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

Metabolic regulator ERRγ governs gastric stem cell differentiation into acid-secreting parietal cells

Parietal cells (PCs) produce gastric acid to kill pathogens and aid digestion. Dysregulated PC census is common in disease, yet how PCs differentiate is unclear. Here, we identify the PC progenitors arising from isthmal stem cells, using mouse models and human gastric cells, and show that they preferentially express cell-metabolism regulator and orphan nuclear receptor Estrogen-related receptor gamma (Esrrg, encoding ERRγ). Esrrg expression facilitated the tracking of stepwise molecular, cellular, and ultrastructural stages of PC differentiation. EsrrgP2ACreERT2 lineage tracing revealed that Esrrg expression commits progenitors to differentiate into mature PCs. scRNA-seq indicated the earliest Esrrg+ PC progenitors preferentially express SMAD4 and SP1 transcriptional targets and the GTPases regulating acid-secretion signal transduction. As progenitors matured, ERRγ-dependent metabolic transcripts predominated. Organoid and mouse studies validated the requirement of ERRγ for PC differentiation. Our work chronicles stem cell differentiation along a single lineage in vivo and suggests ERRγ as a therapeutic target for PC-related disorders.

Ez-Metastasizing: The Crucial Roles of Ezrin in Metastasis

Ezrin is the cytoskeletal organizer and functions in the modulation of membrane-cytoskeleton interaction, maintenance of cell shape and structure, and regulation of cell-cell adhesion and movement, as well as cell survival. Ezrin plays a critical role in regulating tumor metastasis through interaction with other binding proteins. Notably, Ezrin has been reported to interact with immune cells, allowing tumor cells to escape immune attack in metastasis. Here, we review the main functions of Ezrin, the mechanisms through which it acts, its role in tumor metastasis, and its potential as a therapeutic target.

Phosphorylation of CENP-R by Aurora B regulates kinetochore-microtubule attachment for accurate chromosome segregation

Error-free mitosis depends on accurate chromosome attachment to spindle microtubules via a fine structure called the centromere that is epigenetically specified by the enrichment of CENP-A nucleosomes. Centromere maintenance during mitosis requires CENP-A-mediated deposition of constitutive centromere-associated network that establishes the inner kinetochore and connects centromeric chromatin to spindle microtubules during mitosis. Although previously proposed to be an adaptor of retinoic acid receptor, here, we show that CENP-R synergizes with CENP-OPQU to regulate kinetochore-microtubule attachment stability and ensure accurate chromosome segregation in mitosis. We found that a phospho-mimicking mutation of CENP-R weakened its localization to the kinetochore, suggesting that phosphorylation may regulate its localization. Perturbation of CENP-R phosphorylation is shown to prevent proper kinetochore-microtubule attachment at metaphase. Mechanistically, CENP-R phosphorylation disrupts its binding with CENP-U. Thus, we speculate that Aurora B-mediated CENP-R phosphorylation promotes the correction of improper kinetochore-microtubule attachment in mitosis. As CENP-R is absent from yeast, we reasoned that metazoan evolved an elaborate chromosome stability control machinery to ensure faithful chromosome segregation in mitosis.

Pathophysiological Roles of Actin-Binding Scaffold Protein, Ezrin

Ezrin is one of the members of the ezrin/radixin/moesin (ERM) family of proteins. It was originally discovered as an actin-binding protein in the microvilli structure about forty years ago. Since then, it has been revealed as a key protein with functions in a variety of fields including cell migration, survival, and signal transduction, as well as functioning as a structural component. Ezrin acts as a cross-linker of membrane proteins or phospholipids in the plasma membrane and the actin cytoskeleton. It also functions as a platform for signaling molecules at the cell surface. Moreover, ezrin is regarded as an important target protein in cancer diagnosis and therapy because it is a key protein involved in cancer progression and metastasis, and its high expression is linked to poor survival in many cancers. Small molecule inhibitors of ezrin have been developed and investigated as candidate molecules that suppress cancer metastasis. Here, we wish to comprehensively review the roles of ezrin from the pathophysiological points of view.

