Achlorhydria by ezrin knockdown: defects in the formation/expansion of apical canaliculi in gastric parietal cells

Rho kinase phosphorylation promotes ezrin-mediated metastasis in hepatocellular carcinoma

During progression of hepatocellular carcinoma, multiple genetic and epigenetic alterations act to posttranslationally modulate the function of proteins that promote cancer invasion and metastasis. To define such abnormalities that contribute to liver cancer metastasis, we carried out a proteomic comparison of primary hepatocellular carcinoma and samples of intravascular thrombi from the same patient. Mass spectrometric analyses of the liver cancer samples revealed a series of acidic phospho-isotypes associated with the intravascular thrombi samples. In particular, we found that Thr567 hyperphosphorylation of the cytoskeletal protein ezrin was tightly correlated to an invasive phenotype of clinical hepatocellular carcinomas and to poor outcomes in tumor xenograft assays. Using phospho-mimicking mutants, we showed that ezrin phosphorylation at Thr567 promoted in vitro invasion by hepatocarcinoma cells. Phospho-mimicking mutant ezrinT567D, but not the nonphosphorylatable mutant ezrinT567A, stimulated formation of membrane ruffles, suggesting that Thr567 phosphorylation promotes cytoskeletal-membrane remodeling. Importantly, inhibition of Rho kinase, either by Y27632 or RNA interference, resulted in inhibition of Thr567 phosphorylation and a blockade to cell invasion, implicating Rho kinase-ezrin signaling in hepatocellular carcinoma cell invasion. Our findings suggest a strategy to reduce liver tumor metastasis by blocking Rho kinase-mediated phosphorylation of ezrin.

Kir4.1 channel expression is essential for parietal cell control of acid secretion

Kir4.1 channels were found to colocalize with the H(+)/K(+)-ATPase throughout the parietal cell (PC) acid secretory cycle. This study was undertaken to explore their functional role. Acid secretory rates, electrophysiological parameters, PC ultrastructure, and gene and protein expression were determined in gastric mucosae of 7-8-day-old Kir4.1-deficient mice and WT littermates. Kir4.1(-/-) mucosa secreted significantly more acid and initiated secretion significantly faster than WT mucosa. No change in PC number but a relative up-regulation of H(+)/K(+)-ATPase gene and protein expression (but not of other PC ion transporters) was observed. Electron microscopy revealed fully fused canalicular membranes and a lack of tubulovesicles in resting state Kir4.1(-/-) PCs, suggesting that Kir4.1 ablation may also interfere with tubulovesicle endocytosis. The role of this inward rectifier in the PC apical membrane may therefore be to balance between K(+) loss via KCNQ1/KCNE2 and K(+) reabsorption by the slow turnover of the H(+)/K(+)-ATPase, with consequences for K(+) reabsorption, inhibition of acid secretion, and membrane recycling. Our results demonstrate that Kir4.1 channels are involved in the control of acid secretion and suggest that they may also affect secretory membrane recycling.

Soybean-derived beta-conglycinin affects proteome expression in pig intestinal cells in vivo and in vitro

It is well known that β-conglycinin, a soybean allergen, induces allergies and causes intestinal damage in fetuses and neonates. However, the underlying mechanisms responsible for the adverse effects of β-conglycinin remain elusive. In particular, it is unknown whether or not this dietary substance causes direct damage affecting the proliferation and integrity of intestinal cells. This study evaluated the effect of different concentrations of β-conglycinin (0 to 1,500 µg/mL) and the duration of culture (48 or 72 h) on the proliferation and proteome of porcine intestinal epithelial cells. Eight individually housed piglets (10 d old; initial BW, 3.79 ± 0.07 kg) were randomly divided into 2 groups (n = 4) and challenged with or without β-conglycinin via oral administration d 10 through 28. After the last administration of β-conglycinin or PBS, piglets were killed and jejuna mucosal samples were collected for proteomic analysis. Supplementing β-conglycinin to either culture medium or weanling pigs increased (P < 0.05) the expression of proteins related to apoptosis, stress, and inflammation, but decreased (P < 0.05) the expression of proteins related to cytoskeleton and nucleus replication in intestinal cells. Further analysis confirmed an increase in caspase-3 expression in the cells exposed to β-conglycinin in vivo and in vitro. Collectively, these novel results indicate that β-conglycinin directly induces intestinal damage by depressing intestinal-cell growth, damaging the cytoskeleton, and causing apoptosis in the piglet intestine.

Emerging role for ERM proteins in cell adhesion and migration

The highly related ERM (Ezrin, Radixin, Moesin) proteins provide a regulated linkage between the membrane and the underlying actin cytoskeleton. They also provide a platform for the transmission of signals in responses to extracellular cues. Studies in different model organisms and in cultured cells have highlighted the importance of ERM proteins in the generation and maintenance of specific domains of the plasma membrane. A central question is how do ERM proteins coordinate actin filament organization and membrane protein transport/stability with signal transduction pathways to build up complex structures? Through their interaction with numerous partners including membrane proteins, actin cytoskeleton and signaling molecules, ERM proteins have the ability to organize multiprotein complexes in specific cellular compartments. Likewise, ERM proteins participate in diverse functions including cell morphogenesis, endocytosis/exocytosis, adhesion and migration. This review focuses on aspects still poorly understood related to the function of ERM proteins in epithelial cell adhesion and migration.

Conformational switching in ezrin regulates morphological and cytoskeletal changes required for B cell chemotaxis

B cell chemotaxis occurs in response to specific chemokine gradients and is critical for homeostasis and immune response. The molecular regulation of B cell membrane-actin interactions during migration is poorly understood. In this study, we report a role for ezrin, a member of the membrane-cytoskeleton cross-linking ezrin-radixin-moesin proteins, in the regulation of the earliest steps of B cell polarization and chemotaxis. We visualized chemokine-induced changes in murine B cell morphology using scanning electron microscopy and spatiotemporal dynamics of ezrin in B cells using epifluorescence and total internal reflection microscopy. Upon chemokine stimulation, ezrin is transiently dephosphorylated to assume an inactive conformation and localizes to the lamellipodia. B cells expressing a phosphomimetic conformationally active mutant of ezrin or those in which ezrin dephosphorylation was pharmacologically inhibited displayed impaired microvillar dynamics, morphological polarization, and chemotaxis. Our data suggest a 2-fold involvement of ezrin in B cell migration, whereby it first undergoes chemokine-induced dephosphorylation to facilitate membrane flexibility, followed by relocalization to the actin-rich lamellipodia for dynamic forward protrusion of the cells.

