Human Embryonic Stem Cell Communication Network Discovered


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.

ERK_pathway

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.

ELK1 Interaction with ERK2

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.

Model of the Transcriptional Regulatory Network of ERK2 Signaling in hESCsTranscription factors such as ELK1 link ERK2 to sequence-specific regulation of gene expression. ERK2 and ELK1 colocalization defines three distinct modules that target different sets of genes. In this model, combinatorial binding of ERK2 and ELK1 with transcription factors, chromatin regulators, and the basal transcriptional machinery integrates external signaling into the cell-type-specific regulatory network. In hESCs, ERK2 and ELK1 participate in the regulation of pluripotency and self-renewal pathways, whereas differentiation genes are repressed.
Model of the Transcriptional Regulatory Network of ERK2 Signaling in hESCsTranscription factors such as ELK1 link ERK2 to sequence-specific regulation of gene expression. ERK2 and ELK1 colocalization defines three distinct modules that target different sets of genes. In this model, combinatorial binding of ERK2 and ELK1 with transcription factors, chromatin regulators, and the basal transcriptional machinery integrates external signaling into the cell-type-specific regulatory network. In hESCs, ERK2 and ELK1 participate in the regulation of pluripotency and self-renewal pathways, whereas differentiation genes are repressed.

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.”

Mesenchymal Stem Cell Transplantation Improves Heart Remodeling After a Heart Attack


Stem cell scientists from the University of Maryland, Baltimore have used bone marrow mesenchymal stem cells (MSCs) to treat sheep that had suffered a heart attack. They found that the injected stem cells prevented the heart from deteriorating.

This work was a collaboration between the laboratories of Mark Pittenger, ZhonGjun Wu and Bartley Griffith from the Department of Surgery and the Artificial Organ Laboratory.

After a heart attack, the region of the heart that was deprived of oxygen undergoes cell death and is replaced by a heart scar. However, the region next to the dead cells also undergo problematic changes. The cells in these regions adjacent to dead region must contract more forcibly in order to compensate for the noncontracting dead region. These cells enlarge, but some undergo cell death due to inadequate blood supply. There are other changes that can occur, such as abnormalities in Calcium ion handling and poor contractability.

Thus, the problems that result from a heart attack can spread throughout the heart and cause heart failure. In this experiment, the U of Maryland scientists injected MSCs into the sheep hearts four hours after a heart attack to determine if the stem cells could prevent the region adjacent to the dead heart cells from deteriorating.

In this experiment, bone marrow MSCs were isolated from sheep bone marrow and put through a battery of tests to ensure that they could differentiate into bone, cartilage, and fat. Once the researchers were satisfied that the MSCs were proper MSCs, they induced heart attacks in the sheep, and then injected ~200 million MSCs into the area right next to the region of the heart that died.

After 12 weeks, tissue biopsies from these sheep hearts were taken and examined. Also, the sheep hearts were measured for their heart function and structure.

The sheep that did not receive any MSC injections continued to deteriorate and showed signs of stress. The cells adjacent to the dead region expressed a cadre of genes associated with increased cell stress. Furthermore, there was increased cell death and evidence of scarring in the region adjacent to the death region. There was also evidence of Calcium ion-handling problems in the adjacent tissue and increased cell death.

On the other hand, the hearts of the sheep that had received injections of MSCs into the area adjacent to the dead region showed a reduced expression of those genes associated with increased cell stress. Also, these hearts contracted better than those that had not received stem cell injections. There was also less cell death, less scarring, and no evidence of Calcium ion-handling problems.

Changes that occur in the heart after a heart attack are collectively referred to as “remodeling.” Remodeling begins regionally, in those areas near the dead heart cells, but these deleterious changes spread to the rest of the heart, resulting in heart failure. The injections of MSCs into the area next to the dead region clearly prevented remodeling from occurring.

This pre-clinical study is a remarkable study for another reason: the MSCs used in this study were allogeneic. Allogeneic is a fancy way of saying that they did not come from the same animal that suffered the heart attack, but from some other healthy animal. Therefore, the delivery of a donor’s MSCs into the heart of a heart attack patient could potentially prevent heart remodeling.

The main problem with this experiment is that the MSCs were injected directly into the heart muscle. In humans, such a procedure requires special equipment and carries potential risks that include perforation of the heart wall, rupture of the heart wall, or further damaging the heart muscle. Therefore, if such a technology could be adapted to a more practical delivery system in humans, then certainly human clinical trials should be forthcoming.

See Yunshan Zhao, et al., “Mesenchymal stem cell transplantation improves regional cardiac remodeling following ovine infarction.” Stem Cells Translational Medicine 2012;1:685-95.