SH3P7 is a cytoskeleton adapter protein and is coupled to signal transduction from lymphocyte antigen receptors

Nanoparticles Internalization through HIP-55-Dependent Clathrin Endocytosis Pathway

Nanoparticles are promising tools for biomedicine. Many nanoparticles are internalized to function. Clathrin-mediated endocytosis is one of the most important mechanisms for nanoparticle internalization. However, the regulatory mechanism of clathrin-mediated nanoparticle endocytosis is still unclear. Here, we report that the adapter protein HIP-55 regulates clathrin-mediated nanoparticle endocytosis. CdSe/ZnS quantum dots (QDs), a typical nanoparticle, enter cells through the HIP-55-dependent clathrin endocytosis pathway. Both pharmacological inhibitor and genetic intervention demonstrate that QDs enter cells through clathrin-mediated endocytosis. HIP-55 can interact with clathrin and promote clathrin-mediated QDs endocytosis. Furthermore, HIP-55 ΔADF which is defective in F-actin binding fails to promote QDs endocytosis, indicating HIP-55 promotes clathrin-mediated QDs endocytosis depending on interaction with F-actin. In vivo, HIP-55 knockout also inhibits endocytosis of QDs. These findings reveal that HIP-55 acts as an intrinsic regulator for clathrin-mediated nanoparticle endocytosis, providing new insight into the nanoparticle internalization and a new strategy for nanodrug enrichment in target cells.

Dimerization of Human Drebrin-like Protein Governs Its Biological Activity

Drebrin-like protein (DBNL) is a multidomain F-actin-binding protein, which also interacts with other molecules within different intracellular pathways. Here, we present quantitative measurements on the size and conformation of human DBNL. Using dual-focus fluorescence correlation spectroscopy, we determined the hydrodynamic radius of the DBNL monomer. Native gel electrophoresis and dual-color fluorescence cross-correlation spectroscopy show that both endogenous DBNL and recombinant DBNL exist as dimers under physiological conditions. We demonstrate that C-terminal truncations of DBNL downstream of the coiled-coil domain result in its oligomerization at nanomolar concentrations. In contrast, the ADF-H domain alone is a monomer, which displays a concentration-dependent self-assembly. In vivo FLIM-FRET imaging shows that the presence of only actin-binding domains is not sufficient for DBNL to localize properly at the actin filament inside the cell. In summary, our work provides detailed insight into the structure-function relationship of human drebrin-like protein.

Multi-omic analysis reveals HIP-55-dependent regulation of cytokines release

HIP-55 (HPK1 [hematopoietic progenitor kinase 1] -interacting protein of 55 kDa) contains an actin-depolymerizing factor homology (ADF-H) domain at the N-terminus and a src homology 3 (SH3) domain at the C-terminus, which plays an important role in the T cell receptor (TCR) and B-cell receptor (BCR) signaling and immune system. In our previous studies, HIP-55 was found to be highly expressed in several types of tumors and function as a novel oncogenic signaling hub that regulates tumor progression and metastasis through defined functional domains, actin-binding and SH3 modules. However, the wider functions and mechanisms of HIP-55 are still unclear. Here, multi-omic analysis revealed that one of the main biofunctions of HIP-55 is the regulation of cytokines release. Furthermore, to investigate the role of HIP-55 in the cytokine production, a series Cytokine Antibody Arrays were performed to detect differentially expressed cytokines between control and HIP-55 knockdown cells. A total of 97 differentially expressed cytokines were identified from 300 cytokines in A549 cell. Bioinformatics analysis showed these differentially cytokines were mainly enriched in cancer signal pathways and IL-6 is the most critical hub in the integrated network. Analysis of RNAseq data from lung cancer patients showed that there is a strong negative correlation between HIP-55 and interleukin-6 (IL-6) in samples from lung adenocarcinoma patients. Our data indicated that HIP-55 may participate in cancer progression and metastasis via regulating cytokines release.

Identification of serine residues in the connexin43 carboxyl tail important for BCR-mediated spreading of B-lymphocytes

B-lymphocytes recognize antigen via B-cell antigen receptors (BCRs). This binding induces signaling, leading to B-cell activation, proliferation and differentiation. Early events of BCR signaling include reorganization of actin and membrane spreading, which facilitates increased antigen gathering. We have previously shown that the gap junction protein connexin43 (Cx43; also known as GJA1) is phosphorylated upon BCR signaling, and its carboxyl tail (CT) is important for BCR-mediated spreading. Here, specific serine residues in the Cx43 CT that are phosphorylated following BCR stimulation were identified. A chimeric protein containing the extracellular and transmembrane domains of CD8 fused to the Cx43 CT was sufficient to support cell spreading. Cx43 CT truncations showed that the region between amino acids 246-307 is necessary for B-cell spreading. Site-specific serine-to-alanine mutations (S255A, S262A, S279A and S282A) resulted in differential effects on both BCR signaling and BCR-mediated spreading. These serine residues can serve as potential binding sites for actin remodeling mediators and/or BCR signaling effectors; therefore, our results may reflect unique roles for each of these serines in terms of linking the Cx43 CT to actin remodeling.

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.

