Cathepsin B promotes the progression of pancreatic ductal adenocarcinoma in mice

OBJECTIVE

The lysosomal protease cathepsin B is upregulated in human pancreatic ductal adenocarcinoma (PDA) and represents a potential therapeutic target. Loss of cathepsin B delays tumour progression in mouse models of islet, mammary and intestinal carcinoma and decreases invasion and metastasis. This study examines the role of cathepsin B in the initiation, progression and metastasis of PDA.

METHODS

Cathepsin B germline knockout mice were crossed with animals expressing an endogenous Kras(G12D) allele in the pancreas, and mice were aged to evaluate the role of cathepsin B in pancreatic intraepithelial neoplasia (PanIN). A survival study was also performed with mice carrying an additional heterozygous conditional Trp53(R172H) allele. Cell lines derived from tumours were used to investigate the role of cathepsin B in vitro, and subcutaneous allografts investigated the cell autonomous and non-cell autonomous roles of cathepsin B in pancreatic cancer.

RESULTS

Constitutive cathepsin B loss resulted in delayed progression of both PanIN and PDA and a significant survival advantage in mice. Cathepsin B-deficient PDA cells and PanIN showed decreased proliferation and mitogen-activated protein (MAP) kinase signalling. The reconstitution of deficient cells with cathepsin B reversed these findings, which correlated with decreased levels of the active forms of the related protease cathepsin L. Conversely, acute ablation of cathepsin L activated the MAP kinase cascade in PDA cells.

CONCLUSIONS

These results confirm that cathepsin B plays an important cell autonomous role in the progression of PDA and suggest that the regulation of cathepsin L by cathepsin B may be a means of stimulating cell proliferation in neoplasia.

Human cathepsin V protease participates in production of enkephalin and NPY neuropeptide neurotransmitters

Proteases are required for processing precursors into active neuropeptides that function as neurotransmitters for cell-cell communication. This study demonstrates the novel function of human cathepsin V protease for producing the neuropeptides enkephalin and neuropeptide Y (NPY). Cathepsin V is a human-specific cysteine protease gene. Findings here show that expression of cathepsin V in neuroendocrine PC12 cells and human neuronal SK-N-MC cells results in production of (Met)enkephalin from proenkephalin. Gene silencing of cathepsin V by siRNA in human SK-N-MC cells results in reduction of (Met)enkephalin by more than 80%, illustrating the prominent role of cathepsin V for neuropeptide production. In vitro processing of proenkephalin by cathepsin V occurs at dibasic residue sites to generate enkephalin-containing peptides and an ∼24-kDa intermediate present in human brain. Cathepsin V is present in human brain cortex and hippocampus where enkephalin and NPY are produced and is present in purified human neuropeptide secretory vesicles. Colocalization of cathepsin V with enkephalin and NPY in secretory vesicles of human neuroblastoma cells was illustrated by confocal microscopy. Furthermore, expression of cathepsin V with proNPY results in NPY production. These findings indicate the unique function of human cathepsin V for producing enkephalin and NPY neuropeptides required for neurotransmission in health and neurological diseases.

Cathepsins D and L reduce the toxicity of advanced glycation end products

Advanced glycation end product-modified proteins are known for accumulating during aging and in several pathological conditions such as diabetes, renal failure, and neurodegenerative disorders. There is little information about the intracellular fate of endocytosed advanced glycation end products (AGEs) and their influence on proteolytic systems. However, it is known that the lysosomal system is impaired during aging. Therefore, undegraded material may accumulate and play a considerable role in the development of diverse diseases. To investigate if AGEs can be degraded and to test whether they accumulate because of impaired lysosomal proteases we studied the effects of advanced glycation end products on the endosomal-lysosomal system. Five different types of AGEs were generated by bovine serum albumin incubation with glyoxal, methylglyoxal, glucose, fructose, and ribose. The first experiments revealed the uptake of AGEs by the macrophage cell line RAW 264.7. Further investigations demonstrated an increase in cathepsin D and L activity and an increase in mature cathepsins D and L. Increased activities were accompanied by the presence of more lysosomes, measured by staining with LysoTracker blue. To specify the roles of cathepsins D and L we used knockout cells to test the roles of both cathepsins on the toxicity of advanced glycation end products. In summary we conclude that both cathepsins are required for a reduction in advanced glycation end product-induced cytotoxicity.

