The function of the major cytoskeletal components of the brush border

WASp is required for the correct temporal morphogenesis of rhabdomere microvilli

Microvilli are actin-based fingerlike membrane projections that form the basis of the brush border of enterocytes and the Drosophila melanogaster photoreceptor rhabdomere. Although many microvillar cytoskeletal components have been identified, the molecular basis of microvillus formation is largely undefined. Here, we report that the Wiskott-Aldrich syndrome protein (WASp) is necessary for rhabdomere microvillus morphogenesis. We show that WASp accumulates on the photoreceptor apical surface before microvillus formation, and at the time of microvillus initiation WASp colocalizes with amphiphysin and moesin. The loss of WASp delays the enrichment of F-actin on the apical photoreceptor surface, delays the appearance of the primordial microvillar projections, and subsequently leads to malformed rhabdomeres.

Role of two dileucine-like motifs in insulin receptor anchoring to microvilli

In the absence of ligand, the insulin receptor is maintained on microvilli on the cell surface. A dileucine motif (LL(986-987)) is necessary but not sufficient for this anchoring, which also required the presence of additional sequence(s) downstream of position 1000. The aim of the present study was to identify this (these) additional sequence(s). First, exons 16 or 17 were fused to the extracellular and transmembrane domains of complement receptor 1 and stably expressed in Chinese hamster ovary cells. Results obtained indicate that exon 17 is sufficient for anchoring to microvilli. Second, analysis of insulin receptor mutants truncated within exon 17 demonstrated that whereas receptors truncated at position 1000 showed no preferential association with microvilli, receptors truncated at position 1012 displayed a level of association identical to that of the full-length insulin receptor. Third, mutation of a diisoleucine motif (II(1006-1007)) present within this 12-amino acid stretch abrogated the preferential association of the receptor with microvilli. These results indicate that the domain required for association of insulin receptor with microvilli is contained within the region encoded by exon 17 and that, within this sequence, two dileucine-like motifs (LL(986-987) and II(1006-1007)) play a crucial role.

p56Lck anchors CD4 to distinct microdomains on microvilli

Cell-surface microvilli play a central role in adhesion, fusion, and signaling processes. Some adhesion and signaling receptors segregate on microvilli but the determinants of this localization remain mostly unknown. In this study, we considered CD4, a receptor involved in immune response and HIV infection, and p56(Lck), a CD4-associated tyrosine kinase. Analysis of CD4 trafficking reveals that p56(Lck) binds tightly to CD4 independently of its activation state and inhibits CD4 internalization. Electron microscopy analysis established that p56(Lck) mediates CD4 association with microvilli whereas biochemical data indicate that p56(Lck) expression renders CD4 insoluble by the nonionic detergent Triton X-100. In addition, cytoskeleton-disrupting agent increased CD4 solubility, suggesting the involvement of cytoskeletal elements in CD4 anchoring to microvilli. This concept was supported further by the observation that the lateral mobility of CD4 within the plasma membrane was decreased in cells expressing p56(Lck). Finally, isolation of detergent-resistant membranes revealed that the complex CD4-p56(Lck) is enriched within these domains as opposed to conditions in which CD4 does not interact with p56(Lck). In conclusion, our results show that p56(Lck) targets CD4 to specialized lipid microdomains preferentially localized on microvilli. This localization, which prevents CD4 internalization, might facilitate CD4-mediated adhesion processes and could correspond to the signaling site of the receptor.

Contribution of villous atrophy to reduced intestinal maltase in infants with malnutrition

BACKGROUND

It has been known for many years that small intestinal maltase activities are reduced in malnourished infants and in other patients with villous atrophy. The recent availability of human maltase-glucoamylase cDNA provides the opportunity to test the hypothesis that villous atrophy accounts for the reduced maltase enzyme activity in malnourished infants.

METHODS

Mucosal biopsy specimens obtained for clinical evaluation of malnourished infants with poor responses to refeeding were examined by quantitative methods for enzyme activity and mRNA levels.

RESULTS

Maltase activity and maltase-glucoamylase mRNA were reduced (approximately 45% of normal). When maltase-glucoamylase message was normalized to villin message, a structural protein expressed only in enterocytes, a preservation of maltase messages in surviving enterocytes was documented. The luminal glucose transporter-villin message was also preserved.

CONCLUSIONS

The loss of maltase-glucoamylase message paralleled the reduction in villin message and degree of villous atrophy. The reduced maltase-glucoamylase message also paralleled sucrase-isomaltase message, previously found to be decreased in proportion to villous atrophy of malnourished infants. The data directly demonstrate, for the first time, that the terminal steps of starch 1-4 starch digestion and sucrase-isomaltase 1-6 starch digestion are decreased in malnourished infants, secondary to villous atrophy. These data in prior and present reports suggest that mechanisms underlying the chronic villous atrophy of malnutrition should be a priority for investigations in malnourished infants with slower than expected weight gain during refeeding.

Pathophysiology and functional significance of apical membrane disruption during ischemia

The characteristic structure of polarized proximal tubule cells is drastically altered by the onset of ischemic acute renal failure. Distinctive apical brush border microvilli disruption occurs rapidly and in a duration-dependent fashion. Microvillar membranes internalize into the cytosol of the cell or are shed into the lumen as blebs. The microvillar actin core disassembles concurrent with or preceding these membrane changes. Actin and its associated binding proteins no longer interact to form these highly regulated apical membrane structures necessary for microvilli. The resultant epithelial cells have a reduced apical membrane surface that is not polarized either structurally, biochemically or physiologically. Furthermore, the changes in the apical microvilli result in tubular obstruction, reduced Na+ absorption, and partly explain the reduction in glomerular filtration rate. Recent evidence suggests these actin surface membrane alterations induced by ischemia are secondary to activation and relocation of the actin-associated protein, actin depolymerizing factor/cofilin, to the apical membrane domain. Activated (dephosphorylated) actin depolymerizing factor/cofilin proteins bind filamentous actin, increasing subunit treadmilling rates and filament severing. Once activated, the diffuse cytoplasmic distribution of the actin depolymerizing factor/cofilin protein relocalizes to the luminal membrane blebs. During recovery the actin depolymerizing factor/cofilin proteins are again phosphorylated and reassume their normal diffuse cytoplasmic localization. This evidence strongly supports the hypothesis that actin depolymerizing factor/cofilin proteins play a significant role in ischemia-induced injury in the proximal tubule cells.

