Membrane and protein recycling associated with gastric HCl secretion

Membrane Transport across Polarized Epithelia

Polarized epithelial cells line diverse surfaces throughout the body forming selective barriers between the external environment and the internal milieu. To cross these epithelial barriers, large solutes and other cargoes must undergo transcytosis, an endocytic pathway unique to polarized cell types, and significant for the development of cell polarity, uptake of viral and bacterial pathogens, transepithelial signaling, and immunoglobulin transport. Here, we review recent advances in our knowledge of the transcytotic pathway for proteins and lipids. We also discuss briefly the promise of harnessing the molecules that undergo transcytosis as vehicles for clinical applications in drug delivery.

KCNQ1 is the luminal K+ recycling channel during stimulation of gastric acid secretion

Parietal cell (PC) proton secretion via H(+)/K(+)-ATPase requires apical K(+) recycling. A variety of K(+) channels and transporters are expressed in the PC and the molecular nature of the apical K(+) recycling channel is under debate. This study was undertaken to delineate the exact function of KCNQ1 channels in gastric acid secretion. Acid secretory rates and electrophysiological parameters were determined in gastric mucosae of 7- to 8-day-old KCNQ1(+/+), (+/-) and (-/-) mice. Parietal cell ultrastructure, abundance and gene expression levels were quantified. Glandular structure and PC abundance, and housekeeping gene expression did not differ between the KCNQ1(-/-) and (+/+) mucosae. Microvillar secretory membranes were intact, but basal acid secretion was absent and forskolin-stimulated acid output reduced by approximately 90% in KCNQ1(-/-) gastric mucosa. Application of a high K(+) concentration to the luminal membrane restored normal acid secretory rates in the KCNQ1(-/-) mucosa. The study demonstrates that the KCNQ1 channel provides K(+) to the extracellular K(+) binding site of the H(+)/K(+)-ATPase during acid secretion, and no other gastric K(+) channel can substitute for this function.

The recycling and transcytotic pathways for IgG transport by FcRn are distinct and display an inherent polarity

The Fc receptor FcRn traffics immunoglobulin G (IgG) in both directions across polarized epithelial cells that line mucosal surfaces, contributing to host defense. We show that FcRn traffics IgG from either apical or basolateral membranes into the recycling endosome (RE), after which the actin motor myosin Vb and the GTPase Rab25 regulate a sorting step that specifies transcytosis without affecting recycling. Another regulatory component of the RE, Rab11a, is dispensable for transcytosis, but regulates recycling to the basolateral membrane only. None of these proteins affect FcRn trafficking away from lysosomes. Thus, FcRn transcytotic and recycling sorting steps are distinct. These results are consistent with a single structurally and functionally heterogeneous RE compartment that traffics FcRn to both cell surfaces while discriminating between recycling and transcytosis pathways polarized in their direction of transport.

Tetraspan proteins: regulators of renal structure and function

PURPOSE OF REVIEW

Members of the tetraspan family are widely expressed and poorly understood. An emerging literature suggests that through their interactions with other membrane proteins they play central or regulatory roles in a wide variety of physiological processes. This review will discuss selected tetraspan complexes and highlight their relevance to epithelial cells and the kidney.

RECENT FINDINGS

Tetraspans regulate the signaling and trafficking properties of their partner proteins. Tetraspan complexes with integrin molecules, for example, modulate cell adhesion and mobility. Perturbations of tetraspan-integrin assemblies can have dramatic impacts on renal tissue morphogenesis, resulting in a disruption of normal glomerular architecture and selectivity. Tetraspan interactions with renal ion transport proteins appear to affect transporter function by enhancing or inhibiting the endocytic internalization of their transport protein partners.

SUMMARY

Tetraspans constitute a novel class of proteins whose capacity to alter the cell biological and functional properties of their membrane protein partners is likely to have wide ranging and important physiological ramifications.

