Abstract
The epithelial sodium channel (ENaC) is located at the apical membrane of many epithelial cells throughout the body and mediates Na+ influx into epithelial cells. Classical ENaC is made up of three similar subunits, α, β and γENaC subunits. ENaC provides the rate-limiting step for Na+ reabsorption in the distal nephron and therefore controls the extracellular fluid volume and blood pressure. ENaC activity at the cell surface is tightly regulated to maintain Na+ and fluid homeostasis in the body. A number of regulators of ENaC have been identified, including COMMD1 (copper metabolism Murr1 domain containing 1). COMMD1 belongs to a ten-member COMMD (copper metabolism Murr1 domain containing) family whose members all share a C-terminal COMM domain.
Previous work in our laboratory has reported that COMMD1 interacts with the α, β, and γENaC subunits via its COMM domain and down-regulates the amiloride-sensitive Na+ currents generated by αβγENaC. Thus, it was hypothesised that other COMMD family members might also interact with ENaC and down-regulate ENaC activity. Therefore, the aims of this study were (1) to study the interaction between the COMMD family proteins and the ENaC subunits. (2) To investigate the effect of COMMD3 and COMMD9 on ENaC activity and the mechanism by which COMMD3 and COMMD9 regulate ENaC activity. (3) To explore the localisation of endogenous COMMD3 and COMMD9 in cultured cell lines and rat tissues and further ask whether COMMD9 colocalises with αENaC in rat kidney.
Here protein-protein interaction studies demonstrated that all COMMD family members interacted with ENaC. Functionally, COMMD3 and COMMD9 were able to down-regulate the αβγENaC current through reducing the expression of ENaC at the cell surface. Western blot studies were used to show that endogenously expressed COMMD3 and COMMD9 were detected in HEK 293 cells and some rat tissues including kidney. Here immunocytochemistry studies in HEK 293 cells were used to show that COMMD3 was endogenously expressed in a perinuclear manner, while endogenous COMMD9 was present throughout HEK 293 cells. In the rat kidney, endogenous COMMD9 was found to be mainly expressed in the cortical and medullary collecting duct cells using single label immunohistochemistry. Moreover, COMMD9 was colocalised with αENaC in the collecting duct cells in the rat kidney.
In summary, this study identified the interaction between COMMDs and ENaC and discovered the inhibitory effects of COMMD3 and COMMD9 on ENaC activity. In addition, the regulatory mechanism of COMMD3 and COMMD9 on ENaC was found to be through reducing ENaC surface expression. Immunohistochemistry results provide in vivo evidence for the interaction of COMMD9 and ENaC. Therefore, these data obtained in this study suggest COMMD3 and COMMD9 might be endogenous regulators of ENaC and they may be involved in Na+ and fluid homeostasis and therefore ENaC-related disease, such as hypertension. Lastly, this study may provide an insight into a new target point to treat hypertension.