Direct involvement of ezrin/radixin/moesin (ERM)-binding membrane proteins in the organization of microvilli in collaboration with activated ERM proteins

Exclusion of CD43 from the immunological synapse is mediated by phosphorylation-regulated relocation of the cytoskeletal adaptor moesin

Formation of the immunological synapse requires TCR signal-dependent protein redistribution. However, the specific molecular mechanisms controlling protein relocation are not well defined. Moesin is a widely expressed phospho-protein that links many transmembrane molecules to the cortical actin cytoskeleton. Here, we demonstrate that TCR-induced exclusion of the large sialoprotein CD43 from the synapse is an active event mediated by its reversible binding to moesin. Our results also reveal that relocalization of moesin is associated with changes in the phosphorylation status of this cytoskeletal adaptor protein. Finally, these findings raise the possibility that the change in moesin localization resulting from TCR engagement modifies the overall topology of the lymphocyte membrane and facilitates molecular interactions at the site of presenting cell contact.

Exploitation of host factors for efficient infection by Shigella

Shigellosis is a worldwide endemic ulcerating disease of the large intestine caused by enteroinvasive bacteria. Shigella takes the route via M-cells and macrophages to access the basolateral pole of enterocytes. After invasion of and cell-to-cell spread within the epithelial cell layer, the bacterium multiplies within the cytoplasm of enterocytes. Induced by a limited number of bacterial effector proteins, Shigella makes use of established signaling pathways of the host cell to achieve internalization, transcytosis, apoptosis or cell-to-cell spread. This review addresses the host factors required for efficient infection focusing on Shigella-induced cytoskeletal rearrangements and associated signaling.

Delta regulates keratinocyte spreading and motility independently of differentiation

In human interfollicular epidermis stem cells lie in clusters surrounded by their differentiated daughters, transit amplifying cells, an arrangement that reflects differences in cell cohesiveness and motility. Keratinocytes expressing a dominant negative Delta1 mutant, Delta(T), lacking most of the cytoplasmic domain, acquired the motile behaviour of transit cells while retaining their stem cell identity. Conversely, overexpression of Delta1 promoted keratinocyte cohesiveness. The adhesive effects of Delta1 and Delta(T) were independent of SuH-dependent Notch signalling. Delta(T) increased motility and spreading of individual keratinocytes and stimulated lamellipodia formation. Delta and Delta(T) colocalised with cortical actin and redistributed on Latrunculin treatment. We propose that Delta promotes keratinocyte cohesiveness by restricting motility and discuss potential mechanisms by which Delta could interact with the actin cytoskeleton.

Defects in the MITF(mi/mi) apical surface are associated with a failure of outer segment elongation

The loss of MITF function in the MITF(mi/mi)mouse affects not only RPE differentiation, but also the development of rod photoreceptor outer segments. Our data indicate that opsin immunoreactivity is detected in the cell membrane and along the ONL/RPE border of developing MITF(mi/mi)rod photoreceptors and that rod outer segment morphogenesis is initiated. Although molecules associated with the outer segment continued to be expressed, outer segments did not elongate and develop stacked organized discs perpendicular to the RPE. The MITF(mi/mi)RPE also failed to form apical microvilli and lacked the apical network of the phosphoprotein ezrin seen in wild type tissue. The MITF(mi/mi)RPE basal surface was loosely organized and retained ezrin labelling which indicated some degree of differentiation. The correlation seen in our data suggest that there may be a link between the failure of the RPE apical domain to form and lack of rod outer segment elongation.

