Identification and characterization of a family of Rab11-interacting proteins

Rab11a is a small GTP-binding protein enriched in the pericentriolar plasma membrane recycling systems. We hypothesized that Rab11a-binding proteins exist as downstream effectors of its action. Here we define a family of four Rab11-interacting proteins: Rab11-Family Interacting Protein 1 (Rab11-FIP1), Rab11-Family Interacting Protein 2 (Rab11-FIP2), Rab11-Family Interacting Protein 3 (Rab11-FIP3), and pp75/Rip11. All four interacting proteins associated with wild type Rab11a and dominant active Rab11a (Rab11aS20V) as well as Rab11b and Rab25. Rab11-FIP2 also interacted with dominant negative Rab11a (Rab11aS25N) and the tail of myosin Vb. The binding of Rab11-FIP1, Rab11-FIP2, and Rab11-FIP3 to Rab11a was dependent upon a conserved carboxyl-terminal amphipathic alpha-helix. Rab11-FIP1, Rab11-FIP2, and pp75/Rip11 colocalized with Rab11a in plasma membrane recycling systems in both non-polarized HeLa cells and polarized Madin-Darby canine kidney cells. GFP-Rab11-FIP3 also colocalized with Rab11a in HeLa cells. Rab11-FIP1, Rab11-FIP2, and pp75/Rip11 also coenriched with Rab11a and H(+)K(+)-ATPase on parietal cell tubulovesicles, and Rab11-FIP1 and Rab11-FIP2 translocated with Rab11a and the H(+)K(+)-ATPase upon stimulating parietal cells with histamine. The results suggest that the function of Rab11a in plasma membrane recycling systems is dependent upon a compendium of protein effectors.

Brefeldin A rapidly disrupts plasma membrane polarity by blocking polar sorting in common endosomes of MDCK cells

Recent studies showing thorough intermixing of apical and basolateral endosomes have demonstrated that endocytic sorting is critical to maintaining the plasma membrane polarity of epithelial cells. Our studies of living, polarized cells show that disrupting endocytosis with brefeldin-A rapidly destroys the polarity of transferrin receptors in MDCK cells while having no effect on tight junctions. Brefeldin-A treatment induces tubulation of endosomes, but the sequential compartments and transport steps of the transcytotic pathway remain intact. Transferrin is sorted from LDL, but is then missorted from common endosomes to the apical recycling endosome, as identified by its nearly neutral pH, and association with GFP chimeras of Rabs 11a and 25. From the apical recycling endosome, transferrin is then directed to the apical plasma membrane. These data are consistent with a model in which polarized sorting of basolateral membrane proteins occurs via a brefeldin-A-sensitive process of segregation into basolateral recycling vesicles. Although disruption of polar sorting correlates with dissociation of γ-adaptin from endosomes, γ-adaptin does not appear to be specifically involved in sorting into recycling vesicles, as we find it associated with the transcytotic pathway, and particularly to the post-sorting transcytotic apical recycling endosome.

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Myosin vb is associated with plasma membrane recycling systems

Myosin Va is associated with discrete vesicle populations in a number of cell types, but little is known of the function of myosin Vb. Yeast two-hybrid screening of a rabbit parietal cell cDNA library with dominant active Rab11a (Rab11aS20V) identified myosin Vb as an interacting protein for Rab11a, a marker for plasma membrane recycling systems. The isolated clone, corresponding to the carboxyl terminal 60 kDa of the myosin Vb tail, interacted with all members of the Rab11 family (Rab11a, Rab11b, and Rab25). GFP-myosin Vb and endogenous myosin Vb immunoreactivity codistributed with Rab11a in HeLa and Madin-Darby canine kidney (MDCK) cells. As with Rab11a in MDCK cells, the myosin Vb immunoreactivity was dispersed with nocodazole treatment and relocated to the apical corners of cells with taxol treatment. A green fluorescent protein (GFP)-myosin Vb tail chimera overexpressed in HeLa cells retarded transferrin recycling and caused accumulation of transferrin and the transferrin receptor in pericentrosomal vesicles. Expression of the myosin Vb tail chimera in polarized MDCK cells stably expressing the polymeric IgA receptor caused accumulation of basolaterally endocytosed polymeric IgA and the polymeric IgA receptor in the pericentrosomal region. The myosin Vb tail had no effects on transferrin trafficking in polarized MDCK cells. The GFP-myosin Va tail did not colocalize with Rab11a and had no effects on recycling system vesicle distribution in either HeLa or MDCK cells. The results indicate myosin Vb is associated with the plasma membrane recycling system in nonpolarized cells and the apical recycling system in polarized cells. The dominant negative effects of the myosin Vb tail chimera indicate that this unconventional myosin is required for transit out of plasma membrane recycling systems.

Targeted disruption of the Kvlqt1 gene causes deafness and gastric hyperplasia in mice

The KvLQT1 gene encodes a voltage-gated potassium channel. Mutations in KvLQT1 underlie the dominantly transmitted Ward-Romano long QT syndrome, which causes cardiac arrhythmia, and the recessively transmitted Jervell and Lange-Nielsen syndrome, which causes both cardiac arrhythmia and congenital deafness. KvLQT1 is also disrupted by balanced germline chromosomal rearrangements in patients with Beckwith-Wiedemann syndrome (BWS), which causes prenatal overgrowth and cancer. Because of the diverse human disorders and organ systems affected by this gene, we developed an animal model by inactivating the murine Kvlqt1. No electrocardiographic abnormalities were observed. However, homozygous mice exhibited complete deafness, as well as circular movement and repetitive falling, suggesting imbalance. Histochemical study revealed severe anatomic disruption of the cochlear and vestibular end organs, suggesting that Kvlqt1 is essential for normal development of the inner ear. Surprisingly, homozygous mice also displayed threefold enlargement by weight of the stomach resulting from mucous neck cell hyperplasia. Finally, there were no features of BWS, suggesting that Kvlqt1 is not responsible for BWS.

Regulation of vesicle trafficking in madin-darby canine kidney cells by Rab11a and Rab25

Polarized epithelial cells maintain the polarized distribution of basolateral and apical membrane proteins through a process of receptor-mediated endocytosis, sorting, and then recycling to the appropriate membrane domain. We have previously shown that the small GTP-binding proteins, Rab11a and Rab25, are associated with the apical recycling system of Madin-Darby canine kidney cells. Here we have utilized inducible expression of wild-type, dominant negative, and constitutively active mutants to directly compare the functions of Rab25 and Rab11a in postendocytic vesicular transport. We found that a Rab11a mutant deficient in GTP binding, Rab11aS25N, potently inhibited both transcytosis and apical recycling yet failed to inhibit transferrin recycling. Similarly, expression of either wild type Rab25 or the active mutant Rab25S21V inhibited both apical recycling and transcytosis of IgA by greater than 50% but had no effect on basolateral recycling of transferrin. Interestingly, the GTPase-deficient mutant Rab11aS20V inhibited basolateral to apical transcytosis of IgA, but had no effect on either apical or basolateral recycling. These results indicate that neither Rab11a nor Rab25 function in the basolateral recycling of transferrin in polarized Madin-Darby canine kidney cells cells, consistent with recent morphological observations by others. Thus, transferrin receptors must be recycled to the plasma membrane prior to sorting of apically directed cargoes into Rab11a/Rab25-positive apical recycling endosomes.

Reversible drug-induced oxyntic atrophy in rats

BACKGROUND & AIMS

Oxyntic atrophy is the hallmark of chronic gastritis. Many studies have sought to develop animal models for oxyntic atrophy, but none of them are reversible. We now report that rats administered high doses of DMP 777 demonstrate reversible oxyntic atrophy.

METHODS

DMP 777 was administered to CD-1 rats by oral gavage (200 mg. kg(-1). day(-1)). Serum gastrin level, in vivo acid secretion, and gastric histological changes were evaluated in DMP 777-dosed animals. Direct effects of DMP 777 on parietal cells were evaluated by assessment of aminopyrine accumulation into isolated rabbit parietal cells, as well as by assessment of DMP 777 effects on acridine orange fluorescence and H(+),K(+)-adenosine triphosphatase (ATPase) activity in isolated tubulovesicles.

