InteractiveFly: GeneBrief

Abelson Interacting Protein: Biological Overview | References


Gene name - Abelson Interacting Protein

Synonyms - UAP56

Cytological map position - 88A9-88A9

Function - signaling

Keywords - Modulation of WASP-/WAVE F-actin formation and Abelson tyrosine kinase activity

Symbol - Abi

FlyBase ID: FBgn0020510

Genetic map position - 3R:9,944,982..9,948,142 [-]

Classification - Src homology 3 domains, Abl-interactor HHR

Cellular location - cytoplasmic



NCBI links: Precomputed BLAST | EntrezGene
BIOLOGICAL OVERVIEW

One of the central regulators coupling tyrosine phosphorylation with cytoskeletal dynamics is the Abelson interactor (Abi). Its activity regulates WASP-/WAVE (see SCAR) mediated F-actin formation and in addition modulates the activity of the Abelson tyrosine kinase (Abl). Drosophila Abi is capable of promoting bristle development in a wasp dependent fashion. This study reports that Drosophila Abi induces sensory organ development by modulating EGFR signaling. Expression of a membrane-tethered activated Abi protein (AbiMyr) leads to an increase in MAPK activity. Additionally, suppression of EGFR activity inhibits the induction of extra-sensory organs by AbiMyr, whereas co-expression of activated AbiMyr and EGFR dramatically enhances the neurogenic phenotype. In agreement with this observation Abi is able to associate with the EGFR in a common complex. Furthermore, Abi binds the Abl tyrosine kinase. A block of Abl kinase-activity reduces Abi protein stability and strongly abrogates ectopic sensory organ formation induced by AbiMyr. Concomitantly, changes were noted in tyrosine phosphorylation supporting previous reports that Abi protein stability is linked to tyrosine phosphorylation mediated by Abl (Stephan, 2008).

A variety of morphogenetic events during metazoan development such as cell fate determination, proliferation and differentiation require a highly regulated network of signaling pathways resulting in distinct cellular responses. Cell-surface receptors and tyrosine kinases often play an important role in the control of these processes. Among the growth factor receptors, the best understood example is the epidermal growth factor receptor (EGF receptor), which plays important roles in physiological and pathological processes. Unlike the vertebrate homologues the Drosophila genome contains only a single gene encoding a member of the EGF receptor family, called DER/EGFR. In Drosophila, the EGFR pathway is required at multiple times during fly development such as oogenesis, axis determination and sensory organ formation during embryonic and imaginal disc development (Stephan, 2008).

The emergence of Drosophila sensory organs depends on a selection process called lateral inhibition where only one cell is singled out of a group of equivalent proneural cells to become a sensory organ precursor (SOP). The Drosophila notum bears many mechano-sensory bristles, 22 large macrochaetes and about 200 smaller microchaetes. In both cases, bristle formation and spacing requires cell–cell communication mediated by lateral inhibition. The microchaetes on the notum are arranged in rows and are variable in number and position within the rows. Unlike the microchaetes, the large macrochaetes are not arranged in rows, their number is constant and they are formed from proneural clusters at fixed stereotypic positions (Stephan, 2008 and references therein).

Once determined, the SOP undergoes a series of stereotyped asymmetric cell divisions giving rise to the different cells comprising an individual sensory organ. In Drosophila, two major antagonistic signaling pathways regulate the formation of SOPs, the Notch and the EGFR signaling pathways. Notch is pivotal for the process of lateral inhibition which restricts the formation of neural cells. Alternatively, activation of the EGF receptor (EGFR) positively regulates SOP formation. Loss of EGFR function leads to a loss of sensory macrochaetes whereas gain of EGFR-function evokes the formation of additional macrochaetes by stimulating proneural gene autoregulation (Stephan, 2008).

Upon EGFR activation in Drosophila a membrane-localized protein complex is formed acting through the Ras-signaling. The activation of Ras results in the translocation of di-phosphorylated MAPK from the cell surface to the nucleus, where it leads to the expression of the proneural gene scute, which in turn promotes sensory organ development (Stephan, 2008).

This study shows that Abi regulates sensory organ development by modulating EGFR signaling. Following expression of an activated Abi (AbiMyr) in proneural clusters, an increase was noted in MAPK activity. Inhibition of the EGFR pathway suppresses the formation of ectopic sensory organs evoked by AbiMyr. Furthermore, co-expression of activated AbiMyr and EGFR dramatically enhances the induction of extra-sensory organs. This effect also depends on the presence of active Abl tyrosine kinase, which stabilizes and further stimulates Abi by tyrosine phosphorylation (Juang, 1999). The physical interaction of Abi and EGFR depends on the membrane recruitment whereas the ability of the complex to induce neural development depends on both the membrane localization and the C-terminal SH3 domain of Abi, which can bind Wasp as well as the Abl kinase. These data suggest that once Abi is recruited to the membrane, EGFR signaling becomes activated to promote SOP development (Stephan, 2008).

Thus, overexpression of the Abelson kinase and its activator Abi is sufficient to promote the formation of ectopic sensory organs. Previous work has shown a partial loss of sensory organs in adult flies expressing a double-stranded abi RNA (Bogdan, 2005). However, expression of double-stranded RNA can lead to off target effects and confirmation of the mutant phenotypes by genetic analysis is needed. To address this question abi null alleles were generated. However, animals lacking zygotic abi function die relatively late during development as early pupae and no defects can be noted in the SOP pattern. This suggests that either abi is not required for SOP development or that its function is obscured by maternal contributions. In support for this notion, residual Abi protein can be detected in homozygous abi mutant larvae. In addition as previously reported for the abl kinase gene, abi might be integrated into a complex genetic network that masks direct requirements. The role of Abl is only exposed by the simultaneous loss of interaction partners such as disabled or failed axon connections (Stephan, 2008).

The Abi proteins are versatile molecules comprised of multiple protein-protein interaction domains allowing the formation of different protein complexes. Abi does not only function as an activator of the Abelson kinase but in turn Abl positively regulates Abi stability through phosphorylation (Juang, 1999; Huang, 2007). Moreover Abi plays an important role in regulating F-actin dynamics by activating Wave and Wasp proteins, two main activators of Arp2/3-dependent actin polymerization (Bogdan, 2005; Dai, 1995; Innocenti, 2005; Innocenti, 2004; Kunda, 2003; Stephan, 2008 and references therein).

How can alterations in actin dynamics modulate EGFR signaling? Previous studies have shown that cytoskeletal rearrangements are likely to induce the asymmetric distribution and clustering of the EGFR in neuronal stem cells to generate diverse CNS progenitor cells. Since Abi functions as a potent activator of Wasp (Bogdan, 2005; Innocenti, 2005), the enhanced formation of F-actin may lead to clustering of the EGF receptor at the membrane to stimulate its sustained activation (Stephan, 2008).

The enhancement of actin polymerization may also affect endocytosis of the EGFR, which is F-actin dependent and as recently shown can be promoted by Abi and Wasp activity (Benesch, 2005; Innocenti, 2005; Sirotkin, 2005). Although it has long been assumed that endocytosis is the main pathway involved in down-regulation of the EGFR, endocytosis is also directly linked to EGFR activation. Block of endocytosis of the EGFR specifically abrogates MAPK activity. Thus Abi could be involved establishing active signaling compartments. Unfortunately, at present the molecular tools to determine the subcellular localization of the activated EGFR in the developing fly are unavailable (Stephan, 2008).

In addition, activation of the Abi complex may recruit kinases such as Abl to the EGF receptor. Abi acts as a positive regulator of the Abl kinase activity promoting the phosphorylation of several substrates including Mena/Ena, Wave, N-Wasp, the B-cell adaptor for phosphoinositide 3-kinase (BCAP) or Cdc2 (Burton, 2005; Leng, 2005; Lin, 2004; Maruoka, 2005; Tani, 2003). Interestingly, Abi is also a substrate of Abl resulting in increased protein stability (Juang, 1999; Huang, 2007). This positive regulatory loop could hint to an important mechanism for regulating the level of Abl kinase activity during sensory organ development. Indeed, co-expression of AbiMyr with a kinase-defective Abl suppresses tyrosine hyperphosphorylation as well as reduces the level of the AbiMyr protein expression, corresponding to a strong suppression of the number of ectopic sensory organs induced by AbiMyr (Stephan, 2008).

However, the induction of ectopic sensory bristles by Abl does not depend on an intact EGFR signaling or on an activation of the Ras/Raf/MAPK pathway. Co-expression of Abl and EGFR results in an increase of sensory organs but inhibition of EGFR by co-expression of Kekkon1 or heterozygous loss of pole hole function are not sufficient to suppress the Abl-induced neurogenic phenotype suggesting that Abi and Abl act in parallel. In conclusion, it is proposed that the complex of Abi and the EGFR is joined by Abl, which stabilizes not only Abi but also stimulates Wasp function, highlighting the intricate interaction between cytoskeletal dynamics and signaling during sensory organ development (Stephan, 2008).

The WAVE regulatory complex links diverse receptors to the actin cytoskeleton

The WAVE regulatory complex (WRC) controls actin cytoskeletal dynamics throughout the cell by stimulating the actin-nucleating activity of the Arp2/3 complex at distinct membrane sites. However, the factors that recruit the WRC to specific locations remain poorly understood. This study has identified a large family of potential WRC ligands, consisting of approximately 120 diverse membrane proteins, including protocadherins, ROBOs, netrin receptors, neuroligins, GPCRs, and channels. Structural, biochemical, and cellular studies reveal that a sequence motif that defines these ligands binds to a highly conserved interaction surface of the WRC formed by the Sarah and Abi subunits. Mutating this binding surface in flies resulted in defects in actin cytoskeletal organization and egg morphology during oogenesis, leading to female sterility. These findings directly link diverse membrane proteins to the WRC and actin cytoskeleton and have broad physiological and pathological ramifications in metazoans (B. Chen, 2014).

A consensus peptide motif, WIRS, specifically binds to a unique surface formed by the Sra and Abi subunits of the WRC. Strict conservation of the binding surface suggests that this interaction is broadly important to metazoans. The WIRS motif defines a novel class of WRC ligands that contains ~120 diverse membrane proteins. Genetic data further show that mutating the WIRS binding site of the WRC in Drosophila disrupts actin cytoskeleton organization and egg morphology during oogenesis, leading to female sterility, and also disrupts development of the visual system. In summary, these data characterize a widespread and conserved interaction that may link numerous membrane proteins to the WRC and the actin cytoskeleton (B. Chen, 2014).

The WIRS binding surface is contributed by both the Sra and Abi subunits of the WRC and therefore is only present in the fully assembled complex. Consequently, the WIRS interaction is unique to the intact WRC and cannot occur with individual subunits or subcomplexes. This may have important functional implications because, in cells, individual WRC subunits may form complexes with other proteins. For example, Sra1 binds the fragile-X mental retardation syndrome protein FMRP, along with the translation initiation factor eIF4E, using an interaction surface that is normally buried within the WRC. Moreover, Abi has been shown to interact with other proteins independent of its assembly into the WRC, including another member of the Wiskott-Aldrich syndrome protein WASP and the Diaphanous-related formin. Finally, the Nap1 ortholog Hem1 was suggested to exist in large complexes distinct from the WRC. These various complexes likely have distinct cellular functions. For example, the Sra1-FMRPeIF4E complex regulates mRNA localization and protein translation, and the Abi complexes were shown to regulate the actin cytoskeleton in processes distinct from those regulated by the WRC. Therefore, the multisubunit nature of the WIRS binding site may provide a mechanism to specifically regulate the intact WRC (B. Chen, 2014).

