Real-time analysis of osteoclast resorption and fusion dynamics in response to bone resorption inhibitors.

Cathepsin K (CatK), an essential collagenase in osteoclasts (OCs), is a potential therapeutic target for the treatment of osteoporosis. Using live-cell imaging, we monitored the bone resorptive behaviour of OCs during dose-dependent inhibition of CatK by an ectosteric (Tanshinone IIA sulfonate) and an active site inhibitor (odanacatib). CatK inhibition caused drastic reductions in the overall resorption speed of OCs. At IC50 CatK-inhibitor concentration, OCs reduced about 40% of their trench-forming capacity and at fourfold IC50 concentrations, a > 95% reduction was observed. The majority of CatK-inhibited OCs (~ 75%) were involved in resorption-migration-resorption episodes forming adjacent pits, while ~ 25% were stagnating OCs which remained associated with the same excavation. We also observed fusions of OCs during the resorption process both in control and inhibitor-treated conditions, which increased their resorption speeds by 30-50%. Inhibitor IC50-concentrations increased OC-fusion by twofold. Nevertheless, more fusion could not counterweigh the overall loss of resorption activity by inhibitors. Using an activity-based probe, we demonstrated the presence of active CatK at the resorbing front in pits and trenches. In conclusion, our data document how OCs respond to CatK-inhibition with respect to movement, bone resorption activity, and their attempt to compensate for inhibition by activating fusion.

Extracellular Matrix Remodeling in Atopic Dermatitis Harnesses the Onset of an Asthmatic Phenotype and Is a Potential Contributor to the Atopic March.

The development of atopic dermatitis in infancy, and subsequent allergies, such as asthma in later childhood, is known as the atopic march. The mechanism is largely unknown, however the course of disease indicates an inter-epithelial crosstalk, through the onset of inflammation in the skin and progression to other mucosal epithelia. In this study, we investigated if and how skin-lung epithelial crosstalk contributes to the development of the atopic march. First, we emulated inter-epithelial crosstalk through indirect coculture of bioengineered atopic-like skin disease models and three-dimensional bronchial epithelial models triggering an asthma-like phenotype in the latter. A subsequent secretome analysis identified thrombospondin-1, CD44, complement factor C3, fibronectin, and syndecan-4 as potentially relevant skin-derived mediators. Because these mediators are extracellular matrix-related proteins, we then studied the involvement of the extracellular matrix, unveiling distinct proteomic, transcriptomic, and ultrastructural differences in atopic samples. The latter indicated extracellular matrix remodeling triggering the release of the above-mentioned mediators. In vivo mouse data showed that exposure to these mediators dysregulated activated circadian clock genes which are increasingly discussed in the context of atopic diseases and asthma development. Our data point toward the existence of a skin-lung axis that could contribute to the atopic march driven by skin extracellular matrix remodeling.

Expression of elastolytic cathepsins in human skin and their involvement in age-dependent elastin degradation.

BACKGROUND: Skin ageing is associated with structure-functional changes in the extracellular matrix, which is in part caused by proteolytic degradation. Since cysteine cathepsins are major matrix protein-degrading proteases, we investigated the age-dependent expression of elastolytic cathepsins K, S, and V in human skin, their in vitro impact on the integrity of the elastic fibre network, their cleavage specificities, and the release of bioactive peptides. METHODS: Cathepsin-mediated degradation of human skin elastin samples was assessed from young to very old human donors using immunohistochemical and biochemical assays, scanning electron microscopy, and mass spectrometry. RESULTS: Elastin samples derived from patients between 10 and 86 years of age were analysed and showed an age-dependent deterioration of the fibre structure from a dense network of thinner fibrils into a beaded and porous mesh. Reduced levels of cathepsins K, S, and V were observed in aged skin with a predominant epidermal expression. Cathepsin V was the most potent elastase followed by cathepsin K and S. Biomechanical analysis of degraded elastin fibres corroborated the destructive activity of cathepsins. Mass spectrometric determination of the cleavage sites in elastin revealed that all three cathepsins predominantly cleaved in hydrophobic domains. The degradation of elastin was efficiently inhibited by an ectosteric inhibitor. Furthermore, the degradation of elastin fibres resulted in the release of bioactive peptides, which have previously been associated with various pathologies. CONCLUSION: Cathepsins are powerful elastin-degrading enzymes and capable of generating a multitude of elastokines. They may represent a viable target for intervention strategies to reduce skin ageing.