Tissue and cell distribution of the multidrug resistance-associated protein (MRP) in mouse intestine and kidney

The multidrug resistance-associated protein (MRP) that is involved in drug resistance and the export of glutathione-conjugated substrates may not have the same epithelial cell membrane distribution as the P-glycoprotein encoded by the MDR gene. Because intestinal and kidney epithelial cells are polarized cells endowed distinct secreting and absorptive ion and protein transport capacities, we investigated the tissue and cell distribution of MRP in adult mouse small intestine, colon, and kidney by immunohistochemistry. Western blot analyses revealed the 190-kD MRP protein in these tissues. MRP was found in the basolateral membranes of intestinal crypt cells, mainly Paneth cells, but not in differentiated enterocytes. All the cells lining the crypt-villous axis of the colon wall contained MRP. MRP was found in the glomeruli, ascending limb cells, and basolateral membranes of the distal and collecting tubule cells of the kidney but not in proximal tubule cells. Cultured mouse intestinal m-ICcl2 cells and renal distal mpkDCT cells that have retained the features typical of intestinal crypt and renal distal epithelial cells, respectively, also possess MRP in their basolateral membranes. The patterns of subcellular and cellular distribution indicate that MRP may have a specific role in the basolateral transport of endogenous compounds in Paneth, renal distal, and collecting tubule cells.

Plasticity of the gastrointestinal epithelium: the M cell paradigm and opportunism of pathogenic microorganisms

The maintenance during adult life of a large spectrum of pluripotency by stem cells originating from the endoderm seems to be the grounds for the striking plasticity of the digestive epithelium, which is able to drastically modify its differentiation pattern depending on the microenvironment. As a paradigm, Peyer's patch M cell development appears to be induced by crosstalk between lymphoid cells and/or microorganisms. Examples of pathological transdifferentiation of epithelia, also described as 'metaplasia' and affecting various organs, support the concept of intestinal plasticity. Though, the molecular processes involved in epithelial transdifferentiation have not been identified, histological analyses of these metaplastic tissues and experimental induction of transdifferentiation of normal epithelia provide lines of evidence suggesting that a modification of the local environment, such as occurs during contact of the epithelium with lymphoid cells or microorganisms, plays a key role in this process.

Morphological transformation induced by activation of the mitogen-activated protein kinase pathway requires suppression of the T-type Ca2+ channel

Transformation of fibroblasts by various oncogenes, including ras, mos, and src accompanies with characteristic morphological changes from flat to round (or spindle) shapes. Such morphological change is believed to play an important role in establishing malignant characteristics of cancer cells. Activation of the mitogen-activated protein kinase (MAPK) pathway is a converging downstream event of transforming activities of many oncogene products commonly found in human cancers. Intracellular calcium is known to regulate cellular morphology. In fibroblasts, Ca2+ influx is primarily controlled by two types of Ca2+ channels (T- and L-types). Here, we report that the T-type current was specifically inhibited in cells expressing oncogenically activated Ras as well as gain-of-function mutant MEK (MAPK/extracellular signal-regulated kinase (ERK) kinase, a direct activator of MAPK), whereas treatment of ras-transformed cells with a MEK-specific inhibitor restored T-type Ca2+ channel activity. Using a T-type Ca2+ channel antagonist, we further found that suppression of the T-type Ca2+ channel by the activated MAPK pathway is a prerequisite event for the induction and/or maintenance of transformation-associated morphological changes.

CFTR regions containing duodenum specific DNase I hypersensitive sites drive expression in intestinal crypt cells but not in fibroblasts

We have investigated CFTR specific intestinal expression by transfection assays in mouse cultured fibroblasts and transimmortalized intestinal crypt m-ICc12 cells using the beta-galactosidase gene linked to rat CFTR non-coding regions. Two constructs were studied, one encompassing a 5.3 kb region 5' to the gene where numerous duodenum-specific DNase I hypersensitive sites (DHSs) were previously mapped and the other including a 1.3 kb 3' region in which novel DHSs had been identified. In transient transfection assays, transgenes were expressed in m-ICc12 cells but not in fibroblasts. In m-ICc12 cells, the pattern of expression of the chromosomally integrated transgenes paralleled the endogenous expression of CFTR and beta-galactosidase activity was detected in cells containing villin and forming domes. Thus, a 6.6 kb region encompassing 5' and 3' non-coding parts of rat CFTR is able to drive specific expression of a reporter gene in cultured mouse intestinal cells having kept a crypt phenotype.

Advillin (p92): a new member of the gelsolin/villin family of actin regulatory proteins

A new member of the gelsolin/villin family of actin regulatory proteins was initially identified by screening an adult murine brain cDNA library with a probe for bovine adseverin. The predicted amino acid sequence of the 92 kDa murine protein p92 (advillin) is 75% homologous to villin and 65% homologous to gelsolin and adseverin. It shares a six domain structure with other gelsolin family members and has a carboxy-terminal headpiece, similar to, yet distinct from, villin. Northern blot analysis shows a high level of mRNA expression in murine uterus and human intestine. In situ mRNA analysis of adult murine tissues demonstrates that the message is most highly expressed in the endometrium of the uterus, the intestinal lining, and at the surface of the tongue. In murine embryonic development, strong expression of the message is observed by day 14.5 in dorsal root ganglia and trigeminal ganglia. Expression is also noted at day 16.5 in cerebral cortex. We propose that p92 (advillin) has unique functions in the morphogenesis of neuronal cells which form ganglia, and that it may compensate to explain the near normal phenotype observed in villin-deficient mice.