Identification of a new type of haematopoietic progenitor kinase-interacting protein (HIP-55) in Aedes aegypti mosquito haemocytes and its involvement in immunity-like functions in mosquito: a molecular study

In this study, we characterize the HIP-55 protein in the mosquito Aedes aegypti for the first time. HIP-55 is a 55-kDa HPK1-interacting protein that is also called SH3P7. HIP-55 constitutively binds HPK1 'via' an HPK1 proline-rich motif 2(PR2) through its C-terminal SH3 domain. HIP-55 critically interacts with ZAP-70, and this interaction was induced by TCR signalling. ZAP-70 phosphorylated HIP-55 at Tyr-334 and Tyr-344 in vitro and in vivo. In our previous findings, AaZAP gene expression strongly proved that AaZAP-70 was involved in immunity-like functions in mosquito. Northern blot analysis of HIP-55 mRNA expression confirmed that it is only expressed in the abdomen and haemocyte tissues; this prediction correlates 100% and a polyclonal antibody also confirmed its localization in haemocytes and the abdomen. We prepared extracts to show the cytoplasmic expression (CE) of this protein. Previous results had proven that this protein is secreted from the cytoplasm; thus, we confirmed here that the protein is a cytoplasmic adaptor protein in mosquitoes and mammalian systems. Furthermore, our polyclonal antibody against HIP-55 also demonstrated that this protein is found in haemocytes and abdomen tissues, which assumes that the protein may be involved in phagocytic-like functions. RNAi (siRNA) silencing studies were used to degrade mosquito HIP-55; however, silencing only slightly affected the HIP-55 sequence and the gene transcriptional level. To characterize this protein, we cloned 609 bp from the 1.6-kb full-length cDNA using a pET28 vector for polyclonal antibody production. Graphical abstract.

Drebrin-like (Dbnl) Controls Neuronal Migration via Regulating N-Cadherin Expression in the Developing Cerebral Cortex

The actin cytoskeleton is crucial for neuronal migration in the mammalian developing cerebral cortex. The adaptor protein Drebrin-like (Dbnl) plays important roles in reorganization of the actin cytoskeleton, dendrite formation, and endocytosis by interacting with F-actin, cobl, and dynamin. Although Dbnl is known to be expressed in the brain, the functions of this molecule during brain development are largely unknown. In this study, to examine the roles of Dbnl in the developing cerebral cortex, we conducted experiments using mice of both sexes with knockdown of Dbnl, effected by in utero electroporation, in the migrating neurons of the embryonic cortex. Time-lapse imaging of the Dbnl-knockdown neurons revealed that the presence of Dbnl is a prerequisite for appropriate formation of processes in the multipolar neurons in the multipolar cell accumulation zone or the deep part of the subventricular zone, and for neuronal polarization and entry into the cortical plate. We found that Dbnl knockdown decreased the amount of N-cadherin protein expressed on the plasma membrane of the cortical neurons. The defect in neuronal migration caused by Dbnl knockdown was rescued by moderate overexpression of N-cadherin and αN-catenin or by transfection of the phospho-mimic form (Y337E, Y347E), but not the phospho-resistant form (Y337F, Y347F), of Dbnl. These results suggest that Dbnl controls neuronal migration, neuronal multipolar morphology, and cell polarity in the developing cerebral cortex via regulating N-cadherin expression.SIGNIFICANCE STATEMENT Disruption of neuronal migration can cause neuronal disorders, such as lissencephaly and subcortical band heterotopia. During cerebral cortical development, the actin cytoskeleton plays a key role in neuronal migration; however, the mechanisms of regulation of neuronal migration by the actin cytoskeleton still remain unclear. Herein, we report that the novel protein Dbnl, an actin-binding protein, controls multiple events during neuronal migration in the developing mouse cerebral cortex. We also showed that this regulation is mediated by phosphorylation of Dbnl at tyrosine residues 337 and 347 and αN-catenin/N-cadherin, suggesting that the Dbnl-αN-catenin/N-cadherin pathway is important for neuronal migration in the developing cortex.

T Cell-Dependent Affinity Maturation and Innate Immune Pathways Differentially Drive Autoreactive B Cell Responses in Rheumatoid Arthritis

OBJECTIVE

Rheumatoid arthritis (RA) is characterized by the activation of B cells that produce anti-citrullinated protein antibodies (ACPAs) and rheumatoid factors (RFs), but the mechanisms by which tolerance is broken in these B cells remain incompletely understood. We undertook this study to investigate whether ACPA+ and RF+ B cells break tolerance through distinct molecular mechanisms.

METHODS

We developed antigen-tetramers to isolate ACPA+ and RF+ B cells and performed single-cell RNA sequencing on 2,349 B cells from 6 RA patients and 1 healthy donor to analyze their immunoglobulin repertoires and transcriptional programs. Prominent immunoglobulins were expressed as monoclonal antibodies and tested for autoantigen reactivity.

RESULTS

ACPA+ and RF+ B cells were enriched in the peripheral blood of RA patients relative to healthy controls. Characterization of patient-derived monoclonal antibodies confirmed ACPA and RF targeting of tetramer-specific B cells at both antigen-inexperienced and affinity-matured B cell stages. ACPA+ B cells used more class-switched isotypes and exhibited more somatic hypermutations relative to RF+ B cells, and these differences were accompanied by down-regulation of CD72 and up-regulation of genes that promote class-switching and T cell-dependent responses. In contrast, RF+ B cells expressed transcriptional programs that stimulate rapid memory reactivation through multiple innate immune pathways. Coexpression analysis revealed that ACPA+ and RF+ B cell-enriched genes belong to distinct transcriptional regulatory networks.