Characterization of cathepsin S exosites that govern its elastolytic activity.

We have previously determined that the elastolytic activities of cathepsins (Cat) K and V require two exosites sharing the same structural localization on both enzymes. The structural features involved in the elastolytic activity of CatS have not yet been identified. We first mutated the analogous CatK and V putative exosites of CatS into the elastolytically inactive CatL counterparts. The modification of the exosite 1 did not affect the elastase activity of CatS whilst mutation of the Y118 of exosite 2 decreased the cleavage of elastin by ∼70% without affecting the degradation of other macromolecular substrates (gelatin, thyroglobulin). T06, an ectosteric inhibitor that disrupt the elastolytic activity of CatK, blocked ∼80% of the elastolytic activity of CatS without blocking the cleavage of gelatin and thyroglobulin. Docking studies showed that T06 preferentially interacts with a binding site located on the Right domain of the enzyme, outside of the active site. The structural examination of this binding site showed that the loop spanning the L174N175G176K177 residues of CatS is considerably different from that of CatL. Mutation of this loop into the CatL-like equivalent decreased elastin degradation by ∼70% and adding the Y118 mutation brought down the loss of elastolysis to ∼80%. In addition, the Y118 mutation selectively reduced the cleavage of the basement membrane component laminin by ∼50%. In summary, our data show that the degradation of elastin by CatS requires two exosites where one of them is distinct from those of CatK and V whilst the cleavage of laminin requires only one exosite.

Elastolytic activity of cysteine cathepsins K, S, and V promotes vascular calcification.

Elastin plays an important role in maintaining blood vessel integrity. Proteolytic degradation of elastin in the vascular system promotes the development of atherosclerosis, including blood vessel calcification. Cysteine cathepsins have been implicated in this process, however, their role in disease progression and associated complications remains unclear. Here, we showed that the degradation of vascular elastin by cathepsins (Cat) K, S, and V directly stimulates the mineralization of elastin and that mineralized insoluble elastin fibers were ~25-30% more resistant to CatK, S, and V degradation when compared to native elastin. Energy dispersive X-ray spectroscopy investigations showed that insoluble elastin predigested by CatK, S, or V displayed an elemental percentage in calcium and phosphate up to 8-fold higher when compared to non-digested elastin. Cathepsin-generated elastin peptides increased the calcification of MOVAS-1 cells acting through the ERK1/2 pathway by 34-36%. We made similar observations when cathepsin-generated elastin peptides were added to ex vivo mouse aorta rings. Altogether, our data suggest that CatK-, S-, and V-mediated elastolysis directly accelerates the mineralization of the vascular matrix by the generation of nucleation points in the elastin matrix and indirectly by elastin-derived peptides stimulating the calcification by vascular smooth muscle cells. Both processes inversely protect against further extracellular matrix degradation.

Identification of substrate-specific inhibitors of cathepsin K through high-throughput screening.

Cathepsin K (CatK) is a cysteine protease and drug target for skeletal disorders that is known for its potent collagenase and elastase activity. The formation of oligomeric complexes of CatK in the presence of glycosaminoglycans has been associated with its collagenase activity. Inhibitors that disrupt these complexes can selectively block the collagenase activity without interfering with the other regulatory proteolytic activities of the enzyme. Here, we have developed a fluorescence polarization (FP) assay to screen 4761 compounds for substrate-specific ectosteric collagenase inhibitors of CatK. A total of 38 compounds were identified that block the collagenase activity without interfering with the hydrolysis of active site substrates such as the synthetic peptide substrate, benzyloxycarbonyl-Phe-Arg-7-amido-4-methylcoumarin, and gelatin. The identified inhibitors can be divided into two main classes, negatively charged and polyaromatic compounds which suggest the binding to different ectosteric sites. Two of the inhibitors were highly effective in preventing the bone-resorption activity of CatK in osteoclasts. Interestingly, some of the ectosteric inhibitors were capable of differentiating between the collagenase and elastase activity of CatK depending on the ectosteric site utilized by the compound. Owing to their substrate-specific selectivity, ectosteric inhibitors represent a viable alternative to side effect-prone active site-directed inhibitors.

