Sequence analysis of coding and 3' and 5' flanking regions of the epithelial sodium channel alpha, beta, and gamma genes in Dahl S versus R rats

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

Role of the epithelial sodium channel in salt-sensitive hypertension

The epithelial sodium channel (ENaC) is a heteromeric channel composed of three similar but distinct subunits, α, β and γ. This channel is an end-effector in the rennin-angiotensin-aldosterone system and resides in the apical plasma membrane of the renal cortical collecting ducts, where reabsorption of Na(+) through ENaC is the final renal adjustment step for Na(+) balance. Because of its regulation and function, the ENaC plays a critical role in modulating the homeostasis of Na(+) and thus chronic blood pressure. The development of most forms of hypertension requires an increase in Na(+) and water retention. The role of ENaC in developing high blood pressure is exemplified in the gain-of-function mutations in ENaC that cause Liddle's syndrome, a severe but rare form of inheritable hypertension. The evidence obtained from studies using animal models and in human patients indicates that improper Na(+) retention by the kidney elevates blood pressure and induces salt-sensitive hypertension.

Enhanced expression of epithelial sodium channels in the renal medulla of Dahl S rats

Inner medullary collecting duct (IMCD) cells from salt-sensitive (S) Dahl rats transport twice as much Na+ as cells from salt-resistant (R) rats, possibly related to dysregulation of the renal epithelial sodium channel (ENaC). The effect of a high-salt diet on ENaC expression in the inner medulla of S versus R rats has not yet been studied. Young, male S and R rats were placed on a regular-salt (0.3%) or high-salt (8%) diet for 2 or 4 weeks. mRNA and protein expression of ENaC subunits were studied by real-time PCR and immunoblotting. Intracellular distribution of the subunits in the IMCD was evaluated by immunohistochemistry. On regular salt, the abundance of the mRNA of β and γENaC was higher in the medulla of S rats than R rats. This was associated with a greater protein abundance of 90 kDa γENaC and higher immunoreactivity for both α and γ ENaC. High salt did not affect mRNA abundance in either strain and decreased apical staining of βENaC in IMCD of R rats. In contrast, high salt did not affect the higher apical localization of αENaC and increased the apical membrane staining for β and γENaC in the IMCD of S rats. Expression of ENaC subunits is enhanced in the medulla of S vs. R rats on regular salt, and further increased on high salt. The persistent high expression of αENaC and increase in apical localization of β and γENaC may contribute to greater retention of sodium in S rats on a high-salt diet.

The Epithelial Sodium Channel alpha subunit (alpha ENaC) alternatively spliced form "b" in Dahl rats: What's next?

Background

The amiloride-sensitive Epithelial Sodium Channel (ENaC) is critical in maintaining Na⁺ balance, extracellular fluid volume and long term blood pressure control. ENaC is composed of three main subunits α, β, & γ. While α ENaC is critical for channel functionality, β & γ ENaC maximize channel function. To date, there are four alternatively spliced forms of the α subunit of ENaC (α ENaC-a, -b, -c, & -d) that have been published in rats, in addition to the major α ENaC transcript. While α ENaC-a, -c & -d transcripts are low abundance transcripts compared to full-length α ENaC, α ENaC-b is a higher abundance and salt-sensitive transcript compared to full-length α ENaC.

Presentation of the hypothesis

α ENaC-b protein, which is preferentially produced in Dahl R rats, to a greater extent on high salt diet, exerts a dominant negative effect on full-length α ENaC subunit by physically binding to and trapping full-length α ENaC subunit in the endoplasmic reticulum, and finally accelerating full-length α ENaC proteolytic degradation in a dose-dependent manner.

Testing the hypothesis

1) To examine the mRNA and protein abundance of α ENaC-b relative to α ENaC full-length in kidney, lung, and taste tissues of Dahl rats. 2) To compare the expression (mRNA and protein) of α ENaC-b in kidneys of Dahl S and R rats on regular and high salt diet. 3) To examine the putative binding of α ENaC-b proteins to full-length α ENaC in vitro and to determine the impact of such binding on full-length α ENaC expression in vitro.

Implications of the hypothesis

Our studies will be the first to demonstrate the over-expression of salt-sensitive α ENaC-b spliced form in kidney tissues of Dahl R rats at the expense of full-length α ENaC. The current proposal will provide highly novel insights into the putative mechanisms leading to ENaC hypoactivity in high-salt-fed Dahl R rats. Finally, findings from the present proposal will uncover a new mechanism by which alternative splicing may control the regulation of ENaC expression/function.

Article Commentary: The Alternatively Spliced Form “b” of the Epithelial Sodium Channel α subunit (α ENaC): Any Prior Evidence of its Existence?

