Syk-mediated tyrosine phosphorylation is required for the association of hematopoietic lineage cell-specific protein 1 with lipid rafts and B cell antigen receptor signalosome complex

B cell receptor-induced protein dynamics and the emerging role of SUMOylation revealed by proximity proteomics

Successful B cell activation, which is critical for high-affinity antibody production, is controlled by the B cell antigen receptor (BCR). However, we still lack a comprehensive protein-level view of the very dynamic multi-branched cellular events triggered by antigen binding. Here, we employed APEX2 proximity biotinylation to study antigen-induced changes, 5-15 min after receptor activation, at the vicinity of the plasma membrane lipid rafts, wherein BCR enriches upon activation. The data reveals dynamics of signaling proteins, as well as various players linked to the subsequent processes, such as actin cytoskeleton remodeling and endocytosis. Interestingly, our differential expression analysis identified dynamic responses in various proteins previously not linked to early B cell activation. We demonstrate active SUMOylation at the sites of BCR activation in various conditions and report its functional role in BCR signaling through the AKT and ERK1/2 axes.

The Actin Regulators Involved in the Function and Related Diseases of Lymphocytes

Actin is an important cytoskeletal protein involved in signal transduction, cell structure and motility. Actin regulators include actin-monomer-binding proteins, Wiskott-Aldrich syndrome (WAS) family of proteins, nucleation proteins, actin filament polymerases and severing proteins. This group of proteins regulate the dynamic changes in actin assembly/disassembly, thus playing an important role in cell motility, intracellular transport, cell division and other basic cellular activities. Lymphocytes are important components of the human immune system, consisting of T-lymphocytes (T cells), B-lymphocytes (B cells) and natural killer cells (NK cells). Lymphocytes are indispensable for both innate and adaptive immunity and cannot function normally without various actin regulators. In this review, we first briefly introduce the structure and fundamental functions of a variety of well-known and newly discovered actin regulators, then we highlight the role of actin regulators in T cell, B cell and NK cell, and finally provide a landscape of various diseases associated with them. This review provides new directions in exploring actin regulators and promotes more precise and effective treatments for related diseases.

3D-STED Super-Resolution Microscopy Reveals Distinct Nanoscale Organization of the Hematopoietic Cell-Specific Lyn Substrate-1 (HS1) in Normal and Leukemic B Cells

HS1, the hematopoietic homolog of cortactin, acts as a versatile actin-binding protein in leucocytes. After phosphorylation, it is involved in GTPase and integrin activation, and in BCR, TCR, and CXCR4 downstream signaling. In normal and leukemic B cells, HS1 is a central cytoskeletal interactor and its phosphorylation and expression are prognostic factors in chronic lymphocytic leukemia (CLL) patients. We here introduce for the first time a super-resolution imaging study based on single-cell 3D-STED microscopy optimized for revealing and comparing the nanoscale distribution of endogenous HS1 in healthy B and CLL primary cells. Our study reveals that the endogenous HS1 forms heterogeneous nanoclusters, similar to those of YFP-HS1 overexpressed in the leukemic MEC1 cell line. HS1 nanoclusters in healthy and leukemic B cells form bulky assemblies at the basal sides, suggesting the recruitment of HS1 for cell adhesion. This observation agrees with a phasor-FLIM-FRET and STED colocalization analyses of the endogenous MEC1-HS1, indicating an increased interaction with Vimentin at the cell adhesion sites. In CLL cells isolated from patients with poor prognosis, we observed a larger accumulation of HS1 at the basal region and a higher density of HS1 nanoclusters in the central regions of the cells if compared to good-prognosis CLL and healthy B cells, suggesting a different role for the protein in the cell types analyzed. Our 3D-STED approach lays the ground for revealing tiny differences of HS1 distribution, its functionally active forms, and colocalization with protein partners.

Ecm29-Dependent Proteasome Localization Regulates Cytoskeleton Remodeling at the Immune Synapse

The formation of an immune synapse (IS) enables B cells to capture membrane-tethered antigens, where cortical actin cytoskeleton remodeling regulates cell spreading and depletion of F-actin at the centrosome promotes the recruitment of lysosomes to facilitate antigen extraction. How B cells regulate both pools of actin, remains poorly understood. We report here that decreased F-actin at the centrosome and IS relies on the distribution of the proteasome, regulated by Ecm29. Silencing Ecm29 decreases the proteasome pool associated to the centrosome of B cells and shifts its accumulation to the cell cortex and IS. Accordingly, Ecm29-silenced B cells display increased F-actin at the centrosome, impaired centrosome and lysosome repositioning to the IS and defective antigen extraction and presentation. Ecm29-silenced B cells, which accumulate higher levels of proteasome at the cell cortex, display decreased actin retrograde flow in lamellipodia and enhanced spreading responses. Our findings support a model where B the asymmetric distribution of the proteasome, mediated by Ecm29, coordinates actin dynamics at the centrosome and the IS, promoting lysosome recruitment and cell spreading.

