Regulation of protein and vesicle trafficking at the apical membrane of epithelial cells

miR-143-3p modulates depressive-like behaviors via Lasp1 in the mouse ventral hippocampus

Depression is a prevalent and intricate mental disorder. The involvement of small RNA molecules, such as microRNAs in the pathogenesis and neuronal mechanisms underlying the depression have been documented. Previous studies have demonstrated the involvement of microRNA-143-3p (miR-143-3p) in the process of fear memory and pathogenesis of ischemia; however, the relationship between miR-143-3p and depression remains poorly understood. Here we utilized two kinds of mouse models to investigate the role of miR-143-3p in the pathogenesis of depression. Our findings reveal that the expression of miR-143-3p is upregulated in the ventral hippocampus (VH) of mice subjected to chronic restraint stress (CRS) or acute Lipopolysaccharide (LPS) treatment. Inhibiting the expression of miR-143-3p in the VH effectively alleviates depressive-like behaviors in CRS and LPS-treated mice. Furthermore, we identify Lasp1 as one of the downstream target genes regulated by miR-143-3p. The miR-143-3p/Lasp1 axis primarily affects the occurrence of depressive-like behaviors in mice by modulating synapse numbers in the VH. Finally, miR-143-3p/Lasp1-induced F-actin change is responsible for the synaptic number variations in the VH. In conclusion, this study enhances our understanding of microRNA-mediated depression pathogenesis and provides novel prospects for developing therapeutic approaches for this intractable mood disorder.

The emergence of nebulin repeats and evolution of lasp family proteins

The LIM and SH3 domain protein (lasp) family, the smallest proteins in the nebulin superfamily, consists of vertebrate lasp-1 expressed in various non-muscle tissues, vertebrate lasp-2 expressed in the brain and cardiac muscle, and invertebrate lasp whose functions have been analyzed in Ascidiacea and Insecta. Gene evolution of the lasp family proteins was investigated by multiple alignments, comparison of gene structure, and synteny analyses in eukaryotes in which mRNA expression was confirmed. All invertebrates analyzed in this study belonging to the clade Filasterea, with the exception of Placozoa, have at least one lasp gene. The minimal actin-binding region (LIM domain and first nebulin repeat) and SH3 domain detected in vertebrate lasp-2 were found to be conserved among the lasp family proteins, and we showed that nematode lasp has actin-binding activity. The linker sequences vary among invertebrate lasp proteins, implying that the lasp family proteins have universal and diverse functions. Gene structures and syntenic analyses suggest that a gene fragment encoding two nebulin repeats and a linker emerged in Filasterea or Holozoa, and the first lasp gene was generated following combination of three gene fragments encoding the LIM domain, two nebulin repeats with a linker, and the SH3 domain. This article is protected by copyright. All rights reserved.

An update on the LIM and SH3 domain protein 1 (LASP1): a versatile structural, signaling, and biomarker protein

The gene encoding the LIM and SH3 domain protein (LASP1) was cloned two decades ago from a cDNA library of breast cancer metastases. As the first protein of a class comprising one N-terminal LIM and one C-terminal SH3 domain, LASP1 founded a new LIM-protein subfamily of the nebulin group. Since its discovery LASP1 proved to be an extremely versatile protein because of its exceptional structure allowing interaction with various binding partners, its ubiquitous expression in normal tissues, albeit with distinct expression patterns, and its ability to transmit signals from the cytoplasm into the nucleus. As a result, LASP1 plays key roles in cell structure, physiological processes, and cell signaling. Furthermore, LASP1 overexpression contributes to cancer aggressiveness hinting to a potential value of LASP1 as a cancer biomarker.

In this review we summarize published data on structure, regulation, function, and expression pattern of LASP1, with a focus on its role in human cancer and as a biomarker protein. In addition, we provide a comprehensive transcriptome analysis of published microarrays (n=2,780) that illustrates the expression profile of LASP1 in normal tissues and its overexpression in a broad range of human cancer entities.

