Abstract (eng)
The epithelial sodium channel (ENaC), particularly in the kidney, is crucial for the sodium and water balance of the whole body. The reabsorption of sodium and water in the distal nephron is regulated by aldosterone through mineralocorticoid receptor-associated (MCR) changes in the transcription of the Na+/K+-ATPase and the α-subunit of the epithelial sodium channel. A loss-of-function of ENaC leads to different hereditary diseases due to its wide tissue distribution such as colon, kidney, alveoli, salivary glands and sweat glands. One of these channelopathies is pseudohypoaldosteronism type 1B (PHA1B), a very rare, salt-wasting disease characterized by symptoms like dehydration, hypotension, vomiting, metabolic acidosis and failure to thrive. Its manifestation persists into adulthood and patients affected by this disease need a lifelong medical treatment which so far has only been symptomatic. Although these symptoms are typical of hypoaldosteronism, the aldosterone and renin levels are elevated. The activity of the epithelial sodium channel is regulated by its presence in the cell membrane and its open probability.
The cyclic peptides AP301 (INN: solnatide) and AP318, which mimic the lectin-like domain (TIP) of TNF-α, have been shown to increase the amiloride-sensitive sodium current of loss-of-function phenotype ENaC. The aim of my thesis was to investigate if these synthetic TIP-peptides alter the cell surface expression of the frameshift mutants αY447fs, αR438fs, αS243fs, αP197fs, γV543fs despite their lacking carboxy terminus, which has previously been reported of being one site of interaction of the TIP-domain with ENaC. Although these mutations lead to a loss-of-function, the cell surface expression of these mutants in the α-subunit was always higher than wild type ENaC. But still, after the treatment with the TIP-peptides the cell surface abundance varied a lot, and mostly only a transient increase in the cell surface abundance was indicated. The effect on the γ-mutant, regarding the cell surface expression as well as the amiloride-sensitive sodium current, indicated a complete loss-of-function of this mutant. Nevertheless patch-clamp experiments on these loss-of-function mutations in the α-subunit were promising and showed a restoration of the channel function. Therefore, one can deduce that the restoration of ENaC function is above all due to a higher open probability and not because of a higher cell surface expression. The activity of loss-of-function phenotype ENaC can be restored, hence AP301 and AP318 are promising substances for the treatment of PHA1B.