Inactivation of Merlin in malignant mesothelioma cells and the Hippo signaling cascade dysregulation

Malignant mesothelioma (MM) is an aggressive tumor arising primarily from pleural or peritoneal cavities, which is caused by asbestos exposure after long latency. One of the most frequently mutated genes detected in MM cells is the neurofibromatosis type 2 (NF2) tumor suppressor gene which is located at chromosome 22q12. The NF2 gene encodes Merlin, an ERM (Ezrin/Radixin/Moesin) protein. The underphosphorylated form of Merlin is active and acts as a tumor suppressor by regulating several distinct cellular signaling pathways. One of the downstream pathways regulated by Merlin is the Hippo signaling pathway, which is conserved from Drosophila to mammalian cells and plays important roles in organ size control and cancer development. Recent studies have identified alterations of the components in the Hippo signaling cascade in MM cells, including overexpression of Yes-associated protein (YAP) and inactivation of large tumor suppressor homolog 2 (LATS2). Dysregulation of the Merlin-Hippo signaling cascade is one of the frequent and key events of MM cell development and/or progression. Thus, a strategy to normalize this signaling cascade may be the rationale for developing a new target therapy against MM.     

Increased expression of the NF2 tumor suppressor gene product, merlin, impairs cell motility, adhesionand spreading

The neurofibromatosis 2 ( NF2 ) gene product, merlin, is a tumor suppressor protein mutated in schwanno-mas and several other tumors. Merlin, which shares significant homology with the actin-associated proteins ezrin, radixin and moesin (ERM proteins), inhibits cell growth when overexpressed in cell lines. The similarities between merlin and ERM proteins suggest that merlin's growth-regulatory capabilities may be due to alterations in cytoskeletal function. We examined this possibility in rat schwannoma cell lines overexpressing wild-type merlin isoforms and mutant merlin proteins. We found that overexpression of wild-type merlin resulted in transient alterations in F-actin organization, cell spreading and cell attachment. Merlin overexpression also impaired cell motility as measured in an in vitro motility assay. These effects were only observed in cells overexpressing a merlin isoform capable of inhibiting cell growth and not with mutant merlin molecules (NF2 patient mutations) or a merlin splice variant (isoform II) lacking growth-inhibitory activity. These data indicate that merlin may function to maintain normal cytoskeletal organization, and suggest that merlin's influence on cell growth depends on specific cytoskeletal rearrangements.      

Re-defining ERM function in lymphocyte activation and migration

Lymphocyte activation and migration involve large-scale actin cytoskeletal remodeling. The Ezrin–Radixin–Moesin (ERM) family proteins reversibly link the plasma membrane and cortical actin meshwork and mediate the dynamic nature of the membrane-cytoskeletal interface to facilitate remodeling. The reversibility of this linkage is controlled by the conformation of ERM proteins and depends on the phosphorylation of a conserved threonine residue in the actin-binding domain. Disruption of the phospho-cycling nature of ERM proteins through dominant negative and constitutively active mutants results in impaired lymphocyte migration and activation. In recent years, a novel role has emerged for ERM proteins as signaling scaffolds that can modulate B and T-cell activation through additional posttranslational modifications at tyrosine residues. Here, we highlight recent studies that have redefined the role of ERM proteins in lymphocyte activation and migration. We discuss how lymphocyte-specific knockouts of ERM proteins and high resolution imaging techniques have identified a novel function for them as rheostats that modulate the strength of antigen receptor signaling in B cells. Finally, we describe scenarios in which ERM protein function is coopted by pathogens for their own transmission and speculate on the potential of ERM proteins for regulating undesirable lymphocyte behaviors such as autoimmunity and malignancy.     

埃兹蛋白参与调控的信号转导通路研究进展

埃兹蛋白（Ezrin）是细胞骨架连接蛋白（Ezrin-Radixin-Moesin,ERM）家族成员之一,主要分布于细胞皮层。Ezrin作为膜蛋白和肌动蛋白连接蛋白,在调控细胞的形态、生长、生存、黏附、增殖和迁徙等生物学功能中发挥重要作用。其作用机制复杂,涉及Rho、PKA、PKC、MAPK及细胞凋亡等多条信号传导通路。因此,研究Ezrin相关的信号转导,对认识疾病的发展及治疗具有重要意义。     

NF2 deficiency promotes tumorigenesis and metastasis by destabilizing adherens junctions

