Below is a summary of a couple Cell Stem Cell papers that offer map-fragments to one of stem cells Holy Grails: culturing the cells that give rise to blood. This could lead to more broadly applicable alternatives to treatments that now use cord blood or bone marrow transplants. This will become a formal highlight next week.
Also, an article published yesterday in Nature Medicine shows how embryonic stem cells can be used to evauluate mutations implicated in breast cancer.
Developing a way to reliably produce hematopoietic stem cells (HSCs) is a bloody tough problem. Unlike most tissues, cells of the hematopoietic system emerge from several embryonic sites and then circulate through the body. This mobility has perplexed researchers who hope that mimicking the in vivo environment will help them culture HSCs. Now though, two British research teams report complementary techniques for isolating HSCs in Cell Stem Cell. These could form the lifeblood of techniques creating easier alternatives to bone marrow transplantation.
Alexander Medvinsky and his colleagues at the University of Edinburgh went straight to the heart of HSC development — the aorta-gonad-mesonephros (AGM) region, where the first multipotent HSCs are thought to arise in the embryo. By dissociating and reconstructing the mouse AGM’s three-dimensional structure, they developed a new method to expand and track the development of HSCs, and tracked down a population of cells containing markers (VE-cadherin and CD45) generally found on separate types of cells1.
“Medvinsky characterized a cell that’s wearing two hats: It’s an endothelial cell and a blood cell,” says M. William Lensch, of Children’s Hospital Boston and the Harvard Medical School. “When you see a cell like this, it lends credibility that they develop directly from the vasculature,” rather than from elsewhere in the developing embryo.
Medvinsky’s population of cells contained one marker associated with the inner lining of blood vessels and another specific to blood cells themselves, and he thought that these cells might constitute “pre-HSC” progenitors capable of acquiring stem cell function. Indeed, when he injected the VE-cadherin+CD45+ cells in irradiated mice, they rapidly developed into a large pool of definitive HSCs that that restored hematopoiesis. Since these cells mature within the AGM microenvironment, this constitutes an active niche that drives the specification of fully mature HSCs, says Medvinsky.
Rather than trying to isolate HSCs directly, a team led by Majlinda Lako of Newcastle University developed a relatively efficient way of coaxing hematopoietic differentiation from human embryonic stem cells (hESCs). They co-cultured hESCs with AGM-derived stromal cell lines, and found that hematopoietic activity increased at least 31-fold compared to previous co-culturing methods2. They then injected the induced-hESCs into the femurs of immunocompromised mice, and found substantially greater engraftment efficiencies than previously reported — up to 16% for cells co-cultured with the best cell line. Finally, Lako’s group screened around 40 signaling molecules for positive enhancers of hematopoietic differentiation, and flagged the transcription factors TGF-β1 and TGF-β3 as the most efficient inducers of hematopoiesis.
Together, the studies show that nascent cells must mature within the proper context to become definitive HSCs, regardless of whether you start with pre-HSCs or hESCs, says Hanna Mikkola, of the University of California, Los Angeles. “The message from both papers is you really need to have the correct embryonic environment for functional maturation in culture.”
The question of how that maturation occurs remains unanswered. The TGF family members identified by Lako are probably involved, Mikkola says, but she doubts these factors tell the whole story. Lako agrees. As a follow-up, Lako’s group is currently sifting through a library of other candidate factors, including calcium signaling molecules and insulin-like growth factors, for other key regulators of HSC development.
Lako’s results are impressive, says Medvinsky. But he thinks that co-culturing hESCs with his VE-cadherin+CD45+ cells could be even more successful. “With our system we might be able to produce a better outcome.” Lako, however, suspects her stromal cell lines may already contain some of Medvinsky’s “niche” factors. “It’s very likely that we’re using the same signals to induce our human ES cells,” she says. In either case, both authors recognize that more work will be needed to nail down the molecular cues before fully transplantable HSCs can be cultured. “Because you can do this all in a lab dish now, you have the ability to really focus on what the molecules are,” notes Lensch. These two studies now inject new blood into achieving that goal.
1. Taoudi, S. et al. Extensive hematopoietic stem cell generation in the AGM region via maturation of VE-cadherin+CD45+ pre-definitive HSCs. Cell Stem Cell 3: 99–108 (July 2008).
2. Ledran, M.H. et al. Efficient hematopoietic differentiation of human embryonic stem cells on stromal cells derived from hematopoietic niches. Cell Stem Cell 3: 85–98 (July 2008).
Elie Dolgin is a Canadian science writer currently residing in Milwaukee, Wisconsin.