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

Gefitinib induces anoikis in cervical cancer cells

Gefitinib exerts anticancer effects on various types of cancer, such as lung, ovarian, breast, and colon cancers. However, the therapeutic effects of gefitinib on cervical cancer and the underlying mechanisms remain unclear. Thus, this study aimed to explore whether gefitinib can be used to treat cervical cancer and elucidate the underlying mechanisms. Results showed that gefitinib induced a caspase-dependent apoptosis of HeLa cells, which consequently became round and detached from the surface of the culture plate. Gefitinib induced the reorganization of actin cytoskeleton and downregulated the expression of p-FAK, integrin β1 and E-cadherin, which are important in cell-extracellular matrix adhesion and cell-cell interaction, respectively. Moreover, gefitinib hindered cell reattachment and spreading and suppressed interactions between detached cells in suspension, leading to poly (ADP-ribose) polymerase cleavage, a hallmark of apoptosis. It also induced detachment-induced apoptosis (anoikis) in C33A cells, another cervical cancer cell line. Taken together, these results suggest that gefitinib triggers anoikis in cervical cancer cells. Our findings may serve as a basis for broadening the range of anticancer drugs used to treat cervical cancer. [BMB Reports 2024; 57(2): 104-109].

The impact of the BCR-ABL oncogene in the pathology and treatment of chronic myeloid leukemia

Chronic Myeloid Leukemia (CML) is characterized by chromosomal aberrations involving the fusion of the BCR and ABL genes on chromosome 22, resulting from a reciprocal translocation between chromosomes 9 and 22. This fusion gives rise to the oncogenic BCR-ABL, an aberrant tyrosine kinase identified as Abl protein. The Abl protein intricately regulates the cell cycle by phosphorylating protein tyrosine residues through diverse signaling pathways. In CML, the BCR-ABL fusion protein disrupts the first exon of Abl, leading to sustained activation of tyrosine kinase and resistance to deactivation mechanisms. Pharmacological interventions, such as imatinib, effectively target BCR-ABL's tyrosine kinase activity by binding near the active site, disrupting ATP binding, and inhibiting downstream protein phosphorylation. Nevertheless, the emergence of resistance, often attributed to cap structure mutations, poses a challenge to imatinib efficacy. Current research endeavours are directed towards overcoming resistance and investigating innovative therapeutic strategies. This article offers a comprehensive analysis of the structural attributes of BCR-ABL, emphasizing its pivotal role as a biomarker and therapeutic target in CML. It underscores the imperative for ongoing research to refine treatment modalities and enhance overall outcomes in managing CML.

c-Abl controls BCR signaling and B cell differentiation by promoting B cell metabolism

As a non-receptor tyrosine kinase, c-Abl was first studied in chronic myelogenous leukemia, and its role in lymphocytes has been well characterized. c-Abl is involved in B cell development and CD19 associated B cell antigen receptor (BCR) signaling. Although c-Abl regulates different metabolic pathways, the role of c-Abl is still unknown in B cell metabolism. In this study, B cell specific c-Abl knockout (KO) mice (Mb1^Cre+/- c-Abl^fl/fl ) were used to investigate how c-Abl regulates B cell metabolism and BCR signaling. We found that the levels of activation positive BCR signaling proximal molecules, phosphorylated spleen tyrosine kinase (pSYK) and phosphorylated Bruton tyrosine kinase (pBTK), were decreased, while the level of key negative regulator, phosphorylated SH2-containing inositol phosphatase (pSHIP1), was increased in Mb1^Cre+/- c-Abl^fl/fl mice. Furthermore, we found c-Abl deficiency weakened the B cell spreading, formation of BCR signalosomes, and the polymerization of actin during BCR activation, and also impaired the differentiation of germinal center (GC) B cells both in quiescent condition and after immunization. Moreover, B cell mitochondrial respiration and the expression of B cell metabolism regulating molecules were downregulated in c-Abl deficiency mice. Overall, c-Abl, which involved in actin remodeling and B cell metabolism, positively regulates BCR signaling and promotes GC differentiation. This article is protected by copyright. All rights reserved.

The Role of WAVE2 Signaling in Cancer

The Wiskott–Aldrich syndrome protein (WASP) and WASP family verprolin-homologous protein (WAVE)—WAVE1, WAVE2 and WAVE3 regulate rapid reorganization of cortical actin filaments and have been shown to form a key link between small GTPases and the actin cytoskeleton. Upon receiving upstream signals from Rho-family GTPases, the WASP and WAVE family proteins play a significant role in polymerization of actin cytoskeleton through activation of actin-related protein 2/3 complex (Arp2/3). The Arp2/3 complex, once activated, forms actin-based membrane protrusions essential for cell migration and cancer cell invasion. Thus, by activation of Arp2/3 complex, the WAVE and WASP family proteins, as part of the WAVE regulatory complex (WRC), have been shown to play a critical role in cancer cell invasion and metastasis, drawing significant research interest over recent years. Several studies have highlighted the potential for targeting the genes encoding either part of or a complete protein from the WASP/WAVE family as therapeutic strategies for preventing the invasion and metastasis of cancer cells. WAVE2 is well documented to be associated with the pathogenesis of several human cancers, including lung, liver, pancreatic, prostate, colorectal and breast cancer, as well as other hematologic malignancies. This review focuses mainly on the role of WAVE2 in the development, invasion and metastasis of different types of cancer. This review also summarizes the molecular mechanisms that regulate the activity of WAVE2, as well as those oncogenic pathways that are regulated by WAVE2 to promote the cancer phenotype. Finally, we discuss potential therapeutic strategies that target WAVE2 or the WAVE regulatory complex, aimed at preventing or inhibiting cancer invasion and metastasis.

Molecular insights on ABL kinase activation using tree-based machine learning models and molecular docking

Abelson kinase (c-Abl) is a non-receptor tyrosine kinase involved in several biological processes essential for cell differentiation, migration, proliferation, and survival. This enzyme's activation might be an alternative strategy for treating diseases such as neutropenia induced by chemotherapy, prostate, and breast cancer. Recently, a series of compounds that promote the activation of c-Abl has been identified, opening a promising ground for c-Abl drug development. Structure-based drug design (SBDD) and ligand-based drug design (LBDD) methodologies have significantly impacted recent drug development initiatives. Here, we combined SBDD and LBDD approaches to characterize critical chemical properties and interactions of identified c-Abl's activators. We used molecular docking simulations combined with tree-based machine learning models—decision tree, AdaBoost, and random forest to understand the c-Abl activators' structural features required for binding to myristoyl pocket, and consequently, to promote enzyme and cellular activation. We obtained predictive and robust models with Matthews correlation coefficient values higher than 0.4 for all endpoints and identified characteristics that led to constructing a structure–activity relationship model (SAR).

