Biosynthesis of a lysosomal enzyme. Partial structure of two transient and functionally distinct NH2-terminal sequences in cathepsin D

Effect of accelerated aging on shelf-stability, product loss, sensory and biochemical characteristics in 2 lower quality beef cuts

The aim of this study was to determine the impact of accelerated aging (AA) on shelf stability, product loss, sensory and biochemical characteristics of 2 lower quality beef cuts. Triceps brachii (TB) and semimembranosus (SM) were collected and fabricated from 10 USDA Choice beef carcasses and assigned to 1 of 6 treatments: 3 d cooler aged (control), 21 d cooler aged, AA 49 °C for 2 h, AA 49 °C for 3 h, AA 54 °C for 2 h, and AA 54 °C for 3 h. The results showed that AA can decrease APC counts on steak surface and in purge and redness, but increase lightness and product loss of the steaks (P < 0.01). Lower shear force was also found for AA steaks compared to those from the control (P < 0.01), with the AA 54 °C treatments being comparable to 21 d cooler aging. However, the trained sensory panel determined AA steaks were less juicy and flavorful than those from the control and 21 d cooler aged samples (P < 0.05). There was no off-flavor detected in AA steaks though lipid oxidation was higher in AA samples than those in the control steaks (P < 0.01). The AA treatments stimulated cathepsin activity (P < 0.05), which may have enhanced the solubilization of stromal proteins and led to a different troponin-T degradation pattern compared to those from the 21 d aged samples (P < 0.01). Although AA is an economical and time-efficient method to increase tenderness of lower-quality beef cuts, further research is needed to determine strategies to mitigate the decrease in juiciness from AA treatments.

Mechanisms regulating the intracellular trafficking and release of CLN5 and CTSD

Ceroid lipofuscinosis neuronal 5 (CLN5) and cathepsin D (CTSD) are soluble lysosomal enzymes that also localize extracellularly. In humans, homozygous mutations in CLN5 and CTSD cause CLN5 disease and CLN10 disease, respectively, which are two subtypes of neuronal ceroid lipofuscinosis (commonly known as Batten disease). The mechanisms regulating the intracellular trafficking of CLN5 and CTSD and their release from cells are not well understood. Here, we used the social amoeba Dictyostelium discoideum as a model system to examine the pathways and cellular components that regulate the intracellular trafficking and release of the D. discoideum homologs of human CLN5 (Cln5) and CTSD (CtsD). We show that both Cln5 and CtsD contain signal peptides for secretion that facilitate their release from cells. Like Cln5, extracellular CtsD is glycosylated. In addition, Cln5 release is regulated by the amount of extracellular CtsD. Autophagy induction promotes the release of Cln5, and to a lesser extent CtsD. Release of Cln5 requires the autophagy proteins Atg1, Atg5, and Atg9, as well as autophagosomal-lysosomal fusion. Atg1 and Atg5 are required for the release of CtsD. Together, these data support a model where Cln5 and CtsD are actively released from cells via their signal peptides for secretion and pathways linked to autophagy. The release of Cln5 and CtsD from cells also requires microfilaments and the D. discoideum homologs of human AP-3 complex mu subunit, the lysosomal-trafficking regulator LYST, mucopilin-1, and the Wiskott-Aldrich syndrome-associated protein WASH, which all regulate lysosomal exocytosis in this model organism. These findings suggest that lysosomal exocytosis also facilitates the release of Cln5 and CtsD from cells. In addition, we report the roles of ABC transporters, microtubules, osmotic stress, and the putative D. discoideum homologs of human sortilin and cation-independent mannose-6-phosphate receptor in regulating the intracellular/extracellular distribution of Cln5 and CtsD. In total, this study identifies the cellular mechanisms regulating the release of Cln5 and CtsD from D. discoideum cells and provides insight into how altered trafficking of CLN5 and CTSD causes disease in humans.

Dissecting Regulators of Aging and Age-Related Macular Degeneration in the Retinal Pigment Epithelium

Age-related macular degeneration (AMD), the leading cause of blindness in elderly populations, involves the loss of central vision due to progressive dysfunction of the retinal pigment epithelium (RPE) and subsequent loss of light-sensing photoreceptors. While age is a key risk factor, not every aged individual develops AMD. Thus, the critical question is what specific cellular changes tip the balance from healthy aging to disease. To distinguish between changes associated with aging and AMD, we compared the RPE proteome in human eye bank tissue from nondiseased donors during aging (n = 50, 29-91 years) and in donors with AMD (n = 36) compared to age-matched donors without disease (n = 28). Proteins from RPE cells were separated on two-dimensional gels, analyzed for content, and identified using mass spectrometry. A total of 58 proteins displayed significantly altered content with either aging or AMD. Proteins involved in metabolism, protein turnover, stress response, and cell death were altered with both aging and AMD. However, the direction of change was predominantly opposite. With aging, we detected an overall decrease in metabolism and reductions in stress-associated proteins, proteases, and chaperones. With AMD, we observed upregulation of metabolic proteins involved in glycolysis, TCA, and fatty acid metabolism, with a concurrent decline in oxidative phosphorylation, suggesting a reprogramming of energy utilization. Additionally, we detected upregulation of proteins involved in the stress response and protein turnover. Predicted upstream regulators also showed divergent results, with inhibition of inflammation and immune response with aging and activation of these processes with AMD. Our results support the idea that AMD is not simply advanced aging but rather the culmination of perturbed protein homeostasis, defective bioenergetics, and increased oxidative stress within the aging RPE, exacerbated by environmental factors and the genetic background of an individual.

