Parkin reverses intracellular beta-amyloid accumulation and its negative effects on proteasome function

AAV-aMTD-Parkin, a Therapeutic Gene Delivery Cargo, Enhances Motor and Cognitive Functions in Parkinson's and Alzheimer's Diseases

Neurodegenerative disorders, such as Parkinson's disease (PD) and Alzheimer's disease (AD), have a global prevalence and profoundly impact both motor and cognitive functions. Although adeno-associated virus (AAV)-based gene therapy has shown promise, its application for treating central nervous system (CNS) diseases faces several challenges, including effective delivery of AAV vectors across the blood-brain barrier, determining optimal dosages, and achieving targeted distribution. To address these challenges, we have developed a fusion delivery therapeutic cargo called AAV-aMTD-Parkin, which combines a hydrophobic cell-penetrating peptide sequence with the DNA sequences of AAV and Parkin. By employing this fusion delivery platform at lower dosages compared to zolgensma, we have achieved significant enhancements in cell and tissue permeability, while reducing the occurrence of common pathological protein aggregates. Consequently, motor and cognitive functions were restored in animal models of PD and AD. With its dual functionality in addressing PD and AD, AAV-aMTD-Parkin holds immense potential as a novel class of therapeutic biologics for prevalent CNS diseases.

Changes in brain proteasome dynamics associated with aging

In the nervous system, proteasomes are important for proteolysis and cellular homeostasis of neurons and glial cells and for brain health. Proteasome function declines with age in many tissues, including the nervous system, and this decline affects many of the nervous system processes important to brain health and may be related to age-related cognitive decline. Therefore, we analyzed the factors that contribute to this decline in function using the brain of mice from different months of life. Peptidase activity of proteasomes in crude extracts decreased with aging, while ubiquitinated proteins increased with aging. Additionally, there was a tendency for the number of subunits that form proteasomes to decrease slightly with age. On the other hand, ump1, which is required for proteasome formation, accumulated with age. Therefore, analysis of proteasome dynamics in each month revealed that proteasome formation decreased with aging. This study suggests that with aging, not only 20S proteasome function but also 26 proteasome function decreases, the decline in proteasome function is due to the lack of proteasome formation, the PA28-20S-PA700 complex, which is involved in immunity, increases in the brain, and one factor in this lack of proteasome formation is that the proteins called UMP1.

Proteostasis in neurodegenerative diseases

Proteostasis is essential for normal function of proteins and vital for cellular health and survival. Proteostasis encompasses all stages in the "life" of a protein, that is, from translation to functional performance and, ultimately, to degradation. Proteins need native conformations for function and in the presence of multiple types of stress, their misfolding and aggregation can occur. A coordinated network of proteins is at the core of proteostasis in cells. Among these, chaperones are required for maintaining the integrity of protein conformations by preventing misfolding and aggregation and guide those with abnormal conformation to degradation. The ubiquitin-proteasome system (UPS) and autophagy are major cellular pathways for degrading proteins. Although failure or decreased functioning of components of this network can lead to proteotoxicity and disease, like neuron degenerative diseases, underlying factors are not completely understood. Accumulating misfolded and aggregated proteins are considered major pathomechanisms of neurodegeneration. In this chapter, we have described the components of three major branches required for proteostasis-chaperones, UPS and autophagy, the mechanistic basis of their function, and their potential for protection against various neurodegenerative conditions, like Alzheimer's, Parkinson's, and Huntington's disease. The modulation of various proteostasis network proteins, like chaperones, E3 ubiquitin ligases, proteasome, and autophagy-associated proteins as therapeutic targets by small molecules as well as new and unconventional approaches, shows promise.

