Repair of the Injured Adult Heart Involves Resident Cardiac Stem Cell Derived New Myocytes

Abstract

The ability of the adult heart to generate new myocytes after injury is not established. Our purpose was to determine if the adult heart has the capacity to generate new myocytes after injury, and to gain insight into their source. Cardiac injury was induced in the adult feline heart by infusing Isoproterenol (ISO) for 10 days with minipumps and then animals were allowed to recover for 7 or 28 days. Cardiac function was measured with echocardiography and proliferative cells were identified by nuclear incorporation of 5-bromodeoxyuridine (BrdU; 7 day minipump infusion). BrdU was infused for 7 days before euthanasia at Day 10 (injury), Day 17 (early recovery), and Day 38 (late recovery) and, with a separate group of animals, was infused during injury and removed at Day 10, with animals euthanized at Day 38 for a pulse-chase experiment. Isoproterenol caused a reduction in cardiac function with evidence of myocyte loss from necrosis. During the injury phase there was a significant increase in the number of proliferative cells in the atria and ventricle, including an increase in cKit+/BrdU+ proliferative cardiac precursor cells, but there was no increase in the number of BrdU+ new myocytes (Day 10). During the first seven days of recovery (Day 17) there was a significant reduction in cellular proliferation (total BrdU+ nuclei, including cKit+/BrdU+ proliferative cardiac precursor cells) but a significant increase in BrdU+ myocytes. There was modest improvement in cardiac structure and function during recovery. At Day 38 (late recovery), overall cell proliferation (BrdU+ cells) was not different than control (BrdU infused from Days 31-38); however, increased numbers of ("bright") BrdU+ myocytes were found at Day 38 in the pulse-chase experiment, when BrdU was infused during injury (and removed at Day 10). Some of the newly formed myocytes (from the pulse-chase group; Day 38), derived from BrdU+ cardiac precursors appear to be transiently proliferative (between Days 10-38) producing a population of "dimly" BrdU+ myocytes in our pulse-chase protocol (BrdU infused during injury, Days 3-10, and removed at Day 10, with heart explant at Day 38). No significant numbers of "dimly" BrdU+ nuclei were found in any of the hearts in which BrdU was infused for 7 days prior to the animal being euthanized (Control, Day 10, Day 17, Day 38). These observations are most consistent with the conclusions stated. Our results also suggest that myocyte regeneration, as defined by BrdU+ myocytes, was more robust in the atria than the ventricle. The reasons for these differences are not clear and deserve additional study. If true, our findings suggest that cardiac precursors isolated and expanded from atrial tissue might be a better source of cells for autologous cardiac cell therapy. In summary, our data shows that the adult heart has the ability to generate new myocytes after injury, suggests that ISO injury activates cardiac precursor cells that can differentiate into new myocytes during cardiac repair, but that the environment of the ISO injured heart blunts the differentiation of cardiac precursors into functional new myocytes. The contribution of new myocytes to improved function of the ventricle would appear to be small, unless we have underestimated the number of these cells. This is quite possible, and further study is warranted to incorporate the number of "dimly" BrdU+ myocytes that may have undergone a proliferative phase as a progenitor cell and/or as an immature cardiac myocyte. Further understanding the factors that limit endogenous new myocyte formation could significantly contribute to new therapeutic applications and improve the quality of life, and potentially the lifespan, of patients in heart failure.