Drosophila fascin mutants are rescued by overexpression of the villin-like protein, quail

Actin bundle assembly in specialized structures such as microvilli on intestinal epithelia and Drosophila bristles requires two actin bundling proteins. In these systems, the distinct biochemical properties and temporal localization of actin bundling proteins suggest that these proteins are not redundant. During Drosophila oogenesis, the formation of cytoplasmic actin bundles in nurse cells requires two actin bundling proteins, fascin encoded by the singed gene and a villin-like protein encoded by the quail gene. singed and quail mutations are fully recessive and each mutation disrupts nurse cell cytoplasmic actin bundle formation. We used P-element mediated germline transformation to overexpress quail in singed mutants and test whether these proteins have redundant functions in vivo. Overexpression of quail protein in a sterile singed background restores actin bundle formation in egg chambers. The degree of rescue by quail depends on the level of quail protein overexpression, as well as residual levels of fascin function. In nurse cells that contain excess quail but no fascin, the cytoplasmic actin network initially appears wild type but then becomes disorganized in the final stages of nurse cell cytoplasm transport. The ability of quail overexpression to compensate for the absence of fascin demonstrates that fascin is partially redundant with quail in the Drosophila germline. Quail appears to function as a bundle initiator while fascin provides bundle organization.

Prominin, a novel microvilli-specific polytopic membrane protein of the apical surface of epithelial cells, is targeted to plasmalemmal protrusions of non-epithelial cells

Using a new mAb raised against the mouse neuroepithelium, we have identified and cDNA-cloned prominin, an 858-amino acid-containing, 115-kDa glycoprotein. Prominin is a novel plasma membrane protein with an N-terminal extracellular domain, five transmembrane segments flanking two short cytoplasmic loops and two large glycosylated extracellular domains, and a cytoplasmic C-terminal domain. DNA sequences from Caenorhabditis elegans predict the existence of a protein with the same features, suggesting that prominin is conserved between vertebrates and invertebrates. Prominin is found not only in the neuroepithelium but also in various other epithelia of the mouse embryo. In the adult mouse, prominin has been detected in the brain ependymal layer, and in kidney tubules. In these epithelia, prominin is specific to the apical surface, where it is selectively associated with microvilli and microvilli-related structures. Remarkably, upon expression in CHO cells, prominin is preferentially localized to plasma membrane protrusions such as filopodia, lamellipodia, and microspikes. These observations imply that prominin contains information to be targeted to, and/or retained in, plasma membrane protrusions rather than the planar cell surface. Moreover, our results show that the mechanisms underlying targeting of membrane proteins to microvilli of epithelial cells and to plasma membrane protrusions of non-epithelial cells are highly related.

A complete epithelial organization of Caco-2 cells induces I-FABP and potentializes apolipoprotein gene expression

The culture of Caco-2 cells on plastic support impairs the expression of several genes involved in lipid metabolism. We describe culture conditions that permit the expression of the I-FABP gene and better expression of the apolipoprotein A-I, C-III, and A-IV genes. Basal lamina deposited on filters as well as the nature of nutrients on the apical side differentially modulated mRNA expression of I-FABP, APOBEC-1, and apolipoprotein genes. Growing cells on a filter led to functional polarization, illustrated by a secretion of apo B at the basal side, which induced the expression of the I-FABP, APOBEC-1, and apo A-IV genes and highly increased the expression of the apo C-III gene. Moreover, basal lamina deposited on the filter enhances the mRNA expression of apo A-I. Apo C-III and A-IV mRNA levels were decreased when cells were grown on a filter covered with basal lamina in the presence of a medium deprived of protein and lipid on the apical side, whereas these conditions had no effect on I-FABP, apo A-I, and APOBEC-1 mRNA levels. The addition of lipid micelles on the apical side had various effects, according to the genes. Caco-2 cells cultured under the conditions described here closely resembled enterocytes and represent a useful tool for studying the regulation of genes involved in lipid metabolism.

Epithelial cell growth and differentiation. IV. Controlled spatiotemporal expression of transgenes: new tools to study normal and pathological states

The gut epithelium represents a dynamic, well-organized developmental system for examining self-renewal, differentiation, repair, and tumorigenesis. The apical pole of the enterocytes, the brush border, is composed of an array of well-organized actin microfilaments that support the plasma membrane. Villin, one actin-binding protein that contributes to the assembly and dynamics of the microvillus bundle, exhibits special features such as restricted tissue specificity and early expression in the immature crypt cells. The regulatory elements of the villin gene are suitable to control the expression of transgenes in intestinal cells. Engineering genetically modified animals by classic transgenesis using the villin promoter or by gene targeting in the villin locus will allow the establishment of animal models that may recapitulate human intestinal disorders.

The structure, function, and assembly of actin filament bundles

The cellular organization, function, and molecular composition of selected biological systems with prominent actin filament bundles are reviewed. An overall picture of the great variety of functions served by actin bundles emerges from this overview. A unifying theme is that the actin cross-linking proteins are conserved throughout the eukaryotic kingdom and yet assembled in a variety of combinations to produce actin bundles of differing functions. Mechanisms of actin bundle formation in vitro are considered illustrating the variety of physical and chemical driving forces in this exceedingly complex process. Our limited knowledge regarding the formation of actin filament bundles in vivo is contrasted with the elegant biophysical studies performed in vitro but nonetheless reveals that interactions with membranes, nucleation sites, and other organizational components must contribute to formation of actin bundles in vivo.

