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(Circulation. 2004;110:3803-3807.)
© 2004 American Heart Association, Inc.
Heart Failure |
From the Departments of Cardiology (S.Z., S.R., E.T.H.Y.) and Bioimmunotherapy (Z.E.), University of Texas-M.D. Anderson Cancer Center; the Research Center for Cardiovascular Diseases, Institute of Molecular Medicine for the Prevention of Human Diseases, University of Texas-Houston Health Science Center (D.W., J.T.W., E.T.H.Y.); and the Texas Heart Institute/St. Lukes Episcopal Hospital (J.T.W., E.T.H.Y.), Houston, Tex.
Correspondence to Edward T.H. Yeh, MD, University of Texas-M.D. Anderson Cancer Center, Faculty Bldg, Room 2036, 3rd Floor, Box 449, 1515 Holcombe Blvd, Houston, TX 77030. E-mail etyeh{at}mdanderson.org
Received June 2, 2004; revision received September 1, 2004; accepted September 21, 2004.
| Abstract |
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Methods and Results We performed flow cytometry analyses of cells isolated from the hearts of mice that received human CD34+ cells. Human HLA-ABC antigen and cardiac troponin T or Nkx2.5 were used as markers for cardiomyocytes derived from human CD34+ cells, and HLA-ABC and VE-cadherin were used to identify the transformed endothelial cells. The double-positive cells were collected and interphase fluorescence in situ hybridization was used to detect the expression of human and mouse X chromosomes in these cells. We found that 73.3% of nuclei derived from HLA+ and troponin T+ or Nkx2.5+ cardiomyocytes contain both human and mouse X chromosomes and 23.7% contain only human X chromosome. In contrast, the nuclei of HLA, troponin T+ cells contain only mouse X chromosomes. Furthermore, 97.3% of endothelial cells derived from CD34+ cells contained human X chromosome only.
Conclusions Thus, both cell fusion and transdifferentiation may account for the transformation of peripheral blood CD34+ cells into cardiomyocytes in vivo.
Key Words: antigens, CD34 stem cell cardiomyocyte cell fusion
| Introduction |
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| Methods |
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Induction of Myocardial Infarction and CD34+ Cell Transplantation
Experimental myocardial infarction (MI) was induced via a method described previously.11 Two million human peripheral CD34+ cells were injected into the left ventricles of mice (n=7) 30 to 40 minutes after MI was induced. Control mice (n=3) received the same MI procedure but no cell transplantation.
Isolation of CD34+ Cells From Human Peripheral Blood
We performed CD34+ fractionation using immunomagnetic beads as previously described.11,15
Tissue Harvesting and Cell Isolation
The hearts were harvested 60 days after cell transplantation or MI (control animals). A published method16 was modified slightly and used to isolate cells from the heart. Briefly, the heart was cut into 6 pieces and placed in a 10-mL beaker containing 2 mL of the enzymatic solution, ADS buffer containing elastase (0.3 mg/mL, Worthington Biochemical), type II collagenase (0.21 mg/mL, Worthington Biochemical), and pancreatin (0.6 mg/mL, Sigma). The tissues were then incubated repeatedly at 37°C for 15 minutes each time. The isolated cells were filtered through a cell strainer with a pore size of 70 µm, fixed (20 minutes at 4°C) with the Cytofix/Cytoperm kit (BD Biosciences Pharmingen), and stored at 4°C overnight before fluorescence-activated cell sorter (FACS) analysis.
