The Abl-related gene (Arg) nonreceptor tyrosine kinase uses two F-actin-binding domains to bundle F-actin

The FABD domain is critical for the oncogenicity of BCR/ABL in chronic myeloid leukaemia

BACKGROUND

Abnormally expressed BCR/ABL protein serves as the basis for the development of chronic myeloid leukaemia (CML). The F-actin binding domain (FABD), which is a crucial region of the BCR/ABL fusion protein, is also located at the carboxyl end of the c-ABL protein and regulates the kinase activity of c-ABL. However, the precise function of this domain in BCR/ABL remains uncertain.

METHODS

The FABD-deficient adenovirus vectors Ad-BCR/ABL△FABD, wild-type Ad-BCR/ABL and the control vector Adtrack were constructed, and 32D cells were infected with these adenoviruses separately. The effects of FABD deletion on the proliferation and apoptosis of 32D cells were evaluated by a CCK-8 assay, colony formation assay, flow cytometry and DAPI staining. The levels of phosphorylated BCR/ABL, p73, and their downstream signalling molecules were detected by western blot. The intracellular localization and interaction of BCR/ABL with the cytoskeleton-related protein F-actin were identified by immunofluorescence and co-IP. The effect of FABD deletion on BCR/ABL carcinogenesis in vivo was explored in CML-like mouse models. The degree of leukaemic cell infiltration was observed by Wright‒Giemsa staining and haematoxylin and eosin (HE) staining.

RESULTS

We report that the loss of FABD weakened the proliferation-promoting ability of BCR/ABL, accompanied by the downregulation of BCR/ABL downstream signals. Moreover, the deletion of FABD resulted in a change in the localization of BCR/ABL from the cytoplasm to the nucleus, accompanied by an increase in cell apoptosis due to the upregulation of p73 and its downstream proapoptotic factors. Furthermore, we discovered that the absence of FABD alleviated leukaemic cell infiltration induced by BCR/ABL in mice.

CONCLUSIONS

These findings reveal that the deletion of FABD diminished the carcinogenic potential of BCR/ABL both in vitro and in vivo. This study provides further insight into the function of the FABD domain in BCR/ABL.

Knockout of Sorbin And SH3 Domain Containing 2 (Sorbs2) in Cardiomyocytes Leads to Dilated Cardiomyopathy in Mice

Background

Sorbin and SH3 domain containing 2 (Sorbs2) protein is a cytoskeletal adaptor with an emerging role in cardiac biology and disease; yet, its potential relevance to adult‐onset cardiomyopathies remains underexplored. Sorbs2 global knockout mice display lethal arrhythmogenic cardiomyopathy; however, the causative mechanisms remain unclear. Herein, we examine Sorbs2 dysregulation in heart failure, characterize novel Sorbs2 cardiomyocyte‐specific knockout mice (Sorbs2‐cKO), and explore associations between Sorbs2 genetic variations and human cardiovascular disease.

Methods and Results

Bioinformatic analyses show myocardial Sorbs2 mRNA is consistently upregulated in humans with adult‐onset cardiomyopathies and in heart failure models. We generated Sorbs2‐cKO mice and report that they develop progressive systolic dysfunction and enlarged cardiac chambers, and they die with congestive heart failure at about 1 year old. After 3 months, Sorbs2‐cKO mice begin to show atrial enlargement and P‐wave anomalies, without dysregulation of action potential–associated ion channel and gap junction protein expressions. After 6 months, Sorbs2‐cKO mice exhibit impaired contractility in dobutamine‐treated hearts and skinned myofibers, without dysregulation of contractile protein expressions. From our comprehensive survey of potential mechanisms, we found that within 4 months, Sorbs2‐cKO hearts have defective microtubule polymerization and compensatory upregulation of structural cytoskeletal and adapter proteins, suggesting that this early intracellular structural remodeling is responsible for contractile dysfunction. Finally, we identified genetic variants that associate with decreased Sorbs2 expression and human cardiac phenotypes, including conduction abnormalities, atrial enlargement, and dilated cardiomyopathy, consistent with Sorbs2‐cKO mice phenotypes.

Conclusions

Our studies show that Sorbs2 is essential for maintaining structural integrity in cardiomyocytes, likely through strengthening the interactions between microtubules and other cytoskeletal proteins at cross‐link sites.

c-Abl kinase-mediated phosphorylation of γ-tubulin promotes γ-tubulin ring complexes assembly and microtubule nucleation

Cytoskeletal microtubules (MTs) are nucleated from γ-tubulin ring complexes (γTuRCs) located at MT organizing centers (MTOCs), such as the centrosome. However, the exact regulatory mechanism of γTuRC assembly is not fully understood. Here, we showed that the nonreceptor tyrosine kinase c-Abl was associated with and phosphorylated γ-tubulin, the essential component of the γTuRC, mainly on the Y443 residue by in vivo (immunofluorescence and immunoprecipitation) or in vitro (surface plasmon resonance) detection. We further demonstrated that phosphorylation deficiency significantly impaired γTuRC assembly, centrosome construction, and MT nucleation. c-Abl/Arg deletion and γ-tubulin Y443F mutation resulted in an abnormal morphology and compromised spindle function during mitosis, eventually causing uneven chromosome segregation. Our findings reveal that γTuRC assembly and nucleation function are regulated by Abl kinase-mediated γ-tubulin phosphorylation, revealing a fundamental mechanism that contributes to the maintenance of MT function.

Role of the ABL tyrosine kinases in the epithelial-mesenchymal transition and the metastatic cascade

The ABL kinases, ABL1 and ABL2, promote tumor progression and metastasis in various solid tumors. Recent reports have shown that ABL kinases have increased expression and/or activity in solid tumors and that ABL inactivation impairs metastasis. The therapeutic effects of ABL inactivation are due in part to ABL-dependent regulation of diverse cellular processes related to the epithelial to mesenchymal transition and subsequent steps in the metastatic cascade. ABL kinases target multiple signaling pathways required for promoting one or more steps in the metastatic cascade. These findings highlight the potential utility of specific ABL kinase inhibitors as a novel treatment paradigm for patients with advanced metastatic disease. Video abstract.

The Effect of Prenatal Food Restriction on Brain Proteome in Appropriately Grown and Growth Restricted Male Wistar Rats

Background

Fetal growth restriction (FGR) has been associated with a higher risk of developing adverse perinatal outcomes and distinct neurodevelopmental and neurobehavioral disorders. The aim of the present study was to investigate the impact of prenatal food restriction on the brain proteome in both FGR and appropriately grown rats and to identify potential pathways connecting maternal malnutrition with altered brain development.

Methods

Ten time-dated pregnant Wistar rats were housed individually at their 12th day of gestation. On the 15th day of gestation, the rats were randomly divided into two groups, namely the food restricted one (n = 6) and the control group (n = 4). From days 15 to 21 the control group had unlimited access to food and the food restricted group was given half the amount of food that was on average consumed by the control group, based on measurements taken place the day before. On the 21st day of gestation, all rats delivered spontaneously and after birth all newborn pups of the food restricted group were weighed and matched as appropriately grown (non-FGR) or growth restricted (FGR) and brain tissues were immediately collected. A multiplex experiment was performed analyzing brain tissues from 4 FGR, 4 non-FGR, and 3 control male offspring. Differentially expressed proteins (DEPs) were subjected to bioinformatics analysis in order to identify over-represented processes.

Results

Proteomic analysis resulted in the profiling of 3,964 proteins. Gene ontology analysis of the common DEPs using DAVID (https://david.ncifcrf.gov/) showed significant enrichment for terms related to cellular morphology, learning, memory and positive regulation of NF-kappaB signaling. Ingenuity Pathway Analysis showed significant induction of inflammation in FGR pups, whereas significant induction of cell migration and cell spreading were observed in non-FGR pups.

Conclusion

This study demonstrated that in both FGR and non-FGR neonates, a range of adaptive neurodevelopmental processes takes place, which may result in altered cellular morphology, chronic stress, poor memory and learning outcomes. Furthermore, this study highlighted that not only FGR, but also appropriately grown pups, which have been exposed to prenatal food deprivation may be at increased risk for impaired cognitive and developmental outcomes.

