Aging of the Hematopoietic Stem Cell Niche: New Tools to Answer an Old Question

Metabolic Alterations in HSCs during Aging and Leukemogenesis

Aging is a multifaceted process associated with a functional decline in cellular function over time, affecting all lifeforms. During the aging process, metabolism, a fundamental hallmark of life (1), is profoundly altered. In the context of hematopoiesis, the proper function of hematopoietic stem cells, at the apex of the blood system, is tightly linked to their energy metabolism, which in turn shapes hematopoietic output. Here, we review the latest developments in our understanding of the metabolic states and changes in aged hematopoietic stem cells, molecular players and pathways involved in aged hematopoietic stem cell metabolism, the consequences of perturbed metabolism on clonal hematopoiesis and leukemogenesis, and pharmacologic/genetic strategies to reverse or rejuvenate altered metabolic phenotypes.

The Role of Aging and Senescence in Bone Marrow Transplantation Outcome

Bone marrow transplantation is considered a cornerstone in the treatment of hematologic malignancies and blood disorders. While it may offer the possibility of a cure through the use of high-dose chemotherapy and radiation, outcomes are significantly impacted by biological and medical factors. Herein, aging is associated with reduced hematopoiesis, immune function, and overall regenerative capacity of tissues. Growth arrest, a crucial property of cellular senescence, inhibits bone marrow function, lowers immune surveillance in aged adults, and reduces the efficiency of bone marrow transplantation. The clinical course for older recipients is further complicated by the presence of prolonged immunosuppression, slower recovery, and higher complication rates, including life-threatening graft-versus-host disease. Accordingly, there is increasing interest in explaining how aging, cellular senescence, and transplant outcomes are interrelated. The current chapter outlines the mechanisms whereby aging and senescence contribute to the immunological dysregulation and poor bone marrow transplantation outcomes observed in elderly cancer patients. The authors' goal is to suggest therapeutic approaches that will enhance the quality of life and survival rates of elderly bone marrow transplant recipients.

How age affects human hematopoietic stem and progenitor cells and the strategies to mitigate aging

Hematopoietic stem cells (HSCs) are central to blood formation and play a pivotal role in hematopoietic and systemic aging. With aging, HSCs undergo significant functional changes, such as an increased stem cell pool, declined homing and reconstitution capacity, and skewed differentiation toward myeloid and megakaryocyte/platelet progenitors. These phenotypic alterations are likely due to the expansion of certain clones, known as clonal hematopoiesis (CH), which leads to disrupted hematopoietic homeostasis, including anemia, impaired immunity, higher risks of hematological malignancies, and even associations with cardiovascular disease, highlighting the broader impact of HSC aging on overall health. HSC aging is driven by a range of mechanisms involving both intrinsic and extrinsic factors, such as DNA damage accumulation, epigenetic remodeling, inflammaging and metabolic regulation. In this review, we summarize the updated understanding of age-related changes in hematopoietic stem and progenitor cells (HSPCs) and the mechanisms underlying the aging process in mammalian models, especially in human study. Additionally, we provide insights into potential therapeutic strategies to counteract aging process and enhance HSC regenerative capacity, which will support therapeutic interventions and promote healthy aging.

Hyperlipidemia impairs bone repair and regeneration via miR-193a-3p/STMN1/PI3K/Akt axis

Hyperlipidemia, a metabolic disease characterized by excessive blood lipid, disturbs bone metabolism by shifting cell fate of bone marrow stromal cells (BMSCs) towards adipogenic differentiation, thus resulting in poor bone regeneration and osseointegration of implants. Among numerous factors affecting hyperlipidemic bone metabolism, non-coding RNAs play an essential role in post-transcriptional regulation. Our previous study has shown that miR-193a-3p levels were elevated in hyperlipidemia, which hindered implant osseointegration and BMSCs function. However, the downstream targets and pathways of miR-193a-3p warrant further investigation. In this study, we identified STMN1 as the target of miR-193a-3p by miRNA databases and validated their interaction through dual luciferase reporter assays. Models of hyperlipidemia were established in vitro using a high-fat medium and in vivo with a high-fat diet to study these molecular interactions. Besides, miRNA array and PCR analyses confirmed the level of miR-193a and STMN1 in both rats with hyperlipidemia and high-fat-cultured BMSCs. Calvarial defects were used to evaluate STMN1's impact on bone repair and regeneration. As a result, miR-193a-3p levels were highly elevated in hyperlipidemic conditions, whereas the STMN1 levels were reduced sharply. The elevated miR-193a targeted STMN1 and disabled it from activating the PI3K/Akt pathway, thus resulting in delayed bone repair and poor bone regeneration. Additionally, common lipid-lowering drug simvastatin blunted hyperlipidemia's adverse effect on this axis. Our findings underscore the miR-193a-3p/STMN1/PI3K/Akt axis as a novel and promising therapeutic target for hyperlipidemic osteopenia, offering insights into the molecular mechanisms underlying bone metabolism disorders in hyperlipidemia and paving the way for innovative treatments.

