Abi-2, a novel SH3-containing protein interacts with the c-Abl tyrosine kinase and modulates c-Abl transforming activity

Bcr-Abl induces abnormal cytoskeleton remodeling, beta1 integrin clustering and increased cell adhesion to fibronectin through the Abl interactor 1 pathway

Hematopoietic cells isolated from patients with Bcr-Abl-positive leukemia exhibit multiple abnormalities of cytoskeletal and integrin function. These abnormalities are thought to play a role in the pathogenesis of leukemia; however, the molecular events leading to these abnormalities are not fully understood. We show here that the Abi1 pathway is required for Bcr-Abl to stimulate actin cytoskeleton remodeling, integrin clustering and cell adhesion. Expression of Bcr-Abl induces tyrosine phosphorylation of Abi1. This is accompanied by a subcellular translocation of Abi1/WAVE2 to a site adjacent to membrane, where an F-actin-enriched structure containing the adhesion molecules such as beta1-integrin, paxillin and vinculin is assembled. Bcr-Abl-induced membrane translocation of Abi1/WAVE2 requires direct interaction between Abi1 and Bcr-Abl, but is independent of the phosphoinositide 3-kinase pathway. Formation of the F-actin-rich complex correlates with an increased cell adhesion to fibronectin. More importantly, disruption of the interaction between Bcr-Abl and Abi1 by mutations either in Bcr-Abl or Abi1 not only abolished tyrosine phosphorylation of Abi1 and membrane translocation of Abi1/WAVE2, but also inhibited Bcr-Abl-stimulated actin cytoskeleton remodeling, integrin clustering and cell adhesion to fibronectin. Together, these data define Abi1/WAVE2 as a downstream pathway that contributes to Bcr-Abl-induced abnormalities of cytoskeletal and integrin function.

Expression of sialidase Neu2 in leukemic K562 cells induces apoptosis by impairing Bcr-Abl/Src kinases signaling

Chronic myeloid leukemia is a hematopoietic stem cell cancer, originated by the perpetually "switched on" activity of the tyrosine kinase Bcr-Abl, leading to uncontrolled proliferation and insensitivity to apoptotic stimuli. The genetic phenotype of myeloid leukemic K562 cells includes the suppression of cytosolic sialidase Neu2. Neu2 transfection in K562 cells induced a marked decrease (-30% and -80%) of the mRNA of the anti-apoptotic factors Bcl-XL and Bcl-2, respectively, and an almost total disappearance of Bcl-2 protein. In addition, gene expression and activity of Bcr-Abl underwent a 35% diminution, together with a marked decrease of Bcr-Abl-dependent Src and Lyn kinase activity. Thus, the antiapoptotic axis Bcr-Abl, Src, and Lyn, which stimulates the formation of Bcl-XL and Bcl-2, was remarkably weakened. The ultimate consequences of these modifications were an increased susceptibility to apoptosis of K562 cells and a marked reduction of their proliferation rate. The molecular link between Neu2 activity and Bcr-Abl signaling pathway may rely on the desialylation of some cytosolic glycoproteins. In fact, three cytosolic glycoproteins, in the range 45-66 kDa, showed a 50-70% decrease of their sialic acid content upon Neu2 expression, supporting their possible role as modulators of the Bcr-Abl complex.

A WAVE-1 and WRP signaling complex regulates spine density, synaptic plasticity, and memory

The scaffolding protein WAVE-1 (Wiskott-Aldrich syndrome protein family member 1) directs signals from the GTPase Rac through the Arp2/3 complex to facilitate neuronal actin remodeling. The WAVE-associated GTPase activating protein called WRP is implicated in human mental retardation, and WAVE-1 knock-out mice have altered behavior. Neuronal time-lapse imaging, behavioral analyses, and electrophysiological recordings from genetically modified mice were used to show that WAVE-1 signaling complexes control aspects of neuronal morphogenesis and synaptic plasticity. Gene targeting experiments in mice demonstrate that WRP anchoring to WAVE-1 is a homeostatic mechanism that contributes to neuronal development and the fidelity of synaptic connectivity. This implies that signaling through WAVE-1 complexes is essential for neural plasticity and cognitive behavior.

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

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

Changes in transcription within the CA1 field of the hippocampus are associated with age-related spatial learning impairments

Aged rats display a broad range of behavioral performance in spatial learning. The aim of this study was to identify candidate genes that are associated with learning and memory impairments. We first categorized aged-superior learners and age learning-impaired rats based on their performance in the Morris water maze (MWM) and then isolated messenger RNA from the CA1 hippocampal region of each animal to interrogate Affymetrix microarrays. Microarray analysis identified a set of 50 genes that was transcribed differently in aged-superior learners that had successfully learned the spatial strategy in the MWM compared to aged learning-impaired animals that were unable to learn and a variety of groups designed to control for all non-learning aspects of exposure to the water maze paradigm. A detailed analysis of the navigation patterns of the different groups of animals during acquisition and probe trials of the MWM task was performed. Young animals used predominantly an allocentric (spatial) search strategy and aged-superior learners appeared to use a combination of allocentric and egocentric (response) strategies, whereas aged-learning impaired animals displayed thigmotactic behavior. The significant 50 genes that we identified were tentatively classified into four groups based on their putative role in learning: transcription, synaptic morphology, ion conductivity and protein modification. Thus, this study has potentially identified a set of genes that are responsible for the learning impairments in aged rats. The role of these genes in the learning impairments associated with aging will ultimately have to be validated by manipulating gene expression in aged rats. Finally, these 50 genes were functioning in the context of an aging CA1 region where over 200 genes was found to be differentially expressed compared to a young CA1.

WAVE signalling: from biochemistry to biology

The small GTPases Rho, Rac and Cdc42 (cell-division cycle 42) function as molecular switches to modulate the actin cytoskeleton. They achieve this by modulating the activity of downstream cellular targets. One group of Rho GTPase effectors, WAVE (Wiskott-Aldrich syndrome protein verprolin homologous)-1, WAVE-2 and WAVE-3, function as scaffolds for actin-based signalling complexes. The present review highlights current knowledge regarding the biochemistry of the WAVE signalling complexes and their biological significance.

Role for the Abi/wave protein complex in T cell receptor-mediated proliferation and cytoskeletal remodeling

BACKGROUND

The molecular reorganization of signaling molecules after T cell receptor (TCR) activation is accompanied by polymerization of actin at the site of contact between a T cell and an antigen-presenting cell (APC), as well as extension of actin-rich lamellipodia around the APC. Actin polymerization is critical for the fidelity and efficiency of the T cell response to antigen. The ability of T cells to polymerize actin is critical for several steps in T cell activation including TCR clustering, mature immunological synapse formation, calcium flux, IL-2 production, and proliferation. Activation of the Rac GTPase has been linked to regulation of actin polymerization after TCR stimulation. However, the molecules required for TCR-mediated actin polymerization downstream of activated Rac have remained elusive. Here we identify a novel role for the Abi/Wave protein complex, which signals downstream of activated Rac, in the regulation of actin polymerization and T cell activation in response to TCR stimulation.

