Transcriptional enhancers control cell-type-specific gene expression. Primed enhancers are marked by histone H3 lysine 4 (H3K4) mono/di-methylation (H3K4me1/2). Active enhancers are further marked by H3K27 acetylation (H3K27ac). Mixed-lineage leukemia 4 (MLL4/KMT2D) is a major enhancer H3K4me1/2 methyltransferase with functional redundancy with MLL3 (KMT2C). However, its role in cell fate maintenance and transition is poorly understood. Here, we show in mouse embryonic stem cells (ESCs) that MLL4 associates with, but is surprisingly dispensable for the maintenance of, active enhancers of cell-identity genes. As a result, MLL4 is dispensable for cell-identity gene expression and self-renewal in ESCs. In contrast, MLL4 is required for enhancer-binding of H3K27 acetyltransferase p300, enhancer activation, and induction of cell-identity genes during ESC differentiation. MLL4 protein, rather than MLL4-mediated H3K4 methylation, controls p300 recruitment to enhancers. We also show that, in somatic cells, MLL4 is dispensable for maintaining cell identity but essential for reprogramming into induced pluripotent stem cells. These results indicate that, although enhancer priming by MLL4 is dispensable for cell-identity maintenance, it controls cell fate transition by orchestrating p300-mediated enhancer activation.enhancer | MLL4/KMT2D | H3K4 methyltransferase | cell fate transition | p300 I n mammalian cells, enhancers coordinate with promoters to precisely control cell-type-specific gene transcription, which determines the cell identity (1, 2). Comprehensive genome-wide studies have provided insights into the chromatin signatures of enhancers. Primed enhancers are marked by H3K4me1/2. Active enhancers (AEs) are further marked by the histone acetyltransferases CBP/p300-mediated H3K27ac (3). Recent studies classify AEs into typical enhancers and superenhancers. Superenhancers are clusters of AEs bound by lineage-determining transcription factors (TFs). Compared with typical enhancers, superenhancers are more cell-lineage-specific and control cell identity (4). Enhancer activation is orchestrated through a regulatory network involving lineage-determining TFs and chromatinmodifying complexes (2). However, how chromatin-modifying complexes regulate enhancer activation and cell fate transition is poorly understood.Embryonic stem cells (ESCs) derived from blastocysts are capable of unlimited replication in vitro, a property known as ESC self-renewal. ESC identity is maintained during self-renewal. ESC self-renewal is controlled by a core circuitry of TFs including Oct4, Sox2, and Nanog (4). ESCs can rapidly respond to environmental cues and differentiate into three germ layers-ectoderm, endoderm, and mesoderm-which consist of all cell lineages within days (5). Differentiated somatic cells can also be converted back to the pluripotent stage by ectopic expression of Oct4 and Sox2 together with Klf4 and c-Myc, a process known as somatic cell reprogramming into induced pluripotent stem cells (iPSCs) (6). The dramatic changes of ce...

Understanding adipogenesis, the process of adipocyte development, may provide new ways to treat obesity and related metabolic diseases. Adipogenesis is controlled by coordinated actions of lineage-determining transcription factors and epigenomic regulators. Peroxisome proliferator-activated receptor gamma (PPAR␥) and C/EBP␣ are master "adipogenic" transcription factors. In recent years, a growing number of studies have reported the identification of novel transcriptional and epigenomic regulators of adipogenesis. However, many of these novel regulators have not been validated in adipocyte development in vivo and their working mechanisms are often far from clear. In this minireview, we discuss recent advances in transcriptional and epigenomic regulation of adipogenesis, with a focus on factors and mechanisms shared by both white adipogenesis and brown adipogenesis. Studies on the transcriptional regulation of adipogenesis highlight the importance of investigating adipocyte differentiation in vivo rather than drawing conclusions based on knockdown experiments in cell culture. Advances in understanding of epigenomic regulation of adipogenesis have revealed critical roles of histone methylation/demethylation, histone acetylation/deacetylation, chromatin remodeling, DNA methylation, and microRNAs in adipocyte differentiation. We also discuss future research directions that may help identify novel factors and mechanisms regulating adipogenesis.

The epigenomic reader Brd4 is an important drug target for cancers. However, its role in cell differentiation and animal development remains largely unclear. Using two conditional knockout mouse strains and derived cells, we demonstrate that Brd4 controls cell identity gene induction and is essential for adipogenesis and myogenesis. Brd4 co-localizes with lineage-determining transcription factors (LDTFs) on active enhancers during differentiation. LDTFs coordinate with H3K4 mono-methyltransferases MLL3/MLL4 (KMT2C/KMT2D) and H3K27 acetyltransferases CBP/p300 to recruit Brd4 to enhancers activated during differentiation. Brd4 deletion prevents the enrichment of Mediator and RNA polymerase II transcription machinery, but not that of LDTFs, MLL3/MLL4-mediated H3K4me1, and CBP/p300-mediated H3K27ac, on enhancers. Consequently, Brd4 deletion prevents enhancer RNA production, cell identity gene induction and cell differentiation. Interestingly, Brd4 is dispensable for maintaining cell identity genes in differentiated cells. These findings identify Brd4 as an enhancer epigenomic reader that links active enhancers with cell identity gene induction in differentiation.