Emerging roles of serine proteinases in tissue turnover in arthritis

The emerging role of tranexamic acid and its principal target, plasminogen, in skeletal health

The worldwide burden of skeletal diseases such as osteoporosis, degenerative joint disease and impaired fracture healing is steadily increasing. Tranexamic acid (TXA), a plasminogen inhibitor and anti-fibrinolytic agent, is used to reduce bleeding with high effectiveness and safety in major surgical procedures. With its widespread clinical application, the effects of TXA beyond anti-fibrinolysis have been noticed and prompted renewed interest in its use. Some clinical trials have characterized the effects of TXA on reducing postoperative infection rates and regulating immune responses in patients undergoing surgery. Also, several animal studies suggest potential therapeutic effects of TXA on skeletal diseases such as osteoporosis and fracture healing. Although a direct effect of TXA on the differentiation and function of bone cells in vitro was shown, few mechanisms of action have been reported. Here, we summarize recent findings of the effects of TXA on skeletal diseases and discuss the underlying plasminogen-dependent and -independent mechanisms related to bone metabolism and the immune response. We furthermore discuss potential novel indications for TXA application as a treatment strategy for skeletal diseases.

Inhibition of fibroblast activation protein ameliorates cartilage matrix degradation and osteoarthritis progression

Fibroblast activation protein (Fap) is a serine protease that degrades denatured type I collagen, α2-antiplasmin and FGF21. Fap is highly expressed in bone marrow stromal cells and functions as an osteogenic suppressor and can be inhibited by the bone growth factor Osteolectin (Oln). Fap is also expressed in synovial fibroblasts and positively correlated with the severity of rheumatoid arthritis (RA). However, whether Fap plays a critical role in osteoarthritis (OA) remains poorly understood. Here, we found that Fap is significantly elevated in osteoarthritic synovium, while the genetic deletion or pharmacological inhibition of Fap significantly ameliorated posttraumatic OA in mice. Mechanistically, we found that Fap degrades denatured type II collagen (Col II) and Mmp13-cleaved native Col II. Intra-articular injection of rFap significantly accelerated Col II degradation and OA progression. In contrast, Oln is expressed in the superficial layer of articular cartilage and is significantly downregulated in OA. Genetic deletion of Oln significantly exacerbated OA progression, which was partially rescued by Fap deletion or inhibition. Intra-articular injection of rOln significantly ameliorated OA progression. Taken together, these findings identify Fap as a critical pathogenic factor in OA that could be targeted by both synthetic and endogenous inhibitors to ameliorate articular cartilage degradation.

Tissue-Protective and Anti-Inflammatory Landmark of PRP-Treated Mesenchymal Stromal Cells Secretome for Osteoarthritis

Bone-marrow-mesenchymal-stromal-cells (BMSCs)- and platelet-rich-plasma (PRP)-based therapies have shown potential for treating osteoarthritis (OA). Recently, the combination of these two approaches was proposed, with results that overcame those observed with the separate treatments, indicating a possible role of PRP in ameliorating BMSCs' regenerative properties. Since a molecular fingerprint of BMSCs cultivated in the presence of PRP is missing, the aim of this study was to characterize the secretome in terms of soluble factors and extracellular-vesicle (EV)-embedded miRNAs from the perspective of tissues, pathways, and molecules which frame OA pathology. One hundred and five soluble factors and one hundred eighty-four EV-miRNAs were identified in the PRP-treated BMSCs' secretome, respectively. Several soluble factors were related to the migration of OA-related immune cells, suggesting the capacity of BMSCs to attract lympho-, mono-, and granulocytes and modulate their inflammatory status. Accordingly, several EV-miRNAs had an immunomodulating role at both the single-factor and cell level, together with the ability to target OA-characterizing extracellular-matrix-degrading enzymes and cartilage destruction pathways. Overall, anti-inflammatory and protective signals far exceeded inflammation and destruction cues for cartilage, macrophages, and T cells. This study demonstrates that BMSCs cultivated in the presence of PRP release therapeutic molecules and give molecular ground for the use of this combined and innovative therapy for OA treatment.

The Association of uPA, uPAR, and suPAR System with Inflammation and Joint Damage in Rheumatoid Arthritis: suPAR as a Biomarker in the Light of a Personalized Medicine Perspective

BACKGROUND

In recent years, the involvement of the soluble urokinase Plasminogen Activator Receptor (suPAR) in the pathophysiological modulation of Rheumatoid Arthritis (RA) has been documented, resulting in the activation of several intracellular inflammatory pathways.

METHODS

We investigated the correlation of urokinase Plasminogen Activator (uPA)/urokinase Plasminogen Activator Receptor (uPAR) expression and suPAR with inflammation and joint damage in RA, evaluating their potential role in a precision medicine context.

RESULTS

Currently, suPAR has been shown to be a potential biomarker for the monitoring of Systemic Chronic Inflammation (SCI) and COVID-19. However, the effects due to suPAR interaction in immune cells are also involved in both RA onset and progression. To date, the literature data on suPAR in RA endorse its potential application as a biomarker of inflammation and subsequent joint damage.

CONCLUSION

Available evidence about suPAR utility in the RA field is promising, and future research should further investigate its use in clinical practice, resulting in a big step forward for precision medicine. As it is elevated in different types of inflammation, suPAR could potentially work as an adjunctive tool for the screening of RA patients. In addition, a suPAR system has been shown to be involved in RA pathogenesis, so new data about the therapeutic response to Jak inhibitors can represent a possible way to develop further studies.

Allosteric inhibition of HTRA1 activity by a conformational lock mechanism to treat age-related macular degeneration

The trimeric serine protease HTRA1 is a genetic risk factor associated with geographic atrophy (GA), a currently untreatable form of age-related macular degeneration. Here, we describe the allosteric inhibition mechanism of HTRA1 by a clinical Fab fragment, currently being evaluated for GA treatment. Using cryo-EM, X-ray crystallography and biochemical assays we identify the exposed LoopA of HTRA1 as the sole Fab epitope, which is approximately 30 Å away from the active site. The cryo-EM structure of the HTRA1:Fab complex in combination with molecular dynamics simulations revealed that Fab binding to LoopA locks HTRA1 in a non-competent conformational state, incapable of supporting catalysis. Moreover, grafting the HTRA1-LoopA epitope onto HTRA2 and HTRA3 transferred the allosteric inhibition mechanism. This suggests a conserved conformational lock mechanism across the HTRA family and a critical role of LoopA for catalysis, which was supported by the reduced activity of HTRA1-3 upon LoopA deletion or perturbation. This study reveals the long-range inhibition mechanism of the clinical Fab and identifies an essential function of the exposed LoopA for activity of HTRA family proteases.

Induction of IDO1 and Kynurenine by Serine Proteases Subtilisin, Prostate Specific Antigen, CD26 and HtrA: A New Form of Immunosuppression?

