Growth plate compressions and altered hematopoiesis in collagen X null mice

The first report of the presence of collagen X in mammalian dentinal matrix

Collagen X is an extracellular matrix protein, usually found in the hypertrophic cartilage destined to be mineralized. It is intimately associated with the mineralization process of the mammalian hard tissues, and particularly, regulating the compartmentalization of matrix components. Despite the fact that the dentine of the tooth is highly mineralized, there are no previous reports to indicate the presence of collagen X in this connective tissue. Here we report, for the first time, its presence in mammalian dentine based on micromorphological and immunohistochemical data. We hypothesize that the collagen X in dentine may in the long term arrest the progression of the mineralization front towards the soft tissue components of the pulp that are not destined to be mineralized.

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.

Molecular Modulation of Fetal Liver Hematopoietic Stem Cell Mobilization into Fetal Bone Marrow in Mice

Development of hematopoietic stem cells is a complex process, which has been extensively investigated. Hematopoietic stem cells (HSCs) in mouse fetal liver are highly expanded to prepare for mobilization of HSCs into the fetal bone marrow. It is not completely known how the fetal liver niche regulates HSC expansion without loss of self-renewal ability. We reviewed current progress about the effects of fetal liver niche, chemokine, cytokine, and signaling pathways on HSC self-renewal, proliferation, and expansion. We discussed the molecular regulations of fetal HSC expansion in mouse and zebrafish. It is also unknown how HSCs from the fetal liver mobilize, circulate, and reside into the fetal bone marrow niche. We reviewed how extrinsic and intrinsic factors regulate mobilization of fetal liver HSCs into the fetal bone marrow, which provides tools to improve HSC engraftment efficiency during HSC transplantation. Understanding the regulation of fetal liver HSC mobilization into the fetal bone marrow will help us to design proper clinical therapeutic protocol for disease treatment like leukemia during pregnancy. We prospect that fetal cells, including hepatocytes and endothelial and hematopoietic cells, might regulate fetal liver HSC expansion. Components from vascular endothelial cells and bones might also modulate the lodging of fetal liver HSCs into the bone marrow. The current review holds great potential to deeply understand the molecular regulations of HSCs in the fetal liver and bone marrow in mammals, which will be helpful to efficiently expand HSCs in vitro.

Collagen: quantification, biomechanics, and role of minor subtypes in cartilage

Collagen is a ubiquitous biomaterial in vertebrate animals. Although each of its 28 subtypes contributes to the functions of many different tissues in the body, most studies on collagen or collagenous tissues have focussed on only one or two subtypes. With recent developments in analytical chemistry, especially mass spectrometry, significant advances have been made toward quantifying the different collagen subtypes in various tissues; however, high-throughput and low-cost methods for collagen subtype quantification do not yet exist. In this Review, we introduce the roles of collagen subtypes and crosslinks, and describe modern assays that enable a deep understanding of tissue physiology and disease states. Using cartilage as a model tissue, we describe the roles of major and minor collagen subtypes in detail; discuss known and unknown structure-function relationships; and show how tissue engineers may harness the functional characteristics of collagen to engineer robust neotissues.

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.

Phenotypic instability of chondrocytes in osteoarthritis: on a path to hypertrophy

Articular chondrocytes are quiescent, fully differentiated cells responsible for the homeostasis of adult articular cartilage by maintaining cellular survival functions and the fine-tuned balance between anabolic and catabolic functions. This balance requires phenotypic stability that is lost in osteoarthritis (OA), a disease that affects and involves all joint tissues and especially impacts articular cartilage structural integrity. In OA, articular chondrocytes respond to the accumulation of injurious biochemical and biomechanical insults by shifting toward a degradative and hypertrophy-like state, involving abnormal matrix production and increased aggrecanase and collagenase activities. Hypertrophy is a necessary, transient developmental stage in growth plate chondrocytes that culminates in bone formation; in OA, however, chondrocyte hypertrophy is catastrophic and it is believed to initiate and perpetuate a cascade of events that ultimately result in permanent cartilage damage. Emphasizing changes in DNA methylation status and alterations in NF-κB signaling in OA, this review summarizes the data from the literature highlighting the loss of phenotypic stability and the hypertrophic differentiation of OA chondrocytes as central contributing factors to OA pathogenesis.

