Chondroadherin expression changes in skeletal development

Proteomic signatures for identification of impaired glucose tolerance

The implementation of recommendations for type 2 diabetes (T2D) screening and diagnosis focuses on the measurement of glycated hemoglobin (HbA1c) and fasting glucose. This approach leaves a large number of individuals with isolated impaired glucose tolerance (iIGT), who are only detectable through oral glucose tolerance tests (OGTTs), at risk of diabetes and its severe complications. We applied machine learning to the proteomic profiles of a single fasted sample from 11,546 participants of the Fenland study to test discrimination of iIGT defined using the gold-standard OGTTs. We observed significantly improved discriminative performance by adding only three proteins (RTN4R, CBPM and GHR) to the best clinical model (AUROC = 0.80 (95% confidence interval: 0.79-0.86), P = 0.004), which we validated in an external cohort. Increased plasma levels of these candidate proteins were associated with an increased risk for future T2D in an independent cohort and were also increased in individuals genetically susceptible to impaired glucose homeostasis and T2D. Assessment of a limited number of proteins can identify individuals likely to be missed by current diagnostic strategies and at high risk of T2D and its complications.

Quantitative proteomic analysis of Bi Zhong Xiao decoction against collagen-induced arthritis rats in the early and late stages

Background

Rheumatoid arthritis (RA) is a chronic, progressive, systemic autoimmune inflammatory disease. Bi Zhong Xiao decoction (BZXD) performs multiple functions for rheumatoid arthritis (RA) treatment for decades. In this study, we aimed to study the protein alterations of BZXD in the early and late stages of RA.

Methods

Sprague–Dawley rats were randomly divided into the Control, collagen-induced arthritis (CIA) and BZXD groups. Clinical assessment, paw thickness, weight changes and serum inflammatory cytokine levels were used to evaluate anti-inflammatory effects. Histopathological tests were performed to assess the improvement of inflammation and synovial hyperplasia. Moreover, we analyzed the proteins profiling of synovial tissue samples with different time intervals after BZXD treatment by Isobaric Tag for Relative Absolute (ITRAQ) quantitative proteomics technology. To further explore the interrelationships among differentially expressed proteins (DEPs), we used DAVID Bioinformatics Resources v6.8 and STRING 11.0 for bioinformatics analysis. Besides, the western blot and immunohistochemistry were exerted to verify related proteins.

Results

In our study, BZXD ameliorated joint inflammation, and suppressed the pathological changes in arthrosis of CIA rats. The proteomic analysis demonstrated that CIA rats were mainly involved in two significant pathways (the focal adhesion and the ECM-receptor interaction) in the early stage. BZXD down-regulated the expression of proteins involved in these pathways, such as CAV1, CHAD, COL3A1, COL5A2, COL6A1, and COL6A5. Additionally, BZXD exerts anti-inflammatory effects in the late stage mainly by increasing the expression of FASN and affecting fatty acid metabolism.

Conclusion

BZXD exerts therapeutic effects on RA through multi-pathways in the early and late stages. This work may provide proteomic clues for treating RA by BZXD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-022-03663-5.

Bone formation transcripts dominate the differential gene expression profile in an equine osteoporotic condition associated with pulmonary silicosis

Osteoporosis has been associated with pulmonary silicosis in California horses exposed to soils rich in cytotoxic silica dioxide crystals, a syndrome termed silicate associated osteoporosis (SAO). The causal mechanism for the development of osteoporosis is unknown. Osteoporotic lesions are primarily located in bone marrow-rich sites such as ribs, scapula and pelvis. Gene transcription patterns within bone marrow and pulmonary lymph nodes of affected horses may offer clues to disease pathobiology. Bone marrow core and tracheobronchial lymph node tissue samples harvested postmortem from affected and unaffected horses were examined histologically and subjected to RNA sequencing (RNA-seq). Sequenced data were analyzed for differential gene expression and gene ontology. Metatranscriptomic and metagenomic assays evaluated samples for infectious agents. Thirteen of 17 differentially expressed transcripts in bone marrow were linked to bone and cartilage formation such as integrin binding bone sialoprotein (log2FC = 3.39, PFDR = 0.013) and chondroadherin (log2FC = 4.48, PFDR = 0.031). Equus caballus solute carrier family 9, subfamily A2 (log2FC = 3.77, PFDR = 0.0034) was one of the four differentially expressed transcripts linked to osteoclast activity. Osteoblasts were hyperplastic and hypertrophic in bone marrow from affected horses. Biological pathways associated with skeletal morphogenesis were significantly enriched in affected horses. The 30 differentially expressed genes in affected lymph nodes were associated with inflammatory responses. Evidence of infectious agents was not found. The SAO affected bone marrow molecular signature demonstrated increased transcription and heightened activation of osteoblasts. Increased osteoblastic activity could be part of the pathological mechanism for osteoporosis or a compensatory response to the accelerated osteolysis. Transcriptome data offer gene targets for inquiries into the role of osteocytes and osteoblasts in SAO pathogenesis. Viral or bacterial infectious etiology in SAO is less likely based on metatranscriptomic and metagenomic data but cannot be completely ruled out.

Crystal structure of human chondroadherin: solving a difficult molecular-replacement problem usingde novomodels

Chondroadherin (CHAD) is a cartilage matrix protein that mediates the adhesion of isolated chondrocytes. Its protein core is composed of 11 leucine-rich repeats (LRR) flanked by cysteine-rich domains. CHAD makes important interactions with collagen as well as with cell-surface heparin sulfate proteoglycans and α2β1integrins. The integrin-binding site is located in a region of hitherto unknown structure at the C-terminal end of CHAD. Peptides based on the C-terminal human CHAD (hCHAD) sequence have shown therapeutic potential for treating osteoporosis. This article describes a still-unconventional structure solution by phasing withde novomodels, the first of a β-rich protein. Structure determination of hCHAD using traditional, though nonsystematic, molecular replacement was unsuccessful in the hands of the authors, possibly owing to a combination of low sequence identity to other LRR proteins, four copies in the asymmetric unit and weak translational pseudosymmetry. However, it was possible to solve the structure by generating a large number ofde novomodels for the central LRR domain usingRosettaand multiple parallel molecular-replacement attempts usingAMPLE. The hCHAD structure reveals an ordered C-terminal domain belonging to the LRRCT fold, with the integrin-binding motif (WLEAK) being part of a regular α-helix, and suggests ways in which experimental therapeutic peptides can be improved. The crystal structure itself and docking simulations further support that hCHAD dimers form in a similar manner to other matrix LRR proteins.

