Induced pluripotency: history, mechanisms, and applications

Abstract

The generation of induced pluripotent stem cells (iPSCs) from somatic cells demonstrated that adult mammalian cells can be reprogrammed to a pluripotent state by the enforced expression of a few embryonic transcription factors. This discovery has raised fundamental questions about the mechanisms by which transcription factors influence the epigenetic conformation and differentiation potential of cells during reprogramming and normal development. In addition, iPSC technology has provided researchers with a unique tool to derive disease-specific stem cells for the study and possible treatment of degenerative disorders with autologous cells. In this review, we summarize the progress that has been made in the iPSC field over the last 4 years, with an emphasis on understanding the mechanisms of cellular reprogramming and its potential applications in cell therapy.

Figure 1.

Models of cellular reprogramming. (A) Mature cells, such as lymphocytes, reprogram into iPSCs at lower efficiencies than immature cells, such as hematopoietic stem cells. This may be due to a lower number of stochastic epigenetic events (represented by circled numbers and arrows) that are required in immature cells to acquire pluripotency. The precise number and nature of such changes is unclear (represented by “n”). (B) Scheme summarizing major changes that characterize the transition of somatic cells into iPSCs. The early steps are reversible, as indicated by the dashed reverse arrows. “Immature iPSCs” are defined as cells that have already acquired pluripotency but still retain an epigenetic memory of their cell type of origin, while “mature iPSCs” have lost this memory. The wavelines below indicate assumed reprogramming roadblocks that cells are facing at different stages. Failure to pass any of these roadblocks may result in cells that arrest at that stage or, alternatively, undergo senescence or apoptosis.

Figure 2.

Putative role of reprogramming factors during iPSC formation. (A) Scheme depicting the expression of exogenous (red circles) and endogenous (dark-green circles) pluripotency factors at the protein level during different stages of reprogramming. The reprogramming process is initiated predominantly by the exogenous factors, which are gradually replaced by endogenous proteins as well as their targets, such as Nanog (N) or as-yet-unidentified factors (X) (light-green circles). The endogenous loci of some reprogramming factors (such as c-Myc, Klf4, and Sox2) are expressed in some somatic cell types, and the corresponding endogenous proteins might thus become available before activation of the Oct4 locus. (B) Scheme illustrating how the reprogramming factors may exert the rapid repression of somatic genes and the gradual activation of pluripotency (ESC) genes, two processes assumed to be mediated largely by Klf4, Sox2, and Oct4. Somatic gene silencing is associated by single-factor binding to promoter regions, while ESC gene activation involves the establishment of multiprotein complexes. The initial loss of repressive marks (such as DNA methylation and H3K27 histone trimethylation) at ESC promoters might be a passive process driven by multiple rounds of cell division. (C) Scheme showing activation of genes promoting cell division (such as cyclins) by c-Myc and repression of the Ink4a/Arf tumor suppressor locus conferring immortality by an as-yet-undefined combination of reprogramming factors.

Figure 3.

Transitions between alternative pluripotent states. (A) Model showing early developmental stages of the mouse embryo, from zygote to blastocyst and, subsequently, to post-implantation epiblast. ESCs are derived from the ICM (orange crescent) of the blastocyst and require LIF and BMP4 for indefinite self-renewal in vitro (indicated by the curved red arrow). EpiSCs are derived from epiblast stage embryos and require bFGF and activin for their propagation. ESCs readily differentiate into EpiSCs upon the switch to appropriate culture conditions, while the reverse transition is rare but can be enhanced significantly by enforced expression of Klf4. (B) Explant cultures of blastocysts from nonpermissive mouse strains (such as NOD) do not give rise to stable ESC lines in LIF and BMP4. However, a metastable ESC-like state can be attained by forced expression of Klf4 and c-Myc, or by repression of both MAPK and GSK3 signaling (“2i condition”). (C) Human ESCs resemble mouse EpiSCs in their epigenetic configuration (one inactive X chromosome [Xi], and one active X chromosome [Xa]) and marker gene expression (such as Fgf5). A metastable murine ESC-like state can be induced in these cells by overexpression of KLF2, KLF4, and OCT4. Active genes are shown in green, and inactive genes are shown in red.

Figure 4.

Potential applications of iPSCs. Shown are the potential applications of iPSC technology for cell therapy and disease modeling using SMA as an example. In SMA patients, motor neurons are afflicted and die, causing the devastating symptoms of the disease. SMA-specific iPSCs could be coaxed into motor neurons in vitro in order to establish a culture model of the disease that may lead to the identification of novel drugs that prevent the abnormal death of motor neurons in patients. Alternatively, if known, the disease-causing mutation could be repaired (in this case the SMA gene) in iPSCs by gene targeting prior to their differentiation into healthy motor neurons, followed by transplantation into the patient's brain.