enabled: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

Gene name - enabled

Synonyms -

Cytological map position - 56B

Function - Cytoskeletal adaptor protein

Keyword(s) - Cytoskeletal adaptor protein, Axon guidance

Symbol - ena

FlyBase ID:FBgn0000578

Genetic map position - 2-87

Classification - VASP homolog and SH3 domain protein

Cellular location - cytoplasmic



NCBI links: Precomputed BLAST | Entrez Gene
Recent literature
Sakuma, C., Saito, Y., Umehara, T., Kamimura, K., Maeda, N., Mosca, T. J., Miura, M. and Chihara, T. (2016). The Strip-Hippo pathway regulates synaptic terminal formation by modulating actin organization at the Drosophila neuromuscular synapses. Cell Rep 16: 2289-2297. PubMed ID: 27545887
Summary:
Synapse formation requires the precise coordination of axon elongation, cytoskeletal stability, and diverse modes of cell signaling. The underlying mechanisms of this interplay, however, remain unclear. This study demonstrates that Strip, a component of the striatin-interacting phosphatase and kinase (STRIPAK) complex that regulates these processes, is required to ensure the proper development of synaptic boutons at the Drosophila neuromuscular junction. In doing so, Strip negatively regulates the activity of the Hippo (Hpo) pathway, an evolutionarily conserved regulator of organ size whose role in synapse formation is currently unappreciated. Strip functions genetically with Enabled, an actin assembly/elongation factor and the presumptive downstream target of Hpo signaling, to modulate local actin organization at synaptic termini. This regulation occurs independently of the transcriptional co-activator Yorkie, the canonical downstream target of the Hpo pathway. This study identifies a previously unanticipated role of the Strip-Hippo pathway in synaptic development, linking cell signaling to actin organization.
Segal, D., Dhanyasi, N., Schejter, E. D. and Shilo, B. Z. (2016). Adhesion and fusion of muscle cells are promoted by filopodia. Dev Cell 38: 291-304. PubMed ID: 27505416
Summary:
Indirect flight muscles (IFMs) in Drosophila are generated during pupariation by fusion of hundreds of myoblasts with larval muscle templates (myotubes). Live observation of these muscles during the fusion process revealed multiple long actin-based protrusions that emanate from the myotube surface and require Enabled and IRSp53 for their generation and maintenance. Fusion is blocked when formation of these filopodia is compromised. While filopodia are not required for the signaling process underlying critical myoblast cell-fate changes prior to fusion, myotube-myoblast adhesion appears to be filopodia dependent. Without filopodia, close apposition between the cell membranes is not achieved, the cell-adhesion molecule Duf is not recruited to the myotube surface, and adhesion-dependent actin foci do not form. It is therefore proposed that the filopodia are necessary to prime the heterotypic adhesion process between the two cell types, possibly by recruiting the cell-adhesion molecule Sns to discrete patches on the myoblast cell surface.
Kannan, R., Song, J.K., Karpova, T., Clarke, A., Shivalkar, M., Wang, B., Kotlyanskaya, L., Kuzina, I., Gu, Q. and Giniger, E. (2017). The Abl pathway bifurcates to balance Enabled and Rac signaling in axon patterning in Drosophila. Development [Epub ahead of print]. PubMed ID: 28087633
Summary:
The Abl tyrosine kinase signaling network controls cell migration, epithelial organization, axon patterning and other aspects of development. While individual components are known, the relationships among them remain mysterious. This study used FRET measurements of pathway activity, analysis of protein localization and genetic epistasis to dissect the structure of this network in Drosophila. It was found that the adaptor protein Disabled stimulates Abl kinase activity. Abl suppresses the actin regulatory factor Enabled, and 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, a key target of Abl signaling in axons is the WAVE complex that promotes formation of branched actin networks. Thus, Abl constitutes a bifurcating network, suppressing Ena activity in parallel with stimulation of WAVE. The study suggests that the balancing of linear and branched actin networks by Abl is likely to be central to its regulation of axon patterning.