RESULTS

Oral dosing with DMP 777 caused a rapid increase in serum gastrin levels and severe hypochlorhydria. DMP 777 inhibited aminopyrine accumulation into rabbit parietal cells stimulated with either histamine or forskolin. DMP 777 reversed a stimulated proton gradient in isolated parietal cell tubulovesicles. Oral dosing with DMP 777 led to rapid loss of parietal cells from the gastric mucosa. In response to the acute loss of parietal cells, there was an increase in the activity of the progenitor zone along with rapid expansion of the foveolar cell compartment. DMP 777 treatment also led to the emergence of bromodeoxyuridine-labeled cells and cells positive for periodic acid-Schiff in the basal region of fundic glands. With extended dosing over 3-6 months, foveolar hyperplasia and oxyntic atrophy were sustained while chief cell, enterochromaffin-like cell, and somatostatin cell populations were decreased. No histological evidence of neoplastic transformation was observed with dosing up to 6 months. Withdrawal of the drug after 3 or 6 months of dosing led to complete restitution of the normal mucosal lineages within 3 months.

CONCLUSIONS

DMP 777 acts as a protonophore with specificity for parietal cell acid-secretory membranes. DMP 777 in high doses leads to the specific loss of parietal cells. Foveolar hyperplasia, loss of normal gland lineages, and the emergence of basal mucous cells appear as sequelae of the absence of parietal cells. The results suggest that parietal cells are critical for the maintenance of the normal mucosal lineage repertoire.

Genomic structure of murine Rab11 family members

Rab11a, Rab11b, and Rab25 in mammals are thought to comprise a subfamily of Rab proteins, although Rab25 has two amino acid differences in its effector domain. We have isolated and characterized the genomic sequences of murine Rab11a and Rab25 and compared them with those of previously characterized mammalian Rab genes. The Rab11a gene spans 29 kb and Rab25 spans 9 kb. The genes have TATA-less promoters, but contain GC-rich areas in their upstream 5' regions. Both genes have 5 exons, with the introns containing characteristic repeats. Rab11a has an unusually long 8. 5-kb fourth intron. The Rab11a and Rab25 genes are localized to chromosomes 9C and 3E3/F1, respectively. The overall organization of the Rab11a, Rab11b, and Rab25 genes is similar, with homologous exon-intron boundaries, and differs markedly from those of Rab3A and Rab1A. These results confirm that Rab11A, Rab11b, and Rab25 represent a closely related gene family.

Synergistic interaction between hypergastrinemia and Helicobacter infection in a mouse model of gastric cancer

BACKGROUND & AIMS

Hypergastrinemia occurs frequently in association with acid suppression and Helicobacter infection, but its role in the progression to gastric atrophy and gastric cancer has not been well defined.

METHODS

The effects of hypergastrinemia, and possible synergy with Helicobacter felis infection, were investigated in insulin-gastrin (INS-GAS) transgenic mice.

RESULTS

INS-GAS mice initially showed mild hypergastrinemia, increased maximal gastric acid secretion, and increased parietal cell number but later progressed to decreased parietal cell number and hypochlorhydria. Development of gastric atrophy was associated with increased expression of growth factors, heparin-binding epidermal growth factor and transforming growth factor alpha. At 20 months of age, INS-GAS mice showed no evidence of increased enterochromaffin-like cell number, but instead exhibited gastric metaplasia, dysplasia, carcinoma in situ, and gastric cancer with vascular invasion. Invasive gastric carcinoma was observed in 6 of 8 INS-GAS mice that were >20 months old. Helicobacter felis infection of INS-GAS mice led to accelerated (< or = 8 mo) development of intramucosal carcinoma (85%), with submucosal invasion (54%) and intravascular invasion (46%; P < or = 0.05).

CONCLUSIONS

These findings support the unexpected conclusion that chronic hypergastrinemia in mice can synergize with Helicobacter infection and contribute to eventual parietal cell loss and progression to gastric cancer.

Expansion of Pdx1-expressing pancreatic epithelium and islet neogenesis in transgenic mice overexpressing transforming growth factor alpha

BACKGROUND & AIMS

The progenitor cells responsible for transforming growth factor (TGF)-alpha-induced pancreatic ductal metaplasia and neoplasia remain uncharacterized. During pancreatic development, differentiated cell types arise from ductal progenitor cells expressing the Pdx1 homeodomain transcription factor. The aims of this study were, first, to evaluate the role of Pdx1-expressing stem cells in MT-TGFalpha transgenic mice, and second, to further characterize cell proliferation and differentiation in this model.

METHODS

To assess Pdx1 gene expression in normal and metaplastic epithelium, we performed in vivo reporter gene analysis using heterozygous Pdx1(lacZ/+) and bigenic Pdx1(lacZ/+)/MT-TGFalpha mice.

RESULTS

Pdx1(lacZ/+)/MT-TGFalpha bigenics showed up-regulated Pdx1 expression in premalignant metaplastic ductal epithelium. In addition to Pdx1 gene activation, TGF-alpha-induced metaplastic epithelium demonstrated a pluripotent differentiation capacity, as evidenced by focal expression of Pax6 and initiation of islet cell neogenesis. The majority of Pdx1-positive epithelial cells showed no expression of insulin, similar to the pattern observed during embryonic development.

CONCLUSIONS

Overexpression of TGF-alpha induces expansion of a Pdx1-expressing epithelium characterized by focal expression of Pax6 and initiation of islet neogenesis. These findings suggest that premalignant events induced by TGF-alpha in mouse pancreas may recapitulate a developmental program active during embryogenesis.

VAP-33 localizes to both an intracellular vesicle population and with occludin at the tight junction

Tight junctions create a regulated intercellular seal between epithelial and endothelial cells and also establish polarity between plasma membrane domains within the cell. Tight junctions have also been implicated in many other cellular functions, including cell signaling and growth regulation, but they have yet to be directly implicated in vesicle movement. Occludin is a transmembrane protein located at tight junctions and is known to interact with other tight junction proteins, including ZO-1. To investigate occludin's role in other cellular functions we performed a yeast two-hybrid screen using the cytoplasmic C terminus of occludin and a human liver cDNA library. From this screen we identified VAP-33 which was initially cloned from Aplysia by its ability to interact with VAMP/synaptobrevin and thus was implicated in vesicle docking/fusion. Extraction characteristics indicated that VAP-33 was an integral membrane protein. Antibodies to the human VAP-33 co-localized with occludin at the tight junction in many tissues and tissue culture cell lines. Subcellular fractionation of liver demonstrated that 83% of VAP-33 co-isolated with occludin and DPPIV in a plasma membrane fraction and 14% fractionated in a vesicular pool. Thus, both immunofluorescence and fractionation data suggest that VAP-33 is present in two distinct pools in the cells. In further support of this conclusion, a GFP-VAP-33 chimera also distributed to two sites within MDCK cells and interestingly shifted occludin's localization basally. Since VAP-33 has previously been implicated in vesicle docking/fusion, our results suggest that tight junctions may participate in vesicle targeting at the plasma membrane or alternatively VAP-33 may regulate the localization of occludin.

Identification of a metaplastic cell lineage associated with human gastric adenocarcinoma

Metaplastic cell lineages arising in response to chronic injury are precursors for the evolution of dysplasia and adenocarcinoma. Although a subtype of intestinal metaplasia has been associated with gastric adenocarcinoma, the link between this lineage and the evolution of gastric adenocarcinoma has remained unclear. Wang et al (1998) have reported that an aberrant metaplastic cell lineage with morphological characteristics similar to Brunner's glands of the duodenum develops in the fundic mucosa of mice infected with Helicobacter felis. This metaplastic lineage expresses the trefoil peptide spasmolytic polypeptide (SP). Given the epidemiological association of Helicobacter species infection with gastric cancer, we hypothesized that this SP-expressing metaplastic (SPEM) lineage may represent a precursor to or appear commensurate with gastric adenocarcinoma. The SPEM lineage was present in 68% of fundic biopsies from patients with fundic Helicobacterpylori-associated gastritis, but was absent in biopsies of fundic mucosa from patients without H. pylori infection. In a review of archival samples from 22 resected gastric adenocarcinomas, we found the SPEM lineage in 91% of cases, typically located in mucosa adjacent to the carcinoma or areas of dysplasia. Importantly, 59% of resections showed SP immunoreactivity within dysplastic cells. These data indicate a strong association of the SPEM lineage with both chronic H. pylori infection and gastric adenocarcinoma.