RESULTS

Here we show that Abi and Wave rapidly translocate from the T cell cytoplasm to the T cell:B cell contact site in the presence of antigen. Abi and Wave colocalize with actin at the T cell:B cell conjugation site. Moreover, Wave and Abi are necessary for actin polymerization after T cell activation, and loss of Abi proteins in mice impairs TCR-induced cell proliferation and IL-2 production in primary T cells. Significantly, the impairment in actin polymerization in cells lacking Abi proteins is due to the inability of Wave proteins to localize to the T cell:B cell contact site in the presence of antigen, rather than the destabilization of the components of the Wave protein complex.

CONCLUSIONS

The Abi/Wave complex is a novel regulator of TCR-mediated actin dynamics, IL-2 production, and proliferation.

The expression of Abl interactor 2 in leiomyoma and myometrium and regulation by GnRH analogue and transforming growth factor-β

BACKGROUND

Abelson (Abl) interactor 2 (Abi-2) has been considered as a key regulator of cell/tissue structural organization and is differentially expressed in leiomyomas. The objective of this study was to evaluate the expression of Abi-2 in leiomyoma/myometrium during the menstrual cycle and following GnRH analogue (GnRHa) therapy, as well as regulation by transforming growth factor (TGF)-beta1 in leiomyoma and myometrial smooth muscle cells (LSMC and MSMC).

METHODS

We used real-time PCR, Western blotting and immunohistochemistry to determine the expression of Abi-2 in paired leiomyoma and myometrium (n = 27) from proliferative (n = 8) and secretory (n = 12) phases of the menstrual cycle and from patients who received GnRHa therapy (n = 7). Time-dependent action of TGF-beta1 (2.5 ng/ml) and GnRHa (0.1 microM) on Abi-2 expression was determined in LSMC and MSMC.

RESULTS

Leiomyomas express elevated levels of Abi-2 as compared with myometrium from the proliferative but not the secretory phase of the menstrual cycle, with a significant reduction following GnRHa therapy (P < 0.05). Western blotting showed a similar trend in Abi-2 protein expression in leiomyoma/myometrial tissue extracts, which was immunolocalized in LSMC and MSMC, connective tissue fibroblasts and arterial walls. The expression of Abi-2 in LSMC and MSMC was increased by TGF-beta1 (2.5 ng/ml) and was inhibited by GnRHa (0.1 microM) in a time- and cell-dependent manner, and pretreatment with Smad3 SiRNA and U0126, an MEK-1/2 inhibitor, respectively, reversed their actions.

CONCLUSION

Based on the menstrual cycle-dependent expression, the influence of GnRHa therapy, and regulation by TGF-beta in LSMC/MSMC, we conclude that Abi-2 may have a key regulatory function in leiomyomas cellular/tissue structural organization during growth and regression.

Protein complexes regulating Arp2/3-mediated actin assembly

Key steps in regulating actin dynamics are the de novo nucleation and elongation of actin filaments, which can be catalysed by a limited number of proteins and protein complexes. Among these, Arp2/3 complex and formins are the best studied. Arp2/3-complex activity is controlled through signalling-dependent association with nucleation-promoting factors, such as the WASP/WAVE family proteins. A common theme for these molecules, which is well established for WAVEs but is only just beginning to emerge for WASPs, is that they act as coincident detectors of a variety of signalling pathways through the formation of large multi-molecular complexes.

Arp2/3 and SCAR: plants move to the fore

The actin-nucleating Arp2/3 complex is essential for life in yeast and animals, but not in plants, in which mutants of Arp2/3 complex components show relatively minor developmental abnormalities. Animal cells control the activity of the Arp2/3 complex through the suppressor of cyclic AMP receptor (SCAR) complex to achieve cell motility. Amazingly, plants have also retained the SCAR cell-motility pathway, and now provide a unique model for the study of new aspects of SCAR function in the absence of cell motility.

SHP-2 Phosphatase Regulates DNA Damage-induced Apoptosis and G2/M Arrest in Catalytically Dependent and Independent Manners, Respectively

SHP-2, a tyrosine phosphatase implicated in diverse signaling pathways induced by growth factors and cytokines, is also involved in DNA damage-triggered signaling and cellular responses. We previously demonstrated that SHP-2 played an important role in DNA damage-induced apoptosis and G2/M cell cycle checkpoint. In the present studies, we have provided evidence that SHP-2 functions in DNA damage apoptosis and G2/M arrest in catalytically dependent and independent manners, respectively. Mutant embryonic fibroblasts with the Exon 3 deletion mutation in SHP-2 showed decreased apoptosis and diminished G2/M arrest in response to cisplatin treatment. Wild type (WT), but not catalytically inactive mutant SHP-2 (SHP-2 C459S), rescued the apoptotic response of the mutant cells. Interestingly, both WT and SHP-2 C459S efficiently restored the G2/M arrest response. Furthermore, inhibition of the catalytic activity of endogenous SHP-2 in WT cells by overexpression of SHP-2 C459S greatly decreased cell death but not G2/M arrest induced by cisplatin. Biochemical analyses revealed that activation of c-Abl kinase was decreased in SHP-2 C459S-overexpressing cells. However, DNA damage-induced translocation of Cdc25C from the nucleus to the cytoplasm was fully restored in both WT and SHP-2 C459S "rescued" cells. Additionally, we demonstrated that the role of SHP-2 in DNA damage-induced cellular responses was independent of the tumor suppressor p53. Embryonic stem cells with the SHP-2 deletion mutation showed markedly decreased sensitivity to cisplatin-induced apoptosis, attributed to impaired induction of p73 but not p53. In agreement with these results, DNA damage-induced apoptosis and G2/M arrest were also decreased in SHP-2/p53 double mutant embryonic fibroblasts. Collectively, these studies have further defined the mechanisms by which SHP-2 phosphatase regulates DNA damage responses.

RIN1 is an ABL tyrosine kinase activator and a regulator of epithelial-cell adhesion and migration

BACKGROUND

ABL tyrosine kinases control actin remodeling in development and in response to environmental stimuli. These changes affect cell adhesion, cell migration, and cell-cell contact. Little is known, however, about upstream mechanisms regulating ABL protein activation.

RESULTS

We report that the RAS effector RIN1 is an activator of ABL tyrosine kinases. RIN1 expression in fibroblasts promotes the formation of membrane spikes; similar effects have been reported for ABL overexpression. RIN1 binds to the ABL SH3 and SH2 domains, and these interactions stimulate ABL2 catalytic activity. This leads to increased phosphorylation of CRK and CRKL, inhibiting these cytoskeletal regulators by promoting intramolecular over intermolecular associations. Activated RAS participates in a stable RAS-RIN1-ABL2 complex and stimulates the tyrosine kinase-activation function of RIN1. Deletion of the RAS binding domain (RBD) strongly stimulated the ABL2 activation function of RIN1, suggesting that RAS activation results from the relief of RIN1 autoinhibition. The ABL binding domain of RIN1 (RIN1-ABD) increased the activity of ABL2 immune complexes and purified RIN1-ABD-stimulated ABL2 kinase activity toward CRK. Mammary epithelial cells (MECs) from Rin1-/- mice showed accelerated cell adhesion and increased motility in comparison to wild-type cells. Knockdown of RIN1 in epithelial-cell lines blocked the induction of CRKL phosphorylation, confirming that RIN1 normally functions as an inhibitor of cell motility.

