Procongopain from Trypanosoma congolense is processed at basic pH: an unusual feature among cathepsin L-like cysteine proteases

Cysteine cathepsin activity regulation by glycosaminoglycans

Cysteine cathepsins are a group of enzymes normally found in the endolysosomes where they are primarily involved in intracellular protein turnover but also have a critical role in MHC II-mediated antigen processing and presentation. However, in a number of pathologies cysteine cathepsins were found to be heavily upregulated and secreted into extracellular milieu, where they were found to degrade a number of extracellular proteins. A major role in modulating cathepsin activities play glycosaminoglycans, which were found not only to facilitate their autocatalytic activation including at neutral pH, but also to critically modulate their activities such as in the case of the collagenolytic activity of cathepsin K. The interaction between cathepsins and glycosaminoglycans will be discussed in more detail.

Activation route of the Schistosoma mansoni cathepsin B1 drug target: structural map with a glycosaminoglycan switch

Cathepsin B1 (SmCB1) is a digestive protease of the parasitic blood fluke Schistosoma mansoni and a drug target for the treatment of schistosomiasis, a disease that afflicts over 200 million people. SmCB1 is synthesized as an inactive zymogen in which the N-terminal propeptide blocks the active site. We investigated the activation of the zymogen by which the propeptide is proteolytically removed and its regulation by sulfated polysaccharides (SPs). We determined crystal structures of three molecular forms of SmCB1 along the activation pathway: the zymogen, an activation intermediate with a partially cleaved propeptide, and the mature enzyme. We demonstrate that SPs are essential for the autocatalytic activation of SmCB1, as they interact with a specific heparin-binding domain in the propeptide. An alternative activation route is mediated by an S. mansoni asparaginyl endopeptidase (legumain) which is downregulated by SPs, indicating that SPs act as a molecular switch between both activation mechanisms.

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.

Binding of chondroitin 4-sulfate to cathepsin S regulates its enzymatic activity

Human cysteine cathepsin S (catS) participates in distinct physiological and pathophysiological cellular processes and is considered as a valuable therapeutic target in autoimmune diseases, cancer, atherosclerosis, and asthma. We evaluated the capacity of negatively charged glycosaminoglycans (heparin, heparan sulfate, chondroitin 4/6-sulfates, dermatan sulfate, and hyaluronic acid) to modulate the activity of catS. Chondroitin 4-sulfate (C4-S) impaired the collagenolytic activity (type IV collagen) and inhibited the peptidase activity (Z-Phe-Arg-AMC) of catS at pH 5.5, obeying a mixed-type mechanism (estimated Ki = 16.5 ± 6 μM). Addition of NaCl restored catS activity, supporting the idea that electrostatic interactions are primarly involved. Furthermore, C4-S delayed in a dose-dependent manner the maturation of procatS at pH 4.0 by interfering with the intermolecular processing pathway. Binding of C4-S to catS was demonstrated by gel-filtration chromatography, and its affinity was measured by surface plasmon resonance (equilibrium dissociation constant Kd = 210 ± 40 nM). Moreover, C4-S induced subtle conformational changes in mature catS as observed by intrinsic fluorescence spectroscopy analysis. Molecular docking predicted three specific binding sites on catS for C4-S that are different from those found in the crystal structure of the cathepsin K-C4-S complex. Overall, these results describe a novel glycosaminoglycan-mediated mechanism of catS inhibition and suggest that C4-S may modulate the collagenase activity of catS in vivo.

Optimizing dentin bond durability: control of collagen degradation by matrix metalloproteinases and cysteine cathepsins

OBJECTIVES

Contemporary adhesives lose their bond strength to dentin regardless of the bonding system used. This loss relates to the hydrolysis of collagen matrix of the hybrid layers. The preservation of the collagen matrix integrity is a key issue in the attempts to improve the dentin bonding durability.

METHODS

Dentin contains collagenolytic enzymes, matrix metalloproteinases (MMPs) and cysteine cathepsins, which are responsible for the hydrolytic degradation of collagen matrix in the bonded interface.

RESULTS

The identities, roles and function of collagenolytic enzymes in mineralized dentin has been gathered only within last 15 years, but they have already been demonstrated to have an important role in dental hard tissue pathologies, including the degradation of the hybrid layer. Identifying responsible enzymes facilitates the development of new, more efficient methods to improve the stability of dentin-adhesive bond and durability of bond strength.

