Claudin 2 deficiency reduces bile flow and increases susceptibility to cholesterol gallstone disease in mice

Background & Aims Bile formation and secretion are essential functions of the hepatobiliary system. Bile flow is generated by transepithelial transport of water and ionic/nonionic solutes via transcellular and paracellular pathways that is mainly driven by osmotic pressure. We examined the role of tight junction-based paracellular transport in bile secretion. Claudins are cell-cell adhesion molecules in tight junctions that create the paracellular barrier. The claudin family has 27 reported members, some of which have paracellular ion- and/or water-channel-like functions. Claudin 2 is a paracellular channel-forming protein that is highly expressed in hepatocytes and cholangiocytes; we examined the hepatobiliary system of claudin 2 knockout (Cldn2^-/-) mice.

Methods We collected liver and biliary tissues from Cldn2^-/- and Cldn2^+/+ mice and performed histologic, biochemical, and electrophysiologic analyses. We measured osmotic movement of water and/or ions in Cldn2^-/- and Cldn2^+/+ hepatocytes and bile ducts. Mice were placed on lithogenic diets for 4 weeks and development of gallstone disease was assessed.

Results The rate of bile flow in Cldn2^-/- mice was half that of Cldn2^+/+ mice, resulting in significantly more concentrated bile in livers of Cldn2^-/- mice. Consistent with these findings, osmotic gradient-driven water flow was significantly reduced in hepatocyte bile canaliculi and bile ducts isolated from Cldn2^-/- mice, compared with Cldn2^+/+ mice. After 4 weeks on lithogenic diets, all Cldn2^-/- mice developed macroscopically visible gallstones; the main component of the gallstones was cholesterol (>98%). In contrast, none of the Cldn2^+/+ mice placed on lithogenic diets developed gallstones.

Conclusions Based on studies of Cldn2^-/- mice, claudin 2 regulates paracellular ion and water flow required for proper regulation of bile composition and flow. Dysregulation of this process increases susceptibility to cholesterol gallstone disease in mice.