Tissue specificity, localization in brain, and cell-free translation of mRNA encoding the A3 isoform of Na+,K+-ATPase

Differential expression patterns of sodium potassium ATPase alpha and beta subunit isoforms in mouse brain during postnatal development

The sodium potassium ATPase (Na⁺/K⁺ ATPase) is essential for the maintenance of a low intracellular Na⁺ and a high intracellular K⁺ concentration. Loss of function of the Na⁺/K⁺ ATPase due to mutations in Na⁺/K⁺ ATPase genes, anoxic conditions, depletion of ATP or inhibition of the Na⁺/K⁺ ATPase function using cardiac glycosides such as digitalis, causes a depolarization of the resting membrane potential. While in non-excitable cells, the uptake of glucose and amino acids is decreased if the function of the Na⁺/K⁺ ATPase is compromised, in excitable cells the symptoms range from local hyper-excitability to inactivating depolarization. Although several studies have demonstrated the differential expression of the various Na⁺/K⁺ ATPase alpha and beta isoforms in the brain tissue of rodents, their expression profile during development has yet to be thoroughly investigated. An immunohistochemical analysis of postnatal day 19 mouse brain showed ubiquitous expression of Na⁺/K⁺ ATPase isoforms α1, β1 and β2 in both neurons and glial cells, whereas α2 was expressed mostly in glial cells and the α3 and β3 isoforms were expressed in neurons. Furthermore, we examined potential changes in the relative expression of the different Na⁺/K⁺ ATPase isoforms in different brain areas of postnatal day 6 and in adult 9 months old animals using immunoblot analysis. Our results show a significant up-regulation of the α1 isoform in cortex, hippocampus and cerebellum, whereas, the α2 isoform was significantly up-regulated in midbrain. The β3 isoform showed a significant up-regulation in all brain areas investigated. The up-regulation of the α3 isoform matched that of the β2 isoform which were both significantly up-regulated in cortex, hippocampus and midbrain, suggesting that the increased maturation of the neuronal network is accompanied by an increase in expression of α3/β2 complexes in these brain structures.

The Influence of Na(+), K(+)-ATPase on Glutamate Signaling in Neurodegenerative Diseases and Senescence

Decreased Na(+), K(+)-ATPase (NKA) activity causes energy deficiency, which is commonly observed in neurodegenerative diseases. The NKA is constituted of three subunits: α, β, and γ, with four distinct isoforms of the catalytic α subunit (α1-4). Genetic mutations in the ATP1A2 gene and ATP1A3 gene, encoding the α2 and α3 subunit isoforms, respectively can cause distinct neurological disorders, concurrent to impaired NKA activity. Within the central nervous system (CNS), the α2 isoform is expressed mostly in glial cells and the α3 isoform is neuron-specific. Mutations in ATP1A2 gene can result in familial hemiplegic migraine (FHM2), while mutations in the ATP1A3 gene can cause Rapid-onset dystonia-Parkinsonism (RDP) and alternating hemiplegia of childhood (AHC), as well as the cerebellar ataxia, areflexia, pescavus, optic atrophy and sensorineural hearing loss (CAPOS) syndrome. Data indicates that the central glutamatergic system is affected by mutations in the α2 isoform, however further investigations are required to establish a connection to mutations in the α3 isoform, especially given the diagnostic confusion and overlap with glutamate transporter disease. The age-related decline in brain α2∕3 activity may arise from changes in the cyclic guanosine monophosphate (cGMP) and cGMP-dependent protein kinase (PKG) pathway. Glutamate, through nitric oxide synthase (NOS), cGMP and PKG, stimulates brain α2∕3 activity, with the glutamatergic N-methyl-D-aspartate (NMDA) receptor cascade able to drive an adaptive, neuroprotective response to inflammatory and challenging stimuli, including amyloid-β. Here we review the NKA, both as an ion pump as well as a receptor that interacts with NMDA, including the role of NKA subunits mutations. Failure of the NKA-associated adaptive response mechanisms may render neurons more susceptible to degeneration over the course of aging.

Developmental expression analysis of Na, K-ATPase α subunits in Xenopus

Na, K-ATPase is an integral membrane protein complex responsible for maintaining the ionic gradients of Na(+) and K(+) across the plasma membrane and has a variety of cellular functions including neuronal activity. Studies in several organisms have shown that this protein complex regulates multiple aspects of embryonic development and is responsible for the pathogenesis of several human diseases. Here, we report the cloning and expression of Na, K-ATPase α2 (atp1a2) and α3 (atp1a3) subunits during Xenopus development and compare the expression patterns of each subunit. Using in situ hybridization in whole embryos and on sections, we show that all three α subunits are co-expressed in the pronephric kidney, with varying expression in neurogenic derivatives. The atp1a2 has a unique expression in the ependymal cell layer of the developing brain that is not shared with other α subunits. The Na, K-ATPase α1 (atp1a1), and atp1a3 share many expression domains in placode derivatives, including the otic vesicle, lens, ganglion of the anterodorsal lateral line nerve, and ganglia of the facial and anteroventral lateral line nerve and olfactory cells. All the subunits share a common expression domain, the myocardium.

Effects of ouabain on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rats and in decerebrate and arterially perfused in situ preparation from juvenile rats

The significance of Na/K-ATPase on respiratory rhythm generation is not well understood. We investigated the effects of the Na/K-ATPase blocker, ouabain, on respiratory rhythm. Experiments were performed with brainstem-spinal cord preparation from 0 to 3-day-old Wistar rats and with decerebrate and arterially perfused in situ preparation from juvenile rats (postnatal day 11-13). Newborn rat preparations were superfused at a rate of 3.0 ml/min with artificial cerebrospinal fluid, equilibrated with 95% O2 and 5% CO2, pH 7.4, at 26-27 °C. Inspiratory activity was monitored from the fourth cervical ventral root (C4). Application of ouabain (15-20 min) resulted in a dose-dependent increase in the burst rate of C4 inspiratory activity. After washout, the burst rate further increased to reach quasi-maximum values under each condition (e.g. 183% of control in 1 μM, 253% in 10 μM, and 303% in 20 μM at 30 min washout). Inspiratory or pre-inspiratory neurons in the rostral ventrolateral medulla were depolarized. We obtained similar results (i.e. increased phrenic burst rate) in an in situ perfused preparation of juvenile rats. Genes encoding the Na/K-ATPase α subunit were expressed in the region of the parafacial respiratory group (pFRG) in neonatal rats, suggesting that cells (neurons and/or glias) in the pFRG were one of the targets of ouabain. We concluded that Na/K-ATPase activity could be an important factor in respiratory rhythm modulation.

