A membrane-cytoskeletal complex containing Na+,K+-ATPase, ankyrin, and fodrin in Madin-Darby canine kidney (MDCK) cells: implications for the biogenesis of epithelial cell polarity

The C-terminal tail of the metabotropic glutamate receptor subtype 7 is necessary but not sufficient for cell surface delivery and polarized targeting in neurons and epithelia

Complex neuronal functions rely upon the precise sorting, targeting, and restriction of receptors to specific synaptic microdomains. Little is known, however, of the molecular signals responsible for mediating these selective distributions. Here we report that metabotropic glutamate receptor subtype 7a (mGluR7a) is polarized at the basolateral surface when expressed in Madin-Darby canine kidney (MDCK) epithelial cells but is not polarized when expressed in cultured hippocampal neurons. Truncation of the mGluR7 cytoplasmic tail produces a protein that is restricted to a perinuclear intracellular compartment in both neurons and MDCK cells, where this protein colocalizes with a trans-Golgi network antigen. The mGluR7 cytoplasmic domain appended to the transmembrane portion of the vesicular stomatitis virus G protein and the ectodomain of human placental alkaline phosphatase is distributed over the entire cell surface in cultured neurons. When expressed in MDCK cells, this construct remains in an intracellular compartment distinct from endosomes or lysosomes. Thus, the cytoplasmic tail domain of mGluR7 is necessary but not sufficient for polarized targeting in MDCK monolayers, whereas in neurons the cytoplasmic tail is sufficient for cell surface expression but not polarization. Additional mechanisms are likely required to mediate mGluR7 neuronal polarization and synaptic clustering.

Na,K-ATPase in skeletal muscle: two populations of beta-spectrin control localization in the sarcolemma but not partitioning between the sarcolemma and the transverse tubules

We used immunological approaches to study the factors controlling the distribution of the Na,K-ATPase in fast twitch skeletal muscle of the rat. Both alpha subunits of the Na,K-ATPase colocalize with beta-spectrin and ankyrin 3 in costameres, structures at the sarcolemma that lie over Z and M-lines and in longitudinal strands. In immunoprecipitates, the alpha1 and alpha2 subunits of the Na,K-ATPase as well as ankyrin 3 associate with beta-spectrin/alpha- fodrin heteromers and with a pool of beta-spectrin at the sarcolemma that does not contain alpha-fodrin. Myofibers of mutant mice lacking beta-spectrin (ja/ja) have a more uniform distribution of both the alpha1 and alpha2 subunits of the Na,K-ATPase in the sarcolemma, supporting the idea that the rectilinear sarcomeric pattern assumed by the Na,K-ATPase in wild-type muscle requires beta-spectrin. The Na,K-ATPase and beta-spectrin are distributed normally in muscle fibers of the nb/nb mouse, which lacks ankyrin 1, suggesting that this isoform of ankyrin is not necessary to link the Na,K-ATPase to the spectrin-based membrane skeleton. In immunofluorescence and subcellular fractionation experiments, the alpha2 but not the alpha1 subunit of the Na,K-ATPase is present in transverse (t-) tubules. The alpha1 subunit of the pump is not detected in increased amounts in the t-tubules of muscle from the ja/ja mouse, however. Our results suggest that the spectrin-based membrane skeleton, including ankyrin 3, concentrates both isoforms of the Na,K-ATPase in costameres, but that it does not play a significant role in restricting the entry of the alpha1 subunit into the t-tubules.

Modification of the composition of polycystin-1 multiprotein complexes by calcium and tyrosine phosphorylation

Mutations in the PKD1 gene are responsible for >85% of autosomal dominant polycystic kidney disease (ADPKD). The protein product of PKD1, polycystin-1, is a large, modular membrane protein, with putative ligand-binding motifs in the extracelluar N-terminal portion, 9-11 transmembrane domains and an intracellular C-terminal portion with phosphorylation sites. A role for polycystin-1 as a cell surface receptor involved in cell-matrix and cell-cell interactions has been proposed. In this study, we have analyzed polycystin-1 and associated protein distribution in normal human epithelial cells and examined the role of cell-matrix versus cell-cell interactions in regulation of the assembly of polycystin-1 multiprotein complexes. Immunocytochemistry, sucrose density gradient sedimentation, co-immunoprecipitation analyses and in vitro binding assays have shown that polycystin-1 associates with the focal adhesion proteins talin, vinculin, p130Cas, FAK, alpha-actinin, paxillin and pp60c-src in subconfluent normal human fetal collecting tubule (HFCT) epithelia when cell-matrix interactions predominate. Polycystin-1 also forms higher S value complexes with the cell-cell adherens junction proteins E-cadherin, beta- and gamma-catenins in confluent cultures when cell-cell interactions are predominant. Polycystin-1 multiprotein complexes can be disrupted by cytochalasin D but not by colchicine, suggesting involvement of the actin cytoskeleton. Although inhibition of tyrosine phosphorylation by tyrphostin inhibits polycystin-1-FAK interactions, E-cadherin interactions are enhanced. High calcium treatment also increases polycystin-1-E-cadherin interactions.

Ankyrins

This review is focused on ankyrin which is a protein linker between the integral membrane proteins and spectrin-based cytoskeleton. Structure and distribution of different ankyrin isoforms that are products of alternative-spliced genes are described. Interaction of ankyrins with various membranes is considered. Special attention is paid to ankyrin participation in signal transduction and in assembly of integral membrane proteins in specialized membrane domains.

Intracellular redirection of plasma membrane trafficking after loss of epithelial cell polarity

In polarized Madin-Darby canine kidney epithelial cells, components of the plasma membrane fusion machinery, the t-SNAREs syntaxin 2, 3, and 4 and SNAP-23, are differentially localized at the apical and/or basolateral plasma membrane domains. Here we identify syntaxin 11 as a novel apical and basolateral plasma membrane t-SNARE. Surprisingly, all of these t-SNAREs redistribute to intracellular locations when Madin-Darby canine kidney cells lose their cellular polarity. Apical SNAREs relocalize to the previously characterized vacuolar apical compartment, whereas basolateral SNAREs redistribute to a novel organelle that appears to be the basolateral equivalent of the vacuolar apical compartment. Both intracellular plasma membrane compartments have an associated prominent actin cytoskeleton and receive membrane traffic from cognate apical or basolateral pathways, respectively. These findings demonstrate a fundamental shift in plasma membrane traffic toward intracellular compartments while protein sorting is preserved when epithelial cells lose their cell polarity.

Spectrin tethers and mesh in the biosynthetic pathway

The paradox of how the Golgi and other organelles can sort a continuous flux of protein and lipid but maintain temporal and morphological stability remains unresolved. Recent discoveries highlight a role for the cytoskeleton in guiding the structure and dynamics of organelles. Perhaps one of the more striking, albeit less expected, of these discoveries is the recognition that a spectrin skeleton associates with many organelles and contributes to the maintenance of Golgi structure and the efficiency of protein trafficking in the early secretory pathway. Spectrin interacts directly with phosphoinositides and with membrane proteins. The small GTPase ARF, a key player in Golgi dynamics, regulates the assembly of the Golgi spectrin skeleton through its ability to control phosphoinositide levels in Golgi membranes, whereas adapter molecules such as ankyrin link spectrin to other membrane proteins. Direct interactions of spectrin with actin and centractin (ARP1) provide a link to dynein, myosin and presumably other motors involved with intracellular transport. Building on the recognized ability of spectrin to organize macromolecular complexes of membrane and cytosolic proteins into a multifaceted scaffold linked to filamentous structural elements (termed linked mosaics), recent evidence supports a similar role for spectrin in organelle function and the secretory pathway. Two working models accommodate much of the available data: the Golgi mesh hypothesis and the spectrin ankyrin adapter protein tethering system (SAATS) hypothesis.

