Epitope and mimotope for an antibody to the Na, K-ATPase

Proteolytic fragmentation for epitope mapping

The use of antigen fragments generated by specific proteolytic cleavage is a relatively simple "library" approach for epitope mapping in which possible overlapping fragments are screened with the antibody on Western blots. Proteolytic fragmentation with numerous proteases having different cleavage specificites can be carried out on native and denaturated proteins, generating a small and large number of fragments, respectively. To determine the antigenic site of a monoclonal antibody, we have examined the limited proteolytic digestion of the transducin alpha -subunit with four different proteases and detected antibody binding to fragments by Western blot. Using this approach, the epitope for this antibody was localized within the amino-terminal 17 residues of transducin alpha -subunit.

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.

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.

Effect of ethacrynic acid on sodium pump alpha isoforms in SH-SY5Y cells

BACKGROUND

Ethacrynic acid (ECA), a diuretic that has several cellular actions, increases expression of the sodium and potassium-activated adenosine triphosphatase (Na, K-ATPase or Na pump) in normal lymphocytes, but not in lymphocytes of bipolar patients. While this has been proposed to be important in the pathophysiology of bipolar illness, the response of neural tissues to ECA is unknown.

METHODS

Human neuroblastoma SH-SY5Y cells differentiated with 10-microM retinoic acid were treated with various ECA concentrations for 3 days, and changes in Na-pump alpha-isoform expression were quantified with densitometric analysis of Western bands.

RESULTS

Expression of alpha1 and alpha3 Na pump isoforms significantly increased with 10-5 M ECA. Cells treated with 10-6 or 10-7 M ECA showed no change in Na-pump expression, while cells treated with 10-4 M ECA died. The alpha2 isoform could not be detected in differentiated SH-SY5Y cells.

CONCLUSIONS

The effect of ECA on alpha1-isoform in neural tissue is similar to that observed in lymphocytes. As alpha3 isoform is not expressed in lymphocytes, however, we conclude that lymphocytes are an incomplete model of neural tissue.

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.

The Na,K-ATPase alpha 2 isoform is expressed in neurons, and its absence disrupts neuronal activity in newborn mice

Na,K-ATPase is an ion transporter that impacts neural and glial physiology by direct electrogenic activity and the modulation of ion gradients. Its three isoforms in brain have cell-type and development-specific expression patterns. Interestingly, our studies demonstrate that in late gestation, the alpha2 isoform is widely expressed in neurons, unlike in the adult brain, in which alpha2 has been shown to be expressed primarily in astrocytes. This unexpected distribution of alpha2 isoform expression in neurons is interesting in light of our examination of mice lacking the alpha2 isoform which fail to survive after birth. These animals showed no movement; however, defects in gross brain development, muscle contractility, neuromuscular transmission, and lung development were ruled out. Akinesia suggests a primary neuronal defect and electrophysiological recordings in the pre-Bötzinger complex, the brainstem breathing center, showed reduction of respiratory rhythm activity, with less regular and smaller population bursts. These data demonstrate that the Na,K-ATPase alpha2 isoform could be important in the modulation of neuronal activity in the neonate.

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.

Downregulation of cardiac myocyte Na(+)-K(+)-ATPase by adenovirus-mediated expression of an alpha-subunit fragment

Cultured rat cardiac myocytes and A7r5 cells were transfected with an adenoviral vector used earlier for in vivo expression of functional alpha(2)-isoform of the catalytic subunit of rat Na(+)-K(+)-ATPase. Expressions of truncated forms of alpha(2), but little or no intact alpha(2), were detected, suggesting the rapid degradation of alpha(2) in these cultured cells. In neonatal myocytes normally containing the alpha(1)- and the alpha(3)-isoforms, expression of the alpha(2)-fragment led to 1) a significant decrease in the level of endogenous alpha(1)-protein and a modest decrease in alpha(3)-protein, 2) decreases in mRNAs of alpha(1) and alpha(3), 3) decrease in Na(+)-K(+)-ATPase function measured as ouabain-sensitive Rb(+) uptake, 4) increase in intracellular Ca(2+) concentration similar to that induced by ouabain, and 5) eventual loss of cell viability. These findings indicate that the alpha(2)-fragment downregulates endogenous Na(+)-K(+)- ATPase most likely by dominant negative interference either with folding and/or assembly of the predominant housekeeping alpha(1)-isoform or with signal transducing function of the enzyme. Demonstration of rise in intracellular Ca(2+) resulting from alpha(1)-downregulation 1) does not support the previously suggested special roles of less abundant alpha(2)- and alpha(3)-isoforms in the regulation of cardiac Ca(2+), 2) lends indirect support to proposals that observed decrease in total Na(+)-K(+)-ATPase of the failing heart may be a mechanism to compensate for impaired cardiac contractility, and 3) suggests the potential therapeutic utility of dominant negative inhibition of Na(+)-K(+)-ATPase.

