Cathepsin D: a cellular roadmap

The physiological role of the unfolded protein response in the nervous system

The unfolded protein response (UPR) is a cellular stress response pathway activated when the endoplasmic reticulum, a crucial organelle for protein folding and modification, encounters an accumulation of unfolded or misfolded proteins. The UPR aims to restore endoplasmic reticulum homeostasis by enhancing protein folding capacity, reducing protein biosynthesis, and promoting protein degradation. It also plays a pivotal role in coordinating signaling cascades to determine cell fate and function in response to endoplasmic reticulum stress. Recent research has highlighted the significance of the UPR not only in maintaining endoplasmic reticulum homeostasis but also in influencing various physiological processes in the nervous system. Here, we provide an overview of recent findings that underscore the UPR's involvement in preserving the function and viability of neuronal and myelinating cells under physiological conditions, and highlight the critical role of the UPR in brain development, memory storage, retinal cone development, myelination, and maintenance of myelin thickness.

Pro-cathepsin D prevents aberrant protein aggregation dependent on endoplasmic reticulum protein CLN6

We previously expressed a chimeric protein in which the small heat-shock protein αB-crystallin (αBC) is fused at its N-terminus to the C-terminus of the first transmembrane segment of the endoplasmic reticulum (ER) protein mitsugumin 23 and confirmed its localization to the ER. Moreover, overexpression of this N-terminally modified αBC was shown to prevent the aggregation of the coexpressed R120G αBC variant, which is highly aggregation-prone and associated with the hereditary myopathy αB-crystallinopathy. To uncover a molecular mechanism by which the ER-anchored αBC negatively regulates the protein aggregation, we isolated proteins that bind to the ER-anchored αBC and identified the lysosomal protease cathepsin D (CTSD) as one such interacting protein. Proteolytically active CTSD is produced by multi-step processing of pro-cathepsin D (proCTSD), which is initially synthesized in the ER and delivered to lysosomes. When overexpressed, CTSD itself prevented the coexpressed R120G αBC variant from aggregating. This anti-aggregate activity was also elicited upon overexpression of the W383C CTSD variant, which is predominantly sequestered in the ER and consequently remains unprocessed, suggesting that proCTSD, rather than mature CTSD, serves to suppress the aggregation of the R120G αBC variant. Meanwhile, overexpression of the A58V CTSD variant, which is identical to wild-type CTSD except for the Ala58Val substitution within the pro-peptide, did not suppress the protein aggregation, indicating that the integrity of the pro-peptide is required for proCTSD to exert its anti-aggregate activity. Based on our previous finding that overexpression of the ER transmembrane protein CLN6 (ceroid-lipofuscinosis, neuronal 6), identified as an interacting protein of the ER-anchored αBC, prevents the R120G αBC variant from aggregating, the CLN6-proCTSD coupling was hypothesized to underpin the functionality of proCTSD within the ER. Indeed, CTSD, when overexpressed in CLN6-depleted cells, was unable to exert its anti-aggregate activity, supporting our view. Collectively, we show here that proCTSD prevents the protein aggregation through the functional association with CLN6 in the microenvironment surrounding the ER membrane, shedding light on a novel aspect of proCTSD and its potential involvement in CTSD-related disorders characterized by the accumulation of aberrant protein aggregates.

Two-Step Enrichment Facilitates Background Reduction for Proteomic Analysis of Lysosomes

Lysosomes constitute the main degradative compartment of most mammalian cells and are involved in various cellular functions. Most of them are catalyzed by lysosomal proteins, which typically are low abundant, complicating their analysis by mass spectrometry-based proteomics. To increase analytical performance and to enable profiling of lysosomal content, lysosomes are often enriched. Two approaches have gained popularity in recent years, namely, superparamagnetic iron oxide nanoparticles (SPIONs) and immunoprecipitation from cells overexpressing a 3xHA-tagged version of TMEM192 (TMEM-IP). The effect of these approaches on the lysosomal proteome has not been investigated to date. We addressed this topic through a combination of both techniques and proteomic analysis of lysosome-enriched fractions. For SPIONs treatment, we identified altered cellular iron homeostasis and moderate changes of the lysosomal proteome. For overexpression of TMEM192, we observed more pronounced effects in lysosomal protein expression, especially for lysosomal membrane proteins and those involved in protein trafficking. Furthermore, we established a combined strategy based on the sequential enrichment of lysosomes with SPIONs and TMEM-IP. This enabled increased purity of lysosome-enriched fractions and, through TMEM-IP-based lysosome enrichment from SPIONs flow-through and eluate fractions, additional insights into the properties of individual approaches. All data are available via ProteomeXchange with PXD048696.

Lnc-HIBADH-4 Regulates Autophagy-Lysosome Pathway in Amyotrophic Lateral Sclerosis by Targeting Cathepsin D

Amyotrophic lateral sclerosis (ALS) is the most prevalent and lethal class of severe motor neuron diseases (MND) with no efficacious treatment. The pathogenic mechanisms underlying ALS remain unclear. Nearly 90% of patients exhibit sporadic onset (sALS). Therefore, elucidating the pathophysiology of ALS is imperative. Long non-coding RNA (lncRNA) is a large class of non-coding RNAs that regulate transcription, translation, and post-translational processes. LncRNAs contribute to the pathogenesis of diverse neurodegenerative disorders and hold promise as targets for interference in the realm of neurodegeneration. However, the mechanisms of which lncRNAs are involved in ALS have not been thoroughly investigated. We identified and validated a downregulated lncRNA, lnc-HIBADH-4, in ALS which correlated with disease severity and overall survival. Lnc-HIBADH-4 acted as a "molecular sponge" regulating lysosomal function through the lnc-HIBADH-4/miR-326/CTSD pathway, thereby impacting autophagy-lysosome dynamics and the levels of cell proliferation and apoptosis. Therefore, this study discovered and revealed the role of lnc-HIBADH-4 in the pathogenesis of ALS. With further research, lnc-HIBADH-4 is expected to provide a new biomarker in the diagnosis and treatment of ALS.

Translocational attenuation mediated by the PERK-SRP14 axis is a protective mechanism of unfolded protein response

The unfolded protein response (UPR) relieves endoplasmic reticulum (ER) stress through multiple strategies, including reducing protein synthesis, increasing protein folding capabilities, and enhancing misfolded protein degradation. After a multi-omics analysis, we find that signal recognition particle 14 (SRP14), an essential component of the SRP, is markedly reduced in cells undergoing ER stress. Further experiments indicate that SRP14 reduction requires PRKR-like ER kinase (PERK)-mediated eukaryotic translation initiation factor 2α (eIF2α) phosphorylation but is independent of ATF4 or ATF3 transcription factors. The decrease of SRP14 correlates with reduced translocation of fusion proteins and endogenous cathepsin D. Enforced expression of an SRP14 variant with elongation arrest capability prevents the reduced translocation of cathepsin D in stressed cells, whereas an SRP14 mutant without the activity does not. Finally, overexpression of SRP14 augments the UPR and aggravates ER-stress-induced cell death. These data suggest that translocational attenuation mediated by the PERK-SRP14 axis is a protective measure for the UPR to mitigate ER stress.

Impaired cathepsin D in retinal pigment epithelium cells mediates Stargardt disease pathogenesis

Recessive Stargardt disease (STGD1) is an inherited juvenile maculopathy caused by mutations in the ABCA4 gene, for which there is no suitable treatment. Loss of functional ABCA4 in the retinal pigment epithelium (RPE) alone, without contribution from photoreceptor cells, was shown to induce STGD1 pathology. Here, we identified cathepsin D (CatD), the primary RPE lysosomal protease, as a key molecular player contributing to endo-lysosomal dysfunction in STGD1 using a newly developed "disease-in-a-dish" RPE model from confirmed STGD1 patients. Induced pluripotent stem cell (iPSC)-derived RPE originating from three STGD1 patients exhibited elevated lysosomal pH, as previously reported in Abca4-/- mice. CatD protein maturation and activity were impaired in RPE from STGD1 patients and Abca4-/- mice. Consequently, STGD1 RPE cells have reduced photoreceptor outer segment degradation and abnormal accumulation of α-synuclein, the natural substrate of CatD. Furthermore, dysfunctional ABCA4 in STGD1 RPE cells results in intracellular accumulation of autofluorescent material and phosphatidylethanolamine (PE). The altered distribution of PE associated with the internal membranes of STGD1 RPE cells presumably compromises LC3-associated phagocytosis, contributing to delayed endo-lysosomal degradation activity. Drug-mediated re-acidification of lysosomes in the RPE of STGD1 restores CatD functional activity and reduces the accumulation of immature CatD protein loads. This preclinical study validates the contribution of CatD deficiencies to STGD1 pathology and provides evidence for an efficacious therapeutic approach targeting RPE cells. Our findings support a cell-autonomous RPE-driven pathology, informing future research aimed at targeting RPE cells to treat ABCA4-mediated retinopathies.