Banerjee, A. and Roy, J. K. (2018). Bantam regulates the axonal geometry of Drosophila larval brain by modulating actin regulator Enabled. Invert Neurosci 18(2): 7. PubMed ID: 29777401
Summary:
During development, axonogenesis, an integral part of neurogenesis, is based on well-concerted events comprising generation, rearrangement, migration, elongation, and adhesion of neurons. Actin, specifically the crosstalk between the guardians of actin polymerization, like enabled, chickadee, capping protein plays an essential role in crafting several events of axonogenesis. Recent evidences reflect multifaceted role of microRNA during axonogenesis. This study investigated the role of bantam miRNA, a well-established miRNA in Drosophila, in regulating the actin organization during brain development. Immunofluorescence studies showed altered arrangement of neurons and actin filaments whereas both qPCR and western blot revealed elevated expression of enabled, one of the actin modulators in bantam mutant background. Collectively, these results clearly demonstrate that bantam plays an instrumental role in shaping the axon architecture regulating the actin geometry through its modulator enabled.
Harker, A. J., Katkar, H. H., Bidone, T. C., Aydin, F., Voth, G. A., Applewhite, D. A. and Kovar, D. R. (2019). Ena/VASP processive elongation is modulated by avidity on actin filaments bundled by the filopodia crosslinker fascin. Mol Biol Cell: mbcE18080500. PubMed ID: 30601697
Summary:
Ena/VASP tetramers are processive actin elongation factors that localize to diverse F-actin networks composed of filaments bundled by different crosslinking proteins, such as filopodia (fascin), lamellipodia (fimbrin), and stress fibers (alpha-actinin). Previous work has show that Ena takes approximately 3-fold longer processive runs on trailing barbed ends of fascin-bundled F-actin. This study used single-molecule TIRFM and developed a kinetic model to further dissect Ena/VASP's processive mechanism on bundled filaments. Ena's enhanced processivity on trailing barbed ends is specific to fascin bundles, with no enhancement on fimbrin or alpha-actinin bundles. Notably, Ena/VASP's processive run length increases with the number of both fascin-bundled filaments and Ena 'arms', revealing avidity facilitates enhanced processivity. Consistently, Ena tetramers form more filopodia than mutant dimer and trimers in Drosophila culture cells. Moreover, enhanced processivity on trailing barbed ends of fascin-bundled filaments is an evolutionarily conserved property of Ena/VASP homologs, including human VASP and C. elegans UNC-34. These results demonstrate that Ena tetramers are tailored for enhanced processivity on fascin bundles and avidity of multiple arms associating with multiple filaments is critical for this process. Furthermore, a novel regulatory process was discovered whereby bundle size and bundling protein specificity control activities of a processive assembly factor.
Davidson, A. J., Millard, T. H., Evans, I. R. and Wood, W. (2019). Ena orchestrates remodelling within the actin cytoskeleton to drive robust Drosophila macrophage chemotaxis. J Cell Sci. PubMed ID: 30718364
Summary:
The actin cytoskeleton is the engine that powers the inflammatory chemotaxis of immune cells to sites of tissue damage or infection. This study combined genetics with live, in vivo imaging to investigate how cytoskeletal rearrangements drive macrophage recruitment to wounds in Drosophila. The actin-regulatory protein Ena is a master regulator of lamellipodial dynamics in migrating macrophages where it remodels the cytoskeleton to form linear filaments that can then be bundled together by the cross-linker Fascin. In contrast, the formin Dia generates rare, probing filopods for specialised functions that are not required for migration. Ena's role in lamellipodial bundling is so fundamental that its over-expression increases bundling even in the absence of Fascin by marshalling the remaining cross-linking proteins to compensate. This reorganisation of the lamellipod generates cytoskeletal struts that push against the membrane to drive leading edge advancement and boost cell speed. Thus, Ena-mediated remodeling extracts the most from the cytoskeleton to power robust macrophage chemotaxis during their inflammatory recruitment to wounds.
Harker, A. J., Katkar, H. H., Bidone, T. C., Aydin, F., Voth, G. A., Applewhite, D. A. and Kovar, D. R. (2019). Ena/VASP processive elongation is modulated by avidity on actin filaments bundled by the filopodia cross-linker fascin. Mol Biol Cell 30(7): 851-862. PubMed ID: 30601697
Summary:
Ena/VASP tetramers are processive actin elongation factors that localize to diverse F-actin networks composed of filaments bundled by different cross-linking proteins, such as filopodia (fascin), lamellipodia (fimbrin), and stress fibers (alpha-actinin). Ena takes approximately threefold longer processive runs on trailing barbed ends of fascin-bundled F-actin. This study used single-molecule TIRFM (total internal reflection fluorescence microscopy) and developed a kinetic model to further dissect Ena/VASP's processive mechanism on bundled filaments. Ena's enhanced processivity on trailing barbed ends was found to be specific to fascin bundles, with no enhancement on fimbrin or alpha-actinin bundles. Notably, Ena/VASP's processive run length increases with the number of both fascin-bundled filaments and Ena "arms," revealing avidity facilitates enhanced processivity. Consistently, Ena tetramers form more filopodia than mutant dimer and trimers in Drosophila culture cells. Moreover, enhanced processivity on trailing barbed ends of fascin-bundled filaments is an evolutionarily conserved property of Ena/VASP homologues, including human VASP and Caenorhabditis elegans UNC-34. These results demonstrate that Ena tetramers are tailored for enhanced processivity on fascin bundles and that avidity of multiple arms associating with multiple filaments is critical for this process. Furthermore, this study discovered a novel regulatory process whereby bundle size and bundling protein specificity control activities of a processive assembly factor.
Myat, M. M., Louis, D., Mavrommatis, A., Collins, L., Mattis, J., Ledru, M., Verghese, S. and Su, T. T. (2019). Regulators of cell movement during development and regeneration in Drosophila. Open Biol 9(5): 180245. PubMed ID: 31039676
Summary:
Cell migration is a fundamental cell biological process essential both for normal development and for tissue regeneration after damage. Cells can migrate individually or as a collective. To better understand the genetic requirements for collective migration, RNA interference (RNAi) was expressed against 30 genes in the Drosophila embryonic salivary gland cells that are known to migrate collectively. The genes were selected based on their effect on cell and membrane morphology, cytoskeleton and cell adhesion in cell culture-based screens or in Drosophila tissues other than salivary glands. Of these, eight disrupted salivary gland migration, targeting: Rac2, Rab35 and Rab40 GTPases, MAP kinase-activated kinase-2 (MAPk-AK2), RdgA diacylglycerol kinase, Cdk9, the PDSW subunit of NADH dehydrogenase (ND-PDSW) and actin regulator Enabled (Ena). The same RNAi lines were used to determine their effect during regeneration of X-ray-damaged larval wing discs. Cells translocate during this process, but it remained unknown whether they do so by directed cell divisions, by cell migration or both. RNAi targeting Rac2, MAPk-AK2 and RdgA was found to disrupt cell translocation during wing disc regeneration, but RNAi against Ena and ND-PDSW had little effect. We conclude that, in Drosophila, cell movements in development and regeneration have common as well as distinct genetic requirements.
BIOLOGICAL OVERVIEW

Enabled is cytoskeletal regulator that facilitates continued actin polymerization at the barbed ends of actin filaments, induces cellular projections when overexpressed, and functions together with several different receptors (including Robo and UNC-40/DCC) implicated in axon guidance. This overview of Enabled starts with ABL, one of a number of mammalian cancer causing proteins (oncoproteins). ABL is a highly conserved nonreceptor tyrosine kinase containing SH3 and SH2 protein interaction domains. In less technical terms, ABL functions to modify other proteins by phosphorylation, and it attaches to other proteins by means of SH3 and SH2, two different protein interaction domains. One target of Drosophila ABL is Enabled, a protein that binds to ABL and other oncoproteins; Ena possess the Src homology 3 (SH3) protein interaction domain. SH2 and SH3 domains are found in many "adaptor" proteins that bind to other proteins in the progress of carrying out their function.