Acetylation of ezrin regulates membrane-cytoskeleton interaction underlying CCL18-elicited cell migration

Ezrin, a membrane–cytoskeleton linker protein, plays an essential role in cell polarity establishment, cell migration, and division. Recent studies show that ezrin phosphorylation regulates breast cancer metastasis by promoting cancer cell survivor and promotes intrahepatic metastasis via cell migration. However, it was less characterized whether there are additional post-translational modifications and/or post-translational crosstalks on ezrin underlying context-dependent breast cancer cell migration and invasion. Here we show that ezrin is acetylated by p300/CBP-associated factor (PCAF) in breast cancer cells in response to CCL18 stimulation. Ezrin physically interacts with PCAF and is a cognate substrate of PCAF. The acetylation site of ezrin was mapped by mass spectrometric analyses, and dynamic acetylation of ezrin is essential for CCL18-induced breast cancer cell migration and invasion. Mechanistically, the acetylation reduced the lipid-binding activity of ezrin to ensure a robust and dynamic cycling between the plasma membrane and cytosol in response to CCL18 stimulation. Biochemical analyses show that ezrin acetylation prevents the phosphorylation of Thr567. Using atomic force microscopic measurements, our study revealed that acetylation of ezrin induced its unfolding into a dominant structure, which prevents ezrin phosphorylation at Thr567. Thus, these results present a previously undefined mechanism by which CCL18-elicited crosstalks between the acetylation and phosphorylation on ezrin control breast cancer cell migration and invasion. This suggests that targeting PCAF signaling could be a potential therapeutic strategy for combating hyperactive ezrin-driven cancer progression.

Celastrol inhibits ezrin-mediated migration of hepatocellular carcinoma cells

Progression of hepatocellular carcinoma involves multiple genetic and epigenetic alterations that promote cancer invasion and metastasis. Our recent study revealed that hyperphosphorylation of ezrin promotes intrahepatic metastasis in vivo and cell migration in vitro. Celastrol is a natural product from traditional Chinese medicine which has been used in treating liver cancer. However, the mechanism of action underlying celastrol treatment was less clear. Here we show that ROCK2 is a novel target of celastrol and inhibition of ROCK2 suppresses elicited ezrin activation and liver cancer cell migration. Using cell monolayer wound healing, we carried out a phenotype-based screen of natural products and discovered the efficacy of celastrol in inhibiting cell migration. The molecular target of celastrol was identified as ROCK2 using celastrol affinity pull-down assay. Our molecular docking analyses indicated celastrol binds to the active site of ROCK2 kinase. Mechanistically, celastrol inhibits the ROCK2-mediated phosphorylation of ezrin at Thr567 which harnesses liver cancer cell migration. Our findings suggest that targeting ROCK2-ezrin signaling is a potential therapeutic niche for celastrol-based intervention of cancer progression in hepatocellular carcinoma.

ACAP4 interacts with CrkII to promote the recycling of integrin β1

ACAP4, a GTPase-activating protein (GAP) for the ADP-ribosylation factor 6 (ARF6), plays import roles in cell migration, cell polarity, vesicle trafficking and tumorigenesis. Similarly, the ubiquitously expressed adaptor protein CrkII functions in a wide range of cellular activities, including cell proliferation, T cell adhesion and activation, tumorigenesis, and bacterial pathogenesis. Here, we demonstrate that ACAP4 physically interacts with CrkII. Biochemical experiments revealed that ACAP4^550-660 and the SH3N domain of CrkII are responsible for the interaction. Functional characterization showed that the interaction is required for the recruitment of ACAP4 to the plasma membrane where ACAP4 functions to regulate the recycling of the signal transducer integrin β1. Thus, we suggest that the CrkII-ACAP4 complex may be involved in regulation of cell adhesion.

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.

