Repulsive axon guidance: Abelson and Enabled play opposing roles downstream of the roundabout receptor

In Vivo Functional Analysis of Drosophila Robo1 Fibronectin Type-III Repeats

The repellant ligand Slit and its Roundabout (Robo) family receptors regulate midline crossing of axons during development of the embryonic central nervous system (CNS). Slit proteins are produced at the midline and signal through Robo receptors to repel axons from the midline. Disruption of Slit-Robo signaling causes ectopic midline-crossing phenotypes in the CNS of a broad range of animals, including insects and vertebrates. While previous studies have investigated the roles of Drosophila melanogaster Robo1's five Immunoglobulin-like (Ig) domains, little is known about the importance of the three evolutionarily conserved Fibronectin (Fn) type-III repeats. We have individually deleted each of Drosophila Robo1's three Fn repeats, and then tested these Robo1 variants in vitro to determine their ability to bind Slit in cultured Drosophila cells and in vivo to investigate the requirement for each domain in regulating Robo1's embryonic expression pattern, axonal localization, midline repulsive function, and sensitivity to Commissureless (Comm) downregulation. We demonstrate that the Fn repeats are not required for Robo1 to bind Slit or for proper expression of Robo1 in Drosophila embryonic neurons. When expressed in a robo1 mutant background, these variants are able to restore midline repulsion to an extent equivalent to full-length Robo1. We identify a novel requirement for Fn3 in the exclusion of Robo1 from commissures and downregulation of Robo1 by Comm. Our results indicate that each of the Drosophila Robo1 Fn repeats are individually dispensable for the protein's role in midline repulsion, despite the evolutionarily conserved "5 + 3" protein structure.

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.

Robo Ig4 Is a Dimerization Domain

Robo receptors play pivotal roles in axonal guidance as well as in neurogenesis, angiogenesis, cell migration, and cancer progression and invasiveness. They are considered to be attractive drug targets for the treatment of cancer, ocular neovascular disorders, chronic kidney diseases, and more. Despite their great importance, the mechanisms by which Robo receptors switch from their "off" to "on" states remain obscure. One possibility involves a monomer-to-dimer or dimer-to-monomer transition that facilitates the recruitment and activation of enzymatic effectors to instigate intracellular signaling. However, it is not known which domains mediate Robo dimerization, or the structural properties of the dimeric interactions. Here, we identify the extracellular Ig4 (D4) as a Robo dimerization domain. We have determined the crystal structure of the tandem Ig4-5 domains (D4-5) of human Robo2 and found that a hydrophobic surface on D4 mediates close homotypic contacts with a reciprocal D4. Analytical ultracentrifugation measurements of intact and mutated D4-5 shows that dimerization through the D4 interface is specific and has a dimerization dissociation constant of 16.9μM in solution. Direct fluorescence resonance energy transfer dimerization measurements in HEK293 cells corroborate the dimerization of transmembrane hRobo2 through D4, and a functional COS-7 cell collapse assay links D4-mediated dimerization with Robo intracellular signaling. The high level of conservation in the D4 dimerization interface throughout all Robo orthologs and paralogs implies that D4-mediated dimerization is a central hallmark in Robo activation and signaling.

ENA/VASP proteins regulate exocytosis by mediating myosin VI-dependent recruitment of secretory granules to the cortical actin network

In neurosecretory cells, myosin VI associated with secretory granules (SGs) mediates their activity-dependent recruitment to the cortical actin network and is necessary to sustain exocytosis. The mechanism by which myosin VI interacts with SGs is unknown. Using a myosin VI pull-down assay and mass spectrometry we identified Mena, a member of the ENA/VASP family, as a myosin VI binding partner in PC12 cells, and confirmed that Mena colocalized with myosin VI on SGs. Using a knock-sideways approach to inactivate the ENA/VASP family members by mitochondrial relocation, we revealed a concomitant redistribution of myosin VI. This was ensued by a reduction in the association of myosin VI with SGs, a decreased SG mobility and density in proximity to the plasma membrane as well as decreased evoked exocytosis. These data demonstrate that ENA/VASP proteins regulate SG exocytosis through modulating the activity of myosin VI.

Amplification of F-Actin Disassembly and Cellular Repulsion by Growth Factor Signaling

Extracellular cues that regulate cellular shape, motility, and navigation are generally classified as growth promoting (i.e., growth factors/chemoattractants and attractive guidance cues) or growth preventing (i.e., repellents and inhibitors). Yet, these designations are often based on complex assays and undefined signaling pathways and thus may misrepresent direct roles of specific cues. Here, we find that a recognized growth-promoting signaling pathway amplifies the F-actin disassembly and repulsive effects of a growth-preventing pathway. Focusing on Semaphorin/Plexin repulsion, we identified an interaction between the F-actin-disassembly enzyme Mical and the Abl tyrosine kinase. Biochemical assays revealed Abl phosphorylates Mical to directly amplify Mical Redox-mediated F-actin disassembly. Genetic assays revealed that Abl allows growth factors and Semaphorin/Plexin repellents to combinatorially increase Mical-mediated F-actin disassembly, cellular remodeling, and repulsive axon guidance. Similar roles for Mical in growth factor/Abl-related cancer cell behaviors further revealed contexts in which characterized positive effectors of growth/guidance stimulate such negative cellular effects as F-actin disassembly/repulsion.

A Requirement for Mena, an Actin Regulator, in Local mRNA Translation in Developing Neurons

During neuronal development, local mRNA translation is required for axon guidance and synaptogenesis, and dysregulation of this process contributes to multiple neurodevelopmental and cognitive disorders. However, regulation of local protein synthesis in developing axons remains poorly understood. Here, we uncover a novel role for the actin-regulatory protein Mena in the formation of a ribonucleoprotein complex that involves the RNA-binding proteins HnrnpK and PCBP1 and regulates local translation of specific mRNAs in developing axons. We find that translation of dyrk1a, a Down syndrome- and autism spectrum disorders-related gene, is dependent on Mena, both in steady-state conditions and upon BDNF stimulation. We identify hundreds of additional mRNAs that associate with the Mena complex, suggesting that it plays broader role(s) in post-transcriptional gene regulation. Our work establishes a dual role for Mena in neurons, providing a potential link between regulation of actin dynamics and local translation.

