Ezrin has properties to self-associate at the plasma membrane

Ezrin Contributes to the Damage of Airway Epithelial Barrier Related to Diabetes Mellitus

Background

Diabetes mellitus predisposes individuals to respiratory infections. The airway epithelial barrier provides defense against inhaled antigens and pathogens. Ezrin, is a component of the membrane-cytoskeleton that maintains the cellular morphology, intercellular adhesion, and barrier function of epithelial cells. This study aimed to explore the role of ezrin in airway epithelial barrier damage and correlate its expression and activation with diabetes mellitus.

Methods

This study was performed in a murine model of diabetes mellitus and with human bronchial epithelial BEAS-2B cells using real-time PCR, Western blotting, immunohistochemical and immunofluorescence staining. Ezrin was knocked down in BEAS-2B cells using siRNA. Ezrin phosphorylation levels were measured to determine activation status. The integrity of the airway epithelial barrier was assessed in vivo by characterizing morphological structure, and in vitro in BEAS-2B cells by measuring tight junction protein expression, transepithelial electrical resistance (TER) and permeability.

Results

We demonstrated that ezrin expression levels were lower in the lung tissue and airway epithelium of diabetic mice than those in control mice. The morphological structure of the airway epithelium was altered in diabetic mice. High glucose levels downregulated the expression and distribution of ezrin and connexin 43, reduced the expression of tight junction proteins, and altered the epithelial barrier characteristics of BEAS-2B cells. Ezrin knockdown had effects similar to those of high glucose levels. Moreover, a specific inhibitor of ezrin Thr567 phosphorylation (NSC305787) inhibited epithelial barrier formation.

Conclusion

These results demonstrate that ezrin expression and activation are associated with airway epithelial damage in diabetes mellitus. These findings provide new insights into the molecular pathogenesis of pulmonary infections in diabetes mellitus and may lead to novel therapeutic interventions for airway epithelial barrier damage.

Subcellular mechano-regulation of cell migration in confined extracellular microenvironment

Cell migration is a highly coordinated cellular event that determines diverse physiological and pathological processes in which the continuous interaction of a migrating cell with neighboring cells or the extracellular matrix is regulated by the physical setting of the extracellular microenvironment. In confined spaces, cell migration occurs differently compared to unconfined open spaces owing to the additional forces that limit cell motility, which create a driving bias for cells to invade the confined space, resulting in a distinct cell motility process compared to what is expected in open spaces. Moreover, cells in confined environments can be subjected to elevated mechanical compression, which causes physical stimuli and activates the damage repair cycle in the cell, including the DNA in the nucleus. Although cells have a self-restoring system to repair damage from the cell membrane to the genetic components of the nucleus, this process may result in genetic and/or epigenetic alterations that can increase the risk of the progression of diverse diseases, such as cancer and immune disorders. Furthermore, there has been a shift in the paradigm of bioengineering from the development of new biomaterials to controlling biophysical cues and fine-tuning cell behaviors to cure damaged/diseased tissues. The external physical cues perceived by cells are transduced along the mechanosensitive machinery, which is further channeled into the nucleus through subcellular molecular linkages of the nucleoskeleton and cytoskeleton or the biochemical translocation of transcription factors. Thus, external cues can directly or indirectly regulate genetic transcriptional processes and nuclear mechanics, ultimately determining cell fate. In this review, we discuss the importance of the biophysical cues, response mechanisms, and mechanical models of cell migration in confined environments. We also discuss the effect of force-dependent deformation of subcellular components, specifically focusing on subnuclear organelles, such as nuclear membranes and chromosomal organization. This review will provide a biophysical perspective on cancer progression and metastasis as well as abnormal cellular proliferation.

Role of Ezrin in Asthma-Related Airway Inflammation and Remodeling

Ezrin is an actin binding protein connecting the cell membrane and the cytoskeleton, which is crucial to maintaining cell morphology, intercellular adhesion, and cytoskeleton remodeling. Asthma involves dysfunction of inflammatory cells, cytokines, and airway structural cells. Recent studies have shown that ezrin, whose function is affected by extensive phosphorylation and protein interactions, is closely associated with asthma, may be a therapeutic target for asthma treatment. In this review, we summarize studies on ezrin and discuss its role in asthma-related airway inflammation and remodeling.

Random nature of epithelial cancer cell monolayers

Although the polygonal shape of epithelial cells has been drawing the attention of scientists for several centuries, only a decade and a half ago it was demonstrated that distributions of polygon types (DOPTs) are similar in proliferative epithelia of many different plant and animal species. In this study, we show that hyper-proliferation of cancer cells disrupts this universal paradigm and results in randomly organized epithelial structures. Examining non-synchronized and synchronized HeLa cervix cells, we suppose that the spread of cell sizes is the main parameter controlling the DOPT in the cancer cell monolayers. To test this hypothesis, we develop a theory of morphologically similar random polygonal packings. By analysing differences between tumoural and normal epithelial cell monolayers, we conclude that the latter have more ordered structures because of their lower proliferation rates and, consequently, more effective relaxation of mechanical stress associated with cell division and growth. To explain the structural features of normal proliferative epithelium, we take into account the spread of cell sizes in the monolayer. The proposed theory also rationalizes some highly ordered unconventional post-mitotic epithelia.

Pathophysiological Roles of Actin-Binding Scaffold Protein, Ezrin

Ezrin is one of the members of the ezrin/radixin/moesin (ERM) family of proteins. It was originally discovered as an actin-binding protein in the microvilli structure about forty years ago. Since then, it has been revealed as a key protein with functions in a variety of fields including cell migration, survival, and signal transduction, as well as functioning as a structural component. Ezrin acts as a cross-linker of membrane proteins or phospholipids in the plasma membrane and the actin cytoskeleton. It also functions as a platform for signaling molecules at the cell surface. Moreover, ezrin is regarded as an important target protein in cancer diagnosis and therapy because it is a key protein involved in cancer progression and metastasis, and its high expression is linked to poor survival in many cancers. Small molecule inhibitors of ezrin have been developed and investigated as candidate molecules that suppress cancer metastasis. Here, we wish to comprehensively review the roles of ezrin from the pathophysiological points of view.

Ca2+-activated cleavage of ezrin visualised dynamically in living myeloid cells during cell surface area expansion

The intracellular events underlying phagocytosis, a crucial event for innate immunity, are still unresolved. In order to test whether the reservoir of membrane required for the formation of the phagocytic pseudopodia is maintained by cortical ezrin, and that its cleavage is a key step in releasing this membrane, the cleavage of cortical ezrin was monitored within living phagocytes (the phagocytically competent cell line RAW264.7) through expressing two ezrin constructs with fluorescent protein tags located either inside the FERM or at the actin-binding domains. When ezrin is cleaved in the linker region by the Ca2+-activated protease calpain, separation of the two fluorophores would result. Experimentally induced Ca2+ influx triggered cleavage of peripherally located ezrin, which was temporally associated with cell expansion. Ezrin cleavage was also observed in the phagocytic pseudopodia during phagocytosis. Thus, our data demonstrates that peripheral ezrin is cleaved during Ca2+-influx-induced membrane expansion and locally within the extending pseudopodia during phagocytosis. This is consistent with a role for intact ezrin in maintaining folded membrane on the cell surface, which then becomes available for cell spreading and phagocytosis.

Reconstruction and signal propagation analysis of the Syk signaling network in breast cancer cells

The ability to build in-depth cell signaling networks from vast experimental data is a key objective of computational biology. The spleen tyrosine kinase (Syk) protein, a well-characterized key player in immune cell signaling, was surprisingly first shown by our group to exhibit an onco-suppressive function in mammary epithelial cells and corroborated by many other studies, but the molecular mechanisms of this function remain largely unsolved. Based on existing proteomic data, we report here the generation of an interaction-based network of signaling pathways controlled by Syk in breast cancer cells. Pathway enrichment of the Syk targets previously identified by quantitative phospho-proteomics indicated that Syk is engaged in cell adhesion, motility, growth and death. Using the components and interactions of these pathways, we bootstrapped the reconstruction of a comprehensive network covering Syk signaling in breast cancer cells. To generate in silico hypotheses on Syk signaling propagation, we developed a method allowing to rank paths between Syk and its targets. We first annotated the network according to experimental datasets. We then combined shortest path computation with random walk processes to estimate the importance of individual interactions and selected biologically relevant pathways in the network. Molecular and cell biology experiments allowed to distinguish candidate mechanisms that underlie the impact of Syk on the regulation of cortactin and ezrin, both involved in actin-mediated cell adhesion and motility. The Syk network was further completed with the results of our biological validation experiments. The resulting Syk signaling sub-networks can be explored via an online visualization platform.

