Induced pluripotent stem cell


Induced pluripotent stem cells are a type of pluripotent stem cell that can be generated directly from a somatic cell. The iPSC technology was pioneered by Shinya Yamanaka and Kazutoshi Takahashi in Kyoto, Japan, who together showed in 2006 that the introduction of four specific genes, collectively known as Yamanaka factors, encoding transcription factors could convert somatic cells into pluripotent stem cells. Shinya Yamanaka was awarded the 2012 Nobel Prize along with Sir John Gurdon "for the discovery that mature cells can be reprogrammed to become pluripotent."
Pluripotent stem cells hold promise in the field of regenerative medicine. Because they can propagate indefinitely, as well as give rise to every other cell type in the body, they represent a single source of cells that could be used to replace those lost to damage or disease.
The best-known type of pluripotent stem cell is the embryonic stem cell. However, since the generation of embryonic stem cells involves the destruction of a pre-implantation stage embryo, there has been much controversy surrounding their use. Patient-matched embryonic stem cell lines can now be derived using somatic cell nuclear transfer.
Since iPSCs can be derived directly from adult tissues, they not only bypass the need for embryos, but can be made in a patient-matched manner, which means that each individual could have their own pluripotent stem cell line. These unlimited supplies of autologous cells could be used to generate transplants without the risk of immune rejection. While the iPSC technology has not yet advanced to a stage where therapeutic transplants have been deemed safe, iPSCs are readily being used in personalized drug discovery efforts and understanding the patient-specific basis of disease.
Yamanaka named iPSCs with a lower case "i" due to the popularity of the iPod and other Apple products.
In his Nobel seminar, Yamanaka cited the earlier seminal work of Harold Weintraub on the role of myoblast determination protein 1 in reprogramming cell fate to a muscle lineage as an important precursor to the discovery of iPSCs.

Production

iPSCs are typically derived by introducing products of specific sets of pluripotency-associated genes, or "reprogramming factors", into a given cell type. The original set of reprogramming factors are the transcription factors Oct4, Sox2, Klf4 and cMyc. While this combination is most conventional in producing iPSCs, each of the factors can be functionally replaced by related transcription factors, miRNAs, small molecules, or even non-related genes such as lineage specifiers. It is also clear that pro-mitotic factors such as C-MYC/L-MYC or repression of cell cycle checkpoints, such as p53, are conduits to creating a compliant cellular state for iPSC reprogramming.
iPSC derivation is typically a slow and inefficient process, taking one–two weeks for mouse cells and three–four weeks for human cells, with efficiencies around 0.01–0.1%. However, considerable advances have been made in improving the efficiency and the time it takes to obtain iPSCs. Upon introduction of reprogramming factors, cells begin to form colonies that resemble pluripotent stem cells, which can be isolated based on their morphology, conditions that select for their growth, or through expression of surface markers or reporter genes.

First generation (mouse)

Induced pluripotent stem cells were first generated by Shinya Yamanaka and Kazutoshi Takahashi at Kyoto University, Japan, in 2006. They hypothesized that genes important to embryonic stem cell function might be able to induce an embryonic state in adult cells. They chose twenty-four genes previously identified as important in ESCs and used retroviruses to deliver these genes to mouse fibroblasts. The fibroblasts were engineered so that any cells reactivating the ESC-specific gene, Fbx15, could be isolated using antibiotic selection.
Upon delivery of all twenty-four factors, ESC-like colonies emerged that reactivated the Fbx15 reporter and could propagate indefinitely. To identify the genes necessary for reprogramming, the researchers removed one factor at a time from the pool of twenty-four. By this process, they identified four factors, Oct4, Sox2, cMyc, and Klf4, which were each necessary and together sufficient to generate ESC-like colonies under selection for reactivation of Fbx15.

Second generation (mouse)

In June 2007, three separate research groups, including that of Yamanaka's, a Harvard/University of California, Los Angeles collaboration, and a group at MIT, published studies that substantially improved on the reprogramming approach, giving rise to iPSCs that were indistinguishable from ESCs. Unlike the first generation of iPSCs, these second generation iPSCs produced viable chimeric mice and contributed to the mouse germline, thereby achieving the 'gold standard' for pluripotent stem cells.
These second-generation iPSCs were derived from mouse fibroblasts by retroviral-mediated expression of the same four transcription factors. However, instead of using Fbx15 to select for pluripotent cells, the researchers used Nanog, a gene that is functionally important in ESCs. By using this different strategy, the researchers created iPSCs that were functionally identical to ESCs.

Human induced pluripotent stem cells

Generation from human fibroblasts

Reprogramming of human cells to iPSCs was reported in November 2007 by two independent research groups: Shinya Yamanaka of Kyoto University, Japan, who pioneered the original iPSC method, and James Thomson of University of Wisconsin-Madison who was the first to derive human embryonic stem cells. With the same principle used in mouse reprogramming, Yamanaka's group successfully transformed human fibroblasts into iPSCs with the same four pivotal genes, Oct4, Sox2, Klf4, and cMyc, using a retroviral system, while Thomson and colleagues used a different set of factors, Oct4, Sox2, Nanog, and Lin28, using a lentiviral system.