Risk factors and biomarkers of age-related macular degeneration

A biomarker can be a substance or structure measured in body parts, fluids or products that can affect or predict disease incidence. As age-related macular degeneration (AMD) is the leading cause of blindness in the developed world, much research and effort has been invested in the identification of different biomarkers to predict disease incidence, identify at risk individuals, elucidate causative pathophysiological etiologies, guide screening, monitoring and treatment parameters, and predict disease outcomes. To date, a host of genetic, environmental, proteomic, and cellular targets have been identified as both risk factors and potential biomarkers for AMD. Despite this, their use has been confined to research settings and has not yet crossed into the clinical arena. A greater understanding of these factors and their use as potential biomarkers for AMD can guide future research and clinical practice. This article will discuss known risk factors and novel, potential biomarkers of AMD in addition to their application in both academic and clinical settings.

Extracellular matrix and developing growth plate

Growth plate is a specialized cartilaginous structure that mediates the longitudinal growth of skeletal bones. It consists of ordered zones of chondrocytes that secrete an extracellular matrix (ECM) composed of specific types of collagens and proteoglycans. Several heritable human skeletal dysplasias are caused by mutations in these ECM components and this review focuses on the roles of type II, IX, X, and XI collagens, aggrecan, matrilins, perlecan, and cartilage oligomeric matrix protein in the growth plate as deduced from human disease phenotypes and mouse models. Substantial advances have been achieved in deciphering the interaction networks and individual roles of these components in the construction of the growth plate ECM. Furthermore, ER stress and other cellular responses have been identified as key downstream effects of the ECM mutations contributing to abnormal growth plate development. The next challenge is to utilize the molecular level knowledge for the development of potential therapeutics.

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.

Depletion of annexin A5, annexin A6, and collagen X causes no gross changes in matrix vesicle-mediated mineralization, but lack of collagen X affects hematopoiesis and the Th1/Th2 response

Numerous biochemical studies have pointed to an essential role of annexin A5 (AnxA5), annexin A6 (AnxA6), and collagen X in matrix vesicle-mediated biomineralization during endochondral ossification and in osteoarthritis. By binding to the extracellular matrix protein collagen X and matrix vesicles, annexins were proposed to anchor matrix vesicles in the extracellular space of hypertrophic chondrocytes to initiate the calcification of cartilage. However, mineralization appears to be normal in mice lacking AnxA5 and AnxA6, whereas collagen X-deficient mice show only subtle alterations in the growth plate organization. We hypothesized that the simultaneous lack of AnxA5, AnxA6, and collagen X in vivo induces more pronounced changes in the growth plate development and the initiation of mineralization. In this study, we generated and analyzed mice deficient for AnxA5, AnxA6, and collagen X. Surprisingly, mice were viable, fertile, and showed no obvious abnormalities. Assessment of growth plate development indicated that the hypertrophic zone was expanded in Col10a1(-/-) and AnxA5(-/-) AnxA6(-/-) Col10a1(-/-) newborns, whereas endochondral ossification and mineralization were not affected in 13-day- and 1-month-old mutants. In peripheral quantitative computed tomography, no changes in the degree of biomineralization were found in femora of 1-month- and 1-year-old mutants even though the diaphyseal circumference was reduced in Col10a1(-/-) and AnxA5(-/-) AnxA6(-/-) Col10a1(-/-) mice. The percentage of naive immature IgM(+) /IgM(+) B cells and peripheral T-helper cells were increased in Col10a1(-/-) and AnxA5(-/-) AnxA6(-/-) Col10a1(-/-) mutants, and activated splenic T cells isolated from Col10a1(-/-) mice secreted elevated levels of IL-4 and GM-CSF. Hence, collagen X is needed for hematopoiesis during endochondral ossification and for the immune response, but the interaction of annexin A5, annexin A6, and collagen X is not essential for physiological calcification of growth plate cartilage. Therefore, annexins and collagen X may rather fulfill functions in growth plate cartilage not directly linked to the mineralization process.

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.

Differential expression of cartilage and bone-related proteins in pediatric and adult diseased aortic valves