CONCLUSION

Our findings suggest that ACPA+ and RF+ B cells are imprinted with distinct transcriptional programs, which suggests that these autoantibodies associated with increased inflammation in RA arise from 2 different molecular mechanisms.

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.

The role of the drebrin/EB3/Cdk5 pathway in dendritic spine plasticity, implications for Alzheimer's disease

The drebrin/EB3/Cdk5 intracellular signalling pathway couples actin filaments to dynamic microtubules in cellular settings where cells are changing shape. The pathway has been most intensively studied in neuronal development, particularly neuritogenesis and neuronal migration, and in synaptic plasticity at dendritic spines in mature neurons. Drebrin is an actin filament side-binding and bundling protein that stabilises actin filaments. The end-binding (EB) proteins are microtubule plus-end tracking proteins (+TIPs) that localise to the growing plus-ends of dynamic microtubules and regulate their behavior and the binding of other +TIP proteins. EB3 binds specifically to drebrin when drebrin is bound to actin filaments, for example at the base of a growth cone filopodium, and EB3 is located at the plus-end of a growing microtubule inserting into the filopodium. This interaction therefore forms the basis for coupling dynamic microtubules to actin filaments in growth cones of developing neurons. Appropriate responses to growth cone guidance cues depend on actin filament/microtubule co-ordination in the growth cone, although the role of the drebrin/EB3/Cdk5 pathway in this context has not been directly tested. A similar cytoskeleton coupling pathway operates in dendritic spines in mature neurons where the activity-dependent insertion of dynamic microtubules into dendritic spines is facilitated by drebrin binding to EB3. Microtubule insertion into dendritic spines drives spine maturation during long-term potentiation and therefore has a role in synaptic plasticity and memory formation. In Alzheimer's disease and related chronic neurodegenerative diseases, there is an early and dramatic loss of drebrin from dendritic spines that precedes synapse loss and neurodegeneration and might contribute to a failure of synaptic plasticity and hence to cognitive decline.

Muscleblind-like 1 suppresses breast cancer metastatic colonization and stabilizes metastasis suppressor transcripts

Post-transcriptional deregulation is a defining feature of metastatic cancer. While many microRNAs have been implicated as regulators of metastatic progression, less is known about the roles and mechanisms of RNA-binding proteins in this process. We identified muscleblind-like 1 (MBNL1), a gene implicated in myotonic dystrophy, as a robust suppressor of multiorgan breast cancer metastasis. MBNL1 binds the 3' untranslated regions (UTRs) of DBNL (drebrin-like protein) and TACC1 (transforming acidic coiled-coil containing protein 1)-two genes that we implicate as metastasis suppressors. By enhancing the stability of these genes' transcripts, MBNL1 suppresses cell invasiveness. Consistent with these findings, elevated MBNL1 expression in human breast tumors is associated with reduced metastatic relapse likelihood. Our findings delineate a post-transcriptional network that governs breast cancer metastasis through RNA-binding protein-mediated transcript stabilization.

Intracellular signalling during neutrophil recruitment

Recruitment of leucocytes such as neutrophils to the extravascular space is a critical step of the inflammation process and plays a major role in the development of various diseases including several cardiovascular diseases. Neutrophils themselves play a very active role in that process by sensing their environment and responding to the extracellular cues by adhesion and de-adhesion, cellular shape changes, chemotactic migration, and other effector functions of cell activation. Those responses are co-ordinated by a number of cell surface receptors and their complex intracellular signal transduction pathways. Here, we review neutrophil signal transduction processes critical for recruitment to the site of inflammation. The two key requirements for neutrophil recruitment are the establishment of appropriate chemoattractant gradients and the intrinsic ability of the cells to migrate along those gradients. We will first discuss signalling steps required for sensing extracellular chemoattractants such as chemokines and lipid mediators and the processes (e.g. PI3-kinase pathways) leading to the translation of extracellular chemoattractant gradients to polarized cellular responses. We will then discuss signal transduction by leucocyte adhesion receptors (e.g. tyrosine kinase pathways) which are critical for adhesion to, and migration through the vessel wall. Finally, additional neutrophil signalling pathways with an indirect effect on the neutrophil recruitment process, e.g. through modulation of the inflammatory environment, will be discussed. Mechanistic understanding of these pathways provide better understanding of the inflammation process and may point to novel therapeutic strategies for controlling excessive inflammation during infection or tissue damage.

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.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.

