Changes in cell, matrix compartment, and fibrillar collagen volumes between growth-plate zones

Increased FGFR3 is involved in T-2 toxin-induced lesions of hypertrophic cartilage associated with endemic osteoarthritis

This study evaluated the effect of fibroblast growth factor receptor 3 (FGFR3) on damaged hypertrophic chondrocytes of Kashin-Beck disease (KBD). Immunohistochemical staining was used to evaluate FGFR3 expression in growth plates from KBD rat models and engineered cartilage. In vitro study, hypertrophic chondrocytes were pretreated by FGFR3 binding inhibitor (BGJ398) for 24 h before incubation at different T-2 toxin concentrations. Differentiation -related genes (Runx2, Sox9, and Col Ⅹ) and ECM degradation -related genes (MMP-13, Col Ⅱ) in the hypertrophic chondrocytes were analyzed using RT-PCR, and the corresponding proteins were analyzed using western blotting. Hypertrophic chondrocytes death was detected by the Annexin V/PI double staining assay. The integrated optical density of FGFR3 staining was increased in knee cartilage of rats and engineered cartilage treated with T-2 toxin. Both protein and mRNA levels of Runx2, Sox9, Col Ⅱ, and Col Ⅹ were decreased in a dose-dependent manner when exposed to the T-2 toxin and significantly upregulated by 1 μM BGJ398. The expression of MMP-1, MMP-9, and MMP-13 increased in a dose-dependent manner when exposed to T-2 toxin and significantly reduced by 1 μM BGJ398. 1 μM BGJ398 could prevent early apoptosis and necrosis induced by the T-2 toxin. Inhibiting the FGFR3 signal could alleviate extracellular matrix degradation, abnormal chondrocytes differentiation, and excessive cell death in T-2 toxin-induced hypertrophic chondrocytes.

Functionalized Microscaffold-Hydrogel Composites Accelerating Osteochondral Repair through Endochondral Ossification

Osteochondral regeneration remains a key challenge because of the limited self-healing ability of the bone and its complex structure and composition. Biomaterials based on endochondral ossification (ECO) are considered an attractive candidate to promote bone repair because they can effectively address the difficulties in establishing vascularization and poor bone regeneration via intramembranous ossification (IMO). However, its clinical application is limited by the complex cellular behavior of ECO and the long time required for induction of the cell cycle. Herein, functionalized microscaffold-hydrogel composites are developed to accelerate the developmental bone growth process via recapitulating ECO. The design comprises arginine-glycine-aspartic acid (RGD)-peptide-modified microscaffolds loaded with kartogenin (KGN) and wrapped with a layer of RGD- and QK-peptide-comodified alginate hydrogel. These microscaffolds enhance the proliferation and aggregation behavior of the human bone marrow mesenchymal stem cells (hBMSCs); the controlled release of kartogenin induces the differentiation of hBMSCs into chondrocytes; and the hydrogel grafted with RGD and QK peptide facilitates chondrocyte hypertrophy, which creates a vascularized niche for osteogenesis and finally accelerates osteochondral repair in vivo. The findings provide an efficient bioengineering approach by sequentially modulating cellular ECO behavior for osteochondral defect repair.

A critical bioenergetic switch is regulated by IGF2 during murine cartilage development

Long bone growth requires the precise control of chondrocyte maturation from proliferation to hypertrophy during endochondral ossification, but the bioenergetic program that ensures normal cartilage development is still largely elusive. We show that chondrocytes have unique glucose metabolism signatures in these stages, and they undergo bioenergetic reprogramming from glycolysis to oxidative phosphorylation during maturation, accompanied by an upregulation of the pentose phosphate pathway. Inhibition of either oxidative phosphorylation or the pentose phosphate pathway in murine chondrocytes and bone organ cultures impaired hypertrophic differentiation, suggesting that the appropriate balance of these pathways is required for cartilage development. Insulin-like growth factor 2 (IGF2) deficiency resulted in a profound increase in oxidative phosphorylation in hypertrophic chondrocytes, suggesting that IGF2 is required to prevent overactive glucose metabolism and maintain a proper balance of metabolic pathways. Our results thus provide critical evidence of preference for a bioenergetic pathway in different stages of chondrocytes and highlight its importance as a fundamental mechanism in skeletal development.

Nutrition in adolescent growth and development

During adolescence, growth and development are transformative and have profound consequences on an individual's health in later life, as well as the health of any potential children. The current generation of adolescents is growing up at a time of unprecedented change in food environments, whereby nutritional problems of micronutrient deficiency and food insecurity persist, and overweight and obesity are burgeoning. In a context of pervasive policy neglect, research on nutrition during adolescence specifically has been underinvested, compared with such research in other age groups, which has inhibited the development of adolescent-responsive nutritional policies. One consequence has been the absence of an integrated perspective on adolescent growth and development, and the role that nutrition plays. Through late childhood and early adolescence, nutrition has a formative role in the timing and pattern of puberty, with consequences for adult height, muscle, and fat mass accrual, as well as risk of non-communicable diseases in later life. Nutritional effects in adolescent development extend beyond musculoskeletal growth, to cardiorespiratory fitness, neurodevelopment, and immunity. High rates of early adolescent pregnancy in many countries continue to jeopardise the growth and nutrition of female adolescents, with consequences that extend to the next generation. Adolescence is a nutrition-sensitive phase for growth, in which the benefits of good nutrition extend to many other physiological systems.

Application of 3D MAPs pipeline identifies the morphological sequence chondrocytes undergo and the regulatory role of GDF5 in this process

The activity of epiphyseal growth plates, which drives long bone elongation, depends on extensive changes in chondrocyte size and shape during differentiation. Here, we develop a pipeline called 3D Morphometric Analysis for Phenotypic significance (3D MAPs), which combines light-sheet microscopy, segmentation algorithms and 3D morphometric analysis to characterize morphogenetic cellular behaviors while maintaining the spatial context of the growth plate. Using 3D MAPs, we create a 3D image database of hundreds of thousands of chondrocytes. Analysis reveals broad repertoire of morphological changes, growth strategies and cell organizations during differentiation. Moreover, identifying a reduction in Smad 1/5/9 activity together with multiple abnormalities in cell growth, shape and organization provides an explanation for the shortening of Gdf5 KO tibias. Overall, our findings provide insight into the morphological sequence that chondrocytes undergo during differentiation and highlight the ability of 3D MAPs to uncover cellular mechanisms that may regulate this process.

