Distinct bone marrow blood vessels differentially regulate haematopoiesis

Into the Blue: Exploring genetic mechanisms behind the evolution of baleen whales

Marine ecosystems are ideal for studying evolutionary adaptations involved in lineage diversification due to few physical barriers and reduced opportunities for strict allopatry compared to terrestrial ecosystems. Cetaceans (whales, dolphins, and porpoises) are a diverse group of mammals that successfully adapted to various habitats within the aquatic environment around 50 million years ago. While the overall adaptive transition from terrestrial to fully aquatic species is relatively well understood, the radiation of modern whales is still unclear. Here high-quality genomes derived from previously published data were used to identify genomic regions that potentially underpinned the diversification of baleen whales (Balaenopteridae). A robust molecular phylogeny was reconstructed based on 10,159 single copy and complete genes for eight mysticetes, seven odontocetes and two cetacean outgroups. Analysis of positive selection across 3,150 genes revealed that balaenopterids have undergone numerous idiosyncratic and convergent genomic variations that may explain their diversification. Genes associated with aging, survival and homeostasis were enriched in all species. Additionally, positive selection on genes involved in the immune system were disclosed for the two largest species, blue and fin whales. Such genes can potentially be ascribed to their morphological evolution, allowing them to attain greater length and increased cell number. Further evidence is presented about gene regions that might have contributed to the extensive anatomical changes shown by cetaceans, including adaptation to distinct environments and diets. This study contributes to our understanding of the genomic basis of diversification in baleen whales and the molecular changes linked to their adaptive radiation, thereby enhancing our understanding of cetacean evolution.

Protective effects of Gumibao recipe on glucocorticoid-included bone microcirculatory endothelial cell injury and the underlying mechanism

OBJECTIVE

To investigate the protective effects of Gumibao recipe on glucocorticoid-included bone microcirculatory endothelial cell (BMEC) injury, and elucidate the possible underlying mechanism.

METHODS

BMECs were treated with different concentrations of hydrocortisone at different time points, and the viability as well as migration of BMECs were evaluated; furthermore, the release of LDH, levels of VEGF, PAI-1, t-PA, and the content of NO by BMECs have been evaluated by commercially available kits; moreover, the expressions of eNOS, p-PI3K, p-Akt and p-mTOR in BMECs were examined by WB methods. Next, hydrocortisone treated BMECs were co-treated with Gumibao recipe, and the viability, migration and autophagy of BMECs were evaluated.

RESULTS

0.2 mg/ml and 0.3 mg/ml hydrocortisone significantly decreased viability and migration ability of BMECs, and also impeded the endothelial function of BMECs by decreasing the levels of VEGF, t-PA, the content of NO, and increasing the level of PAI-1. Gumibao medicated serum markedly increased the viability and migration of BMECs, and also increased the levels of VEGF, t-PA, the content of NO, meanwhile decreased the level of PAI-1 in 0.3 mg/ml hydrocortisone treated BMECs; moreover, glucocorticoids inhibited the autophagy of BMECs, and Gumibao recipe significantly increased the autophagy of BMECs; meanwhile, autophagy inhibitor 3-MA partially blocked the protective effects of Gumibao recipe. Finally, gumibao recipe partially abrogated the inhibitory effects of hydrocortisone on the activation of PI3K/Akt/mTOR singling, and these effects were further counteracted by PI3K and mTOR inhibitor NVP-BEZ235.

CONCLUSIONS

We reported for the first time the protective effects of Gumibao recipe on glucocorticoid-included BMECs injury, and the possible underlying mechanism may be regulating the autophagy of BMECs via PI3K/AKT/mTOR signaling pathway.

Hematopoietic stem cell metabolism within the bone marrow niche - insights and opportunities

Hematopoiesis unfolds within the bone marrow niche where hematopoietic stem cells (HSCs) play a central role in continually replenishing blood cells. The hypoxic bone marrow environment imparts peculiar metabolic characteristics to hematopoietic processes. Here, we discuss the internal metabolism of HSCs and describe external influences exerted on HSC metabolism by the bone marrow niche environment. Importantly, we suggest that the metabolic environment and metabolic cues are intertwined with HSC cell fate, and are crucial for hematopoietic processes. Metabolic dysregulation within the bone marrow niche during acute stress, inflammation, and chronic inflammatory conditions can lead to reduced HSC vitality. Additionally, we raise questions regarding metabolic stresses imposed on HSCs during implementation of stem cell protocols such as allo-SCT and gene therapy, and the potential ramifications. Enhancing our comprehension of metabolic influences on HSCs will expand our understanding of pathophysiology in the bone marrow and improve the application of stem cell therapies.

p21/Zbtb18 repress the expression of cKit to regulate the self-renewal of hematopoietic stem cells

The maintenance of hematopoietic stem cells (HSCs) is a complex process involving numerous cell-extrinsic and -intrinsic regulators. The first member of the cyclin-dependent kinase family of inhibitors to be identified, p21, has been reported to perform a wide range of critical biological functions, including cell cycle regulation, transcription, differentiation, and so on. Given the previous inconsistent results regarding the functions of p21 in HSCs in a p21-knockout mouse model, we employed p21-tdTomato (tdT) mice to further elucidate its role in HSCs during homeostasis. The results showed that p21-tdT+ HSCs exhibited increased self-renewal capacity compared to p21-tdT- HSCs. Zbtb18, a transcriptional repressor, was upregulated in p21-tdT+ HSCs, and its knockdown significantly impaired the reconstitution capability of HSCs. Furthermore, p21 interacted with ZBTB18 to co-repress the expression of cKit in HSCs and thus regulated the self-renewal of HSCs. Our data provide novel insights into the physiological role and mechanisms of p21 in HSCs during homeostasis independent of its conventional role as a cell cycle inhibitor.

Polyclonal regeneration of mouse bone marrow endothelial cells after irradiative conditioning

Bone marrow endothelial cells (BM-ECs) are the essential components of the BM niche and support the function of hematopoietic stem cells (HSCs). However, conditioning for HSC transplantation causes damage to the recipients' BM-ECs and may lead to transplantation-related morbidity. Here, we investigated the cellular and clonal mechanisms of BM-EC regeneration after irradiative conditioning. Using single-cell RNA sequencing, imaging, and flow cytometry, we revealed how the heterogeneous pool of BM-ECs changes during regeneration from irradiation stress. Next, we developed a single-cell in vitro clonogenic assay and demonstrated that all EC fractions hold a high potential to reenter the cell cycle and form vessel-like structures. Finally, we used Rainbow mice and a machine-learning-based model to show that the regeneration of BM-ECs after irradiation is mostly polyclonal and driven by the broad fraction of BM-ECs; however, the cell output among clones varies at later stages of regeneration.

Optimized intravital three-photon imaging of intact mouse tibia links plasma cell motility to functional states

Intravital deep bone marrow imaging is crucial to studying cellular dynamics and functions but remains challenging, and minimally invasive methods are needed. We employed a high pulse-energy 1650 nm laser to perform three-photon microscopy in vivo, reaching ≈400 μm depth in intact mouse tibia. Repetition rates of 3 and 4 MHz allowed us to analyze motility patterns of fast and rare cells within unperturbed marrow and to identify a bi-modal migratory behavior for plasma cells. Third harmonic generation (THG) was identified as a label-free marker for cellular organelles, particularly endoplasmic reticulum, indicating protein synthesis capacity. We found a strong THG signal, suggesting high antibody secretion, in one-third of plasma cells while the rest showed low signals. We discovered an inverse relationship between migratory behavior and THG signal, linking motility to functional plasma cell states. This method may enhance our understanding of marrow microenvironment effects on cellular functions.