Structural conversion between open and closed forms of radixin: low-angle shadowing electron microscopy

The function of ERM (ezrin/radixin/moesin) proteins as general cross-linkers between actin filaments and plasma membranes is regulated downstream of Rho, through the transition between active and inactive forms. To directly examine the conformational change between the active and inactive forms of ERM proteins, we applied low-angle rotary-shadowing electron microscopy to the radixin molecules, wild-type, T564A-non-phosphorylated-type, and T564E-phosphorylated-type, since most of the active forms are reportedly stabilized in cells by the C-terminal threonine phosphorylation. As a result, the T564A- and wild-type radixin molecules yielded the globular closed forms, approximately 8-14 nm in diameter, with some striations on their surfaces. In contrast, the T564E-radixin molecules tended to take elongated open forms, in which two globular structures measuring approximately 8 nm and approximately 5 nm in diameter were associated with both ends of the filamentous structures. The filamentous structure took either a approximately 20-25 nm-long straight course or a folded course. Taken together with the biochemical and the crystal structural results obtained to date, the closed and open forms represent the inactive and active forms of radixin as cross-linkers between actin filaments and plasma membranes.

Ezrin-dependent promotion of glioma cell clonogenicity, motility, and invasion mediated by BCL-2 and transforming growth factor-beta2

Ezrin belongs to the ezrin-radixin-moesin family proteins, which cross-link actin cytoskeleton and plasma membrane. Malignant glioma cells are paradigmatic for their strong migratory and invasive properties. Here, we report that the expression of dominant-negative ezrins inhibits clonogenicity, migration, and invasiveness of human malignant glioma cells. Furthermore, dominant-negative ezrins block hepatocyte growth factor (HGF)-mediated stimulation of clonogenicity and migration, without altering HGF-induced protein kinase B/Akt and focal adhesion kinase phosphorylation. Glioma cells expressing dominant-negative ezrins exhibit a shift of the BCL-2/BAX rheostat toward apoptosis, reduced alpha(V)beta(3) integrin expression and reduced matrix metalloproteinase (MMP) expression and activity. These changes are associated with a dramatic loss of transforming growth factor beta(2) (TGF-beta(2)) release. Exogenous supplementation of TGF-beta(2) overcomes the inhibitory effects of dominant-negative ezrins on migration and clonogenicity. A neutralizing TGF-beta(2) antibody mimics the effects of dominant-negative ezrins on clonogenicity and migration. Exogenous HGF markedly induces TGF-beta(2) protein levels, and a neutralizing TGF-beta(2) antibody abolishes the HGF-mediated increase in glioma cell motility. Finally, TGF-beta(2) does not modulate BCL-2 or BAX expression, but BCL-2 gene transfer increases the levels of latent and active TGF-beta(2). Intracranial xenografts of U87MG glioma cells transfected with the dominant-negative ezrins in athymic mice grow to significantly smaller volumes, and the median survival of these mice is 50 d compared with 28 d in the control group. These data define a novel pathway for HGF-induced glioma cell migration and invasion, which requires ezrin, changes in the BCL-2/BAX rheostat, and the induction of TGF-beta(2) expression in vitro, and underscore the important role of HGF signaling in vivo.

Ezrin-dependent promotion of glioma cell clonogenicity, motility, and invasion mediated by BCL-2 and transforming growth factor-beta2

Ezrin belongs to the ezrin-radixin-moesin family proteins, which cross-link actin cytoskeleton and plasma membrane. Malignant glioma cells are paradigmatic for their strong migratory and invasive properties. Here, we report that the expression of dominant-negative ezrins inhibits clonogenicity, migration, and invasiveness of human malignant glioma cells. Furthermore, dominant-negative ezrins block hepatocyte growth factor (HGF)-mediated stimulation of clonogenicity and migration, without altering HGF-induced protein kinase B/Akt and focal adhesion kinase phosphorylation. Glioma cells expressing dominant-negative ezrins exhibit a shift of the BCL-2/BAX rheostat toward apoptosis, reduced alpha(V)beta(3) integrin expression and reduced matrix metalloproteinase (MMP) expression and activity. These changes are associated with a dramatic loss of transforming growth factor beta(2) (TGF-beta(2)) release. Exogenous supplementation of TGF-beta(2) overcomes the inhibitory effects of dominant-negative ezrins on migration and clonogenicity. A neutralizing TGF-beta(2) antibody mimics the effects of dominant-negative ezrins on clonogenicity and migration. Exogenous HGF markedly induces TGF-beta(2) protein levels, and a neutralizing TGF-beta(2) antibody abolishes the HGF-mediated increase in glioma cell motility. Finally, TGF-beta(2) does not modulate BCL-2 or BAX expression, but BCL-2 gene transfer increases the levels of latent and active TGF-beta(2). Intracranial xenografts of U87MG glioma cells transfected with the dominant-negative ezrins in athymic mice grow to significantly smaller volumes, and the median survival of these mice is 50 d compared with 28 d in the control group. These data define a novel pathway for HGF-induced glioma cell migration and invasion, which requires ezrin, changes in the BCL-2/BAX rheostat, and the induction of TGF-beta(2) expression in vitro, and underscore the important role of HGF signaling in vivo.

