SIRT2 interferes with autophagy-mediated degradation of protein aggregates in neuronal cells under proteasome inhibition

Abnormal protein aggregates have been suggested as a common pathogenesis of many neurodegenerative diseases. Two well-known protein degradation pathways are responsible for protein homeostasis by balancing protein biosynthesis and degradative processes: the ubiquitin–proteasome system (UPS) and autophagy-lysosomal system. UPS serves as the primary route for degradation of short-lived proteins, but large-size protein aggregates cannot be degraded by UPS. Autophagy is a unique cellular process that facilitates degradation of bulky protein aggregates by lysosome. Recent studies have demonstrated that autophagy plays a crucial role in the pathogenesis of neurodegenerative diseases characterized by abnormal protein accumulation, suggesting that regulation of autophagy may be a valuable therapeutic strategy for the treatment of various neurodegenerative diseases. Sirtuin-2 (SIRT2) is a class III histone deacetylase that is expressed abundantly in aging brain tissue. Here, we report that SIRT2 increases protein accumulation in murine cholinergic SN56 cells and human neuroblastoma SH-SY5Y cells under proteasome inhibition. Overexpression of SIRT2 inhibits lysosome-mediated autophagic turnover by interfering with aggresome formation and also makes cells more vulnerable to accumulated protein-mediated cytotoxicity by MG132 and amyloid beta. Moreover, MG132-induced accumulation of ubiquitinated proteins and p62 as well as cytotoxicity are attenuated in siRNA-mediated SIRT2-silencing cells. Taken together, these results suggest that regulation of SIRT2 could be a good therapeutic target for a range of neurodegenerative diseases by regulating autophagic flux.     

Lysosomal dysfunction in neurodegeneration: The role of ATP13A2/PARK9

Neuronal homeostasis and survival critically depend on an efficient autophagy-lysosomal degradation pathway, especially since neurons cannot reduce the concentration of misfolded proteins and damaged organelles by cell division. While increasing evidence implicates lysosomal dysfunction in the pathogenesis of neurodegenerative disorders, the molecular underpinnings of the role of lysosomes in neurodegeneration remain largely unknown. To this end, studies of neurodegenerative disorders caused by mutations in lysosomal proteins offer an opportunity to elucidate such mechanisms and potentially identify specific therapeutic targets. One of these disorders is Kufor-Rakeb syndrome, caused by mutations in the lysosomal protein ATP13A2/PARK9 and characterized by early-onset Parkinsonism, pyramidal degeneration and dementia. We found that loss of ATP13A2 function results in impaired lysosomal function and, consequently, accumulation of SNCA/α-synuclein and neurotoxicity. Our results suggest that targeting of ATP13A2 to lysosomes to enhance lysosomal function may result in neuroprotection in Kufor-Rakeb syndrome. From a broader perspective, these findings, together with other recent studies of lysosomal dysfunction in neurodegeneration, suggest that strategies to upregulate lysosomal function in neurons represent a promising therapeutic approach for neurodegenerative disorders.     

Lysosomal dysfunction in neurodegeneration

Neuronal homeostasis and survival critically depend on an efficient autophagy-lysosomal degradation pathway, especially since neurons cannot reduce the concentration of misfolded proteins and damaged organelles by cell division. While increasing evidence implicates lysosomal dysfunction in the pathogenesis of neurodegenerative disorders, the molecular underpinnings of the role of lysosomes in neurodegeneration remain largely unknown. To this end, studies of neurodegenerative disorders caused by mutations in lysosomal proteins offer an opportunity to elucidate such mechanisms and potentially identify specific therapeutic targets. One of these disorders is Kufor-Rakeb syndrome, caused by mutations in the lysosomal protein ATP13A2/PARK9 and characterized by early-onset Parkinsonism, pyramidal degeneration and dementia. We found that loss of ATP13A2 function results in impaired lysosomal function and, consequently, accumulation of SNCA/α-synuclein and neurotoxicity. Our results suggest that targeting of ATP13A2 to lysosomes to enhance lysosomal function may result in neuroprotection in Kufor-Rakeb syndrome. From a broader perspective, these findings, together with other recent studies of lysosomal dysfunction in neurodegeneration, suggest that strategies to upregulate lysosomal function in neurons represent a promising therapeutic approach for neurodegenerative disorders.     

