Analysis of a cortical cytoskeletal structure: a role for ezrin-radixin-moesin (ERM proteins) in the marginal band of chicken erythrocytes

Ezrin is a target for oncogenic Kit mutants in murine erythroleukemia

The model of erythroleukemia caused by Spi-1/PU.1 transgenesis in mice is a multistage disease. A preleukemic step is characterized by an acute proliferation of proerythroblasts due to the arrest of differentiation provoked by Spi-1/PU.1. Later on, a blastic crisis occurs associated with somatic oncogenic mutations in the stem cell factor (SCF) receptor kit. To gain insights into the mechanisms of the leukemic progression, we performed proteomic profiling analyses of proerythroblasts isolated at the 2 stages of the disease. Our results indicate that the level of ezrin, a membrane cytoskeletal crosslinker, is increased in the leukemic cells. We show that Kit oncogenic forms are responsible for ezrin phosphorylation and that phosphorylation rather than overexpression is essential in the leukemic proerythroblasts. Using expression of dominant-negative forms of ezrin, we show that phosphorylation of ezrin on residue Y353 participates in apoptosis resistance, whereas phosphorylation on residue Y145 promotes proliferation of the leukemic cells in vitro and in vivo. Another recurrent oncogenic form of tyrosine kinases (Flt3) most frequently involved in human myeloid leukemia was also able to phosphorylate ezrin. These findings point to a new role for ezrin as signaling player in the development of leukemia, being a downstream effector of oncogenic tyrosine kinases in leukemic blasts.

Ferritin associates with marginal band microtubules

We characterized chicken erythrocyte and human platelet ferritin by biochemical studies and immunofluorescence. Erythrocyte ferritin was found to be a homopolymer of H-ferritin subunits, resistant to proteinase K digestion, heat stable, and contained iron. In mature chicken erythrocytes and human platelets, ferritin was localized at the marginal band, a ring-shaped peripheral microtubule bundle, and displayed properties of bona fide microtubule-associated proteins such as tau. Red blood cell ferritin association with the marginal band was confirmed by temperature-induced disassembly-reassembly of microtubules. During erythrocyte differentiation, ferritin co-localized with coalescing microtubules during marginal band formation. In addition, ferritin was found in the nuclei of mature erythrocytes, but was not detectable in those of bone marrow erythrocyte precursors. These results suggest that ferritin has a function in marginal band formation and possibly in protection of the marginal band from damaging effects of reactive oxygen species by sequestering iron in the mature erythrocyte. Moreover, our data suggest that ferritin and syncolin, a previously identified erythrocyte microtubule-associated protein, are identical. Nuclear ferritin might contribute to transcriptional silencing or, alternatively, constitute a ferritin reservoir.

Comparative biology of the ubiquitous Na+/H+ exchanger, NHE1: lessons from erythrocytes

By virtue of their electroneutral exchange of intracellular H+ for extracellular Na+, the Na+/H+ exchangers (NHE1-NHE8) play a pivotal role in many physiological processes. This review focuses on the ubiquitous plasma membrane isoform, NHE1. Particular attention is given to the roles and regulation of NHE1 in erythrocytes, in their own right and as model systems, but pertinent findings from non-erythroid cells are also discussed. NHE1 plays a key role in the regulation of cell volume and pH, and consequently in the control of such diverse processes as blood O2/CO2 transport, and cell proliferation, motility, and survival. Disturbances in NHE1 function are involved in important pathological states such as hypoxic cell damage and cancer development. NHE1 has a predicted topology of 12 transmembrane domains, and a hydrophilic C-terminus thought to be the major site for NHE1 regulation. NHE1 is highly conserved throughout the vertebrate phylum, particularly in the transmembrane region and the proximal part of the C-terminus. In non-erythroid, and probably also in erythroid cells, this part of the hydrophilic C-terminus interacts with multiple binding partners important for NHE1 function. Erythrocyte NHE1s from mammalian, amphibian, and teleost species are activated by cell shrinkage, decreased pH(i), inhibition of Ser/Thr protein phosphatases, and activation of Ser/Thr protein kinases, i.e., many of the stimuli activating NHE1 in non-erythroid cells. In erythrocytes of many lower vertebrates, NHE1 is activated during hypoxia and is an important modulator of hemoglobin oxygen affinity. Sensitivity of NHE1 to oxygenation status has recently been described also in non-erythroid mammalian cells.

A NHERF binding site links the betaPDGFR to the cytoskeleton and regulates cell spreading and migration

The Na(+)/H(+) exchanger regulatory factor, NHERF, is a multifunctional adapter protein involved in a wide range of physiological activities. NHERF associates with merlin and the ezrin/radixin/moesin (MERM) family of membrane-actin cytoskeletal linker proteins through its C-terminus and is capable of interacting via its PDZ1 domain to the betaPDGF receptor (betaPDGFR). Thus, NHERF, potentially links the betaPDGFR to the actin cytoskeleton through its interaction with MERM proteins. In the present study, we have examined whether abolishing the interaction of betaPDGFR with NHERF results in actin cytoskeletal rearrangements. We have stably expressed a wild-type betaPDGFR, a mutant betaPDGFR (L1106A) that is incapable of interacting with NHERF, as well as a kinase defective mutant receptor (K634R), in PDGFR-deficient mouse embryonic fibroblasts. Our observations indicate that cells expressing betaPDGFR (L1106A) were impaired in their ability to spread and migrate on fibronectin compared with wild-type and K634R cells. L1106A mutant cells also revealed an increased number of focal adhesions, a condensed F-actin ring at the cell periphery and a decrease in total focal adhesion kinase (FAK) tyrosine phosphorylation. Further, we show that NHERF and MERM proteins could act as intermediary bridging proteins between betaPDGFR and FAK. Thus, the interaction of betaPDGFR with NHERF may provide an essential link between the cell membrane and the cortical actin cytoskeleton independent of receptor activity.