Approximately 5 million people are affected with aortic valve disease (AoVD) in the United States. The most common treatment is aortic valve (AoV) replacement surgery, however, replacement valves are susceptible to failure, necessitating additional surgeries. The molecular mechanisms underlying disease progression and late AoV calcification are not well understood. Recent studies suggest that genes involved in bone and cartilage development play an active role in osteogenic-like calcification in human calcific AoVD (CAVD). In an effort to define the molecular pathways involved in AoVD progression and calcification, expression of markers of valve mesenchymal progenitors, chondrogenic precursors, and osteogenic differentiation was compared in pediatric non-calcified and adult calcified AoV specimens. Valvular interstitial cell (VIC) activation, extracellular matrix (ECM) disorganization, and markers of valve mesenchymal and skeletal chondrogenic progenitor cells were observed in both pediatric and adult AoVD. However, activated BMP signaling, increased expression of cartilage and bone-type collagens, and increased expression of the osteogenic marker Runx2 are observed in adult diseased AoVs. They are not observed in the majority of pediatric diseased valves, representing a marked distinction in the molecular profile between pediatric and adult diseased AoVs. The combined evidence suggests that an actively regulated osteochondrogenic disease process underlies the pathological changes affecting AoVD progression, ultimately resulting in stenotic AoVD. Both pediatric and adult diseased AoVs express protein markers of valve mesenchymal and chondrogenic progenitor cells while adult diseased AoVs also express proteins involved in osteogenic calcification. These findings provide specific molecular indicators of AoVD progression, which may lead to identification of early disease markers and the development of potential therapeutics.

Transcriptional regulation of endochondral ossification by HIF-2alpha during skeletal growth and osteoarthritis development

Chondrocyte hypertrophy followed by cartilage matrix degradation and vascular invasion, characterized by expression of type X collagen (COL10A1), matrix metalloproteinase-13 (MMP-13) and vascular endothelial growth factor (VEGF), respectively, are central steps of endochondral ossification during normal skeletal growth and osteoarthritis development. A COL10A1 promoter assay identified hypoxia-inducible factor-2alpha (HIF-2alpha, encoded by EPAS1) as the most potent transactivator of COL10A1. HIF-2alpha enhanced promoter activities of COL10A1, MMP13 and VEGFA through specific binding to the respective hypoxia-responsive elements. HIF-2alpha, independently of oxygen-dependent hydroxylation, was essential for endochondral ossification of cultured chondrocytes and embryonic skeletal growth in mice. HIF-2alpha expression was higher in osteoarthritic cartilages versus nondiseased cartilages of mice and humans. Epas1-heterozygous deficient mice showed resistance to osteoarthritis development, and a functional single nucleotide polymorphism (SNP) in the human EPAS1 gene was associated with knee osteoarthritis in a Japanese population. The EPAS1 promoter assay identified RELA, a nuclear factor-kappaB (NF-kappaB) family member, as a potent inducer of HIF-2alpha expression. Hence, HIF-2alpha is a central transactivator that targets several crucial genes for endochondral ossification and may represent a therapeutic target for osteoarthritis.

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.

Smurf2 induces degradation of GSK-3beta and upregulates beta-catenin in chondrocytes: a potential mechanism for Smurf2-induced degeneration of articular cartilage

We have previously demonstrated that Smurf2 is highly expressed in human osteoarthritis (OA) tissue, and overexpression of Smurf2 under the control of the type II collagen promoter (Col2a1) induces an OA-like phenotype in aged Col2a1-Smurf2 transgenic mice, suggesting that Smurf2 is located upstream of a signal cascade which initiates OA development. However, the factors downstream of Smurf2 in this signal cascade and how Smurf2-induced OA is initiated are largely unknown. In this study, we further characterized the phenotypic changes in Col2a1-Smurf2 transgenic and WT articular cartilage from the postnatal stage to adulthood. We found that the articular cartilage degeneration occurring at the cartilage surface in 6 month-old Col2a1-Smurf2 transgenic mice progressed from an expanded hypertrophic domain in the basal layer of the deep articular cartilage at 2.5 weeks of age, which may lead to an accelerated calcification and ectopic ossification of this region at 1 month of age, and aggregation and maturation of articular chondrocytes in the middle and deep zones at 2 months and 4.5 months of age, respectively. Furthermore, we discovered that ectopically expressed Smurf2 interacted with GSK-3beta and induced its ubiquitination and subsequent proteasomal degradation, and hence upregulated beta-catenin in Col2a1-Smurf2 transgenic chondrocytes ex vivo. It is therefore likely that Smurf2-mediated upregulation of beta-catenin through induction of proteasomal degradation of GSK-beta in chondrocytes may activate articular chondrocyte maturation and associated alteration of gene expression, the early events of OA.

