The second sodium pump: from the function to the gene

Abstract

Transepithelial Na+ transport is mediated by passive Na+ entry across the luminal membrane and exit through the basolateral membrane by two active mechanisms: the Na+/K+ pump and the second sodium pump. These processes are associated with the ouabain-sensitive Na+/K+-ATPase and the ouabain-insensitive, furosemide-inhibitable Na+-ATPase, respectively. Over the last 40 years, the second sodium pump has not been successfully associated with any particular membrane protein. Recently, however, purification and cloning of intestinal α-subunit of the Na+-ATPase from guinea pig allowed us to define it as a unique biochemical and molecular entity. The Na+- and Na+/K+-ATPase genes are at the same locus, atp1a1, but have independent promoters and some different exons. Herein, we spotlight the functional characteristics of the second sodium pump, and the associated Na+-ATPase, in the context of its role in transepithelial transport and its response to a variety of physiological and pathophysiological conditions. Identification of the Na+-ATPase gene (atna) allowed us, using a bioinformatics approach, to explore the tertiary structure of the protein in relation to other P-type ATPases and to predict regulatory sites in the promoter region. Potential regulatory sites linked to inflammation and cellular stress were identified in the atna gene. In addition, a human atna ortholog was recognized. Finally, experimental data obtained using spontaneously hypertensive rats suggest that the Na+-ATPase could play a role in the pathogenesis of essential hypertension. Thus, the participation of the second sodium pump in transepithelial Na+ transport and cellular Na+ homeostasis leads us to reconsider its role in health and disease.

Altered signaling pathways linked to angiotensin II underpin the upregulation of renal Na+-ATPase in chronically undernourished rats

“…A further issue was to see whether phosphorylatable domains exist in the recently identified sequence of Na + -ATPase [6,7], which could be targeted by PKC or PKA. With the NetPhos 2 tool [35], we first found 15 Ser and 5 Thr cytosolic potentially phosphorylatable residues, which were located in the center of sequences containing 9 amino acids with a phosphorylation score ≥0.70.…”

“…10 presents the structure and topology of key functional domains that could be influenced by PKC-and PKA-mediated regulatory phosphorylations. The primary sequence of Na + -ATPase used in the in silico analyses was that recently described by Rocafull et al [7]. Fig.…”

“…Those chosen correspond to experimentally validated kinase-mediated phosphorylations of Ser and Thr residues [35]. The source of the Na + -ATPase sequence used for in silico analysis was the α-subunit (atna) from Cavia porcellus, described by Rocafull et al [7] (GenBank accession number: EF489487. 2, GI:283442232; available in: http://www.ncbi.nlm.nih.gov/nuccore/ EF489487.2).…”

“…2, GI:283442232; available in: http://www.ncbi.nlm.nih.gov/nuccore/ EF489487.2). Structural domains were predicted by Rocafull et al [7], using the CPHmodels 3.0 software that aligned the 3D structure of ATNA_CAVPO with a segment of the rat (Na + + K + )ATPase α 1 -subunit (AT1A1_RAT: P06685) in the E 2 conformation [37], as template. We modeled the same EF489487.2 sequence using CPHmodels 3.2 and the Sus scrofa sequence of (Na + + K + )ATPase α 1 -subunit (PDB 3WGU) in an analog of the E 1~P · ADP · 3Na + intermediate of the catalytic cycle [38], as template.…”

“…Primary compromise of kidney functioning by chronic malnutrition impacts the cardiovascular system, increasing the risk of developing hypertension and atherosclerosis [reviewed in 4]. We recently demonstrated that chronically undernourished rats present simultaneous losartan-sensitive alterations in renal and cardiac Na + handling [5], in which upregulation of the recently cloned and purified ouabain-resistant, furosemide-inhibitable Na + -ATPase is pivotal [6,7]. This pump is considered to be the fine-tune mechanism of Na + reabsorption by proximal tubule cells [8], and its altered kinetic and regulatory properties constitute a hallmark of kidney alterations that involve deregulated local angiotensin II (Ang II) levels, inflammation and reactive oxygen species (ROS) signaling [9,10].…”