Processing of human cathepsin D in lysosomes in vitro

Involvement of Endolysosomes and Aurora Kinase A in the Regulation of Amyloid β Protein Levels in Neurons

Aurora kinase A (AURKA) is a serine/threonine-protein kinase that regulates microtubule organization during neuron migration and neurite formation. Decreased activity of AURKA was found in Alzheimer's disease (AD) brain samples, but little is known about the role of AURKA in AD pathogenesis. Here, we demonstrate that AURKA is expressed in primary cultured rat neurons, neurons from adult mouse brains, and neurons in postmortem human AD brains. AURKA phosphorylation, which positively correlates with its activity, is reduced in human AD brains. In SH-SY5Y cells, pharmacological activation of AURKA increased AURKA phosphorylation, acidified endolysosomes, decreased the activity of amyloid beta protein (Aβ) generating enzyme β-site amyloid precursor protein cleaving enzyme (BACE-1), increased the activity of the Aβ degrading enzyme cathepsin D, and decreased the intracellular and secreted levels of Aβ. Conversely, pharmacological inhibition of AURKA decreased AURKA phosphorylation, de-acidified endolysosomes, decreased the activity of cathepsin D, and increased intracellular and secreted levels of Aβ. Thus, reduced AURKA activity in AD may contribute to the development of intraneuronal accumulations of Aβ and extracellular amyloid plaque formation.

Extracellular proteolysis in cancer: Proteases, substrates, and mechanisms in tumor progression and metastasis

A vast ensemble of extracellular proteins influences the development and progression of cancer, shaped and reshaped by a complex network of extracellular proteases. These proteases, belonging to the distinct classes of metalloproteases, serine proteases, cysteine proteases, and aspartic proteases, play a critical role in cancer. They often become dysregulated in cancer, with increases in pathological protease activity frequently driven by the loss of normal latency controls, diminished regulation by endogenous protease inhibitors, and changes in localization. Dysregulated proteases accelerate tumor progression and metastasis by degrading protein barriers within the extracellular matrix (ECM), stimulating tumor growth, reactivating dormant tumor cells, facilitating tumor cell escape from immune surveillance, and shifting stromal cells toward cancer-promoting behaviors through the precise proteolysis of specific substrates to alter their functions. These crucial substrates include ECM proteins and proteoglycans, soluble proteins secreted by tumor and stromal cells, and extracellular domains of cell surface proteins, including membrane receptors and adhesion proteins. The complexity of the extracellular protease web presents a significant challenge to untangle. Nevertheless, technological strides in proteomics, chemical biology, and the development of new probes and reagents are enabling progress and advancing our understanding of the pivotal importance of extracellular proteolysis in cancer.

Exploring lysosomal biology: current approaches and methods

Lysosomes are the degradation centers and signaling hubs in the cell. Lysosomes undergo adaptation to maintain cell homeostasis in response to a wide variety of cues. Dysfunction of lysosomes leads to aging and severe diseases including lysosomal storage diseases (LSDs), neurodegenerative disorders, and cancer. To understand the complexity of lysosome biology, many research approaches and tools have been developed to investigate lysosomal functions and regulatory mechanisms in diverse experimental systems. This review summarizes the current approaches and tools adopted for studying lysosomes, and aims to provide a methodological overview of lysosomal research and related fields.

NUAK1 coordinates growth factor-dependent activation of mTORC2 and Akt signaling

BACKGROUND

mTORC2 is a critical regulator of cytoskeleton organization, cell proliferation, and cancer cell survival. Activated mTORC2 induces maximal activation of Akt by phosphorylation of Ser-473, but regulation of Akt activity and signaling crosstalk upon growth factor stimulation are still unclear.

RESULTS

We identified that NUAK1 regulates growth factor-dependent activation of Akt by two mechanisms. NUAK1 interacts with mTORC2 components and regulates mTORC2-dependent activation of Akt by controlling lysosome positioning and mTOR association with this organelle. A second mechanism involves NUAK1 directly phosphorylating Akt at Ser-473. The effect of NUAK1 correlated with a growth factor-dependent activation of specific Akt substrates. NUAK1 induced the Akt-dependent phosphorylation of FOXO1/3a (Thr-24/Thr-32) but not of TSC2 (Thr-1462). According to a subcellular compartmentalization that could explain NUAK1's differential effect on the Akt substrates, we found that NUAK1 is associated with early endosomes but not with plasma membrane, late endosomes, or lysosomes. NUAK1 was required for the Akt/FOXO1/3a axis, regulating p21CIP1, p27KIP1, and FoxM1 expression and cancer cell survival upon EGFR stimulation. Pharmacological inhibition of NUAK1 potentiated the cell death effect induced by Akt or mTOR pharmacological blockage. Analysis of human tissue data revealed that NUAK1 expression positively correlates with EGFR expression and Akt Ser-473 phosphorylation in several human cancers.

CONCLUSIONS

Our results showed that NUAK1 kinase controls mTOR subcellular localization and induces Akt phosphorylation, demonstrating that NUAK1 regulates the growth factor-dependent activation of Akt signaling. Therefore, targeting NUAK1, or co-targeting it with Akt or mTOR inhibitors, may be effective in cancers with hyperactivated Akt signaling.

Regulation of Peptidase Activity beyond the Active Site in Human Health and Disease

This comprehensive review addresses the intricate and multifaceted regulation of peptidase activity in human health and disease, providing a comprehensive investigation that extends well beyond the boundaries of the active site. Our review focuses on multiple mechanisms and highlights the important role of exosites, allosteric sites, and processes involved in zymogen activation. These mechanisms play a central role in shaping the complex world of peptidase function and are promising potential targets for the development of innovative drugs and therapeutic interventions. The review also briefly discusses the influence of glycosaminoglycans and non-inhibitory binding proteins on enzyme activities. Understanding their role may be a crucial factor in the development of therapeutic strategies. By elucidating the intricate web of regulatory mechanisms that control peptidase activity, this review deepens our understanding in this field and provides a roadmap for various strategies to influence and modulate peptidase activity.

Role of lysosomes in insulin signaling and glucose uptake in cultured rat podocytes

Podocytes are sensitive to insulin, which governs the functional and structural integrity of podocytes that are essential for proper function of the glomerular filtration barrier. Lysosomes are acidic organelles that are implicated in regulation of the insulin signaling pathway. Cathepsin D (CTPD) and lysosome-associated membrane protein 1 (LAMP1) are major lysosomal proteins that reflect the functional state of lysosomes. However, the effect of insulin on lysosome activity and role of lysosomes in the regulation of insulin-dependent glucose uptake in podocytes are unknown. Our studies showed that the short-term incubation of podocytes with insulin decreased LAMP1 and CTPD mRNA levels. Insulin and bafilomycin A1 reduced both the amounts of LAMP1 and CTPD proteins and activity of CTPD, which were associated with a decrease in the fluorescence intensity of lysosomes that were labeled with LysoTracker. Bafilomycin A1 inhibited insulin-dependent endocytosis of the insulin receptor and increased the amounts of the insulin receptor and glucose transporter 4 on the cell surface of podocytes. Bafilomycin A1 also inhibited insulin-dependent glucose uptake despite an increase in the amount of glucose transporter 4 in the plasma membrane of podocytes. These results suggest that lysosomes are signaling hubs that may be involved in the coupling of insulin signaling with the regulation of glucose uptake in podocytes. The dysregulation of this mechanism can lead to the dysfunction of podocytes and development of insulin resistance.

