Proteinases participating in the processing and activation of prolegumain in primary cultured rat macrophages

Legumain is a predictor of all-cause mortality and potential therapeutic target in acute myocardial infarction

The prognostic impact of extracellular matrix (ECM) modulation and its regulatory mechanism post-acute myocardial infarction (AMI), require further clarification. Herein, we explore the predictive role of legumain—which showed the ability in ECM degradation—in an AMI patient cohort and investigate the underlying mechanisms. A total of 212 AMI patients and 323 healthy controls were enrolled in the study. Moreover, AMI was induced in mice by permanent ligation of the left anterior descending artery and fibroblasts were adopted for mechanism analysis. Based on the cut-off value for the receiver-operating characteristics curve, AMI patients were stratified into low (n = 168) and high (n = 44) plasma legumain concentration (PLG) groups. However, PLG was significantly higher in AMI patients than that in the healthy controls (median 5.9 μg/L [interquartile range: 4.2–9.3 μg/L] vs. median 4.4 μg/L [interquartile range: 3.2–6.1 μg/L], P < 0.001). All-cause mortality was significantly higher in the high PLG group compared to that in the low PLG group (median follow-up period, 39.2 months; 31.8% vs. 12.5%; P = 0.002). Multivariate Cox regression analysis showed that high PLG was associated with increased all-cause mortality after adjusting for clinical confounders (HR = 3.1, 95% confidence interval (CI) = 1.4–7.0, P = 0.005). In accordance with the clinical observations, legumain concentration was also increased in peripheral blood, and infarcted cardiac tissue from experimental AMI mice. Pharmacological blockade of legumain with RR-11a, improved cardiac function, decreased cardiac rupture rate, and attenuated left chamber dilation and wall thinning post-AMI. Hence, plasma legumain concentration is of prognostic value in AMI patients. Moreover, legumain aggravates cardiac remodelling through promoting ECM degradation which occurs, at least partially, via activation of the MMP-2 pathway.

Counter Selection Substrate Library Strategy for Developing Specific Protease Substrates and Probes

Legumain (AEP) is a lysosomal cysteine protease that was first characterized in leguminous seeds and later discovered in higher eukaryotes. AEP upregulation is linked to a number of diseases including inflammation, arteriosclerosis, and tumorigenesis. Thus this protease is an excellent molecular target for the development of new chemical markers. We deployed a hybrid combinatorial substrate library (HyCoSuL) approach to obtain P1-Asp fluorogenic substrates and biotin-labeled inhibitors that targeted legumain. Since this approach led to probes that were also recognized by caspases, we introduced a Counter Selection Substrate Library (CoSeSuL) approach that biases the peptidic scaffold against caspases, thus delivering highly selective legumain probes. The selectivity of these tools was validated using M38L and HEK293 cells. We also propose that the CoSeSuL methodology can be considered as a general principle in the design of selective probes for other protease families where selectivity is difficult to achieve by conventional sequence-based profiling.

The asparaginyl endopeptidase legumain is essential for functional recovery after spinal cord injury in adult zebrafish

Unlike mammals, adult zebrafish are capable of regenerating severed axons and regaining locomotor function after spinal cord injury. A key factor for this regenerative capacity is the innate ability of neurons to re-express growth-associated genes and regrow their axons after injury in a permissive environment. By microarray analysis, we have previously shown that the expression of legumain (also known as asparaginyl endopeptidase) is upregulated after complete transection of the spinal cord. In situ hybridization showed upregulation of legumain expression in neurons of regenerative nuclei during the phase of axon regrowth/sprouting after spinal cord injury. Upregulation of Legumain protein expression was confirmed by immunohistochemistry. Interestingly, upregulation of legumain expression was also observed in macrophages/microglia and neurons in the spinal cord caudal to the lesion site after injury. The role of legumain in locomotor function after spinal cord injury was tested by reducing Legumain expression by application of anti-sense morpholino oligonucleotides. Using two independent anti-sense morpholinos, locomotor recovery and axonal regrowth were impaired when compared with a standard control morpholino. We conclude that upregulation of legumain expression after spinal cord injury in the adult zebrafish is an essential component of the capacity of injured neurons to regrow their axons. Another feature contributing to functional recovery implicates upregulation of legumain expression in the spinal cord caudal to the injury site. In conclusion, we established for the first time a function for an unusual protease, the asparaginyl endopeptidase, in the nervous system. This study is also the first to demonstrate the importance of legumain for repair of an injured adult central nervous system of a spontaneously regenerating vertebrate and is expected to yield insights into its potential in nervous system regeneration in mammals.