Liquid-phase ASEM imaging of cellular and structural details in cartilage and bone formed during endochondral ossification: Keap1-deficient osteomalacia

Chondrogenesis and angiogenesis drive endochondral ossification. Using the atmospheric scanning electron microscopy (ASEM) without decalcification and dehydration, we directly imaged angiogenesis-driven ossification at different developmental stages shortly after aldehyde fixation, using aqueous radical scavenger glucose solution to preserve water-rich structures. An embryonic day 15.5 mouse femur was fixed and stained with phosphotungstic acid (PTA), and blood vessel penetration into the hypertrophic chondrocyte zone was visualised. We observed a novel envelope between the perichondrium and proliferating chondrocytes, which was lined with spindle-shaped cells that could be borderline chondrocytes. At postnatal day (P)1, trabecular and cortical bone mineralisation was imaged without staining. Additional PTA staining visualised surrounding soft tissues; filamentous connections between osteoblast-like cells and osteocytes in cortical bone were interpreted as the osteocytic lacunar-canalicular system. By P10, resorption pits had formed on the tibial trabecular bone surface. The applicability of ASEM for pathological analysis was addressed using knockout mice of Keap1, an oxidative-stress sensor. In Keap1-/- femurs, we observed impaired calcification and angiogenesis of epiphyseal cartilage, suggesting impaired bone development. Overall, the quick ASEM method we developed revealed mineralisation and new structures in wet bone tissue at EM resolution and can be used to study mineralisation-associated phenomena of any hydrated tissue.

NFI-C Is Required for Epiphyseal Chondrocyte Proliferation during Postnatal Cartilage Development

Stringent regulation of the chondrocyte cell cycle is required for endochondral bone formation. During the longitudinal growth of long bones, mesenchymal stem cells condense and differentiate into chondrocytes. Epiphyseal chondrocytes sequentially differentiate to form growth- plate cartilage, which is subsequently replaced with bone. Although the importance of nuclear factor 1C (Nfic) in hard tissue formation has been extensively studied, knowledge regarding its biological roles and molecular mechanisms in this process remains insufficient. Herein, we demonstrated that Nfic deficiency affects femoral growth-plate formation. Chondrocyte proliferation was downregulated and the number of apoptotic cell was increased in the growth plates of Nfic-/- mice. Further, the expression of the cell cycle inhibitor p21 was upregulated in the primary chondrocytes of Nfic-/- mice, whereas that of cyclin D1 was downregulated. Our findings suggest that Nfic may contribute to postnatal chondrocyte proliferation by inhibiting p21 expression and by increasing the stability of cyclin D1 protein.

Endochondral ossification and the evolution of limb proportions

Mammals have remarkably diverse limb proportions hypothesized to have evolved adaptively in the context of locomotion and other behaviors. Mechanistically, evolutionary diversity in limb proportions is the result of differential limb bone growth. Longitudinal limb bone growth is driven by the process of endochondral ossification, under the control of the growth plates. In growth plates, chondrocytes undergo a tightly orchestrated life cycle of proliferation, matrix production, hypertrophy, and cell death/transdifferentiation. This life cycle is highly conserved, both among the long bones of an individual, and among homologous bones of distantly related taxa, leading to a finite number of complementary cell mechanisms that can generate heritable phenotype variation in limb bone size and shape. The most important of these mechanisms are chondrocyte population size in chondrogenesis and in individual growth plates, proliferation rates, and hypertrophic chondrocyte size. Comparative evidence in mammals and birds suggests the existence of developmental biases that favor evolutionary changes in some of these cellular mechanisms over others in driving limb allometry. Specifically, chondrocyte population size may evolve more readily in response to selection than hypertrophic chondrocyte size, and extreme hypertrophy may be a rarer evolutionary phenomenon associated with highly specialized modes of locomotion in mammals (e.g., powered flight, ricochetal bipedal hopping). Physical and physiological constraints at multiple levels of biological organization may also have influenced the cell developmental mechanisms that have evolved to produce the highly diverse limb proportions in extant mammals. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Comparative Development and Evolution > Regulation of Organ Diversity Comparative Development and Evolution > Organ System Comparisons Between Species.

Proteome analysis of human mesenchymal stem cells undergoing chondrogenesis when exposed to the products of various magnesium-based materials degradation

Treatment of physeal fractures (15%–30% of all paediatric fractures) remains a challenge as in approximately 10% of the cases, significant growth disturbance may occur. Bioresorbable Magnesium-based implants represent a strategy to minimize damage (i.e., load support until bone healing without second surgery). Nevertheless, the absence of harmful effects of magnesium-implants and their degradation products on the growth plate should be confirmed. Here, the proteome of human mesenchymal stem cells undergoing chondrogenesis was evaluated when exposed to the products of various Magnesium-based materials degradation. The results of this study indicate that the materials induced regulation of proteins associated with cell chondrogenesis and cartilage formation, which should be beneficial for cartilage regeneration.

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Degradation products from Mg-based materials generated changes in protein expression.

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Relevant proteins involved in cartilage formation were upregulated.

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Potential application of especially Pure-Mg and Mg-10Gd for cartilage regeneration.

The impact of immune response on endochondral bone regeneration

Tissue engineered cartilage substitutes, which induce the process of endochondral ossification, represent a regenerative strategy for bone defect healing. Such constructs typically consist of multipotent mesenchymal stromal cells (MSCs) forming a cartilage template in vitro, which can be implanted to stimulate bone formation in vivo. The use of MSCs of allogeneic origin could potentially improve the clinical utility of the tissue engineered cartilage constructs in three ways. First, ready-to-use construct availability can speed up the treatment process. Second, MSCs derived and expanded from a single donor could be applied to treat several patients and thus the costs of the medical interventions would decrease. Finally, it would allow more control over the quality of the MSC chondrogenic differentiation. However, even though the envisaged clinical use of allogeneic cell sources for bone regeneration is advantageous, their immunogenicity poses a significant obstacle to their clinical application. The aim of this review is to increase the awareness of the role played by immune cells during endochondral ossification, and in particular during regenerative strategies when the immune response is altered by the presence of implanted biomaterials and/or cells. More specifically, we focus on how this balance between immune response and bone regeneration is affected by the implantation of a cartilaginous tissue engineered construct of allogeneic origin.

