Mechanism That Triggers Differentiation Of Embryo Cells Discovered

The mechanism whereby embryonic cells stop being flexible and turn into more mature cells that can develop into specific tissues has been discovered by scientists at the Hebrew University of Jerusalem. The discovery has significant consequences towards furthering research that will eventually make possible medical cell replacement therapy based on the use of embryonic cells.

At a very early stage of human development, all cells of the embryo are identical, but unlike adult cells are very flexible and carry within them the potential to become any tissue type, whether it be muscle, skin, liver or brain.

This cell differentiation process begins at about the time that the embryo settles into the uterus. In terms of the inner workings of the cell, this involves two main control mechanisms. On the one hand, the genes that keep the embryo in their fully potent state are turned off, and at the same time, tissue-specific genes are turned on. By activating a certain set of genes, the embryo can make muscle cells. By turning on a different set, these same immature cells can become liver. Other gene sets are responsible for additional tissues.

In a recent paper, published in the journal, Nature Structural and Molecular Biology, Professors Yehudit Bergman and Howard Cedar of the Hebrew University-Hadassah Medical School have deciphered the mechanism whereby embryonic cells stop being flexible and turn into more mature cells that can differentiate into specific tissues. Bergman is the Morley Goldblatt Professor of Cancer Research and Experimental Medicine and Cedar is the Harry and Helen L. Brenner Professor of Molecular Biology at the Medical School.

They found in their experiments, using embryos from laboratory mice and cells that grow in culture, that this entire process is actually controlled by a single gene, called G9a, which itself is capable of directing a whole program of changes that involves turning off a large set of genes so that they remain locked for the entire lifetime of the organism, thereby unable to activate any further cell flexibility.

Their studies shed light not only on this central process, but also can have consequences for medical treatment. One of the biggest challenges today is to generate new tissues for replacing damaged cells in a variety of different diseases, such as Parkinson’s disease or diabetes. Many efforts have been aimed at “reprogramming” readily-available adult cells, but scientists have discovered that it is almost impossible to do this, mainly because normal tissues are locked in their fixed program and have lost their ability to convert back to fully potent, flexible, embryonic cells.

Now, with the new information discovered by Bergman and Cedar, the molecular program that is responsible for turning off cell flexibility has been identified, and this may clear the way towards developing new approaches to program cells in a controlled and specific manner.

Artificial Human Bone Marrow Created In A Test Tube

This development could lead to simpler pharmaceutical drug testing, closer study of immune system defects and a continuous supply of blood for transfusions.

The substance grows on a 3-D scaffold that mimics the tissues supporting bone marrow in the body, said Nicholas Kotov, a professor in the U-M departments of Chemical Engineering; Materials Science and Engineering; and Biomedical Engineering.

The marrow is not made to be implanted in the body, like most 3-D biomedical scaffolds. It is designed to function in a test tube.

Kotov, principal investigator, is an author of a paper about the research currently published online in the journal Biomaterials. Joan Nichols, professor from the University of Texas Medical Branch, collaborated on many aspects of the project.

"This is the first successful artificial bone marrow," Kotov said. "It has two of the essential functions of bone marrow. It can replicate blood stem cells and produce B cells. The latter are the key immune cells producing antibodies that are important to fighting many diseases."

Blood stem cells give rise to blood as well as several other types of cells. B cells, a type of white blood cell, battle colds, bacterial infections, and other foreign or abnormal cells including some cancers.

Cancer-fighting chemotherapy drugs can strongly suppress bone marrow function, leaving the body more susceptible to infection. The new artificial marrow could allow researchers to test how a new drug at certain potencies would affect bone marrow function, Kotov said. This could assist in drug development and catch severe side effects before human drug trials.

Bone marrow is a complicated organ to replicate, Kotov said. Vital to the success of this new development is the three-dimensional scaffold on which the artificial marrow grows. This lattice had to have a high number of precisely-sized pores to stimulate cellular interaction.

The scaffolds are made out of a transparent polymer that nutrients can easily pass through. To create the scaffolds, scientists molded the polymer with tiny spheres ordered like billiard balls. Then, they dissolved the spheres to leave the perfect geometry of pores in the scaffold.

The scaffolds were then seeded with bone marrow stromal cells and osteoblasts, another type of bone marrow cell.

"The geometrical perfection of the polymer molded by spheres is very essential for reproducibility of the drug tests and evaluation of potential drug candidates," Kotov said. "The scaffold for this work had to be designed from scratch closely mimicking real bone marrow because there are no suitable commercially products.

"Certain stem cells that are essential for immunity and blood production are able to grow, divide and differentiate efficiently in these scaffolds due to the close similarity of the pores in the scaffold and the pores in actual bone marrow."