AKAP350, a multiply spliced protein kinase A-anchoring protein associated with centrosomes

Protein kinase A-anchoring proteins (AKAPs) localize the second messenger response to particular subcellular domains by sequestration of the type II protein kinase A. Previously, AKAP120 was identified from a rabbit gastric parietal cell cDNA library; however, a monoclonal antibody raised against AKAP120 labeled a 350-kDa band in Western blots of parietal cell cytosol. Recloning has now revealed that AKAP120 is a segment of a larger protein, AKAP350. We have now obtained a complete sequence of human gastric AKAP350 as well as partial cDNA sequences from human lung and rabbit parietal cells. The genomic region containing AKAP350 is found on chromosome 7q21 and is multiply spliced, producing at least three distinct AKAP350 isoforms as well as yotiao, a protein associated with the N-methyl-D-aspartate receptor. Rabbit parietal cell AKAP350 is missing a sequence corresponding to a single exon in the middle of the molecule located just after the yotiao homology region. Two carboxyl-terminal splice variants were also identified. Both of the major splice variants showed tissue- and cell-specific expression patterns. Immunofluorescence microscopy demonstrated that AKAP350 was associated with centrosomes in many cell types. In polarized Madin-Darby canine kidney cells, AKAP350 localized asymmetrically to one pole of the centrosome, and nocodazole did not alter its localization. During the cell cycle, AKAP350 was associated with the centrosomes as well as with the cleavage furrow during anaphase and telophase. Several epithelial cell types also demonstrated noncentrosomal pools of AKAP350, especially parietal cells, which contained multiple cytosolic immunoreactive foci throughout the cells. The localization of AKAP350 suggests that it may regulate centrosomal and noncentrosomal cytoskeletal systems in many different cell types.

Association of Rab25 and Rab11a with the apical recycling system of polarized Madin-Darby canine kidney cells

Recent evidence suggests that apical and basolateral endocytic pathways in epithelia converge in an apically located, pericentriolar endosomal compartment termed the apical recycling endosome. In this compartment, apically and basolaterally internalized membrane constituents are thought to be sorted for recycling back to their site of origin or for transcytosis to the opposite plasma membrane domain. We report here that in the epithelial cell line Madin-Darby Canine Kidney (MDCK), antibodies to Rab11a label an apical pericentriolar endosomal compartment that is dependent on intact microtubules for its integrity. Furthermore, this compartment is accessible to a membrane-bound marker (dimeric immunoglobulin A [IgA]) internalized from either the apical or basolateral pole, functionally defining it as the apical recycling endosome. We have also examined the role of a closely related epithelial-specific Rab, Rab25, in the regulation of membrane recycling and transcytosis in MDCK cells. When cDNA encoding Rab25 was transfected into MDCK cells, the protein colocalized with Rab11a in subapical vesicles. Rab25 transfection also altered the distribution of Rab11a, causing the coalescence of immunoreactivity into multiple denser vesicular structures not associated with the centrosome. Nevertheless, nocodazole still dispersed these vesicles, and dimeric IgA internalized from either the apical or basolateral membrane was detected in endosomes labeled with antibodies to both Rab11a and Rab25. Overexpression of Rab25 decreased the rate of IgA transcytosis and of apical, but not basolateral, recycling of internalized ligand. Conversely, expression of the dominant-negative Rab25T26N did not alter either apical recycling or transcytosis. These results indicate that both Rab11a and Rab25 associate with the apical recycling system of epithelial cells and suggest that Rab25 may selectively regulate the apical recycling and/or transcytotic pathways.

Rab11 in dysplasia of Barrett's epithelia

Barrett's esophagus predisposes affected patients to the development of esophageal adenocarcinoma. The development of adenocarcinoma proceeds along a progression through low- and high-grade dysplasia. Surveillance of Barrett's patients requires serial endoscopic investigations and grading mucosal biopsies. Unfortunately, grading of biopsies by conventional hematoxylin and eosin staining is fraught with significant interobserver variations. We have found in both biopsy and resection specimens that immunostaining for the small GTP binding protein Rab11 is increased in low-grade dysplastic cells. This staining is lost in high-grade dysplastic cells. These results suggest that low-grade dysplastic cells undergo an apical trafficking blockade, which is released as cells progress to the less differentiated phenotype of high-grade dysplasia and adenocarcinoma. Examination of the SKGT-4 esophageal adenocarcinoma cell line demonstrated prominent mRNA and protein expression for Rab11. Rab11 immunostaining was present in SKGT-4 cells as a perinuclear nidus of punctate staining along with a more diffuse punctate pattern. Thus, Rab11 expression was present in a esophageal adenocarcinoma cells in culture. Markers of vesicle trafficking may be critical factors for grading of mucosal dysplastic transitions leading to adenocarcinoma.

Rab11a redistributes to apical secretory canaliculus during stimulation of gastric parietal cells

Previous investigations in several systems have demonstrated that Rab3 family members redistribute to soluble fractions on fusion of secretory granules with target plasma membranes. Rab proteins are then recycled back onto mature secretory vesicles after reinternalization of the membrane. Although this cycle is well established for Rab3, far less is known about redistribution of other Rab proteins during vesicle fusion and recycling. In the gastric parietal cell, Rab11a is associated with H-K-ATPase-containing tubulovesicles, which fuse with the apical plasma membrane (secretory canaliculus) in response to agonists such as histamine. We have analyzed distribution of Rab11a and other tubulovesicle proteins in resting and histamine-stimulated rabbit parietal cells. Stimulation of isolated gastric glands in the presence of 100 microM histamine and 100 microM 3-isobutyl-1-methylxanthine did not cause a significant increase in soluble Rab11a. H-K-ATPase, Rab11a, Rab25, syntaxin 3, and SCAMPs increased immunoreactivity in stimulus-associated vesicles prepared from rabbits treated with histamine compared with those from ranitidine-treated animals. The large GTPase dynamin was found in both vesicle preparations, but there was no change in amount of immunoreactivity. Immunofluorescence staining of resting and histamine-stimulated primary cultures of parietal cells demonstrated redistribution of H-K-ATPase and Rab11a to F-actin-rich canalicular membranes. Dynamin was present on canalicular membranes in resting and stimulated cells. These results indicate that Rab11a does not cycle off the membrane during the process of tubulovesicle fusion with the secretory canaliculus. Thus Rab11a may remain associated with recycling apical membrane vesicle populations.

Localization and significance of pp55, a gastric mucosal membrane protein with tyrosine kinase activity

In Fischer 344 rats, induction of gastric mucosal proliferative activity, whether the result of aging or injury or occurring after administration of epidermal growth factor, gastrin, or bombesin, is associated with a rise in tyrosine kinase activity and tyrosine phosphorylation of several mucosal proteins, including a protein with a molecular mass of 53-55 kDa. We hypothesized that this phosphotyrosine membrane protein (referred to as pp55) may play a role in regulating gastric mucosal cell proliferation and differentiation. Purification and subsequent immunoprecipitation studies now show that pp55 is a tyrosine kinase. In addition, the enzyme activity in the gastric mucosa is found to be fourfold higher in aged rats than in young rats. Incubation of gastric mucosal membranes with transforming growth factor-alpha (2 x 10(-8) M) stimulates tyrosine kinase activity of pp55. Immunolocalization studies reveal that pp55 immunoreactivity is predominantly present in mucous cells that are located just above the proliferative zone and spasmolytic peptide-immunoreactive mucous neck cells. Tyrosine kinase activity as well as expression of pp55 are also greatly increased in the gastric mucosa after hypertonic saline-induced injury, a condition that results in stimulation of surface mucosal cell proliferation and differentiation. Our current data suggest that pp55 is a tyrosine kinase, likely localized to pre-surface cells. The presence of pp55 in pre-surface mucous cells and the expression and tyrosine kinase activity of this protein, which can be stimulated during mucosal cell proliferation and differentiation, strongly suggest a role for pp55 in differentiation of gastric surface mucous cells.