Decoding the biology and clinical implication of neutrophils in intracranial aneurysm

OBJECTIVE

Abundant neutrophils have been identified in both ruptured and unruptured intracranial aneurysm (IA) domes, with their function and clinical implication being poorly characterized.

MATERIALS AND METHODS

We employed single-cell RNA sequencing (scRNA-Seq) datasets of both human and murine model, and external bulk mRNA sequencing datasets to thoroughly explore the features and functional heterogeneous of neutrophils infiltrating the IA dome.

RESULTS

We found that both unruptured and ruptured IA dome contain a substantial population of neutrophils, characterized by FCGR3B, G0S2, CSF3R, and CXCR2. These cells exhibited heterogeneity in terms of function and differentiation. Despite similar transcriptional activation, neutrophils in IA dome expressed a repertoire of gene programs that mimicked transcriptomic alterations observed from bone marrow to peripheral blood, showing self-similarity. In addition, the recruitment of neutrophils in unruptured IA was primarily mediated by monocytes/macrophages, and once ruptured, both neutrophils, and a specific subset of inflammatory smooth muscle cells (SMCs) were involved in the process. The receiver operator characteristic curve (ROC) analysis indicated that distinct neutrophil subclusters were associated with IA formation and rupture, respectively. By reviewing current studies, we found that neutrophils play a detrimental role to IA wall integrity through secreting specific ligands, ferroptosis driven by ALOX5AP and PTGS2, and the formation of neutrophil extracellular traps (NETs) mediated by PADI4.

INTERPRETATION

This study delineated the biology and potential clinical implications of neutrophils in IA dome and provided a reliable basis for future researches.

Monocytes Release Pro-Cathepsin D to Drive Blood-to-Brain Transcytosis in Diabetes

BACKGROUND

Microvascular complications are the major outcome of type 2 diabetes progression, and the underlying mechanism remains to be determined.

METHODS

High-throughput RNA sequencing was performed using human monocyte samples from controls and diabetes. The transgenic mice expressing human CTSD (cathepsin D) in the monocytes was constructed using CD68 promoter. In vivo 2-photon imaging, behavioral tests, immunofluorescence, transmission electron microscopy, Western blot analysis, vascular leakage assay, and single-cell RNA sequencing were performed to clarify the phenotype and elucidate the molecular mechanism.

RESULTS

Monocytes expressed high-level CTSD in patients with type 2 diabetes. The transgenic mice expressing human CTSD in the monocytes showed increased brain microvascular permeability resembling the diabetic microvascular phenotype, accompanied by cognitive deficit. Mechanistically, the monocytes release nonenzymatic pro-CTSD to upregulate caveolin expression in brain endothelium triggering caveolae-mediated transcytosis, without affecting the paracellular route of brain microvasculature. The circulating pro-CTSD activated the caveolae-mediated transcytosis in brain endothelial cells via its binding with low-density LRP1 (lipoprotein receptor-related protein 1). Importantly, genetic ablation of CTSD in the monocytes exhibited a protective effect against the diabetes-enhanced brain microvascular transcytosis and the diabetes-induced cognitive impairment.

CONCLUSIONS

These findings uncover the novel role of circulatory pro-CTSD from monocytes in the pathogenesis of cerebral microvascular lesions in diabetes. The circulatory pro-CTSD is a potential target for the intervention of microvascular complications in diabetes.

Super-Resolution Microscopy Unveils Synergistic Structural Changes of Organelles Upon Point Mutation

Ethyl methanesulphonate (EMS), is a widely used chemical mutagen that causes high-frequency germline null mutation by inserting an alkyl group into the nucleotide guanine in eukaryotic cells. The effect of EMS on the dynamics of the aneuploid genome, increased cellular instability, and carcinogenicity in relation to benign and malignant tumors are reported, but the molecular level understanding of morphological changes of higher-order chromatin structure has poorly been understood. This is due to a lack of sufficient resolution in conventional microscopic techniques to see small structures below the diffraction limit. Here, using super-resolution radial fluctuation, a largely fragmented, decompaction, and less dense heterochromatin structure upon EMS treatment to HEK 293A cells without any change in nuclear DNA domains is observed. This result suggests an early stage of carcinogenicity happened due to the point mutation. In addition, the distinct structural changes with an elongated morphology of lysosomes are also observed. On the other hand, fragmented and increased heterogeneous populations with an increased cytoplasmic occupancy of mitochondria are observed.

Differential patterns of lysosomal dysfunction are seen in the clinicopathological forms of primary progressive aphasia

Increasing evidence implicates endo-lysosomal dysfunction in frontotemporal dementia (FTD). 18 proteins were quantified using a mass spectrometry assay panel in the cerebrospinal fluid of 36 people with the language variant of FTD, primary progressive aphasia (PPA) (including 13 with non-fluent variant (nfvPPA), 11 with semantic variant (svPPA), and 12 with logopenic variant (lvPPA)) and 19 healthy controls. The concentrations of the cathepsins (B, D, F, L1, and Z) as well as AP-2 complex subunit beta, ganglioside GM2 activator, beta-hexosaminidase subunit beta, tissue alpha L-fucosidase, and ubiquitin were decreased in nfvPPA compared with controls. In contrast, the concentrations of amyloid beta A4 protein, cathepsin Z, and dipeptidyl peptidase 2 were decreased in svPPA compared with controls. No proteins were abnormal in lvPPA. These results indicate a differential alteration of lysosomal proteins in the PPA variants, suggesting those with non-Alzheimer's pathologies are more likely to show abnormal lysosomal function.

Overexpression of ameloblastin in secretory ameloblasts results in demarcated, hypomineralized opacities in enamel

Introduction: Developmental defects of the enamel manifest before tooth eruption and include amelogenesis imperfecta, a rare disease of underlying gene mutations, and molar-incisor hypomineralization (MIH), a prevalent disease in children originating from environmental and epigenetic factors. MIH enamel presents as the abnormal enamel marked by loss of translucency, demarcation between the healthy and affected enamel, and reduced mineral content. The pathophysiology of opaque, demarcated enamel lesions is not understood; however, the retention of enamel proteins in the matrix has been suggested. Ameloblastin (Ambn) is an enamel protein of the secreted calcium-binding phosphoproteins (SCPPs) critical for enamel formation. When the Ambn gene is mutated or deleted, teeth are affected by hypoplastic amelogenesis imperfecta. Methods: In this study, enamel formation in mice was analyzed when transgenic Ambn was overexpressed from the amelogenin promoter encoding full-length Ambn. Ambn was under- and overexpressed at six increasing concentrations in separate mouse lines. Results: Mice overexpressing Ambn displayed opaque enamel at low concentrations and demarcated lesions at high concentrations. The severity of enamel lesions increased starting from the inner enamel close to the dentino-enamel junction (DEJ) to span the entire width of the enamel layer in demarcated areas. Associated with the opaque enamel were 17-kDa Ambn cleavage products, a prolonged secretory stage, and a thin basement membrane in the maturation stage. Ambn accumulations found in the innermost enamel close to the DEJ and the mineralization front correlated with reduced mineral content. Demarcated enamel lesions were associated with Ambn species of 17 kDa and higher, prolonged secretory and transition stages, a thin basement membrane, and shortened maturation stages. Hypomineralized opacities were delineated against the surrounding mineralized enamel and adjacent to ameloblasts detached from the enamel surface. Inefficient Ambn cleavage, loss of contact between ameloblasts, and the altered basement membrane curtailed the endocytic activity; thus, enamel proteins remained unresorbed in the matrix. Ameloblasts have the ability to distinguish between Ambn concentration and Ambn cleavage products through finely tuned feedback mechanisms. The under- or overexpression of Ambn in murine secretory ameloblasts results in either hypoplastic amelogenesis imperfecta or hypomineralization with opaque or sharply demarcated boundaries of lesions, similar to MIH.