Flow Cytometry Detection and Cell Sorting
The filtered cells were permeabilized with the Cytoperm wash kit (BD Biosciences Pharmingen) for 20 minutes at 4°C and incubated with a monoclonal antibody against cardiac troponin T (1:200, clone 1A11, Advanced ImmunoChemical, Long Beach, Calif) or with nonspecific mouse IgG-2b for 30 minutes at 4°C. After 3 washes, the cells were incubated with goat anti-mouse IgG (1:1000) conjugated with Alexa Fluor 488 (Molecular Probes) for 30 minutes at 4°C. In a separate set of experiments, mouse heart cells were incubated with a polyclonal antibody (goat, Santa Cruz Biotechnology, Santa Cruz, Calif) against a cardiac-specific transcription factor, Nkx2.5, or with nonspecific goat IgG for 30 minutes at 4°C. Cells were washed and incubated with chicken anti-goat IgG (1:1000) conjugated with Alexa Fluor 488 for 30 minutes at 4°C. After 3 washes, cells incubated with anticardiac troponin or with anti-Nkx2.5 were incubated again with phycoerythrin (PE)-conjugated anti-human HLA-ABC (clone W6/32, Cedarlane Laboratories, Homby, Ontario, Canada) or PE-conjugated mouse IgG-2a for 30 minutes. For endothelial cell detection, isolated cells were incubated first with antivascular endothelium (VE)cadherin (1:100, Bender MedSystems, Calif), then with the secondary antibody conjugated with Alexa Fluor 488. Cells were analyzed and sorted on a FACSAria flow cytometer (BD Biosciences). Gates were established by nonspecific immunoglobulin binding in each experiment. Approximately 30% to 40% of the entire population was sorted for the double-positive cells.
Polymerase Chain Reaction Analysis of Sorted Cells
Genomic DNA was isolated from heart cells sorted by expression of cardiac troponin T or by dual expression of human HLA-ABC and cardiac troponin T and was used to detect HLA expression by polymerase chain reaction (PCR). PCR analysis was performed with the primers derived from HLA-B exon 57 with the Advantage cDNA PCR kit (BD Biosciences Clontech). The following touchdown PCR program was used: 3 minutes at 94°C; 5 cycles of 30 seconds each at 94°C, 40 seconds each at 66°C, and 50 seconds each at 72°C; 30 cycles of 30 seconds each at 94°C, 40 seconds each at 64°C, and 50 seconds each at 72°C; and 1 cycle of 5 minutes at 72°C. The PCR products were analyzed by electrophoresis of 1% agarose gel, and the size of PCR amplicon derived from the HLA-B gene was
280 bp.
Interphase Fluorescence In Situ Hybridization
The collected double-positive cells were spun onto a slide and fixed immediately with 3:1 methanol:acetic acid solution for 30 minutes. To quench the residual fluorescence from cell sorting, the slides were exposed to white light for 120 hours at 4°C. Complete diminishment of the residual fluorescence was confirmed by examination under an epifluorescence microscope (Nikon Eclipse TE2000U). Slides were briefly fixed in 3:1 methanol:acetic acid again and were predenatured, dehydrated, and denatured according to the manufacturers protocol. Slides were hybridized with a fluorescein isothiocyanateconjugated DNA probe for mouse X chromosomes (ID Labs) and a PE-conjugated probe for human X chromosomes (Qbiogene) overnight at 37°C in a humidified chamber. After posthybridization wash, slides were counterstained with 4',6-diamidino-2-phenylindole (0.02 µg/mL) and examined with an epifluorescence microscope (Nikon Eclipse TE2000U).
| Results |
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1% of cells were double-positive, suggesting that these cardiomyocytes originated from the transplanted human cells. To ensure that immunostaining accurately reflects the genotype of isolated cells, DNA from the sorted double-positive cells and the cells stained only with anticardiac troponin was extracted for PCR detection of the human HLA-B gene. HLA-B fragment was amplified only in double-positive cells (Figure 1D). Thus, it is clear that the double-positive cells are of human origin.
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Interphase Fluorescence In Situ Hybridization Analysis of Cardiomyocytes Developed From Transplanted CD34+ Cells
The population of double-positive cells was collected by cell sorting and examined with fluorescence in situ hybridization (FISH) analysis, in which specific probes for human and mouse X chromosomes were used simultaneously. The specificity of the probes was tested in mouse heart cells and human Hela cells by incubating these cells with both probes, and we confirmed that these 2 probes did not cross-react (Figure 2A). In the nuclei derived from cells that were troponin T+ but HLA, only mouse X chromosomes were detected (Figure 2B). Because the recipient mice were female, 2 X chromosomes were observed in each nucleus. In troponin T+ and HLA+ cells,
70% of the nuclei contained both human and mouse X chromosomes (Table 1, Figure 2C), suggesting that cell fusion had occurred. Because the human donor is male, 1 human X chromosome was paired with 2 mouse X chromosomes in each nucleus (Figure 2C); however,
30% of the nuclei of troponin T+, HLA+ cells contained only human X chromosomes (Table 1, Figure 2D), suggesting that transdifferentiation of CD34+ cells has also taken place. Analysis was also performed on HLA and Nkx2.5 sorted cells in 3 mice and similar findings were obtained (Table 1); however, only human X chromosomes were detected in
97% of cells stained positive to both antiHLA-ABC and antiVE-cadherin (Table 2).
