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

Picofluidic Electro-Osmosis Measurement of Cell Membrane Mechanical Properties

Cells connect with their internal and external environments through plasma membranes, and the mechanical properties of cell membranes govern numerous biological events. Membrane detection techniques such as optical or magnetic tweezers have revealed mechanical strength by membrane-anchored modifications, but it remains challenging to develop label-free methods to reduce the influence of exogenous interference. Here picofluidic electro-osmosis measurement (PEOM), which enables direct and efficient sensing of cell membrane mechanical properties by using a glass nanopipette without labeling, is presented. By generating a picoliter electroosmotic fluid at the nanopipette tip, periodic cell membrane vibration modes are observed from current traces, which carry information on membrane mechanical properties to indicate its biological state. Based on characteristic peaks in the frequency domain, a theoretical framework to describe the vibration modes, which contains two ideal spring vibrator models corresponding to stretching and bending vibrations of cell membrane respectively, is developed. Notably, the PEOM strategy represents a label-free approach to reveal the mechanical properties of living cell membranes from two dimensions, which is completely different from other methods. Additionally, the exciting potential of PEOM is demonstrated for label-free observation of membrane mechanical property changes during different bioprocesses, including cytoskeletal alteration, membrane tension change, and mechanical polarization.

Endosomes and Microtubles are Required for Productive Infection in Aquareovirus

Grass carp reovirus (GCRV), the genus Aquareovirus in family Reoviridae, is viewed as the most pathogenic aquareovirus. To understand the molecular mechanism of how aquareovirus initiates productive infection, the roles of endosome and microtubule in cell entry of GCRV are investigated by using quantum dots (QDs)-tracking in combination with biochemical approaches. We found that GCRV infection and viral protein synthesis were significantly inhibited by pretreating host cells with endosome acidification inhibitors NH₄Cl, chloroquine and bafilomycin A1 (Bafi). Confocal images indicated that GCRV particles could colocalize with Rab5, Rab7 and lysosomes in host cells. Further ultrastructural examination validated that viral particle was found in late endosomes. Moreover, disruption of microtubules with nocodazole clearly blocked GCRV entry, while no inhibitory effects were observed with cytochalasin D treated cells in viral infection, hinting that intracellular transportation of endocytic uptake in GCRV infected cells is via microtubules but not actin filament. Notably, viral particles were observed to transport along microtubules by using QD-labeled GCRV. Altogether, our results suggest that GCRV can use endosomes and microtubules to initiate productive infection.

Cyclin-dependent kinase 5 regulates proliferation, migration, tyrosinase activity, and melanin production in B16-F10 melanoma cells via the essential regulator p-CREB

Melanoma is an aggressive cancer with increasing incidence and a growing lifetime risk that arises from normal melanocytes or their precursors. A thorough understanding of the molecular mechanism of melanomagenesis and melanoma biology is essential for the diagnosis, prognostication, and therapy of melanoma. Cyclin-dependent protein kinase 5 (Cdk5) is one of the proteins highly expressed in B16-F10 melanoma cells that controls melanoma cell motility, invasiveness, and metastatic spread and might be a promising novel therapeutic target. The effect of Cdk5 on proliferation and migration, which are important for carcinogenesis, has not been reported. In the current study, we found that siRNA-mediated knockdown of Cdk5 in B16-F10 melanoma cells inhibited melanoma cell proliferation through downregulation of the CaMK4-p-CREB pathway, inhibited migration through downregulation of p-CREB, integrin beta 1, and integrin beta 5, and also inhibited tyrosinase activity and melanin production through p-CREB-MITF regulation. The results indicate that Cdk5 controls melanoma development, with an essential regulatory role for p-CREB.

Positioning of the Centrosome and Golgi Complex

For over a century, the centrosome has been an organelle more easily tracked than understood, and the study of its peregrinations within the cell remains a chief underpinning of its functional investigation. Increasing attention and new approaches have been brought to bear on mechanisms that control centrosome localization in the context of cleavage plane determination, ciliogenesis, directional migration, and immunological synapse formation, among other cellular and developmental processes. The Golgi complex, often linked with the centrosome, presents a contrasting case of a pleiomorphic organelle for which functional studies advanced somewhat more rapidly than positional tracking. However, Golgi orientation and distribution has emerged as an area of considerable interest with respect to polarized cellular function. This chapter will review our current understanding of the mechanism and significance of the positioning of these organelles.

Genetically induced microtubule disruption in the mouse intestine impairs intracellular organization and transport

In most differentiated cells, microtubules reorganize into noncentrosomal arrays that are cell-type specific. In the columnar absorptive enterocytes of the intestine, microtubules form polarized apical–basal arrays that have been proposed to play multiple roles. However, in vivo testing of these hypotheses has been hampered by a lack of genetic tools to specifically perturb microtubules. Here we analyze mice in which microtubules are disrupted by conditional inducible expression of the microtubule-severing protein spastin. Spastin overexpression resulted in multiple cellular defects, including aberrations in nuclear and organelle positioning and deficient nutrient transport. However, cell shape, adhesion, and polarity remained intact, and mutant mice continued to thrive. Notably, the phenotypes of microtubule disruption are similar to those induced by microtubule disorganization upon loss of CAMSAP3/Nezha. These data demonstrate that enterocyte microtubules have important roles in organelle organization but are not essential for growth under homeostatic conditions.

Mechanisms of DRA recycling in intestinal epithelial cells: effect of enteropathogenic E. coli

Enteropathogenic Escherichia coli (EPEC) is a food-borne pathogen that causes infantile diarrhea worldwide. EPEC decreases the activity and surface expression of the key intestinal Cl(-)/HCO3(-) exchanger SLC26A3 [downregulated in adenoma (DRA)], contributing to the pathophysiology of early diarrhea. Little is known about the mechanisms governing membrane recycling of DRA. In the current study, Caco-2 cells were used to investigate DRA trafficking under basal conditions and in response to EPEC. Apical Cl(-)/HCO3(-) exchange activity was measured as DIDS-sensitive (125)I(-) uptake. Cell surface biotinylation was performed to assess DRA endocytosis and exocytosis. Inhibition of clathrin-mediated endocytosis by chlorpromazine (60 μM) increased apical Cl(-)/HCO3(-) exchange activity. Dynasore, a dynamin inhibitor, also increased function and surface levels of DRA via decreased endocytosis. Perturbation of microtubules by nocodazole revealed that intact microtubules are essential for basal exocytic (but not endocytic) DRA recycling. Mice treated with colchicine showed a decrease in DRA surface levels as visualized by confocal microscopy. In response to EPEC infection, DRA surface expression was reduced partly via an increase in DRA endocytosis and a decrease in exocytosis. These effects were dependent on the EPEC virulence genes espG1 and espG2. Intriguingly, the EPEC-induced decrease in DRA function was unaltered in the presence of dynasore, suggesting a clathrin-independent internalization of surface DRA. In conclusion, these studies establish the role of clathrin-mediated endocytosis and microtubules in the basal surface expression of DRA and demonstrate that the EPEC-mediated decrease in DRA function and apical expression in Caco-2 cells involves decreased exocytosis.

Turnover of the actomyosin complex in zebrafish embryos directs geometric remodelling and the recruitment of lipid droplets

Lipid droplets (LDs), reservoirs of cholesterols and fats, are organelles that hydrolyse lipids in the cell. In zebrafish embryos, the actomyosin complex and filamentous microtubules control the periodic regulation of the LD geometry. Contrary to the existing hypothesis that LD transport involves the kinesin-microtubule system, we find that their recruitment to the blastodisc depends on the actomyosin turnover and is independent of the microtubules. For the first time we report the existence of two distinct states of LDs, an inactive and an active state, that occur periodically, coupled weakly to the cleavage cycles. LDs are bigger, more circular and more stable in the inactive state in which the geometry of the LDs is maintained by actomyosin as well as microtubules. The active state has smaller and irregularly shaped LDs that show shape fluctuations that are linked to actin depolymerization. Because most functions of LDs employ surface interactions, our findings on the LD geometry and its regulation bring new insights to the mechanisms associated with specific functions of LDs, such as their storage capacity for fats or proteins, lipolysis etc.

