Cell cycle quiescence is a critical feature contributing to haematopoietic stem cell (HSC) maintenance. Although various candidate stromal cells have been identified as potential HSC niches, the spatial localization of quiescent HSCs in the bone marrow (BM) remains unclear. Here, using a novel approach that combines whole-mount confocal immunofluorescence imaging techniques and computational modelling to analyse significant tridimensional associations among vascular structures, stromal cells and HSCs, we show that quiescent HSCs associate specifically with small arterioles that are preferentially found in endosteal BM. These arterioles are ensheathed exclusively by rare NG2+ pericytes, distinct from sinusoid-associated LepR+ cells. Pharmacological or genetic activation of HSC cell cycle alters the distribution of HSCs from NG2+ peri-arteriolar niches to LepR+ peri-sinusoidal niches. Conditional depletion of NG2+ cells induces HSC cycling and reduces functional long-term repopulating HSCs in BM. These results thus indicate that arteriolar niches are indispensable to maintain HSC quiescence.

A subset of human Nestin+ mesenchymal stem cells expresses PDGFRα and CD51, and these markers can be used for prospective isolation of these cells.

Mesenchymal stem and progenitor cells (MSPCs) contribute to bone marrow (BM) homeostasis by generating multiple types of stromal cells. MSPCs can be labeled in the adult BM by Nestin-GFP, whereas committed osteoblast progenitors are marked by Osterix expression. However, the developmental origin and hierarchical relationship of stromal cells remain largely unknown. Here, using a lineage-tracing system, we describe three distinct waves of contributions of Osterix+ cells in the BM. First, Osterix+ progenitors in the fetal BM contribute to nascent bone tissues and transient stromal cells that are replaced in the adult marrow. Second, Osterix-expressing cells perinatally contribute to osteolineages and long-lived BM stroma, which have characteristics of Nestin-GFP+ MSPCs. Third, Osterix labeling in the adult marrow is osteolineage-restricted, devoid of stromal contribution. These results uncover a broad expression profile of Osterix and raise the intriguing possibility that distinct waves of stromal cells, primitive and definitive, may organize the developing BM.

The signaling molecule Wnt regulates bone homeostasis through β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. Impairment of canonical Wnt signaling causes bone loss in arthritis and osteoporosis; however, it is unclear how noncanonical Wnt signaling regulates bone resorption. Wnt5a activates noncanonical Wnt signaling through receptor tyrosine kinase-like orphan receptor (Ror) proteins. We showed that Wnt5a-Ror2 signaling between osteoblast-lineage cells and osteoclast precursors enhanced osteoclastogenesis. Osteoblast-lineage cells expressed Wnt5a, whereas osteoclast precursors expressed Ror2. Mice deficient in either Wnt5a or Ror2, and those with either osteoclast precursor-specific Ror2 deficiency or osteoblast-lineage cell-specific Wnt5a deficiency showed impaired osteoclastogenesis. Wnt5a-Ror2 signals enhanced receptor activator of nuclear factor-κB (RANK) expression in osteoclast precursors by activating JNK and recruiting c-Jun on the promoter of the gene encoding RANK, thereby enhancing RANK ligand (RANKL)-induced osteoclastogenesis. A soluble form of Ror2 acted as a decoy receptor of Wnt5a and abrogated bone destruction in mouse arthritis models. Our results suggest that the Wnt5a-Ror2 pathway is crucial for osteoclastogenesis in physiological and pathological environments and represents a therapeutic target for bone diseases, including arthritis.

Summary Perivascular mesenchymal stem and progenitor cells (MSPCs) are critical to form a healthy hematopoietic stem cell (HSC) niche. However, the interactions and influence of acute myelogenous leukemia (AML) stem cells with the microenvironment remain largely unexplored. We have found, unexpectedly, that neuropathy of the sympathetic nervous system (SNS) promotes leukemic bone marrow infiltration in an MLL-AF9 AML model. Development of AML disrupts SNS nerves and the quiescence of Nestin+ niche cells, leading to an expansion of phenotypic MSPCs primed for osteoblastic differentiation, at the expense of HSC-maintaining NG2+ periarteriolar niche cells. Adrenergic signaling maintaining niche quiescence is transduced by the β2, but not β3, adrenergic receptor expressed on stromal cells of leukemic bone marrow. These results indicate that sympathetic neuropathy may represent a mechanism for the malignancy to co-opt the microenvironment and suggest separate mesenchymal niche activities for malignant and healthy hematopoietic stem cells in the bone marrow.

No abstract

A single hematopoietic stem cell (HSC) is capable of reconstituting hematopoiesis and maintaining homeostasis by balancing self-renewal and cell differentiation. The mechanisms of HSC division balance, however, are not yet defined. Here we demonstrate, by characterizing at the single-cell level a purified and minimally heterogeneous murine Tie2+ HSC population, that these top hierarchical HSCs preferentially undergo symmetric divisions. The induction of mitophagy, a quality control process in mitochondria, plays an essential role in self-renewing expansion of Tie2+ HSCs. Activation of the PPAR (peroxisome proliferator–activated receptor)–fatty acid oxidation pathway promotes expansion of Tie2+ HSCs through enhanced Parkin recruitment in mitochondria. These metabolic pathways are conserved in human TIE2+ HSCs. Our data thus identify mitophagy as a key mechanism of HSC expansion and suggest potential methods of cell-fate manipulation through metabolic pathways.

Background: It is increasingly recognized that gut microbiota play a pivotal role in the development of atherosclerotic cardiovascular disease. Previously, we have reported that the abundance of genus Bacteroides is lower in patients with coronary artery disease (CAD) than in patients without CAD with coronary risk factors or in healthy volunteers. However, it remains unclear which and how specific gut bacteria contribute to the progression of atherosclerosis. Methods: We recruited patients with CAD patients and controls without CAD with coronary risk factors. We then compared gut microbial composition using 16S ribosomal RNA gene sequencing in fecal samples to detect species with differential abundance between 2 groups. Subsequently, we used atherosclerosis-prone mice to study the mechanisms underlying the relationship between such species and atherosclerosis. Results: Human fecal 16S ribosomal RNA gene sequencing revealed a significantly lower abundance of Bacteroides vulgatus and Bacteroides dorei in patients with CAD. This significant differential abundance was confirmed by quantitative polymerase chain reaction. Gavage with live B. vulgatus and B. dorei attenuated atherosclerotic lesion formation in atherosclerosis-prone mice, markedly ameliorating endotoxemia followed by decreasing gut microbial lipopolysaccharide production, effectively suppressing proinflammatory immune responses. Furthermore, fecal lipopolysaccharide levels in patients with CAD were significantly higher and negatively correlated with the abundance of B. vulgatus and B. dorei . Conclusions: Our translational research findings identify a previously unknown link between specific gut bacteria and atherosclerosis. Treatment with live B. vulgatus and B. dorei may help prevent CAD. Clinical Trial Registration: URL: https://upload.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000018051 . Unique identifier: UMIN000015703.