Abelson kinase's intrinsically disordered region plays essential roles in protein function and protein stability

Background

The non-receptor tyrosine kinase Abelson (Abl) is a key player in oncogenesis, with kinase inhibitors serving as paradigms of targeted therapy. Abl also is a critical regulator of normal development, playing conserved roles in regulating cell behavior, brain development and morphogenesis. Drosophila offers a superb model for studying Abl’s normal function, because, unlike mammals, there is only a single fly Abl family member. In exploring the mechanism of action of multi-domain scaffolding proteins like Abl, one route is to define the roles of their individual domains. Research into Abl’s diverse roles in embryonic morphogenesis revealed many surprises. For instance, kinase activity, while important, is not crucial for all Abl activities, and the C-terminal F-actin binding domain plays a very modest role. This turned our attention to one of Abl’s least understood features—the long intrinsically-disordered region (IDR) linking Abl’s kinase and F-actin binding domains. The past decade revealed unexpected, important roles for IDRs in diverse cell functions, as sites of posttranslational modifications, mediating multivalent interactions and enabling assembly of biomolecular condensates via phase separation. Previous work deleting conserved regions in Abl’s IDR revealed an important role for a PXXP motif, but did not identify any other essential regions.

Methods

Here we extend this analysis by deleting the entire IDR, and asking whether Abl∆IDR rescues the diverse roles of Abl in viability and embryonic morphogenesis in Drosophila.

Results

This revealed that the IDR is essential for embryonic and adult viability, and for cell shape changes and cytoskeletal regulation during embryonic morphogenesis, and, most surprisingly, revealed a role in modulating protein stability.

Conclusion

Our data provide new insights into the role of the IDR in an important signaling protein, the non-receptor kinase Abl, suggesting that it is essential for all aspects of protein function during embryogenesis, and revealing a role in protein stability. These data will stimulate new explorations of the mechanisms by which the IDR regulates Abl stability and function, both in Drosophila and also in mammals. They also will stimulate further interest in the broader roles IDRs play in diverse signaling proteins.

Ponatinib and other CML Tyrosine Kinase Inhibitors in Thrombosis

Abl1 kinase has important biological roles. The Bcr-Abl1 fusion protein creates undesired kinase activity and is pathogenic in 95% of chronic myeloid leukemia (CML) and 30% of acute lymphoblastic leukemia (ALL) patients. Targeted therapies to these diseases are tyrosine kinase inhibitors. The extent of a tyrosine kinase inhibitor’s targets determines the degree of biologic effects of the agent that may influence the well-being of the patient. This fact is especially true with tyrosine kinase inhibitor effects on the cardiovascular system. Thirty-one percent of ponatinib-treated patients, the tyrosine kinase inhibitor with the broadest inhibitory spectrum, have thrombosis associated with its use. Recent experimental investigations have indicated the mechanisms of ponatinib-associated thrombosis. Further, an antidote to ponatinib is in development by re-purposing an FDA-approved medication.

Transcriptome changes underlie alterations in behavioral traits in different types of chicken

In recent decades, artificial selection has contributed greatly to meeting the demands for animal meat, eggs, and milk. However, it has also resulted in changes in behavior, metabolic and digestive function, and alterations in tissue development, including the brain and skeleton. Our study aimed to profile the behavioral traits and transcriptome pattern of chickens (broilers, layers, and dual-purpose breeds) in response to artificial selection. Broilers spent less time gathered as a group in a novel arena (P < 0.01), suggesting reduced fearfulness in these birds. Broilers also showed a greater willingness to approach a model predator during a vigilance test but had a greater behavioral response when first exposed to the vocalization of the predator. Genes found to be upregulated and downregulated in previous work on chickens divergently selected for fear responses also showed consistent differences in expression between breeds in our study and indicated a reduction in fearfulness in broilers. Gene ACTB_G1 (actin) was differentially expressed between breeds and is a candidate gene involved with skeletal muscle growth and disease susceptibility in broilers. Furthermore, breed-specific alterations in the chicken domestic phenotype leading to differences in growth and egg production were associated with behavioral changes, which are probably underpinned by alterations in gene expression, gene ontology terms, and Kyoto Encyclopedia of Genes and Genomes pathways. The results highlight the change in behavior and gene expression of the broiler strain relative to the layer and a dual-purpose native breed.

Molecular convergence and positive selection associated with the evolution of symbiont transmission mode in stony corals

Heritable symbioses have been critical for the evolution of life. The genetic consequences of evolving a heritable symbiosis from the perspective of the symbiont are well established, but concomitant changes in the host remain unresolved. In stony corals, heritable, vertical transmission has evolved repeatedly, providing a unique opportunity to investigate the genomic basis of this complex trait. We conducted a comparative analysis of 25 coral transcriptomes to identify orthologous genes exhibiting signatures of positive selection and convergent amino acid substitutions in vertically transmitting lineages. The frequency of convergence events tends to be higher among vertically transmitting lineages, consistent with the proposed role of selection in driving the evolution of convergent transmission mode phenotypes. Of 10 774 orthologous genes, 403 exhibited at least one molecular convergence event and evidence of positive selection in at least one vertically transmitting lineage. Functional enrichments among these top candidate genes include processes previously implicated in symbiosis including endocytosis, immune response, cytoskeletal protein binding and cytoplasmic membrane-bounded vesicles. Finally, several novel candidates were identified among 100 genes showing evidence of positive selection at the particular convergence event, highlighting the value of our approach for generating new insight into host mechanisms associated with the evolution of heritable symbioses.

Hematopoietic cell-specific lyn substrate (HCLS1 or HS1): A versatile actin-binding protein in leukocytes

Leukocytes are constantly produced in the bone marrow and released into the circulation. Many different leukocyte subpopulations exist that exert distinct functions. Leukocytes are recruited to sites of inflammation and combat the cause of inflammation via many different effector functions. Virtually all of these processes depend on dynamic actin remodeling allowing leukocytes to adhere, migrate, phagocytose, and release granules. However, actin dynamics are not possible without actin-binding proteins (ABP) that orchestrate the balance between actin polymerization, branching, and depolymerization. The homologue of the ubiquitous ABP cortactin in hematopoietic cells is hematopoietic cell-specific lyn substrate-1, often called hematopoietic cell-specific protein-1 (HCLS1 or HS1). HS1 has been reported in different leukocytes to regulate Arp2/3-dependent migration. However, more evidence is emerging that HS1 functions go far beyond just being a direct actin modulator. For example, HS1 is important for the activation of GTPases and integrins, and mediates signaling downstream of many receptors including BCR, TCR, and CXCR4. In this review, we summarize current knowledge on HS1 functions and discuss them in a pathophysiologic context.

miR-34a is a microRNA safeguard for Citrobacter-induced inflammatory colon oncogenesis

Inflammation often induces regeneration to repair the tissue damage. However, chronic inflammation can transform temporary hyperplasia into a fertile ground for tumorigenesis. Here, we demonstrate that the microRNA miR-34a acts as a central safeguard to protect the inflammatory stem cell niche and reparative regeneration. Although playing little role in regular homeostasis, miR-34a deficiency leads to colon tumorigenesis after Citrobacter rodentium infection. miR-34a targets both immune and epithelial cells to restrain inflammation-induced stem cell proliferation. miR-34a targets Interleukin six receptor (IL-6R) and Interleukin 23 receptor (IL-23R) to suppress T helper 17 (Th17) cell differentiation and expansion, targets chemokine CCL22 to hinder Th17 cell recruitment to the colon epithelium, and targets an orphan receptor Interleukin 17 receptor D (IL-17RD) to inhibit IL-17-induced stem cell proliferation. Our study highlights the importance of microRNAs in protecting the stem cell niche during inflammation despite their lack of function in regular tissue homeostasis.