Acetylation of ACAP4 regulates CCL18-elicited breast cancer cell migration and invasion

Tumor metastasis represents the main causes of cancer-related death. Our recent study showed that chemokine CCL18 secreted from tumor-associated macrophages regulates breast tumor metastasis, but the underlying mechanisms remain less clear. Here, we show that ARF6 GTPase-activating protein ACAP4 regulates CCL18-elicited breast cancer cell migration via the acetyltransferase PCAF-mediated acetylation. CCL18 stimulation elicited breast cancer cell migration and invasion via PCAF-dependent acetylation. ACAP4 physically interacts with PCAF and is a cognate substrate of PCAF during CCL18 stimulation. The acetylation site of ACAP4 by PCAF was mapped to Lys311 by mass spectrometric analyses. Importantly, dynamic acetylation of ACAP4 is essential for CCL18-induced breast cancer cell migration and invasion, as overexpression of the persistent acetylation-mimicking or non-acetylatable ACAP4 mutant blocked CCL18-elicited cell migration and invasion. Mechanistically, the acetylation of ACAP4 at Lys311 reduced the lipid-binding activity of ACAP4 to ensure a robust and dynamic cycling of ARF6-ACAP4 complex with plasma membrane in response to CCL18 stimulation. Thus, these results present a previously undefined mechanism by which CCL18-elicited acetylation of the PH domain controls dynamic interaction between ACAP4 and plasma membrane during breast cancer cell migration and invasion.

DDEFL1 correlated with Rho GTPases activity in breast cancer

DDEFL1 is related to maintaining a limiting amount of ARF6 in GTP-loaded form by accelerating its GTP hydrolysis activity, which has been implicated in hepatocellular cancer pathogenesis and lung cancer development. We investigated DDEFL1 expression in breast cancer and paired normal breast tissues by immunohistochemistry and found that DDEFL1 expression was significantly associated with tumor size, lymph node metastasis, high content of elastosis and TNM stage but not with menopausal status or age. We detected the mRNA and protein expression of DDEFL1 in breast cancer cell lines by Western blotting and quantitative real-time PCR (qRT-PCR). DDEFL1 was obvious in MDA-MB-435s and MDA-MB-231 but very weak in ZR-75-1. Further experiments were conducted to evaluate the effect of DDEFL1 small interfering RNA (siRNA) transfection on the biological behavior of MDA-MB-231. After transfection, the effects of DDEFL1 inhibition on expression of mRNA and protein were also analyzed by Western blotting and qRT-PCR. Increased apoptosis and invasive ability, decreased cellular proliferation was found in MDA-MB-231 with successful DDEFL1 siRNA transient transfection (p < 0.05). Western blotting and qRT-PCR results showed that the DDEFL1 inhibition up-regulated Caspase-3, Apaf-1, cytochrome c, and Bax expression and down-regulated Bcl-2 expression. The DDEFL1 inhibition also down-regulated the mRNA and protein expression of Rho, CDC42 and Rac1. Our study provided a functional linkage through DDEFL1 with breast cancer biological behaviours by Rho GTPases. Possible implication of our main finding for the DDEFL1 role in breast cancer and the downstream signaling pathways for the treatment of breast cancer.

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.

MST4 kinase phosphorylates ACAP4 protein to orchestrate apical membrane remodeling during gastric acid secretion

Digestion in the stomach depends on acidification of the lumen. Histamine-elicited acid secretion is triggered by activation of the PKA cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. Our recent study revealed the functional role of PKA-MST4-ezrin signaling axis in histamine-elicited acid secretion. However, it remains uncharacterized how the PKA-MST4-ezrin signaling axis operates the insertion of H,K-ATPases into the apical plasma membranes of gastric parietal cells. Here we show that MST4 phosphorylates ACAP4, an ARF6 GTPase-activating protein, at Thr⁵⁴⁵ Histamine stimulation activates MST4 and promotes MST4 interaction with ACAP4. ACAP4 physically interacts with MST4 and is a cognate substrate of MST4 during parietal cell activation. The phosphorylation site of ACAP4 by MST4 was mapped to Thr⁵⁴⁵ by mass spectrometric analyses. Importantly, phosphorylation of Thr⁵⁴⁵ is essential for acid secretion in parietal cells because either suppression of ACAP4 or overexpression of non-phosphorylatable ACAP4 prevents the apical membrane reorganization and proton pump translocation elicited by histamine stimulation. In addition, persistent overexpression of MST4 phosphorylation-deficient ACAP4 results in inhibition of gastric acid secretion and blockage of tubulovesicle fusion to the apical membranes. Significantly, phosphorylation of Thr⁵⁴⁵ enables ACAP4 to interact with ezrin. Given the location of Thr⁵⁴⁵ between the GTPase-activating protein domain and the first ankyrin repeat, we reason that MST4 phosphorylation elicits a conformational change that enables ezrin-ACAP4 interaction. Taken together, these results define a novel molecular mechanism linking the PKA-MST4-ACAP4 signaling cascade to polarized acid secretion in gastric parietal cells.