Destabilization of plasma membrane structure by prevention of actin polymerization. Microtubule-dependent tubulation of the plasma membrane

Electron microscopy of thick (0.2-1.0 micron) sections of cytochalasin D-treated cells fixed in the presence of Ruthenium red revealed an extensive, surface-connected tubular compartment in HEp-2 cells. The tubules measured 120–220 nm in diameter and at least up to 6 microns in length. Morphometric analysis showed that in control cells about 0.2% of the total plasma membrane area (defined as all Ruthenium red-labeled membrane) appeared as vesicular or tubular profiles beneath the cell surface. However, after 15–30 minutes of cytochalasin D incubation about 4% of the total plasma membrane area is tubulated, and after 60–105 minutes as much as about 15% of the total plasma membrane appears as tubules. Clathrin-coated pits and caveolae-like structures were occasionally associated with the tubular membrane. Moreover, immunogold labeling showed that the tubular membrane contained transferrin receptors at about the same density as the nontubulated plasma membrane. Examination of cells in which endosomes and lysosomes were labeled with horseradish peroxidase before or after exposure to cytochalasin D showed that these organelles remained spherical, and that no horseradish peroxidase was present in the tubules. Moreover, the surface to volume ratio remained constant with increasing time of cytochalasin D incubation. Accordingly, the surface-connected tubules were not derived from endocytic structures but were formed by invagination of the plasma membrane. The tubule formation is reversible. When microtubules are depolymerized by nocodazole or colchicine treatment before the cells are exposed to cytochalasine D, tubule formation is strongly inhibited. Hence, the cytochalasin D-induced plasma membrane tubulation depends on intact microtubules.

Pathogenicity of the diffusely adhering strain Escherichia coli C1845: F1845 adhesin-decay accelerating factor interaction, brush border microvillus injury, and actin disassembly in cultured human intestinal epithelial cells

The diffusely adhering Escherichia coli strain C1845 harboring the fimbrial F1845 adhesin can infect cultured human intestinal epithelial cells. The mechanism by which E. coli C1845 induces diarrheal illness remains unknown. This study investigated the injuries of cultured human intestinal cells promoted by E. coli C1845. Membrane-associated decay accelerating factor was identified as the intestinal receptor for the F1845 fimbrial adhesin of the E. coli C1845 strain by using purified F1845 adhesin, antibody directed against the F1845 adhesin, and monoclonal antibodies directed against the decay accelerating factor. Using monolayers of Caco-2 cells apically infected with E. coli C1845 and examined by scanning and transmission electron microscopy, we observed that strain C1845 induced injury to microvilli (MV) characterized by elongation and nucleation of the MV. We observed that infection of T84 and Caco-2 cells by E. coli C1845 was followed by disassembly of the actin network in the apical and basal cell domains. MV injury was differentiation related: E. coli C1845 promoted MV injury only when the cells were fully differentiated. The disassembly of the actin network occurred in poorly differentiated and fully differentiated Caco-2 cells but not in undifferentiated cells. Moreover, apical actin disassembly was observed in fully differentiated Caco-2 cells infected with the laboratory strain E. coli HB101(pSSS1) expressing the F1845 adhesin. In conclusion, E. coli C1845 promotes MV lesion in human epithelial intestinal cells, resulting from disassembly of the actin network.

Establishment of hepatic cell polarity in the rat hepatoma-human fibroblast hybrid WIF-B9. A biphasic phenomenon going from a simple epithelial polarized phenotype to an hepatic polarized one

By immunofluorescence and freeze fracture methods, we have studied the establishment of hepatic cell polarity in WIF-B9 cells, a subclone of the WIF-B rat hepatoma-derived hybrid cell line. As previously shown (Ihrke et al. (1993) J. Cell Biol. 123, 1761–1775; Shanks et al. (1994) J. Cell Sci. 107, 813–825), these cells are a suitable model for in vitro studies of various hepatic functions, particularly polarity: in confluent cultures, the majority of cells form bile canaliculus-like structures; membrane domains are settled, according to plasma membrane protein localization similar to rat hepatocytes in situ. We here report that the establishment of WIF-B9 cell polarity is a slow progressive biphasic phenomenon. During the first days of culture, the majority of cells do not make bile canaliculus-like structures. However, they display a polarity similar to that of simple epithelial cells: apical membrane proteins and villin are found at the cell apex; basolateral ones, excluded from this area, are expressed in the remaining membrane area; the tight junction-associated protein ZO-1 and actin are concentrated at the boundary of these two poles, whereas E-cadherin is present at the lateral pole just under the apex. With time in culture, the number of cells expressing this simple epithelial polarized phenotype decreases progressively and, after 10–15 days, depending on the plating density, nearly all the cells express the typical hepatic polarized phenotype. The expression of these two phenotypes is mutually exclusive. Freeze-fracture replicas of both types of polarized cells show either macula occludens, fascia occludens (simple epithelial polarity) or zonula occludens (hepatic polarity), associated with gap junctions. In this last case, two or three continuous strands are generally present all around the bile canaliculus-like structures.

Major epiplasmic proteins of ciliates are articulins: cloning, recombinant expression, and structural characterization

The cytoskeleton of certain protists comprises an extensive membrane skeleton, the epiplasm, which contributes to the cell shape and patterning of the species-specific cortical architecture. The isolated epiplasm of the ciliated protist Pseudomicrothorax dubius consists of two major groups of proteins with molecular masses of 78-80 kD and 11-13 kD, respectively. To characterize the structure of these proteins, peptide sequences of two major polypeptides (78-80 kD) as well as a cDNA representing the entire coding sequence of a minor and hitherto unidentified component (60 kD; p60) of the epiplasm have been determined. All three polypeptides share sequence similarities. They contain repeated valine- and proline-rich motifs of 12 residues with the consensus VPVP--V-V-V-. In p60 the central core domain consists of 24 tandemly repeated VPV motifs. Within the repeat motifs positively and negatively charged residues, when present, show an alternating pattern in register with the V and P positions. Recombinant p60 was purified in 8 M urea and dialyzed against buffer. Infrared spectroscopic measurements indicate 30% beta-sheet. Electron microscopy reveals short filamentous polymers with a rather homogenous diameter (approximately 15-20 nm), but variable lengths. The small polymers form thicker filaments, ribbons, and larger sheets or tubes. A core domain similar to that of P. dubius p60 is also found in the recently described epiplasmic proteins of the flagellate Euglena, the so-called articulins. Our results show that the members of this protein family are not restricted to flagellates, but are also present in the distantly related ciliates where they are major constituents of the epiplasm. Comparison of flagellate and ciliate articulins highlights common features of this novel family of cytoskeletal proteins.