WIRS proteins can directly recruit the WRC to membranes, likely in cooperation with the other classes of WRC ligands. WIRS proteins may also have additional effects on the biochemical activity of the WRC. For example, this study has demonstrated that, although the minimal WIRS motif does not activate the WRC, sequences flanking the motif can potentiate (as in PCDH10) or inhibit (as in PCDH17) activity of the WRC in vitro. Therefore, WIRS proteins may exert different effects on the activity of the assembly, again likely in cooperation with other WRC ligands such as Rac1 or kinases. Alternatively, WIRS proteins could act as a scaffold and modulate WRC activity by coordinately recruiting the complex and other ligands. For example, the cytoplasmic tail of the NMDA receptor subunit NR2B could potentially corecruit cyclin-dependent kinase 5 (Cdk5) and the WRC to facilitate phosphorylation and consequent activation of WAVE. In fact, many WIRS-containing proteins are thought to function as scaffolds, including APC, Ankyrin, WTX/Amer1, Shroom, and Shank. Finally, many WIRS proteins are cell-cell adhesion receptors, which are often densely clustered at the plasma membrane. Such clustering would locally concentrate the WRC, a process known to increase the activity of WASP proteins toward the Arp2/3 complex (B. Chen, 2014).

Finally, WIRS/WRC interactions themselves are likely regulated. In fact, the data suggest that the WIRS motif (F-x-T/S-FX- X) could be modulated by phosphorylation. High-affinity binding requires Thr or Ser at the third position of the WIRS motif. No other residues examined were tolerated. Thus, it is likely that Thr/Ser phosphorylation at this position would block binding as well. Indeed, phosphorylation of various WIRS sites has been identified not only in global phosphoproteome studies but also by site-specific mutagenesi. Together, these various mechanisms could bring a large range of regulatory dynamics to locally tune WRC activity and consequent actin assembly in vivo (B. Chen, 2014).

The conservation of the WIRS binding surface in virtually all metazoans suggests that the WIRS/WRC interaction is broadly important and unique to animals because it is absent from other eukaryotes, including protists, fungi, and plants. It is notable that the WIRS binding surface is found even in nonmetazoan choanoflagellates, suggesting that WIRS/WRC interactions appeared more than 700 million years ago in an early ancestor that predates metazoans. Choanoflagellates are considered to be the closest living relatives to metazoans because they encode many metazoan-specific protein domains, including various cell adhesion molecules and proteins enriched in the nervous system. Although choanoflagellates are generally considered unicellular organisms, they can form simple colonies, leading to the possibility that the WIRS interaction arose to maintain multicellularity. However, this interaction may not be strictly necessary for multicellularity, as the WIRS binding surface is not found in the placozoan T. adhaerens, a primitive, amoeboid- like metazoan that lacks tissues or organs but is made up of distinct cell types. Moreover, a significant number of nonadhesion proteins also contain WIRS motifs, indicating that the WIRS interaction likely developed additional functions (B. Chen, 2014).

In this study, the search was limited to proteins whose WIRS motifs were conserved in four of seven representative species. Among the ~120 WIRS proteins, some display high conservation of their WIRS motifs. These include netrin receptors and ROBO proteins, whose WIRS motifs are conserved from human to C. elegans, despite a significant divergence in the overall sequences of their cytoplasmic tails. The WIRS motifs of many other proteins, including protocadherins and neuroligins, are conserved in all vertebrates examined (from human to zebrafish). It is noted that, by using conservation as a criterion in the search, other bona fide WIRS ligands that are less conserved might have been missed (B. Chen, 2014).

This study has demonstrated biological functions of WIRS/WRC interactions in animals by using Drosophila oogenesis as a model system. Defects observed by disrupting the WIRS binding surface, which resulted in defective egg morphology, disrupted actin cytoskeleton, and female sterility, resemble defects that arise from knocking out the WRC, suggesting that the WIRS interaction plays a major role in regulating WRC function during oogenesis in flies. Additionally, it was observed that the WIRS binding site is also important to the WRC in its non-cell-autonomous function of regulating photoreceptor axonal targeting in developing optic lobes. It is believed that many more WIRS-mediated regulatory functions are yet to be discovered. In support of this assertion, in C. elegans, WIRS-mediated interaction of the neuronal adhesion receptor SYG-1 with the WRC has been shown to regulate actin assembly at presynaptic sites in the neuromuscular junction of the egglaying motor neuron HSN and consequently is critical in initiating both synaptogenesis and axonal branching. It has been proposed that WIRS/WRC interactions are of general and diverse importance to animals throughout development (B. Chen, 2014).

Future studies are needed to reveal which specific WIRScontaining ligands are important to particular processes. Prior data in the literature suggest candidate WIRS proteins during oogenesis. Two membrane-associated proteins, P08630 (Tec29 tyrosine kinase) and Q9VCX1 (locomotion defects protein, Loco), both contain WIRS motifs and have been shown to regulate nurse cell dumping. Loco was also found to regulate the cortical actin cytoskeleton in glia. Phenotypic analysis also reveals an opposite oogenesis defect, which is similar to those observed in kugelei mutants deficient for dFAT2, another WIRS-containing protein. It remains to be determined whether these proteins or others are directly linked to the WRC during this process (B. Chen, 2014).

A variety of evidence also exists in the literature, suggesting functional roles of the WIRS interaction in other biological processes. In addition to PCDH10 and PCDH19, the WIRS proteins DCC and ROBO and the epithelial sodium channel ENaC (γ subunit) have been genetically linked to the WRC. DCC and ROBO differentially regulate the abundance and subcellular localization of the WRC to control the actin cytoskeleton in C. elegans embryonic epidermis. The WRC and Rac1 were found to be essential in regulating the activity of ENaC. The current data suggest that these genetic interactions may be due to direct physical interactions of WIRS motifs with the WRC. The functions of many other WIRS proteins, only a few of which have been previously linked to the actin cytoskeleton (e.g., glutamate receptor NR2B and the postsynaptic cell adhesion molecule Neuroligin1), may also depend on an interaction with the WRC. As a notable example, a 21 amino acid sequence of the Neuroligin1 cytoplasmic tail harboring a WIRS motif (PGIQPLHTFNTFTGGQNNTLP, WIRS bold is required for presynaptic terminal maturation (B. Chen, 2014).

Although it is still very premature to link WIRS/WRC interactions to any disease, several cases are suggestive. For example, seven cases of epilepsy and mental retardation in females (EFMR) were reported to arise from truncations of the cytoplasmic tail of PCDH19, all resulting in the loss of its WIRS motif. Additionally, partial truncation of the DCC cytoplasmic tail, along with its WIRS motif, caused congenital mirror movement in four affected members of a three generation Italian family. Finally, a point mutation (S1359C) that disrupts the WIRS site (LDSFES, S1359) in the adenomatous polyposis coli (APC) protein was associated with three unrelated cases of hepatoblastoma (B. Chen, 2014).

In summary, this study has defined and characterized a large family of potential WRC ligands unique to metazoans. A large and diverse set of membrane proteins comprises this class, many with important biological functions. These findings provide a mechanistic framework to understand how these proteins signal downstream to the actin cytoskeleton via direct interaction with the WRC and how their mutations may ultimately lead to disease (B. Chen, 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 (X. 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) (X. 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 (X. 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 (X. 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 (X. 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 (X. 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 (X. 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 (X. 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 (X. 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 (X. 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 (X. 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 (X. Chen, 2014).

Lamellipodin and the Scar/WAVE complex cooperate to promote cell migration in vivo

Cell migration is essential for development, but its deregulation causes metastasis. The Scar/WAVE complex is absolutely required for lamellipodia and is a key effector in cell migration, but its regulation in vivo is enigmatic. Lamellipodin (Lpd) controls lamellipodium formation through an unknown mechanism. This study reports that Lpd directly binds active Rac, which regulates a direct interaction between Lpd and the Scar/WAVE complex via Abi. Consequently, Lpd controls lamellipodium size, cell migration speed, and persistence via Scar/WAVE in vitro. Moreover, Lpd knockout mice display defective pigmentation because fewer migrating neural crest-derived melanoblasts reach their target during development. Consistently, Lpd regulates mesenchymal neural crest cell migration cell autonomously in Xenopus laevis via the Scar/WAVE complex. Further, Lpd's Drosophila melanogaster orthologue Pico binds Scar, and both regulate collective epithelial border cell migration. Pico also controls directed cell protrusions of border cell clusters in a Scar-dependent manner. Taken together, Lpd is an essential, evolutionary conserved regulator of the Scar/WAVE complex during cell migration in vivo (Law, 2013).

This study reveals that Lpd colocalizes with the Scar/WAVE complex at the very edge of lamellipodia and directly interacts with this complex by binding to the Abi-SH3 domain. Active Rac directly binds Lpd, thereby regulating the interaction between Lpd and the Scar/WAVE complex. It is therefore postulated that Lpd acts as a platform to link active Rac and the Scar/WAVE complex at the leading edge of cells to regulate Scar/WAVE-Arp2/3 activity and thereby lamellipodium formation and cell migration (Law, 2013).

Knockdown of Lpd expression or KO of Lpd highly impaired lamellipodium formation, phenocopying the effect of Scar/WAVE complex knockdown on lamellipodium formation. Conversely, it was observed that overexpression of Lpd increased lamellipodia size in Xenopus NC cells, and this was dependent on the interaction with Abi, linking it to the Scar/WAVE complex. Overexpression of Pico, the Lpd fly orthologue, aberrantly increased the number and frequency of cellular protrusions at the rear of border cell clusters in a Scar-dependent manner, which suggests that the regulation of Scar/WAVE by Lpd is evolutionary conserved. Collectively, these data suggest that Lpd functions to generate lamellipodia via the Scar/WAVE complex (Law, 2013).

Lpd or Pico knockdown or Lpd KO impaired cell migration in vitro and in vivo in Drosophila, Xenopus, and mice. Lpd KO or knockdown cells were unable to migrate via lamellipodia but instead migrated very slowly by extending filopodia. The same residual migration mode had been observed for Arp2/3 knockdown cells. Arp2/3 is activated by the Scar/WAVE complex to regulate cell migration. It was also observed that both Lpd and Abi knockdown impaired NC migration in vivo. Consistently, it was found that Lpd and Abi-Scar/WAVE are in the same pathway regulating cell migration. This is consistent with recent studies suggesting that the Lpd orthologue in C. elegans, mig-10, genetically interacts with abi-1 to regulate axon guidance, synaptic vesicle clustering, and excretory canal outgrowth in C. elegans (Stavoe, 2012; Xu, 2012; McShea, 2013). Collectively, these results suggest that Lpd functions in cell migration via the Scar/WAVE complex in mammalian cells, Xenopus NC cells, and Drosophila border cells (Law, 2013).