Presynaptic Ca2+ buffers control the strength of a fast post-tetanic hyperpolarization mediated by the alpha3 Na(+)/K(+)-ATPase

The excitability of CNS presynaptic terminals after a tetanic burst of action potentials is important for synaptic plasticity. The mechanisms that regulate excitability, however, are not well understood. Using direct recordings from the rat calyx of Held terminal, we found that a fast Na(+)/K(+)-ATPase (NKA)-mediated post-tetanic hyperpolarization (PTH) controls the probability and precision of subsequent firing. Notably, increasing the concentration of internal Ca(2+) buffers or decreasing Ca(2+) influx led to larger PTH amplitudes, indicating that an increase in [Ca(2+)](i) regulates PTH via inhibition of NKAs. The characterization for the first time of a presynaptic NKA pump current, combined with immunofluorescence staining, identified the alpha3-NKA isoform on calyx terminals. Accordingly, the increased ability of the calyx to faithfully fire during a high-frequency train as it matures is paralleled by a larger expression of alpha3-NKA during development. We propose that this newly discovered Ca(2+) dependence of PTH is important in the post-burst excitability of nerve terminals.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Xenopus Na,K-ATPase: primary sequence of the beta2 subunit and in situ localization of alpha1, beta1, and gamma expression during pronephric kidney development

The osmoregulatory function of the pronephric kidney, the first excretory organ of the vertebrate embryo, is essential for embryonic survival. The transport systems engaged in pronephric osmotic regulation are however poorly understood. The Na,K-ATPase is the key component in renal solute transport and water homeostasis. In the present study, we characterized the alpha, beta, and gamma subunits of the Na,K-ATPase of the developing Xenopus embryo. In addition to the known alpha1, beta1, beta3 and gamma subunits, we report here the identification of a novel cDNA encoding the Xenopus beta2 subunit. We demonstrate by in situ hybridization that each Xenopus Na,K-ATPase subunit exhibits a distinct tissue-specific and developmentally regulated expression pattern. We found that the developing pronephric kidney expresses alpha1, beta1, and gamma subunits uniformly along the entire length of the nephron. Onset of pronephric Na,K-ATPase subunit expression occurred in a coordinated fashion indicating that a common regulatory mechanism may initiate pronephric transcription of these genes. The ability to engage in active Na+ reabsorption appears to be established early in pronephric development, since Na,K-ATPase expression was detected well before the completion of pronephric organogenesis. Furthermore, Na,K-ATPase expression defines at the molecular level the onset of maturation phase during pronephric kidney organogenesis. Taken together, our studies reveal a striking conservation of Na,K-ATPase subunit expression between pronephric and metanephric kidneys. The pronephric kidney may therefore represent a simplified model to dissect the regulatory mechanisms underlying renal Na,K-ATPase subunit expression.

Expression, activity and distribution of Na,K-ATPase subunits during in vitro neuronal induction

The expression pattern of the alpha and beta isoforms and the gamma subunit of the Na,K-ATPase was investigated during in vitro induction of pluripotent murine embryonic stem (ES) cells into neuronal cells. alpha1 protein was expressed in undifferentiated ES (UES) cells and throughout all stages studied. In contrast, alpha3 protein was prominent only when neuronal cells have reached full differentiation. In this model, neuron-depleted cultures did not express the alpha3 isoform, indicating its specificity for mature neuronal cells. UES possessed Na,K-ATPase activity consistent with a single isoform (alpha1), whereas in fully mature neuronal cells a ouabain-sensitive isoform (alpha3) accounted for 27+/-4% of the activity, and a ouabain-resistant isoform (alpha1) 66+/-3%. Immunocytochemistry of mature neuronal cells for alpha1 and alpha3 proteins showed a similar distribution, including cell soma and processes, without evidence of polarization. beta1 protein was expressed in uninduced ES, embryonic bodies (EB) and neuronal cells. While proteins of the beta2 and beta3 isoforms were not detected by immunoblots (except for beta2 in UES), their mRNAs were detected in UES and EB (beta2 and beta3), and in immature and fully differentiated neuronal cells (beta3). Message for the beta2 isoform, however, was not present in neuronal cells. gamma subunit mRNA and protein were undetectable at any stage. These results provide further characterization of neuron-like cells obtained by induction of ES cells in vitro, and establish a model for the expression of isoforms of the Na,K-ATPase during neuronal differentiation. The relation to other aspects of neuronal cell development and relevance to a specialised function for the alpha3 subunit in neurons are discussed.

Expression of the beta2-subunit and apical localization of Na+-K+-ATPase in metanephric kidney

During kidney organogenesis, the Na+-K+-ATPase pump is not restricted to the basolateral plasma membrane of the renal epithelial cell but is instead either localized to the apical and lateral membrane sites of the early nephron or expressed in a nonpolarized distribution in the newly formed collecting ducts. The importance of Na+-K+-ATPase beta-subunit expression in the translocation of the Na+-K+-ATPase to the plasma membrane raises the question as to which beta-subunit isoform is expressed during kidney organogenesis. Immunocytochemical, Western analysis and RNase protection studies showed that both beta2-subunit protein and beta2 mRNA are expressed in the early gestation to midgestation human metanephric kidney. In contrast, although beta1 mRNA abundance is equivalent to that of the beta2-subunit in the metanephric kidney, the beta1-subunit protein was not detected in early to midgestation metanephric kidney samples. Immunocytochemical analysis revealed that both alpha1- and beta2-subunits were present in the apical epithelial plasma membranes of distal nephron segments of early stage nephrons, maturing loops of Henle, and collecting ducts during kidney development. We also detected a significant increase in alpha1 and beta1 mRNA after birth with a marked reduction in beta2 mRNA abundance associated with an increase in alpha1- and beta1-subunit proteins and loss of beta2 protein expression. These studies support the conclusion that the expression of the beta2-subunit in the fetal kidney may be an important mechanism controlling polarization of the Na+-K+-ATPase pump in the epithelia of the developing nephron during kidney organogenesis.

Differential regulation of Na,K-ATPase isozymes by protein kinases and arachidonic acid

While several studies have investigated the regulation of the Na, K-ATPase consisting of the alpha1 and beta1 subunits, there is little evidence that intracellular messengers influence the other Na pump isozymes. We studied the effect of different protein kinases and arachidonic acid on the rat Na,K-ATPase isoforms expressed in Sf-9 insect cells. Our results indicate that PKA, PKC, and PKG are able to differentially modify the function of the Na,K-ATPase isozymes. While PKC activation leads to inhibition of all isozymes, PKA activation stimulates the activity of the Na,K-ATPase alpha3 beta1 and decreases that of the alpha1 beta1 and alpha2 beta1 isozymes. In contrast, activation of PKG diminishes the activity of the alpha1 beta1 and alpha3 beta1 isozymes, without altering that of alpha2 beta1. Treatment of cells with arachidonic acid reduced the activities of all the isozymes. The changes in the catalytic capabilities of the Na pump isozymes elicited by PKA and PKC are reflected by changes in the molecular activity of the Na,K-ATPases. One of the mechanisms by which PKA and PKC affect Na pump isozyme activity is through direct phosphorylation of the alpha subunit. In the insect cells, we found a PKA- and PKC-dependent phosphorylation of the alpha1, alpha2 and alpha3 polypeptides. In conclusion, several intracellular messengers are able to modulate the function of the Na,K-ATPase isozymes and some of them in a specific fashion. Because the Na,K-ATPase isozymes have kinetic properties that are unique, this isozyme-specific regulation may be important in adapting Na pump function to the requirements of each cell.

