N-cadherin in osteolineage cells is not required for maintenance of hematopoietic stem cells

Hematopoietic stem cell niche maintenance during homeostasis and regeneration

The bone marrow niche has mystified scientists for many years, leading to widespread investigation to shed light into its molecular and cellular composition. Considerable efforts have been devoted toward uncovering the regulatory mechanisms of hematopoietic stem cell (HSC) niche maintenance. Recent advances in imaging and genetic manipulation of mouse models have allowed the identification of distinct vascular niches that have been shown to orchestrate the balance between quiescence, proliferation and regeneration of the bone marrow after injury. Here we highlight the recently discovered intrinsic mechanisms, microenvironmental interactions and communication with surrounding cells involved in HSC regulation, during homeostasis and in regeneration after injury and discuss their implications for regenerative therapy.

N-cadherin/wnt interaction controls bone marrow mesenchymal cell fate and bone mass during aging

Age-related bone loss is characterized by reduced osteoblastogenesis and excessive bone marrow adipogenesis. The mechanisms governing bone marrow mesenchymal stromal cell (BMSC) differentiation into adipocytes or osteoblasts during aging are unknown. We show here that overexpressing N-cadherin (Cadh2) in osteoblasts increased BMSC adipocyte differentiation and reduced osteoblast differentiation in young transgenic (Tg) mice whereas this phenotype was fully reversed with aging. The reversed phenotype with age was associated with enhanced Wnt5a and Wnt10b expression in osteoblasts and a concomitant increase in BMSC osteogenic differentiation. Consistent with this mechanism, conditioned media from young wild type osteoblasts inhibited adipogenesis and promoted osteoblast differentiation in BMSC from old Cadh2 Tg mice, and this response was abolished by Wnt5a and Wnt10b silencing. Transplantation of BMSC from old Cadh2 Tg mice into young Tg recipients increased Wnt5a and Wnt10b expression and rescued BMSC osteogenic differentiation. In senescent osteopenic mice, blocking the CADH2-Wnt interaction using an antagonist peptide increased Wnt5a and Wnt10b expression, bone formation, and bone mass. The data indicate that Cadh2/Wnt interaction in osteoblasts regulates BMSC lineage determination, bone formation, and bone mass and suggest a therapeutic target for promoting bone formation in the aging skeleton.

Difference in apical and basal growth of the frontal bone primordium in Foxc1ch/ch mice

The frontal and parietal bones form the major part of the calvarium and their primordia appear at the basolateral region of the head and grow apically. A spontaneous loss of Foxc1 function mutant mouse, congenital hydrocephalus (Foxc1(ch/ch)), results in congenital hydrocephalus accompanied by defects in the apical part of the skull vault. We found that during the initiation stage of apical growth of the frontal bone primordium in the Foxc1(ch/ch) mouse, the Runx2 expression domain extended only to the basal side and bone sialoprotein (Bsp) and N-cadherin expression domains appeared only in the basal region. Fluorescent dye (DiI) labeling of the frontal primordium by ex-utero surgery confirmed that apical extension of the frontal bone primordium of the mouse was severely retarded, while extension to the basal side underneath the brain was largely unaffected. Consistent with this observation, decreased cell proliferation activity was seen at the apical tip but not the basal tip of the frontal bone primordium as determined by double detection of Runx2 transcripts and BrdU incorporation. Furthermore, expression of the osteogenic-related genes Bmp4 and-7 was observed only in the basal part of the meninges during the initiation period of primordium growth. These results suggest that a loss of Foxc1 function affects skull bone formation of the apical region and that Bmp expression in the meninges might influence the growth of the calvarial bone primordium.

Concise review: Current concepts in bone marrow microenvironmental regulation of hematopoietic stem and progenitor cells

Hematopoietic stem cell (HSC) behavior is governed in large part by interactions of the blood system with the bone microenvironment. Increasing evidence demonstrates the profound role the local HSC microenvironment or niche plays in normal stem cell function, in therapeutic activation and in the setting of malignancy. A number of cellular and molecular components of the microenvironment have been identified thus far, several of which are likely to provide exciting therapeutic targets in the near future. Clinically effective strategies for niche manipulation, however, require careful study of the interaction of these niche components. Some of the key findings defining these regulatory interactions are explored in this concise review, with special emphasis on potential translational applications.

