Conformational switching in ezrin regulates morphological and cytoskeletal changes required for B cell chemotaxis

Inherited human ezrin deficiency impairs adaptive immunity

BACKGROUND

Inborn errors of immunity (IEI) are a group of monogenic diseases that confer susceptibility to infection, autoimmunity, and cancer. Despite the life-threatening consequences of some IEI, their genetic cause remains unknown in many patients.

OBJECTIVE

We investigated a patient with an IEI of unknown genetic etiology.

METHODS

Whole-exome sequencing identified a homozygous missense mutation of the gene encoding ezrin (EZR), substituting a threonine for an alanine at position 129.

RESULTS

Ezrin is one of the subunits of the ezrin, radixin, and moesin (ERM) complex. The ERM complex links the plasma membrane to the cytoskeleton and is crucial for the assembly of an efficient immune response. The A129T mutation abolishes basal phosphorylation and decreases calcium signaling, leading to complete loss of function. Consistent with the pleiotropic function of ezrin in myriad immune cells, multidimensional immunophenotyping by mass and flow cytometry revealed that in addition to hypogammaglobulinemia, the patient had low frequencies of switched memory B cells, CD4+ and CD8+ T cells, MAIT, γδ T cells, and centralnaive CD4+ cells.

CONCLUSIONS

Autosomal-recessive human ezrin deficiency is a newly recognized genetic cause of B-cell deficiency affecting cellular and humoral immunity.

Glioblastoma Spheroid Invasion through Soft, Brain-Like Matrices Depends on Hyaluronic Acid-CD44 Interactions

Increased secretion of hyaluronic acid (HA), a glycosaminoglycan abundant in the brain extracellular matrix (ECM), correlates with worse clinical outcomes for glioblastoma (GBM) patients. GBM cells aggressively invade the brain parenchyma while encountering spatiotemporal changes in their local ECM, including HA concentration. To investigate how varying HA concentrations affect GBM invasion, patient-derived GBM cells are cultured within a soft, 3D matrix in which HA concentration is precisely varied and cell migration observed. Data demonstrate that HA concentration can determine the invasive activity of patient-derived GBM cells in a biphasic and highly sensitive manner, where the absolute concentration of HA at which cell migration peaked is specific to each patient-derived line. Furthermore, evidence that this response relies on phosphorylated ezrin, which interacts with the intracellular domain of HA-engaged CD44 to effectively link the actin cytoskeleton to the local ECM is provided. Overall, this study highlights CD44-HA binding as a major mediator of GBM cell migration that acts independently of integrins and focal adhesion complexes and suggests that targeting HA-CD44-ezrin interactions represents a promising therapeutic strategy to prevent tumor cell invasion in the brain.

Ezrin Promotes Antigen Receptor Diversity during B Cell Development by Supporting Ig H Chain Variable Gene Recombination

Genome-level rearrangements of Ig genes during B cell development are critical for generation of a diverse repertoire of BCRs that bind to a multitude of foreign Ags and some self Ags. Bone marrow B cell development involves a variety of cell–cell interactions, cell migration, and receptor signaling that likely benefit from the activity of membrane-cytoskeletal reorganizing proteins. However, the specific contribution of such proteins toward BCR repertoire diversification is poorly understood. Ezrin is a membrane-cytoskeletal linker protein that regulates mature B cell activation through spatial organization of the BCR. We employed next-generation sequencing to investigate whether Ezrin plays a role in IgH rearrangements and generation of BCR diversity in developing bone marrow B cells. BCR repertoire development occurred stochastically in B cell progenitors from both control and B cell conditional Ezrin-deficient mice. However, the loss of Ezrin resulted in fewer unique CDRs (CDR3s) in the BCRs and reduced Shannon entropy. Ezrin-deficient pre-B cells revealed similar utilization of joining (J) genes but significantly fewer variable (V) genes, thereby decreasing V-J combinatorial diversity. V-J junctional diversity, measured by CDR3 length and nucleotide additions and deletions, was not altered in Ezrin-deficient pre-B cells. Mechanistically, Ezrin-deficient cells showed a marked decrease in RAG1 gene expression, indicating a less efficient DNA recombination machinery. Overall, our results demonstrate that Ezrin shapes the BCR repertoire through combinatorial diversification.

Executable models of immune signaling pathways in HIV-associated atherosclerosis

Atherosclerosis (AS)-associated cardiovascular disease is an important cause of mortality in an aging population of people living with HIV (PLWH). This elevated risk has been attributed to viral infection, anti-retroviral therapy, chronic inflammation, and lifestyle factors. However, the rates at which PLWH develop AS vary even after controlling for length of infection, treatment duration, and for lifestyle factors. To investigate the molecular signaling underlying this variation, we sequenced 9368 peripheral blood mononuclear cells (PBMCs) from eight PLWH, four of whom have atherosclerosis (AS+). Additionally, a publicly available dataset of PBMCs from persons before and after HIV infection was used to investigate the effect of acute HIV infection. To characterize dysregulation of pathways rather than just measuring enrichment, we developed the single-cell Boolean Omics Network Invariant Time Analysis (scBONITA) algorithm. scBONITA infers executable dynamic pathway models and performs a perturbation analysis to identify high impact genes. These dynamic models are used for pathway analysis and to map sequenced cells to characteristic signaling states (attractor analysis). scBONITA revealed that lipid signaling regulates cell migration into the vascular endothelium in AS+ PLWH. Pathways implicated included AGE-RAGE and PI3K-AKT signaling in CD8+ T cells, and glucagon and cAMP signaling pathways in monocytes. Attractor analysis with scBONITA facilitated the pathway-based characterization of cellular states in CD8+ T cells and monocytes. In this manner, we identify critical cell-type specific molecular mechanisms underlying HIV-associated atherosclerosis using a novel computational method.