Recently a breakthrough in understanding Enabled and Abl has come from the cloning of a Mouse homolog of Enabled (Mena). Both Enabled and Mena are relatives of VASP (Vasodilator-Stimulated Phosphoprotein). A conserved domain of Mena targets it to proteins containing a specific proline-rich motif. The association of Mena with the surface of the intracellular pathogen Listeria monocytogenes and the G-actin binding protein Profilin suggests that Mena may participate in bacterial movement by facilitating actin polymerization. That is, Enabled and Mena, and indirectly ABL, serve to modify the actin based cytoplasm, an important agent in maintaining cell shape, regulating cell migration, and many other cell functions. In Drosophila, ABL and ENA act in the developmental processes of axon pathfinding and eye morphogenesis.

First the relationship of ABL to cancer will be examined because of the insight this provides to an understanding of ABL and Enabled function. Derivatives of ABL protein tyrosine kinase are involved in Philadelphia chromosome positive chronic myelogenous leukemia and acute lymphocytic leukemia in humans, and the pre-B-cell leukemia caused by Abelson murine leukemia virus in mice (from which ABL derives its name). The Philadelphia chromosome is generate by fusion between two normal human chromosomes. These translocations are not found at random, but in the middle of two genes, Abl and Bcr (named after the breakpoint cluster region). The product of this gene fusion is BCR-ABL protein, a hybrid protein able to confer a leukemia phenotype on cells in which it is expressed (Gertler, 1995 and references).

BCR is a multifunctional protein with a pseudo Kinase domain (apparently non-functional), a CDC24 domain and a GAP domain. CDC24 is a yeast gene that when mutated results in defective bud formation. It is believed that CDC24 plays a critical role in the establishment of cell polarity, development of normal cell shape, localization of secretion, and cell-surface deposition. The GAP (GTPase-activating protein) function targets p21 RAC, an important signaling protein in determining and modifying cell shape. Not only does BCR form a hybrid protein with ABL in the Philadelphia translocation, but BCR itself serves to target ABL, resulting in ABL activation (Arlinghaus, 1992 and references). When occuring on a composite protein, the interaction of BCR with ABL causes cell transformation. The chimeric protein products of these oncogenes exhibit elevated tyrosine kinase activity, oligomerization and association with the actin cytoskeleton, tyrosine phosphorylation of BCR sequences, association with signal transducing proteins GRB2, SHC, CBL, SYP and members of the 14-3-3 protein family (Gertler, 1995 and references).

The discovery that the bacterium Listeria recruits the cellular protein VASP, generated great intrigue in the scientific community. Using VASP, Listeria co-opts the cell's cytoskeleton to transport itself within the host cell it infects. VASP is a ligand for profilin, an actin-monomer binding protein that can stimulate the formation of filamentous actin, the non-muscle form of actin that serves as the basis for the actin based cytoskeleton of the cell. Listeria motility results from the rapid polymerization of F-actin at one pole of the bacterium, a process enhanced by host profilin. The properties of VASP make it a candidate host factor that can mediate the recruitment of profilin to the surface of Listeria, and thus promote F-actin assembly. Mena and Evl are two murine proteins highly related to Enabled as well as to VASP. A conserved domain in the N terminal portion of Mena and ENA targets these proteins to proline rich regions of other proteins. VASP interacts with zyxin, a LIM-domain protein localized in focal adhesions that shares a proline-rich motif with vinculin and ActA, two other cytoskeletal proteins. The proline rich region in VASP, which is shared with ENA and Mena, is most likely the profilin-binding site shared by these proteins. Mena itself has been shown to bind to the actin-binding protein Profilin. Localization of Mena to focal adhesions is mediated by its conserved N-terminus, and Mena, like VASP is recruited to the surface of Listera. Mena produces multiple isotypes. One is widely expressed, and a second is enriched in, or specific to, neural cells types. Thus Mena lends some understanding to the possible role of Drosophila ENA in axon pathfinding. (Gertler, 1996).

The well studied migration of neuronal growth cones serves as a paradigm for the actin-driven formation of membrane protrusions (Forscher, 1992). Establishment of proper connections in the central nervous system depends on the ability of neuronal growth cones to guide neurites to their final targets. The ABL-ENA-Profilin pathway is implicated in the process of axonal outgrowth and fasciculation. The mechanism by which the growth cone advances is based on dynamic rearrangement of the actin based cytoskeleton, and it is in this process that Enabled and ABL affect axonogenesis.

Genetic screens for dominant second-site mutations that suppress the lethality of Abl mutations in Drosophila identify alleles of only one gene, enabled. The ENA protein contains proline-rich motifs and binds to ABL and Src SH3 domains. ENA is also a substrate for the Abl kinase. Tyrosine phosphorylation of ENA is increased when it is coexpressed in cells with human or Drosophila Abl, and endogenous ENA tyrosine phosphorylation is reduced in Abl mutant animals. Like Abl, ena is expressed at highest levels in the axons of the embryonic nervous system and ena mutant embryos have defects in axonal architecture. Therefore, it has been concluded that a critical function of Drosophila ABL is to phosphorylate and negatively regulate ENA protein during neural development (Gertler, 1995).

Abi acts cooperatively with Ena to antagonize Abl function in 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. This study shows 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, 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 (Lin, 2009).

