A possible mechanism for ezrin to establish epithelial cell polarity

Membrane Lipids in Epithelial Polarity: Sorting out the PIPs

The development of cell polarity in epithelia, is critical for tissue morphogenesis and vectorial transport between the environment and the underlying tissue. Epithelial polarity is defined by the development of distinct plasma membrane domains: the apical membrane interfacing with the exterior lumen compartment, and the basolateral membrane directly contacting the underlying tissue. The de novo generation of polarity is a tightly regulated process, both spatially and temporally, involving changes in the distribution of plasma membrane lipids, localization of apical and basolateral membrane proteins, and vesicular trafficking. Historically, the process of epithelial polarity has been primarily described in relation to the localization and function of protein 'polarity complexes.' However, a critical and foundational role is emerging for plasma membrane lipids, and in particular phosphoinositide species. Here, we broadly review the evidence for a primary role for membrane lipids in the generation of epithelial polarity and highlight key areas requiring further research. We discuss the complex interchange that exists between lipid species and briefly examine how major membrane lipid constituents are generated and intersect with vesicular trafficking to be preferentially localized to different membrane domains with a focus on some of the key protein-enzyme complexes involved in these processes.

Gastric Parietal Cell Physiology and Helicobacter pylori-Induced Disease

Acidification of the gastric lumen poses a barrier to transit of potentially pathogenic bacteria and enables activation of pepsin to complement nutrient proteolysis initiated by salivary proteases. Histamine-induced activation of the PKA signaling pathway in gastric corpus parietal cells causes insertion of proton pumps into their apical plasma membranes. Parietal cell secretion and homeostasis are regulated by signaling pathways that control cytoskeletal changes required for apical membrane remodeling and organelle and proton pump activities. Helicobacter pylori colonization of human gastric mucosa affects gastric epithelial cell plasticity and homeostasis, promoting epithelial progression to neoplasia. By intervening in proton pump expression, H pylori regulates the abundance and diversity of microbiota that populate the intestinal lumen. We review stimulation-secretion coupling and renewal mechanisms in parietal cells and the mechanisms by which H pylori toxins and effectors alter cell secretory pathways (constitutive and regulated) and organelles to establish and maintain their inter- and intracellular niches. Studies of bacterial toxins and their effector proteins have provided insights into parietal cell physiology and the mechanisms by which pathogens gain control of cell activities, increasing our understanding of gastrointestinal physiology, microbial infectious disease, and immunology.

ASAP3 regulates microvilli structure in parietal cells and presents intervention target for gastric acidity

Gastric acidity-associated disorders such as peptic ulcer and reflux diseases are widespread, and the reported resistance and side effects of currently used medicines suggest an urgent requirement for alternative therapeutic approaches. Here we demonstrate a critical role of ASAP3 in regulating the microvilli structure of parietal cells in vivo, and reveal the feasibility of controlling gastric acidity by targeting ASAP3. Conditional knockout of ASAP3 in mice caused elongation and stacking of microvilli in parietal cells, and substantially decreased gastric acid secretion. These were associated with active assembly of F-actin caused by a higher level of GTP-bound Arf6 GTPase. Consistently, a small molecular compound QS11 inhibited ASAP3 function and significantly reduced gastric acidity in vivo. Of note, the expression of ASAP3 was positively correlated with gastric acid secretion in 90 human cases, and high expression of ASAP3 was associated with reflux disease and peptic ulcer. These results reveal for the first time that ASAP3 regulates the microvilli structures in parietal cells. Our data also suggest ASAP3 as a feasible and drugable therapeutic target for gastric acidity-associated diseases.

Regulation of Transporters and Channels by Membrane-Trafficking Complexes in Epithelial Cells

The vectorial secretion and absorption of fluid and solutes by epithelial cells is dependent on the polarized expression of membrane solute transporters and channels at the apical and basolateral membranes. The establishment and maintenance of this polarized expression of transporters and channels are affected by divers protein-trafficking complexes. Moreover, regulation of the magnitude of transport is often under control of physiological stimuli, again through the interaction of transporters and channels with protein-trafficking complexes. This review highlights the value in utilizing transporters and channels as cargo to characterize core trafficking machinery by which epithelial cells establish and maintain their polarized expression, and how this machinery regulates fluid and solute transport in response to physiological stimuli.

