Inhibition of mammalian legumain by some cystatins is due to a novel second reactive site

A comparison of the activity, lysosomal stability, and efficacy of legumain-cleavable and cathepsin cleavable ADC linkers

1. Over the past two decades antibody-drug conjugates (ADCs) have emerged as a highly effective drug delivery technology. ADCs utilize a monoclonal antibody, a chemical linker, and a therapeutic payload to selectively deliver highly potent pharmaceutical agents to specific cell types.2. Challenges such as premature linker cleavage and clearance due to linker hydrophobicity have adversely impacted the stability and safety of ADCs. While there are various solutions to these challenges, our team has focused on replacement of hydrophobic ValCit linkers (cleaved by CatB) with Asn-containing linkers that are cleaved by lysosomal legumain.3. Legumain is abundantly present in lysosomes and is known to play a role in tumor microenvironment dynamics. Herein, we directly compare the lysosomal cleavage, cytotoxicity, plasma stability, and efficacy of a traditional cathepsin cleavable ADC to a matched Asn-containing legumain-cleavable ADC.4. We demonstrate that Asn-containing linker sequences are specifically cleaved by lysosomal legumain and that Asn-linked MMAE ADCs are broadly active against a variety of tumors, even those with low legumain expression. Finally, we show that AsnAsn-linked ADCs exhibit comparable or improved efficacy to traditional ValCit-linked ADCs. Our study paves the way for replacement of the traditional ValCit linker technology with more hydrophilic Asn-containing peptide linker sequences.

Heparan sulfate selectively inhibits the collagenase activity of cathepsin K

Cathepsin K (CtsK) is a cysteine protease with potent collagenase activity. CtsK is highly expressed by bone-resorbing osteoclasts and plays an essential role in resorption of bone matrix. Although CtsK is known to bind heparan sulfate (HS), the structural details of the interaction, and how HS regulates the biological functions of CtsK, remains largely unknown. In this report, we discovered that HS is a multifaceted regulator of the structure and function of CtsK. Structurally, HS forms a highly stable complex with CtsK and induces its dimerization. Co-crystal structures of CtsK with bound HS oligosaccharides reveal the location of the HS binding site and suggest how HS may support dimerization. Functionally, HS plays a dual role in regulating the enzymatic activity of CtsK. While it preserves the peptidase activity of CtsK by stabilizing its active conformation, it inhibits the collagenase activity of CtsK in a sulfation level-dependent manner. These opposing effects can be explained by our finding that the HS binding site is remote from the active site, which allows HS to specifically inhibit the collagenase activity without affecting the peptidase activity. At last, we show that structurally defined HS oligosaccharides effectively block osteoclast resorption of bone in vitro without inhibiting osteoclast differentiation, which suggests that HS-based oligosaccharide might be explored as a new class of selective CtsK inhibitor for many diseases involving exaggerated bone resorption.

Heparan sulfate selectively inhibits the collagenase activity of cathepsin K

Cathepsin K (CtsK) is a cysteine protease with potent collagenase activity. CtsK is highly expressed by bone-resorbing osteoclasts and plays an essential role in bone remodeling. Although CtsK is known to bind heparan sulfate (HS), the structural details of the interaction, and how HS ultimately regulates the biological functions of CtsK, remains largely unknown. In this report, we determined that CtsK preferably binds to larger HS oligosaccharides, such as dodecasaccharides (12mer), and that the12mer can induce monomeric CtsK to form a stable dimer in solution. Interestingly, while HS has no effect on the peptidase activity of CtsK, it greatly inhibits the collagenase activity of CtsK in a manner dependent on sulfation level. By forming a complex with CtsK, HS was able to preserve the full peptidase activity of CtsK for prolonged periods, likely by stabilizing its active conformation. Crystal structures of Ctsk with a bound 12mer, alone and in the presence of the endogenous inhibitor cystatin-C reveal the location of HS binding is remote from the active site. Mutagenesis based on these complex structures identified 6 basic residues of Ctsk that play essential roles in mediating HS-binding. At last, we show that HS 12mers can effectively block osteoclast resorption of bone in vitro. Combined, we have shown that HS can function as a multifaceted regulator of CtsK and that HS-based oligosaccharide might be explored as a new class of selective CtsK inhibitor in many diseases that involve exaggerated bone resorption.

Phytocystatin 6 is a context-dependent, tight-binding inhibitor of Arabidopsis thaliana legumain isoform β

Plant legumains are crucial for processing seed storage proteins and are critical regulators of plant programmed cell death. Although research on legumains boosted recently, little is known about their activity regulation. In our study, we used pull-down experiments to identify AtCYT6 as a natural inhibitor of legumain isoform β (AtLEGβ) in Arabidopsis thaliana. Biochemical analysis revealed that AtCYT6 inhibits both AtLEGβ and papain-like cysteine proteases through two separate cystatin domains. The N-terminal domain inhibits papain-like proteases, while the C-terminal domain inhibits AtLEGβ. Furthermore, we showed that AtCYT6 interacts with legumain in a substrate-like manner, facilitated by a conserved asparagine residue in its reactive center loop. Complex formation was additionally stabilized by charged exosite interactions, contributing to pH-dependent inhibition. Processing of AtCYT6 by AtLEGβ suggests a context-specific regulatory mechanism with implications for plant physiology, development, and programmed cell death. These findings enhance our understanding of AtLEGβ regulation and its broader physiological significance.

Type 2 Cystatins and Their Roles in the Regulation of Human Immune Response and Cancer Progression

Cystatins are a family of intracellular and extracellular protease inhibitors that inhibit cysteine cathepsins-a group of lysosomal cysteine proteases that participate in multiple biological processes, including protein degradation and post-translational cleavage. Cysteine cathepsins are associated with the development of autoimmune diseases, tumor progression, and metastasis. Cystatins are categorized into three subfamilies: type 1, type 2, and type 3. The type 2 cystatin subfamily is the largest, containing 10 members, and consists entirely of small secreted proteins. Although type 2 cystatins have many shared biological roles, each member differs in structure, post-translational modifications (e.g., glycosylation), and expression in different cell types. These distinctions allow the type 2 cystatins to have unique biological functions and properties. This review provides an overview of type 2 cystatins, including their biological similarities and differences, their regulatory effect on human immune responses, and their roles in tumor progression, immune evasion, and metastasis.

Protease-bound structure of Ricistatin provides insights into the mechanism of action of tick salivary cystatins in the vertebrate host

Tick saliva injected into the vertebrate host contains bioactive anti-proteolytic proteins from the cystatin family; however, the molecular basis of their unusual biochemical and physiological properties, distinct from those of host homologs, is unknown. Here, we present Ricistatin, a novel secreted cystatin identified in the salivary gland transcriptome of Ixodes ricinus ticks. Recombinant Ricistatin inhibited host-derived cysteine cathepsins and preferentially targeted endopeptidases, while having only limited impact on proteolysis driven by exopeptidases. Determination of the crystal structure of Ricistatin in complex with a cysteine cathepsin together with characterization of structural determinants in the Ricistatin binding site explained its restricted specificity. Furthermore, Ricistatin was potently immunosuppressive and anti-inflammatory, reducing levels of pro-inflammatory cytokines IL-6, IL-1β, and TNF-α and nitric oxide in macrophages; IL-2 and IL-9 levels in Th9 cells; and OVA antigen-induced CD4+ T cell proliferation and neutrophil migration. This work highlights the immunotherapeutic potential of Ricistatin and, for the first time, provides structural insights into the unique narrow selectivity of tick salivary cystatins determining their bioactivity.

