Spectrum of ocular disease in children aged between 0 and 3 years at an Australian paediatric tertiary hospital

BACKGROUND

Childhood ocular disease can be a significant health burden to the child, family and society. Previous studies have examined the spectrum of paediatric ocular disease presenting to tertiary hospitals; however, these studies have broader age ranges, smaller sample sizes, and are largely based in developing countries. This study aims to assess the spectrum of ocular disease in the first 3 years of life presenting to the eye department of an Australian tertiary paediatric hospital.

METHODS

The records of 3337 children who had their initial presentation at the eye clinic between the age of 0 and 36 months were reviewed, spanning 6.5 years from 1st July 2012 to 31st December 2018.

RESULTS

The most common primary diagnoses overall were strabismic amblyopia (6.0%), retinopathy of prematurity (5.0%) and nasolacrimal duct obstruction (4.5%). Bilateral visual impairment was more common in younger children, while unilateral visual impairment was more common in older children. The proportion of all children presenting with visual impairment was 10.3%, with 5.7% of all children presenting with bilateral visual impairment and 4.6% presenting with unilateral visual impairment. In children with visual impairment, the most common sites of primary abnormality were lens (21.4%), retina (17.3%), and cerebral and visual pathways (12.1%). The most common primary diagnoses in children with visual impairment were cataract (21.4%), strabismic amblyopia (9.3%) and retinoblastoma (6.5%).

CONCLUSIONS

The spectrum of eye disease and vision impairment presenting in the first 3 years of life facilitates health care planning, greater community education about vision impairment and importance of early intervention, and guidance for appropriate resource allocation. Health systems can apply these findings to aid in early identification and intervention to reduce preventable blindness and institute appropriate rehabilitation services.

Ocular and electrophysiological findings in a patient with Sly syndrome

BACKGROUND

Sly syndrome (Mucopolysaccharidosis Type VII) is an autosomal recessive metabolic storage disorder due to mutations in the GUSB gene encoding the enzyme beta-glucuronidase. Deficiency of this lysosomal enzyme impairs the body's ability to break down the glycosaminoglycans - dermatan, heparan and chondroitin sulphate. Coarse facial features and macrocephaly are typically seen along with bony and skeletal abnormalities, including joint contractures and short stature. Widespread involvement occurs in many other tissues including cardiopulmonary, gastrointestinal, and neurological systems. In view of the rarity of Sly syndrome the ophthalmic features have not been well described.

MATERIALS AND METHODS

Case report of a 16-year-old boy with Sly syndrome with serial OCT, ocular ultrasound, and electroretinogram (ERG).

RESULTS

Corneal clouding was present but there was no evidence of glaucoma or optic neuropathy. Despite no clinical evidence of retinopathy, electrophysiology showed reduced photopic and scotopic responses, particularly involving the b-wave which appears progressive. OCT showed normal foveal architecture and normal retinal nerve fiber thickness.

CONCLUSION

Corneal clouding was noted in this patient and there is no evidence of glaucoma or optic neuropathy. Although retinopathy has not been previously described in Sly syndrome, the ERG changes in this patient suggest that retinopathy may be a feature of MPS VII.

Bufo marinus bladder H-K-ATPase carries out electroneutral ion transport

Bufo marinus bladder H-K-ATPase belongs to the Na-K-ATPase and H-K-ATPase subfamily of oligomeric P-type ATPases and is closely related to rat and human nongastric H-K-ATPases. It has been demonstrated that this ATPase transports K(+) into the cell in exchange for protons and sodium ions, but the stoichiometry of this cation exchange is not yet known. We studied the electrogenic properties of B. marinus bladder H-K-ATPase expressed in Xenopus laevis oocytes. In a HEPES-buffered solution, K(+) activation of the H-K-ATPase induced a slow-onset inward current that reached an amplitude of approximately 20 nA after 1-2 min. When measurements were performed in a solution containing 25 mM HCO at a PCO(2) of 40 Torr, the negative current activated by K(+) was reduced. In noninjected oocytes, intracellular alkalization activated an inward current similar to that due to B. marinus H-K-ATPase. We conclude that the transport activity of the nongastric B. marinus H-K-ATPase is not intrinsically electrogenic but that the inward current observed in oocytes expressing this ion pump is secondary to intracellular alkalization induced by proton transport.

The functional role of beta subunits in oligomeric P-type ATPases

Na,K-ATPase and gastric and nongastric H,K-ATPases are the only P-type ATPases of higher organisms that are oligomeric and are associated with a beta subunit, which is obligatory for expression and function of enzymes. Topogenesis studies suggest that beta subunits have a fundamental and unique role in K+-transporting P-type ATPases in that they facilitate the correct membrane integration and packing of the catalytic a subunit of these P-type ATPases, which is necessary for their resistance to cellular degradation, their acquisition of functional properties, and their routing to the cell surface. In addition to this chaperone function, beta subunits also participate in the determination of intrinsic transport properties of Na,K- and H,K-ATPases. Increasing experimental evidence suggests that beta assembly is a highly ordered, beta isoform-specific process, which is mediated by multiple interaction sites that contribute in a coordinate, multistep process to the structural and functional maturation of Na,K- and H,K-ATPases.

Regulation of Ca2+ channel expression at the cell surface by the small G-protein kir/Gem

Voltage-dependent calcium (Ca2+) channels are involved in many specialized cellular functions, and are controlled by intracellular signals such as heterotrimeric G-proteins, protein kinases and calmodulin (CaM). However, the direct role of small G-proteins in the regulation of Ca2+ channels is unclear. We report here that the GTP-bound form of kir/Gem, identified originally as a Ras-related small G-protein that binds CaM, inhibits high-voltage-activated Ca2+ channel activities by interacting directly with the beta-subunit. The reduced channel activities are due to a decrease in alpha1-subunit expression at the plasma membrane. The binding of Ca2+/CaM to kir/Gem is required for this inhibitory effect by promoting the cytoplasmic localization of kir/Gem. Inhibition of L-type Ca2+ channels by kir/Gem prevents Ca2+-triggered exocytosis in hormone-secreting cells. We propose that the small G-protein kir/Gem, interacting with beta-subunits, regulates Ca2+ channel expression at the cell surface.

Structural and functional features of the transmembrane domain of the Na,K-ATPase beta subunit revealed by tryptophan scanning

In oligomeric P2-ATPases such as Na,K- and H,K-ATPases, beta subunits play a fundamental role in the structural and functional maturation of the catalytic alpha subunit. In the present study we performed a tryptophan scanning analysis on the transmembrane alpha-helix of the Na,K-ATPase beta1 subunit to investigate its role in the stabilization of the alpha subunit, the endoplasmic reticulum exit of alpha-beta complexes, and the acquisition of functional properties of the Na,K-ATPase. Single or multiple tryptophan substitutions in the beta subunits transmembrane domain had no significant effect on the structural maturation of alpha subunits expressed in Xenopus oocytes nor on the level of expression of functional Na,K pumps at the cell surface. Furthermore, tryptophan substitutions in regions of the transmembrane alpha-helix containing two GXXXG transmembrane helix interaction motifs or a cysteine residue, which can be cross-linked to transmembrane helix M8 of the alpha subunit, had no effect on the apparent K(+) affinity of Na,K-ATPase. On the other hand, substitutions by tryptophan, serine, alanine, or cysteine, but not by phenylalanine of two highly conserved tyrosine residues, Tyr(40) and Tyr(44), on another face of the transmembrane helix, perturb the transport kinetics of Na,K pumps in an additive way. These results indicate that at least two faces of the beta subunits transmembrane helix contribute to inter- or intrasubunit interactions and that two tyrosine residues aligned in the beta subunits transmembrane alpha-helix are determinants of intrinsic transport characteristics of Na,K-ATPase.

