Involvement of cathepsin E in exogenous antigen processing in primary cultured murine microglia

A new approach for distinguishing cathepsin E and D activity in antigen-processing organelles

Cathepsin E (CatE) and D (CatD) are the major aspartic proteinases in the endolysosomal pathway. They have similar specificity and therefore it is difficult to distinguish between them, as known substrates are not exclusively specific for one or the other. In this paper we present a substrate-based assay, which is highly relevant for immunological investigations because it detects both CatE and CatD in antigen-processing organelles. Therefore it could be used to study the involvement of these proteinases in protein degradation and the processing of invariant chain. An assay combining a new monospecific CatE antibody and the substrate, MOCAc-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys(Dnp)-D-Arg-NH2[where MOCAc is (7-methoxycoumarin-4-yl)acetyl and Dnp is dinitrophenyl], is presented. This substrate is digested by both proteinases and therefore can be used to detect total aspartic proteinase activity in biological samples. After depletion of CatE by immunoprecipitation, the remaining activity is due to CatD, and the decrease in activity can be assigned to CatE. The activity of CatE and CatD in cytosolic, endosomal and lysosomal fractions of B cells, dendritic cells and human keratinocytes was determined. The data clearly indicate that CatE activity is mainly located in endosomal compartments, and that of CatD in lysosomal compartments. Hence this assay can also be used to characterize subcellular fractions using CatE as an endosomal marker, whereas CatD is a well-known lysosomal marker. The highest total aspartic proteinase activity was detected in dendritic cells, and the lowest in B cells. The assay presented exhibits a lower detection limit than common antibody-based methods without lacking the specificity.

Interferon-gamma regulates cathepsin G activity in microglia-derived lysosomes and controls the proteolytic processing of myelin basic protein in vitro

The serine protease cathepsin (Cat) G dominates the proteolytic processing of the multiple sclerosis (MS)-associated autoantigen myelin basic protein (MBP) in lysosomes from primary human B cells and dendritic cells. This is in contrast to B-lymphoblastoid cell lines, where the asparagine endopeptidase (AEP) is responsible for this task. We have analysed microglia-derived lysosomal proteases for their ability to process MBP in vitro. In lysosomes derived from primary murine microglia, CatD, CatS, AEP and CatG were involved in the processing of MBP. Interestingly, when microglia were treated with interferon-gamma to mimic a T helper type 1-biased cytokine milieu in MS, CatG was drastically down-regulated, in contrast to CatS, CatB, CatL, CatD or AEP. This resulted in significantly increased stability of MBP and a selective lack of CatG-derived proteolytic fragments; however, it did not affect the gross pattern of MBP processing. Inhibition of serine proteases eliminated the processing differences between lysosomal extracts from resting microglia compared to interferon-stimulated microglia. Thus, the cytokine environment modulates lysosomal proteases in microglia by a selective down-regulation of CatG, leading to decreased MBP-processing by microglia-derived lysosomal proteases in vitro.

Selection-by-function: efficient enrichment of cathepsin E inhibitors from a DNA library

A method for efficient enrichment of protease inhibitors out of a DNA library was developed by introducing SF-link technology. A two-step selection strategy was designed consisting of the initial enrichment of aptamers based on binding function while the second enrichment step was based on the inhibitory activity to a protease, cathepsin E (CE). The latter was constructed by covalently linking of a biotinylated peptide substrate to each of the ssDNA molecule contained in the preliminarily selected DNA library, generating 'SF-link'. Gradual enrichment of inhibitory DNAs was attained in the course of selection. One molecule, SFR-6-3, showed an IC(50) of around 30 nM, a K(d) of around 15 nM and high selectivity for CE. Sequence and structure analysis revealed a C-rich sequence without any guanine and possibly an i-motif structure, which must be novel to be found in in vitro-selected aptamers. SF-link technology, which is novel as the screening technology, provided a remarkable enrichment of specific protease inhibitors and has a potential to be further developed.

Cathepsin E Deficiency Induces a Novel Form of Lysosomal Storage Disorder Showing the Accumulation of Lysosomal Membrane Sialoglycoproteins and the Elevation of Lysosomal pH in Macrophages

