Abstract 907: Cardiac Progenitor Cell Migration after Myocardial Infarction
Local administration of growth factors (GFs) promotes migration and homing of cardiac progenitor cells (CPCs) to the infarcted myocardium and activates cardiac regeneration. However, the molecular pathway that mediates the translocation of CPCs to the ischemic myocardium is unknown. We injected a retroviral vector expressing enhanced green fluorescent protein (EGFP) in the atria of the rat heart to label cycling primitive cells that are clustered in this region. Two days later, myocardial infarction was induced and HGF and/or IGF-1 were delivered at increasing doses from the atria to the border zone to create a chemotactic gradient towards the infarct. By two-photon microscopy, it was established that CPCs advanced within the myocardial tissue at a speed of ~80 um/hour and cell locomotion did not occur via the coronary vasculature but through the interstitium. To identify the molecules involved in the interaction between CPCs and extracellular matrix, infarcted rats treated with GFs were sacrificed at 1, 3, 7 and 14 days. In one group of animals, hearts were fixed and studied by confocal microscopy and, in the other group of rats, c-kitPOS CPCs were isolated by enzymatic digestion and immunomagnetic selection, and studied biochemically. CPC stemness was associated with the expression of α4 integrin and a shift in integrin subunits was apparent following GF-administration. CPCs stimulated and mobilized by GFs expressed the phosphorylated forms of focal adhesion kinase (FAK) and paxillin. Additionally, αvα3 and α6α4 integrins were present in these cells while α4 integrin was decreased. Similarly, the formation of protein complexes between c-Met, the receptor of HGF, and α4 integrin was enhanced in GF-treated in CPCs. The chemotactic properties of c-kitPOS CPCs were then assessed in vitro. In the presence of HGF, clonogenic c-kitPOS CPCs migrated through the membrane of a modified Boyden chamber pointing to a critical role played by HGF in cell motility. siRNA silencing of α4, αv and α3 inhibited the effects of HGF on CPC migration on laminin or fibronectin substrates. In conclusion, the FAK pathway and αvα3 and α6α4 integrins are required for the translocation of CPCs from the surviving tissue to the infarct and the restoration of the necrotic myocardium.