Actin-binding protein 1 regulates B cell receptor-mediated antigen processing and presentation in response to B cell receptor activation

Regulation of Cx43 Gap Junction Intercellular Communication by Bruton's Tyrosine Kinase and Interleukin-2-Inducible T-Cell Kinase

T and B cell receptor signaling involves the activation of Akt, MAPKs, and PKC as well as an increase in intracellular Ca²⁺ and calmodulin activation. While these coordinate the rapid turnover of gap junctions, also implicated in this process is Src, which is not activated as part of T and B cell receptor signaling. An in vitro kinase screen identified that Bruton's tyrosine kinase (BTK) and interleukin-2-inducible T-cell kinase (ITK) phosphorylate Cx43. Mass spectroscopy revealed that BTK and ITK phosphorylate Cx43 residues Y247, Y265, and Y313, which are identical to the residues phosphorylated by Src. Overexpression of BTK or ITK in the HEK-293T cells led to increased Cx43 tyrosine phosphorylation as well as decreased gap junction intercellular communication (GJIC) and Cx43 membrane localization. In the lymphocytes, activation of the B cell receptor (Daudi cells) or T cell receptor (Jurkat cells) increased the BTK and ITK activity, respectively. While this led to increased tyrosine phosphorylation of Cx43 and decreased GJIC, the cellular localization of Cx43 changed little. We have previously identified that Pyk2 and Tyk2 also phosphorylate Cx43 at residues Y247, Y265, and Y313 with a similar cellular fate to that of Src. With phosphorylation critical to Cx43 assembly and turnover, and kinase expression varying between different cell types, there would be a need for different kinases to achieve the same regulation of Cx43. The work presented herein suggests that in the immune system, ITK and BTK have the capacity for the tyrosine phosphorylation of Cx43 to alter the gap junction function in a similar manner as Pyk2, Tyk2, and Src.

The Ins and Outs of Antigen Uptake in B cells

A review of our current knowledge of B cell antigen uptake mechanisms, the relevance of these processes to pathology, and outstanding questions in the field. Specific antigens induce B cell activation through the B cell receptor (BCR) which initiates downstream signaling and undergoes endocytosis. While extensive research has shed light on the signaling pathways in health and disease, the endocytic mechanisms remain largely uncharacterized. Given the importance of BCR-antigen internalization for antigen presentation in initiating adaptive immune responses and its role in autoimmunity and malignancy, understanding the molecular mechanisms represents critical, and largely untapped, potential therapeutics. In this review, we discuss recent advancements in our understanding of BCR endocytic mechanisms and the role of the actin cytoskeleton and post-translational modifications in regulating BCR uptake. We discuss dysregulated BCR endocytosis in the context of B cell malignancies and autoimmune disorders. Finally, we pose several outstanding mechanistic questions which will critically advance our understanding of the coordination between BCR endocytosis and B cell activation.

B Cells Adapt Their Nuclear Morphology to Organize the Immune Synapse and Facilitate Antigen Extraction

Upon interaction with immobilized antigens, B cells form an immune synapse where actin remodeling and re-positioning of the microtubule-organizing center (MTOC) together with lysosomes can facilitate antigen extraction. B cells have restricted cytoplasmic space, mainly occupied by a large nucleus, yet the role of nuclear morphology in the formation of the immune synapse has not been addressed. Here we show that upon activation, B cells re-orientate and adapt the size of their nuclear groove facing the immune synapse, where the MTOC sits, and lysosomes accumulate. Silencing the nuclear envelope proteins Nesprin-1 and Sun-1 impairs nuclear reorientation towards the synapse and leads to defects in actin organization. Consequently, B cells are unable to internalize the BCR after antigen activation. Nesprin-1 and Sun-1-silenced B cells also fail to accumulate the tethering factor Exo70 at the center of the synaptic membrane and display defective lysosome positioning, impairing efficient antigen extraction at the immune synapse. Thus, changes in nuclear morphology and positioning emerge as critical regulatory steps to coordinate B cell activation.

Endophilin A2 regulates B-cell endocytosis and is required for germinal center and humoral responses

Antigen‐specific B‐cell responses require endosomal trafficking to regulate antigen uptake and presentation to helper T cells, and to control expression and signaling of immune receptors. However, the molecular composition of B‐cell endosomal trafficking pathways and their specific roles in B‐cell responses have not been systematically investigated. Here, we report high‐throughput identification of genes regulating B‐cell receptor (BCR)‐mediated antigen internalization using genome‐wide functional screens. We show that antigen internalization depends both on constitutive, clathrin‐mediated endocytosis and on antigen‐induced, clathrin‐independent endocytosis mediated by endophilin A2. Although endophilin A2‐mediated endocytosis is dispensable for antigen presentation, it is selectively required for metabolic support of B‐cell proliferation, in part through regulation of iron uptake. Consequently, endophilin A2‐deficient mice show defects in GC B‐cell responses and production of high‐affinity IgG. The requirement for endophilin A2 highlights a unique importance of clathrin‐independent intracellular trafficking in GC B‐cell clonal expansion and antibody responses.

