The Abelson tyrosine kinase, the Trio GEF and Enabled interact with the Netrin receptor Frazzled in Drosophila

Inter-plane feedback coordinates cell morphogenesis and maintains 3D tissue organization in the Drosophila pupal retina

How complex organs coordinate cellular morphogenetic events to achieve three-dimensional (3D) form is a central question in development. The question is uniquely tractable in the late Drosophila pupal retina, where cells maintain stereotyped contacts as they elaborate the specialized cytoskeletal structures that pattern the apical, basal and longitudinal planes of the epithelium. In this study, we combined cell type-specific genetic manipulation of the cytoskeletal regulator Abelson (Abl) with 3D imaging to explore how the distinct cellular morphogenetic programs of photoreceptors and interommatidial pigment cells (IOPCs) organize tissue pattern to support retinal integrity. Our experiments show that photoreceptor and IOPC terminal differentiation is unexpectedly interdependent, connected by an intercellular feedback mechanism that coordinates and promotes morphogenetic change across orthogonal tissue planes to ensure correct 3D retinal pattern. We propose that genetic regulation of specialized cellular differentiation programs combined with inter-plane mechanical feedback confers spatial coordination to achieve robust 3D tissue morphogenesis.

Enabled primarily controls filopodial morphology, not actin organization, in the TSM1 growth cone in Drosophila

Ena/VASP proteins are processive actin polymerases that are required throughout animal phylogeny for many morphogenetic processes, including axon growth and guidance. Here we use in vivo live imaging of morphology and actin distribution to determine the role of Ena in promoting the growth of the TSM1 axon of the Drosophila wing. Altering Ena activity causes stalling and misrouting of TSM1. Our data show that Ena has a substantial impact on filopodial morphology in this growth cone but exerts only modest effects on actin distribution. This is in contrast to the main regulator of Ena, Abl tyrosine kinase, which was shown previously to have profound effects on actin and only mild effects on TSM1 growth cone morphology. We interpret these data as suggesting that the primary role of Ena in this axon may be to link actin to the morphogenetic processes of the plasma membrane, rather than to regulate actin organization itself. These data also suggest that a key role of Ena, acting downstream of Abl, may be to maintain consistent organization and reliable evolution of growth cone structure, even as Abl activity varies in response to guidance cues in the environment.

A tug of war between DCC and ROBO1 signaling during commissural axon guidance

Dynamic and coordinated axonal responses to changing environments are critical for establishing neural connections. As commissural axons migrate across the CNS midline, they are suggested to switch from being attracted to being repelled in order to approach and to subsequently leave the midline. A molecular mechanism that is hypothesized to underlie this switch in axonal responses is the silencing of Netrin1/Deleted in Colorectal Carcinoma (DCC)-mediated attraction by the repulsive SLIT/ROBO1 signaling. Using in vivo approaches including CRISPR-Cas9-engineered mouse models of distinct Dcc splice isoforms, we show here that commissural axons maintain responsiveness to both Netrin and SLIT during midline crossing, although likely at quantitatively different levels. In addition, full-length DCC in collaboration with ROBO3 can antagonize ROBO1 repulsion in vivo. We propose that commissural axons integrate and balance the opposing DCC and Roundabout (ROBO) signaling to ensure proper guidance decisions during midline entry and exit.

Orthogonal coupling of a 3D cytoskeletal scaffold coordinates cell morphogenesis and maintains tissue organization in the Drosophila pupal retina

How complex three-dimensional (3D) organs coordinate cellular morphogenetic events to achieve the correct final form is a central question in development. The question is uniquely tractable in the late Drosophila pupal retina where cells maintain stereotyped contacts as they elaborate the specialized cytoskeletal structures that pattern the apical, basal and longitudinal planes of the epithelium. In this study, we combined cell type-specific genetic manipulation of the cytoskeletal regulator Abelson (Abl) with 3D imaging to explore how the distinct cellular morphogenetic programs of photoreceptors and interommatidial pigment cells coordinately organize tissue pattern to support retinal integrity. Our experiments revealed an unanticipated intercellular feedback mechanism whereby correct cellular differentiation of either cell type can non-autonomously induce cytoskeletal remodeling in the other Abl mutant cell type, restoring retinal pattern and integrity. We propose that genetic regulation of specialized cellular differentiation programs combined with inter-plane mechanical feedback confers spatial coordination to achieve robust 3D tissue morphogenesis.

Robo recruitment of the Wave regulatory complex plays an essential and conserved role in midline repulsion

The Roundabout (Robo) guidance receptor family induces axon repulsion in response to its ligand Slit by inducing local cytoskeletal changes; however, the link to the cytoskeleton and the nature of these cytoskeletal changes are poorly understood. Here, we show that the heteropentameric Scar/Wave Regulatory Complex (WRC), which drives Arp2/3-induced branched actin polymerization, is a direct effector of Robo signaling. Biochemical evidence shows that Slit triggers WRC recruitment to the Robo receptor's WRC-interacting receptor sequence (WIRS) motif. In Drosophila embryos, mutants of the WRC enhance Robo1-dependent midline crossing defects. Additionally, mutating Robo1's WIRS motif significantly reduces receptor activity in rescue assays in vivo, and CRISPR-Cas9 mutagenesis shows that the WIRS motif is essential for endogenous Robo1 function. Finally, axon guidance assays in mouse dorsal spinal commissural axons and gain-of-function experiments in chick embryos demonstrate that the WIRS motif is also required for Robo1 repulsion in mammals. Together, our data support an essential conserved role for the WIRS-WRC interaction in Robo1-mediated axon repulsion.

Trio family proteins as regulators of cell migration and morphogenesis in development and disease - mechanisms and cellular contexts

The well-studied members of the Trio family of proteins are Trio and kalirin in vertebrates, UNC-73 in Caenorhabditis elegans and Trio in Drosophila Trio proteins are key regulators of cell morphogenesis and migration, tissue organization, and secretion and protein trafficking in many biological contexts. Recent discoveries have linked Trio and kalirin to human disease, including neurological disorders and cancer. The genes for Trio family proteins encode a series of large multidomain proteins with up to three catalytic activities and multiple scaffolding and protein-protein interaction domains. As such, Trio family proteins engage a wide array of cell surface receptors, substrates and interaction partners to coordinate changes in cytoskeletal regulatory and protein trafficking pathways. We provide a comprehensive review of the specific mechanisms by which Trio family proteins carry out their functions in cells, highlight the biological and cellular contexts in which they occur, and relate how alterations in these functions contribute to human disease.

Kalirin and Trio: RhoGEFs in Synaptic Transmission, Plasticity, and Complex Brain Disorders

Changes in the actin cytoskeleton are a primary mechanism mediating the morphological and functional plasticity that underlies learning and memory. The synaptic Ras homologous (Rho) guanine nucleotide exchange factors (GEFs) Kalirin and Trio have emerged as central regulators of actin dynamics at the synapse. The increased attention surrounding Kalirin and Trio stems from the growing evidence for their roles in the etiology of a wide range of neurodevelopmental and neurodegenerative disorders. In this Review, we discuss recent findings revealing the unique and diverse functions of these paralog proteins in neurodevelopment, excitatory synaptic transmission, and plasticity. We additionally survey the growing literature implicating these proteins in various neurological disorders.

The first quarter of the C-terminal domain of Abelson regulates the WAVE regulatory complex and Enabled in axon guidance

Background

Abelson tyrosine kinase (Abl) plays a key role in axon guidance in linking guidance receptors to actin dynamics. The long C-terminal domain (CTD) of Drosophila Abl is important for this role, and previous work identified the ‘first quarter’ (1Q) of the CTD as essential. Here, we link the physical interactions of 1Q binding partners to Abl’s function in axon guidance.

Methods

Protein binding partners of 1Q were identified by GST pulldown and mass spectrometry, and validated using axon guidance assays in the embryonic nerve cord and motoneurons. The role of 1Q was assessed genetically, utilizing a battery of Abl transgenes in combination with mutation or overexpression of the genes of pulled down proteins, and their partners in actin dynamics. The set of Abl transgenes had the following regions deleted: all of 1Q, each half of 1Q (‘eighths’, 1E and 2E) or a PxxP motif in 2E, which may bind SH3 domains.