Crk adaptor proteins mediate actin-dependent T cell migration and mechanosensing induced by the integrin LFA-1

T cell entry into inflamed tissue involves firm adhesion, spreading, and migration of the T cells across endothelial barriers. These events depend on "outside-in" signals through which engaged integrins direct cytoskeletal reorganization. We investigated the molecular events that mediate this process and found that T cells from mice lacking expression of the adaptor protein Crk exhibited defects in phenotypes induced by the integrin lymphocyte function-associated antigen 1 (LFA-1), namely, actin polymerization, leading edge formation, and two-dimensional cell migration. Crk protein was an essential mediator of LFA-1 signaling-induced phosphorylation of the E3 ubiquitin ligase c-Cbl and its subsequent interaction with the phosphatidylinositol 3-kinase (PI3K) subunit p85, thus promoting PI3K activity and cytoskeletal remodeling. In addition, we found that Crk proteins were required for T cells to respond to changes in substrate stiffness, as measured by alterations in cell spreading and differential phosphorylation of the force-sensitive protein CasL. These findings identify Crk proteins as key intermediates coupling LFA-1 signals to actin remodeling and provide mechanistic insights into how T cells sense and respond to substrate stiffness.

Coronavirus S protein-induced fusion is blocked prior to hemifusion by Abl kinase inhibitors

Enveloped viruses gain entry into host cells by fusing with cellular membranes, a step that is required for virus replication. Coronaviruses, including the severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV) and infectious bronchitis virus (IBV), fuse at the plasma membrane or use receptor-mediated endocytosis and fuse with endosomes, depending on the cell or tissue type. The virus spike (S) protein mediates fusion with the host cell membrane. We have shown previously that an Abelson (Abl) kinase inhibitor, imatinib, significantly reduces SARS-CoV and MERS-CoV viral titres and prevents endosomal entry by HIV SARS S and MERS S pseudotyped virions. SARS-CoV and MERS-CoV are classified as BSL-3 viruses, which makes experimentation into the cellular mechanisms involved in infection more challenging. Here, we use IBV, a BSL-2 virus, as a model for studying the role of Abl kinase activity during coronavirus infection. We found that imatinib and two specific Abl kinase inhibitors, GNF2 and GNF5, reduce IBV titres by blocking the first round of virus infection. Additionally, all three drugs prevented IBV S-induced syncytia formation prior to the hemifusion step. Our results indicate that membrane fusion (both virus-cell and cell-cell) is blocked in the presence of Abl kinase inhibitors. Studying the effects of Abl kinase inhibitors on IBV will be useful in identifying the host cell pathways required for coronavirus infection. This will provide an insight into possible therapeutic targets to treat infections by current as well as newly emerging coronaviruses.

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.

The Abl-1 Kinase is Dispensable for NK Cell Inhibitory Signalling and is not Involved in Murine NK Cell Education

Natural killer (NK) cell responsiveness in the mouse is determined in an education process guided by inhibitory Ly49 and NKG2A receptors binding to MHC class I molecules. It has been proposed that inhibitory signalling in human NK cells involves Abl-1 (c-Abl)-mediated phosphorylation of Crk, lowering NK cell function via disruption of a signalling complex including C3G and c-Cbl, suggesting that NK cell education might involve c-Abl. Mice deficient in c-Abl expression specifically in murine NK cells displayed normal inhibitory and activating receptor repertoires. Furthermore, c-Abl-deficient NK cells fluxed Ca²⁺ normally after triggering of ITAM receptors, killed YAC-1 tumour cells efficiently and showed normal, or even slightly elevated, capacity to produce IFN-γ after activating receptor stimulation. Consistent with these results, c-Abl deficiency in NK cells did not affect NK cell inhibition via the receptors Ly49G2, Ly49A and NKG2A. We conclude that signalling downstream of murine inhibitory receptors does not involve c-Abl and that c-Abl plays no major role in NK cell education in the mouse.

Aberrant Th2 inflammation drives dysfunction of alveolar macrophages and susceptibility to bacterial pneumonia

The ubiquitin ligase, Itch, is required to prevent autoinflammatory disease in mice and humans. Itch-deficient mice develop lethal pulmonary inflammation characterized by the production of Th2 cytokines (for example, interleukin-4 (IL-4)); however, the contribution of Itch to immune defense against respiratory pathogens has not been determined. We found that Itch-deficient mice were highly susceptible to intranasal infection with the respiratory pathogen Klebsiella pneumoniae. Infected Itch-deficient mice exhibited increased immune cell infiltration, cytokine levels and bacterial burden in the respiratory tract compared with control mice. However, numbers of resident alveolar macrophages were reduced in the lungs from Itch-deficient mice both before and after infection. High levels of Th2 cytokines in the respiratory tract correlated with deceased alveolar macrophages, and genetic ablation of IL-4 restored alveolar macrophages and host defense to K. pneumoniae in Itch-deficient mice, suggesting that loss of alveolar macrophages occurred as a consequence of Th2 inflammation. Adoptive transfer of Itch^-/- CD4⁺ T cells into Rag^-/- mice was sufficient to drive reduction in numbers of Itch-replete alveolar macrophages. Finally, we found that Stat6 signaling downstream of the IL-4 receptor directly reduced fitness of alveolar macrophages when these cells were exposed to the Itch^-/- inflamed respiratory tract. These data suggest that Th2 inflammation directly impairs alveolar macrophage fitness in Itch^-/- mice, and elucidate a previously unappreciated link between Th2 cells, alveolar macrophages and susceptibility to bacterial infection.

Tyrosine kinase c-Abl regulates the survival of plasma cells

Tyrosine kinase c-Abl plays an important role in early B cell development. Its deletion leads to reduced pro- and pre-B cell generation in mice. However, its function in B cell terminal differentiation remains unexplored. Here, we used c-Abl^f/f Aicda^cre/+ mice, in which c-Abl is ablated only in antigen-activated B cells, to study the role of c-Abl in germinal center (GC) B and antibody-secreting plasma cell formation. Upon challenge with a model antigen, we found normal GC and memory B but reduced plasma cells and antigen-specific antibody response in the mutant mice. In-vitro studies revealed that plasma cells lacking c-Abl could be generated but did not accumulate in culture, indicative of survival defect. They also exhibited impaired STAT3 phosphorylation. The plasma cell defects could be rectified by introduction of Bim-deficiency or delivery of colivelin, a STAT3 activator, into c-Abl^f/f Aicda^cre/+ mice. Hence, c-Abl signalling regulates the survival of plasma cells.

Abelson kinase acts as a robust, multifunctional scaffold in regulating embryonic morphogenesis

The importance of Abl kinase activity, the F-actin–binding site, and scaffolding ability in Abl’s many cell biological roles during Drosophila morphogenesis is examined. Abl is a robust multidomain scaffold with different protein motifs and activities contributing differentially to diverse cellular behaviors.