STING couples with PI3K to regulate actin reorganization during BCR activation

The adaptor protein, STING (stimulator of interferon genes), has been rarely studied in adaptive immunity. We used Sting KO mice and a patient's mutated STING cells to study the effect of STING deficiency on B cell development, differentiation, and BCR signaling. We found that STING deficiency promotes the differentiation of marginal zone B cells. STING is involved in BCR activation and negatively regulates the activation of CD19 and Btk but positively regulates the activation of SHIP. The activation of WASP and accumulation of F-actin were enhanced in Sting KO B cells upon BCR stimulation. Mechanistically, STING uses PI3K mediated by the CD19-Btk axis as a central hub for controlling the actin remodeling that, in turn, offers feedback to BCR signaling. Overall, our study provides a mechanism of how STING regulates BCR signaling via feedback from actin reorganization, which contributes to positive regulation of STING on the humoral immune response.

The regulators of BCR signaling during B cell activation

B lymphocytes produce antibodies under the stimulation of specific antigens, thereby exerting an immune effect. B cells identify antigens by their surface B cell receptor (BCR), which upon stimulation, directs the cell to activate and differentiate into antibody generating plasma cells. Activation of B cells via their BCRs involves signaling pathways that are tightly controlled by various regulators. In this review, we will discuss three major BCR mediated signaling pathways (the PLC-γ2 pathway, PI3K pathway and MAPK pathway) and related regulators, which were roughly divided into positive, negative and mutual-balanced regulators, and the specific regulators of the specific signaling pathway based on regulatory effects.

Role of Drebrin at the Immunological Synapse

Although drebrin was first described in neurons, it is also expressed in cells of the immune system, such as T lymphocytes and mast cells. Another member of the drebrin family of proteins, mammalian actin-binding protein 1 (mAbp-1) is more widely expressed and plays important roles in the function of macrophages, polymorphonuclear neutrophils, and B lymphocytes. We will briefly discuss on the function of mAbp-1 and drebrin in immune cells with emphasis on T cells. Specifically, drebrin enables the immune responses of CD4⁺ T lymphocytes. T cells are activated after the recognition of an antigen presented by antigen-presenting cells through cognate cell-cell contacts called immunological synapses (IS). In CD4⁺ T cells, drebrin associates with the chemokine receptor CXCR4, and both molecules redistribute to the IS displaying similar dynamics. Through its interaction with CXCR4 and the actin cytoskeleton, drebrin regulates T cell activation. CD4⁺ T cells are one of the main targets for the human immunodeficiency virus (HIV)-1. This virus utilizes the IS structure to be transmitted to uninfected cells, forming cell-cell contacts called virological synapses (VS). Interestingly, drebrin negatively regulates HIV-1 infection of CD4⁺ T lymphocytes, by regulating actin polymerization at the VS.

Transgelin-2 in B-Cells Controls T-Cell Activation by Stabilizing T Cell - B Cell Conjugates

The immunological synapse (IS), a dynamic and organized junction between T-cells and antigen presenting cells (APCs), is critical for initiating adaptive immunity. The actin cytoskeleton plays a major role in T-cell reorganization during IS formation, and we previously reported that transgelin-2, an actin-binding protein expressed in T-cells, stabilizes cortical F-actin, promoting T-cell activation in response to antigen stimulation. Transgelin-2 is also highly expressed in B-cells, although no specific function has been reported. In this study, we found that deficiency in transgelin-2 (TAGLN2-/-) in B-cells had little effect on B-cell development and activation, as measured by the expression of CD69, MHC class II molecules, and CD80/86. Nevertheless, in B-cells, transgelin-2 accumulated in the IS during the interaction with T-cells. These results led us to hypothesize that transgelin-2 may also be involved in IS stability in B-cells, thereby influencing T-cell function. Notably, we found that transgelin-2 deficiency in B-cells reduced T-cell activation, as determined by the release of IL-2 and interferon-γ and the expression of CD69. Furthermore, the reduced T-cell activation was correlated with reduced B-cell-T-cell conjugate formation. Collectively, these results suggest that actin stability in B-cells during IS formation is critical for the initiation of adaptive T-cell immunity.