Transcriptional regulation of the ezrin gene during rat intestinal development and epithelial differentiation

Polarized intestinal epithelial cells are characterized by the presence of a brush border at their apical surface. The brush border cytoskeleton is assembled during cell differentiation and is composed of parallel actin bundles, held together by specific actin-binding proteins. Using specific cDNA probes we have studied the expression of the mRNAs encoding ezrin and moesin, two members of a class of proteins that connect the microvillar cytoskeleton to the plasma membrane, during the process of enterocyte maturation that occurs both in the embryonic and in the adult small intestine, along the crypt-villus axis. The steady state levels of ezrin mRNA were found to increase in the fetal gut epithelium between day 15 and day 20 of gestation and during the first week after birth, in parallel with the morphogenetic process that leads to cell polarization and brush border assembly. On the contrary, moesin mRNA is expressed at very low levels in the mature small intestine, with a sudden drop in transcription occurring at birth. In the continuously renewing epithelium of adult animals, ezrin mRNA levels are higher in the differentiated villus cells of the distal portions of the gastrointestinal tract and very low in undifferentiated crypt cells. These data demonstrate that the expression of the ezrin gene is regulated at the level of mRNA abundance during development and differentiation of the intestinal epithelium.

Formation of stable microspikes containing actin and the 55 kDa actin bundling protein, fascin, is a consequence of cell adhesion to thrombospondin-1: implications for the anti-adhesive activities of thrombospondin-1

The organisation of the actin cytoskeleton was examined in H9c2 and human intestinal smooth muscle cells adherent on fibronectin or thrombospondin-1. Whereas cells adherent on fibronectin adopted a polygonal shape and rapidly assembled prominent stress fibres and focal contacts, cells adherent on thrombospondin-1 assumed a more irregular morphology with large lamellae containing radial actin microspikes. Focal contacts were not detected in cells adherent on thrombospondin-1, as determined by indirect immunofluorescence staining for vinculin and other focal contact components. Instead, the radial microspikes stained positively for the actin-bundling protein, 55 kDa/fascin, and myosins. In cells adherent on fibronectin, 55 kDa/fascin immunoreactivity was diffuse and tended to be concentrated in the perinuclear region. In long-term adherent cells cultured in serum-containing medium, 55 kDa/fascin was detected in membrane ruffles, in stress fibres and in the perinuclear region. The microspikes formed within 40 minutes of plating cells on thrombospondin-1 and remained present when cells were treated with sodium orthovandate and hydrogen peroxide to increase intracellular phosphotyrosine levels. Indeed, although vanadate-treated cells tended to retract, the microspikes became more prominent and showed an increased intensity of staining for fascin. Under these conditions, a proportion of the microspikes did not appear to be in contact with the substratum: these spikes stained weakly for focal adhesion kinase, talin and vinculin. Cells treated with genistein also spread and formed fascin-containing microspikes which tended to be more slender than those of control cells. In contrast, cells adherent on fibronectin displayed a complex rearrangement of the actin cytoskeleton and a transient enrichment of 55 kDa/fascin-containing structures at the cell surface when treated with sodium orthovanadate and hydrogen peroxide. These observations indicate that cell interactions with fibronectin or thrombospondin-1 send distinct organisational signals to the actin cytoskeleton and may offer a mechanistic framework for further investigations of the anti-adhesive properties of thrombospondin-1.

Cytoskeletal functions during Drosophila oogenesis

Organismal morphogenesis is driven by a complex series of developmentally coordinated changes in cell shape, size, and number. These changes in cell morphology are in turn dependent on alterations in basic cytoarchitecture. Elucidating the mechanisms of development thus requires an understanding of the cytoskeletal elements that organize the cytoplasm of differentiating cells. Drosophila oogenesis has emerged as a versatile system for the study of cytoskeletal function during development. A series of highly coordinated changes in cytoskeletal organization are required to produce a mature Drosophila oocyte, and these cytoskeletal transformations are amenable to a variety of experimental approaches. Genetic, molecular, and cytological studies have shed light on the specific functions of the cytoskeleton during oogenesis. The results of these studies are reviewed here, and their mechanistic implications are considered.

Exploitation of microfilament proteins by Listeria monocytogenes: microvillus-like composition of the comet tails and vectorial spreading in polarized epithelial sheets

Effective cell-to-cell spreading of the facultative intracellular pathogen Listeria monocytogenes requires the interaction between bacteria and the microfilament system of the host cell. By recruiting actin filaments into a ‘comet tail’ localized at one pole of the bacterial cell wall, Listeria become mobile and propel themselves through the cytoplasm. They create protrusions at the plasma membrane that can invaginate adjacent cells. In this work, we have analysed the structural composition of Listeria-recruited microfilaments in various epithelial cell lines by immunofluorescence microscopy. The microfilament-crosslinking proteins alpha-actinin, fimbrin and villin were localized around bacteria as soon as actin filaments could be detected on the bacterial surface. Surprisingly, the same was found for ezrin/radixin, proteins involved in linking microfilaments to the plasma membrane. We found that in a polarized cell line derived from brush border kidney epithelium (LLC-PK1), the actin filaments surrounding intracytoplasmic motile bacteria show the same immunoreactivity as the brush border-like microvilli, when analysed by a specific actin antibody. The successful invasion of polarized LLC-PK1 islets is vectorial, i.e. it progresses predominantly from the periphery of the islets towards the centre. Infection of the peripheral cells is sufficient for infiltration of the entire cellular islets, without any further contact with the extracellular milieu. This is in contrast to nonpolarized epithelial sheets, which can be invaded from the apical surface of any individual cell. The importance of active bacterial motility in this vectorial spreading is emphasized by our finding that an isogenic Listeria mutant that is unable to recruit actin filaments cannot colonize polarized epithelial layers but accumulates in the peripheral cells of the islets.