Regulation of Drosophila Brain Wiring by Neuropil Interactions via a Slit-Robo-RPTP Signaling Complex

The axonal wiring molecule Slit and its Round-About (Robo) receptors are conserved regulators of nerve cord patterning. Robo receptors also contribute to wiring brain circuits. Whether molecular mechanisms regulating these signals are modified to fit more complex brain wiring processes is unclear. We investigated the role of Slit and Robo receptors in wiring Drosophila higher-order brain circuits and identified differences in the cellular and molecular mechanisms of Robo/Slit function. First, we find that signaling by Robo receptors in the brain is regulated by the Receptor Protein Tyrosine Phosphatase RPTP69d. RPTP69d increases membrane availability of Robo3 without affecting its phosphorylation state. Second, we detect no midline localization of Slit during brain development. Instead, Slit is enriched in the mushroom body, a neuronal structure covering large areas of the brain. Thus, a divergent molecular mechanism regulates neuronal circuit wiring in the Drosophila brain, partly in response to signals from the mushroom body.

•

In the Drosophila brain, mushroom bodies are a source of the Slit guidance cue

•

Slit regulates axon growth in the vicinity of mushroom bodies via Robo receptors

•

The phosphatase RPTP69D regulates Robo signaling in the brain

•

RPTP69D regulates Robo3 membrane presentation independent of its enzymatic activity

Slit-Robo Repulsive Signaling Extrudes Tumorigenic Cells from Epithelia

Cells dynamically interact throughout animal development to coordinate growth and deter disease. For example, cell-cell competition weeds out aberrant cells to enforce homeostasis. In Drosophila, tumorigenic cells mutant for the cell polarity gene scribble (scrib) are actively eliminated from epithelia when surrounded by wild-type cells. While scrib cell elimination depends critically on JNK signaling, JNK-dependent cell death cannot sufficiently explain scrib cell extirpation. Thus, how JNK executed cell elimination remained elusive. Here, we show that repulsive Slit-Robo2-Ena signaling exerts an extrusive force downstream of JNK to eliminate scrib cells from epithelia by disrupting E-cadherin. While loss of Slit-Robo2-Ena in scrib cells potentiates scrib tumor formation within the epithelium, Robo2-Ena hyperactivation surprisingly triggers luminal scrib tumor growth following excess extrusion. This extrusive signaling is amplified by a positive feedback loop between Slit-Robo2-Ena and JNK. Our observations provide a potential causal mechanism for Slit-Robo dysregulation in numerous human cancers.

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.

Shape of my heart: Cell-cell adhesion and cytoskeletal dynamics during Drosophila cardiac morphogenesis

The fruit fly Drosophila melanogaster has recently emerged as an excellent system to investigate the genetics of cardiovascular development and disease. Drosophila provides an inexpensive and genetically-tractable in vivo system with a large number of conserved features. In addition, the Drosophila embryo is transparent, and thus amenable to time-lapse fluorescence microscopy, as well as biophysical and pharmacological manipulations. One of the conserved aspects of heart development from Drosophila to humans is the initial assembly of a tube. Here, we review the cellular behaviours and molecular dynamics important for the initial steps of heart morphogenesis in Drosophila, with particular emphasis on the cell-cell adhesion and cytoskeletal networks that cardiac precursors use to move, coordinate their migration, interact with other tissues and eventually sculpt a beating heart.

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.

Slit-Robo GTPase-Activating Protein 2 as a metastasis suppressor in osteosarcoma

Osteosarcoma is the most common primary bone tumor, with metastatic disease responsible for most treatment failure and patient death. A forward genetic screen utilizing Sleeping Beauty mutagenesis in mice previously identified potential genetic drivers of osteosarcoma metastasis, including Slit-Robo GTPase-Activating Protein 2 (Srgap2). This study evaluates the potential role of SRGAP2 in metastases-associated properties of osteosarcoma cell lines through Srgap2 knockout via the CRISPR/Cas9 nuclease system and conditional overexpression in the murine osteosarcoma cell lines K12 and K7M2. Proliferation, migration, and anchorage independent growth were evaluated. RNA sequencing and immunohistochemistry of human osteosarcoma tissue samples were used to further evaluate the potential role of the Slit-Robo pathway in osteosarcoma. The effects of Srgap2 expression modulation in the murine OS cell lines support the hypothesis that SRGAP2 may have a role as a suppressor of metastases in osteosarcoma. Additionally, SRGAP2 and other genes in the Slit-Robo pathway have altered transcript levels in a subset of mouse and human osteosarcoma, and SRGAP2 protein expression is reduced or absent in a subset of primary tumor samples. SRGAP2 and other axon guidance proteins likely play a role in osteosarcoma metastasis, with loss of SRGAP2 potentially contributing to a more aggressive phenotype.

srGAP1 mediates the migration inhibition effect of Slit2-Robo1 in colorectal cancer

BACKGROUND

The neuronal guidance molecule Slit2 plays suppressive role in tumorigenesis and progression. We previously showed that Slit2-Robo1 inhibit cell migration in colorectal cancer (CRC). However, little is known about its downstream effectors in CRC. This study tries to identify whether the Slit-Robo Rho GTPase activating protein 1 (srGAP1) could mediate the inhibitory effect of Slit2-Robo1 on CRC cell migration.

METHODS

The protein expression of srGAP1 in clinical CRC tissues was tested by immunohistochemistry staining. Conditioned medium was prepared from HEK293 cells stably expressing Slit2-myc, Robo1-HA or RoboN (a soluble extracellular domain of Robo1). Immunoprecipitation (IP) was applied to check the interaction between Robo1 and srGAP1, and immunofluorescence (IF) was used to observe the subcellular localization of Robo1 and srGAP1. Small GTPase pull-down assay was used to determine the activity of Cdc42. A modified wound healing assay was performed to detect cell migration.

RESULTS

The protein expression of srGAP1 was remarkably decreased in 47.5% of CRC tissues compared with adjacent noncancerous tissues, and the decreased srGAP1 expression was associated with lymphatic invasion, poor tumor differentiation, high TNM stage, and poor survival (P < 0.05). IP and IF assays revealed that srGAP1 was a Robo1-interacting protein and exhibited similar dynamic subcellular distribution after Slit2 treatment in CRC cells. Small GTPase pull-down assay and migration assay indicated that Slit2-Robo1 signaling inhibited Cdc42 activity and CRC cell motility through srGAP1.