Results

GST pulldown identified Hem and Sra-1 as binding partners of 1Q, and our genetic analyses show that both proteins function with Abl in axon guidance, with Sra-1 likely interacting with 1Q. As Hem and Sra-1 are part of the actin-polymerizing WAVE regulatory complex (WRC), we extended our analyses to Abi and Trio, which interact with Abl and WRC members. Overall, the 1Q region (and especially 2E and its PxxP motif) are important for Abl’s ability to work with WRC in axon guidance. These areas are also important for Abl’s ability to function with the actin regulator Enabled. In comparison, 1E contributes to Abl function with the WRC at the midline, but less so with Enabled.

Conclusions

The 1Q region, and especially the 2E region with its PxxP motif, links Abl with the WRC, its regulators Trio and Abi, and the actin regulator Ena. Removing 1E has specific effects suggesting it may help modulate Abl’s interaction with the WRC or Ena. Thus, the 1Q region of Abl plays a key role in regulating actin dynamics during axon guidance.

Dynamic morphogenesis of a pioneer axon in Drosophila and its regulation by Abl tyrosine kinase

The fundamental problem in axon growth and guidance is to understand how cytoplasmic signaling modulates the cytoskeleton to produce directed growth cone motility. We here dissect this process using live imaging of the TSM1 axon of the developing Drosophila wing. We find that the growth cone is almost purely filopodial, and that it extends by a protrusive mode of growth. Quantitative analysis reveals two separate groups of growth cone properties that together account for growth cone structure and dynamics. The core morphological features of the growth cone are strongly correlated with one another and define two discrete morphs. Genetic manipulation of a critical mediator of axon guidance signaling, Abelson (Abl) tyrosine kinase, shows that while Abl weakly modulates the ratio of the two morphs it does not greatly change their properties. Rather, Abl primarily regulates the second group of properties, which report the organization and distribution of actin in the growth cone and are coupled to growth cone velocity. Other experiments dissect the nature of that regulation of actin organization and how it controls the spatial localization of filopodial dynamics and thus axon extension. Together, these observations suggest a novel, probabilistic mechanism by which Abl biases the stochastic fluctuations of growth cone actin to direct axon growth and guidance.

Abl signaling directs growth of a pioneer axon in Drosophila by shaping the intrinsic fluctuations of actin

The fundamental problem in axon growth and guidance is understanding how cytoplasmic signaling modulates the cytoskeleton to produce directed growth cone motility. Live imaging of the TSM1 axon of the developing Drosophila wing has shown that the essential role of the core guidance signaling molecule, Abelson (Abl) tyrosine kinase, is to modulate the organization and spatial localization of actin in the advancing growth cone. Here, we dissect in detail the properties of that actin organization and its consequences for growth cone morphogenesis and motility. We show that advance of the actin mass in the distal axon drives the forward motion of the dynamic filopodial domain that defines the growth cone. We further show that Abl regulates both the width of the actin mass and its internal organization, spatially biasing the intrinsic fluctuations of actin to achieve net advance of the actin, and thus of the dynamic filopodial domain of the growth cone, while maintaining the essential coherence of the actin mass itself. These data suggest a model whereby guidance signaling systematically shapes the intrinsic, stochastic fluctuations of actin in the growth cone to produce axon growth and guidance.

The Crk adapter protein is essential for Drosophila embryogenesis, where it regulates multiple actin-dependent morphogenic events

Small Src homology domain 2 (SH2) and 3 (SH3) adapter proteins regulate cell fate and behavior by mediating interactions between cell surface receptors and downstream signaling effectors in many signal transduction pathways. The CT10 regulator of kinase (Crk) family has tissue-specific roles in phagocytosis, cell migration, and neuronal development and mediates oncogenic signaling in pathways like that of Abelson kinase. However, redundancy among the two mammalian family members and the position of the Drosophila gene on the fourth chromosome precluded assessment of Crk's full role in embryogenesis. We circumvented these limitations with short hairpin RNA and CRISPR technology to assess Crk's function in Drosophila morphogenesis. We found that Crk is essential beginning in the first few hours of development, where it ensures accurate mitosis by regulating orchestrated dynamics of the actin cytoskeleton to keep mitotic spindles in syncytial embryos from colliding. In this role, it positively regulates cortical localization of the actin-related protein 2/3 complex (Arp2/3), its regulator suppressor of cAMP receptor (SCAR), and filamentous actin to actin caps and pseudocleavage furrows. Crk loss leads to the loss of nuclei and formation of multinucleate cells. We also found roles for Crk in embryonic wound healing and in axon patterning in the nervous system, where it localizes to the axons and midline glia. Thus, Crk regulates diverse events in embryogenesis that require orchestrated cytoskeletal dynamics.

The interaction between PLEKHG2 and ABL1 suppresses cell growth via the NF-κB signaling pathway in HEK293 cells

The Rho family small GTPases mediate cell responses through actin cytoskeletal rearrangement. We previously reported that PLEKHG2, a Rho-specific guanine nucleotide exchange factor, is regulated via interaction with several proteins. We found that PLEKHG2 interacted with non-receptor tyrosine kinase ABL1, but the cellular function remains unclear. Here, we show that the interaction between PLEKHG2 and ABL1 attenuated the PLEKHG2-induced serum response element-dependent gene transcription in a tyrosine phosphorylation-independent manner. PLEKHG2 and ABL1 were co-localized and accumulated within cells co-expressing PLEKHG2 and ABL1. The cellular fractionation analysis suggested that the accumulation involved actin cytoskeletal reorganization. We also revealed that the co-expression of PLEKHG2 with ABL1, but not BCR-ABL, suppressed cell growth and synergistically enhanced NF-κB-dependent gene transcription. The cell growth suppression was canceled by co-expression with IκBα, a member of the NF-κB inhibitor protein family. This study suggests that the interaction between PLEKHG2 and ABL1 suppresses cell growth through intracellular protein accumulation via the NF-κB signaling pathway.

Regulators of Rho GTPases in the Nervous System: Molecular Implication in Axon Guidance and Neurological Disorders

One of the fundamental steps during development of the nervous system is the formation of proper connections between neurons and their target cells-a process called neural wiring, failure of which causes neurological disorders ranging from autism to Down's syndrome. Axons navigate through the complex environment of a developing embryo toward their targets, which can be far away from their cell bodies. Successful implementation of neuronal wiring, which is crucial for fulfillment of all behavioral functions, is achieved through an intimate interplay between axon guidance and neural activity. In this review, our focus will be on axon pathfinding and the implication of some of its downstream molecular components in neurological disorders. More precisely, we will talk about axon guidance and the molecules implicated in this process. After, we will briefly review the Rho family of small GTPases, their regulators, and their involvement in downstream signaling pathways of the axon guidance cues/receptor complexes. We will then proceed to the final and main part of this review, where we will thoroughly comment on the implication of the regulators for Rho GTPases-GEFs (Guanine nucleotide Exchange Factors) and GAPs (GTPase-activating Proteins)-in neurological diseases and disorders.

The Role of Apoptotic Signaling in Axon Guidance

Navigating growth cones are exposed to multiple signals simultaneously and have to integrate competing cues into a coherent navigational response. Integration of guidance cues is traditionally thought to occur at the level of cytoskeletal dynamics. Drosophila studies indicate that cells exhibit a low level of continuous caspase protease activation, and that axon guidance cues can activate or suppress caspase activity. We base a model for axon guidance on these observations. By analogy with other systems in which caspase signaling has non-apoptotic functions, we propose that caspase signaling can either reinforce repulsion or negate attraction in response to external guidance cues by cleaving cytoskeletal proteins. Over the course of an entire trajectory, incorrectly navigating axons may pass the threshold for apoptosis and be eliminated, whereas axons making correct decisions will survive. These observations would also explain why neurotrophic factors can act as axon guidance cues and why axon guidance systems such as Slit/Robo signaling may act as tumor suppressors in cancer.