The raft cytoskeleton binding protein complexes personate functional regulators in cell behaviors

Several cytoskeleton proteins interact with raft proteins to form raft-cytoskeleton binding protein complexes (RCPCs) that control cell migration and adhesion. The purpose of this paper is to review the latest research on the modes and mechanisms by which a RCPC controls different cellular functions. This paper discusses RCPC composition and its role in cytoskeleton reorganization, as well as the latest developments in molecular mechanisms that regulate cell adhesion and migration under normal conditions. In addition, the role of some external stimuli (such as stress and chemical signals) in this process is further debated, and meanwhile potential mechanisms for RCPC to regulate lipid raft fluidity is proposed. Thus, this review mainly contributes to the understanding of RCPC signal transduction in cells. Additionally, the targeted signal transduction of RCPC and its mechanism connection with cell behaviors will provide a logical basis for the development of unified interventions to combat metastasis related dysfunction and diseases.

A biophysical perspective of the regulatory mechanisms of ezrin/radixin/moesin proteins

Many signal transductions resulting from ligand-receptor interactions occur at the cell surface. These signaling pathways play essential roles in cell polarization, membrane morphogenesis, and the modulation of membrane tension at the cell surface. However, due to the large number of membrane-binding proteins, including actin-membrane linkers, and transmembrane proteins present at the cell surface, the molecular mechanisms underlying the regulation at the cell surface are yet unclear. Here, we describe the molecular functions of one of the key players at the cell surface, ezrin/radixin/moesin (ERM) proteins from a biophysical point of view. We focus our discussion on biophysical properties of ERM proteins revealed by using biophysical tools in live cells and in vitro reconstitution systems. We first describe the structural properties of ERM proteins and then discuss the interactions of ERM proteins with PI(4,5)P₂ and the actin cytoskeleton. These properties of ERM proteins revealed by using biophysical approaches have led to a better understanding of their physiological functions in cells and tissues.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-021-00928-0.

A Murine Model of X-Linked Moesin-Associated Immunodeficiency (X-MAID) Reveals Defects in T Cell Homeostasis and Migration

X-linked moesin associated immunodeficiency (X-MAID) is a primary immunodeficiency disease in which patients suffer from profound lymphopenia leading to recurrent infections. The disease is caused by a single point mutation leading to a R171W amino acid change in the protein moesin (moesinR171W). Moesin is a member of the ERM family of proteins, which reversibly link the cortical actin cytoskeleton to the plasma membrane. Here, we describe a novel mouse model with global expression of moesinR171W that recapitulates multiple facets of patient disease, including severe lymphopenia. Further analysis reveals that these mice have diminished numbers of thymocytes and bone marrow precursors. X-MAID mice also exhibit systemic inflammation that is ameliorated by elimination of mature lymphocytes through breeding to a Rag1-deficient background. The few T cells in the periphery of X-MAID mice are highly activated and have mostly lost moesinR171W expression. In contrast, single-positive (SP) thymocytes do not appear activated and retain high expression levels of moesinR171W. Analysis of ex vivo CD4 SP thymocytes reveals defects in chemotactic responses and reduced migration on integrin ligands. While chemokine signaling appears intact, CD4 SP thymocytes from X-MAID mice are unable to polarize and rearrange cytoskeletal elements. This mouse model will be a valuable tool for teasing apart the complexity of the immunodeficiency caused by moesinR171W, and will provide new insights into how the actin cortex regulates lymphocyte function.

Ezrin Is a Novel Protein Partner of Aquaporin-5 in Human Salivary Glands and Shows Altered Expression and Cellular Localization in Sjögren's Syndrome

Sjögren’s syndrome (SS) is an exocrinopathy characterized by the hypofunction of salivary glands (SGs). Aquaporin-5 (AQP5); a water channel involved in saliva formation; is aberrantly distributed in SS SG acini and contributes to glandular dysfunction. We aimed to investigate the role of ezrin in AQP5 mislocalization in SS SGs. The AQP5–ezrin interaction was assessed by immunoprecipitation and proteome analysis and by proximity ligation assay in immortalized human SG cells. We demonstrated, for the first time, an interaction between ezrin and AQP5. A model of the complex was derived by computer modeling and in silico docking; suggesting that AQP5 interacts with the ezrin FERM-domain via its C-terminus. The interaction was also investigated in human minor salivary gland (hMSG) acini from SS patients (SICCA-SS); showing that AQP5–ezrin complexes were absent or mislocalized to the basolateral side of SG acini rather than the apical region compared to controls (SICCA-NS). Furthermore, in SICCA-SS hMSG acinar cells, ezrin immunoreactivity was decreased at the acinar apical region and higher at basal or lateral regions, accounting for altered AQP5–ezrin co-localization. Our data reveal that AQP5–ezrin interactions in human SGs could be involved in the regulation of AQP5 trafficking and may contribute to AQP5-altered localization in SS patients

