Regulation of the epithelial sodium channel (ENaC) by membrane trafficking

Abstract

The epithelial Na(+) channel (ENaC) is a major regulator of salt and water reabsorption in a number of epithelial tissues. Abnormalities in ENaC function have been directly linked to several human disease states including Liddle syndrome, psuedohypoaldosteronism, and cystic fibrosis and may be implicated in salt-sensitive hypertension. ENaC activity in epithelial cells is regulated both by open probability and channel number. This review focuses on the regulation of ENaC in the cells of the kidney cortical collecting duct by trafficking and recycling. The trafficking of ENaC is discussed in the broader context of epithelial cell vesicle trafficking. Well-characterized pathways and protein interactions elucidated using epithelial model cells are discussed, and the known overlap with ENaC regulation is highlighted. In following the life of ENaC in CCD epithelial cells the apical delivery, internalization, recycling, and destruction of the channel will be discussed. While a number of pathways presented still need to be linked to ENaC regulation and many details of the regulation of ENaC trafficking remain to be elucidated, knowledge of these mechanisms may provide further insights into ENaC activity in normal and disease states.

Figure 1

Schematic diagram outlining steps in the apical delivery of proteins in epithelial cells. There are a number of possible routes proteins can follow out of the Golgi to the apical surface depicted schematically by the arrows. Proteins can move directly from the Golgi, bypassing the trans-Golgi network (TGN) (arrow furthest right); traffic via lipid-rafts (rafts); traffic directly from the TGN (yellow vesicle) or move into a recycling vesicle first. For apical membrane proteins this typically involved trafficking to the apical recycling endosome (ARE), but can involve reorganization from the basolateral surface (pink vesicles) to the common recycling endosome (CRE) before being directed to the apical surface. Finally SNARE proteins facilitate the docking and vesicle fusion events that facilitate protein delivery (see text for details). Abbreviations: AEE – apical early endosome, BEE – basolateral early endosome, CRE - common recycling endosome.

Figure 2

a) A schematic representation of the endocytic and recycling pathways commony found in epithelial cells. Proteins involved in each step in the pathway (as discussed in the text) are presented in black, with the relevant GTPase adaptor or effector proteins in red italics. Abbreviations: AEE – apical early endosome, AP-2 - adaptor protein 2, EEA1 - early endosome antigen 1, FIP2 - Rab11 family of interacting proteins 2, LE – late endosomes; LYS – lysosome, PI5K - Phosphatidylinositol 4-phosphate 5-kinase, PIP2 - Phosphatidylinositol 4,5-bisphosphate. b) The ENaC specific endocytosis and recycling pathway is presented in this schematic diagram. Those proteins demonstrated to be involved in ENaC trafficking are depicted. ENaC internalization is initiated by the binding of Nedd4-2 to the intracellular amino-terminus (NH₂) of each subunit and the addition of ubiquitin (Ub) moieties to the carboxy-tail (COOH) of the ENaC subunits. ENaC is then internalized by clathrin-dependant endocytosis through an interaction with epsin and passes through an early endosomal compartment (EEA1 positive). If ENaC is deubiquitinated by a deubiquitinating enzyme (Dub) it will be transferred to the apical recycling endosome (ARE) for recycling to the apical membrane. If not it is likely trafficked via the late endosomes (LE) to the lysosomes (Lys) for degration. Rab proteins (Rab11, 27) deliver ENaC back to the apical surface. Details of the mechanisms of ENaC delivery are provided in the text. Abbreviations: Dub – deubiquitinating enzyme, Nedd4-2 - neural precursor cell expressed, developmentally down-regulated 4-2, UCH-L3 - ubiquitin carboxyl-terminal hydrolase isozyme L3

Figure 2

a) A schematic representation of the endocytic and recycling pathways commony found in epithelial cells. Proteins involved in each step in the pathway (as discussed in the text) are presented in black, with the relevant GTPase adaptor or effector proteins in red italics. Abbreviations: AEE – apical early endosome, AP-2 - adaptor protein 2, EEA1 - early endosome antigen 1, FIP2 - Rab11 family of interacting proteins 2, LE – late endosomes; LYS – lysosome, PI5K - Phosphatidylinositol 4-phosphate 5-kinase, PIP2 - Phosphatidylinositol 4,5-bisphosphate. b) The ENaC specific endocytosis and recycling pathway is presented in this schematic diagram. Those proteins demonstrated to be involved in ENaC trafficking are depicted. ENaC internalization is initiated by the binding of Nedd4-2 to the intracellular amino-terminus (NH₂) of each subunit and the addition of ubiquitin (Ub) moieties to the carboxy-tail (COOH) of the ENaC subunits. ENaC is then internalized by clathrin-dependant endocytosis through an interaction with epsin and passes through an early endosomal compartment (EEA1 positive). If ENaC is deubiquitinated by a deubiquitinating enzyme (Dub) it will be transferred to the apical recycling endosome (ARE) for recycling to the apical membrane. If not it is likely trafficked via the late endosomes (LE) to the lysosomes (Lys) for degration. Rab proteins (Rab11, 27) deliver ENaC back to the apical surface. Details of the mechanisms of ENaC delivery are provided in the text. Abbreviations: Dub – deubiquitinating enzyme, Nedd4-2 - neural precursor cell expressed, developmentally down-regulated 4-2, UCH-L3 - ubiquitin carboxyl-terminal hydrolase isozyme L3