Cytoskeletal control of nuclear morphology and chromatin organization

The nucleus is sculpted toward various morphologies during cellular differentiation and development. Alterations in nuclear shape often result in changes to chromatin organization and genome function. This is thought to be reflective of its role as a cellular mechanotransducer. Recent evidence has highlighted the importance of cytoskeletal organization in defining how nuclear morphology regulates chromatin dynamics. However, the mechanisms underlying cytoskeletal control of chromatin remodeling are not well understood. We demonstrate here the differential influence of perinuclear actin- and microtubule-driven assemblies on nuclear architecture using pharmacological inhibitors and targeted RNA interference knockdown of cytoskeleton components in Drosophila cells. We find evidence that the loss of perinuclear actin assembly results in basolateral enhancement of microtubule organization and this is reflected functionally by enhanced nuclear dynamics. Cytoskeleton reorganization leads to nuclear lamina deformation that influences heterochromatin localization and core histone protein mobility. We also show that modulations in actin-microtubule assembly result in differential gene expression patterns. Taken together, we suggest that perinuclear actin and basolateral microtubule organization exerts mechanical control on nuclear morphology and chromatin dynamics.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

The role of the Golgi apparatus in oxidative stress: is this organelle less significant than mitochondria?

Reactive oxygen species (ROS)/reactive nitrogen species (RNS) and ROS/RNS-mediated oxidative stress have well-established roles in many physiological and pathological processes and are associated with the pathogenesis of many diseases, such as hypertension, ischemia/reperfusion injury, diabetes mellitus, atherosclerosis, stroke, cancer, and neurodegenerative disorders. It is generally accepted that mitochondria play an essential role in oxidative stress because they are responsible for the primary generation of superoxide radicals. Little attention, however, has been paid to the importance of the Golgi apparatus (GA) in this process. The GA is a pivotal organelle in cell metabolism and participates in modifying, sorting, and packaging macromolecules for cell secretion or use within the cell. It is inevitably involved in the process of oxidative stress, which can cause modification and damage of lipids, proteins, DNA, and other structural constituents. Here we discuss the connections between the GA and oxidative stress and highlight the role of the GA in oxidative stress-related Ca(2+)/Mn(2+) homeostasis, cell apoptosis, sphingolipid metabolism, signal transduction, and antioxidation. We also provide a novel perspective on the subcellular significance of oxidative stress and its pathological implications and present "GA stress" as a new concept to explain the GA-specific stress response.

KIF17 stabilizes microtubules and contributes to epithelial morphogenesis by acting at MT plus ends with EB1 and APC

Epithelial polarization is associated with selective stabilization and reorganization of microtubule (MT) arrays. However, upstream events and downstream consequences of MT stabilization during epithelial morphogenesis are still unclear. We show that the anterograde kinesin KIF17 localizes to MT plus ends, stabilizes MTs, and affects epithelial architecture. Targeting of KIF17 to plus ends of growing MTs requires kinesin motor activity and interaction with EB1. In turn, KIF17 participates in localizing adenomatous polyposis coli (APC) to the plus ends of a subset of MTs. We found that KIF17 affects MT dynamics, polymerization rates, and MT plus end stabilization to generate posttranslationally acetylated MTs. Depletion of KIF17 from cells growing in three-dimensional matrices results in aberrant epithelial cysts that fail to generate a single central lumen and to polarize apical markers. These findings implicate KIF17 in MT stabilization events that contribute to epithelial polarization and morphogenesis.

Lovastatin inhibits the thrombin-induced loss of barrier integrity in bovine corneal endothelium

PURPOSE

Increased actomyosin contraction of the dense band of actin cytoskeleton at the apical junctional complex (perijunctional actomyosin ring, PAMR) breaks down the barrier integrity of corneal endothelium. This study has investigated the efficacy of statins, which inhibit activation of RhoA, in opposing the thrombin-induced loss of barrier integrity of monolayers of cultured bovine corneal endothelium.

METHODS

Myosin light chain (MLC) phosphorylation, a biochemical measure of actomyosin contraction, was assayed by urea-glycerol gel electrophoresis, followed by western blot analysis. The locus of MLC phosphorylation and changes in the organization of the PAMR were visualized by immunostaining. Phosphorylation of MYPT1, a regulatory subunit of myosin light-chain phosphatase (MLCP), was assessed by Western blot analysis to determine down-regulation of RhoA. The barrier integrity was assessed in terms of trans-endothelial electrical resistance (TER), and further confirmed by determining permeability to FITC dextran (10 kDa) and distribution of ZO-1, a marker of tight junctional assembly.

RESULTS

Lovastatin, a prototype of lipophilic statins, induced MLC dephosphorylation under basal conditions. It opposed increase in phosphorylation of MLC and MYPT1 in response to thrombin and nocodazole, agents known to activate RhoA in the endothelium. Pretreatment with the statin opposed the thrombin- and nocodazole-induced disruption of the PAMR and the thrombin-induced decline in TER. Lovastatin also opposed the thrombin- and nocodazole-induced increase in permeability to FITC dextran and redistribution of ZO-1. However, upon supplementation with GGPP (geranylgeranyl pyrophosphate), lovastatin failed to oppose the effects of thrombin and nocodazole on the PAMR, ppMLC, and ZO-1 distribution.

CONCLUSIONS

Lovastatin attenuates RhoA activation in the corneal endothelium presumably by reducing its isoprenylation. This underlies the suppression of the thrombin-induced loss in barrier integrity of the corneal endothelium.

PH-domain-dependent selective transport of p75 by kinesin-3 family motors in non-polarized MDCK cells

A key process during epithelial polarization involves establishment of polarized transport routes from the Golgi to distinct apical and basolateral membrane domains. To do this, the machinery involved in selective trafficking must be regulated during differentiation. Our previous studies showed that KIF5B selectively transports vesicles containing p75-neurotrophin receptors to the apical membrane of polarized, but not non-polarized MDCK cells. To identify the kinesin(s) responsible for p75 trafficking in non-polarized MDCK cells we expressed KIF-specific dominant-negative constructs and assayed for changes in post-Golgi transport of p75 by time-lapse fluorescence microscopy. Overexpression of the tail domains of kinesin-3 family members that contain a C-terminal pleckstrin homology (PH) domain, KIF1A or KIF1Bbeta, attenuated the rate of p75 exit from the Golgi in non-polarized MDCK cells but not in polarized cells. Analysis of p75 post-Golgi transport in cells expressing KIF1A or KIF1Bbeta with their PH domains deleted revealed that vesicle transport by these motors depends on the PH domains. Furthermore, purified KIF1A and KIF1Bbeta tails interact with p75 vesicles and these interactions require the PH domain. Knockdown of canine KIF1A also inhibited exit of p75 from the Golgi, and this was rescued by expression of human KIF1A. Together these data demonstrate that post-Golgi transport of p75 in non-polarized epithelial cells is mediated by kinesin-3 family motors in a PH-domain-dependent process.

Microtubule disassembly breaks down the barrier integrity of corneal endothelium

Increased contractility of the peri-junctional actomyosin ring (PAMR) breaks down the barrier integrity of corneal endothelium. This study has examined the effects of microtubule disassembly on Myosin Light Chain (MLC) phosphorylation, a biochemical marker of actomyosin contraction, and barrier integrity in monolayers of cultured bovine corneal endothelial cells (BCEC). Exposure to nocodazole, which readily induced microtubule disassembly, led to disruption of the characteristically dense assembly of cortical actin cytoskeleton at the apical junctional complex (i.e., PAMR) and dispersion of ZO-1 from its normal locus. Nocodazole also led to an increase in phosphorylation of MLC. Concomitant with these changes, nocodazole caused an increase in permeability to HRP and FITC dextran (10 kDa) and a decrease in trans-endothelial electrical resistance (TER). Y-27632 (a Rho kinase inhibitor) and forskolin (known to inhibit activation of RhoA through direct elevation of cAMP) opposed the nocodazole-induced MLC phosphorylation, decrease in TER, and dispersion of ZO-1. Thrombin, which breaks down the barrier integrity of BCEC monolayers, also induced microtubule disassembly and MLC phosphorylation. Pre-treatment with paclitaxel to stabilize microtubules opposed the thrombin effects. These results suggest that microtubule disassembly breaks down the barrier integrity of BCEC through activation of RhoA and subsequent disruption of the PAMR. The thrombin effect also highlights that signaling downstream of GPCRs can also influence the organization of microtubules.