Abelson family kinases (Abls) are key regulators of cell behavior and the cytoskeleton during development and in leukemia. Abl’s SH3, SH2, and tyrosine kinase domains are joined via a linker to an F-actin–binding domain (FABD). Research on Abl’s roles in cell culture led to several hypotheses for its mechanism of action: 1) Abl phosphorylates other proteins, modulating their activity, 2) Abl directly regulates the cytoskeleton via its cytoskeletal interaction domains, and/or 3) Abl is a scaffold for a signaling complex. The importance of these roles during normal development remains untested. We tested these mechanistic hypotheses during Drosophila morphogenesis using a series of mutants to examine Abl’s many cell biological roles. Strikingly, Abl lacking the FABD fully rescued morphogenesis, cell shape change, actin regulation, and viability, whereas kinase-dead Abl, although reduced in function, retained substantial rescuing ability in some but not all Abl functions. We also tested the function of four conserved motifs in the linker region, revealing a key role for a conserved PXXP motif known to bind Crk and Abi. We propose that Abl acts as a robust multidomain scaffold with different protein motifs and activities contributing differentially to diverse cellular behaviors.

The Src kinases Hck, Fgr and Lyn activate Arg to facilitate IgG-mediated phagocytosis and Leishmania infection

Leishmaniasis is a devastating disease that disfigures or kills nearly two million people each year. Establishment and persistence of infection by the obligate intracellular parasite Leishmania requires repeated uptake by macrophages and other phagocytes. Therefore, preventing uptake could be a novel therapeutic strategy for leishmaniasis. Amastigotes, the life cycle stage found in the human host, bind Fc receptors and enter macrophages primarily through immunoglobulin-mediated phagocytosis. However, the host machinery that mediates amastigote uptake is poorly understood. We have previously shown that the Arg (also known as Abl2) non-receptor tyrosine kinase facilitates L. amazonensis amastigote uptake by macrophages. Using small-molecule inhibitors and primary macrophages lacking specific Src family kinases, we now demonstrate that the Hck, Fgr and Lyn kinases are also necessary for amastigote uptake by macrophages. Src-mediated Arg activation is required for efficient uptake. Interestingly, the dual Arg and Src kinase inhibitor bosutinib, which is approved to treat cancer, not only decreases amastigote uptake, but also significantly reduces disease severity and parasite burden in Leishmania-infected mice. Our results suggest that leishmaniasis could potentially be treated with host-cell-active agents such as kinase inhibitors.

Role played by paxillin and paxillin tyrosine phosphorylation in hepatocyte growth factor/sphingosine-1-phosphate-mediated reactive oxygen species generation, lamellipodia formation, and endothelial barrier function

Paxillin is a multifunctional and multidomain focal adhesion adaptor protein. It serves as an important scaffolding protein at focal adhesions by recruiting and binding to structural and signaling molecules. Paxillin tyrosine phosphorylation at Y31 and Y118 is important for paxillin redistribution to focal adhesions and angiogenesis. Hepatocyte growth factor (HGF) and sphingosine-1-phosphate (S1P) are potent stimulators of lamellipodia formation, a prerequisite for endothelial cell migration. The role played by paxillin and its tyrosine phosphorylated forms in HGF- or S1P-induced lamellipodia formation and barrier function is unclear. HGF or S1P stimulated lamellipodia formation, tyrosine phosphorylation of paxillin at Y31 and Y118, and c-Abl in human lung microvascular endothelial cells (HLMVECs). Knockdown of paxillin with small interfering RNA (siRNA) or transfection with paxillin mutants (Y31F or Y118F) mitigated HGF- or S1P-induced lamellipodia formation, translocation of p47 (phox) to lamellipodia, and reactive oxygen species (ROS) generation in HLMVECs. Furthermore, exposure of HLMVECs to HGF or S1P stimulated c-Abl-mediated tyrosine phosphorylation of paxillin at Y31 and Y118 in a time-dependent fashion, and down-regulation of c-Abl with siRNA attenuated HGF- or S1P-mediated lamellipodia formation, translocation of p47 (phox) to lamellipodia, and endothelial barrier enhancement. In vivo, knockdown of paxillin with siRNA in mouse lungs attenuated ventilator-induced lung injury. Together, these results suggest that c-Abl-mediated tyrosine phosphorylation of paxillin at Y31 and Y118 regulates HGF- or S1P-mediated lamellipodia formation, ROS generation in lamellipodia, and endothelial permeability.

Critical role of actin-associated proteins in smooth muscle contraction, cell proliferation, airway hyperresponsiveness and airway remodeling

Asthma is characterized by airway hyperresponsiveness and airway remodeling, which are largely attributed to increased airway smooth muscle contractility and cell proliferation. It is known that both chemical and mechanical stimulation regulates smooth muscle contraction. Recent studies suggest that contractile activation and mechanical stretch induce actin cytoskeletal remodeling in smooth muscle. However, the mechanisms that control actin cytoskeletal reorganization are not completely elucidated. This review summarizes our current understanding regarding how actin-associated proteins may regulate remodeling of the actin cytoskeleton in airway smooth muscle. In particular, there is accumulating evidence to suggest that Abelson tyrosine kinase (Abl) plays a critical role in regulating airway smooth muscle contraction and cell proliferation in vitro, and airway hyperresponsiveness and remodeling in vivo. These studies indicate that Abl may be a novel target for the development of new therapy to treat asthma.

CRK proteins selectively regulate T cell migration into inflamed tissues

Effector T cell migration into inflamed sites greatly exacerbates tissue destruction and disease severity in inflammatory diseases, including graft-versus-host disease (GVHD). T cell migration into such sites depends heavily on regulated adhesion and migration, but the signaling pathways that coordinate these functions downstream of chemokine receptors are largely unknown. Using conditional knockout mice, we found that T cells lacking the adaptor proteins CRK and CRK-like (CRKL) exhibit reduced integrin-dependent adhesion, chemotaxis, and diapedesis. Moreover, these two closely related proteins exhibited substantial functional redundancy, as ectopic expression of either protein rescued defects in T cells lacking both CRK and CRKL. We determined that CRK proteins coordinate with the RAP guanine nucleotide exchange factor C3G and the adhesion docking molecule CASL to activate the integrin regulatory GTPase RAP1. CRK proteins were required for effector T cell trafficking into sites of inflammation, but not for migration to lymphoid organs. In a murine bone marrow transplantation model, the differential migration of CRK/CRKL-deficient T cells resulted in efficient graft-versus-leukemia responses with minimal GVHD. Together, the results from our studies show that CRK family proteins selectively regulate T cell adhesion and migration at effector sites and suggest that these proteins have potential as therapeutic targets for preventing GVHD.

Nonstructural protein 1 (NS1)-mediated inhibition of c-Abl results in acute lung injury and priming for bacterial co-infections: insights into 1918 H1N1 pandemic?

BACKGROUND

Nonstructural protein 1 (NS1) proteins from avian influenza viruses like the 1918 pandemic NS1 are capable of inhibiting the key signaling integrator c-Abl (Abl1), resulting in massive cytopathic cell alterations.