Lymphocyte egress signal sphingosine-1-phosphate promotes ERM-guided, bleb-based migration

Ezrin, radixin, and moesin (ERM) family proteins regulate cytoskeletal responses by tethering the plasma membrane to the underlying actin cortex. Mutations in ERM proteins lead to severe combined immunodeficiency, but the function of these proteins in T cells remains poorly defined. Using mice in which T cells lack all ERM proteins, we demonstrate a selective role for these proteins in facilitating S1P-dependent egress from lymphoid organs. ERM-deficient T cells display defective S1P-induced migration in vitro, despite normal responses to standard protein chemokines. Analysis of these defects revealed that S1P promotes a fundamentally different mode of migration than chemokines, characterized by intracellular pressurization and bleb-based motility. ERM proteins facilitate this process, controlling directional migration by limiting blebbing to the leading edge. We propose that the distinct modes of motility induced by S1P and chemokines are specialized to allow T cell migration across lymphatic barriers and through tissue stroma, respectively.

Pharmacologic Inhibition of Ezrin-Radixin-Moesin Phosphorylation is a Novel Therapeutic Strategy in Rhabdomyosarcoma

Intermediate and high-risk rhabdomyosarcoma (RMS) patients have poor prognosis with available treatment options, highlighting a clear unmet need for identification of novel therapeutic strategies. Ezrin-radixin-moesin (ERM) family members are membrane-cytoskeleton linker proteins with well-defined roles in tumor metastasis, growth, and survival. ERM protein activity is regulated by dynamic changes in the phosphorylation at a conserved threonine residue in their C-terminal actin-binding domain. Interestingly, ERM family member, ezrin, has elevated expression in the RMS tissue. Despite this, the translational scope of targeting ERM family proteins in these tumors through pharmacological inhibition has never been considered. This study investigates the inhibition of ERM phosphorylation using a small molecule pharmacophore NSC668394 as a potential strategy against RMS. Upon in vitro treatment with NSC668394, RMS cells exhibit a dose-dependent decrease in cell viability and proliferation, with induction of caspase-3 cleavage and apoptosis. siRNA-mediated knockdown of individual ERM protein expression revealed that each regulates RMS survival to a different degree. In vivo administration of NSC668394 in RMS xenografts causes significant decrease in tumor growth, with no adverse effect on body weight. Collectively, this study highlights the importance of the active conformation of ERM proteins in RMS progression and survival and supports pharmacologic inhibition of these proteins as a novel therapeutic approach.

Surfing on Membrane Waves: Microvilli, Curved Membranes, and Immune Signaling

Microvilli are finger-like membrane protrusions, supported by the actin cytoskeleton, and found on almost all cell types. A growing body of evidence suggests that the dynamic lymphocyte microvilli, with their highly curved membranes, play an important role in signal transduction leading to immune responses. Nevertheless, challenges in modulating local membrane curvature and monitoring the high dynamicity of microvilli hampered the investigation of the curvature-generation mechanism and its functional consequences in signaling. These technical barriers have been partially overcome by recent advancements in adapted super-resolution microscopy. Here, we review the up-to-date progress in understanding the mechanisms and functional consequences of microvillus formation in T cell signaling. We discuss how the deformation of local membranes could potentially affect the organization of signaling proteins and their biochemical activities. We propose that curved membranes, together with the underlying cytoskeleton, shape microvilli into a unique compartment that sense and process signals leading to lymphocyte activation.

C-terminal phosphorylation modulates ERM-1 localization and dynamics to control cortical actin organization and support lumen formation during Caenorhabditiselegans development

ERM proteins are conserved regulators of cortical membrane specialization that function as membrane-actin linkers and molecular hubs. The activity of ERM proteins requires a conformational switch from an inactive cytoplasmic form into an active membrane- and actin-bound form, which is thought to be mediated by sequential PIP2 binding and phosphorylation of a conserved C-terminal threonine residue. Here, we use the single Caenorhabditiselegans ERM ortholog, ERM-1, to study the contribution of these regulatory events to ERM activity and tissue formation in vivo Using CRISPR/Cas9-generated erm-1 mutant alleles, we demonstrate that a PIP2-binding site is crucially required for ERM-1 function. By contrast, dynamic regulation of C-terminal T544 phosphorylation is not essential but modulates ERM-1 apical localization and dynamics in a tissue-specific manner, to control cortical actin organization and support lumen formation in epithelial tubes. Our work highlights the dynamic nature of ERM protein regulation during tissue morphogenesis and the importance of C-terminal phosphorylation in fine-tuning ERM activity in a tissue-specific context.