ERM-Merlin and EBP50 protein families in plasma membrane organization and function

The ezrin-radixin-moesin (ERM) family of proteins have emerged as key regulatory molecules in linking F-actin to specific membrane proteins, especially in cell surface structures. Merlin, the product of the NF2 tumor suppressor gene, has sequence similarity to ERM proteins and binds to some of the same membrane proteins, but lacks a C-terminal F-actin binding site. In this review we discuss how ERM proteins and merlin are negatively regulated by an intramolecular association between their N- and C-terminal domains. Activation of at least ERM proteins can be accomplished by C-terminal phosphorylation in the presence of PIP2. We also discuss membrane proteins to which ERM and merlin bind, including those making an indirect linkage through the PDZ-containing adaptor molecules EBP50 and E3KARP. Finally, the function of these proteins in cortical structure, endocytic traffic, signal transduction, and growth control is discussed.

The C terminus of annexin II mediates binding to F-actin

Annexin II heterotetramer (AIIt) is a multifunctional Ca(2+)-binding protein composed of two 11-kDa subunits and two annexin II subunits. The annexin II subunit contains the binding sites for anionic phospholipids, heparin, and F-actin, whereas the p11 subunit provides a regulatory function. The F-actin-binding site is presently unknown. In the present study we have utilized site-directed mutagenesis to create annexin II mutants with truncations in the C terminus of the molecule. Interestingly, a mutant annexin II lacking its C-terminal 16, 13, or 9 amino acids was unable to bind to F-actin but still retained its ability to interact with both anionic phospholipids and heparin. Recombinant AIIt, composed of wild-type p11 subunits and the mutant annexin II subunits, was also unable to bundle F-actin. This loss of F-actin bundling activity was directly attributable to the inability of mutant AIIt to bind F-actin. These results establish for the first time that the annexin II C-terminal amino acid residues, LLYLCGGDD, participate in F-actin binding.

Prominin: a story of cholesterol, plasma membrane protrusions and human pathology

Prominin is the first identified member of a novel family of polytopic membrane proteins conserved throughout the animal kingdom. It has an unusual membrane topology, containing five transmembrane domains and two large glycosylated extracellular loops. In mammals, prominin is expressed in various embryonic and adult epithelial cells, as well as in nonepithelial cells, such as hematopoietic stem cells. At the subcellular level, prominin is selectively localized in microvilli and other plasma membrane protrusions, irrespective of cell type. At the molecular level, prominin specifically interacts with membrane cholesterol and is a marker of a novel type of cholesterol-based lipid 'raft'. A frameshift mutation in the human prominin gene, which results in a truncated protein that is no longer transported to the cell surface, is associated with retinal degeneration. Given that prominin is concentrated in the plasma membrane evaginations at the base of the outer segment of rod photoreceptor cells, which are essential precursor structures in the biogenesis of photoreceptive disks, it is proposed that prominin has a role in the generation of plasma membrane protrusions, their lipid composition and organization and their membrane-to-membrane interactions.

