Mammalian legumain – A lysosomal cysteine protease with extracellular functions?

A TrkB cleavage fragment in hippocampus promotes Depressive-Like behavior in mice

Decreased hippocampal tropomyosin receptor kinase B (TrkB) level is implicated in the pathophysiology of stress-induced mood disorder and cognitive decline. However, how TrkB is modified and mediates behavioral responses to chronic stress remains largely unknown. Here the effects and mechanisms of TrkB cleavage by asparagine endopeptidase (AEP) were examined on a preclinical murine model of chronic restraint stress (CRS)-induced depression. CRS activated IL-1β-C/EBPβ-AEP pathway in mice hippocampus, accompanied by elevated TrkB 1-486 fragment generated by AEP. Specifi.c overexpression or suppression of AEP-TrkB axis in hippocampal CaMKIIα-positive cells aggravated or relieved depressive-like behaviors, respectively. Mechanistically, in addition to facilitating AMPARs internalization, TrkB 1-486 interacted with peroxisome proliferator-activated receptor-δ (PPAR-δ) and sequestered it in cytoplasm, repressing PPAR-δ-mediated transactivation and mitochondrial function. Moreover, co-administration of 7,8-dihydroxyflavone and a peptide disrupting the binding of TrkB 1-486 with PPAR-δ attenuated depression-like symptoms not only in CRS animals, but also in Alzheimer's disease and aged mice. These findings reveal a novel role for TrkB cleavage in promoting depressive-like phenotype.

Ion Mobility-Based Enrichment-Free N-Terminomics Analysis Reveals Novel Legumain Substrates in Murine Spleen

Aberrant levels of the asparaginyl endopeptidase legumain have been linked to inflammation, neurodegeneration, and cancer, yet our understanding of this protease is incomplete. Systematic attempts to identify legumain substrates have been previously confined to in vitro studies, which fail to mirror physiological conditions and obscure biologically relevant cleavage events. Using high-field asymmetric waveform ion mobility spectrometry (FAIMS), we developed a streamlined approach for proteome and N-terminome analyses without the need for N-termini enrichment. Compared to unfractionated proteomic analysis, we demonstrate FAIMS fractionation improves N-termini identification by >2.5 fold, resulting in the identification of >2882 unique N-termini from limited sample amounts. In murine spleens, this approach identifies 6366 proteins and 2528 unique N-termini, with 235 cleavage events enriched in WT compared to legumain-deficient spleens. Among these, 119 neo-N-termini arose from asparaginyl endopeptidase activities, representing novel putative physiological legumain substrates. The direct cleavage of selected substrates by legumain was confirmed using in vitro assays, providing support for the existence of physiologically relevant extra-lysosomal legumain activity. Combined, these data shed critical light on the functions of legumain and demonstrate the utility of FAIMS as an accessible method to improve depth and quality of N-terminomics studies.

Trichomonas vaginalis Legumain-2, TvLEGU-2, Is an Immunogenic Cysteine Peptidase Expressed during Trichomonal Infection

Trichomonas vaginalis is the causative agent of trichomoniasis, the most prevalent nonviral, neglected sexually transmitted disease worldwide. T. vaginalis has one of the largest degradomes among unicellular parasites. Cysteine peptidases (CPs) are the most abundant peptidases, constituting 50% of the degradome. Some CPs are virulence factors recognized by antibodies in trichomoniasis patient sera, and a few are found in vaginal secretions that show fluctuations in glucose concentrations during infection. The CPs of clan CD in T. vaginalis include 10 genes encoding legumain-like peptidases of the C13 family. TvLEGU-2 is one of them and has been identified in multiple proteomes, including the immunoproteome obtained with Tv (+) patient sera. Thus, our goals were to assess the effect of glucose on TvLEGU-2 expression, localization, and in vitro secretion and determine whether TvLEGU-2 is expressed during trichomonal infection. We performed qRT-PCR assays using parasites grown under different glucose conditions. We also generated a specific anti-TvLEGU-2 antibody against a synthetic peptide of the most divergent region of this CP and used it in Western blot (WB) and immunolocalization assays. Additionally, we cloned and expressed the tvlegu-2 gene (TVAG_385340), purified the recombinant TvLEGU-2 protein, and used it as an antigen for immunogenicity assays to test human sera from patients with vaginitis. Our results show that glucose does not affect tvlegu-2 expression but does affect localization in different parasite organelles, such as the plasma membrane, Golgi complex, hydrogenosomes, lysosomes, and secretion vesicles. TvLEGU-2 is secreted in vitro, is present in vaginal secretions, and is immunogenic in sera from Tv (+) patients, suggesting its relevance during trichomonal infection.