LASP1, a Newly Identified Melanocytic Protein with a Possible Role in Melanin Release, but Not in Melanoma Progression

The LIM and SH3 protein 1 (LASP1) is a focal adhesion protein. Its expression is increased in many malignant tumors. However, little is known about the physiological role of the protein. In the present study, we investigated the expression and function of LASP1 in normal skin, melanocytic nevi and malignant melanoma. In normal skin, a distinct LASP1 expression is visible only in the basal epidermal layer while in nevi LASP1 protein is detected in all melanocytes. Melanoma exhibit no increase in LASP1 mRNA compared to normal skin. In melanocytes, the protein is bound to dynamin and mainly localized at late melanosomes along the edges and at the tips of the cell. Knockdown of LASP1 results in increased melanin concentration in the cells. Collectively, we identified LASP1 as a hitherto unknown protein in melanocytes and as novel partner of dynamin in the physiological process of membrane constriction and melanosome vesicle release.

cDNA cloning and characterization of LASP1 from silkworm, Bombyx mori, involved in cytoplasmic polyhedrosis virus infection

Full-length cDNA of a LIM and SH3 contained protein 1 (named BmLASP1) was identified from the silkworm, Bombyx mori, for the first time by rapid amplification of cDNA ends. The full-length cDNA of BmLASP1 is 2094 bp, consisting of a 5'-terminal untranslated region (UTR) of 117 bp, and a 3'-UTR of 610 bp with two poly-adenylation signal sequence AATAAA and a poly (A) tail. The BmLASP1 cDNA encodes a polypeptide comprising 455 amino acids, including a LIM domain, two nebulin domains and an SH3 domain. The theoretical isoelectric point is 7.07 and the predicted molecular weight is 51.8 kDa. BmLASP1 has no signal peptide but three potential N-glycosylation sites. Sequence similarity and phylogenic analyses indicated that BmLASP1 belonged to the group of insect LASP1 with a longer linker region which is different from vertebrate LASP1. The LASP1 in silkworm contained eight exons in its coding regions, and the last exon-intron boundary was conserved the same as in mammalian and Ciona intestinalis LASP1 genes. By fluorescent quantitative real-time polymerase chain reaction, the mRNA transcripts of BmLASP1 were mainly detected in the gonad, head, and spiracle, and slightly in the silk gland, vasa mucosa, midgut, fat body, and hemocytes. After silkworm larvae were infected by B. mori cytoplasmic polyhedrosis virus (BmCPV), the relative expression level of BmLASP1 was down-regulated in the midgut. This result suggested that BmLASP1 may play an important role in the response of silkworm to BmCPV infection.

Lasp1 gene disruption is linked to enhanced cell migration and tumor formation

Lasp1 is an actin-binding, signaling pathway-regulated phosphoprotein that is overexpressed in several cancers. siRNA knockdown in cell lines retards cell migration, suggesting the possibility that Lasp1 upregulation influences cancer metastasis. Herein, we utilized a recently developed gene knockout model to assess the role of Lasp1 in modulating nontransformed cell functions. Wound healing and tumor initiation progressed more rapidly in Lasp1(-/-) mice compared with Lasp1(+/+) controls. Embryonic fibroblasts (MEFs) derived from Lasp1(-/-) mice also migrated more rapidly in vitro. These MEFs characteristically possessed increased focal adhesion numbers and displayed more rapid attachment compared with wild-type MEFs. Differential microarray analyses revealed alterations in message expression for proteins implicated in cell migration, adhesion, and cytoskeletal organization. Notably, the focal adhesion protein, lipoma preferred partner (LPP), a zyxin family member and putative Lasp1 binding protein, was increased about twofold. Because LPP gene disruption reduces cell migration, we hypothesize that LPP plays a role in enhancing the migratory capacity of Lasp1(-/-) MEFs, perhaps by modifying the subcellular localization of other motility-associated proteins. The striking contrast in the functional effects of loss of Lasp1 in innate cells compared with cell lines reveals distinct differences in mechanisms of motility and attachment in these models.

LIM and SH3 protein 1 (Lasp1) is a novel p53 transcriptional target involved in hepatocellular carcinoma

BACKGROUND/AIMS

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related death worldwide with poor prognosis associated with tumor invasion and metastasis. The tumor suppressor p53 plays critical roles in tumor development, but there is increasing evidence for its involvement in tumor metastasis with the underlying mechanisms largely unexplored.

METHODS

Using combinatorial analysis of a p53 binding database with HCC microarray expression profile, we identified a novel metastasis-related gene Lasp1 as a potential p53 target.