Neurodegenerative Proteinopathies Induced by Environmental Pollutants: Heat Shock Proteins and Proteasome as Promising Therapeutic Tools

Environmental pollutants' (EPs) amount and diversity have increased in recent years due to anthropogenic activity. Several neurodegenerative diseases (NDs) are theorized to be related to EPs, as their incidence has increased in a similar way to human EPs exposure and they reproduce the main ND hallmarks. EPs induce several neurotoxic effects, including accumulation and gradual deposition of misfolded toxic proteins, producing neuronal malfunction and cell death. Cells possess different mechanisms to eliminate these toxic proteins, including heat shock proteins (HSPs) and the proteasome system. The accumulation and deleterious effects of toxic proteins are induced through HSPs and disruption of proteasome proteins' homeostatic function by exposure to EPs. A therapeutic approach has been proposed to reduce accumulation of toxic proteins through treatment with recombinant HSPs/proteasome or the use of compounds that increase their expression or activity. Our aim is to review the current literature on NDs related to EP exposure and their relationship with the disruption of the proteasome system and HSPs, as well as to discuss the toxic effects of dysfunction of HSPs and proteasome and the contradictory effects described in the literature. Lastly, we cover the therapeutic use of developed drugs and recombinant proteasome/HSPs to eliminate toxic proteins and prevent/treat EP-induced neurodegeneration.

The proteasome: A key modulator of nervous system function, brain aging, and neurodegenerative disease

The proteasome is a large multi-subunit protease responsible for the degradation and removal of oxidized, misfolded, and polyubiquitinated proteins. The proteasome plays critical roles in nervous system processes. This includes maintenance of cellular homeostasis in neurons. It also includes roles in long-term potentiation via modulation of CREB signaling. The proteasome also possesses roles in promoting dendritic spine growth driven by proteasome localization to the dendritic spines in an NMDA/CaMKIIα dependent manner. Proteasome inhibition experiments in varied organisms has been shown to impact memory, consolidation, recollection and extinction. The proteasome has been further shown to impact circadian rhythm through modulation of a range of 'clock' genes, and glial function. Proteasome function is impaired as a consequence both of aging and neurodegenerative diseases. Many studies have demonstrated an impairment in 26S proteasome function in the brain and other tissues as a consequence of age, driven by a disassembly of 26S proteasome in favor of 20S proteasome. Some studies also show proteasome augmentation to correct age-related deficits. In amyotrophic lateral sclerosis Alzheimer's, Parkinson's and Huntington's disease proteasome function is impaired through distinct mechanisms with impacts on disease susceptibility and progression. Age and neurodegenerative-related deficits in the function of the constitutive proteasome are often also accompanied by an increase in an alternative form of proteasome called the immunoproteasome. This article discusses the critical role of the proteasome in the nervous system. We then describe how proteasome dysfunction contributes to brain aging and neurodegenerative disease.

The Role of Voltage-Gated Calcium Channels in Basal Ganglia Neurodegenerative Disorders

Calcium (Ca2+) plays a central role in regulating many cellular processes and influences cell survival. Several mechanisms can disrupt Ca2+ homeostasis to trigger cell death, including oxidative stress, mitochondrial damage, excitotoxicity, neuroinflammation, autophagy, and apoptosis. Voltage-gated Ca2+ channels (VGCCs) act as the main source of Ca2+ entry into electrically excitable cells, such as neurons, and they are also expressed in glial cells such as astrocytes and oligodendrocytes. The dysregulation of VGCC activity has been reported in both Parkinson's disease (PD) and Huntington's (HD). PD and HD are progressive neurodegenerative disorders (NDs) of the basal ganglia characterized by motor impairment as well as cognitive and psychiatric dysfunctions. This review will examine the putative role of neuronal VGCCs in the pathogenesis and treatment of central movement disorders, focusing on PD and HD. The link between basal ganglia disorders and VGCC physiology will provide a framework for understanding the neurodegenerative processes that occur in PD and HD, as well as a possible path towards identifying new therapeutic targets for the treatment of these debilitating disorders.

Role of Neddylation in Neurodegenerative Diseases

Neurodegenerative diseases are characterized by progressive loss of neurons in specific regions of the brain. Neuronal death is often associated with the accumulation of misfolded proteins due to genetic mutations or abnormal protein homeostasis. An essential mechanism for regulating the clearance of misfolded proteins is neddylation, a post-translational modification closely related to ubiquitination. Neddylation is brought about by conjugating neural precursor cell-expressed developmentally downregulated protein 8 (NEDD8) to target substrates through a cascade of cellular events. Neddylation is crucial for many biological processes, and dysfunctional neddylation is implicated in several neurodegenerative diseases. This review discusses the current understanding of the role of neddylation pathways in neurodegenerative disorders and the emergence of neddylation signaling as a potential target for drug discovery and development in neurodegenerative diseases.

