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 http://www.translational-medicine.com/content/pdf/1479-5876-7-106.pdf.

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 benefit 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.