Update on the mechanisms of gastric acid secretion

Acid-related disorders represent a major healthcare concern. In recent years, our understanding of the physiologic processes underlying gastric acid secretion has improved notably. The identity of several apical ion transport proteins, which are necessary for acid secretion to take place, has been resolved. The recent developments have uncovered potential therapeutic targets for the treatment of acid-related disorders. This brief review provides an update on the mechanisms of gastric acid secretion, with a particular focus on apical ion transport.

Moesin1 and Ve-cadherin are required in endothelial cells during in vivo tubulogenesis

Endothelial tubulogenesis is a crucial step in the formation of functional blood vessels during angiogenesis and vasculogenesis. Here, we use in vivo imaging of living zebrafish embryos expressing fluorescent fusion proteins of beta-Actin, alpha-Catenin, and the ERM family member Moesin1 (Moesin a), to define a novel cord hollowing process that occurs during the initial stages of tubulogenesis in intersegmental vessels (ISVs) in the embryo. We show that the primary lumen elongates along cell junctions between at least two endothelial cells during embryonic angiogenesis. Moesin1-EGFP is enriched around structures that resemble intracellular vacuoles, which fuse with the luminal membrane during expansion of the primary lumen. Analysis of silent heart mutant embryos shows that initial lumen formation in the ISVs is not dependent on blood flow; however, stabilization of a newly formed lumen is dependent upon blood flow. Zebrafish moesin1 knockdown and cell transplantation experiments demonstrate that Moesin1 is required in the endothelial cells of the ISVs for in vivo lumen formation. Our analyses suggest that Moesin1 contributes to the maintenance of apical/basal cell polarity of the ISVs as defined by adherens junctions. Knockdown of the adherens junction protein Ve-cadherin disrupts formation of the apical membrane and lumen in a cell-autonomous manner. We suggest that Ve-cadherin and Moesin1 function to establish and maintain apical/basal polarity during multicellular lumen formation in the ISVs.

A possible mechanism for ezrin to establish epithelial cell polarity

Ezrin is an important membrane/actin cytoskeleton linker protein, especially in epithelia. Ezrin has two important binding domains: an NH(2)-terminal region that binds to plasma membrane and a COOH-terminal region that binds to F-actin only after a conformational activation by phosphorylation at Thr567 of ezrin. The present experiments were undertaken to investigate the detailed cellular changes in the time course of expression of ezrin-T567 mutants (nonphosphorylatable T567A and permanent phospho-mimic T567D) in parietal cells and to assess ezrin distribution and its influence on the elaborate membrane recruitment processes of these cells. T567A mutant and wild-type (WT) ezrin were consistently localized to the apical plasma membrane, even with overexpression. On the other hand, T567D went first to apical membrane at early times and low expression levels, then accumulated mainly at the basal surface after 24 h. Overexpression of WT or T567A led to incorporation of internal membranes to apical vacuoles, while overexpression of T567D led to large incorporation of apical and intracellular membranes (including H-K-ATPase) to the basal surface. Differences in polar distribution of ezrin suggest a role for the linker protein in promoting formation and plasticity of membrane surface projections, forming the basis for a novel theory for ezrin as an organizer and regulator of membrane recruitment. A model simulating the cellular distribution of ezrin and its associated membrane- and F-actin-binding forms is given to predict redistributions observed with phosphorylation and mutant overexpression, and it can easily be modified as more specific information regarding binding constants and specific sites becomes available.

Organizing the cell cortex: the role of ERM proteins

Specialized membrane domains are an important feature of almost all cells. In particular, they are essential to tissues that have a highly organized cell cortex, such as the intestinal brush border epithelium. The ERM proteins (ezrin, radixin and moesin) have a crucial role in organizing membrane domains through their ability to interact with transmembrane proteins and the cytoskeleton. In doing so, they can provide structural links to strengthen the cell cortex and regulate the activities of signal transduction pathways. Recent studies examining the structure and in vivo functions of ERMs have greatly advanced our understanding of the importance of membrane-cytoskeleton interactions.

Phospho-regulated ACAP4-Ezrin interaction is essential for histamine-stimulated parietal cell secretion

The ezrin-radixin-moesin proteins provide a regulated linkage between membrane proteins and the cortical cytoskeleton and also participate in signal transduction pathways. Ezrin is localized to the apical membrane of parietal cells and couples the protein kinase A activation cascade to the regulated HCl secretion. Our recent proteomic study revealed a protein complex of ezrin-ACAP4-ARF6 essential for volatile membrane remodeling (Fang, Z., Miao, Y., Ding, X., Deng, H., Liu, S., Wang, F., Zhou, R., Watson, C., Fu, C., Hu, Q., Lillard, J. W., Jr., Powell, M., Chen, Y., Forte, J. G., and Yao, X. (2006) Mol. Cell Proteomics 5, 1437-1449). However, knowledge of whether ACAP4 physically interacts with ezrin and how their interaction is integrated into membrane-cytoskeletal remodeling has remained elusive. Here we provide the first evidence that ezrin interacts with ACAP4 in a protein kinase A-mediated phosphorylation-dependent manner through the N-terminal 400 amino acids of ACAP4. ACAP4 locates in the cytoplasmic membrane in resting parietal cells but translocates to the apical plasma membrane upon histamine stimulation. ACAP4 was precipitated with ezrin from secreting but not resting parietal cell lysates, suggesting a phospho-regulated interaction. Indeed, this interaction is abolished by phosphatase treatment and validated by an in vitro reconstitution assay using phospho-mimicking ezrin(S66D). Importantly, ezrin specifies the apical distribution of ACAP4 in secreting parietal cells because either suppression of ezrin or overexpression of non-phosphorylatable ezrin prevents the apical localization of ACAP4. In addition, overexpressing GTPase-activating protein-deficient ACAP4 results in an inhibition of apical membrane-cytoskeletal remodeling and gastric acid secretion. Taken together, these results define a novel molecular mechanism linking ACAP4-ezrin interaction to polarized epithelial secretion.

