Induced pluripotent stem cells and their applications
Our body requires stem cells to grow and renew damaged tissues. Stem cells can be found in many tissues throughout the human body, with the best examples probably being bone marrow and adipose. Stem cells are also essential for reproduction as each embryo if formed from a singular dividing zygote, which is itself a totipotent stem cell. Stem cells are a wonder of nature because they can divide, supposedly, without limit and undergo a process of specialization called differentiation, throughout which a stem cell alters its epigenome to become a specific type of cell. There are different types of stem cells with different ranges of possible specialized cells, and they can be classed into totipotent, pluripotent and somatic stem cells. Totipotent stem cells can differentiate into any cell type and are only found in the blastocyst. Pluripotent stem cells can differentiate into any body cell type (excluding the placenta) and are naturally found in developing embryos. Somatic stem cells are multipotent and can only differentiate into specific cell types, depending on what tissue they are in, these are found throughout the adult body. The focus of this essay is pluripotent stem cell and more specifically, induced pluripotent stem cells or IPSCs.
Contrary to popular belief, it is not actually the genetic code that changes to cause differentiation. Many think it is changes in the DNA of the cell that causes it to become a different specialized cell type. The DNA sequence in all stem cells actually stays the same as they divide and differentiate. This is because every single cell in our body has the same genetic code and can hence, theoretically, become any cell type and perform any function, this is a vital idea for the creation and use of IPSCs. You may ask: okay, if the genome of a stem cell doesn’t change during differentiation, then what does? The secret of differentiation comes down to which genes are expressed in a cell — the cell’s epigenome.
The epigenome of any given cell is the way in which the DNA is epigenetically marked and packaged. The environment around epigenetically marked DNA and the way in which it is packaged is called chromatin. Chromatin, in turn, can be split up into euchromatin and heterochromatin. Euchromatin refers to loosely packaged DNA, providing easy access for transcription factors and generally carries connotations of high gene expression. Heterochromatin refers to tightly packaged DNA, allowing transcription factors very limited access and generally connotes gene suppression. Different chromatin environments are mainly achieved by modifying histone protein octamers. Histone octamers are complexes of eight histone proteins and function as structural support for DNA. Histone proteins are positively charged which causes them to attract the negatively charged DNA molecule. This allows DNA to wrap around histone octamers and wind up into chromosomes. This allows the DNA to fit inside the nucleus and plays a vital role in the epigenetic regulation of gene expression. Histone proteins can be modified with various epigenetic marks to create different chromatin environments. Perhaps, the most important histone modification is histone acetylation. During histone acetylation, a histone acetyltransferase enzyme transfers an acetyl group onto a histone protein which makes the overall charge of the protein more negative, decreasing the strength of its interaction with DNA. This causes DNA in the region to unwind and leads to euchromatin and high levels of gene expression. Another highly important epigenetic mark is DNA methylation. During DNA methylation, a DNA methyltransferase enzyme transfers a methyl group onto a cytosine amino acid. The methyl group acts as a repressive mark by preventing the binding of transcription factors and is generally associated with heterochromatin and low levels of gene expression. The epigenome plays a crucial role in differentiation of stem cells. As a cell starts to become a specialized cell type, gene regions which encode proteins associated with different cell types become heavily methylated and epigenetically suppressed. Whereas, gene regions which encode useful proteins for the specific cell type experience high levels of histone acetylation and epigenetic promotion of gene expression. The extent to which and selection of which genes are suppressed and which genes are expressed determines the final identity and specialization of the cell.
In 2006, Professor Shinya Yamanaka managed to revert human adult fibroblasts to a pluripotent state using 4 key transcription factors — Sox2, Oct4, Klf4 and c-Myc. These factors are heavily expressed in embryonic stem cells and hence are associated with pluripotency. Yamanaka and his team found that the over-expression of these factors in adult body cells can lead to a reversal of differentiation and restoration of the cell to a pluripotent state. This was a great breakthrough in the field as it meant one could now induce a pluripotent state upon adult body cells, artificially, creating much potential for new therapies and treatments using these iPSCs.
It was previously possible to harvest and store pluripotent stem cells, however, they had to be embryonic stem cells. This cause a myriad of ethical issues as one had to destroy an embryo in the process. With iPSCs this was no longer a problem as they could be harvested from adult tissues such as the skin which regenerate naturally anyway, leaving little to no room for controversy.
The potential uses of iPSCs are nearly limitless, with their ability to divide, seemingly infinitely in the presence of enough nutrients, and their ability to differentiate into any cell type, they can be used to replace dying tissues in degenerative diseases or in cases of severe tissue damage. With the ability to direct the differentiation of iPSCs into desired cell types with decent efficiency, one can generate enough cells to replace a portion of tissue an implant the differentiated iPSCs into the body. Multiple animal studies have demonstrated this potential by replacing cardiac tissue in mice with differentiated iPSCs to help recovery after a heart attack with great success. Moreover, another study directed the differentiation of mouse-derived iPSCs into insulin producing β-cells to treat type 1 diabetes. Furthermore, it seems as though iPSCs can be used just like embryonic stem cells to generate nerve cells which can be incredibly useful for treatment of neurodegenerative disorders such as dementia or other conditions such as depression. Lastly, it seems that techniques similar to inducing pluripotency may be used for epigenetic rejuvenation of cells and the reversal of aging. Nevertheless, one has to be very careful with this, attempting to reverse some effects of aging without fully converting adult cells back into a pluripotent state, as the latter can and has lead to cancer in mice. Other treatments have similar limitations — the field is still very much developing and there are still abnormalities in some iPSCs such as irregular epigenetic marks, it is also rather likely that there may be other problems with iPSCs which have gone unnoticed as of yet. For this reason, I believe, extensive research and clinical trials are required before any treatments are used to treat human diseases, especially in vivo. Just like a catastrophe can be a blessing in disguise, a misused blessing can lead to a catastrophe.
References
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