Appearance and persistence of fibronectin in cartilage. Specific interaction of fibronectin with collagen type II

Insights into the implementation of Fibronectin 1 in the cartilage tissue engineering

Recently, cartilage tissue engineering has become a cornerstone to treat cartilage degeneration and osteoarthritis (OA). Fibronectin1 (FN1) is described as multiple functional glycoproteins that play an essential role in chondrogenic and osteogenic differentiation. Few studies reported the potential of FN1 to enhance tissue engineering and reduce the death of chondrocytes in OA. Further, FN1 possesses multiple binding domains including collagen, integrin, and heparin that can interact with heparan sulfate proteoglycans at the surface of chondrocyte leading to promote cell signaling and differentiation. Recent studies suggested that FN1 can promote chondrocyte differentiation by upregulating TGF-β/PI3K/Akt pathways. Further, FN1 can inhibit the apoptosis of chondrocytes by preventing the release of metalloproteinases through lowering the expression of p-PI3K/PI3K and p-AKT/AKT pathways. However, the use of FN1 in cartilage repair studies using animal models or clinical trials was rarely reported. Therefore, this article provides new insights into the importance of FN1 in cartilage tissue engineering to encourage more studies concerning FN1 in cartilage repair studies. Further, we provided new suggestions for advanced applications of FN1 to treat OA and cartilage degeneration.

Characterization and Localization of Citrullinated Proteoglycan Aggrecan in Human Articular Cartilage

BACKGROUND

Rheumatoid arthritis (RA) is an autoimmune disease of the synovial joints. The autoimmune character of RA is underscored by prominent production of autoantibodies such as those against IgG (rheumatoid factor), and a broad array of joint tissue-specific and other endogenous citrullinated proteins. Anti-citrullinated protein antibodies (ACPA) can be detected in the sera and synovial fluids of RA patients and ACPA seropositivity is one of the diagnostic criteria of RA. Studies have demonstrated that RA T cells respond to citrullinated peptides (epitopes) of proteoglycan (PG) aggrecan, which is one of the most abundant macromolecules of articular cartilage. However, it is not known if the PG molecule is citrullinated in vivo in human cartilage, and if so, whether citrulline-containing neoepitopes of PG (CitPG) can contribute to autoimmunity in RA.

METHODS

CitPG was detected in human cartilage extracts using ACPA+ RA sera in dot blot and Western blot. Citrullination status of in vitro citrullinated recombinant G1 domain of human PG (rhG1) was confirmed by antibody-based and chemical methods, and potential sites of citrullination in rhG1 were explored by molecular modeling. CitPG-specific serum autoantibodies were quantified by enzyme-linked immunosorbent assays, and CitPG was localized in osteoarthritic (OA) and RA cartilage using immunohistochemistry.

FINDINGS

Sera from ACPA+ RA patients reacted with PG purified from normal human cartilage specimens. PG fragments (mainly those containing the G1 domain) from OA or RA cartilage extracts were recognized by ACPA+ sera but not by serum from ACPA- individuals. ACPA+ sera also reacted with in vitro citrullinated rhG1 and G3 domain-containing fragment(s) of PG. Molecular modeling suggested multiple sites of potential citrullination within the G1 domain. The immunohistochemical localization of CitPG was different in OA and RA cartilage.

CONCLUSIONS

CitPG is a new member of citrullinated proteins identified in human joints. CitPG could be found in both normal and diseased cartilage specimens. Antibodies against CitPG may trigger or augment arthritis by forming immune complexes with this autoantigen in the joints of ACPA+ RA patients.

From Skeletal Development to Tissue Engineering: Lessons from the Micromass Assay

Damage and degeneration of the skeletal elements due to disease, trauma, and aging lead to a significant health and economical burden. To reduce this burden, skeletal tissue engineering strategies aim to regenerate functional bone and cartilage in the adult body. However, challenges still exist. Such challenges involve the identification of the external cues that determine differentiation, how to control chondrocyte hypertrophy, and how to achieve specific tissue patterns and boundaries. To address these issues, it could be insightful to look at skeletal development, a robust morphogenetic process that takes place during embryonic development and is commonly modeled in vitro by the micromass assay. In this review, we investigate what the tissue engineering field can learn from this assay. By comparing embryonic skeletal precursor cells from different anatomic locations and developmental stages in micromass, the external cues that guide lineage commitment can be identified. The signaling pathways regulating chondrocyte hypertrophy, and the cues required for tissue patterning, can be elucidated by combining the micromass assay with genetic, molecular, and engineering tools. The lessons from the micromass assay are limited by two major differences between developmental and regenerative skeletogenesis: cell type and scale. We highlight an important difference between embryonic and adult skeletal progenitor cells, in that adult progenitors are not able to form mesenchymal condensations spontaneously. Also, the mechanisms of tissue patterning need to be adjusted to the larger tissue engineering constructs. In conclusion, mechanistic insights of skeletal tissue generation gained from the micromass model could lead to improved tissue engineering strategies and constructs.

Effect of preculture and loading on expression of matrix molecules, matrix metalloproteinases, and cytokines by expanded osteoarthritic chondrocytes

OBJECTIVE

One of the pathologic changes that occurs during osteoarthritis (OA) is the degeneration of the pericellular matrix (PCM). Since the PCM is likely to be involved in mechanotransduction, this study was undertaken to investigate the effects of PCM-like matrix accumulation in zonal OA chondrocytes and their influence on chondrocyte response to compression.