Local accumulation of alpha-spectrin-related protein under plasma membrane during capping and phagocytosis in Acanthamoeba

During capping and phagocytosis the interaction between cluster cell surface receptors and the submembraneous actin-based skeleton may be mediated by spectrin-like proteins. To test this possibility we examined the localization of an alpha-spectrin immunoanalogue, that had been previously identified in whole extracts of Acanthamoeba, during capping of Con A receptors and during phagocytosis of Con A-coated yeast. During capping alpha-spectrin and filamentous actin co-migrated with the Con A receptors and accumulated in the region of cap formation, as demonstrated by double immunofluorescence studies. Immunoelectron microscopy revealed submembraneous location of alpha-spectrin in cells exposed to Con A, both at the time of initial cross-linking and during accumulation of alpha-spectrin in the region of the cap. Phagocytosis studies showed that alpha-spectrin and actin filaments were concentrated around phagocytic cups that enclosed ConA-coated yeast upon internalization. The proteins also surrounded nascent phagosomes present in the vicinity of the plasma membrane but were absent at the later time point of phagosome maturation. These data demonstrate a correlation between clustering of cell surface receptors and submembraneous localization of alpha-spectrin, suggesting an involvement of spectrin-like proteins in mediating the interaction of receptor clusters with the actin cytoskeleton.

Caenorhabditis elegans beta-G spectrin is dispensable for establishment of epithelial polarity, but essential for muscular and neuronal function

The Caenorhabditis elegans genome encodes one alpha spectrin subunit, a beta spectrin subunit (beta-G), and a beta-H spectrin subunit. Our experiments show that the phenotype resulting from the loss of the C. elegans alpha spectrin is reproduced by tandem depletion of both beta-G and beta-H spectrins. We propose that alpha spectrin combines with the beta-G and beta-H subunits to form alpha/beta-G and alpha/beta-H heteromers that perform the entire repertoire of spectrin function in the nematode. The expression patterns of nematode beta-G spectrin and vertebrate beta spectrins exhibit three striking parallels including: (1) beta spectrins are associated with the sites of cell-cell contact in epithelial tissues; (2) the highest levels of beta-G spectrin occur in the nervous system; and (3) beta spectrin-G in striated muscle is associated with points of attachment of the myofilament apparatus to adjacent cells. Nematode beta-G spectrin associates with plasma membranes at sites of cell-cell contact, beginning at the two-cell stage, and with a dramatic increase in intensity after gastrulation when most cell proliferation has been completed. Strikingly, depletion of nematode beta-G spectrin by RNA-mediated interference to undetectable levels does not affect the establishment of structural and functional polarity in epidermis and intestine. Contrary to recent speculation, beta-G spectrin is not associated with internal membranes and depletion of beta-G spectrin was not associated with any detectable defects in secretion. Instead beta-G spectrin-deficient nematodes arrest as early larvae with progressive defects in the musculature and nervous system. Therefore, C. elegans beta-G spectrin is required for normal muscle and neuron function, but is dispensable for embryonic elongation and establishment of early epithelial polarity. We hypothesize that heteromeric spectrin evolved in metazoans in response to the needs of cells in the context of mechanically integrated tissues that can withstand the rigors imposed by an active organism.

Sorting proteins to their target membranes

The functional polarity of epithelial cells depends upon the selective insertion of proteins and lipids into distinct plasma membrane domains, and upon the maintenance of these specialized domains once they are established during epithelial development. This polarized distribution of important categories of proteins including membrane transporters, channels, enzymes, cell adhesion molecules and junctional components allows cells to carry out the vectorial transport of fluid, ions and other molecules across the epithelial barrier. Several mechanisms are required to ensure the directed movement of membrane components within the cell, and to control their delivery to the appropriate target membrane. These include specific "targeting" cassettes in the amino acid sequence of the transported proteins (such as PDZ domains and NPXY or YRRF domains), a variety of accessory proteins (including GTP-binding proteins) that associate with carrier vesicles and membrane compartments within the cell, and cytoskeletal elements such as microtubules, microfilaments and the spectrin-ankyrin network. Incorrectly folded proteins are retained and degraded within the cell, and many "chaperones" are involved in ensuring that newly-synthesized proteins assume the correct two- and three-dimensional orientations and oligomerization prior to exiting from the endoplasmic reticulum (ER). Many of the proteins involved in neurotransmitter release (for example, synaptobrevins, syntaxins) have homologs that are found in non-neuronal cells, where they play a key role in vesicle fusion with the plasma membrane. In view of the complexity of these trafficking processes, it is not surprising that a growing number of disease pathologies have been identified that involve defective targeting and trafficking of proteins. These diseases can be grouped under the name "sorting disorders," and they result from abnormal delivery of functionally important proteins to the cell surface. In some cases, the mutated protein is retained and degraded intracellularly, while in others it may not be delivered to the cell surface after the appropriate physiological stimulation.

Targeting of membrane transporters in renal epithelia: when cell biology meets physiology

Epithelial cells in the kidney have highly specialized transport mechanisms that differ among the many tubule segments, and among the different cell types that are present in some regions. The purpose of this brief review is to examine some of the major intracellular mechanisms by which the membrane proteins that participate in these differentiated cellular functions are addressed, sorted, and delivered to specific membrane domains of epithelial cells. Unraveling these processes is important not only for our understanding of normal cellular function but is also critical for the interpretation of pathophysiological dysfunction in the context of newly generated molecular and cellular information concerning hereditary and acquired transporter abnormalities. Among the topics covered are sorting signals on proteins, role of the cytoskeleton, vesicle coat proteins, the fusion machinery, and exo- and endocytosis of recycling proteins. Examples of these events in renal epithelial cells are highlighted throughout this review and are related to the physiology of the kidney.

Inhibition of myogenesis by ouabain: effect on protein synthesis

Ouabain, a specific inhibitor of the sodium- and potassium-activated adenosine triphosphatase, causes reversible inhibition of the fusion of myoblasts to form myotubes. We further examined this observation to investigate whether control of Na/K-ATPase activity may normally contribute to the regulation of myogenesis. In control cultures, fusion was preceded by a small decrease in intracellular sodium concentration, but intracellular sodium and potassium increased significantly during fusion. Levels of ouabain that produce prolonged inhibition of fusion (400 microM) virtually eliminated sodium and potassium gradients. However, lower ouabain levels (10-100 microM) also produced significant changes in intracellular potassium and/or sodium along with little apparent decrease in the eventual extent of fusion. The effect of ouabain on protein synthesis was also examined. Low levels of ouabain (<50 microM) that did not affect myogenesis also did not affect incorporation of radiolabeled amino acids, while higher concentrations produced a decline in protein synthesis that paralleled decreases in the rate of myoblast fusion. Levels of metabolic labeling were reduced 90% in cultures treated with 400 microM ouabain. Inhibition of protein synthesis would prevent membrane remodeling required for fusion and other events in myogenesis. Thus, our results do not support any specific role for the sodium- and potassium-activated adenosine triphosphatase in regulating myogenesis.