Changes in Neuronal Signaling and Cell Stress Response Pathways are Associated with a Multigenic Response of Drosophila melanogaster to DDT Selection

The adaptation of insect populations to insecticidal control is a continual threat to human health and sustainable agricultural practices, but many complex genomic mechanisms involved in this adaption remain poorly understood. This study applied a systems approach to investigate the interconnections between structural and functional variance in response to dichlorodiphenyltrichloroethane (DDT) within the Drosophila melanogaster strain 91-R. Directional selection in 6 selective sweeps coincided with constitutive gene expression differences in DDT resistant flies, including the most highly upregulated transcript, Unc-115 b, which plays a central role in axon guidance, and the most highly downregulated transcript, the angiopoietin-like CG31832, which is involved in directing vascular branching and dendrite outgrowth but likely may be under trans-regulatory control. Direct functions and protein–protein interactions mediated by differentially expressed transcripts control changes in cell migration, signal transduction, and gene regulatory cascades that impact the nervous system. Although changes to cellular stress response pathways involve 8 different cytochrome P450s, stress response, and apoptosis is controlled by a multifacetted regulatory mechanism. These data demonstrate that DDT selection in 91-R may have resulted in genome-wide adaptations that impacts genetic and signal transduction pathways that converge to modify stress response, cell survival, and neurological functions. This study implicates the involvement of a multigenic mechanism in the adaptation to a chemical insecticide, which impact interconnected regulatory cascades. We propose that DDT selection within 91-R might act systemically, wherein pathway interactions function to reinforce the epistatic effects of individual adaptive changes on an additive or nonadditive basis.

Polarized Dock Activity Drives Shh-Mediated Axon Guidance

In the developing spinal cord, Sonic hedgehog (Shh) attracts commissural axons toward the floorplate. How Shh regulates the cytoskeletal remodeling that underlies growth cone turning is unknown. We found that Shh-mediated growth cone turning requires the activity of Docks, which are unconventional GEFs. Knockdown of Dock3 and 4, or their binding partner ELMO1 and 2, abolished commissural axon attraction by Shh in vitro. Dock3/4 and ELMO1/2 were also required for correct commissural axon guidance in vivo. Polarized Dock activity was sufficient to induce axon turning, indicating that Docks are instructive for axon guidance. Mechanistically, we show that Dock and ELMO interact with Boc, the Shh receptor, and that this interaction is reduced upon Shh stimulation. Furthermore, Shh stimulation translocates ELMO to the growth cone periphery and activates Rac1. This identifies Dock/ELMO as an effector complex of non-canonical Shh signaling and demonstrates the instructive role of GEFs in axon guidance.

Revisiting Netrin-1: One Who Guides (Axons)

Proper patterning of the nervous system requires that developing axons find appropriate postsynaptic partners; this entails microns to meters of extension through an extracellular milieu exhibiting a wide range of mechanical and chemical properties. Thus, the elaborate networks of fiber tracts and non-fasciculated axons evident in mature organisms are formed via complex pathfinding. The macroscopic structures of axon projections are highly stereotyped across members of the same species, indicating precise mechanisms guide their formation. The developing axon exhibits directionally biased growth toward or away from external guidance cues. One of the most studied guidance cues is netrin-1, however, its presentation in vivo remains debated. Guidance cues can be secreted to form soluble or chemotactic gradients or presented bound to cells or the extracellular matrix to form haptotactic gradients. The growth cone, a highly specialized dynamic structure at the end of the extending axon, detects these guidance cues via transmembrane receptors, such as the netrin-1 receptors deleted in colorectal cancer (DCC) and UNC5. These receptors orchestrate remodeling of the cytoskeleton and cell membrane through both chemical and mechanotransductive pathways, which result in traction forces generated by the cytoskeleton against the extracellular environment and translocation of the growth cone. Through intracellular signaling responses, netrin-1 can trigger either attraction or repulsion of the axon. Here we review the mechanisms by which the classical guidance cue netrin-1 regulates intracellular effectors to respond to the extracellular environment in the context of axon guidance during development of the central nervous system and discuss recent findings that demonstrate the critical importance of mechanical forces in this process.

A Dendritic Guidance Receptor Complex Brings Together Distinct Actin Regulators to Drive Efficient F-Actin Assembly and Branching

Proper morphogenesis of dendrites plays a fundamental role in the establishment of neural circuits. The molecular mechanism by which dendrites grow highly complex branches is not well understood. Here, using the Caenorhabditis elegans PVD neuron, we demonstrate that high-order dendritic branching requires actin polymerization driven by coordinated interactions between two membrane proteins, DMA-1 and HPO-30, with their cytoplasmic interactors, the RacGEF TIAM-1 and the actin nucleation promotion factor WAVE regulatory complex (WRC). The dendrite branching receptor DMA-1 directly binds to the PDZ domain of TIAM-1, while the claudin-like protein HPO-30 directly interacts with the WRC. On dendrites, DMA-1 and HPO-30 form a receptor-associated signaling complex to bring TIAM-1 and the WRC to close proximity, leading to elevated assembly of F-actin needed to drive high-order dendrite branching. The synergistic activation of F-actin assembly by scaffolding distinct actin regulators might represent a general mechanism in promoting complex dendrite arborization.

Frazzled can act through distinct molecular pathways in epithelial cells to regulate motility, apical constriction, and localisation of E-Cadherin

Netrin receptors of the DCC/NEO/UNC-40/Frazzled family have well established roles in cell migration and axon guidance but can also regulate epithelial features such as adhesion, polarity and adherens junction (AJ) stability. Previously, we have shown that overexpression of Drosophila Frazzled (Fra) in the peripodial epithelium (PE) inhibits wing disc eversion and also generates cellular protrusions typical of motile cells. Here, we tested whether the molecular pathways by which Fra inhibits eversion are distinct from those driving motility. We show that in disc proper (DP) epithelial cells Fra, in addition to inducing F-Actin rich protrusions, can affect localization of AJ components and columnar cell shape. We then show that these phenotypes have different requirements for the three conserved Fra cytoplasmic P-motifs and for downstream genes. The formation of protrusions required the P3 motif of Fra, as well as integrins (mys and mew), the Rac pathway (Rac1, wave and, arpc3) and myosin regulatory light chain (Sqh). In contrast, apico-basal cell shape change, which was accompanied by increased myosin phosphorylation, was critically dependent upon the P1 motif and was promoted by RhoGef2 but inhibited by Rac1. Fra also caused a loss of AJ proteins (DE-Cad and Arm) from basolateral regions of epithelial cells. This phenotype required all 3 P-motifs, and was dependent upon the polarity factor par6. par6 was not required for protrusions or cell shape change, but was required to block eversion suggesting that control of AJ components may underlie the ability of Fra to promote epithelial stability. The results imply that multiple molecular pathways act downstream of Fra in epithelial cells.

Human TUBB3 Mutations Disrupt Netrin Attractive Signaling

Heterozygous missense mutations in human TUBB3 gene result in a spectrum of brain malformations associated with defects in axon guidance, neuronal migration and differentiation. However, the molecular mechanisms underlying mutation-related axon guidance abnormalities are unclear. Recent studies have shown that netrin-1, a canonical guidance cue, induced the interaction of TUBB3 with the netrin receptor deleted in colorectal cancer (DCC). Furthermore, TUBB3 is required for netrin-1-induced axon outgrowth, branching and pathfinding. Here, we provide evidence that TUBB3 mutations impair netrin/DCC signaling in the developing nervous system. The interaction of DCC with most TUBB3 mutants (eight out of twelve) is significantly reduced compared to the wild-type TUBB3. TUBB3 mutants R262C and A302V exhibit decreased subcellular colocalization with DCC in the growth cones of primary neurons. Netrin-1 increases the interaction of endogenous DCC with wild-type human TUBB3, but not R262C or A302V, in primary neurons. Netrin-1 also increases co-sedimentation of DCC with polymerized microtubules (MTs) in primary neurons expressing the wild-type TUBB3, but not R262C or A302V. Expression of either R262C or A302V not only suppresses netrin-1-induced neurite outgrowth, branching and attraction in vitro, but also causes defects in spinal cord commissural axon (CA) projection and pathfinding in ovo. Our study reveals that missense TUBB3 mutations specifically disrupt netrin/DCC-mediated attractive signaling.