Lpd not only interacts with the Scar/WAVE complex but also directly binds to Ena/VASP proteins. Ena/VASP proteins regulate actin filament length by temporarily preventing capping of barbed ends and by recruiting profilin-actin to the growing end of actin filaments. In contrast, the Scar/WAVE-Arp2/3 complexes increase branching of actin filaments. Lamellipodia with a highly branched actin network protrude more slowly but are more persistent, whereas lamellipodia with longer, less branched actin filaments protrude faster but are less stable and quickly turn into ruffles. It was observed that Lpd overexpression increases cell migration in a Scar/WAVE- and not Ena/VASP-dependent manner. This is consistent with a predominant function of Scar/WAVE downstream of Lpd to regulate a highly branched actin network supporting persistent lamellipodia protrusion and cell migration. Other actin-dependent cell protrusions such as axon extension or dorsal ruffles of fibroblasts require Lpd-Ena/VASP-mediated F-actin structures (Law, 2013).

Collective cell migration describes a group of cells that moves together and affect each other, and various types of collective cell migration exists during development and cancer invasion. Xenopus NC cells migrate as loose streams, whereas Drosophila border cells migrate as a cluster of cells with close cell-cell contacts. This study found that Rac regulates Lpd and Scar/WAVE interaction and that both are required for Xenopus NC migration, which is consistent with previous work in which Rac activity mediates this type of migration. Similarly, NC-derived melanoblast migration in the mouse depends on Rac-Scar/WAVE-Arp2/3, and it was found that Lpd functions in this process as well (Law, 2013).

Drosophila border cell clusters migrate through the fly egg chamber in two phases: an early part characterized by large and persistent front extensions, which are regulated predominantly by PVR (the fly PDGF receptor); and a late part characterized by dynamic collective 'tumbling' behavior. Surprisingly, Pico overexpression resulted in the appearance of a higher proportion of rear facing extensions, a phenotype previously observed with dominant-negative PVR, causing premature tumbling of the border cell cluster. This suggests that Pico function is normally tightly controlled to stabilize specific extensions and functions also in guidance of collective cell migration. Because Lpd-Scar/WAVE control single cell migration as well as collective cell migration, this suggests that they function as general regulators of cell migration (Law, 2013).

Collectively, this study has identified a novel pathway in which Lpd functions as an essential, evolutionary conserved regulator of the Scar/WAVE complex during cell migration in vivo (Law, 2013).

Membrane-targeted WAVE mediates photoreceptor axon targeting in the absence of the WAVE complex in Drosophila

A tight spatial-temporal coordination of F-actin dynamics is crucial for a large variety of cellular processes that shape cells. The Abelson interactor (Abi) has a conserved role in Arp2/3-dependent actin polymerization, regulating Wiskott-Aldrich syndrome protein (WASP) and WASP family verprolin-homologous protein (WAVE). This paper reports that Abi exerts nonautonomous control of photoreceptor axon targeting in the Drosophila visual system through WAVE. In abi mutants, WAVE is unstable but restored by reexpression of Abi, confirming that Abi controls the integrity of the WAVE complex in vivo. Remarkably, expression of a membrane-tethered WAVE protein rescues the axonal projection defects of abi mutants in the absence of the other subunits of the WAVE complex, whereas cytoplasmic WAVE only slightly affects the abi mutant phenotype. Thus complex formation not only stabilizes WAVE, but also provides further membrane-recruiting signals, resulting in an activation of WAVE (Stephan, 2011).

This study shows that abi and wave functions are required for early targeting of R-cell axons but are not needed in the R-cells themselves. These observations strongly suggest a nonautonomous role for the Arp2/3 activator WAVE and its regulator Abi in the brain target area, indicating that in their absence proper cellular communications between projecting R-cell axons and neurons in the target area might be disrupted. It is well established that WAVE and its regulatory complex are effectors of the activated GTPase Rac and one might also assume a similar nonautonomous role for Rac, as for WAVE and Abi. Previous analysis of genetic mosaics in the Drosophila brain lacking rac function indeed revealed an unexpected degree of nonautonomous effects in axon guidance and branching (Stephan, 2011).

How might Abi/WAVE control the targeting of retinal axons into the optic lobe? The formation of the photoreceptor projection pattern depends on complex bidirectional interactions between R-cell axons and different populations of glia, as well as neurons in the lamina target field. In wild type, incoming photoreceptor axons induce the outgrowth of scaffold axons, which in turn act as substrates for glia migration. Conversely, lamina glia cells provide an essential stop signal for photoreceptor axons to terminate their outgrowth in the lamina. These findings highlight the importance of the correct organization of the target area in the establishment of the R-cell projection pattern. The abnormal projections of the wg-positive scaffold axons indicate that neuronal Abi function might be required for the correct organization of the target area. The precise organization of the optic lobe by Abi could control axonal targeting directly (neuron–neuron; Sugie, 2010) or indirectly (neuron–glia; Dearborn, 2004; Yoshida, 2005). The failure of the Abi reexpression in the wingless domain to rescue suggests that Abi/WAVE function is needed in additional neurons in the target area (Stephan, 2011).

Loss-of-function studies in different model organisms clearly revealed a conserved function of Abi/WAVE in regulating axon guidance and axonal outgrowth in developing nervous systems. However, the precise role of WAVE-induced, Arp2/3-mediated actin polymerization in neuronal development is still controversial. Inhibition of Arp2/3 activity in cultured hippocampal neurons resulted in increased axon length but no significant effects on growth cone morphology, whereas it has been recently reported that the knockdown of the Arp2/3 complex impairs lamellipodia and filopodia formation in growth cones of hippocampal neurons and neuroblastoma cells. Recent studies using primary Drosophila mutant neurons confirmed an essential role of the Arp2/3 complex in regulating growth cone motility (Stephan, 2011).

Analyzing early retinal axon targeting in abi mutants represents a good experimental paradigm to measure WAVE activity in vivo. The targeting process does not require wasp function, but only wave function, in contrast to other developmental processes. The functional rescue assay in abi mutants allows examination of the activity of WAVE and WAVE variants in the absence of other WAVE complex subunits. Membrane recruitment of WAVE in abi mutants results in a partial but clear rescue of R-cell projection defects. Several conclusions can be made based on these data. Membrane localization is sufficient to confer partial activity to WAVE without regulation by the WAVE complex. Members of the WAVE complex are not only required to control the integrity of WAVE but also provide means for the membrane recruitment of WAVE. It is concluded from rescue experiments that WAVE activated by artificial membrane targeting induces activation of the Arp2/3 complex. It would be interesting to see whether and to what extent an artificial activation of Arp2/3 will rescue the phenotypic traits associated with a loss of wave. As cytoplasmic, full-length WAVE exerts only slight rescue activity, it is proposed that the Abi/WAVE complex might not only control membrane relocalization but also might be required for full activation of WAVE. The finding that membrane recruitment of WAVE leads to a partial activation might also be true for mammalian neurons. It has recently been shown that artificial membrane recruitment of WAVE partially rescues axonal growth defects in rac-deficient cerebellar granule neurons (Stephan, 2011).

Taken together, recruitment of WAVE to the membrane leads to activation of the Arp2/3 complex and is an important step during its activation but not the only one. Other important signals might include a specific state of phosphorylation and interaction with activated Rac. It is proposed that the analysis of Drosophila photoreceptor axon targeting in abi mutants will facilitate investigation of WAVE activity and regulation by the WAVE complex, as well as other signals independent of the WAVE complex in the context of a developmental process in vivo (Stephan, 2011).

Structure and control of the actin regulatory WAVE complex

Members of the Wiskott-Aldrich syndrome protein (WASP) family control cytoskeletal dynamics by promoting actin filament nucleation with the Arp2/3 complex. The WASP relative WAVE regulates lamellipodia formation within a 400-kilodalton, hetero-pentameric WAVE regulatory complex (WRC). The WRC is inactive towards the Arp2/3 complex, but can be stimulated by the Rac GTPase, kinases and phosphatidylinositols. This paper report the 2.3-Å crystal structure of the WRC and complementary mechanistic analyses. The structure shows that the activity-bearing VCA motif of WAVE is sequestered by a combination of intramolecular and intermolecular contacts within the WRC. Rac and kinases appear to destabilize a WRC element that is necessary for VCA sequestration, suggesting the way in which these signals stimulate WRC activity towards the Arp2/3 complex. The spatial proximity of the Rac binding site and the large basic surface of the WRC suggests how the GTPase and phospholipids could cooperatively recruit the complex to membranes (Chen, 2010).

Members of the WASP family are central to the control of cellular actin dynamics. These proteins receive information from multiple signalling pathways and respond by promoting the actin nucleating activity of the ubiquitous Arp2/3 complex. In this way, WASP proteins control actin assembly spatially and temporally in processes including cell migration, polarization, adhesion and vesicle trafficking (Chen, 2010).

The WASP family is defined by a conserved C-terminal VCA motif (for the verprolin-homology, central and acidic regions), which binds and activates the Arp2/3 complex. This element must be tightly regulated to ensure proper spatial and temporal control over actin assembly. In the best-understood family members, WASP and N-WASP, the VCA is autoinhibited by intramolecular interactions with a regulatory element termed the GTPase binding domain (GBD). Various ligands can bind to WASP/N-WASP simultaneously, and destabilize GBD-VCA contacts, leading to activation. Activation of all family members appears to be restricted to membranes. Superimposed on allosteric control and coupled with membrane recruitment, the activity of WASP proteins can be substantially increased by dimerization, or more generally oligomerization/clustering at membranes (Chen, 2010).

Although WASP and N-WASP can exist independently in cells, WAVE proteins are constitutively associated with four additional proteins inside cells: Sra1/Cyfip1, Nap1/Hem-2, Abi and HSPC300. The components of this ~400-kDa pentamer, termed the WRC, have all been implicated in control of Arp2/3-complex-mediated actin assembly in a wide range of systems. Sra1/Cyfip1 also has a distinct role in translational control. WAVE proteins lack an inhibitory GBD, and the mechanism of VCA regulation within the WRC is not known. The WRC can be activated by a wide range of stimuli, including the Rac GTPase and acidic phospholipids, which appear to act cooperatively at the plasma membrane. Furthermore, components of the WRC can be phosphorylated at numerous positions, with some modifications enhancing signalling activity. The mechanisms by which ligands act individually and cooperatively to recruit and activate the WRC are not known (Chen, 2010).

This study reports the 2.3-Å crystal structure of the WRC and complementary biochemical and cell biological analyses. The combined data reveal how the WAVE VCA is inhibited within the complex and provide plausible mechanisms for WRC activation by Rac and phosphorylation, and for cooperative membrane recruitment by Rac and phospholipids. This analyses provide an integrated picture of how the WRC orchestrates multiple signalling pathways to control actin polymerization at the plasma membrane (Chen, 2010).