A Mild Inhibition of Cathepsin K Paradoxically Stimulates the Resorptive Activity of Osteoclasts in Culture.

Cathepsin K (CatK) inhibition allows reducing bone resorption with specific advantages compared to the existing anti-osteoporosis drugs. Its clinical use appears even more promising with the recent development of ectosteric inhibitors. A confusing observation, however, is that a low dose of the active site CatK inhibitor odanacatib (ODN) was reported to decrease bone mineral density and increase serum levels of the bone resorption marker carboxy-terminal collagen crosslinks (CTX). The present study provides a possible explanation for this paradox. The resorptive activity of human osteoclasts seeded on bone slices was inhibited when subjected to ODN at doses of 20 nM, but about 100-fold lower doses induced a significant increase in CTX levels and in eroded surface (12 repeats). This low-dose-induced stimulation was prevented by inhibition of non-CatK cysteine proteinases, thereby indicating that the stimulation results from an interplay between CatK and other cysteine proteinases. Effective interplay between these proteinases was also shown in enzymatic assays where the CatK-mediated degradation of collagen was enhanced upon addition of cathepsins B or L. Furthermore, extracts of osteoclasts subjected to a low dose of ODN showed higher levels of cathepsin B compared with extracts of control osteoclasts. In conclusion, the low-dose-induced stimulation of resorption observed in the clinical study can be reproduced in osteoclasts cultured in the absence of any other cell. Our data support an osteoclast-intrinsic mechanism where a mild inhibition of CatK results in increased levels of other proteinases contributing to the collagen degradation process.

Tanshinones that selectively block the collagenase activity of cathepsin K provide a novel class of ectosteric antiresorptive agents for bone.

BACKGROUND AND PURPOSE: Attempts to generate active site-directed cathepsin K (CatK) inhibitors for the treatment of osteoporosis have failed because of side effects. We have previously shown that an ectosteric tanshinone CatK inhibitor isolated from Salvia miltiorrhiza blocked, selectively, the collagenase activity of CatK, without affecting the active site and demonstrated its bone-preserving activity in vivo. Here, we have characterize the antiresorptive potential of other tanshinones, which may provide a scaffold for side effect-free CatK inhibitors. EXPERIMENTAL APPROACH: Thirty-one tanshinones were tested for their activity against CatK in enzymic and cell-based assays. The inhibitory potency against triple helical and fibrillar collagen degradation was determined in enzymic assays, by scanning electron microscopy and mechanical strength measurements. Human osteoclast assays were used to determine the effects of the inhibitors on bone resorption, its reversibility and osteoclastogenesis. Binding sites were characterized by molecular docking. KEY RESULTS: Twelve compounds showed highly effective anti-collagenase activity and protected collagen against destruction and mechanical instability without inhibiting the hydrolysis of non-collagenous substrates. Six compounds were highly effective in osteoclast bone resorption assays with IC50 values of <500 nM. None of these tanshinones had effects on cell viability, reversibility of bone resorption inhibition and osteoclastogenesis. The core pharmacophore of the tanshinones appears to be the three-ring system with either a para- or ortho-quinone entity. CONCLUSIONS AND IMPLICATIONS: Our study identified several potent ectosteric antiresorptive CatK inhibitors from the medicinal plant, S. miltiorrhiza, which may avoid side effects seen with active site-directed inhibitors in clinical trials.

Aging-associated modifications of collagen affect its degradation by matrix metalloproteinases.