CONCLUSION

Downregulation of srGAP1 in CRC was associated with tumor progression and poor prognosis. srGAP1 is an important downstream molecule of Slit2 signalling in CRC, and mediates the anti-migration function of Slit2 by inhibiting Cdc42.

srGAP1 mediates the migration inhibition effect of Slit2-Robo1 in colorectal cancer

BACKGROUND

The neuronal guidance molecule Slit2 plays suppressive role in tumorigenesis and progression. We previously showed that Slit2-Robo1 inhibit cell migration in colorectal cancer (CRC). However, little is known about its downstream effectors in CRC. This study tries to identify whether the Slit-Robo Rho GTPase activating protein 1 (srGAP1) could mediate the inhibitory effect of Slit2-Robo1 on CRC cell migration.

METHODS

The protein expression of srGAP1 in clinical CRC tissues was tested by immunohistochemistry staining. Conditioned medium was prepared from HEK293 cells stably expressing Slit2-myc, Robo1-HA or RoboN (a soluble extracellular domain of Robo1). Immunoprecipitation (IP) was applied to check the interaction between Robo1 and srGAP1, and immunofluorescence (IF) was used to observe the subcellular localization of Robo1 and srGAP1. Small GTPase pull-down assay was used to determine the activity of Cdc42. A modified wound healing assay was performed to detect cell migration.

RESULTS

The protein expression of srGAP1 was remarkably decreased in 47.5% of CRC tissues compared with adjacent noncancerous tissues, and the decreased srGAP1 expression was associated with lymphatic invasion, poor tumor differentiation, high TNM stage, and poor survival (P < 0.05). IP and IF assays revealed that srGAP1 was a Robo1-interacting protein and exhibited similar dynamic subcellular distribution after Slit2 treatment in CRC cells. Small GTPase pull-down assay and migration assay indicated that Slit2-Robo1 signaling inhibited Cdc42 activity and CRC cell motility through srGAP1.

CONCLUSION

Downregulation of srGAP1 in CRC was associated with tumor progression and poor prognosis. srGAP1 is an important downstream molecule of Slit2 signalling in CRC, and mediates the anti-migration function of Slit2 by inhibiting Cdc42.

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.

SLIT2/ROBO2 signaling pathway inhibits nonmuscle myosin IIA activity and destabilizes kidney podocyte adhesion

The repulsive guidance cue SLIT2 and its receptor ROBO2 are required for kidney development and podocyte foot process structure, but the SLIT2/ROBO2 signaling mechanism regulating podocyte function is not known. Here we report that a potentially novel signaling pathway consisting of SLIT/ROBO Rho GTPase activating protein 1 (SRGAP1) and nonmuscle myosin IIA (NMIIA) regulates podocyte adhesion downstream of ROBO2. We found that the myosin II regulatory light chain (MRLC), a subunit of NMIIA, interacts directly with SRGAP1 and forms a complex with ROBO2/SRGAP1/NMIIA in the presence of SLIT2. Immunostaining demonstrated that SRGAP1 is a podocyte protein and is colocalized with ROBO2 on the basal surface of podocytes. In addition, SLIT2 stimulation inhibits NMIIA activity, decreases focal adhesion formation, and reduces podocyte attachment to collagen. In vivo studies further showed that podocyte-specific knockout of Robo2 protects mice from hypertension-induced podocyte detachment and albuminuria and also partially rescues the podocyte-loss phenotype in Myh9 knockout mice. Thus, we have identified SLIT2/ROBO2/SRGAP1/NMIIA as a potentially novel signaling pathway in kidney podocytes, which may play a role in regulating podocyte adhesion and attachment. Our findings also suggest that SLIT2/ROBO2 signaling might be a therapeutic target for kidney diseases associated with podocyte detachment and loss.

Hes1: the maestro in neurogenesis

The process of neurogenesis is well orchestrated by the harmony of multiple cues in a spatiotemporal manner. In this review, we focus on how a dynamic gene, Hes1, is involved in neurogenesis with the view of its regulation and functional implications. Initially, we have reviewed the immense functional significance drawn by this maestro during neural development in a context-dependent manner. How this indispensable role of Hes1 in conferring the competency for neural differentiation partly relies on the direct/indirect mode of repression mediated by very specific structural and functional arms of this protein has also been outlined here. We also review the detailed molecular mechanisms behind the well-tuned oscillatory versus sustained expression of this antineurogenic bHLH repressor, which indeed makes it a master gene to implement the elusive task of neural progenitor propensity. Apart from the functional aspects of Hes1, we also discuss the molecular insights into the endogenous regulatory machinery that regulates its expression. Though Hes1 is a classical target of the Notch signaling pathway, we discuss here its differential expression at the molecular, cellular, and/or regional level. Moreover, we describe how its expression is fine-tuned by all possible ways of gene regulation such as epigenetic, transcriptional, post-transcriptional, post-translational, and environmental factors during vertebrate neurogenesis.

Potential role of the Slit/Robo signal pathway in angiogenesis

Intensive investigations on angiogenesis and vasculogenesis have increased our understanding of molecular mechanisms of blood vessel formation during pathologic and developmental conditions. However, endothelial cells (ECs), the main component of vasculature, are heterogeneous, as revealed by our phenotypic and molecular biological studies in the laboratory, and it is still hard to adequately understand the molecular mechanisms of angiogenesis and vasculogenesis. Indeed, there are several major ligand/receptor signal pathways: VEGF/VEGFR, Jagged-1/Notch, Wnt ligand/frizzled receptor, and ephrin/Eph; each of which having distinct and independent roles during vascular formation. In this review, we focus on the angiogenic effect of the Slit and Robo signal pathway that was formally known as neuronal axon guidance. Among the existing vascular signals, this pathway is the most recently found ligand/receptor vascular signal, and may play important physiological roles as other major receptor/ligand signals do. Here, we briefly address: (1) the background of Slit and Robo families; (2) expression patterns of Slit and Robo; (3) functional roles of the Slit/Robo pathway in vascular formation; and (4) confronting tasks of this novel vascular pathway in the near future. Together, a summary of these data suggest the essential role of the Slit/Robo pathway in angiogenesis, and may explain why multiple vascular signals exist in heterogenic endothelial cells.

POU2F2-oriented network promotes human gastric cancer metastasis

BACKGROUND AND AIMS

Aberrant upregulation of POU2F2 expression has been discovered in metastatic gastric cancer (GC). However, the mechanisms underlying the aberrant upregulation and the potential functions of POU2F2 remain uncertain.

