Phosphorylation- and activation-independent association of the tyrosine kinase Syk and the tyrosine kinase substrates Cbl and Vav with tubulin in B-cells

Tubulin Complexity in Cancer and Metastasis

Tubulin plays a fundamental role in cellular function and as the subject for microtubule-active agents in the treatment of ovarian cancer. Microtubule-binding proteins (e.g., tau, MAP1/2/4, EB1, CLIP, TOG, survivin, stathmin) and posttranslational modifications (e.g., tyrosination, deglutamylation, acetylation, glycation, phosphorylation, polyamination) further diversify tubulin functionality and may permit additional opportunities to understand microtubule behavior in disease and to develop microtubule-modifying approaches to combat ovarian cancer. Tubulin-based structures that project from suspended ovarian cancer cells known as microtentacles may contribute to metastatic potential of ovarian cancer cells and could represent an exciting novel therapeutic target.

Tubulin Post-Translational Modifications and Microtubule Dynamics

Microtubules are hollow tube-like polymeric structures composed of α,β-tubulin heterodimers. They play an important role in numerous cellular processes, including intracellular transport, cell motility and segregation of the chromosomes during cell division. Moreover, microtubule doublets or triplets form a scaffold of a cilium, centriole and basal body, respectively. To perform such diverse functions microtubules have to differ in their properties. Post-translational modifications are one of the factors that affect the properties of the tubulin polymer. Here we focus on the direct and indirect effects of post-translational modifications of tubulin on microtubule dynamics.

Leukaemic cells from chronic lymphocytic leukaemia patients undergo apoptosis following microtubule depolymerization and Lyn inhibition by nocodazole

Functional abnormalities of chronic lymphocytic leukaemia (CLL) cells may be related to the microtubular network of cell cytoskeleton; specifically tubulin involvement in cells after B-cell receptor engagement. As microtubule inhibitors could represent a therapeutic strategy for CLL, this study investigated the capability of nocodazole, a synthetic depolymerizing agent, to kill CLL leukaemic cells. We demonstrated that nocodazole was highly specific for the in vitro induction of apoptosis in leukaemic cells from 90 CLL patients, without affecting the viability of T-cells and/or mesenchymal stromal cells (MSCs) recovered from the same patients. Nocodazole was observed to overcome the pro-survival signals provided by MSCs. Competing with ATP for the nucleotide-binding site, nocodazole has been observed to turn off the high basal tyrosine phosphorylation of leukaemic cells mediated by the Src-kinase Lyn. Considering that most anti-microtubule drugs have limited clinical use because of their strong toxic effects, the high selectivity of nocodazole for leukaemic cells in CLL and its capability to bypass microenvironmental pro-survival stimuli, suggests the use of this inhibitor for designing new therapeutic strategies in CLL treatment.

Identification of direct tyrosine kinase substrates based on protein kinase assay-linked phosphoproteomics

Protein kinases are implicated in multiple diseases such as cancer, diabetes, cardiovascular diseases, and central nervous system disorders. Identification of kinase substrates is critical to dissecting signaling pathways and to understanding disease pathologies. However, methods and techniques used to identify bona fide kinase substrates have remained elusive. Here we describe a proteomic strategy suitable for identifying kinase specificity and direct substrates in high throughput. This approach includes an in vitro kinase assay-based substrate screening and an endogenous kinase dependent phosphorylation profiling. In the in vitro kinase reaction route, a pool of formerly phosphorylated proteins is directly extracted from whole cell extracts, dephosphorylated by phosphatase treatment, after which the kinase of interest is added. Quantitative proteomics identifies the rephosphorylated proteins as direct substrates in vitro. In parallel, the in vivo quantitative phosphoproteomics is performed in which cells are treated with or without the kinase inhibitor. Together, proteins phosphorylated in vitro overlapping with the kinase-dependent phosphoproteome in vivo represents the physiological direct substrates in high confidence. The protein kinase assay-linked phosphoproteomics was applied to identify 25 candidate substrates of the protein-tyrosine kinase SYK, including a number of known substrates and many novel substrates in human B cells. These shed light on possible new roles for SYK in multiple important signaling pathways. The results demonstrate that this integrated proteomic approach can provide an efficient strategy to screen direct substrates for protein tyrosine kinases.