Several serine proteases have been linked to autoimmune disorders and tumour initiation although the mechanisms are not fully understood. Activation of the kynurenine pathway enzyme indoleamine-2,3-dioxygenase (IDO1) modulates cellular activity in the brain, tolerogenesis in the immune system and is a major checkpoint in cancer development. We now report that IDO1 mRNA and IDO1 protein expression (generating kynurenine) are induced in human monocyte-derived macrophages by several chymotryptic serine proteases with direct links to tumorigenesis, including Prostate Specific Antigen (PSA), CD26 (Dipeptidyl-peptidase-4, CD26/DPP-4), High Temperature Requirement protein-A (HtrA), and the bacterial virulence factor subtilisin. These proteases also induce expression of the pro-inflammatory cytokine genes IL1B and IL6. Other serine proteases tested: bacterial glu-C endopeptidase and mammalian Pro-protein Convertase Subtilase-Kexin-3 (PCSK3, furin), urokinase plasminogen activator (uPA), cathepsin G or neutrophil elastase, did not induce IDO1, indicating that the reported effects are not a general property of all serine proteases. The results represent a novel mechanism of activating immunosuppressive IDO1 and inducing kynurenine generation which, together with the production of inflammatory cytokines, would contribute to tumour initiation and progression, providing a new target for drug development. In addition, the proteasomal S20 serine protease inhibitor carfilzomib, used in the treatment of myeloma, prevented the induction of IDO1 and cytokine gene expression, potentially contributing to its clinical anti-cancer activity.

Synthesis and Preliminary Evaluation of 131I-Labeled FAPI Tracers for Cancer Theranostics

As an excellent target for cancer theranostics, fibroblast activation protein (FAP) has become an attractive focus in cancer research. A class of FAP inhibitors (FAPIs) with a N-(4-quinolinoyl)-Gly-(2-cyanopyrrolidine) scaffold were developed, which displayed nanomolar affinity and high selectivity. Compared with ⁹⁰Y, ¹⁷⁷Lu, ²²⁵Ac, and ¹⁸⁸Re, ²¹¹At seems to be more favored as a therapeutic candidate for FAPI tracers which have fast washout and short retention in tumor sites. Thus, the current study reported the synthesis of two FAPI precursors for ²¹¹At and ¹³¹I labeling and the preliminary evaluation of ¹³¹I-labeled FAPI analogues for cancer theranostics. FAPI variants with stannyl precursors were successfully synthesized and labeled with ¹³¹I using a radioiododestannylation reaction. Two radioactive tracers were obtained with high radiochemical purity over 99% and good radiochemical yields of 58.2 ± 1.78 and 59.5 ± 4.44% for ¹³¹I-FAPI-02 and ¹³¹I-FAPI-04, respectively. Both tracers showed high specific binding to U87MG cells in comparison with little binding to MCF-7 cells. Compared to ¹³¹I-FAPI-02, ¹³¹I-FAPI-04 exhibited higher affinity, more intracellular uptake, and longer retention time in vitro. Biodistribution studies revealed that both tracers were mainly excreted through the kidneys as well as the hepatobiliary pathway due to their high lipophilicity. In addition, higher accumulation, longer dwell time, and increased tumor-to-organ ratios were achieved by ¹³¹I-FAPI-04, which was clearly demonstrated by SPECT/CT imaging. Furthermore, intratumor injection of ¹³¹I-FAPI-04 significantly suppressed the tumor growth in U87MG xenograft mice without significant toxicity observed. The above results implied that FAP-targeted alpha endoradiotherapy (specific to ²¹¹At) should be used to treat tumors in the near future, considering the chemical similarity between iodine and astatine can ensure the labeling of the latter onto the designed FAPIs.

Synovial tissue from sites of joint pain in knee osteoarthritis patients exhibits a differential phenotype with distinct fibroblast subsets

BACKGROUND

Synovial inflammation is associated with pain severity in patients with knee osteoarthritis (OA). The aim here was to determine in a population with knee OA, whether synovial tissue from areas associated with pain exhibited different synovial fibroblast subsets, compared to synovial tissue from sites not associated with pain. A further aim was to compare differences between early and end-stage disease synovial fibroblast subsets.

METHODS

Patients with early knee OA (n = 29) and end-stage knee OA (n = 22) were recruited. Patient reported pain was recorded by questionnaire and using an anatomical knee pain map. Proton density fat suppressed MRI axial and sagittal sequences were analysed and scored for synovitis. Synovial tissue was obtained from the medial and lateral parapatellar and suprapatellar sites. Fibroblast single cell RNA sequencing was performed using Chromium 10X and analysed using Seurat. Transcriptomes were functionally characterised using Ingenuity Pathway Analysis and the effect of fibroblast secretome on neuronal growth assessed using rat DRGN.

FINDINGS

Parapatellar synovitis was significantly associated with the pattern of patient-reported pain in knee OA patients. Synovial tissue from sites of patient-reported pain exhibited a differential transcriptomic phenotype, with distinct synovial fibroblast subsets in early OA and end-stage OA. Functional pathway analysis revealed that synovial tissue and fibroblast subsets from painful sites promoted fibrosis, inflammation and the growth and activity of neurons. The secretome of fibroblasts from early OA painful sites induced greater survival and neurite outgrowth in dissociated adult rodent dorsal root ganglion neurons.

INTERPRETATION

Sites of patient-reported pain in knee OA exhibit a different synovial tissue phenotype and distinct synovial fibroblast subsets. Further interrogation of these fibroblast pathotypes will increase our understanding of the role of synovitis in OA joint pain and provide a rationale for the therapeutic targeting of fibroblast subsets to alleviate pain in patients.

FUNDING

This study was funded by Versus Arthritis, UK (21530; 21812).

Targeting Dysregulation of Metalloproteinase Activity in Osteoarthritis

Metalloproteinases were first identified as collagen cleaving enzymes and are now appreciated to play important roles in a wide variety of biological processes. The aberrant activity and dysregulation of the metalloproteinase family are linked to numerous diseases including cardiovascular and pulmonary diseases, chronic wounds, cancer, fibrosis and arthritis. Osteoarthritis (OA) is the most prevalent age-related joint disorder that causes pain and disability, but there are no disease-modifying drugs available. The hallmark of OA is loss of articular cartilage and elevated activities of matrix-degrading metalloproteinases are responsible. These enzymes do not exist in isolation and their activity is tightly regulated by a number of processes, such as transcription, proteolytic activation, interaction with their inhibitors, cell surface and extracellular matrix molecules, and endocytic clearance from the extracellular milieu. Here, we describe the functions and roles of metalloproteinase family in OA pathogenesis. We highlight recent studies that have illustrated novel mechanisms regulating their extracellular activity and impairment of such regulations that lead to the development of OA. We also discuss how to stop or slow down the degenerative processes by targeting aberrant metalloproteinase activity, which may in future become therapeutic interventions for the disease.