CONCLUSIONS

RIN1 is a directly binding ABL tyrosine kinase activator in cells as well as in a defined-component assay. In response to environmental changes, this novel signal pathway mediates actin remodeling associated with adhesion and migration of epithelial cells.

Abi enhances Abl-mediated CDC2 phosphorylation and inactivation

Abelson tyrosine kinase (Abl) is a non-receptor tyrosine kinase which is frequently coupled with adaptor proteins to interact with its substrates for the regulation of cytoskeleton rearrangement, cell growth and apoptosis in response to a variety of biological stimuli. The Abl interactor (Abi) family members were first identified as adaptor proteins of Abl for regulating Abl transforming and kinase activity. In the present study, we used a yeast two-hybrid screen to identify Cdc2 as a novel Abi-binding protein. This finding led us to investigate the role of Abi in linking Abl and Cdc2. These three proteins formed a trimeric complex in Drosophila and mammalian cells. The expression of Abi in cells greatly enhanced the formation of the Abl-Cdc2 complex, suggesting that Abi functions as an adaptor protein facilitating the binding between Abl and Cdc2. We show that Abi promotes Abl-mediated phosphorylation of Cdc2 at tyrosine 15 and inactivation of Cdc2 kinase activity. Furthermore, coexpression of Abl and Abi in Drosophila S2 cells led to suppression of cell growth. These data suggest that Abl signaling may be involved in the downregulation of Cdc2 kinase in cell cycle control.

Dissecting Nck/Dock signaling pathways in Drosophila visual system

The establishment of neuronal connections during embryonic development requires the precise guidance and targeting of the neuronal growth cone, an expanded cellular structure at the leading tip of a growing axon. The growth cone contains sophisticated signaling systems that allow the rapid communication between guidance receptors and the actin cytoskeleton in generating directed motility. Previous studies demonstrated a specific role for the Nck/Dock SH2/SH3 adapter protein in photoreceptor (R cell) axon guidance and target recognition in the Drosophila visual system, suggesting strongly that Nck/Dock is one of the long-sought missing links between cell surface receptors and the actin cytoskeleton. In this review, I discuss the recent progress on dissecting the Nck/Dock signaling pathways in R-cell growth cones. These studies have identified additional key components of the Nck/Dock signaling pathways for linking the receptor signaling to the remodeling of the actin cytoskeleton in controlling growth-cone motility.

Functions of ephrin/Eph interactions in the development of the nervous system: emphasis on the hippocampal system

Ephrins and their Eph receptors are membrane-anchored proteins that have key roles in the development of the Central Nervous System. The main characteristics of ephrin/Eph interactions are that their effect is mediated by cell-to-cell contacts and that they can propagate bidirectional signals downstream of the ligand-receptor complex. These characteristics make ephrins and Eph receptors critical cues in the regulation of migrating cells or axons, and in the establishment of tissue patterns and topographic maps in distinct regions of the developing brain. In addition, ephrins and Eph receptors regulate synapse formation and plasticity. These roles would be promoted by complementary gradual expression of receptors and ligands in the neurons involved. Although, historically, ephrins and Eph receptors have been considered as repulsion signals through barriers or gradients, new evidence indicates that they may be both inhibitory and permissive/active cues depending on expression levels. The expression of distinct ligands and receptors in the developing and mature hippocampus suggests that these proteins are involved in distinct processes during the development and maturation of the hippocampal region. In fact, recent studies have shown that ephrin/Eph signaling participates in the formation of the layer-specific patterns of hippocampal afferents, in synaptogenesis and in plasticity. Therefore, ephrin/Eph interactions should be considered a crucial system in the development and maturation of the brain regions, including the hippocampus.

Inclusion of Scar/WAVE3 in a similar complex to Scar/WAVE1 and 2

BACKGROUND

The Scar/WAVE family of proteins mediates signals to actin assembly by direct activation of the Arp2/3 complex. These proteins have been characterised as major regulators of lamellipodia formation downstream of Rac activation and as members of large protein complexes.

RESULTS

We have investigated the interactions of the three human Scar/WAVE isoforms with several previously described binding partners for Scar/WAVE 1 or 2. We find that all three Scar/WAVE isoforms behave similarly and are likely to participate in the same kinds of protein complexes that regulate actin assembly.

CONCLUSION

Differences between Scar/WAVE proteins are therefore likely to be at the level of tissue distribution or subtle differences in the affinity for specific binding partners.

Mitotic phosphorylation rescues Abl from F-actin-mediated inhibition

We have previously shown that F-actin exerts a negative effect on Abl tyrosine kinase activity. This inhibition results from a direct association of F-actin with the C terminus of Abl and accounts, in part, for the loss of Abl activity in detached fibroblasts. We report here that Abl from mitotic cells or cells treated with the protein phosphatase inhibitor okadaic acid remains active when detached from the extracellular matrix. Aspartic acid substitution of Thr(566), which is phosphorylated in mitotic or okadaic acid-treated cells, is sufficient to abolish F-actin-mediated inhibition and to maintain Abl activity despite cell detachment. A recent crystal structure of the Abl N-terminal region has revealed autoinhibitory interactions among the Src homology 3 (SH3), SH2, and kinase domains. We found that deletion of the SH2 domain also abolished the negative effect of F-actin on kinase activity. Immediately following the kinase domain in Abl is a proline-rich linker (PRL) that binds to several SH3 adaptor proteins. Interestingly, binding of the Crk N-terminal SH3 domain to the PRL also disrupted F-actin-mediated inhibition of Abl kinase. These results suggest that F-actin may reinforce the autoinhibitory interactions to regulate Abl kinase and that inhibition can be relieved through phosphorylation and/or protein interactions with the Abl PRL region.

ABI2-deficient mice exhibit defective cell migration, aberrant dendritic spine morphogenesis, and deficits in learning and memory

The Abl-interactor (Abi) family of adaptor proteins has been linked to signaling pathways involving the Abl tyrosine kinases and the Rac GTPase. Abi proteins localize to sites of actin polymerization in protrusive membrane structures and regulate actin dynamics in vitro. Here we demonstrate that Abi2 modulates cell morphogenesis and migration in vivo. Homozygous deletion of murine abi2 produced abnormal phenotypes in the eye and brain, the tissues with the highest Abi2 expression. In the absence of Abi2, secondary lens fiber orientation and migration were defective in the eye, without detectable defects in proliferation, differentiation, or apoptosis. These phenotypes were consistent with the localization of Abi2 at adherens junctions in the developing lens and at nascent epithelial cell adherens junctions in vitro. Downregulation of Abi expression by RNA interference impaired adherens junction formation and correlated with downregulation of the Wave actin-nucleation promoting factor. Loss of Abi2 also resulted in cell migration defects in the neocortex and hippocampus, abnormal dendritic spine morphology and density, and severe deficits in short- and long-term memory. These findings support a role for Abi2 in the regulation of cytoskeletal dynamics at adherens junctions and dendritic spines, which is critical for intercellular connectivity, cell morphogenesis, and cognitive functions.