Radixin is a constituent of stereocilia in hair cells

Proteins of the ezrin-radixin-moesin family are ubiquitous constituents of the submembrane cortex, especially in epithelial cells. Earlier biochemical results suggested that a protein of this family occurs in the hair bundle, the cluster of actin-filled stereocilia that serves as the mechanoreceptive organelle of each hair cell in the inner ear. We prepared antipeptide antisera directed against chicken radixin and ezrin and demonstrated their specificity and absence of crossreactivity. When used in immunocytochemical studies of isolated hair cells, anti-radixin produced an intense band of labeling at the bases of hair bundles from the chicken, frog, mouse, and zebrafish. Electron microscopic immunocytochemistry disclosed that radixin labeling commenced in the stereociliary taper, peaked in the lower stereociliary shaft, and declined progressively toward the hair bundle's top. Labeling with anti-ezrin produced no signal in hair bundles. Radixin is thus a prominent constituent of stereocilia, where it may participate in anchoring the "pointed" ends of actin filaments to the membrane.

ERMs colocalize transiently with L1 during neocortical axon outgrowth

L1 is a member of the Ig superfamily of cell adhesion molecules (CAMs) that functions in many aspects of neuronal development including axonal outgrowth and neuronal migration. These functions require coordination between L1 and the actin cytoskeleton. Because CAMs and the cytoskeleton do not bind directly, membrane-cytoskeletal linkers (MCLs) such as ankyrin are thought to be crucial to their interactions, but data from a knockout mouse suggest that ankyrin is not necessary for the earliest events attributed to L1 function. Recent findings in hippocampal cell culture show that members of the ERM family of proteins (ezrin, radixin, and moesin) can also serve as MCLs between L1 and actin in neurons. Here, we demonstrate that ERM proteins are expressed in extending neuronal processes in the intermediate zone of the developing cortex, a region that is densely packed with migrating neurons and growing axons. ERMs and L1 are codistributed extensively over a transient time course that coincides with rapid axon growth and cortical expansion. This codistribution is strong at embryonic day 17 and 19 but diminishes by postnatal day 0, at which time ankyrin-L1 codistribution increases dramatically. These findings suggest that in the developing neocortex, ERMs are the predominant MCL for L1 during migration and axon extension, neither of which requires ankyrin function. Furthermore, these data suggest that there is a developmentally regulated switch in MCL function in the developing brain.

Gamma-tubulin in chicken erythrocytes: changes in localization during cell differentiation and characterization of cytoplasmic complexes

The mechanism of marginal band (MB) formation in differentiating erythroid cells is not fully understood, and the proteins involved in nucleation of MB microtubules are largely unknown. To gain insights into the function of gamma-tubulin in MB formation, we have followed its distribution in developing chicken erythrocytes and characterized soluble forms of the protein. In early stages of erythroid cells differentiation, gamma-tubulin was present in microtubule-organizing centers, mitotic spindles, as well as on MB. Its subcellular localization changed in the course of differentiation, and in postnatal peripheral erythrocytes gamma-tubulin was found only in soluble forms. After cold-induced depolymerization gamma-tubulin in erythroid cells formed large clusters that were not observed in matured cells, and re-growth experiments demonstrated that gamma-tubulin was not present in distinct nucleation structures at the cell periphery. Soluble gamma-tubulin formed complexes of various size and large complexes were prone to dissociation in the presence of high salt concentration. Interaction of gamma-tubulin with tubulin dimers was revealed by precipitation experiments. gamma-Tubulin occurred in multiple charge variants whose number increased in the course of erythrocyte differentiation and corresponded with decreased binding to MB. The presented data demonstrate for the first time that gamma-tubulin is a substrate for developmentally regulated posttranslational modifications and that the binding properties of gamma-tubulin or its complexes change during differentiation events.

Ezrin, radixin, and moesin are components of Schwann cell microvilli

Ezrin, radixin, and moesin (ERM proteins), as well as the neurofibromatosis 2 (NF2) tumor suppressor merlin/schwannomin, all belong to the protein 4.1 family, yet only merlin is a tumor suppressor in Schwann cells. To gain insight into the possible functions of ERM proteins in Schwann cells, we examined their localization in peripheral nerve, because we have previously shown that merlin is found in paranodes and in Schmidt-Lanterman incisures. All three ERM proteins were highly expressed in the microvilli of myelinating Schwann cells that surround the nodal axolemma as well as in incisures and cytoplasmic puncta in the vicinity of the node. In all of these locations, ERM proteins were colocalized with actin filaments. In contrast, ERM proteins did not surround nodes in the CNS. The colocalization of ERM proteins with actin indicates that they have functions different from those of merlin in myelinating Schwann cells.

