NC4 Domain of cartilage-specific collagen IX inhibits complement directly due to attenuation of membrane attack formation and indirectly through binding and enhancing activity of complement inhibitors C4B-binding protein and factor H

Beyond the Norm: The emerging interplay of complement system and extracellular matrix in the tumor microenvironment

Ground-breaking awareness has been reached about the intricate and dynamic connection between developing tumors and the host immune system. Being a powerful arm of innate immunity and a functional bridge with adaptive immunity, the complement system (C) has also emerged as a pivotal player in the tumor microenvironment (TME). Its "double-edged sword" role in cancer can find an explanation in the controversial relationship between C capability to mediate tumor cell cytolysis or, conversely, to sustain chronic inflammation and tumor progression by enhancing cell invasion, angiogenesis, and metastasis to distant organs. However, comprehensive knowledge about the actual role of C in cancer progression is impaired by several limitations of the currently available studies. In the current review, we aim to bring a fresh eye to the controversial role of C in cancer by analyzing the interplay between C and extracellular matrix (ECM) components as potential orchestrators of the TME. The interaction of C components with specific ECM components can determine C activation or inhibition and promote specific non-canonical functions, which can, in the tumor context, favor or limit progression based on the cancer setting. An in-depth and tumor-specific characterization of TME composition in terms of C components and ECM proteins could be essential to determine their potential interactions and become a key element for improving drug development, prognosis, and therapy response prediction in solid tumors.

Cadherin dysregulation in gastric cancer: insights into gene expression, pathways, and prognosis

Background

The Cadherin gene family holds immense significance in maintaining the integrity and functionality of stomach tissues, playing crucial roles in cell-cell adhesion, cell migration and differentiation. Dysregulation of cadherin expression and function has been closely associated with various gastric diseases, particularly gastric cancer (GC). Understanding the regulation and clinical implications of cadherin genes in GC is essential to improve our knowledge and to identify new potential prognostic markers and therapeutic targets.

Methods

In this study, we provide an overview on the role of cadherin family genes in GC using bioinformatics analysis. We analyzed the expression, mutational status, and prognostic value of these genes based on available public datasets. Our methodology involved data mining, differential expression analysis, functional enrichment analysis, and survival analysis to explore the association between cadherin gene expression and clinical outcomes in GC patients. Additionally, we investigated the relationship between cadherin expression and immune cell infiltration to gain insights into the tumor microenvironment's role in GC progression.

Results

Our bioinformatics analysis revealed significant differential expression of 16 cadherin genes in GC samples compared to normal tissues. Approximately up to 52% of the analyzed cancer samples exhibited genomic alterations in these cadherins, indicating their potential relevance in GC development. Functional enrichment analysis demonstrated that these differentially expressed cadherins were closely associated with critical cellular processes, including cell adhesion and immune-modulation. Remarkably, lower expression levels of most cadherin genes were linked to improved prognosis in GC patients, suggesting their potential importance as valuable prognostic biomarkers.

Conclusions

The findings deriving from our comprehensive study provide important insights into the dysregulation of cadherin genes in GC and their impact on gene expression, molecular pathways, and prognosis. The associations with clinical outcomes and immune cell infiltration highlight the potential role of cadherin genes as prognostic biomarkers and therapeutic targets in GC.

The role of fibromodulin in inflammatory responses and diseases associated with inflammation

Inflammation is an immune response that the host organism eliminates threats from foreign objects or endogenous signals. It plays a key role in the progression, prognosis as well as therapy of diseases. Chronic inflammatory diseases have been regarded as the main cause of death worldwide at present, which greatly affect a vast number of individuals, producing economic and social burdens. Thus, developing drugs targeting inflammation has become necessary and attractive in the world. Currently, accumulating evidence suggests that small leucine-rich proteoglycans (SLRPs) exhibit essential roles in various inflammatory responses by acting as an anti-inflammatory or pro-inflammatory role in different scenarios of diseases. Of particular interest was a well-studied member, termed fibromodulin (FMOD), which has been largely explored in the role of inflammatory responses in inflammatory-related diseases. In this review, particular focus is given to the role of FMOD in inflammatory response including the relationship of FMOD with the complement system and immune cells, as well as the role of FMOD in the diseases associated with inflammation, such as skin wounding healing, osteoarthritis (OA), tendinopathy, atherosclerosis, and heart failure (HF). By conducting this review, we intend to gain insight into the role of FMOD in inflammation, which may open the way for the development of new anti-inflammation drugs in the scenarios of different inflammatory-related diseases.