DESIGN

The role and mechanism of POU2F2 in GC metastasis were investigated in gastric epithelial cells, GC cell lines and an experimental metastasis animal model by gain of function and loss of function. Upstream and downstream targets of POU2F2 were selected by bioinformatics and identified by luciferase reporter assay, electrophoretic mobility shift assay and chromatin immunoprecipitation PCR. The influence of miR-218 on its putative target genes (POU2F2, ROBO1 and IKK-β) and GC metastasis was further explored via in vitro and in vivo approaches.

RESULTS

Increased POU2F2 expression was detected in metastatic GC cell lines and patient samples. POU2F2 was induced by the activation of nuclear factor (NF)-κB and, in turn, regulated ROBO1 transcription, thus functionally contributing to GC metastasis. Finally, miR-218 was found to suppress GC metastasis by simultaneously mediating multiple molecules in the POU2F2-oriented network.

CONCLUSIONS

This study demonstrated that NF-κB and the SLIT2/ROBO1 interaction network with POU2F2 as the central part may exert critical effects on tumour metastasis. Blocking the activation of the POU2F2-oriented metastasis network using miR-218 precursors exemplified a promising approach that sheds light on new strategies for GC treatment.

Slit-Robo signaling

Slits are secreted proteins that bind to Roundabout (Robo) receptors. Slit-Robo signaling is best known for mediating axon repulsion in the developing nervous system. However, in recent years the functional repertoire of Slits and Robo has expanded tremendously and Slit-Robo signaling has been linked to roles in neurogenesis, angiogenesis and cancer progression among other processes. Likewise, our mechanistic understanding of Slit-Robo signaling has progressed enormously. Here, we summarize new insights into Slit-Robo evolutionary and system-dependent diversity, receptor-ligand interactions, signaling crosstalk and receptor activation.

New congenital anomalies of the kidney and urinary tract and outcomes in Robo2 mutant mice with the inserted piggyBac transposon

BACKGROUND

Disruption of ROBO2 in humans causes vesicoureteral reflux (VUR)/congenital anomalies of the kidney and urinary tract (CAKUT). PiggyBac (PB) is a DNA transposon, and its insertion often reduces-but does not eliminate-gene expression. The Robo2 insertion mutant exhibited non-dilating VUR, ureteropelvic junction obstruction (UPJO) not found in reported models. We studied the incidence and outcomes of VUR/CAKUT in this mutant and explored the relationship between Robo2 gene expression and the occurrence and severity of VUR/CAKUT.

METHODS

The urinary systems of newborn mutants were evaluated via Vevo 770 micro-ultrasound. Some of the normal animals-and all of the abnormal animals-were followed to adulthood and tested for VUR. Urinary obstruction experiments were performed on mice with hydronephrosis. The histology of the kidney and ureter was examined by light microscopy and transmission electron microscopy. Robo2 (PB/PB) mice were crossed with Hoxb7/myr-Venus mice to visualize the location of the ureters relative to the bladder.

RESULTS

In Robo2 (PB/PB) mice, PB insertion led to an approximately 50 % decrease in Robo2 gene expression. The most common (27.07 %, 62/229) abnormality was non-dilating VUR, and no statistically significant differences were found between age groups. Approximately 6.97 % displayed ultrasound-detectable CAKUT, and these mice survived to adulthood without improvement. No severe CAKUT were found in Robo2 (PB/+) mice. The refluxing ureters showed disorganized smooth muscle fibers, reduced muscle cell populations, intercellular edema and intracytoplasmic vacuoles in smooth muscle cells. Both UPJ and UVJ muscle defects were noted in Robo2 (PB/PB) mice.

CONCLUSIONS

Robo2 (PB/PB) mice is the first Robo2-deficient mouse model to survive to adulthood while displaying non-dilating VUR, UPJO, and multiple ureters with blind endings. The genetic background of these mutants may influence the penetrance and severity of the CAKUT phenotypes. VUR and other CAKUT found in this mutant had little chance of spontaneous resolution, and this requires careful follow-up. We reported for the first time that the non-dilated refluxing ureters showed disorganized smooth muscle fibers and altered smooth muscle cell structure, more accurately mimicking the characteristics of human cases. Future studies are required to test the role of Robo2 in the ureteric smooth muscle.

A requirement for filopodia extension toward Slit during Robo-mediated axon repulsion

Axons navigate long distances through complex 3D environments to interconnect the nervous system during development. Although the precise spatiotemporal effects of most axon guidance cues remain poorly characterized, a prevailing model posits that attractive guidance cues stimulate actin polymerization in neuronal growth cones whereas repulsive cues induce actin disassembly. Contrary to this model, we find that the repulsive guidance cue Slit stimulates the formation and elongation of actin-based filopodia from mouse dorsal root ganglion growth cones. Surprisingly, filopodia form and elongate toward sources of Slit, a response that we find is required for subsequent axonal repulsion away from Slit. Mechanistically, Slit evokes changes in filopodium dynamics by increasing direct binding of its receptor, Robo, to members of the actin-regulatory Ena/VASP family. Perturbing filopodium dynamics pharmacologically or genetically disrupts Slit-mediated repulsion and produces severe axon guidance defects in vivo. Thus, Slit locally stimulates directional filopodial extension, a process that is required for subsequent axonal repulsion downstream of the Robo receptor.

Slit holds a strange attraction for filopodia

The guidance cue Slit induces axonal repulsion by directing the extension of growth cone filopodia.

Cell-Autonomous and Non-cell-autonomous Function of Hox Genes Specify Segmental Neuroblast Identity in the Gnathal Region of the Embryonic CNS in Drosophila

During central nervous system (CNS) development neural stem cells (Neuroblasts, NBs) have to acquire an identity appropriate to their location. In thoracic and abdominal segments of Drosophila, the expression pattern of Bithorax-Complex Hox genes is known to specify the segmental identity of NBs prior to their delamination from the neuroectoderm. Compared to the thoracic, ground state segmental units in the head region are derived to different degrees, and the precise mechanism of segmental specification of NBs in this region is still unclear. We identified and characterized a set of serially homologous NB-lineages in the gnathal segments and used one of them (NB6-4 lineage) as a model to investigate the mechanism conferring segment-specific identities to gnathal NBs. We show that NB6-4 is primarily determined by the cell-autonomous function of the Hox gene Deformed (Dfd). Interestingly, however, it also requires a non-cell-autonomous function of labial and Antennapedia that are expressed in adjacent anterior or posterior compartments. We identify the secreted molecule Amalgam (Ama) as a downstream target of the Antennapedia-Complex Hox genes labial, Dfd, Sex combs reduced and Antennapedia. In conjunction with its receptor Neurotactin (Nrt) and the effector kinase Abelson tyrosine kinase (Abl), Ama is necessary in parallel to the cell-autonomous Dfd pathway for the correct specification of the maxillary identity of NB6-4. Both pathways repress CyclinE (CycE) and loss of function of either of these pathways leads to a partial transformation (40%), whereas simultaneous mutation of both pathways leads to a complete transformation (100%) of NB6-4 segmental identity. Finally, we provide genetic evidences, that the Ama-Nrt-Abl-pathway regulates CycE expression by altering the function of the Hippo effector Yorkie in embryonic NBs. The disclosure of a non-cell-autonomous influence of Hox genes on neural stem cells provides new insight into the process of segmental patterning in the developing CNS.

