Linking hematopoiesis to endochondral skeletogenesis through analysis of mice transgenic for collagen X

How cell migration helps immune sentinels

The immune system relies on the migratory capacity of its cellular components, which must be mobile in order to defend the host from invading micro-organisms or malignant cells. This applies in particular to immune sentinels from the myeloid lineage, i.e. macrophages and dendritic cells. Cell migration is already at work during mammalian early development, when myeloid cell precursors migrate from the yolk sac, an extra embryonic structure, to colonize tissues and form the pool of tissue-resident macrophages. Later, this is accompanied by a migration wave of precursors and monocytes from the bone marrow to secondary lymphoid organs and the peripheral tissues. They differentiate into DCs and monocyte-derived macrophages. During adult life, cell migration endows immune cells with the ability to patrol their environment as well as to circulate between peripheral tissues and lymphoid organs. Hence migration of immune cells is key to building an efficient defense system for an organism. In this review, we will describe how cell migratory capacity regulates the various stages in the life of myeloid cells from development to tissue patrolling, and migration to lymph nodes. We will focus on the role of the actin cytoskeletal machinery and its regulators, and how it contributes to the establishment and function of the immune system.

Transcriptomics of Wet Skin Biopsies Predict Early Radiation-Induced Hematological Damage in a Mouse Model

The lack of an easy and fast radiation-exposure testing method with a dosimetric ability complicates triage and treatment in response to a nuclear detonation, radioactive material release, or clandestine exposure. The potential of transcriptomics in radiation diagnosis and prognosis were assessed here using wet skin (blood/skin) biopsies obtained at hour 2 and days 4, 7, 21, and 28 from a mouse radiation model. Analysis of significantly differentially transcribed genes (SDTG; p ≤ 0.05 and FC ≥ 2) during the first post-exposure week identified the glycoprotein 6 (GP-VI) signaling, the dendritic cell maturation, and the intrinsic prothrombin activation pathways as the top modulated pathways with stable inactivation after lethal exposures (20 Gy) and intermittent activation after sublethal (1, 3, 6 Gy) exposure time points (TPs). Interestingly, these pathways were inactivated in the late TPs after sublethal exposure in concordance with a delayed deleterious effect. Modulated transcription of a variety of collagen types, laminin, and peptidase genes underlay the modulated functions of these hematologically important pathways. Several other SDTGs related to platelet and leukocyte development and functions were identified. These results outlined genetic determinants that were crucial to clinically documented radiation-induced hematological and skin damage with potential countermeasure applications.

Mesenchymal lineage cells and their importance in B lymphocyte niches

Early B lymphopoiesis occurs in the bone marrow and is reliant on interactions with numerous cell types in the bone marrow microenvironment, particularly those of the mesenchymal lineage. Each cellular niche that supports the distinct stages of B lymphopoiesis is unique. Different cell types and signaling molecules are important for the progressive stages of B lymphocyte differentiation. Cells expressing CXCL12 and IL-7 have long been recognized as having essential roles in facilitating progression through stages of B lymphopoiesis. Recently, a number of other factors that extrinsically mediate B lymphopoiesis (positively or negatively) have been identified. In addition, the use of transgenic mouse models to delete specific genes in mesenchymal lineage cells has further contributed to our understanding of how B lymphopoiesis is regulated in the bone marrow. This review will cover the current understanding of B lymphocyte niches in the bone marrow and key extrinsic molecules and signaling pathways involved in these niches, with a focus on how mesenchymal lineage cells regulate B lymphopoiesis.

Schmid's Type of Metaphyseal Chondrodysplasia: Diagnosis and Management

OBJECTIVES

There are several types of metaphyseal chondrodysplasia and various clinical types have been differentiated. The Schmid type of metaphyseal chondrodysplasia is the most common. Diffuse metaphyseal flaring, irregularity, and growth plate widening, which are most severe in the knees, are the most striking radiological features of this disease. The Schmid type of metaphyseal dysostosis is characterized by failure of normal mineralization of the zone of provisional calcification, leading to widened physes and enlarged knobby metaphyses, effectively causing shortening of the tubular bones, splaying of the metaphyses, coxa vara, and bow legs. Orthopaedic interventions were primarily performed on the lower extremities.

METHODS

Twelve children (seven girls and five boys) aged 7-10 years were enrolled in this study. Moderate short stature was a uniform feature associated with predominant involvement of the proximal femora and bow legs resulted in the development of angular deformities. A waddling gait was a consequence of coxa vara in eight children. Valgus osteotomy of the proximal femur was planned after physeal closure for the group of children with coxa vara. Hemiepiphysiodesis was performed to re-align the genu varum in three children.

RESULTS

Other forms of metaphyseal dysostosis were ruled based on full clinical and radiographic phenotypes, with confirmation through molecular pathology. Mutations in the COL10A1 gene located on chromosome 6q21-q22.3 were confirmed. Re-alignment was accomplished in our group of patients.

