Megakaryocytes are essential for HSC quiescence through the production of thrombopoietin

Niche heterogeneity in the bone marrow

In adult mammals, hematopoietic stem cells (HSCs) are defined by their abilities to self-renew and to differentiate to form all blood cell lineages. These rare multipotent cells occupy specific locations in the bone marrow (BM) microenvironment. The specific microenvironment regulating HSCs, commonly referred to as the niche, comprises multiple cell types whose exact contributions are under active investigation. Understanding cellular cross talk involving HSCs in the BM microenvironment is of fundamental importance for harnessing therapies against benign and malignant blood diseases. In this review, we summarize and evaluate recent advances in our understanding of niche heterogeneity and its influence on HSC function.

Normal and leukemic stem cell niches: insights and therapeutic opportunities

Hematopoietic stem cells (HSCs) rely on instructive cues from the bone marrow (BM) niche to maintain their quiescence and adapt blood production to the organism's needs. Alterations in the BM niche are commonly observed in blood malignancies and directly contribute to the aberrant function of disease-initiating leukemic stem cells (LSCs). Here, we review recent insights into the cellular and molecular determinants of the normal HSC niche and describe how genetic changes in stromal cells and leukemia-induced BM niche remodeling contribute to blood malignancies. Moreover, we discuss how these findings can be applied to non-cell-autonomous therapies targeting the LSC niche.

BMSCs and hematopoiesis

Recent discoveries have significantly expanded our previous knowledge about the role of bone marrow mesenchymal stem cells (BMSCs) in hematopoiesis. BMSCs and their derivatives modulate blood production and immunity at different levels but a prominent role has emerged for BMSCs in the regulation of hematopoietic stem and progenitor cells (HSPCs). Additionally, BMSC-like cells regulate B and T cell lymphopoiesis and also probably myelopoiesis. Furthermore, BMSCs might also exhibit key regulatory properties in non-physiological conditions. BMSCs in extramedullary sites might provide a permissive microenvironment to allow for transient hematopoiesis. BMSCs might be also involved in the manifestation and/or the development of hematopoietic diseases, as stemming from their emerging roles in the progression of hematological malignancies. Here we review some key molecular pathways, adhesion molecules and ligand/receptor interactions that mediate the crosstalk between BMSCs and hematopoietic stem cells (HSCs) in health and disease. The development of novel markers to visualize and isolate individual cells will help to dissect the stromal-hematopoietic interplay.

The Hematopoietic Niche in Myeloproliferative Neoplasms

Specialized microanatomical areas of the bone marrow provide the signals that are mandatory for the maintenance and regulation of hematopoietic stem cells (HSCs) and progenitor cells. A complex microenvironment adjacent to the marrow vasculature (vascular niche) and close to the endosteum (endosteal niche) harbors multiple cell types including mesenchymal stromal cells and their derivatives such as CAR cells expressing high levels of chemokines C-X-C motif ligand 12 and early osteoblastic lineage cells, endothelial cells, and megakaryocytes. The characterization of the cellular and molecular networks operating in the HSC niche has opened new perspectives for the understanding of the bidirectional cross-talk between HSCs and stromal cell populations in normal and malignant conditions. A structural and functional remodeling of the niche may contribute to the development of myeloproliferative neoplasms (MPN). Malignant HSCs may alter the function and survival of MSCs that do not belong to the neoplastic clone. For example, a regression of nestin⁺ MSCs by apoptosis has been attributed to neuroglial damage in MPN. Nonneoplastic MSCs in turn can promote aggressiveness and drug resistance of malignant cells. In the future, strategies to counteract the pathological interaction between the niche and neoplastic HSCs may offer additional treatment strategies for MPN patients.

Hematopoiesis "awakens": Evolving technologies, the force behind them

Amid the beauty of the Kyoto countryside, leaders in the field of hematology met at the 44th annual International Society for Experimental Hematology (ISEH) meeting in late September 2015. Led by ISEH President Paul Frenette and President-Elect David Traver, the meeting covered many aspects of hematopoiesis with a focus on technology. At the meeting, it became clear that the future of hematology is being shaped by innovations in single-cell "omics" and imaging approaches that will provide answers to age-old questions on cellular identity. In this meeting review, we highlight the advances presented in understanding the hematopoietic stem cell (HSC) niche, heterogeneity, stress response, epigenetics, and how these processes change from birth to old age.

CLEC-2 in megakaryocytes is critical for maintenance of hematopoietic stem cells in the bone marrow

Nakamura-Ishizu et al. report that megakaryocytes function as a niche to maintain HSC quiescence through CLEC-2–mediated production of Thpo and other key regulators of HSC function. These findings could enable manipulation of HSCs for clinical application.

