Shinya Yamanaka

There are different types of stem cells.

These are some types of cells that will help in understanding the material.

The prevalent view during the early 20th century was that mature cells were permanently locked into the differentiated state and cannot return to a fully immature, pluripotent stem cell state. They thought that cellular differentiation can only be a unidirectional process. Therefore, non-differentiated egg/early embryo cells can only develop into specialized cells. However, stem cells with limited potency (adult stem cells) remain in bone marrow, intestine, skin etc. to act as a source of cell replacement.[13]

The fact that differentiated cell types had specific patterns of proteins suggested irreversible epigenetic modifications or genetic alterations to be the cause of unidirectional cell differentiation. So, cells progressively become more restricted in the differentiation potential and eventually lose pluripotency.[14]

In 1962, John B. Gurdon demonstrated that the nucleus from a differentiated frog intestinal epithelial cell can generate a fully functional tadpole via transplantation to an enucleated egg. Gurdon used somatic cell nuclear transfer (SCNT) as a method to understand reprogramming and how cells change in specialization. He concluded that differentiated somatic cell nuclei had the potential to revert to pluripotency. This was a paradigm shift during the time. It showed that a differentiated cell nucleus has retained the capacity to successfully revert to an undifferentiated state, with the potential to restart development (pluripotent capacity).

However, the question still remained whether an intact differentiated cell could be fully reprogrammed to become pluripotent.

Shinya Yamanaka proved that introduction of a small set of transcription factors into a differentiated cell was sufficient to revert the cell to a pluripotent state. Yamanaka focused on factors that are important for maintaining pluripotency in embryonic stem (ES) cells. Knowing that transcription factors were involved in the maintenance of the pluripotent state, he selected a set of 24 ES cell transcriptional factors as candidates to reinstate pluripotency in somatic cells.

First, he collected the 24 candidate factors. When all 24 genes encoding these transcription factors were introduced into skin fibroblasts, few actually generated colonies that were remarkably similar to ES cells. Secondly, further experiments were conducted with smaller numbers of transcription factors added to identify the key factors, through a very simple and yet sensitive assay system. Lastly, he identified the four key genes. They found that 4 transcriptional factors (Myc, Oct3/4, Sox2 and Klf4) were sufficient to convert mouse embryonic or adult fibroblasts to pluripotent stem cells (capable of producing teratomas in vivo and contributing to chimeric mice).

These pluripotent cells are called iPS (induced pluripotent stem) cells; they appeared with very low frequency. iPS cells can be selected by inserting the b-geo gene into the Fbx15 locus. The Fbx15 promoter is active in pluripotent stem cells which induce b-geo expression, which in turn gives rise to G418 resistance; this resistance helps us identify the iPS cells in culture.

Moreover, in 2007, Yamanaka and his colleagues found iPS cells with germline transmission (via selecting for Oct4 or Nanog gene). Also in 2007, they were the first to produce human iPS cells.

However, there are some difficulties to overcome. The first is the issue of the very low production rate of iPS cells, and the other is the fact that the 4 transcriptional factors are shown to be oncogenic.

Nonetheless, this is a truly fundamental discovery. This was the first time an intact differentiated somatic cell could be reprogrammed to become pluripotent. This opened up a completely new research field.

In July 2014, a scandal regarding the research of Haruko Obokata was connected to Yamanaka. She could not find the lab notes from the period in question[15] and was made to apologise.[16][17]

Since the original discovery by Yamanaka, much further research has been done in this field, and many improvements have been made to the technology. Improvements made to Yamanaka's research as well as future prospects of his findings are as follows:

1. The delivery mechanism of pluripotency factors has been improved. At first retroviral vectors, that integrate randomly in the genome and cause deregulation of genes that contribute to tumor formation, were used. However, now, non-integrating viruses, stabilised RNAs or proteins, or episomal plasmids (integration-free delivery mechanism) are used.

2. Transcription factors required for inducing pluripotency in different cell types have been identified (e.g. neural stem cells).

3. Small substitutive molecules were identified, that can substitute for the function of the transcription factors.

4. Transdifferentiation experiments were carried out. They tried to change the cell fate without proceeding through a pluripotent state. They were able to systematically identify genes that carry out transdifferentiation using combinations of transcription factors that induce cell fate switches. They found trandifferentiation within germ layer and between germ layers, e.g., exocrine cells to endocrine cells, fibroblast cells to myoblast cells, fibroblast cells to cardiomyocyte cells, fibroblast cells to neurons

5. Cell replacement therapy with iPS cells is a possibility. Stem cells can replace diseased or lost cells in degenerative disorders and they are less prone to immune rejection. However, there is a danger that it may introduce mutations or other genomic abnormalities that render it unsuitable for cell therapy. So, there are still many challenges, but it is a very exciting and promising research area. Further work is required to guarantee safety for patients.

6. Can medically use iPS cells from patients with genetic and other disorders to gain insights into the disease process. - Amyotrophic lateral sclerosis (ALS), Rett syndrome, spinal muscular atrophy (SMA), α1-antitrypsin deficiency, familial hypercholesterolemia and glycogen storage disease type 1A. - For cardiovascular disease, Timothy syndrome, LEOPARD syndrome, type 1 and 2 long QT syndrome - Alzheimer’s, Spinocerebellar ataxia, Huntington’s etc.

7. iPS cells provide screening platforms for development and validation of therapeutic compounds. For example, kinetin was a novel compound found in iPS cells from familial dysautonomia and beta blockers & ion channel blockers for long QT syndrome were identified with iPS cells.

Yamanaka's research has “opened a new door and the world's scientists have set forth on a long journey of exploration, hoping to find our cells’ true potential.”[18]

In 2013, iPS cells were used to generate a human vascularized and functional liver in mice in Japan. Multiple stem cells were used to differentiate the component parts of the liver, which then self-organized into the complex structure. When placed into a mouse host, the liver vessels connected to the hosts vessels and performed normal liver functions, including breaking down of drugs and liver secretions.[19]