This post was contributed by Kusumika (Kushi) Mukherjee.
The ultimate goal in the field of regenerative medicine is to replace lost or damaged cells. Here, I will discuss the two major processes by which an adult somatic cell is converted to a different cell type for regeneration and repair and situations where one process is favored over the other.
Cell conversion happens via:
A differentiated somatic cell reverts back to a pluripotent state, which then proliferates and is redifferentiated to a different cell type.
In this process, differentiated adult somatic cells are converted to cells of a different lineage, without dedifferentiating into a pluripotent stage.
The reversal of a differentiated cell type to an undifferentiated state and then redifferentiation into the cell type of choice in vitro is known as reprogramming . The process can be divided into two stages:
- Dedifferentiation - Conversion of adult somatic cells into the pluripotent state.
- Redifferentiation - Conversion of the pluripotent cells into differentiated cells of choice.
The dedifferentiation stage involves overexpression of four reprogramming factors- OCT4, SOX2, KLF4, and C-MYC - that induce a differentiated somatic cell to revert back to a pluripotent stage (iPSC formation) [2, 3]. The iPSCs then proliferate and redifferentiate to another cell type of choice. The four reprogramming factors can be delivered and expressed in multiple somatic cells via various methods. Some of the more common delivery methods include retrovirus , lentivirus , adenovirus , Sendai virus , plasmid electroporation (episomal) [7, 8] and mRNA transfection . Many of the plasmids used for these methods can be found on Addgene’s stem cell page. On this page, you can also find a table with a list of methods and the species they were used in. iPSCs have now been generated from many different types of somatic cells. The goal is to use cells that can be easily isolated from donors. Apart from fibroblasts, human keratinocytes from hair pluck, peripheral blood cells, and renal epithelial cells from urine are some of the easily isolated somatic cells that have been reprogrammed to iPSCs successfully [10-12].
The next stage of reprogramming consists of redifferentiation of iPSCs into the cell type of choice. This step is sometimes also referred to as “directed differentiation.” Specific cell media, supplements, bioactive small molecules, and growth factors are used to control the cell fate of iPSCs and differentiate them into different cell lineages . Over the last decade, many cell types have been successfully differentiated from human iPSCs. Below is a list of some of these cell types :
|Germ cells ||Hepatocytes |
|Pancreatic β-cells ||Intestinal tissue |
|Lung epithelial cells ||Red blood cells [19, 20]|
|Osteoclasts and osteoblasts [21, 22]||Cardiomyocytes [23, 24]|
|Smooth muscle cells ||Skeletal myogenic cells |
|Chondrocytes ||Adipocytes |
|Keratinocytes ||Photoreceptors |
|Otic hair cells ||Neurons [32, 33, 34]|
You can find a variety of plasmids for differentiation here.
Dedifferentiation to an intermediate pluripotent state is not always obligatory in cell conversion processes . Rather than reprogram cells all the way back to their most primitive pluripotent stem cell state, through transdifferentiation adult somatic cells are converted directly into a different cell type, bypassing the lengthy processes of reprogramming. The process was first observed in the regenerating lens of the newt over 100 years ago . While natural transdifferentiation is rare in mammals, an example is observed in the pancreas when excess β-cell damage results in the transdifferentiation of glucagon-producing α-cells into insulin-producing β-like-cells [37, 38].
In 1987, Davis et al. reported one of the earliest examples of transdifferentiation in vitro where treatment of mouse fibroblasts with 5-azacytidine led to their conversion into myoblasts . In 2000, Ferber et al. showed for the first time that mouse liver cells could be transdifferentiated in vivo to pancreatic β-like-cells with the expression of pancreatic and duodenal homoeobox gene1 (PDX1) . In recent works, transdifferentiation is usually carried out by expressing transcription factors specific to the lineage of the target cell in the original somatic cells . The in vivo and in vitro methods are similar except that the vectors carrying the transdifferentiation factors are directly injected into the organ of interest for in vivo transdifferentiation. Multiple cell types such as fibroblasts, hepatocytes, and pancreatic exocrine cells have been successfully transdifferentiated into neurons and β-cells [40-42].
Reprogramming and transdifferentiation: When to use one over the other
Both reprogramming and transdifferentiation convert differentiated somatic cells into another cell type. However, these two approaches differ in several ways. Below is a table listing some of critical differences (adapted from Zhou and Melton, 2008, ):
|in vivo or in vitro||in vitro||Both in vitro and in vivo|
|Epigenetic marks||Complete removal to convert to iPSCs and re-establishment after||Partial rearrangement from one cell type to another|
|Time and resources||More (due to additional pluripotency stage)||Less|
|Safety||Less (C-MYC, a known oncogene is one of the reprogramming factors)||More|
|Chances of accumulation of harmful mutations||More, due to selection from more cycles of proliferation||Less|
|Cell types||All types||Less|
|Ease of genetic correction||Easy||Difficult|
Overall, reprogramming is very flexible. It offers unlimited potential to produce all cell types in the body. On the other hand, only few cell types have been currently transdifferentiated successfully, limiting the utility of this process. Moreover, it is much easier to genetically modify cells during the reprogramming process as they are propagated in vitro as part of the process. This opens up a wide range of possibilities in clinical situations. In cases where the objective is to fix a disease-inducing genetic mutation in a patient, trying to transdifferentiate any of the patient’s cells will not alleviate the problem. The best option then would be to dedifferentiate cells from the patient in vitro then correct the damaged gene in the resulting iPSCs before differentiating the cells into the correct lineage and returning them back to the patient.
In this post, I have detailed the two major processes by which cells are converted to replenish and repair cells that are lost or damaged. Both transdifferentiation and reprogramming give researchers the ability to convert a differentiated cell to a different cell type. While transdifferentiation is suited for switching cell types between similar lineages, reprogramming is more versatile and universal.
Many thanks to our guest blogger, Kusumika (Kushi) Mukherjee.
Kusumika (Kushi) Mukherjee is the Editor of Trends in Pharmacological Sciences, a Cell Press reviews journal. She joined Cell Press to pursue a career in science communication and publishing after completing her postdoctoral training from Massachusetts General Hospital and Harvard Medical School. Connect with her on LinkedIn @ https://www.linkedin.com/in/
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