The sodium pump becomes a family

Na+/K+-ATPase: More than an Electrogenic Pump

The sodium pump, or Na+/K+-ATPase (NKA), is an essential enzyme found in the plasma membrane of all animal cells. Its primary role is to transport sodium (Na+) and potassium (K+) ions across the cell membrane, using energy from ATP hydrolysis. This transport creates and maintains an electrochemical gradient, which is crucial for various cellular processes, including cell volume regulation, electrical excitability, and secondary active transport. Although the role of NKA as a pump was discovered and demonstrated several decades ago, it remains the subject of intense research. Current studies aim to delve deeper into several aspects of this molecular entity, such as describing its structure and mode of operation in atomic detail, understanding its molecular and functional diversity, and examining the consequences of its malfunction due to structural alterations. Additionally, researchers are investigating the effects of various substances that amplify or decrease its pumping activity. Beyond its role as a pump, growing evidence indicates that in various cell types, NKA also functions as a receptor for cardiac glycosides like ouabain. This receptor activity triggers the activation of various signaling pathways, producing significant morphological and physiological effects. In this report, we present the results of a comprehensive review of the most outstanding studies of the past five years. We highlight the progress made regarding this new concept of NKA and the various cardiac glycosides that influence it. Furthermore, we emphasize NKA's role in epithelial physiology, particularly its function as a receptor for cardiac glycosides that trigger intracellular signals regulating cell-cell contacts, proliferation, differentiation, and adhesion. We also analyze the role of NKA β-subunits as cell adhesion molecules in glia and epithelial cells.

Polarized retinal pigment epithelium generates electrical signals that diminish with age and regulate retinal pathology

The transepithelial potential difference (TEP) across the retinal pigment epithelial (RPE) is dependent on ionic pumps and tight junction "seals" between epithelial cells. RPE cells release neurotrophic growth factors such as pigment epithelial derived factor (PEDF), which is reduced in age-related macular degeneration (AMD). The mechanisms that control the secretion of PEDF from RPE cells are not well understood. Using the CCL2/CX3CR1 double knockout mouse model (DKO), which demonstrates RPE damage and retinal degeneration, we uncovered an interaction between PEDF and the TEP which is likely to play an important role in retinal ageing and in the pathogenesis of AMD. We found that: (a) the expression of ATP1B1 (the Na+ /K+ -ATPase β1 subunit) was reduced significantly in RPE from aged mice, in patients with CNV (Choroidal Neovascularization) and in DKO mice; (b) the expression of PEDF also was decreased in aged persons and in DKO mice; (c) the TEP across RPE was reduced markedly in RPE cells from DKO mice and (d) an applied electric field (EF) of 50-100 mV/mm, used to mimic the natural TEP, increased the expression and secretion of PEDF in primary RPE cells. In conclusion, the TEP across the RPE depends on the expression of ATP1B1 and this regulates the secretion of PEDF by RPE cells and so may regulate the onset of retinal disease. Increasing the expression of PEDF using an applied EF to replenish a disease or age-reduced TEP may offer a new way of preventing or reversing retinal dysfunction.

The Apical Localization of Na+, K+-ATPase in Cultured Human Retinal Pigment Epithelial Cells Depends on Expression of the β2 Subunit

Na⁺, K⁺-ATPase, or the Na⁺ pump, is a key component in the maintenance of the epithelial phenotype. In most epithelia, the pump is located in the basolateral domain. Studies from our laboratory have shown that the β₁ subunit of Na⁺, K⁺-ATPase plays an important role in this mechanism because homotypic β₁-β₁ interactions between neighboring cells stabilize the pump in the lateral membrane. However, in the retinal pigment epithelium (RPE), the Na⁺ pump is located in the apical domain. The mechanism of polarization in this epithelium is unclear. We hypothesized that the apical polarization of the pump in RPE cells depends on the expression of its β₂ subunit. ARPE-19 cells cultured for up to 8 weeks on inserts did not polarize, and Na⁺, K⁺-ATPase was expressed in the basolateral membrane. In the presence of insulin, transferrin and selenic acid (ITS), ARPE-19 cells cultured for 4 weeks acquired an RPE phenotype, and the Na⁺ pump was visible in the apical domain. Under these conditions, Western blot analysis was employed to detect the β₂ isoform and immunofluorescence analysis revealed an apparent apical distribution of the β₂ subunit. qPCR results showed a time-dependent increase in the level of β₂ isoform mRNA, suggesting regulation at the transcriptional level. Moreover, silencing the expression of the β₂ isoform in ARPE-19 cells resulted in a decrease in the apical localization of the pump, as assessed by the mislocalization of the α₂ subunit in that domain. Our results demonstrate that the apical polarization of Na⁺, K⁺-ATPase in RPE cells depends on the expression of the β₂ subunit.

Signaling function of Na,K-ATPase induced by ouabain against LPS as an inflammation model in hippocampus

BACKGROUND

Ouabain (OUA) is a newly recognized hormone that is synthesized in the adrenal cortex and hypothalamus. Low doses of OUA can activate a signaling pathway by interaction with Na,K-ATPase, which is protective against a number of insults. OUA has central and peripheral anti-inflammatory effects. Lipopolysaccharide (LPS), via toll-like receptor 4 activation, is a widely used model to induce systemic inflammation. This study used a low OUA dose to evaluate its effects on inflammation induced by LPS injection in rats.

METHODS

Adult male Wistar rats received acute intraperitoneal (ip) OUA (1.8 μg/kg) or saline 20 minutes before LPS (200 μg/kg, ip) or saline injection. Some of the animals had their femoral artery catheterized in order to assess arterial blood pressure values before and after OUA administration. Na,K-ATPase activity, cytokine mRNA levels, apoptosis-related proteins, NF-κB activation brain-derived neurotrophic factor BDNF, corticosterone and TNF-α levels were measured.

RESULTS

OUA pretreatment decreased mRNA levels of the pro-inflammatory cytokines, inducible nitric oxide synthase (iNOS) and IL-1β, which are activated by LPS in the hippocampus, but with no effect on serum measures of these factors. None of these OUA effects were linked to Na,K-ATPase activity. The involvement of the inflammatory transcription factor NF-κB in the OUA effect was indicated by its prevention of LPS-induced nuclear translocation of the NF-κB subunit, RELA (p65), as well as the decreased cytosol levels of the NF-κB inhibitor, IKB, in the hippocampus. OUA pretreatment reversed the LPS-induced glial fibrillary acidic protein (GFAP) activation and associated inflammation in the dentate gyrus. OUA also prevented LPS-induced increases in the hippocampal Bax/Bcl2 ratio suggesting an anti-apoptotic action in the brain.