Deep proteome profiling reveals novel pathways associated with pro-inflammatory and alcohol-induced microglial activation phenotypes

Microglia, the resident immune cells of the brain, can exhibit a broad range of activation phenotypes, many of which have been implicated in several diseases and disorders of the central nervous system including those related to alcohol abuse. Given the complexity of global-scale molecular changes that define microglial activation, accurate phenotypic classification in the context of alcohol exposure is still lacking. We employed an optimized method for deep, quantitative proteome profiling of primary microglia in order to characterize their response to acute exposure to alcohol (ethanol) as well as the pro-inflammatory driver and TLR4 agonist, LPS. From this analysis, 5,062 total proteins were identified where 4,857 and 4,928 of those proteins were quantifiable by label-free quantitation in ethanol and LPS treatment groups, respectively. This study highlights the subtle, yet significant proteomic changes that occur in ethanol-treated microglia, which do not align with the robust pro-inflammatory phenotype induced by TLR4 activation. Specifically, our results indicate inhibition of several upstream regulators associated with inflammation, opposing effects on pathways such as phagocytosis upon comparison to TLR4-mediated pro-inflammatory phenotype, and a potential metabolic shift associated with increased expression of proteins related to OXPHOS and lipid homeostasis. Data are available via ProteomeXchange with identifier PXD14466. SIGNIFICANCE: Alcohol abuse has a significant impact on the central nervous system, which includes the pathophysiological mechanisms resulting from glial cell activation. Microglia, in particular, are the resident immune cells of the brain and exhibit a broad range of activation phenotypes. The molecular changes that drive microglial activation phenotype are complex and have yet to be fully characterized in the context of alcohol exposure. Our study highlights the first and most comprehensive characterization of alcohol-induced proteomic changes in primary microglia to date and has shed light on novel immune-related and metabolic pathways that are altered due to alcohol exposure. The results from this study provide an important foundation for future work aimed to understand the complexity of alcohol-induced microglial activation in vivo and other translational models of acute and chronic alcohol exposure.

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.

Identification of RNA-binding Proteins in Macrophages by Interactome Capture

Pathogen components, such as lipopolysaccharides of Gram-negative bacteria that activate Toll-like receptor 4, induce mitogen activated protein kinases and NFκB through different downstream pathways to stimulate pro- and anti-inflammatory cytokine expression. Importantly, post-transcriptional control of the expression of Toll-like receptor 4 downstream signaling molecules contributes to the tight regulation of inflammatory cytokine synthesis in macrophages. Emerging evidence highlights the role of RNA-binding proteins (RBPs) in the post-transcriptional control of the innate immune response. To systematically identify macrophage RBPs and their response to LPS stimulation, we employed RNA interactome capture in LPS-induced and untreated murine RAW 264.7 macrophages. This combines RBP-crosslinking to RNA, cell lysis, oligo(dT) capture of polyadenylated RNAs and mass spectrometry analysis of associated proteins. Our data revealed 402 proteins of the macrophage RNA interactome including 91 previously not annotated as RBPs. A comparison with published RNA interactomes classified 32 RBPs uniquely identified in RAW 264.7 macrophages. Of these, 19 proteins are linked to biochemical activities not directly related to RNA. From this group, we validated the HSP90 cochaperone P23 that was demonstrated to exhibit cytosolic prostaglandin E2 synthase 3 (PTGES3) activity, and the hematopoietic cell-specific LYN substrate 1 (HCLS1 or HS1), a hematopoietic cell-specific adapter molecule, as novel macrophage RBPs. Our study expands the mammalian RBP repertoire, and identifies macrophage RBPs that respond to LPS. These RBPs are prime candidates for the post-transcriptional regulation and execution of LPS-induced signaling pathways and the innate immune response. Macrophage RBP data have been deposited to ProteomeXchange with identifier PXD002890.

Actin nucleation at the centrosome controls lymphocyte polarity

Cell polarity is required for the functional specialization of many cell types including lymphocytes. A hallmark of cell polarity is the reorientation of the centrosome that allows repositioning of organelles and vesicles in an asymmetric fashion. The mechanisms underlying centrosome polarization are not fully understood. Here we found that in resting lymphocytes, centrosome-associated Arp2/3 locally nucleates F-actin, which is needed for centrosome tethering to the nucleus via the LINC complex. Upon lymphocyte activation, Arp2/3 is partially depleted from the centrosome as a result of its recruitment to the immune synapse. This leads to a reduction in F-actin nucleation at the centrosome and thereby allows its detachment from the nucleus and polarization to the synapse. Therefore, F-actin nucleation at the centrosome—regulated by the availability of the Arp2/3 complex—determines its capacity to polarize in response to external stimuli.

