Simón Méndez-Ferrer

Mesenchymal and haematopoietic stem cells form a unique bone marrow niche

The cellular constituents forming the haematopoietic stem cell (HSC) niche in the bone marrow are unclear, with studies implicating osteoblasts, endothelial and perivascular cells. Here we demonstrate that mesenchymal stem cells (MSCs), identified using nestin expression, constitute an essential HSC niche component. Nestin+ MSCs contain all the bone-marrow colony-forming-unit fibroblastic activity and can be propagated as non-adherent ‘mesenspheres’ that can self-renew and expand in serial transplantations. Nestin+ MSCs are spatially associated with HSCs and adrenergic nerve fibres, and highly express HSC maintenance genes. These genes, and others triggering osteoblastic differentiation, are selectively downregulated during enforced HSC mobilization or β3 adrenoreceptor activation. Whereas parathormone administration doubles the number of bone marrow nestin+ cells and favours their osteoblastic differentiation, in vivo nestin+ cell depletion rapidly reduces HSC content in the bone marrow. Purified HSCs home near nestin+ MSCs in the bone marrow of lethally irradiated mice, whereas in vivo nestin+ cell depletion significantly reduces bone marrow homing of haematopoietic progenitors. These results uncover an unprecedented partnership between two distinct somatic stem-cell types and are indicative of a unique niche in the bone marrow made of heterotypic stem-cell pairs.

Haematopoietic stem cell release is regulated by circadian oscillations

Haematopoietic stem cells (HSCs) circulate in the bloodstream under steady-state conditions, but the mechanisms controlling their physiological trafficking are unknown. Here we show that circulating HSCs and their progenitors exhibit robust circadian fluctuations, peaking 5 h after the initiation of light and reaching a nadir 5 h after darkness. Circadian oscillations are markedly altered when mice are subjected to continuous light or to a ‘jet lag’ (defined as a shift of 12 h). Circulating HSCs and their progenitors fluctuate in antiphase with the expression of the chemokine CXCL12 in the bone marrow microenvironment. The cyclical release of HSCs and expression of Cxcl12 are regulated by core genes of the molecular clock through circadian noradrenaline secretion by the sympathetic nervous system. These adrenergic signals are locally delivered by nerves in the bone marrow, transmitted to stromal cells by the β3-adrenergic receptor, leading to a decreased nuclear content of Sp1 transcription factor and the rapid downregulation of Cxcl12. These data indicate that a circadian, neurally driven release of HSC during the animal’s resting period may promote the regeneration of the stem cell niche and possibly other tissues.

Bone marrow CD169 + macrophages promote the retention of hematopoietic stem and progenitor cells in the mesenchymal stem cell niche

Hematopoietic stem cells (HSCs) reside in specialized bone marrow (BM) niches regulated by the sympathetic nervous system (SNS). Here, we have examined whether mononuclear phagocytes modulate the HSC niche. We defined three populations of BM mononuclear phagocytes that include Gr-1hi monocytes (MOs), Gr-1lo MOs, and macrophages (MΦ) based on differential expression of Gr-1, CD115, F4/80, and CD169. Using MO and MΦ conditional depletion models, we found that reductions in BM mononuclear phagocytes led to reduced BM CXCL12 levels, the selective down-regulation of HSC retention genes in Nestin+ niche cells, and egress of HSCs/progenitors to the bloodstream. Furthermore, specific depletion of CD169+ MΦ, which spares BM MOs, was sufficient to induce HSC/progenitor egress. MΦ depletion also enhanced mobilization induced by a CXCR4 antagonist or granulocyte colony-stimulating factor. These results highlight two antagonistic, tightly balanced pathways that regulate maintenance of HSCs/progenitors in the niche during homeostasis, in which MΦ cross talk with the Nestin+ niche cell promotes retention, and in contrast, SNS signals enhance egress. Thus, strategies that target BM MΦ hold the potential to augment stem cell yields in patients that mobilize HSCs/progenitors poorly.

