Expression of human cathepsin L or human cathepsin V in mouse thymus mediates positive selection of T helper cells in cathepsin L knock-out mice

A genetic deficiency of the cysteine protease cathepsin L (Ctsl) in mice results in impaired positive selection of conventional CD4+ T helper cells as a result of an incomplete processing of the MHC class II associated invariant chain or incomplete proteolytic generation of positively selecting peptide ligands. The human genome encodes, in contrast to the mouse genome, for two cathepsin L proteases, namely cathepsin L (CTSL) and cathepsin V (CTSV; alternatively cathepsin L2). In the human thymic cortex, CTSV is the predominately expressed protease as compared to CTSL or other cysteine cathepsins. In order to analyze the functions of CTSL and CTSV in the positive selection of CD4+ T cells we employed Ctsl knock-out mice crossed either with transgenic mice expressing CTSL under the control of its genuine human promoter or with transgenic mice expressing CTSV under the control of the keratin 14 (K14) promoter, which drives expression to the cortical epithelium. Both human proteases are expressed in the thymus of the transgenic mice, and independent expression of both CTSL and CTSV rescues the reduced frequency of CD4+ T cells in Ctsl-deficient mice. Moreover, the expression of the human cathepsins does not change the number of CD4+CD25+Foxp3+ regulatory T cells, but the normalization of the frequency of conventional CD4+ T cell in the transgenic mice results in a rebalancing of conventional T cells and regulatory T cells. We conclude that the functional differences of CTSL and CTSV in vivo are not mainly determined by their inherent biochemical properties, but rather by their tissue specific expression pattern.

Unimpeded skin carcinogenesis in K14-HPV16 transgenic mice deficient for plasminogen activator inhibitor

Angiogenesis, extracellular matrix remodeling and cell migration are associated with cancer progression and involve at least, the plasminogen activating system and its main physiological inhibitor, the plasminogen activator inhibitor-1 (PAI-1). Considering the recognized importance of PAI-1 in the regulation of tumor angiogenesis and invasion in murine models of skin tumor transplantation, we explored the functional significance of PAI-1 during early stages of neoplastic progression in the transgenic mouse model of multistage epithelial carcinogenesis (K14-HPV16 mice). We have studied the effect of genetic deletion of PAI-1 on inflammation, angiogenesis, lymphangiogenesis and tumor progression. In this model, PAI-1 deficiency neither impaired keratinocyte hyperproliferation or tumor development nor affected the infiltration of inflammatory cells and development of angiogenic or lymphangiogenic vasculature. We are reporting evidence for concomitant lymphangiogenic and angiogenic switches independent to PAI-1 status. Taken together, these data indicate that PAI-1 is not rate limiting for neoplastic progression and vascularization during premalignant progression, or that there is a functional redundancy between PAI-1 and other tumor regulators, masking the effect of PAI-1 deficiency in this long-term model of multistage epithelial carcinogenesis.

Cathepsin L inactivates human trypsinogen, whereas cathepsin L-deletion reduces the severity of pancreatitis in mice

BACKGROUND & AIMS

Acute pancreatitis is characterized by an activation cascade of digestive enzymes in the pancreas. The first of these, trypsinogen, can be converted to active trypsin by the peptidase cathepsin B (CTSB). We investigated whether cathepsin L (CTSL) can also process trypsinogen to active trypsin and has a role in pancreatitis.

METHODS

In CTSL-deficient (Ctsl(-/-)) mice, pancreatitis was induced by injection of cerulein or infusion of taurocholate into the pancreatic duct. Human tissue, pancreatic juice, mouse pancreatitis specimens, and recombinant enzymes were studied by enzyme assay, immunoblot, N-terminal sequencing, immunocytochemistry, and electron microscopy analyses. Isolated acini from Ctsl(-/-) and Ctsb(-/-) mice were studied.

RESULTS

CTSL was expressed in human and mouse pancreas, colocalized with trypsinogen in secretory vesicles and lysosomes, and secreted into pancreatic juice. Severity of pancreatitis was reduced in Ctsl(-/-) mice, whereas apoptosis and intrapancreatic trypsin activity were increased. CTSL-induced cleavage of trypsinogen occurred 3 amino acids toward the C-terminus from the CTSB activation site and resulted in a truncated, inactive form of trypsin and an elongated propeptide (trypsinogen activation peptide [TAP]). This elongated TAP was not detected by enzyme-linked immunosorbent assay (ELISA) but was effectively converted to an immunoreactive form by CTSB. Levels of TAP thus generated by CTSB were not associated with disease severity, although this is what the TAP-ELISA is used to determine in the clinic.