Study on the mechanism and pharmacokinetics of HB-NC4 based on C5b-9 target in the treatment of osteoarthritis

Osteoarthritis (OA) is a chronic degenerative disease that mostly occurs in elderly individuals over 60 years old. The detailed pathogenesis of OA is unclear. Medicines available on the market are nonsteroidal anti-inflammatory drugs. Therefore, in this study, a fusion protein was introduced, and the detailed mechanism that could alleviate OA was discussed. As a targeted protein, HB-NC4 showed better binding ability to chondrocytes, and its half-life period was prolonged compared to NC4 alone. In addition, HB-NC4 can not only affect the levels of C3 and C5, but also inhibit the formation of the membrane-attack complex (MAC, C5b-9), thereby further affecting the expression of MAPK signalling pathway-related proteins to achieve the goal of treating OA. Thus, in this study, we demonstrate the pharmacokinetics of HB-NC4 and its mechanism to alleviate OA by regulating the complement system and MAPK signalling pathway. This study provides a new method for OA therapy based on fusion proteins.

Design, Preparation, and Bioactivity Study of New Fusion Protein HB-NC4 in the Treatment of Osteoarthritis

Osteoarthritis (OA) is now becoming the main disease that affects public health. There is no specific medicine used for OA in clinical application until now. Recently, several studies demonstrated that OA is closely related to the complement system, and some complement regulators such as N-terminal non-collagenous domain 4 (NC4) aimed at alleviating OA have shown a promising therapeutic effect. However, targeting ability is the main limitation for NC4. In this study, a fusion protein named heparin-binding domain-N-terminal non-collagenous domain 4 (HB-NC4) was proposed to solve this problem, which could provide a better way for OA treatment. First, HB-NC4 plasmid was constructed using ClonExpress II one-step ligation kit method. And Escherichia coli BL21 was utilized to express the fusion protein, Ni²⁺-sepharose, and a desalting gravity column were introduced to purify HB-NC4. The results showed that 0.84 mg HB-NC4 could be obtained from a 1 L culture medium with a purity higher than 92.6%. Then, the hemolytic assay was introduced to validate the anti-complement activity of HB-NC4; these results demonstrated that both HB-NC4 and NC4 had a similar anti-complement activity, which indicated that heparin-binding (HB) did not affect the NC4 structure. Targeting ability was investigated in vivo. HB-NC4 showed a higher affinity to cartilage tissue than NC4, which could prolong the retention time in cartilage. Finally, the destabilization of the medial meniscus (DMM) model was applied to investigate HB-NC4 pharmacodynamics in vivo. The results indicated that HB-NC4 significantly slowed cartilage degradation during the OA process. In summary, compared with NC4, HB-NC4 had better-targeting ability which could improve its therapeutic effect and prolonged its action time. It could be used as a new complement regulator for the treatment of OA in the future.

The Damage-Associated Molecular Patterns (DAMPs) as Potential Targets to Treat Osteoarthritis: Perspectives From a Review of the Literature

During the osteoarthritis (OA) process, activation of immune systems, whether innate or adaptive, is strongly associated with low-grade systemic inflammation. This process is initiated and driven in the synovial membrane, especially by synovium cells, themselves previously activated by damage-associated molecular patterns (DAMPs) released during cartilage degradation. These fragments exert their biological activities through pattern recognition receptors (PRRs) that, as a consequence, induce the activation of signaling pathways and beyond the release of inflammatory mediators, the latter contributing to the vicious cycle between cartilage and synovial membrane. The primary endpoint of this review is to provide the reader with an overview of these many molecules categorized as DAMPs and the contribution of the latter to the pathophysiology of OA. We will also discuss the different strategies to control their effects. We are convinced that a better understanding of DAMPs, their receptors, and associated pathological mechanisms represents a decisive issue for degenerative joint diseases such as OA.