SLI-1 Cbl inhibits the engulfment of apoptotic cells in C. elegans through a ligase-independent function

The engulfment of apoptotic cells is required for normal metazoan development and tissue remodeling. In Caenorhabditis elegans, two parallel and partially redundant conserved pathways act in cell-corpse engulfment. One pathway, which includes the small GTPase CED-10 Rac and the cytoskeletal regulator ABI-1, acts to rearrange the cytoskeleton of the engulfing cell. The CED-10 Rac pathway is also required for proper migration of the distal tip cells (DTCs) during the development of the C. elegans gonad. The second pathway includes the receptor tyrosine kinase CED-1 and might recruit membranes to extend the surface of the engulfing cell. Cbl, the mammalian homolog of the C. elegans E3 ubiquitin ligase and adaptor protein SLI-1, interacts with Rac and Abi2 and modulates the actin cytoskeleton, suggesting it might act in engulfment. Our genetic studies indicate that SLI-1 inhibits apoptotic cell engulfment and DTC migration independently of the CED-10 Rac and CED-1 pathways. We found that the RING finger domain of SLI-1 is not essential to rescue the effects of SLI-1 deletion on cell migration, suggesting that its role in this process is ubiquitin ligase-independent. We propose that SLI-1 opposes the engulfment of apoptotic cells via a previously unidentified pathway.

Cytoskeleton in mast cell signaling

Mast cell activation mediated by the high affinity receptor for IgE (FcεRI) is a key event in allergic response and inflammation. Other receptors on mast cells, as c-Kit for stem cell factor and G protein-coupled receptors (GPCRs) synergistically enhance the FcεRI-mediated release of inflammatory mediators. Activation of various signaling pathways in mast cells results in changes in cell morphology, adhesion to substrate, exocytosis, and migration. Reorganization of cytoskeleton is pivotal in all these processes. Cytoskeletal proteins also play an important role in initial stages of FcεRI and other surface receptors induced triggering. Highly dynamic microtubules formed by αβ-tubulin dimers as well as microfilaments build up from polymerized actin are affected in activated cells by kinases/phosphatases, Rho GTPases and changes in concentration of cytosolic Ca(2+). Also important are nucleation proteins; the γ-tubulin complexes in case of microtubules or Arp 2/3 complex with its nucleation promoting factors and formins in case of microfilaments. The dynamic nature of microtubules and microfilaments in activated cells depends on many associated/regulatory proteins. Changes in rigidity of activated mast cells reflect changes in intermediate filaments build up from vimentin. This review offers a critical appraisal of current knowledge on the role of cytoskeleton in mast cells signaling.

The Rac activator Tiam1 is required for polarized protrusional outgrowth of primary astrocytes by affecting the organization of the microtubule network

Polarized cell migration is a crucial process in the development and repair of tissues, as well as in pathological conditions, including cancer. Recent studies have elucidated important roles for Rho GTPases in the establishment and maintenance of polarity prior to and during cell migration. Here, we show that Tiam1, a specific activator of the small GTPase Rac, is required for the polarized outgrowth of protrusions in primary astrocytes during the initial phase of cell polarization after scratch-wounding monolayers of cells. Tiam1 deficiency delays closure of wounds in confluent monolayers. Lack of Tiam1 impairs adoption of an asymmetrical cell shape as well as microtubule organization within protrusions. Positioning of the centrosome and Golgi apparatus, however, are independent of Tiam1-Rac signaling. We speculate that the function of Tiam1 in polarized outgrowth of astrocyte protrusions involves regulation of microtubule organization, possibly by stabilizing the microtubule cytoskeleton. Our results add Tiam1 as a player to the growing list of proteins involved in polarized outgrowth of protrusions and further elucidate the signaling pathways leading to cell polarization.

Docetaxel suppresses invasiveness of head and neck cancer cells in vitro

The combination of docetaxel, cisplatin, and fluorouracil significantly enhances the survival of head and neck cancer patients compared to cisplatin and fluorouracil. We hypothesized that docetaxel may affect invasiveness of the head and neck cancer cells in addition to its tumor-killing effect. Two different head and neck cancer cell lines (HEp-2 and Ca9-22) were treated with docetaxel at IC(10) and IC(50) concentrations. Cell migration and invasive growth was evaluated by wound healing assay and three-dimensional (3D) culture of multicellular tumor spheroids, respectively. Expression levels of possible downstream effectors for cell migration/invasiveness were measured by immunoblotting in conditions with or without docetaxel. Docetaxel, but not cisplatin, suppressed filopodia formation compared with no treatment (control) condition. Consistent with this, docetaxel suppressed two-dimensional (2D) cell migration and 3D cell invasion compared with control or cisplatin. Only docetaxel treated cells exhibited thick tubulin bundle and had lower activity of Cdc42, a member of the Rho family of small GTPases. In conclusion, Docetaxel treatment suppressed migration and invasiveness of head and neck cancer cells in vitro, which is likely to be mediated by regulating Cdc42 activity.