Cathepsin E, an endolysosomal aspartic proteinase predominantly expressed in cells of the immune system, has an important role in immune responses. However, little is known about the precise roles of cathepsin E in this system. Here we report that cathepsin E deficiency (CatE(-/-)) leads to a novel form of lysosome storage disorder in macrophages, exhibiting the accumulation of the two major lysosomal membrane sialoglycoproteins LAMP-1 and LAMP-2 and the elevation of lysosomal pH. These striking features were also found in wild-type macrophages treated with pepstatin A and Ascaris inhibitor. Whereas there were no obvious differences in their expression, biosynthesis, and trafficking between wild-type and CatE(-/-) macrophages, the degradation rates of these two membrane proteins were apparently decreased as a result of cathepsin E deficiency. Because there was no difference in the vacuolar-type H(+)-ATPase activity in both cell types, the elevated lysosomal pH in CatE(-/-) macrophages is most likely due to the accumulation of these lysosomal membrane glycoproteins highly modified with acidic monosaccharides, thereby leading to the disruption of non-proton factors controlling lysosomal pH. Furthermore, the selective degradation of LAMP-1 and LAMP-2, as well as LIMP-2, was also observed by treatment of the lysosomal membrane fraction isolated from wild-type macrophages with purified cathepsin E at pH 5. Our results thus suggest that cathepsin E is important for preventing the accumulation of these lysosomal membrane sialoglycoproteins that can induce a new form of lysosomal storage disorder.

Cathepsin E-deficient mice show increased susceptibility to bacterial infection associated with the decreased expression of multiple cell surface Toll-like receptors

Cathepsin E, an intracellular aspartic proteinase, is predominantly localized in the endosomal compartments of immune system cells. In the present study, we investigated the role of cathepsin E in immune defense systems against bacterial infection. Cathepsin E-deficient (CatE(-/-)) mice showed dramatically increased susceptibility to infection with both the Gram-positive bacterium Staphyrococcus aureus, and the Gram-negative bacterium Porphyromonas gingivalis when compared with syngeneic wild-type mice, most likely due to impaired regulation of bacterial elimination. Peritoneal macrophages from CatE(-/-) mice showed significantly impaired tumor necrosis factor-alpha and IL-6 production in response to S. aureus and decreased bactericidal activities toward this bacterium. Moreover, the cell surface levels of Toll-like receptor-2 (TLR2) and TLR4, which recognize specific components of Gram-positive and -negative bacteria, respectively, were decreased in CatE(-/-) macrophages, despite no significant difference in the total cellular expression levels of these receptors between the wild-type and CatE(-/-) macrophages, implying trafficking defects in these surface receptors in the latter. These results indicate an essential role of cathepsin E in immune defense against invading microorganisms, most probably due to regulation of the cell surface expression of TLR family members required for innate immune responses.

Destructive potential of the aspartyl protease cathepsin D in MHC class II-restricted antigen processing

Whether specific proteases influence MHC class II antigen presentation is still not clearly defined. Cathepsin D, one of the most abundant lysosomal proteases, is thought to be dispensable for MHC class II antigen presentation, yet in vitro digestions of antigen substrates with endosomes/lysosomes from antigen-presenting cells sometimes reveal a dominant role for pepstatin-sensitive aspartyl proteases of which cathepsin D is the major representative. We tested whether the aspartyl protease substrate myoglobin requires cathepsin D activity for presentation to T cells. Surprisingly, in dendritic cells (DC) lacking cathepsin D, presentation of two different myoglobin T cell epitopes was enhanced rather than hindered. This paradox is resolved by the finding that pepstatin-sensitive myoglobin processing activity persists in lysosomes from cathepsin D-null DC and that this reduced activity, most likely due to cathepsin E, is closer to the optimum level required for myoglobin antigen presentation. Our results indicate redundancy among lysosomal aspartyl proteases and show that while processing activities can be productive for MHC class II T cell epitope generation at one level, they can become destructive above an optimal level.

Immune cell-specific amplification of interferon signaling by the IRF-4/8-PU.1 complex

Both type I interferon (IFN-alpha/beta) and type II IFN (IFN-gamma) exert many functions that are restricted to immune cells. Thus, they play critical roles in innate and adaptive immunity. IFN regulatory factor-4 (IRF-4) and IRF-8 (formerly PU.1 interaction partner [Pip] and IFN consensus sequence binding domain [ICSBP], respectively) are immune cell-specific members of the IRF family that regulate the development of myeloid, lymphoid, and dendritic cells. They form a heterodimeric complex with another immune cell-specific transcription factor PU.1-Spi-1 and regulate transcription of genes in the immune system. This review describes the role of the IRF-8-PU.1 complex in modulating IFN signaling in an immune cell-specific manner. Our studies revealed that some but not all IFN-gamma-inducible genes carry an IFN-gamma activation site (GAS) element that contains a binding site for the IRF- 8-PU.1 complex. The IRF-8-PU.1 complex can take part in GAS-mediated transcription and amplify expression of IFN-gamma-responsive genes initiated by Stat1 in macrophages. Similarly, some but not all IFN-alpha/beta-responsive genes are shown to carry an IFN-stimulated response element (ISRE) that contains an IRF-8-PU.1 binding site. The participation of IRF-8-PU.1 in ISRE-mediated transcription results in the augmentation of IFN-stimulated gene factor 3 (ISGF3)-induced transcription in macrophages. Thus, GAS and ISRE elements, classically defined as universal IFN-alpha/beta and IFN-gamma response sequences, are not the same, and some harbor an embedded motif for IRF- 8-PU.1 binding that functions only in immune cells. Accordingly, the IRF-8-PU.1complex provides secondary IFN signaling pathways unique to the immune system. Collectively, the contribution of IRF-8 and PU.1 to IFN-regulated gene expression may in part account for immune cell-specific functions of IFNs.