Pathophysiological Roles of Ezrin/Radixin/Moesin Proteins

Ezrin/radixin/moesin (ERM) proteins function as general cross-linkers between plasma membrane proteins and the actin cytoskeleton and are involved in the functional expression of membrane proteins on the cell surface. They also integrate Rho guanosine 5'-triphosphatase (GTPase) signaling to regulate cytoskeletal organization by sequestering Rho-related proteins. They act as protein kinase A (PKA)-anchoring proteins and sequester PKA close to its target proteins for their effective phosphorylation and functional regulation. Therefore, ERM proteins seem to play important roles in the membrane transport of electrolytes by ion channels and transporters. In this review, we focus on the pathophysiological roles of ERM proteins in in vivo studies and introduce the phenotypes of their knockout and knockdown mice.

Upregulation of ASAP3 contributes to colorectal carcinogenesis and indicates poor survival outcome

The function and clinical implication of ArfGAP with SH3 domain, ankyrin repeat, and PH domain 3 (ASAP3) in colorectal cancer (CRC) remains undefined. In the present study, we showed that the expression level of ASAP3 was dramatically increased in CRC and its upregulation was associated with American Joint Committee on Cancer stage (P < 0.001) and poor prognosis (P = 0.0022). The combination of stage and ASAP3 expression improved the prediction of survival in CRC patients. Suppression of ASAP3 inhibited cell proliferation by inducing G₁ phase arrest without influencing apoptosis. ASAP3 promoted growth of colon tumors in mice with colitis, and accelerated cell invasion and migration in vitro. Increased ASAP3 was associated with activation of the nuclear factor‐κB (NF‐κB) canonical pathway in CRC. Upregulation of ASAP3 increased the phosphorylation and nuclear translocation of the p65 NF‐κB subunit. Mechanistically, ASAP3 interacts with NF‐κB essential modulator (NEMO) and could reduce the polyubiquitinylation of NEMO. Overall, ASAP3 might regulate NF‐κB via binding to NEMO. ASAP3 acts as an oncogene in colonic cancer and could be a potential biomarker of colon carcinogenesis.

Tributyltin induces disruption of microfilament in HL7702 cells via MAPK-mediated hyperphosphorylation of VASP

Tributyltin (TBT) has been widely used for various industrial purposes, and it has toxic effects on multiple organs and tissues. Previous studies have found that TBT could induce cytoskeletal disruption, especially of the actin filaments. However, the underlying mechanisms remain unclear. The aim of the present study was to determine whether TBT could induce microfilament disruption using HL7702 cells and then to assess for the total levels of various microfilament-associated proteins; finally, the involvement of the MAPK pathway was investigated. The results showed that after TBT treatment, F-actin began to depolymerize and lost its characteristic filamentous structure. The protein levels of Ezrin and Cofilin remained unchanged, the actin-related protein (ARP) 2/3 levels decreased slightly, and the vasodilator-stimulated phosphoprotein (VASP) decreased dramatically. However, the phosphorylation levels of VASP increased 2.5-fold, and the ratio of phosphorylated-VASP/unphosphorylated-VASP increased 31-fold. The mitogen-activated protein kinases (MAPKs) ERK and JNK were discovered to be activated. Inhibition of ERK and JNK not only largely diminished the TBT-induced hyperphosphorylation of VASP but also recovered the cellular morphology and rescued the cells from death. In summary, this study demonstrates that TBT-induced disruption of actin filaments is caused by the hyperphosphorylation of VASP through MAPK pathways. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1530-1538, 2016.

