Although this has caused much excitement in the field of regenerative medicine, a sobering fact is that ES cells are very scarce as they can only be obtained in the inner cell mass of an early stage embryo. Moreover, the quest for ES cells in humans also raise ethical concerns that have driven ES cell research to a standstill.
Thus, despite the huge potential of ES cells for medical purposes, these technical and ethical issues associated with accessing these ES cells (particularly those from humans) could be a huge impediment in the progression of this field.
In light of increasing demand to “cure” debilitating injuries and degenerative disease, there is a relentless urge for scientists to continue ES cell research. The driving force to re-ignite ES cell research was ironically based on research on amphibians. Amphibians are resilient creatures that have the remarkable ability to regenerate limbs after dismemberment. The limb regeneration arises from the apparent dedifferentiation of muscle fibres (or myotubes) to produce cells with stem cell phenotype. These “stem cells”make up a regenerative structure known as the blastema, and is thought that the subsequent tightly controlled differentiation of the stem cells would produce the regenerating limb.
Evidence supporting this concept came from cell lineage tracing studies, which demonstrated that labelled myotubes does indeed dedifferentiate into stem cells. Although dedifferentiation does not naturally occur in mammalian myotubes, studies in 2001 by McGann et al.revealed that dedifferentiation can be recapitulated upon exposure to the amphibian regenerative extract . This study suggested that mammalian cells are capable of undergoing dedifferentiation as long as the appropriate factors are made available to them.
Based on this study, an incredulous idea emerges. Could mammalian somatic cells be dedifferentiated or “reprogrammed” into pluripotent embryonic stem cells? Studies in 2006 by Shinya Yamanaka at Kyoto University, Japan, demonstrated for the first time that it is indeed possible to reprogram somatic cells into pluripotent cells. Specifically, they were able to generate pluripotent stem cells by reprogramming mouse fibroblasts through retrovirus-mediated delivery of four key transcription factors: c-Myc, Oct3/4, Sox2, and Klf4.
These 4 factors are normally expressed in embryonic stem cells (ES cells)- which are natural pluripotent stem cells found in the inner cell mass of developing embryos. While these factors are normally downregulated in somatic cells, the forced expression of these four factors enables fibroblasts to assume pluripotent properties of ES cells .
4 years since the discovery of IPS technology, its still remains as a compelling hot topic in many research journals. The reason for the excitement is because IPS technology could provide a unique source of pluripotent stem cells that can be easily produced from adult somatic cells. In contrast, natural sources of pluripotent stem cells are only from the inner cell mass of developing blastocysts (early stage embryos) known as embryonic stem cells (ES cells), which as you can imagine are scarce and difficult to obtain in large quantities.
As IPS cells are pluripotent, researchers will no longer be reliant on the scarce source of embryonic stem cells (ES cells) to produce functional tissue or organs to replace those that were lost as a result injuries or diseases . Unlike ES cells, IPS cells can be readily produced from somatic cells of adults, allowing patient specific production of IPS derived tissue and/or organs that could be transplanted without rejection. Moreover, IPS technology can circumvent the ethical problems from collecting ES cells from human embryos.
Excitement was soon hampered as imperfections of IPS cells were uncovered, which some would call genetic flaws or reprogramming glitches that cause IPS cells to behave quite differently compared to ES cells. Sobering studies by William E. Lowry and his colleagues at UCLA revealed that despite similarities between IPS and ES cells, they discovered gene expression differences that not only distinguish IPS from ES cells but can also between IPS populations from early and late passages in culture .
By comparing IPS cells produced from different reprogramming experiments, they discovered that the IPS gene expression signature is quite consistent between reprogramming experiments. This suggests that differences between IPS and ES cells are not likely to arise from genomic instability or abnormalities (confirmed by genomic characterization), but rather as a result of reprogramming flaws in IPS cells.
Remarkably, IPS cells demonstrate profound differences in both DNA methylation and microRNA expression profiles compared to ES cells. As expected, functional studies revealed that IPS cells behave differently from ES cells , and produce neurons at lower efficiency, and demonstrate greater preference to generate glial precursors . Collectively, studies thus far suggest that further epigenetic reprogramming and/or manipulations of IPS cells may be needed to better recapitulate ES cells.
This notion was further supported by recent work by Konrad Hocheldinger at Harvard University, Cambridge, using a unique technology to generate genetically identical IPS cells and ES cells (reported in New York Academy of Science Meeting March 23, 2010). With this system, the scientists pinpointed that the difference between IPS and ES cells is due to profound changes specifically in chromosome 12, which as Elie Dolgan has reported in Nature News, March 2010, encodes “a slew of microRNAs that are consistently active in ES cells but silenced in IPS cells”. The differences in microRNA expression may explain why IPS cells differentiate at lower rates of efficiency compared to ES cells. Importantly, further manipulations of this microRNA cluster could
be the first step to creating IPS cells that better mimic ES cells.
As it turns out, there is evidence that IPS implantation in mice could succumb the animals to cancer. While the mechanism for this phenomenon is unknown, recent studies revealing profound epigenetic differences between IPS and ES cells[7, 8] could be the key to explain why IPS cells could cause cancer. Overall, further investigation into the biology of IPS cells is still needed before IPS cells safely and effectively in clinical applications.
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2. Martin, G.R., Isolation of a pluripotent cell line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells. Proc Natl Acad Sci U S A, 1981. 78(12): p. 7634-8.3. Echeverri, K. and E.M. Tanaka, Mechanisms of muscle dedifferentiation during regeneration. Semin Cell Dev Biol, 2002. 13(5): p. 353-60.
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6. Yamanaka, S., Strategies and new developments in the generation of patient-specific pluripotent stem cells. Cell Stem Cell, 2007. 1(1): p. 39-49.7. Chin, M.H., et al., Induced pluripotent stem cells and embryonic stem cells are distinguished by gene expression signatures. Cell Stem Cell, 2009. 5(1): p. 111-23.
8. Hu, B.Y., et al., Neural differentiation of human induced pluripotent stem cells follows
developmental principles but with variable potency. Proc Natl Acad Sci U S A. 107(9): p. 4335-40.