[BojanaL]

Mature cells could be reprogrammed to become pluripotent again

Until recently, it was believed that only embryonic and autologous stem cells have a potential to differentiate in various cell lineages. Discovery that mature, already differentiated adult cell, can be reverted to the pluripotent stem cell stage brought a Nobel Prize in Physiology and Medicine to a Dr. John B. Gurdon and Dr. Shinya Yamanaka in 2012. Adult somatic cells turned into stem cells are called induced pluripotent cells (iPS).

During embryonic development, cell differentiation potential is dropping progressively. Once differentiated, cell normally doesn’t undergo changes that will result in new type of cell formation (cell fate switch) or back to a pluripotent stage. Conrad Hal Waddington suggested a model of mountain and the valley to describe a journey and energy demand necessary for stem cell to become fully formed adult cell. You can imagine stem cells as a marbles on the top of the mountain, and differentiation stages as processes that are happening while they are gliding toward the valley, where they will become one of the numerous differentiated cell types. After they reach the valley and fully developed stage, cells can’t climb back to the top of the mountain to regain their pluripotency. Only cells in adult organism having potential to differentiate in a couple of lineages are autologues stem cells (when replacing old or damaged cells). However, scientists were always curious about cell potential and they start experimenting with it in the second half of the 20th century.

Frogs were favorite model organisms because of the large egg size and extrauterine embryonic development. First experimental attempt to develop a tadpole using Rana pipiens species wasn’t successful; embryo failed to develop when somatic nucleus was transplanted to an enucleated egg. Almost one decade passed before another experiment proved this method could be successful. Dr. Gurdon used Xenopus laevis (different frog species) and UV irradiation to destroy egg nucleus. Tadpole intestinal epithelium derived nucleus was transplanted to an egg resulting in a successful tadpole formation. In was shown for the first time that somatic nucleus from the already differentiated cell has a potential to drive embryonic development and give rise to a numerous cell lines after seeded in the egg cell. Today, this technique is known as somatic cell nuclear transfer (SCNT) and it’s used for cloning all kind of animal species.

Dr. Yamanaka wanted to discover what factors are essential for cell pluripotency. He knew that fusion of the embryonic cytoplasm and somatic nucleus result in pluripotent cell, so he picked 24 transcription factors from the egg’s cytoplasm as the potential candidates for the restored pluripotent capacity. Using those transcription factors and skin fibroblast, he wanted to see which factor is able to produce cells that will resemble embryonic cells. He eliminated one factor by one and finally ended up with 4 factors that are responsible for cell pluripotency. Combination of the Myc, Oct3/4, Sox2 and Klf4 is what can turn mouse fibroblast to a pluripotent stem cell. Other cells could demand different combination, number or type of transcription factors, but the mechanism of action is now fully revealed. It was astonishing discovery and it opened a gate to a completely new research field.

A lot of medical issues could be solved using iPS cells. That’s the main reason why cell fate switch - result of specific combination of the transcription factors directing cell development, is well studied in various species and different organs. When serious organ damage or malfunction is present, transplantation could be only solution, but immune response is inevitable as well as immunosuppressive therapy. If organ repair could be accomplished using iPS cells, immune response would be skipped as our organism wouldn’t recognize iPS cells as a foreign body. Other interesting medical implementation of the iPS cells is in the investigation of the disease patterns and processes in the rare or severe genetic disorders. After extracting from the patient and turning into iPS cells, they could be used as a platform for toxicology testing or drug development. So far, they are used for successful screening of the drugs for the familial dysautonomia and long QT syndrome. When differentiated in vitro, they could provide novel information on the mechanism of disease and its progress. Amyotrophic lateral sclerosis, Ratt syndrome, type 1 and 2 long QT syndrome, a1 antitrypsin deficiency, spinal muscular atrophy…are just a few examples of the diseases that could be investigated using in vitro iPS cells.

Discovery that mature cell could restore stem cell differentiation potential is one of the biggest revelation in the field of medicine and biology. Those experiments have changed the way we think about fundamental biological processes and I'm sure that they will help in controlling and curing serious illness in the future.