FXYD proteins: new tissue- and isoform-specific regulators of Na,K-ATPase

Large-scale identification and characterization of genes with asymmetric expression patterns in the developing chick retina

To understand the molecular basis of topographic retinotectal projection, an overall view of the asymmetrically expressed molecules in the developing retina is needed. We performed a large-scale screening using restriction landmark cDNA scanning (RLCS) in the embryonic day 8 (E8) chick retina. RLCS is a cDNA display system, in which a large number of cDNA species are displayed as two-dimensional spots with intensities reflecting their expression levels as mRNA. We searched for spots that gave different signal intensities between the nasal and temporal retinas or between the dorsal and ventral retinas, and detected about 200 spots that were preferential on one side in the retina. The asymmetric expression of each gene was verified by Northern blotting and in situ hybridization. By subsequent analyses using molecular cloning, DNA sequencing, and database searching, 33 asymmetric molecules along the nasotemporal (N-T) axis and 20 along the dorsoventral (D-V) axis were identified. These included transcription factors, secretory factors, transmembrane proteins, and intracellular proteins with various putative functions. Their expression profiles revealed by in situ hybridization are highly diverse and individual. Moreover, many of them begin to be expressed in the retina from the early developmental stages, suggesting that they are implicated in the establishment and maintenance of regional specificity in the developing retina. The molecular repertoire revealed by this work will provide candidates for future studies to elucidate the molecular mechanisms of topographic retinotectal map formation.

Dexamethasone responsive element in the rat Na, K-ATPase beta1 gene coding region

The Na, K-ATPase plays an essential role in active alveolar epithelial fluid resorption. In fetal and adult alveolar epithelial cells, glucocorticoids (GC) increase Na, K-ATPase activity, mRNA levels, and transcription rate of the beta(1) subunit. In this study, we describe a glucocorticoid responsive element (GRE) in the coding region of the rat Na, K-ATPase beta(1) gene in a rat lung epithelial cell line. Transient transfection experiments with the beta(1) subunit coding region with or without the 5' and 3' untranslated regions demonstrated responsiveness to dexamethasone induction and also identified a GRE at +434 in exon IV. The +434 GRE conferred dexamethasone responsiveness in a heterologous thymidine kinase promoter irrespective of its orientation to the beta(1) promoter. Transcriptional upregulation by dexamethasone was abolished in +434 mutants. Electrophoretic mobility shift assays (EMSA) demonstrated specific binding of nuclear proteins to the +434 GRE and the presence of the GC receptor. This specific binding was inhibited by a GRE previously described in the rat Na, K-ATPase beta(1) gene at -631. In conclusion, we identified a GRE at +434 in the exon IV of the rat Na, K-ATPase beta(1) gene.

Structure/function studies of the gamma subunit of the Na,K-ATPase

The Na,K-ATPase gamma subunit is present primarily in kidney as two splice variants, gammaa and gammab, which differ only at their extracellular N-termini. Two distinct effects of gamma are seen in biochemical Na,K-ATPase assays of mammalian (HeLa) cells transfected with gammaa or gammab, namely, (i) a decrease in K'(ATP) probably secondary to a shift in steady-state E(1) <--> E(2) poise in favor of E(1) and (ii) an increase in cytoplasmic K(+)/Na(+) antagonism seen as an increase in K'(Na) at high K(+) concentration. Mutagenesis experiments involving alterations in extramembranous regions of gamma indicate that different regions mediate the aforementioned distinct effects and that the effects appear to be long range. Studies of ouabain-sensitive fluxes with intact cells confirm the gamma effects seen with membranes and also suggest an additional effect (increase) in apparent affinity for extracellular K(+). Alteration in gamma function was also evidenced in the behavior of a G41 -->R mutation within the transmembrane domain of gamma. G41R is associated with autosomal dominant renal magnesium wasting. Our studies show that this mutation in the gammab variant retards trafficking of gamma, but not alphabeta pumps, to the cell surface and abolishes functional effects of gamma, consistent with the conclusion that the Mg(2+) transport defect is secondary to loss of gamma modulation of Na,K-ATPase function.