Abl2:Cortactin Interactions Regulate Dendritic Spine Stability via Control of a Stable Filamentous Actin Pool

Dendritic spines act as the receptive contacts at most excitatory synapses. Spines are enriched in a network of actin filaments comprised of two kinetically distinct pools. The majority of spine actin is highly dynamic and regulates spine size, structural plasticity, and postsynaptic density organization. The remainder of the spine actin network is more stable, but the function of this minor actin population is not well understood, as tools to study it have not been available. Previous work has shown that disruption of the Abl2/Arg nonreceptor tyrosine kinase in mice compromises spine stability and size. Here, using cultured hippocampal neurons pooled from both sexes of mice, we provide evidence that binding to cortactin tethers Abl2 in spines, where Abl2 and cortactin maintain the small pool of stable actin required for dendritic spine stability. Using fluorescence recovery after photobleaching of GFP-actin, we find that disruption of Abl2:cortactin interactions eliminates stable actin filaments in dendritic spines, significantly reducing spine density. A subset of spines remaining after Abl2 depletion retain their stable actin pool and undergo activity-dependent spine enlargement, associated with increased cortactin and GluN2B levels. Finally, tonic increases in synaptic activity rescue spine loss following Abl2 depletion by promoting cortactin enrichment in vulnerable spines. Together, our findings strongly suggest that Abl2:cortactin interactions promote spine stability by maintaining pools of stable actin filaments in spines.SIGNIFICANCE STATEMENT Dendritic spines contain two kinetically distinct pools of actin. The more abundant, highly dynamic pool regulates spine shape, size, and plasticity. The function of the smaller, stable actin network is not well understood, as tools to study it have not been available. We demonstrate here that Abl2 and its substrate and interaction partner, cortactin, are essential to maintain the stable pool in spines. Depletion of the stable actin pool via disruption of Abl2 or cortactin, or interactions between the proteins, significantly reduces spine stability. We also provide evidence that tonic increases in synaptic activity promote spine stability via enrichment of cortactin in spines, suggesting that synaptic activity acts on the stable actin pool to stabilize dendritic spines.

The NSs Protein Encoded by the Virulent Strain of Rift Valley Fever Virus Targets the Expression of Abl2 and the Actin Cytoskeleton of the Host, Affecting Cell Mobility, Cell Shape, and Cell-Cell Adhesion

Rift Valley fever virus (RVFV) is a highly pathogenic zoonotic arbovirus endemic in many African countries and the Arabian Peninsula. Animal infections cause high rates of mortality and abortion among sheep, goats, and cattle. In humans, an estimated 1 to 2% of RVFV infections result in severe disease (encephalitis, hepatitis, or retinitis) with a high rate of lethality when associated with hemorrhagic fever. The RVFV NSs protein, which is the main virulence factor, counteracts the host innate antiviral response to favor viral replication and spread. However, the mechanisms underlying RVFV-induced cytopathic effects and the role of NSs in these alterations remain for the most part unknown. In this work, we have analyzed the effects of NSs expression on the actin cytoskeleton while conducting infections with the NSs-expressing virulent (ZH548) and attenuated (MP12) strains of RVFV and the non-NSs-expressing avirulent (ZH548ΔNSs) strain, as well as after the ectopic expression of NSs. In macrophages, fibroblasts, and hepatocytes, NSs expression prevented the upregulation of Abl2 (a major regulator of the actin cytoskeleton) expression otherwise induced by avirulent infections and identified here as part of the antiviral response. The presence of NSs was also linked to an increased mobility of ZH548-infected cells compared to ZH548ΔNSs-infected fibroblasts and to strong changes in cell morphology in nonmigrating hepatocytes, with reduction of lamellipodia, cell spreading, and dissolution of adherens junctions reminiscent of the ZH548-induced cytopathic effects observed in vivo Finally, we show evidence of the presence of NSs within long actin-rich structures associated with NSs dissemination from NSs-expressing toward non-NSs-expressing cells.IMPORTANCE Rift Valley fever virus (RVFV) is a dangerous human and animal pathogen that was ranked by the World Health Organization in 2018 as among the eight pathogens of most concern for being likely to cause wide epidemics in the near future and for which there are no, or insufficient, countermeasures. The focus of this work is to address the question of the mechanisms underlying RVFV-induced cytopathic effects that participate in RVFV pathogenicity. We demonstrate here that RVFV targets cell adhesion and the actin cytoskeleton at the transcriptional and cellular level, affecting cell mobility and inducing cell shape collapse, along with distortion of cell-cell adhesion. All these effects may participate in RVFV-induced pathogenicity, facilitate virulent RVFV dissemination, and thus constitute interesting potential targets for future development of antiviral therapeutic strategies that, in the case of RVFV, as with several other emerging arboviruses, are presently lacking.

Epidermal Growth Factor Receptor and Abl2 Kinase Regulate Distinct Steps of Human Papillomavirus 16 Endocytosis

Human papillomavirus 16 (HPV16), the leading cause of cervical cancer, exploits a novel endocytic pathway during host cell entry. This mechanism shares many requirements with macropinocytosis but differs in the mode of vesicle formation. Previous work indicated a role of the epidermal growth factor receptor (EGFR) in HPV16 endocytosis. However, the functional outcome of EGFR signaling and its downstream targets during HPV16 uptake are not well characterized. Here, we analyzed the functional importance of signal transduction via EGFR and its downstream effectors for endocytosis of HPV16. Our findings indicate two phases of EGFR signaling as follows: a-likely dispensable-transient activation with or shortly after cell binding and signaling required throughout the process of asynchronous internalization of HPV16. Interestingly, EGFR inhibition interfered with virus internalization and strongly reduced the number of endocytic pits, suggesting a role for EGFR signaling in the induction of HPV16 endocytosis. Moreover, we identified the Src-related kinase Abl2 as a novel regulator of virus uptake. Inhibition of Abl2 resulted in an accumulation of misshaped endocytic pits, indicating Abl2's importance for endocytic vesicle maturation. Since Abl2 rather than Src, a regulator of membrane ruffling during macropinocytosis, mediated downstream signaling of EGFR, we propose that the selective effector targeting downstream of EGFR determines whether HPV16 endocytosis or macropinocytosis is induced.IMPORTANCE Human papillomaviruses are small, nonenveloped DNA viruses that infect skin and mucosa. The so-called high-risk HPVs (e.g., HPV16, HPV18, HPV31) have transforming potential and are associated with various anogenital and oropharyngeal tumors. These viruses enter host cells by a novel endocytic pathway with unknown cellular function. To date, it is unclear how endocytic vesicle formation occurs mechanistically. Here, we addressed the role of epidermal growth factor receptor signaling, which has previously been implicated in HPV16 endocytosis and identified the kinase Abl2 as a novel regulator of virus uptake. Since other viruses, such as influenza A virus and lymphocytic choriomeningitis virus, possibly make use of related mechanisms, our findings shed light on fundamental strategies of virus entry and may in turn help to develop new host cell-targeted antiviral strategies.

Targeting ABL1 or ARG Tyrosine Kinases to Restrict HIV-1 Infection in Primary CD4+ T-Cells or in Humanized NSG Mice

BACKGROUND

Previous studies support dasatinib as a potent inhibitor of HIV-1 replication. However, a functional distinction between 2 kinase targets of the drug, ABL1 and ARG, has not been assessed.

SETTING

We used primary CD4 T-cells, CD8-depleted peripheral blood mononuclear cells (PBMCs) from a treatment naïve HIV-1 patient, and a humanized mouse model of HIV-1 infection. We assessed the roles of ABL1 and ARG during HIV-1 infection and use of dasatinib as a potential antiviral against HIV-1 in humanized mice.

METHODS

Primary CD4 T-cells were administered siRNA targeting ABL1 or ARG, then infected with HIV-1 containing luciferase reporter viruses. Quantitative polymerase chain reaction of viral integration of 4 HIV-1 strains was also assessed. CD8-depleted PBMCs were treated for 3 weeks with dasatinib. NSG mice were engrafted with CD34 pluripotent stem cells from human fetal cord blood, and infected with Ba-L virus after 19 weeks. Mice were treated daily with dasatinib starting 5 weeks after infection.