Novel techniques to quantitatively assess age-dependent alterations in biophysical properties of HSPCs and bone marrow niche

The present knowledge on hematopoietic stem and progenitor cell (HSPC) biology and aging is based largely on studies in mouse models. Although mouse models are invaluable, they are not without limitations for defining how physical properties of HSPCs and their niche change with age. The bone marrow (BM) niche is a complex, interactive environment with multiple cell types. The structure and organization of the BM niche, especially the extracellular matrix (ECM), change with age. Provided with recent advances in quantitative analytical techniques and in vitro niche models, we have developed novel tools to quantitatively assess the impact of specific biochemical and physical cues on homing, adhesion, and migration of HSPCs. Recent developments in in vitro niche models have also provided new insights into the interactions between HSPCs and their niche, particularly the role of matrix stiffness. Further research is needed to integrate physical biomarkers into comprehensive mathematical models of age-dependent HSPC-niche interactions. The key is to use mouse models in conjunction with direct analyses in in vitro niche models to achieve a more comprehensive understanding of age-dependent alterations in niche function and regulation.

A Senescent Cluster in Aged Human Hematopoietic Stem Cell Compartment as Target for Senotherapy

To identify the differences between aged and young human hematopoiesis, we performed a direct comparison of aged and young human hematopoietic stem and progenitor cells (HSPCs). Alterations in transcriptome profiles upon aging between humans and mice were then compared. Human specimens consist of CD34+ cells from bone marrow, and mouse specimens of hematopoietic stem cells (HSCs; Lin- Kit+ Sca1+ CD150+). Single-cell transcriptomic studies, functional clustering, and developmental trajectory analyses were performed. A significant increase in multipotent progenitor 2A (MPP2A) cluster is found in the early HSC trajectory in old human subjects. This cluster is enriched in senescence signatures (increased telomere attrition, DNA damage, activation of P53 pathway). In mouse models, the accumulation of an analogous subset was confirmed in the aged LT-HSC population. Elimination of this subset has been shown to rejuvenate hematopoiesis in mice. A significant activation of the P53-P21WAF1/CIP1 pathway was found in the MPP2A population in humans. In contrast, the senescent HSCs in mice are characterized by activation of the p16Ink4a pathway. Aging in the human HSC compartment is mainly caused by the clonal evolution and accumulation of a senescent cell cluster. A population with a similar senescence signature in the aged LT-HSCs was confirmed in the murine aging model. Clearance of this senescent population with senotherapy in humans is feasible and potentially beneficial.

The role of the haematopoietic stem cell niche in development and ageing

Blood production depends on rare haematopoietic stem cells (HSCs) and haematopoietic stem and progenitor cells (HSPCs) that ultimately take up residence in the bone marrow during development. HSPCs and HSCs are subject to extrinsic regulation by the bone marrow microenvironment, or niche. Studying the interactions between HSCs and their niche is critical for improving ex vivo culturing conditions and genetic manipulation of HSCs, which is pivotal for improving autologous HSC therapies and transplantations. Additionally, understanding how the complex molecular network in the bone marrow is altered during ageing is paramount for developing novel therapeutics for ageing-related haematopoietic disorders. HSCs are unique amongst stem and progenitor cell pools in that they engage with multiple physically distinct niches during their ontogeny. HSCs are specified from haemogenic endothelium in the aorta, migrate to the fetal liver and, ultimately, colonize their final niche in the bone marrow. Recent studies employing single-cell transcriptomics and microscopy have identified novel cellular interactions that govern HSC specification and engagement with their niches throughout ontogeny. New lineage-tracing models and microscopy tools have raised questions about the numbers of HSCs specified, as well as the functional consequences of HSCs interacting with each developmental niche. Advances have also been made in understanding how these niches are modified and perturbed during ageing, and the role of these altered interactions in haematopoietic diseases. In this Review, we discuss these new findings and highlight the questions that remain to be explored.