Association of L-arginine transporters with fodrin: implications for hypoxic inhibition of arginine uptake

In this study, we investigated the possible interaction between the cationic amino acid transporter (CAT)-1 arginine transporter and ankyrin or fodrin. Because ankyrin and fodrin are substrates for calpain and because hypoxia increases calpain expression and activity in pulmonary artery endothelial cells (PAEC), we also studied the effect of hypoxia on ankyrin, fodrin, and CAT-1 contents in PAEC. Exposure to long-term hypoxia (24 h) inhibited L-arginine uptake by PAEC, and this inhibition was prevented by calpain inhibitor 1. The effects of hypoxia and calpain inhibitor 1 were not associated with changes in CAT-1 transporter content in PAEC plasma membranes. However, hypoxia stimulated the hydrolysis of ankyrin and fodrin in PAEC, and this could be prevented by calpain inhibitor 1. Incubation of solubilized plasma membrane proteins with anti-fodrin antibodies resulted in a 70% depletion of CAT-1 immunoreactivity and in a 60% decrease in L-arginine transport activity in reconstituted proteoliposomes (3,291 +/- 117 vs. 8,101 +/- 481 pmol. mg protein(-1). 3 min(-1) in control). Incubation with anti-ankyrin antibodies had no effect on CAT-1 content or L-arginine transport in reconstituted proteoliposomes. These results demonstrate that CAT-1 arginine transporters in PAEC are associated with fodrin, but not with ankyrin, and that long-term hypoxia decreases L-arginine transport by a calpain-mediated mechanism that may involve fodrin proteolysis.

Intracellular trafficking of variant chicken kidney AE1 anion exchangers: role Of alternative NH(2) termini in polarized sorting and Golgi recycling

The variant chicken kidney AE1 anion exchangers differ only at the NH(2) terminus of their cytoplasmic domains. Transfection studies have indicated that the variant chicken AE1-4 anion exchanger accumulates in the basolateral membrane of polarized MDCK kidney epithelial cells, while the AE1-3 variant, which lacks the NH(2)-terminal 63 amino acids of AE1-4, primarily accumulates in the apical membrane. Mutagenesis studies have shown that the basolateral accumulation of AE1-4 is dependent upon two tyrosine residues at amino acids 44 and 47 of the polypeptide. Interestingly, either of these tyrosines is sufficient to direct efficient basolateral sorting of AE1-4. However, in the absence of both tyrosine residues, AE1-4 accumulates in the apical membrane of MDCK cells. Pulse-chase studies have shown that after delivery to the cell surface, newly synthesized AE1-4 is recycled to the Golgi where it acquires additional N-linked sugar modifications. This Golgi recycling activity is dependent upon the same cytoplasmic tyrosine residues that are required for the basolateral sorting of this variant transporter. Furthermore, mutants of AE1-4 that are defective in Golgi recycling are unable to associate with the detergent insoluble actin cytoskeleton and are rapidly turned over. These studies, which represent the first description of tyrosine-dependent cytoplasmic sorting signal for a type III membrane protein, have suggested a critical role for the actin cytoskeleton in regulating AE1 anion exchanger localization and stability in this epithelial cell type.

Vesicular stomatitis virus G protein acquires pH-independent fusion activity during transport in a polarized endometrial cell line

Entry of vesicular stomatitis virus (VSV), the prototype member of the rhabdovirus family, occurs by receptor-mediated endocytosis. Subsequently, during traversal through the endosomal compartments, the VSV G protein acquires a low-pH-induced fusion-competent form, allowing for fusion of the viral membrane with endosomal and lysosomal membranes. This fusion event releases genomic RNA into the cytoplasm of the cell. Here we provide evidence that the VSV G protein acquires a fusion-competent form during exocytosis in a polarized endometrial cell line, HEC-1A. VSV infection of HEC-1A cells results in high viral yields and giant cell formation. Syncytium formation is blocked in a concentration-dependent manner by treatment with the lysosomotropic weak base ammonium chloride, which raises intravesicular pH. Virus release is somewhat delayed by treatment with ammonium chloride, but virus yields gradually reach those of control cells. In addition, inhibition of vacuolar H(+)-ATPases by treatment with bafilomycin A1 also inhibited cell to cell fusion without altering virus yields. Virions released from infected HEC cells were themselves not fusion competent, since viral entry required an active H(+)-ATPase and a low-pH-induced conformational change in the viral G protein. Thus, the conformation change leading to fusion competence during exocytotic transport is reversible and reverts during or after release of the virion from the infected cell.

Relationships between the actin cytoskeleton and cell volume regulation

The actin cytoskeleton mediates a variety of essential biological functions in cells, including division, shape changes, and movement. A number of studies have suggested that the abundant submembranous actin cytoskeleton present in the cortex of many cell types is involved in the regulation of cell volume. This relationship is supported by numerous works which document the changes in the structural organization of the actin cytoskeleton which accompany cell volume changes and the F-actin-dependence of the regulatory volume responses. In addition, other studies demonstrate structural and functional relationships between the actin cytoskeleton and the membrane transporters known to be involved in cell volume homeostasis. This review provides a summary of the current level of knowledge in this area and discusses the mechanisms which may underlie the linkage between the actin cytoskeleton and cell volume regulation.

A role for intracellular calcium in tight junction reassembly after ATP depletion-repletion

The integrity of the tight junction (TJ), which is responsible for the permeability barrier of the polarized epithelium, is disrupted during ischemic injury and must be reestablished for recovery. Recently, with the use of an ATP depletion-repletion model for ischemia and reperfusion injury in Madin-Darby canine kidney cells, TJ proteins such as zonula occludens-1 (ZO-1) were shown to reversibly form large complexes and associate with cytoskeletal proteins (T. Tsukamoto and S. K. Nigam, J. Biol. Chem. 272: 16133-16139, 1997). In this study, we examined the role of intracellular calcium in TJ reassembly after ATP depletion-repletion by employing the cell-permeant calcium chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM). Lowering intracellular calcium during ATP depletion is associated with significant inhibition of the reestablishment of the permeability barrier following ATP repletion as measured by transepithelial electrical resistance and mannitol flux, marked alterations in the subcellular localization of occludin by immunofluorescent analysis, and decreased solubility of ZO-1 and other TJ proteins by Triton X-100 extraction assay, suggesting that lowering intracellular calcium potentiates the interaction of TJ proteins with the cytoskeleton. Coimmunoprecipitation studies indicated that decreased solubility may partly result from the stabilization of large TJ protein-containing complexes with fodrin. Although ionic detergents (SDS and deoxycholate) appeared to cause a dissociation of ZO-1-containing complexes from the cytoskeleton, sucrose gradient analyses of the solubilized proteins suggested that calcium chelation leads to self-association of these complexes. Together, these results raise the possibility that intracellular calcium plays an important facilitatory role in the reassembly of the TJ damaged by ischemic insults. Calcium appears to be necessary for the dissociation of TJ-cytoskeletal complexes, thus permitting functional TJ reassembly and paracellular permeability barrier recovery.

Translocation of MARCKS and reorganization of the cytoskeleton by PMA correlates with the ion selectivity, the confluence, and transformation state of kidney epithelial cell lines

The role of protein kinase C (PKC) in the regulation of the cytoskeleton of epithelial cells with tightly sealed contacts, poor contacts, and without contacts were investigated by incubating them with a protein kinase C activator phorbol myristoyl acetate (PMA). The morphology and organization of the membrane skeleton and stress fibers as well as the localization of an actin-bundling PKC substrate MARCKS in confluent MDCK cells originating from the distal tubulus of dog kidney, LLC-PK1 cells originating from the proximal tubulus of pig kidney, src-transformed MDCK cells, epidermoid carcinoma A431 cells, and MDCK cells grown in low calcium medium (LC medium) in low density were visualized with phase contrast and immunofluorescence microscopy. Four different responses to the PMA-treatment in actin-based structures of cultured epithelial cells were observed: 1) disintegration of the membrane skeleton in confluent MDCK cells; 2) depolymerization of the stress fibers in confluent MDCK and LLC-PK1 cells; 3) formation of the membrane skeleton in A431 cells, and 4) formation of the stress fibers and membrane skeleton in LC-MDCK cells. Thus, it seems that in fully confluent tightly sealed epithelium, activation of PKC has a deleterious effect on actin-based structures, whereas in cells without contacts or loose contacts, activation of PKC by PMA results in improvement of actin-based cytoskeletal structures. The main difference between the two kidney cell lines used is their selectivity to ion transport: the monolayer of LLC-PK1 cells is anion selective and MDCK cells cation selective. We propose a model where alterations in the ionic milieu within the MDCK cells by means of cation channels affect the disintegration of the membrane skeleton. The distribution of MARCKS followed the distribution of fodrin in both cell lines upon PMA-treatment, suggesting that phosphorylation of MARCKS by PKC may contribute in the regulation of the integrity of the membrane skeleton.