CONCLUSION

The most striking clinical features of Schmid metaphyseal chondrodysplasia which appear within the first 2-3 years of life are: moderate short limbs and short stature, a waddling gait, and increasing shortness of stature with age. The Schmid type of metaphyseal chondrodysplasia is a disorder that arises from defective type X collagen, which is typically found in the hypertrophic zone of the physes. Moderate short stature and a waddling gait associated with pain are the most common clinical presentations. Osteotomies to correct bow legs are sometimes combined with lengthening procedures. Recurrence of the deformities with growth is not uncommon; therefore, hemiepiphysiodesis or stapling might be indicated in some cases.

Col11a1 Regulates Bone Microarchitecture during Embryonic Development

Collagen XI alpha 1 (Col11a1) is an extracellular matrix molecule required for embryonic development with a role in both nucleating the formation of fibrils and regulating the diameter of heterotypic fibrils during collagen fibrillar assembly. Although found in many different tissues throughout the vertebrate body, Col11a1 plays an essential role in endochondral ossification. To further understand the function of Col11a1 in the process of bone formation, we compared skeletal mineralization in wild-type (WT) mice and Col11a1-deficient mice using X-ray microtomography (micro-CT) and histology. Changes in trabecular bone microstructure were observed and are presented here. Additionally, changes to the periosteal bone collar of developing long bones were observed and resulted in an increase in thickness in the case of Col11a1-deficient mice compared to WT littermates. Vertebral bodies were incompletely formed in the absence of Col11a1. The data demonstrate that Col11a1 depletion results in alteration to newly-formed bone and is consistent with a role for Col11a1 in mineralization. These findings indicate that expression of Col11a1 in the growth plate and perichondrium is essential for trabecular bone and bone collar formation during endochondral ossification. The observed changes to mineralized tissues further define the function of Col11a1.

Novel insights into the function and dynamics of extracellular matrix in liver fibrosis

Emerging evidence suggests that altered components and posttranslational modifications of proteins in the extracellular matrix (ECM) may both initiate and drive disease progression. The ECM is a complex grid consisting of multiple proteins, most of which play a vital role in containing the essential information needed for maintenance of a sophisticated structure anchoring the cells and sustaining normal function of tissues. Therefore, the matrix itself may be considered as a paracrine/endocrine entity, with more complex functions than previously appreciated. The aims of this review are to 1) explore key structural and functional components of the ECM as exemplified by monogenetic disorders leading to severe pathologies, 2) discuss selected pathological posttranslational modifications of ECM proteins resulting in altered functional (signaling) properties from the original structural proteins, and 3) discuss how these findings support the novel concept that an increasing number of components of the ECM harbor signaling functions that can modulate fibrotic liver disease. The ECM entails functions in addition to anchoring cells and modulating their migratory behavior. Key ECM components and their posttranslational modifications often harbor multiple domains with different signaling potential, in particular when modified during inflammation or wound healing. This signaling by the ECM should be considered a paracrine/endocrine function, as it affects cell phenotype, function, fate, and finally tissue homeostasis. These properties should be exploited to establish novel biochemical markers and antifibrotic treatment strategies for liver fibrosis as well as other fibrotic diseases.

How to build an inducible cartilage-specific transgenic mouse

Transgenic mice are used to study the roles of specific proteins in an intact living system. Use of transgenic mice to study processes in cartilage, however, poses some challenges. First of all, many factors involved in cartilage homeostasis and disease are also crucial factors in embryogenesis. Therefore, meddling with these factors often leads to death before birth, and mice who do survive cannot be considered normal. The build-up of cartilage in these mice is altered, making it nearly impossible to truly interpret the role of a protein in adult cartilage function.An elegant way to overcome these limitations is to make transgenic mice time- and tissue-specific, there by omitting side-effects in tissues other than cartilage and during embryology. This review discusses the potential building blocks for making an inducible cartilage-specific transgenic mouse. We review which promoters can be used to gain chondrocyte-specificity - all chondrocytes or a specific subset thereof - as well as different systems that can be used to enable inducibility of a transgene.

Programmable 3D silk bone marrow niche for platelet generation ex vivo and modeling of megakaryopoiesis pathologies

We present a programmable bioengineered 3-dimensional silk-based bone marrow niche tissue system that successfully mimics the physiology of human bone marrow environment allowing us to manufacture functional human platelets ex vivo. Using stem/progenitor cells, megakaryocyte function and platelet generation were recorded in response to variations in extracellular matrix components, surface topography, stiffness, coculture with endothelial cells, and shear forces. Millions of human platelets were produced and showed to be functional based on multiple activation tests. Using adult hematopoietic progenitor cells our system demonstrated the ability to reproduce key steps of thrombopoiesis, including alterations observed in diseased states. A critical feature of the system is the use of natural silk protein biomaterial allowing us to leverage its biocompatibility, nonthrombogenic features, programmable mechanical properties, and surface binding of cytokines, extracellular matrix components, and endothelial-derived proteins. This in turn offers new opportunities for the study of blood component production ex vivo and provides a superior tissue system for the study of pathologic mechanisms of human platelet production.