Mutation of the Neurofibromatosis 2 (NF2) tumor suppressor gene leads to cancer development in humans and mice. Recent studies suggest that Nf2 loss also contributes to tumor metastasis. The Nf2-encoded protein, merlin, is related to the ERM (ezrin, radixin, and moesin) family of membrane:cytoskeleton-associated proteins. However, the cellular mechanism whereby merlin controls cell proliferation from this location is not known. Here we show that the major cellular consequence of Nf2 deficiency in primary cells is an inability to undergo contact-dependent growth arrest and to form stable cadherin-containing cell:cell junctions. Merlin colocalizes and interacts with adherens junction (AJ) components in confluent wild-type cells, suggesting that the lack of AJs and contact-dependent growth arrest in Nf2(-/-) cells directly results from the absence of merlin at sites of cell:cell contact. Our studies indicate that merlin functions as a tumor and metastasis suppressor by controlling cadherin-mediated cell:cell contact.     

Ezrin and BCAR1/p130Cas mediate breast cancer growth as 3-D spheroids

CAS proteins and Ezrin, Radixin, Moesin (ERM) family members act as intracellular scaffolds and are involved in interactions with the cytoskeleton, respectively. Both protein families have previously been associated with metastasis and poor prognosis in cancer. Our group recently reported on the overexpression of EZR/VIL2 and BCAR1 and their protein products in breast carcinoma effusions compared to primary breast carcinoma. In the present study, the role of these two proteins was studied in semi-normal MCF10A cells and metastatic MDA-MB-231 breast carcinoma cells cultured in tri-dimensional (3-D) conditions that were hypothesized to reproduce the in vivo conditions of breast cancer metastasis. MCF10A cells formed spheroid-shaped colonies without any Matrigel invasion, while MDA-MB-231 cells displayed an invasive phenotype and showed satellite projections that bridged multiple cell colonies in 3-D culture. E-cadherin was expressed in MCF10A, but not in MDA-MB-231 cells. The temporal expression of ezrin and BCAR1/p130Cas at the mRNA and protein level differed in the two cell lines upon 3-D culturing on Matrigel. Upregulation of BCAR1/p130cas was observed in the transition of MDA-MB-231 from attached to detached culture. Silencing of Ezrin and p130Cas in MDA-MB-231 cells by short hairpin RNA resulted in decreased invasive potential, and p130Cas silencing further resulted in smaller spheroid/colony formation. Our data show that MCF10A and MDA-MB-231 cells differ in their ability to form spheroids, in expression of E-cadherin and in the expression of Ezrin and BCAR1/p130Cas in 3-D cultures on Matrigel, suggesting a role in tumor progression in breast carcinoma.     

Ezrin在肿瘤转移中的作用

如何预防和控制肿瘤转移是提高肿瘤患者生存率的关键。Ezrin,作为连接膜和细胞骨架的蛋白,通过组成膜细胞骨架相关复合体和形成特殊膜结构对细胞活动进行调节,如细胞存活、粘附和运动,而细胞的这一切活动都与肿瘤的发生、发展和转移有重要关系。Ezrin的高表达可以促进肿瘤细胞的转移。本文就Ezrin在肿瘤转移中的作用及其可能的分子机制作一综述。     

Membrane organization and tumorigenesis--the NF2 tumor suppressor, Merlin

The NF2 tumor-suppressor gene was cloned more than a decade ago, but the function of its encoded protein, Merlin, remains elusive. Merlin, like the closely related ERM proteins, appears to provide regulated linkage between membrane-associated proteins and the actin cytoskeleton and is therefore poised to function in receiving and interpreting signals from the extracellular milieu. Recent studies suggest that Merlin may coordinate the processes of growth-factor receptor signaling and cell adhesion. Varying use of this organizing activity by different types of cells could provide an explanation for the unique spectrum of tumors associated with NF2 deficiency in mammals.     