RESULTS

siRNA knockdown of ABL1 or ARG had no effect on viral reverse transcripts, but increased 2-LTR circles 2- to 4-fold and reduced viral integration 2- to 12-fold. siRNA knockdown of ARG increased SAMHD1 activation, whereas knockdown of either kinase reduced RNA polymerase II activation. Treating CD8-depleted PBMCs from a treatment-naïve patient with 50 nM of dasatinib for 3 weeks reduced p24 levels by 99.8%. Ba-L (R5)-infected mice injected daily with dasatinib showed a 95.1% reduction in plasma viral load after 2 weeks of treatment.

CONCLUSIONS

We demonstrate a novel nuclear role for ABL1 and ARG in ex vivo infection experiments, and proof-of-principle use of dasatinib in a humanized mouse model of HIV-1 infection.

Regulation of MT dynamics via direct binding of an Abl family kinase

Genetic studies revealed that Abl family kinases interact functionally with microtubules, but the mechanism by which Abl kinases regulate microtubules remains unclear. Hu et al. provide the first evidence that the Abl family kinase Abl2 directly binds microtubules to regulate microtubule dynamics.

Abl family kinases are essential regulators of cell shape and movement. Genetic studies revealed functional interactions between Abl kinases and microtubules (MTs), but the mechanism by which Abl family kinases regulate MTs remains unclear. Here, we report that Abl2 directly binds to MTs and regulates MT behaviors. Abl2 uses its C-terminal half to bind MTs, an interaction mediated in part through electrostatic binding to tubulin C-terminal tails. Using purified proteins, we found that Abl2 binds growing MTs and promotes MT polymerization and stability. In cells, knockout of Abl2 significantly impairs MT growth, and this defect can be rescued via reexpression of Abl2. Stable reexpression of an Abl2 fragment containing the MT-binding domain alone was sufficient to restore MT growth at the cell edge. These results show Abl2 uses its C-terminal half to bind MTs and directly regulate MT dynamics.

A Role for the Non-Receptor Tyrosine Kinase Abl2/Arg in Experimental Neuroinflammation

Multiple sclerosis is a neuroinflammatory degenerative disease, caused by activated immune cells infiltrating the CNS. The disease etiology involves both genetic and environmental factors. The mouse genetic locus, Eae27, linked to disease development in the experimental autoimmune encephalomyelitis (EAE) model for multiple sclerosis, was studied in order to identify contributing disease susceptibility factors and potential drug targets for multiple sclerosis. Studies of an Eae27 congenic mouse strain, revealed that genetic variation within Eae27 influences EAE development. The Abl2 gene, encoding the non-receptor tyrosine kinase Arg, is located in the 4,1 megabase pair long Eae27 region. The Arg protein plays an important role in cellular regulation and is, in addition, involved in signaling through the B- and T-cell receptors, important for the autoimmune response. The presence of a single nucleotide polymorphism causing an amino acid change in a near actin-interacting domain of Arg, in addition to altered lymphocyte activation in the congenic mice upon immunization with myelin antigen, makes Abl2/Arg a candidate gene for EAE. Here we demonstrate that the non-synonymous SNP does not change Arg's binding affinity for F-actin but suggest a role for Abl kinases in CNS inflammation pathogenesis by showing that pharmacological inhibition of Abl kinases ameliorates EAE, but not experimental arthritis.

Long disordered regions of the C-terminal domain of Abelson tyrosine kinase have specific and additive functions in regulation and axon localization

Abelson tyrosine kinase (Abl) is a key regulator of actin-related morphogenetic processes including axon guidance, where it functions downstream of several guidance receptors. While the long C-terminal domain (CTD) of Abl is required for function, its role is poorly understood. Here, a battery of mutants of Drosophila Abl was created that systematically deleted large segments of the CTD from Abl or added them back to the N-terminus alone. The functionality of these Abl transgenes was assessed through rescue of axon guidance defects and adult lethality in Abl loss-of-function, as well as through gain-of-function effects in sensitized slit or frazzled backgrounds that perturb midline guidance in the Drosophila embryonic nerve cord. Two regions of the CTD play important and distinct roles, but additive effects for other regions were also detected. The first quarter of the CTD, including a conserved PxxP motif and its surrounding sequence, regulates Abl function while the third quarter localizes Abl to axons. These regions feature long stretches of intrinsically disordered sequence typically found in hub proteins and are associated with diverse protein-protein interactions. Thus, the CTD of Abl appears to use these disordered regions to establish a variety of different signaling complexes required during formation of axon tracts.

Phosphoproteomics reveals network rewiring to a pro-adhesion state in annexin-1-deficient mammary epithelial cells

Background

Annexin-1 (ANXA1) plays pivotal roles in regulating various physiological processes including inflammation, proliferation and apoptosis, and deregulation of ANXA1 functions has been associated with tumorigenesis and metastasis events in several types of cancer. Though ANXA1 levels correlate with breast cancer disease status and outcome, its distinct functional involvement in breast cancer initiation and progression remains unclear. We hypothesized that ANXA1-responsive kinase signaling alteration and associated phosphorylation signaling underlie early events in breast cancer initiation events and hence profiled ANXA1-dependent phosphorylation changes in mammary gland epithelial cells.

Methods

Quantitative phosphoproteomics analysis of mammary gland epithelial cells derived from ANXA1-heterozygous and ANXA1-deficient mice was carried out using stable isotope labeling with amino acids in cell culture (SILAC)-based mass spectrometry. Kinase and signaling changes underlying ANXA1 perturbations were derived by upstream kinase prediction and integrated network analysis of altered proteins and phosphoproteins.

Results

We identified a total of 8110 unique phosphorylation sites, of which 582 phosphorylation sites on 372 proteins had ANXA1-responsive changes. A majority of these phosphorylation changes occurred on proteins associated with cytoskeletal reorganization spanning the focal adhesion, stress fibers, and also the microtubule network proposing new roles for ANXA1 in regulating microtubule dynamics. Comparative analysis of regulated global proteome and phosphoproteome highlighted key differences in translational and post-translational effects of ANXA1, and suggested closely coordinated rewiring of the cell adhesion network. Kinase prediction analysis suggested activity modulation of calmodulin-dependent protein kinase II (CAMK2), P21-activated kinase (PAK), extracellular signal-regulated kinase (ERK), and IκB kinase (IKK) upon loss of ANXA1. Integrative analysis revealed regulation of the WNT and Hippo signaling pathways in ANXA1-deficient mammary epithelial cells, wherein there is downregulation of transcriptional effects of TEA domain family (TEAD) suggestive of ANXA1-responsive transcriptional rewiring.

Conclusions

The phosphoproteome landscape uncovered several novel perspectives for ANXA1 in mammary gland biology and highlighted its involvement in key signaling pathways modulating cell adhesion and migration that could contribute to breast cancer initiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-017-0924-4) contains supplementary material, which is available to authorized users.

Constitutively active ABL family kinases, TEL/ABL and TEL/ARG, harbor distinct leukemogenic activities in vivo

ABL (ABL1) and ARG (ABL2) are highly homologous to each other in overall domain structure and amino-acid sequence, with the exception of their C termini. As with ABL, translocations that fuse ARG to ETV6/TEL have been identified in patients with leukemia. To assess the in vivo leukemogenic activity of constitutively active ABL and ARG, we generated a bone marrow (BM) transplantation model using the chimeric forms TEL/ABL and TEL/ARG, which have comparable kinase activities. TEL/ABL rapidly induced fatal myeloid leukemia in recipient mice, whereas recipients of TEL/ARG-transduced cells did not develop myeloid leukemia, instead, they succumbed to a long-latency infiltrative mastocytosis that could be adoptively transferred to secondary recipients. Swapping of the C termini of ABL and ARG altered disease latency and phenotypes. In a detailed in vitro study, TEL/ARG strongly promoted mast cell differentiation in response to stem cell factor or interleukin-3, whereas TEL/ABL preferentially induced myeloid differentiation of hematopoietic stem/progenitor cells. These results indicate that ABL and ARG kinase activate distinct differentiation pathways to induce specific diseases in vivo, that is, myeloid leukemia and mastocytosis, respectively. Further elucidation of the differences in their properties should provide important insight into the pathogenic mechanisms of oncogenes of the ABL kinase family.