Megakaryocytes contribute to the bone marrow-matrix environment by expressing fibronectin, type IV collagen, and laminin

Megakaryocytes associate with the bone marrow vasculature where they convert their cytoplasm into proplatelets that protrude through the vascular endothelium into the lumen and release platelets. The extracellular matrix (ECM) microenvironment plays a critical role in regulating these processes. In this work we demonstrate that, among bone marrow ECM components, fibronectin, type IV collagen, and laminin are the most abundant around bone marrow sinusoids and constitute a pericellular matrix surrounding megakaryocytes. Most importantly, we report, for the first time, that megakaryocytes express components of the basement membrane and that these molecules contribute to the regulation of megakaryocyte development and bone marrow ECM homeostasis both in vitro and in vivo. In vitro, fibronectin induced a threefold increase in the proliferation rate of mouse hematopoietic stem cells leading to higher megakaryocyte output with respect to cells treated only with thrombopoietin or other matrices. However, megakaryocyte ploidy level in fibronectin-treated cultures was significantly reduced. Stimulation with type IV collagen resulted in a 1.4-fold increase in megakaryocyte output, while all tested matrices supported proplatelet formation to a similar extent in megakaryocytes derived from fetal liver progenitor cells. In vivo, megakaryocyte expression of fibronectin and basement membrane components was upregulated during bone marrow reconstitution upon 5-fluorouracil induced myelosuppression, while only type IV collagen resulted upregulated upon induced thrombocytopenia. In conclusion, this work demonstrates that ECM components impact megakaryocyte behavior differently during their differentiation and highlights a new role for megakaryocyte as ECM-producing cells for the establishment of cell niches during bone marrow regeneration.

Roles of osteoclasts in the control of medullary hematopoietic niches

Bone marrow is the major site of hematopoiesis in mammals. The bone marrow environment plays an essential role in the regulation of hematopoietic stem and progenitor cells by providing specialized niches in which these cells are maintained. Many cell types participate to the composition and regulation of hematopoietic stem cell (HSC) niches, integrating complex signals from the bone, immune and nervous systems. Among these cells, the bone-resorbing osteoclasts (OCLs) have been described as main regulators of HSC niches. They are not limited to carving space for HSCs, but they also provide signals that affect the molecular and cellular niche components. However, their exact role in HSC niches remains unclear because of the variety of models, signals and conditions used to address the question. The present review will discuss the importance of the implication of OCLs focusing on the formation of HSC niches, the maintenance of HSCs in these niches and the mobilization of HSCs from the bone marrow. It will underline the importance of OCLs in HSC niches.

The humerus of Eusthenopteron: a puzzling organization presaging the establishment of tetrapod limb bone marrow

Because of its close relationship to tetrapods, Eusthenopteron is an important taxon for understanding the establishment of the tetrapod body plan. Notably, it is one of the earliest sarcopterygians in which the humerus of the pectoral fin skeleton is preserved. The microanatomical and histological organization of this humerus provides important data for understanding the evolutionary steps that built up the distinctive architecture of tetrapod limb bones. Previous histological studies showed that Eusthenopteron's long-bone organization was established through typical tetrapod ossification modalities. Based on a three-dimensional reconstruction of the inner microstructure of Eusthenopteron's humerus, obtained from propagation phase-contrast X-ray synchrotron microtomography, we are now able to show that, despite ossification mechanisms and growth patterns similar to those of tetrapods, it also retains plesiomorphic characters such as a large medullary cavity, partly resulting from the perichondral ossification around a large cartilaginous bud as in actinopterygians. It also exhibits a distinctive tubular organization of bone-marrow processes. The connection between these processes and epiphyseal structures highlights their close functional relationship, suggesting that either bone marrow played a crucial role in the long-bone elongation processes or that trabecular bone resulting from the erosion of hypertrophied cartilage created a microenvironment for haematopoietic stem cell niches.

Defective endochondral ossification-derived matrix and bone cells alter the lymphopoietic niche in collagen X mouse models

Despite the appreciated interdependence of skeletal and hematopoietic development, the cell and matrix components of the hematopoietic niche remain to be fully defined. Utilizing mice with disrupted function of collagen X (ColX), a major hypertrophic cartilage matrix protein associated with endochondral ossification, our data identified a cytokine defect in trabecular bone cells at the chondro-osseous hematopoietic niche as a cause for aberrant B lymphopoiesis in these mice. Specifically, analysis of ColX transgenic and null mouse chondro-osseous regions via micro-computed tomography revealed an altered trabecular bone environment. Additionally, cocultures with hematopoietic and chondro-osseous cell types highlighted impaired hematopoietic support by ColX transgenic and null mouse derived trabecular bone cells. Further, cytokine arrays with conditioned media from the trabecular osteoblast cocultures suggested an aberrant hematopoietic cytokine milieu within the chondro-osseous niche of the ColX deficient mice. Accordingly, B lymphopoiesis was rescued in the ColX mouse derived trabecular osteoblast cocultures with interlukin-7, stem cell factor, and stromal derived factor-1 supplementation. Moreover, B cell development was restored in vivo after injections of interlukin-7. These data support our hypothesis that endrochondrally-derived trabecular bone cells and matrix constituents provide cytokine-rich niches for hematopoiesis. Furthermore, this study contributes to the emerging concept that niche defects may underlie certain immuno-osseous and hematopoietic disorders.