The potential utility of [68 Ga]Ga-DOTA-FAPI-04 as a novel broad-spectrum oncological and non-oncological imaging agent-comparison with [18F]FDG

PURPOSE

This study aimed to compare the detection performance of [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT in the patients with various oncological and non-oncological lesions.

METHODS

A total of 123 patients underwent contemporaneous [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT were included in this prospective study. The maximum standard uptake value (SUVmax) was measured to compare oncological and non-oncological lesion uptake. The sensitivity, specificity, predictive values, and accuracy of [¹⁸F]FDG and [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT for detecting primary, metastatic, and non-oncological lesions were calculated and compared to evaluate the detection efficacy.

RESULTS

The study subjects consisted of 123 patients (69 men and 54 women; mean age 56.11 ± 11.94 years). Among the 102 patients with either newly diagnosed (82 patients) or previously treated solid tumor (20 patients), a total of 88 solid primary malignant tumors in 84/102 patients were detected. Two patients had two primary tumors each and 1 patient had three primary tumors. Among them, 58/102 and 43/102 patients had nodal (376 lesions) and distant metastases (406 lesions), respectively. Eight patients had hematological neoplasm. No malignant oncological diseases were detected in the remaining 13 patients. A total of 145 non-oncological lesions and benign tumors in 52/123 patients were detected incidentally. [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT demonstrated a significantly higher uptake and detection rate for the primary (SUVmax 10.98 ± 5.83 vs. 8.36 ± 6.43, p < 0.001; sensitivity 97.67 vs. 84.89%; and accuracy 96.59 vs. 82.95%, X² = 0.538, p = 0.021), nodal (SUVmax 10.50 ± 5.98 vs. 8.20 ± 6.29, p = 0.011; sensitivity 97.59 vs. 84.72%; and accuracy 97.34 vs. 84.31%, X² = 2.067, p < 0.001), and distant metastatic lesions (SUVmax 9.64 ± 6.45 vs. 6.74 ± 4.83; p < 0.001; sensitivity 98.01 vs. 65.59%; and accuracy 97.04 vs. 65.51%, X² = 4.897, p < 0.001) of solid tumor than did [¹⁸F]FDG PET/CT. [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT demonstrated a lower activity (SUVmax: 6.84 ± 4.67 vs. 13.09 ± 7.29, p < 0.001) and detection rate (sensitivity 50.65 vs. 96.75%, and accuracy 51.28 vs. 95.51%, X² = 5.166, p < 0.001) for multiple myeloma and lymphoma compared to [¹⁸F]FDG PET/CT. [⁶⁸ Ga]Ga-DOTA-FAPI-04 and [¹⁸F]FDG PET/CT PET/CT demonstrated a comparative activity (SUVmax 6.40 ± 3.95 vs. 5.74 ± 15.78, p = 0.729) and detection efficacy (sensitivity 86.52 vs. 72.34%, and accuracy 84.83 vs. 72.41%, X² = 9.460, p = 0.007) for non-oncological lesion and benign tumor detection.

CONCLUSIONS

Except for myeloma and lymphoma, [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT showed a superior detection efficacy for detecting various primary and metastatic lesions than [¹⁸F]FDG PET/CT. A comparative detection utility for non-oncological lesion was obtained with both tracers. [⁶⁸ Ga]Ga-DOTA-FAPI-04 could be used as a broad-spectrum tumor and inflammatory imaging agent in the clinical especially for various solid tumors and non-oncological lesions.

New Insights From Single-Cell Sequencing Data: Synovial Fibroblasts and Synovial Macrophages in Rheumatoid Arthritis

Single-cell RNA sequencing (scRNA-seq) technology can analyze the transcriptome expression level of cells with high-throughput from the single cell level, fully show the heterogeneity of cells, and provide a new way for the study of multicellular biological heterogeneity. Synovitis is the pathological basis of rheumatoid arthritis (RA). Synovial fibroblasts (SFs) and synovial macrophages are the core target cells of RA, which results in the destruction of articular cartilage, as well as bone. Recent scRNA-seq technology has made breakthroughs in the differentiation and development of two types of synovial cells, identification of subsets, functional analysis, and new therapeutic targets, which will bring remarkable changes in RA treatment.

Serpins in cartilage and osteoarthritis: what do we know?

Serpins (serine proteinase inhibitors) are an ancient superfamily of structurally similar proteins, the majority of which use an elegant suicide inhibition mechanism to target serine proteinases. Despite likely evolving from a single common ancestor, the 36 human serpins have established roles regulating diverse biological processes, such as blood coagulation, embryonic development and extracellular matrix (ECM) turnover. Genetic mutations in serpin genes underpin a host of monogenic disorders — collectively termed the ‘serpinopathies’ — but serpin dysregulation has also been shown to drive pathological mechanisms in many common diseases. Osteoarthritis is a degenerative joint disorder, characterised by the progressive destruction of articular cartilage. This breakdown of the cartilage is driven by the metalloproteinases, and it has long been established that an imbalance of metalloproteinases to their inhibitors is of critical importance. More recently, a role for serine proteinases in cartilage destruction is emerging; including the activation of latent matrix metalloproteinases and cell-surface receptors, or direct proteolysis of the ECM. Serpins likely regulate these processes, as well as having roles beyond serine proteinase inhibition. Indeed, serpins are routinely observed to be highly modulated in osteoarthritic tissues and fluids by ‘omic analysis, but despite this, they are largely ignored. Confusing nomenclature and an underappreciation for the role of serine proteinases in osteoarthritis (OA) being the likely causes. In this narrative review, serpin structure, biochemistry and nomenclature are introduced, and for the first time, their putative importance in maintaining joint tissues — as well as their dysregulation in OA — are explored.

The 10q26 Risk Haplotype of Age-Related Macular Degeneration Aggravates Subretinal Inflammation by Impairing Monocyte Elimination

A minor haplotype of the 10q26 locus conveys the strongest genetic risk for age-related macular degeneration (AMD). Here, we examined the mechanisms underlying this susceptibility. We found that monocytes from homozygous carriers of the 10q26 AMD-risk haplotype expressed high amounts of the serine peptidase HTRA1, and HTRA1 located to mononuclear phagocytes (MPs) in eyes of non-carriers with AMD. HTRA1 induced the persistence of monocytes in the subretinal space and exacerbated pathogenic inflammation by hydrolyzing thrombospondin 1 (TSP1), which separated the two CD47-binding sites within TSP1 that are necessary for efficient CD47 activation. This HTRA1-induced inhibition of CD47 signaling induced the expression of pro-inflammatory osteopontin (OPN). OPN expression increased in early monocyte-derived macrophages in 10q26 risk carriers. In models of subretinal inflammation and AMD, OPN deletion or pharmacological inhibition reversed HTRA1-induced pathogenic MP persistence. Our findings argue for the therapeutic potential of CD47 agonists and OPN inhibitors for the treatment of AMD.