Deciphering the evolving niche interactome of human hematopoietic stem cells from ontogeny to aging

Hematopoietic stem cells (HSC) reside within specialized microenvironments that undergo dynamic changes throughout development and aging to support HSC function. However, the evolving cell-cell communication networks within these niches remain largely unexplored. This study integrates single-cell RNA sequencing datasets to systematically characterize the HSC niche interactome from ontogeny to aging. We reconstructed single-cell atlases of HSC niches at different developmental stages, revealing stage-specific cellular compositions and interactions targeting HSC. During HSC maturation, our analysis identified distinct patterns of ligand-receptor interactions and signaling pathways that govern HSC emergence, expansion, and maintenance. HSC aging was accompanied by a decrease in supportive niche interactions, followed by an adaptive increase in interaction strength in old adult bone marrow. This complex aging process involved the emergence of interactions associated with inflammation, altered stem cell function, and a decline in the efficacy of key signaling pathways. Our findings provide a comprehensive understanding of the dynamic remodeling of the HSC niche interactome throughout life, paving the way for targeted interventions to maintain HSC function and promote healthy aging. This study offers valuable insights into the intricate cell-cell communication networks that govern HSC behavior and fate, with implications for hematological disorders and regenerative medicine.

A p38 MAPK-ROS axis fuels proliferation stress and DNA damage during CRISPR-Cas9 gene editing in hematopoietic stem and progenitor cells

Ex vivo activation is a prerequisite to reaching adequate levels of gene editing by homology-directed repair (HDR) for hematopoietic stem and progenitor cell (HSPC)-based clinical applications. Here, we show that shortening culture time mitigates the p53-mediated DNA damage response to CRISPR-Cas9-induced DNA double-strand breaks, enhancing the reconstitution capacity of edited HSPCs. However, this results in lower HDR efficiency, rendering ex vivo culture necessary yet detrimental. Mechanistically, ex vivo activation triggers a multi-step process initiated by p38 mitogen-activated protein kinase (MAPK) phosphorylation, which generates mitogenic reactive oxygen species (ROS), promoting fast cell-cycle progression and subsequent proliferation-induced DNA damage. Thus, p38 inhibition before gene editing delays G1/S transition and expands transcriptionally defined HSCs, ultimately endowing edited cells with superior multi-lineage differentiation, persistence throughout serial transplantation, enhanced polyclonal repertoire, and better-preserved genome integrity. Our data identify proliferative stress as a driver of HSPC dysfunction with fundamental implications for designing more effective and safer gene correction strategies for clinical applications.

Emerging insights into epigenetics and hematopoietic stem cell trafficking in age-related hematological malignancies

BACKGROUND

Hematopoiesis within the bone marrow (BM) is a complex and tightly regulated process predominantly influenced by immune factors. Aging, diabetes, and obesity are significant contributors to BM niche damage, which can alter hematopoiesis and lead to the development of clonal hematopoiesis of intermediate potential (CHIP). Genetic/epigenetic alterations during aging could influence BM niche reorganization for hematopoiesis or clonal hematopoiesis. CHIP is driven by mutations in genes such as Tet2, Dnmt3a, Asxl1, and Jak2, which are associated with age-related hematological malignancies.

OBJECTIVE

This literature review aims to provide an updated exploration of the functional aspects of BM niche cells within the hematopoietic microenvironment in the context of age-related hematological malignancies. The review specifically focuses on how immunological stressors modulate different signaling pathways that impact hematopoiesis.

METHODS

An extensive review of recent studies was conducted, examining the roles of various BM niche cells in hematopoietic stem cell (HSC) trafficking and the development of age-related hematological malignancies. Emphasis was placed on understanding the influence of immunological stressors on these processes.

RESULTS

Recent findings reveal a significant microheterogeneity and temporal stochasticity of niche cells across the BM during hematopoiesis. These studies demonstrate that niche cells, including mesenchymal stem cells, osteoblasts, and endothelial cells, exhibit dynamic interactions with HSCs, significantly influenced by the BM microenvironment as the age increases. Immunosurveillance plays a crucial role in maintaining hematopoietic homeostasis, with alterations in immune signaling pathways contributing to the onset of hematological malignancies. Novel insights into the interaction between niche cells and HSCs under stress/aging conditions highlight the importance of niche plasticity and adaptability.

CONCLUSION

The involvement of age-induced genetic/epigenetic alterations in BM niche cells and immunological stressors in hematopoiesis is crucial for understanding the development of age-related hematological malignancies. This comprehensive review provides new insights into the complex interplay between niche cells and HSCs, emphasizing the potential for novel therapeutic approaches that target niche cell functionality and resilience to improve hematopoietic outcomes in the context of aging and metabolic disorders.