Pro-oncogenic function of HIP-55/Drebrin-like (DBNL) through Ser269/Thr291-phospho-sensor motifs

HIP-55 (HPK1-interacting protein of 55 kDa, also named DBNL, SH3P7, and mAbp1) is a multidomain adaptor protein that is critical for organ development and the immune response. Here, we report the coupling of HIP-55 to cell growth control through its 14-3-3-binding phospho-Ser/Thr-sensor sites. Using affinity chromatography, we found HIP-55 formed a complex with 14-3-3 proteins, revealing a new node in phospho-Ser/Thr-mediated signaling networks. In addition, we demonstrated that HIP-55 is required for proper cell growth control. Enforced HIP-55 expression promoted proliferation, colony formation, migration, and invasion of lung cancer cells while silencing of HIP-55 reversed these effects. Importantly, HIP-55 was found to be upregulated in lung cancer cell lines and in tumor tissues of lung cancer patients. Upregulated HIP-55 was required to promote the growth of tumors in a xenograft animal model. However, tumors with S269A/T291A-mutated HIP-55, which ablates 14-3-3 binding, exhibited significantly reduced sizes, supporting a vital role of the HIP-55/14-3-3 protein interaction node in transmitting oncogenic signals. Mechanistically, HIP-55-mediated tumorigenesis activity appears to be in part mediated by antagonizing the tumor suppressor function of HPK1. Thus, the HIP-55–mediated oncogenic pathway, through S269/T291, may be exploited for the development of new therapeutic strategies.

Adaptor regulation of LFA-1 signaling in T lymphocyte migration: Potential druggable targets for immunotherapies?

The integrin lymphocyte function associated antigen-1 (LFA-1) plays a key role in leukocyte trafficking and in adaptive immune responses through interactions with adhesive ligands, such as ICAM-1. Specific blockade of these interactions has validated LFA-1 as a therapeutic target in many chronic inflammatory diseases, however LFA-1 antagonists have not been clinically successful due to the development of a general immunosuppression, causing fatal side effects. Growing evidence has now established that LFA-1 mediates an array of intracellular signaling pathways by triggering a number of downstream molecules. In this context, a class of multimodular domain-containing proteins capable of recruiting two or more effector molecules, collectively known as "adaptor proteins," has emerged as important mediators in LFA-1 signal transduction. Here, we provide an overview of the adaptor proteins involved in the intracellular signaling cascades by which LFA-1 regulates T-cell motility and immune responses. The complexity of the LFA-1-associated signaling delineated in this review suggests that it may be an important and challenging focus for future research, enabling the identification of "tunable" targets for the development of immunotherapies.

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.

Functional roles of Syk in macrophage-mediated inflammatory responses

Inflammation is a series of complex biological responses to protect the host from pathogen invasion. Chronic inflammation is considered a major cause of diseases, such as various types of inflammatory/autoimmune diseases and cancers. Spleen tyrosine kinase (Syk) was initially found to be highly expressed in hematopoietic cells and has been known to play crucial roles in adaptive immune responses. However, recent studies have reported that Syk is also involved in other biological functions, especially in innate immune responses. Although Syk has been extensively studied in adaptive immune responses, numerous studies have recently presented evidence that Syk has critical functions in macrophage-mediated inflammatory responses and is closely related to innate immune response. This review describes the characteristics of Syk-mediated signaling pathways, summarizes the recent findings supporting the crucial roles of Syk in macrophage-mediated inflammatory responses and diseases, and discusses Syk-targeted drug development for the therapy of inflammatory diseases.

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.

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.

Drebrin contains a cryptic F-actin-bundling activity regulated by Cdk5 phosphorylation

Drebrin activity in F-actin bundling and filopodia induction relies on two adjacent F-actin binding sites and a Cdk5 phosphorylation-regulated intramolecular inhibitory interaction.

Drebrin is an actin filament (F-actin)–binding protein with crucial roles in neuritogenesis and synaptic plasticity. Drebrin couples dynamic microtubules to F-actin in growth cone filopodia via binding to the microtubule-binding +TIP protein EB3 and organizes F-actin in dendritic spines. Precisely how drebrin interacts with F-actin and how this is regulated is unknown. We used cellular and in vitro assays with a library of drebrin deletion constructs to map F-actin binding sites. We discovered two domains in the N-terminal half of drebrin—a coiled-coil domain and a helical domain—that independently bound to F-actin and cooperatively bundled F-actin. However, this activity was repressed by an intramolecular interaction relieved by Cdk5 phosphorylation of serine 142 located in the coiled-coil domain. Phospho-mimetic and phospho-dead mutants of serine 142 interfered with neuritogenesis and coupling of microtubules to F-actin in growth cone filopodia. These findings show that drebrin contains a cryptic F-actin–bundling activity regulated by phosphorylation and provide a mechanistic model for microtubule–F-actin coupling.

Development of an orally-administrative MELK-targeting inhibitor that suppresses the growth of various types of human cancer

We previously reported MELK (maternal embryonic leucine zipper kinase) as a novel therapeutic target for breast cancer. MELK was also reported to be highly upregulated in multiple types of human cancer. It was implied to play indispensable roles in cancer cell survival and indicated its involvement in the maintenance of tumor-initiating cells. We conducted a high-throughput screening of a compound library followed by structure-activity relationship studies, and successfully obtained a highly potent MELK inhibitor OTSSP167 with IC₅₀ of 0.41 nM. OTSSP167 inhibited the phosphorylation of PSMA1 (proteasome subunit alpha type 1) and DBNL (drebrin-like), which we identified as novel MELK substrates and are important for stem-cell characteristics and invasiveness. The compound suppressed mammosphere formation of breast cancer cells and exhibited significant tumor growth suppression in xenograft studies using breast, lung, prostate, and pancreas cancer cell lines in mice by both intravenous and oral administration. This MELK inhibitor should be a promising compound possibly to suppress the growth of tumor-initiating cells and be applied for treatment of a wide range of human cancer.