Achondroplasia: Development, pathogenesis, and therapy

Autosomal dominant mutations in fibroblast growth factor receptor 3 (FGFR3) cause achondroplasia (Ach), the most common form of dwarfism in humans, and related chondrodysplasia syndromes that include hypochondroplasia (Hch), severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN), and thanatophoric dysplasia (TD). FGFR3 is expressed in chondrocytes and mature osteoblasts where it functions to regulate bone growth. Analysis of the mutations in FGFR3 revealed increased signaling through a combination of mechanisms that include stabilization of the receptor, enhanced dimerization, and enhanced tyrosine kinase activity. Paradoxically, increased FGFR3 signaling profoundly suppresses proliferation and maturation of growth plate chondrocytes resulting in decreased growth plate size, reduced trabecular bone volume, and resulting decreased bone elongation. In this review, we discuss the molecular mechanisms that regulate growth plate chondrocytes, the pathogenesis of Ach, and therapeutic approaches that are being evaluated to improve endochondral bone growth in people with Ach and related conditions. Developmental Dynamics 246:291-309, 2017. © 2016 Wiley Periodicals, Inc.

Distinct requirements of wls, wnt9a, wnt5b and gpc4 in regulating chondrocyte maturation and timing of endochondral ossification

Formation of the mandible requires progressive morphologic change, proliferation, differentiation and organization of chondrocytes preceding osteogenesis. The Wnt signaling pathway is involved in regulating bone development and maintenance. Chondrocytes that are fated to become bone require Wnt to polarize and orientate appropriately to initiate the endochondral ossification program. Although the canonical Wnt signaling has been well studied in the context of bone development, the effects of non-canonical Wnt signaling in regulating the timing of cartilage maturation and subsequent bone formation in shaping ventral craniofacial structure is not fully understood.. Here we examined the role of the non-canonical Wnt signaling pathway (wls, gpc4, wnt5b and wnt9a) in regulating zebrafish Meckel's cartilage maturation to the onset of osteogenic differentiation. We found that disruption of wls resulted in a significant loss of craniofacial bone, whereas lack of gpc4, wnt5b and wnt9a resulted in severely delayed endochondral ossification. This study demonstrates the importance of the non-canonical Wnt pathway in regulating coordinated ventral cartilage morphogenesis and ossification.

Extracellular matrix motion and early morphogenesis

For over a century, embryologists who studied cellular motion in early amniotes generally assumed that morphogenetic movement reflected migration relative to a static extracellular matrix (ECM) scaffold. However, as we discuss in this Review, recent investigations reveal that the ECM is also moving during morphogenesis. Time-lapse studies show how convective tissue displacement patterns, as visualized by ECM markers, contribute to morphogenesis and organogenesis. Computational image analysis distinguishes between cell-autonomous (active) displacements and convection caused by large-scale (composite) tissue movements. Modern quantification of large-scale 'total' cellular motion and the accompanying ECM motion in the embryo demonstrates that a dynamic ECM is required for generation of the emergent motion patterns that drive amniote morphogenesis.

FGF signaling in the osteoprogenitor lineage non-autonomously regulates postnatal chondrocyte proliferation and skeletal growth

Fibroblast growth factor (FGF) signaling is important for skeletal development; however, cell-specific functions, redundancy and feedback mechanisms regulating bone growth are poorly understood. FGF receptors 1 and 2 (Fgfr1 and Fgfr2) are both expressed in the osteoprogenitor lineage. Double conditional knockout mice, in which both receptors were inactivated using an osteoprogenitor-specific Cre driver, appeared normal at birth; however, these mice showed severe postnatal growth defects that include an ∼50% reduction in body weight and bone mass, and impaired longitudinal bone growth. Histological analysis showed reduced cortical and trabecular bone, suggesting cell-autonomous functions of FGF signaling during postnatal bone formation. Surprisingly, the double conditional knockout mice also showed growth plate defects and an arrest in chondrocyte proliferation. We provide genetic evidence of a non-cell-autonomous feedback pathway regulating Fgf9, Fgf18 and Pthlh expression, which led to increased expression and signaling of Fgfr3 in growth plate chondrocytes and suppression of chondrocyte proliferation. These observations show that FGF signaling in the osteoprogenitor lineage is obligately coupled to chondrocyte proliferation and the regulation of longitudinal bone growth.

Delayed hypertrophic differentiation of epiphyseal chondrocytes contributes to failed secondary ossification in mucopolysaccharidosis VII dogs

Mucopolysaccharidosis (MPS) VII is a lysosomal storage disorder characterized by deficient β-glucuronidase activity, which leads to the accumulation of incompletely degraded glycosaminoglycans (GAGs). MPS VII patients present with severe skeletal abnormalities, which are particularly prevalent in the spine. Incomplete cartilage-to-bone conversion in MPS VII vertebrae during postnatal development is associated with progressive spinal deformity and spinal cord compression. The objectives of this study were to determine the earliest postnatal developmental stage at which vertebral bone disease manifests in MPS VII and to identify the underlying cellular basis of impaired cartilage-to-bone conversion, using the naturally-occurring canine model. Control and MPS VII dogs were euthanized at 9 and 14 days-of-age, and vertebral secondary ossification centers analyzed using micro-computed tomography, histology, qPCR, and protein immunoblotting. Imaging studies and mRNA analysis of bone formation markers established that secondary ossification commences between 9 and 14 days in control animals, but not in MPS VII animals. mRNA analysis of differentiation markers revealed that MPS VII epiphyseal chondrocytes are unable to successfully transition from proliferation to hypertrophy during this critical developmental window. Immunoblotting demonstrated abnormal persistence of Sox9 protein in MPS VII cells between 9 and 14 days-of-age, and biochemical assays revealed abnormally high intra and extracellular GAG content in MPS VII epiphyseal cartilage at as early as 9 days-of-age. In contrast, assessment of vertebral growth plates and primary ossification centers revealed no significant abnormalities at either age. The results of this study establish that failed vertebral bone formation in MPS VII can be traced to the failure of epiphyseal chondrocytes to undergo hypertrophic differentiation at the appropriate developmental stage, and suggest that aberrant processing of Sox9 protein may contribute to this cellular dysfunction. These results also highlight the importance of early diagnosis and therapeutic intervention to prevent the progression of debilitating skeletal disease in MPS patients.

