Mesenchymal stem cells (MSCs) generate various stromal cells in the hematopoietic stem cell (HSCs) niche. As a major components of the HSC niche, multiple subpopulations of MSCs have been identified so far. MSCs can serve as an important cell therapy to facilitate hematopoietic stem cell transplantation (HSCT). Their use facilitates ex vivo expansion of HSCs, hematopoietic engraftment, and stimulation of residual hematopoietic tissues to restore hematopoiesis. In the bone marrow microenvironment, MSCs control HSC survival, proliferation, migration, and differentiation through close contact, secretion of soluble factors, and regulation of differentiation states. MSC-derived extracellular vesicles (EVs) are a prominent intercellular communication pathway, containing bioactive factors such as proteins, lipids, and miRNAs. Similar to MSCs, they regulate the fate of HSCs and represent a promising adjunctive therapeutic tool for HSCT. Furthermore, intercellular communication between MSCs and HSCs is modulated by diverse signaling pathways and gene expression. A growing number of studies have explored the mechanisms by which MSCs regulate HSCs, but further studies are needed to clarify the underlying mechanisms. Here, we reviewed the research progress on the mechanisms by which MSCs regulate HSCs. A deeper understanding of the MSC-HSC interaction network not only reveals the fundamental principles for maintaining hematopoietic homeostasis but also provides new insights for optimizing HSCT and treating bone marrow failure diseases.

Notch signalling and stem cell ageing: insights from the ReSinAge project Elderly cancer patients suffer high haematopoietic toxicity upon chemotherapy. ReSinAge aims to boost haematopoietic recovery and increase the survival after chemotherapy in the elderly by improving the regenerative capacity of the aged haematopoietic stem cell niche. In detail, we aim to disclose the functional interplay between blood stem cells and their niche, and its targeting as an innovative strategy to overcome chemotherapy toxicity and increase the survival of elderly cancer patients.

Gene therapy for hemoglobinopathies: Clinical trial results and biology of hematopoietic stem cell and the bone marrow niche.

This work introduces stem cell activity as a central factor contributing to the pleiotropic effects of IFN-γ and TGF-β1, as well as to the fluctuations of autoimmune diseases (AIDs) between flares and remissions. Analysis of published data on hair follicle immune privilege indicates that immune protection is not an inherent feature of quiescent stem cells, as previously proposed, but instead depends on the specific pathways that regulate quiescence. While both IFN-γ and high levels of TGF-β1 induce stem cell quiescence, they exert opposite effects on immune privilege: IFN-γ upregulates MHC-I expression, whereas TGF-β1 downregulates it. Similar mechanisms apply to hematopoietic stem cell niches in the bone marrow. Moreover, cytokines such as IGF-1 and α-MSH, which enhance stem cell activity, also downregulate MHC-I. Different concentrations and combinations of these cytokines can promote or suppress stem cell activity and preserve or disrupt immune privilege, underscoring their multifaceted nature. Two mechanisms may contribute to the pleiotropic effects of IFN-γ and TGF-β1: opposing effects on bone marrow activity, with IFN-γ and high TGF-β1 acting in contrast, and differential effects of IFN-γ on immune attack intensity in the bone marrow versus the target tissue during AID. Stem cell dynamics also shape the course of AIDs: high stem cell activity supports tissue regeneration and remission, whereas quiescence together with tissue destruction by autoimmune attacks drives flares. A clear correlation emerges between the effects of various agents on stem cell activity and clinical outcomes in AIDs, highlighting the central role of stem cell activity in their pathogenesis. A proposed TGF-β1 gradient between protected stem cell reservoirs (hair follicle bulge, bone endosteal niches) and less protected regions enables simultaneous preservation of stem cells and regeneration of damaged tissue.

The fate of hematopoietic stem cells (HSCs) is determined by a complex regulatory network supporting self-renewal and quiescence within a niche. Umbilical cord mesenchymal stromal cells (UC-MSCs) are classified as an alternative niche for the expansion of hematopoietic stem and progenitor cells (HSPCs). The molecular mechanisms by which UC-MSCs regulate hematopoiesis are still not fully understood. In this study, the cocultures of UC-MSCs and umbilical cord blood CD34+ (UCB-CD34+) cells were established. Immunophenotype, cell proliferation, and hematopoietic function of UCB-CD34+ cells were evaluated on days 0 to 7. UC-MSCs promoted UCB-CD34+ cell proliferation but were less effective at preserving their stemness. Notably, UC-MSCs promoted the myeloid lineage commitment, significantly observed on day 3. Integrative transcriptomic analysis highlighted the molecular signature and regulatory networks of UC-MSCs. The long non-coding RNA (lncRNA)-RNA binding protein (RBP) interaction network and lncRNA cis- and trans-regulatory networks were evident. The significant 3-gene modules and a set of 10-hub genes were identified in the protein-protein interaction (PPI) network, including RPS16, CD74, RPL35, COX7C, RPL38, RPS28, RPS27, RPS10, TARDBP, and TOMM7. These findings exemplify the niche activity of UC-MSCs in regulating cell differentiation, genomic stability maintenance, and modulation of the hematopoietic supportive niche. The transcriptional landscape, together with the identified regulatory networks, gene modules, and key hub genes provide new insights into the molecular mechanisms of UC-MSCs and establish a basis for refining ex vivo culture systems for therapeutic HSC expansion.