Arg Deficiency Does not Influence the Course of Myelin Oligodendrocyte Glycoprotein (MOG35-55)-induced Experimental Autoimmune Encephalomyelitis

Background

Inhibition of Abl kinases has an ameliorating effect on the rodent model for multiple sclerosis, experimental autoimmune encephalomyelitis, and arrests lymphocyte activation. The family of Abl kinases consists of the Abl1/Abl and Abl2/Arg tyrosine kinases. While the Abl kinase has been extensively studied in immune activation, roles for Arg are incompletely characterized. To investigate the role for Arg in experimental autoimmune encephalomyelitis, we studied disease development in Arg^-/- mice.

Methods

Arg^-/- and Arg^+/+ mice were generated from breeding of Arg^+/- mice on the C57BL/6 background. Mice were immunized with the myelin oligodendrocyte glycoprotein (MOG)35-55 peptide and disease development recorded. Lymphocyte phenotypes of wild type Arg^+/+ and Arg^-/- mice were studied by in vitro stimulation assays and flow cytometry.

Results

The breeding of Arg^+/+ and Arg^-/- mice showed skewing in the frequency of born Arg^-/- mice. Loss of Arg function did not affect development of experimental autoimmune encephalomyelitis, but reduced the number of splenic B-cells in Arg^-/- mice following immunization with MOG peptide.

Conclusions

Development of MOG-induced experimental autoimmune encephalomyelitis is not dependent on Arg, but Arg plays a role for the number of B cells in immunized mice. This might suggest a novel role for the Arg kinase in B-cell trafficking or regulation. Furthermore, the results suggest that Arg is important for normal embryonic development.

Misfolding, Aggregation, and Disordered Segments in c-Abl and p53 in Human Cancer

The current understanding of the molecular mechanisms that lead to cancer is not sufficient to explain the loss or gain of function in proteins related to tumorigenic processes. Among them, more than 100 oncogenes, 20-30 tumor-suppressor genes, and hundreds of genes participating in DNA repair and replication have been found to play a role in the origins of cancer over the last 25 years. The phosphorylation of serine, threonine, or tyrosine residues is a critical step in cellular growth and development and is achieved through the tight regulation of protein kinases. Phosphorylation plays a major role in eukaryotic signaling as kinase domains are found in 2% of our genes. The deregulation of kinase control mechanisms has disastrous consequences, often leading to gains of function, cell transformation, and cancer. The c-Abl kinase protein is one of the most studied targets in the fight against cancer and is a hotspot for drug development because it participates in several solid tumors and is the hallmark of chronic myelogenous leukemia. Tumor suppressors have the opposite effects. Their fundamental role in the maintenance of genomic integrity has awarded them a role as the guardians of DNA. Among the tumor suppressors, p53 is the most studied. The p53 protein has been shown to be a transcription factor that recognizes and binds to specific DNA response elements and activates gene transcription. Stress triggered by ionizing radiation or other mutagenic events leads to p53 phosphorylation and cell-cycle arrest, senescence, or programed cell death. The p53 gene is the most frequently mutated gene in cancer. Mutations in the DNA-binding domain are classified as class I or class II depending on whether substitutions occur in the DNA contact sites or in the protein core, respectively. Tumor-associated p53 mutations often lead to the loss of protein function, but recent investigations have also indicated gain-of-function mutations. The prion-like aggregation of mutant p53 is associated with loss-of-function, dominant-negative, and gain-of-function effects. In the current review, we focused on the most recent insights into the protein structure and function of the c-Abl and p53 proteins that will provide us guidance to understand the loss and gain of function of these misfolded tumor-associated proteins.

Abl2/Abl-related gene stabilizes actin filaments, stimulates actin branching by actin-related protein 2/3 complex, and promotes actin filament severing by cofilin

Both Arp2/3 complex and the Abl2/Arg nonreceptor tyrosine kinase are essential to form and maintain diverse actin-based structures in cells, including cell edge protrusions in fibroblasts and cancer cells and dendritic spines in neurons. The ability of Arg to promote cell edge protrusions in fibroblasts does not absolutely require kinase activity, raising the question of how Arg might modulate actin assembly and turnover in the absence of kinase function. Arg has two distinct actin-binding domains and interacts physically and functionally with cortactin, an activator of the Arp2/3 complex. However, it was not known whether and how Arg influences actin filament stability, actin branch formation, or cofilin-mediated actin severing or how cortactin influences these reactions of Arg with actin. Arg or cortactin bound to actin filaments stabilizes them from depolymerization. Low concentrations of Arg and cortactin cooperate to stabilize filaments by slowing depolymerization. Arg stimulates formation of actin filament branches by Arp2/3 complex and cortactin. An Arg mutant lacking the C-terminal calponin homology actin-binding domain stimulates actin branch formation by the Arp2/3 complex, indicative of autoinhibition. ArgΔCH can stimulate the Arp2/3 complex even in the absence of cortactin. Arg greatly potentiates cofilin severing of actin filaments, and cortactin attenuates this enhanced severing. The ability of Arg to stabilize filaments, promote branching, and increase severing requires the internal (I/L)WEQ actin-binding domain. These activities likely underlie important roles that Arg plays in the formation, dynamics, and stability of actin-based cellular structures.

Two amino acid residues confer different binding affinities of Abelson family kinase SRC homology 2 domains for phosphorylated cortactin

The closely related Abl family kinases, Arg and Abl, play important non-redundant roles in the regulation of cell morphogenesis and motility. Despite similar N-terminal sequences, Arg and Abl interact with different substrates and binding partners with varying affinities. This selectivity may be due to slight differences in amino acid sequence leading to differential interactions with target proteins. We report that the Arg Src homology (SH) 2 domain binds two specific phosphotyrosines on cortactin, a known Abl/Arg substrate, with over 10-fold higher affinity than the Abl SH2 domain. We show that this significant affinity difference is due to the substitution of arginine 161 and serine 187 in Abl to leucine 207 and threonine 233 in Arg, respectively. We constructed Abl SH2 domains with R161L and S187T mutations alone and in combination and find that these substitutions are sufficient to convert the low affinity Abl SH2 domain to a higher affinity "Arg-like" SH2 domain in binding to a phospho-cortactin peptide. We crystallized the Arg SH2 domain for structural comparison to existing crystal structures of the Abl SH2 domain. We show that these two residues are important determinants of Arg and Abl SH2 domain binding specificity. Finally, we expressed Arg containing an "Abl-like" low affinity mutant Arg SH2 domain (L207R/T233S) and find that this mutant, although properly localized to the cell periphery, does not support wild type levels of cell edge protrusion. Together, these observations indicate that these two amino acid positions confer different binding affinities and cellular functions on the distinct Abl family kinases.

There is more than one way to model an elephant. Experiment-driven modeling of the actin cytoskeleton

Mathematical modeling has established its value for investigating the interplay of biochemical and mechanical mechanisms underlying actin-based motility. Because of the complex nature of actin dynamics and its regulation, many of these models are phenomenological or conceptual, providing a general understanding of the physics at play. But the wealth of carefully measured kinetic data on the interactions of many of the players in actin biochemistry cries out for the creation of more detailed and accurate models that could permit investigators to dissect interdependent roles of individual molecular components. Moreover, no human mind can assimilate all of the mechanisms underlying complex protein networks; so an additional benefit of a detailed kinetic model is that the numerous binding proteins, signaling mechanisms, and biochemical reactions can be computationally organized in a fully explicit, accessible, visualizable, and reusable structure. In this review, we will focus on how comprehensive and adaptable modeling allows investigators to explain experimental observations and develop testable hypotheses on the intracellular dynamics of the actin cytoskeleton.

Arg kinase signaling in dendrite and synapse stabilization pathways: memory, cocaine sensitivity, and stress

The Abl2/Arg nonreceptor tyrosine kinase is enriched in dendritic spines where it is essential for maintaining dendrite and synapse stability in the postnatal mouse brain. Arg is activated downstream of integrin α3β1 receptors and it regulates the neuronal actin cytoskeleton by directly binding F-actin and via phosphorylation of substrates including p190RhoGAP and cortactin. Neurons in mice lacking Arg or integrin α3β1 develop normally through postnatal day 21 (P21), however by P42 mice exhibit major reductions in dendrite arbor size and complexity, and lose dendritic spines and synapses. As a result, mice with loss of Arg and Arg-dependent signaling pathways have impairments in memory tasks, heightened sensitivity to cocaine, and vulnerability to corticosteroid-induced neuronal remodeling. Therefore, understanding the molecular mechanisms of Arg regulation may lead to therapeutic approaches to treat human psychiatric and neurodegenerative diseases in which neuronal structure is destabilized.