Disruption of Rab8a and Rab11a causes formation of basolateral microvilli in neonatal enteropathy

Misplaced formation of microvilli to basolateral domains and intracellular inclusions in enterocytes are pathognomonic features in congenital enteropathy associated with mutation of the apical plasma membrane receptor syntaxin 3 (STX3). Although the demonstrated binding of Myo5b to the Rab8a and Rab11a small GTPases in vitro implicates cytoskeleton-dependent membrane sorting, the mechanisms underlying the microvillar location defect remain unclear. By selective or combinatory disruption of Rab8a and Rab11a membrane traffic in vivo, we demonstrate that transport of distinct cargo to the apical brush border rely on either individual or both Rab regulators, whereas certain basolateral cargos are redundantly transported by both factors. Enterocyte-specific Rab8a and Rab11a double-knockout mouse neonates showed immediate postnatal lethality and more severe enteropathy than single knockouts, with extensive formation of microvilli along basolateral surfaces. Notably, following an inducible Rab11a deletion from neonatal enterocytes, basolateral microvilli were induced within 3 days. These data identify a potentially important and distinct mechanism for a characteristic microvillus defect exhibited by enterocytes of patients with neonatal enteropathy.

A single transcription factor is sufficient to induce and maintain secretory cell architecture

Here, Lo et al. demonstrate that cell architecture can be controlled by a developmentally regulated transcriptional program independent of the program that specifies cell identity. They show that MIST1 (BHLHA15) is a “scaling factor” that universally establishes secretory morphology in cells that perform regulated secretion, and targeted deletion of MIST1 causes dismantling of the secretory apparatus of diverse exocrine cells.

We hypothesized that basic helix–loop–helix (bHLH) MIST1 (BHLHA15) is a “scaling factor” that universally establishes secretory morphology in cells that perform regulated secretion. Here, we show that targeted deletion of MIST1 caused dismantling of the secretory apparatus of diverse exocrine cells. Parietal cells (PCs), whose function is to pump acid into the stomach, normally lack MIST1 and do not perform regulated secretion. Forced expression of MIST1 in PCs caused them to expand their apical cytoplasm, rearrange mitochondrial/lysosome trafficking, and generate large secretory granules. Mist1 induced a cohort of genes regulated by MIST1 in multiple organs but did not affect PC function. MIST1 bound CATATG/CAGCTG E boxes in the first intron of genes that regulate autophagosome/lysosomal degradation, mitochondrial trafficking, and amino acid metabolism. Similar alterations in cell architecture and gene expression were also caused by ectopically inducing MIST1 in vivo in hepatocytes. Thus, MIST1 is a scaling factor necessary and sufficient by itself to induce and maintain secretory cell architecture. Our results indicate that, whereas mature cell types in each organ may have unique developmental origins, cells performing similar physiological functions throughout the body share similar transcription factor-mediated architectural “blueprints.”

Ezrin is a Major Regulator of Membrane Tension in Epithelial Cells

Plasma membrane tension is responsible for a variety of cellular functions such as motility, cell division, and endocytosis. Since membrane tension is dominated by the attachment of the actin cortex to the inner leaflet of the plasma membrane, we investigated the importance of ezrin, a major cross-linker of the membrane-cytoskeleton interface, for cellular mechanics of confluent MDCK II cells. For this purpose, we carried out ezrin depletion experiments and also enhanced the number of active ezrin molecules at the interface. Mechanical properties were assessed by force indentation experiments followed by membrane tether extraction. PIP₂ micelles were injected into individual living cells to reinforce the linkage between plasma membrane and actin-cortex, while weakening of this connection was reached by ezrin siRNA and administration of the inhibitors neomycin and NSC 668394, respectively. We observed substantial stiffening of cells and an increase in membrane tension after addition of PIP₂ micelles. In contrast, reduction of active ezrin led to a decrease of membrane tension accompanied by loss of excess surface area, increase in cortical tension, remodelling of actin cytoskeleton, and reduction of cell height. The data confirm the importance of the ezrin-mediated connection between plasma membrane and cortex for cellular mechanics and cell morphology.