Regulation of actin-based cell migration by cAMP/PKA

A wide variety of soluble signaling substances utilize the cyclic AMP-dependent protein kinase (PKA) pathway to regulate cellular behaviors including intermediary metabolism, ion channel conductivity, and transcription. A growing literature suggests that integrin-mediated cell adhesion may also utilize PKA to modulate adhesion-associated events such as actin cytoskeletal dynamics and migration. PKA is dynamically regulated by integrin-mediated cell adhesion to extracellular matrix (ECM). Furthermore, while some hallmarks of cell migration and cytoskeletal organization require PKA activity (e.g. activation of Rac and Cdc42; actin filament assembly), others are inhibited by it (e.g. activation of Rho and PAK; interaction of VASP with the c-Abl tyrosine kinase). Also, cell migration and invasion can be impeded by either inhibition or hyper-activation of PKA. Finally, a number of A-kinase anchoring proteins (AKAPs) serve to associate PKA with various components of the actin cytoskeleton, thereby enhancing and/or specifying cAMP/PKA signaling in those regions. This review discusses the growing literature that supports the hypothesis that PKA plays a central role in cytoskeletal regulation and cell migration.

Thyroid hormone responsive protein (THRP) mediates thyroid hormone-induced cytotoxicity in primary neuronal cultures

The thyroid hormone responsive protein (THRP) is a novel gene product that remains responsive to thyroid hormone (TH) in the cerebral cortex of adult rats. To study the effects of THRP on neuronal cell survival, primary neurons cultured from rats at embryonic day 19 were treated with either 10(-7) mol L(-1) 3,5,3'-triiodothyronine (T(3)), or 10(-7) mol L(-1) L: -thyroxine (T(4)). This resulted in decreasing neuronal cell number starting 48 h after treatment. T(3) -related cytotoxicity was also documented by measurement of lactate dehydrogenase release into the medium and by propidium iodide staining. Treatment of cells with 10(-7) mol L(-1) T(3) resulted in a significant increase in THRP mRNA levels as early as 24 h of treatment in a concentration-dependent manner. T(3) treatment did not alter glyceraldehyde 3-phosphate dehydrogenase (G3PDH) mRNA levels. Exogenous expression of THRP by transfecting cells with a THRP expression construct (pSVL-THRP) was associated with a significant increase in cell death as measured by the increased number of propidium iodide staining cells (18.0+/-2.1 cells per field) compared with mock-transfected cells (3.3+/-0.2), P<0.002. To further document THRP-induced cytotoxicity, the cells were either transfected with pSVL (empty vector)+pSV2neo (neomycin resistance vector for cell labeling), pSVL-THRP+pSV2neo, or pSVL-THRP+pc-Abl (cAbl tyrosine kinase expressing vector)+pSV2neo. After 24 h the cells were treated with 500 microg mL(-1) G418 (a congener of neomycin) to eliminate the non-transfected cells. Transfection with pSVL-THRP reduced neuronal survival relative to cells transfected with pSVL (356+/-15.6 compared with 145+/-16.9, P<0.05). Co-transfection of THRP with wild-type c-Abl did not alter the effect of THRP on cell survival. It is concluded that THRP is an important factor in TH-induced neuronal cell death.

WAVE/SCAR, a multifunctional complex coordinating different aspects of neuronal connectivity

Although it is well established that the WAVE/SCAR complex transduces Rac1 signaling to trigger Arp2/3-dependent actin nucleation, regulatory mechanisms of this complex and its versatile function in the nervous system are poorly understood. Here we show that the Drosophila proteins SCAR, CYFIP and Kette, orthologs of WAVE/SCAR complex components, all show strong accumulation in axons of the central nervous system and indeed form a complex in vivo. Neuronal defects of SCAR, CYFIP and Kette mutants are, despite the initially proposed function of CYFIP and Kette as SCAR silencers, indistinguishable and are as diverse as ectopic midline crossing and nerve branching as well as synapse undergrowth at the larval neuromuscular junction. The common phenotypes of the single mutants are readily explained by the finding that loss of any one of the three proteins leads to degradation of its partners. As a consequence, each mutant is unambiguously to be judged as defective in multiple components of the complex even though each component affects different signaling pathways. Indeed, SCAR-Arp2/3 signaling is known to control axonogenesis whereas CYFIP signaling to the Fragile X Mental Retardation Protein fly ortholog contributes to synapse morphology. Thus, our results identify the Drosophila WAVE/SCAR complex as a multifunctional unit orchestrating different pathways and aspects of neuronal connectivity.

The BCR-ABL story: bench to bedside and back

The twenty-first century is beginning with a sharp turn in the field of cancer therapy. Molecular targeted therapies against specific oncogenic events are now possible. The BCR-ABL story represents a notable example of how research from the fields of cytogenetics, retroviral oncology, protein phosphorylation, and small molecule chemical inhibitors can lead to the development of a successful molecular targeted therapy. Imatinib mesylate (Gleevec, STI571, or CP57148B) is a direct inhibitor of ABL (ABL1), ARG (ABL2), KIT, and PDGFR tyrosine kinases. This drug has had a major impact on the treatment of chronic myelogenous leukemia (CML) as well as other blood neoplasias and solid tumors with etiologies based on activation of these tyrosine kinases. Analysis of CML patients resistant to BCR-ABL suppression by Imatinib mesylate coupled with the crystallographic structure of ABL complexed to this inhibitor have shown how structural mutations in ABL can circumvent an otherwise potent anticancer drug. The successes and limitations of Imatinib mesylate hold general lessons for the development of alternative molecular targeted therapies in oncology.

Abl interactor 1 (Abi-1) wave-binding and SNARE domains regulate its nucleocytoplasmic shuttling, lamellipodium localization, and wave-1 levels

The Abl interactor 1 (Abi-1) protein has been implicated in the regulation of actin dynamics and localizes to the tips of lamellipodia and filopodia. Here, we show that Abi-1 binds the actin nucleator protein Wave-1 through an amino-terminal Wave-binding (WAB) domain and that disruption of the Abi-1-Wave-1 interaction prevents Abi-1 from reaching the tip of the lamellipodium. Abi-1 binds to the Wave homology domain of Wave-1, a region that is required for translocation of Wave-1 to the lamellipodium. Mouse embryo fibroblasts that lack one allele of Abi-1 and are homozygous null for the related Abi-2 protein exhibit decreased Wave-1 protein levels. This phenotype is rescued by Abi-1 proteins that retain Wave-1 binding but not by Abi-1 mutants that cannot bind to Wave-1. Moreover, we uncovered an overlapping SNARE domain in the amino terminus of Abi-1 that interacts with Syntaxin-1, a SNARE family member. Further, we demonstrated that Abi-1 shuttles in and out of the nucleus in a leptomycin B (LMB)-dependent manner and that complete nuclear translocation of Abi-1 in the absence of LMB requires the combined inactivation of the SNARE, WAB, and SH3 domains of Abi-1. Thus, Abi-1 undergoes nucleocytoplasmic shuttling and functions at the leading edge to regulate Wave-1 localization and protein levels.

