Isoform-specific monoclonal antibodies to Na,K-ATPase alpha subunits. Evidence for a tissue-specific post-translational modification of the alpha subunit

Three-dimensional architecture of the subepithelial corpuscular nerve ending in the rat epiglottis reconstructed by array tomography with scanning electron microscopy

In the rat laryngeal mucosa, subepithelial corpuscular nerve endings, called laminar nerve endings, are distributed in the epiglottis and arytenoid region and are activated by the pressure changes of the laryngeal cavity. They are also suggested to play a role in efferent regulation because of secretory vesicles in the axoplasm. In the present study, the laminar nerve endings in the rat laryngeal mucosa were analyzed by 3D reconstruction from serial ultrathin sections in addition to immunohistochemistry for synapsin 1. In the light microscopy, synapsin 1-immunoreactive flattened or bulbous terminal parts of the laminar endings were also immunoreactive with VGLUT1, and were surrounded by S100- or S100B-immunoreactive Schwann cells and vimentin-immunoreactive fibroblasts. In the electron microscopy, 3D reconstruction views showed that laminar endings were composed of flattened terminal parts sized 2-5 μm in longitudinal length, overlapping in three to five multiple layers. The terminal parts of the endings were incompletely wrapped by flat cytoplasmic processes of the Schwann cells. In addition, the fibroblast network surrounded the complex of nerve endings and the Schwann cells. Several terminal parts entered through the basement membrane into the epithelial layer and attached to the basal epithelial cells, suggesting that interaction between epithelial cells and laminar nerve endings plays an important role in sensing the pressure changes in the laryngeal cavity. Secretory vesicles were unevenly distributed throughout the terminal part of the laminar nerve endings. The secretory vesicles were frequently observed in the peripheral limb of the terminal parts. It suggests that the laminar nerve endings in the larynx may release glutamate to maintain continuous discharge during the stretching of the laryngeal mucosa.

Impact of manganese accumulation on Na,K-ATPase expression and function in the cerebellum and striatum of C57Bl/6 mice

Overexposure to Mn causes a neurological disorder-manganism-with motor symptoms that overlap closely with disorders associated with haploinsufficiency in the gene encoding for α3 isoform of Na+,K+-ATPase (NKA). The present study was designed to test the hypothesis that behavioral changes in the mouse model of manganism may be associated with changes in the expression and activity of α3 NKA in the cerebellum (CB) and striatum (STR)-the key brain structures responsible for motor control in adult mice. C57Bl/6 mice were exposed to MnCl2 at 0.5 g/L (in drinking water) for up to eight weeks. After four weeks of Mn consumption, Mn levels were increased in the CB only. Behavioral tests demonstrated decreased performance of Mn-treated mice in the shuttle box test (third through sixth weeks), and the inclined grid walking test (first through sixth weeks), suggesting the development of learning impairment, decreased locomotion, and motor discoordination. The activity of NKA significantly decreased, and the expression of α1-α3 isoforms of NKA increased in the second week in the CB only. Thus, signs of learning and motor disturbances developing in this model of manganism are unlikely to be directly linked to disturbances in the expression or activity of NKA in the CB or STR. Whether these early changes may contribute to the pathogenesis of later behavioral deficits remains to be determined.

Immunohistochemical distribution of proteins involved in glutamate release in subepithelial sensory nerve endings of rat epiglottis

To elucidate the efferent functions of sensory nerve endings, the distribution of calretinin and vesicular glutamate transporter 1 (VGLUT1) in laryngeal laminar nerve endings and the immunohistochemical distribution of proteins associated with synaptic vesicle release, i.e., t-SNARE (SNAP25 and syntaxin 1), v-SNARE (VAMP1 and VAMP2), synaptotagmin 1 (Syt1), bassoon, and piccolo, were examined. Subepithelial laminar nerve endings immunoreactive for Na⁺-K⁺-ATPase α₃-subunit (NKAα₃) were largely distributed in the whole-mount preparation of the epiglottic mucosa, and several endings were also immunoreactive for calretinin. VGLUT1 immunoreactivity was observed within terminal part near the outline of the small processes of NKAα₃-immunoreactive nerve ending. SNAP25, syntaxin 1, and VAMP1 immunoreactivities were detected in terminal parts of calretinin-immunoreactive endings, whereas VAMP2 immunoreactivity was only observed in a few terminals. Terminal parts immunoreactive for calretinin and/or VGLUT1 also exhibited immunoreactivities for Syt1, Ca²⁺ sensor for membrane trafficking, and for bassoon and piccolo, presynaptic scaffold proteins. The presence of vesicular release-related proteins, including SNARE proteins, in the terminals of laryngeal laminar endings indicate that intrinsic glutamate modulates their afferent activity in an autocrine-like manner.

Comparative description of the mRNA expression profile of Na+ /K+ -ATPase isoforms in adult mouse nervous system

Mutations in genes encoding Na⁺ /K⁺ -ATPase α1, α2, and α3 subunits cause a wide range of disabling neurological disorders, and dysfunction of Na⁺ /K⁺ -ATPase may contribute to neuronal injury in stroke and dementia. To better understand the pathogenesis of these diseases, it is important to determine the expression patterns of the different Na⁺ /K⁺ -ATPase subunits within the brain and among specific cell types. Using two available scRNA-Seq databases from the adult mouse nervous system, we examined the mRNA expression patterns of the different isoforms of the Na⁺ /K⁺ -ATPase α, β and Fxyd subunits at the single-cell level among brain regions and various neuronal populations. We subsequently identified specific types of neurons enriched with transcripts for α1 and α3 isoforms and elaborated how α3-expressing neuronal populations govern cerebellar neuronal circuits. We further analyzed the co-expression network for α1 and α3 isoforms, highlighting the genes that positively correlated with α1 and α3 expression. The top 10 genes for α1 were Chn2, Hpcal1, Nrgn, Neurod1, Selm, Kcnc1, Snrk, Snap25, Ckb and Ccndbp1 and for α3 were Sorcs3, Eml5, Neurod2, Ckb, Tbc1d4, Ptprz1, Pvrl1, Kirrel3, Pvalb, and Asic2.

Gestation changes sodium pump isoform expression, leading to changes in ouabain sensitivity, contractility, and intracellular calcium in rat uterus

Developmental and tissue‐specific differences in isoforms allow Na⁺, K⁺‐ATPase function to be tightly regulated, as they control sensitivity to ions and inhibitors. Uterine contraction relies on the activity of the Na⁺, K⁺ATPase, which creates ionic gradients that drive excitation‐contraction coupling. It is unknown whether Na⁺, K⁺ATPase isoforms are regulated throughout pregnancy or whether they have a direct role in modulating uterine contractility. We hypothesized that gestation‐dependent differential expression of isoforms would affect contractile responses to Na⁺, K⁺ATPase α subunit inhibition with ouabain. Our aims were therefore: (1) to determine the gestation‐dependent expression of mRNA transcripts, protein abundance and tissue distribution of Na⁺, K⁺ATPase isoforms in myometrium; (2) to investigate the functional effects of differential isoform expression via ouabain sensitivity; and (3) if changes in contractile responses can be explained by changes in intracellular [Ca²⁺]. Changes in abundance and distribution of the Na⁺, K⁺ATPase α, β and FXYD1 and 2 isoforms, were studied in rat uterus from nonpregnant, and early, mid‐, and term gestation. All α, β subunit isoforms (1,2,3) and FXYD1 were detected but FXYD2 was absent. The α1 and β1 isoforms were unchanged throughout pregnancy, whereas α2 and α3 significant decreased at term while β2 and FXYD1 significantly increased from mid‐term onwards. These changes in expression correlated with increased functional sensitivity to ouabain, and parallel changes in intracellular Ca²⁺, measured with Indo‐1. In conclusion, gestation induces specific regulatory changes in expression of Na⁺, K⁺ATPase isoforms in the uterus which influence contractility and may be related to the physiological requirements for successful pregnancy and delivery.