Volume density, distribution, and ultrastructure of secretory and basolateral membranes and mitochondria predict parietal cell secretory (dys)function

Acid secretion in gastric parietal cells requires highly coordinated membrane transport and vesicle trafficking. Histologically, consensus defines acid secretion as the ratio of the volume density (Vd) of canalicular and apical membranes (CAMs) to tubulovesicular (TV) membranes, a value which varies widely under normal conditions. Examination of numerous achlorhydric mice made it clear that this paradigm is discrepant when used to assess most mice with genetic mutations affecting acid secretion. Vd of organelles in parietal cells of 6 genetically engineered mouse strains was obtained to identify a stable histological phenotype of acid secretion. We confirmed that CAM to TV ratio fairly represented secretory activity in untreated and secretion-inhibited wild-type (WT) mice and in NHE2−/− mice as well, though the response was significantly attenuated in the latter. However, high CAM to TV ratios wrongly posed as active acid secretion in AE2−/−, GHKAα−/−, and NHE4−/− mice. Achlorhydric genotypes also had a significantly higher Vd of basolateral membrane than WT mice, and reduced Vd of mitochondria and canaliculi. The Vd of mitochondria, and ratio of the Vd of basolateral membranes/Vd of mitochondria were preferred predictors of the level of acid secretion. Alterations in acid secretion, then, cause significant changes not only in the Vd of secretory membranes but also in mitochondria and basolateral membranes.

Aberrant localization of ezrin correlates with salivary acini disorganization in Sjogren's Syndrome

OBJECTIVES

To analyse whether the alterations in the structure and organization of microvilli in salivary acinar cells from SS patients are linked to changes in the expression and/or cellular localization of ezrin.

METHODS

Salivary gland (SG) acini from controls and SS patients were used to evaluate ezrin expression by western blot and localization of total and activated (phospho-Thr567) ezrin by IF and EM.

RESULTS

In acini from control labial SGs, ezrin was located predominantly at the apical pole and to a lesser extent at the basal region of these cells. Conversely, in acini extracts from SS patients, ezrin showed significantly elevated levels, which were accompanied with localization mostly at the basal region. Moreover, F-actin maintained its distribution in both the apical region and basolateral cortex; however, it was also observed in the acinar cytoplasm. Phospho-ezrin (active form) was located exclusively at the apical pole of acinar cells from control subjects and abundantly located at the basal cytoplasm in SS samples. These results were confirmed by immunogold studies.

CONCLUSIONS

The decrease of ezrin and phospho-ezrin at the apical pole and the cytoplasmic redistribution of F-actin suggest an altered interaction between the F-actin-cytoskeleton and plasma membrane in SS patient acini, which may explain the microvilli disorganization. These alterations could eventually contribute to SG hyposecretion in SS patients.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.

Internalization of NK cells into tumor cells requires ezrin and leads to programmed cell-in-cell death

Cytotoxic lymphocytes are key players in the orchestration of immune response and elimination of defective cells. We have previously reported that natural killer (NK) cells enter target tumor cells, leading to either target cell death or self-destruction within tumor cells. However, it has remained elusive as to the fate of NK cells after internalization and whether the heterotypic cell-in-cell process is different from that of the homotypic cell-in-cell event recently named entosis. Here, we show that NK cells undergo a cell-in-cell process with the ultimate fate of apoptosis within tumor cells and reveal that the internalization process requires the actin cytoskeletal regulator, ezrin. To visualize how NK cells enter into tumor cells, we carried out real-time dual color imaging analyses of NK cell internalization into tumor cells. Surprisingly, most NK cells commit to programmed cell death after their entry into tumor cells, which is distinctively different from entosis observed in the homotypic cell-in-cell process. The apoptotic cell death of the internalized NK cells was evident by activation of caspase 3 and DNA fragmentation. Furthermore, NK cell death after internalization is attenuated by the caspase inhibitor, Z-VAD-FMK, confirming apoptosis as the mode of NK cell death within tumor cells. To determine protein factors essential for the entry of NK cells into tumor cells, we carried out siRNA-based knockdown analysis and discovered a critical role of ezrin in NK cell internalization. Importantly, PKA-mediated phosphorylation of ezrin promotes the NK cell internalization process. Our findings suggest a novel regulatory mechanism by which ezrin governs NK cell internalization into tumor cells.

Tethering, recycling and activation of the epithelial sodium–proton exchanger, NHE3

NHE3 is a sodium–proton exchanger expressed predominantly in the apical membrane of renal and intestinal epithelia, where it plays a key role in salt and fluid absorption and pH homeostasis. It performs these functions through the exchange of luminal sodium for cytosolic protons. Acute regulation of NHE3 function is mediated by altering the total number of exchangers in the plasma membrane as well as their individual activity. Traffic between endomembrane and plasmalemmal pools of NHE3 dictates the density of exchangers available at the cell surface. The activity of the plasmalemmal pool, however,is not fixed and can be altered by the association with modifier proteins, by post-translational alterations (such as cAMP-mediated phosphorylation) and possibly also via interaction with specific plasmalemmal phospholipids. Interestingly, association with cytoskeletal components affects both levels of regulation, tethering NHE3 molecules at the surface and altering their intrinsic activity. This paper reviews the role of proteins and lipids in the modulation of NHE3 function.

Novel insights of the gastric gland organization revealed by chief cell specific expression of moesin

ERM (ezrin, radixin, and moesin) proteins play critical roles in epithelial and endothelial cell polarity, among other functions. In gastric glands, ezrin is mainly expressed in acid-secreting parietal cells, but not in mucous neck cells or zymogenic chief cells. In looking for other ERM proteins, moesin was found lining the lumen of much of the gastric gland, but it was not expressed in parietal cells. No significant radixin expression was detected in the gastric glands. Moesin showed an increased gradient of expression from the neck to the base of the glands. In addition, the staining pattern of moesin revealed a branched morphology for the gastric lumen. This pattern of short branches extending from the glandular lumen was confirmed by using antibody against zonula occludens-1 (ZO-1) to stain tight junctions. With a mucous neck cell probe (lectin GSII, from Griffonia simplicifolia) and a chief cell marker (pepsinogen C), immunohistochemistry revealed that the mucous neck cells at the top of the glands do not express moesin, but, progressing toward the base, mucous cells showing decreased GSII staining had low or moderate level of moesin expression. The level of moesin expression continued to increase toward the base of the glands and reached a plateau in the base where chief cells and parietal cells abound. The level of pepsinogen expression also increased toward the base. Pepsinogen C was located on cytoplasmic granules and/or more generally distributed in chief cells, whereas moesin was exclusively expressed on the apical membrane. This is a clear demonstration of distinctive cellular expression of two ERM family members in the same tissue. The results provide the first evidence that moesin is involved in the cell biology of chief cells. Novel insights on gastric gland morphology revealed by the moesin and ZO-1 staining provide the basis for a model of cell maturation and migration within the gland.

