Skeletal control of hematopoiesis: Impact of the stromal-vascular bone marrow niche in the support of fetal and adult hematopoiesis

Bone is not only crucial for allowing movement, providing support and protection for vital organs and maintaining mineral homeostasis, but the skeleton also represents the prime site for blood cell production or hematopoiesis. To ensure constant blood cell levels under homeostatic conditions and rapid responses to emergencies or injuries, the balance between self-renewal and differentiation of hematopoietic stem cells (HSCs) needs to be tightly regulated. HSC maintenance and regulation are controlled by both intrinsic factors and external signals within the bone marrow (BM) niche. Crucial components of this niche, including specific stromal, osteogenic and angiogenic compartments, have been intensively studied for their implication in both the maintenance of the stem cell pool and the differentiation of more committed hematopoietic progenitors. Hematological diseases and anti-cancer therapy, including irradiation or chemotherapy followed by HSC transplantation, induce dramatic changes in the cellular and molecular composition of the BM niche. Increasing our knowledge on how these niche alterations impact HSCs and hematopoiesis can help us to better understand disease transformation and progression and to provide new therapeutic strategies. In this thesis, we studied how the communication between stromal, osteolineage and/or vascular cells regulates the supportive niche function for HSCs and hematopoiesis during fetal development, postnatal homeostasis and following injuries caused by irradiation or chemotherapy. Specifically, we altered (i) the osteo-angiogenic signaling control or (ii) the cell-intrinsic adhesion machinery of skeletal progenitors and assessed how these changes induced alterations in other niche components and how these affected the hematopoietic system. In Chapter 4 we first generated a model of increased vascular endothelial growth factor (VEGF) secretion by osteoprogenitor cells, by using conditional transgenic mice for the main isoform VEGF164 crossed to an osteoprogenitor-directed Osx-Cre:GFP driver strain. Via overexpression of this potent osteo-angiogenic growth factor, we focused on the importance of proper signaling control in the stromal-vascular niche for the homing of hematopoietic stem and progenitor cells (HSPCs) towards the fetal bone. Little is known about the characteristics of the fetal BM-environment controlling initial homing and settlement of HSPCs in bone. Studying this process might bring new knowledge on the recipient tissue's requirements for proper HSC homing and therefore might provide new therapeutic strategies to improve the efficiency and outcome of HSC transplantation. We found that mice in which VEGF signaling was increased in bone, displayed skeletal malformations associated with a strong deficit in the amount of HSPCs in the fetal bones. Structural, cellular and molecular characterization of the mutant mouse bones revealed that VEGF stimulation led to expansion of the stromal and vascular compartments in the developing bone marrow cavity, impaired the integrity of the vascular niche, and changed the metabolic status of the bone marrow stroma, resulting in increased oxidative stress in the BM milieu. These alterations in the stromal-vascular niche were associated with impaired HSC homing to the bones, suggesting that controlled stromal-derived angiogenic signaling is a critical determinant of the capacity to support HSC homing during embryogenesis. In Chapter 5 we pursued our investigations of the impact of osteo-angiogenic communication in the niche by studying the repercussions of increased VEGF signaling in adult bones on the composition of the BM niche and the functioning of the hematopoietic system. We found that VEGF overexpression led to increased trabecular bone density, expansion of the stromal and vascular populations in the metaphyseal bone region, and alteration of the platelet-derived growth factor receptor beta (PDGFRβ)+ stromal population in the central marrow. These niche changes were associated with a specific and severe deficit in circulating B lymphocytes in the mutant mice. Further investigation revealed that the lack of B cells originated from a deficit in early B cell progenitors in the BM, and particularly, a blockage in the transition from the pre-pro- to pro-B cell stage of differentiation. This model and further research based on these findings can help unravel new important environmental requirements for proper B cell development in the BM. In the last chapter, we investigated a mouse model in which αE-catenin, an intracellular component of the cell-cell adhesion complexes involving cadherins, was deleted from the Prx1-Cre-expressing skeletal stem and progenitor cell (SSPC) population and their descendant bone cells. This deletion created mice in which bone mass was changed, BM adiposity was increased, and skeletal vascularization was altered. Yet, these changes in the BM environment did not alter the HSC frequency or perturb hematopoiesis under homeostatic conditions. We next tested whether challenging the mutant mice with chemotherapy or irradiation followed by HSC transplantation, would further affect the BM environment and/or impact the hematopoietic response to these treatments. Challenged conditional αE-catenin knockout mice displayed again an altered niche composition, dominated by an increase in BM adipose tissue. Surprisingly, these changes again did not disturb the hematopoietic recovery or the HSC repopulation capacity after transplantation, showing us the flexibility of the hematopoietic system and its capacity to adapt to changes in the BM environment in order to provide the necessary circulating blood cells. Altogether, the data presented in this thesis reveal that changing the cellular distribution and/or molecular composition of a compartment in the BM impacts on the interplay between the osteogenic, stromal and vascular populations, which all interact with and depend on each other to form the niche supporting hematopoiesis. Our results also reveal both the sensitivity of the hematopoietic system to environmental changes and its flexibility to adapt to those changes, underscoring the complexity of the interactions between the heterogeneous BM network and the hematopoietic system.

Publication year:2021

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