Chorein addiction in VPS13A overexpressing rhabdomyosarcoma cells

Chorein encoded by VPS13A (vacuolar protein sorting-associated protein 13A) is defective in chorea-acanthocytosis. Chorein fosters neuronal cell survival, cortical actin polymerization and cell stiffness. In view of its anti-apoptotic effect in neurons, we explored whether chorein is expressed in cancer cells and influences cancer cell survival. RT-PCR was employed to determine transcript levels, specific siRNA to silence chorein, FACS analysis to follow apoptosis and Western blotting to quantify protein abundance. Chorein transcripts were detected in various cancer cell types. The mRNA coding for chorein and chorein protein were most abundant in drug resistant, poorly differentiated human rhabdomyosarcoma cells. Chorein silencing significantly reduced the ratio of phosphorylated (and thus activated) to total phosphoinositide 3 kinase (PI-3K), pointing to inactivation of this crucial pro-survival signaling molecule. Moreover, chorein silencing diminished transcript levels and protein expression of anti-apoptotic BCL-2 and enhanced transcript levels of pro-apoptotic Bax. Silencing of chorein in rhabdomyosarcoma cells was followed by mitochondrial depolarization, caspase 3 activation and stimulation of early and late apoptosis. In conclusion, chorein is expressed in various cancer cells. In cells with high chorein expression levels chorein silencing promotes apoptotic cell death, an effect paralleled by down-regulation of PI-3K activity and BCL-2/Bax expression ratio.

Mammalian actin-binding protein 1/HIP-55 is essential for the scission of clathrin-coated pits by regulating dynamin-actin interaction

Actin and dynamin work cooperatively to drive the invagination and scission of clathrin-coated pits (CCPs). However, little is known about the mechanism that orchestrates the spatiotemporal recruitment of dynamin and actin. Here, we have identified the mammalian actin-binding protein 1 (mAbp1; also called HIP-55 or SH3P7), which could bind to clathrin, actin, as well as dynamin, as an adaptor that links the dynamic recruitment of dynamin and actin for the scission of CCPs. Live-cell imaging reveals that mAbp1 is specifically recruited at a late stage of the long-lived CCPs. mAbp1 knockdown impaired CCP scission by reducing dynamin recruitment at the plasma membrane. However, actin disruption remarkably eliminates mAbp1 recruitment and thus dynamin recruitment. These data suggest that by binding to both clathrin and F-actin, mAbp1 is specifically recruited at a late stage of CCP formation, which subsequently recruits dynamin to CCPs.

Molecular Mechanisms of B Cell Antigen Gathering and Endocytosis

Generation of high-affinity, protective antibodies requires B cell receptor (BCR) signaling, as well as antigen internalization and presentation to helper T cells. B cell antigen internalization is initiated by antigen capture, either from solution or from immune synapses formed on the surface of antigen-presenting cells, and proceeds via clathrin-dependent endocytosis and intracellular routing to late endosomes. Although the components of this pathway are still being discovered, it has become clear that antigen internalization is actively regulated by BCR signaling at multiple steps and, vice versa, that localization of the BCR along the endocytic pathway modulates signaling. Accordingly, defects in BCR internalization or trafficking contribute to enhanced B cell activation in models of autoimmune diseases and in B cell lymphomas. In this review, we discuss how BCR signaling complexes regulate each of the steps of this endocytic process and why defects along this pathway manifest as hyperactive B cell responses in vivo.

Actin-binding protein 1 links B-cell antigen receptors to negative signaling pathways

Prolonged or uncontrolled B-cell receptor (BCR) signaling is associated with autoimmunity. We previously demonstrated a role for actin in BCR signal attenuation. This study reveals that actin-binding protein 1 (Abp1/HIP-55/SH3P7) is a negative regulator of BCR signaling and links actin to negative regulatory pathways of the BCR. In both Abp1(-/-) and bone marrow chimeric mice, in which only B cells lack Abp1 expression, the number of spontaneous germinal center and marginal zone B cells and the level of autoantibody are significantly increased. Serum levels of T-independent antibody responses and total antibody are elevated, whereas T-dependent antibody responses are markedly reduced and fail to undergo affinity maturation. Upon activation, surface BCR clustering is enhanced and B-cell contraction delayed in Abp1(-/-) B cells, concurrent with slow but persistent increases in F-actin at BCR signalosomes. Furthermore, BCR signaling is enhanced in Abp1(-/-) B cells compared with wild-type B cells, including Ca(2+) flux and phosphorylation of B-cell linker protein, the mitogen-activated protein kinase kinase MEK1/2, and ERK, coinciding with reductions in recruitment of the inhibitory signaling molecules hematopoietic progenitor kinase 1 and SH2-containing inositol 5-phosphatase to BCR signalosomes. Our results indicate that Abp1 negatively regulates BCR signaling by coupling actin remodeling to B-cell contraction and activation of inhibitory signaling molecules, which contributes to the regulation of peripheral B-cell development and antibody responses.

Actin-mediated feedback loops in B-cell receptor signaling

Upon recognizing cognate antigen, B cells mobilize multiple cellular apparatuses to propagate an optimal response. Antigen binding is transduced into cytoplasmic signaling events through B-cell antigen receptor (BCR)-based signalosomes at the B-cell surface. BCR signalosomes are dynamic and transient and are subsequently endocytosed for antigen processing. The function of BCR signalosomes is one of the determining factors for the fate of B cells: clonal expansion, anergy, or apoptosis. Accumulating evidence underscores the importance of the actin cytoskeleton in B-cell activation. We have begun to appreciate the role of actin dynamics in regulating BCR-mediated tonic signaling and the formation of BCR signalosomes. Our recent studies reveal an additional function of the actin cytoskeleton in the downregulation of BCR signaling, consequently contributing to the generation and maintenance of B-cell self-tolerance. In this review, we discuss how actin remodels its organization and dynamics in close coordination with BCR signaling and how actin remodeling in turn amplifies the activation and subsequent downregulation process of BCR signaling, providing vital feedback for optimal BCR activation.