Histone Deacetylases in Bone Development and Skeletal Disorders

Histone deacetylases (Hdacs) are conserved enzymes that remove acetyl groups from lysine side chains in histones and other proteins. Eleven of the 18 Hdacs encoded by the human and mouse genomes depend on Zn(2+) for enzymatic activity, while the other 7, the sirtuins (Sirts), require NAD2(+). Collectively, Hdacs and Sirts regulate numerous cellular and mitochondrial processes including gene transcription, DNA repair, protein stability, cytoskeletal dynamics, and signaling pathways to affect both development and aging. Of clinical relevance, Hdacs inhibitors are United States Food and Drug Administration-approved cancer therapeutics and are candidate therapies for other common diseases including arthritis, diabetes, epilepsy, heart disease, HIV infection, neurodegeneration, and numerous aging-related disorders. Hdacs and Sirts influence skeletal development, maintenance of mineral density and bone strength by affecting intramembranous and endochondral ossification, as well as bone resorption. With few exceptions, inhibition of Hdac or Sirt activity though either loss-of-function mutations or prolonged chemical inhibition has negative and/or toxic effects on skeletal development and bone mineral density. Specifically, Hdac/Sirt suppression causes abnormalities in physiological development such as craniofacial dimorphisms, short stature, and bone fragility that are associated with several human syndromes or diseases. In contrast, activation of Sirts may protect the skeleton from aging and immobilization-related bone loss. This knowledge may prolong healthspan and prevent adverse events caused by epigenetic therapies that are entering the clinical realm at an unprecedented rate. In this review, we summarize the general properties of Hdacs/Sirts and the research that has revealed their essential functions in bone forming cells (e.g., osteoblasts and chondrocytes) and bone resorbing osteoclasts. Finally, we offer predictions on future research in this area and the utility of this knowledge for orthopedic applications and bone tissue engineering.

Fibroblast growth factor signaling in skeletal development and disease

Fibroblast growth factor (FGF) signaling pathways are essential regulators of vertebrate skeletal development. FGF signaling regulates development of the limb bud and formation of the mesenchymal condensation and has key roles in regulating chondrogenesis, osteogenesis, and bone and mineral homeostasis. This review updates our review on FGFs in skeletal development published in Genes & Development in 2002, examines progress made on understanding the functions of the FGF signaling pathway during critical stages of skeletogenesis, and explores the mechanisms by which mutations in FGF signaling molecules cause skeletal malformations in humans. Links between FGF signaling pathways and other interacting pathways that are critical for skeletal development and could be exploited to treat genetic diseases and repair bone are also explored.

Dynamic imaging of the growth plate cartilage reveals multiple contributors to skeletal morphogenesis

The diverse morphology of vertebrate skeletal system is genetically controlled, yet the means by which cells shape the skeleton remains to be fully illuminated. Here we perform quantitative analyses of cell behaviours in the growth plate cartilage, the template for long bone formation, to gain insights into this process. Using a robust avian embryonic organ culture, we employ time-lapse two-photon laser scanning microscopy to observe proliferative cells' behaviours during cartilage growth, resulting in cellular trajectories with a spreading displacement mainly along the tissue elongation axis. We build a novel software toolkit of quantitative methods to segregate the contributions of various cellular processes to the cellular trajectories. We find that convergent-extension, mitotic cell division, and daughter cell rearrangement do not contribute significantly to the observed growth process; instead, extracellular matrix deposition and cell volume enlargement are the key contributors to embryonic cartilage elongation.

CB1 cannabinoid receptors mediate endochondral skeletal growth attenuation by Δ9-tetrahydrocannabinol

The endocannabinoid (EC) system regulates bone mass. Because cannabis use during pregnancy results in stature shorter than normal, we examined the role of the EC system in skeletal elongation. We show that CB1 and CB2 cannabinoid receptors are expressed specifically in hypertrophic chondrocytes of the epiphyseal growth cartilage (EGC), which drives vertebrate growth. These cells also express diacylglycerol lipases, critical biosynthetic enzymes of the main EC, and 2-arachidonoylglycerol (2-AG), which is present at significant levels in the EGC. Femora of CB1- and/or CB2-deficient mice at the end of the rapid growth phase are longer compared to wild-type (WT) animals. We find that Δ(9) -tetrahydrocannabinol (THC) slows skeletal elongation of female WT and CB2-, but not CB1-, deficient mice, which is reflected in femoral and lumbar vertebral body length. This in turn results in lower body weight, but unaltered fat content. THC inhibits EGC chondrocyte hypertrophy in ex vivo cultures and reduces the hypertrophic cell zone thickness of CB1-, but not CB2-, deficient mice. These results demonstrate a local growth-restraining EC system in the EGC. The relevance of the present findings to humans remains to be studied.

Growth and differentiation of a long bone in limb development, repair and regeneration

Repair from traumatic bone fracture is a complex process that includes mechanisms of bone development and bone homeostasis. Thus, elucidation of the cellular/molecular basis of bone formation in skeletal development would provide valuable information on fracture repair and would lead to successful skeletal regeneration after limb amputation, which never occurs in mammals. Elucidation of the basis of epimorphic limb regeneration in amphibians would also provide insights into skeletal regeneration in mammals, since the epimorphic regeneration enables an amputated limb to re-develop the three-dimensional structure of bones. In the processes of bone development, repair and regeneration, growth of the bone is achieved through several events including not only cell proliferation but also aggregation of mesenchymal cells, enlargement of cells, deposition and accumulation of extracellular matrix, and bone remodeling.