SARS-CoV-2 non-structural protein 6 triggers NLRP3-dependent pyroptosis by targeting ATP6AP1

A recent mutation analysis suggested that Non-Structural Protein 6 (NSP6) of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is a key determinant of the viral pathogenicity. Here, by transcriptome analysis, we demonstrated that the inflammasome-related NOD-like receptor signaling was activated in SARS-CoV-2-infected lung epithelial cells and Coronavirus Disease 2019 (COVID-19) patients' lung tissues. The induction of inflammasomes/pyroptosis in patients with severe COVID-19 was confirmed by serological markers. Overexpression of NSP6 triggered NLRP3/ASC-dependent caspase-1 activation, interleukin-1β/18 maturation, and pyroptosis of lung epithelial cells. Upstream, NSP6 impaired lysosome acidification to inhibit autophagic flux, whose restoration by 1α,25-dihydroxyvitamin D₃, metformin or polydatin abrogated NSP6-induced pyroptosis. NSP6 directly interacted with ATP6AP1, a vacuolar ATPase proton pump component, and inhibited its cleavage-mediated activation. L37F NSP6 variant, which was associated with asymptomatic COVID-19, exhibited reduced binding to ATP6AP1 and weakened ability to impair lysosome acidification to induce pyroptosis. Consistently, infection of cultured lung epithelial cells with live SARS-CoV-2 resulted in autophagic flux stagnation, inflammasome activation, and pyroptosis. Overall, this work supports that NSP6 of SARS-CoV-2 could induce inflammatory cell death in lung epithelial cells, through which pharmacological rectification of autophagic flux might be therapeutically exploited.

Mechanisms regulating the sorting of soluble lysosomal proteins

Lysosomes are key regulators of many fundamental cellular processes such as metabolism, autophagy, immune response, cell signalling and plasma membrane repair. These highly dynamic organelles are composed of various membrane and soluble proteins, which are essential for their proper functioning. The soluble proteins include numerous proteases, glycosidases and other hydrolases, along with activators, required for catabolism. The correct sorting of soluble lysosomal proteins is crucial to ensure the proper functioning of lysosomes and is achieved through the coordinated effort of many sorting receptors, resident ER and Golgi proteins, and several cytosolic components. Mutations in a number of proteins involved in sorting soluble proteins to lysosomes result in human disease. These can range from rare diseases such as lysosome storage disorders, to more prevalent ones, such as Alzheimer’s disease, Parkinson’s disease and others, including rare neurodegenerative diseases that affect children. In this review, we discuss the mechanisms that regulate the sorting of soluble proteins to lysosomes and highlight the effects of mutations in this pathway that cause human disease. More precisely, we will review the route taken by soluble lysosomal proteins from their translation into the ER, their maturation along the Golgi apparatus, and sorting at the trans-Golgi network. We will also highlight the effects of mutations in this pathway that cause human disease.

OAB-14 Effectively Ameliorates the Dysfunction of the Endosomal-Autophagic-Lysosomal Pathway in APP/PS1 Transgenic Mice

In Alzheimer's disease (AD), damaged Aβ clearance contributes to elevated levels of Aβ that cause a series of cytotoxic cascade reactions. Thus, targeting Aβ clearance has now been considered a valid therapeutic approach for AD. Cellular uptake and degradation are important mechanisms for Aβ clearance, which are mainly performed by the endosomal-autophagic-lysosomal (EAL) pathway. Our previous study showed that OAB-14, a novel small molecule designed with bexarotene as the lead compound, treatment for 3 months significantly alleviated cognitive disorders and remarkably reduced the deposition of Aβ without affecting its production in APP/PS1 transgenic mice. Here, we further revealed that enhancement of the EAL activity is one of the mechanisms that increases Aβ clearance after OAB-14 administration for 3 months. OAB-14 facilitates receptor-mediated endocytosis and restores autophagy flux via the AMPK/mTOR pathway. Meanwhile, OAB-14 enhances the lysosomal activity, and reduced Aβ accumulation in lysosomes was observed in OAB-14-treated AD mice. These results suggest that OAB-14 may promote Aβ clearance in lysosomes by alleviating the EAL dysfunction in AD mice.

Inhibition of GSK-3 ameliorates the pathogenesis of Huntington's disease

In Huntington's disease (HD), the mutant huntingtin (mHtt) accumulates as toxic aggregates in the striatum tissue, with deleterious effects on motor-coordination and cognitive functions. Reducing the levels of mHtt is therefore a promising therapeutic strategy. We have previously reported that GSK-3 is a negative regulator of the autophagy/lysosome pathway, which is responsible for intracellular degradation, and is critically important for maintaining neuronal vitality. Thus, we hypothesized that inhibition of GSK-3 may trigger mHtt clearance thereby reducing mHtt cytotoxicity and improving HD symptoms. Here, we demonstrate that depletion or suppression of autophagy results in a massive accumulation of mHtt aggregates. Accordingly, mHtt aggregates were localized in lysosomes, but, mostly mislocalized from lysosomes in the absence of functional autophagy. Overexpression of GSK-3, particularly the α isozyme, increased the number of mHtt aggregates, while silencing GSK-3α/β, or treatment with a selective GSK-3 inhibitor, L807mts, previously described by us, reduced the amounts of mHtt aggregates. This effect was mediated by increased autophagic and lysosomal activity. Treating R6/2 mouse model of HD with L807mts, reduced striatal mHtt aggregates and elevated autophagic and lysosomal markers. The L807mts treatment also reduced hyperglycemia and improved motor-coordination functions in these mice. In addition, L807mts restored the expression levels of Sirt1, a critical neuroprotective factor in the HD striatum, along with its targets BDNF, DRPP-32, and active Akt, all provide neurotrophic/pro-survival support and typically decline in the HD brain. Our results provide strong evidence for a role for GSK-3 in the regulation of mHtt dynamics, and demonstrate the benefits of GSK-3 inhibition in reducing mHtt toxicity, providing neuroprotective support, and improving HD symptoms.

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

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

Retromer regulates the lysosomal clearance of MAPT/tau

The macroautophagy/autophagy-lysosome axis enables the clearance and degradation of cytoplasmic components including protein aggregates, damaged organelles and invading pathogens. Protein aggregation and lysosomal system dysfunction in the brain are common features of several late-onset neurological disorders including Alzheimer disease. Spatial overlap between depletion of the endosomal-sorting complex retromer and MAPT/tau aggregation in the brain have been previously reported. However, whether retromer dysfunction plays a direct role in mediating MAPT aggregation remains unclear. Here, we demonstrate that the autophagy-lysosome axis is the primary mode for the clearance of aggregated species of MAPT using both chemical and genetic approaches in cell models of amyloid MAPT aggregation. We show that depletion of the central retromer component VPS35 causes a block in the resolution of autophagy. We establish that this defect underlies marked accumulation of cytoplasmic MAPT aggregates upon VPS35 depletion, and that VPS35 overexpression has the opposite effect. This work illustrates how retromer complex integrity regulates the autophagy-lysosome axis to suppress MAPT aggregation and spread.

