A research team at the Ansary Stem Cell Institute and Weill Cornell Medical College led by Shabin Rafii has succeeded in reprogramming amniotic stem cells into mature blood vessel cells. These blood vessel cells can be banked and potentially used for human therapeutic treatments.
Rafii’s lab has been interested in endothelial cells (the cells that compose blood vessels) for many years, and they used amniotic stem cells for these experiments. Other researchers have shown that introducing three different transcription factors into pluripotent stem cells (ETV2, FLI1 and ERG1), can drive the stem cells to differentiate into induced vascular endothelial cells (iVECs). These cells, however, were immature and were not completely differentiated versions of endothelial cells. Furthermore, the iVECs were unstable because they tended to differentiate into non-endothelial cell types while in culture. Therefore, Rafii was sure this regiment was on the right track, but it needed tweaking.
To perfect this protocol, Rafii and co-workers chose to work with amniotic stem cells. Amniotic stem cells are a wonderfully robust stem cell population that have the ability to form a wide variety of cell types and do not form tumors. During development, the embryo is surrounded by a thin membrane called the amniotic membrane. At Carnegie Stage 6, at the end of the 2nd week of development (day 13-14), the embryo is about 0.2 millimeters long. A veil of tissue grows over the disc-like structure at the very top of the embryo. This veil of tissue is called the amnion and the cavity is generates is the amniotic cavity. The embryo grows within this cavity, suspended in amniotic fluid, and as the embryo grows, the amnion grows with it, as does the size of the amniotic cavity within which he embryo remains suspended all the way through embryonic and fetal development until birth.
First of all, Rafii and colleagues transiently expressed ETV2 in the amniotic stem cells and found that they differentiated into iVECs. Therefore, they tried co-expressing FLI1/ERG1 with ETV2, and found that the cells expressed several vascular-specific proteins and assumed a shape that matched mature endothelial cells (ECs). Next, they briefly shut off TGFβ signaling. This pushed the cells over the edge and they became endothelial cells. Their success rate was around 20 percent, which is astounding, since most reprogramming protocols usually sport as success rate of around 1 percent.
When Rafii and his colleagues examined the gene expression profile of the endothelial cells derived from amniotic stem cells, they found that their cultured endothelial cells were very similar to adult endothelial cells: vascular-specific genes were expressed and nonvascular genes were silenced.
Functional assays further confirmed that they are converted amniotic stem cells into endothelial cells. When Raffi and others gave these cells a gel-like matrix called Matrigel, they formed a filigree of blood vessels in the Matrigel plug. When they transplanted their cultured endothelial cells in a living animal, they were able to regenerate the internal sinuses and vasculature of a sick liver. Thus this protocol reprogrammed mature amniotic stem cells into fully functional endothelial cells clinical-scale expansion potential.
The therapeutic potential of this work is certainly not lost on Rafii. He explained, “There is no curative treatment available for patients with vascular diseases, and the common denominator to all these disorders is dysfunction of blood vessels, specifically endothelial cells that are the building blocks of the vessels.”
These cultured endothelial cells, however, do more than just make blood vessels. They produce growth factors such as vascular endothelial growth factor (VEGF) that promote the maintenance, repair, and regeneration of the vasculature. Damaged blood vessels may not be able to stimulate the repair of the organs they service, but newly infused endothelial cells could.
Also tissue engineers tend to grow tissues and artificial organs in porous three-dimensional scaffolds. These cultured endothelial cells could easily form functional blood vessels on such surfaces if they were introduced into an injured organ. Rafii noted that his cultured endothelial cells could be a huge boost for translational vascular medicine. He optimistically predicts that four years of preclinical work could persuade the FDA to approve human clinical trials in which his cells are used to treat vascular disorders. Also banking tissue-typed amnion-derived vascular endothelial cells could establish an inventory of cells for the treatment of diverse disorders.
This work also suggests that amniotic stem cells could potentially be reprogrammed to efficiently form other cell types as well.