Role of ABL family kinases in cancer: from leukaemia to solid tumours

The Abelson (ABL) family of nonreceptor tyrosine kinases, ABL1 and ABL2, transduces diverse extracellular signals to protein networks that control proliferation, survival, migration and invasion. ABL1 was first identified as an oncogene required for the development of leukaemias initiated by retroviruses or chromosome translocations. The demonstration that small-molecule ABL kinase inhibitors could effectively treat chronic myeloid leukaemia opened the door to the era of targeted cancer therapies. Recent reports have uncovered roles for ABL kinases in solid tumours. Enhanced ABL expression and activation in some solid tumours, together with altered cell polarity, invasion or growth induced by activated ABL kinases, suggest that drugs targeting these kinases may be useful for treating selected solid tumours.

Molecular mechanisms of dendrite stability

In the developing brain, dendrite branches and dendritic spines form and turn over dynamically. By contrast, most dendrite arbors and dendritic spines in the adult brain are stable for months, years and possibly even decades. Emerging evidence reveals that dendritic spine and dendrite arbor stability have crucial roles in the correct functioning of the adult brain and that loss of stability is associated with psychiatric disorders and neurodegenerative diseases. Recent findings have provided insights into the molecular mechanisms that underlie long-term dendrite stabilization, how these mechanisms differ from those used to mediate structural plasticity and how they are disrupted in disease.

Distinct functional domains of the Abelson tyrosine kinase control axon guidance responses to Netrin and Slit to regulate the assembly of neural circuits

To develop a functional nervous system, axons must initially navigate through a complex environment, directed by guidance ligands and receptors. These receptors must link to intracellular signaling cascades to direct axon pathfinding decisions. The Abelson tyrosine kinase (Abl) plays a crucial role in multiple Drosophila axon guidance pathways during development, though the mechanism by which Abl elicits a diverse set of guidance outputs is currently unknown. We identified Abl in a genetic screen for genes that contribute to Netrin-dependent axon guidance in midline-crossing (commissural) neurons. We find that Abl interacts both physically and genetically with the Netrin receptor Frazzled, and that disrupting this interaction prevents Abl from promoting midline axon crossing. Moreover, we find that Abl exerts its diverse activities through at least two different mechanisms: (1) a partly kinase-independent, structural function in midline attraction through its C-terminal F-actin binding domain (FABD) and (2) a kinase-dependent inhibition of repulsive guidance pathways that does not require the Abl C terminus. Abl also regulates motor axon pathfinding through a non-overlapping set of functional domains. These results highlight how a multifunctional kinase can trigger diverse axon guidance outcomes through the use of distinct structural motifs.

β1 integrin regulates Arg to promote invadopodial maturation and matrix degradation

β1 integrin is a major regulator of invadopodium maturation. Studies reveal that β1 integrin–mediated adhesion is a key upstream switch that induces Arg-dependent cortactin phosphorylation, actin polymerization, and MMP recruitment to invadopodia for extracellular matrix degradation.

β1 integrin has been shown to promote metastasis in a number of tumor models, including breast, ovarian, pancreatic, and skin cancer; however, the mechanism by which it does so is poorly understood. Invasive membrane protrusions called invadopodia are believed to facilitate extracellular matrix degradation and intravasation during metastasis. Previous work showed that β1 integrin localizes to invadopodia, but its role in regulating invadopodial function has not been well characterized. We find that β1 integrin is required for the formation of mature, degradation-competent invadopodia in both two- and three-dimensional matrices but is dispensable for invadopodium precursor formation in metastatic human breast cancer cells. β1 integrin is activated during invadopodium precursor maturation, and forced β1 integrin activation enhances the rate of invadopodial matrix proteolysis. Furthermore, β1 integrin interacts with the tyrosine kinase Arg and stimulates Arg-dependent phosphorylation of cortactin on tyrosine 421. Silencing β1 integrin with small interfering RNA completely abrogates Arg-dependent cortactin phosphorylation and cofilin-dependent barbed-end formation at invadopodia, leading to a significant decrease in the number and stability of mature invadopodia. These results describe a fundamental role for β1 integrin in controlling actin polymerization–dependent invadopodial maturation and matrix degradation in metastatic tumor cells.

Abl family kinases regulate FcγR-mediated phagocytosis in murine macrophages

Phagocytosis of Ab-coated pathogens is mediated through FcγRs, which activate intracellular signaling pathways to drive actin cytoskeletal rearrangements. Abl and Arg define a family of nonreceptor tyrosine kinases that regulate actin-dependent processes in a variety of cell types, including those important in the adaptive immune response. Using pharmacological inhibition as well as dominant negative and knockout approaches, we demonstrate a role for the Abl family kinases in phagocytosis by macrophages and define a mechanism whereby Abl kinases regulate this process. Bone marrow-derived macrophages from mice lacking Abl and Arg kinases exhibit inefficient phagocytosis of sheep erythrocytes and zymosan particles. Treatment with the Abl kinase inhibitors imatinib and GNF-2 or overexpression of kinase-inactive forms of the Abl family kinases also impairs particle internalization in murine macrophages, indicating Abl kinase activity is required for efficient phagocytosis. Further, Arg kinase is present at the phagocytic cup, and Abl family kinases are activated by FcγR engagement. The regulation of phagocytosis by Abl family kinases is mediated in part by the spleen tyrosine kinase (Syk). Loss of Abl and Arg expression or treatment with Abl inhibitors reduced Syk phosphorylation in response to FcγR ligation. The link between Abl family kinases and Syk may be direct, as purified Arg kinase phosphorylates Syk in vitro. Further, overexpression of membrane-targeted Syk in cells treated with Abl kinase inhibitors partially rescues the impairment in phagocytosis. Together, these findings reveal that Abl family kinases control the efficiency of phagocytosis in part through the regulation of Syk function.

Arg/Abl2 modulates the affinity and stoichiometry of binding of cortactin to F-actin

The Abl family nonreceptor tyrosine kinase Arg/Abl2 interacts with cortactin, an Arp2/3 complex activator, to promote actin-driven cell edge protrusion. Both Arg and cortactin bind directly to filamentous actin (F-actin). While protein-protein interactions between Arg and cortactin have well-characterized downstream effects on the actin cytoskeleton, it is unclear whether and how Arg and cortactin affect each other's actin binding properties. We employ actin cosedimentation assays to show that Arg increases the stoichiometry of binding of cortactin to F-actin at saturation. Using a series of Arg deletion mutants and fragments, we demonstrate that the Arg C-terminal calponin homology domain is necessary and sufficient to increase the stoichiometry of binding of cortactin to F-actin. We also show that interactions between Arg and cortactin are required for optimal affinity between cortactin and the actin filament. Our data suggest a mechanism for Arg-dependent stimulation of binding of cortactin to F-actin, which may facilitate the recruitment of cortactin to sites of local actin network assembly.

The Abl and Arg kinases mediate distinct modes of phagocytosis and are required for maximal Leishmania infection

Leishmania, an obligate intracellular parasite, binds several receptors to trigger engulfment by phagocytes, leading to cutaneous or visceral disease. These receptors include complement receptor 3 (CR3), used by promastigotes, and the Fc receptor (FcR), used by amastigotes. The mechanisms mediating uptake are not well understood. Here we show that Abl family kinases mediate both phagocytosis and the uptake of Leishmania amazonensis by macrophages (Ms). Imatinib, an Abl/Arg kinase inhibitor, decreases opsonized polystyrene bead phagocytosis and Leishmania uptake. Interestingly, phagocytosis of IgG-coated beads is decreased in Arg-deficient Ms, while that of C3bi-coated beads is unaffected. Conversely, uptake of C3bi-coated beads is decreased in Abl-deficient Ms, but that of IgG-coated beads is unaffected. Consistent with these results, Abl-deficient Ms are inefficient at C3bi-opsonized promastigote uptake, and Arg-deficient Ms are defective in IgG1-opsonized amastigote uptake. Finally, genetic loss of Abl or Arg reduces infection severity in murine cutaneous leishmaniasis, and imatinib treatment results in smaller lesions with fewer parasites than in controls. Our studies are the first to demonstrate that efficient phagocytosis and maximal Leishmania infection require Abl family kinases. These results highlight Abl family kinase-mediated signaling pathways as potential therapeutic targets for leishmaniasis.