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.

Spatiotemporal regulation of Src and its substrates at invadosomes

In the past decade, substantial progress has been made in understanding how Src family kinases regulate the formation and function of invadosomes. Invadosomes are organized actin-rich structures that contain an F-actin core surrounded by an adhesive ring and mediate invasive migration. Src kinases orchestrate, either directly or indirectly, each phase of the invadosome life cycle including invadosome assembly, maturation and matrix degradation and disassembly. Complex arrays of Src effector proteins are involved at different stages of invadosome maturation and their spatiotemporal activity must be tightly regulated to achieve effective invasive migration. In this review, we highlight some recent progress and the challenges of understanding how Src is regulated temporally and spatially to orchestrate the dynamics of invadosomes and mediate cell invasion.

The importance of conserved features of yeast actin-binding protein 1 (Abp1p): the conditional nature of essentiality

Saccharomyces cerevisiae Actin-Binding Protein 1 (Abp1p) is a member of the Abp1 family of proteins, which are in diverse organisms including fungi, nematodes, flies, and mammals. All proteins in this family possess an N-terminal Actin Depolymerizing Factor Homology (ADF-H) domain, a central Proline-Rich Region (PRR), and a C-terminal SH3 domain. In this study, we employed sequence analysis to identify additional conserved features of the family, including sequences rich in proline, glutamic acid, serine, and threonine amino acids (PEST), which are found in all family members examined, and two motifs, Conserved Fungal Motifs 1 and 2 (CFM1 and CFM2), that are conserved in fungi. We also discovered that, similar to its mammalian homologs, Abp1p is phosphorylated in its PRR. This phosphorylation is mediated by the Cdc28p and Pho85p kinases, and it protects Abp1p from proteolysis mediated by the conserved PEST sequences. We provide evidence for an intramolecular interaction between the PRR region and SH3 domain that may be affected by phosphorylation. Although deletion of CFM1 alone caused no detectable phenotype in any genetic backgrounds or conditions tested, deletion of this motif resulted in a significant reduction of growth when it was combined with a deletion of the ADF-H domain. Importantly, this result demonstrates that deletion of highly conserved domains on its own may produce no phenotype unless the domains are assayed in conjunction with deletions of other functionally important elements within the same protein. Detection of this type of intragenic synthetic lethality provides an important approach for understanding the function of individual protein domains or motifs.

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.

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.

The adapter protein Nck: role of individual SH3 and SH2 binding modules for protein interactions in T lymphocytes

Nck is a ubiquitously expressed, primarily cytosolic adapter protein consisting of one SH2 domain and three SH3 domains. It links receptor and nonreceptor tyrosine kinases to actin cytoskeleton reorganizing proteins. In T lymphocytes, Nck is a crucial component of signaling pathways for T cell activation and effector function. It recruits actin remodeling proteins to T cell receptor (TCR)-associated activation clusters and thereby initiates changes in cell polarity and morphology. Moreover, Nck is crucial for the TCR-induced mobilization of secretory vesicles to the cytotoxic immunological synapse. To identify the interactome of Nck in human T cells, we performed a systematic screen for interaction partners in untreated or pervanadate-treated cells. We used GST fusion proteins containing full length Nck, the combined SH3 domains or the individual SH3 and SH2 domains to precipitate putative Nck interactors from cellular lysates. Protein bands were excised from gels, processed by tryptic in-gel digestion and analyzed by mass spectrometry. Using this approach, we confirmed previously established interactions (e.g., with Slp76, CD3 epsilon, WASP, and WIPF1) and identified several novel putative Nck-binding proteins. We subsequently verified the SH2 domain binding to the actin-binding protein HIP55 and to FYB/ADAP, and the SH3-mediated binding to the nuclear proteins SFPQ/NONO. Using laser scanning microscopy, we provide new evidence for a nuclear localization of Nck in human T cells. Our data highlight the fundamental role of Nck in the TCR-to-cytoskeleton crosstalk and point to yet unknown nuclear functions of Nck also in T lymphocytes.

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.

Proteomic analysis of resting and thrombin-stimulated platelets reveals the translocation and functional relevance of HIP-55 in platelets

The platelet surface is a dynamic interface that changes rapidly in response to stimuli to co-ordinate the formation of thrombi at sites of vascular injury. Tight control is essential as loss of organisation may result in the inappropriate formation of thrombi (thrombosis) or excessive bleeding. In this paper we describe the comparative analysis of resting and thrombin-stimulated platelet membrane proteomes and associated proteins to identify proteins important to platelet function. Surface proteins were labelled using a biotin tag and isolated by NeurtrAvidin affinity chromatography. Liquid phase IEF and SDS-PAGE were used to separate proteins, and bands of increased intensity in the stimulated platelet fractions were digested and identified by FT-ICR mass spectrometry. Novel proteins were identified along with proteins known to be translocated to the platelet surface. Furthermore, many platelet proteins revealed changes in location associated with function, including G6B and Hip-55. HIP-55 is an SH3-binding protein important in T-cell receptor signalling. Further analysis of HIP-55 revealed that this adaptor protein becomes increasingly associated with both Syk and integrin beta3 upon platelet activation. Analysis of HIP-55 deficient platelets revealed reduced fibrinogen binding upon thrombin stimulation, suggesting HIP-55 to be an important regulator of platelet function.