SAP-1 is a microvillus-specific protein tyrosine phosphatase that modulates intestinal tumorigenesis

SAP-1 (PTPRH) is a receptor-type protein tyrosine phosphatase (RPTP) with a single catalytic domain in its cytoplasmic region and fibronectin type III-like domains in its extracellular region. The cellular localization and biological functions of this RPTP have remained unknown, however. We now show that mouse SAP-1 mRNA is largely restricted to the gastrointestinal tract and that SAP-1 protein localizes to the microvilli of the brush border in gastrointestinal epithelial cells. The expression of SAP-1 in mouse intestine is minimal during embryonic development but increases markedly after birth. SAP-1-deficient mice manifested no marked changes in morphology of the intestinal epithelium. In contrast, SAP-1 ablation inhibited tumorigenesis in mice with a heterozygous mutation of the adenomatous polyposis coli gene. These results thus suggest that SAP-1 is a microvillus-specific RPTP that regulates intestinal tumorigenesis.

Expression, localization, and binding activity of the ezrin/radixin/moesin proteins in the mouse testis

The ezrin, radixin, and moesin (ERM) proteins represent a family of adaptor proteins linking transmembrane proteins to the cytoskeleton. The seminiferous epithelium undergoes extensive changes in cellular composition, location, and shape, implicating roles of the membrane-cytoskeleton interaction. It remains unknown, however, whether the ERM proteins are expressed and play significant roles in the testis. In the present study, we examined the spatiotemporal expression of ERM proteins in the mouse testis by Western blotting and immunohistochemistry. Ezrin immunoreactivity was demonstrated in the cytoplasm of steps 15 and 16 spermatids from 5 weeks postpartum through adulthood, whereas radixin immunoreactivity was in the apical cytoplasm of Sertoli cells from 1 week through 2 weeks postpartum. No immunoreactivity for moesin was detected at any age. Immunoprecipitation demonstrated that ezrin was bound to the cytoskeletal component actin, whereas radixin was bound to both actin and tubulin. Of the transmembrane proteins known to interact with ERM proteins, only cystic fibrosis transmembrane conductance regulator, a chloride transporter, was bound to ezrin in elongated spermatids. These results suggest that ezrin is involved in spermiogenesis whereas radixin is involved in the maturation of Sertoli cells, through interaction with different sets of membrane proteins and cytoskeletal components.

TUDCA prevents cholestasis and canalicular damage induced by ischemia-reperfusion injury in the rat, modulating PKCalpha-ezrin pathway

Cholestasis, induced by liver ischemia-reperfusion injury (IRI), is characterized by dilatation of bile canaliculi and loss of microvilli. Tauroursodeoxycholic acid (TUDCA) is an anti-cholestatic agent, modulating protein kinase C (PKC) alpha pathway. PKC reduces ischemic damage in several organs, its isoform alpha modulates ezrin, a key protein in the maintenance of cell lamellipoidal extensions. We evaluated the effects of TUDCA on cholestasis, canalicular changes and PKCalpha-ezrin expression in a rat model of liver IRI. Livers flushed and stored with Belzer solution or Belzer + 10 mm TUDCA (4 degrees C for 6 h) were reperfused (37 degrees C with O(2)) with Krebs-Ringer bicarbonate + 2.5 micromol/min of Taurocholate or TUDCA. Bile was harvested for bile flow assessment. Liver tissue was employed for Electron Microscopy (EM) and for PKCalpha and ezrin immunoblot and immunofluorescence. The same experiments were conducted with the PKCalpha inhibitor Go-6976. TUDCA-treated livers showed increased bile flow (0.25+/-0.17 vs. 0.042+/-0.02 microl/min/g liver, P<0.05) and better preservation of microvilli and bile canalicular area at EM. These effects were associated with increased PKCalpha and ezrin expression (P=0.03 and P=0.04 vs. control respectively), as also confirmed by immunofluorescence data. PKCalpha inhibition abolished these TUDCA effects. TUDCA administration during IRI reduces cholestasis and canalicular damage in the liver modulating PKCalpha-ezrin pathway.

Helicobacter pylori VacA disrupts apical membrane-cytoskeletal interactions in gastric parietal cells

Helicobacter pylori persistently colonize the human stomach and have been linked to atrophic gastritis and gastric carcinoma. Although it is well known that H. pylori infection can result in hypochlorhydria, the molecular mechanisms underlying this phenomenon remain poorly understood. Here we show that VacA permeabilizes the apical membrane of gastric parietal cells and induces hypochlorhydria. The functional consequences of VacA infection on parietal cell physiology were studied using freshly isolated rabbit gastric glands and cultured parietal cells. Secretory activity of parietal cells was judged by an aminopyrine uptake assay and confocal microscopic examination. VacA permeabilization induces an influx of extracellular calcium, followed by activation of calpain and subsequent proteolysis of ezrin at Met(469)-Thr(470), which results in the liberation of ezrin from the apical membrane of the parietal cells. VacA treatment inhibits acid secretion by preventing the recruitment of H,K-ATPase-containing tubulovesicles to the apical membrane of gastric parietal cells. Electron microscopic examination revealed that VacA treatment disrupts the radial arrangement of actin filaments in apical microvilli due to the loss of ezrin integrity in parietal cells. Significantly, expression of calpain-resistant ezrin restored the functional activity of parietal cells in the presence of VacA. Proteolysis of ezrin in VacA-infected parietal cells is a novel mechanism underlying H. pylori-induced inhibition of acid secretion. Our results indicate that VacA disrupts the apical membrane-cytoskeletal interactions in gastric parietal cells and thereby causes hypochlorhydria.

Changes in gene expression of gastric mucosa during therapeutic acid inhibition

BACKGROUND

Long-term therapy with potent acid inhibitors is a common treatment for gastro-esophageal reflux disease. Administration of proton pump inhibitors (PPIs) causes profound and continuous hypochlorhydria by inhibition of the proton pump in gastric parietal cells. Long-term hypergastrinaemia increases mucosal thickness and enterochromaffin-like cell density in oxyntic mucosa.

