The extracellular matrix of the hematopoietic microenvironment

The bone marrow microenvironment plays an important role in promoting hematopoietic progenitor cell proliferation and differentiation and the controlled egress of these developing hematopoietic cells. The establishment of long-term bone marrow cultures, which are thought to mimic hematopoiesis in vitro, and various stromal cell lines has greatly facilitated the analysis of the functions of this microenvironment. Extracellular matrix (ECM) molecules of all three categories (collagens, proteoglycans and glycoproteins) have been identified as part of this microenvironment and have been shown to be involved in, different biological functions such as cell adhesion and anti-adhesion, binding and presentation of various cytokines and regulation of cell growth. It is suggested that these matrix molecules in combination with cytokines are crucial for compartmentalization of the bone marrow. Although many cell adhesion molecules have been characterized on the surface of hematopoietic progenitor cells, the nature of cellular receptors for the ECM components is less well defined. During leukemia, many immature blood cells are released from bone marrow, but it is not yet known whether these abnormalities in hematopoiesis are also caused by an altered microenvironment or altered composition of its extracellular matrix. The elucidation of the involvement of specific ECM-isoforms and as yet not characterized ECM components and their receptors in the bone marrow will certainly help towards a better understanding of these phenomena.     

Regulation of myelopoiesis by proinflammatory cytokines in infectious diseases

Hematopoiesis is hierarchically orchestrated by a very small population of hematopoietic stem cells (HSCs) that reside in the bone-marrow niche and are tightly regulated to maintain homeostatic blood production. HSCs are predominantly quiescent, but they enter the cell cycle in response to inflammatory signals evoked by severe systemic infection or injury. Thus, hematopoietic stem and progenitor cells (HSPCs) can be activated by pathogen recognition receptors and proinflammatory cytokines to induce emergency myelopoiesis during infection. This emergency myelopoiesis counterbalances the loss of cells and generates lineage-restricted hematopoietic progenitors, eventually replenishing mature myeloid cells to control the infection. Controlled generation of such signals effectively augments host defense, but dysregulated stimulation by these signals is harmful to HSPCs. Such hematopoietic failure often results in blood disorders including chronic inflammatory diseases and hematological malignancies. Recently, we found that interleukin (IL)-27, one of the IL-6/IL-12 family cytokines, has a unique ability to directly act on HSCs and promote their expansion and differentiation into myeloid progenitors. This process resulted in enhanced production of neutrophils by emergency myelopoiesis during the blood-stage mouse malaria infection. In this review, we summarize recent advances in the regulation of myelopoiesis by proinflammatory cytokines including type I and II interferons, IL-6, IL-27, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, and IL-1 in infectious diseases.     

Eph-dependent cell-cell adhesion and segregation in development and cancer

Numerous studies attest to essential roles for Eph receptors and their ephrin ligands in controlling cell positioning and tissue patterning during normal and oncogenic development. These studies suggest multiple, sometimes contradictory, functions of Eph-ephrin signalling, which under different conditions can promote either spreading and cell-cell adhesion or cytoskeletal collapse, cell rounding, de-adhesion and cell-cell segregation. A principle determinant of the balance between these two opposing responses is the degree of receptor/ligand clustering and activation. This equilibrium is likely altered in cancers and modulated by somatic mutations of key Eph family members that have emerged as candidate cancer markers in recent profiling studies. In addition, cross-talk amongst Ephs and with other signalling pathways significantly modulates cell-cell adhesion, both between and within Eph- and ephrin-expressing cell populations. This review summarises our current understanding of how Eph receptors control cell adhesion and morphology, and presents examples demonstrating the importance of these events in normal development and cancer.     