ATM loss disrupts the autophagy-lysosomal pathway

ATM (ataxia telangiectasia mutated) protein is found associated with multiple organelles including synaptic vesicles, endosomes and lysosomes, often in cooperation with ATR (ataxia telangiectasia and Rad3 related). Mutation of the ATM gene results in ataxia-telangiectasia (A-T), an autosomal recessive disorder with defects in multiple organs including the nervous system. Precisely how ATM deficiency leads to the complex phenotypes of A-T, however, remains elusive. Here, we reported that part of the connection may lie in autophagy and lysosomal abnormalities. We found that ATM was degraded through the autophagy pathway, while ATR was processed by the proteasome. Autophagy and lysosomal trafficking were both abnormal in atm^-/- neurons and the deficits impacted cellular functions such as synapse maintenance, neuronal survival and glucose uptake. Upregulated autophagic flux was observed in atm^-/- lysosomes, associated with a more acidic pH. Significantly, we found that the ATP6V1A (ATPase, H+ transporting, lysosomal V1 subunit A) proton pump was an ATM kinase target. In atm^-/- neurons, lysosomes showed enhanced retrograde transport and accumulated in the perinuclear regions. We attributed this change to an unexpected physical interaction between ATM and the retrograde transport motor protein, dynein. As a consequence, SLC2A4/GLUT4 (solute carrier family 4 [facilitated glucose transporter], member 4) translocation to the plasma membrane was inhibited and trafficking to the lysosomes was increased, leading to impaired glucose uptake capacity. Together, these data underscored the involvement of ATM in a variety of neuronal vesicular trafficking processes, offering new and therapeutically useful insights into the pathogenesis of A-T.Abbreviations: 3-MA: 3-methyladenine; A-T: ataxia-telangiectasia; ALG2: asparagine-linked glycosylation 2 (alpha-1,3-mannosyltransferase); AMPK: adenosine 5'-monophosphate (AMP)-activated protein kinase; ATG5: autophagy related 5; ATM: ataxia telangiectasia mutated; ATP6V1A: ATPase, H+ transporting, lysosomal V1 subunit A; ATR: ataxia-telangiectasia and Rad3 related; BFA1: bafilomycin A₁; CC3: cleaved-CASP3; CGN: cerebellar granule neuron; CLQ: chloroquine; CN: neocortical neuron; CTSB: cathepsin B; CTSD: cathepsin D; DYNLL1: the light chain1 of dynein; EIF4EBP1/4E-BP1: eukaryotic translation initiation factor 4E binding protein 1; Etop: etoposide; FBS: fetal bovine serum; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; HBS: HEPES-buffered saline; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; HOMER1: homer protein homolog 1; KU: KU-60019; LAMP1: lysosomal-associated membrane protein 1; LC3B-II: LC3-phosphatidylethanolamine conjugate; Lyso: lysosome; LysopH-GFP: lysopHluorin-GFP; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MAP2: microtubule associated protein 2; MAPK14: mitogen-activated protein kinase 14; MAPK8/JNK1: mitogen-activated protein kinase 8; MCOLN1/TRPML1: mucolipin 1; OSBPL1A: oxysterol binding protein like 1A; PIKK: phosphatidylinositol 3 kinase related kinase; Rapa: rapamycin; RILP: rab interacting lysosomal protein; ROS: reactive oxygen species; SEM: standard error of mean; SLC2A4/GLUT4: solute carrier family 2 (facilitated glucose transporter), member 4; TSC2/tuberin: TSC complex subunit 2; ULK1: unc-51 like kinase 1; UPS: ubiquitin-proteasome system; VE: VE-822; WCL: whole-cell lysate; WT: wild type.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

Mutation-Dependent Pathomechanisms Determine the Phenotype in the Bestrophinopathies

Best vitelliform macular dystrophy (BD), autosomal dominant vitreoretinochoroidopathy (ADVIRC), and the autosomal recessive bestrophinopathy (ARB), together known as the bestrophinopathies, are caused by mutations in the bestrophin-1 (BEST1) gene affecting anion transport through the plasma membrane of the retinal pigment epithelium (RPE). To date, while no treatment exists a better understanding of BEST1-related pathogenesis may help to define therapeutic targets. Here, we systematically characterize functional consequences of mutant BEST1 in thirteen RPE patient cell lines differentiated from human induced pluripotent stem cells (hiPSCs). Both BD and ARB hiPSC-RPEs display a strong reduction of BEST1-mediated anion transport function compared to control, while ADVIRC mutations trigger an increased anion permeability suggesting a stabilized open state condition of channel gating. Furthermore, BD and ARB hiPSC-RPEs differ by the degree of mutant protein turnover and by the site of subcellular protein quality control with adverse effects on lysosomal pH only in the BD-related cell lines. The latter finding is consistent with an altered processing of catalytic enzymes in the lysosomes. The present study provides a deeper insight into distinct molecular mechanisms of the three bestrophinopathies facilitating functional categorization of the more than 300 known BEST1 mutations that result into the distinct retinal phenotypes.

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.

Enzyme replacement therapy with recombinant pro-CTSD (cathepsin D) corrects defective proteolysis and autophagy in neuronal ceroid lipofuscinosis

CTSD (cathepsin D) is one of the major lysosomal proteases indispensable for the maintenance of cellular proteostasis by turning over substrates of endocytosis, phagocytosis and autophagy. Consequently, CTSD deficiency leads to a strong impairment of the lysosomal-autophagy machinery. In mice and humans CTSD dysfunction underlies the congenital variant (CLN10) of neuronal ceroid lipofuscinosis (NCL). NCLs are distinct lysosomal storage disorders (LSDs) sharing various hallmarks, namely accumulation of protein aggregates and ceroid lipofuscin leading to neurodegeneration and blindness. The most established and clinically approved approach to treat LSDs is enzyme replacement therapy (ERT) aiming to replace the defective hydrolase with an exogenously applied recombinant protein. Here we reveal that recombinant human pro-CTSD produced in a mammalian expression system can be efficiently taken up by a variety of cell models, is correctly targeted to lysosomes and processed to the active mature form of the protease. In proof-of-principle experiments we provide evidence that recombinant human CTSD (rhCTSD) can improve the biochemical phenotype of CTSD-deficient hippocampal slice cultures in vitro and retinal cells in vivo. Furthermore, we demonstrate that dosing of rhCTSD in the murine CLN10 model leads to a correction of lysosomal hypertrophy, storage accumulation and impaired autophagic flux in the viscera and central nervous system (CNS). We establish that direct delivery of the recombinant protease to the CNS is required for improvement of neuropathology and lifespan extension. Together these data support the continuation of the pre-clinical studies for the application of rhCTSD in the treatment of NCL.Abbreviations: AIF1/IBA1: allograft inflammatory factor 1; BBB: blood brain barrier; CNS: central nervous system; CTSB: cathepsin B; CTSD: cathepsin D; CTSL: cathepsin L; ERT: enzyme replacement therapy; GFAP: glial fibrillary acidic protein; INL: inner nuclear layer; LAMP1: lysosomal-associated membrane protein 1; LAMP2: lysosomal-associated membrane protein 2; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; LDL: low-density lipoprotein; LRP1: low density lipoprotein receptor-related protein 1; LSD: lysosomal storage disorder; MEFs: mouse embryonic fibroblasts; M6P: mannose 6-phosphate; mCTSD: mature CTSD; NCL: neuronal ceroid lipofuscinosis; ONL: outer nuclear layer; PB: phosphate buffer; proCTSD: pro-cathepsin D; LRPAP1: low density lipoprotein receptor-related protein associated protein 1; rhCTSD: human recombinant CTSD; SAPC: saposin C; SAPD: saposin D; ATP5G1: ATP synthase, H+ transporting, mitochondrial F0 complex, subunit C1 (subunit 9); SQSTM1/p62: sequestosome 1; TPP1: tripeptidyl peptidase I.