Quantification of the alpha(3) subunit of the Na(+)/K(+)-ATPase in developing rat cerebellum

Cerebellar Purkinje neurons of rats have been shown to exhibit a progressive increase in resting membrane potential as the animals develop postnatally. The magnitude of this increase was equivalent in magnitude to the increase in the depolarizing action of ouabain, consistent with a role for the Na(+)/K(+)-pump in the hyperpolarization. Ouabain binding sites in whole cerebellum also increased with age. The present study was undertaken to confirm that the increases in ouabain binding and the electrophysiological responses to ouabain were a consequence of increases in the sodium pump and to determine whether the changes seen at the whole organ level were reflective of changes taking place at the cellular level. Using antibodies directed against the alpha(1), alpha(2), and alpha(3) subunits of the Na(+)/K(+)-ATPase, rats between 13 and 19 days of age exhibited a statistically significant increase in the relative amount of the alpha(3) subunit at the level of the whole organ, as determined by Western and slot blot analyses, with no change in the levels of either the alpha(1) or the alpha(2) subunit. Using immunohistochemistry, the alpha(3) subunit was shown to increase in both the Purkinje cell layer and the white matter during this postnatal time period, while the alpha(1) subunit increased in the granular layer. These results support and extend previous work, which pointed to a role for the electrogenic sodium pump in the developmental increase in Purkinje cell membrane potential. Furthermore, the data provide a cellular mechanism underlying the increase in resting membrane potential, that is, by the specific modulation of the alpha(3) subunit isoform.

Down-regulation of Na(+)/K(+)-ATPase alpha(2) isoform in denervated rat vas deferens

In the rat vas deferens, an organ richly innervated by peripheral sympathetic neurons, we have demonstrated recently the expression of alpha(1) and alpha(2), but not alpha(3) isoforms of the alpha subunit of Na(+)/K(+)-ATPase (EC 3.6.1.37), a membrane-bound enzyme of vital function for living cells (Noël et al., Biochem Pharmacol 55: 1531-1535, 1998). In the present work, we characterized, qualitatively and quantitatively, Na(+)/K(+)-ATPase alpha isoforms in denervated rat vasa deferentia. [(3)H]Ouabain binding at concentrations defined for high-affinity isoforms (alpha(2) and/or alpha(3)) detected only one class of specific binding sites in control (C) and denervated (D) vas deferens. Although the dissociation constant was similar for both groups [K(d) = 138 +/- 14 nM (C) and 125 +/- 8 nM (D)], a marked decrease in density was observed after denervation [716 +/- 81 fmol.mg protein(-1) (C) and 445 +/- 34 fmol.mg protein(-1) (D), P < 0.05]. In addition, western blotting revealed that denervated vasa deferentia produce the alpha(1) and alpha(2) isoforms but not alpha(3), just as we reported for the controls previously (Noël et al., Biochem Pharmacol 55: 1531-1535, 1998). Densitometric analysis showed a decrease of the alpha(2) isoform by about 40% in denervated organs, in very good agreement with what was shown with the [(3)H]ouabain binding technique, but no significant change in alpha(1) isoform density. Truncated alpha(1) (alpha(1)T), an isoform suggested to exist in the guinea pig vas deferens, was not detected. Altogether, our results demonstrated that Na(+)/K(+)-ATPase alpha(2) is down-regulated after sympathetic denervation of the rat vas deferens.