Altered protein secretion in Batten disease

The neuronal ceroid lipofuscinoses (NCLs), collectively known as Batten disease, are a group of neurological diseases that affect all ages and ethnicities worldwide. There are 13 different subtypes of NCL, each caused by a mutation in a distinct gene. The NCLs are characterized by the accumulation of undigestible lipids and proteins in various cell types. This leads to progressive neurodegeneration and clinical symptoms including vision loss, progressive motor and cognitive decline, seizures, and premature death. These diseases have commonly been characterized by lysosomal defects leading to the accumulation of undigestible material but further research on the NCLs suggests that altered protein secretion may also play an important role. This has been strengthened by recent work in biomedical model organisms, including Dictyostelium discoideum, mice, and sheep. Research in D. discoideum has reported the extracellular localization of some NCL-related proteins and the effects of NCL-related gene loss on protein secretion during unicellular growth and multicellular development. Aberrant protein secretion has also been observed in mammalian models of NCL, which has allowed examination of patient-derived cerebrospinal fluid and urine for potential diagnostic and prognostic biomarkers. Accumulated evidence links seven of the 13 known NCL-related genes to protein secretion, suggesting that altered secretion is a common hallmark of multiple NCL subtypes. This Review highlights the impact of altered protein secretion in the NCLs, identifies potential biomarkers of interest and suggests that future work in this area can provide new therapeutic insight.

Summary: This Review discusses work in different model systems and humans, examining the impact of altered protein secretion in the neuronal ceroid lipofuscinoses group of diseases to provide novel therapeutic insights.

Cathepsins B, D, and G Are Expressed in Metastatic Head and Neck Cutaneous Squamous Cell Carcinoma

Aim

We have previously demonstrated the presence of two cancer stem cell (CSC) subpopulations within metastatic head and neck cutaneous squamous cell carcinoma (mHNcSCC) expressing components of the renin-angiotensin system (RAS), which promotes tumorigenesis. Cathepsins B, D and G are enzymes that constitute bypass loops for the RAS. This study investigated the expression and localization of cathepsins B, D, and G in relation to CSC subpopulations within mHNcSCC.

Methods

Immunohistochemical staining was performed on mHNcSCC tissue samples from 20 patients to determine the expression and localization of cathepsins B, D, and G. Immunofluorescence staining was performed on two of these mHNcSCC tissue samples by co-staining of cathepsins B and D with OCT4 and SOX2, and cathepsin G with mast cell markers tryptase and chymase. Western blotting and quantitative reverse transcription polymerase chain reaction (RT-qPCR) were performed on five mHNcSCC samples and four mHNcSCC-derived primary cell lines, to determine protein and transcript expression of these three cathepsins, respectively. Enzyme activity assays were performed on mHNcSCC tissue samples to determine whether these cathepsins were active.

Results

Immunohistochemical staining demonstrated the presence of cathepsins B, D and G in in all 20 mHNcSCC tissue samples. Immunofluorescence staining showed that cathepsins B and D were localized to the CSCs both within the tumor nests and peri-tumoral stroma (PTS) and cathepsin G was localized to the phenotypic mast cells within the PTS. Western blotting demonstrated protein expression of cathepsin B and D, and RT-qPCR demonstrated transcript expression of all three cathepsins. Enzyme activity assays showed that cathepsin B and D to be active.

Conclusion

The presence of cathepsins B and D on the CSCs and cathepsin G on the phenotypic mast cells suggest the presence of bypass loops for the RAS which may be a potential novel therapeutic target for mHNcSCC.

Identification and structure-activity relationship studies of small molecule inhibitors of the human cathepsin D

Cathepsin D, an aspartyl protease, is an attractive therapeutic target for various diseases, primarily cancer and osteoarthritis. However, despite several small molecule cathepsin D inhibitors being developed, that are highly potent, most of them show poor microsomal stability, which in turn limits their clinical translation. Herein, we describe the design, optimization and evaluation of a series of novel non-peptidic acylguanidine based small molecule inhibitors of cathepsin D. Optimization of our hit compound 1a (IC₅₀ = 29 nM) led to the highly potent mono sulphonamide analogue 4b (IC₅₀ = 4 nM), however with poor microsomal stability (HLM: 177 and MLM: 177 μl/min/mg). To further improve the microsomal stability while retaining the potency, we carried out an extensive structure-activity relationship screen which led to the identification of our optimised lead 24e (IC₅₀ = 45 nM), with an improved microsomal stability (HLM: 59.1 and MLM: 86.8 μl/min/mg). Our efforts reveal that 24e could be a good starting point or potential candidate for further preclinical studies against diseases where Cathepsin D plays an important role.

Design, synthesis, and bioactivities of tasiamide B derivatives as cathepsin D inhibitors

Cathepsin D (Cath D) is overexpressed and hypersecreted by malignant tumors and involved in the progress of tumor invasion, proliferation, metastasis, and apoptosis. Cath D has been considered as a potential target to treat cancer. Our previous studies revealed that tasiamide B derivatives TB-9 and TB-11 exhibited high potent inhibition against Cath D and other aspartic proteases, but their molecular weights are still high, and the role of each residue is unknown yet. Based on this, two series of tasiamide B derivatives have been designed, synthesized, and evaluated for their inhibitory activity against Cath D/Cath E/BACE1. Enzymatic assays revealed that the target compound 1 with lower molecule weight showed good inhibitory activity against Cath D with IC₅₀ of 3.29 nM and satisfactory selectivity over Cath E (72-fold) and BACE1 (295-fold), which could be a valuable template for the design of highly potent and selective Cath D inhibitors.

Progranulin Stimulates the In Vitro Maturation of Pro-Cathepsin D at Acidic pH

Single-copy loss-of-function mutations in the progranulin gene (PGRN) underlie the neurodegenerative disease frontotemporal lobar degeneration, while homozygous loss-of-function of PGRN results in the lysosomal storage disorder neuronal ceroid lipofuscinosis. Despite evidence that normal PGRN levels are critical for neuronal health, the function of this protein is not yet understood. Here, we show that PGRN stimulates the in vitro maturation of the lysosomal aspartyl protease cathepsin D (CTSD). CTSD is delivered to the endolysosomal system as an inactive precursor (proCTSD) and requires sequential cleavage steps via intermediate forms to achieve the mature state (matCTSD). In co-immunoprecipitation experiments, PGRN interacts predominantly with immature pro- and intermediate forms of CTSD. PGRN enhances in vitro conversion of proCTSD to matCTSD in a concentration-dependent manner. Differential scanning fluorimetry shows a destabilizing effect induced by PGRN on proCTSD folding (∆T_m = -1.7 °C at a 3:1 molar ratio). We propose a mechanism whereby PGRN binds to proCTSD, destabilizing the propeptide from the enzyme catalytic core and favoring conversion to mature forms of the enzyme. Further understanding of the role of PGRN in CTSD maturation will assist in the development of targeted therapies for neurodegenerative disease.

