The potent regenerative capabilities of planaria, newts, and zebrafish are largely lost in mammals, an observation that has a tangible impact on human health. Compromised tissue regeneration likely contributes to disorders such as liver cirrhosis or inflammatory bowel diseases (IBD). Limited regeneration in liver disease settings can also preclude the ability to perform large surgical resections for cancer. The relative lack of regenerative therapies could in part be because discovering new targets is difficult and is dependent on in vivo systems that cannot be replicated in the culture dish. To identify new pathways that can be targeted to enhance regeneration, we established an in vivo CRISPR-Cas9 screening platform to evaluate genes in the mouse liver. Using this high-throughput system, we assessed the impact of 152 genes encoding epigenetic machines such as histone readers, writers, and erasers [2]. This identified two imitation SWI/SNF (ISWI) chromatin remodeling complex subunits encoded by Baz2a and Baz2b, genes that were not previously known to regulate regeneration. The interaction between BAZ2 proteins with the SMARCA5 enzyme defines the nucleolar remodeling complex (NoRC), one of five subtypes of ISWI complexes that uses ATP hydrolysis to remodel nucleosomes, particularly at ribosomal DNA (rDNA) loci [3]. Our validation studies showed that in vivo Cas9 deletion of either Baz2a or Baz2b increased liver regeneration in a hepatocyte repopulation model. Similarly, chemical inhibition of BAZ2A and BAZ2B using the specific bromodomain inhibitor GSK2801 resulted in increased liver regeneration [4]. Moreover, we also found that GSK2801 could promote intestinal recovery in a mouse model of colitis. These data suggest that BAZ2 containing ISWI/NoRC chromatin remodeling complexes are important for organ regeneration and promising therapeutic targets for multiple diseases. Our central hypothesis is that the imitation SWI components BAZ2A and BAZ2B limit efficient tissue regeneration by restricting increases in protein synthesis via suppression of rRNA transcription. We will test different aspects of this hypothesis by validating Baz2a and Baz2b genes as bona fide regeneration regulators (Aim 1), by understanding the global epigenetic activities of BAZ2 containing complexes (Aim 2), and by determining if increased protein synthesis is a key mechanism by which BAZ2 inhibition promotes regeneration (Aim 3). This project will define the regenerative phenotypes and molecular mechanisms associated with ISWI chromatin remodeling. Our studies will facilitate clinical drug development of small molecule inhibitors of BAZ2 proteins for use in enhancing tissue regeneration in the liver and intestine.
We have discovered that the imitation SWI chromatin remodeling pathway can be modulated to promote regeneration in various tissue contexts. This pathway has putative drug targets in BAZ2A and BAZ2B and we have tested small molecules that can mimic the genetic effects of deleting Baz2 genes. We endeavor to fully explore the phenotypic consequences of Baz2a and Baz2b deletion as well as the mechanisms underlying the inhibition of these genes in regeneration.
