Nanoparticles restore lysosomal acidification defects: Implications for Parkinson and other lysosomal-related diseases

Degradable Nanoparticles Restore Lysosomal pH and Autophagic Flux in Lipotoxic Pancreatic Beta Cells

Chronic exposure to high levels of fatty acids (lipotoxicity) in pancreatic beta cells (β-cells) decreases lysosomal acidity and inhibits autophagic flux. Today, there are a lack of approaches to modify lysosomal acidity to determine whether impaired lysosomal acidification is causally inhibiting autophagic flux and cellular functions. Biodegradable poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) with diameters of ≈100 nm localize to lysosomes and serve as an ideal method to deliver lactic and glycolic acid to lysosomes upon NP polymer degradation. In this study, the ability of PLGA NPs to lower lysosomal pH and restore autophagic flux is investigated in pancreatic insulin secreting (INS1) β-cells. PLGA NPs display a concentration dependent performance with higher concentrations of PLGA NPs, lowering lysosomal pH, as well as restoring autophagic flux and insulin secretion in pancreatic β-cells. These results document that acidifying the lysosome, via an external perturbation, in lipotoxic pancreatic β-cells affords a specific biological outcome of improved cellular degradative activity.

Dysfunction of Cellular Proteostasis in Parkinson's Disease

Despite decades of research, current therapeutic interventions for Parkinson’s disease (PD) are insufficient as they fail to modify disease progression by ameliorating the underlying pathology. Cellular proteostasis (protein homeostasis) is an essential factor in maintaining a persistent environment for neuronal activity. Proteostasis is ensured by mechanisms including regulation of protein translation, chaperone-assisted protein folding and protein degradation pathways. It is generally accepted that deficits in proteostasis are linked to various neurodegenerative diseases including PD. While the proteasome fails to degrade large protein aggregates, particularly alpha-synuclein (α-SYN) in PD, drug-induced activation of autophagy can efficiently remove aggregates and prevent degeneration of dopaminergic (DA) neurons. Therefore, maintenance of these mechanisms is essential to preserve all cellular functions relying on a correctly folded proteome. The correlations between endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) that aims to restore proteostasis within the secretory pathway are well-established. However, while mild insults increase the activity of chaperones, prolonged cell stress, or insufficient adaptive response causes cell death. Modulating the activity of molecular chaperones, such as protein disulfide isomerase which assists refolding and contributes to the removal of unfolded proteins, and their associated pathways may offer a new approach for disease-modifying treatment. Here, we summarize some of the key concepts and emerging ideas on the relation of protein aggregation and imbalanced proteostasis with an emphasis on PD as our area of main expertise. Furthermore, we discuss recent insights into the strategies for reducing the toxic effects of protein unfolding in PD by targeting the ER UPR pathway.

Selective autophagy as a potential therapeutic target for neurodegenerative disorders

Cells rely on surveillance systems such as autophagy to handle protein alterations and organelle damage. Dysfunctional autophagy, an evolutionarily conserved cellular mechanism for degradation of intracellular components in lysosomes, frequently leads to neurodegeneration. The neuroprotective effect of autophagy stems from its ability to eliminate pathogenic forms of proteins such as α-synuclein or tau. However, the same pathogenic proteins often affect different types and steps of the autophagic process. Furthermore, genetic studies have shown that some proteins related to neurodegeneration, such as huntingtin, participate in autophagy as one of their physiological functions. This complex interplay between autophagy and neurodegeneration suggests that targeting autophagy as a whole might have limited applicability in neurodegenerative diseases, and that future efforts should focus instead on targeting specific types and steps of the autophagic process. This change of strategy in the modulation of autophagy might hold promise for future disease-modifying therapies for patients with neurodegenerative disorders.

Lysosomal Dysfunction in Down Syndrome Is APP-Dependent and Mediated by APP-βCTF (C99)

Lysosomal failure underlies pathogenesis of numerous congenital neurodegenerative disorders and is an early and progressive feature of Alzheimer's disease (AD) pathogenesis. Here, we report that lysosomal dysfunction in Down ayndrome (trisomy 21), a neurodevelopmental disorder and form of early onset AD, requires the extra gene copy of amyloid precursor protein (APP) and is specifically mediated by the β cleaved carboxy terminal fragment of APP (APP-βCTF, C99). In primary fibroblasts from individuals with DS, lysosomal degradation of autophagic and endocytic substrates is selectively impaired, causing them to accumulate in enlarged autolysosomes/lysosomes. Direct measurements of lysosomal pH uncovered a significant elevation (0.6 units) as a basis for slowed LC3 turnover and the inactivation of cathepsin D and other lysosomal hydrolases known to be unstable or less active when lysosomal pH is persistently elevated. Normalizing lysosome pH by delivering acidic nanoparticles to lysosomes ameliorated lysosomal deficits, whereas RNA sequencing analysis excluded a transcriptional contribution to hydrolase declines. Cortical neurons cultured from the Ts2 mouse model of DS exhibited lysosomal deficits similar to those in DS cells. Lowering APP expression with siRNA or BACE1 inhibition reversed cathepsin deficits in both fibroblasts and neurons. Deleting one Bace1 allele from adult Ts2 mice had similar rescue effects in vivo The modest elevation of endogenous APP-βCTF needed to disrupt lysosomal function in DS is relevant to sporadic AD where APP-βCTF, but not APP, is also elevated. Our results extend evidence that impaired lysosomal acidification drives progressive lysosomal failure in multiple forms of AD.SIGNIFICANCE STATEMENT Down syndrome (trisomy 21) (DS) is a neurodevelopmental disorder invariably leading to early-onset Alzheimer's disease (AD). We showed in cells from DS individuals and neurons of DS models that one extra copy of a normal amyloid precursor protein (APP) gene impairs lysosomal acidification, thereby depressing lysosomal hydrolytic activities and turnover of autophagic and endocytic substrates, processes vital to neuronal survival. These deficits, which were reversible by correcting lysosomal pH, are mediated by elevated levels of endogenous β-cleaved carboxy-terminal fragment of APP (APP-βCTF). Notably, similar endosomal-lysosomal pathobiology emerges early in sporadic AD, where neuronal APP-βCTF is also elevated, underscoring its importance as a therapeutic target and underscoring the functional and pathogenic interrelationships between the endosomal-lysosomal pathway and genes causing AD.