Cellular and subcellular specification of Na,K-ATPase alpha and beta isoforms in the postnatal development of mouse retina

The Na,K-ATPase is a dominant factor in retinal energy metabolism, and unique combinations of isoforms of its alpha and beta subunits are expressed in different cell types and determine its functional properties. We used isoform-specific antibodies and fluorescence confocal microscopy to determine the expression of Na,K-ATPase alpha and beta subunits in the mouse and rat retina. In the adult retina, alpha1 was found in Müller and horizontal cells, alpha2 in some Müller glia, and alpha3 in photoreceptors and all retinal neurons. beta1 was largely restricted to horizontal, amacrine, and ganglion cells; beta2 was largely restricted to photoreceptors, bipolar cells, and Müller glia; and beta3 was largely restricted to photoreceptors. Photoreceptor inner segments have the highest concentration of Na,K-ATPase in adult retinas. Isoform distribution exhibited marked changes during postnatal development. alpha3 and beta2 were in undifferentiated photoreceptor somas at birth but only later were targeted to inner segments and synaptic terminals. beta3, in contrast, was expressed late in photoreceptor differentiation and was immediately targeted to inner segments. A high level of beta1 expression in horizontal cells preceded migration, whereas increases in beta2 expression in bipolar cells occurred very late, coinciding with synaptogenesis in the inner plexiform layer. Most of the spatial specification of Na,K-ATPase isoform expression was completed before eye opening and the onset of electroretinographic responses on postnatal day 13 (P13), but quantitative increase continued until P22 in parallel with synaptogenesis.

Cellular and subcellular specification of Na,K-ATPase alpha and beta isoforms in the postnatal development of mouse retina

The Na,K-ATPase is a dominant factor in retinal energy metabolism, and unique combinations of isoforms of its alpha and beta subunits are expressed in different cell types and determine its functional properties. We used isoform-specific antibodies and fluorescence confocal microscopy to determine the expression of Na,K-ATPase alpha and beta subunits in the mouse and rat retina. In the adult retina, alpha1 was found in Müller and horizontal cells, alpha2 in some Müller glia, and alpha3 in photoreceptors and all retinal neurons. beta1 was largely restricted to horizontal, amacrine, and ganglion cells; beta2 was largely restricted to photoreceptors, bipolar cells, and Müller glia; and beta3 was largely restricted to photoreceptors. Photoreceptor inner segments have the highest concentration of Na,K-ATPase in adult retinas. Isoform distribution exhibited marked changes during postnatal development. alpha3 and beta2 were in undifferentiated photoreceptor somas at birth but only later were targeted to inner segments and synaptic terminals. beta3, in contrast, was expressed late in photoreceptor differentiation and was immediately targeted to inner segments. A high level of beta1 expression in horizontal cells preceded migration, whereas increases in beta2 expression in bipolar cells occurred very late, coinciding with synaptogenesis in the inner plexiform layer. Most of the spatial specification of Na,K-ATPase isoform expression was completed before eye opening and the onset of electroretinographic responses on postnatal day 13 (P13), but quantitative increase continued until P22 in parallel with synaptogenesis.

Rat skeletal muscle in culture expresses the alpha1 but not the alpha2 protein subunit isoform of the Na+/K+ pump

Studies from this laboratory have shown that the physiological expression of the Na+/K+ pump in primary cultures of rat skeletal muscle increases with development. The molecular mechanisms underlying these changes are not known. Therefore, we have examined the expression of alpha and beta subunits of the Na+/K+ pump at both the protein and mRNA levels during myogenesis of primary skeletal muscle cell cultures obtained from newborn rats. Protein isoforms were identified by Western blotting techniques with specific monoclonal and polyclonal antibodies and subunit mRNA was studied with specific cDNA probes. Freshly isolated skeletal muscle from newborn rats expressed both alpha1 and alpha2 protein subunits. From day 1 after plating, primary cultures expressed only the alpha1 protein isoform. In contrast, both beta1 and beta2 isoforms were expressed in freshly isolated muscle and in primary cultures, with beta1 expression being stronger in both preparations. Studies on RNA expression showed that mRNA for alpha1, alpha2, beta1, and beta2 isoforms was identified both in freshly isolated muscle and after plating of cells in culture. These findings indicate that the lack of alpha2 protein expression in primary muscle cell cultures reflects a form of posttranscriptional regulation. There did not appear to be a quantitative difference in isoform expression as a function of age or of fusion in spite of developmental increases in Na+/K+ pump activity and its dependence on cell fusion. The lack of expression of the alpha2 subunit isoform suggests that the developmental changes in physiological expression of the Na+/K+ pump in primary cultures of skeletal muscle may be attributable either to the changes in activity of the alpha1 subunit or to differential activities of alphabeta complexes involving either of the beta subunits.

