During development, cells of the epicardium undergo epithelial-mesenchymal transition (EMT) to generate multipotent progenitor cells (epicardium-derived cells, EPDCs) that contribute to multiple lineages of the cardiovascular system. Critical to this process is the epicardially-expressed Wilms’ tumour 1 (Wt1) gene. In the adult, the epicardium is largely quiescent and no longer expresses Wt1. Upon myocardial infarction (MI), however, the adult epicardium is reactivated; it re-expresses Wt1, reverts to its embryonic potency, and contributes to cardiac repair. Previous work showed that this reactivation is enhanced by ‘priming’ mice with the G-actin-binding protein thymosin β4 (Tβ4), although the molecular mechanisms were not defined. Here we investigated Wt1 regulation in the epicardium during development and post-MI and found Wt1 to be regulated by the SWI/SNF chromatin remodelling complex in combination with Tβ4. During development, Wt1-expressing cells in the epicardium expressed the SWI/SNF ATPases, Brg1 and Brm. Both Wt1 and SWI/SNF were largely absent from the uninjured adult epicardium, but the re-expression of Wt1 post-MI was accompanied by substantial upregulation of both Brg1 and Brm. Exogenous Tβ4 in the form of priming increased the post-MI expression of Wt1, Brg1 and Brm, whilst endogenous Tβ4 was found to be required for maximal Wt1 reactivation. Furthermore, we found that Tβ4 physically interacts with the SWI/SNF complex and that it potentiates SWI/SNF-mediated activation of specific regulatory regions of the Wt1 promoter and intron 1, suggesting that this interaction is functional. Loss of Brg1 in the epicardium using Gata5Cre;Brg1flox embryos showed that Brg1 is not required for Wt1 expression, which we postulate is due to compensation from Brm, whilst loss of Brg1 in Wt1-expressing lineages (using the tamoxifen-inducible Wt1CreERT2) caused no obvious phenotype. Further understanding of the mechanisms underlying Wt1 expression, during both development and epicardial reactivation post-MI, is essential in our quest to utilise resident-cell-based therapy in cardiac repair.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:626794 |
| Date | January 2014 |
| Creators | Howard, S. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1419273/ |
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