Proximal Tubule Autophagy Differs in Type 1 and 2 Diabetes

BACKGROUND

Evidence of a protective role of autophagy in kidney diseases has sparked interest in autophagy as a potential therapeutic strategy. However, understanding how the autophagic process is altered in each disorder is critically important in working toward therapeutic applications.

METHODS

Using cultured kidney proximal tubule epithelial cells (PTECs) and diabetic mouse models, we investigated how autophagic activity differs in type 1 versus type 2 diabetic nephropathy. We explored nutrient signals regulating starvation-induced autophagy in PTECs and used autophagy-monitoring mice and PTEC-specific autophagy-deficient knockout mice to examine differences in autophagy status and autophagy's role in PTECs in streptozotocin (STZ)-treated type 1 and db/db type 2 diabetic nephropathy. We also examined the effects of rapamycin (an inhibitor of mammalian target of rapamycin [mTOR]) on vulnerability to ischemia-reperfusion injury.

RESULTS

Administering insulin or amino acids, but not glucose, suppressed autophagy by activating mTOR signaling. In db/db mice, autophagy induction was suppressed even under starvation; in STZ-treated mice, autophagy was enhanced even under fed conditions but stagnated under starvation due to lysosomal stress. Using knockout mice with diabetes, we found that, in STZ-treated mice, activated autophagy counteracts mitochondrial damage and fibrosis in the kidneys, whereas in db/db mice, autophagic suppression jeopardizes kidney even in the autophagy-competent state. Rapamycin-induced pharmacologic autophagy produced opposite effects on ischemia-reperfusion injury in STZ-treated and db/db mice.

CONCLUSIONS

Autophagic activity in PTECs is mainly regulated by insulin. Consequently, autophagic activity differs in types 1 and 2 diabetic nephropathy, which should be considered when developing strategies to treat diabetic nephropathy by modulating autophagy.

Lysosomal dysfunction in proteinopathic neurodegenerative disorders: possible therapeutic roles of cAMP and zinc

A number of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, share intra- and/or extracellular deposition of protein aggregates as a common core pathology. While the species of accumulating proteins are distinct in each disease, an increasing body of evidence indicates that defects in the protein clearance system play a crucial role in the gradual accumulation of protein aggregates. Among protein degradation systems, the endosome-autophagosome-lysosome pathway (EALP) is the main degradation machinery, especially for large protein aggregates. Lysosomal dysfunction or defects in fusion with vesicles containing cargo are commonly observed abnormalities in proteinopathic neurodegenerative diseases. In this review, we discuss the available evidence for a mechanistic connection between components of the EALP-especially lysosomes-and neurodegenerative diseases. We also focus on lysosomal pH regulation and its significance in maintaining flux through the EALP. Finally, we suggest that raising cAMP and free zinc levels in brain cells may be beneficial in normalizing lysosomal pH and EALP flux.

Ubiquilins regulate autophagic flux through mTOR signalling and lysosomal acidification

Although the aetiology of amyotrophic lateral sclerosis (ALS) remains poorly understood, impaired proteostasis is a common feature of different forms of ALS. Mutations in genes encoding ubiquilins, UBQLN2 and UBQLN4, cause familial ALS. The role of ubiquilins in proteasomal degradation is well established, but their role in autophagy-lysosomal clearance is poorly defined. Here, we describe a crosstalk between endoplasmic reticulum stress, mTOR signalling and autophagic flux in Drosophila and mammalian cells lacking ubiquilins. We found that loss of ubiquilins leads to endoplasmic reticulum stress, impairs mTORC1 activity, promotes autophagy and causes the demise of neurons. We show that ubiquilin mutants display defective autophagic flux due to reduced lysosome acidification. Ubiquilins are required to maintain proper levels of the V0a/V100 subunit of the vacuolar H+-ATPase and lysosomal pH. Feeding flies acidic nanoparticles alleviates defective autophagic flux in ubiquilin mutants. Hence, our studies reveal a conserved role for ubiquilins as regulators of autophagy by controlling vacuolar H+-ATPase activity and mTOR signalling.

Emerging new roles of the lysosome and neuronal ceroid lipofuscinoses

Neuronal Ceroid Lipofuscinoses (NCLs), commonly known as Batten disease, constitute a group of the most prevalent neurodegenerative lysosomal storage disorders (LSDs). Mutations in at least 13 different genes (called CLNs) cause various forms of NCLs. Clinically, the NCLs manifest early impairment of vision, progressive decline in cognitive and motor functions, seizures and a shortened lifespan. At the cellular level, all NCLs show intracellular accumulation of autofluorescent material (called ceroid) and progressive neuron loss. Despite intense studies the normal physiological functions of each of the CLN genes remain poorly understood. Consequently, the development of mechanism-based therapeutic strategies remains challenging. Endolysosomal dysfunction contributes to pathogenesis of virtually all LSDs. Studies within the past decade have drastically changed the notion that the lysosomes are merely the terminal degradative organelles. The emerging new roles of the lysosome include its central role in nutrient-dependent signal transduction regulating metabolism and cellular proliferation or quiescence. In this review, we first provide a brief overview of the endolysosomal and autophagic pathways, lysosomal acidification and endosome-lysosome and autophagosome-lysosome fusions. We emphasize the importance of these processes as their dysregulation leads to pathogenesis of many LSDs including the NCLs. We also describe what is currently known about each of the 13 CLN genes and their products and how understanding the emerging new roles of the lysosome may clarify the underlying pathogenic mechanisms of the NCLs. Finally, we discuss the current and emerging therapeutic strategies for various NCLs.