Cell polarity is marked by re-orientation of the centrosome, but the mechanisms governing centrosome polarization are poorly understood. Here Obino et al. show that in lymphocytes centrosome-associated Arp2/3 nucleates actin that tethers the centrosome to the nucleus; activation depletes Arp2/3 from the centrosome and frees it from the nucleus.

phoP, SPI1, SPI2 and aroA mutants of Salmonella Enteritidis induce a different immune response in chickens

Poultry is the most frequent reservoir of non-typhoid Salmonella enterica for humans. Understanding the interactions between chickens and S. enterica is therefore important for vaccine design and subsequent decrease in the incidence of human salmonellosis. In this study we therefore characterized the interactions between chickens and phoP, aroA, SPI1 and SPI2 mutants of S. Enteritidis. First we tested the response of HD11 chicken macrophage-like cell line to S. Enteritidis infection monitoring the transcription of 36 genes related to immune response. All the mutants and the wild type strain induced inflammatory signaling in the HD11 cell line though the response to SPI1 mutant infection was different from the rest of the mutants. When newly hatched chickens were inoculated, the phoP as well as the SPI1 mutant did not induce an expression of any of the tested genes in the cecum. Despite this, such chickens were protected against challenge with wild-type S. Enteritidis. On the other hand, inoculation of chickens with the aroA or SPI2 mutant induced expression of 27 and 18 genes, respectively, including genes encoding immunoglobulins. Challenge of chickens inoculated with these two mutants resulted in repeated induction of 11 and 13 tested genes, respectively, including the genes encoding immunoglobulins. In conclusion, SPI1 and phoP mutants induced protective immunity without inducing an inflammatory response and antibody production. Inoculation of chickens with the SPI2 and aroA mutants also led to protective immunity but was associated with inflammation and antibody production. The differences in interaction between the mutants and chicken host can be used for a more detailed understanding of the chicken immune system.

Rho and Rap guanosine triphosphatase signaling in B cells and chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) cells proliferate predominantly in niches in the lymph nodes, where signaling from the B cell receptor (BCR) and the surrounding microenvironment are critical for disease progression. In addition, leukemic cells traffic constantly from the bloodstream into the lymph nodes, migrate within lymphatic tissues and egress back to the bloodstream. These processes are driven by chemokines and their receptors, and depend on changes in cell migration and integrin-mediated adhesion. Here we describe how Rho and Rap guanosine triphosphatases (GTPases) contribute to both BCR signaling and chemokine receptor signaling, particularly by regulating cytoskeletal dynamics and integrin activity. We propose that new inhibitors of BCR-activated kinases are likely to affect CLL cell trafficking via Rho and Rap GTPases, and that upstream regulators or downstream effectors could be good targets for therapeutic intervention in CLL.

Chicken innate immune response to oral infection with Salmonella enterica serovar Enteritidis

The characterization of the immune response of chickens to Salmonella infection is usually limited to the quantification of expression of genes coding for cytokines, chemokines or antimicrobial peptides. However, processes occurring in the cecum of infected chickens are likely to be much more diverse. In this study we have therefore characterized the transcriptome and proteome in the chicken cecum after infection with Salmonella Enteritidis. Using a combination of 454 pyrosequencing, protein mass spectrometry and quantitative real-time PCR, we identified 48 down- and 56 up-regulated chicken genes after Salmonella Enteritidis infection. The most inducible gene was that coding for MMP7, exhibiting a 5952 fold induction 9 days post-infection. An induction of greater than 100 fold was observed for IgG, IRG1, SAA, ExFABP, IL-22, TRAP6, MRP126, IFNγ, iNOS, ES1, IL-1β, LYG2, IFIT5, IL-17, AVD, AH221 and SERPIN B. Since prostaglandin D2 synthase was upregulated and degrading hydroxyprostaglandin dehydrogenase was downregulated after the infection, prostaglandin must accumulate in the cecum of chickens infected with Salmonella Enteritidis. Finally, above mentioned signaling was dependent on the presence of a SPI1-encoded type III secretion system in Salmonella Enteritidis. The inflammation lasted for 2 weeks after which time the expression of the “inflammatory” genes returned back to basal levels and, instead, the expression of IgA and IgG increased. This points to an important role for immunoglobulins in the restoration of homeostasis in the cecum after infection.

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.

