PDGFRβ+ cells play a dual role as hematopoietic precursors and niche cells during mouse ontogeny

Single-cell RNA sequencing reveals vascularization-associated cell subpopulations in dental pulp: PDGFRβ+ DPSCs with activated PI3K/AKT pathway

BACKGROUND

This study aims to address challenges in dental pulp regeneration therapy. The heterogeneity of DPSCs poses challenges, especially in stem cell transplantation for clinical use, particularly when sourced from donors of different ages and conditions.

METHODS

Pseudotime analysis was employed to analyze single-cell sequencing data, and immunohistochemical studies were conducted to investigate the expression of fibronectin 1 (FN1). We performed in vitro sorting of PDGFRβ+ DPSCs using flow cytometry. A series of functional assays, including cell proliferation, scratch, and tube formation assays, were performed to experimentally validate the vasculogenic capabilities of the identified PDGFRβ+ DPSC subset. Furthermore, gene-edited mouse models were utilized to demonstrate the importance of PDGFRβ+ DPSCs. Transcriptomic sequencing was conducted to compare the differences between PDGFRβ+ DPSCs and P1-DPSCs.

RESULTS

Single-cell sequencing analysis unveiled a distinct subset, PDGFRβ+ DPSCs, characterized by significantly elevated FN1 expression during dental pulp development. Subsequent cell experiments demonstrated that this subset possesses remarkable abilities to promote HUVEC proliferation, migration, and tube formation. Gene-edited mouse models confirmed the vital role of PDGFRβ+ DPSCs in dental pulp development. Transcriptomic sequencing and in vitro experiments demonstrated that the PDGFR/PI3K/AKT signaling pathway is a crucial factor mediating the proliferation rate and pro-angiogenic properties of PDGFRβ+ DPSCs.

CONCLUSION

We defined a new subset, PDGFRβ+ DPSCs, characterized by strong proliferative activity and pro-angiogenic capabilities, demonstrating significant clinical translational potential.

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.

Baicalein attenuates oxidative damage in mice haematopoietic cells through regulation of PDGFRβ

Platelet-derived growth factor receptor β (PDGFRβ) plays a crucial role in murine haematopoiesis. Baicalein (BAI), a naturally occurring flavonoid, can alleviate disease damage through anti-oxidative, anti-apoptotic, and anti-inflammatory mechanisms. However, whether BAI attenuates oxidative damage in murine haematopoietic cells by PDGFRβ remains unexplored. In this study, we utilized a tert-butyl hydroperoxide (TBHP)-induced BaF3 cell injury model and an ionising radiation (IR)-induced mice injury model to investigate the impact of the presence or absence of PDGFRβ on the pharmacological effects of BAI. In addition, the BAI-PDGFRβ interaction was characterized by molecular docking and dynamics simulations. The results show that a specific concentration of BAI led to increased cell viability, reduced reactive oxygen species (ROS) content, upregulated nuclear factor erythroid 2-related factor 2 (NRF2) expression, and its downstream target genes heme oxygenase 1 (HO-1) and NAD(P)H Quinone Dehydrogenase 1 (NQO1), and activated protein kinase B (AKT) pathway in cells expressing PDGFRβ plasmid and experiencing damage. Similarly, BAI elevated lineage-Sca1+cKIT+ (LSK) cell proportion, promoted haematopoietic restoration, enhanced NRF2-mediated antioxidant response in PDGFRβ+/+ mice. However, despite BAI usage, PDGFRβ knockout mice (PDGFRβ-/-) showed lower LSK proportion and less antioxidant capacity than the total body irradiation (TBI) group. Furthermore, we demonstrated an interaction between BAI and PDGFRβ at the molecular level. Collectively, our results indicate that BAI attenuates oxidative stress injury and helps promote haematopoietic cell recovery through regulation of PDGFRβ.

Capturing embryonic hematopoiesis in temporal and spatial dimensions

Hematopoietic stem cells (HSCs) possess the ability to sustain the continuous production of all blood cell types throughout an organism's lifespan. Although primarily located in the bone marrow of adults, HSCs originate during embryonic development. Visualization of the birth of HSCs, their developmental trajectory, and the specific interactions with their successive niches have significantly contributed to our understanding of the biology and mechanics governing HSC formation and expansion. Intravital techniques applied to live embryos or non-fixed samples have remarkably provided invaluable insights into the cellular and anatomical origins of HSCs. These imaging technologies have also shed light on the dynamic interactions between HSCs and neighboring cell types within the surrounding microenvironment or niche, such as endothelial cells or macrophages. This review delves into the advancements made in understanding the origin, production, and cellular interactions of HSCs, particularly during the embryonic development of mice and zebrafish, focusing on studies employing (live) imaging analysis.