Effects of ezrin knockdown on the structure of gastric glandular epithelia

Ezrin, an adaptor protein that cross-links plasma membrane-associated proteins with the actin cytoskeleton, is concentrated on apical surfaces of epithelial cells, especially in microvilli of the small intestine and stomach. In the stomach, ezrin is predominantly expressed on the apical canalicular membrane of parietal cells. Transgenic ezrin knockdown mice in which the expression level of ezrin was reduced to <7% compared with the wild-type suffered from achlorhydria because of impairment of membrane fusion between tubulovesicles and apical membranes. We observed, for the first time, hypergastrinemia and foveolar hyperplasia in the gastric fundic region of the knockdown mice. Dilation of fundic glands was observed, the percentage of parietal and chief cells was reduced, and that of mucous-secreting cells was increased. The parietal cells of knockdown mice contained dilated tubulovesicles and abnormal mitochondria, and subsets of these cells contained abnormal vacuoles and multilamellar structures. Therefore, lack of ezrin not only causes achlorhydria and hypergastrinemia but also changes the structure of gastric glands, with severe perturbation of the secretory membranes of parietal cells.

Cell Polarity Kinase MST4 Cooperates with cAMP-dependent Kinase to Orchestrate Histamine-stimulated Acid Secretion in Gastric Parietal Cells

The digestive function of the stomach depends on acidification of the gastric lumen. Acid secretion into the lumen is triggered by activation of the PKA cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. A coupling protein is ezrin, whose phosphorylation at Ser-66 by PKA is required for parietal cell activation. However, little is known regarding the molecular mechanism(s) by which this signaling pathway operates in gastric acid secretion. Here we show that PKA cooperates with MST4 to orchestrate histamine-elicited acid secretion by phosphorylating ezrin at Ser-66 and Thr-567. Histamine stimulation activates PKA, which phosphorylates MST4 at Thr-178 and then promotes MST4 kinase activity. Interestingly, activated MST4 then phosphorylates ezrin prephosphorylated by PKA. Importantly, MST4 is important for acid secretion in parietal cells because either suppression of MST4 or overexpression of non-phosphorylatable MST4 prevents the apical membrane reorganization and proton pump translocation elicited by histamine stimulation. In addition, overexpressing MST4 phosphorylation-deficient ezrin results in an inhibition of gastric acid secretion. Taken together, these results define a novel molecular mechanism linking the PKA-MST4-ezrin signaling cascade to polarized epithelial secretion in gastric parietal cells.

Hyperphosphorylation of microfilament-associated proteins is involved in microcystin-LR-induced toxicity in HL7702 cells

Microcystin-LR (MC-LR) has been regarded as a hepatotoxin, which can cause cytoskeletal reorganization, especially of the actin filaments. However, the underlying mechanisms remain unclear. In this study, whether MC-LR could induce microfilaments disruption was verified in the normal human liver cell line HL7702; and then the transcription, translation, and phosphorylation levels of major microfilament-associated proteins were measured; finally, the underlying mechanisms was investigated. After treatment with MC-LR, the actin filaments lost their characteristic filamentous organization in the cells, demonstrating increased actin depolymerization. The mRNA and protein levels of ezrin, vasodilator-stimulated phosphoprotein (VASP), actin-related protein2/3, and cofilin remained unchanged. However, the phosphorylation levels of ezrin and VASP were increased, when treated with 10 μM MC-LR. Moreover, P38 and ERK1/2 were involved in MC-LR-induced hyperphosphorylation of microfilament-associated proteins. In summary, this study demonstrates that MC-LR can cause disruption of actin filaments in HL7702 cells due to MC-LR-induced mitogen-activated protein kinase pathway activation and hyperphosphorylation of different types of microfilament-associated proteins.