High expression of hematopoietic cell specific Lyn substrate-1 (HS1) predicts poor survival of B-cell chronic lymphocytic leukemia patients

B-cell chronic lymphocytic leukemia (B-CLL) is the most common leukemia in adults in western countries. HS1 protein regulates leukemic cell migration and homing, and can indirectly promote disease progression and influence patient survival. The aim of this study was to evaluate HS1 expression in CLL patients in connection with other known prognostic factors and patients' survival.

METHODS

90 untreated CLL patients were included into the study. The control group consisted of 28 healthy matched people. HS1 detection was performed by western-blotting. Mutational status of IgVH, as well as CD38 and ZAP70 expression was also analyzed.

RESULTS

HS1 expression was significantly higher in CLL patients comparing to controls. Positive correlation was shown between HS1 and: age (p=0.0454), Rai stage (p=0.0412), leukocytosis (p=0.0129) and beta-2-microglobulin (p=0.0342). Patients with lymphocyte doubling time shorter or equal to 6 months had higher expression of HS1. Patients with higher HS1 expression had shorter survival than those with lower HS1 expression (p=0.0329).

CONCLUSIONS

We showed, that high HS1 expression predicts poor survival of chronic lymphocytic leukemia patients.

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.

The cytoskeleton coordinates the early events of B-cell activation

B cells contribute to protective adaptive immune responses through generation of antibodies and long-lived memory cells, following engagement of the B-cell receptor (BCR) with specific antigen. Recent imaging investigations have offered novel insights into the ensuing molecular and cellular events underlying B-cell activation. Following engagement with antigen, BCR microclusters form and act as sites of active signaling through the recruitment of intracellular signaling molecules and adaptors. Signaling through these "microsignalosomes" is propagated and enhanced through B-cell spreading in a CD19-dependent manner. Subsequently, the mature immunological synapse is formed, and functions as a platform for antigen internalization, enabling the antigen presentation to helper T cells required for maximal B-cell activation. In this review, we discuss the emerging and critical role for the cytoskeleton in the coordination and regulation of these molecular events during B-cell activation.

Identification of BCAR-1 as a new substrate of Syk tyrosine kinase through a determination of amino acid sequence preferences surrounding the substrate tyrosine residue

Syk, a non-receptor tyrosine kinase, is an essential signaling molecule in B cells and other hematopoietic cells. Recently, its unexpected diverse functions were recognized in the regulation of cellular adhesion, innate immune recognition, vascular development, and carcinogenesis. Despite its pleiotropic role, only a few substrate proteins have been identified. To find new substrate proteins for Syk, we performed a systemic in vitro kinase assay using GST fusion peptides to determine the substrate specificity surrounding the tyrosine residue to be phosphorylated. Substitution of amino acid residues surrounding tyrosine 178 of BLNK, a principal Syk substrate in B cell receptor-mediated signaling, revealed that acidic residues at sites -5 to -1 were necessary for phosphorylation by Syk. Valine at site +1 was also influential in phosphorylation and a substitution of Pro on site +3 to a basic amino acid residue, Lys, resulted in attenuated phosphorylation. On the basis of these results, a general consensus phosphorylation motif for Syk was determined and several new candidate target proteins were identified in protein database searches. Of the candidate proteins, BCAR-1 (breast cancer anti-estrogen resistance 1) was confirmed to be phosphorylated by Syk in an in vitro kinase assay using a full-length protein of BCAR-1. Furthermore, BCAR-1 was tyrosine phosphorylated upon the overexpression of Syk in HEK-293T cells. These results suggest that more Syk substrates can be found using an in vitro kinase approach and show for the first time that BCAR-1 is a physiological substrate of Syk.

HS1 has a central role in the trafficking and homing of leukemic B cells

The function of the intracellular protein hematopoietic cell-specific Lyn substrate-1 (HS1) in B lymphocytes is poorly defined. To investigate its role in migration, trafficking, and homing of leukemic B lymphocytes we have used B cells from HS1(-/-) mice, the HS1-silenced human chronic lymphocytic leukemia (CLL) MEC1 cell line and primary leukemic B cells from patients with CLL. We have used both in vitro and in vivo models and found that the lack of expression of HS1 causes several important functional effects. In vitro, we observed an impaired cytoskeletal remodeling that resulted in diminished cell migration, abnormal cell adhesion, and increased homotypic aggregation. In vivo, immunodeficient Rag2(-/-)γ(c)(-/-) mice injected with HS1-silenced CLL B cells showed a decreased organ infiltration with the notable exception of the bone marrow (BM). The leukemic-prone Eμ-TCL1 transgenic mice crossed with HS1-deficient mice were compared with Eμ-TCL1 mice and showed an earlier disease onset and a reduced survival. These findings show that HS1 is a central regulator of cytoskeleton remodeling that controls lymphocyte trafficking and homing and significantly influences the tissue invasion and infiltration in CLL.