PU.1 regulates cathepsin S expression in professional APCs

Cathepsin S (CTSS) is a cysteine protease that is constitutively expressed in APCs and mediates processing of MHC class II-associated invariant chain. CTSS and the Ets family transcription factor PU.1 are highly expressed in cells of both myeloid (macrophages and dendritic cells) and lymphoid (B lymphocytes) lineages. Therefore, we hypothesized that PU.1 participates in the transcriptional regulation of CTSS in these cells. In A549 cells (a human epithelial cell line that does not express either CTSS or PU.1), the expression of PU.1 enhances CTSS promoter activity approximately 5- to 10-fold. In RAW cells (a murine macrophage-like cell line that constitutively expresses both CTSS and PU.1), the expression of a dominant-negative PU.1 protein and a short-interfering RNA PU.1 construct attenuates basal CTSS promoter activity, mRNA levels, and protein expression. EMSAs show binding of PU.1 to oligonucleotides derived from the CTSS promoter at two different Ets consensus binding elements. Mutation of these sites decreases the baseline CTSS activity in RAW cells that constitutively express PU.1. Chromatin immunoprecipitation experiments show binding of PU.1 with the CTSS promoter in this same region. Finally, the expression of PU.1, in concert with several members of the IFN regulatory factor family, enhances CTSS promoter activity beyond that achieved by PU.1 alone. These data indicate that PU.1 participates in the regulation of CTSS transcription in APCs. Thus, manipulation of PU.1 expression may directly alter the endosomal proteolytic environment in these cells.

Contribution of extracellular proteolysis and microglia to intracerebral hemorrhage

Proteases, such as tissue plasminogen activator, thrombin, metalloproteinases, and cathepsins, have complex functions in the mammalian brain under both normal and pathological conditions. Some of these proteases are expressed by neuronal cells, whereas others are made by the immunocompetent, macrophage-like cells of the brain: the microglia. This article reviews the physiological and pathological functions of these proteinases in the brain as well as recent findings linking extracellular proteases with neuronal cell death in ischemic or hemorrhagic stroke. Better understanding of protease expression and signaling, microglial activation, and their relationship with neuronal cell death during stroke injury could contribute to the development of relevant inhibitors as novel neuroprotective agents for treating ischemic stroke and intracerebral hemorrhage.

Involvement of prostaglandin E2 released from leptomeningeal cells in increased expression of transforming growth factor-β in glial cells and cortical neurons during systemic inflammation

The leptomeninges play a central role in the antiinflammatory response through the glia-neuron interaction during systemic inflammation. In the present study, we examined the possible production of two potent antiinflammatory mediators, prostaglandin E(2) (PGE(2)) and transforming growth factor-beta1 (TGF-beta1) by leptomeningeal cells during systemic inflammation. After immunization with the complete Freund's adjuvant (CFA), cyclooxygenase (COX)-2 and membrane-bound PGE synthase-1 (mPGES-1) were induced in the leptomeninges. Primary cultured leptomeningeal cells secreted PGE(2) after treatment with lipopolysaccharide (LPS) or proinflammatory cytokines. The LPS-induced release of PGE(2) was depressed by a selective COX-2 inhibitor, NS-398. On the other hand, TGF-beta1 and TGF-beta receptor II (TGF-betaRII) both markedly increased in the leptomeninges and the parenchymal cells after the CFA injection. Double-staining immunohistochemistry demonstrated TGF-beta1 to be induced in both glial cells and cortical neurons, whereas TGF-betaRII was induced only in cortical neurons. Furthermore, the conditioned medium prepared from the leptomeningeal cells after LPS stimulation was able to induce an increased expression of TGF-beta1 and TGF-betaRII in the primary cultured glial cells and cortical neurons. This increased expression was suppressed by NS-398. PGE(2) was found to increase directly the production of TGF-beta1 and TGF-betaRII in the primary cultured cells. These observations strongly suggest that PGE(2), which is biosynthesized by the leptomeninges, mainly regulates the production of TGF-beta1 by glial cells and cortical neuron, thus playing a protective role in the cortical neurons during systemic inflammation. Furthermore, TGF-beta1 may also exert a protective effect directly on the cortical neurons.