ERM Proteins at the Crossroad of Leukocyte Polarization, Migration and Intercellular Adhesion

Ezrin, radixin and moesin proteins (ERMs) are plasma membrane (PM) organizers that link the actin cytoskeleton to the cytoplasmic tail of transmembrane proteins, many of which are adhesion receptors, in order to regulate the formation of F-actin-based structures (e.g., microspikes and microvilli). ERMs also effect transmission of signals from the PM into the cell, an action mainly exerted through the compartmentalized activation of the small Rho GTPases Rho, Rac and Cdc42. Ezrin and moesin are the ERMs more highly expressed in leukocytes, and although they do not always share functions, both are mainly regulated through phosphatidylinositol 4,5-bisphosphate (PIP₂) binding to the N-terminal band 4.1 protein-ERM (FERM) domain and phosphorylation of a conserved Thr in the C-terminal ERM association domain (C-ERMAD), exerting their functions through a wide assortment of mechanisms. In this review we will discuss some of these mechanisms, focusing on how they regulate polarization and migration in leukocytes, and formation of actin-based cellular structures like the phagocytic cup-endosome and the immune synapse in macrophages/neutrophils and lymphocytes, respectively, which represent essential aspects of the effector immune response.

Intrinsic properties of human germinal center B cells set antigen affinity thresholds

Protective antibody responses to vaccination or infection depend on affinity maturation, a process by which high-affinity germinal center (GC) B cells are selected on the basis of their ability to bind, gather, and present antigen to T follicular helper (T_fh) cells. Here, we show that human GC B cells have intrinsically higher-affinity thresholds for both B cell antigen receptor (BCR) signaling and antigen gathering as compared with naïve B cells and that these functions are mediated by distinct cellular structures and pathways that ultimately lead to antigen affinity- and T_fh cell-dependent differentiation to plasma cells. GC B cells bound antigen through highly dynamic, actin- and ezrin-rich pod-like structures that concentrated BCRs. The behavior of these structures was dictated by the intrinsic antigen affinity thresholds of GC B cells. Low-affinity antigens triggered continuous engagement and disengagement of membrane-associated antigens, whereas high-affinity antigens induced stable synapse formation. The pod-like structures also mediated affinity-dependent antigen internalization by unconventional pathways distinct from those of naïve B cells. Thus, intrinsic properties of human GC B cells set thresholds for affinity selection.

The transmembrane adaptor protein NTAL limits mast cell chemotaxis toward prostaglandin E2

Chemotaxis of mast cells is one of the crucial steps in their development and function. Non–T cell activation linker (NTAL) is a transmembrane adaptor protein that inhibits the activation of mast cells and B cells in a phosphorylation-dependent manner. Here, we studied the role of NTAL in the migration of mouse mast cells stimulated by prostaglandin E2 (PGE2). Although PGE2 does not induce the tyrosine phosphorylation of NTAL, unlike IgE immune complex antigens, we found that loss of NTAL increased the chemotaxis of mast cells toward PGE2. Stimulation of mast cells that lacked NTAL with PGE2 enhanced the phosphorylation of AKT and the production of phosphatidylinositol 3,4,5-trisphosphate. In resting NTAL-deficient mast cells, phosphorylation of an inhibitory threonine in ERM family proteins accompanied increased activation of β1-containing integrins, which are features often associated with increased invasiveness in tumors. Rescue experiments indicated that only full-length, wild-type NTAL restored the chemotaxis of NTAL-deficient cells toward PGE2. Together, these data suggest that NTAL is a key inhibitor of mast cell chemotaxis toward PGE2, which may act through the RHOA/ERM/β1-integrin and PI3K/AKT axes.

Cutting Edge: Deletion of Ezrin in B Cells of Lyn-Deficient Mice Downregulates Lupus Pathology

Genetic deletion of the Src family tyrosine kinase Lyn in mice recapitulates human systemic lupus erythematosus, characterized by hyperactive BCR signaling, splenomegaly, autoantibody generation, and glomerulonephritis. However, the molecular regulators of autoimmunity in Lyn-deficient mice and in human lupus remain poorly characterized. In this study, we report that conditional deletion of the membrane-cytoskeleton linker protein ezrin in B cells of Lyn-deficient mice (double knockout [DKO] mice) ameliorates B cell activation and lupus pathogenesis. B cells from DKO mice respond poorly to BCR stimulation, with severe downregulation of major signaling pathways. DKO mice exhibit reduced splenomegaly as well as significantly lower levels of autoantibodies against a variety of autoantigens, including dsDNA, histone, and chromatin. Leukocyte infiltration and deposition of IgG and complement component C3 in the kidney glomeruli of DKO mice are markedly reduced. Our data demonstrate that ezrin is a novel molecular regulator of B cell-associated lupus pathology.

Wnt signalling pathways in chronic lymphocytic leukaemia and B-cell lymphomas

In this review, we discuss the intricate roles of the Wnt signalling network in the development and progression of mature B-cell-derived haematological malignancies, with a focus on chronic lymphocytic leukaemia (CLL) and related B-cell lymphomas. We review the current literature and highlight the differences between the β-catenin-dependent and -independent branches of Wnt signalling. Special attention is paid to the role of the non-canonical Wnt/planar cell polarity (PCP) pathway, mediated by the Wnt-5-receptor tyrosine kinase-like orphan receptor (ROR1)-Dishevelled signalling axis in CLL. This is mainly because the Wnt/PCP co-receptor ROR1 was found to be overexpressed in CLL and the Wnt/PCP pathway contributes to numerous aspects of CLL pathogenesis. We also discuss the possibilities of therapeutically targeting the Wnt signalling pathways as an approach to disrupt the crucial interaction between malignant cells and their micro-environment. We also advocate the need for research in this direction for other lymphomas, namely, diffuse large B-cell lymphoma, Hodgkin lymphoma, mantle cell lymphoma, Burkitt lymphoma and follicular lymphoma where the Wnt signalling pathway probably plays a similar role.