Ezrin has a COOH-terminal actin-binding site that is conserved in the ezrin protein family

Ezrin, previously also known as cytovillin, p81, and 80K, is a cytoplasmic protein enriched in microvilli and other cell surface structures. Ezrin is postulated to have a membrane-cytoskeleton linker role. Recent findings have also revealed that the NH2-terminal domain of ezrin is associated with the plasma membrane and the COOH-terminal domain with the cytoskeleton (Algrain, M., O. Turunen, A. Vaheri, D. Louvard, and M. Arpin. 1993. J. Cell Biol. 120: 129-139). Using bacterially expressed fragments of ezrin we now demonstrate that ezrin has an actin-binding capability. We used glutathione-S-transferase fusion proteins of truncated ezrin in affinity chromatography to bind actin from the cell extract or purified rabbit muscle actin. We detected a binding site for filamentous actin that was localized to the COOH-terminal 34 amino acids of ezrin. No binding of monomeric actin was detected in the assay. The region corresponding to the COOH-terminal actin-binding site in ezrin is highly conserved in moesin, actin-capping protein radixin and EM10 protein of E. multilocularis, but not in merlin/schwannomin. Consequently, this site is a potential actin-binding site also in the other members of the protein family. Furthermore, the actin-binding site in ezrin shows sequence homology to the actin-binding site in the COOH terminus of the beta subunit of the actin-capping protein CapZ and one of the potential actin-binding sites in myosin heavy chain. The actin-binding capability of ezrin supports its proposed role as a membrane-cytoskeleton linker.

Molecular motors are differentially distributed on Golgi membranes from polarized epithelial cells

Microtubules (MT) are required for the efficient transport of membranes from the trans-Golgi and for transcytosis of vesicles from the basolateral membrane to the apical cytoplasm in polarized epithelia. MTs in these cells are primarily oriented with their plus ends basally near the Golgi and their minus-ends in the apical cytoplasm. Here we report that isolated Golgi and Golgi-enriched membranes from intestinal epithelial cells possess the actin based motor myosin-I, the MT minus-end-directed motor cytoplasmic dynein and its in vitro motility activator dynactin (p150/Glued). The Golgi can be separated into stacks, possessing features of the Golgi cisternae, and small membranes enriched in the trans-Golgi network marker TGN 38/41. Whereas myosin-I is present on all membranes in the Golgi fraction, dynein is present only on the small membrane fraction. Dynein, like myosin-I, is associated with membranes as a cytoplasmic peripheral membrane protein. Dynein and myosin-I coassociate with membranes that bind to MTs and cross-link actin filaments and MTs in a nucleotide-dependent manner. We propose that cytoplasmic dynein moves Golgi membranes along MTs to the cell cortex where myosin-I provides local delivery through the actin-rich cytoskeleton to the apical membrane.

The villin-like protein encoded by the Drosophila quail gene is required for actin bundle assembly during oogenesis

Mutations in the Drosophila quail gene result in female sterility due to the disruption of cytoplasmic transport from the nurse cells into the oocyte late in oogenesis. Nurse cells from quail mutant egg chambers fail to assemble cytoplasmic actin filament bundles correctly. We have cloned the quail gene and found that it encodes a protein with homology to the vertebrate actin-regulating protein villin. Unlike vertebrate villin, which is restricted to specialized absorptive epithelial cells, the villin-like protein encoded by quail is germline specific in adult flies. Antibodies directed against the quail protein show a striking colocalization with filamentous actin in the nurse cells and the oocyte. Our results demonstrate that the villin-like product of quail is required for the formation of cytoplasmic actin filament bundles in nurse cells, possibly by regulating both the polymerization and organization of actin filaments as demonstrated for vertebrate villin in vitro.

Brush border myosin-I microinjected into cultured cells is targeted to actin-containing surface structures

The isolated intestinal microvillus cytoskeleton (core) consists of four major proteins: actin, villin, fimbrin and brush border myosin-I. These proteins can assemble in vitro into structures resembling native microvillus cores. Of these components, villin and brush border myosin-I show tissue-specific expression, so they may be involved in the morphogenesis of intestinal microvilli. When introduced into cultured cells that normally lack the protein, villin induces a reorganization of the actin filaments to generate large surface microvilli. Here we examine the consequences of microinjecting brush border myosin-I either alone or together with villin into cultured fibroblasts. Injection of brush border myosin-I has no discernible effect on the overall morphology of the cells, but does become localized to either normal or villin-induced microvilli and other surface structures containing an actin cytoskeleton. Since some endogenous myosin-Is have been found associated with cytoplasmic vesicles, these results show that brush border myosin-I has a domain that specifically targets it to the plasma membrane in both intestinal and cultured cell systems. Ultrastructural examination of microvilli on control cultured cells revealed that they contain a far more highly ordered bundle of microfilaments than had been previously appreciated. The actin filaments in microvilli of villin-injected cells appeared to be more tightly cross-linked when examined by thin-section electron microscopy. In intestinal microvilli, the core bundle is separated from the plasma membrane by about 30 nm due to the presence of brush border myosin-I.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the role of actin filaments in regulating cell swelling

Actin filaments could play a role in regulation of cell swelling by two distinct mechanisms. One is by a tensile mechanism involving the coordinated interaction of actin and actin-associated proteins with all plasma membrane domains. The actin-membrane linkage would restrain cell swelling in the event of the influx of water. In shark rectal gland cells, conditions that cause massive cell swelling (i.e., high K medium, exposure to mercurials) are associated with disruption of membrane-associated actin filaments. Under conditions that result in only moderate swelling (Na-pump inhibition, Li substitution) the actin filaments remain associated with the cell membrane. Thus, in this cell type, disruption of the actin-membrane organization is correlated with increased swelling. Another mechanism by which actin could limit cell swelling is via regulation of ion transport proteins that are activated by cell swelling. This could be accomplished by a vesicle transport and insertion mechanism that delivers ion transport units to the cell membrane or by interaction with transport proteins already present in the membrane. Cell-attached patch clamp studies of RCCT-28A cells exposed to hypotonic medium demonstrated the activation of Cl-channel activity coincident with an alteration in actin. Activation of the channel was mimicked by stretching the membrane. Exposure of inside-out patches to cytochalasins also increased Cl-channel activity. Treatment of isolated patches with phalloidin inhibited stretch-induced activation. Thus, regulation of a volume-sensitive Cl-channel appears to be directly related to the state of organization of actin filaments.