Discerning the mechanism of action of HtrA4: a serine protease implicated in the cell death pathway

High-temperature requirement protease A4 (HtrA4) is a secretary serine protease whose expression is up-regulated in pre-eclampsia (PE) and hence is a possible biomarker of PE. It has also been altered in cancers such as glioblastoma, breast carcinoma, and prostate cancer making it an emerging therapeutic target. Among the human HtrAs, HtrA4 is the least characterized protease pertaining to both structure and its functions. Although the members of human HtrA family share a significant structural and functional conservation, subtle structural changes have been associated with certain distinct functional requirements. Therefore, intricate dissection of HtrA4 structural and functional properties becomes imperative to understand its role in various biological and pathophysiological processes. Here, using inter-disciplinary approaches including in silico, biochemical and biophysical studies, we have done a domain-wise dissection of HtrA4 to delineate the roles of the domains in regulating oligomerization, stability, protease activity, and specificity. Our findings distinctly demonstrate the importance of the N-terminal region in oligomerization, stability and hence the formation of a functional enzyme. In silico structural comparison of HtrA4 with other human HtrAs, enzymology studies and cleavage assays with X-linked inhibitor of apoptosis protein (XIAP) show overall structural conservation and allosteric mode of protease activation, which suggest functional redundancy within this protease family. However, significantly lower protease activity as compared with HtrA2 indicates an additional mode of regulation of the protease activity in the cellular milieu. Overall, these studies provide first-hand information on HtrA4 and its interaction with antiapoptotic XIAP thus implicating its involvement in the apoptotic pathway.

Recent advances in understanding the regulation of metalloproteinases

Metalloproteinases remain important players in arthritic disease, in part because members of this large enzymatic family, namely matrix metalloproteinase-1 (MMP-1) and MMP-13, are responsible for the irreversible degradation of articular cartilage collagen. Although direct inhibition of MMPs fell out of vogue with the initial clinical disappointment of the first generation of compounds, interest in other mechanisms that control these important enzymes has always been maintained. Since these enzymes are critically important for tissue homeostasis, their expression and activity are tightly regulated at many levels, not just by direct inhibition by their endogenous inhibitors the tissue inhibitors of metalloproteinases (TIMPs). Focussing on MMP-13, we discuss recent work that highlights new discoveries in the transcriptional regulation of this enzyme, from defined promoter functional analysis to how more global technologies can provide insight into the enzyme’s regulation, especially by epigenetic mechanisms, including non-coding RNAs. In terms of protein regulation, we highlight recent findings into enzymatic cascades involved in MMP-13 regulation and activation. Importantly, we highlight a series of recent studies that describe how MMP-13 activity, and in fact that of other metalloproteinases, is in part controlled by receptor-mediated endocytosis. Together, these new discoveries provide a plethora of novel regulatory mechanisms, besides direct inhibition, which with renewed vigour could provide further therapeutic opportunities for regulating the activity of this class of important enzymes.

uPA/uPAR signaling in rheumatoid arthritis: Shedding light on its mechanism of action

Rheumatoid arthritis (RA) is a systemic and chronic autoimmune inflammatory disorder affecting multiple joints. Various cytokines, chemokines and growth factors synergistically modulate the joint physiology leading to bone erosion and cartilage degradation. Other than these conventional mediators that are well established in the past, the newly identified plasminogen activator (PA) family of proteins have been witnessed to possess a multifactorial approach in mediating RA pathogenesis. One such family of proteins comprises of the urokinase-type plasminogen activator (uPA) and its receptor (uPAR)/soluble-type plasminogen activator receptor (suPAR). PA family of proteins are classified into two types namely: uPA and tissue type plasminogen activator (tPA). Both these subtypes have been implicated to play a key role in RA disease progression. However during RA pathogenesis, uPA secreted by neutrophils, chondrocytes, and monocytes are designated to interact with uPAR expressed on macrophages, fibroblast-like synoviocytes (FLS), chondrocytes and endothelial cells. Interaction of uPA/uPAR promotes the disease progression of RA through secretion of several cytokines, chemokines, growth factors and matrix metalloproteinases (MMPs). Moreover, uPA/uPAR initiates inflammatory responses in macrophages and FLS through activation of PI3K/Akt signaling pathways. Furthermore, uPAR plays a dual role in osteoclastogenesis under the presence/absence of growth factors like monocyte-colony stimulating factor (M-CSF). Overall, this review emphasizes the role of uPA/uPAR on various immune cells, signaling pathways and osteoclastogenesis involved in RA pathogenesis.

Serine proteinases in the turnover of the cartilage extracellular matrix in the joint: implications for therapeutics

Cartilage destruction is a key characteristic of arthritic disease, a process now widely established to be mediated by metzincins such as MMPs. Despite showing promise in preclinical trials during the 1990s, MMP inhibitors for the blockade of extracellular matrix turnover in the treatment of cancer and arthritis failed clinically, primarily due to poor selectivity for target MMPs. In recent years, roles for serine proteinases in the proteolytic cascades leading to cartilage destruction have become increasingly apparent, renewing interest in the potential for new therapeutic strategies that utilize pharmacological inhibitors against this class of proteinases. Herein, we describe key serine proteinases with likely importance in arthritic disease and highlight recent advances in this field. LINKED ARTICLES: This article is part of a themed section on Translating the Matrix. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.1/issuetoc.

The serine proteinase hepsin is an activator of pro-matrix metalloproteinases: molecular mechanisms and implications for extracellular matrix turnover

Increasing evidence implicates serine proteinases in the proteolytic cascades leading to the pathological destruction of extracellular matrices such as cartilage in osteoarthritis (OA). We have previously demonstrated that the type II transmembrane serine proteinase (TTSP) matriptase acts as a novel initiator of cartilage destruction via the induction and activation of matrix metalloproteinases (MMPs). Hepsin is another TTSP expressed in OA cartilage such that we hypothesized this proteinase may also contribute to matrix turnover. Herein, we demonstrate that addition of hepsin to OA cartilage in explant culture induced significant collagen and aggrecan release and activated proMMP-1 and proMMP-3. Furthermore, hepsin directly cleaved the aggrecan core protein at a novel cleavage site within the interglobular domain. Hepsin expression correlated with synovitis as well as tumour necrosis factor α expression, and was induced in cartilage by a pro-inflammatory stimulus. However, a major difference compared to matriptase was that hepsin demonstrated markedly reduced capacity to activate proteinase-activated receptor-2. Overall, our data suggest that hepsin, like matriptase, induces potent destruction of the extracellular matrix whilst displaying distinct efficiencies for the cleavage of specific substrates.

Matriptase Induction of Metalloproteinase-Dependent Aggrecanolysis In Vitro and In Vivo: Promotion of Osteoarthritic Cartilage Damage by Multiple Mechanisms

Objective

To assess the ability of matriptase, a type II transmembrane serine proteinase, to promote aggrecan loss from the cartilage of patients with osteoarthritis (OA) and to determine whether its inhibition can prevent aggrecan loss and cartilage damage in experimental OA.