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.

The actin-binding protein Abp1 controls dendritic spine morphology and is important for spine head and synapse formation

Polymerization and organization of actin into complex superstructures, including those found in dendritic spines, is indispensable for structure and function of neuronal networks. Here we show that the filamentous actin (F-actin)-binding protein 1 (Abp1), which controls Arp2/3 complex-mediated actin nucleation and binds to postsynaptic scaffold proteins of the ProSAP (proline-rich synapse-associated protein 1)/Shank family, has a profound impact on synaptic organization. Overexpression of the two Abp1 F-actin-binding domains increases the length of thin, filopodia-like and mushroom-type spines but dramatically reduces mushroom spine density, attributable to lack of the Abp1 Src homology 3 (SH3) domain. In contrast, overexpression of full-length Abp1 increases mushroom spine and synapse density. The SH3 domain alone has a dominant-negative effect on mushroom spines, whereas the density of filopodia and thin, immature spines remains unchanged. This suggests that both actin-binding and SH3 domain interactions are crucial for the role of Abp1 in spine maturation. Indeed, Abp1 knockdown significantly reduces mushroom spine and synapse density. Abp1 hereby works in close conjunction with ProSAP1/Shank2 and ProSAP2/Shank3, because Abp1 effects were suppressed by ProSAP2 RNA interference and the ProSAP/Shank-induced increase of spine head width is further promoted by Abp1 cooverexpression and reduced on Abp1 knockdown. Also, interfering with the formation of functional Abp1-ProSAP protein complexes prevents ProSAP-mediated spine head extension. Spine head extension furthermore depends on local Arp2/3 complex-mediated actin polymerization, which is controlled by Abp1 via the Arp2/3 complex activator N-WASP (neural Wiskott-Aldrich syndrome protein). Abp1 thus plays an important role in the formation and morphology control of synapses by making a required functional connection between postsynaptic density components and postsynaptic actin dynamics.

Tubulin Polymerization Modulates Interleukin-2 Receptor Signal Transduction in Human T Cells

Few data exist on the modulation of cytokine receptor signaling by the actin or tubulin cytoskeleton. Therefore, we studied interleukin-2 receptor (IL-2R) signaling in phytohemagglutinine (PHA)-pretreated human T cells in the context of alterations in the cytoskeletal system induced by cytochalasin D (CyD), jasplaklinolide (Jas), taxol (Tax), or colchicine (Col). We found that changes in cytoskeletal tubulin polymerization altered the strength of several IL-2-triggered signals. Moreover, Tax-induced tubulin hyperpolymerization augmented the surface expression of the IL-2R ss -chain and enhanced the association of the IL-2R beta -chain with cytoskeletal tubulin. The IL-2R beta-chain, in turn, was constitutively associated with tubulin and, more weakly, actin. To exclude the possibility that these associations are artifacts caused by PHA, we confirmed them in T cells from TCR-transgenic DO 11.10 mice stimulated with their nominal antigen. We conclude that altered polymerization of cytoskeletal components, especially tubulin, is accompanied by modulation of IL-2 signaling at the receptor level.

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

The BCR serves as both signal transducer and Ag transporter. Binding of Ags to the BCR induces signaling cascades and Ag processing and presentation, two essential cellular events for B cell activation. BCR-initiated signaling increases BCR-mediated Ag-processing efficiency by increasing the rate and specificity of Ag transport. Previous studies showed a critical role for the actin cytoskeleton in these two processes. In this study, we found that actin-binding protein 1 (Abp1/HIP-55/SH3P7) functioned as an actin-binding adaptor protein, coupling BCR signaling and Ag-processing pathways with the actin cytoskeleton. Gene knockout of Abp1 and overexpression of the Src homology 3 domain of Abp1 inhibited BCR-mediated Ag internalization, consequently reducing the rate of Ag transport to processing compartments and the efficiency of BCR-mediated Ag processing and presentation. BCR activation induced tyrosine phosphorylation of Abp1 and translocation of both Abp1 and dynamin 2 from the cytoplasm to plasma membrane, where they colocalized with the BCR and cortical F-actin. Mutations of the two tyrosine phosphorylation sites of Abp1 and depolymerization of the actin cytoskeleton interfered with BCR-induced Abp1 recruitment to the plasma membrane. The inhibitory effect of a dynamin proline-rich domain deletion mutant on the recruitment of Abp1 to the plasma membrane, coimmunoprecipitation of dynamin with Abp1, and coprecipitation of Abp1 with GST fusion of the dyanmin proline-rich domain demonstrate the interaction of Abp1 with dynamin 2. These results demonstrate that the BCR regulates the function of Abp1 by inducing Abp1 phosphorylation and actin cytoskeleton rearrangement, and that Abp1 facilitates BCR-mediated Ag processing by simultaneously interacting with dynamin and the actin cytoskeleton.