Biocompatible Polymer Nanoparticles for Drug Delivery Applications in Cancer and Neurodegenerative Disorder Therapies

Polymer nanoparticles (NPs) represent one of the most innovative non-invasive approaches for drug delivery applications. NPs main objective is to convey the therapeutic molecule be they drugs, proteins, or nucleic acids directly into the target organ or tissue. Many polymers are used for the synthesis of NPs and among the currently most employed materials several biocompatible synthetic polymers, namely polylactic acid (PLA), poly lactic-co-glycolic acid (PLGA), and polyethylene glycol (PEG), can be cited. These molecules are made of simple monomers which are naturally present in the body and therefore easily excreted without being toxic. The present review addresses the different approaches that are most commonly adopted to synthetize biocompatible NPs to date, as well as the experimental strategies designed to load them with therapeutic agents. In fact, drugs may be internalized in the NPs or physically dispersed therein. In this paper the various types of biodegradable polymer NPs will be discussed with emphasis on their applications in drug delivery. Close attention will be devoted to the treatment of cancer, where both active and passive targeting is used to enhance efficacy and reduce systemic toxicity, and to diseases affecting the central nervous system, inasmuch as NPs can be modified to target specific cells or cross membrane barriers.

Monitoring the death of single BaF3 cells under plasmonic photothermal heating induced by ultrasmall gold nanorods

Morphological changes and trypan-blue staining are temporally tracked in single cells via optical microscopy after plasmonic photothermal heating.

Endosomal/lysosomal location of organically modified silica nanoparticles following caveolae-mediated endocytosis

Organically modified silica (ORMOSIL) nanoparticles (NPs) are widely used in biomedicine.

Harnessing Lysosomal pH through PLGA Nanoemulsion as a Treatment of Lysosomal-Related Neurodegenerative Diseases

Most neurodegenerative disorders are characterized by deposits of misfolded proteins and neuronal degeneration in specific brain regions. Growing evidence indicates that lysosomal impairment plays a primary pathogenic role in these diseases, in particular, the occurrence of increased lysosomal pH. Thus, therapeutic development aiming at restoring lysosomal function represents a novel, precise, and promising strategy for the treatment of these pathologies. Herein we demonstrate that acidic oil-in-water nanoemulsions loaded with poly(dl-lactide- co-glycolide) (PLGA) are able to rescue impaired lysosomal pH in genetic cellular models of Parkinson's disease. For in vivo assays, nanoemulsions were labeled with an original synthetic hydrophobic far red-emitting dye to allow fluorescence monitoring. Following stereotaxic injection in the mouse brain, widespread diffusion of the nanocarrier was observed, up to 500 μm from the injection site, as well as internalization into the lysosomal compartment in brain cells. Finally, promising preliminary assays of systemic administration demonstrate that a fraction of the formulation crosses the blood brain barrier, penetrates the brain parenchyma, is internalized by cells, and colocalizes with lysosomal markers. Overall, these results suggest the feasibility and the therapeutic potential of this new nanoformulation as an effective drug delivery tool to the brain, with the potential to rescue pathological lysosomal deficits.

New Perspectives on Roles of Alpha-Synuclein in Parkinson's Disease

Parkinson’s disease (PD) is one of the synucleinopathies spectrum of disorders typified by the presence of intraneuronal protein inclusions. It is primarily composed of misfolded and aggregated forms of alpha-synuclein (α-syn), the toxicity of which has been attributed to the transition from an α-helical conformation to a β-sheetrich structure that polymerizes to form toxic oligomers. This could spread and initiate the formation of “LB-like aggregates,” by transcellular mechanisms with seeding and subsequent permissive templating. This hypothesis postulates that α-syn is a prion-like pathological agent and responsible for the progression of Parkinson’s pathology. Moreover, the involvement of the inflammatory response in PD pathogenesis has been reported on the excessive microglial activation and production of pro-inflammatory cytokines. At last, we describe several treatment approaches that target the pathogenic α-syn protein, especially the oligomers, which are currently being tested in advanced animal experiments or are already in clinical trials. However, there are current challenges with therapies that target α-syn, for example, difficulties in identifying varying α-syn conformations within different individuals as well as both the cost and need of long-duration large trials.

Promoting the clearance of neurotoxic proteins in neurodegenerative disorders of ageing

Neurodegenerative disorders of ageing (NDAs) such as Alzheimer disease, Parkinson disease, frontotemporal dementia, Huntington disease and amyotrophic lateral sclerosis represent a major socio-economic challenge in view of their high prevalence yet poor treatment. They are often called 'proteinopathies' owing to the presence of misfolded and aggregated proteins that lose their physiological roles and acquire neurotoxic properties. One reason underlying the accumulation and spread of oligomeric forms of neurotoxic proteins is insufficient clearance by the autophagic-lysosomal network. Several other clearance pathways are also compromised in NDAs: chaperone-mediated autophagy, the ubiquitin-proteasome system, extracellular clearance by proteases and extrusion into the circulation via the blood-brain barrier and glymphatic system. This article focuses on emerging mechanisms for promoting the clearance of neurotoxic proteins, a strategy that may curtail the onset and slow the progression of NDAs.

