Stem Cells Dev. 2015 Apr 15;24(8):938-47.
Uterine-Derived Stem Cells Reconstitute the Bone Marrow of Irradiated Mice
Sun Z, Wu J, Li SH, Zhang Y, Xaymardan M, Wen XY, Weisel RD, Keating A, Li RK.
Division of Cardiovascular Surgery, Toronto General Research Institute, University Health Network, Toronto, Canada.
Hematopoietic stem cells (HSCs) can be found in several tissues of mesodermal origin. Uterine tissue contains stem cells and can regenerate during each menstrual cycle with robust new tissue formation. Stem cells may play a role in this regenerative potential. Here, we report that transplantation of cells isolated from murine uterine tissue can rescue lethally irradiated mice and reconstitute the major hematopoietic lineages. Donor cells can be detected in the blood and hematopoietic tissues such as spleen and bone marrow (BM) of recipient mice. Uterine tissue contains a significant percentage of cells that are Sca-1(+), Thy 1.2(+), or CD45(+) cells, and uterine cells (UCs) were able to give rise to hematopoietic colonies in methylcellulose. Using secondary reconstitution, a key test for hematopoietic potential, we found that the UCs exhibited HSC-like reconstitution of BM and formation of splenic nodules. In a sensitive assay for cell fusion, we used a mixture of cells from Cre and loxP mice for reconstitution and demonstrated that hematopoietic reconstitution by UCs is not a function of fusion with donor BM cells. We also showed that the hematopoietic potential of the uterine tissue was not a result of BM stem cells residing in the uterine tissue. In conclusion, our data provide novel evidence that cells isolated from mesodermal tissues such as the uterus can engraft into the hematopoietic system of irradiated recipients and give rise to multiple hematopoietic lineages. Thus, uterine tissue could be considered an important source of stem cells able to support hematopoiesis.
Stem cells are undifferentiated cells that can proliferate and differentiate into specialized cells or divide to produce more stem cells. Adult stem cells can be found in various tissue and organs such as in bone marrow, adipose tissue, and blood. These cells possess the important functions of repairing and replenishing adult tissues. Stem cells have been clinically used in bone marrow transplantation in the past, and recently expanded to treat other diseases such as spinal cord injury, liver cirrhosis, chronic limb ischemia, and end stage heart failure. Although multi-potent adult stem cells are usually lineage-restricted, new research related to these cells suggests that multi-potent stem cells may be capable of conversion into unrelated cell types. For example, fibroblasts and umbilical cord blood stem cells were reported to convert into functional neurons .
In mammals, the lining of the uterus, known as the endometrium, regenerates during each menstrual cycle such that new tissue forms each month. Stem cells may play a role in the regenerative capacity of the uterus and previous work has identified the existence of uterine stem cells (USCs)  . Recently, our research group showed that the uterus contains precursor cells known as hemangioblasts that can differentiate into either blood or endothelial (vessel lining) cells  . The stem cells that differentiate into blood cells are called hematopoietic stem cells (HSCs) and they can be found in bone marrow or possibly in other tissues. A fundamental characteristic of HSCs is their ability to reconstitute all blood cell lines after transplantation into a lethally-irradiated recipient  . Thus, in the current study, we investigated whether stem cells obtained from uterine tissue could rescue lethally-irradiated mice by reconstituting their blood cell lines.
Figure 1: Flow chart depicting methodological steps and results of study. A. Uterine stem cells (USCs) were extracted from healthy green fluorescence positive (GFP+) mice and transplanted into lethally-irradiated primary recipient mice, significantly extending their lifespan and restoring their blood cell lines. Eight weeks after transplantation, GFP expression was detected in the blood, bone marrow, and spleen of primary recipients. B. Secondary reconstitution was carried out by transplanting bone marrow (BM) cells extracted from primary recipient mice that were previously reconstituted with USCs into secondary lethally-irradiated recipients, successfully restoring their blood cell lines and increasing their lifespan. GFP expression was detected in the blood, BM, and spleen of the secondary recipients for at least 9 weeks.
In our study, USCs were extracted from the uteri of green fluorescence protein (GFP)-positive mice, whose cells glow green under a fluorescence microscope. 1) When these USCs were transplanted into lethally-irradiated mice, they significantly extended their lifespan, suggesting that they contributed to short-term hematopoietic function in these mice (Figure 1A). 2) When USCs were mixed with whole bone marrow from wild-type mice and transplanted, the recipient survival rate reached 95%, which was higher than the 80% survival rate reached by recipients that received whole bone marrow cells only. Eight weeks later, approximately 33-42% of cells in the blood, bone marrow, and spleen of recipient mice that received a mixture of UCs and whole bone marrow were GFP-positive, indicating the presence of donor USCs. Importantly, these GFP-positive cells in the bone marrow and spleen were found to express the hematopoietic marker CD45 and the markers typically associated with hematopoietic progenitors or stem cell populations, including Sca-1, CD34 and AC-133. This suggests that the transplanted USCs contributed to the restoration of the hematopoietic cell lineage of the recipient. 3) Another defining characteristic of HSCs is their potential for long-term self-renewal. This capability is typically tested by secondary reconstitution (Figure 1B) in which stem cells obtained from a previously-reconstituted individual are transplanted into another lethally-irradiated recipient. To determine whether USCs possess this HSC-like trait, we injected BM cells isolated from mice that were previously reconstituted with a mixture of GFP-positive USCs and whole bone marrow cells into lethally-irradiated wild-type mice. Importantly, GFP expression was detected in the blood, BM, and spleen of the secondary recipients for at least 9 weeks, suggesting that uterine tissue contains cells with an HSC-like ability to establish long-term self-renewal.
Conclusions: This study has demonstrated that USCs are multi-potent stem cells and shown that the uterus contains HSC-like stem cells that are capable of reconstituting the bone marrow of irradiated recipients. Cells from the uterus may represent a potent novel source of stem cells that could be used in future cell therapies.
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This research was supported by a grant from the Canadian Institutes of Health Research (MOP86661) awarded to R-KL.
Ren-Ke Li, MD, PhD Senior Scientist
University Health Network
MaRS Centre, Toronto Medical Discovery Tower Room 3-702 101 College Street Toronto, Ontario, Canada M5G 1L7 firstname.lastname@example.org