The gamma subunit of the Na,K-ATPase induces cation channel activity

The gamma subunit of the Na,K-ATPase is a hydrophobic protein of approximately 10 kDa. The gamma subunit was expressed in Sf-9 insect cells and Xenopus oocytes to ascertain its role in Na,K-ATPase function. Immunoblotting has shown that the gamma subunit is expressed in Sf-9 cells infected with recombinant baculovirus containing the cDNA for the human gamma subunit. Confocal microscopy demonstrates that the gamma subunit can be delivered to the plasma membrane of Sf-9 cells independently of the other Na,K-ATPase subunits and that gamma colocalizes with alpha1 when these proteins are coexpressed. When Sf-9 cells were coinfected with alpha1 and gamma, antibodies to the gamma subunit were able to coimmunoprecipitate the alpha1 subunit, suggesting that gamma is able to associate with alpha1. The gamma subunit is a member of a family of single-pass transmembrane proteins that induces ion fluxes in Xenopus oocytes. Evidence that the gamma subunit is a functional component was supported by experiments showing gamma-induced cation channel activity when expressed in oocytes and increases in Na+ and K+ uptake when expressed in Sf-9 cells.

Cytochemical localization of ouabain-sensitive, K(+)-dependent p-nitrophenylphosphatase activity in the facial nerve of reserpinized guinea pigs

Ion-transporting Na,K-ATPase plays an essential role in nerve conduction. To clarify the cytochemical effects of reserpine on transport Na,K-ATPase activity, the localization of ouabain-sensitive, K(+)-dependent p-nitrophenylphosphatase (K-NPPase) activity was investigated in the facial nerves of normal and reserpinized guinea pigs using a cerium-based method. In the normal facial nerve, the reaction product of K-NPPase activity was observed on the internodal axolemma and Schmidt-Lanterman incisures. In the Ranvier nodes, enzyme activity was localized to the paranodal and nodal axolemma. In the reserpinized nerves, reaction product was detectable on the nodal axolemma but was undetectable on the other parts of the axolemma. Nodal K-NPPase was not affected by reserpine treatment. Therefore, the transport Na,K-ATPase on the nodal axolemma might differ from that on the other parts of the axolemma. Allowing reserpinized animals to survive. Two different ouabain-sensitive K-NPPase reactivities, "reserpine-sensitive" and "reserpine-resistant," might be present in the facial nerve of guinea pigs.

Parallel detection of Na,K-ATPase alpha subunit isoforms by pan-specific monoclonal mAb 9A7

While emphasis has been placed upon those proteins which either mediate or respond to the rapid influx of calcium following depolarization, there has been little emphasis upon those proteins which aid in the reequilibration of the membrane potential. In an effort to identify presynaptic membrane proteins implicated in neurosecretion, monoclonal antibodies were screened against proteins which cosegregated with neuronal voltage-dependent calcium channels (VDCC) following immunoprecipitation. One monoclonal antibody (mAb 9A7) identified a 110-kDa protein. Micropeptide sequencing of (i) the mAb 9A7 immunoaffinity purified antigen and (ii) the 110-kDa protein present in the neuronal (N-type) VDCC preparation (McEnery et al., 1991, Proc. Natl. Acad. Sci. 88, 11095-11099) indicated identity with the alpha subunit(s) of the Na,K-ATPase. Further characterization by Western blotting, immunochemical localization, and immunoaffinity purification indicated that mAb 9A7 not only recognized the alpha3 isoform which is predominant in neuronal tissues but also identified the alpha1 and alpha2 isoforms. mAb 9A7 exhibited a wide cross-species reactivity and recognized human, rat, and mouse alpha subunit isoforms at an internal epitope. The pan-specificity of mAb 9A7 and the differential mobility of the alpha1 isoform relative to the alpha2 and alpha3 permitted parallel detection of multiple alpha isoforms. Western blot analysis of undifferentiated rat pheochromocytoma cell line (PC12) and human neuroblastoma (IMR32) cells indicated coexpression of the alpha1 and alpha3 isozymes. Upon differentiation of IMR32 cells by dibutrylyl-cAMP, a substantial increase in the alpha3 relative to the alpha1 isoform was observed. While the enrichment of total Na,K-ATPase may reflect the increased demand for ATP-dependent ion transport as IMR32 cells become more excitable, the specific increase in the alpha3 isoform suggests a unique role of this isoform during IMR32 cell differentiation.

Relevance of Na,K-ATPase to local extracellular potassium homeostasis and modulation of synaptic transmission

The ion gradients generated by the Na,K-ATPase are essential for Na+-coupled transport systems, osmoregulation and restoration of ion concentrations in excitable tissues. Indirectly, the sodium pump controls intracellular Ca2+ concentration through the Na/Ca exchanger. In the nervous system various neurotransmitters can modulate Na,K-ATPase activity. The great diversity of Na,K-ATPase subunit isoforms, their complex spatial and temporal regulation of expression and their cellular localisation imply a functional role of the sodium pump in different regulatory pathways. Among these, potassium homeostasis and modulation of synaptic transmission are discussed here.

Isoforms of Na,K-ATPase alpha and beta subunits in the rat cerebellum and in granule cell cultures

There are multiple isoforms of the Na,K-ATPase in the nervous system, three isoforms of the alpha subunit, and at least two of the beta subunit. The alpha subunit is the catalytic subunit. The beta subunit has several roles. It is required for enzyme assembly, it has been implicated in neuron-glia adhesion, and the experimental exchange of beta subunit isoforms modifies enzyme kinetics, implying that it affects functional properties. Here we describe the specificities of antibodies against the Na,K-ATPase beta subunit isoforms beta1 and beta2. These antibodies, along with antibodies against the alpha subunit isoforms, were used to stain sections of the rat cerebellum and cultures of cerebellar granule cells to ascertain expression and subcellular distribution in identifiable cells. Comparison of alpha and beta isoform distribution with double-label staining demonstrated that there was no preferential association of particular alpha subunits with particular beta subunits, nor was there an association with excitatory or inhibitory neurotransmission modes. Isoform composition differences were seen when Purkinje, basket, and granule cells were compared. Whether beta1 and beta2 are specific for neurons and glia, respectively, has been controversial, but expression of both beta subunit types was seen here in granule cells. In rat cerebellar astrocytes, in sections and in culture, alpha2 expression was prominent, yet the expression of either beta subunit was low in comparison. The complexity of Na,K-ATPase isoform distribution underscores the subtlety of its regulation and physiological role in excitable cells.

