Intracellular degradation of misfolded proteins in polyglutamine neurodegenerative diseases

A number of neurodegenerative diseases, including Alzheimer's, Parkinson's, and polyglutamine diseases, are characterized by the age-dependent formation of intracellular protein aggregates and neurodegeneration. Although there is some debate surrounding the role of these aggregates in neurotoxicity, the formation of aggregates is known to reflect the accumulation of misfolded and toxic proteins. The degradation of misfolded proteins occurs mainly via the ubiquitin-proteasome and autophagy pathways. In neuronal cells, polyglutamine protein inclusions are present predominantly in the nucleus, which is not accessible to autophagy. It remains unclear how the ubiquitin-proteasomal and autophagy pathways remove misfolded proteins in the different subcellular regions of neurons, where disease proteins become misfolded and aggregated in an age-dependent manner. Here we discuss the key findings to date about the roles of the ubiquitin-proteasome system and autophagy in polyglutamine diseases. Understanding how these two pathways function to clear mutant polyglutamine proteins will further the development of effective treatments for polyglutamine and other neurodegenerative diseases.     

The role of the ubiquitin-proteasomal pathway in Parkinson's disease and other neurodegenerative disorders

A unifying feature of neurodegenerative diseases is the abnormal accumulation and processing of mutant or damaged intra- and extracellular proteins; this leads to selective neuronal vulnerability and dysfunction. The ubiquitin-proteasomal pathway (UPP) is poised to play a central role in the processing of damaged and toxic proteins by ubiquitin-dependent proteolysis. The UPP can be overwhelmed in several neurodegenerative diseases. This results in the accumulation of toxic proteins and the formation of inclusions, and ultimately to neuronal dysfunction and cell death. Further analysis of the cellular and molecular mechanisms by which the UPP influences the detoxification of damaged and toxic proteins in neurodegenerative diseases could provide novel concepts and targets for the treatment and understanding of the pathogenesis of these devastating disorders.     

Taking out the garbage: cathepsin D and calcineurin in neurodegeneration

Cellular homeostasis requires a tightly controlled balance between protein synthesis, folding and degradation. Especially long-lived, post-mitotic cells such as neurons depend on an efficient proteostasis system to maintain cellular health over decades. Thus, a functional decline of processes contributing to protein degradation such as autophagy and general lysosomal proteolytic capacity is connected to several age-associated neurodegenerative disorders, including Parkinson's, Alzheimer's and Huntington's diseases. These so called proteinopathies are characterized by the accumulation and misfolding of distinct proteins, subsequently driving cellular demise. We recently linked efficient lysosomal protein breakdown via the protease cathepsin D to the Ca~(2+)/calmodulin-dependent phosphatase calcineurin. In a yeast model for Parkinson's disease, functional calcineurin was required for proper trafficking of cathepsin D to the lysosome and for recycling of its endosomal sorting receptor to allow further rounds of shuttling. Here, we discuss these findings in relation to present knowledge about the involvement of cathepsin D in proteinopathies in general and a possible connection between this protease, calcineurin signalling and endosomal sorting in particular. As dysregulation of Ca~(2+) homeostasis as well as lysosomal impairment is connected to a plethora of neurodegenerative disorders, this novel interplay might very well impact pathologies beyond Parkinson's disease.     

Proteinaceous intracellular inclusions in neurodegenerative disorders

Neurodegenerative disorders are characterized by accumulation of "toxic", pathologic proteins in brain cells. Mutations in genes coding these proteins result in conformational disturbances of the protein structure and their accumulation and aggregation manifesting at the level of light microscope as various intracellular inclusions. This paper is an attempt of approach cellular mechanisms underlying neurodegenerative disorders with special attention to mechanisms of protein elimination.     