Characterization of rat cathepsin E and mutants with changed active-site residues and lacking propeptides and N-glycosylation, expressed in human embryonic kidney 293T cells

To study the roles of the catalytic activity, propeptide, and N-glycosylation of the intracellular aspartic proteinase cathepsin E in biosynthesis, processing, and intracellular trafficking, we constructed various rat cathepsin E mutants in which active-site Asp residues were changed to Ala or which lacked propeptides and N-glycosylation. Wild-type cathepsin E expressed in human embryonic kidney 293T cells was mainly found in the LAMP-1-positive endosomal organelles, as determined by immunofluorescence microscopy. Consistently, pulse-chase analysis revealed that the initially synthesized pro-cathepsin E was processed to the mature enzyme within a 24 h chase. This process was completely inhibited by brefeldin A and bafilomycin A, indicating its transport from the endoplasmic reticulum (ER) to the endosomal acidic compartment. Mutants with Asp residues in the two active-site consensus motifs changed to Ala and lacking the propeptide (Leu23-Phe58) and the putative ER-retention sequence (Ser59-Asp98) were neither processed nor transported to the endosomal compartment. The mutant lacking the ER-retention sequence was rapidly degraded in the ER, indicating the importance of this sequence in correct folding. The single (N92Q or N324D) and double (N92Q/N324D) N-glycosylation-deficient mutants were neither processed into a mature form nor transported to the endosomal compartment, but were stably retained in the ER without degradation. These data indicate that the catalytic activity, propeptides, and N-glycosylation of this protein are all essential for its processing, maturation, and trafficking.

The Role of the Cathepsin E Propeptide in Correct Folding, Maturation and Sorting to the Endosome

Cathepsin E (CE) is an endosomal aspartic proteinase of the A1 family that is highly homologous to the lysosomal aspartic proteinase cathepsin D (CD). Newly synthesized CE undergoes several proteolytic processing events to yield mature CE, from which the N-terminal propeptide usually comprising 39 amino acids is removed. To define the role of the propeptide of CE in its biosynthesis and processing, we constructed two fusion proteins using chimeric DNAs encoding the CE propeptide fused to the mature CD tagged with HA at the COOH terminus (termed ED-HA) and encoding the CD propeptide fused to the mature CE (termed DE). Pulse-chase analysis revealed that wild-type CE expressed in human embryonic kidney cells is autoproteolytically processed into mature CE within a 12-h chase, whereas the chimeric DE failed to be converted into mature CE even after a 24-h chase. The DE chimera was nevertheless capable of acid-dependent autoactivation in vitro to yield a catalytically active form, although its specificity constants (kcat/Km) were considerably high but less (35%) than those of the wild-type CE. By contrast, the chimeric ED-HA expressed in HeLa cells underwent neither processing into a catalytically active enzyme nor acid-dependent autoactivation in vitro. The ED-HA protein was less stable than wt-CD-HA, as determined on pulse-chase analysis and on trypsin digestion. These data indicate that the propeptide of CE is essential for the correct folding, maturation, and targeting of this protein to its final destination.

Purification and characterization of recombinant human cathepsin E expressed in human kidney cell line 293

A cDNA encoding human prepro-cathepsin E was introduced into the adenovirus-transformed HEK-293 (human embryonic kidney) cell line. The construct contained both a V5 peptide epitope and histidine tags at the carboxy terminus. Transfected cells efficiently secreted recombinant pro-cathepsin E into the culture medium. The secreted pro-cathepsin E was purified in a single step using Ni affinity chromatography yielding a protein of about 92 kDa under non-reducing conditions. The amino-terminal sequence of the purified protein began at Ser20, suggesting human cathepsin E accumulated in the culture supernatant as the pro-enzyme. The purified protein was rapidly and completely converted to the active form by treatment at pH 4.0 or below. Steady state kinetic parameters for hydrolysis of the fluorogenic peptide substrate MOCAc-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys(Dnp)-d-Arg-NH2 (cleavage at the Phe-Phe bond) were consistent with previously reported values for purified human enzyme (kc/Ki= 53 x 10(6) M(-1) s(-1), Km= 6.3 microM, and kcat= 3 x 10(2) s(-1)). The activated protein was potently inhibited by pepstatin with Ki= 0.2 nM, as well as a reported beta secretase inhibitor. This work demonstrates the potential for producing large quantities of highly purified human cathepsin E from HEK-293 cells in quantities to support both biochemical and structural characterization of the enzyme.

