Microtubule distribution in cultured cells and intact tissues: improved immunolabeling resolution through the use of reversible embedment cytochemistry

Characterization of human Ccser2 as a protein tracking the plus-ends of microtubules

OBJECTIVE

Microtubules, which are closely related to cell proliferation, have been the promising therapeutic target of cancer. Therefore, it is necessary to understand the intracellular control mechanisms of microtubules, the whole picture of which is still unclear though. Intracellular dynamics of microtubules are regulated by various microtubule-associated proteins, one group of which is microtubule plus-end-tracking proteins (+ TIPs), localizing to the extending tips of microtubules. Here, we report the identification and analysis of Ccser2 as a new + TIP in human breast cancer MCF-7 cells.

RESULTS

Ccser2 was found to be a member of + TIPs by microscopic observations including time-lapse imaging. The C-terminal region of Ccser2, including two SxIP motifs, was likely to be important for the tracking function. In MCF-7 cells, endogenous Ccser2 was mainly detected in the peripheral regions of microtubule fibers, suggesting that Ccser2 functions in cell projections.

The murine Gcap14 gene encodes a novel microtubule binding and bundling protein

Microtubules form flexible fibers, which are utilized in cell proliferation and differentiation. Although the flexibility of microtubules was shown to be regulated by various microtubule-associated proteins, this regulation is still far from complete understanding. Here, we report a new potential regulator of microtubules in mammals. Gcap14 colocalizes with microtubules in mammalian cells transfected with Gcap14 expression vector. Association of Gcap14 with microtubules was confirmed by biochemical subcellular fractionation. Recombinant Gcap14 protein cosedimented with pure microtubules, indicating a direct binding between the two. Furthermore, recombinant Gcap14 was shown to have the ability of inducing microtubule bundling in vitro.

A generative model of microtubule distributions, and indirect estimation of its parameters from fluorescence microscopy images

The microtubule network plays critical roles in many cellular processes, and quantitative models of how its organization varies across cell types and conditions are required for understanding those roles and as input to cell simulations. High-throughput image acquisition technologies are potentially valuable for this purpose, but do not provide sufficient resolution for current analysis methods that rely on tracing of individual microtubules. We describe a parametric conditional model of microtubule distribution that can generate a microtubule network in intact cells using a persistent random walk approach. The model parameters are physically meaningful as they directly describe the spatial distribution of microtubules and include the number of microtubules as well as the mean of the length distribution. We also present an indirect method for estimating the parameters of the model from three-dimensional fluorescence microscope images of cells that relies on comparing acquired images with simulated images generated from the model. Our results show that our method can reasonably recover parameters for a given query image, and we present the distributions of parameters estimated by our method for a collection of HeLa cell images. (c) 2010 International Society for Advancement of Cytometry.

The actin cytoskeleton in whole mount preparations and sections

In non-muscle cells, the actin cytoskeleton plays a key role by providing a scaffold contributing to the definition of cell shape, force for driving cell motility, cytokinesis, endocytosis, and propulsion of pathogens, as well as tracks for intracellular transport. A thorough understanding of these processes requires insight into the spatial and temporal organisation of actin filaments into diverse higher-order structures, such as networks, parallel bundles, and contractile arrays. Transmission and scanning electron microscopy can be used to visualise the actin cytoskeleton, but due to the delicate nature of actin filaments, they are easily affected by standard preparation protocols, yielding variable degrees of ultrastructural preservation. In this chapter, we describe different conventional and cryo-approaches to visualise the actin cytoskeleton using transmission electron microscopy and discuss their specific advantages and drawbacks. In the first part, we present three different whole mount techniques, which allow visualisation of actin in the peripheral, thinly spread parts of cells grown in monolayers. In the second part, we describe specific issues concerning the visualisation of actin in thin sections. Techniques for three-dimensional visualisation of actin, protein localisation, and correlative light and electron microscopy are also included.

