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

Pleiotropic Ankyrins: Scaffolds for Ion Channels and Transporters

The ankyrin proteins (Ankyrin-R, Ankyrin-B, and Ankyrin-G) are a family of scaffolding, or membrane adaptor proteins necessary for the regulation and targeting of several types of ion channels and membrane transporters throughout the body. These include voltage-gated sodium, potassium, and calcium channels in the nervous system, heart, lungs, and muscle. At these sites, ankyrins recruit ion channels, and other membrane proteins, to specific subcellular domains, which are then stabilized through ankyrin's interaction with the submembranous spectrin-based cytoskeleton. Several recent studies have expanded our understanding of both ankyrin expression and their ion channel binding partners. This review provides an updated overview of ankyrin proteins and their known channel and transporter interactions. We further discuss several potential avenues of future research that would expand our understanding of these important organizational proteins.

Multiple Na,K-ATPase Subunits Colocalize in the Brush Border of Mouse Choroid Plexus Epithelial Cells

(1) Background: The unusual accumulation of Na,K-ATPase complexes in the brush border membrane of choroid plexus epithelial cells have intrigued researchers for decades. However, the full range of the expressed Na,K-ATPase subunits and their relation to the microvillus cytoskeleton remains unknown. (2) Methods: RT-PCR analysis, co-immunoprecipitation, native PAGE, mass spectrometry, and differential centrifugation were combined with high-resolution immunofluorescence histochemistry, proximity ligase assays, and stimulated emission depletion (STED) microscopy on mouse choroid plexus cells or tissues in order to resolve these issues. (3) Results: The choroid plexus epithelium expresses Na,K-ATPase subunits α1, α2, β1, β2, β3, and phospholemman. The α1, α2, β1, and β2, subunits are all localized to the brush border membrane, where they appear to form a complex. The ATPase complexes may stabilize in the brush border membrane via anchoring to microvillar actin indirectly through ankyrin-3 or directly via other co-precipitated proteins. Aquaporin 1 (AQP1) may form part of the proposed multi-protein complexes in contrast to another membrane protein, the Na-K-2Cl cotransporter 1 (NKCC1). NKCC1 expression seems necessary for full brush border membrane accumulation of the Na,K-ATPase in the choroid plexus. (4) Conclusion: A multitude of Na,K-ATPase subunits form molecular complexes in the choroid plexus brush border, which may bind to the cytoskeleton by various alternative actin binding proteins.

Correlative dual-color dSTORM/AFM reveals protein clusters at the cytoplasmic side of human bronchial epithelium membranes

The organization of a cell membrane is vital for various functions, such as receptor signaling and membrane traffic. However, the understanding of membrane organization remains insufficient, especially the localizations of specific proteins in the cell membrane. Here, we used correlative super-resolution fluorescence/atomic force microscopy to correlate the distributions of specific proteins Na+/K+-ATPase (NKA, an integral membrane protein) and ankyrin G (AnkG, a scaffolding protein) with the topography of the cytoplasmic side of human bronchial epithelium membranes. Our data showed that NKA and AnkG proteins preferred to localize in the protein islands of membranes. Interestingly, we also found that functional domains composed of specific proteins with a few hundreds of nanometers were formed by assembling protein islands with a few tens of nanometers.

SerThr-PhosphoProteome of Brain from Aged PINK1-KO+A53T-SNCA Mice Reveals pT1928-MAP1B and pS3781-ANK2 Deficits, as Hub between Autophagy and Synapse Changes

Hereditary Parkinson’s disease (PD) can be triggered by an autosomal dominant overdose of alpha-Synuclein (SNCA) as stressor or the autosomal recessive deficiency of PINK1 Serine/Threonine-phosphorylation activity as stress-response. We demonstrated the combination of PINK1-knockout with overexpression of SNCA^A53T in double mutant (DM) mice to exacerbate locomotor deficits and to reduce lifespan. To survey posttranslational modifications of proteins underlying the pathology, brain hemispheres of old DM mice underwent quantitative label-free global proteomic mass spectrometry, focused on Ser/Thr-phosphorylations. As an exceptionally strong effect, we detected >300-fold reductions of phosphoThr1928 in MAP1B, a microtubule-associated protein, and a similar reduction of phosphoSer3781 in ANK2, an interactor of microtubules. MAP1B depletion is known to trigger perturbations of microtubular mitochondria trafficking, neurite extension, and synaptic function, so it was noteworthy that relevantly decreased phosphorylation was also detected for other microtubule and microfilament factors, namely MAP2^S1801, MARK1^S394, MAP1A^T1794, KIF1A^S1537, 4.1N^S541, 4.1G^S86, and ADD2^S528. While the MAP1B heavy chain supports regeneration and growth cones, its light chain assists DAPK1-mediated autophagy. Interestingly, relevant phosphorylation decreases of DAPK2^S299, VPS13D^S2429, and VPS13C^S2480 in the DM brain affected regulators of autophagy, which are implicated in PD. Overall, significant downregulations were enriched for PFAM C2 domains, other kinases, and synaptic transmission factors upon automated bioinformatics, while upregulations were not enriched for selective motifs or pathways. Validation experiments confirmed the change of LC3 processing as reflection of excessive autophagy in DM brain, and dependence of ANK2/MAP1B expression on PINK1 levels. Our new data provide independent confirmation in a mouse model with combined PARK1/PARK4/PARK6 pathology that MAP1B/ANK2 phosphorylation events are implicated in Parkinsonian neurodegeneration. These findings expand on previous observations in Drosophila melanogaster that the MAP1B ortholog futsch in the presynapse is a primary target of the PARK8 protein LRRK2, and on a report that MAP1B is a component of the pathological Lewy body aggregates in PD patient brains. Similarly, ANK2 gene locus variants are associated with the risk of PD, ANK2 interacts with PINK1/Parkin-target proteins such as MIRO1 or ATP1A2, and ANK2-derived peptides are potent inhibitors of autophagy.

Directional Fluid Transport across Organ-Blood Barriers: Physiology and Cell Biology

Directional fluid flow is an essential process for embryo development as well as for organ and organism homeostasis. Here, we review the diverse structure of various organ-blood barriers, the driving forces, transporters, and polarity mechanisms that regulate fluid transport across them, focusing on kidney-, eye-, and brain-blood barriers. We end by discussing how cross talk between barrier epithelial and endothelial cells, perivascular cells, and basement membrane signaling contribute to generate and maintain organ-blood barriers.

An Adaptable Spectrin/Ankyrin-Based Mechanism for Long-Range Organization of Plasma Membranes in Vertebrate Tissues

Ankyrins are membrane-associated proteins that together with their spectrin partners are responsible for micron-scale organization of vertebrate plasma membranes, including those of erythrocytes, excitable membranes of neurons and heart, lateral membrane domains of columnar epithelial cells, and striated muscle. Ankyrins coordinate functionally related membrane transporters and cell adhesion proteins (15 protein families identified so far) within plasma membrane compartments through independently evolved interactions of intrinsically disordered sequences with a highly conserved peptide-binding groove formed by the ANK repeat solenoid. Ankyrins are coupled to spectrins, which are elongated organelle-sized proteins that form mechanically resilient arrays through cross-linking by specialized actin filaments. In addition to protein interactions, cellular targeting and assembly of spectrin/ankyrin domains also critically depend on palmitoylation of ankyrin-G by aspartate-histidine-histidine-cysteine 5/8 palmitoyltransferases, as well as interaction of beta-2 spectrin with phosphoinositide lipids. These lipid-dependent spectrin/ankyrin domains are not static but are locally dynamic and determine membrane identity through opposing endocytosis of bulk lipids as well as specific proteins. A partnership between spectrin, ankyrin, and cell adhesion molecules first emerged in bilaterians over 500 million years ago. Ankyrin and spectrin may have been recruited to plasma membranes from more ancient roles in organelle transport. The basic bilaterian spectrin-ankyrin toolkit markedly expanded in vertebrates through gene duplications combined with variation in unstructured intramolecular regulatory sequences as well as independent evolution of ankyrin-binding activity by ion transporters involved in action potentials and calcium homeostasis. In addition, giant vertebrate ankyrins with specialized roles in axons acquired new coding sequences by exon shuffling. We speculate that early axon initial segments and epithelial lateral membranes initially were based on spectrin-ankyrin-cell adhesion molecule assemblies and subsequently served as "incubators," where ion transporters independently acquired ankyrin-binding activity through positive selection.

