Recent intriguing observations indicate that stem cell plasticity may exist. Rodent bone marrow cells have been shown to contribute to liver, skeletal and cardiac muscle. These extremely exciting findings could impact health care in a dramatic way in the future, if applicable to the human system. Marrow stem cells could theoretically be harvested from a patient and used to repair his or her damaged heart, muscle tissue, or liver. In order to realize this potential, the scientific community must determine the phenotype(s) of the human counterparts to the murine cells that have displayed plasticity, must determine whether a single totipotent stem cell from human marrow is capable of differentiation into tissue and blood, and must optimize isolation and transplantation of the totipotent cells. We hypothesize that there are true totipotent cells that reside in the adult human and mouse bone marrow and in neonatal umbilical cord blood. We hypothesize that common stem cells exist for blood and liver, and that in the human, their phenotype will be plastic non-adherent CD34-/lin-, and in the mouse their phenotype will be plastic non- adherent SP or c-kit+Thy-1.1(lo)Lin-sca-1+. We predict that human and murine cells that can repair muscle are actually plastic adherent mesenchymal stem cells (MSC) and will not generate blood cells. We will use our novel immunodeficient nude/NOD/SCID mouse strain, which has a lifespan of two years, as the recipient for marked stem cells of defined phenotype, isolated from murine bone marrow, human marrow, and human umbilical cord blood. Retroviral and lentiviral vector marked MSC and HSC fractions from each source will be assessed in vivo for their capacity to form blood cells and muscle, following muscle injury, or blood cells and liver, following liver injury to the recipient. Clonal integration analysis at the single cell level, with sequencing, will determine whether the progeny were derived from the same precursor, or from discrete stem cells. We will also determine whether it is possible to modulate the survival, differentiation, and recruitment of candidate murine and human totipotent stem cells to specific tissues by expression of supraphysiological levels of Hepatocyte Growth Factor/Scatter Factor (HGF/SF). HGF is a chemoattractant and viability factor that is elevated in injured liver, cardiac, and skeletal muscle. HGF affects the motility and maintains the viability of myoblasts, hepatic oval cells, and hematopoietic stem cells. We hypothesize that HGF, secreted locally in response to tissue injury, is a major factor in recruiting totipotent stem cells from the circulation into the site of muscle or liver injury. We hypothesize that HGF may maintain the viability of recruited, totipotent stem cells, while they are directed by other inductive, tissue-specific factors in the local microenvironment to differentiate, to regenerate the damage tissue. The impact of HGF on the recruitment, survival and differentiation of human and murine totipotent stem cell candidates will be tested in vitro and in vivo. Our studies will provide definitive proof that stem cell plasticity exists, will identify the phenotypes of the murine and human cells that can generate liver or muscle tissue in addition to blood cells, and will determine methods to enhance the recruitment of those cells to damaged tissue.
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