Microtubule-dependent vesicle transport: modulation of channel and transporter activity in liver and kidney

Microtubule-based vesicle transport driven by kinesin and cytoplasmic dynein motor proteins facilitates several membrane-trafficking steps including elements of endocytosis and exocytosis in many different cell types. Most early studies on the role of microtubule-dependent vesicle transport in membrane trafficking focused either on neurons or on simple cell lines. More recently, other work has considered the role of microtubule-based vesicle transport in other physiological systems, including kidney and liver. Investigation of the role of microtubule-based vesicle transport in membrane trafficking in cells of the kidney and liver suggests a major role for microtubule-based vesicle transport in the rapid and directed movement of ion channels and transporters to and from the apical plasma membranes, events essential for kidney and liver function and homeostasis. This review discusses the evidence supporting a role for microtubule-based vesicle transport and the motor proteins, kinesin and cytoplasmic dynein, in different aspects of membrane trafficking in cells of the kidney and liver, with emphasis on those functions such as maintenance of ion channel and transporter composition in apical membranes that are specialized functions of these organs. Evidence that defects in microtubule-based transport contribute to diseases of the kidney and liver is also discussed.

AMF-R tubules concentrate in a pericentriolar microtubule domain after MSV transformation of epithelial MDCK cells

Autocrine motility factor receptor (AMF-R) is localized to an intracellular microtubule-associated membranous organelle, the AMF-R tubule. In well-spread untransformed MDCK epithelial cells, the microtubules originate from a broad perinuclear region and AMF-R tubules extend throughout the cytoplasm of the cells. In Moloney sarcoma virus (mos)-transformed MDCK (MSV-MDCK) cells, microtubules accumulate around the centrosome, forming a microtubule domain rich in stabilized detyrosinated microtubules. AMF-R tubules are quantitatively associated with this pericentriolar microtubule domain and the rough endoplasmic reticulum and lysosomes also co-distribute with the pericentriolar mass of microtubules. The Golgi apparatus is closely associated with the microtubule organizing center (MTOC) within the juxtanuclear mass of AMF-R tubules, and no co-localization of AMF-R tubules with the Golgi marker beta-COP could be detected by confocal microscopy. After nocodazole treatment and washout, microtubule nucleation occurs exclusively at the centrosome of MSV-MDCK cells, and only after microtubule extension to the cell periphery does the microtubule cytoskeleton reorganize to generate the pericentriolar microtubule domain after 30-60 min. AMF-R tubules dispersed by nocodazole treatment concentrate in the pericentriolar region in parallel with the reorganization of the microtubule cytoskeleton. MSV transformation of epithelial MDCK cells results in the stabilization of a pericentriolar microtubule domain responsible for the concentration and polarized distribution of AMF-R tubules.

Cytoskeletal organization in clusters of isolated polarized skate hepatocytes: structural and functional evidence for microtubule-dependent transcytosis

Isolated hepatocytes from the marine vertebrate Raja erinacea (the little skate) retain their structural and functional integrity as clusters of cells formed around a single tubular bile canaliculus, and therefore can be used as a model of polarized hepatocytes in situ. In this study we used confocal and conventional epifluorescence microscopy in conjunction with fluorescent markers and immunocytochemistry to examine the structure and function of the cytoskeleton in these cells. Actin filaments in the hepatocyte clusters were found cortically and also concentrated in a pericanalicular array, while microtubules appeared to radiate away from a concentration near the apical membrane of the biliary pole towards the basolateral sinusoidal surfaces. Treatment of clusters with the microtubule disrupting agent, nocodazole, resulted in the microtubules depolymerizing from the basolateral surfaces towards the apical surface, indicating that the microtubules were oriented with their plus ends at the basolateral surface and their minus ends at the apical surface. Nocodazole was also found to disrupt the ability of clusters to transcytose a fluorescent bile salt derivative into their canalicular lumens. We detected cytoplasmic dynein in skate hepatocyte homogenates by Western blotting using an anti-dynein intermediate chain antibody, and immunofluorescent staining of intact hepatocytes revealed a punctate vesicular pattern. The polarized arrangement of microtubules, the presence of cytoplasmic dynein, and the inhibition of bile salt secretion by nocodozole are consistent with the microtubule cytoskeleton playing a fundamental role in the mediation of transcytosis, endocytosis, and bile excretory function in these hepatocytes. These polarized isolated skate hepatocytes represent an excellent experimental model for the in vitro study of hepatic transport, and allow for important comparative studies aimed at elucidating the evolutionarily conserved nature of various hepatocyte structures amongst the vertebrates.