Trafficking to the apical and basolateral membranes in polarized epithelial cells

Renal epithelial cells must maintain distinct protein compositions in their apical and basolateral membranes in order to perform their transport functions. The creation of these polarized protein distributions depends on sorting signals that designate the trafficking route and site of ultimate functional residence for each protein. Segregation of newly synthesized apical and basolateral proteins into distinct carrier vesicles can occur at the trans-Golgi network, recycling endosomes, or a growing assortment of stations along the cellular trafficking pathway. The nature of the specific sorting signal and the mechanism through which it is interpreted can influence the route a protein takes through the cell. Cell type-specific variations in the targeting motifs of a protein, as are evident for Na,K-ATPase, demonstrate a remarkable capacity to adapt sorting pathways to different developmental states or physiologic requirements. This review summarizes our current understanding of apical and basolateral trafficking routes in polarized epithelial cells.

Polarization of membrane associated proteins in the choroid plexus epithelium from normal and slc4a10 knockout mice

The choroid plexus epithelium (CPE) has served as a model-epithelium for cell polarization and transport studies and plays a crucial role for cerebrospinal fluid (CSF) production. The normal luminal membrane expression of Na(+),K(+)-ATPase, aquaporin-1 and Na(+)/H(+) exchanger 1 in the choroid plexus is severely affected by deletion of the slc4a10 gene that encodes the bicarbonate transporting protein Ncbe/NBCn2. The causes for these deviations from normal epithelial polarization and redistribution following specific gene knockout are unknown, but may be significant for basic epithelial cell biology. Therefore, a more comprehensive analysis of cell polarization in the choroid plexus is warranted. We find that the cytoskeleton in the choroid plexus contains αI-, αII-, βI-, and βII-spectrin isoforms along with the anchoring protein ankyrin-3, most of which are mainly localized in the luminal membrane domain. Furthermore, we find α-adducin localized near the plasma membranes globally, but with only faint expression in the luminal membrane domain. In slc4a10 knockout mice, the abundance of β1 Na(+),K(+)-ATPase subunits in the luminal membrane is markedly reduced. Anion exchanger 2 abundance is increased in slc4a10 knockout and its anchor protein, α-adducin is almost exclusively found near the basolateral domain. The αI- and βI-spectrin abundances are also decreased in the slc4a10 knockout, where the basolateral domain expression of αI-spectrin is exchanged for a strictly luminal domain localization. E-cadherin expression is unchanged in the slc4a10 knockout, while small decreases in abundance are observed for its probable adaptor proteins, the catenins. Interestingly, the abundance of the tight junction protein claudin-2 is significantly reduced in the slc4a10 knockouts, which may critically affect paracellular transport in this epithelium. The observations allow the generation of new hypotheses on basic cell biological paradigms that can be tested experimentally in future studies.

Na(+), K(+)-ATPase subunit composition in a human chondrocyte cell line; evidence for the presence of α1, α3, β1, β2 and β3 isoforms

Membrane transport systems participate in fundamental activities such as cell cycle control, proliferation, survival, volume regulation, pH maintenance and regulation of extracellular matrix synthesis. Multiple isoforms of Na(+), K(+)-ATPase are expressed in primary chondrocytes. Some of these isoforms have previously been reported to be expressed exclusively in electrically excitable cells (i.e., cardiomyocytes and neurons). Studying the distribution of Na(+), K(+)-ATPase isoforms in chondrocytes makes it possible to document the diversity of isozyme pairing and to clarify issues concerning Na(+), K(+)-ATPase isoform abundance and the physiological relevance of their expression. In this study, we investigated the expression of Na(+), K(+)-ATPase in a human chondrocyte cell line (C-20/A4) using a combination of immunological and biochemical techniques. A panel of well-characterized antibodies revealed abundant expression of the α1, β1 and β2 isoforms. Western blot analysis of plasma membranes confirmed the above findings. Na(+), K(+)-ATPase consists of multiple isozyme variants that endow chondrocytes with additional homeostatic control capabilities. In terms of Na(+), K(+)-ATPase expression, the C-20/A4 cell line is phenotypically similar to primary and in situ chondrocytes. However, unlike freshly isolated chondrocytes, C-20/A4 cells are an easily accessible and convenient in vitro model for the study of Na(+), K(+)-ATPase expression and regulation in chondrocytes.

"The Lower Threshold" phenomenon in tumor cells toward endogenous digitalis-like compounds: Responsible for tumorigenesis?

Since their first discovery as potential anti-cancer drugs decades ago, there is increasing evidence that digitalis-like compounds (DLC) have anti-tumor effects. Less is known about endogenous DLC (EDLC) metabolism and regulation. As stress hormones synthesized in and secreted from the adrenal gland, they likely take part in the hypothalamo-pituitary-adrenal (HPA) axis. In a previous study, we revealed reduced EDLC concentrations in plasma and organs from immune-compromised animals and proposed that a similar situation of a deregulated HPA axis with "adrenal EDLF exhaustion" may contribute to tumorigenesis in chronic stress situations. Here, we put forward the hypothesis that a lowered EDLC response threshold of tumor cells as compared with normal cells increases the risk of tumorigenesis, especially in those individuals with reduced EDLC plasma concentrations after chronic stress exposure. We will evaluate this hypothesis by (a) summarizing the effects of different DLC concentrations on tumor as compared with normal cells and (b) reviewing some essential differences in the Na/K-ATPase of tumor as compared with normal cells (isoform pattern, pump activity, mutations of other signalosome receptors). We will conclude that (1) tumor cells, indeed, seem to have their individual "physiologic" EDLC response range that already starts at pmolar levels and (2) that individuals with markedly reduced (pmolar) EDLC plasma levels are predisposed to cancer because these EDLC concentrations will predominantly stimulate the proliferation of tumor cells. Finally, we will summarize preliminary results from our department supporting this hypothesis.

A marked animal-vegetal polarity in the localization of Na(+),K(+) -ATPase activity and its down-regulation following progesterone-induced maturation

The stage-VI Xenopus oocyte has a very distinct animal-vegetal polarity with structural and functional asymmetry. In this study, we show the expression and distribution pattern of Na(+),K(+) -ATPase in stage-VI oocytes, and its changes following progesterone-induced maturation. Using enzyme-specific electron microscopy phosphatase histochemistry, [(3) H]-ouabain autoradiography, and immunofluorescence cytochemistry at light microscopic level, we find that Na(+),K(+) -ATPase activity is mainly confined to the animal hemisphere. Electron microscopy histochemical results also suggest that polarized distribution of Na(+),K(+) -ATPase activity persists following progesterone-induced maturation, and it becomes gradually more polarized towards the animal pole. The time course following progesterone-induced maturation suggests that there is an initial up-regulation and then gradual down-regulation of Na(+),K(+) -ATPase activity leading to germinal vesicle breakdown (GVBD). By GVBD, the Na(+),K(+) -ATPase activity is completely down-regulated due to endocytotic removal of pump molecules from the plasma membrane into the sub-cortical region of the oocyte. This study provides the first direct evidence for a marked asymmetric localization of Na(+),K(+) -ATPase activity in any vertebrate oocyte. Here, we propose that such asymmetry in Na(+),K(+) -ATPase activity in stage-VI oocytes, and their down-regulation following progesterone-induced maturation, is likely to have a role in the active state of the germinal vesicle in stage-VI oocytes and chromosomal condensation after GVBD.