Evidence for an interaction between adducin and Na(+)-K(+)-ATPase: relation to genetic hypertension

Adducin point mutations are associated with genetic hypertension in Milan hypertensive strain (MHS) rats and in humans. In transfected cells, adducin affects actin cytoskeleton organization and increases the Na(+)-K(+)-pump rate. The present study has investigated whether rat and human adducin polymorphisms differently modulate rat renal Na(+)-K(+)-ATPase in vitro. We report the following. 1) Both rat and human adducins stimulate Na(+)-K(+)-ATPase activity, with apparent affinity in tens of nanomolar concentrations. 2) MHS and Milan normotensive strain (MNS) adducins raise the apparent ATP affinity for Na(+)-K(+)-ATPase. 3) The mechanism of action of adducin appears to involve a selective acceleration of the rate of the conformational change E(2) (K) --> E(1) (Na) or E(2)(K). ATP --> E(1)Na. ATP. 4) Apparent affinities for mutant rat and human adducins are significantly higher than those for wild types. 5) Recombinant human alpha- and beta-adducins stimulate Na(+)-K(+)-ATPase activity, as do the COOH-terminal tails, and the mutant proteins display higher affinities than the wild types. 6) The cytoskeletal protein ankyrin, which is known to bind to Na(+)-K(+)-ATPase, also stimulates enzyme activity, whereas BSA is without effect; the effects of adducin and ankyrin when acting together are not additive. 7) Pig kidney medulla microsomes appear to contain endogenous adducin; in contrast with purified pig kidney Na(+)-K(+)-ATPase, which does not contain adducin, added adducin stimulates the Na(+)-K(+)-ATPase activity of microsomes only about one-half as much as that of purified Na(+)-K(+)-ATPase. Our findings strongly imply the existence of a direct and specific interaction between adducin and Na(+)-K(+)-ATPase in vitro and also suggest the possibility of such an interaction in intact renal membranes.

Association of the epithelial sodium channel with Apx and alpha-spectrin in A6 renal epithelial cells

Recent molecular cloning of the epithelial sodium channel (ENaC) provides the opportunity to identify ENaC-associated proteins that function in regulating its cell surface expression and activity. We have examined whether ENaC is associated with Apx (apical protein Xenopus) and the spectrin-based membrane cytoskeleton in Xenopus A6 renal epithelial cells. We have also addressed whether Apx is required for the expression of amiloride-sensitive Na(+) currents by cloned ENaC. Sucrose density gradient centrifugation of A6 cell detergent extracts showed co-sedimentation of xENaC, alpha-spectrin, and Apx. Immunoblot analysis of proteins co-immunoprecipitating under high stringency conditions from peak Xenopus ENaC/Apx-containing gradient fractions indicate that ENaC, Apx, and alpha-spectrin are associated in a macromolecular complex. To examine whether Apx is required for the functional expression of ENaC, alphabetagamma mENaC cRNAs were coinjected into Xenopus oocytes with Apx sense or antisense oligodeoxynucleotides. The two-electrode voltage clamp technique showed there was a marked reduction in amiloride-sensitive current in oocytes coinjected with antisense oligonucleotides when to compared with oocytes coinjected with sense oligonucleotides. These studies indicate that ENaC is associated in a macromolecular complex with Apx and alpha-spectrin in A6 cells and suggest that Apx is required for the functional expression of ENaC in Xenopus epithelia.

Role for the third intracellular loop in cell surface stabilization of the alpha2A-adrenergic receptor

Previous studies have shown that alpha2A-adrenergic receptor (alpha2A-AR) retention at the basolateral surface of polarized MDCKII cells involves its third intracellular (3i loop). The present studies examining mutant alpha2A-ARs possessing short deletions of the 3i loop indicate that no single region can completely account for the accelerated surface turnover of the Delta3ialpha2A-AR, suggesting that the entire 3i loop is involved in basolateral retention. Both wild-type and Delta3i loop alpha2A-ARs are extracted from polarized Madin-Darby canine kidney (MDCK) cells with 0.2% Triton X-100 and with a similar concentration/response profile, suggesting that Triton X-100-resistant interactions of the alpha2A-AR with cytoskeletal proteins are not involved in receptor retention on the basolateral surface. The indistinguishable basolateral t(1)/(2) for either the wild-type or nonsense 3i loop alpha2A-AR suggests that the stabilizing properties of the alpha2A-AR 3i loop are not uniquely dependent on a specific sequence of amino acids. The accelerated turnover of Delta3i alpha2A-AR cannot be attributed to alteration in agonist-elicited alpha2A-AR redistribution, because alpha2A-ARs are not down-regulated in response to agonist. Taken together, the present studies show that stabilization of the alpha2A-AR on the basolateral surface of MDCKII cells involves multiple mechanisms, with the third intracellular loop playing a central role in regulating these processes.

Immunolocalization of the fodrin, E-cadherin, and beta-catenin adhesion complex in infiltrating ductal carcinoma of the breast-comparison with an in vitro model

Fodrin, E-cadherin, and beta-catenin immunolocalization was studied in 54 cases of infiltrating ductal carcinoma of the breast and compared with an in vitro model in order to study the dynamic relationship between these components of an adhesion complex. In low-grade tumours, the staining patterns were similar for both fodrin and E-cadherin, with localization of these proteins to the cell membranes. beta-Catenin showed reduced membrane staining compared with non-neoplastic epithelium. High-grade tumours displayed strong membranous as well as cytoplasmic immunolocalization of fodrin, while E-cadherin staining was fragmented or lost from the membranes, with only occasional weak intracellular staining. beta-Catenin showed fragmented membrane staining and cytoplasmic accumulation. In addition, nuclear staining of beta-catenin was occasionally observed. In a v-src-transformed MDCK cell line, following 15min of src activation, beta-catenin began to detach from the cell membrane and localize to the cytoplasm, while fodrin and E-cadherin remained unchanged. After 30-45min of src activation, the cells lost their cuboidal shape and began to lose cell-to-cell contact. Fodrin staining remained mostly membranous while that of E-cadherin and beta-catenin was fragmented and spiky. After 60min of src activation, fodrin localized completely in the cell cytoplasm, while E-cadherin and beta-catenin were partly cytoplasmic with fragmented and spiky membranous staining. Occasionally, beta-catenin was seen in the nucleus. Both in vivo and in vitro findings clearly demonstrated a disruption of the E-cadherin/beta-catenin/fodrin/cytoskeleton linkage concomitant with the loss of cell-to-cell adhesion and change in cell shape, from epithelioid to a fibroblastoid phenotype. Membranous localization of E-cadherin showed a positive correlation with oestrogen and progesterone expression, whereas loss of membranous E-cadherin and cytoplasmic accumulation of fodrin was more often observed in high-grade carcinomas and showed a positive correlation with p53 expression.