Change in localization of alkaline phosphatase and mannosidase II by colchicine treatment of primary cultures of fetal rat hepatocytes

We examined the changes in localization of alkaline phosphatase (ALP) and mannosidase II (man II), a Golgi marker, after colchicine treatment of primary cultures of fetal rat hepatocytes, using double immunofluorescence staining and confocal laser microscopy. In hepatocytes cultured in basal medium, ALP was localized in the perinuclear cytoplasm, and man II was observed in the Golgi region of the cytoplasm. When hepatocytes were cultured in dexamethasone-supplemented medium, ALP was also localized in the plasma membrane surrounding the bile canaliculus-like structure that was formed between adjacent cells. In hepatocytes cultured in the same medium containing colchicine, the structure of microtubules in the cytoplasm was lost, man II exhibited granular distribution scattering throughout the cytoplasm, and ALP was localized in coarse granular sites of the cytoplasm. However, ALP was not colocalized at the same sites as man II. The present study indicated that colchicine inhibits the dexamethasone-promoted translocation of ALP to the plasma membrane surrounding the bile canaliculus-like structure in primary cultures of fetal rat hepatocytes by disassembling microtubules and discomposing the Golgi complex.

Fragmentation of the Golgi apparatus: an early apoptotic event independent of the cytoskeleton

The Golgi apparatus undergoes irreversible fragmentation during apoptosis, in part as a result of caspase-mediated cleavage of several Golgi-associated proteins. However, Golgi structure and orientation is also regulated by the cytoskeleton and cytoskeletal changes have been implicated in inducing apoptosis. Consequently, we have analyzed the role of actin filaments and microtubules in apoptotic Golgi fragmentation. We demonstrate that in Fas receptor-activated cells, fragmentation of the Golgi apparatus was an early event that coincided with release of cytochrome c from mitochondria. Significantly, Golgi fragmentation preceded major changes in the organization of both the actin cytoskeleton and microtubules. In staurosporine-treated cells, actin filament organization was rapidly disrupted; however, the Golgi apparatus maintained its juxtanuclear localization and underwent complete fragmentation only at later times. Attempts to stabilize actin filaments with jasplakinolide prior to treatment with staurosporine did not prevent Golgi fragmentation. Finally, in response to Fas receptor activation or staurosporine treatment the levels of beta-actin or alpha-tubulin remained unaltered, whereas several Golgi proteins, p115 and golgin-160, underwent caspase-mediated cleavage. Our data demonstrate that breakdown of the Golgi apparatus is an early event during apoptosis that occurs independently of major changes to the actin and tubulin cytoskeleton.

Polarization-dependent selective transport to the apical membrane by KIF5B in MDCK cells

Microtubule-based vesicular transport is well documented in epithelial cells, but the specific motors involved and their regulation during polarization are largely unknown. We demonstrate that KIF5B mediates post-Golgi transport of an apical protein in epithelial cells, but only after polarity has developed. Time-lapse imaging of EB1-GFP in polarized MDCK cells showed microtubule plus ends growing toward the apical membrane, implying that plus end-directed N-kinesins might be used to transport apical proteins. Indeed, time-lapse microscopy revealed that expression of a KIF5B dominant negative or microinjection of function-blocking KIF5 antibodies inhibited selectively post-Golgi transport of the apical marker, p75-GFP, after polarization of MDCK cells. Expression of other KIF dominant negatives did not alter p75-GFP trafficking. Immunoprecipitation experiments demonstrated an interaction between KIF5B and p75-GFP in polarized, but not in subconfluent, MDCK cells. Our results demonstrate that apical protein transport depends on selective microtubule motors and that epithelial cells switch kinesins for post-Golgi transport during acquisition of polarity.

Dystroglycan receptor is involved in integrin activation in intestinal epithelia

The dystroglycans (alpha-DG and beta-DG), which play important roles in the formation of basement membranes, have been well studied in skeletal muscle and nerve, but their expression and localization in intestinal epithelial cells has not been previously investigated. Here, we demonstrated that the DG complex, composed of alpha-DG, beta-DG, and utrophin, is specifically expressed in the basolateral membrane of the Caco-2-BBE monolayer. The DG complex coprecipitated with beta(1)-integrin, suggesting a possible interaction among these proteins. In addition, we observed that activation of DG receptors by laminin-1 enhanced the interaction between beta(1)-integrin and laminin-1, whereas activation of DG receptors by laminin-2 reduced the interaction between beta(1)-integrin and laminin-2. Finally, we demonstrated that the intracellular COOH-terminal tail of beta-DG and its binding to the DG binding domain of utrophin are crucial for the interactions between laminin-1/-2 and beta(1)-integrin. Collectively, these novel results indicate that dystroglycans play important roles in the regulation of interactions between intestinal epithelial cells and the extracellular matrix.

Copper-dependent interaction of dynactin subunit p62 with the N terminus of ATP7B but not ATP7A

The P-type ATPase affected in Wilson disease, ATP7B, is a key liver protein required to regulate and maintain copper homeostasis. When hepatocytes are exposed to elevated copper levels, ATP7B traffics from the trans-Golgi network toward the biliary canalicular membrane to excrete excess copper into bile. The N-terminal region of ATP7B contains six metal-binding sites (MBS), each with the copper-binding motif MXCXXC. These sites are required for the activity and copper-regulated intracellular redistribution of ATP7B. Two proteins are known to interact with the ATP7B N-terminal region: the copper chaperone ATOX1 that delivers copper to ATP7B, and COMMD1 (MURR1) that is potentially involved in vesicular copper sequestration. To identify additional proteins that interact with ATP7B and hence are involved in copper homeostasis, a yeast two-hybrid approach was employed to screen a human liver cDNA library. The dynactin subunit p62 (dynactin 4; DCTN4) was identified as an interacting partner, and this interaction was confirmed by co-immunoprecipitation from mammalian cells. The dynactin complex binds cargo, such as vesicles and organelles, to cytoplasmic dynein for retrograde microtubule-mediated trafficking and could feasibly be involved in the copper-regulated trafficking of ATP7B. The ATP7B/p62 interaction required copper, the metal-binding CXXC motifs, and the region between MBS 4 and MBS 6 of ATP7B. The p62 subunit did not interact with the related copper ATPase, ATP7A. We propose that the ATP7B interaction with p62 is a key component of the copper-induced trafficking pathway that delivers ATP7B to subapical vesicles of hepatocytes for the removal of excess copper into bile.

Enteropathogenic Escherichia coli EspG disrupts microtubules and in conjunction with Orf3 enhances perturbation of the tight junction barrier

EspG, a secreted effector of enteropathogenic Escherichia coli (EPEC), as well as its homologue Orf3, has been shown to disrupt microtubules (MTs) in fibroblasts and non-polarized epithelial cells. The roles of MTs and the effects of MT disruption in these cell types differ significantly. The aim of this study was to investigate the effects of EspG on polarized, host target intestinal epithelial cells. Immunofluorescent labelling of tubulin showed that EPEC caused progressive fragmentation and loss of the MT network in cells harbouring attached organisms. Immunoblots of proteins extracted from EPEC-infected cells showed a corresponding loss of alpha-tubulin. Type III secretion system (TTSS)-deficient strains had no effect on MT suggesting TTSS dependence. Mutation of espG, but not espF or map, ablated EPEC's effects on MTs for up to 6 h. Ectopic expression of EspG in HeLa cells caused MT disruption. While deletion of espG alone had no effect on the EPEC-induced decrease in transepithelial electrical resistance (TER), mutation of both espG and orf3 significantly delayed the kinetics of this response. Complementation of the double mutant with espG alone restored the kinetics of TER drop to that of wild type. Herein, we describe a previously unrecognized phenotype for the EPEC effectors EspG and Orf3.