OBJECTIVE

The aim of this study was to see whether this very common clinical intervention induces significant changes in the gastric mucosal gene expression pattern.

METHODS

Seven patients suffering from gastro-esophageal reflux disease were included in this study. Endoscopic biopsies were taken from the corpus mucosa before and toward the end of a 3-month treatment with the PPI esomeprazole.

RESULTS

Microarray analysis identified 186 differentially expressed genes. A high proportion of genes with changed gene expression levels during PPI treatment are involved in proliferation, apoptosis, and stress response.

CONCLUSION

This study identified many genes that were not previously known to be affected by inhibition of gastric acid secretion. Further characterization of the functional roles of genes whose expression is modulated by potent acid inhibition may give new insight into the biological responses to potent acid inhibition, including the mucosal response to the moderately increased gastrin levels encountered in clinical practice.

Identification of ezrin as a target of gastrin in immature mouse gastric parietal cells

The gastric acid-secreting parietal cell exhibits profound morphological changes on stimulation. Studies in gastrin null (Gas-KO) mice indicate that maturation of parietal cell function depends on the hormone gastrin acting at the G-protein-coupled cholecystokinin 2 receptor. The relevant cellular mechanisms are unknown. The application of differential mRNA display to samples of the gastric corpus of wild-type (C57BL/6) and Gas-KO mice identified the cytoskeletal linker protein, ezrin, as a previously unsuspected target of gastrin. Gastrin administered in vivo or added to gastric glands in vitro increased ezrin abundance in Gas-KO parietal cells. In parietal cells of cultured gastric glands from wild-type mice treated with gastrin, histamine or carbachol, ezrin was localized to vesicular structures resembling secretory canaliculi. In contrast, in cultured parietal cells from Gas-KO mice, ezrin was typically distributed in the cytosol, and this did not change after incubation with gastrin, histamine or carbachol. However, priming with gastrin for approximately 24 h, either in vivo prior to cell culture or by addition to cultured gastric glands, induced the capacity for secretagogue-stimulated localization of ezrin to large vesicular structures in Gas-KO mice. Similarly, in a functional assay based on measurement of intracellular pH, cultured parietal cells from Gas-KO mice were refractory to gastrin unless primed. The priming effect of gastrin was not attributable to the paracrine mediator histamine, but was prevented by inhibitors of protein kinase C and transactivation of the epidermal growth factor receptor. We conclude that in gastrin null mice there is reduced ezrin expression and a defect in ezrin subcellular distribution in gastric parietal cells, and that both can be reversed by priming with gastrin.

Comparative study of ezrin phosphorylation among different tissues: more is good; too much is bad

In a comparison of three different tissues, the membrane cytoskeleton linker protein ezrin was found to assume high levels of phosphorylation on threonine-567 (T567) in the brush border membranes of renal proximal tubule cells and small intestine enterocytes, in contrast to the apical canalicular membrane of gastric parietal cells. Together with an earlier observation that increased T567 phosphorylation is associated with more elaborate microvilli in parietal cells, this comparative study suggested a higher phosphorylation level requirement for the denser and more uniform distribution of microvilli at brush border surfaces. Using a kinase inhibitor, staurosporin, and metabolic inhibitor, sodium azide, relatively high turnover of ezrin T567 phosphorylation was observed in all three epithelia. Aiming to understand the role of phosphorylation turnover in these tissues, detergent extraction analysis of gastric glands and proximal tubules revealed that an increased phosphorylation on ezrin T567 greatly enhanced its association with F-actin, while ezrin-membrane interaction persisted regardless of the changes of phosphorylation level on ezrin T567. Finally, expression of Thr567Asp mutant ezrin, which mimics the phospho-ezrin state but does not allow turnover, caused aberrant growth of membrane projections in cultured proximal tubule cells, consistent with what had previously been observed in several cell lines and gastric parietal cells. These results fit into a model of surface plasticity, which posits that the turnover of phosphorylation on T567 empowers ezrin to relax and reposition membrane to the underlying cytoskeleton under varying conditions of filament growth or rapid membrane expansion (or depletion).

Direct AKAP-mediated protein-protein interactions as potential drug targets

A-kinase-anchoring proteins (AKAPs) are a diverse family of about 50 scaffolding proteins. They are defined by the presence of a structurally conserved protein kinase A (PKA)-binding domain. AKAPs tether PKA and other signalling proteins such as further protein kinases, protein phosphatases and phosphodiesterases by direct protein-protein interactions to cellular compartments. Thus, AKAPs form the basis of signalling modules that integrate cellular signalling processes and limit these to defined sites. Disruption of AKAP functions by gene targeting, knockdown approaches and, in particular, pharmacological disruption of defined AKAP-dependent protein-protein interactions has revealed key roles of AKAPs in numerous processes, including the regulation of cardiac myocyte contractility and vasopressin-mediated water reabsorption in the kidney. Dysregulation of such processes causes diseases, including cardiovascular and renal disorders. In this review, we discuss AKAP functions elucidated by gene targeting and knockdown approaches, but mainly focus on studies utilizing peptides for disruption of direct AKAP-mediated protein-protein interactions. The latter studies point to direct AKAP-mediated protein-protein interactions as targets for novel drugs.

Interaction of ezrin with the novel guanine nucleotide exchange factor PLEKHG6 promotes RhoG-dependent apical cytoskeleton rearrangements in epithelial cells

The mechanisms underlying functional interactions between ERM (ezrin, radixin, moesin) proteins and Rho GTPases are not well understood. Here we characterized the interaction between ezrin and a novel Rho guanine nucleotide exchange factor, PLEKHG6. We show that ezrin recruits PLEKHG6 to the apical pole of epithelial cells where PLEKHG6 induces the formation of microvilli and membrane ruffles. These morphological changes are inhibited by dominant negative forms of RhoG. Indeed, we found that PLEKHG6 activates RhoG and to a much lesser extent Rac1. In addition we show that ezrin forms a complex with PLEKHG6 and RhoG. Furthermore, we detected a ternary complex between ezrin, PLEKHG6, and the RhoG effector ELMO. We demonstrate that PLEKHG6 and ezrin are both required in macropinocytosis. After down-regulation of either PLEKHG6 or ezrin expression, we observed an inhibition of dextran uptake in EGF-stimulated A431 cells. Altogether, our data indicate that ezrin allows the local activation of RhoG at the apical pole of epithelial cells by recruiting upstream and downstream regulators of RhoG and that both PLEKHG6 and ezrin are required for efficient macropinocytosis.

