Impaired Bile Secretion Promotes Chronic Liver Injury in Sickle Cell Disease

Hepatic crisis is an emergent complication affecting sickle cell disease (SCD) patients, however, the molecular mechanism of sickle cell hepatobiliary crisis remains poorly understood. We examined the liver pathophysiology of SCD using a humanized mouse model (townes SCD mice; homozygous for Hba^tm1(HBA)Tow, homozygous for Hbb^{tm2(HBG1,HBB*)Tow}). These mice have the major features (irreversibly sickled red cells, splenomegaly, anemia, multiorgan pathology) found in humans with SCD and, as such, represent a useful in vivo system to study hepatobiliary changes in SCD disease.

SCD mice manifested progressive hepatomegaly, liver injury and hyperbilirubinemia. RNA-sequence analysis of total RNA from SCD mouse liver compared to control (AS) identified dysregulation of genes encoding proteins responsible for fibrosis, bile acid synthesis, bile transport and cholesterol metabolism. Immunohistochemical analysis confirmed inflammation, fibrosis and increased ductular reaction in SCD mice. To mechanistically address the cholestatic phenotype in SCD mice, we used our recently developed multi-photon-excitation (MPE) enabled in vivo real-time fluorescence microscopy of intact liver in live mice. We used Texas-Red (TXR) dextran to visualize the blood flow through liver sinusoids and carboxyfluorescein (used as a surrogate of bile flow) to visualize bile flow through biliary canaliculi. Real time imaging show that sinusoidal ischemia occurs in the liver of transgenic-humanized SCD mice under basal condition. Intravital imaging also revealed impaired bile secretion into the bile canaliculi, which was associated with loss of apical bile acid transporters and bile acid biosynthetic enzymes, hepatic bile accumulation, and activation of Farnesoid X receptor (FXR) and small heterodimer partner (Shp) in the liver of SCD mice. These findings are the first to identify that impaired bile acid synthesis and misexpression of bile transporters promote intrahepatic bile accumulation and impaired bile secretion, leading to hepatobiliary injury of SCD. Improved understanding of these processes could potentially benefit the development of new therapies to treat sickle cell hepatic crisis.