The in vivo role of Abi with respect to Abl has remained enigmatic. Abi was first identified as an Abl kinase substrate, functioning in modulating the transformation activity of oncogenic Abl in human cancers. Intriguingly, Abi also functions as an activator of Abl kinase activity. Moreover, the interaction of Abl and Abi can trigger an array of biochemical and functional changes in Abi, including protein phosphorylation, stability and subcellular localization, which might ultimately lead to the control of a particular biological process in vivo. Although both Abi and Abl proteins are highly expressed in the mammalian and Drosophila nervous systems, the role of Abi in modulating the function of Abl in developing nervous systems has remained unclear. In this investigation, genetic and functional studies were conducted to advance the understanding of how abi and Abl interact in vivo. To do this, abi loss-of-function alleles were generated and characterized for genetic and functional studies in Drosophila. Immunohistochemical analysis revealed that Abi is primarily expressed in the developing CNS. Consistent with this finding, phenotypic analysis suggested that mutations in abi resulted in axonal guidance defects in the CNS. In an analysis of Abl mutants, it was found Abl to be crucial for restricting synaptic overgrowth in the larval NMJ. Importantly, further studies of the genetic interaction found a functional link between abi and Abl in axonogenesis and synaptogenesis. Moreover, Abi and Ena were found to cooperate in modulating the function of Abl in NMJ growth. Finally, based on additional cellular biology studies, it is proposed that the functional interactions between Abi, Ena and Abl might be mediated through the modulation of actin cytoskeleton reorganization (Lin, 2009).

Accumulated evidence suggests that the highly conserved actin-regulatory pathways are essential for synaptogenesis and synaptic plasticity. Abi, Ena and Abl proteins are all involved in actin dynamics. Using the Drosophila NMJ as a model system, it is proposed that the abi-ena-Abl interaction in synaptogenesis might be associated with actin cytoskeleton reorganization. In fact, several actin regulatory molecules associated with both Abl and Abi have been implicated in synaptic growth. For example, Wiskott-Aldrich Syndrome protein (Wasp) is a kinase substrate of Abl and also a binding partner for Abi. The mutations in wasp result in phenotypes very similar to those present in Abl mutants, with synaptic overgrowth and hyperbranching at the NMJ. Another example is that of Diaphanous (Dia), which also interacts with both Abl and Abi, and has recently been found to modulate synaptic growth of the Drosophila NMJ. dia mutant heterozygotes have been found to be able to enhance the cellularization phenotype of an Abl maternal-null mutant, suggesting that Dia might be involved in the regulation of Abl signaling for actin reorganization. Consistent with this idea, the interaction of Dia with Abi protein has been found to be important in regulating the formation and stability of cell-cell junctions in mammalian cells. Future studies investigating whether and how Wasp and/or Dia can participate in Abl-Abi signaling for the regulation of Drosophila synaptogenesis could be interesting (Lin, 2009).

Besides Wasp and Dia, other actin regulators might also contribute to Abl-Abi signaling during nervous system development, for, as another study has suggested, Abl may be a key regulator in modulating different types and sites of actin polymerization within the cells. Abi has been shown to play a key role in the activation of the SCAR/WAVE complex, which relays signaling from Rac1 to the Arp2/3 complex for actin cytoskeleton remodeling. Genetic studies have shown that the heterozygosity of scar, but not of kette, suppressed Abl NMJ phenotypes. A current model suggests that eliminating any component from the SCAR/WAVE complex induces the breakdown of other complex components and subsequently results in abnormal lamellipodia formation. Genetic study using the Drosophila NMJ as a model does not appear to fully support this idea. The results suggested that only a subcomplex of SCAR/WAVE might be involved in synaptogenesis. In fact, recent studies have demonstrated that some components of the SCAR/WAVE complex might work outside the complex to regulate various biological processes, including neutrophil chemotaxis, cell motility and adhesion, and the formation of cell-cell junctions. Thus, it is possible that Kette is not in a complex with Abi and Scar to modulate the function of Abl in NMJ growth. To explore this hypothesis, it will be important to examine the genetic interactions between abi and scar or kette in NMJ morphogenesis (Lin, 2009).

This study found strong in vivo evidence for an antagonistic relationship between Abl and Abi in axonogenesis and synaptogenesis. Supporting this model, one very recent study has demonstrated that Abl can inhibit the role of Abi in the engulfment of apoptotic cells in C. elegans (Hurwitz, 2009). Given that Abi is the Abl kinase substrate and that it also functions as an adaptor protein for Abl in regulating other downstream effectors, it is feasible that Abi might act downstream of Abl in modulating NMJ growth. If so, the removal of both copies of abi could conceivably further suppress Abl-/- NMJ phenotypes. Preliminary morphological and functional data both suggest that the minor NMJ defects of Abl-/- abi+/- are further rescued in Abl-/- abi-/- mutants. However, Abl-/- abi-/- double mutants showed early lethality and defects in axonal innervations, rendering the finding inconclusive. Further epistasis analysis combining abi and abl gain-of-function and loss-of-function mutations are needed to test this hypothesis (Lin, 2009).

Since the data suggest an antagonistic interaction between abi and Abl for the CNS and NMJ phenotypes, it is speculated that Abl heterozygosity would suppress the semilethal phenotype of abi mutants. Surprisingly, preliminary data showed that the lethality of abi hypomorphic mutants (abiP1/KO and abiP1/Df) is further increased by Abl+/-. This result does not seem to support a general bidirectional antagonistic relationship between Abl and abi for the biological processes involved during development. Thus, a complex genetic interaction network between Abl and abi might be present in development processes (Lin, 2009).