Age-related changes in Na, K-ATPase expression, subunit isoform selection and assembly in the stria vascularis lateral wall of mouse cochlea

Due to the critical role of cochlear ion channels for hearing, the focus of the present study was to examine age-related changes of Na, K-ATPase (NKA) subunits in the lateral wall of mouse cochlea. We combined qRT-PCR, western blot and immunocytochemistry methodologies in order to determine gene and protein expression levels in the lateral wall of young and aged CBA/CaJ mice. Of the seven NKA subunits, only the mRNA expressions of α1, β1 and β2 subunit isoforms were detected in the lateral wall of CBA/CaJ mice. Aging was accompanied by dys-regulation of gene and protein expression of all three subunits detected. Hematoxylin and eosin (H&E) staining revealed atrophy of the cochlear stria vascularis (SV). The SV atrophy rate (20%) was much less than the ∼80% decline in expression of all three NKA isoforms, indicating lateral wall atrophy and NKA dys-regulation are independent factors and that there is a combination of changes involving the morphology of SV and NKA expression in the aging cochlea which may concomitantly affect cochlear function. Immunoprecipitation assays showed that the α1-β1 heterodimer is the selective preferential heterodimer over the α1-β2 heterodimer in cochlea lateral wall. Interestingly, in vitro pathway experiments utilizing cultured mouse cochlear marginal cells from the SV (SV-K1 cells) indicated that decreased mRNA and protein expressions of α1, β1 and β2 subunit isoforms are not associated with reduction of NKA activity following in vitro application of ouabain, but ouabain did disrupt the α1-β1 heterodimer interaction. Lastly, the association between the α1 and β1 subunit isoforms was present in the cochlear lateral wall of young adult mice, but this interaction could not be detected in old mice. Taken together, these data suggest that in the young adult mouse there is a specific, functional selection and assembly of NKA subunit isoforms in the SV lateral wall, which is disrupted and dys-regulated with age. Interventions for this age-linked ion channel disruption may have the potential to help diagnose, prevent, or treat age-related hearing loss.

Cold-water immersion after training sessions: effects on fiber type-specific adaptations in muscle K+ transport proteins to sprint-interval training in men

Effects of regular use of cold-water immersion (CWI) on fiber type-specific adaptations in muscle K⁺ transport proteins to intense training, along with their relationship to changes in mRNA levels after the first training session, were investigated in humans. Nineteen recreationally active men (24 ± 6 yr, 79.5 ± 10.8 kg, 44.6 ± 5.8 ml·kg^-1·min^-1) completed six weeks of sprint-interval cycling, either without (passive rest; CON) or with training sessions followed by CWI (15 min at 10°C; COLD). Muscle biopsies were obtained before and after training to determine abundance of Na⁺, K⁺-ATPase isoforms (α_1-3, β_1-3) and phospholemman (FXYD1) and after recovery treatments (+0 h and +3 h) on the first day of training to measure mRNA content. Training increased ( P < 0.05) the abundance of α₁ and β₃ in both fiber types and β₁ in type-II fibers and decreased FXYD1 in type-I fibers, whereas α₂ and α₃ abundance was not altered by training ( P > 0.05). CWI after each session did not influence responses to training ( P > 0.05). However, α₂ mRNA increased after the first session in COLD (+0 h, P < 0.05) but not in CON ( P > 0.05). In both conditions, α₁ and β₃ mRNA increased (+3 h; P < 0.05) and β₂ mRNA decreased (+3 h; P < 0.05), whereas α₃, β₁, and FXYD1 mRNA remained unchanged ( P > 0.05) after the first session. In summary, Na⁺,K⁺-ATPase isoforms are differently regulated in type I and II muscle fibers by sprint-interval training in humans, which, for most isoforms, do not associate with changes in mRNA levels after the first training session. CWI neither impairs nor improves protein adaptations to intense training of importance for muscle K⁺ regulation. NEW & NOTEWORTHY Although cold-water immersion (CWI) after training and competition has become a routine for many athletes, limited published evidence exists regarding its impact on training adaptation. Here, we show that CWI can be performed regularly without impairing training-induced adaptations at the fiber-type level important for muscle K⁺ handling. Furthermore, sprint-interval training invoked fiber type-specific adaptations in K⁺ transport proteins, which may explain the dissociated responses of whole-muscle protein levels and K⁺ transport function to training previously reported.

Quantification of endogenous and exogenous protein expressions of Na,K-ATPase with super-resolution PALM/STORM imaging

Transient transfection of fluorescent fusion proteins is a key enabling technology in fluorescent microscopy to spatio-temporally map cellular protein distributions. Transient transfection of proteins may however bypass normal regulation of expression, leading to overexpression artefacts like misallocations and excess amounts. In this study we investigate the use of STORM and PALM microscopy to quantitatively monitor endogenous and exogenous protein expression. Through incorporation of an N-terminal hemagglutinin epitope to a mMaple3 fused Na,K-ATPase (α₁ isoform), we analyze the spatial and quantitative changes of plasma membrane Na,K-ATPase localization during competitive transient expression. Quantification of plasma membrane protein density revealed a time dependent increase of Na,K-ATPase, but no increase in size of protein clusters. Results show that after 41h transfection, the total plasma membrane density of Na,K-ATPase increased by 63% while the endogenous contribution was reduced by 16%.

Mutations in ATP1A1 Cause Dominant Charcot-Marie-Tooth Type 2

Although mutations in more than 90 genes are known to cause CMT, the underlying genetic cause of CMT remains unknown in more than 50% of affected individuals. The discovery of additional genes that harbor CMT2-causing mutations increasingly depends on sharing sequence data on a global level. In this way-by combining data from seven countries on four continents-we were able to define mutations in ATP1A1, which encodes the alpha1 subunit of the Na⁺,K⁺-ATPase, as a cause of autosomal-dominant CMT2. Seven missense changes were identified that segregated within individual pedigrees: c.143T>G (p.Leu48Arg), c.1775T>C (p.Ile592Thr), c.1789G>A (p.Ala597Thr), c.1801_1802delinsTT (p.Asp601Phe), c.1798C>G (p.Pro600Ala), c.1798C>A (p.Pro600Thr), and c.2432A>C (p.Asp811Ala). Immunostaining peripheral nerve axons localized ATP1A1 to the axolemma of myelinated sensory and motor axons and to Schmidt-Lanterman incisures of myelin sheaths. Two-electrode voltage clamp measurements on Xenopus oocytes demonstrated significant reduction in Na⁺ current activity in some, but not all, ouabain-insensitive ATP1A1 mutants, suggesting a loss-of-function defect of the Na⁺,K⁺ pump. Five mutants fall into a remarkably narrow motif within the helical linker region that couples the nucleotide-binding and phosphorylation domains. These findings identify a CMT pathway and a potential target for therapy development in degenerative diseases of peripheral nerve axons.

Mutations in ATP1A1 Cause Dominant Charcot-Marie-Tooth Type 2

Although mutations in more than 90 genes are known to cause CMT, the underlying genetic cause of CMT remains unknown in more than 50% of affected individuals. The discovery of additional genes that harbor CMT2-causing mutations increasingly depends on sharing sequence data on a global level. In this way-by combining data from seven countries on four continents-we were able to define mutations in ATP1A1, which encodes the alpha1 subunit of the Na⁺,K⁺-ATPase, as a cause of autosomal-dominant CMT2. Seven missense changes were identified that segregated within individual pedigrees: c.143T>G (p.Leu48Arg), c.1775T>C (p.Ile592Thr), c.1789G>A (p.Ala597Thr), c.1801_1802delinsTT (p.Asp601Phe), c.1798C>G (p.Pro600Ala), c.1798C>A (p.Pro600Thr), and c.2432A>C (p.Asp811Ala). Immunostaining peripheral nerve axons localized ATP1A1 to the axolemma of myelinated sensory and motor axons and to Schmidt-Lanterman incisures of myelin sheaths. Two-electrode voltage clamp measurements on Xenopus oocytes demonstrated significant reduction in Na⁺ current activity in some, but not all, ouabain-insensitive ATP1A1 mutants, suggesting a loss-of-function defect of the Na⁺,K⁺ pump. Five mutants fall into a remarkably narrow motif within the helical linker region that couples the nucleotide-binding and phosphorylation domains. These findings identify a CMT pathway and a potential target for therapy development in degenerative diseases of peripheral nerve axons.

Na,K-ATPase Isozymes in Colorectal Cancer and Liver Metastases

The goal of this study was to define Na,K-ATPase α and β subunit isoform expression and isozyme composition in colorectal cancer cells and liver metastases. The α1, α3, and β1 isoforms were the most highly expressed in tumor cells and metastases; in the plasma membrane of non-neoplastic cells and mainly in a cytoplasmic location in tumor cells. α1β1 and α3β1 isozymes found in tumor and metastatic cells exhibit the highest and lowest Na⁺ affinity respectively and the highest K⁺ affinity. Mesenchymal cell isozymes possess an intermediate Na⁺ affinity and a low K⁺ affinity. In cancer, these ions are likely to favor optimal conditions for the function of nuclear enzymes involved in mitosis, especially a high intra-nuclear K⁺ concentration. A major and striking finding of this study was that in liver, metastasized CRC cells express the α3β1 isozyme. Thus, the α3β1 isozyme could potentially serve as a novel exploratory biomarker of CRC metastatic cells in liver.

