Stem cells are a class of pluripotent cells with self-renewing capabilities. Under certain conditions, stem cells differentiation can become a variety of functional cells. According to the developmental stage of stem cells, they are divided into embryonic stem cells (ES cells) and adult stem cells (somatic stem cells). There are three types of stem cells according to their developmental potential: totipotent stem cells (TSCs), pluripotent stem cells, and unipotent stem cells (multipotent stem cells). Stem cell is a kind of under-differentiated and immature cell that has the potential to regenerate various tissues and organs and the human body. It is called "universal cell" in the medical profession.
Induced pluripotent stem cells (iPSCs) were originally developed by Japanese scientists using viral vectors to transfect four transcription factors Oct4, Sox2, Klf4, and c-Myc into differentiated somatic cells and reprogram them. Embryonic stem cells are a similar cell type, and their discovery proves that stem cell differentiation into somatic cells is a reversible process. The research of iPSCs mainly focuses on the selection of somatic cells and reprogramming factors, the establishment of reprogramming factor-mediated carrier systems, the promotion of induction of transformation rate, and the establishment of disease-specific iPSC models.
1. The selection of somatic cells
In terms of somatic cell selection, research has found that in addition to fibroblasts, mouse liver cells, gastric cells, B lymphocytes, neural stem cells, and human bone marrow mesenchymal stem cells, keratinocytes, peripheral blood cells, and skin fibroblasts, embryonic fibroblasts, adipose stem cells, etc. can produce iPSC. Post-mitotic neurons and terminally differentiated lymphocytes can also induce the formation of iPSCs. It can be seen that all somatic cells have the potential to be induced into iPSCs by reprogramming, but different cell types have great differences in the efficiency of iPSC production, the effect of inducing factors, and the safety of iPSCs obtained. It can be seen that it seems that all somatic cells have the potential to be induced into iPSC after reprogramming, but different cell types have great efficiency in the production of iPSC, the effect of inducing factors, and the safety of the obtained iPSC difference. For example, human keratinocytes are more efficient at reprogramming than fibroblasts; mouse gastric and hepatocytes are more efficient than fibroblasts when they activate the ESC-specific Fbx15 gene, and they integrate fewer viruses; different types of somatic cells ( mouse tail skin cells, stomach cells, and embryonic skin cells) induce iPSC, which can lead to different tumor probabilities in mice.
2. The selection of reprogramming factors
Although Oct4, Sox2, c-Myc, and Klf4 were the first to be found that can reprogram fibroblasts into four iPSC reprogramming factors, with the deepening of the research, it was found that in some cases, some factors were adjusted or replaced. It can also obtain good induction effects, such as Oct4, Sox2, Nanog, Lin28 can also reprogram human fibroblasts into iPSC; Oct4 and Sox2 can reprogram human cord blood stem cells into iPSC; Human fibroblasts can endogenously express c-Myc and Klf4. During the reprogramming transformation process, iPSC can be successfully obtained without the involvement of exogenous c-Myc; for nerves that endogenously overexpress Sox2 and c-Myc stem cells can be reprogrammed into iPSC using only Oct4. These findings suggest multiple signaling pathways in reprogramming of mature somatic cells.
3. Reprogramming factor mediator
Lentivirus and retrovirus are the two main vectors used to mediate reprogramming factors in the initial stages of iPSC technology research. However, with the deepening of research, it was found that lentiviral vectors and retroviral vectors may cause foreign aid oncogenes to integrate into the cell genome and cause insertion mutations. The activation of viral genes has caused tumors in mice produced by iPSC differentiation. This result undoubtedly introduces small risks to clinical transformation and application of iPSC, and may even lead to adverse consequences. It is precisely because of the discovery that the use of the vector or the inducing gene itself can cause tumorigenesis. Many research groups are working to find ways to generate non-integrated iPSCs to reduce the risk of tumorigenesis. These methods include adenovirus, Sendai virus, plasmid, synthetic RNA, protein, piggy Bac transposon and small molecule induced reprogramming, etc. These methods have been implemented at the laboratory level.
4. The iPSC-induced transformation rate
Although the establishment of the iPSC can be repeated, the efficiency is very low, and the reprogramming efficiency will be accompanied by a decrease in the security of the iPSC. Therefore, improving the efficiency of iPSC reprogramming has also been the focus of research in this field. Although the molecular mechanism of iPSC formation is not very clear, research has found that many factors, such as cell type, reprogramming factors, vectors, and even culture conditions and microenvironment, are related to the induced conversion rate of iPSC. For example, the efficiency of reprogramming adipose stem cells with Oct4, Sox2, c-Myc, and Klf4 factors is 0.2%, and it is 20 times that of skin fibroblasts under the same conditions. When using retrovirus as a vector to induce reprogramming of human juvenile keratinocytes, the efficiency should be 100 times that of fibroblasts, and the production efficiency of iPSC cultured in hypoxia microenvironment can be increased to 2.5-4.2 times.
The role of iPSC in regenerative medicine
Regenerative medicine is a discipline that studies the repair and regeneration of damaged tissues and organs. When certain organs and tissues of humans are damaged and cannot be repaired in time, human life is in danger. Hundreds of millions of people around the world suffer from different forms of tissue and organ trauma every year, so there is a huge demand for various tissues and organs for transplantation. However, the sources of organs available for transplantation are limited and difficult. At the same time, the clinical application of organ transplantation is limited due to the constraints of immune rejection and transplantation costs. Therefore, using stem cells or progenitor / precursor cells that can be expanded in large numbers has the potential for differentiation. Through engineering means and combined with new biological materials, there is a broad space for development of technical means that can replace the tissues that can adapt to the human biochemical and physiological conditions.
To be continued in Part II…