Phosphorylation of ezrin on Thr567 is required for the synergistic activation of cell spreading by EPAC1 and protein kinase A in HEK293T cells

Recent studies have demonstrated that the actin binding protein, ezrin, and the cAMP-sensor, EPAC1, cooperate to induce cell spreading in response to elevations in intracellular cAMP. To investigate the mechanisms underlying these effects we generated a model of EPAC1-dependent cell spreading based on the stable transfection of EPAC1 into HEK293T (HEK293T-EPAC1) cells. We found that direct activation of EPAC1 with the EPAC-selective analogue, 8-pCPT-2'-O-Me-cAMP (007), promoted cell spreading in these cells. In addition, co-activation of EPAC1 and PKA, with a combination of the adenylate cyclase activator, forskolin, and the cAMP phosphodiesterase inhibitor, rolipram, was found to synergistically enhance cell spreading, in association with cortical actin bundling and mobilisation of ezrin to the plasma membrane. PKA activation was also associated with phosphorylation of ezrin on Thr567, as detected by an electrophoretic band mobility shift during SDS-PAGE. Inhibition of PKA activity blocked ezrin phosphorylation and reduced the cell spreading response to cAMP elevation to levels induced by EPAC1-activation alone. Transfection of HEK293T-EPAC1 cells with inhibitory ezrin mutants lacking the key PKA phosphorylation site, ezrin-Thr567Ala, or the ability to associate with actin, ezrin-Arg579Ala, promoted cell arborisation and blocked the ability of EPAC1 and PKA to further promote cell spreading. The PKA phospho-mimetic mutants of ezrin, ezrin-Thr567Asp had no effect on EPAC1-driven cell spreading. Our results indicate that association of ezrin with the actin cytoskeleton and phosphorylation on Thr567 are required, but not sufficient, for PKA and EPAC1 to synergistically promote cell spreading following elevations in intracellular cAMP.

Moesin expression in fibrosis in the mouse cornea after sterile mechanical trauma or infection

PURPOSE

The aim of this study was to compare the expression patterns of 3 important biochemical characteristics of fibrosis-moesin, transforming growth factor (TGF)-β1, and α-smooth muscle actin (SMA) in the mouse cornea with fibrosis induced by common etiologies-sterile mechanical injury and infection.

METHODS

Corneas of 8-week-old C57BL6 mice were either wounded after an anterior keratectomy or were infected by Pseudomonas aeruginosa, and the animals were killed on days 2 and 7, and on weeks 2 and 4 after the procedure. Western blot and immunofluorescence were used to analyze the expression of moesin and phospho-moesin, and the presence of myofibroblasts identified by the expression of α-SMA in the corneal stroma. The expression of TGF-β1 was analyzed by immunofluorescence.

RESULTS

By immunofluorescent analysis, TGF-β1, α-SMA, and phospho-moesin were not detected in the normal corneal stroma. However, after either treatment, TGF-β1 expression increased, along with phospho-moesin in the wounded corneal stroma until day 7, and decreased after week 2. No expression of TGF-β1 and phospho-moesin was found at postoperative week 4. Moesin expression increased until week 2. Myofibroblasts positive for α-SMA were detected on day 2 until week 4 and peaked at week 2. Western blot analysis confirmed the immunofluorescent data for moesin, phospho-moesin, and α-SMA.

CONCLUSIONS

The similar expression pattern of moesin, phospho-moesin, TGF-β1, and α-SMA in the mouse cornea with fibrosis caused by sterile mechanical injury or infection indicated a role for moesin signaling in corneal fibrosis. Interference with the action of moesin may be a potential approach for intervention strategies to avert fibrosis after infection or mechanical injury.

Desmoglein 3: a help or a hindrance in cancer progression?

