Induced pluripotent stem cells (iPSCs) have many of the characteristics of embryonic stem cells, but are made from mature cells by means of a process called cell reprogramming. To reprogram cells, particular genes are delivered into mature cells, which are then cultured until they h:ave the growth properties of pluripotent cells. Further tests are required to demonstrate that the growing cells actually are iPSCs, but once they pass these tests, these cells can be grown in culture indefinitely and, ideally, differentiated into just about any cell type in our bodies (caveat: some iPSC lines can only differentiate into particular cell lineages). Theoretically, any cell type can be reprogrammed into iPSCs, but work from many laboratories has demonstrated that the identity of the founder cell influences the type of cell into which it can be reprogrammed.
Founder cells can be easily acquired from a donor and come in one of four types: fibroblasts (in skin), keratinocytes (also from skin), peripheral and umbilical cord blood, and dental pulp cells (from baby teeth). A variety of laboratories from around the world have made iPSC lines from a gaggle of different founder cells. Because of the significant influence of founder cells for iPSC characteristics, the use of iPSCs for regenerative medicine and other medical applications requires that the desired iPSC line should be selected based on the founder cell type and the characteristics of the iPSC line.
However, the founder cell identity is not the only factor that affects the characteristics of derived iPSC lines. The methods by which the founder cells are reprogrammed can also profoundly contribute to the differentiation efficiency of iPSC lines. According to Yoshinori Yoshida, Associate Professor at the Center for iPSC Research and Application (CiRA) at Kyoto University, the most commonly used methods of cell reprogramming utilize retroviruses, episomal/plasmids, and Sendai viruses to move genes into cells.
The cells found in blood represent a diverse group of cells that includes red blood cells that carry oxygen, platelets that heal wounds, and white blood cells that fight off infection. All the cells in blood are made by bone marrow-specific stem cells called “hematopoietic stem cells.” The production of clinical grade blood has remained a kind of “holy grail” for cellular reprogramming studies. Some scientists have argued that in order to make good-quality hematopoietic cells, the best founder cells are hematopoietic cells. Is this true? Yoshida and his colleagues examined a very large number of iPSC lines that were made from different founder cells and with differing reprogramming methods. The results of these experiments were published in the journal Cell Stem Cell (doi:10.1016/j.stem.2016.06.019).
Remarkably, Yoshida and his crew discovered that neither of these factors has a significant effect. What did have a significant effect were the expression of certain genes and the position of particular DNA methylations. These two factors were better indicators of the efficiency at which an iPSC line could differentiate into the hematopoietic stem cells.
“We found the IGF2 (Insulin-like Growth Factor-2) gene marks the beginning of reprogramming to hematopoietic cells”, said Dr. Masatoshi Nishizawa, a hematologist who works in Yoshida’s lab and is the first author of this new study. Higher expression of the IGF2 gene is indicative of iPSCs initiating differentiation into hematopoietic cells. Even though IGF2 itself is not directly related to hematopoiesis, its uptake corresponded to an increase in the expression of those genes involved in directing differentiation into hematopoietic stem cells.
Although IGF2 marked the beginnings of differentiation to hematopoietic lineage, the completion of differentiation was marked by the methylation profiles of the iPS cell DNA. “DNA methylation has an effect on a cell staying pluripotent or differentiating,” explained Yoshida. Completion of the final stages of differentiation was highly correlated with less aberrant methylation during the reprogramming process. Blood founder cells showed a much lesser tendency to display aberrant DNA methylation patterns than did other iPSC lines made from other founder cells. This probably explains why past experiments seemed to indicate that the founder cell contributes to the effectiveness of differentiating iPS cells to the hematopoietic stem cell lineage.
These findings reveal molecular factors that can be used to evaluate the differentiation potential of different iPSC lines, which should, hopefully, expedite the progression of iPSCs to clinical use. Nishizawa expects this work to provide the basis for evaluating iPSC lines for the preparation of other cell types. “I think each cell type will have its own special patterns,” he said.