enabled, a dosage-sensitive suppressor of mutations in the Drosophila Abl tyrosine kinase, encodes an Abl substrate with SH3 domain-binding properties

A quantitative method to analyze Drosophila pupal eye patterning

BACKGROUND

The Drosophila pupal eye has become a popular paradigm for understanding morphogenesis and tissue patterning. Correct rearrangement of cells between ommatidia is required to organize the ommatidial array across the eye field. This requires cell movement, cell death, changes to cell-cell adhesion, signaling and fate specification.

METHODOLOGY

We describe a method to quantitatively assess mis-patterning of the Drosophila pupal eye and objectively calculate a 'mis-patterning score' characteristic of a specific genotype. This entails step-by-step scoring of specific traits observed in pupal eyes dissected 40-42 hours after puparium formation and subsequent statistical analysis of this data.

SIGNIFICANCE

This method provides an unbiased quantitative score of mis-patterning severity that can be used to compare the impact of different genetic mutations on tissue patterning.

Abi plays an opposing role to Abl in Drosophila axonogenesis and synaptogenesis

Abl tyrosine kinase (Abl) regulates axon guidance by modulating actin dynamics. Abelson interacting protein (Abi), originally identified as a kinase substrate of Abl, also plays a key role in actin dynamics, yet its role with respect to Abl in the developing nervous system remains unclear. Here we show that mutations in abi disrupt axonal patterning in the developing Drosophila central nervous system (CNS). However, reducing abi gene dosage by half substantially rescues Abl mutant phenotypes in pupal lethality, axonal guidance defects and locomotion deficits. Moreover, we show that mutations in Abl increase synaptic growth and spontaneous synaptic transmission frequency at the neuromuscular junction. Double heterozygosity for abi and enabled (ena) also suppresses the synaptic overgrowth phenotypes of Abl mutants, suggesting that Abi acts cooperatively with Ena to antagonize Abl function in synaptogenesis. Intriguingly, overexpressing Abi or Ena alone in cultured cells dramatically redistributed peripheral F-actin to the cytoplasm, with aggregates colocalizing with Abi and/or Ena, and resulted in a reduction in neurite extension. However, co-expressing Abl with Abi or Ena redistributed cytoplasmic F-actin back to the cell periphery and restored bipolar cell morphology. These data suggest that abi and Abl have an antagonistic interaction in Drosophila axonogenesis and synaptogenesis, which possibly occurs through the modulation of F-actin reorganization.

Enabled and Capping protein play important roles in shaping cell behavior during Drosophila oogenesis

During development, cells craft an impressive array of actin-based structures, mediating events as diverse as cytokinesis, apical constriction, and cell migration. One challenge is to determine how cells regulate actin assembly and disassembly to carry out these cell behaviors. During Drosophila oogenesis diverse cell behaviors are seen in the soma and germline. We used oogenesis to explore developmental roles of two important actin regulators: Enabled/VASP proteins and Capping protein. We found that Enabled plays an important role in cortical integrity of nurse cells, formation of robust bundled actin filaments in late nurse cells that facilitate nurse cell dumping, and migration of somatic border cells. During nurse cell dumping, Enabled localizes to barbed ends of the nurse cell actin filaments, suggesting its mechanism of action. We further pursued this mechanism using mutant Enabled proteins, each affecting one of its protein domains. These data suggest critical roles for the EVH2 domain and its tetramerization subdomain, while the EVH1 domain appears less critical. Enabled appears to be negatively regulated during oogenesis by Abelson kinase. We also explored the function of Capping protein. This revealed important roles in oocyte determination, nurse cell cortical integrity and nurse cell dumping, and support the idea that Capping protein and Enabled act antagonistically during dumping. Together these data reveal places that these actin regulators shape oogenesis.

C. elegans CARMIL negatively regulates UNC-73/Trio function during neuronal development

Whereas many molecules that promote cell and axonal growth cone migrations have been identified, few are known to inhibit these processes. In genetic screens designed to identify molecules that negatively regulate such migrations, we identified CRML-1, the C. elegans homolog of CARMIL. Although mammalian CARMIL acts to promote the migration of glioblastoma cells, we found that CRML-1 acts as a negative regulator of neuronal cell and axon growth cone migrations. Genetic evidence indicates that CRML-1 regulates these migrations by inhibiting the Rac GEF activity of UNC-73, a homolog of the Rac and Rho GEF Trio. The antagonistic effects of CRML-1 and UNC-73 can control the direction of growth cone migration by regulating the levels of the SAX-3 (a Robo homolog) guidance receptor. Consistent with the hypothesis that CRML-1 negatively regulates UNC-73 activity, these two proteins form a complex in vivo. Based on these observations, we propose a role for CRML-1 as a novel regulator of cell and axon migrations that acts through inhibition of Rac signaling.

The cytoskeleton-associated Ena/VASP proteins are unanticipated partners of the PMR1 mRNA endonuclease

The PMR1 mRNA endonuclease catalyzes the selective decay of a limited number of mRNAs. It participates in multiple complexes, including one containing c-Src, its activating kinase, and one containing its substrate mRNA. This study used tandem affinity purification (TAP) chromatography to identify proteins in HeLa cell S100 associated with the mature 60-kDa form of Xenopus PMR1 (xPMR60). Unexpectedly, this identified a number of cytoskeleton-associated proteins, most notably the Ena family proteins mammalian Enabled (Mena) and vasodilator-stimulated phosphoprotein (VASP). These are regulators of actin dynamics that distribute throughout the cytoplasm and concentrate along the leading edge of the cell. xPMR60 interacts with Mena and VASP in vivo, overexpression of Mena has no impact on mRNA decay, and Mena and VASP are recovered together with xPMR60 in each of the major complexes of PMR1-mRNA decay. In a wound-healing experiment induced expression of active xPMR60 in stably transfected cells resulted in a twofold increase in cell motility compared with uninduced cells or cells expressing inactive xPMR60 degrees . Under these conditions xPMR60 colocalizes with VASP along one edge of the cell.

Diversity of polyproline recognition by EVH1 domains

Enabled/VASP Homology-1 (EVH1) domains function primarily as interaction modules that link signaling proteins by binding to proline-rich sequences. EVH1 domains are ~115 residues in length and adopt the pleckstrin homology (PH) fold. Four different protein families contain EVH1 domains: Ena/VASP, Homer, WASP and SPRED. Except for the SPRED domains, for which no binding partners are known, EVH1 domains use a conserved hydrophobic cleft to bind a four-residue motif containing 2-4 prolines. Conserved aromatic residues, including an invariant tryptophan, create a wedge-shaped groove on the EVH1 surface that matches the triangular profile of a polyproline type II helix. Hydrophobic residues adjacent to the polyproline motif dock into complementary sites on the EVH1 domain to enhance ligand binding specificity. Pseudosymmetry in the polyproline type II helix allows peptide ligands to bind in either of two N-to-C terminal orientations, depending on interactions between sequences flanking the prolines and the EVH1 domain. EVH1 domains also recognize non-proline motifs, as illustrated by the structure of an EVH1:LIM3 complex and the extended EVH1 ligands of the verprolin family.

