Expression and localization of villin, fimbrin, and myosin I in differentiating mouse F9 teratocarcinoma cells

The actin-bundling protein L-plastin-A double-edged sword: Beneficial for the immune response, maleficent in cancer

The dynamic organization of the actin cytoskeleton into bundles and networks is orchestrated by a large variety of actin-binding proteins. Among them, the actin-bundling protein L-plastin is normally expressed in hematopoietic cells, where it is involved in the immune response. However, L-plastin is also often ectopically expressed in malignant cancer cells of non-hematopoietic origin and is even considered as a marker for cancer progression. Post-translational modification modulates L-plastin activity. In particular, L-plastin Ser5 phosphorylation has been shown to be important for the immune response in leukocytes as well as for invasion and metastasis formation of carcinoma cells. This chapter discusses the physiological and pathological role of L-plastin with a special focus on the importance of L-plastin Ser5 phosphorylation for the protein functions. The potential use of Ser5 phosphorylated L-plastin as a biomarker and/or therapeutic target will be evoked.

The role of actin bundling proteins in the assembly of filopodia in epithelial cells

The goal of this review is to highlight how emerging new models of filopodia assembly, which include tissue specific actin-bundling proteins, could provide more comprehensive representations of filopodia assembly that would describe more adequately and effectively the complexity and plasticity of epithelial cells.  This review also describes how the true diversity of actin bundling proteins must be considered to predict the far-reaching significance and versatile functions of filopodia in epithelial cells.

A blockade in Wnt signaling is activated following the differentiation of F9 teratocarcinoma cells

Aberrant activation of the Wnt signaling pathway is a common event in human tumor progression. Wnt signaling has also been implicated in maintaining a variety of adult and embryonic stem cells by imposing a restraint to differentiation. To understand the effect of Wnt signaling on the differentiation of epithelial cells, we used mouse teratocarcinoma F9 cells as a model. The F9 cells can be differentiated into visceral endoderm (VE) resembling absorptive columnar epithelial cells. We performed comparative gene expression analysis on retinoic acid-differentiated and undifferentiated F9 cells and confirmed that markers of VE and intestinal epithelium were induced upon differentiation. The induction of these markers by retinoic acid was reduced in the presence of Wnt, although Wnt alone did not change their expression. This suggests that Wnt signaling inhibited the differentiation of F9 cells by altering gene expression. This inhibition was also reflected in the morphology of the F9 cells as their apical-basal polarity was disrupted by inclusion of Wnt during differentiation. These results support a model in which Wnt modulates the expression of genes required for normal terminal differentiation of the stem cells. However, it follows that progenitor cells must escape from Wnt signaling to attain the differentiated state. Accordingly, we found that differentiated F9 cells no longer responded to Wnt and that a blockade in Wnt signaling occurred upstream of Axin. Consistent with this, Wnt negative regulators, such as Dickkopf-1 and Disabled-2, were induced upon the differentiation of F9 cells. We propose that a similar system to produce Wnt inhibitors regulates homeostasis of certain stem cell compartments in vivo.

NMR structure of an F-actin-binding "headpiece" motif from villin

A growing family of F-actin-bundling proteins harbors a modular F-actin-binding headpiece domain at the C terminus. Headpiece provides one of the two F-actin-binding sites essential for filament bundling. Here, we report the first structure of a functional headpiece domain. The NMR structure of chicken villin headpiece (HP67) reveals two subdomains that share a tightly packed hydrophobic core. The N-terminal subdomain contains bends, turns, and a four-residue alpha-helix as well as a buried histidine residue that imparts a pH-dependent folding. The C-terminal subdomain is composed of three alpha-helices and its folding is pH-independent. Two residues previously implicated in F-actin-binding form a buried salt-bridge between the N and C-terminal subdomains. The rest of the identified actin-binding residues are solvent-exposed and map onto a unique F-actin-binding surface.

Volume of liquid below the epithelium of an F9 cell as a signal for differentiation into visceral endoderm

