Cells use a variety of mechanisms to talk to each other. These signaling pathways are called “signal transduction” pathways, and they vary extensively from one cell type to another.
Therefore, it should be no surprise that human embryonic stem cells signal to each other. The precise signal transduction pathway that human embryonic stem cells use to communicate with each other is the subject of a research project from a laboratory in Singapore.
Human embryonic stem cells or hESCs can differentiate into any adult cell type. The factors that keep hESCs in their pluripotent state are of interest to stem cell scientists because they might allow them to better direct the differentiation of hESCs or even grow them in culture better.
Cell-to-cell communication is vitally important to multicellular organisms. The coordinated development of tissues in the embryo that culminate in the formation of specific organs requires that cells receive signals and respond accordingly. If there are errors in these signals, the cells will respond differently and the embryo will either be grossly abnormal, or the cell might divide uncontrollably to make a tumor.
Human ESCs communicate by means of a signal transduction pathway known as the extracellular regulated kinase or ERK pathway. The ERK signal transduction pathway begins with the binding of a growth factor receptor by a growth factor. These growth factors are almost always bound to the extracellular matrix, which is the goo that surrounds cells and provides a structure in which the cells live. The binding of the receptor causes the receptor to pair with another copy of itself, and that activates the bits of the receptor found inside the cell (tyrosine kinase domain for the interested). The activated receptor attaches phosphates to itself, which causes particular proteins to find and bind the receptor, which recruits particular proteins to the cell membrane. One of the recruited proteins is a protein kinase called RAF. RAF attaches phosphate groups to the protein kinase MEK, and MEK attaches phosphate groups to the protein kinase ERK. Once ERK has a phosphate attached to it, it can move into the nucleus and regulate transcription factors involved in the control of gene expression. Thus a phenomenon that began at the cell membrane culminates in a change in gene expression.
Stem cell scientists a A*STAR’s Genomic Institute of Singapore and the Max Planck Institute of Molecular Genetics (MPIMG) in Berlin, Germany studied how genetic information is accessed in hESCs. To do this they mapped the kinase interactions across the entire human genome (kinases are enzymes that attach phosphate groups to other molecules) and discovered that ERK2, a protein that belongs to the ERK signal transduction pathway targets important sites such as non-coding genes, and histones, cell cycle, metabolism, and stem cell-specific genes.
The ERK signaling pathway involves an additional protein called ELK1 that interacts with ERK2. However, this research team discovered that ELK1 has a second, totally opposite function. At genomic sites not targeted by ERK signaling, ELK1 silences genetic information, which keeps the cell in its undifferentiated state.
The authors propose a model that integrates this bi-directional control to keep the cell in the stem cell state, in which genes necessary for differentiation are repressed by ELK1 that is not associated with ERK2, and cell-cycle, translation and other pluripotency genes are activated by ELK1 in association with ERK2 or ERK2 plus other transcription factors.
First author Jonathan Göke from Stem Cell and Developmental Biology at the GIS said, “The ERK signaling pathway has been known for many years, but this is the first time we are able to see the full spectrum of the response in the genome of stem cells. We have found many biological processes that are associated with this signaling pathway, but we also found new and unexpected patterns such as this dual-mode of ELK1. It will be interesting to see how this communication network changes in other cells, tissues, or in disease.”
A co-author of this study, Martin Vingron said, “A remarkable feature of this study is, how information was extracted by computational means from the data.”
Professor Ng Huck Hui, managing author of this paper, added, “This is an important study because it describes the cell’s signaling network and its integration into the general regulatory network. Understanding the biology of embryonic stem cells is a first step to understanding the capabilities and caveats of stem cells in future medical applications.”