Endosomal dysfunction contributes to cerebellar deficits in spinocerebellar ataxia type 6

Spinocerebellar ataxia type 6 (SCA6) is a rare disease that is characterized by cerebellar dysfunction. Patients have progressive motor coordination impairment, and postmortem brain tissue reveals degeneration of cerebellar Purkinje cells and a reduced level of cerebellar brain-derived neurotrophic factor (BDNF). However, the pathophysiological changes underlying SCA6 are not fully understood. We carried out RNA-sequencing of cerebellar vermis tissue in a mouse model of SCA6, which revealed widespread dysregulation of genes associated with the endo-lysosomal system. Since disruption to endosomes or lysosomes could contribute to cellular deficits, we examined the endo-lysosomal system in SCA6. We identified alterations in multiple endosomal compartments in the Purkinje cells of SCA6 mice. Early endosomes were enlarged, while the size of the late endosome compartment was reduced. We also found evidence for impaired trafficking of cargo to the lysosomes. As the proper functioning of the endo-lysosomal system is crucial for the sorting and trafficking of signaling molecules, we wondered whether these changes could contribute to previously identified deficits in signaling by BDNF and its receptor tropomyosin kinase B (TrkB) in SCA6. Indeed, we found that the enlarged early endosomes in SCA6 mice accumulated both BDNF and TrkB. Furthermore, TrkB recycling to the cell membrane in recycling endosomes was reduced, and the late endosome transport of BDNF for degradation was impaired. Therefore, mis-trafficking due to aberrant endo-lysosomal transport and function could contribute to SCA6 pathophysiology through alterations to BDNF-TrkB signaling, as well as mishandling of other signaling molecules. Deficits in early endosomes and BDNF localization were rescued by chronic administration of a TrkB agonist, 7,8-dihydroxyflavone, that we have previously shown restores motor coordination and cerebellar TrkB expression. The endo-lysosomal system is thus both a novel locus of pathophysiology in SCA6 and a promising therapeutic target.

Cholesterol Oxidation Products: Potential Adverse Effect and Prevention of Their Production in Foods

Cholesterol oxidation products (COPs) are a group of substances formed during food processing. COPs in diet is a health concern because they may affect human health in association with the risk of various diseases including atherosclerosis, Alzheimer's disease, age-related macular degeneration, diabetes, and chronic gastrointestinal inflammatory colitis. Production of COPs in foods can be affected by many factors such as temperature, pH, light, oxygen, water, carbohydrates, fatty acids, proteins, and metal cations. The key issue is preventing its generation in foods. Some COPs can also be produced in vivo by both nonenzymatic and enzymatic-catalyzed oxidation reactions. Currently, a number of natural antioxidants such as catechins, flavonoids, and other polyphenols have been proven to inhibit the generation of COPs. In addition, measures taken during food processing can also minimize the production of COPs, such as the Maillard reaction and marinating food with plant polyphenol-rich seasonings. In conclusion, a comprehensive approach encompassing the suppression on COPs generation and implementation of processing measures is imperative to safeguard human health against the production of COPs in the food chain.

Adaptive responses of neuronal cells to chronic endoplasmic reticulum (ER) stress

Accumulation of misfolded proteins or perturbation of calcium homeostasis leads to endoplasmic reticulum (ER) stress and is linked to the pathogenesis of neurodegenerative diseases. Hence, understanding the ability of neuronal cells to cope with chronic ER stress is of fundamental interest. Interestingly, several brain areas uphold functions that enable them to resist challenges associated with neurodegeneration. Here, we established novel clonal mouse hippocampal (HT22) cell lines that are resistant to prolonged (chronic) ER stress induced by thapsigargin (TgR) or tunicamycin (TmR) as in vitro models to study the adaption to ER stress. Morphologically, we observed a significant increase in vesicular und autophagosomal structures in both resistant lines and 'giant lysosomes', especially striking in TgR cells. While autophagic activity increased under ER stress, lysosomal function appeared slightly impaired; in both cell lines, we observed enhanced ER-phagy. However, proteomic analyses revealed that various protein clusters and signaling pathways were differentially regulated in TgR versus TmR cells in response to chronic ER stress. Additionally, bioenergetic analyses in both resistant cell lines showed a shift toward aerobic glycolysis ('Warburg effect') and a defective complex I of the oxidative phosphorylation (OXPHOS) machinery. Furthermore, ER stress-resistant cells differentially activated the unfolded protein response (UPR) comprising IRE1α and ATF6 pathways. These findings display the wide portfolio of adaptive responses of neuronal cells to chronic ER stress. ER stress-resistant neuronal cells could be the basis to uncover molecular modulators of adaptation, resistance, and neuroprotection as potential pharmacological targets for preventing neurodegeneration.

Evolutionary Adaptation of Protein Turnover in White Muscle of Stenothermal Antarctic Fish: Elevated Cold Compensation at Reduced Thermal Responsiveness

Protein turnover is highly energy consuming and overall relates to an organism's growth performance varying largely between species, e.g., due to pre-adaptation to environmental characteristics such as temperature. Here, we determined protein synthesis rates and capacity of protein degradation in white muscle of the cold stenothermal Antarctic eelpout (Pachycara brachycephalum) and its closely related temperate counterpart, the eurythermal common eelpout (Zoarces viviparus). Both species were exposed to acute warming (P. brachycephalum, 0 °C + 2 °C day-1; Z. viviparus, 4 °C + 3 °C day-1). The in vivo protein synthesis rate (Ks) was monitored after injection of 13C-phenylalanine, and protein degradation capacity was quantified by measuring the activity of cathepsin D in vitro. Untargeted metabolic profiling by nuclear magnetic resonance (NMR) spectroscopy was used to identify the metabolic processes involved. Independent of temperature, the protein synthesis rate was higher in P. brachycephalum (Ks = 0.38-0.614 % day-1) than in Z. viviparus (Ks= 0.148-0.379% day-1). Whereas protein synthesis remained unaffected by temperature in the Antarctic species, protein synthesis in Z. viviparus increased to near the thermal optimum (16 °C) and tended to fall at higher temperatures. Most strikingly, capacities for protein degradation were about ten times higher in the Antarctic compared to the temperate species. These differences are mirrored in the metabolic profiles, with significantly higher levels of complex and essential amino acids in the free cytosolic pool of the Antarctic congener. Together, the results clearly indicate a highly cold-compensated protein turnover in the Antarctic eelpout compared to its temperate confamilial. Constant versus variable environments are mirrored in rigid versus plastic functional responses of the protein synthesis machinery.

Tutorial review for peptide assays: An ounce of pre-analytics is worth a pound of cure

The recent increase in peptidomimetic-based medications and the growing interest in peptide hormones has brought new attention to the quantification of peptides for diagnostic purposes. Indeed, the circulating concentrations of peptide hormones in the blood provide a snapshot of the state of the body and could eventually lead to detecting a particular health condition. Although extremely useful, the quantification of such molecules, preferably by liquid chromatography coupled to mass spectrometry, might be quite tricky. First, peptides are subjected to hydrolysis, oxidation, and other post-translational modifications, and, most importantly, they are substrates of specific and nonspecific proteases in biological matrixes. All these events might continue after sampling, changing the peptide hormone concentrations. Second, because they include positively and negatively charged groups and hydrophilic and hydrophobic residues, they interact with their environment; these interactions might lead to a local change in the measured concentrations. A phenomenon such as nonspecific adsorption to lab glassware or materials has often a tremendous effect on the concentration and needs to be controlled with particular care. Finally, the circulating levels of peptides might be low (pico- or femtomolar range), increasing the impact of the aforementioned effects and inducing the need for highly sensitive instruments and well-optimized methods. Thus, despite the extreme diversity of these peptides and their matrixes, there is a common challenge for all the assays: the need to keep concentrations unchanged from sampling to analysis. While significant efforts are often placed on optimizing the analysis, few studies consider in depth the impact of pre-analytical steps on the results. By working through practical examples, this solution-oriented tutorial review addresses typical pre-analytical challenges encountered during the development of a peptide assay from the standpoint of a clinical laboratory. We provide tips and tricks to avoid pitfalls as well as strategies to guide all new developments. Our ultimate goal is to increase pre-analytical awareness to ensure that newly developed peptide assays produce robust and accurate results.

The UPR Maintains Proteostasis and the Viability and Function of Hippocampal Neurons in Adult Mice

The unfolded protein response (UPR), which comprises three branches: PERK, ATF6α, and IRE1, is a major mechanism for maintaining cellular proteostasis. Many studies show that the UPR is a major player in regulating neuron viability and function in various neurodegenerative diseases; however, its role in neurodegeneration is highly controversial. Moreover, while evidence suggests activation of the UPR in neurons under normal conditions, deficiency of individual branches of the UPR has no major effect on brain neurons in animals. It remains unclear whether or how the UPR participates in regulating neuronal proteostasis under normal and disease conditions. To determine the physiological role of the UPR in neurons, we generated mice with double deletion of PERK and ATF6α in neurons. We found that inactivation of PERK and ATF6α in neurons caused lysosomal dysfunction (as evidenced by decreased expression of the V0a1 subunit of v-ATPase and decreased activation of cathepsin D), impairment of autophagic flux (as evidenced by increased ratio of LC3-II/LC3-I and increased p62 level), and accumulation of p-tau and Aβ42 in the hippocampus, and led to impairment of spatial memory, impairment of hippocampal LTP, and hippocampal degeneration in adult mice. These results suggest that the UPR is required for maintaining neuronal proteostasis (particularly tau and Aβ homeostasis) and the viability and function of neurons in the hippocampus of adult mice.