Regulatory T Cells in Bioactive Peptides-Induced Oral Tolerance; a Two-Edged Sword Related to the Risk of Chronic Diseases: A Systematic Review

This systematic review assesses the literature regarding beneficial and potential detrimental effects of bioactive peptides (BPs), focusing on evidence of regulatory T cells (T-regs) mediated oral tolerance (OT), collagen hydrolysate (CH) supplementation in osteoarthritis (OA) and the association of T-regs with chronic disease. The systematic search was done for articles published from inception to April 2019 using the PubMed and Scopus databases. About 3081 papers were identified by three different search strategies and screened against inclusion criteria which resulted in the inclusion of 22 articles. From the included articles, 12 papers were related to treatment of different disease in vivo by oral administration of BPs, six articles evaluated the effects of CH supplementation, as a rich source of BPs, on OA pain-relief and four observational studies assessed the association of circulating T-regs and risk of cancer and cardiovascular disease (CVD). The evidence obtained from first search strategy, indicated that oral administration of BPs improve clinical manifestations of animal models of allergy, arthritis, atherosclerosis, ulcerative colitis and allograft rejection by T-regs expansion; while, observational studies showed that although higher levels of circulating T-regs reduced risk of CVD and allergy, but, increased risk of solid cancers.

Identifying effector molecules, cells, and cytokines of innate immunity in OA

Inflammatory changes are observed in affected joints of osteoarthritis (OA) patients and are thought to be involved in the pathology that develops along OA progression. This narrative review provides an overview of the various cell types that are present in the joint during OA and which alarmins, cytokines, chemokines, growth factors, and other mediators they produce. Moreover, the involvement of more systemic processes like inflammaging and its associated cellular senescence in the context of OA are discussed.

[Role and progress of innate immunity in pathogenesis of osteoarthritis]

Objective

To review and summarize the role and progress of innate immunity in the pathogenesis of osteoarthritis (OA).

Methods

The domestic and foreign literature in recent years was reviewed. The role of innate immune-mediated inflammation, macrophages, T cells, and complement systems in the pathogenesis of OA, potential therapeutic targets, and the latest research progress were summarized.

Results

With the deepening of research, OA is gradually considered as a low-grade inflammation, in which innate immunity plays an important role. The polarization of synovial macrophage subpopulation in OA has been studied extensively. Current data shows that the failure of transformation from M1 subtype to M2 subtype is a key link in the progression of OA. T cells and complement system are also involved in the pathological process of OA.

Conclusion

At present, the role of innate immunity in the progress of OA has been played in the spotlight, whereas the specific mechanism has not been clear. The macrophage subtype polarization is a potential therapeutic target for early prevention and treatment of OA.

Novel potential inhibitors of complement system and their roles in complement regulation and beyond

The complement system resembles a double-edged sword since its activation can either benefit or harm the host. Thus, regulation of this system is of utmost importance and performed by several circulating and membrane-bound complement inhibitors. The pool of well-established regulators has recently been enriched with proteins that either share structural homology to known complement inhibitors such as Sushi domain-containing (SUSD) protein family and Human CUB and Sushi multiple domains (CSMD) families or extracellular matrix (ECM) macromolecules that interact with and modulate complement activity. In this review, we summarize the current knowledge about newly discovered complement inhibitors and discuss their implications in complement regulation, as well as in processes beyond complement regulation such cancer development. Understanding the behavior of these proteins will introduce new mechanisms of complement regulation and may provide new avenues in the development of novel therapies.

Osteoarthritis and the Complement Cascade

Accumulating evidence demonstrates that complement activation is involved in the pathogenesis of osteoarthritis (OA). However, the intimate complement regulation and cross talk with other signaling pathways in joint-associated tissues remain incompletely understood. Recent insights are summarized and discussed here, to put together a more comprehensive picture of complement involvement in OA pathogenesis. Complement is regulated by several catabolic and inflammatory mediators playing a key role in OA. It seems to be involved in many processes observed during OA development and progression, such as extracellular cartilage matrix (ECM) degradation, chondrocyte and synoviocyte inflammatory responses, cell lysis, synovitis, disbalanced bone remodeling, osteophyte formation, and stem cell recruitment, as well as cartilage angiogenesis. In reverse, complement can be activated by various ECM components and their cleavage products, which are released during OA-associated cartilage degradation. There are, however, some other cartilage ECM components that can inhibit complement, underlining the diverse effects of ECM on the complement activation. It is hypothesized that complement might also be directly activated by mechanical stress, thereby contributing to OA. The question arises whether keeping the complement activation in balance could represent a future therapeutic strategy in OA treatment and in the prevention of its progression.