Identification of clathrin and clathrin adaptors on tubulovesicles of gastric acid secretory (oxyntic) cells

gamma-Adaptin and clathrin heavy chain were identified on tubulovesicles of gastric oxyntic cells with the anti-gamma-adaptin monoclonal antibody (MAb) 100/3 and an anti-clathrin heavy chain MAb (MAb 23), respectively. In Western blots, crude gastric microsomes from rabbit and rat and density gradient-purified, H-K-ATPase-rich microsomes from these same species were immunoreactive for gamma-adaptin and clathrin. In immunofluorescent labeling of isolated rabbit gastric glands, anti-gamma-adaptin and anti-clathrin heavy chain immunoreactivity appeared to be concentrated in oxyntic cells. In primary cultures of rabbit oxyntic cells, the immunocytochemical distribution of gamma-adaptin immunoreactivity was similar to that of the tubulovesicular membrane marker in oxyntic cells, the H-K-ATPase. Further biochemical characterization of the tubulovesicular gamma-adaptin-containing complex suggested that it has a subunit composition that is typical of that for a clathrin adaptor: in addition to the gamma-adaptin subunit, it contains a beta-adaptin subunit and other subunits of apparent molecular masses of 50 kDa and 19 kDa. From solubilized gastric microsomes from rabbit, gamma-adaptin could be copurified with the major cargo protein of tubulovesicles, the H-K-ATPase. Thus this tubulovesicular coat may bind directly to the H-K-ATPase and may thereby mediate the regulated trafficking of the H-K-ATPase at the apical membrane of the oxyntic cell during the gastric acid secretory cycle. Given the similarities of the regulated trafficking of the H-K-ATPase with recycling of cargo through the apical recycling endosome of many epithelial cells, we propose that tubulovesicular clathrin and adaptors may regulate some part of an apical recycling pathway in other epithelial cells.

Mice lacking secretory phospholipase A2 show altered apoptosis and differentiation with Helicobacter felis infection

BACKGROUND & AIMS

Infection with Helicobacter pylori uniformly leads to a chronic superficial gastritis that may progress to atrophic gastritis, a premalignant process. A mouse model of Helicobacter felis infection was used to study possible genetic determinants of the response to infection.

METHODS

Three inbred mouse strains with known secretory phospholipase A2 (sPLA2) genotypes [BALB/c (+/+), C3H/HeJ (+/+), and C57BL/6 (-/-)] were orally infected with H. felis and examined longitudinally using routine histology, immunocytochemistry, electron microscopy, proliferating cell nuclear antigen, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling, and Northern and Western blot studies.

RESULTS

Only the C57BL/6 strain showed increased gastric fundic proliferation and apoptosis in response to infection. In addition, the C57BL/6 mouse showed a marked loss of parietal and chief cells, along with a marked expansion of an aberrant gastric mucous cell lineage that stained positive for spasmolytic polypeptide. In contrast, no significant change in these cell types was observed in BALB/c and C3H/HeJ strains. Increased expression of sPLA2 was observed in BALB/c and C3H/HeJ after H. felis infection, whereas sPLA2 expression was absent in C57BL/6 mice.

CONCLUSIONS

H. felis infection leads to increased apoptosis and altered cellular differentiation in the C57BL/6 mouse, a strain that lacks gastric sPLA2 expression. Because sPLA2 has been identified recently as the MOM1 (modifier of MIN) locus that influences polyp formation in the colon, these studies suggest that sPLA2 may also influence the gastric epithelial response to Helicobacter infection.

Mapping of ezrin dimerization using yeast two-hybrid screening

Ezrin, a membrane-cytoskeleton linker protein, is involved in the recruitment of H+/K(+)-ATPase-containing tubulovesicles to the canalicular membrane during acid secretion in the parietal cell. Ezrin exists as monomers and head-to-tail dimers in vivo, and oligomerization is presumably important for activation. In this study, we mapped regions of ezrin-ezrin interaction using the yeast two-hybrid assay. We observed that the N-terminal 283 amino acids are sufficient for interaction with the carboxyl terminal 140 amino acids. The region 333.446 inhibits this association. However, the inclusion of amino acids 283-310 appears to release the inhibition. These specific interactions may play a critical role in the formation of dimerization-competent ezrin molecules.

Increased immunoreactivity for Rab11, a small GTP-binding protein, in low-grade dysplastic Barrett's epithelia

Adenocarcinoma of the esophagus develops from metaplastic Barrett's columnar epithelia through the evolution of dysplastic epithelial intermediates. Although the role of dysplasia leading to adenocarcinoma is well established, far less is known regarding the cellular changes involved in this process. Because the development of dysplasia is characterized by the loss of apical secretory specializations, we hypothesized that changes in apical trafficking might be involved in the dysplastic process. We have sought to evaluate the expression of an important candidate regulator of apical trafficking, the small GTP-binding protein, Rab11, in resection and biopsy tissue from patients with Barrett's esophagus. Sections from esophageal resection specimens from 4 patients and endoscopic biopsies from 60 patients were stained with antibodies against Rab11 and Rab25 as well as protein markers of the Golgi apparatus and p53 protein. Rab11 staining in low-grade dysplastic regions was similar to that observed with monoclonal antibodies against Rab25 and gamma-adaptin and colocalized with staining for the Golgi marker, the mannose-6-phosphate receptor. In the esophageal adenocarcinoma resections, prominent Rab11 immunostaining was observed in the supranuclear region of low-grade dysplastic cells. In contrast, regions of high-grade dysplasia demonstrating strong nuclear p53 staining showed only diffuse or absent Rab11 staining. In endoscopic biopsies, 91% of biopsies that were read unanimously as low-grade dysplasia demonstrated supranuclear Rab11 staining. Fourteen percent of biopsies unanimously graded as being without dysplasia demonstrated perinuclear Rab11 staining. No p53 immunostaining was observed in any of the low-grade dysplasia biopsy specimens. An increase in Rab11 immunoreactivity seems to correlate with low-grade dysplasia, whereas p53 immunostaining correlates with high-grade dysplasia. The colocalization of Rab11 staining with increased immunoreactivity for markers of the trans-Golgi system is consistent with a defect in apical trafficking due to an expansion of either the trans-Golgi compartment or the apical recycling vesicle system.

Rab11 is associated with transferrin-containing recycling compartments in K562 cells

3'RACE PCR was used to survey Rab transcripts synthesized by the human hematopoietic K562 cell line. Among the identified GTP-binding proteins, Rab11 was discovered. This result was unexpected since Rab11 previously had been found associated with polarized and secretory cells. Rab11 mRNA was abundant compared to that for other Rabs in K562 cells; protein levels represented 0.05-0.1% of total membrane protein. Localization of Rab11 using confocal immunofluoresence microscopy revealed extensive overlap with transferrin in recycling and/or exocytic compartments and suggests that Rab11 in non-polarized and non-secretory cells may play a role in the trafficking and recycling of internalized proteins.

Gastrin deficiency results in altered gastric differentiation and decreased colonic proliferation in mice

BACKGROUND & AIMS

Gastrin is a peptide hormone important in the regulation of both acid secretion and differentiation of oxyntic mucosal cells of the stomach. To further elucidate the role of gastrin in the growth and development of the gastrointestinal tract, we have generated mice that are deficient in gastrin.

METHODS

Gastrin-deficient mice were generated through targeted gene disruption. Gastric and colonic architecture were determined by routine histology and immunohistochemical techniques. Proliferation was assessed by 5-bromo-2'-deoxyuridine incorporation.