Distribution of Na,K-ATPase α subunits in rat vestibular sensory epithelia

The afferent encoding of vestibular stimuli depends on molecular mechanisms that regulate membrane potential, concentration gradients, and ion and neurotransmitter clearance at both afferent and efferent relays. In many cell types, the Na,K-ATPase (NKA) is essential for establishing hyperpolarized membrane potentials and mediating both primary and secondary active transport required for ion and neurotransmitter clearance. In vestibular sensory epithelia, a calyx nerve ending envelopes each type I hair cell, isolating it over most of its surface from support cells and posing special challenges for ion and neurotransmitter clearance. We used immunofluorescence and high-resolution confocal microscopy to examine the cellular and subcellular patterns of NKAα subunit expression within the sensory epithelia of semicircular canals as well as an otolith organ (the utricle). Results were similar for both kinds of vestibular organ. The neuronal NKAα3 subunit was detected in all afferent endings-both the calyx afferent endings on type I hair cells and bouton afferent endings on type II hair cells-but was not detected in efferent terminals. In contrast to previous results in the cochlea, the NKAα1 subunit was detected in hair cells (both type I and type II) but not in supporting cells. The expression of distinct NKAα subunits by vestibular hair cells and their afferent endings may be needed to support and shape the high rates of glutamatergic neurotransmission and spike initiation at the unusual type I-calyx synapse.

Na+/K+ ATPase α1 and α3 isoforms are differentially expressed in α- and γ-motoneurons

The Na(+)/K(+) ATPase (NKA) is an essential membrane protein underlying the membrane potential in excitable cells. Transmembrane ion transport is performed by the catalytic α subunits (α1-4). The predominant subunits in neurons are α1 and α3, which have different affinities for Na(+) and K(+), impacting on transport kinetics. The exchange rate of Na(+)/K(+) markedly influences the activity of the neurons expressing them. We have investigated the distribution and function of the main isoforms of the α subunit expressed in the mouse spinal cord. NKAα1 immunoreactivity (IR) displayed restricted labeling, mainly confined to large ventral horn neurons and ependymal cells. NKAα3 IR was more widespread in the spinal cord, again being observed in large ventral horn neurons, but also in smaller interneurons throughout the dorsal and ventral horns. Within the ventral horn, the α1 and α3 isoforms were mutually exclusive, with the α3 isoform in smaller neurons displaying markers of γ-motoneurons and α1 in α-motoneurons. The α3 isoform was also observed within muscle spindle afferent neurons in dorsal root ganglia with a higher proportion at cervical versus lumbar regions. We confirmed the differential expression of α subunits in motoneurons electrophysiologically in neonatal slices of mouse spinal cord. γ-Motoneurons were excited by bath application of low concentrations of ouabain that selectively inhibit NKAα3 while α-motoneurons were insensitive to these low concentrations. The selective expression of NKAα3 in γ-motoneurons and muscle spindle afferents, which may affect excitability of these neurons, has implications in motor control and disease states associated with NKAα3 dysfunction.

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

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

Spatial distribution of Na+-K+-ATPase in dendritic spines dissected by nanoscale superresolution STED microscopy

BACKGROUND

The Na+,K+-ATPase plays an important role for ion homeostasis in virtually all mammalian cells, including neurons. Despite this, there is as yet little known about the isoform specific distribution in neurons.

RESULTS

With help of superresolving stimulated emission depletion microscopy the spatial distribution of Na+,K+-ATPase in dendritic spines of cultured striatum neurons have been dissected. The found compartmentalized distribution provides a strong evidence for the confinement of neuronal Na+,K+-ATPase (α3 isoform) in the postsynaptic region of the spine.

CONCLUSIONS

A compartmentalized distribution may have implications for the generation of local sodium gradients within the spine and for the structural and functional interaction between the sodium pump and other synaptic proteins. Superresolution microscopy has thus opened up a new perspective to elucidate the nature of the physiological function, regulation and signaling role of Na+,K+-ATPase from its topological distribution in dendritic spines.

Agrin regulation of alpha3 sodium-potassium ATPase activity modulates cardiac myocyte contraction

Drugs that inhibit Na,K-ATPases, such as digoxin and ouabain, alter cardiac myocyte contractility. We recently demonstrated that agrin, a protein first identified at the vertebrate neuromuscular junction, binds to and regulates the activity of alpha3 subunit-containing isoforms of the Na,K-ATPase in the mammalian brain. Both agrin and the alpha3 Na,K-ATPase are expressed in heart, but their potential for interaction and effect on cardiac myocyte function was unknown. Here we show that agrin binds to the alpha3 subunit of the Na,K-ATPase in cardiac myocyte membranes, inducing tyrosine phosphorylation and inhibiting activity of the pump. Agrin also triggers a rapid increase in cytoplasmic Na(+) in cardiac myocytes, suggesting a role in cardiac myocyte function. Consistent with this hypothesis, spontaneous contraction frequencies of cultured cardiac myocytes prepared from mice in which agrin expression is blocked by mutation of the Agrn gene are significantly higher than in the wild type. The Agrn mutant phenotype is rescued by acute treatment with recombinant agrin. Furthermore, exposure of wild type myocytes to an agrin antagonist phenocopies the Agrn mutation. These data demonstrate that the basal frequency of myocyte contraction depends on endogenous agrin-alpha3 Na,K-ATPase interaction and suggest that agrin modulation of the alpha3 Na,K-ATPase is important in regulating heart function.

Differential responses in gills of euryhaline tilapia, Oreochromis mossambicus, to various hyperosmotic shocks

Euryhaline tilapia (Oreochromis mossambicus) survived in brackish water (BW; 20 per thousand) but died in seawater (SW; 35 per thousand) within 6 h when transferred directly from fresh water (FW). The purpose of this study was to clarify responses in gills of FW tilapia to various hyperosmotic shocks induced by BW or SW. In FW-acclimated tilapia, scanning electron micrographs of gills revealed three subtypes of MR cell apical surfaces: wavy-convex (subtype I), shallow-basin (subtype II), and deep-hole (subtype III). Density of apical surfaces of mitochondrion-rich (MR) cell in gills of the BW-transfer tilapia decreased significantly within 3 h post-transfer due to disappearance of subtype I cells, but increased from 48 h post-transfer because of increasing density of subtype III cells. SW-transfer individuals, however, showed decreased density of MR cell openings after 1 h post-transfer because subtype I MR cell disappeared. On the other hand, relative branchial Na+/K+-ATPase (NKA) alpha1-subunit mRNA levels, protein abundance, and NKA activity of the BW-transfer group increased significantly at 6, 12, and 12 h post-transfer, respectively. In the SW-transfer group, relative mRNA and protein abundance of gill NKA alpha1-subunit did not change while NKA activity declined before dying in 5 h. Upon SW transfer, dramatic increases (nearly 2-fold) of plasma osmolality, [Na+], and [Cl(-)] were found prior to death. For the BW-transfer group, plasma osmolality was eventually controlled by 96 h post-transfer by enhancement of NKA expression and subtype III MR cell. The success or failure of NKA activation from gene to functional protein as well as the development of specific SW subtype in gills were crucial for the survival of euryhaline tilapia to various hyperosmotic shocks.

Distribution of the Na,K-ATPase alpha subunit in the rat spiral ganglion and organ of corti

Processing of sound in the cochlea involves both afferent and efferent innervation. The Na,K-ATPase (NKA) is essential for cells that maintain hyperpolarized membrane potentials and sodium and potassium concentration gradients. Heterogeneity of NKA subunit expression is one mechanism that tailors physiology to particular cellular demands. Therefore, to provide insight into molecular differences that distinguish the various innervation pathways in the cochlea, we performed a variety of double labeling experiments with antibodies against three of the alpha isoforms of the NKA (NKA alpha 1-3) and markers identifying particular subsets of neurons or supporting cells in whole mount preparations of the organ of Corti and spiral ganglion. We found that the NKA alpha 3 is abundantly expressed within the membranes of the spiral ganglion somata, the type I afferent terminals contacting the inner hair cells, and the medial efferent terminals contacting the outer hair cells. We also found expression of the NKA alpha 1 in the supporting cells that neighbor the inner hair cells and express the glutamate transporter GLAST. These findings suggest that both the NKA alpha 1 and NKA alpha 3 are poised to play an essential role in the regulation of the type I afferent synapses, the medial efferent synapses, and also glutamate transport from the afferent-inner hair cell synapse.