Isoforms of Na,K-ATPase alpha and beta subunits in the rat cerebellum and in granule cell cultures

There are multiple isoforms of the Na,K-ATPase in the nervous system, three isoforms of the alpha subunit, and at least two of the beta subunit. The alpha subunit is the catalytic subunit. The beta subunit has several roles. It is required for enzyme assembly, it has been implicated in neuron-glia adhesion, and the experimental exchange of beta subunit isoforms modifies enzyme kinetics, implying that it affects functional properties. Here we describe the specificities of antibodies against the Na,K-ATPase beta subunit isoforms beta1 and beta2. These antibodies, along with antibodies against the alpha subunit isoforms, were used to stain sections of the rat cerebellum and cultures of cerebellar granule cells to ascertain expression and subcellular distribution in identifiable cells. Comparison of alpha and beta isoform distribution with double-label staining demonstrated that there was no preferential association of particular alpha subunits with particular beta subunits, nor was there an association with excitatory or inhibitory neurotransmission modes. Isoform composition differences were seen when Purkinje, basket, and granule cells were compared. Whether beta1 and beta2 are specific for neurons and glia, respectively, has been controversial, but expression of both beta subunit types was seen here in granule cells. In rat cerebellar astrocytes, in sections and in culture, alpha2 expression was prominent, yet the expression of either beta subunit was low in comparison. The complexity of Na,K-ATPase isoform distribution underscores the subtlety of its regulation and physiological role in excitable cells.

Location and effect of obesity on putative anorectic binding sites in the rat brain

Anorectic drugs such as mazindol bind to a class of low-affinity, sodium-sensitive sites in the brain which are affected by ambient glucose concentrations and a predisposition to develop diet-induced obesity (DIO). This study used quantitative autoradiography of 10 nM 3H-mazindol binding to identify the cellular location of these putative anorectic binding sites in the brain and to assess the way in which the development of DIO affected their binding. We previously showed that chow-fed, obesity-prone rats have widespread increases in brain 3H-mazindol binding to these low-affinity sites as compared with diet-resistant (DR) rats. Here, low-affinity 3H-mazindol binding was assessed in the brains of eight rats which developed DIO vs. eight which were DR after three months on a high-energy diet. DIO rats gained 89% more weight and had 117% higher plasma insulin levels but no difference in plasma glucose levels compared with DR rats. Along with these differences, low-affinity 3H-mazindol binding in DIO rats was identical to that in DR rats in all of the 23 brain areas assessed. This suggested that this binding was downregulated by the development of obesity in DIO rats. In other chow-fed rats, stereotaxic injections of 5,7-dihydroxytryptamine and 6-hydroxydopamine (6OHDA) to ablate serotonin and catecholamine nerve terminals in the ventromedial nucleus of the hypothalamus (VMN) had no effect on 3H-mazindol binding. However, ibotenic acid injected into the VMN, substantia nigra, pars reticulata, and pars compacta destroyed intrinsic neurons and/or their local processes and decreased low-affinity 3H-mazindol binding by 13%-22%. Destruction of dopamine neurons in the substantia nigra, pars compacta, and noradrenergic neurons in the locus ceruleus with 6OHDA also reduced 3H-mazindol binding in those areas by 9% and 12%, respectively. This suggested that up to 22% of putative anorectic binding sites may be located on the cell bodies of dopamine, norepinephrine, and other neurons, but not on serotonin or catecholamine nerve terminals in the brain. Binding to these sites may be downregulated by the development of DIO, possibly as a result of the concomitant hyperinsulinemia.

Translational regulation of Na,K-ATPase alpha1 and beta1 polypeptide expression in epithelial cells

To investigate the regulation of the Na,K-ATPase, we have studied the expression of the Na,K-ATPase polypeptides in several mammalian cell lines using the vaccinia virus/T7 RNA polymerase expression system. Infection of several fibroblast-like cell lines with viral recombinants containing the Na,K-ATPase alpha and beta isoforms, the glucose transporters, GLUT 1 and GLUT 4, or the capsid protein of the Sindbis virus all result in the production of the appropriate protein products. However, all epithelial cell lines tested fail to synthesize the Na,K-ATPase viral recombinants, yet they efficiently express the other virally directed polypeptides. While Madin-Darby canine kidney (MDCK) epithelial cells infected with the Na,K-ATPase alpha1 or beta1 recombinant viruses produce both mRNAs, the messages are inefficiently translated. Furthermore, the RNA from infected MDCK cells does not direct the in vitro synthesis of the beta1 polypeptide, whereas the message from infected fibroblast-like BSC 40 cells is efficiently translated both in vivo and in vitro. Moreover, the synthesis of the H,K-ATPase alpha subunit is also limited in MDCK cells, although the H,K-ATPase beta subunit is efficiently expressed. Expression of chimeras constructed between the Na+ pump beta1 isoform and the H,K-ATPase beta subunit indicates that sequences in the 5' coding region of the beta1 message have an inhibitory effect; however, the stringent translational regulation of the beta1 isoform in MDCK cells requires the 5' and 3' regions of the coding sequence. The ability of the polarized cell lines to limit the synthesis of the Na+ pump polypeptides while expressing other vaccinia recombinants at high levels suggests that the polarized cells possess a stringent mechanism for the specific translational regulation of a select set of messages.

Differential effects of hypothyroidism on Na-K-ATPase mRNA alpha isoforms in the developing rat brain

In the developing rat cerebrum, the level of different isoforms of Na-K-ATPase mRNA increases significantly during the first three postnatal weeks, which represent the critical period of synaptogenesis and myelination-the two thyroid hormone-sensitive maturational events. To determine the possible functional relationship of these isoforms with maturational events in the developing brain and their mode of regulation by T3, we have examined the effect of hypothyroidism on the expression of the different alpha-isoforms (alpha 1, alpha 2, and alpha 3) of Na-K-ATPase mRNA covering the first 3 wk of postnatal development. Quantitation of these mRNAs from cerebra of 1-, 5-, 10-, 15-, and 20-d-old normal and hypothyroid rats by Northern blot analysis indicate that alpha 3 mRNA is not only predominantly expressed throughout this entire period of study but also represents the species which is most severely affected in the hypothyroid brain. The relative sensitivity for the expression of these mRNAs to T3 were alpha 3 > alpha 1 > alpha 2. These results, together with the report of predominant expression of the alpha 3 isoform in neuronal cells, suggest specific functional involvement of this isoform with the decisive maturational events in the rat brain. Kinetic studies on in vivo induction of Na-K-ATPase alpha-mRNAs by T3 in the 15-d-old hypothyroid rat shows clear stimulation of all the isoforms within 1 h of the administration of the optimal dose (200 micrograms T3/100 g body wt) suggesting a direct, possibly transcriptional effect of the hormone on the expression of these genes.

Na, K-ATPase isoform gene expression in normal and hypertrophied dog heart

OBJECTIVES

The catalytic alpha subunit of the sodium-potassium ATPase, the target of digitalis glycosides, has three isoforms; the expression of these isoforms is tissue-specific and developmentally regulated. While the effect of pressure overload on Na, K-ATPase isoform expression has been studied in rodent heart, there are no systematic data on this question in hearts of larger animals, which differ from those of rodents both in isoform composition and in glycoside sensitivity. Thus, we investigated the expression of Na, K-ATPase isoforms in normal dog heart; we also examined the effect of experimental left ventricular hypertrophy on isoform expression.

