Purified recombinant human prosaposin forms oligomers that bind procathepsin D and affect its autoactivation

Human Saposin B Ligand Binding and Presentation to α-Galactosidase A

Sphingolipid activator protein B (saposin B; SapB) is an essential activator of globotriaosylceramide (Gb3) catabolism by α-galactosidase A. However, the manner by which SapB stimulates α-galactosidase A activity remains unknown. To uncover the molecular mechanism of SapB presenting Gb3 to α-galactosidase A, we subjected the fluorescent substrate globotriaosylceramide-nitrobenzoxidazole (Gb3-NBD) to a series of biochemical and structural assays involving SapB. First, we showed that SapB stably binds Gb3-NBD using a fluorescence equilibrium binding assay, isolates Gb3-NBD from micelles, and facilitates α-galactosidase A cleavage of Gb3-NBD in vitro. Second, we crystallized SapB in the presence of Gb3-NBD and validated the ligand-bound assembly. Third, we captured transient interactions between SapB and α-galactosidase A by chemical cross-linking. Finally, we determined the crystal structure of SapB bound to α-galactosidase A. These findings establish general principles for molecular recognition in saposin:hydrolase complexes and highlight the utility of NBD reporter lipids in saposin biochemistry and structural biology.

Sortilin knock-down alters the expression and distribution of cathepsin D and prosaposin and up-regulates the cation-dependent mannose-6-phosphate receptor in rat epididymal cells

The selective transport to lysosomes can be mediated by either mannose-6-phosphate receptors (CD-MPR and CI-MPR) or sortilin. In mammalian epididymis, some lysosomal proteins are secreted into the lumen through unknown mechanisms. To investigate the underlying mechanisms of lysosomal protein transport in epididymal cells we studied the expression and distribution of cathepsin D (CatD) and prosaposin (PSAP) in a sortilin knocked down RCE-1 epididymal cell line (RCE-1 KD) in comparison with non-transfected RCE-1 cells. In RCE-1 cells, CatD was found in the perinuclear zone and co-localize with sortilin, whereas in RCE-1 KD cells, the expression, distribution and processing of the enzyme were altered. In turn, PSAP accumulated intracellularly upon sortilin knock-down and redistributed from LAMP-1-positive compartment to a perinuclear location, remaining co-localized with CatD. Interestingly, the sortilin knock-down induced CD-MPR overexpression and a redistribution of the receptor from the perinuclear zone to a dispersed cytoplasmic location, accompanied by an increased co-localization with CatD. The increase in CD-MPR could result from a compensatory response for the proper delivery of CatD to lysosomes in epididymal cells. The intracellular pathway taken by lysosomal proteins could be an approach for addressing further studies to understand the mechanism of exocytosis and therefore the role of these proteins in the epididymis.

Cab45 deficiency leads to the mistargeting of progranulin and prosaposin and aberrant lysosomal positioning

The trans-Golgi Network (TGN) sorts molecular "addresses" and sends newly synthesized proteins to their destination via vesicular transport carriers. Despite the functional significance of packaging processes at the TGN, the sorting of soluble proteins remains poorly understood. Recent research has shown that the Golgi resident protein Cab45 is a significant regulator of secretory cargo sorting at the TGN. Cab45 oligomerizes upon transient Ca2+ influx, recruits soluble cargo molecules (clients), and packs them in sphingomyelin-rich transport carriers. However, the identity of client molecules packed into Cab45 vesicles is scarce. Therefore, we used a precise and highly efficient secretome analysis technology called hiSPECs. Intriguingly, we observed that Cab45 deficient cells manifest hypersecretion of lysosomal hydrolases. Specifically, Cab45 deficient cells secrete the unprocessed precursors of prosaposin (PSAP) and progranulin (PGRN). In addition, lysosomes in these cells show an aberrant perinuclear accumulation suggesting a new role of Cab45 in lysosomal positioning. This work uncovers a yet unknown function of Cab45 in regulating lysosomal function.

The role of lysosomal cathepsins in neurodegeneration: Mechanistic insights, diagnostic potential and therapeutic approaches

Lysosomes are ubiquitous organelles with a fundamental role in maintaining cellular homeostasis by mediating degradation and recycling processes. Cathepsins are the most abundant lysosomal hydrolyses and are responsible for the bulk degradation of various substrates. A correct autophagic function is essential for neuronal survival, as most neurons are post-mitotic and thus susceptible to accumulate cellular components. Increasing evidence suggests a crucial role of the lysosome in neurodegeneration as a key regulator of aggregation-prone and disease-associated proteins, such as α-synuclein, β-amyloid and huntingtin. Particularly, alterations in lysosomal cathepsins CTSD, CTSB and CTSL can contribute to the pathogenesis of neurodegenerative diseases as seen for neuronal ceroid lipofuscinosis, synucleinopathies (Parkinson's disease, Dementia with Lewy Body and Multiple System Atrophy) as well as Alzheimer's and Huntington's disease. In this review, we provide an overview of recent evidence implicating CTSD, CTSB and CTSL in neurodegeneration, with a special focus on the role of these enzymes in α-synuclein metabolism. In addition, we summarize the potential role of lysosomal cathepsins as clinical biomarkers in neurodegenerative diseases and discuss potential therapeutic approaches by targeting lysosomal function.