Polarity and nucleation of microtubules in polarized epithelial cells

Microtubules oriented in the apicobasal axis of columnar epithelial cells are arranged with a uniform polarity with minus ends toward the apical surface, suggesting that these cytoskeletal filaments might serve as a substrate for polarized movement of membrane vesicles within the cell. It is not known whether hepatocytes, a cuboidal epithelium in which transcellular transport is a requisite step in normal apical membrane biogenesis, contain microtubules arranged with a similar polarity. In the present study, we explore the question of microtubule polarity and possible mechanisms for nucleation in the epithelial cell lines WIF-B (hepatocyte), Caco-2 (intestine), and Madin-Darby canine kidney (MDCK). Caco-2 microtubules in the apicobasal axis had uniform polarity with minus ends nearest the apical surface. After cold and nocodazole-induced depolymerization, microtubule regrowth initiated in the apical region in all three cell types. The apex of WIF-B and Caco-2 cells contained two pools of gamma-tubulin: one associated with centrosomes and the other delocalized under the apical membrane. Non-centrosomal gamma-tubulin was present in complexes that sedimented between 10S and 29S; both forms could bind microtubules. The presence of both centrosomal and noncentrosomal gamma-tubulin in apical cytoplasm suggest multiple mechanisms by which microtubule nucleation might occur in epithelial cells.

Ultrastructural filament organization of hepatocyte cytoplasm revealed by embedment-free sections of rat liver

The cytoplasmic matrix of rat liver cells was examined by embedment-free electron microscopy of polyethylene glycol (PEG) embedded tissue from which the PEG had been removed subsequently. Non-ionic detergent extraction of fresh blocks of tissue permitted the major cytoskeletal components to be visualized in relatively thick embedment-free sections. Immunolabelling confirmed the presence of the major cytoskeletal proteins (actin, cytokeratin and tubulin) in these sections. Hence, this method permits the examination of the ultrastructural organization of the cytoskeleton of the hepatocyte in blocks of fresh hepatic tissue.

In vitro microtubule-nucleating activity of spindle pole bodies in fission yeast Schizosaccharomyces pombe: cell cycle-dependent activation in xenopus cell-free extracts

The spindle pole body (SPB) is the equivalent of the centrosome in fission yeast. In vivo it nucleates microtubules (MTs) during mitosis, but, unlike animal centrosomes, does not act as a microtubule organizing center (MTOC) during interphase. We have studied the MT-nucleating activity of SPBs in vitro and have found that SPBs in permeabilized cells retain in vivo characteristics. SPBs in cells permeabilized during mitosis can nucleate MTs, and are recognized by two antibodies: anti-gamma-tubulin and MPM-2 which recognizes phosphoepitopes. SPBs in cells permeabilized during interphase cannot nucleate MTs and are only recognized by anti-gamma-tubulin. Interphase SPBs which cannot nucleate can be converted to a nucleation competent state by incubation in cytostatic factor (CSF)-arrested Xenopus egg extracts. After incubation, they are recognized by MPM-2, and can nucleate MTs. The conversion does not occur in Xenopus interphase extract, but occurs in Xenopus interphase extract driven into mitosis by preincubation with exogenous cyclin B. The conversion is ATP dependent and inhibited by protein kinase inhibitors and alkaline phosphatase. Purified, active, cdc2 kinase/cyclin B complex in itself is not effective for activation of MT nucleation, although some interphase SPBs are now stained with MPM-2. These results suggest that the ability of SPBs in vitro to nucleate MTs after exposure to CSF-arrested extracts is activated through a downstream pathway which is regulated by cdc2 kinase.

pH-induced microtubule-dependent redistribution of late endosomes in neuronal and epithelial cells

The interaction between late endocytic structures and microtubules in polarized cells was studied using a procedure previously shown to cause microtubule-dependent redistribution of lysosomes in fibroblasts and macrophages (Heuser, J. 1989. J. Cell Biol. 108:855-864). In cultured rat hippocampal neurons, low cytoplasmic pH caused cation-independent mannose-6-phosphate receptor-enriched structures to move out of the cell body and into the processes. In filter grown MDCK cells lowering the cytosolic pH to approximately 6.5 caused late endosomes to move to the base of the cell and this process was shown to be microtubule dependent. Alkalinization caused a shift in distribution towards the apical pole of the cell. The results are consistent with low pH causing the redistribution of late endosomes towards the plus ends of the microtubules. In MDCK cells the microtubules orientated vertically in the cell may play a role in this process. The shape changes that accompanied the redistribution of the late endosomes in MDCK cells were examined by electron microscopy. On low pH treatment fragmentation of the late endosomes was observed whereas after microtubule depolymerization individual late endosomal structures appeared to fuse together. The late endosomes of the MDCK cell appear to be highly pleomorphic and dependent on microtubules for their form and distribution in the cell.