When retinoic acid-primed F9 cells are allowed to aggregate, they form embryoid bodies with an outer layer of (α)-fetoprotein-producing visceral endoderm cells and an internal cavity. I show that maturation of the visceral endoderm is dependent on the size of F9 aggregates. Size fractionation of aggregates of retinoic acid-primed F9 cells on Percoll density gradients revealed that only aggregates with diameters larger than 180 microm developed into embryoid bodies with an endoderm layer secreting (α)-fetoprotein. Size dependent alpha-fetoprotein-secretion was also observed when retinoic acid-primed F9 cells were cultured on porous microcarrier beads larger than 185 microm. Retinoic acid-primed F9 cells on flat microporous membranes did not differentiate and secrete alpha-fetoprotein unless exposed to a limited volume of medium at their basolateral surface. This suggested that maturation of the visceral endoderm is signaled by the volume of liquid phase below the epithelium. I postulate that the epithelial layer of an F9 aggregate encloses liquid and forms a barrier to diffusion of some critical factor(s). The concentration of such a factor may reach a threshold due to enlargement of the liquid phase during growth of the F9 aggregate and thereby signal maturation of the outer layer of cells into visceral endoderm.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Developmental expression of extracellular glutathione peroxidase suggests antioxidant roles in deciduum, visceral yolk sac, and skin

Extracellular glutathione peroxidase (EGPx) is a secreted selenium-dependent enzyme that reduces hydroperoxides and organic hydroperoxides. Selenium deficiency in females is associated with infertility and spontaneous abortion, suggesting a role for selenium-requiring proteins during embryonic development. To gain insight into functions of EGPx in vivo, we determined sites of murine EGPx synthesis by in situ hybridization during embryogenesis and in adult tissues. At E7.5 of development, high EGPx expression was found in the maternally derived deciduum, with lower levels of accumulation in the embryonic visceral endoderm. At E9.5, the major sites of expression were the yolk sac endoderm and heart musculature. By E16.5, EGPx mRNA expression persisted in yolk sac endoderm but also accumulated significantly in atrially derived myocytes, ossification centers, adipose tissue, intestinal epithelium, and in a ventral-to-dorsal gradient in developing skin. Glutathione peroxidase activity due to EGPx protein was identified in the fluids surrounding the developing mouse embryo at midgestation. The expression of EGPx in tissues at the maternal-fetal interface--deciduum, visceral yolk sac, and skin--suggests that EGPx may serve to protect the embryo from oxidant damage. In adult mice, we identified the S1 segment of the kidney proximal tubules as the primary site of EGPx mRNA accumulation, with lower EGPx levels in atrial cardiac muscle, intestine, skin, and adipose tissue. These findings suggest that EGPx may serve a wider antioxidant role than previously recognized in the interstitium of multiple localized tissues, particularly those associated with the active transport of lipids.

Targeted disruption of the mouse villin gene does not impair the morphogenesis of microvilli

The small intestine is functionally dependent on the presence of the brush border, a tightly packed array of microvilli that forms the amplified apical surface of absorptive cells. In the core of each microvillus, actin filaments are bundled by two proteins, villin and fimbrin. Previous in vitro studies using antisense approaches indicated that villin plays an important role in the morphogenesis of microvilli. To examine the in vivo consequences of villin deficiency, we disrupted the mouse villin gene by targeted recombination in mouse embryonic stem cells. A beta-galactosidase cDNA was also introduced into the villin locus by the targeting event. Homozygous villin-deficient mice are viable, fertile, and display no gross abnormalities. Intact microvilli are present in the small intestine, colon, kidney proximal tubules, and liver bile canaliculi. Although subtle ultrastructural abnormalities can be detected in the actin cores of small intestinal microvilli, localization of sucrase isomaltase, brush border myosin I, and zonula occludens I to the microvillar surface of the small intestine is normal. Thus, in vivo, villin plays a minor or redundant role in the generation of microvilli in multiple absorptive tissues.

Long-term in vitro culture and characterisation of avian embryonic stem cells with multiple morphogenetic potentialities

Petitte, J.N., Clarck, M.E., Verrinder Gibbins, A. M. and R. J. Etches (1990; Development 108, 185–189) demonstrated that chicken early blastoderm contains cells able to contribute to both somatic and germinal tissue when injected into a recipient embryo. However, these cells were neither identified nor maintained in vitro. Here, we show that chicken early blastoderm contains cells characterised as putative avian embryonic stem (ES) cells that can be maintained in vitro for long-term culture. These cells exhibit features similar to those of murine ES cells such as typical morphology, strong reactivity toward specific antibodies, cytokine-dependent extended proliferation and high telomerase activity. These cells also present high capacities to differentiate in vitro into various cell types including cells from ectodermic, mesodermic and endodermic lineages. Production of chimeras after injection of the cultivated cells reinforced the view that our culture system maintains in vitro some avian putative ES cells.