Molecular mechanisms in exercise-induced cardioprotection

Physical inactivity is increasingly recognized as modifiable behavioral risk factor for cardiovascular diseases. A partial list of proposed mechanisms for exercise-induced cardioprotection include induction of heat shock proteins, increase in cardiac antioxidant capacity, expression of endoplasmic reticulum stress proteins, anatomical and physiological changes in the coronary arteries, changes in nitric oxide production, adaptational changes in cardiac mitochondria, increased autophagy, and improved function of sarcolemmal and/or mitochondrial ATP-sensitive potassium channels. It is currently unclear which of these protective mechanisms are essential for exercise-induced cardioprotection. However, most investigations focus on sarcolemmal KATP channels, NO production, and mitochondrial changes although it is very likely that other mechanisms may also exist. This paper discusses current information about these aforementioned topics and does not consider potentially important adaptations within blood or the autonomic nervous system. A better understanding of the molecular basis of exercise-induced cardioprotection will help to develop better therapeutic strategies.

Shear stress-induced changes of membrane transporter localization and expression in mouse proximal tubule cells

Our previous studies of microperfused single proximal tubule showed that flow-dependent Na(+) and HCO(3)(-) reabsorption is due to a modulation of both NHE3 and vacuolar H(+)-ATPase (V-ATPase) activity. An intact actin cytoskeleton was indicated to provide a structural framework for proximal tubule cells to transmit mechanical forces and subsequently modulate cellular functions. In this study, we have used mouse proximal tubule (MPT) cells as a model to study the role of fluid shear stress (FSS) on apical NHE3 and V-ATPase and basolateral Na/K-ATPase trafficking and expression. Our hypothesis is that FSS stimulates both apical and basolateral transporter expression and trafficking, which subsequently mediates salt and volume reabsorption. We exposed MPT cells to 0.2 dynes/cm(2) FSS for 3 h and performed confocal microscopy and Western blot analysis to compare the localization and expression of both apical and basolateral transporters in control cells and cells subjected to FSS. Our findings show that FSS leads to an increment in the amount of protein expression, and a translocation of apical NHE3 and V-ATPase from the intracellular compartment to the apical plasma membrane and Na/K-ATPase to the basolateral membrane. Disrupting actin by cytochalasin D blocks the FSS-induced changes in NHE3 and Na/K-ATPase, but not V-ATPase. In contrast, FSS-induced V-ATPase redistribution and expression are largely inhibited by colchicine, an agent that blocks microtubule polymerization. Our findings suggest that the actin cytoskeleton plays an important role in FSS-induced NHE3 and Na/K-ATPase trafficking, and an intact microtubule network is critical in FSS-induced modulation of V-ATPase in proximal tubule cells.

The spectrin-ankyrin-4.1-adducin membrane skeleton: adapting eukaryotic cells to the demands of animal life

The cells in animals face unique demands beyond those encountered by their unicellular eukaryotic ancestors. For example, the forces engendered by the movement of animals places stresses on membranes of a different nature than those confronting free-living cells. The integration of cells into tissues, as well as the integration of tissue function into whole animal physiology, requires specialisation of membrane domains and the formation of signalling complexes. With the evolution of mammals, the specialisation of cell types has been taken to an extreme with the advent of the non-nucleated mammalian red blood cell. These and other adaptations to animal life seem to require four proteins--spectrin, ankyrin, 4.1 and adducin--which emerged during eumetazoan evolution. Spectrin, an actin cross-linking protein, was probably the earliest of these, with ankyrin, adducin and 4.1 only appearing as tissues evolved. The interaction of spectrin with ankyrin is probably a prerequisite for the formation of tissues; only with the advent of vertebrates did 4.1 acquires the ability to bind spectrin and actin. The latter activity seems to allow the spectrin complex to regulate the cell surface accumulation of a wide variety of proteins. Functionally, the spectrin-ankyrin-4.1-adducin complex is implicated in the formation of apical and basolateral domains, in aspects of membrane trafficking, in assembly of certain signalling and cell adhesion complexes and in providing stability to otherwise mechanically fragile cell membranes. Defects in this complex are manifest in a variety of hereditary diseases, including deafness, cardiac arrhythmia, spinocerebellar ataxia, as well as hereditary haemolytic anaemias. Some of these proteins also function as tumor suppressors. The spectrin-ankyrin-4.1-adducin complex represents a remarkable system that underpins animal life; it has been adapted to many different functions at different times during animal evolution.

Interactions of the proteins of neuronal ceroid lipofuscinosis: clues to function

Neuronal ceroid lipofuscinoses (NCL) are caused by mutations in eight different genes, are characterized by lysosomal accumulation of autofluorescent storage material, and result in a disease that causes degeneration of the central nervous system (CNS). Although functions are defined for some of the soluble proteins that are defective in NCL (cathepsin D, PPT1, and TPP1), the primary function of the other proteins defective in NCLs (CLN3, CLN5, CLN6, CLN7, and CLN8) remain poorly defined. Understanding the localization and network of interactions for these proteins can offer clues as to the function of the NCL proteins and also the pathways that will be disrupted in their absence. Here, we present a review of the current understanding of the localization, interactions, and function of the proteins associated with NCL.

A Golgi-associated protein 4.1B variant is required for assimilation of proteins in the membrane

The archetypal membrane skeleton is that of the erythrocyte, consisting predominantly of spectrin, actin, ankyrin R and protein 4.1R. The presence in the Golgi of a membrane skeleton with a similar structure has been inferred, based on the identification of Golgi-associated spectrin and ankyrin. It has long been assumed that a Golgi-specific protein 4.1 must also exist, but it has not previously been found. We demonstrate here that a hitherto unknown form of protein 4.1, a 200 kDa 4.1B, is associated with the Golgi of Madin-Darby canine kidney (MDCK) and human bronchial epithelial (HBE) cells. This 4.1B variant behaves like a Golgi marker after treatment with Brefeldin A and during mitosis. Depletion of the protein in HBE cells by siRNA resulted in disruption of the Golgi structure and failure of Na(+)/K(+)-ATPase, ZO-1 and ZO-2 to migrate to the membrane. Thus, this newly identified Golgi-specific protein 4.1 appears to have an essential role in maintaining the structure of the Golgi and in assembly of a subset of membrane proteins.

Water and ion channels: crucial in the initiation and progression of apoptosis in central nervous system?

Programmed cell death (PCD), is a highly regulated and sophisticated cellular mechanism that commits cell to isolated death fate. PCD has been implicated in the pathogenesis of numerous neurodegenerative disorders. Countless molecular events underlie this phenomenon, with each playing a crucial role in death commitment. A precedent event, apoptotic volume decrease (AVD), is ubiquitously observed in various forms of PCD induced by different cellular insults. Under physiological conditions, cells when subjected to osmotic fluctuations will undergo regulatory volume increase/decrease (RVI/RVD) to achieve homeostatic balance with neurons in the brain being additionally protected by the blood-brain-barrier. However, during AVD following apoptotic trigger, cell undergoes anistonic shrinkage that involves the loss of water and ions, particularly monovalent ions e.g. K(+), Na(+) and Cl(-). It is worthwhile to concentrate on the molecular implications underlying the loss of these cellular components which posed to be significant and crucial in the successful propagation of the apoptotic signals. Microarray and real-time PCR analyses demonstrated several ion and water channel genes are regulated upon the onset of lactacystin (a proteosomal inhibitor)-mediated apoptosis. A time course study revealed that gene expressions of water and ion channels are being modulated just prior to apoptosis, some of which are aquaporin 4 and 9, potassium channels and chloride channels. In this review, we shall looked into the molecular protein machineries involved in the execution of AVD in the central nervous system (CNS), and focus on the significance of movements of each cellular component in affecting PCD commitment, thus provide some pharmacological advantages in the global apoptotic cell death.