TOM1L1 is a Lyn substrate involved in FcepsilonRI signaling in mast cells

Protein-tyrosine kinase Lyn and Syk are critical for antigen-receptor-induced signal transduction in mast cells. To identify novel Lyn/Syk substrates, we screened an RBL-2H3 bacterial expression library for proteins that were tyrosine phosphorylated with baculoviral expressed Lyn or Syk. Five clones as potential Lyn substrates and eight clones as Syk substrates were identified including known substrates such as SLP-76, LAT, and alpha-tubulin. A potential substrate of Lyn identified was the molecule TOM1L1, which has several domains thought to be important for membrane trafficking and protein-protein interactions. Because the function of TOM1L1 is unclear, the rat TOM1L1 full-length cDNA was isolated and used to express the protein in COS-1 and RBL-2H3 mast cells. In COS-1 cells, the co-transfection of TOM1L1 and Lyn, but not Syk, resulted in the tyrosine phosphorylation of TOM1L1. In RBL-2H3 mast cells, the overexpressed TOM1L1 was strongly tyrosine phosphorylated in non-stimulated cells, and this phosphorylation was enhanced by FcepsilonRI aggregation. By subcellular fractionation, wild-type TOM1L1 was mainly in the cytoplasm with a small fraction constitutively associated with the membrane; this association was markedly reduced in deletion mutants lacking several of the protein interaction domains. The overexpression of TOM1L1 enhanced antigen-induced tumor necrosis factor (TNF) alpha generation and release. Both protein interaction domains (VHS and the coiled-coil domains) were required for the increased TNFalpha release, but not the increased TNFalpha generation. These results suggest that TOM1L1 is a novel protein involved in the FcepsilonRI signal transduction for the generation of cytokines.

Abl-SH3 binding protein 2, 3BP2, interacts with CIN85 and HIP-55

The adapter 3BP2 is involved in leukocyte signaling downstream Src/Syk-kinases coupled immunoreceptors. Here, we show that 3BP2 directly interacts with the endocytic scaffold protein CIN85 and the actin-binding protein HIP-55. 3BP2 co-localized with CIN85 and HIP-55 in T cell rafts and at the T cell/APC synapse, an active zone of receptors and proteins recycling. A binding region of CIN85 SH3 domains on 3BP2 was mapped to a PVPTPR motif in the first proline-rich region of 3BP2, whereas the C-terminal SH3 domain of HIP-55 bound a more distal proline-rich domain of 3BP2. Together, our data suggest an unexpected role of 3BP2 in endocytic and cytoskeletal regulation through its interaction with CIN85 and HIP-55.

SH3P7/mAbp1 deficiency leads to tissue and behavioral abnormalities and impaired vesicle transport

The intracellular adaptor protein SH3P7 is the mammalian ortholog of yeast actin-binding protein 1 and thus alternatively named as mAbp1 (or HIP55). Structural properties, biochemical analysis of its interaction partners and siRNA studies implicated mAbp1 as an accessory protein in clathrin-mediated endocytosis (CME). Here, we describe the generation and characterization of mice deficient for SH3P7/mAbp1 owing to targeted gene disruption in embryonic stem cells. Mutant animals are viable and fertile without obvious deficits during the first weeks of life. Abnormal structure and function of organs including the spleen, heart, and lung is observed at about 3 months of age in both heterozygous and homozygous mouse mutants. A moderate reduction of both receptor-mediated and synaptic endocytosis is observed in embryonic fibroblasts and in synapses of hippocampal neurons, respectively. Recycling of synaptic vesicles in hippocampal boutons is severely impaired and delayed four-fold. The presynaptic defect of SH3P7/mAbp1 mouse mutants is associated with their constricted physical capabilities and disturbed neuromotoric behaviour. Our data reveal a nonredundant role of SH3P7/mAbp1 in CME and places its function downstream of vesicle fission.

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.

HIP-55 is important for T-cell proliferation, cytokine production, and immune responses

Engagement of the T-cell receptor (TCR) triggers a series of signaling events that lead to the activation of T cells. HIP-55 (SH3P7 or mAbp1), an actin-binding adaptor protein, interacts with and is tyrosine phosphorylated by ZAP-70, which is a crucial proximal protein tyrosine kinase for TCR signaling. HIP-55 is important for JNK and HPK1 activation induced by TCR signaling. In this study, we report the generation and characterization of HIP-55 knockout mice. We found that HIP-55 knockout mice were viable and fertile but showed decreased body weight and increased occurrence of death within the first 4 weeks after birth. The lymphoid organs in HIP-55 knockout mice showed cellularity and T-cell development comparable to that of the wild-type mice. HIP-55 knockout T cells displayed defective T-cell proliferation, decreased cytokine production, and decreased up-regulation of the activation markers induced by TCR stimulation. TCR internalization was slightly increased in HIP-55 knockout T cells. These phenotypes were accompanied by reduced immune responses, including antigen-specific antibody production and T-cell proliferation in HIP-55 knockout mice. The TCR-induced signaling events, including LAT/phospholipase Cgamma1 phosphorylation and HPK1/JNK activation, were partially defective in HIP-55 knockout T cells. These results demonstrate the importance of HIP-55 as an adaptor protein in the TCR signaling and immune system.