Methods

Aggrecan release from human OA cartilage explants and human stem cell–derived cartilage discs was evaluated, and cartilage‐conditioned media were used for Western blotting. Gene expression was analyzed by real‐time polymerase chain reaction. Murine OA was induced by surgical destabilization of the medial meniscus, and matriptase inhibitors were administered via osmotic minipump or intraarticular injection. Cartilage damage was scored histologically and aggrecan cleavage was visualized immunohistochemically using specific neoepitope antibodies.

Results

The addition of soluble recombinant matriptase promoted a time‐dependent release of aggrecan (and collagen) from OA cartilage, which was sensitive to metalloproteinase inhibition and protease‐activated receptor 2 antagonism. Although engineered human (normal) cartilage discs failed to release aggrecan following matriptase addition, both matrix metalloproteinase– and aggrecanase‐mediated cleavages of aggrecan were detected in human OA cartilage. Additionally, while matriptase did not directly degrade aggrecan, it promoted the accumulation of low‐density lipoprotein receptor–related protein 1 (LRP‐1) in conditioned media of the OA cartilage explants. Matriptase inhibition via neutralizing antibody or small molecule inhibitor significantly reduced cartilage damage scores in murine OA, which was associated with reduced generation of metalloproteinase‐mediated aggrecan cleavage.

Conclusion

Matriptase potently induces the release of metalloproteinase‐generated aggrecan fragments as well as soluble LRP‐1 from OA cartilage. Therapeutic targeting of matriptase proteolytic activity reduces metalloproteinase activity, further suggesting that this serine proteinase may have potential as a disease‐modifying therapy in OA.

Theranostic Probes for Targeting Tumor Microenvironment: An Overview

Long gone is the time when tumors were thought to be insular masses of cells, residing independently at specific sites in an organ. Now, researchers gradually realize that tumors interact with the extracellular matrix (ECM), blood vessels, connective tissues, and immune cells in their environment, which is now known as the tumor microenvironment (TME). It has been found that the interactions between tumors and their surrounds promote tumor growth, invasion, and metastasis. The dynamics and diversity of TME cause the tumors to be heterogeneous and thus pose a challenge for cancer diagnosis, drug design, and therapy. As TME is significant in enhancing tumor progression, it is vital to identify the different components in the TME such as tumor vasculature, ECM, stromal cells, and the lymphatic system. This review explores how these significant factors in the TME, supply tumors with the required growth factors and signaling molecules to proliferate, invade, and metastasize. We also examine the development of TME-targeted nanotheranostics over the recent years for cancer therapy, diagnosis, and anticancer drug delivery systems. This review further discusses the limitations and future perspective of nanoparticle based theranostics when used in combination with current imaging modalities like Optical Imaging, Magnetic Resonance Imaging (MRI) and Nuclear Imaging (Positron Emission Tomography (PET) and Single Photon Emission Computer Tomography (SPECT)).

The Involvement of Protease Nexin-1 (PN1) in the Pathogenesis of Intervertebral Disc (IVD) Degeneration

Protease nexin-1 (PN-1) is a serine protease inhibitor belonging to the serpin superfamily. This study was undertaken to investigate the regulatory role of PN-1 in the pathogenesis of intervertebral disk (IVD) degeneration. Expression of PN-1 was detected in human IVD tissue of varying grades. Expression of both PN-1 mRNA and protein was significantly decreased in degenerated IVD, and the expression levels of PN-1 were correlated with the grade of disc degeneration. Moreover, a decrease in PN-1 expression in primary NP cells was confirmed. On induction by IL-1β, the expression of PN-1 in NP cells was decreased at day 7, 14, and 21, as shown by western blot analysis and immunofluorescence staining. PN-1 administration decreased IL-1β-induced MMPs and ADAMTS production and the loss of Agg and Col II in NP cell cultures through the ERK1/2/NF-kB signaling pathway. The changes in PN-1 expression are involved in the pathogenesis of IVD degeneration. Our findings indicate that PN-1 administration could antagonize IL-1β-induced MMPs and ADAMTS, potentially preventing degeneration of IVD tissue. This study also revealed new insights into the regulation of PN-1 expression via the ERK1/2/NF-kB signaling pathway and the role of PN-1 in the pathogenesis of IVD degeneration.

Determinants of amyloid fibril degradation by the PDZ protease HTRA1

Excessive aggregation of proteins has a major impact on cell fate and is a hallmark of amyloid diseases in humans. To resolve insoluble deposits and to maintain protein homeostasis, all cells use dedicated protein disaggregation, protein folding and protein degradation factors. Despite intense recent research, the underlying mechanisms controlling this key metabolic event are not well understood. Here, we analyzed how a single factor, the highly conserved serine protease HTRA1, degrades amyloid fibrils in an ATP-independent manner. This PDZ protease solubilizes protein fibrils and disintegrates the fibrillar core structure, allowing productive interaction of aggregated polypeptides with the active site for rapid degradation. The aggregate burden in a cellular model of cytoplasmic tau aggregation is thus reduced. Mechanistic aspects of ATP-independent proteolysis and its implications in amyloid diseases are discussed.

Proteomic analysis of cerebrospinal fluid in canine cervical spondylomyelopathy

STUDY DESIGN

Prospective study.

OBJECTIVE

To identify proteins with differential expression in the cerebrospinal fluid (CSF) from 15 clinically normal (control) dogs and 15 dogs with cervical spondylomyelopathy (CSM).

SUMMARY OF BACKGROUND DATA

Canine CSM is a spontaneous, chronic, compressive cervical myelopathy similar to human cervical spondylotic myelopathy. There is a limited knowledge of the molecular mechanisms underlying these conditions. Differentially expressed CSF proteins may contribute with novel information about the disease pathogenesis in both dogs and humans.

METHODS

Protein separation was performed with 2-dimensional electrophoresis. A Student t test was used to detect significant differences between groups (P < 0.05). Three comparisons were made: (1) control versus CSM-affected dogs, (2) control versus non-corticosteroid-treated CSM-affected dogs, and (3) non-corticosteroid-treated CSM-affected versus corticosteroid-treated CSM-affected dogs. Protein spots exhibiting at least a statistically significant 1.25-fold change between groups were selected for subsequent identification with capillary-liquid chromatography tandem mass spectrometry.

RESULTS

A total of 96 spots had a significant average change of at least 1.25-fold in 1 of the 3 comparisons. Compared with the CSF of control dogs, CSM-affected dogs demonstrated increased CSF expression of 8 proteins including vitamin D-binding protein, gelsolin, creatine kinase B-type, angiotensinogen, α-2-HS-glycoprotein, SPARC (secreted protein, acidic, rich in cysteine), calsyntenin-1, and complement C3, and decreased expression of pigment epithelium-derived factor, prostaglandin-H2 D-isomerase, apolipoprotein E, and clusterin. In the CSF of CSM-affected dogs, corticosteroid treatment increased the expression of haptoglobin, transthyretin isoform 2, cystatin C-like, apolipoprotein E, and clusterin, and decreased the expression of angiotensinogen, α-2-HS-glycoprotein, and gelsolin.