Parkin: A potential target to promote healthy aging

Parkin is an E3 ubiquitin ligase mostly known for its role in regulating the removal of defective mitochondria via mitophagy. However, increasing experimental evidence that Parkin regulates several other aspects of mitochondrial biology in addition to its role in mitophagy has emerged over the past two decades. Indeed, Parkin has been shown to regulate mitochondrial biogenesis and dynamics and mitochondrial-derived vesicle formation, suggesting that Parkin plays key roles in maintaining healthy mitochondria. While Parkin is commonly described as a cytosolic E3 ubiquitin ligase, Parkin was also detected in other cellular compartments, including the nucleus, where it regulates transcription factors and acts as a transcription factor itself. New evidence also suggests that Parkin overexpression can be leveraged to delay aging. In D. melanogaster, for example, Parkin overexpression extends lifespan. In mammals, Parkin overexpression delays hallmarks of aging in several tissues and cell types. Parkin overexpression also confers protection in various models of cellular senescence and neurological disorders closely associated with aging, such as Alzheimer's and Parkinson's diseases. Recently, Parkin overexpression has also been shown to suppress tumor growth. In this review, we discuss newly emerging biological roles of Parkin as a modulator of cellular homeostasis, survival, and healthy aging, and we explore potential mechanisms through which Parkin exerts its beneficial effects on cellular health. Abstract figure legend Parkin: A potential target to promote healthy aging Illustration of key aspects of Parkin biology, including Parkin function and cellular localization and key roles in the regulation of mitochondrial quality control. The organs and systems in which Parkin overexpression was shown to exert protective effects relevant to the promotion of healthy aging are highlighted in the black rectangle at the bottom of the Figure. This article is protected by copyright. All rights reserved.

Genetic and pharmacologic proteasome augmentation ameliorates Alzheimer's-like pathology in mouse and fly APP overexpression models

The proteasome has key roles in neuronal proteostasis, including the removal of misfolded and oxidized proteins, presynaptic protein turnover, and synaptic efficacy and plasticity. Proteasome dysfunction is a prominent feature of Alzheimer's disease (AD). We show that prevention of proteasome dysfunction by genetic manipulation delays mortality, cell death, and cognitive deficits in fly and cell culture AD models. We developed a transgenic mouse with neuronal-specific proteasome overexpression that, when crossed with an AD mouse model, showed reduced mortality and cognitive deficits. To establish translational relevance, we developed a set of TAT-based proteasome-activating peptidomimetics that stably penetrated the blood-brain barrier and enhanced 20S/26S proteasome activity. These agonists protected against cell death, cognitive decline, and mortality in cell culture, fly, and mouse AD models. The protective effects of proteasome overexpression appear to be driven, at least in part, by the proteasome's increased turnover of the amyloid precursor protein along with the prevention of overall proteostatic dysfunction.

Parkin as a Molecular Bridge Linking Alzheimer’s and Parkinson’s Diseases?

Alzheimer’s (AD) and Parkinson’s (PD) diseases are two distinct age-related pathologies that are characterized by various common dysfunctions. They are referred to as proteinopathies characterized by ubiquitinated protein accumulation and aggregation. This accumulation is mainly due to altered lysosomal and proteasomal clearing processes and is generally accompanied by ER stress disturbance, autophagic and mitophagic defects, mitochondrial structure and function alterations and enhanced neuronal cell death. Genetic approaches aimed at identifying molecular triggers responsible for familial forms of AD or PD have helped to understand the etiology of their sporadic counterparts. It appears that several proteins thought to contribute to one of these pathologies are also likely to contribute to the other. One such protein is parkin (PK). Here, we will briefly describe anatomical lesions and genetic advances linked to AD and PD as well as the main cellular processes commonly affected in these pathologies. Further, we will focus on current studies suggesting that PK could well participate in AD and thereby act as a molecular bridge between these two pathologies. In particular, we will focus on the transcription factor function of PK and its newly described transcriptional targets that are directly related to AD- and PD-linked cellular defects.