Structural characterization suggests models for monomeric and dimeric forms of full-length ezrin

Ezrin is a member of the ERM (ezrin–radixin–moesin) family of proteins that have been conserved through metazoan evolution. These proteins have dormant and active forms, where the latter links the actin cytoskeleton to membranes. ERM proteins have three domains: an N-terminal FERM [band Four-point-one (4.1) ERM] domain comprising three subdomains (F1, F2, and F3); a helical domain; and a C-terminal actin-binding domain. In the dormant form, FERM and C-terminal domains form a stable complex. We have determined crystal structures of the active FERM domain and the dormant FERM:C-terminal domain complex of human ezrin. We observe a bistable array of phenylalanine residues in the core of subdomain F3 that is mobile in the active form and locked in the dormant form. As subdomain F3 is pivotal in binding membrane proteins and phospholipids, these transitions may facilitate activation and signaling. Full-length ezrin forms stable monomers and dimers. We used small-angle X-ray scattering to determine the solution structures of these species. As expected, the monomer shows a globular domain with a protruding helical coiled coil. The dimer shows an elongated dumbbell structure that is twice as long as the monomer. By aligning ERM sequences spanning metazoan evolution, we show that the central helical region is conserved, preserving the heptad repeat. Using this, we have built a dimer model where each monomer forms half of an elongated antiparallel coiled coil with domain-swapped FERM:C-terminal domain complexes at each end. The model suggests that ERM dimers may bind to actin in a parallel fashion.

Tailor-made ezrin actin binding domain to probe its interaction with actin in-vitro

Ezrin, a member of the ERM (Ezrin/Radixin/Moesin) protein family, is an Actin-plasma membrane linker protein mediating cellular integrity and function. In-vivo study of such interactions is a complex task due to the presence of a large number of endogenous binding partners for both Ezrin and Actin. Further, C-terminal actin binding capacity of the full length Ezrin is naturally shielded by its N-terminal, and only rendered active in the presence of Phosphatidylinositol bisphosphate (PIP2) or phosphorylation at the C-terminal threonine. Here, we demonstrate a strategy for the design, expression and purification of constructs, combining the Ezrin C-terminal actin binding domain, with functional elements such as fusion tags and fluorescence tags to facilitate purification and fluorescence microscopy based studies. For the first time, internal His tag was employed for purification of Ezrin actin binding domain based on in-silico modeling. The functionality (Ezrin-actin interaction) of these constructs was successfully demonstrated by using Total Internal Reflection Fluorescence Microscopy. This design can be extended to other members of the ERM family as well.

Human ezrin-moesin-radixin proteins modulate hepatitis C virus infection

Host cytoskeletal proteins of the ezrin-moesin-radixin (EMR) family have been shown to modulate single-stranded RNA virus infection through regulating stable microtubule formation. Antibody engagement of CD81, a key receptor for hepatitis C virus (HCV) entry, induces ezrin phosphorylation. Here we tested the role of EMR proteins in regulating HCV infection and explored potential therapeutic targets. We show that HCV E2 protein induces rapid ezrin phosphorylation and its cellular redistribution with F-actin by way of spleen tyrosine kinase (SYK). Therapeutically blocking the functional roles of SYK or F-actin reorganization significantly reduced Huh7.5 cell susceptibility to HCV J6/JFH-1 infection. Using gene regulation, real-time quantitative polymerase chain reaction, western blot, and fluorescent microscopy analysis, we found that proteins of the EMR family differentially regulate HCV infection in the J6/JFH-1/Huh7.5 cell system. Moesin and radixin, but not ezrin, expression were significantly decreased in chronic HCV J6/JFH-1-infected Huh7.5 cells and HCV-infected patient liver biopsies compared to controls. The decreases in moesin and radixin in HCV J6/JFH-1-infected Huh7.5 cells were associated with a significant increase in stable microtubules. Ezrin knockdown inhibited immediate postentry events in HCV infection. Overexpression of moesin or radixin significantly reduced HCV protein expression. In contrast, transient knockdown of moesin or radixin augmented HCV infection. Making use of the Con1 HCV replicon system, we tested the effect of EMR proteins on HCV replication. We found that transient knockdown of moesin increased HCV RNA expression while overexpression of EMR showed no significant effect on HCV replication.

CONCLUSION

Our findings demonstrate the important role of EMR proteins during HCV infection at the postentry level and highlight possible novel targets for HCV treatment.

Knockdown of Ezrin by RNA interference reverses malignant behavior of human pancreatic cancer cells in vitro

BACKGROUND

Pancreatic cancer is one of the most aggressive tumors with a dismal prognosis. The membrane cytoskeletal crosslinker Ezrin participates in several functions including cell proliferation, adhesion, motility and survival. There is increasing evidence that Ezrin is overexpressed in vast majority of malignant tumors and regulates tumor progression. However, its roles in pancreatic cancer remain elusive.

METHODS

Three pairs of specific Ezrin siRNAs were designed and synthetized and screened to determine the most efficient one for construction of a hairpin RNA plasmid targeting Ezrin. After transfection into the Panc-1 pancreatic cancer cell line, real-time quantitative PCR and Western blotting were performed to examine the expression of mRNA and protein. The MTT method was applied to examine the proliferation and the drug sensibility to Gemcitabine. Flow cytometry was used to assess the cycle and apoptosis, while capacity for invasion was determined with transwell chambers. Furthermore, we detected phosphorylated-Erk1/2 protein and phosphorylated-Akt protein by Western blotting.

RESULTS

Real-time quantitative PCR and Western blotting revealed that Ezrin expression was notably down-regulated at both mRNA and protein levels by RNA interference (P< 0.01). Proliferation was inhibited and drug resistance to gemcitabine was improved (P< 0.05). Flow cytometry showed that the proportion of cells in the G1/G0 phase increased (P< 0.01), and in G2/M and S phases decreased (P< 0.05), with no apparent differences in apoptosis (P> 0.05). The capacity for invasion was markedly reduced (P< 0.01). In addition, down-regulating Ezrin expression had no effect on phosphorylated-Akt protein (P>0.05), but could decrease the level of phosphorylated-Erk1/2 protein (P< 0.05).

CONCLUSIONS

RNA interference of Ezrin could inhibit its expression in the pancreatic cancer cells line Panc-1, leading to a potent suppression of malignant behavior in vitro. Assessment of potential as a target for pancreatic cancer treatment is clearly warranted.

Analysis of SRC oncogenic signaling in colorectal cancer by stable isotope labeling with heavy amino acids in mouse xenografts

The non-receptor tyrosine kinase SRC is frequently deregulated in human colorectal cancer (CRC), and SRC increased activity has been associated with poor clinical outcomes. In nude mice engrafted with human CRC cells, SRC over-expression favors tumor growth and is accompanied by a robust increase in tyrosine phosphorylation in tumor cells. How SRC contributes to this tumorigenic process is largely unknown. We analyzed SRC oncogenic signaling in these tumors by means of a novel quantitative proteomic analysis. This method is based on stable isotope labeling with amino acids of xenograft tumors by the addition of [(13)C(6)]-lysine into mouse food. An incorporation level greater than 88% was obtained in xenograft tumors after 30 days of the heavy lysine diet. Quantitative phosphoproteomic analysis of these tumors allowed the identification of 61 proteins that exhibited a significant increase in tyrosine phosphorylation and/or association with tyrosine phosphorylated proteins upon SRC expression. These mainly included molecules implicated in vesicular trafficking and signaling and RNA binding proteins. Most of these proteins were specific targets of SRC signaling in vivo, as they were not identified by analysis via stable isotope labeling by amino acids in cell culture (SILAC) of the same CRC cells in culture. This suggests that oncogenic signaling induced by SRC in tumors significantly differs from that induced by SRC in cell culture. We next confirmed this notion experimentally with the example of the vesicular trafficking protein and SRC substrate TOM1L1. We found that whereas TOM1L1 depletion only slightly affected SRC-induced proliferation of CRC cells in vitro, it drastically decreased tumor growth in xenografted nude mice. We thus concluded that this vesicular trafficking protein plays an important role in SRC-induced tumor growth. Overall, these data show that SILAC analysis in mouse xenografts is a valuable approach for deciphering tyrosine kinase oncogenic signaling in vivo.