Microtubule organization and function in epithelial cells

Microtubules are essential for many aspects of polarity in multicellular organisms, ranging from the asymmetric distribution of cell-fate determinants in the one-cell embryo to the transient polarity generated in migrating fibroblasts. Epithelial cells exhibit permanent cell polarity characterized by apical and basolateral surface domains of distinct protein and lipid composition that are segregated by tight junctions. They are also endowed with a microtubule network that reflects the asymmetry of their cell surface: microtubule minus-ends face the apical- and microtubule plus-ends the basal domain. Strikingly, the formation of distinct surface domains during epithelial differentiation is accompanied by the re-organization of microtubules from a uniform array focused at the centrosome to the noncentrosomal network that aligns along the apico-basolateral polarity axis. The significance of this coincidence for epithelial morphogenesis and the signaling mechanisms that drive microtubule repolymerization in developing epithelia remain major unresolved questions that we are only beginning to address. Studies in cultured polarized epithelial cells have established that microtubules serve as tracks that facilitate targeted vesicular transport. Novel findings suggest, moreover, that microtubule-based transport promotes protein sorting, and even the generation of transport carriers in the endo- and exocytic pathways.

Herpes simplex virus type 1 infection of polarized epithelial cells requires microtubules and access to receptors present at cell–cell contact sites

Mucosal epithelia are invaded from the apical surface during a primary infection by herpes simplex virus type 1 (HSV-1). HSV-1 progeny virus, synthesized from latently infected peripheral neurons that innervate such epithelia, reinfects the epithelia most likely from the basolateral surface. The epithelial cell lines MDCK and Caco-2 can be inducedin vitroto differentiate into polarized cells with distinct apical and plasma membrane domains separated by tight junctions if they are cultured on porous membrane filters. Our data using these culture systems showed that highly polarized epithelial cells were not susceptible to apical HSV-1 infection. However, HSV-1 infected these cells if added from the basolateral surface or if a depletion of extracellular Ca2+had weakened the strength of the cell–cell contacts. Basolateral infection and apical infection after the Ca2+switch required an intact microtubule network for genome targeting to the nucleus. This system can be used to identify the microtubule motors that HSV-1 uses during virus entry in polarized epithelial cells.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Microtubule-associated epithelial protein E-MAP-115 is localized in the spermatid manchette

A microtubule-associated protein E-MAP-115 has been originally isolated and characterized from HeLa cells. Because of its predominant expression in cultured cells of epithelial origin, it has been suggested to be involved in the regulation of cell polarization. The present immunocytochemical, Northern blot and in situ hybridization analysis of E-MAP-115 in the mouse and rat seminiferous epithelium indicates its distinct association with the spermatid manchette, a unique microtubular structure which appears in the cytoplasm of spermatids at step 8 when nuclear polarization and elongation starts. At steps 15-16 when manchette has been disassembled, immunoreactivity for E-MAP-115 disappeared. At immunoelectron microscopical level, E-MAP-15 was associated with the microtubules of the manchette. In the Western and Northern blot analysis, a distinct stage-dependent expression of a single E-MAP-115 polypeptide and two mRNA species (3.4 and 2.4 kb) could be identified. MTEST 60, a spermatid-specific transcript, showed a 100% homology over region of 68-193 bp of E-MAP-115 sequence. The reported specific localization of E-MAP-115 to the spermatid manchette strongly supports its role as a regulator of cell polarization. This, in turn, supports the hypotheses concerning the dynamic function of the manchette during spermiogenesis.

Three-dimensional analysis of post-Golgi carrier exocytosis in epithelial cells

Targeted delivery of proteins to distinct plasma membrane domains is critical to the development and maintenance of polarity in epithelial cells. We used confocal and time-lapse total internal reflection fluorescence microscopy (TIR-FM) to study changes in localization and exocytic sites of post-Golgi transport intermediates (PGTIs) carrying GFP-tagged apical or basolateral membrane proteins during epithelial polarization. In non-polarized Madin Darby Canine Kidney (MDCK) cells, apical and basolateral PGTIs were present throughout the cytoplasm and were observed to fuse with the basal domain of the plasma membrane. During polarization, apical and basolateral PGTIs were restricted to different regions of the cytoplasm and their fusion with the basal membrane was completely abrogated. Quantitative analysis suggested that basolateral, but not apical, PGTIs fused with the lateral membrane in polarized cells, correlating with the restricted localization of Syntaxins 4 and 3 to lateral and apical membrane domains, respectively. Microtubule disruption induced Syntaxin 3 depolarization and fusion of apical PGTIs with the basal membrane, but affected neither the lateral localization of Syntaxin 4 or Sec6, nor promoted fusion of basolateral PGTIs with the basal membrane.

Infection of polarized epithelial cells with flavivirus West Nile: polarized entry and egress of virus occur through the apical surface

Both polarized epithelial Vero (C1008) and non-polarized Vero (control) cells were grown on permeable cell culture inserts and infected either apically or basolaterally with West Nile (WN) or Kunjin (KUN) virus. KUN virus (closely related to WN virus) was used as a comparison. Using indirect immunofluorescence and plaque assays of productive virus titres, entry of WN and KUN viruses was confined to the apical surface of polarized epithelial cells. For the first time, these results provided evidence on the distribution of flavivirus-specific receptor(s) in polarized epithelial cells; that is to say that receptor expression was shown to be predominant at the apical surface. In addition, the release of these viruses from polarized Vero C1008 epithelial cells was also examined. Egress of WN virus strain Sarafend (S) was observed to occur predominantly at the apical surface of Vero C1008 cells. In contrast, the release of KUN virus was bi-directional from polarized Vero C1008 cells. Furthermore, disruption of the cellular microtubule network was shown to inhibit the apical release of WN (S) virus but had no effect on the release of KUN virus. Hence, the difference in the release of these closely related viruses suggested the involvement of a microtubule-dependent, polarized sorting mechanism for WN virus proteins but not for KUN virus proteins in polarized epithelial cells.

Microtubule integrity is essential for apical polarization and epithelial morphogenesis in the thyroid

In this study, we examined the contribution of microtubules to epithelial morphogenesis in primary thyroid cell cultures. Thyroid follicles consist of a single layer of polarized epithelial cells surrounding a closed compartment, the follicular lumen. Freshly isolated porcine thyroid cells aggregate and reorganize to form follicles when grown in primary cultures. Follicular reorganization is principally a morphogenetic process that entails the assembly of biochemically distinct apical and basolateral membrane domains, delimited by tight junctions. The establishment of cell surface polarity during folliculogenesis coincided with the polarized redistribution of microtubules, predominantly in the developing apical poles of cells. Disruption of microtubule integrity using either colchicine or nocodazole caused loss of defined apical membrane domains, tight junctions and follicular lumina. Apical membrane and tight junction markers became randomly distributed at the outer surfaces of aggregates. In contrast, the basolateral surface markers, E-cadherin and Na(+),K(+)-ATPase, remained correctly localized at sites of cell-cell contact and at the free surfaces of cell aggregates. These findings demonstrate that microtubules play a necessary role in thyroid epithelial morphogenesis. Specifically, microtubules are essential to preserve the correct localization of apical membrane components within enclosed cellular aggregates, a situation that is also likely to pertain where lumina must be formed from solid aggregates of epithelial precursors.

Listeriolysin O-induced stimulation of mucin exocytosis in polarized intestinal mucin-secreting cells: evidence for toxin recognition of membrane-associated lipids and subsequent toxin internalization through caveolae

Lysteriolysin O (LLO) induces a microtubule-dependent activation of mucin exocytosis in the human mucin-secreting HT29-MTX. Cholesterol inhibits the LLO-induced mucin exocytosis, whereas the oxidized form of cholesterol had no inhibitory effect. LLO-induced mucin exocytosis inhibited by cholesterol can be restored by enzymatic treatment with cholesterol oxidase. Inhibition of cholesterol synthesis in HT29-MTX cells results in a decrease in the LLO-induced mucin exocytosis. Other lipids such as gangliosides are able to inhibit the LLO-induced mucin exocytosis, suggesting that the binding of the toxin occurs at a multiplicity of membrane-associated lipids acting as receptors. Incubation of the toxin with lipids such as cholesterol or gangliosides does not decrease binding of LLO to target membranes. The present work also provides evidence that the LLO-induced mucin exocytosis develops independently of the pore-forming activity of the toxin. Finally, we demonstrated that the toxin associates with detergent-insoluble glycolipid microdomains (DIGs) containing VIP/21 caveolin, allowing internalization of the toxin and subsequent activation of the mucin exocytosis.