Cathepsin L activity is essential to elastase perfusion-induced abdominal aortic aneurysms in mice

OBJECTIVE

The development of abdominal aortic aneurysms (AAA) requires extensive aortic wall matrix degradation. Human AAA lesions express high levels of cathepsin L (CatL), one of the most potent mammalian elastases. Whether this protease participates directly in AAA pathogenesis, however, is unknown.

METHODS AND RESULTS

We generated experimental AAA with aortic elastase perfusion in mice and established an essential role of CatL in AAA formation. After 14 days postperfusion, most wild-type (Ctsl(+/+)) mice developed AAA, but none of the CatL-deficient (Ctsl(-/-)) mice did. AAA lesion macrophage contents, CD4(+) T cell numbers, CD31(+) and laminin-5 angiogenic fragment γ2(+) microvessel numbers, and elastin fragmentation were all significantly lower in Ctsl(-/-) mice than in Ctsl(+/+) mice. While lesions from Ctsl(-/-) mice contained fewer Ki67(+) proliferating cells than did Ctsl(+/+) mice, the absence of CatL did not affect lesion apoptotic cell contents or medial smooth-muscle cell loss significantly. Mechanistic studies indicated that the absence of CatL reduced lesion chemokine monocyte chemotactic protein-1 content, macrophage and T-cell in vitro transmigration, and angiogenesis, and altered the expression and activities of matrix metalloproteinases and other cysteinyl cathepsins in inflammatory cells, vascular cells, and AAA lesions.

CONCLUSION

CatL contributes to AAA formation by promoting lesion inflammatory cell accumulation, angiogenesis, and protease expression.

Contribution of cathepsin L to secretome composition and cleavage pattern of mouse embryonic fibroblasts

The endolysosomal cysteine endoprotease cathepsin L is secreted from cells in a variety of pathological conditions such as cancer and arthritis. We compared the secretome composition and extracellular proteolytic cleavage events in cell supernatants of cathepsin L-deficient and wild-type mouse embryonic fibroblasts (MEFs). Quantitative proteomic comparison of cell conditioned media indicated that cathepsin L deficiency affects, albeit in a limited manner, the abundances of extracellular matrix (ECM) components, signaling proteins, and further proteases as well as endogenous protease inhibitors. Immunodetection corroborated that cathepsin L deficiency results in decreased abundance of the ECM protein periostin and elevated abundance of matrix metalloprotease (MMP)-2. While mRNA levels of MMP-2 were not affected by cathepsin L ablation, periostin mRNA levels were reduced, potentially indicating a downstream effect. To characterize cathepsin L contribution to extracellular proteolysis, we performed terminal amine isotopic labeling of substrates (TAILS), an N-terminomic technique for the identification and quantification of native and proteolytically generated protein N-termini. TAILS identified >1500 protein N-termini. Cathepsin L deficiency predominantly reduced the magnitude of collagenous cleavage sites C-terminal to a proline residue. This contradicts cathepsin L active site specificity and indicates altered activity of further proteases as a result of cathepsin L ablation.

Gene targeting of the cysteine peptidase cathepsin H impairs lung surfactant in mice

Background

The 11 human cysteine cathepsins are proteases mainly located in the endolysosomal compartment of all cells and within the exocytosis pathways of some secretory cell types. Cathepsin H (Ctsh) has amino- and endopeptidase activities. In vitro studies have demonstrated Ctsh involvement in the processing and secretion of the pulmonary surfactant protein B (SP-B). Furthermore, Ctsh is highly expressed in the secretory organelles of alveolar type II pneumocytes where the surfactant proteins are processed.

Methodology/Principal Findings

Hence, we generated Ctsh null mice by gene targeting in embryonic stem cells to investigate the role of this protease in surfactant processing in vivo. The targeting construct contains a ß-galactosidase (lacZ) reporter enabling the visualisation of Ctsh expression sites. Ctsh-deficiency was verified by northern blot, western blot, and measurement of the Ctsh aminopeptidase activity. Ctsh^−/− mice show no gross phenotype and their development is normal without growth retardation. Broncho-alveolar lavage (BAL) from Ctsh^−/− mice contained lower levels of SP-B indicating reduced SP-B secretion. The BAL phospholipid concentration was not different in Ctsh^+/+ and Ctsh^−/− mice, but measurement of surface tension by pulsating bubble surfactometry revealed an impairment of the tension reducing function of lung surfactant of Ctsh^−/− mice.