Spatial periodicity in growth plate shear mechanical properties is disrupted by vitamin D deficiency

The growth plate is a highly organized section of cartilage in the long bones of growing children that is susceptible to mechanical failure as well as structural and functional disruption caused by a dietary deficiency of vitamin D. The shear mechanical properties of the proximal tibial growth plate of rats raised either on normal or vitamin D and calcium deficient diets were measured. A sinusoidal oscillating shear load was applied to small excised growth plate specimens perpendicular to the direction of growth while imaging the deformation in real time with a fast confocal microscope. Local deformations and shear strains were quantified using image correlation. The proliferative zone of the growth plate bores the majority of the shear strain and the resting, hypertrophic and calcification zones deformed less. Surprisingly, we regularly observed discontinuous deformations in the proliferative zone in both groups that resembled cell columns sliding past one another in the direction of growth. These discontinuities manifested as regions of concentrated longitudinal shear strain. Furthermore, these shear strain concentrations were spaced evenly in the proliferative zone and the spacing between them was similar across growth plate regions and across control specimens. In contrast to the healthy controls, the vitamin D deficient growth plate exhibited larger variations in the size and orientation of cellular columns in the proliferative and hypertrophic zones. High strains were observed between columns, much as they were in the controls. However, the regular spacing of shear strain concentrations was not preserved, echoing the observation of decreased structural organization.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Control of chondrocyte gene expression by actin dynamics: a novel role of cholesterol/Ror-alpha signalling in endochondral bone growth

Elucidating the signalling pathways that regulate chondrocyte differentiation, such as the actin cytoskeleton and Rho GTPases, during development is essential for understanding of pathological conditions of cartilage, such as chondrodysplasias and osteoarthritis. Manipulation of actin dynamics in tibia organ cultures isolated from E15.5 mice results in pronounced enhancement of endochondral bone growth and specific changes in growth plate architecture. Global changes in gene expression were examined of primary chondrocytes isolated from embryonic tibia, treated with the compounds cytochalasin D, jasplakinolide (actin modifiers) and the ROCK inhibitor Y27632. Cytochalasin D elicited the most pronounced response and induced many features of hypertrophic chondrocyte differentiation. Bioinformatics analyses of microarray data and expression validation by real-time PCR and immunohistochemistry resulted in the identification of the nuclear receptor retinoid related orphan receptor-α (Ror-α) as a novel putative regulator of chondrocyte hypertrophy. Expression of Ror-α target genes, (Lpl, fatty acid binding protein 4 [Fabp4], Cd36 and kruppel-like factor 5 [Klf15]) were induced during chondrocyte hypertrophy and by cytochalasin D and are cholesterol dependent. Stimulation of Ror-α by cholesterol results in increased bone growth and enlarged, rounded cells, a phenotype similar to chondrocyte hypertrophy and to the changes induced by cytochalasin D, while inhibition of cholesterol synthesis by lovastatin inhibits cytochalasin D induced bone growth. Additionally, we show that in a mouse model of cartilage specific (Col2-Cre) Rac1, inactivation results in increased Hif-1α (a regulator of Rora gene expression) and Ror-α⁺ cells within hypertrophic growth plates. We provide evidence that cholesterol signalling through increased Ror-α expression stimulates chondrocyte hypertrophy and partially mediates responses of cartilage to actin dynamics.

Hdac-mediated control of endochondral and intramembranous ossification

Histone deacetylases (Hdacs) remove acetyl groups (CH3CO-) from ε-amino groups in lysine residues within histones and other proteins. This posttranslational (de) modification alters protein stability, protein-protein interactions, and chromatin structure. Hdac activity plays important roles in the development of all organs and tissues, including the mineralized skeleton. Bone is a dynamic tissue that forms and regenerates by two processes: endochondral and intramembranous ossification. Chondrocytes and osteoblasts are responsible for producing the extracellular matrices of skeletal tissues. Several Hdacs contribute to the molecular pathways and chromatin changes that regulate tissue-specific gene expression during chondrocyte and osteoblast specification, maturation, and terminal differentiation. In this review, we summarize the roles of class I and class II Hdacs in chondrocytes and osteoblasts. The effects of small molecule Hdac inhibitors on the skeleton are also discussed.

Expression of Runx2 and type X collagen in vertebral growth plate of patients with adolescent idiopathic scoliosis

The different expression of type X collagen and Runx2 between the convex and concave side of vertebral growth plate in scoliosis may help to improve our understanding of the role that growth plate tissue play in the development or progression of idiopathic scoliosis. In this investigation, there were significant differences of the total expression of type X collagen, Runx2 protein, and Runx2 mRNA between convex side and concave side growth plates of the apex vertebrae (p < 0.05). The total expression of type X collagen in the concave side growth plates of the lower end vertebrae was higher than that in the same side growth plates of apex (p < 0.05). The total expression of Runx2 in the concave side growth plates in the upper and lower end vertebrae were higher than that in the concave side growth plates of apex (p < 0.05). The expression of type X collagen, Runx2, and Runx2 mRNA, the cell density of type X collagen and Runx2 positive chondrocytes, and histological changes between convex side and concave side of the vertebral growth plate indicated that the vertebral growth plate was affected by mechanical forces, which was a secondary change and could contribute to progression of adolescent idiopathic scoliosis.

Uncoupling of growth plate maturation and bone formation in mice lacking both Schnurri-2 and Schnurri-3

Formation and remodeling of the skeleton relies on precise temporal and spatial regulation of genes expressed in cartilage and bone cells. Debilitating diseases of the skeletal system occur when mutations arise that disrupt these intricate genetic regulatory programs. Here, we report that mice bearing parallel null mutations in the adapter proteins Schnurri2 (Shn2) and Schnurri3 (Shn3) exhibit defects in patterning of the axial skeleton during embryogenesis. Postnatally, these compound mutant mice develop a unique osteochondrodysplasia. The deletion of Shn2 and Shn3 impairs growth plate maturation during endochondral ossification but simultaneously results in massively elevated trabecular bone formation. Hence, growth plate maturation and bone formation can be uncoupled under certain circumstances. These unexpected findings demonstrate that both unique and redundant functions reside in the Schnurri protein family that are required for proper skeletal patterning and remodeling.

Recapitulation of endochondral bone formation using human adult mesenchymal stem cells as a paradigm for developmental engineering

Mesenchymal stem/stromal cells (MSC) are typically used to generate bone tissue by a process resembling intramembranous ossification, i.e., by direct osteoblastic differentiation. However, most bones develop by endochondral ossification, i.e., via remodeling of hypertrophic cartilaginous templates. To date, endochondral bone formation has not been reproduced using human, clinically compliant cell sources. Here, we aimed at engineering tissues from bone marrow-derived, adult human MSC with an intrinsic capacity to undergo endochondral ossification. By analogy to embryonic limb development, we hypothesized that successful execution of the endochondral program depends on the initial formation of hypertrophic cartilaginous templates. Human MSC, subcutaneously implanted into nude mice at various stages of chondrogenic differentiation, formed bone trabeculae only when they had developed in vitro hypertrophic tissue structures. Advanced maturation in vitro resulted in accelerated formation of larger bony tissues. The underlying morphogenetic process was structurally and molecularly similar to the temporal and spatial progression of limb bone development in embryos. In particular, Indian hedgehog signaling was activated at early stages and required for the in vitro formation of hypertrophic cartilage. Subsequent development of a bony collar in vivo was followed by vascularization, osteoclastic resorption of the cartilage template, and appearance of hematopoietic foci. This study reveals the capacity of human MSC to generate bone tissue via an endochondral program and provides a valid model to study mechanisms governing bone development. Most importantly, this process could generate advanced grafts for bone regeneration by invoking a "developmental engineering" paradigm.