METHODS

Superficial and middle/deep zone chondrocytes from macroscopically normal cartilage of OA knees were expanded and encapsulated in alginate gels. The effects of compression (short-term or long-term) and preculture on chondrocyte expression of various matrix molecules, cytokines, and matrix metalloproteinases (MMPs) were assessed. Additionally, nonexpanded chondrocytes were encapsulated in alginate and cultured in the presence or absence of transforming growth factor β1 (TGFβ1) and dexamethasone and analyzed following short-term compression experiments.

RESULTS

Expanded OA chondrocytes (superficial and middle/deep zone) that were precultured for 2 weeks under free-swelling conditions prior to dynamic compression responded more sensitively to loading and had increased matrix accumulation, increased interleukin-1β (IL-1β) and IL-4 levels, and decreased levels of MMP-2 (in the middle/deep zone) compared to the nonloaded controls. Compression also decreased MMP-3 and MMP-13 levels even without preculture. Nonexpanded chondrocytes did not respond to compression, but differences in gene expression were found depending on the zone of harvest, time in culture, and medium composition.

CONCLUSION

Our findings demonstrate that with predeposited PCM-like matrix, compressive stimulation can enhance matrix protein accumulation in expanded OA chondrocytes. Investigations into how PCM or other matrix components differentially affect this balance under mechanical loading may provide invaluable insight into OA pathogenesis and the use of expanded cells in tissue engineering and regenerative medicine-based applications.

Genetic polymorphisms associated with intervertebral disc degeneration

BACKGROUND CONTEXT

Disc degeneration (DD) is a multifaceted chronic process that alters the structure and function of the intervertebral discs and can lead to painful conditions. The pathophysiology of degeneration is not well understood, but previous studies suggest that certain genetic polymorphisms may be important contributing factors leading to an increased risk of DD.

PURPOSE

To review the genetic factors in DD with a focus on polymorphisms and their putative role in the pathophysiology of degeneration. Elucidating the genetic components that are associated with degeneration could provide insights into the mechanism of the process. Furthermore, defining these relationships and eventually using them in a clinical setting may allow an identification and early intervention for those who are at a high risk for painful DD.

STUDY DESIGN

Literature review.

METHODS

This literature review focused on the studies concerning genetic polymorphisms and their associations with DD.

RESULTS

Genetic polymorphisms in 20 genes have been analyzed in association with DD, including vitamin D receptor, growth differentiation factor 5 (GDF5), aggrecan, collagen Types I, IX, and XI, fibronectin, hyaluronan and proteoglycan link protein 1 (HAPLN1), thrombospondin, cartilage intermediate layer protein (CILP), asporin, MMP1, 2, and 3, parkinson protein 2, E3 ubiquitin protein ligase (PARK2), proteosome subunit β type 9 (PSMB9), tissue inhibitor of metalloproteinase (TIMP), cyclooxygenase-2 (COX2), and IL1α, IL1β, and IL6. Each genetic polymorphism codes for a protein that has a functional role in the pathogenesis of DD.

CONCLUSIONS

There are known associations between several genetic polymorphisms and DD. Of the 20 genes analyzed, polymorphisms in vitamin D receptor, aggrecan, Type IX collagen, asporin, MMP3, IL1, and IL6 show the most promise as functional variants. Genetic studies are crucial for understanding the mechanism of the degeneration. This genetic information could eventually be used as a predictive model for determining a patient's risk for symptomatic DD.

Effect of exercise on articular cartilage

This review primarily focuses on how the macromolecular composition and architecture of articular cartilage and its unique biomechanical properties play a pivotal role in the ability of articular cartilage to withstand mechanical loads several magnitudes higher than the weight of the individual. Current findings on short-term and long-term effects of exercise on human articular cartilage are reviewed, and the importance of appropriate exercises for individuals with normal and diseased or aberrated cartilage is discussed.

Fibronectin splicing variants in human intervertebral disc and association with disc degeneration

STUDY DESIGN

In this study, normal intervertebral disc (IVD) tissues and degenerative human IVD tissues were compared for presence of fibronectin (FN) mRNA splice variants and for FN fragments (FN-f).

OBJECTIVE

To further understand FN RNA splice forms and protein fragments in disc degeneration.

SUMMARY OF BACKGROUND DATA

FN splice variants play important roles in regulating cell-matrix and matrix-matrix interactions in skeletogenesis and skeletal function in limbs and other sites. However, presence and possible roles of FN splice variants and fragments in human IVD have not been determined.

METHODS

Normal infant and adult IVD tissues were obtained from organ donors, and degenerative human IVD tissues were obtained from patients undergoing spinal surgeries. FN splice patterns were assessed by reverse transcriptase polymerase chain reaction. Relative expression levels were semiquantified by densitometry. FN and its fragments were studied by Western blot analysis.

RESULTS

Both the EDB and EDB splice variants were present in normal and degenerative IVD tissues. The EDB to EDB ratio was highest in moderately degenerative tissue. The EDA domain was only expressed in infant but not adult tissue. Variable-region (V) splice forms were present in all tissues studied. A splice form with the entire V-region, the 15th type III domain, and 10th type I domain adjacent to the 3'end of V region omitted (referred to as [V+III-15+I-10], also known as [V+C] splice form) was present at higher levels in adult than in infant samples. FN-f were also detected in degenerating tissue, but not in normal IVD tissue samples.

CONCLUSION

The data indicate that higher levels of EDB isoform and FN-f are associated with IVD degeneration. This shift in alternative splicing may reflect an attempt of tissue repair and remodeling. Novel information gathered in this study will lead to a better understanding of pathologic processes associated with disc malfunction and degeneration.