CONCLUSIONS

CTSL inactivates trypsinogen and counteracts the ability of CTSB to form active trypsin. In mouse models of pancreatitis, absence of CTSL induces apoptosis and reduces disease severity.

Deficiency for the cysteine protease cathepsin L promotes tumor progression in mouse epidermis

To define a functional role for the endosomal/lysosomal cysteine protease cathepsin L (Ctsl) during squamous carcinogenesis, we generated mice harboring a constitutive Ctsl deficiency in addition to epithelial expression of the human papillomavirus type 16 oncogenes (human cytokeratin 14 (K14)-HPV16). We found enhanced tumor progression and metastasis in the absence of Ctsl. As tumor progression in K14-HPV16 mice is dependent on inflammation and angiogenesis, we examined immune cell infiltration and vascularization without finding any effect of the Ctsl genotype. In contrast, keratinocyte-specific transgenic expression of cathepsin V, the human orthologue of mouse Ctsl, in otherwise Ctsl-deficient K14-HPV16 mice restored the phenotype observed in the control HPV16 skin. To better understand this phenotype at the molecular level, we measured several oncogenic signal transduction pathways in primary keratinocytes on stimulation with keratinocyte-conditioned cell culture medium. We found increased activation of protein kinase B/Akt and mitogen-activated protein kinase pathways in protease-deficient cells, especially if treated with media conditioned by Ctsl-deficient keratinocytes. Similarly, the level of active GTP-Ras was increased in Ctsl-deficient epidermis. We conclude that Ctsl is critical for the termination of growth factor signaling in the endosomal/lysosomal compartment of keratinocytes and, therefore, functions as an anti-tumor protease.

CA/C1 peptidases of the malaria parasites Plasmodium falciparum and P. berghei and their mammalian hosts--a bioinformatical analysis

In genome-wide screens we studied CA/C1 peptidases of malaria-causing plasmodia and their hosts (man and mouse). For Plasmodium falciparum and P. berghei, several new CA/C1 peptidase genes encoding proteases of the L- and B-family with specific promoter modules were identified. In addition, two new human CA/C1 peptidase loci and one new mouse gene locus were found; otherwise, the sets of CA/C1 peptidase genes in man and mouse seem to be complete now. In each species studied there is a multitude of CA/C1 peptidases with lysosomal localization signals and partial functional overlap according to similar but subfamily-specific structures. Individual target structures in plasmodia include residues specifically different in CA/C1 peptidase subsite 2. This is of medical interest considering CA/C1 peptidase inhibition for chemotherapy in malaria, malignancies and other diseases. Promoter structures and mRNA regulation differ widely among CA/C1 peptidase subfamilies and between mammals and plasmodia. We characterized promoter modules conserved in mouse and man for the CA/C1 peptidase families B and L (with the L-like subfamily, F-like subfamily and mouse-specific J-like subfamily). RNA motif searches revealed conserved regulatory elements such as GAIT elements; plasmodial CA/C1 peptidase mRNA elements include ARE elements and mammalian mRNAs contain 15-lox DICE elements.

Cathepsin L participates in dynorphin production in brain cortex, illustrated by protease gene knockout and expression

Dynorphin opioid neuropeptides mediate neurotransmission for analgesia and behavioral functions. Dynorphin A, dynorphin B, and alpha-neoendorphin are generated from prodynorphin by proteolytic processing. This study demonstrates the significant role of the cysteine protease cathepsin L for producing dynorphins. Cathepsin L knockout mouse brains showed extensive decreases in dynorphin A, dynorphin B, and alpha-neoendorphin that were reduced by 75%, 83%, and 90%, respectively, compared to controls. Moreover, cathepsin L in brain cortical neurons was colocalized with dynorphins in secretory vesicles, the primary site of neuropeptide production. Cellular coexpression of cathepsin L with prodynorphin in PC12 cells resulted in increased production of dynorphins A and B. Comparative studies of PC1/3 and PC2 convertases showed that PC1/3 knockout mouse brains had a modest decrease in dynorphin A, and PC2 knockout mice showed a minor decrease in alpha-neoendorphin. Overall, these results demonstrate a prominent role for cathepsin L, jointly with PC1/3 and PC2, for production of dynorphins in brain.