Weighted gene co-expression network analysis and machine learning identified the lipid metabolism-related gene LGMN as a novel biomarker for keloid

The aetiology of keloid formation remains unclear, and existing treatment modalities have not definitively established a successful approach. Therefore, it is necessary to identify reliable and novel keloid biomarkers as potential targets for therapeutic interventions. In this study, we performed differential expression analysis and functional enrichment analysis on the keloid related datasets, and found that multiple metabolism-related pathways were associated with keloid formation. Subsequently, the differentially expressed genes (DEGs) were intersected with the results of weighted gene co-expression network analysis (WGCNA) and the lipid metabolism-related genes (LMGs). Then, three learning machine algorithms (SVM-RFE, LASSO and Random Forest) together identified legumain (LGMN) as the most critical LMGs. LGMN was overexpressed in keloid and had a high diagnostic performance. The protein-protein interaction (PPI) network related to LGMN was constructed by GeneMANIA database. Functional analysis of indicated PPI network was involved in multiple immune response-related biological processes. Furthermore, immune infiltration analysis was conducted using the CIBERSORT method. M2-type macrophages were highly infiltrated in keloid tissues and were found to be significantly and positively correlated with LGMN expression. Gene set variation analysis (GSVA) indicated that LGMN may be related to promoting fibroblast proliferation and inhibiting their apoptosis. Moreover, eight potential drug candidates for keloid treatment were predicted by the DSigDB database. Western blot, qRT-PCR and immunohistochemistry staining results confirmed that LGMN was highly expressed in keloid. Collectively, our findings may identify a new biomarker and therapeutic target for keloid and contribute to the understanding of the potential pathogenesis of keloid.

Asparagine endopeptidase protects podocytes in adriamycin-induced nephropathy by regulating actin dynamics through cleaving transgelin

Focal segmental glomerulosclerosis (FSGS) is the most common glomerular disorder causing end-stage renal diseases worldwide. Central to the pathogenesis of FSGS is podocyte dysfunction, which is induced by diverse insults. However, the mechanism governing podocyte injury and repair remains largely unexplored. Asparagine endopeptidase (AEP), a lysosomal protease, regulates substrates by residue-specific cleavage or degradation. We identified the increased AEP expression in the primary proteinuria model which was induced by adriamycin (ADR) to mimic human FSGS. In vivo, global AEP knockout mice manifested increased injury-susceptibility of podocytes in ADR-induced nephropathy (ADRN). Podocyte-specific AEP knockout mice exhibited much more severe glomerular lesions and podocyte injury after ADR injection. In contrast, podocyte-specific augmentation of AEP in mice protected against ADRN. In vitro, knockdown and overexpression of AEP in human podocytes revealed the cytoprotection of AEP as a cytoskeleton regulator. Furthermore, transgelin, an actin-binding protein regulating actin dynamics, was cleaved by AEP, and, as a result, removed its actin-binding regulatory domain. The truncated transgelin regulated podocyte actin dynamics and repressed podocyte hypermotility, compared to the native full-length transgelin. Together, our data reveal a link between lysosomal protease AEP and podocyte cytoskeletal homeostasis, which suggests a potential therapeutic role for AEP in proteinuria disease.

