Stem Cells
The discovery that cells from differentiated tissues can be reprogrammed into embryonic-like stem cells, called inducible pluripotent stem cells (iPS), is a breakthrough on the way to create disease and patient specific stem cells and provided an alternative preventing ethical issues associated with embryonic stem cells derived from human embryos. In 2008 "Reprogramming Cells" was designated 'Breakthrough of the Year' by the journal Science with specific mentioning of the Tet Technology in the corresponding editorial. Since then, the Tet Technology was successfully applied by many laboratories to further advance the iPS field.
The original reprogramming protocols involved viral transfer of a set of 4 transcription factors into somatic cells, a tedious and inefficient process (Takahashi and Yamanaka, 2006). A significant step forward to increase efficiency and safety of reprogramming was reported by the Jaenisch group in 2008 (Wernig et al., 2008) when the Tet-On Advanced System was used to control expression of reprogramming factors. iPS-cells emerging after Dox-induced reprogramming were used to generate chimeric mice and of these, genetically identical "secondary iPS" which will allow studies on the process of reprogramming under standardized conditions. Further, it allows generation of large numbers of cells for high-throughput screens to identify small molecules to replace the original biological reprogramming factors.
Therapeutic use of cell reprogramming still seemed limited due to the viral delivery of reprogramming factors raising safety concerns for its clinical use. Two elegant studies published back-to-back in 2009 (Kaji et al., Woltjen et al., 2009) combined the use of the Tet Technology with the piggyback (PB) transposon delivery system. Upon genomic integration and Dox-inducible activation of reprogramming factors, stable iPS cells were produced in human and mouse fibroblasts. Subsequent transient expression of the transposase enzyme removed the integrated DNA which was not required anymore in a seamless and highly efficient fashion, giving rise to genetically non-modified iPS cells, ideal starting material for regenerative medicine.
Last year another progress in cellular reprogramming was achieved when scientists from Stanford University published how they converted mouse fibroblasts directly into functional neurons bypassing the pluripotent state. Vierbuchen and colleagues (Vierbuchen et al., 2010) infected mouse fibroblasts with a lentiviral pool of Tet-controlled neuronal transcription factors. By sequential reduction of the number of transcription factors they eventually identified a combination of just 3 genes sufficient for conversion of embryonal and postnatal tail tip fibroblasts into neuronal cells called induced neuronal (iN) cells. These findings mark another breakthrough in biomedical stem cell science. It may be possible to apply this approach to the generation of other cell types when their key transcription factors are identified. Furthermore, if such conversion is possible with human cells, it will help to understand the mechanisms of cell fate determination and it may be applied to create disease specific cells for disease modeling and drug discovery.