Chicken erythroid AE1 anion exchangers associate with the cytoskeleton during recycling to the Golgi

Chicken erythroid AE1 anion exchangers receive endoglycosidase F (endo F)-sensitive sugar modifications in their initial transit through the secretory pathway. After delivery to the plasma membrane, anion exchangers are internalized and recycled to the Golgi where they acquire additional N-linked modifications that are resistant to endo F. During recycling, some of the anion exchangers become detergent insoluble. The acquisition of detergent insolubility correlates with the association of the anion exchanger with cytoskeletal ankyrin. Reagents that inhibit different steps in the endocytic pathway, including 0.4 M sucrose, ammonium chloride, and brefeldin A, block the acquisition of endo F-resistant sugars and the acquisition of detergent insolubility by newly synthesized anion exchangers. The inhibitory effects of ammonium chloride on anion exchanger processing are rapidly reversible. Furthermore, AE1 anion exchangers become detergent insoluble more rapidly than they acquire endo F-resistant modifications in cells recovering from an ammonium chloride block. This suggests that the cytoskeletal association of the recycling anion exchangers occurs after release from the compartment where they accumulate due to ammonium chloride treatment, and prior to their transit through the Golgi. The recycling pool of newly synthesized anion exchangers is reflected in the steady-state distribution of the polypeptide. In addition to plasma membrane staining, anion exchanger antibodies stain a perinuclear compartment in erythroid cells. This perinuclear AE1-containing compartment is also stained by ankyrin antibodies and partially overlaps the membrane compartment stained by NBD C6-ceramide, a Golgi marker. Detergent extraction of erythroid cells in situ has suggested that a substantial fraction of the perinuclear pool of AE1 is cytoskeletal associated. The demonstration that erythroid anion exchangers interact with elements of the cytoskeleton during recycling to the Golgi suggests the cytoskeleton may be involved in the post-Golgi trafficking of this membrane transporter.

Neural cell adhesion molecule (N-CAM) is required for cell type segregation and normal ultrastructure in pancreatic islets

Classical cell dissociation/reaggregation experiments with embryonic tissue and cultured cells have established that cellular cohesiveness, mediated by cell adhesion molecules, is important in determining the organization of cells within tissue and organs. We have employed N-CAM-deficient mice to determine whether N-CAM plays a functional role in the proper segregation of cells during the development of islets of Langerhans. In N-CAM-deficient mice the normal localization of glucagon-producing alpha cells in the periphery of pancreatic islets is lost, resulting in a more randomized cell distribution. In contrast to the expected reduction of cell-cell adhesion in N-CAM-deficient mice, a significant increase in the clustering of cadherins, F-actin, and cell-cell junctions is observed suggesting enhanced cadherin-mediated adhesion in the absence of proper N-CAM function. These data together with the polarized distribution of islet cell nuclei and Na+/K+-ATPase indicate that islet cell polarity is also affected. Finally, degranulation of beta cells suggests that N-CAM is required for normal turnover of insulin-containing secretory granules. Taken together, our results confirm in vivo the hypothesis that a cell adhesion molecule, in this case N-CAM, is required for cell type segregation during organogenesis. Possible mechanisms underlying this phenomenon may include changes in cadherin-mediated adhesion and cell polarity.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Human calcitonin receptor is directly targeted to and retained in the basolateral surface of MDCK cells

The human calcitonin receptor (hCTR) is expressed in polarized cells of the kidney, bone, and nervous system. In the kidney, hCTRs are found in cells of the distal nephron to which blood-borne calcitonin has access only at the basolateral surface. We expressed hCTR subtypes 1 and 2 in Madin-Darby canine kidney (MDCK) cells to establish a cell model useful for delineating the molecular mechanisms underlying hCTR polarity. Selective cell surface incubation demonstrated functional polarity of hCTRs by equilibrium binding or cross-linking of radioiodinated salmon calcitonin (125I-sCT) and cAMP accumulation stimulated by sCT. We estimated that at the steady state there are 40-fold more hCTRs on the basolateral than on the apical side. Domain-selective cell surface biotinylation followed by immunoblotting of streptavidin-agarose-fractionated biotinylated glycoproteins independently confirmed the polarized distribution of FLAG epitope-tagged hCTR-2 in the basolateral domain. Confocal microscopy of immunostained receptors revealed that hCTRs are concentrated on a lateral subdomain of the basolateral membrane. Cell surface arrival assay of newly formed receptors demonstrated that direct delivery to the basolateral domain is the mechanism by which hCTRs become polarized. Measurement of receptor turnover on the basolateral surface showed that retention contributes to hCTR distribution at the steady state.

Biogenesis of polarized epithelial cells during kidney development in situ: roles of E-cadherin-mediated cell-cell adhesion and membrane cytoskeleton organization

Organization of proteins into structurally and functionally distinct plasma membrane domains is an essential characteristic of polarized epithelial cells. Based on studies with cultured kidney cells, we have hypothesized that a mechanism for restricting Na/K-ATPase to the basal-lateral membrane involves E-cadherin-mediated cell-cell adhesion and integration of Na/K-ATPase into the Triton X-100-insoluble ankyrin- and spectrin-based membrane cytoskeleton. In this study, we examined the relevance of these in vitro observations to the generation of epithelial cell polarity in vivo during mouse kidney development. Using differential detergent extraction, immunoblotting, and immunofluorescence histochemistry, we demonstrate the following. First, expression of the 220-kDa splice variant of ankyrin-3 correlates with the development of resistance to Triton X-100 extraction for Na/K-ATPase, E-cadherin, and catenins and precedes maximal accumulation of Na/K-ATPase. Second, expression of the 190-kDa slice variant of ankyrin-3 correlates with maximal accumulation of Na/K-ATPase. Third, Na/K-ATPase, ankyrin-3, and fodrin specifically colocalize at the basal-lateral plasma membrane of all epithelial cells in which they are expressed and during all stages of nephrogenesis. Fourth, the relative immunofluorescence staining intensities of Na/K-ATPase, ankyrin-3, and fodrin become more similar during development until they are essentially identical in adult kidney. Thus, renal epithelial cells in vivo regulate the accumulation of E-cadherin-mediated adherens junctions, the membrane cytoskeleton, and Na/K-ATPase through sequential protein expression and assembly on the basal-lateral membrane. These results are consistent with a mechanism in which generation and maintenance of polarized distributions of these proteins in vivo and in vitro involve cell-cell adhesion, assembly of the membrane cytoskeleton complex, and concomitant integration and retention of Na/K-ATPase in this complex.

Polarization of the Na+, K(+)-ATPase in epithelia derived from the neuroepithelium

The neuroepithelium generates a fascinating group of epithelia. One of their intriguing properties is how they polarize the distribution of the Na+, K(+)-ATPase. Typically, this ion pump is concentrated in the basolateral membrane, but it is concentrated in the apical membranes of the retinal pigment epithelium and the epithelium of the choroid plexus. A comparison of their development with that of systemic epithelia yields insights into how cells polarize the distribution of this and other membrane proteins. The polarization of the Na+, K(+)-ATPase depends upon the interplay between different sorting signals and different types of polarity mechanisms. These include intracellular targeting signals that direct the delivery of newly synthesized proteins, and maintenance signals that stabilize proteins in the proper membrane domain. Conflicting signals appear to be arranged in a hierarchy that can be rearranged as cells respond to certain environmental stimuli. Part of this response is mediated by changes in the distribution and composition of the cortical cytoskeleton.

Rotavirus infection reduces sucrase-isomaltase expression in human intestinal epithelial cells by perturbing protein targeting and organization of microvillar cytoskeleton

Rotavirus infection is the most common cause of severe infantile gastroenteritis worldwide. These viruses infect mature enterocytes of the small intestine and cause structural and functional damage, including a reduction in disaccharidase activity. It was previously hypothesized that reduced disaccharidase activity resulted from the destruction of rotavirus-infected enterocytes at the villus tips. However, this pathophysiological model cannot explain situations in which low disaccharidase activity is observed when rotavirus-infected intestine exhibits few, if any, histopathologic changes. In a previous study, we demonstrated that the simian rotavirus strain RRV replicated in and was released from human enterocyte-like Caco-2 cells without cell destruction (N. Jourdan, M. Maurice, D. Delautier, A. M. Quero, A. L. Servin, and G. Trugnan, J. Virol. 71:8268-8278, 1997). In the present study, to reinvestigate disaccharidase expression during rotavirus infection, we studied sucrase-isomaltase (SI) in RRV-infected Caco-2 cells. We showed that SI activity and apical expression were specifically and selectively decreased by RRV infection without apparent cell destruction. Using pulse-chase experiments and cell surface biotinylation, we demonstrated that RRV infection did not affect SI biosynthesis, maturation, or stability but induced the blockade of SI transport to the brush border. Using confocal laser scanning microscopy, we showed that RRV infection induces important alterations of the cytoskeleton that correlate with decreased SI apical surface expression. These results lead us to propose an alternate model to explain the pathophysiology associated with rotavirus infection.