Dual-Targeting PET Tracers Enable Enzyme-Mediated Self-Assembly for the PET Imaging of Legumain Activity

Legumain, a lysosomal cysteine protease overexpressed in a variety of tumors, has been considered a promising biomarker for various cancers. Precise detection of legumain activity in the lysosome represents an important strategy for early diagnosis and prognosis of tumors. Small-molecule probes with the property of target-enabled self-assembly hold great potential for molecular imaging. In this study, we reported two dual-targeting radiotracers ([18F]SF-AAN-M and [18F]SF-AAN-HEM) with a property of legumain-mediated self-assembly for positron emission tomography (PET) imaging. Both the radiotracers were synthesized with high labeling yield (>50%) and the radiochemical purity was over 99% via one-step straightforward 18F-labeling. Both tracers were efficiently activated by the reducing agent and legumain to self-assemble into aggregates and showed enhanced retention in legumain-overexpressed MDA-MB-468 cells and tumors, indicating that the introduction of lysosome-targeting morpholine increased the tumor uptake and extended the retention of radiotracers in legumain-overexpressed tumors. In addition, [18F]SF-AAN-HEM with a hydrophilic (histidine-glutamate)3 tag displayed significantly reduced liver uptake with no conspicuous reduction in tumor uptake, affording high signal-to-noise ratios (tumor/liver and tumor/muscle). All of these results suggest that dual-targeting tracer [18F]SF-AAN-HEM could provide a promising tool for in vivo monitoring legumain activity in tumors.

Monocyte-derived macrophages orchestrate multiple cell-type interactions to repair necrotic liver lesions in disease models

The liver can fully regenerate after partial resection, and its underlying mechanisms have been extensively studied. The liver can also rapidly regenerate after injury, with most studies focusing on hepatocyte proliferation; however, how hepatic necrotic lesions during acute or chronic liver diseases are eliminated and repaired remains obscure. Here, we demonstrate that monocyte-derived macrophages (MoMFs) were rapidly recruited to and encapsulated necrotic areas during immune-mediated liver injury and that this feature was essential in repairing necrotic lesions. At the early stage of injury, infiltrating MoMFs activated the Jagged1/notch homolog protein 2 (JAG1/NOTCH2) axis to induce cell death-resistant SRY-box transcription factor 9+ (SOX9+) hepatocytes near the necrotic lesions, which acted as a barrier from further injury. Subsequently, necrotic environment (hypoxia and dead cells) induced a cluster of complement 1q-positive (C1q+) MoMFs that promoted necrotic removal and liver repair, while Pdgfb+ MoMFs activated hepatic stellate cells (HSCs) to express α-smooth muscle actin and induce a strong contraction signal (YAP, pMLC) to squeeze and finally eliminate the necrotic lesions. In conclusion, MoMFs play a key role in repairing the necrotic lesions, not only by removing necrotic tissues, but also by inducing cell death-resistant hepatocytes to form a perinecrotic capsule and by activating α-smooth muscle actin-expressing HSCs to facilitate necrotic lesion resolution.