Assessing the osteoblast transcriptome in a model of enhanced bone formation due to constitutive Gs-G protein signaling in osteoblasts

G protein-coupled receptor (GPCR) signaling in osteoblasts (OBs) is an important regulator of bone formation. We previously described a mouse model expressing Rs1, an engineered constitutively active Gs-coupled GPCR, under the control of the 2.3 kb Col I promoter. These mice showed a dramatic age-dependent increase in trabecular bone of femurs. Here, we further evaluated the effects of enhanced Gs signaling in OBs on intramembranous bone formation by examining calvariae of 1- and 9-week-old Col1(2.3)/Rs1 mice and characterized the in vivo gene expression specifically occurring in osteoblasts with activated Gs G protein-coupled receptor signaling, at the cellular level rather than in a whole bone. Rs1 calvariae displayed a dramatic increase in bone volume with partial loss of cortical structure. By immunohistochemistry, Osterix was detected in cells throughout the inter-trabecular space while Osteocalcin was expressed predominantly in cells along bone surfaces, suggesting the role of paracrine mediators secreted from OBs driven by 2.3 kb Col I promoter could influence early OB commitment, differentiation, and/or proliferation. Gene expression analysis of calvarial OBs revealed that genes affected by Rs1 signaling include those encoding proteins important for cell differentiation, cytokines and growth factors, angiogenesis, coagulation, and energy metabolism. The set of Gs-GPCRs and other GPCRs that may contribute to the observed skeletal phenotype and candidate paracrine mediators of the effect of Gs signaling in OBs were also determined. Our results identify novel detailed in vivo cellular changes of the anabolic response of the skeleton to Gs signaling in mature OBs.

Proteoglycan Expression Is Influenced by Mechanical Load in TMJ Discs

The expression and assembly of the extracellular matrix are profoundly associated with adaptive and pathological responses of the temporomandibular joint (TMJ). To better understand the adaptive responses of the TMJ disc to mechanical loading, we examined the expression of 2 modular proteoglycans and 10 small leucine-rich proteoglycans (SLRPs) at the mRNA and protein levels and determined the contents of proteoglycan-related glycosaminoglycans (GAGs) in rat TMJ discs in response to altered mechanical loading caused by an incisal bite plane. One hundred thirty 7-week-old male Wistar rats were assigned to control and bite plane groups. TMJ disc thickness and the intensity of toluidine blue staining of metachromasia increased in the posterior band after 2 weeks of wearing the bite plane. GAG content increased significantly in the bite plane group after 2 weeks. Quantitative real-time RT-PCR (reverse transcription polymerase chain reaction) analysis indicated that biglycan and chondroadherin mRNA levels increased after 2 weeks and that the level of decorin mRNA increased at 4 weeks. Versican mRNA levels increased after 3 weeks, particularly for the V0 and V1 versican isoforms, which carry more GAG attachment sites than do the V2 and V3 isoforms. Western analysis demonstrated a corresponding increase in the levels of versican, biglycan, and decorin core proteins at 4 weeks in the bite plane group. These results indicate that mechanical loading differentially influences proteoglycan mRNA expression and protein accumulation in the TMJ disc. The change in proteoglycan mRNA and protein levels may lead to the modulation of matrix–matrix and cell–matrix interactions and has important biological significance for adaptation to complicated biomechanical requirements and for tissue maintenance in the TMJ disc.

Cell and matrix modulation in prenatal and postnatal equine growth cartilage, zones of Ranvier and articular cartilage

Formation of synovial joints includes phenotypic changes of the chondrocytes and the organisation of their extracellular matrix is regulated by different factors and signalling pathways. Increased knowledge of the normal processes involved in joint development may be used to identify similar regulatory mechanisms during pathological conditions in the joint. Samples of the distal radius were collected from prenatal and postnatal equine growth plates, zones of Ranvier and articular cartilage with the aim of identifying Notch signalling components and cells with stem cell-like characteristics and to follow changes in matrix protein localisation during joint development. The localisation of the Notch signalling components Notch1, Delta4, Hes1, Notch dysregulating protein epidermal growth factor-like domain 7 (EGFL7), the stem cell-indicating factor Stro-1 and the matrix molecules cartilage oligomeric matrix protein (COMP), fibromodulin, matrilin-1 and chondroadherin were studied using immunohistochemistry. Spatial changes in protein localisations during cartilage maturation were observed for Notch signalling components and matrix molecules, with increased pericellular localisation indicating new synthesis and involvement of these proteins in the formation of the joint. However, it was not possible to characterise the phenotype of the chondrocytes based on their surrounding matrix during normal chondrogenesis. The zone of Ranvier was identified in all horses and characterised as an area expressing Stro-1, EGFL7 and chondroadherin with an absence of COMP and Notch signalling. Stro-1 was also present in cells close to the perichondrium, in the articular cartilage and in the fetal resting zone, indicating stem cell-like characteristics of these cells. The presence of stem cells in the articular cartilage will be of importance for the repair of damaged cartilage. Perivascular chondrocytes and hypertrophic cells of the cartilage bone interface displayed positive staining for EGFL7, which is a novel finding and suggests a role of EGFL7 in the vascular infiltration of growth cartilage.

Clinical and molecular description of a 17q21.33 microduplication in a girl with severe kyphoscoliosis and developmental delay

High proportion of disease-associated copy number variant maps to chromosome 17. Genomic studies have provided an insight into its complex genomic structure such as relative abundance of segmental duplication and intercepted repetitive elements. 17q21.31, 17q11.2 and 17q12 loci are well known on this chromosome and are associated with microdeletion and microduplication syndrome. No syndrome associated with 17q21.33 locus have been described. We report clinical, cytogenetic and molecular investigations of a 13 years-old girl admitted for evaluation of microcephaly, scoliosis, skeletal defects and learning difficulties. We carried out detailed analysis of the clinical phenotype of this patient and investigated the genetic basis using Agilent 180K Array Comparative Genomic Hybridization. We identified a ∼0.9 Mb de novo microduplication on chromosome 17q21.33. Four genes, COL1A1, SGCA, PPP1R9B and CHAD located within the duplicated region are possible candidates for clinical features present in our patients. Gene expression studies by real-time RT-PCR assay only showed an overexpression of SGCA (P < 0.01), a component of the dystrophin glycoprotein complex. Defect of SGCA was previously shown to lead to severe childhood autosomal recessive muscular dystrophy (LGMD2D) which result in progressive muscle weakness and can also be associated with hyperlordosis or scoliosis. Further cases with similar duplications are expected to be diagnosed. This will contribute to the delineation of this potential new microduplication syndrome and to improve genetic counseling.

