Revealing roles of S-layer protein (SlpA) in Clostridioides difficile pathogenicity by generating the first slpA gene deletion mutant

Clostridioides difficile infection (CDI) with high morbidity and high mortality is an urgent threat to public health, and C. difficile pathogenesis studies are eagerly required for CDI therapy. The major surface layer protein, SlpA, was supposed to play a key role in C. difficile pathogenesis; however, a lack of isogenic slpA mutants has greatly hampered analysis of SlpA functions. In this study, the whole slpA gene was successfully deleted for the first time via CRISPR-Cas9 system. Deletion of slpA in C. difficile resulted in smaller, smother-edged colonies, shorter bacterial cell size, and aggregation in suspension. For life cycle, the mutant demonstrated lower growth (changes of optical density at 600 nm, OD600) but higher cell density (colony-forming unit, CFU), decreased toxins production, and inhibited sporulation. Moreover, the mutant was more impaired in motility, more sensitive to vancomycin and Triton X-100-induced autolysis, releasing more lactate dehydrogenase. In addition, SlpA deficiency led to robust biofilm formation but weak adhesion to human host cells.IMPORTANCEClostridioides difficile infection (CDI) has been the most common hospital-acquired infection, with a high rate of antibiotic resistance and recurrence incidences, become a debilitating public health threat. It is urgently needed to study C. difficile pathogenesis for developing efficient strategies as CDI therapy. SlpA was indicated to play a key role in C. difficile pathogenesis. However, analysis of SlpA functions was hampered due to lack of isogenic slpA mutants. Surprisingly, the first slpA deletion C. difficile strain was generated in this study via CRISPR-Cas9, further negating the previous thought about slpA being essential. Results in this study will provide direct proof for roles of SlpA in C. difficile pathogenesis, which will facilitate future investigations for new targets as vaccines, new therapeutic agents, and intervention strategies in combating CDI.

Pharmacologic targeting of Cdc42 GTPase by a small molecule Cdc42 activity-specific inhibitor prevents platelet activation and thrombosis

Gene targeting of Cdc42 GTPase has been shown to inhibit platelet activation. In this study, we investigated a hypothesis that inhibition of Cdc42 activity by CASIN, a small molecule Cdc42 Activity-Specific INhibitor, may down regulate platelet activation and thrombus formation. We investigated the effects of CASIN on platelet activation in vitro and thrombosis in vivo. In human platelets, CASIN, but not its inactive analog Pirl7, blocked collagen induced activation of Cdc42 and inhibited phosphorylation of its downstream effector, PAK1/2. Moreover, addition of CASIN to washed human platelets inhibited platelet spreading on immobilized fibrinogen. Treatment of human platelets with CASIN inhibited collagen or thrombin induced: (a) ATP secretion and platelet aggregation; and (b) phosphorylation of Akt, ERK and p38-MAPK. Pre-incubation of platelets with Pirl7, an inactive analog of CASIN, failed to inhibit collagen induced aggregation. Washing of human platelets after incubation with CASIN eliminated its inhibitory effect on collagen induced aggregation. Intraperitoneal administration of CASIN to wild type mice inhibited ex vivo aggregation induced by collagen but did not affect the murine tail bleeding times. CASIN administration, prior to laser-induced injury in murine cremaster muscle arterioles, resulted in formation of smaller and unstable thrombi compared to control mice without CASIN treatment. These data suggest that pharmacologic targeting of Cdc42 by specific and reversible inhibitors may lead to the discovery of novel antithrombotic agents.

Molecular identity of cardiac mitochondrial chloride intracellular channel proteins

Emerging evidences demonstrate significance of chloride channels in cardiac function and cardioprotection from ischemia-reperfusion (IR) injury. Unlike mitochondrial potassium channels sensitive to calcium (BKCa) and ATP (KATP), molecular identity of majority of cardiac mitochondrial chloride channels located at the inner membrane is not known. In this study, we report the presence of unique dimorphic chloride intracellular channel (CLIC) proteins namely CLIC1, CLIC4 and CLIC5 as abundant CLICs in the rodent heart. Further, CLIC4, CLIC5, and an ortholog present in Drosophila (DmCLIC) localize to adult cardiac mitochondria. We found that CLIC4 is enriched in the outer mitochondrial membrane, whereas CLIC5 is present in the inner mitochondrial membrane. Also, CLIC5 plays a direct role in regulating mitochondrial reactive oxygen species (ROS) generation. Our study highlights that CLIC5 is localized to the cardiac mitochondria and directly modulates mitochondrial function.