Functional interactions of HPK1 with adaptor proteins

Hematopoietic progenitor kinase 1 (HPK1 or MAP4K1) is a hematopoietic-specific mammalian STE20-like protein serine/threonine kinase, comprised of a STE20-like kinase domain in its N-terminus, four proline-rich motifs, a caspase cleavage site, and a distal C-terminal Citron homology domain. HPK1 is involved in many cellular signaling cascades that include MAPK signaling, antigen receptor signaling, apoptosis, growth factor signaling, and cytokine signaling. HPK1 binds many adaptor proteins including members of the Grb2 family, Nck family, Crk family, SLP-76 family, and actin-binding adaptors like HIP-55. HPK1 tyrosine phosphorylation and kinase activation depend on the presence of adaptor proteins. Adaptor proteins are required not only for linking HPK1 to cell surface receptors like the EGFR, but also for downstream gene transcription like NFAT, AP-1 and IL-2. The HPK1 association with Crk, CrkL, and HIP-55 mediate HPK1-dependent c-Jun N-terminal kinase (JNK) activation, while the association of HPK1 with SLP-76, Gads, CrkL, Grb2, and Grap affect T- and B-cell dependent gene transcription. Interestingly, HPK1 has been implicated in both increasing and decreasing NFAT, AP-1, and IL-2 gene transcription in T-cells where adaptor proteins play a key role. Lastly, HPK1 will phosphorylate Crk and CrkL, in vitro, which presents a novel possibility for the regulation of adaptor proteins and downstream signaling events.

Fyn binding protein, Fyb, interacts with mammalian actin binding protein, mAbp1

The immune cell specific protein Fyn-T binding protein (Fyb) has been identified as a target of the Yersinia antiphagocytic effector Yersinia outer protein H (YopH), but its role in macrophages is unknown. By using Fyb domains as bait to screen a mouse lymphoma cDNA library, we identified a novel interaction partner, mammalian actin binding protein 1 (mAbp1). We show that mAbp1 binds the Fyb N-terminal via its C-terminally located src homology 3 domain. The interaction between Fyb and mAbp1 is detected in macrophage lysates and the proteins co-localize with F-actin in the leading edge. Hence, mAbp1 is likely to constitute a downstream effector of Fyb involved in F-actin dynamics.

Serological identification of immunogenic antigens in acute monocytic leukemia

In order to improve disease-free survival and potentially a cure, it is necessary to identify more potent leukemia antigen. Here, we defined the acute monocytic leukemia-associated antigen (LAA) recognized by the humoral immune system for the first time. We have applied the method of serologic analysis of recombinant cDNA expression library (SEREX) on acute monocytic leukemia (FAB M5), followed by DNA sequencing and analyzing of positive clones. Then, the reactivity of normal and other leukemia sera with positive clones were performed. Thirty-five distinct novel antigens reactive with autologous IgG were identified by SEREX analysis on an acute monocytic leukemia patient and were characterized according to cDNA sequence and the reactivity with allogeneic sera. Twenty of the 35 antigens identified in this study were recognized by IgG antibodies in normal sera, and the remaining 15 were recognized exclusively by sera from allogeneic leukemia patients but not by normal donor sera, suggested that the immune response to these 15 antigens are leukemia related. The 15 immunogenic antigens detected by immune responses in the autologous host facilitate the identification of epitopes recognized by antigen-specific cytotoxic T lymphocytes (CTL) and are potential candidates for diagnosis and immunotherapy in acute myeloid leukemia (AML).

PRAM-1 potentiates arsenic trioxide-induced JNK activation

The promyelocytic leukemia RARalpha target gene encoding an adaptor molecule-1 (PRAM-1) is involved in a signaling pathway induced by retinoic acid in acute promyelocytic leukemia (APL) cells. To better understand the function of PRAM-1, we have undertaken the identification of its partners through a yeast two-hybrid screen. Here, we show that the proline-rich domain of PRAM-1 interacted with the Src homology 3 (SH3) domain of hematopoietic progenitor kinase 1 (HPK-1)-interacting protein of 55 kDa (HIP-55, also called SH3P7 and Abp1) known to stimulate the activity of HPK-1 and c-Jun N-terminal kinase (JNK). Overexpression of PRAM-1 in the NB4 APL cell line increased arsenic trioxide-induced JNK activation through a caspase 3-like-dependent activity. Dissociation of the SH3 domain from the rest of the HIP-55 protein was observed in the NB4 APL cell line treated with arsenic trioxide due to specific cleavage by caspase 3-like enzymes. The cleavage of HIP-55 correlated with the induction of PRAM-1 mRNA and protein expression. Taken together, our results suggest that the caspase 3-cleaved SH3 domain of HIP-55 is likely involved in PRAM-1-mediated JNK activation upon arsenic trioxide-induced differentiation of NB4 cells.

Cellular mechanisms of bacterial internalization counteracted by Yersinia

Upon host-cell contact, human pathogenic Yersinia species inject Yop virulence effectors into the host through a Type III secretion-and-translocation system. These virulence effectors cause a block in phagocytosis (YopE, YopT, YpkA, and YopH) and suppression of inflammatory mediators (YopJ). The Yops that block phagocytosis either interfere with the host cell actin regulation of Rho GTPases (YopE, YopT, and YpkA) or specifically and rapidly inactivate host proteins involved in signaling from the receptor to actin (YopH). The block in uptake has been shown to be activated following binding to Fc, Complement, and beta1-integrin receptors in virtually any kind of host cell. Thus, the use of Yersinia as a model system to study Yersinia-host cell interactions provides a good tool to explore signaling pathways involved in phagocytosis.