Regulation of a multigenic invasion programme by the transcription factor, AP-1: re-expression of a down-regulated gene, TSC-36, inhibits invasion

The transcription factor AP-1 (activator protein-1) is required for transformation by many oncogenes, which function upstream of it in the growth factor-ras signal transduction pathway. Previously, we proposed that one role of AP-1 in transformation is to regulate the expression of a multigenic invasion programme. As a test of this proposal we sought to identify AP-1 regulated genes based upon their differential expression in 208F rat fibroblasts transformed by FBR-v-fos (FBR), and to determine if they functioned in the invasion programme. Subtracted cDNA libraries specific for up- or down-regulated genes in FBRs compared to 208Fs were constructed and analysed. Northern analysis revealed that the cDNAs in both libraries represented differentially expressed genes. Nucleic acid sequence analysis of randomly selected cDNA clones from each library coupled with searches of nucleic acid and amino acid sequence databases determined that many of the cDNAs represented proteins that function in various aspects of the invasion process. Functional analysis of one the down-regulated genes, TSC-36/follistatin-related protein (TSC-36/Frp), which has not previously been associated with invasion, demonstrated that its expression in FBRs inhibited in vitro invasion. These results support the proposal that AP-1 in transformed cells regulates a multigenic invasion programme.

Subcellular distribution of envoplakin and periplakin: insights into their role as precursors of the epidermal cornified envelope

Envoplakin and periplakin are two plakins that are precursors of the epidermal cornified envelope. We studied their distribution and interactions by transfection of primary human keratinocytes and other cells. Full-length periplakin localized to desmosomes, the interdesmosomal plasma membrane and intermediate filaments. Full length envoplakin also localized to desmosomes, but mainly accumulated in nuclear and cytoplasmic aggregates with associated intermediate filaments. The envoplakin rod domain was required for aggregation and the periplakin rod domain was necessary and sufficient to redistribute envoplakin to desmosomes and the cytoskeleton, confirming earlier predictions that the proteins can heterodimerize. The linker domain of each protein was required for intermediate filament association. Like the NH(2) terminus of desmoplakin, that of periplakin localized to desmosomes; however, in addition, the periplakin NH(2) terminus accumulated at cell surface microvilli in association with cortical actin. Endogenous periplakin was redistributed from microvilli when keratinocytes were treated with the actin disrupting drug Latrunculin B. We propose that whereas envoplakin and periplakin can localize independently to desmosomes, the distribution of envoplakin at the interdesmosomal plasma membrane depends on heterodimerization with periplakin and that the NH(2) terminus of periplakin therefore plays a key role in forming the scaffold on which the cornified envelope is assembled.

Microvillar ion channels: cytoskeletal modulation of ion fluxes

The recently presented theory of microvillar Ca(2+)signaling [Lange, K. (1999) J. Cell. Physiol.180, 19-35], combined with Manning's theory of "condensed counterions" in linear polyelectrolytes [Manning, G. S. (1969). J. Chem. Phys.51, 924-931] and the finding of cable-like ion conductance in actin filaments [Lin, E. C. & Cantiello, H. F. (1993). Biophys. J.65, 1371-1378], allows a systematic interpretation of the role of the actin cytoskeleton in ion channel regulation. Ion conduction through actin filament bundles of microvilli exhibits unique nonlinear transmission properties some of which closely resemble that of electronic semiconductors: (1) bundles of microfilaments display significant resistance to cation conduction and (2) this resistance is decreased by supply of additional energy either as thermal, mechanical or electromagnetic field energy. Other transmission properties, however, are unique for ionic conduction in polyelectrolytes. (1) Current pulses injected into the filaments were transformed into oscillating currents or even into several discrete charge pulses closely resembling that of single-channel recordings. Discontinuous transmission is due to the existence of counterion clouds along the fixed anionic charge centers of the polymer, each acting as an "ionic capacitor". (2) The conductivity of linear polyelectrolytes strongly decreases with the charge number of the counterions; thus, Ca(2+)and Mg(2+)are effective modulator of charge transfer through linear polyelectrolytes. Field-dependent formation of divalent cation plugs on either side of the microvillar conduction line may generate the characteristic gating behavior of cation channels. (3) Mechanical movement of actin filament bundles, e.g. bending of hair cell microvilli, generates charge translocations along the filament structure (mechano-electrical coupling). (4) Energy of external fields, by inducing molecular dipoles within the polyelectrolyte matrix, can be transformed into mechanical movement of the system (electro-mechanical coupling). Because ionic transmission through linear polyelectrolytes is very slow compared with electronic conduction, only low-frequency electromagnetic fields can interact with the condensed counterion systems of linear polyelectrolytes. The delineated characteristics of microvillar ion conduction are strongly supported by the phenomenon of electro-mechanical coupling (reverse transduction) in microvilli of the audioreceptor (hair) cells and the recently reported dynamics of Ca(2+)signaling in microvilli of audio- and photoreceptor cells. Due to the cell-specific expression of different types and combinations of ion channels and transporters in the microvillar tip membrane of differentiated cells, the functional properties of this cell surface organelle are highly variable serving a multitude of different cellular functions including receptor-mediated effects such as Ca(2+)signaling, regulation of glucose and amino acid transport, as well as modulation of membrane potential. Even mechanical channel activation involved in cell volume regulation can be deduced from the systematic properties of the microvillar channel concept. In addition, the specific ion conduction properties of microfilaments combined with their proposed role in Ca(2+)signaling make microvilli the most likely cellular site for the interaction with external electric and magnetic fields.