Cadherin-11 regulates both mesenchymal stem cell differentiation into smooth muscle cells and the development of contractile function in vivo

Although soluble factors, such as transforming growth factor β1 (TGF-β1), induce mesenchymal stem cell (MSC) differentiation towards the smooth muscle cell (SMC) lineage, the role of adherens junctions in this process is not well understood. In this study, we found that cadherin-11 but not cadherin-2 was necessary for MSC differentiation into SMCs. Cadherin-11 regulated the expression of TGF-β1 and affected SMC differentiation through a pathway that was dependent on TGF-β receptor II (TGFβRII) but independent of SMAD2 or SMAD3. In addition, cadherin-11 activated the expression of serum response factor (SRF) and SMC proteins through the Rho-associated protein kinase (ROCK) pathway. Engagement of cadherin-11 increased its own expression through SRF, indicative of the presence of an autoregulatory feedback loop that committed MSCs to the SMC fate. Notably, SMC-containing tissues (such as aorta and bladder) from cadherin-11-null (Cdh11(-/-)) mice showed significantly reduced levels of SMC proteins and exhibited diminished contractility compared with controls. This is the first report implicating cadherin-11 in SMC differentiation and contractile function in vitro as well as in vivo.

N-Cadherin and Tie2 positive CD34⁺CD38⁻CD123⁺ leukemic stem cell populations can develop acute myeloid leukemia more effectively in NOD/SCID mice

Emerging studies suggest that the population of malignant cells found in human acute myelogenous leukemia (AML) arises from a rare population of leukemic stem cells (LSCs). A lot of investigators have reported the identification of cell surface markers, such as CD123. Here, we report the identification of N-cadherin and Tie2 as LSCs markers. Inoculation of CD34(+)CD38(-)CD123(+)N-cadherin(+) and CD34(+)CD38(-)CD123(+) Tie2(+) population can induce leukemia in NOD/SCID mice. The leukemic blast cells from the primary leukemic mice could also induce leukemia in the secondary transplantation. These findings suggested that N-cadherin and Tie2 were the important markers that can assist in leukemia development.

The bone marrow niche for haematopoietic stem cells

Niches are local tissue microenvironments that maintain and regulate stem cells. Haematopoiesis provides a model for understanding mammalian stem cells and their niches, but the haematopoietic stem cell (HSC) niche remains incompletely defined and beset by competing models. Recent progress has been made in elucidating the location and cellular components of the HSC niche in the bone marrow. The niche is perivascular, created partly by mesenchymal stromal cells and endothelial cells and often, but not always, located near trabecular bone. Outstanding questions concern the cellular complexity of the niche, the role of the endosteum and functional heterogeneity among perivascular microenvironments.

Cellular complexity of the bone marrow hematopoietic stem cell niche

The skeleton serves as the principal site for hematopoiesis in adult terrestrial vertebrates. The function of the hematopoietic system is to maintain homeostatic levels of all circulating blood cells, including myeloid cells, lymphoid cells, red blood cells, and platelets. This action requires the daily production of more than 500 billion blood cells. The vast majority of these cells are synthesized in the bone marrow, where they arise from a limited number of hematopoietic stem cells (HSCs) that are multipotent and capable of extensive self-renewal. These attributes of HSCs are best demonstrated by marrow transplantation, where even a single HSC can repopulate the entire hematopoietic system. HSCs are therefore adult stem cells capable of multilineage repopulation, poised between cell fate choices which include quiescence, self-renewal, differentiation, and apoptosis. While HSC fate choices are in part determined by multiple stochastic fluctuations of cell autonomous processes, according to the niche hypothesis, signals from the microenvironment are also likely to determine stem cell fate. While it had long been postulated that signals within the bone marrow could provide regulation of hematopoietic cells, it is only in the past decade that advances in flow cytometry and genetic models have allowed for a deeper understanding of the microenvironmental regulation of HSCs. In this review, we will highlight the cellular regulatory components of the HSC niche.

