Studies on the role of inflammation and stem cells in cardiac ischaemic injury

Abstract

The regenerative capacity of human myocardium is unable to compensate adequately forth significant loss of heart muscle that occurs in acute myocardial infarction (Ml). However, recent studies indicate that the adult heart may contain populations of cells that have stem cell characteristics and which could have regenerative potential. In order to better examine the biology of such cells, this study developed a mouse model of MI that combined low mortality, reproducible infarct size and the opportunity for reperfusion studies. One potential barrier to the expression of regenerative capacity at the time of acute Ml is inflammation, which, as well as contributing to myocardial injury, may prevent the activation and function of putative stem cells. The C5/C5a axis in the complement cascade is a key mediator of the inflammatory events involved in MI and therefore represents an attractive target. An antibody to the C5a receptor was used in vivo in the murine model of MI. Although the neutrophil infiltration at the time of MI was significantly reduced (p<0.05), the size of MI was unchanged in comparison to control, suggesting that other regenerative strategies must be explored. Therefore in pursuit of a mechanism by which the heart may regenerate, cells were isolated and assayed in vitro for their stem cell capacity. Cells expressing platelet derived growth factor receptor alpha (PDGFRA) were candidates for such stem cells, both as these are present in the proepicardium and epicardium in the embryo, and that Pdgfra(^+) mice demonstrate cardiac abnormalities. A combined approach was employed, using genetically modified mice expressing Green Fluorescence Protein (GFP) under the transcriptional control of the Pdgfra locus, expression of surface antigens as well as the ability of candidate stem cells to demonstrate self renewal and proliferation in a colony forming unit fibroblast assay (CFU-F). A Sca-1(^+) /PECAM˙/PDGFRA(^GFP+) fraction isolated from the adult heart was found to be enriched for colony forming activity. Two populations were found which formed colonies, the PDGFRA(^GFPHIGH) population which had 'stem cell' like properties in terms of proliferation, prolonged self renewal and multipotent in vitro differentiation, and the PDGFRA (^GFPMEDIUM) population which had 'progenitor cell' like properties in that although it had proliferative ability, the potential of this population to continue to self renew and differentiate into multiple lineages was limited. The number of candidate stem cells and progenitor cells in the heart increased post infarct as evidenced by an increase in number of PDGFRA (^GFPHIGH) and PDGFRA (^GFPMEDIUM) cells, together with a greater colony forming ability of the PDGFRA (^GEPHIGH) cells. These effects were magnified in a proregenerative transgenic model, Pdgfra (^GFP/+) ;mlGF, in which there was over expression of insulin-like growth factor 1 (IGF-1), supporting the argument that Sca-1(^+) /PECAM˙/PDGFRA(^GFP+) cells may have a role to play in the regenerative capacity of the adult heart.