Connective tissue growth factor (CTGF) acts as a downstream mediator of TGF-beta1 to induce mesenchymal cell condensation

Mesenchymal cell (MC) condensation or the aggregation of MCs precedes chondrocyte differentiation and is required for subsequent cartilage formation during endochondral ossification. In this study, we used micromass cultures of C3H10T1/2 cells as an in vitro model system for studying MC condensation and the events important for this process. Transforming growth factor beta1 (TGF-beta1) served as the initiator of MC condensation in our model system and we were interested in determining whether CTGF functions as a downstream mediator of TGF-beta1. CTGF is a matricellular protein that has been found to be expressed in MC condensations and in the perichondrium. Micromass cultures of C3H10T1/2 cells condensed under TGF-beta1 stimulation concomitant with dramatic up-regulation of CTGF mRNA and protein levels. CTGF silencing by either CTGF siRNA or CTGF antisense oligonucleotide approaches showed that TGF-beta1-induced condensation was CTGF dependent. Furthermore, silencing of CTGF expression resulted in significant reductions in cell proliferation and migration, events that are crucial during MC condensation. In addition, up-regulation of Fibronectin (FN) and suppression of Sox9 expression by TGF-beta1 was also found to be mediated by CTGF. Immunofluorescence of developing mouse vertebrae showed that CTGF, TGF-beta1 and FN were co-expressed in condensations of MCs, while Sox9 expression was low at this stage. During subsequent chondrogenesis, Sox9 expression was high in chondrocytes while CTGF expression was limited to the perichondrium. Thus, CTGF is an essential downstream mediator of TGF-beta1-induced MC condensation through its effects on cell proliferation and migration. CTGF is also involved in up-regulating FN and suppressing Sox9 expression during TGF-beta1 induced MC condensation.

The role of human HtrA1 in arthritic disease

Human HtrA1 belongs to a widely conserved family of serine proteases involved in various aspects of protein quality control and cell fate. Although HtrA1 has been implicated in the pathology of several diseases, its precise biological functions remain to be established. Through identification of potential HtrA1 targets, studies presented herein propose that within the context of arthritis pathology HtrA1 contributes to cartilage degradation. Elevated synovial HtrA1 levels were detected in fluids obtained from rheumatoid and osteoarthritis patients, with synovial fibroblasts identified as a major source of secreted HtrA1. Mass spectrometry analysis of potential HtrA1 substrates within synovial fluids identified fibronectin as a candidate target, and treatment of fibronectin with recombinant HtrA1 led to the generation of fibronectin-degradation products that may be involved in cartilage catabolism. Consistently, treatment of synovial fibroblasts with HtrA1 or HtrA1-generated fibronectin fragments resulted in the specific induction of matrix metalloprotease 1 and matrix metalloprotease 3 expression, suggesting that HtrA1 contributes to the destruction of extracellular matrix through both direct and indirect mechanisms.

Assessing self-renewal and differentiation in human embryonic stem cell lines

Like other cell populations, undifferentiated human embryonic stem cells (hESCs) express a characteristic set of proteins and mRNA that is unique to the cells regardless of culture conditions, number of passages, and methods of propagation. We sought to identify a small set of markers that would serve as a reliable indicator of the balance of undifferentiated and differentiated cells in hESC populations. Markers of undifferentiated cells should be rapidly downregulated as the cells differentiate to form embryoid bodies (EBs), whereas markers that are absent or low during the undifferentiated state but that are induced as hESCs differentiate could be used to assess the presence of differentiated cells in the cultures. In this paper, we describe a list of markers that reliably distinguish undifferentiated and differentiated cells. An initial list of approximately 150 genes was generated by scanning published massively parallel signature sequencing, expressed sequence tag scan, and microarray datasets. From this list, a subset of 109 genes was selected that included 55 candidate markers of undifferentiated cells, 46 markers of hESC derivatives, four germ cell markers, and four trophoblast markers. Expression of these candidate marker genes was analyzed in undifferentiated hESCs and differentiating EB populations in four different lines by immunocytochemistry, reverse transcription-polymer-ase chain reaction (RT-PCR), microarray analysis, and quantitative RT-PCR (qPCR). We show that qPCR, with as few as 12 selected genes, can reliably distinguish differentiated cells from undifferentiated hESC populations.

The influence of the pericellular microenvironment on the chondrocyte response to osmotic challenge

OBJECTIVE

To examine whether differences in the pericellular microenvironment of different chondron preparations influence the chondrocyte volume regulatory response to experimental osmotic challenge.

DESIGN

Mechanically extracted chondrons (MC), enzymatically extracted chondrons (EC) and isolated chondrocytes (IC) were seeded into agarose and sampled at 1, 3 and 7 days. Samples mounted in a perfusion chamber were subjected to osmotic challenge. The cross-sectional areas of the chondrocyte and pericellular microenvironment were measured under isotonic, hypertonic and hypotonic conditions, and percentage change calculated. Separate samples were immunolabeled for type VI collagen and keratan sulfate.

RESULTS

Initially, the microenvironment of MC represented 60% of the chondron area and was occupied by type VI collagen and keratan sulfate. In EC, the microenvironment comprised 18% of the chondron area with narrow bands of type VI collagen and keratan sulfate. IC had no visible microenvironment, with small amounts of type VI collagen and keratan sulfate present. All preparations sequestered additional pericellular macromolecules during culture. Under isotonic conditions, the EC and IC chondrocytes were larger than those of MC. All chondrocytes shrank under hypertonic conditions and swelled under hypotonic conditions. MC were the least responsive, displaying the most efficient volume regulation. IC showed the largest response initially but this decreased with time. EC exhibited intermediate responses that decreased as the microenvironment increased in size.