Parkinson's disease: convergence on synaptic homeostasis

Parkinson's disease, the second most common neurodegenerative disorder, affects millions of people globally. There is no cure, and its prevalence will double by 2030. In recent years, numerous causative genes and risk factors for Parkinson's disease have been identified and more than half appear to function at the synapse. Subtle synaptic defects are thought to precede blunt neuronal death, but the mechanisms that are dysfunctional at synapses are only now being unraveled. Here, we review recent work and propose a model where different Parkinson proteins interact in a cell compartment-specific manner at the synapse where these proteins regulate endocytosis and autophagy. While this field is only recently emerging, the work suggests that the loss of synaptic homeostasis may contribute to neurodegeneration and is a key player in Parkinson's disease.

Colloidal stability as a determinant of nanoparticle behavior in the brain

Drug delivery to the brain is challenging due to a highly regulated blood-brain barrier (BBB) and a complex brain microenvironment. Nanoparticles, due to their tailorability, provide promising platforms to enhance therapeutic delivery and achieve controlled release and disease-specific localization in the brain. However, we have yet to fully understand the complex interactions between nanoparticles and the biological environments in which they operate. It is important to perform a systematic study to characterize nanoparticle behavior as a function of ion composition, concentration, and pH in cerebrospinal fluid (CSF). These could alter nanoparticle biological identity and influence diffusive capability and cellular uptake. In this study, poly(ethylene glycol) (PEG)-coated and carboxyl-coated polystyrene (PS-PEG and PS-COOH respectively) nanoparticles (NPs) were used to evaluate the aggregation kinetics, colloidal stability, and diffusive capability of nanoparticles in conditions relevant to the brain microenvironment. Size, surface charge, and surface coating were varied in a range of CSF ion concentrations and compositions, pH conditions, and temperatures. Small changes in calcium concentration and pH destabilize nanoparticles in CSF. However, PS-PEG NPs remain stable over a wider variety of conditions than PS-COOH NPs, and have higher diffusion capabilities in both agarose gels, an in vitro model of the brain microenvironment, and an organotypic brain tissue slice model. These results demonstrate the need for steric stabilization to maintain nanoparticle colloidal stability in a wide range of conditions. Importantly, colloidal stabilization allows for increased diffusive capability and can be used to predict diffusive behavior in the brain microenvironment.

Malfunctioning of Chaperone-Mediated Autophagy in Parkinson's Disease: Feats, Constraints, and Flaws of Modulators

Homeostatic regulation of class II programmed cell death/autophagy for the degradation and elimination of substandard organelles and defective proteins is decisive for the survival of dopaminergic neurons. Chaperone-mediated autophagy (CMA), one of the most highly dedicated self-sacrificing events, is accountable for the partial elimination of redundant soluble cytoplasmic proteins in Parkinson's disease (PD). CMA is characterized by the selective delivery of superfluous protein containing lysine-phenylalanine-glutamate-arginine-glutamine (KFERQ)/KFERQ-like motif to the lysosome through molecular chaperones, such as heat shock cognate-70 (Hsc-70). KFERQ/KFERQ-like motif present in the poor quality cytoplasmic substrate protein and Hsc-70 complex is recognized by a janitor protein, which is referred to as the lysosome-associated membrane protein-2A (LAMP-2A). This protein is known to facilitate an entry of substrate-chaperone complex in the lumen for hydrolytic cleavage of substrate and elimination of end-products. Impaired CMA is repeatedly blamed for an accumulation of surplus soluble proteins. However, it is still an enigma if CMA is a bonus or curse for PD. Case-control studies and cellular and animal models have deciphered the contribution of impaired CMA in PD. Current article updates the role of CMA in toxicant models and recapitulates the evidences that have highlighted a link between impaired CMA and PD. Although PD is an irreversible happening and CMA is a dual edging phenomenon, it is anticipated that fine-tuning of the latter encounters the former to a certain extent. Besides, the truth, embellishment, and propaganda regarding the issue are also emphasized in the final segment of the article.

Lysosome trafficking and signaling in health and neurodegenerative diseases

Lysosomes, single-membrane organelles defined by a uniquely strong acidic lumenal pH and high content of acid hydrolases, are the shared degradative compartments of the endocytic and autophagic pathways. These pathways, and especially lysosomes, are points of particular vulnerability in many neurodegenerative diseases. Beyond the role of lysosomes in substrate degradation, new findings have ascribed to lysosomes the leading role in sensing and responding to cellular nutrients, growth factors and cellular stress. This review aims to integrate recent concepts of basic lysosome biology and pathobiology as a basis for understanding neurodegenerative disease pathogenesis. Here, we discuss the newly recognized signaling functions of lysosomes and specific aspects of lysosome biology in neurons while re-visiting the classical defining criteria for lysosomes and the importance of preserving strict definitions. Our discussion emphasizes dynein-mediated axonal transport of maturing degradative organelles, with further consideration of their roles in synaptic function. We finally examine how distinctive underlying disturbances of lysosomes in various neurodegenerative diseases result in unique patterns of auto/endolysosomal mistrafficking. The rapidly emerging understanding of lysosomal trafficking and disruptions in lysosome signaling is providing valuable clues to new targets for disease-modifying therapies.

Alzheimer's disease and the autophagic-lysosomal system

Age-related neurodegenerative diseases are of critical concern to the general population and research/medical community due to their health impact and socioeconomic consequences. A feature of most, if not all, neurodegenerative disorders is the presence of proteinopathies, in which misfolded or conformationally altered proteins drive disease progression and are often used as a primary neuropathological marker of disease. In particular, Alzheimer's disease (AD) is characterized by abnormal accumulation of protein aggregates, primarily extracellular plaques composed of the Aβ peptide and intracellular tangles comprised of the tau protein, both of which may indicate a primary defect in protein clearance. Protein degradation is a key cellular mechanism for protein homeostasis and is essential for cell survival but is disrupted in neurodegenerative diseases. Dysregulation in proteolytic pathways - mainly the autophagic-lysosomal system (A-LS) and the ubiquitin-proteasome system (UPS) - has been increasingly associated with proteinopathies in neurodegenerative diseases. Here we review the role of dysfunctional autophagy underlying AD-related proteinopathy and discuss how to model this aspect of disease, as well as summarize recent advances in translational strategies for targeted A-LS dysfunction in AD.