Deep-etching immunogold replica electron microscopy of cytoskeletal elements in cultured hamster heart cells

A procedure has been developed for the three-dimensional immunoelectron microscopic localization of cytoskeletal filaments by a deep-etching replica method in combination with immunogold labeling and/or myosin subfragment 1 (S1) decoration techniques. Neonatal hamster heart cells grown on glass coverslips were extracted with Triton X-100 or physically permeabilized by breaking open the cell membranes. S1 decoration was performed on some specimens immediately after the permeabilization. After prefixation in formaldehyde, samples were immunostained with poly- or monoclonal antibodies to desmin or vimentin, and indirectly tagged with colloidal gold probes by the biotin-streptavidin method. After postfixation with glutaraldehyde, tannic acid and osmium tetroxide, the cells were freeze-etched and rotary-replicated with platinum and carbon in a freeze-fracture apparatus. Replicas were viewed with a transmission electron microscope using a tilting specimen stage to obtain stereo images. The procedure made it possible to identify the specific filaments within the complex cytoskeletal networks in cultured hamster heart muscle and nonmuscle cells at high resolution and in three dimensions. The method has advantages in its three-dimensionality and feasibility to evaluate the data by comparing them with those obtained by alternative light microscopic methods. Details of the protocol and a description of the results of using three different antibodies are given.

Microtubules are stabilized in confluent epithelial cells but not in fibroblasts

Rhodamine-tagged tubulin was microinjected into epithelial cells (MDCK) and fibroblasts (Vero) to characterize the dynamic properties of labeled microtubules in sparse and confluent cells. Fringe pattern fluorescence photobleaching revealed two components with distinct dynamic properties. About one-third of the injected tubulin diffused rapidly in the cytoplasm with a diffusion coefficient of 1.3-1.6 x 10(-8) cm2/s. This pool of soluble cytoplasmic tubulin was increased to greater than 80% when cells were treated with nocodazole, or reduced to approximately 20% upon treatment of cells with taxol. Fluorescence recovery of the remaining two-thirds of labeled tubulin occurred with an average half-time (t1/2) of 9-11 min. This pool corresponds to labeled tubulin associated with microtubules, since it was sensitive to treatment of cells with nocodazole and since taxol increased its average t1/2 to greater than 22 min. Movement of photobleached microtubules in the cytoplasm with rates of several micrometers per minute was shown using very small interfringe distances. A significant change in the dynamic properties of microtubules occurred when MDCK cells reached confluency. On a cell average, microtubule half-life was increased about twofold to approximately 16 min. In fact, two populations of cells were detected with respect to their microtubule turnover rates, one with a t1/2 of approximately 9 min and one with a t1/2 of greater than 25 min. Correspondingly, the rate of incorporation of microinjected tubulin into interphase microtubules was reduced about twofold in confluent MDCK cells. In contrast to the MDCK cells, no difference in microtubule dynamics was observed in sparse and confluent populations of Vero fibroblasts, where the average microtubule half-life was approximately 10 min. Thus, microtubules are significantly stabilized in epithelial but not fibroblastic cells grown to confluency.

Differential microtubule requirements for transcytosis in MDCK cells

Given the role of microtubules in directing the transport of many intracellular organelles, we investigated whether intact microtubules were also required for transcytosis across epithelia. Using polarized MDCK cells expressing receptors for the Fc domain of IgG (FcRII-B2) or polymeric immunoglobulin (pIg-R), we examined the involvement of microtubules in apical to basolateral and basolateral to apical transcytosis, respectively. While depolymerization of microtubules with nocodozole had no effect on apical to basolateral transcytosis via FcR, basolateral to apical transcytosis of dimeric IgA via pIg-R was almost completely blocked. Inhibition due to nocodozole was selective for basolateral to apical transcytosis, since neither endocytosis nor receptor recycling was significantly affected at either plasma membrane domain. As shown by confocal microscopy, the block in transcytosis was due to the inability of MDCK cells to translocate IgA-containing vesicles from the basolateral to the apical cytoplasm in the absence of an intact microtubule network. The nocodazole sensitive step could be partially by-passed, however, by allowing cells to internalize IgA at 17 degrees C prior to nocodazole treatment. Although incubation at 17 degrees C blocked release of IgA into the apical medium, it did not prevent translocation of IgA-containing vesicles to the apical cytoplasm. Thus, receptor-mediated transcytosis in opposite directions exhibits distinct requirements for microtubules, a feature which reflects the spatial organization of MDCK cells.