Inhibition of TGF-β signaling in bone marrow endothelial cells promotes hematopoietic recovery in acute myeloid leukemia patients

Although it is an effective treatment for acute myeloid leukemia (AML), chemotherapy leads to myelosuppression and poor hematopoietic reconstruction. Hematopoiesis is regulated by bone marrow (BM) endothelial cells (ECs), and BM ECs are dysfunctional in acute leukemia patients with poor hematopoietic reconstitution after allogenic hematopoietic stem cell transplantation. Thus, it is crucial to explore the underlying mechanism of EC impairment and establish strategies for targeted therapy. TGF-β signaling was found to be upregulated in ECs from AML patients in complete remission (CR ECs) and led to CR EC damage. Administration of a TGF-β inhibitor rescued the dysfunction of ECs caused by TGF-β1 expression in vitro, especially their hematopoiesis-supporting ability. Moreover, inhibition of TGF-β expression repaired the BM EC damage triggered by chemotherapy in both AML patients in vitro and in an AML-CR murine model, and restored normal hematopoiesis without promoting AML progression. Mechanistically, our data reveal alterations in the transcriptomic pattern of damaged BM ECs, accompanied by the overexpression of downstream molecules TGF-βR1, pSmad2/3, and functional genes related to adhesion, angiogenesis suppression and pro-apoptosis. Collectively, our findings reveal for the first time that the activation of TGF-β signaling leads to BM EC dysfunction and poor hematopoietic reconstitution. Targeting TGF-β represents a potential therapeutic strategy to promote multilineage hematopoiesis, thereby benefiting more cancer patients who suffer from myelosuppression after chemotherapy.

Skeletal stem and progenitor cells in bone physiology, ageing and disease

Skeletal stem cells (SSCs) and related progenitors with osteogenic potential, collectively termed skeletal stem and/or progenitor cells (SSPCs), are crucial for providing osteoblasts for bone formation during homeostatic tissue turnover and fracture repair. Besides mediating normal bone physiology, they also have important roles in various metabolic bone diseases, including osteoporosis. SSPCs are of tremendous interest because they represent prime future targets for osteoanabolic therapies and bone regenerative medicine. Remarkable progress has been made in characterizing various SSC and SSPC populations in postnatal bone. SSPCs exist in the periosteum and within the bone marrow stroma, including subsets localizing around arteriolar and sinusoidal blood vessels; they can display osteogenic, chondrogenic, adipogenic and/or fibroblastic potential, and exert critical haematopoiesis-supportive functions. However, much remains to be clarified. By the current markers, bona fide SSCs are commonly contained within broader SSPC populations characterized by considerable heterogeneity and overlap, whose common versus specific functions in health and disease have not been fully unravelled. Here, we review the present knowledge of the identity, fates and relationships of SSPC populations in the postnatal bone environment, their contributions to bone maintenance, the changes observed upon ageing, and the effect of metabolic diseases such as osteoporosis and diabetes mellitus.

An Endothelial Cell Is Not Simply an Endothelial Cell

Endothelial cells (ECs) are a multifaceted component of the vascular system with roles in immunity, maintaining tissue fluid balance, and vascular tone. Dysregulation or dysfunction of ECs can have far-reaching implications, leading pathologies ranging from cardiovascular diseases, such as hypertension and atherosclerosis, ischemia, chronic kidney disease, blood-brain barrier integrity, dementia, and tumor metastasis. Recent advancements in regenerative medicine have highlighted the potential of stem cell-derived ECs, particularly from induced pluripotent stem cells, to treat ischemic tissues, as well as models of vascular integrity. This review summarizes what is known in the generation of ECs with an emphasis on tissue-specific ECs and EC subphenotypes important in the development of targeted cell-based therapies for patient treatment.

Engineered and hybrid human megakaryocytic extracellular vesicles for targeted non-viral cargo delivery to hematopoietic (blood) stem and progenitor cells

Native and engineered extracellular vesicles generated from human megakaryocytes (huMkEVs) or from the human megakaryocytic cell line CHRF (CHEVs) interact with tropism delivering their cargo to both human and murine hematopoietic stem and progenitor cells (HSPCs). To develop non-viral delivery vectors to HSPCs based on MkEVs, we first confirmed, using NOD-scid IL2Rγnull (NSG™) mice, the targeting potential of the large EVs, enriched in microparticles (huMkMPs), chosen for their large cargo capacity. 24 h post intravenous infusion into NSG mice, huMkEVs induced a nearly 50% increase in murine platelet counts. PKH26-labeled huMkEVs or CHEVs localized to the HSPC-rich bone marrow preferentially interacting with murine HSPCs, thus confirming their receptor-mediated tropism for NSG HSPCs, and their potential to treat thromobocytopenias. We explored this tropism to functionally deliver synthetic cargo, notably plasmid DNA coding for a fluorescent reporter, to NSG HSPCs both in vitro and in vivo. We loaded huMkEVs with plasmid DNA either through electroporation or by generating hybrid particles with preloaded liposomes. Both methods facilitated successful functional targeted delivery of pDNA, as tissue weight-normalized fluorescence intensity of the expressed fluorescent reporter was significantly higher in bone marrow than other tissues. Furthermore, the fraction of fluorescent CD117+ HSPCs was nearly 19-fold higher than other cell types within the bone marrow 72-h following administration of the hybrid particles, further supporting that HSPC tropism is retained when using hybrid particles. These data demonstrate the potential of these EVs as a non-viral, HSPC-specific cargo vehicle for gene therapy applications to treat hematological diseases.

The role of vascular and lymphatic networks in bone and joint homeostasis and pathology

The vascular and lymphatic systems are integral to maintaining skeletal homeostasis and responding to pathological conditions in bone and joint tissues. This review explores the interplay between blood vessels and lymphatic vessels in bones and joints, focusing on their roles in homeostasis, regeneration, and disease progression. Type H blood vessels, characterized by high expression of CD31 and endomucin, are crucial for coupling angiogenesis with osteogenesis, thus supporting bone homeostasis and repair. These vessels facilitate nutrient delivery and waste removal, and their dysfunction can lead to conditions such as ischemia and arthritis. Recent discoveries have highlighted the presence and significance of lymphatic vessels within bone tissue, challenging the traditional view that bones are devoid of lymphatics. Lymphatic vessels contribute to interstitial fluid regulation, immune cell trafficking, and tissue repair through lymphangiocrine signaling. The pathological alterations in these networks are closely linked to inflammatory joint diseases, emphasizing the need for further research into their co-regulatory mechanisms. This comprehensive review summarizes the current understanding of the structural and functional aspects of vascular and lymphatic networks in bone and joint tissues, their roles in homeostasis, and the implications of their dysfunction in disease. By elucidating the dynamic interactions between these systems, we aim to enhance the understanding of their contributions to skeletal health and disease, potentially informing the development of targeted therapeutic strategies.

Influences on the Hematopoietic Stem Cell Niche

Hematopoietic stem cells (HSCs) are supported by the bone marrow microenvironment to maintain normal production of blood cells. The niche may be considered an "ecosystem" that support the function of HSCs and other supportive cells. Alterations in the bone marrow niche are commonly observed in hematologic malignancies. Here, we review recent insights into the location and the molecular and cellular components of the bone marrow niche. Moreover, we discuss how the niche interacts with HSCs to drive the pathogenesis of hematopoietic malignancies. Overall, a better understanding of the influences on the HSC niche may drive therapeutic development targeting defective and aberrant hematopoiesis.