Dynein Dysfunction Reproduces Age-Dependent Retromer Deficiency: Concomitant Disruption of Retrograde Trafficking Is Required for Alteration in β-Amyloid Precursor Protein Metabolism

It is widely accepted that β-amyloid (Aβ) protein plays a pivotal role in Alzheimer disease pathogenesis, and accumulating evidence suggests that endocytic dysfunction is involved in Aβ pathology. Retromer, a conserved multisubunit complex, mediates the retrograde transport of numerous kinds of cargo from endosomes to the trans-Golgi network. Several studies have found that retromer deficiency enhances Aβ pathology both in vitro and in vivo. Cytoplasmic dynein, a microtubule-based motor protein, mediates minus-end-directed vesicle transport via interactions with dynactin, another microtubule-associated protein that also interacts with retromer. Aging attenuates the dynein-dynactin interaction, and dynein dysfunction reproduces age-dependent endocytic disturbance, resulting in the intracellular accumulation of beta-amyloid precursor protein (APP) and its β-cleavage products, including Aβ. Here, we report that aging itself affects retromer trafficking in cynomolgus monkey brains. In addition, dynein dysfunction reproduces this type of age-dependent retromer deficiency (ie, the endosomal accumulation of retromer-related proteins and APP. Moreover, we found that knockdown of Rab7, Rab9, or Rab11 did not alter endogenous APP metabolism, such as that observed in aged monkey brains and in dynein-depleted cells. These findings suggest that dynein dysfunction can cause retromer deficiency and that concomitant disruption of retrograde trafficking may be the key factor underlying age-dependent Aβ pathology.

Structure-based optimization of non-peptidic Cathepsin D inhibitors

We discovered a novel series of non-peptidic acylguanidine inhibitors of Cathepsin D as target for osteoarthritis. The initial HTS-hits were optimized by structure-based design using CatD X-ray structures resulting in single digit nanomolar potency in the biochemical CatD assay. However, the most potent analogues showed only micromolar activities in an ex vivo glycosaminoglycan (GAG) release assay in bovine cartilage together with low cellular permeability and suboptimal microsomal stability. This new scaffold can serve as a starting point for further optimization towards in vivo efficacy.

Lysosomal alkalization and dysfunction in human fibroblasts with the Alzheimer's disease-linked presenilin 1 A246E mutation can be reversed with cAMP

Mutation in presenilin 1 (PS1) is one of the leading causes of familial Alzheimer's disease (fAD). PS1 mutation exacerbates the autophagic and lysosomal pathology in AD patients, leading to accumulation of partially degraded material in bloated lysosomes and autophagosomes - a pathology that bears some resemblance to other diseases characterized by elevated lysosomal pH, like age-related macular degeneration. In this study, we examined the effect of the PS1-fAD mutation A246E on lysosomal pH and lysosomal function, and asked whether restoration of lysosomal pH could reverse some of these changes. Lysosomal pH was elevated by 0.2-0.3 pH units in human fibroblasts with the PS1-fAD mutation. The lysosomal alkalization in PS1-fAD fibroblasts was supported by a reduction in the pH-dependent cleavage of cathepsin D and by a reduction in binding of boron-dipyrromethene (BODIPY) FL-pepstatin A to the cathepsin D active site. PS1-fAD cells had increased LC3B-II/-I ratios and p62 levels, consistent with impaired lysosomal degradation and analogous to changes induced by lysosomal alkalinization with chloroquine. PS1-fAD fibroblasts had increased expression of ATP6V1B2, ATG5, BECN1 TFEB mRNA, and of ATP6V1B2, ATG5 and beclin at the protein level, consistent with chronic impairment of autophagic and lysosomal functions in the mutant cells. Critically, cyclic adenosine monophosphate (cAMP) treatment reacidified lysosomal pH in mutant PS1-fAD; cAMP also increased the availability of active cathepsin D and lowered the LC3B-II/-I ratio. These results confirm a small elevation in the lysosomal pH of human PS1-fAD fibroblasts, demonstrate that this lysosomal alkalization is associated with chronic changes in autophagy and degradation, and suggest that treatment to reacidify the lysosomes with cAMP can reverse these changes.

Similarities and differences in the biogenesis, processing and lysosomal targeting between zebrafish and human pro-Cathepsin D: functional implications

The lysosomal protease Cathepsin D (CD) plays a role in neurodegenerative diseases, cancer, and embryo-fetus abnormalities. It is therefore of interest to know how this protein is synthesized in animal species used for modeling human diseases. Zebrafish (Danio rerio) is emerging as a valuable 'in vivo' vertebrate model for several human diseases. We have characterized the biogenetic pathways of zebrafish and human CD transgenically expressed in both human SH-SY5Y cells and zebrafish PAC2 cells. Differently from human CD, zebrafish CD was synthesized as a mono-glycosylated precursor (pro-CD) that was eventually processed into a single-chain mature polypeptide. In PAC2 cells, ammonium chloride and chloroquine impaired the N-glycosylation, and greatly stimulated the secretion, of pro-CD; still, a portion of un-glycosylated pro-CD reached the lysosomes and was processed to mature CD. The treatment with tunicamycin, which abrogates N-glycosylation, resulted in a similar effect. Zebrafish pro-CD was correctly processed when expressed in human cells, and its glycosylation, transport and maturation were not impaired by ammonium chloride. On the contrary, the transport and processing of human pro-CD expressed in zebrafish cells were profoundly altered: while the intermediate single-chain was not detectable, a small amount of double-chain mature CD still formed. This fact indicates that the enzyme machinery for single- to double-chain processing of mammal CD is present in zebrafish. Our data highlight the respective impact of the information imparted by the primary sequence and of the cellular transport and processing machineries in the biogenesis of lysosomal CD.