Isoform specificity of Na-K-ATPase-mediated ouabain signaling

The ion transporter Na-K-ATPase functions as a cell signal transducer that mediates ouabain-induced activation of protein kinases, such as ERK. While Na-K-ATPase composed of the alpha(1)-polypeptide is involved in cell signaling, the role of other alpha-isoforms (alpha(2), alpha(3), and alpha(4)) in transmitting ouabain effects is unknown. We have explored this using baculovirus-directed expression of Na-K-ATPase polypeptides in insect cells and ERK phosphorylation as an indicator of ouabain-induced signaling. Ouabain addition to Sf-9 cells coexpressing Na-K-ATPase alpha(1)- and beta(1)-isoforms stimulated ERK phosphorylation. In contrast, expression of the alpha(1)- and beta(1)-polypeptides alone resulted in no effect, indicating that the alphabeta-complex is necessary for Na-K-ATPase signaling. Moreover, the ouabain effect was sensitive to genistein, suggesting that Na-K-ATPase-mediated tyrosine kinase activation is a critical event in the intracellular cascade leading to ERK phosphorylation. In addition, the Na-K-ATPases alpha(3)beta(1)- and alpha(4)beta(1)-isozymes, but not alpha(2)beta(1), responded to ouabain treatment. In agreement with the differences in ouabain affinity of the alpha-polypeptides, alpha(1)beta(1) required 100- to 1,000-fold more ouabain to signal than did alpha(4)beta(1) and alpha(3)beta(1), respectively. These results confirm the role of the Na-K-ATPase in ouabain signal transduction, show that there are important isoform-specific differences in Na-K-ATPase signaling, and demonstrate the suitability of the baculovirus expression system for studying Na-K-ATPase-mediated ouabain effects.

Phylogenetic preservation of alpha3 Na+,K+-ATPase distribution in vertebrate peripheral nervous systems

The alpha(3) isoform of Na(+),K(+)-ATPase is uniquely expressed in afferent and efferent neurons innervating muscle spindles in the peripheral nervous system (PNS) of adult rats, but the distribution pattern of this isoform in other species has not been investigated. We compared expression of alpha(3) Na(+),K(+)-ATPase in lumbar dorsal root ganglia (DRG), spinal roots, and skeletal muscle samples of amphibian (frog), reptilian (turtle), avian (pigeon and chicken), and mammalian (mouse and human) species. In all species studied, the alpha(3) Na(+),K(+)-ATPase isoform was nonuniformly expressed in peripheral ganglia and nerves. In spinal ganglia, only 5-20% of neurons expressed this isoform, and, in avian and mammalian species, these alpha(3) Na(+),K(+)-ATPase-expressing neurons belonged to a subpopulation of large DRG neurons. In ventral root fibers of pigeons, mice, and humans, the alpha(3) Na(+),K(+)-ATPase was abundantly expressed predominantly in small myelinated axons. In skeletal muscle samples from turtles, pigeons, mice, and humans, alpha(3) Na(+),K(+)-ATPase was detected in intramuscular myelinated axons and in profiles of nerve terminals associated with the equatorial and polar regions of muscle spindle intrafusal fibers. These results show that the expression profiles for alpha(3) Na(+),K(+)-ATPase in the peripheral nervous system of a wide variety of vertebrate species are similar to the profile of rats and suggest that stretch receptor-associated expression of alpha(3) Na(+),K(+)-ATPase is preserved through vertebrate evolution.

The Na,K-ATPase alpha4 isoform from humans has distinct enzymatic properties and is important for sperm motility

In the rat, the Na,K-ATPase alpha4 isoform exhibits unique enzymatic characteristics and is important for sperm motility. In this work, we studied expression, localization and function of alpha4 in human spermatozoa. We show two catalytically active Na,K-ATPase alpha polypeptides with different ouabain affinity and identified expression of alpha1, alpha4, beta1 and beta3 isoforms in the gametes. In addition, human sperm presented two Na,K-ATPases composed of alpha4, alpha4beta1 and alpha4beta3. Kinetic analysis of these isozymes produced in insect cells showed that, compared with human alpha1beta1, alpha4beta1 and alpha4beta3 exhibit higher Na(+) and lower K(+) affinity and higher sensitivity to ouabain. These particular enzymatic properties suggested a role for alpha4 in sperm function. Using computer-assisted sperm analysis (CASA), we found that ouabain inhibition of alpha4 significantly decreased percentage sperm motility. In contrast, ouabain did not affect linearity of forward progression, amplitude of lateral head displacement, beat cross frequency and sperm straight-line, curvilinear or average path velocities. This suggests a primary role of alpha4 in flagellar motility. Accordingly, we found alpha4 in the sperm tail, predominating in the mid-piece of the flagellum. Therefore, similar to the rat ortholog, human Na,K-ATPase alpha4 isoform has a distinct activity that is essential for sperm function.

Subunit composition and role of Na+,K+-ATPases in ventricular myocytes

Na+,K+-ATPases are composed of one alpha and one beta subunit; four alpha and three beta isoforms have been found to date. We elucidated which alpha and beta subunits were present in the ventricular myocytes of rat and guinea-pig and what roles the Na+,K(+)-ATPase isozymes play in cardiac contraction. The presence of the alpha1, alpha2, and alpha3 subunits and the beta1 and beta2 subunits in rat and guinea-pig hearts were confirmed at the protein or mRNA level. Immunocytochemistry showed a patchy presence of alpha1 in the transverse tubules and surface sarcolemma, whereas alpha2 was distributed continuously in the transverse tubules alone. The alpha3 isoform was expressed prominently in the guinea-pig intercalated disc and slightly in the rat. On the other hand, the beta1 isoform was located in the transverse tubules and surface sarcolemma, whereas the beta2 was mainly located in the intercalated disc. The immunocytochemistry and immunoprecipitation findings indicated that the alpha1 and alpha2 form heterodimers with beta1 and the alpha3 with beta2 in ventricular myocytes. The application of low concentrations of ouabain enhanced the amplitudes of twitch without a change in resting tension in rat and guinea-pig ventricular stripts, whereas that of high concentrations resulted in a decrease in twitch with an increase in the resting tension. We thus conclude that the alpha2beta1 and alpha3beta2 isozymes are selectively located in the transverse tubules and intercalated disc of the ventricular myocytes, respectively, and the alpha2beta1 is involved in the regulation of the Ca2+ contents in the SR.

Sensory receptors in the airways: neurochemical coding of smooth muscle-associated airway receptors and pulmonary neuroepithelial body innervation

Mainly due to the lack of conclusive morphological data, correlation between functionally and morphologically defined lung receptors has so far been unsatisfactory. In the present study, multiple immunocytochemical stainings with a panel of markers for (mechanso)sensory nerve fibres were performed in order to visualise putative receptor terminals in rat intrapulmonary airways. We first focussed on determining the location, morphology and neurochemical coding of subepithelial receptor-like structures that have been sporadically reported in the wall of large diameter airways. Immunostaining with antibodies against Na+/K+-ATPase alpha3, vesicular glutamate transporter 1 (VGLUT1) and VGLUT2 revealed branching laminar subepithelial receptor endings associated with airway smooth muscle. The latter nerve terminals appeared to further express calbindin D28k (CB), and the ATP receptor P2X3, but were calcitonin gene-related peptide (CGRP)-negative. The nerve fibres that give rise to these terminals were shown to be myelinated and have a vagal sensory origin. Because of the close association between the laminar terminals of this receptor-like structures and airway smooth muscle, we will further refer to these clearly morphologically identifiable sensory end organs as 'smooth muscle-associated airway receptors (SMARs)'. Secondly, we further explored the sensory innervation of pulmonary neuroepithelial bodies (NEBs). NEBs are intraepithelial groups of neuroendocrine cells, contacted by several nerve fibre populations, at least three of which are sensory. The spinal sensory innervation of NEBs expresses CGRP and substance P, contacts NEBs at their basal pole, and is capsaicin-sensitive. The intraepithelial vagal sensory innervation of NEBs, on the other hand, appears to be myelinated and could be labelled by antibodies against VGLUT1, VGLUT2, CB and P2X3 receptors. Na+/K+-ATPase alpha3 immunostaining additionally labelled part of the vagal sensory innervation of rat pulmonary NEBs. The neurochemical coding and receptor-like appearance of SMARs and of the complex vagal sensory innervation of NEBs appeared to be almost identical and reminiscent of mechanosensors. Both SMARs and vagal nodose nerve terminals in NEBs therefore likely represent the morphological counterparts of subgroups of the extensive population of physiologically characterised myelinated vagal airway receptors, the majority of which are mechanosensitive. Electrophysiological data based on 'local' stimuli should be interpreted with caution, because of the regular close apposition of SMARs and NEBs and the very similar characteristics of their nerve terminals.