SIGNIFICANCE

Understanding the nature and role of proteolytic degradation of dentin-adhesive interfaces has improved immensely and has practically grown to a scientific field of its own within only 10 years, holding excellent promise that stable resin-dentin bonds will be routinely available in a daily clinical setting already in a near future.

Expression, purification and characterisation of two variant cysteine peptidases from Trypanosoma congolense with active site substitutions

Congopain, the major cysteine peptidase of Trypanosoma congolense is an attractive candidate for an anti-disease vaccine and target for the design of specific inhibitors. A complicating factor for the inclusion of congopain in a vaccine is that multiple variants of congopain are present in the genome of the parasite. In order to determine whether the variant congopain-like genes code for peptidases with enzymatic activities different to those of congopain, two variants were cloned and expressed. Two truncated catalytic domain variants were recombinantly expressed in Pichia pastoris. The two expressed catalytic domain variants differed slightly from one another in substrate preferences and also from that of C2 (the recombinant truncated form of congopain). Surprisingly, a variant with the catalytic triad Ser(25), His(159) and Asn(175) was shown to be active against classical cysteine peptidase substrates and inhibited by E-64, a class-specific cysteine protease inhibitor. Both catalytic domain clones and C2 had pH optima of either 6.0 or 6.5 implying that these congopain-like proteases are likely to be expressed and active in the bloodstream of the host animal.

Modulation of the immunogenicity of the Trypanosoma congolense cysteine protease, congopain, through complexation with alpha(2)-macroglobulin

The protozoan parasite Trypanosoma congolense is the main causative agent of livestock trypanosomosis. Congopain, the major lysosomal cysteine proteinase of T. congolense, contributes to disease pathogenesis, and antibody-mediated inhibition of this enzyme may contribute to mechanisms of trypanotolerance. The potential of different adjuvants to facilitate the production of antibodies that would inhibit congopain activity was evaluated in the present study. Rabbits were immunised with the recombinant catalytic domain of congopain (C2), either without adjuvant, with Freund’s adjuvant or complexed with bovine or rabbit α₂-macroglobulin (α₂M). The antibodies were assessed for inhibition of congopain activity. Rabbits immunised with C2 alone produced barely detectable anti-C2 antibody levels and these antibodies had no effect on recombinant C2 or native congopain activity. Rabbits immunised with C2 and Freund’s adjuvant produced the highest levels of anti-C2 antibodies. These antibodies either inhibited C2 and native congopain activity to a small degree, or enhanced their activity, depending on time of production after initial immunisation. Rabbits receiving C2-α₂M complexes produced moderate levels of anti-C2 antibodies and these antibodies consistently showed the best inhibition of C2 and native congopain activity of all the antibodies, with maximum inhibition of 65%. Results of this study suggest that antibodies inhibiting congopain activity could be raised in livestock with a congopain catalytic domain-α₂M complex. This approach improves the effectiveness of the antigen as an anti-disease vaccine candidate for African trypanosomosis.

Glycosaminoglycans facilitate procathepsin B activation through disruption of propeptide-mature enzyme interactions

Lysosomal cysteine cathepsin B participates in numerous diverse cellular processes. In acquiring its activity, the proregion, which blocks the substrate-binding site in the proenzyme, needs to be cleaved off. Here we demonstrate that polyanionic polysaccharides, glycosaminoglycans (GAGs), can accelerate the autocatalytic removal of the propeptide and subsequent activation of cathepsin B. We show that naturally occurring GAGs such as chondroitin sulfates and heparin, as well as the synthetic analog dextran sulfate, accelerate the processing in a concentration-dependent manner. Heparin oligosaccharides down to the size of tetrasaccharides were efficient in accelerating the procathepsin B processing, whereas disaccharides were without effect. Further, the ability of the GAGs to accelerate procathepsin B processing was sensitive to increasing NaCl concentrations, indicating that electrostatic interaction between the GAGs and procathepsin B are operative in the accelerating effect. Also the processing of the catalytic procathepsin B mutant by wild type cathepsin B was enhanced in the presence of GAGs, suggesting that GAGs induce a conformational change in procathepsin B, converting it into a better substrate. Site-directed mutagenesis showed that His(28), Lys(39), and Arg(40), located within the procathepsin B propeptide, have significant roles in the acceleration of procathepsin B activation induced by short GAGs. Because procathepsin B and GAGs often co-localize in vivo, we propose that GAGs may play a physiological role in the activation of procathepsin B.