VIP enhances the differentiation of retinal pigment epithelium in culture: from cAMP and pp60(c-src) to melanogenesis and development of fluid transport capacity

The retinal pigment epithelium (RPE) is a single cell layer juxtaposed between the neural retina and the choroid and functions as a blood-retina barrier. The RPE performs functions essential for photoreceptor (PR) survival. Although the regulation of these functions has remained unknown, it is a distinct possibility that the RPE is under constant regulation by signaling molecules coming from the choroid and the retina. Vasoactive intestinal peptide (VIP), a 28-amino acid neuropeptide present in the retina and in the choroid, has been shown to promote the growth and differentiation of a variety of cells in tissue and organ cultures. In cultured RPE cells, VIP is the one most effective stimulator of the cAMP signaling pathway among a long list of neurotransmitters and modulators tested. For example, VIP, at 1 microM, stimulates the intracellular cAMP to 80-100- and 20-fold in 3 min in RPE cells cultured from chick embryos and adult human donor eyes, respectively. In cultured chick embryonic RPE, VIP is also shown to be a potent and effective modulator of pp60(c-src), the non-receptor tyrosine kinase present in differentiating and terminally differentiated cells. VIP stimulates both overall phosphorylation at unknown sites and phosphotyrosine dephosphorylation in pp60(c-src). A 190-kDa microtubule-associated protein is known to be one of the downstream targets in VIP-modulated signaling pathways. At the cellular level, VIP stimulates cell proliferation modestly and melanogenesis pronouncedly in growing chick embryonic RPE cultures. Ultimately, the differentiation goal of RPE cells in vivo is to perform functions that are essential for photoreceptor survival. On bare permeable supports (that is, without biological material coating), the chick embryonic RPE cells grow to become RPE sheets with a cytoarchitecture that allows the display of two of the RPE functions. These cultures demonstrate structural polarity and are functionally polarized, allowing for proper macromolecule secretion and fluid transport. VIP is shown to stimulate macromolecule secretion at the apical surface (retina facing) and the development of the capacity for fluid transport from the apical to the basal surface of the RPE sheet. In conclusion, studies in our laboratory indicate that VIP is a differentiation promotor during the development of a functional RPE. Recent advances in the molecular biology of melanogenesis and the fluid transport-linked Na-K-2Cl cotransporter in other cells will allow future studies of VIP modulated events in the RPE at the molecular level. Finally, identification of RPE differentiation factors may prove essential for the ultimate success of RPE transplantation, thus promoting the rescue of photoreceptor cells in retinal degeneration.

Microtubule configurations and post-translational alpha-tubulin modifications during mammalian spermatogenesis

The mechanisms underlying cell cycle progression and differentiation are tightly entwined with changes associated in the structure and composition of the cytoskeleton. Mammalian spermatogenesis is a highly intricate process that involves differentiation and polarization of the round spermatid. We found that pachytene spermatocytes and round spermatids have most of the microtubules randomly distributed in a cortical network without any apparent centrosome. The Golgi apparatus faces the acrosomal vesicle and some microtubules contact its surface. In round spermatids, at step 7, there is an increase in short microtubules around and over the nucleus. These microtubules are located between the rims of the acrosome and may be the very first sign in the formation of the manchette. This new microtubular configuration is correlated with the beginning of the migration of the Golgi apparatus from the acrosomal region towards the opposite pole of the cell. Next, the cortical microtubules form a bundle running around the nucleus perpendicular to the main axis of the cell. At later stages, the nuclear microtubules increase in size and a fully formed manchette appears at stage 9. On the other hand, acetylated tubulin is present in a few microtubules in pachytene spermatocytes and in the axial filament (precursor of the sperm tail) in round spermatids. Our results suggest that at step 7, the spermatid undergoes a major microtubular reordering that induces or allows organelle movement and prepares the cell for the formation of the manchette and further nuclear shaping. This new microtubular configuration is associated with an increase in short microtubules over the nucleus that may correspond to the initial step of the manchette formation. The new structure of the cytoskeleton may be associated with major migratory events occurring at this step of differentiation.

Uniform apicobasal polarity of microtubules and apical location of gamma-tubulin in polarized intestinal epithelium in situ

Polarized differentiation of the intestinal epithelium has been previously shown to depend on an intact microtubular system that is essential for vectorial delivery of apical membrane proteins to the apical cell surfaces. Uniform alignment and polarization of microtubules have been suggested to provide the ultrastructural basis for vectorial transport between the Golgi apparatus and the apical cell surface. In the present study we applied the hook decoration technique to analyse the polarity of microtubules in the rat jejunal epithelium. By immunocytochemistry we studied the subcellular location of gamma-tubulin, an essential component of microtubule-organizing centers. Microtubules were found to be mainly aligned parallel to the apicobasal axis of the cells and to extend from the subterminal space underneath the apical terminal web down to the cellular basis. We found that 98% out of 1122 decorated microtubules displayed uniform apicobasal polarity with the growing ends (plus ends) pointing basally and the non-growing ends (minus ends) pointing towards the cellular apex. No differences were observed with respect to microtubular polarity between the apical, perinuclear and infranuclear cellular portions. Immunostaining specific for gamma-tubulin was restricted to the apical subterminal space underneath the rootlets of microvilli. These findings indicate that the apical subterminal space of enterocytes serves as the predominant if not exclusive microtubule-organizing compartment from which uniformly polarized microtubules grow out with their plus ends towards the cellular basis.

Regeneration and maintenance of bile canalicular networks in collagen-sandwiched hepatocytes

The morphological and cytoskeletal reorganization of collagen-sandwiched rat hepatocytes during the de novo formation of complete canalicular networks was examined by phase, fluorescence and electron microscopy. During the initial stages of membrane repolarization, there was a marked accumulation of both microfilaments and microtubules at the sites of canalicular generation. Microtubule-disrupting agents (colchicine, nocodazole) inhibited the localization of actin filaments at cell margins and the initiation and branching of canalicular networks. After removal of microtubule-disrupting agents, microfilaments relocalized to the canalicular borders and microtubules nucleated along the margins of the bile canaliculi at sites distinct from the peri-canalicular actin networks. Microfilament-perturbing agents (cytochalasin D, phalloidin) did not affect the de novo initiation of bile canaliculi and only slightly impaired the development of canalicular lumina into networks. In established cultures with complete canalicular networks, subsequent treatment with microtubule-disrupting agents did not acutely affect the integrity of preformed canalicular networks. In contrast, treatment with microfilament-perturbing agents caused a marked dilation of most canaliculi. These results illustrate the differential role of the cytoskeleton in the regeneration and maintenance of bile canalicular networks by collagen-sandwiched hepatocytes. Moreover, this study shows the utility of this system as an in vitro model for examining the regulation of cell and membrane polarity.

Translocation of Yersinia entrocolitica across reconstituted intestinal epithelial monolayers is triggered by Yersinia invasin binding to beta1 integrins apically expressed on M-like cells

Yersinia enterocolitica cross the intestinal epithelium via translocation through M cells, which are located in the follicle-associated epithelium (FAE) of Peyer's patches (PP). To investigate the molecular basis of this process, studies were performed using a recently developed in vitro model, in which the enterocyte-like cell line Caco-2 and PP lymphocytes are co-cultured in order to establish FAE-like structures including M cells. Here, we demonstrate that Y. enterocolitica does not adhere significantly to the apical membrane of differentiated enterocyte-like Caco-2 cells that express binding sites for Ulex europaeus agglutinin (UEA)-1. In contrast, Y. enterocolitica adhered to, and was internalized by, cells that lacked UEA-1 binding sites and displayed a disorganized brush border. These cells were considered to be converted to M-like cells. Further analysis revealed that part of these cells expressed beta1 integrins at their apical surface and, as revealed by comparison of wild-type and mutant strains, interacted with invasin of Y. enterocolitica. Consistently, anti-beta1 integrin antibodies significantly inhibited internalization of inv-expressing yersiniae. Experiments with Yersinia mutant strains deficient in YadA or Yop secretion revealed that these virulence factors play a minor role in this process. After internalization, yersiniae were transported within LAMP-1-negative vacuoles to, and released at, the basal surface. Internalization and transport of yersiniae was inhibited by cytochalasin D, suggesting that F-actin assembly is required for this process. These results provide direct evidence that expression of beta1 integrins at the apical surface of M cells enables interaction with the invasin of Y. enterocolitica, and thereby initiates internalization and translocation of bacteria.