Regulation of cell volume via microvillar ion channels

A novel mechanism of cellular volume regulation is presented, which ensues from the recently introduced concept of transport and ion channel regulation via microvillar structures (Lange K, 1999, J Cell Physiol 180:19-35). According to this notion, the activity of ion channels and transporter proteins located on microvilli of differentiated cells is regulated by changes in the structural organization of the bundle of actin filaments in the microvillar shaft region. Cells with microvillar surfaces represent two-compartment systems consisting of the cytoplasm on the one side and the sum of the microvillar tip (or, entrance) compartments on the other side. The two compartments are separated by the microvillar actin filament bundle acting as diffusion barrier ions and other solutes. The specific organization of ion and water channels on the surface of microvillar cell types enables this two-compartment system to respond to hypo- and hyperosmotic conditions by activation of ionic fluxes along electrochemical gradients. Hypotonic exposure results in swelling of the cytoplasmic compartment accompanied by a corresponding reduction in the length of the microvillar diffusion barrier, allowing osmolyte efflux and regulatory volume decrease (RVD). Hypertonic conditions, which cause shortening of the diffusion barrier via swelling of the entrance compartment, allow osmolyte influx for regulatory volume increase (RVI). Swelling of either the cytoplasmic or the entrance compartment, by using membrane portions of the microvillar shafts for surface enlargement, activates ion fluxes between the cytoplasm and the entrance compartment by shortening of microvilli. The pool of available membrane lipids used for cell swelling, which is proportional to length and number of microvilli per cell, represents the sensor system that directly translates surface enlargements into activation of ion channels. Thus, the use of additional membrane components for osmotic swelling or other types of surface-expanding shape changes (such as the volume-invariant cell spreading or stretching) directly regulates influx and efflux activities of microvillar ion channels. The proposed mechanism of ion flux regulation also applies to the physiological main functions of epithelial cells and the auxiliary action of swelling-induced ATP release. Furthermore, the microvillar entrance compartment, as a finely dispersed ion-accessible peripheral space, represents a cellular sensor for environmental ionic/osmotic conditions able to detect concentration gradients with high lateral resolution. Volume regulation via microvillar surfaces is only one special aspect of the general property of mechanosensitivity of microvillar ionic pathways.

Morphogenic effects of ezrin require a phosphorylation-induced transition from oligomers to monomers at the plasma membrane

ERM (ezrin, radixin, moesin) proteins act as linkers between the plasma membrane and the actin cytoskeleton. An interaction between their NH(2)- and COOH-terminal domains occurs intramolecularly in closed monomers and intermolecularly in head-to-tail oligomers. In vitro, phosphorylation of a conserved threonine residue (T567 in ezrin) in the COOH-terminal domain of ERM proteins disrupts this interaction. Here, we have analyzed the role of this phosphorylation event in vivo, by deriving stable clones producing wild-type, T567A, and T567D ezrin from LLC-PK1 epithelial cells. We found that T567A ezrin was poorly associated with the cytoskeleton, but was able to form oligomers. In contrast, T567D ezrin was associated with the cytoskeleton, but its distribution was shifted from oligomers to monomers at the membrane. Moreover, production of T567D ezrin induced the formation of lamellipodia, membrane ruffles, and tufts of microvilli. Both T567A and T567D ezrin affected the development of multicellular epithelial structures. Collectively, these results suggest that phosphorylation of ERM proteins on this conserved threonine regulates the transition from membrane-bound oligomers to active monomers, which induce and are part of actin-rich membrane projections.