Generation from additional cell types

Obtaining fibroblasts to produce iPSCs involves a skin biopsy, and there has been a push towards identifying cell types that are more easily accessible. In 2008, iPSCs were derived from human keratinocytes, which could be obtained from a single hair pluck. In 2010, iPSCs were derived from peripheral blood cells, and in 2012, iPSCs were made from renal epithelial cells in the urine.
Other considerations for starting cell type include mutational load, time it takes to expand the population of starting cells, and the ability to differentiate into a given cell type.

Genes used to produce iPSCs

The generation of induced pluripotent cells is crucially dependent on the transcription factors used for the induction.
Oct-3/4 and certain products of the Sox gene family have been identified as crucial transcriptional regulators involved in the induction process whose absence makes induction impossible. Additional genes, however, including certain members of the Klf family, the Myc family, Nanog, and LIN28, have been identified to increase the induction efficiency.
  • Oct-3/4 Oct-3/4 is one of the family of octamer transcription factors, and plays a crucial role in maintaining pluripotency. The absence of Oct-3/4 in Oct-3/4+ cells, such as blastomeres and embryonic stem cells, leads to spontaneous trophoblast differentiation, and presence of Oct-3/4 thus gives rise to the pluripotency and differentiation potential of embryonic stem cells. Various other genes in the "Oct" family, including Oct-3/4's close relatives, Oct1 and Oct6, fail to elicit induction, thus demonstrating the exclusiveness of Oct-3/4 to the induction process. Schöler showed that Oct4 overexpression during reprogramming causes epigenetic changes deteriorating the quality of iPSCs. Comparing to OSKM new SKM reprogramming generates iPSCs with developmental potential equivalent to embryonic stem cell, as determined by their ability to generate all-iPSC mice through tetraploid embryo complementation. iPSCs with higher developmental potential could also be generated by enhancing dimerization between Oct4 and Sox2 using a chimeric Sox factor.
  • Sox family: The Sox family of transcription factors is associated with maintaining pluripotency similar to Oct-3/4, although it is associated with multipotent and unipotent stem cells in contrast with Oct-3/4, which is exclusively expressed in pluripotent stem cells. While Sox2 was the initial gene used for induction by Yamanaka et al., Jaenisch et al., and Thomson et al., other transcription factors in the Sox family have been found to work as well in the induction process. Sox1 yields iPSCs with a similar efficiency as Sox2, and genes Sox3, Sox15, and Sox18 also generate iPSCs, although with decreased efficiency. Velychko et al. engineered a chimeric super-reprogramming factor, Sox2-17 or "super-Sox", which enhanced or allowed generation of mouse, human, cynomolgus monkey, porcine and bovine iPSCs.
  • Klf family: Klf4 of the Klf family of transcription factors was initially identified by Yamanaka et al. and confirmed by Jaenisch et al. As a factor for the generation of mouse iPS cells and was demonstrated by Yamanaka et al. as a factor for generation of human iPS cells. However, Thomson et al. reported that Klf4 was unnecessary for generation of human iPS cells and in fact failed to generate human iPS cells. Klf2 and Klf4 were found to be factors capable of generating iPS cells, and related genes Klf1 and Klf5 did as well, although with reduced efficiency.
  • Myc family: The Myc family of transcription factors are proto-oncogenes implicated in cancer. Yamanaka et al. and Jaenisch et al. demonstrated that c-myc is a factor implicated in the generation of mouse iPS cells and Yamanaka et al. demonstrated it was a factor implicated in the generation of human iPS cells. However, Thomson et al., Yamanaka et al. usage of the "myc" family of genes in induction of iPS cells is troubling for the eventuality of iPS cells as clinical therapies, as 25% of mice transplanted with c-myc-induced iPS cells developed lethal teratomas. N-myc and L-myc have been identified to induce instead of c-myc with similar efficiency.
  • Nanog: In embryonic stem cells, Nanog, along with Oct-3/4 and Sox2, is necessary in promoting pluripotency. Therefore, it was surprising when Yamanaka et al. reported that Nanog was unnecessary for induction although Thomson et al. has reported it is possible to generate iPS cells with Nanog as one of the factors.
  • LIN28: LIN28 is an mRNA binding protein expressed in embryonic stem cells and embryonic carcinoma cells associated with differentiation and proliferation. Thomson et al. demonstrated that LIN28 is a factor in iPSC generation in combination with OCT4, SOX2, and NANOG.
  • Glis1: Glis1 is transcription factor that can be used with Oct-3/4, Sox2 and Klf4 to induce pluripotency. It poses numerous advantages when used instead of C-myc.