The researchers demonstrated that the artificial marrow gives a human-like response to an infectious New Caledonia/99/H1N1 flu virus. This is believed to be a first.

To determine whether the substance behaves like real bone marrow, the scientists implanted it in mice with immune deficiencies. The mice produced human immune cells and blood vessels grew through the substance.

Scientists Probe Limits Of 'Cancer Stem-cell Model'

One of the most promising new ideas about the causes of cancer, known as the cancer stem-cell model, must be reassessed because it is based largely on evidence from a laboratory test that is surprisingly flawed when applied to some cancers, University of Michigan researchers have concluded.

By upgrading the lab test, the U-M scientists showed that melanoma---the deadliest form of skin cancer---does not follow the conventional cancer stem-cell model, as prior reports had suggested.

The findings, to be published as the cover article in the Dec. 4 edition of Nature, also raise questions about the model's application to other cancers, said Sean Morrison, director of the Center for Stem Cell Biology at the U-M Life Sciences Institute.

"I think the cancer stem-cell model will, in the end, hold up for some cancers," Morrison said. "But other cancers, like melanoma, probably won't follow a cancer stem-cell model at all. The field will have to be reassessed after more time is spent to optimize the methods used to detect cancer stem cells."

The cancer stem-cell model has steadily gained supporters over the last decade. It states that a handful of rogue stem cells drive the formation and growth of malignant tumors in many cancers. Proponents of the controversial idea have been pursuing new treatments that target these rare stem cells, instead of trying to kill every cancer cell in a patient's body.

But in a series of experiments involving human melanoma cells transplanted into mice, Morrison's team found that the tumor-forming cells aren't rare at all. They're quite common, in fact, but standard laboratory tests failed to detect most of them.

Scientists previously estimated that only one in 1 million melanoma cells has the ability to run wild, exhibiting the kind of unchecked proliferation that leads to new tumors. These aggressive interlopers are the cancer stem cells, according to backers of the model.

But after updating and improving the laboratory tests used to detect these aberrant cells, Morrison's team determined that at least one-quarter of melanoma cells are "tumorigenic," meaning they have the ability to form new tumors. The laboratory tests are known as assays.

"The assay on which the field is based misses most of the cancer cells that can proliferate to form tumors," Morrison said. "Our data suggest that it's not going to be possible to cure melanoma by targeting a small sub-population of cells."

Melanoma kills more than 8,000 Americans each year. The human melanoma cells used in the mouse experiments were provided---with the patients' consent---by a team from the U-M's Multidisciplinary Melanoma Program, one of the country's largest melanoma programs and part of the U-M Comprehensive Cancer Center.

"People were looking to the cancer stem-cell model as an exciting new source for the development of life-saving cures for advanced melanoma," said Dr. Timothy Johnson, director of the U-M melanoma program and a co-author of the Nature paper. "Unfortunately, our results show that melanoma does not strictly follow this model.

"So we'll need to redirect our scientific efforts and remain focused on the fundamental biological processes underlying the growth of melanomas in humans," said Johnson, a cutaneous oncologist. "And as we pursue new treatments for advanced melanoma, we'll have to consider that a high proportion of cancer cells may need to be killed."

Morrison and Johnson stressed that the team's findings do not broadly invalidate the cancer stem-cell model. Cancer stem cells likely do exist in some forms of cancer but are "probably much more common than people have been estimating," Morrison said.

The standard technique used to detect tumor-causing cancer cells in mouse transplants is called the NOD/SCID assay. NOD/SCID mice have defective immune systems. Scientists use the severely immunocompromised mice because the rodents don't reject transplanted human cancer cells the way normal mice would.

However, while the immune system in NOD/SCID mice is impaired, it's not completely inoperative. The mice lack T and B immune cells but still possess natural killer cells, which attack and destroy many of the transplanted human cancer cells.

Morrison's team replaced NOD/SCID mice with mice that lacked T cells, B cells and natural killer cells---and made a few other improvements to the assay. Using the modified assay, they found that about one in four transplanted melanoma cells formed tumors in the mice.

They concluded that previous studies using NOD/SCID mice vastly underestimated the number of tumor-causing melanoma cells, partly because natural killer cells wiped out many of the cancer cells. But once the natural killer cells were eliminated, the "more permissive conditions" allowed many of the transplanted melanoma cells to survive and thrive, the authors wrote.

Co-lead authors of the Nature paper are Life Sciences Institute research fellows Elsa Quintana and Mark Shackleton. In addition to Morrison and Johnson, other co-authors are U-M surgical oncologist Dr. Michael Sabel and U-M dermatopathologist Dr. Douglas Fullen.

The work was supported by the Howard Hughes Medical Institute, the Allen H. Blondy Research Fellowship and the Lewis and Lillian Becker gift.