Ultralow-dose irradiation enables engraftment and intravital tracking of disease initiating niches in clonal hematopoiesis

Recent advances in imaging suggested that spatial organization of hematopoietic cells in their bone marrow microenvironment (niche) regulates cell expansion, governing progression, and leukemic transformation of hematological clonal disorders. However, our ability to interrogate the niche in pre-malignant conditions has been limited, as standard murine models of these diseases rely largely on transplantation of the mutant clones into conditioned mice where the marrow microenvironment is compromised. Here, we leveraged live-animal microscopy and ultralow dose whole body or focal irradiation to capture single cells and early expansion of benign/pre-malignant clones in the functionally preserved microenvironment. 0.5 Gy whole body irradiation (WBI) allowed steady engraftment of cells beyond 30 weeks compared to non-conditioned controls. In-vivo tracking and functional analyses of the microenvironment showed no change in vessel integrity, cell viability, and HSC-supportive functions of the stromal cells, suggesting minimal inflammation after the radiation insult. The approach enabled in vivo imaging of Tet2+/- and its healthy counterpart, showing preferential localization within a shared microenvironment while forming discrete micro-niches. Notably, stationary association with the niche only occurred in a subset of cells and would not be identified without live imaging. This strategy may be broadly applied to study clonal disorders in a spatial context.

Vascular smooth muscle cell-derived exosomes promote osteoblast-to-osteocyte transition via β-catenin signaling

Blood vessel growth and osteogenesis in the skeletal system are coupled; however, fundamental aspects of vascular function in osteoblast-to-osteocyte transition remain unclear. Our study demonstrates that vascular smooth muscle cells (VSMCs), but not endothelial cells, are sufficient to drive bone marrow mesenchymal stromal cell-derived osteoblast-to-osteocyte transition via β-catenin signaling and exosome-mediated communication. We found that VSMC-derived exosomes are loaded with transcripts encoding proteins associated with the osteocyte phenotype and members of the WNT/β-catenin signaling pathway. In contrast, endothelial cell-derived exosomes facilitated mature osteoblast differentiation by reprogramming the TGFB1 gene family and osteogenic transcription factors osterix (SP7) and RUNX2. Notably, VSMCs express significant levels of tetraspanins (CD9, CD63, and CD81) and drive the intracellular trafficking of exosomes with a lower membrane zeta potential than those from other cells. Additionally, the high ATP content within these exosomes supports mineralization mechanisms, as ATP is a substrate for alkaline phosphatase. Osteocyte function was further validated by RNA sequencing, revealing activity in genes related to intermittent mineralization and sonic hedgehog signaling, alongside a significant increase in TNFSF11 levels. Our findings unveil a novel role of VSMCs in promoting osteoblast-to-osteocyte transition, thus offering new insights into bone biology and homeostasis, as well as in bone-related diseases. Clinically, these insights could pave the way for innovative therapeutic strategies targeting VSMC-derived exosome pathways to treat bone-related disorders such as osteoporosis. By manipulating these signaling pathways, it may be possible to enhance bone regeneration and improve skeletal health in patients with compromised bone structure and function.

The ATF4-RPS19BP1 axis modulates ribosome biogenesis to promote erythropoiesis

ABSTRACT

Hematopoietic differentiation is controlled by intrinsic regulators and the extrinsic hematopoietic niche. Activating transcription factor 4 (ATF4) plays a crucial role in the function of fetal and adult hematopoietic stem cell maintenance. However, the precise function of ATF4 in the bone marrow (BM) niche and the mechanism by which ATF4 regulates adult hematopoiesis remain largely unknown. Here, we used 4 cell-type-specific mouse Cre lines to achieve conditional knockout of Atf4 in Cdh5+ endothelial cells, Prx1+ BM stromal cells, Osx+ osteoprogenitor cells, and Mx1+ hematopoietic cells and uncovered the role of Atf4 in niche cells and hematopoiesis. Intriguingly, depletion of Atf4 in niche cells did not affect hematopoiesis; however, Atf4-deficient hematopoietic cells exhibited erythroid differentiation defects, leading to hypoplastic anemia. Mechanistically, ATF4 mediated direct regulation of Rps19bp1 transcription, which is, in turn, involved in 40 S ribosomal subunit assembly to coordinate ribosome biogenesis and promote erythropoiesis. Finally, we demonstrate that under conditions of 5-fluorouracil-induced stress, Atf4 depletion impedes the recovery of hematopoietic lineages, which requires efficient ribosome biogenesis. Taken together, our findings highlight the indispensable role of the ATF4-RPS19BP1 axis in the regulation of erythropoiesis.

Bone marrow niches for hematopoietic stem cells

Hematopoietic stem cells (HSCs) are the cornerstone of the hematopoietic system. HSCs sustain the continuous generation of mature blood derivatives while self-renewing to preserve a relatively constant pool of progenitors throughout life. Yet, long-term maintenance of functional HSCs exclusively takes place in association with their native tissue microenvironment of the bone marrow (BM). HSCs have been long proposed to reside in fixed and identifiable anatomical units found in the complex BM tissue landscape, which control their identity and fate in a deterministic manner. In the last decades, tremendous progress has been made in the dissection of the cellular and molecular fabric of the BM, the structural organization governing tissue function, and the plethora of interactions established by HSCs. Nonetheless, a holistic model of the mechanisms controlling HSC regulation in their niche is lacking to date. Here, we provide an overview of our current understanding of BM anatomy, HSC localization, and crosstalk within local cellular neighborhoods in murine and human tissues, and highlight fundamental open questions on how HSCs functionally integrate in the BM microenvironment.

Interplay Between Skeletal and Hematopoietic Cells in the Bone Marrow Microenvironment in Homeostasis and Aging

PURPOSE OF THE REVIEW

In this review, we discuss the most recent scientific advances on the reciprocal regulatory interactions between the skeletal and hematopoietic stem cell niche, focusing on immunomodulation and its interplay with the cell's mitochondrial function, and how this impacts osteoimmune health during aging and disease.

RECENT FINDINGS

Osteoimmunology investigates interactions between cells that make up the skeletal stem cell niche and immune system. Much work has investigated the complexity of the bone marrow microenvironment with respect to the skeletal and hematopoietic stem cells that regulate skeletal formation and immune health respectively. It has now become clear that these cellular components cooperate to maintain homeostasis and that dysfunction in their interaction can lead to aging and disease. Having a deeper, mechanistic appreciation for osteoimmune regulation will lead to better research perspective and therapeutics with the potential to improve the aging process, skeletal and hematologic regeneration, and disease targeting.

The crosstalk between lung cancer and the bone marrow niche fuels emergency myelopoiesis

Modest response rates to immunotherapy observed in advanced lung cancer patients underscore the need to identify reliable biomarkers and targets, enhancing both treatment decision-making and efficacy. Factors such as PD-L1 expression, tumor mutation burden, and a 'hot' tumor microenvironment with heightened effector T cell infiltration have consistently been associated with positive responses. In contrast, the predictive role of the abundantly present tumor-infiltrating myeloid cell (TIMs) fraction remains somewhat uncertain, partly explained by their towering variety in terms of ontogeny, phenotype, location, and function. Nevertheless, numerous preclinical and clinical studies established a clear link between lung cancer progression and alterations in intra- and extramedullary hematopoiesis, leading to emergency myelopoiesis at the expense of megakaryocyte/erythroid and lymphoid differentiation. These observations affirm that a continuous crosstalk between solid cancers such as lung cancer and the bone marrow niche (BMN) must take place. However, the BMN, encompassing hematopoietic stem and progenitor cells, differentiated immune and stromal cells, remains inadequately explored in solid cancer patients. Subsequently, no clear consensus has been reached on the exact breadth of tumor installed hematopoiesis perturbing cues nor their predictive power for immunotherapy. As the current era of single-cell omics is reshaping our understanding of the hematopoietic process and the subcluster landscape of lung TIMs, we aim to present an updated overview of the hierarchical differentiation process of TIMs within the BMN of solid cancer bearing subjects. Our comprehensive overview underscores that lung cancer should be regarded as a systemic disease in which the cues governing the lung tumor-BMN crosstalk might bolster the definition of new biomarkers and druggable targets, potentially mitigating the high attrition rate of leading immunotherapies for NSCLC.