The expression and function of cathepsin E in dendritic cells

Cathepsin E is an aspartic proteinase that has been implicated in Ag processing within the class II MHC pathway. In this study, we document the presence of cathepsin E message and protein in human myeloid dendritic cells, the preeminent APCs of the immune system. Cathepsin E is found in a perinuclear compartment, which is likely to form part of the endoplasmic reticulum, and also a peripheral compartment just beneath the cell membrane, with a similar distribution to that of Texas Red-dextran within 2 min of endocytosis. To investigate the function of cathepsin E in processing, a new soluble targeted inhibitor was synthesized by linking the microbial aspartic proteinase inhibitor pepstatin to mannosylated BSA via a cleavable disulfide linker. This inhibitor was shown to block cathepsin D/E activity in cell-free assays and within dendritic cells. The inhibitor blocked the ability of dendritic cells from wild-type as well as cathepsin D-deficient mice to present intact OVA, but not an OVA-derived peptide, to cognate T cells. The data therefore support the hypothesis that cathepsin E has an important nonredundant role in the class II MHC Ag processing pathway within dendritic cells.

A new selective substrate for cathepsin E based on the cleavage site sequence of α2-macroglobulin

Cathepsin E is an intracellular aspartic proteinase of the pepsin family predominantly expressed in cells of the immune system and believed to contribute to homeostasis by participating in host defense mechanisms. Studies on its enzymatic properties, however, have been limited by a lack of sensitive and selective substrates. For a better understanding of the importance of this enzymein vivo, we designed and synthesized a highly sensitive peptide substrate for cathepsin E based on the sequence of the specific cleavage site of α2-macroglobulin. The substrate constructed, MOCAc-Gly-Ser-Pro-Ala-Phe-Leu-Ala-Lys(Dnp)-D-Arg-NH2[where MOCAc is (7-methoxycoumarin-4-yl)acetyl and Dnp is dinitrophenyl], derived from the cleavage site sequence of human α2-macroglobulin, was the most sensitive and selective for cathepsin E, withkcat/Kmvalues of 8–11 μM-1S-1, whereas it was resistant to hydrolysis by the analogous aspartic proteinases cathepsin D and pepsin, as well as the lysosomal cysteine proteinases cathepsins B, L, and H. The assay allows the detection of a few fmol of cathepsin E, even in the presence of plasma and cell lysate, and gives accurate results over a wide enzyme concentration range. This substrate might represent a useful tool for monitoring and accurately quantifying cathepsin E, even in crude enzyme preparations.

Accumulation of catalytically active proteases in lacrimal gland acinar cell endosomes during chronic ex vivo muscarinic receptor stimulation

Chronic muscarinic stimulation induces functional quiescence (Scand J Immunol 2003;58:550-65) and alters the traffic of immature cathepsin B (Exp Eye Res 2004;79:665-75) in lacrimal acinar cells. To test whether active proteases aberrantly accumulate in the endosomes, cell samples were cultured 20 h with and without 10-microm carbachol (CCh), incubated with [125I]-bovine serum albumin and then lysed and analysed by subcellular fractionation. CCh decreased total cysteine protease and cathepsin S activities in the isolated lysosome, redistributing them to early endocytic and biosynthetic compartments. CCh decreased [125I] accumulation in all compartments of cells loaded in the absence of protease inhibitors; the cysteine protease inhibitor, leupeptin, prevented the endosomal decrease but not the lysosomal decrease. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis and autoradiography demonstrated [125I]-labelled proteolytic products in endomembrane compartments of both control and CCh-stimulated cells, even in the presence of leupeptin, but analysis indicated that CCh increased the amount in endosomes. Two-dimensional fractionation analyses suggest that the CCh-induced redistributions result from blocks in traffic to the late endosome from both the early endosome and the trans-Golgi network. Therefore, we conjecture that chronic muscarinic acetylcholine receptor stimulation leads to aberrant proteolytic processing of autoantigens in endosomes, from whence previously cryptic epitopes may be secreted to the underlying interstitial space.

Trafficking of exogenous peptides into proteasome-dependent major histocompatibility complex class I pathway following enterotoxin B subunit-mediated delivery