Regulatory Role of Ubiquitin Specific Protease-13 (USP13) in Misfolded Protein Clearance in Neurodegenerative Diseases

Ubiquitin Specific Protease (USP)-13 is a de-ubiquitinase member of the cysteine-dependent protease superfamily that cleaves ubiquitin off protein substrates to reverse ubiquitin-mediated protein degradation. Several findings implicate USPs in neurodegeneration. Ubiquitin targets proteins to major degradation pathways, including the proteasome and the lysosome. In melanoma cells, USP13 regulates the degradation of several proteins primarily via ubiquitination and de-ubiquitination. However, the significance of USP13 in regulating protein clearance in neurodegeneration is largely unknown. This mini-review summarizes the most recent evidence pertaining to the role of USP13 in protein clearance via autophagy and the proteasome in neurodegenerative diseases.

The Roles of the Ubiquitin-Proteasome System in the Endoplasmic Reticulum Stress Pathway

The endoplasmic reticulum (ER) is a highly dynamic organelle in eukaryotic cells, which is essential for synthesis, processing, sorting of protein and lipid metabolism. However, the cells activate a defense mechanism called endoplasmic reticulum stress (ER stress) response and initiate unfolded protein response (UPR) as the unfolded proteins exceed the folding capacity of the ER due to the environmental influences or increased protein synthesis. ER stress can mediate many cellular processes, including autophagy, apoptosis and senescence. The ubiquitin-proteasome system (UPS) is involved in the degradation of more than 80% of proteins in the cells. Today, increasing numbers of studies have shown that the two important components of UPS, E3 ubiquitin ligases and deubiquitinases (DUBs), are tightly related to ER stress. In this review, we summarized the regulation of the E3 ubiquitin ligases and DUBs in ER stress.

Interaction between Parkin and α-Synuclein in PARK2-Mediated Parkinson's Disease

Parkin and α-synuclein are two key proteins involved in the pathophysiology of Parkinson's disease (PD). Neurotoxic alterations of α-synuclein that lead to the formation of toxic oligomers and fibrils contribute to PD through synaptic dysfunction, mitochondrial impairment, defective endoplasmic reticulum and Golgi function, and nuclear dysfunction. In half of the cases, the recessively inherited early-onset PD is caused by loss of function mutations in the PARK2 gene that encodes the E3-ubiquitin ligase, parkin. Parkin is involved in the clearance of misfolded and aggregated proteins by the ubiquitin-proteasome system and regulates mitophagy and mitochondrial biogenesis. PARK2-related PD is generally thought not to be associated with Lewy body formation although it is a neuropathological hallmark of PD. In this review article, we provide an overview of post-mortem neuropathological examinations of PARK2 patients and present the current knowledge of a functional interaction between parkin and α-synuclein in the regulation of protein aggregates including Lewy bodies. Furthermore, we describe prevailing hypotheses about the formation of intracellular micro-aggregates (synuclein inclusions) that might be more likely than Lewy bodies to occur in PARK2-related PD. This information may inform future studies aiming to unveil primary signaling processes involved in PD and related neurodegenerative disorders.

The Role of Sphingolipids and Specialized Pro-Resolving Mediators in Alzheimer's Disease

Alzheimer’s disease (AD) is the leading cause of dementia worldwide giving rise to devastating forms of cognitive decline, which impacts patients’ lives and that of their proxies. Pathologically, AD is characterized by extracellular amyloid deposition, neurofibrillary tangles and chronic neuroinflammation. To date, there is no cure that prevents progression of AD. In this review, we elaborate on how bioactive lipids, including sphingolipids (SL) and specialized pro-resolving lipid mediators (SPM), affect ongoing neuroinflammatory processes during AD and how we may exploit them for the development of new biomarker panels and/or therapies. In particular, we here describe how SPM and SL metabolism, ranging from ω-3/6 polyunsaturated fatty acids and their metabolites to ceramides and sphingosine-1-phosphate, initiates pro- and anti-inflammatory signaling cascades in the central nervous system (CNS) and what changes occur therein during AD pathology. Finally, we discuss novel therapeutic approaches to resolve chronic neuroinflammation in AD by modulating the SPM and SL pathways.