Immune Contributions to Osteoarthritis

PURPOSE OF THE REVIEW

Mounting evidence supports a role of low-grade inflammation in the pathophysiology of osteoarthritis (OA). We review and discuss the role of synovitis, complement activation, cytokines, and immune cell population in OA.

RECENT FINDINGS

Using newer imaging modalities, synovitis is found in the majority of knees with OA. Complement activation and pro-inflammatory cytokines play a significant role in the development of cartilage destruction and synovitis. Immune cell infiltration of OA synovial tissue by sub-populations of T cells and activated macrophages correlates with OA disease progression and pain. The innate and acquired immune system plays a key role in the low-grade inflammation found associated with OA. Targets of these pathways my hold promise for future disease-modifying osteoarthritis drugs (DMOADs).

Novel therapeutic interventions for pseudoachondroplasia

Pseudoachondroplasia (PSACH), a severe short-limbed dwarfing condition, is associated with life-long joint pain and early onset osteoarthritis. PSACH is caused by mutations in cartilage oligomeric matrix protein (COMP), a pentameric matricellular protein expressed primarily in cartilage and other musculoskeletal tissues. Mutations in COMP diminish calcium binding and as a result perturb protein folding and export to the extracellular matrix. Mutant COMP is retained in the endoplasmic reticulum (ER) of growth plate chondrocytes resulting in massive intracellular COMP retention. COMP trapped in the ER builds an intracellular matrix network that may prevent the normal cellular clearance mechanisms. We have shown that accumulation of intracellular matrix in mutant-COMP (MT-COMP) mice stimulates intense unrelenting ER stress, inflammation and oxidative stress. This cytotoxic stress triggers premature death of growth plate chondrocytes limiting long-bone growth. Here, we review the mutant COMP pathologic mechanisms and anti-inflammatory/antioxidant therapeutic approaches to reduce ER stress. In MT-COMP mice, aspirin and resveratrol both dampen the mutant COMP chondrocyte phenotype by decreasing intracellular accumulation, chondrocyte death and inflammatory marker expression. This reduction in chondrocyte stress translates into an improvement in long-bone growth in the MT-COMP mice. Our efforts now move to translational studies targeted at reducing the clinical consequences of MT-COMP and painful sequelae associated with PSACH.

Complement factor H in host defense and immune evasion

Complement is the major humoral component of the innate immune system. It recognizes pathogen- and damage-associated molecular patterns, and initiates the immune response in coordination with innate and adaptive immunity. When activated, the complement system unleashes powerful cytotoxic and inflammatory mechanisms, and thus its tight control is crucial to prevent damage to host tissues and allow restoration of immune homeostasis. Factor H is the major soluble inhibitor of complement, where its binding to self markers (i.e., particular glycan structures) prevents complement activation and amplification on host surfaces. Not surprisingly, mutations and polymorphisms that affect recognition of self by factor H are associated with diseases of complement dysregulation, such as age-related macular degeneration and atypical haemolytic uremic syndrome. In addition, pathogens (i.e., non-self) and cancer cells (i.e., altered-self) can hijack factor H to evade the immune response. Here we review recent (and not so recent) literature on the structure and function of factor H, including the emerging roles of this protein in the pathophysiology of infectious diseases and cancer.

The complement system is activated in synovial fluid from subjects with knee injury and from patients with osteoarthritis

Background

The complement system is suggested to be involved in the pathogenesis of osteoarthritis (OA), and proinflammatory cytokines may play a role in OA development by inducing proteases. The association between complement factors, cytokines and OA has not been investigated. The aim of the present study was to explore the involvement of the complement system after knee trauma and in OA.

Methods

C4d, C3bBbP and soluble terminal complement complex (sTCC) resulting from complement activation were immunoassayed in synovial fluid from subjects with healthy knees (reference), OA, rheumatoid arthritis (RA; positive control), pyrophosphate arthritis (PPA; positive control) and knee injury; other biomarkers were previously assessed. Magnetic resonance imaging was used to assess joint injuries.