HIP-55/DBNL-dependent regulation of adrenergic receptor mediates the ERK1/2 proliferative pathway

The activation of β-adrenergic receptors (β-ARs) plays a key role in regulating cardiac function. However, the detailed regulatory mechanisms of β-AR-induced fibrosis are still unclear. We used a proteomics approach to analyze the changes in protein expression patterns in cardiac fibrosis with β-AR stimulation. HIP-55 (also called debrin-like; DBNL) was revealed as a novel regulator in the signaling regulatory network with β-AR activation. Further studies of both HIP-55-overexpressed and -deficient cardiac fibroblasts indicated that HIP-55 negatively regulated β-AR-activated cardiac fibroblast proliferation and the proliferative signaling pathway may be associated with the extracellular signal-regulated protein kinase (ERK) activation. Our data provide a new mechanistic insight into the role of HIP-55 in β-AR-induced cardiac fibroblast proliferation and suggest a new treatment strategy for proliferative disorders.

Phosphosite mapping of HIP-55 protein in mammalian cells

In the present study, hematopoietic progenitor kinase 1 (HPK1)-interacting protein of 55 kDa (HIP-55) protein was over-expressed in HEK293 cells, which was genetically attached with 6x His tag. The protein was purified by nickel-charged resin and was then subjected to tryptic digestion. The phosphorylated peptides within the HIP-55 protein were enriched by TiO₂ affinity chromatography, followed by mass spectrometry analysis. Fourteen phosphorylation sites along the primary structure of HIP-55 protein were identified, most of which had not been previously reported. Our results indicate that bio-mass spectrometry coupled with manual interpretation can be used to successfully identify the phosphorylation modification in HIP-55 protein in HEK293 cells.

How B cells capture, process and present antigens: a crucial role for cell polarity

B cells are key components of the adaptive immune response. Their differentiation into either specific memory B cells or antibody-secreting plasma cells is a consequence of activation steps that involve the processing and presentation of antigens. The engagement of B cell receptors by surface-tethered antigens leads to the formation of an immunological synapse that coordinates cell signalling events and that promotes antigen uptake for presentation on MHC class II molecules. In this Review, we discuss membrane trafficking and the associated molecular mechanisms that are involved in antigen extraction and processing at the B cell synapse, and we highlight how B cells use cell polarity to coordinate the complex events that ultimately lead to efficient humoral responses.

N-wasp is essential for the negative regulation of B cell receptor signaling

Negative regulation of receptor signaling is essential for controlling cell activation and differentiation. In B-lymphocytes, the down-regulation of B-cell antigen receptor (BCR) signaling is critical for suppressing the activation of self-reactive B cells; however, the mechanism underlying the negative regulation of signaling remains elusive. Using genetically manipulated mouse models and total internal reflection fluorescence microscopy, we demonstrate that neuronal Wiskott-Aldrich syndrome protein (N-WASP), which is coexpressed with WASP in all immune cells, is a critical negative regulator of B-cell signaling. B-cell-specific N-WASP gene deletion causes enhanced and prolonged BCR signaling and elevated levels of autoantibodies in the mouse serum. The increased signaling in N-WASP knockout B cells is concurrent with increased accumulation of F-actin at the B-cell surface, enhanced B-cell spreading on the antigen-presenting membrane, delayed B-cell contraction, inhibition in the merger of signaling active BCR microclusters into signaling inactive central clusters, and a blockage of BCR internalization. Upon BCR activation, WASP is activated first, followed by N-WASP in mouse and human primary B cells. The activation of N-WASP is suppressed by Bruton's tyrosine kinase-induced WASP activation, and is restored by the activation of SH2 domain-containing inositol 5-phosphatase that inhibits WASP activation. Our results reveal a new mechanism for the negative regulation of BCR signaling and broadly suggest an actin-mediated mechanism for signaling down-regulation.

The actin cytoskeleton coordinates the signal transduction and antigen processing functions of the B cell antigen receptor

The B cell antigen receptor (BCR) is the sensor on the B cell surface that surveys foreign molecules (antigen) in our bodies and activates B cells to generate antibody responses upon encountering cognate antigen. The binding of antigen to the BCR induces signaling cascades in the cytoplasm, which provides the first signal for B cell activation. Subsequently, BCRs internalize and target bound antigen to endosomes, where antigen is processed into T cell recognizable forms. T helper cells generate the second activation signal upon binding to antigen presented by B cells. The optimal activation of B cells requires both signals, thereby depending on the coordination of BCR signaling and antigen transport functions. Antigen binding to the BCR also induces rapid remodeling of the cortical actin network of B cells. While being initiated and controlled by BCR signaling, recent studies reveal that this actin remodeling is critical for both the signaling and antigen processing functions of the BCR, indicating a role for actin in coordinating these two pathways. Here we will review previous and recent studies on actin reorganization during BCR activation and BCR-mediated antigen processing, and discuss how actin remodeling translates BCR signaling into rapid antigen uptake and processing while providing positive and negative feedback to BCR signaling.