Cadherin-mediated cell-cell adhesion and signaling in the skeleton

Direct cell-to-cell interactions via cell adhesion molecules, in particular cadherins, are critical for morphogenesis, tissue architecture, and cell sorting and differentiation. Partially overlapping, yet distinct roles of N-cadherin (cadherin-2) and cadherin-11 in the skeletal system have emerged from mouse genetics and in vitro studies. Both cadherins are important for precursor commitment to the osteogenic lineage, and genetic ablation of Cdh2 and Cdh11 results in skeletal growth defects and impaired bone formation. While Cdh11 defines the osteogenic lineage, persistence of Cdh2 in osteoblasts in vivo actually inhibits their terminal differentiation and impairs bone formation. The action of cadherins involves both cell-cell adhesion and interference with intracellular signaling, and in particular the Wnt/β-catenin pathway. Both cadherin-2 and cadherin-11 bind to β-catenin, thus modulating its cytoplasmic pools and transcriptional activity. Recent data demonstrate that cadherin-2 also interferes with Lrp5/6 signaling by sequestering these receptors in inactive pools via axin binding. These data extend the biologic action of cadherins in bone forming cells, and provide novel mechanisms for development of therapeutic strategies aimed at enhancing bone formation.

Regulation of hematopoietic stem cells by bone marrow stromal cells

Hematopoietic stem cells (HSCs) reside in specialized microenvironments (niches) in the bone marrow. The stem cell niche is thought to provide signals that support key HSC properties, including self-renewal capacity and long-term multilineage repopulation ability. The stromal cells that comprise the stem cell niche and the signals that they generate that support HSC function are the subjects of intense investigation. Here, we review the complex and diverse stromal cell populations that reside in the bone marrow and examine their contribution to HSC maintenance. We highlight recent data suggesting that perivascular chemokine CXC ligand (CXCL)12-expressing mesenchymal progenitors and endothelial cells are key cellular components of the stem cell niche in the bone marrow.

Cadherins and Wnt signalling: a functional link controlling bone formation

Cadherins are calcium-dependent cell adhesion molecules that have a major role in morphogenesis and tissue formation. In bone, cadherins control osteoblast differentiation by mediating cell-cell adhesion and signals that promote phenotypic osteoblast gene expression. Furthermore, cadherins can interact with Wnt signalling to modulate osteoblastogenesis. One mechanism involves the interaction of N-cadherin with β-catenin at the cell membrane, resulting in β-catenin sequestration, reduction of the cytosolic β-catenin pool and inhibition of Wnt signalling. In addition to modulating the β-catenin pool, N-cadherin can regulate osteoblasts by interacting with the Wnt coreceptors LRP5 or LRP6. We showed that the functional interaction between N-cadherin and LRP5/6 in osteoblasts promotes β-catenin degradation and reduces canonical Wnt signalling. This crosstalk between N-cadherin and Wnt signalling has a negative impact on osteoblast proliferation, differentiation and survival, independently of cell-cell adhesion, which results in decreased bone formation and delayed bone accrual in mice. The identification of this crosstalk between N-cadherin and Wnt signalling may have therapeutic implications, as a disruption of the N-cadherin-LRP5/6 interaction using a competitor peptide can increase Wnt/β-catenin signalling without affecting cell-cell adhesion, and this effect results in increased osteoblastogenesis and bone tissue formation in vivo. In this review, we summarize our current knowledge of the key crosstalks between cadherins and Wnt signalling that impact osteoblast function, bone formation and bone mass, and the possible therapeutic implications of such interactions for promoting osteoblastogenesis, bone formation and bone mass.