CHIP: a clonal odyssey of the bone marrow niche

Clonal hematopoiesis of indeterminate potential (CHIP) is characterized by the selective expansion of hematopoietic stem and progenitor cells (HSPCs) carrying somatic mutations. While CHIP is typically asymptomatic, it has garnered substantial attention due to its association with the pathogenesis of multiple disease conditions, including cardiovascular disease (CVD) and hematological malignancies. In this Review, we will discuss seminal and recent studies that have advanced our understanding of mechanisms that drive selection for mutant HSPCs in the BM niche. Next, we will address recent studies evaluating potential relationships between the clonal dynamics of CHIP and hematopoietic development across the lifespan. Next, we will examine the roles of systemic factors that can influence hematopoietic stem cell (HSC) fitness, including inflammation, and exposures to cytotoxic agents in driving selection for CHIP clones. Furthermore, we will consider how - through their impact on the BM niche - lifestyle factors, including diet, exercise, and psychosocial stressors, might contribute to the process of somatic evolution in the BM that culminates in CHIP. Finally, we will review the role of old age as a major driver of selection in CHIP.

Endothelial and macrophage interactions in the angiogenic niche

The interactions between vascular cells, especially endothelial cells, and macrophages play a pivotal role in maintaining the subtle balance of vascular biology, which is crucial for angiogenesis in both healthy and diseased states. These cells are central to ensuring a harmonious balance between tissue repair and preventing excessive angiogenic activity, which could lead to pathological conditions. Recent advances in sophisticated genetic engineering vivo models and novel sequencing approaches, such as single-cell RNA-sequencing, in immunobiology have significantly enhanced our understanding of the gene expression and behavior of macrophages. These insights offer new perspectives on the role macrophages play not only in development but also across various health conditions. In this review, we explore the complex interactions between multiple types of macrophages and endothelium, focusing on their impact on new blood vessel formation. By understanding these intricate interactions, we aim to provide insights into new methods for managing angiogenesis in various diseases, thereby offering hope for the development of novel therapeutic approaches.

Age and dose dependent changes to the bone and bone marrow microenvironment after cytotoxic conditioning with busulfan

Preparative regimens before Hematopoietic Cell Transplantation (HCT) damage the bone marrow (BM) microenvironment, potentially leading to secondary morbidity and even mortality. The precise effects of cytotoxic preconditioning on bone and BM remodeling, regeneration, and subsequent hematopoietic recovery over time remain unclear. Moreover, the influence of recipient age and cytotoxic dose have not been fully described. In this study, we longitudinally investigated bone and BM remodeling after busulfan treatment with low intensity (LI) and high intensity (HI) regimens as a function of animal age. As expected, higher donor chimerism was observed in young mice in both LI and HI regimens compared to adult mice. Noticeably in adult mice, significant engraftment was only observed in the HI group. The integrity of the blood-bone marrow barrier in calvarial BM blood vessels was lost after busulfan treatment in the young mice and remained altered even 6 weeks after HCT. In adult mice, the severity of vascular leakage appeared to be dose-dependent, being more pronounced in HI compared to LI recipients. Interestingly, no noticeable change in blood flow velocity was observed following busulfan treatment. Ex vivo imaging of the long bones revealed a reduction in the frequency and an increase in the diameter and density of the blood vessels shortly after treatment, a phenomenon that largely recovered in young mice but persisted in older mice after 6 weeks. Furthermore, analysis of bone remodeling indicated a significant alteration in bone turnover at 6 weeks compared to earlier timepoints in both young and adult mice. Overall, our results reveal new aspects of bone and BM remodeling, as well as hematopoietic recovery, which is dependent on the cytotoxic dose and recipient age.

Chemical Coaxing of Mesenchymal Stromal Cells by Drug Repositioning for Nestin Induction

Mesenchymal stromal cells (MSCs) display heterogeneity in origin and functional role in tissue homeostasis. Subsets of MSCs derived from the neural crest express nestin and serve as niches in bone marrow, but the possibility of coaxing MSCs into nestin-expresing cells for enhanced supportive activity is unclear. In this study, as an approach to the chemical coaxing of MSC functions, we screened libraries of clinically approved chemicals to identify compounds capable of inducing nestin expression in MSCs. Out of 2000 clinical compounds, we chose vorinostat as a candidate to coax the MSCs into neural crest-like fates. When treated with vorinostat, MSCs exhibited a significant increase in the expression of genes involved in the pluripotency and epithelial-mesenchymal transition (EMT), as well as nestin and CD146, the markers for pericytes. In addition, these nestin-induced MSCs exhibited enhanced differentiation towards neuronal cells with the upregulation of neurogenic markers, including SRY-box transcription factor 2 (Sox2), SRY-box transcription factor 10 (Sox10) and microtubule associated protein 2 (Map2) in addition to nestin. Moreover, the coaxed MSCs exhibited enhanced supporting activity for hematopoietic progenitors without supporting leukemia cells. These results demonstrate the feasibility of the drug repositioning of MSCs to induce neural crest-like properties through the chemical coaxing of cell fates.

Vascular smooth muscle cells exhibit elevated hypoxia-inducible Factor-1α expression in human blood vessel organoids, influencing osteogenic performance

Considering the importance of alternative methodologies to animal experimentation, we propose an organoid-based biological model for in vitro blood vessel generation, achieved through co-culturing endothelial and vascular smooth muscle cells (VSMCs). Initially, the organoids underwent comprehensive characterization, revealing VSMCs (α-SMA + cells) at the periphery and endothelial cells (CD31+ cells) at the core. Additionally, ephrin B2 and ephrin B4, genes implicated in arterial and venous formation respectively, were used to validate the obtained organoid. Moreover, the data indicates exclusive HIF-1α expression in VSMCs, identified through various methodologies. Subsequently, we tested the hypothesis that the generated blood vessels have the capacity to modulate the osteogenic phenotype, demonstrating the ability of HIF-1α to promote osteogenic signals, primarily by influencing Runx2 expression. Overall, this study underscores that the methodology employed to create blood vessel organoids establishes an experimental framework capable of producing a 3D culture model of both venous and arterial endothelial tissues. This model effectively guides morphogenesis from mesenchymal stem cells through paracrine signaling, ultimately leading to an osteogenic acquisition phenotype, with the dynamic involvement of HIF-1α.

Bioengineered niches that recreate physiological extracellular matrix organisation to support long-term haematopoietic stem cells

Long-term reconstituting haematopoietic stem cells (LT-HSCs) are used to treat blood disorders via stem cell transplantation. The very low abundance of LT-HSCs and their rapid differentiation during in vitro culture hinders their clinical utility. Previous developments using stromal feeder layers, defined media cocktails, and bioengineering have enabled HSC expansion in culture, but of mostly short-term HSCs and progenitor populations at the expense of naive LT-HSCs. Here, we report the creation of a bioengineered LT-HSC maintenance niche that recreates physiological extracellular matrix organisation, using soft collagen type-I hydrogels to drive nestin expression in perivascular stromal cells (PerSCs). We demonstrate that nestin, which is expressed by HSC-supportive bone marrow stromal cells, is cytoprotective and, via regulation of metabolism, is important for HIF-1α expression in PerSCs. When CD34+ve HSCs were added to the bioengineered niches comprising nestin/HIF-1α expressing PerSCs, LT-HSC numbers were maintained with normal clonal and in vivo reconstitution potential, without media supplementation. We provide proof-of-concept that our bioengineered niches can support the survival of CRISPR edited HSCs. Successful editing of LT-HSCs ex vivo can have potential impact on the treatment of blood disorders.