Characterization of proteases in the seminal plasma and spermatozoa of llama

Camelids' semen has peculiar characteristics that differentiate it from other species, including the highly viscous aspect of seminal plasma that greatly difficult sperm manipulation and the development of techniques such as cryopreservation, artificial insemination, and/or in vitro fertilization. The presence of proteases in the seminal plasma is responsible for semen liquefaction, and sperm functionality to achieve fertilization. The enzymatic and molecular composition of the semen of llama remains unknown. Therefore, the goal of the study was to characterize the protease activity and composition of the seminal plasma and sperm of llama semen. The proteolytic activity was performed using gelatine zymography and the composition by mass-spectrometry. Metallo-proteases were the major source of gelatinolytic activity in seminal plasma, while serine-peptidases were the main enzymes of sperm cells. Matrix Metalloproteinase 2 (MMP2) was the most prominent metallo-protease of llama seminal plasma characterized under the exposure of different inhibitors (EDTA and benzamidine) and by a specific immunodetection. Moreover, the prostate and epididymis were identified as potential sites of its synthesis and secretion. Outstandingly, this metalloproteinase was undetectable in llama sperm. Regarding, the molecular composition of semen by mass-spectrometry, 4 metallo-, 9 serine-, 8 threonine-, and 1 aspartic-peptidases were identified alongside 15 regulators in the sperm cell; where 24 were directly or indirectly interacting. Whereas 6 metallo-, 12 serine-, 3 cysteine-, and 1 aspartic-peptidases were identified, besides 7 inhibitors and 5 regulators in llama seminal plasma where 30 of them were directly or indirectly interconnected. This is the first study describing a partial degradome of llama seminal plasma and spermatozoa suggesting significant differences especially the absence of MMP2 in spermatozoa in contrast to data observed in other species. The characterization of proteases in llama semen will provide a better understanding of the molecular mechanisms involved in the in vivo or in vitro fertilization process in this species.

Structural and functional studies of legumain-mycocypin complexes revealed a competitive, exosite-regulated mode of interaction

Under pathophysiologic conditions such as Alzheimer’s disease and cancer, the endolysosomal cysteine protease legumain was found to translocate to the cytosol, the nucleus, and the extracellular space. These noncanonical localizations demand for a tight regulation of legumain activity, which is in part conferred by protein inhibitors. While there is a significant body of knowledge on the interaction of human legumain with endogenous cystatins, only little is known on its regulation by fungal mycocypins. Mycocypins are characterized by (i) versatile, plastic surface loops allowing them to inhibit different classes of enzymes and (ii) a high resistance toward extremes of pH and temperature. These properties make mycocypins attractive starting points for biotechnological and medical applications. In this study, we show that mycocypins utilize an adaptable reactive center loop to target the active site of legumain in a substrate-like manner. The interaction was further stabilized by variable, isoform-specific exosites, converting the substrate recognition into inhibition. Additionally, we found that selected mycocypins were capable of covalent complex formation with legumain by forming a disulfide bond to the active site cysteine. Furthermore, our inhibition studies with other clan CD proteases suggested that mycocypins may serve as broad-spectrum inhibitors of clan CD proteases. Our studies uncovered the potential of mycocypins as a new scaffold for drug development, providing the basis for the design of specific legumain inhibitors.

Active legumain promotes invasion and migration of neuroblastoma by regulating epithelial-mesenchymal transition

Neuroblastoma (NB) is a commonly occurring malignancy in children. Epithelial-mesenchymal transition (EMT) is an adaptive change in promoting tumor metastasis. As an important factor in regulating tumor metastasis, whether legumain could promote metastasis of NB by EMT is still unexplored. Legumain is the active form of prolegumain, abundant in tumor plasma. So in the current study, different forms of legumain were identified in NB. Second, correlation analysis of N-cadherin and active legumain was identified by western blot analysis. Third, legumain gene amplification or gene knockdown were proceeded to examine the effect of legumain on EMT by scratch and transwell assay; meanwhile, active mature legumain or its asparagine endopeptidase (AEP) inhibitor was also added in. Finally, legumain can be detected differently in NB cells. Changes in legumain could influence NB metastasis by regulating EMT markers (e.g., N-cadherin, vimentin, and slug). Besides, the effect of legumain on EMT by its AEP activity was proved by intervention experiment of AEP gene transfection and gene knockdown experiments or adding recombinant human legumain suspension or specific inhibitor of AEP in NB cells (p < 0.05). These results suggest that legumain can promote invasion and migration of NB by regulating EMT, and EMT of NB is regulated by AEP activity of legumain, which can be inhibited by a specific AEP inhibitor.