RESULTS

In this study, we demonstrate that Lasp1 is indeed a bona fide p53 target by validating the functional repression effect of p53 on Lasp1 via a p53 response element. Transient transfection of wild-type p53 but not the mutant form suppressed Lasp1 in Hep3B (p53-/-) cells, while p53 siRNA up-regulated its expression in HepG2 (p53+/+) cells. p53 mutations at key residues involved in DNA binding abrogates the p53-mediated suppression of Lasp1 expression. In addition, Lasp1 regulates HCC cell growth as well as cell migration and invasion ability.

CONCLUSIONS

p53 transcriptionally represses Lasp1, which is a partner protein in affecting HCC cell motility. This suggests that p53 may play a role in influencing tumor metastasis through Lasp1.

The actin-binding protein Lasp promotes Oskar accumulation at the posterior pole of the Drosophila embryo

During Drosophila oogenesis, Oskar mRNA is transported to the posterior pole of the oocyte, where it is locally translated and induces germ-plasm assembly. Oskar protein recruits all of the components necessary for the establishment of posterior embryonic structures and of the germline. Tight localization of Oskar is essential, as its ectopic expression causes severe patterning defects. Here, we show that the Drosophila homolog of mammalian Lasp1 protein, an actin-binding protein previously implicated in cell migration in vertebrate cell culture, contributes to the accumulation of Oskar protein at the posterior pole of the embryo. The reduced number of primordial germ cells in embryos derived from lasp mutant females can be rescued only with a form of Lasp that is capable of interacting with Oskar, revealing the physiological importance of the Lasp-Oskar interaction.

Heterogeneity of acid secretion induced by carbachol and histamine along the gastric gland axis and its relationship to [Ca2+]i

The gastric glands of the mammalian fundic mucosa are constituted by different cell types. Gastric fluid is a mixture of acid, alkali, ions, enzymes, and mucins secreted by parietal, chief, and mucous cells. We studied activation of acid secretion using LysoSensor Yellow/Blue in conjunction with fluo 3 to measure changes in pH and Ca(2+) in isolated rabbit gastric glands. We evidenced a spatial heterogeneity in the amplitude of acid response along the gland axis under histamine and cholinergic stimulation. Carbachol induced a transitory pH increase before acidification. This relative alkalinization may be related to granule release from other cell types. Omeprazole inhibited the acid component but not the rise in pH. Histamine stimulated acid secretion without increase of lumen pH. We studied the relationship between Ca(2+) release and/or entry and H(+) secretion in glands stimulated by carbachol. Ca(2+) release was associated with a fast and transient components of H(+) secretion. We found a linear relationship between Ca(2+) release and H(+) secretion. Ca(2+) entry was associated with a second slow and larger component of acid secretion. The fast component may be the result of activation of Cl(-) and K(+) channels and hence H(+)/K(+) pumps already present in the membrane, whereas the slow component might be associated with translocation of H(+)/K(+) pumps to the canaliculi. In conclusion, with cholinergic stimulation, gastric glands secrete a mixture of acid and other product(s) with a pH above 4.2, both triggered by Ca(2+) release. Maintenance of acid secretion depends on Ca(2+) entry and perhaps membrane fusion.

Targeted disruption of the Lasp-1 gene is linked to increases in histamine-stimulated gastric HCl secretion

Lasp-1 (LIM and SH3 domain protein 1) is a multidomain actin-binding protein that is differentially expressed within epithelial tissues and brain. In the gastric mucosa, Lasp-1 is highly expressed in the HCl-secreting parietal cell, where it is prominently localized within the F-actin-rich subcellular regions. Histamine-induced elevation of parietal cell [cAMP]i increases Lasp-1 phosphorylation, which is correlated with activation of HCl secretion. To determine whether Lasp-1 is involved in the regulation of HCl secretion in vivo, we generated a murine model with a targeted disruption of the Lasp-1 gene. Lasp-1-null mice had slightly lower body weights but developed normally and had no overt phenotypic abnormalities. Basal HCl secretion was unaffected by loss of Lasp-1, but histamine stimulation induced a more robust acid secretory response in Lasp-1-null mice compared with wild-type littermates. A similar effect of histamine was observed in isolated gastric glands on the basis of measurements of accumulation of the weak base [14C]aminopyrine. In addition, inhibition of the acid secretory response to histamine by H2 receptor blockade with ranitidine proceeded more slowly in glands from Lasp-1-null mice. These findings support the conclusion that Lasp-1 is involved in the regulation of parietal HCl secretion. We speculate that cAMP-dependent phosphorylation of Lasp-1 alters interactions with F-actin and/or endocytic proteins that interact with Lasp-1, thereby regulating the trafficking/activation of the H+, K+-ATPase (proton pump).

