Cholestatic liver diseases are a significant cause of morbidity and mortality and the leading indication for pediatric liver transplant. These include diseases such as biliary atresia, Alagille syndrome, progressive intrahepatic cholestasis entities, ductal plate abnormalities including Caroli syndrome and congenital hepatic fibrosis, primary sclerosing cholangitis, bile acid synthesis defects, and certain metabolic disease. Medical management of these patients typically includes supportive care for complications of chronic cholestasis including malnutrition, pruritus, and portal hypertension. However, there are limited effective interventions to prevent progressive liver damage in these diseases, leaving clinicians to ultimately rely on liver transplantation in many cases. Agents such as ursodeoxycholic acid, bile acid sequestrants, and rifampicin have been mainstays of treatment for years with the understanding that they may decrease or alter the composition of the bile acid pool, though clinical response to these medications is frequently insufficient and their effects on disease progression remain limited. Recently, animal and human studies have identified potential new therapeutic targets which may disrupt the enterohepatic circulation of bile acids, alter the expression of bile acid transporters or decrease the production of bile acids. In this article, we will review bile formation, bile acid signaling, and the relevance for current and newer therapies for pediatric cholestasis. We will also highlight further areas of potential targets for medical intervention for pediatric cholestatic liver diseases.

Background & Aims: The extrahepatic bile duct is the primary tissue initially affected by the cholangiopathy biliary atresia. Biliary atresia affects neonates exclusively and current animal models suggest that the developing bile duct is uniquely susceptible to damage. In this study, we aimed to define the anatomical and functional differences between the neonatal and adult mouse extrahepatic bile ducts.Methods: We studied mouse passaged cholangiocytes, mouse BALB/c neonatal and adult primary cholangiocytes and isolated extrahepatic bile ducts, and a collagen reporter mouse. Methods included transmission electron microscopy, lectin staining, immunostaining, rhodamine uptake assays, bile acid toxicity assays, and in vitro modeling of the matrix.Results: The cholangiocyte monolayer of the neonatal extrahepatic bile duct was immature, lacking the uniform apical glycocalyx and mature cell-cell junctions typical of adult cholangiocytes. Functional studies showed that the glycocalyx protected against bile acid injury and that neonatal cholangiocyte monolayers were more permeable than adult monolayers. In adult ducts, the submucosal space was filled with collagen I, elastin, hyaluronic acid, and proteoglycans. In contrast, the neonatal submucosa had little collagen I and elastin, although both increased rapidly after birth.In vitro modeling suggested that the composition of the neonatal submucosa relative to the adult submucosa led to increased diffusion of bile. A Col-GFP reporter mouse showed that cells in the neonatal but not adult submucosa were actively producing collagen. Conclusion:We identified four key differences between the neonatal and adult extrahepatic bile duct. We showed that these features may have functional implications, suggesting the neonatal extrahepatic bile ducts are particularly susceptible to injury and fibrosis.Lay Summary Biliary atresia is a disease that affects newborns and is characterized by extrahepatic bile duct injury and obstruction with resulting liver injury. We identify four key differences between the epithelial and submucosal layers of the neonatal and adult extrahepatic bile duct and show that these may render the neonatal duct particularly susceptible to injury. Highlights 1) The apical glycocalyx is thin and patchy in neonatal compared to adult cholangiocytes 2) Neonatal cholangiocytes have immature cell-cell junctions and increased permeability 3) The neonatal submucosal space has minimal collagen I or elastin 4) The neonatal submucosal space contains many actively collagen-secreting cells Graphical abstract

Background and Aims The unfolded protein response (UPR) is a coordinated cellular response to endoplasmic reticulum (ER) stress that functions to maintain cellular homeostasis. When ER stress is unresolved, the UPR can trigger apoptosis. Pathways within the UPR influence bile acid metabolism in adult animal models and adult human liver diseases, however, the UPR has not been studied in young animal models or pediatric liver diseases. In this study we sought to determine whether weanling age mice had altered UPR activation compared with adult mice, which could lead to increased bile acid–induced hepatic injury. Approach and Results We demonstrate that after 7 days of cholic acid (CA) feeding to wild‐type animals, weanling age mice have a 2‐fold greater serum alanine aminotransferase (ALT) levels compared with adult mice, with increased hepatic apoptosis. Weanling mice fed CA have increased hepatic nuclear X‐box binding protein 1 spliced (XBP1s) expression, but cannot increase expression of its protective downstream target’s ER DNA J domain‐containing protein 4 and ER degradation enhancing α‐mannoside. In response to tunicamycin induced ER stress, young mice have blunted expression of several UPR pathways compared with adult mice. CA feeding to adult liver‐specific XBP1 knockout (LS‐XBP1−/−) mice, which are unable to resolve hepatic ER stress, leads to increased serum ALT and CCAAT/enhancer binding homologous protein, a proapoptotic UPR molecule, expression to levels similar to CA‐fed LS‐XBP1−/− weanlings. Conclusions Weanling mice have attenuated hepatic XBP1 signaling and impaired UPR activation with resultant increased susceptibility to bile acid–induced injury.