Another interesting issue is that the abi mutants did not display obvious defects in synaptic bouton number or synaptic transmission, although they exhibited midline crossing defects in the embryonic CNS. Because other members of SCAR complex, including Scar, Kette, Sra-1 and HSPC300, exhibit both CNS and NMJ phenotypes, it is still possible that abi mutants might show minor morphological or functional abnormalities if different phenotypic characteristics are studied. Detailed morphological assays are required to investigate other phenotypic traits of the NMJ in larval or later developmental stages. Alternatively, one could reason that the roles of Abi in synaptic growth and axonal guidance are not exactly identical. Results similar to this finding have been observed for the loss of spastin, a gene enriched in axons and synaptic connections, as spastin mutants only exhibit NMJ but not CNS defects (Lin, 2009).

This work also suggested that the synaptic overgrowth phenotypes in Abl mutants could be completely rescued by expressing Abl in the presynaptic nerve cells but not in the postsynaptic muscles, suggesting that the presynaptic Abl is more crucial than the postsynaptic population for Drosophila larval NMJ formation. However, studies in mammalian Abl and Arg (also known as Abl2) have shown that both proteins localize to the presynaptic terminals and dendritic spines of synapses in the hippocampal CA1 area. Abl and Arg have also been shown to be essential for the agrin-induced clustering of acetylcholine receptors (AChRs) on the postsynaptic membrane of the mammalian NMJ, suggesting that Abl function is required in the postsynaptic region of the mammalian NMJ. However, these reports do not exclude the possibility that Drosophila Abl might also function in postsynaptic regions of the developing brain. The reason for this speculation is that the mammalian NMJ uses acetylcholine as the neurotransmitter, unlike the Drosophila NMJ, which uses glutamate as a transmitter. Since acetylcholine receptors also function in the developing brain of Drosophila, it would be important to investigate whether Drosophila Abl also plays a role in the postsynaptic region of the neurons, where acetylcholine receptors are expressed (Lin, 2009).

In conclusion, these genetic studies in Drosophila suggest that Abi and Abl play opposing roles in axonogenesis and synaptogenesis. This conclusion is further supported by a series of biochemical, immunocytochemical and morphological studies in cultured cells. These findings offer new insights into the functional interaction between Abl, Abi and Ena in nervous system development (Lin, 2009).

Enabled signaling pathway regulates Golgi architecture in Drosophila photoreceptor neurons

The golgi apparatus is optimized separately in different tissues for efficient protein trafficking, little is known of how cell signaling shapes this organelle. This study finds 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 of 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, it was demonstrated 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 (Kannan, 2014).

The Abl tyrosine kinase signaling pathway controls golgi morphology and localization in Drosophila photoreceptors through its regulation of the actin cytoskeleton. Ena, the main effector of Abl in morphogenesis, is associated with the cis-golgi compartment, and it regulates golgi localization and dynamics under the control of Abl and its interacting adaptor protein, Dab. Reducing the levels of Abl or Dab, or overexpressing Ena, led to similar defects in golgi fragmentation state and subcellular distribution. During golgi biogenesis, Abl increases the frequency of fusion of golgi cisternae, and decreases fission events. Abl evidently controls golgi organization through its regulation of actin structure, as the effect of Abl signaling on golgi could be blocked by modulating actin structure genetically or pharmacologically. Collectively, these data reveal an unexpected link between a fundamental tyrosine kinase signaling pathway in neuronal cells and the structure of the golgi compartment (Kannan, 2014).

The data reported here suggest that the Abl signaling pathway controls golgi morphology and localization through its control of actin structure. This is consistent with previous reports that altering the levels of actin modulators perturbs the structure and function of the golgi apparatus. A variety of proteins that modulate actin dynamics have been localized to golgi. Ultra-structural studies established the association of actin filaments with golgi membranes and the association of β and γ actin with the golgi. In cultured cell models, including neurons, actin depolymerization leads to golgi compactness, fragmentation and altered subcellular distribution. It is noted, moreover, that the reported golgi-associated signaling proteins include several that have been linked to Abl signaling, including the Abl target Abi, the Abi binding partner WAVE, and various effectors of Rac GTPase including ADF/cofilin, WASH and Arp2/3. Thus, for example, Abi and WAVE have been implicated in actin dependent golgi stack reorganization and in scission of the golgi at cell division to allow faithful inheritance of golgi complex to daughter cells in Drosophila S2 cell cycles (Kondylis, 2007). These data reinforce the importance of actin-regulating signaling pathways for controlling golgi biogenesis (Kannan, 2014).

Two lines of evidence suggest that the increase observed in golgi number in Abl pathway mutants is due primarily to net fragmentation of pre-existing golgi cisternae and not to de novo synthesis of golgi. First, live imaging of golgi dynamics in neurons of the Drosophila eye disc reveals that the steady-state number of golgi cisternae reflects an ongoing balance of fusion and fission events, much as observed previously in yeast. Quantification of these events in wildtype vs Abl mutant tissue demonstrated directly that loss of Abl significantly increased the frequency of fission events, and reduced the frequency of fusions. Second, the absolute volume of cis-golgi in Abl mutant photoreceptors was not substantially greater than that in wildtype, as judged by direct measurement of the volume of GM130- immunoreactive material in deconvoluted image stacks of photoreceptor clusters. While a small apparent increase was observed in golgi volume in the mutants (~55%, based on pixel counts), it is noted that golgi cisternae are small on the length scale of the point spread function of visible light, such that the fluorescent signal from a single cisterna extends into the surrounding cytoplasm. The increase in apparent golgi volume is therefore within the range expected due simply to fluorescence 'spillover' from the three-fold greater number of separate golgi cisternae in the mutants (Kannan, 2014).