Conclusions/Significance

We conclude that cathepsin H is involved in the SP-B production and reduced SP-B levels impair the physical properties of the lung surfactant. However, Ctsh defiency does not reproduce the severe phenotype of SP-B deficient mice. Hence, other proteases of the secretory pathway of type II pneumocytes, i.e. cathepsins C or E, are still able to produce surfactant of sufficient quality in absence of Ctsh.

Contribution of cathepsin L to secretome composition and cleavage pattern of mouse embryonic fibroblasts

The endolysosomal cysteine endoprotease cathepsin L is secreted from cells in a variety of pathological conditions such as cancer and arthritis. We compared the secretome composition and extracellular proteolytic cleavage events in cell supernatants of cathepsin L-deficient and wild-type mouse embryonic fibroblasts (MEFs). Quantitative proteomic comparison of cell conditioned media indicated that cathepsin L deficiency affects, albeit in a limited manner, the abundances of extracellular matrix (ECM) components, signaling proteins, and further proteases as well as endogenous protease inhibitors. Immunodetection corroborated that cathepsin L deficiency results in decreased abundance of the ECM protein periostin and elevated abundance of matrix metalloprotease (MMP)-2. While mRNA levels of MMP-2 were not affected by cathepsin L ablation, periostin mRNA levels were reduced, potentially indicating a downstream effect. To characterize cathepsin L contribution to extracellular proteolysis, we performed terminal amine isotopic labeling of substrates (TAILS), an N-terminomic technique for the identification and quantification of native and proteolytically generated protein N-termini. TAILS identified >1500 protein N-termini. Cathepsin L deficiency predominantly reduced the magnitude of collagenous cleavage sites C-terminal to a proline residue. This contradicts cathepsin L active site specificity and indicates altered activity of further proteases as a result of cathepsin L ablation.

Disrupted in renal carcinoma 2 (DIRC2), a novel transporter of the lysosomal membrane, is proteolytically processed by cathepsin L

DIRC2 (Disrupted in renal carcinoma 2) has been initially identified as a breakpoint-spanning gene in a chromosomal translocation putatively associated with the development of renal cancer. The DIRC2 protein belongs to the MFS (major facilitator superfamily) and has been previously detected by organellar proteomics as a tentative constituent of lysosomal membranes. In the present study, lysosomal residence of overexpressed as well as endogenous DIRC2 was shown by several approaches. DIRC2 is proteolytically processed into a N-glycosylated N-terminal and a non-glycosylated C-terminal fragment respectively. Proteolytic cleavage occurs in lysosomal compartments and critically depends on the activity of cathepsin L which was found to be indispensable for this process in murine embryonic fibroblasts. The cleavage site within DIRC2 was mapped between amino acid residues 214 and 261 using internal epitope tags, and is presumably located within the tentative fifth intralysosomal loop, assuming the typical MFS topology. Lysosomal targeting of DIRC2 was demonstrated to be mediated by a N-terminal dileucine motif. By disrupting this motif, DIRC2 can be redirected to the plasma membrane. Finally, in a whole-cell electrophysiological assay based on heterologous expression of the targeting mutant at the plasma membrane of Xenopus oocytes, the application of a complex metabolic mixture evokes an outward current associated with the surface expression of full-length DIRC2. Taken together, these data strongly support the idea that DIRC2 is an electrogenic lysosomal metabolite transporter which is subjected to and presumably modulated by limited proteolytic processing.

CD2AP in mouse and human podocytes controls a proteolytic program that regulates cytoskeletal structure and cellular survival

Kidney podocytes are highly differentiated epithelial cells that form interdigitating foot processes with bridging slit diaphragms (SDs) that regulate renal ultrafiltration. Podocyte injury results in proteinuric kidney disease, and genetic deletion of SD-associated CD2-associated protein (CD2AP) leads to progressive renal failure in mice and humans. Here, we have shown that CD2AP regulates the TGF-β1-dependent translocation of dendrin from the SD to the nucleus. Nuclear dendrin acted as a transcription factor to promote expression of cytosolic cathepsin L (CatL). CatL proteolyzed the regulatory GTPase dynamin and the actin-associated adapter synaptopodin, leading to a reorganization of the podocyte microfilament system and consequent proteinuria. CD2AP itself was proteolyzed by CatL, promoting sustained expression of the protease during podocyte injury, and in turn increasing the apoptotic susceptibility of podocytes to TGF-β1. Our study identifies CD2AP as the gatekeeper of the podocyte TGF-β response through its regulation of CatL expression and defines a molecular mechanism underlying proteinuric kidney disease.

