Cathepsin B: an alternative protease for the generation of an aggrecan 'metalloproteinase' cleavage neoepitope

Natural Compounds: Potential Therapeutics for the Inhibition of Cartilage Matrix Degradation in Osteoarthritis

Osteoarthritis (OA) is the most common degenerative joint disease characterized by enzymatic degradation of the cartilage extracellular matrix (ECM) causing joint pain and disability. There is no disease-modifying drug available for the treatment of OA. An ideal drug is expected to stop cartilage ECM degradation and restore the degenerated ECM. The ECM primarily contains type II collagen and aggrecan but also has minor quantities of other collagen fibers and proteoglycans. In OA joints, the components of the cartilage ECM are degraded by matrix-degrading proteases and hydrolases which are produced by chondrocytes and synoviocytes. Matrix metalloproteinase-13 (MMP-13) and a disintegrin and metalloproteinase with thrombospondin motifs 4 and 5 (ADAMTS5) are the major collagenase and aggrecanase, respectively, which are highly expressed in OA cartilage and promote cartilage ECM degradation. Current studies using various in vitro and in vivo approaches show that natural compounds inhibit the expression and activity of MMP-13, ADAMTS4, and ADAMTS5 and increase the expression of ECM components. In this review, we have summarized recent advancements in OA research with a focus on natural compounds as potential therapeutics for the treatment of OA with emphasis on the prevention of cartilage ECM degradation and improvement of joint health.

Cysteine cathepsins: A long and winding road towards clinics

Biomedical research often focuses on properties that differentiate between diseased and healthy tissue; one of the current focuses is elevated expression and altered localisation of proteases. Among these proteases, dysregulation of cysteine cathepsins can frequently be observed in inflammation-associated diseases, which tips the functional balance from normal physiological to pathological manifestations. Their overexpression and secretion regularly exhibit a strong correlation with the development and progression of such diseases, making them attractive pharmacological targets. But beyond their mostly detrimental role in inflammation-associated diseases, cysteine cathepsins are physiologically highly important enzymes involved in various biological processes crucial for maintaining homeostasis and responding to different stimuli. Consequently, several challenges have emerged during the efforts made to translate basic research data into clinical applications. In this review, we present both physiological and pathological roles of cysteine cathepsins and discuss the clinical potential of cysteine cathepsin-targeting strategies for disease management and diagnosis.

The Impact of Inflammatory Stimuli on Xylosyltransferase-I Regulation in Primary Human Dermal Fibroblasts

Inflammation plays a vital role in regulating fibrotic processes. Beside their classical role in extracellular matrix synthesis and remodeling, fibroblasts act as immune sentinel cells participating in regulating immune responses. The human xylosyltransferase-I (XT-I) catalyzes the initial step in proteoglycan biosynthesis and was shown to be upregulated in normal human dermal fibroblasts (NHDF) under fibrotic conditions. Regarding inflammation, the regulation of XT-I remains elusive. This study aims to investigate the effect of lipopolysaccharide (LPS), a prototypical pathogen-associated molecular pattern, and the damage-associated molecular pattern adenosine triphosphate (ATP) on the expression of XYLT1 and XT-I activity of NHDF. We used an in vitro cell culture model and mimicked the inflammatory tissue environment by exogenous LPS and ATP supplementation. Combining gene expression analyses, enzyme activity assays, and targeted gene silencing, we found a hitherto unknown mechanism involving the inflammasome pathway components cathepsin B (CTSB) and caspase-1 in XT-I regulation. The suppressive role of CTSB on the expression of XYLT1 was further validated by the quantification of CTSB expression in fibroblasts from patients with the inflammation-associated disease Pseudoxanthoma elasticum. Altogether, this study further improves the mechanistic understanding of inflammatory XT-I regulation and provides evidence for fibroblast-targeted therapies in inflammatory diseases.

Behind every smile there's teeth: Cathepsin B's function in health and disease with a kidney view

Cathepsin B (CatB) is a very abundant lysosomal protease with endo- and carboxydipeptidase activities and even ligase features. In this review, we will provide a general characterization of CatB and describe structure, structure-derived properties and location-dependent proteolytic actions. We depict CatB action within lysosome and its important roles in lysosomal biogenesis, lysosomal homeostasis and autophagy rendering this protease a key player in orchestrating lysosomal functions. Lysosomal leakage and subsequent escape of CatB into the cytosol lead to harmful actions, e.g. the role in activating the NLPR3 inflammasome, affecting immune responses and cell death. The second focus of this review addresses CatB functions in the kidney, i.e. the glomerulus, the proximal tubule and collecting duct with strong emphasis of its role in pathology of the respective segment. Finally, observations regarding CatB functions that need to be considered in cell culture will be discussed. In conclusion, CatB a physiologically important molecule may, upon aberrant expression in different cellular context, become a harmful player effectively showing its teeth behind its smile.

Spatial and chronological localization of septoclasts in the mouse Meckel's cartilage

Meckel's cartilage (MC) in the first branchial arch of mammals is a transient structure that disappears before birth, except for the most anterior and posterior portions. Recent studies reported that some congenital abnormalities in craniofacial regions are linked with the persistence or dysplasia of MC. However, the mechanisms underlying the resorption of MC have not been elucidated. Cartilage resorption in endochondral ossification is performed by multinuclear osteoclasts/chondroclasts as well as mononuclear septoclasts, which were newly added to the list of cartilage phagocytes. Septoclasts located exclusively at the chondro-osseous junction of the growth plate resorb the uncalcified cartilage matrix. We hypothesized that septoclasts participate in the resorption of MC and attempted to clarify the localization and roles of septoclasts in MC of mouse using a specific immunohistochemistry marker, epidermal type-fatty acid-binding protein (E-FABP/FABP5). E-FABP-immunopositive septoclasts were detected for the first time at the beginning of MC resorption and localized along the resorption surface. Septoclasts of MC in embryonic mice possessed several processes that elongated toward the uncalcified cartilage matrix, expressed cathepsin B, and exhibited characteristic pericapillary localization. Additionally, they localized between hypertrophied cartilage and osteoclasts/chondroclasts in the resorption surface. Confocal laser-scanning microscopy revealed a decrease in the numbers of septoclasts and their processes with the progression of MC disappearance before birth. The present study showed that E-FABP-immunopositive septoclasts participated in the disappearance of MC through the resorption of the uncalcified cartilage matrix and that they have different roles from osteoclasts/chondroclasts.

Cathepsins in neuronal plasticity

Proteases comprise a variety of enzymes defined by their ability to catalytically hydrolyze the peptide bonds of other proteins, resulting in protein lysis. Cathepsins, specifically, encompass a class of at least twenty proteases with potent endopeptidase activity. They are located subcellularly in lysosomes, organelles responsible for the cell’s degradative and autophagic processes, and are vital for normal lysosomal function. Although cathepsins are involved in a multitude of cell signaling activities, this chapter will focus on the role of cathepsins (with a special emphasis on Cathepsin B) in neuronal plasticity. We will broadly define what is known about regulation of cathepsins in the central nervous system and compare this with their dysregulation after injury or disease. Importantly, we will delineate what is currently known about the role of cathepsins in axon regeneration and plasticity after spinal cord injury. It is well established that normal cathepsin activity is integral to the function of lysosomes. Without normal lysosomal function, autophagy and other homeostatic cellular processes become dysregulated resulting in axon dystrophy. Furthermore, controlled activation of cathepsins at specialized neuronal structures such as axonal growth cones and dendritic spines have been positively implicated in their plasticity. This chapter will end with a perspective on the consequences of cathepsin dysregulation versus controlled, localized regulation to clarify how cathepsins can contribute to both neuronal plasticity and neurodegeneration.