CONCLUSIONS

The composition and structural integrity of the pericellular microenvironment do influence the cellular response to experimental osmotic challenge. This suggests that the microenvironment functions in situ to mediate the chondrocyte response to physicochemical changes associated with joint loading.

Fibronectin fragments and their role in inflammatory arthritis

OBJECTIVES

To identify potential immunopathogenic links between fibronectin (Fn) fragmentation and the inflammatory response in chronic joint disease.

METHODS

Scientific papers involving studies of Fn fragments and inflammatory processes important in the pathogenesis of arthritis, including chondrolysis, synoviocyte growth and adhesion, polymorphonuclear leukocyte (PMN) and monocyte function, proteolysis, and immune complex activation were reviewed. In addition, reports identifying Fn fragments in synovial fluid (SF) were assessed.

RESULTS

A series of Fn fragments have been identified in arthritic SF by several investigators. Fn and fragments ranging from 30 to 200 kd are present in elevated concentrations in inflammatory SF. SF Fn fragments display reduced affinity for fibrin and collagen. The 29- and 50-kd amino terminal fragments mediate release of proteoglycan from articular cartilage by RGD-independent mechanisms. Fn fragments can induce fibroblast gene expression of metalloproteinases or can act as proteinases themselves. A 90-kd plasmin generated fragment possesses homology with streptokinase. Fragments mediate PMN chemotaxis and enhance proliferation of CD4+ lymphocytes as well as binding to the C1q component of complement and influencing the behavior of immune complexes.

CONCLUSIONS

Fn fragments can be functionally and biochemically characterized in diseased SF. Modification of fragment formation and inhibition of fragment function may have potential therapeutic value in the interruption of chronic synovial inflammation.

Confocal analysis of the molecular heterogeneity in the pericellular microenvironment produced by adult canine chondrocytes cultured in agarose gel

Adult articular chondrocytes are each surrounded by a heterogeneous microenvironment and together form the chondron. Since little is known of chondron development, agarose gel culture, confocal immunohistochemistry and image analysis have been used to characterize the molecular anatomy and temporal development of the chondrocyte pericellular microenvironment in vitro. Two structurally distinct domains were identified during the 12-week culture period. The first comprised a narrow glycocalyx, 1-3 microns in width, which consolidated over time and was rich in collagen types II, VI, IX and XI, fibronectin, decorin and the aggrecan epitopes, 5D4 and HABR. The second region emerged after 4-6 weeks in culture and progressively developed a broad territorial region up to 12 microns wide around the chondrocyte and pericellular glycocalyx. Co-localization studies confirmed the dominance of aggrecan epitopes 2B6, EFG-4, 5D4 and HABR in the territorial domain, whereas surface density mapping with NIH image revealed two patterns of staining, one punctate and stippled, the other more uniform in distribution. The pericellular differentiation identified appeared analogous to the chondrons of adult articular cartilage, and provides an appropriate in vitro model for further studies of cell surface receptor function in the orchestration of pericellular matrix assembly.

Articular cartilage chondrons: form, function and failure

The chondrocyte and its pericellular microenvironment together represent the chondron, historically considered the primary structural, functional and metabolic unit of articular and other hyaline cartilages. This review summarises research over the last 10 years to establish the molecular anatomy, functional properties and metabolic contribution of the chondron in articular cartilage homeostasis, and its failure during the initiation and progression of degenerative osteoarthritis.

Structural colocalisation of type VI collagen and fibronectin in agarose cultured chondrocytes and isolated chondrons extracted from adult canine tibial cartilage

Cell-matrix and matrix-matrix interactions are of critical importance in regulating the development, maintenance and repair of articular cartilage. In this study, we examined the structural colocalisation of type VI collagen and fibronectin in isolated chondrons and long-term agarose cultured chondrocytes extracted from normal adult canine articular cartilage. Using double labelling immunohistochemistry in conjunction with dual channel confocal microscopy and digital image processing we demonstrate that type VI collagen and fibronectin are distributed in a similar staining pattern and are colocalised at the surface of cultured chondrocytes and isolated chondrons. The results suggest that type VI collagen and fibronectin may play a role in both cell-matrix adhesion and matrix-matrix cohesion in the pericellular microenvironment surrounding articular cartilage chondrocytes.

Fibronectin mRNA alternative splicing is temporally and spatially regulated during chondrogenesis in vivo and in vitro

Fibronectin, a component of the extracellular matrix in a variety of tissues, participates in many critical cellular processes, including differentiation, adhesion, and migration. A positive correlation exists between the presence of fibronectin and the onset of chondrogenesis, the differentiation of mesenchyme into cartilage. Heterogeneity in the structure of fibronectin is largely due to the alternative splicing of at least three exons (IIIB, IIIA, and V) during processing of a single primary transcript. We have previously shown that the fibronectin mRNA splicing patterns change during chondrogenesis (Bennett et al. [1991] J. Biol. Chem, 266:5918-5924). All of the fibronectin mRNAs from prechondrogenic chick limb mesenchyme contain exons IIIB, IIIA, and V (B + A + V +), whereas all of the fibronectin mRNAs from chick cartilage contain exons IIIB and V but do not contain exon IIIA (B + A - V +). In this study, we show that fibronectin mRNAs containing exon IIIA (FN-A) and/or the mRNAs containing exon IIIB (FN-B) are expressed in a specific and different spatiotemporal manner in the developing chick limb in vivo, as well as in limb mesenchymal cells undergoing chondrogenesis in vitro. Specifically, in situ hybridization reveals that FN-B mRNAs are present throughout the various stages (HH 20-30) of limb cartilage development in vivo, whereas FN-A mRNAs disappear following the condensation phase of chondrogenesis and absent from the resulting cartilage, Chick limb cartilage fibronectin mRNAs are therefore B + A-, as in other embryonic cartilage tissues. Furthermore, limb mesenchymal cells undergoing chondrogenesis in vitro lose FN-A mRNAs immediately following condensation, recapitulating the events that occur during chondrogenesis in vivo. These results suggest an important role for fibronectin mRNA alternative splicing during chondrogenic differentiation.