The novel tubulin-targeting agent pyrrolo-1,5-benzoxazepine-15 induces apoptosis in poor prognostic subgroups of chronic lymphocytic leukemia

Pyrrolo-1,5-benzoxazepine-15 (PBOX-15) is a novel microtubule depolymerization agent that induces cell cycle arrest and subsequent apoptosis in a number of cancer cell lines. Chronic lymphocytic leukemia (CLL) is characterized by clonal expansion of predominately nonproliferating mature B cells. Here, we present data suggesting PBOX-15 is a potential therapeutic agent for CLL. We show activity of PBOX-15 in samples taken from a cohort of CLL patients (n = 55) representing both high-risk and low-risk disease. PBOX-15 exhibited cytotoxicity in CLL cells (n = 19) in a dose-dependent manner, with mean IC(50) of 0.55 micromol/L. PBOX-15 significantly induced apoptosis in CLL cells (n = 46) including cells with poor prognostic markers: unmutated IgV(H) genes, CD38 and zeta-associated protein 70 (ZAP-70) expression, and fludarabine-resistant cells with chromosomal deletions in 17p. In addition, PBOX-15 was more potent than fludarabine in inducing apoptosis in fludarabine-sensitive cells. Pharmacologic inhibition and small interfering RNA knockdown of caspase-8 significantly inhibited PBOX-15-induced apoptosis. Pharmacologic inhibition of c-jun NH(2)-terminal kinase inhibited PBOX-15-induced apoptosis in mutated IgV(H) and ZAP-70(-) CLL cells but not in unmutated IgV(H) and ZAP-70(+) cells. PBOX-15 exhibited selective cytotoxicity in CLL cells compared with normal hematopoietic cells. Our data suggest that PBOX-15 represents a novel class of agents that are toxic toward both high-risk and low-risk CLL cells. The need for novel treatments is acute in CLL, especially for the subgroup of patients with poor clinical outcome and drug-resistant disease. This study identifies a novel agent with significant clinical potential.

LC/MS identification of 12 intracellular cytoskeletal and inflammatory proteins from monocytes adherent on surface-adsorbed fibronectin-derived peptides

The extent and duration of the host response determines device efficacy, yet the mechanism is poorly understood. U937 promonocytic cells were cultured on peptide-adsorbed tissue-culture polystyrene to better understand surface-modulated intracellular events. Phosphotyrosine proteins were enriched by immunoprecipitation and analyzed by nanospray HPLC-coupled tandem mass spectrometry (LC/MS). Tyrosine-phosphorylated proteins were chosen based on physiological significance and previous densitometry results, which identified a set of proteins ranging from approximately 200 to approximately 23 kDa showing altered phosphorylation levels in response to various surface-adsorbed ligands and phosphorylation inhibitor AG18. Although LC/MS has been used for nearly a decade, its application to the field of biomaterials is relatively novel. Twelve intracellular proteins identified by nanospray LC/MS are potentially related to the host response. Eight of the twelve proteins are related to the cytoskeleton including: moesin, heat shock protein 90beta, alpha-tubulin, elongation factor 1alpha, beta actin, vimentin, plasminogen activator inhibitor 2, and heterogeneous ribonuclear protein A2. The remaining four proteins: high mobility group box 1, caspase recruitment domain 5, glycoprotein 96, and heterogeneous nuclear ribonucleoprotein D0 modulate inflammation. The specific effect each peptide has upon modulating the phosphorylation state of these proteins cannot be determined from this work; however, 12 viable targets have been identified for further investigation into the role each plays in the surface-mediated monocyte response.

The Cbl interactome and its functions

Cbl proteins are ubiquitin ligases and multifunctional adaptor proteins that are implicated in the regulation of signal transduction in various cell types and in response to different stimuli. Cbl-associated proteins can assemble together at a given time or space inside the cell, and such an interactome can form signal competent networks that control many physiological processes. Dysregulation of spatial or temporal constraints in the Cbl interactome results in the development of human pathologies such as immune diseases, diabetes and cancer.