METHODS

In the current study, we addressed the consequences of NS1-mediated alteration of c-Abl on acute lung injury and pathogenicity in an in vivo mouse model.

RESULTS

Comparing isogenic strains that differ only in their ability to inhibit c-Abl, we observed elevated pathogenicity for the c-Abl-inhibiting virus. NS1-mediated blockade of c-Abl resulted in severe lung pathology and massive edema formation and facilitated secondary bacterial pneumonia. This phenotype was independent of differences in replication and immune responses, defining it as an NS1 virulence mechanism distinct from its canonical functions. Microarray analysis revealed extensive downregulation of genes involved in cell integrity and vascular endothelial regulation.

CONCLUSIONS

NS1 protein-mediated blockade of c-Abl signaling drives acute lung injury and primes for bacterial coinfections revealing potential insights into the pathogenicity of the 1918 pandemic virus.

WASp family verprolin-homologous protein-2 (WAVE2) and Wiskott-Aldrich syndrome protein (WASp) engage in distinct downstream signaling interactions at the T cell antigen receptor site

T cell antigen receptor (TCR) engagement has been shown to activate pathways leading to actin cytoskeletal polymerization and reorganization, which are essential for lymphocyte activation and function. Several actin regulatory proteins were implicated in regulating the actin machinery, such as members of the Wiskott-Aldrich syndrome protein (WASp) family. These include WASp and the WASp family verprolin-homologous protein-2 (WAVE2). Although WASp and WAVE2 share several structural features, the precise regulatory mechanisms and potential redundancy between them have not been fully characterized. Specifically, unlike WASp, the dynamic molecular interactions that regulate WAVE2 recruitment to the cell membrane and specifically to the TCR signaling complex are largely unknown. Here, we identify the molecular mechanism that controls the recruitment of WAVE2 in comparison with WASp. Using fluorescence resonance energy transfer (FRET) and novel triple-color FRET (3FRET) technology, we demonstrate how WAVE2 signaling complexes are dynamically regulated during lymphocyte activation in vivo. We show that, similar to WASp, WAVE2 recruitment to the TCR site depends on protein-tyrosine kinase, ZAP-70, and the adaptors LAT, SLP-76, and Nck. However, in contrast to WASp, WAVE2 leaves this signaling complex and migrates peripherally together with vinculin to the membrane leading edge. Our experiments demonstrate that WASp and WAVE2 differ in their dynamics and their associated proteins. Thus, this study reveals the differential mechanisms regulating the function of these cytoskeletal proteins.

Avian influenza viruses inhibit the major cellular signalling integrator c-Abl

The non-structural protein 1 (NS1) of influenza A viruses (IAV) encodes several src homology (SH) binding motifs (bm) (one SH2bm, up to two SH3bm), which mediate interactions with host cell proteins. In contrast to NS1 of human IAV, NS1 of avian strains possess the second SH3bm (SH3(II)bm) consensus sequence. Since our former studies demonstrated an NS1-CRK interaction, mediated by this motif, here, we addressed the regulatory properties of this SH3bm for cellular signalling. Initially, we observed a reduced basal CRK phosphorylation upon infection with avian IAV harbouring an NS1 with an SH3(II)bm in contrast to human IAV. Reduced activity of the tyrosine kinase c-Abl was identified to be responsible for reduced CRK phosphorylation. Further, binding of NS1 to c-Abl was determined, and mutational manipulation of the SH3(II)bm illustrated the necessity of this motif for c-Abl inhibition. Interestingly, Abl kinase inhibition resulted in impaired avian IAV propagation and pathogenicity and mutational analysis linked the pronounced inhibition of c-Abl to cytopathogenic cell alterations upon avian IAV infections. Taken together, NS1 proteins of avian IAV interfere with the kinase activity of c-Abl, a major cellular signalling integrator that controls multiple signalling processes and cell fate regulations apparently including IAV infections.

SDF1 reduces interneuron leading process branching through dual regulation of actin and microtubules

Normal cerebral cortical function requires a highly ordered balance between projection neurons and interneurons. During development these two neuronal populations migrate from distinct progenitor zones to form the cerebral cortex, with interneurons originating in the more distant ganglionic eminences. Moreover, deficits in interneurons have been linked to a variety of neurodevelopmental disorders underscoring the importance of understanding interneuron development and function. We, and others, have identified SDF1 signaling as one important modulator of interneuron migration speed and leading process branching behavior in mice, although how SDF1 signaling impacts these behaviors remains unknown. We previously found SDF1 inhibited leading process branching while increasing the rate of migration. We have now mechanistically linked SDF1 modulation of leading process branching behavior to a dual regulation of both actin and microtubule organization. We find SDF1 consolidates actin at the leading process tip by de-repressing calpain protease and increasing proteolysis of branched-actin-supporting cortactin. Additionally, SDF1 stabilizes the microtubule array in the leading process through activation of the microtubule-associated protein doublecortin (DCX). DCX stabilizes the microtubule array by bundling microtubules within the leading process, reducing branching. These data provide mechanistic insight into the regulation of interneuron leading process dynamics during neuronal migration in mice and provides insight into how cortactin and DCX, a known human neuronal migration disorder gene, participate in this process.

Lymphocytes from chronic lymphocytic leukaemia undergo ABL1-linked amoeboid motility and homotypic interaction as an early adaptive change to ex vivo culture

BACKGROUND

Those stimuli that together promote the survival, differentiation and proliferation of the abnormal B-lymphocytes of chronic lymphocytic leukaemia (CLL) are encountered within tissues, where together they form the growth-supporting microenvironment. Different tissue-culture systems promote the survival of the neoplastic lymphocytes from CLL, partly replicating the in vivo tissue environment of the disorder. In the present study, we focussed on the initial adaptive changes to the tissue culture environment focussing particularly on migratory behaviour and cellular interactions.

METHODS

A high-density CLL culture system was employed to test CLL cell-responses using a range of microscopic techniques and flow cytometric analyses, supported by mathematical measures of cell shape-change and by biochemical techniques. The study focussed on the evaluation of changes to the F-actin cytoskeleton and cell behaviour and on ABL1 signalling processes.

RESULTS

We showed that the earliest functional response by the neoplastic lymphocytes was a rapid shape-change caused through rearrangement of the F-actin cytoskeleton that resulted in amoeboid motility and promoted frequent homotypic interaction between cells. This initial response was functionally distinct from the elongated motility that was induced by chemokine stimulation, and which also characterised heterotypic interactions between CLL lymphocytes and accessory cells at later culture periods. ABL1 is highly expressed in CLL lymphocytes and supports their survival, it is also recognised however to have a major role in the control of the F-actin cytoskeleton. We found that the cytoplasmic fraction of ABL1 became co-localised with F-actin structures of the CLL lymphocytes and that the ABL1 substrate CRKL became phosphorylated during initial shape-change. The ABL-inhibitor imatinib mesylate prevented amoeboid movement and markedly reduced homotypic interactions, causing cells to acquire a globular shape to rearrange F-actin to a microvillus form that closely resembled that of CLL cells isolated directly from circulation.