Ezrin ubiquitylation by the E3 ubiquitin ligase, WWP1, and consequent regulation of hepatocyte growth factor receptor activity

The membrane cytoskeleton linker ezrin participates in several functions downstream of the receptor Met in response to Hepatocyte Growth Factor (HGF) stimulation. Here we report a novel interaction of ezrin with a HECT E3 ubiquitin ligase, WWP1/Aip5/Tiul1, a potential oncogene that undergoes genomic amplification and overexpression in human breast and prostate cancers. We show that ezrin binds to the WW domains of WWP1 via the consensus motif PPVY⁴⁷⁷ present in ezrin’s C-terminus. This association results in the ubiquitylation of ezrin, a process that requires an intact PPVY⁴⁷⁷ motif. Interestingly ezrin ubiquitylation does not target the protein for degradation by the proteasome. We find that ezrin ubiquitylation by WWP1 in epithelial cells leads to the upregulation of Met level in absence of HGF stimulation and increases the response of Met to HGF stimulation as measured by the ability of the cells to heal a wound. Interestingly this effect requires ubiquitylated ezrin since it can be rescued, after depletion of endogenous ezrin, by wild type ezrin but not by a mutant of ezrin that cannot be ubiquitylated. Taken together our data reveal a new role for ezrin in Met receptor stability and activity through its association with the E3 ubiquitin ligase WWP1. Given the role of Met in cell proliferation and tumorigenesis, our results may provide a mechanistic basis for understanding the role of ezrin in tumor progression.

Tunneling nanotubes provide a unique conduit for intercellular transfer of cellular contents in human malignant pleural mesothelioma

Tunneling nanotubes are long, non-adherent F-actin-based cytoplasmic extensions which connect proximal or distant cells and facilitate intercellular transfer. The identification of nanotubes has been limited to cell lines, and their role in cancer remains unclear. We detected tunneling nanotubes in mesothelioma cell lines and primary human mesothelioma cells. Using a low serum, hyperglycemic, acidic growth medium, we stimulated nanotube formation and bidirectional transfer of vesicles, proteins, and mitochondria between cells. Notably, nanotubes developed between malignant cells or between normal mesothelial cells, but not between malignant and normal cells. Immunofluorescent staining revealed their actin-based assembly and structure. Metformin and an mTor inhibitor, Everolimus, effectively suppressed nanotube formation. Confocal microscopy with 3-dimensional reconstructions of sectioned surgical specimens demonstrated for the first time the presence of nanotubes in human mesothelioma and lung adenocarcinoma tumor specimens. We provide the first evidence of tunneling nanotubes in human primary tumors and cancer cells and propose that these structures play an important role in cancer cell pathogenesis and invasion.

Ezrin regulates microvillus morphogenesis by promoting distinct activities of Eps8 proteins

The membrane cytoskeleton linker ezrin differentially regulates the activity of Eps8 and Eps8L1a in microvillar actin-F assembly. Eps8L1a displays F-actin capping activity, therefore controlling microvillus length, whereas, as previously shown, Eps8 displays bundling activity.

The mechanisms that regulate actin filament polymerization resulting in the morphogenesis of the brush border microvilli in epithelial cells remain unknown. Eps8, the prototype of a family of proteins capable of capping and bundling actin filaments, has been shown to bundle the microvillar actin filaments. We report that Eps8L1a, a member of the Eps8 family and a novel ezrin-interacting partner, controls microvillus length through its capping activity. Depletion of Eps8L1a leads to the formation of long microvilli, whereas its overexpression has the opposite effect. We demonstrate that ezrin differentially modulates the actin-capping and -bundling activities of Eps8 and Eps8L1a during microvillus assembly. Coexpression of ezrin with Eps8 promotes the formation of membrane ruffles and tufts of microvilli, whereas expression of ezrin and Eps8L1a induces the clustering of actin-containing structures at the cell surface. These distinct morphological changes are neither observed when a mutant of ezrin defective in its binding to Eps8/Eps8L1a is coexpressed with Eps8 or Eps8L1a nor observed when ezrin is expressed with mutants of Eps8 or Eps8L1a defective in the actin-bundling or -capping activities, respectively. Our data show a synergistic effect of ezrin and Eps8 proteins in the assembly and organization of actin microvillar filaments.

An ezrin-rich, rigid uropod-like structure directs movement of amoeboid blebbing cells

Melanoma cells can switch between an elongated mesenchymal-type and a rounded amoeboid-type migration mode. The rounded 'amoeboid' form of cell movement is driven by actomyosin contractility resulting in membrane blebbing. Unlike elongated A375 melanoma cells, rounded A375 cells do not display any obvious morphological front-back polarisation, although polarisation is thought to be a prerequisite for cell movement. We show that blebbing A375 cells are polarised, with ezrin (a linker between the plasma membrane and actin cytoskeleton), F-actin, myosin light chain, plasma membrane, phosphatidylinositol (4,5)-bisphosphate and β1-integrin accumulating at the cell rear in a uropod-like structure. This structure does not have the typical protruding shape of classical leukocyte uropods, but, as for those structures, it is regulated by protein kinase C. We show that the ezrin-rich uropod-like structure (ERULS) is an inherent feature of polarised A375 cells and not a consequence of cell migration, and is necessary for cell invasion. Furthermore, we demonstrate that membrane blebbing is reduced at this site, leading to a model in which the rigid ezrin-containing structure determines the direction of a moving cell through localised inhibition of membrane blebbing.

Activation of F-actin binding capacity of ezrin: synergism of PIP₂ interaction and phosphorylation

Ezrin is a membrane-cytoskeleton linker protein that can bind F-actin in its active conformation. Several means of regulation of ezrin's activity have been described including phosphorylation of Thr-567 and binding of L-α-phosphatidylinositol-4,5-bisphosphate (PIP(2)). However, the relative contributions of these events toward activation of the protein and their potential interdependence are not known. We developed an assay based on solid-supported membranes, to which different ezrin mutants (ezrin T567A (inactive mutant), wild-type, and T567D (active pseudophosphorylated mutant)) were bound, that enabled us to analyze the influence of phosphorylation and PIP(2) binding on ezrin's activation state in vitro. The lipid bilayers employed contained either DOGS-NTA-Ni to bind the proteins via an N-terminal His-tag, or PIP(2), to which ezrin binds via specific binding sites located in the N-terminal region of the protein. Quantitative analysis of the binding behavior of all three proteins to the two different receptor lipids revealed that all three bind with high affinity and specificity to the two receptor lipids. Fluorescence microscopy on ezrin-decorated solid-supported membranes showed that, dependent on the mode of binding and the phosphorylation state, ezrin is capable of binding actin filaments. A clear synergism between phosphorylation and the receptor lipid PIP(2) was observed, suggesting a conformational switch from the dormant to the active, F-actin binding state by recognition of PIP(2), which is enhanced by the phosphorylation.

Ezrin promotes actin assembly at the phagosome membrane and regulates phago-lysosomal fusion

Phagosome maturation is defined as the process by which phagosomes fuse sequentially with endosomes and lysosomes to acquire an acidic pH and hydrolases that degrade ingested particles. While the essential role of actin cytoskeleton remodeling during particle internalization is well established, its role during the later stages of phagosome maturation remains largely unknown. We have previously shown that purified mature phagosomes assemble F-actin at their membrane, and that the ezrin-radixin-moesin (ERM) proteins ezrin and moesin participate in this process. Moreover, we provided evidence that actin assembly on purified phagosomes stimulates their fusion with late endocytic compartments in vitro. In this study, we further investigated the role of ezrin in phagosome maturation. We engineered a structurally open form of ezrin and demonstrated that ezrin binds directly to the actin assembly promoting factor N-WASP (Neural Wiskott-Aldrich Syndrome Protein) by its FERM domain. Using a cell-free system, we found that ezrin stimulates F-actin assembly on purified phagosomes by recruiting the N-WASP-Arp2/3 machinery. Accordingly, we showed that the down-regulation of ezrin activity in macrophages by a dominant-negative approach caused reduced F-actin accumulation on maturing phagosomes. Furthermore, using fluorescence and electron microscopy, we found that ezrin is required for the efficient fusion between phagosomes and lysosomes. Live-cell imaging analysis supported the notion that ezrin is necessary for the fusogenic process itself, promoting the transfer of the lysosome content into the phagosomal lumen.