Early Events in B Cell Activation

B cell activation is initiated by the ligation of the B cell receptor (BCR) with antigen and ultimately results in the production of protective antibodies against potentially pathogenic invaders. Here we review recent literature concerned with the spatiotemporal dynamic characterization of the early molecular events of B cell activation, including the initiation of BCR triggering, the formation of BCR microclusters, and the dynamic regulation of BCR signaling. Because these events involve the considerable reorganization of molecules within the membrane, an important role for the cytoskeleton is emerging in the regulation of B cell activation. At each stage we highlight the role of the cytoskeleton, establishing its pivotal position during the initiation and regulation of B cell activation.

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.

Hematopoietic lineage cell-specific protein-1 (HS1) regulates PAR-mediated ERK activation and thromboxane generation in platelets

Thrombin-induced platelet activation leads to tyrosine phosphorylation of hematopoietic lineage cell-specific protein-1 (HS1), a 75 kDa adapter protein expressed exclusively in cells of hematopoietic lineage. We have shown HS1 to be a functionally important signaling molecule downstream of PAR-4 and GPVI collagen receptor. We have thus begun to elucidate PAR signaling pathway of HS1 phosphorylation, and its functional implications. PAR-1 and PAR-4 activating peptides (SFLLRN and AYPGKF, respectively) induced HS1 phosphorylation in a Gq-dependent manner as shown by incubation with the Gq inhibitor, YM254890. Consistently, HS1 phosphorylation was abolished in platelets from Gq deficient mice upon AYPGKF stimulation. Treatment with ADP receptor antagonists did not affect HS1 phosphorylation. Pretreatment of platelets with Src kinase inhibitors abolished HS1 phosphorylation. Further Syk activation, as measured by tyrosine phosphorylation of Syk (residues 525/526), in response to PAR activation was abolished in the presence of Src inhibitors. HS1 null mice show inhibition of PAR-mediated thromboxane A2 generation compared to wild type littermates. Phosphorylation of Erk, a key signaling molecule in thromboxane generation, was also diminished in HS1 null mice platelets. Based on these findings, we conclude that tyrosine phosphorylation of HS1 occurs downstream of both PAR-1 and PAR-4. HS1 phosphorylation is a Gq mediated response regulated by Src kinases. Thus, HS1 may mediate PAR-induced thromboxane generation through regulation of Erk phosphorylation.

Contributions of F-BAR and SH2 domains of Fes protein tyrosine kinase for coupling to the FcepsilonRI pathway in mast cells

This study investigates the roles of Fer-CIP4 homology (FCH)-Bin/amphiphysin/Rvs (F-BAR) and SH2 domains of Fes protein tyrosine kinase in regulating its activation and signaling downstream of the high-affinity immunoglobulin G (IgE) receptor (FcepsilonRI) in mast cells. Homology modeling of the Fes F-BAR domain revealed conservation of some basic residues implicated in phosphoinositide binding (R113/K114). The Fes F-BAR can bind phosphoinositides and induce tubulation of liposomes in vitro. Mutation of R113/K114 to uncharged residues (RK/QQ) caused a significant reduction in phosphoinositide binding in vitro and a more diffuse cytoplasmic localization in transfected COS-7 cells. RBL-2H3 mast cells expressing full-length Fes carrying the RK/QQ mutation show defects in FcepsilonRI-induced Fes tyrosine phosphorylation and degranulation compared to cells expressing wild-type Fes. This correlated with reduced localization to Lyn kinase-containing membrane fractions for the RK/QQ mutant compared to wild-type Fes in mast cells. The Fes SH2 domain also contributes to Fes signaling in mast cells, via interactions with the phosphorylated FcepsilonRI beta chain and the actin regulatory protein HS1. We show that Fes phosphorylates C-terminal tyrosine residues in HS1 implicated in actin stabilization. Thus, coordinated actions of the F-BAR and SH2 domains of Fes allow for coupling to FcepsilonRI signaling and potential regulation the actin reorganization in mast cells.

New insights into the early molecular events underlying B cell activation

The appropriate activation of B cells is critical for the development and operation of immune responses and is dependent on the extensive coordination of intra- and intercellular communications in response to antigen stimulation. An accurate description of the B cell-activation process requires investigation of these interactions within their correct cellular context both at high resolution and in real time. Here, we discuss a number of recent studies that have offered insight into the early molecular events of B cell activation. We suggest that segregation within the B cell membrane triggers localized cytoskeleton reorganisation and signaling, allowing the formation of B cell receptor (BCR) microclusters. These BCR microclusters are the sites for the coordinated recruitment of the signalosome and are propagated during B cell spreading. We discuss the recent identification of a critical role for CD19 in the B cell response to membrane-bound antigen and suggest a mechanism involving BCR microclusters by which it mediates its stimulatory function. Finally, we consider research that has taken advantage of recent technological advances in multiphoton microscopy that have allowed its application to the investigation of the dynamics of membrane-bound antigen presentation and subsequent B cell activation in lymph nodes in vivo.