Loss of Christianson Syndrome Na+/H+ Exchanger 6 (NHE6) Causes Abnormal Endosome Maturation and Trafficking Underlying Lysosome Dysfunction in Neurons

Loss-of-function mutations in endosomal Na⁺/H⁺ exchanger 6 (NHE6) cause the X-linked neurologic disorder Christianson syndrome. Patients exhibit symptoms associated with both neurodevelopmental and neurodegenerative abnormalities. While loss of NHE6 has been shown to overacidify the endosome lumen, and is associated with endolysosome neuropathology, NHE6-mediated mechanisms in endosome trafficking and lysosome function have been understudied. Here, we show that NHE6-null mouse neurons demonstrate worsening lysosome function with time in culture, likely as a result of defective endosome trafficking. NHE6-null neurons exhibit overall reduced lysosomal proteolysis despite overacidification of the endosome and lysosome lumen. Akin to Nhx1 mutants in Saccharomyces cerevisiae, we observe decreased endosome-lysosome fusion in NHE6-null neurons. Also, we find premature activation of pH-dependent cathepsin D (CatD) in endosomes. While active CatD is increased in endosomes, CatD activation and CatD protein levels are reduced in the lysosome. Protein levels of another mannose 6-phosphate receptor (M6PR)-dependent enzyme, β-N-acetylglucosaminidase, were also decreased in lysosomes of NHE6-null neurons. M6PRs accumulate in late endosomes, suggesting defective M6PR recycling and retromer function in NHE6-null neurons. Finally, coincident with decreased endosome-lysosome fusion, using total internal reflection fluorescence, we also find a prominent increase in fusion between endosomal multivesicular bodies and the plasma membrane, indicating enhanced exosome secretion from NHE6-null neurons. In summary, in addition to overacidification of endosomes and lysosomes, loss of NHE6 leads to defects in endosome maturation and trafficking, including enhanced exosome release, contributing to lysosome deficiency and potentially leading to neurodegenerative disease.

SIGNIFICANCE STATEMENT Loss-of-function mutations in the endosomal Na⁺/H⁺ exchanger 6 (NHE6) cause Christianson syndrome, an X-linked neurologic disorder. Loss of NHE6 has been shown to overacidify endosomes; however, endosome trafficking mechanisms have been understudied, and the mechanisms leading to neurodegeneration are largely unknown. In NHE6-null mouse neurons in vitro, we find worsening lysosome function with days in culture. Notably, pH-dependent lysosome enzymes, such as cathepsin D, have reduced activity in lysosomes yet increased, precocious activity in endosomes in NHE6-null neurons. Further, endosomes show reduced fusion to lysosomes, and increased fusion to the plasma membrane with increased exosome release. This study identifies new mechanisms involving defective endosome maturation and trafficking that impair lysosome function in Christianson syndrome, likely contributing to neurodegeneration.

Loss of Lysosomal Proteins Progranulin and Prosaposin Associated with Increased Neurofibrillary Tangle Development in Alzheimer Disease

Alzheimer disease (AD) is a progressive neurodegenerative disease causing cognitive decline in the aging population. To develop disease-modifying treatments, understanding the mechanisms behind the pathology is important, which should include observations using human brain samples. We reported previously on the association of lysosomal proteins progranulin (PGRN) and prosaposin (PSAP) with amyloid plaques in non-demented aged control and AD brains. In this study, we investigated the possible involvement of PGRN and PSAP in tangle formation using human brain tissue sections of non-demented aged control subjects and AD cases and compared with cases of frontotemporal dementia with granulin (GRN) mutations. The study revealed that decreased amounts of PGRN and PSAP proteins were detected even in immature neurofibrillary tangles, while colocalization was still evident in adjacent neurons in all cases. Results suggest that neuronal loss of PGRN preceded loss of PSAP as tangles developed and matured. The GRN mutation cases exhibited almost complete absence of PGRN in most neurons, while PSAP signal was preserved. Although based on correlative data, we suggest that reduced levels of PGRN and PSAP and their interaction in neurons might predispose to accumulation of p-Tau protein.

Castration causes an increase in lysosomal size and upregulation of cathepsin D expression in principal cells along with increased secretion of procathepsin D and prosaposin oligomers in adult rat epididymis

In the epididymis, lysosomal proteins of the epithelial cells are normally targeted from the Golgi apparatus to lysosomes for degradation, although their secretion into the epididymal lumen has been documented and associated with sperm maturation. In this study, cathepsin D (CatD) and prosaposin (PSAP) were examined in adult epididymis of control, and 2-day castrated rats without (Ct) and with testosterone replacement (Ct+T) to evaluate their expression and regulation within epididymal epithelial cells. By light microscope-immunocytochemistry, a quantitative increase in size of lysosomes in principal cells of Ct animals was noted from the distal initial segment to the proximal cauda. Androgen replacement did not restore the size of lysosomes to control levels. Western blot analysis revealed a significant increase in CatD expression in the epididymis of Ct animals, which suggested an upregulation of its expression in principal cells; androgens restored levels of CatD to that of controls. In contrast, PSAP expression in Ct animals was not altered from controls. Additionally, an increase in procathepsin D levels was noted from samples of the epididymal fluid of Ct compared to control animals, accompanied by an increased complex formation with PSAP. Moreover, an increased oligomerization of prosaposin was observed in the epididymal lumen of Ct rats, with changes reverted to controls in Ct+T animals. Taken together these data suggest castration causes an increased uptake of substrates that are acted upon by CatD in lysosomes of principal cells and in the lumen by procathepsin D. These substrates may be derived from apoptotic cells noted in the lumen of proximal regions and possibly by degenerating sperm in distal regions of the epididymis of Ct animals. Exploring the mechanisms by which lysosomal enzymes are synthesized and secreted by the epididymis may help resolve some of the issues originating from epididymal dysfunctions with relevance to sperm maturation.

Putative second hit rare genetic variants in families with seemingly GBA-associated Parkinson's disease

Rare variants in the beta-glucocerebrosidase gene (GBA1) are common genetic risk factors for alpha synucleinopathy, which often manifests clinically as GBA-associated Parkinson's disease (GBA-PD). Clinically, GBA-PD closely mimics idiopathic PD, but it may present at a younger age and often aggregates in families. Most carriers of GBA variants are, however, asymptomatic. Moreover, symptomatic PD patients without GBA variant have been reported in families with seemingly GBA-PD. These observations obscure the link between GBA variants and PD pathogenesis and point towards a role for unidentified additional genetic and/or environmental risk factors or second hits in GBA-PD. In this study, we explored whether rare genetic variants may be additional risk factors for PD in two families segregating the PD-associated GBA1 variants c.115+1G>A (ClinVar ID: 93445) and p.L444P (ClinVar ID: 4288). Our analysis identified rare genetic variants of the HSP70 co-chaperone DnaJ homolog subfamily B member 6 (DNAJB6) and lysosomal protein prosaposin (PSAP) as additional factors possibly influencing PD risk in the two families. In comparison to the wild-type proteins, variant DNAJB6 and PSAP proteins show altered functions in the context of cellular alpha-synuclein homeostasis when expressed in reporter cells. Furthermore, the segregation pattern of the rare variants in the genes encoding DNAJB6 and PSAP indicated a possible association with PD in the respective families. The occurrence of second hits or additional PD cosegregating rare variants has important implications for genetic counseling in PD families with GBA1 variant carriers and for the selection of PD patients for GBA targeted treatments.