NOVELTY STATEMENT

This review introduces novel concepts regarding the plasticity and adaptability of BM niche cells in response to immunological stressors and epigenetics. It proposes that targeted therapeutic strategies aimed at enhancing niche cell resilience could mitigate the adverse effects of aging, diabetes, and obesity on hematopoiesis and clonal hematopoiesis. Additionally, the review suggests that understanding the precise temporal and spatial dynamics of niche-HSC interactions and epigenetics influence may lead to innovative treatments for age-related hematological malignancies.

Integrating Microarray Data and Single-Cell RNA-Seq Reveals Key Gene Involved in Spermatogonia Stem Cell Aging

The in vitro generation of spermatogonial stem cells (SSCs) from embryonic stem cells (ESCs) offers a viable approach for addressing male infertility. A multitude of molecules participate in this intricate process, which requires additional elucidation. Despite the decline in SSCs in aged testes, SSCs are deemed immortal since they can multiply for three years with repeated transplantation. Nonetheless, the examination of aging is challenging due to the limited quantity and absence of precise indicators. Using a microarray, we assessed genome-wide transcripts (about 55,000 transcripts) of fibroblasts and SSCs. The WGCNA approach was then used to look for SSC-specific transcription factors (TFs) and hub SSC-specific genes based on ATAC-seq, DNase-seq, RNA-seq, and microarray data from the GEO databases as well as gene expression data (RNA-seq and microarray data). The microarray analysis of three human cases with different SSCs revealed that 6 genes were upregulated, and the expression of 23 genes was downregulated compared to the normal case in relation to aging genes. To reach these results, online assessments of Enrich Shiny GO, STRING, and Cytoscape were used to forecast the molecular and functional connections of proteins before identifying the master routes. The biological process and molecular function keywords of cell-matrix adhesion, telomerase activity, and telomere cap complex were shown to be significantly altered in upregulated differentially expressed genes (DEGs) by the functional enrichment analysis. According to our preliminary research, cell-specific TFs and TF-mediated GRNs are involved in the creation of SSCs. In order to maximize the induction efficiency of ESC differentiation into SSCs in vitro, hub SSC-specific genes and important SSC-specific TFs were identified, and sophisticated network regulation was proposed. According to our research, these genes and the hub proteins that they interact with may be able to shine a light on the pathophysiologies of infertility and aberrant germ cells.

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.

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.

Reduced protein intake and aging affects the sustainment of hematopoiesis by impairing bone marrow mesenchymal stem cells

Protein malnourishment (PM) is common among the elderly, but how aging and PM impact hematopoiesis is not fully understood. This study aimed to assess how aging and PM affect the hematopoietic regulatory function of bone marrow (BM) mesenchymal stem cells (MSCs). Young and aged male C57BL/6J mice were fed with normoproteic or hypoproteic diets and had their nutritional, biochemical, and hematological parameters evaluated. BM MSCs were characterized and had their secretome, gene expression, autophagy, reactive oxygen species production (ROS), and DNA double-stranded breaks evaluated. The modulation of hematopoiesis by MSCs was assayed using in vitro and in vivo models. Lastly, BM invasiveness and mice survival were evaluated after being challenged with leukemic cells of the C1498 cell line. Aging and PM alter biochemical parameters, changing the peripheral blood and BM immunophenotype. MSC autophagy was affected by aging and the frequencies for ROS and DNA double-stranded breaks. Regarding the MSCs' secretome, PM and aging affected CXCL12, IL-6, and IL-11 production. Aging and PM up-regulated Akt1 and PPAR-γ while down-regulating Cdh2 and Angpt-1 in MSCs. Aged MSCs increased C1498 cell proliferation while reducing their colony-forming potential. PM and aging lowered mice survival, and malnourishment accumulated C1498 cells at the BM. Finally, aged and/or PM MSCs up-regulated Sox2, Nanog, Pou5f1, and Akt1 expression while down-regulating Cdkn1a in C1498 cells. Together, aging and PM can induce cell-intrinsic shifts in BM MSCs, creating an environment that alters the regulation of hematopoietic populations and favoring the development of malignant cells.

Regulators of clonal hematopoiesis and physiological consequences of this condition

Clonal hematopoiesis (CH) is a prevalent condition that results from somatic mutations in hematopoietic stem cells. When these mutations occur in "driver" genes, they can potentially confer fitness advantages to the affected cells, leading to a clonal expansion. While most clonal expansions of mutant cells are generally considered to be asymptomatic since they do not impact overall blood cell numbers, CH carriers face long-term risks of all-cause mortality and age-associated diseases, including cardiovascular disease and hematological malignancies. While considerable research has focused on understanding the association between CH and these diseases, less attention has been given to exploring the regulatory factors that contribute to the expansion of the driver gene clone. This review focuses on the association between environmental stressors and inherited genetic risk factors in the context of CH development. A better understanding of how these stressors impact CH development will facilitate mechanistic studies and potentially lead to new therapeutic avenues to treat individuals with this condition.