Rho-kinase modulates the function of STEF, a Rac GEF, through its phosphorylation

Rho family GTPases are key regulators of various physiological processes. Several recent studies indicated that the antagonistic relationship between Rho and Rac is essential for cell polarity and that the Rac activity is negatively regulated by Rho. In this study, we found that Rho-kinase, an effector of Rho, counteracted the Rac GEF STEF-induced Rac1 activation in COS7 cells. Rho-kinase phosphorylated STEF at Thr1662 in vitro, and Y-27632, a Rho-kinase inhibitor, suppressed lysophosphatidic acid-induced phosphorylation of STEF in PC12D cells. STEF interacted with specific molecules such as microtubule-associated protein 1B, and the phosphorylation of STEF by Rho-kinase diminished its interaction with these molecules. STEF promoted nerve growth factor-induced neurite outgrowth in PC12D cells, while the phosphomimic mutant of STEF had a weakened ability to enhance neurite outgrowth. Taken together, these results suggest that the phosphorylation of STEF by Rho-kinase exerts the inhibitory effect on the function of STEF.

Regulation of microtubule formation in activated mast cells by complexes of gamma-tubulin with Fyn and Syk kinases

Aggregation of the high-affinity IgE receptors (FcepsilonRIs) on the surface of granulated mast cells initiates a chain of signaling events culminating in the release of allergy mediators. Although microtubules are involved in mast cell degranulation, the molecular mechanism that controls microtubule rearrangement after FcepsilonRI triggering is poorly understood. In this study, we show that the activation of bone marrow-derived mast cells (BMMCs) induced by FcepsilonRI aggregation or treatment with pervanadate leads to a rapid polymerization of microtubules. This polymerization was not dependent on the presence of Lyn kinase as determined by experiments with BMMCs isolated from Lyn-negative mice. One of the key regulators of microtubule polymerization is gamma-tubulin. Immunoprecipitation experiments revealed that gamma-tubulin from activated cells formed complexes with Fyn and Syk protein tyrosine kinases and several tyrosine phosphorylated proteins from both wild-type and Lyn(-/-) BMMCs. Pretreatment of the cells with Src-family or Syk-family selective tyrosine kinase inhibitors, PP2 or piceatannol, respectively, inhibited the formation of microtubules and reduced the amount of tyrosine phosphorylated proteins in gamma-tubulin complexes, suggesting that Src and Syk family kinases are involved in the initial stages of microtubule formation. This notion was corroborated by pull-down experiments in which gamma-tubulin complex bounds to the recombinant Src homology 2 and Src homology 3 domains of Fyn kinase. We propose that Fyn and Syk kinases are involved in the regulation of binding properties of gamma-tubulin and/or its associated proteins, and thus modulate the microtubule nucleation in activated mast cells.

Nocodazole, a microtubule depolymerising agent, induces apoptosis of chronic lymphocytic leukaemia cells associated with changes in Bcl-2 phosphorylation and expression

Microtubule active drugs are used in the treatment of malignancies and their mechanism of action in cycling cells is to produce mitotic arrest followed by apoptosis. In this study, we investigate in detail the specificity and mechanism by which a microtubule de-polymerising agent, nocodazole, induces apoptosis in non-cyclingm, i.e. G(0)/G(1), chronic lymphocytic leukaemia (CLL) B-cells. The majority of cases of CLL are sensitive (IC(50)<or=16 microM) but normal peripheral blood B-cells, which are also in G(0)/G(1), are resistant to the maximum in vitro concentration of this agent. Taxol, a microtubule stabilising drug does not kill CLL cells suggesting a specific effect of nocodazole. The mechanism of apoptosis involves mitochondrial membrane depolarisation, activation of caspases and cleavage of PARP. Nocodazole causes two patterns of change to Bcl-2 expression. In one there is increase in expression of the serine-70 phosphorylated form of Bcl-2 and in the other total Bcl-2 expression is reduced. Collectively the data shows that sensitivity to nocodazole-induced apoptosis is a feature of chronic lymphocytic leukaemia and suggests that newer microtubule active agents be systematically investigated for their effectiveness in this condition.

The Syk tyrosine kinase: a new negative regulator in tumor growth and progression

The spleen tyrosine kinase Syk was long thought to be a hematopoietic cell-specific signaling molecule. Recent evidence demonstrated that it is also expressed by many non-hematopoietic cell types and that it plays a negative role in cancer. A significant drop in its expression was first observed during breast cancer progression, but an anomalous Syk expression has now also been evidenced in many other tumor types. Mechanistic studies using Syk re-expression demonstrated its suppressive function in tumorigenesis and metastasis formation, which is surprising for a tyrosine kinase. Loss of Syk expression is regulated, albeit not exclusively, by its promoter hypermethylation. The molecular mechanism of its tumor-suppressive function remains largely unknown; the identification of its activators and effectors in non-hematopoietic cells will be a challenge for the years to come. An increasing number of clinical studies reveal a correlation between reduced Syk expression and an increased risk for metastasis formation, and assign Syk as a potential new prognostic marker in different tumor types.

