376.5 - In the Absence of YAP, TAZ Contributes to Hepatocyte Adaptation in Chronic Cholestasis in Females

Laura Molina (University of Pittsburgh School of Medicine), Adelya Gabdulkhakova (University of Pittsburgh School of Medicine), Junjie Zhu (University of Pittsburgh School of Pharmacy), Silvia Liu (University of Pittsburgh School of Medicine, University of Pittsburgh School of Medicine), Minakshi Poddar (University of Pittsburgh School of Medicine), Sucha Singh (University of Pittsburgh School of Medicine), Xiaochao Ma (University of Pittsburgh School of Pharmacy), Aatur Singhi (University of Pittsburgh School of Medicine, University of Pittsburgh School of Medicine), Sungjin Ko (University of Pittsburgh School of Medicine, University of Pittsburgh School of Medicine), Satdarshan Monga (University of Pittsburgh School of Medicine, University of Pittsburgh School of Medicine, University of Pittsburgh School of Medicine)

We previously demonstrated that loss of Yes-associated protein 1 (YAP1) in early liver development (YAP1 KO) leads to an Alagille syndrome-like phenotype with absence of intrahepatic bile ducts and severe cholestasis accompanied by chronic hepatocyte adaptations to reduce liver injury. We noticed that TAZ, a paralog of YAP1, was significantly upregulated in hepatocytes of YAP1 KO mice and bound to TEAD transcription factors, suggesting compensatory activity in hepatocytes.

We next queried the relationship between YAP1 and TAZ by deleting both genes during early liver development using the Foxa3-Cre promoter, similar to the YAP1 KO mice, resulting in multiple allele combinations. Interestingly, we show that YAP1/TAZ double knock-out (DKO) mice are embryonic lethal and stillborn, the embryos surviving almost until birth with no grossly visible abnormalities. The liver morphology during development does not show gross abnormalities, suggesting an extrahepatic source of lethality. This suggests that while YAP1 loss has profound impact on biliary development, YAP1 and TAZ together play a co-dependent critical role in foregut endoderm development yet to be determined.

Next, when we examined TAZ heterozygosity in YAP1 KO mice, we found significant embryonic lethality among males, with insufficient samples to analyze a potential cause at this time. However, among females, loss of one copy of TAZ from YAP1 KO mice resulted in a similar gross phenotype to YAP1 KO, with a complete lack of intrahepatic bile ducts resulting in severe chronic cholestatic disease. While serum bilirubin levels were comparable after TAZ heterozygosity, liver transaminases were more elevated than in YAP1 KO alone, suggesting worsening hepatocyte injury. Hepatocyte proliferation levels were not affected by TAZ heterozygosity. Global gene expression patterns as detected by RNA-sequencing were grossly similar as compared to YAP1 KO mice, although subtle alterations were identified in pathways related to apoptosis, cell division, inflammatory signaling, and metabolism, and we also show that TAZ is needed to regulate canonical YAP1 targets in YAP1 KO hepatocytes such as CTGF and CYR61. We also found significant elevation in serum cholesterol compared to YAP1 KO alone, and xanthomas were observed at later stages on these mice, suggesting worsening metabolic derangements caused by TAZ heterozygosity beyond what was observed in YAP1 KO alone. Interestingly, bile acid profiles in both liver and serum as detected by mass spectrometry showed no difference in YAP1 KO with or without TAZ heterozygosity. Ongoing work aims to further identify the role of TAZ in metabolic adaptations to chronic cholestasis with potential implications for clinical regulation of hypercholesterolemia and long-term liver metabolic health. 

Support or Funding Information

Funding provided by 2T32EB001026-16A1 and 1F30DK121393-01A1 to L.M., 5R01CA204586-05, 1R01DK62277 and Endowed Chair for Experimental Pathology to S.P.M., and NIH grant 1P30DK120531-01 to Pittsburgh Liver Research Center (Clinical Biospecimen Repository and Processing Core).