Phospholipase C-gamma2 and Vav cooperate within signaling microclusters to propagate B cell spreading in response to membrane-bound antigen

B cell receptor (BCR) recognition of membrane-bound antigen initiates a spreading and contraction response, the extent of which is controlled through the formation of signaling-active BCR-antigen microclusters and ultimately affects the outcome of B cell activation. We followed a genetic approach to define the molecular requirements of BCR-induced spreading and microcluster formation. We identify a key role for phospholipase C-γ2 (PLCγ2), Vav, B cell linker, and Bruton's tyrosine kinase in the formation of highly coordinated “microsignalosomes,” the efficient assembly of which is absolutely dependent on Lyn and Syk. Using total internal reflection fluorescence microscopy, we examine at high resolution the recruitment of PLCγ2 and Vav to microsignalosomes, establishing a novel synergistic relationship between the two. Thus, we demonstrate the importance of cooperation between components of the microsignalosome in the amplification of signaling and propagation of B cell spreading, which is critical for appropriate B cell activation.

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

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

Studies on the actin-binding protein HS1 in platelets

BACKGROUND

The platelet cytoskeleton mediates the dramatic change in platelet morphology that takes place upon activation and stabilizes thrombus formation. The Arp2/3 complex plays a vital role in these processes, providing the protrusive force for lamellipodia formation. The Arp2/3 complex is highly regulated by a number of actin-binding proteins including the haematopoietic-specific protein HS1 and its homologue cortactin. The present study investigates the role of HS1 in platelets using HS1-/- mice.

RESULTS

The present results demonstrate that HS1 is not required for platelet activation, shape change or aggregation. Platelets from HS1-/- mice spread normally on a variety of adhesion proteins and have normal F-actin and Arp2/3 complex distributions. Clot retraction, an actin-dependent process, is also normal in these mice. Platelet aggregation and secretion is indistinguishable between knock out and littermates and there is no increase in bleeding using the tail bleeding assay.

CONCLUSION

This study concludes that HS1 does not play a major role in platelet function. It is possible that a role for HS1 is masked by the presence of cortactin.

T-cell-receptor-dependent actin regulatory mechanisms

Following stimulation, T cells undergo marked changes in actin architecture that are required for productive immune responses. T-cell-receptor-dependent reorganization of the actin cytoskeleton is necessary for the formation of the immunological synapse at the T-cell-antigen-presenting-cell contact site and the distal pole complex at the opposite face of the T cell. Convergence of specific signaling molecules within these two plasma membrane domains facilitates downstream signaling events leading to full T-cell activation. Recent studies have identified many of the relevant actin-regulatory proteins, and significant progress has been made in our understanding of how these proteins choreograph molecular movements associated with T-cell activation. Proteins such as WASp, WAVE2, HS1 and cofilin direct the formation of a cortical actin scaffold at the immune synapse, while actin-binding proteins such as ezrin and moesin direct binding of signaling molecules to actin filaments within the distal pole complex.

Hematopoietic lineage cell specific protein 1 (HS1) is a functionally important signaling molecule in platelet activation

Collagen activates platelets through an intracellular signaling cascade downstream of glycoprotein VI (GPVI). We have investigated the contribution of hematopoietic lineage cell-specific protein 1 (HS1) downstream of GPVI in platelet activation. Stimulation of GPVI leads to tyrosine phosphorylation of HS1, which is blocked by Src-family kinase inhibitors. Coimmunoprecipitation experiments revealed that HS1 associates with Syk and phosphatidylinositol 3-kinases. HS1-null mice displayed increased bleeding times and increased time to occlusion in the FeCl(3) in vivo thrombosis model compared with their wild-type littermates. In addition, aggregation and secretion responses were diminished in HS1-null mouse platelets after stimulation of GPVI and protease-activated receptor 4 (PAR-4) agonists compared with wild-type littermate mouse platelets. Finally, Akt phosphorylation was diminished after GPVI or PAR-4 stimulation in platelets from HS1-null mice compared with their wild-type littermates. These results demonstrate that phosphorylation of the HS1 protein occurs downstream of GPVI stimulation and that HS1 plays a significant functional role in platelet activation downstream of GPVI and PARs.