LINKED ARTICLES

This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc.

ERM Proteins Play Distinct Roles in Cell Invasion by Extracellular Amastigotes of Trypanosoma cruzi

The protozoan parasite Trypanosoma cruzi is the causative agent of Chagas' disease. In mammalian hosts, T. cruzi alternates between trypomastigote and amastigote forms. Additionally, trypomastigotes can differentiate into amastigotes in the extracellular environment generating infective extracellular amastigotes (EAs). Ezrin-radixin-moesin (ERM) are key proteins linking plasma membrane to actin filaments, the major host cell component responsible for EA internalization. Our results revealed that depletion of host ezrin and radixin but not moesin inhibited EAs invasion in HeLa cells. ERM are recruited and colocalize with F-actin at EA invasion sites as shown by confocal microscopy. Invasion assays performed with cells overexpressing ERM showed increased EAs invasion in ezrin and radixin but not moesin overexpressing cells. Finally, time-lapse experiments have shown altered actin dynamics leading to delayed EA internalization in ezrin and radixin depleted cells when compared to control or moesin depleted cells. Altogether, these findings show distinct roles of ERM during EAs invasion, possibly regulating F-actin dynamics and plasma membrane interplay.

Ezrin links CFTR to TLR4 signaling to orchestrate anti-bacterial immune response in macrophages

Macrophages (MΦs) with mutations in cystic fibrosis transmembrane conductance regulator (CFTR) have blunted induction of PI3K/AKT signaling in response to TLR4 activation, leading to hyperinflammation, a hallmark of cystic fibrosis (CF) disease. Here, we show that Ezrin links CFTR and TLR4 signaling, and is necessary for PI3K/AKT signaling induction in response to MΦ activation. Because PI3K/AKT signaling is critical for immune regulation, Ezrin-deficient MΦs are hyperinflammatory and have impaired Pseudomonas aeruginosa phagocytosis, phenocopying CF MΦs. Importantly, we show that activated CF MΦs have reduced protein levels and altered localization of the remaining Ezrin to filopodia that form during activation. In summary, we have described a direct link from CFTR to Ezrin to PI3K/AKT signaling that is disrupted in CF, and thus promotes hyper-inflammation and weakens phagocytosis.

Ezrin directly interacts with AQP2 and promotes its endocytosis

The water channel aquaporin-2 (AQP2) is a major regulator of water homeostasis in response to vasopressin (VP). Dynamic trafficking of AQP2 relies on its close interaction with trafficking machinery proteins and the actin cytoskeleton. Here, we report the identification of ezrin, an actin-binding protein from the ezrin/radixin/moesin (ERM) family as an AQP2-interacting protein. Ezrin was first detected in a co-immunoprecipitation (co-IP) complex using an anti-AQP2 antibody in a proteomic analysis. Immunofluorescence staining revealed the co-expression of ezrin and AQP2 in collecting duct principal cells, and VP treatment caused redistribution of both proteins to the apical membrane. The ezrin-AQP2 interaction was confirmed by co-IP experiments with an anti-ezrin antibody, and by pulldown assays using purified full-length and FERM domain-containing recombinant ezrin. By using purified recombinant proteins, we showed that ezrin directly interacts with AQP2 C-terminus through its N-terminal FERM domain. Knocking down ezrin expression with shRNA resulted in increased membrane accumulation of AQP2 and reduced AQP2 endocytosis. Therefore, through direct interaction with AQP2, ezrin facilitates AQP2 endocytosis, thus linking the dynamic actin cytoskeleton network with AQP2 trafficking.

The ezrin-radixin-moesin family of proteins in the regulation of B-cell immune response

Dynamic reorganization of the cortical cytoskeleton is essential for numerous cellular processes, including B- and T-cell activation and migration. The ezrin-radixin-moesin (ERM) family of proteins plays structural and regulatory roles in the rearrangement of plasma membrane flexibility and protrusions through its members' reversible interaction with cortical actin filaments and the plasma membrane. Recent studies demonstrated that ERM proteins not only are involved in cytoskeletal organization but also offer a platform for the transmission of signals in response to a variety of extracellular stimuli through their ability to cross-link transmembrane receptors with downstream signaling components. In this review, we summarize present knowledge relating to ERMs and recent progress made toward elucidating a novel role for them in the regulation of B-cell function in health and disease.