The central nervous system (CNS) needs to be subdivided into functionally specified regions. In the developing CNS of Drosophila, each neural stem cell, called neuroblasts (NB), acquires a unique identity according to its anterior-posterior and dorso-ventral position to generate a specific cell lineage. Along the anterior-posterior body axis, Hox genes of the Bithorax-Complex convey segmental identities to NBs in the trunk segments. In the derived gnathal and brain segments, the mechanisms specifying segmental NB identities are largely unknown. We investigated the role of Hox genes of the Antennapedia-Complex in the gnathal CNS. In addition to cell-autonomous Hox gene function, we unexpectedly uncovered a parallel non-cell-autonomous pathway in mediating segmental specification of embryonic NBs in gnathal segments. Both pathways restrict the expression of the cell cycle gene CyclinE, ensuring the proper specification of a glial cell lineage. Whereas the Hox gene Deformed mediates this cell-autonomously, labial and Antennapedia influence the identity via transcriptional regulation of the secreted molecule Amalgam (and its downstream pathway) in a non-cell-autonomous manner. These findings shed new light on the role of the highly conserved Hox genes during segmental patterning of neural stem cells in the CNS.

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.

Suppression of Slit2/Robo1 mediated HUVEC migration by Robo4

Slit proteins and their receptors, the Roundabout (Robo) family, are known to have a pivotal role in the vascular system. Slit2/Robo1 regulates the migration of human umbilical vein endothelial cells (HUVECs) and tumor-associated endothelial cells. Robo4, the endothelial-specific Robo, is also considered to be involved in vascular cell migration. However, the Slit/Robo signaling pathway is still unclear. Using a Boyden chamber assay, we found that Slit2 induces the migration of HUVECs under a Robo4 knockdown condition. This effect disappeared in Robo1 knockdown cells. The co-existence of the N-terminal extracellular portion of Robo1 blocked the Slit2-evoked migration of HUVECs, while that of Robo4 caused no effect. These results show that the Slit2 signal is transduced through Robo1, while the negative regulation of Robo4 is an intracellular event. Targeted proteomics using an anti-Robo1 monoclonal antibody identified CdGAP, an adhesion-localized Rac1-and Cdc42-specific GTPase activating protein, as a candidate for Slit2/Robo1 signaling. Robo1 and CdGAP were co-immunoprecipitated from CHO cells co-transfected with Robo1 and CdGAP genes. These results suggest that Slit2/Robo1 binding exerts an effect on cell migration, which is negatively regulated by Robo4, and Robo1 may function by interacting with CdGAP in HUVECs.

The emerging role of Slit-Robo pathway in gastric and other gastro intestinal cancers

Gastric cancer remains one of the most common cancers worldwide and one of the leading cause for cancer-related deaths. Due to the high frequency of metastasis, it is still one of the most lethal malignancies in which kinds of signaling pathways are involved in. The Roundabout (ROBO) receptors and their secreted SLIT glycoprotein ligands, which were originally identified as important axon guidance molecules, have implication in the regulation of neurons and glia, leukocytes, and endothelial cells migration. Recent researches also put high emphasis on the important roles of the Slit-Robo pathway in tumorigenesis, cancer progression and metastasis. Herein we provide a comprehensive review on the role of these molecules and their associated signaling pathway in gastric and other gastrointestinal cancers. Improved knowledge of the Slit-Robo signaling pathway in gastric carcinoma will be useful for deep understanding the mechanisms of tumor development and identifying ideal targets of anticancer therapy in gastric carcinoma.

Neuritic complexity of hippocampal neurons depends on WIP-mediated mTORC1 and Abl family kinases activities

INTRODUCTION

Neuronal morphogenesis is governed mainly by two interconnected processes, cytoskeletal reorganization, and signal transduction. The actin-binding molecule WIP (Wiskott-Aldrich syndrome protein [WASP]-interacting protein) was identified as a negative regulator of neuritogenesis. Although WIP controls activity of the actin-nucleation-promoting factor neural WASP (N-WASP) during neuritic differentiation, its implication in signal transduction remains unknown.

METHODS

Using primary neurons from WIP-deficient and wild-type mice we did an immunofluorescence, morphometric, and biochemical analysis of the signaling modified by WIP deficiency.

RESULTS

Here, we describe the WIP contribution to the regulation of neuritic elaboration and ramification through modification in phosphorylation levels of several kinases that participate in the mammalian target of rapamycin complex 1 (mTORC1)-p70S6K (phosphoprotein 70 ribosomal protein S6 kinase, S6K) intracellular signaling pathway. WIP deficiency induces an increase in the number of neuritic bifurcations and filopodial protrusions in primary embryonic neurons. This phenotype is not due to modifications in the activity of the phosphoinositide 3 kinase (PI3K)-Akt pathway, but to reduced phosphorylation of the S6K residues Ser(411) and Thr(389). The resulting decrease in kinase activity leads to reduced S6 phosphorylation in the absence of WIP. Incubation of control neurons with pharmacological inhibitors of mTORC1 or Abl, two S6K regulators, conferred a morphology resembling that of WIP-deficient neurons. Moreover, the preferential co-distribution of phospho-S6K with polymerized actin is altered in WIP-deficient neurons.

CONCLUSION

These experiments identify WIP as a member of a signaling cascade comprised of Abl family kinases, mTORC1 and S6K, which regulates neuron development and specifically, neuritic branching and complexity. Thus, we postulated a new role for WIP protein.