The migration of hematopoietic progenitors from the fetal liver to the fetal bone marrow: lessons learned and possible clinical applications

The ontogeny of hematopoietic stem cells (HSCs) is complex, with multiple sites of embryonic origin as well as several locations of expansion and maturation in the embryo and the adult. Hematopoietic progenitors (HPs) with diverse developmental potential are first found in the yolk sac, aorta-gonad-mesonephros region and placenta. These progenitors then colonize the fetal liver (FL), where they undergo expansion and maturation. HSCs from the FL colonize the fetal bone marrow (FBM), governed by a complex orchestration of transcription programs including migratory molecules with chemotactic activity, adhesion molecules, and molecules that modulate the extracellular matrix. Understanding the mechanisms that regulate the patterns of HSC migration between FL and FBM could improve the engraftment potential of embryonic stem cell-derived HPs, because these cells might display a migratory behavior more similar to early HPs than to adult HSCs. Understanding the changes in migratory behavior in the context of FL to FBM HSC migration could lead to new approaches in the treatment of blood malignancies. We will review the current knowledge in the field of FL to the FBM HSCs migration during development, focusing on changes in expression of molecules important for this process and exploring its clinical applications.

COL10A1 expression is elevated in diverse solid tumor types and is associated with tumor vasculature

AIM

The identification of molecular markers that are upregulated in multiple tumor types could lead to novel diagnostic and therapeutic strategies. The authors screened a panel of RNAs prepared from diverse tumors and tumor cell lines, and compared them with normal tissues and cultured somatic cell types, in order to identify candidate genes expressed in a broad spectrum of tumor types.

MATERIALS & METHODS

Gene expression microarray analysis was carried out on 128 individual tumor samples representing over 20 tumor types, 85 samples representing 31 diverse normal tissue types, 68 tumor cell lines and 97 diverse normal primary cell cultures. Genes were ranked for elevated expression across a large number and variety of tumors relative to normal tissues.

RESULTS & CONCLUSION

COL10A1 was identified as a gene with restricted expression in most normal tissues and elevated expression in many diverse tumor types. By contrast, COL10A1 expression was undetectable in the 68 tumor cell lines surveyed in this study. Immunofluorescence studies localized collagen, type X, α-1 (collagen X) staining to tumor vasculature in breast tumors, whereas the vasculature of normal breast tissue was either collagen X-negative or had markedly lower levels. The tumor microenvironment-specific expression of collagen X, together with its localization in the vasculature, may facilitate its use as a novel target for the diagnosis and treatment of diverse solid tumor types.

Association of cartilage-specific deletion of peroxisome proliferator-activated receptor γ with abnormal endochondral ossification and impaired cartilage growth and development in a murine model

OBJECTIVE

Long bones develop through the strictly regulated process of endochondral ossification within the growth plate, resulting in the replacement of cartilage by bone. Defects in this process can result in skeletal abnormalities and a predisposition to degenerative joint diseases such as osteoarthritis (OA). Studies suggest that activation of the transcription factor peroxisome proliferator-activated receptor γ (PPARγ) is an important therapeutic target in OA. To devise PPARγ-related therapies in OA, it is critical to identify the role of this transcription factor in cartilage biology. Therefore, this study sought to determine the in vivo role of PPARγ in endochondral ossification and cartilage development, using cartilage-specific PPARγ-knockout (KO) mice.

METHODS

Cartilage-specific PPARγ-KO mice were generated using the Cre/loxP system. Histomorphometric and immunohistochemical analyses were performed to assess the patterns of ossification, proliferation, differentiation, and hypertrophy of chondrocytes, skeletal organization, bone density, and calcium deposition in the KO mice.

RESULTS

PPARγ-KO mice exhibited reductions in body length, body weight, length of the long bones, skeletal growth, cellularity, bone density, calcium deposition, and trabecular bone thickness, abnormal organization of the growth plate, loss of columnar organization, shorter hypertrophic zones, and delayed primary and secondary ossification. Immunohistochemical analyses for Sox9, 5-bromo-2'-deoxyuridine, p57, type X collagen, and platelet endothelial cell adhesion molecule 1 revealed reductions in the differentiation, proliferation, and hypertrophy of chondrocytes and in vascularization of the growth plate in mutant mice. Isolated chondrocytes and cartilage explants from mutant mice showed aberrant expression of Sox9 and extracellular matrix markers, including aggrecan, type II collagen, and matrix metalloproteinase 13. In addition, chondrocytes from mutant mice exhibited enhanced phosphorylation of p38 and decreased expression of Indian hedgehog.

CONCLUSION

The presence of PPARγ is required for normal endochondral ossification and cartilage development in vivo.