Results

Compared with levels in the reference group, the median concentrations of C4d, C3bBbP and sTCC in the OA, RA, PPA and knee injury groups were 2- to 34-fold increased (p < 0.001 to p = 0.044). For the knee injury group, the median concentrations of C4d, C3bBbP and sTCC were 5- to 12-fold increased (p < 0.001) at the day of injury; after 3–12 weeks, C3bBbP and sTCC concentrations were similar to reference levels; and C4d was still increased several years after injury. In the 0–12 weeks period after injury, the concentrations of C4d, C3bBbP and sTCC correlated positively with levels of interleukin (IL)-1β, IL-6 and tumour necrosis factor α (r_s range 0.232–0.547); none of the measured complement factors correlated with proteolytic fragments of aggrecan or cartilage oligomeric matrix protein. Knees with osteochondral fracture, with or without disrupted cortical bone, had higher concentrations of C4d (p = 0.014, p = 0.004) and sTCC (p = 0.004, p < 0.001) compared with knees without fractures.

Conclusions

The complement system is activated in OA and after knee injury. Following knee injury, this activation is instant and associated with inflammation as well as with the presence of osteochondral fractures.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-1123-x) contains supplementary material, which is available to authorized users.

Recent advances in the understanding of molecular mechanisms of cartilage degeneration, synovitis and subchondral bone changes in osteoarthritis

Osteoarthritis (OA), the most common form of degenerative joint disease, is linked to high morbidity. It is predicted to be the single greatest cause of disability in the general population by 2030. The development of disease-modifying therapy for OA currently face great obstacle mainly because the onset and development of the disease involve complex molecular mechanisms. In this review, we will comprehensively summarize biological and pathological mechanisms of three key aspects: degeneration of articular cartilage, synovial immunopathogenesis, and changes in subchondral bone. For each tissue, we will focus on the molecular receptors, cytokines, peptidases, related cell, and signal pathways. Agents that specifically block mechanisms involved in synovial inflammation, degeneration of articular cartilage, and subchondral bone remodeling can potentially be exploited to produce targeted therapy for OA. Such new comprehensive agents will benefit affected patients and bring exciting new hope for the treatment of OA.

Inflammation in joint injury and post-traumatic osteoarthritis

Inflammation is a variable feature of osteoarthritis (OA), associated with joint symptoms and progression of disease. Signs of inflammation can be observed in joint fluids and tissues from patients with joint injuries at risk for development of post-traumatic osteoarthritis (PTOA). Furthermore, inflammatory mechanisms are hypothesized to contribute to the risk of OA development and progression after injury. Animal models of PTOA have been instrumental in understanding factors and mechanisms involved in chronic progressive cartilage degradation observed after a predisposing injury. Specific aspects of inflammation observed in humans, including cytokine and chemokine production, synovial reaction, cellular infiltration and inflammatory pathway activation, are also observed in models of PTOA. Many of these models are now being utilized to understand the impact of post-injury inflammatory response on PTOA development and progression, including risk of progressive cartilage degeneration and development of chronic symptoms post-injury. As evidenced from these models, a vigorous inflammatory response occurs very early after joint injury but is then sustained at a lower level at the later phases. This early inflammatory response contributes to the development of PTOA features including cartilage erosion and is potentially modifiable, but specific mediators may also play a role in tissue repair. Although the optimal approach and timing of anti-inflammatory interventions after joint injury are yet to be determined, this body of work should provide hope for the future of disease modification tin PTOA.

An inflammatory equine model demonstrates dynamic changes of immune response and cartilage matrix molecule degradation in vitro

The molecular aspects of inflammation were investigated in equine articular cartilage explants using quantitative proteomics. Articular cartilage explants were stimulated with interleukin (IL)-1β in vitro for 25 days, and proteins released into cell culture media were chemically labeled with isobaric mass tags and analyzed by liquid chromatography-tandem mass spectrometry. A total of 127 proteins were identified and quantified in media from explants. IL-1β-stimulation resulted in an abundance of proteins related to inflammation, including matrix metalloproteinases, acute phase proteins, complement components and IL-6. Extracellular matrix (ECM) molecules were released at different time points, and fragmentation of aggrecan and cartilage oligomeric matrix protein was observed at days 3 and 6, similar to early-stage OA in vivo. Degradation products of the collagenous network were observed at days 18 and 22, similar to late-stage OA. This model displays a longitudinal quantification of released molecules from the ECM of articular cartilage. Identification of dynamic changes of extracellular matrix molecules in the secretome of equine explants stimulated with IL-1β over time may be useful for identifying components released at different time points during the spontaneous OA process.