Studies of intestinal morphology and cathepsin B expression in a transgenic mouse aiming at intestine-specific expression of Cath B-EGFP

Cathepsin B has been shown to not only reside within endo-lysosomes of intestinal epithelial cells, but it was also secreted into the extracellular space of intestinal mucosa in physiological and pathological conditions. In an effort to further investigate the function of this protease in the intestine, we generated a transgenic mouse model that would enable us to visualize the localization of cathepsin B in vivo. Previously we showed that the A33-antigen promoter could be successfully used in vitro in order to express cathepsin B-green fluorescent protein chimeras in cells that co-expressed the intestine-specific transcription factor Cdx1. In this study an analog approach was used to express chimeric cathepsin B specifically in the intestine of transgenic animals. No overt phenotype was observed for the transgenic mice that reproduced normally. Biochemical and morphological studies confirmed that the overall intestinal phenotype including the structure and polarity of this tissue as well as cell numbers and differentiation states were not altered in the A33-CathB-EGFP mice when compared to wild type animals. However, transgenic expression of chimeric cathepsin B could not be visualized because it was not translated in situ although the transgene was maintained over several generations.

Ferri-liposomes as an MRI-visible drug-delivery system for targeting tumours and their microenvironment

The tumour microenvironment regulates tumour progression and the spread of cancer in the body. Targeting the stromal cells that surround cancer cells could, therefore, improve the effectiveness of existing cancer treatments. Here, we show that magnetic nanoparticle clusters encapsulated inside a liposome can, under the influence of an external magnet, target both the tumour and its microenvironment. We use the outstanding T2 contrast properties (r2=573-1,286 s(-1) mM(-1)) of these ferri-liposomes, which are ∼95 nm in diameter, to non-invasively monitor drug delivery in vivo. We also visualize the targeting of the tumour microenvironment by the drug-loaded ferri-liposomes and the uptake of a model probe by cells. Furthermore, we used the ferri-liposomes to deliver a cathepsin protease inhibitor to a mammary tumour and its microenvironment in a mouse, which substantially reduced the size of the tumour compared with systemic delivery of the same drug.

Specific functions of lysosomal proteases in endocytic and autophagic pathways

Endolysosomal vesicles form a highly dynamic multifunctional cellular compartment that contains multiple highly potent proteolytic enzymes. Originally these proteases have been assigned to cooperate solely in executing the unselective ‘bulk proteolysis’ within the acidic milieu of the lysosome. Although to some degree this notion still holds true, evidence is accumulating for specific and regulatory functions of individual ‘acidic’ proteases in many cellular processes linked to the endosomal/lysosomal compartment. Here we summarize and discuss the functions of individual endolysosomal proteases in such diverse processes as the termination of growth factor signaling, lipoprotein particle degradation, infection, antigen presentation, and autophagy. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Cellular senescence induced by cathepsin X downregulation

Cellular senescence represents a powerful tumor suppressor mechanism to prevent proliferation and invasion of malignant cells. Since tumor cells as well as primary fibroblasts lacking the lysosomal cysteine-type carboxypeptidase cathepsin X exhibit a reduced invasive capacity, we hypothesized that the underlying reason may be the induction of cellular senescence. To investigate the cellular and molecular mechanisms leading to diminished migration/invasion of cathepsin X-deficient cells, we have analyzed murine embryonic fibroblasts (MEF) derived from cathepsin X-deficient mice and neonatal human dermal fibroblasts (NHDF) transfected with siRNAs targeting cathepsin X. Remarkably, both cell types exhibited a flattened and enlarged cell body, a characteristic phenotype of senescent cells. Additional evidence for accelerated senescence was obtained by detection of the common senescence marker β-galactosidase. Further examination revealed increased expression levels of senescence-associated genes such as p16, p21, p53, and caveolin in these cells along with a reduced proliferation rate. The accelerated cellular senescence induced by cathepsin X deficiency was rescued by simultaneous expression of exogenous cathepsin X. Finally, cell cycle analysis confirmed a marked reduction of the synthesis rate and prolongation of the S-phase, while susceptibility to apoptosis of cathepsin X-deficient cells remained unchanged. In conclusion, cathepsin X deficiency leads to accelerated cellular senescence and consequently to diminished cellular proliferation and migration/invasion implying a potential role of cathepsin X in bypassing cellular senescence.