Cutting Edge: Ezrin Regulates Inflammation by Limiting B Cell IL-10 Production

IL-10 produced by B cells is important for controlling inflammation, thus underscoring the need to identify mechanisms regulating its production. In this study, we demonstrate that conditional deletion of ezrin in B cells increases IL-10 production induced by TLR4 ligation. The MyD88-independent Toll/IL-1R domain-containing adapter inducing IFN-β-IFN regulatory factor 3 pathway is required for Ezrin-deficient B cells to produce higher IL-10 upon LPS stimulation. Treatment of B cells with a novel small-molecule inhibitor of ezrin induces its dephosphorylation and increases LPS-induced NF-κB and IFN regulatory factor 3 activation and IL-10 secretion, indicating a role for threonine 567 phosphorylation of ezrin in limiting IL-10. Loss of ezrin in B cells results in dampened proinflammatory response to a sublethal dose of LPS in vivo, which is dependent on increased IL-10 production. Taken together, our data yield new insights into molecular and membrane-cytoskeletal regulation of B cell IL-10 production and reveal ezrin as a potential therapeutic target in inflammatory diseases.

Identification of Ezrin-Radixin-Moesin proteins as novel regulators of pathogenic B-cell receptor signaling and tumor growth in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is a hematological cancer associated with an aggressive clinical course. The predominant subtypes of DLBCL display features of chronic or tonic B-cell antigen receptor (BCR) signaling. However, it is not known whether the spatial organization of the BCR contributes to the regulation of pro-survival signaling pathways and cell growth. Here, we show that primary DLBCL tumors and patient-derived DLBCL cell lines contain high levels of phosphorylated Ezrin-Radixin-Moesin (ERM) proteins. The surface BCRs in both activated B cell and germinal B cell subtype DLBCL cells co-segregate with phosphoERM suggesting that the cytoskeletal network may support localized BCR signaling and contribute to pathogenesis. Indeed, ablation of membrane-cytoskeletal linkages by dominant-negative mutants, pharmacological inhibition and knockdown of ERM proteins disrupted cell surface BCR organization, inhibited proximal and distal BCR signaling, and reduced the growth of DLBCL cell lines. In vivo administration of the ezrin inhibitor retarded the growth of DLBCL tumor xenografts, concomitant with reduction in intratumor phosphoERM levels, dampened pro-survival signaling and induction of apoptosis. Our results reveal a novel ERM-based spatial mechanism that is coopted by DLBCL cells to sustain tumor cell growth and survival.

ERM proteins at a glance

The cell cortex is a dynamic and heterogeneous structure that governs cell identity and behavior. The ERM proteins (ezrin, radixin and moesin) are major architects of the cell cortex, and they link plasma membrane phospholipids and proteins to the underlying cortical actin cytoskeleton. Recent studies in several model systems have uncovered surprisingly dynamic and complex molecular activities of the ERM proteins and have provided new mechanistic insight into how they build and maintain cortical domains. Among many well-established and essential functions of ERM proteins, this Cell Science at a Glance article and accompanying poster will focus on the role of ERMs in organizing the cell cortex during cell division and apical morphogenesis. These examples highlight an emerging appreciation that the ERM proteins both locally alter the mechanical properties of the cell cortex, and control the spatial distribution and activity of key membrane complexes, establishing the ERM proteins as a nexus for the physical and functional organization of the cell cortex and making it clear that they are much more than scaffolds. This article is part of a Minifocus on Establishing polarity.

Ezrin tunes the magnitude of humoral immunity

Ezrin is a member of the ezrin-radixin-moesin family of membrane-actin cytoskeleton cross-linkers that participate in a variety of cellular processes. In B cells, phosphorylation of ezrin at different sites regulates multiple processes, such as lipid raft coalescence, BCR diffusion, microclustering, and endosomal JNK activation. In this study, we generated mice with conditional deletion of ezrin in the B cell lineage to investigate the physiological significance of ezrin's function in Ag receptor-mediated B cell activation and humoral immunity. B cell development, as well as the proportion and numbers of major B cell subsets in peripheral lymphoid organs, was unaffected by the loss of ezrin. Using superresolution imaging methods, we show that, in the absence of ezrin, BCRs respond to Ag binding by accumulating into larger and more stable signaling microclusters. Loss of ezrin led to delayed BCR capping and accelerated lipid raft coalescence. Although proximal signaling proteins showed stronger activation in the absence of ezrin, components of the distal BCR signaling pathways displayed distinct effects. Ezrin deficiency resulted in increased B cell proliferation and differentiation into Ab-secreting cells ex vivo and stronger T cell-independent and -dependent responses to Ag in vivo. Overall, our data demonstrate that ezrin regulates amplification of BCR signals and tunes the strength of B cell activation and humoral immunity.

Sphingolipid regulation of ezrin, radixin, and moesin proteins family: implications for cell dynamics

A key but poorly studied domain of sphingolipid functions encompasses endocytosis, exocytosis, cellular trafficking, and cell movement. Recently, the ezrin, radixin and moesin (ERM) family of proteins emerged as novel potent targets regulated by sphingolipids. ERMs are structural proteins linking the actin cytoskeleton to the plasma membrane, also forming a scaffold for signaling pathways that are used for cell proliferation, migration and invasion, and cell division. Opposing functions of the bioactive sphingolipid ceramide and sphingosine-1-phosphate (S1P), contribute to ERM regulation. S1P robustly activates whereas ceramide potently deactivates ERM via phosphorylation/dephosphorylation, respectively. This recent dimension of cytoskeletal regulation by sphingolipids opens up new avenues to target cell dynamics, and provides further understanding of some of the unexplained biological effects mediated by sphingolipids. In addition, these studies are providing novel inroads into defining basic mechanisms of regulation and action of bioactive sphingolipids. This review describes the current understanding of sphingolipid regulation of the cytoskeleton, it also describes the biologies in which ERM proteins have been involved, and finally how these two large fields have started to converge. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Sphingosine 1-phosphate induces filopodia formation through S1PR2 activation of ERM proteins