CONCLUSION

Our results suggest that a low dose of OUA has an important anti-inflammatory effect in the rat hippocampus. This effect was associated with decreased GFAP induction by LPS in the dentate gyrus, a brain area linked to adult neurogenesis.

Zebrafish as a novel model to assess Na+/K(+)-ATPase-related neurological disorders

Modeling neurological disorders using zebrafish increases rapidly as this model system allows easy access to all developmental stages and imaging of pathological processes. A surprising degree of functional conservation has been demonstrated between human genes implicated in neurodegenerative diseases and their zebrafish orthologues. Zebrafish offers rapid high throughput screening of therapeutic compounds and live imaging of pathogenic mechanisms in vivo. Several recent zebrafish studies functionally assessed the role of the sodium-potassium pump (Na(+)/K(+)-ATPase). The Na(+)/K(+)-ATPase maintains the electrochemical gradients across the plasma membrane, essential for e.g. signaling, secondary active transport, glutamate re-uptake and neuron excitability in animal cells. Na(+)/K(+)-ATPase mutations are associated with neurological disorders, where mutations in the Na(+)/K(+)-ATPase α2 and α3 isoforms cause Familial hemiplegic migraine type 2 (FHM2) and Rapid-onset dystonia-parkinsonism (RDP)/Alternating hemiplegic childhood (AHC), respectively. In zebrafish, knock-down of Na(+)/K(+)-ATPase isoforms included skeletal and heart muscle defects, impaired embryonic motility, depolarized Rohon-beard neurons and abrupt brain ventricle development. In this review, we discuss zebrafish as a model to assess Na(+)/K(+)-ATPase isoform functions. Furthermore, studies investigating proteomic changes in both α2- and α3-isoform deficient embryos and their potential connections to the Na(+)/K(+)-ATPase functions will be discussed.

RNAi reveals the key role of Nervana 1 in cockroach oogenesis and embryo development

Na(+), K(+)-ATPases is a heterodimer protein consisting of α- and β-subunits that control the ion transport through cell membranes. In insects the β-subunit of the Na(+), K(+)-ATPase, known as Nervana, was characterized as a nervous system-specific glycoprotein antigen from adult Drosophila melanogaster heads. Nervana is expressed ubiquitously in all insect tissues, and in epithelial cells appeared located in a basolateral position as part of the septate junctions. Herein we study two Nervana isoforms from Blattella germanica, a cockroach species with panoistic ovaries. The sequencing and the phylogenetic analysis results suggest that these two isoforms are orthologs of D. melanogaster Nervana 1 and Nervana 2, respectively. Nervana 1 is highly expressed in the ovary of B. germanica, and depleting its expression results in changes in oocyte shape that do not impair oviposition. However, the resulting embryos show different defects and never hatch. These findings highlight the importance of this type of membrane pump in insect oogenesis as well as in embryo development, and its possible regulation by juvenile hormone.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

Optical probing of neuronal circuit dynamics: genetically encoded versus classical fluorescent sensors

During the past few decades, optical methods for imaging activity in networks composed of thousands of neurons have been developed. These techniques rely mainly on organic-chemistry-based dyes as indicators of Ca(2+) and membrane potential. However, recently a new generation of probes, genetically encoded fluorescent protein sensors, has emerged for use by physiologists studying the operation of neuronal circuits. We critically review the development of these new probes, and analyze objectives and experimental conditions in which classical probes are likely to prevail and where the fluorescent protein sensors will open paths to previously unexplored territories of functional neuroimaging.

Sodium/potasium ATPase (Na+, K+-ATPase) and ouabain/related cardiac glycosides: a new paradigm for development of anti- breast cancer drugs?

Prolonged exposure to 17beta-estradiol (E2) is a key etiological factor for human breast cancer. The biological effects and carcinogenic effects of E2 are mediated via estrogen receptors (ERs), ERalpha and ERbeta. Anti-estrogens, e.g. tamoxifen, and aromatase inhibitors have been used to treat ER-positive breast cancer. While anti-estrogen therapy is initially successful, a major problem is that most tumors develop resistance and the disease ultimately progresses, pointing to the need of developing alternative drugs targeting to other critical targets in breast cancer cells. We have identified that Na+, K+-ATPase, a plasma membrane ion pump, has unique/valuable properties that could be used as a potentially important target for breast cancer treatment: (a) it is a key player of cell adhesion and is involved in cancer progression; (b) it serves as a versatile signal transducer and is a target for a number of hormones including estrogens and (d) its aberrant expression and activity are implicated in the development and progression of breast cancer. There are several lines of evidence indicating that ouabain and related digitalis (the potent inhibitors of Na+, K+-ATPase) possess potent anti-breast cancer activity. While it is not clear how the suggested anti-cancer activity of these drugs work, several observations point to ouabain and digitalis as being potential ER antagonists. We critically reviewed many lines of evidence and postulated a novel concept that Na+, K+-ATPase in combination with ERs could be important targets of anti-breast cancer drugs. Modulators, e.g. ouabain and related digitalis could be useful to develop valuable anti-breast cancer drugs as both Na+, K+-ATPase inhibitors and ER antagonists.

Na,K-ATPase Subunit Heterogeneity as a Mechanism for Tissue-Specific Ion Regulation

The Na,K-ATPase comprises a family of isozymes that catalyze the active transport of cytoplasmic Na+ for extracellular K+ at the plasma membrane of cells. Isozyme diversity for the Na,K-ATPase results from the association of different molecular forms of the alpha (alpha1, alpha2, alpha3, and alpha4) and beta (beta1, beta2, and beta3) subunits that constitute the enzyme. The various isozymes are characterized by unique enzymatic properties and a highly regulated pattern of expression that depends on cell type, developmental stage, and hormonal stimulation. The molecular complexity of the Na,K-ATPase goes beyond its alpha and beta isoforms and, in certain tissues, other accessory proteins associate with the enzyme. These small membrane-bound polypeptides, known as the FXYD proteins, modulate the kinetic characteristics of the Na,K-ATPase. The experimental evidence available suggests that the molecular and functional heterogeneity of the Na,K-ATPase is a physiologically relevant event that serves the specialized functions of cells. This article focuses on the functional properties, regulation, and the biological relevance of the Na,K-ATPase isozymes as a mechanism for the tissue-specific control of Na+ and K+ homeostasis.