How is Big Data reshaping preclinical aging research?

The exponential scientific and technological progress during the past 30 years has favored the comprehensive characterization of aging processes with their multivariate nature, leading to the advent of Big Data in preclinical aging research. Spanning from molecular omics to organism-level deep phenotyping, Big Data demands large computational resources for storage and analysis, as well as new analytical tools and conceptual frameworks to gain novel insights leading to discovery. Systems biology has emerged as a paradigm that utilizes Big Data to gain insightful information enabling a better understanding of living organisms, visualized as multilayered networks of interacting molecules, cells, tissues and organs at different spatiotemporal scales. In this framework, where aging, health and disease represent emergent states from an evolving dynamic complex system, context given by, for example, strain, sex and feeding times, becomes paramount for defining the biological trajectory of an organism. Using bioinformatics and artificial intelligence, the systems biology approach is leading to remarkable advances in our understanding of the underlying mechanism of aging biology and assisting in creative experimental study designs in animal models. Future in-depth knowledge acquisition will depend on the ability to fully integrate information from different spatiotemporal scales in organisms, which will probably require the adoption of theories and methods from the field of complex systems. Here we review state-of-the-art approaches in preclinical research, with a focus on rodent models, that are leading to conceptual and/or technical advances in leveraging Big Data to understand basic aging biology and its full translational potential.

The Ins and Outs of Endosteal Niche Disruption in the Bone Marrow: Relevance for Myeloma Oncogenesis

Multiple Myeloma (MM) and its preexisting stage, termed Monoclonal Gammopathy of Undetermined Significance (MGUS), have long been considered mainly as genomic diseases. However, the bone changes observed in both conditions have led to a reassessment of the role of the bone microenvironment, mainly the endosteal niche in their genesis. Here, we consider the disruption of the endosteal niche in the bone marrow, that is, the shift of the endosteal niche from an osteoblastic to an osteoclastic profile produced by bone senescence and inflammaging, as the key element. Thus, this disrupted endosteal niche is proposed to represent the permissive microenvironment necessary not only for the emergence of MM from MGUS but also for the emergence and maintenance of MGUS. Moreover, the excess of osteoclasts would favor the presentation of antigens (Ag) into the endosteal niche because osteoclasts are Ag-presenting cells. As such, they could significantly stimulate the presentation of some specific Ag and the clonal expansion of the stimulated cells as well as favor the expansion of such selected clones because osteoclasts are immunosuppressive. We also discuss this scenario in the Gaucher disease, in which the high incidence of MGUS and MM makes it a good model both at the bone level and the immunological level. Finally, we envisage that this endosteal niche disruption would increase the stochasticity (epigenetic and genetic instability) in the selected clones, according to our Tissue Disruption-induced cell Stochasticity (TiDiS) theory.

Unraveling Heterogeneity in the Aging Hematopoietic Stem Cell Compartment: An Insight From Single-cell Approaches

Specific cell types and, therefore, organs respond differently during aging. This is also true for the hematopoietic system, where it has been demonstrated that hematopoietic stem cells alter a variety of features, such as their metabolism, and accumulate DNA damage, which can lead to clonal outgrowth over time. In addition, profound changes in the bone marrow microenvironment upon aging lead to senescence in certain cell types such as mesenchymal stem cells and result in increased inflammation. This heterogeneity makes it difficult to pinpoint the molecular drivers of organismal aging gained from bulk approaches, such as RNA sequencing. A better understanding of the heterogeneity underlying the aging process in the hematopoietic compartment is, therefore, needed. With the advances of single-cell technologies in recent years, it is now possible to address fundamental questions of aging. In this review, we discuss how single-cell approaches can and indeed are already being used to understand changes observed during aging in the hematopoietic compartment. We will touch on established and novel methods for flow cytometric detection, single-cell culture approaches, and single-cell omics.

DRAG in situ barcoding reveals an increased number of HSPCs contributing to myelopoiesis with age

Ageing is associated with changes in the cellular composition of the immune system. During ageing, hematopoietic stem and progenitor cells (HSPCs) that produce immune cells are thought to decline in their regenerative capacity. However, HSPC function has been mostly assessed using transplantation assays, and it remains unclear how HSPCs age in the native bone marrow niche. To address this issue, we present an in situ single cell lineage tracing technology to quantify the clonal composition and cell production of single cells in their native niche. Our results demonstrate that a pool of HSPCs with unequal output maintains myelopoiesis through overlapping waves of cell production throughout adult life. During ageing, the increased frequency of myeloid cells is explained by greater numbers of HSPCs contributing to myelopoiesis rather than the increased myeloid output of individual HSPCs. Strikingly, the myeloid output of HSPCs remains constant over time despite accumulating significant transcriptomic changes throughout adulthood. Together, these results show that, unlike emergency myelopoiesis post-transplantation, aged HSPCs in their native microenvironment do not functionally decline in their regenerative capacity.

