Activation of cardiac fibroblast phenotypes following myocardial ischemia-reperfusion injury in transgenic mouse models

Following myocardial injury, cardiac fibroblasts are believed to play important roles in inflammatory and wound healing processes. We hypothesized that activation of cardiac fibroblasts and myofibroblasts in response to ischemia‐reperfusion (I/R) injury could be detected with transgenic mouse strains expressing fibroblast phenotype‐sensitive promoters linked to expression of fluorescent reporter proteins (Collagen I promoter:Enhanced Green Fluorescent Protein, Coll I:EGFP or Smooth Muscle α‐Actin promoter:Red Fluorescent Protein, SMA:RFP). Activation of cardiac fibroblasts was monitored by reporter fluorescence in frozen tissue sections and by FACS analysis of enzymatically dispersed fibroblast populations. Frozen sections from hearts of reporter mice sacrificed 4 days after I/R injury induced by transient coronary artery (LAD) occlusion showed robust enhancement in cells expressing CollI:EGFP and modest increases in SMA:RFP fluorescence. Fibroblast activation was localized to the infarct zone, and was not observed in control non‐injured mice. FACS analysis of cardiac fibroblasts isolated after I/R injury showed elevations of fluorescent reporter expression (EGFP>RFP). The respective fibroblast phenotypes were maintained under culture conditions. In conclusion, fibroblast phenotype‐sensitive reporters show activation of fibroblasts in the injured myocardium in vivo.