The WRC is typically densely clustered at its sites of action in cells. This is believed to be necessary for spatially restricted actin assembly during, for example, polarized cell movement. Clustering is mediated by the combined actions of phosphoinositide lipids and Rac, as well as various SH3-containing proteins. The polybasic region of WAVE2 (equivalent to residues 172-184 of WAVE1) can bind phosphoinositide lipids in vitro, and is essential for membrane recruitment of the WRC and formation of lamellipodia in cells. Surface electrostatic calculations show that the face containing the WAVE1:Abi2:HSPC300 four-helix bundle is negatively charged, whereas much of the face of the complex adjacent to the polybasic region is positively charged. This polar distribution suggests that when the WRC is recruited to the plasma membrane, the side covered by the four-helix bundle is exposed to the cytoplasm, and the opposite side contacts the membrane. In this orientation, Rac would bind approximately to the side of the WRC, and then its C-terminal isoprene group, the polybasic region of WAVE and the basic surface of the Sra1/Nap1 dimer could all be directed towards the plasma membrane. The meander region and the VCA motif of WAVE would face the cytoplasm, making them accessible to other regulators (for example, kinases), and to the Arp2/3 complex and actin. This organization would allow simultaneous phosphoinositide and Rac binding, cooperatively recruiting the WRC to membranes and enhancing allosteric activation. Self-association of the WRC at membranes, and consequent enhanced activity, could be mediated by intercomplex binding of the N-terminal helix of Sra1 with the WAVE/Abi/HSPC300 trimer, as observed in the crystal lattice (Chen, 2010).

Sra1 was recently reported to support translation inhibition through simultaneous binding to the translational regulator FMRP and the translation initiation factor eIF4E. However, the putative mode of eIF4E binding is incompatible with the WRC structure. Thus, eIF4E may bind to isolated Sra1, but not the WRC, consistent with the finding that eIF4E co-immunoprecipitates with Sra1 but not WAVE. These observations suggest that Sra1 may partition between the WRC, which regulates actin dynamics, and a free (or alternatively complexed) state that regulates translation. Similar arguments have also been made regarding different pools of Nap1 and Abi. Interestingly, defects in Sra1 or its ligands in both pathways -- protocadherin-10, which binds the WRC, and FMRP -- are implicated in autism and other mental disorders, suggesting that an appropriate balance of these pathways or their joint action may be needed for proper neuronal development and function. Future studies of the intact WRC and its separate components will reveal how this system coordinates multiple processes in normal and abnormal cellular function (Chen, 2010).

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. 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).

Abi activates WASP to promote sensory organ development

Actin polymerization is a key process for many cellular events during development. To a large extent, the formation of filamentous actin is controlled by the WASP and WAVE proteins that activate the Arp2/3 complex in different developmental processes. WAVE function is regulated through a protein complex containing Sra1, Kette and Abi. Using biochemical, cell biological and genetic tools, this study shows that the Abi protein also has a central role in activating WASP-mediated processes. Abi binds WASP through its carboxy-terminal domain and acts as a potent stimulator of WASP-dependent F-actin formation. To elucidate the biological function of abi in Drosophila, bristle development, a process known to require wasp function, was studied. Reduction of abi function leads to a loss of bristles similar to that observed in wasp mutants. Activation of Abi results in the formation of ectopic bristles, a phenotype that is suppressed by a reduction of wasp activity but is not affected by the reduction of wave function. Thus, in vivo Abi may set the balance between WASP and WAVE in different actin-based developmental processes (Bogdan, 2005). In a mammalian system Abi also binds and activates N-WASP, suggesting a functional conservation of this central player (Innocenti, 2005).

Developmental processes involving movement and cell shape changes are based on a highly dynamic reorganization of the actin cytoskeleton. The Arp2/3 complex is an important regulator of actin polymerization. Its activation is mediated by members of the Wiskott-Aldrich syndrome proteins (N-WASP/WAVE). WASP and N-WASP (neural WASP) are specific effectors of Cdc42 that are characterized by a CRIB (Cdc42/Rac interactive binding) domain and an NH2-terminal WASP homology domain. By contrast, WAVE proteins lack these sequence motifs but possess a highly specific WAVE homology domain (WHD) and transduce Rac signalling to the Arp2/3 complex (Bogdan, 2005).

The in vivo roles of WASP and WAVE proteins are as different as their molecular architectures. This has been clearly established both in vertebrates and Drosophila. Whereas WAVE acts as the main Arp2/3 regulator during axonal growth, oogenesis and eye development, WASP is required for cell fate decisions during sensory organ development. This last function is independent of Cdc42 but is mediated through Arp2/3 (Bogdan, 2005).

To activate WASP, an intramolecular inhibition has to be released by factors such as Cdc42, phosphatidylinositol-4,5-bisphosphate, or several SH3-domain-containing proteins. In contrast, WAVE proteins are constitutively active in vitro. Activity is regulated by a multi-protein complex comprising Kette (NAP1, HEM), Sra-1 (CYFIP, PIR121), Abi (E3b1) and HSPC300. Initial work had suggested that WAVE1 is trans-inhibited and cannot stimulate Arp2/3 nucleation activity in vitro. This finding was supported by genetic data, showing that Kette or PIR121 antagonize WAVE function. However, new data clearly show that a reconstituted pentameric WAVE complex is constitutively active in stimulating Arp2/3 function and remains stable, even after Rac stimulation. This argues against a trans-inhibition model of WAVE function and raises the question as to whether in vivo additional proteins or post-translational modifications are required to control WAVE activity (Bogdan, 2005).

Surprisingly, Sra1 and Kette are also involved in the regulation of WASP. These unforeseen genetic interactions prompted an attempt to elucidate the functional relationship of these genes. Biochemical, cell biological and genetic data presented in this study clearly identify Abi as a central player in orchestrating the regulation of WASP, and thus define a new key component in directing cytoskeletal dynamics (Bogdan, 2005).

The Abi protein has a relative molecular mass of 52,000, and was first identified as a substrate of the Abelson kinase. During embryonic development, Abi shows a prominent coexpression with known members of the WAVE complex, Kette and Sra-1. It is maternally expressed and later in development becomes concentrated in the nervous system. Abi contains a C-terminal SH3 domain, which can interact with the Abelson kinase and the amino-terminal WAVE-interacting domain (WAB domain), which is required for binding and recruiting WAVE (Bogdan, 2005).

In addition, Abi binds Kette and WASP. Yeast two-hybrid experiments show that a deletion of the N-terminal 85 amino acids of the Drosophila Abi protein (AbiDeltaN) abrogates its interaction with WAVE but does not affect the interaction with Kette or WASP. Conversely, the C-terminal SH3 domain of Abi is necessary and sufficient for WASP interaction, and deletion of this domain blocks the interaction with WASP but not with WAVE. These deletion constructs also show that Abi binds Kette through its central domain (Bogdan, 2005).

To further test the interaction between Abi and WASP, glutathione S-transferase (GST) pull-down experiments were performed. Affinity purification of Abi from S2R+ cells using GST-WASP coupled to glutathione-Sepharose beads results in the copurification of Kette, which directly binds Abi, and Sra-1, which binds Kette. Significant amounts of WAVE were also found to be present in the protein complex bound to GST-WASP. Disruption of F-actin by Latrunculin A does not affect this coprecipitation. Furthermore, in WAVE pull-down experiments, some WASP protein could be detected in the precipitate. As expected, Abi is able to coprecipitate WAVE as well as WASP. Since WASP cannot bind directly to WAVE in a yeast two-hybrid assay, these findings suggest that Abi is in a position to coordinate the activity of two main regulators of the Arp2/3 complex (Bogdan, 2005).

As described for mammalian cells, endogenous WAVE is distributed relatively uniformly at the leading edge in Drosophila S2R+ cells. Abi and WASP show a more dotted expression pattern. Thus, Abi, WASP and WAVE are all localized at the leading edge of the cell, as reported for N-WASP and WAVE2 in lamellipodia of mouse myoblasts (Bogdan, 2005).

To determine the functional relevance of the proteins, RNA interference (RNAi) experiments were performed. RNAi-mediated silencing of abi, kette or sra-1 in S2 cells leads to a collapse of all lamellipodia-like structures and induces a starfish-like morphology. Reduction of WASP expression results in a more complex phenotype and the most sensitive phenotypic trait appeared to be the cell size. Twenty per cent of the cells showed a normal F-actin pattern but were consistently smaller than control cells. In 55% of the cells, lamellipodia appeared to collapse, leaving isolated F-actin-rich cell extensions. A requirement of the mammalian N-WASP for lamellipodia formation has also been observed in mammalian cell lines. Twenty-five per cent of the cells showed a starfish-like phenotype, which is similar to the abi knockdown phenotype. These effects are dependent on the wasp RNAi dose and require efficient protein silencing (Bogdan, 2005).

In contrast to the loss-of-function phenotype, even high levels of Abi expression did not affect cell morphology and F-actin organization. Ectopically expressed Abi is distributed similarly to the endogenous protein, but increased levels are associated with F-actin. It has been shown that membrane recruitment of Kette or Sra-1 results in their activation and induces WASP-dependent phenotypes in vivo. Expression of membrane-tethered Abi (AbiMyr) or of a N-terminal deletion variant lacking the WAVE interaction domain Abi (AbiDeltaN) results in extensive filopodia formation in about 50% of the transfected cells. However, deletion of the C-terminal SH3 domain from AbiMyr, which mediates binding to WASP, abrogates this effect. If AbiMyr acts through WASP, then a membrane-tethered WASP should also induce the formation of filopodia-like structures. Indeed, WASPMyr expression induces a regularly spaced array of filopodia-like structures in about 80% of the cells. Whereas wild-type S2R+ cells are characterized by membrane ruffling, both AbiMyr and WASPMyr induce small and dynamic cell extensions. In contrast, reduction of abi function by RNAi leads to less dynamic structures and finally cell collapse. To test whether the AbiMyr phenotype depends on WASP or WAVE, RNAi experiments were performed in cells expressing AbiMyr. In both cases the knockdown phenotype of WAVE or WASP is epistatic to the AbiMyr-induced phenotype, indicating that in S2R+ cells WAVE either performs vital functions, or that WAVE-dependent formation of lamellipodia is a prerequisite for AbiMyr-induced filopodia formation (Bogdan, 2005).

Because Abi binds WASP and can induce WASP-dependent phenotypes, whether the regulation of F-actin formation can be reconstituted in vitro was tested. Drosophila WASP and Abi proteins expressed in Escherichia coli were purified and their ability to activate mammalian Arp2/3 was tested in an actin polymerization assay. Drosophila WASP has only a small effect on Arp2/3-dependent actin polymerization kinetics by itself. The addition of Abi significantly increases this basal activity in a dose-dependent manner (Bogdan, 2005).

These findings prompted similar experiments in vivo. The expression of different Abi transgenes were induced using the Gal4-UAS system. Overexpression of AbiMyr in wing imaginal discs results in a significant elevation in the level of F-actin. The en-Gal4 driver allows the anterior compartment of the wing imaginal disc to be used as a negative control. Upon expression of high levels of AbiMyr, F-actin fibres are rearranged and the free G-actin pool appears to be reduced. When WASP function is present, F-actin fibres extend into AbiMyr-expressing processes. The elevation in actin polymerization was neither observed upon overexpression of wild-type Abi nor upon overexpression of AbiMyr lacking the C-terminal SH3 domain, suggesting that both membrane localization and interaction with WASP are required for F-actin formation. To further test the requirement for WASP, a C-terminal truncation (WASPDeltaCA) was used that still contains the proline-rich sequences binding to Abi but lacks sequences required for Arp2/3 complex binding. Expression of WASPDeltaCA results in a wasp-like mutant phenotype. Coexpression of membrane-tethered Abi and the dominant-negative WASPDeltaCA construct clearly suppressed ectopic F-actin formation, indicating that AbiMyr stimulates actin polymerization via WASP. Similarly, the effects caused by AbiMyr expression could be suppressed by reducing the wasp gene dose, clearly showing that Abi positively regulates WASP activity in vitro and in vivo. Expression of WASPDeltaCA alone does not have any effect on F-actin or G-actin distribution. The specificity of F-actin and G-actin detection was demonstrated by Latrunculin A treatment (Bogdan, 2005).

