Chromatin remodeling and myogenesis

Abstract

<p>The lineage identity of a particular cell within a multi-cellular organism is defined by the repertoire of mRNA it expresses. Changes to the transcript repertoire can affect cell identity and stage of differentiation. Chromatin as the biologically relevant target of transcription factors has a direct impact on cell identity and differentiation, with changes to its structure central to lineage choice, cellular identity and differentiation. The requirement of chromatin remodeling in these developmental processes is evident throughout myogenesis, with changes to the cell's chromatin structure occurring during both the specification of myoblasts and the differentiation of specified myoblasts to myotubes. This thesis examines three distinct aspects of chromatin remodeling in relation to myogenesis. Firstly, the role of Snf2h in MyoD-mediated repression of target genes during myoblast proliferation was investigated. Snf2h was identified as a MyoD-interacting protein in a repressed transactivator yeast two-hybrid screen. Overexpression of either an active or inactive form of human SNF2H in myoblasts accelerated the differentiation process without affecting either growth rate or cell cycle kinetics. Furthermore, the chromatin structure surrounding the myogenin E boxes was more open in myoblasts expressing inactive human SNF2H compared to control myoblasts suggesting functional Snf2h complexes are required to repress differentiation specific genes during growth and subsequently maintain the myoblast proliferative state. From these and published results, a model describing the role of Snf2h in myogenesis is suggested. This model proposes that Snf2h activity facilitates the repression of MyoD target genes by histone deacetylases. In addition, genome-wide changes to histone H4 acetylation and histone H3-K9 dimethylation during myoblast differentiation were assessed. The patterns of genome-wide histone modifications in proliferating myoblasts were distinct from the patterns observed in myotubes, suggesting distinct mechanisms are targeting histone-modifying activity in myoblasts and myotubes. Furthermore, in our study, hyperacetylation of histone H4 and hypomethylation of histone H3 did not correlate strongly with transcriptional status. Our results support the hypothesis of the Histone Code Model, which theorizes that a combination of distinct histone modifications and not individual histone modifications defines the transcriptional competence of a particular gene. (Abstract shortened by UMI.)</p>