Characterization of EVL-I as a protein kinase D substrate

EVL-I is a splice variant of EVL (Ena/VASP like protein), whose in vivo function and regulation are still poorly understood. We found that Protein Kinase D (PKD) interacts in vitro and in vivo with EVL-I and phosphorylates EVL-I in a 21 amino acid alternately-included insert in the EVH2 domain. Following knockdown of the capping protein CPbeta and spreading on laminin, phosphorylated EVL-I can support filopodia formation and the phosphorylated EVL-I is localized at filopodial tips. Furthermore, we found that the lamellipodial localization of EVL-I is unaffected by phosphorylation, but that impairment of EVL-I phosphorylation is associated with ruffling of lamellipodia upon PDBu stimulation. Besides the lamellipodial and filopodial localization of phosphorylated EVL-I in fibroblasts, we determined that EVL-I is hyperphosphorylated and localized in the cell-cell contacts of certain breast cancer cells and mouse embryo keratinocytes. Taken together, our results show that phosphorylated EVL-I is present in lamellipodia, filopodia and cell-cell contacts and suggest the existence of signaling pathways that may affect EVL-I via phosphorylation of its EVH2 domain.

A functional misexpression screen uncovers a role for enabled in progressive neurodegeneration

Drosophila is a well-established model to study the molecular basis of neurodegenerative diseases. We carried out a misexpression screen to identify genes involved in neurodegeneration examining locomotor behavior in young and aged flies. We hypothesized that a progressive loss of rhythmic activity could reveal novel genes involved in neurodegenerative mechanisms. One of the interesting candidates showing progressive arrhythmicity has reduced enabled (ena) levels. ena down-regulation gave rise to progressive vacuolization in specific regions of the adult brain. Abnormal staining of pre-synaptic markers such as cystein string protein (CSP) suggest that axonal transport could underlie the neurodegeneration observed in the mutant. Reduced ena levels correlated with increased apoptosis, which could be rescued in the presence of p35, a general Caspase inhibitor. Thus, this mutant recapitulates two important features of human neurodegenerative diseases, i.e., vulnerability of certain neuronal populations and progressive degeneration, offering a unique scenario in which to unravel the specific mechanisms in an easily tractable organism.

Ena/VASP: proteins at the tip of the nervous system

The emergence of neurites from a symmetrical cell body is an essential feature of nervous system development. Neurites are the precursors of axons and dendrites and are tipped by growth cones, motile structures that guide elongating axons in the developing nervous system. Growth cones steer the axon along a defined path to its appropriate target in response to guidance cues. This navigation involves the dynamic extension and withdrawal of actin-filled finger-like protrusions called filopodia that continuously sample their environment. Ena/VASP proteins, a conserved family of actin-regulatory proteins, are crucial for filopodia formation and function downstream of several guidance cues. Here we review recent findings into Ena/VASP function in neurite initiation, axon outgrowth and guidance.

Enabled plays key roles in embryonic epithelial morphogenesis in Drosophila

Studies in cultured cells and in vitro have identified many actin regulators and begun to define their mechanisms of action. Among these are Enabled (Ena)/VASP proteins, anti-Capping proteins that influence fibroblast migration, growth cone motility, and keratinocyte cell adhesion in vitro. However, partially redundant family members in mammals and maternal Ena contribution in Drosophila previously prevented assessment of the roles of Ena/VASP proteins in embryonic morphogenesis in flies or mammals. We used several approaches to remove maternal and zygotic Ena function, allowing us to address this question. We found that inactivating Ena does not disrupt cell adhesion or epithelial organization, suggesting its role in these processes is cell type-specific. However, Ena plays an important role in many morphogenetic events, including germband retraction, segmental groove retraction and head involution, whereas it is dispensable for other morphogenetic movements. We focused on dorsal closure, analyzing mechanisms by which Ena acts. Ena modulates filopodial number and length, thus influencing the speed of epithelial zippering and the ability of cells to match with correct neighbors. We also explored filopodial regulation in cultured Drosophila cells and embryos. These data provide new insights into developmental and mechanistic roles of this important actin regulator.

Large scale hippocampal cellular distress may explain the behavioral consequences of repetitive traumatic experiences--a proteomic approach

Early life traumatic experiences are associated with psychopathology in adulthood. This may be due in part to the effects of trauma on hippocampal development and protein expression. The purpose of the study was to investigate the effects of early life trauma and adult re-stress on ventral hippocampal protein expression. Adolescent rats (n = 19) were subjected to a triple stressor on post-natal day 28 followed 7 days later by the first re-stress session and 25 days later (post-natal day 60 = adulthood) by the second re-stress session. Ventral hippocampi were collected on post-natal day 68 for protein expression determinations using protein arrays and 2D-gel electrophoresis with liquid chromatography tandem mass spectrometry. Compared to controls, traumatized animals showed an increase in Ca(2+) homeostatic proteins, dysregulated signaling pathways and energy metabolism enzymes, cytoskeletal protein changes, a decrease in neuroplasticity regulators, energy metabolism enzymes and an increase in apoptotic initiator proteins. These results indicate the extensive impact of trauma on adult brain development and behavior.

Role of Ena/VASP proteins in homeostasis and disease

The actin cytoskeleton is required for many important processes during embryonic development. In later stages of life, important homeostatic processes depend on the actin cytoskeleton, such as immune response, haemostasis and blood vessel preservation. Therefore, the function of the actin cytoskeleton must be tightly regulated, and aberrant regulation may cause disease. A growing number of proteins have been described to bind and regulate the actin cytoskeleton. Amongst them, Ena/VASP proteins function as anti-capping proteins, thereby directly modulating the actin ultrastructure. Ena/VASP function is regulated by their recruitment into protein complexes downstream of plasma membrane receptors and by phosphorylation. As regulators of the actin ultrastructure, Ena/VASP proteins are involved in crucial cellular functions, such as shape change, adhesion, migration and cell-cell interaction and hence are important targets for therapeutic intervention. In this chapter, we will first describe the structure, function and regulation of Ena/VASP proteins. Then, we will review the involvement of Ena/VASP proteins in the development of human diseases. Growing evidence links Ena/VASP proteins to important human diseases, such as thrombosis, cancer, arteriosclerosis, cardiomyopathy and nephritis. Finally, present and future perspectives for the development of therapeutic molecules interfering with Ena/VASP-mediated protein-protein interactions are presented.