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 Adhesion Molecule on Glia (AMOG) Is Widely Expressed by Astrocytes in Developing and Adult Mouse Brain

Adhesion molecule on glia (AMOG) is a 45 - 50 kD cell surface glycoprotein structurally similar to the Na, K-ATPase beta-subunit and associated with the catalytic subunit of this enzyme. Previous immunofluorescence results had suggested that AMOG is transiently expressed on Bergmann glia during mouse cerebellar development, and antibody-inhibition results have implicated it in the migration of granule neurons. We report that, while AMOG mRNA is detected in Bergmann glia during the migratory period, this astrocyte derivative continues to express AMOG mRNA at similar levels in adult mice suggesting a functional role for AMOG in adulthood. Evidence from RNA and protein blot analyses that AMOG is present before birth, increasing about ten fold in adult mouse brain and cerebellum is also provided. RNA blot analysis of astrocyte-enriched cell populations and in situ hybridization results show that astrocytes synthesize AMOG mRNA in all regions of the developing and adult brain. In the adult, AMOG mRNA is more abundant in grey than white matter and, among grey matter regions, highest in cerebellar cortex. These results indicate a relationship between density of neuronal elements and AMOG expression. It is further speculated that AMOG is part of a Na,K-ATPase complex expressed preferentially by astrocytes in mouse brain.

Endogenous digitalis-like ligands of the sodium pump: possible involvement in mood control and ethanol addiction

This review addresses possible involvement of endogenous digitalis-like sodium pump ligands (SPL) in the mood control and ethanol addiction. Endogenous SPL include cardenolide and bufadienolide classes. Multiple SPL and multiple isoforms of the Na/K-ATPase, one of the key membrane enzymes, comprise a complex regulatory system. In the nervous system, pattern of expression of Na/K-ATPase is based on multiple alpha/beta isoform combinations. Clinical studies demonstrate changes in the activity of Na/K-ATPase in patients with bipolar and unipolar mood disorders. The effects of ethanol on the Na/K-ATPase are concentration-dependent and are associated with both inhibition and activation of enzyme activity. Reinforcing effect of ethanol as well as its voluntary consumption may be affected by digitalis glycosides and endogenous SPL.

A hybrid between Na+,K+-ATPase and H+,K+-ATPase is sensitive to palytoxin, ouabain, and SCH 28080

Na(+),K(+)-ATPase is inhibited by cardiac glycosides such as ouabain, and palytoxin, which do not inhibit gastric H(+),K(+)-ATPase. Gastric H(+),K(+)-ATPase is inhibited by SCH28080, which has no effect on Na(+),K(+)-ATPase. The goal of the current study was to identify amino acid sequences of the gastric proton-potassium pump that are involved in recognition of the pump-specific inhibitor SCH 28080. A chimeric polypeptide consisting of the rat sodium pump alpha3 subunit with the peptide Gln(905)-Val(930) of the gastric proton pump alpha subunit substituted in place of the original Asn(886)-Ala(911) sequence was expressed together with the gastric beta subunit in the yeast Saccharomyces cerevisiae. Yeast cells that express this subunit combination are sensitive to palytoxin, which interacts specifically with the sodium pump, and lose intracellular K(+) ions. The palytoxin-induced K(+) efflux is inhibited by the sodium pump-specific inhibitor ouabain and also by the gastric proton pump-specific inhibitor SCH 28080. The IC(50) for SCH 28080 inhibition of palytoxin-induced K(+) efflux is 14.3 +/- 2.4 microm, which is similar to the K(i) for SCH 28080 inhibition of ATP hydrolysis by the gastric H(+),K(+)-ATPase. In contrast, palytoxin-induced K(+) efflux from cells expressing either the native alpha3 and beta1 subunits of the sodium pump or the alpha3 subunit of the sodium pump together with the beta subunit of the gastric proton pump is inhibited by ouabain but not by SCH 28080. The acquisition of SCH 28080 sensitivity by the chimera indicates that the Gln(905)-Val(930) peptide of the gastric proton pump is likely to be involved in the interactions of the gastric proton-potassium pump with SCH 28080.

Pretreatment with cationic lipid-mediated transfer of the Na+K+-ATPase pump in a mouse model in vivo augments resolution of high permeability pulmonary oedema

Resolution of pulmonary oedema is mediated by active absorption of liquid across the alveolar epithelium. A key component of this process is the sodium-potassium ATPase (Na+K+-ATPase) enzyme located on the basolateral surface of epithelial cells and up-regulated during oedema resolution. We hypothesised that lung liquid clearance could be further up-regulated by lipid-mediated transfer and expression of exogenous Na+K+-ATPase cDNA. We demonstrate proof of this principle in a model of high permeability pulmonary oedema induced by intraperitoneal injection of thiourea (2.5 mg/kg) in C57/BL6 mice. Pretreatment of mice (24 h before thiourea) by nasal sniffing of cationic liposome (lipid #67)-DNA complexes encoding the alpha and beta subunits of Na+K+-ATPase (160 microg per mouse), significantly (P<0.01) decreased the wet:dry weight ratios measured 2 h after thiourea injection compared with control animals, pretreated with an equivalent dose of an irrelevant gene. Whole lung Na+K+-ATPase activity was significantly (P<0.05) increased in mice pretreated with Na+K+-ATPase cDNA compared both with untreated control animals as well as animals pretreated with the irrelevant gene. Nested RT-PCR on whole lung homogenates confirmed gene transfer by detection of vector-specific mRNA in three of four mice studied 24 h after gene transfer. This demonstration of a significant reduction in pulmonary oedema following in vivo gene transfer raises the possibility of gene therapy as a novel, localised approach for pulmonary oedema in clinical settings such as ARDS and lung transplantation.