The C-terminal domain of chondroadherin: a new regulator of osteoclast motility counteracting bone loss

Chondroadherin (CHAD) is a leucine-rich protein promoting cell attachment through binding to integrin α2 β1 and syndecans. We observed that CHAD mRNA and protein were lower in bone biopsies of 50-year-old to 65-year-old osteoporotic women and in bone samples of ovariectomized mice versus gender/age-matched controls, suggesting a role in bone metabolism. By the means of an internal cyclic peptide (cyclicCHAD), we observed that its integrin binding sequence impaired preosteoclast migration through a nitric oxide synthase 2-dependent mechanism, decreasing osteoclastogenesis and bone resorption in a concentration-dependent fashion, whereas it had no effect on osteoblasts. Consistently, cyclicCHAD reduced transcription of two nitric oxide downstream genes, migfilin and vasp, involved in cell motility. Furthermore, the nitric oxide donor, S-nitroso-N-acetyl-D,L-penicillamine, stimulated preosteoclast migration and prevented the inhibitory effect of cyclicCHAD. Conversely, the nitric oxide synthase 2 (NOS2) inhibitor, N5-(1-iminoethyl)-l-ornithine, decreased both preosteoclast migration and differentiation, confirming a role of the nitric oxide pathway in the mechanism of action triggered by cyclicCHAD. In vivo, administration of cyclicCHAD was well tolerated and increased bone volume in healthy mice, with no adverse effect. In ovariectomized mice cyclicCHAD improved bone mass by both a preventive and a curative treatment protocol, with an effect in line with that of the bisphosphonate alendronate, that was mimicked by the NOS2 inhibitor [L-N6-(1-Iminoethyl)-lysine.2 dihydrochloride]. In both mouse models, cyclicCHAD reduced osteoclast and bone resorption without affecting osteoblast parameters and bone formation. In conclusion, CHAD is a novel regulator of bone metabolism that, through its integrin binding domain, inhibits preosteoclast motility and bone resorption, with a potential translational impact for the treatment of osteoporosis.

Nanomechanical phenotype of chondroadherin-null murine articular cartilage

Chondroadherin (CHAD), a class IV small leucine rich proteoglycan/protein (SLRP), was hypothesized to play important roles in regulating chondrocyte signaling and cartilage homeostasis. However, its roles in cartilage development and function are not well understood, and no major osteoarthritis-like phenotype was found in the murine model with CHAD genetically deleted (CHAD^−/−). In this study, we used atomic force microscopy (AFM)-based nanoindentation to quantify the effects of CHAD deletion on changes in the biomechanical function of murine cartilage. In comparison to wild-type (WT) mice, CHAD-deletion resulted in a significant≈70–80% reduction in the indentation modulus, E_ind, of the superficial zone knee cartilage of 11 weeks, 4 months and 1 year old animals. This mechanical phenotype correlates well with observed increases in the heterogeneity collagen fibril diameters in the surface zone. The results suggest that CHAD mainly plays a major role in regulating the formation of the collagen fibrillar network during the early skeletal development. In contrast, CHAD-deletion had no appreciable effects on the indentation mechanics of middle/deep zone cartilage, likely due to the dominating role of aggrecan in the middle/deep zone. The presence of significant rate dependence of the indentation stiffness in both WT and CHAD^−/− knee cartilage suggested the importance of both fluid flow induced poroelasticity and intrinsic viscoelasticity in murine cartilage biomechanical properties. Furthermore, the marked differences in the nanomechanical behavior of WT versus CHAD^−/− cartilage contrasted sharply with the relative absence of overt differences in histological appearance. These observations highlight the sensitivity of nanomechanical tools in evaluating structural and mechanical phenotypes in transgenic mice.

Proteomic analysis profile of engineered articular cartilage with chondrogenic differentiated adipose tissue-derived stem cells loaded polyglycolic acid mesh for weight-bearing area defect repair

The present study was designed to investigate the possibility of full-thickness defects repair in porcine articular cartilage (AC) weight-bearing area using chondrogenic differentiated autologous adipose-derived stem cells (ASCs) with a follow-up of 3 and 6 months, which is successive to our previous study on nonweight-bearing area. The isolated ASCs were seeded onto the phosphoglycerate/polylactic acid (PGA/PLA) with chondrogenic induction in vitro for 2 weeks as the experimental group prior to implantation in porcine AC defects (8 mm in diameter, deep to subchondral bone), with PGA/PLA only as control. With follow-up time being 3 and 6 months, both neo-cartilages of postimplantation integrated well with the neighboring normal cartilage and subchondral bone histologically in experimental group, whereas only fibrous tissue in control group. Immunohistochemical and toluidine blue staining confirmed similar distribution of COL II and glycosaminoglycan in the regenerated cartilage to the native one. A vivid remolding process with repair time was also witnessed in the neo-cartilage as the compressive modulus significantly increased from 70% of the normal cartilage at 3 months to nearly 90% at 6 months, which is similar to our former research. Nevertheless, differences of the regenerated cartilages still could be detected from the native one. Meanwhile, the exact mechanism involved in chondrogenic differentiation from ASCs seeded on PGA/PLA is still unknown. Therefore, proteome is resorted leading to 43 proteins differentially identified from 20 chosen two-dimensional spots, which do help us further our research on some committed factors. In conclusion, the comparison via proteome provided a thorough understanding of mechanisms implicating ASC differentiation toward chondrocytes, which is further substantiated by the present study as a perfect supplement to the former one in nonweight-bearing area.

Regulation of Stem Cell Fate in a Three-Dimensional Micropatterned Dual-Crosslinked Hydrogel System

Micropatterning technology is a powerful tool for controlling the cellular microenvironment and investigating the effects of physical parameters on cell behaviors, such as migration, proliferation, apoptosis, and differentiation. Although there have been significant developments in regulating the spatial and temporal distribution of physical properties in various materials, little is known about the role of the size of micropatterned regions of hydrogels with different crosslinking densities on the response of encapsulated cells. In this study, novel alginate hydrogel system is engineered that can be micropatterned three-dimensionally to create regions that are crosslinked by a single mechanism or dual mechanisms. By manipulating micropattern size while keeping the overall ratio of single- to dual-crosslinked hydrogel volume constant, the physical properties of the micropatterned alginate hydrogels are spatially tunable. When human adipose-derived stem cells (hASCs) are photoencapsulated within micropatterned hydrogels, their proliferation rate is a function of micropattern size. Additionally, micropattern size dictates the extent of osteogenic and chondrogenic differentiation of photoencapsulated hASC. The size of 3D micropatterned physical properties in this new hydrogel system introduces a new design parameter for regulating various cellular behaviors, and this dual-crosslinked hydrogel system provides a new platform for studying proliferation and differentiation of stem cells in a spatially controlled manner for tissue engineering and regenerative medicine applications.

