Induced Pluripotent stem cells (iPSCs) are cells which have the potential to proliferate to generate cell lines with the innate potential to differentiate into any other cell type found in the organism of extract. In contrast to embryonic stem cells, iPSCs are generated from a fully differentiated adult somatic cell. This negates the requirement for finding and extracting the rare cell types capable of pluripotency and circumvents the need for embryonic stem cells with the benefits of being patient specific as well as harbouring significantly less ethical implications particularly in relation to ESCs.

iPSCs offer the potential for breakthrough applications in drug discovery, disease modelling and especially regenerative medicine to replace cells where damage or disease has occurred. Their inherent non-immunogenic nature relative to the donor makes them ideal for treating patients with a theoretically unlimited supply of healthy cells. They also provide a potential platform for development of drugs uniquely tailored to the individual from which the iPSC used in their development was derived.

iPSCs not only have the ability to proliferate to form an embryo and ultimately an entire organism, they can also be added to a differentiated cell mass or tissue and contribute to form the same tissue typology. They have even been experimentally proven to form germ cells from which sperm and eggs are generated.

iPSCs were first induced by a method devised by John Gurdon. In which a frog egg was irradiated in order to destroy the nucleus. Then the nucleus of a frog fibroblast skin cell was isolated and inserted into the aforementioned enucleated egg. The fact that the cell could subsequently form a tadpole was evidence for its pluripotency.

This demonstrated that a nucleus isolated from the differentiated adult cell contains all the genes present in an embryonic cell and therefore the entire organism and it was concluded that the cytoplasm of the egg must contain the necessary factors which stimulate pluripotent cell development.

The transcription factors essential for pluripotency were later identified, with only four proven to be essential for pluripotency. These include the Oct4, Sox2, cMyc, and Klf4 and are known as Yamanaka factors after the scientist Shinya Yamanaka. If these factors are over-expressed in somatic cells then pluripotency is induced. Without these four factors, pluripotency is not attained to any significant level. This Proves their integral role in the regulation of signalling in the development of embryonic stem cell pluripotency (Liu et al, 2008).

However, the method used by Yamanka using fibroblasts generated a 1% yield. It was later found that other cell types such as keratinocytes, liver, stomach or neural stem cells provided greater yield as they had more rapid expression times for the critical Yamanaka factors. It was since also observed that substituting cMyc and Klf4 with factors Nanog and Lin28 also yielded iPSCs (Yu, J., Vodyanik, M.A., Smuga-Otto, K., et al. 2007).

The original approach used by Yamanaka to generate iPSCs was via a retrovirus vector to insert the four transcription factors into the host cell genome. This elicited long lasting gene expression but with an inherent risk of metagenesis due to their method of transduction.

Many different approaches have since been developed to improve efficiency of reprogramming cells to induce pluripotency. The main aims being to reduce or phase out transduction and its corresponding low success rate and mutagenic risk. Such methods include chemical induction or using polycistron which reduces the frequency of transduction and mutagenesis. Another approach is to use direct RNA delivery via microRNAs which replaces transgene sequence integration completely.

iPSCs provide the ideal platform for many potential breakthroughs in drug discovery and healthcare therapies. However, there seem to be such a diverse range of ways to generate iPSCs that for successful widespread use in these fields standardization of generation techniques is critical to employ.

 

Liu, X., Huang, J., Chen, T., Wang, Y., Xin, S., Li, J., Pei, G., Kang, J.(2008).Yamanaka factors critically regulate the developmental signalling network in mouse embryonic stem cells. Cell Research. 2008 Dec;18(12):1177-89.

Yu J, Vodyanik MA, Smuga-Otto K, et al. (2007) Induced pluripotent stem cell lines derived from human somatic cells. Science. 2007;318:1917–1920.