The molecular and cellular hematopoietic stem cell specification niche

Hematopoietic stem cells (HSCs) are a population of tissue-specific stem cells that reside in the bone marrow of adult mammals, where they self-renew and continuously regenerate the adult hematopoietic lineages over the life of the individual. Prominence as a stem cell model and clinical usefulness have driven interest in understanding the physiologic processes that lead to the specification of HSCs during embryonic development. High-efficiency directed differentiation of HSCs by the instruction of defined progenitor cells using sequentially defined instructive molecules and conditions remains impossible, indicating that comprehensive knowledge of the complete set of precursor intermediate identities and required inductive inputs remains incompletely understood. Recently, interest in the molecular and cellular microenvironment where HSCs are specified from endothelial precursors-the "specification niche"-has increased. Here we review recent progress in understanding these niche spaces across vertebrate phyla, as well as how a better characterization of the origin and molecular phenotypes of the niche cell populations has helped inform and complicate previous understanding of signaling required for HSC emergence and maturation.

A multistep computational approach reveals a neuro-mesenchymal cell population in the embryonic hematopoietic stem cell niche

The first hematopoietic stem and progenitor cells (HSPCs) emerge in the Aorta-Gonad-Mesonephros (AGM) region of the mid-gestation mouse embryo. However, the precise nature of their supportive mesenchymal microenvironment remains largely unexplored. Here, we profiled transcriptomes of laser micro-dissected aortic tissues at three developmental stages and individual AGM cells. Computational analyses allowed the identification of several cell subpopulations within the E11.5 AGM mesenchyme, with the presence of a yet unidentified subpopulation characterized by the dual expression of genes implicated in adhesive or neuronal functions. We confirmed the identity of this cell subset as a neuro-mesenchymal population, through morphological and lineage tracing assays. Loss of function in the zebrafish confirmed that Decorin, a characteristic extracellular matrix component of the neuro-mesenchyme, is essential for HSPC development. We further demonstrated that this cell population is not merely derived from the neural crest, and hence, is a bona fide novel subpopulation of the AGM mesenchyme.

A life-time of hematopoietic cell function: ascent, stability, and decline

Aging is a set of complex processes that occur temporally and continuously. It is generally a unidirectional progression of cellular and molecular changes occurring during the life stages of cells, tissues and ultimately the whole organism. In vertebrate organisms, this begins at conception from the first steps in blastocyst formation, gastrulation, germ layer differentiation, and organogenesis to a continuum of embryonic, fetal, adolescent, adult, and geriatric stages. Tales of the "fountain of youth" and songs of being "forever young" are dominant ideas informing us that growing old is something science should strive to counteract. Here, we discuss the normal life stages of the blood system, particularly the historical recognition of its importance in the early growth stages of vertebrates, and what this means with respect to progressive gain and loss of hematopoietic function in the adult.

Runx1+ vascular smooth muscle cells are essential for hematopoietic stem and progenitor cell development in vivo

Hematopoietic stem cells (HSCs) produce all essential cellular components of the blood. Stromal cell lines supporting HSCs follow a vascular smooth muscle cell (vSMC) differentiation pathway, suggesting that some hematopoiesis-supporting cells originate from vSMC precursors. These pericyte-like precursors were recently identified in the aorta-gonad-mesonephros (AGM) region; however, their role in the hematopoietic development in vivo remains unknown. Here, we identify a subpopulation of NG2+Runx1+ perivascular cells that display a sclerotome-derived vSMC transcriptomic profile. We show that deleting Runx1 in NG2+ cells impairs the hematopoietic development in vivo and causes transcriptional changes in pericytes/vSMCs, endothelial cells and hematopoietic cells in the murine AGM. Importantly, this deletion leads also to a significant reduction of HSC reconstitution potential in the bone marrow in vivo. This defect is developmental, as NG2+Runx1+ cells were not detected in the adult bone marrow, demonstrating the existence of a specialised pericyte population in the HSC-generating niche, unique to the embryo.

