University of Wisconsin at Madison Stem Cell researchers led by Igor Slukvin discovered two genetic programs that can convert pluripotent stem cells into the wide array of white and red blood cells found in human blood (pluripotent means “capable of developing into more than one organ or tissue and not fixed as to potential development).
This research has ferreted out the actual pathway used by the developing human body to make blood-based cells at the early stages of development.
During embryonic development, blood formation, which includes the formation of blood cells and blood vessels from the same progenitor cell; a cell called a hemangioblast. This begins in week three of development in the extraembryonic mesoderm or the primary embryonic umbilical sac, which is also known as the yolk sac. Also, the connecting stalk and chorion contain blood islands as well. These blood islands are rich in particular growth factors such as vascular endothelial growth factor (VEGF) and placental growth factor (PIGF). The blood islands form clusters with two cell populations; peripheral cells (angioblasts) that form the endothelial cells that form vessels. These networks of vessels extend and fuse together to form a robust a network. The cores of the blood islands (hemocytoblasts) form blood cells. Initially all vessels (arteries and veins) look the same. Blood formation occurs later in week 5, and occurs throughout the embryonic mesenchyme (connective tissue), and then moves to the liver, and then the spleen, and then bone marrow.
Hematopoietic stem cells (HSCs), the stem cells that form the blood cells, form from the wall of the aorta, which is the major blood vessel in the embryo. In the aortic wall, cells called hemogenic endothelial cells bud off progenitor cells that become HSCs.
A course of transcription factors have now been identified by Slukvin and his team as the triggers that switch these cells into HSCs. Two groups of transcriptional regulators can induce distinct developmental programs from pluripotent stem cells. The first developmental program, directed by the transcription factors ETV2 and GATA2, the pan-myeloid pathway, switches cells into the myeloid lineage (the myeloid lineage includes red blood cells, platelets, neutrophils, macrophages, basophils and eosinophils). The second developmental pathway, directed by the transcription factors GATA2 and TAL1, directs cells into the erythro-megakaryocytic pathway. In either cases, these transcription factors directly convert human pluripotent stem cells into an endothelium, which subsequently transform into blood cells with pan-myeloid or erythro-megakaryocytic potential.
In Slukvin’s laboratory, treatment of either ETV2 and GATA2 or GATA2 and TAL1 induced cells to make the complete range of human blood cells. Slukvin said of these experiments, “This is the first demonstration of the production of different kinds of cells from human pluripotent stem cells using transcription factors.” Transcription factors bind to DNA at specific sites and regulate gene expression.
Slukvin continued: “By overexpressing just two transcription factors, we can, in the laboratory dish, reproduce the sequence of events we see in the embryo.”
Slukvin and his co-workers showed that his technique produced blood cells by the millions. For every million stem cells, it was possible to produce 30 million blood cells.
Slukvin and his colleagues did not use viruses to genetically modify these stem cells. Instead they used modified RNA to induce overexpression of these transcription factors. Such a technique avoids genetic modification of cells and is inherently safer.
“You can do it without a virus, and genome integrity is not affected,” said Slukvin. This technique might also work to differentiate pluripotent stem cells into other cell types, such as pancreatic beta cells, brain-specific cells, or liver cells.
Despite these successes, Slukvin says that the “Holy grail” of hematopoietic research is to differentiate pluripotent stem cells into HSCs. Since HSC transplants are used to treat multiple myeloma and other types of blood-based cancers as well, making HSCs in the laboratory remains a significant goal and challenge as well.
“We still don’t know how to do that,” said Slukin, “but our new approach to making blood cells will give us an opportunity to model their development in a dish and identify novel hematopoietic stem cell factors.”