Production and processing of a recombinant Fasciola hepatica cathepsin B-like enzyme (FhcatB1) reveals potential processing mechanisms in the parasite

The liver fluke,Fasciola hepatica, apparently uses a number of cysteine proteases during its life cycle, most likely for feeding, immune evasion and invasion of tissues. A cathepsin B-like enzyme (herein referred to as FhcatB1) appears to be a major enzyme secreted by the invasive, newly excysted juvenile flukes of this parasite. To examine the processing mechanisms for this enzyme, a recombinant form was expressed inPichia pastorisand purified to yield a homogenous pool of the enzyme. The purified enzyme could be autoactivated at low pH via a bi-molecular mechanism, a process that was greatly accelerated by the presence of large, negatively charged molecules such as dextran sulfate. The enzyme could also apparently be processed to the correct size by an asparaginyl endopeptidase via cleavage in an unusual insertion N-terminal to the normal cleavage site used to yield the active form of the enzyme. Thus, there appear to be a number of ways in which this enzyme can be processed to its optimally active form prior to secretion byF. hepatica.

Heterologous Expression, Purification, Refolding, and Structural-Functional Characterization of EP-B2, a Self-Activating Barley Cysteine Endoprotease

We describe the heterologous expression in Escherichia coli of the proenzyme precursor to EP-B2, a cysteine endoprotease from germinating barley seeds. High yields (50 mg/l) of recombinant proEP-B2 were obtained from E. coli inclusion bodies in shake flask cultures following purification and refolding. The zymogen was rapidly autoactivated to its mature form under acidic conditions at a rate independent of proEP-B2 concentration, suggesting a cis mechanism of autoactivation. Mature EP-B2 was stable and active over a wide pH range and efficiently hydrolyzed a recombinant wheat gluten protein, alpha2-gliadin, at sequences with known immunotoxicity in celiac sprue patients. The X-ray crystal structure of mature EP-B2 bound to leupeptin was solved to 2.2 A resolution and provided atomic insights into the observed subsite specificity of the endoprotease. Our findings suggest that orally administered proEP-B2 may be especially well suited for treatment of celiac sprue.

Glycosaminoglycans Modulate Activation, Activity, and Stability of Tripeptidyl-peptidase I in Vitro and in Vivo

Tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal exopeptidase that sequentially removes tripeptides from the N termini of polypeptides and shows a minor endoprotease activity. Mutations in TPP I lead to classic late-infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disease. TPP I proenzyme is converted in lysosomes into a mature enzyme with the assistance of another protease and is able to autoactivate in acidic pH in vitro via a unimolecular mechanism. Because autoactivation in vitro at the pH values reported for lysosomes generated inactive enzyme, we intended to determine whether physiologically relevant factors can modify this process to also make it plausible in vivo. Here, we report that high ionic strength and glycosaminoglycans (GAGs) increase yields (ionic strength) or yields and rates (GAGs) of activation, enhance degradation of liberated TPP I prosegment fragments, and switch effective autoactivation of TPP I proenzyme toward less acidic pH values (up to pH 6.0). Although ionic strength and GAGs also inhibited TPP I activity in vitro and in living cells, the degree of inhibition (from 20 to 60%) appears to be of rather limited functional significance. Importantly, binding to GAGs improved thermal stability of TPP I and protected the enzyme against alkaline pH-induced denaturation in vitro (t((1/2)) of mature enzyme at pH 7.4 increased by approximately 8-fold in the presence of heparin) and in vivo ( approximately 2-fold higher loss of TPP I in cells deficient in GAGs than in control cells after bafilomycin A1 treatment). These findings elucidate a potent physiologically relevant mechanism of TPP I regulation by GAGs and suggest that generation of the active enzyme via autoactivation can be accomplished not only in vitro but in vivo as well.

Recombinant human procathepsin S is capable of autocatalytic processing at neutral pH in the presence of glycosaminoglycans

Cathepsin S is unique among mammalian cysteine cathepsins in being active and stable at neutral pH. We show that autocatalytic activation of procathepsin S at low pH is a bimolecular process that is considerably accelerated (approximately 20-fold) by glycosaminoglycans and polysaccharides such as dextran sulfate, chondroitin sulfates A and E, and dermatan sulfate through electrostatic interaction with the proenzyme. Procathepsin S is also shown to undergo autoactivation at neutral pH in the presence of dextran sulfate with t1/2 of approximately 20 min at pH 7.5. This novel property of procathepsin S may have implications in pathological conditions associated with the appearance of active cathepsins outside lysosomes.