METHODS

hypertrophy was produced by aortic banding. Expression was assessed by quantitative Northern and Western blotting, immunofluorescence, and 3H-ouabain binding.

RESULTS

RNA blotting indicated that the alpha 3 isoform represented 11% of Na, K-ATPase mRNA in normal dog LV. Normal dog LV expressed alpha 1 and alpha 3 protein, but no detectable alpha 2; immunoreactive alpha 1 and alpha 3 protein were also present in Purkinje fibers. There was a statistically significant decrease in total expression of all alpha isoform mRNA's in hypertrophied dog LV, resulting in a greater proportion of alpha 1. The expression level of the alpha 3 isoform mRNA and protein was lower in hypertrophied hearts.

CONCLUSIONS

These results indicate a greater proportion of alpha 1 isoform pumps in experimental canine hypertrophy. Thus, shifts in NA, K-ATPase isoforms occur in pressure-overloaded heart in large animals as well as rodents.

Comparison of the enzymatic properties of the Na,K-ATPase alpha 3 beta 1 and alpha 3 beta 2 isozymes

The coexpression of multiple isoforms of the alpha and beta subunits of the Na,K-ATPase in mammalian tissues gives rise to the complex molecular heterogeneity that characterizes the Na pump. The expression of the different Na,K-ATPase isoforms in insect cells using recombinant baculoviruses represents a useful system for the analysis of Na,K-ATPase isoform function. In the present study, we use this system to direct the expression of the rat Na,K-ATPase alpha 3 beta 1 and alpha 3 beta 2 in sf-9 cells, a cell line derived from the ovary of the fall armyworm, Spodoptera frugiperda. The association of alpha 3 with either beta 1 or beta 2 results in catalytically competent Na,K-ATPase isozymes. Analysis of the kinetic characteristics of these enzymes demonstrates that the accompanying beta subunit isoform does not drastically affect the properties of the alpha 3 polypeptide. This is evidenced by the similar turnover numbers, apparent affinities for K+ and ATP, and the comparable high sensitivity to ouabain exhibited by both isozymes. The kinetic dependence on Na+, however, is different for both isozymes, with alpha 3 beta 2 displaying a 1.6-fold higher apparent affinity for the cation than alpha 3 beta 1. Comparison with other Na,K-ATPase isozymes shows that the apparent Na+ affinity of alpha 3 beta 2 is similar to that of the alpha 1 beta 1 Na pump widely expressed in every tissue; nevertheless, its reactivity toward K+, ATP, and ouabain are characteristic of the alpha 3 isoform. The most pronounced kinetic differences in Na,K-ATPase function are a result of variations in alpha isoform composition.(ABSTRACT TRUNCATED AT 250 WORDS)

The presence of both negative and positive elements in the 5'-flanking sequence of the rat Na,K-ATPase alpha 3 subunit gene are required for brain expression in transgenic mice

The Na,K-ATPase is an integral plasma membrane protein consisting of alpha and beta subunits, each of which has discrete isoforms expressed in a tissue-specific manner. Of the three functional alpha isoform genes, the one encoding the alpha 3 isoform is the most tissue-restricted in its expression, being found primarily in the brain. To identify regions of the alpha 3 isoform gene that are involved in directing expression in the brain, a 1.6 kb 5'-flanking sequence was attached to a reporter gene, chloramphenicol acetyltransferase (CAT). The alpha 3-CAT chimeric gene construct was microinjected into fertilized mouse eggs, and transgenic mice were produced. Analysis of adult transgenic mice from different lines revealed that the transgene is expressed primarily in the brain. To further delineate regions that are needed for conferring expression in this tissue, systematic deletions of the 5'-flanking sequence of the alpha 3-CAT fusion constructs were made and analyzed, again using transgenic mice. The results from these analyses indicate that DNA sequences required for mediating brain-specific expression of the alpha 3 isoform gene are present within 210 bp upstream of the transcription initiation site. alpha 3-CAT promoter constructs containing scanning mutations in this region were also assayed in transgenic mice. These studies have identified both a functional neural-restrictive silencer element as well as a positively acting cis element.

Immunodetection and enzymatic characterization of the alpha 3-isoform of Na,K-ATPase in dog heart

The expression of the canine alpha 2 and 3 subunit isoenzymes of NA,K-ATPase has been investigated in plasma membranes isolated from dog heart, brain and kidney by immunoblotting, employing polyclonal anti rat fusion protein, and enzymological techniques. Western blot analysis revealed with purified dog membrane Na,K-ATPase preparations, one immunoreactive signal with rat specific alpha 3 antisera in cardiac tissues, and two immunoreactive signals with rat alpha 2 and alpha 3 antisera in cerebral tissues. These findings suggested the specific expression of alpha 3 polypeptide in dog heart (99 kDa), whereas dog brain expressed the alpha 2 and 3 polypeptides. The stained bands were superimposed. The antibody to rat brain alpha 1 fusion protein did not cross-react with dog antigens whatever the three tissues tested. Expression of the alpha 3-subunit isoform in dog heart membranes was consistent with a high affinity digitoxigenin-sensitive class of Na,K-ATPase (IC50 = 7 +/- 2 nM). A single component with low affinity to digitoxigenin (IC50 = 110 +/- 10 nM) characterized the alpha 1 kidney form. The mixture of alpha 2 and alpha 3 isoforms in dog brain exhibited an apparent affinity for digitoxigenin (IC50 = 17 +/- 5 nM) lower than the heart. The sodium dependences of the high affinity digitoxigenin sites were for the cardiac alpha 3 form (K0.5 = 10 +/- 1.9 mM) and for the cerebral alpha 2 and alpha 3 mixture (K0.5 19.6 +/- 4.9 mM). The sensitivities for Na+ of the low affinity sites (alpha 1) were: 6.7 +/- 1.4 mM, 6.3 +/- 1.2 mM and 11.6 +/- 2.9 mM in heart, brain and kidney respectively. This is the first report of the catalytic characteristics of the alpha 3 subunit isoenzyme in canine cardiac plasma membranes.