The role of innate immunity in osteoarthritis: when our first line of defense goes on the offensive

Although osteoarthritis (OA) has existed since the dawn of humanity, its pathogenesis remains poorly understood. OA is no longer considered a "wear and tear" condition but rather one driven by proteases where chronic low-grade inflammation may play a role in perpetuating proteolytic activity. While multiple factors are likely active in this process, recent evidence has implicated the innate immune system, the older or more primitive part of the body's immune defense mechanisms. The roles of some of the components of the innate immune system have been tested in OA models in vivo including the roles of synovial macrophages and the complement system. This review is a selective overview of a large and evolving field. Insights into these mechanisms might inform our ability to identify patient subsets and give hope for the advent of novel OA therapies.

Role of Complement on Broken Surfaces After Trauma

Activation of both the complement and coagulation cascade after trauma and subsequent local and systemic inflammatory response represent a major scientific and clinical problem. After severe tissue injury and bone fracture, exposure of innate immunity to damaged cells and molecular debris is considered a main trigger of the posttraumatic danger response. However, the effects of cellular fragments (e.g., histones) on complement activation remain enigmatic. Furthermore, direct effects of "broken" bone and cartilage surfaces on the fluid phase response of complement and its interaction with key cells of connective tissues are still unknown. Here, we summarize data suggesting direct and indirect complement activation by extracellular and cellular danger associated molecular patterns. In addition, key complement components and the corresponding receptors (such as C3aR, C5aR) have been detected on "exposed surfaces" of the damaged regions. On a cellular level, multiple effects of complement activation products on osteoblasts, osteoclasts, chondrocytes and mesenchymal stem cells have been found.In conclusion, the complement system may be activated by trauma-altered surfaces and is crucially involved in connective tissue healing and posttraumatic systemic inflammatory response.

Biomarkers and proteomic analysis of osteoarthritis

Our friend and colleague, Dr. Dick Heinegård, contributed greatly to the understanding of joint tissue biochemistry, the discovery and validation of arthritis-related biomarkers and the establishment of methodology for proteomic studies in osteoarthritis (OA). To date, discovery of OA-related biomarkers has focused on cartilage, synovial fluid and serum. Methods, such as affinity depletion and hyaluronidase treatment have facilitated proteomics discovery research from these sources. Osteoarthritis usually involves multiple joints; this characteristic makes it easier to detect OA with a systemic biomarker but makes it hard to delineate abnormalities of individual affected joints. Although the abundance of cartilage proteins in urine may generally be lower than other tissue/sample sources, the protein composition of urine is much less complex and its collection is non-invasive thereby facilitating the development of patient friendly biomarkers. To date however, relatively few proteomics studies have been conducted in OA urine. Proteomics strategies have identified many proteins that may relate to pathological mechanisms of OA. Further targeted approaches to validate the role of these proteins in OA are needed. Herein we summarize recent proteomic studies related to joint tissues and the cohorts used; a clear understanding of the cohorts is important for this work as we expect that the decisive discoveries of OA-related biomarkers rely on comprehensive phenotyping of healthy non-OA and OA subjects. Besides the common phenotyping criteria that include, gender, age, and body mass index (BMI), it is essential to collect data on symptoms and signs of OA outside the index joints and to bolster this with objective imaging data whenever possible to gain the most precise appreciation of the total burden of disease. Proteomic studies on systemic biospecimens, such as serum and urine, rely on comprehensive phenotyping data to unravel the true meaning of the proteomic results.