The Ins and Outs of Cathepsins: Physiological Function and Role in Disease Management

Cathepsins are the most abundant lysosomal proteases that are mainly found in acidic endo/lysosomal compartments where they play a vital role in intracellular protein degradation, energy metabolism, and immune responses among a host of other functions. The discovery that cathepsins are secreted and remain functionally active outside of the lysosome has caused a paradigm shift. Contemporary research has unraveled many versatile functions of cathepsins in extralysosomal locations including cytosol and extracellular space. Nevertheless, extracellular cathepsins are majorly upregulated in pathological states and are implicated in a wide range of diseases including cancer and cardiovascular diseases. Taking advantage of the differential expression of the cathepsins during pathological conditions, much research is focused on using cathepsins as diagnostic markers and therapeutic targets. A tailored therapeutic approach using selective cathepsin inhibitors is constantly emerging to be safe and efficient. Moreover, recent development of proteomic-based approaches for the identification of novel physiological substrates offers a major opportunity to understand the mechanism of cathepsin action. In this review, we summarize the available evidence regarding the role of cathepsins in health and disease, discuss their potential as biomarkers of disease progression, and shed light on the potential of extracellular cathepsin inhibitors as safe therapeutic tools.

The Role of Cysteine Cathepsins in Cancer Progression and Drug Resistance

Cysteine cathepsins are lysosomal enzymes belonging to the papain family. Their expression is misregulated in a wide variety of tumors, and ample data prove their involvement in cancer progression, angiogenesis, metastasis, and in the occurrence of drug resistance. However, while their overexpression is usually associated with highly aggressive tumor phenotypes, their mechanistic role in cancer progression is still to be determined to develop new therapeutic strategies. In this review, we highlight the literature related to the role of the cysteine cathepsins in cancer biology, with particular emphasis on their input into tumor biology.

Cysteine Cathepsins and their Extracellular Roles: Shaping the Microenvironment

For a long time, cysteine cathepsins were considered primarily as proteases crucial for nonspecific bulk proteolysis in the endolysosomal system. However, this view has dramatically changed, and cathepsins are now considered key players in many important physiological processes, including in diseases like cancer, rheumatoid arthritis, and various inflammatory diseases. Cathepsins are emerging as important players in the extracellular space, and the paradigm is shifting from the degrading enzymes to the enzymes that can also specifically modify extracellular proteins. In pathological conditions, the activity of cathepsins is often dysregulated, resulting in their overexpression and secretion into the extracellular space. This is typically observed in cancer and inflammation, and cathepsins are therefore considered valuable diagnostic and therapeutic targets. In particular, the investigation of limited proteolysis by cathepsins in the extracellular space is opening numerous possibilities for future break-through discoveries. In this review, we highlight the most important findings that establish cysteine cathepsins as important players in the extracellular space and discuss their roles that reach beyond processing and degradation of extracellular matrix (ECM) components. In addition, we discuss the recent developments in cathepsin research and the new possibilities that are opening in translational medicine.

Modulation of Receptor Protein Tyrosine Phosphatase Sigma Increases Chondroitin Sulfate Proteoglycan Degradation through Cathepsin B Secretion to Enhance Axon Outgrowth

Severed axon tips reform growth cones following spinal cord injury that fail to regenerate, in part, because they become embedded within an inhibitory extracellular matrix. Chondroitin sulfate proteoglycans (CSPGs) are the major axon inhibitory matrix component that is increased within the lesion scar and in perineuronal nets around deafferented neurons. We have recently developed a novel peptide modulator (intracellular sigma peptide) of the cognate receptor of CSPGs, protein tyrosine phosphatase σ (RPTPσ), which has been shown to markedly improve sensorimotor function, micturition, and coordinated locomotor behavior in spinal cord contused rats. However, the mechanism(s) underlying how modulation of RPTPσ mediates axon outgrowth through inhibitory CSPGs remain unclear. Here, we describe how intracellular sigma peptide modulation of RPTPσ induces enhanced protease Cathepsin B activity. Using DRG neurons from female Sprague Dawley rats cultured on an aggrecan/laminin spot assay and a combination of biochemical techniques, we provide evidence suggesting that modulation of RPTPσ regulates secretion of proteases that, in turn, relieves CSPG inhibition through its digestion to allow axon migration though proteoglycan barriers. Understanding the mechanisms underlying RPTPσ modulation elucidates how axon regeneration is impaired by proteoglycans but can then be facilitated following injury.SIGNIFICANCE STATEMENT Following spinal cord injury, chondroitin sulfate proteoglycans (CSPGs) upregulate and potently inhibit axon regeneration and functional recovery. Protein tyrosine phosphatase σ (RPTPσ) has been identified as a critical cognate receptor of CSPGs. We have previously characterized a synthetic peptide (intracellular sigma peptide) that targets the regulatory intracellular domain of the receptor to allow axons to regenerate despite the presence of CSPGs. Here, we have found that one important mechanism by which peptide modulation of the receptor enhances axon outgrowth is through secretion of a protease, Cathepsin B, which enables digestion of CSPGs. This work links protease secretion to the CSPG receptor RPTPσ for the first time with implications for understanding the molecular mechanisms underlying neural regeneration and plasticity.

Keratan sulfate, a complex glycosaminoglycan with unique functional capability

From an evolutionary perspective keratan sulfate (KS) is the newest glycosaminoglycan (GAG) but the least understood. KS is a sophisticated molecule with a diverse structure, and unique functional roles continue to be uncovered for this GAG. The cornea is the richest tissue source of KS in the human body but the central and peripheral nervous systems also contain significant levels of KS and a diverse range of KS-proteoglycans with essential functional roles. KS also displays important cell regulatory properties in epithelial and mesenchymal tissues and in bone and in tumor development of diagnostic and prognostic utility. Corneal KS-I displays variable degrees of sulfation along the KS chain ranging from non-sulfated polylactosamine, mono-sulfated and disulfated disaccharide regions. Skeletal KS-II is almost completely sulfated consisting of disulfated disaccharides interrupted by occasional mono-sulfated N-acetyllactosamine residues. KS-III also contains highly sulfated KS disaccharides but differs from KS-I and KS-II through 2-O-mannose linkage to serine or threonine core protein residues on proteoglycans such as phosphacan and abakan in brain tissue. Historically, the major emphasis on the biology of KS has focused on its sulfated regions for good reason. The sulfation motifs on KS convey important molecular recognition information and direct cell behavior through a number of interactive proteins. Emerging evidence also suggest functional roles for the poly-N-acetyllactosamine regions of KS requiring further investigation. Thus further research is warranted to better understand the complexities of KS.

Origin and development of septoclasts in endochondral ossification of mice

Septoclasts are mononuclear spindle-shaped phagocytes with their long processes in uncalcified cartilage matrices and locate adjacent to the capillary endothelium at the chondro-osseous junction of the growth plate. We have previously revealed a selective expression of epidermal-type fatty acid-binding protein (E-FABP/FABP5) in septoclasts. Although, pericytes are known to distribute along capillaries and directly surround their endothelial cells in a situation similar to septoclasts, no clear evidence is available on the relationship between septoclasts and pericytes. We investigated the chronological localization and morphological change of septoclasts during development of the tibia of mice to clarify the development of septoclasts and the immune-localization of pericyte markers in septoclasts to clarify the origin of septoclasts. E-FABP-immunoreactive septoclasts emerged at the perichondrium in the middle of the cartilaginous templates of the tibia in prenatal development. Septoclasts migrated to the surface of the cartilage adjacent to invading blood vessels. Processes of septoclasts became longer and their apexes attached to Von Kossa-negative uncalcified matrices during the formation process of the primary ossification center. Not only platelet-derived growth factor receptor beta, but also neuron-glial antigen 2 was localized in septoclasts of mice from E15 (embryonic day 15) to P6w (postnatal 6 week). Our results suggest that septoclasts are originated from pericytes and involved in the blood vessel invasion during formation of the primary ossification center.