The pivotal position of the actin cytoskeleton in the initiation and regulation of B cell receptor activation

The actin cytoskeleton is a dynamic cellular network known for its function in cell morphology and motility. Recent studies using high resolution and real time imaging techniques have revealed that actin plays a critical role in signal transduction, primarily by modulating the dynamics and organization of membrane-associated receptors and signaling molecules. This review summarizes what we have learned so far about a regulatory niche of the actin cytoskeleton in the signal transduction of the B cell receptor (BCR). The activation of the BCR is initiated and regulated by a close coordination between the dynamics of surface BCRs and the cortical actin network. The actin cytoskeleton is involved in regulating the signaling threshold of the BCR to antigenic stimulation, the kinetics and amplification of BCR signaling activities, and the timing and kinetics of signaling downregulation. Actin exerts its regulatory function by controlling the kinetics, magnitude, subcellular location, and nature of BCR clustering and BCR signaling complex formation at every stage of signaling. The cortical actin network is remodeled by initial detachment from the plasma membrane, disassembly and subsequent reassembly into new actin structures in response to antigenic stimulation. Signaling responsive actin regulators translate BCR stimulatory and inhibitory signals into a series of actin remodeling events, which enhance signaling activation and down-regulation by modulating the lateral mobility and spatial organization of surface BCR. The mechanistic understanding of actin-mediated signaling regulation in B cells will help us explore B cell-specific manipulations of the actin cytoskeleton as treatments for B cell-mediated autoimmunity and B cell cancer. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Chorein sensitivity of cytoskeletal organization and degranulation of platelets

Chorea-acanthocytosis (ChAc), a lethal disease caused by defective chorein, is characterized by neurodegeneration and erythrocyte acanthocytosis. The functional significance of chorein in other cell types remained ill-defined. The present study revealed chorein expression in blood platelets. As compared to platelets from healthy volunteers, platelets from patients with ChAc displayed a 47% increased globular/filamentous actin ratio, indicating actin depolymerization. Moreover, phosphoinositide-3-kinase subunit p85 phosphorylation, p21 protein-activated kinase (PAK1) phosphorylation, as well as vesicle-associated membrane protein 8 (VAMP8) expression were significantly reduced in platelets from patients with ChAc (by 17, 22, and 39%, respectively) and in megakaryocytic (MEG-01) cells following chorein silencing (by 16, 54, and 11%, respectively). Activation-induced platelet secretion from dense granules (ATP release) and α granules (P-selectin exposure) were significantly less (by 55% after stimulation with 1 μg/ml CRP and by 33% after stimulation with 5 μM TRAP, respectively) in ChAc platelets than in control platelets. Furthermore, platelet aggregation following stimulation with different platelet agonists was significantly impaired. These observations reveal a completely novel function of chorein, i.e., regulation of secretion and aggregation of blood platelets.

Regulation of late endosomal/lysosomal maturation and trafficking by cortactin affects Golgi morphology

Cortactin is a branched actin regulator and tumor-overexpressed protein that promotes vesicular trafficking at a variety of cellular sites, including endosomes and the trans-Golgi network. To better understand its role in secretory trafficking, we investigated its function in Golgi homeostasis. Here, we report that knockdown (KD) of cortactin leads to a dramatic change in Golgi morphology by light microscopy, dependent on binding the Arp2/3 actin-nucleating complex. Surprisingly, there was little effect of cortactin-KD on anterograde trafficking of the constitutive cargo vesicular stomatitis virus glycoprotein (VSVG), Golgi assembly from endoplasmic reticulum membranes upon Brefeldin A washout, or Golgi ultrastructure. Instead, electron microscopy studies revealed that cortactin-KD cells contained a large number of immature-appearing late endosomal/lysosomal (LE/Lys) hybrid organelles, similar to those found in lysosomal storage diseases. Consistent with a defect in LE/Lys trafficking, cortactin-KD cells also exhibited accumulation of free cholesterol and retention of the retrograde Golgi cargo mannose-6-phosphate receptor in LE. Inhibition of LE maturation by treatment of control cells with Rab7 siRNA or chloroquine led to a compact Golgi morphology similar to that observed in cortactin-KD cells. Furthermore, the Golgi morphology defects of cortactin-KD cells could be rescued by removal of cholesterol-containing lipids from the media, suggesting that buildup of cholesterol-rich membranes in immature LE/Lys induced disturbances in retrograde trafficking. Taken together, these data reveal that LE/Lys maturation and trafficking are highly sensitive to cortactin-regulated branched actin assembly and suggests that cytoskeletal-induced Golgi morphology changes can be a consequence of altered trafficking at late endosomes.