Intersubunit interactions in human X,K-ATPases: role of membrane domains M9 and M10 in the assembly process and association efficiency of human, nongastric H,K-ATPase alpha subunits (ATP1al1) with known beta subunits

Na,K- and H,K-ATPase (X,K-ATPase) alpha subunits need association with a beta subunit for their maturation, but the authentic beta subunit of nongastric H,K-ATPase alpha subunits has not been identified. To better define alpha-beta interactions in these ATPases, we coexpressed human, nongastric H,K-ATPase alpha (AL1) and Na,K-ATPase alpha1 (alpha1NK) as well as AL1-alpha1 and alpha1-AL1 chimeras, which contain exchanged M9 and M10 membrane domains, together with each of the known beta subunits in Xenopus oocytes and followed their resistance to cellular and proteolytic degradation and their ER exit. We show that all beta subunits (gastric betaHK, beta1NK, beta2NK, beta3NK, or Bufo bladder beta) can associate efficiently with alpha1NK, but only gastric betaHK, beta2NK, and Bufo bladder beta can form stably expressed AL1-beta complexes that can leave the ER. The trypsin resistance and the forces of subunit interaction, probed by detergent resistance, are lower for AL1-beta complexes than for alpha1NK-beta complexes. Furthermore, chimeric alpha1-AL1 can be stabilized by beta subunits, but alpha1-AL1-gastric betaHK complexes are retained in the ER. On the other hand, chimeric AL1-alpha1 cannot be stabilized by any beta subunit. In conclusion, these results indicate that (1) none of the known beta subunits is the real partner subunit of AL1 but an as yet unidentified, authentic beta should have structural features resembling gastric betaHK, beta2NK, or Bufo bladder beta and (2) beta-mediated maturation of alpha subunits is a multistep process which depends on the membrane insertion properties of alpha subunits as well as on several discrete events of intersubunit interactions.

Determinants of topogenesis and glycosylation of type II membrane proteins. Analysis of Na,K-ATPase beta 1 AND beta 3 subunits by glycosylation mapping

The structural and molecular determinants that govern the correct membrane insertion and folding of membrane proteins are still ill-defined. By following the addition of sugar chains to engineered glycosylation sites (glycosylation mapping) in Na,K-ATPase beta isoforms expressed in vitro and in Xenopus oocytes, in combination with biochemical techniques, we have defined the C-terminal end of the transmembrane domain of these type II proteins. N-terminal truncation and the removal of a single charged residue at the N-terminal start of the putative transmembrane domain influence the proper positioning of the transmembrane domain in the membrane as reflected by a repositioning of the transmembrane domain, the exposure of a putative cryptic signal peptidase cleavage site, and the production of protein species unable to insert into the membrane. Glycosylation mapping in vivo revealed that the degree of glycosylation at acceptor sites located close to the membrane increases with the time proteins spend in the endoplasmic reticulum. Furthermore, core sugars added to such acceptor sites cannot be processed to fully glycosylated species even when the protein is transported to the cell surface. Thus, the glycosylation mapping strategy applied in intact cells is a useful tool for the study of determinants for the correct membrane insertion of type II and probably other membrane proteins, as well as for the processing of sugar chains in glycoproteins.

Endoplasmic reticulum quality control of oligomeric membrane proteins: topogenic determinants involved in the degradation of the unassembled Na,K-ATPase alpha subunit and in its stabilization by beta subunit assembly

The molecular nature of determinants that mediate degradation of unassembled, polytopic subunits of oligomeric membrane proteins and their stabilization after partner subunit assembly is largely unknown. Expressing truncated Na,K-ATPase alpha subunits alone or together with beta subunits, we find that in unassembled alpha subunits neither the four N-terminal transmembrane segments acting as efficient alternating signal anchor-stop transfer sequences nor the large, central cytoplasmic loop exposes any degradation signal, whereas poor membrane insertion efficiency of C-terminal membrane domains M5, M7, and M9 coincides with the transient exposure of degradation signals to the cytoplasmic side. beta assembly with an alpha domain comprising at least D902 up to Y910 in the extracytoplasmic M7/M8 loop is necessary to stabilize Na,K-ATPase alpha subunits by favoring M7/M8 membrane pair formation and by protecting a degradation signal recognized from the endoplasmic reticulum (ER) lumenal side. Thus our results suggest that ER degradation of Na,K-ATPase alpha subunits is 1) mainly mediated by folding defects caused by inefficient membrane insertion of certain membrane domains, 2) a multistep process, which involves proteolytic and/or chaperone components acting from the ER lumenal side in addition to cytosolic, proteasome-related factors, and 3) prevented by partner subunit assembly because of direct protection and retrieval of degradation signals from the cytoplasm to the ER lumenal side. These results likely represent a paradigm for the ER quality control of unassembled, polytopic subunits of oligomeric membrane proteins.

Transport and pharmacological properties of nine different human Na, K-ATPase isozymes

Na,K-ATPase plays a crucial role in cellular ion homeostasis and is the pharmacological receptor for digitalis in man. Nine different human Na,K-ATPase isozymes, composed of 3 alpha and beta isoforms, were expressed in Xenopus oocytes and were analyzed for their transport and pharmacological properties. According to ouabain binding and K(+)-activated pump current measurements, all human isozymes are functional but differ in their turnover rates depending on the alpha isoform. On the other hand, variations in external K(+) activation are determined by a cooperative interaction mechanism between alpha and beta isoforms with alpha2-beta2 complexes having the lowest apparent K(+) affinity. alpha Isoforms influence the apparent internal Na(+) affinity in the order alpha1 > alpha2 > alpha3 and the voltage dependence in the order alpha2 > alpha1 > alpha3. All human Na,K-ATPase isozymes have a similar, high affinity for ouabain. However, alpha2-beta isozymes exhibit more rapid ouabain association as well as dissociation rate constants than alpha1-beta and alpha3-beta isozymes. Finally, isoform-specific differences exist in the K(+)/ouabain antagonism which may protect alpha1 but not alpha2 or alpha3 from digitalis inhibition at physiological K(+) levels. In conclusion, our study reveals several new functional characteristics of human Na,K-ATPase isozymes which help to better understand their role in ion homeostasis in different tissues and in digitalis action and toxicity.

Is phosphorylation of the alpha1 subunit at Ser-16 involved in the control of Na,K-ATPase activity by phorbol ester-activated protein kinase C?

The alpha1 subunit of Na,K-ATPase is phosphorylated at Ser-16 by phorbol ester-sensitive protein kinase(s) C (PKC). The role of Ser-16 phosphorylation was analyzed in COS-7 cells stably expressing wild-type or mutant (T15A/S16A and S16D-E) ouabain-resistant Bufo alpha1 subunits. In cells incubated at 37 degrees C, phorbol 12, 13-dibutyrate (PDBu) inhibited the transport activity and decreased the cell surface expression of wild-type and mutant Na,K-pumps equally ( approximately 20-30%). This effect of PDBu was mimicked by arachidonic acid and was dependent on PKC, phospholipase A(2), and cytochrome P450-dependent monooxygenase. In contrast, incubation of cells at 18 degrees C suppressed the down-regulation of Na,K-pumps and revealed a phosphorylation-dependent stimulation of the transport activity of Na,K-ATPase. Na,K-ATPase from cells expressing alpha1-mutants mimicking Ser-16 phosphorylation (S16D or S16E) exhibited an increase in the apparent Na affinity. This finding was confirmed by the PDBu-induced increase in Na sensitivity of the activity of Na,K-ATPase measured in permeabilized nontransfected COS-7 cells. These results illustrate the complexity of the regulation of Na,K-ATPase alpha1 isozymes by phorbol ester-sensitive PKCs and reveal 1) a phosphorylation-independent decrease in cell surface expression and 2) a phosphorylation-dependent stimulation of the transport activity attributable to an increase in the apparent Na affinity.