Abi1 is essential for the formation and activation of a WAVE2 signalling complex

WAVE2 belongs to a family of proteins that mediates actin reorganization by relaying signals from Rac to the Arp2/3 complex, resulting in lamellipodia protrusion. WAVE2 displays Arp2/3-dependent actin nucleation activity in vitro, and does not bind directly to Rac. Instead, it forms macromolecular complexes that have been reported to exert both positive and negative modes of regulation. How these complexes are assembled, localized and activated in vivo remains to be established. Here we use tandem mass spectrometry to identify an Abi1-based complex containing WAVE2, Nap1 (Nck-associated protein) and PIR121. Abi1 interacts directly with the WHD domain of WAVE2, increases WAVE2 actin polymerization activity and mediates the assembly of a WAVE2-Abi1-Nap1-PIR121 complex. The WAVE2-Abi1-Nap1-PIR121 complex is as active as the WAVE2-Abi1 sub-complex in stimulating Arp2/3, and after Rac activation it is re-localized to the leading edge of ruffles in vivo. Consistently, inhibition of Abi1 by RNA interference (RNAi) abrogates Rac-dependent lamellipodia protrusion. Thus, Abi1 orchestrates the proper assembly of the WAVE2 complex and mediates its activation at the leading edge in vivo.

Abl-dependent tyrosine phosphorylation of Sos-1 mediates growth-factor-induced Rac activation

The non-receptor tyrosine kinase Abl participates in receptor tyrosine kinase (RTK)-induced actin cytoskeleton remodelling, a signalling pathway in which the function of Rac is pivotal. More importantly, the activity of Rac is indispensable for the leukaemogenic ability of the BCR-Abl oncoprotein. Thus, Rac might function downstream of Abl and be activated by it. Here, we elucidate the molecular mechanisms through which Abl signals to Rac in RTK-activated pathways. We show that Sos-1, a dual guanine nucleotide-exchange factor (GEF), is phosphorylated on tyrosine, after activation of RTKs, in an Abl-dependent manner. Sos-1 and Abl interact in vivo, and Abl-induced tyrosine phosphorylation of Sos-1 is sufficient to elicit its Rac-GEF activity in vitro. Genetic or pharmacological interference with Abl (and the related kinase Arg) resulted in a marked decrease in Rac activation induced by physiological doses of growth factors. Thus, our data identify the molecular connections of a pathway RTKs-Abl-Sos-1-Rac that is involved in signal transduction and actin remodelling.

Abi, Sra1, and Kette control the stability and localization of SCAR/WAVE to regulate the formation of actin-based protrusions

BACKGROUND

In animal cells, GTPase signaling pathways are thought to generate cellular protrusions by modulating the activity of downstream actin-regulatory proteins. Although the molecular events linking activation of a GTPase to the formation of an actin-based process with a characteristic morphology are incompletely understood, Rac-GTP is thought to promote the activation of SCAR/WAVE, whereas Cdc42 is thought to initiate the formation of filopodia through WASP. SCAR and WASP then activate the Arp2/3 complex to nucleate the formation of new actin filaments, which through polymerization exert a protrusive force on the membrane.

RESULTS

Using RNAi to screen for genes regulating cell form in an adherent Drosophila cell line, we identified a set of genes, including Abi/E3B1, that are absolutely required for the formation of dynamic protrusions. These genes delineate a pathway from Cdc42 and Rac to SCAR and the Arp2/3 complex. Efforts to place Abi in this signaling hierarchy revealed that Abi and two components of a recently identified SCAR complex, Sra1 (p140/PIR121/CYFIP) and Kette (Nap1/Hem), protect SCAR from proteasome-mediated degradation and are critical for SCAR localization and for the generation of Arp2/3-dependent protrusions.

CONCLUSIONS

In Drosophila cells, SCAR is regulated by Abi, Kette, and Sra1, components of a conserved regulatory SCAR complex. By controlling the stability, localization, and function of SCAR, these proteins may help to ensure that Arp2/3 activation and the generation of actin-based protrusions remain strictly dependant on local GTPase signaling.

How do Abl family kinases regulate cell shape and movement?

Genetic analysis and studies of normal and leukemia cells in culture have shown that Abl family nonreceptor tyrosine kinases regulate cell morphogenesis and motility. Abl family kinases, which include Drosophila (D-) Abl and the vertebrate Abl and Arg proteins, relay signals from cell surface growth-factor and adhesion receptors to promote cytoskeletal rearrangements. Recent biochemical and crystallographic analyses have clarified the mechanisms by which growth-factor and adhesion receptors might regulate the activity of Abl family kinases. When activated, Abl family kinases can regulate cytoskeletal dynamics by phosphorylating several known cytoskeletal regulatory proteins. In addition, the C-terminal half of Abl family kinases has several domains that bind to cytoskeletal components. Emerging evidence suggests that Abl family kinases can use these domains to directly organize cytoskeletal structure in vivo.

Signaling at the growth cone: ligand-receptor complexes and the control of axon growth and guidance

The guidance of axons during the establishment of the nervous system is mediated by a variety of extracellular cues that govern cytoskeletal dynamics in axonal growth cones. A large number of these guidance cues and their cell-surface receptors have now been identified, and the intracellular signaling pathways by which these cues induce cytoskeletal rearrangements are becoming defined. This review summarizes our current understanding of the major families of axon guidance cues and their receptors, with a particular emphasis on receptor signaling mechanisms. We also discuss recent advances in understanding receptor cross talk and how the activities of guidance cues and their receptors are modulated during neural development.

Cytoplasmic c-Abl provides a molecular 'Rheostat' controlling carcinoma cell survival and invasion

Tumor cell metastasis involves the coordinated activation of migration and survival mechanisms necessary for cell invasion of foreign tissues. Here, we report that cytoplasmic c-Abl tyrosine kinase determines whether a cell invades the ECM or commits suicide. c-Abl phosphorylates the cytoskeleton-associated adaptor protein, Crk, at tyrosine 221, causing disassociation of Crk from the Crk-associated substrate (CAS) and disassembly of Crk/CAS complexes. c-Abl-induced disruption of Crk/CAS complexes inhibits cell migration and promotes apoptosis in normal cells, and is deregulated in highly invasive carcinoma cells. c-Abl-mediated disassembly of Crk/CAS complexes and induction of death occur via disruption of the cytoskeleton, which is distinct from nuclear c-Abl-induced apoptosis in response to DNA-damaging agents. Inhibition of c-Abl kinase activity or Crk binding to Abl's polyproline region prevents Crk phosphorylation and apoptosis, leading to increased cell survival and invasion of the extracellular matrix. Together, these data illustrate that c-Abl prevents aberrant motility and survival through Crk 221 phosphorylation and modulation of Crk/CAS complexes, and that deregulation of this pathway contributes to cell metastasis.

PIR121 regulates pseudopod dynamics and SCAR activity in Dictyostelium

BACKGROUND

The WASP/SCAR family of adaptor proteins coordinates actin reorganization by coupling different signaling molecules, including Rho-family GTPases, to the activation of the Arp2/3 complex. WASP binds directly to Cdc42 through its GTPase binding domain (GBD), but SCAR does not contain a GBD, and no direct binding has been found. However, SCAR has recently been found to copurify with four other proteins in a complex. One of these, PIR121, binds directly to Rac.