Desmoglein 3 is one of seven desmosomal cadherins that mediate cell-cell adhesion in desmosomes. Desmosomes are the intercellular junctional complexes that anchor the intermediate filaments of adjacent cells and confer strong cell adhesion thus are essential in the maintenance of tissue architecture and structural integrity. Like adherens junctions, desmosomes function as tumour suppressors and are down regulated in the process of epithelial-mesenchymal transition and in tumour cell invasion and metastasis. However, recently several studies have shown that various desmosomal components, including desmoglein 3, are up-regulated in cancer with increased levels of expression correlating with the clinical stage of malignancy, implicating their potentiality to serve as a diagnostic and prognostic marker. Furthermore, in vitro studies have demonstrated that overexpression of desmoglein 3 in cancer cell lines activates several signal pathways that have an impact on cell morphology, adhesion and locomotion. These additional signalling roles of desmoglein 3 may not be associated to its adhesive function in desmosomes but rather function outside of the junctions, acting as a key regulator in the control of actin based cellular processes. This review will discuss recent advances which support the role of desmoglein 3 in cancer progression.

MiR-199a inhibits the ability of proliferation and migration by regulating CD44-Ezrin signaling in cutaneous squamous cell carcinoma cells

Cutaneous squamous cell carcinoma (cSCC), the second most common form of human cancer, is an epithelial skin tumor, which can result in metastasis with lethal consequences accounting for about 20% of all skin cancer-related deaths. The metastasis is the main reason for cSCC-related deaths with an overall 5-year survival rate < 30% in cases that spread systemically. The role of miRNAs has been involved in SCC of different origins. Recent data have revealed that the expression of miRNA-199a was changed in many human cancers. In this study, we found that miR-199a was significantly decreased in cSCC tissues, which had an inverse relationship with CD44. MiR-199a specifically regulated the expression of CD44 at mRNA and protein levels, and the interaction between CD44 and Ezrin in cSCC cells. Moreover, the suppressive role of miR-199a in cell migration in cSCC cells was also associated with the activity of MMP2 and MMP9. Taken together, our data indicated that increased expression of endogenous mature miR-199a might prevent the growth and migration of human cSCC via decreasing the expression of CD44 and regulating the interaction between CD44 and Ezrin, which may provide a potentially important therapeutic target for human cSCC.

Ezrin regulates the expression of Mrp2/Abcc2 and Mdr1/Abcb1 along the rat small intestinal tract

Multidrug resistance-associated protein 2 (MRP2)/ATP-binding cassette protein C2 (ABCC2) and multidrug resistance protein 1 (MDR1)/ABCB1 are well-known efflux transporters located on the brush border membrane of the small intestinal epithelia, where they limit the absorption of a broad range of substrates. The expression patterns of MRP2/ABCC2 and MDR1/ABCB1 along the small intestinal tract are tightly regulated. Several reports have demonstrated the participation of ERM (ezrin/radixin/moesin) proteins in the posttranslational modulation of MRP2/ABCC2 and MDR1/ABCB1, especially with regard to their membrane localization. The present study focused on the in vivo expression profiles of MRP2/ABCC2, MDR1/ABCB1, ezrin, and phosphorylated ezrin to further elucidate the relationship between the efflux transporters and the ERM proteins. The current results showed good correlation between the phosphorylation status of ezrin and Mrp2/Abcc2 expression along the gastrointestinal tract of rats and between the expression profiles of both ezrin and Mdr1/Abcb1 in the small intestine. We also demonstrated the involvement of conventional protein kinase C isoforms in the regulation of ezrin phosphorylation. Furthermore, experiments conducted with wild-type (WT) ezrin and a T567A (Ala substituted Thr) dephosphorylated mutant showed a decrease in membrane surface-localized and total expressed MRP2/ABCC2 in T567A-expressing vs. WT ezrin-expressing Caco-2 cells. In contrast, T567A- and WT-expressing cells both showed an increase in membrane surface-localized and total expressed MDR1/ABCB1. These findings suggest that the phosphorylation status and the expression profile of ezrin differentially direct MRP2/ABCC2 and MDR1/ABCB1 expression, respectively, along the small intestinal tract.