Insulin-induced stimulation of Na+,K(+)-ATPase activity in kidney proximal tubule cells depends on phosphorylation of the alpha-subunit at Tyr-10

Phosphorylation of the alpha-subunit of Na+,K(+)-ATPase plays an important role in the regulation of this pump. Recent studies suggest that insulin, known to increase solute and fluid reabsorption in mammalian proximal convoluted tubule (PCT), is stimulating Na+,K(+)-ATPase activity through the tyrosine phosphorylation process. This study was therefore undertaken to evaluate the role of tyrosine phosphorylation of the Na+,K(+)-ATPase alpha-subunit in the action of insulin. In rat PCT, insulin and orthovanadate (a tyrosine phosphatase inhibitor) increased tyrosine phosphorylation level of the alpha-subunit more than twofold. Their effects were not additive, suggesting a common mechanism of action. Insulin-induced tyrosine phosphorylation was prevented by genistein, a tyrosine kinase inhibitor. The site of tyrosine phosphorylation was identified on Tyr-10 by controlled trypsinolysis in rat PCTs and by site-directed mutagenesis in opossum kidney cells transfected with rat alpha-subunit. The functional relevance of Tyr-10 phosphorylation was assessed by 1) the abolition of insulin-induced stimulation of the ouabain-sensitive (86)Rb uptake in opossum kidney cells expressing mutant rat alpha1-subunits wherein tyrosine was replaced by alanine or glutamine; and 2) the similarity of the time course and dose dependency of the insulin-induced increase in ouabain-sensitive (86)Rb uptake and tyrosine phosphorylation. These findings indicate that phosphorylation of the Na+,K(+)-ATPase alpha-subunit at Tyr-10 likely participates in the physiological control of sodium reabsorption in PCT.

Role of the intracellular domain of the beta subunit in Na,K pump function

The catalytic alpha subunit of the (Na,K)- and (H,K)-ATPases needs to be coexpressed with a beta subunit in order to produce cation transport activity. Although the isoform of the beta subunit is known to influence the functional characteristics of the Na,K pump, the role of the different domains of the beta subunit is not fully understood. We have studied the function of a Na,K pump resulting from the expression of a wild-type alpha subunit with a N-terminally truncated mutant of the beta subunit using the two-electrode voltage clamp and the cut-open oocyte techniques. While the maximal activity, measured as the K+-activated outward current, was not significantly altered, the beta N-terminal truncation induced an ouabain-sensitive conductance in the absence of extracellular K+. The voltage dependence of the ouabain-sensitive charge distribution indicated that in the Na/Na exchange conditions, the E1-E2 conformation equilibrium was shifted towards the E2 conformation, a change resulting from alteration of both the forward and the backward reaction rate. Removal of the intracellular domain of the beta subunit modifies several aspects of the whole enzyme function by a mechanism that must imply the state of the extracellular and/or transmembrane parts of the alpha/beta subunit complex.

beta-subunit assembly is essential for the correct packing and the stable membrane insertion of the H,K-ATPase alpha-subunit

The alpha-subunits of H,K-ATPase (HKAalpha) and Na,K-ATPase require a beta-subunit for maturation. We investigated the role of the beta-subunit in the membrane insertion and stability of the HKAalpha expressed in Xenopus oocytes. Individual membrane segments M1, M2, M3, M4, and M9 linked to a glycosylation reporter act as signal anchor (SA) motifs, and M10 acts as a partial stop transfer motif. In combined HKAalpha constructs, M2 acts as an efficient stop transfer sequence, and M3 acts as a SA sequence. However, M5 and M9 have only partial SA function, and M7 has no SA function. Consistent with the membrane insertion properties of segments in combined alpha constructs, M1-3 alpha-proteins are resistant to cellular degradation, and M1-5 up to M1-10 alpha-proteins are not resistant to cellular degradation. However, co-expression with beta-subunits increases the membrane insertion of M9 in a M1-9 alpha-protein and completely protects M1-10 alpha-proteins against cellular degradation. Our results indicate that HKAalpha N-terminal (M1-M4) membrane insertion and stabilization are mediated by intrinsic molecular characteristics; however, the C-terminal (M5-M10) membrane insertion and thus the stabilization of the entire alpha-subunit depend on intramolecular and intermolecular beta-subunit interactions that are similar but not identical to data obtained for the Na,K-ATPase alpha-subunit.

Role of beta-subunit domains in the assembly, stable expression, intracellular routing, and functional properties of Na,K-ATPase

The beta-subunit of Na,K-ATPase (betaNK) interacts with the catalytic alpha-subunit (alphaNK) in the ectodomain, the transmembrane, and the cytoplasmic domain. The functional significance of these different interactions was studied by expressing alphaNK in Xenopus oocytes along with N-terminally modified betaNK or with chimeric betaNK/betaH,K-ATPase (betaHK). Complete truncation of the betaNK N terminus allows for cell surface-expressed, functional Na,K-pumps that exhibit, however, reduced apparent K+ and Na+ affinities as assessed by electrophysiological measurements. A mutational analysis suggests that these functional effects are not related to a direct interaction of the beta N terminus with the alphaNK but rather that N-terminal truncation induces a conformational change in another functionally relevant beta domain. Comparison of the functional properties of alphaNK.betaNK, alphaNK.betaHK, or alphaNK. betaNK/betaHK complexes shows that the effect of the betaNK on K+ binding is mainly mediated by its ectodomain. Finally, betaHK/NK containing the transmembrane domain of betaHK produces stable but endoplasmic reticulum-retained alphaNK.beta complexes, while alphaNK/betaHK complexes can leave the ER but exhibit reduced ouabain binding capacity and transport function. Thus, interactions of both the transmembrane and the ectodomain of betaNK with alphaNK are necessary to form correctly folded Na,K-ATPase complexes that can be targeted to the plasma membrane and/or become functionally competent. Furthermore, the beta N terminus plays a role in the beta-subunit's folding necessary for correct interactions with the alpha-subunit.

Membrane integration of Na,K-ATPase alpha-subunits and beta-subunit assembly

The control of membrane insertion of polytopic proteins is still poorly understood. We carried out in vivo translation/insertion experiments in Xenopus oocytes with combined wild type or mutant membrane segments of the alpha-subunit of the heterodimeric Na, K-ATPase linked to a glycosylation reporter sequence. We confirm that the four N-terminal hydrophobic segments of the alpha-subunit behave as alternating signal anchor/stop transfer motifs necessary for two lipid-inserted membrane pairs. For the six C-terminal membrane segments, however, proper packing depends on specific sequence information and association with the beta-subunit. M5 is a very inefficient signal anchor sequence due to the presence of prolines and polar amino acids. Its correct membrane insertion is probably mediated by posttranslational hairpin formation with M6, which is favored by a proline pair in the connecting loop. M7 has partial signal anchor function, which may be mediated by the presence of glycine and glutamine residues. The formation of a transmembrane M7/M8 pair requires the association of the beta-subunit, which induces a conformational change in the connecting extracytoplasmic loop that favors M7/M8 packing. The formation of the M9/M10 pair appears to be predominantly mediated by the efficient stop transfer function of M10. Mutations that provide signal anchor function to M5, M7, and M9 abolish or impede the transport activity of the enzyme. These data illustrate the importance of specific amino acids near or within hydrophobic regions as well as of subunit oligomerization for correct topographical alignment that is necessary for proper folding and/or activity of oligomeric membrane proteins.