Long disordered regions of the C-terminal domain of Abelson tyrosine kinase have specific and additive functions in regulation and axon localization

Abelson tyrosine kinase (Abl) is a key regulator of actin-related morphogenetic processes including axon guidance, where it functions downstream of several guidance receptors. While the long C-terminal domain (CTD) of Abl is required for function, its role is poorly understood. Here, a battery of mutants of Drosophila Abl was created that systematically deleted large segments of the CTD from Abl or added them back to the N-terminus alone. The functionality of these Abl transgenes was assessed through rescue of axon guidance defects and adult lethality in Abl loss-of-function, as well as through gain-of-function effects in sensitized slit or frazzled backgrounds that perturb midline guidance in the Drosophila embryonic nerve cord. Two regions of the CTD play important and distinct roles, but additive effects for other regions were also detected. The first quarter of the CTD, including a conserved PxxP motif and its surrounding sequence, regulates Abl function while the third quarter localizes Abl to axons. These regions feature long stretches of intrinsically disordered sequence typically found in hub proteins and are associated with diverse protein-protein interactions. Thus, the CTD of Abl appears to use these disordered regions to establish a variety of different signaling complexes required during formation of axon tracts.

Midline axon guidance in the Drosophila embryonic central nervous system

Studies in the fruit fly Drosophila melanogaster have provided many fundamental insights into the genetic regulation of neural development, including the identification and characterization of evolutionarily conserved axon guidance pathways and their roles in important guidance decisions. Due to its highly organized and fast-developing embryonic nervous system, relatively small number of neurons, and molecular and genetic tools for identifying, labeling, and manipulating individual neurons or small neuronal subsets, studies of axon guidance in the Drosophila embryonic CNS have allowed researchers to dissect these genetic mechanisms with a high degree of precision. In this review, we discuss the major axon guidance pathways that regulate midline crossing of axons and the formation and guidance of longitudinal axon tracts, two processes that contribute to the development of the precise three-dimensional structure of the insect nerve cord. We focus particularly on recent insights into the roles and regulation of canonical midline axon guidance pathways, and on additional factors and pathways that have recently been shown to contribute to axon guidance decisions at and near the midline.

Motor axon guidance in Drosophila

The Drosophila motor system starts to assemble during embryonic development. It is composed of 30 muscles per abdominal hemisegment and 36 motor neurons assembling into nerve branches to exit the CNS, navigate within the muscle field and finally establish specific connections with their target muscles. Several families of guidance molecules that play a role controlling this process as well as transcriptional regulators that program the behavior of specific motor neuron have been identified. In this review we summarize the role of both groups of molecules in the motor system as well as their relationship where known. It is apparent that partially redundant guidance protein families and membrane molecules with different functional output direct guidance decisions cooperatively. Some distinct transcriptional regulators seem to control guidance of specific nerve branches globally directing the expression of groups of pathfinding molecules in all motor neurons within the same motor branch.

New perspectives on the roles of Abl tyrosine kinase in axon patterning

The Abelson tyrosine kinase (Abl) lies at the heart of one of the small set of ubiquitous, conserved signal transduction pathways that do much of the work of development and physiology. Abl signaling is essential to epithelial integrity, motility of autonomous cells such as blood cells, and axon growth and guidance in the nervous system. However, though Abl was one of the first of these conserved signaling machines to be identified, it has been among the last to have its essential architecture elucidated. Here we will first discuss some of the challenges that long delayed the dissection of this pathway, and what they tell us about the special problems of investigating dynamic processes like motility. We will then describe our recent experiments that revealed the functional organization of the Abl pathway in Drosophila neurons. Finally, in the second part of the review we will introduce a different kind of complexity in the role of Abl in motility: the discovery of a previously unappreciated function in protein secretion and trafficking. We will provide evidence that the secretory function of Abl also contributes to its role in axon growth and guidance, and finally end with a discussion of the challenges that Abl pleiotropy provide for the investigator, but the opportunities that it provides for coordinating biological regulation.

The Abl pathway bifurcates to balance Enabled and Rac signaling in axon patterning in Drosophila

The Abl tyrosine kinase signaling network controls cell migration, epithelial organization, axon patterning and other aspects of development. Although individual components are known, the relationships among them remain unresolved. We now use FRET measurements of pathway activity, analysis of protein localization and genetic epistasis to dissect the structure of this network in Drosophila We find that the adaptor protein Disabled stimulates Abl kinase activity. Abl suppresses the actin-regulatory factor Enabled, and we find that Abl also acts through the GEF Trio to stimulate the signaling activity of Rac GTPase: Abl gates the activity of the spectrin repeats of Trio, allowing them to relieve intramolecular repression of Trio GEF activity by the Trio N-terminal domain. Finally, we show that a key target of Abl signaling in axons is the WAVE complex that promotes the formation of branched actin networks. Thus, we show that Abl constitutes a bifurcating network, suppressing Ena activity in parallel with stimulation of WAVE. We suggest that the balancing of linear and branched actin networks by Abl is likely to be central to its regulation of axon patterning.

A Sonic hedgehog coreceptor, BOC regulates neuronal differentiation and neurite outgrowth via interaction with ABL and JNK activation

Neurite outgrowth is a critical step for neurogenesis and remodeling synaptic circuitry during neuronal development and regeneration. An immunoglobulin superfamily member, BOC functions as Sonic hedgehog (Shh) coreceptor in canonical and noncanonical Shh signaling in neuronal development and axon outgrowth/guidance. However signaling mechanisms responsible for BOC action during these processes remain unknown. In our previous studies, a multiprotein complex containing BOC and a closely related protein CDO promotes myogenic differentiation through activation of multiple signaling pathways, including non-receptor tyrosine kinase ABL. Given that ABL and Jun. N-terminal kinase (JNK) are implicated in actin cytoskeletal dynamics required for neurogenesis, we investigated the relationship between BOC, ABL and JNK during neuronal differentiation. Here, we demonstrate that BOC and ABL are induced in P19 embryonal carcinoma (EC) cells and cortical neural progenitor cells (NPCs) during neuronal differentiation. BOC-depleted EC cells or Boc^-/- NPCs exhibit impaired neuronal differentiation with shorter neurite formation. BOC interacts with ABL through its putative SH2 binding domain and seems to be phosphorylated in an ABL activity-dependent manner. Unlike wildtype BOC, ABL-binding defective BOC mutants exhibit impaired JNK activation and neuronal differentiation. Finally, Shh treatment enhances JNK activation which is diminished by BOC depletion. These data suggest that BOC interacts with ABL and activates JNK thereby promoting neuronal differentiation and neurite outgrowth.

Mushroom body defect is required in parallel to Netrin for midline axon guidance in Drosophila

The outgrowth of many neurons within the central nervous system is initially directed towards or away from the cells lying at the midline. Recent genetic evidence suggests that a simple model of differential sensitivity to the conserved Netrin attractants and Slit repellents is insufficient to explain the guidance of all axons at the midline. In the Drosophila embryonic ventral nerve cord, many axons still cross the midline in the absence of the Netrin genes (NetA and NetB) or their receptor frazzled. Here we show that mutation of mushroom body defect (mud) dramatically enhances the phenotype of Netrin or frazzled mutants, resulting in many more axons failing to cross the midline, although mutations in mud alone have little effect. This suggests that mud, which encodes a microtubule-binding coiled-coil protein homologous to NuMA and LIN-5, is an essential component of a Netrin-independent pathway that acts in parallel to promote midline crossing. We demonstrate that this novel role of Mud in axon guidance is independent of its previously described role in neural precursor development. These studies identify a parallel pathway controlling midline guidance in Drosophila and highlight a novel role for Mud potentially acting downstream of Frizzled to aid axon guidance.

Equivalent Activities of Repulsive Axon Guidance Receptors

Receptors on the growth cone at the leading edge of elongating axons play critical guidance roles by recognizing cues via their extracellular domains and transducing signals via their intracellular domains, resulting in changes in direction of growth. An important concept to have emerged in the axon guidance field is the importance of repulsion as a major guidance mechanism. Given the number and variety of different repulsive receptors, it is generally thought that there are likely to be qualitative differences in the signals they transduce. However, the nature of these possible differences is unknown. By creating chimeras using the extracellular and intracellular domains of three different Drosophila repulsive receptors, Unc5, Roundabout (Robo), and Derailed (Drl) and expressing them in defined cells within the embryonic nervous system, we examined the responses elicited by their intracellular domains systematically. Surprisingly, we found no qualitative differences in growth cone response or axon growth, suggesting that, despite their highly diverged sequences, each intracellular domain elicits repulsion via a common pathway. In terms of the signaling pathway(s) used by the repulsive receptors, mutations in the guanine nucleotide exchange factor Trio strongly enhance the repulsive activity of all three intracellular domains, suggesting that repulsion by Unc5, Robo, and Drl, and perhaps repulsion in general, involves Trio activity.