CONCLUSION

We suggest that ABL1-induced amoeboid motility and homotypic interaction represent a distinctive early response to the tissue environment by CLL lymphocytes. This response is separate from that induced by chemokine or during heterotypic cell-contact, and may play a role in the initial entry and interactions of CLL lymphocytes in tissues.

The capable ABL: what is its biological function?

The mammalian ABL1 gene encodes the ubiquitously expressed nonreceptor tyrosine kinase ABL. In response to growth factors, cytokines, cell adhesion, DNA damage, oxidative stress, and other signals, ABL is activated to stimulate cell proliferation or differentiation, survival or death, retraction, or migration. ABL also regulates specialized functions such as antigen receptor signaling in lymphocytes, synapse formation in neurons, and bacterial adhesion to intestinal epithelial cells. Although discovered as the proto-oncogene from which the Abelson leukemia virus derived its Gag-v-Abl oncogene, recent results have linked ABL kinase activation to neuronal degeneration. This body of knowledge on ABL seems confusing because it does not fit the one-gene-one-function paradigm. Without question, ABL capabilities are encoded by its gene sequence and that molecular blueprint designs this kinase to be regulated by subcellular location-dependent interactions with inhibitors and substrate activators. Furthermore, ABL shuttles between the nucleus and the cytoplasm where it binds DNA and actin--two biopolymers with fundamental roles in almost all biological processes. Taken together, the cumulated results from analyses of ABL structure-function, ABL mutant mouse phenotypes, and ABL substrates suggest that this tyrosine kinase does not have its own agenda but that, instead, it has evolved to serve a variety of tissue-specific and context-dependent biological functions.

Role of Abl in airway hyperresponsiveness and airway remodeling

BACKGROUND

Asthma is a chronic disease that is characterized by airway hyperresponsiveness and airway remodeling. The underlying mechanisms that mediate the pathological processes are not fully understood. Abl is a non-receptor protein tyrosine kinase that has a role in the regulation of smooth muscle contraction and smooth muscle cell proliferation in vitro. The role of Abl in airway hyperresponsiveness and airway remodeling in vivo is largely unknown.

METHODS

To evaluate the role of Abl in asthma pathology, we assessed the expression of Abl in airway tissues from the ovalbumin sensitized and challenged mouse model, and human asthmatic airway smooth muscle cells. In addition, we generated conditional knockout mice in which Abl expression in smooth muscle was disrupted, and then evaluated the effects of Abl conditional knockout on airway resistance, smooth muscle mass, cell proliferation, IL-13 and CCL2 in the mouse model of asthma. Furthermore, we determined the effects of the Abl pharmacological inhibitors imatinib and GNF-5 on these processes in the animal model of asthma.

RESULTS

The expression of Abl was upregulated in airway tissues of the animal model of asthma and in airway smooth muscle cells of patients with severe asthma. Conditional knockout of Abl attenuated airway resistance, smooth muscle mass and staining of proliferating cell nuclear antigen in the airway of mice sensitized and challenged with ovalbumin. Interestingly, conditional knockout of Abl did not affect the levels of IL-13 and CCL2 in bronchoalveolar lavage fluid of animals treated with ovalbumin. However, treatment with imatinib and GNF-5 inhibited the ovalbumin-induced increase in IL-13 and CCL2 as well as airway resistance and smooth muscle growth in animals.

CONCLUSIONS

These results suggest that the altered expression of Abl in airway smooth muscle may play a critical role in the development of airway hyperresponsiveness and airway remodeling in asthma. Our findings support the concept that Abl may be a novel target for the development of new therapy to treat asthma.

Phosphoregulation of the WAVE regulatory complex and signal integration

The WAVE2 regulatory complex (WRC) induces actin polymerization by activating the actin nucleator Arp2/3. Polymerizing actin pushes against the cell membrane and induces dramatic edge protrusions. In order to properly control such changes in cell morphology and function, cells have evolved multiple methods to tightly regulate WRC and Arp2/3 activity in space and time. Of these mechanisms, phosphorylation plays a fundamental role in transmitting extracellular and intracellular signals to the WRC and the actin cytoskeleton. This review discusses the phosphorylation-based regulatory inputs into the WRC. Signaling pathways that respond to growth factors, chemokines, hormones, and extracellular matrix converge upon the WAVE and ABI components of the WRC. The Abl, Src, ERK, and PKA kinases promote complex activation through a WRC conformation change that permits interaction with the Arp2/3 complex and through WRC translocation to the cell edge. The neuron-specific CDK5 and constitutively active CK2 kinases inhibit WRC activation. These regulatory signals are integrated in space and time as they coalesce upon the WRC. The combination of WRC phosphorylation events and WRC activity is controlled by stimulus, cell type, and cell cycle-specific pathway activation and via pathway cross-inhibition and cross-activation.

c-Abl tyrosine kinase plays a critical role in β2 integrin-dependent neutrophil migration by regulating Vav1 activity

The recruitment and migration of neutrophils are critical for innate immunity and acute inflammatory responses. However, the mechanism that regulates the recruitment and migration of neutrophils has not been well characterized. We here reveal a novel function of c-Abl kinase in regulating neutrophil migration. Our results demonstrate that c-Abl kinase is required for neutrophil recruitment in vivo and migration in vitro, and the inhibition of c-Abl kinase activity has a significant impact on neutrophil migratory behavior. Moreover, c-Abl kinase activation depends on β2 integrin engagement, and the activated c-Abl kinase further regulates actin polymerization and membrane protrusion dynamics at the extended leading edges during neutrophil migration. In addition, we identify the Rho GEF Vav1 as a major downstream effector of c-Abl kinase. The C-terminal SH3-SH2-SH3 domain and proline-rich region of Vav1 are required for its interaction with c-Abl kinase, and c-Abl kinase probably regulates the activity of Vav1 by direct phosphorylation at Tyr-267 in the DH domain. Together, these results indicate that c-Abl kinase plays a critical role in β2 integrin-dependent neutrophil migration by regulating Vav1 activity.

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.

Interactions among HCLS1, HAX1 and LEF-1 proteins are essential for G-CSF-triggered granulopoiesis

We found that hematopoietic cell-specific Lyn substrate 1 (HCLS1 or HS1) is highly expressed in human myeloid cells and that stimulation with granulocyte colony-stimulating factor (G-CSF) leads to HCLS1 phosphorylation. HCLS1 binds the transcription factor lymphoid-enhancer binding factor 1 (LEF-1), transporting LEF-1 into the nucleus upon G-CSF stimulation and inducing LEF-1 autoregulation. In patients with severe congenital neutropenia, inherited mutations in the gene encoding HCLS1-associated protein X-1 (HAX1) lead to profound defects in G-CSF-triggered phosphorylation of HCLS1 and subsequently to reduced autoregulation and expression of LEF-1. Consistent with these results, HCLS1-deficient mice are neutropenic. In bone marrow biopsies of the majority of tested patients with acute myeloid leukemia, HCLS1 protein expression is substantially elevated, associated with high levels of G-CSF synthesis and, in some individuals, a four-residue insertion in a proline-rich region of HCLS1 protein known to accelerate intracellular signaling. These data demonstrate the importance of HCLS1 in myelopoiesis in vitro and in vivo.

