Interaction of cartilage matrix protein with aggrecan. Increased covalent cross-linking with tissue maturation

Cartilage-Specific Cre Recombinase Transgenes/Alleles in the Mouse

Cartilage is a specialized skeletal tissue with a unique extracellular matrix elaborated by its resident cells, chondrocytes. The tissue presents in several forms, including growth plate and articular cartilage, wherein chondrocytes follow a differential differentiation program and have different fates. The induction of gene modifications in cartilage specifically relies on mouse transgenes and knockin alleles taking advantages of transcriptional elements primarily active in chondrocytes at a specific differentiation stage or in a specific cartilage type. These transgenes/alleles have been widely used to study the roles of specific genes in cartilage development, adult homeostasis, and pathology. As cartilage formation is critical for postnatal life, the inactivation or significant alteration of key cartilaginous genes is often neonatally lethal and therefore hampers postnatal studies. Gold standard approaches to induce postnatal chondrocyte-specific gene modifications include the Cre-loxP and Tet-ON/OFF systems. Selecting the appropriate promoter/enhancer sequences to drive Cre expression is of crucial importance and determines the specificity of conditional gain- or loss-of-function models. In this chapter, we discuss a series of transgenes and knockin alleles that have been developed for gene manipulation in cartilage and we compare their expression patterns and efficiencies.

Mice Lacking the Matrilin Family of Extracellular Matrix Proteins Develop Mild Skeletal Abnormalities and Are Susceptible to Age-Associated Osteoarthritis

Matrilins (MATN1, MATN2, MATN3 and MATN4) are adaptor proteins of the cartilage extracellular matrix (ECM), which bridge the collagen II and proteoglycan networks. In humans, dominant-negative mutations in MATN3 lead to various forms of mild chondrodysplasias. However, single or double matrilin knockout mice generated previously in our laboratory do not show an overt skeletal phenotype, suggesting compensation among the matrilin family members. The aim of our study was to establish a mouse line, which lacks all four matrilins and analyze the consequence of matrilin deficiency on endochondral bone formation and cartilage function. Matn1-4^−/− mice were viable and fertile, and showed a lumbosacral transition phenotype characterized by the sacralization of the sixth lumbar vertebra. The development of the appendicular skeleton, the structure of the growth plate, chondrocyte differentiation, proliferation, and survival were normal in mutant mice. Biochemical analysis of knee cartilage demonstrated moderate alterations in the extractability of the binding partners of matrilins in Matn1-4^−/− mice. Atomic force microscopy (AFM) revealed comparable compressive stiffness but higher collagen fiber diameters in the growth plate cartilage of quadruple mutant compared to wild-type mice. Importantly, Matn1-4^−/− mice developed more severe spontaneous osteoarthritis at the age of 18 months, which was accompanied by changes in the biomechanical properties of the articular cartilage. Interestingly, Matn4^−/− mice also developed age-associated osteoarthritis suggesting a crucial role of MATN4 in maintaining the stability of the articular cartilage. Collectively, our data provide evidence that matrilins are important to protect articular cartilage from deterioration and are involved in the specification of the vertebral column.

The extracellular matrix as a multitasking player in disease

Extracellular matrices (ECMs) are highly specialized and dynamic three-dimensional (3D) scaffolds into which cells reside in tissues. ECM is composed of a variety of fibrillar components, such as collagens, fibronectin, and elastin, and non-fibrillar molecules as proteoglycans, hyaluronan, and glycoproteins including matricellular proteins. These macromolecular components are interconnected forming complex networks that actively communicate with cells through binding to cell surface receptors and/or matrix effectors. ECMs exert diverse roles, either providing tissues with structural integrity and mechanical properties essential for tissue functions or regulating cell phenotype and functions to maintain tissue homeostasis. ECM molecular composition and structure vary among tissues, and is markedly modified during normal tissue repair as well as during the progression of various diseases. Actually, abnormal ECM remodeling occurring in pathologic circumstances drives disease progression by regulating cell-matrix interactions. The importance of matrix molecules to normal tissue functions is also highlighted by mutations in matrix genes that give rise to genetic disorders with diverse clinical phenotypes. In this review, we present critical and emerging issues related to matrix assembly in tissues and the multitasking roles for ECM in diseases such as osteoarthritis, fibrosis, cancer, and genetic diseases. The mechanisms underlying the various matrix-based diseases are also discussed. Research focused on the highly dynamic 3D ECM networks will help to discover matrix-related causative abnormalities of diseases as well as novel diagnostic tools and therapeutic targets.

Deficient Mechanical Activation of Anabolic Transcripts and Post-Traumatic Cartilage Degeneration in Matrilin-1 Knockout Mice

Matrilin-1 (Matn1), a cartilage-specific peri-cellular and extracellular matrix (ECM) protein, has been hypothesized to regulate ECM interactions and transmit mechanical signals in cartilage. Since Matn1 knock-out (Matn1^-/-) mice exhibit a normal skeleton, its function in vivo is unclear. In this study, we found that the anabolic Acan and Col2a transcript levels were significantly higher in wildtype (Matn1^+/+) mouse cartilage than that of MATN1^-/- mice in vivo. However, such difference was not observed between Matn1^+/+ and MATN1^-/- chondrocytes cultured under stationary conditions in vitro. Cyclic loading significantly stimulated Acan and Col2a transcript levels in Matn1^+/+ but not in MATN1^-/- chondrocytes. This suggests that, while Matn1^+/+ chondrocytes increase their anabolic gene expression in response to mechanical loading, the MATN1^-/- chondrocytes fail to do so because of the deficiency in mechanotransduction. We also found that altered elastic modulus of cartilage matrix in Matn1^-/- mice, suggesting the mechanotransduction has changed due to the deficiency of Matn1. To understand the impact of such deficiency on joint disease, mechanical loading was altered in vivo by destabilization of medial meniscus. While Matn1^+/+ mice exhibited superficial fissures and clefts consistent with mechanical damage to the articular joint, Matn1^-/- mice presented more severe cartilage lesions characterized by proteoglycan loss and disorganization of cells and ECM. This suggests that Matn1 deficiency affects pathogenesis of post-traumatic osteoarthritis by failing to up-regulate anabolic gene expression. This is the first demonstration of Matn1 function in vivo, which suggests its protective role in cartilage degeneration under altered mechanical environment.

How to build an inducible cartilage-specific transgenic mouse

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

Insulin-like growth factor binding proteins 4 and 7 released by senescent cells promote premature senescence in mesenchymal stem cells

Cellular senescence is the permanent arrest of cell cycle, physiologically related to aging and aging-associated diseases. Senescence is also recognized as a mechanism for limiting the regenerative potential of stem cells and to protect cells from cancer development. The senescence program is realized through autocrine/paracrine pathways based on the activation of a peculiar senescence-associated secretory phenotype (SASP). We show here that conditioned media (CM) of senescent mesenchymal stem cells (MSCs) contain a set of secreted factors that are able to induce a full senescence response in young cells. To delineate a hallmark of stem cells SASP, we have characterized the factors secreted by senescent MSC identifying insulin-like growth factor binding proteins 4 and 7 (IGFBP4 and IGFBP7) as key components needed for triggering senescence in young MSC. The pro-senescent effects of IGFBP4 and IGFBP7 are reversed by single or simultaneous immunodepletion of either proteins from senescent-CM. The blocking of IGFBP4/7 also reduces apoptosis and promotes cell growth, suggesting that they may have a pleiotropic effect on MSC biology. Furthermore, the simultaneous addition of rIGFBP4/7 increased senescence and induced apoptosis in young MSC. Collectively, these results suggest the occurrence of novel-secreted factors regulating MSC cellular senescence of potential importance for regenerative medicine and cancer therapy.