STF-62247 accumulates in lysosomes and blocks late stages of autophagy to selectively target von Hippel-Lindau-inactivated cells

Autophagy is a highly conserved, homeostatic process by which cytosolic components reach lysosomes for degradation. The roles played by different autophagic processes in cancer are complex and remain cancer type and stage dependent. Renal cell carcinoma (RCC) is the most common subtype of kidney cancer and is characterized by the inactivation of the von Hippel-Lindau (VHL) tumor suppressor. Our previous study identified a small compound, STF-62247, as an autophagy-modulating molecule causing selective cytotoxicity for VHL-inactivated cells. This present study investigates the effects of STF-62247 specifically on the macroautophagic flux to better characterize its mechanism of action in RCC. Our results clearly demonstrate that this compound is a potent blocker of late stages of autophagy. We show that inhibiting autophagy by CRISPR knockouts of autophagy-related genes rendered VHL-deficient cells insensitive to STF-62247, uncovering the importance of the autophagic pathway in STF-selective cell death. By exploiting the autofluorescence of STF-62247, we pinpointed its cellular localization to lysosomes. Finally, in response to prolonged STF treatments, we show that VHL-proficient cells are able to surmount the block in late stages of autophagy by restoring their lysosome numbers. Conversely, an increase in autophagic vesicles accompanied by a time-dependent decrease in lysosomes was observed in VHL-deficient cells. This is the first mechanistic study investigating STF-62447’s effects on the autophagic flux in RCC. Importantly, our study reclassifies STF-62247 as a blocker of later stages of autophagy and highlights the possibility of blocking this process through lysosome disruption in VHL-mutated RCCs.

The phytochemical polydatin ameliorates non-alcoholic steatohepatitis by restoring lysosomal function and autophagic flux

Impaired autophagic degradation of intracellular lipids is causally linked to the development of non‐alcoholic steatohepatitis (NASH). Pharmacological agents that can restore hepatic autophagic flux could therefore have therapeutic potentials for this increasingly prevalent disease. Herein, we investigated the effects of polydatin, a natural precursor of resveratrol, in a murine nutritional model of NASH and a cell line model of steatosis. Results showed that oral administration of polydatin protected against hepatic lipid accumulation and alleviated inflammation and hepatocyte damage in db/db mice fed methionine‐choline deficient diet. Polydatin also alleviated palmitic acid‐induced lipid accumulation in cultured hepatocytes. In both models, polydatin restored lysosomal function and autophagic flux that were impaired by NASH or steatosis. Mechanistically, polydatin inhibited mTOR signalling and up‐regulated the expression and activity of TFEB, a known master regulator of lysosomal function. In conclusion, polydatin ameliorated NASH through restoring autophagic flux. The polydatin‐regulated autophagy was associated with inhibition of mTOR pathway and restoration of lysosomal function by TFEB. Our study provided affirmative preclinical evidence to inform future clinical trials for examining the potential anti‐NASH effect of polydatin in humans.

BEST1 protein stability and degradation pathways differ between autosomal dominant Best disease and autosomal recessive bestrophinopathy accounting for the distinct retinal phenotypes

Mutations in bestrophin-1 (BEST1) are associated with distinct retinopathies, notably three forms with autosomal dominant inheritance and one condition with an autosomal recessive mode of transmission. The molecular mechanisms underlying their distinct retinal phenotypes are mostly unknown. Although heterozygous missense mutations in BEST1 reveal dominant-negative effects in patients with autosomal dominant Best disease (BD), heterozygous mutations associated with autosomal recessive bestrophinopathy (ARB) display no disease phenotype. Here we show that the recessive mutations trigger a strong and fast protein degradation process in the endoplasmic reticulum (ER), thereby favoring a decreased stoichiometry of mutant versus normal BEST1 subunits in the assembly of the homo-pentameric BEST1 chloride channel. In contrast, dominant mutations escape ER-associated degradation and are subjected to a slightly delayed post-ER degradation via the endo-lysosomal degradation pathway. As a result, increased formation of a non-functional BEST1 channel occurs due to a roughly equimolar incorporation of normal and mutant BEST1 subunits into the channel complex. Taken together, our data provide insight into the molecular pathways of dominantly and recessively acting BEST1 missense mutations suggesting that the site of subcellular protein quality control as well as the rate and degree of mutant protein degradation are ultimately responsible for the distinct retinal disease phenotypes in BD and ARB.

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.

Cation-dependent mannose-6-phosphate receptor expression and distribution are influenced by estradiol in MCF-7 breast cancer cells

Cancer cells secrete procathepsin D, and its secretion is enhanced by estradiol. Although alterations in the pro-enzyme intracellular transport have been reported, the mechanism by which it is secreted remains poorly understood. In this work, we have studied the influence of estradiol on the expression and distribution of the cation-dependent mannose-6-phosphate receptor (CD-MPR), which would be a key molecule to ensure the proper localization of the enzyme to lysosomes in breast cancer cells. Immunoblotting studies demonstrated that the expression of CD-MPR is higher in MCF-7 cells, as compared to other breast cancer and non-tumorigenic cells. This expression correlated with high levels of cathepsin D (CatD) in these cells. By immunofluorescence, this receptor mostly co-localized with a Golgi marker in all cell types, exhibiting an additional peripheral labelling in MCF-7 cells. In addition, CD-MPR showed great differences regarding to cation-independent mannose-6-phosphate receptor. On the other hand, the treatment with estradiol induced an increase in CD-MPR and CatD expression and a re-distribution of both proteins towards the cell periphery. These effects were blocked by the anti-estrogen tamoxifen. Moreover, a re-distribution of CD-MPR to plasma membrane-enriched fractions, analyzed by gradient centrifugation, was observed after estradiol treatment. We conclude that, in hormone-responsive breast cancer cells, CD-MPR and CatD are distributed together, and that their expression and distribution are influenced by estradiol. These findings strongly support the involvement of the CD-MPR in the pro-enzyme transport in MCF-7 cells, suggesting the participation of this receptor in the procathepsin D secretion previously reported in breast cancer cells.

Reduced lysosomal clearance of autophagosomes promotes survival and colonization of Helicobacter pylori

Evasion of autophagy is key for intracellular survival of bacteria in host cells, but its involvement in persistent infection by Helicobacter pylori, a bacterium identified to invade gastric epithelial cells, remains obscure. The aim of this study was to functionally characterize the role of autophagy in H. pylori infection. Autophagy was assayed in H. pylori-infected human gastric epithelium and the functional role of autophagy was determined via genetic or pharmacological ablation of autophagy in mouse and cell line models of H. pylori infection. Here, we showed that H. pylori inhibited lysosomal function and thereby promoted the accumulation of autophagosomes in gastric epithelial cells. Importantly, inhibiting autophagosome formation by pharmacological inhibitors or genetic ablation of BECN1 or ATG5 reduced H. pylori intracellular survival, whereas inhibition of lysosomal functions exerted an opposite effect. Further experiments demonstrated that H. pylori inhibited lysosomal acidification and the retrograde trafficking of mannose-6-phosphate receptors, both of which are known to positively regulate lysosomal function. We conclude that H. pylori subverts autophagy into a pro-survival mechanism through inhibition of lysosomal clearance of autophagosomes. Disruption of autophagosome formation offers a novel strategy to reduce H. pylori colonization in human stomachs. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Membrane detachment is not essential for COG complex function

COG is a multisubunit vesicle tethering complex in the Golgi. We demonstrate that both COG subcomplexes can be permanently attached to Golgi membranes and that major COG functions do not require cycling between the membrane and cytosol.

The conserved oligomeric Golgi (COG) complex is a vesicle tether of the “complexes associated with tethering containing helical rods” family, which functions on the cytoplasmic side of Golgi. It is currently unknown whether COG function, or function of any multisubunit vesicular tether, depends on cycling between the membrane and cytosol. Therefore, we permanently anchored key subunits of COG subcomplexes (COG4, COG7, and COG8) to Golgi membranes using transmembrane protein TMEM115 (TMEM-COG). All TMEM-COG subunits tested were Golgi localized, integrated into the COG complex, and stabilized membrane association of endogenous subunits. Interestingly, TMEM-COG4 and TMEM-COG7 equally rescued COG function in organization of Golgi markers, glycosylation, and abundance of COG-sensitive proteins. In contrast, TMEM-COG8 was not as effective, indicating that N-terminal attachment of COG8 interfered with overall COG structure and function, and none of the TMEM-COG subunits rescued the abnormal Golgi architecture caused by COG knockout. Collectively, these data indicate that both subcomplexes of the COG complex can perform most of COG function when permanently attached to membranes and that the cytosolic pool of COG is not completely essential to COG function.