Functional mechanisms and roles of adaptor proteins in abl-regulated cytoskeletal actin dynamics

Abl is a nonreceptor tyrosine kinase and plays an essential role in the modeling and remodeling of F-actin by transducing extracellular signals. Abl and its paralog, Arg, are unique among the tyrosine kinase family in that they contain an unusual extended C-terminal half consisting of multiple functional domains. This structural characteristic may underlie the role of Abl as a mediator of upstream signals to downstream signaling machineries involved in actin dynamics. Indeed, a group of SH3-containing accessory proteins, or adaptor proteins, have been identified that bind to a proline-rich domain of the C-terminal portion of Abl and modulate its kinase activity, substrate recognition, and intracellular localization. Moreover, the existence of signaling cascade and biological outcomes unique to each adaptor protein has been demonstrated. In this paper, we summarize functional roles and mechanisms of adaptor proteins in Abl-regulated actin dynamics, mainly focusing on a family of adaptor proteins, Abi. The mechanism of Abl's activation and downstream signaling mediated by Abi is described in comparison with those by another adaptor protein, Crk.

The cytoplasmic tyrosine kinase Arg regulates gastrulation via control of actin organization

Coordinated cell movements are crucial for vertebrate gastrulation and are controlled by multiple signals. Although many factors are shown to mediate non-canonical Wnt pathways to regulate cell polarity and intercalation during gastrulation, signaling molecules acting in other pathways are less investigated and the connections between various signals and cytoskeleton are not well understood. In this study, we show that the cytoplasmic tyrosine kinase Arg modulates gastrulation movements through control of actin remodeling. Arg is expressed in the dorsal mesoderm at the onset of gastrulation, and both gain- and loss-of-function of Arg disrupted axial development in Xenopus embryos. Arg controlled migration of anterior mesendoderm, influenced cell decision on individual versus collective migration, and modulated spreading and protrusive activities of anterior mesendodermal cells. Arg also regulated convergent extension of the trunk mesoderm by influencing cell intercalation behaviors. Arg modulated actin organization to control dynamic F-actin distribution at the cell-cell contact or in membrane protrusions. The functions of Arg required an intact tyrosine kinase domain but not the actin-binding motifs in its carboxyl terminus. Arg acted downstream of receptor tyrosine kinases to regulate phosphorylation of endogenous CrkII and paxillin, adaptor proteins involved in activation of Rho family GTPases and actin reorganization. Our data demonstrate that Arg is a crucial cytoplasmic signaling molecule that controls dynamic actin remodeling and mesodermal cell behaviors during Xenopus gastrulation.

Short-term weightlessness produced by parabolic flight maneuvers altered gene expression patterns in human endothelial cells

This study focused on the effects of short-term microgravity (22 s) on the gene expression and morphology of endothelial cells (ECs) and evaluated gravisensitive signaling elements. ECs were investigated during four German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt) parabolic flight campaigns. Hoechst 33342 and acridine orange/ethidium bromide staining showed no signs of cell death in ECs after 31 parabolas (P31). Gene array analysis revealed 320 significantly regulated genes after the first parabola (P1) and P31. COL4A5, COL8A1, ITGA6, ITGA10, and ITGB3 mRNAs were down-regulated after P1. EDN1 and TNFRSF12A mRNAs were up-regulated. ADAM19, CARD8, CD40, GSN, PRKCA (all down-regulated after P1), and PRKAA1 (AMPKα1) mRNAs (up-regulated) provide a very early protective mechanism of cell survival induced by 22 s microgravity. The ABL2 gene was significantly up-regulated after P1 and P31, TUBB was slightly induced, but ACTA2 and VIM mRNAs were not changed. β-Tubulin immunofluorescence revealed a cytoplasmic rearrangement. Vibration had no effect. Hypergravity reduced CARD8, NOS3, VASH1, SERPINH1 (all P1), CAV2, ADAM19, TNFRSF12A, CD40, and ITGA6 (P31) mRNAs. These data suggest that microgravity alters the gene expression patterns and the cytoskeleton of ECs very early. Several gravisensitive signaling elements, such as AMPKα1 and integrins, are involved in the reaction of ECs to altered gravity.

Abelson family tyrosine kinases regulate the function of nicotinic acetylcholine receptors and nicotinic synapses on autonomic neurons

Abelson family kinases (AFKs; Abl1, Abl2) are non-receptor tyrosine kinases (NRTKs) implicated in cancer, but they also have important physiological roles that include regulating synaptic structure and function. Recent studies using Abl-deficient mice and the antileukemia drug STI571 [imatinib mesylate (Gleevec); Novartis], which potently and selectively blocks Abl kinase activity, implicate AFKs in regulating presynaptic neurotransmitter release in hippocampus and postsynaptic clustering of nicotinic acetylcholine receptors (nAChRs) in muscle. Here, we tested whether AFKs are relevant for regulating nAChRs and nAChR-mediated synapses on autonomic neurons. AFK immunoreactivity was detected in ciliary ganglion (CG) lysates and neurons, and STI571 application blocked endogenous Abl tyrosine kinase activity. With similar potency, STI571 specifically reduced whole-cell current responses generated by both nicotinic receptor subtypes present on CG neurons (α3*- and α7-nAChRs) and lowered the frequency and amplitude of α3*-nAChR-mediated excitatory postsynaptic currents. Quantal analysis indicated that the synaptic perturbations were postsynaptic in origin, and confocal imaging experiments revealed they were unaccompanied by changes in nAChR clustering or alignment with presynaptic terminals. The results indicate that in autonomic neurons, Abl kinase activity normally supports postsynaptic nAChR function to sustain nAChR-mediated neurotransmission. Such consequences contrast with the influence of Abl kinase activity on presynaptic function and synaptic structure in hippocampus and muscle, respectively, demonstrating a cell-specific mechanism of action. Finally, because STI571 potently inhibits Abl kinase activity, the autonomic dysfunction side effects associated with its use as a chemotherapeutic agent may result from perturbed α3*- and/or α7-nAChR function.

ABL tyrosine kinases: evolution of function, regulation, and specificity

ABL-family proteins comprise one of the best conserved branches of the tyrosine kinases. Each ABL protein contains an SH3-SH2-TK (Src homology 3-Src homology 2-tyrosine kinase) domain cassette, which confers autoregulated kinase activity and is common among nonreceptor tyrosine kinases. This cassette is coupled to an actin-binding and -bundling domain, which makes ABL proteins capable of connecting phosphoregulation with actin-filament reorganization. Two vertebrate paralogs, ABL1 and ABL2, have evolved to perform specialized functions. ABL1 includes nuclear localization signals and a DNA binding domain through which it mediates DNA damage-repair functions, whereas ABL2 has additional binding capacity for actin and for microtubules to enhance its cytoskeletal remodeling functions. Several types of posttranslational modifications control ABL catalytic activity, subcellular localization, and stability, with consequences for both cytoplasmic and nuclear ABL functions. Binding partners provide additional regulation of ABL catalytic activity, substrate specificity, and downstream signaling. Information on ABL regulatory mechanisms is being mined to provide new therapeutic strategies against hematopoietic malignancies caused by BCR-ABL1 and related leukemogenic proteins.

Abi1/Hssh3bp1 pY213 links Abl kinase signaling to p85 regulatory subunit of PI-3 kinase in regulation of macropinocytosis in LNCaP cells

Macropinocytosis is regulated by Abl kinase via an unknown mechanism. We previously demonstrated that Abl kinase activity is, itself, regulated by Abi1 subsequent to Abl kinase phosphorylation of Abi1 tyrosine 213 (pY213) [1]. Here we show that blocking phosphorylation of Y213 abrogated the ability of Abl to regulate macropinocytosis, implicating Abi1 pY213 as a key regulator of macropinocytosis. Results from screening the human SH2 domain library and mapping the interaction site between Abi1 and the p85 regulatory domain of PI-3 kinase, coupled with data from cells transfected with loss-of-function p85 mutants, support the hypothesis that macropinocytosis is regulated by interactions between Abi1 pY213 and the C-terminal SH2 domain of p85-thereby linking Abl kinase signaling to p85-dependent regulation of macropinocytosis.

