Disease relevance of SCNN1G

• Liddle's syndrome of hypertension and pseudoaldosteronism has been shown to arise from mutations in SCNN1B and SCNN1G [1].
• Pseudohypoaldosteronism type 1 (PHA1, OMIM 264350) is an uncommon inherited disorder characterized by salt-wasting and end-organ unresponsiveness to mineralocorticoids [2].
• Sixteen-month-old transgenic rescue mice that were kept under a regular salt diet that contains a small amount of sodium (0.3% Na(+)) displayed a compensated PHA-1 phenotype with normal body weight, normal kidney index, normal blood pressure, but 6.3-fold elevated plasma aldosterone levels compared with the age-matched control group [3].
• Temporal Fourier analysis was applied to Kr-81m ventilation scintigraphy to determine the amplitude (AMP1) and phase (PHA1) of the first harmonic of a single composite respiratory cycle and to compare regional patterns in subjects with obstructive pulmonary disease (COPD) and nonobstructed subjects [4].

High impact information on SCNN1G

• These two chromosomal regions harbour the genes encoding the three subunits of the human amiloride sensitive epithelial sodium channel (hENaC): SCNN1B and SCNN1G on 16p and SCNN1A on 12p [2].
• A complete genome search using homozygosity mapping in eleven consanguineous families with PHA1 provided conclusive evidence of linkage with heterogeneity [2].
• The gammaENaC (-/-) newborn exhibits a phenotype that resembles the clinical manifestations of human neonatal PHA1 [5].
• Transcriptional ENaC induction by butyrate led to synthesis of gamma-ENaC subunits, but correct targeting of ENaC channels to the apical cell membrane was dependent on corticosteroid hormones [6].
• RESULTS: Butyrate up-regulated beta- and gamma-ENaC mRNA expression in HT-29/B6 cells and induced transcription from beta- and gamma-ENaC promoter constructs [6].

Chemical compound and disease context of SCNN1G

• Syndromes of aldosterone resistance have been also characterized molecularly and mutations in the gene MLR, encoding mineralocorticoid receptor, and in the genes SNCC1A, SNCC1B, and SCNN1G, encoding subunits of the epithelial Na+ channel, have been found in dominant and recessive forms of pseudohypoaldosteronism type 1, respectively [7].

Biological context of SCNN1G

• In the SCNN1G gene, we identified three missense mutations, A578V (n=1), P603S (n=1), and L609F (n=1) in a heterozygous state in addition to two synonymous ones, Ile550 (n=1) and Leu649 (n= 91, heterozygous; n=2, homozygous) [8].
• To study gene regulation, the promoters of beta- and gamma-ENaC were analyzed in reporter gene assays [9].
• Promoter analysis revealed that down-regulation of beta- and gamma-ENaC gene expression was primarily induced by tumor necrosis factor alpha [9].
• Furthermore, 2 base substitutions in exon 13 were present in all the Chinese subjects compared with the published European SCNN1G DNA sequence [10].
• Although the effects of the A(-173) allele were recessive and although the AA genotype was found in just 0.7% of our study population, we observed that this variation of human SCNN1G had significant effects on blood pressure [11].

Anatomical context of SCNN1G

• Individually substituting C terminus-truncated alpha-, beta-, or gamma-ENaC subunits for their wild-type counterparts reversed the S1A-induced inhibition of I(Na), and oocytes expressing ENaC comprised of three C terminus-truncated subunits showed no S1A inhibition of I(Na) [12].
• The results showed that T lymphocytes express the ENaC gamma subunit mRNA, and B lymphocytes the ENaC beta subunit mRNA [13].
• Confocal microscopy of Madin-Darby canine kidney cell monolayers stably transfected with wild type, rat isoforms of alpha-, beta-, and gamma-ENaC revealed co-localization of alpha-ENaC with the cortical F-actin cytoskeleton both at the apical membrane and within the subapical cytoplasm [14].
• Immunoblotting showed increased protein expression of alpha-ENaC, the 70-kDa form of gamma-ENaC, and the Na-Cl cotransporter (NCC) in kidney cortex in aldosterone-treated rats, whereas spironolactone decreased alpha-ENaC and NCC compared with control rats treated with lithium alone [15].
• At the level of the single channel, submicromolar concentrations of hNE increased activity of near-silent ENaC approximately 108-fold in patches from NIH-3T3 cells expressing rat alpha-, beta-, and gamma-ENaC subunits [16].