RESULTS

Targeted disruption of the gastrin gene resulted in mice incapable of expressing gastrin messenger RNA (mRNA) or producing gastrin peptide. This deficiency led to a marked change in gastric architecture, with a decrease in number of parietal and enterochromaffin-like cells and an increase in number of mucous neck cells. There was no difference in the proliferation labeling index of the stomach in gastrin-deficient mice (3.04% +/- 0.33%) compared with wild-type littermates (3.15% +/- 0.18%). The colon of gastrin-deficient mice seemed normal histologically, although there was a decreased proliferation labeling index (2.97% +/- 0.52%) compared with wild-type littermates (4.71% +/- 0.44%; P < 0.01).

CONCLUSIONS

Gastrin is important in regulating the differentiation of the gastric mucosa and is a trophic factor for the colonic mucosa.

Two Rab proteins, vesicle-associated membrane protein 2 (VAMP-2) and secretory carrier membrane proteins (SCAMPs), are present on immunoisolated parietal cell tubulovesicles

The tubulovesicles of gastric parietal cells sequester H+/K+-ATPase molecules within resting parietal cells. Stimulation of parietal cell secretion elicits delivery of intracellular H+/K+-ATPase to the apically oriented secretory canaliculus. Previous investigations have suggested that this process requires the regulated fusion of intracellular tubulovesicles with the canalicular target membrane. We have sought to investigate the presence of critical putative regulators of vesicle fusion on immunoisolated gastric parietal cell tubulovesicles. Highly purified tubulovesicles were prepared by gradient fractionation and immunoisolation on magnetic beads coated with monoclonal antibodies against the alpha subunit of H+/K+-ATPase. Western blot analysis revealed the presence of Rab11, Rab25, vesicle-associated membrane protein 2 (VAMP-2) and secretory carrier membrane proteins (SCAMPs) on immunoisolated vesicles. The same cohort of proteins was recovered on vesicles immunoisolated with monoclonal antibodies against SCAMPs and VAMP-2. In contrast, whereas immunoreactivities for syntaxin 1A/1B and synaptosome-associated protein (SNAP-25) were present in gradient-isolated vesicles, none of the immunoreactivity was associated with immunoisolated vesicles. The observation of VAMP-2 and two Rab proteins on immunoisolated H+/K+-ATPase-containing tubulovesicles supports the role for tubulovesicles in a regulated vesicle fusion process. In addition, the presence of SCAMPs along with Rab11 and Rab25 implicates the tubulovesicles as a critical apical recycling vesicle population.

Inhibition of parietal cell acid secretion is mediated by the classical epidermal growth factor receptor

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) inhibit gastric acid secretion both in vivo and in vitro. Previous studies have indicated that EGF and TGF-alpha bind to the same EGF/TGF-alpha receptor. Nevertheless, we and others have previously demonstrated that inhibition of acid secretion by these growth factors requires concentrations of the peptides that are 10-fold higher than those necessary for induction of mitogenesis. Therefore, we have sought to investigate whether gastric parietal cells may possess a second EGF/TGF-alpha receptor class. Two systems were studied: First, [125I]TGF-alpha was cross-linked to the receptor in isolated rabbit parietal cell membranes, and labeled species were resolved on SDS-PAGE. Second, acid secretion was evaluated in pylorus-ligated waved-2 mutant mice, which carry a disabling point mutation in their classical EGF/TGF-alpha receptor. In isolated parietal cells, [125I]TGF-alpha was cross-linked into a single species of 170 kDa. Cross-linking was inhibited in the presence of unlabeled TGF-alpha with an IC50 of 80 nM. In the pylorus-ligated mice, control littermate mice demonstrated a dose-dependent inhibition of acid secretion by EGF with an IC50 of 20 micrograms/kg. In contrast, EGF had no inhibitory effect on acid secretion in waved-2 mice at concentrations up to 100 micrograms/kg. No alterations in parietal cell or gastrin cell numbers were observed. These results in both isolated rabbit parietal cells and waved-2 mice support the existence of only a single class of EGF/TGF-alpha receptors in parietal cells. Differences in growth factor affinity are likely due to the modification of the receptor or one of its coordinate regulators.

Cyclic nucleotide-dependent vasorelaxation is associated with the phosphorylation of a small heat shock-related protein

Activation of cyclic nucleotide-dependent signaling pathways leads to the relaxation of various smooth muscles. One of the major phosphorylation events associated with cyclic nucleotide-dependent vasorelaxation in bovine trachealis and carotid artery smooth muscle is the phosphorylation of two 20-kDa phosphoproteins with pI values of 5.7 and 5.9 (previously designated pp8 and pp3, respectively). The present studies sought to determine the identities of pp3 and pp8 in vascular smooth muscle. The phosphopeptide maps for the pp8 and pp3 proteins were similar. Preparative two-dimensional gel electrophoresis and amino acid sequencing of a peptide fragment of the pp3 protein revealed a sequence identical to a 20-kDa heat shock-related protein (HSP20) previously purified from skeletal muscle. Western blot and immunoprecipitation analysis with anti-HSP20 antibodies demonstrated that the pp3 and pp8 proteins are phosphorylated forms of HSP20. In addition, HSP20 could be phosphorylated in vitro by both cAMP-dependent protein kinase and cGMP-dependent protein kinase. These data suggest that the phosphorylation of the heat shock-related protein HSP20 is associated with cyclic nucleotide-dependent relaxation of vascular smooth muscle.

Identification and characterization of a novel A-kinase-anchoring protein (AKAP120) from rabbit gastric parietal cells

The type-II cAMP-dependent protein kinase (A-Kinase) partitions primarily into the particulate fraction in gastric parietal cells. Localization of this kinase to particular subcellular domains is mediated through the binding of the regulatory subunit (RII) dimer to A-Kinase-anchoring proteins (AKAPs). Using a [32P]RII overlay assay, we have screened a rabbit gastric parietal cell cDNA library and have isolated a single RII-binding protein clone. Sequence analysis revealed an open reading frame coding for 1022 amino acids (AKAP120). Recombinant fragments of the full-length clone were prepared and the RII-binding region mapped to an area between amino acids 489 and 549. This area contained a putative alpha-helical RII-binding region between amino acids 503 and 516. Incubation of [32P]RII with a synthetic peptide of AKAP120-(489-522) completely inhibited the binding of [32P]RII to the recombinant AKAP120 fragments that demonstrated RII binding. In vitro RII-binding affinity studies indicated a high-affinity interaction between AKAP120 and RII with a Kapp between 50 and 120 nM for the three recombinant fragments that bound [32P]RII. RNase-protection analysis revealed that AKAP120 is a widely distributed protein, with the highest levels of mRNA observed in gastric fundus. The presence of this novel high-affinity AKAP in gastric parietal cells suggests that it may regulate RII subcellular sequestration in this cell type.

Ezrin is a cyclic AMP-dependent protein kinase anchoring protein

cAMP-dependent protein kinase (A-kinase) anchoring proteins (AKAPs) are responsible for the subcellular sequestration of the type II A-kinase. Previously, we identified a 78 kDa AKAP which was enriched in gastric parietal cells. We have now purified the 78 kDa AKAP to homogeneity from gastric fundic mucosal supernates using type II A-kinase regulatory subunit (RII) affinity chromatography. The purified 78 kDa AKAP was recognized by monoclonal antibodies against ezrin, the canalicular actin-associated protein. Recombinant ezrin produced in either Sf9 cells or bacteria also bound RII. Recombinant radixin and moesin, ezrin-related proteins, also bound RII in blot overlay. Analysis of recombinant truncations of ezrin mapped the RII binding site to a region between amino acids 373 and 439. This region contained a 14-amino-acid amphipathic alpha-helical putative RII binding region. A synthetic peptide containing the amphipathic helical region (ezrin409-438) blocked RII binding to ezrin, but a peptide with a leucine to proline substitution at amino acid 421 failed to inhibit RII binding. In mouse fundic mucosa, RII immunoreactivity redistributed from a predominantly cytosolic location in resting parietal cells, to a canalicular pattern in mucosa from animals stimulated with gastrin. These results demonstrate that ezrin is a major AKAP in gastric parietal cells and may function to tether type II A-kinase to a region near the secretory canaliculus.

