Abstract 521: Cardiac Fibroblasts are Activated During Postnatal Extracellular Matrix Remodeling

Abstract

Objectives: During the postnatal period, P0 to P30 in the mouse, the myocardial extracellular matrix (ECM) transitions to a mature profile necessary for increased cardiac output. We hypothesize that, during the postnatal period, cardiac fibroblasts (CF) become activated, as indicated by Periostin (Postn) expression, and remodel the ECM, followed by quiescence in the mature heart. Our goal is to study CF activation and its role in postnatal heart development. Methods: CF activation was monitored by Postn MerCreMer(MCM) ;Rosa GFP lineage analysis and the resident CF were studied using a TCF21 MCM ;R26 GFP model. Tamoxifen induction of Cre MCM was initiated in the postnatal period and lineage-specific CF were quantified. Cardiac ECM maturation and CF proliferation, together with bone marrow-derived and endothelial cells, were assessed by Immunofluorescence (IF). TCF21 , Postn , Col1a1 , TnnI and CD31 transcripts were examined by RNAscope, and Fibronectin (FN) compartmentalization, an indicator of ECM maturation, also was examined. Results: FN, Postn, and Postn MCM R26 GFP lineage cells were more prominent at P0-7 than at P30. Postn -expressing cells also co-express Tcf21 and Col1a1 , but not TnnI (myocytes) or PECAM (endothelial cells), as determined by RNAscope. In agreement, Postn MCM R26 GFP CF do not express CD31, CD45, or the myofibroblast marker alphaSMA at P7. At P30, Postn, FN, and Col1a1 expression is reduced in CF, but TCF21 MCM ;R26 GFP is maintained, suggesting a transient period of CF activation followed by quiescence. Similarly, proliferation rates of the Postn MCM R26 GFP cells are higher than TCF21 MCM ;R26 GFP CF at P7, followed by decreased proliferation at P30. Conclusions: Activation and proliferation of CFs and ECM gene expression is increased in the week after birth relative to quiescent CFs at P30. Postn+ cells also peak at P7 but are not detected in appreciable numbers at P30, suggesting a critical role in ECM remodeling and myocardial maturation in the postnatal period. Studies are ongoing to determine the codependence of postnatal activated CF, ECM maturation and myocardial regeneration.