Associations of SCNN1G with chemical compounds

• Most steroid hormone insensitivity syndromes arise from mutations in the corresponding receptor, but available genetic evidence is against involvement of the mineralocorticoid receptor gene, MLR, in PHA1 [1].
• After expression of exogenous gamma-ENaC, aldosterone again reduced the frequency of observance of the 21-pS alpha-ENaC channel but induced the appearance of a 5-pS channel, presumably a alphabetagamma-ENaC channel [17].
• Inhibition of ENaC activity was predominantly attributed to a doxorubicin-dependent decrease in gamma-ENaC subunit mRNA expression and an increase in gamma-ENaC promoter methylation [18].
• Proteins were present for the alpha-, beta-, and gamma-subunits of ENaC and Sgk1, and expression of alpha- and gamma-ENaC was upregulated by dexamethasone [19].
• The gamma-ENaC promoter could also be induced by the SCFA propionate but not by acetate [6].

Physical interactions of SCNN1G

• Both beta- and gamma-ENaC participate in negative regulation by interacting with Nedd4-2, an E3 ubiquitin-ligase [20].
• Syntaxin 1A was able to coimmunoprecipitate in vitro-translated gamma-ENaC, but not alpha- or beta-ENaC [21].

Regulatory relationships of SCNN1G

• Reverse transcriptase polymerase chain reaction and Northern blots showed that human B lymphocytes express messages for both alpha- and beta- but not gamma-ENaC [17].

Other interactions of SCNN1G

• Stable transfection of GR-alpha restored intestine-specific glucocorticoid upregulation of beta- and gamma-ENaC in HT-29/B6 cells, which was followed by intact targeting of ENaC channels to the apical cell membrane and dose-dependent induction of electrogenic sodium absorption [22].
• Indirect immunofluorescence microscopy revealed that delta-ENaC is co-expressed with alpha-, beta-, and gamma-ENaC in H441 cells at the protein level [23].
• Also, using antibodies raised against alpha-, beta-, or gamma-ENaC, we detected syntaxin 1A in immunoprecipitates from Madin-Darby canine kidney cells stably transfected with alphabetagamma-ENaC [21].
• Finally, the whole kidney abundances of alpha-1 Na-K-ATPase, alpha- beta-, and gamma-ENaC (70-kDa band) positively correlated with net sodium balance, whereas NKCC2 was negatively correlated to FE(Na) [24].
• Ribonuclease protection assays revealed that the steady-state levels of alpha-, beta-, and gamma-ENaC mRNAs were similar in CF and normal airway superficial epithelia [25].

Analytical, diagnostic and therapeutic context of SCNN1G

• We now report the molecular cloning of the human lung beta (SCNN1B) and gamma (SCNN1G) chains [26].
• Western blots showed that both alpha- and beta- but not gamma-ENaC proteins are expressed and strongly reduced by antisense oligonucleotides [17].
• Although both transcription factors increased butyrate-mediated gamma-ENaC transcription upon overexpression, chromatin immunoprecipitation revealed that only Sp3 binds to the gamma-ENaC promoter in vivo and that Sp3 binding is enhanced by butyrate [6].
• The bulk of whole cell gamma-ENaC was degraded within 1 h after treatment with inhibitors of synthesis; however, a significant pool was "protected" from inhibitors for up to 12 h [27].

References