Retinoic acid receptors are required for skeletal growth, matrix homeostasis and growth plate function in postnatal mouse

The retinoic acid receptors alpha, beta and gamma (RARalpha, RARbeta and RARgamma) are nuclear hormone receptors that regulate fundamental processes during embryogenesis, but their roles in skeletal development and growth remain unclear. To study skeletal-specific RAR function, we created conditional mouse mutants deficient in RAR expression in cartilage. We find that mice deficient in RARalpha and RARgamma (or RARbeta and RARgamma) exhibit severe growth retardation obvious by about 3 weeks postnatally. Their growth plates are defective and, importantly, display a major drop in aggrecan expression and content. Mice deficient in RARalpha and RARbeta, however, are virtually normal, suggesting that RARgamma is essential. In good correlation, we find that RARgamma is the most strongly expressed RAR in mouse growth plate and its expression characterizes the proliferative and pre-hypertrophic zones where aggrecan is strongly expressed also. By being avascular, those zones lack endogenous retinoids as indicated by previous RARE reporter mice and our direct biochemical measurements and thus, RARgamma is likely to exert ligand-less repressor function. Indeed, our data indicate that: aggrecan production is enhanced by RARgamma over-expression in chondrocytes under retinoid-free culture conditions; production is further boosted by co-repressor Zac1 or pharmacologic agents that enhance RAR repressor function; and RAR/Zac1 function on aggrecan expression may involve Sox proteins. In sum, our data reveal that RARs, and RARgamma in particular, exert previously unappreciated roles in growth plate function and skeletal growth and regulate aggrecan expression and content. Since aggrecan is critical for growth plate function, its deficiency in RAR-mutant mice is likely to have contributed directly to their growth retardation.

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.

Non-MHC ligands for inhibitory immune receptors: novel insights and implications for immune regulation

Regulation of cellular responses by inhibitory receptors is crucial for proper function of the immune system. The prototype inhibitory immune receptors are major histocompatibility complex (MHC) class I binding killer-Ig like receptors (KIRs) present on effector cells such as natural killer (NK) cells and effector T cells. However, the recent identification of non-MHC class I ligands for inhibitory immune receptors, such as KLRG1, KLRB1 and LAIR-1, indicates that also MHC class I-independent inhibitory immune receptors play crucial roles in inducing peripheral tolerance. The presence of these receptors on many other immune cell types besides effector cells suggests that tight regulation of cell activation is necessary in all facets of the immune response in both normal and diseased tissue. Here, we review novel insights and implications of non-MHC class I ligand binding to inhibitory immune receptors. We give an overview of the known ligand-receptor pairs by grouping the ligands according to their properties and discuss implications of these interactions for the maintenance of immune balance and for the defense against tumors and pathogens.

Agrin is highly expressed by chondrocytes and is required for normal growth

Agrin is a heparan sulfate proteoglycan that is best known for its crucial involvement in the organization and maintenance of postsynaptic structures at the neuromuscular junction. Consistent with this role, mice deficient of agrin die at birth due to respiratory failure. Here we examined the early postnatal development of agrin-deficient mice in which perinatal death was prevented by transgenic expression of neural agrin in motor neurons. Such transgenic, agrin-deficient mice were born at Mendelian ratio but exhibited severe postnatal growth retardation. Growth plate morpholgy was markedly altered in these mice, with changes being most prominent in the hypertrophic zone. Compression of this zone was not caused by reduced viability of hypertrophic chondrocytes, as no differences in the apoptosis rates could be observed. Furthermore, deposition of the major cartilage matrix components collagen type II and aggrecan was slightly reduced in these mice. Consistent with a role for agrin in skeletal development, we show for the first time that agrin is highly expressed by chondrocytes and localizes to the growth plate in wild-type mice. Our data show that agrin is expressed in cartilage and that it plays a critical role in normal skeletal growth.

Mutations of COL10A1 in Schmid metaphyseal chondrodysplasia

Schmid metaphyseal chondrodysplasia (SMCD) is a dominantly inherited cartilage disorder caused by mutations in the gene for the hypertrophic cartilage extracellular matrix structural protein, collagen X (COL10A1). Thirty heterozygous mutations have been described, about equally divided into two mutation types, missense mutations, and mutations that introduce premature termination signals. The COL10A1 mutations are clustered (33/36) in the 3' region of exon 3, which codes for the C-terminal NC1 trimerization domain. The effect of COL10A1 missense mutations have been examined by in vitro expression and assembly assays and cell transfection studies, which suggest that a common consequence is the disruption of collagen X trimerization and secretion, with consequent intracellular degradation. The effect of COL10A1 nonsense mutations in cartilage tissue has been examined in two patients, demonstrating that the mutant mRNA is completely removed by nonsense mediated mRNA decay. Thus for both classes of mutations, functional haploinsufficiency is the most probable cause of the clinical phenotype in SMCD.

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.

