Evolution of the epithelial sodium channel and the sodium pump as limiting factors of aldosterone action on sodium transport.

Despite large changes in salt intake, the mammalian kidney is able to maintain the extracellular sodium concentration and osmolarity within very narrow margins, thereby controlling blood volume and blood pressure. In the aldosterone-sensitive distal nephron (ASDN), aldosterone tightly controls the activities of epithelial sodium channel (ENaC) and Na,K-ATPase, the two limiting factors in establishing transepithelial sodium transport. It has been proposed that the ENaC/degenerin gene family is restricted to Metazoans, whereas the α- and β-subunits of Na,K-ATPase have homologous genes in prokaryotes. This raises the question of the emergence of osmolarity control. By exploring recent genomic data of diverse organisms, we found that: 1) ENaC/degenerin exists in all of the Metazoans screened, including nonbilaterians and, by extension, was already present in ancestors of Metazoa; 2) ENaC/degenerin is also present in Naegleria gruberi, an eukaryotic microbe, consistent with either a vertical inheritance from the last common ancestor of Eukaryotes or a lateral transfer between Naegleria and Metazoan ancestors; and 3) The Na,K-ATPase β-subunit is restricted to Holozoa, the taxon that includes animals and their closest single-cell relatives. Since the β-subunit of Na,K-ATPase plays a key role in targeting the α-subunit to the plasma membrane and has an additional function in the formation of cell junctions, we propose that the emergence of Na,K-ATPase, together with ENaC/degenerin, is linked to the development of multicellularity in the Metazoan kingdom. The establishment of multicellularity and the associated extracellular compartment ("internal milieu") precedes the emergence of other key elements of the aldosterone signaling pathway.

Hormonal regulation of (Na+,K+)-ATPase biosynthesis in the toad bladder. Effect of aldosterone and 3,5,3'-triiodo-L-thyronine.

Aldosterone stimulates transepithelial Na+ transport in the toad bladder, and thyroid hormone antagonizes this mineralocorticoid action. In the present study, we assessed the influence of these two hormones on the biosynthesis of (Na+,K+)ATPase, the major driving force of Na+ transport. Rates of enzyme synthesis were estimated by immunoprecipitation with monospecific alpha (96,000 daltons) and beta (60,000 daltons) subunit antibodies. After a 30-min pulse of intact tissue with [35S]methionine, the anti-alpha-serum recognized the 96,000-dalton alpha subunit and the anti-beta-serum, a 42,000-dalton protein, in total cell extracts. The biosynthesis rates of both these proteins were increased 2.8- and 2.4-fold respectively, over controls by 80 nM aldosterone after 18 h of hormone treatment. The hormonal effect was not apparent up to 3 h of incubation and was dose dependent between 0.2 and 20 nM aldosterone. The hormonal induction was antagonized by spironolactone (500-fold excess) but not by amiloride. The action of aldosterone thus seems to be a receptor-mediated process and a primary event independent of the Na+ permeability of the apical membrane. Thyroid hormone, on the other hand, had no effect on either basal or aldosterone-stimulated synthesis rates of both enzyme proteins. The results demonstrate a direct effect of aldosterone on gene expression of the (Na+,K+)-ATPase. Ultimately, this phenomenon could be linked to the late mineralocorticoid action of this hormone. On the other hand, thyroid hormone, in contrast to the situation in mammals, does not stimulate de novo enzyme synthesis in amphibia. Neither can the antimineralocorticoid action of thyroid hormone in the toad bladder be explained by an inhibition of the (Na+,K+)-ATPase synthesis.

Epithelial sodium transport and its control by aldosterone: the story of our internal environment revisited.