Structural and functional characterization of an epitope in the conserved C-terminal region of HIV-1 gp120

Through an integrated study of the reactivity of a monoclonal antibody, 803-15.6, with synthetic peptides and native recombinant HIV-1 envelope glycoprotein gp120, we have obtained structure-functional information on a region of rgp120 not yet elucidated by X-ray crystallography. mAb 803-15.6 binds with high affinity and broad cross-clade specificity to the conserved C-terminal region (amino acids 502-516) of HIV-1 rgp120. Phage display selection from a random peptide library identified the core binding motif as AXXKXRH, homologous to residues 502-508. Using quantitative binding analyses, the affinity of mAb 803-15.6 for native, monomeric recombinant gp120HXB2 (rgp120) was found to be similar to that for the synthetic gp120 peptide (502-516). Circular dichroism studies indicate that the synthetic peptide largely has a random coil conformation in solution. The results therefore suggest that the 803-15.6 epitope is fully accessible on rgp120 and that this region of rgp120 is as flexible as the synthetic peptide. Residues 502-504 are on the edge of a putative gp41 binding site that has been postulated to change conformation on CD4 binding. However, the affinity of mAb 803-15.6 for rgp120 is not affected by binding of CD4 and vice-versa. These results suggest either that the 502-504 region does not change conformation upon CD4 binding, or that recombinant gp120 does not undergo the same changes as occur in the native viral gp120-gp41 oligomer. The detailed characterization of the 803-15.6 epitope may be useful for further study of the role of the C5 region of gp120 in the viral attachment and fusion process.

Plasticity of Na,K-ATPase isoform expression in cultures of flat astrocytes: species differences in gene expression

The Na,K-ATPase plays an active role in glial physiology, contributing to K+ uptake as well as to the Na+ gradients used by other membrane carriers. There are multiple isoforms of Na,K-ATPase alpha and beta subunits, and different combinations result in different affinities for Na+ and K+. Isoform choice should thus influence K+ and Na+ homeostasis in astrocytes. Prior studies of astrocyte Na,K-ATPase subunit composition have produced apparently conflicting results, suggesting plasticity of gene expression. Purified flat astrocytes from the cerebral cortex and cerebellum of both mouse and rat were systematically investigated here. Using antibodies specific for the alpha1, alpha2, alpha3, beta1, beta2, and beta3 subunits, isoform level was assessed with Western blots, and cellular distribution was visualized with immunofluorescence. Although alpha1 was always expressed, differences were observed in the expression of alpha2 and beta2, subunits that can be expressed in astrocytes in vivo and in coculture with neurons. In addition, abundant alpha subunit was expressed in rat astrocytes and in mouse cerebellar astrocytes without an equivalent level of any of the known beta isoforms, suggesting that an additional beta subunit important for glia is yet to be discovered. Conditions that have been shown to increase Na,K-ATPase activity in astrocyte cultures, such as dibutyryl cAMP, high extracellular K+, and glutamate, did not specifically induce missing subunits, suggesting that cellular interactions are required to alter the ion transporter phenotype.

Topological mimicry and epitope duplication in the guanylyl cyclase C receptor

Guanylyl cyclase C (GCC) is the receptor for the gastrointestinal hormones, guanylin, and uroguanylin, in addition to the bacterial heat-stable enterotoxins, which are one of the major causes of watery diarrhea the world over. GCC is expressed in intestinal cells, colorectal tumor tissue and tumors originating from metastasis of the colorectal carcinoma. We have earlier generated a monoclonal antibody to human GCC, GCC:B10, which was useful for the immunohistochemical localization of the receptor in the rat intestine (Nandi A et al., 1997, J Cell Biochem 66:500-511), and identified its epitope to a 63-amino acid stretch in the intracellular domain of GCC. In view of the potential that this antibody has for the identification of colorectal tumors, we have characterized the epitope for GCC:B10 in this study. Overlapping peptide synthesis indicated that the epitope was contained in the sequence HIPPENIFPLE. This sequence was unique to GCC, and despite a short stretch of homology with serum amyloid protein and pertussis toxin, no cross reactivity was detected. The core epitope was delineated using a random hexameric phage display library, and two categories of sequences were identified, containing either a single, or two adjacent proline residues. No sequence identified by phage display was identical to the epitope present in GCC, indicating that phage sequences represented mimotopes of the native epitope. Alignment of these sequences with HIPPENIFPLE suggested duplication of the recognition motif, which was confirmed by peptide synthesis. These studies allowed us not only to define the requirements of epitope recognition by GCC:B10 monoclonal antibody, but also to describe a novel means of epitope recognition involving topological mimicry and probable duplication of the cognate epitope in the native guanylyl cyclase C receptor sequence.