New Approach Speeds Up Differentiation of Stem Cells
Medgadget.com, January 22, 2010
Endothelial cells comprise the lining of the blood
vessels. They play a fundamental role in overall health due to their ability
to: a) act as a selective barrier between circulation and tissue, allowing
certain cells to enter tissue from the blood stream while not allowing others;
b) keeping the blood vessels free of clotting; and c) controlling contraction of
the blood vessels. Diseases such as atherosclerosis are believed to occur
because of injury to the endothelial cells. The Ross classic paradigm states
that seemingly disparate cardiovascular risk factors act on a final common
pathway that culminates in endothelial-cell injury (Ross. The pathogenesis of
atherosclerosis: a perspective for the 1990s. Nature 1993;362:801-9).
The body has naturally occurring stem cells that regenerate
the endothelium called “endothelial progenitor cells” (EPC). These cells are
found in circulation and their levels are associated with decreased risk of
cardiovascular disease (Hill et al. New England Journal of Medicine 348;7).
Additionally, known cardiovascular risk factors such as smoking have been
demonstrated to directly decrease circulating EPC numbers, which normalize upon
cessation, as seen in this video
http://www.youtube.com/watch?v=DA2box-mP4g. Food supplements such as Stem-Kine
have been demonstrated to increase the number of circulating EPC, and therefore
possibly reducing cardiovascular risk factors
An alternative method of increasing endothelial health is
by administration of EPC from exogenous sources. For example, in patients with
an advanced form of peripheral artery disease, termed critical limb ischemia,
administration of bone marrow cells has been demonstrated to cause therapeutic
http://www.youtube.com/watch?v=OwIOL13vXQ4. This is believed to occur at
least in part due to the high concentration of EPC in this tissue.
Theoretically the administration of EPC would be therapeutically beneficial in
that high numbers of progenitor cells may be administered on a as-needed basis.
The major limitation of EPC administration is that it is difficult to generate
large number of these cells.
A recent study (James et al. Expansion and maintenance
of human embryonic stem cell-derived endothelial cells by TGFbeta inhibition is
Id1 dependent. Nat Biotechnol. 2010 Jan 17) reported a novel method of
screening for agents that cause stem cells to develop into EPC. Investigators
from the Howard Hughes Medical Institute, the Ansary Stem Cell Institute, and
the Department of Genetic Medicine, Weill Cornell Medical College constructed
embryonic stem cell lines that glow green upon differentiation into endothelial
cells. Specifically, they used genetic engineering to insert the endothelial
cell-specific VE-cadherin promoter into embryonic stem cells, so that when the
promoter is active it drives expression of green fluorescent protein (GFP),
which causes only endothelial cells to glow green.
Out of numerous approaches screened, it was found that the
inhibition of the growth factor TGF-beta resulted in profound increases in
formation of endothelial cells from embryonic stem cells. However the blockade
of TGF-beta had to be performed at certain timepoints of the in vitro culture in
order to attain maximum yield.
Dr James, who was the lead author of the study stated
“Blocking TGF-beta at just the right time during cell culturing dramatically
increased the number of vascular endothelial cells produced. Previously,
researchers needed to start with five stem cells for every endothelial cell they
hoped to generate. But with the new method, starting with five stem cells
generated 40 endothelial cells“.
The ability to screen stem cells for drugs that stimulate
differentiation will allow for use of high-throughput approaches towards
identifying agents that selectively modulate generation of endothelium. This is
a very powerful tool that can be used for development of compounds which, like
Stem-Kine, may be useful in increasing generation of new blood vessels. This
would have major implications in the treatment of cardiovascular disease.