Nuclear legumain activity in colorectal cancer

The cysteine protease legumain is involved in several biological and pathological processes, and the protease has been found over-expressed and associated with an invasive and metastatic phenotype in a number of solid tumors. Consequently, legumain has been proposed as a prognostic marker for certain cancers, and a potential therapeutic target. Nevertheless, details on how legumain advances malignant progression along with regulation of its proteolytic activity are unclear. In the present work, legumain expression was examined in colorectal cancer cell lines. Substantial differences in amounts of pro- and active legumain forms, along with distinct intracellular distribution patterns, were observed in HCT116 and SW620 cells and corresponding subcutaneous xenografts. Legumain is thought to be located and processed towards its active form primarily in the endo-lysosomes; however, the subcellular distribution remains largely unexplored. By analyzing subcellular fractions, a proteolytically active form of legumain was found in the nucleus of both cell lines, in addition to the canonical endo-lysosomal residency. In situ analyses of legumain expression and activity confirmed the endo-lysosomal and nuclear localizations in cultured cells and, importantly, also in sections from xenografts and biopsies from colorectal cancer patients. In the HCT116 and SW620 cell lines nuclear legumain was found to make up approximately 13% and 17% of the total legumain, respectively. In similarity with previous studies on nuclear variants of related cysteine proteases, legumain was shown to process histone H3.1. The discovery of nuclear localized legumain launches an entirely novel arena of legumain biology and functions in cancer.

Intra- and extracellular regulation of activity and processing of legumain by cystatin E/M

Legumain, an asparaginyl endopeptidase, is up-regulated in tumour and tumour-associated cells, and is linked to the processing of cathepsin B, L, and proMMP-2. Although legumain is mainly localized to the endosomal/lysosomal compartments, legumain has been reported to be localized extracellularly in the tumour microenvironment and associated with extracellular matrix and cell surfaces. The most potent endogenous inhibitor of legumain is cystatin E/M, which is a secreted protein synthesised with an export signal. Therefore, we investigated the cellular interplay between legumain and cystatin E/M. As a cell model, HEK293 cells were transfected with legumain cDNA, cystatin E/M cDNA, or both, and over-expressing monoclonal cell lines were selected (termed M38L, M4C, and M3CL, respectively). Secretion of prolegumain from M38L cells was inhibited by treatment with brefeldin A, whereas bafilomycin A1 enhanced the secretion. Cellular processing of prolegumain to the 46 and 36 kDa enzymatically active forms was reduced by treatment with either substance alone. M38L cells showed increased, but M4C cells decreased, cathepsin L processing suggesting a crucial involvement of legumain activity. Furthermore, we observed internalization of cystatin E/M and subsequently decreased intracellular legumain activity. Also, prolegumain was shown to internalize followed by increased intracellular legumain processing and activation. In addition, in M4C cells incomplete processing of the internalized prolegumain was observed, as well as nuclear localized cystatin E/M. Furthermore, auto-activation of secreted prolegumain was inhibited by cystatin E/M, which for the first time shows a regulatory role of cystatin E/M in controlling both intra- and extracellular legumain activity.

Autophagic activity measured in whole rat hepatocytes as the accumulation of a novel BHMT fragment (p10), generated in amphisomes by the asparaginyl proteinase, legumain

To investigate the stepwise autophagic-lysosomal processing of hepatocellular proteins, the abundant cytosolic enzyme, betaine:homocysteine methyltransferase (BHMT) was used as a probe. Full-length (45 kDa) endogenous BHMT was found to be cleaved in an autophagy-dependent (3-methyladenine-sensitive) manner in isolated rat hepatocytes to generate a novel N-terminal 10-kDa fragment (p10) identified and characterized by mass spectrometry. The cleavage site was consistent with cleavage by the asparaginyl proteinase, legumain and indeed a specific inhibitor of this enzyme (AJN-230) was able to completely suppress p10 formation in intact cells, causing instead accumulation of a 42-kDa intermediate. To prevent further degradation of p10 or p42 by the cysteine proteinases present in autophagic vacuoles, the proteinase inhibitor leupeptin had to be present. Asparagine, an inhibitor of amphisome-lysosome fusion, did not detectably impede either p42 or p10 formation, indicating that BHMT processing primarily takes place in amphisomes rather than in lysosomes. Lactate dehydrogenase (LDH) was similarly degraded primarily in amphisomes by leupeptin-sensitive proteolysis, but some additional leupeptin-resistant LDH degradation in lysosomes was also indicated. The autophagic sequestration of BHMT appeared to be nonselective, as the accumulation of p10 (in the presence of leupeptin) or of its precursors (in the additional presence of AJN-230) proceeded at approximately the same rate as the model autophagic cargo, LDH. The complete lack of a cytosolic background makes p10 suitable for use in a "fragment assay" of autophagic activity in whole cells. Incubation of hepatocytes with ammonium chloride, which neutralizes amphisomes as well as lysosomes, caused rapid, irreversible inhibition of legumain activity and stopped all p10 formation. The availability of several methods for selective targeting of legumain in intact cells may facilitate functional studies of this enigmatic enzyme, and perhaps suggest novel ways to reduce its contribution to cancer cell metastasis or autoimmune disease.

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

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