RESULTS

We have identified four of the members of this complex in Dictyostelium and disrupted the pirA gene, which encodes PIR121. The resulting mutant cells are unusually large, maintain an excessive proportion of their actin in a polymerized state and display severe defects in movement and chemotaxis. They also continually extend new pseudopods by widening and splitting existing leading edges rather than by initiating new pseudopods. Comparing these cells to scar null mutants shows behavior that is broadly consistent with overactivation of SCAR. Deletion of the pirA gene in a scar(-) mutant resulted in cells resembling their scar(-) parents with no obvious changes, confirming that PIR121 mainly acts through SCAR in vivo. Surprisingly given their hyperactive phenotype, we find that pirA(-) mutants contain very little intact SCAR protein despite normal levels of mRNA, suggesting a posttranscriptional downregulation of activated SCAR.

CONCLUSIONS

Our results demonstrate a genetic connection between the pirA and scar genes. PIR121 appears to inhibit the activity of SCAR in the absence of activating signals. The location of the newly formed protrusions indicates that unregulated SCAR is acting at the edges of existing pseudopods, not elsewhere in the cell. We suggest that active SCAR protein released from the inhibitory complex is rapidly removed and that this is an important and novel mechanism for controlling actin dynamics.

Specific targeted therapy of chronic myelogenous leukemia with imatinib

Chronic myeloid leukemia (CML) is characterized by the Philadelphia translocation that fuses BCR sequences from chromosome 22 upstream of the ABL gene on chromosome 9. The chimerical Bcr-Abl protein expressed by CML cells has constitutive tyrosine kinase activity, which is essential for the pathogenesis of the disease. Imatinib, an ATP-competitive selective inhibitor of Bcr-Abl, has unprecedented efficacy for the treatment of CML. Most patients with early stage disease achieve durable complete hematological and complete cytogenetic remissions, with minimal toxicity. In contrast, responses are less stable in patients with advanced CML. This review highlights the pathogenesis of CML, its clinical features, and the development of imatinib as a specific molecularly targeted therapy. Aspects of disease monitoring and side effects are covered as well as resistance to imatinib and strategies to overcome resistance, such as alternative signal transduction inhibitors and drug combinations. Perspectives for further development are also discussed.

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

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

Abl interactor 1 promotes tyrosine 296 phosphorylation of mammalian enabled (Mena) by c-Abl kinase

Mammalian Enabled (Mena) is a mammalian homologue of Drosophila Enabled (Ena), which genetically interacts with Drosophila Abl tyrosine kinase. The signaling pathway involving c-Abl and Mena (Ena) is not fully understood. To find molecules that participate in the c-Abl/Mena pathway, we searched for Mena-binding proteins using a yeast two-hybrid system. We identified Abl interactor 1 (Abi-1), which is known to interact with c-Abl, as a binding protein for Mena. Binding analysis revealed that the Ena/Vasp homology 1 domain of Mena and the polyproline structure of Abi-1 are necessary for the interaction. The interaction between Mena and Abi-1 was also observed in a mammalian expression system. Importantly, Abi-1 dramatically promoted c-Abl-mediated tyrosine phosphorylation of Mena but not other substrates such as c-Cbl. Mutational analysis demonstrated that the phosphorylation site of Mena is Tyr-296. Our results suggest that Abi-1 regulates c-Abl-mediated phosphorylation of Mena by interacting with both proteins.

Effect of thyroid hormone responsive protein (THRP) expression on PC12 cell survival

The thyroid hormone responsive protein (THRP) is a novel gene product that remains responsive to thyroid hormone in the cerebral cortex of adult rats. The biological effects of THRP are currently unknown. Since thyroid hormones (TH) are known to cause cell death in primary neuronal cultures, the effect of exogenous THRP expression on PC12 cell viability was investigated. Co-transfection of the THRP expression plasmid with the selectable marker pSV2neo resulted in a lower number of surviving PC12 cells compared to transfection with pSV2neo and the empty vector, pSVL. Similar results were observed when PC12 cells were transfected with the plasmid pCMV. SPORT beta-gal with and without pSVL-THRP. However, expression of exogenous THRP in the colonic epithelial cell line Caco-2 and the glial cell line U251 had no effect on cell viability. Coexpression of THRP with either the wild-type (WT)-c-Abl or a kinase-defective mutant c-Abl (K290R) did not alter the cell viability changes induced by THRP alone. Under these experimental conditions the predominant form of cell death was necrosis as evidenced by in situ analyses, such as terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) and staining with membrane permeating and non-permeating nuclear dyes, Hoechst 33342 and propidium iodide respectively. In addition cell cycle arrest induced by THRP was demonstrated by reduced (3)H-thymidine incorporation into cellular DNA. The number of PC12 cells treated with 10(-7) M of l-3, 5, 3'-triiodothyronine (T(3)) was significantly reduced after the fourth day of culture. Treatment of the cells with T(3 )resulted in a dose dependent induction of THRP mRNA. It is concluded that: (1). THRP expression induces PC12 cell death; (2). under these experimental conditions the form of cell death is predominantly necrosis although cell cycle arrest may also occur; (3). the effect of THRP on cell viability is not modulated by c-Abl tyrosine kinase; and (4). the effect of T(3 )treatment on PC12 cell survival may be mediated by THRP.

Role of SHP-2 tyrosine phosphatase in the DNA damage-induced cell death response

SHP-2, a ubiquitously expressed Src hmology 2 (SH2) domain-containing tyrosine phosphatase, plays a critical role in the regulation of growth factor and cytokine signal transduction. Here we report a novel function of this phosphatase in DNA damage-induced cellular responses. Mutant embryonic fibroblast cells lacking functional SHP-2 showed significantly decreased apoptosis in response to DNA damage. Following cisplatin treatment, induction of p73 and its downstream effector p21(Cip1) was essentially blocked in SHP-2 mutant cells. Further investigation revealed that activation of the nuclear tyrosine kinase c-Abl, an essential mediator in DNA damage induction of p73, was impaired in the mutant cells, suggesting a functional requirement of SHP-2 in c-Abl activation. Consistent with this observation, the effect of overexpression of c-Abl kinase in SHP-2 mutant cells on sensitizing the cells to DNA damage-induced death was abolished. Additionally, we found that in embryonic fibroblast cells 30-40% of SHP-2 was localized in the nuclei, and that a fraction of nuclear SHP-2 was constitutively associated with c-Abl via its SH3 domain. Phosphatase activity of nuclear but not cytoplasmic SHP-2 was significantly enhanced in response to DNA damage. These results together suggest a novel nuclear function for SHP-2 phosphatase in the regulation of DNA damage-induced apoptotic responses.

The extreme carboxyl terminus of v-Abl is required for lymphoid cell transformation by Abelson virus

The v-Abl protein tyrosine kinase encoded by Abelson murine leukemia virus (Ab-MLV) induces transformation of pre-B cells in vivo and in vitro and can transform immortalized fibroblast cell lines in vitro. Although the kinase activity of the protein is required for these events, most previously studied mutants encoding truncated v-Abl proteins that lack the extreme carboxyl terminus retain high transforming capacity in NIH 3T3 cells but transform lymphocytes poorly. To understand the mechanisms responsible for poor lymphoid transformation, mutants expressing a v-Abl protein lacking portions of the COOH terminus were compared for their ability to transform pre-B cells. Although all mutants lacking sequences within the COOH terminus were compromised for lymphoid transformation, loss of amino acids in the central region of the COOH terminus, including those implicated in JAK interaction and DNA binding, decreased transformation twofold or less. In contrast, loss of the extreme COOH terminus rendered the protein unstable and led to rapid proteosome-mediated degradation, a feature that was more prominent when the protein was expressed in Ab-MLV-transformed lymphoid cells. These data indicate that the central portion of the COOH terminus is not essential for lymphoid transformation and reveal that one important function of the COOH terminus is to stabilize the v-Abl protein in lymphoid cells.