Increased adhesiveness in cultured endometrial-derived cells is related to the absence of moesin expression

Human endometrial epithelial cells (EECs) are nonadhesive for embryos throughout most of the menstrual cycle. During the so-called implantation window, the apical plasma membrane of EECs acquire adhesive properties by undergoing a series of morphological and biochemical changes. The human endometrial-derived epithelial cell line, RL95-2, serves as an in vitro model for receptive uterine epithelium because of its high adhesiveness for trophoblast-derived cells. In contrast, the HEC-1-A cell line, which displays poor adhesive properties for trophoblast cells, is considered to be less receptive. The ezrin, radixin, and moesin protein family members, which are present underneath the apical plasma membrane, potentially act to link the cytoskeleton and membrane proteins. In the present study, we have further investigated the adhesive features in these two unrelated endometrial-derived cell lines using an established in vitro model for embryonic adhesion. We have also analyzed the protein pattern and mRNA expression of ezrin and moesin in RL95-2 cells versus HEC-1-A cells. The results demonstrate that RL95-2 cells were indeed more receptive (81% blastocyst adhesion) compared with HEC-1-A cells (46% blastocyst adhesion). An intermediate adhesion rate was found in primary EECs cultured on extracellular matrix gel, thus allowing a partial polarization of these cells (67% blastocyst adhesion). Furthermore, we found that moesin was absent from RL95-2 cells. In contrast, ezrin is expressed in both cell lines, yet it is reduced in adherent RL95-2 cells. Data are in agreement with the hypothesis that uterine receptivity requires down-regulation or absence of moesin, which is a less-polarized actin cytoskeleton.

Activities of the EM10 protein from Echinococcus multilocularis in cultured mammalian cells demonstrate functional relationships to ERM family members

The ezrin-radixin-moesin (ERM) homolog EM10 is expressed by the larval stage of the parasite E. multilocularis and shows 46.9% overall identity in the primary structure with human ezrin. To determine whether EM10 has similar activities to ERM proteins, we investigated properties of the protein expressed in mammalian cells. In particular, we transiently expressed haemagglutinin-tagged (HA-tagged) versions of the full-length EM10 as well as the amino- and the carboxy-terminal halves of EM10 in HtTA-1 cells. In addition we stably transfected NIH-3T3 cells with untagged full-length EM10. The data demonstrate that EM10 polypeptides behave like their corresponding portions of radixin when transiently expressed in mammalian cells. The full-length and amino-terminal EM10 polypeptides were localized to cortical structures. Cells expressing the carboxy-terminal polypeptide of EM10 showed long actin-filled protrusions. Cells expressing full-length EM10 showed a reduction in endogenous moesin-staining at cortical structures. In stably transfected NIH-3T3 cells EM10 was not crisply localized but rather was diffuse throughout the cytoplasm. These cells showed a conspicuous loss of stress-fibers, a phenotype that was not seen in analogous experiments with ERM proteins. The results demonstrate both similarities and differences between the functional properties of EM10 and ERM proteins expressed in vertebrate cells.

Elliptical versus circular erythrocyte marginal bands: isolation, shape conversion, and mechanical properties

Differentiation of nucleated erythrocytes involves transformation from spheroids to flattened discoids to mature flattened ellipsoids. The marginal band (MB) of microtubules is required for this process and continues to play a role in maintaining mature ellipsoidal cell shape. One hypothesis for MB function is that cell ellipticity is generated and maintained by asymmetric application of force across a flexible, circular MB frame by the membrane skeleton or other transverse elements. This is based on an earlier finding that isolated erythrocyte MBs are much more circular than MBs in situ. However, our present studies of salamander erythrocyte MBs isolated by a detergent-based method challenge this hypothesis. Most of these isolated MBs are initially elliptical, even though they lack transverse material (= E-MBs). They can be stabilized in that form for long periods and can be converted experimentally into the circular form (= C-MBs) by extended incubation in isolation medium or by treatment with elastase or subtilisin. We have tested an alternative hypothesis for generation and maintenance of ellipsoidal MBs, one based on intrinsic differential bending resistance and supported by construction of models. Using laser microsurgical transection to compare mechanical responses of isolated E-MBs and C-MBs, we have found their behavior to be quite different. Whereas C-MBs linearize, most E-MBs do not, instead retaining considerable curvature. These results are incompatible with the differential bending resistance hypothesis, which predicts both C-MB and E-MB linearization. However, they are consistent with a third model, in which material bound to the MB stabilizes it in the mature ellipsoidal form, and indicate that the mechanism for maintenance of MB ellipticity differs from that involved in its generation.

Actin-dependent anterograde movement of growth-cone-like structures along growing hippocampal axons: a novel form of axonal transport?