SIGNIFICANCE STATEMENT

A prevailing concept that has emerged in the axon guidance field is the importance of repulsion as a guidance mechanism for steering axons to their appropriate targets. Given the number and variety of different repulsive receptors, it is generally thought that there are differences in the signals that they transduce. However, this has never been tested directly. We have used the advanced genetics of Drosophila to compare directly the outputs of different repulsive receptors. Surprisingly, we found no qualitative differences in receptor-mediated repulsion, suggesting that, despite their highly diverged domain structure, each receptor couples to a common repulsive pathway. We went on to show that this common pathway involves Trio, a guanine nucleotide exchange factor known to promote cytoskeletal remodeling.

The H3K4me3/2 histone demethylase RBR-2 controls axon guidance by repressing the actin-remodeling gene wsp-1

The dynamic regulation of histone modifications is important for modulating transcriptional programs during development. Aberrant H3K4 methylation is associated with neurological disorders, but how the levels and the recognition of this modification affect specific neuronal processes is unclear. Here, we show that RBR-2, the sole homolog of the KDM5 family of H3K4me3/2 demethylases in Caenorhabditis elegans, ensures correct axon guidance by controlling the expression of the actin regulator wsp-1. Loss of rbr-2 results in increased levels of H3K4me3 at the transcriptional start site of wsp-1, with concomitant higher wsp-1 expression responsible for defective axon guidance. In agreement, overexpression of WSP-1 mimics rbr-2 loss, and its depletion restores normal axon guidance in rbr-2 mutants. NURF-1, an H3K4me3-binding protein and member of the chromatin-remodeling complex NURF, is required for promoting aberrant wsp-1 transcription in rbr-2 mutants and its ablation restores wild-type expression of wsp-1 and axon guidance. Thus, our results establish a precise role for epigenetic regulation in neuronal development by demonstrating a functional link between RBR-2 activity, H3K4me3 levels, the NURF complex and the expression of WSP-1.

Dysregulated Dscam levels act through Abelson tyrosine kinase to enlarge presynaptic arbors

Increased expression of Down Syndrome Cell Adhesion Molecule (Dscam) is implicated in the pathogenesis of brain disorders such as Down syndrome (DS) and fragile X syndrome (FXS). Here, we show that the cellular defects caused by dysregulated Dscam levels can be ameliorated by genetic and pharmacological inhibition of Abelson kinase (Abl) both in Dscam-overexpressing neurons and in a Drosophila model of fragile X syndrome. This study offers Abl as a potential therapeutic target for treating brain disorders associated with dysregulated Dscam expression.

DOI:http://dx.doi.org/10.7554/eLife.05196.001

Information is transmitted through the brain by cells called neurons, which are connected into specific circuits and networks. As the brain develops, several different signaling molecules control how the connections between neurons develop. If these signals occur at the wrong time or wrong place, or in the wrong amount, the neurons may not connect in the right way; this is the cause of several brain disorders.

One of the signaling molecules that helps neural circuits to form in the developing brain is the Dscam protein. Having too much Dscam has been linked to neurons with enlarged presynaptic terminals. Presynaptic terminals are the parts of each neuron that send information on to the next cell, and when they are enlarged it results in the neuron not being able to communicate with other neurons in a targeted way. People with brain disorders including Down syndrome, epilepsy and possibly fragile X syndrome often have excessive amounts of Dscam.

It was not known precisely how Dscam signals within neurons. Sterne, Kim and Ye have now investigated this by exploring the effects of Dscam on a group of well-known neurons in the larvae of the fruit fly species Drosophila. The presynaptic terminals of single neurons in this group were labeled in the larvae using a genetic marker. This revealed that the neurons of larvae that had been engineered to produce too much Dscam had larger presynaptic terminals than normal.

Further investigation showed that for Dscam to influence how a presynaptic terminal grows, it must interact with another signaling protein called Abelson tyrosine kinase (or Abl for short). Therefore, the larger presynaptic terminals seen in larvae that produce too much Dscam are a result of the Dscam protein activating too much Abl.

There are several drugs that are approved for use in humans that suppress the activity of Abl. Sterne, Kim and Ye used two of these to treat fruit fly larvae, and found that they reversed the detrimental effects of extra Dscam on the larvae's neural circuit. Furthermore, the drugs fixed neural defects in a fruit fly model designed to reproduce the symptoms of fragile X syndrome.

Overall, the results presented by Sterne, Kim and Ye suggest that suppressing the abnormally high activity of the Abl protein could be a way of treating the brain disorders caused by having excessive amounts of the Dscam protein. The next step is to test whether Dscam and Abl interact in the same way in mammals and whether the proposed treatment is effective in treating mammalian models of disorders that involve dysregulated Dscam signaling.

DOI:http://dx.doi.org/10.7554/eLife.05196.002

Netrins and Frazzled/DCC promote the migration and mesenchymal to epithelial transition of Drosophila midgut cells

Mesenchymal-epithelial transitions (METs) are important in both development and the growth of secondary tumours. Although the molecular basis for epithelial polarity is well studied, less is known about the cues that induce MET. Here we show that Netrins, well known as chemotropic guidance factors, provide a basal polarising cue during the Drosophila midgut MET. Both netrinA and netrinB are expressed in the visceral mesoderm, the substrate upon which midgut cells migrate, while their receptor frazzled (fra) is expressed in midgut cells. Netrins are required to polarise Fra to the basal surface, and Netrins and Fra undergo mutually-dependent endocytosis, with Fra subsequently trafficking to late endosomes. Mutations to fra and netrins affect both migration and MET but to different degrees. Loss of fra strongly delays migration, midgut cells fail to extend protrusions, and apico-basal polarisation of proteins and epithelium formation is inhibited. In netrin mutants, the migration phenotype is weaker and cells still extend protrusions. However, apico-basal polarisation of proteins, including Fra, and FActin is greatly disrupted and a monolayer fails to form. Delocalised accumulations of FActin are prevalent in netrin mutants but not fra mutants suggesting delocalised Fra may disrupt the MET. βPS localisation is also affected in netrin mutants in that a basal gradient is reduced while localisation to the midgut/VM interface is increased. Since a similar effect is seen when endocytosis is inhibited, Netrin and Fra may regulate Integrin turnover. The results suggest Netrin-dependent basal polarisation of Fra is critical for the formation of an epithelium.

The UNC-6/Netrin receptors UNC-40/DCC and UNC-5 inhibit growth cone filopodial protrusion via UNC-73/Trio, Rac-like GTPases and UNC-33/CRMP

UNC-6/Netrin is a conserved axon guidance cue that can mediate both attraction and repulsion. We previously discovered that attractive UNC-40/DCC receptor signaling stimulates growth cone filopodial protrusion and that repulsive UNC-40–UNC-5 heterodimers inhibit filopodial protrusion in C. elegans. Here, we identify cytoplasmic signaling molecules required for UNC-6-mediated inhibition of filopodial protrusion involved in axon repulsion. We show that the Rac-like GTPases CED-10 and MIG-2, the Rac GTP exchange factor UNC-73/Trio, UNC-44/Ankyrin and UNC-33/CRMP act in inhibitory UNC-6 signaling. These molecules were required for the normal limitation of filopodial protrusion in developing growth cones and for inhibition of growth cone filopodial protrusion caused by activated MYR::UNC-40 and MYR::UNC-5 receptor signaling. Epistasis studies using activated CED-10 and MIG-2 indicated that UNC-44 and UNC-33 act downstream of the Rac-like GTPases in filopodial inhibition. UNC-73, UNC-33 and UNC-44 did not affect the accumulation of full-length UNC-5::GFP and UNC-40::GFP in growth cones, consistent with a model in which UNC-73, UNC-33 and UNC-44 influence cytoskeletal function during growth cone filopodial inhibition.