Neurochemical characterisation of sensory receptors in airway smooth muscle: comparison with pulmonary neuroepithelial bodies

Descriptions of morphologically well-defined sensory airway receptors are sparse, in contrast to the multiplicity of airway receptors that have been identified electrophysiologically. The present study aimed at further determining the location, morphology and neurochemical coding of subepithelial receptor-like structures that have been sporadically reported in the wall of large diameter airways. The results were compared with those obtained for pulmonary neuroepithelial bodies (NEBs), which are complex intraepithelial sensory airway receptors. Multiple immunocytochemical staining showed branching laminar subepithelial receptor-like endings, which were found to intercalate in the smooth muscle layer of intrapulmonary conducting airways in rats. Because of the consistent intimate association with the airway smooth muscle, the laminar terminals will further be referred to as 'smooth muscle-associated airway receptors (SMARs)'. SMARs were characterised by their Na(+)/K(+)-ATPase alpha3, vesicular glutamate transporter 1 (VGLUT1) and VGLUT2-immunoreactivity, expression of the ATP receptor P2X(3), and the presence of calcium-binding proteins. Nerve fibres giving rise to SMARs were shown to be myelinated and to have a vagal origin. Interestingly, the neurochemical coding and receptor-like appearance of SMARs appeared to be almost identical to at least part of the complex vagal sensory terminals in NEBs. Intraepithelial nerve endings in pulmonary NEBs were indeed also shown to originate from myelinated vagal afferent nerve fibres, and to express Na(+)/K(+)-ATPase alpha3, VGLUT1, VGLUT2, P2X(3) and calcium-binding proteins. Since several of the latter proteins have been reported as markers for mechanoreceptor terminals in other organs, both SMARs and the vagal nodose nerve terminals in NEBs seem good candidates to represent the morphological counterparts of at least subsets of the extensive population of physiologically characterised myelinated vagal airway mechanoreceptors. The observation that SMARs and NEBs are regularly found in each other's immediate neighbourhood, and the very similar characteristics of their nerve terminals, point out that the interpretation of electrophysiological data based on 'local' stimuli should be made with great caution.

Decreased expression of both the alpha1- and alpha2-subunits of the Na-K-ATPase reduces maximal alveolar epithelial fluid clearance

Impaired epithelial sodium channel function predisposes to delayed resorption of pulmonary edema and more severe experimental lung injury, whereas even a small fraction of the normal Na-K-ATPase activity is thought to be sufficient to maintain normal ion transport. However, direct proof is lacking. Therefore, we studied baseline and cAMP stimulated alveolar fluid clearance (AFC) in mice with a 50% decrease in lung protein expression of the alpha(1)- and/or alpha(2)-subunit of the Na-K-ATPase. There was no difference in basal and stimulated AFC in alpha(1)(+/-) or alpha(2)(+/-) mice compared with wild-type littermates. Also, the compound heterozygous mice (alpha(1)(+/-)/alpha(2)(+/-)) had normal basal AFC. However, the combined alpha(1)(+/-)/alpha(2)(+/-) mice showed a significant decrease in cAMP-stimulated AFC compared with wild-type littermates (11.1 +/- 1.0 vs. 14.9 +/- 1.8%/30 min, P < 0.001). When exposed to 96 h of >95% hyperoxia, the decrease in stimulated AFC in the alpha(1)(+/-)/alpha(2)(+/-) mice was not associated with more lung edema compared with wild-type littermates (lung wet-to-dry weight ratio 6.6 +/- 0.9 vs. 5.9 +/- 1.1, respectively; P = not significant). Thus a 50% decrease in protein expression of the alpha(1)- or alpha(2)-subunits of the Na-K-ATPase does not impair basal or stimulated AFC. However, a 50% protein reduction in both the alpha(1)- and alpha(2)-subunits of the Na-K-ATPase produces a submaximal stimulated AFC, suggesting a synergistic role for alpha(1)- and alpha(2)-subunits in cAMP-dependent alveolar epithelial fluid clearance.

The human Na,K-ATPase alpha4 isoform is a ouabain-sensitive alpha isoform that is expressed in sperm

The Na,K-ATPase generates electrochemical gradients across the plasma membrane that are responsible for numerous cellular and physiological processes. The active Na,K-ATPase is minimally composed of an alpha and a beta subunit and families of isoforms for both subunits exist. Recent studies have identified a physiological role for the rat Na,K-ATPase alpha4 isoform in sperm motility. However, very little is known about the human Na,K-ATPase alpha4 isoform other than its genomic sequence and structure and its mRNA expression pattern. Here, the human alpha4 isoform of the Na,K-ATPase is cloned, expressed, and characterized. Full length cDNAs encoding the putative human alpha4 isoform of the Na,K-ATPase were identified from a number of ESTs and a protein product corresponding to this isoform was shown to be expressed from these cDNAs. The human Na,K-ATPase alpha4 isoform protein was found to be expressed in mature sperm in human testes sections and it is localized specifically to the principle piece of human sperm. In addition, the presence of the Na,K-ATPase alpha4 isoform is absent in immature testes however its expression appears coincident with sexual maturity. And finally, the human Na,K-ATPase alpha4 isoform was shown to be as sensitive to cardiac glycoside inhibition as the human Na,K-ATPase alpha1 isoform. Considering the important role of the rat Na,K-ATPase alpha4 isoform in rat sperm motility, the demonstration that the human alpha4 isoform is a sperm-specific protein localized to the flagellum suggests a role for the human Na,K-ATPase alpha4 isoform in human sperm physiology.

Effects of warm acclimation on Na+,K+-ATPase α-subunit expression in chloride cells of Antarctic fish

The teleosts Trematomus bernacchii thrive in southern oceanic waters with temperatures below 0 degrees C. These fish have serum osmolalities almost double those found in fish of temperate waters, thereby lowering their serum's freezing point and the energy needed for ionic homeostasis. Upon warm acclimation to 4 degrees C, T. bernacchii decrease their serum osmolality and increase the Na+,K+-ATPase activity in their gills. Na+,K+-ATPase alpha1-, alpha2-, and alpha3-subunit isoforms are expressed in the gills of T. bernacchii and it is thought that Na+,K+-ATPase subunit composition in chloride cells changes with warm acclimation. Using immunohistochemistry, we compared the number of chloride cells expressing various alpha-isoforms of the Na+,K+-ATPase in the gills of cold- and warm-acclimated T. bernacchii. We found no change in the number of alpha2- or alpha3-immunopositive cells in warm-acclimated fish gills or in the number of cells immunopositive for the Na+,K+,2Cl- cotransporter. However, the number of pan-alpha-immunopositive (recognizing all three alpha-isoforms) and alpha1-immunopositive cells both increased in warm-acclimated fish. This suggests that changes in the number of alpha1-isoform-expressing chloride cells could contribute to the increased Na+,K+-ATPase activity that occurs with warm-acclimation.

Brain Na+,K+-ATPase isozyme activity and protein expression in ouabain-induced hypertension

In normotensive rats, chronic infusion of exogenous ouabain causes hypertension involving central mechanisms. To determine whether ouabain-induced hypertension is associated with specific changes in brain Na+,K+-ATPase activity and expression, we assessed brain Na+,K+-ATPase isozyme activity and protein expression in rats treated with ouabain (50 microg/day s.c. or 10 microg/day i.c.v. for 14 days). Resting mean arterial pressure (MAP) was higher in s.c.- and i.c.v.-ouabain-treated animals vs. control (124+/-2 vs. 105+/-2 and 130+/-2 vs. 109+/-2, respectively, p<0.01). Ouabain infused s.c. or i.c.v. for 14 days had no effect on Na+,K+-ATPase isozyme activity in hypothalamic, pontine/medullary or cortical microsomes. However, the percent increase in total Na+,K+-ATPase activity produced in vitro by antibody Fab fragments that bind ouabain with high affinity (Digibind) was two-fold greater in s.c.- and i.c.v.-ouabain-treated rats vs. control, but only in hypothalamic microsomes. Thus, ouabain infused s.c. or i.c.v. does appear to directly inhibit Na+,K+-ATPase activity in the hypothalamus. On the other hand, in the hypothalamus, s.c.- and i.c.v.-ouabain infusions tended to increase alpha3 (by 30-44%), but had no effect on alpha1 or alpha2 Na+,K+-ATPase isozyme protein expression. In addition, ouabain was found to partially dissociate from the Na+,K+-ATPase enzyme following sample processing. Thus, the inability to detect a decrease in enzyme activity in the hypothalamus in response to ouabain may be due, in part, to an increase in enzyme expression and the dissociation of ouabain during sample processing.

Up-regulation of Na(+),K(+)-ATPase alpha 3-isoform and down-regulation of the alpha1-isoform in human colorectal cancer

We investigated expression levels of Na(+),K(+)-ATPase alpha-isoforms and their ATPase activities in human colorectal cancer tissue and the accompanying normal mucosa. A decrease in expression of the alpha1-isoform protein was observed in all sampled cancer tissues compared with the normal mucosae. The level of ouabain (5 microM)-sensitive Na(+),K(+)-ATPase activity in carcinomas was 81+/-5% that of in the normal mucosae. The mRNA expression of alpha2- and alpha 4-isoforms was decreased in almost all the carcinoma samples. Interestingly, the expression level of the alpha 3-isoform protein in the cancer tissue was higher than that of the normal mucosa. These results indicate that a decrease in the alpha1-isoform expression and an increase in the alpha 3-isoform expression may be associated with colorectal cancer.