Gene targeting implicates Cdc42 GTPase in GPVI and non-GPVI mediated platelet filopodia formation, secretion and aggregation

Background

Cdc42 and Rac1, members of the Rho family of small GTPases, play critical roles in actin cytoskeleton regulation. We have shown previously that Rac1 is involved in regulation of platelet secretion and aggregation. However, the role of Cdc42 in platelet activation remains controversial. This study was undertaken to better understand the role of Cdc42 in platelet activation.

Methodology/Principal Findings

We utilized the Mx-cre;Cdc42^lox/lox inducible mice with transient Cdc42 deletion to investigate the involvement of Cdc42 in platelet function. The Cdc42-deficient mice exhibited a significantly reduced platelet count than the matching Cdc42^+/+ mice. Platelets isolated from Cdc42^−/−, as compared to Cdc42^+/+, mice exhibited (a) diminished phosphorylation of PAK1/2, an effector molecule of Cdc42, (b) inhibition of filopodia formation on immobilized CRP or fibrinogen, (c) inhibition of CRP- or thrombin-induced secretion of ATP and release of P-selectin, (d) inhibition of CRP, collagen or thrombin induced platelet aggregation, and (e) minimal phosphorylation of Akt upon stimulation with CRP or thrombin. The bleeding times were significantly prolonged in Cdc42^−/− mice compared with Cdc42^+/+ mice.

Conclusion/Significance

Our data demonstrate that Cdc42 is required for platelet filopodia formation, secretion and aggregation and therefore plays a critical role in platelet mediated hemostasis and thrombosis.

Proteomic analysis of lung tissues from patients with pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension is a disorder of vascular remodeling causing increased resistance to pulmonary blood flow. The expression of proteins in lungs from pulmonary arterial hypertension patients was investigated in an unbiased approach to further understand the pathobiology of this disease.

METHODS AND RESULTS

Label-free liquid chromatography tandem mass spectrometry was used to compare protein profiles in surgical samples of lungs from 8 patients with pulmonary arterial hypertension and 8 control subjects. More than 300 proteins were detected. On the basis of robust criteria, the levels of 25 proteins varied between the 2 groups. The majority of upregulated proteins were associated with cell growth, proliferation, and cell metabolism. Novel findings included an increased expression of chloride intracellular channel 4, receptor for advanced glycation end products, and periostin. Increased expression of chloride intracellular channel 4, a multifunctional protein involved in angiogenesis, and several signaling pathways implicated in pulmonary arterial hypertension--transforming growth factor-β, vascular endothelial growth factor, and bone morphogenetic protein--was confirmed by Western blotting and localized predominantly to endothelial cells in occlusive and plexiform vascular lesions.

CONCLUSIONS

Label-free proteomics identified differences in the expression of several proteins in the pulmonary arterial hypertension lung, many of which are relevant to the disease process. Increased expression of chloride intracellular channel 4 may be pertinent to the disorganized angiogenesis of plexiform lesions.

CLIC5 mutant mice are resistant to diet-induced obesity and exhibit gastric hemorrhaging and increased susceptibility to torpor

Chloride intracellular channel 5 (CLIC5) and other CLIC isoforms have been implicated in a number of biological processes, but their specific functions are poorly understood. The association of CLIC5 with ezrin and the actin cytoskeleton led us to test its possible involvement in gastric acid secretion. Clic5 mutant mice exhibited only a minor reduction in acid secretion, Clic5 mRNA was expressed at only low levels in stomach, and Clic5 mutant parietal cells were ultrastructurally normal, negating the hypothesis that CLIC5 plays a major role in acid secretion. However, the mutants exhibited gastric hemorrhaging in response to fasting, reduced monocytes and granulocytes suggestive of immune dysfunction, behavioral and social disorders suggestive of neurological dysfunction, and evidence of a previously unidentified metabolic defect. Wild-type and mutant mice were maintained on normal and high-fat diets; plasma levels of various hormones, glucose, and lipids were determined; and body composition was studied by quantitative magnetic resonance imaging. Clic5 mutants were lean, hyperphagic, and highly resistant to diet-induced obesity. Plasma insulin and glucose levels were reduced, and leptin levels were very low; however, plasma triglycerides, cholesterol, phospholipids, and fatty acids were normal. Indirect calorimetry revealed increased peripheral metabolism and greater reliance on carbohydrate metabolism. Because Clic5 mutants were unable to maintain energy reserves, they also exhibited increased susceptibility to fasting-induced torpor, as indicated by telemetric measurements showing episodes of reduced body temperature and heart rate. These data reveal a requirement for CLIC5 in the maintenance of normal systemic energy metabolism.