HS1, a Lyn kinase substrate, is abnormally expressed in B-chronic lymphocytic leukemia and correlates with response to fludarabine-based regimen

In B-Chronic Lymphocytic Leukemia (B-CLL) kinase Lyn is overexpressed, active, abnormally distributed, and part of a cytosolic complex involving hematopoietic lineage cell-specific protein 1 (HS1). These aberrant properties of Lyn could partially explain leukemic cells' defective apoptosis, directly or through its substrates, for example, HS1 that has been associated to apoptosis in different cell types. To verify the hypothesis of HS1 involvement in Lyn-mediated leukemic cell survival, we investigated HS1 protein in 71 untreated B-CLL patients and 26 healthy controls. We found HS1 overexpressed in leukemic as compared to normal B lymphocytes (1.38±0.54 vs 0.86±0.29, p<0.01), and when HS1 levels were correlated to clinical parameters we found a higher expression of HS1 in poor-prognosis patients. Moreover, HS1 levels significantly decreased in ex vivo leukemic cells of patients responding to a fludarabine-containing regimen. We also observed that HS1 is partially localized in the nucleus of neoplastic B cells. All these data add new information on HS1 study, hypothesizing a pivotal role of HS1 in Lyn-mediated modulation of leukemic cells' survival and focusing, one more time, the attention on the BCR-Lyn axis as a putative target for new therapeutic strategies in this disorder.

Src-mediated phosphorylation of mammalian Abp1 (DBNL) regulates podosome rosette formation in transformed fibroblasts

Podosomes are dynamic actin-based structures that mediate adhesion to the extracellular matrix and localize matrix degradation to facilitate cell motility and invasion. Drebrin-like protein (DBNL), which is homologous to yeast mAbp1 and is therefore known as mammalian actin-binding protein 1 (mAbp1), has been implicated in receptor-mediated endocytosis, vesicle recycling and dorsal ruffle formation. However, it is not known whether mAbp1 regulates podosome formation or cell invasion. In this study, we found that mAbp1 localizes to podosomes and is necessary for the formation of podosome rosettes in Src-transformed fibroblasts. Despite their structural similarity, mAbp1 and cortactin play distinct roles in podosome regulation. Cortactin was necessary for the formation of podosome dots, whereas mAbp1 was necessary for the formation of organized podosome rosettes in Src-transformed cells. We identified specific Src phosphorylation sites, Tyr337 and Tyr347 of mAbp1, which mediate the formation of podosome rosettes and degradation of the ECM. In contrast to dorsal ruffles, the interaction of mAbp1 with WASP-interacting protein (WIP) was not necessary for the formation of podosome rosettes. Finally, we showed that depletion of mAbp1 increased invasive cell migration, suggesting that mAbp1 differentially regulates matrix degradation and cell invasion. Collectively, our findings identify a role for mAbp1 in podosome rosette formation and cell invasion downstream of Src.

Endocytosed BCRs sequentially regulate MAPK and Akt signaling pathways from intracellular compartments

Binding of antigen to the B cell antigen receptor (BCR) triggers both BCR signaling and endocytosis. How endocytosis regulates BCR signaling remains unknown. Here we report that BCR signaling was not extinguished by endocytosis of BCRs; instead, BCR signaling initiated at the plasma membrane continued as the BCR trafficked intracellularly with the sequential phosphorylation of kinases. Blocking the endocytosis of BCRs resulted in the recruitment of both proximal and downstream kinases to the plasma membrane, where mitogen-activated protein kinases (MAPKs) were hyperphosphorylated and the kinase Akt and its downstream target Foxo were hypophosphorylated, which led to the dysregulation of gene transcription controlled by these pathways. Thus, the cellular location of the BCR serves to compartmentalize kinase activation to regulate the outcome of signaling.

Actin-depolymerizing factor homology domain: a conserved fold performing diverse roles in cytoskeletal dynamics

Actin filaments form contractile and protrusive structures that play central roles in many processes such as cell migration, morphogenesis, endocytosis, and cytokinesis. During these processes, the dynamics of the actin filaments are precisely regulated by a large array of actin-binding proteins. The actin-depolymerizing factor homology (ADF-H) domain is a structurally conserved protein motif, which promotes cytoskeletal dynamics by interacting with monomeric and/or filamentous actin, and with the Arp2/3 complex. Despite their structural homology, the five classes of ADF-H domain proteins display distinct biochemical activities and cellular roles, only parts of which are currently understood. ADF/cofilin promotes disassembly of aged actin filaments, whereas twinfilin inhibits actin filament assembly via sequestering actin monomers and interacting with filament barbed ends. GMF does not interact with actin, but instead binds Arp2/3 complex and promotes dissociation of Arp2/3-mediated filament branches. Abp1 and drebrin are multidomain proteins that interact with actin filaments and regulate the activities of other proteins during various actin-dependent processes. The exact function of coactosin is currently incompletely understood. In this review article, we discuss the biochemical functions, cellular roles, and regulation of the five groups of ADF-H domain proteins.