The role of endocytosis in activating and regulating signal transduction

Endocytosis is increasingly understood to play crucial roles in most signaling pathways, from determining which signaling components are activated, to how the signal is subsequently transduced and/or terminated. Whether a receptor-ligand complex is internalized via a clathrin-dependent or clathrin-independent endocytic route, and the complexes' subsequent trafficking through specific endocytic compartments, to then be recycled or degraded, has profound effects on signaling output. This review discusses the roles of endocytosis in three markedly different signaling pathways: the Wnt, Notch, and Eph/Ephrin pathways. These offer fundamentally different signaling systems: (1) diffusible ligands inducing signaling in one cell, (2) membrane-tethered ligands inducing signaling in a contacting receptor cell, and (3) bi-directional receptor-ligand signaling in two contacting cells. In each of these systems, endocytosis controls signaling in fascinating ways, and comparison of their similarities and dissimilarities will help to expand our understanding of endocytic control of signal transduction across multiple signaling pathways.     

Eph- and ephrin-dependent mechanisms in tumor and stem cell dynamics

The erythropoietin-producing hepatocellular (Eph) receptors comprise the largest family of receptor tyrosine kinases (RTKs). Initially regarded as axon-guidance and tissue-patterning molecules, Eph receptors have now been attributed with various functions during development, tissue homeostasis, and disease pathogenesis. Their ligands, ephrins, are synthesized as membrane-associated molecules. At least two properties make this signaling system unique: (1) the signal can be simultaneously transduced in the receptor- and the ligand-expressing cell, (2) the signaling outcome through the same molecules can be opposite depending on cellular context. Moreover, shedding of Eph and ephrin ectodomains as well as ligand-dependent and -independent receptor crosstalk with other RTKs, proteases, and adhesion molecules broadens the repertoire of Eph/ephrin functions. These integrated pathways provide plasticity to cell–microenvironment communication in varying tissue contexts. The complex molecular networks and dynamic cellular outcomes connected to the Eph/ephrin signaling in tumor–host communication and stem cell niche are the main focus of this review.     

Oncogenic protein tyrosine kinases

FLT3, a member of the class III receptor tyrosine kinases (RTKs), is preferentially expressed on the cell surface of hematopoietic progenitors, and the ligand of FLT3 (FL) is expressed as a membrane-bound or soluble form by bone marrow stroma cells. It has been disclosed that FL-FLT3 interaction plays an important role in the maintenance, proliferation and differentiation of hematopoiesis. FLT3 is also expressed in a high proportion of acute myeloid leukemia (AML) and B-lineage acute lymphoblastic leukemia cells. Activating mutations of FLT3 are the most frequent genetic lesions in AML, and AML patients with FLT3 mutations have a worse prognosis than those with normal FLT3. Exploring the mechanism by which FLT3 mutations cause autoactivation and uncontrolled signaling might lead to a better understanding of how FLT3 becomes oncogenic and provide insights for the development of new drugs.     

Factor(s) from nonmacrophage bone marrow stromal cells inhibit Lewis lung carcinoma and B16 melanoma growth in mice

Bone marrow stroma produces positive and negative growth regulators which constitute the hematopoietic microenvironment. As many tumors metastasize to the bones, these regulators may also influence tumor growth. Hematopoietic cytokines may indeed exert both positive and negative effect on tumor growth. We report that, when mixed with tumor cells. adherent bone marrow cells inhibit primary tumor growth and metastases formation in mice transplanted with Lewis lung carcinoma or B16 melanoma. Peritoneal macrophages or lymph node cells did not exert any influence. The tumor inhibition was apparently due to soluble factor(s) released by marrow stromal cells. In cocultures with B16 melanoma cells, adherent bone marrow cells exerted a significant antiproliferative effect which was increased by previous culture of the bone marrow cells with granulocyte-macrophage colony-stimulating factor but not with macrophage colony-stimulating factor. Neither neutralizing antibodies against tumor necrosis factor-alpha, transforming growth factor-beta or interferon alpha/beta nor addition of Escherichia coli lipopolysaccharide to generate inflammatory cytokines could affect the antiproliferative effect of bone marrow stromal cells. The bone marrow stroma factor(s) which inhibit tumor growth might, therefore, be a novel growth regulator.     