Developmental cell-specific regulation of Na(+)-K(+)-ATPase alpha 1-, alpha 2-, and alpha 3-isoform gene expression

Na(+)-K(+)-activated adenosine triphosphatase (Na(+)-K(+)-ATPase) is the integral membrane protein that maintains the Na(+)-K(+) electrochemical gradient across the plasma membrane. Because of the importance of the Na(+)-K(+) electrochemical gradient to fundamental and specialized cell functions, we investigated the cell-specific modulation of Na(+)-K(+)-ATPase alpha-subunit isoform (alpha 1, alpha 2, and alpha 3) gene expression in different stages of postimplantation mouse embryos and neonatal rat tissues by in situ hybridization with use of isoform-specific rat-derived antisense RNA probes. At early organogenesis (9.5-10.5 days postcoitus), we demonstrated generalized coexpression of alpha 1- and alpha 2-isoforms throughout the mouse embryo with greater levels in the developing but already functional heart, in contrast to the distinct spatially restricted alpha 3-isoform gene expression in the early developing neural tube. At midorganogenesis (15.5-16.5 days postcoitus), differential spatial variation in alpha 1-, alpha 2-, and alpha 3-isoform gene expression was already evident in all organs. Interestingly, region-specific expression patterns within single cell types were noted throughout development and were exemplified by 1) alpha 3-isoform gene expression in marginal cells of the 10.5-day-postcoitus developing neural tube; 2) alpha 1-, alpha 2-, and alpha 3-isoform gene expression in cerebellar granular cells of the 4-day-old rat brain; and 3) alpha 1- and alpha 3-isoform gene expression in 4-day-old rat ventricular cardiomyocytes. These isoform-specific changes in cellular and regional Na(+)-K(+)-ATPase alpha-isoform gene expression may play an active role in development and specialized cell functions.

Catalytic subunit isoforms of mammalian lens Na,K-ATPase

To identify the Na,K-ATPase isoforms present in the mammalian lens, seven antisera were prepared to selected peptide sequences of the catalytic (alpha) subunit. Three antisera were prepared to peptide sequences at the N-terminus of the three sequenced rat alpha isoforms. There is < 53% sequence homology among the isoforms in this region. Three antisera were prepared to peptide sequences at the ouabain binding site in the extracellular loop between membrane spanning sequences 1 and 2 of the sequenced rat alpha isoforms; sequence homology among the isoforms in this region is < 69%. An antiserum was also prepared to the carboxyl terminal region of the alpha 2 rat isoform. The sequenced isoforms (rat and human) in this region are > 94% homologous. The results from stains of Western blots of SDS-PAGE separations of lens membranes are presented. Alpha 1 is the predominant isoform of the epithelium. It is not found in cells of the central epithelium but is present in cells located more toward the equator. Alpha 3 is the catalytic subunit of the central 43% of the epithelium. The lens fiber cell membranes have a catalytic subunit that is related to the alpha 2 isoform. In the fiber cell a 98-100 kDa band stains with the antiserum to the alpha 2 N-terminus and the antiserum to the alpha 2 ouabain site. The antiserum to the alpha 2 C-terminus does not stain the 98-100 kDa band. (Preliminary reports of these results were presented at the 1992 and 1993 meetings of the Association for Research in Vision and Ophthalmology).

Identification of a functional thyroid hormone response element in the upstream flanking region of the human Na,K-ATPase beta 1 gene

The human Na,K-ATPase beta 1 subunit gene promoter activity is stimulated by thyroid hormone (T3) in the human intestinal Caco-2 cells. To identify potential cis-acting transcriptional regulatory elements involved in this process, chimeric plasmids containing varying lengths of the 5' flanking region of the human beta 1 Na,K-ATPase gene linked to the firefly luciferase reporter gene were introduced into Caco-2 cells by transient transfection. Analysis of T3-regulated luciferase activity of cells carrying these plasmids, and subsequent use of site-directed mutagenesis revealed that a region from -459 to -438 (relative to the transcriptional start site) is required for the induction of the beta 1 Na,K-ATPase gene by T3. An oligonucleotide containing this sequence from -465 to -433 confers T3 responsiveness to a heterologous promoter. Gel mobility shift assays showed specific binding of nuclear proteins of Caco-2 cells to this region and immunoreactive T3 receptor was identified in one of these complexes. These data demonstrate that there is a cis-acting thyroid hormone responsive element in the 5' flanking region of the human beta 1 Na,K-ATPase gene and induction of transcription of this gene by T3 involves specific binding of the thyroid hormone receptor to the TRE located at position -459 to -438.

Molecular cloning and immunological characterization of the gamma polypeptide, a small protein associated with the Na,K-ATPase

The gamma subunit of the Na,K-ATPase is a small membrane protein that copurifies with the alpha and beta subunits of the enzyme. Strong evidence that the gamma subunit is a component of the Na,K-ATPase comes from studies indicating that the subunit is involved in forming the site for cardiac glycoside binding. We have isolated and characterized the cDNAs coding the gamma subunit from several species. The gamma subunit is a highly conserved protein consisting of 58 amino acids with a molecular weight of 6500. Hydropathy analysis reveals the presence of a single hydrophobic domain that is sufficient to cross the membrane. There are no sites for N-linked glycosylation. Northern blot analysis revealed that the gamma subunit mRNA is expressed in a tissue-specific fashion and is present in all tissues characterized. gamma-specific antibodies have been used to verify that the sequenced protein is the same protein labeled by [3H]nitroazidobenzoyl-ouabain (NAB-ouabain), and that this protein, the gamma subunit of the Na,K-ATPase, has a distribution pattern along nephron segments that is identical with the alpha subunit. In addition, coimmunoprecipitation of the alpha, beta and gamma subunits demonstrate specific association of the subunits. These results are consistent with the notion that the gamma subunit is specifically associated with and may be an important component of the Na,K-ATPase.

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.

Chimeric rat Na,K-ATPase alpha 1/alpha 3* isoforms. Analysis of the structural basis for differences in Na+ requirements in the alpha 1 and alpha 3* isoforms

Na,K-ATPase molecules containing the alpha 1, alpha 2*, and alpha 3* isoforms expressed in HeLa cells exhibit a two- to threefold difference in their K0.5 for Na+ (alpha 1 = alpha 2* < alpha 3*). To investigate the structural basis for this difference, chimeric alpha 1/alpha 3* isoform cDNAs were constructed and expressed in HeLa cells. Na,K-ATPase containing each alpha isoform chimera was analyzed for its Na+ dependence properties. Results of these experiments do not reveal a region in the alpha 1 or alpha 3* isoform that is clearly responsible for the apparent affinity for Na+. It is possible that molecular interactions involving amino acids that span virtually the entire Na,K-ATPase molecule contribute to the determination of this parameter.

Housekeeping Na,K-ATPase alpha 1 subunit gene promoter is composed of multiple cis elements to which common and cell type-specific factors bind

Na,K-ATPase alpha 1 subunit gene (ATP1A1) is one of the housekeeping genes involved in homeostasis of Na+ and K+ in all animal cells. We identified and characterized the cis-acting elements that regulate the expression of ATP1A1. The region between -155 and -49 was determined as a positive regulatory region in five cultured cell lines of different tissue origins (MDCK, B103, L6, 3Y1, and HepG2). The region was divided into three subregions: from -120 to -106 (including the Sp1 binding site), from -102 to -61, and from -58 to -49 (including an Sp1 consensus sequence). Cell type-specific factors binding to the middle subregion (from -102 to -61) were detected by gel retardation analysis, using nuclear extracts prepared from MDCK and B103 cells. Two gel retardation complexes were formed in the B103 nuclear extract, and three were formed in the MDCK nuclear extract. DNA binding regions of these factors were located at -88 to -69 and differed from each other in DNase I footprinting experiments. These factors also showed different binding characteristics in gel retardation competition and methylation interference experiments. The identified cis element was named the ATP1A1 regulatory element. The core sequence of this element is found in several other genes involved in cellular energy metabolism, suggesting that the sequence is a common regulatory element responsive to the state of energy metabolism.