CLIC5A, a component of the ezrin-podocalyxin complex in glomeruli, is a determinant of podocyte integrity

The chloride intracellular channel 5A (CLIC5A) protein, one of two isoforms produced by the CLIC5 gene, was isolated originally as part of a cytoskeletal protein complex containing ezrin from placental microvilli. Whether CLIC5A functions as a bona fide ion channel is controversial. We reported previously that a CLIC5 transcript is enriched approximately 800-fold in human renal glomeruli relative to most other tissues. Therefore, this study sought to explore CLIC5 expression and function in glomeruli. RT-PCR and Western blots show that CLIC5A is the predominant CLIC5 isoform expressed in glomeruli. Confocal immunofluorescence and immunogold electron microscopy reveal high levels of CLIC5A protein in glomerular endothelial cells and podocytes. In podocytes, CLIC5A localizes to the apical plasma membrane of foot processes, similar to the known distribution of podocalyxin and ezrin. Ezrin and podocalyxin colocalize with CLIC5A in glomeruli, and podocalyxin coimmunoprecipitates with CLIC5A from glomerular lysates. In glomeruli of jitterbug (jbg/jbg) mice, which lack the CLIC5A protein, ezrin and phospho-ERM levels in podocytes are markedly lower than in wild-type mice. Transmission electron microscopy reveals patchy broadening and effacement of podocyte foot processes as well as vacuolization of glomerular endothelial cells. These ultrastructural changes are associated with microalbuminuria at baseline and increased susceptibility to adriamycin-induced glomerular injury compared with wild-type mice. Together, the data suggest that CLIC5A is required for the development and/or maintenance of the proper glomerular endothelial cell and podocyte architecture. We postulate that the interaction between podocalyxin and subjacent filamentous actin, which requires ezrin, is compromised in podocytes of CLIC5A-deficient mice, leading to dysfunction under unfavorable genetic or environmental conditions.

Spatiotemporal regulation of chloride intracellular channel protein CLIC4 by RhoA

Chloride intracellular channel (CLIC) 4 is a soluble protein structurally related to omega-type glutathione-S-transferases (GSTs) and implicated in various biological processes, ranging from chloride channel formation to vascular tubulogenesis. However, its function(s) and regulation remain unclear. Here, we show that cytosolic CLIC4 undergoes rapid but transient translocation to discrete domains at the plasma membrane upon stimulation of G(13)-coupled, RhoA-activating receptors, such as those for lysophosphatidic acid, thrombin, and sphingosine-1-phosphate. CLIC4 recruitment is strictly dependent on Galpha(13)-mediated RhoA activation and F-actin integrity, but not on Rho kinase activity; it is constitutively induced upon enforced RhoA-GTP accumulation. Membrane-targeted CLIC4 does not seem to enter the plasma membrane or modulate transmembrane chloride currents. Mutational analysis reveals that CLIC4 translocation depends on at least six conserved residues, including reactive Cys35, whose equivalents are critical for the enzymatic function of GSTs. We conclude that CLIC4 is regulated by RhoA to be targeted to the plasma membrane, where it may function not as an inducible chloride channel but rather by displaying Cys-dependent transferase activity toward a yet unknown substrate.

S100A4 and bone morphogenetic protein-2 codependently induce vascular smooth muscle cell migration via phospho-extracellular signal-regulated kinase and chloride intracellular channel 4

RATIONALE

S100A4/Mts1 is implicated in motility of human pulmonary artery smooth muscle cells (hPASMCs), through an interaction with the RAGE (receptor for advanced glycation end products).