Altered matrix at the chondro-osseous junction leads to defects in lymphopoiesis

The collagen X transgenic and null (ColX-Tg/KO) mice have revealed a link between endochondral ossification (EO) and hematopoiesis, and thus serve as model systems to study hematopoietic niches. The altered collagen X function in ColX-Tg/KO mice resulted not only in skeletal defects, which included changes in growth plate ultrastructure, altered localization of heparan sulfate proteoglycans (HSPG), and reduced trabecular bone, but also in hematopoietic defects, which included reduced B lymphocyte numbers throughout life without associated increases in B cell apoptosis. Consequently, the ColX-Tg/KO mice exhibited diminished in vitro and in vivo immune responses. Moreover, reduced expression of several hematopoietic and B lymphopoietic cytokines were measured from ColX-KO-derived hypertrophic chondrocyte and trabecular osteoblast cultures. Together, these data expand the current hematopoietic niche model by including the EO-derived extracellular matrix, for example, the collagen X/HSPG network, as well as the EO-derived hypertrophic chondrocytes and trabecular osteoblasts as hematopoietic signal mediating cells.

Growth-plate cartilage in chronic renal failure

Bone growth occurs in the growth-plate cartilage located at the ends of long bones. Changes in the architecture, abnormalities in matrix organization, reduction in protein staining and RNA expression of factors involved in cell signaling have been described in the growth-plate cartilage of nephrectomized animals. These changes can lead to a smaller growth plate associated with decrease in chondrocyte proliferation, delayed hypertrophy, and prolonged initiation of mineralization and vascular invasion. As a result, chronic renal failure can result in stunted body growth and skeletal deformities. Multiple etiologic factors can contribute to impaired bone growth in renal failure, including suboptimal nutrition, metabolic acidosis, and secondary hyperparathyroidism. Recent findings have also shown the tight connection between chondro/osteogenesis, hematopoiesis, and immunogenesis.

Congenic mice confirm that collagen X is required for proper hematopoietic development

The link between endochondral skeletal development and hematopoiesis in the marrow was established in the collagen X transgenic (Tg) and null (KO) mice. Disrupted function of collagen X, a major hypertrophic cartilage matrix protein, resulted in skeletal and hematopoietic defects in endochondrally derived tissues. Manifestation of the disease phenotype was variable, ranging from perinatal lethality in a subset of mice, to altered lymphopoiesis and impaired immunity in the surviving mice. To exclude contribution of strain specific modifiers to this variable manifestation of the skeleto-hematopoietic phenotype, C57Bl/6 and DBA/2J collagen X congenic lines were established. Comparable disease manifestations confirmed that the skeleto-hematopoietic alterations are an inherent outcome of disrupted collagen X function. Further, colony forming cell assays, complete blood count analysis, serum antibody ELISA, and organ outgrowth studies established altered lymphopoiesis in all collagen X Tg and KO mice and implicated opportunistic infection as a contributor to the severe disease phenotype. These data support a model where endochondral ossification-specific collagen X contributes to the establishment of a hematopoietic niche at the chondro-osseous junction.

Bone growth during rapamycin therapy in young rats

BACKGROUND

Rapamycin is an effective immunosuppressant widely used to maintain the renal allograft in pediatric patients. Linear growth may be adversely affected in young children since rapamycin has potent anti-proliferative and anti-angiogenic properties.

METHODS

Weanling three week old rats were given rapamycin at 2.5 mg/kg daily by gavage for 2 or 4 weeks and compared to a Control group given equivalent amount of saline. Morphometric measurements and biochemical determinations for serum calcium, phosphate, iPTH, urea nitrogen, creatinine and insulin-growth factor I (IGF-I) were obtained. Histomorphometric analysis of the growth plate cartilage, in-situ hybridization experiments and immunohistochemical studies for various proteins were performed to evaluate for chondrocyte proliferation, chondrocyte differentiation and chondro/osteoclastic resorption.

RESULTS

At the end of the 2 weeks, body and tibia length measurements were shorter after rapamycin therapy associated with an enlargement of the hypertrophic zone in the growth plate cartilage. There was a decrease in chondrocyte proliferation assessed by histone-4 and mammalian target of rapamycin (mTOR) expression. A reduction in parathyroid hormone/parathyroid hormone related peptide (PTH/PTHrP) and an increase in Indian hedgehog (Ihh) expression may explain in part, the increase number of hypertrophic chondrocytes. The number of TRAP positive multinucleated chondro/osteoclasts declined in the chondro-osseous junction with a decrease in the receptor activator of nuclear factor kappa beta ligand (RANKL) and vascular endothelial growth factor (VEGF) expression. Although body and tibial length remained short after 4 weeks of rapamycin, changes in the expression of chondrocyte proliferation, chondrocyte differentiation and chondro/osteoclastic resorption which were significant after 2 weeks of rapamycin improved at the end of 4 weeks.

CONCLUSION

When given to young rats, 2 weeks of rapamycin significantly decreased endochondral bone growth. No catch-up growth was demonstrated at the end of 4 weeks, although markers of chondrocyte proliferation and differentiation improved. Clinical studies need to be done to evaluate these changes in growing children.

Heparan sulfate proteoglycans: a GAGgle of skeletal-hematopoietic regulators

This review summarizes our current understanding of the presence and function of heparan sulfate proteoglycans (HSPGs) in skeletal development and hematopoiesis. Although proteoglycans (PGs) comprise a large and diverse group of cell surface and matrix molecules, we chose to focus on HSPGs owing to their many proposed functions in skeletogenesis and hematopoiesis. Specifically, we discuss how HSPGs play predominant roles in establishing and regulating niches during skeleto-hematopoietic development by participating in distinct developmental processes such as patterning, compartmentalization, growth, differentiation, and maintenance of tissues. Special emphasis is placed on our novel hypothesis that mechanistically links endochondral skeletogenesis to the establishment of the hematopoietic stem cell (HSC) niche in the marrow. HSPGs may contribute to these developmental processes through their unique abilities to establish and mediate morphogen, growth factor, and cytokine gradients; facilitate signaling; provide structural stability to tissues; and act as molecular filters and barriers.