膜-细胞骨架连接分子Ezrin与肿瘤转移

Ezrin是ERM(Ezrin/Rad ixin/Moesin)家族的重要成员,是真核细胞膜蛋白-细胞骨架的桥梁分子。参与细胞形态学,细胞黏附、运动、细胞骨架的重塑及信号转导过程。Ezrin在不同肿瘤组织中的表达异常,推测它可能参与肿瘤细胞的侵袭转移,但表达的不一致提示Ezrin在不同肿瘤的进展和转移中发挥着不同的作用。     

Loss of γ-tubulin,GCP-WD/NEDD1 and CDK5RAP2 from the Centrosome of Neurons in Developing Mouse Cerebral and Cerebellar Cortex

It has been recently reported that the centrosome of neurons does not have microtubule nucleating activity. Microtubule nucleation requires γ-tubulin as well as its recruiting proteins, GCP-WD/NEDD1 and CDK5RAP2 that anchor γ-tubulin to the centrosome. Change in the localization of these proteins during in vivo development of brain, however, has not been well examined. In this study we investigate the localization of γ-tubulin, GCP-WD and CDK5RAP2 in developing cerebral and cerebellar cortex with immunofluorescence. We found that γ-tubulin and its recruiting proteins were localized at centrosomes of immature neurons, while they were lost at centrosomes in mature neurons. This indicated that the loss of microtubule nucleating activity at the centrosome of neurons is due to the loss of γ-tubulin-recruiting proteins from the centrosome. RT-PCR analysis revealed that these proteins are still expressed after birth, suggesting that they have a role in microtubule generation in cell body and dendrites of mature neurons. Microtubule regrowth experiments on cultured mature neurons showed that microtubules are nucleated not at the centrosome but within dendrites. These data indicated the translocation of microtubule-organizing activity from the centrosome to dendrites during maturation of neurons, which would explain the mixed polarity of microtubules in dendrites.     

CD44 is required for two consecutive steps in HGF/c-Met signaling

The tyrosine kinase receptor c-Met and its ligand HGF/SF, ezrin, and splice variants of CD44 have independently been identified as tumor metastasis-associated proteins. We now show that these proteins cooperate. A CD44 isoform containing variant exon v6 sequences is strictly required for c-Met activation by HGF/SF in rat and human carcinoma cells, in established cell lines as well as in primary keratinocytes. CD44v6-deficient tumor cells were unable to activate c-Met unless they were transfected with a CD44v6-bearing isoform. Antibodies to two v6-encoded epitopes inhibited autophosphorylation of c-Met by interfering with the formation of a complex formed by c-Met, CD44v6, and HGF/SF. In addition, signal transduction from activated c-Met to MEK and Erk required the presence of the cytoplasmic tail of CD44 including a binding motif for ERM proteins. This suggests a role for ERM proteins and possibly their link to the cortical actin cytoskeleton in signal transfer.     

Ezrin蛋白与肿瘤转移

众所周知,转移是恶性肿瘤患者死亡的主要原因,如何预防和控制肿瘤转移是提高肿瘤患者生存率的关键,Ezrin作为连接使者,将细胞的膜与细胞骨架连接起来,调节诸如细胞存活、黏附和运动等活动,促进肿瘤的发生与发展。Ezrin的高表达促进肿瘤细胞的转移。本文就Ezrin的作用机制作一综述。     

Ezrin, Radixin, and Moesin (ERM) proteins function as pleiotropic regulators of human immunodeficiency virus type 1 infection

Ezrin, radixin, and moesin (ERM) proteins supply functional linkage between integral membrane proteins and cytoskeleton in mammalian cells to regulate membrane protein dynamisms and cytoskeleton rearrangement. To assess potential role of the ERM proteins in HIV-1 lifecycle, we examined if suppression of ERM function in human cells expressing HIV-1 infection receptors influences HIV-1 envelope (Env)-mediated HIV-1-vector transduction and cell-cell fusion. Expression of an ezrin dominant negative mutant or knockdown of ezrin, radixin, or moesin with siRNA uniformly decreased transduction titers of HIV-1 vectors having X4-tropic Env. In contrast, transduction titers of R5-tropic Env HIV-1 vectors were decreased only by radixin knockdown: ezrin knockdown had no detectable effects and moesin knockdown rather increased transduction titer. Each of the ERM suppressions had no detectable effects on cell surface expression of CD4, CCR5, and CXCR4 or VSV-Env-mediated HIV-1 vector transductions. Finally, the individual knockdown of ERM mRNAs uniformly decreased efficiency of cell-cell fusion mediated by X4- or R5-tropic Env and HIV-1 infection receptors. These results suggest that (i) the ERM proteins function as positive regulators of infection by X4-tropic HIV-1, (ii) moesin additionally functions as a negative regulator of R5-tropic HIV-1 virus infection at the early step(s) after the membrane fusion, and (iii) receptor protein dynamisms are regulated differently in R5- and X4-tropic HIV-1 infections.     