Molecular separation of two signaling pathways for the receptor, Notch

Notch is required for many aspects of cell fate specification and morphogenesis during development, including neurogenesis and axon guidance. We here provide genetic and biochemical evidence that Notch directs axon growth and guidance in Drosophila via a "non-canonical", i.e. non-Su(H)-mediated, signaling pathway, characterized by association with the adaptor protein, Disabled, and Trio, an accessory factor of the Abl tyrosine kinase. We find that forms of Notch lacking the binding sites for its canonical effector, Su(H), are nearly inactive for the cell fate function of the receptor, but largely or fully active in axon patterning. Conversely, deletion from Notch of the binding site for Disabled impairs its action in axon patterning without disturbing cell fate control. Finally, we show by co-immunoprecipitation that Notch protein is physically associated in vivo with both Disabled and Trio. Together, these data provide evidence for an alternate Notch signaling pathway that mediates a postmitotic, morphogenetic function of the receptor.

The receptor protein tyrosine phosphatase PTP69D antagonizes Abl tyrosine kinase to guide axons in Drosophila

During Drosophila embryogenesis, both the cytoplasmic Abelson tyrosine kinase (Abl) and the membrane bound tyrosine phosphatase PTP69D are required for proper guidance of CNS and motor axons. We provide evidence that PTP69D modulates signaling by Abl and its antagonist, Ena. An Abl loss-of function mutation dominantly suppresses most Ptp69D mutant phenotypes including larval/pupal lethality and CNS and motor axon defects, while increased Abl and decreased Ena expression dramatically increase the expressivity of Ptp69D axonal defects. In contrast, Ptp69D mutations do not affect Abl mutant phenotypes. These results support the hypothesis that PTP69D antagonizes the Abl/Ena genetic pathway, perhaps as an upstream regulator. We also find that mutation of the gene encoding the cytoplasmic Src64B tyrosine kinase exacerbates Ptp69D phenotypes, suggesting that two different cytoplasmic tyrosine kinases, Abl and Src64B, modify PTP69D-mediated axon patterning in quite different ways.

Using Bcr-Abl to examine mechanisms by which abl kinase regulates morphogenesis in Drosophila

Signaling by the nonreceptor tyrosine kinase Abelson (Abl) plays key roles in normal development, whereas its inappropriate activation helps trigger the development of several forms of leukemia. Abl is best known for its roles in axon guidance, but Abl and its relatives also help regulate embryonic morphogenesis in epithelial tissues. Here, we explore the role of regulation of Abl kinase activity during development. We first compare the subcellular localization of Abl protein and of active Abl, by using a phosphospecific antibody, providing a catalog of places where Abl is activated. Next, we explore the consequences for morphogenesis of overexpressing wild-type Abl or expressing the activated form found in leukemia, Bcr-Abl. We find dose-dependent effects of elevating Abl activity on morphogenetic movements such as head involution and dorsal closure, on cell shape changes, on cell protrusive behavior, and on the organization of the actin cytoskeleton. Most of the effects of Abl activation parallel those caused by reduction in function of its target Enabled. Abl activation leads to changes in Enabled phosphorylation and localization, suggesting a mechanism of action. These data provide new insight into how regulated Abl activity helps direct normal development and into possible biological functions of Bcr-Abl.

Structural basis for the recruitment of profilin-actin complexes during filament elongation by Ena/VASP

Cells sustain high rates of actin filament elongation by maintaining a large pool of actin monomers above the critical concentration for polymerization. Profilin-actin complexes constitute the largest fraction of polymerization-competent actin monomers. Filament elongation factors such as Ena/VASP and formin catalyze the transition of profilin-actin from the cellular pool onto the barbed end of growing filaments. The molecular bases of this process are poorly understood. Here we present structural and energetic evidence for two consecutive steps of the elongation mechanism: the recruitment of profilin-actin by the last poly-Pro segment of vasodilator-stimulated phosphoprotein (VASP) and the binding of profilin-actin simultaneously to this poly-Pro and to the G-actin-binding (GAB) domain of VASP. The actin monomer bound at the GAB domain is proposed to be in position to join the barbed end of the growing filament concurrently with the release of profilin.

Ena/VASP function in retinal axons is required for terminal arborization but not pathway navigation

The Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) family of proteins is required for filopodia formation in growth cones and plays a crucial role in guidance cue-induced remodeling of the actin cytoskeleton. In vivo studies with pharmacological inhibitors of actin polymerization have previously provided evidence for the view that filopodia are needed for growth cone navigation in the developing visual pathway. Here we have re-examined this issue using an alternative strategy to generate growth cones without filopodia in vivo by artificially targeting Xena/XVASP (Xenopus homologs of Ena/VASP) proteins to mitochondria in retinal ganglion cells (RGCs). We used the specific binding of the EVH1 domain of the Ena/VASP family of proteins with the ligand motif FP4 to sequester the protein at the mitochondria surface. RGCs with reduced function of Xena/XVASP proteins extended fewer axons out of the eye and possessed dynamic lamellipodial growth cones missing filopodia that advanced slowly in the optic tract. Surprisingly, despite lacking filopodia, the axons navigated along the optic pathway without obvious guidance errors, indicating that the Xena/XVASP family of proteins and filopodial protrusions are non-essential for pathfinding in retinal axons. However, depletion of Xena/XVASP proteins severely impaired the ability of growth cones to form branches within the optic tectum, suggesting that this protein family, and probably filopodia, plays a key role in establishing terminal arborizations.

Spitz from the retina regulates genes transcribed in the second mitotic wave, peripodial epithelium, glia and plasmatocytes of the Drosophila eye imaginal disc

Proliferation, differentiation, and other processes must be coordinated during the development of multi-cellular animals. A discrete and regulated cell division, the Second Mitotic Wave (SMW), occurs concomitantly with early cell fate decisions in the Drosophila developing retina. Signals from the Epidermal Growth Factor Receptor (EGFR) are required to promote cell cycle arrest of specified cells and antagonize S-phase entry in the SMW. Cells that do not receive any EGFR activity enter S-phase in the SMW in response to the Notch pathway. To identify genes with potential roles in the SMW, we used microarrays and genetic manipulation of the EGFR pathway to seek transcripts regulated during the SMW. RNA in situ hybridization of 126 differentially transcribed genes revealed genes that have novel expression patterns in cells closely associated with the SMW. In addition, other genes' transcripts were regulated in the differentiating photoreceptor cells, retinal basal glia, the peripodial epithelium and blood cells (plasmatocytes) associated with the developing retina. These novel targets suggest that during eye development, EGFR activity coordinates transcriptional programs in other tissues with retinal differentiation.

Drosophila as a genetic and cellular model for studies on axonal growth

One of the most fascinating processes during nervous system development is the establishment of stereotypic neuronal networks. An essential step in this process is the outgrowth and precise navigation (pathfinding) of axons and dendrites towards their synaptic partner cells. This phenomenon was first described more than a century ago and, over the past decades, increasing insights have been gained into the cellular and molecular mechanisms regulating neuronal growth and navigation. Progress in this area has been greatly assisted by the use of simple and genetically tractable invertebrate model systems, such as the fruit fly Drosophila melanogaster. This review is dedicated to Drosophila as a genetic and cellular model to study axonal growth and demonstrates how it can and has been used for this research. We describe the various cellular systems of Drosophila used for such studies, insights into axonal growth cones and their cytoskeletal dynamics, and summarise identified molecular signalling pathways required for growth cone navigation, with particular focus on pathfinding decisions in the ventral nerve cord of Drosophila embryos. These Drosophila-specific aspects are viewed in the general context of our current knowledge about neuronal growth.