Curcumin Suppresses Proliferation and Migration of MDA-MB-231 Breast Cancer Cells through Autophagy-Dependent Akt Degradation

Previous studies have evidenced that the anticancer potential of curcumin (diferuloylmethane), a main yellow bioactive compound from plant turmeric was mediated by interfering with PI3K/Akt signaling. However, the underlying molecular mechanism is still poorly understood. This study experimentally revealed that curcumin treatment reduced Akt protein expression in a dose- and time-dependent manner in MDA-MB-231 breast cancer cells, along with an activation of autophagy and suppression of ubiquitin-proteasome system (UPS) function. The curcumin-reduced Akt expression, cell proliferation, and migration were prevented by genetic and pharmacological inhibition of autophagy but not by UPS inhibition. Additionally, inactivation of AMPK by its specific inhibitor compound C or by target shRNA-mediated silencing attenuated curcumin-activated autophagy. Thus, these results indicate that curcumin-stimulated AMPK activity induces activation of the autophagy-lysosomal protein degradation pathway leading to Akt degradation and the subsequent suppression of proliferation and migration in breast cancer cell.     

PrPSc accumulation in neuronal plasma membranes links Notch-1 activation to dendritic degeneration in prion diseases

The pathological changes occurring in Parkinson's and several other neurodegenerative diseases are complex and poorly understood, but all clearly involve protein aggregation. Also frequently appearing in neurodegeneration is mitochondrial dysfunction which may precede, coincide or follow protein aggregation. These observations led to the concept that protein aggregation and mitochondrial dysfunction either arise from the same etiological factors or are interactive. Understanding the mechanisms and regulation of processes that lead to protein aggregation or mitochondrial dysfunction may therefore contribute to the design of better therapeutics. Clearance of protein aggregates and dysfunctional organelles is dependent on macroautophagy which is the process through which aged or damaged proteins and organelles are first degraded by the lysosome and then recycled. The macroautophagy-lysosomal pathway is essential for maintaining protein and energy homeostasis. Not surprisingly, failure of the lysosomal system has been implicated in diseases that have features of protein aggregation and mitochondrial dysfunction. This review summarizes 3 major topics: 1) the current understanding of Parkinson's disease pathogenesis in terms of accumulation of damaged proteins and reduction of cellular bioenergetics; 2) evolving insights into lysosomal function and biogenesis and the accumulating evidence that lysosomal dysfunction may cause or exacerbate Parkinsonian pathology and finally 3) the possibility that enhancing lysosomal function may provide a disease modifying therapy.     

Neurodegenerative lysosomal disorders: a continuum from development to late age

Neuronal survival requires continuous lysosomal turnover of cellular constituents delivered by autophagy and endocytosis. Primary lysosomal dysfunction in inherited congenital "lysosomal storage" disorders is well known to cause severe neurodegenerative phenotypes associated with accumulations of lysosomes and autophagic vacuoles (AVs). Recently, the number of inherited adult-onset neurodegenerative diseases caused by proteins that regulate protein sorting and degradation within the endocytic and autophagic pathways has grown considerably. In this Perspective, we classify a group of neurodegenerative diseases across the lifespan as disorders of lysosomal function, which feature extensive autophagic-endocytic-lysosomal neuropathology and may share mechanisms of neurodegeneration related to degradative failure and lysosomal destabilization. We highlight Alzheimer's disease as a disease within this group and discuss how each of the genes and other risk factors promoting this disease contribute to progressive lysosomal dysfunction and neuronal cell death.     