The future of the pump

Since it was discovered 3 decades ago the H,K-ATPase has come to be recognized as the key both to the generation and pharmacologic suppression of gastric acid secretion. Although 30 years of concerted research has answered many questions, it is perhaps not surprising that these efforts have raised many new and crucial issues that await elucidation. These can be divided into 5 broad categories that relate to structure, mechanism, regulation, trafficking, and macromolecular interactions. It is probably safe to predict that the growing sophistication of x-ray crystallographic techniques will yield a picture of the pump's molecular structure in the near future. These insights will, in turn, illuminate the details of the process through which enzymatic hydrolysis is coupled to ion translocation with unprecedented clarity. The gastric parietal cell employs an extremely complicated system of receptors, kinases, and second messengers to maintain tight control over pump function. Upon activation, this cell also performs a massive and elegant membrane trafficking transformation that plays a critical role in the regulatory process. Finally, it is becoming clear that every ion transport protein is a component in a large macromolecular complex whose constituents help to determine all of the transport system's fundamental physiologic properties. These are the major topics that will drive H,K pump research in the future, and it is likely that their resolution will create the foundations for the next generation of therapies aimed at controlling gastric acid secretion and its clinical consequences.

Perturbation of gastric mucosa in mice expressing the temperature-sensitive mutant of SV40 large T antigen. Potential for establishment of an immortalised parietal cell line

Gastric parietal cells have a unique secretory membrane system that undergoes a profound transformation when the parietal cell is stimulated to secrete acid. Understanding this process has been hindered by the lack of an immortalised parietal cell line. Here we have explored a strategy for the development of a parietal cell line by the generation of transgenic mice bearing the temperature-sensitive mutant of the SV40 large T antigen (SV40 tsA58) under the control of the regulatory sequences of the gastric H+/K+ ATPase beta-subunit (H/Kbeta-tsA58). Three H/ Kbeta-tsA58 transgenic mouse lines were established, namely 218, 224 and 228, all of which expressed the tsA58 T antigen in the gastric mucosa. Unexpectedly, the gastric mucosae of all lines were hypertrophic indicating that the temperature-sensitive large T antigen was partially active at 37 degrees C. Immunofluorescence together with light and electron microscopic studies revealed that mature parietal and zymogenic cells were absent in H/Kbeta-tsA58 transgenic lines 218 and 224, and small undifferentiated cells were the dominant cell type in the gastric units. On the other hand, a few mature parietal cells were detected in line 228 together with an increased proportion of undifferentiated cells and, normally rare, pre-parietal cells. As line 228 represented a rich source of pre-parietal cells, gastric cells from line 228 were isolated and cultured at 33 degrees C, the permissive temperature for tsA58. Gastric epithelial cells, expressing the T antigen, were maintained in culture for over 6 weeks. Upon a temperature shift to 39 C the cultured gastric cells developed characteristics of differentiated parietal cells, including the presence of a nascent canaliculus and dramatically increased production of the gastric H+/K+ ATPase beta-subunit. Therefore, this system shows the potential to generate an immature parietal cell line that can be induced to differentiate in vitro.

ERM-Merlin and EBP50 protein families in plasma membrane organization and function

The ezrin-radixin-moesin (ERM) family of proteins have emerged as key regulatory molecules in linking F-actin to specific membrane proteins, especially in cell surface structures. Merlin, the product of the NF2 tumor suppressor gene, has sequence similarity to ERM proteins and binds to some of the same membrane proteins, but lacks a C-terminal F-actin binding site. In this review we discuss how ERM proteins and merlin are negatively regulated by an intramolecular association between their N- and C-terminal domains. Activation of at least ERM proteins can be accomplished by C-terminal phosphorylation in the presence of PIP2. We also discuss membrane proteins to which ERM and merlin bind, including those making an indirect linkage through the PDZ-containing adaptor molecules EBP50 and E3KARP. Finally, the function of these proteins in cortical structure, endocytic traffic, signal transduction, and growth control is discussed.

