Turning back the clock and revolutionizing a science. The 2012 Nobel Prize in Physiology or Medicine goes to Developmental Biology. John Bertrand Gurdon, Nobel Laureate (2012)

The 2012 Nobel Prize for Medicine has been awarded to Drs. John B. Gurdon and Shinya Yamanaka for their creative discoveries in nuclear reprogramming. Although a clinical product derived from reprogrammed nuclei is not yet available, these pioneers have changed the face of stem cell science and presented us with exciting possibilities for the future. In July 2006, I was an attendee at the fourth meeting of the International Society for Stem Cell Research (ISSCR) held in Toronto, Canada. Dr. Gurdon was invited to speak in the Plenary Session on Regeneration and Reprogramming. In 1962, Dr. Gurdon had published the defi nitive nuclear transfer experiments using the vertebrate model Xenopus laevis, proving that differentiated somatic cells could be reprogrammed to a totipotent state and that adult frogs arising from transplanted nuclei were capable of reproduction (Gurdon, 1962a, 1962b). This remarkable achievement proved that the DNA of a single nucleus retained the full complement of genes necessary for recreating a healthy, fertile adult when reprogrammed by the egg. A scientist not named on the invited speaker list that year was Dr. Yamanaka. Dr. Yamanaka's landmark paper demonstrating that the egg was not the only strategy for nuclear reprogramming was on the brink of publication in the journal Cell (Takahashi et al., 2006). Dr. Yamanaka was invited to present his unpublished work, which involved retroviral transduction of fibroblasts with transcription factors that are highly expressed in embryonic stem cells. This approach yielded the generation of a new stem cell type, which he called induced pluripotent stem cells (iPSCs; see image to right). For those of us who were not privy to this work, we were completely astounded at the scientific feat. I still think about that day in Canada, and remember it as being one of those rare moments in my career where I was fortunate enough to witness the origins of a paradigm shift in science. In just six short years since we were first introduced to induced reprogramming, it has become a widely used approach for generating pluripotent stem cells. Critically, iPSCs diminishes the ethical and moral barriers of destroying precious human embryos to acquire embryonic stem cell lines. The induced reprogramming technology enabled the new field of ‘disease in a dish’ to emerge, where adult cells could be removed from an individual with a disease, and after reprogramming, the developmental ontogeny of the disease could be studied. Biotech companies now sell ready-to-use, “reprogramming kits” for research purposes. Finally, the promise of manufacturing an autologous cell product to cure or ameliorate life-threatening diseases intractable with current therapy is the potential jewel in the crown of this line of research. This important and richly deserved recognition of nuclear reprogramming does not mean that the hard work is over. It also does not mean that embryonic stem cell work is finished, as the safety and quality if induced pluripotent stem cells relative to embryonic stem cells remains unclear. To this stem cell scientist, however, it is evident that we are entering into a new era with the wind at our backs, and expectations high as we work towards making the promise of pluripotent stem cells a reality. Amander Clark Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research, University of California, Los Angeles References Gurdon, JB. (1962). Dev. Biol. 4:256-273. Gurdon J. (1962). J Embryol. Exp. Morph. 10:622–640. Takahashi K, Yamanaka S. (2006) Cell 126(4):663-676. The decision by the Nobel Prize Committee to split the 2012 Prize in Physiology or Medicine between John Gurdon and Shinya Yamanaka creates an absolutely brilliant juxtaposition. Given the stem-cell era we live in, people are subjected to news about stem cells all the time, so they have at least a glimmer of insight into the signifi cance of what Yamanaka's laboratory discovered. But what do they understand about what Gurdon did and why he too deserves the Prize? The headline in the New York Times article on October 9, 2012 was “Cloning and Stem Cell Work Earns Nobel”. And indeed, Gurdon is known to the wider public as “the Person Who Cloned Frogs”. But cloning is not what he set out to do. I was a graduate student at the time, working with Jim Ebert at the Carnegie Institution's Department of Embryology in Baltimore, with a Master's Degree in my pocket from work performed with Bob Briggs at Indiana University. Gurdon's experiment was an earth-shaker for me. He did not just “clone frogs”. What he did was experimentally address a key question in developmental biology at the time, namely, “Is the nucleus irreversibly altered during development?” At this time, the differential gene expression theory of development was in full fl ower. Its essence was that all the nuclei in all the cells of the body had equivalent genomes and that what made those cells different was the developmentally imposed expression of different genes in different tissues. But nobody knew how differential expression was imposed - nobody knew whether it might involve subtle structural changes in the genome or in chromatin or at some other level of organization in the nucleus. Gurdon's approach was to ask, “To what extent can the nucleus of a definitively differentiated cell support the repertoire of functions required in the development of an individual?” He used as nuclear donors Xenopus tadpole intestinal epithelial cells (not “adult”, as is often reported [even by the New York Times], but terminally differentiated in any case, which was of course the point). As recipients he used Xenopus oocytes enucleated at meiosis II, when fertilization would normally occur. Most failed, but a few developed to the tadpole stage. The fact that he only got tadpoles, not adult frogs, was