The Ins and Outs of Cathepsins: Physiological Function and Role in Disease Management

Cathepsins are the most abundant lysosomal proteases that are mainly found in acidic endo/lysosomal compartments where they play a vital role in intracellular protein degradation, energy metabolism, and immune responses among a host of other functions. The discovery that cathepsins are secreted and remain functionally active outside of the lysosome has caused a paradigm shift. Contemporary research has unraveled many versatile functions of cathepsins in extralysosomal locations including cytosol and extracellular space. Nevertheless, extracellular cathepsins are majorly upregulated in pathological states and are implicated in a wide range of diseases including cancer and cardiovascular diseases. Taking advantage of the differential expression of the cathepsins during pathological conditions, much research is focused on using cathepsins as diagnostic markers and therapeutic targets. A tailored therapeutic approach using selective cathepsin inhibitors is constantly emerging to be safe and efficient. Moreover, recent development of proteomic-based approaches for the identification of novel physiological substrates offers a major opportunity to understand the mechanism of cathepsin action. In this review, we summarize the available evidence regarding the role of cathepsins in health and disease, discuss their potential as biomarkers of disease progression, and shed light on the potential of extracellular cathepsin inhibitors as safe therapeutic tools.

Roles of Plant-Specific Inserts in Plant Defense

Ubiquitously expressed in plants, the plant-specific insert (PSI) of typical plant aspartic proteases (tpAPs) has been associated with plant development, stress response, and defense processes against invading pathogens. Despite sharing high sequence identity, structural studies revealed possible different mechanisms of action among species. The PSI induces signaling pathways of defense hormones in vivo and demonstrates broad-spectrum activity against phytopathogens in vitro. Recent characterization of the PSI-tpAP relationship uncovered novel, nonconventional intracellular protein transport pathways and improved tpAP production yields for industrial applications. In spite of research to date, relatively little is known about the structure-function relationships of PSIs. A comprehensive understanding of their biological roles may benefit plant protection strategies against virulent phytopathogens.

Exploring Extracellular Vesicles Biogenesis in Hypothalamic Cells through a Heavy Isotope Pulse/Trace Proteomic Approach

Studies have shown that the process of extracellular vesicles (EVs) secretion and lysosome status are linked. When the lysosome is under stress, the cells would secrete more EVs to maintain cellular homeostasis. However, the process that governs lysosomal activity and EVs secretion remains poorly defined and we postulated that certain proteins essential for EVs biogenesis are constantly synthesized and preferentially sorted to the EVs rather than the lysosome. A pulsed stable isotope labelling of amino acids in cell culture (pSILAC) based quantitative proteomics methodology was employed to study the preferential localization of the newly synthesized proteins into the EVs over lysosome in mHypoA 2/28 hypothalamic cell line. Through proteomic analysis, we found numerous newly synthesized lysosomal enzymes—such as the cathepsin proteins—that preferentially localize into the EVs over the lysosome. Chemical inhibition against cathepsin D promoted EVs secretion and a change in the EVs protein composition and therefore indicates its involvement in EVs biogenesis. In conclusion, we applied a heavy isotope pulse/trace proteomic approach to study EVs biogenesis in hypothalamic cells. The results demonstrated the regulation of EVs secretion by the cathepsin proteins that may serve as a potential therapeutic target for a range of neurological disorder associated with energy homeostasis.

Cathepsin D in the Tumor Microenvironment of Breast and Ovarian Cancers

Cancer remains a major and leading health problem worldwide. Lack of early diagnosis, chemoresistance, and recurrence of cancer means vast research and development are required in this area. The complexity of the tumor microenvironment in the biological milieu poses greater challenges in having safer, selective, and targeted therapies. Existing strategies such as chemotherapy, radiotherapy, and antiangiogenic therapies moderately improve progression-free survival; however, they come with side effects that reduce quality of life. Thus, targeting potential candidates in the microenvironment, such as extracellular cathepsin D (CathD) which has been known to play major pro-tumorigenic roles in breast and ovarian cancers, could be a breakthrough in cancer treatment, specially using novel treatment modalities such as immunotherapy and nanotechnology-based therapy. This chapter discusses CathD as a pro-cancerous, more specifically a proangiogenic factor, that acts bi-functionally in the tumor microenvironment, and possible ways of targeting the protein therapeutically.

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.

The lysosomal function of progranulin, a guardian against neurodegeneration

Progranulin (PGRN), encoded by the GRN gene in humans, is a secreted growth factor implicated in a multitude of processes ranging from regulation of inflammation to wound healing and tumorigenesis. The clinical importance of PGRN became especially evident in 2006, when heterozygous mutations in the GRN gene, resulting in haploinsufficiency, were found to be one of the main causes of frontotemporal lobar degeneration (FTLD). FTLD is a clinically heterogenous disease that results in the progressive atrophy of the frontal and temporal lobes of the brain. Despite significant research, the exact function of PGRN and its mechanistic relationship to FTLD remain unclear. However, growing evidence suggests a role for PGRN in the lysosome-most striking being that homozygous GRN mutation leads to neuronal ceroid lipofuscinosis, a lysosomal storage disease. Since this discovery, several links between PGRN and the lysosome have been established, including the existence of two independent lysosomal trafficking pathways, intralysosomal processing of PGRN into discrete functional peptides, and direct and indirect regulation of lysosomal hydrolases. Here, we summarize the cellular functions of PGRN, its roles in the nervous system, and its link to multiple neurodegenerative diseases, with a particular focus dedicated to recent lysosome-related mechanistic developments.