Procathepsin D and cancer: From molecular biology to clinical applications

Procathepsin D (pCD) is overexpressed and secreted by cells of various tumor types including breast and lung carcinomas. pCD affects multiple features of tumor cells including proliferation, invasion, metastases and apoptosis. Several laboratories have previously shown that the mitogenic effect of pCD on cancer cells is mediated via its propeptide part (APpCD). However, the exact mechanism of how pCD affects cancer cells has not been identified. Recent observations have also revealed the possible use of pCD/APpcD as a marker of cancer progression. The purpose of this review is to summarize the three major potentials of pCD-tumor marker, potential drug, and screening agent.

Export of a Toxoplasma gondii rhoptry neck protein complex at the host cell membrane to form the moving junction during invasion

One of the most conserved features of the invasion process in Apicomplexa parasites is the formation of a moving junction (MJ) between the apex of the parasite and the host cell membrane that moves along the parasite and serves as support to propel it inside the host cell. The MJ was, up to a recent period, completely unknown at the molecular level. Recently, proteins originated from two distinct post-Golgi specialised secretory organelles, the micronemes (for AMA1) and the neck of the rhoptries (for RON2/RON4/RON5 proteins), have been shown to form a complex. AMA1 and RON4 in particular, have been localised to the MJ during invasion. Using biochemical approaches, we have identified RON8 as an additional member of the complex. We also demonstrated that all RON proteins are present at the MJ during invasion. Using metabolic labelling and immunoprecipitation, we showed that RON2 and AMA1 were able to interact in the absence of the other members. We also discovered that all MJ proteins are subjected to proteolytic maturation during trafficking to their respective organelles and that they could associate as non-mature forms in vitro. Finally, whereas AMA1 has previously been shown to be inserted into the parasite membrane upon secretion, we demonstrated, using differential permeabilization and loading of RON-specific antibodies into the host cell, that the RON complex is targeted to the host cell membrane, where RON4/5/8 remain associated with the cytoplasmic face. Globally, these results point toward a model of MJ organization where the parasite would be secreting and inserting interacting components on either side of the MJ, both at the host and at its own plasma membranes.

A unique feature of apicomplexan parasites is the formation of an intimate contact between the apex of the parasite and the host cell membrane called the moving junction that moves along the parasite during invasion. Proteins originated from two distinct secretory organelles, the microneme for AMA1 and the rhoptry neck for RON2/4/5 proteins, are associated to form the junction. Here, we have furthered the characterization of the MJ complex by describing RON8, an additional protein component. AMA1 has previously been shown to be inserted into the parasite membrane upon secretion. Our study demonstrates that all the RON proteins are translocated into the host cell, where RON4/5/8 remain associated with the cytoplasmic face of the host cell plasma membrane. Furthermore, we identified a privileged interaction between transmembrane MJ proteins AMA1 and RON2 in vitro. Overall, this led us to propose the first model describing the putative MJ organisation at the interface between the host cell and Toxoplasma. In this original concept, the parasite would export its own receptor (RON2) and ligand (AMA1) on either side of the MJ.

Cathepsin D--many functions of one aspartic protease

For years, it has been held that cathepsin D (CD) is involved in rather non-specific protein degradation in a strongly acidic milieu of lysosomes. Studies with CD knock-out mice revealed that CD is not necessary for embryonal development, but it is indispensable for postnatal tissue homeostasis. Mutation that abolishes CD enzymatic activity causes neuronal ceroid lipofuscinosis (NCL) characterized by severe neurodegeneration, developmental regression, visual loss and epilepsy in both animals and humans. In the last decade, however, an increasing number of studies demonstrated that enzymatic function of CD is not restricted solely to acidic milieu of lysosomes with important consequences in regulation of apoptosis. In addition to CD enzymatic activity, it has been shown that apoptosis is also regulated by catalytically inactive mutants of CD which suggests that CD interacts with other important molecules and influences cell signaling. Moreover, procathepsin D (pCD), secreted from cancer cells, acts as a mitogen on both cancer and stromal cells and stimulates their pro-invasive and pro-metastatic properties. Numerous studies found that pCD/CD level represents an independent prognostic factor in a variety of cancers and is therefore considered to be a potential target of anti-cancer therapy. Studies dealing with functions of cathepsin D are complicated by the fact that there are several simultaneous forms of CD in a cell-pCD, intermediate enzymatically active CD and mature heavy and light chain CD. It became evident that these forms may differently regulate the above-mentioned processes. In this article, we review the possible functions of CD and its various forms in cells and organisms during physiological and pathological conditions.

Human cathepsin D

A literature survey was performed of human cathepsin D gene, cathepsin D biosynthesis, posttranslatory modifications, transport within the cell, substrate specificity and catalytic effect. Methods used to determine the activity and level of this proteinase as well as its role in the biochemistry and pathobiochemistry of cells, tissues and organs were considered.

Functional biology of the neuronal ceroid lipofuscinoses (NCL) proteins

Neuronal ceroid lipofucinoses (NCLs) are a group of severe neurodegenerative disorders characterized by accumulation of autofluorescent ceroid lipopigment in patients' cells. The different forms of NCL share many similar pathological features but result from mutations in different genes. The genes affected in NCLs encode both soluble and transmembrane proteins and are localized to ER or to the endosomes/lysosomes. Due to selective vulnerability of the central nervous system in the NCL disorders, the corresponding proteins are proposed to have important, tissue specific roles in the brain. The pathological similarities of the different NCLs have led not only to the grouping of these disorders but also to suggestion that the NCL proteins function in the same biological pathway. Despite extensive research, including the development of several model organisms for NCLs and establishment of high-throughput techniques, the precise biological function of many of the NCL proteins has remained elusive. The aim of this review is to summarize the current knowledge of the functions, or proposed functions, of the different NCL proteins.