Establishment of renal proximal tubule cell lines by targeted oncogenesis in transgenic mice using the L-pyruvate kinase-SV40 (T) antigen hybrid gene

Targeted oncogenesis allowed us to obtain two cell lines which have been derived from the proximal tubule of kidney from transgenic mice harbouring the simian virus (SV40) large T and small t antigens placed under the control of the 5′ regulatory sequence from the rat L-type pyruvate kinase (L-PK) gene. The cell lines (PKSV-PCT and PKSV-PR cells) were derived from early (PCT) and late (Pars Recta, PR) microdissected proximal tubules grown in D-glucose-enriched medium. In such conditions of culture, both cell lines exhibited L-PK transcripts, a stable expression of SV40-encoded nuclear large T antigen, a prolonged life span but failed to induce tumors when injected sub-cutaneously into athymic (nu-nu) mice. Confluent cells, grown on plastic support or porous filters, were organized as monolayers of polarized cuboid cells with well developed apical microvilli and formed domes. Both cell lines exhibited morphological features of proximal tubule cells with villin located in the apical brush-border and substantial amounts of hydrolase activity. By immunofluorescence studies using specific antibodies, aminopeptidase N appeared restricted to the apical microvillar domain, whereas the H2 histocompatibility antigen was distributed in the cytoplasm and lateral membranes. These results demonstrate that the proximal morphological phenotype has been fully preserved in these cultured cells derived from tissue-specific targeted oncogenesis in transgenic mice.

Actin microfilaments play a critical role in endocytosis at the apical but not the basolateral surface of polarized epithelial cells

Treatment with cytochalasin D, a drug that acts by inducing the depolymerization of the actin cytoskeleton, selectively blocked endocytosis of membrane bound and fluid phase markers from the apical surface of polarized MDCK cells without affecting the uptake from the basolateral surface. Thus, in MDCK cell transformants that express the VSV G protein, cytochalasin blocked the internalization of an anti-G mAb bound to apical G molecules, but did not reduce the uptake of antibody bound to the basolateral surface. The selective effect of cytochalasin D on apical endocytosis was also demonstrated by the failure of the drug to reduce the uptake of 125I-labeled transferrin, which occurs by receptor-mediated endocytosis, via clathrin-coated pits, almost exclusively from the basolateral surface. The actin cytoskeleton appears to play a critical role in adsorptive as well as fluid phase apical endocytic events, since treatment with cytochalasin D prevented the apical uptake of cationized ferritin, that occurs after the marker binds to the cell surface, as well as uptake of Lucifer yellow, a fluorescent soluble dye. Moreover, the drug efficiently blocked infection of the cells with influenza virus, when the viral inoculum was applied to the apical surface. On the other hand, it did not inhibit the basolateral uptake of Lucifer yellow, nor did it prevent infection with VSV from the basolateral surface, or with influenza when this virus was applied to monolayers in which the formation of tight junctions had been prevented by depletion of calcium ions. EM demonstrated that cytochalasin D leads to an increase in the number of coated pits in the apical surface where it suppresses the pinching off of coated vesicles. In addition, in drug-treated cells cationized ferritin molecules that were bound to microvilli were not cleared from the microvillar surface, as is observed in untreated cells. These findings indicate that there is a fundamental difference in the process by which endocytic vesicles are formed at the two surfaces of polarized epithelial cells and that the integrity and/or the polymerization of actin filaments are required at the apical surface. Actin filaments in microvilli may be part of a mechanochemical motor that moves membrane components along the microvillar surface towards intermicrovillar spaces, or provides the force required for converting a membrane invagination or pit into an endocytic vesicle within the cytoplasm.

Amphibian intestinal villin: isolation and expression during embryonic and larval development

An actin-binding protein of M(r) 105,000 has been isolated from anuran amphibian intestinal mucosa. Polyclonal antibodies directed against chicken and pig intestinal villins and anti-porcine villin headpiece monoclonal antibody crossreact with the amphibian M(r) 105,000 protein. Furthermore, the latter possesses an NH2-terminal sequence that is very homologous to those of avian and mammalian villins. In addition, polyclonal antibodies directed against amphibian intestinal M(r) 105,000 protein crossreact with chicken and mouse intestinal epithelial cell villins. These data indicate that the amphibian intestinal M(r) 105,000 protein is immunologically and structurally related to villin, an actin-binding protein expressed in specific epithelial tissues in vertebrates. Morphological, immunocytochemical and immunoblotting techniques were then used to investigate the expression of villin during embryonic and larval intestinal development of Xenopus laevis. Villin is not found in the egg or the endoderm of the early embryo. It is first detected just before hatching in the apical domain of endodermal cells at a time when few surface microvilli are visible by transmission electron microscopy. In the newly hatched larva, villin accumulates as these cells differentiate. These results provide a detailed developmental profile of Xenopus intestinal villin expression and demonstrate that this protein is a useful marker for the presumptive intestinal endoderm.

