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Bone marrow stem cells (BMSC) from adult mice are now believed to generate non-hematopoietic cell types. This newly defined property is referred to as stem cell plasticity. We tested the potential of lineage negative c-kit positive (Lin- c-kit+), GFP+ BMSC to differentiate into cardiac myocytes in myocardial infarcts produced by ligation of the left coronary artery. At 9 days post-transplant the hearts showed a band of developing GFP+ myocytes within the damaged myocardium. These GFP+ myocytes were positive for cardiac specific myosin and early expressed transcription factors. Endothelial cells and smooth muscle cells also developed from the donor bone marrow cells. Left ventricular end diastolic pressure (LVEDP) and left ventricular developed pressure (LVDP) were improved. Lin-c- kit- cells did not regenerate myocardium. We next tested the ability of cytokine-mobilized BMSC to regenerate myocardium. Nuclei in regenerating cardiomyocytes were positive for Csx/Nkx 2.5, GATA-4 and MEF2. Cytoplasmic proteins included desmin, nestin and connexin 43. Regenerating arterioles consisted of endothelial cells and smooth muscle cells positive for Ki67, and flkl. These regenerating vessels contained circulating TER119 positive red blood cells. Repair of infarcted myocardium resulted in improved heart function and survival. At day 27 after cytokine treatment and surgery, 11 of 15 mice survived compared with 9 of 52 non-treated mice. Left ventricular ejection fraction in infarcted hearts in cytokine-treated mice was 48%, 62% and 114% higher than the ejection fraction in non-treated mice at 9, 16 and 26 days following coronary artery occlusion. These findings demonstrate that circulating autologous stem cells traffic to the ischemic, infarcted myocardium and undergo differentiation into cardiomyocytes and vascular structures. We conclude that adult BMSC have the potential for repair in acute, ischemic heart disease.

Several 2'-5' oligoadenylate (2-5A) core molecules were tested for their inhibitory effect on erythroid colony-forming units (CFU-E). These compounds were introduced into the CFU-E microenvironment by vascular perfusion of isolated rat hind limbs. The marrow microvasculature did not appear to be damaged by these procedures when examined by electron microscopy. After a brief in situ exposure to 2-5A core, the marrow cells were grown in methylcellulose medium with added erythropoietin (Ep). Maximum percent inhibition of CFU-E was obtained with 2-5A trimer core. The tetramer core was less effective, and no inhibition of colony formation was seen in cultures of marrow perfused with 2-5A dimer core. From this it was concluded that the inhibitory effect of 2-5A trimer core was highly specific. Furthermore, this effect was of long duration, since when 2-5A trimer core was given just prior to Ep the CFU-E were blocked in their differentiative response to Ep for 48-72 h.

Adult BM stem cells are being investigated for their potential to regenerate injured tissues by a process referred to as plasticity or transdifferentiation. Although data supporting stem cell plasticity is extensive, a controversy has emerged based on findings that propose cell-cell fusion as a more appropriate interpretation for this phenomenon. A major focus of this controversy is the claim that acutely infarcted myocardium in adult hearts can be regenerated by BM stem cells. Many researchers consider the adult heart to be a post-mitotic organ, whereas others believe that a low level of cardiomyocyte renewal occurs throughout life. If renewal occurs, it may be in response to cardiac stem cell activity or to stem cells that migrate from distant tissues. Post-mortem microscopic analysis of experimentally induced myocardial infarctions in several rodent models suggests that cardiomyocyte renewal is achieved by stem cells that infiltrate the damaged tissue. For a better understanding of the possible involvement of stem cells in myocardial regeneration, it is important to develop appropriate technologies to monitor myocardial repair over time with an emphasis on large animal models. Studies on non-human primate, swine and canine models of acute myocardial infarctions would enable investigators to utilize clinical quality cell-delivery devices, track labeled donor cells after precision transplantation and utilize non-invasive imaging for functional assays over time with clinical accuracy. In addition, if stem cell plasticity is to reach the next level of acceptance, it is important to identify the environmental cues needed for stem cell trafficking and to define the genetic and cellular mechanisms that initiate transdifferentiation. Only then will it be possible to determine if, and to what extent, BM stem cells are involved in myocardial regeneration and to begin to regulate precisely tissue repair. [ABSTRACT FROM AUTHOR]

Presents information on the short-term and long-term reconstituting stem cells in mouse myocardium. Developmental potential of stem cells that emerge during late embryonic and fetal development; Reconstitution of the entire hematopoietic system; Overview of myocardial infarction in hear disease.