Polarized membrane distribution of potassium-dependent ion pumps in epithelial cells: different roles of the N-glycans of their beta subunits

The Na,K-ATPases and the H,K-ATPases are two potassium-dependent homologous heterodimeric P2-type pumps that catalyze active transport of Na+ in exchange for K+ (Na,K-ATPase) or H+ in exchange for K+ (H,K-ATPase). The ubiquitous Na,K-ATPase maintains intracellular ion balance and membrane potential. The gastric H,K-ATPase is responsible for acid secretion by the parietal cell of the stomach. Both pumps consist of a catalytic alpha-subunit and a glycosylated beta-subunit that is obligatory for normal pump maturation and trafficking. Individual N-glycans linked to the beta-subunits of the Na,K-ATPase and H,K-ATPase are important for stable membrane integration of their respective alpha subunits, folding, stability, subunit assembly, and enzymatic activity of the pumps. They are also essential for the quality control of unassembled beta-subunits that results in either the exit of the subunits from the ER or their ER retention and subsequent degradation. Overall, the importance of N-glycans for the maturation and quality control of the H,K-ATPase is greater than that of the Na,K-ATPase. The roles of individual N-glycans of the beta-subunits in the post-ER trafficking, membrane targeting and plasma membrane retention of the Na,K-ATPase and H,K-ATPase are different. The Na,K-ATPase beta1-subunit is the major beta-subunit isoform in cells with lateral location of the pump. All three N-glycans of the Na,K-ATPase beta1-subunit are important for the lateral membrane retention of the pump due to glycan-mediated interaction between the beta1-subunits of the two neighboring cells in the cell monolayer and cytosolic linkage of the alpha-subunit to the cytoskeleton. This intercellular beta1-beta1 interaction is also important for formation of cell-cell contacts. In contrast, the N-glycans unique to the Na,K-ATPase beta2-subunit,which has up to eight N-glycosylation sites, contain apical sorting information. This is consistent with the apical location of the Na,K-ATPase in normal and malignant epithelial cells with high abundance of the beta2-subunit. Similarly, all seven N-glycans of the gastric H,K-ATPase beta-subunit determine apical sorting of this subunit.

Adducin promotes micrometer-scale organization of beta2-spectrin in lateral membranes of bronchial epithelial cells

Adducin promotes assembly of spectrin-actin complexes, and is a target for regulation by calmodulin, protein kinase C, and rho kinase. We demonstrate here that adducin is required to stabilize preformed lateral membranes of human bronchial epithelial (HBE) cells through interaction with beta2-spectrin. We use a Tet-on regulated inducible small interfering RNA (siRNA) system to deplete alpha-adducin from confluent HBE cells. Depletion of alpha-adducin resulted in increased detergent solubility of spectrin after normal membrane biogenesis during mitosis. Conversely, depletion of beta2-spectrin resulted in loss of adducin from the lateral membrane. siRNA-resistant alpha-adducin prevented loss of lateral membrane, but only if alpha-adducin retained the MARCKS domain that mediates spectrin-actin interactions. Phospho-mimetic versions of adducin with S/D substitutions at protein kinase C phosphorylation sites in the MARCKS domain were not active in rescue. We find that adducin modulates long-range organization of the lateral membrane based on several criteria. First, the lateral membrane of adducin-depleted cells exhibited reduced height, increased curvature, and expansion into the basal surface. Moreover, E-cadherin-GFP, which normally is restricted in lateral mobility, rapidly diffuses over distances up to 10 microm. We conclude that adducin acting through spectrin provides a novel mechanism to regulate global properties of the lateral membrane of bronchial epithelial cells.

Ankyrin-G is a molecular partner of E-cadherin in epithelial cells and early embryos

E-cadherin is a ubiquitous component of lateral membranes in epithelial tissues and is required to form the first lateral membrane domains in development. Here, we identify ankyrin-G as a molecular partner of E-cadherin and demonstrate that ankyrin-G and beta-2-spectrin are required for accumulation of E-cadherin at the lateral membrane in both epithelial cells and early embryos. Ankyrin-G binds to the cytoplasmic domain of E-cadherin at a conserved site distinct from that of beta-catenin. Ankyrin-G also recruits beta-2-spectrin to E-cadherin-beta-catenin complexes, thus providing a direct connection between E-cadherin and the spectrin/actin skeleton. In addition to restricting the membrane mobility of E-cadherin, ankyrin-G and beta-2-spectrin also are required for exit of E-cadherin from the trans-Golgi network in a microtubule-dependent pathway. Ankyrin-G and beta-2-spectrin co-localize with E-cadherin in preimplantation mouse embryos. Moreover, knockdown of either ankyrin-G or beta-2-spectrin in one cell of a two-cell embryo blocks accumulation of E-cadherin at sites of cell-cell contact. E-cadherin thus requires both ankyrin-G and beta-2-spectrin for its cellular localization in early embryos as well as cultured epithelial cells. We have recently reported that ankyrin-G and beta-2-spectrin collaborate in biogenesis of the lateral membrane ( Kizhatil, K., Yoon, W., Mohler, P. J., Davis, L. H., Hoffman, J. A., and Bennett, V. (2007) J. Biol. Chem. 282, 2029-2037 ). Together with the current findings, these data suggest a ankyrin/spectrin-based mechanism for coordinating membrane assembly with extracellular interactions of E-cadherin at sites of cell-cell contact.

Cardiotonic steroids on the road to anti-cancer therapy

The sodium pump, Na(+)/K(+)-ATPase, could be an important target for the development of anti-cancer drugs as it serves as a versatile signal transducer, it is a key player in cell adhesion and its aberrant expression and activity are implicated in the development and progression of different cancers. Cardiotonic steroids, known ligands of the sodium pump have been widely used for the treatment of heart failure. However, early epidemiological evaluations and subsequent demonstration of anti-cancer activity in vitro and in vivo have indicated the possibility of developing this class of compound as chemotherapeutic agents in oncology. Their development to date as anti-cancer agents has however been impaired by a narrow therapeutic margin resulting from their potential to induce cardiovascular side-effects. The review will thus discuss (i) sodium pump structure, function, expression in diverse cancers and its chemical targeting and that of its sub-units, (ii) reported in vitro and in vivo anti-cancer activity of cardiotonic steroids, (iii) managing the toxicity of these compounds and the limitations of existing preclinical models to adequately predict the cardiotoxic potential of new molecules in man and (iv) the potential of chemical modification to reduce the cardiovascular side-effects and improve the anti-cancer activity of new molecules.

The future of the pump

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

Endogenous and exogenous cardiac glycosides: their roles in hypertension, salt metabolism, and cell growth

Cardiotonic steroids (CTS), long used to treat heart failure, are endogenously produced in mammals. Among them are the hydrophilic cardenolide ouabain and the more hydrophobic cardenolide digoxin, as well as the bufadienolides marinobufagenin and telecinobufagin. The physiological effects of endogenous ouabain on blood pressure and cardiac activity are consistent with the "Na(+)-lag" hypothesis. This hypothesis assumes that, in cardiac and arterial myocytes, a CTS-induced local increase of Na(+) concentration due to inhibition of Na(+)/K(+)-ATPase leads to an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) via a backward-running Na(+)/Ca(2+) exchanger. The increase in [Ca(2+)](i) then activates muscle contraction. The Na(+)-lag hypothesis may best explain short-term and inotropic actions of CTS. Yet all data on the CTS-induced alteration of gene expression are consistent with another hypothesis, based on the Na(+)/K(+)-ATPase "signalosome," that describes the interaction of cardiac glycosides with the Na(+) pump as machinery activating various signaling pathways via intramembrane and cytosolic protein-protein interactions. These pathways, which may be activated simultaneously or selectively, elevate [Ca(2+)](i), activate Src and the ERK1/2 kinase pathways, and activate phosphoinositide 3-kinase and protein kinase B (Akt), NF-kappaB, and reactive oxygen species. A recent development indicates that new pharmaceuticals with antihypertensive and anticancer activities may be found among CTS and their derivatives: the antihypertensive rostafuroxin suppresses Na(+) resorption and the Src-epidermal growth factor receptor-ERK pathway in kidney tubule cells. It may be the parent compound of a new principle of antihypertensive therapy. Bufalin and oleandrin or the cardenolide analog UNBS-1450 block tumor cell proliferation and induce apoptosis at low concentrations in tumors with constitutive activation of NF-kappaB.