It is striking that both increase and decrease of Ena led to net fragmentation of golgi. Why might this be? It is known that both fission and fusion of membranes requires actin dynamics: at scission, polymerization provides force for separating membranes, while in fusion, actin polymerization is essential for bringing membranes together and for supplying membrane vesicles, among other things. As a result, altering actin dynamics is apt to change the probabilities of multiple aspects of both fission and fusion events, making it impossible to predict a priori how the balance will be altered by a given manipulation, just as either increase or decrease of Ena can inhibit cell or axon motility, depending on the details of the experiment, due to the non-linear nature of actin dynamics. Indeed, this study also observed net golgi fragmentation when actin was stabilized with jasplakinolide, just as was done from depolymerization with cytochalasin or latrunculin. More direct experiments will be necessary to fully understand this dynamic, however. deficits selectively disrupt dendritic morphogenesis but not axogenesis, and perhaps consistent with this, Abl/Ena function is essential for dendrite arborization in these cells but has not been reported to affect their axon patterning. Finally, in some contexts, neuronal development requires local translation of guidance molecules in the growth cone rather than translation in the cell soma. It is likely that the need for actin dynamics to target different subcellular compartments in different cell types will be reflected in different patterns of Abl/Ena protein localization (Kannan, 2014).

This study reports the role of Abl/Ena-dependent regulation of actin structure on overall golgi structure and localization but there may be more subtle effects on golgi function as well. For example, recent evidence supports a role for actin-dependent regulation of the specificity of protein sorting in the golgi complex. Preferential sorting of cargos is achieved by nucleation of distinct actin filaments at the golgi complex. In Hela cells, for example, Arp2/3 mediated nucleation of actin branches at cis-golgi regulates retrograde trafficking of the acid hydroxylase receptor CI-MPR, while Formin family mediated nucleation of linear actin filaments at golgi regulates selective trafficking of the lysosomal enzyme cathepsin D. Similarly, the actin-severing protein ADF/cofilin, the mammalian ortholog of Drosophila twinstar, sculpts an actin-based sorting domain at the trans-golgi network for selective cargo sorting. It will be important to investigate whether the effects of Abl/Ena on golgi morphology have functional consequences on bulk secretion or protein sorting (Kannan, 2014).

Protein trafficking and membrane addition in neurons need to be coordinated with the growth requirements of the axonal and dendritic plasma membranes, but the mechanisms that do so have been obscure. Abl pathway proteins associate with many of the ubiquitous guidance receptors that direct axon growth and guidance throughout phylogeny, including Netrin, Roundabout, the receptor tyrosine phosphatase DLAR, Notch and others. The data therefore suggest a potential link between the regulatory machinery that senses guidance information and the secretory machinery that helps execute those patterning choices. Indeed, preliminary experiments suggest that some of the axonal defects of Abl pathway mutants may arise from alterations in golgi function. Beyond this, Abl signaling is essential in neuronal migration, epithelial polarity and integrity, cell adhesion, hematopoiesis and oncogenesis, among other processes The data reported in this study now compel a reexamination of the many functions of Abl to ascertain whether some of these effects arise, at least in part, from regulation of secretory function (Kannan, 2014).

Ena/VASP proteins cooperate with the WAVE complex to regulate the actin cytoskeleton

Ena/VASP proteins and the WAVE regulatory complex (WRC) regulate cell motility by virtue of their ability to independently promote actin polymerization. This study demonstrates that Ena/VASP and the WRC control actin polymerization in a cooperative manner through the interaction of the Ena/VASP EVH1 domain with an extended proline rich motif in Abi. This interaction increases cell migration and enables VASP to cooperatively enhance WRC stimulation of Arp2/3 complex-mediated actin assembly in vitro in the presence of Rac. Loss of this interaction in Drosophila macrophages results in defects in lamellipodia formation, cell spreading, and redistribution of Ena to the tips of filopodia-like extensions. Rescue experiments of abi mutants also reveals a physiological requirement for the Abi:Ena interaction in photoreceptor axon targeting and oogenesis. These data demonstrate that the activities of Ena/VASP and the WRC are intimately linked to ensure optimal control of actin polymerization during cell migration and development (Chen, 2014).

Ena/VASP proteins regulate cell migration by promoting actin polymerization at the plasma membrane via antagonizing actin filament capping and acting as processive actin polymerases. Each family member consists of an N-terminal EVH1 domain, a central proline-rich region, and a C-terminal EVH2 domain. The EVH2 domain, which contains monomeric and F-actin binding sites, is responsible for promoting actin polymerization. In contrast, the EVH1 domain mediates intracellular targeting of Ena/VASP proteins by interacting with a sequence (D/E)FPPPPX(D/E)(D/E), which is referred to as the 'FPPPP' motif because these residues are essential for binding. Ena/VASP proteins are recruited to focal adhesions by zyxin, which contains four 'FPPPP' motifs. The ability of Ena/VASP proteins to control cell migration, however, depends on their recruitment to the leading edge, by 'FPPPP' motif containing MRL proteins (Mig10, RIAM, and Lamellipodin) (Chen, 2014).

Of all the proteins interacting with the EVH1 domain of Ena/ VASP proteins, Testin (Tes), a focal adhesion protein, stands out as the only one that lacks an 'FPPPP' motif. Tes negatively regulates the localization of Mena at focal adhesions and also inhibits Mena-dependent cell migration. Tes interacts with Mena via its C-terminal LIM3 domain and is unique in being the only protein that binds a single Ena/VASP family member. Given the interaction of Tes with Mena, additional atypical EVH1 binding partners that also lack 'FPPPP' motifs were sought. The EVH1 domain interacts directly with Abi, a component of the WAVE regulatory complex (WRC), which plays an essential role in driving cell migration by activating the Arp2/3 complex in response to Rac signaling. These observations demonstrate that the EVH1:Abi interaction enhances cell migration and the ability of Rac-activated WRC to promote Arp2/3- mediated actin polymerization as well as the function of WRC in vivo in Drosophila (Chen, 2014).