OBJECTIVE

We hypothesized that S100A4/Mts1-mediated hPASMC motility might be enhanced by loss of function of bone morphogenetic protein (BMP) receptor (BMPR)II, observed in pulmonary arterial hypertension.

METHODS AND RESULTS

Both S100A4/Mts1 (500 ng/mL) and BMP-2 (10 ng/mL) induce migration of hPASMCs in a novel codependent manner, in that the response to either ligand is lost with anti-RAGE or BMPRII short interference (si)RNA. Phosphorylation of extracellular signal-regulated kinase is induced by both ligands and is required for motility by inducing matrix metalloproteinase 2 activity, but phospho-extracellular signal-regulated kinase 1/2 is blocked by anti-RAGE and not by BMPRII short interference RNA. In contrast, BMPRII short interference RNA, but not anti-RAGE, reduces expression of intracellular chloride channel (CLIC)4, a scaffolding molecule necessary for motility in response to S100A4/Mts1 or BMP-2. Reduced CLIC4 expression does not interfere with S100A4/Mts1 internalization or its interaction with myosin heavy chain IIA, but does alter alignment of myosin heavy chain IIA and actin filaments creating the appearance of vacuoles. This abnormality is associated with reduced peripheral distribution and/or delayed activation of RhoA and Rac1, small GTPases required for retraction and extension of lamellipodia in motile cells.

CONCLUSIONS

Our studies demonstrate how a single ligand (BMP-2 or S100A4/Mts1) can recruit multiple cell surface receptors to relay signals that coordinate events culminating in a functional response, ie, cell motility. We speculate that this carefully controlled process limits signals from multiple ligands, but could be subverted in disease.

Chloride intracellular channel 4 is involved in endothelial proliferation and morphogenesis in vitro

New capillaries are formed through angiogenesis and an integral step in this process is endothelial tubulogenesis. The molecular mechanisms driving tube formation during angiogenesis are not yet delineated. Recently, the chloride intracellular channel 4 (CLIC4)-orthologue EXC-4 was found to be necessary for proper development and maintenance of the Caenorhabditis elegans excretory canal, implicating CLIC4 as a regulator of tubulogenesis. Here, we studied the role of CLIC4 in angiogenesis and endothelial tubulogenesis. We report the effects of inhibiting or inducing CLIC4 expression on distinct aspects of endothelial cell behavior in vitro. Our experiments utilized RNA interference to establish cultured human endothelial cell lines with significant reduction of CLIC4 expression, and a CLIC4-expressing lentiviral plasmid was used to establish CLIC4 overexpression in endothelial cells. We observed no effect on cell migration and a modest effect on cell survival. Reduced CLIC4 expression decreased cell proliferation, capillary network formation, capillary-like sprouting, and lumen formation. This suggests that normal endogenous CLIC4 expression is required for angiogenesis and tubulogenesis. Accordingly, increased CLIC4 expression promoted proliferation, network formation, capillary-like sprouting, and lumen formation. We conclude that CLIC4 functions to promote endothelial cell proliferation and to regulate endothelial morphogenesis, and is thus involved in multiple steps of in vitro angiogenesis.

The chloride intracellular channel protein CLIC5 is expressed at high levels in hair cell stereocilia and is essential for normal inner ear function

Although CLIC5 is a member of the chloride intracellular channel protein family, its association with actin-based cytoskeletal structures suggests that it may play an important role in their assembly or maintenance. Mice homozygous for a new spontaneous recessive mutation of the Clic5 gene, named jitterbug (jbg), exhibit impaired hearing and vestibular dysfunction. The jbg mutation is a 97 bp intragenic deletion that causes skipping of exon 5, which creates a translational frame shift and premature stop codon. Western blot and immunohistochemistry results confirmed the predicted absence of CLIC5 protein in tissues of jbg/jbg mutant mice. Histological analysis of mutant inner ears revealed dysmorphic stereocilia and progressive hair cell degeneration. In wild-type mice, CLIC5-specific immunofluorescence was detected in stereocilia of both cochlear and vestibular hair cells and also along the apical surface of Kolliker's organ during cochlear development. Refined immunolocalization in rat and chicken vestibular hair cells showed that CLIC5 is limited to the basal region of the hair bundle, similar to the known location of radixin. Radixin immunostaining appeared reduced in hair bundles of jbg mutant mice. By mass spectrometry and immunoblotting, CLIC5 was shown to be expressed at high levels in stereocilia of the chicken utricle, in an approximate 1:1 molar ratio with radixin. These results suggest that CLIC5 associates with radixin in hair cell stereocilia and may help form or stabilize connections between the plasma membrane and the filamentous actin core.