The interaction between progranulin and prosaposin is mediated by granulins and the linker region between saposin B and C

The frontotemporal lobar degeneration (FTLD) protein progranulin (PGRN) is essential for proper lysosomal function. PGRN localizes in the lysosomal compartment within the cell. Prosaposin (PSAP), the precursor of lysosomal saposin activators (saposin A, B, C, D), physically interacts with PGRN. Previously, we have shown that PGRN and PSAP facilitate each other's lysosomal trafficking. Here, we report that the interaction between PSAP and PGRN requires the linker region of saposin B and C (BC linker). PSAP protein with the BC linker mutated, fails to interact with PGRN and deliver PGRN to lysosomes in the biosynthetic and endocytic pathways. On the other hand, PGRN interacts with PSAP through multiple granulin motifs. Granulin D and E bind to PSAP with similar affinity as full-length PGRN. Read the Editorial Comment for this article on page 154.

Lysosomal storage diseases

Lysosomes are cytoplasmic organelles that contain a variety of different hydrolases. A genetic deficiency in the enzymatic activity of one of these hydrolases will lead to the accumulation of the material meant for lysosomal degradation. Examples include glycogen in the case of Pompe disease, glycosaminoglycans in the case of the mucopolysaccharidoses, glycoproteins in the cases of the oligosaccharidoses, and sphingolipids in the cases of Niemann-Pick disease types A and B, Gaucher disease, Tay-Sachs disease, Krabbe disease, and metachromatic leukodystrophy. Sometimes, the lysosomal storage can be caused not by the enzymatic deficiency of one of the hydrolases, but by the deficiency of an activator protein, as occurs in the AB variant of GM2 gangliosidosis. Still other times, the accumulated lysosomal material results from failed egress of a small molecule as a consequence of a deficient transporter, as in cystinosis or Salla disease. In the last couple of decades, enzyme replacement therapy has become available for a number of lysosomal storage diseases. Examples include imiglucerase, taliglucerase and velaglucerase for Gaucher disease, laronidase for Hurler disease, idursulfase for Hunter disease, elosulfase for Morquio disease, galsulfase for Maroteaux-Lamy disease, alglucosidase alfa for Pompe disease, and agalsidase alfa and beta for Fabry disease. In addition, substrate reduction therapy has been approved for certain disorders, such as eliglustat for Gaucher disease. The advent of treatment options for some of these disorders has led to newborn screening pilot studies, and ultimately to the addition of Pompe disease and Hurler disease to the Recommended Uniform Screening Panel (RUSP) in 2015 and 2016, respectively.

Lysosomal cathepsins and their regulation in aging and neurodegeneration

Lysosomes and lysosomal hydrolases, including the cathepsins, have been shown to change their properties with aging brain a long time ago, although their function was not really understood. The first biochemical and clinical studies were followed by a major expansion in the last 20 years with the development of animal disease models and new approaches leading to a major advancement of understanding of the role of physiological and degenerative processes in the brain at the molecular level. This includes the understanding of the major role of autophagy and the cathepsins in a number of diseases, including its critical role in the neuronal ceroid lipofuscinosis. Similarly, cathepsins and some other lysosomal proteases were shown to have important roles in processing and/or degradation of several important neuronal proteins, thereby having either neuroprotective or harmful roles. In this review, we discuss lysosomal cathepsins and their regulation with the focus on cysteine cathepsins and their endogenous inhibitors, as well as their role in several neurodegenerative diseases.

The role of Sema4A in angiogenesis, immune responses, carcinogenesis, and retinal systems

Semaphorins were originally identified as axon guidance cues that regulate the functional activity of axons in the nervous system. In addition, accumulating evidence indicates that semaphorins have multiple functions in physiological and pathogenic processes, including vascular development, tumor progression, and immune responses. Sema4A is a semaphorin expressed in immune cells, and is thus termed an “immune semaphorin.” Sema4A has 4 types of receptors: Plexin D family, Plexin B family, Tim-2, and Nrp-1. Recent studies suggest that Sema4A plays critical roles in many processes including cell–cell interactions, immune-cell activation, differentiation, and migration. In other studies, Sema4A is also associated with carcinogenesis and retinal systems. In this review, we summarize current knowledge regarding the biology of Sema4A in relation to angiogenesis, immune responses, colorectal cancer, and the retina.

Procathepsin E is highly abundant but minimally active in pancreatic ductal adenocarcinoma tumors

The cathepsin family of lysosomal proteases is increasingly being recognized for their altered expression in cancer and role in facilitating tumor progression. The aspartyl protease cathepsin E is overexpressed in several cancers and has been investigated as a biomarker for pancreatic ductal adenocarcinoma (PDAC). Here we show that cathepsin E expression in mouse PDAC tumors is increased by more than 400-fold when compared to healthy pancreatic tissue. Cathepsin E accumulates over the course of disease progression and accounts for more than 3% of the tumor protein in mice with end-stage disease. Through immunoblot analysis we determined that only procathepsin E exists in mouse PDAC tumors and cell lines derived from these tumors. By decreasing the pH, this procathepsion E is converted to the mature form, resulting in an increase in proteolytic activity. Although active site inhibitors can bind procathepsin E, treatment of PDAC mice with the aspartyl protease inhibitor ritonavir did not decrease tumor burden. Lastly, we used multiplex substrate profiling by mass spectrometry to identify two synthetic peptides that are hydrolyzed by procathepsin E near neutral pH. This work represents a comprehensive analysis of procathepsin E in PDAC and could facilitate the development of improved biomarkers for disease detection.

Prosaposin facilitates sortilin-independent lysosomal trafficking of progranulin

Prosaposin directly interacts with progranulin and facilitates progranulin lysosomal trafficking via the trafficking receptors M6PR and LRP1, independent of the previously identified progranulin trafficking pathway mediated by sortilin.

Mutations in the progranulin (PGRN) gene have been linked to two distinct neurodegenerative diseases, frontotemporal lobar degeneration (FTLD) and neuronal ceroid lipofuscinosis (NCL). Accumulating evidence suggests a critical role of PGRN in lysosomes. However, how PGRN is trafficked to lysosomes is still not clear. Here we report a novel pathway for lysosomal delivery of PGRN. We found that prosaposin (PSAP) interacts with PGRN and facilitates its lysosomal targeting in both biosynthetic and endocytic pathways via the cation-independent mannose 6-phosphate receptor and low density lipoprotein receptor-related protein 1. PSAP deficiency in mice leads to severe PGRN trafficking defects and a drastic increase in serum PGRN levels. We further showed that this PSAP pathway is independent of, but complementary to, the previously identified PGRN lysosomal trafficking mediated by sortilin. Collectively, our results provide new understanding on PGRN trafficking and shed light on the molecular mechanisms behind FTLD and NCL caused by PGRN mutations.