Chondroadherin fragmentation mediated by the protease HTRA1 distinguishes human intervertebral disc degeneration from normal aging

Chondroadherin, a member of the leucine-rich repeat family, has previously been demonstrated to be fragmented in some juveniles with idiopathic scoliosis. This observation led us to investigate adults with disc degeneration. Immunoblotting analysis demonstrated that non-degenerate discs from three different age groups show no chondroadherin fragmentation. Furthermore, the chondroadherin fragments in adult degenerate disc and the juvenile scoliotic disc were compared via immunoblot analysis and appeared to have a similar size. We then investigated whether or not chondroadherin fragmentation increases with the severity of disc degeneration. Three different samples with different severities were chosen from the same disc, and chondroadherin fragmentation was found to be more abundant with increasing severity of degeneration. This observation led us to the creation of a neoepitope antibody to the cleavage site observed. We then observed that the cleavage site in adult degenerate discs and juvenile scoliotic discs was identical as confirmed by the neoepitope antibody. Consequently, investigation of the protease capable of cleaving chondroadherin at this site was necessary. In vitro digests of disc tissue demonstrated that ADAMTS-4 and -5; cathepsins K, B, and L; and MMP-3, -7, -12, and -13 were incapable of cleavage of chondroadherin at this site and that HTRA1 was indeed the only protease capable. Furthermore, increased protein levels of the processed form of HTRA1 were demonstrated in degenerate disc tissues via immunoblotting. The results suggest that chondroadherin fragmentation can be used as a biomarker to distinguish the processes of disc degeneration from normal aging.

The skeletal phenotype of chondroadherin deficient mice

Chondroadherin, a leucine rich repeat extracellular matrix protein with functions in cell to matrix interactions, binds cells via their α2β1 integrin as well as via cell surface proteoglycans, providing for different sets of signals to the cell. Additionally, the protein acts as an anchor to the matrix by binding tightly to collagens type I and II as well as type VI. We generated mice with inactivated chondroadherin gene to provide integrated studies of the role of the protein. The null mice presented distinct phenotypes with affected cartilage as well as bone. At 3–6 weeks of age the epiphyseal growth plate was widened most pronounced in the proliferative zone. The proteome of the femoral head articular cartilage at 4 months of age showed some distinct differences, with increased deposition of cartilage intermediate layer protein 1 and fibronectin in the chondroadherin deficient mice, more pronounced in the female. Other proteins show decreased levels in the deficient mice, particularly pronounced for matrilin-1, thrombospondin-1 and notably the members of the α1-antitrypsin family of proteinase inhibitors as well as for a member of the bone morphogenetic protein growth factor family. Thus, cartilage homeostasis is distinctly altered. The bone phenotype was expressed in several ways. The number of bone sialoprotein mRNA expressing cells in the proximal tibial metaphysic was decreased and the osteoid surface was increased possibly indicating a change in mineral metabolism. Micro-CT revealed lower cortical thickness and increased structure model index, i.e. the amount of plates and rods composing the bone trabeculas. The structural changes were paralleled by loss of function, where the null mice showed lower femoral neck failure load and tibial strength during mechanical testing at 4 months of age. The skeletal phenotype points at a role for chondroadherin in both bone and cartilage homeostasis, however, without leading to altered longitudinal growth.

The glycosylation profile of osteoadherin alters during endochondral bone formation

Endochondral bone formation involves the dynamic interplay between the cells and their extracellular environment to facilitate the deposition of a calcified matrix. Numerous molecules are involved within this process, including collagens and non-collagenous proteins, and their post-translational modifications have been shown to effect their biomolecular interactions. Osteoadherin (OSAD), a keratin sulfate (KS)-substituted small leucine-rich proteoglycan has been isolated from mineralized tissues and is considered to be a mineralized tissue-specific protein. However, to date, information is limited concerning the dynamic expression and role of this proteoglycan during bone formation and the biomineralization process. The current study aimed to examine the dynamic expression of this protein throughout mouse metatarsal long bone development, from the cartilage anlagen (E15) to the fully formed bone (Adult). Using quantitative gene expression analysis we observed that OSAD was produced with the onset of mineralization and the formation of the ossification center. This finding was reflected in the localization studies, using both light and electron microscopy, and showed that initial OSAD localization was restricted to the endosteal surfaces of the diaphysis and forming metaphysis. Furthermore, we analyzed protein extracts, both mineral and non-mineral associated fractions, and showed that OSAD was substituted with varying patterns of glycosylation during bone development. Sequential enzymatic digestions of the non-mineral bound protein extracts demonstrated that OSAD lacked any KS chains throughout all development stages. Whereas, in the mineral bound fractions, with long bone maturation the substitution with KS became more apparent with development. Therefore, it can be concluded that different pools of OSAD are produced during endochondral bone formation and these may have specific roles in directing the mineralization process.

A novel de novo 1.8 Mb microdeletion of 17q21.33 associated with intellectual disability and dysmorphic features

We report on a de novo 17q21.33 microdeletion, 1.8 Mb in size, detected in a patient with mild intellectual disability, growth retardation, poor weight gain, microcephaly, long face, large beaked nose, thick lower lip, micrognathia and other dysmorphic features. The deletion was detected by whole-genome genotyping BeadChip assay and involves the genomic region between 45,682,246 and 47,544,816 bp on chromosome 17. Among the 24 RefSeq genes included in this deletion are the CA10 and CACNA1G genes that are involved in brain development and neurological processes. A possible candidate gene for the prenatal and postnatal growth retardation is the CHAD gene, which product chondroadherin is a cartilage protein with cell binding properties. These three genes may be responsible for the patient's phenotype.

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.

Proteomic analysis of Col11a1-associated protein complexes

Cartilage plays an essential role during skeletal development within the growth plate and in articular joint function. Interactions between the collagen fibrils and other extracellular matrix molecules maintain structural integrity of cartilage, orchestrate complex dynamic events during embryonic development, and help to regulate fibrillogenesis. To increase our understanding of these events, affinity chromatography and liquid chromatography/tandem mass spectrometry were used to identify proteins that interact with the collagen fibril surface via the amino terminal domain of collagen α1(XI) a protein domain that is displayed at the surface of heterotypic collagen fibrils of cartilage. Proteins extracted from fetal bovine cartilage using homogenization in high ionic strength buffer were selected based on affinity for the amino terminal noncollagenous domain of collagen α1(XI). MS was used to determine the amino acid sequence of tryptic fragments for protein identification. Extracellular matrix molecules and cellular proteins that were identified as interacting with the amino terminal domain of collagen α1(XI) directly or indirectly, included proteoglycans, collagens, and matricellular molecules, some of which also play a role in fibrillogenesis, while others are known to function in the maintenance of tissue integrity. Characterization of these molecular interactions will provide a more thorough understanding of how the extracellular matrix molecules of cartilage interact and what role collagen XI plays in the process of fibrillogenesis and maintenance of tissue integrity. Such information will aid tissue engineering and cartilage regeneration efforts to treat cartilage tissue damage and degeneration.