Functional activity of epidermal growth factor receptors in autosomal recessive polycystic kidney disease

Evidence from a number of laboratories suggests a potential role for the epidermal growth factor (EGF)-transforming growth factor-alpha-epidermal growth factor receptor (EGF-R) axis in promoting epithelial hyperplasia and cyst formation in autosomal recessive polycystic kidney disease (ARPKD). As previously reported, in the C57BL-6Jcpk/cpk (CPK), BALB/c-bpk/bpk (BPK), and C3H-orpk/orpk (ORPK) murine models of ARPKD, as well as in human ARPKD and human ADPKD, the EGF-R is mislocated to the apical surface of cystic collecting tubule (CT) epithelial cells. The present studies demonstrate that cells from cystic and control CTs can be isolated and that these cells maintain their in vivo EGF-R phenotype in vitro. Domain-specific high-affinity ligand binding was assessed by standard Scatchard analysis, and selective ligand stimulation of apical vs. basolateral EGF-R in these cells was followed by measurement of receptor autophosphorylation and determination of cell proliferation. These studies demonstrate that in vitro apically expressed EGF-Rs exhibit high-affinity binding for EGF, autophosphorylate in response to EGF, and transmit a mitogenic signal when stimulated by the appropriate ligand.

Adducin in essential hypertension

In Milan hypertensive rats (MHS) the sequence of events going from renal function to cell membrane ion transport abnormalities and finally to the molecular defect responsible of hypertension has been established. A polymorphism of the cytoskeletal protein adducin has been identified as a likely culprit for hypertension in these rats. Two point mutations in MHS alpha- (F316Y) and beta- (Q529R) adducin genes have been shown to be associated with hypertension in genetic crosses of MHS and MNS rats. Also in humans, a polymorphism of alpha-adducin gene (Gly460Trp) has been found to be significantly associated both to hypertension and salt sensitivity. Studies aimed at clarifying the functional role of alpha-adducin variants have shown that adducin from the MHS rats is able to stimulate Na-KATPase activity both after transfection in renal tubular cells and after incubation with the enzyme in a cell-free system. Also the human hypertensive alpha-adducin variant displays the same activity of MHS adducin in a cell-free system. Therefore, both in humans and in rats, adducin polymorphisms may affect blood pressure and kidney function by modulating the overall capacity of tubular epithelial cells to transport ions, through variations of the Na-KATPase activity. However adducin polymorphisms account for only a portion of hypertension both in humans and rats. Therefore additive or epistatic interactions with other genes involved in renal sodium handling need to be studied.

Host cell dependence of viral morphology

The morphology of influenza virions was found to depend on cellular determinants. Influenza viral filaments up to 30 microm in length were observed to form at high levels on surfaces of various polarized epithelial cell types infected with the A/Udorn/72 virus. In contrast, virions produced by nonpolarized cell types infected with this virus were almost exclusively of spherical morphology. Disruption of the actin microfilament array by cytochalasin D treatment of polarized MDCK cells had a profound effect on viral morphology. Although virus titers and release of spherical particles were not reduced in the presence of cytochalasin D, we observed a 15-fold reduction in the release of filamentous particles. In contrast, the ratio of filaments to spheres produced by infected MDCK cells was not altered by the microtubule-disrupting agent nocodazole. These observations indicate that the polarized cell phenotype and the integrity of the actin microfilament network are important cellular determinants of the morphology of a filamentous influenza virus.

Migrating transformed MDCK cells are able to structurally polarize a voltage-activated K+ channel

Cell migration of transformed renal epithelial cells (MDCK-F) depends-in addition to cytoskeletal mechanisms-on the polarized activity of a Ca2+-sensitive K+ channel in the rear part of the cells. However, because of the lack of specific markers for this channel we are not able to determine whether a polarized distribution of the channel protein underlies its functional polarization. To determine whether the migrating MDCK-F cells have retained the ability to target K+ channels to distinct membrane areas we stably transfected the cells with the voltage-dependent K+ channel Kv1.4. Stable expression and insertion into the plasma membrane could be shown by reverse transcription-PCR, genomic PCR, Western blot, and patch-clamp techniques, respectively. The distribution of Kv1.4 was assessed with indirect immunofluorescence by using conventional and confocal microscopy. These experiments revealed that Kv1.4 is expressed only in transfected cells where it elicits the typical voltage-dependent, rapidly inactivating K+ current. The Kv1.4 protein is clustered at the leading edge of protruding lamellipodia of migrating MDCK-F cells. This characteristic distribution of Kv1.4 provides strong evidence that migrating MDCK-F cells are able to insert ion channels into the plasma membrane in an asymmetric way, which reflects the polarization of migrating cells in the plane of movement. These findings suggest that not only epithelial cells and nerve cells, but also migrating cells, can create functionally distinct plasma membrane areas.

Anion exchanger 2 (AE2) binds to erythrocyte ankyrin and is colocalized with ankyrin along the basolateral plasma membrane of human gastric parietal cells

The hydrochloric acid secreting parietal cells of the human stomach mucosa have been shown to express anion exchanger 2 (AE2). AE2 is restricted to the basolateral membrane domain and is responsible for the basolateral uptake of Cl- and release of HCO3-. It is unknown which mechanism is responsible for the basolateral positioning of AE2 in parietal cells. We raised the question whether AE2 might be immobilized at the cell surface by linkage via ankyrin to the spectrin/actin-based membrane cytoskeleton. In the present study we communicate two observations that support this hypothesis, namely that in parietal cells ankyrin is localized with AE2 along the basolateral cell surface and, secondly, that purified erythrocyte ankyrin binds to the in vitro-translated cytoplasmic domain of AE2. We conclude from these observations that AE2 in parietal cells might be linked via ankyrin to the basolateral membrane cytoskeleton and that this type of linkage might play a role in immobilizing AE2 in a non-random fashion along the basolateral membrane domain.

In vitro and in vivo evidence for actin association of the naphthylphthalamic acid-binding protein from zucchini hypocotyls

The N-1-naphthylphthalamic acid (NPA)-binding protein is part of the auxin efflux carrier, the protein complex that controls polar auxin transport in plant tissues. This study tested the hypothesis that the NPA-binding protein (NBP) is associated with the actin cytoskeleton in vitro and that an intact actin cytoskeleton is required for polar auxin transport in vivo. Cytoskeletal polymerization was altered in extracts of zucchini hypocotyls with reagents that stabilized either the polymeric or monomeric forms of actin or tubulin. Phalloidin treatment altered actin polymerization, as demonstrated by immunoblot analyses following native and denaturing electrophoresis. Phalloidin increased both filamentous actin (F-actin) and NPA-binding activity, while cytochalasin D and Tris decreased both F-actin and NPA-binding activity in cytoskeletal pellets. The microtubule stabilizing drug taxol increased pelletable tubulin, but did not alter either the amount of pelletable actin or NPA-binding activity. Treatment of etiolated zucchini hypocotyls with cytochalasin D decreased the amount of auxin transport and its regulation by NPA. These experimental results are consistent with an in vitro actin cytoskeletal association of the NPA-binding protein and with the requirement of an intact actin cytoskeleton for maximal polar auxin transport in vivo.