Convergent extension movements in growth plate chondrocytes require gpi-anchored cell surface proteins

Proteins that are localized to the cell surface via glycosylphosphatidylinositol (gpi) anchors have been proposed to regulate cell signaling and cell adhesion events involved in tissue patterning. Conditional deletion of Piga, which encodes the catalytic subunit of an essential enzyme in the gpi-biosynthetic pathway, in the lateral plate mesoderm results in normally patterned limbs that display chondrodysplasia. Analysis of mutant and mosaic Piga cartilage revealed two independent cell autonomous defects. First, loss of Piga function interferes with signal reception by chondrocytes as evidenced by delayed maturation. Second, the proliferative chondrocytes, although present, fail to flatten and arrange into columns. We present evidence that the abnormal organization of mutant proliferative chondrocytes results from errors in cell intercalation. Collectively, our data suggest that the distinct morphological features of the proliferative chondrocytes result from a convergent extension-like process that is regulated independently of chondrocyte maturation.

Mechanisms of bone repair and regeneration

Bone problems can have a highly deleterious impact on life and society, therefore understanding the mechanisms of bone repair is important. In vivo studies show that bone repair processes in adults resemble normal development of the skeleton during embryogenesis, which can thus be used as a model. In addition, recent studies of skeletal stem cell biology have underlined several crucial molecular and cellular processes in bone formation. Hedgehog, parathyroid hormone-related protein, Wnt, bone morphogenetic proteins and mitogen-activated protein kinases are the main molecular players, and osteoclasts and mesenchymal stem cells are the main cells involved in these processes. However, questions remain regarding the precise mechanisms of bone formation, how the different molecular processes interact, and the real identity of regenerative cells. Here, we review recent studies of bone regeneration and repair. A better understanding of the underlying mechanisms is expected to facilitate the development of new strategies for improving bone repair.

Age and pattern of the onset of differential growth among growth plates in rats

Differential growth is the phenomenon whereby growth plates in the same individual at the same time all have uniquely different axial growth velocities. Differential growth is clearly present in the adolescent skeleton. In this study we ask two questions. When and by what pattern does the phenomenon of differential growth begin? Second, to what extent are the development of differential growth velocities correlated with changes in hypertrophic chondrocyte volume and/or with changes in chondrocytic production/turnover? Four growth plates (proximal and distal radial; proximal and distal tibial) were studied at 24 different time points in Long-Evans rats between the 17th gestational day (when differential growth does not exist) and postnatal day 27 (when differential growth is well established). Growth velocities were measured using fluorochrome labeling. Using stereological methodology, multiple chondrocytic kinetic parameters were measured for all growth plates. Elongation of the proximal radial growth plate decreases relative to elongation in the other three growth plates in the late fetal phase. Differential growth is fully expressed at postnatal day 13 when the other three growth plates start to decrease daily elongation at different rates. Differential growth is primarily associated with differences in hypertrophic cell volume manifested when growth deceleration occurs. This study also illustrates that differential growth is superimposed on systemic regulators that affect all growth plates simultaneously. The most dramatic illustration of this is the sharp decline in growth velocity in all four growth plates that occurs perinatally.

Chondrocyte aggregation in suspension culture is GFOGER-GPP- and beta1 integrin-dependent

Isolated chondrocytes form aggregates in suspension culture that maintain chondrocyte phenotype in a physiological pericellular environment. The molecular mechanisms involved in chondrocyte aggregation have not been previously identified. Using this novel suspension culture system, we performed mRNA and protein expression analysis along with immunohistochemistry for potential cell adhesion molecules and extracellular matrix integrin ligands. Inhibition of aggregation assays were performed using specific blocking agents. We found that: (i) direct cell-cell interactions were not involved in chondrocyte aggregation, (ii) chondrocytes in aggregates were surrounded by a matrix rich in collagen II and cartilage oligomeric protein (COMP), (iii) aggregation depends on a beta1-integrin, which binds a triple helical GFOGER sequence found in collagens, (iv) integrin alpha10-subunit is the most highly expressed alpha-subunit among those tested, including alpha5, in aggregating chondrocytes. Taken together, this body of evidence suggests that the main molecular interaction involved in aggregation of phenotypically stable chondrocytes is the alpha10beta1-collagen II interaction.

Histomorphometric analysis of an adolescent distal tibial physis prior to growth plate closure

PURPOSE

Our current understanding of the rate and pattern of physeal closure is based on roentgenographic, magnetic resonance imaging, and qualitative histological studies. The purpose of this report is to provide a detailed histomorphometric/stereological analysis of a distal tibial human growth plate in the process of physiological epiphysiodesis.

METHODS

A human distal tibial growth plate was sampled in three regions (anterior, central, and posterior), with each region further separated medially, in the middle, and laterally. The regions were assessed for the location and extent of bony bar formation as well as for physeal height. Companion sections from optimally fixed tissue in the distal 100 microm of the hypertrophic zone were analyzed for hypertrophic chondrocytic volumes.

RESULTS

Physis closure started in the middle of the central region of the growth plate, with 46% of the volume in this area occupied by trans-physeal bridging bone. The growth plate was also narrowed with the lowest physeal heights evident in the middle of the central and anterior regions of the physis. Disruption of the regular columns of the physis was evident with the cells arranged in clusters with intervening areas of acellularity. The average hypertrophic cell volume was 5,900 microm(3) and did not significantly differ between different areas of the physis.

CONCLUSIONS

This is the first characterization of closure in a human distal tibial growth plate via optimum fixation and stereological techniques. The studied physis was during the earliest phases of closure and provides stereological support that the distal tibial physis closes in a central to medial direction.