Mechanisms of autoinhibition and STI-571/imatinib resistance revealed by mutagenesis of BCR-ABL

The Bcr-Abl fusion protein kinase causes chronic myeloid leukemia and is targeted by the signal transduction inhibitor STI-571/Gleevec/imatinib (STI-571). Sequencing of the BCR-ABL gene in patients who have relapsed after STI-571 chemotherapy has revealed a limited set of kinase domain mutations that mediate drug resistance. To obtain a more comprehensive survey of the amino acid substitutions that confer STI-571 resistance, we performed an in vitro screen of randomly mutagenized BCR-ABL and recovered all of the major mutations previously identified in patients and numerous others that illuminate novel mechanisms of acquired drug resistance. Structural modeling implies that a novel class of variants acts allosterically to destabilize the autoinhibited conformation of the ABL kinase to which STI-571 preferentially binds. This screening strategy is a paradigm applicable to a growing list of target-directed anti-cancer agents and provides a means of anticipating the drug-resistant amino acid substitutions that are likely to be clinically problematic.

Post-transcriptional mechanisms in BCR/ABL leukemogenesis: role of shuttling RNA-binding proteins

Shuttling hnRNPs control the fate of eukaryotic mRNAs throughout their journey from the active site of transcription to that of translation; thus, gain or loss of their function in hematopoietic cells might result in altered hematopoiesis and/or be associated with the process of leukemogenesis. In BCR/ABL-expressing cells, there is a marked increase in the protein levels FUS, hnRNP A1 and hnRNP E2, three RNA-binding proteins involved in the regulation of mRNA processing, nucleocytoplasmic export, and translation. Ectopic expression and/or inhibition of the activity of these RNA-binding proteins affects proliferation, survival, and differentiation of normal and BCR/ABL-expressing cells, suggesting that enhanced expression/activity of certain RNA-binding proteins plays an important, but as yet unrecognized, role in BCR/ABL leukemogenesis. The identification of the mRNA subsets associated with RNA-binding proteins upregulated in BCR/ABL-expressing cells should functionally link the process of leukemogenesis with alteration of mRNA metabolism.

Interaction between UV-damaged DNA binding activity proteins and the c-Abl tyrosine kinase

The c-Abl tyrosine kinase is activated by some forms of DNA damage, including ionizing radiation, but not UV radiation. The functions of this activation in the damage response pathways remain obscure. To identify potential targets of c-Abl kinase, we utilized the yeast two-hybrid system to screen a murine cDNA library. One c-Abl binding protein of particular interest was the large subunit (DDB1) of the heterodimeric complex with UV-damaged DNA binding activity (UV-DDB). This complex binds with high specificity to DNA damaged by UV, is absent in a subset of xeroderma pigmentosum group E cells, and is required for global genomic repair of UV-induced damage. The association of c-Abl with DDB1 required the kinase domain of c-Abl and preserved the interaction between DDB1 and the small subunit (DDB2) of the UV-DDB complex. Significantly, overexpression of c-Abl increased tyrosine phosphorylation of DDB2 and suppressed UV-DDB activity. Conversely, a dominant negative, kinase-deficient allele of c-Abl decreased tyrosine phosphorylation of DDB2 and dramatically stimulated UV-DDB activity. These results suggest that one role of c-Abl may be to negatively regulate UV-DDB activity by phosphorylation of DDB2.

Ahi-1, a novel gene encoding a modular protein with WD40-repeat and SH3 domains, is targeted by the Ahi-1 and Mis-2 provirus integrations

The Ahi-1 locus was initially identified as a common helper provirus integration site in Abelson pre-B-cell lymphomas and shown to be closely linked to the c-myb proto-oncogene. Since no significant alteration of c-myb expression was found in Abelson murine leukemia virus-induced pre-B-lymphomas harboring a provirus inserted within the Ahi-1 locus, this suggested that it harbors another gene whose dysregulation is involved in tumor formation. Here we report the identification of a novel gene (Ahi-1) targeted by these provirus insertional mutations and the cloning of its cDNA. The Ahi-1 proviral insertions were found at the 3' end of the gene, in an inverse transcriptional orientation, with most of them located around and downstream of the last exon, whereas another insertion was within intron 22. In addition, another previously identified provirus insertion site, Mis-2, was found to map within the 16th intron of the Ahi-1 gene. The Ahi-1 cDNA encodes a 1,047-amino-acid protein. The predicted Ahi-1 protein is a modular protein that contains one SH3 motif and seven WD40 repeats. The Ahi-1 gene is conserved in mammals and encodes two major RNA species of 5 and 4.2 kb and several other shorter splicing variants. The Ahi-1 gene is expressed in mouse embryos and in several organs of the mouse and rat, notably at high levels in the brain and testes. In tumor cells harboring insertional mutations in Ahi-1, truncated Ahi-1/viral fused transcripts were identified, including some splicing variants with deletion of the SH3 domain. Therefore, Ahi-1 is a novel gene targeted by provirus insertion and encoding a protein that exhibits several features of a signaling molecule. Thus, Ahi-1 may play an important role in signal transduction in normal cells and may be involved in tumor development, possibly in cooperation with other oncogenes (such as v-abl and c-myc) or with a tumor suppressor gene (Nf1), since Ahi-1 insertion sites were identified in tumors harboring v-abl defective retroviruses or a c-myc transgene or in tumors exhibiting deletion of Nf1.

Functional involvement of Akt signaling downstream of Jak1 in v-Abl-induced activation of hematopoietic cells

Activation of intracellular signaling pathways is important for cellular transformation and tumorigenesis. The nonreceptor tyrosine kinases Jak1 and Jak3, which bind to the v-Abl oncoprotein, are constitutively activated in cells transformed with the Abelson murine leukemia virus. A mutant of p160 v-Abl lacking the Jak1-binding region (v-Abl Delta858-1080) has a significant defect in Jak/STAT (signal transducers and activators of transcription) activation, cytokine-independent cell growth/survival, and tumorigenesis. To identify the pathways downstream of Jak kinases in v-Abl-mediated signaling, we examined the activation of several signaling molecules by p160 v-Abl or the v-Abl Delta858-1080 mutant. We demonstrate that, in addition to the decreased Ras activation, signaling through phosphatidylinositol-3 kinase and Akt are impaired in cells expressing mutant v-Abl. The proliferative defect of v-Abl Delta858-1080 was rescued by activated v-Akt and was also moderately rescued by activated v-H-Ras. However, constitutive active phosphatidylinositol-3 kinase (p110CAAX) did not complement this effect. Cells expressing v-Abl Delta858-1080 demonstrated reduced tumor formation in nude mice. In contrast, cells coexpressing v-Akt with v-Abl Delta858-1080 demonstrated reduced latency and increased frequency of tumor formation in nude nice compared with cells expressing v-Abl Delta858-1080 alone, whereas v-H-Ras or p110CAAX had minimum effects on tumor formation. These results suggest that Jak1-dependent Akt activation is important in v-Abl-mediated transformation.