A novel role for GADD45beta as a mediator of MMP-13 gene expression during chondrocyte terminal differentiation

The growth arrest and DNA damage-inducible 45beta (GADD45beta) gene product has been implicated in the stress response, cell cycle arrest, and apoptosis. Here we demonstrated the unexpected expression of GADD45beta in the embryonic growth plate and uncovered its novel role as an essential mediator of matrix metalloproteinase-13 (MMP-13) expression during terminal chondrocyte differentiation. We identified GADD45beta as a prominent early response gene induced by bone morphogenetic protein-2 (BMP-2) through a Smad1/Runx2-dependent pathway. Because this pathway is involved in skeletal development, we examined mouse embryonic growth plates, and we observed expression of Gadd45beta mRNA coincident with Runx2 protein in pre-hypertrophic chondrocytes, whereas GADD45beta protein was localized prominently in the nucleus in late stage hypertrophic chondrocytes where Mmp-13 mRNA was expressed. In Gadd45beta(-/-) mouse embryos, defective mineralization and decreased bone growth accompanied deficient Mmp-13 and Col10a1 gene expression in the hypertrophic zone. Transduction of small interfering RNA-GADD45beta in epiphyseal chondrocytes in vitro blocked terminal differentiation and the associated expression of Mmp-13 and Col10a1 mRNA in vitro. Finally, GADD45beta stimulated MMP-13 promoter activity in chondrocytes through the JNK-mediated phosphorylation of JunD, partnered with Fra2, in synergy with Runx2. These observations indicated that GADD45beta plays an essential role during chondrocyte terminal differentiation.

A highly conserved enhancer in mammalian type X collagen genes drives high levels of tissue-specific expression in hypertrophic cartilage in vitro and in vivo

Previously we have identified a cis-acting regulatory domain in the human type X collagen gene upstream of the transcription start site which acts as a strong enhancer in hypertrophic, but not in resting chondrocytes. Here we show that this enhancer is highly conserved also in the murine and bovine Col10a1 genes, but not found in the known promoter sequences of chicken Col10a1. It contains a functionally active AP-1 site (TPA Responsive Element, TRE) which is essential for the high transcriptional activity of the COL10A1 enhancer in transiently transfected hypertrophic chondrocytes. Gel-shift experiments with nuclear extracts of hypertrophic chondrocytes revealed FosB and Fra-1 as candidates regulating AP-1 factors binding to the TRE site. In fact, coexpression of FosB and Fra-1 in reporter gene assays greatly stimulated transcriptional activity of enhancer bearing reporter genes. Quantitative analysis of AP-1 factor mRNA levels in distinct fractions of fetal bovine epiphyseal chondrocytes by real-time PCR confirmed significant levels of FosB and Fra-1 mRNA besides other AP-1 factors in hypertrophic chondrocytes. A key role of the enhancer element in regulating tissue-specific expression of the Col10a1 gene was shown by establishing transgenic mouse lines with a reporter gene containing a 4.6 kb murine Col10a1 promoter fragment which included the enhancer, exon 1, part of exon 2 and the first intron. Reporter gene expression was seen exclusively in hypertrophic cartilages in the growth plates of long bones, ribs, vertebrae, sternum and mandibles of 17.5-18.5 dpc embryos, confirming that the 4.6 kb promoter is able to drive specific expression of Col10a1 in hypertrophic cartilage. These established transgenic lines should facilitate the genetic analysis of regulatory pathways of chondrocyte maturation and Col10a1 gene expression in the future.

SP3/SP1 transcription activity regulates specific expression of collagen type X in hypertrophic chondrocytes

Previously, we have shown that two non-canonical specificity protein (SP)-binding sites within the proximal promoter (nucleotide (nt) -139 to +5) of the chicken Col10a1 gene are involved in conferring tissue-specific expression of type X collagen to hypertrophic chondrocytes. In the present study, we examined the role of SP3/SP1 transcription factors in the regulation of the Col10a1 promoter. The SP3/SP1 ratio is higher in hypertrophic versus non-hypertrophic chondrocytes, due to the significant decrease in SP1 in hypertrophic cells detected by real-time PCR and Western blot analyses. Functional analyses by transfection-mediated overexpression of SP1 and SP3 suggest that SP1 inhibits the Col10a1 promoter. This effect is negated by an interaction with SP3 in hypertrophic chondrocytes. Additionally, mutation analysis showed that the 40-bp intervening sequence (nt -115 to -75) is required for expression of the Col10a1 gene. In this sequence, a binding site for Dlx5/6 transcription factors (nt -99 to -87) retards a protein specific for hypertrophic chondrocytes in electrophoretic mobility shift assay. Endogenous levels of Dlx5 are 3-fold higher in hypertrophic versus non-hypertrophic cells by real-time PCR analysis, and overexpression of Dlx5 in non-hypertrophic chondrocytes activates the proximal Col10a1 promoter 3-fold. These results indicate that the SP3/SP1 ratio and Dlx5 are important regulators of the proximal Col10a1 promoter in hypertrophic cartilage and suggest that interactions between SP3 and SP1 regulate expression of different types of collagen during chondrocyte differentiation.