CONCLUSION

Many of the differentially expressed proteins are associated with damaged neural tissue, bone turnover, and/or compromised blood-spinal cord barrier. The knowledge of the protein changes that occur in CSM and upon corticosteroid treatment of CSM-affected patients will aid in further understanding the pathomechanisms underlying this disease.

LEVEL OF EVIDENCE

N/A.

A conserved activation cluster is required for allosteric communication in HtrA-family proteases

In E. coli, outer-membrane stress causes a transcriptional response through a signaling cascade initiated by DegS cleavage of a transmembrane antisigma factor. Each subunit of DegS, an HtrA-family protease, contains a protease domain and a PDZ domain. The trimeric protease domain is autoinhibited by the unliganded PDZ domains. Allosteric activation requires binding of unassembled outer-membrane proteins (OMPs) to the PDZ domains and protein substrate binding. Here, we identify a set of DegS residues that cluster together at subunit-subunit interfaces in the trimer, link the active sites and substrate binding sites, and are crucial for stabilizing the active enzyme conformation in response to OMP signaling. These residues are conserved across the HtrA-protease family, including orthologs linked to human disease, supporting a common mechanism of allosteric activation. Indeed, mutation of residues at homologous positions in the DegP quality-control protease also eliminates allosteric activation.

Deficiency of fibroblast activation protein alpha ameliorates cartilage destruction in inflammatory destructive arthritis

Introduction

Inflammatory destructive arthritis, like rheumatoid arthritis (RA), is characterized by invasion of synovial fibroblasts (SF) into the articular cartilage and erosion of the underlying bone, leading to progressive joint destruction. Because fibroblast activation protein alpha (FAP) has been associated with cell migration and cell invasiveness, we studied the function of FAP in joint destruction in RA.

Methods

Expression of FAP in synovial tissues and fibroblasts from patients with osteoarthritis (OA) and RA as well as from wild-type and arthritic mice was evaluated by immunohistochemistry, fluorescence microscopy and polymerase chain reaction (PCR). Fibroblast adhesion and migration capacity was assessed using cartilage attachment assays and wound-healing assays, respectively. For in vivo studies, FAP-deficient mice were crossed into the human tumor necrosis factor transgenic mice (hTNFtg), which develop a chronic inflammatory arthritis. Beside clinical assessment, inflammation, cartilage damage, and bone erosion were evaluated by histomorphometric analyses.

Results

RA synovial tissues demonstrated high expression of FAP whereas in OA samples only marginal expression was detectable. Consistently, a higher expression was detected in arthritis SF compared to non-arthritis OA SF in vitro. FAP-deficiency in hTNFtg mice led to less cartilage degradation despite unaltered inflammation and bone erosion. Accordingly, FAP^−/− hTNFtg SF demonstrated a lower cartilage adhesion capacity compared to hTNFtg SF in vitro.

Conclusions

These data point to a so far unknown role of FAP in the attachment of SF to cartilage, promoting proteoglycan loss and subsequently cartilage degradation in chronic inflammatory arthritis.

Activity-modulating monoclonal antibodies to the human serine protease HtrA3 provide novel insights into regulating HtrA proteolytic activities

Mammalian HtrA (high temperature requirement factor A) proteases, comprising 4 multi-domain members HtrA1-4, play important roles in a number of normal cellular processes as well as pathological conditions such as cancer, arthritis, neurodegenerative diseases and pregnancy disorders. However, how HtrA activities are regulated is not well understood, and to date no inhibitors specific to individual HtrA proteins have been identified. Here we investigated five HtrA3 monoclonal antibodies (mAbs) that we have previously produced, and demonstrated that two of them regulated HtrA3 activity in an opposing fashion: one inhibited while the other stimulated. The inhibitory mAb also blocked HtrA3 activity in trophoblast cells and enhanced migration and invasion, confirming its potential in vivo utility. To understand how the binding of these mAbs modulated HtrA3 protease activity, their epitopes were visualized in relation to a 3-dimensional HtrA3 homology model. This model suggests that the inhibitory HtrA3 mAb blocks substrate access to the protease catalytic site, whereas the stimulatory mAb may bind to the PDZ domain alone or in combination with the N-terminal and protease domains. Since HtrA1, HtrA3 and HtrA4 share identical domain organization, our results establish important foundations for developing potential therapeutics to target these HtrA proteins specifically for the treatment of a number of diseases, including cancer and pregnancy disorders.

Extended structure-activity relationship and pharmacokinetic investigation of (4-quinolinoyl)glycyl-2-cyanopyrrolidine inhibitors of fibroblast activation protein (FAP)

Fibroblast activation protein (FAP) is a serine protease related to dipeptidyl peptidase IV (DPPIV). It has been convincingly linked to multiple disease states involving remodeling of the extracellular matrix. FAP inhibition is investigated as a therapeutic option for several of these diseases, with most attention so far devoted to oncology applications. We previously discovered the N-4-quinolinoyl-Gly-(2S)-cyanoPro scaffold as a possible entry to highly potent and selective FAP inhibitors. In the present study, we explore in detail the structure-activity relationship around this core scaffold. We report extensively optimized compounds that display low nanomolar inhibitory potency and high selectivity against the related dipeptidyl peptidases (DPPs) DPPIV, DPP9, DPPII, and prolyl oligopeptidase (PREP). The log D values, plasma stabilities, and microsomal stabilities of selected compounds were found to be highly satisfactory. Pharmacokinetic evaluation in mice of selected inhibitors demonstrated high oral bioavailability, plasma half-life, and the potential to selectively and completely inhibit FAP in vivo.

Genetically Engineered Mouse Models Reveal the Importance of Proteases as Osteoarthritis Drug Targets

More than two decades of research has revealed a combination of proteases that determine cartilage degradation in osteoarthritis. These include metalloproteinases, which degrade the major macromolecules in cartilage, aggrecan and type II collagen, serine proteases, and cysteine proteases, for example cathepsin K. This review summarizes the function of proteases in osteoarthritis progression, as revealed by studies of genetically engineered mouse models. A brief overview of the biochemical characteristics and features of several important proteases is provided, with the objective of increasing understanding of their function. Published data reveal at least three enzymes to be major targets for osteoarthritis drug development: ADAMTS-5, MMP-13, and cathepsin K. In surgical models of osteoarthritis, mice lacking these enzymes are protected from cartilage damage and, to varying degrees, from bone changes. In-vivo studies targeting these proteases with selective small-molecule inhibitors have been performed for a variety of animal models. Mouse models will provide opportunities for future tests of the therapeutic effect of protease inhibitors, both on progression of structural damage to the joint and on associated pain.