Hematopoietic Stem Cells and the Immune System in Development and Aging

Hematopoietic stem cells (HSCs) support haematopoiesis throughout life and give rise to the whole variety of cells of the immune system. Developing in the early embryo, passing through the precursor stage, and maturing into the first HSCs, they undergo a fairly large number of divisions while maintaining a high regenerative potential due to high repair activity. This potential is greatly reduced in adult HSCs. They go into a state of dormancy and anaerobic metabolism to maintain their stemness throughout life. However, with age, changes occur in the pool of HSCs that negatively affect haematopoiesis and the effectiveness of immunity. Niche aging and accumulation of mutations with age reduces the ability of HSCs to self-renew and changes their differentiation potential. This is accompanied by a decrease in clonal diversity and a disturbance of lymphopoiesis (decrease in the formation of naive T- and B-cells) and the predominance of myeloid haematopoiesis. Aging also affects mature cells, regardless of HSC, therefore, phagocytic activity and the intensity of the oxidative burst decrease, and the efficiency of processing and presentation of antigens by myeloid cells is impaired. Aging cells of innate and adaptive immunity produce factors that form a chronic inflammatory background. All these processes have a serious negative impact on the protective properties of the immune system, increasing inflammation, the risk of developing autoimmune, oncological, and cardiovascular diseases with age. Understanding the mechanisms of reducing the regenerative potential in a comparative analysis of embryonic and aging HSCs, the features of inflammatory aging will allow us to get closer to deciphering the programs for the development, aging, regeneration and rejuvenation of HSCs and the immune system.

Establishing a human bone marrow single cell reference atlas to study ageing and diseases

Introduction

Ageing in the human bone marrow is associated with immune function decline that results in the elderly being vulnerable to illnesses. A comprehensive healthy bone marrow consensus atlas can serve as a reference to study the immunological changes associated with ageing, and to identify and study abnormal cell states.

Methods

We collected publicly available single cell transcriptomic data of 145 healthy samples encompassing a wide spectrum of ages ranging from 2 to 84 years old to construct our human bone marrow atlas. The final atlas has 673,750 cells and 54 annotated cell types.

Results

We first characterised the changes in cell population sizes with respect to age and the corresponding changes in gene expression and pathways. Overall, we found significant age-associated changes in the lymphoid lineage cells. The naïve CD8+ T cell population showed significant shrinkage with ageing while the effector/memory CD4+ T cells increased in proportion. We also found an age-correlated decline in the common lymphoid progenitor population, in line with the commonly observed myeloid skew in haematopoiesis among the elderly. We then employed our cell type-specific ageing gene signatures to develop a machine learning model that predicts the biological age of bone marrow samples, which we then applied to healthy individuals and those with blood diseases. Finally, we demonstrated how to identify abnormal cell states by mapping disease samples onto the atlas. We accurately identified abnormal plasma cells and erythroblasts in multiple myeloma samples, and abnormal cells in acute myeloid leukaemia samples.

Discussion

The bone marrow is the site of haematopoiesis, a highly important bodily process. We believe that our healthy bone marrow atlas is a valuable reference for studying bone marrow processes and bone marrow-related diseases. It can be mined for novel discoveries, as well as serve as a reference scaffold for mapping samples to identify and investigate abnormal cells.

G‑CSF delays tooth extraction socket bone healing via the inhibition of bone turnover in mice

Granulocyte colony-stimulating factor (G-CSF) regulates the survival, proliferation and differentiation of all cells in the neutrophil lineage, and is consequently used for neutropenic conditions. Upon G-CSF administration, osteoblasts and osteocytes are suppressed, and the support system allowing hematopoietic stem cells to remain in the microenvironment is diminished. The present study focused on and investigated G-CSF as a regulatory factor of bone remodeling. The aim of the present study was to investigate the effect of G-CSF administration on the bone healing of tooth extraction sockets. Significant differences in the bone volume fraction, and trabecular separation of the proximal femurs and alveolar septa were observed between the G-CSF and control (saline-treated) groups. The trabecular bone of the femur and alveolar septa was reduced in the G-CSF group compared with that in the control group. In addition, serum procollagen type 1 N-terminal propeptide levels, a marker of bone formation, were lower in the G-CSF group compared with in the control group. Fibrous connective tissues and immature bone were observed in the extraction socket, and bone healing was delayed in the G-CSF group compared with that in the control group. The bone area in the extraction socket 6 days after tooth extraction was significantly smaller in the G-CSF group (23.6%) than that in the control group (45.1%). Furthermore, G-CSF administration reduced the number of canaliculi per osteocyte and inhibited the connection of osteocyte networks. Consequently, osteoblast activation was inhibited and bone remodeling changed to a state of low bone turnover in the G-CSG group. Analysis of bone formation parameters revealed that the G-CSF group exhibited a lower mineral apposition rate compared with in the control group. In conclusion, these findings indicated that G-CSF may delay bone healing of the socket after tooth extraction.