Development of an intact intervertebral disc organ culture system in which degeneration can be induced as a prelude to studying repair potential

The present work describes a novel bovine disc organ culture system with long-term maintenance of cell viability, in which degenerative changes can be induced as a prelude to studying repair. Discs were isolated with three different techniques: without endplates (NEP), with bony endplates (BEP) and with intact cartilage endplates (CEP). Swelling, deformation, and cell viability were evaluated in unloaded cultures. Degeneration was induced by a single trypsin injection into the center of the disc and the effect on cell viability and matrix degradation was followed. Trypsin-treated discs were exposed to TGFβ to evaluate the potential to study repair in this system. NEP isolated discs showed >75% maintained cell viability for up to 10 days but were severely deformed, BEP discs on the other hand maintained morphology but failed to retain cell viability having only 27% viable cells after 10 days. In CEP discs, both cell viability and morphology were maintained for at least 4 weeks where >75% of the cells were still viable. To mimic proteoglycan loss during disc degeneration, a single trypsin injection was administered to the center of the disc. This resulted in 60% loss of aggrecan, after 7 days, without affecting cell viability. When TGFβ was injected to validate that the system can be used to study a repair response following injection of a bio-active substance, proteoglycan synthesis nearly doubled compared to baseline synthesis. Trypsin-treated bovine CEP discs therefore provide a model system for studying repair of the degenerate disc, as morphology, cell viability and responsiveness to bio-active substances were maintained.

Microscale versus nanoscale scaffold architecture for mesenchymal stem cell chondrogenesis

Nanofiber scaffolds, produced by the electrospinning technique, have gained widespread attention in tissue engineering due to their morphological similarities to the native extracellular matrix. For cartilage repair, studies have examined their feasibility; however these studies have been limited, excluding the influence of other scaffold design features. This study evaluated the effect of scaffold design, specifically examining a range of nano to micron-sized fibers and resulting pore size and mechanical properties, on human mesenchymal stem cells (MSCs) derived from the adult bone marrow during chondrogenesis. MSC differentiation was examined on these scaffolds with an emphasis on temporal gene expression of chondrogenic markers and the pluripotent gene, Sox2, which has yet to be explored for MSCs during chondrogenesis and in combination with tissue engineering scaffolds. Chondrogenic markers of aggrecan, chondroadherin, sox9, and collagen type II were highest for cells on micron-sized fibers (5 and 9 μm) with pore sizes of 27 and 29 μm, respectively, in comparison to cells on nano-sized fibers (300 nm and 600 to 1400 nm) having pore sizes of 2 and 3 μm, respectively. Undifferentiated MSCs expressed high levels of the Sox2 gene but displayed negligible levels on all scaffolds with or without the presence of inductive factors, suggesting that the physical features of the scaffold play an important role in differentiation. Micron-sized fibers with large pore structures and mechanical properties comparable to the cartilage ECM enhanced chondrogenesis, demonstrating architectural features as well as mechanical properties of electrospun fibrous scaffolds enhance differentiation.

Identification and characterization of the integrin alpha2beta1 binding motif in chondroadherin mediating cell attachment

Chondroadherin is a leucine-rich repeat protein known to mediate adhesion of isolated cells via the integrin α(2)β(1) and to interact with collagen. In this work, we show that cell adhesion to chondroadherin leads to activation of MAPKs but does not result in cell spreading and division. This is in contrast to the spreading and dividing of cells grown on collagen, although the binding is mediated via the same α(2)β(1) receptor. We identified a cell binding motif, CQLRGLRRWLEAK(318) by mass spectrometry after protease digestion of chondroadherin. Cells adhering to the synthetic peptide CQLRGLRRWLEAK(318) remained round, as was observed when they bound to the intact protein. The peptide added in solution was able to inhibit cell adhesion to the intact protein in a dose-dependent manner and was also verified to bind to the α(2)β(1) integrin. A cyclic peptide, CQLRGLRRWLEAKASRPDATC(326), mimicking the structural constraints of this sequence in the intact protein, showed similar efficiency in inhibiting binding to chondroadherin. The unique peptide motif responsible for cellular binding is primarily located in the octamer sequence LRRWLEAK(318). Binding of cells to the active peptide or to chondroadherin immobilized on cell culture plates rapidly induces intracellular signaling (i.e. ERK phosphorylation). Thus, chondroadherin interaction with cells may be central for maintaining the adult chondrocyte phenotype and cartilage homeostasis. The peptides, particularly the more stable cyclic peptide, open new opportunities to modulate cell behavior in situations of tissue pathology.

Identification of SOX9 interaction sites in the genome of chondrocytes

BACKGROUND

Our previous work has provided strong evidence that the transcription factor SOX9 is completely needed for chondrogenic differentiation and cartilage formation acting as a "master switch" in this differentiation. Heterozygous mutations in SOX9 cause campomelic dysplasia, a severe skeletal dysmorphology syndrome in humans characterized by a generalized hypoplasia of endochondral bones. To obtain insights into the logic used by SOX9 to control a network of target genes in chondrocytes, we performed a ChIP-on-chip experiment using SOX9 antibodies.

METHODOLOGY/PRINCIPAL FINDINGS

The ChIP DNA was hybridized to a microarray, which covered 80 genes, many of which are involved in chondrocyte differentiation. Hybridization peaks were detected in a series of cartilage extracellular matrix (ECM) genes including Col2a1, Col11a2, Aggrecan and Cdrap as well as in genes for specific transcription factors and signaling molecules. Our results also showed SOX9 interaction sites in genes that code for proteins that enhance the transcriptional activity of SOX9. Interestingly, a strong SOX9 signal was also observed in genes such as Col1a1 and Osx, whose expression is strongly down regulated in chondrocytes but is high in osteoblasts. In the Col2a1 gene, in addition to an interaction site on a previously identified enhancer in intron 1, another strong interaction site was seen in intron 6. This site is free of nucleosomes specifically in chondrocytes suggesting an important role of this site on Col2a1 transcription regulation by SOX9.

CONCLUSIONS/SIGNIFICANCE

Our results provide a broad understanding of the strategies used by a "master" transcription factor of differentiation in control of the genetic program of chondrocytes.

