Summary

Successful reprogramming of differentiated human somatic cells into a pluripotent state would allow creation of patient- and disease-specific stem cells. We previously reported generation of induced pluripotent stem (iPS) cells, capable of germline transmission, from mouse somatic cells by transduction of four defined transcription factors. Here, we demonstrate the generation of iPS cells from adult human dermal fibroblasts with the same four factors: Oct3/4, Sox2, Klf4, and c-Myc. Human iPS cells were similar to human embryonic stem (ES) cells in morphology, proliferation, surface antigens, gene expression, epigenetic status of pluripotent cell-specific genes, and telomerase activity. Furthermore, these cells could differentiate into cell types of the three germ layers in vitro and in teratomas. These findings demonstrate that iPS cells can be generated from adult human fibroblasts.

Embryonic stem (ES) cells, derived from the inner cell mass of mammalian blastocysts, have the ability to grow indefinitely while maintaining pluripotency (Evans and Kaufman, 1981andMartin, 1981). These properties have led to expectations that human ES cells might be useful to understand disease mechanisms, to screen effective and safe drugs, and to treat patients of various diseases and injuries, such as juvenile diabetes and spinal cord injury (Thomson etal., 1998). Use of human embryos, however, faces ethical controversies that hinder the applications of human ES cells. In addition, it is difficult to generate patient- or disease-specific ES cells, which are required for their effective application. One way to circumvent these issues is to induce pluripotent status in somatic cells by direct reprogramming (Yamanaka, 2007).

We showed that induced pluripotent stem (iPS) cells can be generated from mouse embryonic fibroblasts (MEF) and adult mouse tail-tip fibroblasts by the retrovirus-mediated transfection of four transcription factors, namely Oct3/4, Sox2, c-Myc, and Klf4 (Takahashi and Yamanaka, 2006). Mouse iPS cells are indistinguishable from ES cells in morphology, proliferation, gene expression, and teratoma formation. Furthermore, when transplanted into blastocysts, mouse iPS cells can give rise to adult chimeras, which are competent for germline transmission (Maherali etal., 2007, Okita etal., 2007andWernig etal., 2007). These results are proof of principle that pluripotent stem cells can be generated from somatic cells by the combination of a small number of factors.

In the current study, we sought to generate iPS cells from adult human somatic cells by optimizing retroviral transduction in human fibroblasts and subsequent culture conditions. These efforts have enabled us to generate iPS cells from adult human dermal fibroblasts and other human somatic cells, which are comparable to human ES cells in their differentiation potential in vitro and in teratomas.

Induction of iPS cells from mouse fibroblasts requires retroviruses with high transduction efficiencies (Takahashi and Yamanaka, 2006). We, therefore, optimized transduction methods in adult human dermal fibroblasts (HDF). We first introduced green fluorescent protein (GFP) into adult HDF with amphotropic retrovirus produced in PLAT-A packaging cells. As a control, we introduced GFP to mouse embryonic fibroblasts (MEF) with ecotropic retrovirus produced in PLAT-E packaging cells(Morita etal., 2000). In MEF, more than 80% of cells expressed GFP (FigureS1). In contrast, less than 20% of HDF expressed GFP with significantly lower intensity than in MEF. To improve the transduction efficiency, we introduced the mouse receptor for retroviruses, Slc7a1 (Verrey etal., 2004) (also known as mCAT1), into HDF with lentivirus. We then introduced GFP to HDF-Slc7a1 with ecotropic retrovirus. This strategy yielded a transduction efficiency of 60%, with a similar intensity to that in MEF.

The protocol for human iPS cell induction is summarized inFigure1A. We introduced the retroviruses containing human Oct3/4, Sox2, Klf4, and c-Myc into HDF-Slc7a1 (Figure1B; 8 105 cells per 100 mm dish). The HDF derived from facial dermis of 36-year-old Caucasian female. Six days after transduction, the cells were harvested by trypsinization and plated onto mitomycin C-treated SNL feeder cells (McMahon and Bradley, 1990) at 5 104 or 5 105 cells per 100 mm dish. The next day, the medium (DMEM containing 10% FBS) was replaced with a medium for primate ES cell culture supplemented with 4 ng/ml basic fibroblast growth factor (bFGF).

Induction of iPS Cells from Adult HDF

(A) Time schedule of iPS cell generation.

(B) Morphology of HDF.

(C) Typical image of non-ES cell-like colony.

(D) Typical image of hES cell-like colony.

(E) Morphology of established iPS cell line at passage number 6 (clone 201B7).

(F) Image of iPS cells with high magnification.

(G) Spontaneously differentiated cells in the center part of human iPS cell colonies.

(HN) Immunocytochemistry for SSEA-1 (H), SSEA-3 (I), SSEA-4 (J), TRA-1-60 (K), TRA-1-81 (L), TRA-2-49/6E (M), and Nanog (N). Nuclei were stained with Hoechst 33342 (blue). Bars = 200 m (BE, G), 20 m (F), and 100 m (HN).

Approximately two weeks later, some granulated colonies appeared that were not similar to hES cells in morphology (Figure1C). Around day 25, we observed distinct types of colonies that were flat and resembled hES cell colonies (Figure1D). From 5 104 fibroblasts, we observed 10 hES cell-like colonies and 100 granulated colonies (7/122, 8/84, 8/171, 5/73, 6/122, and 11/213 in six independent experiments, summarized in Table S1). At day 30, we picked hES cell-like colonies and mechanically disaggregated them into small clumps without enzymatic digestion. When starting with 5 105 fibroblasts, the dish was nearly covered with more than 300 granulated colonies. We occasionally observed some hES cell-like colonies in between the granulated cells, but it was difficult to isolate hES cell-like colonies because of the high density of granulated cells. The nature of the non-hES-like cells remains to be determined.

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