Localization of capping protein in chicken epithelial cells by immunofluorescence and biochemical fractionation

We have localized capping protein in epithelial cells of several chicken tissues using affinity-purified polyclonal antibodies and immunofluorescence. Capping protein has a distribution in each tissue coincident with proteins of the cell-cell junctional complex, which includes the zonula adherens, zonula occludens, and desmosome. "En face" views of the epithelial cells showed capping protein distributed in a polygonal pattern coincident with cell boundaries in intestinal epithelium, sensory epithelium of the cochlea, and the pigmented epithelium of the retina and at regions of cell-cell contact between chick embryo kidney cells in culture. "Edge-on" views obtained by confocal microscopy of intact single intestinal epithelial cells and of retinal pigmented epithelium showed that capping protein is located in the apical region of the epithelial cells coincident with the junctional complexes. These images do not resolve the individual types of junctions of the junctional complex. Immunolabeling of microvilli or stereocilia was faint or not detectable. Capping protein was also detected in the cytoplasm of intact intestinal epithelial cells and in nuclei of cells in the pigmented retina and in the kidney cell cultures, but not in nuclei of cells of the intestinal epithelium or sensory epithelium. Biochemical fractionation of isolated intestinal epithelial cells shows capping protein in the brush border fraction, which contains the junctional complexes, and in the soluble fraction. These results are consistent with the results of the immunolabeling experiments. Highly purified microvilli of the brush borders also contained capping protein; this result was unexpected based on the low intensity of immunofluorescence staining of microvilli and stereocilia. The microvilli were not contaminated with junctional complexes, as defined by the absence of several markers for cell junctions. The cause and significance of this discrepancy is not certain at this time. Since capping protein binds the barbed end of actin filaments in vitro, we hypothesize that capping protein is bound to the barbed ends of actin filaments associated with one or more of the junctions of the junctional complex.

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

F9 embryonic carcinoma cells are a multipotent cell line which can be induced to differentiate into cells resembling the visceral endoderm, an extraembryonic absorptive epithelium characterized by apical microvilli. We have examined the role of villin, fimbrin, and myosin I, the major actin-binding proteins in the intestinal and visceral yolk sac microvilli, in the development of epithelial polarity and the assembly of the microvillus cytoskeleton in differentiating F9 cells. By immunoblot analysis villin was first detected at 4 days of differentiation. Confocal microscopy localized villin at Day 4 to the apical surface and by Day 6 to the basolateral surfaces as well. In comparison, fimbrin and myosin I were both present in undifferentiated F9 cells and became associated with the apical surface after villin during differentiation to visceral endoderm. The accumulation of villin, fimbrin, and myosin I at the apical surface in differentiating F9 cells correlated with the appearance of microvilli containing organized actin filament bundles. Two mouse villin cDNAs were isolated and characterized to examine villin expression during F9 differentiation. Mouse villin was encoded by two transcripts (3.8 and 3.4 kb) which differ in their 3'-noncoding region. Both villin mRNAs were first detected by Day 4 of differentiation and their appearance coincided with expression of the visceral endoderm marker alpha-fetoprotein. The pattern of expression and order of accumulation of villin, fimbrin, and myosin I in differentiating F9 cells are common to developing gut and yolk sac epithelium. This suggests that microvillus assembly is directed by a sequence of temporally and spatially regulated localizations of these actin-binding proteins.

Localization of actin in the retina of the crayfishProcambarus clarkii

The distribution of actin in the retina of the crayfish was investigated at the LM level using FITC-phalloidin. Fluorescent staining was associated with the main rhabdom and eighth cell rhabdom, the zonula adherens junctions between retinula cells, and the basement membrane of the retina. EM and S1 decoration were used to confirm the presence of actin and identify its structural relationships. Phalloidin staining of the rhabdom and S1 decoration of actin filaments in the rhabdom microvilli confirmed earlier findings that actin is a component of the microvillus cytoskeleton in the crayfish. At the zonula adherens junctions, actin filaments, identified by S1 decoration, run longitudinally within the plaque of the junction. At the extreme proximal end of the rhabdom, actin filaments associated with the junctions fill each small area of retinula cell cytoplasm. In the basement membrane, EM and S1 decoration show that basilar cells contain large bundles of actin filaments which are associated with cell-matrix adherens junctions. Foot cells which lie immediately below the rhabdom also contain similar junctions and actin is tentatively identified in these cells. The functional role of actin at these various locations is discussed in relation to retinal organization in the crayfish and other invertebrates.

Cytoskeleton and other differentiation markers in the colon

Differentiation of intestinal epithelial cells involves a complex process of establishment of cell polarity, commitment to cell lineage, and inhibition of cell division. Polarized epithelial cells are characterized by specific junctional complexes and cytoskeletal proteins which produce specific membrane domains. Intestinal cytoskeletal proteins are often preserved in neoplastic colonic tissues, and can be used to identify the cell of origin of poorly differentiated cancers. In this context, these proteins are markers of organ-specific differentiation. In addition, since loss of cytoskeletal polarity commonly occurs in transformed cells, aberrant expression of these proteins may be used as a marker of neoplasia in the colon. Normal polarization of basolateral proteins (secretory component) and apical proteins such as brush border hydrolases, cytoskeletal proteins (villin, fodrin), and carcinoembryonic antigen can become disrupted in adenomas and cancers. Cytoskeletal intermediate filaments (cytokeratins) demonstrate increased immunoreactivity in villous adenomas and cancers compared with normal colonic crypts. Altered actin bundles are found in preneoplastic mucosa such as colon from patients with familial polyposis coli. Molecular mechanisms responsible for altered cytoskeletal structures remain unclear; however, altered protein phosphorylation most likely plays a role. For example, the phosphorylation status of cytoskeletal and junctional complex proteins appears to influence their solubility and interactive properties, which may result in altered cell polarity. Markers of altered cytoskeletal structure and polarity can identify neoplastic colonocytes; however, the extent to which they can be used as intermediate markers of colonic neoplasia remains to be determined.

Modular organization of actin crosslinking proteins

A family of actin-crosslinking proteins share a conserved 125 residue sequence that lies within a 250 residue actin-binding domain. This domain is combined with spacer segments consisting of a variable number of repeated alpha-helical or beta-sheet motifs and other functional domains, which generate proteins that differ in their ability to form actin bundles or networks and to associate with the plasma membrane. These functional domains are not in other actin-crosslinking proteins, one of which is elongation factor 1a (EF-1a) suggesting there are several pathways for the evolution of actin-crosslinking function.