Embryonic stem cells (ESC) have the potential to self-replicate and differentiate into various functional cells of the human body. Its unique biological characteristics make it widely used in various fields of biological research. At the same time, its potential medical value has become a current research hotspot. However, ESC originates from early embryos before implantation. hESC has been controversial since its birth. Its ethical issues and sources have restricted the development of related research and stem cell therapy. In recent years, the emergence of iPSC has the potential to replace ESC. iPSC has similar functions to ESC, but it bypasses the ethical and legal obstacles that ESC research has always faced, so its application prospects are very broad.
The iPSC reprogramming technology was firstly proposed by the team of Japanese scientists Takahashi and Yamanaka in 2006. They selected four factors, including transcription factors "OCT3/4 and SOX2" and upregulated genes in tumors "c-MYC and KLF4" and cloned them into viral vectors which were subsequently transduced into mouse embryonic and adult fibroblast cultures. These fibroblasts were induced to an embryonic-like state under ES cell culture conditions. These new cells are similar to embryonic stem cells in terms of morphology, gene and protein expression, epigenetic modification status, cell multiplication ability, embryoid body and teratoma formation ability, and differentiation ability. Accordingly, Takahashi and Yamanaka designated these new cells as iPSC. Henceforth, the use of mature cells to reprogram them into iPSC with the potential to differentiate into multiple cells has become a research hot topic, and has been applied in the fields of disease treatment and organ regeneration.
Fig.1 The morphology of ESCs, iPSCs, and mouse embryonic fibroblasts (MEFs). (Takahashi, 2006)
Since Takahashi and Yamanaka reported the successful iPSC reprogramming in 2006, they generated iPSCs from adult human dermal fibroblasts with the same four factors (OCT3/4, SOX2, c-MYC, and KLF4) in 2007. Yu et al. demonstrated that the utilization of different four factors "OCT4, SOX2, NANOG, and LIN28" could also induce iPSC from diploid IMR90 human fetal fibroblasts. Methods of factor reprogramming fall into two broad categories: chemical and transgene reprogramming. Lately, the induction of pluripotency in multiple kinds of somatic cells has been reported. For example, iPSC has been established from hematopoietic stem cells, T and B lymphocytes, renal epithelial cells, pancreatic beta cells, fibrocytes, keratinocytes, mesenchymal stem cells, bone marrow cells, and albumin-expressing hepatocytes, etc. Taken together, all cells in the human body seem to have the potential to be reprogrammed into iPSC.
In fact, only a small number of cells could be successfully reprogrammed into iPSC, thus the inefficiency becomes the major problem for this attractive technology. This low efficiency could be explained by two models: the elite model and the stochastic model. In the elite model, only elite cells (progenitor cells and stem cells) are capable of being reprogrammed into iPSC. In the stochastic model, all cells have potential, while only a small portion of them could be finally induced to be iPSC. Accordingly, the whole process of reprogramming has been broken down into phases, and successful reprogramming for a single cell requires the completion of all phases. Generally, the process of successful reprogramming can be divided into the early phase and late phase. In the early phase, somatic cells undergo the following events: transition of cellular metabolic status from oxidative phosphorylation to glycolysis, suppression of transition from mesenchymal to epithelial, down-regulated gene expression. In the late phase, the pluripotency-associated genes need to be activated while the tissue-specific transcription factors and developmental genes need to be suppressed. Besides, the H3K4me3 and H3K27me3 are methylated on high CpG promoter regions.
Fig.2 The elite model (top) and the stochastic model (bottom). (Karagiannis, 2019)
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References
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