The ERM proteins interact with the HOPS complex to regulate the maturation of endosomes

In the degradative pathway, the progression of cargos through endosomal compartments involves a series of fusion and maturation events. The HOPS (homotypic fusion and protein sorting) complex is part of the machinery that promotes the progression from early to late endosomes and lysosomes by regulating the exchange of small GTPases. We report that an interaction between subunits of the HOPS complex and the ERM (ezrin, radixin, moesin) proteins is required for the delivery of EGF receptor (EGFR) to lysosomes. Inhibiting either ERM proteins or the HOPS complex leads to the accumulation of the EGFR into early endosomes, delaying its degradation. This impairment in EGFR trafficking observed in cells depleted of ERM proteins is due to a delay in the recruitment of Rab7 on endosomes. As a consequence, the maturation of endosomes is perturbed as reflected by an accumulation of hybrid compartments positive for both early and late endosomal markers. Thus, ERM proteins represent novel regulators of the HOPS complex in the early to late endosomal maturation.

Ezrin promotes invasion and metastasis of pancreatic cancer cells

BACKGROUND

Pancreatic cancer has a high mortality rate because it is usually diagnosed when metastasis have already occurred (microscopic and gross disease). Ezrin plays important roles in cell motility, invasion and tumor progression, and it is especially crucial for metastasis. However, its function in pancreatic cancer remains elusive.

METHODS AND RESULTS

We found that ezrin overexpression promoted cell protrusion, microvillus formation, anchorage-independent growth, motility and invasion in a pancreatic cancer cell line, MiaPaCa-2, whereas ezrin silencing resulted in the opposite effects. Ezrin overexpression also increased the number of metastatic foci (6/8 vs. 1/8) in a spontaneous metastasis nude mouse model. Furthermore, ezrin overexpression activated Erk1/2 in MiaPaCa-2 cells, which might be partially related to the alteration of cell morphology and invasion. Immunohistochemical analysis showed that ezrin was overexpressed in pancreatic ductal adenocarcinoma (PDAC) (91.4%) and precancerous lesions, i.e. the tubular complexes in chronic pancreatitis (CP) and pancreatic intraepithelial neoplasm (PanIN) (85.7% and 97.1%, respectively), compared to normal pancreatic tissues (0%). Ezrin was also expressed in intercalated ducts adjacent to the adenocarcinoma, which has been considered to be the origin of ducts and acini, as well as the starting point of pancreatic ductal carcinoma development.

CONCLUSIONS

We propose that ezrin might play functional roles in modulating morphology, growth, motility and invasion of pancreatic cancer cells, and that the Erk1/2 pathway may be involved in these roles. Moreover, ezrin may participate in the early events of PDAC development and may promote its progression to the advanced stage.

Interleukin-7 compartmentalizes its receptor signaling complex to initiate CD4 T lymphocyte response

Interleukin (IL)-7 is a central cytokine that controls homeostasis of the CD4 T lymphocyte pool. Here we show on human primary cells that IL-7 binds to preassembled receptors made up of proprietary chain IL-7Ralpha and the common chain gammac shared with IL-2, -4, -9, -15, and -21 receptors. Upon IL-7 binding, both chains are driven in cholesterol- and sphingomyelin-rich rafts where associated signaling proteins Jak1, Jak3, STAT1, -3, and -5 are found to be phosphorylated. Meanwhile the IL-7.IL-7R complex interacts with the cytoskeleton that halts its diffusion as measured by single molecule fluorescence autocorrelated spectroscopy monitored by microimaging. Comparative immunoprecipitations of IL-7Ralpha signaling complex from non-stimulated and IL-7-stimulated cells confirmed recruitment of proteins such as STATs, but many others were also identified by mass spectrometry from two-dimensional gels. Among recruited proteins, two-thirds are involved in cytoskeleton and raft formation. Thus, early events leading to IL-7 signal transduction involve its receptor compartmentalization into membrane nanodomains and cytoskeleton recruitment.

G protein-coupled receptor kinase 2 activates radixin, regulating membrane protrusion and motility in epithelial cells

Ezrin/radixin/moesin (ERM) proteins are membrane-cytoskeleton linkers that also have roles in signal transduction. Here we show that G protein-coupled receptor kinase 2 (GRK2) regulates membrane protrusion and cell migration during wound closure in Madin-Darby canine kidney (MDCK) epithelial cell monolayers at least partly through activating phosphorylation of radixin on a conserved, regulatory C-terminal Thr residue. GRK2 phosphorylated radixin exclusively on Thr 564 in vitro. Expression of a phosphomimetic (Thr-564-to-Asp) mutant of radixin resulted in increased Rac1 activity, membrane protrusion and cell motility in MDCK cells, suggesting that radixin functions "upstream" of Rac1, presumably as a scaffolding protein. Phosphorylation of ERM proteins was highest during the most active phase of epithelial cell sheet migration over the course of wound closure. In view of these results, we explored the mode of action of quinocarmycin/quinocarcin analog DX-52-1, an inhibitor of cell migration and radixin function with considerable selectivity for radixin over the other ERM proteins, finding that its mechanism of inhibition of radixin does not appear to involve binding and antagonism at the site of regulatory phosphorylation.

The Janus-faced role of ezrin in "linking" cells to either normal or metastatic phenotype

In the majority of eukaryotic cells, the ezrin, radixin and moesin (ERM) proteins are involved in many physiologic functions including regulation of actin cytoskeleton, control of cell shape, adhesion, motility and modulation of signal transduction pathways. In a previous study, we used a dominant negative ezrin-mutant to address ezrin involvement in remodeling of actin cytoskeleton and subsequently we depicted ezrin key role in melanoma cell migration and progression. Herein, we highlight recent advances on ezrin involvement in the metastatic phenomenon, including also some more neglected ezrin-related functions. Novel molecular processes driven by ezrin activation include: phagocytosis, acquisition of resistance to chemotherapeutics and triggering of programmed cell death signals. Recent data support an integrated role of ezrin also in development of tumor malignancy. On one hand, ezrin may be responsible of deranged execution of specific known functions such as adhesion and motility and on the other, it may also participate to unique metastatic determinants, through the establishment of aberrant linkages with tumor-related proteins. For instance, ezrin misslocalization, absence or deranged activity has started to be correlated with tumor progression in many tumors of different species, including humans. Concomitantly, ezrin may act simultaneously as a regulatory or deregulatory chaperon in both normal and tumor cells. It is still to be established whether this Janus-faced feature of ezrin is due to some unknown transforming Zelig-like property or to the fact that a tumor-associated molecule preferentially links to ezrin thus distracting it from its normal connections. However, the contribution of ezrin functional deregulation to the acquisition of the metastatic phenotype appears clear and ezrin or ezrin aberrant associations may represent good candidates for future anti-tumor therapies.

Coordinate Gα13 and Wnt6-β-catenin signaling in F9 embryonal carcinoma cells is required for primitive endoderm differentiation

The mouse F9 embryonal carcinoma cell line is ideally suited to study the epithelial-to-mesenchymal transition accompanying the differentiation of primitive to parietal extraembryonic endoderm. In F9 cells, the application of exogenous agents including retinoic acid or activation of signal transduction cascades downstream of G-proteins triggers widespread changes in gene expression and leads to the formation of primitive endoderm. The epithelial-to-mesenchymal transition is completed and parietal endoderm develops as of result of increasing PKA activity in primitive endoderm cells. Expression of a constitutively active form of Gα13(Q226L) is sufficient to induce F9 cells into parietal endoderm and a model is emerging that a signaling axis linking G-protein signaling to RhoA and the ERM protein moesin is required for differentiation. In this study, we found that expression of either p115RhoGEF or a constitutively active, GTPase-deficient form of RhoA(L63) promoted primitive, but not parietal, endoderm formation. The overexpression of Gα13(Q226L) or p115RhoGEF, but not Rho(L63), caused β-catenin to translocate to the nucleus. Surprisingly, the stimulation of the Wnt-β-catenin pathway was accompanied by nuclear β-catenin and primitive endoderm formation, even when a dominant negative was used to block the signaling axis at the level of p115RhoGEF or when ROCK activity was inhibited using the pharmacological agent Y-27632. Together, results indicate that the coordinate signaling by two independent pathways, one involving canonical Wnt-β-catenin activation of target genes and the other with Gα13 signaling to ERM proteins to modulate cytoarchitectural changes, is required during the retinoic acid induced differentiation of F9 cells to primitive endoderm.

