Researchers at Johns Hopkins, using human embryonic
stem cells, have uncovered the molecular
underpinnings of one of the earliest steps in human
development. 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 is 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 it 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 the 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," said Linzhao
Cheng, 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," Cheng said. 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 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," Cheng said.
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 the 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 Johns Hopkins.