The natural aging process and various pathologies correlate with alterations in the composition and the structural and mechanical integrity of the connective tissue. Collagens represent the most abundant matrix proteins and provide for the overall stiffness and resilience of tissues. The structural changes of collagens and their susceptibility to degradation are associated with skin wrinkling, bone and cartilage deterioration, as well as cardiovascular and respiratory malfunctions. Here, matrix metalloproteinases (MMPs) are major contributors to tissue remodeling and collagen degradation. During aging, collagens are modified by mineralization, accumulation of advanced glycation end-products (AGEs), and the depletion of glycosaminoglycans (GAGs), which affect fiber stability and their susceptibility to MMP-mediated degradation. We found a reduced collagenolysis in mineralized and AGE-modified collagen fibers when compared to native fibrillar collagen. GAGs had no effect on MMP-mediated degradation of collagen. In general, MMP digestion led to a reduction in the mechanical strength of native and modified collagen fibers. Successive fiber degradation with MMPs and the cysteine-dependent collagenase, cathepsin K (CatK), resulted in their complete degradation. In contrast, MMP-generated fragments were not or only poorly cleaved by non-collagenolytic cathepsins such as cathepsin V (CatV). In conclusion, our data indicate that aging and disease-associated collagen modifications reduce tissue remodeling by MMPs and decrease the structural and mechanic integrity of collagen fibers, which both may exacerbate extracellular matrix pathology.

A composite docking approach for the identification and characterization of ectosteric inhibitors of cathepsin K.

Cathepsin K (CatK) is a cysteine protease that plays an important role in mammalian intra- and extracellular protein turnover and is known for its unique and potent collagenase activity. Through studies on the mechanism of its collagenase activity, selective ectosteric sites were identified that are remote from the active site. Inhibitors targeting these ectosteric sites are collagenase selective and do not interfere with other proteolytic activities of the enzyme. Potential ectosteric inhibitors were identified using a computational approach to screen the druggable subset of and the entire 281,987 compounds comprising Chemical Repository library of the National Cancer Institute-Developmental Therapeutics Program (NCI-DTP). Compounds were scored based on their affinity for the ectosteric site. Here we compared the scores of three individual molecular docking methods with that of a composite score of all three methods together. The composite docking method was up to five-fold more effective at identifying potent collagenase inhibitors (IC50 < 20 µM) than the individual methods. Of 160 top compounds tested in enzymatic assays, 28 compounds revealed blocking of the collagenase activity of CatK at 100 µM. Two compounds exhibited IC50 values below 5 µM corresponding to a molar protease:inhibitor concentration of <1:12. Both compounds were subsequently tested in osteoclast bone resorption assays where the most potent inhibitor, 10-[2-[bis(2-hydroxyethyl)amino]ethyl]-7,8-diethylbenzo[g]pteridine-2,4-dione, (NSC-374902), displayed an inhibition of bone resorption with an IC50-value of approximately 300 nM and no cell toxicity effects.

Effect of conditioning solutions containing ferric chloride on dentin bond strength and collagen degradation.

OBJECTIVE: To investigate the effects of conditioning solutions containing ferric chloride (FeCl3) on resin-dentin bond strength; on protection of dentin collagen against enzymatic degradation and on cathepsin-K (CT-K) activity. METHODS: Conditioning solutions were prepared combining citric acid (CA) and anhydrous ferric chloride (FeCl3) in different concentrations. The solutions were applied to etch flat dentin surfaces followed by bonding with adhesive resin. Phosphoric acid (PA) gel etchant was used as control. The microtensile bond strength (µTBS) was tested after 24h of storage in water and after 9 months of storage in phosphate buffer saline. Dentin slabs were demineralized in 0.5M EDTA, pre-treated or not with FeCl3 and incubated with CT-K. The collagenase activity on dentin collagen matrix was examined and characterized by SEM. Additional demineralized dentin slabs were treated with the conditioning solutions, and the amount of Fe bound to collagen was determined by EDX. The activity of CT-K in the presence of FeCl3 was monitored fluorimetrically. Data were analyzed by ANOVA followed by post-hoc tests as required (α=5%). RESULTS: Slightly higher bond strengths were obtained when dentin was conditioned with 5% CA/0.6% FeCl3 and 5% CA-1.8%FeCl3 regardless of storage time. Bond strengths reduced significantly for all tested conditioners after 9 months of storage. Treating dentin with 1.8% FeCl3 was effective to preserve the structure of collagen against CT-K. EDX analysis revealed binding of Fe-ions to dentin collagen after 15s immersion of demineralized dentin slabs into FeCl3 solutions. FeCl3 at concentration of 0.08% was able to suppress CT-K activity. SIGNIFICANCE: This study shows that FeCl3 binds to collagen and offers protection against Cat-K degradation. Mixed solutions of CA and FeCl3 may be used as alternative to PA to etch dentin in resin-dentin bonding with the benefits of preventing collagen degradation.