Loss-of-function mutations in CST6 cause dry skin, desquamation and abnormal keratosis without hypotrichosis

Cystatin M/E (encoded by the CST6 gene) is a cysteine protease inhibitor, that exerts regulatory and protective effects against uncontrolled proteolysis mainly by directly regulating cathepsin V, cathepsin L, and legumain activities. Previous studies have suggested that CST6 may exert a regulatory role in epidermal differentiation and hair follicle formation by inhibiting the activity of respective cognate target proteases. However, until recently, studies have revealed that loss- or gain-of-function of the CST6 gene causes dry skin with hypotrichosis in humans. Here, we reported two siblings of Chinese origin with dry skin, desquamation and abnormal keratosis without hypotrichosis. By applying whole-exome sequencing, we identified homozygous loss-of-function mutation c.251G > A (p.Gly84Asp) in the CST6 gene as the underlying genetic cause. Further fluorimetric enzyme assays demonstrated the mutant cystatin M/E protein lost its inhibitory function on the protease activity of cathepsins. Moreover, the corresponding mutation in mice resulted in excessive cornification, desquamation, impaired skin barrier function, and abnormal proliferation and differentiation of keratinocytes. In conclusion, the homozygous missense mutation c.251G > A in CST6 gene resulted in dry skin, desquamation, as well as abnormal keratosis of the skin, promoting our understanding of the role of protease-antiprotease balance in human skin disorders.

An evolutionary molecular adaptation of an unusual stefin from the liver fluke Fasciola hepatica redefines the cystatin superfamily

Fasciolosis is a worldwide parasitic disease of ruminants and an emerging human disease caused by the liver fluke Fasciola hepatica. The cystatin superfamily of cysteine protease inhibitors is composed of distinct families of intracellular stefins and secreted true cystatins. FhCyLS-2 from F. hepatica is an unusual member of the superfamily, where our sequence and 3D structure analyses in this study revealed that it combines characteristics of both families. The protein architecture demonstrates its relationship to stefins, but FhCyLS-2 also contains the secretion signal peptide and disulfide bridges typical of true cystatins. The secretion status was confirmed by detecting the presence of FhCyLS-2 in excretory/secretory products, supported by immunolocalization. Our high-resolution crystal structure of FhCyLS-2 showed a distinct disulfide bridging pattern and functional reactive center. We determined that FhCyLS-2 is a broad specificity inhibitor of cysteine cathepsins from both the host and F. hepatica, suggesting a dual role in the regulation of exogenous and endogenous proteolysis. Based on phylogenetic analysis that identified several FhCyLS-2 homologues in liver/intestinal foodborne flukes, we propose a new group within the cystatin superfamily called cystatin-like stefins.

The Mammalian Cysteine Protease Legumain in Health and Disease

The cysteine protease legumain (also known as asparaginyl endopeptidase or δ-secretase) is the only known mammalian asparaginyl endopeptidase and is primarily localized to the endolysosomal system, although it is also found extracellularly as a secreted protein. Legumain is involved in the regulation of diverse biological processes and tissue homeostasis, and in the pathogenesis of various malignant and nonmalignant diseases. In addition to its proteolytic activity that leads to the degradation or activation of different substrates, legumain has also been shown to have a nonproteolytic ligase function. This review summarizes the current knowledge about legumain functions in health and disease, including kidney homeostasis, hematopoietic homeostasis, bone remodeling, cardiovascular and cerebrovascular diseases, fibrosis, aging and senescence, neurodegenerative diseases and cancer. In addition, this review addresses the effects of some marketed drugs on legumain. Expanding our knowledge on legumain will delineate the importance of this enzyme in regulating physiological processes and disease conditions.

Legumain Activity Is Controlled by Extended Active Site Residues and Substrate Conformation

Legumain is a lysosomal cysteine protease with strict specificity for cleaving after asparagine residues. By sequence comparison, legumain belongs to MEROPS clan CD of the cysteine proteases, which indicates its structural and mechanistic relation to caspases. Contrasting caspases, legumain harbors a pH-dependent ligase activity in addition to the protease activity. Although we already have a significant body of knowledge on the catalytic activities of legumain, many mechanistic details are still elusive. In this study, we provide evidence that extended active site residues and substrate conformation are steering legumain activities. Biochemical experiments and bioinformatics analysis showed that the catalytic Cys189 and His148 residues are regulated by sterically close Glu190, Ser215 and Asn42 residues. While Glu190 serves as an activity brake, Ser215 and Asn42 have a favorable effect on legumain protease activity. Mutagenesis studies using caspase-9 as model enzyme additionally showed that a similar Glu190 activity brake is also implemented in the caspases. Furthermore, we show that the substrate's conformational flexibility determines whether it will be hydrolyzed or ligated by legumain. The functional understanding of the extended active site residues and of substrate prerequisites will allow us to engineer proteases with increased enzymatic activity and better ligase substrates, with relevance for biotechnological applications.

The Asparaginyl Endopeptidase Legumain: An Emerging Therapeutic Target and Potential Biomarker for Alzheimer’s Disease

Alzheimer’s disease (AD) is incurable dementia closely associated with aging. Most cases of AD are sporadic, and very few are inherited; the pathogenesis of sporadic AD is complex and remains to be elucidated. The asparaginyl endopeptidase (AEP) or legumain is the only recognized cysteine protease that specifically hydrolyzes peptide bonds after asparagine residues in mammals. The expression level of AEPs in healthy brains is far lower than that of peripheral organs. Recently, growing evidence has indicated that aging may upregulate and overactivate brain AEPs. The overactivation of AEPs drives the onset of AD through cleaving tau and amyloid precursor proteins (APP), and SET, an inhibitor of protein phosphatase 2A (PP2A). The AEP-mediated cleavage of these peptides enhances amyloidosis, promotes tau hyperphosphorylation, and ultimately induces neurodegeneration and cognitive impairment. Upregulated AEPs and related deleterious reactions constitute upstream events of amyloid/tau toxicity in the brain, and represent early pathological changes in AD. Thus, upregulated AEPs are an emerging drug target for disease modification and a potential biomarker for predicting preclinical AD. However, the presence of the blood–brain barrier greatly hinders establishing body-fluid-based methods to measure brain AEPs. Research on AEP-activity-based imaging probes and our recent work suggest that the live brain imaging of AEPs could be used to evaluate its predictive efficacy as an AD biomarker. To advance translational research in this area, AEP imaging probes applicable to human brain and AEP inhibitors with good druggability are urgently needed.

Review: Unraveling the origin of the structural and functional diversity of plant cystatins

The regulation of protease activity is a critical factor for the physiological balance during plant growth and development. Among the proteins involved in controlling protease activity are the cystatins, well-described inhibitors of cysteine proteases present in viruses, bacteria and most Eukaryotes. Plant cystatins, commonly called phytocystatins, display unique structural and functional diversity and are classified according to their molecular weight as type-I, -II, and -III. Their gene structure is highly conserved across Viridiplantae and provides insights into their evolutionary relationships. Many type-I phytocystatins with introns share sequence similarities with type-II phytocystatins. New data shows that they could have originated from recent losses of the carboxy-terminal extension present in type-II phytocystatins. Intronless type-I phytocystatins originated from a single event shared by flowering plants. Pieces of evidence show multiple events of gene duplications, intron losses, and gains throughout the expansion and diversity of the phytocystatin family. Gene duplication events in Gymnosperms and Eudicots resulted in inhibitors with amino acid substitutions that may modify their interaction with target proteases and other proteins. This review brings a phylogenomic analysis of plant cystatin evolution and contributes to a broader understanding of their origins. A complete functional genomic analysis among phytocystatins and their roles in plant development and responses to abiotic and biotic stresses remains a question to be fully solved.