Biosynthesis and secretion of the mannose 6-phosphate receptor and its ligands in polarized Caco-2 cells

We have analyzed the transport of newly synthesized mannose 6-phosphate (Man-6-P)-bearing proteins (i.e., lysosomal enzymes) in the polarized human colon adenocarcinoma cell line, Caco-2, by subjecting filter-grown cells to a pulse-chase labeling protocol using [(35)S]methionine, and the resulting cell lysate, apical medium, and basolateral medium were immunoprecipitated with insulin-like growth factor II/Man-6-P receptor (IGF-II/MPR)-specific antisera. The results showed that the majority of secreted lysosomal enzymes accumulated in the apical medium at >2 h of chase and that this polarized distribution was facilitated by the IGF-II/MPR selectively endocytosing lysosomal enzymes from the basolateral surface. Treatment with various agents known to affect vesicular transport events demonstrated that incubations at 16 degrees C or incubations with brefeldin A inhibited the secretion of lysosomal enzymes from both the apical and basolateral surface, whereas treatment with nocodazole selectively blocked apical secretion. In contrast, incubation with NH4Cl or nocodazole had a stimulatory effect on basolateral secretion. Taken together, these results demonstrate that the sorting of Man-6-P-containing proteins into the apical and basolateral secretory pathways is regulated by distinct components of the intracellular trafficking machinery.

Cytoplasmic dynein and dynactin as likely candidates for microtubule-dependent apical targeting of pancreatic zymogen granules

The critical role of microtubules in vectorial delivery of post-Golgi carrier vesicles to the apical cell surface has been established for various polarized epithelial cell types. In the present study we used secretory granules of the rat and chicken pancreas, termed zymogen granules, as model system for apically bound post-Golgi carrier vesicles that underlie the regulated exocytotic pathway. We found that targeting of zymogen granules to the apical cell surface requires an intact microtubule system which contains its colchicine-resistant organizing center and, thus, the microtubular minus ends close to the apical membrane domain. Purified zymogen granules and their membranes were found to be associated with cytoplasmic dynein intermediate and heavy chain and to contain the major components of the dynein activator complex, dynactin, i.e. p150Glued, p62, p50, Arp1, and beta-actin. Kinesin heavy chain and the kinesin receptor, 160 kD kinectin, were not detected as components of zymogen granules. Immunofluorescence staining showed a zymogen granule-like distribution for dynein and dynactin (p150Glued, p62, p50, Arpl) in the apical cytoplasm, whereas kinesin and kinectin were largely concentrated in the basal half of the cells in a pattern similar to the distribution of calreticulin, a component of the endoplasmic reticulum. Secretory granules of non-polarized chromaffin cells of the bovine adrenal medulla, that are assumed to underlie microtubular plus end targeting from the Golgi apparatus to the cell periphery, were not found to be associated with dynein or dynactin. To our knowledge, this is the first demonstration of major components of the dynein-dynactin complex associated with the membrane of a biochemically and functionally well-defined organelle which is considered to underlie a vectorial minus end-driven microtubular transport critically involved in precise delivery of digestive enzymes to the apically located acinar lumen.

Microtubules and signal transduction

Although molecular components of signal transduction pathways are rapidly being identified, how elements of these pathways are positioned spatially and how signals traverse the intracellular environment from the cell surface to the nucleus or to other cytoplasmic targets are not well understood. The discovery of signaling molecules that interact with microtubules (MTs), as well as the multiple effects on signaling pathways of drugs that destabilize or hyperstabilize MTs, indicate that MTs are likely to be critical to the spatial organization of signal transduction. MTs themselves are also affected by signaling pathways and this may contribute to the transmission of signals to downstream targets.

Role of microtubules in the organization of the Golgi complex

The Golgi complex of mammalian cells is composed of cisternal stacks that function in processing and sorting of membrane and luminal proteins during transport from the site of synthesis in the endoplasmic reticulum to lysosomes, secretory vacuoles, and the cell surface. Even though exceptions are found, the Golgi stacks are usually arranged as an interconnected network in the region around the centrosome, the major organizing center for cytoplasmic microtubules. A close relation thus exists between Golgi elements and microtubules (especially the stable subpopulation enriched in detyrosinated and acetylated tubulin). After drug-induced disruption of microtubules, the Golgi stacks are disconnected from each other, partly broken up, dispersed in the cytoplasm, and redistributed to endoplasmic reticulum exit sites. Despite this, intracellular protein traffic is only moderately disturbed. Following removal of the drugs, scattered Golgi elements move along reassembling microtubules back to the centrosomal region and reunite into a continuous system. The microtubule-dependent motor proteins cytoplasmic dynein and kinesin bind to Golgi membranes and have been implicated in vesicular transport to and from the Golgi complex. Microinjection of dynein heavy chain antibodies causes dispersal of the Golgi complex, and the Golgi complex of cells lacking cytoplasmic dynein is likewise spread throughout the cytoplasm. In a similar manner, kinesin antibodies have been found to inhibit Golgi-to-endoplasmic reticulum transport in brefeldin A-treated cells and scattering of Golgi elements along remaining microtubules in cells exposed to a low concentration of nocodazole. The molecular mechanisms in the interaction between microtubules and membranes are, however, incompletely understood. During mitosis, the Golgi complex is extensively reorganized in order to ensure an equal partitioning of this single-copy organelle between the daughter cells. Mitosis-promoting factor, a complex of cdc2 kinase and cyclin B, is a key regulator of this and other events in the induction of cell division. Cytoplasmic microtubules depolymerize in prophase and as a result thereof, the Golgi stacks become smaller, disengage from each other, and take up a perinuclear distribution. The mitotic spindle is thereafter put together, aligns the chromosomes in the metaphase plate, and eventually pulls the sister chromatids apart in anaphase. In parallel, the Golgi stacks are broken down into clusters of vesicles and tubules and movement of protein along the exocytic and endocytic pathways is inhibited. Using a cell-free system, it has been established that the fragmentation of the Golgi stacks is due to a continued budding of transport vesicles and a concomitant inhibition of the fusion of the vesicles with their target membranes. In telophase and after cytokinesis, a Golgi complex made up of interconnected cisternal stacks is recreated in each daughter cell and intracellular protein traffic is resumed. This restoration of a normal interphase morphology and function is dependent on reassembly of a radiating array of cytoplasmic microtubules along which vesicles can be carried and on reactivation of the machinery for membrane fusion.

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.

From cytoskeleton to polarity and chemoreception in the gut epithelium

The membrane surface of polarized epithelial cells can be separated in apical and basolateral domains that differ in molecular composition and function. Components of the cytoskeleton obviously play an important role in both generation and maintenance of epithelial polarity. The microtubular system is uniformly aligned in enterocytes and pancreatic acinar cells with the minus ends of the microtubules located apically and the plus ends basally. Drug-induced disassembly of microtubules results in mistargeting of pancreatic zymogen granules and of apical but not basolateral membrane proteins of enterocytes. Association of zymogen granules with the minus end motor protein, cytoplasmic dynein, and components of its receptor, dynactin, indicates that microtubules are important for vectorial delivery of apical but not basolateral post-Golgi vesicles. The lateral membrane domain of the gut epithelium is scaffolded by a spectrin-based membrane cytoskeleton consisting basically of actin, spectrin, and ankyrin. Ankyrin binds to different integral membrane proteins, such as the sodium pump in glandular and kidney epithelial cells or to the anion exchanger (AE2) of gastric parietal cells, thereby probably playing a critical role in maintenance and/or generation of the polarized distribution of these basolateral membrane proteins. Scattered epithelial cells sharing apical cytoskeletal features of gustatory receptor cells were identified as brush cells (tufted cells). These cells are rich in nitric oxide synthase and contain in their apical brush border the gustatory trimeric G-protein, alpha-gustducin, indicating that brush cells are involved in chemoreceptive signaling.