Previously we demonstrated that the sphingolipids ceramide and S1P (sphingosine 1-phosphate) regulate phosphorylation of the ERM (ezrin/radixin/moesin) family of cytoskeletal proteins [Canals, Jenkins, Roddy, Hernande-Corbacho, Obeid and Hannun (2010) J. Biol. Chem. 285, 32476-3285]. In the present article, we show that exogenously applied or endogenously generated S1P (in a sphingosine kinase-dependent manner) results in significant increases in phosphorylation of ERM proteins as well as filopodia formation. Using phosphomimetic and non-phosphorylatable ezrin mutants, we show that the S1P-induced cytoskeletal protrusions are dependent on ERM phosphorylation. Employing various pharmacological S1PR (S1P receptor) agonists and antagonists, along with siRNA (small interfering RNA) techniques and genetic knockout approaches, we identify the S1PR2 as the specific and necessary receptor to induce phosphorylation of ERM proteins and subsequent filopodia formation. Taken together, the results demonstrate a novel mechanism by which S1P regulates cellular architecture that requires S1PR2 and subsequent phosphorylation of ERM proteins.

Ezrin and moesin are required for efficient T cell adhesion and homing to lymphoid organs

T cell trafficking between the blood and lymphoid organs is a complex, multistep process that requires several highly dynamic and coordinated changes in cyto-architecture. Members of the ezrin, radixin and moesin (ERM) family of actin-binding proteins have been implicated in several aspects of this process, but studies have yielded conflicting results. Using mice with a conditional deletion of ezrin in CD4+ cells and moesin-specific siRNA, we generated T cells lacking ERM proteins, and investigated the effect on specific events required for T cell trafficking. ERM-deficient T cells migrated normally in multiple in vitro and in vivo assays, and could undergo efficient diapedesis in vitro. However, these cells were impaired in their ability to adhere to the β1 integrin ligand fibronectin, and to polarize appropriately in response to fibronectin and VCAM-1 binding. This defect was specific for β1 integrins, as adhesion and polarization in response to ICAM-1 were normal. In vivo, ERM-deficient T cells showed defects in homing to lymphoid organs. Taken together, these results show that ERM proteins are largely dispensable for T cell chemotaxis, but are important for β1 integrin function and homing to lymphoid organs.

Cross-talk between tetraspanin CD9 and transmembrane adaptor protein non-T cell activation linker (NTAL) in mast cell activation and chemotaxis

Chemotaxis, a process leading to movement of cells toward increasing concentrations of chemoattractants, is essential, among others, for recruitment of mast cells within target tissues where they play an important role in innate and adaptive immunity. Chemotaxis is driven by chemoattractants, produced by various cell types, as well as by intrinsic cellular regulators, which are poorly understood. In this study we prepared a new mAb specific for the tetraspanin CD9. Binding of the antibody to bone marrow-derived mast cells triggered activation events that included cell degranulation, Ca(2+) response, dephosphorylation of ezrin/radixin/moesin (ERM) family proteins, and potent tyrosine phosphorylation of the non-T cell activation linker (NTAL) but only weak phosphorylation of the linker for activation of T cells (LAT). Phosphorylation of the NTAL was observed with whole antibody but not with its F(ab)(2) or Fab fragments. This indicated involvement of the Fcγ receptors. As documented by electron microscopy of isolated plasma membrane sheets, CD9 colocalized with the high-affinity IgE receptor (FcεRI) and NTAL but not with LAT. Further tests showed that both anti-CD9 antibody and its F(ab)(2) fragment inhibited mast cell chemotaxis toward antigen. Experiments with bone marrow-derived mast cells deficient in NTAL and/or LAT revealed different roles of these two adaptors in antigen-driven chemotaxis. The combined data indicate that chemotaxis toward antigen is controlled in mast cells by a cross-talk among FcεRI, tetraspanin CD9, transmembrane adaptor proteins NTAL and LAT, and cytoskeleton-regulatory proteins of the ERM family.

Spatial coupling of JNK activation to the B cell antigen receptor by tyrosine-phosphorylated ezrin

The ezrin-radixin-moesin proteins regulate B lymphocyte activation via their effect on BCR diffusion and microclustering. This relies on their ability to dynamically tether the plasma membrane with actin filaments that is in turn facilitated by phosphorylation of the conserved threonine residue in the actin-binding domain. In this study, we describe a novel function of ezrin in regulating JNK activation that is mediated by phosphorylation of a tyrosine (Y353) residue that is unconserved with moesin and radixin. BCR, but not CD40, TLR4, or CXCR5 stimulation, induced phosphorylation of ezrin at Y353 in mouse splenic B cells. Ezrin existed in a preformed complex with Syk in unstimulated B cells and underwent Syk-dependent phosphorylation upon anti-IgM stimulation. Y353-phosphorylated ezrin colocalized with the BCR within minutes of stimulation and cotrafficked with the endocytosed BCRs through the early and late endosomes. The T567 residue of ezrin was rephosphorylated in late endosomes and at the plasma membrane at later times of BCR stimulation. Expression of a nonphosphorylatable Y353F mutant of ezrin specifically impaired JNK activation. BCR crosslinking induced the association of Y353-phosphorylated ezrin with JNK and its kinase MAPKK7, as well as spatial colocalization with phosphorylated JNK in the endosomes. The yellow fluorescent protein-tagged Y353F mutant displayed reduced colocalization with the endocytosed BCR as compared with wild-type ezrin-yellow fluorescent protein. Taken together, our data identify a novel role for ezrin as a spatial adaptor that couples JNK signaling components to the BCR signalosome, thus facilitating JNK activation.