Detecting cathepsin activity in human osteoarthritis via activity-based probes

Introduction

Lysosomal cathepsins have been reported to contribute to Osteoarthritis (OA) pathophysiology due to their increase in pro-inflammatory conditions. Given the causal role of cathepsins in OA, monitoring their specific activity could provide means for assessing OA severity. To this end, we herein sought to assess a cathepsin activity-based probe (ABP), GB123, in vitro and in vivo.

Methods

Protein levels and activity of cathepsins B and S were monitored by immunoblot analysis and GB123 labeling in cultured primary chondrocytes and conditioned media, following stimuli with tumor necrosis factor alpha (TNFα) and/or Interleukin 1 beta (IL-1β). Similarly, cathepsin activity was examined in sections of intact cartilage (IC) and degraded cartilage (DC) regions of OA. Finally, synovial fluid (SF) and serum from donors with no signs of diseases, early OA, late OA and rheumatoid arthritis (RA) patients were analyzed with GB123 to detect distinct activity levels of cathepsin B and S.

Results

Cathepsin activity in cell lysates, conditioned media explants and DC sections showed enhanced enzymatic activity of cathepsins B and S. Further histological analysis revealed that cathepsin activity was found higher in superficial zones of DC than in IC. Examining serum and SF revealed that cathepsin B is significantly elevated with OA severity in serum and SF, yet levels of cathepsin S are more correlated with synovitis and RA.

Conclusions

Based on our data, cathepsin activity monitored by ABPs correlated well with OA severity and joint inflammation, directing towards a novel etiological target for OA, which possesses significant translational potential in developing means for non-invasive detection of early signs of OA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0586-5) contains supplementary material, which is available to authorized users.

Chopping off the chondrocyte proteome

The progressive nature of osteoarthritis is manifested by the dynamic increase of degenerated articular cartilage, which is one of the major characteristics of this debilitating disease. As articular chondrocytes become exposed to inflammatory stress they enter a pro-catabolic state, which leads to the secretion and activation of a plethora of proteases. In aim to detect the disease before massive areas of cartilage are destroyed, various protein and non-protein biomarkers have been examined in bodily fluids and correlated with disease severity. This review will discuss the widely research extracellular degraded products as well as products generated by affected cellular pathways upon increased protease activity. While extracellular components could be more abundant, cleaved cellular proteins are less abundant and are suggested to possess a significant effect on cell metabolism and cartilage secretome. Subtle changes in cell secretome could potentially act as indicators of the chondrocyte metabolic and biological state. Therefore, it is envisioned that combined biomarkers composed of both cell and extracellular-degraded secretome could provide a valuable platform for testing drug efficacy to halt disease progression at its early stages.

Alarmins S100A8/S100A9 aggravate osteophyte formation in experimental osteoarthritis and predict osteophyte progression in early human symptomatic osteoarthritis

OBJECTIVE

Alarmins S100A8 and S100A9 are major products of activated macrophages regulating cartilage damage and synovial activation during murine and human osteoarthritis (OA). In the current study, we investigated whether S100A8 and S100A9 are involved in osteophyte formation during experimental OA and whether S100A8/A9 predicts osteophyte progression in early human OA.

METHODS

OA was elicited in S100A9-/- mice in two experimental models that differ in degree of synovial activation. Osteophyte size, S100A8, S100A9 and VDIPEN neoepitope was measured histologically. Chondrogenesis was induced in murine mesenchymal stem cells in the presence of S100A8. Levels of S100A8/A9 were determined in plasma of early symptomatic OA participants of the Cohort Hip and Cohort Knee (CHECK) cohort study and osteophytes measured after 2 and 5 years.

RESULTS

Osteophyte size was drastically reduced in S100A9-/- mice in ligaments and at medial femur and tibia on days 21 and 42 of collagenase-induced OA, in which synovial activation is high. In contrast, osteophyte size was not reduced in S100A9-/- mice during destabilised medial meniscus OA, in which synovial activation is scant. S100A8 increased expression and activation of matrix metalloproteinases during micromass chondrogenesis, thereby possibly increasing cartilage matrix remodelling allowing for larger osteophytes. Interestingly, early symptomatic OA participants of the CHECK study with osteophyte progression after 2 and 5 years had elevated S100A8/A9 plasma levels at baseline, while C-reactive protein, erythrocyte sedimentation rate and cartilage oligomeric matrix protein were not elevated at baseline.

CONCLUSIONS

S100A8/A9 aggravate osteophyte formation in experimental OA with high synovial activation and may be used to predict osteophyte progression in early symptomatic human OA.

Cysteine cathepsins and extracellular matrix degradation

BACKGROUND

Cysteine cathepsins are normally found in the lysosomes where they are involved in intracellular protein turnover. Their ability to degrade the components of the extracellular matrix in vitro was first reported more than 25years ago. However, cathepsins were for a long time not considered to be among the major players in ECM degradation in vivo. During the last decade it has, however, become evident that abundant secretion of cysteine cathepsins into extracellular milieu is accompanying numerous physiological and disease conditions, enabling the cathepsins to degrade extracellular proteins.

SCOPE OF VIEW

In this review we will focus on cysteine cathepsins and their extracellular functions linked with ECM degradation, including regulation of their activity, which is often enhanced by acidification of the extracellular microenvironment, such as found in the bone resorption lacunae or tumor microenvironment. We will further discuss the ECM substrates of cathepsins with a focus on collagen and elastin, including the importance of that for pathologies. Finally, we will overview the current status of cathepsin inhibitors in clinical development for treatment of ECM-linked diseases, in particular osteoporosis.

MAJOR CONCLUSIONS

Cysteine cathepsins are among the major proteases involved in ECM remodeling, and their role is not limited to degradation only. Deregulation of their activity is linked with numerous ECM-linked diseases and they are now validated targets in a number of them. Cathepsins S and K are the most attractive targets, especially cathepsin K as a major therapeutic target for osteoporosis with drugs targeting it in advanced clinical trials.