Regulation of actin polymerization and adhesion-dependent cell edge protrusion by the Abl-related gene (Arg) tyrosine kinase and N-WASp

Extracellular cues stimulate the Abl family nonreceptor tyrosine kinase Arg to promote actin-based cell edge protrusions. Several Arg-interacting proteins are potential links to the actin cytoskeleton, but exactly how Arg stimulates actin polymerization and cellular protrusion has not yet been fully elucidated. We used affinity purification to identify N-WASp as a novel binding partner of Arg. N-WASp activates the Arp2/3 complex and is an effector of Abl. We find that the Arg SH3 domain binds directly to N-WASp. Arg phosphorylates N-WASp on Y256, modestly increasing the affinity of Arg for N-WASp, an interaction that does not require the Arg SH2 domain. The Arg SH3 domain stimulates N-WASp-dependent actin polymerization in vitro, and Arg phosphorylation of N-WASp weakly stimulates this effect. Arg and N-WASp colocalize to adhesion-dependent cell edge protrusions in vivo. The cell edge protrusion defects of arg-/- fibroblasts can be complemented by re-expression of an Arg-yellow fluorescent protein (YFP) fusion, but not by an N-WASp binding-deficient Arg SH3 domain point mutant. These results suggest that Arg promotes actin-based protrusions in response to extracellular stimuli through phosphorylation of and physical interactions with N-WASp.

Regulation of cell migration and morphogenesis by Abl-family kinases: emerging mechanisms and physiological contexts

The Abl-family non-receptor tyrosine kinases are essential regulators of the cytoskeleton. They transduce diverse extracellular cues into cytoskeletal rearrangements that have dramatic effects on cell motility and morphogenesis. Recent biochemical and genetic studies have revealed several mechanisms that Abl-family kinases use to mediate these effects. Abl-family kinases stimulate actin polymerization through the activation of cortactin, hematopoietic lineage cell-specific protein (HS1), WASp- and WAVE-family proteins, and Rac1. They also attenuate cell contractility by inhibiting RhoA and altering adhesion dynamics. These pathways impinge on several physiological processes, including development and maintenance of the nervous and immune systems, and epithelial morphogenesis. Elucidating how Abl-family kinases are regulated, and where and when they coordinate cytoskeletal changes, is essential for garnering a better understanding of these complex processes.

Arg interacts with cortactin to promote adhesion-dependent cell edge protrusion

The molecular mechanisms by which the Abelson (Abl) or Abl-related gene (Arg) kinases interface with the actin polymerization machinery to promote cell edge protrusions during cell–matrix adhesion are unclear. In this study, we show that interactions between Arg and the Arp2/3 complex regulator cortactin are essential to mediate actin-based cell edge protrusion during fibroblast adhesion to fibronectin. Arg-deficient and cortactin knockdown fibroblasts exhibit similar defects in adhesion-dependent cell edge protrusion, which can be restored via reexpression of Arg and cortactin. Arg interacts with cortactin via both binding and catalytic events. The cortactin Src homology (SH) 3 domain binds to a Pro-rich motif in the Arg C terminus. Arg mediates adhesion-dependent phosphorylation of cortactin, creating an additional binding site for the Arg SH2 domain. Mutation of residues that mediate Arg–cortactin interactions abrogate the abilities of both proteins to support protrusions, and the Nck adapter, which binds phosphocortactin, is also required. These results demonstrate that interactions between Arg, cortactin, and Nck1 are critical to promote adhesion-dependent cell edge protrusions.

Unc119 protects from Shigella infection by inhibiting the Abl family kinases

Background

Bacteria engage cell surface receptors and intracellular signaling molecules to enter the cell. Unc119 is an adaptor protein, which interacts with receptors and tyrosine kinases. Its role in bacterial invasion of cells is unknown.

Methodology/Principal Findings

We used biochemical, molecular and cell biology approaches to identify the binding partners of Unc119, and to study the effect of Unc119 on Abl family kinases and Shigella infection. We employed loss-of-function and gain-in-function approaches to study the effect of Unc119 in a mouse model of pulmonary shigellosis. Unc119 interacts with Abl family kinases and inhibits their kinase activity. As a consequence, it inhibits Crk phosphorylation, which is essential for Shigella infection. Unc119 co-localizes with Crk and Shigella in infected cells. Shigella infectivity increases in Unc119-deficient epithelial and macrophage cells. In a mouse model of shigellosis cell-permeable TAT-Unc119 inhibits Shigella infection. Conversely, Unc119 knockdown in vivo results in enhanced bacterial invasion and increased lethality. Unc119 is an inducible protein. Its expression is upregulated by probacteria and bacterial products such as lipopolysacharide and sodium butyrate. The latter inhibits Shigella infection in mouse lungs but is ineffective in Unc119 deficiency.

Conclusions

Unc119 inhibits signaling pathways that are used by Shigella to enter the cell. As a consequence it provides partial but significant protection from Shigella infections. Unc119 induction in vivo boosts host defense against infections.

Abl tyrosine kinases in T-cell signaling

Stimulation of the T-cell antigen receptor (TCR) leads to the activation of signaling pathways that are essential for T-cell development and the response of mature T cells to antigens. The TCR has no intrinsic catalytic activity, but TCR engagement results in tyrosine phosphorylation of downstream targets by non-receptor tyrosine kinases. Three families of tyrosine kinases have long been recognized to play critical roles in TCR-dependent signaling. They are the Src, zeta-associated protein of 70 kDa, and Tec families of kinases. More recently, the Abelson (Abl) tyrosine kinases have been shown to be activated by TCR engagement and to be required for maximal TCR signaling. Using T-cell conditional knockout mice deficient for Abl family kinases, Abl (Abl1) and Abl-related gene (Arg) (Abl2), it was recently shown that loss of Abl kinases results in defective T-cell development and a partial block in the transition to the CD4(+)CD8(+) stage. Abl/Arg double null T cells exhibit impaired TCR-induced signaling, proliferation, and cytokine production. Moreover, conditional knockout mice lacking Abl and Arg in T cells exhibit impaired CD8(+) T-cell expansion in vivo upon Listeria monocytogenes infection. Thus, Abl kinase signaling is required for both T-cell development and mature T-cell function.

Reciprocal regulation of Abl and receptor tyrosine kinases

Previously, we showed that Abl kinases (c-Abl, Arg) are activated downstream of PDGF in a manner dependent on Src kinases and PLC-gamma1, and promote PDGF-mediated proliferation and migration of fibroblasts. We additionally demonstrated that Abl kinases bind directly to PDGFR-beta via their SH2 domains.In this study, we extend these findings by demonstrating that Abl kinases also are activated downstream of aPDGF autocrine growth loop in glioblastoma cells, indicating that the PDGFR-Abl signaling pathway also is likely to be important in glioblastoma development and/or progression.We recently showed that Abl kinases are highly active in many breast cancer cell lines, and the Her-2 receptor tyrosine kinase contributes to c-Abl and Arg kinase activation. In this study, we show that Abl kinase SH2 domains bind directly to Her-2, and like PDGFR-beta , Her-2 directly phosphorylates c-Abl. Previously, we demonstrated that PDGFR-beta directly phosphorylates Abl kinases in vitro, and Abl kinases reciprocally phosphorylate PDGFR-beta . Here, we show that PDGFR-beta-phosphorylation of Abl kinases has functional consequences as PDGFR-beta phosphorylates Abl kinases on Y245 and Y412, sites known to be required for activation of Abl kinases. Moreover, PDGFR-beta phosphorylates Arg on two additional unique sites whose function is unknown. Importantly, we also show that Abl-dependent phosphorylation of PDGFR-beta has functional and biological significances. c-Abl phosphorylates three tyrosine residues on PDGFR-beta (Y686, Y934, Y970), while Arg only phosphorylatesY686. Y686 and Y934 reside in PDGFR-beta catalytic domains, while Y970 is in the C-terminal tail. Using site-directed mutagenesis, we show that Abl-dependent phosphorylation of PDGFR-beta activates PDGFR-beta activity, in vitro, but serves to downregulate PDGFR-mediated chemotaxis. These data are exciting as they indicate that Abl kinases not only are activated by PDGFR and promote PDGFR-mediated proliferation and migration,but also act in an intricate negative feedback loop to turn-off PDGFR-mediated chemotaxis.

