80K-H and induces neuritogenesis

Tripartite motif (TRIM)-containing proteins, which are defined by the presence of a common domain structure composed of a RING finger, one or two B-box motifs and a coiled-coil motif, are involved in many biological processes including innate immunity, viral infection, carcinogenesis, and development. Here we show that TRIM67, which has a TRIM motif, an FN3 domain and a SPRY domain, is highly expressed in the cerebellum and that TRIM67 interacts with PRG-1 and 80K-H, which is involved in the Ras-mediated signaling pathway. Ectopic expression of TRIM67 results in degradation of endogenous 80K-H and attenuation of cell proliferation and enhances neuritogenesis in the neuroblastoma cell line N1E-115. Furthermore, morphological and biological changes caused by knockdown of 80K-H are similar to those observed by overexpression of TRIM67. These findings suggest that TRIM67 regulates Ras signaling via degradation of 80K-H, leading to neural differentiation including neuritogenesis.

The microtubule-associated C-I subfamily of TRIM proteins and the regulation of polarized cell responses

TRIM proteins are multidomain proteins that typically assemble into large molecular complexes, the composition of which likely explains the diverse functions that have been attributed to this group of proteins. Accumulating data on the roles of many TRIM proteins supports the notion that those that share identical C-terminal domain architectures participate in the regulation of similar cellular processes. At least nine different C-terminal domain compositions have been identified. This chapter will focus on one subgroup that possess a COS motif, FNIII and SPRY/B30.2 domain as their C-terminal domain arrangement. This C-terminal domain architecture plays a key role in the interaction of all six members of this subgroup with the microtubule cytoskeleton. Accumulating evidence on the functions of some of these proteins will be discussed to highlight the emerging similarities in the cellular events in which they participate.

Different levels of the Tripartite motif protein, Anomalies in sensory axon patterning (Asap), regulate distinct axonal projections of Drosophila sensory neurons

The axonal projection pattern of sensory neurons typically is regulated by environmental signals, but how different sensory afferents can establish distinct projections in the same environment remains largely unknown. Drosophila class IV dendrite arborization (C4da) sensory neurons project subtype-specific axonal branches in the ventral nerve cord, and we show that the Tripartite motif protein, Anomalies in sensory axon patterning (Asap) is a critical determinant of the axonal projection patterns of different C4da neurons. Asap is highly expressed in C4da neurons with both ipsilateral and contralateral axonal projections, but the Asap level is low in neurons that have only ipsilateral projections. Mutations in asap cause a specific loss of contralateral projections, whereas overexpression of Asap induces ectopic contralateral projections in C4da neurons. We also show by biochemical and genetic analysis that Asap regulates Netrin signaling, at least in part by linking the Netrin receptor Frazzled to the downstream effector Pico. In the absence of Asap, the sensory afferent connectivity within the ventral nerve cord is disrupted, resulting in specific larval behavioral deficits. These results indicate that different levels of Asap determine distinct patterns of axonal projections of C4da neurons by modulating Netrin signaling and that the Asap-mediated axonal projection is critical for assembly of a functional sensory circuit.