Autophagy and neurodegenerative disorders

Accumulation of aberrant proteins and inclusion bodies are hallmarks in most neurodegenerative diseases. Consequently, these aggregates within neurons lead to toxic effects, overproduction of reactive oxygen species and oxidative stress. Autophagy is a significant intracellular mechanism that removes damaged organelles and misfolded proteins in order to maintain cell homeostasis. Excessive or insufficient autophagic activity in neurons leads to altered homeostasis and influences their survival rate, causing neurodegeneration. The review article provides an update of the role of autophagic process in representative chronic and acute neurodegenerative disorders.     

The neuroprotective effect of the GSK-3β inhibitor and influence on the extrinsic apoptosis in the ALS transgenic mice

The age-related neurodegenerative disorders such as Alzheimer's, Parkinson's, and Huntington's diseases are characterized by the abnormal accumulation or aggregation of proteins. Advanced glycation end products (AGEs) are proteins or lipids that become glycated after exposure to sugars. The formation of AGEs promotes the deposition of proteins due to the protease resistant crosslinking between the peptides and proteins. Several proteins implicated in neurodegenerative diseases such as amyloid β, tau, α-synuclein, and prions are glycated and the extent of glycation is correlated with the pathologies of the patients. These data suggest that AGEs contribute to the development of neurodegenerative diseases. In this review we summarize recent advances on the investigation of the roles of AGEs in neurodegenerative diseases, with special focus on Alzheimer's and Parkinson's diseases. It is clear that AGEs modification triggers the abnormal deposition and accumulation of the modified proteins, which in turn sustain the local oxidative stress and inflammatory response, eventually leading to the pathological and clinical aspects of neurodegenerative diseases. Further characterization of the molecular mechanisms responsible for AGEs mediated neurotoxicity will provide important clues on the development of novel strategies for the prevention and treatment of neurodegenerative diseases.     

Control of autophagy as a therapy for neurodegenerative disease

Autophagy is an intracellular degradation process that clears long-lived proteins and organelles from the cytoplasm. It involves the formation of double-membraned structures called autophagosomes that can engulf portions of cytoplasm containing oligomeric protein complexes and organelles, such as mitochondria. Autophagosomes fuse with lysosomes and their contents then are degraded. Failure of autophagy in neurons can result in the accumulation of aggregate-prone proteins and neurodegeneration. Pharmacological induction of autophagy can enhance the clearance of intracytoplasmic aggregate-prone proteins, such as mutant forms of huntingtin, and ameliorate pathology in cell and animal models of neurodegenerative diseases. In this Review, the autophagic machinery and the signaling pathways that regulate the induction of autophagy are described. The ways in which dysfunctions at multiple stages in the autophagic pathways contribute to numerous neurological disorders are highlighted through the use of examples of Mendelian and complex conditions, including Alzheimer disease, Parkinson disease and forms of motor neuron disease. The different ways in which autophagic pathways might be manipulated for the therapeutic benefit of patients with neurodegenerative disorders are also considered.     

Autophagy in neurodegenerative disorders: pathogenic roles and therapeutic implications

Autophagy is a highly conserved intracellular pathway involved in the elimination of proteins and organelles by lysosomes. Known originally as an adaptive response to nutrient deprivation in mitotic cells, autophagy is now recognized as an arbiter of neuronal survival and death decisions in neurodegenerative diseases. Studies using postmortem human tissue, genetic and toxin-induced animal and cellular models indicate that many of the etiological factors associated with neurodegenerative disorders can perturb the autophagy process. Emerging data support the view that dysregulation of autophagy might play a critical role in the pathogenesis of neurodegenerative disorders. In this review, we highlight the pathophysiological roles of autophagy and its potential therapeutic implications in debilitating neurodegenerative disorders, including amyotrophic lateral sclerosis and Alzheimer's, Parkinson's and Huntington's diseases.     