Proteomic approaches to understanding the role of the cytoskeleton in host-defense mechanisms

The cytoskeleton is a cellular scaffolding system whose functions include maintenance of cellular shape, enabling cellular migration, division, intracellular transport, signaling and membrane organization. In addition, in immune cells, the cytoskeleton is essential for phagocytosis. Following the advances in proteomics technology over the past two decades, cytoskeleton proteome analysis in resting and activated immune cells has emerged as a possible powerful approach to expand our understanding of cytoskeletal composition and function. However, so far there have only been a handful of studies of the cytoskeleton proteome in immune cells. This article considers promising proteomics strategies that could augment our understanding of the role of the cytoskeleton in host-defense mechanisms.

Immune pathology associated with altered actin cytoskeleton regulation

The actin cytoskeleton plays a crucial role in a variety of important cellular processes required for normal immune function, including locomotion, intercellular interactions, endocytosis, cytokinesis, signal transduction, and maintenance of cell morphology. Recent studies have uncovered not only many of the components and mechanisms that regulate the cortical actin cytoskeleton but have also revealed significant immunopathological consequences associated with genetic alteration of actin cytoskeletal regulatory genes. These advances have provided new insights into the role of cortical actin cytoskeletal regulation in a number of immune cell functions and have identified cytoskeletal regulatory proteins critical for normal immune system activity and susceptibility to autoimmunity.

A myosin IK-Abp1-PakB circuit acts as a switch to regulate phagocytosis efficiency

Actin dynamics and myosin (Myo) contractile forces are necessary for formation and closure of the phagocytic cup. In Dictyostelium, the actin-binding protein Abp1 and myosin IK are enriched in the closing cup and especially at an actin-dense constriction furrow formed around the neck of engulfed budded yeasts. This phagocytic furrow consists of concentric overlapping rings of MyoK, Abp1, Arp3, coronin, and myosin II, following an order strikingly reminiscent of the overall organization of the lamellipodium of migrating cells. Mutation analyses of MyoK revealed that both a C-terminal farnesylation membrane anchor and a Gly-Pro-Arg domain that interacts with profilin and Abp1 were necessary for proper localization in the furrow and efficient phagocytosis. Consequently, we measured the binding affinities of these interactions and unraveled further interactions with profilins, dynamin A, and PakB. Due to the redundancy of the interaction network, we hypothesize that MyoK and Abp1 are restricted to regulatory roles and might affect the dynamic of cup progression. Indeed, phagocytic uptake was regulated antagonistically by MyoK and Abp1. MyoK is phosphorylated by PakB and positively regulates phagocytosis, whereas binding of Abp1 negatively regulates PakB and MyoK. We conclude that a MyoK-Abp1-PakB circuit acts as a switch regulating phagocytosis efficiency of large particles.

Dynamin 2 and human diseases

Dynamin 2 (DNM2) mutations cause autosomal dominant centronuclear myopathy, a rare form of congenital myopathy, and intermediate and axonal forms of Charcot-Marie-Tooth disease, a peripheral neuropathy. DNM2 is a large GTPase mainly involved in membrane trafficking through its function in the formation and release of nascent vesicles from biological membranes. DNM2 participates in clathrin-dependent and clathrin-independent endocytosis and intracellular membrane trafficking (from endosomes and Golgi apparatus). Recent studies have also implicated DNM2 in exocytosis. DNM2 belongs to the machinery responsible for the formation of vesicles and regulates the cytoskeleton providing intracellular vesicle transport. In addition, DNM2 tightly interacts with and is involved in the regulation of actin and microtubule networks, independent from membrane trafficking processes. We summarize here the molecular, biochemical, and functional data on DNM2 and discuss the possible pathophysiological mechanisms via which DNM2 mutations can lead to two distinct neuromuscular disorders.

Actin-binding protein-1 interacts with WASp-interacting protein to regulate growth factor-induced dorsal ruffle formation

The authors show that the mammalian actin binding protein-1 (mAbp1) is required for PDGF-induced dorsal ruffle formation. mAbp1 interacts directly with WASp Interacting Protein (WIP) through its SH3 domain, and this interaction is important for regulating dorsal ruffle formation.

Growth factor stimulation induces the formation of dynamic actin structures known as dorsal ruffles. Mammalian actin-binding protein-1 (mAbp1) is an actin-binding protein that has been implicated in regulating clathrin-mediated endocytosis; however, a role for mAbp1 in regulating the dynamics of growth factor–induced actin-based structures has not been defined. Here we show that mAbp1 localizes to dorsal ruffles and is necessary for platelet-derived growth factor (PDGF)-mediated dorsal ruffle formation. Despite their structural similarity, we find that mAbp1 and cortactin have nonredundant functions in the regulation of dorsal ruffle formation. mAbp1, like cortactin, is a calpain 2 substrate and the preferred cleavage site occurs between the actin-binding domain and the proline-rich region, generating a C-terminal mAbp1 fragment that inhibits dorsal ruffle formation. Furthermore, mAbp1 directly interacts with the actin regulatory protein WASp-interacting protein (WIP) through its SH3 domain. Finally, we demonstrate that the interaction between mAbp1 and WIP is important in regulating dorsal ruffle formation and that WIP-mediated effects on dorsal ruffle formation require mAbp1. Taken together, these findings identify a novel role for mAbp1 in growth factor–induced dorsal ruffle formation through its interaction with WIP.