Gene expression regulating blastocyst formation

Development of embryos to the blastocyst stage is a critical event in the early lives of all eutherian mammalian species. Blastocyst formation is essential for implantation and is the principal morphological determinant of embryo quality prior to embryo transfer. The physiological events and roles of specific gene families that regulate blastocyst formation are subjects of intense research Recent findings have demonstrated that bovine embryos express multiple members of the Na/K-ATPase ion transporter gene family. Two members of this family have been co-localized to bovine trophectoderm, but each becomes largely confined to opposing cell membrane margins. Bovine blastocysts display a greater sensitivity to ouabain (potent inhibitor of the Na/K-ATPase) than murine blastocysts, and enzyme activity (ouabain sensitive 86Rb+ uptake) undergoes a 9-fold increase from the bovine morula to the blastocyst stage. Disruption of Na/K-ATPase gene expression by antisense oligodeoxynucleotide inhibition abolishes blastocyst formation. These results have implicated the Na/K-ATPase as a key regulator of bovine blastocyst formation and have provided insights necessary for the production of healthy bovine embryos by the application of in vitro maturation, in vitro fertilization and in vitro culture methods.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

Na/K-ATPase-mediated 86Rb+ uptake and asymmetrical trophectoderm localization of alpha1 and alpha3 Na/K-ATPase isoforms during bovine preattachment development

This study evaluated Na/K-ATPase alpha 1- and alpha 3-subunit isoform polypeptide expression and localization during bovine preattachment development. Na/K-ATPase cation transport activity from the one-cell to blastocyst stage was also determined by measuring ouabain-sensitive 86Rb+ uptake. Both alpha1- and alpha 3-subunit polypeptides were detected by immunofluorescence to encircle the entire cell margins of each blastomere of inseminated zygotes, cleavage stage embryos, and morulae. Immunofluorescent localization of alpha1-subunit polypeptide in bovine blastocysts revealed an alpha1 immunofluorescence signal confined to the basolateral membrane margins of the trophectoderm and encircling the cell periphery of each inner cell mass (ICM) cell. In contrast, alpha 3-subunit polypeptide immunofluorescence was localized primarily to the apical cell surfaces of the trophectoderm with a reduced signal present in basolateral trophectoderm regions. There was no apparent alpha 3-subunit signal in the ICM. Analysis of 86Rb+ transport in vitro demonstrated ouabain-sensitive activity throughout development from the one-cell to the six- to eight-cell stage of bovine development. 86Rb+ uptake by morulae (day 6 postinsemination) did not vary significantly from uptake detected in cleavage stage embryos; however, a significant increase was measured at the blastocyst stage (P < 0.05). Treatment of embryos with cytochalasin D (5 micrograms/ml) did not influence 86Rb+ uptake in cleavage stage embryos. Cytochalasin D treatment however was associated with a significant rise in ion transport in morulae and blastocysts (13.49 and 61.57 fmol/embryo/min, respectively) compared to untreated controls (2.65 and 22.83 fmol/embryo/min, respectively). Our results, for the first time, demonstrate that multiple Na/K-ATPase alpha-subunit isoforms are distributed throughout the first week of mammalian development and raise the possibility that multiple isozymes of the Na/K-ATPase contribute to blastocyst formation.

The electrogenic effects of Na(+)-K(+)-ATPase in rat auditory thalamus

1. The electrogenic effects of the Na(+)-K(+)-ATPase in thalamic neurones were investigated by means of intracellular and whole-cell patch-clamp recording techniques in rat medial geniculate body (MGB) maintained in vitro. 2. In twenty-six out of thirty-one neurones recorded intracellularly, application of the Na(+)-K+ pump inhibitor strophanthidin induced two different types of membrane depolarization: a small, reversible depolarization with a peak amplitude of 4 +/- 2.6 mV or a prolonged depolarization of large amplitude (48.6 +/- 9.0 mV) with or without a decrease in apparent membrane resistance. Blockade of glutamate receptors with kynurenic acid or 6-cyano-7-nitroquinoxaline-2,3-dione and (+/-)-2-amino-5-phosphonopentanoic acid did not prevent either type of pump response, but the large depolarization was not seen when the medium contained the sodium channel blocker TTX. 3. Whole-cell patch-clamp recording showed that the small membrane depolarization is mediated by an inward membrane current (39.00 +/- 5.70 pA) that exhibited a weak voltage dependence. An inward current of similar amplitude was also induced in MGB cells when the pipette solution contained nominally zero Na+ or when K+ was temporarily omitted from the extracellular medium. The large membrane depolarization or the corresponding membrane current was not observed in whole-cell conditions. 6. Transient inhibition of the electrogenic Na(+)-K(+)-ATPase consistently led to a change in the mode of synaptic transmission in MGB cells, during which the synaptically evoked burst response was either blocked or converted into a single spike discharge. 7. Taken together, these data suggest that blockade of the electrogenic pump produces a dual membrane effect in mammalian thalamic neurones: a small electrogenic membrane depolarization and a large depolarization response that can be prevented by artificially maintaining the transmembrane ionic gradients. The electrogenic activity of the Na(+)-K(+)-ATPase may play an important role in setting the mode of synaptic transmission in sensory thalamus.

Ouabain sensitivity and expression of Na/K-ATPase alpha- and beta-subunit isoform genes during bovine early development

The fluid movements that arise during blastocyst formation (cavitation) are, at least in part, driven by the Na/K-ATPase. In this study, the reverse transcriptase-polymerase chain reaction (RT-PCR) was used to survey bovine pre-attachment embryos for transcripts encoding known isoforms of the Na/K-ATPase alpha- and beta-subunits, including isoforms not previously detected during the first week of mammalian development. Transcripts encoding the Na-K-ATPase alpha 1, alpha 2, alpha 3 and beta 2 isoforms were detected throughout bovine preattachment development. This is the first indication that alpha 2, alpha 3 and beta 2 mRNAs are expressed during this early developmental interval. As in the mouse, beta 1-subunit transcripts were not detected until the morula stage and were also present in blastocysts. Thus, in two mammalian species an increase in abundance of beta 1 isoform transcripts in the morula stage is coincident with the onset of cavitation. Transcripts encoding the recently characterized alpha 4 isoform were not detected. The sensitivity of bovine blastocysts to ouabain (a potent inhibitor of Na/K-ATPase) was determined by assessing the ability of bovine blastocysts to recover in ouabain supplemental culture medium following cytochalasin-induced blastocyst collapse. Re-expansion of bovine blastocysts was inhibited in all ouabain concentrations down to 10(-9) M. Mouse blastocysts, in contrast, were sensitive to ouabain at or above 10(-3)M. These results have established that transcripts encoding multiple isoforms of both the alpha and beta subunits of the Na/K-ATPase are expressed throughout early bovine development and that bovine blastocysts display a greater sensitivity to ouabain than murine blastocysts. Future analysis will determine the possible individual and collective roles of these isoforms during blastocyst formation.