GENERAL SIGNIFICANCE

Due to their major role in ECM remodeling cysteine cathepsins have emerged as an important group of therapeutic targets for a number of ECM-related diseases, including, osteoporosis, cancer and cardiovascular diseases. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

The effect of neonatal gene therapy on skeletal manifestations in mucopolysaccharidosis VII dogs after a decade

Mucopolysaccharidosis (MPS) VII is a lysosomal storage disease due to deficient activity of β-glucuronidase (GUSB), and results in glycosaminoglycan accumulation. Skeletal manifestations include bone dysplasia, degenerative joint disease, and growth retardation. One gene therapy approach for MPS VII involves neonatal intravenous injection of a gamma retroviral vector expressing GUSB, which results in stable expression in liver and secretion of enzyme into blood at levels predicted to be similar or higher to enzyme replacement therapy. The goal of this study was to evaluate the long-term effect of neonatal gene therapy on skeletal manifestations in MPS VII dogs. Treated MPS VII dogs could walk throughout their lives, while untreated MPS VII dogs could not stand beyond 6 months and were dead by 2 years. Luxation of the coxofemoral joint and the patella, dysplasia of the acetabulum and supracondylar ridge, deep erosions of the distal femur, and synovial hyperplasia were reduced, and the quality of articular bone was improved in treated dogs at 6 to 11 years of age compared with untreated MPS VII dogs at 2 years or less. However, treated dogs continued to have osteophyte formation, cartilage abnormalities, and an abnormal gait. Enzyme activity was found near synovial blood vessels, and there was 2% as much GUSB activity in synovial fluid as in serum. We conclude that neonatal gene therapy reduces skeletal abnormalities in MPS VII dogs, but clinically-relevant abnormalities remain. Enzyme replacement therapy will probably have similar limitations long-term.

Pathogenesis of lumbar spine disease in mucopolysaccharidosis VII

Mucopolysaccharidosis type VII (MPS VII) is characterized by deficient β-glucuronidase (GUSB) activity, which leads to accumulation of chondroitin, heparan and dermatan sulfate glycosaminoglycans (GAGs), and multisystemic disease. MPS VII patients can develop kypho-scoliotic deformity and spinal cord compression due to disease of intervertebral disks, vertebral bodies, and associated tissues. We have previously demonstrated in MPS VII dogs that intervertebral disks degenerate, vertebral bodies have irregular surfaces, and vertebral body epiphyses have reduced calcification, but the pathophysiological mechanisms underlying these changes are unclear. We hypothesized that some of these manifestations could be due to upregulation of destructive proteases, possibly via the binding of GAGs to Toll-like receptor 4 (TLR4), as has been proposed for other tissues in MPS models. In this study, the annulus fibrosus of the intervertebral disk of 6-month-old MPS VII dogs had cathepsin B and K activities that were 117- and 2-fold normal, respectively, which were associated with elevations in mRNA levels for these cathepsins as well as TLR4. The epiphyses of MPS VII dogs had a marked elevation in mRNA for the cartilage-associated gene collagen II, consistent with a developmental delay in the conversion of the cartilage to bone in this region. The spine obtained at autopsy from a young man with MPS VII exhibited similar increased cartilage in the vertebral bodies adjacent to the end plates, disorganization of the intervertebral disks, and irregular vertebral end plate morphology. These data suggest that the pathogenesis of destructive changes in the spine in MPS VII may involve upregulation of cathepsins. Inhibition of destructive proteases, such as cathepsins, might reduce spine disease in patients with MPS VII or related disorders.

MMP proteolysis of the human extracellular matrix protein aggrecan is mainly a process of normal turnover

Although it has been shown that aggrecanases are involved in aggrecan degradation, the role of MMP (matrix metalloproteinase) aggrecanolysis is less well studied. To investigate MMP proteolysis of human aggrecan, in the present study we used neoepitope antibodies against MMP cleavage sites and Western blot analysis to identify MMP-generated fragments in normal and OA (osteoarthritis/osteoarthritic) cartilage, and in normal, knee injury and OA and SF (synovial fluid) samples. MMP-3 in vitro digestion showed that aggrecan contains six MMP cleavage sites, in the IGD (interglobular domain), the KS (keratan sulfate) region, the border between the KS region and CS (chondroitin sulfate) region 1, the CS1 region, and the border between the CS2 and the G3 domain, and kinetic studies showed a specific order of digestion where the cleavage between CS2 and the G3 domain was the most preferred. In vivo studies showed that OA cartilage contained (per dry weight) 3.4-fold more MMP-generated FFGV fragments compared with normal cartilage, and although aggrecanase-generated SF-ARGS concentrations were increased 14-fold in OA and knee-injured patients compared with levels in knee-healthy reference subjects, the SF-FFGV concentrations did not notably change. The results of the present study suggest that MMPs are mainly involved in normal aggrecan turnover and might have a less-active role in aggrecan degradation during knee injury and OA.

Calpain is involved in C-terminal truncation of human aggrecan

Mature aggrecan is generally C-terminally truncated at several sites in the CS (chondroitin sulfate) region. Aggrecanases and MMPs (matrix metalloproteinases) have been suggested to be responsible for this digestion. To identify whether calpain, a common intracellular protease, has a specific role in the proteolysis of aggrecan we developed neoepitope antibodies (anti-PGVA, anti-GDLS and anti-EDLS) against calpain cleavage sites and used Western blot analysis to identify calpain-generated fragments in normal and OA (osteoarthritis) knee cartilage and SF (synovial fluid) samples. Our results showed that human aggrecan contains six calpain cleavage sites: one in the IGD (interglobular domain), one in the KS (keratan sulfate) region, two in the CS1 and two in the CS2 region. Kinetic studies of calpain proteolysis against aggrecan showed that the aggrecan molecule was cleaved in a specific order where cuts in CS1 was the most preferred and cuts in KS region was the second most preferred cleavage. OA and normal cartilage contained low amounts of a calpain-generated G1–PGVA fragment (0.5–2%) compared with aggrecanase-generated G1–TEGE (71–76%) and MMP-generated G1–IPEN (23–29%) fragments. Significant amounts of calpain-generated GDLS and EDLS fragments were found in OA and normal cartilage, and a ARGS–EDLS fragment was detected in arthritic SF samples. The results of the present study indicate that calpains are involved in the C-terminal truncation of aggrecan and might have a minor role in arthritic diseases.

Western blot quantification of aggrecan fragments in human synovial fluid indicates differences in fragment patterns between joint diseases

OBJECTIVE

To develop a Western blot method for quantification of multiple aggrecan fragments in human synovial fluids (SFs).

METHOD

SF aggrecan fragments were prepared from knee healthy (reference), knee injury and arthritis subjects by CsCl gradient centrifugations collecting D1 fractions. Samples were analyzed by Western blot, using antibodies against the N-terminal epitope ARGS and the G3 domain, and fragments were quantified using a digital luminescence image analyzer.

RESULTS

The method had a coefficients of variation of 10-30%, and a high correlation (r(S)=0.86) with a corresponding enzyme-linked immunosorbent assay (ELISA). The SFs from reference, knee injured and arthritic subjects contained two major ARGS fragments, ARGS-SELE and ARGS-CS1, and three major G3 fragments (GRGT-G3, GLGS-G3 and AGEG-G3). Compared to the reference, the acute arthritis and acute joint injury groups had a 30-fold elevated concentration of ARGS fragments, and both groups had a higher proportion of the aggrecan in joint fluid as ARGS fragments compared to the other groups. The reference and chronic injury groups had an excess of ARGS-CS1 fragments over ARGS-SELE fragments, while subjects with acute arthritis or osteoarthritis had a more even distribution between these fragments.

CONCLUSIONS

We have developed a novel Western blot quantification method for quantification of SF aggrecan fragments which can differentiate fragments of different sizes sharing the same epitope. The anti-ARGS and anti-G3 quantitative Western blots provided information important for a better understanding of the proteolytic pathways in aggrecan breakdown, information that discriminates between different joint diseases, and may aid in identification of new biomarkers.