LIMP-2 links late phagosomal trafficking with the onset of the innate immune response to Listeria monocytogenes: a role in macrophage activation

The innate immune response to Listeria monocytogenes depends on phagosomal bacterial degradation by macrophages. Here, we describe the role of LIMP-2, a lysosomal type III transmembrane glycoprotein and scavenger-like protein, in Listeria phagocytosis. LIMP-2-deficient mice display a macrophage-related defect in Listeria innate immunity. They produce less acute phase pro-inflammatory cytokines/chemokines, MCP-1, TNF-α, and IL-6 but normal levels of IL-12, IL-10, and IFN-γ and a 25-fold increase in susceptibility to Listeria infection. This macrophage defect results in a low listericidal potential, poor response to TNF-α activation signals, impaired phago-lysosome transformation into antigen-processing compartments, and uncontrolled LM cytosolic growth that fails to induce normal levels of acute phase pro-inflammatory cytokines. LIMP-2 transfection of CHO cells confirmed that LIMP-2 participates in the degradation of Listeria within phagosomes, controls the late endosomal/lysosomal fusion machinery, and is linked to the activation of Rab5a. Therefore, the role of LIMP-2 appears to be connected to the TNF-α-dependent and early activation of Listeria macrophages through internal signals linking the regulation of late trafficking events with the onset of the innate Listeria immune response.

Cathepsin D is one of the major enzymes involved in intracellular degradation of AGE-modified proteins

Oxidized and cross-linked modified proteins are known to accumulate in ageing. Little is known about whether the accumulation of proteins modified by advanced glycation end products (AGEs) is due to an affected intracellular degradation. Therefore, this study was designed to determine whether the intracellular enzymes cathepsin B, cathepsin D and the 20S proteasome are able to degrade AGE-modified proteins in vitro. It shows that AGE-modified albumin is degraded by cathepsin D, while cathepsin B was less effective in the degradation of aldehyde-modified albumin and the 20S proteasome was completely unable to degrade them. Mouse primary embryonic fibroblasts isolated from a cathepsin D knockout animals were found to have an extensive intracellular AGE-accumulation, mainly in lysosomes, and a reduction of AGE-modified protein degradation compared to cells isolated from wild type animals. In summary, it can be assumed that cathepsin D plays a significant role in the removal of AGE-modified proteins.

Deletion of cathepsin H perturbs angiogenic switching, vascularization and growth of tumors in a mouse model of pancreatic islet cell cancer

Proteases can regulate many aspects of tumor development as their actions, which include degradation of the extracellular matrix, proteolytic processing of chemokines and activation of other enzymes, influence several key tumorigenic processes. Members of one protease class, the cysteine cathepsins, have received increasing recognition for their involvement in cancer development, and numerous clinical studies have reported correlations between elevated cathepsin levels and malignant progression. This is also the case for cathepsin H, a member of the cysteine cathepsin family, and its utility as a prognostic marker has been analyzed extensively. However, there is limited information available on its specific functions in tumor development and progression. To gain further insight into the role of this protease in cancer, we crossed cathepsin H-deficient mice with the RIP1-Tag2 model of pancreatic islet carcinogenesis. Deletion of cathepsin H significantly impaired angiogenic switching of the pre-malignant hyperplastic islets and resulted in a reduction in the subsequent number of tumors that formed. Moreover, the tumor burden in cathepsin H null RT2 mice was significantly reduced, in association with defects in the blood vasculature and increased apoptosis. Thus, we demonstrate here for the first time important tumor-promoting roles for cathepsin H in vivo using a mouse model of human cancer.