Time-course changes in the expression of Na, K-ATPase and the morphometry of mitochondrion-rich cells in gills of euryhaline tilapia (Oreochromis mossambicus) during freshwater acclimation

Changes in expression of Na, K-ATPase (NKA) and morphometry of mitochondrion-rich (MR) cells in gills of tilapia were investigated on a 96-hr time course following transfer from seawater (SW) to fresh water (FW). A transient decline in plasma osmolality and Na+, Cl- concentrations occurred from 3 hrs onward. Gills responded to FW transfer by decreasing NKA activity as early as 3 hrs from transfer. This response was followed by a significant decrease in the NKA isoform alpha1-mRNA abundance, which was detected by real-time PCR at 6 hrs post transfer. Next, a decrease of alpha1-protein amounts were observed from 6 hrs until 24 hrs post transfer. Additionally, during the time course of FW transfer, modifications in number and size of subtypes of gill MR cells were observed although no significant difference was found in densities of all subtypes of MR cells. These modifications were found as early as 3 hrs, evident at 6 hrs (exhibition of 3 subtypes of MR cells), and mostly completed by 24 hrs post transfer. Such rapid responses (in 3 hrs) as concurrent changes in branchial NKA expression and modifications of MR cell subtypes are thought to improve the osmoregulatory capacity of tilapia in acclimation from hypertonic SW to hypotonic FW.

Intense exercise up-regulates Na+,K+-ATPase isoform mRNA, but not protein expression in human skeletal muscle

Characterization of expression of, and consequently also the acute exercise effects on, Na(+),K(+)-ATPase isoforms in human skeletal muscle remains incomplete and was therefore investigated. Fifteen healthy subjects (eight males, seven females) performed fatiguing, knee extensor exercise at approximately 40% of their maximal work output per contraction. A vastus lateralis muscle biopsy was taken at rest, fatigue and 3 and 24 h postexercise, and analysed for Na(+),K(+)-ATPase alpha(1), alpha(2), alpha(3), beta(1), beta(2) and beta(3) mRNA and crude homogenate protein expression, using Real-Time RT-PCR and immunoblotting, respectively. Each individual expressed gene transcripts and protein bands for each Na(+),K(+)-ATPase isoform. Each isoform was also expressed in a primary human skeletal muscle cell culture. Intense exercise (352 +/- 69 s; mean +/-s.e.m.) immediately increased alpha(3) and beta(2) mRNA by 2.4- and 1.7-fold, respectively (P < 0.05), whilst alpha(1) and alpha(2) mRNA were increased by 2.5- and 3.5-fold at 24 h and 3 h postexercise, respectively (P < 0.05). No significant change occurred for beta(1) and beta(3) mRNA, reflecting variable time-dependent responses. When the average postexercise value was contrasted to rest, mRNA increased for alpha(1), alpha(2), alpha(3), beta(1), beta(2) and beta(3) isoforms, by 1.4-, 2.2-, 1.4-, 1.1-, 1.0- and 1.0-fold, respectively (P < 0.05). However, exercise did not alter the protein abundance of the alpha(1)-alpha(3) and beta(1)-beta(3) isoforms. Thus, human skeletal muscle expresses each of the Na(+),K(+)-ATPase alpha(1), alpha(2), alpha(3), beta(1), beta(2) and beta(3) isoforms, evidenced at both transcription and protein levels. Whilst brief exercise increased Na(+),K(+)-ATPase isoform mRNA expression, there was no effect on isoform protein expression, suggesting that the exercise challenge was insufficient for muscle Na(+),K(+)-ATPase up-regulation.

Quantification of α-subunit isoforms of Na,K-ATPase in rat resistance vessels

AIM

Rat mesenteric resistance vessels (RV) were characterized with respect to concentration of individual alpha-subunit isoforms of Na,K-ATPase.

METHODS

Total vessel homogenates were used to avoid any loss or subfractionation of membranes. They were applied to sodium dodecyl sulphate gels and, for calibration, in parallel lanes were run purified rat Na,K-ATPase preparations with known isoform distribution and content. The capacity per mg protein for Na+-dependent 32P-phosphorylation of Na,K-ATPase isolated from rat kidney was used for alpha1 calibration and that for high-affinity (3H)ouabain binding of Na,K-ATPase isolated from rat brain was used for (alpha2 + alpha3) calibration. Western blots containing homogenate proteins and reference enzyme were incubated with isoform-specific antibodies and radiolabelled secondary antibodies. The signals from adjacent alpha spots were used for qualitative and quantitative characterization of rat vessels.

RESULTS

A concentration of 100.7 +/- 14.4 pmol (n = 11) per g wet weight of the alpha1-isoform containing Na,K-ATPase was found in RV from 12-14-week rats. A much lower and more unreliable content of alpha2- and alpha3-isoforms was found. These ouabain-sensitive isoforms seem to represent a maximum of 5-10% each compared with the ouabain-insensitive rat alpha1-isoform.

CONCLUSIONS

The isoform pattern in RV, in which the isoform with high/intermediate Na+-affinity is the absolutely dominating one representing nearly all sodium pumps in this tissue, is very different from that seen in rat skeletal muscles. Due to the high content of the ouabain-insensitive alpha1-isoform in rat RV this species would seem a less relevant model in studies addressing a role of cardiac glycosides and putative endogenous ouabain-like factors in hypertension.

Expression and cellular localization of Na,K-ATPase isoforms in the rat ventral prostate

OBJECTIVE

To determine the expression and plasma membrane domain location of isoforms of Na,K-ATPase in the rat ventral prostate.

MATERIALS AND METHODS

Ventral prostate glands from adult male rats were dissected, cryosectioned (7 micro m) and attached to poly-l-lysine coated glass slides. The sections were then fixed in methanol and subjected to indirect immunofluorescence and immunoperoxidase procedures using a panel of well-characterized monoclonal and polyclonal antibodies raised against known Na,K-ATPase subunit isoforms. Immunofluorescence micrographs were digitally captured and analysed by image analysis software.

RESULTS

There was expression of Na,K-ATPase alpha1, beta1, beta2 and beta3 subunit isoforms in the lateral and basolateral plasma membrane domains of prostatic epithelial cells. The alpha1 isoform was abundant but there was no evidence of alpha2, alpha3 or gamma isoform expression in epithelial cells. The alpha3 isoform was not detected, but there was a relatively low level of alpha2 isoform expression in the smooth muscle and stroma.

CONCLUSION

Rat prostate Na,K-ATPase consists of alpha1/beta1, alpha1/beta2 and alpha1/beta3 isoenzymes. These isoform proteins were located in the lateral and basolateral plasma membrane domains of ventral prostatic epithelial cells. The distribution and subcellular localization of Na,K-ATPase is different in rodent and human prostate. Basolateral Na,K-ATPase probably contributes to the establishment of transepithelial ionic gradients that are a prerequisite for the uptake of metabolites by secondary active transport mechanisms and active citrate secretion.

Na(+) pump alpha 2-isoform specifically couples to contractility in vascular smooth muscle: evidence from gene-targeted neonatal mice

The relative expression of alpha(1)- and alpha(2)-Na(+)/K(+)-ATPase isoforms found in vascular smooth muscle is developmentally regulated and under hormonal and neurogenic control. The physiological roles of these isoforms in vascular function are not known. It has been postulated that the alpha(1)-isoform serves a "housekeeping" role, whereas the alpha(2)-isoform localizes to a subsarcolemmal compartment and modulates contractility. To test this hypothesis, isoform-specific gene-targeted mice in which the mRNA for either the alpha(1)- or the alpha(2)-Na(+)/K(+)-ATPase isoform was ablated were utilized. Both of these knockouts, alpha(1)(-/-) and alpha(2)(-/-), are lethal; the latter dies at birth, which allows this neonatal aorta to be studied. Isometric force in alpha(2)(-/-)-aorta was more sensitive to contractile agonists and less sensitive to the vasodilators forskolin and sodium nitroprusside (SNP) than wild-type (WT) aorta; alpha(2)(+/-)-aortas had intermediate values. In contrast, neonatal alpha(1)(+/-)-aorta was similar to WT. Western blot analysis indicated a population of 70% alpha(1)- and 30% alpha(2)-isoforms in the WT. Thus in terms of the total Na(+)/K(+)-ATPase protein, the alpha(2)(-/-)-aorta (at 70%) would be similar to the alpha(1)(+/-)-aorta (at 65%) but with a dramatically different phenotype. These data suggest that individual alpha-isoforms of the Na(+)/K(+)-ATPase differ functionally and that the alpha(2)-isoform couples more strongly to activation-relaxation pathways. Three-dimensional image-acquisition and deconvolution analyses suggest that the alpha(2)-isoform is distributed differently than the alpha(1)-isoform. Importantly, these isoforms do not localize to the same regions.