Cathepsin L plays a major role in cholecystokinin production in mouse brain cortex and in pituitary AtT-20 cells: protease gene knockout and inhibitor studies

Cholecystokinin (CCK) is a peptide neurotransmitter whose production requires proteolytic processing of the proCCK precursor to generate active CCK8 neuropeptide in brain. This study demonstrates the significant role of the cysteine protease cathepsin L for CCK8 production. In cathepsin L knockout (KO) mice, CCK8 levels were substantially reduced in brain cortex by an average of 75%. To evaluate the role of cathepsin L in producing CCK in the regulated secretory pathway of neuroendocrine cells, pituitary AtT-20 cells that stably produce CCK were treated with the specific cathepsin L inhibitor, CLIK-148. CLIK-148 inhibitor treatment resulted in decreased amounts of CCK secreted from the regulated secretory pathway of AtT-20 cells. CLIK-148 also reduced cellular levels of CCK9 (Arg-CCK8), consistent with CCK9 as an intermediate product of cathepsin L, shown by the decreased ratio of CCK9/CCK8. The decreased CCK9/CCK8 ratio also suggests a shift in the production to CCK8 over CCK9 during inhibition of cathepsin L. During reduction of the PC1/3 processing enzyme by siRNA, the ratio of CCK9/CCK8 was increased, suggesting a shift to the cathepsin L pathway for the production of CCK9. The changes in ratios of CCK9 compared to CCK8 are consistent with dual roles of the cathepsin L protease pathway that includes aminopeptidase B to remove NH2-terminal Arg or Lys, and the PC1/3 protease pathway. These results suggest that cathepsin L functions as a major protease responsible for CCK8 production in mouse brain cortex, and participates with PC1/3 for CCK8 production in pituitary cells.

Cathepsin B release from rodent intestine mucosa due to mechanical injury results in extracellular matrix damage in early post-traumatic phases

An in vivo model was used to investigate the role of cathepsins in mouse intestine after mechanical manipulation. Inspection of different intestine segments by immunofluorescence microscopy provided evidence for a local release of cathepsin B from cells of individual gut sections shortly after traumatic injury. Densitometry of immunoblots ruled out alterations in cathepsin B expression levels. Because similar results were obtained with both mouse and rat intestine trauma models, we were interested in identifying potential targets of released cathepsin B in early post-traumatic phases. Immunoblotting revealed initial declines followed by an increase in protein levels of claudin-1 and E-cadherin, indicating that tight junctions and cell-cell adhesions were only transiently compromised by surgical trauma. Apical aminopeptidase N and dipeptidyl peptidase IV were only slightly affected, whereas basolateral low-density lipoprotein receptors were strongly up-regulated in response to trauma. As potential targets of cathepsin B released from injured cells, we identified collagen IV and laminin of the basement membrane that was damaged during initial post-traumatic stages. Because increased collagen IV expression was observed in the intestine of cathepsin B-deficient animals, we propose a direct role of cathepsin B in that it contributes to acute post-traumatic extracellular matrix damage and may thereby facilitate onset of post-operative ileus.

Genetic cathepsin B deficiency reduces beta-amyloid in transgenic mice expressing human wild-type amyloid precursor protein

Neurotoxic beta-amyloid (Abeta) peptides participate in Alzheimer's disease (AD); therefore, reduction of Abeta generated from APP may provide a therapeutic approach for AD. Gene knockout studies in transgenic mice producing human Abeta may identify targets for reducing Abeta. This study shows that knockout of the cathepsin B gene in mice expressing human wild-type APP (hAPPwt) results in substantial decreases in brain Abeta40 and Abeta42 by 67% and decreases in levels of the C-terminal beta-secretase fragment (CTFbeta) derived from APP. In contrast, knockout of cathepsin B in mice expressing hAPP with the rare Swedish (Swe) and Indiana (Ind) mutations had no effect on Abeta. The difference in reduction of Abeta in hAPPwt mice, but not in hAPPSwe/Ind mice, shows that the transgenic model can affect cathepsin B gene knockout results. Since most AD patients express hAPPwt, these data validate cathepsin B as a target for development of inhibitors to lower Abeta in AD.