Human and mouse perforin are processed in part through cleavage by the lysosomal cysteine proteinase cathepsin L

The pore-forming protein perforin is synthesized as an inactive precursor in natural killer (NK) cells and cytotoxic T lymphocytes (CTLs), and becomes active when a short C-terminal peptide is cleaved within acidic lysosome-like cytotoxic granules. Although it was shown more than a decade ago that this cleavage is pH dependent and can be inhibited by the generic cysteine cathepsin inhibitor E-64d, no protease capable of processing the perforin C terminus has been identified. Neither is it known whether a single protease is responsible or the processing has inbuilt redundancy. Here, we show that incubation of human NK cells and primary antigen-restricted mouse CTLs with the cathepsin L (CatL) inhibitor L1 resulted in a marked inhibition of perforin-dependent target cell death and reduced perforin processing. In vitro, CatL preferentially cleaved a site on full-length recombinant perforin close to its C terminus. The NK cells of mice deficient in CatL showed a reduction but not a complete absence of processed perforin, indicating that cysteine proteases other than CatL are also able to process perforin. We conclude that granule-bound cathepsins are essential for processing perforin to its active form, and that CatL is an important, but not exclusive, participant in this process.

Specialized roles for cysteine cathepsins in health and disease

Cathepsins were originally identified as proteases that act in the lysosome. Recent work has uncovered nontraditional roles for cathepsins in the extracellular space as well as in the cytosol and nucleus. There is strong evidence that subspecialized and compartmentalized cathepsins participate in many physiologic and pathophysiologic cellular processes, in which they can act as both digestive and regulatory proteases. In this review, we discuss the transcriptional and translational control of cathepsin expression, the regulation of intracellular sorting of cathepsins, and the structural basis of cathepsin activation and inhibition. In particular, we highlight the emerging roles of various cathepsin forms in disease, particularly those of the cardiac and renal systems.

Cathepsin X-deficient gastric epithelial cells in co-culture with macrophages: characterization of cytokine response and migration capability after Helicobacter pylori infection

Our previous studies have shown an association between Helicobacter pylori infection, the strong up-regulation of cathepsin X (CTSX, also called cathepsin Z/P), and the development of gastric cancer. In the present study, we analyzed primary and conventional gastric epithelial cell lines to establish an optimal in vitro mouse model system for the examination of H. pylori-induced overexpression of Ctsx in a functional way. Gastric epithelial cells were isolated from stomachs of wild-type C57BL6/N and Ctsx(-/-) mice and compared with the gastric cancer cell line CLS103. Indirect co-cultures of epithelial cells and macrophages were infected with H. pylori strain SS1 and analyzed for the expression of cathepsins, cytokines, and adhesion factors. Cellular interactions, migration capability, and adherence of H. pylori were assessed using time-lapse video microscopy and colony-forming assays. Isolated primary cells from wild-type and transgenic mice revealed qualities and expression profiles similar to those of corresponding tissue samples. Adherence of H. pylori was significantly higher in primary compared with commercially cells. Thus, induction of cathepsins, cytokines, and adhesion proteins was detected solely in primary cells and co-cultured macrophages. Microarray and migration experiments indicated that Ctsx is involved in B/T-cell proliferation/migration and adhesion of macrophages. Primary epithelial cells from stomach of Ctsx(-/-) mice represent an excellent model of H. pylori gastritis to elaborate the special functions of Ctsx in regulating the immune response to H. pylori.

Impaired turnover of autophagolysosomes in cathepsin L deficiency

Some of the phenotypes of mice deficient for the lysosomal cysteine endopeptidase cathepsin L (Ctsl) are characterized by large dysmorphic vesicles in the cytoplasm. Specifically, the heart (dilative cardiomyopathy), the thyroid (impaired thyroglobulin processing) and keratinocytes (periodic hair loss and epidermal hyperproliferation) are affected. We hypothesized that the formation of aberrant vesicles is owing to defects in macroautophagy. Therefore, primary mouse embryonic fibroblasts (MEF), which were derived from Ctsl(-/-) animals crossed with mice transgenic for the autophagy marker GFP-LC3, were investigated. Ctsl(-/-) MEF show increased number and size of vesicular structures belonging to the 'acidic' cellular compartment and are also characterized by GFP-LC3. Induction of autophagy by nutrient starvation or rapamycin treatment showed no significant impairment of the initiation of autophagy, the formation of autophagosomes or autophagosome-lysosome fusion in Ctsl(-/-) MEF, but co-localization of GFP-LC3 and Lamp1 revealed unusually large autophagolysosomes filled with Lamp1. Furthermore, the soluble lysosomal enzyme cathepsin D was elevated in Ctsl(-/-) MEF. Thus, degradation of autophagolysosomal content is impaired in the absence of Ctsl. This could slow the turnover of autophagolysosomes and result in accumulation of the dysmorphic and 'acidic' vesicles that were previously described in the context of the pathological phenotypes of Ctsl(-/-) mice.