Sphingolipid regulation of ezrin, radixin, and moesin proteins family: implications for cell dynamics

A key but poorly studied domain of sphingolipid functions encompasses endocytosis, exocytosis, cellular trafficking, and cell movement. Recently, the ezrin, radixin and moesin (ERM) family of proteins emerged as novel potent targets regulated by sphingolipids. ERMs are structural proteins linking the actin cytoskeleton to the plasma membrane, also forming a scaffold for signaling pathways that are used for cell proliferation, migration and invasion, and cell division. Opposing functions of the bioactive sphingolipid ceramide and sphingosine-1-phosphate (S1P), contribute to ERM regulation. S1P robustly activates whereas ceramide potently deactivates ERM via phosphorylation/dephosphorylation, respectively. This recent dimension of cytoskeletal regulation by sphingolipids opens up new avenues to target cell dynamics, and provides further understanding of some of the unexplained biological effects mediated by sphingolipids. In addition, these studies are providing novel inroads into defining basic mechanisms of regulation and action of bioactive sphingolipids. This review describes the current understanding of sphingolipid regulation of the cytoskeleton, it also describes the biologies in which ERM proteins have been involved, and finally how these two large fields have started to converge. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Desmoglein 3 promotes cancer cell migration and invasion by regulating activator protein 1 and protein kinase C-dependent-Ezrin activation

Desmoglein 3 (Dsg3), the pemphigus vulgaris antigen, has recently been shown to be upregulated in squamous cell carcinoma (SCC) and has been identified as a good tumor-specific marker for clinical staging of cervical sentinel lymph nodes in head and neck SCC. However, little is known about its biological function in cancer. The actin-binding protein Ezrin and the activator protein 1 (AP-1) transcription factor are implicated in cancer progression and metastasis. Here, we report that Dsg3 regulates the activity of c-Jun/AP-1 as well as protein kinase C (PKC)-mediated phosphorylation of Ezrin-Thr567, which contributes to the accelerated motility of cancer cells. Ectopic expression of Dsg3 in cancer cell lines caused enhanced phosphorylation at Ezrin-Thr567 with concomitant augmented membrane protrusions, cell spreading and invasive phenotype. We showed that Dsg3 formed a complex with Ezrin at the plasma membrane that was required for its proper function of interacting with F-actin and CD44 as Dsg3 knockdown impaired these associations. The increased Ezrin phosphorylation in Dsg3-overexpressing cells could be abrogated substantially by various pharmacological inhibitors for Ser/Thr kinases, including PKC and Rho kinase that are known to activate Ezrin. Furthermore, a marked increase in c-Jun S63 phosphorylation, among others, was found in Dsg3-overexpressing cells and the activation of c-Jun/AP-1 was further supported by a luciferase reporter assay. Taken together, our study identifies a novel Dsg3-mediated c-Jun/AP-1 regulatory mechanism and PKC-dependent Ezrin phosphorylation that could be responsible for Dsg3-associated cancer metastasis.

Intracellular lumen extension requires ERM-1-dependent apical membrane expansion and AQP-8-mediated flux

Many unicellular tubes such as capillaries form lumens intracellularly, a process that is not well understood. Here we show that the cortical membrane organizer ERM-1 is required to expand the intracellular apical/lumenal membrane and its actin undercoat during single-cell C.elegans excretory canal morphogenesis. We characterize AQP-8, identified in an ERM-1 overexpression (ERM-1[++]) suppressor screen, as a canalicular aquaporin that interacts with ERM-1 in lumen extension in a mercury-sensitive manner, implicating water-channel activity. AQP-8 is transiently recruited to the lumen by ERM-1, co-localizing in peri-lumenal cuffs interspaced along expanding canals. An ERM-1[++]-mediated increase in the number of lumen-associated canaliculi is reversed by AQP-8 depletion. We propose that the ERM-1-AQP-8 interaction propels lumen extension by translumenal flux, suggesting a direct morphogenetic effect of water-channel-regulated fluid pressure.