Functional links between membrane transport and the spectrin cytoskeleton

Membrane transporters precisely regulate which molecules cross the plasma membrane and when they can cross. In many cases it is also important to regulate where substances can cross the plasma membrane. Consequently, cells have evolved mechanisms to confine and stabilize membrane transport proteins within specific subdomains of the plasma membrane. A number of different transporters (including ion pumps, channels and exchangers) are known to physically associate with the spectrin cytoskeleton, a submembrane complex of spectrin and ankyrin. These proteins form a protein scaffold that assembles within discrete subdomains of the plasma membrane in polarized cells. Recent genetic studies in humans and model organisms have provided the opportunity to test the hypothesis that the spectrin cytoskeleton has a direct role in restricting transporters to specialized domains. Remarkably, genetic defects in spectrin and ankyrin can produce effects on cell physiology that are comparable to knockouts of the transporters themselves.

The role of the beta1 subunit of the Na,K-ATPase and its glycosylation in cell-cell adhesion

Based on recent data showing that overexpression of the Na,K-ATPase beta(1) subunit increased cell-cell adhesion of nonpolarized cells, we hypothesized that the beta(1) subunit can also be involved in the formation of cell-cell contacts in highly polarized epithelial cells. In support of this hypothesis, in Madin-Darby canine kidney (MDCK) cells, the Na,K-ATPase alpha(1) and beta(1) subunits were detected as precisely co-localized with adherens junctions in all stages of the monolayer formation starting from the initiation of cell-cell contact. The Na,K-ATPase and adherens junction protein, beta-catenin, stayed partially co-localized even after their internalization upon disruption of intercellular contacts by Ca(2+) depletion of the medium. The Na,K-ATPase subunits remained co-localized with the adherens junctions after detergent treatment of the cells. In contrast, the heterodimer formed by expressed unglycosylated Na,K-ATPase beta(1) subunit and the endogenous alpha(1) subunit was easily dissociated from the adherens junctions and cytoskeleton by the detergent extraction. The MDCK cell line in which half of the endogenous beta(1) subunits in the lateral membrane were substituted by unglycosylated beta(1) subunits displayed a decreased ability to form cell-to-cell contacts. Incubation of surface-attached MDCK cells with an antibody against the extracellular domain of the Na,K-ATPase beta(1) subunit specifically inhibited cell-cell contact formation. We conclude that the Na,K-ATPase beta(1) subunit is involved in the process of intercellular adhesion and is necessary for association of the heterodimeric Na,K-ATPase with the adherens junctions. Further, normal glycosylation of the Na,K-ATPase beta(1) subunit is essential for the stable association of the pump with the adherens junctions and plays an important role in cell-cell contact formation.

Cell cycle deregulation and loss of stem cell phenotype in the subventricular zone of TGF-beta adaptor elf-/- mouse brain

The mammalian forebrain subependyma contains neural stem cells and other proliferating progenitor cells. Recent studies have shown the importance of TGF-beta family members and their adaptor proteins in the inhibition of proliferation in the nervous system. Previously, we have demonstrated that TGF-beta induces phosphorylation and association of ELF (embryonic liver fodrin) with Smad3 and Smad4 resulting in nuclear translocation. Elf(-/-) mice manifest abnormal neuronal differentiation, with loss of neuroepithelial progenitor cell phenotype in the subventricular zone (SVZ) with dramatic marginal cell hyperplasia and loss of nestin expression. Here, we have analyzed the expression of cell cycle-associated proteins cdk4, mdm2, p21, and pRb family members in the brain of elf(-/-) mice to verify the role of elf in the regulation of neural precursor cells in the mammalian brain. Increased proliferation in SVZ cells of the mutant mice coincided with higher levels of cdk4 and mdm2 expression. A lesser degree of apoptosis was observed in the mutant mice compared to the wild-type control. Elf(-/-) embryos showed elevated levels of hyperphosphorylated forms of pRb, p130 and p107 and decreased level of p21 compared to the wild-type control. These results establish a critical role for elf in the development of a SVZ neuroepithelial stem cell phenotype and regulation of neuroepithelial cell proliferation, suggesting that a mutation in the elf locus renders the cells susceptible to a faster entry into S phase of cell cycle and resistance to senescence and apoptotic stimuli.

Critical interactions between TGF-beta signaling/ELF, and E-cadherin/beta-catenin mediated tumor suppression

Inactivation of the transforming growth factor-beta (TGF-beta) pathway occurs often in malignancies of the gastrointestinal (GI) system. However, only a fraction of sporadic GI tumors exhibit inactivating mutations in early stages of cancer formation, suggesting that other mechanisms play a critical role in the inactivation of this pathway. Here, we show a wide range of GI tumors, including those of the stomach, liver and colon in elf+/- and elf+/- / Smad4+/- mutant mice. We found that embryonic liver fodrin (ELF), a beta-Spectrin originally identified in endodermal stem/progenitor cells committed to foregut lineage, possesses potent antioncogenic activity and is frequently inactivated in GI cancers. Specifically, E-cadherin accumulation at cell-cell contacts and E-cadherin-beta-catenin-dependent epithelial cell-cell adhesion is disrupted in elf+/- / Smad4+/- mutant gastric epithelial cells, and could be rescued by ectopic expression of full-length elf, but not Smad3 or Smad4. Subcellular fractionation revealed that E-cadherin is expressed mainly at the cell membrane after TGF-beta stimulation. In contrast, elf+/- / Smad4+/- mutant tissues showed abnormal distribution of E-cadherin that could be rescued by overexpression of ELF but not Smad3 or Smad4. Our results identify a group of common lethal malignancies in which inactivation of TGF-beta signaling, which is essential for tumor suppression, is disrupted by inactivation of the ELF adaptor protein.

Inactivation of ELF/TGF-beta signaling in human gastrointestinal cancer

TGF-beta/Smads regulate a wide variety of biological responses through transcriptional regulation of target genes. ELF, a beta-spectrin, plays a key role in the transmission of TGF-beta-mediated transcriptional response through Smads. ELF was originally identified as a key protein involved in endodermal stem/progenitor cells committed to foregut lineage. Also, as a major dynamic adaptor and scaffolding protein, ELF is important for the generation of functionally distinct membranes, protein sorting and the development of polarized differentiated epithelial cells. Disruption of elf results in the loss of Smad3/Smad4 activation and, therefore, a disruption of the TGF-beta pathway. These observations led us to pursue the function of ELF in gastrointestinal (GI) epithelial cell-cell adhesion and tumor suppression. Here, we show a significant loss of ELF and reduced Smad4 expression in human gastric cancer tissue samples. Also, of the six human gastric cancer cell lines examined, three show deficient ELF expression. Furthermore, we demonstrate the rescue of E-cadherin-dependent homophilic cell-cell adhesion by ectopic expression of full-length elf. Our results suggest that ELF has an essential role in tumor suppression in GI cancers.

Recombinant addition of N-glycosylation sites to the basolateral Na,K-ATPase beta1 subunit results in its clustering in caveolae and apical sorting in HGT-1 cells

In most polarized cells, the Na,K-ATPase is localized on the basolateral plasma membrane. However, an unusual location of the Na,K-ATPase was detected in polarized HGT-1 cells (a human gastric adenocarcinoma cell line). The Na,K-ATPase alpha1 subunit was detected along with the beta2 subunit predominantly on the apical membrane, whereas the Na,K-ATPase beta1 subunit was not found in HGT-1 cells. However, when expressed in the same cell line, a yellow fluorescent protein-linked Na,K-ATPase beta1 subunit was localized exclusively to the basolateral surface and resulted in partial redistribution of the endogenous alpha1 subunit to the basolateral membrane. The human beta2 subunit has eight N-glycosylation sites, whereas the beta1 isoform has only three. Accordingly, up to five additional N-glycosylation sites homologous to the ones present in the beta2 subunit were successively introduced in the beta1 subunit by site-directed mutagenesis. The mutated beta1 subunits were detected on both apical and basolateral membranes. The fraction of a mutant beta1 subunit present on the apical membrane increased in proportion to the number of glycosylation sites inserted and reached 80% of the total surface amount for the beta1 mutant with five additional sites. Clustered distribution and co-localization with caveolin-1 was detected by confocal microscopy for the endogenous beta2 subunit and the beta1 mutant with additional glycosylation sites but not for the wild type beta1 subunit. Hence, the N-glycans linked to the beta2 subunit of the Na,K-ATPase contain apical sorting information, and the high abundance of the beta2 subunit isoform, which is rich in N-glycans, along with the absence of the beta1 subunit, is responsible for the unusual apical location of the Na,K-ATPase in HGT-1 cells.

