Ellis, J.L., Bove, K.E., Schuetz, E.G., Leino, D., Valencia, C.A., Schuetz, J.D., Miethke, A., Yin, C. (2017) Zebrafish abcb11b mutant reveals novel strategies to restore bile excretion impaired by bile salt export pump deficiency. Hepatology (Baltimore, Md.). 67(4):1531-1545.
Bile salt export pump BSEP (ABCB11) is a liver-specific adenosine triphosphate-cassette binding transporter that mediates canalicular bile salt excretion from hepatocytes. Human mutations in ABCB11 cause progressive familial intrahepatic cholestasis type II (PFIC2). Although over 150 ABCB11 variants have been reported, our understanding of their biological consequences is limited by the lack of experimental model that recapitulates the patient phenotypes. We applied CRISPR/Cas9-based genome editing technology to knockout abcb11b, the ortholog of human ABCB11, in zebrafish and found that these mutants died prematurely. Histological and ultrastructural analyses showed that abcb11b mutant zebrafish exhibited hepatocyte injury similar to that seen in patients with PFIC2. Hepatocytes of the mutant zebrafish failed to excrete the fluorescently tagged bile acid that is a substrate of human BSEP. Multi-drug resistance protein 1 (MDR1), which is thought to play a compensatory role in Abcb11 knockout mice, was mislocalized to the hepatocyte cytoplasm in abcb11b mutant zebrafish and in a patient lacking BSEP protein due to nonsense mutations in ABCB11. We discovered that BSEP deficiency induced autophagy in both human and zebrafish hepatocytes. Treatment with rapamycin restored bile acid excretion, attenuated hepatocyte damage, and extended lifespan of abcb11b mutant zebrafish, correlating with the recovery of canalicular Mdr1 localization.
Conclusions Collectively, these data suggest a model that rapamycin rescues BSEP-deficient phenotypes by prompting alternative transporters to excreting bile salts. MDR1 is a candidate for such alternative transporter. This article is protected by copyright. All rights reserved.