From the Cover: Potentiation of Drug-Induced Phospholipidosis In Vitro through PEGlyated Graphene Oxide as the Nanocarrier

Cationic amphiphilic drugs (CADs) are small molecules that can induce phospholipidosis (PLD), causing the intracellular accumulation of phospholipid in the lamellar bodies. Nanotechnology based drug delivery systems have been used widely, while it is unknown if drug-induced PLD (DIP) can be potentiated through drug retention by indigestible nanocarriers. Due to the high drug loading capacity of graphene, we investigated if PEGylated graphene oxide (PEG-GO) loaded with CAD could potentiate DIP. Tamoxifen induced the accumulation of NBD-PE, a fluorescence labeled phospholipid in human hepatoma HepG2 cells, while PEG-GO loaded with tamoxifen (PEG-GO/tamoxifen) further potentiated PLD. PEG-GO/tamoxifen induced more gene expression of PLD marker than tamoxifen alone. PEG-GO enhanced DIP was also observed for other CAD, indicating that nanocarrier potentiated DIP could be universal. More lamellar bodies were observed in PEG-GO/tamoxifen treated cells than tamoxifen alone by transmission electron microscopy. When compared with tamoxifen alone, PEG-GO/tamoxifen showed a delayed but potent PLD. In addition, the retarded PLD recovery by PEG-GO/tamoxifen indicated that the reversibility of DIP was interfered. Confocal microscopy revealed the increased number of lysosomes, greater expression of lysosomal associated membrane protein 2 (LAMP2) (a PLD marker), and an increase in the co-localization between lysosome/LAMP2 and NBD-PE by PEG-GO/tamoxifen rather than tamoxifen alone. Finally, we found that PEG-GO or/and tamoxifen-induced PLD seemed to have no correlation with autophagy. This research suggests pharmaceutical companies and regulatory agencies that if nanoparticles are used as the vectors for drug delivery, the adverse drug effects may be further potentiated probably through the long-term accumulation of nanocarriers.

Dysfunction of autophagy and endosomal-lysosomal pathways: Roles in pathogenesis of Down syndrome and Alzheimer's Disease

Individuals with Down syndrome (DS) have an increased risk of early-onset Alzheimer's Disease (AD), largely owing to a triplication of the APP gene, located on chromosome 21. In DS and AD, defects in endocytosis and lysosomal function appear at the earliest stages of disease development and progress to widespread failure of intraneuronal waste clearance, neuritic dystrophy and neuronal cell death. The same genetic factors that cause or increase AD risk are also direct causes of endosomal-lysosomal dysfunction, underscoring the essential partnership between this dysfunction and APP metabolites in AD pathogenesis. The appearance of APP-dependent endosome anomalies in DS beginning in infancy and evolving into the full range of AD-related endosomal-lysosomal deficits provides a unique opportunity to characterize the earliest pathobiology of AD preceding the classical neuropathological hallmarks. Facilitating this characterization is the authentic recapitulation of this endosomal pathobiology in peripheral cells from people with DS and in trisomy mouse models. Here, we review current research on endocytic-lysosomal dysfunction in DS and AD, the emerging importance of APP/βCTF in initiating this dysfunction, and the potential roles of additional trisomy 21 genes in accelerating endosomal-lysosomal impairment in DS. Collectively, these studies underscore the growing value of investigating DS to probe the biological origins of AD as well as to understand and ameliorate the developmental disability of DS.

Mini-review on initiatives to interfere with the propagation and clearance of alpha-synuclein in Parkinson's disease

In this mini-review, we summarize recent findings relating to the prion-like propagation of α-synuclein (α-syn) and the development of novel therapeutic strategies to target synucleinopathy in Parkinson’s disease (PD). We link the Braak’s staging hypothesis of PD with the recent evidence from in-vivo and in-vitro studies for the prion-like cell-to-cell propagation of α-syn (via exocytosis and endocytosis). The classical accumulation of aggregated α-syn in PD may result from an increased production or a failure in the mechanisms of clearance of α-syn. We discuss novel agents, currently in clinical trial for PD including the ones that impact the aggregation of α-syn and others that interfere with α-syn endocytosis as a means to target the progression of the disease.

The physiological determinants of drug-induced lysosomal stress resistance

Many weakly basic, lipophilic drugs accumulate in lysosomes and exert complex, pleiotropic effects on organelle structure and function. Thus, modeling how perturbations of lysosomal physiology affect the maintenance of lysosomal ion homeostasis is necessary to elucidate the key factors which determine the toxicological effects of lysosomotropic agents, in a cell-type dependent manner. Accordingly, a physiologically-based mathematical modeling and simulation approach was used to explore the dynamic, multi-parameter phenomenon of lysosomal stress. With this approach, parameters that are either directly involved in lysosomal ion transportation or lysosomal morphology were transiently altered to investigate their downstream effects on lysosomal physiology reflected by the changes they induce in lysosomal pH, chloride, and membrane potential. In addition, combinations of parameters were simultaneously altered to assess which parameter was most critical for recovery of normal lysosomal physiology. Lastly, to explore the relationship between organelle morphology and induced stress, we investigated the effects of parameters controlling organelle geometry on the restoration of normal lysosomal physiology following a transient perturbation. Collectively, our results indicate a key, interdependent role of V-ATPase number and membrane proton permeability in lysosomal stress tolerance. This suggests that the cell-type dependent regulation of V-ATPase subunit expression and turnover, together with the proton permeability properties of the lysosomal membrane, is critical to understand the differential sensitivity or resistance of different cell types to the toxic effects of lysosomotropic drugs.