Localization of cystic fibrosis transmembrane conductance regulator signaling complexes in human salivary gland striated duct cells

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-dependent protein kinase (PKA)-regulated Cl(-) channel, crucial for epithelial cell regulation of salt and water transport. Previous studies showed that ezrin, an actin binding and A-kinase anchoring protein (AKAP), facilitates association of PKA with CFTR. We used immunohistochemistry and immunogold transmission electron microscopy to localize CFTR, ezrin, and PKA type II regulatory (RII) and catalytic (C) subunits in striated duct cells of human parotid and submandibular glands. Immunohistochemistry localized the four proteins mainly to the apical membrane and the apical cytoplasm of striated duct cells. In acinar cells, ezrin localized to the luminal membrane, and PKA RII subunits were present in secretory granules, as previously described. Immunogold labeling showed that CFTR and PKA RII and C subunits were localized to the luminal membrane and associated with apical granules and vesicles of striated duct cells. Ezrin was present along the luminal membrane, on microvilli and along the junctional complexes between cells. Double labeling showed specific protein associations with apical granules and vesicles and along the luminal membrane. Ezrin, CFTR, and PKA RII and C subunits are co-localized in striated duct cells, suggesting the presence of signaling complexes that serve to regulate CFTR activity.

Spatial control of proton pump H,K-ATPase docking at the apical membrane by phosphorylation-coupled ezrin-syntaxin 3 interaction

The digestive function of the stomach depends on acidification of the gastric lumen. Acid secretion into the lumen is triggered by activation of a cAMP-dependent protein kinase (PKA) cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. A coupling protein is ezrin whose phosphorylation at Ser-66 by PKA is required for parietal cell activation. However, little is known regarding the molecular mechanism(s) by which ezrin operates in gastric acid secretion. Here we show that phosphorylation of Ser-66 induces a conformational change of ezrin that enables its association with syntaxin 3 (Stx3) and provides a spatial cue for H,K-ATPase trafficking. This conformation-dependent association is specific for Stx3, and the binding interface is mapped to the N-terminal region. Biochemical analyses show that inhibition of ezrin phosphorylation at Ser-66 prevents ezrin-Stx3 association and insertion of H,K-ATPase into the apical plasma membrane of parietal cells. Using atomic force microscopic analyses, our study revealed that phosphorylation of Ser-66 induces unfolding of ezrin molecule to allow Stx3 binding to its N terminus. Given the essential role of Stx3 in polarized secretion, our study presents the first evidence in which phosphorylation-induced conformational rearrangement of the ezrin molecule provides a spatial cue for polarized membrane trafficking in epithelial cells.

Loss of ASAP3 destabilizes cytoskeletal protein ACTG1 to suppress cancer cell migration

ArfGAP with SH3 domain, ankyrin repeat and PH domain 3 (ASAP3), previously known as ACAP4, DDEFL1 and UPLC1, is considered to be an important regulator in cancer cell migration/invasion and actin-based cytoskeletal remodeling. However, the underlying mechanisms through which ASAP3 mediates these processes are not well-elucidated. This study reported that in certain types of cancer cells, loss of ASAP3 suppressed cell migration/invasion, in part by destabilizing γ-actin-1 (ACTG1), a cytoskeletal protein considered to be an integral component of the cell migratory machinery, essential for the rearrangement of the dynamic cytoskeletal networks and important in diseases, such as brain malformation, hearing loss and cancer development. The data, for the first time, link ASAP3 with ACTG1 in the regulation of cytoskeletal maintenance and cell motility.