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| Discussion |
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In our study, we used an animal model in which SCID mice were transplanted with human peripheral blood CD34+ cells after the mouse hearts were injured by experimental MI. The specific antigen (human HLA-ABC) of the donor cells allowed us to track these cells accurately and rapidly using immunodetection methods, such as FACS analysis. Instead of evaluating tissue sections of the heart, we were able to examine quantitatively the entire heart and collect populations of cells of interest by using FACS sorting. Our approach, therefore, enables us to focus on an individual cell population of the heart and to perform further analysis, such as FISH, on the population.
Alvarez-Dolado et al10 reported that cell fusion in a mouse model was responsible for transformation of bone marrowderived cells into hepatocytes, Purkinje neurons, and cardiomyocytes. No transdifferentiation was observed in their study. Beltrami et al14 demonstrated that adult cardiac stem cells differentiated into mature cardiomyocytes by transdifferentiation. Our results indicated that both cell fusion (73.3%) and transdifferentiation (23.7%) were involved in the transformation of transplanted stem cells into cardiomyocytes. The discrepancies between our results and those from the above 2 research groups could come from differences in the presence or absence of heart injury. The animals used in the study by Alvarez-Dolado et al were uninjured,10 and the frequency of transdifferentiation was expected to be extremely low11; alternatively, the results might relate to the number of cells analyzed. Both the Beltrami and Alvarez-Dolado groups analyzed tissue sections. In the study by Alvarez-Dolado and colleagues, 25 heart sections of a total thickness of 0.25 to 1.25 mm, which represented only a small portion of the heart, were analyzed.10 The methods used to detect cell fusion might explain the differences in study results. Alvarez-Dolados team used a method based on Cre/lox recombination to detect fusion.10 Beltrami et al14 excluded the existence of cell fusion based on the number, location, size, and DNA content of the transplanted cells. In our study, the detection method was highly specific because we used FISH to detect specific human X chromosomes in a population of human cellderived cardiomyocytes selected by FACS. Finally, the sources of transplanted stem cells could explain the differences that were found. In Alvarez-Dolado and colleagues study, hematopoietic cells from the bone marrow transformed into cardiomyocytes via fusion. Beltrami et al used adult cardiac stem cells. We used human peripheral blood CD34+ cells. It is unclear, however, whether stem cells from different origins convert into the cells of recipient phenotype through different mechanisms. Oh et al have reported both fusion and transdifferentiation in a mouse model of ischemia reperfusion injury followed by the injection of Sca-1+ cells.13 Our experimental approach is closer to that of Oh et al and our results also are similar to theirs. We found that 97.3% of the endothelial cells derived from human CD34+ cells contained only human X chromosomes. This is not surprising because the CD34+ cell population contains endothelial progenitors that can differentiate into the mature endothelial cells naturally. Two groups have reported failures to observe transformation of bone marrow stem cells into cardiomyocytes in animal models.17,18 The reason for their inability to detect cardiomyocyte transformation is not clear, but it could be related to differences in cell origin, cell preparation, and detection methodology.
In conclusion, our results suggest that human peripheral blood CD34+ cells develop into cardiomyocytes in the injured hearts of SCID mice through both cell fusion and transdifferentiation. A number of groups have already embarked on the application of human bone marrow mesenchymal stem cells to repair damaged myocardium.1921 Our studies, thus, provide mechanistic insight on how human stem cells can transform into cardiomyocytes in vivo.
| Footnotes |
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| References |
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