Current theory indicates that mitochondria were obtained 1.5 billion years ago from an ancient prokaryote. The mitochondria provided the capacity for aerobic respiration, the creation of the eukaryotic cell, and eventually complex multicellular organisms. Recent reports have found that mitochondria play essential roles in aging and determining lifespan. A variety of heritable and acquired diseases are linked to mitochondrial dysfunction. We report here that mitochondria are more dynamic than previously considered: mitochondria or mtDNA can move between cells. The active transfer from adult stem cells and somatic cells can rescue aerobic respiration in mammalian cells with nonfunctional mitochondria.human bone marrow ͉ nonhematopoietic ͉ stem͞progenitor cells ͉ ischemia M itochondria are essential organelles in plant and animal cells that are from a prokaryotic ancestor and play a key role in processes such as oxidative phosphorylation, aerobic metabolism of glucose and fat, calcium signaling, and apoptosis (1, 2). The human mitochondrial genome is 16,568 bp and encodes a limited number of mitochondria-specific proteins, rRNAs, and tRNAs (3). All other mitochondrial proteins are encoded in the nucleus. The mitochondrial genome is maternally inherited and undergoes a high rate of mutation because mtDNA is not protected by histones, is inefficiently repaired (4), and is exposed to oxygen radicals generated by oxidative phosphorylation (1).Nearly every tissue contains stem-like progenitor cells that repair tissues after damage (5), but local repair can be supplemented by stem-like progenitor cells from the bone marrow. Stem͞progenitor cells repair tissues by differentiating to replace lost cells, providing cytokines and growth factors, and by cell fusion with endogenous cells.In this article, we ask whether stem͞progenitor cells or other somatic cells can repair cells with nonfunctional mitochondria by transfer of functional mitochondria or mtDNA. We used cells that were pretreated with ethidium bromide so that the mtDNA became mutated and depleted and the cells became incapable of aerobic respiration and growth (A549 °cells), except in a permissive medium containing uridine and pyruvate to supplement anaerobic glycolysis (6, 7). The A549 °cells were cocultured with either adult nonhematopoietic stem͞progenitor cells from human bone marrow (hMSCs) or with skin fibroblasts. The cocultures produced clones of rescued A549 °cells with functional mitochondria. ResultsRescue of A549 °Cells by Coculture with hMSCs. PCR assays indicated that a number of mitochondrial genes and DNA sequences were absent or depleted in A549 °cells: the genes for cytochrome oxidase I and II, the gene for tRNA leucine, and hypervariable regions I and II of the D loop (Fig. 1A). Some of the PCR products may have been generated by nonfunctional nuclear pseudogenes in the A549 °cells that are only detectable after mtDNA depletion (8). Sequencing of the PCR products demonstrated additional extensive damage and mutations of A549 °mtDNA (data not sho...

We tested the hypothesis that multipotent stromal cells from human bone marrow (hMSCs) can provide a potential therapy for human diabetes mellitus. Severe but nonlethal hyperglycemia was produced in NOD͞scid mice with daily low doses of streptozotocin on days 1-4, and hMSCs were delivered via intracardiac infusion on days 10 and 17. The hMSCs lowered blood glucose levels in the diabetic mice on day 32 relative to untreated controls (18.34 mM ؎ 1.12 SE vs. 27.78 mM ؎ 2.45 SE, P ‫؍‬ 0.0019). ELISAs demonstrated that blood levels of mouse insulin were higher in the hMSC-treated as compared with untreated diabetic mice, but human insulin was not detected. PCR assays detected human Alu sequences in DNA in pancreas and kidney on day 17 or 32 but not in other tissues, except heart, into which the cells were infused. In the hMSC-treated diabetic mice, there was an increase in pancreatic islets and ␤ cells producing mouse insulin. Rare islets contained human cells that colabeled for human insulin or PDX-1. Most of the ␤ cells in the islets were mouse cells that expressed mouse insulin. In kidneys of hMSC-treated diabetic mice, human cells were found in the glomeruli. There was a decrease in mesangial thickening and a decrease in macrophage infiltration. A few of the human cells appeared to differentiate into glomerular endothelial cells. Therefore, the results raised the possibility that hMSCs may be useful in enhancing insulin secretion and perhaps improving the renal lesions that develop in patients with diabetes mellitus.insulin ͉ pancreas ͉ streptozotocin ͉ transplantation P revious publications presented conflicting observations as to whether cells from bone marrow can provide a potential therapy for diabetes mellitus. One strategy (1-4) was to differentiate plastic adherent marrow cells in culture into insulin-secreting cells. A second strategy was to transplant diabetic mice with genetically labeled marrow and to search for labeled insulinproducing cells in the recipient mice. One study using a CRELoxP-GFP system found that 1.7-3% of the cells in islets of the recipient mice were marrow-derived and that GFP-labeled donor cells isolated from the islets expressed insulin, glucose transporter 2, and transcription factors typically found in ␤ cells (5). Three subsequent reports in which mice were transplanted with GFPexpressing bone marrow did not find evidence of marrow cells becoming insulin-producing cells in the pancreas of recipient mice (6-8), but in the reports it was difficult to exclude the possibility that the GFP gene was inactivated or that GFP-labeled cells were destroyed as they engrafted into islets. A third strategy was to determine whether systemically administered marrow cells enhanced regeneration of pancreatic insulin-producing cells in diabetic models. Hess et al. (9) reported that in NOD͞scid mice in which diabetes was induced with streptozotocin (STZ), partial marrow ablation followed by transplantation of either GFP-labeled whole-marrow or GFP-labeled c-kit ϩ cells from murine marrowenhanced r...

The past decade has seen an explosion of research directed toward better understanding of the mechanisms of mesenchymal stem/stromal cell (MSC) function during rescue and repair of injured organs and tissues. In addition to delineating cell–cell signaling and molecular controls for MSC differentiation, the field has made particular progress in defining several other mechanisms through which administered MSCs can promote tissue rescue/repair. These include: 1) paracrine activity that involves secretion of proteins/peptides and hormones; 2) transfer of mitochondria by way of tunneling nanotubes or microvesicles; and 3) transfer of exosomes or microvesicles containing RNA and other molecules. Improved understanding of MSC function holds great promise for the application of cell therapy and also for the development of powerful cell-derived therapeutics for regenerative medicine. Focusing on these three mechanisms, we discuss MSC-mediated effects on immune cell responses, cell survival, and fibrosis and review recent progress with MSC-based or MSC-derived therapeutics.

Adult stem cells from human bone marrow stroma, referred to as mesenchymal stem cells or marrow stromal cells (hMSCs), are attractive candidates for clinical use. The optimal conditions for hMSC expansion require medium supplemented with fetal calf serum (FCS). Some forms of cell therapy will involve multiple doses, raising a concern over immunological reactions caused by medium-derived FCS proteins. By a sensitive fluorescence-based assay we determined that 7 to 30 mg of FCS proteins are associated with a standard preparation of 100 million hMSCs, a dosage that probably will be needed for clinical therapies. Here we present ex vivo growth conditions for hMSCs that reduce the FCS proteins to less than 100 ng per 100 million hMSCs, approximately a 100,000-fold reduction. The cells maintain their proliferative capacity and sustain their ability for multilineage differentiation. Experiments in rats demonstrate that rat MSCs grown in 20% FCS induce a substantial humoral response after repeated administrations, whereas cells grown under the conditions described in this study reduce the immunogenicity in terms of IgG response over 1000-fold to barely detectable levels. Our results have the potential to dramatically improve cellular and genetic therapies using hMSCs and perhaps other cells.