The overall objective is to advance our understanding of the relationship of the Apical Sodium-dependent bile acid transporter (ASBT) and Organic Solute Transporter alpha-beta (OSTab) to the pathogenesis of intestinal and hepatobiliary disease. Collectively, our findings strongly support the concept that in addition to its essential role in maintaining bile acid (BA) homeostasis, ASBT-OSTab functions to protect the ileal epithelium against BA-induced injury. Moreover, the concept of a protective role for ASBT-OSTab can potentially be extended to other BA-transporting epithelium and the cholehepatic shunt pathway by our identification of a dysfunctional mutation in OSTb (SLC51B) in two pediatric patients with congenital diarrhea and features of liver disease. However, despite progress, the role of BAs and toxic bile in the pathogenesis of human disease and the opportunities for highly effective therapeutic intervention remain elusive. Guided by the applicants’ recently published studies and strong preliminary data, three specific aims are proposed to interrogate ASBT-OSTab’s role in the pathogenesis of disease and the mechanism of action of new BA-based therapies. Specific Aim 1 is designed to elucidate the molecular mechanisms underlying the ileal injury associated with inactivation of OSTa. This will be accomplished by examining the roles for BAs and reactive oxygen species (ROS) in the intestinal injury and restitution response in Osta null mice, and the roles of the Nox1 and Nrf2 in that process. Specific Aim 2 is designed to elucidate the role of the ASBT in the cholehepatic shunting of BAs and the actions of therapeutic and cytotoxic BAs. This will be accomplished by examining the requirement for ASBT in cholehepatic shunting of BAs, the requirement for ASBT in the bicarbonate-rich hypercholeresis induced by therapeutic BAs such as UDCA and norUDCA, and the role of biliary ASBT in models of obstructive cholestasis. Specific Aim 3 is designed to test the hypothesis that OSTab functions to protect human hepatocytes and/or cholangiocytes from BA-induced injury. This will be accomplished using hepatocyte and cholangiocyte in vitro models. These innovative studies will yield novel insights to the the pathways underlying BA-induced injury and role of cholehepatic shunting of BAs in health and disease, with the goal of translating those insights into new preventive measures and treatments.