Hematopoietic stem cells (HSCs) have an amazing capacity to maintain and repopulate all the blood cells – from platelets to red blood cells to immune cells – for a person’s entire lifetime. This regenerative capacity is carefully orchestrated by signals that tell the cells when to go to sleep and when to wake up and make more blood cells. Many of these signals come from the hematopoietic niche, or the microenvironment in bone marrow where most blood cell production occurs. When these signals go haywire, it can lead to blood ailments like leukemia. Studying which signals coordinate HSC sleepiness and blood cell production can help researchers understand the molecular abnormalities of these diseases.
The Termini Lab in the Translational Sciences and Therapeutics Division is characterizing new mechanisms of HSC regulation. Specifically, they study the role of the cell surface molecule syndecan-2 in this process. Syndecan-2 is a proteoglycan, meaning that it has a transmembrane protein core that is decorated by long carbohydrate chains. These chains wave around outside the cell to grab the attention of signaling molecules so that they can act on the cell. Previous work from Dr. Termini has demonstrated that HSCs with a lot of syndecan-2 can repopulate the blood system and self-renew for longer than those without syndecan-2. Work from others has shown that bone marrow niche cells like stromal and endothelial cells are critical for proper HSC function, but whether and how syndecan-2 expression on these niche cells changes HSC ability to repopulate the blood system is unclear.
To answer this question, researchers in the Termini lab started by characterizing how much syndecan-2 was present on these supportive niche cells. The group next isolated primary endothelial, stromal, and hematopoietic bone marrow cells from mice to compare their levels of syndecan-2 expression. They found that stromal and endothelial cells expressed much higher levels of syndecan-2 mRNA and protein relative to hematopoietic cells from the same mice, indicating that bone marrow cell populations express syndecan-2 differently. While intriguing, questions about the potential role of niche syndecan-2 in HSC function remained, so the team turned to mouse models to knock out syndecan-2 in the bone marrow niche cells.
The group generated mice with a syndecan-2 knockout in either stromal or endothelial cells. They found that mice lacking stromal syndecan-2 had significantly more B cells, and mice without endothelial syndecan-2 had significantly more B and T cells than control mice, highlighting a potential role for niche syndecan-2 in blood cell balance. They also assessed the frequency of HSCs in the bone marrow of the knockout mice, but there were no differences between the knockout and control mice. “Even though niche syndecan-2 did not impact how many stem cells were in each mouse,” explained Dr. Matt Hagen, lead author of the study, “there was still a potential that the lack of syndecan-2 changed HSC function in some way.”
The team set out to test HSC function by performing competitive transplantation assays. To do this, they isolated whole bone marrow from one knockout mouse (donor cells), mixed it with whole bone marrow from another mouse (competitor cells), and injected the mixture into an irradiated recipient mouse. After four months, they found no differences in the blood system between any groups. The group next isolated their cells of interest from the first group of recipient mice and transplanted them into a secondary group of recipient mice to test the long-term function of the blood stem cells. They found that bone marrow from the stromal syndecan-2 knockout mice failed to reconstitute the blood system of the recipient mice. They also found fewer long-living HSCs and other hematopoietic progenitor cells in the bone marrow of these recipients. When they repeated this assay with only stem cells from the knockout mice, they found almost none of the donor cells persisted in mice over the course of the experiment. These striking results suggest that stromal syndecan-2 is essential for proper long-term HSC function.
While these results were exciting, the team worried that transplanting the cells into an irradiated mouse could be impacting their results. To get around this potential limitation, they knocked down syndecan-2 in a human stromal cell line. They then cultured mouse HSCs with the radiation-free knockdown or control cells and analyzed the proportion of differentiated blood cells and HSCs. When grown with syndecan-2-knockout stromal cell lines, the group observed more differentiated blood cells and fewer HSCs relative to the HSCs growth with control cells. Taken together, the team’s results suggest that stromal syndecan-2 prevents HSC exhaustion.
Syndecan-2 is known to interact with various growth factors in other body systems. To establish this paradigm in the blood system, the group tested the ability of syndecan-2 to bind to the bone marrow growth factors and found that syndecan-2 bound all of them with a very high affinity. Mechanistically, this suggests that stromal syndecan-2 is important for niche cell-growth factor interactions, and that these interactions control HSC fate. In the future, the group hopes to more thoroughly define these interactions and continue characterizing the role of proteoglycans in normal hematopoiesis.
Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium Members Drs. Brandon Hadland and Tina Termini contributed to this research.
This work was supported by funding from the National Institutes of Health, the American Heart Association, the American Society of Gene and Cell Therapy, the Andy Hill CARE Foundation, the V Foundation, the Fred Hutch Office of Faculty Affairs, the National Science Foundation, and the Damon Runyon Cancer Research Foundation.
Hagen MW, Setiawan NJ, Wellington R, Woodruff KA, Newman C, Billings TM, Nazzaro MN, Hadland B, Termini CM. 2025. Depletion of microenvironmental syndecan-2 impairs hematopoietic stem cell self-renewal and cytokine responses. bioRxiv. https://doi.org/10.1101/2025.11.07.687077
Kelsey Woodruff is a PhD candidate in the Termini Lab at Fred Hutch Cancer Center. She studies how acute myeloid leukemia cells remodel the sugars on their membranes to reprogram cancer cell signaling. Originally from Indiana, she holds a bachelor's degree in Biochemistry from Ball State University. Outside of lab, you can find her crocheting and enjoying the Seattle summers.
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