Abnormal bone quality in cartilage oligomeric matrix protein and matrilin 3 double-deficient mice caused by increased tissue inhibitor of metalloproteinases 3 deposition and delayed aggrecan degradation

OBJECTIVE

Cartilage oligomeric matrix protein (COMP) and matrilin 3 are extracellular matrix proteins that are abundant in cartilage. As adaptor molecules, both proteins bridge and stabilize macromolecular networks consisting of fibrillar collagens and proteoglycans. Mutations in the genes coding for COMP and matrilin 3 have been linked to human chondrodysplasias, while in mice, deficiency in COMP or matrilin 3 does not cause any pronounced skeletal abnormalities. Given the similar functions of COMP and matrilin 3 in the assembly and stabilization of the extracellular matrix, our aim was to determine whether these proteins could functionally compensate for each other.

METHODS

To assess this putative redundancy of COMP and matrilin 3, we generated COMP/matrilin 3 double-deficient mice and performed an in-depth analysis of their skeletal development.

RESULTS

At the newborn stage, the overall skeletal morphology of the double mutants was normal, but at 1 month of age, the long bones were shortened and the total body length reduced. Peripheral quantitative computed tomography revealed increased metaphyseal trabecular bone mineral density in the femora. Moreover, the degradation of aggrecan in the cartilage remnants in the metaphyseal trabecular bone was delayed, paralleled by increased deposition of tissue inhibitor of metalloproteinases 3 (TIMP-3). The structure and morphology of the growth plate were grossly normal, but in the center, focal closures were observed, a phenotype very similar to that described in matrix metalloproteinase 13 (MMP-13)-deficient mice.

CONCLUSION

We propose that a lack of COMP and matrilin 3 leads to increased deposition of TIMP-3, which causes partial inactivation of MMPs, including MMP-13, a mechanism that would explain the similarities in phenotype between COMP/matrilin 3 double-deficient and MMP-13-deficient mice.

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.

Matrilin-1 expression is increased in the vertebral column of Atlantic salmon (Salmo salar L.) individuals displaying spinal fusions

We have previously characterized the development of vertebral fusions induced by elevated water temperature in Atlantic salmon. Molecular markers of bone and cartilage development together with histology were used to understand the complex pathology and mechanism in the development of this spinal malformation. In this study, we wanted to use proteomics, a non-hypothetical approach to screen for possible new markers involved in the fusion process. Proteins extracted from non-deformed and fused vertebrae of Atlantic salmon were therefore compared by two-dimensional electrophoresis (2DE) and MALDI-TOF analysis. Data analysis of protein spots in the 2DE gels demonstrated matrilin-1, also named cartilage matrix protein, to be the most highly up-regulated protein in fused compared with non-deformed vertebrae. Furthermore, real-time PCR analysis showed strong up-regulation of matrilin-1 mRNA in fused vertebrae. Immunohistochemistry demonstrated induced matrilin-1 expression in trans-differentiating cells undergoing a metaplastic shift toward chondrocytes in fusing vertebrae, whereas abundant expression was demonstrated in cartilaginous tissue and chordocytes of both non-deformed and fused vertebrae. These results identifies matrilin-1 as a new interesting candidate in the fusion process, and ratify the use of proteomic as a valuable technique to screen for markers involved in vertebral pathogenesis.

Comparative analysis with collagen type II distinguishes cartilage oligomeric matrix protein as a primary TGFβ-responsive gene

OBJECTIVE

This study aims to investigate the regulation of expression of Cartilage oligomeric matrix protein (COMP), which is predominately expressed by chondrocytes and functions to organize the extracellular matrix. Mutations in COMP cause two skeletal dysplasias: pseudoachondroplasia and multiple epiphyseal dysplasia. The mechanism controlling COMP expression during chondrocyte differentiation is still poorly understood.

DESIGN

Primary human bone marrow-derived stem cells were induced to differentiate into chondrocyte by pellet cultures. We then compared the temporal expression of COMP with the well-characterized cartilage-specific Type II collagen (Col2a1), and their response to transforming growth factor (TGF)β and Sox trio (Sox5, 6, and 9) stimulation.

RESULTS

COMP and Col2a1 expression are differentially regulated by three distinct mechanisms. First, upregulation of COMP mRNA precedes Col2a1 by several days during chondrogenesis. Second, COMP expression is independent of high cell density but requires TGF-β1. Induction of COMP mRNA by TGF-β1 is detected within 2h in the absence of protein synthesis and is blocked by specific inhibitors of the TGFβ signaling pathway; and therefore, COMP is a primary TFGβ-response gene. Lastly, while Col2a1 expression is intimately controlled by the Sox trio, overexpression of Sox trio fails to activate the COMP promoter.

CONCLUSION

COMP and Col2a1 expression are regulated differently during chondrogenesis. COMP is a primary response gene of TGFβ and its fast induction during chondrogenesis suggests that COMP is suitable for rapidly accessing the chondrogenic potential of stem cells.

Cartilage intermediate layer protein 2 (CILP-2) is expressed in articular and meniscal cartilage and down-regulated in experimental osteoarthritis

Using transcriptome profiling to determine differential gene expression between the permanent mouse articular cartilage and the transient growth plate cartilage, we identified a highly expressed gene, Cilp2, which is expressed differentially by articular chondrocytes. CILP-2 is highly homologous to CILP-1 (cartilage intermediate layer protein 1), which is expressed in the intermediate zone of articular cartilage and has been linked to cartilage degenerative diseases. We demonstrated that Cilp2 has a restricted mRNA distribution at the surface of the mouse articular cartilage during development, becoming localized to the intermediate zone of articular cartilage and meniscal cartilage with maturity. Although the extracellular CILP-2 protein localization is broadly similar to CILP-1, CILP-2 appears to be more localized in the deeper intermediate zone of the articular cartilage extracellular matrix at maturity. CILP-2 was shown to be proteolytically processed, N-glycosylated, and present in human articular cartilage. In surgically induced osteoarthritis in mice, Cilp1 and Cilp2 gene expression was dysregulated. However, whereas Cilp1 expression was increased, Cilp2 gene expression was down-regulated demonstrating a differential response to mechanically induced joint destabilization. CILP-2 protein was reduced in the mouse osteoarthritic cartilage. Ultrastructural analysis also suggested that CILP-2 may be associated with collagen VI microfibrils and thus may mediate interactions between matrix components in the territorial and inter-territorial articular cartilage matrix. mRNA expression analysis indicated that whereas Cilp1 and Cilp2 are expressed most abundantly in cartilaginous tissues, expression can be detected in muscle and heart.

Matrilin-4 is processed by ADAMTS-5 in late Golgi vesicles present in growth plate chondrocytes of defined differentiation state

The two aggrecanases ADAMTS-4 and ADAMTS-5 have been shown to not only play roles in the breakdown of cartilage extracellular matrix in osteoarthritis, but also mediate processing of matrilins in the secretory pathway. The matrilins are adaptor proteins with a function in connecting fibrillar and network-like components in the cartilage extracellular matrix. Cleavage resulting in processed matrilins with fewer ligand-binding subunits could make these less efficient in providing matrix cohesion. In this study, the processing and degradation of matrilin-4 during cartilage remodeling in the growth plate of the developing mouse long bones were studied in greater detail. We show that ADAMTS-5 and a matrilin-4 neoepitope, revealed upon ADAMTS cleavage, colocalize in prehypertrophic/hypertrophic chondrocytes while they are not detected in proliferating chondrocytes of the growth plate. ADAMTS-5 and the cleaved matrilin-4 are preferentially detected in vesicles derived from the Golgi apparatus. The matrilin-4 neoepitope was not observed in the growth plate of ADAMTS-5 deficient mice. We propose that in the growth plate ADAMTS-5, and not ADAMTS-4, has a physiological function in the intracellular processing of matrilins and potentially of other extracellular matrix proteins.