Ephrins in reverse, park and drive

Eph receptors and their membrane-anchored ephrin ligands are thought to orchestrate cell movements by transducing bidirectional tyrosine-kinase-mediated signals into both cells expressing the receptors and cells expressing the ligands. Whether the resulting event is repulsion of an axonal growth cone, directing the orderly segmentation of hindbrain rhombomere cells or controlling angiogenic remodelling, such elaborate and diverse cell movements require intricate changes in the actin cytoskeleton, as well as precise regulation of cellular adhesion. Recent work by several groups has begun to link ephrin reverse signals to intracellular pathways that regulate actin dynamics and might help to explain how these ligands function as receptors to direct cell movement, adhesion and de-adhesion events.

mRNA differential display identification of thyroid hormone-responsive protein (THRP) gene in association with early phase of long-term potentiation

The process of long-term potentiation (LTP) consists of the early induction and late maintenance phases. Few studies have examined the cellular mechanisms underlying these two phases; their respective mRNA expression profiles have not yet been elucidated. Here we used the technique of PCR differential display to identify genes that are differentially expressed between the early and late phases of LTP in vivo. Our results indicated that the cDNA fragment corresponding to one mRNA with preferentially increased expression during the early, but not late, phase of LTP encodes the rat thyroid hormone-responsive protein (THRP) gene. In situ hybridization analysis confirmed the results obtained from the PCR differential display. Prior NMDA receptor blockade with MK801 prevented induction of LTP and decreased THRP mRNA expression in the dentate gyrus, as assayed by quantitative RT-PCR analysis. THRP antisense oligonucleotide treatment before tetanic stimulation also prevented induction of LTP. However, when THRP antisense oligonucleotide was administered after induction of LTP, it did not affect expression and maintenance of LTP. THRP is known to be responsive to thyroid hormone. Our results indicate that direct thyroid hormone (T3) injection into the dentate gyrus produces a long-lasting enhancement of synaptic efficacy of these neurons. T3 injection also markedly increased THRP mRNA expression in the dentate gyrus. Taken together, our results suggest that THRP mRNA expression plays an important role in the early phase, but not the late phase, of LTP and that both THRP and thyroid hormone are involved in synaptic plasticity in hippocampal neurons.

Tyrosine kinase oncogenes in normal hematopoiesis and hematological disease

Tyrosine kinase oncogenes are formed as a result of mutations that induce constitutive kinase activity. Many of these tyrosine kinase oncogenes that are derived from genes, such as c-Abl, c-Fes, Flt3, c-Fms, c-Kit and PDGFRbeta, that are normally involved in the regulation of hematopoiesis or hematopoietic cell function. Despite differences in structure, normal function, and subcellular location, many of the tyrosine kinase oncogenes signal through the same pathways, and typically enhance proliferation and prolong viability. They represent excellent potential drug targets, and it is likely that additional mutations will be identified in other kinases, their immediate downstream targets, or in proteins regulating their function.

Partial characterization of a cerebral thyroid hormone-responsive protein

The thyroid hormone-responsive protein (THRP) is expressed in rat cerebral tissue and has 83% overall sequence homology with c-Abl interactor protein, Abi-2, which is a substrate for the tyrosine kinase activity of c-Abl. Within the core region of the two proteins, the sequence similarity approaches 99%. To determine whether THRP is a rat homologue of Abi-2 or is a distinct protein with unique properties, the tissue distribution of THRP and Abi-2 mRNA's was examined using a sensitive ribonuclease protection assay and probes specific for THRP and Abi-2, respectively. The THRP mRNA content of cerebral tissue (1340.0 +/- 126.5 arbitrary units) was 2.3-fold higher than Abi-2 mRNA (581.3 +/- 73.7), while the ratio of hepatic content of THRP mRNA (209.0 +/- 49.1) to hepatic Abi-2 mRNA (2923.0 +/- 378.7) was only 0.07 (P < 0.004). Very low levels of Abi-2 mRNA, but not THRP mRNA, were also found in the heart and small intestine. Experiments with PC12 cells transfected with the full-length THRP cDNA and grown in the presence or absence of a tyrosine kinase inhibitor, along with experiments where PC12 cells were cotransfected with the THRP cDNA with or without the wild-type or mutant (tyrosine kinase deficient) c-Abl cDNA, showed that THRP is tyrosine phosphorylated; however, it is not a substrate for c-Abl. These studies demonstrate that THRP and Abi-2 have distinct tissue distribution and distinct biological properties.

Mutational analysis of the regulatory function of the c-Abl Src homology 3 domain

The catalytic activity of the c-Abl tyrosine kinase is tightly regulated by its Src homology 3 (SH3) domain through a complex mechanism that may involve intramolecular binding to Pro242 in the linker region between the SH2 and catalytic domains as well as interactions with a trans-inhibitor. We analysed the effect of mutation or replacement of SH3 on c-Abl tyrosine kinase activity and transformation. Random mutagenesis of SH3 identified several novel point mutations that dysregulated c-Abl kinase activity in vivo, but the RT loop was insensitive to mutational activation. Activating SH3 mutations abolished binding of proline-rich SH3 ligands in vitro, while mutations at Ser140 in the connector between the SH3 and SH2 domains activated Abl kinase activity in vivo and in vitro but did not impair SH3 ligand-binding. Abl was regulated efficiently when its SH3 domain was replaced with a heterologous SH3 from c-Src that binds a different spectrum of proline-rich ligands, but not by substitution of a modular WW domain with similar ligand-binding specificity. These results suggest that the SH3 domain regulates Abl principally by binding to the atypical intramolecular ligand Pro242 rather than a canonical PxxP ligand. Coordination between the SH3 and SH2 domains mediated by the connector region may be required for regulation of Abl even in the absence of SH2 ligand binding.

The SH2/SH3 adaptor Grb4 transduces B-ephrin reverse signals

Bidirectional signals mediated by membrane-anchored ephrins and Eph receptor tyrosine kinases have important functions in cell-cell recognition events, including those that occur during axon pathfinding and hindbrain segmentation. The reverse signal that is transduced into B-ephrin-expressing cells is thought to involve tyrosine phosphorylation of the signal's short, conserved carboxy-terminal cytoplasmic domain. The Src-homology-2 (SH2) domain proteins that associate with activated tyrosine-phosphorylated B-subclass ephrins have not been identified, nor has a defined cellular response to reverse signals been described. Here we show that the SH2/SH3 domain adaptor protein Grb4 binds to the cytoplasmic domain of B ephrins in a phosphotyrosine-dependent manner. In response to B-ephrin reverse signalling, cells increase FAK catalytic activity, redistribute paxillin, lose focal adhesions, round up, and disassemble F-actin-containing stress fibres. These cellular responses can be blocked in a dominant-negative fashion by expression of the isolated Grb4 SH2 domain. The Grb4 SH3 domains bind a unique set of other proteins that are implicated in cytoskeletal regulation, including the Cbl-associated protein (CAP/ponsin), the Abl-interacting protein-1 (Abi-1), dynamin, PAK1, hnRNPK and axin. These data provide a biochemical pathway whereby cytoskeletal regulators are recruited to Eph-ephrin bidirectional signalling complexes.