Regulation of expression and function by subunits of oligomeric P-type ATPases

Na,K-ATPase activity must be finely controlled to meet the constantly changing physiological demands and to avoid destabilization of body homeostasis. Recent experimental evidence suggests that certain regulatory mechanisms are closely linked to the multisubunit structure of the Na,K-pump molecule. Na,K-ATPase is composed of a catalytic alpha and a glycoprotein beta subunit and sometimes of a third component, the gamma subunit. The beta subunit is a fundamental element of Na,K-ATPase in that its assembly in the ER is required for the structural and functional maturation of the catalytic alpha subunit and in consequence the beta subunit controls the expression of functional pumps at the cell surface. Furthermore, beta subunits influence the transport properties of the mature catalytic alpha subunits. Distinct interaction sites mediate the two functions of the beta subunit. Recently, we have started to characterize the gamma subunit, the functional role of which is yet not known. Immuno-radiolabeling of epitope-tagged gamma subunits expressed in Xenopus oocytes shows that the gamma subunits is a type I membrane protein which specifically associates only with Na,K-ATPase but not with other oligomeric P-type ATPases. The gamma peptide does not influence the formation or the cell surface expression of functional alpha-beta complexes. On the other hand, the gamma peptide itself needs association with Na,K-ATPase to be stably expressed and to be efficiently transported to the plasma membrane. Finally, the gamma subunit can modulate the K activation of Na,K-pumps. In conclusion, processes such as subunit assembly or the subunit composition of the cell surface expressed Na,K-pumps appear to cooperate with hormones in the control of the expression and the activity of Na,K-ATPase.

Unusual degradation of alpha-beta complexes in Xenopus oocytes by beta-subunits of Xenopus gastric H-K-ATPase

The catalytic alpha-subunit of oligomeric P-type ATPases such as Na-K-ATPase and H-K-ATPase requires association with a beta-subunit after synthesis in the endoplasmic reticulum (ER) to become stably expressed and functionally active. In this study, we have expressed the beta-subunit of Xenopus gastric H-K-ATPase (betaHK) in Xenopus oocytes together with alpha-subunits of H-K-ATPase (alphaHK) or Na-K-ATPase (alphaNK) and have followed the biosynthesis, assembly, and cell surface expression of functional pumps. Immunoprecipitations of Xenopus betaHK from metabolically labeled oocytes show that it is well expressed and, when synthesized without alpha-subunits, can leave the ER and become fully glycosylated. Xenopus betaHK can associate with both coexpressed alphaHK and alphaNK, but the alpha-beta complexes formed are degraded rapidly in or close to the ER and do not produce functional pumps at the cell surface as assessed by 86Rb uptake. A possible explanation of these results is that Xenopus betaHK may contain a tissue-specific signal that is important in the formation or correct targeting of functional alpha-beta complexes in the stomach but that cannot be recognized in Xenopus oocytes and in consequence leads to cellular degradation of the alpha-beta complexes in this experimental system.

Na,K-ATPase

Na,K-ATPase is the driving force for renal Na+ reabsorption and is thus critically implicated in the control of extracellular volume and blood pressure. This review focuses on most recent advances in the elucidation of intrinsic structural features that are important for Na,K-ATPase function and in the identification of regulatory mechanisms that are implicated in the modulation of Na,K-ATPase activity in the kidney under physiological or pathophysiological conditions.

The gamma subunit is a specific component of the Na,K-ATPase and modulates its transport function

The role of small, hydrophobic peptides that are associated with ion pumps or channels is still poorly understood. By using the Xenopus oocyte as an expression system, we have characterized the structural and functional properties of the gamma peptide which co-purifies with Na,K-ATPase. Immuno-radiolabeling of epitope-tagged gamma subunits in intact oocytes and protease protection assays show that the gamma peptide is a type I membrane protein lacking a signal sequence and exposing the N-terminus to the extracytoplasmic side. Co-expression of the rat or Xenopus gamma subunit with various proteins in the oocyte reveals that it specifically associates only with isozymes of Na,K-ATPase. The gamma peptide does not influence the formation and cell surface expression of functional Na,K-ATPase alpha-beta complexes. On the other hand, the gamma peptide itself needs association with Na,K-ATPase in order to be stably expressed in the oocyte and to be transported efficiently to the plasma membrane. Gamma subunits do not associate with individual alpha or beta subunits but only interact with assembled, transport-competent alpha-beta complexes. Finally, electrophysiological measurements indicate that the gamma peptide modulates the K+ activation of Na,K pumps. These data document for the first time the membrane topology, the specificity of association and a potential functional role for the gamma subunit of Na,K-ATPase.

Role of glycosylation and disulfide bond formation in the beta subunit in the folding and functional expression of Na,K-ATPase

Initial folding is a prerequisite for subunit assembly in oligomeric proteins. In this study, we have compared the role of co-translational modifications in the acquisition of an assembly-competent conformation of the beta subunit, the assembly of which is required for the structural and functional maturation of the catalytic Na,K-ATPase alpha subunit. Cysteine or asparagine residues implicated in disulfide bond formation or N-glycosylation, respectively, in the Xenopus beta1 subunit were eliminated by site-directed mutagenesis, and the assembly efficiency of the mutants and the functional expression of Na+,K+ pumps were studied after expression in Xenopus oocytes. Our results show that lack of each one of the two most C-terminal disulfide bonds indeed permits short term but completely abolishes long term assembly of the beta subunit. On the other hand, lack of the most N-terminal disulfide bonds allows the expression of a small number of functional Na+,K+ pumps at the cell surface. Elimination of all three but not of one or two glycosylation sites produces beta subunits that remain stably expressed in the endoplasmic reticulum, in association with binding protein but not as irreversible aggregates. The assembly efficiency of nonglycosylated beta subunits is decreased but a reduced number of functional Na+,K+ pumps is expressed at the cell surface. The lack of sugars does not influence the apparent K+ or ouabain affinity of the Na+,K+ pumps. Thus, these data show that disulfide bond formation and N-glycosylation may play important but qualitatively distinct roles in the initial folding of oligomeric protein subunits. Moreover, the results suggest that an endoplasmic reticulum degradation pathway exists, which is glycosylation-dependent.

Phorbol 12-myristate 13-acetate down-regulates Na,K-ATPase independent of its protein kinase C site: decrease in basolateral cell surface area

The effect of protein kinase C (PKC) stimulation on the pump current (Ip) generated by the Na,K-ATPase was measured in A6 epithelia apically permeabilized with amphotericin B. Phorbol 12-myristate 13-acetate (PMA) produced a decrease in Ip carried by sodium pumps containing the endogenous Xenopus laevis or transfected Bufo marinus alpha 1 subunits (approximately 30% reduction within 25 min, maximum after 40 min) independent of the PKC phosphorylation site (T15A/S16A). In addition to this major effect of PMA, which was independent of the intracellular sodium concentration and was prevented by the PKC inhibitor bisindolylmaleimide GF 109203X (BIM), another BIM-resistant, PKC site-independent decrease was observed when the Ip was measured at low sodium concentrations (total reduction approximately 50% at 5 mM sodium). Using ouabain binding and cell surface biotinylation, stimulation of PKC was shown to reduce surface Na,K-ATPase by 14 to 20% within 25 min. The same treatment stimulated fluid phase endocytosis sevenfold and decreased by 16.5% the basolateral cell surface area measured by transepithelial capacitance measurements. In conclusion, PKC stimulation produces a decrease in sodium pump function which can be attributed, to a large extent, to a withdrawal of sodium pumps from the basolateral cell surface independent of their PKC site. This reduction of the number of sodium pumps is parallel to a decrease in basolateral membrane area.

alpha 1 but not alpha 2 or alpha 3 isoforms of Na,K-ATPase are efficiently phosphorylated in a novel protein kinase C motif