Expression of ezrin in glial tubes in the adult subventricular zone and rostral migratory stream

Ezrin is a member of the ERM (ezrin-radixin-moesin) family of membrane-cytoskeletal linking proteins. ERM proteins are involved in a wide variety of cellular functions including cell motility, signal transduction, cell-cell interaction and cell-matrix recognition. A recent in situ hybridization study showed that the mRNA encoding ezrin is expressed in neurogenic regions of the mature brain including the subventricular zone (SVZ) and rostral migratory stream (RMS); however, the specific cell types expressing ezrin and their relationship to migrating and proliferating cells in these regions have not been characterized previously. In this study, we used immunocytochemistry to perform double labeling with a variety of cell-type specific markers to characterize the expression of ezrin in the SVZ and RMS of adult mice. Ezrin was expressed at high levels in both the SVZ and RMS where ezrin-immunopositive processes formed a trabecular network surrounding the proliferating and migrating cells. Ezrin-positive cells co-labeled with the glial makers S100beta and GFAP (glial fibrillary acidic protein), but only minimally with the early neuronal markers beta III tubulin and polysialylated form of neural cell adhesion molecule 1 (PSA-NCAM), indicating that ezrin was expressed primarily in the glial tube cells. Ezrin positive cells also expressed beta-catenin, a membrane-complex protein previously implicated in the regulation of stem-cell proliferation and neuronal migration. Glial tube cells act as both precursors of, and a physical channel for, migrating neuroblasts. Bi-directional signals between glial tube cells and migrating neuroblasts have been shown to regulate the rates of both proliferation of the precursor cells and migration of the newly generated neuroblasts. Our finding that ezrin and beta-catenin are both present at the cell membrane of the glial tube cells suggests that these proteins may be involved in those signaling processes.     

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Background: The tumor suppressor protein merlin is thought to regulate cell proliferation and cell adhesion through interaction with protein partners. Loss of merlin is associated with Neurofibromatosis Type 2 (NF2) tumors. NHERF1 or EBP50 is a scaffolding protein that functions in apical organization of polarized cells. Merlin and NHERF1 have been shown to interact in vitro in vertebrates. We investigate how the Drosophila NHERF1 orthologue, Sip1, and Merlin function to regulate cell proliferation and adhesion. Results: We identify two conserved arginine residues (R325 and R335) in Merlin which, in addition to the FERM domain, are required for interaction with Sip1. Mutation of the arginine residues result in reduced Sip1 binding to Merlin and loss of Merlin growth suppressor function. Over‐expression of Merlin^R325A and/or Merlin^R335L in Drosophila wings result in increased proliferation in the adult wing (increase in size), which is rescued by co‐over‐expression of constitutively active Merlin protein. Reduced Sip1 binding to Merlin also produces defects in adhesion in follicle epithelial cells. Conclusions: Sip1 facilitates the activation of Merlin as a tumor suppressor protein. Thus, our work provides insight into how Merlin functions as a tumor suppressor and in adhesion and this provides insight into the mechanism of NF2 pathogenesis. Developmental Dynamics 243:1554–1570, 2014. © 2014 Wiley Periodicals, Inc.     

Regulation of cell proliferation and adhesion by means of a novel region of drosophila merlin interacting with Sip1

Background: The tumor suppressor protein merlin is thought to regulate cell proliferation and cell adhesion through interaction with protein partners. Loss of merlin is associated with Neurofibromatosis Type 2 (NF2) tumors. NHERF1 or EBP50 is a scaffolding protein that functions in apical organization of polarized cells. Merlin and NHERF1 have been shown to interact in vitro in vertebrates. We investigate how the Drosophila NHERF1 orthologue, Sip1, and Merlin function to regulate cell proliferation and adhesion. Results: We identify two conserved arginine residues (R325 and R335) in Merlin which, in addition to the FERM domain, are required for interaction with Sip1. Mutation of the arginine residues result in reduced Sip1 binding to Merlin and loss of Merlin growth suppressor function. Over‐expression of Merlin^R325A and/or Merlin^R335L in Drosophila wings result in increased proliferation in the adult wing (increase in size), which is rescued by co‐over‐expression of constitutively active Merlin protein. Reduced Sip1 binding to Merlin also produces defects in adhesion in follicle epithelial cells. Conclusions: Sip1 facilitates the activation of Merlin as a tumor suppressor protein. Thus, our work provides insight into how Merlin functions as a tumor suppressor and in adhesion and this provides insight into the mechanism of NF2 pathogenesis. Developmental Dynamics 243:1554–1570, 2014. © 2014 Wiley Periodicals, Inc.     

