iPSCs have been derived from many different species, such as humans, mice, pigs, rabbits, rats, marmosets, and rhesus monkeys. The successfully reprogrammed cell types contain fibroblasts, marrow mesenchymal cells, gastric/intestinal epithelial cells, keratinocytes, hepatocytes, stomach cells, neural stem cells, pancreatic cells, blood/liver/neural progenitor cells, cord/peripheral blood cells, adipose stem cells, B and T lymphocytes, and so on [
45‐
48], of which fibroblasts are the most commonly used parental somatic cell type for the generation of iPSCs. Between 2010 and 2012, the first reports were published on the production of iPS cell lines from human gingival fibroblasts and periodontal ligament fibroblasts by reprogramming using a retroviral transduction cocktail of OCT3/4, SOX2, KLF4, and c-MYC [
49‐
51]. These induced cell lines expressed the human-ES (hES) cell-associated cell-surface antigens like SSEA3, SSEA4, GCTM-2, TG30 (CD9), and Tra-1-60, as well as the hESC marker genes OCT4, NANOG, and GDF3 [
52]. Interestingly, in 2012, another research group established iPSCs from human periodontal ligament fibroblasts by introducing the human-ESC (hESC) markers OCT3/4, SOX2, NANOG, KLF4, and Lin28 through retrovirus transduction, even without the oncogene c-MYC [
51], which was found to be responsible for tumors in iPSC chimeric mice [
53]. In 2015, Umezaki et al. [
54] demonstrated that human gingival integration-free iPSCs could be generated using episomal plasmid vectors without retroviral transduction. Compared with skin, gingival tissue is obtained more easily and gingival wound after sampling heals more quickly. Furthermore, reprogramming efficiency of mouse gingival fibroblasts is higher than that of mouse dermal fibroblasts [
55]. These discoveries suggest that iPSCs derived from GFs represent an optimal and more practical cell-based tissue-regenerative treatment for periodontal diseases.
Since the first establishment of iPS cell line by Yamanaka in 2006, many scientists have made efforts to improve the efficiency and safety of the reprogramming process. Generally, the approaches for factor reprogramming include transgene and chemical reprogramming while methods for transgene reprogramming can be classified into three groups: RNA-based, DNA-based, and protein transduction (direct cell transduction). RNA-based reprogramming can be achieved by transfection of synthetic RNA, modified RNA, and micro RNA [
56••]. DNA-based ways are most widely used, which include the use of viruses and plasmids. The very first way for cell reprogramming was by retroviral delivery of four transcription factors (
Oct4,
Sox2,
Klf4, and
Myc). In 2009, some researchers found direct protein transduction can improve inducing efficiency, but it is easily affected by the quality of recombinant proteins [
57‐
59]. As for chemical cell reprogramming, the use of small-molecule compounds has been developed [
51], and several reviews on small molecule drugs used for improving the generation of iPSCs are available [
60,
61]. A variety of reprogramming methods to derive iPSCs and their advantages and disadvantages are shown in Table
1. Among available reprogramming methods, the easiest and most efficient method by now is the integration of reprogramming factors into the genome by retroviral or lentiviral transduction [
43,
75].
Table 1
Advantages and disadvantages reprogramming methods to derive iPSCs
| Forms | Vectors | Fibroblasts, neural stem cells, stomach cells, liver cells, keratinocytes, amniotic cells, blood cells, adipose cells, melanocytes, human T cells, β cells | No genomic integration, no premature silencing, inexpensive | Low efficiency, sequence-sensitive RNA replicase, difficulty in purging cells of replicating virus Genomic recombination, insertional mutagenesis | |
Transgene (OSKM) | DNA-based | Viruses (lentivirus, adenovirus, Sendai virus ) |
Transposon | Fibroblasts, mouse ESCs | Host-factor independent, wide chromosomal distribution, high efficiency | Gene mutations, genomic rearrangements | |
Minicircle DNA | Human ASCs | High expression in mammalian cells, high transfection efficiency, stable ectopic transgene expression | Low expressions for transcription factors | |
RNA-based | Micro RNA | Mouse fibroblasts, Human fibroblasts | Nonviral, nontranscription-factor | Multiple transfection, low efficiency | |
Modified RNAs | Human fibroblasts | Avoid the endogenous antiviral cell defense, very high efficiency | Technically complex | |
Synthetic RNAs | Murine EFs, human epidermal keratinocytes | High efficiency | High and dose-dependent cytotoxicity | |
Recombinant Proteins | Human fibroblasts, mouse fibroblasts | Nongenome integration, easily controlled | Very low efficiency, unstable, easily affected by quality of proteins | |
Chemical approaches (small molecule compounds) | Mouse EFs | Nonimmunogenic, cost-effective, easy to handle, structural versatility, faster, more efficient, self-renewal promotion, controllable microenvironment | Time and dosage of specific biochemicals need to be optimized | |