In order to convert cells from almost any tissue in our bodies into induced pluripotent stem cells (iPSCs) requires a detailed knowledge of the reprogramming process. Initiating the reprogramming process differs from one cell type to another, but the cellular and genetic mechanics of reprogramming might be largely the same.
A research team at the Spanish National Cancer Research Center headquartered in Madrid, Spain, and headed by Ralph P. Schneider from the Telomeres and Telomerase Group, which is led by Maria A. Blasco, have discovered that a gene called TRF1 is essential for nuclear reprogramming.
TRF1 or telomere repeat binding factor 1 is a member of a complex of proteins called the “shelterin complex” that binds to the ends of chromosomes (known as telomeres) and protects them. Mouse embryos that lack TRF1 die very early during embryonic life and if an adult tissue is missing TRF1, it shrinks and stops working (organ atrophy).
A variety of observations have established that pluripotent cells have long, intact telomeres. Furthermore, pluripotent cells have a very active telomerase enzyme, which is the enzyme that synthesizes the telomere ends of each chromosome. Telomeres not only protect the structural integrity of the chromosomes, but they also serve as a template or starting point for the replication and extension of the telomerase by telomerase.
In the cell, the telomere does not exist in isolation, but it is embedded in a complex of DNA and the shelterin complex proteins, of which TRF1 is a member. Pluripotent cells have very long telomeres, but it is uncertain if the shelterin complex components are necessary to maintain the pluripotent state (see Marión RM, Blasco MA. Curr Opin Genet Dev. 2010 Apr;20(2):190-6).
To investigate this question, Schneider and others constructed a version of TRF1 that was fused to a glowing proteins in order to track its function during reprogramming. Then they injected this construct into mouse embryonic stem cells and made genetically engineered mice that carried this glowing version of TRF1.
When they tracked TRF1 function in adult cells, embryonic cells, and stem cells, it was clear that TRF1 is a superb marker for stem cells. It distinguishes adult stem cells from non-stem and is also indispensable for stem cell function. In fact, TRF1 is such a good marker for stem cells that it can be used to isolated stem cells from surrounding cells.
Pluripotent stem cells show the highest levels of TRF1 expression. In fact, in iPSCs, the expression of TRF1 goes from very low to rather high. This led Schneider and his colleagues to suggest that TRF1 is an indicator of pluripotency. To corroborate their hypothesis, Schneider and others showed that the more pluripotent the iPSC stem cell line, the higher the levels of expression of TRF1. Also, TRF1 is required to maintain pluripotency and is also required for the induction of pluripotency. TRF1 inhibits cell death and the expression of TRF1 is directly activated by the pro-pluripotency gene Oct4.
Thus TRF1 is another gene required for iPSC production. It also seems to be required for iPSC production regardless of the tissue from which is comes from.