Housekeeping Na,K-ATPase alpha 1 subunit gene promoter is composed of multiple cis elements to which common and cell type-specific factors bind

Na,K-ATPase alpha 1 subunit gene (ATP1A1) is one of the housekeeping genes involved in homeostasis of Na+ and K+ in all animal cells. We identified and characterized the cis-acting elements that regulate the expression of ATP1A1. The region between -155 and -49 was determined as a positive regulatory region in five cultured cell lines of different tissue origins (MDCK, B103, L6, 3Y1, and HepG2). The region was divided into three subregions: from -120 to -106 (including the Sp1 binding site), from -102 to -61, and from -58 to -49 (including an Sp1 consensus sequence). Cell type-specific factors binding to the middle subregion (from -102 to -61) were detected by gel retardation analysis, using nuclear extracts prepared from MDCK and B103 cells. Two gel retardation complexes were formed in the B103 nuclear extract, and three were formed in the MDCK nuclear extract. DNA binding regions of these factors were located at -88 to -69 and differed from each other in DNase I footprinting experiments. These factors also showed different binding characteristics in gel retardation competition and methylation interference experiments. The identified cis element was named the ATP1A1 regulatory element. The core sequence of this element is found in several other genes involved in cellular energy metabolism, suggesting that the sequence is a common regulatory element responsive to the state of energy metabolism.

Low K+ increases Na(+)-K(+)-ATPase alpha- and beta-subunit mRNA and protein abundance in cultured renal proximal tubule cells

Studies from this laboratory demonstrate that LLC-PK1/Cl4 cells, a cultured renal cell line, respond to incubation in low-K+ medium by coordinately increasing abundance of both alpha- and beta-subunits of Na(+)-K(+)-ATPase but increase only beta- and not alpha-mRNA levels (Lescale-Matys et al. J. Biol. Chem. 265: 17935-17940, 1990) and that alpha-abundance is likely increased as a result of increased efficiency of alpha-mRNA translation (L. Lescale-Matys and A. A. McDonough. J. Cell Biol. 111: 311A, 1990). The aim of this report was to determine if nontransformed kidney cells would respond to low K+ in a similar manner. We incubated primary cultures of rat proximal tubule cells in low K+ (0.25 mM) for up to 24 h to address this aim. Na(+)-K(+)-ATPase activity, measured enzymatically, and abundance of alpha- and beta-subunits, measured by immunoblot, were increased significantly and coordinately by 8 h of low K+, and, by 24 h of low K+, these parameters were increased to 2.17 +/- 0.34 (activity), 2.03 +/- 0.21 (alpha), and 2.39 +/- 0.48 (beta)-fold over control. Pretranslationally, beta-mRNA, measured by Northern blot analysis, increased to 1.76 +/- 0.35 after 3 h of low K+ and to 3.4 +/- 0.75-fold over control after 24 h of low K+. The increase in alpha-mRNA was smaller and delayed compared with the beta-mRNA response, but it was sufficient to account for the observed increase in alpha-protein and Na(+)-K(+)-ATPase activity at steady state: alpha-mRNA increased to 1.27 +/- 0.09 after 6 h and to 1.91 +/- 0.41-fold over control after 24 h in low K+. We conclude that the accumulation of sodium pumps in cultured renal proximal tubule cells, unlike LLC-PK1 cells, can be accounted for by increases in both alpha- and beta-subunit mRNA levels.

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.

The A2 isoform of rat Na+,K(+)-adenosine triphosphatase is active and exhibits high ouabain affinity when expressed in transfected fibroblasts

The alpha isoforms of the Na+,K(+)-ATPase (Na+ pump) are expressed with developmental and tissue heterogeneity in rodents and possess different sensitivity to inhibition by ouabain. We directly characterized the ouabain sensitivity of the rat A2 (alpha 2) isoform by transfecting NIH 3T3 cells with rat A2. The treated cells exhibit high affinity (40 nM) ouabain binding with a density of 2 pmol/mg protein. 86Rb+ flux studies confirm that A2 is functional in this system and that A2 is inhibited by submicromolar concentrations of ouabain. These findings are consistent with measurements of ouabain affinity in tissues which express the A2 isoform.

Thyroid hormone induction of rat myocardial Na(+)-K(+)-ATPase: alpha 1-, alpha 2-, and beta 1-mRNA and -protein levels at steady state

In this study, we measured the time courses of change in Na(+)-K(+)-ATPase alpha 1-, alpha 2-, and beta 1-subunit mRNA and protein abundance in cardiac myocytes isolated from euthyroid, hypothyroid, and hyperthyroid (hypothyroids injected daily with 1 microgram T3/g body wt) rats. In hypothyroids, alpha 1-, alpha 2-, and beta 1-protein levels were decreased to 0.55, 0.42, and 0.57 of euthyroids, predicting the decrease in Na(+)-K(+)-ATPase activity to 0.53 of control. There was no change in these subunits' mRNA levels, indicating that the decreases in protein levels were not due to decreased subunit transcription rates. In hyperthyroids, the alpha 1-mRNA increased to a steady state of threefold over hypothyroid by 1 day of T3 treatment, and the alpha 1-protein levels increased to twofold over hypothyroid by 4 days of T3. alpha 2-mRNA increased to 5-fold over hypothyroid by 2 days, whereas the alpha 2-protein levels increased to 14-fold above hypothyroid by 4 days of T3. Beta 1-mRNA increased to 12-fold above hypothyroid by 1 day of T3 treatment, whereas beta 1-protein increased to only 2.5-fold over hypothyroid by 4 days of T3. The discoordinate changes in alpha 2- and beta 1-mRNA vs. protein can be reconciled with the hypothesis that beta 1 is rate limiting for assembly in eu- and hypothyroids, and favors assembly with alpha 1, while excess unassembled alpha 2 is degraded. In the hyperthyroids we predict beta 1 is not rate limiting and there is increased alpha 2 beta 1-assembly. We calculate that T3 decreases the alpha 1-to-alpha 2 ratio from 24:1 in hypothyroid to 3.4:1 in hyperthyroid cardiomyocytes.

