Induced Pluripotent Stem Cell

A somatic cell that has been reprogrammed to exhibit pluripotent stem cell properties. Many somatic cells have been successfully reprogrammed; however, fibroblast cells are commonly utilized because they can be easily extracted from a patient using a safe and non-invasive skin biopsy and are easy to culture in a lab.

The successful creation of induced pluripotent stem cells proved to the world that cellular differentiation is not a unidirectional process--with the proper instruction, cellular differentiation is bidirectional. However, the hypothesis that cells only differentiate unidirectionally towards terminal differentiation was first challenged in 1962 by John Gurdon's pioneering work in nuclear transfer and also with the historic birth of Dolly the Sheep in February of 1997 via somatic cell nuclear transfer (SCNT). The success of SCNT stimulated developmental biologists to begin exploring the possibility of creating pluripotent stem cells (e.g. induced pluripotent stem cells) by reprogramming fully differentiated somatic cells to an embryonic-like state.

The groundbreaking discovery came in 2006 when Kazutoshi Takahashi and Shinya Yamanaka demonstrated that mouse fibroblasts can be reprogrammed to pluripotent "embryonic like" stem cells by overexpressing specific genetic factors. The team hypothesized that a select group of 24 pluripotency-related genes, when over-expressed in mouse somatic cells, can induce pluripotency. Of the 24 genes screened, only four were necessary to reprogram mouse fibroblasts into pluripotent stem cells—otherwise known as induced pluripotent stem (iPS) cells. The Yamanaka team determined the essential stemness genes to be Oct-4, SOX2, c-Myc, Klf-4 -- four genes that have important functions in the regulation of pluripotency in embryonic stem cells. One year later, the team reported that the same four factors are capable of inducing pluripotency in human somatic cells. This discovery revolutionized the field of stem cell science and regenerative medicine because it opened the door for development of patient-specific autologous pluripotent stem cells for clinical use.

A number of methods are used to artificially over-express genes in a cell. Stem cell scientists are also exploring the use of small molecules and other technologies to reprogram cells without the use of viral vectors. The Yamanaka group utilized retroviral vectors to deliver the transgenes encoded for the four reprogramming factors: Oct-4, SOX2, Klf-4 and c-Myc. The retroviral vectors deliver the four transgenes into the cells. The transgenes are then integrated into the host’s genome, thereby permitting long term expression of the transgenes.  If all four transgenes successfully integrate into the fibroblast’s genome, they will then begin to express the transgenes as functional proteins. The four reprogramming factors, Oct-4, SOX2, Klf-4 and c-Myc function as transcription factors, meaning that they are capable of binding to the DNA to control the transcription of a unique set of genes. Together, Oct-4, SOX2, Klf-4 and c-Myc induce the expression of genes that are not normally expressed in the fibroblast but are expressed in pluripotent stem cells. The four transcription factors continue to drive transcription of their downstream genes, leading to the activation of other transcriptional networks, inducing a cascade of transcriptional activity. Over the period of two to three weeks, the gene expression profile of the fibroblast changes and begins to express a repertoire of genes that are commonly identified in pluripotent embryonic stem cells. At approximately the same time, the reprogrammed fibroblasts undergo morphological changes. Soon after, the reprogrammed cells begin to grow as a tightly packed cluster of cells known as a colony, which mirrors how undifferentiated embryonic stem cells grow in culture. The emergence of these colonies is the first sign that the fibroblasts have been reprogrammed into induced pluripotent stem cells. From here, the iPS cell colonies are isolated and expanded so that they may be used to further our understanding of human development and mechanisms involved in many devastating human diseases.