Clathrin and AP-1 regulate apical polarity and lumen formation during C. elegans tubulogenesis

Clathrin coats vesicles in all eukaryotic cells and has a well-defined role in endocytosis, moving molecules away from the plasma membrane. Its function on routes towards the plasma membrane was only recently appreciated and is thought to be limited to basolateral transport. Here, an unbiased RNAi-based tubulogenesis screen identifies a role of clathrin (CHC-1) and its AP-1 adaptor in apical polarity during de novo lumenal membrane biogenesis in the C. elegans intestine. We show that CHC-1/AP-1-mediated polarized transport intersects with a sphingolipid-dependent apical sorting process. Depleting each presumed trafficking component mislocalizes the same set of apical membrane molecules basolaterally, including the polarity regulator PAR-6, and generates ectopic lateral lumens. GFP::CHC-1 and BODIPY-ceramide vesicles associate perinuclearly and assemble asymmetrically at polarized plasma membrane domains in a co-dependent and AP-1-dependent manner. Based on these findings, we propose a trafficking pathway for apical membrane polarity and lumen morphogenesis that implies: (1) a clathrin/AP-1 function on an apically directed transport route; and (2) the convergence of this route with a sphingolipid-dependent apical trafficking path.

Acid secretion-associated translocation of KCNJ15 in gastric parietal cells

Potassium ions are required for gastric acid secretion. Several potassium channels have been implicated in providing K(+) at the apical membrane of parietal cells. In examining the mRNA expression levels between gastric mucosa and liver tissue, KCNJ15 stood out as the most highly specific K(+) channel in the gastric mucosa. Western blot analysis confirmed that KCNJ15 is abundant in the stomach. Immunofluorescence staining of isolated gastric glands indicated that KCNJ15 was expressed in parietal cells and chief cells, but not in mucous neck cells. In resting parietal cells, KCNJ15 was mainly found in puncta throughout the cytoplasm but was distinct from H(+)-K(+)-ATPase. Upon stimulation, KCNJ15 and H(+)-K(+)-ATPase become colocalized on the apical membranes, as suggested by immunofluorescence staining. Western blot analysis of the resting and the stimulated membrane fractions confirmed this observation. From nonsecreting preparations, KCNJ15-containing vesicles sedimented after a 4-h centrifugation at 100,000 g, but not after a 30-min spin, which did sediment most of the H(+)-K(+)-ATPase-containing tubulovesicles. Most of the KCNJ15 containing small vesicle population was depleted upon stimulation of parietal cells, as indicated by the fact that the KCNJ15 signal was shifted to a large membrane fraction that sedimented at 4,000 g. Our results demonstrate that, in nonsecreting parietal cells, KCNJ15 is stored in vesicles distinct from the H(+)-K(+)-ATPase-enriched tubulovesicles. Furthermore, upon stimulation, KCNJ15 and H(+)-K(+)-ATPase both translocate to the apical membrane for active acid secretion. Thus KCNJ15 can be added to the family of apical K(+) channels in gastric parietal cells.

Phosphorylation of radixin regulates cell polarity and Mrp-2 distribution in hepatocytes

Radixin, the dominant ezrin-radixin-moesin (ERM) protein in hepatocytes, has two important binding domains: an NH2-terminal region that binds to plasma membrane and a COOH-terminal region that binds to F-actin after a conformational activation by phosphorylation at Thr564. The present studies were undertaken to investigate the cellular changes in expression of radixin in WIF-B cells and to assess radixin distribution and its influence on cell polarity. We used a recombinant adenoviral expression system encoding radixin wild-type and Thr564 mutants fused to cyan fluorescent protein (CFP), as well as conventional immunostaining procedures. Functional analyses were characterized quantitatively. Similar to endogenous radixin, adenovirus-infected radixin-CFP-wild type and nonphosphorylatable radixin-CFP-T564A were found to be expressed heavily in the compartment of canalicular membrane vacuoles, typically colocalizing with multidrug resistance-associated protein 2 (Mrp-2). Expression of radixin-CFP-T564D, which mimics constant phosphorylation, was quite different, being rarely associated with canalicular membranes. The WIF-B cells were devoid of a secretory response, T567D radixin became predominantly redistributed to the basolateral membrane, usually in the form of dense, long spikes and fingerlike projections, and the altered cell polarity involved changes in apical membrane markers. Differences in polar distribution of radixin suggest a role for the linker protein in promoting formation and plasticity of membrane surface projections and also suggest that radixin might be an organizer and regulator of Mrp-2 and cell polarity in hepatocytes.