Proteolytic processing causes extensive heterogeneity of tissue matrilin forms

The matrilins are a family of multidomain extracellular matrix proteins with adapter functions. The oligomeric proteins have a bouquet-like structure and bind to a variety of different ligands whereby the avidity of their interactions is dependent on the number of subunits and domains present. Here we show the contribution of post-translational proteolytic processing to the heterogeneity of matrilins seen in tissue extracts and cell culture supernatants. A cleavage site after two glutamate residues in the hinge region close to the C-terminal coiled-coil oligomerization domain is conserved among the matrilins. Cleavage at this site yields molecules that lack almost complete subunits. The processing is least pronounced in matrilin-1 and particularly complex in matrilin-2, which contains additional cleavage sites. Replacement of the hinge region in matrilin-4 by the matrilin-1 hinge region had no marked effect on the processing. A detailed study revealed that matrilin-4 is processed already in the secretory pathway and that the activation of the responsible enzymes is dependent on proprotein convertase activity. Matrilin-3 and -4, but not matrilin-1 subunits present in matrilin-1/-3 hetero-oligomers, were identified as substrates for ADAMTS4 and ADAMTS5, whereas ADAMTS1 did not cleave any matrilin. A neo-epitope antibody raised against the N terminus of the C-terminal cleavage product of matrilin-4 detected processed matrilin-4 in cultures of primary chondrocytes as well as on cartilage sections showing that the conserved cleavage site is used in vivo.

An immunohistochemical study of the extracellular matrix of entheses associated with the human pisiform bone

The immunohistochemical labelling patterns of the extracellular matrix at the insertion of the flexor carpi ulnaris tendon and the entheses at both ends of the pisometacarpal and pisohamate ligaments were compared in order to relate the molecular composition of the attachment sites to their mechanical environment. Tissue was obtained from elderly dissecting room cadavers and labelled with a panel of monoclonal antibodies directed against collagens, glycosaminoglycans, proteoglycans and matrix proteins. All entheses were fibrocartilaginous and labelled positively for molecules typically associated with articular cartilage (type II collagen, chondroitin 6 sulphate, aggrecan and link protein). Labelling for type II collagen was most conspicuous at the attachment of the flexor carpi ulnaris tendon. In the ligaments, type II collagen labelling was always greater at the pisiform end. Matrilin 1 was universally present at all five entheses examined and fibromodulin labelling was most intense around the tidemark. Fibromodulin may thus be involved in anchorage and/or the control of mineralization at the hard-soft tissue interface of entheses. The greater prominence of fibrocartilage at the pisiform enthesis of the flexor carpi ulnaris tendon than at any ligament attachment may relate to the marked change in the tendon insertional angle that occurs with wrist movements. We also suggest that the more fibrocartilaginous character of the proximal compared with the distal ends of the ligaments relates to the fact that the pisiform is anchored in position and is thus at the centre of rotation of any movement of ligaments attached to it.

Abnormal collagen fibrils in cartilage of matrilin-1/matrilin-3-deficient mice

Matrilins are oligomeric extracellular matrix adaptor proteins mediating interactions between collagen fibrils and other matrix constituents. All four matrilins are expressed in cartilage and mutations in the human gene encoding matrilin-3 (MATN3) are associated with different forms of chondrodysplasia. Surprisingly, however, Matn3-null as well as Matn1- and Matn2-null mice do not show an overt skeletal phenotype, suggesting a dominant negative pathomechanism for the human disorders and redundancy/compensation among the family members in the knock-out situation. Here, we show that mice lacking both matrilin-1 and matrilin-3 develop an apparently normal skeleton, but exhibit biochemical and ultrastructural abnormalities of the knee joint cartilage. At the protein level, an altered SDS-PAGE band pattern and a clear up-regulation of the homotrimeric form of matrilin-4 were evident in newborn Matn1/Matn3 and Matn1 knock-out mice, but not in Matn3-null mice. The ultrastructure of the cartilage matrix after conventional chemical fixation was grossly normal; however, electron microscopy of high pressure frozen and freeze-substituted samples, revealed two consistent observations: 1) moderately increased collagen fibril diameters throughout the epiphysis and the growth plate in both single and double mutants; and 2) increased collagen volume density in Matn1(-/-)/Matn3(-/-) and Matn3(-/-) mice. Taken together, our results demonstrate that matrilin-1 and matrilin-3 modulate collagen fibrillogenesis in cartilage and provide evidence that biochemical compensation might exist between matrilins.

Matrilins mediate weak cell attachment without promoting focal adhesion formation

The matrilins form a family of non-collagenous adaptor proteins in the extracellular matrix. The extracellular ligand interactions of matrilins have been studied in some detail, while the potential interplay between matrilins and cells has been largely neglected. Except for matrilin-4, all matrilins mediate cell attachment, but only for matrilin-1 and -3 the binding is clearly dose dependent and seen already at moderate coating concentrations. Even so, much higher concentrations of matrilin-1 or -3 than of fibronectin are required for cell attachment to reach plateau values. Integrins contribute to the matrilin-mediated cell attachment, but the binding does not lead to formation of focal contacts and reorganisation of the actin cytoskeleton. Cells deficient in beta1 integrins are able to adhere, although weaker, and matrilins do not bind the soluble integrin alpha1beta1 and alpha2beta1 ectodomains. Cell surface proteoglycans may promote the attachment, as cells deficient in glycosaminoglycan biosynthesis adhere less well to matrilin-3. Even so, exogenous glycosaminoglycans are not able to compete for the attachment of HaCaT cells to matrilins.

Collagen IX is indispensable for timely maturation of cartilage during fracture repair in mice

Fracture repair recapitulates in adult organisms the sequence of cell biological events of endochondral ossification during skeletal development and growth. After initial inflammation and deposition of granulation tissue, a cartilaginous callus is formed which, subsequently, is remodeled into bone. In part, bone formation is influenced also by the properties of the extracellular matrix of the cartilaginous callus. Deletion of individual macromolecular components can alter extracellular matrix suprastructures, and hence stability and organization of mesenchymal tissues. Here, we took advantage of the collagen IX knockout mouse model to better understand the role of this collagen for organization, differentiation and maturation of a cartilaginous template during formation of new bone. Although a seemingly crucial component of cartilage fibrils is missing, collagen IX-deficient mice develop normally, but are predisposed to premature joint cartilage degeneration. However, we show here that lack of collagen IX alters the time course of callus differentiation during bone fracture healing. The maturation of cartilage matrix was delayed in collagen IX-deficient mice calli as judged by collagen X expression during the repair phase and the total amount of cartilage matrix was reduced. Entering the remodeling phase of fracture healing, Col9a1(-/-) calli retained a larger percentage of cartilage matrix than in wild type indicating also a delayed formation of new bone. We concluded that endochondral bone formation can occur in collagen IX knockout mice but is impaired under conditions of stress, such as the repair of an unfixed fractured long bone.

Proteomic analysis of mouse growth plate cartilage

Cartilage is a highly specialized load-bearing tissue with a small number of cells and a high proportion of extracellular matrix (ECM). The abundance of heavily sulfated proteoglycans and a poorly soluble collagenous ECM presents a major technical challenge to 2-DE. Here we report proteomic analysis of mouse growth plate cartilage using novel methodology for tissue dissection and sample prefractionation. We have successfully resolved cartilage tissue extracts by 2-DE for the first time and identified cartilage ECM proteins by Western blotting and MS/MS.

Pericellular matrilins regulate activation of chondrocytes by cyclic load-induced matrix deformation

Pericellular matrix is at the ideal location to be involved in transmitting mechanical signals from the microenvironment to a cell. We found that changes of the content of matrilins that link various pericellular molecules surrounding chondrocytes affect mechanical stimulation of chondrocyte proliferation and gene expression. Thus, pericellular matrilins may play a role in chondrocyte mechanotransduction.