In time-lapse video recordings of hippocampal neurons in culture, we have identified previously uncharacterized structures, nicknamed "waves," that exhibit lamellipodial activity closely resembling that of growth cones, but which periodically emerge at the base of axons and travel distally at an average rate of 3 microm/min. In electron micrographs of identified waves, the cortical region of the axon appears expanded to either side, forming lamellipodia like those at growth cones. No other gross differences were noted in the ultrastructural features of the axon shaft at the site of a wave. Immunocytochemistry revealed that waves contain a marked concentration of F-actin, GAP-43, cortactin, and ezrin or a related protein, constituents that are also concentrated in growth cones. Treatment with the actin-disrupting agent cytochalasin B caused a reversible collapse of lamellipodia and cessation of the forward movement of individual waves along the axon, indicating that their anterograde transport is dependent on intact actin filaments. Treatment with the microtubule-depolymerizing agent nocodazole led to a rapid disorganization of wave structure and a subsequent suppression of wave activity that may reflect a role of microtubules in actin organization. The results suggest that actin and other cytoskeletal components concentrated in growth cones may be transported together as growth-cone-like structures from the cell body to the axon tip via an actin-dependent mechanism.

Tau is required for neurite outgrowth and growth cone motility of chick sensory neurons

The role of the microtubule-associated protein (MAP) tau in axon growth remains controversial. Antisense experiments have suggested that tau is required for axon outgrowth, whereas genetic knockout and immunodepletion studies have suggested that tau plays no role in this process. To investigate the role of tau in both neurite outgrowth and growth cone motility, we have used a different approach, the chromophore-assisted laser inactivation (CALI) of tau in chick dorsal root ganglion (DRG) neurons in culture. This approach generates an acute loss of tau function that is not subject to compensation by other MAPs. Inactivation of tau in whole DRG neurons (including cell body and neurites) reduced neurite number and length. Inactivation of tau within regions of growth cones using micro-scale CALI caused a decrease in neurite extension rate by approximately 2-fold. Surprisingly, it also caused a approximately 20% decrease in the lamellipodial size within the inactivation region, whereas the filopodial motility was not affected. These results suggest that tau is required in neurite outgrowth and that tau also functions in lamellipodial motility at the growth cone leading edge.

The effect of mechanical strain on hyaluronan metabolism in embryonic fibrocartilage cells

The development of the synovial joint cavity between the cartilage anlagen of the long bones is thought to be mediated by differential matrix synthesis at the developing articular surfaces. In addition, many studies have shown that removal of movement-induced mechanical stimuli from developing diarthrodial joints prevents cavity formation or produces a secondary fusion of previously cavitated joints. Herein, we describe an inductive influence of mechanical strain on hyaluronan metabolism and the expression of hyaluronan-binding proteins in cultured cells isolated from the articular surface of the distal tibial condyles of 18-day chick embryos. The effect of 10 min of mechanical strain on hyaluronan release into culture media, intracellular uridine diphospho-glucose dehydrogenase activity (an enzyme required for hyaluronan saccharide precursor production), cell surface hyaluronan-binding protein expression and HA synthase mRNA expression were analysed up to 24 h later. Six hours after the application of strain, there was a significant increase in the accumulation of hyaluronan released into tissue culture media by strained fibrocartilage cells compared with controls, an effect still detectable after 24 h. Strained cells also showed increased activity for uridine diphospho-glucose dehydrogenase and expressed higher levels of the hyaluronan-binding protein CD44 at 24 h. In addition, at 24 h mRNA for HA synthase 2 was expressed in all samples whereas mRNA for HA synthase 3 was only expressed in strained cells. These results further highlight the role for movement-induced stimuli in differential extracellular matrix metabolism during joint development and also show that strain may facilitate differential HA synthase gene expression.

A functional role for ezrin during Shigella flexneri entry into epithelial cells

Shigella flexneri is an enteroinvasive bacterium responsible for bacillary dysentery in humans. Bacterial entry into epithelial cells is a crucial step for the establishment of the infection. It is characterized by a transient reorganization of the host cell cytoskeleton at the site of bacterial interaction with the cell membrane, which leads to bacterial engulfment by a macropinocytic process. We show in this study that the membrane-cytoskeleton linker, ezrin, a member of the ERM (ezrin, radixin, moesin) family, plays an active role in the process of Shigella uptake. Ezrin is highly enriched in cellular protrusions induced by the bacterium and is found in close association with the plasma membrane. In addition, Shigella entry is significantly reduced in cells transfected with a dominant negative allele of ezrin with entry foci showing much shorter cellular protrusions. These results indicate that ezrin not only acts as a membrane-cytoskeleton linker, but may also mediate extension of cellular projections in the presence of signals such as those elicited by invading microorganisms.

Radixin is involved in lamellipodial stability during nerve growth cone motility

Immunocytochemistry and in vitro studies have suggested that the ERM (ezrin-radixin-moesin) protein, radixin, may have a role in nerve growth cone motility. We tested the in situ role of radixin in chick dorsal root ganglion growth cones by observing the effects of its localized and acute inactivation. Microscale chromophore-assisted laser inactivation (micro-CALI) of radixin in growth cones causes a 30% reduction of lamellipodial area within the irradiated region whereas all control treatments did not affect lamellipodia. Micro-CALI of radixin targeted to the middle of the leading edge often split growth cones to form two smaller growth cones during continued forward movement (>80%). These findings suggest a critical role for radixin in growth cone lamellipodia that is similar to ezrin function in pseudopodia of transformed fibroblasts. They are consistent with radixin linking actin filaments to each other or to the membrane during motility.