Regulation of fish gill Na(+)-K(+)-ATPase by selective sulfatide-enriched raft partitioning during seawater adaptation

Na(+)-K(+)-ATPase is arguably the most important enzyme in the animal cell plasma membrane, but the role of the membrane in its regulation is poorly understood. We investigated the relationship between Na(+)-K(+)-ATPase and membrane microdomains or "lipid rafts" enriched in sulfatide (sulfogalactosylceramide/SGC), a glycosphingolipid implicated as a cofactor for this enzyme, in the basolateral membrane of rainbow trout gill epithelium. Our studies demonstrated that when trout adapt to seawater (33 ppt), Na(+)-K(+)-ATPase relocates to these structures. Arylsulfatase-induced desulfation of basolateral membrane SGC prevented this relocation and significantly reduced Na(+)-K(+)-ATPase activity in seawater but not freshwater trout. We contend that Na(+)-K(+)-ATPase partitions into SGC-enriched rafts to help facilitate the up-regulation of its activity during seawater adaptation. We also suggest that differential partitioning of Na(+)-K(+)-ATPase between these novel SGC-enriched regulatory platforms results in two distinct, physiological Na(+) transport modes. In addition, we extend the working definition of cholesterol-dependent raft integrity to structural dependence on the sulfate moiety of SGC in this membrane.

The polarized expression of Na+,K+-ATPase in epithelia depends on the association between beta-subunits located in neighboring cells

The polarized distribution of Na+,K+-ATPase plays a paramount physiological role, because either directly or through coupling with co- and countertransporters, it is responsible for the net movement of, for example, glucose, amino acids, Ca2+, K+, Cl-, and CO3H- across the whole epithelium. We report here that the beta-subunit is a key factor in the polarized distribution of this enzyme. 1) Madin-Darby canine kidney (MDCK) cells (epithelial from dog kidney) express the Na+,K+-ATPase over the lateral side, but not on the basal and apical domains, as if the contact with a neighboring cell were crucial for the specific membrane location of this enzyme. 2) MDCK cells cocultured with other epithelial types (derived from human, cat, dog, pig, monkey, rabbit, mouse, hamster, and rat) express the enzyme in all (100%) homotypic MDCK/MDCK borders but rarely in heterotypic ones. 3) Although MDCK cells never express Na+,K+-ATPase at contacts with Chinese hamster ovary (CHO) cells, they do when CHO cells are transfected with beta1-subunit from the dog kidney (CHO-beta). 4) This may be attributed to the adhesive property of the beta1-subunit, because an aggregation assay using CHO (mock-transfected) and CHO-beta cells shows that the expression of dog beta1-subunit in the plasma membrane does increase adhesiveness. 5) This adhesiveness does not involve adherens or tight junctions. 6) Transfection of beta1-subunit forces CHO-beta cells to coexpress endogenous alpha-subunit. Together, our results indicate that MDCK cells express Na+,K+-ATPase at a given border provided the contacting cell expresses the dog beta1-subunit. The cell-cell interaction thus established would suffice to account for the polarized expression and positioning of Na+,K+-ATPase in epithelial cells.

The membrane skeleton

One important element that defines cell shape is the membrane skeleton. This filamentous network is closely apposed to the cytoplasmic face of the plasma membrane where it gives mechanical support to the membrane, provides specific attachment sites for cytoskeletal components and helps to organize some integral membrane proteins into domains. The membrane skeleton of erythrocytes has been studied extensively by biochemical and ultrastructural methods, but similar structures in other cell types are just beginning to be defined. In this review, David Pumplin and Robert Bloch draw attention to these nonerythroid skeletons and compare and contrast them with the erythrocyte model.

Plasma membrane-cytoskeleton-endoplasmic reticulum complexes in neurons and astrocytes

The possibility that certain integral plasma membrane (PM) proteins involved in Ca(2+) homeostasis form junctional units with adjacent endoplasmic reticulum (ER) in neurons and glia was explored using immunoprecipitation and immunocytochemistry. Rat brain membranes were solubilized with the mild, non-ionic detergent, IGEPAL CA-630. Na(+)/Ca(2+) exchanger type 1 (NCX1), a key PM Ca(2+) transporter, was immunoprecipitated from the detergent-soluble fraction. Several abundant PM proteins co-immunoprecipitated with NCX1, including the alpha2 and alpha3 isoforms of the Na(+) pump catalytic (alpha) subunit, and the alpha2 subunit of the dihydropyridine receptor. The adaptor protein, ankyrin 2 (Ank 2), and the cytoskeletal proteins, alpha-fodrin and beta-spectrin, also selectively co-immunoprecipitated with NCX1, as did the ER proteins, Ca(2+) pump type 2 (SERCA 2), and inositol-trisphosphate receptor type 1 (IP(3)R-1). In contrast, a number of other abundant PMs, adaptors, and cytoskeletal proteins did not co-immunoprecipitate with NCX1, including the Na(+) pump alpha1 isoform, PM Ca(2+) pump type 1 (PMCA1), beta-fodrin, and Ank 3. In reciprocal experiments, immunoprecipitation with antibodies to the Na(+) pump alpha2 and alpha3 isoforms, but not alpha1, co-immunoprecipitated NCX1; the antibodies to alpha1 did, however, co-immunoprecipitate PMCA1. Antibodies to Ank 2, alpha-fodrin, beta-spectrin and IP(3)R-1 all co-immunoprecipitated NCX1. Immunocytochemistry revealed partial co-localization of beta-spectrin with NCX1, Na(+) pump alpha3, and IP(3)R-1 in neurons and of alpha-fodrin with NCX1 and SERCA2 in astrocytes. The data support the idea that in neurons and glia PM microdomains containing NCX1 and Na(+) pumps with alpha2 or alpha3 subunits form Ca(2+) signaling complexes with underlying ER containing SERCA2 and IP(3)R-1. These PM and ER components appear to be linked through the cytoskeletal spectrin network, to which they are probably tethered by Ank 2.

Golgi localization of Syne-1

We have previously identified a Golgi-localized spectrin isoform by using an antibody to the beta-subunit of erythrocyte spectrin. In this study, we show that a screen of a lambdagt11 expression library resulted in the isolation of an approximately 5-kb partial cDNA from a Madin-Darby bovine kidney (MDBK) cell line, which encoded a polypeptide of 1697 amino acids with low, but detectable, sequence homology to spectrin (37%). A blast search revealed that this clone overlaps with the 5' end of a recently identified spectrin family member Syne-1B/Nesprin-1beta, an alternately transcribed gene with muscle-specific forms that bind acetylcholine receptor and associate with the nuclear envelope. By comparing the sequence of the MDBK clone with sequence data from the human genome database, we have determined that this cDNA represents a central portion of a very large gene ( approximately 500 kb), encoding an approximately 25-kb transcript that we refer to as Syne-1. Syne-1 encodes a large polypeptide (8406 amino acids) with multiple spectrin repeats and a region at its amino terminus with high homology to the actin binding domains of conventional spectrins. Golgi localization for this spectrin-like protein was demonstrated by expression of epitope-tagged fragments in MDBK and COS cells, identifying two distinct Golgi binding sites, and by immunofluorescence microscopy by using several different antibody preparations. One of the Golgi binding domains on Syne-1 acts as a dominant negative inhibitor that alters the structure of the Golgi complex, which collapses into a condensed structure near the centrosome in transfected epithelial cells. We conclude that the Syne-1 gene is expressed in a variety of forms that are multifunctional and are capable of functioning at both the Golgi and the nuclear envelope, perhaps linking the two organelles during muscle differentiation.

