Abstract 224: Cardiac Stem Cell Ablation Blocks Regeneration, Exacerbates Cardiac Remodeling and Increases Mortality after Diffuse Myocardial Injury.
Under normal physiological conditions the adult heart is an extremely slow-proliferative organ. c-kitpos cardiac stem cells (CSCs) are present in the adult myocardium (>95% quiescent) and are involved in maintaining cardiac cell homeostasis. We have previously shown that following diffuse myocardial damage, the myocardium responds with rapid CSC activation, proliferation and differentiation into immature small myocytes with a fetal phenotype. These regenerating myocytes can undergo a few rounds of replication before becoming mature and terminally differentiated. These cellular events directly correlate with recovery from heart failure and restoration of normal LV function in less than 2 weeks. To establish a cause-effect relationship we evaluated the functional contribution of CSC activation to myocyte regeneration and LV recovery following myocardial injury. Four days after a single injection of isoproterenol to male Wistar rats (ISO; 5 mgkg−1; which kills ~8 % ventricular myocytes in a diffuse pattern), >95% CSCs have re-entered the cell cycle. To ablate the CSC myocardial pool, the anti-mitotic agent 5-fluorouracil (5-FU; 10mgkg−1), was administered in 3 cycles of 5 days/wk starting at the 4th day post ISO injection. Animals were sacrificed 7, 14 and 28 days later. LV function was measured by echocardiography. The increase in CSC number normally found after ISO was completely abolished by 5-FU treatment. As a direct consequence, new myocyte formation (Ki67pos/BrdU labeled) was reduced by >92% and was associated with lack of LV functional recovery, as compared to the ISO/non 5-FU-treated animals. The lack of CSC amplification and consequent myocyte replacement resulted in a disproportionate maladaptative hypertrophy of the spared myocytes followed by their progressive drop-out by necrosis and apoptosis after ISO in 5-FU-treated rats. This resulted in progressive worsening of LV function, cardiac failure and increased mortality at 28 days. The myocyte death and LV dysfunction was not a toxic effect of 5-FU because it was not seen in the 5-FU treated control animals. These results identify CSC activation and new myocyte formation as an integral and essential component of the endogenous myocardial response to diffuse myocardial injury.