INTRODUCTION

Chondrocytes reside in a capsule of pericellular matrix (chondron), which has been hypothesized to play a critical role in transducing mechanical signals to the cell. In this study, we test the hypothesis that the levels of matrilin (MATN)-1 and -3, major components of the chondrocyte pericellular matrix network, regulate activation of chondrocyte proliferation and differentiation by cyclic load-induced matrix deformation.

MATERIALS AND METHODS

Functional matrilins were decreased by expressing a dominant negative mini-MATN in primary chondrocytes or by using MATN1-null chondrocytes. The abundance of matrilins was also increased by expressing a wildtype MATN1 or MATN3 in chondrocytes. Chondrocytes were cultured in a 3D sponge subjected to cyclic deformation at 1 Hz. Chondrocyte gene expression was quantified by real-time RT-PCR and by Western blot analysis. Matrilin pericellular matrix assembly was examined by immunocytochemistry.

RESULTS

Elimination of functional matrilins from pericellular matrix abrogated mechanical activation of Indian hedgehog signaling and abolished mechanical stimulation of chondrocyte proliferation and differentiation. Excessive or reduced matrilin content decreased mechanical response of chondrocytes.

CONCLUSIONS

Normal content of matrilins is essential to optimal activation of chondrocytes by mechanical signals. Our data suggest that the sensitivity of chondrocytes to the changes in the microenvironment can be adjusted by altering the content of matrilins in pericellular matrix. This finding supports a critical role of pericellular matrix in chondrocyte mechano-transduction and has important implications in cartilage tissue engineering and mechanical adaptation.

Functional knockout of the matrilin-3 gene causes premature chondrocyte maturation to hypertrophy and increases bone mineral density and osteoarthritis

Mutations in the gene encoding matrilin-3 (MATN3), a noncollagenous extracellular matrix protein, have been reported in a variety of skeletal diseases, including multiple epiphyseal dysplasia, which is characterized by irregular ossification of the epiphyses and early-onset osteoarthritis, spondylo-epimetaphyseal dysplasia, and idiopathic hand osteoarthritis. To assess the role of matrilin-3 in the pathogenesis of these diseases, we generated Matn3 functional knockout mice using embryonic stem cell technology. In the embryonic growth plate of the developing long bones, Matn3 null chondrocytes prematurely became prehypertrophic and hypertrophic, forming an expanded zone of hypertrophy. This expansion was attenuated during the perinatal period, and Matn3 homozygous null mice were viable and showed no gross skeletal malformations at birth. However, by 18 weeks of age, Matn3 null mice had a significantly higher total body bone mineral density than Matn1 null mice or wild-type littermates. Aged Matn3 null mice were much more predisposed to develop severe osteoarthritis than their wild-type littermates. Here, we show that matrilin-3 plays a role in modulating chondrocyte differentiation during embryonic development, in controlling bone mineral density in adulthood, and in preventing osteoarthritis during aging. The lack of Matn3 does not lead to postnatal chondrodysplasia but accounts for higher incidence of osteoarthritis.

Approaching theProteoglycome: Molecular Interactions of Proteoglycans and Their Functional Output

[Image: see text] Through their diverse core protein modules and glycan/glycosaminoglycan moieties, proteoglycans may engage in numerous cellular and molecular interactions which are dispensable during embryogenesis, are essential for the maintenance of a healthy state and are prone to modulation in pathological conditions. Proteoglycan interactions may involve binding to other structural components of the ECM, to cell surface receptors, to membrane-associated components, and to soluble signaling molecules, which through this interaction may become entrapped in the ECM or sequestered at the cell surface. Understanding of these multiple interplays is therefore of paramount importance and requires a detailed mapping through what we define as the proteoglycome.

WARP is a novel multimeric component of the chondrocyte pericellular matrix that interacts with perlecan

WARP is a novel member of the von Willebrand factor A domain superfamily of extracellular matrix proteins that is expressed by chondrocytes. WARP is restricted to the presumptive articular cartilage zone prior to joint cavitation and to the articular cartilage and fibrocartilaginous elements in the joint, spine, and sternum during mouse embryonic development. In mature articular cartilage, WARP is highly specific for the chondrocyte pericellular microenvironment and co-localizes with perlecan, a prominent component of the chondrocyte pericellular region. WARP is present in the guanidine-soluble fraction of cartilage matrix extracts as a disulfide-bonded multimer, indicating that WARP is a strongly interacting component of the cartilage matrix. To investigate how WARP is integrated with the pericellular environment, we studied WARP binding to mouse perlecan using solid phase and surface plasmon resonance analysis. WARP interacts with domain III-2 of the perlecan core protein and the heparan sulfate chains of the perlecan domain I with K(D) values in the low nanomolar range. We conclude that WARP forms macromolecular structures that interact with perlecan to contribute to the assembly and/or maintenance of "permanent" cartilage structures during development and in mature cartilages.

Association of matrilin-3 polymorphisms with spinal disc degeneration and osteoarthritis of the first carpometacarpal joint of the hand

BACKGROUND

Seven polymorphisms in the matrilin-3(MATN3) gene were previously tested for genetic association with hand osteoarthritis in an Icelandic cohort. One of the variants, involving a conserved amino acid substitution (T303M; SNP5), was related to idiopathic hand osteoarthritis.

OBJECTIVES

To investigate SNP5 and two other promising polymorphisms (rs2242190; SNP3, rs8176070; SNP6) for association with radiographic and symptomatic hand osteoarthritis phenotypes, as well as other heritable phenotypes.

METHODS

Polymorphisms were examined in two distinct cohorts of subjects: a population based sample from the Rotterdam study (n = 809), and affected siblings from the genetics, osteoarthrosis and progression (GARP) study (n = 382).

RESULTS

The originally described association of T303M with the hand osteoarthritis phenotype was not observed in the populations studied. In the Rotterdam sample, however, carrying the T allele of T303M conferred an odds ratio of 2.9 (95% confidence interval (CI), 1.2 to 7.3; p = 0.02) for spinal disc degeneration. In the GARP study, carriers of the A allele of SNP6 had an odds ratio of 2.0 (95% CI, 1.3 to 3.1, p = 0.004) for osteoarthritis of the first carpometacarpal joint (CMC1) as compared with the Rotterdam sample as a control group. Subsequent haplotype analysis showed that a common haplotype, containing the risk allele of SNP6, conferred a significant risk in sibling pairs with CMC1 osteoarthritis (odds ratio = 1.7 (95% CI, 1.1 to 2.7, p = 0.02)).

CONCLUSIONS

These associations suggest that the MATN3 region also determines susceptibility to spinal disc degeneration and CMC1 osteoarthritis.

Altered integration of matrilin-3 into cartilage extracellular matrix in the absence of collagen IX

The matrilins are a family of four noncollagenous oligomeric extracellular matrix proteins with a modular structure. Matrilins can act as adapters which bridge different macromolecular networks. We therefore investigated the effect of collagen IX deficiency on matrilin-3 integration into cartilage tissues. Mice harboring a deleted Col9a1 gene lack synthesis of a functional protein and produce cartilage fibrils completely devoid of collagen IX. Newborn collagen IX knockout mice exhibited significantly decreased matrilin-3 and cartilage oligomeric matrix protein (COMP) signals, particularly in the cartilage primordium of vertebral bodies and ribs. In the absence of collagen IX, a substantial amount of matrilin-3 is released into the medium of cultured chondrocytes instead of being integrated into the cell layer as in wild-type and COMP-deficient cells. Gene expression of matrilin-3 is not affected in the absence of collagen IX, but protein extraction from cartilage is greatly facilitated. Matrilin-3 interacts with collagen IX-containing cartilage fibrils, while fibrils from collagen IX knockout mice lack matrilin-3, and COMP-deficient fibrils exhibit an intermediate integration. In summary, the integration of matrilin-3 into cartilage fibrils occurs both by a direct interaction with collagen IX and indirectly with COMP serving as an adapter. Matrilin-3 can be considered as an interface component, capable of interconnecting macromolecular networks and mediating interactions between cartilage fibrils and the extrafibrillar matrix.