Bone Marrow Mesenchymal Stem and Progenitor Cells Induce Monocyte Emigration in Response to Circulating Toll-like Receptor Ligands

Inflammatory (Ly6C(hi) CCR2+) monocytes provide defense against infections but also contribute to autoimmune diseases and atherosclerosis. Monocytes originate from bone marrow and their entry into the bloodstream requires stimulation of CCR2 chemokine receptor by monocyte chemotactic protein-1 (MCP1). How monocyte emigration from bone marrow is triggered by remote infections remains unclear. We demonstrated that low concentrations of Toll-like receptor (TLR) ligands in the bloodstream drive CCR2-dependent emigration of monocytes from bone marrow. Bone marrow mesenchymal stem cells (MSCs) and their progeny, including CXC chemokine ligand (CXCL)12-abundant reticular (CAR) cells, rapidly expressed MCP1 in response to circulating TLR ligands or bacterial infection and induced monocyte trafficking into the bloodstream. Targeted deletion of MCP1 from MSCs impaired monocyte emigration from bone marrow. Our findings suggest that bone marrow MSCs and CAR cells respond to circulating microbial molecules and regulate bloodstream monocyte frequencies by secreting MCP1 in proximity to bone marrow vascular sinuses.

Neuropathy of haematopoietic stem cell niche is essential for myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are diseases caused by mutations in the haematopoietic stem cell (HSC) compartment. Most MPN patients have a common acquired mutation of Janus kinase 2 (JAK2) gene in HSCs that renders this kinase constitutively active, leading to uncontrolled cell expansion. The bone marrow microenvironment might contribute to the clinical outcomes of this common event. We previously showed that bone marrow nestin+ mesenchymal stem cells (MSCs) innervated by sympathetic nerve fibres regulate normal HSCs. Here we demonstrate that abrogation of this regulatory circuit is essential for MPN pathogenesis. Sympathetic nerve fibres, supporting Schwann cells and nestin+ MSCs are consistently reduced in the bone marrow of MPN patients and mice expressing the human JAK2(V617F) mutation in HSCs. Unexpectedly, MSC reduction is not due to differentiation but is caused by bone marrow neural damage and Schwann cell death triggered by interleukin-1β produced by mutant HSCs. In turn, in vivo depletion of nestin+ cells or their production of CXCL12 expanded mutant HSC number and accelerated MPN progression. In contrast, administration of neuroprotective or sympathomimetic drugs prevented mutant HSC expansion. Treatment with β3-adrenergic agonists that restored the sympathetic regulation of nestin+ MSCs prevented the loss of these cells and blocked MPN progression by indirectly reducing the number of leukaemic stem cells. Our results demonstrate that mutant-HSC-driven niche damage critically contributes to disease manifestation in MPN and identify niche-forming MSCs and their neural regulation as promising therapeutic targets.

The neural crest is a source of mesenchymal stem cells with specialized hematopoietic stem cell niche function

Mesenchymal stem cells (MSCs) and osteolineage cells contribute to the hematopoietic stem cell (HSC) niche in the bone marrow of long bones. However, their developmental relationships remain unclear. In this study, we demonstrate that different MSC populations in the developing marrow of long bones have distinct functions. Proliferative mesoderm-derived nestin− MSCs participate in fetal skeletogenesis and lose MSC activity soon after birth. In contrast, quiescent neural crest-derived nestin+ cells preserve MSC activity, but do not generate fetal chondrocytes. Instead, they differentiate into HSC niche-forming MSCs, helping to establish the HSC niche by secreting Cxcl12. Perineural migration of these cells to the bone marrow requires the ErbB3 receptor. The neonatal Nestin-GFP+ Pdgfrα− cell population also contains Schwann cell precursors, but does not comprise mature Schwann cells. Thus, in the developing bone marrow HSC niche-forming MSCs share a common origin with sympathetic peripheral neurons and glial cells, and ontogenically distinct MSCs have non-overlapping functions in endochondrogenesis and HSC niche formation. DOI: http://dx.doi.org/10.7554/eLife.03696.001

Cooperation of β2‐ and β3‐adrenergic receptors in hematopoietic progenitor cell mobilization