The functional relevance of abi was examined in the developing fly, focusing on the development of the sensory organs. Drosophila harbours only one gene encoding a WASP-like protein. Homozygous zygotic wasp mutants survive until adulthood and show a loss of bristles. Further analyses showed that WASP governs cell-fate decisions of specific external sensory organs that develop from proneural clusters. Expression of the dominant-negative WASPDeltaCA by the sca-GAL4 driver, which activates expression in the proneural clusters, leads to a loss of thoracic bristles similar to but not as extreme as is seen in wasp mutant flies. Likewise, reduction of abi function by in vivo expression of double-stranded abi RNA results in a similar bristle phenotype. In addition, the bristles are shorter and thinner compared with the wild type. In contrast, expression of AbiMyr resulted in the formation of additional bristles, independent of the GAL4 driver used (ptc-GAL4) and the wave gene dosage. Expression of an AbiMyr protein lacking the N-terminal WAVE interaction domain still induces the formation of extra bristles. These experiments show that Abi does not act through WAVE during Drosophila bristle development. Moreover, heterozygous loss of wasp function or coexpression of the dominant-negative waspDeltaCA transgene partially suppresses the formation of ectopic bristles induced by AbiMyr, which is most clearly seen for the notopleural bristles. By contrast, deletion of the C-terminal WASP-binding domain of Abi abrogates the ability of AbiMyr to induce bristles. Thus, in vivo Abi acts through WASP (Bogdan, 2005).

During development of Drosophila sensory organs, a single cell is selected out of a group of equivalent proneural cells (proneural cluster) by lateral inhibition to become a sensory organ precursor (SOP). The SOP undergoes a series of stereotyped asymmetric cell divisions giving rise to the different cells comprising an individual sensory organ. Zygotic wasp mutants show defects in asymmetric SOP division and lack bristles. Because a WASP protein lacking the CA domain is not able to rescue this phenotype, Arp2/3-dependent processes are involved in establishing cell fate (Bogdan, 2005).

To test directly whether Abi affects asymmetric cell division or promotes the allocation of neural fate the number of SOPs was determined using neuralized-lacZ or hindsight expression. Wild-type wing imaginal discs are characterized by a stereotypic patterning of SOPs. Expression of abi RNAi in proneural clusters results in a loss of many SOPs, which is similar to the wasp mutant phenotype. In contrast, expression of AbiMyr in proneural clusters leads to the formation of additional hindsight-expressing cells, indicating the formation of additional SOPs; all dorsocentral, scutelar and notopleural SOPS are affected; on average, three (instead of one) SOPs are found (Bogdan, 2005).

In summary, these findings place the Kette-Sra-1-Abi complex at a key position coordinating the formation of F-actin by regulating WAVE and WASP proteins. Furthermore, Abi-WASP is involved in SOP specification. The formation of SOPs is regulated by two main antagonistic pathways. Whereas Notch restricts the formation of neural cells, activation of the epidermal growth factor receptor (EGFR) promotes SOP formation. WASP forms a complex with EGFR and transmits Nck-dependent signals from EGFR to polarize cortical actin filaments. In turn, cytoskeletal rearrangements lead to receptor clustering and their subsequent activation. These experiments highlight the possibility that reverse signalling may occur in vivo and point to the interwoven relationship between receptor signalling and actin dynamics that is mediated by Abi and WASP. Thus, Abi functions as a central regulator that directs WASP- or WAVE-dependent developmental processes, presumably with the help of additional factors (Bogdan, 2005).

The involvement of Abl and PTP61F in the regulation of Abi protein localization and stability and lamella formation in Drosophila S2 cells.

Most aspects of cellular events are regulated by a series of protein phosphorylation and dephosphorylation processes. Abi (Abl interactor protein) functions as a substrate adaptor protein for Abl and a core member of the WAVE complex, relaying signals from Rac to Arp2/3 complex and regulating actin dynamics. It is known that the recruitment of Abi into the lamella promotes polymerization of actin, although how it does this is unclear. PTP61F, a Drosophila homolog of mammalian PTP1B, can reverse the Abl phosphorylation of Abi and colocalizes with Abi in Drosophila S2 cells. Abi can be translocalized from the cytosol to the cell membrane by either increasing Abl or reducing endogenous PTP61F. This reciprocal regulation of Abi phosphorylation is also involved in modulating Abi protein level, which is thought to affect the stability of the WAVE complex. Using mass spectrometry, several important tyrosine phosphorylation sites were identified in Abi. The translocalization and protein half-life of wild type (wt) and phosphomutant Abi and their abilities to restore the lamellipodia structure of the Abi-reduced cells were compared. The phosphomutant was found to have reduced ability to translocalize and to have a protein half-life shorter than that of wt Abi. Although the wt Abi could fully restore the lamellipodia structure, the phosphomutant could not. Together, these findings suggest that the reciprocal regulation of Abi phosphorylation by Abl and PTP61F may regulate the localization and stability of Abi and may regulate the formation of lamella (Huang, 2007).

Tyrosine phosphorylation/dephosphorylation is a common and important post-translational modification of key signaling proteins. Although Abi was originally identified as a kinase substrate of Abl, little is known about how phosphorylation contributes to its biological significance. This study demonstrates that the level of Abi tyrosine phosphorylation is balanced between phosphorylation by Abl and dephosphorylation by PTP61F. This conclusion is based on several lines of evidence. First, RNAi knockdown of PTP61F expression greatly elevates the tyrosine phosphorylation level of endogenous Abi in the Drosophila S2 cells. PTP61F was shown to be a major physiological phosphatase of Abi by substrate trapping, GST pulldown, and colocalization experiments. Second, RNAi experiments also suggested that Abl is the major kinase mediator of tyrosine phosphorylation of Abi. Finally and most importantly, Abl-mediated tyrosine phosphorylation of Abi was shown to be counteracted by the dephosphorylation of a wild-type but not the catalytically inactive PTP61F proteins. Taken together, these results indicate that the coordinated phosphorylating action of Abl and dephosphorylating action of PTP61F together coordinate to regulate the tyrosine phosphorylation level of Abi (Huang, 2007).

Because many adaptor proteins undergo protein relocalization in response to tyrosine phosphorylation and protein-protein interaction, whether the balance of tyrosine phosphorylation in Abi affected its localization subcellularly was investigated. In cells ectopically expressing Abl and Abi, both Abl and Abi proteins relocalize to the same cortex cell regions, where active actin polymerization occurs. The relocalization appears to be dependent, as least in part, on the kinase activity of Abl because the kinase inactive Abl would exhibit markedly reduced ability to relocalize Abi from a cytosolic punctate position to a plasma membrane position. This finding was further supported by additional experiments showing that a reduction in endogenous PTP61F could increase the relocalization of GFP-Abi from cytosol to the plasma membrane, and when Abi4YF phosphomutant protein was expressed, there was a decrease in this translocation activity. It is thought that the Abl/PTP61F-mediated phosphorylation/dephosphorylation of Abi is critical in regulating Abi functions by relocalizing Abi from the cytoplasm to the cell membrane. The importance of Abi phosphorylation is also evident in a study of Abi protein stability. Using overexpression or RNAi knockdown approach to control Abl or PTP61F protein levels, it was demonstrated that both protein level and activity of these two enzymes are essential in the control of Abi protein stability. Furthermore, it was shown that the protein turnover rate of Abi4YF is much faster than that of wt Abi, further supporting the idea that the tyrosine phosphorylation of Abi is critical in modulating its protein stability. Together, these results highlight the importance of the Abi phosphorylation/dephosphorylation in modulating its localization and protein stability (Huang, 2007).

Although the data indicate that Abl phosphorylation of Abi is critical in modulating Abi subcellular localization, the underlying mechanism behind this process remains elusive. Nevertheless, the Abl protein is known to contain a myristoyl group, a common module for anchoring a protein to a cell membrane. Moreover, recent structural studies, which found that removal of the myristoyl group activates c-Abl kinase activity, suggest that by binding myristoyl group to the kinase pocket of Abl, an inactive conformation may be promoted. Because Abi functions as a positive regulator of Abl kinase, the Abl-Abi interaction might induce a conformational change through which the buried region that bears an N-myristoylation signal is exposed. This would result in targeting the Abl·Abi complex to cell membrane. In fact, the data showed that the interaction of a kinase-defective Abl with Abi was also somewhat able to trigger the Abi relocalization from the cytosol to the cell membrane. It was also found that Abl phosphorylation of Abi in the complex further promoted the relocalization of Abi to cell membrane. One recent structural and biochemical study has shown that a phosphotyrosine ligand can further convert the Abl protein into a more 'open' conformation. It might be possible that the phosphotyrosine residues of Abi further opened the semi-unfolded position of myristoyl group in Abl, further promoting the relocalization of Abl·Abi complex (Huang, 2007).

Another intriguing issue is the unique subcellular localization of PTP61F. Immunocytochemistry experiments showed that most PTP61F colocalizes with Abi in the perinuclear region, presumably where the endoplasmic reticulum (ER) is situated, but not at the leading edge of the spreading cells, where Abi is preferentially localized. These finds regarding the subcellular localization of PTP61F in S2 cells is similar to those reported for its mammalian counterpart, PTP1B. PTP1B largely resides on the external face of the ER. Given the finding that PTP61F counteracts Abl phosphorylation of Abi, the spatial separation of PTP61F from Abi in the leading edge might suggest that the phosphorylation of Abi is less affected by PTP61F in this region and that Abi residing at the leading edge might exert a higher tyrosine phosphorylation signal than Abi in the perinuclear region. In fact, several groups have reported that a number of proteins localized to the leading edge of motile cells are tyrosine-phosphorylated. Nevertheless, the substrates of PTP targeting the cell periphery ultimately have to be dephosphorylated to terminate its role in signaling. To elucidate the mechanism underlying this process, three models have been proposed. First, the receptor tyrosine kinases, such as epidermal growth factor receptor and platelet-derived growth factor receptor, can be directed from plasma membrane to ER by vesicle trafficking for dephosphorylation by PTP1B. Second, the ER membrane-anchored PTP1B can be cleaved by the calcium-dependent neutral protease calpain to relocate its catalytic domain from membranes to the cytosol. Last, by reaching the ER network to the cell periphery, PTP1B at the local attachment site can interact with its substrates in the plasma membrane. These models together with the current findings suggest that there is a possible link between PTP61F and Abl signaling in the regulation of Abi phosphorylation at the leading edge of the plasma membrane (Huang, 2007).