Extracellular matrix molecules: potential targets in pharmacotherapy

The extracellular matrix (ECM) consists of numerous macromolecules classified traditionally into collagens, elastin, and microfibrillar proteins, proteoglycans including hyaluronan, and noncollagenous glycoproteins. In addition to being necessary structural components, ECM molecules exhibit important functional roles in the control of key cellular events such as adhesion, migration, proliferation, differentiation, and survival. Any structural inherited or acquired defect and/or metabolic disturbance in the ECM may cause cellular and tissue alterations that can lead to the development or progression of disease. Consequently, ECM molecules are important targets for pharmacotherapy. Specific agents that prevent the excess accumulation of ECM molecules in the vascular system, liver, kidney, skin, and lung; alternatively, agents that inhibit the degradation of the ECM in degenerative diseases such as osteoarthritis would be clinically beneficial. Unfortunately, until recently, the ECM in drug discovery has been largely ignored. However, several of today's drugs that act on various primary targets affect the ECM as a byproduct of the drugs' actions, and this activity may in part be beneficial to the drugs' disease-modifying properties. In the future, agents and compounds targeting directly the ECM will significantly advance the treatment of various human diseases, even those for which efficient therapies are not yet available.

Comprehensive profiling of cartilage extracellular matrix formation and maturation using sequential extraction and label-free quantitative proteomics

Articular cartilage is indispensable for joint function but has limited capacity for self-repair. Engineering of neocartilage in vitro is therefore a major target for autologous cartilage repair in arthritis. Previous analysis of neocartilage has targeted cellular organization and specific molecular components. However, the complexity of extracellular matrix (ECM) development in neocartilage has not been investigated by proteomics. To redress this, we developed a mouse neocartilage culture system that produces a cartilaginous ECM. Differential analysis of the tissue proteome of 3-week neocartilage and 3-day postnatal mouse cartilage using solubility-based protein fractionation targeted components involved in neocartilage development, including ECM maturation. Initially, SDS-PAGE analysis of sequential extracts revealed the transition in protein solubility from a high proportion of readily soluble (NaCl-extracted) proteins in juvenile cartilage to a high proportion of poorly soluble (guanidine hydrochloride-extracted) proteins in neocartilage. Label-free quantitative mass spectrometry (LTQ-Orbitrap) and statistical analysis were then used to filter three significant protein groups: proteins enriched according to extraction condition, proteins differentially abundant between juvenile cartilage and neocartilage, and proteins with differential solubility properties between the two tissue types. Classification of proteins differentially abundant between NaCl and guanidine hydrochloride extracts (n = 403) using bioinformatics revealed effective partitioning of readily soluble components from subunits of larger protein complexes. Proteins significantly enriched in neocartilage (n = 78) included proteins previously not reported or with unknown function in cartilage (integrin-binding protein DEL1; coiled-coil domain-containing protein 80; emilin-1 and pigment epithelium derived factor). Proteins with differential extractability between juvenile cartilage and neocartilage included ECM components (nidogen-2, perlecan, collagen VI, matrilin-3, tenascin and thrombospondin-1), and the relationship between protein extractability and ECM ultrastructural organization was supported by electron microscopy. Additionally, one guanidine extract-specific neocartilage protein, protease nexin-1, was confirmed by immunohistochemistry as a novel component of developing articular cartilage in vivo. The extraction profile and matrix-associated immunostaining implicates protease nexin-1 in cartilage development in vitro and in vivo.

Variation in chondroadherin abundance and fragmentation in the human scoliotic disc

STUDY DESIGN

Variation in abundance and structure of chondroadherin (CHAD) were studied in the extracellular matrix (ECM) of scoliotic and normal human discs.

OBJECTIVE

To determine whether CHAD abundance and fragmentation vary between different sides of the scoliotic disc and between scoliotic and normal discs.

SUMMARY OF BACKGROUND DATA

Scoliosis involves curvature of the spine including wedging of the intervertebral discs (IVDs), resulting in altered mechanical loading, which can influence cell metabolism and matrix structure in the IVD. A protein such as CHAD that can influence both cell metabolism and ECM organization could influence disc pathology in scoliosis.

METHODS

IVDs were obtained from patients with scoliosis and from normal individuals. A proteomic analysis was performed to identify molecules that exhibit side-specific variations in abundance. In addition, changes in the abundance and fragmentation of CHAD and other members of the leucine-rich repeat protein family were studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. Aggrecan fragmentation was used as an indicator of proteinase action.

RESULTS

The relative amount of CHAD was consistently lower on the concave side of the discs in all patients studied. In addition, proteolytic degradation of CHAD occurred in some patients with scoliosis, but not in normal IVDs. The presence of aggrecan fragments provided evidence for both aggrecanase and metalloproteinase activity in the scoliotic discs although no side-specific difference was found. Other members of the leucine-rich repeat family of proteins did not show evidence of the same consistent variation in abundance between the 2 sides and did not show signs of degradation.

CONCLUSION

As CHAD can interact with both the ECM and the cells, it can provide a mechanism for regulating cell metabolism and ECM structure, and so play a role in promoting matrix homeostasis. Thus, changes in CHAD abundance or structure could be associated with the pathologic changes occurring in the scoliotic IVD.

Purification, crystallization and preliminary X-ray diffraction analysis of human chondroadherin

Chondroadherin is a cartilage matrix protein that is known to mediate the adhesion of isolated chondrocytes. Its protein core is composed of 11 leucine-rich repeats flanked by cysteine-rich domains at the N- and C-terminal ends. Recombinant human chondroadherin was crystallized using the sitting-drop vapour-diffusion method. The crystals belong to the monoclinic space group P2(1), with unit-cell parameters a = 56.4, b = 111.3, c = 128.5 A, beta = 92.2, and are most likely to contain four molecules in the asymmetric unit. The crystals diffracted to at least 2.3 A using synchrotron radiation, but structure determination using molecular replacement has so far been unsuccessful.

Growth plate zonal microarray analysis shows upregulation of extracellular matrix genes and downregulation of metalloproteinases and cathepsins following irradiation

Although the growth plate matrix area fraction increases after irradiation, extracellular matrix (ECM) gene expression in this context has not been studied. The hypothesis was that normally expressed ECM genes would be upregulated after irradiation. The right limbs of six Sprague-Dawley 5-week-old rats were irradiated with the left limbs as controls. Half of the animals were harvested after 1 week and half after 2. Microarray was conducted from normal and irradiated tibial growth plate proliferative zone (PZ) and hypertrophic zone (HZ) chondrocytes separated by laser microdissection at each time point. In situ hybridization (ISH) and real-time polymerase chain reaction (PCR) were used to confirm expression of selected genes. At 1 and 2 weeks after irradiation, both normally expressed ECM genes and others not highly expressed in the normal growth plate showed upregulation. Metalloproteinases and cathepsins were downregulated. PZ gene expression after irradiation exhibited features of the normal HZ, suggesting premature terminal differentiation. ECM genes not highly expressed in the normal growth plate included several members of the small leucine-rich proteins and the ezrin-radixin-moesin family. The effects of irradiation on cathepsin K (Ctsk), integrin binding sialoprotein (Ibsp), and procollagen II alpha 1 (Col2a1), as determined by ISH and real-time PCR, were highly correlated with the microarray results. Accumulation of matrix following radiation injury to the growth plate correlated well with changes in gene expression. Upregulation of genes not normally highly expressed in the noninjured growth plate suggests their importance in the injury and repair response.