The Potential Role of the Proteases Cathepsin D and Cathepsin L in the Progression and Metastasis of Epithelial Ovarian Cancer

Epithelial ovarian cancer (EOC) is the leading cause of death from gynecologic malignancies and has a poor prognosis due to relatively unspecific early symptoms, and thus often advanced stage, metastasized cancer at presentation. Metastasis of EOC occurs primarily through the transcoelomic route whereby exfoliated tumor cells disseminate within the abdominal cavity, particularly to the omentum. Primary and metastatic tumor growth requires a pool of proangiogenic factors in the microenvironment which propagate new vasculature in the growing cancer. Recent evidence suggests that proangiogenic factors other than the widely known, potent angiogenic factor vascular endothelial growth factor may mediate growth and metastasis of ovarian cancer. In this review we examine the role of some of these alternative factors, specifically cathepsin D and cathepsin L.

Alteration of cathepsin D trafficking induced by hypoxia and extracellular acidification in MCF-7 breast cancer cells

The microenvironment that surrounds tumor cells is characterized by hypoxic conditions and extracellular acidity. These hostile conditions induce crucial changes in cell behavior and can promote the secretion of many soluble factors such as growth factors, cytokines and enzymes. The lysosomal aspartyl-endopeptidase cathepsin D (CD) is a marker of poor prognosis in breast cancer and is associated with a metastatic risk. In this study, the transport of CD was investigated in a model of breast cancer cells line (MCF-7) cultivated under hypoxia and acidification of media. CD secretion was assessed using Western blot analysis and protease activity was measured in conditioned culture media. We demonstrate that cultured MCF-7 cells secrete an active 52 kDa pCD precursor and report that under hypoxia there was an increased amount of pCD secreted. More surprisingly, extracellular acidification (pH 6 and 5.6) induced the secretion of the fully-mature and active (34 kDa + 14 kDa) double chain CD. Our findings reflect the fact that chemical anomalies influence the secretion path of CD in a breast cancer cell model, resulting in altered trafficking of the mature form. This important result may provide new arguments in favor of the role of extracellular CD in the degradation of the matrix proteins that constitute the breast tumor microenvironment.

Two Faces of Cathepsin D: Physiological Guardian Angel and Pathological Demon

Since its discovery as a lysosomal hydrolase, Cathepsin D (CatD) has been the subject of intensive scrutiny by numerous scientists. Those accumulated efforts have defined its biosynthetic pathway, structure, and companion proteins in the context of its perceived “house keeping” function. However, in the past two decades CatD has emerged as a multifunctional enzyme, involved in myriad biological processes beyond its original “housekeeping” role. CatD is responsible for selective and limited cleavage (quite distinct from non-specific protein degradation) of particular substrates vital to proper cellular function. These proteolytic events are critical in the control of biological processes, including cell cycle progression, differentiation and migration, morphogenesis and tissue remodeling, immunological processes, ovulation, fertilization, neuronal outgrowth, angiogenesis, and apoptosis. Consistent with the biological relevance of CatD, its deficiency, altered regulation or post-translational modification underlie important pathological conditions such as cancer, atherosclerosis, neurological and skin disorders. Specifically, deregulated synthesis, post-translational modifications and hyper-secretion of CatD, along with its mitogenic effects, are established hallmarks of cancer. More importantly, but less studied, is its significance in regulating the sensitivity to anticancer drugs.

This review outlines CatD’s post-translational modifications, cellular trafficking, secretion and protein binding partners in normal mammary gland, and restates the “site-specific” function of CatD which is most probably dictated by its post-translational modifications and binding partners. Noteworthy, CatD’s association with one of its binding partners in the context of drug sensitivity is highlighted, with the optimism that it could contribute to the development of more effective chemotherapeutic agent(s) tailored for individual patients.

Cathepsin D activity and selectivity in the acidic conditions of a tumor microenvironment: Utilization in the development of a novel Cathepsin D substrate for simultaneous cancer diagnosis and therapy

Pro-Cathepsin D (pCD) is an aspartyl endopeptidase which is over expressed in many cancers. This over expression generally led to its secretion into the extracellular culture medium of cancer cells. Moreover, pCD can auto activate and cleave its substrates at an acidic pH compatible with that found in tumor microenvironments (TME). Thus, exploiting these two pathological characteristics of TME offers the opportunity to develop new protease-activated vector on the basis of their specific substrate structures. The aim of this study was to validate new pCD substrates in the extracellular pH conditions of TME. As a first step, we investigated the effect of pH on the catalytic activity and selectivity of mature Cathepsin D (CD). It was found that the increase in the pH of the media led to a decrease in the reaction rate. However, the specificity of mature CD was not affected by a variation in pH. In the second step, the effect of the substrate structure was studied. We demonstrated that the substrate structure had a significant effect on the catalytic activity of CD. In fact, some modifications in peptide structure induced a change in the catalytic behavior that involved a substrate activation phenomenon. We suggest that this activation may be related to the amphiphilic nature of the modified peptide that may induce an interfacial activation mechanism. Finally, pCD, which is the major form found in the extracellular culture medium of cancer cells, was used. We demonstrated that the proform of CD cleave the modified peptide 5 at pH 6.5 with the same cleavage selectivity obtained with the mature form of the protease. These data provide a better understanding of CD behavior in tumor microenvironment conditions and this knowledge can be used to develop more specific tools for diagnosis and drug delivery.