The epithelial sodium channel (ENaC) is critical in maintaining sodium balance across aldosterone-responsive epithelia. ENaC is a combined channel formed of three subunits (αβγ) with α ENaC subunit being the most critical for channel functionality. In a previous report, we have demonstrated the existence and mRNA expression levels of four alternatively spliced forms of the α ENaC subunit denoted by -a, -b, -c and -d in kidney cortex of Dahl S and R rats. Of the four alternatively spliced forms presently identified, α ENaC-b is considered the most interesting for the following reasons: Aside from being a salt-sensitive transcript, α ENaC-b mRNA expression is ∼32 fold higher than α ENaC wildtype in kidney cortex of Dahl rats. Additionally, the splice site used to generate α ENaC-b is conserved across species. Finally, α ENaC-b mRNA expression is significantly higher in salt-resistant Dahl R rats versus salt-sensitive Dahl S rats. As such, this commentary aims to highlight some of the previously published research articles that described the existence of an additional protein band on α ENaC western blots that could account for α ENaC-b in other rat species.

Aberrant ENaC activation in Dahl salt-sensitive rats

BACKGROUND

The epithelial sodium channel (ENaC) plays an important role in the regulation of blood pressure by modulating Na reabsorption in the kidney. Dahl salt-sensitive rats on high-salt diet develop severe hypertension, and high-salt diet has been reported to stimulate ENaC mRNA expression in the kidney abnormally in Dahl salt-sensitive rats despite a suppressed plasma aldosterone concentration (PAC).

METHODS

We investigated the effect of high-salt diet on ENaC protein expression in Dahl salt-resistant and Dahl salt-sensitive rats, and examined the effect of amiloride (5 mg/kg per day) and eplerenone (0.125% diet) on blood pressure and renal injury in Dahl salt-sensitive rats.

RESULTS

Dahl salt-sensitive rats developed hypertension and renal damage following 4 weeks of treatment with high-salt diet. Although PAC and kidney aldosterone content were all suppressed by the high-salt diet in Dahl salt-sensitive rats, both beta and gammaENaC mRNA expression and protein abundance were significantly increased. The molecular weight shift of gammaENaC from 85 to 70 kDa, an indication of ENaC activation, was clearly increased in Dahl salt-sensitive rats on high-salt diet compared with the low-salt group or Dahl salt-resistant rats on high-salt diet. Four weeks of treatment with amiloride, but not eplerenone, significantly ameliorated hypertension and kidney injury in Dahl salt-sensitive rats fed high-salt diet, suggesting aberrant aldosterone-independent activation of ENaC.

CONCLUSION

These results suggest that inappropriate expression and activation of ENaC could be one of the underlying mechanisms by which Dahl salt-sensitive rats develop salt-sensitive hypertension and organ damage, and indicate a therapeutic benefit of amiloride in salt-sensitive hypertension where ENaC is excessively activated.

A Novel Mechanism in Regulating the Alpha-Subunit of the Epithelial Sodium Channel (α ENaC) by the Alternatively Spliced Form α ENaC-b

Introduction

In Dahl rats’ kidney cortex, the alternatively spliced form of the epithelial sodium channel α subunit (α ENaC-b) is the most abundant mRNA transcript (32+/-3 fold > α ENaC-wt) as was investigated by quantitative RT-PCR analysis. α ENaC-b mRNA levels were significantly higher in Dahl R versus S rats, and were further augmented by high salt diet.

Objectives

In the present study, we described the molecular cloning and searched for a possible role of α ENaC-b by testing its potential expression in COS7 cells as well as its impact on α ENaC-wt expression levels when co-expressed in COS7 cells in a dose-dependent manner.

Methods

Using RT-PCR strategy, the full-length wildtype α ENaC transcript and the alternatively spliced form α ENaC-b were amplified, sequenced, cloned, subcloned into PCMV-sport6 expression vector, expressed and co-expressed into COS7 cells in a dose-dependent manner. A combination of denaturing and native western blotting techniques was employed to examine the expression of α ENaC-b in vitro, and to determine if an interaction between α ENaC-b and α ENaC-wt occurs in vitro, and finally to demonstrate if degradation of α ENaC-wt protein does occur.

Results

α ENaC-b is translated in COS7 cells. Co-expression of α ENaC-b together with α ENaC-wt reduced α ENaC-wt levels in a dose-dependent manner. α ENaC-wt and α ENaC-b appear to form a complex that enhances the degradation of α ENaC-wt.

Conclusions

Western blots suggest a novel mechanism in α ENaC regulation whereby α ENaC-b exerts a dominant negative effect on α ENaC-wt expression. This is potentially by sequestering α ENaC-wt, enhancing its proteolytic degradation, and possibly explaining the mechanism of salt-resistance in Dahl R rats.

Characterization of the epithelial sodium channel alpha subunit coding and non-coding transcripts and their corresponding mRNA expression levels in Dahl R versus S rat kidney cortex on normal and high salt diet

Aims/hypothesis

The α subunit of the amiloride-sensitive epithelial sodium channel (α ENaC) is critical for the expression of functional channels. In humans and rats, non functional alternatively spliced forms of α ENaC have been proposed to act as negative regulatory components for ENaC. The purpose of this study was to examine the presence and consequently investigate the mRNA expression levels of alternatively spliced forms of α ENaC in kidney cortex of Dahl salt-resistant rats (R) versus Dahl salt-sensitive rats (S) on high salt and normal diets.