Increased CD44 expression in human bronchial epithelial repair after damage or plating at low cell densities

We have investigated the effect of mechanical damage, cell density, and cell-derived soluble mediators on CD44 expression in a model of bronchial epithelial repair. CD44 (all isoforms) and variant-containing isoforms (CD44v3, CD44v6, and CD44v9) were identified with flow cytometry and immunocytochemistry with image analysis. After mechanical damage, CD44 expression increased up to 500 microm from the wound edge and for up to 48 h in two human bronchial epithelium-derived cell lines, 16HBE14o- and NCI-H292. CD44 expression was unchanged by interferon-gamma and increased by <50% by tumor necrosis factor-alpha. To exclude other soluble factors, a Vaseline spacer was used to temporarily divide petri dishes, with cells at high density on one side and those at low density on the other. After the spacer was removed, the cells at low cell density growing in the shared medium expressed up to fourfold higher CD44, although cell proliferation was unchanged. Thus increased CD44 expression at low cell density was not mediated by soluble factors and may reflect functional involvement in cell motility, dedifferentiation, or altered cell-substrate adhesion in epithelial repair.

Heparan sulfate proteoglycan isoforms of the CD44 hyaluronan receptor induced in human inflammatory macrophages can function as paracrine regulators of fibroblast growth factor action

The CD44 glycoprotein is expressed in multiple isoforms on a variety of cell types where it functions as a receptor for hyaluronan-mediated motility. Recently, interest has centered on CD44 heparan sulfate proteoglycan (HSPG) isoforms because of their potential to sequester heparin-binding growth factors and chemokines. Expression of these isoforms on ectodermal cells has recently been shown to regulate limb morphogenesis via presentation of fibroblast growth factor (FGF) 4/FGF 8 while expression on tumor cells was shown to sequester hepatocyte growth factor and promote tumor dissemination. To date, however, CD44 HSPG expression in tissue macrophages and lymphocytes has not been adequately investigated, despite the fact these cells actively synthesize growth factors and chemokines and indirect evidence that monocyte CD44 sequesters macrophage inflammatory protein-1beta. Here we show primary human monocytes rather than lymphocytes express CD44 HSPGs, but only following in vitro differentiation to macrophages or activation with the proinflammatory cytokine interleukin-1alpha or bacterial lipopolysaccharide. Furthermore, we show these isoforms are preferentially modified with heparan rather than chondroitin sulfate, bind the macrophage-derived growth factors FGF-2, vascular endothelial growth factor, and heparin-binding epidermal growth factor with varying affinities (K(d) 25-330 nM) and in the case of FGF-2, can stimulate productive binding to the high affinity tyrosine kinase FGF receptor 1 (FGFR1). In contrast, we find no evidence for significant binding to C-C chemokines. Last, we confirm by immunofluorescent antibody staining that inflamed synovial membrane macrophages express CD44 HSPGs and that expression is greatest in cells containing high FGF-2 levels. These results suggest a paracrine role for macrophage CD44 HSPG isoforms in the regulation of growth factor action during inflammation.