Autophagy and misfolded proteins in neurodegeneration

The accumulation of misfolded proteins in insoluble aggregates within the neuronal cytoplasm is one of the common pathological hallmarks of most adult-onset human neurodegenerative diseases. The clearance of these misfolded proteins may represent a promising therapeutic strategy in these diseases. The two main routes for intracellular protein degradation are the ubiquitin-proteasome and the autophagy-lysosome pathways. In this review, we will focus on the autophagic pathway, by providing some examples of how impairment at different steps in this degradation pathway is related to different neurodegenerative diseases. We will also consider that upregulating autophagy may be useful in the treatment of some of these diseases. Finally, we discuss how antioxidants, which have been considered to be beneficial in neurodegenerative diseases, can block autophagy, thus potentially compromising their therapeutic potential.     

Autophagy in the central nervous system: implications for neurodegenerative disorders

The autophagy-lysosomal pathway is a major proteolytic pathway that in mammalian systems mainly comprises of macroautophagy and chaperone-mediated autophagy. The former is relatively non-selective and involves bulk degradation of proteins and organelles, whereas the latter is selective for certain cytosolic proteins. These autophagy pathways are important in development, differentiation, cellular remodeling and survival during nutrient starvation. Autophagy is crucial for neuronal homeostasis and acts as a local housekeeping process, since neurons are post-mitotic cells and require effective protein degradation to prevent accumulation of toxic aggregates. A growing body of evidence now suggests that dysfunction of autophagy causes accumulation of abnormal proteins and/or damaged organelles. Such accumulation has been linked to synaptic dysfunction, cellular stress and neuronal death. Abnormal autophagy may be involved in the pathology of both chronic nervous system disorders, such as proteinopathies (Alzheimer's, Parkinson's, Huntington's disease) and acute brain injuries. Although autophagy is generally beneficial, its aberrant activation may also exert a detrimental role in neurological diseases depending on the environment and the insult, leading to autophagic neuronal death. In this review we summarize the current knowledge regarding the role of autophagy-lysosomal pathway in the central nervous system and discuss the implication of autophagy dysregulation in human neurological diseases and animal models.     

Protein misfolding in neurodegenerative diseases

A common pathogenic mechanism shared by diverse neurodegenerative disorders, like Alzheimer's disease, Parkinson's disease, Huntington's disease and transmissible spongiform encephalopathies, may be altered protein homeostasis leading to protein misfolding and aggregation of a wide variety of different proteins in the form of insoluble fibrils. Mutations in the genes encoding protein constituents of these aggregates have been linked to the corresponding diseases, thus a reasonable scenario of pathogenesis was based on misfolding of a neurone-specific protein that forms insoluble fibrils that subsequently kill neuronal cells. However, during the past 5 years accumulating evidence has revealed the neurotoxic role of prefibrillar intermediate forms (soluble oligomers and protofibrils) produced during fibril formation. Many think these may be the predominant neurotoxic species, whereas microscopically visible fibrillar aggregates may not be toxic. Large protein aggregates may rather be simply inactive, or even represent a protective state that sequesters and inactivates toxic oligomers and protofibrils. Further understanding of the biochemical mechanisms involved in protein misfolding and fibrillization may optimize the planning of common therapeutic approaches for neurodegenerative diseases, directed towards reversal of protein misfolding, blockade of protein oligomerization and interference with the action of toxic proteins.     

Protein accumulation in traumatic brain injury

Traumatic brain injury (TBI) is one of the most devastating diseases in our society, accounting for a high percentage of mortality and disability. A major consequence of TBI is the rapid and long-term accumulation of proteins. This process largely reflects the interruption of axonal transport as a result of extensive axonal injury. Although many proteins are found accumulating after TBI, three have received particular attention; β-amyloid precursor protein and its proteolytic products, amyloid-β (Aβ) peptides, neurofilament proteins, and synuclein proteins. Massive coaccumulations of all of these proteins are found in damaged axons throughout the white matter after TBI. Additionally, these proteins form aggregates in other neuronal compartments and in brain parenchyma after brain trauma. Interestingly, TBI is also an epigenetic risk factor for developing neurodegenerative disorders, such as Alzheimer’s disease and Parkinson’s disease. Here, the similarities and differences of these accumulations with pathologies of neurodegenerative diseases will be explored. In addition, the potential deleterious roles of protein accumulations on functional outcome and progressive neurodegeneration following TBI will be examined.     