The exon encoding the fibronectin type III-9 repeat is constitutively included in the mRNA from chick limb mesenchyme and cartilage

The fibronectin monomer is comprised of three types of homologous repeating units, the types I, II, and III elements. Each type III repeat is encoded by two exons except for the two type III repeats involved in alternative splicing (IIIB and IIIA) and the type III-9 repeat which are all encoded by one exon. The fact that the type III-9 repeat is the only other type III repeat encoded by one exon has led to speculation that this exon may also be alternatively spliced. However, no evidence exists for alternative splicing of this exon in any tissues examined to date. The recent localization of a cell adhesion synergy site within the type III-9 repeat increases the likelihood of functional ramifications if the exon encoding this repeat is alternatively spliced in specific cells or tissues. We have shown previously that chick cartilage contains an unusual fibronectin mRNA splicing pattern and that the pattern changes during chondrogenesis from B+A+V+ to B+A-V+. In order to completely characterize the fibronectin mRNA in cartilage and other mesenchymal tissues for all possible alternative splicing events, we have determined whether or not the exon encoding the type III-9 repeat is alternatively spliced in these tissues. RNase protection and RT/PCR assays indicate that the fibronectin mRNA in all of these tissues, including cartilage, contains the type III-9 repeat as a constitutively included exon. Thus the exon encoding the type III-9 repeat will serve as a useful control exon for examining the regulation of tissue-specific alternative splicing during chondrogenesis.

Ultrastructural immunolocalization of fibronectin in epiphyseal and metaphyseal bone of young rats

Fibronectin is a well known glycoprotein of extracellular connective tissue matrices due to a specific amino acid-sequence (RGD) suggested to act as an attachment factor in cell-cell or cell-matrix interactions. Although also present in bone, little is known about the role of fibronectin in this tissue. To obtain data for discussions on function we used ultrastructural immunolocalization techniques to quantitatively examine the distribution of fibronectin in various bone matrix compartments. The study was focused on three different stages of endochondral ossification in growing long bones of young rats. The results show large amounts of fibronectin in mature bone tissue. At a higher magnification, an obvious fibronectin association to individual fibrils of collagen type I was demonstrated. Intracellular labeling was observed in Golgi-related vesicles in some active osteoblasts of metaphyseal bone, indicating local synthesis of fibronectin. In contrast to previous suggestions based on light microscopic observations, the labeling of bone or cartilage matrices facing the surface of all cell types were low. The pattern is clearly different from that of osteopontin and bone sialoprotein, two other bone matrix proteins with the same cell-binding sequence. Our results indicate that fibronectin at these stages of development participates in matrix organization rather than being an important link between cartilage or bone matrix and adjacent cells.

Splicing patterns of fibronectin mRNA from normal and osteoarthritic human articular cartilage

Fibronectin, a large extracellular glycoprotein, mediates the interaction of cells with the extracellular matrix. Heterogeneity in the structure of fibronectin is largely due to the alternative splicing of three exons (IIIB, IIIA and V) during processing of the fibronectin primary transcript. Osteoarthritis, a degenerative disease of synovial joints, is characterized by a progressive loss of the articular cartilage eventually resulting in pain and loss of joint function. In contrast to the loss of most cartilage matrix proteins accompanying this process, osteoarthritic cartilage contains more fibronectin than disease-free cartilage. We examined the splicing patterns of fibronectin mRNA from adult human articular cartilage of normal and osteoarthritic joints by RNase protection (exon IIIA and exon IIIB) and reversed transcription-polymerase chain reaction (exon V) assays to determine whether or not the increased fibronectin content in osteoarthritic cartilage is also associated with differences in the splicing patterns of these three alternatively spliced exons. The results revealed no gross differences in splicing of these exons between the fibronectin mRNA isolated from adult human articular normal and osteoarthritic cartilage. Thus alterations in the structure of cartilage fibronectin do not appear to correlate with the increased level of fibronectin protein associated with osteoarthritis.

Mechanism of longitudinal bone growth and its regulation by growth plate chondrocytes

Growth plate chondrocytes play a pivotal role in promoting longitudinal bone growth. The current review represents a brief survey of the phenomena involved in this process at the cellular level; it delineates the contributions made by various activities during the course of the chondrocyte life cycle, notably proliferation and hypertrophy, and illustrates how the relative contributions may be modulated according to the particular needs of an organism at critical phases of growth. The cellular mechanisms by which a few well characterized growth-promoting substances exert their influences are discussed in the light of recent findings pertaining to epiphyseal plate chondrocytes in vivo.