SNARE proteins and rab3A contribute to canalicular formation in parietal cells

SNARE proteins - rab3A - parietal cells - H+/K+-ATPase When stimulated by histamine, acetylcholine, or gastrin the luminal compartments of oxyntic parietal cells display conspicuous morphological changes. The luminal plasma membrane surface becomes greatly expanded, while the cytoplasmic tubulovesicles are decreased in parallel. Due to these membrane rearrangements the H+/K(+)-ATPase obtains access to the luminal surface, where proton secretion occurs. The stimulation-induced translocation of H+/K(+)-ATPase involves a fusion process. Exocytotic membrane fusion in neurons is achieved by the highly regulated interaction of mainly three proteins, the vesicle protein synaptobrevin and the plasma membrane proteins syntaxin and SNAP25 (synaptosomal-associated protein of 25 kDa), also referred to as SNARE proteins. Using immunofluorescence microscopy we analysed the subcellular distribution of neuronal synaptic proteins and rab3A in resting and stimulated parietal cells from pig and rat. In resting cells all synaptic proteins colocalized with the H+/ K(+)-ATPase trapped in the tubulovesicular compartment. After stimulation, translocated H+/K(+)-ATPase showed a typical canalicular distribution. Syntaxin, synaptobrevin, SNAP25 and rab3A underwent a similar redistribution in stimulated cells and consequently localized to the canalicular compartment. Using immunoprecipitation we found that the SNARE complex consisting of synaptobrevin, syntaxin and SNAP25, which is a prerequisite for membrane fusion in neurons, is also assembled in parietal cells. In addition the parietal cell-derived synaptobrevin could be proteolytically cleaved by tetanus toxin light chain. These data may provide evidence that SNARE proteins and rab3A are functionally involved in the stimulation-induced translocation of the H+/K(+)-ATPase.

Role of actin in EGF-induced alterations in enterocyte SGLT1 expression

Na+-glucose cotransporter (SGLT1) expression and the role of actin in epidermal growth factor (EGF)-induced alterations in glucose transport and brush-border surface area were examined in New Zealand White rabbit jejunal loops. In separate experiments, EGF or EGF concurrent with cytochalasin D, an inhibitor of actin polymerization, was administered to the experimental loop and compared with its vehicle control. SGLT1 expression was measured by Western blot in brush-border membrane vesicles (BBMV) after 5-min and 1-h exposure. Glucose kinetics were determined by a rapid filtration technique, and brush-border surface area was examined by electron microscopy after 1-h exposure. The effect of cytochalasin D alone on BBMV glucose kinetics and brush-border surface area was also assessed. EGF resulted in a significant increase in BBMV SGLT1 expression (P < 0.05), glucose maximal uptake (Vmax; P < 0.001), and absorptive brush-border surface area (P < 0.001). These effects were abolished with concurrent cytochalasin D treatment. Cytochalasin D alone had no effect on glucose transport or brush-border surface area. The findings suggest that EGF acutely upregulates jejunal brush-border surface area and the Vmax for jejunal glucose uptake via the recruitment and insertion of SGLT1 from an internal pool into the brush border by a mechanism that is dependent on actin polymerization.

A basolateral sorting signal is encoded in the alpha-subunit of Na-K-ATPase

Na-K-ATPase and H-K-ATPase are highly homologous ion pumps that exhibit distinct plasma membrane distributions in epithelial cells. We have studied the alpha-subunits of these heterodimeric pumps to identify the protein domains responsible for their polarized sorting. A chimeric alpha-subunit construct (N519H) was generated in which the first 519 amino acid residues correspond to the Na-K-ATPase sequence and the remaining 500 amino acids are derived from the H-K-ATPase sequence. In stably transfected LLC-PK1 cell lines, we found that the N519H chimera is restricted to the basolateral surface under steady-state conditions, suggesting that residues within the NH2-terminal 519 amino acids of the Na-K-ATPase alpha-subunit contain a basolateral sorting signal. H-K-ATPase beta-subunit expressed alone in LLC-PK1 cells accumulates at the apical surface. When coexpressed with N519H, the H-K-ATPase beta-subunit assembles with this chimera and accompanies it to the basolateral surface. Thus the NH2-terminal basolateral signal in the Na-K-ATPase alpha-subunit masks or is dominant over any apical sorting information present in the beta-polypeptide. In gastric parietal cells, the H-K-ATPase beta-subunit targets the H-K-ATPase to an intracellular vesicular compartment which fuses with the plasma membrane in response to secretagogue stimulation. To test whether the chimera-H-K-ATPase beta-subunit complex is directed to a similar compartment in LLC-PK1 cells, we treated transfected cells with drugs that raise intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels. Elevation of cytosolic cAMP increased the surface expression of both the N519H chimera and the H-K-ATPase beta-subunit. This increase in surface expression, however, appears to be the result of transcriptional upregulation and not recruitment of chimera to the surface from a cAMP-inducible compartment.