Vav1 and Vav2 play different roles in macrophage migration and cytoskeletal organization

Vav family proteins act as guanine nucleotide exchange factors for Rho family proteins, which are known to orchestrate cytoskeletal changes and cell migration in response to extracellular stimuli. Using mice deficient for Vav1, Vav2 and/or Vav3, overlapping and isoform-specific functions of the three Vav proteins have been described in various hematopoietic cell types, but their roles in regulating cell morphology and migration have not been studied in detail. To investigate whether Vav isoforms have redundant or unique functions in regulating adhesion and migration, we investigated the properties of Vav1-deficient and Vav2-deficient macrophages. Both Vav1-deficient and Vav2-deficient cells have a smaller adhesive area; yet, only Vav1-deficient cells have a reduced migration speed, which coincides with a lower level of microtubules. Vav2-deficient macrophages display a high level of constitutive membrane ruffling, but neither Vav1 nor Vav2 is required for colony stimulating factor-1-induced membrane ruffling and cell spreading. Our results suggest that the migration speed of macrophages is regulated independently of spread area or membrane ruffling and that Vav1 is selectively required to maintain a normal migration speed.

The multidomain protooncogenic protein c-Cbl binds to tubulin and stabilizes microtubules

The protooncogenic protein c-Cbl is known to regulate the actin cytoskeleton. In this study, we present results indicating that c-Cbl can also regulate the microtubular network. We have shown that c-Cbl binds to tubulin and microtubules through its tyrosine kinase binding (TKB) domain. However, the character of the interactions described in this report is novel, since the G306E mutation, which disrupts the ability of c-Cbl's TKB to bind to tyrosine-phosphorylated proteins, does not affect the observed interaction between c-Cbl and microtubules. Furthermore, overexpression of c-Cbl in human pulmonary artery endothelial cells and COS-7 cells leads to microtubule stabilization. We demonstrate that this effect of c-Cbl is mediated by TKB, and, like c-Cbl binding to microtubules, is independent of the ability of TKB to bind to tyrosine-phosphorylated proteins. Finally, we have shown that c-Cbl directly polymerizes microtubules in vitro, and that TKB is necessary and sufficient for this effect of c-Cbl. In this last phenomenon, as well as in the previous ones, the effect of TKB is not sensitive to the inactivating G306E mutation. Overall, the results presented in this report suggest a novel function for c-Cbl-microtubule binding and stabilization.

PAK4 mediates morphological changes through the regulation of GEF-H1

Precise spatial and temporal regulation of Rho GTPases is required in controlling F-actin-based changes in cell morphology. The molecular mechanisms through which microtubules (MTs) modulate the activity of RhoGTPases and regulate the actin cytoskeleton are unclear. Here we show that p21-activated-kinase 4 (PAK4) mediates morphological changes through its association with the Rho-family guanine nucleotide exchange factor (GEF), GEF-H1. We show that this association is dependent upon a novel GEF-H1 interaction domain (GID) within PAK4. Further, we show that PAK4-mediated phosphorylation of Ser810 acts as a switch to block GEF-H1-dependent stress fiber formation while promoting the formation of lamellipodia in NIH-3T3 cells. We found that the endogenous PAK4-GEF-H1 complex associates with MTs and that PAK4 phosphorylation of MT-bound GEF-H1 releases it into the cytoplasm of NIH-3T3 cells, which coincides with the dissolution of stress fibers. Our observations propose a novel role for PAK4 in GEF-H1-dependent crosstalk between MTs and the actin cytoskeleton.