The hematopoietic stem-cell niche in health and leukemia

Research in the last decade has shown that hematopoietic stem cells (HSCs) interact with and are modulated by a complex multicellular microenvironment in the bone marrow, which includes both the HSC progeny and multiple non-hematopoietic cell types. Intense work is gradually throwing light on the composition of the HSC niche and the molecular cues exchanged between its components, which has implications for HSC production, maintenance and expansion. In addition, it has become apparent that bidirectional interactions between leukemic cells and their niche play a previously unrecognized role in the initiation and development of hematological malignancies. Consequently, targeting of the malignant niche holds considerable promise for more specific antileukemic therapies. Here we summarize the latest insights into HSC niche biology and recent work showing multiple connections between hematological malignancy and alterations in the bone marrow microenvironment.     

Exploring oligodendrocyte guidance: ‘to boldly go where no cell has gone before’

Oligodendrocytes, the myelinating cells of the central nervous system (CNS), originate early in the formation of the brain in specific foci, and migrate throughout the parenchyma. The instructional cues guiding the migration of these progenitor cells must be encoded into their developing environment. Soluble factors as well as membrane-bound cues most likely synergize to create a complex thoroughfare needed to sculpt and organize the brain into a functional organ with white and gray matter. Classically, the focus of many guidance related studies in the CNS has been limited to neuron physiology. However, It is becoming increasingly clear that their lifelong partners, oligodendrocytes, express both ligands and receptors able to both present and respond to these classical cues. In this short review, some recent findings in the Semaphorin and Eph fields will be presented with respect to oligodendrocyte expression and function.Received 4 November 2004; received after revision 1 December 2004; accepted 7 December 2004     

ADAM proteases: ligand processing and modulation of the Notch pathway

ADAM metalloproteases play important roles in development and disease. One of the key functions of ADAMs is the proteolytic processing of Notch receptors and their ligands. ADAM-mediated cleavage of Notch represents the first step in regulated intramembrane proteolysis of the receptor, leading to activation of the Notch pathway. Recent reports indicate that the transmembrane Notch ligands also undergo ADAM-mediated processing in cultured cells and in vivo. The proteolytic processing of Notch ligands modulates the strength and duration of Notch signals, leads to generation of soluble intracellular domains of the ligands, and may support a bi-directional signaling between cells.     

Evidence for the existence of an agent in the serum of the cyclic hematopoietic dog which influences hemoglobin synthesis

Summary Serum samples collected through the cycle of a cyclic hematopoietic (CH) dog under reduced atmospheric conditions, were assayed for their ability to affect hemoglobin synthesis by normal canine bone marrow. Varying levels of hemoglobin synthesis in the presence of different serum samples suggest an agent cycles in the serum of CH dogs which influences hemoglobin synthesis.     

Nuclear receptors in neural stem/progenitor cell homeostasis

In the central nervous system, embryonic and adult neural stem/progenitor cells (NSCs) generate the enormous variety and huge numbers of neuronal and glial cells that provide structural and functional support in the brain and spinal cord. Over the last decades, nuclear receptors and their natural ligands have emerged as critical regulators of NSC homeostasis during embryonic development and adult life. Furthermore, substantial progress has been achieved towards elucidating the molecular mechanisms of nuclear receptors action in proliferative and differentiation capacities of NSCs. Aberrant expression or function of nuclear receptors in NSCs also contributes to the pathogenesis of various nervous system diseases. Here, we review recent advances in our understanding of the regulatory roles of steroid, non-steroid, and orphan nuclear receptors in NSC fate decisions. These studies establish nuclear receptors as key therapeutic targets in brain diseases.     

Vascular stem/progenitor cells: functions and signaling pathways

Vascular stem/progenitor cells (VSCs) are an important source of all types of vascular cells needed to build, maintain, repair, and remodel blood vessels. VSCs, therefore, play critical roles in the development, normal physiology, and pathophysiology of numerous diseases. There are four major types of VSCs, including endothelial progenitor cells (EPCs), smooth muscle progenitor cells (SMPCs), pericytes, and mesenchymal stem cells (MSCs). VSCs can be found in bone marrow, circulating blood, vessel walls, and other extravascular tissues. During the past two decades, considerable progress has been achieved in the understanding of the derivation, surface markers, and differentiation of VSCs. Yet, the mechanisms regulating their functions and maintenance under normal and pathological conditions, such as in eye diseases, remain to be further elucidated. Owing to the essential roles of blood vessels in human tissues and organs, understanding the functional properties and the underlying molecular basis of VSCs is of critical importance for both basic and translational research.     