Gangliosides protection against lysosomal pathology of synucleinopathies

Gangliosides are abundantly expressed in the nervous system, and deregulated expression or activity of gangliosides is associated with the progression of various disorders, including lysosomal storage diseases, Guillian-Barre syndrome, and Alzheimer disease. By contrast, previous studies show that GM1 ganglioside may act in a protective manner in the drug (e.g., MPTP and 6-OHDA)-induced Parkinsonian models, although the precise mechanisms have not been well addressed. In our recent publication, dementia with Lewy bodies (DLB)-linked neuroblastoma cells were treated with D-Threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), an inhibitor of glycosyl ceramide synthetase. These PDMP-treated cells develop lysosomal diseases characterized by reduced lysosomal activity, enhanced lysosomal permeability and cytotoxicity. Furthermore, PDMP-mediated inhibition of autophagy-lysosomal pathway result in both accumulation of α-synuclein and mutant β-synuclein. Finally, these phenotypes are reversed by ganglioside treatment. Taken together, our results suggest that endogenous gangliosides may play a protective role against the lysosomal pathology of synucleinopathies.     

Snapin deficiency is associated with developmental defects of the central nervous system

The autophagy-lysosomal pathway is an intracellular degradation process essential for maintaining neuronal homoeostasis. Defects in this pathway have been directly linked to a growing number of neurodegenerative disorders. We recently revealed that Snapin plays a critical role in co-ordinating dynein-driven retrograde transport and late endosomal-lysosomal trafficking, thus maintaining efficient autophagy-lysosomal function. Deleting snapin in neurons impairs lysosomal proteolysis and reduces the clearance of autolysosomes. The role of the autophagy-lysosomal system in neuronal development is, however, largely uncharacterized. Here, we report that snapin deficiency leads to developmental defects in the central nervous system. Embryonic snapin-/- mouse brain showed reduced cortical plates and intermediate zone cell density, increased apoptotic death in the cortex and third ventricle, enhanced membrane-bound LC3-II staining associated with autophagic vacuoles and an accumulation of polyubiquitinated proteins in the cortex and hippocampus. Thus our results provide in vivo evidence for the essential role of late endocytic transport and autophagy-lysosomal function in maintaining neuronal survival and development of the mammalian central nervous system. In addition, our study supports the existence of a functional interplay between the autophagy-lysosome and ubiquitin-proteasome systems in the protein quality-control process.     

The Protein Quality Control Machinery Regulates Its Misassembled Proteasome Subunits

Cellular toxicity introduced by protein misfolding threatens cell fitness and viability. Failure to eliminate these polypeptides is associated with various aggregation diseases. In eukaryotes, the ubiquitin proteasome system (UPS) plays a vital role in protein quality control (PQC), by selectively targeting misfolded proteins for degradation. While the assembly of the proteasome can be naturally impaired by many factors, the regulatory pathways that mediate the sorting and elimination of misassembled proteasomal subunits are poorly understood. Here, we reveal how the dysfunctional proteasome is controlled by the PQC machinery. We found that among the multilayered quality control mechanisms, UPS mediated degradation of its own misassembled subunits is the favored pathway. We also demonstrated that the Hsp42 chaperone mediates an alternative pathway, the accumulation of these subunits in cytoprotective compartments. Thus, we show that proteasome homeostasis is controlled through probing the level of proteasome assembly, and the interplay between UPS mediated degradation or their sorting into distinct cellular compartments.     