Lysosomal response in relation to α-synuclein pathology differs between Parkinson's disease and multiple system atrophy

Intracellular deposition of pathologically altered α-synuclein mostly in neurons characterises Parkinson's disease (PD), while its accumulation predominantly in oligodendrocytes is a feature of multiple system atrophy (MSA). Recently a prion-like spreading of pathologic α-synuclein has been suggested to play a role in the pathogenesis of PD and MSA. This implicates a role of protein processing systems, including lysosomes, supported also by genetic studies in PD. However, particularly for MSA, the mechanism of cell-to-cell propagation of α-synuclein is yet not fully understood. To evaluate the significance of lysosomal response, we systematically compared differently affected neuronal populations in PD, MSA, and non-diseased brains using morphometric immunohistochemistry (cathepsin D), double immunolabelling (cathepsin D/α-synuclein) laser confocal microscopy, and immunogold electron microscopy for the disease associated α-synuclein. We found that i) irrespective of the presence of neuronal inclusions, the volume density of cathepsin D immunoreactivity significantly increases in affected neurons of the pontine base in MSA brains; ii) volume density of cathepsin D immunoreactivity increases in nigral neurons in PD without inclusions and with non-ubiquitinated pre-aggregates of α-synuclein, but not in neurons with Lewy bodies; iii) cathepsin D immunoreactivity frequently colocalises with α-synuclein pre-aggregates in nigral neurons in PD; iv) ultrastructural observations confirm disease-associated α-synuclein in neuronal and astrocytic lysosomes in PD; v) lysosome-associated α-synuclein is observed in astroglia and rarely in oligodendroglia and in neurons in MSA. Our observations support a crucial role for the neuronal endosomal-lysosomal system in the processing of α-synuclein in PD. We suggest a distinct contribution of lysosomes to the pathogenesis of MSA, including the possibility of oligodendroglial and eventually neuronal uptake of exogenous α-synuclein in MSA.

FMNL2 and -3 regulate Golgi architecture and anterograde transport downstream of Cdc42

The Rho-family small GTPase Cdc42 localizes at plasma membrane and Golgi complex and aside from protrusion and migration operates in vesicle trafficking, endo- and exocytosis as well as establishment and/or maintenance of cell polarity. The formin family members FMNL2 and -3 are actin assembly factors established to regulate cell edge protrusion during migration and invasion. Here we report these formins to additionally accumulate and function at the Golgi apparatus. As opposed to lamellipodia, Golgi targeting of these proteins required both their N-terminal myristoylation and the interaction with Cdc42. Moreover, Golgi association of FMNL2 or -3 induced a phalloidin-detectable actin meshwork around the Golgi. Importantly, functional interference with FMNL2/3 formins by RNAi or CRISPR/Cas9-mediated gene deletion invariably induced Golgi fragmentation in different cell lines. Furthermore, absence of these proteins led to enlargement of endosomes as well as defective maturation and/or sorting into late endosomes and lysosomes. In line with Cdc42 - recently established to regulate anterograde transport through the Golgi by cargo sorting and carrier formation - FMNL2/3 depletion also affected anterograde trafficking of VSV-G from the Golgi to the plasma membrane. Our data thus link FMNL2/3 formins to actin assembly-dependent functions of Cdc42 in anterograde transport through the Golgi apparatus.

Leishmania donovani resides in modified early endosomes by upregulating Rab5a expression via the downregulation of miR-494

Several intracellular pathogens arrest the phagosome maturation in the host cells to avoid transport to lysosomes. In contrast, the Leishmania containing parasitophorous vacuole (PV) is shown to recruit lysosomal markers and thus Leishmania is postulated to be residing in the phagolysosomes in macrophages. Here, we report that Leishmania donovani specifically upregulates the expression of Rab5a by degrading c-Jun via their metalloprotease gp63 to downregulate the expression of miR-494 in THP-1 differentiated human macrophages. Our results also show that miR-494 negatively regulates the expression of Rab5a in cells. Subsequently, L. donovani recruits and retains Rab5a and EEA1 on PV to reside in early endosomes and inhibits transport to lysosomes in human macrophages. Similarly, we have also observed that Leishmania PV also recruits Rab5a by upregulating its expression in human PBMC differentiated macrophages. However, the parasite modulates the endosome by recruiting Lamp1 and inactive pro-CathepsinD on PV via the overexpression of Rab5a in infected cells. Furthermore, siRNA knockdown of Rab5a or overexpression of miR-494 in human macrophages significantly inhibits the survival of the parasites. These results provide the first mechanistic insights of parasite-mediated remodeling of endo-lysosomal trafficking to reside in a specialized early endocytic compartment.

Progranulin regulates lysosomal function and biogenesis through acidification of lysosomes

Progranulin (PGRN) haploinsufficiency resulting from loss-of-function mutations in the PGRN gene causes frontotemporal lobar degeneration accompanied by TDP-43 accumulation, and patients with homozygous mutations in the PGRN gene present with neuronal ceroid lipofuscinosis. Although it remains unknown why PGRN deficiency causes neurodegenerative diseases, there is increasing evidence that PGRN is implicated in lysosomal functions. Here, we show PGRN is a secretory lysosomal protein that regulates lysosomal function and biogenesis by controlling the acidification of lysosomes. PGRN gene expression and protein levels increased concomitantly with the increase of lysosomal biogenesis induced by lysosome alkalizers or serum starvation. Down-regulation or insufficiency of PGRN led to the increased lysosomal gene expression and protein levels, while PGRN overexpression led to the decreased lysosomal gene expression and protein levels. In particular, the level of mature cathepsin D (CTSDmat) dramatically changed depending upon PGRN levels. The acidification of lysosomes was facilitated in cells transfected with PGRN. Then, this caused degradation of CTSDmat by cathepsin B. Secreted PGRN is incorporated into cells via sortilin or cation-independent mannose 6-phosphate receptor, and facilitated the acidification of lysosomes and degradation of CTSDmat. Moreover, the change of PGRN levels led to a cell-type-specific increase of insoluble TDP-43. In the brain tissue of FTLD-TDP patients with PGRN deficiency, CTSD and phosphorylated TDP-43 accumulated in neurons. Our study provides new insights into the physiological function of PGRN and the role of PGRN insufficiency in the pathogenesis of neurodegenerative diseases.

Dehydropachymic acid decreases bafilomycin A1 induced β-Amyloid accumulation in PC12 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Fuling, the sclerotium of Poria cocos, was frequently used in traditional Chinese medicine (TCM) formulae for Alzheimer's disease (AD) intervention over the past 10 centuries. And its extracts exhibited significant effects in both cellular and animal models of AD in previous studies. However, its mechanisms on prevention and treatment of AD have not been well elucidated yet.

AIM OF THE STUDY

To investigate the effect and corresponding mechanisms of dehydropachymic acid, which is one of the major triterpenes in P. cocos, on the clearance of β-amyloid accumulation in bafilomycin A1 induced PC12 cells.

MATERIALS AND METHODS

MTT assay was used to examine the DPA effect on the viability of PC12 cells stable transfected with pCB6-APP (PC12-APP). PC12-APP cells were treated with DPA at the concentration of 6.25, 12.5, 25μg/mL for 4h, and then co-treated with 50nmol/L bafilomycin A1 for 48h except the controls. The Aβ_1-42 content in culture medium was determined by ELISA. The intracellular amount of APP, Aβ_1-42, LC3, cathepsin D was measured by Western blotting and normalized to GAPDH loading control. The PC12 cells stable transfected with pSelect-LC3-GFP (PC12-LC3-GFP) was used in the fluorescence microscopy estimation of autophagosomes accumulation. The internal pH in lysosome was detected by LysoTracker Red staining.