Beta4 integrin promotes osteosarcoma metastasis and interacts with ezrin

The development of pulmonary metastasis is the major cause of death in osteosarcoma, and its molecular basis is poorly understood. In this study, we show that beta4 integrin is highly expressed in human osteosarcoma cell lines and tumor samples. Furthermore, highly metastatic MNNG-HOS cells have increased levels of beta4 integrin. Suppression of beta4 integrin expression by shRNA and disruption of beta4 integrin function by transfection of dominant-negative beta4 integrin was sufficient to revert this highly metastatic phenotype in the MNNG-HOS model without significantly affecting primary tumor growth. These findings suggest a role for beta4 integrin expression in the metastatic phenotype in human osteosarcoma cells. In addition, we identified a previously uncharacterized interaction between beta4 integrin and ezrin, a membrane-cytoskeletal linker protein that is implicated in the metastatic behavior of osteosarcoma. The beta4 integrin-ezrin interaction appears to be critical for maintenance of beta4 integrin expression. These data begin to integrate ezrin and beta4 integrin expression into a model of action for the mechanism of osteosarcoma metastases.

Quantitative phosphoproteomics reveals a cluster of tyrosine kinases that mediates SRC invasive activity in advanced colon carcinoma cells

The nonreceptor tyrosine kinase Src is frequently overexpressed and/or activated in human colorectal carcinoma (CRC), and its increased activity has been associated with a poor clinical outcome. Src has been implicated in growth and invasion of these cancer cells by still not well-known mechanisms. Here, we addressed Src oncogenic signaling using quantitative phosphoproteomics. Src overexpression increased growth and invasiveness of metastatic SW620 CRC cells. Stable isotope labeling with amino acids in cell culture in combination with liquid chromatography tandem mass spectrometry allowed the identification of 136 proteins which exhibited a significant increase in and/or association with tyrosine phosphorylation upon Src expression. These mainly include signaling, cytoskeleton, and vesicular-associated proteins. Interestingly, Src also phosphorylated a cluster of tyrosine kinases, i.e., the receptors Met and EphA2, the cytoplasmic tyrosine kinase Fak, and pseudo-tyrosine kinase SgK223, which were required for its invasive activity. Similar results were obtained with metastatic Colo205 CRC cells that exhibit high endogenous Src activity. We concluded that Src uses a tyrosine kinases network to promote its invasive activity in CRC and this implicates a reverse signaling via tyrosine kinase receptors. Targeting these tyrosine kinases may be of significant therapeutic value in this cancer.

Activated ezrin promotes cell migration through recruitment of the GEF Dbl to lipid rafts and preferential downstream activation of Cdc42

Establishment of polarized cell morphology is a critical factor for migration and requires precise spatial and temporal activation of the Rho GTPases. Here, we describe a novel role of the actin-binding ezrin/radixin/moesin (ERM)-protein ezrin to be involved in recruiting Cdc42, but not Rac1, to lipid raft microdomains, as well as the subsequent activation of this Rho GTPase and the downstream effector p21-activated kinase (PAK)1, as shown by fluorescence lifetime imaging microscopy. The establishment of a leading plasma membrane and the polarized morphology necessary for random migration are also dependent on ERM function and Cdc42 in motile breast carcinoma cells. Mechanistically, we show that the recruitment of the ERM-interacting Rho/Cdc42-specific guanine nucleotide exchange factor Dbl to the plasma membrane and to lipid raft microdomains requires the phosphorylated, active conformer of ezrin, which serves to tether the plasma membrane or its subdomains to the cytoskeleton. Together these data suggest a mechanism whereby precise spatial guanine nucleotide exchange of Cdc42 by Dbl is dependent on functional ERM proteins and is important for directional cell migration.

Ezrin/moesin in motile Walker 256 carcinosarcoma cells: Signal-dependent relocalization and role in migration

Rat Walker 256 carcinosarcoma cells spontaneously develop front-tail polarity and migrate in the absence of added stimuli. Constitutive activation of phosphatidylinositol-3 kinase (PI 3-kinase), Rac, Rho and Rho kinase are essential for these processes. Ezrin and moesin are putative targets of these signaling pathways leading to spontaneous migration. To test this hypothesis, we used specific siRNA probes that resulted in a downregulation of ezrin and moesin by about 70% and in a similar reduction in the fraction of migrating cells. Spontaneous polarization however was not affected, indicating a more subtle role of ezrin and moesin in migration. We provide furthermore evidence that endogenous ezrin and moesin colocalize with F-actin at the contracted tail of polarized cells, similar to ectopically expressed green fluorescent protein-tagged ezrin. Our results suggest that myosin light chain and ezrin are markers of front and tail, respectively, even in the absence of morphological polarization. We further show that endogenous ezrin and moesin are phosphorylated and that activities of PI-3 kinase, Rho and Rac, but not of Rho-kinase, are required for this C-terminal phosphorylation. Activation of protein kinase C in contrast suppressed phosphorylation of ezrin and moesin. Inhibition of ezrin phosphorylation prevented its membrane association.

Gem associates with Ezrin and acts via the Rho-GAP protein Gmip to down-regulate the Rho pathway

Gem is a protein of the Ras superfamily that plays a role in regulating voltage-gated Ca2+ channels and cytoskeletal reorganization. We now report that GTP-bound Gem interacts with the membrane-cytoskeleton linker protein Ezrin in its active state, and that Gem binds to active Ezrin in cells. The coexpression of Gem and Ezrin induces cell elongation accompanied by the disappearance of actin stress fibers and collapse of most focal adhesions. The same morphological effect is elicited when cells expressing Gem alone are stimulated with serum and requires the expression of ERM proteins. We show that endogenous Gem down-regulates the level of active RhoA and actin stress fibers. The effects of Gem downstream of Rho, i.e., ERM phosphorylation as well as disappearance of actin stress fibers and most focal adhesions, require the Rho-GAP partner of Gem, Gmip, a protein that is enriched in membranes under conditions in which Gem induced cell elongation. Our results suggest that Gem binds active Ezrin at the plasma membrane-cytoskeleton interface and acts via the Rho-GAP protein Gmip to down-regulate the processes dependent on the Rho pathway.

Gene replacement reveals a specific role for E-cadherin in the formation of a functional trophectoderm

During mammalian embryogenesis the trophectoderm represents the first epithelial structure formed. The cell adhesion molecule E-cadherin is ultimately necessary for the transition from compacted morula to the formation of the blastocyst to ensure correct establishment of adhesion junctions in the trophectoderm. Here, we analyzed to what extent E-cadherin confers unique adhesion and signaling properties in trophectoderm formation in vivo. Using a gene replacement approach, we introduced N-cadherin cDNA into the E-cadherin genomic locus. We show that the expression of N-cadherin driven from the E-cadherin locus reflects the expression pattern of endogenous E-cadherin. Heterozygous mice co-expressing E- and N-cadherin are vital and show normal embryonic development. Interestingly, N-cadherin homozygous mutant embryos phenocopy E-cadherin-null mutant embryos. Upon removal of the maternal E-cadherin, we demonstrate that N-cadherin is able to provide sufficient cellular adhesion to mediate morula compaction, but is insufficient for the subsequent formation of a fully polarized functional trophectoderm. When ES cells were isolated from N-cadherin homozygous mutant embryos and teratomas were produced, these ES cells differentiated into a large variety of tissue-like structures. Importantly, different epithelial-like structures expressing N-cadherin were formed, including respiratory epithelia, squamous epithelia with signs of keratinization and secretory epithelia with goblet cells. Thus, N-cadherin can maintain epithelia in differentiating ES cells,but not during the formation of the trophectoderm. Our results point to a specific and unique function for E-cadherin during mouse preimplantation development.