Expression of a Na,K-ATPase beta 3 subunit during development of the zebrafish central nervous system

Zebrafish beta 3, a full length cDNA clone encoding a zebrafish Na,K-ATPase beta subunit, was isolated. The protein shares highest homology with the beta 3 subunits of amphibians and mammals, slightly less homology with the beta 2 subunits, and is distinct from the beta 1 subunits. The fish beta subunit co-assembled with alpha subunits to form Na,K-ATPase enzymes when expressed in Xenopus oocytes. Embryonic expression was first detected by whole-mount in situ hybridization between 8-12 hr post-fertilization (hpf) in the head mesoderm. Subsequently, and up to 24 hpf, the mRNA was confined to four dorsal domains in the anterior neural tube. After a transient downregulation during the second day, expression was again conspicuous in the nervous system of 3-day-old larvae. Based on its distribution pattern, the fish beta subunit could be involved in setting up regional identities in the developing fish CNS and in the differentiation of distinct cell types.

Activity-induced internalization and rapid degradation of sodium channels in cultured fetal neurons

A regulatory mechanism for neuronal excitability consists in controlling sodium channel density at the plasma membrane. In cultured fetal neurons, activation of sodium channels by neurotoxins, e.g., veratridine and alpha-scorpion toxin (alpha-ScTx) that enhance the channel open state probability induced a rapid down-regulation of surface channels. Evidence that the initial step of activity-induced sodium channel down-regulation is mediated by internalization was provided by using 125I-alpha-ScTx as both a channel probe and activator. After its binding to surface channels, the distribution of 125I-alpha-ScTx into five subcellular compartments was quantitatively analyzed by EM autoradiography. 125I-alpha-ScTx was found to accumulate in tubulovesicular endosomes and disappear from the cell surface in a time-dependent manner. This specific distribution was prevented by addition of tetrodotoxin (TTX), a channel blocker. By using a photoreactive derivative to covalently label sodium channels at the surface of cultured neurons, we further demonstrated that they are degraded after veratridine-induced internalization. A time-dependent decrease in the amount of labeled sodium channel alpha subunit was observed after veratridine treatment. After 120 min of incubation, half of the alpha subunits were cleaved. This degradation was prevented totally by TTX addition and was accompanied by the appearance of an increasing amount of a 90-kD major proteolytic fragment that was already detected after 45-60 min of veratridine treatment. Exposure of the photoaffinity-labeled cells to amphotericin B, a sodium ionophore, gave similar results. In this case, degradation was prevented when Na+ ions were substituted by choline ions and not blocked by TTX. After veratridine- or amphotericin B-induced internalization of sodium channels, breakdown of the labeled alpha subunit was inhibited by leupeptin, while internalization was almost unaffected. Thus, cultured fetal neurons are capable of adjusting sodium channel density by an activity-dependent endocytotic process that is triggered by Na+ influx.

Channel activators reduce the expression of sodium channel alpha-subunit mRNA in developing neurons

The expression of rat brain sodium channel alpha-subunit (Na+I, Na+II and Na+III) and beta 1-subunit mRNAs was examined in rat fetal brain neurons in culture. A combined technique of reverse transcription and polymerase chain reaction (RT-PCR) was used. Two different PCR primer sets were designed to obtain simultaneous amplification of the three alpha-subunit mRNAs. All three molecules were detected in fetal neurons but the expression pattern (Na+III > Na+II > > Na+I) was different than that observed in adult tissue (Na+II > Na+I > Na+III). Expression of the beta 1-subunit mRNA was detected using a specific PCR primer set. Doublet bands were amplified, from fetal cells and adult brain mRNA. To get further insight into the molecular mechanism that underlie activity dependent plasticity of sodium channels, we studied the effect on the expression of sodium channel subunits mRNA of a 60 h incubation of cells in the presence of a scorpion neurotoxin that blocks channel inactivation. An overall decrease in the expression of all three alpha-subunit mRNAs was observed whereas the beta 1-subunit mRNA was unaffected by the same treatment. When cells were incubated with the scorpion neurotoxin together with tetrodotoxin, to block Na+ influx through channels, the decrease in mRNA expression was not observed. Finally, a 60 h continuous depolarization of cells induced by application of a high concentration KC1 solution did not mimic the effect of the scorpion toxin. These observations suggest that a persistent activation of the sodium channels is able to down-regulate mRNA expression for alpha-subunits but not for the beta 1-subunit.

Coupling of the Na+/Ca2+ exchanger, Na+/K+ pump and sarcoplasmic reticulum in smooth muscle

The Na+/Ca2+ exchanger, driven by a transmembrane Na+ gradient, plays a key role in regulating Ca2+ concentration in many cells. Although the exchanger influences Ca2+ concentration, its activity in smooth muscle appears to be closely coupled to Ca2+ availability from intracellular stores. This linkage might result if the exchanger were positioned close to Ca2+ storage sites within the sarcoplasmic reticulum. To test this hypothesis we have developed methods to assess the relative three-dimensional distribution of proteins involved in Na+/K+ pumping, Na+/Ca2+ exchange, Ca2+ storage within the sarcoplasmic reticulum, and attachment of contractile filaments to the membrane in smooth muscle. Here we report that the Na+/Ca2+ exchanger is largely co-distributed with the Na+/K+ pump on unique regions of the plasma membrane in register with, and close to, calsequestrin-containing regions of the sarcoplasmic reticulum in sites distinct from the sites where contractile filaments attach to the membrane. This molecular organization suggests that the plasma membrane is divided into at least two functional domains, and appear to provide a mechanism for the strong linkage seen in smooth muscle between Na+/K+ pumping and Na+/Ca2+ exchange, and between Na+/Ca2+ exchange and Ca2+ release from the sarcoplasmic reticulum.

