COMMD1 downregulates δ epithelial sodium channel (δENaC) through trafficking and ubiquitination

The epithelial sodium channel (ENaC) is a key regulator of salt homeostasis and blood pressure. The classic ENaC consists of three subunits: α, β and γ, which are highly expressed in epithelial tissues such as kidney, colon and lung. Despite close similarity, a fourth ENaC subunit called δENaC is highly expressed in brain, heart, ovary, testis, and pancreas, and its physiological significance remains unknown. A δENaC binding partner called COMMD1/Murr1, which is a regulator of intracellular copper concentration, was identified in a yeast‐two hybrid screen. A Xenopus oocyte functional assay showed that COMMD1 downregulates the channel activity of δENaC, and that direct interaction between δENaC and COMMD1 is necessary for the inhibition. The present study investigates the underlying mechanism of COMMD1‐mediated δENaC downregulation. Here we show in glutathione S‐transferase (GST) pulldown studies that direct interaction between δENaC and Murr1 occurs through the COMM domain of COMMD1. Immunocytochemical and confocal microscopy studies suggest that both δENaC and COMMD1 are localised to intracellular compartments in cultured cells. With the use of organelle markers, it was found that δENaC and COMMD1 are primarily colocalised in the Golgi apparatus. Ubiquitination assays suggest that COMMD1 downregulates δENaC channel activity through enhanced ubiquitination of δENaC, which would lead to a decrease in δENaC surface expression. Together, these findings are consistent with the hypothesis that COMMD1 downregulates δENaC through direct interaction, altering its intracellular trafficking, inducing δENaC ubiquitination, leading to reduction of δENaC surface expression and activity.