The hematopoietic stem cell (HSC) niche is a specific microenvironment in the bone marrow that maintains the ability of HSCs to differentiate and self-renew. It comprises two interconnected sub-niches: the vascular and the intraosseous. This distinction is particularly relevant in the context of homing, as hematopoietic stem cells sequentially interact with both niches during the engraftment process. The components of the bone marrow niche are divided into cellular and extracellular elements. All of them are crucial for maintaining niche homeostasis and, consequently, are essential for the success of HSC transplantation and subsequent engraftment.Homing is the process of active migration of hematopoietic stem cells into the bone marrow, which occurs during bone marrow transplantation — a common treatment for hematopoietic tissue tumors. However, a significant proportion of the transplanted cells fail to reach their niche, leading to various side effects and complications of this procedure. Currently, there is active research focused on improving the efficacy of HSC transplantation. The approaches under investigation include both methods to directly enhance cell migration and strategies to preemptively increase the number of transplantable hematopoietic stem cells. Homing itself is a key target for new technologies, as improving its efficiency can reduce the time required for blood cell recovery after transplantation. Advancements in this field have the potential to transform current HSC transplantation practices and significantly increase patient survival rates following the procedure.

Adrenergic signaling and hematopoietic stem cell function in the bone marrow niche rely on peri-arteriolar NADPH oxidase (NOX) and connexin gap junctions.

IFN-γ drives long-term bone marrow niche dysfunction following chemotherapy

Single cell transcriptional profiling reveals distinct subsets of human megakaryocytes in myelofibrosis

Macrophages mediate hematopoietic stem cell trans-endocytosis to coordinate regenerative stress and aging

The publication describes complex support structures and scaffolds for stem cells and organoids. Consideration of geometric and structural parameters has an influence on stem cell development and organogenesis comparable to that of molecular genetics and biochemical parameters. Two essential representatives are discussed here in more detail: hematopoietic stem cells (HSCs) and brain organoids. Due to their ability to fully regenerate the blood system, HSCs are used for stem cell transplantations. Therefore, efficient approaches to create an artificial but close-to-nature stem cell niche in vitro for amplification of HSCs are highly desirable. Apart from biochemical and biological factors, geometrical and biomechanical parameters are important. Biotechnological multiscale engineering is able to mimic the HSC niche, improving their amplification. Another highly dynamic process underlying hierarchic orders is organogenesis. Entire organs develop from individual cells that are spatially arranged in precise patterns and exposed to chemical, mechanical, and structural stimuli. The significance of the different scales during their development is explained using human brain organoids. Here, geometrically suitable structures improve biochemical differentiation protocols. Such technical hybrid systems can foster research of a rather inaccessible organ and possibly serve as a platform for more energy-efficient computing devices, such as organoid automata, hence, orgamats.

Hematopoietic stem cells (HSCs) can reconstitute the human blood system. In vivo, HSCs are localized in and regulated by distinct bone marrow (BM) microenvironments, or niches, like the vascular HSC niches near fenestrated sinusoidal blood vessels. These delicate structures, comprising a single-layered endothelium and a discontinuous basement membrane, pose challenges in soft tissue engineering. In this study, the basement membrane in vascular niches is mimicked using nanofibrous fibrinogen scaffolds. A novel clamping system enables handling the scaffold as a membrane and seeding both sides-one with microvascular endothelial cells (HMEC-1) and the other with mesenchymal stem and stroma cells (iMSC#3). Subsequently, HSCs and their progenitors (HSPCs) are introduced from both sides to emulate their niche dynamics (residency, exit, and homing). The study reveals that the fibrinogen scaffolds are highly cytocompatible and show good cell-adhesive properties. In addition, HSPCs are able to migrate through the scaffolds, validating them as fenestrated basement membrane mimetics. This in vitro model offers insights into HSPC behavior in the vascular niche and can serve as a drug testing platform in future studies. Moreover, beyond HSCs, the presented scaffold-based mimetic of the basement membrane offers new opportunities for mimicking and studying vasculature in tissue engineering approaches.