Progressive loss of Syk and abnormal proliferation in breast cancer cells

The tumor suppressor gene Syk tyrosine kinase is absent or reduced in invasive breast cancer tissues and cell lines; its loss in breast tissues is linked to poor prognosis and metastasis. Also, evidence shows that in vitro Syk is involved in regulating proliferation. Here, we show by in situ hybridization on breast tissue sections that the loss of Syk expression is progressive during tumor development. Strikingly, Syk is already partially lost in normal epithelial tissue adjacent to the cancer lesion. In vivo, cell proliferation (as measured by the proliferative index Ki67) increased from normal to ductal carcinoma in situ to invasive, whereas Syk in situ staining in the same tissues decreased. In vitro, the presence of Syk was associated with reduced cell proliferation in an epidermal growth factor receptor-overexpressing breast cancer cell line, BT549, whereas changes in apoptosis were undetected. Concomitantly, the kinase activity of the proto-oncogene Src was reduced by approximately 30%. A 5-fold increase in abnormal mitoses was observed in the Syk-transfected cells compared with vector control. We propose that Syk is involved in the regulation of cell proliferation, possibly by controlling mechanisms of mitosis and cytokinesis via Src signal transduction pathway(s). Because of its progressive and early loss during tumor onset and development, monitoring of Syk loss in breast epithelial cells by noninvasive techniques such as ductal lavage may be a powerful tool for screening purposes.

Rac2 specificity in macrophage integrin signaling: potential role for Syk kinase

Herein we report that, despite the similarity of Rac2 to Rac1 (92% amino acid identity), macrophages derived from Rac2-/- mice, which continue to express Rac1, display a marked defect in alphavbeta3/alphavbeta5 and alpha4beta1 integrin-directed migration measured on vitronectin and fibronectin fragments (FN-H296), respectively. In contrast, mouse embryo fibroblasts derived from the Rac2 knockout mice utilize Rac1 for migration via alphavbeta3/alphavbeta5 and alpha4beta1. The genetic reconstitution of bone marrow-derived macrophages (BMM) with Rac2 restores the integrin-dependent migration of Rac2-deficient macrophages on vitronectin (VN) and FN-H296. The levels of GTP-Rac2 generated upon specific integrin engagement in wild type macrophages parallels the phenotypic defect observed in Rac2-deficient macrophages; i.e. FN-H296, alpha4beta1 > VN, alphavbeta3/alphavbeta5 > FN-CH271, alpha5beta1 > intact FN. In a COS7 cell system, the expression of Syk kinase alone is sufficient to convert the alpha4beta1 migration response to Rac2 dependence. Therefore, we present the first evidence that the alpha4beta1 receptor in blood cells has evolved a Syk-Rac2 signaling axis to transmit signals required for integrin-directed migration suggesting that Syk kinase in part encodes myeloid Rac2 specificity in vivo.

Microtubule-actin cross-talk at focal adhesions

Focal adhesions are dynamic structures in which traction forces are exerted against the substratum during cell migration and are sites for the organization of signaling complexes. Palazzo and Gundersen discuss how focal adhesions may also be the site of cross-talk between the actin-based and microtubule-based cytoskeletons. Microtubules appear to deliver factors that can regulate the formation and dissolution of focal adhesions, whereas focal adhesions contribute to microtubule localization and stability.

Prolactin receptor signal transduction

Within the immune system, multiple isoforms of the human prolactin receptor (PRLr) serve to mediate the effects of its ligand (PRL). Now numbering four, these isoforms are structurally and functionally distinct, demonstrating significant differences in ligand affinities, kinetics of transduction and the transduction proteins activated. The proximal transduction pathways activated during PRLr-associated signaling include the tyrosine kinases Jak2, Fyn and Tec, the phosphatase SHP-2, the guanine nucleotide exchange factor Vav, and the signaling suppressor SOCS. Differential activation of these pathways may contribute to the pleiotropism of PRL action in tissues of the immune system.

Beyond the RING: CBL proteins as multivalent adapters

Following discovery of c-Cbl, a cellular form of the transforming retroviral protein v-Cbl, multiple Cbl-related proteins have been identified in vertebrate and invertebrate organisms. c-Cbl and its homologues are capable of interacting with numerous proteins involved in cell signaling, including various molecular adapters and protein tyrosine kinases. It appears that Cbl proteins play several functional roles, acting both as multivalent adapters and inhibitors of various protein tyrosine kinases. The latter function is linked, to a substantial extent, to the E3 ubiquitin-ligase activity of Cbl proteins. Experimental evidence for these functions, interrelations between them, and their biological significance are addressed in this review, with the main accent placed on the adapter functions of Cbl proteins.