Sequence analysis of DNA encoding an avian Na+,K(+)-ATPase beta 2-subunit

The DNA encoding a chicken Na+,K(+)-ATPase beta 2-subunit was cloned and sequenced. The deduced amino acid sequence has structural features common to all known Na+,K(+)-ATPase beta-subunits. It is proposed to belong to the beta 2-isoform family, though the amino acid sequence has significantly diverged from mammalian beta 2-subunit sequences. Similar to other Na+,K(+)-ATPase beta 2-isoforms, the chicken beta 2-isoform mRNA is predominantly expressed in brain tissue.

Control of the Na+,K(+)-ATPase under normal and pathological conditions

The Na+,K(+)-ATPase is an important enzyme in determining the ionic milieu of the cerebromicrovasculature and neurons. The effect of hypertension or aging on this enzyme, as well as its susceptibility to regulation by fatty acids or aluminum, is the focus of this study. A significant increase (34%) in the apparent affinity constant (KD) but no change in the maximum binding capacity (Bmax) for [3H]ouabain binding to the cerebromicrovascular Na+,K(+)-ATPase occurs after induction of acute hypertension. In addition, long chain unsaturated fatty acids stimulate the binding of [3H]ouabain to the enzyme in microvessels from normotensive and hypertensive rats. The synaptosomal Na+,K(+)-ATPase is sensitive to aluminum. AlCl3 (1-100 microM) inhibits the K(+)-dependent-p-nitrophenylphosphatase (K(+)-NPPase) activity of the Na+,K(+)-ATPase in a dose-dependent manner. AlCl3 (100 microM) decreases the Vmax by 14% but does not alter the KM, suggestive of non-competitive inhibition. The enzyme from aged brain displays a greater Vmax, but shows the same susceptibility to AlCl3 as the enzyme from younger brain. In summary, disruption of the Na+,K(+)-ATPase may underlie, at least in part, abnormalities of nerve and vascular cell function in disorders where elevated concentrations of fatty acids or metal ions are involved.

The cell biology of blastocyst development

Preimplantation development encompasses the "free"-living period of mammalian embryogenesis, which culminates in the formation of a fluid-filled structure, the blastocyst. Cavitation (blastocyst formation) is accompanied by the expression of a novel set of gene products that contribute directly to the attainment of cell polarity with the trophectoderm, which is both the first epithelium of development and the outer cell layer encircling the inner cell mass of the blastocyst. Several of these gene products have been identified and include the tight junction (ZO-1), Na/K-ATPase (alpha and beta subunits), uvomorulin, gap junction (connexin43), and growth factors such as transforming growth factor-alpha (TGF-alpha) and epidermal growth factor (EGF). This review will examine the role(s) of each of these gene products during the onset and progression of blastocyst formation. The trophectodermal tight junctional permeability seal regulates the leakage of blastocoel fluid and also assists in the maintenance of a polarized Na/K-ATPase distribution to the basolateral plasma membrane domain of the mural trophectoderm. The polarized distribution of the Na/K-ATPase plays an integral role in the establishment of a trans-trophectoderm Na+ gradient, which drives the osmotic accumulation of water across the epithelium into the nascent blastocoelic cavity. The cell adhesion provided by uvomorulin is necessary for the establishment of the tight junctional seal, as well as the maintenance of the polarized Na/K-ATPase distribution. Growth factors such as TGF-alpha and EGF stimulate an increase in the rate of blastocoel expansion, which could, in part, be mediated by secondary messengers that result in an increase in Na/K-ATPase activity. Insight into the mechanism of cavitation has, therefore, directly linked blastocyst formation to trophectoderm cell differentiation, which arises through fundamental cell biological processes that are directly involved in the attainment of epithelial cell polarity.

Loss and recovery of activities of alpha+ and alpha isozymes of (Na(+) + K+)-ATPase in cortical focal ischemia: GM1 ganglioside protects plasma membrane structure and function

Alterations in cellular membrane structure and the subsequent failure of its function after CNS ischemia were monitored by analyzing changes in the plasma membrane marker enzyme (Na(+) + K(+)-ATPase. The levels of two isozymes of (Na(+) + K(+)-ATPase, alpha+ and alpha, which have distinct cellular and anatomical distributions, were studied to determine if differential cellular damage occurs in primary and peri-ischemic injury areas. The efficacy of monosialoganglioside (GM1) treatment was assessed, since this glycosphingolipid has been shown to reduce ischemic injury by protecting cell membrane structure/function. Using a rat model of cortical focal ischemia, levels of both ATPase isozyme activities were assayed in total membrane fractions from primary ischemic tissue (parietal cortex) and three peri-ischemic tissue areas (frontal, occipital, and temporal cortex) at 1, 3, 5, 7, and 14 days after ischemia. No significant loss of either isozyme's activity occurred in any tissue area at 1 day after ischemia. At 5 days, in the primary ischemic area, both isozyme activity levels decreased by 70-75%. The alpha+ enzyme activity loss persisted up to 14 days, while a 17% recovery in alpha activity occurred. In the three peri-ischemic tissue areas, enzyme activity losses ranged from 42%-59% at 3 days after ischemia. A complete restoration of both isozyme activities was seen at 14 days. After three days of GM1 ganglioside treatment there was no loss of total (Na*+) + K(+)-ATPase activity in the three peri-ischemic areas, and a significantly reduced loss in the primary infarct tissue. An autoradiographic analysis of brain coronal sections using 3H-ouabain supports the enzymatic data and GM1 effects. Reductions in 3H-ouabain binding in all cortical layers at 3 days after ischemia were visualized. GM1 treatment significantly reduced these 3H-ouabain binding losses. In summary, time-dependent quantitative changes in activity levels of ATPase isozymes (alpha+ and alpha) reflect the different degree of membrane damage that occurs in primary vs. peri-ischemic tissues (e.g., irreversible vs. reversible membrane damage), and that ischemia affects cell membranes of all neural elements in a largely similar fashion. GM1 ganglioside was found to reduce plasma membrane damage in all CNS cell types.