Phosphorylation of the Bin, Amphiphysin, and RSV161/167 (BAR) domain of ACAP4 regulates membrane tubulation

ArfGAP With Coiled-Coil, Ankyrin Repeat And PH Domains 4 (ACAP4) is an ADP-ribosylation factor 6 (ARF6) GTPase-activating protein essential for EGF-elicited cell migration. However, how ACAP4 regulates membrane dynamics and curvature in response to EGF stimulation is unknown. Here, we show that phosphorylation of the N-terminal region of ACAP4, named the Bin, Amphiphysin, and RSV161/167 (BAR) domain, at Tyr34 is necessary for EGF-elicited membrane remodeling. Domain structure analysis demonstrates that the BAR domain regulates membrane curvature. EGF stimulation of cells causes phosphorylation of ACAP4 at Tyr34, which subsequently promotes ACAP4 homodimer curvature. The phospho-mimicking mutant of ACAP4 demonstrates lipid-binding activity and tubulation in vitro, and ARF6 enrichment at the membrane is associated with ruffles of EGF-stimulated cells. Expression of the phospho-mimicking ACAP4 mutant promotes ARF6-dependent cell migration. Thus, the results present a previously undefined mechanism by which EGF-elicited phosphorylation of the BAR domain controls ACAP4 molecular plasticity and plasma membrane dynamics during cell migration.

Conundrum, an ARHGAP18 orthologue, regulates RhoA and proliferation through interactions with Moesin

RhoA, a small GTPase, regulates epithelial integrity and morphogenesis by controlling filamentous actin assembly and actomyosin contractility. Another important cytoskeletal regulator, Moesin (Moe), an ezrin, radixin, and moesin (ERM) protein, has the ability to bind to and organize cortical F-actin, as well as the ability to regulate RhoA activity. ERM proteins have previously been shown to interact with both RhoGEF (guanine nucleotide exchange factors) and RhoGAP (GTPase activating proteins), proteins that control the activation state of RhoA, but the functions of these interactions remain unclear. We demonstrate that Moe interacts with an unusual RhoGAP, Conundrum (Conu), and recruits it to the cell cortex to negatively regulate RhoA activity. In addition, we show that cortically localized Conu can promote cell proliferation and that this function requires RhoGAP activity. Surprisingly, Conu's ability to promote growth also appears dependent on increased Rac activity. Our results reveal a molecular mechanism by which ERM proteins control RhoA activity and suggest a novel linkage between the small GTPases RhoA and Rac in growth control.

Nesfatin-1 inhibits gastric acid secretion by cultured rat gastric mucosa cells

AIM: To clarify the effect of nesfatin-1 on gastric acid secretion and the expression of the H+/K+-ATPase mRNA and protein in rat gastric mucosa cells in vitro.

METHODS: Gastric mucosa cells were isolated from SD rats by enzymolysis and identified by immunofluorescence staining. Cultured rat gastric mucosa cells were divided into control group and nesfatin-1 group, and the nesfatin-1 group was pretreated with different concentrations (0, 10^-4, 10^-3, 10^-2, 10^-1 μmol/L) of nesfatin-1 for different durations (0, 1, 2, 3, 4 h). The effect of nesfatin-1 on gastric acid secretion was investigated by monitoring ¹⁴C-aminopyrine (¹⁴C-AP) accumulation, and the expression of H⁺/K⁺-ATPase α and β subunit mRNA and protein was examined by real-time PCR and Western blot.

RESULTS: Pretreatment with nesfatin-1 at a dose of 10^-1 or 10^-2 μmol/L for 2 or 3 h inhibited gastric acid secretion, but nesfatin-1 at a dose of 10^-3 or 10^-4 μmol/L had no such effect. Nesfatin-1 at a dose of 10^-1 μmol/L inhibited the expression of H⁺/K⁺-ATPase α subunit mRNA after pretreatment for 1, 2, or 3 h and inhibited the expression of H⁺/K⁺-ATPase β subunit mRNAs after pretreatment for 1 or 2 h. In the dose range between 10^-4 to 10^-1 μmol/L, nesfatin-1 dose-dependently inhibited the expression of H⁺/K⁺-ATPase α subunit and β subunit mRNA after pretreatment for 2 h. Nesfatin-1 at a dose of 10^-1 μmmol/L inhibited H⁺/K⁺-ATPase α subunit protein expression after pretreatment for 1, 2 or 3 h and inhibited H⁺/K⁺-ATPase β subunit protein expression after pretreatment for 2 or 3 h. In the dose range between 10^-3 to 10^-1 μmol/L, nesfatin-1 dose-dependently inhibited H⁺/K⁺-ATPase α and β subunit protein expression after pretreatment for 2 h.

