Researchers at Johns Hopkins have uncovered the molecular underpinnings of one of the earliest steps in human development using human embryonic stem cells.
Their identification of a critical signal mediated by the protein BMP-4 that drives the differentiation of stem cells into what will become the placenta, will be published in the April issue of Cell Stem Cell.
The finding, they say, also highlights one aspect of human cell biology that has not been replicated in other animal model systems. And is virtually impossible to use anything other than human embryonic stem cells to gather information of this kind.
One reason for the excitement, the investigators say, is that the system can provide a research model to study very early human development, including the formation of placenta which develops from the same early embryo.
“The finding was serendipitous and at the same time a very important addition to our understanding of early human development,” says Linzhao Cheng, Ph.D., an associate professor of gynecology and obstetrics and co-director of the stem cell program of the Johns Hopkins Institute for Cell Engineering. “This is one area of stem cell biology where human and mouse differ significantly and we never would have discovered this if we had limited our studies to using only mouse embryonic stem cells. Adult human stem cells just didn't work for this.”
The research team uncovered their find during efforts to study a rare human blood disorder caused by mutations in a gene called PIG-A. According to Cheng, a good model to study the disease does not exist as engineered mice without the gene either die before birth, or do not reproduce symptoms found in patients.
So using a conventional genetic engineering tool, the researchers tried for years-literally-to knock out PIG-A in adult stem cells, without success. They then turned to knocking out PIG-A in human embryonic stem cells.
“Only with the human embryonic stem cells could we grow out the rare cells engineered to lack PIG-A,” says Cheng. The result was the growth of two human embryonic stem cell lines that lack PIG-A, and therefore do not contain any proteins known as glycosylphosphatidylinositol (GPI) anchor proteins on the cell's surface. GPI anchor proteins attach many different types of proteins involved in cell communication to a cell's outside surface. Without certain GPI proteins, cells may not function properly.
Then the researchers took one more step to verify that their engineered embryonic stem cells behaved like normal stem cells. “We just wanted to make sure that our knockout cells could still differentiate and specialize,” says Cheng.
One of the earliest steps of embryonic stem cell differentiation in normal embryonic development is the development of the trophoblast, a layer of seed cells that later develops into the placenta.
Trophoblast differentiation, according to Cheng, occurs when embryonic stem cells are exposed to BMP-4 protein, either naturally or in lab.
To their surprise, however, when they treated their knockout cells with BMP-4, the cells did not become trophoblasts.
Only when they added the PIG-A gene back into their cells did BMP-4 do its work and cause the cells to become trophoblasts, allowing the researchers to conclude that trophoblast differentiation depends on certain cell surface proteins to receive the BMP-4 signal.
The research was funded by the National Institutes of Health and Johns Hopkins Institute for Cell Engineering.
Authors on the paper are Guibin Chen, Zhaohui Ye, Xiaobing Yu, Jizhong Zou, Prashant Mali, Robert Brodsky and Cheng, all of Hopkins.