Specific expression of ezrin, a cytoskeletal-membrane linker protein, in a subset of chick retinotectal and sensory projections

Lamina-specific neuronal connections are a fundamental feature in many parts of the vertebrate central nervous system. In the chick, the optic tectum is the primary visual centre, and it has a multilaminated structure consisting of 15 laminae, of which only three or four receive retinal projections. Each of the retinorecipient laminae establishes synaptic connections selectively from one of a few subsets of retinal ganglion cells (RGCs). We have generated a series of monoclonal antibodies that appear to stain only one of the retinorecipient laminae. One of these, TB4, stained lamina F which receives inputs from a subpopulation of approximately 10-20% of RGCs which express the presynaptic acetylcholine receptor beta2-subunit. TB4 recognized a single 79-kDa protein on immunoblotting. cDNA cloning and immunochemical analysis revealed that the TB4 antigen molecule was ezrin, a cytoskeletal-membrane linker molecule belonging to the ezrin-radixin-moesin family. Unilateral enucleation of the eye, both prior to and after the establishment of retinotectal projections, attenuated the lamina-selective staining with TB4 in the contralateral tectum, suggesting that ezrin is anterogradely transported from RGCs to lamina F. Ezrin was thus expressed in a subset of RGCs that project to lamina F. Similar subset-selective expression and resultant lamina-selective distribution of ezrin were also observed in the lamina-specific central projections from the dorsal root ganglia. The staining pattern with TB4 in the dorsal root ganglia and spinal cord indicated that high expression of ezrin was restricted in cutaneous sensory neurons, but not in muscle sensory neurons. Since ezrin modulates cell morphology and cell adhesion profiles by linking membrane proteins with the cytoskeleton, it was suggested that ezrin is involved in the formation and/or maintenance of lamina-specific connections for neuronal subpopulations in the visual and somatosensory systems.

Suppression of radixin and moesin alters growth cone morphology, motility, and process formation in primary cultured neurons

In this study we have examined the cellular functions of ERM proteins in developing neurons. The results obtained indicate that there is a high degree of spatial and temporal correlation between the expression and subcellular localization of radixin and moesin with the morphological development of neuritic growth cones. More importantly, we show that double suppression of radixin and moesin, but not of ezrin-radixin or ezrin-moesin, results in reduction of growth cone size, disappearance of radial striations, retraction of the growth cone lamellipodial veil, and disorganization of actin filaments that invade the central region of growth cones where they colocalize with microtubules. Neuritic tips from radixin-moesin suppressed neurons displayed high filopodial protrusive activity; however, its rate of advance is 8-10 times slower than the one of growth cones from control neurons. Radixin-moesin suppressed neurons have short neurites and failed to develop an axon-like neurite, a phenomenon that appears to be directly linked with the alterations in growth cone structure and motility. Taken collectively, our data suggest that by regulating key aspects of growth cone development and maintenance, radixin and moesin modulate neurite formation and the development of neuronal polarity.

CD43 Interacts With Moesin and Ezrin and Regulates Its Redistribution to the Uropods of T Lymphocytes at the Cell-Cell Contacts

Chemokines as well as the signaling through the adhesion molecules intercellular adhesion molecule (ICAM)-3 and CD43 are able to induce in T lymphocytes their switching from a spherical to a polarized motile morphology, with the formation of a uropod at the rear of the cell. We investigated here the role of CD43 in the regulation of T-cell polarity, CD43-cytoskeletal interactions, and lymphocyte aggregation. Pro-activatory anti-CD43 monoclonal antibody (MoAb) induced polarization of T lymphocytes with redistribution of CD43 to the uropod and the CCR2 chemokine receptor to the leading edge of the cell. Immunofluorescence analysis showed that all three ezrin-radixin-moesin (ERM) actin-binding proteins localized in the uropod of both human T lymphoblasts stimulated with anti-CD43 MoAb and tumor-infiltrating T lymphocytes. Radixin localized at the uropod neck, whereas ezrin and moesin colocalized with CD43 in the uropod. Biochemical analyses showed that ezrin and moesin coimmunoprecipitated with CD43 in T lymphoblasts. Furthermore, in these cells, the CD43-associated moesin increased after stimulation through CD43. The interaction of moesin and ezrin with CD43 was specifically mediated by the cytoplasmic domain of CD43, as shown by precipitation of both ERM proteins with a GST-fusion protein containing the CD43 cytoplasmic tail. Videomicroscopy analysis of homotypic cell aggregation induced through CD43 showed that cellular uropods mediate cell-cell contacts and lymphocyte recruitment. Immunofluorescence microscopy performed in parallel showed that uropods enriched in CD43 and moesin localized at the cell-cell contact areas of cell aggregates. The polarization and homotypic cell aggregation induced through CD43 was prevented by butanedione monoxime, indicating the involvement of myosin cytoskeleton in these phenomena. Altogether, these data indicate that CD43 plays an important regulatory role in remodeling T-cell morphology, likely through its interaction with actin-binding proteins ezrin and moesin. In addition, the redistribution of CD43 to the uropod region of migrating lymphocytes and during the formation of cell aggregates together with the enhancing effect of anti-CD43 antibodies on lymphocyte cell recruitment suggest that CD43 plays a key role in the regulation of cell-cell interactions during lymphocyte traffic.