Microtubule-dependent formation of podosomal adhesion structures in primary human macrophages

Podosomes are unique actin-rich adhesion structures of monocyte-derived cells such as macrophages and osteoclasts. They clearly differ from other substratum-contacting organelles like focal adhesions in morphological and functional regards. Formation of podosomes has been shown to be dependent on the small GTPase CDC42Hs and its effector Wiskott-Aldrich syndrome protein (WASp). In this study, we investigated the functional relation between podosomes and the microtubule system in primary human macrophages. We demonstrate that, in contrast to focal adhesions, assembly of podosomes in macrophages and their monocytic precursors is dependent on an intact microtubule system. In contrast, experiments using Wiskott-Aldrich syndrome (WAS) macrophages indicate that the microtubule system is not reciprocally dependent on podosomes. A potential linker between podosomes and microtubules may be WASp itself, considering that microinjection of the WASp polyproline domain prevents podosome reassembly. This polyproline domain is thought to link WASp to microtubules via CDC42 interacting protein 4 (CIP4). Consistently, macrophages microinjected with CIP4 constructs deficient in either the microtubule- or the WASp-binding domain also fail to reassemble podosomes. In sum, our findings show that microtubules are essential for podosome formation in primary human macrophages and that WASp and CIP4 may be involved in this phenomenon.

Syk-dependent phosphorylation of microtubules in activated B-lymphocytes

Syk is a protein-tyrosine kinase that is essential for B-lymphocyte development and B-cell signaling. Syk phosphorylates tubulin on tyrosine both in vitro and in intact lymphocytes. Here we show that (alpha)-tubulin present within the cytoskeletal microtubule network was phosphorylated in a Syk-dependent manner following the activation of B-cells by engagement of the B-cell antigen receptor or by treatment with the phosphotyrosine phosphatase inhibitor, pervanadate. Immunofluorescence staining of microtubule cytoskeletons and western blotting studies with antibodies to phosphotyrosine confirmed the phosphorylation of polymerized tubulin in Syk-expressing, but not Syk-deficient, cells. At low concentrations of pervanadate, centrosomes appeared to be preferentially tyrosine-phosphorylated. Tubulin phosphorylated to a high stoichiometry on tyrosine assembled into microtubules in vitro, and preassembled microtubules were also phosphorylated by Syk kinase in vitro. Thus, Syk has the capacity to interact with microtubule networks within the B-lymphocyte and catalyzes the phosphorylation of the (alpha)-tubulin subunit. Syk-dependent phosphorylation of microtubules may affect the ability of the microtubule cytoskeleton to serve as a platform upon which signaling complexes are assembled.

Observing FcepsilonRI signaling from the inside of the mast cell membrane

We have determined the membrane topography of the high-affinity IgE receptor, FcstraightepsilonRI, and its associated tyrosine kinases, Lyn and Syk, by immunogold labeling and transmission electron microscopic (TEM) analysis of membrane sheets prepared from RBL-2H3 mast cells. The method of Sanan and Anderson (Sanan, D.A., and R.G.W. Anderson. 1991. J. Histochem. Cytochem. 39:1017-1024) was modified to generate membrane sheets from the dorsal surface of RBL-2H3 cells. Signaling molecules were localized on the cytoplasmic face of these native membranes by immunogold labeling and high-resolution TEM analysis. In unstimulated cells, the majority of gold particles marking both FcepsilonRI and Lyn are distributed as small clusters (2-9 gold particles) that do not associate with clathrin-coated membrane. Approximately 25% of FcepsilonRI clusters contain Lyn. In contrast, there is essentially no FcepsilonRI-Syk colocalization in resting cells. 2 min after FcepsilonRI cross-linking, approximately 10% of Lyn colocalizes with small and medium-sized FcepsilonRI clusters (up to 20 gold particles), whereas approximately 16% of Lyn is found in distinctive strings and clusters at the periphery of large receptor clusters (20-100 gold particles) that form on characteristically osmiophilic membrane patches. While Lyn is excluded, Syk is dramatically recruited into these larger aggregates. The clathrin-coated pits that internalize cross-linked receptors bud from membrane adjacent to the Syk-containing receptor complexes. The sequential association of FcstraightepsilonRI with Lyn, Syk, and coated pits in topographically distinct membrane domains implicates membrane segregation in the regulation of FcstraightepsilonRI signaling.

Cytoskeletal polarization and redistribution of cell-surface molecules during T cell antigen recognition

T cell antigen recognition is accompanied by cytoskeletal polarization towards the APC and large-scale redistribution of cell surface molecules into 'supramolecular activation clusters' (SMACs), forming an organized contact interface termed the 'immunological synapse' (IS). Molecules are arranged in the IS in a micrometer scale bull's eye pattern with a central accumulation of TCR/peptide-MHC (the cSMAC) surrounded by a peripheral ring of adhesion molecules (the pSMAC). We propose that segregation of cell surface molecules on a much smaller scale initiates TCR triggering, which drives the formation of the IS by active transport processes. IS formation may function as a checkpoint for full T cell activation, integrating information on the presence and quality of TCR ligands and the nature and activation state of the APC.