A double-headed cathepsin B inhibitor devoid of warhead

Most synthetic inhibitors of peptidases have been targeted to the active site for inhibiting catalysis through reversible competition with the substrate or by covalent modification of catalytic groups. Cathepsin B is unique among the cysteine peptidase for the presence of a flexible segment, known as the occluding loop, which can block the primed subsites of the substrate binding cleft. With the occluding loop in the open conformation cathepsin B acts as an endopeptidase, and it acts as an exopeptidase when the loop is closed. We have targeted the occluding loop of human cathepsin B at its surface, outside the catalytic center, using a high-throughput docking procedure. The aim was to identify inhibitors that would interact with the occluding loop thereby modulating enzyme activity without the help of chemical warheads against catalytic residues. From a large library of compounds, the in silico approach identified [2-[2-(2,4-dioxo-1,3-thiazolidin-3-yl)ethylamino]-2-oxoethyl] 2-(furan-2-carbonylamino) acetate, which fulfills the working hypothesis. This molecule possesses two distinct binding moieties and behaves as a reversible, double-headed competitive inhibitor of cathepsin B by excluding synthetic and protein substrates from the active center. The kinetic mechanism of inhibition suggests that the occluding loop is stabilized in its closed conformation, mainly by hydrogen bonds with the inhibitor, thus decreasing endoproteolytic activity of the enzyme. Furthermore, the dioxothiazolidine head of the compound sterically hinders binding of the C-terminal residue of substrates resulting in inhibition of the exopeptidase activity of cathepsin B in a physiopathologically relevant pH range.

Cathepsin B is essential for regeneration of scratch-wounded normal human epidermal keratinocytes

Migration, proliferation and differentiation of keratinocytes are important processes during tissue regeneration and wound healing of the skin. Here, we focussed on proteases that contribute to extracellular matrix (ECM) remodeling as a prerequisite of keratinocyte migration. In particular, we assessed the significance of the mammalian cysteine peptidase cathepsin B for human keratinocytes during regeneration from scratch wounding. We describe the construction of a scratch apparatus that allows applying scratches of defined length, width and depth to cultured cells in a reproducible fashion. The rationale for our approach derived from our previous work where we have shown that HaCaT keratinocytes secrete cathepsin B into the extracellular space during spontaneous and induced migration. Here, we observed rapid removal of type IV collagen from underneath lamellipodial extensions of keratinocytes at the advancing fronts of regenerating monolayers, indicating that proteolytic ECM remodeling starts upon initiation of keratinocyte migration. Furthermore, we verified our previous results with HaCaT cells by using normal human epidermal keratinocytes (NHEK) and show that non-cell-permeant cathepsin B-specific inhibitors delayed full regeneration of the monolayers from scratch wounding in both cell systems, HaCaT and NHEK. Application of a single dose of cathepsin B inhibitor directly after scratch wounding of keratinocytes demonstrated that cathepsin B is essential during initial stages of wound healing, while its contribution to the subsequent processes of proliferation and differentiation of keratinocytes was of less significance. This notion was supported by our observation that the cathepsin B inhibitors used in this study did not affect proliferation rates of keratinocytes of regenerating cultures. Thus, we conclude that cathepsin B is indeed involved in ECM remodeling after its secretion from migrating keratinocytes. Cathepsin B might directly cleave ECM constituents or it may initiate proteolytic cascades that involve other proteases with the ability to degrade ECM components. Because cathepsin B is important for enabling migration of both, HaCaT cells and NHEK, our results support the notion that HaCaT keratinocytes represent an excellent cell culture model for analysis of human epidermal skin keratinocyte migration.

Aggrecanases and aggrecanase-generated fragments in the human intervertebral disc at early and advanced stages of disc degeneration

STUDY DESIGN

A comparative study of aggrecanases and aggrecan fragmentation profile in the human intervertebral disc at early and advanced stages of disc degeneration.

OBJECTIVE

To determine differences in the content of the aggrecanases and the profile of aggrecan fragmentation in early and advanced stages of disc degeneration using cadaveric human intervertebral discs.

SUMMARY OF BACKGROUND DATA

Aggrecanases and aggrecanase-generated aggrecan fragments have been found in human degenerated discs. However, the association between the grade of disc degeneration and the content of the aggrecanases and the profile of aggrecan fragments has not been well studied.

METHODS

A total of 108 cadaveric donor spines were assessed by MRI T2 imaging and graded based on the Thompson scale. Twelve donor spines (average age, 63 years), each specifically exhibiting 2 different stages (Grade 2 and Grade 4) of disc degeneration at different disc levels, were included in this study. After harvesting the preselected discs, tissue samples were obtained from the center of the nucleus pulposus (NP) and the middle zone of the anulus fibrosus (AF). The amount of the aggrecanases, specifically ADAMTS-4 and ADAMTS-5, and the pattern of aggrecan fragmentation in the isolated tissues were assessed by western blot using specific antibodies.

RESULTS

In both NP and the AF tissues, the amount of ADAMTS-4 detected was higher in disc tissues with a higher level of degeneration (Grade 4) than in Grade 2 disc tissues with a lower level of degeneration. However, the amount of ADAMTS-5 detected did not differ between the 2 disc tissue grades. The aggrecan fragmentation analysis of these samples demonstrated the presence of aggrecanase-mediated fragmentation in both groups; however, there was no apparent difference in the aggrecan fragmentation profile between discs at early and advanced stages of disc degeneration.

CONCLUSION

Aggrecanases are involved in aggrecanolysis at both the early and advanced stages of disc degeneration. The aggrecan fragmentation profile analysis demonstrates the involvement of aggrecanases, as well as that of matrix metalloproteinases and/or cathepsins, during disc degeneration.

Interleukin-6 and cyclic AMP stimulate release of cathepsin B in human osteoblasts

Previous studies have suggested that cathepsin B participates in the joint destruction associated with rheumatoid arthritis (RA). This study examined the activity of cathepsin B (a lysosomal cysteine protease) in human osteoblasts along with its regulation by cyclic AMP and Interleukin-6 (IL-6). Cyclic AMP elevating agents activate cathepsin B and stimulate the secretion of cathepsin B via the secretion of IL-6, a potent mediator of RA. This study investigated the induction of cathepsin B using the proinflammatory cytokine in human osteoblasts (MG-63) in relation to p38 mitogen-activated protein kinase (MAPK) and nuclear factor (NF)-kappa B transcription factor. When added to MG-63 cells, IL-6 stimulated the production of cathepsin B, which was reduced significantly by the addition of SB203580, a specific p38 MAPK inhibitor. In addition, the release of IL-6 was also inhibited by either pyrrolidine dithiocarbamate (PDTC) or NF-kappaB SN50, which are potent NF-kappaB inhibitors. Both NF-kappaB inhibitors had a larger inhibitory effect on the activity of cathepsin B in the presence of SB203580. IL-6 stimulated the NF-kappaB binding affinity as well as the activation of p38 MAP kinase, leading to the release of cathepsin B. However, SB203580 had no effect on the IL-6-induced activation of NF-kappaB, and neither of the NF-kappaB inhibitors decreased the level of p38 MAPK activation in the IL-6-stimulated osteoblasts. Moreover, IL-6 increased the activity of urokinase type plasminogen activator (uPA) in MG-63 cells, which was inhibited by SB203580, PDTC and NF-kappaB SN50. This strongly suggests that p38 MAPK and NF-kappaB are essential to the IL-6-induced activation of cathepsin B or uPA and that these two IL-6-activated pathways can act independently.

Calcium signaling leads to mitochondrial depolarization in impact-induced chondrocyte death in equine articular cartilage explants

OBJECTIVE

Chondrocyte apoptosis is an important factor in the progression of osteoarthritis. This study aimed to elucidate the mechanisms involved upstream of caspase 9 activation and, in particular, calcium signaling and mitochondrial depolarization.