The tyrosine kinase Abl is a component of macrophage podosomes and is required for podosome formation and function

Myeloid leukocytes form actin-based plasma membrane protrusions, called podosomes, that are implicated in myeloid cell recruitment into tissues and cell migration within the interstitium. In this study, we show that tyrosine kinases of the Abl family are present in podosomes formed by murine and human macrophages. Silencing of Abl expression in bone marrow-derived macrophages and monocyte-derived macrophages by siRNA or Abl enzymatic inhibition with imatinib resulted in the disassembly of macrophage podosomes and the reduction of their capacity to degrade an extracellular matrix and migrate through matrigel matrices and endothelial cell monolayers. Additionally, macrophages deficient in Src-family kinases, which cross-talk with Abl in regulating macrophage migration, also demonstrated podosome disassembly. These findings suggest that podosome disassembly induced by Abl targeting may inhibit podosome-dependent functions such as leukocyte recruitment into inflammatory sites and osteoclast-dependent bone resorption.

Tyrosine kinases EnAbling adaptor molecules for chemokine-induced Rap1 activation in T cells

Chemokines regulate T cell trafficking into secondary lymphoid organs and migration across endothelial cells in response to inflammatory signals. The small guanosine triphosphatase Rap1 is a critical regulator of chemokine signaling in T cells, but how chemokines activate Rap1 has been unclear. A study showed that Abl family tyrosine kinases were essential for chemokine-induced Rap1 activation, T cell polarization, and migration. Abl family kinases promoted Rap1 activation by phosphorylating the adaptor protein human enhancer of filamentation 1 (HEF1), thus establishing a critical Abl-HEF1-Rap1 signaling axis for chemokine-induced T cell migration.

Abl family kinases modulate T cell-mediated inflammation and chemokine-induced migration through the adaptor HEF1 and the GTPase Rap1

Chemokine signaling is critical for T cell function during homeostasis and inflammation and directs T cell polarity and migration through the activation of specific intracellular pathways. Here, we uncovered a previously uncharacterized role for the Abl family tyrosine kinases Abl and Arg in the regulation of T cell-dependent inflammatory responses and showed that the Abl family kinases were required for chemokine-induced T cell polarization and migration. Our data demonstrated that Abl and Arg were activated downstream of chemokine receptors and mediated the chemokine-induced tyrosine phosphorylation of human enhancer of filamentation 1 (HEF1), an adaptor protein that is required for the activity of the guanosine triphosphatase Rap1, which mediates cell adhesion and migration. Phosphorylation of HEF1 by Abl family kinases and activation of Rap1 were required for chemokine-induced T cell migration. Mouse T cells that lacked Abl and Arg exhibited defective homing to lymph nodes and impaired migration to sites of inflammation. These findings suggest that Abl family kinases are potential therapeutic targets for the treatment of T cell-dependent immune disorders that are characterized by chemokine-mediated inflammation.

c-Abl promotes osteoblast expansion by differentially regulating canonical and non-canonical BMP pathways and p16INK4a expression

Defects in stem cell renewal or progenitor cell expansion underlie ageing-related diseases such as osteoporosis. Yet much remains unclear about the mechanisms regulating progenitor expansion. Here we show that the tyrosine kinase c-Abl plays an important role in osteoprogenitor expansion. c-Abl interacts with and phosphorylates BMPRIA and the phosphorylation differentially influences the interaction of BMPRIA with BMPRII and the Tab1-Tak1 complex, leading to uneven activation of Smad1/5/8 and Erk1/2, the canonical and non-canonical BMP pathways that direct the expression of p16(INK4a). c-Abl deficiency shunts BMP signalling from Smad1/5/8 to Erk1/2, leading to p16(INK4a) upregulation and osteoblast senescence. Mouse genetic studies revealed that p16(INK4a) controls mesenchymal stem cell maintenance and osteoblast expansion and mediates the effects of c-Abl deficiency on osteoblast expansion and bone formation. These findings identify c-Abl as a regulator of BMP signalling pathways and uncover a role for c-Abl in p16(INK4a) expression and osteoprogenitor expansion.

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.

Actin engine in immunological synapse

T cell activation and function require physical contact with antigen presenting cells at a specialized junctional structure known as the immunological synapse. Once formed, the immunological synapse leads to sustained T cell receptor-mediated signalling and stabilized adhesion. High resolution microscopy indeed had a great impact in understanding the function and dynamic structure of immunological synapse. Trends of recent research are now moving towards understanding the mechanical part of immune system, expanding our knowledge in mechanosensitivity, force generation, and biophysics of cell-cell interaction. Actin cytoskeleton plays inevitable role in adaptive immune system, allowing it to bear dynamic and precise characteristics at the same time. The regulation of mechanical engine seems very complicated and overlapping, but it enables cells to be very sensitive to external signals such as surface rigidity. In this review, we focus on actin regulators and how immune cells regulate dynamic actin rearrangement process to drive the formation of immunological synapse.

Drug-interaction studies evaluating T-cell proliferation reveal distinct activity of dasatinib and imatinib in combination with cyclosporine A

Development of small molecule tyrosine kinase inhibitors for the treatment of chronic myeloid leukemia has been astonishingly successful; however, their off-target effects have generated both challenges and opportunities for extending their clinical application. Dasatinib and imatinib are two of the most commonly used tyrosine kinase inhibitors and both have been shown to impact T-cell function. Due to this activity, their use as potential immune suppressants has been proposed. In this report, we investigated drug interactions with cyclosporine A in suppressing T-cell proliferation. Dasatinib and imatinib were titrated against varying concentrations of cyclosporine in the cultures and T-cell proliferation assessed by 5-6-carboxyfluorescein diacetate, succinimidyl ester dye dilution. These proliferation data were then used to determine the combination index to evaluate additive, synergistic, or antagonistic interactions between the drugs. This analysis uncovered a number of different drug interactions affecting T-cell proliferation. Cyclosporine had an additive or synergistic effect on T-cell proliferation when combined with dasatinib and imatinib for 3 of the 4 methods of stimulating T-cell proliferation. However, when T cells were stimulated with anti-CD3 and anti-CD28 antibodies, this interaction was found to be strongly antagonistic at low dasatinib concentrations. In contrast, this strong antagonism was not observed when imatinib was used in combination with cyclosporine A. This study suggests drug interactions affecting T cells may need to be carefully taken into account when using tyrosine kinase inhibitors. Furthermore, the technique to evaluate drug interactions is novel, and applicable to study any interaction affecting proliferation.

Kinase activity profiling of pneumococcal pneumonia

Background

Pneumonia represents a major health burden. Previous work demonstrated that although the induction of inflammation is important for adequate host defense against pneumonia, an inability to regulate the host's inflammatory response within the lung later during infection can be detrimental. Intracellular signaling pathways commonly rely on activation of kinases, and kinases play an essential role in the regulation of the inflammatory response of immune cells.