Self-Organization of Sinusoidal Vessels in Pluripotent Stem Cell-derived Human Liver Bud Organoids

The induction of tissue-specific vessels in in vitro living tissue systems remains challenging. Here, we directly differentiated human pluripotent stem cells into CD32b+ putative liver sinusoidal progenitors (iLSEP) by dictating developmental pathways. By devising an inverted multilayered air-liquid interface (IMALI) culture, hepatic endoderm, septum mesenchyme, arterial and sinusoidal quadruple progenitors self-organized to generate and sustain hepatocyte-like cells neighbored by divergent endothelial subsets composed of CD32blowCD31high, LYVE1+STAB1+CD32bhighCD31lowTHBD-vWF-, and LYVE1-THBD+vWF+ cells. Wnt2 mediated sinusoidal-to-hepatic intercellular crosstalk potentiates hepatocyte differentiation and branched endothelial network formation. Intravital imaging revealed iLSEP developed fully patent human vessels with functional sinusoid-like features. Organoid-derived hepatocyte- and sinusoid-derived coagulation factors enabled correction of in vitro clotting time with Factor V, VIII, IX, and XI deficient patients' plasma and rescued the severe bleeding phenotype in hemophilia A mice upon transplantation. Advanced organoid vascularization technology allows for interrogating key insights governing organ-specific vessel development, paving the way for coagulation disorder therapeutics.

Complexities of modeling the bone marrow microenvironment to facilitate hematopoietic research

Hematopoiesis occurs in the bone marrow (BM), within a specialized microenvironment referred to as the stem cell niche, where the hematopoietic stem cells (HSCs) reside and are regulated for quiescence, self-renewal and differentiation through intrinsic and extrinsic mechanisms. The BM contains at least two distinctive HSC-supportive niches: an endosteal osteoblastic niche that supports quiescence and self-renewal and a more vascular/perisinusoidal niche that promotes proliferation and differentiation. Both associate with supporting mesenchymal stromal cells. Within the more hypoxic osteoblastic niche, HSCs specifically interact with the osteoblasts that line the endosteal surface, which secrete several important HSC quiescence and maintenance regulatory factors. In vivo imaging indicates that the HSCs and progenitors located further away, in the vicinity of sinusoidal endothelial cells, are more proliferative. Here, HSCs interact with endothelial cells via specific cell adhesion molecules. Endothelial cells also secrete several factors important for HSC homeostasis and proliferation. In addition, HSCs and mesenchymal stromal cells are embedded within the extracellular matrix (ECM), an important network of proteins such as collagen, elastin, laminin, proteoglycans, vitronectin, and fibronectin. The ECM provides mechanical characteristics such as stiffness and elasticity important for cell behavior regulation. ECM proteins are also able to bind, sequester, display, and distribute growth factors across the BM, thus directly affecting stem cell fate and regulation of hematopoiesis. These important physical and chemical features of the BM require careful consideration when creating three-dimensional models of the BM.

Hematopoietic Stem Cells and Their Niche in Bone Marrow

Extensive research has explored the functional correlation between stem cells and progenitor cells, particularly in blood. Hematopoietic stem cells (HSCs) can self-renew and regenerate tissues within the bone marrow, while stromal cells regulate tissue function. Recent studies have validated the role of mammalian stem cells within specific environments, providing initial empirical proof of this functional phenomenon. The interaction between bone and blood has always been vital to the function of the human body. It was initially proposed that during evolution, mammalian stem cells formed a complex relationship with the surrounding microenvironment, known as the niche. Researchers are currently debating the significance of molecular-level data to identify individual stromal cell types due to incomplete stromal cell mapping. Obtaining these data can help determine the specific activities of HSCs in bone marrow. This review summarizes key topics from previous studies on HSCs and their environment, discussing current and developing concepts related to HSCs and their niche in the bone marrow.

Skeleton-derived extracellular vesicles in bone and whole-body aging: From mechanisms to potential applications

The skeleton serves as a supportive and protective organ for the body. As individuals age, their bone tissue undergoes structural, cellular, and molecular changes, including the accumulation of senescent cells. Extracellular vesicles (EVs) play a crucial role in aging through the cellular secretome and have been found to induce or accelerate age-related dysfunction in bones and to contribute further via the circulatory system to the aging of phenotypes of other bodily systems. However, the extent of these effects and their underlying mechanisms remain unclear. Therefore, this paper attempts to give an overview of the current understanding of age-related alteration in EVs derived from bones. The role of EVs in mediating communications among bone-related cells and other body parts is discussed, and the significance of bones in the whole-body aging process is highlighted. Ultimately, it is hoped that gaining a clearer understanding of the relationship between EVs and aging mechanisms may serve as a basis for new treatment strategies for age-related degenerative diseases in the skeleton and other systems.

Systemic and local regulation of hematopoietic homeostasis in health and disease

Hematopoietic stem cells (HSCs) generate all blood cell lineages responsible for tissue oxygenation, life-long hematopoietic homeostasis and immune protection. In adulthood, HSCs primarily reside in the bone marrow (BM) microenvironment, consisting of diverse cell types that constitute the stem cell 'niche'. The adaptability of the hematopoietic system is required to respond to the needs of the host, whether to maintain normal physiology or during periods of physical, psychosocial or environmental stress. Hematopoietic homeostasis is achieved by intricate coordination of systemic and local factors that orchestrate the function of HSCs throughout life. However, homeostasis is not a static process; it modulates HSC and progenitor activity in response to circadian rhythms coordinated by the central and peripheral nervous systems, inflammatory cues, metabolites and pathologic conditions. Here, we review local and systemic factors that impact hematopoiesis, focusing on the implications of aging, stress and cardiovascular disease.

Neutrophils in glioma microenvironment: from immune function to immunotherapy

Glioma is a malignant tumor of the central nervous system (CNS). Currently, effective treatment options for gliomas are still lacking. Neutrophils, as an important member of the tumor microenvironment (TME), are widely distributed in circulation. Recently, the discovery of cranial-meningeal channels and intracranial lymphatic vessels has provided new insights into the origins of neutrophils in the CNS. Neutrophils in the brain may originate more from the skull and adjacent vertebral bone marrow. They cross the blood-brain barrier (BBB) under the action of chemokines and enter the brain parenchyma, subsequently migrating to the glioma TME and undergoing phenotypic changes upon contact with tumor cells. Under glycolytic metabolism model, neutrophils show complex and dual functions in different stages of cancer progression, including participation in the malignant progression, immune suppression, and anti-tumor effects of gliomas. Additionally, neutrophils in the TME interact with other immune cells, playing a crucial role in cancer immunotherapy. Targeting neutrophils may be a novel generation of immunotherapy and improve the efficacy of cancer treatments. This article reviews the molecular mechanisms of neutrophils infiltrating the central nervous system from the external environment, detailing the origin, functions, classifications, and targeted therapies of neutrophils in the context of glioma.

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

BACKGROUND

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has yet to be elucidated.

METHODS

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 h post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

RESULTS

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

CONCLUSIONS

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

Integrative analyses of genetic characteristics associated with skeletal endothelial cells

The osseous vascular endothelium encompasses a vast intricate framework that regulates bone remodeling. Osteoporosis, an age-associated systemic bone disease, is characterized by the degeneration of the vascular architecture. Nevertheless, the precise mechanisms underpinning the metamorphosis of endothelial cells (ECs) with advancing age remain predominantly enigmatic. In this study, we conducted a systematic analysis of differentially expressed genes (DEGs) and the associated pathways in juvenile and mature femoral ECs, utilizing data sourced from the Gene Expression Omnibus (GEO) repositories (GSE148804) and employing bioinformatics tools. Through this approach, we successfully discerned six pivotal genes, namely Adamts1, Adamts2, Adamts4, Adamts14, Col5a1, and Col5a2. Subsequently, we constructed a miRNA-mRNA network based on miRNAs displaying differential expression between CD31hiEMCNhi and CD31lowEMCNlow ECs, utilizing online repositories for prediction. The expression of miR-466i-3p and miR-466i-5p in bone marrow ECs exhibited an inverse correlation with age. Our in vivo experiments additionally unveiled miR-466i-5p as a pivotal regulator in osseous ECs and a promising therapeutic target for age-related osteoporosis.