Identification and characterization of enhancer elements controlling cell type-specific and signalling dependent chromatin programming during hematopoietic development

The development of multi-cellular organisms from a single fertilized egg requires to differentially execute the information encoded in our DNA. This complex process is regulated by the interplay of transcription factors with a chromatin environment, both of which provide the epigenetic information maintaining cell-type specific gene expression patterns. Moreover, transcription factors and their target genes form vast interacting gene regulatory networks which can be exquisitely stable. However, all developmental processes originate from pluripotent precursor cell types. The production of terminally differentiated cells from such cells, therefore, requires successive changes of cell fates, meaning that genes relevant for the next stage of differentiation must be switched on and genes not relevant anymore must be switched off. The stimulus for the change of cell fate originates from extrinsic signals which set a cascade of intracellular processes in motion that eventually terminate at the genome leading to changes in gene expression and the development of alternate gene regulatory networks. How developmental trajectories are encoded in the genome and how the interplay between intrinsic and extrinsic processes regulates development is one of the major questions in developmental biology. The development of the hematopoietic system has long served as model to understand how changes in gene regulatory networks drive the differentiation of the various blood cell types. In this review, we highlight the main signals and transcription factors and how they are integrated at the level of chromatin programming and gene expression control. We also highlight recent studies identifying the cis-regulatory elements such as enhancers at the global level and explain how their developmental activity is regulated by the cooperation of cell-type specific and ubiquitous transcription factors with extrinsic signals.

Protocol to analyze and validate transcriptomic changes in PDGFRβ-KO mesenchymal stem cell osteogenic potential in the mouse embryo

Mesenchymal stem/stromal cells (MSCs) can differentiate into osteoblasts under appropriate conditions. PDGFRβ signaling controls MSC osteogenic potential both transcriptomically and in culture. Here, we present a "computer to the bench" protocol to analyze changes in MSC osteogenic potential at transcriptomic and cellular level in the absence of PDGFRβ. We detail the preparation of cells from mouse embryos, the analysis of transcriptomic changes from single-cell RNA-sequencing data, the procedure for MSC derivation and culture, and an osteogenic assay for functional validation. For complete details on the use and execution of this protocol, please refer to Sá da Bandeira et al. (2022).1.

The (intra-aortic) hematopoietic cluster cocktail: what is in the mix?

The adult-definitive hematopoietic hierarchy and hematopoietic stem cells (HSCs) residing in the bone marrow are established during embryonic development. In mouse, human, and many other mammals, it is the sudden formation of so-called intra-aortic/arterial hematopoietic clusters (IAHCs) that best signifies and visualizes this de novo generation of HSCs and hematopoietic progenitor cells (HPCs). Cluster cells arise through an endothelial-to-hematopoietic transition and, for some time, express markers/genes of both tissue types, whilst acquiring more hematopoietic features and losing endothelial ones. Among several hundreds of IAHC cells, the midgestation mouse embryo contains only very few bona fide adult-repopulating HSCs, suggestive of a challenging cell fate to achieve. Most others are HPCs of various types, some of which have the potential to mature into HSCs in vitro. Based on the number of cells that reveal hematopoietic function, a fraction of IAHC cells is uncharacterized. This review aims to explore the current state of knowledge on IAHC cells. We will describe markers useful for isolation and characterization of these fleetingly produced, yet vitally important, cells and for the refined enrichment of the HSCs they contain, and speculate on the role of some IAHC cells that are as-yet functionally uncharacterized.

Dorsal aorta polarization and haematopoietic stem cell emergence

Recent studies have highlighted the crucial role of the aorta microenvironment in the generation of the first haematopoietic stem cells (HSCs) from specialized haemogenic endothelial cells (HECs). Despite more than two decades of investigations, we require a better understanding of the cellular and molecular events driving aorta formation and polarization, which will be pivotal to establish the mechanisms that operate during HEC specification and HSC competency. Here, we outline the early mechanisms involved in vertebrate aorta formation by comparing four different species: zebrafish, chicken, mouse and human. We highlight how this process, which is tightly controlled in time and space, requires a coordinated specification of several cell types, in particular endothelial cells originating from distinct mesodermal tissues. We also discuss how molecular signals originating from the aorta environment result in its polarization, creating a unique entity for HSC generation.