ESRP2-miR122 axis regulates postnatal onset of polyploidization

Postnatal period of development is critical for mammalian tissues and is coordinated through precise activation of genetic programs that govern differentiation, growth and maturation. Here, we describe a cell type- and developmental stage-specific program of alternative splicing that drives sequential replacement of fetal-to-adult protein isoforms in the mouse liver. Using deep transcriptome analysis of loss- & gain-of-function models we identified Epithelial Splicing regulatory protein 2 (ESRP2) as the major regulator for these developmental splicing decisions. Targeted deletion of ESRP2 in mice resulted in the failure of fetal-to-adult switch in splicing for hundreds of RNA transcripts that encode proteins involved in functional competence of hepatocytes. To delineate its role in activation of adult splicing program, we generated transgenic mice with tetracycline-inducible and hepatocyte-specific expression of ESRP2. Remarkably, premature expression of ESRP2 in the livers of newborn pups forced an earlier-than-normal onset of adult splicing program. To determine the in vivo ESRP2 RNA binding landscape within hepatocytes, we used CRISPR-Cas9 technology to FLAG-tag the endogenous locus of ESRP2 in mice and performed FLAG-eCLIP to identify genomewide binding sites. We identified ESRP2 as a regulator of miR-122 levels in hepatocytes, wherein this axis balances the polyploidization and proliferation states in postnatal hepatocytes.