Anti-Aβ antibodies bound to neuritic plaques enhance microglia activity and mitigate tau pathology

The brain pathology of Alzheimer's disease (AD) is characterized by the misfolding and aggregation of both the amyloid beta (Aβ) peptide and hyperphosphorylated forms of the tau protein. Initial Aβ deposition is considered to trigger a sequence of deleterious events contributing to tau pathology, neuroinflammation and ultimately causing the loss of synapses and neurons. To assess the effect of anti-Aβ immunization in this context, we generated a mouse model by overexpressing the human tau protein in the hippocampus of 5xFAD mice. Aβ plaque deposition combined with human tau overexpression leads to an array of pathological manifestations including the formation of tau-positive dystrophic neurites and accumulation of hyperphosphorylated tau at the level of neuritic plaques. Remarkably, the presence of human tau reduces microglial clustering in proximity to the Aβ plaques, which may affect the barrier role of microglia. In this mouse model, continuous administration of anti-Aβ antibodies enhances the clustering of microglial cells even in the presence of tau. Anti-Aβ immunization increases plaque compaction, reduces the spread of tau in the hippocampal formation and prevents the formation of tau-positive dystrophic neurites. However, the treatment does not significantly reduce tau-induced neurodegeneration in the dentate gyrus. These results highlight that anti-Aβ immunization is able to enhance microglial activity around neuritic plaques, mitigating part of the tau-induced pathological manifestations.

An integrated approach to unravel a putative crosstalk network in Alzheimer's disease and Parkinson's disease

Integration of multiple profiling data and construction of functional regulatory networks provide a powerful approach to uncover functional relationships and significant molecular entities from transcriptomic data, highlighting the molecular mechanisms of complex diseases. Despite having an overlap in the neuropathologies of AD and PD, the molecular entities overlapped and mechanisms behind them are less known. Here we used an integrated strategy to analyze miRNA and gene transcriptomic data to understand the role of miRNAs and genes in regulatory activities taking place in cells, and find transcriptomic signatures linking AD and PD. We preprocessed and analyzed publicly available microarray datasets and identified 97 DEGs and 21 DEmiRs that may be involved in the overlapped mechanisms between these two disorders. Among the DEGs, we found HSPA9, PGK1, SDHC, FH, DLD, YWHAZ and ACLY as the major protein-coding genes involved in the crosstalk for AD-PD pathogenesis. Further we integrated these DEGs and DEmiRs with regulatory TFs to construct an overlapped dysregulated network of AD and PD. In the network, miR-27a-3p, miR-148a-3p and miR-15a-5p were found to be the most relevant with maximum interactions, describing their significance in the potential crosstalk. We also looked into the dysregulated biological processes and pathways overlapped in AD and PD. In conclusion, we highlighted the DEGs, DEmiRs, their interactions and related pathways overlapped in AD and PD pathogenesis, also describing a potential crosstalk at molecular level. Besides, our findings can further be used for molecular studies to reveal an assured AD-PD crosstalk.

The proteasome as a druggable target with multiple therapeutic potentialities: Cutting and non-cutting edges

Ubiquitin Proteasome System (UPS) is an adaptable and finely tuned system that sustains proteostasis network under a large variety of physiopathological conditions. Its dysregulation is often associated with the onset and progression of human diseases; hence, UPS modulation has emerged as a promising new avenue for the development of treatments of several relevant pathologies, such as cancer and neurodegeneration. The clinical interest in proteasome inhibition has considerably increased after the FDA approval in 2003 of bortezomib for relapsed/refractory multiple myeloma, which is now used in the front-line setting. Thereafter, two other proteasome inhibitors (carfilzomib and ixazomib), designed to overcome resistance to bortezomib, have been approved for treatment-experienced patients, and a variety of novel inhibitors are currently under preclinical and clinical investigation not only for haematological malignancies but also for solid tumours. However, since UPS collapse leads to toxic misfolded proteins accumulation, proteasome is attracting even more interest as a target for the care of neurodegenerative diseases, which are sustained by UPS impairment. Thus, conceptually, proteasome activation represents an innovative and largely unexplored target for drug development. According to a multidisciplinary approach, spanning from chemistry, biochemistry, molecular biology to pharmacology, this review will summarize the most recent available literature regarding different aspects of proteasome biology, focusing on structure, function and regulation of proteasome in physiological and pathological processes, mostly cancer and neurodegenerative diseases, connecting biochemical features and clinical studies of proteasome targeting drugs.