Inhibition of Signaling Through the B Cell Antigen Receptor by the Protooncogene Product, c-Cbl, Requires Syk Tyrosine 317 and the c-Cbl Phosphotyrosine-Binding Domain

The Syk protein-tyrosine kinase couples the B cell Ag receptor (BCR) to intracellular biochemical pathways. Syk becomes phosphorylated on multiple tyrosine residues upon receptor cross-linking. Tyrosine 317 is a site of phosphorylation located within the linker region of Syk that separates the amino-terminal, tandem pair of SH2 domains from the carboxyl-terminal catalytic domain. The amino acid sequence surrounding phosphotyrosine 317 matches the consensus sequence for recognition by the phosphotyrosine-binding (PTB) domain of the protooncogene product, c-Cbl. The overexpression of c-Cbl in DT40 B cells inhibits Ag receptor-mediated activation of the NF-AT transcription factor. The ability of overexpressed c-Cbl to inhibit signaling requires both Syk tyrosine 317 and a functional c-Cbl PTB domain. Mutant forms of Syk lacking tyrosine 317 exhibit an enhanced ability to couple the BCR to pathways leading to the activation of both NF-AT and Elk-1. Coimmunoprecipitation experiments indicate that Syk phosphotyrosine 317 and the c-Cbl PTB domain enhance, but are not required for, all interactions between these two proteins. In unstimulated cells, c-Cbl and Syk can be isolated in a complex that also contains tubulin. A mutant form of Syk lacking tyrosine at position 317 exhibits an enhanced ability to interact with a diphosphopeptide modeled on the immunoreceptor tyrosine-based activation motif of the CD79a component of the Ag receptor. These studies indicate that c-Cbl may contribute to the regulation of BCR signaling by modulating the ability of Syk to associate with the BCR and couple the receptor to intracellular signaling pathways.

Microtubule targeting of substrate contacts promotes their relaxation and dissociation

We recently showed that substrate contact sites in living fibroblasts are specifically targeted by microtubules (Kaverina, I., K. Rottner, and J.V. Small. 1998. J. Cell Biol. 142:181-190). Evidence is now provided that microtubule contact targeting plays a role in the modulation of substrate contact dynamics. The results are derived from spreading and polarized goldfish fibroblasts in which microtubules and contact sites were simultaneously visualized using proteins conjugated with Cy-3, rhodamine, or green fluorescent protein. For cells allowed to spread in the presence of nocodazole the turnover of contacts was retarded, as compared with controls and adhesions that were retained under the cell body were dissociated after microtubule reassembly. In polarized cells, small focal complexes were found at the protruding cell front and larger adhesions, corresponding to focal adhesions, at the retracting flanks and rear. At retracting edges, multiple microtubule contact targeting preceded contact release and cell edge retraction. The same effect could be observed in spread cells, in which microtubules were allowed to reassemble after local disassembly by the application of nocodazole to one cell edge. At the protruding front of polarized cells, focal complexes were also targeted and as a result remained either unchanged in size or, more rarely, were disassembled. Conversely, when contact targeting at the cell front was prevented by freezing microtubule growth with 20 nM taxol and protrusion stimulated by the injection of constitutively active Rac, peripheral focal complexes became abnormally enlarged. We further found that the local application of inhibitors of myosin contractility to cell edges bearing focal adhesions induced the same contact dissociation and edge retraction as observed after microtubule targeting. Our data are consistent with a mechanism whereby microtubules deliver localized doses of relaxing signals to contact sites to retard or reverse their development. We propose that it is via this route that microtubules exert their well-established control on cell polarity.

Microtubule growth activates Rac1 to promote lamellipodial protrusion in fibroblasts

Microtubules are involved in actin-based protrusion at the leading-edge lamellipodia of migrating fibroblasts. Here we show that the growth of microtubules induced in fibroblasts by removal of the microtubule destabilizer nocodazole activates Rac1 GTPase, leading to the polymerization of actin in lamellipodial protrusions. Lamellipodial protrusions are also activated by the rapid growth of a disorganized array of very short microtubules induced by the microtubule-stabilizing drug taxol. Thus, neither microtubule shortening nor long-range microtubule-based intracellular transport is required for activating protrusion. We suggest that the growth phase of microtubule dynamic instability at leading-edge lamellipodia locally activates Rac1 to drive actin polymerization and lamellipodial protrusion required for cell migration.