Calpain regulates actin remodeling during cell spreading

Previous studies suggest that the Ca2+-dependent proteases, calpains, participate in remodeling of the actin cytoskeleton during wound healing and are active during cell migration. To directly test the role that calpains play in cell spreading, several NIH-3T3- derived clonal cell lines were isolated that overexpress the biological inhibitor of calpains, calpastatin. These cells stably overexpress calpastatin two- to eightfold relative to controls and differ from both parental and control cell lines in morphology, spreading, cytoskeletal structure, and biochemical characteristics. Morphologic characteristics of the mutant cells include failure to extend lamellipodia, as well as abnormal filopodia, extensions, and retractions. Whereas wild-type cells extend lamellae within 30 min after plating, all of the calpastatin-overexpressing cell lines fail to spread and assemble actin-rich processes. The cells genetically altered to overexpress calpastatin display decreased calpain activity as measured in situ or in vitro. The ERM protein ezrin, but not radixin or moesin, is markedly increased due to calpain inhibition. To confirm that inhibition of calpain activity is related to the defect in spreading, pharmacological inhibitors of calpain were also analyzed. The cell permeant inhibitors calpeptin and MDL 28, 170 cause immediate inhibition of spreading. Failure of the intimately related processes of filopodia formation and lamellar extension indicate that calpain is intimately involved in actin remodeling and cell spreading.

An essential role for the interaction between hyaluronan and hyaluronan binding proteins during joint development

We studied the expression of hyaluronan binding proteins (HABPs) during the development of embryonic chick joints, using immunocytochemistry and biotinylated HA. The expression of actin capping proteins and of actin itself was also studied because the cytoskeleton is important in controlling HA-HABP interactions. Three cell surface HABPs were localized in the epiphyseal cartilage, articular fibrocartilage, and interzone that comprise the developing joint. Of these three HABPs, CD44 was associated with the articular fibrocartilages and interzone, whereas RHAMM and the IVd4 epitope were associated with all three tissues. Biotinylated HA was localized to interzone and articular fibrocartilages before cavity formation and within epiphyseal chondrocytes post cavitation. Actin filament bundles were observed at the developing joint line, as was the expression of the actin capping protein moesin. Manipulation of joint cavity development, using oligosaccharides of HA, disrupted joint formation and was associated with decreases in CD44 and actin filament expression as well as decreased hyaluronan synthetic capability. These results suggest that HA is actively bound by CD44 at the developing joint line and that HA-HABP interactions play a major role in the initial separation events occurring during joint formation.

Epithelial and fibroblastoid cells contain numerous cell-type specific putative microtubule-regulating proteins, among which are ezrin and fodrin

Upon cell junction formation, the microtubules of polarizing epithelial cells become reorganized by unknown signaling mechanisms and regulating proteins. Microtubule-associated (MAPs) and other types of proteins are likely to be involved in this process, but most of these are unknown. Such proteins are called here collectively microtubule-regulating proteins (MRPs). As a first step towards their characterization, we used co-sedimentation of cytosolic proteins of MDCK cells and A72, a dog fibroblastoid line, with an excess of taxol-stabilized MTs, to obtain a cell fraction enriched in putative MRPs ("MRPs"). Additional tests have led to the inventory of around 40 "MRPs" among the 80 proteins present in the microtubule pellet. We also found that "MRPs" are recovered in higher amounts from MDCK cytosol, and that half of these are cell-type specific. These results corroborate data from yeast cells and insect eggs, and show that in mammalian somatic cells too, a large number of proteins seems to be involved in microtubule regulation, and that different cell types use a specific set of MRPs. "MRPs" found in both cell types are the intermediate chain of cytoplasmic dynein, Arp1, the major subunit of the dynactin complex, and CLIP-170. Two MDCK-specific "MRPs" were identified as the actin-binding proteins ezrin and alpha-fodrin. These results are discussed with regard to a possible involvement of ezrin and fodrin in morphogenetic interactions of microtubules with the membrane cytoskeleton in polarizing epithelia upon junction formation.

Regulation of the neurofibromatosis type 2 tumor suppressor protein, merlin, by adhesion and growth arrest stimuli

The neurofibromatosis type 2 tumor suppressor gene is inactivated in the development of familial and sporadic schwannomas and meningiomas. The encoded protein, Merlin, is closely related to the Ezrin, Radixin, and Moesin family of membrane/cytoskeletal linker proteins. Examination of Merlin in several cell lines revealed that the protein migrates as two distinct species near 70 kDa. Phosphatase treatment and orthophosphate labeling demonstrated that the species with decreased mobility is phosphorylated. Given Merlin's localization to cortical actin structures, we examined the effect of cell-cell contact or other forms of growth arrest on Merlin expression and post-translational modification. Under conditions of confluency or serum deprivation, the levels of phosphorylated and unphosphorylated Merlin species increased significantly. Cells arrested in G1 by other methods or other phases of the cell cycle did not show changes in Merlin levels. Furthermore, loss of adhesion resulted in a nearly complete dephosphorylation of Merlin, which was reversed upon re-plating of cells, suggesting Merlin phosphorylation may be responsive to cell spreading or changes in cell shape. Thus, the tumor suppressor function of Merlin may involve the regulation of cellular responses to cues such as cell-cell contact, growth factor microenvironment, or changes in cell shape.