Ca2+-dependent membrane bound protein fraction from rabbit gastric mucosa contains a protein whose histidyl residue is phosphorylated

We found an autophosphorylated protein with a molecular weight of 40 kDa (p40) in the crude annexin fraction of rabbit gastric mucosa, i.e., the materials released by EGTA from the membrane fraction obtained in the presence of Ca2+. This protein was enriched in chief cells in the gastric glands, and also found in the heart and the liver by Western blotting. The protein bound to phenyl-Sepharose in the presence of Ca2+ and showed extremely basic nature. The phosphorylation site of p40 was considered to be histidyl residue based on the stability to the various agents, the synthesizing activity of ATP from ADP, and the results of phosphoamino acid analysis. The autophosphorylation of p40 was augmented several tenth fold by GDP, Ras, myelin basic protein, or H1 histone at micromolar range. The phosphorylated form was rapidly dephosphorylated in the presence of cold ATP, succinate, and CoA, suggesting that p40 has succinyl-CoA synthetase activity. In fact, a peptide fragment from p40 showed a striking homology with the alpha subunits of succinyl-CoA synthetases from Escherichia coli, Dictyostelium discoideum, and rat liver. These results suggest that p40 is extramitochondrial alpha subunit of succinyl-CoA synthetase or its homologue.

Fast freeze-fixation/freeze-substitution reveals the secretory membranes of the gastric parietal cell as a network of helically coiled tubule. A new model for parietal cell transformation

The parietal cell of the gastric mucosa undergoes rapid morphological transformation when it is stimulated to produce hydrochloric acid. In chemically fixed cells, this process is seen as a reduction in number of cytoplasmic ‘tubulovesicles’ as the apical surface of the cell progressively invaginates to increase the secretory surface area. It is widely believed that the tubulovesicles represent stored secretory membrane in the cytoplasm of the unstimulated cell, which is incorporated into the apical membrane upon stimulation, because they share H+,K+-ATPase activity with the apical membrane. However, fusion of tubulovesicles with the apical membrane concomitant with parietal cell activation has never been convincingly demonstrated. We have used fast freeze-fixation and freeze-substitution to study stages of morphological transformation in these cells. Tubulovesicles were not seen in the cytoplasm of any of our cryoprepared cells. Instead, the cytoplasm of the unstimulated cell contained numerous and densely packed helical coils of tubule, each having an axial core of cytoplasm. The helical coils were linked together by connecting tubules, lengths of relatively straight tubule. Lengths of straight connecting tubule also extended from coils lying adjacent to the apical and canalicular surfaces and ended at the apical and canaliculus membranes. Immunogold labelling with alpha- and beta-subunit-specific antibodies showed that the gastric H+,K+-ATPase was localized to the membranes of this tubular system, which therefore represented the configuration of the secretory membrane in the cytoplasm of the unstimulated parietal cell. Stimulation of the cells with histamine and isobutylmethylxanthine lead to modification of the tubular membrane system, correlated with progressive invagination of the apical membrane. The volume of the tubule lumen increased and, as this occurred, the tight spiral twist of the helical coils was lost, indicating that tubule distension was accounted for by partial unwinding. This exposed the cores of cytoplasm in the axes of the coils as rod-shaped elements of a three-dimensional reticulum, resembling a series of microvilli in random thin sections. Conversely, treatment with the H2 antagonist cimetidine caused severe contraction of the tubular membrane system and intracellular canaliculi. Our results indicate that tubulovesicles are an artifact of chemical fixation; consequently, they cannot have a role in parietal cell transformation. From our findings we propose an alternative model for morphological transformation in the parietal cell. This model predicts cytoskeleton-mediated control over expansion and contraction of the tubular membrane network revealed by cryopreparation. The model is compatible with the localization of cytoskeletal components in these cells.