Ena/VASP proteins have an anti-capping independent function in filopodia formation

Filopodia have been implicated in a number of diverse cellular processes including growth-cone path finding, wound healing, and metastasis. The Ena/VASP family of proteins has emerged as key to filopodia formation but the exact mechanism for how they function has yet to be fully elucidated. Using cell spreading as a model system in combination with small interfering RNA depletion of Capping Protein, we determined that Ena/VASP proteins have a role beyond anticapping activity in filopodia formation. Analysis of mutant Ena/VASP proteins demonstrated that the entire EVH2 domain was the minimal domain required for filopodia formation. Fluorescent recovery after photobleaching data indicate that Ena/VASP proteins rapidly exchange at the leading edge of lamellipodia, whereas virtually no exchange occurred at filopodial tips. Mutation of the G-actin-binding motif (GAB) partially compromised stabilization of Ena/VASP at filopodia tips. These observations led us to propose a model where the EVH2 domain of Ena/VASP induces and maintains clustering of the barbed ends of actin filaments, which putatively corresponds to a transition from lamellipodial to filopodial localization. Furthermore, the EVH1 domain, together with the GAB motif in the EVH2 domain, helps to maintain Ena/VASP at the growing barbed ends.

A macromolecular complex involving the amyloid precursor protein (APP) and the cytosolic adapter FE65 is a negative regulator of axon branching

Several studies suggest a role for the amyloid precursor protein (APP) in neurite outgrowth and synaptogenesis, but the downstream interactions that mediate the function of APP during neuron development are unknown. By introducing interaction-deficient FE65 into cultured hippocampal neurons using adenovirus, we show that a complex including APP, FE65 and an additional protein is involved in neurite outgrowth at early stages of neuronal development. Both FE65 that is unable to interact with APP (PID2 mutants) or a WW mutant increased axon branching. Although the FE65 mutants did not affect total neurite output, both mutants decreased axon segment length, consistent with an overall slowing of axonal growth cones. FE65 mutants did not alter the localization of either APP or FE65 in axonal growth cones, suggesting that the effects on neurite outgrowth are achieved by alterations in local complex formation within the axonal growth cone.

Abelson kinase (Abl) and RhoGEF2 regulate actin organization during cell constriction in Drosophila

Morphogenesis involves the interplay of different cytoskeletal regulators. Investigating how they interact during a given morphogenetic event will help us understand animal development. Studies of ventral furrow formation, a morphogenetic event during Drosophila gastrulation, have identified a signaling pathway involving the G-protein Concertina (Cta) and the Rho activator RhoGEF2. Although these regulators act to promote stable myosin accumulation and apical cell constriction, loss-of-function phenotypes for each of these pathway members is not equivalent, suggesting the existence of additional ventral furrow regulators. Here, we report the identification of Abelson kinase (Abl) as a novel ventral furrow regulator. We find that Abl acts apically to suppress the accumulation of both Enabled (Ena) and actin in mesodermal cells during ventral furrow formation. Further, RhoGEF2 also regulates ordered actin localization during ventral furrow formation, whereas its activator, Cta, does not. Taken together, our data suggest that there are two crucial preconditions for apical constriction in the ventral furrow:myosin stabilization/activation, regulated by Cta and RhoGEF2; and the organization of apical actin, regulated by Abl and RhoGEF2. These observations identify an important morphogenetic role for Abl and suggest a conserved mechanism for this kinase during apical cell constriction.

NESH (Abi-3) is present in the Abi/WAVE complex but does not promote c-Abl-mediated phosphorylation

Abl interactor (Abi) was identified as an Abl tyrosine kinase-binding protein and subsequently shown to be a component of the macromolecular Abi/WAVE complex, which is a key regulator of Rac-dependent actin polymerization. Previous studies showed that Abi-1 promotes c-Abl-mediated phosphorylation of Mammalian Enabled (Mena) and WAVE2. In addition to Abi-1, mammals possess Abi-2 and NESH (Abi-3). In this study, we compared the three Abi proteins in terms of the promotion of c-Abl-mediated phosphorylation and the formation of Abi/WAVE complex. Although Abi-2, like Abi-1, promoted the c-Abl-mediated phosphorylation of Mena and WAVE2, NESH (Abi-3) had no such effect. This difference was likely due to their binding abilities as to c-Abl. Immunoprecipitation revealed that NESH (Abi-3) is present in the Abi/WAVE complex. Our results suggest that NESH (Abi-3), like Abi-1 and Abi-2, is a component of the Abi/WAVE complex, but likely plays a different role in the regulation of c-Abl.

Engrailed controls the organization of the ventral nerve cord through frazzled regulation

In Drosophila, the ventral nerve cord (VNC) architecture is built from neuroblasts that are specified during embryonic development, mainly by transcription factors. Here we show that Engrailed, a homeodomain transcription factor known to be involved in the establishment of neuroblast identity, is also directly implicated in the regulation of axonal guidance cues. Posterior commissures (PC) are missing in engrailed mutant embryos, and axonal pathfinding defects are observed when Engrailed is ectopically expressed at early stages, prior to neuronal specification. We also show that frazzled, enabled, and trio, all of which are potential direct targets of Engrailed and are involved in axonal navigation, interact genetically with engrailed to form posterior commissures in the developing VNC. The regulation of frazzled expression in engrailed-expressing neuroblasts contributes significantly to the formation of the posterior commissures by acting on axon growth. Finally, we identified a small genomic fragment within intron 1 of frazzled that can mediate activation by Engrailed in vivo when fused to a GFP reporter. These results indicate that Engrailed's function during the segregation of the neuroblasts is crucial for regulating different actors that are later involved in axon guidance.

Abl deregulates Cdk5 kinase activity and subcellular localization in Drosophila neurodegeneration

Although Abl functions in mature neurons, work to date has not addressed Abl's role on Cdk5 in neurodegeneration. We found that beta-amyloid (Abeta42) initiated Abl kinase activity and that blockade of Abl kinase rescued both Drosophila and mammalian neuronal cells from cell death. We also found activated Abl kinase to be necessary for the binding, activation, and translocalization of Cdk5 in Drosophila neuronal cells. Conversion of p35 into p25 was not observed in Abeta42-triggered Drosophila neurodegeneration, suggesting that Cdk5 activation and protein translocalization can be p25-independent. Our genetic studies also showed that abl mutations repressed Abeta42-induced Cdk5 activity and neurodegeneration in Drosophila eyes. Although Abeta42 induced conversion of p35 to p25 in mammalian cells, it did not sufficiently induce Cdk5 activation when c-Abl kinase activity was suppressed. Therefore, we propose that Abl and p35/p25 cooperate in promoting Cdk5-pY15, which deregulates Cdk5 activity and subcellular localization in Abeta42-triggered neurodegeneration.