Mechanisms of photo-induced vitreous liquefaction

To understand the mechanisms of photo-induced vitreous liquefaction, this study investigated the effects of free radicals on collagen and hyaluronic acid (HA). Bovine vitreous collagen or HA was irradiated by visible light in the presence of riboflavin (RF) as a photosensitizer. The changes in the molecular weight of collagen and HA were monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high-performance liquid chromatography, respectively. Free radicals were shown to cause an increase in the high-molecular-weight components and insolubilization of the vitreous collagen and a decrease in the molecular weight of HA. The change in molecular properties of the vitreous collagen could be attributed to extensive crosslinks of the molecules. Since RF is present in the vitreous, which is irradiated by visible light over a lifetime, both cross-links of vitreous collagen and degradation of HA may contribute to age-related vitreous liquefaction.

Effect of pregnancy on proteoglycan-induced progressive polyarthritis in BALB/c mice: remission of disease activity

Proteoglycan-induced arthritis is a murine autoimmune model displaying many similarities to human rheumatoid arthritis and ankylosing spondylitis, as has been documented by clinical, immunological and histopathological studies. Since the onset of arthritis correlates with the serum antibody level to mouse cartilage proteoglycan (PG), it is believed that these autoreactive antibodies may play crucial roles in the pathological mechanisms of PG-induced arthritis. We have found that fertility in these PG-induced arthritic mice had been reduced but, unlike collagen-induced arthritis, had not been completely lost. Moreover, pregnancy had a beneficial effect upon the clinical symptoms with very little or no influence on serum antibody levels. Although fertility was retained and arthritic mothers delivered healthy offspring, the birth frequency was significantly less than in non-arthritic age-matched controls. Furthermore, the presence of anti-PG autoantibodies (predominantly IgG1 subclass) transmitted from arthritic mothers to infants transplacentally and by milk during the lactation period did not render these offspring either resistant or more sensitive to subsequent induction of arthritis. Subsequent immunization of infants with 'arthritogenic' PG revealed an unaltered susceptibility to arthritis induction.

Age-related changes in the molecular properties of vitreous collagen

To understand the molecular events underlying rheological vitreous changes, age-related changes in the molecular properties of bovine and human vitreous collagen were monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The adult bovine vitreous showed an increase in the high molecular-weight components of the collagen compared with the young samples. In addition, the apparent molecular size of the alpha-chain of the adult type II collagen was slightly larger than that of the young samples. The same tendency also was observed in human vitreous. The increase of high-molecular-weight components of the vitreous collagen could be attributed to intermolecular cross-links of the peptide chains.

Fibronectin, cartilage, and osteoarthritis

Fibronectin is a multifunctional glycoprotein present at low levels in the extracellular matrix of normal cartilage. In this tissue, as in others, it may be a component of a cell matrix adhesion complex together with cell-surface proteoglycans but also may play a role in the organization of the extracellular matrix. In osteoarthritis (OA), fibronectin content is markedly increased in the altered matrix because of an increased synthesis by the chondrocytes and accumulation in the extracellular matrix. At least part of the fibronectins synthesized in one degenerated cartilage is composed of isoforms more sensitive to proteolytic cleavage that are absent in normal cartilage. This increased content of fibronectin during osteoarthritic processes might entail several consequences related to the multiple functions of fibronectin and its generated fragments, namely a change in chondrocyte phenotype, a switch in synthesis of collagen type, an increased activity of locally secreted metalloproteases, and induction of self-proteolytic activities against gelatin and fibronectin. However, it is not yet clearly understood whether the early increased synthesis of fibronectin in OA acts as an "agent" in an attempt to repair cartilage by the chondrocytes or whether it acts as a deleterious "agent."

Functional assessment of joint use in experimental inflammatory murine arthritis

A select group of cartilage proteoglycans (fetal human, porcine, and canine articular cartilages and human osteophytes, all depleted of chondroitin sulfate) produces progressive polyarthritis and spondylitis in BALB/c mice. The development of the disease in this murine strain is dependent on the expression of both cell-mediated and humoral immunities to host mouse cartilage proteoglycan. Autoantibodies have been detected in sera of arthritis animals from the fifth to sixth week after immunization, and their appearance precedes the development of the first clinical symptoms by a few days in animals with passively transferred arthritis. In this preliminary experiment, we describe several functional tests and gait analyses in normal mice, in acutely and chronically arthritic mice, and in randomly selected mice with proteoglycan-induced and collagen-induced arthritis. The procedures revealed that changes in joint use and gait could predate by weeks the appearance of the first clinical symptoms (joint swelling, redness, and joint stiffness) of arthritis in mice. Moreover, abnormalities measured by functional tests, such as strength of grip and maintenance of posture on sandpaper, wood, or vinyl surfaces at three different tilt angles (30, 45, and 60 degrees), and gait analysis preceded the appearance of autoantibodies in sera of immunized animals; this indicates that such measurements could provide a noninvasive and simple method to assess joint function accurately during the development of arthritis.

Fibronectin and keratan sulfate synthesis by canine articular chondrocytes in culture is modulated by dibutyryl cyclic adenosine monophosphate

The ability of cyclic adenosine monophosphate (cAMP) to maintain differentiated properties of canine articular chondrocytes in culture is reported. Treatment with 0.5 mM dibutyryl cAMP (DBcAMP) caused the cells to adopt a more rounded morphology. This change in morphology seems to have no effect on the overall biosynthetic rates of the cells. After a pulse with 35S-methionine, there was no difference in the concentration of labeled proteins between cultures treated with DBcAMP and control cultures. After 6 days, the amount of fibronectin (FN) in the media of DBcAMP-treated cultures detected by an enzyme-linked immunosorbent assay was specifically reduced by 30%. The amount of 35S-FN purified by gelatin-affinity chromatography decreased 33%. Moreover, the percentage of FN containing the extra domain A sequence was reduced from 19.4 +/- 8.7% in control cultures to 9.6 +/- 4.2%. Concomitant with the decrease in FN, there was an increase in the concentration of keratan sulfate in the media of DBcAMP-treated cultures. After 6 days, treated cultures had 47% more keratan sulfate than controls did. These changes appear not to be the result of a change in the deposition of FN or keratan sulfate, because the amount of these molecules that could be extracted from the cell layer was typically below the limit of detection of the assays. Instead, it seems there is a phenotypic change in the chondrocytes pertaining to the production of FN and keratan sulfate.