Surface receptors and functional interactions of human natural killer cells: from bench to the clinic

The past 10years have witnessed dramatic progress in our understanding of how natural killer (NK) cells function and their role in innate immunity. Thanks to an array of inhibitory receptors specific for different HLA class I molecules, human NK cells can sense the decrease or loss of even single alleles at the cell surface. This represents a typical condition of a potential danger, i.e. the presence of tumor or virally infected cells. NK cell triggering and lysis of these cells is mediated by several activating receptors and coreceptors that have recently been identified and cloned. While normal cells are usually resistant to NK-mediated attack, a remarkable exception is represented by dendritic cells (DCs). In their immature form they are susceptible to NK-mediated lysis because of the expression of low levels of surface HLA class I molecules. The process of DC maturation (mDCs) is characterized by the surface expression of high levels of HLA class I molecules. Accordingly, mDCs become resistant to NK cells. A recent major breakthrough highlighted the role played by donor NK cells in allogenic bone marrow transplantation to cure acute myeloid leukemias. Alloreactive NK cells derived from donor hematopoietic precursors not only prevented leukemic relapses, but also prevented graft rejection and graft-versus-host disease.Received 12 March 2003; received after revision 18 April 2003; accepted 30 April 2003     

Donor origin of the in vitro hematopoietic microenvironment after marrow transplantation in mice

Bone marrow stroma from radiochimeric mice was established in culture. The polymorphic enzyme glucose phosphate isomerase (GPI) was used to determine the proportions of donor and recipient present in the original bone marrow and in cultured stroma. Bone marrow initially containing 95% donor GPI, when cultured and subsequently passaged for up to 8 weeks remained about 70% donor GPI. We conclude that many cultured stromal cells are donor derived in our radiochimeras and these are probably of hematopoietic origin.     

Mesenchymal stem cells or cardiac progenitors for cardiac repair? A comparative study

In the past, clinical trials transplanting bone marrow–derived mononuclear cells reported a limited improvement in cardiac function. Therefore, the search for stem cells leading to more successful stem cell therapies continues. Good candidates are the so-called cardiac stem cells (CSCs). To date, there is no clear evidence to show if these cells are intrinsic stem cells from the heart or mobilized cells from bone marrow. In this study we performed a comparative study between human mesenchymal stem cells (hMSCs), purified c-kit⁺ CSCs, and cardiosphere-derived cells (CDCs). Our results showed that hMSCs can be discriminated from CSCs by their differentiation capacity towards adipocytes and osteocytes and the expression of CD140b. On the other hand, cardiac progenitors display a greater cardiomyogenic differentiation capacity. Despite a different isolation protocol, no distinction could be made between c-kit⁺ CSCs and CDCs, indicating that they probably derive from the same precursor or even are the same cells.     

<Emphasis Type="Italic">Cis</Emphasis>–<Emphasis Type="Italic">trans</Emphasis> interactions of cell surface receptors: biological roles and structural basis

Cell surface receptors bind ligands expressed on other cells (in trans) in order to communicate with neighboring cells. However, an increasing number of cell surface receptors are found to also interact with ligands expressed on the same cell (in cis). These observations raise questions regarding the biological role of such cis interactions. Specifically, it is important to know whether cis and trans binding have distinct functional effects and, if so, how a single cell discriminates between interactions in cis versus trans. Further, what are the structural features that allow certain cell surface receptors to engage ligand both on the same as well as on an apposed cell membrane? Here, we summarize known examples of receptors that display cis–trans binding and discuss the emerging diversity of biological roles played by these unconventional two-way interactions, along with their structural basis.