Regulation of somitogenesis by Ena/VASP proteins and FAK during Xenopus development

The metameric organization of the vertebrate body plan is established during somitogenesis as somite pairs sequentially form along the anteroposterior axis. Coordinated regulation of cell shape, motility and adhesion are crucial for directing the morphological segmentation of somites. We show that members of the Ena/VASP family of actin regulatory proteins are required for somitogenesis in Xenopus. Xenopus Ena (Xena) localizes to the cell periphery in the presomitic mesoderm (PSM), and is enriched at intersomitic junctions and at myotendinous junctions in somites and the myotome, where it co-localizes with β1-integrin, vinculin and FAK. Inhibition of Ena/VASP function with dominant-negative mutants results in abnormal somite formation that correlates with later defects in intermyotomal junctions. Neutralization of Ena/VASP activity disrupts cell rearrangements during somite rotation and leads to defects in the fibronectin (FN) matrix surrounding somites. Furthermore, inhibition of Ena/VASP function impairs FN matrix assembly, spreading of somitic cells on FN and autophosphorylation of FAK, suggesting a role for Ena/VASP proteins in the modulation of integrin-mediated processes. We also show that inhibition of FAK results in defects in somite formation, blocks FN matrix deposition and alters Xena localization. Together, these results provide evidence that Ena/VASP proteins and FAK are required for somite formation in Xenopus and support the idea that Ena/VASP and FAK function in a common pathway to regulate integrin-dependent migration and adhesion during somitogenesis.

Abelson, enabled, and p120 catenin exert distinct effects on dendritic morphogenesis in Drosophila

Neurons exhibit diverse dendritic branching patterns that are important for their function. However, the signaling pathways that control the formation of different dendritic structures remain largely unknown. To address this issue in vivo, we use the peripheral nervous system (PNS) of Drosophila as a model system. Through both loss-of-function and gain-of-function analyses in vivo, we show here that the nonreceptor tyrosine kinase Abelson (Abl), an important regulator of cytoskeleton dynamics, inhibits dendritic branching of dendritic arborization (DA) sensory neurons in Drosophila. Enabled (Ena), a substrate for Abl, promotes the formation of both dendritic branches and actin-rich spine-like protrusions of DA neurons, an effect opposite to that of Abl. In contrast, p120 catenin (p120 ctn) primarily enhances the development of spine-like protrusions. These results suggest that Ena is a key regulator of dendritic branching and that different regulators of the actin cytoskeleton exert distinct effects on dendritic morphogenesis.

Characterization of human Enah gene

Enabled homolog (Enah) is a mammalian ortholog of Drosophila Enabled (Ena), which is genetically linked to the Drosophila Abl tyrosine phosphorylation signaling cascade and is required for normal neural development. Vertebrates have three Ena-related genes: Enah, VASP (vasodilator-stimulated phosphoprotein) and Ena/VASP like (EVL). These genes play an important role in linking signal transduction pathways to localized remodeling of the actin cytoskeleton. We isolated and sequenced a cDNA encoding human Enah. Comparison of the amino acid sequences of mouse (Mus musculus) and human (Homo sapiens) species shows 86.6% identity. The human protein appears longer than the mouse and additional amino acids are concentrated in a region containing repeats of the amino acid sequence LERER. The complete gene is about 157 kb and consists of 14 exons. Analysis of multiple tissue northern blot revealed a major transcript of about 4.8 kb in all tissue examined. Alternatively spliced isoforms were isolated by RT-PCR. The gene is differentially expressed and to gain insight factors affecting its expression we cloned and preliminarily characterized human Enah gene promoter.

Ena/VASP proteins can regulate distinct modes of actin organization at cadherin-adhesive contacts

Functional interactions between classical cadherins and the actin cytoskeleton involve diverse actin activities, including filament nucleation, cross-linking, and bundling. In this report, we explored the capacity of Ena/VASP proteins to regulate the actin cytoskeleton at cadherin-adhesive contacts. We extended the observation that Ena/vasodilator-stimulated phosphoprotein (VASP) proteins localize at cell-cell contacts to demonstrate that E-cadherin homophilic ligation is sufficient to recruit Mena to adhesion sites. Ena/VASP activity was necessary both for F-actin accumulation and assembly at cell-cell contacts. Moreover, we identified two distinct pools of Mena within individual homophilic adhesions that cells made when they adhered to cadherin-coated substrata. These Mena pools localized with Arp2/3-driven cellular protrusions as well as at the tips of cadherin-based actin bundles. Importantly, Ena/VASP activity was necessary for both modes of actin activity to be expressed. Moreover, selective depletion of Ena/VASP proteins from the tips of cadherin-based bundles perturbed the bundles without affecting the protrusive F-actin pool. We propose that Ena/VASP proteins may serve as higher order regulators of the cytoskeleton at cadherin contacts through their ability to modulate distinct modes of actin organization at those contacts.

Cloning and developmental expression of Xenopus Enabled (Xena)

Regulation of actin dynamics, organization, and interaction with cell surface adhesion proteins is critical for tissue morphogenesis during development. The Ena/VASP family of actin-binding proteins function in several cellular processes that involve dynamic regulation of the actin cytoskeleton, including axon guidance, platelet aggregation, cell migration, and cell adhesion. The vertebrate Ena/VASP family is composed of three genes: Ena (Enabled), VASP (Vasodilator Stimulated Phosphoprotein), and Evl (Ena/VASP-Like). To better understand the role of Ena/VASP proteins during vertebrate development, we have cloned and characterized the developmental expression of Ena in Xenopus laevis. Analysis of the temporal expression of Xenopus Ena (Xena) demonstrates that multiple isoforms of Xena are detected throughout embryogenesis and that the presence of different isoforms is developmentally regulated. In situ hybridization analyses reveal that Xena is broadly expressed throughout development. During gastrulation and neurulation, Xena is detected in the neuroepithelium, notochord, and somites. In tadpoles, Xena expression is restricted to dorsal regions of the brain, whereas it is expressed at lower levels throughout the spinal cord. Xena expression is also detected in the notochord, myotome, heart, pronephros, and cranial placodes, including the olfactory and otic placodes. Analysis of the subcellular localization of Xena using a GFP fusion protein revealed that Xena localizes to adherens junctions and focal adhesions in Xenopus animal caps and NIH3T3 fibroblasts, respectively. These results define spatiotemporal windows in which Xena may function during early Xenopus development to modulate actin-dependent processes such as cell adhesion and migration.