Oxidative stress induces lysosomal membrane permeabilization and ceramide accumulation in retinal pigment epithelial cells

Oxidative stress has been implicated in the pathogenesis of age-related macular degeneration, the leading cause of blindness in older adults, with retinal pigment epithelium (RPE) cells playing a key role. To better understand the cytotoxic mechanisms underlying oxidative stress, we used cell culture and mouse models of iron overload, as iron can catalyze reactive oxygen species formation in the RPE. Iron-loading of cultured induced pluripotent stem cell-derived RPE cells increased lysosomal abundance, impaired proteolysis and reduced the activity of a subset of lysosomal enzymes, including lysosomal acid lipase (LIPA) and acid sphingomyelinase (SMPD1). In a liver-specific Hepc (Hamp) knockout murine model of systemic iron overload, RPE cells accumulated lipid peroxidation adducts and lysosomes, developed progressive hypertrophy and underwent cell death. Proteomic and lipidomic analyses revealed accumulation of lysosomal proteins, ceramide biosynthetic enzymes and ceramides. The proteolytic enzyme cathepsin D (CTSD) had impaired maturation. A large proportion of lysosomes were galectin-3 (Lgals3) positive, suggesting cytotoxic lysosomal membrane permeabilization. Collectively, these results demonstrate that iron overload induces lysosomal accumulation and impairs lysosomal function, likely due to iron-induced lipid peroxides that can inhibit lysosomal enzymes.

A differential proteomics study of cerebrospinal fluid from individuals with Niemann-Pick disease, Type C1

Niemann-Pick, type C1 (NPC1) is a fatal, neurodegenerative disease, which belongs to the family of lysosomal diseases. In NPC1, endo/lysosomal accumulation of unesterified cholesterol and sphingolipids arise from improper intracellular trafficking resulting in multi-organ dysfunction. With the proximity between the brain and cerebrospinal fluid (CSF), performing differential proteomics provides a means to shed light to changes occurring in the brain. In this study, CSF samples obtained from NPC1 individuals and unaffected controls were used for protein biomarker identification. A subset of these individuals with NPC1 are being treated with miglustat, a glycosphingolipid synthesis inhibitor. Of the 300 identified proteins, 71 proteins were altered in individuals with NPC1 compared to controls including cathepsin D, and members of the complement family. Included are a report of 10 potential markers for monitoring therapeutic treatment. We observed that pro-neuropeptide Y (NPY) was significantly increased in NPC1 individuals relative to healthy controls; however, individuals treated with miglustat displayed levels comparable to healthy controls. In further investigation, NPY levels in a NPC1 mouse model corroborated our findings. We posit that NPY could be a potential therapeutic target for NPC1 due to its multiple roles in the central nervous system such as attenuating neuroinflammation and reducing excitotoxicity.

Maintenance of cathepsin D-dependent autophagy-lysosomal function protects against cardiac ischemia/reperfusion injury

Cardiac ischemia/reperfusion(I/R) induced-cardiac vascular endothelial injury is an important pathological process that appears in the early stage of cardiac I/R injury. The autophagy-lysosomal pathway is essential for the maintenance of cellular homeostasis. However, in cardiac I/R injury, the role of the autophagy-lysosomal pathway is controversial. The present study aimed to use oxygen-glucose deprivation/oxygen-glucose resupply(OGD/OGR) in human coronary artery endothelial cells(HCAECs) with I/R injury to assess the role of the autophagy-lysosomal pathway in I/R-induced endothelial injury. The results revealed lysosomal dysfunction and impaired autophagic flux in endothelial cells exposed to OGD/OGR. Meanwhile, our data showed that the levels of cathepsin D(CTSD) decreased time-dependently. Knockdown of CTSD caused lysosomal dysfunction and impaired autophagic flux. Conversely, restoration of CTSD levels protected HCAECs against OGD/OGR induced-defects in autophagy-lysosomal function and cellular damage. Our findings indicated that I/R induced-impaired autophagic flux, rather than excessive autophagic initiation, mediates endothelial cells injury. The maintenance of autophagy-lysosomal function is critical to protect endothelial cells against I/R injury, and CTSD is a key regulator. Thus, strategies focused on restoring CTSD function are potentially novel treatments for cardiac reperfusion injury.

Identification of a mitochondrial targeting sequence in cathepsin D and its localization in mitochondria

Cathepsin D (CTSD) is a major lysosomal protease harboring an N-terminal signal peptide (amino acids 1-20) to enable vesicular transport from endoplasmic reticulum to lysosomes. Here, we report the possibility of a mitochondrial targeting sequence and mitochondrial localization of CTSD in cells. Live-cell imaging analysis with C-terminal enhanced green fluorescent protein-tagged CTSD (EGFP-CTSD) indicated that CTSD localizes to mitochondria. CTSD amino acids 21-35 are responsible for its mitochondrial localization, which exhibit typical features of mitochondrial targeting sequences, and are evolutionarily conserved. A proteinase K protection assay and sucrose gradient analysis showed that a small population of endogenous CTSD molecules exists in mitochondria. These results suggest that CTSD is a dual-targeted protein that may localize in both lysosomes and mitochondria.

Azithromycin, a potent autophagy inhibitor for cancer therapy, perturbs cytoskeletal protein dynamics

BACKGROUND

Autophagy plays an important role in tumour cell growth and survival and also promotes resistance to chemotherapy. Hence, autophagy has been targeted for cancer therapy. We previously reported that macrolide antibiotics including azithromycin (AZM) inhibit autophagy in various types of cancer cells in vitro. However, the underlying molecular mechanism for autophagy inhibition remains unclear. Here, we aimed to identify the molecular target of AZM for inhibiting autophagy.

METHODS

We identified the AZM-binding proteins using AZM-conjugated magnetic nanobeads for high-throughput affinity purification. Autophagy inhibitory mechanism of AZM was analysed by confocal microscopic and transmission electron microscopic observation. The anti-tumour effect with autophagy inhibition by oral AZM administration was assessed in the xenografted mice model.

RESULTS

We elucidated that keratin-18 (KRT18) and α/β-tubulin specifically bind to AZM. Treatment of the cells with AZM disrupts intracellular KRT18 dynamics, and KRT18 knockdown resulted in autophagy inhibition. Additionally, AZM treatment suppresses intracellular lysosomal trafficking along the microtubules for blocking autophagic flux. Oral AZM administration suppressed tumour growth while inhibiting autophagy in tumour tissue.

CONCLUSIONS

As drug-repurposing, our results indicate that AZM is a potent autophagy inhibitor for cancer treatment, which acts by directly interacting with cytoskeletal proteins and perturbing their dynamics.

Gouty arthritis involves impairment of autophagic degradation via cathepsin D inactivation-mediated lysosomal dysfunction that promotes apoptosis in macrophages

This study examined autophagy-lysosome pathway (ALP) perturbations in synovial monocytes/macrophages from patients with gouty arthritis (GA) and the associations of ALP perturbations with cell death. Synovial fluid mononuclear cells (SFMCs) and synovial tissues (STs) from patients with GA, as well as monosodium urate (MSU) crystal-exposed macrophages, underwent immunoblotting, quantitative polymerase chain reaction, and immunofluorescence analyses of markers linked to the ALP (microtubule-associated protein 1 light chain 3B [LC3B], p62, cathepsin D [CTSD], and lysosome-associated membrane protein 2 [LAMP2]) and cell death (caspase-3). GA STs underwent immunohistochemistry and immunofluorescence analyses to determine the distributions of LC3B-positive autophagosomes and macrophages. GA SFMCs and STs exhibited impaired autophagic degradation, indicated by elevated levels of LC3B and p62, along with CTSD upregulation and caspase-3 activation. Macrophages from GA STs exhibited significant accumulation of LC3B-positive autophagosomes. The temporal effects of MSU crystals on the ALP and the associations of these effects with cell death were investigated using a macrophage model of GA. MSU crystal-exposed macrophages exhibited early (2 h) autophagosome formation but later (6-24 h) autophagic flux impairment, demonstrated by p62 accumulation, lysosomal inhibitor failure to increase LC3B accumulation, and LC3B colocalization with p62. These macrophages exhibited autophagic flux impairment because of CTSD inactivation-mediated lysosomal dysfunction, which caused immature CTSD to accumulate within damaged LAMP2-positive lysosomes. This accumulation coincided with caspase-3-dependent cell death (24 h) that was unaffected by CTSD inhibition. These findings indicate that GA involves MSU crystal-induced impairment of autophagic degradation via CTSD inactivation-mediated lysosomal dysfunction, which promotes apoptosis in macrophages.