Transforming the Niche: The Emerging Role of Extracellular Vesicles in Acute Myeloid Leukaemia Progression

Acute myeloid leukaemia (AML) management remains a significant challenge in oncology due to its low survival rates and high post-treatment relapse rates, mainly attributed to treatment-resistant leukaemic stem cells (LSCs) residing in bone marrow (BM) niches. This review offers an in-depth analysis of AML progression, highlighting the pivotal role of extracellular vesicles (EVs) in the dynamic remodelling of BM niche intercellular communication. We explore recent advancements elucidating the mechanisms through which EVs facilitate complex crosstalk, effectively promoting AML hallmarks and drug resistance. Adopting a temporal view, we chart the evolving landscape of EV-mediated interactions within the AML niche, underscoring the transformative potential of these insights for therapeutic intervention. Furthermore, the review discusses the emerging understanding of endothelial cell subsets' impact across BM niches in shaping AML disease progression, adding another layer of complexity to the disease progression and treatment resistance. We highlight the potential of cutting-edge methodologies, such as organ-on-chip (OoC) and single-EV analysis technologies, to provide unprecedented insights into AML-niche interactions in a human setting. Leveraging accumulated insights into AML EV signalling to reconfigure BM niches and pioneer novel approaches to decipher the EV signalling networks that fuel AML within the human context could revolutionise the development of niche-targeted therapy for leukaemia eradication.

Endothelial SMAD1/5 signaling couples angiogenesis to osteogenesis in juvenile bone

Skeletal development depends on coordinated angiogenesis and osteogenesis. Bone morphogenetic proteins direct bone formation in part by activating SMAD1/5 signaling in osteoblasts. However, the role of SMAD1/5 in skeletal endothelium is unknown. Here, we found that endothelial cell-conditional SMAD1/5 depletion in juvenile mice caused metaphyseal and diaphyseal hypervascularity, resulting in altered trabecular and cortical bone formation. SMAD1/5 depletion induced excessive sprouting and disrupting the morphology of the metaphyseal vessels, with impaired anastomotic loop formation at the chondro-osseous junction. Endothelial SMAD1/5 depletion impaired growth plate resorption and, upon long-term depletion, abrogated osteoprogenitor recruitment to the primary spongiosa. Finally, in the diaphysis, endothelial SMAD1/5 activity was necessary to maintain the sinusoidal phenotype, with SMAD1/5 depletion inducing formation of large vascular loops and elevated vascular permeability. Together, endothelial SMAD1/5 activity sustains skeletal vascular morphogenesis and function and coordinates growth plate remodeling and osteoprogenitor recruitment dynamics in juvenile mouse bone.

Endosteal stem cells at the bone-blood interface: A double-edged sword for rapid bone formation: Bone marrow endosteal stem cells provide a robust source of bone-making osteoblasts both in normal and abnormal bone formation

Endosteal stem cells are a subclass of bone marrow skeletal stem cell populations that are particularly important for rapid bone formation occurring in growth and regeneration. These stem cells are strategically located near the bone surface in a specialized microenvironment of the endosteal niche. These stem cells are abundant in young stages but eventually depleted and replaced by other stem cell types residing in a non-endosteal perisinusoidal niche. Single-cell molecular profiling and in vivo cell lineage analyses play key roles in discovering endosteal stem cells. Importantly, endosteal stem cells can transform into bone tumor-making cells when deleterious mutations occur in tumor suppressor genes. The emerging hypothesis is that osteoblast-chondrocyte transitional identities confer a special subset of endosteal stromal cells with stem cell-like properties, which may make them susceptible for tumorigenic transformation. Endosteal stem cells are likely to represent an important therapeutic target of bone diseases caused by aberrant bone formation.

MarShie: a clearing protocol for 3D analysis of single cells throughout the bone marrow at subcellular resolution

Analyzing immune cell interactions in the bone marrow is vital for understanding hematopoiesis and bone homeostasis. Three-dimensional analysis of the complete, intact bone marrow within the cortex of whole long bones remains a challenge, especially at subcellular resolution. We present a method that stabilizes the marrow and provides subcellular resolution of fluorescent signals throughout the murine femur, enabling identification and spatial characterization of hematopoietic and stromal cell subsets. By combining a pre-processing algorithm for stripe artifact removal with a machine-learning approach, we demonstrate reliable cell segmentation down to the deepest bone marrow regions. This reveals age-related changes in the marrow. It highlights the interaction between CX3CR1+ cells and the vascular system in homeostasis, in contrast to other myeloid cell types, and reveals their spatial characteristics after injury. The broad applicability of this method will contribute to a better understanding of bone marrow biology.

Thrombopoietin mimetic stimulates bone marrow vascular and stromal niches to mitigate acute radiation syndrome

Background

Acute radiation syndrome (ARS) manifests after exposure to high doses of radiation in the instances of radiologic accidents or incidents. Facilitating the regeneration of the bone marrow (BM), namely the hematopoietic stem and progenitor cells (HSPCs), is a key in mitigating ARS and multi-organ failure. JNJ-26366821, a PEGylated thrombopoietin mimetic (TPOm) peptide, has been shown as an effective medical countermeasure (MCM) to treat hematopoietic-ARS (H-ARS) in mice. However, the activity of TPOm on regulating BM vascular and stromal niches to support HSPC regeneration has not yet been elucidated.

Methods

C57BL/6J mice (9-14 weeks old) received sublethal or lethal total body irradiation (TBI), a model for H-ARS, by 137Cs or X-rays. At 24 hours post-irradiation, mice were subcutaneously injected with a single dose of TPOm (0.3 mg/kg or 1.0 mg/kg) or PBS (vehicle). At homeostasis and on days 4, 7, 10, 14, 18, and 21 post-TBI with and without TPOm treatment, BM was harvested for histology, BM flow cytometry of HSPCs, endothelial (EC) and mesenchymal stromal cells (MSC), and whole-mount confocal microscopy. For survival, irradiated mice were monitored and weighed for 30 days. Lastly, BM triple negative cells (TNC; CD45-, TER-119-, CD31-) were sorted for single-cell RNA-sequencing to examine transcriptomics after TBI with or without TPOm treatment.

Results

At homeostasis, TPOm expanded the number of circulating platelets and HSPCs, ECs, and MSCs in the BM. Following sublethal TBI, TPOm improved BM architecture and promoted recovery of HSPCs, ECs, and MSCs. Furthermore, TPOm elevated VEGF-C levels in normal and irradiated mice. Following lethal irradiation, mice improved body weight recovery and 30-day survival when treated with TPOm after 137Cs and X-ray exposure. Additionally, TPOm reduced vascular dilation and permeability. Finally, single-cell RNA-seq analysis indicated that TPOm increased the expression of collagens in MSCs to enhance their interaction with other progenitors in BM and upregulated the regeneration pathway in MSCs.

Conclusions

TPOm interacts with BM vascular and stromal niches to locally support hematopoietic reconstitution and systemically improve survival in mice after TBI. Therefore, this work warrants the development of TPOm as a potent radiation MCM for the treatment of ARS.

Endothelial-derived small extracellular vesicles support B-cell acute lymphoblastic leukemia development

PURPOSE

The bone marrow niche plays an important role in leukemia development. However, the contributions of different niche components to leukemia development and their underlying mechanisms remain largely unclear.

METHOD

Cre/LoxP-based conditional knockout technology was used to delete VPS33B or ANGPTL2 gene in niche cells. Murine B-ALL model was established by overexpressing the N-Myc oncogene in hematopoietic stem progenitor cells. The frequency of leukemia cells and immunophenotypic B220+ CD43+ LICs was detected by flow cytometry. SEVs was isolated by sequential centrifugation and mass spectrometry was performed to analyze the different components of SEVs. Immunoprecipitation and western blot were used to measure the interaction of VPS33B and ANGPTL2.