Protein stability is also a critical factor for regulating Abi functions in actin dynamics. When Abi expression is lost in S2 cells, there are severe defects in the formation of lamella. However, Abi alone does not affect the formulation of lamella. Abi forms a tight complex with WAVE, Hem, and Sra-1 to regulate the formation of lamellipodia (Steffen, 2004; Stradal, 2001). Moreover, the lamellipodia formation is dependent on the integrity of a functioning WAVE macromolecular complex (WAVE·Abi·Hem·Sra-1) (Kunda, 2003; Rogers, 2003), but the down-regulation of Abi expression impairs the stability of WAVE complex in both mammalian and Drosophila cells (Stradal, 2006). Thus, the protein stability of Abi is highly correlated with its regulation of the formation of lamella. A previous studies demonstrated that the interaction between Abl and Abi leads to an increased protein level of Abi (Juang, 1999). This study has extended this finding by showing that the impaired Abl-mediated phosphorylation of Abi results in protein instability of Abi in S2 cells. It was also demonstrated that the ectopic expression of a wild type, but not an inactive, PTP61F also dramatically diminishes the Abi protein stability. Thus, these results indicate that tyrosine phosphorylation/dephosphorylation by Abl/PTP61F is critical in controlling Abi protein turnover. Because it has been suggested that Abi undergoes an ubiquitin-mediated proteolysis (Dai, 1998), it might be possible that that the Abl phosphorylation of Abi represses the proteolysis degradation. One recent study has demonstrated that the Abl phosphorylation of c-Jun blocks the access of E3 ubiquitin ligase, Itch, to its binding epitope of c-Jun and thus prevents c-Jun from proteosome-mediated proteolysis (Huang, 2007).

Although tyrosine phosphorylation of Abi is important in stabilizing Abi protein level, the protein interaction between Abi and Abl might also attribute to Abi protein stability. This study showed that the coexpression of a kinase-defective c-Abl and Abi resulted in a 1.8-fold increase of Abi protein level. Moreover, this protective effect probably is a common feature of both the Abi·Abl and Abi·WAVE·Hem·Sra-1 complexes. Preliminary data showed that the ectopic coexpression of Abi, Hem, and WAVE could enhance Abi protein level and that the reduction of Hem or WAVE markedly decreased the endogenous Abi protein level, suggesting that a constitutive complex of Abi with WAVE, Hem, and Sra-1 could protect Abi from post-translational degradation. This finding is also comparable with what was documented in the scenario of WAVE/Scar stability (Huang, 2007).

Following this logic, it is reasonable to speculate that a sizeable fraction of the ectopically expressed Abi proteins would be short of match protein members to form the protein complex. Under this circumstance, the Abi would be more dependent on the phosphorylation regulation mechanism to modulate its protein stability. Supporting this view, it was shown that the protein stability of exogenous Abi was particularly sensitive to the phosphorylation regulation. Although component proteins of the WAVE/Scar complex are thought to form a constitutive multiprotein complex in cells, a recent study showed that a small population of WAVE2 exists as a monomeric form without forming a complex with other component proteins in A431 cells. In addition to WAVE2, Hem/Nap1 was also reported to regulate cell motility through a WAVE/Scar-independent pathway in Dictyostelium and human neutrophil. These studies suggest that the component proteins of WAVE/Scar complex may present as a 'noncomplex' form of protein in regulating cell motility. Accordingly, although exogenous Abi is relatively independent of the physiological state, it is more amenable to the study of protein stability (Huang, 2007).

Another interesting issue related to the Abi stabilization by Abl is linked to a possibility that the stabilized Abi protein might in return further enhance the Abl kinase activity and Abi phosphorylation, thus establishing a positive feedback loop between Abl activity and Abi phosphorylation/stability. Because Abl kinase activity is tightly regulated in cells, this positive feedback loop should be eventually interrupted by a third molecule. PTP61F might be such a suppressor and function as a brake on the overactivated Abl-Abi signaling pathway by dephosphorylating Abi and facilitating protein degradation (Huang, 2007).

Finally, this study investigated whether a phosphomutant of Abi, which is not phosphorylated by Abl, could disrupt the lamella formation in motile cells. The experiments suggest that the expression of Abi4YF was unable to rescue the lamella defects that were caused by treatment with Abi3'-UTR RNAi, supporting the idea that Abl might act positively in promoting Abi regulation of the lamella formation. Two recent reports have demonstrated that the mutation on a critical Abl phosphorylation residue of WAVE can disrupt its role in actin-dependent cytoskeleton remodeling. The data might suggest that Abl phosphorylation on Abi promotes actin polymerization. Therefore, it was asked whether Abl also phosphorylates all the other WAVE members and whether that would in turn modulate their roles in actin dynamics. Preliminary data showed that Abl, via the linkage of Abi, could tyrosine phosphorylate each protein member of the WAVE complex and that process was also counteracted by PTP61F (Huang, 2007).

Although additional experiments are required to determine the physiological significance of the phosphorylation/dephosphorylation of WAVE complex proteins, it is possible that the opposing actions of Abl and PTP61F in all of the other WAVE members might play a role in fine-tuning actin polymerization in cells. To resolve this possibility, it will be important to investigate whether Abl/PTP61F phosphorylation/dephosphorylation of the WAVE complex is occurred in a consecutive or synchronized manner in actin polymerization (Huang, 2007).

This work has demonstrated that Abi undergoes protein modification by Abl and PTP61F via reversible tyrosine phosphorylation/dephosphorylation. This process regulates its relocalization to the cell membrane and protein stability, and ultimately it regulates the formation of lamella in motile cells. Therefore, the balance between Abl kinase and PTP61F phosphatase in regulating tyrosine phosphorylation of Abi may mediate the function of WAVE complex in cell periphery. This study furthers understanding of how and where the reciprocal regulatory processes of an actin-associated protein function within the cells (Huang, 2007).

The golgi comprises a paired stack that is separated at G2 by modulation of the actin cytoskeleton through Abi and Scar/WAVE

During the cell cycle, the Golgi, like other organelles, has to be duplicated in mass and number to ensure its correct segregation between the two daughter cells. It remains unclear, however, when and how this occurs. This study shows that in Drosophila S2 cells, the Golgi likely duplicates in mass to form a paired structure during G1/S phase and remains so until G2 when the two stacks separate, ready for entry into mitosis. Pairing requires an intact actin cytoskeleton which in turn depends on Abi/Scar but not WASP. This actin-dependent pairing is not limited to flies but also occurs in mammalian cells. It is further shown that preventing the Golgi stack separation at G2 blocks entry into mitosis, suggesting that this paired organization is part of the mitotic checkpoint, similar to what has been proposed in mammalian cells (Kondylis, 2007).

During the cell cycle, the Golgi, like other organelles, has to duplicate in mass and/or number to ensure its correct segregation between the two daughter cells. It remains unclear, however, when and how this occurs. The process of Golgi duplication and inheritance in mammalian cells is still debated, as different modes of Golgi biogenesis have been proposed. One reason why this issue is not yet settled could be due to the elaborate organization of the Golgi stacks, which are interconnected to form a single-copy organelle capping the nucleus, thus impeding clear visualization of organelle duplication and segregation. Therefore, this study has exploited the relatively small number of Golgi stacks in Drosophila tissue-cultured S2 cells to revisit this issue (Kondylis, 2007).

In S2 cells, the Golgi stacks are found in close proximity to transitional endoplasmic reticulum (tER) sites, forming tER-Golgi units (Kondylis, 2003; Herpers, 2004). Their number nearly doubles at G2 phase. In an effort to identify factors mediating this process, focus was placed on cytoskeletal elements that have been involved in the organization of the mammalian Golgi apparatus. Microtubules are involved in mammalian Golgi ribbon maintenance, as their depolymerization leads to its reorganization into individual Golgi stacks in close proximity to ER exit sites (Kondylis, 2007 and references therein).

F-actin has also been implicated in the organization of the mammalian Golgi apparatus; its depolymerization leads to a compact appearance of this organelle without disruption of cisternal stacking. A key regulator of actin polymerization is the Arp2/3 complex. Its F-actin nucleation activity is triggered both by Wiskott-Aldrich syndrome protein (WASP) and WASP family verprolin-homologous (WAVE/Scar) proteins, which are in turn regulated by Rho small GTPases. WASP exists in an autoinhibited state that is released by the cooperative action of Cdc42, PI(4,5)P, and other SH3-containing proteins. In contrast, WAVE/Scar proteins, together with Sra-1, Kette (Nap1), Abi, and HSPC300, form a stable complex, which is itself regulated by Rac (Kondylis, 2007 and references therein).

Rho GTPases have recently been implicated in maintaining Golgi architecture. Cdc42 has been localized on the Golgi membrane and shown to recruit the Arp2/3 complex around this organelle via ARHGAP10. Furthermore, suppression of the brain-specific Rho-binding protein Citron-N in neurons was shown to lead to fragmentation of the Golgi apparatus, and Rho1 was proposed to exert its local effect on F-actin by regulating ROCK and profilin activity (Kondylis, 2007 and references therin).

This study shows that drug-induced F-actin depolymerization in S2 cells leads to doubling of the number of tER-Golgi units independent of anterograde transport. Using live cell imaging, electron microscopy, and three-dimensional (3D) electron tomography, this study shows that each Golgi is organized as a pair of stacks held together by an actin-based mechanism, both in Drosophila and in human cells. In S2 cells, this is mediated by Abi and Scar, suggesting a novel role for the Rac signaling cascade in Golgi architecture. Last, it was shown that the Golgi stacks undergo separation at G2 through modulation of Abi and Scar, and that blocking this separation prevents cells from entering mitosis, supporting the existence of a G2/M checkpoint related to Golgi structural organization (Kondylis, 2007).

The two Golgi stacks could be physically linked without displaying membrane continuity or being interconnected, for instance through intercisternal tubular connections, either permanent or transient. Tubules connecting cisternae of adjacent stacks are involved in the formation of the Golgi ribbon in mammalian cells and, recently, GM130 and GRASP65 have been proposed to be required for their integrity. However, the putative tubules connecting the two stacks in the pair would have different molecular requirements, at least in Drosophila, since depletion of dGM130 or dGRASP does not lead to their separation (Kondylis, 2003; Kondylis, 2005; Kondylis, 2007 and references therein).

F-actin could provide a physical link holding the paired Golgi stacks together, or it could help in the formation/maintenance of intercisternal tubules. In addition, short actin filaments have been proposed to link spectrin mosaics leading to the formation of a skeleton that surrounds the Golgi complex. One of its functions could be to hold the two Golgi stacks close enough to allow the formation and fusion of the tubules. It could also surround the tubules themselves, thus providing membrane stability. The localization of Abi and Scar at the periphery of the tER-Golgi units and between the two stacks in a pair is consistent with both proposed functions. These tomography studies so far have not revealed clear membrane continuities between Golgi cisternae, though examples have been found of a tubular network which is shared by the paired stacks (Kondylis, 2007).

tER sites behave similarly to the Golgi, as they also separate at G2 and upon F-actin depolymerization. Because little is known about the mechanism regulating the biogenesis of tER sites, it is difficult to envisage how the two parts could be held together. The spectrin-actin mesh described above could be common to Golgi and tER sites, and Golgi and tER site scission could be achieved in a synchronized fashion. Alternatively, either of these organelles could split first and lead to the scission of the other, perhaps by providing positional information. Recently, the centrosome component centrin 2 that is also localized to tER sites in Trypanosoma has been shown to give such positioning information. A more in-depth study combining immunogold labeling and 3D tomography would be required to elucidate such fine details of tER-Golgi structural organization (Kondylis, 2007).

