Pausing the biological clock could boost lab-grown blood stem cells

A decade ago, Raquel Espin Palazon discovered that inflammatory signaling pathways must be activated for embryos to produce blood stem cells. Her lab’s latest work shows the potential value of keeping those same signals turned off after they’re initially activated.

New research by a team led by Espin Palazon and Clyde Campbell, assistant professors of genetics, development and cell biology at Iowa State University, will contribute to efforts to develop lab-grown, patient-derived blood stem cells. This promising but unrealized advance in regenerative medicine could eliminate the need for bone marrow transplants to treat blood disorders such as leukemia, lymphoma and anemia by injecting stem cells.

Timing is key to the results, published September 6 in Nature Communications. Espin Palazon’s previous work established that NF-kB, a well-studied network of proteins that helps trigger inflammation, the set of responses our bodies use to fight infection, injury, and other perceived dangers, was essential to the formation of blood stem cells. Understanding when and why inflammatory signals appear will help replicate the process.

“This is really a big step forward for the laboratory production of blood stem cells. These protocols will be more precise and more efficient,” said Espin Palazon.

Signals come in waves

Stem cells are the source of all new cells, and allow organisms to grow, repair, and renew themselves. Some stem cells can create any type of cell, while others are specialized. The hundreds of billions of new blood cells a human makes each day come from specialized hematopoietic stem cells in their bone marrow. A reserve of hematopoietic stem cells is created before birth in an embryo.

Espin Palazon and Campbell’s research group works with zebrafish, a subject frequently studied in medical research because they are genetically similar to humans and lay fast-developing eggs outside their bodies. By inserting reporter genes that produce fluorescence, scientists can make certain types of proteins and gene expression glow visibly.

By tracking real-time expression of NF-kB in zebrafish embryos using a signaling line that only lights up briefly, the Iowa State University-led team found that inflammatory signaling activates in two waves. The two-fold activation and deactivation acts like a biological clock, coordinating a progression that converts some of an embryo’s vascular cells into blood stem cells.

If the first wave doesn’t arrive, the cells aren’t prepared for the transition. If the second wave doesn’t arrive, the newly created stem cells don’t detach from the blood vessels and rush to duty. Between waves, blood stem cells are born and proliferate modestly. But if the second wave is delayed, they continue to proliferate, the researchers found.

“By manipulating this signaling, we can create a massive amount of blood stem cells,” Espin Palazon said.

“I almost fell off my chair”

The researchers had no idea that inhibiting the reactivation phase could restart blood stem cell production, Campbell said.

“I was the first person to see it under the microscope, and I almost fell off my chair. I yelled at Raquel, ‘What is this?’ It’s one of those moments that you love in science. There are few moments where you see something that blows your mind. I expected to see maybe eight stem cells, and instead I saw hundreds,” Campbell said.

Finding a way to increase yield is exciting because existing methods for culturing blood stem cells produce few functional cells, Espin Palazon said. The process involves genetically reprogramming mature cells to behave like embryonic stem cells and then using these induced pluripotent stem cells to generate blood stem cells. Further study is needed to determine how a better understanding of the timing of inflammatory signals can be used to improve protocols for creating blood stem cells in the lab.

“Now it’s all about optimizing and integrating those signals,” Campbell said.

And then?

For years, Espin Palazon and Campbell have collaborated with researchers at Children’s Hospital of Philadelphia, who are doing extensive work on induced pluripotent stem cells. The hospital researchers confirmed that NF-kB signaling exhibits similar timing patterns and effects in laboratory efforts to produce human blood stem cells.

While continuing their collaboration with the hospital, ISU researchers will soon be able to study the process in-house, in a new cell culture lab on the Iowa State campus that can generate induced pluripotent stem cells. Campbell, who spent a month learning the protocols at the Philadelphia hospital, said the lab should be up and running by the end of the year.

Espin Palazon and Campbell said they expect the methods and results of their latest work to translate to the study of other types of stem cells, the aging process and patient-derived immunotherapy to treat cancer. Scientists continue to learn more about the different functions of inflammatory signaling throughout life.

“Inflammatory networks are necessary for life to begin, they keep us alive by fighting infections and viruses, and ultimately can cause our demise,” Campbell said.