Identification of chloride intracellular channel proteins in spermatozoa

We have identified for the first time the presence of chloride intracellular channel (CLIC) proteins in bovine epididymal spermatozoa. CLIC1 was discovered during microsequencing of proteins that co-purified with protein phosphatase 1, PP1gamma2, in sperm extracts. In addition to CLIC1, Western blot showed that two additional CLIC family members, CLIC4 and CLIC5, are also present in spermatozoa. CLIC fusion proteins, GST-CLIC1, GST-CLIC4 and GST-CLIC5, were all able to bind to PP1gamma2 in sperm extracts during pull-down assays. Immunofluorescence microscopy revealed that each of the three isoforms occupies a distinct location within the cell. Given that PP1gamma2 is a key enzyme regulating sperm motility, PP1gamma2-binding proteins, such as the CLIC proteins, are likely to play significant roles in sperm function.

The chloride channel inhibitor NS3736 [corrected] prevents bone resorption in ovariectomized rats without changing bone formation

Chloride channel activity is essential for osteoclast function. Consequently, inhibition of the osteoclastic chloride channel should prevent bone resorption. Accordingly, we tested a chloride channel inhibitor on bone turnover and found that it inhibits bone resorption without affecting bone formation. This study indicates that chloride channel inhibitors are highly promising for treatment of osteoporosis.

INTRODUCTION

The chloride channel inhibitor, NS3736, blocked osteoclastic acidification and resorption in vitro with an IC50 value of 30 microM. When tested in the rat ovariectomy model for osteoporosis, daily treatment with 30 mg/kg orally protected bone strength and BMD by approximately 50% 6 weeks after surgery. Most interestingly, bone formation assessed by osteocalcin, mineral apposition rate, and mineralized surface index was not inhibited.

MATERIALS AND METHODS

Analysis of chloride channels in human osteoclasts revealed that ClC-7 and CLIC1 were highly expressed. Furthermore, by electrophysiology, we detected a volume-activated anion channel on human osteoclasts. Screening 50 different human tissues showed a broad expression for CLIC1 and a restricted immunoreactivity for ClC-7, appearing mainly in osteoclasts, ovaries, appendix, and Purkinje cells. This highly selective distribution predicts that inhibition of ClC-7 should specifically target osteoclasts in vivo. We suggest that NS3736 is inhibiting ClC-7, leading to a bone-specific effect in vivo.

RESULTS AND CONCLUSION

In conclusion, we show for the first time that chloride channel inhibitors can be used for prevention of ovariectomy-induced bone loss without impeding bone formation. We speculate that the coupling of bone resorption to bone formation is linked to the acidification of the resorption lacunae, thereby enabling compounds that directly interfere with this process to be able to positive uncouple this process resulting in a net bone gain.

CLIC-5A functions as a chloride channel in vitro and associates with the cortical actin cytoskeleton in vitro and in vivo

CLIC-5A is a member of the chloride intracellular channel protein family, which is comprised of six related human genes encoding putative chloride channels. In this study, we found that reconstitution of purified recombinant CLIC-5A into artificial liposomes resulted in a dose-dependent chloride efflux that was sensitive to the chloride channel blocker IAA-94. CLIC-5A was originally isolated as a component of an ezrin-containing cytoskeletal complex from human placental microvilli. Here we show that similar protein complexes can be isolated using either immobilized CLIC-5A or the C-terminal F-actin-binding domain of ezrin and that actin polymerization is required for de novo assembly of these complexes. To investigate the behavior of CLIC-5A in vivo, JEG-3 placental choriocarcinoma cells were stably transfected with epitope-tagged CLIC-5A. In fixed cells, CLIC-5A displayed a polarized distribution and colocalized with ezrin in apical microvilli. Microvillar localization of CLIC-5A was retained after Triton X-100 extraction and was disrupted by treatment with latrunculin B. In transient transfections assays, we mapped a region between residues 20 and 54 of CLIC-5A that is required for targeting of CLIC-5A to microvilli in JEG-3 cells. Interestingly, expression of CLIC-5A in JEG-3 cells did not enhance the rate of iodide efflux in intact cells, suggesting that if CLIC-5A is a chloride channel, its channel activity may be restricted to intracellular membrane compartments in these cells. Regardless of its role in ion transport, CLIC-5A, like ezrin, may play an important role in the assembly or maintenance of F-actin-based structures at the cell cortex.