Cartilage oligomeric matrix protein and its binding partners in the cartilage extracellular matrix: interaction, regulation and role in chondrogenesis

Thrombospondins (TSPs) are widely known as a family of five calcium-binding matricellular proteins. While these proteins belong to the same family, they are encoded by different genes, regulate different cellular functions and are localized to specific regions of the body. TSP-5 or Cartilage Oligomeric Matrix Protein (COMP) is the only TSP that has been associated with skeletal disorders in humans, including pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED). The pentameric structure of COMP, the evidence that it interacts with multiple cellular proteins, and the recent reports of COMP acting as a 'lattice' to present growth factors to cells, inspired this review of COMP and its interacting partners. In our review, we have compiled the interactions of COMP with other proteins in the cartilage extracellular matrix and summarized their importance in maintaining the structural integrity of cartilage as well as in regulating cellular functions.

Inhibition of complement component C5 protects porcine chondrocytes from xenogeneic rejection

OBJECTIVE

Tissue-based xenografts such as cartilage are rejected within weeks by humoral and cellular mechanisms that preclude its clinical application in regenerative medicine. The problem could be overcome by identifying key molecules triggering rejection and the development of genetic-engineering strategies to counteract them. Accordingly, high expression of α1,2-fucosyltransferase (HT) in xenogeneic cartilage reduces the galactose α1,3-galactose (Gal) antigen and delays rejection. Yet, the role of complement activation in this setting is unknown.

DESIGN

To determine its contribution, we assessed the effect of inhibiting C5 complement component in α1,3-galactosyltransferase-knockout (Gal KO) mice transplanted with porcine cartilage and studied the effect of human complement on porcine articular chondrocytes (PAC).

RESULTS

Treatment with an anti-mouse C5 blocking antibody for 5 weeks enhanced graft survival by reducing cellular rejection. Moreover, PAC were highly resistant to complement-mediated lysis and primarily responded to human complement by releasing IL-6 and IL-8. This occurred even in the absence of anti-Gal antibody and was mediated by both C5a and C5b-9. Indeed, C5a directly triggered IL-6 and IL-8 secretion and up-regulated expression of swine leukocyte antigen I (SLA-I) and adhesion molecules on chondrocytes, all processes that enhance cellular rejection. Finally, the use of anti-human C5/C5a antibodies and/or recombinant expression of human complement regulatory molecule CD59 (hCD59) conferred protection in correspondence with their specific functions.

CONCLUSIONS

Our study demonstrates that complement activation contributes to rejection of xenogeneic cartilage and provides valuable information for selecting approaches for complement inhibition.

Interactions of the complement system with molecules of extracellular matrix: relevance for joint diseases

Rheumatoid arthritis (RA) is a highly disabling disease affecting all structures of the joint. Understanding the pathology behind the development of RA is essential for developing targeted therapeutic strategies as well as for developing novel markers to predict disease onset. Several molecules normally hidden within the cartilage tissue are exposed to complement components in the synovial fluid upon cartilage breakdown. Some of these have been shown to activate complement and toll-like receptors, which may enhance an already existing inflammatory response, thereby worsening the course of disease. Other cartilage-resident molecules have in contrast shown to possess complement-inhibitory properties. Knowledge about mechanisms behind pathological complement activation in the joints will hopefully lead to methods which allow us to distinguish patients with pathological complement activation from those where other inflammatory pathways are predominant. This will help to elucidate which patients will benefit from complement inhibitory therapies, which are thought to aid a specific subset of patients or patients at a certain stage of disease. Future challenges are to target the complement inhibition specifically to the joints to minimize systemic complement blockade.

The regulatory roles of small leucine-rich proteoglycans in extracellular matrix assembly

Small leucine-rich proteoglycans (SLRPs) are involved in a variety of biological and pathological processes. This review focuses on their regulatory roles in matrix assembly. SLRPs have protein cores and hypervariable glycosylation with multivalent binding abilities. During development, differential interactions of SLRPs with other molecules result in tissue-specific spatial and temporal distributions. The changing expression patterns play a critical role in the regulation of tissue-specific matrix assembly and therefore tissue function. SLRPs play significant structural roles within extracellular matrices. In addition, they play regulatory roles in collagen fibril growth, fibril organization and extracellular matrix assembly. Moreover, they are involved in mediating cell-matrix interactions. Abnormal SLRP expression and/or structures result in dysfunctional extracellular matrices and pathophysiology. Altered expression of SLRPs has been found in many disease models, and structural deficiency also causes altered matrix assembly. SLRPs regulate assembly of the extracellular matrix, which defines the microenvironment, modulating both the extracellular matrix and cellular functions, with an impact on tissue function.