Modulation of mTOR signaling as a strategy for the treatment of Pompe disease

Mechanistic target of rapamycin (mTOR) coordinates biosynthetic and catabolic processes in response to multiple extracellular and intracellular signals including growth factors and nutrients. This serine/threonine kinase has long been known as a critical regulator of muscle mass. The recent finding that the decision regarding its activation/inactivation takes place at the lysosome undeniably brings mTOR into the field of lysosomal storage diseases. In this study, we have examined the involvement of the mTOR pathway in the pathophysiology of a severe muscle wasting condition, Pompe disease, caused by excessive accumulation of lysosomal glycogen. Here, we report the dysregulation of mTOR signaling in the diseased muscle cells, and we focus on potential sites for therapeutic intervention. Reactivation of mTOR in the whole muscle of Pompe mice by TSC knockdown resulted in the reversal of atrophy and a striking removal of autophagic buildup. Of particular interest, we found that the aberrant mTOR signaling can be reversed by arginine. This finding can be translated into the clinic and may become a paradigm for targeted therapy in lysosomal, metabolic, and neuromuscular diseases.

Central neurotoxicity induced by the instillation of ZnO and TiO2 nanoparticles through the taste nerve pathway

AIM

To explore whether nanoparticles (NPs) can be transported into the CNS via the taste nerve pathway.

MATERIALS & METHODS

ZnO and TiO₂ NPs were tongue-instilled to male Wistar rats. Toxicity was assessed by Zn/Ti biodistribution, histopathological examination, oxidative stress assay, quantitative reverse-transcriptase PCR analysis, learning and memory capabilities.

RESULTS

ZnO NPs and TiO₂ NPs significantly deposited in the nerves and brain, respectively. The histopathological examination indicated a slight injury in the cerebral cortex and hippocampus. Ultrastructural changes and an imbalanced oxidative stress were observed. The Morris water maze results showed that the learning and memory of rats were impaired.

CONCLUSION

NPs can enter the CNS via the taste nerve translocation pathway and induce a certain adverse effect.

Effects of mild running on substantia nigra during early neurodegeneration

Moderate physical exercise acts at molecular and behavioural levels, such as interfering in neuroplasticity, cell death, neurogenesis, cognition and motor functions. Therefore, the aim of this study is to analyse the cellular effects of moderate treadmill running upon substantia nigra during early neurodegeneration. Aged male Lewis rats (9-month-old) were exposed to rotenone 1mg/kg/day (8 weeks) and 6 weeks of moderate treadmill running, beginning 4 weeks after rotenone exposure. Substantia nigra was extracted and submitted to proteasome and antioxidant enzymes activities, hydrogen peroxide levels and Western blot to evaluate tyrosine hydroxylase (TH), alpha-synuclein, Tom-20, PINK1, TrkB, SLP1, CRMP-2, Rab-27b, LC3II and Beclin-1 level. It was demonstrated that moderate treadmill running, practiced during early neurodegeneration, prevented the increase of alpha-synuclein and maintained the levels of TH unaltered in substantia nigra of aged rats. Physical exercise also stimulated autophagy and prevented impairment of mitophagy, but decreased proteasome activity in rotenone-exposed aged rats. Physical activity also prevented H₂O₂ increase during early neurodegeneration, although the involved mechanism remains to be elucidated. TrkB levels and its anterograde trafficking seem not to be influenced by moderate treadmill running. In conclusion, moderate physical training could prevent early neurodegeneration in substantia nigra through the improvement of autophagy and mitophagy.

Lysosomal enzyme replacement therapies: Historical development, clinical outcomes, and future perspectives

Lysosomes and lysosomal enzymes play a central role in numerous cellular processes, including cellular nutrition, recycling, signaling, defense, and cell death. Genetic deficiencies of lysosomal components, most commonly enzymes, are known as "lysosomal storage disorders" or "lysosomal diseases" (LDs) and lead to lysosomal dysfunction. LDs broadly affect peripheral organs and the central nervous system (CNS), debilitating patients and frequently causing fatality. Among other approaches, enzyme replacement therapy (ERT) has advanced to the clinic and represents a beneficial strategy for 8 out of the 50-60 known LDs. However, despite its value, current ERT suffers from several shortcomings, including various side effects, development of "resistance", and suboptimal delivery throughout the body, particularly to the CNS, lowering the therapeutic outcome and precluding the use of this strategy for a majority of LDs. This review offers an overview of the biomedical causes of LDs, their socio-medical relevance, treatment modalities and caveats, experimental alternatives, and future treatment perspectives.

N370S-GBA1 mutation causes lysosomal cholesterol accumulation in Parkinson's disease

BACKGROUND

Heterozygous mutations in the GBA1 gene, which encodes the lysosomal enzyme β-glucocerebrosidase-1, increase the risk of developing Parkinson's disease, although the underlying mechanisms remain unclear. The aim of this study was to explore the impact of the N370S-GBA1 mutation on cellular homeostasis and vulnerability in a patient-specific cellular model of PD.

METHODS

We isolated fibroblasts from 4 PD patients carrying the N370S/wild type GBA1 mutation and 6 controls to study the autophagy-lysosome pathway, endoplasmic reticulum stress, and Golgi apparatus structure by Western blot, immunofluorescence, LysoTracker and Filipin stainings, mRNA analysis, and electron microscopy. We evaluated cell vulnerability by apoptosis, reactive oxygen species and mitochondrial membrane potential with flow cytometry.

RESULTS

The N370S mutation produced a significant reduction in β-glucocerebrosidase-1 protein and enzyme activity and β-glucocerebrosidase-1 retention within the endoplasmic reticulum, which interrupted its traffic to the lysosome. This led to endoplasmic reticulum stress activation and triggered unfolded protein response and Golgi apparatus fragmentation. Furthermore, these alterations resulted in autophagosome and p62/SQSTM1 accumulation. This impaired autophagy was a result of dysfunctional lysosomes, indicated by multilamellar body accumulation probably caused by increased cholesterol, enlarged lysosomal mass, and reduced enzyme activity. This phenotype impaired the removal of damaged mitochondria and reactive oxygen species production and enhanced cell death.

