Profile of John Gurdon and Shinya Yamanaka, 2012 Nobel Laureates in Medicine or Physiology

We celebrate the 2012 Nobel Prize for Medicine awarded to Sir John Gurdon and Shinya Yamanaka for their groundbreaking contributions to the field of cell reprogramming. In 1962, in a series of experiments inspired by Briggs and King (1), Gurdon demonstrated that the nucleus of a frog somatic cell could be reprogrammed to behave like the nucleus of a fertilized frog egg (2). By inserting the nuclei of intestinal epithelial cells into enucleated eggs, Gurdon was able to create healthy swimming tadpoles. These experiments were the first successful instances of somatic cell nuclear transfer (SCNT) using genetically normal cells. In 2006, Yamanaka made a further conceptual leap. With four defined transcription factors he induced intact mouse somatic cells to revert to a pluripotent state without an egg or embryo as intermediary (3). These cells, dubbed induced pluripotent stem (iPS) cells, have the capacity to turn into all of the cells of the adult mouse. This result, now replicated with human cells, has enormous implications for basic research, clinical medicine, and reproduction. In the 44-year-long gap between Gurdon and Yamanaka’s respective discoveries, recombinant DNA technology emerged, SCNT was successfully performed in mammals, and embryonic stem (ES) cell research took flight.

Sir John B. Gurdon (Left) and Shinya Yamanaka (Right) during an interview with Nobelprize.org on December 6, 2012. Image Copyright Nobel Media AB 2012/Niklas Elmehed.

In the early 1960s there were divergent views about how a one-cell fertilized animal egg could give rise to the more than 200 different cell types that comprise the adult body. It was obvious that germ cells safeguarded the full complement of genes but what about somatic cells? One prevailing view was that redundant genes could be lost or permanently inactivated as somatic cells became specialized for certain activities. For example, a muscle cell might lose the genes for neural activity or bone formation. A competing view, the one that Gurdon’s experiments in the frog ultimately confirmed, was that somatic cells retained a full retinue of genes and it was selective gene expression that accounted for cell fate choices. Although the initial “cloning” experiments generated swimming tadpoles, it wasn’t until Gurdon showed that these tadpoles could mature into fertile adults (4) that it became clear that the frog somatic cell nucleus contained all of the genes needed for full development. The technical inefficiencies of his experiments begged the question of whether the frogs created by Gurdon using SCNT in fact arose from unspecialized cell donors present in the epithelial population. Such doubts were dispelled when skin nuclei that were demonstrably specialized were used successfully by Gurdon et al. in an exacting series of experiments performed in the early 1970s, when I was a doctoral student in the laboratory (5). Thus, as well as yielding profound insight into mechanisms of development, these frog experiments also proved that the genome of at least some specialized cell types could be reprogrammed to a totipotent state. Clearly, the frog egg cytoplasm contained factors capable of orchestrating the necessary changes in an incoming nucleus. Interestingly, nuclei from adult frog cells, unlike those taken from tadpoles, were never able to generate viable adult progeny.

Nuclear transfer experiments by others in rabbits, mice, cows, and sheep followed Gurdon’s initial work with frogs. Many animals were generated but none through the use of somatic cell donors. Nevertheless, these experiments laid the groundwork for a breakthrough in 1996 when Campbell et al. in Edinburgh reported the generation of two lambs by transfer of nuclei from an established, differentiated cell line derived from nine-day-old sheep embryos (6). This same technology was used one year later in a collaboration between my group and the Edinburgh team using adult mammary cells. The birth of Dolly the Sheep proved that mammalian clones could be made from adult cell nuclei (7).

The series of reprogramming successes that Gurdon’s work inspired involved radical disruption to cell integrity; this was also true of the reprogramming seen when embryonic stem cells were fused to other differentiated cell types (8). Restoring the pluripotent state in an intact differentiated cell seemed an altogether more daunting proposition, but not to Yamanaka. Armed with knowledge of ES cell biology, the history of frog and mammalian SCNT, and the demonstration in 1987 by Davis et al. (9) that the enforced expression of a single, added transcription factor (TF) gene could change fibroblasts into muscle-like cells, Yamanaka set out to reprogram an intact differentiated somatic cell back to the pluripotent state. His “pluripotent” reference point was the ES cell. Transcriptional analysis of mouse ES cells allowed him to compile a hit-list of 24 TFs whose expression seemed to be associated with the maintenance of the pluripotent state. All these TF genes were assembled into retrovirus vectors and introduced into mouse fibroblasts in various combinations, ranging from individual factors to the complete set of 24. The infected cells were cultured in conditions known to support ES cell growth. This experimental approach struck many scientists working in the area as utterly naive. Extraordinarily, it worked and four TF genes were identified that, in combination, reprogrammed the fibroblasts into iPS cells. These first iPS cells had similar although not identical properties to murine ES cells. Subsequently, using improved assays, Yamanaka and colleagues (10), as well as other groups, were able to confirm that the canonical set of four TFs could not only reprogram fibroblasts to an ES-like pluripotent state, but also that the same procedure would work on many different adult cell types from mice and humans.

It is salutary to note that in an era of intense competition between scientists, Gurdon and Yamanaka’s place in Nobel history is neither begrudged nor disputed. The impact of their respective contributions to the field of reprogramming is clear and will endure. However, our knowledge of the reprogramming process remains patchy; for example, it is unclear how much overlap, if any, there is between the reprogramming triggered by SCNT and that seen using defined factors. Back to the bench!