Changes in mRNA gene expression during growth in the femoral head of the young rat

The rate of physeal growth slows as an animal matures with changes in mRNA gene expression due to the altered cellular activity. To measure the change in gene expression during the juvenile growth period, the femoral head, enclosing the proximal femoral physis, primary spongiosa, and articular cartilage, was collected from both femora of 16 female Sprague-Dawley rats between 4 and 10 weeks of age. One femur of each rat had had a mid-diaphyseal femoral fracture at 4 weeks of age. RNA was extracted and hybridized to 16 Affymetrix Rat Genomic 230 2.0 GeneChip microarrays with probe sets for 31,000 genes of which 18,200 were expressed. Of these, 8002 genes had a significant change in gene expression during growth, about half increasing and half decreasing. These changes included up-regulation with time of genes related to cartilage, blood vessels, osteoprotegerin, osteomodulin, and most ribosomal proteins. There was down-regulation with maturity of genes related to bone, growth-promoting cytokines, G proteins, GTPase-mediated signal transduction factors, cytokine receptors, mitosis, integrin-linked kinase, and the cytoskeleton. In summary, the slowing of growth with maturity was associated with changes in mRNA gene expression in the femoral head for a large number of genes. These changes in gene expression between young and mature rats suggest factors which are important for the support of the rapid linear growth during early life.

Evidence for overgrowth after midfemoral fracture via increased RNA for mitosis

Middiaphyseal femoral fractures in children and young rats stimulate linear femoral growth, a phenomenon commonly attributed to increased vascularity. To test for changes in mRNA expression of genes related to blood vessels, nerve fibers, cartilage, bone, and cell metabolism, we measured mRNA gene expression for all known rat genes in the physis at various times after diaphyseal fracture. Female Sprague-Dawley rats, 4 weeks of age at surgery, were subjected to a unilateral, simple, transverse, middiaphyseal femoral fracture stabilized with an intramedullary rod. At 0 (intact), 0.1, 0.4, 1, 2, 3, 4, and 6 weeks after fracture, the femoral head with the proximal physis was collected from fractured and intact femora. The RNA was extracted, processed to biotinlabeled cRNA, and hybridized to Affymetrix Rat 230 2.0 GeneChip microarrays. Transcripts from fracture-induced lengthening of the injured femora were compared to those of the intact contralateral femur. In the proximal physis, transcripts related to blood vessels and cartilage formation were down-regulated by fracture. Transcripts related to bone remodeling, nerve axon elongation, cell division, and protein synthesis were up-regulated by fracture. The data support increased mitotic activity in the physis after a midshaft fracture and not increased vascularity.

Differential adherence of osteoarthritis and rheumatoid arthritis synovial fibroblasts to cartilage and bone matrix proteins and its implication for osteoarthritis pathogenesis

In osteoarthritis (OA), cartilage and bone fragments have been described within the synovial tissue which are surrounded by synovial cells (i.e. detritus synovitis). These cells appear to attach actively to the cartilage and bone fragments. In rheumatoid arthritis (RA), on the other hand, synovial fibroblasts (SF) have also been shown to be localized at sites of invasion into cartilage and bone and to degrade extracellular matrix (ECM) by secreting proteolytic enzymes. One prerequisite for exerting their aggressive properties is the attachment to cartilage and bone ECM. This attachment appears to be mediated by the expression of different adhesion molecules for which corresponding binding sites on ECM components are known. As it has not been addressed to which ECM proteins SF adhere and with which affinity this process takes place, we investigated the adherence of SF from patients with OA and RA to different cartilage and bone matrix proteins. Synovial tissue samples were obtained during synovectomy or arthroplastic surgery and used for isolating and culturing SF. Synovial cells attaching to cartilage/bone fragments were characterized using immunohistochemistry. The adherence of SF to ECM proteins was examined using an adhesion assay with the following proteins coated on 96-well plates: aggrecan (AGG), bone sialoprotein (BSP), cartilage oligomeric matrix protein (COMP), collagen type I, II and VI, proline arginine-rich, end leucine-rich repeat protein (PRELP), osteopontin (OPN) and recombinant chondroadherin (CHAD). Bovine serum albumin was used as negative control. In addition, adhering fibroblasts were photographed using a phase-contrast microscope. As compared with RA-SF, significantly higher numbers of OA-SF adhering to collagen type II, OPN and CHAD could be detected (P < 0.05). In contrast, RA-SF showed increased attachment to collagen type II, OPN and BSP. Adhesion to AGG, COMP and PRELP appeared not to be significantly increased and differed widely among the SF samples, and, apart from one exception (BSP), OA-SF adhered in higher numbers to the matrix proteins than did RA-SF. Using immunohistochemistry, synovial cells attached to cartilage/bone fragments could be shown to predominantly express CD68 (>/=50%). The CD68-negative population was of the fibroblast phenotype (AS02 positive). The study demonstrates that the binding pattern of OA-SF and RA-SF to ECM proteins differs considerably and therefore provides novel insights into the difficult pathophysiology of OA and RA. In general, it appeared that SF adhere primarily to ECM proteins that contain known binding sites for adhesion molecules (e.g. integrins: collagen/integrin alpha(2)beta(1)) and that higher numbers of OA-SF adhered to the cartilage and bone matrix proteins than did RA-SF.

Fibronectin fragments cause release and degradation of collagen-binding molecules from equine explant cultures

OBJECTIVE

Previous experiments have shown that addition of fragmented fibronectin can induce cartilage chondrolysis. In this study we investigated the fate of the collagen- and cell-binding molecules Cartilage oligomeric matrix protein (COMP) and chondroadherin.

DESIGN

Equine articular cartilage explants were stimulated with the C-terminal and the N-terminal heparin-binding fragments of fibronectin respectively, and the conditioned media were analysed by both quantitative (ELISA) and qualitative (mass spectrometry, Western blots) methods.