The different roles of aggrecan interaction domains

The aggregating proteoglycans of the lectican family are important components of extracellular matrices. Aggrecan is the most well studied of these and is central to cartilage biomechanical properties and skeletal development. Key to its biological function is the fixed charge of the many glycosaminoglycan chains, that provide the basis for the viscoelastic properties necessary for load distribution over the articular surface. This review is focused on the globular domains of aggrecan and their role in anchoring the proteoglycans to other extracellular matrix components. The N-terminal G1 domain is vital in that it binds the proteoglycan to hyaluronan in ternary complex with link protein, retaining the proteoglycan in the tissue. The importance of the C-terminal G3 domain interactions has recently been emphasized by two different human hereditary disorders: autosomal recessive aggrecan-type spondyloepimetaphyseal dysplasia and autosomal dominant familial osteochondritis dissecans. In these two conditions, different missense mutations in the aggrecan C-type lectin repeat have been described. The resulting amino acid replacements affect the ligand interactions of the G3 domain, albeit with widely different phenotypic outcomes.

The role of synovitis in osteoarthritis pathogenesis

Research into the pathophysiology of osteoarthritis (OA) has focused on cartilage and peri-articular bone, but there is increasing recognition that OA affects all of the joint tissues, including the synovium (SM). Under normal physiological conditions the synovial lining consists of a thin layer of cells with phenotypic features of macrophages and fibroblasts. These cells and the underlying vascularized connective tissue stroma form a complex structure that is an important source of synovial fluid (SF) components that are essential for normal cartilage and joint function. The histological changes observed in the SM in OA generally include features indicative of an inflammatory "synovitis"; specifically they encompass a range of abnormalities, such as synovial lining hyperplasia, infiltration of macrophages and lymphocytes, neoangiogenesis and fibrosis. The pattern of synovial reaction varies with disease duration and associated metabolic and structural changes in other joint tissues. Imaging modalities including magnetic resonance (MRI) and ultrasound (US) have proved useful in detecting and quantifying synovial abnormalities, but individual studies have varied in their methods of evaluation. Despite these differences, most studies have concluded that the presence of synovitis in OA is associated with more severe pain and joint dysfunction. In addition, synovitis may be predictive of faster rates of cartilage loss in certain patient populations. Recent studies have provided insights into the pathogenic mechanisms underlying the development of synovitis in OA. Available evidence suggests that the inflammatory process involves engagement of Toll-like receptors and activation of the complement cascade by degradation products of extracellular matrices of cartilage and other joint tissues. The ensuing synovial reaction can lead to synthesis and release of a wide variety of cytokines and chemokines. Some of these inflammatory mediators are detected in joint tissues and SF in OA and have catabolic effects on chondrocytes. These inflammatory mediators represent potential targets for therapeutic interventions designed to reduce both symptoms and structural joint damage in OA. This article is part of a Special Issue entitled "Osteoarthritis".

PRELP protein inhibits the formation of the complement membrane attack complex

PRELP is a 58-kDa proteoglycan found in a variety of extracellular matrices, including cartilage and at several basement membranes. In rheumatoid arthritis (RA), the cartilage tissue is destroyed and fragmented molecules, including PRELP, are released into the synovial fluid where they may interact with components of the complement system. In a previous study, PRELP was found to interact with the complement inhibitor C4b-binding protein, which was suggested to locally down-regulate complement activation in joints during RA. Here we show that PRELP directly inhibits all pathways of complement by binding C9 and thereby prevents the formation of the membrane attack complex (MAC). PRELP does not interfere with the interaction between C9 and already formed C5b-8, but inhibits C9 polymerization thereby preventing formation of the lytic pore. The alternative pathway is moreover inhibited already at the level of C3-convertase formation due to an interaction between PRELP and C3. This suggests that PRELP may down-regulate complement attack at basement membranes and on damaged cartilage and therefore limit pathological complement activation in inflammatory disease such as RA. The net outcome of PRELP-mediated complement inhibition will highly depend on the local concentration of other complement modulating molecules as well as on the local concentration of available complement proteins.