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.

Spatial transcriptomes and microbiota reveal immune mechanism that respond to pathogen infection in the posterior intestine of Sebastes schlegelii

The intestine is a site of immune cell priming at birth. Therefore, spatial transcriptomes were performed to define how the transcriptomic landscape was spatially organized in the posterior intestine of Sebastes schlegelii following Edwardsiella piscicida infection. In the healthy condition, we identified a previously unappreciated molecular regionalization of the posterior intestine. Following bacterial infection, most immune-related genes were identified in mucosa layer. Moreover, investigation of immune-related genes and genes in immune-related KEGG pathways based on spatial transcriptomes shed light on which sections of these genes are in the posterior intestine. Meanwhile, the high expression of genes related to regeneration also indicated that the posterior intestine was responding to the invasion of pathogens by constantly proliferating new cells. In addition, the increasing microbiota communities indicated that these bacteria maintained posterior intestine integrity and shaped the mucosal immune system. Taken together, spatial transcriptomes and microbiota compositions have significant implications for understanding the immune mechanism that responds to E. piscicida infection in the posterior intestine of S. schlegelii, which also provides a theoretical basis for the spatial distribution of immune genes and changes in bacterial flora in other teleosts in the process of resisting pathogens.

Cocaine perturbs mitovesicle biology in the brain

Cocaine, an addictive psychostimulant, has a broad mechanism of action, including the induction of a wide range of alterations in brain metabolism and mitochondrial homeostasis. Our group recently identified a subpopulation of non-microvesicular, non-exosomal extracellular vesicles of mitochondrial origin (mitovesicles) and developed a method to isolate mitovesicles from brain parenchyma. We hypothesised that the generation and secretion of mitovesicles is affected by mitochondrial abnormalities induced by chronic cocaine exposure. Mitovesicles from the brain extracellular space of cocaine-administered mice were enlarged and more numerous when compared to controls, supporting a model in which mitovesicle biogenesis is enhanced in the presence of mitochondrial alterations. This interrelationship was confirmed in vitro. Moreover, cocaine affected mitovesicle protein composition, causing a functional alteration in mitovesicle ATP production capacity. These data suggest that mitovesicles are previously unidentified players in the biology of cocaine addiction and that target therapies to fine-tune brain mitovesicle functionality may be beneficial to mitigate the effects of chronic cocaine exposure.

Macrophage Resistance to Ionizing Radiation Exposure Is Accompanied by Decreased Cathepsin D and Increased Transferrin Receptor 1 Expression

PURPOSE

To identify a molecular signature of macrophages exposed to clinically relevant ionizing radiation (IR) doses, mirroring radiotherapy sessions.

METHODS

Human monocyte-derived macrophages were exposed to 2 Gy/ fraction/ day for 5 days, mimicking one week of cancer patient's radiotherapy. Protein expression profile by proteomics was performed.

RESULTS

A gene ontology analysis revealed that radiation-induced protein changes are associated with metabolic alterations, which were further supported by a reduction of both cellular ATP levels and glucose uptake. Most of the radiation-induced deregulated targets exhibited a decreased expression, as was the case of cathepsin D, a lysosomal protease associated with cell death, which was validated by Western blot. We also found that irradiated macrophages exhibited an increased expression of the transferrin receptor 1 (TfR1), which is responsible for the uptake of transferrin-bound iron. TfR1 upregulation was also found in tumor-associated mouse macrophages upon tumor irradiation. In vitro irradiated macrophages also presented a trend for increased divalent metal transporter 1 (DMT1), which transports iron from the endosome to the cytosol, and a significant increase in iron release.

CONCLUSIONS

Irradiated macrophages present lower ATP levels and glucose uptake, and exhibit decreased cathepsin D expression, while increasing TfR1 expression and altering iron metabolism.

Insights into the Genetic Profile of Two Siblings Affected by Unverricht-Lundborg Disease Using Patient-Derived hiPSCs

Unverricht-Lundborg disease (ULD), also known as progressive myoclonic epilepsy 1 (EPM1), is a rare autosomal recessive neurodegenerative disorder characterized by a complex symptomatology that includes action- and stimulus-sensitive myoclonus and tonic-clonic seizures. The main cause of the onset and development of ULD is a repeat expansion of a dodecamer sequence localized in the promoter region of the gene encoding cystatin B (CSTB), an inhibitor of lysosomal proteases. Although this is the predominant mutation found in most patients, the physio-pathological mechanisms underlying the disease complexity remain largely unknown. In this work, we used patient-specific iPSCs and their neuronal derivatives to gain insight into the molecular and genetic machinery responsible for the disease in two Italian siblings affected by different phenotypes of ULD. Specifically, fragment length analysis on amplified CSTB promoters found homozygous status for dodecamer expansion in both patients and showed that the number of dodecamer repeats is the same in both. Furthermore, the luciferase reporter assay showed that the CSTB promoter activity was similarly reduced in both lines compared to the control. This information allowed us to draw important conclusions: (1) the phenotypic differences of the patients do not seem to be strictly dependent on the genetic mutation around the CSTB gene, and (2) that some other molecular mechanisms, not yet clearly identified, might be taken into account. In line with the inhibitory role of cystatin B on cathepsins, molecular investigations performed on iPSCs-derived neurons showed an increased expression of lysosomal cathepsins (B, D, and L) and a reduced expression of CSTB protein. Intriguingly, the increase in cathepsin expression does not appear to be correlated with the residual amount of CSTB, suggesting that other mechanisms, in addition to the regulation of cathepsins, could be involved in the pathological complexity of the disease.

Mass Spectrometric-Based Proteomics for Biomarker Discovery in Osteosarcoma: Current Status and Future Direction

Due to a lack of novel therapies and biomarkers, the clinical outcomes of osteosarcoma patients have not significantly improved for decades. The advancement of mass spectrometry (MS), peptide quantification, and downstream pathway analysis enables the investigation of protein profiles across a wide range of input materials, from cell culture to long-term archived clinical specimens. This can provide insight into osteosarcoma biology and identify candidate biomarkers for diagnosis, prognosis, and stratification of chemotherapy response. In this review, we provide an overview of proteomics studies of osteosarcoma, indicate potential biomarkers that might be promising therapeutic targets, and discuss the challenges and opportunities of mass spectrometric-based proteomics in future osteosarcoma research.

FRET-based cathepsin probes for simultaneous detection of cathepsin B and D activities

Fluorescent cathepsin probes were prepared by modification of peptidic substrates for cathepsin B (CTSB) and cathepsin D (CTSD) with FRET pairs. Fluorophores with distinguishable emission characteristics were applied to CTSB and CTSD probes with their appropriate quenchers to simultaneously monitor the activity of CTSB and/or CTSD. Conjugation of both the CTSB and CTSD probes with short single-stranded DNA drastically increased their reactivity to cathepsins over the parent probes possibly by improving their solubility. The activity of CTSB and CTSD were simultaneously detected by using these orthogonal FRET-based cathepsin probes.