A Novel Mechanism of G Protein-dependent Phosphorylation of Vasodilator-stimulated Phosphoprotein

Vasodilator-stimulated phosphoprotein (VASP) is a major substrate of protein kinase A (PKA). Here we described the novel mechanism of VASP phosphorylation via cAMP-independent PKA activation. We showed that in human umbilical vein endothelial cells (HUVECs) alpha-thrombin induced phosphorylation of VASP. Specific inhibition of Galpha13 protein by the RGS domain of a guanine nucleotide exchange factor, p115RhoGEF, inhibited thrombin-dependent phosphorylation of VASP. More importantly, Galpha13-induced VASP phosphorylation was dependent on activation of RhoA and mitogen-activated protein kinase kinase kinase, MEKK1, leading to the stimulation of the NF-kappaB signaling pathway. alpha-Thrombin-dependent VASP phosphorylation was inhibited by small interfering RNA-mediated knockdown of RhoA, whereas Galpha13-dependent VASP phosphorylation was inhibited by a specific RhoA inhibitor botulinum toxin C3 and by a dominant negative mutant of MEKK1. We determined that Galpha13-dependent VASP phosphorylation was also inhibited by specific PKA inhibitors, PKI and H-89. In addition, the expression of phosphorylation-deficient IkappaB and pretreatment with the proteasome inhibitor MG-132 abolished Galpha13- and alpha-thrombin-induced VASP phosphorylation. In summary, we have described a novel pathway of Galpha13-induced VASP phosphorylation that involves activation of RhoA and MEKK1, phosphorylation and degradation of IkappaB, release of PKA catalytic subunit from the complex with IkappaB and NF-kappaB, and subsequent phosphorylation of VASP.

Influence of Notch on dorsoventral compartmentalization and actin organization in the Drosophila wing

Compartment boundaries play key roles in tissue organization by separating cell populations. Activation of the Notch receptor is required for dorsoventral (DV) compartmentalization of the Drosophila wing, but the nature of its requirement has been controversial. Here, we provide additional evidence that a stripe of Notch activation is sufficient to establish a sharp separation between cell populations, irrespective of their dorsal or ventral identities. We further find that cells at the DV compartment boundary are characterized by a distinct shape, a smooth interface, and an accumulation of F-actin at the adherens junction. Genetic manipulation establishes that a stripe of Notch activation is both necessary and sufficient for this DV boundary cell phenotype, and supports the existence of a non-transcriptional branch of the Notch pathway that influences F-actin. Finally, we identify a distinct requirement for a regulator of actin polymerization, capulet, in DV compartmentalization. These observations imply that Notch effects compartmentalization through a novel mechanism, which we refer to as a fence, that does not depend on the establishment of compartment-specific cell affinities, but does depend on the organization of the actin cytoskeleton.

Molecular cloning and expression of Ena/Vasp-like (Evl) during Xenopus development

Ena/VASP proteins are actin-binding proteins implicated in the regulation of axon guidance, platelet aggregation, cell motility, and cell adhesion. The vertebrate Ena/VASP family is comprised of three genes: Ena (Enabled), VASP (Vasodilator Stimulated Phosphoprotein), and Evl (Ena/VASP-Like). We have cloned and characterized cDNAs encoding three alternatively spliced isoforms of Xenopus laevis Evl, designated Xevl, Xevl-I and Xevl-H. Analysis of the temporal expression of Xevl, Xevl-I and Xevl-H demonstrates that transcripts for each isoform are first detectable at low levels at stage 18, show increased abundance by stage 23, and persist throughout the remainder of embryogenesis. In situ hybridization analyses using a probe that detects all three Xevl isoforms or a probe that specifically detects the Xevl-H isoform revealed expression in the cement gland, brain, neural tube, myotome, and neural placodes, including the otic, lateral line, and olfactory placodes. These results suggest roles for Xevl in regulating actin dynamics and cell adhesion in neural and mesodermal tissues during later stages of Xenopus development.

Abl tyrosine kinase and its substrate Ena/VASP have functional interactions with kinesin-1

Relatively little is known about how microtubule motors are controlled or about how the functions of different cytoskeletal systems are integrated. A yeast two-hybrid screen for proteins that bind to Drosophila Enabled (Ena), an actin polymerization factor that is negatively regulated by Abl tyrosine kinase, identified kinesin heavy chain (Khc), a member of the kinesin-1 subfamily of microtubule motors. Coimmunoprecipitation from Drosophila cytosol confirmed a physical interaction between Khc and Ena. Kinesin-1 motors can carry organelles and other macromolecular cargoes from neuronal cell bodies toward terminals in fast-axonal-transport. Ena distribution in larval axons was not affected by mutations in the Khc gene, suggesting that Ena is not itself a fast transport cargo of Drosophila kinesin-1. Genetic interaction tests showed that in a background sensitized by reduced Khc gene dosage, a reduction in Abl gene dosage caused distal paralysis and axonal swellings. A concomitant reduction in ena dosage rescued those defects. These results suggest that Ena/VASP, when not inhibited by the Abl pathway, can bind Khc and reduce its transport activity in axons.

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

The attractive Netrin receptor Frazzled (Fra), and the signaling molecules Abelson tyrosine kinase (Abl), the guanine nucleotide-exchange factor Trio,and the Abl substrate Enabled (Ena), all regulate axon pathfinding at the Drosophila embryonic CNS midline. We detect genetic and/or physical interactions between Fra and these effector molecules that suggest that they act in concert to guide axons across the midline. Mutations in Abland trio dominantly enhance fra and Netrin mutant CNS phenotypes, and fra;Abl and fra;trio double mutants display a dramatic loss of axons in a majority of commissures. Conversely,heterozygosity for ena reduces the severity of the CNS phenotype in fra, Netrin and trio,Abl mutants. Consistent with an in vivo role for these molecules as effectors of Fra signaling, heterozygosity for Abl, trio or ena reduces the number of axons that inappropriately cross the midline in embryos expressing the chimeric Robo-Fra receptor. Fra interacts physically with Abl and Trio in GST-pulldown assays and in co-immunoprecipitation experiments. In addition, tyrosine phosphorylation of Trio and Fra is elevated in S2 cells when Abl levels are increased. Together, these data suggest that Abl, Trio, Ena and Fra are integrated into a complex signaling network that regulates axon guidance at the CNS midline.