Transcription and translation require a high concentration of potassium across the entire tree of life. The conservation of a high intracellular potassium was an absolute requirement for the evolution of life on Earth. This was achieved by the interplay of P- and V-ATPases that can set up electrochemical gradients across the cell membrane, an energetically costly process requiring the synthesis of ATP by F-ATPases. In animals, the control of an extracellular compartment was achieved by the emergence of multicellular organisms able to produce tight epithelial barriers creating a stable extracellular milieu. Finally, the adaptation to a terrestrian environment was achieved by the evolution of distinct regulatory pathways allowing salt and water conservation. In this review we emphasize the critical and dual role of Na(+)-K(+)-ATPase in the control of the ionic composition of the extracellular fluid and the renin-angiotensin-aldosterone system (RAAS) in salt and water conservation in vertebrates. The action of aldosterone on transepithelial sodium transport by activation of the epithelial sodium channel (ENaC) at the apical membrane and that of Na(+)-K(+)-ATPase at the basolateral membrane may have evolved in lungfish before the emergence of tetrapods. Finally, we discuss the implication of RAAS in the origin of the present pandemia of hypertension and its associated cardiovascular diseases.

The Aldosterone/Renin Ratio as a Diagnostic Tool for the Diagnosis of Primary Hypoaldosteronism in Newborns and Infants.

BACKGROUND/AIMS: Primary hypoaldosteronism is a rare inborn disorder with life-threatening symptoms in newborns and infants due to an aldosterone synthase defect. Diagnosis is often difficult as the plasma aldosterone concentration (PAC) can remain within the normal range and thus lead to misinterpretation and delayed initiation of life-saving therapy. We aimed to test the eligibility of the PAC/plasma renin concentration (PRC) ratio as a tool for the diagnosis of primary hypoaldosteronism in newborns and infants. Meth ods: Data of 9 patients aged 15 days to 12 months at the time of diagnosis were collected. The diagnosis of primary hypoaldosteronism was based on clinical and laboratory findings over a period of 12 years in 3 different centers in Switzerland. To enable a valid comparison, the values of PAC and PRC were correlated to reference methods. RESULTS: In 6 patients, the PAC/PRC ratio could be determined and showed constantly decreased values <1 (pmol/l)/(mU/l). In 2 patients, renin was noted as plasma renin activity (PRA). PAC/PRA ratios were also clearly decreased. The diagnosis was subsequently genetically confirmed in 8 patients. CONCLUSION: A PAC/PRC ratio <1 pmol/mU and a PAC/PRA ratio <28 (pmol/l)/(ng/ml × h) are reliable tools to identify primary hypoaldosteronism in newborns and infants and help to diagnose this life-threatening disease faster. © 2015 S. Karger AG, Basel.

Alternatively spliced proline-rich cassettes link WNK1 to aldosterone action.

The thiazide-sensitive NaCl cotransporter (NCC) is important for renal salt handling and blood-pressure homeostasis. The canonical NCC-activating pathway consists of With-No-Lysine (WNK) kinases and their downstream effector kinases SPAK and OSR1, which phosphorylate NCC directly. The upstream mechanisms that connect physiological stimuli to this system remain obscure. Here, we have shown that aldosterone activates SPAK/OSR1 via WNK1. We identified 2 alternatively spliced exons embedded within a proline-rich region of WNK1 that contain PY motifs, which bind the E3 ubiquitin ligase NEDD4-2. PY motif-containing WNK1 isoforms were expressed in human kidney, and these isoforms were efficiently degraded by the ubiquitin proteasome system, an effect reversed by the aldosterone-induced kinase SGK1. In gene-edited cells, WNK1 deficiency negated regulatory effects of NEDD4-2 and SGK1 on NCC, suggesting that WNK1 mediates aldosterone-dependent activity of the WNK/SPAK/OSR1 pathway. Aldosterone infusion increased proline-rich WNK1 isoform abundance in WT mice but did not alter WNK1 abundance in hypertensive Nedd4-2 KO mice, which exhibit high baseline WNK1 and SPAK/OSR1 activity toward NCC. Conversely, hypotensive Sgk1 KO mice exhibited low WNK1 expression and activity. Together, our findings indicate that the proline-rich exons are modular cassettes that convert WNK1 into a NEDD4-2 substrate, thereby linking aldosterone and other NEDD4-2-suppressing antinatriuretic hormones to NCC phosphorylation status.