Identification of a Steric Zipper Motif in the Amyloidogenic Core of Human Cystatin C and Its Use for the Design of Self-Assembling Peptides

Amyloid fibrils have been known for many years. Unfortunately, their fame stems from negative aspects related to amyloid diseases. Nevertheless, due to their properties, they can be used as interesting nanomaterials. Apart from their remarkable stability, amyloid fibrils may be regarded as a kind of a storage medium and as a source of active peptides. In many cases, their structure may guarantee a controlled and slow release of peptides in their active form; therefore, they can be used as a potential nanomaterial in drug delivery systems. In addition, amyloid fibrils display controllable stiffness, flexibility, and satisfactory mechanical strength. In addition, they can be modified and functionalized very easily. Understanding the structure and genesis of amyloid assemblies derived from a broad range of amyloidogenic proteins could help to better understand and use this unique material. One of the factors responsible for amyloid aggregation is the steric zipper. Here, we report the discovery of steric zipper-forming peptides in the sequence of the amyloidogenic protein, human cystatin C (HCC). The ability of short peptides derived from this fragment of HCC to form fibrillar structures with defined self-association characteristics and the factors influencing this aggregation are also presented in this paper.

In Silico, In Vitro, and Clinical Investigations of Cathepsin B and Stefin A mRNA Expression and a Correlation Analysis in Kidney Cancer

The cysteine protease Cathepsin B (CtsB) plays a critical role in multiple signaling pathways, intracellular protein degradation, and processing. Endogenous inhibitors regulate its enzymatic activity, including stefins and other cystatins. Recent data proved that CtsB is implicated in tumor extracellular matrix remodeling, cell invasion, and metastasis: a misbalance between cathepsins and their natural inhibitors is often considered a sign of disease progression. In the present study, we investigated CtsB and stefin A (StfA) expression in renal cell carcinoma (RCC). mRNA analysis unveiled a significant CTSB and STFA increase in RCC tissues compared to adjacent non-cancerogenic tissues and a higher CtsB expression in malignant tumors than in benign renal neoplasms. Further analysis highlighted a positive correlation between CtsB and StfA expression as a function of patient sex, age, tumor size, grade, lymph node invasion, metastasis occurrence, and survival. Alternative overexpression and silencing of CtsB and StfA confirmed the correlation expression between these proteins in human RCC-derived cells through protein analysis and fluorescent microscopy. Finally, the ectopic expression of CtsB and StfA increased RCC cell proliferation. Our data strongly indicated that CtsB and StfA expression play an important role in RCC development by mutually stimulating their expression in RCC progression.

Lysosomal peptidases – Intriguing roles in cancer progression and neurodegeneration

Lysosomal peptidases are hydrolytic enzymes capable of digesting waste proteins that are targeted to lysosomes via endocytosis and autophagy. Besides intracellular protein catabolism, they play more specific roles in several other cellular processes and pathologies, either within lysosomes, upon secretion into the cell cytoplasm or extracellular space, or bound to the plasma membrane. In cancer, lysosomal peptidases are generally associated with disease progression, as they participate in crucial processes leading to changes in cell morphology, signaling, migration, and invasion, and finally metastasis. However, they can also enhance the mechanisms resulting in cancer regression, such as apoptosis of tumor cells or antitumor immune responses. Lysosomal peptidases have also been identified as hallmarks of aging and neurodegeneration, playing roles in oxidative stress, mitochondrial dysfunction, abnormal intercellular communication, dysregulated trafficking, and the deposition of protein aggregates in neuronal cells. Furthermore, deficiencies in lysosomal peptidases may result in other pathological states, such as lysosomal storage disease. The aim of this review was to highlight the role of lysosomal peptidases in particular pathological processes of cancer and neurodegeneration and to address the potential of lysosomal peptidases in diagnosing and treating patients.

The Mechanism of Asparagine Endopeptidase in the Progression of Malignant Tumors: A Review

Asparagine endopeptidase (AEP), also called legumain, is currently the only known cysteine protease that specifically cleaves peptide bonds in asparaginyl residue in the mammalian genome. Since 2003, AEP has been reported to be widely expressed in a variety of carcinomas and is considered a potential therapeutic target. In the following years, researchers intensively investigated the substrates of AEP and the mechanism of AEP in partial tumors. With the identification of substrate proteins such as P53, integrin αvβ3, MMP-2, and MMP-9, the biochemical mechanism of AEP in carcinomas is also more precise. This review will clarify the probable mechanisms of AEP in the progression of breast carcinoma, glioblastoma, gastric carcinoma, and epithelial ovarian carcinoma. This review will also discuss the feasibility of targeted therapy with AEP inhibitor (AEPI) in these carcinomas.

Externally added cystatin C reduces growth of A375 melanoma cells by increasing cell cycle time

Some secreted cysteine protease inhibitors of the cystatin family appear to affect intracellular proteolysis and growth of human cells, as a result of internalization. Here, we studied the effects of external addition of the most abundant human cystatin, cystatin C, on viability and proliferation of cancer cells in culture. A dose‐dependent decrease in viable cells was seen for A375 melanoma, MCF‐7 breast cancer, and PC‐3 prostate cancer cells cultured in 1–5 µm cystatin C after 24 h. Real‐time assessment of growth rates in A375 cell cultures for 48 h by digital holographic microscopy showed an increased doubling time for cells cultured in the presence of 5 µm cystatin C (20.1 h) compared with control cells (14.7 h). A prolonged doubling time was already observed during the first 12 h, indicating a rapid general decrease in cell proliferation at the population level. Tracking of individual cells in phase holographic images showed that dividing cells incubated with 5 µm cystatin C underwent fewer mitoses during 48 h than control cells. In addition, the time between cell divisions was longer, especially for the first cell cycle. Incubation with the variant W106F‐cystatin C (with high cellular uptake rate) resulted in a lower number of viable cells and a prolonged doubling time than when cells were incubated with wild‐type cystatin C, but no effect was observed for (R24A,R25A)‐cystatin C (low cellular uptake). Thus, cystatin C causes prolonged cell division leading to decreased proliferation of melanoma cells, and internalization seems to be a prerequisite for this effect.

Cystatin M/E (Cystatin 6): A Janus-Faced Cysteine Protease Inhibitor with Both Tumor-Suppressing and Tumor-Promoting Functions

Alongside its contribution in maintaining skin homeostasis and its probable involvement in fetal and placental development, cystatin M/E (also known as cystatin 6) was first described as a tumor suppressor of breast cancer. This review aims to provide an update on cystatin M/E with particular attention paid to its role during tumorigenesis. Cystatin M/E, which is related to type 2 cystatins, displays the unique property of being a dual tight-binding inhibitor of both legumain (also known as asparagine endopeptidase) and cysteine cathepsins L, V and B, while its expression level is epigenetically regulated via the methylation of the CST6 promoter region. The tumor-suppressing role of cystatin M/E was further reported in melanoma, cervical, brain, prostate, gastric and renal cancers, and cystatin M/E was proposed as a biomarker of prognostic significance. Contrariwise, cystatin M/E could have an antagonistic function, acting as a tumor promoter (e.g., oral, pancreatic cancer, thyroid and hepatocellular carcinoma). Taking into account these apparently divergent functions, there is an urgent need to decipher the molecular and cellular regulatory mechanisms of the expression and activity of cystatin M/E associated with the safeguarding homeostasis of the proteolytic balance as well as its imbalance in cancer.