DCC regulates cell adhesion in human colon cancer derived HT-29 cells and associates with ezrin

The deleted in colorectal cancer (DCC) gene encodes a 170- to 190-kDa protein of the Immunoglobulin superfamily. Firstly identified as a tumor suppressor gene in human colorectal carcinomas, the main function for DCC has been described in the nervous system as part of a receptor complex for netrin-1. Moreover, roles in mucosecretory cell differentiation and as inducer of apoptosis have also been reported. DCC knockout mice supported a crucial role for this gene in axonal migration, yet questioned its implication in tumor suppression and mucosecretory differentiation. The work presented here demonstrates that a DCC-transfected HT-29 colonic human cell line (HT-29/DCC) displays an increase in cell-cell adhesion to the detriment of cell-matrix interactions: HT-29/DCC cells exhibit more and better-structured desmosomes while focal adhesions and hemidesmosomes are disrupted. HT-29/DCC cells show no changes in adherent junctions but upon treatment with TPA, HT-29/DCC cells show resistance to scattering, and maintain E-cadherin in the membrane. In addition, the actin cytoskeleton is affected in HT-29/DCC cells: stress fibers are disrupted while cortical actin remains intact. We identified a putative ERM-M (ezrin/radixin/moesin and merlin) binding domain in the juxtamembrane region of the DCC protein. In vitro pull-down assays demonstrate the interaction of the DCC cytoplasmic domain with the N-terminal region of ezrin and merlin, and co-immunoprecipitation assays in transiently DCC-transfected COS-1 cells showed that the interaction between DCC and ezrin also takes place in vivo. Altogether, our results suggest that DCC could regulate cell adhesion and migration through its association with ERM-M proteins.

The membrane-cytoskeleton organizer ezrin is necessary for hepatocellular carcinoma cell growth and invasiveness

PURPOSE

The change of cell mobility is one of the preconditions of tumor metastasis. Cell skeleton alteration and rearrangement of F-actin was closely related to cell mobility. Ezrin is a membrane-cytoskeleton organizer that can mediate the rearrangement and the function of F-actin. In this paper, we investigated the effect of ezrin on hepatocellular carcinoma cell growth and invasiveness.

METHODS

Hepatocellular carcinoma cell lines such as MHCC-1, MHCC97-H, SF7721, SMMC7721, Hep3B, and HepG2 were chosen in this study. We first examined the expression and the distribution of ezrin and F-actin in these cell lines using immunofluorescence, RT-PCR, and the western blot. Next we used small interfering RNA (siRNA) to down-regulate ezrin expression in MHCC-1, MHCC97-H, SF7721, and HepG2 to investigate the role of ezrin in tumor cell growth and invasiveness.

RESULTS

Our preliminary results showed that the expression of ezrin and gamma-actin in MHCC-1, MHCC97-H, and SF7721 with higher metastatic potential were obviously up-regulated than those in SMMC7721, Hep3B, and HepG2 with lower potential. No different expression of beta-actin was found in the above tumor cell lines. The outcome of RNAi indicated that decreasing ezrin expression can notably inhibit the proliferation of the four hepatocellular carcinoma cell lines (p < 0.01, n = 10). The proportion of cells in G2-M phase also decreased after RNAi. The number of pseudopods decreased as well after RNAi treatment (p < 0.01, n = 5). The mobility and invasiveness of cancer cells decreased with decreasing ezrin expression tested by transwell assay (p < 0.01, n = 8).

CONCLUSION

Ezrin plays an important role in the process of hepatocellular carcinoma cell proliferation, migration, and invasiveness.

Mutational and immunohistochemical analysis of ezrin-, radixin-, moesin (ERM) molecules in epilepsy-associated glioneuronal lesions

Glioneuronal lesions are frequently observed in biopsy specimens obtained from patients with pharmacoresistant epilepsies, comprising focal cortical dysplasias (FCD) and gangliogliomas. Recent findings point to the phosphoinositide 3-kinase (PI3K) pathway and tuberin/hamartin signaling cascade as being compromised in these lesions. Ezrin, radixin and moesin (ERM-/band-4.1 proteins) genes represent downstream effectors of the PI3K pathway, are involved in cytoskeleton-membrane interference, cell growth, migration and differentiation, and harbor tumor suppressor motifs. Accumulation of band-4.1 proteins has been identified in cortical tubers of tuberous sclerosis patients, which share neuropathological similarities with FCD and gangliogliomas. Here, we have studied the immunohistochemical distribution pattern of ERMs, as well as allelic variants, occurring in gangliogliomas (n=20) and FCDs (FCD(IIa), n=7; FCD(IIb), n=37). Aberrant accumulation of ERMs was observed in dysplastic neurons of FCDs and gangliogliomas as well as in balloon cells. Adjacent brain tissue without structural abnormalities was used as control and showed only faint neuropil staining. Mutational screening revealed silent polymorphisms in the ezrin gene in two individuals suffering from FCD(IIb). A transition from G to A in radixin exon 2 resulted in an exchange of valine by isoleucine at codon 50 in an additional FCD(IIb) specimen. Such sequence alterations were not found in controls. The present data suggest accumulation of ERM expression in dysplastic cellular components but do not favor mutational events of ERM in the pathogenesis of FCDs or gangliogliomas. Aberrant expression of ERMs is, however, compatible with compromised PI3K-pathway signaling in glioneuronal lesions characterized by abnormal cellular differentiation and aberrant network formation.

Vezatin, a protein associated to adherens junctions, is required for mouse blastocyst morphogenesis

Cell-cell interactions play a major role during preimplantation development of the mouse embryo. The formation of adherens junctions is a major feature of compaction, the first morphogenetic event that takes place at the 8-cell stage. Then, during the following two cell cycles, tight junctions form, and the outer layer of cells differentiate into a functional epithelium, leading to the formation of the blastocoel cavity. Until now, E-cadherin was the only transmembrane molecule localized in adherens junctions and required for early development. Vezatin is a transmembrane protein of adherens junctions, interacting with the E-cadherin-catenins complex. Here, we show that vezatin is expressed very early during mouse preimplantation development. It co-localizes with E-cadherin throughout development, being found all around the cell cortex before compaction and basolaterally in adherens junctions thereafter. In addition, vezatin is also detected in nuclei during most of the cell cycle. Finally, using a morpholino-oligonucleotide approach to inhibit vezatin function during preimplantation development, we observed that inhibition of vezatin synthesis leads to a cell cycle arrest with limited cell-cell interactions. This phenotype can be rescued when mRNAs coding for vezatin missing the 5'UTR are co-injected with the anti-vezatin morpholino-oligonucleotide. Cells derived from blastomeres injected with morpholino-oligonucleotide had a reduced amount of vezatin concomitantly with a decrease in the quantity of E-cadherin and beta-catenin localized in the areas of intercellular contact. Shift in E-cadherin cortical distribution was correlated with a strong decrease in E-cadherin mRNA and protein contents. Altogether, these observations demonstrate that vezatin is required for morphogenesis of the preimplantation mouse embryo.

A network of PDZ-containing proteins regulates T cell polarity and morphology during migration and immunological synapse formation

T cell shape is dictated by the selective recruitment of molecules to different regions of the cell (polarity) and is integral to every aspect of T cell function, from migration to cytotoxicity. This study describes a mechanism for the regulation of T cell polarity. We show that T cells contain a network of asymmetrically distributed proteins with the capacity to dictate the subcellular localization of both cell surface receptors and morphological determinants in T cells. Proteins from the Scribble, Crumbs3, and Par3 complexes, previously shown to regulate epithelial polarity, were polarized in T cells containing either uropods or immunological synapses. Reduction in Scribble expression prevented the polarization of cell surface receptors and prevented morphological changes associated with uropod formation, migration, and antigen presentation. By dynamically coordinating molecular distribution throughout the T cell, this network provides a mechanism by which T cell function and polarity are linked.