Drosophila Rho1 is unlikely to have a role in holding the two Golgi stacks together. The overexpression of the Rho1 constitutively inactive mutant or treatment of S2 cells with ROCK or myosin light chain inhibitors (Y27632 and blebbistatin) did not affect the Golgi number. Cdc42 is also unlikely to participate as the depletion of its downstream effector WASP did not lead to Golgi separation, although the overexpression of the Cdc42T17N dominant negative did. However, this effect could be due to nonspecific sequestration of the guanine nucleotide exchange factor involved in maintaining the paired Golgi stacks and may be shared with other small GTPases (Kondylis, 2007).

Interestingly, the results are consistent with a role for Rac GTPases in Drosophila Golgi architecture. Expression of the constitutively inactive form of Rac1 led to a near-doubling in the Golgi number, and depletion of Scar/WAVE or Abi, which are regulated by Rac GTPases, led to a similar phenotype. The identical results obtained in Scar and Abi RNAi suggest that this well-established Scar/WAVE pentameric complex is involved in holding the paired Golgi stacks together by promoting F-actin polymerization. These data indicate that the Rac signaling pathway is involved. However, the Scar/Abi complex has recently been shown to also stimulate Arp2/3 and F-actin polymerization independently of Rac. This would need to be investigated further (Kondylis, 2007).

This study shows that the separation of the paired Golgi stacks occurs at G2, prior to mitosis. A similar phenomenon has already been reported during cell division in Toxoplasma gondii, where a single Golgi stack grows as a duplicated organelle and is separated as the cell divides. However, the mechanism underlying this separation is not known (Kondylis, 2007).

The Golgi doubling in number at G2 phase resembles many aspects of this observed upon F-actin depolymerization. In both cases, a similar increase in Golgi number and decrease in their size are observed. Furthermore, this study has shown that it is the modulation of the F-actin cytoskeleton and the activity of Abi/Scar at G2 that lead to Golgi stack separation. (1) It was found that both Scar and Abi localized to the Golgi, strongly arguing for having a role in actin remodeling around this organelle. (2) The Golgi stacks in G2 cells remain insensitive to F-actin depolymerization. (3) Cells depleted of Abi and Scar that exhibit separated Golgi stacks do not split them further at G2. (4) The overexpression of Abi prevents Golgi separation at G2. This strongly suggests that the F-actin/Abi/Scar-mediated link of the two stacks has been severed in a G2-specific manner, perhaps by kinases such as Polo (Kondylis, 2007).

Because tER sites and the Golgi apparatus ultimately disperse later in mitosis, both in mammalian and Drosophila S2 cells, the Golgi stack separation prior to dispersion might be part of the proposed Golgi G2/M checkpoint. Indeed, reagents that interfere with the GRASP65/55 phosphorylation by Polo and ERK/MEK, respectively, arrest or delay the cell cycle at the G2/M transition. This study shows that blocking Golgi separation at G2 by overexpressing Abi also prevents S2 cells from entering mitosis. This strengthens the relationship between Golgi organization and mitotic entry, although it cannot formally be excluded that the mitotic block observed is partly due to additional effects of Abi overexpression, for instance at the plasma membrane (Kondylis, 2007).

It is proposed that at G2, the paired stacks are separated along with the adjacent tER sites. As the cell enters mitosis, the Golgi membrane and the tER sites disperse, and are segregated into the two daughter cells, where the tER-Golgi units are rebuilt. The Golgi could be rebuilt as a very small paired stack in close association with Scar, Abi, and F-actin, or as a single stack that will duplicate by a mechanism that still needs to be unraveled. Since G1 cells are all sensitive to F-actin depolymerization, this suggests that the formation of the paired Golgi stack starts just after the exit from mitosis and persists until S phase, when the Golgi seems to grow significantly. A more detailed understanding will come from EM study of S and G2 cells (Kondylis, 2007).

One of the remaining questions regards the impact of the Abi/Scar role on Golgi organization during development. Using Scar/WAVE, Abi, Kette, and Sra-1 mutants, as well as transgenic flies carrying inducible RNAi constructs, it will be possible to assess whether any of the observed phenotypes (defects in oogenesis, cell and organ morphology, neuroanatomical malformations, and failure in cell migration) is in part due to defects in Golgi organization (Kondylis, 2007).

Drosophila abelson interacting protein (dAbi) is a positive regulator of abelson tyrosine kinase activity

Human and mouse Abelson interacting proteins (Abi) are SH3-domain containing proteins that bind to the proline-rich motifs of the Abelson protein tyrosine kinase. A new member of this gene family, a Drosophila Abi (dAbi), is a substrate for Abl kinase and co-immunoprecipitates with Abl if the Abi SH3 domain is intact. A new function has been identified for both dAbi and human Abi-2 (hAbi-2). Both proteins activate the kinase activity of Abl as assayed by phosphorylation of the Drosophila Enabled (Ena) protein. Removal of the dAbi SH3 domain eliminates dAbi's activation of Abl kinase activity. dAbi is an unstable protein in cells and is present at low steady state levels but its protein level is increased coincident with phosphorylation by Abl kinase. Expression of the antisense strand of dAbi reduces dAbi protein levels and abolishes activation of Abl kinase activity. Modulation of Abi protein levels may be an important mechanism for regulating the level of Abl kinase activity in the cell (Juang, 1999).

Many critical cellular functions, such as cell growth and differentiation, are regulated by the kinase activity of protein tyrosine kinases (PTKs) and deregulated kinase activity of PTKs is associated with oncogenic activity. Relatively little is known about the normal regulation of the kinase activity of the Abelson (Abl) non-receptor PTK. Regulation of Abl activity is important as suggested by the consequences of improper regulation. Oncogenic forms of Abl such as BcrAbl in chronic myelogenous leukemia (CML) and v-Abl in Abelson murine leukemia virus have increased, unregulated kinase activity that phosphorylate several cellular substrates. This increased phosphorylation is believed to activate signal transduction pathways that participate in the uncontrolled cell growth of the tumor cells. Mutation of the SH3 domain of c-Abl also activates the oncogenic potential of Abl, producing elevated kinase activity, predominantly cytoplasmic localization, and cell transformation. It has been proposed that the SH3 domain of c-Abl may have a function in the negative regulation of Abl's activity. Overexpression of c-Abl in cells results in autophosphorylation of c-Abl, suggesting that there might be a limited concentration of a cellular modulator that suppresses c-Abl activity in cells. Thus, one of the models which has been proposed for the inhibition of the c-Abl tyrosine kinase invokes a trans-acting cellular inhibitor interacting with the Abl SH3 domain (Juang, 1999).

Proteins that interact with the SH3 domain of Abl and that might thereby modulate kinase activity of Abl have recently been identified. Initial reports identified three mammalian Abi proteins; murine Abi-1, hAbi-2 and human ArgBP1 (Dai, 1995; Shi, 1995; Wang, 1996). The Abi proteins physically associate with Abl and Abelson-related (Arg) kinases. The sequence motifs of Abi family members include an SH3 domain, proline-rich regions, PEST regions, and a homeodomain homologous region (Dai, 1995; Shi, 1995). Abi proteins are phosphorylated by the Abl PTKs. Overexpressing full length mouse Abi-1 in NIH3T3 cells suppresses v-Abl transforming activity (Shi, 1995). Stable expression of the full length hAbi-2 is toxic in NIH3T3 cells, but expression of a truncated form of hAbi-2, which lacks the first 157 amino acids including a proline-rich domain required for binding the Abl-SH3 domain, activates c-Abl transforming activity in NIH3T3 cells (Dai, 1995). It was proposed (Dai, 1995; Shi, 1995) that Abi is a tumor suppressor in mammalian cells. Hypothesized regulatory functions of Abi include stabilization of the inactive form of c-Abl or, alternatively, inhibiting access of Abl to protein substrates and thereby negatively regulating Abl activity (Juang, 1999).

The Abl gene has been conserved through evolution and related genes have been found in human, cat, mouse, Drosophila and C. elegans. This study used Drosophila genetics to understand the regulatory molecules of the conserved Abl pathway. In order to determine if Abi regulation of Abl is a mechanism that has been conserved through evolution, attempts were made to isolate a Drosophila member of the Abi family. This paper reports the identification of dAbi. Analysis of the dAbi protein shows very similar structural features to that of mammalian proteins. dAbi behaves similarly to mammalian Abi proteins by associating with Abl kinase and serving as a substrate for the Abl kinase. Two new properties were revealed by the biochemical analysis of both dAbi and hAbi-2 proteins. First, dAbi and hAbi-2 act as potent activators of Abl kinase activity. Second, dAbi is an unstable protein that is stabilized coincident with phosphorylation. It is proposed that regulation of Abi protein levels may be a mechanism for modulating Abl kinase activity that has been well conserved during evolution (Juang, 1999).

Degenerate oligonucleotides were designed based on the conserved SH3 domain peptide sequences of mammalian Abi proteins and used for PCR of Drosophila embryonic cDNA libraries. A fragment of 146 bp, the expected size of the SH3 domain, was isolated, cloned and sequenced. The predicted open reading frame was most similar in sequence to the mammalian Abi SH3 domain. The DNA fragment was used as a probe for high stringency screening of the library and hybridized to a 1.7 Kb Drosophila cDNA. The cDNA encodes a predicted open reading frame of 473 amino acids. A BLAST search of the GenBank non-redundant peptide sequence database (nr) identified a number of Abi and Abi-related proteins. dAbi appeared most similar to human e3B1/hssh3bp1, sharing 42% amino acid sequence identity and 54% amino acid similarity. Human e3B1 was originally identified in a screen for proteins that bound the SH3 domain of eps8, a substrate of receptor tyrosine kinases including the epidermal growth factor receptor (EGFR) tyrosine kinase (Biesova, 1997). In vitro assays with GST fusion proteins showed that e3B1 also bound the SH3 domain Abl (Biesova, 1997). Subsequently, one isoform of a candidate spectrin SH3 domain-binding protein was found to be identical to e3B1. The Drosophila sequence shows similar levels of amino acid identity when compared to other human Abi proteins. Pairwise sequence comparisons showed 40% amino-acid identity and 54% amino-acid similarity between dAbi and human ArgBP1A whereas comparison of dAbi and hAbi-2 gave similar results showing 39% amino-acid identity and 52% amino-acid similarity. The Drosophila sequence has motifs similar to those found in the human Abi proteins including a carboxy-terminal SH3 domain with 76% identity to hAbi-2. The amino-terminal domain includes a homeo-domain similarity, and the middle portion includes three polyproline motifs and a PEST domain. It is proposed that this sequence be called Drosophila Abi (dAbi) (Juang, 1999).

