10. The Promise of Induced Pluripotent Stem Cells (iPSCs …

Charles A. Goldthwaite, Jr., Ph.D.

In 2006, researchers at Kyoto University in Japan identified conditions that would allow specialized adult cells to be genetically "reprogrammed" to assume a stem cell-like state. These adult cells, called induced pluripotent stem cells (iPSCs), were reprogrammed to an embryonic stem cell-like state by introducing genes important for maintaining the essential properties of embryonic stem cells (ESCs). Since this initial discovery, researchers have rapidly improved the techniques to generate iPSCs, creating a powerful new way to "de-differentiate" cells whose developmental fates had been previously assumed to be determined.

Although much additional research is needed, investigators are beginning to focus on the potential utility of iPSCs as a tool for drug development, modeling of disease, and transplantation medicine. The idea that a patient's tissues could provide him/ her a copious, immune-matched supply of pluripotent cells has captured the imagination of researchers and clinicians worldwide. Furthermore, ethical issues associated with the production of ESCs do not apply to iPSCs, which offer a non-controversial strategy to generate patient-specific stem cell lines. As an introduction to this exciting new field of stem cell research, this chapter will review the characteristics of iPSCs, the technical challenges that must be overcome before this strategy can be deployed, and the cells' potential applications to regenerative medicine.

As noted in other chapters, stem cells represent a precious commodity. Although present in embryonic and adult tissues, practical considerations such as obtaining embryonic tissues and isolating relatively rare cell types have limited the large-scale production of populations of pure stem cells (see the Chapter, "Alternate Methods for Preparing Pluripotent Stem Cells" for details). As such, the logistical challenges of isolating, culturing, purifying, and differentiating stem cell lines that are extracted from tissues have led researchers to explore options for "creating" pluripotent cells using existing non-pluripotent cells. Coaxing abundant, readily available differentiated cells to pluripotency would in principle eliminate the search for rare cells while providing the opportunity to culture clinically useful quantities of stem-like cells.

One strategy to accomplish this goal is nuclear reprogramming, a technique that involves experimentally inducing a stable change in the nucleus of a mature cell that can then be maintained and replicated as the cell divides through mitosis. These changes are most frequently associated with the reacquisition of a pluripotent state, thereby endowing the cell with developmental potential. The strategy has historically been carried out using techniques such as somatic cell nuclear transfer (SCNT),1,2 altered nuclear transfer (ANT),3,4 and methods to fuse somatic cells with ESCs5,6 (see "Alternate Methods for Preparing Pluripotent Stem Cells" for details of these approaches). From a clinical perspective, these methods feature several drawbacks, such as the creation of an embryo or the development of hybrid cells that are not viable to treat disease. However, in 2006, these efforts informed the development of nuclear reprogramming in vitro, the breakthrough method that creates iPSCs.

This approach involves taking mature "somatic" cells from an adult and introducing the genes that encode critical transcription factor proteins, which themselves regulate the function of other genes important for early steps in embryonic development (See Fig. 10.1). In the initial 2006 study, it was reported that only four transcription factors (Oct4, Sox2, Klf4, and c-Myc) were required to reprogram mouse fibroblasts (cells found in the skin and other connective tissue) to an embryonic stem celllike state by forcing them to express genes important for maintaining the defining properties of ESCs.7 These factors were chosen because they were known to be involved in the maintenance of pluripotency, which is the capability to generate all other cell types of the body. The newly-created iPSCs were found to be highly similar to ESCs and could be established after several weeks in culture.7,8 In 2007, two different research groups reached a new milestone by deriving iPSCs from human cells, using either the original four genes9 or a different combination containing Oct4, Sox2, Nanog, and Lin28.10 Since then, researchers have reported generating iPSCs from somatic tissues of the monkey11 and rat.12,13

However, these original methods of reprogramming are inefficient, yielding iPSCs in less than 1% of the starting adult cells.14,15 The type of adult cell used also affects efficiency; fibroblasts require more time for factor expression and have lower efficiency of reprogramming than do human keratinocytes, mouse liver and stomach cells, or mouse neural stem cells.1419

Several approaches have been investigated to improve reprogramming efficiency and decrease potentially detrimental side effects of the reprogramming process. Since the retroviruses used to deliver the four transcription factors in the earliest studies can potentially cause mutagenesis (see below), researchers have investigated whether all four factors are absolutely necessary. In particular, the gene c-Myc is known to promote tumor growth in some cases, which would negatively affect iPSC usefulness in transplantation therapies. To this end, researchers tested a three-factor approach that uses the orphan nuclear receptor Esrrb with Oct4 and Sox2, and were able to convert mouse embryonic fibroblasts to iPSCs.20 This achievement corroborates other reports that c-Myc is dispensable for direct reprogramming of mouse fibroblasts.21 Subsequent studies have further reduced the number of genes required for reprogramming,2226 and researchers continue to identify chemicals that can either substitute for or enhance the efficiency of transcription factors in this process.27 These breakthroughs continue to inform and to simplify the reprogramming process, thereby advancing the field toward the generation of patient-specific stem cells for clinical application. However, as the next section will discuss, the method by which transcription factors are delivered to the somatic cells is critical to their potential use in the clinic.

Figure 10.1. Generating Induced Pluripotent Stem Cells (iPSCs).

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