Proteoglycan-targeted antibodies as markers on non-Hodgkin lymphoma xenografts

A family of mono- and polyclonal antibodies raised against proteoglycans or their "subcomponents" served as novel markers to characterize the phenotypes of three non-Hodgkin lymphoma xenograft lines (HT 58 lymphoblastic, HT 117 centroblastic, HT 130 centrocytic) together with normal, human peripheral blood B lymphocytes. These xenografted NHL lines, maintained by serial transplantations on artificially immunosuppressed mice, expressed very similar B-cell-related antigens and differences on the cell surface (HT 58 greater than HT 117 greater than HT 130 greater than B cells) when they were exposed to monoclonal antibodies (mAb) to cartilage proteoglycans. Anti-proteoglycan antibodies used in this study recognize complex epitopes of core protein segment associated with carbohydrate, shared by human cartilage proteoglycans and certain lymphoma cells. The binding of these antibodies was independent of cell-cycle phase. The results suggest that the anti-proteoglycan mAbs could be used as new phenotypic markers to individualize non-Hodgkin lymphomas.

Exogenous glycosaminoglycans modulate chondrogenesis, cyclic AMP level and cell growth in limb bud mesenchyme cultures

Effects of hyaluronate, heparin and chondroitin-6-sulfate were studied on micromass cultures of chick limb bud mesenchyme (Hamburger and Hamilton stages 23-24). Histochemical, electron microscopical, biochemical and radiochemical investigations of day 4 cultures revealed dose-dependent inhibitory effects of these glycosaminoglycans on chondrogenesis, cyclic AMP level and growth of cells. In addition, hyaluronate with 100 micrograms/ml dose caused a displacement of newly formed proteoglycan from cultures into the medium. It is supposed that exogenous glycosaminoglycans influence ionic equilibrium in the immediate vicinity of cells and disturb the organization of the prechondrogenic extracellular matrix resulting in alterations of cell membrane--cytoskeleton associations. These alterations may provoke a reduction in cyclic AMP level and DNA synthesis. It is suggested that a reduction in cyclic AMP level preceding the expression of cartilage phenotype results in the inhibition of chondrogenesis.

Experimental osteoarthritic articular cartilage is enriched in guanidine soluble type VI collagen

Experimental osteoarthritis was surgically induced in the right knee joint of dogs; the left knee served as a control. Articular cartilage was extracted with 4 M guanidinium chloride, 0.05 M sodium acetate, pH 6.0, containing proteinase inhibitors and the proteins purified by associative CsCl density gradient centrifugation. Equal quantities of protein were electrophoresed in agarose-acrylamide gradient gels and the high molecular weight type VI collagen bands detected in immunoblots with a polyclonal antiserum. Type VI collagen bands between 185 and 220 kDa were evident in the pathological specimens of dogs sacrificed 3, 5, and 7 months after surgery and were either absent or only very weakly visible in the controls. These results demonstrate that experimental osteoarthritic cartilage is enriched in 4 M guanidine-soluble type VI collagen.

Isolation of proteoglycan-specific T lymphocytes from patients with ankylosing spondylitis

Three T-cell lines and clones of the OKT4 phenotype have been isolated from the peripheral blood of three patients with ankylosing spondylitis. Antigen specificities of T cells were determined with purified protein derivative-(PPD) and cartilage-derived antigens, namely proteoglycans from human articular cartilage and intervertebral disc, bovine nasal cartilage, and rat chondrosarcoma and human type II collagen from cartilage. A cell line from one patient reacted with proteoglycans from human articular cartilage and human intervertebral disc, but the other two cell lines (each from a different patient) and four clones from one of the latter two lines proved to be highly specific for the human articular cartilage proteoglycan. From a study of four proteoglycan specific clones isolated from one patient, it is clear that removal of chondroitin sulfate had no effect on immunoreactivity but digestion of proteoglycan with pronase or alkali/sodium borohydride treatment abolished all reactivity. A OKT4-positive T-cell clone isolated from a healthy adult which was reactive to PPD was used to compare the antigen specificity of cells: this clone showed no reactivity to any of the other putative antigens listed above.

Extracellular matrix alterations during endochondral ossification in humans

Immunohistochemical methods were employed to examine alterations in the cartilage extracellular matrix constituents associated with endochondral ossification in humans. The distributions of chondroitin 4- and 6-sulfate and keratan sulfate proteoglycan (PG) determinants, cartilage PG link protein, collagen types I and II, and fibronectin were determined in iliac crest growth-plate specimens using the avidin-biotin-horseradish peroxidase system. Collagen type II was distributed throughout the growth plate, providing a framework within which chondrocytes divided and formed clusters of differentiating (hypertrophic) cells. The septa between these clusters and their subchondral extensions into underlying bone trabeculae were rich in PG, PG link protein, and collagen type II and resembled the extracellular matrix of reserve cartilage. The territorial matrix associated with the differentiating cells within the clusters contained reduced amounts of collagen type II, PG link protein, and possibly cartilage PG. Collagen type I and fibronectin were detected within the cytoplasm of the maturing and degenerating cells, and fibronectin localized intensely to the pericellular matrix envelopes of these cells. These alterations presumably facilitate the degradation of the matrix associated with the cell clusters by invading vascular tissue, while the septa, which retain the characteristics of more typical cartilage matrix, are not degraded and firmly anchor the cartilage to the subchondral bone.