Endocytosed HIV-1 Envelope Glycoprotein Traffics to Rab14+ Late Endosomes and Lysosomes to Regulate Surface Levels in T-Cell Lines

Production of infectious HIV-1 particles requires incorporation of the viral envelope glycoprotein (Env) at the plasma membrane (PM) of infected CD4+ T cells. Env trafficking to the PM exposes viral epitopes that can be exploited by the host immune system; however, HIV-1 can evade this response by endocytosis of excess Env from the PM. The fate of Env after internalization remains unclear, with evidence suggesting several different vesicular trafficking steps may be involved, including recycling pathways. To date, there have been very few studies documenting the trafficking pathways of native Env in infected T cells. Furthermore, it remains unclear whether there are T-cell-specific endosomal pathways regulating the fate of endocytic Env. Here, we use a pulse-labeling approach with a monovalent anti-Env Fab probe to characterize the trafficking of internalized Env within infected CD4+ T-cell lines, together with CRISPR/Cas9-mediated endogenous protein tagging, to assess the role of host cell Rab GTPases in Env trafficking. We show that endocytosed Env traffics to Rab14+ compartments that possess hallmarks of late endosomes and lysosomes. We also demonstrate that Env can recycle back to the PM, although we find that recycling does not occur at high rates when compared to the model recycling protein transferrin. These results help to resolve open questions about the fate and relevance of endocytosed Env in HIV-infected cells and suggest a novel role for Rab14 in a cell-type-specific late-endosomal/lysosomal trafficking pathway in T cells. IMPORTANCE HIV-1 envelope glycoprotein (Env) evades immune neutralization through many mechanisms. One immune evasion strategy may result from the internalization of excess surface-exposed Env to prevent antibody-dependent cellular cytotoxicity or neutralization. Characterization of the fate of endocytosed Env is critical to understand which vesicular pathways could be targeted to promote display of Env epitopes to the immune system. In this study, we characterize the endocytic fate of native Env, expressed from infected human T-cell lines. We demonstrate that Env is rapidly trafficked to a late-endosome/lysosome-like compartment and can be recycled to the cell surface for incorporation into virus assembly sites. This study implicates a novel intracellular compartment, marked by host-cell Rab14 GTPases, for the sequestration of Env. Therapeutic approaches aimed at mobilizing this intracellular pool of Env could lead to stronger immune control of HIV-1 infection via antibody-dependent cell-mediated cytotoxicity.

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.

LAMP3 inhibits autophagy and contributes to cell death by lysosomal membrane permeabilization

ABBREVIATIONS

A253-control: A253 control for LAMP3 stable overexpression; A253- LAMP3: A253 LAPM3 stable overexpression; CASP1: caspase 1; CASP3: caspase 3; CHX: cycloheximide; CTSB: cathepsin B; CTSD: cathepsin D; CQ: chloroquine; DCs: dendritic cells; ER: endoplasmic reticulum; LGALS3: galectin 3; HCV: hepatitis C virus; HSG-control: HSG control for LAMP3 stable overexpression; HSG-LAMP3: HSG LAMP3 stable overexpression; HSP: heat shock protein; HTLV-1: human T-lymphocyte leukemia virus-1; IXA: ixazomib; LAMP: lysosomal associated membrane protein; MHC: major histocompatibility complex; mAb: monoclonal antibody; OE: overexpression; pepA: pepstatin A; pAb: polyclonal antibody; pSS: primary Sjögren syndrome; qRT-PCR: quantitative real- time reverse transcriptase polymerase chain reaction; SLE: systemic lupus erythematosus; SS: Sjögren syndrome; UPR: unfolded protein response; V-ATPase: vacuolar-type proton- translocating ATPase; Y-VAD: Ac-YVAD-cmk; Z-DEVD; Z-DEVD-fmk; Z-VAD: Z-VAD- fmk.

Cathepsin D interacts with adenosine A2A receptors in mouse macrophages to modulate cell surface localization and inflammatory signaling

Adenosine A_2A receptor (A_2AR)–dependent signaling in macrophages plays a key role in the regulation of inflammation. However, the processes regulating A_2AR targeting to the cell surface and degradation in macrophages are incompletely understood. For example, the C-terminal domain of the A_2AR and proteins interacting with it are known to regulate receptor recycling, although it is unclear what role potential A_2AR-interacting partners have in macrophages. Here, we aimed to identify A_2AR-interacting partners in macrophages that may effect receptor trafficking and activity. To this end, we performed a yeast two-hybrid screen using the C-terminal tail of A_2AR as the “bait” and a macrophage expression library as the “prey.” We found that the lysosomal protease cathepsin D (CtsD) was a robust hit. The A_2AR–CtsD interaction was validated in vitro and in cellular models, including RAW 264.7 and mouse peritoneal macrophage (IPMΦ) cells. We also demonstrated that the A_2AR is a substrate of CtsD and that the blockade of CtsD activity increases the density and cell surface targeting of A_2AR in macrophages. Conversely, we demonstrate that A_2AR activation prompts the maturation and enzymatic activity of CtsD in macrophages. In summary, we conclude that CtsD is a novel A_2AR-interacting partner and thus describe molecular and functional interplay that may be crucial for adenosine-mediated macrophage regulation in inflammatory processes.

Recombinant pro-CTSD (cathepsin D) enhances SNCA/α-Synuclein degradation in α-Synucleinopathy models

Parkinson disease (PD) is a neurodegenerative disorder characterized by the abnormal intracellular accumulation of SNCA/α-synuclein. While the exact mechanisms underlying SNCA pathology are not fully understood, increasing evidence suggests the involvement of autophagy as well as lysosomal deficiencies. Because CTSD (cathepsin D) has been proposed to be the major lysosomal protease involved in SNCA degradation, its deficiency has been linked to the presence of insoluble SNCA conformers in the brain of mice and humans as well as to the transcellular transmission of SNCA aggregates. We here postulate that SNCA degradation can be enhanced by the application of the recombinant human proform of CTSD (rHsCTSD). Our results reveal that rHsCTSD is efficiently endocytosed by neuronal cells, correctly targeted to lysosomes and matured to an enzymatically active protease. In dopaminergic neurons derived from induced pluripotent stem cells (iPSC) of PD patients harboring the A53T mutation within the SNCA gene, we confirm the reduction of insoluble SNCA after treatment with rHsCTSD. Moreover, we demonstrate a decrease of pathological SNCA conformers in the brain and within primary neurons of a ctsd-deficient mouse model after dosing with rHsCTSD. Boosting lysosomal CTSD activity not only enhanced SNCA clearance in human and murine neurons as well as tissue, but also restored endo-lysosome and autophagy function. Our findings indicate that CTSD is critical for SNCA clearance and function. Thus, enzyme replacement strategies utilizing CTSD may also be of therapeutic interest for the treatment of PD and other synucleinopathies aiming to decrease the SNCA burden.Abbreviations: aa: amino acid; SNCA/α-synuclein: synuclein alpha; APP: amyloid beta precursor protein; BBB: blood brain barrier; BF: basal forebrain; CBB: Coomassie Brilliant Blue; CLN: neuronal ceroid lipofuscinosis; CNL10: neuronal ceroid lipofuscinosis type 10; Corr.: corrected; CTSD: cathepsin D; CTSB: cathepsin B; DA: dopaminergic; DA-iPSn: induced pluripotent stem cell-derived dopaminergic neurons; dox: doxycycline; ERT: enzyme replacement therapy; Fx: fornix, GBA/β-glucocerebrosidase: glucosylceramidase beta; h: hour; HC: hippocampus; HT: hypothalamus; i.c.: intracranially; IF: immunofluorescence; iPSC: induced pluripotent stem cell; KO: knockout; LAMP1: lysosomal associated membrane protein 1; LSDs: lysosomal storage disorders; MAPT: microtubule associated protein tau; M6P: mannose-6-phosphate; M6PR: mannose-6-phosphate receptor; MB: midbrain; mCTSD: mature form of CTSD; neurofil.: neurofilament; PD: Parkinson disease; proCTSD: proform of CTSD; PRNP: prion protein; RFU: relative fluorescence units; rHsCTSD: recombinant human proCTSD; SAPC: Saposin C; SIM: structured illumination microscopy; T-insol: Triton-insoluble; T-sol: Triton-soluble; TEM: transmission electron microscopy, TH: tyrosine hydroxylase; Thal: thalamus.

Intravitreal gene therapy restores the autophagy-lysosomal pathway and attenuates retinal degeneration in cathepsin D-deficient mice

Loss of vision due to progressive retinal degeneration is a hallmark of neuronal ceroid lipofuscinoses (NCL), a group of fatal neurodegenerative lysosomal storage diseases. Enzyme substitution therapies represent promising treatment options for NCLs caused by dysfunctions of soluble lysosomal enzymes. Here, we compared the efficacy of a cell-based enzyme substitution strategy and a gene therapy approach to attenuate the retinal pathology in cathepsin D- (CTSD) deficient mice, an animal model of CLN10 disease. Levels of enzymatically active CTSD in mutant retinas were significantly higher after an adeno-associated virus vector-mediated CTSD transfer to retinal glial cells and retinal pigment epithelial cells than after intravitreal transplantations of a CTSD overexpressing clonal neural stem cell line. In line with this finding, the gene therapy treatment restored the disrupted autophagy-lysosomal pathway more effectively than the cell-based approach, as indicated by a complete clearance of storage, significant attenuation of lysosomal hypertrophy, and normalized levels of the autophagy marker sequestosome 1/p62 and microtubule-associated protein 1 light chain 3-II. While the cell-based treatment did not prevent the rapidly progressing loss of various retinal cell types, the gene therapy approach markedly attenuated retinal degeneration as demonstrated by a pronounced rescue of photoreceptor cells and rod bipolar cells.