2,3-butanedione monoxime (BDM), a potent inhibitor of actin-myosin interaction, induces ion and fluid transport in MDCK monolayers

Membrane-cytoskeleton interactions have been shown to be crucial to modulate polarity, cell shape and the paracellular pathway in epithelial MDCK cell monolayers. In particular, actin organization and myosin-dependent contractility play an important role in the regulation of these functions. Participation of myosin in vectorial transport, expressed as formation of domes, was investigated in confluent monolayers of high transepithelial electrical resistance (TER) plated on non-permeable supports. Cells exposed to 2,3-butanedione monoxime, a selective inhibitor of myosin ATPase, showed a remarkable increase in the number of domes. Replacement of extracellular Na+ and Cl- and inhibition of Na+-K+-ATPase blocked the induction of domes. The monoxime also caused a reduction of the TER leading to an increase in the paracellular flux of small molecular weight dextran. However, immunofluorescence microscopy of drug-treated cells showed that the localization and staining pattern of tight junction proteins ZO-1, occludin, and claudin 1, or the actin-myosin ring at the zonula adherens, were not modified. Treatment with the drug produced striking re-arrangements of actin filaments at the microvilli and at the basal level of the cells. Our data show that disruption of actin-myosin interaction at several cellular sites contributed importantly to the increased transport activity and the formation of the domes. These results point to the relevant role or actin-myosin dynamics and actin organization in the regulation of ion and water channel activity in these cells.

Distinct distribution of specific members of protein 4.1 gene family in the mouse nephron

BACKGROUND

Protein 4.1 is an adapter protein that links the actin cytoskeleton to various transmembrane proteins. These 4.1 proteins are encoded by four homologous genes, 4.1R, 4.1G, 4.1N, and 4.1B, which undergo complex alternative splicing. Here we performed a detailed characterization of the expression of specific 4.1 proteins in the mouse nephron.

METHODS

Distribution of renal 4.1 proteins was investigated by staining of paraformaldehyde-fixed mouse kidney sections with antibodies highly specific for each 4.1 protein. Major 4.1 splice forms, amplified from mouse kidney marathon cDNA, were expressed in transfected COS-7 cells in order to assign species of known exon composition to proteins detected in kidney.

RESULTS

A 105 kD 4.1R splice form, initiating at ATG-2 translation initiation site and lacking exon 16, but including exon 17B, was restricted to thick ascending limb of Henle's loop. A 95 kD 4.1N splice form, lacking exons 15 and 17D, was expressed in either descending or ascending thin limb of Henle's loop, distal convoluted tubule, and all regions of the collecting duct system. A major 108 kD 4.1B splice form, initiating at a newly characterized ATG translation initiation site, and lacking exons 15, 17B, and 21, was present only in Bowman's capsule and proximal convoluted tubule (PCT). There was no expression of 4.1G in kidney.

CONCLUSION

Distinct distribution of 4.1 proteins along the nephron suggests their involvement in targeting of selected transmembrane proteins in kidney epithelium and, therefore, in regulation of specific kidney functions.

Multiple cytoplasmic signals direct the intracellular trafficking of chicken kidney AE1 anion exchangers in MDCK cells

AE1/F(c) receptor chimeras have been used to define the sequences that direct the basolateral sorting, recycling and cytoskeletal association of the chicken AE1-4 anion exchanger in MDCK cells. These analyses revealed that amino acids 1-63 of AE1-4 were sufficient to redirect a cytoplasmic tailless murine IgG F(c)RII B2 receptor from the apical to the basolateral membrane of MDCK cells, where F(c)1-63 associated with elements of the actin cytoskeleton. In contrast to F(c)1-63, chimeras containing amino acids 1-37 (F(c)1-37) or 38-63 (F(c)38-63) of AE1-4 accumulated in intracellular membrane compartments that overlapped late endosomes and the trans-Golgi network (TGN), respectively. Internalization assays indicated that the patterns of localization observed for F(c)1-37 and F(c)38-63 resulted from the recycling of these chimeras from the cell surface. These assays further indicated that F(c)1-37 and F(c)38-63 each possess a basolateral sorting activity. Mutagenesis studies revealed that the endocytic and basolateral sorting activities in F(c)1-37 are dependent upon serine 25, which is located in a sequence similar to a sorting signal in the polymeric immunoglobulin receptor. In addition, the sorting activities associated with F(c)38-63 were dependent upon tyrosine 47 and leucine 50. These residues resided within the sequence, YVEL, which matches the YXXPhi motif (where X is any amino acid and Phi is a hydrophobic residue) that functions as an endocytic and TGN recycling signal for other membrane proteins. Our data indicate that amino acids 1-63 of AE1-4 contain sorting and cytoskeletal binding activities that account for most of the properties previously associated with AE1-4 in MDCK cells. Furthermore, the alternative localization patterns exhibited by chimeras containing various combinations of these activities suggest that interplay between these cytoplasmic activities is critical for specifying AE1-4 localization in epithelial cells.

Interaction of cofilin with triose-phosphate isomerase contributes glycolytic fuel for Na,K-ATPase via Rho-mediated signaling pathway

We reported previously that cofilin, an actin-binding protein, interacts with Na,K-ATPase and enhances its activity (Lee, K., Jung, J., Kim, M., and Guidotti, G. (2001) Biochem. J. 353, 377-385). To understand the nature of this interaction and the role of cofilin in the regulation of Na,K-ATPase activity, we searched for cofilin-binding proteins in the rat skeletal muscle cDNA library using the yeast two-hybrid system. Several cDNA clones were isolated, some of which coded for triose-phosphate isomerase, a glycolytic enzyme. The interaction of cofilin with triose-phosphate isomerase as well as Na,K-ATPase was confirmed by immunoprecipitation and confocal microscopy in HeLa cells. Cofilin was translocated to the plasma membrane along with triose-phosphate isomerase by the Rho activator lysophosphatidic acid but not by the p160 Rho-associated kinase inhibitor Y-27632, suggesting that the phosphorylated form of cofilin bound to TPI interacts with Na,K-ATPase. Ouabain-sensitive (86)Rb(+) uptake showed that Na,K-ATPase activity was increased by the overexpression of cofilin and lysophosphatidic acid treatment, but not by the overexpression of mutant cofilin S3A and Y-27632 treatment. Pretreatment with the glycolytic inhibitor iodoacetic acid caused a remarkable reduction of Na,K-ATPase activity, whereas pretreatment with the oxidative inhibitor carbonyl cyanide m-chlorophenylhydrazone caused no detectable changes, suggesting that the phosphorylated cofilin is involved in feeding glycolytic fuel for Na,K-ATPase activity. These findings provide a novel molecular mechanism for the regulation of Na,K-ATPase activity and for the nature of the functional coupling of cellular energy transduction.

System y+ localizes to different membrane subdomains in the basolateral plasma membrane of epithelial cells

We report here that the system y+ cationic amino acid transporter ATRC1 localized to clusters within the basolateral membrane of polarized Madin-Darby canine kidney and human embryonic kidney (HEK) cells, suggesting that the transporters are restricted to discrete membrane microdomains in epithelial cells. Based on solubility in nonionic detergents, two populations of ATRC1 molecules existed: approximately half of the total ATRC1 in HEK cells associated with the actin membrane cytoskeleton, whereas another one-fourth resided in detergent-resistant membranes (DRM). In agreement with these findings, cytochalasin D reduced the amount of ATRC1 associated with the actin membrane cytoskeleton. Although some ATRC1 clusters in HEK cells colocalized with caveolin, the majority of ATRC1 did not colocalize with this marker protein for a type of DRM called caveolae. This distribution of ATRC1 is somewhat different from that reported for pulmonary artery endothelial cells in which transporters cluster predominantly in caveolae, suggesting that differences in the proportion of ATRC1 in specific membrane microdomains correlate with differences in the physiological role of the transporter in polarized kidney epithelial vs. vascular endothelial cells.