Protein kinase C (PKC) phosphorylates the catalytic alpha 1 subunit of Na,K-ATPase in purified enzyme preparations and in intact cells. Little is known, however, whether all three known alpha isoforms are substrates for PKC and whether direct phosphorylation is implicated in the modulation of the transport activity of the different Na,K-ATPase isozymes. In this study, we investigated the structural requirements for PKC phosphorylation of alpha 1, alpha 2, and alpha 3 isoforms of different species after expression in Xenopus oocytes. By using a combination of site-directed mutagenesis and computer-assisted protein modeling, we characterized a novel Ser-X-His motif which in concert with more distantly located basic residues acts as an efficient substrate for PKC-mediated phosphorylation in the N-terminus of most Na,K-ATPase alpha 1 isoforms. As indicated by controlled proteolysis, alpha 2 isoforms are also phosphorylated in the N-terminus but to a much lower extent than alpha 1 isoforms containing the Ser-X-His motif. Phosphorylation and phosphoamino acid analysis of fusion proteins containing the wild-type or mutant N-terminus of alpha 2 reveal that Thr-Thr-Ser-X-Asn or Thr-Thr-Ala-X-Asn motifs represent weak targets for PKC phosphorylation. Finally, our data suggest that, with the exception of rat alpha 3, all alpha 3 isoforms from other species are not substrates for PKC. On the basis of the phosphorylation efficiency, we may speculate that only alpha 1 but not alpha 2 or alpha 3 isoforms of Na,K-ATPase are likely candidates for regulatory PKC phosphorylation.

Degradation and endoplasmic reticulum retention of unassembled alpha- and beta-subunits of Na,K-ATPase correlate with interaction of BiP

Assembly of alpha- and beta-subunits in the endoplasmic reticulum is a prerequisite for the structural and functional maturation of oligomeric P-type ATPases. In Xenopus oocytes, overexpressed, unassembled alpha- and beta-subunits of Xenopus Na,K-ATPase are retained in the endoplasmic reticulum (ER) and are degraded with different kinetics, while unassembled beta-subunits of gastric H, K-ATPase leave the ER. In this study, we have investigated the role of the immunoglobulin-binding protein, BiP, in the folding, assembly, and ER retention of ATPase subunits. We determined the primary sequence of Xenopus BiP and used polyclonal antibodies to examine the interaction with BiP of various wild type and mutant alpha- and beta-subunits overexpressed in Xenopus oocytes. Our results show that ER-retained, unassembled Na,K-ATPase beta-subunits, but not transport-competent H,K-ATPase beta-subunits, efficiently associate with BiP until assembly with alpha-subunits occurs. Furthermore, the kinetics of BiP interaction with unassembled wild type and with mutant Na,K-ATPase beta-subunits parallels their respective stability against cellular degradation. Finally, alpha-subunits that are overexpressed in oocytes and are rapidly degraded and endogenous oocyte alpha-subunits that are stably expressed as individual assembly-competent proteins also interact with oocyte or exogenous BiP, and the interaction time correlates with the protein's stability. These data demonstrate for the first time that BiP might be involved in a long term maturation arrest and/or in the ER quality control of a multimembrane-spanning protein and lend support for a universal chaperone function of BiP.

Improved monitoring of preterm infants by Fuzzy Logic

Keeping the oxygenation status of newborn infants within physiologic limits is a crucial task in intensive care. For this purpose several vital parameters are supervised routinely by monitors, such as electrocardiograph, transcutaneous partial oxygen pressure monitor and pulse oximeter. Each monitor issues an alarm signal whenever an upper or lower limit of the parameter(s) measured is exceeded. However, in practice it turns out, that a considerable amount of false alarms is generated by artefacts, which are attributed mostly to movements of the infants. Eliminating these false alarms would be of benefit to the staff as well as the patients of the intensive care unit. Accordingly, an automated system based on Fuzzy Logic was developed, which is capable of distinguishing between critical situations and artefacts. The system is based on a Transputer IMS T425 in a PC, which collects the data from the monitors, plots it on a colour screen, saves it to hard disk and analyses it by Fuzzy Logic. Fuzzy algorithms were developed to generate more reliable alarms. All vital parameters of eight infants, who either moved often and/or frequently produced real alarm situations, were recorded. Synchronously the infants' movements and care procedures were video taped. The data and video were analysed off line with the help of an experienced neonatologist. His judgement was compared to the analysis of the Fuzzy Logic system. The results show that it is possible to improve the reliability of the monitored data with the aid of an evaluation strategy based on Fuzzy Logic and hence distinguish between real alarm situations and movement artefacts to the extent that an application in an intensive care unit under routine conditions becomes conceivable.

Oligomerization and maturation of Na,K-ATPase: functional interaction of the cytoplasmic NH2 terminus of the beta subunit with the alpha subunit

Subunit assembly plays an essential role in the maturation of oligomeric proteins. In this study, we have characterized the main structural and functional consequences of the assembly of alpha and beta subunits of Na,K-ATPase. Xenopus oocytes injected with alpha and/or beta cRNA were treated with brefeldin A, which permitted the accumulation of individual subunits or alpha-beta complexes in the ER. Only alpha subunits that are associated with beta subunits become resistant to trypsin digestion and cellular degradation. Similarly, assembly with beta subunits is necessary and probably sufficient for the catalytic alpha subunit to acquire its main functional properties at the level of the ER, namely the ability to adopt different ligand-dependent conformations and to hydrolyze ATP in an Na(+)- and K(+)-dependent, ouabain-inhibitable fashion. Not only the alpha but also the beta subunit undergoes a structural change after assembly, which results in a global increase in its protease resistance. Furthermore, extensive and controlled proteolysis assays on wild-type and NH2-terminally modified beta subunits revealed a K(+)-dependent interaction of the cytoplasmic NH2 terminus of the beta subunit with the alpha subunit, which is likely to be involved in the modulation of the K(+)-activation of the Na,K-pump transport activity. Thus, we conclude that the ER assembly process not only establishes the basic structural interactions between individual subunits, which are required for the maturation of oligomeric proteins, but also distinct, functional interactions, which are involved in the regulation of functional properties of mature proteins.

Adrenergic, dopaminergic, and muscarinic receptor stimulation leads to PKA phosphorylation of Na-K-ATPase

Na-K-adenosinetriphosphatase (Na-K-ATPase) is a potential target for phosphorylation by protein kinase A (PKA) and C (PKC). We have investigated whether the Na-K-ATPase alpha-subunit becomes phosphorylated at its PKA or PKC phosphorylation sites upon stimulation of G protein-coupled receptors primarily linked either to the PKA or the PKC pathway. COS-7 cells, transiently or stably expressing Bufo marinus Na-K-ATPase wild-type alpha- or mutant alpha-subunits affected in its PKA or PKC phosphorylation site, were transfected with recombinant DNA encoding beta 2- or alpha 1-adrenergic (AR), dopaminergic (D1A-R), or muscarinic cholinergic (M1-AChR) receptor subspecies. Agonist stimulation of beta 2-AR or D1A-R led to phosphorylation of the wild-type alpha-subunit, as well as the PKC mutant, but not of the PKA mutant, indicating that these receptors can phosphorylate the Na-K-ATPase via PKA activation. Surprisingly, stimulation of the alpha 1B-AR, alpha 1C-AR, and M1-AChR also increased the phosphorylation of the wild-type alpha-subunit and its PKC mutant but not of its PKA mutant. Thus the phosphorylation induced by these primarily phospholipase C-linked receptors seems mainly mediated by PKA activation. These data indicate that the Na-K-ATPase alpha-subunit can act as an ultimate target for PKA phosphorylation in a cascade starting with agonist-receptor interaction and leading finally to a phosphorylation-mediated regulation of the enzyme.