The LIM and SH3 domain protein family: structural proteins or signal transducers or both?

LIM and SH3 Protein 1 (LASP-1) was initially identified from a cDNA library of metastatic axillary lymph nodes (MLN) more than a decade ago. It was found to be overexpressed in human breast and ovarian cancer and became the first member of a newly defined LIM-protein subfamily of the nebulin group characterized by the combined presence of LIM and SH3 domains. LASP2, a novel LASP1-related gene was first identified and characterized in silico. Subsequently it proved to be a splice variant of the Nebulin gene and therefore was also termed LIM/nebulette. LASP-1 and -2 are highly conserved in their LIM, nebulin-like and SH3 domains but differ significantly at their linker regions. Both proteins are ubiquitously expressed and involved in cytoskeletal architecture, especially in the organization of focal adhesions. Here we present the first systematic review to summarize all relevant data concerning their domain organization, expression profiles, regulating factors and function. We compile evidence that both, LASP-1 and LASP-2, are important during early embryo- and fetogenesis and are highly expressed in the central nervous system of the adult. However, only LASP-1 seems to participate significantly in neuronal differentiation and plays an important functional role in migration and proliferation of certain cancer cells while the role of LASP-2 is more structural. The increased expression of LASP-1 in breast tumours correlates with high rates of nodal-metastasis and refers to a possible relevance as a prognostic marker.

MicroRNA and proteome expression profiling in early-symptomatic α-synuclein(A30P)-transgenic mice

The α-synuclein has been implicated in the pathophysiology of Parkinson's disease (PD), because mutations in the alpha-synuclein gene cause autosomal-dominant hereditary PD and fibrillary aggregates of alpha-synuclein are the major component of Lewy bodies. Since presynaptic accumulation of α-synuclein aggregates may trigger synaptic dysfunction and degeneration, we have analyzed alterations in synaptosomal proteins in early symptomatic α-synuclein(A30P)-transgenic mice by two-dimensional differential gel electrophoresis. Moreover, we carried out microRNA expression profiling using microfluidic chips, as microRNA have recently been shown to regulate synaptic plasticity in rodents and to modulate polyglutamine-induced protein aggregation and neurodegeneration in flies. Differentially expressed proteins in α-synuclein(A30P)-transgenic mice point to alterations in mitochondrial function, actin dynamics, iron transport, and vesicle exocytosis, thus partially resembling findings in PD patients. Oxygen consumption of isolated brain mitochondria, however, was not reduced in mutant mice. Levels of several microRNA (miR-10a, -10b, -212, -132, -495) were significantly altered. One of them (miR-132) has been reported to be highly inducible by growth factors and to be a key regulator of neurite outgrowth. Moreover, miR-132-recognition sequences were detected in the mRNA transcripts of two differentially expressed proteins. MicroRNA may thus represent novel biomarkers for neuronal malfunction and potential therapeutic targets for human neurodegenerative diseases.

H,K-ATPase protein localization and Kir4.1 function reveal concordance of three axes during early determination of left-right asymmetry

Consistent laterality is a fascinating problem, and study of the Xenopus embryo has led to molecular characterization of extremely early steps in left-right patterning: bioelectrical signals produced by ion pumps functioning upstream of asymmetric gene expression. Here, we reveal a number of novel aspects of the H+/K+-ATPase module in chick and frog embryos. Maternal H+/K+-ATPase subunits are asymmetrically localized along the left-right, dorso-ventral, and animal-vegetal axes during the first cleavage stages, in a process dependent on cytoskeletal organization. Using a reporter domain fused to molecular motors, we show that the cytoskeleton of the early frog embryo can provide asymmetric, directional information for subcellular transport along all three axes. Moreover, we show that the Kir4.1 potassium channel, while symmetrically expressed in a dynamic fashion during early cleavages, is required for normal LR asymmetry of frog embryos. Thus, Kir4.1 is an ideal candidate for the K+ ion exit path needed to allow the electroneutral H+/K+-ATPase to generate voltage gradients. In the chick embryo, we show that H+/K+-ATPase and Kir4.1 are expressed in the primitive streak, and that the known requirement for H+/K+-ATPase function in chick asymmetry does not function through effects on the circumferential expression pattern of Connexin43. These data provide details crucial for the mechanistic modeling of the physiological events linking subcellular processes to large-scale patterning and suggest a model where the early cytoskeleton sets up asymmetric ion flux along the left-right axis as a system of planar polarity functioning orthogonal to the apical-basal polarity of the early blastomeres.

