| Transplanted skeletal myoblasts can form large intramyocardial grafts to replace scar tissue and improve left ventricular function following myocardial infarction. However, graft size is difficult to control reproducibly even when the number of cells administered and the injection technique are kept constant. Alternatively, injecting a “sub-therapeutic” dose of cells followed by a period of reversible in situ proliferation may provide greater control over graft size, enhance cellular integration with host tissue, and ultimately improve the efficacy of cell-mediated therapy. Basic fibroblast growth factor (bFGF) stimulates skeletal myoblast proliferation, but its mitogenic effect on tissue fibroblasts may exacerbate fibrosis if administered to the injured heart in vivo. To induce myoblast proliferation without adding exogenous bFGF, we adapted a chemically induced dimerization strategy to activate the bFGF receptor, fibroblast growth factor receptor-1 (FGFR-1). To achieve this aim, we generated a retroviral vector encoding a chimeric receptor, F36Vfgfr-1, consisting of a modified FK506 binding protein domain, F36V (which binds a bivalent synthetic ligand, AP20187), fused with the cytoplasmic domain of FGFR-1. Mouse MM14 myoblasts were transduced with this construct and treated with AP20187 (“dimerizer”) to induce receptor dimerization. Dimerizer treatment stimulated proliferation, blocked differentiation, and activated the MAP kinase pathway in transfected myoblasts, similar to bFGF treatment, whereas the dimerizer had no effect on non-transfected cells. Preliminary studies indicate that dimerizer treatment also blocks differentiation in primary skeletal myoblasts from human muscle, although the effects of dimerizer on their proliferation are not yet clear. The in vivo effects of dimerizer treatment on myoblasts expressing F36Vfgfr-1 were studied by injecting transduced MM14 cells subcutaneously into nude mice and administering dimerizer from implanted osmotic pumps. A greater percentage of graft myoblasts incorporated BrdU in mice treated with dimerizer-eluting pumps compared with vehicle-loaded pumps. These results provide evidence that chemically induced dimerization of the FGFR-1 cytoplasmic domain can selectively stimulate myoblast proliferation in vitro and in vivo. We therefore hypothesize that in vivo administration of AP20187 following myoblast grafting may allow control over graft size and ultimately improve cardiac function following myocardial infarction. |
