Coaxing a stem cell into the desired adult cell type can be reminiscent of helicopter parenting. Just the right ingredients, at just the right time, are necessary to steer the cell down exactly the path desired by the researchers – whatever the path might be.
Two papers out this week describe a method to coax highly pluripotent cells into more specialized blood-forming stem cells that is also popular in certain parts of the helicopter parent world: exposing it to a heartbeat.
In parenting, heartbeats are supposed to have a calming influence on the infant by reminding it of the womb. In the generation of blood stem cells, what makes a heartbeat helpful is more prosaic: shear stress and nitric oxide.
In the May 13 issues of Cell and Nature, separate teams show that blood flow regulates the formation of blood cell progenitors via nitric oxide signaling and increased expression of the protein runx-1, which is the key regulator of blood formation.
The two papers are "complementary to each other," Cell paper senior author Leonard Zon told Medical Device Daily's sister publication, BioWorld Today.
Zon said that the strengths of his team's approach lies in the use of the zebrafish (which, just for the record, has a reproductive rate of 200 eggs per week and so is not a species that tends towards helicopter parenting.)
"The beauty of our experiments was that we were screening in a real animal, looking at a real aorta," he said.
Zon and his colleagues used zebrafish in a chemical genetics screen, screening a total of about 8,000 embryos per week with 2,500 compounds with a known mechanism of action – roughly a third of them FDA-approved drugs. The work which has been ongoing for several years, has already netted one compound – a stable analog of prostaglandin E – that Zon's team originally reported in 2007 as contributing to blood stem cell formation.
That analog, Zon said, is on the cusp of entering clinical trials. But at the time of the original screening, Zon said, the researchers found "a whole other class of chemicals that were surprising to us at the time" – compounds that had an effect on blood flow or heartbeat.
The researchers tested whether these compounds were indeed acting on stem cells through their effects on blood flow by testing them at different time points in embryonic development. Zebrafish embryos develop a heartbeat at 23 hours of age, and circulation is established by 36 hours of age.
When his team tested the compounds after circulation was established, "they all worked well" to increase the numbers of blood stem cells, Zon said. But if they were administered before the embryo developed a heartbeat, "none of them worked except for nitric oxide."
The authors tested nitric oxide in a mutant zebrafish that develops neither a heartbeat nor circulation; Zon explained that such mutants are not nearly as impaired overall as one might suspect, because the it is small enough so that cells can get oxygen and necessary nutrients through diffusion. They do not, however, develop many blood stem cells. But when the scientists treated the mutants with nitric oxide, the molecule was able to compensate for the lack of heartbeat or circulation and make blood stem cells.
Finally, the researchers extended their findings to mammals by feeding nitric oxide to pregnant mice; their embryos, Zon said, also had a reduced number of stem cells in the aorta.
Both runx-1 and nitric oxide are also implicated in the development of blood cells in the second paper, published in Nature, which investigated the formation of blood cells from embryonic stem cells. The Nature team also specifically implicated the friction of blood flow against arterial walls in the process of blood cell formation, showing the authors write in their paper, that "biomechanical forces stimulate embryonic haematopoiesis," or blood formation.
The evidence found in zebrafish and mice may lead to new methods to coax very early-stage stem cells, such as embryonic stem cells or iPS cells, into more specialized but still pluripotent blood stem cells. Zon said that it was "easy" to make outright blood cells, but that the production of blood stem cells – which could be used for bone marrow transplants in leukemia patients without a matched bone marrow donor – has been more challenging.
The papers also answer two long-standing scientific questions. First, they can explain why the heart starts beating very early in embryonic development, at a time when the embryo's nutritional needs can be met by diffusion.
Secondly, Zon said, it explains why blood stem cells form in the aorta: "It is an ideal place to sense changes in blood flow to time blood stem cells for the production of blood in the future."