Flamingo, a seven-pass transmembrane cadherin, cooperates with Netrin/Frazzled in Drosophila midline guidance

During central nervous system development, several guidance cues and receptors, as well as cell adhesion molecules, are required for guiding axons across the midline and along the anterior-posterior axis. In Drosophila, commissural axons sense the midline attractants Netrin A and B (Net) through Frazzled (Fra) receptors. Despite their importance, lack of Net or fra affects only some commissures, suggesting that additional molecules can fulfill this function. Recently, planar cell polarity (PCP) proteins have been implicated in midline axon guidance in both vertebrate and invertebrate systems. Here, we report that the atypical cadherin and PCP molecule Flamingo/Starry night (Fmi/Stan) acts jointly with Net/Fra signaling during midline development. Additional removal of fmi strongly increases the guidance defects in Net/fra mutants. Rescue and domain deletion experiments suggest that Fmi signaling facilitates commissural pathfinding potentially by mediating axonal fasciculation in a partly homophilic manner. Altogether, our results indicate that contact-mediated cell adhesion via Fmi acts in addition to the Net/Fra guidance system during axon pathfinding across the midline, underlining the importance of PCP molecules during vertebrates and invertebrates midline development.

Promotion of colorectal cancer invasion and metastasis through activation of NOTCH-DAB1-ABL-RHOGEF protein TRIO

We have recently identified a metastasis suppressor gene for colorectal cancer: AES/Aes, which encodes an endogenous inhibitor of NOTCH signaling. When Aes is knocked out in the adenomatous epithelium of intestinal polyposis mice, their tumors become malignant, showing marked submucosal invasion and intravasation. Here, we show that one of the genes induced by NOTCH signaling in colorectal cancer is DAB1/Dab1. Genetic depletion of DAB1 suppresses cancer invasion and metastasis in the NOTCH signaling-activated mice. DAB1 is phosphorylated by ABL tyrosine kinase, which activates ABL reciprocally. Consistently, inhibition of ABL suppresses cancer invasion in mice. Furthermore, we show that one of the targets of ABL is the RAC/RHOGEF protein TRIO, and that phosphorylation at its Tyr residue 2681 (pY2681) causes RHO activation in colorectal cancer cells. Its unphosphorylatable mutation TRIO Y2681F reduces RHOGEF activity and inhibits invasion of colorectal cancer cells. Importantly, TRIO pY2681 correlates with significantly poorer prognosis of patients with colorectal cancer after surgery.

SIGNIFICANCE

These results indicate that TRIO pY2681 is one of the downstream effectors of NOTCH signaling activation in colorectal cancer, and can be a prognostic marker, helping to determine the therapeutic modality of patients with colorectal cancer.

Rho-guanine nucleotide exchange factors during development: Force is nothing without control

The development of multicellular organisms is associated with extensive rearrangements of tissues and cell sheets. The driving force for these rearrangements is generated mostly by the actin cytoskeleton. In order to permit the reproducible development of a specific body plan, dynamic reorganization of the actin cytoskeleton must be precisely coordinated in space and time. GTP-exchange factors that activate small GTPases of the Rho family play an important role in this process. Here we review the role of this class of cytoskeletal regulators during important developmental processes such as epithelial morphogenesis, cytokinesis, cell migration, cell polarity, neuronal growth cone extension and phagocytosis in different model systems.

The Abl/enabled signaling pathway regulates Golgi architecture in Drosophila photoreceptor neurons

The Golgi apparatus is optimized separately in different tissues for efficient protein trafficking, but we know little of how cell signaling shapes this organelle. We now find that the Abl tyrosine kinase signaling pathway controls the architecture of the Golgi complex in Drosophila photoreceptor (PR) neurons. The Abl effector, Enabled (Ena), selectively labels the cis-Golgi in developing PRs. Overexpression or loss of function of Ena increases the number of cis- and trans-Golgi cisternae per cell, and Ena overexpression also redistributes Golgi to the most basal portion of the cell soma. Loss of Abl or its upstream regulator, the adaptor protein Disabled, lead to the same alterations of Golgi as does overexpression of Ena. The increase in Golgi number in Abl mutants arises in part from increased frequency of Golgi fission events and a decrease in fusions, as revealed by live imaging. Finally, we demonstrate that the effects of Abl signaling on Golgi are mediated via regulation of the actin cytoskeleton. Together, these data reveal a direct link between cell signaling and Golgi architecture. Moreover, they raise the possibility that some of the effects of Abl signaling may arise, in part, from alterations of protein trafficking and secretion.

Function and regulation of the Rho guanine nucleotide exchange factor Trio

Rho GTPases oscillate between an inactive GDP-bound state and an active GTP-bound state. They are activated by Rho Guanine nucleotide Exchange Factors (GEF), which accelerate the GDP to GTP exchange. RhoGEFs fall into two different classes: the Dbl family and the DOCK family of proteins. In this review, we focus on the function and regulation of the Dbl family RhoGEF Trio. Trio and its paralog Kalirin are unique within this family in that they display two GEF domains of distinct specificity. Trio is a major regulator of neuronal development, and its function is conserved through evolution. Moreover, Trio plays an important role in cell adhesion and in signaling pathways elicited by Gαq protein-coupled receptors. Combined, these observations suggest that Trio has a major role in cellular physiology. Of note, Trio is an essential gene for mouse development, with a prominent role in the development of the nervous system. Finally, Trio expression is significantly increased in different types of tumors and it has been proposed that it could participate in oncogenesis.

Control of male and female fertility by the netrin axon guidance genes

The netrin axon guidance genes have previously been implicated in fertility in C. elegans and in vertebrates. Here we show that adult Drosophila lacking both netrin genes, NetA and NetB, have fertility defects in both sexes together with an inability to fly and reduced viability. NetAB females produce fertilized eggs at a much lower rate than wild type. Oocyte development and ovarian innervation are unaffected in NetAB females, and the reproductive tract appears normal. A small gene, hog, that resides in an intron of NetB does not contribute to the NetAB phenotype. Restoring endogenous NetB expression rescues egg-laying, but additional genetic manipulations, such as restoration of netrin midline expression and inhibition of cell death have no effect on fertility. NetAB males induce reduced egg-laying in wild type females and display mirror movements of their wings during courtship. Measurement of courtship parameters revealed no difference compared to wild type males. Transgenic manipulations failed to rescue male fertility and mirror movements. Additional genetic manipulations, such as removal of the enabled gene, a known suppressor of the NetAB embryonic CNS phenotype, did not improve the behavioral defects. The ability to fly was rescued by inhibition of neuronal cell death and pan-neural NetA expression. Based on our results we hypothesize that the adult fertility defects of NetAB mutants are due to ovulation defects in females and a failure to properly transfer sperm proteins in males, and are likely to involve multiple neural circuits.

Distinct functional domains of the Abelson tyrosine kinase control axon guidance responses to Netrin and Slit to regulate the assembly of neural circuits

To develop a functional nervous system, axons must initially navigate through a complex environment, directed by guidance ligands and receptors. These receptors must link to intracellular signaling cascades to direct axon pathfinding decisions. The Abelson tyrosine kinase (Abl) plays a crucial role in multiple Drosophila axon guidance pathways during development, though the mechanism by which Abl elicits a diverse set of guidance outputs is currently unknown. We identified Abl in a genetic screen for genes that contribute to Netrin-dependent axon guidance in midline-crossing (commissural) neurons. We find that Abl interacts both physically and genetically with the Netrin receptor Frazzled, and that disrupting this interaction prevents Abl from promoting midline axon crossing. Moreover, we find that Abl exerts its diverse activities through at least two different mechanisms: (1) a partly kinase-independent, structural function in midline attraction through its C-terminal F-actin binding domain (FABD) and (2) a kinase-dependent inhibition of repulsive guidance pathways that does not require the Abl C terminus. Abl also regulates motor axon pathfinding through a non-overlapping set of functional domains. These results highlight how a multifunctional kinase can trigger diverse axon guidance outcomes through the use of distinct structural motifs.

The Abelson tyrosine kinase regulates Notch endocytosis and signaling to maintain neuronal cell fate in Drosophila photoreceptors

The development of a functional organ requires coordinated programs of cell fate specification, terminal differentiation and morphogenesis. Whereas signaling mechanisms that specify individual cell fates are well documented, little is known about the pathways and molecules that maintain these fates stably as normal development proceeds or how their dysregulation may contribute to altered cell states in diseases such as cancer. In Drosophila, the tyrosine kinase Abelson (Abl) interfaces with multiple signaling pathways to direct epithelial and neuronal morphogenesis during embryonic and retinal development. Here we show that Abl is required for photoreceptor cell fate maintenance, as Abl mutant photoreceptors lose neuronal markers during late pupal stages but do not re-enter a proliferative state or undergo apoptosis. Failure to maintain the differentiated state correlates with impaired trafficking of the Notch receptor and ectopic Notch signaling, and can be suppressed by reducing the genetic dose of Notch or of its downstream transcriptional effector Suppressor of Hairless. Together, these data reveal a novel mechanism for maintaining the terminally differentiated state of Drosophila photoreceptors and suggest that neuronal fates in the fly retina retain plasticity late into development. Given the general evolutionary conservation of developmental signaling mechanisms, Abl-mediated regulation of Notch could be broadly relevant to cell fate maintenance and reprogramming during normal development, regeneration and oncogenic transformation.

