The bone marrow microenvironment is a critical regulator of haematopoietic stem cell self-renewal and fate1. Although it is appreciated that ageing, chronic inflammation and other insults compromise bone marrow function and thereby negatively affect haematopoiesis2, it is not known whether different bone compartments exhibit distinct microenvironmental properties and functional resilience. Here we use imaging, pharmacological approaches and mouse genetics to uncover specialized properties of bone marrow in adult and ageing skull. Specifically, we show that the skull bone marrow undergoes lifelong expansion involving vascular growth, which results in an increasing contribution to total haematopoietic output. Furthermore, skull is largely protected against major hallmarks of ageing, including upregulation of pro-inflammatory cytokines, adipogenesis and loss of vascular integrity. Conspicuous rapid and dynamic changes to the skull vasculature and bone marrow are induced by physiological alterations, namely pregnancy, but also pathological challenges, such as stroke and experimental chronic myeloid leukaemia. These responses are highly distinct from femur, the most extensively studied bone marrow compartment. We propose that skull harbours a protected and dynamically expanding bone marrow microenvironment, which is relevant for experimental studies and, potentially, for clinical treatments in humans.
© 2024. The Author(s).

Hematopoietic stem cells (HSCs) reside in specific microenvironments that facilitate their regulation through both internal mechanisms and external cues. Bone marrow endothelial cells (BMECs), which are found in one of these microenvironments, play a vital role in controlling the self-renewal and differentiation of HSCs during hematological stress. We previously showed that 27-hydroxycholesterol (27HC) administration of exogenous 27HC negatively affected the population of HSCs and progenitor cells by increasing the reactive oxygen species levels in the bone marrow. However, the effect of 27HC on BMECs is unclear. To determine the function of 27HC in BMECs, we employed magnetic-activated cell sorting to isolate CD31+ BMECs and CD31- cells. We demonstrated the effect of 27HC on CD31+ BMECs and HSCs. Treatment with exogenous 27HC led to a decrease in the number of BMECs and reduced the expression of adhesion molecules that are crucial for maintaining HSCs. Our results demonstrate that BMECs are sensitively affected by 27HC and are crucial for HSC survival.

The bone marrow microenvironment is a critical regulator of hematopoietic stem cell self-renewal and fate. While it is appreciated that aging, chronic inflammation and other insults compromise bone marrow function and thereby negatively affect hematopoiesis, it is not known whether different bone compartments exhibit distinct microenvironmental properties and functional resilience. Here, we have employed imaging, pharmacological approaches, and mouse genetics to uncover specialized and highly surprising properties of bone marrow in adult and aging skull. Specifically, we show that the skull bone marrow undergoes lifelong expansion involving vascular growth, which results in an increasing contribution to total hematopoietic output. Furthermore, skull is largely protected against major hallmarks of aging, including upregulation of inflammatory cytokines, adipogenesis and loss of vascular integrity. Striking dynamic and rapid changes to the skull vasculature and bone marrow are induced by physiological alterations, namely pregnancy, but also pathological challenges, such as stroke and experimental chronic myeloid leukemia. These responses are highly distinct from femur, the most extensively studied bone marrow compartment. We propose that skull harbors a protected and dynamically expanding bone marrow microenvironment, which is relevant for experimental studies but, potentially, also clinical treatments in humans.

Chronic myeloid leukemia (CML) is a blood cancer characterized by dysregulated production of maturing myeloid cells driven by the product of the Philadelphia chromosome, the BCR-ABL1 tyrosine kinase. Tyrosine kinase inhibitors (TKIs) have proved effective in treating CML, but there is still a cohort of patients who do not respond to TKI therapy even in the absence of mutations in the BCR-ABL1 kinase domain that mediate drug resistance. To discover novel strategies to improve TKI therapy in CML, we developed a nonlinear mathematical model of CML hematopoiesis that incorporates feedback control and lineage branching. Cell-cell interactions were constrained using an automated model selection method together with previous observations and new in vivo data from a chimeric BCR-ABL1 transgenic mouse model of CML. The resulting quantitative model captures the dynamics of normal and CML cells at various stages of the disease and exhibits variable responses to TKI treatment, consistent with those of CML patients. The model predicts that an increase in the proportion of CML stem cells in the bone marrow would decrease the tendency of the disease to respond to TKI therapy, in concordance with clinical data and confirmed experimentally in mice. The model further suggests that, under our assumed similarities between normal and leukemic cells, a key predictor of refractory response to TKI treatment is an increased maximum probability of self-renewal of normal hematopoietic stem cells. We use these insights to develop a clinical prognostic criterion to predict the efficacy of TKI treatment and design strategies to improve treatment response. The model predicts that stimulating the differentiation of leukemic stem cells while applying TKI therapy can significantly improve treatment outcomes.
© 2023, Rodriguez, Iniguez et al.

Practical procedures for sorting and analysis of leukemia stem cells (LSCs) are to improve our understanding of chronic myelogenous leukemia (CML). Here, we present a detailed magnetic-bead-based sorting and flow-cytometry-based analysis protocol for LSCs in BCR-ABL-driven CML mice. We describe steps for sorting and functional analysis of BCR-ABL-expressing c-Kit+ cells (GFP+c-Kit+) from CML mice as well as antibody staining and gating strategies for characterization of leukemia stem/progenitor cells and myeloid leukemia cells. For complete details on the use and execution of this protocol, please refer to Liu et al. (2022).1.
Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.