title: MLL5, a trithorax homolog, indirectly regulates H3K4 methylation, represses cyclin A2 expression, and promotes myogenic differentiation.
abstract: Most cells in adult tissues are nondividing. In skeletal muscle, differentiated myofibers have exited the cell cycle permanently, whereas satellite stem cells withdraw transiently, returning to active proliferation to repair damaged myofibers. We have examined the epigenetic mechanisms operating in conditional quiescence by analyzing the function of a predicted chromatin regulator mixed lineage leukemia 5 (MLL5) in a culture model of reversible arrest. MLL5 is induced in quiescent myoblasts and regulates both the cell cycle and differentiation via a hierarchy of chromatin and transcriptional regulators. Knocking down MLL5 delays entry of quiescent myoblasts into S phase, but hastens S-phase completion. Cyclin A2 (CycA) mRNA is no longer restricted to S phase, but is induced throughout G(0)/G(1), with activation of the cell cycle regulated element (CCRE) in the CycA promoter. Overexpressed MLL5 physically associates with the CCRE and impairs its activity. MLL5 also regulates CycA indirectly: Cux, an activator of CycA promoter and S phase is induced in RNAi cells, and Brm/Brg1, CCRE-binding repressors that promote differentiation are repressed. In knockdown cells, H3K4 methylation at the CCRE is reduced, reflecting quantitative global changes in methylation. MLL5 appears to lack intrinsic histone methyl transferase activity, but regulates expression of histone-modifying enzymes LSD1 and SET7/9, suggesting an indirect mechanism. Finally, expression of muscle regulators Pax7, Myf5, and myogenin is impaired in MLL5 knockdown cells, which are profoundly differentiation defective. Collectively, our results suggest that MLL5 plays an integral role in novel chromatin regulatory mechanisms that suppress inappropriate expression of S-phase-promoting genes and maintain expression of determination genes in quiescent cells.
authors.affiliation: Centre for Cellular and Molecular Biology, Hyderabad, 500 007 India.
authors.forename: Soji
authors.initials: S
authors.lastname: Sebastian
authors.forename: Prethish
authors.initials: P
authors.lastname: Sreenivas
authors.forename: Ramkumar
authors.initials: R
authors.lastname: Sambasivan
authors.forename: Sirisha
authors.initials: S
authors.lastname: Cheedipudi
authors.forename: Prashanth
authors.initials: P
authors.lastname: Kandalla
authors.forename: Grace K
authors.initials: GK
authors.lastname: Pavlath
authors.forename: Jyotsna
authors.initials: J
authors.lastname: Dhawan
mesh_terms: D000818:Animals; D002454:Cell Differentiation; D002868:Chromosomal Proteins, Non-Histone / Q000737:chemistry* / Q000378:metabolism; D019925:Cyclin A / Q000235:genetics / Q000378:metabolism*; D056751:Cyclin A2; D029721:Drosophila Proteins / Q000737:chemistry*; D016193:G1 Phase; D055785:Gene Knockdown Techniques; D011495:Histone-Lysine N-Methyltransferase / Q000172:deficiency / Q000378:metabolism*; D006657:Histones / Q000378:metabolism*; D008239:Lysine / Q000378:metabolism*; D008745:Methylation; D051379:Mice; D032446:Myoblasts / Q000166:cytology* / Q000201:enzymology; D011485:Protein Binding; D034622:RNA Interference; D012097:Repressor Proteins / Q000378:metabolism; D020218:Response Elements / Q000235:genetics; D016196:S Phase; D017386:Sequence Homology, Amino Acid; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic
chemical_list: C508184:CCNA2 protein, mouse; D002868:Chromosomal Proteins, Non-Histone; D019925:Cyclin A; D056751:Cyclin A2; D029721:Drosophila Proteins; D006657:Histones; D012097:Repressor Proteins; C445179:SWI-SNF-B chromatin-remodeling complex; D014157:Transcription Factors; C512571:trx protein, Drosophila; D011495:Histone-Lysine N-Methyltransferase; C539231:MLL5 protein, mouse; D008239:Lysine