Tyrosine phosphorylation of the Rho guanine nucleotide exchange factor Trio regulates netrin-1/DCC-mediated cortical axon outgrowth

The chemotropic guidance cue netrin-1 mediates attraction of migrating axons during central nervous system development through the receptor Deleted in Colorectal Cancer (DCC). Downstream of netrin-1, activated Rho GTPases Rac1 and Cdc42 induce cytoskeletal rearrangements within the growth cone. The Rho guanine nucleotide exchange factor (GEF) Trio is essential for Rac1 activation downstream of netrin-1/DCC, but the molecular mechanisms governing Trio activity remain elusive. Here, we demonstrate that Trio is phosphorylated by Src family kinases in the embryonic rat cortex in response to netrin-1. In vitro, Trio was predominantly phosphorylated at Tyr(2622) by the Src kinase Fyn. Though the phospho-null mutant Trio(Y2622F) retained GEF activity toward Rac1, its expression impaired netrin-1-induced Rac1 activation and DCC-mediated neurite outgrowth in N1E-115 neuroblastoma cells. Trio(Y2622F) impaired netrin-1-induced axonal extension in cultured cortical neurons and was unable to colocalize with DCC in growth cones, in contrast to wild-type Trio. Furthermore, depletion of Trio in cortical neurons reduced the level of cell surface DCC in growth cones, which could be restored by expression of wild-type Trio but not Trio(Y2622F). Together, these findings demonstrate that Trio(Y2622) phosphorylation is essential for the regulation of the DCC/Trio signaling complex in cortical neurons during netrin-1-mediated axon outgrowth.

c-Jun N-terminal kinase 1 (JNK1) is required for coordination of netrin signaling in axon guidance

The JNK family of MAPKs is involved in a large variety of physiological and pathological processes in brain development, such as neural survival, migration, and polarity as well as axon regeneration. However, whether JNK activation is involved in axon guidance remains unknown. Here, we provide evidence indicating the JNK pathway is required for Netrin signaling in the developing nervous system. Netrin-1 increased JNK1, not JNK2 or JNK3, activity in the presence of deleted in colorectal cancer (DCC) or Down syndrome cell adhesion molecule (DSCAM), and expression of both of them further enhanced Netrin-1-induced JNK1 activity in vitro. Inhibition of JNK signaling either by a JNK inhibitor, SP600125, or expression of a dominant negative form of MKK4, a JNK upstream activator, blocked Netrin-1-induced JNK1 activation in HEK293 cells. Netrin-1 increased endogenous JNK activity in primary neurons. Netrin-1-induced JNK activation was inhibited either by the JNK inhibitor or an anti-DCC function-blocking antibody. Combination of the anti-DCC function-blocking antibody with expression of DSCAM shRNA in primary neurons totally abolished Netrin-1-induced JNK activation, whereas knockdown of DSCAM partially inhibited the Netrin-1 effect. In the developing spinal cord, phospho-JNK was strongly expressed in commissural axons before and as they crossed the floor plate, and Netrin-1 stimulation dramatically increased the level of endogenous phospho-JNK in commissural axon growth cones. Inhibition of JNK signaling either by JNK1 RNA interference (RNAi) or the JNK inhibitor suppressed Netrin-1-induced neurite outgrowth and axon attraction. Knockdown of JNK1 in ovo caused defects in spinal cord commissural axon projection and pathfinding. Our study reveals that JNK1 is important in the coordination of DCC and DSCAM in Netrin-mediated attractive signaling.

MIG-10 functions with ABI-1 to mediate the UNC-6 and SLT-1 axon guidance signaling pathways

Extracellular guidance cues steer axons towards their targets by eliciting morphological changes in the growth cone. A key part of this process is the asymmetric recruitment of the cytoplasmic scaffolding protein MIG-10 (lamellipodin). MIG-10 is thought to asymmetrically promote outgrowth by inducing actin polymerization. However, the mechanism that links MIG-10 to actin polymerization is not known. We have identified the actin regulatory protein ABI-1 as a partner for MIG-10 that can mediate its outgrowth-promoting activity. The SH3 domain of ABI-1 binds to MIG-10, and loss of function of either of these proteins causes similar axon guidance defects. Like MIG-10, ABI-1 functions in both the attractive UNC-6 (netrin) pathway and the repulsive SLT-1 (slit) pathway. Dosage sensitive genetic interactions indicate that MIG-10 functions with ABI-1 and WVE-1 to mediate axon guidance. Epistasis analysis reveals that ABI-1 and WVE-1 function downstream of MIG-10 to mediate its outgrowth-promoting activity. Moreover, experiments with cultured mammalian cells suggest that the interaction between MIG-10 and ABI-1 mediates a conserved mechanism that promotes formation of lamellipodia. Together, these observations suggest that MIG-10 interacts with ABI-1 and WVE-1 to mediate the UNC-6 and SLT-1 guidance pathways.

To form neural circuits, axons must navigate through the developing nervous system to reach their correct targets. Axon navigation is led by the growth cone, a structure at the tip of the growing axon that responds to extracellular guidance cues. Many of these guidance cues and their receptors have been identified. However, much less is known about the internal signaling events that give rise to the structural changes required for growth cone steering. A key component of the internal response is MIG-10, a protein that becomes asymmetrically localized in response to the extracellular cues. MIG-10 is thought to serve as a scaffold that can spatially control outgrowth-promoting proteins within the growth cone. However, we do not know the identity of the outgrowth-promoting proteins that associate with MIG-10. Here we report that MIG-10 associates physically with the actin regulatory protein ABI-1. We present genetic evidence indicating that ABI-1 functions downstream of MIG-10 to mediate its outgrowth-promoting activity. Additional genetic evidence indicates that these proteins function in both attractive and repulsive guidance signaling pathways. We also present evidence suggesting that the connection between MIG-10 and ABI-1 represents a phylogenetically conserved mechanism for the control of cellular outgrowth.

The many faces of the guanine-nucleotide exchange factor trio

Small Rho-GTPases are enzymes that are bound to GDP or GTP, which determines their inactive or active state, respectively. The exchange of GDP for GTP is catalyzed by so-called Rho-guanine nucleotide exchange factors (GEFs). Rho-GEFs are characterized by a Dbl-homology (DH) and adjacent Pleckstrin-homology (PH) domain that serves as enzymatic unit for the GDP/GTP exchange. Rho-GEFs show different GTPase specificities, meaning that a particular GEF can activate either multiple GTPases or only one specific GTPase. We recently reported that the Rho-GEF Trio, known to be able to exchange GTP on Rac1, RhoG and RhoA, regulates lamellipodia formation to mediate cell spreading and migration in a Rac1-dependent manner. In this commentary, we review the current knowledge of Trio in several aspects of cell biology.

Down syndrome cell adhesion molecule (DSCAM) associates with uncoordinated-5C (UNC5C) in netrin-1-mediated growth cone collapse

In the developing nervous system, neuronal growth cones explore the extracellular environment for guidance cues, which can guide them along specific trajectories toward their targets. Netrin-1, a bifunctional guidance cue, binds to deleted in colorectal cancer (DCC) and DSCAM mediating axon attraction, and UNC5 mediating axon repulsion. Here, we show that DSCAM interacts with UNC5C and this interaction is stimulated by netrin-1 in primary cortical neurons and postnatal cerebellar granule cells. DSCAM partially co-localized with UNC5C in primary neurons and brain tissues. Netrin-1 induces axon growth cone collapse of mouse cerebellum external granule layer (EGL) cells, and the knockdown of DSCAM or UNC5C by specific shRNAs or blocking their signaling by overexpressing dominant negative mutants suppresses netrin-1-induced growth cone collapse. Similarly, the simultaneous knockdown of DSCAM and UNC5C also blocks netrin-1-induced growth cone collapse in EGL cells. Netrin-1 increases tyrosine phosphorylation of endogenous DSCAM, UNC5C, FAK, Fyn, and PAK1, and promotes complex formation of DSCAM with these signaling molecules in primary postnatal cerebellar neurons. Inhibition of Src family kinases efficiently reduces the interaction of DSCAM with UNC5C, FAK, Fyn, and PAK1 and tyrosine phosphorylation of these proteins as well as growth cone collapse of mouse EGL cells induced by netrin-1. The knockdown of DSCAM inhibits netrin-induced tyrosine phosphorylation of UNC5C and Fyn as well as the interaction of UNC5C with Fyn. The double knockdown of both receptors abolishes the induction of Fyn tyrosine phosphorylation by netrin-1. Our study reveals the first evidence that DSCAM coordinates with UNC5C in netrin-1 repulsion.