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.

The control of chondrogenesis

Chondrogenesis is the earliest phase of skeletal development, involving mesenchymal cell recruitment and migration, condensation of progenitors, and chondrocyte differentiation, and maturation and resulting in the formation of cartilage and bone during endochondral ossification. This process is controlled exquisitely by cellular interactions with the surrounding matrix, growth and differentiation factors, and other environmental factors that initiate or suppress cellular signaling pathways and transcription of specific genes in a temporal-spatial manner. Vertebrate limb development is controlled by interacting patterning systems involving prominently the fibroblast growth factor (FGF), bone morphogenetic protein (BMP), and hedgehog pathways. Both positive and negative signaling kinases and transcription factors, such as Sox9 and Runx2, and interactions among them determine whether the differentiated chondrocytes remain within cartilage elements in articular joints or undergo hypertrophic maturation prior to ossification. The latter process requires extracellular matrix remodeling and vascularization controlled by mechanisms that are not understood completely. Recent work has revealed novel roles for mediators such as GADD45beta, transcription factors of the Dlx, bHLH, leucine zipper, and AP-1 families, and the Wnt/beta-catenin pathway that interact at different stages during chondrogenesis.

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.

Expression and activity of the CDK inhibitor p57Kip2 in chondrocytes undergoing hypertrophic differentiation

Growth plates of p57-null mice exhibit several abnormalities, including loss of collagen type X (CollX) expression. The phenotypic consequences of p57 expression were assessed in an in vitro model of hypertrophic differentiation. Adenoviral p57 expression was not sufficient for CollX expression but did augment induction of CollX by BMP-2.

INTRODUCTION

During hypertrophic differentiation, chondrocytes pass from an actively proliferative state to a postmitotic, hypertrophic phenotype. The induction of growth arrest is a central feature of this phenotypic transition. Mice lacking the cyclin dependent-kinase inhibitor p57Kip2 exhibit several developmental abnormalities including chondrodysplasia. Although growth plate chondrocytes in p57-null mice undergo growth arrest, they do not express collagen type X, a specific marker of the hypertrophic phenotype. This study was carried out to investigate the link between p57 expression and the induction of collagen type X in chondrocytes and to determine whether p57 overexpression is sufficient for the induction of hypertrophic differentiation.

MATERIALS AND METHODS

Neonatal rat epiphyseal or growth plate chondrocytes were maintained in an aggregate culture model, in defined, serum-free medium. Protein and mRNA levels were monitored by Western and Northern blot analyses, respectively. Proliferative activity was assessed by fluorescent measurement of total DNA and by 3H-thymidine incorporation rates. An adenoviral vector was used to assess the phenotypic consequences of p57 expression.

RESULTS AND CONCLUSIONS

During in vitro hypertrophic differentiation, levels of p57 mRNA and protein were constant despite changes in chondrocyte proliferative activity and the induction of hypertrophic-specific genes in response to bone morphogenetic protein (BMP)-2. Adenoviral p57 overexpression induced growth arrest in prehypertrophic epiphyseal chondrocytes in a dose-dependent manner but was not sufficient for the induction of collagen type X, either alone or when coexpressed with the related CDKI p21Cip1. Similar results were obtained with more mature tibial growth plate chondrocytes. p57 overexpression did augment collagen type X induction by BMP-2. These data indicate that p57-mediated growth arrest is not sufficient for expression of the hypertrophic phenotype, but rather it occurs in parallel with other aspects of the differentiation pathway. Our findings also suggest a contributing role for p57 in the regulation of collagen type X expression in differentiating chondrocytes.