Caenorhabditis elegans WASP and Ena/VASP proteins play compensatory roles in morphogenesis and neuronal cell migration

We report here that WASP and Ena/VASP family proteins play overlapping roles in C. elegans morphogenesis and neuronal cell migration. Specifically, these studies demonstrate that UNC-34/Ena plays a role in morphogenesis that is revealed only in the absence of WSP-1 function and that WSP-1 has a role in neuronal cell migration that is revealed only in the absence of UNC-34/Ena activity. To identify additional genes that act in parallel to unc-34/ena during morphogenesis, we performed a screen for synthetic lethals in an unc-34 null mutant background utilizing an RNAi feeding approach. To our knowledge, this is the first reported RNAi-based screen for genetic interactors. As a result of this screen, we identified a second C. elegans WASP family protein, wve-1, that is most homologous to SCAR/WAVE proteins. Animals with impaired wve-1 function display defects in gastrulation, fail to undergo proper morphogenesis, and exhibit defects in neuronal cell migrations and axon outgrowth. Reducing wve-1 levels in either unc-34/ena or wsp-1 mutant backgrounds also leads to a significant enhancement of the gastrulation and morphogenesis defects. Thus, unc-34/ena, wsp-1, and wve-1 play overlapping roles during embryogenesis and unc-34/ena and wsp-1 play overlapping roles in neuronal cell migration. These observations show that WASP and Ena/VASP proteins can compensate for each other in vivo and provide the first demonstration of a role for Ena/VASP proteins in gastrulation and morphogenesis. In addition, our results provide the first example of an in vivo role for WASP family proteins in neuronal cell migrations and cytokinesis in metazoans.

The Netrin family of guidance factors: emphasis on Netrin-1 signalling

During the development of the nervous system, neurons respond to the coordinated action of a variety of attractive and repulsive signals from the embryonic environment. Netrins form a family of extracellular proteins that regulate the migration of neurons and axonal growth cones. These proteins are bifunctional signals that are chemoattractive for some neurons and chemorepellent for others. Netrins mainly interact with the specific receptors DCC and UNC-5 family. To date, several Netrins have been described in mouse and humans: Netrin-1, -3/NTL2, -4/beta and G-Netrins. Netrin-1 is the most studied member of the family. It is involved in the development many projections of the nervous system. When Netrin-1 interacts with its specific receptors, a cascade of local cytoplasmic events is triggered. Several signal transduction pathways and effector molecules have been implicated in the response to Netrin-1: small Rho-GTPases, MAP-Kinases, second messengers and the Microtubule Associated Protein 1B (MAP1B).

Palladin is a novel binding partner for Ena/VASP family members

Palladin is an actin-associated protein that contains proline-rich motifs within its amino-terminal sequence that are similar to motifs found in zyxin, vinculin, and the Listeria protein ActA. These motifs are known to be potential binding sites for the Vasodilator-Stimulated Phosphoprotein (VASP). Here, we demonstrate that palladin is an additional direct binding partner for VASP, by using co-immunoprecipitation and blot overlay techniques with both endogenous palladin and recombinant myc-tagged palladin. These results show that VASP binds to full-length palladin and also to the amino-terminal half of palladin, where the polyproline motifs are located. Using a synthetic peptide array, two discrete binding sites for VASP were identified within palladin's proline-rich amino-terminal domain. Using double-label immunofluorescence staining of fully-spread and actively-spreading fibroblasts, the extent of co-localization of palladin and VASP was explored. These proteins were found to strongly co-localize along stress fibers, and partially co-localize in focal adhesions, lamellipodia, and focal complexes. These results suggest that the recently described actin-associated protein palladin may play an important role in recruiting VASP to sites of actin filament growth, anchorage, and crosslinking.

Loss of c-abl facilitates anchorage-independent growth of p53- and RB- deficient primary mouse embryonic fibroblasts

The c-abl tyrosine kinase is the proto-oncogene of the v-abl oncogene of the Abelson murine leukemia virus. Although mutational variants of c-Abl can exhibit gain of function and can produce a transformed phenotype, the function of c-Abl in transformation remained unclear. Here, we report that the loss of c-abl facilitates transformation. c-abl-knockout mouse embryonic fibroblasts (MEFs) immortalized by SV40 T antigen acquired anchorage-independent growth, and by constructing mutational variants of T antigen we showed that binding of large T antigen to p53 and RB was necessary to induce anchorage-independent growth. Although c-abl/p53 double-knockout MEFs did not undergo anchorage-independent growth, those expressing human papilloma virus 16 E7, which mainly inactivates RB, did. Our results show that the loss of c-abl facilitates anchorage-independent growth in the context of p53 and RB deficiency, and suggest that loss of function of c-abl facilitates some types of transformation.

The microtubule plus end tracking protein Orbit/MAST/CLASP acts downstream of the tyrosine kinase Abl in mediating axon guidance

Axon guidance requires coordinated remodeling of actin and microtubule polymers. Using a genetic screen, we identified the microtubule-associated protein Orbit/MAST as a partner of the Abelson (Abl) tyrosine kinase. We find identical axon guidance phenotypes in orbit/MAST and Abl mutants at the midline, where the repellent Slit restricts axon crossing. Genetic interaction and epistasis assays indicate that Orbit/MAST mediates the action of Slit and its receptors, acting downstream of Abl. We find that Orbit/MAST protein localizes to Drosophila growth cones. Higher-resolution imaging of the Orbit/MAST ortholog CLASP in Xenopus growth cones suggests that this family of microtubule plus end tracking proteins identifies a subset of microtubules that probe the actin-rich peripheral growth cone domain, where guidance signals exert their initial influence on cytoskeletal organization. These and other data suggest a model where Abl acts as a central signaling node to coordinate actin and microtubule dynamics downstream of guidance receptors.

Critical role of Ena/VASP proteins for filopodia formation in neurons and in function downstream of netrin-1

Ena/VASP proteins play important roles in axon outgrowth and guidance. Ena/VASP activity regulates the assembly and geometry of actin networks within fibroblast lamellipodia. In growth cones, Ena/VASP proteins are concentrated at filopodia tips, yet their role in growth cone responses to guidance signals has not been established. We found that Ena/VASP proteins play a pivotal role in formation and elongation of filopodia along neurite shafts and growth cone. Netrin-1-induced filopodia formation was dependent upon Ena/VASP function and directly correlated with Ena/VASP phosphorylation at a regulatory PKA site. Accordingly, Ena/VASP function was required for filopodial formation from the growth cone in response to global PKA activation. We propose that Ena/VASP proteins control filopodial dynamics in neurons by remodeling the actin network in response to guidance cues.

Ena/VASP proteins: regulators of the actin cytoskeleton and cell migration

Ena/VASP proteins are a conserved family of actin regulatory proteins made up of EVH1, EVH2 domains, and a proline-rich central region. They have been implicated in actin-based processes such as fibroblast migration, axon guidance, and T cell polarization and are important for the actin-based motility of the intracellular pathogen Listeria monocytogenes. Mechanistically, these proteins associate with barbed ends of actin filaments and antagonize filament capping by capping protein (CapZ). In addition, they reduce the density of Arp2/3-dependent actin filament branches and bind Profilin at sites of actin polymerization. Vertebrate Ena/VASP proteins are substrates for PKA/PKG serine/threonine kinases. Phosphorylation by these kinases appears to modulate Ena/VASP function within cells, although the mechanism underlying this regulation remains to be determined.