Tributyltin inhibits autophagy by decreasing lysosomal acidity in SH-SY5Y cells

Tributyltin (TBT) is an environmental pollutant that remains in marine sediments and is toxic to mammals. For example, TBT elicits neurotoxic and immunosuppressive effects on rats. However, it is not entirely understood how TBT causes toxicity. Autophagy plays a pivotal role in protein quality control and eliminates aggregated proteins and damaged organelles. We previously reported that TBT dephosphorylates mammalian target of rapamycin (mTOR), which may be involved in enhancement of autophagosome synthesis, in primary cultures of cortical neurons. Autophagosomes can accumulate due to enhancement of autophagosome synthesis or inhibition of autophagic degradation, and we did not clarify whether TBT alters autophagic flux. Here, we investigated the mechanism by which TBT causes accumulation of autophagosomes in SH-SY5Y cells. TBT inhibited autophagy without affecting autophagosome-lysosome fusion before it caused cell death. TBT dramatically decreased the acidity of lysosomes without affecting lysosomal membrane integrity. TBT decreased the mature protein level of cathepsin B, and this may be related to the decrease in lysosomal acidity. These results suggest that TBT inhibits autophagic degradation by decreasing lysosomal acidity. Autophagy impairment may be involved in the mechanism underlying neuronal death and/or T-cell-dependent thymus atrophy induced by TBT.

OUP accepted manuscript

Recessive mutations in the CTNS gene encoding the lysosomal transporter cystinosin cause cystinosis, a lysosomal storage disease leading to kidney failure and multisystem manifestations. A Ctns knockout mouse model recapitulates features of cystinosis, but the delayed onset of kidney manifestations, phenotype variability and strain effects limit its use for mechanistic and drug development studies. To provide a better model for cystinosis, we generated a Ctns knockout rat model using CRISPR/Cas9 technology. The Ctns^−/− rats display progressive cystine accumulation and crystal formation in multiple tissues including kidney, liver and thyroid. They show an early onset and progressive loss of urinary solutes, indicating generalized proximal tubule dysfunction, with development of typical swan-neck lesions, tubulointerstitial fibrosis and kidney failure, and decreased survival. The Ctns^−/− rats also present crystals in the cornea, and bone and liver defects, as observed in patients. Mechanistically, the loss of cystinosin induces a phenotype switch associating abnormal proliferation and dedifferentiation, loss of apical receptors and transporters, and defective lysosomal activity and autophagy in the cells. Primary cultures of proximal tubule cells derived from the Ctns^−/− rat kidneys confirmed the key changes caused by cystine overload, including reduced endocytic uptake, increased proliferation and defective lysosomal dynamics and autophagy. The novel Ctns^−/− rat model and derived proximal tubule cell system provide invaluable tools to investigate the pathogenesis of cystinosis and to accelerate drug discovery.

Dysregulated endolysosomal trafficking in cells arrested in the G1 phase of the host cell cycle impairs Salmonella vacuolar replication

Modulation of the host cell cycle has emerged as a common theme among the pathways regulated by bacterial pathogens, arguably to promote host cell colonization. However, in most cases the exact benefit ensuing from such interference to the infection process remains unclear. Previously, we have shown that Salmonella actively induces G₂/M arrest of host cells, and that infection is severely inhibited in cells arrested in G₁. In this study, we demonstrate that Salmonella vacuolar replication is inhibited in host cells blocked in G₁, whereas the cytosolic replication of the closely related pathogen Shigella is not affected. Mechanistically, we show that cells arrested in G₁, but not cells arrested in G₂, present dysregulated endolysosomal trafficking, displaying an abnormal accumulation of vesicles positive for late endosomal and lysosomal markers. In addition, the macroautophagic/autophagic flux and degradative lysosomal function are strongly impaired. This endolysosomal trafficking dysregulation results in sustained activation of the SPI-1 type III secretion system and lack of vacuole repair by the autophagy pathway, ultimately compromising the maturation and integrity of the Salmonella-containing vacuole. As such, Salmonella is released in the host cytosol. Collectively, our findings demonstrate that the modulation of the host cell cycle occurring during Salmonella infection is related to a disparity in the permissivity of cells arrested in G₁ and G₂/M, due to their intrinsic characteristics.

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.

Targeting Lysosomes to Reverse Hydroquinone-Induced Autophagy Defects and Oxidative Damage in Human Retinal Pigment Epithelial Cells

In age-related macular degeneration (AMD), hydroquinone (HQ)-induced oxidative damage in retinal pigment epithelium (RPE) is believed to be an early event contributing to dysregulation of inflammatory cytokines and vascular endothelial growth factor (VEGF) homeostasis. However, the roles of antioxidant mechanisms, such as autophagy and the ubiquitin-proteasome system, in modulating HQ-induced oxidative damage in RPE is not well-understood. This study utilized an in-vitro AMD model involving the incubation of human RPE cells (ARPE-19) with HQ. In comparison to hydrogen peroxide (H₂O₂), HQ induced fewer reactive oxygen species (ROS) but more oxidative damage as characterized by protein carbonyl levels, mitochondrial dysfunction, and the loss of cell viability. HQ blocked the autophagy flux and increased proteasome activity, whereas H₂O₂ did the opposite. Moreover, the lysosomal membrane-stabilizing protein LAMP2 and cathepsin D levels declined with HQ exposure, suggesting loss of lysosomal membrane integrity and function. Accordingly, HQ induced lysosomal alkalization, thereby compromising the acidic pH needed for optimal lysosomal degradation. Pretreatment with MG132, a proteasome inhibitor and lysosomal stabilizer, upregulated LAMP2 and autophagy and prevented HQ-induced oxidative damage in wildtype RPE cells but not cells transfected with shRNA against ATG5. This study demonstrated that lysosomal dysfunction underlies autophagy defects and oxidative damage induced by HQ in human RPE cells and supports lysosomal stabilization with the proteasome inhibitor MG132 as a potential remedy for oxidative damage in RPE and AMD.

Release of Iron-Loaded Ferritin in Sodium Iodate-Induced Model of Age Related Macular Degeneration: An In-Vitro and In-Vivo Study

To evaluate the role of iron in sodium iodate (NaIO3)-induced model of age-related macular degeneration (AMD) in ARPE-19 cells in-vitro and in mouse models in-vivo. ARPE-19 cells, a human retinal pigment epithelial cell line, was exposed to 10 mM NaIO3 for 24 h, and the expression and localization of major iron modulating proteins was evaluated by Western blotting (WB) and immunostaining. Synthesis and maturation of cathepsin-D (cat-D), a lysosomal enzyme, was evaluated by quantitative reverse-transcriptase polymerase chain reaction (RT-qPCR) and WB, respectively. For in-vivo studies, C57BL/6 mice were injected with 40 mg/kg mouse body weight of NaIO3 intraperitoneally, and their retina was evaluated after 3 weeks as above. NaIO3 induced a 10-fold increase in ferritin in ARPE-19 cells, which co-localized with LC3II, an autophagosomal marker, and LAMP-1, a lysosomal marker. A similar increase in ferritin was noted in retinal lysates and retinal sections of NaIO3-injected mice by WB and immunostaining. Impaired synthesis and maturation of cat-D was also noted. Accumulated ferritin was loaded with iron, and released from retinal pigmented epithelial (RPE) cells in Perls' and LAMP-1 positive vesicles. NaIO3 impairs lysosomal degradation of ferritin by decreasing the transcription and maturation of cat-D in RPE cells. Iron-loaded ferritin accumulates in lysosomes and is released in lysosomal membrane-enclosed vesicles to the extracellular milieu. Accumulation of ferritin in RPE cells and fusion of ferritin-containing vesicles with adjacent photoreceptor cells is likely to create an iron overload, compromising their viability. Moreover, reduced activity of cat-D is likely to promote accumulation of other cellular debris in lysosomal vesicles, contributing to AMD-like pathology.

The c.863A>G (p.Glu288Gly) variant of the CTSD gene is not associated with CLN10 disease

Background

Cathepsin D is a lysosomal aspartic protease encoded by the CTSD gene. It plays important roles in many biological processes. Biallelic loss‐of‐function mutation of CTSD is considered a cause of CLN10 disease. CLN10 is a rare autosomal recessive disorder that is one of 14 types of neuronal ceroid lipofuscinoses (NCLs). To date, only a few cases of CLN10 and 12 disease‐causing mutations have been reported worldwide.