The Ubiquitin–Proteasome System in Retinal Health and Disease

The ubiquitin–proteasome system (UPS) is the main intracellular pathway for modulated protein turnover, playing an important role in the maintenance of cellular homeostasis. It also exerts a protein quality control through degradation of oxidized, mutant, denatured, or misfolded proteins and is involved in many biological processes where protein level regulation is necessary. This system allows the cell to modulate its protein expression pattern in response to changing physiological conditions and provides a critical protective role in health and disease. Impairments of UPS function in the central nervous system (CNS) underlie an increasing number of genetic and idiopathic diseases, many of which affect the retina. Current knowledge on the UPS composition and function in this tissue, however, is scarce and dispersed. This review focuses on UPS elements reported in the retina, including ubiquitinating and deubiquitinating enzymes (DUBs), and alternative proteasome assemblies. Known and inferred roles of protein ubiquitination, and of the related, SUMO conjugation (SUMOylation) process, in normal retinal development and adult homeostasis are addressed, including modulation of the visual cycle and response to retinal stress and injury. Additionally, the relationship between UPS dysfunction and human neurodegenerative disorders affecting the retina, including Alzheimer's, Parkinson's, and Huntington's diseases, are dealt with, together with numerous instances of retina-specific illnesses with UPS involvement, such as retinitis pigmentosa, macular degenerations, glaucoma, diabetic retinopathy (DR), and aging-related impairments. This information, though still basic and limited, constitutes a suitable framework to be expanded in incoming years and should prove orientative toward future therapy design targeting sight-affecting diseases with a UPS underlying basis.     

Fighting disease by selective autophagy of aggregate-prone proteins

Ubiquitinated protein aggregates are hallmarks of a range of human diseases, including neurodegenerative, liver and muscle disorders. These protein aggregates are typically positive for the autophagy receptor p62. Whereas the ubiquitin-proteasome system (UPS) degrades shortlived and misfolded ubiquitinated proteins that are small enough to enter the narrow pore of the barrel-shaped proteasome, the lysosomal pathway of autophagy can degrade larger structures including entire organelles or protein aggregates. This degradation requires autophagy receptors that link the cargo with the molecular machinery of autophagy and is enhanced by certain posttranslational modifications of the cargo. In this review we focus on how autophagy clears aggregate-prone proteins and the relevance of this process to protein aggregate associated diseases.     

Expression of mutant or cytosolic PrP in transgenic mice and cells is not associated with endoplasmic reticulum stress or proteasome dysfunction

The cellular pathways activated by mutant prion protein (PrP) in genetic prion diseases, ultimately leading to neuronal dysfunction and degeneration, are not known. Several mutant PrPs misfold in the early secretory pathway and reside longer in the endoplasmic reticulum (ER) possibly stimulating ER stress-related pathogenic mechanisms. To investigate whether mutant PrP induced maladaptive responses, we checked key elements of the unfolded protein response (UPR) in transgenic mice, primary neurons and transfected cells expressing two different mutant PrPs. Because ER stress favors the formation of untranslocated PrP that might aggregate in the cytosol and impair proteasome function, we also measured the activity of the ubiquitin proteasome system (UPS). Molecular, biochemical and immunohistochemical analyses found no increase in the expression of UPR-regulated genes, such as Grp78/Bip, CHOP/GADD153, or ER stress-dependent splicing of the mRNA encoding the X-box-binding protein 1. No alterations in UPS activity were detected in mutant mouse brains and primary neurons using the Ub(G76V)-GFP reporter and a new fluorogenic peptide for monitoring proteasomal proteolytic activity in vivo. Finally, there was no loss of proteasome function in neurons in which endogenous PrP was forced to accumulate in the cytosol by inhibiting cotranslational translocation. These results indicate that neither ER stress, nor perturbation of proteasome activity plays a major pathogenic role in prion diseases.     