CONCLUSIONS

Our results support a connection between the loss of β-glucocerebrosidase-1 function, cholesterol accumulation, and the disruption of cellular homeostasis in GBA1-PD. Our work reveals new insights into the cellular pathways underlying PD pathogenesis, providing evidence that GBA1-PD shares common features with lipid-storage diseases. © 2017 International Parkinson and Movement Disorder Society.

[Autophagy and brain: the case of neurodegenerative diseases]

The autophagy-lysosome system is an essential pathway to get rid of unwanted cellular components (proteins and organelles). The brain, and specifically neurons, are very sensitive to abnormalities of the proteome because altered proteins or damaged organelles cannot be diluted by cell division that does not occur in these cells. Most neurodegenerative disorders are characterized by accumulation of undegraded misfolded proteins and are currently associated with autophagy-lysosome dysfunctions. Recent studies have highlighted the modulation of this complex pathway as a putative therapeutic strategy. This review provides an update on the brain-related specificities and dysfunctions of this pathway and discusses the autophagy-based therapies couteracting neurodegeneration.

Misrouting of v-ATPase subunit V0a1 dysregulates lysosomal acidification in a neurodegenerative lysosomal storage disease model

Defective lysosomal acidification contributes to virtually all lysosomal storage disorders (LSDs) and to common neurodegenerative diseases like Alzheimer's and Parkinson's. Despite its fundamental importance, the mechanism(s) underlying this defect remains unclear. The v-ATPase, a multisubunit protein complex composed of cytosolic V1-sector and lysosomal membrane-anchored V0-sector, regulates lysosomal acidification. Mutations in the CLN1 gene, encoding PPT1, cause a devastating neurodegenerative LSD, INCL. Here we report that in Cln1^−/− mice, which mimic INCL, reduced v-ATPase activity correlates with elevated lysosomal pH. Moreover, v-ATPase subunit a1 of the V0 sector (V0a1) requires palmitoylation for interacting with adaptor protein-2 (AP-2) and AP-3, respectively, for trafficking to the lysosomal membrane. Notably, treatment of Cln1^−/− mice with a thioesterase (Ppt1)-mimetic, NtBuHA, ameliorated this defect. Our findings reveal an unanticipated role of Cln1 in regulating lysosomal targeting of V0a1 and suggest that varying factors adversely affecting v-ATPase function dysregulate lysosomal acidification in other LSDs and common neurodegenerative diseases.

Lysosomal acidification defects have been implicated in various neurodegenerative disorders. Bagh et al. show that the V0a1 subunit of v-ATPase requires palmitoylation for correct sorting and trafficking to the lysosome membrane, and that such a process is impaired in a mouse model of a devastating neurodegenerative lysosomal storage disease, INCL.

Therapeutic potential of autophagy-enhancing agents in Parkinson's disease

Converging evidence from genetic, pathological and experimental studies have increasingly suggested an important role for autophagy impairment in Parkinson’s Disease (PD). Genetic studies have identified mutations in genes encoding for components of the autophagy-lysosomal pathway (ALP), including glucosidase beta acid 1 (GBA1), that are associated with increased risk for developing PD. Observations in PD brain tissue suggest an aberrant regulation of autophagy associated with the aggregation of α-synuclein (α-syn). As autophagy is one of the main systems involved in the proteolytic degradation of α-syn, pharmacological enhancement of autophagy may be an attractive strategy to combat α-syn aggregation in PD. Here, we review the potential of autophagy enhancement as disease-modifying therapy in PD based on preclinical evidence. In particular, we provide an overview of the molecular regulation of autophagy and targets for pharmacological modulation within the ALP. In experimental models, beneficial effects on multiple pathological processes involved in PD, including α-syn aggregation, cell death, oxidative stress and mitochondrial dysfunction, have been demonstrated using the autophagy enhancers rapamycin and lithium. However, selectivity of these agents is limited, while upstream ALP signaling proteins are involved in many other pathways than autophagy. Broad stimulation of autophagy may therefore cause a wide spectrum of dose-dependent side-effects, suggesting that its clinical applicability is limited. However, recently developed agents selectively targeting core ALP components, including Transcription Factor EB (TFEB), lysosomes, GCase as well as chaperone-mediated autophagy regulators, exert more specific effects on molecular pathogenetic processes causing PD. To conclude, the targeted manipulation of downstream ALP components, rather than broad autophagy stimulation, may be an attractive strategy for the development of novel pharmacological therapies in PD. Further characterization of dysfunctional autophagy in different stages and molecular subtypes of PD in combination with the clinical translation of downstream autophagy regulation offers exciting new avenues for future drug development.

Analysis of Lysosomal pH by Flow Cytometry Using FITC-Dextran Loaded Cells

The acidic environment of the lysosomal lumen provides an optimal milieu for the acid hydrolases and is also essential for fusion/fission of endo-lysosomal compartments and sorting of cargo. Evidence suggests that maintaining lysosomal acidity is essential to avoid disease. In this chapter, we describe a protocol for analyzing the lysosomal pH in cultured cells using the fluorescent probe fluorescein isothiocyanate (FITC)-dextran together with a dual-emission ratiometric technique suitable for flow cytometry. Fluorescence-labeled dextran is endocytosed and accumulated in the lysosomal compartment. FITC shows a pH-dependent variation in fluorescence when analyzed at maximum emission wavelength and no variation when analyzing at the isosbestic point, thereby the ratio can be used to determine the lysosomal pH. A standard curve is obtained by equilibrating intralysosomal pH with extracellular pH using the ionophore nigericin. The protocol also includes information regarding procedures to induce lysosomal alkalinization and lysosomal membrane permeabilization.