An immunohistochemical study of fibronectin in human osteoarthritic and disease free articular cartilage

Fibronectin is a minor component of cartilage connective tissue matrix, which is reported to accumulate in increased amounts in osteoarthritis. The presence of raised levels of fibronectin in human osteoarthritic cartilage by immunoperoxidase localisation is confirmed. Residual femoral head articular cartilage from 17 patients with osteoarthritis contained variable but substantial amounts of fibronectin. This was localised mainly in a band within the matrix of the surface zone. No significant deposits of fibronectin were found in this or any other area of the normal specimens. Intracellular fibronectin was identified in some cells of the surface zone, indicating that it was, in part, synthesised locally. The presence and distribution of locally produced fibronectin in osteoarthritic cartilage suggest that its synthesis is a response by chondrocytes to changes in the cartilage matrix.

Calcifying epithelioma of Malherbe with ossification. Special reference to lectin binding and immunohistochemistry of ossified sites

Lectin binding sites and immunohistochemically detectable fibronectin and carbonic anhydrase II (CA II) were examined in induced bone structure of calcifying epithelioma of Malherbe. Pathological features of ossification were particularly evident in the border zone between tumor epithelium and stroma, and epithelial foci adjacent to sites of bone formation were conspicuously basophilic. The use of lectin binding in these foci coincided with the basophilic epithelial zones: Con A, RCA-1, PNA, SBA, and WGA bound strongly, indicating the presence of glucose (Glu), mannose (Man), galactose (Gal), N-acetyl-D-galactosamine (GalNAC), and N-acetyl-D-glucosamine (GlcNAC). Fibronectin was also detected in the same epithelial-mesenchymal interacting layers as the positive lectin-binding sites. Staining for CA II was strongly positive in giant cells and in epithelial zones in the bone-inducing areas. The bond induction mechanism in the stromal tissue of calcifying epithelioma of Malherbe may initially involve the action CA II in the epithelial-mesenchymal interacting zone, which then brings about ossification of the matrix which is fibronectin-positive and rich in lectin-binding sites.

Immunity to cartilage proteoglycans in BALB/c mice with progressive polyarthritis and ankylosing spondylitis induced by injection of human cartilage proteoglycan

Intraperitoneal injection of human fetal cartilage proteoglycan (depleted of chondroitin sulfate) in Freund's complete or incomplete adjuvant induces a chronic erosive polyarthritis and spondylitis in all female BALB/c mice. This occurrence is strain-specific but not haplotype-specific, and it is sex-related. The development of the arthritis is associated with the natural presence of cellular immunity to the immunizing antigen and to chondroitinase ABC-treated mouse cartilage proteoglycan. In addition, relatively more antibody to the immunizing proteoglycan is elicited in arthritic mice, and antibodies are produced that cross-react with native mouse proteoglycan. This combination of immune responses is not observed in mice that do not develop arthritis. Associated with the arthritis is the development of cytotoxicity to mouse chondrocytes and, in some animals, of rheumatoid factor, immune deposits in joint tissues and kidneys, and the production of autoantibodies to mouse type II collagen. These observations might be related to our earlier demonstration that immunity to human cartilage proteoglycan is observed in some patients with ankylosing spondylitis.

Monoclonal antibodies to different protein-related epitopes of human articular cartilage proteoglycans

Monoclonal antibodies produced against chondroitinase-treated human adult cartilage proteoglycans were selected for their ability to recognize epitopes on native proteoglycans. Binding analyses revealed that four of these monoclonal antibodies (BCD-4, BCD-7, EFG-4 and KPC-190) each recognized a different epitope on the same proteoglycan molecule which represents a subpopulation of a high buoyant density (D1) fraction of human articular cartilage proteoglycans (10, 30, 50 and 60% in fetal-newborn, 1.5 years old, 15 years old and 52-56 years old cartilages, respectively). Analysis of epitope specificities revealed that BCD-7 and EFG-4 monoclonal antibodies recognized epitopes on proteoglycan monomer which are associated with the protein structure in that they are sensitive to cleavage by Pronase, papain and alkali treatment and do not include keratan sulphate, chondroitin sulphate or oligosaccharides. The BCD-4 and KPC-190 epitopes also proved to be sensitive to Pronase or papain digestion or to alkali treatment, but keratanase or endo-beta-galactosidase also reduced the immunoreactivity of these epitopes. These observations indicate that the BCD-4 and KPC-190 epitopes represent peptides substituted with keratan sulphate or keratan sulphate-like structures. The BCD-4 epitope is, however, absent from a keratan sulphate-rich fragment of human adult proteoglycan, while the other three epitopes were detected in this fragment. None of these four epitopes were detected in the link proteins of human cartilage, in the hyaluronic acid-binding region of human newborn cartilage proteoglycan, in Swarm rat chondrosarcoma proteoglycan, in chicken limb bud proteoglycan monomer and in the small dermatan sulphate-proteoglycan of bovine costal cartilage. EFG-4 and KPC-190 epitopes were not detected in human fetal cartilage proteoglycans, although fetal molecules contained trace amounts of epitopes reactive with BCD-4 and BCD-7 antibodies.