Type X collagen gene regulation by Runx2 contributes directly to its hypertrophic chondrocyte-specific expression in vivo

The alpha1(X) collagen gene (Col10a1) is the only known hypertrophic chondrocyte-specific molecular marker. Until recently, few transcriptional factors specifying its tissue-specific expression have been identified. We show here that a 4-kb murine Col10a1 promoter can drive beta-galactosidase expression in lower hypertrophic chondrocytes in transgenic mice. Comparative genomic analysis revealed multiple Runx2 (Runt domain transcription factor) binding sites within the proximal human, mouse, and chick Col10a1 promoters. In vitro transfection studies and chromatin immunoprecipitation analysis using hypertrophic MCT cells showed that Runx2 contributes to the transactivation of this promoter via its conserved Runx2 binding sites. When the 4-kb Col10a1 promoter transgene was bred onto a Runx2(+/-) background, the reporter was expressed at lower levels. Moreover, decreased Col10a1 expression and altered chondrocyte hypertrophy was also observed in Runx2 heterozygote mice, whereas Col10a1 was barely detectable in Runx2-null mice. Together, these data suggest that Col10a1 is a direct transcriptional target of Runx2 during chondrogenesis.

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.

Physiology and pathophysiology of the growth plate

Longitudinal growth of the skeleton is a result of endochondral ossification that occurs at the growth plate. Through a sequential process of cell proliferation, extracellular matrix synthesis, cellular hypertrophy, matrix mineralization, vascular invasion, and eventually apoptosis, the cartilage model is continually replaced by bone as length increases. The regulation of longitudinal growth at the growth plate occurs generally through the intimate interaction of circulating systemic hormones and locally produced peptide growth factors, the net result of which is to trigger changes in gene expression by growth plate chondrocytes. This review highlights recent advances in genetics and cell biology that are illuminating the important regulatory mechanisms governing the structure and biology of the growth plate, and provides selected examples of how studies of human mutations have yielded a wealth of new knowledge regarding the normal biology and pathophysiology of growth plate cartilage.

Skeletal dysplasias caused by a disruption of skeletal patterning and endochondral ossification

Identification of a number of the genes that cause skeletal dysplasias has helped clinicians to provide accurate diagnoses, genetic counseling, and pre-natal diagnosis for this complex group of disorders. This review considers how some of the recent advances in human and murine genetics have led to an increased understanding of normal bone development and, in particular, the processes of skeletal patterning and endochondral ossification.

The molecular genetics of bone formation: implications for therapeutic interventions in bone disorders

Skeletal biology is a complex process involving the developmental commitment and differentiation of chondrocytes and osteoblasts which produce and mineralize cartilage and bone matrix during growth and postnatal life. Several genes are involved in controlling osteogenesis by acting on target cells in a very complex manner. Manipulation of genes in mice and studies of genetic mutations affecting the skeleton in humans have enabled the assessment of the role of transcription factors, bone matrix proteins and regulatory factors involved in the control of chondrocyte and osteoblast differentiation, and have considerably improved our understanding of the bone formation process. Clinical studies and gene polymorphism analyses suggest that the variable expression of particular genes may be linked to clinical osteoporosis. A major challenge in the future will be to develop molecularly targeted approaches to stimulating bone formation and increasing bone mass. The use of mouse strain models and transgenic animals with variable bone density may be useful to identify genetic determinants of bone mass which may serve as a basis for drug discovery and development. On the other hand, the availability of gene microarrays and other emerging genomic techniques are promising tools to identify genes that are distinctly expressed in health and disease. These technologies may also serve to test the mechanisms of action of drugs aimed at increasing bone formation. Genetic studies of the molecular signaling pathways involved in normal and pathological osteogenesis may also help to identify genes that could be targeted for therapeutic intervention. Candidate approaches include selective gene transfection in target cells and the use of drugs acting on gene promoters to selectively enhance gene expression in osteoblasts. The development of these strategies is expected not only to bring new insight into the molecular mechanisms that govern bone formation in normal and pathological situations but, in the long term, may also result in the identification of novel molecular targets for therapeutic interventions for bone formation disorders.

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

Each skeletal element where marrow develops is first defined by a hypertrophic cartilage blueprint. Through programmed tissue substitution, the cartilaginous skeletal model is replaced by trabecular bone and marrow, with accompanying longitudinal tissue growth. During this process of endochondral ossification, hypertrophic cartilage expresses a unique matrix molecule, collagen X. Previously we reported that transgenic mice with dominant interference collagen X mutations develop variable skeleto-hematopoietic abnormalities, manifested as growth plate compressions, diminished trabecular bone, and reduced lymphatic organs (Nature 1993, 365:56). Here, histology and flow cytometry reveal marrow hypoplasia and impaired hematopoiesis in all collagen X transgenic mice. A subset of mice with perinatal lethality manifested the most severe skeletal defects and a reduction of marrow hematopoiesis, highlighted by a lymphocyte decrease. Thymic reduction is accompanied by a paucity of cortical immature T cells, consistent with the marrow's inability to replenish maturing cortical lymphocytes. Diminished spleens exhibit indistinct lymphatic nodules and red pulp depletion; the latter correlates with erythrocyte-filled vascular sinusoids in marrows. All mice display reduced B cells in marrows and spleens, and elevated splenic T cells. These hematopoietic defects underscore an unforeseen link between hypertrophic cartilage, endochondral ossification, and establishment of the marrow microenvironment required for blood cell differentiation.