Histomorphological study of the spinal growth plates from the convex side and the concave side in adolescent idiopathic scoliosis

Asymmetrical growth of the vertebrae has been implicated as one possible etiologic factor in the pathogenesis of adolescent idiopathic scoliosis. The longitudinal vertebral growth derives from the endochondral ossification of the vertebral growth plate. In the present study, the growth plates from the convex and concave side of the vertebrae were characterized by the method of histology and immunohistochemistry to evaluate the growth activity, cell proliferation, and apoptosis. Normal zoned architectures were observed in the convex side of the growth plate and disorganized architectures in the concave side. The histological grades were significantly different between the convex and the concave side of the growth plate in the apex vertebrae (P < 0.05). The histological difference was also found significant statistically between end vertebrae and apex vertebrae in the concave side of vertebral growth plates (P < 0.05). The proliferative potential indexes and apoptosis indexes of chondrocytes in the proliferative and hypertrophic zone in the convex side were significantly higher than that in the concave side in the apex vertebral growth plate (P < 0.05). There was a significant difference of the proliferative potential index (proliferating cell nuclear antigen, PCNA index) between convex side and concave side at the upper end vertebra (P < 0.05). The difference of the proliferative potential index and apoptosis index were found significant statistically in the concave side of the vertebral growth plate between end vertebrae and apex vertebrae (P < 0.05). The same result was also found for the apoptosis index (terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate biotin nick end labeling assay, TUNEL index) in the convex side of vertebral growth plate between end vertebrae and apex vertebrae (P < 0.05). Some correlation were found between radiographic measurements and proliferation and apoptosis indexes. The difference in histological grades and cellular activity between the convex and concave side indicated that the bilateral growth plate of the vertebrae in AIS patients have different growth kinetics which may affect the curve progression.

Distinct developmental changes in the distribution of calcium, phosphorus and sulphur during fetal growth-plate development

Gradients in the concentrations of free phosphate (Pi) and calcium (Ca) exist in fully developed growth zones of long bones and ribs, with the highest concentrations closest to the site of mineralization. As high concentrations of Pi and Ca induce chondrocyte maturation and apoptosis, it has been hypothesized that Ca and Pi drive chondrocyte differentiation in growth plates. This study aimed to determine whether gradients in the important spectral elements phosphorus (P), Ca and sulphur (S) are already present in early stages of development, or whether they gradually develop with maturation of the growth zone. We quantified the concentration profiles of Ca, P, S, chloride and potassium at four different stages of early development of the distal growth plates of the porcine femurs, using particle-induced X-ray emission and forward- and backward-scattering spectrometry with a nuclear microprobe. A Ca concentration gradient towards the mineralized area and a stepwise increase in S was found to develop slowly with tissue maturation. The increase in S co-localizes with the onset of proliferation. A P gradient was not detected in the earliest developmental stages. High Ca levels, which may induce chondrocyte maturation, are present near the mineralization front. As total P concentrations do not correspond with former free Pi measurements, we hypothesize that the increase of free Pi towards the bone-forming site results from enzymatic cleavage of bound phosphate.

Alterations in the growth plate associated with growth modulation by sustained compression or distraction

Sustained mechanical load is known to modulate endochondral growth in the immature skeleton, but it is not known what causes this mechanical sensitivity. This study aimed to quantify alterations in parameters of growth plate performance associated with mechanically altered growth rate. Vertebral and proximal tibial growth plates of immature rats and cattle, and rabbit (proximal tibia only) were subjected to different magnitudes of sustained loading, which altered growth rates by up to 53%. The numbers of proliferative chondrocytes, their rate of proliferation, and the amount of chondrocytic enlargement occurring in the hypertrophic zone were quantified. It was found that reduced growth rate with compression and increased growth rate with distraction were associated with corresponding changes in the number of proliferative chondrocytes per unit width of growth plate, and in the final (maximum) chondrocytic height in the hypertrophic zone (overall correlation coefficients 0.38 and 0.56 respectively). According to multiple linear regression coefficients for these two variables (0.72 and 1.39 respectively), chondrocytic enlargement made a greater contribution to altered growth rates.

Histologic, risser sign, and digital skeletal age evaluation for residual spine growth potential in Chinese female idiopathic scoliosis

STUDY DESIGN

A prospective study.

OBJECTIVE

To ascertain the correlation between histologic grades (HGs) of vertebral growth plates and Risser grades as well as DSA stages in the Chinese female idiopathic scoliosis (IS) patients; to identify whether digital skeletal age (DSA) is a reliable indicator for accurate evaluation of the spinal residual growth potential.

SUMMARY OF BACKGROUND DATA

DSA is considered one of the more important indicators for representing the peak height velocity (PHV) typically and predicting spinal growth potential. The correlation between HGs of growth plates and DSA stages in IS patients is unclear.

METHODS

Thirty-nine Chinese female patients were available for this study. Superior and inferior growth plates were obtained at each level when anterior approach surgeries were performed. Histologic examinations were conducted after the specimens were processed. Of these patients, 28 cases were evaluated by DSA stages in this study. Correlations between histologic grades, Risser grades, menarchal status, and chronologic age were analyzed in 39 patients. Correlations between histologic grades, DSA, menarchal status, and chronologic age were analyzed in 28 patients.

RESULTS

There was a negative correlation between the following: HGs and Risser grades in 39 patients (r = -0.645, P = 0.000-0.05), HGs and menarchal status in patients in Risser 4 (r = -0.710, P = 0.002-0.05), HGs and DSA stages in 28 cases (r = -0.541, P = 0.003-0.05), and HGs and menarchal status in patients in DSA Stage III (r = -0.591, P = 0.006-0.05). Statistical significance of growth activity of growth plates was found between patients in Risser Grades 0 to 1 and those in Risser Grades 2 to 5 (P = 0.020-0.05) and patients in DSA Stage II and those in DSA Stage III (P = 0.014-0.05).

CONCLUSION

DSA may be a reliable indicator for predicting the spinal residual growth potential in IS patients, but it should be correlated with menarchal status and chronologic ages.