A lasp family protein of Ciona intestinalis

Lasp-1 and lasp-2 are actin-binding proteins that contain a LIM domain, nebulin repeats, and an SH3 domain and they are significantly conserved in mammalian and avian. Lasp-1 is widely expressed in nonmuscle tissues and lasp-2 is specifically expressed in the brain. Genes encoding proteins homologous to lasp-1 and lasp-2 were deposited in the genome/cDNA database of invertebrates such as sea urchins, nematodes, and insects; however, function of their proteins have not been studied in detail. In this study, we analyzed the gene structure, actin-binding activity, and expression of the lasp protein of the ascidian Ciona intestinalis (Ci lasp). A single gene encoding lasp protein was found in the ascidian, and the amino acid sequences of Ci lasp and other invertebrate lasp proteins exhibited similarity to vertebrate lasp-1 and lasp-2 to the same extent. A part of the exon-intron boundaries was conserved between the vertebrate lasp-1, the vertebrate lasp-2 and the invertebrate lasp genes. Ci lasp exhibited actin-binding activity in a co-sedimentation assay. In situ hybridization revealed that the expression of Ci lasp mRNA was apparent in nervous system of early embryos and was detected in various tissues in young adults. This suggests that the functions of invertebrate lasp proteins might include the functions of vertebrate lasp-1 and lasp-2.

Drebrin E2 is differentially expressed and phosphorylated in parietal cells in the gastric mucosa

Developmentally regulated brain proteins (drebrins) are highly expressed in brain where they may regulate actin filament formation in dendritic spines. Recently, the drebrin E2 isoform was detected in certain epithelial cell types including the gastric parietal cell. In gastric parietal cells, activation of HCl secretion is correlated with actin filament formation and elongation within intracellular canaliculi, which are the sites of acid secretion. The aim of this study was to define the pattern of drebrin expression in gland units in the intact rabbit oxyntic gastric mucosa and to initiate approaches to define the functions of this protein in parietal cells. Drebrin E2 expression was limited entirely or almost entirely to parietal cells and depended upon the localization of parietal cells along the gland axis. Rabbit drebrin E2 was cloned and found to share 86% identity with human drebrin 1a and to possess a number of cross-species conserved protein-protein interaction and phosphorylation consensus sites. Two-dimensional Western blot and phosphoaffinity column analyses confirmed that drebrin is phosphorylated in parietal cells, and several candidate phosphorylation sites were identified by mass spectrometry. Overexpression of epitope-tagged drebrin E2 led to the formation of microspikes and F-actin-rich ring-like structures in cultured parietal cells and suppressed cAMP-dependent acid secretory responses. In Madin-Darby canine kidney cells, coexpression of epitope-tagged drebrin and the Rho family GTPase Cdc42, which induces filopodial extension, produced an additive increase in the length of microspike projections. Coexpression of dominant negative Cdc42 with drebrin E2 did not prevent drebrin-induced microspike formation. These findings suggest that 1) drebrin can induce the formation of F-actin-rich membrane projections by Cdc42-dependent and -independent mechanisms; and that 2) drebrin plays an active role in directing the secretagogue-dependent formation of F-actin-rich filaments on the parietal cell canalicular membrane. Finally, the differential distribution of drebrin in parietal cells along the gland axis suggests that drebrin E2 may be an important marker of parietal cell differentiation and functionality.

Phosphorylation of mouse LASP-1 on threonine 156 by cAMP- and cGMP-dependent protein kinase