An Ectosteric Inhibitor of Cathepsin K Inhibits Bone Resorption in Ovariectomized Mice.

The potent cathepsin K (CatK) inhibitor, Tanshinone IIA sulfonic sodium (T06), was tested for its in vitro and in vivo antiresorptive activities. T06 binds in an ectosteric site of CatK remote from its active site and selectively inhibits collagen degradation with an IC50 value of 2.7 ± 0.2 µM (CatK:T06 molar ratio of 1:5). However, it does not suppress fluorogenic peptide cleavage and gelatinolysis at a 2500-fold molar excess. Contrary to active site-directed CatK inhibitors, such as odanacatib, T06 suppresses bone resorption in both human and mouse osteoclasts equally well (IC50 value for human and mouse osteoclasts: 237 ± 60 nM and 245 ± 55 nM, respectively) and its antiresorptive activity is fully reversible in both cell types. Moreover, T06 affects neither the metabolic activity of osteoclasts nor osteoclastogenesis. In in vivo studies, 40 mg T06/kg/d given to 12-week-old ovariectomized (OVX) mice for 3 months reduced plasma CTx-1 by 20% and increased osteoblast numbers and plasma P1NP by ∼28% when compared with the OVX control. µCT analysis of T06-treated OVX mice showed a 35% increase in bone mineral density and other femoral trabecular bone parameters when compared with OVX animals. T06 did not alter the number of osteoclasts, had no estrogenic effect on the uterus, did not change plasma estradiol levels, and did not inhibit fibroblast-mediated TGF-ß1 processing or degradation and cognitive functions in OVX mice. This study indicates that the ectosteric inhibitor, T06, is a selective antiresorptive CatK inhibitor that may overcome the shortcomings of side effect-prone active site-directed drugs, which all failed in clinical trials. © 2017 American Society for Bone and Mineral Research.

Cathepsin K osteoporosis trials, pycnodysostosis and mouse deficiency models: Commonalities and differences.

INTRODUCTION: The osteoporosis market reached a value of more than $11 billion in 2015. Current treatments remain mostly antiresorptive and comprise of bisphosphonates, the anti-RANKL antibody, denusomab, and selective estrogen receptor modulators (SERMs). The most promising novel antiresorptives are cathepsin K inhibitors, which selectively target the bone matrix, degrading protease without interfering with osteoclast viability or formation as all other antiresorptives do. AREAS COVERED: This review analyses the current status of cathepsin K inhibitor development, its side effects, and compares the phenotypes of mouse and human cathepsin K deficiencies with drug treatment outcomes. EXPERT OPINION: Several selective cathepsin K inhibitors have been developed and evaluated in preclinical and clinical studies. Although all compounds were effective in reducing bone resorption markers, the development of some compounds was terminated either due to side effects or market competition. The most advanced compound is odanacatib, which significantly reduced bone fracture rates in a 5-year trial but still exhibits safety concerns. The analysis of mouse and human catK deficiencies sheds some light on the consequences of a cathepsin K inhibitor treatment. How predictive the knockout phenotypes are regarding long-term cathepsin K treatment remains unclear. Clearly, more studies are needed to understand the mechanism of the observed side effects and novel approaches are needed to make CatK inhibitors either osteoclast-specific or selective for the inhibition of the collagen matrix without affecting the other activities of the protease.

A novel approach to inhibit bone resorption: exosite inhibitors against cathepsin K.