Three-dimensional polarization sensitizes hepatocytes to Fas/CD95 apoptotic signalling

Maintenance of epithelial cell shape and polarity determines many vital cell functions, including the appropriate response to external stimuli. Murine hepatocytes cultured in a three-dimensional Matrigel matrix formed highly polarized organoids characterized by specific localization of an ERM (ezrin/radixin/moesin) protein, radixin, at microvillus-lined membrane domains. These apical domains surrounded a lumen and were bordered by tight junctions. The hepatocyte organoids were functional as judged by the high level of albumin secretion and accumulation of bilirubin. Stimulation of the Fas/CD95 death receptor, which is highly hepatotoxic in vivo, was a strong inducer of apoptosis in the polarized organoids. This was in sharp contrast to the monolayer hepatocyte cultures, which were protected from death by exacerbated NF-κB signalling following engagement of the death receptors. Thus, hepatocytes in polarized, functional organoids modulate an intracellular signal transduction pathway, allowing the recapitulation of their physiological response to an apoptotic stimulus.

Aging affects choroidal proteins involved in CSF production in Sprague-Dawley rats

Aging is currently associated with progressive declines of cerebral functions. From these, a decreased resistance to dehydration suggested alteration in choroidal control of brain homeostasis and reduced cerebrospinal fluid (CSF) production in old subjects. In the present study, choroid plexuses of 20-month old Sprague-Dawley rats were compared with those of 3- and 10-month old rats. Using ultrastructure analysis and immunodetection of ezrin, a protein associating cytoskeleton to membranes, we showed that progressive loss of microvilli and strong decrease in apical ezrin are evident in 20-month old rats. Using immunolabeling and confocal microscopy, we found reduction in expression of two choroidal proteins, carbonic anhydrase II and aquaporin 1, involved in CSF secretion. In addition, we confirmed previous studies indicating that choroidal Na,K-ATPase decreased with age. In situ hybridization analyses showed that mRNA levels for Na,K-ATPase and aquaporin 1 were significantly lowered in choroid plexus of old rats. These findings are consistent with a reduced secretory activity in choroid plexus and suggest that massive disorders could affect choroidal CSF production in aged rats.

Ezrin oligomers are the membrane-bound dormant form in gastric parietal cells

Ezrin is a member of ezrin, radixin, moesin (ERM) protein family that links F-actin to membranes. The NH(2)- and COOH-terminal association domains of ERM proteins, known respectively as N-ERMAD and C-ERMAD, participate in interactions with membrane proteins and F-actin, and intramolecular and intermolecular interactions within and among ERM proteins. In gastric parietal cells, ezrin is heavily represented on the apical membrane and is associated with cell activation. Ezrin-ezrin interactions are presumably involved in functional regulation of ezrin and thus became a subject of our study. Fluorescence resonance energy transfer (FRET) was examined with cyan fluorescent protein (CFP)- and yellow fluorescent protein (YFP)-tagged ezrin incorporated into HeLa cells and primary cultures of parietal cells. Constructs included YFP at the NH(2) terminus of ezrin (YFP-Ez), CFP at the COOH terminus of ezrin (Ez-CFP), and double-labeled ezrin (N-YFP-ezrin-CFP-C). FRET was probed using fluorescence microscopy and spectrofluorometry. Evidence of ezrin oligomer formation was found using FRET in cells coexpressing Ez-CFP and YFP-Ez and by performing coimmunoprecipitation of endogenous ezrin with fluorescent protein-tagged ezrin. Thus intermolecular NH(2)- and COOH-terminal association domain (N-C) binding in vivo is consistent with the findings of earlier in vitro studies. After the ezrin oligomers were separated from monomers, FRET was observed in both forms, indicating intramolecular and intermolecular N-C binding. When the distribution of native ezrin as oligomers vs. monomers was examined in resting and maximally stimulated parietal cells, a shift of ezrin oligomers to the monomeric form was correlated with stimulation, suggesting that ezrin oligomers are the membrane-bound dormant form in gastric parietal cells.

Phosphorylation of ezrin on threonine T567 plays a crucial role during compaction in the mouse early embryo

The preimplantation development of the mouse embryo leads to the divergence of the first two cell lineages, the inner cell mass and the trophectoderm. The formation of a microvillus pole during compaction at the eight-cell stage and its asymmetric inheritance during mitosis are key events in the emergence of these two cell populations. Ezrin, a member of the ERM protein family, seems to be involved in the formation and stabilization of this apical microvillus pole. To further characterize its function in early development, we mutated the key residue T567, which was reported to be essential for regulation of ezrin function through phosphorylation. Here, we show that expression of ezrin mutants in which the COOH-terminal threonine T567 was replaced by an aspartate (to mimic a phosphorylated residue; T567D) or by an alanine (to avoid phosphorylation; T567A) interferes with E-cadherin function and disrupts the first morphogenetic events of development: compaction and cavitation. The active mutant ezrin-T567D induces the formation of numerous and abnormally long microvilli at the surface of blastomeres. Moreover, it localizes all around the cell cortex and inhibits cell-cell adhesion and cell polarization at the eight-cell stage. During the following stages, only half of the embryos are able to compact and finally to cavitate. In those embryos, the amount of ezrin-T567D decreases in the basolateral areas, while the proportion of adherens junctions increases. The reverse inactive mutant ezrin-T567A is mainly cytoplasmic and does not perturb compaction at the eight-cell stage. However, at the 16-cell stage, it relocalizes at the basolateral cortex, leading to a strong decrease in the surface of adherens junctions, and finally, embryos abort development. Our results show that ezrin is directly involved in the formation of microvilli in the early mouse embryo. Moreover, they indicate that maintenance of ezrin in basolateral areas prevents microvilli breakdown and inhibits the formation of normal cell-cell contacts mediated by E-cadherin, thereby impairing blastomeres polarization and morphogenesis of the blastocyst.

Shiga toxin binding to globotriaosyl ceramide induces intracellular signals that mediate cytoskeleton remodeling in human renal carcinoma-derived cells

Shiga toxin is a bacterial toxin consisting of A and B subunits. Generally, the essential cytotoxicity of the toxin is thought to be mediated by the A subunit, which possesses RNA cleavage activity and thus induces protein synthesis inhibition. We previously reported, however, that the binding of the Shiga toxin 1-B subunit to globotriaosyl ceramide, a functional receptor for Shiga toxin, induces intracellular signals in a manner that is dependent on glycolipid-enriched membrane domains, or lipid rafts. Although the precise role of this signaling mechanism is not known, here we report that Shiga-toxin-mediated intracellular signals induce cytoskeleton remodeling in ACHN cells derived from renal tubular epithelial carcinoma. Using confocal laser scanning microscopy, we observed that Shiga toxin 1-B treatment induces morphological changes in ACHN cells in a time-dependent manner. In addition, the morphological changes were accompanied by the redistribution of a number of proteins, including actin, ezrin, CD44, vimentin, cytokeratin, paxillin, FAK, and α- and γ-tubulins, all of which are involved in cytoskeletal organization. The transient phosphorylation of ezrin and paxillin was also observed during the course of protein redistribution. Experiments using inhibitors for a variety of kinases suggested the involvement of lipid rafts, Src family protein kinase, PI 3-kinase, and RHO-associated kinase in Shiga toxin 1-B-induced ezrin phosphorylation. Shiga toxin 1-B-induced cytoskeletal remodeling should provide an in vitro model that can be used to increase our understanding of the pathogenesis of Shiga-toxin-mediated cell injury and the role of lipid-raft-mediated cell signaling in cytoskeletal remodeling.

Delta-promoted filopodia mediate long-range lateral inhibition in Drosophila

Drosophila thoracic mechanosensory bristles originate from cells that are singled out from 'proneural' groups of competent epithelial cells. Neural competence is restricted to individual sensory organ precursors (SOPs) by Delta/Notch-mediated 'lateral inhibition', whereas other cells in the proneural field adopt an epidermal fate. The precursors of the large macrochaetes differentiate separately from individual proneural clusters that comprise about 20-30 cells or as heterochronic pairs from groups of more than 100 cells, whereas the precursors of the small regularly spaced microchaetes emerge from even larger proneural fields. This indicates that lateral inhibition might act over several cell diameters; it was difficult to reconcile with the fact that the inhibitory ligand Delta is membrane-bound until the observation that SOPs frequently extend thin processes offered an attractive hypothesis. Here we show that the extension of these planar filopodia--a common attribute of wing imaginal disc cells--is promoted by Delta and that their experimental suppression reduces Notch signalling in distant cells and increases bristle density in large proneural groups, showing that these membrane specializations mediate long-range lateral inhibition.

