A method to generate human induced pluripotent stem cells (iPSCs) via retrovirus-mediated ectopic expression of OCT4, SOX2, KLF4 and MYC is described. A practical way to identify human iPSC colonies based on GFP expression is also discussed.
Human embryonic stem cells (hESCs) are pluripotent and an invaluable cellular sources for in vitro disease modeling and regenerative medicine1. It has been previously shown that human somatic cells can be reprogrammed to pluripotency by ectopic expression of four transcription factors (Oct4, Sox2, Klf4 and Myc) and become induced pluripotent stem cells (iPSCs)2-4 . Like hESCs, human iPSCs are pluripotent and a potential source for autologous cells. Here we describe the protocol to reprogram human fibroblast cells with the four reprogramming factors cloned into GFP-containing retroviral backbone4. Using the following protocol, we generate human iPSCs in 3-4 weeks under human ESC culture condition. Human iPSC colonies closely resemble hESCs in morphology and display the loss of GFP fluorescence as a result of retroviral transgene silencing. iPSC colonies isolated mechanically under a fluorescence microscope behave in a similar fashion as hESCs. In these cells, we detect the expression of multiple pluripotency genes and surface markers.
1. Reprogramming by Retrovirus Expressing Reprogramming Factors
2. Isolation and Expansion of iPSCs
3. Immunofluorescence Analysis of Pluripotent Markers
4. Quantitative Real-time PCR Assay for the Pluripotent Markers
5. Representative Results
Figure 1. Morphological changes of retrovirus-infected human fibroblasts. (A-D) Progressive morphological change in colonies from Detroit-551 fibroblasts infected with reprogramming factors. Day 5 (A), Day 10 (B), Day 14 (C), day 21 (D). Cells show the hESC-like morphology after 21 days.
Figure 2. Representative GFP fluorescent expression in cells undergoing reprogramming. BJ and Detroit 551 fibroblasts were infected with retrovirus expressing four reprogramming factors4, and incubated in hESC medium for four weeks. BJ fibroblasts (A, B) and Detroit 551 (C, D) show similar morphological. From day 21, GFP negative colonies start to form, which represent the bona fide iPSCs10. (E, F) show transformed Detroit 551 cells that have not undergone proper reprogramming. (A, C, E) colonies under phase contrast view. (B, D) properly reprogrammed cells that show the GFP silencing. (F) bright GFP expression from transformed colony.
Figure 3. Characterization of the human induced pluripotent stem cells. (A) Human 551-iPS-K1 cell colonies express markers common to pluripotent cells. DAPI staining indicates the total cell content per field. (B) Quantitative real time-PCR (RT-qPCR) for expression of OCT4, SOX2, KLF4, MYC in parental fibroblast, 551-iPS-K1 iPSCs, and H9 human embryonic stem cells (hESCs). Data were normalized against β-actin housekeeping gene and plotted relative to the expression level in the parental fibroblast cells4.
Expression of four transcription factors reprograms human fibroblasts to iPSCs. Many attempts were made to generate human iPSCs using non-integrating or non-genetic approaches to generate clinically safe iPSCs. So far, these methods show extremely low efficiency and require further optimization to improve the reproducibility11-14. Retro- or lentiviral methods are readily used to derive and apply iPSCs for human in vitro disease models, which are less dependent on the safety issues caused by viral integration. Reprogramming method described here is available for the efficient derivation of human iPSCs. Selection of human iPSCs is primarily based on colony morphology that resembles human ESCs. More importantly, our method takes advantage of the feature of silencing of the retroviral long terminal repeats (LTR) in pluripotent stem cells7,15. The retroviral vector used in this protocol contains the GFP gene linked to the reprogramming factors via an Internal Ribosome Entry Sequence (IRES) and5. GFP expression is driven by proviral LTR. The fibroblasts infected with these viruses initially show the bright GFP expression. Once fully reprogrammed, iPSCs will lose GFP expression, which is easily visualized under a fluorescence microscope. In this protocol, we described the generation of iPSCs from fetal and neonatal fibroblasts (Detroit 551 and BJ1). However, this retroviral vector has been used to generate iPSCs from normal adult fibroblasts as well as a variety of patients with Mendelian and complex disorders4,16,17.
Previously we have analyzed the change in cellular surface markers during human somatic cell reprogramming10. There is a progressive change in cell surface markers. Fibroblasts express CD13, which is repressed by the expression of reprogramming. Cells undergoing reprogramming start to express SSEA4 together with GFP. Then, they lose the expression of GFP via proviral silecing and express additional pluripotency maker TRA1-6010. The expression of TRA1-60 is well correlated with GFP silencing and the well-developed teratoma formation, suggesting that TRA1-60 is a marker for faithfully reprogrammed iPSCs. GFP silencing is the alternative marker for TRA1-60 expression and allows the identification of iPSC colonies without laborious live cell staining. Using the loss of GFP expression as a marker for iPSCs, stem cell scientists who have no prior experience in reprogramming will readily and consistently isolate iPSCs.
The authors have nothing to disclose.
This work was funded by Yale School of Medicine and Child Health Research Award from Charles Hood Foundation.
Name | Concentration | Company | Catalogue Number |
hESC medium | |||
DMEM/F12 | 80% | Invitrogen | 11330057 |
Knockout Serum Replacer | 20% | Invitrogen | 10828-028 |
L-Glutamine (200 mM) | 2 mM | Invitrogen | 25030081 |
Nonessential Amino Acids (10 mM) | 0.1 mM | Invitrogen | 11140050 |
β-Mercaptoethanol (14.3 M) or MTG | 0.1 mM | Invitrogen | M-6250 |
bFGF-2 10 μg/ml | 4 ng/ml | GIBCO/BRL | GF003AF |
Penicillin/Streptomycin | 1% | Millipore | 15140-122 |
Fiboblasts Medium | |||
DMEM | 90% | Invitrogen | 11965118 |
FBS | 10% | Invitrogen | 10407028 |
Penicillin/Streptomycin | 1% | Millipore | 15140-122 |
Table 1. Culture Medium
Name | Concentration | Company | Catalogue Number |
Antibodies | |||
OCT4 | 1:500 | Abcam | Ab19857 |
SSEA3 | 1:100 | Milipore | MAB4303 |
SSEA4 | 1:100 | BD Biosciences | BD560218 |
Tra-1-81 | 1:100 | BD Biosciences | BD560173 |
Tra-1-60 | 1:100 | BD Biosciences | BD560174 |
NANOG | 1:500 | Abcam | Ab21624 |
Alexa-Flur 488 | 1:1000 | Invitrogen | A11008 |
Alexa-Flur 555 | 1:1000 | Invitrogen | A21422 |
DAPI | 1:5000 | Invitrogen | D1306 |
Plasmids | |||
pMIG-OCT4 | Addgene | 17225 | |
pMIG-SOX2 | Addgene | 17226 | |
pMIG-KLF4 | Addgene | 17227 | |
pMIG-MYC | Addgene | 18119 | |
Other Materials | |||
Collagenase type IV | 1mg/ml | Invitrogen | 17104019 |
Gelatin, Porcine | 0.1% | Sigma | G 1890 |
Triton | 0.2% | Sigma | X100-500ML |
Paraformaldehyde | 4% | Sigma | 47608 |
BSA | 3% | American Bioanalytical | AB01800 |
MEF feeder cells | Millipore | PMEF-N | |
Cell Lifter | Corning | 3008 | |
Equipment | |||
Fluorescent microscopy: inverted microscope with GFP filter |
Table 2. Reagents and equipment.