Role of membrane trafficking in plasma membrane solute transport

Cells can rapidly and reversibly alter solute transport rates by changing the kinetics of transport proteins resident within the plasma membrane. Most notably, this can be brought about by reversible phosphorylation of the transporter. An additional mechanism for acute regulation of plasma membrane transport rates is by the regulated exocytic insertion of transport proteins from intracellular vesicles into the plasma membrane and their subsequent regulated endocytic retrieval. Over the past few years, the number of transporters undergoing this regulated trafficking has increased dramatically, such that what was once an interesting translocation of a few transporters has now become a widespread modality for regulating plasma membrane solute permeabilities. The aim of this article is to review the models proposed for the regulated trafficking of transport proteins and what lines of evidence should be obtained to document regulated exocytic insertion and endocytic retrieval of transport proteins. We highlight four transporters, the insulin-responsive glucose transporter, the antidiuretic hormone-responsive water channel, the urinary bladder H(+)-ATPase, and the cystic fibrosis transmembrane conductance regulator Cl- channel, and discuss the various approaches taken to document their regulated trafficking. Finally, we discuss areas of uncertainty that remain to be investigated concerning the molecular mechanisms involved in regulating the trafficking of proteins.

Membrane-actin microfilament connections: an increasing diversity of players related to band 4.1

The cortical actin cytoskeleton participates in various membrane-based processes which necessitate a large amount of plasticity in the molecular components involved in these interactions. A family of proteins homologous to band 4.1 is involved in the reorganization of the actin cytoskeleton in response to various stimuli, and probably plays a role in transmembrane signalling. This family includes tyrosine phosphatases, substrates of tyrosine kinases and a candidate for a tumor-suppressor gene.

Localization of brush border cytoskeletal proteins in gastric oxynticopeptic cells from the bullfrog Rana catesbeiana

The contribution of brush border cytoskeletal proteins (actin, villin, fimbrin, and brush border myosin-1) to organization of the cytoskeletal network underlying apical plications of oxynticopeptic cells was examined by immunohistochemical techniques in frozen sections of gastric mucosa from the bullfrog, Rana catesbeiana. Apical localization of F-actin with phalloidin in oxynticopeptic cells inhibited with cimetidine revealed small, punctate domains within the apical cytoplasm that were consistent with the presence of short microvilli revealed by electron microscopy. Localization of F-actin in cells stimulated with forskolin was limited to a wide continuous band of cytoplasm corresponding to the location of numerous long surface folds. Inhibition of protein synthesis with cycloheximide did not prevent acid secretion or formation of actin filaments within surface folds in stimulated oxynticopeptic cells, suggesting that the formation of filaments does not require actin synthesis. Staining of gastric mucosae with fluorescent DNase-1 demonstrated that oxynticopeptic cells possess an unusually large pool of non-filamentous actin. Taken together, these results suggest that actin-filament formation in stimulated cells occurs by polymerization of an existing pool of non-filamentous actin. Localization of antibodies specific for villin and fimbrin revealed that these proteins were present within intestinal absorptive cells and gastric surface and neck cells but were not present within inhibited or stimulated oxynticopeptic cells. Brush border myosin-1, present in intestinal absorptive cells, was not present in gastric epithelium. Thus, we propose that actin-containing projections in oxynticopeptic cells are not organized like intestinal microvilli and that filament formation occurs after stimulation by modulating intracellular pools of filamentous and non-filamentous actin.

Moesin: a member of the protein 4.1-talin-ezrin family of proteins

Moesin (membrane-organizing extension spike protein, pronounced mó ez in) has previously been isolated from bovine uterus and characterized as a possible receptor protein for heparan sulfate. We now have cloned and sequenced its complete cDNA, which represents a single 4.2-kilobase mRNA encoding a protein of 577 amino acids. It contains no apparent signal peptide or transmembrane domain. In addition, the protein shows significant sequence identity (72%) to ezrin (cytovillin, p81), as well as similarity to protein 4.1 and talin. All of the latter proteins have been postulated to serve as structural links between the plasma membrane and the cytoskeleton. A similar role for moesin is implied by structure and domain predictions derived from the cDNA-deduced peptide sequence. Furthermore, our data indicate that moesin is identical to the 77-kDa band that copurifies with ezrin in its isolation from human placenta [Bretscher, A. (1989) J. Cell Biol. 108, 921-930].