Characterization of ouabain high-affinity binding to rat cerebral cortex. Modulation by melatonin

High-affinity [3H]ouabain binding to membrane preparations of rat cerebral cortex was examined using a rapid filtration procedure. At 37 degrees C, binding reached equilibrium in about 60 min. Scatchard analyses of the data at equilibrium revealed a single population of binding sites with a dissociation constant of KD = 3.1 +/- 0.36 nM and a binding site concentration of Bmax = 246.4 +/- 18.4 fmol/mg protein. Kinetic analyses of the association and dissociation curves indicated a kinetic KD = 4.6 nM, which is in good agreement with the value obtained at equilibrium. When various digitalis compounds were tested for their ability to inhibit [3H]ouabain binding, the following Ki values (nM) were obtained: ouabain (3.9); digoxin (18); acetyl-digitoxin (66); k-strophanthin (95); digitoxin (236). When melatonin was added to the incubation medium, the ability of ouabain to inhibit [3H]ouabain binding increased in a dose-related manner to yield the following Ki values (nM): melatonin 10 nM (2); melatonin 20 nM (1.2); melatonin 40 nM (0.8). These data suggest the existence in the rat cerebral cortex of high-affinity ouabain binding sites which may be a locus for the molecular action of melatonin.

Cloning of a cDNA encoding an Artemia franciscana Na/K ATPase alpha-subunit

Clones of cDNA that code for an isoform of the Artemia franciscana Na/K ATPase alpha subunit (NaKA alpha) have been isolated. The sequence of the longest of these clones (pArATNa136) is 3595 nucleotides; it codes for a 1004-amino acid protein whose sequence is identical to that of two previously sequenced Artemia NaKA alpha peptides. The encoded protein is over 73% identical to Drosophila melanogaster and vertebrate NaKA alpha s, and 73.8% identical to another Artemia NaKA alpha isoform previously described (named alpha 2850 in this article). The two Artemia cDNA clones code for mRNAs of different size; the clone pArATNa136 codes for a 4.5-kb mRNA while the alpha 2850 clone codes for a 3.6-kb mRNA. The degree of homology and the different size of the mRNAs encoded by both cDNAs suggest that they code for two different isoforms of the protein.

Sodium and potassium ATPase of the teleost fish Catostomus commersoni. Sequence, protein structure and evolutionary conservation of the alpha-subunit

The alpha-subunit of a Na+/K+ ATPase has been cloned by analysing a lambda gt11 library constructed from polyA+ RNA from the hypothalamic region of the teleost fish Catostomus commersoni (white sucker). The cDNA clone consists of 3853 bp and predicts a protein of 1027 amino-acid residues. Alignment of the sucker sequence with protein sequences previously published for alpha-subunits from various species reveals a high degree of homology throughout the entire sequence containing five potential sites for N-glycosylation, a phosphorylation site and a site for binding fluorescein 5'-isothiocyanate (FITC). A hydropathy profile predicts a secondary structure of the Na+/K+ ATPase alpha-subunit with at least eight membrane-spanning domains. Northern and southern blot analyses suggest the existence of two distinct Na+/K+ ATPase alpha-subunit genes in the sucker genome.

Partial purification and properties of (Na+ + K+)-ATPase from Potamon potamios

1. The tissue distribution of the (Na+ + K+)-ATPase in the freshwater/land crab Potamon Potamios was studied. 2. Gills were found to display the highest total activity in the whole animal (47%) but the highest specific activity was detected in the heart (15.15 mumol Pi/mg protein/min). 3. All other organs tested were found to have low enzyme activity. 4. The freshwater/land crab ATPase enzyme was inhibited by ouabain with a Ki of 0.5 mM.Km values for ATP, Mg2+ and K+ were 1.4, 4.0 and 1.2 mM respectively. The enzyme also showed a break in the Arrhenius plot at 23 degrees C. 5. A purification method of microsomal ATPase is described involving ultracentrifugation and electrofocusing.

Modulation of periodic cold receptor activity by ouabain

The effect of ouabain on the periodic discharge pattern of feline cold receptors was studied in order to substantiate a possible contribution of Na/K pump activity to signal transduction. Afferent activity was recorded from the cold fibres of an isolated preparation of the tongue. The periodic pattern consisted of beating activity and of grouped discharges and was characterized by two parameters, the oscillation frequency and the number of impulses initiated per cycle. Ouabain (10(-7)-10(-6)M) induced in all receptors excitatory responses, consisting of a short vigorous increase of activity followed by inhibition. Thus the receptors never stabilized to or maintained a new static level of activity. The ouabain-induced responses occurred repeatedly in several receptors and were produced by remarkable stereotyped modifications of both the oscillation frequency and the number of impulses per cycle. The oscillation frequency attained peak values which increased monotonically with higher static temperatures and which were considerably greater than peak control values. The data indicate that an electrogenic Na/K pump contributes to the transducer process of cold receptors and that inhibition of this pump evidently gives rise to a depolarizing imbalance of the membrane potential, accelerating the oscillation frequency to a maximum value. Thus the oscillation frequency seems to be controlled by temperature and by membrane potential in cold receptors.

Na,K-ATPase isoform expression in sheep red blood cell precursors

Isoform expression of mammalian red cell Na,K-ATPase was analyzed using messenger RNA isolated from red cell precursor-enriched bone marrow of anemic sheep. Expression of the catalytic alpha subunit was analyzed using rat isoform-specific cDNA probes and expression of the beta 1 subunit, using a sheep beta 1-specific cDNA probe. RNA isolated from sheep kidney and brain were analyzed concurrently. In the red cell, as in the kidney, messenger RNA encoding only one isoform (alpha 1) of the catalytic subunit is detected; neither of the other isoforms (alpha 2 or alpha 3) could be detected. This holds true for bone marrow of sheep of either the low potassium or high potassium phenotype. Relative to the expression of alpha 1, beta subunit-specific message (beta 1) was extremely low in the red cell compared to either kidney (less than 5%) or brain (less than 3%). Using a rat cDNA probe specific for a beta 1-like subunit, beta 2, message was detected in brain but not in either kidney or bone marrow.