Highly conserved proximal promoter element harbouring paired Sox9-binding sites contributes to the tissue- and developmental stage-specific activity of the matrilin-1 gene

The matrilin-1 gene has the unique feature that it is expressed in chondrocytes in a developmental stage-specific manner. Previously, we found that the chicken matrilin-1 long promoter with or without the intronic enhancer and the short promoter with the intronic enhancer restricted the transgene expression to the columnar proliferative chondroblasts and prehypertrophic chondrocytes of growth-plate cartilage in transgenic mice. To study whether the short promoter shared by these transgenes harbours cartilage-specific control elements, we generated transgenic mice expressing the LacZ reporter gene under the control of the matrilin-1 promoter between -338 and +67. Histological analysis of the founder embryos demonstrated relatively weak transgene activity in the developing chondrocranium, axial and appendicular skeleton with highest level of expression in the columnar proliferating chondroblasts and prehypertrophic chondrocytes. Computer analysis of the matrilin-1 genes of amniotes revealed a highly conserved Pe1 (proximal promoter element 1) and two less-conserved sequence blocks in the distal promoter region. The inverted Sox motifs of the Pe1 element interacted with chondrogenic transcription factors Sox9, L-Sox5 and Sox6 in vitro and another factor bound to the spacer region. Point mutations in the Sox motifs or in the spacer region interfered with or altered the formation of nucleoprotein complexes in vitro and significantly decreased the reporter gene activity in transient expression assays in chondrocytes. In vivo occupancy of the Sox motifs in genomic footprinting in the expressing cell type, but not in fibroblasts, also supported the involvement of Pe1 in the tissue-specific regulation of the gene. Our results indicate that interaction of Pe1 with distal DNA elements is required for the high level, cartilage- and developmental stage-specific transgene expression.

Zebrafish (Danio rerio) matrilins: shared and divergent characteristics with their mammalian counterparts

We have cloned the cDNAs of the zebrafish (Danio rerio) members of the matrilin family of extracellular adaptor proteins. In contrast to mammals, no orthologue of matrilin-2 was found in zebrafish, either by RT (reverse-transcriptase) PCR using degenerated primers or by screening the databases (Ensembl and NCBI); however, two forms of matrilin-3, matrilin-3a and -3b, were present. The identity with the mammalian matrilins is from more than 70% for the VWA (von Willebrand factor A)-like domains to only 28% for the coiled-coil domains of matrilin-3a and -3b. In all zebrafish matrilins we found a greater variety of splice variants than in mammals, with splicing mainly affecting the number of EGF (epidermal growth factor)-like repeats. The exon-intron organization is nearly identical with that of mammals, and also the characteristic AT-AC intron interrupting the exons coding for the coiled-coil domain is conserved. In the matrilin-3b gene a unique exon codes for a proline- and serine/threonine-rich domain, possibly having mucin-like properties. The matrilin-1 and -3a genes were mapped to chromosome 19 and 20 respectively by the radiation hybrid method. The temporal and spatial expression of zebrafish matrilins is similar to that seen in the mouse. Zebrafish matrilin-4 is highly expressed as early as 24 hpf (h post fertilization), whereas the other matrilins show peak expression at 72 hpf. By immunostaining of whole mounts and sections, we found that matrilin-1 and -3a show predominantly skeletal staining, whereas matrilin-4 is more widespread, with the protein also being present in loose connective tissues and epithelia.

The matrilins--adaptor proteins in the extracellular matrix

The matrilins form a four-member family of modular, multisubunit matrix proteins, which are expressed in cartilage but also in many other forms of extracellular matrix. They participate in the formation of fibrillar or filamentous structures and are often associated with collagens. It appears that they mediate interactions between collagen-containing fibrils and other matrix constituents, such as aggrecan. This adaptor function may be modulated by physiological proteolysis that causes the loss of single subunits and thereby a decrease in binding avidity. Attempts to study matrilin function by gene inactivation in mouse have been frustrating and so far not yielded pronounced phenotypes, presumably because of the extensive redundancy within the family allowing compensation by one family member for another. However, mutations in matrilin-3 in humans cause different forms of chondrodysplasias and perhaps also hand osteoarthritis. As loss of matrilin-3 is not critical in mouse, these phenotypes are likely to be caused by dominant negative effects.

Expression pattern of matrilins and other extracellular matrix proteins characterize distinct stages of cell differentiation during antler development

Deer antler regeneration is a uniquely intense and complex process, which involves chondrogenic and intramembranous ossification. Cell differentiation in the developing antler of red deer, Cervus elaphus, was characterized with extracellular matrix markers. Expression of the four matrilin genes was monitored by immunohistochemistry and in situ hybridization and compared to cartilage markers collagen II and cartilage link protein, the bone component collagen I, and the endothelial basement membrane constituent laminin. The mesenchyme layer at the very tip of the velvet antler was enriched in link protein, indicative of the role of hyaluronan in apical morphogenesis. Matrilin-2, formerly described as a component of hard and soft connective tissue matrices, was identified here also as a marker of cells with high differentiation potential: it is expressed predominantly by mesenchyme cells, prechondrocytes and preosteoblasts. In addition to matrilin-3, documented as a component of the bony extracellular matrix, expression of the other three matrilin genes was observed in osteoprogenitor cells and osteoblasts. A layer of presumed osteoprogenitor cells, which surrounded the perivascular channels, expressed all four matrilins and collagen I. As a consequence, all four matrilins, including matrilin-1, previously detected in the skeleton only in cartilage, were found associated to collagen I-rich structures in a thin layer bordering the columns of hypertrophic chondrocytes. Cells with similar morphology and expression pattern were identified in the periosteum. Altogether all cell types of the chondrogenic and osteogenic lineage that expressed the four matrilins were in a separate study [Faucheux, C., Nicholls, B.M., Allen, S., Danks, J.A, Horton, M.A., Price, J.S., 2004. Recapitulation of the parathyroid hormone-related peptide-Indian hedgehog pathway in the regenerating deer antler. Dev. Dyn. 231, 88-97] positive for parathyroid hormone-related peptide and its receptor.

Mature bovine articular cartilage contains abundant aggrecan that is C-terminally truncated at Ala719-Ala720, a site which is readily cleaved by m-calpain

Extracts of normal mature articular cartilage contain aggrecan molecules which bear the G1 domain (the N-terminal globular domain of aggrecan) and are C-terminally truncated by proteolysis at a number of sites. A proportion of these molecules are generated by an aggrecanase and/or matrix-metalloproteinase-mediated cleavage in the IGD (interglobular domain between the G1 and G2 domains of aggrecan). However, the proteinase(s) responsible for formation of the majority of the larger G1-G2 and glycosaminoglycan-bearing truncated species is (are) unknown. N-terminal sequencing of aggrecan core fragments generated by m-calpain digestion of bovine aggrecan has identified four novel cleavage sites: one within the CS (chondroitin sulphate)-1 domain (at one or more of the bonds Ser1229-Val1230, Ser1249-Val1250, Ser1287-Val1288, Gly1307-Val1308 and Ser1346-Val1347), two within the IGD (at bonds Ala474-Ala475 and Gly365-Gly366) and one within the KS (keratan sulphate) domain (at Ala719-Ala720). A new monoclonal antibody (SK-28) to the C-terminal neoepitope at M710VTQVGPGVA719 showed that aggrecan products generated by this cleavage are present in high abundance in mature bovine articular cartilage extracts. We conclude that m-calpain, or an unidentified proteinase with the capacity to cleave at the same site, is active during aggrecan biosynthesis/secretion by mature chondrocytes or in the matrix of mature bovine articular cartilage in vivo.