Altered endochondral ossification in collagen X mouse models leads to impaired immune responses

Disruption of collagen X function in hypertrophic cartilage undergoing endochondral ossification was previously linked to altered hematopoiesis in collagen X transgenic (Tg) and null (KO) mice (Jacenko et al., [2002] Am J Pathol 160:2019-2034). Mice displayed altered growth plates, diminished trabecular bone, and marrow hypoplasia with an aberrant lymphocyte profile throughout life. This study identifies altered B220+, CD4+, and CD8+ lymphocyte numbers, as well as CD4+/fox3P+ T regulatory cells in the collagen X mice. Additionally, diminished in vitro splenocyte responses to mitogens and an inability of mice to survive a challenge with Toxoplasma gondii, confirm impaired immune responses. In concert, ELISA and protein arrays identify aberrant levels of inflammatory, chemo-attractant, and matrix binding cytokines in collagen X mouse sera. These data link the disruption of collagen X function in the chondro-osseous junction to an altered hematopoietic stem cell niche in the marrow, resulting in impaired immune function.

Deconstructing the hematopoietic stem cell niche: revealing the therapeutic potential

Development of the hematopoietic system is a stage-specific process where the bone marrow eventually becomes the principal source of hematopoiesis in the adult mammalian organism. Sustained hematopoiesis in the bone marrow, however, depends on the self-renewal of the resident hematopoietic stem cells (HSCs). The region where these HSCs are hypothesized to self renew is called the stem cell ‘niche.’ Recent studies have identified components of the HSC niche in the bone marrow, including cells of the osteoblastic lineage, extracellular matrix molecules and molecular signaling interactions between the stem cells and niche cells. Specific pharmacological targeting of these niche components has led to beneficial HSC effects, demonstrating a new therapeutic approach where stem cell function is altered through targeting of the niche.

Exploitation of stem cell homing for gene delivery

Stem cells have been the focus of numerous investigations to treat diseases as far ranging as diabetes, chronic heart failure and multiple sclerosis over the past decade. The process of stem-cell-based repair of acute injury involves homing and engrafting of the stem cell of interest to the site of injury followed by either differentiation of the stem cell to indigenous end-organ cells or liberation of paracrine factors that lead to preservation and/or optimization of organ function. Recognition of the ability of stem cells to home to sites of acute injury suggests that, if appropriately defined and harnessed, stem cell homing could serve as a means of local drug delivery through the infusion of genetically engineering stem cells that secrete gene products of interest. The authors have recently demonstrated the use of this approach in preclinical studies of acute myocardial function. In addition, the use of engineered cells that home to appropriate niches have been used to correct genetic deficiency states (i.e., severe combined immunodeficiency, diabetes mellitus) in patients with otherwise chronic debilitating diseases. This review focuses on exploiting stem cell homing for gene transfer and on the state of the art and the challenges that face the field.

Regulation of hematopoietic niches by sympathetic innervation

Once hematopoiesis is established in the bone marrow, a continuous egress of hematopoietic stem cells (HSCs) to the periphery occurs at a low frequency. It has been proposed that this phenomenon is part of a regenerative homeostatic mechanism that ensures the maintenance of hematopoiesis through the life of the individual. The administration of certain cytotoxic drugs or cytokines can enhance the mobilization of hematopoietic progenitors to the periphery. During the past 15 years, granulocyte-colony stimulating factor (G-CSF) has been used as a standard cytokine for mobilization protocols in experimental models and in humans. Despite extensive efforts by multiple groups, a definitive mechanism explaining its role in mobilization has not been provided. In a recent paper, Katayama et al., through a series of clever associations supported by well-defined experimental systems, proposed that signals through the sympathetic nervous system modify the activity of the hematopoietic niche, acting as regulators of the mobilization of hematopoietic progenitors. This surprising finding adds a new level of complexity to the cellular milieu responsible for generation and maintenance of the hematopoietic niche.