Autophagy in neurodegenerative disease: friend, foe or turncoat?

Autophagy, a lysosomal pathway for degrading organelles and long-lived proteins, is becoming recognized as a key adaptive response that can preclude death in stressed or diseased cells. However, during development strong induction of autophagy in specific cell populations mediates a type of programmed cell death that has distinctive 'autophagic' morphology and a requirement for autophagy activity. The recent identification of autophagosomes in neurons in a growing number of neurodegenerative disorders has, therefore, sparked controversy about whether these structures are contributing to neuronal cell death or protecting against it. Emerging evidence supports the view that induction of autophagy is a neuroprotective response and that inadequate or defective autophagy, rather than excessive autophagy, promotes neuronal cell death in most of these disorders. In this review, we consider possible mechanisms underlying autophagy-associated cell death and their relationship to pathways mediating apoptosis and necrosis.     

Mechanisms of Disease: astrocytes in neurodegenerative disease

The term neurodegenerative disease refers to the principal pathology associated with disorders such as amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease and Parkinson's disease, and it is presumed that neurodegeneration results in the clinical findings seen in patients with these diseases. Decades of pathological and physiological studies have focused on neuronal abnormalities in these disorders, but it is becoming increasingly evident that astrocytes are also important players in these and other neurological disorders. Our understanding of the normative biology of astrocytes has been aided by the development of animal models in which astrocyte-specific proteins and pathways have been manipulated, and mouse models of neurodegenerative diseases have also revealed astrocyte-specific pathologies that contribute to neurodegeneration. These models have led to the development of targeted therapies for pathways in which astrocytes participate, and this research should ultimately influence the clinical treatment of neurodegenerative disorders.     

A hypothesis on prion disorders: are infectious, inherited, and sporadic causes so distinct?

Prion diseases include a group of either sporadic, inherited or infectious disorders characterized by spongiform neurodegeneration and reactive glyosis in several brain regions. Whatever the origin, the neuropathological hallmark of prion diseases is the presence of brain aggregates containing an altered isoform of a cellular protein, named prion protein. Recent findings show the potential toxicity of the normal cellular prion protein, which occurs when its physiological metabolism is altered. In particular, several studies demonstrate that accumulation of the prion protein in the cytosol can be a consequence of an increased amount of misfolded prion proteins, a derangement of the correct protein trafficking or a reduced activity of the ubiquitin-proteasome system. The same effects can be a consequence of a mutation in the gene coding for the prion protein. In all these conditions, one assists to accumulation and self-replication of insoluble prion proteins which leads to a severe disease resembling what observed following typical "prion infections". This article provides an opinion aimed at reconciling the classic Prusiner's theory concerning the "prion concepts" with the present knowledge arising from experimental studies on neurodegenerative disorders, suggesting a few overlapping steps in the pathogenesis of these diseases.     

Autophagy in motor neuron disease: Key pathogenetic mechanisms and therapeutic targets

Autophagy is a lysosome-dependant intracellular degradation process that eliminates long-lived proteins as well as damaged organelles from the cytoplasm. An increasing body of evidence suggests that dysregulation of this system plays a pivotal role in the etiology and/or progression of neurodegenerative diseases including motor neuron disorders. Herein, we review the latest findings that highlight the involvement of autophagy in the pathogenesis of amyotrophic lateral sclerosis (ALS) and the potential role of this pathway as a target of therapeutic purposes. Autophagy promotes the removal of toxic, cytoplasmic aggregate-prone pathogenetic proteins, enhances cell survival, and modulates inflammation. The existence of several drugs targeting this pathway can facilitate the translation of basic research to clinical trials for ALS and other motor neuron diseases.     