Osteoimmunology: the little niche with the big impact.

Thrombopoietin (TPO), predominantly produced by the liver, is the key regulator for platelet production and the hematopoietic stem cell niche. Our earlier report demonstrated that platelet GPIbα is required for hepatocellular TPO generation, which is the major resource of TPO in the blood circulation. However, how hepatocytes physically contact circulating sinusoidal platelets across the liver endothelium for this process is unknown. Kupffer cells reside in contact with both sinusoidal blood and underlying hepatocytes, and mediate senescent platelet clearance, but their role in TPO regulation has never been explored. Here, we found Kupffer cell depletion via either clodronate liposomes or specific transgenic models abrogated circulating TPO. Kupffer cell depletion also prevented TPO level increase in GPIbα-deficient mice following wild-type (GPIbα+) platelet transfusion, signifying an interdependent mechanism for TPO regulation. Mice treated with arsenite had significantly decreased liver endothelial fenestrations and hepatocyte sinusoidal protrusions as well as TPO levels. This effect was exacerbated by Kupffer cell depletion, and Kupffer cells were identified to enhance liver endothelial fenestrations. Electron microscopy and immunofluorescence analysis of the liver revealed platelets arrested on Kupffer cell surface were in contact with hepatocyte protrusions. Thus, we elucidated that Kupffer cells promote endothelial fenestrae and hepatocyte protrusions, accumulate circulating platelets, and facilitate cellular interactions between hepatocytes and platelets, which drive TPO generation. This connection between platelet clearance and thrombopoiesis should have broad implications for hematology and pathologies such as Bernard-Soulier syndrome, thrombocytopenias, as well as liver diseases.

BackgroundAging is linked to various dysfunctions of the immune system, including the decline of its primary developmental source: the hematopoietic stem cell (HSC) niche. This decline leads to chronic inflammation, increased vulnerability to infections, cancer, autoimmune diseases, and reduced vaccine efficacy. As individuals age, the HSC niche undergoes significant changes, including greater adipocyte accumulation and alterations in the molecular microenvironment, which may influence the development and function of immune cells. Among these cells, the impact of the aging HSC niche on dendritic cell (DC) function is less understood. Heterochronic autologous HSC transplantation is a promising intervention to prevent age-related disorders, contributing to the extension of healthspan and longevity, however, several murine experiments failed to produce the expected results, which led us to presume that the problem lies within the old HSC niche. Therefore, we created in vitro models of young and old HSC niches and examined how these microenvironments affect the differentiation and maturation and functionality of BM-derived DCs (BMDCs).ResultsAn analysis of the conditioned media from young and aged HSC niches revealed that the environment of aged niches exhibited an increased presence of adiponectin. This media was subsequently utilized in BMDC differentiation and maturation protocols, with their effects closely monitored. Our results indicate that the old HSC niche microenvironment promotes premature BMDC activation, characterized by elevated MHC class II expression and enhanced allostimulatory capacity of BMDCs at their immature stage. Additionally, LPS stimulation of BMDCs, used to induce DC maturation, significantly increased CD86 expression on BMDCs from the aged niche. However, these cells did not show superior allostimulatory capacity compared to their counterparts from the young niche environment. By analyzing the BMDC cytokine profile, we observed that when cultured in aged niche-conditioned media, the BMDCs secreted significantly higher levels of IL-6, indicating a heightened proinflammatory activation state.ConclusionsCollectively, our findings suggest that aging-related changes within the HSC niche can considerably alter DC functionality by disrupting their normal development from BM precursors. These results emphasize the significance of this phenomenon and its implications for immunosenescence.

Skeletal harboring of hematopoietic stem cells (HSCs) is generally considered as vertebrate-specific innovation during water-to-land transition. However, this long-standing view has not been rigorously evaluated as hematopoietic sites remain poorly understood in most invertebrate groups. We report, to our knowledge, the first discovery of abundant HSCs in adult mollusk shells, an invertebrate hematopoietic niche resembling vertebrate bone marrow (BM). Cell-lineage analysis and functional assays reveal the developmental origin of HSCs during larval shell formation and their participation in hemocyte-mediated shell regeneration and soft-body blood supply. Widespread skeleton-related HSC-like cells are found in diverse invertebrate groups and bony fish group, suggesting skeletons as a universal niche for animal HSCs and HSC-skeleton association preceding vertebrate water-to-land transition. Comparison of invertebrate and vertebrate skeletal HSCs enables the macroevolutionary profiling of a core-set of animal HSC regulators. Our findings would boost fundamental paradigm shifts for hematopoiesis and stem cell research in invertebrates and provide the redefined understanding of vertebrate BM evolution and water-to-land transition.

