Lung symptoms in pseudohypoaldosteronism type 1 are associated with deficiency of the α-subunit of the epithelial sodium channel

P170 The mineralocorticoid hormone, aldosterone increases renal tubule Na absorption via increases in the protein abundances of the α-subunit of the epithelial sodium channel (ENaC) and the 70 kDa form of the γ- subunit of ENaC (JCI 104:R19-R23). This study assesses the affect of dietary salt restriction on the regulation of the epithelial sodium channel (ENaC) in the lung and distal colon, in addition to kidney, using semiquantitative immunoblotting. Rats were placed initially on either a control Na intake (0.02 meq/day), or a low Na intake (0.2 meq/day) for 10 days. The low salt treated rats demonstrated an increase in plasma aldosterone levels at day 10 (control = 0.78 + 0.32 nM; Na restricted = 3.50 + 1.30 nM). In kidney homogenates, there were marked increases in the band density of the α-subunit of ENaC (286 % of control) and the 70 kDa form of γ-subunit of ENaC (262 % of control), but no increase in the abundance of the β-subunit of ENaC. In lung homogenates, there was no significant change in the band densities of the α, β, or γ subunits of ENaC. In distal colon, there was an increase in the band density of the β-subunit of ENaC (311 % of control) and an increase in both the 85 kDa (2355% of control) and 70 kDa (843 % of control) form of the γ subunit of ENaC in response to dietary Na restriction. However, there was no significant difference in the band density of the α-subunit of ENaC. These findings demonstrate tissue specific regulation of the three subunits of ENaC in response to dietary salt restriction.

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TMPRSS4-dependent activation of the epithelial sodium channel requires cleavage of the γ-subunit distal to the furin cleavage site

AJP Renal Physiology ◽

10.1152/ajprenal.00330.2011 ◽

2012 ◽

Vol 302 (1) ◽

pp. F1-F8 ◽

Cited By ~ 30

Author(s):

Christopher J. Passero ◽

Gunhild M. Mueller ◽

Michael M. Myerburg ◽

Marcelo D. Carattino ◽

Rebecca P. Hughey ◽

...

Keyword(s):

Sodium Channel ◽

Cleavage Site ◽

Epithelial Sodium Channel ◽

Cleavage Sites ◽

Furin Cleavage ◽

Channel Activation ◽

Α Subunit ◽

Γ Subunit ◽

Furin Cleavage Site ◽

Unique Mechanism

The epithelial sodium channel (ENaC) is activated by a unique mechanism, whereby inhibitory tracts are released by proteolytic cleavage within the extracellular loops of two of its three homologous subunits. While cleavage by furin within the biosynthetic pathway releases one inhibitory tract from the α-subunit and moderately activates the channel, full activation through release of a second inhibitory tract from the γ-subunit requires cleavage once by furin and then at a distal site by a second protease, such as prostasin, plasmin, or elastase. We now report that coexpression of mouse transmembrane protease serine 4 (TMPRSS4) with mouse ENaC in Xenopus oocytes was associated with a two- to threefold increase in channel activity and production of a unique ∼70-kDa carboxyl-terminal fragment of the γ-subunit, similar to the ∼70-kDa γ-subunit fragment that we previously observed with prostasin-dependent channel activation. TMPRSS4-dependent channel activation and production of the ∼70-kDa fragment were partially blocked by mutation of the prostasin-dependent cleavage site (γRKRK186QQQQ). Complete inhibition of TMPRSS4-dependent activation of ENaC and γ-subunit cleavage was observed when three basic residues between the furin and prostasin cleavage sites were mutated (γK173Q, γK175Q, and γR177Q), in addition to γRKRK186QQQQ. Mutation of the four basic residues associated with the furin cleavage site (γRKRR143QQQQ) also prevented TMPRSS4-dependent channel activation. We conclude that TMPRSS4 primarily activates ENaC by cleaving basic residues within the tract γK173-K186 distal to the furin cleavage site, thereby releasing a previously defined key inhibitory tract encompassing γR158-F168 from the γ-subunit.

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

Physiological Genomics ◽

10.1152/physiolgenomics.00002.2011 ◽

2011 ◽

Vol 43 (13) ◽

pp. 844-854 ◽

Cited By ~ 29

Author(s):

Romain A. Studer ◽

Emilie Person ◽

Marc Robinson-Rechavi ◽

Bernard C. Rossier

Keyword(s):

Sodium Channel ◽

Sodium Transport ◽

Salt Intake ◽

Epithelial Sodium Channel ◽

Cell Junctions ◽

Limiting Factors ◽

Last Common Ancestor ◽

Β Subunit ◽

Α Subunit ◽

Additional Function

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.

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A homozygous missense mutation in SCNN1A is responsible for a transient neonatal form of pseudohypoaldosteronism type 1

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00066.2011 ◽

2011 ◽

Vol 301 (3) ◽

pp. E467-E473 ◽

Cited By ~ 25

Author(s):

Mirjam Dirlewanger ◽

Delphine Huser ◽

Maria-Christina Zennaro ◽

Eric Girardin ◽

Laurent Schild ◽

...

Keyword(s):

Missense Mutation ◽

Functional Domain ◽

Loss Of Function ◽

Dietary Salt ◽

Dominant Form ◽

Α Subunit ◽

Homozygous Missense Mutation ◽

Pseudohypoaldosteronism Type

Pseudohypoaldosteronism type 1 (PHA1) is a monogenic disorder of mineralocorticoid resistance characterized by salt wasting, hyperkalemia, high aldosterone levels, and failure to thrive. An autosomal recessive form (AR-PHA1) is caused by mutations in the epithelial sodium channel ENaC with usually severe and persisting multiorgan symptoms. The autosomal dominant form of PHA1 (AD-PHA1) is due to mutations in the mineralocorticoid receptor causing milder and transient symptoms restricted to the kidney. We identified a homozygous missense mutation in the SCNN1A gene (c.727T>C/p.Ser243Pro), encoding α-subunit of ENaC (α-ENaC) in a prematurely born boy with a severe salt-losing syndrome. The patient improved rapidly under treatment, and dietary salt supplementation could be stopped after 6 mo. Interestingly, the patient's sibling born at term and harboring the same homozygous Ser243Pro mutation showed no symptom of salt-losing nephropathy. In vitro expression of the αSer243Pro ENaC mutant revealed a slight but significant decrease in ENaC activity that is exacerbated in the presence of high Na+ load. Our study provides the first evidence that ENaC activity is critical for the maintenance of salt balance in the immature kidney of preterm babies. Together with previous studies, it shows that, when the kidney is fully mature, the severity of the symptoms of AR-PHA1 is related to the degree of the ENaC loss of function. Finally, this study identifies a novel functional domain in the extracellular loop of ENaC.

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