The scaffolding protein synapse-associated protein 97 is required for enhanced signaling through isotype-switched IgG memory B cell receptors

After their first encounter with a foreign antigen, naïve B cells that have immunoglobulin M (IgM) B cell receptors (BCRs) trigger the primary antibody response and the generation of memory B cells with IgG BCRs. When these memory B cells reencounter the same antigen, the cell surface IgG BCRs stimulate their rapid differentiation into plasma cells that release large amounts of IgG antibodies. We showed that the conserved cytoplasmic tail of the IgG BCR, which contains a putative PDZ (postsynaptic density 95/disc large/zona occludens 1)-binding motif, associated with synapse-associated protein 97 (SAP97), a PDZ domain-containing scaffolding molecule that is involved in controlling receptor density and signal strength at neuronal synapses. SAP97 accumulated and bound to IgG BCRs in the immunological synapses that formed in response to B cell engagement with antigen. Knocking down SAP97 in IgG⁺ B cells or mutating the putative PDZ-binding motif in the BCR tail impaired formation of the immunological synapse, initiation of IgG BCR signaling, and downstream activation of the mitogen-activated protein kinase p38. Thus, heightened B cell memory responses are encoded, in part, by a mechanism that involves SAP97 serving as a scaffolding protein in the IgG BCR immunological synapse.

Expression and activation of the membrane-cytoskeleton protein ezrin during the normal endometrial cycle

OBJECTIVE

To examine total ezrin expression (ezrin and phospho-ezrin) through the normal endometrial cycle and to correlate ezrin activation and localization with cytologic changes.

DESIGN

Experimental laboratory study.

SETTING

University medical centers.

PATIENT(S)

Reproductive-age women.

INTERVENTION(S)

A total of 36 samples of normal early, mid-, and late proliferative- and secretory-phase endometrium were studied for immunoreactive total ezrin (ir-T-ezrin) and phospho-ezrin (ir-p-ezrin) expression by histology, immunohistochemistry, and Western blotting.

MAIN OUTCOME MEASURE(S)

Total ezrin and phospho-ezrin expressions through the normal endometrial cycle.

RESULT(S)

Throughout the cycle ir-T-ezrin is present in the epithelium. The intensity and localization of both ir-ezrin and ir-p-ezrin vary greatly throughout the cycle. The main findings include the following: lateral localization of ir-ezrin/ir-p-ezrin in association with membrane specializations; dense staining around secretory vacuoles (secretory phase); dense staining of the apical surfaces, including microvilli and pinopodes of epithelial cells, especially during the mid- to late secretory phases; and the presence of ezrin in the glandular secretions. Immunoreactive total ezrin and ir-p-ezrin were not expressed by stromal fibroblasts.

CONCLUSION(S)

Ezrin is a prominent protein in the cycling endometrium. The most striking findings were the gravitation of ir-ezrin/ir-p-ezrin to the periphery of secretory vacuoles, localization on apical surfaces of the luminal epithelium, dense ezrin staining in secretory-phase epithelial cell plumes, and the presence of ir-ezrin/ir-p-ezrin in secretory-phase luminal secretions. These findings may have functional implications, especially for implantation biology.

Constitutively active ezrin increases membrane tension, slows migration, and impedes endothelial transmigration of lymphocytes in vivo in mice

ERM (ezrin, radixin moesin) proteins in lymphocytes link cortical actin to plasma membrane, which is regulated in part by ERM protein phosphorylation. To assess whether phosphorylation of ERM proteins regulates lymphocyte migration and membrane tension, we generated transgenic mice whose T-lymphocytes express low levels of ezrin phosphomimetic protein (T567E). In these mice, T-cell number in lymph nodes was reduced by 27%. Lymphocyte migration rate in vitro and in vivo in lymph nodes decreased by 18% to 47%. Lymphocyte membrane tension increased by 71%. Investigations of other possible underlying mechanisms revealed impaired chemokine-induced shape change/lamellipod extension and increased integrin-mediated adhesion. Notably, lymphocyte homing to lymph nodes was decreased by 30%. Unlike most described homing defects, there was not impaired rolling or sticking to lymph node vascular endothelium but rather decreased migration across that endothelium. Moreover, decreased numbers of transgenic T cells in efferent lymph suggested defective egress. These studies confirm the critical role of ERM dephosphorylation in regulating lymphocyte migration and transmigration. Of particular note, they identify phospho-ERM as the first described regulator of lymphocyte membrane tension, whose increase probably contributes to the multiple defects observed in the ezrin T567E transgenic mice.