Sesame water-soluble proteins fraction contains endopeptidases and exopeptidases with high activity: A natural source for plant proteases

Recently, the interest in the plant proteases has greatly increased. However, only a few of proteases are isolated from the hugely produced oilseeds for the practical utilizations. In this study, the raw sesame milk prepared from peeled sesame seeds was separated into floating, skim, and precipitate fractions by centrifugation. The predominant aspartic endopeptidases and serine carboxypeptidases, which exerted high synergetic activity at pH 4.5-5 and 50-60 °C, were identified in the skim by the liquid chromatography tandem mass spectrometry, Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis, protease inhibitor assay, trichloroacetic acid-nitrogen soluble index (TCA-NSI), and free amino acid analyses. By incubating the mixture (protein content, 2%) of skim and precipitate at pH 4.5 and 50 °C for 6 h, the TCA-NSI and free amino acids achieved to 38.42% and 3148 mg/L, respectively. Moreover, these proteases efficiently degraded the proteins from soybean, peanut, and bovine milk.

pH-dependent and dynamic interactions of cystatin C with heparan sulfate

Cystatin C (Cst-3) is a potent inhibitor of cysteine proteases with diverse biological functions. As a secreted protein, the potential interaction between Cst-3 and extracellular matrix components has not been well studied. Here we investigated the interaction between Cst-3 and heparan sulfate (HS), a major component of extracellular matrix. We discovered that Cst-3 is a HS-binding protein only at acidic pH. By NMR and site-directed mutagenesis, we identified two HS binding regions in Cst-3: the highly dynamic N-terminal segment and a flexible region located between residue 70-94. The composition of the HS-binding site by two highly dynamic halves is unique in known HS-binding proteins. We further discovered that HS-binding severely impairs the inhibitory activity of Cst-3 towards papain, suggesting the interaction could actively regulate Cst-3 activity. Using murine bone tissues, we showed that Cst-3 interacts with bone matrix HS at low pH, again highlighting the physiological relevance of our discovery.

Differential seminal plasma proteome signatures of boars with high and low resistance to hypothermic semen preservation at 5°C

BACKGROUND

Hypothermic storage at 5°C has been investigated as an alternative to promote the prudent use of antibiotics for boar artificial insemination doses. However, this temperature is challenging for some ejaculates or boars.

OBJECTIVE

The present study aimed to identify putative biomarkers for semen resistance to hypothermic storage at 5°C by comparing the seminal plasma proteomes of boars with high and low seminal resistance to preservation at 5°C.

MATERIALS AND METHODS

From an initial group of 34 boars, 15 were selected based on the following criteria: ejaculate with ≤20% abnormal spermatozoa and at least 70% progressive motility at 120 hours of storage at 17°C. Then, based on the response to semen hypothermic storage at 5°C, boars were classified into two categories: high resistance-progressive motility of >75% in the three collections (n = 3); and low resistance-progressive motility of <75% in the three collections (n = 3). Seminal plasma proteins were analyzed in pools, and differential proteomics was performed using Multidimensional Protein Identification Technology.

RESULTS

Progressive motility was lower at 120 hours of storage in low resistance, compared to high resistance boars (P < .05). Acrosome and plasma membrane integrity were not affected by the boar category, storage time, or their interaction (P ≥ .104). Sixty-five proteins were considered for differential proteomics. Among the differentially expressed and exclusive proteins, the identification of proteins such cathepsin B, legumain, and cystatin B suggests significant changes in key enzymes (eg, metalloproteinases) involved in spermatogenesis, sperm integrity, and fertilizing potential.

DISCUSSION AND CONCLUSION

Differences in the seminal plasma suggest that proteins involved in the proteolytic activation of metalloproteinases and proteins related to immune response modulation could disrupt key cellular pathways during spermatogenesis and epididymal maturation, resulting in altered resistance to chilling injury. Further in vivo studies focusing on the immunological crosstalk between epithelial cells and gametes might explain how the immune regulators influence sperm resistance to hipothermic storage.

The Influence of the Mixed DPC:SDS Micelle on the Structure and Oligomerization Process of the Human Cystatin C

Human cystatin C (hCC), a member of the superfamily of papain-like cysteine protease inhibitors, is the most widespread cystatin in human body fluids. Physiologically active hCC is a monomer, which dimerization and oligomerization lead to the formation of the inactive, insoluble amyloid form of the protein, strictly associated with cerebral amyloid angiopathy, a severe state causing death among young patients. It is known, that biological membranes may accelerate the oligomerization processes of amyloidogenic proteins. Therefore, in this study, we describe an influence of membrane mimetic environment-mixed dodecylphosphocholine:sodium dodecyl sulfate (DPC:SDS) micelle (molar ratio 5:1)-on the effect of the hCC oligomerization. The hCC-micelle interactions were analyzed with size exclusion chromatography, circular dichroism, and nuclear magnetic resonance spectroscopy. The experiments were performed on the wild-type (WT) cystatin C, and two hCC variants-V57P and V57G. Collected experimental data were supplemented with molecular dynamic simulations, making it possible to highlight the binding interface and select the residues involved in interactions with the micelle. Obtained data shows that the mixed DPC:SDS micelle does not accelerate the oligomerization of protein and even reverses the hCC dimerization process.

Crouching Tiger, Hidden Protein: Searching for Insecticidal Toxins in Venom of the Red Tiger Assassin Bug (Havinthus rufovarius)

Assassin bugs are venomous insects that prey on other arthropods. Their venom has lethal, paralytic, and liquifying effects when injected into prey, but the toxins responsible for these effects are unknown. To identify bioactive assassin bug toxins, venom was harvested from the red tiger assassin bug (Havinthus rufovarius), an Australian species whose venom has not previously been characterised. The venom was fractionated using reversed-phase high-performance liquid chromatography, and four fractions were found to cause paralysis and death when injected into sheep blowflies (Lucilia cuprina). The amino acid sequences of the major proteins in two of these fractions were elucidated by comparing liquid chromatography/tandem mass spectrometry data with a translated venom-gland transcriptome. The most abundant components were identified as a solitary 12.8 kDa CUB (complement C1r/C1s, Uegf, Bmp1) domain protein and a 9.5 kDa cystatin. CUB domains are present in multidomain proteins with diverse functions, including insect proteases. Although solitary CUB domain proteins have been reported to exist in other heteropteran venoms, such as that of the bee killer assassin bug Pristhesancus plagipennis, their function is unknown, and they have not previously been reported as lethal or paralysis-inducing. Cystatins occur in the venoms of spiders and snakes, but again with an unknown function. Reduction and alkylation experiments revealed that the H. rufovarius venom cystatin featured five cysteine residues, one of which featured a free sulfhydryl group. These data suggest that solitary CUB domain proteins and/or cystatins may contribute to the insecticidal activity of assassin bug venom.

Characterization of endogenous endopeptidases and exopeptidases and application for the limited hydrolysis of peanut proteins

Research concerning the utilization of oilseed endogenous proteases is scarce. Herein, we investigated the peanut proteases and their effects on peanut proteins. Liquid chromatography tandem mass spectrometry analysis showed that peanut contained several endopeptidases and exopeptidases. Protease inhibitor assay and analysis of cleavage sites showed that the obvious proteolytic activity at pH 2-5 and 20-60 °C was from aspartic endopeptidases (optimal at pH 3) and one legumain (pH 4). The above endopeptidases destroyed five and six IgE-binding epitopes of Ara h 1 at pH 3 and 4, respectively. Ara h 1 (>95%) and arachin (50-60%) could be hydrolyzed to generate 10-20 kDa and <4 kDa peptides at pH 3, which was enhanced by the pH 3 → 4 incubation. Further, the limited hydrolysis improved the gel-forming ability and in vitro digestibility (approximately 15%) of peanut proteins. Free amino acid analysis showed that the activity of exopeptidases was low at pH 2-5.