Hematopoietic stem cells (HSCs) depend on the bone marrow (BM) niche for their maintenance, proliferation, and differentiation. The BM niche is composed of nonhematopoietic and mature hematopoietic cells, including megakaryocytes (Mks). Thrombopoietin (Thpo) is a crucial cytokine produced by BM niche cells. However, the cellular source of Thpo, upon which HSCs primarily depend, is unclear. Moreover, no specific molecular pathway for the regulation of Thpo production in the BM has been identified. Here, we demonstrate that the membrane protein C-type lectin-like receptor-2 (CLEC-2) mediates the production of Thpo and other factors in Mks. Mice conditionally deleted for CLEC-2 in Mks (Clec2^MkΔ/Δ) produced lower levels of Thpo in Mks. CLEC-2–deficient Mks showed down-regulation of CLEC-2–related signaling molecules Syk, Lcp2, and Plcg2. Knockdown of these molecules in cultured Mks decreased expression of Thpo. Clec2^MkΔ/Δ mice exhibited reduced BM HSC quiescence and repopulation potential, along with extramedullary hematopoiesis. The low level of Thpo production may account for the decline in HSC potential in Clec2^MkΔ/Δ mice, as administration of recombinant Thpo to Clec2^MkΔ/Δ mice restored stem cell potential. Our study identifies CLEC-2 signaling as a novel molecular mechanism mediating the production of Thpo and other factors for the maintenance of HSCs.

Hypoxia regulates the hematopoietic stem cell niche

Bone marrow, the site of hematopoiesis throughout adulthood, is a physiologically hypoxic organ. Thus, various biological oxygen sensors and their signaling cascades play a pivotal role in hematopoietic systems in the bone marrow under both physiologic and pathologic conditions. Hypoxia-inducible factors (HIFs) are hypoxic stress sensor proteins that are stabilized under homeostatic or stress-induced hypoxia. In the hypoxic bone marrow, HIFs play crucial roles in hematopoietic stem cells (HSCs) and in the cells of the HSC niche. The signals downstream of the HIFs maintain HSC quiescence, survival, and metabolic homeostasis through both cell-autonomous and non-cell-autonomous mechanisms. Leukemic stem cells (LSCs) hijack these delicate hypoxia-sensing mechanisms to sustain their self-renewal potential, promoting disease progression and drug resistance even under normoxic conditions. This review focuses on HIF-mediated oxygen-sensing mechanisms of adult HSCs and LSCs and their niche cells in the hypoxic bone marrow.

IL-1α induces thrombopoiesis through megakaryocyte rupture in response to acute platelet needs

An alternative pathway triggering enhanced platelet release from bone marrow megakaryocytes via a rupture-based mechanism is regulated by IL-1α in response to acute platelet requirements.

Intravital visualization of thrombopoiesis revealed that formation of proplatelets, which are cytoplasmic protrusions in bone marrow megakaryocytes (MKs), is dominant in the steady state. However, it was unclear whether this is the only path to platelet biogenesis. We have identified an alternative MK rupture, which entails rapid cytoplasmic fragmentation and release of much larger numbers of platelets, primarily into blood vessels, which is morphologically and temporally different than typical FasL-induced apoptosis. Serum levels of the inflammatory cytokine IL-1α were acutely elevated after platelet loss or administration of an inflammatory stimulus to mice, whereas the MK-regulator thrombopoietin (TPO) was not elevated. Moreover, IL-1α administration rapidly induced MK rupture–dependent thrombopoiesis and increased platelet counts. IL-1α–IL-1R1 signaling activated caspase-3, which reduced plasma membrane stability and appeared to inhibit regulated tubulin expression and proplatelet formation, and ultimately led to MK rupture. Collectively, it appears the balance between TPO and IL-1α determines the MK cellular programming for thrombopoiesis in response to acute and chronic platelet needs.

Making Blood: The Haematopoietic Niche throughout Ontogeny

Approximately one-quarter of all cells in the adult human body are blood cells. The haematopoietic system is therefore massive in scale and requires exquisite regulation to be maintained under homeostatic conditions. It must also be able to respond when needed, such as during infection or following blood loss, to produce more blood cells. Supporting cells serve to maintain haematopoietic stem and progenitor cells during homeostatic and pathological conditions. This coalition of supportive cell types, organised in specific tissues, is termed the haematopoietic niche. Haematopoietic stem and progenitor cells are generated in a number of distinct locations during mammalian embryogenesis. These stem and progenitor cells migrate to a variety of anatomical locations through the conceptus until finally homing to the bone marrow shortly before birth. Under stress, extramedullary haematopoiesis can take place in regions that are typically lacking in blood-producing activity. Our aim in this review is to examine blood production throughout the embryo and adult, under normal and pathological conditions, to identify commonalities and distinctions between each niche. A clearer understanding of the mechanism underlying each haematopoietic niche can be applied to improving ex vivo cultures of haematopoietic stem cells and potentially lead to new directions for transplantation medicine.

Bone marrow stem cells: current and emerging concepts

The interactions of stromal cells with hematopoietic cells in the bone marrow have long been a subject of research, but only recently have technologies allowed us to dissect them at the stem cell level. On the other hand, limitations of these technical tools might explain numerous discrepancies in this field. It is becoming increasingly clear that mesenchymal stem cells (MSCs) represent an important component of the hematopoietic stem cell (HSC) niche in the bone marrow. However, there is heterogeneity among HSCs, and many putatively different mesenchymal progenitors identified in the bone marrow using Cre recombinase-driven mouse lines seem to exhibit HSC niche properties. Development of better reporter lines has demonstrated that some of these Cre lines do not always specifically mark the expected cells. Also, characterization of different cell populations has often been partial, and issues of redundancy and compensation might explain apparently contradictory results. Recognizing and overcoming these limitations, while also clearly defining the distinctions between subgroups of mesenchymal cells, will be essential to advance the field.