AKAP350 at the Golgi apparatus. II. Association of AKAP350 with a novel chloride intracellular channel (CLIC) family member

AKAP350 can scaffold a number of protein kinases and phosphatases at the centrosome and the Golgi apparatus. We performed a yeast two-hybrid screen of a rabbit parietal cell library with a 3.2-kb segment of AKAP350 (nucleotides 3611-6813). This screen yielded a full-length clone of rabbit chloride intracellular channel 1 (CLIC1). CLIC1 belongs to a family of proteins, all of which contain a high degree of homology in their carboxyl termini. All CLIC family members were able to bind a 133-amino acid domain within AKAP350 through the last 120 amino acids in the conserved CLIC carboxyl termini. Antibodies developed against a bovine CLIC, p64, immunoprecipitated AKAP350 from HCA-7 colonic adenocarcinoma cell extracts. Antibodies against CLIC proteins recognized at least five CLIC species including a novel 46-kDa CLIC protein. We isolated the human homologue of bovine p64, CLIC5B, from HCA-7 cell cDNA. A splice variant of CLIC5, the predicted molecular mass of CLIC5B corresponds to the molecular mass of the 46-kDa CLIC immunoreactive protein in HCA-7 cells. Antibodies against CLIC5B colocalized with AKAP350 at the Golgi apparatus with minor staining of the centrosomes. AKAP350 and CLIC5B association with Golgi elements was lost following brefeldin A treatment. Furthermore, GFP-CLIC5B-(178-410) targeted to the Golgi apparatus in HCA-7 cells. The results suggest that AKAP350 associates with CLIC proteins and specifically that CLIC5B interacts with AKAP350 at the Golgi apparatus in HCA-7 cells.

Identification of a novel member of the chloride intracellular channel gene family (CLIC5) that associates with the actin cytoskeleton of placental microvilli

The chloride intracellular channel (CLIC) gene family has been implicated in chloride ion transport within various subcellular compartments. We report here the molecular, biochemical, and cellular characterization of a new member of this gene family termed CLIC5. CLIC5 was isolated from extracts of placental microvilli as a component of a multimeric complex consisting of several known cytoskeletal proteins, including actin, ezrin, alpha-actinin, gelsolin, and IQGAP1. We cloned human cDNAs and generated antibodies specific for CLIC5, CLIC1/NCC27, and CLIC4/huH1/p64H1. CLIC5 shares 52-76% overall identity with human CLIC1, CLIC2, CLIC3, and CLIC4. Northern blot analysis showed that CLIC5 has a distinct pattern of expression compared with CLIC1 and CLIC4. Immunoblot analysis of extracts from placental tissues demonstrated that CLIC4 and CLIC5 are enriched in isolated placental microvilli, whereas CLIC1 is not. Moreover, in contrast to CLIC1 and CLIC4, CLIC5 is associated with the detergent-insoluble cytoskeletal fraction of microvilli. Indirect immunofluorescence microscopy revealed that CLIC4 and CLIC5 are concentrated within the apical region of the trophoblast, whereas CLIC1 is distributed throughout the cytoplasm. These studies suggest that CLIC1, CLIC4, and CLIC5 play distinct roles in chloride transport and that CLIC5 interacts with the cortical actin cytoskeleton in polarized epithelial cells.