a distracting but not damning detail because, clearly, the nucleus of a differentiated cell could support development of a functional, feeding animal. While his results were entirely consistent with the notions of the day, there was resistance to their generalizability, and Gurdon remained “the Guy Who Cloned Frogs”. The discovery of genomic imprinting (nuclear differentiation by another name?), along with a lot of negative results, led to a school of thought that the frog findings were not repeatable in mammals. Dolly the sheep, assisted by Ian Wilmut and co-workers, changed all that. And the world changed. The discovery that mammals could be cloned from somatic cell nuclei verifi ed the generalizability of Gurdon's results. While progressive alterations in the structure and function of nuclear components unquestionably occur during development, they are by and large reversible. Yamanaka's brilliant experiments involving reprogramming of somatic cells with a packet of transcription factors now can be seen as entirely consistent with Gurdon's results using nuclear transplantation. One approach reprograms somatic cell developmental potential with a defined molecular recipe in vitro, the other reprograms somatic cell nuclei by placing them in a friendly new environment in vivo. Marvelous. John R. Coleman Professor Emeritus of Biology Brown University John B. Gurdon's contribution to our understanding of differentiation is certainly deserving of a Nobel Prize. Prior to his studies in the 1960s, it was thought that all differentiation went downhill, much like a marble rolling downhill, and from the moment of fertilization, that process was fi xed and not reversible except through gametogenesis. Gurdon's studies with nuclear transfer certainly challenged that concept as he showed that a nucleus could be both remodeled (morphologically) by the oocyte cytoplasm and be subsequently reprogrammed (metabolically) to behave as though it were a zygotic nucleus. The early embryo, in contrast to a somatic cell, has a unique morphology, metabolism, and large nuclei and inactive nucleoli; the nuclei of early amphibian embryos are devoid of nucleoli. Remodeling of a small somatic cell nucleus containing numerous nucleoli occurred after transfer to the cytoplasm of an oocyte, as illustrated by a swelling of the nucleus to the diameter of a zygotic nucleus and by a disappearance of the nucleoli (Gurdon, 1964; Gurdon and Brown, 1965). Loss of the nucleoli in the oocyte is indirect evidence that rRNA synthesis was shut off, whereas the nucleoli then would reappear at the appropriate developmental time in the resulting embryo. This nuclear remodeling is accomplished by an exchange of proteins between the cytoplasm and the transferred nucleus. In the 1980s, the technology became available to study individual genes and thus to better understand nuclear reprogramming. Two examples stand out that illustrate the fi delity of reprogramming that can occur after transfer of a somatic nucleus to the cytoplasm of an enucleated, activated frog oocyte. The first is muscle-specific actin, which is normally transcribed as the myotome cells of the gastrula stage embryo begin to differentiate. Transcription of muscle-specifi c actin immediately ceases after the transfer of a myotome cell nucleus back to an enucleated, activated oocyte. It is not until the resulting embryo develops to the gastrula stage, the stage when the myotome cells begin to differentiate, that transcription of the muscle-specific actin gene begins only in the developing myotome cells (Gurdon et al., 1984). The second example is of the 5Sooc gene, which is is transcribed and translated for only a short time at the late blastula stage. When a nucleus beyond this stage (when the 5Sooc gene is not transcribed) is transferred back to an enucleated activated oocyte, the developing embryo does not transcribe the 5Sooc gene except for a short time as it passes through the late blastula stage (Wakefield and Gurdon, 1983). These studies contributed to a transformation of the way we think about differentiation: It was no longer a one-way street that goes downhill. The frog egg was large and was thought to contain an abundance of “remodeling” and “reprogramming” factors. People began to dream of similar application to mammalian embryos. Applications of this technology began first with nuclei from cleavage stage mammalian embryos because it was thought that the mammalian oocyte was not capable of much remodeling and reprogramming. Later, more advanced-stage nuclei were used and those nuclei could also be remodeled and reprogrammed (Whitworth and Prather, 2010) by the cytoplasm of the oocyte to produce a zygote with full developmental potential. This technology not only transformed the way we think about differentiation, but also provided a method to clone individuals from somatic cells. The somatic cell nuclear transfer (SCNT) technology, in combination with genetic engineering of the somatic cell, is now the state-of-the-art for making certain genetic modifi cations to mammals for which embryonic stem cells have not been adequately developed (Han et al., 2010; Gurdon and Wilmut, 2011; Whyte and Prather, 2011). Gurdon's original finding has changed our collective mindset into how cells work. Merged with the amazing induced pluripotent stem cell (iPS) technologies developed by co-awardee Prof. Yamanaka, these results now pave new frontiers for unparalleled scientific and clinical applications. Randall S. Prather, editor References Gurdon JB. 1964. Adv Morpholg 4:1–43. Gurdon JB, Brennan S, Fairman S, Mohun TJ. 1984. Cell 38:691–700. Gurdon JB, Brown DD. 1965. J Mol Biol. 12:27–35. Gurdon JB, Wilmut I. 2011. Cold Spring Harb Perspecr Biol. 3:pii a002659. Han Z, Cheng Y, Liang CG, Latham KE. 2010. Methods Enzymol. 476:171–184. Wakefield L, Gurdon JB. 1983. The EMBO journal. 2:1613–1619. Whitworth KM, Prather RS. 2010. Mol Reprod Dev. 77:1001–1015. Whyte JJ, Prather RS. 2011. Mol Reprod Dev. 78:879–891.