The cystatin M/E-cathepsin L balance is essential for tissue homeostasis in epidermis, hair follicles, and cornea

Cystatin M/E (CST6) is a nonredundant, epithelium-specific protease inhibitor with a presumed role in epidermal differentiation and tumor suppression. We have previously reported that cystatin M/E deficiency in Cst6(-/-) mice causes neonatal lethality because of excessive transepidermal water loss. Biochemical evidence suggests that cystatin M/E controls the activity of legumain, cathepsin L, cathepsin V, and transglutaminase-3. Using a genetic approach we sought to define the role of cystatin M/E in epithelial biology by identification of its target proteases and their downstream functions. Ablation of cathepsin L in a Cst6(-/-) background (Cst6(-/-)Ctsl(-/-) double-knockout mice) restored viability and resulted in normalization of stratum corneum morphology. Ablation of legumain or transglutaminase-3 in Cst6(-/-) mice, however, did not rescue the lethal phenotype. Intriguingly, both Cst6(-/-)Ctsl(-/-) and Cst6(-/-)Ctsl(+/-) mice were viable, but the absence of cystatin M/E caused scarring alopecia in adult animals. In the cornea of Cst6(-/-)Ctsl(+/-) mice, we observed keratitis, hyperplasia, and transition to a cornified epithelium. Evidence is provided that activation of cathepsin D and transglutaminase-1 are downstream events, dependent of cathepsin L activity. We conclude that a tightly regulated balance between cathepsin L and cystatin M/E is essential for tissue integrity in epidermis, hair follicles, and corneal epithelium.

Distinct protease requirements for antigen presentation in vitro and in vivo

Asparagine endopeptidase (AEP) or legumain is a potentially important Ag-processing enzyme that introduces limited cleavages that trigger unfolding and class II MHC binding of different Ag substrates. AEP is necessary and sufficient for optimal processing and presentation of the tetanus toxin C fragment (TTCF) Ag in vitro, but its importance has not been tested in vivo. Surprisingly, virtually normal T cell and Ab responses to TTCF were mounted in AEP-deficient mice when examined 10 d after immunization. This was the case when TTCF was emulsified with CFA, adsorbed onto alum, or expressed within live Salmonella typhimurium. In addition, the dominant Ab and T cell determinants recognized in TTCF were essentially unchanged in AEP-deficient mice. These data are explained, at least in part, by the much lower levels of AEP expressed in primary murine APCs compared with immortalized B cell lines. Even so, the initial in vivo kinetics of TTCF presentation were slower in AEP-deficient mice and, as expected, boosting AEP levels in primary APCs enhanced and accelerated TTCF processing and presentation in vitro. Thus, AEP remains the protease of choice for TTCF processing; however, in its absence, other enzymes can substitute to enable slower, but equally robust, adaptive immune responses. Moreover, clear relationships between Ags and processing proteases identified from short-term in vitro processing and presentation studies do not necessarily predict an absolute in vivo dependency on those processing enzymes, not least because they may be expressed at strikingly different levels in vitro versus in vivo.

Cathepsin H is an additional convertase of pro-granzyme B

The serine protease granzyme B (GrB) is the most potent proapoptotic cytotoxin of the granule exocytosis pathway of cytotoxic lymphocytes. GrB is synthesized as a zymogen (proGrB) and activated in cytotoxic granules by the lysosomal cysteine protease cathepsin C (CatC) which removes the N-terminal dipeptide Gly-Glu. It has been shown recently that mice lacking CatC nonetheless express significant residual GrB activity, indicating the presence of additional proGrB convertases. Here, we describe an assay to assess activation of proGrB and show that the amino-peptidase cathepsin H (CatH) has proGrB convertase activity in vitro, whereas dipeptidylpeptidase II does not. We generated mice lacking both CatC and CatH expression (CatCH(-/-)) and found that their lymphocytes have reduced convertase activity compared with those from CatC-deficient mice. Despite this, cytotoxic lymphocytes from CatCH(-/-) mice retain cytotoxic activity and some residual GrB activity. We conclude that CatH can act as an additional proGrB convertase and that other protease/s (apart from dipeptidylpeptidase II) must also possess convertase activity. This indicates a great deal of functional redundancy in GrB maturation, which would prevent pathogen-mediated immune suppression by via convertase inhibition.