The cardiac conduction system in the rat expresses the alpha 2 and alpha 3 isoforms of the Na+,K(+)-ATPase

The sodium pump is crucial for the function of the heart and of the cardiac conduction system, which initiates the heartbeat. The alpha (catalytic) subunit of this pump has three isoforms; the alpha 1 isoform is ubiquitous, but the alpha 2 and alpha 3 isoforms are localized to excitable tissue. Because rodent alpha 2 and alpha 3 isoforms are relatively sensitive to ouabain, which also slows cardiac conduction, we studied heart-cell-specific expression of pump isoform genes. Multiple conduction-system structures, including sinoatrial node, bundle branches, and Purkinje strands, had prominent, specific hybridization signal for alpha 2 and alpha 3 isoforms compared with adjacent working myocytes. This gene-expression approach may be useful for labeling conduction tissue and also for localizing specific membrane channels and receptors in this system.

Cell-specific expression of mRNAs encoding Na+,K(+)-ATPase alpha- and beta-subunit isoforms within the rat central nervous system

We have used in situ hybridization histochemistry to analyze the subcellular distribution of mRNAs encoding Na,K-ATPase alpha- and beta-subunit isoforms in the rat central nervous system. Substantial differences in the cell-specific pattern of expression were found for the genes encoding three isoforms of the alpha subunit. Transcripts of alpha 1-subunit gene were detected in virtually all cell types and structures examined. Expression of alpha 2-subunit mRNA was characteristic of glia, whereas alpha 3-subunit transcripts were predominant in neurons. Transcripts encoding the beta 1 subunit were detected in neurons, whereas beta 2-subunit mRNA expression was characteristic of glia. mRNA encoding both beta-subunit isoforms was present in choroidal epithelial cells. The distribution pattern of alpha- and beta-subunit mRNAs in structures throughout the central nervous system is consistent with the possibility of six structurally distinct Na+,K(+)-ATPase isoenzymes.

Immunocytochemical demonstration of Na+,K(+)-ATPase in internodal axolemma of myelinated fibers of rat sciatic and optic nerves

We used postembedding electron microscopic immunocytochemistry with colloidal gold to determine the ultrastructural distribution of Na+,K(+)-ATPase in the sciatic and optic nerves of the rat. Using a polyclonal antiserum raised against the denatured catalytic subunit of brain Na+,K(+)-ATPase, we found immunoreactivity along the internodal axolemma of myelinated fibers in both nerves. This antiserum did not produce labeling of nodal axolemma. These results suggest that an important site of energy-dependent sodium-potassium exchange is along the internodal axolemma of myelinated fibers in the mammalian CNS and PNS and that there may be differences between the internodal and nodal forms of the enzyme.

Phosphorylation of brain (Na+,K+)-ATPase alpha catalytic subunits in normal and epileptic cerebral cortex: I. The audiogenic mice and the cat with a freeze lesion

Partially purified (Na+,K+)-ATPase (E.C. 3.6.1.3.) was investigated in the epileptic cortex of audiogenic DBA/2 mice and in the primary and secondary foci of cats with acute or chronic freeze lesions. No differences in specific activities measured at 3 mM K+ were observed between epileptic and control cortex, except an increase of enzymic activities in the primary focus of acutely lesioned cats. The (Na+,K+)-ATPase catalytic subunits were resolved by SDS-gel electrophoresis and their phosphorylation levels were measured in presence of K+ ions and phenytoin. K+ was more effective in inducing maximal dephosphorylation of (Na+,K+)-ATPase in C57/BL, with identical affinity in the two strains. Phenytoin decreased the net phosphorylation level of (Na+,K+)-ATPase by about 50% in C57/BL mice, but only by 20% in DBA/2 mice. Both K+ and phenytoin dephosphorylating influences were decreased in primary and secondary foci of acutely lesioned cats. Those changes were limited to the alpha(-) subunit. In chronic cats, the dephosphorylating step of the (Na+,K+)-ATPase catalytic subunit recovered a normal affinity to K+, but its sensitivity to phenytoin remained decreased. Those differences in K+ and phenytoin influences on brain (Na+,K+)-ATPases between control and epileptic cortex might be responsible for the ictal transformation and seizure spread. In cats, the alteration of the alpha(-) isoform could mainly affect the glial cells.

Hypokalemia decreases Na(+)-K(+)-ATPase alpha 2- but not alpha 1-isoform abundance in heart, muscle, and brain

K+ deficiency has been linked to a loss of K+ from muscle associated with a decrease in ouabain binding and K(+)-dependent phosphatase activity. This study aimed to quantitate the Na(+)-K(+)-ATPase alpha- and beta-isoform-specific responses to hypokalemia in vivo in heart, skeletal muscle, and brain at pre- and posttranslational levels. Two-week dietary K+ restriction resulted in decreases in alpha 2-mRNA in heart and skeletal muscle to 0.60 and 0.65, and in alpha 2-protein abundance to 0.38 and 0.18 of control, respectively. The decrease in alpha 2-protein was greater than the decrease in mRNA in both tissues, suggesting translational and/or posttranslational mechanism(s) of regulation as well as pretranslational regulation in response to hypokalemia. K(+)-dependent p-nitrophenyl phosphatase (pNPPase) activity decreased in heart and skeletal muscle to 0.67 and 0.58, respectively. There were no changes in alpha 1-. or beta-mRNA or protein levels in skeletal muscle or heart. In brain, there was a similar pattern of regulation. While brain alpha 2-mRNA did not change in hypokalemia, protein levels decreased to 0.72 of control. In conclusion, hypokalemia is associated with a large decrease in expression of the alpha 2-isoform of Na(+)-K(+)-ATPase. These results support the hypothesis that in skeletal and heart muscle hypokalemia induces a decrease in Na(+)-K(+)-ATPase activity (measured as K(+)-dependent pNPPase activity) by specifically decreasing the expression of the alpha 2-isoform of Na(+)-K(+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential distribution of (Na, K)-ATPase alpha isoforms in the central nervous system

1. mRNA transcripts for three isoforms of the alpha subunit of (Na,K)-ATPase have been previously identified in the rat nervous system and designated alpha 1, alpha 2 and alpha 3. 2. In order to study the localization and expression of the different alpha isoform mRNAs on a regional and cellular level in the brain, we prepared probes from the unique 3' untranslated region of rat alpha 1 cDNA and from a segment containing a portion of the translated region of rat alpha 3 cDNA. These probes were used in dot blot and in situ hybridization assays of rat brain. 3. alpha 1 mRNA was found predominantly in cerebral cortex, dentate gyrus of hippocampus, and specific isolated brain-stem nuclei such as locus ceruleus and motor nuclei V and VII. In contrast alpha 3 mRNA was found predominantly in pyramidal neurons in the deep layers of cerebral cortex, in both pyramidal and dentate gyrus neurons of the hippocampus, and in neurons of most subcortical structures of the thalamus, basal ganglia, and brain-stem nuclei. 4. In the cerebellum, Purkinje cells showed predominantly alpha 3, as did stellate and basket cells. The granule cells contained predominantly alpha 1. 5. These experiments show that mRNAs for both alpha 1 and alpha 3 isoforms of (Na,K)-ATPase are found in neurons of the CNS. The isoforms have unique cellular and regional distributions, which in some cases overlap.