METHODS

Articular cartilage explants obtained from healthy horses were subjected to a single impact load (500-gm weight dropped from a height of 50 mm) and cultured in vitro for up to 48 hours. Chondrocyte death was quantified by the TUNEL method. Release of proteoglycans was determined by the dimethylmethylene blue assay. Weight change was measured, and mitochondrial depolarization was determined using JC-1 staining. To assess the role of calcium signaling in impact-induced chondrocyte death, explants were preincubated in culture medium containing various concentrations of calcium. Inhibitors were used to assess the role of individual signaling components in impact-induced chondrocyte death.

RESULTS

Calcium quenching, inhibitors of calpains, calcium/calmodulin-regulated kinase II (CaMKII), and mitochondrial depolarization reduced impact-induced chondrocyte death after 48 hours in culture. Transient mitochondrial depolarization was observed 3-6 hours following a single impact load. Mitochondrial depolarization was prevented by calcium quenching, inhibitors of calpain, CaMKII, permeability transition pore formation, ryanodine receptor, and the mitochondrial uniport transporter. Cathepsin B did not appear to be involved in impact-induced chondrocyte death. The calpain inhibitor prevented proteoglycan loss, but the percentage weight gain and proteoglycan loss were unaffected by all treatments used.

CONCLUSION

Following a single impact load, calcium is released from the endoplasmic reticulum via the ryanodine receptor and is taken up by the mitochondria via the uniport transporter, causing mitochondrial depolarization and caspase 9 activation. In addition, calpains and CaMKII play important roles in causing mitochondrial depolarization.

Cysteine cathepsins: cellular roadmap to different functions

Cysteine cathepsins belong to the papain-like family C1 of clan CA cysteine peptidases. These enzymes are ubiquitously expressed and exert their proteolytic activity mainly, but not exclusively within the compartments along the endocytic pathway. Moreover, cysteine cathepsins are active in pericellular environments as soluble enzymes or bound to cell surface receptors at the plasma membrane, and possibly even within secretory vesicles, the cytosol, mitochondria, and within the nuclei of eukaryotic cells. Proteolytic actions performed by cysteine cathepsins are essential in the maintenance of homeostasis and depend heavily upon their correct sorting and trafficking within cells. As a consequence, the numerous and diverse approaches to identification, qualitative and quantitative determination, and visualization of cysteine cathepsin functions in vitro, in situ, and in vivo cover the entire spectrum of biochemistry, molecular and cell biology. This review focuses upon the transport pathways directing cysteine cathepsins to their points of action and thus emphasizes the broader role and functionality of cysteine cathepsins in a number of specific cellular locales. Such understanding will provide a foundation for future research investigating the involvement of these peptidases with their substrates, inhibitors, and the intertwined proteolytic networks at the hubs of complex biological systems.

Aggrecanolysis in human osteoarthritis: confocal localization and biochemical characterization of ADAMTS5-hyaluronan complexes in articular cartilages

OBJECTIVE

Human osteoarthritis (OA) is characterized by aggrecanase-mediated depletion of cartilage aggrecan. We have examined the abundance, location and some biochemical properties of the six known aggrecanases (A disintegrin and metalloproteinase with thrombospondin-like motifs 1 (ADAMTS1) 4, 5, 8, 9 and 15) in normal and OA human cartilages.

METHODS

Formalin-fixed, ethylenediamine tetraacetic acid (EDTA)-decalcified sections of full-depth cartilage from human OA tibial plateaus and normal control samples were studied by confocal imaging. Probes included specific antibodies to aggrecanases and two aggrecan epitopes, as well as biotinylated hyaluronan binding protein (HABP) for hyaluronan (HA) visualization. Cartilage extracts were analyzed by Western blot for the individual proteinases and aggrecan fragments.

RESULTS

ADAMTS5 was present in association with cells throughout normal cartilage and was markedly increased in OA, particularly in clonal groups in the superficial and transitional zones, where it was predominantly co-localized with HA. Consistent with the confocal analysis, a high molecular weight complex of ADAMTS5 and HA was isolated from human OA cartilage by isotonic salt extraction and chromatography on Superose 6. The complex eluted with an apparent molecular size of about 2x10(6) and contained major ADAMTS5 forms of 150, 60, 40 and 30kDa. The yield of most forms on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was markedly enhanced by prior digestion of the complex with either Streptomyces hyaluronidase or chondroitinase ABC.

CONCLUSION

ADAMTS5 abundance and distribution in human OA cartilages is consistent with a central role for this enzyme in destructive aggrecanolysis. HA-dependent sequestration of ADAMTS5 in the pericellular matrix may be a mechanism for regulating the activity of this proteinase in human OA cartilage.

Blocking aggrecanase cleavage in the aggrecan interglobular domain abrogates cartilage erosion and promotes cartilage repair

Aggrecan loss from cartilage in arthritis is mediated by aggrecanases. Aggrecanases cleave aggrecan preferentially in the chondroitin sulfate-2 (CS-2) domain and secondarily at the E(373) downward arrow(374)A bond in the interglobular domain (IGD). However, IGD cleavage may be more deleterious for cartilage biomechanics because it releases the entire CS-containing portion of aggrecan. Recent studies identifying aggrecanase-2 (ADAMTS-5) as the predominant aggrecanase in mouse cartilage have not distinguished aggrecanolysis in the IGD from aggrecanolysis in the CS-2 domain. We generated aggrecan knockin mice with a mutation that rendered only the IGD resistant to aggrecanases in order to assess the contribution of this specific cleavage to cartilage pathology. The knockin mice were viable and fertile. Aggrecanase cleavage in the aggrecan IGD was not detected in knockin mouse cartilage in situ nor following digestion with ADAMTS-5 or treatment of cartilage explant cultures with IL-1 alpha. Blocking cleavage in the IGD not only diminished aggrecan loss and cartilage erosion in surgically induced osteoarthritis and a model of inflammatory arthritis, but appeared to stimulate cartilage repair following acute inflammation. We conclude that blocking aggrecanolysis in the aggrecan IGD alone protects against cartilage erosion and may potentiate cartilage repair.

Do the matrix degrading enzymes cathepsins B and D increase following a high intensity exercise regime?

OBJECTIVE

It has been shown by others that levels of matrix degrading enzymes are increased in osteoarthritis (OA) and so are proposed to be involved in the aetiopathogenesis of the disease, including exercise-associated OA. Therefore we hypothesised that cathepsin B and cathepsin D were increased in cartilage samples previously shown to have early stage OA from 2-year-old Thoroughbred horses, euthanased for reasons other than this study, that had a history of 19-week high intensity exercise (n=6) compared to age and sex-matched horses with a history of low intensity exercise (n=6).

METHODS

Cartilage samples were used from four specific sites within the carpal joints. Standard immunolocalisation protocols and blind counting of positive and negative cells within the articular surface, mid-zone and deep zone (DZ) were used to test our hypothesis.

RESULTS

A high intensity exercise regime did not significantly alter the number of chondrocytes positive for cathepsin B, whereas a significant decrease was found for cathepsin D in the DZ, indicating that these enzymes are regulated differently by mechanical loading. Furthermore, cathepsin D varied according to the topographical location within the joint, reflecting biomechanical differences experienced during a high compared to a low intensity exercise regime.

CONCLUSION

This study disproves our hypothesis that cathepsins B and D are increased following a high intensity exercise regime unlike that reported for other matrix enzymes.