The B-subunit component of Escherichia coli heat-labile enterotoxin (EtxB), which binds to cell surface GM1 ganglioside receptors, was recently shown to be a highly effective vehicle for delivery of conjugated peptides into the major histocompatibility complex (MHC) class I pathway. In this study we have investigated the pathway of epitope delivery. The peptides used contained the epitope either located at the C terminus or with a C-terminal extension. Pretreatment of cells with cholesterol-disrupting agents blocked transport of EtxB conjugates to the Golgi/endoplasmic reticulum, but did not affect EtxB-mediated MHC class I presentation. Under these conditions, EtxB conjugates entered EEA1-positive early endosomes where peptides were cleaved and translocated into the cytosol. Endosome acidification was required for epitope presentation. Purified 20 S immunoproteasomes were able to generate the epitope from peptides in vitro, but 26 S proteasomes were not. Only presentation from the C-terminal extended peptide was proteasome-dependent in cells, and this was found to be significantly slower than presentation from peptides with the epitope at the C terminus. These results implicate the proteasome in the generation of the correct C terminus of the epitope and are consistent with proteasome-independent N-terminal trimming. Epitope presentation was blocked in a TAP-deficient cell line, providing further evidence that conjugated peptides enter the cytosol as well as demonstrating a requirement for the peptide transporter. Our findings demonstrate the utility of EtxB-mediated peptide delivery for rapid and efficient loading of MHC class I epitopes in several different cell types. Conjugated peptides are released from early endosomes into the cytosol where they gain access to proteasomes and TAP in the "classical" pathway of class I presentation.

Crystal Structure of an Activation Intermediate of Cathepsin E

Cathepsin E is an intracellular, non-lysosomal aspartic protease expressed in a variety of cells and tissues. The protease has proposed physiological roles in antigen presentation by the MHC class II system, in the biogenesis of the vasoconstrictor peptide endothelin, and in neurodegeneration associated with brain ischemia and aging. Cathepsin E is the only A1 aspartic protease that exists as a homodimer with a disulfide bridge linking the two monomers. Like many other aspartic proteases, it is synthesized as a zymogen which is catalytically inactive towards its natural substrates at neutral pH and which auto-activates in an acidic environment. Here we report the crystal structure of an activation intermediate of human cathepsin E at 2.35A resolution. The overall structure follows the general fold of aspartic proteases of the A1 family, and the intermediate shares many features with the intermediate 2 on the proposed activation pathway of aspartic proteases like pepsin C and cathepsin D. The pro-sequence is cleaved from the protease and remains stably associated with the mature enzyme by forming the outermost sixth strand of the interdomain beta-sheet. However, different from these other aspartic proteases the pro-sequence of cathepsin E remains intact after cleavage from the mature enzyme. In addition, the active site of cathepsin E in the crystal is occupied by N-terminal amino acid residues of the mature protease in the non-primed binding site and by an artificial N-terminal extension of the pro-sequence from a neighboring molecule in the primed site. The crystal structure of the cathepsin E/pro-sequence complex, therefore, provides further insight into the activation mechanism of aspartic proteases.

Genetic dissection of systemic autoimmune disease in Nrf2-deficient mice

Systemic lupus erythematosus (SLE) is an autoimmune disorder with immune-complex deposition that affects multiple organs. Previous studies have suggested the involvement of oxidative stress and apoptosis in SLE, but no clear link to etiology has been established. Here we show that mice deficient in a transcription factor responsible for controlling the expression of numerous detoxification and antioxidant genes develop an autoimmune disease with multiple organ pathologies that closely resembles human SLE. Aged female mice with a knockout of nuclear factor, erythroid-derived 2, like 2 (nrf2) are prone to develop antibodies against double-stranded DNA and the Smith antigen as well as IgG, IgM, and C3 deposition in kidney, liver, heart, and brain. Prior to the development of autoimmune antibodies and organ pathology, oxidative damage occurs in the liver and kidney as indicated by the increased levels of the DNA oxidation marker 8-hydroxydeoxyguanosine and the later increase in the lipid peroxidation product malondialdehyde. Gene expression profiles demonstrate an early decrease in numerous antioxidant and detoxification genes in the livers and altered levels of cytokines and T and B cell-specific genes in the spleens of nrf2 knockout mice. These data strongly suggest that a deficiency in detoxification and increased oxidative stress can result in the development of a systemic autoimmune disease.

Neuronal and microglial cathepsins in aging and age-related diseases

It has been long believed that cathepsins compensate for each other because of their overlapping substrate specificities. However, there is increasing evidence that disturbance of the normal balance of their enzymatic activities is the first insult in brain aging and age-related diseases. The imbalance of cathepsins may further cause age-related neuropathological changes such as accumulation of autophagic vacuoles and the formation of ceroid-lipofuscin leading to neuronal dysfunction and damage. Leakage of cathepsins due to the fragility of lysosomal membranes during aging also contributes to neurodegeneration. Furthermore, the deficiency of cathepsin D has been recently revealed to provoke a novel type of lysosomal storage disease associated with massive neurodegeneration. In these animals, microglia are activated to initiate inflammatory and cytotoxic responses by binding and phagocytosis of storage neurons. Activated microglia also release some members of cathepsins to induce neuronal death by degrading extracellular matrix proteins. Thus the microglial activation possibly through sensing neuronal storage may also be an important causative factor for neurodegeneration in lysosomal storage diseases and age-related diseases such as Alzheimer's disease. This review describes the pathological roles of neuronal and microglial cathepsins in brain aging and age-related diseases.