CXCL12/SDF‐1 dynamically regulates hematopoietic stem cell (HSC) attraction in the bone marrow (BM). Circadian regulation of bone formation and HSC traffic is relayed in bone and BM by β‐adrenergic receptors (β‐AR) expressed on HSCs, osteoblasts, and mesenchymal stem/progenitor cells. Circadian HSC release from the BM follows rhythmic secretion of norepinephrine from nerve terminals, β3‐AR activation, and Cxcl12 downregulation, possibly from reduced Sp1 nuclear content. Here, we show that β‐AR stimulation in stromal cells causes Sp1 degradation, partially mediated by the 26S proteasome. Inverted trends of circulating hematopoietic progenitors and BM Cxcl12 mRNA levels change acutely after light onset, shown to induce sympathetic efferent activity. In BM stromal cells, activation of β3‐AR downregulates Cxcl12, whereas β2‐AR stimulation induces clock gene expression. Double deficiency in β2‐ and β3‐ARs compromises enforced mobilization. Therefore, β2‐ and β3‐ARs have specific roles in stromal cells and cooperate during progenitor mobilization.

Circadian rhythms influence hematopoietic stem cells

Purpose of reviewHematopoiesis is tightly regulated in the bone marrow through the microenvironment, soluble factors from the circulation, and neural inputs from the autonomic nervous system. Most physiological processes are not uniform but rather vary according to the time of day. There is increasing evidence showing the impact of biological rhythms on the traffic of hematopoietic stem cells (HSCs) and their proliferation and differentiation capacities. Recent findingsRecent evidence supports the role of the sympathetic nervous system in the regulation of HSC behavior, both directly and through supporting stromal cells. In addition, the sympathetic nervous system transduces circadian information from the central pacemaker in the brain, the suprachiasmatic nucleus, to the bone marrow microenvironment, directing circadian oscillations in hematopoiesis and HSC migration. SummaryHSC traffic and hematopoiesis do not escape the circadian regulation that controls most physiological processes. Clinically, the timing of stem cell harvest or infusion may impact the yield or engraftment, respectively, and may result in better therapeutic outcomes.

Hematopoietic stem cell trafficking: Regulated adhesion and attraction to bone marrow microenvironment

Abstract:  Hematopoiesis takes place preferentially within bone cavities, suggesting that bone‐derived factors contribute to blood formation. Hematopoietic stem and progenitor cells (HSPCs) can be mobilized from the bone marrow parenchyma to the circulation by various agonists whose common downstream action leads to alteration in the expression or function of the chemokine CXCL12 and adhesion molecules mediating migration. Granulocyte colony‐stimulating factor (G‐CSF), the most prevalent drug used to mobilize HSPCs, dramatically suppresses osteoblast function. Recent studies suggest that G‐CSF‐mediated suppression requires signals from the sympathetic nervous system (SNS). This review summarizes emerging concepts thought to contribute to stem cell migration.

Self-Renewing Human Bone Marrow Mesenspheres Promote Hematopoietic Stem Cell Expansion

Strategies for expanding hematopoietic stem cells (HSCs) include coculture with cells that recapitulate their natural microenvironment, such as bone marrow stromal stem/progenitor cells (BMSCs). Plastic-adherent BMSCs may be insufficient to preserve primitive HSCs. Here, we describe a method of isolating and culturing human BMSCs as nonadherent mesenchymal spheres. Human mesenspheres were derived from CD45- CD31- CD71- CD146+ CD105+ nestin+ cells but could also be simply grown from fetal and adult BM CD45--enriched cells. Human mesenspheres robustly differentiated into mesenchymal lineages. In culture conditions where they displayed a relatively undifferentiated phenotype, with decreased adherence to plastic and increased self-renewal, they promoted enhanced expansion of cord blood CD34+ cells through secreted soluble factors. Expanded HSCs were serially transplantable in immunodeficient mice and significantly increased long-term human hematopoietic engraftment. These results pave the way for culture techniques that preserve the self-renewal of human BMSCs and their ability to support functional HSCs.