RESULTS

Both COMP and chondroadherin were released in a dose-dependent manner upon stimulation with the Hep II (C-terminal heparin-binding) fragment of fibronectin. The kinetics of release for the two components differed. Moreover, COMP was degraded while no fragments of chondroadherin could be detected. Stimulation with Hep II also induced production of nitric oxide in a dose-dependent manner. We compared effects of the Hep II fragment with that of Hep I (the N-terminal heparin-binding fragment of fibronectin) and found that while Hep I did indeed elicit release of COMP and chondroadherin, the response was less potent, and production of nitric oxide was negligible. The responses to both fragments were elicited within 24h.

CONCLUSIONS

We suggest that the events described here may be early, critical stages in cartilage destruction preceding collagen destruction.

Association of chondroadherin with collagen type II

Chondroadherin is a cell binding, leucine-rich repeat protein found in the territorial matrix of articular cartilage. Several members of the leucine-rich repeat protein family present in the extracellular matrix of e.g. cartilage have been shown to interact with collagen and influence collagen fibrillogenesis. We show that complexes of monomeric collagen type II and chondroadherin can be released under non-denaturing conditions from articular cartilage treated with p-aminophenylmercuric acetate to activate resident matrix metalloproteinases. Purified complexes as well as complexes formed in vitro between recombinant chondroadherin and collagen type II were studied by electron microscopy. Chondroadherin was shown to bind to two sites on collagen type II. The interaction was characterized by surface plasmon resonance analysis showing K(D) values in the nanomolar range. Both chondroadherin and collagen interact with chondrocytes, partly via the same receptor, but give rise to different cellular responses. By also interacting with each other, a complex system is created which may be of functional importance for the communication between the cells and its surrounding matrix and/or in the regulation of collagen fibril assembly.

Chondrogenic differentiation of mesenchymal stem cells from bone marrow: differentiation-dependent gene expression of matrix components

Transforming growth factor (TGF)-beta-induced chondrogenesis of mesenchymal stem cells derived from bone marrow involves the rapid deposition of a cartilage-specific extracellular matrix. The sequential events in this pathway leading from the undifferentiated stem cell to a mature chondrocyte were investigated by analysis of key matrix elements. Differentiation was rapidly induced in cells cultured in the presence of TGF-beta 3 or -beta 2 and was accompanied by the early expression of fibromodulin and cartilage oligomeric matrix protein. An increase in aggrecan and versican core protein synthesis defined an intermediate stage, which also involved the small leucine-rich proteoglycans decorin and biglycan. This was followed by the appearance of type II collagen and chondroadherin. The pathway was also characterized by the appearance of type X collagen, usually associated with hypertrophic cartilage. There was also a change in the pattern of sulfation of chondroitin sulfate, with a progressive increase in the proportion of 6-sulfated species. The major proportion of newly synthesized glycosaminoglycan was part of an aggregating proteoglycan network. These data allow us to define the phenotype of the differentiated cell and to understand in greater detail the sequential process of matrix assembly.

A comparison of the expression pattern of five genes of the family of small leucine-rich proteoglycans during mouse development

For five members of the family of the small leucine-rich proteoglycans (SLRPs), the expression pattern during fetal development was analyzed. RNA in situ hybridization on whole body sections of mouse embryos was performed for biglycan (Bgn), decorin (Dcn), fibromodulin (Fmod), chondroadherin (Chad), and lumican (Lum). Special attention was given to the question of whether these patterns coincide only with sites of collagen secretion in connective tissue during tissue modeling or if expression can be observed at specific sites of organ differentiation also. In general, Fmod, Lum, and Bgn are expressed at sites of cartilage and bone formation and interstitial tissue deposition; Chad is expressed only at sites of cartilage; and Dcn is expressed only at sites of interstitial tissue deposition. However, there are some distinct developmental stages where no collagen secretion is known to occur. For example, this applies for the expression of Fmod in the forming somites of stage 9.5 postconception (p.c.), for Dcn and Lum in later stage embryos in the pituitary gland and dorsal root ganglia, and for Bgn and Dcn during differentiation in the kidney. These studies provide further evidence for a role of these molecules during connective tissue organization but also for an involvement at specific sites of organ differentiation.

Super-motifs and evolution of tandem leucine-rich repeats within the small proteoglycans--biglycan, decorin, lumican, fibromodulin, PRELP, keratocan, osteoadherin, epiphycan, and osteoglycin

Leucine-rich repeats (LRRs) with 20-30 amino acids in unit length are present in many proteins from prokaryotes to eukaryotes. The LRR-containing proteins include a family of nine small proteoglycans, forming three distinct subfamilies: class I contains biglycan/PG-I and decorin/PG-II; class II: lumican, fibromodulin, PRELP, keratocan, and osteoadherin; and class III: epiphycan/PG-Lb and osteoglycin or osteoinductive factor. Comparative sequence analysis of the 34 available protein sequences reveals that these proteoglycans have two types of LRRs, which we call S and T. The type S LRR is 21 residues long and has the consensus sequence of xxaPzxLPxxLxxLxLxxNxI. The type T LRR has 26 residues; its consensus sequence is zzxxaxxxxFxxaxxLxxLxLxxNxL. In both "x" indicates variable residue; "z" is frequently a gap; "a" is Val, Leu, or Ile; and I is Ile or Leu. These type S and TLRRs are ordered into two super-motifs--STT with about 73 residues in classes I and II and ST with about 47 residues in class III. The 12 LRRs in the small proteoglycans of I and II are best represented as (STT)4; the seven LRRs of class III as (ST)T(ST)2. Our analyses indicate that classes I/II and III evolved along different paths after the establishment of the precursor ST, and classes I and II also diverged after the establishment of the precursor (STT)4.

The mouse chondroadherin gene: characterization and chromosomal localization

The mouse chondroadherin gene was isolated from a cosmid genomic library by the use of a rat chondroadherin cDNA probe. Southern blot analysis of mouse genomic DNA revealed a simple pattern of hybridization indicating a single copy gene for chondroadherin. The mouse chondroadherin gene encompasses 4.1 kb and consists of four exons separated by one large intron of 1929 bp followed by two smaller introns of 247 and 225 bp, respectively. Most of the translated region, including the start codon and the main part of a leucine-rich region, is contained within the first exon. Two small exons of 164 and 146 bp encode the rest of the protein. Interestingly, 4 bases from the stop codon, in the 3'-UTR, a third intron is located. A putative promoter region of 669 bp was sequenced and shown to contain a potential TATAA-box signal 29 bp upstream of the transcription start site and several recognition sites for transcription factors. The exon/intron organization of the chondroadherin gene differs from those of the other known genes of the leucine-rich repeat (LRR) family in the extracellular matrix. Taken together with comparison of protein sequences of other members of the LRR family in the extracellular matrix, the data suggest that chondroadherin has evolved along a different pathway. The chondroadherin gene was mapped to mouse chromosome 11, near D11Mit14, by single-strand conformation polymorphism linkage analysis.