927. Human Mesenchymal and Hematopoietic Stem Cell Contribution to Cardiac Tissue Regeneration after Injury

Abstract

Cardiomyocyte regeneration by stem cells from non-cardiac tissues could be a future treatment for cardiovascular diseases. However, there is little direct evidence that adult stem cells can regenerate a damaged heart. In vitro assays of stem cells from the bone marrow compartment using 5-azacytidine suggests that these stem cells can adopt the cardiomyocyte morphology and express cardiac-specific markers. One of the most commonly used in vivo models for myocardial infarction is coronary ligation. In this model, the left anterior descending (LAD) coronary artery is tied off to create an area of ischemia in the left ventricle. Using this model, it remains a controversy as to whether stem cells from the bone marrow, either hematopoietic or mesenchymal, can contribute and/or enhance cardiomyocyte regeneration following damage. Our lab has previously reported that co-transplantation of hematopoietic stem cells (HSC) with mesenchymal stem cells (MSC) greatly enhances the complete hematopoietic lineage reconstitution in myeloablated immunodeficient mice. To address the role of HSC and/or MSC in repair for cardiovascular diseases, we have developed a heart injury model using doxorubicin, a chemotherapeutic drug that has been shown to cause cardiac toxicity. Hematopoietic and mesenchymal cells are injected next to the ischemic area immediately after the LAD surgery, or intraperitoneally/intravenously two days after the doxorubicin injection. LAD-injured mice are then assessed functionally by echocardiography, to obtain ejection fraction, end systolic volume, end diastolic volume and wall motion score indexes. Our preliminary studies showed show low but persistent engraftment of human cells in the cardiac tissue in 100% of surviving transplants. An additional aim of our studies is to ask whether co-transplantation of HSC and MSC could give a functional phenotype suggestive of cardiovascular repair. To distinguish HSC from MSC following their co-transplantation, we transduced HSC with a retroviral construct expressing YFP and MSC either with a monocistronic retroviral construct expressing GFP alone or with a bicistronic retroviral construct expressing a chemotatic/proliferation factor (hepatocyte growth factor) and GFP. The use of the bicistronic construct is to determine whether secretion of HGF in vivo might enhance the survival of either HSC, MSC, or both, in the damaged heart. Moderate but detectable improvements in cardiac function can be seen in these mice. Whether the exogenous human HSC and/or MSC had transdifferentiated into cardiomyocyte-like cells or whether the injected cells indirectly recruit endogenous cardiomyocyte stem cells to proliferate and repair the cardiac damage is currently being investigated. Collectively, the use of retroviral gene transfer of different fluorescent markers to track stem cells in vivo in this cardiac-damaged xenotransplantation model will provide a more definitive answer as to which stem cells of the bone marrow compartment enhance cardiac repair. The use of retroviral gene transfer of specific soluble factors, such as HGF, could provide additional survival and/or differentiation support of stem cells in vivo.