Hematopoietic stem cell transplantation (HSCT) has long served as an unparalleled therapeutic option for numerous hematologic malignancies and genetic disorders. However, the efficacy of HSCT is often limited by suboptimal homing of transplanted stem cells to the bone marrow (BM) niche and insufficient long-term engraftment. Despite the need for effective methods to predict clinical outcomes, a suitable model system remains lacking. In this review, we first examine the biological factors that govern hematopoietic stem cell (HSC) homing and niche interactions, including key chemokine–receptor axes (such as CXCR4–CXCL12) and regulatory molecules that facilitate long-term HSC quiescence and repopulation. We then explore how CRISPR-based gene editing technologies are leveraged to modify HSCs for improved clinical outcomes, while addressing the challenges of DNA damage responses and off-target effects associated with editing. Most importantly, we highlight emerging BM organoid (BMO) systems as next-generation 3-dimensional culture models that recapitulate the human BM microenvironment. BMOs represent a promising platform for preclinical testing of gene-edited HSCs and for investigating human-specific HSC–niche interactions beyond animal models. By integrating gene-edited HSCs with organoid models, researchers can more accurately evaluate HSC homing and engraftment in a precisely mimicked, human-like environment. We also emphasize the potential of combining gene editing with BMO technologies to advance personalized HSCT and pave the way for safer, more effective transplantation procedures. Despite their promise, current BMOs exhibit limitations such as incomplete tissue recapitulation, lack of vascularization, batch variability, and limited long-term stability. Addressing all these challenges will be essential for increasing their physiological relevance and translational potential.

The spleen is a key site for extramedullary hematopoiesis that hosts a rare population of functional hematopoietic stem cells (HSCs). While the microenvironment that supports extramedullary hematopoiesis response has gained interest, a niche for splenic HSCs at steady-state remains undescribed. Here, we have uncovered a red-pulp-specific, myofibroblastic niche that supports murine splenic HSCs within a ≈ 200-μm-wide capsular zone. Detailed spatial-distribution and perturbation analysis showed the importance of myofibroblasts in maintaining HSCs in a quiescent state. Unlike reported for the adult bone marrow, the HSCs in splenic niche were not spatially associated with vascular components. G-CSF-mediated chemokine alteration and 5-FU-induced proliferation resulted in HSCs shifts away from the splenic capsule. Interestingly, upon regaining quiescence, the HSCs re-occupied niches close to capsular myofibroblasts. Proteomic interactome profiles confirmed the relevance of capsular myofibroblasts for splenic HSCs and identified potential niche regulators of HSC maintenance. Together, this study demonstrates a dynamic HSC localization in the spleen and its niche context at homeostasis and under stress. It offers a model to uncover novel regulators crucial for HSC function.

Skeletal stem cells (SSCs) have been isolated from various tissues, including periosteum and bone marrow, where they exhibit key functions in bone biology and hematopoiesis, respectively. The role of periosteal SSCs (P-SSCs) in bone regeneration and healing has been extensively studied, but their ability to contribute to the bone marrow stroma is still under debate. In the present study, we characterized a mouse whole bone transplantation model that mimics the initial bone marrow necrosis and fatty infiltration seen after injury. Using this model and a lineage tracing approach, we observed the migration of P-SSCs into the bone marrow after transplantation. Once in the bone marrow, P-SSCs are phenotypically and functionally reprogrammed into bone marrow mesenchymal stem cells (BM-MSCs) that express high levels of hematopoietic stem cell niche factors such as Cxcl12 and Kitl. In addition, using ex vivo and in vivo approaches, we found that P-SSCs are more resistant to acute stress than BM-MSCs. These results highlight the plasticity of P-SSCs and their potential role in bone marrow regeneration after bone marrow injury.

Skeletal stem cells (SSCs) have been isolated from various tissues, including periosteum and bone marrow, where they exhibit key functions in bone biology and hematopoiesis, respectively. The role of periosteal SSCs (P-SSCs) in bone regeneration and healing has been extensively studied, but their ability to contribute to the bone marrow stroma is still under debate. In the present study, we characterized a mouse whole bone transplantation model that mimics the initial bone marrow necrosis and fatty infiltration seen after injury. Using this model and a lineage tracing approach, we observed the migration of P-SSCs into the bone marrow after transplantation. Once in the bone marrow, P-SSCs are phenotypically and functionally reprogrammed into bone marrow mesenchymal stem cells (BM-MSCs) that express high levels of hematopoietic stem cell niche factors such as Cxcl12 and Kitl. In addition, using ex vivo and in vivo approaches, we found that P-SSCs are more resistant to acute stress than BM-MSCs. These results highlight the plasticity of P-SSCs and their potential role in bone marrow regeneration after bone marrow injury.