Compartmentalized cAMP signalling in regulated exocytic processes in non-neuronal cells

Cyclic adenosine monophosphate (cAMP) is a central second messenger controlling a plethora of vital functions. Studies of cAMP dynamics in living cells have revealed markedly inhomogeneous concentrations of the second messenger in different compartments. Moreover, cAMP effectors such as cAMP-dependent protein kinase (PKA) and cAMP-activated GTP-exchange factors (Epacs) are tethered to specific cellular sites. Both the tailoring of cAMP concentrations, and the activities of cAMP-dependent signalling systems at specific cellular locations are prerequisites for most, if not all, cAMP-dependent processes. This review focuses on the role of compartmentalized cAMP signalling in exocytic processes in non-neuronal cells. Particularly, the insertion of aquaporin-2 into the plasma membrane of renal principal cells as an example for a cAMP-dependent exocytic process in a non-secretory cell type, renin secretion from juxtaglomerular cells as a cAMP-triggered exocytosis from an endocrine cell, insulin release from pancreatic beta-cells as a Ca2+-mediated and cAMP-potentiated exocytic processes in an endocrine cell, and cAMP- or Ca2+ -triggered H+ secretion from gastric parietal cells as an exocytic process in an exocrine cell are discussed. The selected examples of cAMP-regulated exocytic pathways are reviewed with regard to key proteins involved: adenylyl cyclases, phosphodiesterases, PKA, A kinase anchoring proteins (AKAPs) and Epacs.

Ezrin and radixin both regulate the apical membrane localization of ABCC2 (MRP2) in human intestinal epithelial Caco-2 cells

Multidrug resistance-associated protein ABCC2 (MRP2) is widely expressed in mammalian tissues including intestine, liver and kidney, and it has been shown to be located exclusively on the apical membrane of polarized cells. Recently, several reports suggest that apical membrane localization of ABCC2 (Mrp2) was regulated by radixin in rodent liver. To investigate the mechanism underlying this apical membrane targeting of MRP2 in human intestine, we chose Caco-2 cells as a model to examine the unique roles of ezrin and radixin. Following immunostaining, radixin and ezrin were found to be concentrated at the apical membrane of Caco-2 cells. Using the RNAi method, radixin and ezrin stable knockdown Caco-2 cells were constructed. A cell surface biotinylation experiment with radixin or ezrin stable knockdown Caco-2 cells showed that radixin or ezrin deficiency caused the loss of ABCC2 (MRP2) from the cell surface. An immunoprecipitation assay showed that radixin and ezrin were associated with ABCC2 (MRP2). These findings indicate that both ezrin and radixin are independently required for the apical membrane localization of ABCC2 (MRP2) in Caco-2 cells. Radixin and ezrin play similar roles in the apical membrane localization of ABCC2 (MRP2) and their expression level and subcellular distribution are important factors in the regulation of ABCC2 (MRP2) at the post-transcriptional level.

High turnover of ezrin T567 phosphorylation: conformation, activity, and cellular function

In its dormant state, the membrane cytoskeletal linker protein ezrin takes on a NH(2) terminal-to-COOH terminal (N-C) binding conformation. In vitro evidence suggests that eliminating the N-C binding conformation by Thr(567) phosphorylation leads to ezrin activation. Here, we found for resting gastric parietal cells that the levels of ezrin phosphorylation on Thr(567) are low and can be increased to a small extent ( approximately 40%) by stimulating secretion via the cAMP pathway. Treatment of cells with protein phosphatase inhibitors led to a rapid, dramatic increase in Thr(567) phosphorylation by 400% over resting levels, prompting the hypothesis that ezrin activity is regulated by turnover of phosphorylation on Thr(567). In vitro and in vivo fluorescence resonance energy transfer analysis demonstrated that Thr(567) phosphorylation opens the N-C interaction. However, even in the closed conformation, ezrin localizes to membranes by an exposed NH(2) terminal binding site. Importantly, the opened phosphorylated form of ezrin more readily cosediments with F-actin and binds more tightly to membrane than the closed forms. Furthermore, fluorescence recovery after photobleaching analysis in live cells showed that the Thr567Asp mutant had longer recovery times than the wild type or the Thr567Ala mutant, indicating the Thr(567)-phosphorylated form of ezrin is tightly associated with F-actin and the membrane, restricting normal activity. These data demonstrate and emphasize the functional importance of reversible phosphorylation of ezrin on F-actin binding. A novel model is proposed whereby ezrin and closely associated kinase and phosphatase proteins represent a motor complex to maintain a dynamic relationship between the varying membrane surface area and filamentous actin length.

Attack and defence in the gastric epithelium - a delicate balance

The gastric epithelium is a complex structure formed into tubular branched gastric glands. The glands contain a wide variety of cell types concerned with the secretion of hydrochloric acid, proteases, mucus and a range of signalling molecules. All cell types originate from stem cells in the neck region of the gland, before migrating and differentiating to assume their characteristic positions and functions. Endocrine and local paracrine mediators are of crucial importance for maintaining structural and functional integrity of the epithelium, in the face of a hostile luminal environment. The first such mediator to be recognized, the hormone gastrin, was identified over a century ago and is now established as the major physiological stimulant of gastric acid secretion. Recent studies, including those using mice that overexpress or lack the gastrin gene, suggest a number of previously unrecognized roles for this hormone in the regulation of cellular proliferation, migration and differentiation. This review focuses on the identification of hitherto unsuspected gastrin-regulated genes and discusses the paracrine cascades that contribute to the maintenance of gastric epithelial architecture and secretory function. Helicobacter infection is also considered in cases where it shares targets and signalling mechanisms with gastrin.

Effects of genetic backgrounds on hyperbilirubinemia in radixin-deficient mice due to different expression levels of Mrp3

ERM (ezrin/radixin/moesin) proteins are organizers of apical actin cortical layer in general. We previously reported that the knockout of radixin resulted in Rdx(-/-) mice with displacement/loss of the canalicular transporter Mrp2, giving rise to Dubin-Johnson syndrome-like conjugated hyperbilirubinemia in the mixed genetic background (C57BL/6-129/Sv) (Kikuchi, et al. (2002) Nature Genetics 31, 320-325). However, when these mice were kept under mixed genetic background for years (late mixed backgrounds; LMB), the conjugated hyperbilirubinemia gradually became inconspicuous, while evidence of liver injury increased. We examined the effect of genetic background by backcrossing LMB Rdx(-/-) mice to C57BL/6 and 129/Sv wild type mice with the result that the Rdx(-/-) congenic mice regained hyperbilirubinemia with reduced hepatocellular damage. As revealed by immunofluorescence and western blots, the localization/expression of apical transporters, Mrp2, CD26, P-gps, and Bsep were not influenced by backcrossing, though those of a basolateral transporter, Mrp3, were strikingly increased by backcrossing.

