Current Molecular Pharmacology - Volume 6, Issue 1, 2013

Cystic fibrosis (CF) is a hereditary disease caused by mutations in the gene encoding the chloride channel “cystic fibrosis transmembrane conductance regulator” (CFTR). The lack of functional CFTR in CF airways leads to impaired ion and fluid homeostasis of the fluid layer which lines the airway surfaces (ASL). The ASL is important for proper ciliary beat and clearance of mucus from the airways. According to the “low volume hypothesis”, CF airway epithelia hyperabsorb sodium via the epithelial sodium channel (ENaC). Although the contribution of ENaC to CF pathogenesis is still under debate, there is convincing data demonstrating that re-hydration of the ASL might improve mucociliary clearance in CF patients. ASL re-hydration might, amongst other things, be achieved by a block of airway transepithelial sodium absorption with inhibitors of ENaC. This mini-review article describes the role of ENaC in ASL fluid homeostasis and rehydration, and summarizes the current state of the art in the discovery and establishment of compounds which inhibit ENaC activity and may represent pharmacological tools for the treatment of CF.

Lungs contain a particular amount of fluid that is crucial for proper lung function. This fluid content is tightly controlled within certain limits. Fluid accumulation in the alveolar airspace impairs gas exchange and represents a lifethreatening condition referred to as pulmonary edema. Ion transport processes by pulmonary epithelia represent a mechanism, responsible for fluid absorption from the airspace. Thus, it is obvious to consider ion transport processes as target for therapeutic interventions in pulmonary edema. The principle mechanism responsible for fluid absorption from the airspace is: Na+ diffuses through luminal Na+ channels into epithelial cells and is extruded by Na+/K+-ATPases at the basolateral side. This process generates an osmotic gradient that represents the driving force for fluid absorption. The rate of Na+ absorption is limited by the number/activity of Na+ channels in the luminal membrane of alveolar epithelial cells. Although different Na+ channels have been identified, the epithelial Na+ channel (ENaC) is a major player that participates in Na+-driven fluid absorption and thus a suitable target for the treatment of pulmonary edema. This article reviews cellular mechanisms by which ENaC activity can be increased in alveolar epithelia (lectins, proteases, β-adrenoceptors, mineralo-/glucocorticoid-receptors). These mechanisms are involved in regulating ENaC-dependent fluid absorption under physiological conditions. Additionally, pre-clinical as well as some preliminary clinical studies revealed that “ENaC-activators/stimulators” (β2-adrenoceptor agonists and mineralo-/glucocorticoid-receptor agonists) could be beneficial for therapeutic interventions in patients with pulmonary edema. However, the outcome of subsequently performed multicenter clinical trials with “ENaC-activators/stimulators” for treatment of patients with pulmonary edema was disappointing.

Epithelial Na+ channels (ENaCs) are comprised of subunits that have large extracellular regions linked to membrane spanning domains where the channel pore and gate reside. A variety of external factors modify channel activity by interacting at sites within extracellular regions that lead to conformational changes that are transmitted to the channel gate and alter channel open probability. Our review addresses two external factors that have important roles in regulating channel activity, proteases and laminar shear stress.

The epithelial sodium channel (ENaC) plays an essential role in transepithelial sodium reabsorption in the renal connecting tubule and collecting duct. Therefore, controlling ENaC activity is an important regulatory event in electrolyte and extracellular volume homeostasis, and thus in the control of blood pressure. Many independent signaling pathways converge on ENaC, although the most important for its physiological role is the enhancement of channel activity by the steroid hormone aldosterone. In this review, we briefly summarize current knowledge about ENaC regulation and the different chemical compounds available to directly or indirectly modify channel function. In addition, current and possible clinical uses of ENaC and aldosterone antagonists are highlighted.

The epithelial sodium channel/degenerin (ENaC/deg) family of ion channels is formed by a large number of genes with variable tissue expression patterns and physiological roles. ENaC is a non-voltage gated, constitutively active channel highly selective for sodium. ENaC is formed by three homologous subunits, α, β and γ, and a fourth subunit (δ) has been found in human and monkeys that can substitute α to form functional channels. The best-characterized role of ENaC is to serve as a rate-limiting step in transepithelial sodium reabsorption in the distal part of the kidney tubule and other tight epithelia. However, ENaC subunits are also found in the peripheral and central nervous system, where their functional roles are only beginning to be understood. In this review, we mainly focus on the putative pathophysiological roles of ENaC channels in the central nervous system and their potential value as drug targets in neurodegenerative disorders and the central control of blood pressure.

Amiloride-sensitive epithelial sodium channels (ENaCs) transport Na+ and are essential for salt and fluid homeostasis across epithelial tissues. Several pathological conditions of renal and pulmonary tissues are associated with abnormal ENaC function. The signalling pathways that regulate ENaC activity utilise a number of kinases. Over recent years, more have been identified and their mechanisms of action expanded. The functions of SGK and PKA are the best understood and both up-regulate ENaC activity. SGK is an important target of PI3K via PDK1 and TORC2 whilst PKA is linked with the activity of other kinases that have complementary effects. CK2, GRK2, IKKβ and PKD1 also up-regulate ENaC. In contrast, PKC, ERK1/2 and AMPK are inhibitory. Two key convergence targets for kinase action persist. These are phosphorylation of Nedd4-2 and the β and γ subunits of ENaC. Depending on the sites targeted, phosphorylation predominantly promotes or decreases association between these proteins to regulate ENaC retrieval and its subsequent abundance in the membrane. Alternative emerging targets include proteins involved in the translocation and recycling of ENaC channels to the membrane. Targeting kinases to modify ENaC function in vivo has shown some promise. Inactivation of SGK has produced mild but positive effects on renal function. Activating PKA has shown potential in lung pathologies. Inhibition of PI3K and PKB may prove useful in diabetic related alterations in renal Na+ handling, as could activation of AMPK, which may also have potential in the treatment of pulmonary pathologies associated with elevated ENaC activity.