Clonal hematopoiesis, somatic mosaicism, and age-associated disease

Somatic mosaicism, the occurrence of multiple genetically distinct cell clones within the same tissue, is an evitable consequence of human aging. The hematopoietic system is no exception to this, where studies have revealed the presence of expanded blood cell clones carrying mutations in preleukemic driver genes and/or genetic alterations in chromosomes. This phenomenon is referred to as clonal hematopoiesis and is remarkably prevalent in elderly individuals. While clonal hematopoiesis represents an early step toward a hematological malignancy, most individuals will never develop blood cancer. Somewhat unexpectedly, epidemiological studies have found that clonal hematopoiesis is associated with an increase in the risk of all-cause mortality and age-related disease, particularly in the cardiovascular system. Studies using murine models of clonal hematopoiesis have begun to shed light on this relationship, suggesting that driver mutations in mature blood cells can causally contribute to aging and disease by augmenting inflammatory processes. Here we provide an up-to-date review of clonal hematopoiesis within the context of somatic mosaicism and aging and describe recent epidemiological studies that have reported associations with age-related disease. We will also discuss the experimental studies that have provided important mechanistic insight into how driver mutations promote age-related disease and how this knowledge could be leveraged to treat individuals with clonal hematopoiesis.

Recent advances in engineering hydrogels for niche biomimicking and hematopoietic stem cell culturing

Hematopoietic stem cells (HSCs) provide a life-long supply of haemopoietic cells and are indispensable for clinical transplantation in the treatment of malignant hematological diseases. Clinical applications require vast quantities of HSCs with maintained stemness characteristics. Meeting this demand poses often insurmountable challenges for traditional culture methods. Creating a supportive artificial microenvironment for the culture of HSCs, which allows the expansion of the cells while maintaining their stemness, is becoming a new solution for the provision of these rare multipotent HSCs. Hydrogels with good biocompatibility, excellent hydrophilicity, tunable biochemical and biophysical properties have been applied in mimicking the hematopoietic niche for the efficient expansion of HSCs. This review focuses on recent progress in the use of hydrogels in this specialized application. Advanced biomimetic strategies use for the creation of an artificial haemopoietic niche are discussed, advances in combined use of hydrogel matrices and microfluidics, including the emerging organ-on-a-chip technology, are summarized. We also provide a brief description of novel stimulus-responsive hydrogels that are used to establish an intelligent dynamic cell microenvironment. Finally, current challenges and future perspectives of engineering hydrogels for HSC biomedicine are explored.

Functionalized 3D scaffolds for engineering the hematopoietic niche

Hematopoietic stem cells (HSCs) reside in a subzone of the bone marrow (BM) defined as the hematopoietic niche where, via the interplay of differentiation and self-renewal, they can give rise to immune and blood cells. Artificial hematopoietic niches were firstly developed in 2D in vitro cultures but the limited expansion potential and stemness maintenance induced the optimization of these systems to avoid the total loss of the natural tissue complexity. The next steps were adopted by engineering different materials such as hydrogels, fibrous structures with natural or synthetic polymers, ceramics, etc. to produce a 3D substrate better resembling that of BM. Cytokines, soluble factors, adhesion molecules, extracellular matrix (ECM) components, and the secretome of other niche-resident cells play a fundamental role in controlling and regulating HSC commitment. To provide biochemical cues, co-cultures, and feeder-layers, as well as natural or synthetic molecules were utilized. This review gathers key elements employed for the functionalization of a 3D scaffold that demonstrated to promote HSC growth and differentiation ranging from 1) biophysical cues, i.e., material, topography, stiffness, oxygen tension, and fluid shear stress to 2) biochemical hints favored by the presence of ECM elements, feeder cell layers, and redox scavengers. Particular focus is given to the 3D systems to recreate megakaryocyte products, to be applied for blood cell production, whereas HSC clinical application in such 3D constructs was limited so far to BM diseases testing.