The discoidin domain receptor DDR2 is a receptor for type X collagen

During endochondral ossification, collagen X is deposited in the hypertrophic zone of the growth plate. Our previous results have shown that collagen X is capable of interacting directly with chondrocytes, primarily via integrin alpha2beta1. In this study, we determined whether collagen X could also interact with the non-integrin collagen receptors, discoidin domain receptors (DDRs), DDR1 or DDR2. The widely expressed DDRs are receptor tyrosine kinases that are activated by a number of different collagen types. Collagen X was found to be a much better ligand for DDR2 than for DDR1. Collagen X bound to the DDR2 extracellular domain with high affinity and stimulated DDR2 autophosphorylation, the first step in transmembrane signalling. Expression of DDR2 in the epiphyseal plate was confirmed by RT-PCR and immunohistochemistry. The spatial expression of DDR2 in the hypertrophic zone of the growth plate is consistent with a physiological interaction of DDR2 with collagen X. Surprisingly, the discoidin domain of DDR2, which fully contains the binding sites for the fibrillar collagens I and II, was not sufficient for collagen X binding. The nature of the DDR2 binding site(s) within collagen X was further analysed. In addition to a collagenous domain, collagen X contains a C-terminal NC1 domain. DDR2 was found to recognise the triple-helical region of collagen X as well as the NC1 domain. Binding to the collagenous region was dependent on the triple-helical conformation. DDR2 autophosphorylation was induced by the collagen X triple-helical region but not the NC1 domain, indicating that the triple-helical region of collagen X contains a specific DDR2 binding site that is capable of receptor activation. Our study is the first to describe a non-fibrillar collagen ligand for DDR2 and will form the basis for further studies into the biological function of collagen X during endochondral ossification.

Skeletal development, bone remodeling, and hematopoiesis

In adult mammals, the bone marrow microenvironment is defined by close interactions between cells derived from mesenchymal progenitors and cells derived from hematopoietic progenitors. The influence that one population of cells has over the other has been a matter of intense study since it was established that hematopoietic stem cells (HSCs) require support of stromal elements to engraft, self-renew, and progress towards lineage commitment. Within the stromal components, cells of the osteoblastic lineage have the ability to interact with HSCs, and it has been proposed that they could be one of the main cell types responsible for the generation and maintenance of hematopoietic niches. Possible molecular mechanisms involved in the interaction between osteoblastic and hematopoietic cells have been described. However, understanding the relative importance of each one of them, their production by defined cells, and their kinetics of appearance have been limited by the lack of in vivo models allowing the physical and/or temporal dissection of the components of the osteoblastic lineage. Here, we provide a summary of the evidence that have established the importance of osteoblasts in hematopoiesis, and we propose new experimental strategies that could help to define the nature of these interactions.

Stem cell engraftment at the endosteal niche is specified by the calcium-sensing receptor

During mammalian ontogeny, haematopoietic stem cells (HSCs) translocate from the fetal liver to the bone marrow, where haematopoiesis occurs throughout adulthood. Unique features of bone that contribute to a microenvironmental niche for stem cells might include the known high concentration of calcium ions at the HSC-enriched endosteal surface. Cells respond to extracellular ionic calcium concentrations through the seven-transmembrane-spanning calcium-sensing receptor (CaR), which we identified as being expressed on HSCs. Here we show that, through the CaR, the simple ionic mineral content of the niche may dictate the preferential localization of adult mammalian haematopoiesis in bone. Antenatal mice deficient in CaR had primitive haematopoietic cells in the circulation and spleen, whereas few were found in bone marrow. CaR-/- HSCs from fetal liver were normal in number, in proliferative and differentiative function, and in migration and homing to the bone marrow. Yet they were highly defective in localizing anatomically to the endosteal niche, behaviour that correlated with defective adhesion to the extracellular matrix protein, collagen I. CaR has a function in retaining HSCs in close physical proximity to the endosteal surface and the regulatory niche components associated with it.

Misfolding of collagen X chains harboring Schmid metaphyseal chondrodysplasia mutations results in aberrant disulfide bond formation, intracellular retention, and activation of the unfolded protein response

Collagen X is a short chain collagen expressed specifically by the hypertrophic chondrocytes of the cartilage growth plate during endochondral bone formation. Accordingly, COL10A1 mutations disrupt growth plate function and cause Schmid metaphyseal chondrodysplasia (SMCD). SMCD mutations are almost exclusively located in the NC1 domain, which is crucial for both trimer formation and extracellular assembly. Several mutations are expected to reduce the level of functional collagen X due to NC1 domain misfolding or exclusion from stable trimer formation. However, other mutations may be tolerated within the structure of the assembled NC1 trimer, allowing mutant chains to exert a dominant-negative impact within the extracellular matrix. To address this, we engineered SMCD mutations that are predicted either to prohibit subunit folding and assembly (NC1del10 and Y598D, respectively) or to allow trimerization (N617K and G618V) and transfected these constructs into 293-EBNA and SaOS-2 cells. Although expected to form stable trimers, G618V and N617K chains (like Y598D and NC1del10 chains) were secreted very poorly compared with wild-type collagen X. Interestingly, all mutations resulted in formation of an unusual SDS-stable dimer, which dissociated upon reduction. As the NC1 domain sulfhydryl group is not solvent-exposed in the correctly folded NC1 monomer, disulfide bond formation would result only from a dramatic conformational change. In cells expressing mutant collagen X, we detected significantly increased amounts of the spliced form of X-box DNA-binding protein mRNA and up-regulation of BiP, two key markers for the unfolded protein response. Our data provide the first clear evidence for misfolding of SMCD collagen X mutants, and we propose that solvent exposure of the NC1 thiol may trigger the recognition and degradation of mutant collagen X chains.

