Scientists discover key molecular switch for blood stem cell regeneration

A single molecular switch is essential for blood stem cells to enter an activated, regenerative state in which they produce new blood cells, according to a preclinical study led by Weill Cornell Medicine investigators. The discovery could lead to more effective bone marrow transplants and gene therapies.

Stem cells are immature cells that have a basic regenerative role in virtually all tissues. They normally exist in a quiescent, slowly dividing state, but after an injury can replace tissue by switching to an activated state in which they multiply rapidly and turn into mature, functional cells.

The researchers, in a study published Feb. 25 in Nature Immunology, found that a DNA transcription-regulating protein called FLI-1 has a critical role in this regenerative process for blood stem cells, which are mostly resident in the bone marrow until they are stimulated or "mobilized" to move into the bloodstream. They showed that transiently producing FLI-1 in quiescent adult mobilized bone marrow stem cells activates them so that they swiftly expand their numbers and have a better chance of being transplanted successfully into a new host.

The approach we outlined in this study could substantially improve the efficiency of marrow transplants and marrow-cell-targeted gene therapies, especially in cases where the donor has a very limited supply of viable blood stem cells."

Dr. Shahin Rafii, study senior author, director of the Hartman Institute for Therapeutic Organ Regeneration and the Ansary Stem Cell Institute, chief of the division of regenerative medicine, and the Arthur B. Belfer Professor in Genetic Medicine at Weill Cornell Medicine

Dr. Rafii is also a member of the Englander Institute for Precision Medicine and the Sandra and Edward Meyer Cancer Center at Weill Cornell Medicine.

Marrow transplants, which include blood stem cells, allow the replenishment of the blood cell and immune cell populations in recipients, and are important elements in the treatment of some cancers. Doctors also sometimes replenish blood cells in cancer patients using healthy blood stem cells drawn from the patients themselves, although these stem cells may be harder to activate and expand if they have already been exposed to chemotherapies and/or radiation treatments. Similarly, some gene therapies, especially for blood disorders such as beta-thalassemia, require the harvesting of patients' blood stem cells, insertion of a therapeutic gene, and expansion of the vulnerable stem cells in the laboratory before re-infusion into the patient. All these applications would be improved if doctors had a safe, reliable method for switching quiescent blood stem cells into a more regenerative state.

In the study, the researchers used single-cell profiling and other techniques to analyze differences in gene activity between quiescent and activated blood stem cells. Eventually they zeroed in on FLI-1, a transcription factor protein that can control the activity of thousands of genes. Its absence, they showed, keeps blood stem cells quiescent, and largely shuts down these cells' interactions with surrounding marrow cells, in particular the specialized endothelial cells that compose the blood vessels. FLI-1's activity, in contrast, restores stem cells' connections and co-adaptability with their microenvironmental endothelial cell niche, also known as the vascular niche. FLI-1 pushes them into an activated, regenerative state—greatly improving their ability to expand and restore the blood cell supply in a new host.

The mutations that drive overactivity of FLI-1 are known drivers of some leukemias. However, the researchers developed a method for stimulating blood stem cells with FLI-1 for only a few days at a time, using an approach similar to that of modified mRNA-based vaccines.

"The stem cells we prime with FLI-1 modified mRNA in this way wake up from hibernation, expand and functionally and durably engraft  in the recipient host, without any evidence of cancer," said study co-first author Dr. Tomer Itkin, who was an instructor of biology in medicine in the Rafii laboratory at the time of the study and currently is the director of Tel Aviv University's Neufeld Cardiovascular Research Institute and is an assistant professor in Tel Aviv University's Sagol Center for Regenerative Medicine.

The team also addressed a long-standing puzzle in the blood stem cell field by showing that the greater regenerative potential of human umbilical cord-derived blood stem cells, compared with adult stem cells isolated from blood, is associated with differences in these cells' levels of FLI-1 activity, affecting their potency to interact with a regenerative vascular niche.

The study involved extensive computational analysis to decipher the role of FLI-1 in stem cell activation and its integration with known signaling pathways that drive stem cell self-renewal and survival. It also clarified the relationship between blood stem cells and their marrow environment, specifically the vascular niche.

"We showed that stem cell activity is not autonomous but also is not fully determined by endothelial cell vascular niche signals—it depends instead on signaling and adaptability between the two," said co-first author Sean Houghton, a bioinformatics analyst in the Rafii laboratory during the study and currently a senior bioinformatics analyst for the Englander Institute for Precision Medicine at Weill Cornell Medicine.

The researchers plan to follow up with further preclinical development and scaling up of their modified mRNA-based method to transiently introduce FLI-1 in the blood stem cells, with the ultimate goal of testing it in human patients. Their approach could set the stage for treating wide range of blood disorders with long-term stable and safe blood production.