Krp1, a novel kelch related protein that is involved in pseudopod elongation in transformed cells

We have previously shown that the transcription factor AP-1 regulates the expression of genes which allow neoplastically transformed rat fibroblasts to become invasive. Searches for further AP-1 target genes led to the identification of a gene encoding a novel rat kelch family member, named kelch related protein 1 (Krp1). Kelch family members are characterized by a series of repeats at their carboxyl terminus and a BTB/POZ domain near their amino terminus. Rat Krp1 has a primarily cytoplasmic localization, and a small fraction appears to accumulate and co-localize with F-actin at membrane ruffle-like structures in the tips of pseudopodia. Overexpression of Krp1 in transformed rat fibroblasts led to the formation of dramatically elongated pseudopodia, while expression of truncated Krp1 polypeptides resulted in a reduction in the length of pseudopodia. We propose that the transformation-specific expression of Krp1 is required for pseudopod elongation, which are structures that are required for cell motility and invasion.

Ezrin promotes morphogenesis of apical microvilli and basal infoldings in retinal pigment epithelium

Ezrin, a member of the ezrin/radixin/moesin (ERM) family, localizes to microvilli of epithelia in vivo, where it bridges actin filaments and plasma membrane proteins. Here, we demonstrate two specific morphogenetic roles of ezrin in the retinal pigment epithelium (RPE), i.e., the formation of very long apical microvilli and of elaborate basal infoldings typical of these cells, and characterize the role of ezrin in these processes using antisense and transfection approaches. In the adult rat RPE, only ezrin (no moesin or radixin) was detected at high levels by immunofluorescence and immunoelectron microscopy at microvilli and basal infoldings. At the time when these morphological differentiations develop, in the first two weeks after birth, ezrin levels increased fourfold to adult levels. Addition of ezrin antisense oligonucleotides to primary cultures of rat RPE drastically decreased both apical microvilli and basal infoldings. Transfection of ezrin cDNA into the RPE-J cell line, which has only trace amounts of ezrin and moesin, sparse and stubby apical microvilli, and no basal infoldings, induced maturation of microvilli and the formation of basal infoldings without changing moesin expression levels. Taken together, the results indicate that ezrin is a major determinant in the maturation of surface differentiations of RPE independently of other ERM family members.

Identification of PA2.26 antigen as a novel cell-surface mucin-type glycoprotein that induces plasma membrane extensions and increased motility in keratinocytes

PA2.26 antigen was identified as a cell-surface protein induced in epidermal carcinogenesis and skin remodeling processes. PA2.26 is expressed in carcinoma cell lines and cultured fibroblasts but absent in nontumorigenic keratinocytes. In tissues, PA2.26 is present in epithelial cells of the choroid plexus, ependyma, glomerulus and alveolus, in mesothelial cells, and in endothelia of lymphatic vessels. Biochemical characterization of PA2.26 protein and sequence analysis of the isolated cDNA demonstrate that PA2.26 antigen is a mucin-like transmembrane glycoprotein. Confocal and immunoelectron microscopy analysis in cultured cells reveal that PA2. 26 is concentrated in actin-rich microvilli and plasma membrane projections, such as filopodia, lamellipodia and ruffles, where it colocalizes with members of the ERM (ezrin, radixin, moesin) family protein. Ezrin and moesin, but not radixin, can be coimmunoprecipitated together with PA2.26 from cell lysates. Ectopic expression of PA2.26 in immortalized, nontumorigenic, keratinocytes induces an epithelial-fibroblastoid morphological conversion with increased plasma membrane extensions, concomitantly to a major reorganization of the actin cytoskeleton, redistribution of ezrin to cell-surface projections, and enhanced motility. These findings suggest an involvement of PA2.26 in cell migration.

Activation of moesin and adducin by Rho-kinase downstream of Rho

The Rho GTPase (Rho) is a member of the Rho family, which belongs to the Ras superfamily of GTP-binding proteins. Like other GTP-binding proteins, Rho exists in two conformational states, an inactive GDP-bound form and an active GTP-bound form. Active Rho interacts with specific effectors to regulate the actin cytoskeleton and to mediate a variety of biological functions in cells. Rho-associated kinase (Rho-kinase) is the most studied Rho-effector, and studies of its biochemical and cell biological functions have provided us with useful information for understanding the molecular mechanisms of the actions of Rho. This review aims to summarize the roles of Rho and Rho-kinase in the regulation of the cytoskeletons.