Murine and human cathepsin B exhibit similar properties: possible implications for drug discovery

Validation of drug targets and subsequent preclinical studies are usually carried out on animal disease models, with mouse being the most commonly used. However, results from mouse models cannot always be directly related to human disease. Major discrepancies between the properties of murine and human variants were observed during the evaluation of compounds targeting cathepsins S and K. It is important, therefore, to know whether similar differences exist between murine and human cathepsin B. Thus, both enzymes were expressed and biochemically characterized. The enzymes exhibited similar biochemical properties, indicating that cathepsin B transgenic mouse models could be useful for studying its role in human pathologies.

Trial of the cysteine cathepsin inhibitor JPM-OEt on early and advanced mammary cancer stages in the MMTV-PyMT-transgenic mouse model

Recent data suggest proteases of the papain-like cysteine cathepsin family as molecular targets for cancer therapy. Here, we report the treatment of polyoma middle T oncogene-induced breast cancers in mice with the cell-permeable broad-spectrum cysteine cathepsin inhibitor JPM-OEt. Up to 100 mg/kg inhibitor was intraperitoneally injected once per day in two trials on early and advanced cancers. In both trials, transient delays in tumour growth were observed. However, at the endpoint of both experiments no significant differences in tumour weights, histopathology and lung metastasis were found between the inhibitor and the control group. The invasive strand formation of collagen I-embedded tumour cell spheroids generated from primary tumours of inhibitor-treated mice in the early cancer trial could be inhibited in vitro by JPM-OEt; a result arguing against induction of resistance to the inhibitor. Measurement of cysteine cathepsin activities in tissue extracts after intraperitoneal injection of JPM-OEt revealed effective inhibition of cysteine cathepsins in pancreas, kidneys and liver, while activities in mammary cancers and in lungs were not significantly affected. We conclude that the pharmacokinetic properties of JPM-OEt, which result in poor bioavailability, may prohibit its use for stand-alone treatment of solid mammary cancers and their lung metastases.

Major role of cathepsin L for producing the peptide hormones ACTH, beta-endorphin, and alpha-MSH, illustrated by protease gene knockout and expression

The pituitary hormones adrenocorticotropic hormone (ACTH), beta-endorphin, and alpha-melanocyte stimulating hormone (alpha-MSH) are synthesized by proteolytic processing of their common proopiomelanocortin (POMC) precursor. Key findings from this study show that cathepsin L functions as a major proteolytic enzyme for the production of POMC-derived peptide hormones in secretory vesicles. Specifically, cathepsin L knock-out mice showed major decreases in ACTH, beta-endorphin, and alpha-MSH that were reduced to 23, 18, and 7% of wild-type controls (100%) in pituitary. These decreased peptide levels were accompanied by increased levels of POMC consistent with proteolysis of POMC by cathepsin L. Immunofluorescence microscopy showed colocalization of cathepsin L with beta-endorphin and alpha-MSH in the intermediate pituitary and with ACTH in the anterior pituitary. In contrast, cathepsin L was only partially colocalized with the lysosomal marker Lamp-1 in pituitary, consistent with its extralysosomal function in secretory vesicles. Expression of cathepsin L in pituitary AtT-20 cells resulted in increased ACTH and beta-endorphin in the regulated secretory pathway. Furthermore, treatment of AtT-20 cells with CLIK-148, a specific inhibitor of cathepsin L, resulted in reduced production of ACTH and accumulation of POMC. These findings demonstrate a prominent role for cathepsin L in the production of ACTH, beta-endorphin, and alpha-MSH peptide hormones in the regulated secretory pathway.