RESULTS

Here, we showed that specific knockout of vascular protein sorting 33b (Vps33b) in endothelial cells (ECs), but not megakaryocytes or mesenchymal stem cells, resulted in a significant decrease in the secretion of small extracellular vesicles (SEVs) and a delay in the development of B-cell lymphoblastic leukemia (B-ALL). Vps33b knockdown endothelial cells contained much lower levels of SEVs that contained angiopoietin-like protein 2 (ANGPTL2) than the control cells. Importantly, conditional knockout of Angptl2 in ECs significantly delayed B-ALL progression. Moreover, C-terminal region of ANGPTL2 (aa247-471) could directly interact with Sec1-like domain 1 of VPS33B (aa1-aa146). We further demonstrated that the point mutations R399H and G402S in ANGPTL2 led to a dramatic decrease in the secretion of ANGPTL2-SEVs. We also showed that wild-type ANGPTL2-containing SEVs, but not mutant ANGPTL2-containing SEVs, significantly enhanced B-ALL development.

CONCLUSION

In summary, our findings indicate that the secretion of ANGPTL2-containing SEVs in ECs sustains the leukemogenic activities of B-ALL cells, which is fine-tuned by the direct interaction of VPS33B and ANGPTL2. These findings reveal that niche-specific SEVs play an important role in B-ALL development.

Stress-protecting harbors for hematopoietic stem cells

Hematopoietic stem cells (HSCs) rely on specialized microenvironments known as niches to maintain their self-renewal and multilineage potential to generate diverse types of blood cells continuously. Over the last two decades, substantial advancements have been made in unraveling the niche cell components and HSC localizations under homeostatic and stressed circumstances. Advances in imaging, combined with the discovery of phenotypic surface markers combinations and single cell sequencing, have greatly facilitated the systematic examination of HSC localizations. This review aims to present a summary of HSC localizations, highlighting potential distinctions between phenotypically and functionally defined HSCs, and explore the functionality of niches in ensuring the integrity and long-term maintenance of HSCs.

Contribution of the TIME in BCP-ALL: the basis for novel approaches therapeutics

The bone marrow (BM) niche is a microenvironment where both immune and non-immune cells functionally interact with hematopoietic stem cells (HSC) and more differentiated progenitors, contributing to the regulation of hematopoiesis. It is regulated by various signaling molecules such as cytokines, chemokines, and adhesion molecules in its microenvironment. However, despite the strict regulation of BM signals to maintain their steady state, accumulating evidence in B-cell precursor acute lymphoblastic leukemia (BCP-ALL) indicates that leukemic cells can disrupt the physiological hematopoietic niche in the BM, creating a new leukemia-supportive microenvironment. This environment favors immunological evasion mechanisms and the interaction of these cells with the development and progression of BCP-ALL. With a growing understanding of the tumor immune microenvironment (TIME) in the development and progression of BCP-ALL, current strategies focused on "re-editing" TIME to promote antitumor immunity have been developed. In this review, we summarize how TIME cells are disrupted by the presence of leukemic cells, evading immunosurveillance mechanisms in the BCP-ALL model. We also explore the crosstalk between TIME and leukemic cells that leads to treatment resistance, along with the most promising immuno-therapy strategies. Understanding and further research into the role of the BM microenvironment in leukemia progression and relapse are crucial for developing more effective treatments and reducing patient mortality.

Hypoxia and the Hypoxia-Inducible Factors in Lymphocyte Differentiation and Function

Molecular oxygen doubles as a biomolecular building block and an element required for energy generation and metabolism in aerobic organisms. A variety of systems in mammalian cells sense the concentration of oxygen to which they are exposed and are tuned to the range present in our blood and tissues. The ability to respond to insufficient O2 in tissues is central to regulation of erythroid lineage cells, but challenges also are posed for immune cells by a need to adjust to very different oxygen concentrations. Hypoxia-inducible factors (HIFs) provide a major means of making such adjustments. For adaptive immunity, lymphoid lineages are initially defined in bone marrow niches; T lineage cells arise in the thymus, and B cells complete maturation in the spleen. Lymphocytes move from these first stops into microenvironments (bloodstream, lymphatics, and tissues) with distinct oxygenation in each. Herein, evidence pertaining to functions of the HIF transcription factors (TFs) in lymphocyte differentiation and function is reviewed. For the CD4+ and CD8+ subsets of T cells, the case is very strong that hypoxia and HIFs regulate important differentiation events and functions after the naïve lymphocytes emerge from the thymus. In the B lineage, the data indicate that HIF1 contributes to a balanced regulation of B-cell fates after antigen (Ag) activation during immunity. A model synthesized from the aggregate literature is that HIF in lymphocytes generally serves to modulate function in a manner dependent on the molecular context framed by other TFs and signals.

Micro-environment alterations through time leading to myeloid malignancies

The micro-environment plays a critical role in haematopoietic stem cell (HSC) development, self-renewal, differentiation and maintenance by providing a supportive cellular framework and essential molecular cues to sustain homeostasis. In ageing and development of age-related clonal haematopoiesis, the combined contribution of intrinsic alterations in haematopoietic stem cells and their surrounding micro-environment can promote myeloid skewing and release of pro-inflammatory cytokines. A pro-inflammatory micro-environment is a common feature in the initiation and sustenance of several myeloid malignancies. Furthermore, remodelling of the micro-environment is recognized to potentiate the survival of malignant over normal cells. This review explores micro-environmental interactions in the haematopoietic system of adults, especially how the bone marrow micro-environment is impacted by ageing, the onset of age-related clonal haematopoiesis and the development of myeloid malignancies. In addition, we also discuss the possible role age-related clonal haematopoiesis and chronic inflammatory conditions play in altering the bone marrow micro-environment dynamics. Finally, we explore the importance of in vitro models that accurately mimic different aspects of the bone marrow micro-environment in order to study normal and malignant haematopoiesis. LINKED ARTICLES: This article is part of a themed issue on Cancer Microenvironment and Pharmacological Interventions. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.2/issuetoc.

Spatial mapping of human hematopoiesis at single-cell resolution reveals aging-associated topographic remodeling

ABSTRACT

The spatial anatomy of hematopoiesis in the bone marrow (BM) has been extensively studied in mice and other preclinical models, but technical challenges have precluded a commensurate exploration in humans. Institutional pathology archives contain thousands of paraffinized BM core biopsy tissue specimens, providing a rich resource for studying the intact human BM topography in a variety of physiologic states. Thus, we developed an end-to-end pipeline involving multiparameter whole tissue staining, in situ imaging at single-cell resolution, and artificial intelligence-based digital whole slide image analysis and then applied it to a cohort of disease-free samples to survey alterations in the hematopoietic topography associated with aging. Our data indicate heterogeneity in marrow adipose tissue (MAT) content within each age group and an inverse correlation between MAT content and proportions of early myeloid and erythroid precursors, irrespective of age. We identify consistent endosteal and perivascular positioning of hematopoietic stem and progenitor cells (HSPCs) with medullary localization of more differentiated elements and, importantly, uncover new evidence of aging-associated changes in cellular and vascular morphologies, microarchitectural alterations suggestive of foci with increased lymphocytes, and diminution of a potentially active megakaryocytic niche. Overall, our findings suggest that there is topographic remodeling of human hematopoiesis associated with aging. More generally, we demonstrate the potential to deeply unravel the spatial biology of normal and pathologic human BM states using intact archival tissue specimens.