The WRC binds and activates the Arp2/3 complex to drive actin polymerization at the plasma membrane in response to Rac signaling during cell migration (Bisi, 2013). In contrast, Ena/VASP proteins stimulate cell migration by antagonizing actin filament capping and acting as processive actin polymerases. This study has now demonstrated that Ena/VASP proteins can be linked to the function of WRC by virtue of a direct interaction between their EVH1 domains and Abi, an integral component of the WRC (Chen, 2014).

The results have confirmed and extended previous yeast two-hybrid data and pull-downs from cell lysates demonstrating that the EVH1 domains of Mena and VASP can interact with human and mouse Abi1. The structure of several EVH1:FPPPP complexes reveals that the 'FPPPP' motif adopts a type II polyproline helix that is coordinated by three aromatic residues present in all Ena/VASP family members. In contrast, the EVH1 domain interacts with an extended proline-rich binding site in human Abi1. Consistent with their ability to bind, Abi2 has an almost identical sequence whereas Abi3 has two 'LPPPP' motifs in this region. In many respects, the extended nature of the Abi1 interaction resembles that of the N-WASP WH1 binding site in WIP, which also involves three regions of contact. In classical EVH1 interactions, the acidic residues flanking the 'FPPPP' motif play an important role in determining the affinity, orientation and specificity of EVH1 binding. In contrast, the EVH1 binding site in human Abi1 contains two pairs of aspartic acid residues flanking the central phenylalanine in the middle of the motif as well as a downstream acidic patch (DYEDEE). The molecular basis of the EVH1 human Abi1 interaction, including the extended peptide orientation and role of acidic residues, must await structural determination of the complex. Nevertheless, the data clearly demonstrate that the EVH1 domain can bind additional proline rich ligands beyond 'FPPPP' motifs (Chen, 2014).

Interestingly, the meander region of WAVE1 contains an 'LPPPP' motif that is capable of interacting with Mena. The ability of Mena to bind Abi in the WRC presumably explains why it still associates with WAVE lacking its proline rich region. Consistent with the presence of 'LPPPP' motifs pull-downs with recombinant proteins demonstrate that the EVH1 domain of Mena can interact with WAVE 1 and 2, but not WAVE 3. These observations, however, suggest that the interaction with Abi is more important for Mena interactions with the WRC than WAVE. Moreover, in vitro assays clearly demonstrate that the ability of Rac to activate WRC-mediated actin polymerization via the Arp2/3 complex is significantly enhanced by VASP binding to Abi. In contrast to the full-length protein, monomeric VASP or its isolated EVH1 domain is unable to activate the WRC to stimulate Arp2/3-mediated actin polymerization even at high concentrations. This difference may reflect the ability of the VASP tetramer to induce oligomerization of the WRC, an effect that would enhance WRC potency toward the Arp2/3 complex. It is possible that the simultaneous engagement of a VASP tetramer with Abi and the 'LPPPP' motif in WAVE increases the activity of the WRC. However, oligomerization alone cannot account for the data because mutating the actin binding elements of VASP, which should have no effect on tetramerization, abrogates activity. Furthermore, the VASP effect does not appear to be simple allosteric activation of the WRC (i.e., release of the VCA), because this should produce activity equal to that of the VCA alone. While not definitive, the collective data are most consistent with a model in which VASP binds the Rac-activated WRC with high affinity based on tetramerization-mediated avidity and also interacts with actin filaments, thus increasing the association of the WRC with filaments. Because both the released WAVE VCA and actin filaments activate the Arp2/3 complex, assembling these two elements should enhance their cooperative actions and increase actin assembly (Chen, 2014).

In contrast to the situation in humans, the interaction between the EVH1 domain of Ena and Abi in Drosophila is mediated by two 'LPPPP' motifs located in a proline rich region of Abi. The loss of these two 'LPPPP' motifs increases the dynamics of the WRC at the plasma membrane but does not affect lamellipodium formation in S2 cells in culture. In contrast, the consequences of disrupting the interaction of Ena with Abi in vivo are more dramatic, as primary macrophages expressing Abi- DEna have reduced lamellipodial membrane protrusions and defects in cell morphology. Unlike the situation in S2 cells, which have been treated with dsRNA and transiently transfected with GFP-tagged expression constructs, the abi transgenes (Abi and AbiDEna) are expressed from the same genomic locus (Chen, 2014).

These in vivo rescue experiments therefore allow for a more precise analysis of the requirement of the interaction between Ena and Abi rather than in the hypomorphic situation in S2 cells. The ability of AbiDEna to rescue lamellipodium formation in S2 cells might reflect an incomplete abi knockdown or a difference in its expression level compared to endogenous Abi in untreated cells. Consistent with this, in macrophages, this study found that strong expression of Abi in earlier larval stages using the da-Gal4 driver results in a more robust rescue of lamellipodia protrusion and cell morphology defects as compared to macrophage-specific expression (hmlD-gal4) at late larval stages. Given that in vitro actin polymerization assays indicate that VASP (Ena) is not an essential activator but rather acts cooperatively with Rac1 to promote WRC activation, it is likely that in vivo the requirement for this interaction depends on the level of Abi. This explanation may also partially account for the more dramatic phenotypes observed in the multicellular context (Chen, 2014).

Remarkably, this study found that the loss of the ability of Abi to interact with Ena resulted in a similar defect in R-cell targeting as the absence of the complete protein. This suggests that Ena has a nonautonomous role in the larval brain, as has been previously shown for WRC function in targeting of early retinal axons (Stephan, 2011). Mosaic mutant analysis further supports a nonautonomous function for Ena in retinal axon targeting. Thus, it is proposed that the interaction between Ena and the WRC is required to regulate actin dynamics in the target area neurons. However, since the precise projection pattern of early retinal axons depends on complex interactions between different populations of glia cells and neurons in the target field, it remains unclear how Ena and the WRC function together in this developmental context. In contrast, Drosophila oogenesis provides an excellent model to study the cell autonomous function of the interaction between Ena and the WRC (Chen, 2014).