BACKGROUND AND PURPOSE: Cathepsin K (CatK) is a major drug target for the treatment of osteoporosis. Potent active site-directed inhibitors have been developed and showed variable success in clinical trials. These inhibitors block the entire activity of CatK and thus may interfere with other pathways. The present study investigates the antiresorptive effect of an exosite inhibitor that selectively inhibits only the therapeutically relevant collagenase activity of CatK. EXPERIMENTAL APPROACH: Human osteoclasts and fibroblasts were used to analyse the effect of the exosite inhibitor, ortho-dihydrotanshinone (DHT1), and the active site inhibitor, odanacatib (ODN), on bone resorption and TGF-ß1 degradation. Cell cultures, Western blot, light and scanning electron microscopy as well as energy dispersive X-ray spectroscopy, molecular modelling and enzymatic assays were used to evaluate the inhibitors. KEY RESULTS: DHT1 selectively inhibited the collagenase activity of CatK, without affecting the viability of osteoclasts. Both inhibitors abolished the formation of resorption trenches, with DHT1 having a slightly higher IC50 value than ODN. Maximal reductions of other resorption parameters by DHT1 and ODN were comparable, respectively 41% and 33% for total resorption surface, 46% and 48% for resorption depths, and 83% and 61% for C-terminal telopetide fragment (CTX) release. DHT1 did not affect the turnover of fibrosis-associated TGF-ß1 in fibroblasts, whereas 500 nM ODN was inhibitory. CONCLUSIONS AND IMPLICATIONS: Our study shows that an exosite inhibitor of CatK can specifically block bone resorption without interfering with other pathways.

Development and characterization of a eukaryotic expression system for human type II procollagen.

BACKGROUND: Triple helical collagens are the most abundant structural protein in vertebrates and are widely used as biomaterials for a variety of applications including drug delivery and cellular and tissue engineering. In these applications, the mechanics of this hierarchically structured protein play a key role, as does its chemical composition. To facilitate investigation into how gene mutations of collagen lead to disease as well as the rational development of tunable mechanical and chemical properties of this full-length protein, production of recombinant expressed protein is required. RESULTS: Here, we present a human type II procollagen expression system that produces full-length procollagen utilizing a previously characterized human fibrosarcoma cell line for production. The system exploits a non-covalently linked fluorescence readout for gene expression to facilitate screening of cell lines. Biochemical and biophysical characterization of the secreted, purified protein are used to demonstrate the proper formation and function of the protein. Assays to demonstrate fidelity include proteolytic digestion, mass spectrometric sequence and posttranslational composition analysis, circular dichroism spectroscopy, single-molecule stretching with optical tweezers, atomic-force microscopy imaging of fibril assembly, and transmission electron microscopy imaging of self-assembled fibrils. CONCLUSIONS: Using a mammalian expression system, we produced full-length recombinant human type II procollagen. The integrity of the collagen preparation was verified by various structural and degradation assays. This system provides a platform from which to explore new directions in collagen manipulation.

Changes in Structural-Mechanical Properties and Degradability of Collagen during Aging-associated Modifications.

During aging, changes occur in the collagen network that contribute to various pathological phenotypes in the skeletal, vascular, and pulmonary systems. The aim of this study was to investigate the consequences of age-related modifications on the mechanical stability and in vitro proteolytic degradation of type I collagen. Analyzing mouse tail and bovine bone collagen, we found that collagen at both fibril and fiber levels varies in rigidity and Young's modulus due to different physiological changes, which correlate with changes in cathepsin K (CatK)-mediated degradation. A decreased susceptibility to CatK-mediated hydrolysis of fibrillar collagen was observed following mineralization and advanced glycation end product-associated modification. However, aging of bone increased CatK-mediated osteoclastic resorption by ∼27%, and negligible resorption was observed when osteoclasts were cultured on mineral-deficient bone. We observed significant differences in the excavations generated by osteoclasts and C-terminal telopeptide release during bone resorption under distinct conditions. Our data indicate that modification of collagen compromises its biomechanical integrity and affects CatK-mediated degradation both in bone and tissue, thus contributing to our understanding of extracellular matrix aging.