Wnt/β-Catenin-Signaling Modulates Megakaryopoiesis at the Megakaryocyte-Erythrocyte Progenitor Stage in the Hematopoietic System

The bone marrow (BM) hematopoietic system (HS) gives rise to blood cells originating from hematopoietic stem cells (HSCs), including megakaryocytes (MKs) and red blood cells (erythrocytes; RBCs). Many steps of the cell-fate decision remain to be elucidated, being important for cancer treatment. To explore the role of Wnt/β-catenin for MK and RBC differentiation, we activated β-catenin signaling in platelet-derived growth factor b (Pdgfb)-expressing cells of the HS using a Cre-lox approach (Ctnnb1BM-GOF). FACS analysis revealed that Pdgfb is mainly expressed by megakaryocytic progenitors (MKPs), MKs and platelets. Recombination resulted in a lethal phenotype in mutants (Ctnnb1BM-GOFwt/fl, Ctnnb1BM-GOFfl/fl) 3 weeks after tamoxifen injection, showing an increase in MKs in the BM and spleen, but no pronounced anemia despite reduced erythrocyte counts. BM transplantation (BMT) of Ctnnb1BM-GOF BM into lethally irradiated wildtype recipients (BMT-Ctnnb1BM-GOF) confirmed the megakaryocytic, but not the lethal phenotype. CFU-MK assays in vitro with BM cells of Ctnnb1BM-GOF mice supported MK skewing at the expense of erythroid colonies. Molecularly, the runt-related transcription factor 1 (RUNX1) mRNA, known to suppress erythropoiesis, was upregulated in Ctnnb1BM-GOF BM cells. In conclusion, β-catenin activation plays a key role in cell-fate decision favoring MK development at the expense of erythroid production.

Bone marrow adipocytes fuel emergency hematopoiesis after myocardial infarction

After myocardial infarction (MI), emergency hematopoiesis produces inflammatory myeloid cells that accelerate atherosclerosis and promote heart failure. Since the balance between glycolysis and mitochondrial metabolism regulates hematopoietic stem cell homeostasis, metabolic cues may influence emergency myelopoiesis. Here, we show in humans and female mice that hematopoietic progenitor cells increase fatty acid metabolism after MI. Blockade of fatty acid oxidation by deleting carnitine palmitoyltransferase (Cpt1A) in hematopoietic cells of Vav1Cre/+Cpt1Afl/fl mice limited hematopoietic progenitor proliferation and myeloid cell expansion after MI. We also observed reduced bone marrow adiposity in humans, pigs and mice following MI. Inhibiting lipolysis in adipocytes using AdipoqCreERT2Atglfl/fl mice or local depletion of bone marrow adipocytes in AdipoqCreERT2iDTR mice also curbed emergency hematopoiesis. Furthermore, systemic and regional sympathectomy prevented bone marrow adipocyte shrinkage after MI. These data establish a critical role for fatty acid metabolism in post-MI emergency hematopoiesis.

Perfusion Window Chambers Enable Interventional Analyses of Tumor Microenvironments

Intravital microscopy (IVM) allows spatial and temporal imaging of different cell types in intact live tissue microenvironments. IVM has played a critical role in understanding cancer biology, invasion, metastases, and drug development. One considerable impediment to the field is the inability to interrogate the tumor microenvironment and its communication cascades during disease progression and therapeutic interventions. Here, a new implantable perfusion window chamber (PWC) is described that allows high-fidelity in vivo microscopy, local administration of stains and drugs, and longitudinal sampling of tumor interstitial fluid. This study shows that the new PWC design allows cyclic multiplexed imaging in vivo, imaging of drug action, and sampling of tumor-shed materials. The PWC will be broadly useful as a novel perturbable in vivo system for deciphering biology in complex microenvironments.

Improved prediction of long-term kidney outcomes in people with type 2 diabetes by levels of circulating haematopoietic stem/progenitor cells

AIM/HYPOTHESIS

We examined whether prediction of long-term kidney outcomes in individuals with type 2 diabetes can be improved by measuring circulating levels of haematopoietic stem/progenitor cells (HSPCs), which are reduced in diabetes and are associated with cardiovascular risk.

METHODS

We included individuals with type 2 diabetes who had a baseline determination of circulating HSPCs in 2004-2019 at the diabetes centre of the University Hospital of Padua and divided them into two groups based on their median value per ml of blood. We collected updated data on eGFR and albuminuria up to December 2022. The primary endpoint was a composite of new-onset macroalbuminuria, sustained ≥40% eGFR decline, end-stage kidney disease or death from any cause. The analyses were adjusted for known predictors of kidney disease in the population with diabetes.

RESULTS

We analysed 342 participants (67.8% men) with a mean age of 65.6 years. Those with low HSPC counts (n=171) were significantly older and had a greater prevalence of hypertension, heart failure and nephropathy (45.0% vs 33.9%; p=0.036), as evidenced by lower eGFR and higher albuminuria at baseline. During a median follow-up of 6.7 years, participants with high vs low HSPC counts had lower rates of the composite kidney outcome (adjusted HR 0.69 [95% CI 0.49, 0.97]), slower decline in eGFR and a similar increase in albuminuria. Adding the HSPC information to the risk score of the CKD Prognosis Consortium significantly improved discrimination of individuals with future adverse kidney outcomes.

CONCLUSIONS/INTERPRETATION

HSPC levels predict worsening of kidney function and improve the identification of individuals with type 2 diabetes and adverse kidney outcomes over and beyond a clinical risk score.

Identifying Bone Marrow Microenvironmental Populations in Myelodysplastic Syndrome and Acute Myeloid Leukemia

The bone marrow microenvironment consists of distinct cell populations, such as mesenchymal stromal cells, endothelial cells, osteolineage cells, and fibroblasts, which provide support for hematopoietic stem cells (HSCs). In addition to supporting normal HSCs, the bone marrow microenvironment also plays a role in the development of hematopoietic stem cell disorders, such as myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). MDS-associated mutations in HSCs lead to a block in differentiation and progressive bone marrow failure, especially in the elderly. MDS can often progress to therapy-resistant AML, a disease characterized by a rapid accumulation of immature myeloid blasts. The bone marrow microenvironment is known to be altered in patients with these myeloid neoplasms. Here, a comprehensive protocol to isolate and phenotypically characterize bone marrow microenvironmental cells from murine models of myelodysplastic syndrome and acute myeloid leukemia is described. Isolating and characterizing changes in the bone marrow niche populations can help determine their role in disease initiation and progression and may lead to the development of novel therapeutics targeting cancer-promoting alterations in the bone marrow stromal populations.

Good Cop, Bad Cop: Profiling the Immune Landscape in Multiple Myeloma

Multiple myeloma (MM) is a dyscrasia of plasma cells (PCs) characterized by abnormal immunoglobulin (Ig) production. The disease remains incurable due to a multitude of mutations and structural abnormalities in MM cells, coupled with a favorable microenvironment and immune suppression that eventually contribute to the development of drug resistance. The bone marrow microenvironment (BMME) is composed of a cellular component comprising stromal cells, endothelial cells, osteoclasts, osteoblasts, and immune cells, and a non-cellular component made of the extracellular matrix (ECM) and the liquid milieu, which contains cytokines, growth factors, and chemokines. The bone marrow stromal cells (BMSCs) are involved in the adhesion of MM cells, promote the growth, proliferation, invasion, and drug resistance of MM cells, and are also crucial in angiogenesis and the formation of lytic bone lesions. Classical immunophenotyping in combination with advanced immune profiling using single-cell sequencing technologies has enabled immune cell-specific gene expression analysis in MM to further elucidate the roles of specific immune cell fractions from peripheral blood and bone marrow (BM) in myelomagenesis and progression, immune evasion and exhaustion mechanisms, and development of drug resistance and relapse. The review describes the role of BMME components in MM development and ongoing clinical trials using immunotherapeutic approaches.

Hematopoietic stem cells through the ages: A lifetime of adaptation to organismal demands

Hematopoietic stem cells (HSCs), which govern the production of all blood lineages, transition through a series of functional states characterized by expansion during fetal development, functional quiescence in adulthood, and decline upon aging. We describe central features of HSC regulation during ontogeny to contextualize how adaptive responses over the life of the organism ultimately form the basis for HSC functional degradation with age. We particularly focus on the role of cell cycle regulation, inflammatory response pathways, epigenetic changes, and metabolic regulation. We then explore how the knowledge of age-related changes in HSC regulation can inform strategies for the rejuvenation of old HSCs.