Enhanced cathepsin L expression is mediated by different Ras effector pathways in fibroblasts and epithelial cells

Ras expression induces increased expression and altered targeting of lysosomal proteases in multiple cell types, but the specific downstream cytoplasmic signaling pathways mediating these changes have not been identified. In this study, we compared the involvement of 3 major Ras effectors, Raf, phosphatidylinositol 3-kinase (PI3K) and Ral guanine nucleotide exchange factor (RalGEF) in the Ras-mediated alteration of lysosomal protease protein expression and targeting in rat 208F fibroblasts and rat ovarian surface epithelial (ROSE) cells. Effector domain mutants of Ras, constitutively activated variants of Raf, PI3K and RalGEF and pharmacologic inhibitors of MEK and PI3K were utilized to determine the role of these downstream pathways in mediating fibroblast transformation and lysosomal protease regulation in the fibroblasts and epithelial cells. We found that Raf activation of the ERK mitogen-activated protein kinase pathway alone was sufficient to cause morphologic and growth transformation of the fibroblasts and was necessary and sufficient to alter cathepsin L expression and targeting. In contrast, transformation and upregulation of cathepsin L expression in the epithelial cells required the activity of all 3 Ras effectors. Increased protease secretion from the epithelial cells was not observed on ectopic expression of Ras, as it was from the fibroblasts, consistent with the utilization of different signaling pathways in the 2 cell types. In neither cell type did Ras expression increase the expression, processing or secretion of 2 other major lysosomal proteases, cathepsin B and cathepsin D. Thus, Ras utilizes different effectors to mediate transformation and to deregulate cathepsin L expression and secretion in fibroblast and epithelial cells.

The plant aspartic proteinase-specific polypeptide insert is not directly related to the activity of oryzasin 1

Many plant aspartic proteinases (APs) are different from animal and microbial APs in that they contain a polypeptide insert, approximately 100 amino acids in length, in the C-terminal region. To interpret the significance of this insert, we constructed an expression system for rice AP oryzasin 1 by linking a pro-oryzasin 1 downstream of glutathione S-transferase (GST). GST-proOS1 expressed the highest degree of hemoglobin-hydrolytic activity when treated at pH 3.3 and incubated for 24 h at room temperature. We carried out a similar experiment using an insert-lacking proOS1 mutant, GST-DeltaproOS1, as the fusion protein, and found it to show similar activity. This result indicates that the insert is not involved in the production of AP activity. We then investigated the autolysis of the two proteins by Western blot analysis. GST-proOS1 was autolyzed into 67- and 64-kDa fragments, while GST-DeltaproOS1 autolyzed to 54- and 52-kDa products. GST-DeltaproOS1 clearly produced two molecular species early in the autolytic process, and not later than 3 h from the start, but no such clear result was observed in the case of GST-proOS1. This suggests that, although the presence of the plant AP-specific insert does not influence the enzyme activity by itself, it apparently has an effect on the autolysis of OS1.

Secretory lysosome biogenesis in cytotoxic T lymphocytes from normal and Chediak Higashi syndrome patients

The lytic proteins mediating target cell killing are stored in the lysosomes of activated cytotoxic T lymphocytes (CTL) and are secreted upon recognition of a target cell. These secretory lysosomes cannot be detected in resting T lymphocytes. Interaction of a resting cell with a target cell activates de novo formation of secretory lysosomes. CTL clones in culture mimic this behaviour, and so provide an ideal system for studying secretory lysosome biogenesis and maturation. In the genetic disease, Chediak Higashi syndrome (CHS), all lysosomes in the cells are enlarged and reduced in number compared with wild-type (WT) cells. We have used CTL from this disease to study secretory lysosome biogenesis and maturation. We show that at early stages after activation the secretory lysosomes are identical in WT and mutant cells, and that delivery of proteins to the secretory lysosome along the biosynthetic and endocytic pathways is normal in the mutant cells. With time, the lysosomes in the mutant cells aggregate, become larger and fewer in number and eventually form giant structures. Our results show that the initial steps of secretory lysosome formation are normal in CHS, but that the organelles subsequently fuse together during cell maturation to form the giant secretory lysosomes.

Iodination of mature cathepsin D in thyrocytes as an indicator for its transport to the cell surface

Thyrocytes are known for their ability to iodinate thyroglobulin from which the thyroid hormones are generated. In the intact thyroid gland the iodination process is almost exclusively executed at the apical plasma membrane of thyroid epithelial cells. Here, we show that freshly isolated thyrocytes iodinated polypeptides other than thyroglobulin and that one of the major iodinated polypeptides was the mature form of the lysosomal protease cathepsin D (CD). The detection of mature CD as an iodinated polypeptide suggested that a fraction of the lysosomally maturated enzyme was delivered to the apical plasma membrane where it became available for iodination. After labeling of thyrocytes with [35S]methionine/cysteine overnight part of the mature CD was released into the culture medium. This was abolished by inhibiting maturation of CD with NH4Cl, indicating that mature CD appeared in the medium after its proteolytic maturation in an acidic compartment. Besides CD other soluble lysosomal polypeptides like the beta-N-acetylhexosaminidase and the sphingolipid-activating protein D (Sap D) were iodinated and partially secreted as mature polypeptides. In contrast, the membrane-associated lysosomal ceramidase was iodinated and partially secreted as immature single-chain enzyme and not as fully maturated two-chain enzyme. These data indicate that a portion of mature CD and other soluble lysosomal enzymes is delivered from lysosomes to the cell surface whereas some membrane-associated enzymes from the terminal lysosomal compartment are efficiently excluded from this process.