The adhesion molecule on glia (AMOG) is a homologue of the beta subunit of the Na,K-ATPase

AMOG (adhesion molecule on glia) is a Ca2(+)-independent adhesion molecule which mediates selective neuron-astrocyte interaction in vitro (Antonicek, H., E. Persohn, and M. Schachner. 1987. J. Cell Biol. 104:1587-1595). Here we report the structure of AMOG and its association with the Na,K-ATPase. The complete cDNA sequence of mouse AMOG revealed 40% amino acid identity with the previously cloned beta subunit of rat brain Na,K-ATPase. Immunoaffinity-purified AMOG and the beta subunit of detergent-purified brain Na,K-ATPase had identical apparent molecular weights, and were immunologically cross-reactive. Immunoaffinity-purified AMOG was associated with a protein of 100,000 Mr. Monoclonal antibodies revealed that this associated protein comprised the alpha 2 (and possibly alpha 3) isoforms of the Na,K-ATPase catalytic subunit, but not alpha 1. The monoclonal AMOG antibody that blocks adhesion was shown to interact with Na,K-ATPase in intact cultured astrocytes by its ability to increase ouabain-inhibitable 86Rb+ uptake. AMOG-mediated adhesion occurred, however, both at 4 degrees C and in the presence of ouabain, an inhibitor of the Na,K-ATPase. Both AMOG and the beta subunit are predicted to be extracellularly exposed glycoproteins with single transmembrane segments, quite different in structure from the Na,K-ATPase alpha subunit or any other ion pump. We hypothesize that AMOG or variants of the beta subunit of the Na,K-ATPase, tightly associated with an alpha subunit, are recognition elements for adhesion that subsequently link cell adhesion with ion transport.

Cholinergic synaptic vesicles contain a V-type and a P-type ATPase

Fifty to eighty-five percent of the ATPase activity in different preparations of cholinergic synaptic vesicles isolated from Torpedo electric organ was half-inhibited by 7 microM vanadate. This activity is due to a recently purified phosphointermediate, or P-type, ATPase, Acetylcholine (ACh) active transport by the vesicles was stimulated about 35% by vanadate, demonstrating that the P-type enzyme is not the proton pump responsible for ACh active transport. Nearly all of the vesicle ATPase activity was inhibited by N-ethylmaleimide. The P-type ATPase could be protected from N-ethylmaleimide inactivation by vanadate, and subsequently reactivated by complexation of vanadate with deferoxamine. The inactivation-protection pattern suggests the presence of a vanadate-insensitive, N-ethylmaleimide-sensitive ATPase consistent with a vacuolar, or V-type, activity expected to drive ACh active transport. ACh active transport was half-inhibited by 5 microM N-ethylmaleimide, even in the presence of vanadate. The presence of a V-type ATPase was confirmed by Western blots using antisera raised against three separate subunits of chromaffin granule vacuolar ATPase I. Both ATPase activities, the P-type polypeptides, and the 38-kilodalton polypeptide of the V-type ATPase precisely copurify with the synaptic vesicles. Solubilization of synaptic vesicles in octaethyleneglycol dodecyl ether detergent results in several-fold stimulation of the P-type activity and inactivation of the V-type activity, thus explaining why the V-type activity was not detected previously during purification of the P-type ATPase. It is concluded that cholinergic vesicles contain a P-type ATPase of unknown function and a V-type ATPase which is the proton pump.

Purification and subunit composition of a cholinergic synaptic vesicle glycoprotein, phosphointermediate-forming ATPase

A glycoprotein ATPase in cholinergic synaptic vesicles of Torpedo electric organ was solubilized with octa-ethylene glycol dodecyl ether detergent. Study of potential stabilizing factors identified crude brain phosphatidylserine, glycerol, dithiothreitol, and protease inhibitors as of value in maintaining activity. The ATPase was purified from the solubilized, stabilized material by glycerol density gradient band sedimentation velocity ultracentrifugation, and hydroxylapatite, wheat germ lectin affinity, and size exclusion chromatographies. The pure ATPase had a specific activity of about 37 mumol ATP hydrolyzed/min/mg protein. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified material typically exhibited three polypeptides of molecular masses 110, 104, and 98 kilodaltons (kDa) and a fourth diffuse polypeptide of 60 kDa. This composition suggests that the ATPase is a member of the P-type, or phosphointermediate-forming, family, but it was shown to be distinct from the ouabain-sensitive Na+,K+- and CA2+-stimulated Mg2+-ATPases. The purified vesicle enzyme was rapidly phosphorylated by [gamma-32P]ATP on about 14% of the subunits with molecular weights of 98,000-110,000. About 16% of the ATPase was phosphorylated in whole-vesicle ghosts in a manner consistent with formation of a phosphointermediate, thus confirming the P-type nature of this enzyme.

Ouabain (or salt solution lacking potassium) mimics the effects of dark pulses on the circadian pacemaker in cultured chick pineal cells

Chick pineal cells in static culture display a persistent photosensitive circadian rhythm of melatonin production and release. Pulses of white light or darkness, in otherwise constant red light, induce phase shifts in subsequent cycles whose magnitude and direction depend on the phase at which the pulse is given. Such 'phase-dependent phase shifts' are mediated by effects on the underlying pacemaker. Here, we describe the effects of ouabain, a specific inhibitor of Na,K-ATPase, and of salt solutions lacking potassium (SS-K), which also inhibit the pump, on the melatonin rhythm. Pulses of ouabain, or of SS-K, induced phase advances and phase delays that were phase and concentration-dependent. The relationship between time of treatment and effect on the subsequent phase of the rhythm (the phase-response curve) for these treatments was essentially the same as that for pulses of darkness.

Advances in Na+,K+-ATPase studies: from protein to gene and back to protein

Complete primary structures of both subunits of Na+,K+-ATPase from various sources have been established by a combination of the methods for molecular cloning and protein chemistry. The gene family homologous to the alpha-subunit cDNA of animal Na+,K+-ATPases has been found in the human genome. Some genes of this family encode the known isoforms (alpha I and alpha II) of the Na+,K+-ATPase catalytic subunit. The proteins coded by other genes can be either new isoforms of the Na+,K+-ATPase catalytic subunit or other ion-transporting ATPases. Expression of the genes of this family proceeds in a tissue-specific manner and changes during the postnatal development and neoplastic transformation. The complete exon-intron structure of one of the genes of this family has been established. This gene codes for the form of the catalytic subunit, the existence of which has been unknown. Apparently, all the genes of the discovered family have a similar intron-exon structure. There is certain correlation between the gene structure and the proposed domain arrangement of the alpha-subunit. The results obtained have become the basis for the experiments which prove the existence of the earlier unknown alpha III isoform of the Na+,K+-ATPase catalytic subunit and have made possible the study of its function.