Structural requirements for the collagenase and elastase activity of cathepsin K and its selective inhibition by an exosite inhibitor.

Human cathepsin K (CatK) is a major drug target for the treatment of osteoporosis. Although its collagenase activity is unique, CatK also exerts a potent elastolytic activity that is shared with human cathepsins V and S. Other members of the cysteine cathepsin family, which are structurally similar, do not exhibit significant collagen and elastin degrading activities. This raises the question of the presence of specific structural elements, exosites, that are required for these activities. CatK has two exosites that control its collagenolytic and elastolytic activity. Modifications of exosites 1 and 2 block the elastase activity of CatK, whereas only exosite-1 alterations prevent collagenolysis. Neither exosite affects the catalytic activity, protease stability, subsite specificity of CatK or the degradation of other biological substrates by this protease. A low-molecular-mass inhibitor that docks into exosite-1 inhibits the elastase and collagenase activity of CatK without interfering with the degradation of other protein substrates. The identification of CatK exosites opens up the prospect of designing highly potent inhibitors that selectively inhibit the degradation of therapeutically relevant substrates by this multifunctional protease.

Structural basis of collagen fiber degradation by cathepsin K.

Cathepsin K is the major collagenolytic protease in bone that facilitates physiological as well as pathological bone degradation. Despite its key role in bone remodeling and for being a highly sought-after drug target for the treatment of osteoporosis, the mechanism of collagen fiber degradation by cathepsin K remained elusive. Here, we report the structure of a collagenolytically active cathepsin K protein dimer. Cathepsin K is organized into elongated C-shaped protease dimers that reveal a putative collagen-binding interface aided by glycosaminoglycans. Molecular modeling of collagen binding to the dimer indicates the participation of nonactive site amino acid residues, Q21 and Q92, in collagen unfolding. Mutations at these sites as well as perturbation of the dimer protein-protein interface completely inhibit cathepsin-K-mediated fiber degradation without affecting the hydrolysis of gelatin or synthetic peptide. Using scanning electron microscopy, we demonstrate the specific binding of cathepsin K at the edge of the fibrillar gap region of collagen fibers, which suggest initial cleavage events at the N- and C-terminal ends of tropocollagen molecules. Edman degradation analysis of collagen fiber degradation products revealed those initial cleavage sites. We propose that one cathepsin K molecule binds to collagen-bound glycosaminoglycans at the gap region and recruits a second protease molecule that provides an unfolding and cleavage mechanism for triple helical collagen. Removal of collagen-associated glycosaminoglycans prevents cathepsin K binding and subsequently fiber hydrolysis. Cathepsin K dimer and glycosaminoglycan binding sites represent novel targeting sites for the development of nonactive site-directed second-generation inhibitors of this important drug target.

Effects of cysteine proteases on the structural and mechanical properties of collagen fibers.

Excessive cathepsin K (catK)-mediated turnover of fibrillar type I and II collagens in bone and cartilage leads to osteoporosis and osteoarthritis. However, little is known about how catK degrades compact collagen macromolecules. The present study is aimed to explore the structural and mechanical consequences of collagen fiber degradation by catK. Mouse tail type I collagen fibers were incubated with either catK or non-collagenase cathepsins. Methods used include scanning electron microscopy, protein electrophoresis, atomic force microscopy, and tensile strength testing. Our study revealed evidence of proteoglycan network degradation, followed by the progressive disassembly of macroscopic collagen fibers into primary structural elements by catK. Proteolytically released GAGs are involved in the generation of collagenolytically active catK-GAG complexes as shown by AFM. In addition to their structural disintegration, a decrease in the tensile properties of fibers was observed due to the action of catK. The Young's moduli of untreated collagen fibers versus catK-treated fibers in dehydrated conditions were 3.2 ± 0.68 GPa and 1.9 ± 0.65 GPa, respectively. In contrast, cathepsin L, V, B, and S revealed no collagenase activity, except the disruption of proteoglycan-GAG interfibrillar bridges, which slightly decreased the tensile strength of fibers.