Expression and characterization of human beta-secretase candidates metalloendopeptidase MP78 and cathepsin D in beta APP-overexpressing cells

Human beta-secretase candidates, MP78 (h-MP78, EC 3.4.24.15) and cathepsin D (Cat D, EC 3.4.23.5), were evaluated for their ability to enhance amyloid-beta-protein (A beta) secretion when overexpressed in beta APP-containing cells. HEK-293 cells stably co-expressing h-MP78 or Cat D and h-beta APP695 were metabolically labeled with [35S]methionine and A beta secretion was quantified in the conditioned media by immunoprecipitation and ELISA without showing any significant increase in A beta production. Because Cat D is known to have a higher affinity for APP-substrate containing the Swedish familial Alzheimer's disease double mutation (SFAD, K595N and M596L substitutions in beta APP695) than for the wild type substrate [Dreyer et al., Eur. J. Biochem., 224 (1994) 265-271], the effect of Cat D overexpression was tested in a HEK293/beta APPSFAD stable cell line. ELISA analysis of the conditioned media from these cells did also not reveal any increase in A beta generation. In addition, recombinant h-MP78 purified from E. coli cleaved an APP-derived substrate spanning the beta-secretase site (ISEVKMD1AEFRHDS) at multiple sites, but the beta-site cleavage was only a minor one; cleavage occurred predominantly at K-M and E-F bonds. Human liver Cat D also cleaved the same substrate at multiple sites, yet the major cleavage at pH 4.0 occurred at the amyloidogenic D1 site. These findings indicate that h-MP78 does not have the cleavage specificity required for a beta-secretase protease and although Cat D fulfilled the amyloidogenic cleavage specificity, the results of the co-expression experiments make both enzymes less likely candidates as relevant beta-secretases.

Expression and the role of cathepsin H in human glioma progression and invasion

Proteinases and their inhibitors may play a role in the development and progression of many cancers. Several studies suggested that lysosomal proteinases cathepsin B, L, and D may be involved in the malignant progression of some human neoplastic diseases. In this study, we determined the levels of cathepsin H in human glioma progression and the significance of cathepsin H in glioma cell invasion. Levels of cathepsin H antigen were found to be significantly higher in glioblastomas and anaplastic astrocytoma when compared with normal brain tissue and low-grade gliomas. Western blotting confirmed the presence of authentic cathepsin H with a doublet at 27 and 25 kDa in normal brain tissue and tumor samples. However, the intensity of the band increased significantly in glioblastoma samples. Cathepsin H antibody inhibited the invasion of glioblastoma cell lines through Matrigel invasion assay. These data suggest that the tumor-specific increase in antigen may be a useful independent marker of tumor progression in central nervous system neoplasms.

Bovine milk procathepsin D and cathepsin D: coagulation and milk protein degradation

Cathepsin D is an indigenous aspartic proteinase in bovine milk. By competitive enzyme-linked immunosorbent assay the amount of immunoreactive cathepsin D and procathepsin D in bovine skim milk was estimated to be 0.4 microgram/ml. Immunoreactive cathepsin D purified from whey consisted of a small fraction of mature cathepsin D, but the major form was the proenzyme procathepsin D. A preparation of bovine milk procathepsin D was, like mature cathepsin D, able to degrade purified alpha s1-, alpha s2-, beta- and kappa-casein and alpha-lactalbumin, while beta-lactoglobulin was resistant to cleavage. The cleavage sites in these proteins were determined and compared with those of chymosin. Cathepsin D was capable of generating the alpha s1-I, beta-I, beta-II and beta-III fragments originally described from the action of chymosin on the respective caseins, and these fragments were subjected to further proteolysis. Cathepsin D was also able to liberate the caseinomacropeptide from purified kappa-casein, and to coagulate bovine skim milk. This demonstrated that milk contains an indigenous coagulation enzyme present mainly in the whey fraction.

Procathepsin D cannot autoactivate to cathepsin D at acid pH

The amino acid sequence of the propart of bovine procathepsin D was determined at the protein level. Incubation of the isolated procathepsin D at pH 3.5-5.0 for 30-120 min leads to a 2 kDa reduction in its molecular mass, as seen by SDS-PAGE. The activation product is pseudocathepsin D and is the result of a proteolytic cleavage between LeuP26 and IleP27 in the propart. Incubation at pH 5.0 for 20 h of either procathepsin D or pseudocathepsin D results in both cases in approximately equal amounts of pseudocathepsin D and a further processed intermediate, nine amino acids shorter than pseudocathepsin D. No reaction products corresponding to cathepsin D with a mature amino terminus were observed, showing that autoproteolysis alone cannot generate the mature form found in the lysosomes.

Molecular cloning and functional characterization of chicken cathepsin D, a key enzyme for yolk formation

Upon receptor-mediated endocytosis of very-low-density lipoprotein (VLDL) and vitellogenin into growing chicken oocytes, the protein moieties of these lipoproteins are proteolytically cleaved. Unlike the complete lysosomal degradation in somatic cells, enzymatic ligand breakdown in oocytes generates a characteristic set of polypeptides, which enter yolk storage compartments for subsequent utilization by the embryo. Here, we demonstrate directly that the catalyst for the intraoocytic processing of both apolipoprotein B and vitellogenin is cathepsin D. The enzyme was purified from oocytic yolk, preovulatory follicle homogenates, and liver by affinity chromatography. When plasma VLDL and vitellogenin were incubated with the purified enzyme, fragments indistinguishable from those found in yolk were generated from both precursors under identical, mildly acidic conditions. Amino-terminal sequencing of the pure enzyme demonstrated 88% identity with mammalian cathepsin Ds over 34 residues. On the basis of this information, a full-length clone specifying chicken preprocathepsin D was isolated from a chicken follicle cDNA library by screening with a human cathepsin D probe. Whereas previous studies have demonstrated that the receptors for lipoproteins in somatic cells and oocytes, respectively, of the chicken are the products of different genes, Southern and Northern blot hybridization experiments showed that the enzymes expressed in oocytes and liver are the product of a single gene, giving rise to a 3.3-kb transcript. The primary structure of the 335-residue mature protein suggests a high degree of conservation of known crucial features of aspartyl proteases; however, the absence of the so-called processing region and of an aromatic residue in a region thought to partake in catalysis raise questions with possible evolutionary implications.