Mice lacking the extracellular matrix adaptor protein matrilin-2 develop without obvious abnormalities

Matrilins are putative adaptor proteins of the extracellular matrix (ECM) which can form both collagen-dependent and collagen-independent filamentous networks. While all known matrilins (matrilin-1, -2, -3, and -4) are expressed in cartilage, only matrilin-2 and matrilin-4 are abundant in non-skeletal tissues. To clarify the biological role of matrilin-2, we have developed a matrilin-2-deficient mouse strain. Matrilin-2 null mice show no gross abnormalities during embryonic or adult development, are fertile, and have a normal lifespan. Histological and ultrastructural analyses indicate apparently normal structure of all organs and tissues where matrilin-2 is expressed. Although matrilin-2 co-localizes with matrilin-4 in many tissues, Northern hybridization, semiquantitative RT-PCR, immunohistochemistry and biochemical analysis reveal no significant alteration in the steady-state level of matrilin-4 expression in homozygous mutant mice. Immunostaining of wild-type and mutant skin samples indicate no detectable differences in the expression and deposition of matrilin-2 binding partners including collagen I, laminin-nidogen complexes, fibrillin-2 and fibronectin. In addition, electron microscopy reveals an intact basement membrane at the epidermal-dermal junction and normal organization of the dermal collagen fibrils in mutant skin. These data suggest that either matrilin-2 and matrilin-2-mediated matrix-matrix interactions are dispensable for proper ECM assembly and function, or that they are efficiently compensated by other matrix components including wild-type levels of matrilin-4.

Interactions between the cartilage oligomeric matrix protein and matrilins. Implications for matrix assembly and the pathogenesis of chondrodysplasias

The cartilage oligomeric matrix protein (COMP) and matrilins are abundant non-collagenous proteins in the cartilage extracellular matrix. In the presence of calcium, COMP and matrilin-1 elute together in the gel filtration of cartilage extracts and can be co-immunoprecipitated. In a screen for ligands of matrilin-1, -3, and -4 using an ELISA-style binding assay, COMP was identified as a prominent binding partner for all three, indicating a conservation of the COMP interaction among matrilins. The interaction of COMP and matrilin-4 is saturable, and an apparent K(D) of 1 nm was determined. However, only the full-length COMP and the full-length matrilin-4 proteins showed a strong interaction, indicating that the oligomeric structures markedly increase the affinity. Mutations in COMP or matrilin-3 cause related forms of human chondrodysplasia, and the COMP mutation D469Delta, which is found in patients with pseudoachondroplasia, has been shown to cause a reduced calcium binding. Despite this, the mutation causes only a slight decrease in matrilin-4 binding. This indicates that impaired binding of COMP to matrilins does not cause the pseudoachondroplasia phenotype but rather that matrilins may be coretained in the rough endoplasmatic reticulum where COMP accumulates in the chondrocytes of patients.

Matrilin-3 is dispensable for mouse skeletal growth and development

Matrilin-3 belongs to the matrilin family of extracellular matrix (ECM) proteins and is primarily expressed in cartilage. Mutations in the gene encoding human matrilin-3 (MATN-3) lead to autosomal dominant skeletal disorders, such as multiple epiphyseal dysplasia (MED), which is characterized by short stature and early-onset osteoarthritis, and bilateral hereditary microepiphyseal dysplasia, a variant form of MED characterized by pain in the hip and knee joints. To assess the function of matrilin-3 during skeletal development, we have generated Matn-3 null mice. Homozygous mutant mice appear normal, are fertile, and show no obvious skeletal malformations. Histological and ultrastructural analyses reveal endochondral bone formation indistinguishable from that of wild-type animals. Northern blot, immunohistochemical, and biochemical analyses indicated no compensatory upregulation of any other member of the matrilin family. Altogether, our findings suggest functional redundancy among matrilins and demonstrate that the phenotypes of MED disorders are not caused by the absence of matrilin-3 in cartilage ECM.

Functional analysis of the regulatory regions of the matrilin-1 gene in transgenic mice reveals modular arrangement of tissue-specific control elements

Matrilin-1 is a non-collagenous protein, which functions in the organization of the extracellular matrix by forming collagen-dependent and -independent filamentous networks. It is secreted primarily by chondrocytes in a characteristic spatial, temporal and developmental stage-specific pattern during skeletogenesis. As a first step to define the tissue- and site-specific regulatory regions of the chicken matrilin-1 gene in vivo, we generated transgenic mice harboring various promoter and intronic fragments fused to the LacZ reporter gene. Histological analysis of the transgene expression pattern during ontogenic development revealed specific X-gal staining in most primordial elements of endochondral bones of transgenic mouse lines carrying either the long promoter between -2011 and +67 or the intronic fragment with a short promoter between -338 and +1819. The cartilage-specific activity of the latter transgene, however, was accompanied with variable ectopic expression pattern in neural and other tissues depending on the site of integration. The presence of both promoter upstream and intronic elements was necessary for the high level transgene activity in all chondrogenic tissues and for the extraskeletal transgene expression pattern resembling the most to that of the chicken matrilin-1 gene, e.g. expression in the eye, and lack of expression in the diminishing notochord and nucleus pulposus. The activity of the transgenes was restricted to the columnar proliferating and pre-hypertrophic chondrocytes visualized by BrdU incorporation and distribution of phosphorylated Sox9, respectively. DNA elements between -2011 and -338 also mediated ectopic LacZ expression in cells of neural crest origin. These results suggest that an interplay of modularly arranged cartilage- and neural crest-specific DNA elements control the expression of the matrilin-1 gene. The dispersal of cartilage-specific elements in the promoter upstream and intronic regions shows similarity to the transcriptional regulation of the Col11a2 gene.

Complexes of matrilin-1 and biglycan or decorin connect collagen VI microfibrils to both collagen II and aggrecan

Native supramolecular assemblies containing collagen VI microfibrils and associated extracellular matrix proteins were isolated from Swarm rat chondrosarcoma tissue. Their composition and spatial organization were characterized by electron microscopy and immunological detection of molecular constituents. The small leucine-rich repeat (LRR) proteoglycans biglycan and decorin were bound to the N-terminal region of collagen VI. Chondroadherin, another member of the LRR family, was identified both at the N and C termini of collagen VI. Matrilin-1, -3, and -4 were found in complexes with biglycan or decorin at the N terminus. The interactions between collagen VI, biglycan, decorin, and matrilin-1 were studied in detail and revealed a biglycan/matrilin-1 or decorin/matrilin-1 complex acting as a linkage between collagen VI microfibrils and aggrecan or alternatively collagen II. The complexes between matrilin-1 and biglycan or decorin were also reconstituted in vitro. Colocalization of collagen VI and the different ligands in the pericellular matrix of cultured chondrosarcoma cells supported the physiological relevance of the observed interactions in matrix assembly.

Identification and characterization of AMACO, a new member of the von Willebrand factor A-like domain protein superfamily with a regulated expression in the kidney

The genes coding for human and mouse AMACO, an extracellular matrix protein containing VWA-like domains related to those in MAtrilins and COllagens, were detected in databases, the cDNAs were cloned, and the primary structures were deduced from the nucleotide sequences. The genes consist of 14 exons and have a similar exon/intron organization. The protein consists of a signal peptide sequence, an N-terminal VWA domain connected to two additional, tandem VWA domains by a cysteine-rich sequence and an epidermal growth factor (EGF)-like domain. The C terminus is made up of another EGF-like domain followed by a unique sequence present in mouse, but absent in human. The predicted molecular weight of the proteins is 79,485 in human and 83,024 in mouse. Full-length AMACO was expressed in 293-EBNA cells, purified by use of an affinity tag and subjected to biochemical characterization. Both monomers and aggregates of AMACO were recovered, as shown by electron microscopy and SDS-PAGE. AMACO was found in the media of a variety of established cell lines of both fibroblast and epithelial origin. In the matrix formed by 293-EBNA cells overexpressing the protein, AMACO was deposited in patchy structures that were often cell-associated. Affinity-purified antibodies detect expression in cartilage and expression associated with certain basement membranes. In the kidney of adult mice, a second promoter located in intron 4 is active. If the resulting transcript is translated it could not yield a secreted protein because of the lack of a signal peptide sequence. The developmental switch from an AMACO mRNA, expressed by the newborn kidney, to the truncated transcript found in the adult kidney indicates an unusual regulation of AMACO expression.

