Bowers M,  Zhang B,  Ho Y, Agarwal P, Chen C, Bhatia R. Osteoblast ablation reduces normal long-term hematopoietic stem cell self-renewal but accelerates leukemia development. Blood 2015;125(17)2678-2688.

Prepared by: Peterson Chao, Advance Stem Cell, Fall 2015

LAY SUMMARY

The paper used a mouse model to activate chronic myelogenous leukemia. The authors were able to determine the effect that osteoblasts on normal and leukemic hematopoiesis.  Osteoblasts are part of the bone marrow niche (microenvironment). Thus, these cells have a major role in controlling the fate of hematopoietic stem cells.  This was proved by deleting the osteoblasts, followed by functional studies with normal and leukemic conditions. The deletion of osteoblasts resulted in decreased bone marrow cellularity, increased in hematopoietic stem and progenitor cells.  Thus, it was concluded that osteoblasts are necessary for the regular development of hematopoiesis.  Next, the authors determine if the stem cells in the osteoblast-depleted mice can be transplanted into irradiated mice. This set of studies was important because it would indicate if the stem cells can home to the correct niche for efficient functions.  As compared to the stem cells derived from the normal mice, engraftment was decreased in the stem cells from the osteoblast-ablated mice. There was a decrease in the multi-lineage potential, and the ability of hematopoietic cells to increase their entry into the cell cycle, resulting in enhanced cellularity and decreased survival. Transplantation studies using leukemic stem cells from normal and osteoblast-depleted mice showed increased survival of the leukemic cells in the osteoblast-ablated leukemic stem cells, and decreased leukemogenic capacity.  The authors concluded that leukemic stem cells needed osteoblasts for survival. Since Jagged-1 is a ligand for the Notch pathway, which is linked to stem cells, its role was studied in transforming normal cells into leukemic cells.  The Jagged-1 ligand inhibited cell cycling.  The authors suggested that the loss of Jagged-1 could contribute reverse quiescence to increase cell cycling, which is characteristic of leukemic cells.

SCIENTIFIC SUMMARY

The hematopoietic stem cell (HSC) niche, which includes osteoblasts (OB), is essential for the maintenance of normal blood cell development.  However, when disturbances such as chronic myelogenous leukemia (CML) alter and deregulate the machinery of this system, there is atypical cellular growth and proliferation.  Using a mouse model which induced the ablation of OB and simulated CML conditions, Bowers et al. were able to elucidate the effect from these two conditions in normal and leukemic mice as well as utilize transplantation studies for additional confirmation. 

Compared to normal mice, OB ablated mice had lower bone marrow cellularity as well as increased progenitor populations of the long term hematopoietic stem cells (LTHSCs).  Increased extramedullary hematopoiesis was also observed in the bone marrow and spleen.  One significant finding is that there were lower counts of a LTHSC phenotype lacking CD229 in OB ablated mice.  Since CD229 is the most strongly overexpressed immune receptor in leukemia, the absence of this phenotype in OB ablated mice indicate that leukemic stem cells utilize the normal HSC niche.  Additionally, transplantation of bone marrow cells from normal and OB ablated mice further supported this finding.  For effective engraftment, HSCs need to home to the correct niche in order to successfully proliferate.  Irradiated primary recipient mice which received cells from OB ablated mice experienced decreased engraftment of LTHSCs, decreased LTHSCs in the G₀ phase, and decreased differentiated cells such as myeloid, B, and T cells.  Irradiated secondary recipient mice which received cells from the primary recipient mice also experienced the same results.  It was concluded that there is reduced quiescence, decreased potential to differentiate, and also decreased engraftment without the presence of OBs. 

Next, the effect of OBs on the leukemic stem cell niche was identified.  When compared to CML mice, the OB ablated CML mice experienced decreased LTHSC count, increased white blood cell, neutrophil, and cell count, and decreased survival over the span of 60 days.  These results indicate that OB ablation resulted in accelerated leukemia development since the characteristics of leukemia were exaggerated in osteoblast ablated CML mice.  Additionally, the experiment further confirmed these results by testing the ability of leukemic stem cells from OB ablated CML mice and regular CML mice to transplant and engraft into primary recipient mice.  When compared to the regular CML mice, the mice receiving cells from the OB ablated CML mice experienced higher survival and decreased neutrophilic leukocytosis.  These results indicated that LTHSCs from OB ablated CML mice show reduced long term engraftment and leukemogenic capacity after transplantation.

Lastly, the role of the Notch pathway Jagged-1 ligand in normal and CML mice was studied using OP9 cells that allowed the differentiation of hematopoietic stem cells.  When compared to normal mice, CML mice showed increased Jagged-1 expression.  Both normal and CML LTHSCs cultured with OP9-Jag cells showed increases in G₀  and decreased cells in other cell cycle phases.  The data was also similar with and without the OP9 immobilized wells.  Additionally, normal and CML LTHSCs generated reduced numbers of cell growth in culture with OP9-Jag compared with control OP9 cells.  These results indicated that there was Jagged-1 was involved in cell quiescence. 

These results indicated that osteoblasts and Jagged-1 are necessary for the regular maintenance of hematopoiesis and leukemia.  Therefore, this experiment proved to be successful in determining that osteoblasts do indeed play a regulatory role in hematopoiesis by affecting the regulatory mechanisms.  Future studies are necessary in elucidate the niche of hematopoietic stem cells in order to realize novel treatments.