The NALP3 inflammasome is involved in the innate immune response to amyloid-beta

The fibrillar peptide amyloid-beta (A beta) has a chief function in the pathogenesis of Alzheimer's disease. Interleukin 1 beta (IL-1 beta) is a key cytokine in the inflammatory response to A beta. Insoluble materials such as crystals activate the inflammasome formed by the cytoplasmic receptor NALP3, which results in the release of IL-1 beta. Here we identify the NALP3 inflammasome as a sensor of A beta in a process involving the phagocytosis of A beta and subsequent lysosomal damage and release of cathepsin B. Furthermore, the IL-1 beta pathway was essential for the microglial synthesis of proinflammatory and neurotoxic factors, and the inflammasome, caspase-1 and IL-1 beta were critical for the recruitment of microglia to exogenous A beta in the brain. Our findings suggest that activation of the NALP3 inflammasome is important for inflammation and tissue damage in Alzheimer's disease.

Cathepsin L participates in the production of neuropeptide Y in secretory vesicles, demonstrated by protease gene knockout and expression

Neuropeptide Y (NPY) functions as a peptide neurotransmitter and as a neuroendocrine hormone. The active NPY peptide is generated in secretory vesicles by proteolytic processing of proNPY. Novel findings from this study show that cathepsin L participates as a key proteolytic enzyme for NPY production in secretory vesicles. Notably, NPY levels in cathepsin L knockout (KO) mice were substantially reduced in brain and adrenal medulla by 80% and 90%, respectively. Participation of cathepsin L in producing NPY predicts their colocalization in secretory vesicles, a primary site of NPY production. Indeed, cathepsin L was colocalized with NPY in brain cortical neurons and in chromaffin cells of adrenal medulla, demonstrated by immunofluorescence confocal microscopy. Immunoelectron microscopy confirmed the localization of cathepsin L with NPY in regulated secretory vesicles of chromaffin cells. Functional studies showed that coexpression of proNPY with cathepsin L in neuroendocrine PC12 cells resulted in increased production of NPY. Furthermore, in vitro processing indicated cathepsin L processing of proNPY at paired basic residues. These findings demonstrate a role for cathepsin L in the production of NPY from its proNPY precursor. These studies illustrate the novel biological role of cathepsin L in the production of NPY, a peptide neurotransmitter, and neuroendocrine hormone.

Reduced tumour cell proliferation and delayed development of high-grade mammary carcinomas in cathepsin B-deficient mice

Expression levels of the papain-like cysteine protease cathepsin B (Ctsb) have been positively correlated with mammary tumour progression and metastasis; however, its roles in the hallmark processes of malignant growth remain poorly defined. Using Ctsb-deficient mice we investigated tumour cell differentiation, proliferation and apoptosis in the Tg(MMTV-PyMT) mouse mammary cancer model. Absence of Ctsb significantly impaired development of high-grade invasive ductal carcinomas and reduced the metastatic burden in the lungs. Mice lacking Ctsb exhibited reduced cell proliferation in mammary carcinomas and their lung metastases. Notably, intravenous injection of primarily isolated, Ctsb-expressing tumour cells into congenic Ctsb-deficient mice revealed impaired cell proliferation in the resulting experimental lung metastases, providing evidence for the involvement of Ctsb in paracrine regulation of cancer cell proliferation. No Ctsb genotype-dependent difference in tumour cell death was observed in vivo or by treatment of isolated PyMT cancer cells with tumour necrosis factor-alpha. However, cancer cells lacking Ctsb exhibited significantly higher resistance to apoptosis induction by the lysosomotropic agent Leu-Leu-OMe. Thus, our results indicate an in vivo role for Ctsb in promoting cellular anaplasia in mammary cancers and proliferation in lung metastases.

Caspase-8 is activated by cathepsin D initiating neutrophil apoptosis during the resolution of inflammation

In the resolution of inflammatory responses, neutrophils rapidly undergo apoptosis. We describe a new proapoptotic pathway in which cathepsin D directly activates caspase-8. Cathepsin D is released from azurophilic granules in neutrophils in a caspase-independent but reactive oxygen species–dependent manner. Under inflammatory conditions, the translocation of cathepsin D in the cytosol is blocked. Pharmacological or genetic inhibition of cathepsin D resulted in delayed caspase activation and reduced neutrophil apoptosis. Cathepsin D deficiency or lack of its translocation in the cytosol prolongs innate immune responses in experimental bacterial infection and in septic shock. Thus, we identified a new function of azurophilic granules that is in addition to their role in bacterial defense mechanisms: to regulate the life span of neutrophils and, therefore, the duration of innate immune responses through the release of cathepsin D.

