Factions of Hematopoietic
Maintaining the balance: blood stem cells at work and rest

Linheng Li, Ph.D., co-leader of Cancer Biology, has spent the last two decades surveying a couple of cell populations that, while not warring gangs, do have opposite agendas. Both are factions within a larger “party” of hematopoietic, or blood-forming stem cells; but where one group busily spawns blood cells by the billions, members of the other are content to maintain a lowkey background presence.

hematopoietic stem cell 

A dividing hematopoietic stem cell (blue and green). 

The process of continually replacing worn-out blood and immune cells – particularly during injury or disease – takes its own toll on actively dividing hematopoietic stem cells (HSCs), Li explains, which is when the quiescent pool of HSCs usually mobilizes as a backup crew. In between, however, both groups’ polar functions have to be kept in check so neither proliferation nor sedentary behavior prevails. The molecular forces manipulating this tenuous yin-yang balance are what Li, one of KU Cancer Center’s 11 joint faculty based at the Stowers Institute of Medical Research in Kansas City, Mo., is most interested in exploring. 

"We think these two groups of HSCs occupy separate niches in the bone marrow, in which different molecular players and signals dominate,” Li says. “But whereas the existence of different HSC populations is now widely recognized, there’s less consensus in the field on precisely how these groups are regulated – whether, for instance, maintenance occurs in distinct microenvironments, as we suggest.”

According to this model, quiescent HSCs make their home in the thin layer of connective tissue, called the endosteum, lining the marrow cavity’s spongy bone walls. Proliferating HSCs, on the other hand, occupy sinusoids, or small blood vessels, in the central marrow. Two recent publications in as many years – one in Cell, the other in Nature – outline what Li and his group have learned so far about how the quiescent stem cell niche, in particular, is regulated. It appears to involve a different arm of the Wnt pathway, a complex network of signaling proteins and receptors by which information passes from the outside to the inside of a cell, than that at work in the active niche. 

This non-canonical arm of the Wnt pathway, Li elaborates, exerts its influence on quiescent HSCs at least partly through two molecules: Flamingo and Frizzled 8. Through a series of experiments, he first determined that both these non-canonical Wnt members were expressed at much higher levels than any canonical Wnt counterparts in the niche where quiescent HSCs hang out. Li and his crew then noted, in mice genetically engineered to lack either Flamingo or Frizzled 8, a significant drop in quiescent HSC numbers along with reduced HSC function by more than 70 percent. They also wiped out actively proliferating HSCs with the drug 5-fluorouracil and, observing a swift reversal of events whereby canonical Wnt signaling trumped that of Frizzled 8 and Flamingo to prod quiescent stem cells into replenishing mode, confirmed that these two representatives of the non-canonical Wnt arm normally tell HSCs to sit still. 

The researchers have since uncovered what Li calls “an additional layer of regulation” ensuring long-term maintenance of the HSC reserve force. It occurs through genomic imprinting, in which a gene’s maternal or paternal copy, but not both, is silenced to ensure appropriate expression. While this happens in only a subset of genes, it’s also a crucial mechanism for controlling growth and development at the fetal stage. 

Using a mouse model, Li and his group devised a molecular situation where H19, a gene that stifles growth and is normally expressed via its maternal copy, was effectively inactivated. This loss of imprinting control led to IGF2, which encourages cell proliferation, being expressed from both its copies, not just the paternal one. With H19 shut off and IGF2 all geared up to go, quiescent HSCs came awake instead of remaining in a sleeping state. 

As it turns out, Li explains, H19 encodes a microRNA that, true to the regulatory nature of these short strings of bases, normally keeps a lid on the levels of the protein receptor that IGF2 needs to promote growth. So genomic imprinting control through H19 is necessary to restrict IGF2’s actions and maintain a quiescent state of affairs in HSCs – potentially, Li thinks, with the added influence of non-canonical Wnt signaling via Flamingo and Frizzled 8 from outside HSCs. Extremism is no more welcome here than in most settings: tipping the scales too far in either direction could result in leukemia and other overgrowth disorders; or premature aging of HSCs alongside declining blood cell production. 

Li sees efforts like his to scope out both players and territory, molecularly speaking, as key in figuring out how to fix matters when the balance between two equally vital groups of stem cells is upset. For instance, speeding up the mobilization of quiescent HSCs in the wake of chemotherapy drugs, which often ravage the actively proliferating cell population, would be valuable in treating cancer. Or, when quiescent stem cells are sometimes found in permanent lockdown, blocking the culprit – overly “noisy” non-canonical Wnt signaling – could unjam the brakes on blood cell production and prevent premature HSC aging. 

“We’ll keep investigating stem cell quiescence and proliferation at deeper levels, including that of epigenetic regulation,” Li says. “It’s one thing to study signaling from the cell surface and downstream, as we’re doing, and another to examine how cells respond – genomic imprinting is one way, but just the start.”

Funding sources for this research

  • Stowers Institute of Medical Research
  • NIH U01DK085501: “Isolation and Characterization of Intestinal Stem Cells”

Relevant publications

  • Linheng Li and Hans Clevers, “Coexistence of Quiescent and Active Adult Stem Cells in Mammals.” Science (Jan 2010).
  • Sugimura, R., He, X.C., Venkatraman, A., Arai, F., Box, A., Semerad, C., Haug, J.S., Peng, L., Zhong, X., Suda, T., and Li, L., “Non-canonical Wnt Signaling Maintains Hematopoietic Stem Cells in the Niche.” Cell (July 2012).
  • Venkatraman A, He XC, Thorvaldsen JL, Sugimura R, Perry JM, Tao F, Zhao M, Christenson MK, Sanchez R, Yu JY, Peng L, Haug JS, Paulson A, Li H, Zhong XB, Clemens TL, Bartolomei MS, Li L, “Maternal-specific imprinting at the H19-Igf2 locus maintains adult hematopoietic stem cells quiescence.” Nature (Aug 2013).