The subcellular organization of Madin-Darby canine kidney cells during the formation of a polarized epithelium

Studies of the developing trophectoderm in the mouse embryo have shown that extensive cellular remodeling occurs during epithelial formation. In this investigation, confocal immunofluorescence microscopy is used to examine the three-dimensional changes in cellular architecture that take place during the polarization of a terminally differentiated epithelial cell line. Madin-Darby canine kidney cells were plated at a low density on permeable filter supports. Antibodies that specifically recognize components of the tight junction, adherens junction, microtubules, centrosomes, and the Golgi complex were used to study the spatial remodeling of the cytoarchitecture during the formation of the polarized cell layer. The immunofluorescence data were correlated with establishment of functional tight junctions as measured by transepithelial resistance and back-exchange of the cell surface, labeled with metabolites of the fluorescent lipid analogue N-(7-[4-nitrobenzo-2-oxa-1,3-diazole]) aminocaproyl sphingosine. 1 d after plating, single cells had microtubules, radiating from a broad region, that contained the centrosomes and the Golgi complex. 2 d after plating, the cells had grown to confluence and had formed functional tight junctions close to the substratum. The centrioles had split and no longer organized the microtubules which were running above and below the nucleus. The Golgi complex had spread around the nucleus. By the fifth day after plating, the final polarized state had been achieved. The junctional complex had moved greater than 10 microns upward from its basal location. The centrioles were together below the apical membrane, and the Golgi complex formed a ribbon-like convoluted structure located in the apical region above the nucleus. The microtubules were organized in an apical web and in longitudinal microtubule bundles in the apical-basal axis of the columnar cell. The longitudinal microtubules were arranged with their minus ends spread over the apical region of the cell and their plus ends toward the basal region. These findings show that there is an extensive remodeling of epithelial cytoarchitecture after formation of cell-cell contacts. Reorganization of the microtubule network results in functional polarization of the cytoplasm.

Nocodazole, a microtubule-active drug, interferes with apical protein delivery in cultured intestinal epithelial cells (Caco-2)

The polarized delivery of membrane proteins to the cell surface and the initial secretion of lysosomal proteins into the culture medium were studied in the polarized human intestinal adenocarcinoma cell line Caco-2 in the presence or absence of the microtubule-active drug nocodazole. The appearance of newly synthesized proteins at the plasma membrane was measured by their sensitivity to proteases added either to the apical or the basolateral surface of cells grown on nitrocellulose filters. Nocodazole was found to reduce the delivery to the cell surface of an apical membrane protein, aminopeptidase N, and to lead to its partial missorting to the basolateral surface, whereas the drug had no influence on the delivery of a basolateral 120-kD membrane protein defined by a monoclonal antibody. Furthermore, nocodazole selectively blocked the apical secretion of two lysosomal proteins, cathepsin D and acid alpha-glucosidase, whereas the drug had no influence on their basolateral secretion. These results suggest that in Caco-2 cells an intact microtubular network is important for the transport of newly synthesized proteins to the apical cell surface.

Nonmitogenic morphoregulatory action of pp60v-src on multicellular epithelial structures

Madin-Darby canine kidney (MDCK) cells form polarized, multicellular epithelial structures in vitro. Low-level expression of pp60v-src in MDCK cells elicits plasticity in these multicellular structures. Plasticity was revealed by the displacement of cells from mechanically stressed regions of the epithelial monolayers; however, the two-dimensional relationship between the cells in the remainder of the monolayer was maintained. Electron microscopy of multicellular structures revealed abnormal separation of the lateral membranes of adjacent cells and selective uncoupling of the junctional complex; the zonula adherens was disrupted, but the zonula occludens and desmosomes were retained. Significantly, this result was not accompanied by transformation of the cells, as judged by the absence of anchorage-independent growth potential. These results demonstrate a nonmitogenic biological activity of pp60v-src which is experimentally dissociable from transformation. This morphoregulatory action on higher-order epithelial structures may reflect a function of related cellular tyrosine kinases.