Role and regulation of vascularization processes in endochondral bones

Adequate vascularization is an absolute requirement for bone development, growth, homeostasis, and repair. Endochondral ossification during fetal skeletogenesis is typified by the initial formation of a prefiguring cartilage template of the future bone, which itself is intrinsically avascular. When the chondrocytes reach terminal hypertrophic differentiation they become invaded by blood vessels. This neovascularization process triggers the progressive replacement of the growing cartilage by bone, in a complex multistep process that involves the coordinated activity of chondrocytes, osteoblasts, and osteoclasts, each standing in functional interaction with the vascular system. Studies using genetically modified mice have started to shed light on the molecular regulation of the cartilage neovascularization processes that drive endochondral bone development, growth, and repair, with a prime role being played by vascular endothelial growth factor and its isoforms. The vasculature of bone remains important throughout life as an intrinsic component of the bone and marrow environment. Bone remodeling, the continual renewal of bone by the balanced activities of osteoclasts resorbing packets of bone and osteoblasts building new bone, takes place in close spatial relationship with the vascular system and depends on signals, oxygen, and cellular delivery via the bloodstream. Conversely, the integrity and functionality of the vessel system, including the exchange of blood cells between the hematopoietic marrow and the circulation, rely on a delicate interplay with the cells of bone. Here, the current knowledge on the cellular relationships and molecular crosstalk that coordinate skeletal vascularization in bone development and homeostasis will be reviewed.

Adhesion in the stem cell niche: biological roles and regulation

Stem cell self-renewal is tightly controlled by the concerted action of stem cell-intrinsic factors and signals within the niche. Niche signals often function within a short range, allowing cells in the niche to self-renew while their daughters outside the niche differentiate. Thus, in order for stem cells to continuously self-renew, they are often anchored in the niche via adhesion molecules. In addition to niche anchoring, however, recent studies have revealed other important roles for adhesion molecules in the regulation of stem cell function, and it is clear that stem cell-niche adhesion is crucial for stem cell self-renewal and is dynamically regulated. Here, we highlight recent progress in understanding adhesion between stem cells and their niche and how this adhesion is regulated.

Redox regulation of stem/progenitor cells and bone marrow niche

Bone marrow (BM)-derived stem and progenitor cell functions including self-renewal, differentiation, survival, migration, proliferation, and mobilization are regulated by unique cell-intrinsic and -extrinsic signals provided by their microenvironment, also termed the "niche." Reactive oxygen species (ROS), especially hydrogen peroxide (H(2)O(2)), play important roles in regulating stem and progenitor cell functions in various physiologic and pathologic responses. The low level of H(2)O(2) in quiescent hematopoietic stem cells (HSCs) contributes to maintaining their "stemness," whereas a higher level of H(2)O(2) within HSCs or their niche promotes differentiation, proliferation, migration, and survival of HSCs or stem/progenitor cells. Major sources of ROS are NADPH oxidase and mitochondria. In response to ischemic injury, ROS derived from NADPH oxidase are increased in the BM microenvironment, which is required for hypoxia and hypoxia-inducible factor-1α expression and expansion throughout the BM. This, in turn, promotes progenitor cell expansion and mobilization from BM, leading to reparative neovascularization and tissue repair. In pathophysiological states such as aging, atherosclerosis, heart failure, hypertension, and diabetes, excess amounts of ROS create an inflammatory and oxidative microenvironment, which induces cell damage and apoptosis of stem and progenitor cells. Understanding the molecular mechanisms of how ROS regulate the functions of stem and progenitor cells and their niche in physiological and pathological conditions will lead to the development of novel therapeutic strategies.

N-cadherin+ HSCs in fetal liver exhibit higher long-term bone marrow reconstitution activity than N-cadherin- HSCs

Adult hematopoietic stem cells (HSCs) are maintained in a microenvironment known as the stem cell niche. The regulation of HSCs in fetal liver (FL) and their niche, however, remains to be elucidated. In this study, we investigated the role of N-cadherin (N-cad) in the maintenance of HSCs during FL hematopoiesis. By using anti-N-cad antibodies (Abs) produced by our laboratory, we detected high N-cad expression in embryonic day 12.5 (E12.5) mouse FL HSCs, but not in E15.5 and E18.5 FL. Immunofluorescence staining revealed that N-cad(+)c-Kit(+) and N-cad(+) endothelial protein C receptor (EPCR)(+) HSCs co-localized with Lyve-1(+) sinusoidal endothelial cells (ECs) in E12.5 FL and that some of these cells also expressed N-cad. However, N-cad(+) HSCs were also observed to detach from the perisinusoidal niche at E15.5 and E18.5, concomitant with a down-regulation of N-cad and an up-regulation of E-cadherin (E-cad) in hepatic cells. Moreover, EPCR(+) long-term (LT)-HSCs were enriched in the N-cad(+)Lin(-)Sca-1(+)c-Kit(+) (LSK) fraction in E12.5 FL, but not in E15.5 or E18.5 FL. In a long-term reconstitution (LTR) activity assay, higher engraftment associated with N-cad(+) LSK cells versus N-cad(-) LSK cells in E12.5 FL when transplanted into lethally irradiated recipient mice. However, the higher engraftment of N-cad(+) LSK cells decreased subsequently in E15.5 and E18.5 FL. It is possible that N-cad expression conferred higher LTR activity to HSCs by facilitating interactions with the perisinusoidal niche, especially at E12.5. The down-regulation of N-cad during FL hematopoiesis may help us better understand the regulation and mobility of HSCs before migration into BM.