Acrosome-specific gene AEP1: identification, characterization and roles in spermatogenesis

Spermatogenesis is a tightly regulated process leading to the development of spermatozoa. To elucidate the molecular spermatogenic mechanisms, we identified an acrosome-specific gene AEP1 in spermatids, which is located in rat chromosome 17p14 with a transcript size of 3,091 bp encoding a signal peptide, zinc finger-like motif, coiled-coil region, several predicted glycosylation and phosphorylation sites. Northern blot and RT-PCR analyses revealed the restricted expression of AEP1 to the testis only. In postnatal rat testes, AEP1 mRNA became detectable from postnatal 25 dpp (round spermatids) and onwards. By using in situ hybridization (ISH) and flow cytometry-fluorescent ISH, only the haploid spermatids yielded the positive AEP1 signal. Immunohistochemistry showed that AEP1 was expressed in the acrosomal cap of late-staged germ cells in rat testis, and co-localized with the acrosomal marker, peanut agglutinin. The spatial expression of AEP1 immunoreactivity in testis was conserved among diverse mammalian species (rat, pig, monkey, human). To further study its roles in spermatogenesis, we showed AEP1 and beta-actin was associated together in complex by co-immunoprecipitation in adult germ cells and by immunofluorescence assay in isolated spermatozoon. In human testes diagnosed with hypospermatogenesis, lower expression of AEP1 was observed, whereas there was no detectable signal in undescended testes. In short, AEP1 is an evolutionary-conserved acrosome-specific gene and likely functions in acrosome-cap formation.

Regulation of cytoskeletal dynamics at the immune synapse: new stars join the actin troupe

Reorganization of actin cytoskeletal dynamics plays a critical role in controlling T-lymphocyte activation and effector functions. Interaction of T-cell receptors (TCR) with appropriate major histocompatibility complex-peptide complexes on antigen-presenting cells results in the activation of signaling cascades, leading to the accumulation of F-actin at the cell-cell contact site. This event is required for the formation and stabilization of the immune synapse (IS), a cellular structure essential for the modulation of T-cell responses. Analysis of actin cytoskeletal dynamics following engagement of the TCR has largely focused on the Arp2/3 regulator, WASp, because of its early identification and its association with human disease. However, recent studies have shown equally important roles for several additional actin regulatory proteins. In this review, we turn the spotlight on the expanding cast of actin regulatory proteins, which co-ordinate actin dynamics at the IS.

HS1 functions as an essential actin-regulatory adaptor protein at the immune synapse

HS1, the leukocyte-specific homolog of cortactin, regulates F-actin in vitro and is phosphorylated in response to TCR ligation, but its role in lymphocyte activation has not been addressed. We demonstrate that HS1-deficient T cells fail to accumulate F-actin at the immune synapse (IS) and, upon TCR ligation, form actin-rich structures that are disordered and unstable. Early TCR activation events are intact in these cells, but Ca2+ influx and IL-2 gene transcription are defective. Importantly, HS1 tyrosine phosphorylation is required for its targeting to the IS and for its function in regulating actin dynamics and IL-2 promoter activity. Phosphorylation also links HS1 to multiple signaling proteins, including Lck, PLCgamma1, and Vav1, and is essential for the stable recruitment of Vav1 to the IS. Taken together, our studies show that HS1 is indispensable for signaling events leading to actin assembly and IL-2 production during T cell activation.

Syk is required for monocyte/macrophage chemotaxis to CX3CL1 (Fractalkine)

CX3CL1 (fractalkine), the only member of the delta subclass of chemokines, is a known chemotactic factor for monocytes/macrophages as well as NK cells and T lymphocytes. In several pathologies, excessive production of CX3CL1 at specific sites leads primarily to monocyte/macrophage recruitment, which causes tissue and vascular damage. Despite their clinical relevance, the mechanisms underlying monocyte/macrophage chemotaxis to CX3CL1 remain poorly documented. The present report addresses this issue and identifies cell signaling crucial for this process. Using the murine monocyte/macrophage RAW cell line, we show that CX3CL1 treatment elicits a rapid and transient increase in F-actin and the formation of F-actin-enriched cell protrusions. CX3CL1 also triggers tyrosine phosphorylation of proteins localized in those protrusions. The protein tyrosine kinase Syk is activated upon CX3CL1 treatment, and reduction of Syk expression using RNA-mediated interference results in a specific and massive impairment of RAW cell migration to CX3CL1. Similar results are obtained using the Syk inhibitor, piceatannol. Cells with reduced Syk expression also exhibit a major defect in CX3CL1-induced cytoskeletal remodeling. These data suggest that in monocytes/macrophages, Syk is essential for proper reorganization of the actin cytoskeleton in response to CX3CL1 and is therefore required for cell chemotaxis to CX3CL1.