Proteolytic processing sites producing the mature form of human cathepsin D

1. The proteolytic processing sites of human lysosomal aspartic protease cathepsin D at which the intermediate single-chain form was converted into the mature two-chain form were determined. 2. The two chains were isolated by reversed-phase HPLC in order to investigate the cleavage sites of the enzyme. 3. Protein sequencing of the heavy chain, which was presumed to be derived from the C-terminal side in the single-chain enzyme, gave an N-terminal Leu 105. In addition, it revealed that there were also minor sequences, which commenced with Gly 106 and Gly 107. 4. A small C-terminal peptide was isolated from the light chain, which had been digested with two kinds of exogenous proteases. Sequence determination of this peptide, which was characterized as a nonapeptide by mass spectrometry, suggested that the C-terminus of the light chain was Ser 98. 5. These results indicate that a Ser 98-Ala 99 bond and an Ala 104-Leu 105 bond are cleaved to release 6 amino acid residues between the two chains.

The early and late processing of lysosomal enzymes: proteolysis and compartmentation

Lysosomal enzymes are subjected to a number of modifications including carbohydrate restructuring and proteolytic maturation. Some of these reactions support lysosomal targeting, others are necessary for activation or keeping the enzyme inactive before being segregated, while still others may be adventitious. The non-segregated fraction of the enzyme is secreted and can be isolated from the medium. It is considered that the secreted lysosomal enzymes fulfill certain physiological and pathophysiological roles. By comparing the secreted and the intracellular enzymes it is possible to distinguish between the reactions that occur before and after the segregation. In this review the reactions that may influence the segregation are referred to as the early processing and those characteristic for the enzymes isolated from lysosomal compartments as the late processing. The early processing is characterized mainly by modifications of carbohydrate side chains. In the late processing, proteolytic fragmentation represents the most conspicuous changes. The review focuses on the compartmentation of the reactions and the proteolytic fragmentation of lysosomal enzyme precursors. While a plethora of proteolytic reactions are involved, our knowledge of the proteinases responsible for the particular maturation reactions remains very limited. The review points also to work with cells from patients affected with lysosomal storage disorders, which contributed to our understanding of the lysosomal apparatus.

Isolation and characterization of a stable activation intermediate of the lysosomal aspartyl protease cathepsin D

Procathepsin D is the intracellular aspartyl protease precursor of cathepsin D, a major lysosomal enzyme. Procathepsin D is rapidly processed inside the cell, and, thus, examination of its proteolyic activation and structure has been difficult. To study this proenzyme, a nonglycosylated form of the human fibroblast procathepsin D was expressed in Escherichia coli, refold in vitro, and purified by affinity chromatography on pepstatinyl agarose. Sequence analysis of the refolded, autoactivated enzyme allowed determination of the autoproteolytic cleavage site. The sequence surrounding this cleavage site between residues LeuP26 and IleP27 (in the "pro" region) resembled the first cleavage site found during activation of other aspartyl proteases. Thus, the autoactivated procathepsin D is analogous to the pepsin activation intermediate, which has been termed pseudopepsin. The enzymatic activity, thermal and pH stability, and fluorescence spectra of pseudocathepsin D were compared to mature, predominantly two-chain, cathepsin D isolated from human placenta. The results indicated that pseudocathepsin D and mature enzyme have a similar Km toward a peptide substrate and cleave a protein substrate at identical sites. Temperature stability of the recombinant enzyme was similar to that of the tissue-derived enzyme. However, the recombinant enzyme had increased stability at low pH when compared to the glycosylated tissue-derived two-chain cathepsin D. Fluorescence spectra of the recombinant and tissue-derived enzymes were identical. Thus, the absence of asparagine-linked oligosaccharides and the presence of the remaining segment of propeptide did not significantly alter the structural and enzymatic properties of the enzyme.

Isolation and sequencing of a cDNA clone encoding rat liver lysosomal cathepsin D and the structure of three forms of mature enzymes

We isolated and sequenced a cDNA clone corresponding to the entire coding sequence of rat liver lysosomal cathepsin D. The deduced amino acid sequence revealed that cathepsin D consists of 407 amino acid residues (Mr 44,608) and the 20 NH2-terminal residues seem to constitute a cleavable signal peptide after which 44 amino acid residues follow as a propeptide. Two putative N-linked glycosylation sites and aspartic acid in the active site are as well conserved as those of human lysosomal cathepsin D. In the NH2-terminal sequence analysis of two isolated heavy chains of the mature enzyme, the termini were assigned as tryptophan (118th residue) and glycine (165th or 166th residue), respectively, hence demonstrates that the two heavy chains derive from a split of the single chain of cathepsin D at position between 117th and 118th or between 164th and 165th or 165th and 166th amino acids. We conclude that cathepsin D in rat liver lysosomes is a mixture of three forms composed of a single and two two-chain forms. However, the amounts of the two two-chain forms are low compared with that of the single chain form. Densidometric determination after SDS-PAGE revealed that the two two-chain forms account for less than 5% of the single chain form. There is a 82% similarity in amino acid level between rat and human liver lysosomal cathepsin D.