Methods

Using quantitative RT-PCR strategy, we examined the mRNA expression levels of previously reported α ENaC-a and -b alternatively spliced forms in kidney cortex of Dahl S and R rats on normal and four-week high salt diet and compared their corresponding abundance to wildtype α ENaC mRNA levels. We identified 2 novel non-coding C-terminus spliced forms and examined their mRNA expression in Dahl R versus S rat kidney cortex. We also tested the presence of five previously reported lung-specific α ENaC spliced forms in Dahl rat kidney cortex (CK479583, CK475461, CK364785, CK475819, and CB690980).

Results

Previously reported α ENaC-a and -b alternatively spliced forms are present in Dahl rat kidney cortex and are significantly higher in Dahl R versus S rats (P < 0.05). Four-week high salt diet significantly increases α ENaC-b (P < 0.05), but not α ENaC-a transcript abundance in Dahl R, but not S rats. Two non-coding α ENaC spliced forms -c and -d are newly identified in the present study, whose levels are comparable in Dahl R and S rats. Compared to α ENaC-wt, α ENaC-a, -c and -d are low abundance transcripts (4 +/- 2, 110 +/- 20, and 10 +/- 2 fold less respectively), in contrast to α ENaC-b abundance that exceeds α ENaC-wt by 32 +/- 3 fold. We could not identify any of the five previously reported lung-specific α ENaC spliced forms (CK479583, CK475461, CK364785, CK475819, and CB690980) in Dahl rat kidney cortex.

Conclusion/interpretation

α ENaC alternative splicing might regulate α ENaC by the formation of coding RNA species (α ENaC-a and -b) and non-coding RNA species (α ENaC-c and -d). α ENaC-a and -b mRNA levels are significantly higher in Dahl R versus S rats. Additionally, α ENaC-b is a salt-sensitive transcript whose levels are significantly higher 4-weeks post high salt diet compared to normal salt diet in Dahl R rats. Among the four α ENaC transcripts (-a, -b, -c and -d), α ENaC-b is a predominant transcript that exceeds α ENaC-wt abundance by ~32 fold. α ENaC-a and -b spliced forms, particularly, α ENaC-b, might potentially act as dominant negative proteins for ENaC activity, thereby rescuing Dahl R rats from developing salt-sensitive hypertension on high salt diet. On the other hand, non-coding α ENaC-c and -d might assist alternative splicing, facilitate RNA processing, or regulate α ENaC as well as each other.

Important genetic checkpoints for insulin resistance in salt-sensitive (S) Dahl rats

Despite the marked advances in research on insulin resistance (IR) in humans and animal models of insulin resistance, the mechanisms underlying high salt-induced insulin resistance remain unclear. Insulin resistance is a multifactorial disease with both genetic and environmental factors (such as high salt) involved in its pathogenesis. High salt triggers insulin resistance in genetically susceptible patients and animal models of insulin resistance. One of the mechanisms by which high salt might precipitate insulin resistance is through its ability to enhance an oxidative stress-induced inflammatory response that disrupts the insulin signaling pathway. The aim of this hypothesis is to discuss two complementary approaches to find out how high salt might interact with genetic defects along the insulin signaling and inflammatory pathways to predispose to insulin resistance in a genetically susceptible model of insulin resistance. The first approach will consist of examining variations in genes involved in the insulin signaling pathway in the Dahl S rat (an animal model of insulin resistance and salt-sensitivity) and the Dahl R rat (an animal model of insulin sensitivity and salt-resistance), and the putative cellular mechanisms responsible for the development of insulin resistance. The second approach will consist of studying the over-expressed genes along the inflammatory pathway whose respective activation might be predictive of high salt-induced insulin resistance in Dahl S rats.

Variations in genes encoding the insulin receptor substrates -1 and/or -2 (IRS-1, -2) and/or genes encoding the glucose transporter (GLUTs) proteins have been found in patients with insulin resistance. To better understand the combined contribution of excessive salt and genetic defects to the etiology of the disease, it is essential to investigate the following question:

Question 1: Do variations in genes encoding the IRS -1 and -2 and/or genes encoding the GLUTs proteins predict high salt-induced insulin resistance in Dahl S rats?

A significant amount of evidence suggested that salt-induced oxidative stress might predict an inflammatory response that upregulates mediators of inflammation such as the nuclear factor- kappa B (NF-kappa B), the tumor necrosis factor-alpha (TNF-α) and the c-Jun Terminal Kinase (JNK). These inflammatory mediators disrupt the insulin signaling pathway and predispose to insulin resistance. Therefore, the following question will be thoroughly investigated:

Question 2: Do variations in genes encoding the NF-kappa B, the TNF-α and the JNK, independently or in synergy, predict an enhanced inflammatory response and subsequent insulin resistance in Dahl S rats in excessive salt environment?

Finally, to better understand the combined role of these variations on glucose metabolism, the following question will be addressed:

Question 3: What are the functional consequences of gene variations on the rate of glucose delivery, the rate of glucose transport and the rate of glucose phosphorylation in Dahl S rats?

The general hypothesis is that "high-salt diet in combination with defects in candidate genes along the insulin signaling and inflammatory pathways predicts susceptibility to high salt-induced insulin resistance in Dahl S rats".