Immunohistochemical study of matrilin-1 in arthritic articular cartilage of the mandibular condyle

BACKGROUND

The objective of this study was to investigate the expression of matrilin-1 in arthritic articular cartilage of the mandibular condyle by means of immunohistochemical methods.

METHODS

Condylar cartilage specimens were obtained from temporomandibular joints (TMJs) of 12 patients with arthritis (osteoarthritis and internal derangement) (mean age 51.8 years; age range 28-71 years) and four patients with TMJ ankylosis (mean age 44.0 years; age range 16-64 years), diagnosed clinically and with imaging examinations. Paraffin sections were immunostained with anti-matrilin-1 antibodies.

RESULTS

Matrilin-1 expression was detected in both patient groups with TMJ ankylosis and arthritis, and the level was remarkably higher in arthritic cartilage. The mean percentage of matrilin-1-producing cells to the total chondrocytes was significantly (P < 0.05) greater in the arthritic group (43.9 +/- 19.2%) than in subjects with TMJ ankylosis (28.0 +/- 8.7%).

CONCLUSIONS

Articular chondrocytes in the TMJ condyle can express matrilin-1 and the expression is enhanced in arthritic cartilage, suggesting a presence of functional or adaptive remodeling in the condyle in response to degenerative changes in the TMJ structures.

Effects of overexpression of membrane-bound transferrin-like protein (MTf) on chondrogenic differentiation in Vitro

Membrane-bound transferrin-like protein (MTf) is expressed in parallel with the expression of cartilage-characteristic genes during differentiation of chondrocytes, and the MTf level is much higher in cartilage than in other tissues. To investigate the role of MTf in cartilage, we examined the effects of growth factors on MTf expression in mouse prechondrogenic ATDC5 cells and the effect of MTf overexpression on differentiation of ATDC5 and mouse pluripotent mesenchymal C3H10T1/2 cells. In ATDC5 cultures, bone morphogenetic protein-2 and transforming growth factor-beta as well as insulin induced MTf mRNA expression when these peptides induced chondrogenic differentiation. Forced expression of rabbit MTf in ATDC5 cells induced aggrecan, type II collagen, matrilin-1, type X collagen mRNAs, and cell-shape changes from fibroblastic cells to spherical chondrocytes. Accordingly, the synthesis and accumulation of proteoglycans were higher in MTf-expressing cultures than in control cultures. These effects of MTf overexpression correlated with the MTf protein level on the cell surface and decreased in the presence of anti-MTf antibody. However, the aggrecan mRNA level in the ATDC5 cells overexpressing MTf was lower than that in wild type ATDC5 cells exposed to 10 microg/ml insulin. MTf overexpression in C3H10T1/2 cells also induced aggrecan and/or type II collagen mRNA but not the spherical phenotype. These findings suggest that the expression of MTf on the cell surface facilitates the differentiation of prechondrogenic cells, although MTf overexpression alone seems to be insufficient to commit pluripotent mesenchymal cells to the chondrocyte lineage.

Matrilin-2 interacts with itself and with other extracellular matrix proteins

Matrilin-2 is a component of extracellular filamentous networks. To study the interactions by which it can be integrated into such assemblies, full-length and truncated forms of matrilin-2 were recombinantly expressed in HEK-293 cells and purified from conditioned medium. The recombinant proteins, when used in interaction assays, showed affinity to matrilin-2 itself, but also to other collagenous and non-collagenous extracellular matrix proteins. The interaction between matrilin-2 and collagen I was studied in greater detail and could be shown to occur at distinct sites on the collagen I molecule and to have a K (D) of about 3 x 10(-8) M. Interactions with some non-collagenous protein ligands were even stronger, with matrilin-2 binding to fibrillin-2, fibronectin and laminin-1-nidogen-1 complexes, with K (D) values in the range of 10(-8)-10(-11) M. Co-localization of matrilin-2 with these ligands in the dermal-epidermal basement membrane, in the microfibrils extending from the basement membrane into the dermis, and in the dermal extracellular matrix, indicates a physiological relevance of the interactions in the assembly of supramolecular extracellular matrix structures.

Comparative genetics and evolution of annexin A13 as the founder gene of vertebrate annexins

Annexin A13 (ANXA13) is believed to be the original founder gene of the 12-member vertebrate annexin A family, and it has acquired an intestine-specific expression associated with a highly differentiated intracellular transport function. Molecular characterization of this subfamily in a range of vertebrate species was undertaken to assess coding region conservation, gene organization, chromosomal linkage, and phylogenetic relationships relevant to its progenitor role in the structure-function evolution of the annexin gene superfamily. Protein diagnostic features peculiar to this subfamily include an alternate isoform containing a KGD motif, an elevated basic amino acid content with polyhistidine expansion in the 5'-translated region, and the conservation of 15% core tetrad residues specific to annexin A13 members. The 12 coding exons comprising the 58-kb human ANXA13 gene were deduced from BAC clone sequencing, whereas internal repetitive elements and neighboring genes in chromosome 8q24.12 were identified by contig analysis of the draft sequence from the human genome project. A unique exon splicing pattern in the annexin A13 gene was corroborated by coanalysis of mouse, rat, zebrafish, and pufferfish genomic DNA and determined to be the most distinct of all vertebrate annexins. The putative promoter region was identified by phylogenetic footprinting of potential binding sites for intestine-specific transcription factors. Mouse annexin A13 cDNA was used to map the gene to an orthologous linkage group in mouse chromosome 15 (between Sdc2 and Myc by backcross analysis), and the zebrafish cDNA permitted its localization to linkage group 24. Comparative analysis of annexin A13 from nine species traced this gene's speciation history and assessed coding region variation, whereas phylogenetic analysis showed it to be the deepest-branching vertebrate annexin, and computational analysis estimated the gene age and divergence rate. The unique, conserved aspects of annexin A13 primary structure, gene organization, and genetic maps identify it as the probable common ancestor of all vertebrate annexins, beginning with the sequential duplication to annexins A7 and A11 approximately 700 MYA, before the emergence of chordates.

Characterization of the matrilin coiled-coil domains reveals seven novel isoforms

Matrilins constitute a family of four oligomeric extracellular proteins that are involved in the development and homeostasis of cartilage and bone. To reveal their homo- and heterotypic oligomerization propensities, we analyzed the four human matrilin coiled-coil domains by biochemical and biophysical methods. These studies not only confirmed the homo- and heterotypic oligomerization states reported for the full-length proteins but revealed seven novel matrilin isoforms. Specific heterotrimeric interactions of variable chain stoichiometries were observed between matrilin-1 and matrilin-2, matrilin-1 and matrilin-4, and matrilin-2 and matrilin-4. In addition, matrilin-1 formed two different specific heterotetramers with matrilin-3. Interestingly, a distinct heterotrimer consisting of three different chains was formed between matrilin-1, matrilin-2, and matrilin-4. No interactions, however, were observed between matrilin-2 and matrilin-3 or between matrilin-3 and matrilin-4. Both homo- and heterotypic oligomers folded into parallel disulfide-linked structures, although coiled-coil formation was not dependent on disulfide bridge formation. Our results indicate that the heterotypic preferences seen for the matrilin coiled-coil domains are the result of the packing of the hydrophobic core rather than ionic interactions. Mass spectrometry revealed that the concentrations of the individual chains statistically determined the stoichiometry of the heteromers, suggesting that formation of the different matrillin chain combinations is controlled by expression levels.