Inhibition of intracellular sorting and processing of lysosomal cathepsins H and L at reduced temperature in primary cultures of rat hepatocytes

Our recent studies with pulse-chase kinetic analysis in primary cultures of rat hepatocytes suggest that newly synthesized lysosomal cathepsins H and L are initially synthesized as larger proform enzymes, and then the precursor molecules are subsequently converted to the mature enzymes by limited proteolysis during the intracellular sorting process. This proteolytic maturation of procathepsins appears to proceed within an acidic environment, and these processing events are closely connected with the activation of enzymes. To further characterize the intracellular processing site for lysosomal cathepsins H and L, the pulse-chase kinetic study was carried out at 20 degrees C in cultured rat hepatocytes, because the transport of the procathepsins was expected to be blocked at the trans-Golgi compartment at 20 degrees C. We show here that the newly synthesized procathepsins are accumulated intracellularly and the processing for lysosomal cathepsins is completely arrested at 20 degrees C along the sorting pathway. The procathepsins thus accumulated in the cell are presumed to be transported to the Golgi complex, since the oligosaccharide moieties of these polypeptides appear to be phosphorylated. When the cells were shifted to 37 degrees C after an incubation for 4 h at 20 degrees C, a gradual increase of the mature forms was found. However, the processing kinetics generating the mature enzymes were slow compared to those in control cells at 37 degrees C. When the NH4Cl was present in the cells after the temperature shift to 37 degrees C, the intracellular processing of procathepsins was considerably retarded and the release of intracellular procathepsins into the extracellular medium was observed. These results indicate that NH4Cl might exert the inhibitory effect on the mannose 6-phosphate receptor-mediated intracellular targeting mechanism for the lysosomal cathepsins. Hence, the intracellular location of procathepsins accumulated at 20 degrees C is considered to be in proximity to the trans-Golgi compartment. Taken together, the present observations suggest that the propeptide-processing step for procathepsins, which is a critical step for generating the active enzymes, proceeds within the prelysosomal compartment or the lysosomes after the enzymes leave the trans-Golgi compartment.

Localization of lysosomal and digestive enzymes in cytoplasmic vacuoles in caerulein-pancreatitis

Intracellular localization and enzymatic activities of lysosomal enzymes (cathepsin B, N-acetyl-beta-glucosaminidase, and beta-glucuronidase) were studied in control rats and after induction of caerulein pancreatitis. In control rats high enzymatic activities were found in the postnuclear 1000 g fraction (purified zymogen granules). The corresponding subcellular fraction in pancreatitis animals additionally contained larger secretory vacuoles and autophagosomes and revealed a marked increase in lysosomal enzyme activities. Immunolabelling studies at the ultrastructural level for trypsinogen and cathepsin B demonstrated a colocalization of lysosomal and digestive enzymes in zymogen granules in healthy controls. After induction of pancreatitis immunolabelling still demonstrated a colocalisation of cathepsin B and trypsinogen in secretory granules and newly formed Golgi-derived secretory vacuoles. Concomitantly appearing autophagosomes were, however, only labelled for cathepsin B. It is concluded that segregation of lysosomal and digestive enzymes is incomplete in normal acinar cells resulting in a colocalization in zymogen granules. In pancreatitis colocalization in secretory granules is maintained, whereas only lysosomal enzymes were sufficiently transferred into autophagic vacuoles. No indication for impaired mechanisms of molecular sorting of lysosomal and digestive enzymes in caerulein-induced pancreatitis was found.

Expression and refolding of recombinant human fibroblast procathepsin D

Procathepsin D is a precursor of the human lysosomal protease cathepsin D. Due to its short half-life, procathepsin D is difficult to obtain in quantities sufficient to allow structural and enzymatic studies. To obtain large quantities of this precursor, procathepsin D was expressed using the T7 promoter vector pET3a in bacteria that carry a chromosomal copy of the T7 RNA polymerase gene under the control of the lac promoter. At high cell density in rich medium, basal levels of T7 RNA polymerase were sufficient to express recombinant procathepsin D without addition of an exogenous inducer of the lac promoter. The recombinant protein, constituting almost half of the total cell protein, accumulated in intracytoplasmic inclusion bodies and was isolated from the insoluble fraction of lysed cells. Antibodies prepared against the purified recombinant protein were shown to crossreact with native human placental and porcine spleen cathepsin D. Recombinant procathepsin D was solubilized in denaturants and was refolded. After extended preincubation of the denatured protein at acid pH to allow folding and activation of the zymogen, pepstatin inhibitable catalytic proteolysis was detected. These data demonstrated that the glycosylated aspartic protease, procathepsin D can be refolded and activated in an unglycosylated form and thus provides a system for the study of procathepsin D structure and function.

Evidence that aspartic proteinase is involved in the proteolytic processing event of procathepsin L in lysosomes

Our recent studies have shown that cathepsin L is first synthesized as an enzymatically inactive proform in endoplasmic reticulum and is successively converted into an active form during intracellular transport and we postulated that aspartic proteinases would be responsible for the intracellular propeptide-processing step of procathepsin L accompanied by the activation of enzyme (Y. Nishimura, T. Kawabata, and K. Kato (1988) Arch. Biochem. Biophys. 261, 64-71). To better understand this proposed mechanism, we investigated the effect of pepstatin, a potent inhibitor of aspartic proteinases, on the intracellular processing kinetics of cathepsin L analyzed by pulse-chase experiments in vivo with [35S]methionine in the primary cultures of rat hepatocytes. In the pepstatin-treated cells, the proteolytic conversion of cellular procathepsin L of 39 kDa to the mature enzyme was significantly inhibited and considerable amounts of proenzyme were found in the cell after 5-h chase periods. Further, the subcellular fractionation experiments demonstrated that the intracellular processing of procathepsin L in the high density lysosomal fraction was significantly inhibited and that considerable amounts of the procathepsin L form were still observed in the light density microsomal fraction after 2 h of chase. These results suggest that pepstatin treatment caused a significant inhibitory effect on the intracellular processing and also on the intracellular movement of procathepsin L from the endoplasmic reticulum to the lysosomes. These findings provide the first evidence showing that aspartic proteinase may play an important role in the intracellular proteolytic processing and activation of lysosomal cathepsin L in vivo. Therefore, we suggest that cathepsin D, a major lysosomal aspartic proteinase, is more likely to be involved in this proposed model in the lysosomes.