Human ATP1AL1 gene encodes a ouabain-sensitive H-K-ATPase

The cDNA for ATP1AL1, the fifth member of the human Na-K-adenosinetriphosphatase (ATPase)/H-K-ATPase gene family, was recently cloned (A. V. Grishin, V. E. Sverdlov, M. B. Kostina, and N. N. Modyanov. FEBS Lett. 349: 144-150, 1994). The encoded protein (ATP1AL1) has all the primary structural features common to the catalytic alpha-subunit of ion-transporting P-type ATPases and is similar (63-64% identity) to the Na-K-ATPase alpha-subunit isoforms and the gastric H-K-ATPase alpha-subunit. In this study, ATP1AL1 was expressed in Xenopus laevis oocytes in combination with the beta-subunit of rabbit gastric H-K-ATPase. The functional properties of the stable alpha/beta-complex were studied by 86Rb+ uptake and demonstrated that ATP1AL1 is a novel human K(+)-dependent ATPase [apparent half-constant activation/(K1/2) for K+ approximately 375 microM)]. ATP1AL1-mediated inward K+ transport was inhibited by ouabain (inhibition constant approximately 13 microM) and was found to be inhibited by high concentrations of SCH-28080 (approximately 70% at 500 microM). ATP1AL1 expression resulted in the alkalinization of the oocytes' cytoplasm and ouabain-sensitive proton extrusion, as measured with pH-sensitive microelectrodes. These data argue that ATP1AL1 is the catalytic alpha-subunit of a human nongastric P-type ATPase capable of exchanging extracellular potassium for intracellular protons.

Labeling of a cysteine in the cardiotonic glycoside binding site by the steroid derivative HDMA

The digoxigenin derivative N-hydroxysuccinimidyl digoxigenin-3-O-methylcarbonyl-epsilon-aminocaproate (HDMA) has been shown to covalently label the ouabain binding site of the Na,K-ATPase epsilon subunit [Antolovic et al. (1995) Eur. J. Biochem. 227, 61-67]. In the present study we observed both, labeling and inactivation of the activity, of wild type Na,K-ATPase overexpressed in Xenopus oocyte. In contrast, no significant inhibition and no labeling could be detected when a Cys-113 of the first transmembrane segment was mutated to serine, although the affinity of this mutant for digoxigenin or HDMA measured in acute inhibition experiments was similar to the wild type. This indicates that after docking of its genin moiety, HDMA can form a thioester bond with Cys-113.

Primary structure and functional expression of the mouse and frog alpha-subunit of the gastric H(+)-K(+)-ATPase

The H(+)-K(+)-ATPase of the gastric parietal cells is responsible for the acidification of the stomach lumen. This heterodimeric protein belongs to the family of cation-translocating P-type ATPases, which includes the closely related Na(+)-ATPase. We have cloned the alpha-subunit cDNA of the Xenopus and murine gastric H(+)-K(+)-ATPase (alpha H-K). We have expressed Xenopus and murine alpha H-K along with the previously cloned gastric H(+)-K(+)-ATPase beta-subunit of rabbit (beta H-K) in Xenopus oocytes by cRNA injection. An antibody directed against the beta H-K coimmunoprecipitates under nondenaturing conditions the alpha H-K of both species, demonstrating assembly of the alpha/beta complex. Additionally, we demonstrate the presence of K(+)-transporting H(+)-K(+)-ATPase in the plasma membrane of oocytes by 86Rb- uptake. The H(+)-K(+)-ATPase-mediated K+ uptake was inhibited by the gastric H(+)-K(+)-ATPase inhibitor Sch-28080, but not by ouabain, and shows K(+)-dependent activation (K1/2 approximately 2 mM). Furthermore, H(+)-K(+)-ATPase-expressing oocytes show a Sch-28080 inhibitable proton extrusion. Our data indicate that the expressed H(+)-K(+)-ATPase behaves functionally in oocytes as in the gastric gland.

Phosphorylation of the Na,K-ATPase alpha-subunit by protein kinase A and C in vitro and in intact cells. Identification of a novel motif for PKC-mediated phosphorylation

Na,K-ATPase is a potential target for regulatory phosphorylation by protein kinase A and C (PKA and PKC). To identify the phosphorylation sites, we have mutated the alpha 1-subunit of Bufo marinus in a highly conservative PKA and in 20 different PKC consensus sequences. The mutants were expressed in Xenopus oocytes and their phosphorylation capacity tested in homogenates upon stimulation of PKA or PKC. While serine 943 (Ser-943) was identified as a unique target site for PKA, none of the PKC consensus serine or threonine residues are implicated in PKC phosphorylation. Controlled trypsinolysis of phosphorylated alpha-subunits of various purified enzyme preparations and of alpha/beta complexes from oocyte homogenates revealed that PKC phosphorylation was exclusively associated with the N terminus. A fusion protein containing the first 32 amino acids of the Bufo alpha-subunit was phosphorylated in vitro and serine and threonine residues (Thr-15 and Ser-16) in this region were identified by site-directed mutagenesis as the PKC phosphorylation sites. Finally, the Bufo alpha-subunit was phosphorylated by protein kinases in transfected COS-7 cells. In intact cells, PKA stimulation induced phosphorylation exclusively on Ser-943 and PKC stimulation mainly on Thr-15 and Ser-16, which are contained in a novel PKC phosphorylation motif.

In vivo phosphorylation of the Na,K-ATPase alpha subunit in sciatic nerves of control and diabetic rats: effects of protein kinase modulators

The phosphorylation state of the Na,K-ATPase alpha subunit has been examined in 32P-labeled sciatic nerves of control and streptozotocin-treated diabetic rats. Intact nerves were challenged with protein kinase (PK) modulators and alpha-subunit 32P labeling was analyzed after immunoprecipitation. In control nerves, the PKC activator phorbol 12-myristate 13-acetate (PMA) had little effect on alpha-subunit 32P labeling. In contrast, staurosporine, a PKC inhibitor, and extracellular calcium omission decreased it. In Ca(2+)-free conditions, PMA restored the labeling to basal levels. The cAMP-raising agent forskolin reduced the 32P labeling of the alpha subunit. The results suggest that nerve Na,K-ATPase is tonically phosphorylated by PKC in a Ca(2+)-dependent manner and that PKA modulates the phosphorylation process. In nerves of diabetic rats, PMA increased 32P labeling of the alpha subunit. In contrast to staurosporine or extracellular calcium omission, the decreased state of phosphorylation seen with forskolin was no longer significant in diabetic nerves. No change in the level of alpha-subunit isoforms (alpha 1 or alpha 2) was detected by Western blot analysis in such nerves. In conclusion, the altered effect of PK activators on Na,K-ATPase phosphorylation state is consistent with the view that a defect in PKC activation exists in diabetic nerves.

Modulation of the Na,K-pump function by beta subunit isoforms

To study the role of the Na,K-ATPase beta subunit in the ion transport activity, we have coexpressed the Bufo alpha 1 subunit (alpha 1) with three different isotypes of beta subunits, the Bufo Na,K-ATPase beta 1 (beta 1NaK) or beta 3 (beta 3NaK) subunit or the beta subunit of the rabbit gastric H,K-ATPase (beta HK), by cRNA injection in Xenopus oocyte. We studied the K+ activation kinetics by measuring the Na,K-pump current induced by external K+ under voltage clamp conditions. The endogenous oocyte Na,K-ATPase was selectively inhibited, taking advantage of the large difference in ouabain sensitivity between Xenopus and Bufo Na,K pumps. The K+ half-activation constant (K1/2) was higher in the alpha 1 beta 3NaK than in the alpha 1 beta 1NaK groups in the presence of external Na+, but there was no significant difference in the absence of external Na+. Association of alpha 1 and beta HK subunits produced active Na,K pumps with a much lower apparent affinity for K+ both in the presence and in the absence of external Na+. The voltage dependence of the K1/2 for external K+ was similar with the three beta subunits. Our results indicate that the beta subunit has a significant influence on the ion transport activity of the Na,K pump. The small structural differences between the beta 1NaK and beta 3NaK subunits results in a difference of the apparent affinity for K+ that is measurable only in the presence of external Na+, and thus appears not to be directly related to the K+ binding site. In contrast, association of an alpha 1 subunit with a beta HK subunit results in a Na,K pump in which the K+ binding or translocating mechanisms are altered since the apparent affinity for external K+ is affected even in the absence of external Na+.