NDST-1 modulates BMPR and PTHrP signaling during endochondral bone formation in a gene knockout model

GlcNAc N-deacetylase/N-sulfotransferase-1 (NDST-1), a member of the enzyme family catalyzing the first modification step in the biosynthesis of heparan sulfate (HS), was knocked out in mice to investigate its role in embryonic development. NDST-1 null mice exhibited delayed endochondral bone formation including shortened calcified zones in limbs, delayed chondrocyte and osteogenetic differentiation, and increased chondrocyte proliferation. In situ HS binding assay revealed that the binding ability of bone morphogenetic protein (BMP) -2, -4, and -6 to endogenous HS was decreased in mutant phalanges, while that of fibroblast growth factor-1 (FGF-1) was not affected. Up-regulation of BMPR-IA, Phospho-Smad1 (P-Smad1) and parathyroid-hormone related protein (PTHrP), but not the Indian hedgehog, Gli1, Gli3, Patched, and FGFR-3, was observed. Furthermore, block of BMPR signaling with noggin rescued the delayed chondrocyte hypertrophic differentiation in NDST-1 (-/-) mice and recovered the expression of both P-Smad1 and PTHrP proteins. These results suggested that NDST-1-dependent heparan sulfate might negatively modulate BMP and its downstream PTHrP signaling, and thus affect endochondral bone development.

Histomorphometric study of the spinal growth plates in idiopathic scoliosis and congenital scoliosis

BACKGROUND

Previous studies have suggested that the relative anterior spinal overgrowth may play an important role in the etiopathogenesis of spinal deformity in adolescent idiopathic scoliosis (AIS). Little is known about the histomorphometry of the anterior and posterior spinal growth plates.

METHODS

In the present study, the growth plates from the anterior and posterior column of the spine of the AIS (n = 9) and the congenital scoliosis (CS; n = 9) were harvested intraoperatively. The growth plates were harvested from apical area in AIS patients and from normal region in CS patients. The biopsies were prepared with routine histological methods for quantitative histomorphometric analysis. Apoptosis and cell proliferation of the growth plate chondrocytes were examined by triphosphate-biotin nick end labeling assay and immunohistochemistry of proliferating cell nuclear antigen antibody.

RESULTS

The growth plates of AIS and CS were shown to have normal architectures as the normal growth plate. However, it is shown that the proliferative and hypertrophic chondrocytes in the anterior column of AIS patients was more active in terms of the zonal area and height, proliferative chondrocytes, and apoptotic chondrocytes than that of the posterior column (P < 0.05). The difference found in AIS patients was not observed in CS patients.

CONCLUSION

The difference in histomorphometry and cellular activity between the anterior and posterior column in AIS and CS patients indicated that these two groups of patients have different growth kinetics which may affect the curve development.

Involvement of cyclic guanosine monophosphate-dependent protein kinase II in chondrocyte hypertrophy during endochondral ossification

During vertebrate skeletal development, the appendicular skeleton forms through endochondral ossification, which involves the intricately regulated multistep differentiation of mesenchymal cells. During this process, mesenchymal condensations initially differentiate into chondrocytes. Then chondrocytes in the center further differentiate into hypertrophic chondrocytes. Hypertrophic chondrocytes express a number of osteogenic factors and induce bone formation. Although numerous studies have provided novel insights into the regulation and function of cartilage development, little is known about the intracellular signaling pathways regulating chondrocyte hypertrophy. Recent study revealed that cyclic guanosine monophosphate (cGMP)-dependent protein kinase II (cGKII) coupled the stop of proliferation and the start of hypertrophic differentiation of chondrocytes. Herein, we review the molecular mechanism of regulation of chondrocyte hypertrophy by cGKII and the interaction between cGKII and other signaling pathways.

Changes in perlecan during chondrocyte differentiation in the fetal bovine rib growth plate

Perlecan is a heparan sulfate proteoglycan present in the growth plate and essential for endochondral ossification. We evaluated the synthesis and structure of perlecan in the different zones of the growth plate. The growth plates from fetal bovine ribs were isolated and sequentially sliced into 1-mm sections containing the hypertrophic zone, lower proliferative zone, upper proliferative zone, intermediate zone, and resting zone, respectively. The slices were then either incubated in culture medium with 35SO4 to measure total sulfated proteoglycan synthesis and perlecan synthesis, extracted for perlecan core protein analysis by Western blot, or extracted for perlecan isolation and subsequent characterization of glycosaminoglycan size and disaccharide composition. 35SO4 incorporation into perlecan was three-fourfold higher in the proliferating/hypertrophic zone than the resting zone. Western blot showed perlecan content was greatest in the lower and upper proliferating zones and that a perlecan fragment lacking portions of the N- and C-terminal domains containing heparan sulfate was also present in all zones. Purified perlecan from the hypertrophic/lower proliferative zone had larger chondroitin sulfate chains and a different composition of CS and HS disaccharides than the perlecan isolated from the resting zone. These results indicate perlecan deposition is increased and is turned over during proliferation to be replaced by a perlecan with a different sulfation pattern.

Modulation of FGF-2 binding to chondrocytes from the developing growth plate by perlecan

FGF-2 is a regulator of chondrocyte proliferation in the developing growth plate and has been shown to bind to perlecan, a heparan sulfate proteoglycan. We evaluated the effect of perlecan isolated from the growth plate on the binding of FGF-2 to its low and high affinity receptors on resting and proliferating chondrocytes. Chondrocytes were isolated by pronase/collagenase digestion of 1 mm thick slices from the resting and proliferating zones of fetal bovine ribs and were plated in serum-free DMEM. Chondrocytes maintained their zone-specific level of DNA and matrix synthesis over a two-day culture period. The collagen, aggrecan, and perlecan components of the matrix produced were associated with the cell layer and were secreted into the medium. Most of the perlecan made by the chondrocytes was secreted into the medium. Western blots showed medium perlecan to contain two high molecular weight core proteins and overlay assays showed only the large core protein bound FGF-2. Cell layer perlecan contained only the smaller core protein. Immunoprecipitation assays of media showed that the medium perlecan bound (125)I-FGF-2, that the bound FGF-2 was eluted from perlecan by 2 M NaCl at pH 7.4, and that this binding was eliminated by prior digestion with heparatinase. This indicates that the perlecan secreted into the medium is a low affinity receptor for FGF-2. (125)I-FGF-2 also bound to the chondrocytes in cell culture. Competition studies showed exogenous FGF-2 reduced (125)I-FGF-2 binding to high affinity receptor but not the low affinity receptor in the cell layer. Exogenous perlecan, however, reduced (125)I-FGF-2 binding to both the low and the high affinity receptors in the cell layer by approximately 60%. The results suggest that perlecan made by growth plate chondrocytes is a low affinity receptor for FGF-2 and acts to sequester FGF-2 away from the high affinity receptor.