A fundamental role of mAbp1 in neutrophils: impact on beta(2) integrin-mediated phagocytosis and adhesion in vivo

The mammalian actin-binding protein 1 (mAbp1, Hip-55, SH3P7) is phosphorylated by the nonreceptor tyrosine kinase Syk that has a fundamental effect for several beta(2) integrin (CD11/CD18)-mediated neutrophil functions. Live cell imaging showed a dynamic enrichment of enhanced green fluorescence protein-tagged mAbp1 at the phagocytic cup of neutrophil-like differentiated HL-60 cells during beta(2) integrin-mediated phagocytosis of serum-opsonized Escherichia coli. The genetic absence of Syk or its pharmacologic inhibition using piceatannol abrogated the proper localization of mAbp1 at the phagocytic cup. The genetic absence or down-regulation of mAbp1 using the RNA interference technique significantly compromised beta(2) integrin-mediated phagocytosis of serum-opsonized E coli or Salmonella typhimurium in vitro as well as clearance of S typhimurium infection in vivo. Moreover, the genetic absence of mAbp1 almost completely abrogated firm neutrophil adhesion under physiologic shear stress conditions in vitro as well as leukocyte adhesion and extravasation in inflamed cremaster muscle venules of mice treated with tumor-necrosis factor alpha. Functional analysis showed that the down-regulation of mAbp1 diminished the number of beta(2) integrin clusters in the high-affinity conformation under flow conditions. These unanticipated results define mAbp1 as a novel molecular player in integrin biology that is critical for phagocytosis and firm neutrophil adhesion under flow conditions.

B cell antigen receptor endocytosis and antigen presentation to T cells require Vav and dynamin

Antigen binding to the B cell antigen receptor (BCR) initiates an array of signaling events. These include endocytosis of ligand-receptor complexes via clathrin-coated pits, trafficking of the internalized ligand to lysosomes, degradation of the associated proteins to peptides, and peptide presentation on nascent major histocompatibility complex class II to T cells. The signal transduction events supporting BCR internalization are not well understood. We have identified a pathway supporting BCR internalization that includes the Vav1 and/or Vav3 isoforms and the GTPase dynamin. Vav1 and -3 are not required for B cell development and maturation, nor for a variety of BCR-induced signaling events nor for BCR signaling leading to major histocompatibility complex class II and CD80 expression, but Vav1 and/or -3 are absolutely required for BCR endocytosis and BCR-induced Rac-GTP loading. This is the first demonstration of a link between Vav and Rac in BCR internalization leading to antigen presentation to T cells.

Dynamics of membrane trafficking downstream of B and T cell receptor engagement: impact on immune synapses

The onset of an adaptive immune response requires the activation of T and B lymphocytes by antigen-presenting cells, through a specialized form of intercellular communication, known as the immunological synapse (IS). In B lymphocytes the IS promotes efficient recognition and acquisition of membrane-bound Ags, while in T cells, it modulates the T cell response upon exposure to peptide-major histocompatibility complexes. In this review, we highlight the similarities that determine B and T cell activation, focusing on immune receptor downstream signaling events that lead to synapse formation. We stress the notion that polarization of T and B lymphocytes characterized by global changes in cytoskeleton and membrane trafficking modulates synapse structure and function, thus determining lymphocyte effector functions and fate.

Btk regulates B cell receptor-mediated antigen processing and presentation by controlling actin cytoskeleton dynamics in B cells

The high efficiency of Ag processing and presentation by B cells requires Ag-induced BCR signaling and actin cytoskeleton reorganization, although the underlying mechanism for such requirements remains elusive. In this study, we identify Bruton's tyrosine kinase (Btk) as a linker connecting BCR signaling to actin dynamics and the Ag transport pathway. Using xid mice and a Btk inhibitor, we show that BCR engagement increases actin polymerization and Wiskott-Aldrich syndrome protein activation in a Btk-dependent manner. Concurrently, we observe Btk-dependent increases in the levels of phosphatidylinositide-4,5-bisphosphate and phosphorylated Vav upon BCR engagement. The rate of BCR internalization, its movement to late endosomes, and efficiency of BCR-mediated Ag processing and presentation are significantly reduced in both xid and Btk inhibitor-treated B cells. Thus, Btk regulates actin dynamics and Ag transport by activating Wiskott-Aldrich syndrome protein via Vav and phosphatidylinositides. This represents a novel mechanism by which BCR-mediated signaling regulates BCR-mediated Ag processing and presentation.