Polyadenylation of Na(+)-K(+)-ATPase beta 1-subunit during early development of Xenopus laevis

In fully grown Xenopus oocytes, the synthesis of beta-subunits is limiting for the formation of functional Na(+)-K(+)-adenosinetriphosphatase alpha/beta-complexes (Geering, K. FEBS Lett. 285: 189-193, 1991). In the present study, we show that during oocyte growth (from stage I to stage VI) alpha 1-, but not beta 1- or beta 3-isoform, mRNAs accumulate. In addition, beta-mRNAs are apparently sequestered in an untranslated pool in fully grown oocytes (stage VI). From fertilization to morulation, the total pools of alpha 1-, beta 1-, or beta 3-mRNAs vary little. Whereas polyadenylated [poly(A)+] alpha 1- and beta 3-isoform mRNAs did not change significantly, poly(A)+ beta 1-mRNA abundance increased three- to fourfold at morulation, accompanied by a parallel increase in beta 1-protein synthesis. After midblastula transition (i.e., at early gastrula) and during neurulation, poly(A)+ alpha 1- and beta 3-mRNAs accumulated rapidly, whereas poly(A)+ beta 1-mRNA accumulation was delayed by approximately 2 h, beginning only at early neurula. Our results indicate that 1) the abundance of poly(A)+ beta 1-mRNA is rate limiting during embryonic development for the assembly of alpha 1/beta 1-heterodimers, shown to be involved in the vectorial transport of sodium in kidney cells, and 2) the polyadenylation of beta 1-mRNA is a rate-limiting factor during morulation for the synthesis and assembly of new sodium pumps at the time of blastocoel fluid formation. The 3'-untranslated region of beta 1-mRNA (but not of alpha 1-mRNA) expresses cytoplasmic polyadenylation elements (CPEs) with the consensus sequence AXX-AUUUU(A/U)(A/U)(A/U). A role of CPE in the differential polyadenylation of alpha 1- and beta 1-mRNA is proposed.

Hydrophobic C-terminal amino acids in the beta-subunit are involved in assembly with the alpha-subunit of Na,K-ATPase

To define the structural basis of oligomerization for the alpha- and beta-subunits of Na,K-ATPase, we have attempted to identify the amino acids in the C-terminus of the beta-subunit that are involved in subunit assembly. We predicted that the last 10 amino acids form a beta-strand-like structure exposing on one side a hydrophilic and on the other side a continuous hydrophobic domain. The relative importance of the two domains in assembly was probed by introducing point mutations in either domain of Xenopus beta 3-subunits and by testing the ability of these mutants to stabilize newly synthesized alpha-subunits expressed in Xenopus oocytes and to form functional alpha-beta complexes at the plasma membrane. All single and double mutants with changes at R268 and/or K272 to either uncharged or negatively charged amino acids associated with coexpressed alpha-subunits and increased the number of ouabain binding sites and Rb uptake into oocytes. On the other hand, mutations affecting the hydrophobic amino acids influenced the assembly efficiency with alpha-subunits to a variable extent. The single mutants V269N and I275N did not influence and the mutant V273N slightly affected the assembly process. On the other hand, the cellular accumulation of alpha-subunits and the expression of functional Na,K pumps was considerably reduced with the mutant F271N and totally abolished with the double mutant V269N/F271N. Finally, replacement of V269 and F271 or V273 and I275 with the less hydrophobic alanine also significantly decreased subunit assembly, which was no longer detectable after replacement of all four amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the transmembrane and extracytoplasmic domain of beta subunits in subunit assembly, intracellular transport, and functional expression of Na,K-pumps

The ubiquitous Na,K- and the gastric H,K-pumps are heterodimeric plasma membrane proteins composed of an alpha and a beta subunit. The H,K-ATPase beta subunit (beta HK) can partially act as a surrogate for the Na,K-ATPase beta subunit (beta NK) in the formation of functional Na,K-pumps (Horisberger et al., 1991. J. Biol. Chem. 257:10338-10343). We have examined the role of the transmembrane and/or the ectodomain of beta NK in (a) its ER retention in the absence of concomitant synthesis of Na,K-ATPase alpha subunits (alpha NK) and (b) the functional expression of Na,K-pumps at the cell surface and their activation by external K+. We have constructed chimeric proteins between Xenopus beta NK and rabbit beta HK by exchanging their NH2-terminal plus transmembrane domain with their COOH-terminal ectodomain (beta NK/HK, beta HK/NK). We have expressed these constructs with or without coexpression of alpha NK in the Xenopus oocyte. In the absence of alpha NK, Xenopus beta NK and all chimera that contained the ectodomain of beta NK were retained in the ER while beta HK and all chimera with the ectodomain of beta HK could leave the ER suggesting that ER retention of unassembled Xenopus beta NK is mediated by a retention signal in the ectodomain. When coexpressed with alpha NK, only beta NK and beta NK/HK chimera assembled efficiently with alpha NK leading to similar high expression of functional Na,K-pumps at the cell surface that exhibited, however, a different apparent K+ affinity. beta HK or chimera with the transmembrane domain of beta HK assembled less efficiently with alpha NK leading to lower expression of functional Na,K-pumps with a different apparent K+ affinity. The data indicate that the transmembrane domain of beta NK is important for efficient assembly with alpha NK and that both the transmembrane and the ectodomain of beta subunits play a role in modulating the transport activity of Na,K-pumps.

Mechanisms of urinary K+ and H+ excretion: primary structure and functional expression of a novel H,K-ATPase

The kidney plays an essential role in regulating potassium and acid balance. A major site for these regulations is in the collecting tubule. In the present study, we report the primary sequence of a novel alpha subunit of the P-ATPase gene family, which we isolated from the urinary bladder epithelium of the toad Bufo marinus, the amphibian equivalent of the mammalian collecting tubule. The cDNA encodes a protein of 1,042 amino acids which shares approximately 67% identity with the alpha 1 subunit of the ouabain-inhibitable Na,K-ATPase and approximately 69% identity with the alpha subunit of the SCH28080-inhibitable gastric H,K-ATPase. When coexpressed in Xenopus oocytes with a beta subunit isolated from the same cDNA library, the ATPase is able to transport rubidium (a potassium surrogate) inward, and hydrogen outward, leading to alkalization of the intracellular compartment and acidification of the external medium. The novel ATPase has a unique pharmacological profile showing intermediate sensitivity to both ouabain and SCH28080. Our findings indicate that the bladder ATPase is a member of a new ion motive P-ATPase subfamily. The bladder ATPase is expressed in the urinary tract but not in the stomach or the colon. This H,K-ATPase may be one of the molecules involved in H+ and K+ homeostasis, mediating the transport of these ions across urinary epithelia and therefore regulating their urinary excretion.

Mutation of a conserved proline residue in the beta-subunit ectodomain prevents Na(+)-K(+)-ATPase oligomerization

A highly conserved sequence motif (4 tyrosines and 1 proline: YYPYY) of the Na(+)-K(+)-adenosinetriphosphatase (ATPase) beta 1-subunit ectodomain has been mutagenized to study its possible role in alpha/beta-assembly and sodium pump function. Single as well as double tyrosine mutants (tyrosine to phenylalanine: Y to F) of Xenopus laevis beta 1-subunits are able to associate with alpha 1-subunits and form functional Na-K pumps at the plasma membrane that are indistinguishable from wild-type alpha 1, beta 1-Na-K pumps (as assessed by measurements of ouabain binding, 86Rb flux, Na-K pump current, and activation by external potassium). In contrast, a single proline mutation (proline to glycine: P244G) reduced by > 90% the proper assembly and function of Na(+)-K(+)-ATPase, despite a normal rate of synthesis and core glycosylation. Our data indicate that proline-244 plays a critical role in the proper folding of the beta-subunit and its ability to associate efficiently with the alpha 1-subunit in the endoplasmic reticulum.

