Bile Salts Activate BLINaC - An Epithelial Na+ Channel in the Liver

Abstract

Brain Liver Intestine Na+ Channel (BLINaC) is an ion channel of the DEG/ENaC gene family of unknown function. Expression of rat BLINaC (rBLINaC) in Xenopus oocytes leads to small unselective currents that are only weakly sensitive to amiloride. rBLINaC is potently inhibited by micromolar concentrations of extracellular Ca2+ and removal of Ca2+ leads to robust currents and increases Na+ selectivity. Recently we identified the fenamate flufenamic acid (FFA) and related compounds as agonists of rBLINaC. Application of millimolar concentrations of FFA to rBLINaC expressing oocytes induces a robust, Na+-selective current, which is partially blocked by amiloride. The identification of FFA as an artificial activator for BLINaC suggests the presence of an endogenous ligand.In rodents, the blinac mRNA is expressed mainly in brain, liver and intestine; in humans, it is mainly expressed in the small intestine. Immunhistochemical stainings of mouse liver revealed prominent expression of the BLINaC protein in the apical membrane of cholangiocytes lining the bile duct of mice, potentially implicating BLINaC in the generation of secondary bile. Guided by this localisation we tested whether the channel is affected by bile. The application of diluted bile to Xenopus oocytes expressing rBLINaC indeed induced a strong and reversible amiloride-sensitive unselective current. We identified bile salts as the BLINaC-activating molecules in bile and are currently trying to unravel the mechanistic basis of this activation. Bile salts are amphiphatic molecules that affect the structure, curvature and fluidity of membranes and might thus be activating BLINaC indirectly via a membrane dependent mechanism. Using a chimeric approach with ASIC1a, a related but bile salt-insensitive channel, we are currently defining the domains of BLINaC that are crucial for sensing membrane modulation induced by bile salts.