N(o)-cadherin role for HSCs

In this issue of Blood, Bromberg et al and Greenbaum et al delete the gene encoding the cell adhesion molecule N-cadherin in osteoblastic cells forming the hematopoietic stem cell (HSC) niche to demonstrate that N-cadherin has no role in regulating hematopoiesis.

Getting blood from bone: an emerging understanding of the role that osteoblasts play in regulating hematopoietic stem cells within their niche

Blood and bone are dynamic tissues that are continuously renewed throughout life. Early observations based upon the proximity of bone and hematopoietic progenitor populations in marrow suggested that interactions between skeletal and hematopoietic elements are likely to be crucial in the development and function of each system. As a result of these morphologic observations, several groups have demonstrated that the osteoblasts play an important role in hematopoiesis by serving as a specific local microenvironment, or niche, for hematopoietic stem cells. Significant new developments in this area of active investigation have emerged since our last examination of this area in 2005. Here we discuss these new insights into the function and morphology of the hematopoietic stem cell niche, with a particular focus on cells of the osteoblastic lineage.

The regulation of normal and leukemic hematopoietic stem cells by niches

The origin and propagation of normal and leukemic hematopoietic cells critically depend on their interplays with the hematopoietic microenvironment (or so-called niche), which represent important biological models for understanding organogenesis and tumorigenesis. Nevertheless, the anatomic and functional characterizations of the niche cells for normal hematopoietic stem cells (HSCs) have proved a formidable task. It is uncertain whether the combinational effects of a few sets of molecular niche elements, behind the long-sought cellular architectures with preferred anatomic locations, actually meets the functional definition of HSC niche. Moreover, even much less is known about the niche components for numerous types of leukemia-stem cells (LSCs) that originate via discrete cellular and molecular transforming mechanisms. However, one interesting scenario is emerging, i.e., the leukemia cells can positively remodel the hematopoietic microenvironment favorable for their competition over the normal hematopoiesis that co-exists within the same eco-system. This property probably represents a previously unappreciated essential trait of a functional LSC. Obviously, the further exploration into how the hematopoietic microenvironment interplay with normal or malignant hematopoiesis will shed light onto the designing of novel types of niche-targeting therapies for leukemia.

Osteoblastic N-cadherin is not required for microenvironmental support and regulation of hematopoietic stem and progenitor cells

Hematopoietic stem cell (HSC) regulation is highly dependent on interactions with the marrow microenvironment. Controversy exists on N-cadherin's role in support of HSCs. Specifically, it is unknown whether microenvironmental N-cadherin is required for normal marrow microarchitecture and for hematopoiesis. To determine whether osteoblastic N-cadherin is required for HSC regulation, we used a genetic murine model in which deletion of Cdh2, the gene encoding N-cadherin, has been targeted to cells of the osteoblastic lineage. Targeted deletion of N-cadherin resulted in an age-dependent bone phenotype, ultimately characterized by decreased mineralized bone, but no difference in steady-state HSC numbers or function at any time tested, and normal recovery from myeloablative injury. Intermittent parathyroid hormone (PTH) treatment is well established as anabolic to bone and to increase marrow HSCs through microenvironmental interactions. Lack of osteoblastic N-cadherin did not block the bone anabolic or the HSC effects of PTH treatment. This report demonstrates that osteoblastic N-cadherin is not required for regulation of steady-state hematopoiesis, HSC response to myeloablation, or for rapid expansion of HSCs through intermittent treatment with PTH.