EPCR Signaling Controls the Activity of Hematopoietic Stem Cells Independent of Coagulation Regulation

Background

All mature blood cells are derived from multipotent hematopoietic stem cells (HSCs) which are activated to meet the demand of the host during inflammation and injury. The endothelial cell protein C receptor (EPCR) is a marker for primitivity and quiescence of HSCs but the relative contributions of EPCR signaling versus anticoagulant functions in HSC maintenance are incompletely defined.

Aims

We aimed to dissect functions of EPCR by studying anticoagulant and signaling function in HSC of EPCR ^C/S mice carrying a single intracellular point mutation abolishing normal trafficking of EPCR through endo-lysosomal compartments. We assessed the contributions of EPCR signaling to stem cell maintenance by analyzing HSC mobilization and leukemia progression.

Methods

We studied the frequency and cell cycle activity of bone marrow (BM) hematopoietic stem and progenitor cells (HSPC) by multicolor flow cytometry. Furthermore, we analyzed changes in hematopoiesis in steady state, after granulocyte colony stimulating factor (G-CSF)-induced mobilization, in the context of aging and in the context of leukemia, using the MLL-AF9-induced acute myeloid leukemia (AML) model.

Results

HSCs, lungs and isolated lung-derived smooth muscle cells of EPCR ^C/S mice showed protein expression levels and anticoagulant function indistinguishable from wildtype (WT). We found an increase of circulating HSCs in the peripheral blood of EPCR ^C/S mice compared to control under steady state conditions. Isolated HSC displayed diminished polarization of CDC42 and VLA-4 (α ₄β ₁ integrin) affinity to VCAM-1 in EPCR ^C/S versus strain-matched EPCR ^wt mice, indicating that EPCR signaling directly controls HSC retention via integrin affinity to the BM niche. In addition, we noticed a higher cell cycle activity in myeloid-restricted progenitors of EPCR ^C/S mice compared to control. G-CSF treatment led to increased mobilization of both BM neutrophils and HSCs into the peripheral blood of EPCR ^C/S mice compared to EPCR ^wt mice. A myeloid bias was also seen in serially transplanted aged mice, resulting in increased frequencies of myeloid-biased progenitors in the BM of EPCR ^C/S mice compared to control mice, accompanied by an increase of circulating neutrophils in the blood. Consistent with higher cell cycle activity of myeloid progenitors and an overall increase of myeloid-biased output in EPCR ^C/S mice, induction of AML by retroviral transduction of EPCR ^C/S BM cells with MLL-AF9-expressing retrovirus resulted in an increase of cell cycle activity of Lin ^- MLL-AF9 ⁺ leukemic BM blasts and a higher leukemic load in the peripheral blood of mice transplanted with MLL-AF9 ⁺ EPCR ^C/S BM compared to control. As a result, MLL-AF9 ⁺ EPCR ^C/S leukemia showed a more aggressive disease with shortened survival times compared to control. In contrast, chemotherapy of MLL-AF9 ⁺ EPCR ^C/S leukemia reduced leukemic load in the peripheral blood and decelerated disease progression. These data demonstrate that increased leukemia cell cycle activity conferred chemosensitivity.

Conclusion

With a site-specific EPCR mutant knock-in mouse, we here demonstrate that EPCR signaling and anticoagulant function can be separated. We provide direct evidence that EPCR signaling plays a crucial role in maintaining HSC retention via VLA-4 affinity to VCAM-1, controls cell cycle activity and myeloid output in normal, stress-induced, and malignant hematopoiesis with implications for therapeutic approaches in acute myeloid leukemia.