Drug delivery strategies for cathepsin inhibitors in joint diseases

Cathepsins play important roles in the development of joint and bone diseases such as osteoporosis, rheumatoid arthritis (RA) and osteoarthritis (OA). Cathepsin inhibitors are presently in development and clinical testing for use as novel disease-modifying drugs for the improved treatment of osteoporosis. They may also be applicable for the treatment of joint diseases. However, some barriers still hamper their clinical applications in these indications. Based on pathophysiological features of RA and OA, the authors discuss six potential drug delivery strategies for the effective delivery of cathepsin inhibitors or other antiarthritic drugs to the arthritic joint tissue. Successful application of these strategies may significantly contribute to a more effective and safe treatment of RA and OA.

Estimation of the identity of proteolytic aggrecan fragments using PAGE migration and Western immunoblot

OBJECTIVE

To develop calculation models, using Western immunoblot, as a tool for the estimation of proteolytic human aggrecan fragment identity.

METHOD

Seven human aggrecan fragments (calibrators), purified by CsCl gradient centrifugation and identified by Western immunoblot of N- and C-terminals, were used to develop calculation models. The models were used for identification of unknown aggrecan fragments each having one of their N- or C-terminals identified.

RESULTS

The calibrator molecular weights (Mw) from sodium dodecyl sulfate (SDS)-gels (m), the Mw of amino acids (a) and the Mw of their carbohydrate substitution (g) were expressed as K = m/(a+g), or as K = 1.085m/(a+g) when compensation for the G1 domain was required. Using these models together with average K-values, 12 out of the 17 immuno-detected aggrecan fragments were calculated to a known protease cleavage site, while five were identified to domain levels.

CONCLUSIONS

With six neoepitope antibodies together with antibodies against the G1- and G3-domain it was possible to predict the identity of several proteolytic fragments from different regions within the aggrecan monomer.

Lack of tissue inhibitor of metalloproteinases-3 results in an enhanced inflammatory response in antigen-induced arthritis

Tissue inhibitor of metalloproteinases-3 (TIMP-3) is known to inhibit matrix metalloproteinases, aggrecanases, and tumor necrosis factor (TNF)-alpha-converting enzyme (TACE, ADAM17). These metalloproteases participate in different aspects of joint destruction in inflammatory arthritis. To determine the relative importance of this inhibitor in joint pathology, wild-type and Timp3-/- mice were immunized with methylated bovine serum albumin followed by arthritis induction by intra-articular injection of the same antigen. Animals were monitored for up to 14 days after challenge, and joint tissues were analyzed by routine and Safranin O staining and for the presence of aggrecan neoepitopes produced by metalloprotease cleavage. Serum TNF-alpha was measured by immunoassay. Compared to wild-type animals, Timp3-/- mice showed a dramatic increase in the initial inflammatory response to intra-articular antigen injection, and serum TNF-alpha levels were greatly elevated in the Timp3-/- animals after immunization. However, these differences in clinical features disappeared by days 7 to 14. No difference in Safranin O staining or aggrecan cleavage site neoepitope abundance was seen. Thus, in inflammatory joint disease TIMP-3 likely dampens the inflammatory response of TNF-alpha by reducing ADAM17 activity.

Cartilage breakdown in rheumatoid arthritis

Rheumatoid arthritis (RA) is a connective tissue disease characterized by destruction of the joint cartilage and subsequently of the underlying bone. Cartilage destruction is due to proteolysis by enzymes called metalloproteinases (MMPs), whose production and expression are regulated by numerous local mediators such as cytokines, growth factors, prostaglandins, oxygen species, and neuropeptides. MMP activation is largely due to a stimulatory effect of cytokines including IL-1beta and TNFalpha. When these cytokines bind to their membrane receptor, they set off signaling cascades, with activation of TGFbeta-activating kinase (TAK-1), of NF-kappaB by Ikappa-B kinase, of mitogen-activated protein kinases (MAP kinases), and finally of activator protein-1 (AP-1). Tissue inhibitors of MMPs (TIMPs) specifically inhibit MMPs. The interrelations between joint inflammation and joint destruction remain poorly understood. Experimental data suggest that IL-1 may be involved chiefly in joint destruction and TNF in joint inflammation. However, TNF antagonists are potent inhibitors of joint destruction in clinical practice. These results suggest that the mediators function as a network and that inhibition of a single mediator can affect the entire web. Insights gained into the innermost mechanisms of cartilage breakdown in patients with RA have led to major therapeutic breakthroughs. Thus, TNF antagonists have proved highly effective in RA. Future progress will no doubt stem from new knowledge about the extracellular mediators and intracellular signaling pathways that lead to the production and activation of enzymes responsible for cartilage degradation.

ADAMTS-1-knockout mice do not exhibit abnormalities in aggrecan turnover in vitro or in vivo

OBJECTIVE

To determine the role of the proteinase ADAMTS-1 in normal and accelerated catabolism of aggrecan in articular and growth plate cartilage of mice.

METHODS

Expression of ADAMTS-1 was determined using reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of RNA isolated from microdissected chondrocytes from different zones of mouse growth plate and articular cartilage. Real-time RT-PCR for ADAMTS-4, ADAMTS-5, and ADAMTS-9 was performed on femoral head cartilage of wild-type (WT) and ADAMTS-1-knockout (KO) mice. Histologic and immunohistologic evaluation of growth plate and articular cartilage was performed in WT and KO mice from birth to 12 weeks of age. The effect of ADAMTS-1 ablation on cartilage proteoglycan loss was studied in antigen-induced arthritis (AIA). Aggrecan catabolism in WT and KO mice was studied in an in vitro model of cartilage degradation, by quantitation of glycosaminoglycan loss and histologic, immunohistologic, and Western immunoblot analyses.

RESULTS

ADAMTS-1 messenger RNA (mRNA) was expressed in normal mouse articular and growth plate cartilage and was up-regulated in terminal hypertrophic differentiation of growth plate chondrocytes. There was no difference in mRNA levels in the cartilage of WT compared with KO mice for the other potential aggrecanases ADAMTS-4, ADAMTS-5, or ADAMTS-9. ADAMTS-1-KO mice were significantly smaller than their WT littermates; however, no morphologic differences between the genotypes were evident in growth plate or articular cartilage from birth to skeletal maturity (12-16 weeks). Similarly, no difference in cartilage aggrecan content or presence of aggrecan degradation products was detected between WT and KO mice. There was no difference between WT and KO mice in the degree of synovial inflammation or depletion of cartilage aggrecan in AIA. There was no difference between WT and KO cartilage in either basal or stimulated aggrecan loss in vitro; however, subtle changes in the aggrecanase-generated aggrecan catabolites were observed in interleukin-1-treated cartilage.

CONCLUSION

Although ADAMTS-1 is expressed in articular and growth plate cartilage and is able to cleave aggrecan at physiologically relevant sites, our results indicate that it does not play a significant nonredundant role in normal cartilage and bone development and growth. Similarly, ablation of ADAMTS-1 offered no protection from accelerated aggrecanolysis in an inflammatory model of arthritis or in an in vitro model of early cartilage degradation. ADAMTS-1 does not appear to be a viable target for treatment of cartilage destruction in arthritis.