Target Enzymes Considered for the Treatment of Alzheimer's Disease and Parkinson's Disease

Various amyloidogenic proteins have been suggested to be involved in the onset and progression of neurodegenerative diseases (ND) such as Alzheimer's disease (AD) and Parkinson's disease (PD). Particularly, the aggregation of misfolded amyloid-β and hyperphosphorylated tau and α-synuclein are linked to the pathogenesis of AD and PD, respectively. In order to care the diseases, multiple small molecules have been developed to regulate the aggregation pathways of these amyloid proteins. In addition to controlling the aggregation of amyloidogenic proteins, maintaining the levels of the proteins in the brain by amyloid degrading enzymes (ADE; neprilysin (NEP), insulin-degrading enzyme (IDE), asparagine endopeptidase (AEP), and ADAM10) is also essential to cure AD and PD. Therefore, numerous biological molecules and chemical agents have been investigated as either inducer or inhibitor against the levels and activities of ADE. Although the side effect of enhancing the activity of ADE could occur, the removal of amyloidogenic proteins could result in a relatively good strategy to treat AD and PD. Furthermore, since the causes of ND are diverse, various multifunctional (multitarget) chemical agents have been designed to control the actions of multiple risk factors of ND, including amyloidogenic proteins, metal ions, and reactive oxygen species. Many of them, however, were invented without considerations of regulating ADE levels and actions. Incorporation of previously created molecules with the chemical agents handling ADE could be a promising way to treat AD and PD. This review introduces the ADE and molecules capable of modulating the activity and expression of ADE.

The potential of cystatin C as a predictive biomarker in breast cancer

INTRODUCTION

Breast cancer (BCa) is the leading cause of cancer-related deaths among women. Numerous efforts are being directed toward identifying novel tissue and/or circulating molecular markers that may help clinicians in detecting early-stage BCa patients and in providing an accurate estimation of the prognosis and prediction of response to clinical treatments. In this setting, emerging evidence has indicated Cystatin C (Cyst C), as the most potent endogenous inhibitor of cysteine cathepsins, as a possible useful marker in the clinical management of BCa patients.

AREAS COVERED

This review analyzes the results of emerging studies underpinning a potential clinical role of Cyst C, as additional marker in BCa.

EXPERT OPINION

Cyst C expression levels have been reported to be altered in tumor tissues and/or in biological fluids of BCa patients. Furthermore, clinical evidence has highlighted a significant correlation between altered Cyst C levels in tumor tissues and/or biological fluids and some clinco-biological parameters of BCa progression. These findings provide evidence for a potential clinical use of Cyst C as a novel marker to improve the clinical and therapeutic management of BCa patients and as a gauge for better clarifying the role of cysteine proteinases in the various steps of BCa progression.

Secreted cystatins decrease proliferation and enhance apoptosis of human leukemic cells

Cysteine proteases are implicated in proteolysis events favoring cancer cell growth, spread, and death by apoptosis. Herein, we have studied whether the net growth and survival of the leukemic cell lines Jurkat, U937, and HL‐60 are affected by external addition of five proteins acting as natural cysteine protease inhibitors. None of the cystatins examined (A, C, D, and E/M) or chagasin showed consistent effects on Fas‐induced apoptosis when evaluated at 1 µm. In contrast, when the intrinsic apoptosis pathway was activated by hydrogen peroxide, addition of cystatin D augmented caspase‐3‐like activity within all three cell lines. Flow cytometric analysis of U937 cells also showed increased numbers of annexin V‐positive cells when hydrogen peroxide was used to initiate apoptosis and cells were cultured in the presence of cystatin D or C. Moreover, stimulation of hydrogen peroxide‐induced apoptotic U937 cells with either cystatin C or D resulted in a dose‐dependent decrease in the number of cells. Cell viability was also decreased when U937 cells were cultured in the presence of cystatin C or D (1–9 µm) only, demonstrating that these cystatins can reduce cell proliferation by themselves in addition to enhancing apoptosis induced by oxidative stress. These effects on U937 cells were paralleled by internalization of cystatins C and D, indicating these effects are caused by downregulation of intracellular proteolysis. External addition of cystatins C and D to HL‐60 and Jurkat cells demonstrated similar degrees of cystatin D uptake and decreased viability as for U937 cells, indicating that these effects are general for leukemic cells.

Structural Characterization of Covalently Stabilized Human Cystatin C Oligomers

Human cystatin C (HCC), a cysteine-protease inhibitor, exists as a folded monomer under physiological conditions but has the ability to self-assemble via domain swapping into multimeric states, including oligomers with a doughnut-like structure. The structure of the monomeric HCC has been solved by X-ray crystallography, and a covalently linked version of HCC (stab-1 HCC) is able to form stable oligomeric species containing 10-12 monomeric subunits. We have performed molecular modeling, and in conjunction with experimental parameters obtained from atomic force microscopy (AFM), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) measurements, we observe that the structures are essentially flat, with a height of about 2 nm, and the distance between the outer edge of the ring and the edge of the central cavity is ~5.1 nm. These dimensions correspond to the height and diameter of one stab-1 HCC subunit and we present a dodecamer model for stabilized cystatin C oligomers using molecular dynamics simulations and experimentally measured parameters. Given that oligomeric species in protein aggregation reactions are often transient and very highly heterogeneous, the structural information presented here on these isolated stab-1 HCC oligomers may be useful to further explore the physiological relevance of different structural species of cystatin C in relation to protein misfolding disease.

Parasite Cystatin: Immunomodulatory Molecule with Therapeutic Activity against Immune Mediated Disorders

The use of parasites or their products for treating chronic inflammation associated diseases (CIADs) has generated significant attention recently. Findings from basic and clinical research have provided valuable information on strengthening the notion that parasites' molecules can be developed as biotherapeutic agents. Completion of the genome, secreotome, and proteome of the parasites has provided an excellent platform for screening and identifying several host immunomodulatory molecules from the parasites and evaluate their therapeutic potential for CIADs. One of the widely studied host immunomodulatory molecules of the parasites is the cysteine protease inhibitor (cystatin), which is primarily secreted by the parasites to evade host immune responses. In this review, we have attempted to summarize the findings to date on the use of helminth parasite-derived cystatin as a therapeutic agent against CIADs. Although several studies suggest a role for alternatively activated macrophages, other regulatory cells, and immunosuppressive molecules, in this immunoregulatory activity of the parasite-derived cystatin, there is still no clear demonstration as to how cystatin induces its anti-inflammatory effect in suppressing CIADs.

Recent advances in the development of legumain-selective chemical probes and peptide prodrugs

Legumain, which is also known as vacuolar processing enzyme (VPE) or asparaginyl endopeptidase (AEP), is a cysteine protease that was first discovered and characterized in the leguminous seeds of the moth bean in the early 1990s. Later, this enzyme was also detected in higher organisms, including eukaryotes. This pH-dependent protease displays the highest activity in acidic endolysosomal compartments; however, legumain also displays nuclear, cytosolic and extracellular activity when stabilized by other proteins or intramolecular complexes. Based on the results from over 25 years of research, this protease is involved in multiple cellular events, including protein degradation and antigen presentation. Moreover, when dysregulated, this protease contributes to the progression of several diseases, with cancer being the well-studied example. Research on legumain biology was undoubtedly facilitated by the use of small molecule chemical tools. Therefore, in this review, I present the historical perspectives and most current strategies for the development of small molecule substrates, inhibitors and activity-based probes for legumain. These tools are of paramount importance in elucidating the roles of legumain in multiple biological processes. Finally, as this enzyme appears to be a promising molecular target for anticancer therapies, the development of legumain-activated prodrugs is also described.