Regulation of apoptosis by E3 ubiquitin ligases in ubiquitin proteasome system

Apoptosis is an organised ATP‐dependent programmed cell death that organisms have evolved to maintain homoeostatic cell numbers and eliminate unnecessary or unhealthy cells from the system. Dysregulation of apoptosis can have serious manifestations culminating into various diseases, especially cancer. Accurate control of apoptosis requires regulation of a wide range of growth enhancing as well as anti‐oncogenic factors. Appropriate regulation of magnitude and temporal expression of key proteins is vital to maintain functional apoptotic signalling. Controlled protein turnover is thus critical to the unhindered operation of the apoptotic machinery, disruption of which can have severe consequences, foremost being oncogenic transformation of cells. The ubiquitin proteasome system (UPS) is one such major cellular pathway that maintains homoeostatic protein levels. Recent studies have found interesting links between these two fundamental cellular processes, wherein UPS depending on the cue can either inhibit or promote apoptosis. A diverse range of E3 ligases are involved in regulating the turnover of key proteins of the apoptotic pathway. This review summarises an overview of key E3 ubiquitin ligases involved in the regulation of the fundamental proteins involved in apoptosis, linking UPS to apoptosis and attempts to emphasize the significance of this relationship in context of cancer.     

Inhibition of Protein Deubiquitination by PR-619 Activates the Autophagic Pathway in OLN-t40 Oligodendroglial Cells

Protein aggregate formation may be the result of an impairment of the protein quality control system, e.g., the ubiquitin proteasome system (UPS) and the lysosomal autophagic pathway. For proteasomal degradation, proteins need to be covalently modified by ubiquitin and deubiquitinated before the substrates are proteolytically degraded. Deubiquitination is performed by a large family of proteases, the deubiquitinating enzymes (DUBs). DUBs display a variety of functions and their inhibition may have pathological consequences. Using the broad specificity DUB inhibitor PR-619 we previously have shown that DUB inhibition leads to an overload of ubiquitinated proteins, to protein aggregate formation and subsequent inhibition of the UPS. This study was undertaken to investigate whether PR-619 modulates autophagic functions to possibly compensate the failure of the proteasomal system. Using the oligodendroglial cell line OLN-t40 and a new oligodendroglial cell line stably expressing GFP-LC3, we show that DUB inhibition leads to the activation of autophagy and to the recruitment of LC3 and of the ubiquitin binding protein p62 to the forming aggresomes without impairing the autophagic flux. Furthermore, PR-619 induced the transport of lysosomes to the forming aggregates in a process requiring an intact microtubule network. Further stimulation of autophagy by rapamycin did not prevent PR-619 aggregate formation but rather exerted cytotoxic effects. Hence, inhibition of DUBs by PR-619 activated the autophagic pathway supporting the hypothesis that the UPS and the autophagy–lysosomal pathway are closely linked together.     

An insight into the mechanistic role of p53-mediated autophagy induction in response to proteasomal inhibition-induced neurotoxicity

The ubiquitin-proteasome system (UPS) and the autophagy-lysosomal pathway (ALP) are the two most important components of cellular mechanisms for protein degradation. In the present study we investigated the functional relationship of the two systems and the interactional role of p53 in vitro. Our study showed that the proteasome inhibitor lactacystin induced an increase in p53 level and autophagy activity, whereas inhibition of p53 by pifithrin-α or small interference RNA (siRNA) of p53 attenuated the autophagy induction and increased protein aggregation. Furthermore, we found that the pretreatment with the autophagy inhibitor 3-methyladenine or Beclin 1 siRNA further activated p53 and its downstream apoptotic pathways, while the autophagy inducer rapamycin showed the opposite effects. Moreover, we demonstrated that rapamycin pretreatment increased tyrosine hydroxylase (TH) protein level in dopamine (DA) neurons, which was associated with its induction of autophagy to degrade aggregated proteins. Our results suggest that p53 can mediate proteasomal inhibition-induced autophagy enhancement which in turn can partially block p53 or its downstream mitochondria-dependent apoptotic pathways. Further autophagy induction with rapamycin protects DA neurons from lactacystin-mediated cell death by downregulating p53 and its related apoptotic pathways and by inducing autophagy to degrade aggregated proteins. Therefore, rapamycin may be a promising drug for protection against neuronal injury relevant to Parkinson’s disease (PD). Our studies thus provide a mechanistic insight into the functional link between the two protein degradation systems.     