Ezrin/radixin/moesin: versatile controllers of signaling molecules and of the cortical cytoskeleton

Ezrin, radixin and moesin (ERM) proteins are widely distributed proteins located in the cellular cortex, in microvilli and adherens junctions. They feature an N-terminal membrane binding domain linked by an alpha-helical domain to the C-terminal actin-binding domain. In the dormant state, binding sites in the N-terminal domain are masked by interactions with the C-terminal region. The alpha-helical domain also contributes to masking of binding sites. A specific sequence of signaling events results in dissociation of these intramolecular interactions resulting in ERM activation. ERM molecules have been implicated in mediating actin-membrane linkage and in regulating signaling molecules. They are involved in cell membrane organization, cell migration, phagocytosis and apoptosis, and may also play cell-specific roles in tumor progression. Their precise involvement in these processes has yet to be elucidated.

The nuclear receptor hepatocyte nuclear factor 4alpha acts as a morphogen to induce the formation of microvilli

Microvilli are actin-based organelles found on apical plasma membranes that are involved in nutrient uptake and signal transduction. Numerous components, including ezrin/radixin/moesin (ERM) proteins, have been identified that link filamentous actins to transmembrane proteins, but the signals driving microvillus biogenesis are not known. In this study, we show that the conditional and/or ectopic expression of a nuclear receptor, hepatocyte nuclear factor 4α (HNF4α), triggers microvillus morphogenesis. We also demonstrate that HNF4α expression induces ERM-binding phosphoprotein 50 (EBP50) expression and that attenuation of EBP50 using RNA interference inhibits microvillus development. We conclude that HNF4α acts as a morphogen to trigger microvillus formation.

Effect of nitric oxide on histamine-induced cytological transformations in parietal cells in isolated human gastric glands

Previous studies have shown that nitric oxide (NO) inhibits histamine-induced gastric acid secretion in isolated human gastric glands. NO synthase has been found to be present in the human oxyntic mucosa and has been suggested to serve as a paracrine regulator of gastric acid secretion. Histamine stimulation of parietal cells induces cytoskeletal rearrangements, recruitment of H+/K+-ATPase-rich tubulovesicles to the apical membrane and expansion of intracellular canaliculi. The aim of the present study was thus to investigate (i) the effect of an NO donor on histamine-induced cytological transformations and (ii) the influence of increased [Ca2+]i on NO-induced morphological changes in human parietal cells. Human gastric glands were isolated and subjected to the NO donor SNAP prior to histamine administration. [Ca2+]i was increased by photolysis of the caged Ca2+ compound NP-EGTA. The distribution of F-actin, ezrin, and H+/K+-ATPase was assessed by confocal microscopy. Ultrastructural analysis was performed using transmission electron microscopy. SNAP did not influence the histamine-induced translocation of F-actin, ezrin, and H+/K+-ATPase but prevented an increase in the canalicular size. Elevation of [Ca2+]i in resting cells was found to mimic histamine-induced intraparietal cell transformations; however, NO-induced parietal cell morphology was unaffected by a rise in [Ca2+]i. These results indicate that NO inhibits secretion of fluid into the canalicular lumen without affecting membrane recruitment and that this effect is Ca2+-insensitive.

Understanding ERM proteins--the awesome power of genetics finally brought to bear

In epithelial cells, the Ezrin, Radixin and Moesin (ERM) proteins are involved in many cellular functions, including regulation of actin cytoskeleton, control of cell shape, adhesion and motility, and modulation of signaling pathways. However, discerning the specific cellular roles of ERMs has been complicated by redundancy between these proteins. Recent genetic studies in model organisms have identified unique roles for ERM proteins. These include the regulation of morphogenesis and maintenance of integrity of epithelial cells, stabilization of intercellular junctions, and regulation of the Rho small GTPase. These studies also suggest that ERMs have roles in actomyosin contractility and vesicular trafficking in the apical domain of epithelial cells. Thus, genetic analysis has enhanced our understanding of these widely expressed membrane-associated proteins.

Radixin is required to maintain apical canalicular membrane structure and function in rat hepatocytes

BACKGROUND & AIMS

Ezrin-radixin-moesin proteins are cross-linkers between the plasma membrane and actin filaments. Radixin, the dominant ezrin-radixin-moesin protein in hepatocytes, has been reported to selectively tether multidrug-resistance-associated protein 2 to the apical canalicular membrane. However, it remains to be determined if this is its primary function.

METHODS

An adenovirus-mediated short interfering RNA (siRNA) was used to down-regulate radixin expression in collagen sandwich-cultured rat hepatocytes and morphologic and functional changes were characterized quantitatively.

RESULTS

In control cultures, an extensive bile canalicular network developed with properly localized apical and basolateral transporters that provided for functional excretion of fluorescent cholephiles into the bile canalicular lumina. siRNA-induced suppression of radixin was associated with a marked reduction in the canalicular membrane structure as observed by differential interference contrast microscopy and F-actin staining, in contrast to control cells exposed to adenovirus encoding scrambled siRNA. Indirect immunofluorescence showed that apical transporters (multidrug-resistance-associated protein 2, bile salt export pump, and multidrug-resistance protein 1) dissociated from their normal location at the apical membrane and were found largely associated with Rab11-containing endosomes. Localization of the basolateral membrane transporter, organic anion transporting polypeptide 2 (Oatp2), was not affected. Consistent with this dislocation of apical transporters, the biliary excretion of glutathione-methylfluorescein and cholylglycylamido-fluorescein was decreased significantly in the radixin-deficient cells, but not in the control siRNA cells.

CONCLUSIONS

Radixin is essential for maintaining the polarized targeting and/or retaining of canalicular membrane transporters and is a critical determinant of the overall structure and function of the apical membrane of hepatocytes.