Calpain 3 is activated through autolysis within the active site and lyses sarcomeric and sarcolemmal components

Calpain 3 (Capn3) is known as the skeletal muscle-specific member of the calpains, a family of intracellular nonlysosomal cysteine proteases. This enigmatic protease has many unique features among the calpain family and, importantly, mutations in Capn3 have been shown to be responsible for limb girdle muscular dystrophy type 2A. Here we demonstrate that the Capn3 activation mechanism is similar to the universal activation of caspases and corresponds to an autolysis within the active site of the protease. We undertook a search for substrates in immature muscle cells, as several lines of evidence suggest that Capn3 is mostly in an inactive state in muscle and needs a signal to be activated. In this model, Capn3 proteolytic activity leads to disruption of the actin cytoskeleton and disorganization of focal adhesions through cleavage of several endogenous proteins. In addition, we show that titin, a previously identified Capn3 partner, and filamin C are further substrates of Capn3. Finally, we report that Capn3 colocalizes in vivo with its substrates at various sites along cytoskeletal structures. We propose that Capn3-mediated cleavage produces an adaptive response of muscle cells to external and/or internal stimuli, establishing Capn3 as a muscle cytoskeleton regulator.

Chemokine stimulation of human peripheral blood T lymphocytes induces rapid dephosphorylation of ERM proteins, which facilitates loss of microvilli and polarization

Lymphocyte microvilli mediate initial rolling-adhesion along endothelium but are lost during transmigration from circulation to tissue. However, the mechanism for resorption of lymphocyte microvilli remains unexplored. We show that chemokine stimulation of human peripheral blood T (PBT) cells is sufficient to induce rapid resorption of microvilli. Microvilli in other cells are regulated by ezrin/radixin/moesin (ERM) proteins, which link the plasma membrane to the cortical F-actin cytoskeleton; maintenance of these linkages requires ERM activation, reflected by phosphorylation at a specific carboxy-terminal threonine residue. Carboxyphosphorylated-ERM (cpERM) proteins in resting PBT cells show a punctate peripheral distribution consistent with localization to microvilli. cpERM dephosphorylation begins within seconds of stimulation by chemokines (stromal derived factor 1 alpha [SDF-1 alpha] or secondary lymphoid tissue cytokine), and ERM proteins lose their punctate distribution with kinetics paralleling the loss of microvilli. The cpERM proteins are preferentially associated with the cytoskeleton at rest and this association is lost with chemokine-induced dephosphorylation. Transfection studies show that a dominant-negative ERM construct destroys microvilli, whereas a construct mimicking cpERM facilitates formation of microvilli, retards chemokine-induced loss of microvilli, and markedly impairs chemokine-induced polarization. Thus, chemokine induces rapid dephosphorylation and inactivation of cpERM, which may in turn facilitate 2 aspects of cytoskeletal reorganization involved in lymphocyte recruitment: loss of microvilli and polarization.

NF2: The wizardry of merlin

Neurofibromatosis type II (NF2) is an autosomal dominant cancer syndrome characterized by the formation of tumors of the nervous system, particularly schwannomas and meningiomas. The NF2 gene is also implicated in the development of sporadic schwannomas and meningiomas, as well as tumor types seemingly unrelated to the NF2 disorder, such as malignant mesotheliomas. Inactivation of NF2 occurs by a "two-hit" mechanism, as proposed by Al Knudson, and the NF2 gene behaves as a classical tumor suppressor gene. The NF2 gene product, merlin, exhibits homology with the ezrin-radixin-moesin family of membrane-cytoskeleton-linking proteins. During the past several years, there has been intensive investigation aimed at elucidating the mechanisms underlying merlin's functions. In this review, we summarize the involvement of NF2 inactivation in tumorigenesis. We also discuss observations implicating merlin in cell motility and cell proliferation, with a focus on recent findings linking merlin to Rac signaling.

Insights into a Single Rod-like Helix in Activated Radixin Required for Membrane−Cytoskeletal Cross-Linking†

The members of the ezrin-radixin-moesin (ERM) family of proteins function as membrane-cytoskeletal cross-linkers in actin-rich cell surface structures. ERM proteins are thereby thought to be essential for cortical cytoskeleton organization, cell motility, adhesion, and proliferation. These modular polypeptides consist of a central helix-rich region, termed the alpha-domain, that connects an N-terminal FERM domain required for membrane binding and a C-terminal region which contains a major actin-binding motif. Conformational regulation of ERM protein function occurs by association of the FERM and C-terminal domains, whereby the membrane- and actin-binding activities are mutually suppressed and the protein is thought to take an inactive "closed" form. Here we report in vitro and in vivo studies of radixin to address the role of the alpha-domain in conformational activation of ERM proteins. Remarkably, an isolated alpha-domain comprised of radixin(311-469) forms a monomeric, stable helical rod that spans 240 A in length from the N-terminus to the C-terminus, most likely stabilized by extensive salt bridge interactions. By fusing green fluorescent protein variants to the FERM and C-terminal domains, we probed in vitroconformational changes impacted by the presence of the alpha-domain using fluorescence resonance energy transfer (FRET). Furthermore, deletion of this unusually long alpha-helical structure (radixin residues 314-411) prevents ERM membrane targeting in vivo.

Identification of a novel Ezrin-binding site in syndecan-2 cytoplasmic domain

ERM (Ezrin/Radixin/Moesin) proteins are crosslinkers between plasma membrane proteins and the actin cytoskeleton, thereby involved in the formation of cell adhesion sites. Earlier work showed that Ezrin links syndecan-2 to the actin cytoskeleton. Here we provide evidence that the Ezrin N-terminal domain binds to the syndecan-2 cytoplasmic domain with an estimated K(D) of 0.71 microM and without the requirement of other proteins. We also studied the regions in the syndecan-2 cytoplasmic domain implicated in the binding to Ezrin. By truncating the syndecan-2 cytoplasmic domain and by oligopeptide competition assays we show that the Ezrin-binding sequence is not located in the positively charged juxtamembrane region (RMRKK), but in the neighboring sequence DEGSYD. We therefore conclude that the consensus sequence for Ezrin binding is unique among membrane proteins, suggesting a distinct regulation.

The role of the CD44/ezrin complex in cancer metastasis

CD44 is a cell adhesion molecule that was traditionally known as 'homing receptor'. This molecule is known to interact with the ezrin family (ERM family) members and form a complex that plays diverse roles within both normal and abnormal cells, particularly cancer cells. CD44 and ezrin and their respective complex have properties suggesting that they may be important in the process of tumour-endothelium interactions, cell migrations, cell adhesion, tumour progression and metastasis. This article reviews the role of CD44, ezrin family and the CD44/ezrin complex in cancer cells and their clinical impact in patients with cancer.

Alveolar echinococcosis: characterization of diagnostic antigen Em18 and serological evaluation of recombinant Em18

The Echinococcus multilocularis protein Em18 is one of the most promising antigens for use in serodiagnosis of alveolar echinococcosis in human patients. Here we identify an antigenic relationship between Em18 and a 65-kDa immunodominant E. multilocularis surface protein previously identified as either EM10 or EmII/3. The NH(2)-terminal sequence of native Em18 was determined, revealing it to be a fragment of EM10. Experiments were undertaken to investigate the effect of proteinase inhibitors on the degradation of EM10 in crude extracts of E. multilocularis protoscoleces. Em18 was found to be the product of degradation of EM10 by cysteine proteinase. A recombinant Em18 (RecEm18, derived from (349)K to (508)K of EM10) was successfully expressed by using Escherichia coli expression system and then evaluated for use in serodiagnosis of alveolar echinococcosis. RecEm18 was recognized by 27 (87.1%) and 28 (90.3%) of 31 serum samples from clinically and/or pathologically confirmed alveolar echinococcosis patients by enzyme-linked immunosorbent assay and immunoblotting, respectively. Of 33 serum samples from cystic echinococcosis patients, 1 was recorded as having a weak positive reaction to RecEm18; however, none of the serum samples which were tested from neurocysticercosis patients (n = 10) or healthy people (n = 15) showed positive reactions. RecEm18 has the potential for use in the differential serodiagnosis of alveolar echinococcosis.