Enhanced in vivo platelet adhesion in vasodilator-stimulated phosphoprotein (VASP)-deficient mice

Platelet adhesion and activation at the vascular wall are the initial steps leading to arterial thrombosis and vascular occlusion. Prostacyclin and nitric oxide inhibit platelet adhesion, acting via cyclic adenosine monophosphate (cAMP)- and cyclic guanosine monophosphate (cGMP)-dependent protein kinases. A major downstream target for both cAMP- and cGMP-dependent protein kinases is the vasodilator-stimulated phosphoprotein (VASP). To test the significance of VASP for the regulation of platelet adhesion in vivo, we studied platelet-vessel wall interactions using VASP-deficient (VASP-/-) mice. Under physiologic conditions, platelet adhesion to endothelial cells was significantly enhanced in VASP null mutants when compared with wild-type mice (P <.05). Platelet recruitment in VASP null mice involved P-selectin and the fibrinogen receptor glycoprotein IIb-IIIa (GPIIb-IIIa). Under pathophysiologic conditions, the loss of VASP increased platelet adhesion to the postischemic intestinal microvasculature, to the atherosclerotic endothelium of ApoE-deficient mice, and to the subendothelial matrix following endothelial denudation (P <.05 vs wild type). Importantly, platelet adhesion in VASP null mutants was unresponsive to nitric oxide. These data show for the first time in vivo that VASP is involved in down-regulation of platelet adhesion to the vascular wall under both physiologic and pathophysiologic conditions.

Fyn is required for oxidative- and hyperosmotic-stress-induced tyrosine phosphorylation of caveolin-1

Caveolin-1 is phosphorylated on Tyr(14) in response to both oxidative and hyperosmotic stress. In the present paper, we show that this phosphorylation requires activation of the Src family kinase Fyn. Stress-induced caveolin phosphorylation was abolished by three Src kinase inhibitors, SU6656, PP2 and PD180970, and was not observed in fibroblasts derived from a Src, Yes and Fyn triple-knockout mouse (SYF-/-). Using cell lines derived from single-kinase-knockout mice (Src-/-, Yes-/- and Fyn-/-), we show that expression of Fyn, but not Src or Yes, is required for stress-induced caveolin phosphorylation. Heterologous expression of Fyn in the SYF-/- and Fyn-/- cells was sufficient to reconstitute stress-induced caveolin phosphorylation, and overexpression of Fyn in wild-type cells induced hyperphosphorylation of caveolin. Fyn was autophosphorylated following oxidative stress, verifying activation of this kinase. Interestingly, there was a concomitant increase in the phosphorylation of Fyn on its Csk (C-terminal Src kinase) site, indicating feedback inhibition. Csk binds to phosphocaveolin [Cao, Courchesne and Mastick (2002) J. Biol. Chem. 277, 8771-8774] and should phosphorylate any co-localized Src-family kinases. Oxidative-stress-induced phosphorylation of caveolin-1 also requires expression of Abl [Sanguinetti and Mastick (2003) Cell Signal. 15, 289-298]. Using inhibitors and cells derived from knockout mice, we verified a requirement for both Abl and Fyn in stress-induced caveolin phosphorylation in a single cell type. Our data suggest a novel mechanism for attenuation of Src-kinase activity by Abl: stable tyrosine phosphorylation of a scaffolding protein, caveolin, and recruitment of Csk. Paxillin, a substrate of both Abl and Src, organizes a similar regulatory complex.

Drosophila pod-1 crosslinks both actin and microtubules and controls the targeting of axons

Actin and microtubules (MTs) are tightly coordinated during neuronal growth cone navigation and are dynamically regulated in response to guidance cues; however, little is known about the underlying molecular mechanisms. Here, we characterize Drosophila pod-1 (dpod1) and show that purified Dpod1 can crosslink both actin and MTs. In cultured S2 cells, Dpod1 colocalizes with lamellar actin and MTs, and overexpression remodels the cytoskeleton to promote dynamic neurite-like actin-dependent projections. Consistent with these observations, Dpod1 localizes to the tips of growing axons, regions where actin and MTs interact, and is especially abundant at navigational choice points. In either the absence or overabundance of Dpod1, growth cone targeting but not outgrowth is disrupted. Taken together, these results reveal novel activities for pod-1 and show that proper levels of Dpod1, an actin/MT crosslinker, must be maintained in the growth cone for correct axon guidance.

Function and regulation of Ena/VASP proteins

Regulation of cytoskeletal dynamics is required to coordinate cell movement, adhesion and shape change. The Ena/VASP protein family is thought to play an important role in linking signaling pathways to remodeling of the actin cytoskeleton. This review will examine the mechanisms by which Ena/VASP function might control actin dynamics and how these proteins are linked to various signaling pathways.

Regulation of F-actin-dependent processes by the Abl family of tyrosine kinases

The F-actin cytoskeleton is a fundamental component of all eukaryotic cells. It provides force and stability and plays an integral role in a diverse array of cellular processes. The spatiotemporal regulation of F-actin dynamics is essential for proper biological output. The basic molecular machinery underlying the assembly and disassembly of filamentous actin is conserved in all eukaryotic cells. Additionally, protein tyrosine kinases, found only in multicellular eukaryotes, provide links between extracellular signals and F-actin-dependent cellular processes. Among the tyrosine kinases, c-Abl and its relative Arg are unique in binding directly to F-actin. Recent results have demonstrated a role for c-Abl in membrane ruffling, cell spreading, cell migration, and neurite extension in response to growth factor and extracellular matrix signals. c-Abl appears to regulate the assembly of F-actin polymers into different structures, depending on the extracellular signal. Interestingly, c-Abl contains nuclear import and export signals, and the nuclear c-Abl inhibits differentiation and promotes apoptosis in response to genotoxic stress. The modular structure and the nuclear-cytoplasmic shuttling of c-Abl suggest that it integrates multiple signals to coordinate F-actin dynamics with the cellular decision to differentiate or to die.

Interactions between the secreted protein Amalgam, its transmembrane receptor Neurotactin and the Abelson tyrosine kinase affect axon pathfinding

Two novel dosage-sensitive modifiers of the Abelson tyrosine kinase (Abl) mutant phenotype have been identified. Amalgam (Ama) is a secreted protein that interacts with the transmembrane protein Neurotactin (Nrt) to promote cell:cell adhesion. We have identified an unusual missense ama allele, ama(M109), which dominantly enhances the Abl mutant phenotype, affecting axon pathfinding. Heterozygous null alleles of ama do not show this dominant enhancement, but animals homozygous mutant for both ama and Abl show abnormal axon outgrowth. Cell culture experiments demonstrate the Ama(M109) mutant protein binds to Nrt, but is defective in mediating Ama/Nrt cell adhesion. Heterozygous null alleles of nrt dominantly enhance the Abl mutant phenotype, also affecting axon pathfinding. Furthermore, we have found that all five mutations originally attributed to disabled are in fact alleles of nrt. These results suggest Ama/Nrt-mediated adhesion may be part of signaling networks involving the Abl tyrosine kinase in the growth cone.