Methods

Exome sequencing was performed on a 15‐year‐old girl with pervasive brain developmental disorder. The effects of the identified variants were investigated through multiple functional experiments.

Results

There were no differences in mRNA and protein expression, intracellular localization, maturation, and proteolytic activity between the cells with the mutant CTSD gene and those with the wild‐type CTSD gene.

Conclusion

These results suggest that the c.863A>G (p.Glu288Gly) homozygous variant is not a pathogenic variation, but a benign variant.

Cathepsin D-Managing the Delicate Balance

Lysosomal proteases play a crucial role in maintaining cell homeostasis. Human cathepsin D manages protein turnover degrading misfolded and aggregated proteins and favors apoptosis in the case of proteostasis disruption. However, when cathepsin D regulation is affected, it can contribute to numerous disorders. The down-regulation of human cathepsin D is associated with neurodegenerative disorders, such as neuronal ceroid lipofuscinosis. On the other hand, its excessive levels outside lysosomes and the cell membrane lead to tumor growth, migration, invasion and angiogenesis. Therefore, targeting cathepsin D could provide significant diagnostic benefits and new avenues of therapy. Herein, we provide a brief overview of cathepsin D structure, regulation, function, and its role in the progression of many diseases and the therapeutic potentialities of natural and synthetic inhibitors and activators of this protease.

Elevated mRNA expression and defective processing of cathepsin D in HeLa cells lacking the mannose 6-phosphate pathway

Disruption of the mannose 6‐phosphate (M‐6‐P) pathway in HeLa cells by inactivation of the GNPTAB gene, which encodes the α/β subunits of GlcNAc‐1‐phosphotransferase, results in missorting of newly synthesized lysosomal acid hydrolases to the cell culture media instead of transport to the endolysosomal system. We previously demonstrated that the majority of the lysosomal aspartyl protease, cathepsin D, is secreted in these GNPTAB^−/− HeLa cells. However, the intracellular content of cathepsin D in these cells was still greater than that of WT HeLa cells which retained most of the protease, indicating a marked elevation of cathepsin D expression in response to abrogation of the M‐6‐P pathway. Here, we demonstrate that HeLa cells lacking GlcNAc‐1‐phosphotransferase show a fivefold increase in cathepsin D mRNA expression over control cells, accounting for the increase in cathepsin D at the protein level. Further, we show that this increase at the mRNA level occurs independent of the transcription factors TFEB and TFE3. The intracellular cathepsin D can still be trafficked to lysosomes in the absence of the M‐6‐P pathway, but fails to undergo proteolytic processing into the fully mature heavy and light chains. Uptake experiments performed by feeding GNPTAB^−/− HeLa cells with various phosphorylated cathepsins reveal that only cathepsin B is capable of partially restoring cleavage, providing evidence for a role for cathepsin B in the proteolytic processing of cathepsin D.

Autophagy triggers CTSD (cathepsin D) maturation and localization inside cells to promote apoptosis

CTSD/CathD/CATD (cathepsin D) is a lysosomal aspartic protease. A distinguishing characteristic of CTSD is its dual functions of promoting cell proliferation via secreting a pro-enzyme outside the cells as a ligand, and promoting apoptosis via the mature form of this enzyme inside cells; however, the regulation of its secretion, expression, and maturation is undetermined. Using the lepidopteran insect Helicoverpa armigera, a serious agricultural pest, as a model, we revealed the dual functions and regulatory mechanisms of CTSD secretion, expression, and maturation. Glycosylation of asparagine 233 (N233) determined pro-CTSD secretion. The steroid hormone 20-hydroxyecdysone (20E) promoted CTSD expression. Macroautophagy/autophagy triggered CTSD maturation and localization inside midgut cells to activate CASP3 (caspase 3) and promote apoptosis. Pro-CTSD was expressed in the pupal epidermis and was secreted into the hemolymph to promote adult fat body endoreplication/endoreduplication, cell proliferation, and association. Our study revealed that the differential expression and autophagy-mediated maturation of CTSD in tissues determine its roles in apoptosis and cell proliferation, thereby determining the cell fates of tissues during lepidopteran metamorphosis.Abbreviations: 20E: 20-hydroxyecdysone; 3-MA: 3-methyladenine; ACTB/β-actin: actin beta; AKT: protein kinase B; ATG1: autophagy-related 1; ATG4: autophagy-related 4; ATG5: autophagy-related 5; ATG7: autophagy-related 7; ATG14: autophagy-related 14; BSA: bovine serum albumin; CASP3: caspase 3; CQ: choroquine; CTSD: cathepsin D; DAPI: 4',6-diamidino-2-phenylindole; DMSO: dimethyl sulfoxide; DPBS: dulbecco's phosphate-buffered saline; DsRNA: double-stranded RNA; EcR: ecdysone receptor; EcRE: ecdysone response element; EdU: 5-ethynyl-2´-deoxyuridine; G-m-CTSD: glycosylated-mautre-CTSD; G-pro-CTSD: glycosylated-pro-CTSD; HaEpi: Helicoverpa armigera epidermal cell line; HE staining: hematoxylin and eosin staining; IgG: immunoglobin G; IM: imaginal midgut; JH: juvenile hormone; Kr-h1: krueppel homologous protein 1; LM: larval midgut; M6P: mannose-6-phosphate; PBS: phosphate-buffered saline; PCD: programmed cell death; PNGase: peptide-N-glycosidase F; RFP: red fluorescent protein; RNAi: RNA interference; SDS-PAGE: sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SYX17: syntaxin 17; USP1: ultraspiracle isoform 1.

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.

Cathepsin B and D deficiency in the mouse pancreas induces impaired autophagy and chronic pancreatitis

The major lysosomal proteases, Cathepsin B (CTSB), Cathepsin D (CTSD) and Cathepsin L (CTSL), are implicated in autophagic activity. To investigate the role of each cathepsin in the exocrine pancreas, we generated mice in which the pancreas was specifically deficient in Ctsb, Ctsd and Ctsl. Each of these gene knockout (KO) and Ctsb;Ctsl and Ctsd;Ctsl double-knockout (DKO) mice were almost normal. However, we found cytoplasmic degeneration in the pancreatic acinar cells of Ctsb;Ctsd DKO mice, similar to autophagy related 5 (Atg5) KO mice. LC3 and p62 (autophagy markers) showed remarkable accumulation and the numbers of autophagosomes and autolysosomes were increased in the pancreatic acinar cells of Ctsb;Ctsd DKO mice. Moreover, these Ctsb;Ctsd DKO mice also developed chronic pancreatitis (CP). Thus, we conclude that both Ctsb and Ctsd deficiency caused impaired autophagy in the pancreatic acinar cells, and induced CP in mice.

Combined Inhibition of p38MAPK and PIKfyve Synergistically Disrupts Autophagy to Selectively Target Cancer Cells

In nutrient-poor conditions, autophagy buffers metabolic stress and counteracts the effects of chemotherapy and radiation on cancer cells, which depend on autophagy for survival. However, clinical trials targeting autophagy have failed to produce successful anticancer treatments using currently available inhibitors. Recent studies have shown that PIKfyve kinase inhibitors disrupt lysosome function in autophagy and can selectively kill certain cancer cells. Analysis of biochemical changes caused by PIKfyve inhibition revealed that resistant cells contain significantly higher levels of cellular p38MAPK protein and phosphorylation. Expression of the lysosomal protein, lysosomal-associated membrane protein 2, carrying phosphomimetic mutations of the p38MAPK phosphorylation sites prevented all effects caused by PIKfyve inhibition-induced lysosome dysfunction. Thus, the activation of p38MAPK in response to PIKfyve inhibition revealed a novel compensatory role in maintaining lysosome function in autophagy. The functional cooperation between the cellular PIKfyve and p38MAPK pathways in regulating lysosome homeostasis was especially important in cancer cells. Combined inhibition of PIKfyve and p38MAPK activities synergistically blocked autophagy-mediated protein degradation, prevented cathepsin maturation, and markedly reduced the viability of multiple cancer cell types without affecting the viability of normal cells. Furthermore, combined PIKfyve and p38MAPK inhibitors synergistically reduced tumor growth in mice bearing xenografts of human colorectal adenocarcinoma, suggesting a novel way to target cancer cells by prolonged inhibition of autophagy using lower drug concentrations. SIGNIFICANCE: This study demonstrates that PIKfyve and p38MAPK cooperate to regulate lysosome homeostasis and their combined inhibition synergistically blocks autophagy to reduce cancer cell viability in vitro and in vivo.