RhoA-dependent phosphorylation and relocalization of ERM proteins into apical membrane/actin protrusions in fibroblasts

The ERM proteins (ezrin, radixin, and moesin) are a group of band 4. 1-related proteins that are proposed to function as membrane/cytoskeletal linkers. Previous biochemical studies have implicated RhoA in regulating the association of ERM proteins with their membrane targets. However, the specific effect and mechanism of action of this regulation is unclear. We show that lysophosphatidic acid stimulation of serum-starved NIH3T3 cells resulted in relocalization of radixin into apical membrane/actin protrusions, which was blocked by inactivation of Rho by C3 transferase. An activated allele of RhoA, but not Rac or CDC42Hs, was sufficient to induce apical membrane/actin protrusions and localize radixin or moesin into these structures in both Rat1 and NIH3T3 cells. Lysophosphatidic acid treatment led to phosphorylation of radixin preceding its redistribution into apical protrusions. Significantly, cotransfection of RhoAV14 or C3 transferase with radixin and moesin revealed that RhoA activity is necessary and sufficient for their phosphorylation. These findings reveal a novel function of RhoA in reorganizing the apical actin cytoskeleton and suggest that this function may be mediated through phosphorylation of ERM proteins.

NHE-RF, a regulatory cofactor for Na(+)-H+ exchange, is a common interactor for merlin and ERM (MERM) proteins

We have identified the human homologue of a regulatory cofactor of Na(+)-H+ exchanger (NHE-RF) as a novel interactor for merlin, the neurofibromatosis 2 tumor suppressor protein. NHE-RF mediates protein kinase A regulation of Na(+)-H+ exchanger NHE3 to which it is thought to bind via one of its two PDZ domains. The carboxyl-terminal region of NHE-RF, downstream of the PDZ domains, interacts with the amino-terminal protein 4.1 domain-containing segment of merlin in yeast two-hybrid assays. This interaction also occurs in affinity binding assays with full-length NHE-RF expressed in COS-7 cells. NHE-RF binds to the related ERM proteins, moesin and radixin. We have localized human NHE-RF to actin-rich structures such as membrane ruffles, microvilli, and filopodia in HeLa and COS-7 cells, where it co-localizes with merlin and moesin. These findings suggest that hNHE-RF and its binding partners may participate in a larger complex (one component of which might be a Na(+)-H+ exchanger) that could be crucial for the actin filament assembly activated by the ERM proteins and for the tumor suppressor function of merlin.

Analysis of molecular domains of epitope-tagged merlin isoforms in Cos-7 cells and primary rat Schwann cells

The Neurofibromatosis 2 gene product, merlin, has striking similarity to ezrin, radixin, and moesin (ERM), members of the protein 4.1 family which have been demonstrated to connect proteins in the plasma membrane to the cytoskeletal components. The recent localization of merlin to the motile regions in cultured cells such as membrane ruffles further supports the notion that merlin represents a new class of tumor suppressors. Here we describe the localization of full-length and truncated polypeptides of merlin expressed as Flag-tagged proteins in transfected cells. Similar to endogenous merlin, the epitope-tagged full-length merlin localizes to the membrane ruffles in transfected Cos-7 cells and rat Schwann cells. In addition, the over-expressed merlin localizes to other actin-rich cortical structures, such as microvilli and filopodia. The amino-terminal half of merlin is seen dispersed throughout the cells and in membrane ruffles. Compared to the amino-terminal half of merlin, its carboxy-terminal half localizes more distinctly to membrane ruffles. The full-length and the carboxy-terminal portion of merlin co-localize with F-actin at the membrane ruffles. However, distinct from the ERM proteins, the carboxy-terminal-truncated merlin and F-actin do not co-localize with each other at the stress fibers. Our results suggest that both the amino- and the carboxy-terminal domains of merlin contribute to its membrane ruffle localization.