Multifocal heterogeneity in villin and Ep-CAM expression in Barrett's esophagus

Barrett's esophagus (BE) is a metaplastic change of the squamous esophageal epithelium to columnar gastric or intestinal-like epithelium. BE is associated with long-standing gastroesophageal reflux disease and carries an increased risk for dysplasia and adenocarcinoma. Little if any is known regarding the differentiation state of esophageal metaplasia and its relationship to carcinogenesis. In this study, we investigated the potential of villin, a cytoskeletal protein, and Ep-CAM, a glandular epithelial glycoprotein, to serve as markers for enterocytic differentiation in BE at the molecular level. Endoscopic mucosal biopsy samples of normal esophagus, BE, stomach and duodenum were collected from 23 patients with BE. Biopsies were analyzed for villin and Ep-CAM expression by immunoblotting, and in some cases for the presence of microvilli by electron microscopy. By mapping of BE segments in 6 patients, correlations were also made between the histologic evidence of metaplasia and villin expression. Villin was uniformly expressed in all duodenal samples but was not detected in normal esophagus and stomach. In BE biopsies, villin expression was limited to the subset of patients whose adjacent biopsies showed microvilli by electron microscopy. In several patients studied, however, the expression of villin and the epithelial glycoprotein Ep-CAM differed among various regions of esophageal metaplasia within the same patient. Mapping studies failed to reveal any correlation among histologic evidence of metaplasia, dysplasia and villin expression and confirmed the multifocal heterogeneity of villin expression in BE. Preliminary data of 4 adenocarcinoma patients studied showed that villin expression was absent in 3 and very low in 1 patient. Ep-CAM was highly expressed in all adenocarcinoma patients. Our results show that BE represents a complex epithelium with significant heterogeneity in antigen expression and ultrastructural morphologic features. This molecular heterogeneity supports the presence of different stages of differentiation within the same epithelium.

Cytoskeletal and calcium-binding proteins in the mammalian organ of Corti: cell type-specific proteins displaying longitudinal and radial gradients

Whole mounts and tissue sections of the organ of Corti from two representative mammalian species, the Mongolian gerbil (Meriones unguiculatus) and the guinea pig (Cavea porcellus) were probed with antibodies to cytoskeletal and calcium-binding proteins (actin, tubulin, including post-translational modifications, spectrin, fimbrin, calmodulin, parvalbumin, calbindin, S-100 and calretinin). All of the proteins tested were expressed in both species. New findings include the following. Actin is present in large accumulations in cell bodies of the Deiters cells under the outer hair cells (OHC), as well as in the filament networks previously described. These accumulations are more prominent in the apical turns. Tubulin is present in sensory cells in the tyrosinated (more dynamic) form, while tubulin in the supporting cells is post-translationally modified, indicating greater stability. Fimbrin, present in the stereocilia of both IHCs and OHCs, is similar to the isoform of fimbrin found in the epithelial cells of the intestine (fimbrin-I), which implies that actin bundling by fimbrin is reduced in the presence of increased calcium. Parvalbumin appears to be an IHC-specific calcium-binding protein in the gerbil as well as in the guinea pig; labeling displays a longitudinal gradient, with hair cells at the apex staining intensely and hair cells at the base staining weakly. Calbindin displays a similar longitudinal gradient, with staining intense in the IHCs and OHCs at the apex and weak to absent in the base. In the middle turns of the guinea pig cochlea, OHCs in the first row near the pillar cells lose immunoreactivity to calbindin before those in the second and third rows. Calmodulin is found throughout the whole cochlea in the IHCs and OHCs in the stereocilia, cuticular plate, and cell body. Calretinin is present in IHCs and Deiters cells in both species, as well as the tectal cell (modified Hensen cell) in the gerbil. S-100 is a supporting cell-specific calcium-binding protein which has not been localized in the sensory cells of these two species. The supporting cells containing S-100 include the inner border, inner phalangeal, pillar, Deiters, tectal (in gerbil) and Hensen cells, where labeling displays a longitudinal gradient decreasing in intensity towards the apex (opposite to what has been seen with labeling for other proteins in the cochlea).