Methodology/Principal Findings

Pneumonia was induced in mice via intranasal instillation of Streptococcus (S.) pneumoniae. Kinomics peptide arrays, exhibiting 1024 specific consensus sequences for protein kinases, were used to produce a systems biology analysis of cellular kinase activity during the course of pneumonia. Several differences in kinase activity revealed by the arrays were validated in lung homogenates of individual mice using western blot. We identified cascades of activated kinases showing that chemotoxic stress and a T helper 1 response were induced during the course of pneumococcal pneumonia. In addition, our data point to a reduction in WNT activity in lungs of S. pneumoniae infected mice. Moreover, this study demonstrated a reduction in overall CDK activity implying alterations in cell cycle biology.

Conclusions/Significance

This study utilizes systems biology to provide insight into the signaling events occurring during lung infection with the common cause of community acquired pneumonia, and may assist in identifying novel therapeutic targets in the treatment of bacterial pneumonia.

Hematopoietic lineage cell-specific protein 1 functions in concert with the Wiskott-Aldrich syndrome protein to promote podosome array organization and chemotaxis in dendritic cells

Dendritic cells (DCs) are professional APCs that reside in peripheral tissues and survey the body for pathogens. Upon activation by inflammatory signals, DCs undergo a maturation process and migrate to lymphoid organs, where they present pathogen-derived Ags to T cells. DC migration depends on tight regulation of the actin cytoskeleton to permit rapid adaptation to environmental cues. We investigated the role of hematopoietic lineage cell-specific protein 1 (HS1), the hematopoietic homolog of cortactin, in regulating the actin cytoskeleton of murine DCs. HS1 localized to lamellipodial protrusions and podosomes, actin-rich structures associated with adhesion and migration. DCs from HS1(-/-) mice showed aberrant lamellipodial dynamics. Moreover, although these cells formed recognizable podosomes, their podosome arrays were loosely packed and improperly localized within the cell. HS1 interacts with Wiskott-Aldrich syndrome protein (WASp), another key actin-regulatory protein, through mutual binding to WASp-interacting protein. Comparative analysis of DCs deficient for HS1, WASp or both proteins revealed unique roles for these proteins in regulating podosomes with WASp being essential for podosome formation and with HS1 ensuring efficient array organization. WASp recruitment to podosome cores was independent of HS1, whereas HS1 recruitment required Src homology 3 domain-dependent interactions with the WASp/WASp-interacting protein heterodimer. In migration assays, the phenotypes of HS1- and WASp-deficient DCs were related, but distinct. WASp(-/y) DCs migrating in a chemokine gradient showed a large decrease in velocity and diminished directional persistence. In contrast, HS1(-/-) DCs migrated faster than wild-type cells, but directional persistence was significantly reduced. These studies show that HS1 functions in concert with WASp to fine-tune DC cytoarchitecture and direct cell migration.

IL-2 induces a WAVE2-dependent pathway for actin reorganization that enables WASp-independent human NK cell function

Wiskott-Aldrich syndrome (WAS) is a primary immunodeficiency associated with an increased susceptibility to herpesvirus infection and hematologic malignancy as well as a deficiency of NK cell function. It is caused by defective WAS protein (WASp). WASp facilitates filamentous actin (F-actin) branching and is required for F-actin accumulation at the NK cell immunological synapse and NK cell cytotoxicity ex vivo. Importantly, the function of WASp-deficient NK cells can be restored in vitro after exposure to IL-2, but the mechanisms underlying this remain unknown. Using a WASp inhibitor as well as cells from patients with WAS, we have defined a direct effect of IL-2 signaling upon F-actin that is independent of WASp function. We found that IL-2 treatment of a patient with WAS enhanced the cytotoxicity of their NK cells and the F-actin content at the immunological synapses formed by their NK cells. IL-2 stimulation of NK cells in vitro activated the WASp homolog WAVE2, which was required for inducing WASp-independent NK cell function, but not for baseline activity. Thus, WAVE2 and WASp define parallel pathways to F-actin reorganization and function in human NK cells; although WAVE2 was not required for NK cell innate function, it was accessible through adaptive immunity via IL-2. These results demonstrate how overlapping cytoskeletal activities can utilize immunologically distinct pathways to achieve synonymous immune function.

Role of c-Abl in L-selectin shedding from the neutrophil surface

L-selectin is a key molecule that participates in neutrophil tethering and subsequent rolling. It is cleaved from the surface of neutrophils activated in the presence of lipopolysaccharides, N-formyl-methionine-leucine-phenylalanine (fMLP), or Interleukin-8 (IL-8). We previously showed that L-selectin is also shed from the neutrophil surface during rolling on sialyl Lewis-x coated surfaces in a force-, ADAM-17 sheddase-, and p38 MAP kinase-dependent manner under flow. c-Abl tyrosine kinase is phosphorylated when L-selectin on the surface of neutrophils is cross-linked with anti-L-selectin antibodies. Here, we study the effect of c-Abl inhibition on L-selectin shedding from primary human neutrophils in static conditions following exposure to fMLP, IL-8, and hypotonic buffer and under flow through sialyl Lewis-x coated microtubes. Results indicate that c-Abl inhibition by STI571 significantly affects neutrophil adhesion via L-selectin, by decreasing the average rolling velocity and increasing the flux of rolling cells. The change in surface receptor expression was verified by flow cytometry. Interestingly, other forms of L-selectin shedding induced by fMLP, IL-8 or osmotic swelling were unaffected by STI571 treatment. These findings implicate the c-Abl signaling molecule in regulating L-selectin mechanical shedding in response to shear stress, setting this type of signaling apart from those triggered by the presence of a hypotonic environment, fMLP, or IL-8. This study sheds light on the role of c-Abl in neutrophil adhesion not previously reported in the literature.

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.

F-actin-binding domain of c-Abl regulates localized phosphorylation of C3G: role of C3G in c-Abl-mediated cell death

The c-Abl tyrosine kinase maintains cellular homeostasis through its ability to regulate apoptosis and actin dynamics. In vivo, c-Abl activity is stringently regulated and mechanisms involved are not fully understood. Here, we identified the Rap1 guanine nucleotide exchange factor, C3G (RapGEF1), as a substrate and an effector of c-Abl-mediated functions. Ectopic expression of c-Abl in mammalian cell lines, known to induce apoptosis, resulted in phosphorylation of endogenous C3G on Y504 coincident with cell detachment and chromatin condensation. Phosphorylation of C3G coincided with restricted c-Abl activation in regions rich in actin, and was dependent on cellular F-actin dynamics. Unlike C3G or c-Abl, p-C3G was resistant to detergent extraction, suggesting its enhanced affinity for the cytoskeleton. Localized C3G phosphorylation and coincidence with cells undergoing cell death was dependent on F-actin-binding domain (FABD) of c-Abl. Activation of endogenous c-Abl by oxidative stress was associated with phosphorylation of cellular C3G on Y504. Inhibition of C3G expression and function using RNAi or dominant-negative approaches inhibited c-Abl-mediated cell death. These findings identify C3G as a novel target of c-Abl and also show that FABD of c-Abl is essential for regulation of its restricted activation to induce apoptosis.