Understanding cell fate and function decisions of cardiac mesenchymal stem cells

The adult heart, albeit the presence of multiple progenitor cell types, is one of the poorly regenerated organs following injury. Extensive characterisation of such progenitor populations is therefore crucial to understanding the potential of the cells to be manipulated leading to better injury resolution and repair. Cardiac colony forming unit fibroblasts (cCFU-Fs) are a resident progenitor cell population, analogous to bone marrow CFU-Fs, giving rise to cardiac mesenchymal stromal cells (cMSCs) in vitro. While being one of the better characterised progenitor populations, intrinsic cell fate decisions driving cCFU-F function in homeostasis and repair are yet to be elucidated. Hence, the aim of this thesis was to initiate understanding the underlying molecular mechanisms that guide cCFU-F and cMSC outcomes. This aim was addressed using three different approaches; (1) by determining the intrinsic lineage commitment of cMSCs using an unbiased in vitro assay, (2) by investigating the role of the cardiogenic transcription factor T-box 20 (TBX20) which is highly expressed in cMSCs and (3) by comparing the cMSCs from atria and ventricles to determine the effect of anatomical location within the heart. Serum reduction in growth media was used as a method to induce unbiased differentiation of cells in vitro. The assay however, did not behave as anticipated to differentiate the cells. Instead, under low serum conditions, cells lost mesenchymal characteristics and acquired a gene signature similar to that of an active epicardium. Transcriptome analysis confirmed the loss of mesenchymal character possibly via downregulation of TGFβ signalling and revealed predominantly the paracrine potential of cMSCs. Deletion of Tbx20 in cCFU-Fs affected clonogenicity of cells while preliminary observations suggested a role in regulating cMSC differentiation. Comparison of atrial and ventricular cCFU-Fs revealed enrichment of cCFU-Fs in the atria which also showed a higher growth rate in vitro. Relative hypoxia induced metabolic changes may have partly contributed to these differences. Additionally, we describe a mouse model to study atrial fibrosis which may also provide a platform to characterise reparative and reactive fibrosis. In summary, this thesis presents the developments made in understanding cell fate and function decisions of cCFU-Fs/cMSCs and potential governing factors