The first characterization of a cystatin and a cathepsin L-like peptidase from Aedes aegypti and their possible role in DENV infection by the modulation of apoptosis

Recently, a salivary gland transcriptome study demonstrated that the transcripts of a putative cystatin gene (SeqID AAEL013287; Aacystatins) from Aedes aegypti were increased in DENV2-infected mosquitoes and that silencing of the Aacystatin gene resulted in an increase in DENV titres. In this work, Aacystatin was biochemically characterized; the purified recombinant inhibitor was able to inhibit typical cysteine proteases with a Ki in the nM range. Pulldown assays using Aag2 cell extracts identified a cathepsin L-like peptidase (AaCatL) as a possible target of Aacystatin. Purified recombinant AaCatL had an optimal pH of 5.0 and displayed a preference for Leu, Val and Phe residues at P2, which is common for other cathepsin L-like peptidases. Transcription analysis of Aacystatin and AaCatL in the salivary glands and midgut of DENV2-infected mosquitoes revealed a negative correlation between DENV2 titres and levels of the inhibitor and peptidase, suggesting their involvement in DENV2-mosquito interactions. Considering that apoptosis may play an important role during viral infections, the possible involvement of Aacystatin in staurosporine-induced apoptosis in Aag2 cells was investigated; the results showed higher expression of the inhibitor in treated cells; moreover, pre incubation with rAacystatin was able to increase Aag2 cell viability.

A novel type 1 cystatin involved in the regulation of Rhipicephalus microplus midgut cysteine proteases

Rhipicephalus microplus is a cattle ectoparasite found in tropical and subtropical regions around the world with great impact on livestock production. R. microplus can also harbor pathogens, such as Babesia sp. and Anaplasma sp. which further compromise cattle production. Blood meal acquisition and digestion are key steps for tick development. In ticks, digestion takes place inside midgut cells and is mediated by aspartic and cysteine peptidases and, therefore, regulated by their inhibitors. Cystatins are a family of cysteine peptidases inhibitors found in several organisms and have been associated in ticks with blood acquisition, blood digestion, modulation of host immune response and tick immunity. In this work, we characterized a novel R. microplus type 1 cystatin, named Rmcystatin-1b. The inhibitor transcripts were found to be highly expressed in the midgut of partially and fully engorged females and they appear to be modulated at different days post-detachment. Purified recombinant Rmcystatin-1b displayed inhibitory activity towards typical cysteine peptidases with high affinity. Moreover, rRmcystatin-1b was able to inhibit native R. microplus cysteine peptidases and RNAi-mediated knockdown of the cystatin transcripts resulted in increased proteolytic activity. Moreover, rRmcystatin-1b was able to interfere with B. bovis growth in vitro. Taken together our data strongly suggest that Rmcystatin-1b is a regulator of blood digestion in R. microplus midgut.

Emerging Alternative Proteinases in APP Metabolism and Alzheimer's Disease Pathogenesis: A Focus on MT1-MMP and MT5-MMP

Processing of amyloid beta precursor protein (APP) into amyloid-beta peptide (Aβ) by β-secretase and γ-secretase complex is at the heart of the pathogenesis of Alzheimer’s disease (AD). Targeting this proteolytic pathway effectively reduces/prevents pathology and cognitive decline in preclinical experimental models of the disease, but therapeutic strategies based on secretase activity modifying drugs have so far failed in clinical trials. Although this may raise some doubts on the relevance of β- and γ-secretases as targets, new APP-cleaving enzymes, including meprin-β, legumain (δ-secretase), rhomboid-like protein-4 (RHBDL4), caspases and membrane-type matrix metalloproteinases (MT-MMPs/η-secretases) have confirmed that APP processing remains a solid mechanism in AD pathophysiology. This review will discuss recent findings on the roles of all these proteinases in the nervous system, and in particular on the roles of MT-MMPs, which are at the crossroads of pathological events involving not only amyloidogenesis, but also inflammation and synaptic dysfunctions. Assessing the potential of these emerging proteinases in the Alzheimer’s field opens up new research prospects to improve our knowledge of fundamental mechanisms of the disease and help us establish new therapeutic strategies.

Characteristics and function of a novel cystatin gene in the pine wood nematode Bursaphelenchus xylophilus

Bursaphelenchus xylophilus is the pathogen that causes pine wilt disease (PWD). The disease has caused significant economic losses and damage to forests. However, the pathogenic mechanism of B. xylophilus remains unclear. Cystatins are involved in various biological processes where they regulate normal proteolysis and also play a role in pathogenicity, but their functions in B. xylophilus are unknown. Therefore, we cloned the full-length cDNA of a cystatin gene of B. xylophilus (Bx-cpi-1) by rapid-amplification of cDNA ends and analyzed its characteristics with bioinformatic methods. In situ mRNA hybridization analyses showed that transcripts of Bx-cpi-1 were abundantly expressed in the reproductive organs of B. xylophilus. The expression of Bx-cpi-1 was investigated using qPCR. Bx-cpi-1 was expressed during each of the different developmental stages of B. xylophilus. The highest gene expression was at the egg stage. After infection of Pinus massoniana, the expression of Bx-cpi-1 increased. The functions of Bx-cpi-1 were verified by RNA interference. The feeding rate, reproduction and pathogenicity of B. xylophilus all decreased as a result of silencing of the Bx-cpi-1 gene. These results revealed that Bx-cpi-1 may be a variant of a type II cystatin gene which is involved in the development and pathogenic process of B. xylophilus.

Summary:Bx-cpi-1, a variant of a type II cystatin gene, was abundantly expressed in the reproductive organs of Bursaphelenchusxylophilus, and involved in the development and pathogenicity of B. xylophilus.

Cystatins in cancer progression: More than just cathepsin inhibitors

Cystatins are endogenous and reversible inhibitors of cysteine peptidases that are important players in cancer progression. Besides their primary role as regulators of cysteine peptidase activity, cystatins are involved in cancer development and progression through proteolysis-independent mechanisms. Mechanistic studies of cystatin function revealed that they affect all stages of cancer progression including tumor growth, apoptosis, invasion, metastasis and angiogenesis. Recently, the involvement of cystatins in the antitumor immune responses was reported. In this review, we discuss molecular mechanisms and clinical aspects of cystatins in cancer. Altered expression of cystatins in cancer resulting in harmful excessive cysteine peptidase activity has been a subject of several studies in order to find correlations with clinical outcome and therapy response. However, involvement in anti-tumor immune response and signaling cascades leading to cancer progression designates cystatins as possible targets for development of new anti-tumor drugs.

Molecular Characterization of a Dirofilaria immitis Cysteine Protease Inhibitor (Cystatin) and Its Possible Role in Filarial Immune Evasion

Infection with canine heartworm (Dirofilaria immitis), spread via mosquito vectors, causes coughing, asthma, pneumonia, and bronchitis in humans and other animals. The disease is especially severe and often fatal in dogs and represents a serious threat to public health worldwide. Cysteine protease inhibitors (CPIs), also known as cystatins, are major immunomodulators of the host immune response during nematode infections. Herein, we cloned and expressed the cystatin Di-CPI from D. immitis. Sequence analysis revealed two specific cystatin-like domains, a Q-x-V-x-G motif, and a SND motif. Phylogenetic analysis indicates that Di-CPI is a member of the second subgroup of nematode type II cystatins. Probing of D. immitis total proteins with anti-rDi-CPI polyclonal antibody revealed a weak signal, and immunofluorescence-based histochemical analysis showed that native Di-CPI is mainly localized in the cuticle of male and female worms and the gut of male worms. Treatment of canine peripheral blood mononuclear cells (PMBCs) with recombinant Di-CPI induced a Th2-type immune response characterized by high expression of the anti-inflammatory factor interleukin-10. Proliferation assays showed that Di-CPI inhibits the proliferation of canine PMBCs by 15%. Together, the results indicate that Di-CPI might be related to cellular hyporesponsiveness in dirofilariasis and may help D. immitis to evade the host immune system.