Cell biology and function of neuronal ceroid lipofuscinosis-related proteins

Neuronal ceroid lipofuscinoses (NCL) comprise a group of inherited lysosomal disorders with variable age of onset, characterized by lysosomal accumulation of autofluorescent ceroid lipopigments, neuroinflammation, photoreceptor- and neurodegeneration. Most of the NCL-related genes encode soluble and transmembrane proteins which localize to the endoplasmic reticulum or to the endosomal/lysosomal compartment and directly or indirectly regulate lysosomal function. Recently, exome sequencing led to the identification of four novel gene defects in NCL patients and a new NCL nomenclature currently comprising CLN1 through CLN14. Although the precise function of most of the NCL proteins remains elusive, comprehensive analyses of model organisms, particularly mouse models, provided new insight into pathogenic mechanisms of NCL diseases and roles of mutant NCL proteins in cellular/subcellular protein and lipid homeostasis, as well as their adaptive/compensatorial regulation at the transcriptional level. This review summarizes the current knowledge on the expression, function and regulation of NCL proteins and their impact on lysosomal integrity. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

Endosomal sorting by Semaphorin 4A in retinal pigment epithelium supports photoreceptor survival

Photoreceptor cell death is the hallmark of a group of human inherited retinal degeneration. Although the causative genetic mutations are often known, the mechanisms leading to photoreceptor degeneration remain poorly defined. Here, we show that Semaphorin 4A (Sema4A), a member of axonal guidance molecule semaphorin, plays a role in Rab11/FIP2-mediated endosomal sorting in retinal pigment epithelial cells to support photoreceptor function. In response to oxidative stress, Sema4A switches the endosomal sorting of the lysosomal precursor protein prosaposin from the lysosome to the exosomal release, which prevents light-induced photoreceptor apoptosis. In the absence of oxidative stress, Sema4A sorts retinoid-binding proteins with retinoids between the cell surface and endoplasmic reticulum, by which 11-cis-retinal, a chromophore for phototransduction, is regenerated and transported back to photoreceptors. Owing to defects in these processes, Sema4A-deficient mice exhibit marked photoreceptor degeneration. Our findings therefore indicate that Sema4A regulates two distinct endosomal-sorting pathways that are critical for photoreceptor survival and phototransduction during the transition between daylight and darkness.

Hepatic deficiency of low density lipoprotein receptor-related protein-1 reduces high density lipoprotein secretion and plasma levels in mice

The low density lipoprotein receptor-related protein-1 (LRP1) is known to serve as a chylomicron remnant receptor in the liver responsible for the binding and plasma clearance of apolipoprotein E-containing lipoproteins. Previous in vitro studies have provided evidence to suggest that LRP1 expression may also influence high density lipoprotein (HDL) metabolism. The current study showed that liver-specific LRP1 knock-out (hLrp1(-/-)) mice displayed lower fasting plasma HDL cholesterol levels when compared with hLrp1(+/+) mice. Lecithin:cholesterol acyl transferase and hepatic lipase activities in plasma of hLrp1(-/-) mice were comparable with those observed in hLrp1(+/+) mice, indicating that hepatic LRP1 inactivation does not influence plasma HDL remodeling. Plasma clearance of HDL particles and HDL-associated cholesteryl esters was also similar between hLrp1(+/+) and hLrp1(-/-) mice. In contrast, HDL secretion from primary hepatocytes isolated from hLrp1(-/-) mice was significantly reduced when compared with that observed with hLrp1(+/+) hepatocytes. Biotinylation of cell surface proteins revealed decreased surface localization of the ATP-binding cassette, subfamily A, member 1 (ABCA1) protein, but total cellular ABCA1 level was not changed in hLrp1(-/-) hepatocytes. Finally, hLrp1(-/-) hepatocytes displayed reduced binding capacity for extracellular cathepsin D, resulting in lower intracellular cathepsin D content and impairment of prosaposin activation, a process that is required for membrane translocation of ABCA1 to facilitate cholesterol efflux and HDL secretion. Taken together, these results documented that hepatic LRP1 participates in cellular activation of lysosomal enzymes and through this mechanism, indirectly modulates the production and plasma levels of HDL.

Proteomic analysis of synaptosomal protein expression reveals that cerebral ischemia alters lysosomal Psap processing

Cerebral ischemia (CI) induces dramatic changes in synaptic structure and function that precedes delayed post-ischemic neuronal death. Here, a proteomic analysis was used to identify the effects of focal CI on synaptosomal protein levels. Contralateral and ipsilateral synaptosomes, prepared from adult mice subjected to 60 min middle cerebral artery occlusion, were isolated following 3, 6 and 20 h of reperfusion. Synaptosomal protein samples (n=3) were labeled using the cleavable ICAT system prior to analysis with nanoLC-MS/MS. Each sample was analyzed by LC-MS to identify differential expressions using InDEPT software and differentially expressed peptides were identified by targeted LC-MS/MS. A total of 62 differentially expressed proteins were identified and Gene Ontology classification (cellular component) indicated that the majority of the proteins were located in the mitochondria and other components consistent with synaptic localization. The observed alterations in synaptic protein levels poorly correlated with gene expression, indicating the involvement of post-transcriptional regulatory mechanisms in determining post-ischemic synaptic protein content. Additionally, immunohistochemistry analysis of prosaposin (Psap) and saposin C (SapC) indicates that CI disrupts Psap processing and glycosphingolipid metabolism. These results demonstrate that the synapse is adversely affected by CI and may play a role in mediating post-ischemic neuronal viability.

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.

The role of UDP-Glc:glycoprotein glucosyltransferase 1 in the maturation of an obligate substrate prosaposin

A natural substrate for UGT1 is confirmed, revealing how the enzyme functions in the calnexin chaperone system as a quality control step in protein folding.

An endoplasmic reticulum (ER) quality control system assists in efficient folding and disposal of misfolded proteins. N-linked glycans are critical in these events because their composition dictates interactions with molecular chaperones. UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1) is a key quality control factor of the ER. It adds glucoses to N-linked glycans of nonglucosylated substrates that fail a quality control test, supporting additional rounds of chaperone binding and ER retention. How UGT1 functions in its native environment is poorly understood. The role of UGT1 in the maturation of glycoproteins at basal expression levels was analyzed. Prosaposin was identified as a prominent endogenous UGT1 substrate. A dramatic decrease in the secretion of prosaposin was observed in ugt1^−/− cells with prosaposin localized to large juxtanuclear aggresome-like inclusions, which is indicative of its misfolding and the essential role that UGT1 plays in its proper maturation. A model is proposed that explains how UGT1 may aid in the folding of sequential domain–containing proteins such as prosaposin.

