Oval cells are adult liver progenitor cells that play a physiologic role in liver regeneration and have attractive therapeutic potential. They are believed to reside in a niche within the periportal region. However, the signals that activate oval cells after injury are not known. The Wnt family of proteins plays important roles in adult stem cell maintenance in a variety of mammalian tissues. Wnt signaling is also involved in liver development and repair. Evidence suggests that Wnt signaling is central to the injury response that activates oval cells. A main unanswered question in tissue repair is how injury leads to activation of the signals necessary for rebuilding the tissue. Pursuing a novel line of research, the Nusse lab has identified a transcriptional control element (called CAIRE, for Cis Acting Injury Responsive Element) that governs the expression of Wnt in response to injury. Through the use of reporter genes linked to this enhancer, CAIRE function has been examined in various tissues. In preliminary work, the lab has shown that it is in the periportal region of the liver and responds to injury. The hypothesis is that CAIRE undergoes activation in response to liver injury that activates the oval cell compartment. It induces oval cell proliferation by driving local Wnt expression, which is required for the regenerative response. A specific signaling pathway exists upstream of CAIRE that controls CAIRE activity and Wnt expression. The proposed studies will address these hypotheses. First, the role of CAIRE in three distinct forms of liver injury that exhibit different degrees of oval cell activation will be examined. CAIRE reporter expression is expected in injury that activates the oval cell population. Next, transgenic mice in which CAIRE conditionally activates the Wnt inhibitor Dkk will be generated to determine if local Wnt signaling driven by CAIRE is required for the oval cell regenerative response. Wnt inhibition is expected to result in decreased oval cell regeneration after injury. Lastly, transcription factors that bind to and activate CAIRE will be identified. Bioinformatics will be used to identify potential transcription factors that bind to CAIRE. These factors will be examined for direct binding to CAIRE. Based on these results, mutations will be made in transcription factor binding sites within CAIRE. Transgenic mice will be made to assess for loss-of- function. These mice are expected to have decreased oval cell regenerative response. These studies will provide the first evidence of an upstream activator of Wnt expression. They will add to the evidence for a key role for Wnt signaling in liver injury and the subsequent regenerative response involving oval cells. Lastly, the results will give insight into the role of the niche in linking the injury response to tissue regeneration.
Cirrhosis is one of the most significant causes of morbidity and mortality in the United States. Oval cells have tremendous potential in cell replacement therapy for cirrhosis. Improved understanding of how signals from the niche regulate oval cell mediated regeneration will be an important step towards realizing the potential of cell based therapy for liver disease.
|Wang, Bruce; Zhao, Ludan; Fish, Matt et al. (2015) Self-renewing diploid Axin2(+) cells fuel homeostatic renewal of the liver. Nature 524:180-5|