Expression of ezrin radixin moesin proteins in the adult subventricular zone and the rostral migratory stream

Continuous proliferation occurs in the adult subventricular zone (SVZ) of the lateral ventricles throughout life. In the SVZ, progenitor cells differentiate into neuroblasts, which migrate tangentially along the rostral migratory stream (RMS) to reach their final destination in the olfactory bulb. These progenitor cells mature and integrate into the existing neural network of the olfactory bulb. Long distance migration of neuroblasts in the RMS requires a highly dynamic cytoskeleton with the ability to respond to surrounding stimuli. Radixin is a member of the ERM (Ezrin, Radixin, Moesin) family, which connect the actin cytoskeleton to the extracellular matrix through transmembrane proteins. The membrane-cytoskeleton linker proteins of the ERM family may regulate cellular events with a high demand on cytoskeleton plasticity, such as cell motility. Recently, specific expression of the ERM protein ezrin was shown in the RMS. Radixin however has not been characterized in this region. Here we used immunohistochemistry and confocal microscopy to examine the expression of radixin in the different cell types of the adult subventricular zone niche and in the RMS. Our findings indicate that radixin is strongly expressed in neuroblasts of the adult RMS and subventricular zone, and also in Olig2-positive cells. We also demonstrate the presence of radixin in the cerebral cortex, striatum, cerebellum, thalamus, hippocampus as well as the granular and periglomerular layers of the olfactory bulb. Our studies also reveal the localization of radixin in neurosphere culture studies and we reveal the specificity of our labeling using Western blotting. The expression pattern demonstrated here suggests a role for radixin in neuronal migration and differentiation in the adult RMS. Understanding how adult neuronal migration is regulated is of importance for the development of new therapeutic interventions using endogenous repair for neurodegenerative diseases.     

Expression of HLA antigens by human thymic epithelial cells

Human thymuses were examined by tissue section staining with antibodies specific for monomorphic and polymorphic HLA-A, B, C, and DR determinants. The principal cell type expressing high levels of HLA antigens has the distribution of epithelial cells. Immunoelectron microscopy confirmed their epithelial nature. As in the mouse, both medullary and cortical epithelial cells express high levels of class II (DR) antigens, a finding that is remarkable in that these antigens were originally thought to be restricted to lymphoid and accessory cells. Class I (A, B, and C) antigens are also present on thymic epithelial cells. They are easily detectable on medullary epithelial cells, but two distinct patterns of cortical straining were observed. One group of antibodies produced intense dendritic staining throughout the cortex: the other group produced only faint or no corticol dendritic staining at all. These different staining patterns do not correlate with known properties of the antibodies and thus appear to be due to intrinsci properties of the different A, B, and C antigens.     

miR-132 靶向调控Ezrin 抑制卵巢癌细胞增殖、促进凋亡的实验研究

目的:探讨微小核糖核酸分子(miRNA)-132 在卵巢癌中生物学作用和作用靶点。方法:收集22 例卵巢癌及癌旁非肿瘤组织标本,RT-PCR 检测miR-132 表达量;RT-PCR 检测人正常卵巢上皮细胞和卵巢癌细胞系中miR-132 表达量;选择miR-132 表达量最高或最低的卵巢癌细胞株,分别转染阴性对照质粒(NC)和miR-132 mimic 质粒,RT-PCR 检测转染后miR-132 表达量;CCK-8 法检测细胞增殖,流式细胞仪检测细胞凋亡,Western blot 检测Ezrin 蛋白表达。结果:卵巢癌组织中miR-132 表达量显著低于癌旁非肿瘤组织,而卵巢癌细胞系中miR-132 表达量显著低于正常卵巢上皮细胞,差异均有统计学意义(P<0.05);选择卵巢癌细胞株中miR-132 表达量最低卵巢癌SKOV3 细胞株进行基因转染,与转染阴性对照质粒相比,转染miR-132 mimic 质粒后miR-132 表达量显著上升,细胞增殖显著降低,细胞凋亡显著增加,差异均有统计学意义(P<0.05);Western blot 结果显示,上调miR-132 表达后,卵巢癌SKOV3 细胞株中Ezrin 蛋白表达显著上升(P<0.05)。结论:在卵巢癌中,miR-132 可能作为一种抑癌基因通过靶向调控Ezrin 抑制卵巢癌细胞增殖、促进凋亡。