Eight full-length abelson related gene (Arg) isoforms are constitutively expressed in caki-1 cell line and cell distribution of two isoforms has been analyzed after transfection

The human Arg (Abl2) nonreceptor tyrosine kinase has a role in cytoskeletal rearrangements by its C-terminal F-actin- and microtubule-binding sequences. We have previously identified Arg transcripts with different 5'- and 3'-ends, named respectively long and short 1A and 1B (1AL, 1AS, 1BL, 1BS) and long and short C-termini (CTL and CTS), that have different expression patterns in various cell types. The combination of the different ends permits to predict eight putative full-length Arg transcripts and corresponding proteins. By Reverse Transcription-Long PCR we show here that all eight full-length transcripts are endogenously expressed in Caki-1 cells and the two bands, approximately 10 kDa different, shown by 1-D Western blots of Hek293T and Caki-1 lysates correspond to the full-length Arg protein isoforms with different C-termini. 2-D Western blot analysis evidenced different high molecular weight and slight acidic specific spots in Hek293T and Caki-1 lysates. The cellular localization of two Arg isoforms (1BLCTL and 1BLCTS) transfected in Caki-1 and Hek293T cells was cytoplasmic, and some differences in cytoskeleton interactions have been evidenced. Moreover, in Hek293T cells only the transfected 1BLCTS isoform gives rise to a large intracytoplasmic cylindrical structure containing phalloidin-positive amorphous actin aggregates. The presence of eight full-length Arg isoforms with different cellular expression may imply a diverse functional role in normal and neoplastic cells.

Abl kinases regulate autophagy by promoting the trafficking and function of lysosomal components

Autophagy is a lysosome-dependent degradative pathway that regulates the turnover of intracellular organelles, parasites, and long-lived proteins. Deregulation of autophagy results in a variety of pathological conditions, but little is known regarding the mechanisms that link normal cellular and pathological signals to the regulation of distinct stages in the autophagy pathway. Here we uncover a novel role for the Abl family kinases in the regulation of the late stages of autophagy. Inhibition, depletion, or knockout of the Abl family kinases, Abl and Arg, resulted in a dramatic reduction in the intracellular activities of the lysosomal glycosidases alpha-galactosidase, alpha-mannosidase and neuraminidase. Inhibition of Abl kinases also reduced the processing of the precursor forms of cathepsin D and cathepsin L to their mature, lysosomal forms, which coincided with the impaired turnover of long-lived cytosolic proteins and accumulation of autophagosomes. Furthermore, defective lysosomal degradation of long-lived proteins in the absence of Abl kinase signaling was accompanied by a perinuclear redistribution of lysosomes and increased glycosylation and stability of lysosome-associated membrane proteins, which are known to be substrates for lysosomal enzymes and play a role in regulating lysosome mobility. Our findings reveal a role for Abl kinases in the regulation of late-stage autophagy and have important implications for therapies that employ pharmacological inhibitors of the Abl kinases.

Abelson tyrosine kinase facilitates Salmonella enterica serovar Typhimurium entry into epithelial cells

The intracellular gram-negative bacterial pathogen Salmonella enterica serovar Typhimurium gains entry into nonphagocytic cells by manipulating the assembly of the host actin cytoskeleton. S. enterica serovar Typhimurium entry requires a functional type III secretion system, a conduit through which bacterial effector proteins are directly translocated into the host cytosol. We and others have previously reported the enhancement of tyrosine kinase activities during Salmonella serovar Typhimurium infection; however, neither specific kinases nor their targets have been well characterized. In this study, we investigated the roles of the cellular Abelson tyrosine kinase (c-Abl) and the related protein Arg in the context of serovar Typhimurium infection. We found that bacterial internalization was inhibited by more than 70% in cells lacking both c-Abl and Arg and that treatment of wild-type cells with a pharmaceutical inhibitor of the c-Abl kinase, STI571 (imatinib), reduced serovar Typhimurium invasion efficiency to a similar extent. Bacterial infection led to enhanced phosphorylation of two previously identified c-Abl substrates, the adaptor protein CT10 regulator of kinase (CrkII) and the Abelson-interacting protein Abi1, a component of the WAVE2 complex. Furthermore, overexpression of the nonphosphorylatable form of CrkII resulted in decreased invasion. Taken together, these findings indicate that c-Abl is activated during S. enterica serovar Typhimurium infection and that its phosphorylation of multiple downstream targets is functionally important in bacterial internalization.

The c-Abl tyrosine kinase regulates actin remodeling at the immune synapse

Actin dynamics during T-cell activation are controlled by the coordinate action of multiple actin regulatory proteins, functioning downstream of a complex network of kinases and other signaling molecules. The c-Abl nonreceptor tyrosine kinase regulates actin responses in nonhematopoietic cells, but its function in T cells is poorly understood. Using kinase inhibitors, RNAi, and conditional knockout mice, we investigated the role of c-Abl in controlling the T-cell actin response. We find that c-Abl is required for normal actin polymerization and lamellipodial spreading at the immune synapse, and for downstream events leading to efficient interleukin-2 production. c-Abl also plays a key role in signaling chemokine-induced T-cell migration. c-Abl is required for the appropriate function of 2 proteins known to be important for controlling actin responses to T-cell receptor (TCR) engagement, the actin-stabilizing adapter protein HS1, and the Rac1-dependent actin polymerizing protein WAVE2. c-Abl binds to phospho-HS1 via its SH2 domains and is required for full tyrosine phosphorylation of HS1 during T-cell activation. In addition, c-Abl is required for normal localization of WAVE2 to the immune synapse (IS). These studies identify c-Abl as a key player in the signaling cascade, leading to actin reorganization during T-cell activation.

RPEL motifs link the serum response factor cofactor MAL but not myocardin to Rho signaling via actin binding

Myocardin (MC) family proteins are transcriptional coactivators for serum response factor (SRF). Each family member possesses a conserved N-terminal region containing three RPEL motifs (the "RPEL domain"). MAL/MKL1/myocardin-related transcription factor A is cytoplasmic, accumulating in the nucleus upon activation of Rho GTPase signaling, which alters interactions between G-actin and the RPEL domain. We demonstrate that MC, which is nuclear, does not shuttle through the cytoplasm and that the contrasting nucleocytoplasmic shuttling properties of MAL and MC are defined by their RPEL domains. We show that the MAL RPEL domain binds actin more avidly than that of MC and that the RPEL motif itself is an actin-binding element. RPEL1 and RPEL2 of MC bind actin weakly compared with those of MAL, while RPEL3 is of comparable and low affinity in the two proteins. Actin binding by all three motifs is required for MAL regulation. The differing behaviors of MAL and MC are specified by the RPEL1-RPEL2 unit, while RPEL3 can be exchanged between them. We propose that differential actin occupancy of multiple RPEL motifs regulates nucleocytoplasmic transport and activity of MAL.

Abl tyrosine kinases regulate cell-cell adhesion through Rho GTPases

Adherens junctions are calcium-dependent cell-cell contacts that link neighboring cells through cadherin receptors. Coordinated regulation of the actin cytoskeleton by the Rho GTPases is required for the formation and dissolution of adherens junctions. However, the pathways that link cadherin signaling to cytoskeletal regulation remain poorly defined. Here we identify the Abl family kinases as critical mediators of cadherin-mediated adhesion. Endogenous Abl family kinases, Abl and Arg, are activated and required for Rac activation after cadherin engagement and regulate the formation and maintenance of adherens junctions in mammalian cells. Significantly, we show that Abl-dependent regulation of the Rho-ROCK-myosin signaling pathway is critical for the maintenance of adherens junctions. Inhibition of the Abl kinases in epithelial sheets results in the activation of Rho and its downstream target ROCK, leading to enhanced phosphorylation of the myosin regulatory light chain. These signaling events result in enhanced stress fiber formation and increased actomyosin contractility, thereby disrupting adherens junctions. Conversely, Arg gain of function promotes adherens junction formation through a Crk-dependent pathway in cells with weak junctions. These data identify the Abl kinases as a regulatory link between the cadherin-catenin adhesion complex and the actin cytoskeleton through regulation of Rac and Rho during adherens junction formation, and also reveal a functional link between Abl and Rho that is essential for adherens junction stability.

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

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