Tumor fibroblast specific activation of a hybrid ferritin nanocage-based optical probe for tumor microenvironment imaging

Ferritin-based FAP-α-responsive fluorescence nanoprobes could be activated immediately as they penetrate the tumor blood vessels and come across the FAP-α molecules on the membrane of CAFs in the tumor microenvironment instead of reaching a certain depth into tumor tissue to interact with tumor cells. This probe may become a promising nanomaterial for highly specific and sensitive tumor imaging through responding to the enzymes in the tumor microenvironment.

Selective Inhibitors of Fibroblast Activation Protein (FAP) with a (4-Quinolinoyl)-glycyl-2-cyanopyrrolidine Scaffold

Fibroblast activation protein (FAP) is a serine protease that is generally accepted to play an important role in tumor growth and other diseases involving tissue remodeling. Currently there are no FAP inhibitors with reported selectivity toward both the closely related dipeptidyl peptidases (DPPs) and prolyl oligopeptidase (PREP). We present the discovery of a new class of FAP inhibitors with a N-(4-quinolinoyl)-Gly-(2-cyanopyrrolidine) scaffold. We have explored the effects of substituting the quinoline ring and varying the position of its sp(2) hybridized nitrogen atom. The most promising inhibitors combined low nanomolar FAP inhibition and high selectivity indices (>10(3)) with respect to both the DPPs and PREP. Preliminary experiments on a representative inhibitor demonstrate that plasma stability, kinetic solubility, and log D of this class of compounds can be expected to be satisfactory.

Emerging Frontiers in cartilage and chondrocyte biology

Articular cartilage is a uniquely ordered tissue that is designed to resist compression and redistribute load, but is poorly equipped for self-repair. The chondrocyte is the only resident cell type, responsible for maintaining a specialised and extensive matrix that is avascular and lacks innervation. These attributes, as well as the slow turnover rate of aggrecan and type II collagen in mature articular cartilage, present a considerable challenge to the tissue engineer. Similarly, those attempting to halt the progression of cartilage erosion must contend with these unusual characteristics. This review explores the gaps in our knowledge of cartilage biology and pathology, including what is known about the relative contribution of collagenases and aggrecanases to cartilage degradation, the need to regulate the chondrocytic phenotype and the putative role of chondrocyte hypertrophy in the pathogenesis of degenerative and rheumatic joint disease. Recent advances in cartilage tissue engineering are also reviewed.

Architecture and regulation of HtrA-family proteins involved in protein quality control and stress response

Protein quality control is vital for all living cells and sophisticated molecular mechanisms have evolved to prevent the excessive accumulation of unfolded proteins. High-temperature requirement A (HtrA) proteases have been identified as important ATP-independent quality-control factors in most species. HtrA proteins harbor a serine-protease domain and at least one peptide-binding PDZ domain to ensure efficient removal of misfolded or damaged proteins. One distinctive property of HtrAs is their ability to assemble into complex oligomers. Whereas all examined HtrAs are capable of forming pyramidal 3-mers, higher-order complexes consisting of up to 24 molecules have been reported. Tight control of chaperone and protease function is of pivotal importance in preventing deleterious HtrA-protease activity. In recent years, structural biology provided detailed insights into the molecular basis of the regulatory mechanisms, which include unique intramolecular allosteric signaling cascades and the dynamic switching of oligomeric states of HtrA proteins. Based on these results, functional models for many family members have been developed. The HtrA protein family represents a remarkable example of how structural and functional diversity is attained from the assembly of simple molecular building blocks.

Allosteric regulation of DegS protease subunits through a shared energy landscape

The PDZ domains of the trimeric DegS protease bind unassembled outer-membrane proteins (OMPs) that accumulate in the E. coli periplasm. This cooperative binding reaction triggers a proteolytic cascade that activates a transcriptional stress response. To dissect the mechanism of allosteric activation, we generated hybrid DegS trimers with different numbers of PDZ domains and/or protease-domain mutations. By studying the chemical reactivity and enzymatic properties of these hybrids, we show that all subunits experience a strongly coupled energetic landscape. For example, OMP-peptide binding to a single PDZ domain stimulates active-site chemical modification and proteolytic cleavage in the attached and neighboring protease domains. OMP-peptide binding relieves inhibitory PDZ interactions, whereas the interfaces between protease domains in the trimeric DegS core mediate positively cooperative activation driven both by substrate binding and inhibition relief.

Human high temperature requirement serine protease A1 (HTRA1) degrades tau protein aggregates

Protective proteases are key elements of protein quality control pathways that are up-regulated, for example, under various protein folding stresses. These proteases are employed to prevent the accumulation and aggregation of misfolded proteins that can impose severe damage to cells. The high temperature requirement A (HtrA) family of serine proteases has evolved to perform important aspects of ATP-independent protein quality control. So far, however, no HtrA protease is known that degrades protein aggregates. We show here that human HTRA1 degrades aggregated and fibrillar tau, a protein that is critically involved in various neurological disorders. Neuronal cells and patient brains accumulate less tau, neurofibrillary tangles, and neuritic plaques, respectively, when HTRA1 is expressed at elevated levels. Furthermore, HTRA1 mRNA and HTRA1 activity are up-regulated in response to elevated tau concentrations. These data suggest that HTRA1 is performing regulated proteolysis during protein quality control, the implications of which are discussed.

The structural basis of mode of activation and functional diversity: a case study with HtrA family of serine proteases

HtrA (High temperature requirement protease A) proteins that are primarily involved in protein quality control belong to a family of serine proteases conserved from bacteria to humans. HtrAs are oligomeric proteins that share a common trimeric pyramidal architecture where each monomer comprises a serine protease domain and one or two PDZ domains. Although the overall structural integrity is well maintained and they exhibit similar mechanism of activation, subtle conformational changes and structural plasticity especially in the flexible loop regions and domain interfaces lead to differences in their active site conformation and hence in their specificity and functions.

Substrate-induced remodeling of the active site regulates human HTRA1 activity

Crystal structures of active and inactive conformations of the human serine protease HTRA1 reveal that substrate binding to the active site is sufficient to stimulate proteolytic activity. HTRA1 attaches to liposomes, digests misfolded proteins into defined fragments and undergoes substrate-mediated oligomer conversion. In contrast to those of other serine proteases, the PDZ domain of HTRA1 is dispensable for activation or lipid attachment, indicative of different underlying mechanistic features.

Alpha-1 antitrypsin protein and gene therapies decrease autoimmunity and delay arthritis development in mouse model

Background

Alpha-1 antitrypsin (AAT) is a multi-functional protein that has anti-inflammatory and tissue protective properties. We previously reported that human AAT (hAAT) gene therapy prevented autoimmune diabetes in non-obese diabetic (NOD) mice and suppressed arthritis development in combination with doxycycline in mice. In the present study we investigated the feasibility of hAAT monotherapy for the treatment of chronic arthritis in collagen-induced arthritis (CIA), a mouse model of rheumatoid arthritis (RA).