LIM and SH3 domain protein (LASP-1) is a specific focal adhesion protein involved in cell migration. Overlay studies demonstrate that LASP-1 directly binds to the proline-rich domains of zyxin, lipoma preferred partner (LPP), and vasodilator-stimulated phosphoprotein (VASP), with zyxin being the most prominent interacting partner. Despite the LIM/zinc-finger domain, hypothesized to be involved in homodimerization, LASP-1 exists as a monomer. In vitro phosphorylation of recombinant mouse LASP-1 by cAMP- and cGMP-dependent protein kinase (PKA and PKG, respectively) occurs at serine 61, serine 99, and threonine 156 whereas in intact cells mouse LASP-1 is phosphorylated only at threonine 156. This site is different from the known in vivo phosphorylation sites in human (serine 146) and rabbit (serine 99 and serine 146). Nevertheless, immunofluorescence of LASP-1 in human and mouse mesangial cells revealed no difference in subcellular distribution. Exposure of the cells to forskolin induced a translocation of both, human and mouse LASP-1, from the focal contacts to the cell interior without affecting F-actin structure. Immunoblotting of LASP-1 in various mouse and human tissues detected a similar prominent expression in non-muscle tissue. Altogether, our data suggest so far no functional differences between human and mouse LASP-1.

N/A

N/A

Membrane dynamics and the regulation of epithelial cell polarity

Plasma membranes of epithelial cells consist of two domains, an apical and a basolateral domain, the surfaces of which differ in composition. The separation of these domains by a tight junction and the fact that specific transport pathways exist for intracellular communication between these domains and distinct intracellular compartments relevant to cell polarity development, have triggered extensive research on issues that focus on how the polarity is generated and maintained. Apart from proper assembly of tight junctions, their potential functioning as landmark for the transport machinery, cell-cell adhesion is obviously instrumental in barrier formation. In recent years, distinct endocytic compartments, defined as subapical compartment or common endosome, were shown to play a prominent role in regulating membrane trafficking to and from polarized membrane domains. Sorting devices remain to be determined but likely include distinct rab proteins, and evidence is accumulating to indicate that signaling events may direct intracellular membrane transport, intimately involved in the biogenesis and maintenance of polarized membrane domains and hence the development of cell polarity.

A novel LIM and SH3 protein (lasp-2) highly expressing in chicken brain

From eluates of F-actin affinity chromatography of chicken brain, we identified a novel actin-binding protein (lasp-2) whose gene was predicted in silico. We cloned cDNA of chicken lasp-2 and analyzed its structure, expression, activity, and localization with lasp-1 (LIM and SH3 protein 1), a previously identified actin-binding protein closely related to lasp-2. Chicken lasp-2 showed high homology to mammalian putative lasp-2. Both chicken lasp-1 and chicken lasp-2 have N-terminal LIM domains, C-terminal SH3 domains, and internal nebulin repeats. However, lasp-2 is greatly different from lasp-1 in the sequence between the second nebulin repeat and a SH3 domain, and the region is conserved in chicken, mouse, and human. As expected from its structural similarity to lasp-1, lasp-2 possessed actin-binding activity and localized with actin filament in filopodia of neuroblastoma. In contrast to lasp-1, which is widely distributed in non-muscle tissues, lasp-2 was highly expressed in brain.

Actin microfilaments et al.--the many components, effectors and regulators of epithelial cell endocytosis

The aim of this review is to introduce the advances made over the past several years regarding the participation of actin and actin-associated proteins in clathrin-mediated endocytosis in simple cell models, and then to consider the evidence for the involvement of these effectors in apical clathrin-mediated endocytosis in epithelial cells. Basic mechanisms of clathrin-mediated endocytosis are initially addressed, followed by a detailed description of the actin cytoskeleton: its organization, function and, most importantly, the essential role played by proteins and signaling pathways responsible for the regulation of actin filament dynamics. Our focus then shifts to the GTPase, dynamin and its pivotal role as a bridge between various components of the clathrin endocytic machinery and the actin cytoskeleton. Mechanisms and effectors of dynamin-dependent endocytosis are then described, with a particular emphasis on novel proteins, which link dynamin to actin filaments. We consider additional effectors proposed to interact with actin to facilitate clathrin-mediated endocytosis in a dynamin-independent manner. The multiple roles which actin filaments are thought to play in endocytosis are addressed followed by a more detailed characterization of actin filament participation specifically in apical endocytosis. We conclude by discussing how these concepts may be integrated to improve drug internalization at the apical plasma membrane of epithelial cells.