Mammalian plasma fetuin-B is a selective inhibitor of ovastacin and meprin metalloproteinases

Vertebrate fetuins are multi-domain plasma-proteins of the cystatin-superfamily. Human fetuin-A is also known as AHSG, α₂-Heremans-Schmid-glycoprotein. Gene-knockout in mice identified fetuin-A as essential for calcified-matrix-metabolism and bone-mineralization. Fetuin-B deficient mice, on the other hand, are female infertile due to zona pellucida ‘hardening’ caused by the metalloproteinase ovastacin in unfertilized oocytes. In wildtype mice fetuin-B inhibits the activity of ovastacin thus maintaining oocytes fertilizable. Here we asked, if fetuins affect further proteases as might be expected from their evolutionary relation to single-domain-cystatins, known as proteinase-inhibitors. We show that fetuin-A is not an inhibitor of any tested protease. In stark contrast, the closely related fetuin-B selectively inhibits astacin-metalloproteinases such as meprins and ovastacin, but not astacins of the tolloid-subfamily, nor any other proteinase. The analysis of fetuin-B expressed in various mammalian cell types, insect cells, and truncated fish-fetuin expressed in bacteria, showed that the cystatin-like domains alone are necessary and sufficient for inhibition. This report highlights fetuin-B as a specific antagonist of ovastacin and meprin-metalloproteinases. Control of ovastacin was shown to be indispensable for female fertility. Meprin inhibition, on the other hand, renders fetuin-B a potential key-player in proteolytic networks controlling angiogenesis, immune-defense, extracellular-matrix-assembly and general cell-signaling, with implications for inflammation, fibrosis, neurodegenerative disorders and cancer.

Cystatin F as a regulator of immune cell cytotoxicity

Cysteine cathepsins are lysosomal peptidases involved in the regulation of innate and adaptive immune responses. Among the diverse processes, regulation of granule-dependent cytotoxicity of cytotoxic T-lymphocytes (CTLs) and natural killer (NK) cells during cancer progression has recently gained significant attention. The function of cysteine cathepsins is regulated by endogenous cysteine protease inhibitors-cystatins. Whereas other cystatins are generally cytosolic or extracellular proteins, cystatin F is present in endosomes and lysosomes and is thus able to regulate the activity of its target directly. It is delivered to endosomal/lysosomal vesicles as an inactive, disulphide-linked dimer. Proteolytic cleavage of its N-terminal part leads to the monomer, the only form that is a potent inhibitor of cathepsins C, H and L, involved in the activation of granzymes and perforin. In NK cells and CTLs the levels of active cathepsin C and of granzyme B are dependent on the concentration of monomeric, active cystatin F. In tumour microenvironment, inactive dimeric cystatin F can be secreted from tumour cells or immune cells and further taken up by the cytotoxic cells. Subsequent monomerization and inhibition of cysteine cathepsins within the endosomal/lysosomal vesicles impairs granzyme and perforin activation, and provokes cell anergy. Further, the glycosylation pattern has been shown to be important in controlling secretion of cystatin F from target cells, as well as internalization by cytotoxic cells and trafficking to endosomal/lysosomal vesicles. Cystatin F is therefore an important mediator used by bystander cells to reduce NK and T-cell cytotoxicity.

Characterization of a novel endogenous cysteine proteinase inhibitor, trichocystatin-3 (TC-3), localized on the surface of Trichomonas vaginalis

Trichomonas vaginalis is a flagellated protist responsible for human trichomoniasis. T. vaginalis has three genes encoding for endogenous cysteine proteinase (CP) inhibitors, known as trichocystatin-1 through trichocystatin-3 (TC-1, TC-2, and TC-3). These inhibitors belong to the cystatin family. In this study, we characterized trichocystatin-3 (TC-3), an endogenous cysteine proteinase (CP) inhibitor of T. vaginalis. TC-3 possesses a signal peptide in the N-terminus and two putative glycosylation sites (typical of family 2, cystatins) but lacks the PW motif and cysteine residues (typical of family 1, stefins). Native TC-3 was recognized as an ∼18 kDa protein band in a T. vaginalis protein extract. By confocal microscopy, endogenous TC-3 was found in the Golgi complex, cytoplasm, large vesicles, and the plasma membrane. These localizations are consistent with an in silico prediction. In addition, the purified recombinant protein (TC-3r) functions as an inhibitor of cathepsin L CPs, such as human liver cathepsin L and trichomonad CPs, present in a proteinase-resistant extract (PRE). Via a pull-down assay using TC-3r as bait and PRE, we identified several trichomonad CPs targeted by TC-3, primarily TvCP3. These CP-TC-3 interactions occur in vesicles, in the cytoplasm, and on the parasite surface. In addition, TC-3r showed a protective effect on HeLa cell monolayers by inhibiting trichomonad surface CPs involved in cellular damage. Our results show that the endogenous inhibitor TC-3 plays a key role in the regulation of endogenous CP proteolytic activity.

The transcription factor C/EBP α controls the role of cystatin F during the differentiation of monocytes to macrophages

Cystatin F is an inhibitor of cysteine peptidases expressed solely in immune cells. It is the only type II cystatin able to enter endosomal/lysosomal vesicles and to regulate directly the activity of intracellular cysteine cathepsins. Its expression in promonocytic U937 and promyeloblastic HL-60 cells is highly upregulated but, after differentiation with phorbol 12-myristate 13-acetate - PMA, its levels drop significantly. In contrast, the activities of intracellular cysteine cathepsins C, L and S are higher in differentiated cells than in non-differentiated ones due, presumably, to the lower inhibitory capacity of cystatin F. Using immunofluorescence confocal microscopy, proximity ligation assay and co-immunoprecipitation, cathepsins C, L and S were confirmed to be the main interacting partners of cystatin F in U937 and HL-60 cells. The promoter region of the cystatin F gene, CST7, contains a unique binding site for transcription factor C/EBP α, one of the main myeloid differentiation instructors. Using the chromatin immunoprecipitation assay, C/EBP α was shown to bind to CST7 gene in U937 cells. Following cell differentiation with PMA, the binding of C/EBP α was decreased significantly. The protein level of C/EBP α was also significantly lower in differentiated than in non-differentiated cells. It was shown that, during monocyte to macrophage differentiation, the endosomal/lysosomal proteolytic activity can be regulated by cystatin F whose expression is under the control of transcriptional factor C/EBP α.

Structural and functional analysis of cystatin E reveals enzymologically relevant dimer and amyloid fibril states

Protein activity is often regulated by altering the oligomerization state. One mechanism of multimerization involves domain swapping, wherein proteins exchange parts of their structures and thereby form long-lived dimers or multimers. Domain swapping has been specifically observed in amyloidogenic proteins, for example the cystatin superfamily of cysteine protease inhibitors. Cystatins are twin-headed inhibitors, simultaneously targeting the lysosomal cathepsins and legumain, with important roles in cancer progression and Alzheimer's disease. Although cystatin E is the most potent legumain inhibitor identified so far, nothing is known about its propensity to oligomerize. In this study, we show that conformational destabilization of cystatin E leads to the formation of a domain-swapped dimer with increased conformational stability. This dimer was active as a legumain inhibitor by forming a trimeric complex. By contrast, the binding sites toward papain-like proteases were buried within the cystatin E dimer. We also showed that the dimers could further convert to amyloid fibrils. Unexpectedly, cystatin E amyloid fibrils contained functional protein, which inhibited both legumain and papain-like enzymes. Fibril formation was further regulated by glycosylation. We speculate that cystatin amyloid fibrils might serve as a binding platform to stabilize the pH-sensitive legumain and cathepsins in the extracellular environment, contributing to their physiological and pathological functions.

