Homeward Bound

Cancer patients who undergo aggressive chemotherapy or radiation treatment often sustain severe damage to their bone marrow, that potent tissue containing the stem cells that give rise to every different type of blood cell in the body. Because these hematopoietic stem cells (HSCs) also naturally circulate, they can be harvested from the peripheral blood, bone marrow or umbilical cord blood of donors and infused into patients to rescue them from further injury and death. But this process is inefficient. In a paper published online March 25 in Nature, Harvard researchers and their colleagues take important steps toward improving the efficiency of HSC transplantation.

Like all stem cells, HSCs need to spend time in specialized microenvironments, or niches, that support their continued function. Like weary travelers returning home to rest and recharge, HSCs must home to the bone marrow. Upon arrival, they break through a blood vessel wall to engraft in the marrow, where they remain until ready to become remobilized in the blood.

Transplanted HSCs also must find their way to the bone marrow to survive and properly function. Unfortunately, only a small number of them actually manage to do this. This means that large numbers of donor cells must be harvested and infused into the patient’s bloodstream. To get enough HSCs from umbilical cord blood to transplant into an adult, for example, it is often necessary to pool multiple donor samples. So increasing the efficiency of HSC homing and engraftment would significantly improve treatment success for patients.

A team of researchers led by David Scadden, the Gerald and Darlene Jordan professor of medicine at the Harvard Stem Cell Institute, Massachusetts General Hospital, and the Department of Stem Cell and Regenerative Biology at HMS and the Faculty of Arts and Sciences, has now identified a key mechanism by which HSCs find their way to the bone marrow. The work was part of an ongoing collaboration with the lab of Henry Kronenberg, HMS professor of medicine at MGH, who studies the signaling pathways that regulate bone formation and remodeling.

As it turns out, bones are much more than containers to hold the marrow. They form part of the niche that supports HSCs. Stimulation of the cells that give rise to bone tissue causes more HSCs to be made. It has long been known that a protein that mediates adrenaline and hormone receptor signaling, called G-alpha-s, is important for bone remodeling. Intriguingly, these signaling pathways have now been shown to also be involved in HSC regulation, adding substance to the link between bone and bone marrow function. When Kronenberg and colleagues made embryos composed of a mixture of normal and G-alpha-s mutant cells, they discovered that the mutants never contributed to embryonic bone marrow, suggesting that HSCs need G-alpha-s to engraft.

To see if this need was also true for adult HSCs, Gregor Adams, a former postdoc in the Scadden lab, infused G-alpha-s mutant HSCs into the bloodstream of normal mice. He discovered that mutant HSCs did not populate the bone marrow. When he transplanted mutant HSCs into lethally irradiated mice, they could not rescue the animals; the mice died within two weeks, confirming that G-alpha-s mutant HSCs cannot engraft into the bone marrow.

Some possible explanations for this failure include a breakdown in the development of the cells and defects in the general machinery involved in cell migration. Adams put these to the test.

He grew mutant HSCs in a dish and let them differentiate, or grow, in the presence of factors that attract HSCs. Cells lacking G-alpha-s behaved normally. Adams and colleagues also tested whether the problem might be in the mutants’ interaction with their environment. By injecting fluorescently labeled HSCs into the tail veins of mice and using advanced microscopy techniques, they were able to make movies of transplanted HSCs as they traveled through blood vessels in the skulls of anesthetized mice. Whereas normal blood stem cells roll gently along the walls of blood vessels, blood stem cells lacking G-alpha-s did not interact with the blood vessel wall; instead, they shot through the vessels like children on a water slide, making it difficult for them to exit the bloodstream and enter the bone marrow.

To see if inactivation of G-alpha-s also affects HSC behavior after engraftment, the researchers transplanted HSCs into mice and inactivated G-alpha-s in these cells after they were engrafted. The team found that mutant HSCs were not efficiently remobilized into the bloodstream.

Together, these findings demonstrate that G-alpha-s signaling is crucial for multiple steps in the life of an HSC: homing to the bone marrow, engraftment in their niche, and remobilization into the bloodstream.

Fortunately, because many currently available drugs already target G-alpha-s–mediated pathways, it might be possible to improve the efficiency of HSC transplantation by stimulating G-alpha-s signaling in donor HSCs before infusing them into patients. The aim would be to enhance homing and engraftment. In a key experiment, Adams and colleagues tested this proposition by treating HSCs with cholera toxin, which permanently activates G-alpha-s, and infusing them into normal adult mice. The researchers observed a striking increase in the number of HSCs that homed to the bone marrow and engrafted, showing that pharmacological treatment could, indeed, enhance the efficiency of HSC transplantation.

The findings have important implications for treatment of cancer patients who have lost bone marrow function. “Now that we have identified specific pathways by which hematopoietic stem cells home to the bone marrow, this opens up opportunities to potentially modulate pathways for transplantation therapies,” said Adams, who currently heads his own lab at the University of Southern California. Scadden’s lab is now investigating how malignant HSCs from leukemia patients interact with their microenvironment, in hopes that differences between normal and malignant cells could be exploited to give normal HSCs an edge over their disease-­causing counterparts.

Students may contact David Scadden at scadden.david@mgh.harvard.edu for more information.

Conflict Disclosure: David Scadden is a consultant and stockholder for Fate Therapeutics.

Funding Sources: The National Institutes of Health; the content of the work is the responsibility solely of the authors.