Slit Binding via the Ig1 Domain Is Essential for Midline Repulsion by Drosophila Robo1 but Dispensable for Receptor Expression, Localization, and Regulation in Vivo

The midline repellant ligand Slit and its Roundabout (Robo) family receptors constitute the major midline repulsive pathway in bilaterians. Slit proteins produced at the midline of the central nervous system (CNS) signal through Robo receptors expressed on axons to prevent them from crossing the midline, and thus regulate connectivity between the two sides of the nervous system. Biochemical structure and interaction studies support a model in which Slit binding to the first immunoglobulin-like (Ig1) domain of Robo receptors activates a repulsive signaling pathway in axonal growth cones. Here, we examine the in vivo functional importance of the Ig1 domain of the Drosophila Robo1 receptor, which controls midline crossing of axons in response to Slit during development of the embryonic CNS. We show that deleting Ig1 from Robo1 disrupts Slit binding in cultured Drosophila cells, and that a Robo1 variant lacking Ig1 (Robo1(∆Ig1)) is unable to promote ectopic midline repulsion in gain-of-function studies in the Drosophila embryonic CNS. We show that the Ig1 domain is not required for proper expression, axonal localization, or Commissureless (Comm)-dependent regulation of Robo1 in vivo, and we use a genetic rescue assay to show that Robo1(∆Ig1) is unable to substitute for full-length Robo1 to properly regulate midline crossing of axons. These results establish a direct link between in vitro biochemical studies of Slit-Robo interactions and in vivo genetic studies of Slit-Robo signaling during midline axon guidance, and distinguish Slit-dependent from Slit-independent aspects of Robo1 expression, regulation, and activity during embryonic development.

Slit-Dependent Endocytic Trafficking of the Robo Receptor Is Required for Son of Sevenless Recruitment and Midline Axon Repulsion

Understanding how axon guidance receptors are activated by their extracellular ligands to regulate growth cone motility is critical to learning how proper wiring is established during development. Roundabout (Robo) is one such guidance receptor that mediates repulsion from its ligand Slit in both invertebrates and vertebrates. Here we show that endocytic trafficking of the Robo receptor in response to Slit-binding is necessary for its repulsive signaling output. Dose-dependent genetic interactions and in vitro Robo activation assays support a role for Clathrin-dependent endocytosis, and entry into both the early and late endosomes as positive regulators of Slit-Robo signaling. We identify two conserved motifs in Robo's cytoplasmic domain that are required for its Clathrin-dependent endocytosis and activation in vitro; gain of function and genetic rescue experiments provide strong evidence that these trafficking events are required for Robo repulsive guidance activity in vivo. Our data support a model in which Robo's ligand-dependent internalization from the cell surface to the late endosome is essential for receptor activation and proper repulsive guidance at the midline by allowing recruitment of the downstream effector Son of Sevenless in a spatially constrained endocytic trafficking compartment.

Lamellipodin promotes actin assembly by clustering Ena/VASP proteins and tethering them to actin filaments

Enabled/Vasodilator (Ena/VASP) proteins promote actin filament assembly at multiple locations, including: leading edge membranes, focal adhesions, and the surface of intracellular pathogens. One important Ena/VASP regulator is the mig-10/Lamellipodin/RIAM family of adaptors that promote lamellipod formation in fibroblasts and drive neurite outgrowth and axon guidance in neurons. To better understand how MRL proteins promote actin network formation we studied the interactions between Lamellipodin (Lpd), actin, and VASP, both in vivo and in vitro. We find that Lpd binds directly to actin filaments and that this interaction regulates its subcellular localization and enhances its effect on VASP polymerase activity. We propose that Lpd delivers Ena/VASP proteins to growing barbed ends and increases their polymerase activity by tethering them to filaments. This interaction represents one more pathway by which growing actin filaments produce positive feedback to control localization and activity of proteins that regulate their assembly.

Robo2 acts in trans to inhibit Slit-Robo1 repulsion in pre-crossing commissural axons

During nervous system development, commissural axons cross the midline despite the presence of repellant ligands. In Drosophila, commissural axons avoid premature responsiveness to the midline repellant Slit by expressing the endosomal sorting receptor Commissureless, which reduces surface expression of the Slit receptor Roundabout1 (Robo1). In this study, we describe a distinct mechanism to inhibit Robo1 repulsion and promote midline crossing, in which Roundabout2 (Robo2) binds to and prevents Robo1 signaling. Unexpectedly, we find that Robo2 is expressed in midline cells during the early stages of commissural axon guidance, and that over-expression of Robo2 can rescue robo2-dependent midline crossing defects non-cell autonomously. We show that the extracellular domains required for binding to Robo1 are also required for Robo2's ability to promote midline crossing, in both gain-of-function and rescue assays. These findings indicate that at least two independent mechanisms to overcome Slit-Robo1 repulsion in pre-crossing commissural axons have evolved in Drosophila.

TIPsy tour guides: how microtubule plus-end tracking proteins (+TIPs) facilitate axon guidance

The growth cone is a dynamic cytoskeletal vehicle, which drives the end of a developing axon. It serves to interpret and navigate through the complex landscape and guidance cues of the early nervous system. The growth cone’s distinctive cytoskeletal organization offers a fascinating platform to study how extracellular cues can be translated into mechanical outgrowth and turning behaviors. While many studies of cell motility highlight the importance of actin networks in signaling, adhesion, and propulsion, both seminal and emerging works in the field have highlighted a unique and necessary role for microtubules (MTs) in growth cone navigation. Here, we focus on the role of singular pioneer MTs, which extend into the growth cone periphery and are regulated by a diverse family of microtubule plus-end tracking proteins (+TIPs). These +TIPs accumulate at the dynamic ends of MTs, where they are well-positioned to encounter and respond to key signaling events downstream of guidance receptors, catalyzing immediate changes in microtubule stability and actin cross-talk, that facilitate both axonal outgrowth and turning events.

USP33 mediates Slit-Robo signaling in inhibiting colorectal cancer cell migration

Originally discovered in neuronal guidance, the Slit-Robo pathway is emerging as an important player in human cancers. However, its involvement and mechanism in colorectal cancer (CRC) remains to be elucidated. Here, we report that Slit2 expression is reduced in CRC tissues compared with adjacent noncancerous tissues. Extensive promoter hypermethylation of the Slit2 gene has been observed in CRC cells, which provides a mechanistic explanation for the Slit2 downregulation in CRC. Functional studies showed that Slit2 inhibits CRC cell migration in a Robo-dependent manner. Robo-interacting ubiquitin-specific protease 33 (USP33) is required for the inhibitory function of Slit2 on CRC cell migration by deubiquitinating and stabilizing Robo1. USP33 expression is downregulated in CRC samples, and reduced USP33 mRNA levels are correlated with increased tumor grade, lymph node metastasis and poor patient survival. Taken together, our data reveal USP33 as a previously unknown tumor-suppressing gene for CRC by mediating the inhibitory function of Slit-Robo signaling on CRC cell migration. Our work suggests the potential value of USP33 as an independent prognostic marker of CRC.