Cell biology of acid secretion by the parietal cell

Acid secretion by the gastric parietal cell is regulated by paracrine, endocrine, and neural pathways. The physiological stimuli include histamine, acetylcholine, and gastrin via their receptors located on the basolateral plasma membranes. Stimulation of acid secretion typically involves an initial elevation of intracellular calcium and/or cAMP followed by activation of a cAMP-dependent protein kinase cascade that triggers the translocation and insertion of the proton pump enzyme, H,K-ATPase, into the apical plasma membrane of parietal cells. Whereas the H,K-ATPase contains a plasma membrane targeting motif, the stimulation-mediated relocation of the H,K-ATPase from the cytoplasmic membrane compartment to the apical plasma membrane is mediated by a SNARE protein complex and its regulatory proteins. This review summarizes the progress made toward an understanding of the cell biology of gastric acid secretion. In particular we have reviewed the early signaling events following histaminergic and cholinergic activation, the identification of multiple factors participating in the trafficking and recycling of the proton pump, and the role of the cytoskeleton in supporting the apical pole remodeling, which appears to be necessary for active acid secretion by the parietal cell. Emphasis is placed on identifying protein factors that serve as effectors for the mechanistic changes associated with cellular activation and the secretory response.

Actin binding of human LIM and SH3 protein is regulated by cGMP- and cAMP-dependent protein kinase phosphorylation on serine 146

Various drugs that elevate cGMP levels and activate cGMP-dependent protein kinase (cGK) inhibit agonist-induced platelet activation. In the present study we identified the LIM and SH3 domain protein (LASP) that was recently cloned from human breast cancer cells (Tomasetto, C., Regnier, C., Moog-Lutz, C., Mattei, M. G., Chenard, M. P., Liderau, R., Basset, P., and Rio, M. C. (1995) Genomics 28, 367-376) as a novel substrate of cGK in human platelets. Recombinant human LASP was phosphorylated by cGMP- and cAMP-dependent protein kinase (cAK) in vitro. Cotransfection of PtK-2 cells with LASP and cGK confirmed phosphorylation of LASP in vivo. Studies with human LASP mutants identified serine 146 as a specific phosphorylation site for cGK and cAK in vivo. LASP is an actin-binding protein, and the phospho-LASP-mimicking mutant S146D showed reduced binding affinity for F-actin in cosedimentation experiments. Immunofluorescence of transfected PtK2 cells demonstrated the localization of LASP in the tips of cell membrane extensions and at cell-cell contacts. Expression of the human LASP mutant S146D resulted in nearly complete relocalization to the cytosol and reduced migration of the cells. Taken together, these data suggest that phosphorylation of LASP by cGK and cAK may be involved in cytoskeletal organization and cell motility.

Localization and function of soluble N-ethylmaleimide-sensitive factor attachment protein-25 and vesicle-associated membrane protein-2 in functioning gastric parietal cells

The soluble N-ethylmaleimide-sensitive factor attachment protein of 25 kDa (SNAP-25) plays an important role in vesicle trafficking. Together with vesicle-associated membrane protein-2 (VAMP-2) and syntaxin, SNAP-25 forms a ternary complex implicated in docking and fusion of secretory vesicles with the plasma membrane during exocytosis. These so-called SNARE proteins are believed to regulate tubulovesicle trafficking and fusion during the secretory cycle of the gastric parietal cell. Here we examined the cellular localization and functional importance of SNAP-25 in parietal cell cultures. Adenoviral constructs were used to express SNAP-25 tagged with cyan fluorescent protein, VAMP-2 tagged with yellow fluorescent protein, and SNAP-25 in which the C-terminal 25 amino acids were deleted (SNAP-25 Delta181-206). Membrane fractionation experiments and fluorescent imaging showed that SNAP-25 is localized to the apical plasma membrane. The expression of the mutant SNAP-25 Delta181-226 inhibited the acid secretory response of parietal cells. Also, SNAP Delta181-226 bound poorly in vitro with recombinant syntaxin-1 compared with wild type SNAP-25, indicating that pairing between syntaxin-1 and SNAP-25 is required for parietal cell activation. Dual expression of SNAP-25 tagged with cyan fluorescent protein and VAMP-2 tagged with yellow fluorescent protein revealed a dynamic change in distribution associated with acid secretion. In resting cells, SNAP-25 is at the apical plasma membrane and VAMP-2 is associated with cytoplasmic H,K-ATPase-rich tubulovesicles. After stimulation, the two proteins co-localize on the apical plasma membrane. These data demonstrate the functional significance of SNAP-25 as a SNARE protein in the parietal cell and show the dynamic stimulation-associated redistribution of VAMP-2 from H,K-ATPase-rich tubulovesicles to co-localize with SNAP-25 on the apical plasma membrane.