Catching Them Early: Framework Parameters and Progress for Prenatal and Childhood Application of Advanced Therapies

Advanced therapy medicinal products (ATMPs) are medicines for human use based on genes, cells or tissue engineering. After clear successes in adults, the nascent technology now sees increasing pediatric application. For many still untreatable disorders with pre- or perinatal onset, timely intervention is simply indispensable; thus, prenatal and pediatric applications of ATMPs hold great promise for curative treatments. Moreover, for most inherited disorders, early ATMP application may substantially improve efficiency, economy and accessibility compared with application in adults. Vindicating this notion, initial data for cell-based ATMPs show better cell yields, success rates and corrections of disease parameters for younger patients, in addition to reduced overall cell and vector requirements, illustrating that early application may resolve key obstacles to the widespread application of ATMPs for inherited disorders. Here, we provide a selective review of the latest ATMP developments for prenatal, perinatal and pediatric use, with special emphasis on its comparison with ATMPs for adults. Taken together, we provide a perspective on the enormous potential and key framework parameters of clinical prenatal and pediatric ATMP application.

Increasing Complexity of Molecular Landscapes in Human Hematopoietic Stem and Progenitor Cells during Development and Aging

The past five decades have seen significant progress in our understanding of human hematopoiesis. This has in part been due to the unprecedented development of advanced technologies, which have allowed the identification and characterization of rare subsets of human hematopoietic stem and progenitor cells and their lineage trajectories from embryonic through to adult life. Additionally, surrogate in vitro and in vivo models, although not fully recapitulating human hematopoiesis, have spurred on these scientific advances. These approaches have heightened our knowledge of hematological disorders and diseases and have led to their improved diagnosis and therapies. Here, we review human hematopoiesis at each end of the age spectrum, during embryonic and fetal development and on aging, providing exemplars of recent progress in deciphering the increasingly complex cellular and molecular hematopoietic landscapes in health and disease. This review concludes by highlighting links between chronic inflammation and metabolic and epigenetic changes associated with aging and in the development of clonal hematopoiesis.

Glucose Metabolism and Aging of Hematopoietic Stem and Progenitor Cells

Comprehensive proteomics studies of human hematopoietic stem and progenitor cells (HSPC) have revealed that aging of the HSPC compartment is characterized by elevated glycolysis. This is in addition to deregulations found in murine transcriptomics studies, such as an increased differentiation bias towards the myeloid lineage, alterations in DNA repair, and a decrease in lymphoid development. The increase in glycolytic enzyme activity is caused by the expansion of a more glycolytic HSPC subset. We therefore developed a method to isolate HSPC into three distinct categories according to their glucose uptake (GU) levels, namely the GU^high, GU^inter and GU^low subsets. Single-cell transcriptomics studies showed that the GU^high subset is highly enriched for HSPC with a differentiation bias towards myeloid lineages. Gene set enrichment analysis (GSEA) demonstrated that the gene sets for cell cycle arrest, senescence-associated secretory phenotype, and the anti-apoptosis and P53 pathways are significantly upregulated in the GU^high population. With this series of studies, we have produced a comprehensive proteomics and single-cell transcriptomics atlas of molecular changes in human HSPC upon aging. Although many of the molecular deregulations are similar to those found in mice, there are significant differences. The most unique finding is the association of elevated central carbon metabolism with senescence. Due to the lack of specific markers, the isolation and collection of senescent cells have yet to be developed, especially for human HSPC. The GU^high subset from the human HSPC compartment possesses all the transcriptome characteristics of senescence. This property may be exploited to accurately enrich, visualize, and trace senescence development in human bone marrow.

Endothelial Senescence and the Chronic Vascular Diseases: Challenges and Therapeutic Opportunities in Atherosclerosis

Atherosclerosis is probably one of the paradigms of disease linked to aging. Underlying the physiopathology of atherosclerosis are cellular senescence, oxidative stress, and inflammation. These factors are increased in the elderly and from chronic disease patients. Elevated levels of oxidative stress affect cellular function and metabolism, inducing senescence. This senescence modifies the cell phenotype into a senescent secretory phenotype. This phenotype activates immune cells, leading to chronic systemic inflammation. Moreover, due to their secretory phenotype, senescence cells present an increased release of highlighted extracellular vesicles that will change nearby/neighborhood cells and paracrine signaling. For this reason, searching for specific senescent cell biomarkers and therapies against the development/killing of senescent cells has become relevant. Recently, senomorphic and senolityc drugs have become relevant in slowing down or eliminating senescence cells. However, even though they have shown promising results in experimental studies, their clinical use is still yet to be determined.