RESULTS

DPA had no significant effect on the cell viability but could significantly decrease Aβ_1-42 content in culture medium and eliminate the intracellular accumulation of APP and Aβ_1-42 in bafilomycin A1 induced PC12-APP cells. Furthermore, DPA lowered the LC3-II/LC3-I ratio and reduced the GFP-labeled LC3 puncta which were elevated by bafilomycin A1. And the increase in internal pH of lysosome and decrease in mCatD amount in Bafilomycin A1 induced PC12-APP cells were restored by DPA treatment. These results indicated that DPA could restore the lysosomal acidification and recover the autophgic flux which is impaired by bafilomycin A1.

CONCLUSIONS

DPA could effectively clear the accumulation of Aβ_1-42 in bafilomycin A1 impaired PC12 cells through restoring the lysosomal acidification and recovering the autophgic flux. And these results highlight its therapeutic potential for AD treatment.

Azithromycin impairs TLR7 signaling in dendritic cells and improves the severity of imiquimod-induced psoriasis-like skin inflammation in mice

BACKGROUND

The activation of Toll-like receptor 7 (TLR7) in dendritic cells (DCs) plays a crucial role in the pathogenesis of psoriasis. The macrolide antibiotic azithromycin (AZM) had been demonstrated to inhibit the TLR4 agonist-induced maturation and activation of murine bone marrow-derived DCs (BMDCs).

OBJECTIVE

To investigate the effects of AZM on the induction of DC maturation and activation by imiquimod (IMQ), a synthetic TLR7 agonist, as well as its potential as a therapeutic agent for psoriasis.

METHODS

The effects of AZM on IMQ-induced DC activation were investigated based on the expression of cell surface markers and cytokine secretion. The lysosomal pH, post-translational processing of TLR7, and TLR7 signaling were also examined in DCs. The therapeutic effects of AZM on psoriasis were evaluated in a murine model of IMQ-induced psoriasis-like skin inflammation.

RESULTS

AZM significantly inhibited the expression of co-stimulatory molecules (CD40 and CD80) and reduced TNF-α, IL-10, IL-12p40, IL-12p70, IL-23p19 in BMDCs and IFN-α production in plasmacytoid DCs. AZM treatment impaired lysosomal acidification, interrupted TLR7 maturation in the lysosome, and ultimately blocked the IMQ-induced NF-κB and IRF-7 nuclear translocation in DCs. AZM treatment decreased signs of IMQ-induced skin inflammation in BALB/c mice. In addition to decreasing keratinocyte hyper-proliferation and restoring their terminal differentiation, AZM treatment decreased the accumulation of DCs as well as CD4, CD8 T cells and IL-17 producing cells in psoriatic skin lesions. AZM treatment improved splenomegaly, decreased the populations of Th17 and γδ T cells, and reduced the expression of cytokines known to be involved in the pathogenesis of psoriasis, such as IL-17A, IL-17F, IL-22 and IL-23, in the skin and spleen.

CONCLUSION

AZM impaired IMQ-induced DC activation by decreasing lysosomal acidification and disrupting TLR7 maturation and signaling. AZM significantly improved the IMQ-induced psoriasis-like inflammation in mice. AZM may be a potential therapeutic candidate for psoriasis treatment.

The GATOR2 Component Wdr24 Regulates TORC1 Activity and Lysosome Function

TORC1 is a master regulator of metabolism in eukaryotes that responds to multiple upstream signaling pathways. The GATOR complex is a newly defined upstream regulator of TORC1 that contains two sub-complexes, GATOR1, which inhibits TORC1 activity in response to amino acid starvation and GATOR2, which opposes the activity of GATOR1. While the GATOR1 complex has been implicated in a wide array of human pathologies including cancer and hereditary forms of epilepsy, the in vivo relevance of the GATOR2 complex remains poorly understood in metazoans. Here we define the in vivo role of the GATOR2 component Wdr24 in Drosophila. Using a combination of genetic, biochemical, and cell biological techniques we demonstrate that Wdr24 has both TORC1 dependent and independent functions in the regulation of cellular metabolism. Through the characterization of a null allele, we show that Wdr24 is a critical effector of the GATOR2 complex that promotes the robust activation of TORC1 and cellular growth in a broad array of Drosophila tissues. Additionally, epistasis analysis between wdr24 and genes that encode components of the GATOR1 complex revealed that Wdr24 has a second critical function, the TORC1 independent regulation of lysosome dynamics and autophagic flux. Notably, we find that two additional members of the GATOR2 complex, Mio and Seh1, also have a TORC1 independent role in the regulation of lysosome function. These findings represent a surprising and previously unrecognized function of GATOR2 complex components in the regulation of lysosomes. Consistent with our findings in Drosophila, through the characterization of a wdr24^-/- knockout HeLa cell line we determined that Wdr24 promotes lysosome acidification and autophagic flux in mammalian cells. Taken together our data support the model that Wdr24 is a key effector of the GATOR2 complex, required for both TORC1 activation and the TORC1 independent regulation of lysosomes.

TORC1 is a conserved multi-protein complex that regulates metabolism and cell growth in response to many upstream inputs including nutrient availability. When amino acids are limiting, the GATOR1 complex inhibits TORC1 activation. The inhibition of TORC1 slows cellular metabolism and promotes cell survival during times of protein scarcity. A second critical response to amino acid limitation is the activation of autophagy. During autophagy cells degrade intracellular components in specialized membrane-bound organelles called autolysosomes that are formed when lysosomes fuse with autophagosomes. In times of nutrient stress, the process of autophagy allows proteins and other building blocks of the cell to be broken down and repurposed for vital cellular functions. Here we demonstrate that Wdr24, a component of the multi-protein GATOR2 complex, has a dual role in the regulation of cellular metabolism in Drosophila. First, Wdr24 is required to oppose the activity of the GATOR1 complex, thus activating TORC1 in a broad array of Drosophila tissues. Second, Wdr24 promotes the acidification of lysosomes and thus facilitates autophagic flux. Our data support the model that Wdr24 uses both TORC1 dependent and independent pathways to regulate cellular metabolism.

Trehalose Alters Subcellular Trafficking and the Metabolism of the Alzheimer-associated Amyloid Precursor Protein

The disaccharide trehalose is commonly considered to stimulate autophagy. Cell treatment with trehalose could decrease cytosolic aggregates of potentially pathogenic proteins, including mutant huntingtin, α-synuclein, and phosphorylated tau that are associated with neurodegenerative diseases. Here, we demonstrate that trehalose also alters the metabolism of the Alzheimer disease-related amyloid precursor protein (APP). Cell treatment with trehalose decreased the degradation of full-length APP and its C-terminal fragments. Trehalose also reduced the secretion of the amyloid-β peptide. Biochemical and cell biological experiments revealed that trehalose alters the subcellular distribution and decreases the degradation of APP C-terminal fragments in endolysosomal compartments. Trehalose also led to strong accumulation of the autophagic marker proteins LC3-II and p62, and decreased the proteolytic activation of the lysosomal hydrolase cathepsin D. The combined data indicate that trehalose decreases the lysosomal metabolism of APP by altering its endocytic vesicular transport.