Inhibition of Autophagy via p53-Mediated Disruption of ULK1 in a SCA7 Polyglutamine Disease Model

Spinocerebellar ataxia type 7 (SCA7) is one of nine neurodegenerative disorders caused by expanded polyglutamine domains. These so-called polyglutamine (polyQ) diseases are all characterized by aggregation. Reducing the level of aggregating polyQ proteins via pharmacological activation of autophagy has been suggested as a therapeutic approach. However, recently, evidence implicating autophagic dysfunction in these disorders has also been reported. In this study, we show that the SCA7 polyglutamine protein ataxin-7 (ATXN7) reduces the autophagic activity via a previously unreported mechanism involving p53-mediated disruption of two key proteins involved in autophagy initiation. We show that in mutant ATXN7 cells, an increased p53–FIP200 interaction and co-aggregation of p53–FIP200 into ATXN7 aggregates result in decreased soluble FIP200 levels and subsequent destabilization of ULK1. Together, this leads to a decreased capacity for autophagy induction via the ULK1–FIP200–Atg13–Atg101 complex. We also show that treatment with a p53 inhibitor, or a blocker of ATXN7 aggregation, can restore the soluble levels of FIP200 and ULK1, as well as increase the autophagic activity and reduce ATXN7 toxicity. Understanding the mechanism behind polyQ-mediated inhibition of autophagy is of importance if therapeutic approaches based on autophagy stimulation should be developed for these disorders.     

Differential Degradation of Full-length and Cleaved Ataxin-7 Fragments in a Novel Stable Inducible SCA7 Model

Spinocerebellar ataxia type 7 (SCA7) is one of nine neurodegenerative disorders caused by expanded polyglutamine repeats, and a common toxic gain-of-function mechanism has been proposed. Proteolytic cleavage of several polyglutamine proteins has been identified and suggested to modulate the polyglutamine toxicity. In this study, we show that full-length and cleaved fragments of the SCA7 disease protein ataxin-7 (ATXN7) are differentially degraded. We found that the ubiquitin-proteosome system (UPS) was essential for the degradation of full-length endogenous ATXN7 or transgenic full-length ATXN7 with a normal or expanded glutamine repeat in both HEK 293T and stable PC12 cells. However, a similar contribution by UPS and autophagy was found for the degradation of proteolytically cleaved ATXN7 fragments. Furthermore, in our novel stable inducible PC12 model, induction of mutant ATXN7 expression resulted in toxicity and this toxicity was worsened by inhibition of either UPS or autophagy. In contrast, pharmacological activation of autophagy could ameliorate the ATXN7-induced toxicity. Based on our findings, we propose that both UPS and autophagy are important for the reduction of mutant ataxin-7-induced toxicity, and enhancing ATXN7 clearance through autophagy could be used as a potential therapeutic strategy in SCA7.     

Frontotemporal dementia and tauopathy

The presence of abundant neurofibrillary lesions made of hyperphosphorylated tau proteins is the characteristic neuropathology of a subset of neurodegenerative disorders classified as "tauopathies." The discovery of mutations in the tau gene in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) constitutes convincing evidence that tau proteins play a key role in the pathogenesis of neurodegenerative disorders. Moreover, it now is known that the most common form of sporadic frontotemporal dementia (FTD), which is characterized by frontotemporal neuron loss, gliosis, and microvacuolar change, also is a tauopathy caused by a loss of tau protein expression. Thus, these discoveries have begun to change the classification and the neuropathologic diagnosis of FTD and tauopathies, as well as current understanding of the disease mechanisms underlying them. Although transgenic mice expressing wild-type human tau or variants thereof with an FTDP-17 mutation result in tau pathologies and brain degeneration similar to that seen in human tauopathies, the precise mechanisms leading to the onset and progression of neurodegenerative disorders remain incompletely understood. Here, we review current understanding of human neurodegenerative tauopathies and prospects for translative recent insights about these into therapeutic interventions to prevent or ameliorate them.