The Rac GTP exchange factor TIAM-1 acts with CDC-42 and the guidance receptor UNC-40/DCC in neuronal protrusion and axon guidance

The mechanisms linking guidance receptors to cytoskeletal dynamics in the growth cone during axon extension remain mysterious. The Rho-family GTPases Rac and CDC-42 are key regulators of growth cone lamellipodia and filopodia formation, yet little is understood about how these molecules interact in growth cone outgrowth or how the activities of these molecules are regulated in distinct contexts. UNC-73/Trio is a well-characterized Rac GTP exchange factor in Caenorhabditis elegans axon pathfinding, yet UNC-73 does not control CED-10/Rac downstream of UNC-6/Netrin in attractive axon guidance. Here we show that C. elegans TIAM-1 is a Rac-specific GEF that links CDC-42 and Rac signaling in lamellipodia and filopodia formation downstream of UNC-40/DCC. We also show that TIAM-1 acts with UNC-40/DCC in axon guidance. Our results indicate that a CDC-42/TIAM-1/Rac GTPase signaling pathway drives lamellipodia and filopodia formation downstream of the UNC-40/DCC guidance receptor, a novel set of interactions between these molecules. Furthermore, we show that TIAM-1 acts with UNC-40/DCC in axon guidance, suggesting that TIAM-1 might regulate growth cone protrusion via Rac GTPases in response to UNC-40/DCC. Our results also suggest that Rac GTPase activity is controlled by different GEFs in distinct axon guidance contexts, explaining how Rac GTPases can specifically control multiple cellular functions.

Axons extend great distances to make precise synaptic connections in the developing nervous system. Axons are guided to their targets by the growth cone, a dynamic structure at the axon distal tip that senses extracellular cues telling the axon where to go. In response to guidance cues, growth cones alter their shape and motility resulting in outgrowth and turning. The cytoskeleton (actin and microtubules) underlies growth cone motility and guidance. The signaling mechanisms linking guidance receptors to cytoskeletal change remain mysterious. Here, we define a new signaling mechanism downstream of the guidance receptor UNC-40/DCC involving the GTPases CDC-42 and Rac, which have long been known to control growth cone protrusion. We show that CDC-42 and Rac act in a linear pathway in axon guidance; CDC-42 acts upstream of the GTPase regulatory molecule TIAM-1, which is a GTP exchange factor specific for Rac and which activates Rac signaling. We also show that TIAM-1 acts with UNC-40/DCC signaling in protrusion and axon guidance. Our results imply that Rac GTPase function in axon guidance is complex and that distinct GEFs (TIAM-1 and UNC-73/Trio) might control Rac GTPases in different aspects of axon guidance.

Single-cell analysis of population context advances RNAi screening at multiple levels

A large set of high-content RNAi screens investigating mammalian virus infection and multiple cellular activities is analysed to reveal the impact of population context on phenotypic variability and to identify indirect RNAi effects.

Cell population context determines phenotypes in RNAi screens of multiple cellular activities (including virus infection, cell size regulation, endocytosis, and lipid homeostasis), which can be accounted for by a combination of novel image analysis and multivariate statistical methods.

Accounting for cell population context-mediated effects strongly changes the reproducibility and consistency of RNAi screens across cell lines as well as of siRNAs targeting the same gene.

Such analyses can identify the perturbed regulation of population context dependent cell-to-cell variability, a novel perturbation phenotype.

Overall, these methods advance the use of large-scale RNAi screening for a systems-level understanding of cellular processes.

Isogenic cells in culture show strong variability, which arises from dynamic adaptations to the microenvironment of individual cells. Here we study the influence of the cell population context, which determines a single cell's microenvironment, in image-based RNAi screens. We developed a comprehensive computational approach that employs Bayesian and multivariate methods at the single-cell level. We applied these methods to 45 RNA interference screens of various sizes, including 7 druggable genome and 2 genome-wide screens, analysing 17 different mammalian virus infections and four related cell physiological processes. Analysing cell-based screens at this depth reveals widespread RNAi-induced changes in the population context of individual cells leading to indirect RNAi effects, as well as perturbations of cell-to-cell variability regulators. We find that accounting for indirect effects improves the consistency between siRNAs targeted against the same gene, and between replicate RNAi screens performed in different cell lines, in different labs, and with different siRNA libraries. In an era where large-scale RNAi screens are increasingly performed to reach a systems-level understanding of cellular processes, we show that this is often improved by analyses that account for and incorporate the single-cell microenvironment.

Netrin instructs synaptic vesicle clustering through Rac GTPase, MIG-10, and the actin cytoskeleton

Netrin is a chemotrophic factor known to regulate a number of neurodevelopmental processes, including cell migration, axon guidance, and synaptogenesis. Although the role of Netrin in synaptogenesis is conserved throughout evolution, the mechanisms by which it instructs synapse assembly are not understood. Here we identify a mechanism by which the Netrin receptor UNC-40/DCC instructs synaptic vesicle clustering in vivo. UNC-40 localized to presynaptic regions in response to Netrin. We show that UNC-40 interacted with CED-5/DOCK180 and instructed CED-5 presynaptic localization. CED-5 in turn signaled through CED-10/Rac1 and MIG-10/Lamellipodin to organize the actin cytoskeleton in presynaptic regions. Localization of this signaling pathway to presynaptic regions was necessary for synaptic vesicle clustering during synapse assembly but not for the subcellular localization of active zone proteins. Thus, vesicle clustering and localization of active zone proteins are instructed by separate pathways downstream of Netrin. Our data indicate that signaling modules known to organize the actin cytoskeleton during guidance can be co-opted to instruct synaptic vesicle clustering.

The RhoGEF trio functions in sculpting class specific dendrite morphogenesis in Drosophila sensory neurons

Background

As the primary sites of synaptic or sensory input in the nervous system, dendrites play an essential role in processing neuronal and sensory information. Moreover, the specification of class specific dendrite arborization is critically important in establishing neural connectivity and the formation of functional networks. Cytoskeletal modulation provides a key mechanism for establishing, as well as reorganizing, dendritic morphology among distinct neuronal subtypes. While previous studies have established differential roles for the small GTPases Rac and Rho in mediating dendrite morphogenesis, little is known regarding the direct regulators of these genes in mediating distinct dendritic architectures.

Methodology/Principal Findings

Here we demonstrate that the RhoGEF Trio is required for the specification of class specific dendritic morphology in dendritic arborization (da) sensory neurons of the Drosophila peripheral nervous system (PNS). Trio is expressed in all da neuron subclasses and loss-of-function analyses indicate that Trio functions cell-autonomously in promoting dendritic branching, field coverage, and refining dendritic outgrowth in various da neuron subtypes. Moreover, overexpression studies demonstrate that Trio acts to promote higher order dendritic branching, including the formation of dendritic filopodia, through Trio GEF1-dependent interactions with Rac1, whereas Trio GEF-2-dependent interactions with Rho1 serve to restrict dendritic extension and higher order branching in da neurons. Finally, we show that de novo dendritic branching, induced by the homeodomain transcription factor Cut, requires Trio activity suggesting these molecules may act in a pathway to mediate dendrite morphogenesis.

Conclusions/Significance

Collectively, our analyses implicate Trio as an important regulator of class specific da neuron dendrite morphogenesis via interactions with Rac1 and Rho1 and indicate that Trio is required as downstream effector in Cut-mediated regulation of dendrite branching and filopodia formation.