Cell type-specific functions of the lysosomal protease cathepsin L in the heart

Deficiency of the lysosomal cysteine protease cathepsin L (Ctsl) in mice results in a phenotype affecting multiple tissues, including thymus, epidermis, and hair follicles, and in the heart develops as a progressive dilated cardiomyopathy (DCM). To understand the role of Ctsl in the maintenance of regular heart morphology and function, it is critical to determine whether the DCM in Ctsl-/- mice is primarily because of the lack of Ctsl expression and activity in the cardiomyocytes or is caused by the additional extracardiac pathologies. Cardiomyocyte-specific expression of Ctsl in Ctsl-/- mice, using an alpha-myosin heavy chain promoter-Ctsl transgene, results in improved cardiac contraction, normal mRNA expression of atrionatriuretic peptide, normal heart weight, and regular ultrastructure of cardiomyocytes. Epithelial expression of cathepsin L2 (CTSL2) by a K14 promoter-CTSL2-transgene resulted in rescue of the Ctsl-/- hair loss phenotype. In these mice, cardiac atrionatriuretic peptide expression and end systolic heart dimensions were also significantly attenuated. However, cardiac contraction was not improved, and increased heart weight as well as the typical changes in lysosomal ultrastructure of Ctsl-/- hearts persisted. Myocardial fibrosis was detected in all Ctsl-/- mice irrespective of transgene-mediated cardiac Ctsl expression or extracardiac CTSL2 expression. Expression of collagen 1 was not enhanced in Ctsl-/- hearts, but a reduced collagenolytic activity suggests a role for Ctsl in collagen turnover by cardiac fibroblasts. We conclude that the DCM of Ctsl-/- mice is primarily caused by absence of the protease in cardiomyocytes, whereas the complex gross phenotype of Ctsl-deficient mice, i.e. the fur defect, results in additional stress to the heart.

A transporter associated with antigen-processing independent vacuolar pathway for the MHC class I-mediated presentation of endogenous transmembrane proteins

MHC class I molecules present peptides derived from the ectodomains of endogenous transmembrane proteins; however, the processing of these Ags is incompletely understood. As model transmembrane Ags we investigated the processing of MHC-I-derived fusion proteins containing the N-terminally extended K(b)-restricted OVA epitope SIINFEKL in the extracytoplasmic domain. In TAP-deficient, nonprofessional APCs, the epitope was cleaved out of various sequence contexts and presented to T cells. Ag presentation was inhibited by acidophilic amines and inhibitors of the vacuolar proton pump, indicating processing in endosomes. Endosomal aspartic-type cathepsins, and to some extent also the trans-Golgi network protease furin, were involved in processing. Clathrin-dependent and independent internalization from the cell surface targeted MHC-I fusion proteins to early and late endosomes, where SIINFEKL/K(b) complexes were detected by immunofluorescence microscopy. Targeting of MHC-I fusion proteins to processing compartments was independent of sequence motifs in the cytoplasmic tail. Not only TAP-deficient cells, but also TAP-competent APCs used the vacuolar pathway for processing of MHC-I fusion proteins. Thus, endosomal processing of internalized endogenous transmembrane proteins represents a novel alternate pathway for the generation of MHC-I-binding peptides.

Emerging roles of cysteine cathepsins in disease and their potential as drug targets

The general view on cysteine cathepsins, which were long believed to be primarily involved in intracellular protein turnover, has dramatically changed in last 10 to 15 years. The discovery of new cathepsins, such as cathepsins K, V, X, F and O, and their tissue distribution suggested that at least some of them are involved in very specific cellular processes. Moreover, gene ablation experiments revealed that cathepsins play a vital role in numerous physiological processes, such as antigen processing and presentation, bone remodelling, prohormone processing and wound healing. Their involvement in several pathologies, including osteoporosis, rheumatoid arthritis, osteoarthritis, bronchial asthma and cancer have also been confirmed and today several of them have been validated as relevant targets for therapies. Compounds targeting cathepsins S and K are already in clinical evaluation, whereas others are in experimental phases. The cathepsin K inhibitor AAE-581 (balicatib) as the most advanced of them passed Phase II clinical trials in 2005. In this review, we discuss the current view on cathepsins as an emerging group of targets for several diseases and the development of cathepsin K and S inhibitors for treatment of osteoporosis and various immune disorders.