Demonstration of microtubules in the terminal web of mature absorptive cells from the small intestine of the rat

The terminal web (TW) region of mature absorptive cells in the small intestine of the rat contains an elaborate cytoskeleton which supports the apical microvillus membrane. In studies regarding the structural organization of the cytoskeleton and associated proteins in the small intestine, microtubules have not been mentioned as components of the TW. By transmission electron microscopy of conventional resin-embedded sections of rat small intestine, we observe many microtubule profiles in the TW of mature absorptive cells. These microtubules are found in various orientations, although most course parallel to the long axis of the cell, and many microtubule profiles are seen in close association with smooth-surfaced vesicles.

Redistribution of microtubules and pericentriolar material during the development of polarity in mouse blastomeres

The distribution of microtubules and microtubule organizing centers (MTOCs) during the development of cell polarity in eight-cell mouse blastomeres was studied by immunofluorescence and immunoelectron microscopy using monoclonal anti-tubulin antibodies and an anti-pericentriolar material (PCM) serum. In early eight-cell blastomeres microtubules were found mainly around the nucleus and in the cell cortex, whereas PCM foci were observed dispersed in the cytoplasm. During the eight-cell stage, microtubules disappeared from the area adjacent to the zone of intercellular contact and accumulated in the apical part of the cell while their number decreased in the basal domain. The PCM also relocalized to the apical domain of the cell, but this occurred after the redistribution of the microtubules by a mechanism that involved the microtubule network. The possible roles of both MTOCs and microtubules in establishing cell polarity are discussed.

Nonmitogenic morphoregulatory action of pp60v-src on multicellular epithelial structures

Madin-Darby canine kidney (MDCK) cells form polarized, multicellular epithelial structures in vitro. Low-level expression of pp60v-src in MDCK cells elicits plasticity in these multicellular structures. Plasticity was revealed by the displacement of cells from mechanically stressed regions of the epithelial monolayers; however, the two-dimensional relationship between the cells in the remainder of the monolayer was maintained. Electron microscopy of multicellular structures revealed abnormal separation of the lateral membranes of adjacent cells and selective uncoupling of the junctional complex; the zonula adherens was disrupted, but the zonula occludens and desmosomes were retained. Significantly, this result was not accompanied by transformation of the cells, as judged by the absence of anchorage-independent growth potential. These results demonstrate a nonmitogenic biological activity of pp60v-src which is experimentally dissociable from transformation. This morphoregulatory action on higher-order epithelial structures may reflect a function of related cellular tyrosine kinases.

Distribution of microtubules and microfilaments in exocrine (ventral prostatic epithelial cells and pancreatic exocrine cells) and endocrine cells (cells of the adenohypophysis and islets of Langerhans). The relationship between cytoskeletons and epithelial-cell polarity

We investigated the distribution of microtubules and microfilaments in some exocrine and endocrine cells in rats. Microtubules were stained by applying an immunofluorescent technique using antibodies against beta-tubulin, while microfilaments were stained with rhodamine-phalloidin, which binds selectively to polymerized actin filaments. In the cytoplasm of some exocrine cells (pancreatic acinar cells and ventral prostatic epithelial cells), the microtubules were distributed longitudinally from the apical region to the basal region, but no microtubules were found in the nuclear region. In exocrine cells, most of the microfilaments were localized beneath the apical plasma membrane. In some endocrine cells (those of the adenohypophysis and the islets of Langerhans), the microtubules exhibited a radial or reticular distribution in the cytoplasm, and intense fluorescence was observed in the perinuclear region. The immunofluorescence produced by the antibodies against beta-tubulin was more intense in endocrine cells than in exocrine cells. The microfilaments observed in the endocrine cells studied were homogenously distributed beneath the plasma membrane. Dot-like rhodamine-phalloidin staining was often observed in the cytoplasm of both the exocrine and endocrine cells. The present study clearly demonstrated marked differences in the distribution of cytoskeletal elements in exocrine and endocrine cells, and these may reflect differences in the secretory direction of such cells as well as in epithelial-cell polarity.