Linking cell surface receptors to microtubules: tubulin folding cofactor D mediates Dscam functions during neuronal morphogenesis

Formation of functional neural networks requires the coordination of cell surface receptors and downstream signaling cascades, which eventually leads to dynamic remodeling of the cytoskeleton. Although a number of guidance receptors affecting actin cytoskeleton remodeling have been identified, it is relatively unknown how microtubule dynamics are regulated by guidance receptors. We used Drosophila olfactory projection neurons to study the molecular mechanisms of neuronal morphogenesis. Dendrites of each projection neuron target a single glomerulus of ∼50 glomeruli in the antennal lobe, and the axons show stereotypical pattern of terminal arborization. In the course of genetic analysis of the dachsous mutant allele (ds(UAO71)), we identified a mutation in the tubulin folding cofactor D gene (TBCD) as a background mutation. TBCD is one of five tubulin-folding cofactors required for the formation of α- and β-tubulin heterodimers. Single-cell clones of projection neurons homozygous for the TBCD mutation displayed disruption of microtubules, resulting in ectopic arborization of dendrites, and axon degeneration. Interestingly, overexpression of TBCD also resulted in microtubule disruption and ectopic dendrite arborization, suggesting that an optimum level of TBCD is crucial for in vivo neuronal morphogenesis. We further found that TBCD physically interacts with the intracellular domain of Down syndrome cell adhesion molecule (Dscam), which is important for neural development and has been implicated in Down syndrome. Genetic analyses revealed that TBCD cooperates with Dscam in vivo. Our study may offer new insights into the molecular mechanism underlying the altered neural networks in cognitive disabilities of Down syndrome.

Slit2-Robo signaling in inflammation and kidney injury

Acute kidney injury is an increasingly common global health problem and is associated with severe morbidity and mortality. In addition to facing high mortality rates, the survivors of acute kidney injury are at increased risk of developing chronic kidney disease and end-stage renal disease. Renal ischemia-reperfusion injury (IRI) is the most common cause of acute kidney injury, and results from impaired delivery of oxygen and nutrients to the kidney. Massive leukocyte influx into the post-ischemic kidney is one of the hallmarks of IRI. The recruited leukocytes exacerbate tissue damage and, if uncontrolled, initiate the progressive changes that lead to renal fibrosis and chronic kidney disease. Early on, recruitment and activation of platelets promotes microthrombosis in the injured kidney, further exacerbating kidney damage. The diversity, complexity, and multiplicity of pathways involved in leukocyte recruitment and platelet activation make it extremely challenging to control these processes, and past efforts have met with limited success in human trials. A generalized strategy to inhibit infiltration of inflammatory leukocytes and platelets, thereby reducing inflammation and injury, may prove to be more beneficial. In this review, we summarize recent findings demonstrating that the neuronal guidance cues, Slit and Roundabout (Robo), prevent the migration of multiple leukocyte subsets towards diverse inflammatory chemoattractants, and have potent anti-platelet functions in vitro and in vivo. These properties uniquely position Slit2 as a novel therapeutic that could be used to prevent acute kidney injury associated with IRI.

Coordinate regulation of stem cell competition by Slit-Robo and JAK-STAT signaling in the Drosophila testis

Stem cells in tissues reside in and receive signals from local microenvironments called niches. Understanding how multiple signals within niches integrate to control stem cell function is challenging. The Drosophila testis stem cell niche consists of somatic hub cells that maintain both germline stem cells and somatic cyst stem cells (CySCs). Here, we show a role for the axon guidance pathway Slit-Roundabout (Robo) in the testis niche. The ligand Slit is expressed specifically in hub cells while its receptor, Roundabout 2 (Robo2), is required in CySCs in order for them to compete for occupancy in the niche. CySCs also require the Slit-Robo effector Abelson tyrosine kinase (Abl) to prevent over-adhesion of CySCs to the niche, and CySCs mutant for Abl outcompete wild type CySCs for niche occupancy. Both Robo2 and Abl phenotypes can be rescued through modulation of adherens junction components, suggesting that the two work together to balance CySC adhesion levels. Interestingly, expression of Robo2 requires JAK-STAT signaling, an important maintenance pathway for both germline and cyst stem cells in the testis. Our work indicates that Slit-Robo signaling affects stem cell function downstream of the JAK-STAT pathway by controlling the ability of stem cells to compete for occupancy in their niche.

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.

USP33, a new player in lung cancer, mediates Slit-Robo signaling

Ubiquitin specific protease 33 (USP33) is a multifunctional protein regulating diverse cellular processes. The expression and role of USP33 in lung cancer remain unexplored. In this study, we show that USP33 is down-regulated in multiple cohorts of lung cancer patients and that low expression of USP33 is associated with poor prognosis. USP33 mediates Slit-Robo signaling in lung cancer cell migration. Downregulation of USP33 reduces the protein stability of Robo1 in lung cancer cells, providing a previously unknown mechanism for USP33 function in mediating Slit activity in lung cancer cells. Taken together, USP33 is a new player in lung cancer that regulates Slit-Robo signaling. Our data suggest that USP33 may be a candidate tumor suppressor for lung cancer with potential as a prognostic marker.

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

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

Abelson phosphorylation of CLASP2 modulates its association with microtubules and actin

The Abelson (Abl) non-receptor tyrosine kinase regulates the cytoskeleton during multiple stages of neural development, from neurulation, to the articulation of axons and dendrites, to synapse formation and maintenance. We previously showed that Abl is genetically linked to the microtubule (MT) plus end tracking protein (+TIP) CLASP in Drosophila. Here we show in vertebrate cells that Abl binds to CLASP and phosphorylates it in response to serum or PDGF stimulation. In vitro, Abl phosphorylates CLASP with a Km of 1.89 µM, indicating that CLASP is a bona fide substrate. Abl-phosphorylated tyrosine residues that we detect in CLASP by mass spectrometry lie within previously mapped F-actin and MT plus end interaction domains. Using purified proteins, we find that Abl phosphorylation modulates direct binding between purified CLASP2 with both MTs and actin. Consistent with these observations, Abl-induced phosphorylation of CLASP2 modulates its localization as well as the distribution of F-actin structures in spinal cord growth cones. Our data suggest that the functional relationship between Abl and CLASP2 is conserved and provides a means to control the CLASP2 association with the cytoskeleton. © 2014 The Authors. Cytoskeleton Published by Wiley Periodicals, Inc.