ERM proteins in epithelial cell organization and functions

ERM (Ezrin, Radixin, Moesin) proteins are membrane-cytoskeleton linkers that regulate the structure and the function of specific domains of the plasma membrane. ERM proteins are expressed in all metazoan analyzed so far. Genetic analysis of ERM protein functions has recently been performed simultaneously in three different organisms, mouse, Drosophila melanogaster and C. elegans. These studies have revealed a remarkable conservation of the protein functions through evolution. Moreover they have shed light on the crucial role these proteins play in various physiological processes that occur in epithelial cells.

Role of SLC xenobiotic transporters and their regulatory mechanisms PDZ proteins in drug delivery and disposition

Various types of xenobiotic (or drug) transporters have been recently identified to play important roles as barriers against toxic compounds and influx pumps to take up nutrients into the body. Since those xenobiotic transporters generally have wide range of recognition specificity and accept various types of compounds as substrates, localization and functional expression of such transporters could be one of the critical factors that affect the disposition and subsequent biological activity of therapeutic agents. Identification and characterization of drug transporters have given us a scientific basis for understanding drug delivery and disposition, as well as the molecular mechanisms of drug interaction and inter-individual/inter-species differences. To precisely understand pharmacological roles of the transporters in the body, it is also important to clarify molecular mechanisms involved in regulation of the transporters. As a first step to clarify the regulatory mechanisms that govern cell-surface expression and/or function of these transporters, recent researches have focused on PDZ (PSD-95/Discs-large/ZO-1) binding motif localized on carboxylic terminus of several types of xenobiotic transporters. Most of the transporters showing direct interaction potential with the PDZ domain-containing proteins are expressed on apical membranes in epithelial cells of kidney and/or small intestine, implying that such protein-protein interaction may play a role in apical localization of the transporters. In this mini-review article, we summarize importance of transporters and their regulatory mechanisms in drug delivery and disposition, focusing on several aspects of transporter-mediated drug targeting.

Microvilli defects in retinas of ezrin knockout mice

Ezrin, a member of the ezrin/moesin/radixin (ERM) family, localizes to microvilli of epithelia in vivo, where it functions as a bridge between actin filaments and plasma membrane proteins. In the eye, ezrin has been localized to both apical microvilli of Müller cells and retinal pigment epithelium (RPE) apical microvilli and basal infoldings. In the present study, we analyze these structures in the eyes of early postnatal ezrin knockout mice. This analysis indicates that the loss of ezrin leads to substantial reductions in the apical microvilli and basal infoldings in RPE cells and in the Müller cell apical microvilli. The absence of apical microvilli in the RPE is accompanied by the presence of microvilli-like inclusions (MIs) in the RPE cytoplasm. Finally, photoreceptors in the ezrin knockout animals show substantial retardation in development as compared to their wild type littermates.

The cryofixation of isolated rat gastric mucosa provides new insights into the functional transformation of gastric parietal cells: an in vitro experimental model study

Cryofixation is currently accepted as the best initial fixation step to preserve not only the fine structure but also the antigenicity of biological samples. To elucidate the functional transformation of gastric parietal cells, we have newly developed an in vitro experimental model, named the isolated gastric mucosa. In this study, acid secretion of the parietal cell was stimulated with histamine or inhibited with cimetidine, and the samples were cryofixed by plunge freezing for light microscopy or high-pressure freezing for electron microscopy. As a result, the organization of glandular cells was well-preserved and quite similar to freshly excised rat gastric mucosa for at least 2 h after isolation. Immunohistochemistry of H+/K+-ATPase demonstrated a translocation of H+/K+-ATPase from the cytoplasm to the apical membrane associated with histamine-stimulation. In cimetidine-treated mucosa, most of the parietal cells were morphologically in the resting state, showing numerous tubulovesicles in their cytoplasm. In contrast, histamine-stimulated parietal cells exhibited well-developed intracellular canaliculi lined with long microvilli. To the best of our knowledge, the present study is first to demonstrate an electron micrograph that strongly suggests a membrane fusion between the tubulovescile and the apical membrane. Moreover, a stimulation-associated translocation of ezrin was clearly shown from the cytoplasm to the apical region, corresponding to apical microvilli development in the isolated gastric mucosa model. We here describe the preparation of the isolated rat gastric mucosa model, which provides new insights into the functional transformation of parietal cells by the application of cryotechniques.

Phosphorylation of ezrin on threonine 567 produces a change in secretory phenotype and repolarizes the gastric parietal cell

Phosphorylation of the membrane-cytoskeleton linker protein ezrin has been functionally linked to acid secretion and vesicle recruitment to the apical secretory membrane in gastric parietal cells. Phosphorylation of the conserved T567 residue of ezrin has been shown to alter the N/C oligomerization of ezrin and promote the formation of actin-rich surface projections in other cells. To test the importance of T567 as a regulatory site for ezrin in parietal cell activation, we incorporated wild-type (WT) and mutant forms of ezrin, including the nonphosphorylatable T567A mutation and a mutant mimicking permanent phosphorylation, T567D. All ezrin constructs included C-terminal cyan-fluorescent protein (CFP) and were incorporated into adenoviral constructs for efficient introduction into cultured parietal cells from rabbit stomach. Fluorescence microscopy was used to localize CFP-ezrin and monitor morphological responses. Accumulation of a weak base (aminopyrine) was used to monitor receptor-mediated acid secretory response of the cultured cells. Similar to endogenous ezrin, WT and T567A CFP-ezrin localized heavily to apical membrane vacuoles with considerably lower levels associated with the surrounding basolateral membrane. Interestingly, H,K-ATPase within cytoplasmic tubulovesicles was incorporated into the apical vacuoles along with WT and T567A mutant ezrin. In these parietal cells secretagogue stimulation produced a striking vacuolar expansion associated with HCl secretion and the secretory phenotype. Expression of T567D CFP-ezrin was quite different, being rarely associated with apical vacuoles. T567D was more typically localized to the basolateral membrane, often associated with long spikes and fingerlike projections. Moreover, the cells did not display secretagogue-dependent morphological changes and, to our surprise, H,K-ATPase was recruited to the T567D CFP-ezrin-enriched basolateral projections. We conclude that T567 phosphorylation, which is probably regulated through Rho signaling pathway, may direct ezrin to membrane-cytoskeletal activity at the basolateral membrane and away from apical secretory activity. The large basolateral expansion is predicted to recruit membranes from sources not normally targeted to that surface.