Question:

Liddle's syndrome (pseudohyperaldosteronism) is caused by gain-of-function mutations in ENaC. ENaC regulation by aldosterone is at least in part mediated by phosphorylation of the ubiquitin ligase NEDD4-2 which inhibits ENaC ubiquitination and channel retrieval. ENaC mutations causing Liddle's syndrome disrupt the physiological NEDD4-2/ENaC interaction and hence mimick the effect of aldosterone. This results in increased ENaC activity, renal Na⁺ retention and hypertension. Previously we reported that ENaC activity in the late distal convoluted tubule/early connecting tubule (DCT2/CNT) is constitutively high and does not depend on circulating aldosterone. This is in contrast to the aldosterone dependence of ENaC activity in the late CNT/early collecting duct (CNT/CCD) (2). This raises the question whether Liddle's syndrome mutations affect ENaC activity differently in the DCT2/CNT and CNT/CCD. Therefore, the aim of the present study was to investigate ENaC regulation in the DCT2/CNT and CNT/CCD in a mouse model of Liddle's syndrome.

Methods:

Whole-cell amiloride-sensitive ENaC currents (δI_ami) were determined using the patch-clamp technique in isolated fragments of DCT2/CNT or CNT/CCD obtained either from wild type (WT) mice or from homozygous mice transgenic for the Liddle's syndrome mutation (L/L). To modify plasma aldosterone levels, mice were maintained on high, normal or low Na⁺ diets.

Results:

In CNT/CCD, δI_ami was significantly higher in L/L mice compared to WT mice maintained either on a high Na⁺ diet (L/L: 71 ± 11 pA; WT: 8 ± 1.5 pA, p < 0.001) or on a standard Na⁺ diet (L/L 68 ± 11 pA; WT: 34 ± 11 pA, p < 0.05). On a low salt diet, δI_ami was not significantly higher in L/L mice than in WT mice (343 ± 60 pA vs 241 ± 69 pA). Interestingly, δI_ami in DCT2/CNT of L/L mice was significantly higher than in WT mice under all dietary conditions: 718 ± 160 pA vs 187 ± 167 pA (p < 0.01) on a high Na⁺ diet, 1136 ± 144 pA vs 295 ± 70 pA (p < 0.0001) on a normal, and 1101 ± 107 vs 326 ± 52 pA (p < 0.0001) on a low salt diet.

Conclusion:

Our data indicate that the primary site of increased Na⁺ reabsorption in mice with Liddle's syndrome is the DCT2/CNT. Compensatory suppression of plasma aldosterone levels, known to occur in L/L mice (1), is likely to mask the stimulatory effect of the Liddle's syndrome mutation on δI_ami in CNT/CCD from mice on a low salt diet and to a lesser extent on a normal salt diet.

References.

Bertog M et al. J Physiol 586: 459-75, 2008.

Nesterov V et al. Am J Physiol Renal Physiol 303: F1289-F1299, 2012.