Cathepsin D in pancreatic acinar cells is implicated in cathepsin B and L degradation, but not in autophagic activity

Cathepsin D (CD) is the major lysosomal aspartic protease and is widely distributed in the cells of various mammalian tissues. CD participates in various physiological events such as regulation of programmed cell death, activation of enzymatic precursors, and metabolic degradation of intracellular proteins through macroautophagy. To investigate the role of CD in pancreatic acinar cells, which constitute the exocrine pancreas, we generated and examined mice specifically deficient for CD in pancreatic acinar cells. CD deficient mice showed normal pancreatic development and autophagic activity, although LC3-II, which is a marker of the autophagosome, accumulates in both physiological and pancreatitis conditions. Moreover, CD deficiency leads to accumulation of matured cathepsin B (CB) and cathepsin L (CL) which are members of the cysteine protease family. We therefore conclude that CD in pancreatic acinar cells is implicated in CB and CL degradation but not in autophagic activity.

2-(3',5'-Dimethoxybenzylidene) cyclopentanone, a novel synthetic small-molecule compound, provides neuroprotective effects against ischemic stroke

2-(3',5'-Dimethoxybenzylidene) cyclopentanone (DMBC) is a novel small-molecule compound synthesized by our group. Here, we found that in rat models of permanent middle cerebral artery occlusion (pMCAO), intraperitoneal injection (ip) of DMBC at 1h after ischemia reduced infarct volume, improved neurological deficits and increased the protein levels of microtubule-associated protein 2 (MAP 2) and glial fibrillary acid protein (GFAP) in the ischemic cortex. Post-treatment of DMBC still produced neuroprotective effects even when administered at 6h after ischemia. In the oxygen-glucose deprivation (OGD)-induced astrocytes or HT22 cell injury, DMBC treatment decreased the OGD-induced lactate dehydrogenase (LDH) leakage and increased the GFAP levels in astrocytes. In addition, Annexin-V-Fluos staining analysis revealed that DMBC treatment attenuated both OGD-induced apoptosis and necrosis in astrocytes. Western blotting analysis showed DMBC treatment inhibited the ischemia or OGD-induced increases in active cathepsin B in the ischemic cortex or in astrocytes or HT22 cells. Immunofluorescence analysis demonstrated that DMBC treatment blocked the ischemia or OGD-induced release of cathepsin B from the lysosomes into the cytoplasm in the ischemic cortex or in astrocytes or HT22 cells. Taken together, our results indicate that DMBC can offer neuroprotective effects against cerebral ischemia with an extended therapeutic window and its mechanism might be associated with inhibition of the cathepsin B activation.

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.

Cathepsin D inhibitors as potential therapeutics for breast cancer treatment: Molecular docking and bioevaluation against triple-negative and triple-positive breast cancers

The main aim of this study was to discover small molecule inhibitors against Cathepsin D (CatD) (EC.3.4.23.5), a clinically proven prognostic marker for breast cancer, and to explore the mechanisms by which CatD could be a useful therapeutic target for triple-positive and triple-negative breast cancers (TPBC & TNBC). The crystal structure of CatD at 2.5 Å resolution (PDB: 1LYB), which was complexed with Pepstatin A, was selected for computer-aided molecular modeling. The methods used in our study were pharmacophore modeling and molecular docking. Virtual screening was performed to identify small molecules from an in-house database and a large commercial chemical library. Cytotoxicity studies were performed on human normal cell line HEK293T and growth inhibition studies on breast adenocarcinoma cell lines, namely MCF-7, MDA-MB-231, SK-BR-3, and MDA-MB-468. Furthermore, RT-PCR analysis, in vitro enzyme assay, and cell cycle analysis ascertained the validity of the selected molecules. A set of 28 molecules was subjected to an in vitro fluorescence-based inhibitory activity assay, and among them six molecules exhibited >50 % inhibition at 25μM. These molecules also exhibited good growth inhibition against TPBC and TNBC cancer types. Among them, molecules 1 and 17 showed single-digit micromolar GI50 values against MCF-7 and MDA-MB-231 cell lines.

Mahogunin regulates fusion between amphisomes/MVBs and lysosomes via ubiquitination of TSG101

Aberrant metabolic forms of the prion protein (PrP), membrane-associated (Ctm)PrP and cytosolic (cyPrP) interact with the cytosolic ubiquitin E3 ligase, Mahogunin Ring Finger-1 (MGRN1) and affect lysosomes. MGRN1 also interacts with and ubiquitinates TSG101, an ESCRT-I protein, involved in endocytosis. We report that MGRN1 modulates macroautophagy. In cultured cells, functional depletion of MGRN1 or overexpression of (Ctm)PrP and cyPrP blocks autophagosome-lysosome fusion, alleviates the autophagic flux and its degradative competence. Concurrently, the degradation of cargo from the endo-lysosomal pathway is also affected. This is significant because catalytic inactivation of MGRN1 alleviates fusion of lysosomes with either autophagosomes (via amphisomes) or late endosomes (either direct or mediated through amphisomes), without drastically perturbing maturation of late endosomes, generation of amphisomes or lysosomal proteolytic activity. The compromised lysosomal fusion events are rescued by overexpression of TSG101 and/or its monoubiquitination in the presence of MGRN1. Thus, for the first time we elucidate that MGRN1 simultaneously modulates both autophagy and heterophagy via ubiquitin-mediated post-translational modification of TSG101.

Cofilin recruits F-actin to SPCA1 and promotes Ca2+-mediated secretory cargo sorting

The cofilin CFL-1 recruits actin to the P-type calcium ATPase SPCA1 at the trans-Golgi network, thereby activating the ATPase, promoting Ca²⁺ influx, and driving secretory cargo sorting.

The actin filament severing protein cofilin-1 (CFL-1) is required for actin and P-type ATPase secretory pathway calcium ATPase (SPCA)-dependent sorting of secretory proteins at the trans-Golgi network (TGN). How these proteins interact and activate the pump to facilitate cargo sorting, however, is not known. We used purified proteins to assess interaction of the cytoplasmic domains of SPCA1 with actin and CFL-1. A 132–amino acid portion of the SPCA1 phosphorylation domain (P-domain) interacted with actin in a CFL-1–dependent manner. This domain, coupled to nickel nitrilotriacetic acid (Ni-NTA) agarose beads, specifically recruited F-actin in the presence of CFL-1 and, when expressed in HeLa cells, inhibited Ca²⁺ entry into the TGN and secretory cargo sorting. Mutagenesis of four amino acids in SPCA1 that represent the CFL-1 binding site also affected Ca²⁺ import into the TGN and secretory cargo sorting. Altogether, our findings reveal the mechanism of CFL-1–dependent recruitment of actin to SPCA1 and the significance of this interaction for Ca²⁺ influx and secretory cargo sorting.

Parkinson's disease-linked mutations in VPS35 induce dopaminergic neurodegeneration

Mutations in the vacuolar protein sorting 35 homolog (VPS35) gene at the PARK17 locus, encoding a key component of the retromer complex, were recently identified as a new cause of late-onset, autosomal dominant Parkinson's disease (PD). Here we explore the pathogenic consequences of PD-associated mutations in VPS35 using a number of model systems. VPS35 exhibits a broad neuronal distribution throughout the rodent brain, including within the nigrostriatal dopaminergic pathway. In the human brain, VPS35 protein levels and distribution are similar in tissues from control and PD subjects, and VPS35 is not associated with Lewy body pathology. The common D620N missense mutation in VPS35 does not compromise its protein stability or localization to endosomal and lysosomal vesicles, or the vesicular sorting of the retromer cargo, sortilin, SorLA and cation-independent mannose 6-phosphate receptor, in rodent primary neurons or patient-derived human fibroblasts. In yeast we show that PD-linked VPS35 mutations are functional and can normally complement VPS35 null phenotypes suggesting that they do not result in a loss-of-function. In rat primary cortical cultures the overexpression of human VPS35 induces neuronal cell death and increases neuronal vulnerability to PD-relevant cellular stress. In a novel viral-mediated gene transfer rat model, the expression of D620N VPS35 induces the marked degeneration of substantia nigra dopaminergic neurons and axonal pathology, a cardinal pathological hallmark of PD. Collectively, these studies establish that dominant VPS35 mutations lead to neurodegeneration in PD consistent with a gain-of-function mechanism, and support a key role for VPS35 in the development of PD.