HaCaT keratinocytes secrete lysosomal cysteine proteinases during migration

Cathepsin B, a lysosomal cysteine proteinase, was detected within vesicles of cellular protrusions forming cell-cell contact sites between keratinocytes of the stratum spinosum of human skin. This observation suggested the possibility that secretion of the protease into the pericellular spaces could be involved in the dissociation of cell-cell contacts to enable intraepidermal keratinocyte migration. To determine whether cathepsin B is indeed secreted from migrating keratinocytes, we first used subconfluent HaCaT cells as a culture model to study spontaneous keratinocyte migration. A cathepsin B-specific fluorescent affinity label proved the association of mature cathepsin B with the surfaces of HaCaT cells at the leading edges of growing cells. Second, we used scratch-wounds of confluent HaCaT monolayers as a model of induced keratinocyte migration. Cathepsin B was detected within lysosomes, i.e. vesicles within the perinuclear region of non-wounded cells. Expression of cathepsin B was up-regulated and cathepsin B-positive vesicles showed a redistribution from perinuclear to peripheral regions of keratinocytes at the wound margins within 4 h after wounding. Enzyme cytochemistry further showed that cell surface-associated cathepsin B was proteolytically active at the leading fronts of migrating keratinocytes. In addition, increased amounts of mature forms of cathepsin B were detected within the conditioned media of HaCaT cells during the first 4 h after scratch-wounding. In contrast, and as a control, the activity of the cytosolic enzyme lactate dehydrogenase was not significantly higher in media of wounded cells as compared with non-wounded controls, arguing for a specific induction of cathepsin B secretion upon wounding and migration of the cells. This was further substantiated by applying various cathepsin B-specific inhibitors after wounding. These experiments showed that the migration ability of keratinocytes was reduced due to the blockage of functional cathepsin B. Thus, our results strongly suggest that cell surface-associated cathepsin B is a protease that contributes to the remodelling of the extracellular matrix and thereby promotes keratinocyte migration during wound healing.

Matrix metalloproteinases are not essential for aggrecan turnover during normal skeletal growth and development

The growth plate is a transitional region of cartilage and highly diversified chondrocytes that controls long bone formation. The composition of growth plate cartilage changes markedly from the epiphysis to the metaphysis, notably with the loss of type II collagen, concomitant with an increase in MMP-13; type X collagen; and the C-propeptide of type II collagen. In contrast, the fate of aggrecan in the growth plate is not clear: there is biosynthesis and loss of aggrecan from hypertrophic cartilage, but the mechanism of loss is unknown. All matrix metalloproteinases (MMPs) cleave aggrecan between amino acids N341 and F342 in the proteinase-sensitive interglobular domain (IGD), and MMPs in the growth plate are thought to have a role in aggrecanolysis. We have generated mice with aggrecan resistant to proteolysis by MMPs in the IGD and found that the mice develop normally with no skeletal deformities. The mutant mice do not accumulate aggrecan, and there is no significant compensatory proteolysis occurring at alternate sites in the IGD. Our studies reveal that MMP cleavage in this key region is not a predominant mechanism for removing aggrecan from growth plate cartilage.

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

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

Altered endochondral bone development in matrix metalloproteinase 13-deficient mice

The assembly and degradation of extracellular matrix (ECM) molecules are crucial processes during bone development. In this study, we show that ECM remodeling is a critical rate-limiting step in endochondral bone formation. Matrix metalloproteinase (MMP) 13 (collagenase 3) is poised to play a crucial role in bone formation and remodeling because of its expression both in terminal hypertrophic chondrocytes in the growth plate and in osteoblasts. Moreover, a mutation in the human MMP13 gene causes the Missouri variant of spondyloepimetaphyseal dysplasia. Inactivation of Mmp13 in mice through homologous recombination led to abnormal skeletal growth plate development. Chondrocytes differentiated normally but their exit from the growth plate was delayed. The severity of the Mmp13- null growth plate phenotype increased until about 5 weeks and completely resolved by 12 weeks of age. Mmp13-null mice had increased trabecular bone, which persisted for months. Conditional inactivation of Mmp13 in chondrocytes and osteoblasts showed that increases in trabecular bone occur independently of the improper cartilage ECM degradation caused by Mmp13 deficiency in late hypertrophic chondrocytes. Our studies identified the two major components of the cartilage ECM, collagen type II and aggrecan, as in vivo substrates for MMP13. We found that degradation of cartilage collagen and aggrecan is a coordinated process in which MMP13 works synergistically with MMP9. Mice lacking both MMP13 and MMP9 had severely impaired endochondral bone, characterized by diminished ECM remodeling, prolonged chondrocyte survival, delayed vascular recruitment and defective trabecular bone formation (resulting in drastically shortened bones). These data support the hypothesis that proper ECM remodeling is the dominant rate-limiting process for programmed cell death, angiogenesis and osteoblast recruitment during normal skeletal morphogenesis.

Chemokines in cartilage degradation

Besides the well-known activities of the prototypical inflammatory cytokines (IL-1beta, TNFalpha), a role for chemokines and their receptors in cartilage degradation in osteoarthritis has recently been reported. Human chondrocytes can produce CC and CXC chemokines and express chemokine receptors for both chemokine subfamilies. Engagement of these receptors can induce the release of matrix degrading enzymes such as matrix metalloproteinases 1, 3, and 13, and N-acetyl-beta-D-glucosaminidase. Furthermore GROalpha, a CXC chemokine acting on CXCR2, can activate an apoptotic pathway in chondrocytes that leads to chondrocyte cell death. These findings suggest that chemokines can act as an autocrine or paracrine loop on chondrocytes and can contribute to many pathophysiological patterns present in osteoarthritis. Chemokines and their downstream signaling pathways can be considered novel therapeutic targets in osteoarthritis.

Detection of aggrecanase- and MMP-generated catabolic neoepitopes in the rat iodoacetate model of cartilage degeneration

OBJECTIVE

To characterize the time course of aggrecan and type II collagen degradation in the rat iodoacetate model of cartilage degeneration in relationship to the temporal sequence that has been described in human osteoarthritis (OA).

DESIGN

Rats were injected intra-articularly in one knee joint with iodoacetate and damage to the tibial plateau was assessed from digitized images captured using an image analyzer. The articular cartilage from the tibial plateau was harvested, extracted and glycosaminoglycan (GAG) content was measured using the dimethylmethylene blue (DMMB) assay. Cartilage aggrecan neoepitopes were detected in cartilage extracts by Western blotting using antibodies recognizing the aggrecanase-generated C-terminal neoepitope NITEGE (BC-13) and the MMP-generated C-terminal neoepitope DIPEN (BC-4). A type II collagen collagenase-generated neoepitope was detected in cartilage extracts by ELISA using the Col2-3/4Cshort antibody; denatured collagen was detected using the Col2-3/4m antibody.

RESULTS

Degenerative joint changes and proteoglycan (GAG) loss progressed with time after iodoacetate injection. Western blotting of cartilage extracts of iodoacetate treated rats demonstrated an increase in both aggrecanase- and MMP-generated epitopes with the NITEGE aggrecanase neoepitope being significantly elevated on days 7, 14 and 21 while DIPEN the MMP neoepitope was significantly elevated on days 7 and 14. The type II collagen neoepitope recognized by Col2-3/4Cshort was significantly increased in cartilage extracts of rats at days 14 and 21 after iodoacetate injection.

CONCLUSION

The proteoglycan fragments extracted from the knee cartilage of rats after the intra-articular injection of iodoacetate appeared to result from cleavage at both aggrecanase and MMP sites. Cleavage of type II collagen by collagenase was also detected after iodoacetate injection and occurred subsequent to the initiation of aggrecan loss. These observations serve to demonstrate similarities in the mechanisms of cartilage degeneration induced by iodoacetate to those seen in articular cartilage in OA.