Beyond pathology: APP, brain development and Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia among the elderly. Research in the AD field has been mostly focused on the biology of the Aβ peptide but increasing evidence is shifting attention toward the physiological role of APP as key to understanding AD pathology. It is becoming apparent that APP plays a central role in the mechanisms that guarantee the accuracy and the robustness of brain wiring. In the present review we explore APP functions with focus on some of the underlying molecular mechanisms.

Ena/VASP Enabled is a highly processive actin polymerase tailored to self-assemble parallel-bundled F-actin networks with Fascin

Filopodia are exploratory finger-like projections composed of multiple long, straight, parallel-bundled actin filaments that protrude from the leading edge of migrating cells. Drosophila melanogaster Enabled (Ena) is a member of the Ena/vasodilator-stimulated phosphoprotein protein family, which facilitates the assembly of filopodial actin filaments that are bundled by Fascin. However, the mechanism by which Ena and Fascin promote the assembly of uniformly thick F-actin bundles that are capable of producing coordinated protrusive forces without buckling is not well understood. We used multicolor evanescent wave fluorescence microscopy imaging to follow individual Ena molecules on both single and Fascin-bundled F-actin in vitro. Individual Ena tetramers increase the elongation rate approximately two- to threefold and inhibit capping protein by remaining processively associated with the barbed end for an average of ∼10 s in solution, for ∼60 s when immobilized on a surface, and for ∼110 s when multiple Ena tetramers are clustered on a surface. Ena also can gather and simultaneously elongate multiple barbed ends. Collectively, these properties could facilitate the recruitment of Fascin and initiate filopodia formation. Remarkably, we found that Ena's actin-assembly properties are tunable on Fascin-bundled filaments, facilitating the formation of filopodia-like F-actin networks without tapered barbed ends. Ena-associated trailing barbed ends in Fascin-bundled actin filaments have approximately twofold more frequent and approximately fivefold longer processive runs, allowing them to catch up with leading barbed ends efficiently. Therefore, Fascin and Ena cooperate to extend and maintain robust filopodia of uniform thickness with aligned barbed ends by a unique mechanistic cycle.

Cell-intrinsic drivers of dendrite morphogenesis

The proper formation and morphogenesis of dendrites is fundamental to the establishment of neural circuits in the brain. Following cell cycle exit and migration, neurons undergo organized stages of dendrite morphogenesis, which include dendritic arbor growth and elaboration followed by retraction and pruning. Although these developmental stages were characterized over a century ago, molecular regulators of dendrite morphogenesis have only recently been defined. In particular, studies in Drosophila and mammalian neurons have identified numerous cell-intrinsic drivers of dendrite morphogenesis that include transcriptional regulators, cytoskeletal and motor proteins, secretory and endocytic pathways, cell cycle-regulated ubiquitin ligases, and components of other signaling cascades. Here, we review cell-intrinsic drivers of dendrite patterning and discuss how the characterization of such crucial regulators advances our understanding of normal brain development and pathogenesis of diverse cognitive disorders.

Roundabout receptors

Roundabout receptors (Robo) and their Slit ligands were discovered in the 1990s and found to be key players in axon guidance. Slit was initially described s an extracellular matrix protein that was expressed by midline glia in Drosophila. A few years later, it was shown that, in vertebrates and invertebrates, Slits acted as chemorepellents for axons crossing the midline. Robo proteins were originally discovered in Drosophila in a mutant screen for genes involved in the regulation of midline crossing. This ligand-receptor pair has since been implicated in a variety of other neuronal and non-neuronal processes ranging from cell migration to angiogenesis, tumourigenesis and even organogenesis of tissues such as kidneys, lungs and breasts.

Interactions between a receptor tyrosine phosphatase and a cell surface ligand regulate axon guidance and glial-neuronal communication

We developed a screening method for orphan receptor ligands, in which cell-surface proteins are expressed in Drosophila embryos from GAL4-dependent insertion lines and ligand candidates identified by the presence of ectopic staining with receptor fusion proteins. Stranded at second (Sas) binds to the receptor tyrosine phosphatase Ptp10D in embryos and in vitro. Sas and Ptp10D can interact in trans when expressed in cultured cells. Interactions between Sas and Ptp10D on longitudinal axons are required to prevent them from abnormally crossing the midline. Sas is expressed on both neurons and glia, whereas Ptp10D is restricted to CNS axons. We conducted epistasis experiments by overexpressing Sas in glia and examining how the resulting phenotypes are changed by removal of Ptp10D from neurons. We find that neuronal Ptp10D restrains signaling by overexpressed glial Sas, which would otherwise produce strong glial and axonal phenotypes.

Cardiac overexpression of Mammalian enabled (Mena) exacerbates heart failure in mice

Mammalian enabled (Mena) is a key regulator of cytoskeletal actin dynamics, which has been implicated in heart failure (HF). We have previously demonstrated that cardiac Mena deletion produced cardiac dysfunction with conduction abnormalities and hypertrophy. Moreover, elevated Mena expression correlates with HF in human and animal models, yet the precise role of Mena in cardiac pathophysiology is unclear. In these studies, we evaluated mice with cardiac myocyte-specific Mena overexpression (TTA/TgTetMena) comparable to that observed in cardiac pathology. We found that the hearts of TTA/TgTetMena mice were functionally and morphologically comparable to wild-type littermates, except for mildly increased heart mass in the transgenic mice. Interestingly, TTA/TgTetMena mice were particularly susceptible to cardiac injury, as these animals experienced pronounced decreases in ejection fraction and fractional shortening as well as heart dilatation and hypertrophy after transverse aortic constriction (TAC). By "turning off" Mena overexpression in TTA/TgTetMena mice either immediately prior to or immediately after TAC surgery, we discovered that normalizing Mena levels eliminated cardiac hypertrophy in TTA/TgTetMena animals but did not preclude post-TAC cardiac functional deterioration. These findings indicate that hearts with increased levels of Mena fare worse when subjected to cardiac injury and suggest that Mena contributes to HF pathophysiology.