Cathepsin E: A Novel Target for Regulation by Class II Transactivator

The aspartic proteinase cathepsin E (CatE) has been implicated in Ag processing. In this study we report that CatE expression is negatively regulated by the MHC class II transactivator (CIITA). CIITA-deficient murine and human B cells expressed greater CatE than wild-type B cells, whereas overexpression of CIITA in a human gastric carcinoma cell line, AGS, resulted in decreased CatE mRNA and protein. AGS cells expressing CIITA also exhibited decreased processing of OVA Ag. Inhibition of CatE expression is specific to the type III CIITA isoform and maps to the acidic and proline/serine/threonine-rich (PST) protein domains of CIITA. We found that CatE expression is inducible by PU.1 and p300, and that this induction can be reversed by CIITA. These findings demonstrate a novel phenomenon: regulation of CatE Ag processing by CIITA in an isoform-dependent manner.

Potentiation of NMDA receptor-mediated synaptic responses by microglia

To study the influence of microglia on glutamatergic synaptic transmission in the acute phase of neuronal injury, we first examined the effects of primary cultured microglia transferred onto the organotypic cortical slice cultures. In these microglia-transferred cortical slice cultures, stimulation of the subcortical white matter induced fast excitatory postsynaptic potentials followed by N-methyl-D-aspartate (NMDA) receptor-mediated plateau-like potentials that were never observed in control slice cultures. A similar potentiation of NMDA receptor-mediated postsynaptic responses was also observed by an application of a microglial-conditioned medium (MCM, 10% v/v) in acute cortical slices. These effects of MCM disappeared after boiling or incubation with proteinase K. After fractionation of MCM by anion-exchange chromatography, the enhancing activity of each fraction was quantitated electrophysiologically. When each fraction was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the fraction 24 which showed the most potent enhancing activity on NMDA receptor-mediated responses contained a relatively strong protein band with a molecular mass of approximately 70 kDa. MCM also enhanced both glutamate- and NMDA-induced inward currents recorded from acutely isolated cortical neurons. It was also noted that glutamate and NMDA induced transient large inward currents during an application of MCM, which were never observed in the control condition. These observations strongly suggest that NMDA receptor-mediated responses can be potentiated by both heat- and protease-labile (presumably 70-kDa proteins) molecules released from microglia.

Pepstatin A induces extracellular acidification distinct from aspartic protease inhibition in microglial cell lines

The extrusion of protons is considered a very general parameter of the activation of many kinds of membrane or intracellular molecules, such as receptors, ion channels, and enzymes. We found that pepstatin A caused a reproducible, concentration-related increase in the extracellular acidification rate in two microglial cell lines, Ra2 and 6-3. Washing abolished pepstatin A-induced acidification immediately. However, pepstatin A did not cause the extracellular acidification in other cell types, such as CHO, C6 glioma, and NIH3T3 cells. These observations strongly suggest that pepstatin A interacts with certain membrane proteins specific to both Ra2 and 6-3 cells from outside. N-methylmaleimide and N,N'-dicyclohexylcarbodiimide, inhibitors of H(+)-ATPase, were found to reduce pepstatin A-induced response strongly, while bafilomycin A1, a vacuolar H(+)-ATPase inhibitor, vanadate, a P-type H(+)-ATPase inhibitor, and NaN3, an F1 ATPase inhibitor, virtually did not. 5-(N-ethyl-N-isopropyl) amiloride, an inhibitor of Na(+)/H(+) exchanger isoform 1, greatly enhanced pepstatin-induced response, while amiloride did not. Zn(2+), a voltage-dependent proton channel blocker, did not affect pepstatin-induced response neither. Staurosporine, a nonspecific inhibitor of protein kinase C, inhibited pepstatin A-induced response, while chelerythrine, more selective inhibitor of protein kinase C, greatly enhanced it. H-7 and H-8 did not affected the response. These findings suggest that pepstatin A induces extracellular acidification in microglia cell lines, Ra2 and 6-3, through an N-methylmaleimide- and N,N'-dicyclohexylcarbodiimide-sensitive, but bafilomycin A1-insensitive, ATPase, which seems to be distinct from protein kinase C-dependent process.