Ontogeny of alpha 1- and beta 1-isoforms of Na(+)-K(+)-ATPase in fetal distal rat lung epithelium

Because immature, in contrast to mature, fetal lungs have ineffective Na transport, we wished to determine the ontogeny of Na(+)-K(+)-ATPase expression in fetal distal lung epithelium (FDLE). FDLE and fibroblasts (FLF) from 17- to 22-day gestational age fetal rats (term = 22 days) were grown in primary culture. Northern and slot-blot analyses utilizing isoform-specific cDNA probes determined that alpha 1- (3.7 kb) and beta 1- (2.7, 2.3, and 1.9 kb) transcripts were present in FDLE at levels approximately fivefold higher than in FLF. alpha 2-, alpha 3-, or beta 2-isoforms of Na(+)-K(+)-ATPase were not detected. In 17-day gestational age FDLE, only small amounts of alpha 1-mRNA levels were detectable, and there were approximately 10-fold less beta 1-isoform transcripts. By 20 days gestational age, the level of alpha 1-transcripts roughly doubled, whereas beta 1-levels increased approximately sixfold. Thus, during the transition from the canalicular to saccular stages of lung development, FDLE have a differentially regulated surge in mRNA levels of alpha 1- and beta 1-Na(+)-K(+)-ATPase isoforms and do not switch isoforms during lung development. Levels for both isoform transcripts then fell before birth, reaching values less than those seen for 17-day gestational age FDLE. FDLE vesicle Na(+)-K(+)-ATPase activity did not increase until 22 days gestational age.

Phosphorylation of Na,K-ATPase alpha-subunits in microsomes and in homogenates of Xenopus oocytes resulting from the stimulation of protein kinase A and protein kinase C

The phosphorylation of the alpha-subunit of Na+/K(+)-transporting ATPase (Na,K-ATPase) by cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) was characterized in purified enzyme preparations of Bufo marinus kidney and duck salt gland and in microsomes of Xenopus oocytes. In addition, we have examined cAMP and phorbol esters, which are stimulators of PKA and PKC, respectively, for their ability to provoke the phosphorylation of alpha-subunits of Na,K-ATPase in homogenates of Xenopus oocytes. In the enzyme from the duct salt gland, phosphorylation by PKA and PKC occurs on serine and threonine residues, whereas in the enzyme from B. marinus kidney and Xenopus oocytes, phosphorylation by PKA occurs only on serine residues. Phosphopeptide analysis indicates that a site phosphorylated by PKA resides in a 12-kDa fragment comprising the C terminus of the polypeptide. Studies of phosphorylation performed on homogenates of Xenopus oocytes show that not only endogenous oocyte Na,K-ATPase but also exogenous Xenopus Na,K-ATPase expressed in the oocyte by microinjection of cRNA can be phosphorylated in response to stimulation of oocyte PKA and PKC. In conclusion, these data are consistent with the possibility that the alpha-subunit of Na,K-ATPase can serve as a substrate for PKA and PKC in vivo.

Beta 1- and beta 3-subunits can associate with presynthesized alpha-subunits of Xenopus oocyte Na,K-ATPase

Oligomerization of newly synthesized alpha- and beta-subunits is a prerequisite for the structural and functional maturation of Na,K-ATPase. In this study, we have tested the competence of presynthesized alpha- and beta-subunits to assemble into functional enzyme complexes. Antisense oligonucleotides complementary to alpha-mRNA were used to inhibit alpha-subunit synthesis in Xenopus oocytes leaving a presynthesized trypsin-sensitive alpha-subunit pool. beta-Subunits expressed in these oocytes from injected cRNA assembled with the preexisting alpha-subunits, rendered them trypsin-resistant, and permitted the expression of more ouabain binding sites at the plasma membrane. Similarly, presynthesized beta 1- or beta 3-subunits produced in Xenopus oocytes by injection of beta-cRNA and later of specific antisense oligonucleotides were stabilized and transported out of the endoplasmic reticulum when alpha-cRNA was injected into oocytes. These data indicate that alpha- and beta-subunits can insert into endoplasmic reticulum membranes independent of each other in an assembly-competent form and retain their ability for oligomerization after synthesis.

Regulation of alpha 1-beta 3-NA(+)-K(+)-ATPase isozyme during meiotic maturation of Xenopus laevis oocytes

During progesterone-induced maturation of Xenopus oocytes, the transport and ouabain binding capacity of Na(+)-K(+)-ATPase at the plasma membrane is completely downregulated. To elucidate the mechanism and the physiological significance of this process, we have followed the fate of oocyte alpha-beta 3-Na(+)-K(+)-ATPase complexes during meiotic maturation and early embryonic development. An immunocytochemical follow-up of the catalytic alpha-subunit, ouabain binding studies, cell surface iodination, and oocyte cell fractionation combined with immunochemical subunit detection provides evidence that following progesterone treatment Na(+)-K(+)-ATPase molecules are retrieved from the oocyte plasma membrane. The enzyme complexes are recovered in an active form in an intracellular compartment in both in vitro and in vivo matured eggs. Exogenous Xenopus alpha 1- and beta 1-complexes expressed in the oocyte from injected cRNAs are regulated by progesterone similar to endogenous Na(+)-K(+)-ATPase complexes. Finally, active Na(+)-K+ pumps internalized during oocyte maturation appear to be redistributed to plasma membrane fractions during blastula formation in Xenopus embryos. In conclusion, our data suggest that endocytosis of alpha 1- and beta 3-complexes during meiotic maturation of Xenopus oocytes is responsible for downregulation of Na(+)-K(+)-ATPase activity and results in an intracellular pool of functional enzymes, which might be reexpressed during early development in response to physiological needs.

Processing, intracellular transport, and functional expression of endogenous and exogenous alpha-beta 3 Na,K-ATPase complexes in Xenopus oocytes

The minimal functional Na,K-ATPase unit is composed of a catalytic alpha-subunit and a glycosylated beta-subunit. So far three putative beta-isoforms have been described, but only beta 1-isoforms have been identified clearly as part of a purified active enzyme complex. In this study we provide evidence that a putative beta 3-isoform might be the functional component of Xenopus oocyte Na,K-ATPase. beta 3-isoforms are expressed in the oocyte plasma membrane together with alpha-subunits, but beta 3-isoforms are synthesized to a lesser extent than alpha-subunits. The unassembled oocyte alpha-subunits accumulate in an immature trypsin-sensitive form most likely in the endoplasmic reticulum (ER). Injection of both beta 1- and beta 3-cRNA into oocytes abolishes the transport constraint of the oocyte alpha-subunit, renders it trypsin-resistant, and finally leads to an increased number of functional pumps at the plasma membrane. In addition, beta 3-isoforms as beta 1-isoforms depend on the concomitant synthesis of alpha-subunits to be able to leave the ER and to become fully glycosylated. Finally, alpha-beta 1 and alpha-beta 3 complexes expressed at the plasma membrane appear to have similar transport properties as assessed by ouabain binding, rubidium uptake, and electrophysiological measurements in oocytes coexpressing exogenous alpha 1- and beta 1- or beta 3-isoforms. Thus our data indicate that beta 3-isoforms have functional qualities similar to beta 1-isoforms. They can assemble and impose a structural reorganization to newly synthesized alpha-subunits which permits the exit from the ER and the expression of functional Na,K-pumps at the plasma membrane.