Ezrin: a protein requiring conformational activation to link microfilaments to the plasma membrane in the assembly of cell surface structures

The cortical cytoskeleton of eucaryotic cells provides structural support to the plasma membrane and also contributes to dynamic processes such as endocytosis, exocytosis, and transmembrane signaling pathways. The ERM (ezrin-radixin-moesin) family of proteins, of which ezrin is the best studied member, play structural and regulatory roles in the assembly and stabilization of specialized plasma membrane domains. Ezrin and related molecules are concentrated in surface projections such as microvilli and membrane ruffles where they link the microfilaments to the membrane. The present knowledge about ezrin is discussed from an historical perspective. Both biochemical and cell biological studies have revealed that ezrin can exist in a dormant conformation that requires activation to expose otherwise masked association sites. Current results indicate that activated ezrin monomers or head-to-tail oligomers associate directly with F-actin through a domain in its C terminus, and with the membrane through its N-terminal domain. The association of ezrin with transmembrane proteins can be direct, as in the case of CD44, or indirect through EBP50. Other binding partners, including the regulatory subunit of protein kinase A and rho-GDI, suggest that ezrin is an integral component of these signaling pathways. Although the membrane-cytoskeletal linking function is clear, further studies are necessary to reveal how the activation of ezrin and its association with different binding partners is regulated.

Identification of EBP50: A PDZ-containing phosphoprotein that associates with members of the ezrin-radixin-moesin family

Members of the ezrin-radixin-moesin (ERM) family of membrane-cytoskeletal linking proteins have NH2- and COOH-terminal domains that associate with the plasma membrane and the actin cytoskeleton, respectively. To search for ERM binding partners potentially involved in membrane association, tissue lysates were subjected to affinity chromatography on the immobilized NH2-terminal domains of ezrin and moesin, which comprise the ezrin-radixin-moesin-association domain (N-ERMAD). A collection of polypeptides at 50-53 kD from human placenta and at 58-59 kD from bovine brain bound directly to both N-ERMADs. The 50-53-kD placental proteins migrated as a major 50-kD species after phosphatase treatment, indicating that the heterogeneity is due to different phosphorylation states. We refer to these polypeptides as ERM-binding phosphoprotein 50 (EBP50). Sequence analysis of human EBP50 was used to identify an approximately 2-kb human cDNA that encodes a 357-residue polypeptide. Recombinant EBP50 binds tightly to the N-ERMADs of ezrin and moesin. Peptide sequences from the brain candidate indicated that it is closely related to EBP50. EBP50 has two PSD-95/DlgA/ZO-1-like (PDZ) domains and is most likely a homologue of rabbit protein cofactor, which is involved in the protein kinase A regulation of the renal brush border Na+/H+ exchanger. EBP50 is widely distributed in tissues, and is particularly enriched in those containing polarized epithelia. Immunofluorescence microscopy of cultured cells and tissues revealed that EBP50 colocalizes with actin and ezrin in the apical microvilli of epithelial cells, and immunoelectron microscopy demonstrated that it is specifically associated with the microvilli of the placental syncytiotrophoblast. Moreover, EBP50 and ezrin can be coimmunoprecipitated as a complex from isolated human placental microvilli. These findings show that EBP50 is a physiologically relevant ezrin binding protein. Since PDZ domains are known to mediate associations with integral membrane proteins, one mode of membrane attachment of ezrin is likely to be mediated through EBP50.

Soluble ezrin purified from placenta exists as stable monomers and elongated dimers with masked C-terminal ezrin-radixin-moesin association domains

Previous work has indicated that ezrin, a membrane-microfilament linking protein, exists largely as a monomeric protein in solution. Here we purify from human placenta two cytosolic ezrin species that chromatography differently on gel filtration, anion, and cation exchange resins. Both species contain only the ezrin polypeptide, yet they do not readily interconvert in vitro as determined by gel filtration analysis. Determination of the physical properties of the two species indicates that one represents the conventional monomer, whereas the other represents highly asymmetric dimers. Chemical cross-linking data support this conclusion. Purified ezrin monomers normally have a masked C-terminal domain (termed a C-ERMAD) that, upon exposure, can associate with an N-terminal domain (termed N-ERMAD) of another ezrin molecule. Here we show that purified ezrin dimers also have masked C-ERMADs. On the basis of these results, we suggest a working model for the molecular organization of ezrin monomers and dimers and propose a hypothesis that explains the stable coexistence of ezrin monomers and dimers in placenta. Since radixin and moesin, the two other members of the closely related ERM protein family, both contain N- and C-ERMADs, the results we have documented and models proposed for ezrin are likely to apply to radixin and moesin as well.