Proteomic analysis of a detergent-resistant membrane skeleton from neutrophil plasma membranes

Plasma membranes are organized into functional domains both by liquid-ordered packing into "lipid rafts," structures that resist Triton extraction, and by attachments to underlying cytoskeletal proteins in assemblies called "membrane skeletons." Although the actin cytoskeleton is implicated in many lipid raft-mediated signaling processes, little is known about the biochemical basis for actin involvement. We show here that a subset of plasma membrane skeleton proteins from bovine neutrophils co-isolates with cholesterol-rich, detergent-resistant membrane fragments (DRMs) that exhibit a relatively high buoyant density in sucrose (DRM-H; d approximately 1.16 g/ml). By using matrix-assisted laser desorption/ionization time of flight and tandem mass spectrometry, we identified 19 major DRM-H proteins. Membrane skeleton proteins include fodrin (nonerythroid spectrin), myosin-IIA, myosin-IG, alpha-actinin 1, alpha-actinin 4, vimentin, and the F-actin-binding protein, supervillin. Other DRM-H components include lipid raft-associated integral membrane proteins (stomatin, flotillin 1, and flotillin 2), extracellular surface-bound and glycophosphatidylinositol-anchored proteins (IgM, membrane-type 6 matrix metalloproteinase), and intracellular dually acylated signaling proteins (Lyn kinase, Galpha(i-2)). Consistent with cytoskeletal association, most DRM-H-associated flotillin 2, Lyn, and Galpha(i-2) also resist extraction with 0.1 m octyl glucoside. Supervillin, myosin-IG, and myosin-IIA resist extraction with 0.1 m sodium carbonate, a treatment that removes all detectable actin, suggesting that these cytoskeletal proteins are proximal to the DRM-H bilayer. Binding of supervillin to the DRM-H fragments is confirmed by co-immunoaffinity purification. In spreading neutrophils, supervillin localizes with F-actin in cell extensions and in discrete basal puncta that partially overlap with Galpha(i) staining. We suggest that the DRM-H fraction represents a membrane skeleton-associated subset of leukocyte signaling domains.

CK2 constitutively associates with and phosphorylates chicken erythroid ankyrin and regulates its ability to bind to spectrin

Previous analyses have shown that the phosphorylation state of chicken erythroid ankyrin regulates its association with the spectrin cytoskeleton in vivo. Treatment of erythroid cells with serine and threonine phosphatase inhibitors stimulates the hyperphosphorylation of ankyrin and its dissociation from spectrin. In this study, we demonstrate that a kinase that directs the phosphorylation of ankyrin in vivo coprecipitates with ankyrin-containing complexes and has properties identical to CK2. Studies using CK2-specific inhibitors have indicated that all of the phosphorylation events associated with erythroid ankyrin in vivo are CK2 dependent. Furthermore, inhibitor studies combined with in vitro binding analyses have indicated that the phosphorylation of erythroid ankyrin by CK2 regulates its ability to associate with spectrin. Additional analyses revealed that CK2 coprecipitates with ankyrin-3-containing complexes isolated from Madin Darby canine kidney epithelial cells and phosphorylates this epithelial ankyrin isoform in vivo. These results are the first demonstration of a kinase constitutively associating with the ankyrin-spectrin cytoskeleton in erythroid and kidney epithelial cells. This association provides a mechanism for rapidly reorganizing the membrane cytoskeleton in these cell types through the phosphorylation of ankyrin.

Secretory pathway quality control operating in Golgi, plasmalemmal, and endosomal systems

Exportable proteins that have significant defects in nascent polypeptide folding or subunit assembly are frequently retained in the endoplasmic reticulum and subject to endoplasmic reticulum-associated degradation by the ubiquitin-proteasome system. In addition to this, however, there is growing evidence for post-endoplasmic reticulum quality control mechanisms in which mutant or non-native exportable proteins may undergo anterograde transport to the Golgi complex and post-Golgi compartments before intracellular disposal. In some instances, these proteins may undergo retrograde transport back to the endoplasmic reticulum with re-targeting to the endoplasmic reticulum-associated degradation pathway; in other typical cases, they are targeted into the endosomal system for degradation by vacuolar/lysosomal proteases. Such quality control targeting is likely to involve recognition of features more commonly expressed in mutant proteins, but may also be expressed by wild-type proteins, especially in cells with perturbation of local environments that are essential for normal protein trafficking and stability in the secretory pathway and at the cell surface.

Identification of the cofilin-binding sites in the large cytoplasmic domain of Na,K-ATPase

Na,K-ATPase, an alpha, beta heterodimer, is found in the plasma membrane of all animal cells. The alpha chain is believed to have 10 transmembrane regions and a large cytoplasmic domain between the 4th and 5th transmembrane regions (H4-H5). In our previous report, the large (3rd) cytoplasmic domains of the alpha1 and alpha2 isoform were found to interact with cofilin, an actin-modulating protein, by the yeast two-hybrid system. Here we show that cofilin interacts only with the 3rd cytoplasmic domain of the alpha2 subunit but not with the 2nd, 4th, and 5th cytoplasmic domains or the cytoplasmic region of the beta subunit of Na,K-ATPase. We also demonstrate that cofilin interacts with the large cytoplasmic domains of the alpha1, alpha2 and alpha3 isoforms of Na,K-ATPase, but not with those of glucose transporter 1, glucose transporter 4, cystic fibrosis transmembrane conductance regulator and plasma membrane Ca-ATPase. We introduced 10 mutations into the 3rd cytoplasmic domain of Na,K-ATPase to identify the binding sites with cofilin. Eight of these mutants were single amino acid substitutions (R417Q, K470Q, K654G, D672A, K691A, R700G, R700A and D710G) and two were double mutant (K654GR700G and K719AK720A). Analysis of the activity of the reporter gene of these mutants shows that residues D672 and R700 of the 3rd cytoplasmic domain of Na,K-ATPase are involved in the interaction with cofilin.

ELF a beta-spectrin is a neuronal precursor cell marker in developing mammalian brain; structure and organization of the elf/beta-G spectrin gene

Spectrins play a pivotal role in axonal transport, neurite extension, the organization of synaptic vesicles, as well as for protein sorting in the Golgi apparatus and cell membrane. Among spectrins there is great variability in sequence composition, tissue distribution, and function, with two known genes encoding the alpha-chain, and at least five encoding the beta-chain. It remains unclear as to whether novel beta-spectrins such as elf1-4 are distinct genes or beta-G-spectrin isoforms. The role for ELF in the developing nervous system has not been identified to date. In this study we demonstrate the genomic structure of elf-3, as well as the expression of ELF in the developing mouse brain using a peptide specific antibody against its distinctive amino-terminal end. Full genomic structural analyses reveal that elf-3 is composed of 31 exons spanning approximately 67 kb, and confirm that elf and mouse brain beta-G-spectrin share multiple exons, with a complex form of exon/intron usage. In embryonic stages, E9-12, anti-ELF localized to the primary brain vesicular cells that also labeled strongly with anti-nestin but not anti-vimentin. At E12-14, anti-ELF localized to axonal sprouts in the developing neuroblasts of cortex and purkinje cell layer of the cerebellum, as well as in cell bodies in the diencephalon and metencephalon. Double labeling identified significant co-localization of anti-ELF, nestin and dystrophin in sub ventricular zone cells and in stellate-like cells of the developing forebrain. These studies define clearly the expression of ELF, a new isoform of beta-G-spectrin in the developing brain. Based on its expression pattern, ELF may have a role in neural stem cell development and is a marker of axonal sprouting in mid stages of embryonic development.

Ion pumps in polarized cells: sorting and regulation of the Na+, K+- and H+, K+-ATPases

The physiologic function of an ion transport protein is determined, in part, by its subcellular localization and by the cellular mechanisms that modulate its activity. The Na(+),K(+)-ATPase and the H(+),K(+)-ATPases are closely related members of the P-type family of ion transporting ATPases. Despite their homology, these pumps are sorted to different domains in polarized epithelial cells, and their enzymatic activities are subject to distinct regulatory pathways. The molecular signals responsible for these properties have begun to be elucidated. It appears that a complex array of inter- and intramolecular interactions govern trafficking, distribution, and catalytic capacities of these proteins.