Co-distribution of calmodulin-dependent protein kinase II and inositol trisphosphate receptors in an apical domain of gastrointestinal mucosal cells

The Type 3 inositol 1,4,5-trisphosphate (InsP3) receptor is expressed at high levels in gastrointestinal tissues. This receptor has 16 potential phosphorylation sites for calcium/calmodulin-dependent protein kinase II (CaM kinase II). To determine if the Type 3 InsP3 receptor is likely to be a physiologic substrate for CaM kinase II, localizations of the Type 3 InsP3 receptor and CaM kinase II were compared in tissues of the gastrointestinal tract. Cellular and subcellular localizations were determined by immunofluorescence microscopy in rat intestine, pancreas, and stomach, and in isolated rabbit gastric glands. Both proteins were found in the apical region of intestinal enterocytes, pancreatic acinar cells, and gastric parietal, chief, and surface mucous cells. CaM kinase II was found throughout the entire intracellular canalicular F-actin domain of parietal cells, whereas the type 3 InsP3 receptor was restricted to the neck region. Thus, in several gastrointestinal tissues the Type 3 InsP3 receptor is specifically localized to a portion of the apical cytoskeletal domain in which resides the calcium-responsive effector CaM kinase II.

Foveolar hyperplasia following partial gastrectomy results from expansion of surface mucous cell compartment

Antrectomy with gastroenterostomy reconstruction is often associated with the development of foveolar hyperplasia and oxyntic atrophy. While a role for bile reflux in the etiology of foveolar hyperplasia is well established, the identity of the mucous cell lineages responsible for this condition have not been well characterized. We have studied three patients who demonstrated foveolar hyperplasia in their gastric remnant following antrectomy and gastroenterostomy. Mucosal samples were stained with antibodies against the trefoil peptides pS2 and hSP, to identify surface mucous and mucous neck cell lineages, respectively. Postoperative biopsies from all three patients showed oxyntic atrophy as documented by staining of parietal cells with antibodies against H/K-ATPase. All three patients demonstrated an exclusive expansion of pS2 immunoreactive mucous cells. The hSP staining cells were located deep in the expanded foveolar region. The results suggest that foveolar hyperplasia following antrectomy with gastroenterostomy results from a reactive hyperplasia of surface mucous cells. This pattern of surface cell hyperplasia is more consistent with a reactive expansion of mucous cells than with a response to chronic injury.

Purification, cloning, and expression of a novel, endogenous, calcium-sensitive, 28-kDa phosphoprotein

In gastric parietal cells, cholinergically induced increases in intracellular free calcium concentrations have been well characterized, but little is known about the signaling events beyond the initial rise in intracellular calcium. In the present study, we report the isolation of a 28-kDa protein, which is rapidly phosphorylated in intact, enriched parietal cells in response to both the cholinergic agonist, carbachol, and the calcium ionophore, ionomycin. A combination of in situ 32P labeling and one- and two-dimensional gel electrophoresis was used to acquire sufficient quantities of protein to obtain partial amino acid sequence. Cloning of the pp28 cDNA revealed a novel protein which we have named CSPP28 based on its calcium-sensitive phosphorylation. There are three CSPP28 mRNA species (1.7, 2.2, and 3.3 kilobases) that are widely distributed throughout a variety of rabbit tissues. Recombinant CSPP28 was phosphorylated by both crude parietal cell homogenate and purified CaM kinase II in a calcium/calmodulin-dependent manner. We propose that CSPP28 may play an important and ubiquitous role in the calcium signaling pathway.

A somatodendritic distribution of Rab11 in rabbit brain neurons

Comparisons of subcellular targeting in neurons and epithelial cells have led to suggestions that apical epithelial antigens localize to axons and nerve terminals. We have studied the distribution in brain of the small GTP-binding protein, Rab11, which is localized to apical vesicular populations in epithelial cells. Sections of rabbit brain were examined by immunohistochemistry using a monoclonal antibody against rabbit Rab11. Rab11 immunoreactivity was present exclusively in neurons. In all regions examined, including forebrain, cerebellum, thalamus and brainstem, Rab11 immunoreactivity was observed in cell bodies and dendrites. No staining was observed in hippocampal neurons. These results indicate that the distribution of Rab11 does not support previous suggestions that apical epithelial markers should localize to axons and synaptic endings.

Overexpression of transforming growth factor-alpha alters differentiation of gastric cell lineages

Overexpression of transforming growth factor-alpha (TGF-alpha) in the gastric fundic mucosa of metallothionein promoter/enhancer-TGF-alpha(MT-TGF-alpha) transgenic mice produces a phenotype of foveolar hyperplasia similar to that observed in Ménétrier's disease. We have investigated the dynamics involved in the alterations of gastric mucosal morphology in the MT-TGF-alpha mouse model. The fundic mucosa of MT-TGF-alpha mice and nontransgenic littermates was evaluated in animals treated with cadmium sulfate. To mark the mucosal proliferative zone, 8-bromodeoxyuridine (BrdU) was administered 2 hr prior to killing. Gastric mucosa was examined by diastase-resistant, periodic acid-Schiff-positive (DR-PAS) staining and immunohistochemistry for H/K-ATPase an BrdU. MT-TGF-alpha mice demonstrated increased numbers of DR-PAS-staining mucous cells and lower parietal cell numbers per gland unit. While the proliferative zone in nontransgenic mice was located in the upper half of the gland, the zone in MT-TGF-alpha mice was located in the basal region. Overexpression of TGF-alpha in MT-TGF-alpha mice leads to an alteration in the development of mucosal lineages from the fundic progenitor zone, which is biased towards the predominant differentiation of foveolar mucous cells.

Genetic ablation of parietal cells in transgenic mice: a new model for analyzing cell lineage relationships in the gastric mucosa

The gastric mucosa of mammalian stomach contains several differentiated cell types specialized for the secretion of acid, digestive enzymes, mucus, and hormones. Understanding whether each of these cell lineages is derived from a common stem cell has been a challenging problem. We have used a genetic approach to analyze the ontogeny of progenitor cells within mouse stomach. Herpes simplex virus 1 thymidine kinase was targeted to parietal cells within the gastric mucosa of transgenic mice, and parietal cells were ablated by treatment of animals with the antiherpetic drug ganciclovir. Ganciclovir treatment produced complete ablation of parietal cells, dissolution of gastric glands, and loss of chief and mucus-producing cells. Termination of drug treatment led to the reemergence of all major gastric epithelial cell types and restoration of glandular architecture. Our results imply the existence of a pluripotent stem cell for the gastric mucosa. Parietal cell ablation should provide a model for analyzing cell lineage relationships within the stomach as well as mechanisms underlying gastric injury and repair.

Rab11 is an apically located small GTP-binding protein in epithelial tissues

Rab proteins are involved in many aspects of dynamic vesicle processing within eukaryotic cells. We have previously identified Rab11 in gastric parietal cell tubulovesicle membranes. We have produced a monoclonal antibody that is specific for Rab11. In all rabbit tissues examined, Rab11 immunoreactivity was highly enriched in epithelial cells. In the gastric fundus, parietal cells were stained in a pattern consistent with localization on tubulovesicles. Surface mucous cells of both the fundus and antrum demonstrated punctate subapical staining. Ileal and proximal colonic enterocyte labeling was observed deep to the brush borders. In the distal colon, staining was observed in the apical regions of mid-crypt cells. In skin and esophagus, punctate immunoreactivity was present in the medial layers of the squamous epithelia. Prominent Rab11 immunoreactivity was present in hepatocytes deep to the bile canaliculi. Punctate subapical staining was observed in pancreatic acinar cells. Apical staining was also observed in collecting duct cells and in the glandular cells of the prostate. These results indicate the Rab11 is expressed in apical vesicular populations in discrete epithelial cell populations.