Biogenesis and proteolytic processing of lysosomal DNase II

Deoxyribonuclease II (DNase II) is a key enzyme in the phagocytic digestion of DNA from apoptotic nuclei. To understand the molecular properties of DNase II, particularly the processing, we prepared a polyclonal antibody against carboxyl-terminal sequences of mouse DNase II. In the present study, partial purification of DNase II using Con A Sepharose enabled the detection of endogenous DNase II by Western blotting. It was interesting that two forms of endogenous DNase II were detected--a 30 kDa form and a 23 kDa form. Neither of those forms carried the expected molecular weight of 45 kDa. Subcellular fractionation showed that the 23 kDa and 30 kDa proteins were localized in lysosomes. The processing of DNase II in vivo was also greatly altered in the liver of mice lacking cathepsin L. DNase II that was extracellularly secreted from cells overexpressing DNase II was detected as a pro-form, which was activated under acidic conditions. These results indicate that DNase II is processed and activated in lysosomes, while cathepsin L is involved in the processing of the enzyme.

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.

Regulation of protein glycosylation and sorting by the Golgi matrix proteins GRASP55/65

The Golgi receives the entire output of newly synthesized cargo from the endoplasmic reticulum, processes it in the stack largely through modification of bound oligosaccharides, and sorts it in the trans-Golgi network. GRASP65 and GRASP55, two proteins localized to the Golgi stack and early secretory pathway, mediate processes including Golgi stacking, Golgi ribbon linking and unconventional secretion. Previously, we have shown that GRASP depletion in cells disrupts Golgi stack formation. Here we report that knockdown of the GRASP proteins, alone or combined, accelerates protein trafficking through the Golgi membranes but also has striking negative effects on protein glycosylation and sorting. These effects are not caused by Golgi ribbon unlinking, unconventional secretion or endoplasmic reticulum stress. We propose that GRASP55/65 are negative regulators of exocytic transport and that this slowdown helps to ensure more complete protein glycosylation in the Golgi stack and proper sorting at the trans-Golgi network.

Inhibition of glycogen synthase kinase-3 ameliorates β-amyloid pathology and restores lysosomal acidification and mammalian target of rapamycin activity in the Alzheimer disease mouse model: in vivo and in vitro studies

Accumulation of β-amyloid (Aβ) deposits is a primary pathological feature of Alzheimer disease that is correlated with neurotoxicity and cognitive decline. The role of glycogen synthase kinase-3 (GSK-3) in Alzheimer disease pathogenesis has been debated. To study the role of GSK-3 in Aβ pathology, we used 5XFAD mice co-expressing mutated amyloid precursor protein and presenilin-1 that develop massive cerebral Aβ loads. Both GSK-3 isozymes (α/β) were hyperactive in this model. Nasal treatment of 5XFAD mice with a novel substrate competitive GSK-3 inhibitor, L803-mts, reduced Aβ deposits and ameliorated cognitive deficits. Analyses of 5XFAD hemi-brain samples indicated that L803-mts restored the activity of mammalian target of rapamycin (mTOR) and inhibited autophagy. Lysosomal acidification was impaired in the 5XFAD brains as indicated by reduced cathepsin D activity and decreased N-glycoyslation of the vacuolar ATPase subunit V0a1, a modification required for lysosomal acidification. Treatment with L803-mts restored lysosomal acidification in 5XFAD brains. Studies in SH-SY5Y cells confirmed that GSK-3α and GSK-3β impair lysosomal acidification and that treatment with L803-mts enhanced the acidic lysosomal pool as demonstrated in LysoTracker Red-stained cells. Furthermore, L803-mts restored impaired lysosomal acidification caused by dysfunctional presenilin-1. We provide evidence that mTOR is a target activated by GSK-3 but inhibited by impaired lysosomal acidification and elevation in amyloid precursor protein/Aβ loads. Taken together, our data indicate that GSK-3 is a player in Aβ pathology. Inhibition of GSK-3 restores lysosomal acidification that in turn enables clearance of Aβ burdens and reactivation of mTOR. These changes facilitate amelioration in cognitive function.

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

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

Lysosomes as a possible target of enniatin B-induced toxicity in Caco-2 cells

Enniatins are cyclic hexadepsipeptidic mycotoxins with ionophoric, antibiotic, and insecticidal activity. Enniatin B (EnnB), the most important analogue, is produced by many Fusarium species and is a common contaminant in grain-based foods. The compound's cytotoxic potential has been shown in different experiments; however, the mode of action has not been detailed so far. In the present study, several mutually confirmative experiments have been performed indicating that EnnB-initiated cytotoxicity could be connected with lysosomal membrane permeabilization (LMP). Lysosomal functionality, as assessed by the Neutral Red assay, was already affected after 3 h of toxin exposure. After 24 h, cell proliferation was decreased, and there was indication for a cell cycle arrest in the G(2)/M phase leading to the initiation of apoptosis or necrosis. Intracellular ROS-production was observed. However, antioxidants did not alter the observed EnnB-induced loss of lysosomal functionality leading to the conclusion that ROS was not an initial factor but one produced later in the event cascade. The collected data suggested that lysosomal destabilization is an upstream event in EnnB-initiated cytotoxicity followed by a certain extent of translocation of cathepsins into the cytosol, which was observed using immunological and proteomic methods. It appeared that cell death induced by EnnB was delayed and occurred not as a massive lysosomal breakdown but was probably progressing and leading to partial and selective LMP, starting a nonapoptotic cell death pathway with morphological features that had been previously considered as necrotic. The molecular mechanism of EnnB-triggered lysosomal destabilization, and the cellular processes leading to mitochondrial permeabilization and cell death are still unknown. They may, however, be connected to the compound's ionophoric properties.

Cathepsin D is partly endocytosed by the LRP1 receptor and inhibits LRP1-regulated intramembrane proteolysis

The aspartic protease cathepsin-D (cath-D) is a marker of poor prognosis in breast cancer that is overexpressed and hypersecreted by human breast cancer cells. Secreted pro-cath-D binds to the extracellular domain of the β-chain of the LDL receptor-related protein-1 (LRP1) in fibroblasts. The LRP1 receptor has an 85-kDa transmembrane β-chain and a noncovalently attached 515-kDa extracellular α-chain. LRP1 acts by (1) internalizing many ligands via its α-chain, (2) activating signaling pathways by phosphorylating the LRP1β-chain tyrosine and (3) modulating gene transcription by regulated intramembrane proteolysis (RIP) of its β-chain. LRP1 RIP involves two cleavages: the first liberates the LRP1 ectodomain to give a membrane-associated form, LRP1β-CTF, and the second generates the LRP1β-intracellular domain, LRP1β-ICD, that modulates gene transcription. Here, we investigated the endocytosis of pro-cath-D by LRP1 and the effect of pro-cath-D/LRP1β interaction on LRP1β tyrosine phosphorylation and/or LRP1β RIP. Our results indicate that pro-cath-D was partially endocytosed by LRP1 in fibroblasts. However, pro-cath-D and ectopic cath-D did not stimulate phosphorylation of the LRP1β-chain tyrosine. Interestingly, ectopic cath-D and its catalytically inactive (D231N)cath-D, and pro-(D231N)cath-D all significantly inhibited LRP1 RIP by preventing LRP1β-CTF production. Thus, cath-D inhibits LRP1 RIP independently of its catalytic activity by blocking the first cleavage. As cath-D triggers fibroblast outgrowth by LRP1, we propose that cath-D modulates the growth of fibroblasts by inhibiting LRP1 RIP in the breast tumor microenvironment.