Blood and bone: two tissues whose fates are intertwined to create the hematopoietic stem-cell niche

The mechanisms of bone and blood formation have traditionally been viewed as distinct, unrelated processes, but compelling evidence suggests that they are intertwined. Based on observations that hematopoietic precursors reside close to endosteal surfaces, it was hypothesized that osteoblasts play a central role in hematopoiesis, and it has been shown that osteoblasts produce many factors essential for the survival, renewal, and maturation of hematopoietic stem cells (HSCs). Preceding these observations are studies demonstrating that the disruption or perturbation of normal osteoblastic function has a profound and central role in defining the operational structure of the HSC niche. These observations provide a glimpse of the dimensions and ramifications of HSC-osteoblast interactions. Although more research is required to secure a broader grasp of the molecular mechanisms that govern blood and bone biology, the central role for osteoblasts in hematopoietic stem cell regulation is reviewed herein from the perspectives of (1) historical context; (2) the role of the osteoblast in supporting stem cell survival, proliferation, and maintenance; (3) the participation, if any, of osteoblasts in the creation of a stem cell niche; (4) the molecules that mediate HSC-osteoblast interactions; (5) the role of osteoblasts in stem cell transplantation; and (6) possible future directions for investigation.

Chicken collagen X regulatory sequences restrict transgene expression to hypertrophic cartilage in mice

Collagen X is produced by hypertrophic cartilage undergoing endochondral ossification. Transgenic mice expressing defective collagen X under the control of 4.7- or 1.6-kb chicken collagen X regulatory sequences yielded skeleto-hematopoietic defects (Jacenko O, LuValle P, Olsen BR: Spondylometaphyseal dysplasia in mice carrying a dominant-negative mutation in a matrix protein specific for cartilage-to-bone transition. Nature 1993, 365:56-61; Jacenko O, Chan D, Franklin A, Ito S, Underhill CB, Bateman JF, Campbell MR: A dominant interference collagen X mutation disrupts hypertrophic chondrocyte pericellular matrix and glycosaminoglycan and proteoglycan distribution in transgenic mice. Am J Pathol 2001, 159:2257-2269; Jacenko O, Roberts DW, Campbell MR, McManus PM, Gress CJ, Tao Z: Linking hematopoiesis to endochondral ossification through analysis of mice transgenic for collagen X. Am J Pathol 2002, 160:2019-2034). Current data indicate that the hematopoietic abnormalities do not result from extraskeletal expression of endogenous collagen X or the transgene. Organs from mice carrying either promoter were screened by immunohistochemistry, in situ hybridization, and Northern blot; transgene and mouse collagen X proteins and messages were detected only in hypertrophic cartilage. Likewise, reverse transcriptase-polymerase chain reaction revealed both transgene and mouse collagen X amplicons only in the endochondral skeleton of mice with the 4.7-kb promoter; however, in mice with the 1.6-kb promoter, multiple organs were transgene-positive. Collagen X and transgene amplicons were also detected in marrow, but likely resulted from contaminating trabecular bone; this was supported by reverse transcriptase-polymerase chain reaction analysis of rat tibial zones free of trabeculae. Our data demonstrate that in mice, the 4.7-kb chicken collagen X promoter restricts transcription temporo-spatially to that of endogenous collagen X, and imply that murine skeleto-hematopoietic defects result from transgene co-expression with collagen X. Moreover, the 4.7-kb hypertrophic cartilage-specific promoter could be used for targeting transgenes to this tissue site in mice.

Partial characterization of cell-type X collagen interactions

Type X collagen is a short-chain non-fibrillar collagen that is deposited exclusively at sites of new bone formation. Although this collagen has been implicated in chondrocyte hypertrophy and endochondral ossification, its precise function remains unclear. One possible function could be to regulate the processes of chondrocyte hypertrophy through direct cell-type X collagen interactions. Adhesions of embryonic chick chondrocytes, and cell lines with known expression of collagen-binding integrins (MG63 and HOS), were assayed on chick type X collagen substrates, including the native, heat-denatured and pepsin-digested collagen, and the isolated C-terminal non-collagenous (NC1) domain. Type X collagen supported the greatest level of adhesion for all cell types tested. The involvement of the alpha2beta1 integrin in type X collagen-cell interaction was demonstrated by adhesion studies in the presence of Mg(2+) and Ca(2+) ions and integrin-function-blocking antibodies. Cells expressing alpha2beta1 integrin (chick chondrocytes and MG63 cells) also adhered to heat-denatured type X collagen and the isolated NC1 domain; however, removal of the non-collagenous domains by limited pepsinization of type X collagen resulted in very low levels of adhesion. Both focal contacts and actin stress-fibre formation were apparent in cells plated on type X collagen. The presence of alpha2 and beta1 integrin subunits in isolated chondrocytes and epiphyseal cartilage was also confirmed by immunolocalization. Our results demonstrate, for the first time, that type X collagen is capable of interacting directly with chondrocytes and other cells, primarily via alpha2beta1 integrin. These findings are atypical from the fibrillar collagen-cell interactions via collagen binding integrins in that: (1) the triple-helical conformation is not strictly required for cell adhesion; (2) the NC1 domain is also involved in the adhesion of alpha2beta1-expressing cells. These data form the basis for further studies into the mechanism and biological significance of type X collagen deposition in the growth plate.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.