Mutations in JAGGED1, a ligand of the Notch pathway, are found in greater than 94% of patients with Alagille syndrome (AGS), with NOTCH2 receptor mutations identified in two families (McDaniell

Mutations in JAGGED1, a ligand of the Notch pathway, are found in greater than 94% of patients with Alagille syndrome (AGS), with NOTCH2 receptor mutations identified in two families (McDaniell et al., 2006; Warthen et al., 2006). AGS is a pleiotropic, autosomal dominant disease characterized in the majority of cases by neonatal jaundice, cholestasis and paucity of intrahepatic bile ducts (IHBDs) (Emerick et al., 1999). Whether the paucity of IHBDs is due to a developmental defect in bile duct morphogenesis, a lack of postnatal branching and elongation or an inability to maintain formed ducts remains unclear (Perrault, 1981; Hadchouel, 1992; Libbrecht et al., 2005). In support of a bile duct maintenance defect, a subset of AGS patients, with clinical indications for progressive liver disease, demonstrate an increase in bile duct paucity from initial to subsequent liver biopsies (Emerick et al., 1999; Libbrecht et al., 2005). Although studies in mouse models have demonstrated a requirement for Notch signaling in IHBD development (Loomes et al., 2007; Geisler et al., 2008; Lozier et al., 2008; Zong et al., 2009; Sparks et al., 2010), the progressive paucity in AGS patients suggests an additional requirement for Notch signaling in the maintenance of IHBDs. Notch signaling is a highly conserved intercellular communication pathway required for cell specification, lineage restriction, and maintenance of stem and progenitor populations during development and in adults (Chiba, 2006). Notch ligands, which are present on the cell surface, bind Notch receptors on the surface of an adjacent cell, resulting in a series of proteolytic cleavages culminating in the -secretase-dependent release of the Notch intracellular domain (NICD) from the cytoplasmic membrane. After release, the NICD translocates to the nucleus, where it interacts with the common DNA-binding partner for all Notch receptors, recombination signal binding protein for immunoglobulin kappa J region (RBP-J). This association converts RBP-J from a transcriptional co-repressor to a co-activator, resulting in target gene expression. In mammals, there are two families of canonical Notch ligands (Jagged1 and 2, and Delta-like-1, -3 and -4) and four Notch receptors (Notch1-4). Previous studies have shown that Notch signaling regulates ductal plate formation and IHBD morphogenesis in mice (Geisler et al., 2008; Lozier et al., 2008; Zong et al., 2009). These studies have not revealed the intact IHBD structure, because the analyses have focused on two-dimensional (2D) morphology and immunohistochemistry in tissue section. Because the mammalian liver is not conducive to in vivo three-dimensional (3D) imaging, we have used a resin casting method to resolve the global 3D