Differential gene expression during early murine yolk sac development

The visceral yolk sac (VYS), composed of extraembryonic mesoderm and visceral endoderm, is the initial site of blood cell development and serves important nutritive and absorptive functions. In the mouse, the visceral endoderm becomes a morphologically distinct tissue at the time of implantation (E4.5), while the extraembryonic mesoderm arises during gastrulation (E6.5-8.5). To isolate genes differentially expressed in the developing yolk sac, polymerase chain reaction (PCR) methods were used to construct cDNA from late primitive streak to neural plate stage (E7.5) murine VYS mesoderm and VYS endoderm tissues. Differential screening led to the identification of six VYS mesoderm-enriched clones: ribosomal protein L13a, the heat shock proteins hsc 70 and hsp 86, guanine-nucleotide binding protein-related gene, cellular nucleic acid binding protein, and alpha-enolase. One VYS endoderm-specific cDNA was identified as apolipoprotein C2. In situ hybridization studies confirmed the differential expression of these genes in E7.5 yolk sac tissues. These results indicate that representative cDNA populations can be obtained from small numbers of cells and that PCR methodologies permit the study of gene expression during early mammalian postimplantation development. While all of the mesoderm-enriched genes were ubiquitously expressed in the embryo proper, apolipoprotein C2 expression was confined to the visceral endoderm. These results are consistent with the hypothesis that at E7.5, the yolk sac endoderm provides differentiated liver-like functions, while the newly developing extraembryonic mesoderm is still a largely undifferentiated tissue.

Recombinant expression of the brush border myosin I heavy chain

Although the specific functions of myosin I motors are not known, their localization to membrane structures suggests a function in membrane motility. Different myosin I isoforms in the same cell or in different cells can possess different localizations. To determine if the localization and biochemical activity of the best-characterized mammalian myosin I, chicken intestinal epithelium brush border myosin I, was dependent on determinants of the membrane or actin cytoskeleton specific to epithelial cells, we transfected the cDNA for the heavy chain of this myosin into COS cells. Transient transfection of COS cells with the chicken brush border myosin heavy chain resulted in the production of recombinant myosin I. Recombinant brush border myosin I localized to protrusions of the plasma membrane, particularly at spreading edges, and also to unknown cytoplasmic structures. Some cells expressing particularly high levels of brush border myosin I possessed a highly irregular surface. Recombinant brush border myosin I purified from COS cells bound to actin filaments in an ATP-dependent manner and decorated actin filaments to form a characteristic appearance. The recombinant myosin also catalyzed calcium-sensitive, actin-activated MgATPase activity similar to that of the native enzyme. Thus, any cellular factor required for the general membrane localization or biochemical activity of brush border myosin I is present in COS cells as well as intestinal epithelium.

Quantitative switch in integrin expression accompanies differentiation of F9 cells treated with retinoic acid

F9 embryonal carcinoma cells resemble epithelial cells when in monolayer culture. After treatment with retinoic acid these cells differentiate into fibroblastic-like cells in a sequence that has been modeled as the mammalian equivalent of the differentiation from stem cells of the inner cell mass to parietal endoderm. This study examined the changes in integrin subtypes that accompany retinoic acid-induced differentiation of F9 cells. Although several integrins were found to be present on the surface of F9 cells and retinoic acid-induced (RA) cells, the two dominant integrins were alpha 3 beta 1 and alpha 5 beta 1. Differentiation of F9 cells resulted in about 10- to 25-fold increase in the amount of alpha 3 beta 1 integrin protein as measured by immunoprecipitation of cell surface labeled material. There was a corresponding several-fold reduction of alpha 5 beta 1 protein. The concentration of alpha 3 mRNA was about the same in F9 and RA cells while the concentration of alpha 5 mRNA dropped several-fold after retinoic acid treatment. Thus alpha 3 regulation appeared to be largely posttranscriptional while the drop in alpha 5 protein may have been a result of transcriptional down-regulation. Quantitative measurement of adhesion suggested that most of the F9 and RA cell-substrate adhesion to fibronectin or laminin is mediated by these integrins. They are the dominant integrins present, and antibodies to either these integrins or to the substrate blocked the adhesion. Despite the large switch in integrin subtype protein expression there was little difference between the two cell types in initial cell interactions when adhesive affinities were measured quantitatively. Also there was no difference between the two phenotypes in rate of initial adhesive strengthening. The phenotypic difference was first observed with later events in the attachment and spreading of the RA-treated cells to the substrate. These results show that retinoic acid treatment alters the amounts of alpha 5 and alpha 3 integrin subunits during the F9 to RA phenotypic switch. The data show that these integrins are important in the cell-substrate adhesion to fibronectin and laminin. They show, however, that the phenotypic changes observed with differentiation are not associated with the initial preferential adhesions to the substrate, but rather with consequences that alter the cytoskeletal architecture of the cell.