Previous phenotypic analyses of mutant egg chambers suggest Ena and WRC have both distinct and overlapping functions during oogenesis. Both are required for the integrity of the cortical actin in nurse cells and mutant egg chambers become multinucleated as the plasma membrane breaks down due to a loss of cortical actin integrity. In contrast, to wave mutant egg chambers, disruption of ena function does not affect ring canal morphology but rather leads to a reduced and delayed formation of cytoplasmic actin filament bundles. Similar to wave germline clones, the loss of abi in the germline results in a dumpless mutant phenotype and female flies are sterile (Zobel, 2013). This study has found that these defects in egg morphology and female fertility cannot be rescued by reexpression of a full-length Abi deficient in Ena binding. AbiDEna mutant egg chambers have defects in the integrity of the nurse cell cortical actin resulting in detached cytoplasmic actin bundles and ring canals. The rupture of nurse cell membranes is even more obvious at later stages when the fast transport of nurse cell contents starts, as recently observed for ena, wave, and abi mutants (Chen, 2014).

In addition to nurse cell dumping defects, a striking egg chamber elongation defect was also observed. Mutant eggs lacking the interaction between Abi and Ena fail to elongate and remain spherical as similarly found in rac or pak mutants. The round egg phenotype observed in flies expressing AbiDEna suggests that there might be a defect in the basal actin cytoskeleton of the follicle cells that drives egg chamber elongation. Consistently, reexpression of AbiDEna in somatic follicle cells (abi, da > UASt-AbiDEna) also results in a round-egg phenotype. These data suggest a requirement of WRC function in follicle cells during egg elongation. Supporting this notion, this study found that a follicle cell-specific knockdown of Sra-1 function results in a strong round-egg phenotype (Chen, 2014).

The rescue experiments additionally imply a more complex interaction network among Ena, Abi, and SH3 interacting proteins. Whereas a minimal Abi fragment lacking the Ena-binding or proline-rich region and the C-terminal SH3 domain is able to rescue substantially abi mutant traits in Drosophila and Dictyostelium, the disruption of Ena-binding alone completely abolishes Abi activity. Thus, a scenario is proposed in which the influence of Ena on WRC activity depends on additional proteins interacting with the Abi-SH3 domain. The most prominent candidate is the nonreceptor tyrosine kinase Abelson (Abl) that binds Abi and Ena proteins. Based on the antagonistic genetic interaction between ena and abl, it has been hypothesized that a precise balance between Abl and Ena activity is required for fly viability. However, it is still unclear how Abl affects the function of Ena, because mutation of all known Abl phosphorylation sites only has a modest effect on Ena function in vivo. Similarly, Abl and Abi have opposing roles in Drosophila. Thus, a model is proposed in which Ena synergizes with Rac to activate the WRC, but also inhibits Abl function. Abl in turn inhibits WRC function. Thus, the disruption of Ena binding to dAbi would simultaneously decrease WRC stimulation by Ena and increase its inhibition by Abl. Such a scenario would explain why loss of Ena binding to Abi (WRC) phenocopies the abi mutants. This also suggests that the interaction among WRC, Abl, and Ena function is of more general relevance for actin-based processes in multicellular contexts. Furthermore, recent data also suggest that lamellipodin, which cooperates with the WRC to promote cell migration in vivo, is also likely to be part of this complex regulatory network, because it can bind both the EVH1 domain of VASP and the SH3 domain of Abi. In summary, these in vitro data clearly demonstrate that Ena/VASP proteins can directly affect the activity of the WAVE complex, whereas the observations in Drosophila have revealed that, in vivo, the function and activity of Ena/VASP proteins and the WAVE complex are intimately linked (Chen, 2014).


GENE STRUCTURE

cDNA clone length - 3.4 kb


PROTEIN STRUCTURE

Amino Acids - 684

Structural Domains

The most notable feature of the predicted ENA protein is a proline-rich core, with 58 proline residues located between amino acids 340-480 and seven matches to the proline-rich consensus site for binding the ABl SH3 domain (Gertler, 1995).

The ActA protein of the intracellular pathogen Listeria monocytogenes induces a dramatic reorganization of the actin-based cytoskeleton. Two profilin binding proteins, VASP and Mena, are the only cellular proteins known so far to bind directly to ActA. This interaction is mediated by a conserved module, the EVH1 domain. E/DFPPPPXD/E, a motif repeated four times within the primary sequence of ActA, is identified as the core of the consensus ligand for EVH1 domains. This motif is also present and functional in at least two cellular proteins, zyxin and vinculin, which are in this respect major eukaryotic analogs of ActA. The functional importance of the novel protein-protein interaction was examined in the Listeria system. Removal of EVH1 binding sites on ActA reduces bacterial motility and strongly attenuates Listeria virulence. ActA-EVH1 binding is a paradigm for a novel class of eukaryotic protein-protein interactions involving a proline-rich ligand that is clearly different from those described for SH3 and WW/WWP domains. This class of interactions appears to be of general importance for processes dependent on rapid actin remodeling (Niebuhr, 1997).

The Enabled/VASP homology 1 (EVH1; also called WH1) domain is an interaction module found in several proteins implicated in actin-based cell motility. EVH1 domains bind the consensus proline-rich motif FPPPP and are required for targeting the actin assembly machinery to sites of cytoskeletal remodeling. The crystal structure of the mammalian Enabled (Mena) EVH1 domain complexed with a peptide ligand reveals a mechanism of recognition distinct from that used by other proline-binding modules. The EVH1 domain fold is unexpectedly similar to that of the pleckstrin homology domain, a membrane localization module. This finding demonstrates the functional plasticity of the pleckstrin homology fold as a binding scaffold and suggests that membrane association may play an auxiliary role in EVH1 targeting (Prehoda, 1999).


enabled: | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References
date revised: 8 dec 96 
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