CONCLUSION: Our data suggest that nesfatin-1 inhibits gastric acid secretion by rat gastric mucosa cells in vitro possibly by down-regulating the expression of H⁺/K⁺-ATPase mRNA and protein..

ACAP4 protein cooperates with Grb2 protein to orchestrate epidermal growth factor-stimulated integrin β1 recycling in cell migration

ARF6 GTPase is an important regulator of membrane trafficking and actin-based cytoskeleton dynamics active at the leading edge of migrating cells. The integrin family heterodimeric transmembrane proteins serve as major receptors for extracellular matrix proteins, which play essential roles in cell adhesion and migration. Our recent proteomic analyses of ARF6 effectors have identified a novel ARF6 GTPase-activating protein, ACAP4, essential for EGF-induced cell migration. However, molecular mechanisms underlying ACAP4-mediated cell migration have remained elusive. Here, we show that ACAP4 regulates integrin β1 dynamics during EGF-stimulated cell migration by interaction with Grb2. Our biochemical study shows that EGF stimulation induces phosphorylation of tyrosine 733, which enables ACAP4 to bind Grb2. This interaction of ACAP4 with Grb2 regulates integrin β1 recycling to the plasma membrane. Importantly, knockdown of ACAP4 by siRNA or overexpression of ACAP4 decreased recycling of integrin β1 to the plasma membrane and reduced integrin-mediated cell migration. Taken together, these results suggest a novel function for ACAP4 in the regulation of cell migration through controlling integrin β1 dynamics.

Gastric secretion

PURPOSE OF REVIEW

The review summarizes the past year's literature regarding the regulation of gastric exocrine and endocrine secretion, both basic science and clinical.

RECENT FINDINGS

Gastric acid secretion is an elaborate and dynamic process that is regulated by neural (efferent and afferent), hormonal (e.g. gastrin), and paracrine (e.g. histamine, ghrelin, somatostatin) pathways as well as mechanical (e.g. distension) and chemical (e.g. amino acids) stimuli. Secretion of hydrochloric acid (HCl) by parietal cells involves translocation of HK-ATPase-containing cytoplasmic tubulovesicles to the apical membrane with subsequent electroneutral transport of hydronium ions in exchange for potassium. The main apical potassium channel is KCNQ1 which, when activated, assembles with its β-subunit KCNE2 to function as a constitutively open, voltage-insensitive, and acid-resistant luminal potassium channel. Proton pump inhibitors block acid secretion by covalently binding to cysteine residues accessible from the luminal surface of the HK-ATPase. Potassium-competitive ATPase blockers (P-CABs) act by competing for K on the luminal surface of HK-ATPase. As they are acid-stable and do not require acid-dependent activation, P-CABs hold promise for rapid and prolonged inhibition of acid secretion.

SUMMARY

We continue to make progress in our understanding of the physiologic regulation of gastric acid secretion. A better understanding of the pathways and mechanisms regulating acid secretion should lead to improved management of patients with acid-induced disorders.

Ezrin, Radixin and Moesin: key regulators of membrane-cortex interactions and signaling

The cell cortex serves as a critical nexus between the extracellular environment/cell membrane and the underlying cytoskeleton and cytoplasm. In many cells, the cell cortex is organized and maintained by the Ezrin, Radixin and Moesin (ERM) proteins, which have the ability to interact with both the plasma membrane and filamentous actin. Although this membrane-cytoskeletal linkage function is critical to stability of the cell cortex, recent studies indicate that this is only a part of what ERMs do in many cells. In addition to their role in binding filamentous actin, ERMs regulate signaling pathways through their ability to bind transmembrane receptors and link them to downstream signaling components. In this review we discuss recent evidence in a variety of cells indicating that ERMs serve as scaffolds to facilitate efficient signal transduction on the cytoplasmic face of the plasma membrane.

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.