Prosaposin down-modulation decreases metastatic prostate cancer cell adhesion, migration, and invasion

Background

Factors responsible for invasive and metastatic progression of prostate cancer (PCa) remain largely unknown. Previously, we reported cloning of prosaposin (PSAP) and its genomic amplification and/or overexpression in several androgen-independent metastatic PCa cell lines and lymph node metastases. PSAP is the lysosomal precursor of saposins, which serve as activators for lysosomal hydrolases involved in the degradation of ceramide (Cer) and other sphingolipids.

Results

Our current data show that, in metastatic PCa cells, stable down-modulation of PSAP by RNA-interference via a lysosomal proteolysis-dependent pathway decreased β_1A-integrin expression, its cell-surface clustering, and adhesion to basement membrane proteins; led to disassembly of focal adhesion complex; and decreased phosphorylative activity of focal adhesion kinase and its downstream adaptor molecule, paxillin. Cathepsin D (CathD) expression and proteolytic activity, migration, and invasion were also significantly decreased in PSAP knock-down cells. Transient-transfection studies with β_1A integrin- or CathD-siRNA oligos confirmed the cause and effect relationship between PSAP and CathD or PSAP and Cer-β_1A integrin, regulating PCa cell migration and invasion.

Conclusion

Our findings suggest that by a coordinated regulation of Cer levels, CathD and β_1A-integrin expression, and attenuation of "inside-out" integrin-signaling pathway, PSAP is involved in PCa invasion and therefore might be used as a molecular target for PCa therapy.

A stretch of 17 amino acids in the prosaposin C terminus is critical for its binding to sortilin and targeting to lysosomes

Prosaposin, the precursor of four lysosomal cofactors required for the hydrolysis of sphingolipids, is transported to the lysosomes via the alternative receptor, sortilin. In this study, we identified a specific domain of 17 amino acids within the C terminus of prosaposin involved in binding to this sorting receptor. We generated six prosaposin deletion constructs and examined the effect of truncation by coimmunoprecipitation and confocal microscopy. The experiments revealed that the first half of the prosaposin C terminus (aa 524-540), containing a saposin-like motif, was required and necessary to bind sortilin and to transport it to the lysosomes. Based on this result, we introduced twelve site-directed point mutations within the first half of the C terminus. Although the interaction of prosaposin with sortilin was pH dependent, the mutation of hydrophilic amino acids that usually modulate pH-dependent protein interactions did not affect the binding of prosaposin to sortilin. Conversely, a tryptophan (W530) and two cysteines (C528 and C536) were essential for its interaction with sortilin and for its transport to the lysosomes. In conclusion, our investigation demonstrates that a saposin-like motif within the first half of the prosaposin C terminus contains the sortilin recognition site.

Genetic modifiers of degeneration in the cathepsin D deficient Drosophila model for neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses (NCLs) are pediatric, neurodegenerative, lysosomal storage disorders. Mutations in cathepsin D result in the most severe, congenital form of NCLs. We have previously generated a cathepsin D deficient Drosophila model, which exhibits the key features of NCLs: progressive intracellular accumulation of autofluorescent storage material and modest neurodegeneration in the brain areas related to visual functions. Here we extend the phenotypic characterization of cathepsin D deficient Drosophila and report that modest degenerative changes are also present in their retinae. Furthermore, by utilizing this phenotype, we examined the possible effect of 17 candidate modifiers, selected based on the results from other cathepsin D deficiency models. We found enhancers of this phenotype that support the involvement of endocytosis-, lipid metabolism- and oxidation-related factors in the cathepsin D deficiency induced degeneration. Our results warrant further investigation of these mechanisms in the pathogenesis of cathepsin D deficiency.

Saposin B-dependent reconstitution of arylsulfatase A activity in vitro and in cell culture models of metachromatic leukodystrophy

Arylsulfatase A (ASA) catalyzes the intralysosomal desulfation of 3-O-sulfogalactosylceramide (sulfatide) to galactosylceramide. The reaction requires saposin B (Sap B), a non-enzymatic proteinaceous cofactor which presents sulfatide to the catalytic site of ASA. The lack of either ASA or Sap B results in a block of sulfatide degradation, progressive intralysosomal accumulation of sulfatide, and the fatal lysosomal storage disease metachromatic leukodystrophy. We studied the coupled Sap B-ASA reaction in vitro using detergent-free micellar and liposomal assay systems and in vivo using cell culture models of metachromatic leukodystrophy. Under in vitro conditions, the reaction had a narrow pH optimum around pH 4.3 and was inhibited by mono- and divalent cations, phosphate and sulfite. Bis(monoacylglycero) phosphate and phosphatidic acid were activators of the reaction, underscoring a significant role of acidic phosphoglycerolipids in sphingolipid degradation. Desulfation was negligible when Sap B was substituted by Sap A, C, or D. Up to a molar ratio between Sap B and sulfatide of 1:5, an elevation of Sap B concentrations caused a sharp increase of sulfatide hydrolysis, indicating the requirement of unexpected high Sap B levels for maximum turnover. Feeding of ASA-deficient, sulfatide-storing primary mouse kidney cells with ASA caused partial clearance of sulfatide. Co-feeding of Sap B or its precursor prosaposin resulted in the lysosomal uptake of the cofactor but did not promote ASA-catalyzed sulfatide hydrolysis. This suggests that Sap B is not a limiting factor of the coupled Sap B-ASA reaction in mouse kidney cells even if sulfatide has accumulated to unphysiologically high levels.

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.

Cathepsin D: a cellular roadmap

Cathepsin D is a normal and major component of lysosomes, it is found in almost all cells and tissues of mammals. Present review describes different events in cellular life of cathepsin D mainly its biosynthesis, co-translational and posttranslational modifications, targeting to lysosomes and proteolytic processing and maturation within lysosomes.

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.