Ischemia-induced phosphorylation of phospholemman directly activates rat cardiac Na/K-ATPase

Regulation of the Na/K ATPase by protein kinases is model-specific. We have observed a profound activation of the sarcolemmal Na/K ATPase during cardiac ischemia, which is masked by an inhibitor of the enzyme in the cytosol. The aim of these studies was to characterize the pathways involved in this activation in the Langendorff-perfused rat heart. Na/K ATPase activity was determined by measuring ouabain-sensitive phosphate generation by cardiac homogenates at 37 degrees C. In isolated sarcolemma, ischemia (30 min) caused a substantial activation of the Na/K ATPase compared with aerobic controls, which was abolished by perfusing the heart with staurosporine or H89. However, the alpha1 subunit of the Na/K ATPase was not phosphorylated during ischemia. The sarcolemmal protein phospholemman (PLM) was found associated with the Na/K ATPase alpha1 and beta1 but not alpha2 subunits, and PLM increased its association with the catalytic subunit of PKA following ischemia. In vitro 14-3-3 binding assays indicated that PLM was phosphorylated following ischemia. These results indicate that the ischemia-induced activation of the Na/K ATPase is indirect, through phosphorylation of PLM, which is an integral part of the Na/K ATPase enzyme complex in the heart. The role of PLM is analogous to phospholamban in regulating the sarcoplasmic reticulum calcium ATPase.

Phospholemman expression in extraglomerular mesangium and afferent arteriole of the juxtaglomerular apparatus

The molecular mechanisms with which the juxtaglomerular apparatus accomplishes its twin functions, acute regulation of glomerular blood flow and secretion of renin, are still not clearly understood. Least understood is the role of the extraglomerular mesangial (EM) cells, also known as lacis or Goormaghtigh cells, which lie sandwiched between the macula densa and the afferent and efferent arterioles. Here, we report that immunoreactivity for phospholemman (FXYD1), a single-span membrane protein homologous to the gamma (gamma) sub-unit of the Na,K-ATPase, is found in the kidney in EM cells with the Na,K-ATPase beta2-subunit and in cortical blood vessels and the afferent arteriole with Na,K-ATPase alpha2 and beta2. Phospholemman's distribution in EM cells is distinct from that of the Na,K-ATPase gamma-subunit, which is found on the basolateral surface of macula densa cells with Na,K-ATPase alpha1 and beta1. Phospholemman is a major kinase target, and its location in the juxtaglomerular apparatus suggests that it is involved in tubuloglomerular feedback.

Epithelial Na, K-ATPase expression is down-regulated in canine prostate cancer; a possible consequence of metabolic transformation in the process of prostate malignancy

BACKGROUND: An important physiological function of the normal prostate gland is the synthesis and secretion of a citrate rich prostatic fluid. In prostate cancer, citrate production levels are reduced as a result of altered cellular metabolism and bioenergetics. Na, K-ATPase is essential for citrate production since the inward Na+ gradients it generates are utilized for the Na+ dependent uptake of aspartate, a major substrate for citrate synthesis. The objective of this study was to compare the expression of previously identified Na, K-ATPase isoforms in normal canine prostate, benign prostatic hyperplasia (BPH) and prostatic adenocarcinoma (PCa) using immunohistochemistry in order to determine whether reduced citrate levels in PCa are also accompanied by changes in Na, K-ATPase expression. RESULTS: Expression of Na, K-ATPase alpha1 and beta1 isoforms was observed in the lateral and basolateral plasma membrane domains of prostatic epithelial cells in normal and BPH prostates. Canine kidney was used as positive control for expression of Na, K-ATPase alpha1 and gamma isoforms. The alpha1 isoform was detected in abundance in prostatic epithelial cells but there was no evidence of alpha2, alpha3 or gamma subunit expression. In advanced PCa, Na, K-ATPase alpha1 isoform expression was significantly lower compared to normal and BPH glands. The abundant basolateral immunostaining observed in normal and BPH tissue was significantly attenuated in PCa. CONCLUSION: The loss of epithelial structure and function and the transformation of normal epithelial cells to malignant cells in the canine prostate have important implications for cellular metabolism and are accompanied by a down regulation of Na, K-ATPase.

Phospholemman, a single-span membrane protein, is an accessory protein of Na,K-ATPase in cerebellum and choroid plexus

Phospholemman (FXYD1) is a homolog of the Na,K-ATPase gamma subunit (FXYD2), a small accessory protein that modulates ATPase activity. Here we show that phospholemman is highly expressed in selected structures in the CNS. It is most abundant in cerebellum, where it was detected in the molecular layer, in Purkinje neurons, and in axons traversing the granule cell layer. Phospholemman was particularly enriched in choroid plexus, the organ that secretes CSF in the ventricles, where it colocalized with Na,K-ATPase in the apical membrane. It was also enriched, with Na,K-ATPase, in certain tanycytes or ependymal cells of the ventricle wall. Two different experimental approaches demonstrated that phospholemman physically associated with the Na,K-ATPase in cerebellum and choroid plexus: the proteins copurified after detergent treatment and co-immunoprecipitated from solubilized crude membranes using either anti-phospholemman or anti-Na,K-ATPase antibodies. Phospholemman antibodies precipitated all three Na,K-ATPase alpha subunit isoforms (alpha1-alpha3) from cerebellum, indicating that the interaction is not specific to a particular alpha isoform and consistent with the presence of phospholemman in both neurons and glia. Antibodies against the C-terminal domain of phospholemman reduced Na,K-ATPase activity in vitro without effect on Na+ affinity. At least two other FXYD family members have been detected in the CNS, suggesting that additional complexity of sodium pump regulation will be found.

Kidney injury molecule-1 expression in murine polycystic kidney disease

Kidney injury molecule-1 (Kim-1) is a type 1 membrane protein maximally upregulated in proliferating and dedifferentiated tubular cells after renal ischemia. Because epithelial dedifferentiation, proliferation, and local ischemia may play a role in the pathophysiology of autosomal dominant polycystic kidney disease, we investigated Kim-1 expression in a mouse model of this disease. In the Pkd2(WS25/-) mouse model for autosomal dominant polycystic kidney disease, cystic kidneys show markedly upregulated Kim-1 levels compared with noncystic control kidneys. Kim-1 is present in a subset of cysts of different sizes and segmental origins and in clusters of proximal tubules near cysts. Kim-1-expressing tubular cells show decreased complexity and quantity of basolateral staining for Na-K-ATPase. Other changes in polarity characteristic of ischemic injury are not present in Kim-1-expressing pericystic tubules. Polycystin-2 expression is preserved in Kim-1-expressing tubules. The interstitium surrounding Kim-1-expressing tubules shows high proliferative activity and staining for smooth muscle alpha-actin, characteristic of myofibroblasts. Although the functional role of the protein in cysts remains unknown, Kim-1 expression in tubules is strongly associated with partial dedifferentiation of epithelial cells and may play a role in the development of interstitial fibrosis.

Changes in brain Na, K-ATPase isoform expression and enzymatic activity after aortic constriction

The objective of the present study was to test the hypothesis that brain Na, K-ATPase expression and/or activity is altered following an increase in blood pressure produced by constriction of the abdominal aorta just proximal to the renal arteries. A suprarenal constriction (SRC) was made to conform to the diameter of a 19-gauge (19-G) or 20-gauge (20-G) needle, while in a sham-operated group (Sham) the aorta was exposed surgically but not constricted. Within 1 week of SRC, mean arterial pressure was increased and remained elevated at 4 weeks post surgery. At 1 week, whole-brain Na, K-ATPase mRNA levels were depressed for all isoforms (alpha1 approximately beta1>alpha2>alpha3). No changes were observed in the hypothalamus. At 4 weeks, the mRNA levels of all alpha isoforms were significantly increased in the whole brain and these changes were paralleled by an increase of alpha2 and alpha3 transcript in the hypothalamus. beta1 mRNA expression was increased in the hypothalamus only. The alpha-isoform protein expression generally changed in the same direction as mRNA changes at both 1 and 4 weeks, as did alpha1 enzyme activity at 1 week and the combined alpha2/alpha3 enzyme activities at 4 weeks. Since inhibition of brain Na, K-ATPase increases sympathetic nervous system (SNS) activity and blood pressure, the decreases in brain Na, K-ATPase expression and activity at 1 week post SRC may contribute to the hypertension during its developmental phase, while the increase in the alpha2/alpha3 brain expression and activity at 4 weeks may be a compensatory response to established hypertension.