Characterization of furazolidone apical-related effects to human polarized intestinal cells

In studying the effects of furazolidone (FZ) on the human intestinal Caco-2 cell line grown on microporous membrane, we have previously demonstrated a higher toxicity when the compound was administered at the apical (AP) side than at the basolateral (BL) side. Moreover, we have also shown the production, in the intact cells, of a nitroanion radical from FZ by a cytochrome c P450 reductase. The aim of the present study was to investigate which specific cell structures and functions are involved in the observed domain-related toxicity. The relevance of alterations in integrity and selective properties of the intestinal barrier as first-pass site for ingested molecules is also discussed. We have confirmed that, as expected, the Caco-2 cells are protected from FZ injury by a specific inhibitor of the cytochrome c P450 reductase, and we have shown that this protection is more active on the apical side of the cells. In sublethal conditions, FZ causes increased permeability to 3H-mannitol and, to a different extent, to 3H-inulin. Again the effect is higher when the cells are apically exposed. We have thus examined the tight junctions morphology: a disruption of the apical perijunctional actin-bound cytoskeleton was detected by rhodamine-phalloidin staining and microtubule disorganization by antitubulin fluoresceinated antibodies. Again, the effect was more evident when the cells were apically treated with FZ. Preferential transport and accumulation of the compound by active transport mechanisms could be excluded, since transport of FZ was linear and no intracellular accumulation was detected either from the AP and or the BL sides. All together these results may suggest that the AP formation of the active metabolite and its possible reactivity with SH groups of perijunctional microfilaments could be responsible of the higher FZ apical toxicity. This study shows that polarized differentiated cells are very interesting in vitro models to investigate specific cellular domains as targets of toxic effects and to detect subtle changes that may be induced, in absence of cell death, in specialized epithelial layers.

Actin filaments and microtubules are involved in different membrane traffic pathways that transport sphingolipids to the apical surface of polarized HepG2 cells

In polarized HepG2 hepatoma cells, sphingolipids are transported to the apical, bile canalicular membrane by two different transport routes, as revealed with fluorescently tagged sphingolipid analogs. One route involves direct, transcytosis-independent transport of Golgi-derived glucosylceramide and sphingomyelin, whereas the other involves basolateral to apical transcytosis of both sphingolipids. We show that these distinct routes display a different sensitivity toward nocodazole and cytochalasin D, implying a specific transport dependence on either microtubules or actin filaments, respectively. Thus, nocodazole strongly inhibited the direct route, whereas sphingolipid transport by transcytosis was hardly affected. Moreover, nocodazole blocked "hyperpolarization," i.e., the enlargement of the apical membrane surface, which is induced by treating cells with dibutyryl-cAMP. By contrast, the transcytotic route but not the direct route was inhibited by cytochalasin D. The actin-dependent step during transcytotic lipid transport probably occurs at an early endocytic event at the basolateral plasma membrane, because total lipid uptake and fluid phase endocytosis of horseradish peroxidase from this membrane were inhibited by cytochalasin D as well. In summary, the results show that the two sphingolipid transport pathways to the apical membrane must have a different requirement for cytoskeletal elements.

GalNAc-alpha-O-benzyl inhibits NeuAcalpha2-3 glycosylation and blocks the intracellular transport of apical glycoproteins and mucus in differentiated HT-29 cells

Exposure for 24 h of mucus-secreting HT-29 cells to the sugar analogue GalNAc-alpha-O-benzyl results in inhibition of Galbeta1-3GalNAc:alpha2,3-sialyltransferase, reduced mucin sialylation, and inhibition of their secretion (Huet, G., I. Kim, C. de Bolos, J.M. Loguidice, O. Moreau, B. Hémon, C. Richet, P. Delannoy, F.X. Real., and P. Degand. 1995. J. Cell Sci. 108:1275-1285). To determine the effects of prolonged inhibition of sialylation, differentiated HT-29 populations were grown under permanent exposure to GalNAc-alpha-O-benzyl. This results in not only inhibition of mucus secretion, but also in a dramatic swelling of the cells and the accumulation in intracytoplasmic vesicles of brush border-associated glycoproteins like dipeptidylpeptidase-IV, the mucin-like glycoprotein MUC1, and carcinoembryonic antigen which are no longer expressed at the apical membrane. The block occurs beyond the cis-Golgi as substantiated by endoglycosidase treatment and biosynthesis analysis. In contrast, the polarized expression of the basolateral glycoprotein GP 120 is not modified. Underlying these effects we found that (a) like in mucins, NeuAcalpha2-3Gal-R is expressed in the terminal position of the oligosaccharide species associated with the apical, but not the basolateral glycoproteins of the cells, and (b) treatment with GalNAc-alpha-O-benzyl results in an impairment of their sialylation. These effects are reversible upon removal of the drug. It is suggested that alpha2-3 sialylation is involved in apical targeting of brush border membrane glycoproteins and mucus secretion in HT-29 cells.

Apiconuclear organization of microtubules does not specify protein delivery from the trans-Golgi network to different membrane domains in polarized epithelial cells

In nonpolarized epithelial cells, microtubules originate from a broad perinuclear region coincident with the distribution of the Golgi complex and extend outward to the cell periphery (perinuclear [PN] organization). During development of epithelial cell polarity, microtubules reorganize to form long cortical filaments parallel to the lateral membrane, a meshwork of randomly oriented short filaments beneath the apical membrane, and short filaments at the base of the cell; the Golgi becomes localized above the nucleus in the subapical membrane cytoplasm (apiconuclear [AN] organization). The AN-type organization of microtubules is thought to be specialized in polarized epithelial cells to facilitate vesicle trafficking between the trans-Golgi Network (TGN) and the plasma membrane. We describe two clones of MDCK cells, which have different microtubule distributions: clone II/G cells, which gradually reorganize a PN-type distribution of microtubules and the Golgi complex to an AN-type during development of polarity, and clone II/J cells which maintain a PN-type organization. Both cell clones, however, exhibit identical steady-state polarity of apical and basolateral proteins. During development of cell surface polarity, both clones rapidly establish direct targeting pathways for newly synthesized gp80 and gp135/170, and E-cadherin between the TGN and apical and basolateral membrane, respectively; this occurs before development of the AN-type microtubule/Golgi organization in clone II/G cells. Exposure of both clone II/G and II/J cells to low temperature and nocodazole disrupts >99% of microtubules, resulting in: 1) 25-50% decrease in delivery of newly synthesized gp135/170 and E-cadherin to the apical and basolateral membrane, respectively, in both clone II/G and II/J cells, but with little or no missorting to the opposite membrane domain during all stages of polarity development; 2) approximately 40% decrease in delivery of newly synthesized gp80 to the apical membrane with significant missorting to the basolateral membrane in newly established cultures of clone II/G and II/J cells; and 3) variable and nonspecific delivery of newly synthesized gp80 to both membrane domains in fully polarized cultures. These results define several classes of proteins that differ in their dependence on intact microtubules for efficient and specific targeting between the Golgi and plasma membrane domains.