Evidence for a regulatory role for histamine in gastric enterochromaffin-like cell proliferation induced by hypergastrinemia

BACKGROUND/AIMS

Hypergastrinemia, induced by sustained suppression of gastric acid secretion, is associated with gastric enterochromaffin-like (ECL) cell hyperplasia and carcinoid tumor formation. We examined the effect of a selective H1-histamine antagonist, terfenadine, on gastric mucosal cell proliferation to determine whether histamine might modulate ECL cell generation.

METHODS

The rodent mastomys received the H2-antagonist loxtidine (2 g/l drinking water) alone or in combination with terfenadine (0.5 g/l or 35 mg/l drinking water) for 120 days. Controls received water or terfenadine alone. Serum gastrin levels and tissue histamine content were assayed by radioimmunoassays, and tissue chromogranin levels determined (Western blot analysis). In vivo cell proliferation was measured by bromodeoxyuridine (BrdU, 200 mg/kg/day, 3 days) incorporation. Gastric mucosal thickness was determined, ECL cell number was assessed, and the percentage of proliferating ECL cells quantitated. To evaluate the direct action on ECL cells we then studied the effect of terfenadine on histamine secretion and DNA synthesis (BrdU uptake) in an isolated preparation (approximately 90% pure) of ECL cells.

RESULTS

Loxtidine increased serum gastrin levels, mucosal thickness, tissue chromogranin levels, tissue histamine content, BrdU incorporation, ECL cell number, and proliferating ECL cells (all parameters p < 0.05). Terfenadine alone, irrespective of dosage, had no significant effect. The high dose in combination with loxtidine significantly inhibited the increase in tissue chromogranin levels, tissue histamine content, ECL cell number and proliferating ECL cells (p < 0.05), but did not alter other parameters, compared to loxtidine alone. The low does did not alter the loxtidine-induced changes. In pure isolated ECL cells, terfenadine did not alter histamine secretion either alone or in combination with gastrin (10 nM). DNA synthesis was significantly inhibited by terfenadine (IC50 10(-10) M).

CONCLUSIONS

Terfenadine specifically inhibited the effect of loxtidine-induced ECL cell proliferation in vivo and significantly inhibited ECL cell DNA synthesis in vitro. We postulate that histamine, through an H1 receptor, positively modulates gastric ECL cell proliferation.

Rab proteins in gastric parietal cells: evidence for the membrane recycling hypothesis

The gastric parietal cell secretes large quantities of HCl into the lumen of the gastric gland in response to secretagogues such as histamine. In the membrane recycling hypothesis, this secretory activity requires the trafficking of the gastric H+/K(+)-ATPase to the cell surface from intracellular tubulovesicles. The Rab subclass of small GTP-binding proteins is thought to confer specificity to vesicle transport throughout the secretory pathway, and previous investigations established that Rab11 is highly expressed in gastric parietal cells. Recent discoveries in intra-Golgi transport and neuronal synaptic vesicle fusion have fortuitously converged on an evolutionarily conserved protein complex involved in vesicle docking and fusion. Recent results indicate that Rab11 is involved in the apical targeting of vesicles in parietal cells and other epithelial cells throughout the gastrointestinal tract. In support of the membrane recycling hypothesis, Rab co-segregates with H+/K(+)-ATPase in parietal cells. The presence of Rab11 on tubulovesicles supports a role for this Rab protein in recycling vesicle trafficking.

Expression of trefoil peptides in the gastric mucosa of transgenic mice overexpressing transforming growth factor-alpha

Overexpression of transforming growth factor-alpha (TGF-alpha) in the gastric mucosa of metallothionein-TGF alpha (MT-TGF alpha) transgenic mice leads to a marked alteration in the ontogeny of the fundic cellular lineages. Induction of the transgene leads to the over-production of mucous cells with a concomitant diminution in the development of parietal cell and chief cell lineages. We have sought to define more precisely the mucous cell lineages involved in the mucous cell hyperplasia in MT-TGF alpha mice by investigating the expression of trefoil peptides in MT-TGF alpha mice. MT-TGF alpha mice and their non-transgenic littermates were treated with cadmium sulfate beginning at 13 days of age. Animals were then sacrificed at intervals over the following 2 weeks and gastric mucosa was examined for expression of trefoil peptides and TGF alpha by immunohistochemistry and in situ hybridization. No TGF alpha mRNA expression could be demonstrated by in situ hybridization in non-transgenic mice. In MT-TGF alpha mice, in situ grains for TGF alpha mRNA were detected at the base of fundic glands in 13 day old animals, whereas the expression was observed more widely in the mucosa of older animals (28 days). TGF alpha immunoreactivity was observed in foveolar mucous cells and residual parietal cells in MT-TGF alpha mice at all ages. By in situ hybridization, pS2 mRNA was detected in the surface mucous cells in normal gastric mucosa. In MT-TGF alpha mice, pS2 mRNA was found throughout the expanded foveolar region. By in situ hybridization, spasmolytic peptide (SP) expression was observed in the region of the progenitor zone in both groups of mice. By immunohistochemistry, SP expression was noted in a broad band of mucous neck cells deep to the progenitor zone. No gastric expression of intestinal trefoil factor (ITF) was noted in either group of mice. The results demonstrate that the expansion of the foveolar mucous cell compartment in MT-TGF alpha mice is due to the hyperplasia of normal surface cells expressing their particular mucin-associated trefoil peptide, pS2.

Distribution of A-kinase anchoring proteins in parietal cells

Recent investigations have suggested that subcellular compartmentalization of second messenger responsive enzyme systems may be responsible for specific patterns of cellular activation. The type II cAMP-dependent kinase (A-kinase) is localized to particular subcellular domains through the binding of the regulatory subunit (RII) dimer to A-kinase anchoring proteins (AKAPs). Using a [32P]RII overlay assay, we have investigated the presence of AKAPs throughout the gastrointestinal tract, with specific emphasis focused on the gastric parietal cell. All gastrointestinal tissues contained at least one detectable AKAP (60 kDa), with five AKAPs (50-140 kDa) in fundic and antral mucosa. Isolated gastric glands contained four AKAPs. Two AKAPs (50 and 78 kDa) were detected in purified parietal cells, with the 78 kDa AKAP (AKAP78) specific to parietal cell enriched populations. RII-binding to all of these AKAPs was abolished by preincubation of [32P]RII with a synthetic peptide representing the RII-binding region of the AKAP, HT-31. AKAP78 was distributed throughout all membrane fractions of subfractionated parietal cells, with the largest amount of RII-binding detected in the light membrane fraction. Identification of A-kinase regulatory subunits by photoaffinity labeling with 8-azido-[32P]cAMP demonstrated that RII segregated into the same parietal cell subfractions as AKAP78. A majority (approximately 60%) of AKAP78 was detected in the Triton X-100-insoluble fraction, suggesting that this protein resides in a cytoskeletal domain. AKAP78 may be involved in localizing the type II A-kinase to specific intracellular locations in the parietal cell.

A novel serine-specific kinase activity associated with exocrine secretory granules

Zymogen secretion from exocrine cells involves an exocytotic process that is highly regulated by the modification of cytoplasmic components at different cellular levels. In the present studies, purified secretory granules were prepared from rabbit gastric chief cells, rat pancreatic acinar cells, and parotid glands to characterize a Mg(2+)-dependent protein kinase activity. In chief cell granules, endogenous pepsinogen, a fortuitous substrate, was phosphorylated at optimal Mg2+ and K+ concentrations of 40 and 50 mM, respectively. Adenosine 3',5'-cyclic monophosphate, Ca2+, and calmodulin had no significant effects on the kinase activity. In contrast, Mn2+ or Zn2+ inhibited the kinase activity. In addition to pepsinogen, the exogenous substrates casein, myelin basic protein, and lysine-rich histone were also phosphorylated by the granule-associated kinase. All substrates were exclusively phosphorylated on serine residues. ATP, but not GTP, served as the donor in the phosphorate transfer reaction. Casein kinase (CK) inhibitors CKI-7 and dibromoribofuransylbenzimidazole at concentrations (10 microM) that significantly inhibited CK activities in the tissue homogenate failed to inhibit the granule-associated kinase activity. The kinase activity was localized to the granule membrane and could be removed from the membrane with either 5 mM EDTA or alkaline carbonate extraction. Furthermore, protease digestion sensitivity revealed that the kinase was localized on the cytoplasmic face of the granules. Our results therefore indicate that the secretory granules of exocrine gastric chief cells, pancreatic acini, and parotid acini possess a unique serine-specific protein kinase activity. The cytoplasmic orientation of the kinase activity suggests a possible role in vesicle processing or the exocytotic process.