Papain-like and legumain-like proteases in rice: genome-wide identification, comprehensive gene feature characterization and expression analysis

BACKGROUND

Papain-like and legumain-like proteases are proteolytic enzymes which play key roles in plant development, senescence and defense. The activities of proteases in both families could be inhibited by a group of small proteins called cystatin. Cystatin family genes have been well characterized both in tobacco and rice, suggesting their potential roles in seed development. However, their potential targets, papain-like and legumain-like proteases, have not been well characterized in plants, especially in rice, a model plant for cereal biology.

RESULTS

Here, 33 papain-like and 5 legumain-like proteases have been identified in rice genome, respectively. Gene structure, distribution in rice chromosome, and evolutionary relationship to their counterparts in other plants have been well characterized. Comprehensive expression profile analysis revealed that two family genes display divergent expression pattern, which are regulated temporally and spatially during the process of seed development and germination. Our experiments also revealed that the expression of most genes in these two families is sensitively responsive to plant hormones and different abiotic stresses.

CONCLUSIONS

Genome-wide identification and comprehensive gene expression pattern analysis of papain-like and legumain-like proteases in rice suggests their multiple and cooperative roles in seed development and response to environmental variations, which provides several useful cues for further in-depth study.

Potential skin involvement in ALS: revisiting Charcot's observation - a review of skin abnormalities in ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease affecting motor neurons of the brain and spinal cord, leading to progressive paralysis and death. Interestingly, many skin changes have been reported in ALS patients, but never as yet fully explained. These observations could be due to the common embryonic origin of the skin and neural tissue known as the ectodermal germ layer. Following the first observation in ALS patients' skin by Dr Charcot in the 19th century, in the absence of bedsores unlike other bedridden patients, other morphological and molecular changes have been observed. Thus, the skin could be of interest in the study of ALS and other neurodegenerative diseases. This review summarizes skin changes reported in the literature over the years and discusses about a novel in vitro ALS tissue-engineered skin model, derived from patients, for the study of ALS.

Host Cell Proteases: Cathepsins

Cathepsins are proteolytic enzymes with a broad spectrum of substrates. They are known to reside within endo-lysosomes where they acquire optimal conditions for proteolytic activity and substrate cleavage. However, cathepsins have been detected in locations other than the canonical compartments of the endocytotic pathway. They are often secreted from cells in either proteolytically inactive proform or as mature and active enzyme; this may happen in both physiological and pathological conditions. Moreover, cytosolic and nuclear forms of cathepsins have been described and are currently an emerging field of research aiming at understanding their functions in such unexpected cellular locations. This chapter summarizes the canonical pathways of biosynthesis and transport of cathepsins in healthy cells. We further describe how cathepsins can reach unexpected locations such as the extracellular space or the cytosol and the nuclear matrix. No matter where viruses and cathepsins encounter, several outcomes can be perceived. Thus, scenarios are discussed on how cathepsins may support virus entry into host cells, involve in viral fusion factor and polyprotein processing in different host cell compartments, or help in packaging of viral particles during maturation. It is of note to mention that this review is not meant to comprehensively cover the present literature on viruses encountering cathepsins but rather illustrates, on some representative examples, the possible roles of cathepsins in replication of viruses and in the course of disease.

A novel type I cystatin of parasite origin with atypical legumain-binding domain

Parasite inhibitors of cysteine peptidases are known to influence a vast range of processes linked to a degradation of either the parasites' own proteins or proteins native to their hosts. We characterise a novel type I cystatin (stefin) found in a sanguinivorous fish parasite Eudiplozoon nipponicum (Platyhelminthes: Monogenea). We have identified a transcript of its coding gene in the transcriptome of adult worms. Its amino acid sequence is similar to other stefins except for containing a legumain-binding domain, which is in this type of cystatins rather unusual. As expected, the recombinant form of E. nipponicum stefin (rEnStef) produced in Escherichia coli inhibits clan CA peptidases - cathepsins L and B of the worm - via the standard papain-binding domain. It also blocks haemoglobinolysis by cysteine peptidases in the worm's excretory-secretory products and soluble extracts. Furthermore, we had confirmed its ability to inhibit clan CD asparaginyl endopeptidase (legumain). The presence of a native EnStef in the excretory-secretory products of adult worms, detected by mass spectrometry, suggests that this protein has an important biological function at the host-parasite interface. We discuss the inhibitor's possible role in the regulation of blood digestion, modulation of antigen presentation, and in the regeneration of host tissues.

Cystatin F Affects Natural Killer Cell Cytotoxicity

Cystatin F is a cysteine peptidase inhibitor which, unlike other cystatin family members, is targeted to endosomal/lysosomal compartments. It is synthesized as an inactive disulfide-linked dimer which is then converted to an active monomer by proteolytic cleavage of 15 N-terminal residues. Cystatin F has been suggested to regulate the cytotoxicity of natural killer (NK) cells by inhibiting the major granzyme convertases, cathepsins C and H. To test this hypothesis, we prepared variants of cystatin F and analyzed their uptake, subcellular trafficking, and peptidase inhibition, as well as their impact on the cytotoxicity of NK-92 cells and primary NK cells. The N-glycosylation pattern is responsible for the secretion, uptake, and subcellular sorting of cystatin F in HeLa and Hek293 cells, whereas the legumain binding site had no effect on these processes. Active, N-terminally truncated, monomeric cystatin F can also be internalized by recipient cells and targeted to endo/lysosomes, affecting also cells lacking the activating peptidase. Cystatin F mutants capable of cell internalization and trafficking through the endo/lysosomal pathway significantly decreased cathepsin C and H activities, both in situ, following transfection and in trans, using conditioned media. Further, incubation of IL-2 stimulated NK-92 and primary NK cells with full-length and N-terminally truncated cystatin F mutants led to suppression of their granule-mediated cytotoxicity. This effect was most significant with the N-terminally truncated mutants. These results suggest that cystatin F can be an important mediator within tumor microenvironment affecting the cytotoxicity of NK cells and consequently antitumor immune response.

Multiple legumain isoenzymes in ticks

By searching nucleotide databases for the North American Lyme disease vector, Ixodes scapularis, we have complemented the previously characterized European Ixodes ricinus legumain IrAE1 with a full set of nine analogous genes (isae1-9). Six of these were PCR confirmed as genes present in all tick genomes tested. The absolute mRNA copy number examined by quantitative (q)PCR enabled expression profiling and an absolute comparison of mRNA levels for individual I. scapularis (Is)AEs in tick tissues. Four IsAEs (1, 2, 4, 9) were expressed solely in the gut and thus are proposed to be involved in host blood digestion. Expression qPCR profiling over developmental stages confirmed IsAE1, the direct analogue of previously characterized I. ricinus IrAE1, as the principle legumain transcript in partially engorged females, and demonstrated its strong regulation by on-host feeding in larvae, nymphs and females. In contrast, IsAE2 was the predominant gut legumain in unfed nymphs, unfed females and males. In-silico, IsAE1 and IsAE2 protein three-dimensional structural models displayed minimal differences in overall proenzyme structures, even in comparison with recently resolved crystal structures of mammalian prolegumain. Three functional studies were performed in I. ricinus with IsAE1/IsAE2 analogues: double IrAE1/IrAE2 RNA interference silencing, feeding of ticks on IrAE1+IrAE2 immunized hosts and in vitro membrane tick feeding on blood containing a legumain-specific inhibitor. The latter experiment led to reduced weights of fully engorged ticks and limited oviposition, and indicated the potential of legumain inhibitors for novel anti-tick interventions.