K+-induced hyperpolarization in rat mesenteric artery: identification, localization and role of Na+/K+-ATPases

1. Mechanisms underlying K(+)-induced hyperpolarizations in the presence and absence of phenylephrine were investigated in endothelium-denuded rat mesenteric arteries (for all mean values, n=4). 2. Myocyte resting membrane potential (m.p.) was -58.8+/-0.8 mV. Application of 5 mM KCl produced similar hyperpolarizations in the absence (17.6+/-0.7 mV) or presence (15.8+/-1.0 mV) of 500 nM ouabain. In the presence of ouabain +30 microM barium, hyperpolarization to 5 mM KCl was essentially abolished. 3. In the presence of 10 microM phenylephrine (m.p. -33.7+/-3 mV), repolarization to 5 mM KCl did not occur in the presence or absence of 4-aminopyridine but was restored (-26.9+/-1.8 mV) on addition of iberiotoxin (100 nM). Under these conditions the K+-induced repolarization was insensitive to barium (30 microM) but abolished by 500 nM ouabain alone. 4. In the presence of phenylephrine + iberiotoxin the hyperpolarization to 5 mM K(+) was inhibited in the additional presence of 300 nM levcromakalim, an action which was reversed by 10 microM glibenclamide. 5. RT-PCR, Western blotting and immunohistochemical techniques collectively showed the presence of alpha(1)-, alpha(2)- and alpha(3)-subunits of Na(+)/K(+)-ATPase in the myocytes. 6. In K(+)-free solution, re-introduction of K(+) (to 4.6 mM) hyperpolarized myocytes by 20.9+/-0.5 mV, an effect unchanged by 500 nM ouabain but abolished by 500 microM ouabain. 7. We conclude that under basal conditions, Na(+)/K(+)-ATPases containing alpha(2)- and/or alpha(3)-subunits are partially responsible for the observed K(+)-induced effects. The opening of myocyte K(+) channels (by levcromakalim or phenylephrine) creates a 'K(+) cloud' around the cells which fully activates Na(+)/K(+)-ATPase and thereby abolishes further responses to [K(+)](o) elevation.

Differential regulation of renal Na,K-ATPase by splice variants of the gamma subunit

Sodium and potassium-exchanging adenosine triphosphatase (Na,K-ATPase) in the kidney is associated with the gamma subunit (gamma, FXYD2), a single-span membrane protein that modulates ATPase properties. Rat and human gamma occur in two splice variants, gamma(a) and gamma(b), with different N termini. Here we investigated their structural heterogeneity and functional effects on Na,K-ATPase properties. Both forms were post-translationally modified during in vitro translation with microsomes, indicating that there are four possible forms of gamma. Site-directed mutagenesis revealed Thr(2) and Ser(5) as potential sites for post-translational modification. Similar modification can occur in cells, with consequences for Na,K-ATPase properties. We showed previously that stable transfection of gamma(a) into NRK-52E cells resulted in reduction of apparent affinities for Na(+) and K(+). Individual clones differed in gamma post-translational modification, however, and the effect on Na(+) affinity was absent in clones with full modification. Here, transfection of gamma(b) also resulted in clones with or without post-translational modification. Both groups showed a reduction in Na(+) affinity, but modification was required for the effect on K(+) affinity. There were minor increases in ATP affinity. The physiological importance of the reduction in Na(+) affinity was shown by the slower growth of gamma(a), gamma(b), and gamma(b') transfectants in culture. The differential influence of the four structural variants of gamma on affinities of the Na,K-ATPase for Na(+) and K(+), together with our previous finding of different distributions of gamma(a) and gamma(b) along the rat nephron, suggests a highly specific mode of regulation of sodium pump properties in kidney.

Novel regulation of cell [Na(+)] in macula densa cells: apical Na(+) recycling by H-K-ATPase

Na-K-ATPase is the nearly ubiquitous enzyme that maintains low-Na(+), high-K(+) concentrations in cells by actively extruding Na(+) in exchange for K(+). The prevailing paradigm in polarized absorbing epithelial cells, including renal nephron segments and intestine, has been that Na-K-ATPase is restricted to the basolateral membrane domain, where it plays a prominent role in Na(+) absorption. We have found, however, that macula densa (MD) cells lack functionally and immunologically detectable amounts of Na-K-ATPase protein. In fact, these cells appear to regulate their cytosolic [Na(+)] via another member of the P-type ATPase family, the colonic form of H-K-ATPase, which is located at the apical membrane in these cells. We now report that this constitutively expressed apical MD colonic H-K-ATPase can function as a Na(H)-K-ATPase and regulate cytosolic [Na(+)] in a novel manner. This apical Na(+)-recycling mechanism may be important as part of the sensor function of MD cells and represents a new paradigm in cell [Na(+)] regulation.

Prevention of kidney ischemia/reperfusion-induced functional injury, MAPK and MAPK kinase activation, and inflammation by remote transient ureteral obstruction

Protection against ischemic kidney injury is afforded by 24 h of ureteral obstruction (UO) applied 6 or 8 days prior to the ischemia. Uremia or humoral factors are not responsible for the protection, since unilateral UO confers protection on that kidney but not the contralateral kidney. Prior UO results in reduced postischemic outer medullary congestion and leukocyte infiltration. Prior UO results in reduced postischemic phosphorylation of c-Jun N-terminal stress-activated protein kinase 1/2 (JNK1/2), p38, mitogen-activated protein kinase (MAPK) kinase 4 (MKK4), and MKK3/6. Very few cells stain positively for proliferating cell nuclear antigen after obstruction, indicating that subsequent protection against ischemia is not related to proliferation with increased numbers of newly formed daughter cells more resistant to injury. UO increases the expression of heat shock protein (HSP)-25 and HSP-72. The increased HSP-25 expression persists for 6 or 8 days, whereas HSP-72 does not. HSP-25 expression is increased in the proximal tubule cells in the outer stripe of the outer medulla postobstruction, prior to, and 24 h after ischemia. In LLC-PK(1) renal epithelial cells, adenovirus-expressed human HSP-27 confers resistance to chemical anoxia and oxidative stress. Increased HSP-27 expression in LLC-PK(1) cells results in reduced H(2)O(2)-induced phosphorylation of JNK1/2 and p38. In conclusion, prior transient UO renders the kidney resistant to ischemia. This resistance to functional consequences of ischemia is associated with reduced postischemic activation of JNK, p38 MAP kinases, and their upstream MAPK kinases. The persistent increase in HSP-25 that occurs as a result of UO may contribute to the reduction in phosphorylation of MAPKs that have been implicated in adhesion molecule up-regulation and cell death.

The alpha1 isoform of Na+,K+-ATPase in rat soleus and extensor digitorum longus

A novel immunochemical method was used for determination of the concentration of Na,K-adenosine triphosphatase (ATPase) containing the ouabain-insensitive alpha1 peptide in rat m. soleus and extensor digitorum longus (EDL). Homogenates of soleus and EDL from 4-week or 10-11-week rats were run on sodium dodecyl sulphate (SDS) gels and in parallel lanes was run a well-characterized preparation of Na,K-ATPase isolated from rat kidney that is known to contain only the alpha1 isoform. After electroblotting to PVDF membranes blots were incubated with the alpha1 specific monoclonal antibody 3B, then with an 125I-coupled secondary antibody, and finally the specific labelling of adjacent alpha spots was analysed by means of an electronic autoradiography system (Packard InstantImager). As the alpha1 content of reference Na,K-ATPase was known from the specific Na+-dependent 32P-phosphorylation capacity, the alpha1 content of adjacent alpha spots in homogenates from soleus and EDL could be calculated. In soleus and EDL from 4-week rats an alpha1 concentration of 135-220 pmol (g tissue)(-1) was found, dependent on the conditions of the experiments but without significant differences between the two types of muscle. In 10-11-week rats a significantly lower concentration of 70-80 and 40-60 pmol (g tissue)(-1) in soleus and EDL, respectively, was found. Ouabain-insensitive Na,K-ATPase containing the alpha1 peptide may thus represent 15-25% of the total number of pumps in skeletal muscle if another 20-30% has to be added to the pool known from (3H)ouabain binding.

Immunocytochemical localization of Na-K-ATPase alpha- and gamma-subunits in rat kidney

The gamma-subunit of the Na-K-ATPase is a single-span membrane protein that alters the kinetic properties of the enzyme. It is expressed in the kidney, but our initial observations indicated that it is not present in all nephron segments (Arystarkhova E, Wetzel RK, Asinovski NK, and Sweadner KJ. J Biol Chem 274: 33183-33185, 1999). Here we used triple-label confocal immunofluorescence microscopy in rat kidney with antibodies to Na-K-ATPase alpha1- and gamma-subunits and nephron segment-specific markers. Na-K-ATPase alpha1-subunit stain was low but unambiguous in proximal segments, moderate in macula densa, connecting tubules, and cortical collecting ducts, high in thick ascending limb and distal convoluted tubules, and nearly undetectable in glomeruli, descending and ascending thin limb, and medullary collecting ducts. The gamma-subunit colocalized at staining levels similar to alpha1-subunit in basolateral membranes in all segments except cortical thick ascending limb and cortical collecting ducts, which had alpha1-subunit but no detectable gamma-subunit stain. Selective gamma-subunit expression may contribute to the variations in Na-K-ATPase properties in different renal segments.