The coiled-coil domain is required for HS1 to bind to F-actin and activate Arp2/3 complex

HS1 (hematopoietic lineage cell-specific protein 1), a substrate of protein tyrosine kinases in lymphocytes, binds to F-actin, and promotes Arp2/3 complex-mediated actin polymerization. However, the mechanism for the interaction between HS1 and F-actin has not yet been fully characterized. HS1 contains 3.5 tandem repeats, a coiled-coil region, and an SH3 domain at the C terminus. Unlike cortactin, which is closely related to HS1 and requires absolutely the repeat domain for F-actin binding, an HS1 mutant with deletion of the repeat domain maintains a significant F-actin binding activity. On the other hand, deletion of the coiled-coil region abolished the ability of HS1 to bind to actin filaments and to activate the Arp2/3 complex for actin nucleation and actin branching. Furthermore, a peptide containing the coiled-coil sequence only was sufficient for F-actin binding. Within cells overexpressing green fluorescent protein-tagged HS1 proteins, wild type HS1 co-localizes with cortical F-actin at the cell leading edge, whereas mutants with deletion of either the coiled-coil region or the repeat domain diffuse in the cytoplasm. Immunoprecipitation analysis reveals that the coiled-coil deletion mutant binds poorly to F-actin, whereas the mutant without the repeat domain fails to bind to both Arp2/3 complex and F-actin. These data suggest that the HS1 coiled-coil region acts synergistically with the repeat domain in the modulation of the Arp2/3 complex-mediated actin polymerization.

Thrombin-induced tyrosine phosphorylation of HS1 in human platelets is sequentially catalyzed by Syk and Lyn tyrosine kinases and associated with the cellular migration of the protein

Thrombin stimulation of platelets triggers Tyr phosphorylation of several signaling proteins, most of which remain unidentified. In this study, we demonstrate for the first time that hematopoietic lineage cell-specific protein 1 (HS1) undergoes a transient Tyr phosphorylation in human platelets stimulated with thrombin. The protein is synergistically phosphorylated by Syk and Lyn tyrosine kinases according to a sequential phosphorylation mechanism. By means of specific inhibitors (PP2, SU6656, and piceatannol) and phosphopeptide-specific antibodies, as well as by coimmunoprecipitation and binding competition experiments, we show that Syk acts as the primary kinase that phosphorylates HS1 at Tyr397 and that Syk phosphorylation is required for HS1 interaction with the Lyn SH2 domain. Upon docking to Syk-phosphorylated HS1, Lyn catalyzes the secondary phosphorylation of the protein at Tyr222. Once the secondary Tyr phosphorylation of HS1 is accomplished the protein dissociates from Lyn and undergoes a dephosphorylation process. HS1 Tyr phosphorylation does not occur when thrombin-induced actin assembly is inhibited by cytochalasin D even under conditions in which Syk and Lyn are still active. Immunofluorescence microscopic analysis shows that the agonist promotes HS1 migration to the plasma membrane and that the inhibition of Lyn-mediated secondary phosphorylation of HS1 abrogates the subcellular translocation of the protein. All together these results indicate that HS1 Tyr phosphorylation catalyzed by Syk and Lyn plays a crucial role in the translocation of the protein to the membrane and is involved in the cytoskeleton rearrangement triggered by thrombin in human platelets.

Lysosome is a primary organelle in B cell receptor-mediated apoptosis: an indispensable role of Syk in lysosomal function

To investigate the mechanism of B cell receptor (BCR)-mediated apoptosis, we utilized immature B cell lines, DT40 and WEHI-231. In both cell lines, BCR-crosslinking caused the increase in lysosomal pH with early apoptotic changes characterized by chromatin condensation and phosphatidylserine exposure. This increase was detected in c-Abl-deficient DT40 cells but not in Syk-deficient cells, which corresponded to the fact that the former cells but not the latter revealed BCR-induced apoptosis. In contrast, BCR-crosslinking caused no apparent change in mitochondrial transmembrane potential. Therefore, the lysosomal change might be a primary event in BCR-induced apoptosis in DT40 cells. The increased activity of cathepsin B and apoptosis-preventing effect of a cathepsin inhibitor suggested a significant role of lysosomal enzymes in this apoptosis. By microscopic studies, lysosomes of wild-type DT40 cells fused to BCR-carrying endosomes became enlarged and accumulated one another. In contrast, these changes of lysosomal dynamics did not occur in Syk-deficient cells but transfer of wild-type Syk restored the lysosomal changes and apoptosis. These results demonstrated that the lysosomal change accompanied with the activation of lysosomal enzymes is a primary step in BCR-crosslinking-mediated apoptosis and Syk is responsible for this step through the fusion of BCR-carrying endosomes to lysosomes.