Lysosomal Acidification in Cultured Astrocytes Using Nanoparticles

Following cellular engulfment, nanoparticles end up in the lysosomes, making them an ideal tool for modifying the lysosomal environment. Here, we describe how acidic nanoparticles can be used to lower the pH of lysosomes in cultured, primary astrocytes and thereby increase their degradation capacity. To guarantee that the cell culture is completely devoid of professional phagocytes, we isolate, expand, and differentiate neural stem cells from embryonic mouse cortex to achieve astrocytes for these experiments. Immunostainings with LAMP2-specific antibodies can be performed to verify the lysosomal localization of the nanoparticles, and the effect on lysosomal acidification can easily be followed with LysoTracker dye.

Lysosomal Re-acidification Prevents Lysosphingolipid-Induced Lysosomal Impairment and Cellular Toxicity

Neurodegenerative lysosomal storage disorders (LSDs) are severe and untreatable, and mechanisms underlying cellular dysfunction are poorly understood. We found that toxic lipids relevant to three different LSDs disrupt multiple lysosomal and other cellular functions. Unbiased drug discovery revealed several structurally distinct protective compounds, approved for other uses, that prevent lysosomal and cellular toxicities of these lipids. Toxic lipids and protective agents show unexpected convergence on control of lysosomal pH and re-acidification as a critical component of toxicity and protection. In twitcher mice (a model of Krabbe disease [KD]), a central nervous system (CNS)-penetrant protective agent rescued myelin and oligodendrocyte (OL) progenitors, improved motor behavior, and extended lifespan. Our studies reveal shared principles relevant to several LSDs, in which diverse cellular and biochemical disruptions appear to be secondary to disruption of lysosomal pH regulation by specific lipids. These studies also provide novel protective strategies that confer therapeutic benefits in a mouse model of a severe LSD.

Disorders of lysosomal acidification-The emerging role of v-ATPase in aging and neurodegenerative disease

Autophagy and endocytosis deliver unneeded cellular materials to lysosomes for degradation. Beyond processing cellular waste, lysosomes release metabolites and ions that serve signaling and nutrient sensing roles, linking the functions of the lysosome to various pathways for intracellular metabolism and nutrient homeostasis. Each of these lysosomal behaviors is influenced by the intraluminal pH of the lysosome, which is maintained in the low acidic range by a proton pump, the vacuolar ATPase (v-ATPase). New reports implicate altered v-ATPase activity and lysosomal pH dysregulation in cellular aging, longevity, and adult-onset neurodegenerative diseases, including forms of Parkinson disease and Alzheimer disease. Genetic defects of subunits composing the v-ATPase or v-ATPase-related proteins occur in an increasingly recognized group of familial neurodegenerative diseases. Here, we review the expanding roles of the v-ATPase complex as a platform regulating lysosomal hydrolysis and cellular homeostasis. We discuss the unique vulnerability of neurons to persistent low level lysosomal dysfunction and review recent clinical and experimental studies that link dysfunction of the v-ATPase complex to neurodegenerative diseases across the age spectrum.

Systems-level thinking for nanoparticle-mediated therapeutic delivery to neurological diseases

Neurological diseases account for 13% of the global burden of disease. As a result, treating these diseases costs $750 billion a year. Nanotechnology, which consists of small (~1-100 nm) but highly tailorable platforms, can provide significant opportunities for improving therapeutic delivery to the brain. Nanoparticles can increase drug solubility, overcome the blood-brain and brain penetration barriers, and provide timed release of a drug at a site of interest. Many researchers have successfully used nanotechnology to overcome individual barriers to therapeutic delivery to the brain, yet no platform has translated into a standard of care for any neurological disease. The challenge in translating nanotechnology platforms into clinical use for patients with neurological disease necessitates a new approach to: (1) collect information from the fields associated with understanding and treating brain diseases and (2) apply that information using scalable technologies in a clinically-relevant way. This approach requires systems-level thinking to integrate an understanding of biological barriers to therapeutic intervention in the brain with the engineering of nanoparticle material properties to overcome those barriers. To demonstrate how a systems perspective can tackle the challenge of treating neurological diseases using nanotechnology, this review will first present physiological barriers to drug delivery in the brain and common neurological disease hallmarks that influence these barriers. We will then analyze the design of nanotechnology platforms in preclinical in vivo efficacy studies for treatment of neurological disease, and map concepts for the interaction of nanoparticle physicochemical properties and pathophysiological hallmarks in the brain. WIREs Nanomed Nanobiotechnol 2017, 9:e1422. doi: 10.1002/wnan.1422 For further resources related to this article, please visit the WIREs website.

Targeting α-synuclein: Therapeutic options

The discovery of the central role of α-synuclein (αSyn) in the pathogenesis of Parkinson's disease (PD) has powered, in the last decade, the emergence of novel relevant models of this condition based on viral vector-mediated expression of the disease-causing protein or inoculation of toxic species of αSyn. Although the development of these powerful tools and models has provided considerable insights into the mechanisms underlying neurodegeneration in PD, it has also been translated into the expansion of the landscape of preclinical therapeutic strategies. Much attention is now brought to the proteotoxic mechanisms induced by αSyn and how to block them using strategies inspired by intrinsic cellular pathways such as the enhancement of cellular clearance by the lysosomal-autophagic system, through proteasome-mediated degradation or through immunization. The important effort undertaken by several laboratories and consortia to tackle these issues and identify novel targets warrants great promise for the discovery not only of neuroprotective approaches but also of restorative strategies for PD and other synucleinopathies. In this viewpoint, we summarize the latest advances in this new area of PD research and will discuss promising approaches and ongoing challenges. © 2016 International Parkinson and Movement Disorder Society.