Methods

DBA/1 mice were immunized with bovine type II collagen (bCII) to induce arthritis. These mice were pretreated either with hAAT protein or with recombinant adeno-associated virus vector expressing hAAT (rAAV-hAAT). Control groups received saline injections. Arthritis development was evaluated by prevalence of arthritis and arthritic index. Serum levels of B-cell activating factor of the TNF-α family (BAFF), antibodies against both bovine (bCII) and mouse collagen II (mCII) were tested by ELISA.

Results

Human AAT protein therapy as well as recombinant adeno-associated virus (rAAV8)-mediated hAAT gene therapy significantly delayed onset and ameliorated disease development of arthritis in CIA mouse model. Importantly, hAAT therapies significantly reduced serum levels of BAFF and autoantibodies against bCII and mCII, suggesting that the effects are mediated via B-cells, at least partially.

Conclusion

These results present a new drug for arthritis therapy. Human AAT protein and gene therapies are able to ameliorate and delay arthritis development and reduce autoimmunity, indicating promising potential of these therapies as a new treatment strategy for RA.

HTRA proteases: regulated proteolysis in protein quality control

Controlled proteolysis underlies a vast diversity of protective and regulatory processes that are of key importance to cell fate. The unique molecular architecture of the widely conserved high temperature requirement A (HTRA) proteases has evolved to mediate critical aspects of ATP-independent protein quality control. The simple combination of a classic Ser protease domain and a carboxy-terminal peptide-binding domain produces cellular factors of remarkable structural and functional plasticity that allow cells to rapidly respond to the presence of misfolded or mislocalized polypeptides.

Membrane-type I matrix metalloproteinase-dependent regulation of rheumatoid arthritis synoviocyte function

In rheumatoid arthritis, the coordinated expansion of the synoviocyte mass is coupled with a pathologic angiogenic response that leads to the destructive remodeling of articular as well as surrounding connective tissues. Although rheumatoid synoviocytes express a multiplicity of proteolytic enzymes, the primary effectors of cartilage, ligament, and tendon damage remain undefined. Herein, we demonstrate that human rheumatoid synoviocytes mobilize the membrane-anchored matrix metalloproteinase (MMP), membrane-type I MMP (MT1-MMP), to dissolve and invade type I and type II collagen-rich tissues. Though rheumatoid synoviocytes also express a series of secreted collagenases, these proteinases are ineffective in mediating collagenolytic activity in the presence of physiologic concentrations of plasma- or synovial fluid-derived antiproteinases. Furthermore, MT1-MMP not only directs the tissue-destructive properties of rheumatoid synoviocytes but also controls synoviocyte-initiated angiogenic responses in vivo. Together, these findings identify MT1-MMP as a master regulator of the pathologic extracellular matrix remodeling that characterizes rheumatoid arthritis as well as the coupled angiogenic response that maintains the aggressive phenotype of the advancing pannus.

Matrix metalloproteinases are less essential for the in-situ gelatinolytic activity in heart muscle than in skeletal muscle

In order to determine whether the contribution of matrix metalloproteinases (MMPs) to the tissue gelatinolytic activity is similar between myocardium and skeletal muscle tissue, in-situ zymography was applied to myoblasts originated from myocardium or skeletal muscle of rodents as well as tissue sections of heart and soleus muscles of rats. Gelatinolyic activity was observed in cytoplasm and nucleus of both heart and skeletal myoblasts. The chelating agent EDTA blocked much of the gelatinolytic activity and the organomercurial activator of MMPs increased the activity in cells of both muscle origins. However, the inhibition of gelatinolytic activity by a broad spectrum MMP inhibitor was less profound in heart myoblasts than that in skeletal myoblasts. Gelatinolytic activity was also expressed in the endomysium and perimysium of tissue sections of heart and soleus muscles. Similar with findings in the cell studies, the gelatinase activity was increased by the MMP activator, mostly blocked by EDTA and partially inhibited by the MMP inhibitor. In the presence of the MMP inhibitor, the remaining gelatinolytic activity in the tissue sections was again higher in myocardium than that in soleus muscle. This observation was further supported by the gelatinolytic activity examined in tissue homogenates. Our findings suggest that other proteinases, in addition to MMPs, are more responsive for the tissue gelatinolytic activity in heart muscle as compared with that in skeletal muscle.

Dipeptidyl peptidase-IV in synovial fluid and in synovial fluid mononuclear cells of patients with rheumatoid arthritis

BACKGROUND

Dipeptidyl peptidase-IV (DPP-IV) enzymatic activity controls biological halftime of multiple local mediators. Its deregulation is associated with pathogenesis of several autoimmune diseases, including rheumatoid arthritis (RA). Although DPP-IV is the canonical representative of the group, a number of other proteins have been shown to have similar enzymatic activity. This study was aimed to identify the molecular source of DPP-IV activity in synovial fluid (SF) and fluid mononuclear cells (FMNC) in patients with RA and osteoarthritis (OA). In addition, the association of DPP-IV and the concentration of stromal cell-derived factor-1alpha (SDF), DPP-IV substrate, were evaluated.

METHODS

DPP-IV activity was measured by the kinetic fluorimetric method. The expression of studied molecules in FMNC and their concentrations in SF were assayed using flow cytometry and ELISA respectively.

RESULTS

DPP-IV activity in SF, dominantly derived from the canonical DPP-IV, does not significantly differ between RA and OA. However, a significantly lower DPP-IV activity and expression in FMNC was found in RA as opposed to OA patients. Negative correlation between SDF concentration in SF and the relative amount of CD3+CD26+ cells was observed.

CONCLUSIONS

We report decreased presence of DPP-IV/CD26 in CD3+ FMNC in RA, which also may participate on impaired balance of SDF concentration in SF.

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

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

Proteinases involved in matrix turnover during cartilage and bone breakdown

The joint is a discrete unit that consists of cartilage, bone, tendon and ligaments. These tissues are all composed of an extracellular matrix made of collagens, proteoglycans and specialised glycoproteins that are actively synthesised, precisely assembled and subsequently degraded by the resident connective tissue cells. A balance is maintained between matrix synthesis and degradation in healthy adult tissues. Different classes of proteinases play a part in connective tissue turnover in which active proteinases can cleave matrix protein during resorption, although the proteinase that predominates varies between different tissues and diseases. The metalloproteinases are potent enzymes that, once activated, degrade connective tissue and are inhibited by tissue inhibitors of metalloproteinases (TIMPs); the balance between active matrix metalloproteinases and TIMPs determines, in many tissues, the extent of extracellular matrix degradation. The serine proteinases are involved in the initiation of activation cascades and some, such as elastase, can directly degrade the matrix. Cysteine proteinases are responsible for the breakdown of collagen in bone following the removal of the osteoid layer and the attachment of osteoclasts to the exposed bone surface. Various growth factors increase the synthesis of matrix and proteinase inhibitors, whereas cytokines (alone or in combination) can inhibit matrix synthesis and stimulate proteinase production and matrix destruction.