Moesin interacts with the cytoplasmic region of intercellular adhesion molecule-3 and is redistributed to the uropod of T lymphocytes during cell polarization

During activation, T lymphocytes become motile cells, switching from a spherical to a polarized shape. Chemokines and other chemotactic cytokines induce lymphocyte polarization with the formation of a uropod in the rear pole, where the adhesion receptors intercellular adhesion molecule-1 (ICAM-1), ICAM-3, and CD44 redistribute. We have investigated membrane-cytoskeleton interactions that play a key role in the redistribution of adhesion receptors to the uropod. Immunofluorescence analysis showed that the ERM proteins radixin and moesin localized to the uropod of human T lymphoblasts treated with the chemokine RANTES (regulated on activation, normal T cell expressed, and secreted), a polarization-inducing agent; radixin colocalized with arrays of myosin II at the neck of the uropods, whereas moesin decorated the most distal part of the uropod and colocalized with ICAM-1, ICAM-3, and CD44 molecules. Two other cytoskeletal proteins, beta-actin and alpha-tubulin, clustered at the cell leading edge and uropod, respectively, of polarized lymphocytes. Biochemical analysis showed that moesin coimmunoprecipitates with ICAM-3 in T lymphoblasts stimulated with either RANTES or the polarization- inducing anti-ICAM-3 HP2/19 mAb, as well as in the constitutively polarized T cell line HSB-2. In addition, moesin is associated with CD44, but not with ICAM-1, in polarized T lymphocytes. A correlation between the degree of moesin-ICAM-3 interaction and cell polarization was found as determined by immunofluorescence and immunoprecipitation analysis done in parallel. The moesin-ICAM-3 interaction was specifically mediated by the cytoplasmic domain of ICAM-3 as revealed by precipitation of moesin with a GST fusion protein containing the ICAM-3 cytoplasmic tail from metabolically labeled Jurkat T cell lysates. The interaction of moesin with ICAM-3 was greatly diminished when RANTES-stimulated T lymphoblasts were pretreated with the myosin-disrupting drug butanedione monoxime, which prevents lymphocyte polarization. Altogether, these data indicate that moesin interacts with ICAM-3 and CD44 adhesion molecules in uropods of polarized T cells; these data also suggest that these interactions participate in the formation of links between membrane receptors and the cytoskeleton, thereby regulating morphological changes during cell locomotion.

A dual involvement of the amino-terminal domain of ezrin in F- and G-actin binding

Human recombinant ezrin, or truncated forms, were coated in microtiter plate and their capacity to bind actin determined. F-actin bound ezrin with a Kd of 504 +/- 230 nM and a molecular stoichiometry of 10.6 actin per ezrin. Ezrin bound both alpha- and beta/gamma-actin essentially as F-form. F-actin binding was totally prevented or drastically reduced when residues 534-586 or 13-30 were deleted, respectively. An actin binding activity was detected in amino-terminal constructs (ezrin 1-310 and 1-333) provided the glutathione S-transferase moiety of the fusion protein was removed. Series of carboxyl-terminal truncations confirmed the presence of this actin-binding site which bound both F- and G-actin. The F- and G-actin-binding sites were differently sensitive to various chemical effectors and distinct specific ezrin antibodies. The internal actin-binding site was mapped between residues 281 and 333. The association of ezrin amino-terminal fragment to full-length ezrin blocked F-actin binding to ezrin. It is proposed that, in full-length ezrin, the F-actin-binding site required the juxtaposition of the distal-most amino- and carboxyl-terminal residues of the ezrin molecule.

Ezrin, radixin and moesin are possible auto-immune antigens in rheumatoid arthritis

In order to deduce which cellular molecules react with the sera from patients with rheumatoid arthritis (RA), human and mouse cellular extracts were fractionated stepwise, by ethanol precipitation and their reactivity analysed by Western blotting. It was found that three cytoplasmic molecules with molecular weights of 80,000, 81,000 and 77,000 were immunoreactive and they were identified as ezrin (E), radixin (R), and moesin (M), respectively, by partial amino acid sequencing. Using cDNA clones of these human molecules, recombinant proteins were produced in Escherichia coli and used to enable the antigens to detect the antibodies in the sera of patients with RA. Of 71 sera tested, 24 sera (33.8%) reacted with at least one of three recombinant antigens, although there was no significant correlation between the presence of the antibodies and clinical manifestations, such as disease duration or stage. There was also no discernible relationship to other auto-antibodies such as antinuclear antibodies (ANA) and rheumatoid factor. The results suggest that ERM proteins are possible novel auto-immune target antigens for RA.

Response of radixin to perturbations of growth cone morphology and motility in chick sympathetic neurons in vitro

The ERM protein--ezrin, radixin, moesin--localize to a variety of cortical structures, where they may participate in connecting the cytoskeleton to components of the plasma membrane. Antibodies that recognize the ERM proteins specifically stain growth cones of various neurons [Goslin et al., 1989: J. Cell Biol. 109:1621-1631; Birgbauer et al., 1991: J. Neurosci. Res. 30:232-241]. To probe the function of ERM proteins in growth cones, we studied the consequences of perturbing growth cone morphology and motility of cultured chick sympathetic neurons. We demonstrate that radixin is present in these growth cones. Withdrawal of nerve growth factor (NGF) induces rapid collapse of the growth cones; concomitantly, radixin staining in these growth cones are greatly diminished. Upon readdition of NGF, rapid growth cone formation is accompanied by relocalization of radixin. Induction of growth cone collapse by either growth cone-growth cone contact or exposure to brain membrane extract results in a similar diminution of radixin staining. We induced a more subtle change in the organization of the growth cones by subjecting them to an electric field. These growth cones rapidly orient toward the cathode. We show that the radixin staining of the growth cones is also asymmetrically localized toward the leading edges in the new direction of growth. The results suggest that the localization of radixin may be essential for the normal expression of growth cone morphology and function.