When the egg is fertilized by a sperm, it is transformed into a single-celled embryo or zygote that is metabolically active and driven to divide and develop. The egg, on the other hand, is a rather inert cell from a metabolic perspective. What is it in the egg that allows it to transform into something so remarkably different?
A new study by Swea-Ling Khaw and others in the laboratory of Ng Shyh-Chang at the Genome Institute of Singapore (GIS) has elucidated two main factors that help rejuvenate the egg and might also help reprogram adult cells into induced pluripotent stem cells (iPSCs).
Eggs express large amounts of a protein called Tcl1. Tcl1 suppresses the function of old, potentially malfunctioning mitochondria (the structure in cells that makes the energy for the cell). This suppression prevents damaged mitochondria from adversely affecting the egg’s transformation from into an embryo.
Remember also that if an adult cell is fused to an egg, it can cause the egg to divide and form an early embryo. Therefore, the egg cytoplasm is able to reprogram adult cells as well, and Tcl1 seems to play a role in this reprogramming capability as well.
In a screen for genes that are important to the reprogramming process, Shyh-Chang’s laboratory isolated two genes, Tcl1 and Tcl1b1. Further investigation of these two proteins showed that Tcl1 affects mitochondria by inhibiting a mitochondrial protein called polynucleotide phosphorylase (PNP). By locking PNP in the cytoplasm rather than the mitochondria, the growth and function of the mitochondria are inhibited. Tcl1b1 activates the Akt kinase, which stimulate cell growth, survival, and metabolism.
In a review article in the journal Stem Cells and Development, Anaïs Wanet and others explain that energy production in pluripotent stem cells is largely by means of glycolysis, which occurs in the cytoplasm. Mitochondria in pluripotent stem cells are immature subfunctional. When adult cells are reprogrammed into iPSCs, mitochondria function is shut down and energy production is largely derived from glycolysis. When the cells differentiate, the mitochondria are remodeled and become functional once again. Tcl1 is the protein that help shut down the mitochondria so that the pluripotent state can ensue and Tcf1b1 gears up the pluripotent stem cells to grow and divide at will.
Given this remarkable finding, can Tcf1 help make better iPSCs? Almost certainly, but how does one use this important factor to make better iPSCs? That awaits further experimentation. Additionally, this finding might also help aging and infertility issues as well. Hopefully this work by Shyh-Chang and her colleagues will lead to many more fruitful and exciting experiments.