Species variability of erythrocyte transport ATPases in mammals

Na+, K(+)-ATPase, and Ca(2+)-ATPase in whole erythrocytes from five species of mammals (rat, mouse, guinea pig, golden hamster, rabbit) after cell treatment with Tween 20 (7.5 mg/ml) varied over a wide range: from 3.0 +/- 0.9 mumol Pi/hr/ml cells in rabbit to 27.3 +/- 4.9 mumol Pi/hr/ml cells in mouse for Na+, K(+)-ATPase and from 8.0 +/- 1.6 mumol Pi/hr/ml cells in hamster to 47.2 +/- 4.9 mumol Pi/hr/ml cells in mouse for Ca(2+)-ATPase. Differences were less pronounced in red cell ghosts. Fatty acid and phospholipid compositions of erythrocyte membranes were similar for all species. Nevertheless, the activity of Ca(2+)-ATPase in ghosts significantly correlated (r = -0.884) with the ratio of PC + SM/PE + PS in red cell membranes. Rabbit membranes had the lowest content of arachidonate. Rat hemolysate activated Na+, K(+)-ATPase in the ghosts from the animals of any species investigated, whereas the enzyme activation by the homohemolysate was characteristic only of the rat, mouse, and guinea pig ghosts. The data obtained suggest that there are differences in both activity and intracellular control of transport ATPase in erythrocytes of different mammals.

Cytoskeletal association is important for differential targeting of glucose transporter isoforms in Leishmania

The major glucose transporter of the parasitic protozoan Leishmania enriettii exists in two isoforms, one of which (iso-1) localizes to the flagellar membrane, while the other (iso-2) localizes to the plasma membrane of the cell body, the pellicular membrane. These two isoforms differ only in their cytosolic NH2-terminal domains. Using immunoblots and immunofluorescence microscopy of detergent-extracted cytoskeletons, we have demonstrated that iso-2 associates with the microtubular cytoskeleton that underlies the cell body membrane, whereas the flagellar membrane isoform iso-1 does not associate with the cytoskeleton. Deletion mutants that remove the first 25 or more amino acids from iso-1 are retargeted from the flagellum to the pellicular membrane, suggesting that these deletions remove a signal required for flagellar targeting. Unlike the full-length iso-1 protein, these deletion mutants associate with the cytoskeleton. Our results suggest that cytoskeletal binding serves as an anchor to localize the iso-2 transporter within the pellicular membrane, and that the flagellar targeting signal of iso-1 diverts this transporter into the flagellar membrane and away from the pellicular microtubules.

Distribution of Na+,K(+)-ATPase in photoreceptor cells of insects

Light stimulation of insect photoreceptors causes opening of cation channels and an inward current that is partially carried by Na+ ions. There is also an efflux of K+ ions upon photostimulation. Na+ and K+ gradients across the photoreceptor membrane are reestablished by the activity of the enzyme Na+,K(+)-ATPase. About two-thirds of the total amount of ATP consumed in response to a light stimulus is attributed to the activity of this ion pump, demonstrating the importance of this enzyme for photoreceptor function. Insect photoreceptor cells are polarized epithelial cells; their plasma membrane is organized into two domains having a distinct morphology, molecular composition, and function. The visual pigment rhodopsin and the molecular components of the transduction machinery are localized in the rhabdomere, an array of densely packed microvilli, whereas Na+,K(+)-ATPase resides in the nonrhabdomeric membrane. Comparative immunolocalization studies on compound eyes of diverse insect species have demonstrated subtle variations in the distribution patterns of Na+,K(+)-ATPase. These may be accounted for by differences in the mechanisms responsible for Na+,K(+)-ATPase positioning.

Redistribution of villin to proximal tubule basolateral membranes after ischemia and reperfusion

After ischemia and reperfusion, severe alterations in the cytoskeletal organization of renal tubular epithelial cells have been reported. These effects, accompanied by a modification in the polarized distribution of some membrane transport proteins, are especially evident in the proximal tubule. In normal proximal tubule cells, actin is concentrated in apical brush border microvilli, along with the actin-binding protein villin. Because villin plays an important role in actin bundling and in microvillar assembly but can also act as an actin-fragmenting protein at higher calcium concentrations, we examined the effects of ischemic injury and reperfusion on the distribution of villin and actin in proximal tubule cells of rat kidney. Using specific antibodies against villin and actin, we show that these proteins redistribute in parallel from the apical to the basolateral plasma membrane within 1 h of reperfusion after ischemia. Ischemia alone had no effect on the staining pattern. Repolarization of villin to the apical membrane begins within hours after reperfusion with enhanced apical localization over time during the period of regeneration. This apical repolarization of villin is accompanied by the migration of actin back to the apical membrane. These results show not only that villin may be involved in the initial disruption of the actin cytoskeleton during reperfusion injury but also that its migration back to the apical domain of these cells accompanies the reestablishment of a normal actin distribution in the brush border.

Cell-cell adhesion molecules and the development of an epithelial phenotype in cultured human retinal pigment epithelial cells

For most epithelial cells, the adherens junction protein E-cadherin is an epithelial morphogen, inducing the development of an epithelial phenotype in vitro after cell contact at confluency. Here retinal pigment epithelial cells (RPE), which lack E-cadherin but express a cadherin that is also found in many non-epithelial cells (N-cadherin), were examined for the ability to produce an epithelial phenotype in vitro. Subpopulations of grossly epithelioid or fusiform cells were selected for analysis from RPE cultures derived from adult human donors. After confluency, epithelioid RPE cells were observed to undergo time-dependent changes that were similar to those previously found in epithelial cells expressing E-cadherin: the cadherin gradually developed a zonular distribution of detergent-resistant protein that co-localized with forming circumferential actin bundles; Na/K ATPase accumulated at cell contact sites, then polarized to its tissue-specific domain (the apical membrane for RPE); the cells formed elevated domes on the impermeant culture substrate. In contrast to cells expressing E-cadherin, these events in RPE required weeks rater than days at confluency. Additional proteins were examined in epithelioid RPE cells revealing that cytokeratins reorganized after confluency producing a zonular array, and several other adhesion proteins (alpha5beta1 integrin, ICAM-1, PECAM-1, NCAM) became enriched at cell-cell contact sites, each developing a distinct pattern at a distinct postconfluency interval. In contrast to epithelioid RPE, in fusiform RPE the adhesion molecules did not develop discrete distribution patterns after confluency, although the same complement of adhesion proteins was expressed. In cells expressing E-cadherin, the absence of epithelial properties is often due to underexpression of the cadherin or of the catenins, adherens junction proteins that link the cadherin to actin. Fusiform RPE, however, were not deficient in these proteins, expressing amounts of N-cadherin, alpha-catenin, beta-catenin, plakoglobin, p120, alpha-actinin and vinculin that were equivalent to epithelioid cells. It appears, therefore, that a subset of epithelial cells that express N-cadherin can produce a highly-developed epithelial phenotype in vitro through a slow morphogenetic process. However, the expression alone of adhesion molecules, including those with a morphoregulatory function in other cells, is insufficient to produce an epithelial phenotype in all cells derived from the pigment epithelium.