Purification and functional characterization of human 11beta hydroxylase expressed in Escherichia coli

The human 11beta-hydroxylase (hCYP11B1) is responsible for the conversion of 11-deoxycortisol into the major mammalian glucocorticoid, cortisol. The reduction equivalents needed for this reaction are provided via a short electron transfer chain consisting of a [2Fe-2S] ferredoxin and a FAD-containing reductase. On the biochemical and biophysical level, little is known about hCYP11B1 because it is very unstable for analyses performed in vitro. This instability is also the reason why it has not been possible to stably express it so far in Escherichia coli and subsequently purify it. In the present study, we report on the successful and reproducible purification of recombinant hCYP11B1 coexpressed with molecular chaperones GroES/GroEL in E. coli. The protein was highly purified to apparent homogeneity, as observed by SDS/PAGE. Upon mass spectrometry, the mass-to-charge ratio (m/z) of the protein was estimated to be 55 761, which is consistent with the value 55 760.76 calculated for the form lacking the translational initiator Met. The functionality of hCYP11B1 was analyzed using different methods (substrate conversion assays, stopped-flow, Biacore). The results clearly demonstrate that the enzyme is capable of hydroxylating its substrates at position 11-beta. Moreover, the determined NADPH coupling percentage for the hCYP11B1 catalyzed reactions using either 11-deoxycortisol or 11-deoxycorticosterone as substrates was approximately 75% in both cases. Biacore and stopped-flow measurements indicate that hCYP11B1 possesses more than one binding site for its redox partner adrenodoxin, possibly resulting in the formation of more than one productive complexes. In addition, we performed CD measurements to obtain information about the structure of hCYP11B1.

Distribution of prosaposin in the rat nervous system

Prosaposin is the precursor of four sphingolipid activator proteins (saposins A, B, C, and D) for lysosomal hydrolases and is abundant in the nervous system and muscle. In addition to its role as a precursor of saposins in lysosomes, intact prosaposin has neurotrophic effects in vivo or in vitro when supplied exogenously. We examined the distribution of prosaposin in the central and peripheral nervous systems and its intracellular distribution. Using a monospecific antisaposin D antibody that crossreacts with prosaposin but not with saposins A, B, or C, immunoblot experiments showed that both the central and peripheral nervous systems express unprocessed prosaposin and little saposin D. Using the antisaposin D antibodies, we demonstrated that prosaposin is abundant in almost all neurons of both the central and peripheral nervous systems, including autonomic nerves, as well as motor and sensory nerves. Immunoelectron microscopy using double staining with antisaposin D and anticathepsin D antibodies showed strong prosaposin immunoreactivity mainly in the lysosomal granules in the neurons in both the central and peripheral nervous systems. The expression of prosaposin mRNA, examined using in situ hybridization, was observed in these same neurons. Our results suggest that prosaposin is synthesized ubiquitously in neurons of both the central and peripheral nervous systems.

Folding, activity and targeting of mutated human cathepsin D that cannot be processed into the double-chain form

The precursor of human cathepsin D (CD) is converted into the single-chain and the double-chain active polypeptides by subsequent proteolysis reactions taking place in the endosomal-lysosomal compartment and involving specific aminoacid sequences. We have mutagenized the region of aminoacids (comprising the beta-hairpin loop) involved in the latter proteolytic maturation step and generated a mutant CD that cannot be converted into the mature double-chain form. This mutant CD expressed in rodent cells reaches the lysosome and is stable as single-chain polypeptide, bears high-mannose type sugars, binds to pepstatin A and is enzymatically active, indicating that it is correctly folded. The present work provides new insights on the aminoacid region involved in the terminal processing of human CD and on the function of the processing beta-hairpin loop.

The catalytically inactive precursor of cathepsin D induces apoptosis in human fibroblasts and HeLa cells

In several reports cathepsin D has been implicated in apoptosis. In some systems the effects of agents considered to be mediated by cathepsin D were inhibited in the presence of pepstatin A, an inhibitor of the enzyme. In other studies the effect of a mutant cathepsin D deprived of activity was indistinguishable from that of the normal enzyme. Here we show that in human fibroblasts and in HeLa cells apoptosis can be induced by microinjecting into cytosol either mature cathepsin D or its inactive precursor procathepsin D. The microinjected precursor remains in the uncleaved form. These results confirm that the proapoptotic effect of cathepsin D in the cytosol is independent of its catalytic activity and suggest that the interaction of cathepsin D with the downstream effector does not involve the active site of the enzyme, since in the proenzyme the active site is masked by the prosequence.

Cathepsin D, a Lysosomal Protease, Regulates ABCA1-mediated Lipid Efflux

To identify genes involved in the regulation of plasma high density lipoprotein (HDL) cholesterol (HDL-C) levels, patients with low HDL-C and age- and sex-matched controls (normal HDL-C) were extensively characterized. Comparative transcriptome analysis was carried out in cholesterol-loaded monocyte-derived macrophages from low HDL subjects segregated into groups with or without cholesterol efflux defects or ABCA1 mutations. Clusters of differentially regulated genes were evident in the low HDL groups as compared with controls. Of particular note, expression of cathepsin D (CTSD), a lysosomal proteinase, was reduced by approximately 50% in monocyte-derived macrophages of low HDL-C subjects, most significantly those with cholesterol efflux defects but without mutations in ABCA1 (p < 0.01). These results were verified by reverse transcription-PCR and replicated in a second cohort. We show here that blocking the activity or expression of CTSD, by pepstatin or CTSD small interfering RNA, respectively, reduced ABCA1 expression and protein abundance in both macrophages and CHO cells and apolipoprotein A-I-mediated lipid efflux by more than 70%. Conversely, expression of CTSD increased both ABCA1 mRNA expression and cellular ABCA1 protein. Consistent with its role in the proteolytic processing of prosaposin, inactivation of CTSD function resulted in the accumulation of glycosphingo-lipid and free cholesterol in late endosomes/lysosomes, a phenotype similar to NPC1 deficiency. Inhibition of CTSD also caused retention of ABCA1 in lysosomal compartments, reducing its trafficking to the plasma membrane. These studies demonstrate a novel and potentially important role for CTSD in intracellular cholesterol trafficking and ABCA1-mediated efflux. Therefore, decreased CTSD expression may contribute to low plasma HDL-C levels.