Peptide YY selectively stimulates expression of the colonocytic phenotype

Peptide YY (PYY) is produced by colonic mucosal endocrine cells and modulates gastrointestinal endocrine activity through specific Y-receptors. The direct effects of PYY on intestinal mucosal growth and differentiation remain uncharacterized. The abundance of PYY in colonic mucosa suggests that PYY acts locally to maintain colonocytic differentiation. We tested this hypothesis in human Caco-2 intestinal epithelial cells, which express alkaline phosphatase (AP) and dipeptidyl dipeptidase (DP), brush-border enzymes differentially concentrated in large and small intestinal mucosa, respectively. The effects of PYY on enzyme specific activity were compared with those of pancreatic polypeptide, neuropeptide-Y, vasoactive intestinal peptide, pentagastrin, bombesin, and selective Y1- and Y2-receptor agonists. Brush-border enzyme activity was assessed by AP and DP specific activity in cell lysates quantitated spectrophotometrically following synthetic substrate digestion. PYY, neuropeptide-Y, pancreatic polypeptide, and vasoactive intestinal peptide (10(-7) mol/L) stimulated AP activity. PYY brought about the greatest increase (38.0%+/-11.0%, n=48). Only PYY decreased DP specific activity (7.9%+/-2.2%, n=48). The Y2-agonist but not the Y1-agonist mimicked these PYY effects (increasing AP 28.3%+/-3.5% and decreasing DP 10.4%+/-3.6%). These data suggest that PYY promotes differentiation toward a colonocytic phenotype in Caco-2 intestinal epithelial cells and that this effect may be mediated through the Y2-receptor subtype.

Disruption of microtubules reveals two independent apical targeting mechanisms for G-protein-coupled receptors in polarized renal epithelial cells

G-protein-coupled receptors demonstrate differing trafficking itineraries in polarized Madin-Darby canine kidney (MDCK II) cells. The alpha2A adrenergic receptor (alpha2AAR) is directly delivered to the basolateral subdomain; the A1 adenosine receptor (A1AdoR) is apically enriched in its targeting; and the alpha2BAR subtype is randomly delivered to both domains but selectively retained basolaterally (Keefer, J. R., and Limbird, L. E. (1993) J. Biol. Chem. 268, 11340-11347; Saunders, C., Keefer, J. R., Kennedy, A. P., Wells, J. N., and Limbird, L. E. (1996) J. Biol. Chem. 271, 995-1002; Wozniak, M., and Limbird, L. E. (1996) J. Biol. Chem. 271, 5017-5024). The present studies explore the role of the polarized cytoskeleton in localization of G-protein-coupled receptors in MDCK II cells. Nocodazole or colchicine, which disrupt microtubules, did not perturb lateral localization of alpha2AR subtypes but led to a relocalization the A1AdoR to the basolateral surface, revealed by immunocytochemical and metabolic labeling strategies. Conversely, the apical component of the random delivery of alpha2BAR was not affected by these agents, suggesting microtubule-dependent and -independent apical targeting mechanisms for G-protein-coupled receptors in polarized cells. Apparent rerouting of the apically targeted A1AdoR was selective for microtubule-disrupting agents, since cytochalasin D, which disrupts actin polymerization, did not alter A1AdoR or alpha2BAR localization or targeting. These data suggest that multiple apical targeting mechanisms exist for G-protein-coupled receptors and that microtubule-disrupting agents serve as tools to probe their different trafficking mechanisms.

Substrate recognition by osteoclast precursors induces C-src/microtubule association

The osteoclast is distinguished from other macrophage polykaryons by its polarization, a feature induced by substrate recognition. The most striking component of the polarized osteoclast is its ruffled membrane, probably reflecting insertion of intracellular vesicles into the bone apposed plasmalemma. The failure of osteoclasts in c-src-/- osteopetrotic mice to form ruffled membranes indicates pp60(c-src) (c-src) is essential to osteoclast polarization. Interestingly, c-src itself is a vesicular protein that targets the ruffled membrane. This being the case, we hypothesized that matrix recognition by osteoclasts, and their precursors, induces c-src to associate with microtubules that traffic proteins to the cell surface. We find abundant c-src associates with tubulin immunoprecipitated from avian marrow macrophages (osteoclast precursors) maintained in the adherent, but not nonadherent, state. Since the two proteins colocalize only within adherent avian osteoclast-like cells examined by double antibody immunoconfocal microscopy, c-src/tubulin association reflects an authentic intracellular event. C-src/tubulin association is evident within 90 min of cell-substrate recognition, and the event does not reflect increased expression of either protein. In vitro kinase assay demonstrates tubulin-associated c-src is enzymatically active, phosphorylating itself as well as exogenous substrate. The increase in microtubule-associated kinase activity attending adhesion mirrors tubulin-bound c-src and does not reflect enhanced specific activity. The fact that microtubule-dissociating drugs, as well as cold, prevent adherence-induced c-src/tubulin association indicates the protooncogene complexes primarily, if not exclusively, with polymerized tubulin. Association of the two proteins does not depend upon protein tyrosine phosphorylation and is substrate specific, as it is induced by vitronectin and fibronectin but not type 1 collagen. Finally, consistent with cotransport of c-src and the osteoclast vacuolar proton pump to the polarized plasmalemma, the H+-ATPase decorates microtubules in a manner similar to the protooncogene, specifically coimmunoprecipitates with c-src from the osteoclast light Golgi membrane fraction, and is present, with c-src, in preparations enriched with acidifying vesicles reconstituted from the osteoclast ruffled membrane.

Bacteroides fragilis toxin rapidly intoxicates human intestinal epithelial cells (HT29/C1) in vitro

Enterotoxigenic Bacteroides fragilis strains associated with childhood diarrhea produce a 20-kDa protein toxin (BFT). Purified BFT causes striking morphologic changes in subconfluent human colonic epithelial cells (HT29/C1). In a 3-h HT29/C1 cell assay, the estimated half-maximal effective concentration of BFT was 12.5 pM, and morphologic effects were detectable as early as 30 min and nearly complete by 1.5 h. Concentrations as low as 0.5 pM could also cause intoxication, but morphologic changes were detectable only when the assay was extended to 18 h. The onset of this intoxication was concentration dependent and rapid, occurring within minutes (<7 min at 0.25 nM, <2 min at 2.5 nM). Notably, the onset of intoxication at 37 degrees C became irreversible to washing within 2 min after exposure to BFT. Morphologic changes were completely inhibited by treatment of HT29/C1 cells with BFT at 4 degrees C but could be demonstrated by subsequent warming to temperatures of 15 degrees C or higher after washing. The time required for the association of BFT with HT29/C1 cells at 4 degrees C was inversely correlated with concentration. Inhibitors of endosomal and Golgi trafficking (NH4Cl and brefeldin A) prevented the intoxication of HT29/C1 cells by Clostridium difficile toxin A and cholera toxin, respectively, but not by BFT. Agents altering microtubule structure did not affect the cellular activity of BFT. These data indicate that a purified toxin from B. fragilis strains associated with diarrhea rapidly and irreversibly intoxicates human intestinal epithelial cells (HT29/C1) in a concentration- and temperature-dependent manner and that the process of intoxication may not involve internalization mechanisms utilizing microtubules or sensitive to pH or brefeldin A.

Involvement of the mutated M protein in altered budding polarity of a pantropic mutant, F1-R, of Sendai virus

Wild-type Sendai virus buds at the apical plasma membrane domain of polarized epithelial MDCK cells, whereas a pantropic mutant, F1-R, buds at both the apical and basolateral domains. In F1-R-infected cells, polarized protein transport and the microtubule network are impaired. It has been suggested that the mutated F and/or M proteins in F1-R are responsible for these changes (M. Tashiro, J. T. Seto, H.-D. Klenk, and R. Rott, J. Virol. 67:5902-5910, 1993). To clarify which gene or mutation(s) was responsible for the microtubule disruption which leads to altered budding of F1-R, MDCK cell lines containing the M gene of either the wild type or F1-R were established. When wild-type M protein was expressed at a level corresponding to that synthesized in virus-infected cells, cellular polarity and the integrity of the microtubules were affected to some extent. On the other hand, expression of the mutated F1-R M protein resulted in the formation of giant cells about 40 times larger than normal MDCK cells. Under these conditions, the effects on the microtubule network were enhanced. The microtubules were disrupted and polarized protein transport was impaired as indicated by the nonpolarized secretion of gp80, a host cell glycoprotein normally secreted from the apical domain, and bipolar budding of wild-type and F1-R Sendai viruses. The mutated F glycoprotein of F1-R was transported bipolarly in cells expressing the F1-R M protein, whereas it was transported predominantly to the apical domain when expressed alone or in cells coexpressing the wild-type M protein. These findings indicate that the M protein of F1-R is involved in the disruption of the microtubular network, leading to impairment of cellular polarity, bipolar transport of the F glycoprotein, and bipolar budding of the virus.