泛素化在调控铁死亡中的作用

铁死亡是一种由脂质过氧化驱动的铁依赖性的新的细胞死亡方式,越来越多的证据表明,铁死亡与各种病理状态有关,如神经退行性疾病、糖尿病肾病、癌症等,脂质过氧化驱动的铁死亡可能促进或抑制这些疾病的发生发展,细胞中抗氧化系统通过抑制脂质过氧化在抵抗铁死亡过程中发挥着重要作用。铁死亡的关键通路有以SLC7A11-GPX4为关键分子的氨基酸代谢通路、以铁蛋白或转铁蛋白为主的铁代谢通路,以及脂质代谢通路。铁死亡的发生受到细胞内蛋白质的调节,这些蛋白质会发生各种翻译后修饰,包括泛素化修饰。泛素-蛋白酶体系统（ubiquitin-proteasome system,UPS）是细胞内主要降解系统之一,通过酶促级联反应催化泛素分子标记待降解蛋白,随后由蛋白酶体识别并降解目标蛋白质。UPS根据其降解底物的不同在调节铁死亡的反应中发挥双重作用。UPS通过促进铁死亡关键分子（如SLC7A11、GPX4、GSH）以及抗氧化系统成分（如NRF2）的泛素化降解从而促进铁死亡,也可以通过促进脂质代谢通路中相关分子（如ACSL4、ALOX15）的泛素化降解从而抑制铁死亡。本综述介绍泛素化修饰在调控铁死亡进程中作用的最新研究进展,总结了已发表的关于E3泛素连接酶和去泛素酶调控铁死亡的研究,归纳了泛素连接酶、去泛素酶调控铁死亡的作用靶点,有助于确定人类疾病中新的预后指标,为这些疾病提供潜在的治疗策略。     

HMGB1 as an autophagy sensor in oxidative stress

High mobility group box 1 (HMGB1) is a DNA-binding nuclear protein, actively released following cytokine stimulation as well as passively during cell injury and death. Autophagy is a tightly regulated cellular stress pathway involving the lysosomal degradation of cytoplasmic organelles or proteins. Organisms respond to oxidative injury by orchestrating stress responses such as autophagy to prevent further damage. Recently, we reported that HMGB1 is an autophagy sensor in the presence of oxidative stress. Hydrogen peroxide (H 2O 2) and loss of superoxide dismutase 1 (SOD1)-mediated oxidative stress promotes cytosolic HMGB1 expression and extracellular release. Inhibition of HMGB1 release or loss of HMGB1 decreases the number of autolysosomes and autophagic flux in human and mouse cell lines under conditions of oxidative stress. These findings provide insight into how HMGB1, a damage associated molecular pattern (DAMP), triggers autophagy as defense mechanism under conditions of cellular stress.     

Autophagy Coupling Interplay: Can Improve Cellular Repair and Aging?

Regular protein synthesis is a needful and complex task for a healthy cell. Improper folding leads to the deposition of misfolded proteins in cells. Autophagy and ubiquitin–proteasome system (UPS) are the conserved intracellular degradation processes of eukaryotic cells. How exactly these two pathways cross talk to each other is unclear. We do not know how the impairment of autophagy or UPS leads to the disturbance in cellular homeostasis and contribute into cellular aging and neurodegeneration. Here in this review, we will focus on the functional interconnections of autophagy and UPS, and why their loss of function results in abnormal aggregation of misfolded proteotoxic species in cells. Finally, we enumerate and discuss the crucial inducers of autophagy pathways and elaborate their intersection steps, which have been considered to be advantageous in aging linked with the abnormal protein aggregation. The final goal of this review is to improve our current understanding about multifaceted properties and interactions of autophagy and UPS, which may provide new insights to identify novel therapeutic strategies for aging and neurodegenerative diseases.