Growth defect in Grg5 null mice is associated with reduced Ihh signaling in growth plates

Gene-targeted disruption of Grg5, a mouse homologue of Drosophila groucho (gro), results in postnatal growth retardation in mice. The growth defect, most striking in approximately half of the Grg5 null mice, occurs during the first 4-5 weeks of age, but most mice recover retarded growth later. We used the nonlinear mixed-effects model to fit the growth data of wild-type, heterozygous, and Grg5 null mice. On the basis of preliminary evidence suggesting an interaction between Grg5 and the transcription factor Cbfa1/Runx2, critical for skeletal development, we further investigated the skeleton in the mice. A long bone growth plate defect was identified, which included shorter zones of proliferative and hypertrophic chondrocytes and decreased trabecular bone formation. This decreased trabecular bone formation is likely caused by a reduced recruitment of osteoblasts into the growth plate region of Grg5 null mice. Like the growth defect, the growth plate and trabecular bone abnormality improved as the mice grew older. The growth plate defect was associated with reduced Indian hedgehog expression and signaling. We suggest that Grg5, a transcriptional coregulator, modulates the activities of transcription factors, such as Cbfa1/Runx2 in vivo to affect Ihh expression and the function of long bone growth plates.

Insight into Schmid metaphyseal chondrodysplasia from the crystal structure of the collagen X NC1 domain trimer

Collagen X is expressed specifically in the growth plate of long bones. Its C1q-like C-terminal NC1 domain forms a stable homotrimer and is crucial for collagen X assembly. Mutations in the NC1 domain cause Schmid metaphyseal chondrodysplasia (SMCD). The crystal structure at 2.0 A resolution of the human collagen X NC1 domain reveals an intimate trimeric assembly strengthened by a buried cluster of calcium ions. Three strips of exposed aromatic residues on the surface of NC1 trimer are likely to be involved in the supramolecular assembly of collagen X. Most internal SMCD mutations probably prevent protein folding, whereas mutations of surface residues may affect the collagen X suprastructure in a dominant-negative manner.

A dominant interference collagen X mutation disrupts hypertrophic chondrocyte pericellular matrix and glycosaminoglycan and proteoglycan distribution in transgenic mice

Collagen X transgenic (Tg) mice displayed skeleto-hematopoietic defects in tissues derived by endochondral skeletogenesis.(1) Here we demonstrate that co-expression of the transgene product containing truncated chicken collagen X with full-length mouse collagen X in a cell-free translation system yielded chicken-mouse hybrid trimers and truncated chicken homotrimers; this indicated that the mutant could assemble with endogenous collagen X and thus had potential for dominant interference. Moreover, species-specific collagen X antibodies co-localized the transgene product with endogenous collagen X to hypertrophic cartilage in growth plates and ossification centers; proliferative chondrocytes also stained diffusely. Electron microscopy revealed a disrupted hexagonal lattice network in the hypertrophic chondrocyte pericellular matrix in Tg growth plates, as well as altered mineral deposition. Ruthenium hexamine trichloride-positive aggregates, likely glycosaminoglycans (GAGs)/proteoglycans (PGs), were also dispersed throughout the chondro-osseous junction. These defects likely resulted from transgene co-localization and dominant interference with endogenous collagen X. Moreover, altered GAG/PG distribution in growth plates of both collagen X Tg and null mice was confirmed by a paucity of staining for hyaluronan and heparan sulfate PG. A provocative hypothesis links the disruption of the collagen X pericellular network and GAG/PG decompartmentalization to the potential locus for hematopoietic failure in the collagen X mice.

Collagens: building blocks at the end of the development line

Collagens are some of the major building blocks of the vertebrate body. In addition to their structural role, they are important for cell guidance during development and for maintaining tissue integrity. In their absence, phenotypes range from lethal to mild. These studies demonstrate that collagens,in their rich array, play important roles in development and are significant elements in reading the developmental code.