Abi proteins were identified in two hybrid screens using the carboxy-terminal domain of Abl or the SH3 domain of Abl. Abi and Abl can potentially interact at two sites, the SH3 domain of Abl with the proline motifs of Abi and the SH3 domain of Abi with the proline motifs of Abl. In addition to binding to Abl, Abi proteins appear to be excellent substrates for the kinase, raising a question of whether Abi proteins are upstream regulators or downstream effectors of the kinase. Abi-1 has been proposed to be a negative regulator of Abl because its full length protein suppresses v-Abl transformation (Shi, 1995). The full length hAbi-2 was also suggested to function as a potential tumor suppressor gene through negative regulation of Abl kinase activity (Dai, 1995). One hypothesis put forth to explain Abi's suppression of Abl's transforming activity is that binding of Abi to the Abl SH3 domain negatively regulates Abl kinase (Dai, 1995). This hypothesis was based in part on observations that structural alterations of Abl that attenuate binding to Abi, e.g. mutation of the Abl SH3, enhance the transforming activity of c-Abl. However, the observation that Abi-1 did not inhibit the overall kinase activity of v-Abl (Shi, 1995) suggests that Abi-1 suppression of v-Abl transforming activity may be due to other mechanisms of action (Juang, 1999).

The identification of a Drosophila Abi protein indicates that the Abl-Abi interaction may have been conserved during evolution. dAbi and hAbi-2 are 39% identical and 52% similar in amino acid sequence and share similar protein motifs. dAbi associates with Abl, as determined by co-immunoprecipitation, and is a substrate for the Abl PTK. The dAbi SH3 domain is required for a detectable interaction with the Abl kinase. Surprisingly, transient coexpression of Abl kinase with either dAbi or hAbi-2 proteins in both Drosophila S2 cells and mammalian COS cells leads to activation of Abl kinase that is dependent upon the presence of an intact Abi SH3 domain (Juang, 1999).

One model to explain how dAbi and hAbi-2 might activate Abl kinase would be that interaction of Abi with Abl generates a conformational change in Abl that results in exposure of its catalytic domain. Data to support this model comes from a recent report on site directed mutations in the Abl catalytic domain, SH3 domain and linker region between the SH2 and catalytic domain (SH2-CD) (Barila, 1998). Mutations in all three domains were found to activate c-Abl kinase implicating intramolecular inhibitory interactions of the Abl SH3 domain with the catalytic domain and with the SH2-CD linker region. In addition, mutation of the SH2-CD linker region caused a conformational change in the protein resulting in altered and increased protease sensitivity of the mutated protein compared with c-Abl (Barila, 1998). Such a model could also reconcile the conflicting observations that dAbi and hAbi-2 activate Abl kinase activity yet Abi proteins are reported to suppress Abl transforming activity since binding of Abi to Abl may also have a role in regulating other biological activities of Abl such as Abl transforming activity (Dai, 1995; Shi, 1995; Juang, 1999 and references therein).

Although the activation of Abl kinase by dAbi implicates dAbi as an upstream regulator of Abl, Abl also has effects on dAbi. Phosphorylation of dAbi leads to a significant increase in the steady state level of dAbi protein. The rapid turnover of dAbi in S2 cells suggests that a potential mechanism for the increase in dAbi level is through a phosphorylation-dependent increase in dAbi protein stability. Although dAbi may have other binding partners than Abl, one possible effect of the increased dAbi protein levels would be to provide an amplified or sustained activation of the Abl kinase. In this role, an increase in dAbi level would result in dAbi binding to Abl, activation of Abl kinase activity, increased phosphorylation of Abl substrates, including dAbi, and an additional increase in dAbi steady state levels that would maintain activation of Abl kinase. Clearly some additional mechanism would be needed to terminate this hypothetical positive regulatory loop (Juang, 1999).

If dAbi is a key regulator of Abl kinase in vivo, it will be important to understand the regulation of dAbi protein turnover. Proteolysis is critical for controlling protein concentrations in other key processes, such as signal cascades and the cell cycle. Reversible protein phosphorylation has also been linked to the regulation of protein degradation. Phosphorylation of proteins, such as the cyclins, leads to ubiquitination and degradation of proteins. However, in other proteins, e.g. c-jun, phosphorylation results in a reduction in ubiquitination and stabilization. Analyses of dAbi and hAbi-2/ArgBP1 sequences have revealed PEST sequences, regions rich in proline (P), glutamic acid (E), serine (S) and threonine (T). Such sequences have been suggested to target rapid intracellular degradation of proteins. Many PEST-containing proteins are important regulatory molecules, such as components of signal transduction pathways, homeotic proteins, and key enzymes. Wang (1996) has reported that a human Abi isoform, ArgBP1A, with three PEST regions, has a consistently lower expression level than another isoform, ArgBP1B, with only two PEST regions, consistent with a role of the PEST motifs in Abi protein stability (Juang, 1999).

Regulation of protein stability is thought to be important in how cells respond to DNA damage. In response to DNA damage, p53 protein turnover is reduced and its level rises. The increased p53 levels are important to cause G1 arrest and provide cells an opportunity to repair the DNA damage, or to prevent further proliferation of the damaged cell by causing apoptosis. Regulation of c-Abl has also been implicated in the DNA damage-induced apoptosis. In response to DNA-damaging agents, c-Abl protein tyrosine kinase down-regulates cdk2 and causes G1 arrest by a p53-dependent mechanism and may also be involved in a p53-independent induction of apoptosis. In response to DNA damage, the ataxia telangiectasis mutant (ATM) protein is reported to phosphorylate and activate c-Abl. It will be interesting to determine whether DNA damage also alters the protein stability of Abi, as this would provide an additional mechanism for activating Abl kinase activity in response to DNA damage (Juang, 1999).

The Abl kinase has been implicated in cancer, regulation of the cell cycle, regulation of cell adhesion and normal developmental processes in Drosophila and mammalian systems. Further study of Abi proteins may provide new insights into how Abl kinase activity is regulated in one or more of these processes. The identification of a dAbi will also permit genetic analysis of Abi in normal development (Juang, 1999).

Synaptic vesicle clustering requires a distinct MIG-10/Lamellipodin isoform and ABI-1 downstream from Netrin

The chemotrophic factor Netrin can simultaneously instruct different neurodevelopmental programs in individual neurons in vivo. How neurons correctly interpret the Netrin signal and undergo the appropriate neurodevelopmental response is not understood. This study identified MIG-10 isoforms as critical determinants of individual cellular responses to Netrin. Distinct MIG-10 isoforms, varying only in their N-terminal motifs, can localize to specific subcellular domains and are differentially required for discrete neurodevelopmental processes in vivo. MIG-10B was identified as an isoform uniquely capable of localizing to presynaptic regions and instructing synaptic vesicle clustering in response to Netrin. MIG-10B interacts with Abl-interacting protein-1 (ABI-1)/Abi1, a component of the WAVE complex, to organize the actin cytoskeleton at presynaptic sites and instruct vesicle clustering through SNN-1/Synapsin. A motif in the MIG-10B N-terminal domain was identified that is required for its function and localization to presynaptic sites. With this motif, a dominant-negative MIG-10B construct was engineered that disrupts vesicle clustering and animal thermotaxis behavior when expressed in a single neuron in vivo. These findings indicate that the unique N-terminal domains confer distinct MIG-10 isoforms with unique capabilities to localize to distinct subcellular compartments, organize the actin cytoskeleton at these sites, and instruct distinct Netrin-dependent neurodevelopmental programs (Stavoe, 2012).

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

Extracellular guidance cues steer axons towards their targets by eliciting morphological changes in the growth cone. A key part of this process is the asymmetric recruitment of the cytoplasmic scaffolding protein MIG-10 (lamellipodin). MIG-10 is thought to asymmetrically promote outgrowth by inducing actin polymerization. However, the mechanism that links MIG-10 to actin polymerization is not known. This study identified the actin regulatory protein ABI-1 as a partner for MIG-10 that can mediate its outgrowth-promoting activity. The SH3 domain of ABI-1 binds to MIG-10, and loss of function of either of these proteins causes similar axon guidance defects. Like MIG-10, ABI-1 functions in both the attractive UNC-6 (netrin) pathway and the repulsive SLT-1 (slit) pathway. Dosage sensitive genetic interactions indicate that MIG-10 functions with ABI-1 and WVE-1 to mediate axon guidance. Epistasis analysis reveals that ABI-1 and WVE-1 function downstream of MIG-10 to mediate its outgrowth-promoting activity. Moreover, experiments with cultured mammalian cells suggest that the interaction between MIG-10 and ABI-1 mediates a conserved mechanism that promotes formation of lamellipodia. Together, these observations suggest that MIG-10 interacts with ABI-1 and WVE-1 to mediate the UNC-6 and SLT-1 guidance pathways (Xu, 2012).

Abelson interactor-1 (ABI-1) interacts with MRL adaptor protein MIG-10 and is required in guided cell migrations and process outgrowth in C. elegans

Directed cell migration and process outgrowth are vital to proper development of many metazoan tissues. These processes are dependent on reorganization of the actin cytoskeleton in response to external guidance cues. During development of the nervous system, the MIG-10/RIAM/Lamellipodin (MRL) signaling proteins are thought to transmit positional information from surface guidance cues to the actin polymerization machinery, and thus to promote polarized outgrowth of axons. In C. elegans, mutations in the MRL family member gene mig-10 result in animals that have defects in axon guidance, neuronal migration, and the outgrowth of the processes or 'canals' of the excretory cell, which is required for osmoregulation in the worm. In addition, mig-10 mutant animals have recently been shown to have defects in clustering of vesicles at the synapse. To determine additional molecular partners of MIG-10, a yeast two-hybrid screen was conducted using isoform MIG-10A as bait, and Abelson-interactor protein-1 (ABI-1) was isolated. ABI-1, a downstream target of Abl non-receptor tyrosine kinase, is a member of the WAVE regulatory complex (WRC) involved in the initiation of actin polymerization. Further analysis using a co-immunoprecipitation system confirmed the interaction of MIG-10 and ABI-1 and showed that it requires the SH3 domain of ABI-1. Single mutants for mig-10 and abi-1 displayed similar phenotypes of incomplete migration of the ALM neurons and truncated outgrowth of the excretory cell canals, suggesting that the ABI-1/MIG-10 interaction is relevant in vivo. Cell autonomous expression of MIG-10 isoforms rescued both the neuronal migration and the canal outgrowth defects, showing that MIG-10 functions autonomously in the ALM neurons and the excretory cell. These results suggest that MIG-10 and ABI-1 interact physically to promote cell migration and process outgrowth in vivo. In the excretory canal, ABI-1 is thought to act downstream of UNC-53/NAV2, linking this large scaffolding protein to actin polymerization during excretory canal outgrowth. abi-1RNAi enhanced the excretory canal truncation observed in mig-10 mutants, while double mutant analysis between unc-53 and mig-10 showed no increased truncation of the posterior canal beyond that observed in mig-10 mutants. Morphological analysis of mig-10 and unc-53 mutants showed that these genes regulate canal diameter as well as its length, suggesting that defective lumen formation may be linked to the ability of the excretory canal to grow out longitudinally. Taken together, these results suggest that MIG-10, UNC-53, and ABI-1 act sequentially to mediate excretory cell process outgrowth (McShea, 2013).


REFERENCES

Search PubMed for articles about Drosophila Abi

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Law, A. L., Vehlow, A., Kotini, M., Dodgson, L., Soong, D., Theveneau, E., Bodo, C., Taylor, E., Navarro, C., Perera, U., Michael, M., Dunn, G. A., Bennett, D., Mayor, R. and Krause, M. (2013). Lamellipodin and the Scar/WAVE complex cooperate to promote cell migration in vivo. J Cell Biol. 203(4): 673-89. PubMed ID: 24247431

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date revised: 15 October 2014

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