Expression of matrilins during maturation of mouse skeletal tissues

The matrilins are a recently discovered family of non-collagenous extracellular matrix proteins. During embryogenesis, all matrilins are expressed in skeletal tissues. Additionally, matrilin-2 and -4 are expressed in the dermis and in connective tissues of internal organs, e.g. of the lung and kidney. After birth, the expression of matrilin-1 and -3 remains specific for cartilage and bone whereas matrilin-2 and -4 display a broader tissue distribution and could be detected in epithelial, muscle, and nervous tissue as well as in loose and dense connective tissue. In epiphyseal cartilage of growing long bones, matrilin-1 and -3 are present in all cartilage regions, in contrast to matrilin-2, which is expressed in the proliferative and the upper hypertrophic zones. Similarly matrilin-4 was detected all over the epiphyseal cartilage, with the weakest expression in the hypertrophic zone. Although it was shown that matrilin-1 and -3 can form hetero-oligomers and are often co-localized in tissue, clear differences in their spatial distribution could be demonstrated by double-immunolabelling. During joint development matrilin-2 and matrilin-4 are present at the developing joint surface, while in articular cartilage of 6-week-old mice all matrilins are only weakly expressed.

Matrilin-3 in human articular cartilage: increased expression in osteoarthritis

OBJECTIVE

Matrilin-3 is a member of the recently described matrilin family of extracellular matrix proteins containing von Willebrand factor A-like domains. The matrilin-3 subunit can form homo-tetramers as well as hetero-oligomers together with subunits of matrilin-1 (cartilage matrix protein). It has a restricted tissue distribution and is strongly expressed in growing skeletal tissues. Detailed information on expression and distribution of extracellular matrix proteins is important to understand cartilage function in health and in disease like osteoarthritis (OA).

METHODS

Normal and osteoarthritic cartilage were systematically analysed for matrilin-3 expression, using immunohistochemistry, Western blot analysis, in situ hybridization, and quantitative PCR.

RESULTS

Our results indicate that matrilin-3 is a mandatory component of mature articular cartilage with its expression being restricted to chondrocytes from the tangential zone and the upper middle cartilage zone. Osteoarthritic cartilage samples with only moderate morphological osteoarthritic degenerations have elevated levels of matrilin-3 mRNA. In parallel, we found an increased deposition of matrilin-3 protein in the cartilage matrix. Matrilin-3 staining was diffusely distributed in the cartilage matrix, with no cellular staining being detectable. In cartilage samples with minor osteoarthritic lesions, matrilin-3 deposition was restricted to the middle zone and to the upper deep zone. A strong correlation was found between enhanced matrilin-3 gene and protein expression and the extent of tissue damage. Sections with severe osteoarthritic degeneration showed the highest amount of matrilin-3 mRNA, strong signals in in situ hybridization, and prominent protein deposition in the middle and deep cartilage zone.

CONCLUSION

We conclude that matrilin-3 is an integral component of human articular cartilage matrix and that the enhanced expression of matrilin-3 in OA may be a cellular response to the modified microenvironment in the disease.

Comparative analysis of the mouse and human genes (Matn2 and MATN2) for matrilin-2, a filament-forming protein widely distributed in extracellular matrices

We previously identified matrilin-2 (MATN2), the largest member of the novel family of matrilins. These filament-forming adapter proteins expressed in a distinct, but partially overlapping, pattern in all tissues were implicated in the organization of the extracellular matrix. Matrilin-2 functions in a great variety of tissues. Here, we present the genomic organization of the highly conserved mouse and human MATN2 loci, which cover >100 kb and 167.167 kb genomic regions, respectively, and are composed of 19 exons. RT-PCR analysis revealed that alternative transcripts with identical protein coding regions are transcribed from two promoters in both species. The upstream, housekeeping type promoter is functional in all tissues and cell types tested. The activity of the downstream, TATA-like promoter preceded with putative motifs for the homeobox transcription factor PRRX2 is restricted to embryonic fibroblasts and certain cell lines. The oligomerization module is split by an U12-type AT-AC intron found in conserved position in all four matrilin genes. We assigned Matn2 to mouse chromosome 15, linked to Trhr and Sntb1 in a region synthenic to human chromosome 8q22-24.

The occurrence of autoantibodies to matrilin 1 reflects a tissue-specific response to cartilage of the respiratory tract in patients with relapsing polychondritis

OBJECTIVE

Relapsing polychondritis (RP) is an inflammatory disease that mainly affects cartilage tissue in the auricle, nose, and lower respiratory tract. When tracheolaryngeal cartilage is involved, the disease is occasionally fatal. Matrilin 1 is a cartilage-specific protein most prominently expressed in tracheal cartilage, but not in joint cartilage. Immunization with the protein in rats and mice induces respiratory distress and nasal destruction, as seen in RP. We investigated the response to matrilin 1 and other cartilage proteins in sera from patients with RP, 4 additional groups of patients with other major connective tissue diseases, and healthy control subjects.

METHODS

Sera were analyzed by enzyme-linked immunosorbent assay (ELISA) for antibody responses to matrilin 1, types II, IX, and XI collagen, and cartilage oligomeric matrix protein (COMP). Titers above the mean + 3SD of controls were considered positive. Specificity of matrilin 1 recognition was further investigated by the capacity of high-titer sera to block the binding of a matrilin 1-specific monoclonal antibody in inhibition ELISAs. In vivo reactivity and specificity were tested by injecting sera into neonatal mice, and antibody binding was detected by immunohistochemical staining.

RESULTS

Serum antibodies from RP patients bound tracheolaryngeal and nasal cartilage in vivo and inhibited the binding of anti-matrilin 1-specific monoclonal antibodies. Thirteen of the 97 RP patients had increased titers of matrilin 1 antibody. Positive titers correlated with respiratory symptoms in 69% of the cases. Significant responses to type II collagen and COMP were also detected.

CONCLUSION

Antibodies to matrilin 1 bind tracheolaryngeal cartilage in vivo and are correlated with an inflammatory attack on tracheolaryngeal cartilage that is often seen in RP.

Characterization of the mouse matrilin-4 gene: a 5' antiparallel overlap with the gene encoding the transcription factor RBP-l

We have isolated and characterized the gene encoding mouse matrilin-4 (Matn4), an extracellular matrix protein present in a broad spectrum of tissues. The gene spanned 16 kb, consisted of 12 exons, and localized to chromosome 2. As in all known matrilin genes, the last intron, separating the exons coding for the coiled-coil domain, did not follow the GT-AG rule and belonged to the subgroup of introns having AT-AC at the ends. Matn4 contained two exons in the 5' UTR that could be alternatively spliced. We localized a major and a minor transcription start site to two different untranslated exons: exon 0a and exon 0b. Matn4 divergently overlapped 5' with the gene encoding RBP-L (for recombining binding protein suppressor of hairless-like; Rbpsuhl), a transcription factor with homology to RBP-JK. Exon 1 of Rbpsuhl was located in the second intron of Matn4, whereas exon 0a, the first exon of Matn4, was located in the second intron of Rbpsuhl. The second exons of the respective genes overlapped in an antisense orientation. We mapped the major transcription start of Rbpsuhl to a position approximately 150 nt upstream of the splice acceptor site of the first intron, leading to the synthesis of a truncated variant of RBP-L probably missing the amino-terminal 121 amino acid residues. We analyzed the expression of the different Matn4 and Rbpsuhl transcripts by quantitative RT-PCR; this showed the highest expression for both genes in lung and brain. In situ hybridization of brain sections showed a partially overlapping expression pattern for the two genes.