An in vitro model for the analysis of intestinal brush border assembly. II. Changes in expression and localization of brush border proteins during cell contact-induced brush border assembly in Caco-2BBe cells

In the companion paper (M. D. Peterson and M. S. Mooseker (1993). J. Cell Sci. 105, 445–460) we describe a method for modeling brush border assembly in the Caco-2BBe clones. In this study we have examined the molecular changes accompanying cell contact-induced brush border assembly. A subset of brush border proteins was tracked throughout brush border assembly by immunoblotting and by immunofluorescent localization using laser scanning confocal microscopy. Actin, fodrin, villin and presumptive unconventional myosin immunogens were distributed at the periphery of depolarized cells. All proteins partitioned primarily with the membrane fraction upon differential sedimentation of depolarized cell lysates; the fractionation patterns were comparable to those of confluent cells. After a monolayer had formed, each protein showed a redistribution to the apical domain in a discrete sequence. Actin and villin began to shift apically at 2 d, while fodrin and the unconventional myosin immunogens did not redistribute until 3 d. Enterocyte-like localization was observed by 5 d for all proteins. Sucrase-isomaltase was not reliably detectable until 9 d by immunofluorescence, after brush border assembly was complete. Quantitative immunoblot analysis of total cell extracts demonstrated an average 10-fold increase in villin levels, while fodrin levels appeared to remain unchanged. Three putative unconventional myosin immunogens of 140 kDa, 130 kDa, and 110 kDa have been detected previously in the C2BBe cells with a head-specific monoclonal antibody to avian brush border myosin I (M. D. Peterson and M. S. Mooseker (1992) J. Cell Sci. 102, 581–600). Each of these immunogens displayed distinct expression patterns during brush border assembly. The 140 kDa species decreased by half, while the 130 kDa immunogen(s) did not change in any consistent fashion. The 110 kDa protein, presumed to be human brush border myosin I, rose on average 8-fold. A ribonuclease protection assay was also performed using a probe for human brush border myosin I. Equal amounts of total RNA from depolarized and confluent cells were assayed; the level of protected product was approximately 9-fold greater in the confluent cells. The expression patterns of the brush border proteins, coupled with the correlation to the ultrastructural features during brush border assembly in C2BBe cells, show that differentiation of the C2BBe cells closely resembles the changes that occur during human fetal intestinal differentiation.

Relative distribution of actin, myosin I, and myosin II during the wound healing response of fibroblasts

Myosin I is present in Swiss 3T3 fibroblasts and its localization reflects a possible involvement in the extension and/or retraction of protrusions at the leading edge of locomoting cells and the transport of vesicles, but not in the contraction of stress fibers or transverse fibers. An affinity-purified polyclonal antibody to brush border myosin I colocalizes with a polypeptide of 120 kD in fibroblast extracts. Within initial protrusions of polarized, migrating fibroblasts, myosin I exhibits a punctate distribution, whereas actin is diffuse and myosin II is absent. Myosin I also exists in linear arrays parallel to the direction of migration in filopodia and microspikes, established protrusions, and within the leading lamellae of migrating cells. Myosin II and actin colocalize along transverse fibers in the lamellae of migrating cells, while myosin I displays no definitive organization along these fibers. During contractions of actin-based fibers, myosin II is concentrated in the center of the cell, while the distribution of myosin I does not change. Thus, myosin I is found at the correct location and time to be involved in the extension and/or retraction of protrusions and the transport of vesicles. Myosin II-based contractions in more posterior cellular regions could generate forces to separate cells, maintain a polarized cell shape, maintain the direction of locomotion, maximize the rate of locomotion, and/or aid in the delivery of cytoskeletal/contractile subunits to the leading edge.

Identification, characterization and cloning of myr 1, a mammalian myosin-I

We have identified, characterized and cloned a novel mammalian myosin-I motor-molecule, called myr 1 (myosin-I from rat). Myr 1 exists in three alternative splice forms: myr 1a, myr 1b, and myr 1c. These splice forms differ in their numbers of putative calmodulin/light chain binding sites. Myr 1a-c were selectively released by ATP, bound in a nucleotide-dependent manner to F-actin and exhibited amino acid sequences characteristic of myosin-I motor domains. In addition to the motor domain, they contained a regulatory domain with up to six putative calmodulin/light chain binding sites and a tail domain. The tail domain exhibited 47% amino acid sequence identity to the brush border myosin-I tail domain, demonstrating that myr 1 is related to the only other mammalian myosin-I motor molecule that has been characterized so far. In contrast to brush border myosin-I which is expressed in mature enterocytes, myr 1 splice forms were differentially expressed in all tested tissues. Therefore, myr 1 is the first mammalian myosin-I motor molecule with a widespread tissue distribution in neonatal and adult tissues. The myr 1a splice form was preferentially expressed in neuronal tissues. Its expression was developmentally regulated during rat forebrain ontogeny and subcellular fractionation revealed an enrichment in purified growth cone particles, data consistent with a role for myr 1a in neuronal development.