Impaired cyclic nucleotide-dependent vasorelaxation in human umbilical artery smooth muscle

Activation of either the adenylate cyclase pathway with forskolin or the guanylate cyclase pathway with sodium nitroprusside fails to induce active relaxation of serotonin-precontracted human umbilical artery smooth muscle (HUASM) but causes active relaxation of serotonin-precontracted bovine carotid artery smooth muscle (BCASM). This difference in response appears to be unique to HUASM in that all other vascular muscles exhibit relaxation in response to these substances. Forskolin and sodium nitroprusside stimulation leads to respective increases in intracellular adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) concentrations in HUASM and BCASM. cAMP- and cGMP-dependent protein kinases are both present in HUASM and can be activated in homogenates of HUASM by the addition of exogenous cAMP and cGMP, respectively. When either forskolin or nitroprusside acts in BCASM, two low-molecular-weight proteins display an increase in the extent of phosphorylation. Neither protein shows such an increase when HUASM is treated with either agent. Thus the inability of HUASM to display active relaxation appears to be secondary to impaired activation of cyclic nucleotide-dependent protein kinases. The refractoriness to active relaxation may contribute to the vasospasm that occurs in the umbilical vasculature with parturition.

PYY inhibition of VIP-stimulated ion transport in the rabbit distal ileum

Vasoactive intestinal polypeptide (VIP) is the pathophysiologic mediator of several small intestinal hypersecretion states. VIP exerts its effect by binding mucosal receptors and ultimately increasing intracellular levels of cAMP. Peptide YY (PYY), a GI hormone concentrated in the distal ileum and colon, has been demonstrated to decrease VIP-mediated secretion in the colon through a specific Y4 mucosal receptor. Characterization of PYY's effect on VIP-stimulated small intestinal secretion may provide a basis for future therapeutic interventions. We hypothesized that ion transport in the small intestine is mediated through a novel Y receptor subtype. We performed Ussing chamber ion transport studies on rabbit ileum using VIP, PYY, and other pancreatic polypeptide (PP)-fold peptides in order to specifically examine: (1) the effects of VIP and PYY on basal and VIP-stimulated short circuit current (Isc), and (2) the changes in VIP-stimulated Isc in response to NPY, PP, leucine31,proline31 neuropeptide Y fragment, ([Leu31,Pro34]NPY) and the carboxy-terminal fragment of NPY (NPY13-36). VIP increased basal Isc in a concentration-dependent manner, while PYY decreased basal Isc. Graded concentrations of PYY decreased VIP-stimulated increases in Isc. PYY added prior to VIP had no effect on VIP-stimulated increases in ISC. Inhibition of VIP-stimulated Isc increases was seen with NPY, but not with [Leu31,Pro34]NPY, PP, or NPY13-36. This distinct pattern of binding affinity characterizes a novel Y receptor subtype. Additionally, increases in Isc by VIP despite pretreatment with PYY suggests that VIP-stimulated ion transport is mediated through mechanisms other than increases in cAMP.

Enrichment of rab11, a small GTP-binding protein, in gastric parietal cells

Parietal cell secretion of acid requires the coordinated fusion of H(+)-K(+)-adenosinetriphosphatase (ATPase)-containing tubulovesicles with a secretory canalicular target membrane. We have previously reported the presence of rab2 on parietal cell tubulovesicles (L. H. Tang, S. A. Stoch, I. M. Modlin, and J. R. Goldenring. Biochem. J. 285: 715-719, 1992). Since 60% of the small GTP-binding protein sequences obtained from parietal cells were > 95% homologous with human rab11 (J. R. Goldenring, K. R. Shen, H. D. Vaughan, and I.M. Modlin. J. Biol. Chem. 268: 18419-18422, 1993), we sought to study rab11 in gastric parietal cells. A complete rab11 sequence was obtained, and the deduced amino acid sequence of rabbit rab11 was identical to that for human. Rab11 mRNA was present throughout the gastrointestinal mucosa. mRNA for both rab11 and rab2 were enriched in isolated parietal cells compared with chief cells. A polyclonal antiserum against rab11 labeled a single 25-kDa band in isolated parietal cells. Immunostaining of rat fundic tissue demonstrated prominent staining of parietal cells. Rab11 staining cosegregated with alpha-H(+)-K(+)-ATPase staining in enriched preparations of rabbit parietal cell tubulovesicles. These results suggest that rab11 is enriched in parietal cells and is associated with intracellular tubulovesicles.

Histamine as an intermediate growth factor in genesis of gastric ECLomas associated with hypergastrinemia in mastomys

Profound and sustained inhibition of gastric acid secretion has been associated with development of carcinoid tumors of the fundic enterochromaffin-like (ECL) cells in rodents. While ECL cell hyperplasia has been recognized in humans, the development of carcinoid tumors is rare and often confined to patients under treatment for gastrinoma related to the multiple endocrine neoplasia type I (MEN1) syndrome. The Mastomys was utilized as a model for the rapid induction of ECLomas by insurmountable acid secretory blockade induced by the pharmacologically irreversible H2-receptor antagonist, loxtidine. Loxtidine-induced ECL cell hyperplasia and neoplasia were compared in the absence of presence of cyproheptadine (0.5 mg/kg), an H1-receptor antagonist. Loxtidine administration resulted in a significant increase in ECL cell hyperplasia and neoplasia as well as an increase in ECL cell number, mucosal thickness, plasma gastrin levels, and stomach weight. Cyproheptadine ameliorated loxtidine-induced ECL cell hyperplasia and neoplasia and significantly decreased loxtidine-stimulated increases in ECL cell number. Nevertheless, cyproheptadine failed to alter the loxtidine-induced increase in plasma gastrin, stomach weight or mucosal height. The results indicate that cyproheptadine, an H1-receptor antagonist, inhibits loxtidine-induced ECL cell hyperplasia independent of any effects on serum gastrin.

Partial agonism by gastrin for a cholecystokinin receptor mediating pepsinogen secretion

Isolated gastric glands from rabbit were used to characterize the functional cholecystokinin (CCK)-like peptide receptors that mediate pepsinogen secretion. Pepsinogen secretion was stimulated by both CCK octapeptide sulfate (CCK-8) and A-71378, a selective CCK-A-type receptor agonist, with similar mean effective doses (1.0 and 0.8 nM, respectively). Compared with CCK-8, gastrin-17 (G-17-I) showed reduced potency and only partial efficacy for stimulation of pepsinogen secretion while inhibiting the maximal CCK-8-stimulated response. The nonpeptide inhibitors, asperlicin and L-364,718, inhibited pepsinogen secretion with identical pA2 values for antagonism of both CCK and gastrin, indicating that both peptides interact with the same functional receptor. Specific binding of [3H]CCK-8 to isolated chief cell membranes was displaced fully by both CCK and gastrin, indicating full receptor occupancy by both peptides. A novel synthetic peptide analogue, pseudogastrin [(Glu)5-Ala-Tyr-Nle-Gly-Trp-Nle-Asp-Phe-NH2], was used to investigate the structural basis for the lower potency and efficacy of G-17-I. The potency of CCK and gastrin analogues for pepsinogen secretion was found to be dependent on both sulfation of a tyrosine residue and the position of the tyrosine residue relative to the COOH-terminal phenylalanine amide. The efficacy appears to be determined partially by the extended NH2-terminal sequence of G-17-I. The results of the present study are interpreted to show that pepsinogen secretion is mediated by a CCK-A-type receptor and gastrin acts at the same receptor as a partial agonist.