Ezrin oligomers are major cytoskeletal components of placental microvilli: a proposal for their involvement in cortical morphogenesis

Ezrin is a component of the microvillus cytoskeleton of a variety of polarized epithelial cells and is believed to function as a membrane-cytoskeletal linker. In this study, we isolated microvilli from human placental syncytiotrophoblast as a model system for biochemical analysis of ezrin function. In contrast to intestinal microvilli, ezrin is a major protein component of placental microvilli, comprising approximately 5% of the total protein mass and present at about one quarter of the molar abundance of actin. Gel filtration and chemical cross-linking studies demonstrated that ezrin exists mainly in the form of noncovalent dimers and higher order oligomers in extracts of placental microvilli. A novel form of ezrin, apparently representing covalently cross-linked adducts, was present as a relatively minor constituent of placental microvilli. Both oligomers and adducts remained associated with the detergent-insoluble cytoskeleton, indicating a tight interaction with actin filaments. Moreover, stimulation of human A431 carcinoma cells with EGF induces the rapid formation of ezrin oligomers in vivo, thus identifying a signal transduction pathway involving ezrin oligomerization coincident with microvillus assembly. In addition to time course studies, experiments with tyrosine kinase and tyrosine phosphatase inhibitors revealed a correlation between the phosphorylation of ezrin on tyrosine and the onset of oligomer formation, consistent with the possibility that phosphorylation of ezrin might be required for the generation of stable oligomers. Based on these observations, a model for the assembly of cell surface structures is proposed.

Beta 2-microglobulin co-distributes with the heavy chain of the intestinal IgG-Fc receptor throughout the transepithelial transport pathway of the neonatal rat

Maternal IgG crosses the proximal small intestine of the suckling rat by receptor-mediated endocytosis and transepithelial transport. The Fc receptor resembles the major histocompatibility complex class I antigens in that it consists of two subunits: a transmembrane glycoprotein (gp50) in association with beta 2-microglobulin. We used immunofluorescence microscopy and quantitative immunogold cytochemistry to study the subcellular distribution of the two subunits. In mature absorptive cells both subunits were colocalized in each of the membrane compartments that mediate transcytosis of IgG. IgG administered in situ apparently caused both subunits to concentrate within endocytic pits of the apical plasma membrane, suggesting that ligand causes redistribution of receptors at this site. These results support a model for transport in which IgG is transferred across the cell as a complex with both subunits. During absorptive cell differentiation, gp50 and beta 2-microglobulin showed nearly identical patterns of increased expression that accompanied the development of the apical endocytic apparatus and terminal web. However, absorptive cells in weanling rats expressed no detectable gp50 and only low levels of beta 2-microglobulin in the Golgi region and on the basolateral plasma membrane where class I antigens would likely reside. Thus, beta 2-microglobulin has a novel distribution unrelated to its function as a subunit of the class I antigens. The co-expression of the two receptor subunits is restricted to neonatal epithelial cells engaged in IgG transport and is coordinately regulated during absorptive cell differentiation and during postnatal intestinal development.

Ezrin is concentrated in the apical microvilli of a wide variety of epithelial cells whereas moesin is found primarily in endothelial cells

Ezrin and moesin are two cytoskeletal proteins originally purified from human placenta that are 74% identical in overall protein sequence. They are believed to be membrane-cytoskeletal linking proteins because they share sequence homology with erythrocyte band 4.1 and colocalize with actin specifically in microvilli and membrane ruffles in cultured cells. To determine if ezrin and moesin share similar distributions in vivo, we studied their localizations with respect to F-actin in tissue sections. Surprisingly, ezrin and moesin exhibited very different cellular distributions. Ezrin was highly concentrated and colocalized with actin on the apical surface of many epithelial cell types. During enterocyte differentiation, the pattern of expression and redistribution of ezrin was consistent with it performing a role in microvillus assembly. Immunoelectron microscopy in differentiated cells revealed that ezrin was restricted mainly to the plasma membrane of microvilli and other actin-rich surface projections. Moesin was found in endothelial cells and was also enriched in the apical microvilli of a restricted set of epithelial cells. All polarized cell types with abundant microvilli contained one or both proteins, suggesting that ezrin and moesin perform related functions. However, the differential expression of ezrin and moesin indicates that they have distinct properties, which are uniquely adapted to specific cell types.