Molecular cloning of preprocathepsin E cDNA from the stomach of bullfrog Rana catesbeiana

A cDNA library was constructed from a poly(A)(+) RNA fraction of the gastric mucosa of bullfrog Rana catesbeiana. We cloned a cDNA encoding preprocathepsin E (Pre-Pro-CE) from the library. The present study is the first demonstration of the Pre-Pro-CE cDNA of lower vertebrate such as amphibian. Amino acid sequence deduced from the cDNA was compared with partial amino acid sequence determined by Edman degradation, suggesting that the cDNA comprises an open reading frame encoding a signal peptide (16 amino acids), a pro-sequence (33 amino acids) and a mature protein region (348 amino acids). Two consensus tri-peptide sequences (FDT and VDT) as active site and positions of seven cysteine residues were conserved in this amphibian CE. Although the bullfrog CE was deduced to contain one potential N-linked glycosylation site, its position (Asn(139)-Leu(140)-Thr(141)) was different from that of mammalian CEs. Molecular phylogenetic analysis showed that the bullfrog Pro-CE belongs to the typical Pro-CE group among various aspartic proteinases.

[Atopic dermatitis and cathepsin E]

Cathepsin E is an intracellular aspartic proteinase expressed predominantly in immune cells and skin. We show that cathepsin E-deficient mice spontaneously develop atopic dermatitis (AD)-like skin lesions comparable to human AD when kept under conventional circumstances, but not under specific pathogen-free conditions. These mice displayed AD-associated phenotypes including eosinophilia; increased serum IgE, IL-18, and IL-1beta; and enhanced production of Th2 cytokines. Cathepsin E deficiency also resulted in greater decrease of the rate of degradation for serum IL-18 and IL-1beta. Interestingly, cathepsin E levels in blood cells were significantly decreased in AD patients and the AD model NC/Nga mice compared to healthy donors and the control mice, respectively. Our results indicate that deficiency or defective production of cathepsin E strongly induces AD in humans and mice, probably due to the systemic accumulation of IL-18 and IL-1beta, leading to stimulation of Th2 responses, and that cathepsin E-deficient mice are a newly discovered model to analyze pathologic mechanisms of human AD.

Microglial functions and proteases

There is accumulating evidence that intracellular and extracellular proteases of microglia contribute to various events in the central nervous system (CNS) through both nonspecific and limited proteolysis. Cathepsin E and cathepsin S, endosomal/lysosomal proteases, have been shown to play important roles in the major histocompatibility complex (MHC) class II-mediated antigen presentation of microglia by processing of exogenous antigens and degradation of the invariant chain associated with MHC class II molecules, respectively. Some members of cathepsins are also involved in neuronal death after secreted from microglia and clearance of phagocytosed amyloid- beta peptides. Tissue-type plasminogen activator, a serine protease, secreted from microglia participates in neuronal death, enhancement of N-methyl-D-aspartate receptor-mediated neuronal responses, and activation of microglia via either proteolytic or nonproteolytic activity. Calpain, a calcium-dependent cysteine protease, has been shown to play a pivotal role in the pathogenesis of multiple sclerosis by degrading myelin proteins extracellulary. Furthermore, matrix metalloproteases secreted from microglia also receive great attention as mediators of inflammation and tissue degradation through processing of pro-inflammatory cytokines and damage to the blood-brain barrier. The growing knowledge about proteolytic events mediated by microglial proteases will not only contribute to better understanding of microglial functions in the CNS but also may aid in the development of protease inhibitors as novel neuroprotective agents.

Disruption of structural and functional integrity of alpha 2-macroglobulin by cathepsin E

alpha 2-Macroglobulin (alpha 2M) is an abundant glycoprotein with the intrinsic capacity for capturing diverse proteins for rapid delivery into cells. After internalization by the receptor- mediated endocytosis, alpha 2M-protein complexes were rapidly degraded in the endolysosome system. Although this is an important pathway for clearance of both alpha 2M and biological targets, little is known about the nature of alpha 2M degradation in the endolysosome system. To investigate the possible involvement of intracellular aspartic proteinases in the disruption of structural and functional integrity of alpha 2M in the endolysosome system, we examined the capacity of alpha 2M for interacting with cathepsin E and cathepsin D under acidic conditions and the nature of its degradation. alpha 2M was efficiently associated with cathepsin E under acidic conditions to form noncovalent complexes and rapidly degraded through the generation of three major proteins with apparent molecular masses of 90, 85 and 30 kDa. Parallel with this reaction, alpha 2M resulted in the rapid loss of its antiproteolytic activity. Analysis of the N-terminal amino-acid sequences of these proteins revealed that alpha 2M was selectively cleaved at the Phe811-Leu812 bond in about 100mer downstream of the bait region. In contrast, little change was observed for alpha 2M treated by cathepsin D under the same conditions. Together, the synthetic SPAFLA peptide corresponding to the Ser808-Ala813 sequence of human alpha 2M, which contains the cathepsin E-cleavage site, was selectively cleaved by cathepsin E, but not cathepsin D. These results suggest the possible involvement of cathepsin E in disruption of the structural and functional integrity of alpha 2M in the endolysosome system.