An infection-induced oxidation site regulates legumain processing and tumor growth

Oxidative stress is a defining feature of most cancers, including those that stem from carcinogenic infections¹. Reactive oxygen species (ROS) can drive tumor formation^2–4, yet the molecular oxidation events that contribute to tumorigenesis are largely unknown. Here we show that inactivation of a single, redox-sensitive cysteine in the host protease legumain, which is oxidized during infection with the gastric cancer-causing bacterium Helicobacter pylori, accelerates tumor growth. By using chemical proteomics to map cysteine reactivity in human gastric cells, we determined that H. pylori infection induces oxidation of legumain at Cys219. Legumain oxidation dysregulates intracellular legumain processing and decreases the activity of the enzyme in H. pylori-infected cells. We further show that the site-specific loss of Cys219 reactivity increases tumor growth and mortality in a xenograft model. Our findings establish a link between an infection-induced oxidation site and tumorigenesis while underscoring the importance of cysteine reactivity in tumor growth.

Asparaginyl endopeptidase protects against podocyte injury in diabetic nephropathy through cleaving cofilin-1

Podocyte injury and loss are critical events in diabetic nephropathy (DN); however, the underlying molecular mechanisms remain unclear. Here, we demonstrate that asparaginyl endopeptidase (AEP) protects against podocyte injury through modulating the dynamics of the cytoskeleton. AEP was highly upregulated in diabetic glomeruli and hyperglycemic stimuli treated-podocytes; however, AEP gene knockout and its compound inhibitor treatment accelerated DN in streptozotocin-induced diabetic mice, whereas specific induction of AEP in glomerular cells attenuated podocyte injury and renal function deterioration. In vitro, elevated AEP was involved in actin cytoskeleton maintenance and anti-apoptosis effects. Mechanistically, we found that AEP directly cleaved the actin-binding protein cofilin-1 after the asparagine 138 (N138) site. The protein levels of endogenous cofilin-1 1-138 fragments were upregulated in diabetic podocytes, consistent with the changes in AEP levels. Importantly, we found that cofilin-1 1-138 fragments were remarkably unphosphorylated than full-length cofilin-1, indicating the enhanced cytoskeleton maintenance activity of cofilin-1 1-138. Then we validated cofilin-1 1-138 could rescue podocytes from cytoskeleton disarrangement and injury in diabetic conditions. Taken together, our data suggest a protective role of elevated AEP in podocyte injury during DN progression through cleaving cofilin-1 to maintain podocyte cytoskeleton dynamics and defend damage.

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.

Bifunctional Therapeutic Peptide Assembled Nanoparticles Exerting Improved Activities of Tumor Vessel Normalization and Immune Checkpoint Inhibition

The effectiveness of cancer immunotherapy is impaired by the dysfunctional vasculature of tumors. Created hypoxia zones and limited delivery of cytotoxic immune cells help to have immune resistance in tumor tissue. Structural and functional normalization of abnormal tumor vasculature provide vessels for more perfusion efficiency and drug delivery that result in alleviating the hypoxia in the tumor site and increasing infiltration of antitumor T cells. Taking advantage of peptide amphiphiles, herein, a novel peptide amphiphile nanoparticle composed of an antiangiogenic peptide (FSEC) and an immune checkpoint blocking peptide (^D PPA) is designed and characterized. FSEC peptide is known to be involved in vessel normalization of tumors in vivo. ^D PPA is an inhibitory peptide of the PD-1/PD-L1 immune checkpoint pathway. The peptide amphiphile nanoparticle sets out to test whether simultaneous modulation of tumor vasculature and immune systems in the tumor microenvironment has a synergistic effect on tumor suppression. Increased intratumoral infiltration of immune cells following vascular normalization, and simultaneously blocking the immune checkpoint function of PD-L1 reactivates effective immune responses to the tumors. In summary, the current study provides a new perspective on the regulation of tumor vessel normalization and immunotherapy based on functional peptide nanoparticles as nanomedicine for improved therapeutic purposes.

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.

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.