Na,K-ATPase transport from endoplasmic reticulum to Golgi requires the Golgi spectrin-ankyrin G119 skeleton in Madin Darby canine kidney cells

Spectrin (betaISigma*) and ankyrin (AnkG119) associate with Golgi membranes and the dynactin complex, but their role in vesicle trafficking remains uncertain. We find that the actin-binding domain and membrane-association domain 1 (MAD1) of betaI spectrin together form a constitutive Golgi targeting signal in transfected MDCK cells. Expression of this signal in transfected cells disrupts the endogenous Golgi spectrin skeleton and blocks transport of alpha- and beta-Na,K-ATPase and vesicular stomatitis virus-G protein from the endoplasmic reticulum (ER) but does not disrupt the formation of Golgi stacks, the distribution of beta-COP, or the transport and surface display of E-cadherin. The Golgi spectrin skeleton is thus required for the transport of a subset of membrane proteins from the ER to the Golgi. We postulate that together with polyfunctional adapter proteins such as AnkG119, Golgi spectrin forms a docking complex that acts prior to the cis-Golgi, presumably with vesicular-tubular clusters (VTCs or ERGIC), to sequester specific membrane proteins into vesicles transiting between the ER and Golgi, and subsequently (probably involving other isoforms of spectrin and ankyrin) to mediate cargo transport within the Golgi and to other membrane compartments. We hypothesize that this vesicular spectrin-ankyrin adapter-protein trafficking (or tethering) system (SAATS) mediates the capture and transport of many membrane proteins and acts in conjunction with vesicle-targeting molecules to effect the efficient transport of cargo proteins.

Anti-brain spectrin immunoreactivity in Alzheimer's disease: degradation of spectrin in an animal model of cholinergic degeneration

In a previous work, we described the existence of anti-brain spectrin auto antibodies in Alzheimer's disease (AD) patients (J. Neuroimmunol. 68 (1996) 39-44). In this report, we further support our previous observations, showing that sera from 9 out of 18 AD patients, but none of 14 control subjects, immunoreacted with spectrin synthesized by PC12 cells. In addition, degradation of brain spectrin was found to be greatly enhanced in the frontal cortex of rats subjected to an animal model of cholinergic degeneration. Our data suggest that spectrin degradation and generation of anti-spectrin auto antibodies may be related to the cholinergic degeneration encountered in AD.

Tight junction proteins form large complexes and associate with the cytoskeleton in an ATP depletion model for reversible junction assembly

A key feature of the ischemic epithelial cell phenotype is the disruption of tight junctions (TJ). In a Manin-Darby canine kidney cell model for ischemia-reperfusion/hypoxia-reoxygenation injury which employs inhibitors of glycolysis (2-deoxy-D-glucose) and oxidative phosphorylation (antimycin A), transepithelial electrical resistance, a measure of TJ integrity, dropped rapidly, correlating well with declining ATP levels. Although immunocytochemical studies revealed only subtle changes in the distribution of the TJ proteins, zonula occludens (ZO)-1, ZO-2, and cingulin, examination of the Triton X-100 solubilities of these proteins, an indicator of cytoskeletal association, revealed a striking shift of all three TJ proteins into the insoluble pool, consistent with increased cytoskeletal interaction during ATP depletion. In addition, rate-zonal centrifugation analysis of a detergent-soluble fraction showed an increase in the amount of ZO-1 and ZO-2 in high density fractions following ATP depletion, providing further evidence for association of TJ proteins into a large complex possibly involving the cytoskeleton. Analysis of immunoprecipitation data from [35S]methionine-labeled cells revealed that ATP depletion led to the association of a 240-kDa protein with the ZO-1-containing complex. Western blots of this protein immunoprecipitated with anti-ZO-1 antibodies confirmed its identity as fodrin, a protein believed to link membrane and other proteins to the actin-based cytoskeleton. Together, our data suggest that in the absence of major immunocytochemical changes, ATP depletion leads TJ proteins to form large insoluble complexes and associate with the cytoskeleton. We propose a model in which a key, potentially regulated, step in the generation of the ischemic epithelial cell phenotype is the interaction between TJ proteins and fodrin and/or other cytoskeletal proteins.

Polarisation-dependent association of plectin with desmoplakin and the lateral submembrane skeleton in MDCK cells

The intermediate filament-binding protein plectin and cytokeratin were localised at the cellular periphery of fully polarised Madin-Darby canine kidney (MDCK) cells, whereas vimentin was primarily found in a perinuclear network. Confocal and immunoelectron microscopy revealed that plectin was restricted to areas underlying the lateral plasma membrane. It colocalised with fodrin, a component of the submembrane skeleton, and was closely associated with desmosomal plaque structures. Biochemically, plectin was shown to interact directly with immunoprecipitated desmoplakin in vitro. Upon loss of cell polarity in low calcium medium, plectin redistributed to a cytoplasmic vimentin- and cytokeratin-related network, clearly distinct from diffusely distributed fodrin and internalised desmoplakin structures. The structural reorganisation of plectin was also reflected by an increased solubility of the protein in Triton X-100/high salt, and a decrease in its half-life from approximately 20 to approximately 5 hours. Furthermore, unlike cytokeratins and vimentin, desmoplakin and fodrin did not associate with plectin attached to magnetic beads in cell lysates of unpolarised cells, while all proteins formed a stable complex in polarised cells. Altogether, these data indicate that plectin is involved in the anchorage of intermediate filaments to desmosomes and to the submembrane skeleton in polarised MDCK cells.

Protein insolubility and late-stage morphogenesis in long-term postconfluent cultures of MDCK epithelial cells

Epithelial morphogenesis in vitro has been studied in cultures soon after cell-cell contact at confluency when several actin-associated proteins (fodrin, adherens junction molecules E-cadherin and catenins) localize to specific subcellular domains and become resistant to extraction with non-ionic detergents. Here we demonstrate that early confluency is followed by a long postconfluent period of several weeks during which these proteins and actin itself become progressively enriched in the detergent-resistant fraction of MDCK epithelial cells. Cultures from another tissue (human retinal pigment epithelium) which produces weakly epithelialized monolayers in culture do not exhibit similar late-stage increases in protein insolubility. After confluency some cells in MDCK cultures undergo additional morphogenetic changes giving rise to cord-like structures, and the MDCK adherens junction becomes more stable to disrupting agents. These results indicate that in vitro morphogenesis is not restricted to early confluency in MDCK cells but rather molecular stabilization and dynamic changes in cell shape occur over a protracted postconfluent interval.

Golgi membrane skeleton: identification, localization and oligomerization of a 195 kDa ankyrin isoform associated with the Golgi complex

To extend our finding of a Golgi-localized form of the membrane skeleton protein spectrin, we have identified an isoform of ankyrin that associates at steady state with the Golgi complex. Immuno-light and -electron microscopy show that this ankyrin isoform localizes to the perinuclear cytoplasm on tubular vesicular structures that co-stain with Golgi marker proteins. An antiserum raised against erythrocyte ankyrin, which was used to identify the Golgi ankyrin, recognized three prominent polypeptides of 220, 213 and 195 kDa in MDCK cells. Affinity purification of this antiserum against each of these MDCK cell ankyrins revealed that only an antibody specific for the 195 kDa form retained the ability to stain the Golgi complex; affinity purified antibody preparations specific for both the 220 and 213 kDa forms stained punctate and reticular cytoplasmic structures distinct from the Golgi complex. Antibody specific for the 195 kDa ankyrin did not recognize a recently identified 119 kDa ankyrin that is also localized to the Golgi. The 195 kDa Golgi ankyrin binds purified erythrocyte spectrin, and rapidly co-sediments with Golgi beta-spectrin during brief, low speed centrifugation of Triton X-100 extracts of MDCK cells. Golgi ankyrin and beta-spectrin are retained on tubular vesicular ‘Golgi ghosts’ following extraction of cultured cells with Triton X-100. Significantly, Golgi ghost tubules containing ankyrin/spectrin are co-linear with individual microtubules, suggesting a role for both Golgi membrane skeleton and microtubules in spatial localization of the Golgi. Golgi ankyrin dissociates from Golgi membranes during mitosis and in cells treated with brefeldin A, indicating that Golgi ankyrin has a dynamic assembly state similar to that of Golgi spectrin and other Golgi membrane coat proteins.