The Effect of Dysfunctional Ubiquitin Enzymes in the Pathogenesis of Most Common Diseases

Ubiquitination is a multi-step enzymatic process that involves the marking of a substrate protein by bonding a ubiquitin and protein for proteolytic degradation mainly via the ubiquitin–proteasome system (UPS). The process is regulated by three main types of enzymes, namely ubiquitin-activating enzymes (E1), ubiquitin-conjugating enzymes (E2), and ubiquitin ligases (E3). Under physiological conditions, ubiquitination is highly reversible reaction, and deubiquitinases or deubiquitinating enzymes (DUBs) can reverse the effect of E3 ligases by the removal of ubiquitin from substrate proteins, thus maintaining the protein quality control and homeostasis in the cell. The dysfunction or dysregulation of these multi-step reactions is closely related to pathogenic conditions; therefore, understanding the role of ubiquitination in diseases is highly valuable for therapeutic approaches. In this review, we first provide an overview of the molecular mechanism of ubiquitination and UPS; then, we attempt to summarize the most common diseases affecting the dysfunction or dysregulation of these mechanisms.

Ubiquitin biology in neurodegenerative disorders: From impairment to therapeutic strategies

The abnormal accumulation of neurotoxic proteins is the typical hallmark of various age-related neurodegenerative disorders (NDDs), including Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic lateral sclerosis and Multiple sclerosis. The anomalous proteins, such as Aβ, Tau in Alzheimer's disease and α-synuclein in Parkinson's disease, perturb the neuronal physiology and cellular homeostasis in the brain thereby affecting the millions of human lives across the globe. Here, ubiquitin proteasome system (UPS) plays a decisive role in clearing the toxic metabolites in cells, where any aberrancy is widely reported to exaggerate the neurodegenerative pathologies. In spite of well-advancement in the ubiquitination research, their molecular markers and mechanisms for target-specific protein ubiquitination and clearance remained elusive. Therefore, this review substantiates the role of UPS in the brain signaling and neuronal physiology with their mechanistic role in the NDD's specific pathogenic protein clearance. Moreover, current and future promising therapies are discussed to target UPS-mediated neurodegeneration for better public health.

Promiscuous Roles of Autophagy and Proteasome in Neurodegenerative Proteinopathies

Alterations in autophagy and the ubiquitin proteasome system (UPS) are commonly implicated in protein aggregation and toxicity which manifest in a number of neurological disorders. In fact, both UPS and autophagy alterations are bound to the aggregation, spreading and toxicity of the so-called prionoid proteins, including alpha synuclein (α-syn), amyloid-beta (Aβ), tau, huntingtin, superoxide dismutase-1 (SOD-1), TAR-DNA-binding protein of 43 kDa (TDP-43) and fused in sarcoma (FUS). Recent biochemical and morphological studies add to this scenario, focusing on the coordinated, either synergistic or compensatory, interplay that occurs between autophagy and the UPS. In fact, a number of biochemical pathways such as mammalian target of rapamycin (mTOR), transcription factor EB (TFEB), Bcl2-associated athanogene 1/3 (BAG3/1) and glycogen synthase kinase beta (GSk3β), which are widely explored as potential targets in neurodegenerative proteinopathies, operate at the crossroad between autophagy and UPS. These biochemical steps are key in orchestrating the specificity and magnitude of the two degradation systems for effective protein homeostasis, while intermingling with intracellular secretory/trafficking and inflammatory pathways. The findings discussed in the present manuscript are supposed to add novel viewpoints which may further enrich our insight on the complex interactions occurring between cell-clearing systems, protein misfolding and propagation. Discovering novel mechanisms enabling a cross-talk between the UPS and autophagy is expected to provide novel potential molecular targets in proteinopathies.

miR-218 Inhibits Mitochondrial Clearance by Targeting PRKN E3 Ubiquitin Ligase

The selective elimination of dysfunctional mitochondria through mitophagy is crucial for preserving mitochondrial quality and cellular homeostasis. The most described mitophagy pathway is regulated by a positive ubiquitylation feedback loop in which the PINK1 (PTEN induced kinase 1) kinase phosphorylates both ubiquitin and the E3 ubiquitin ligase PRKN (Parkin RBR E3 ubiquitin ligase), also known as PARKIN. This event recruits PRKN to the mitochondria, thus amplifying ubiquitylation signal. Here we report that miR-218 targets PRKN and negatively regulates PINK1/PRKN-mediated mitophagy. Overexpression of miR-218 reduces PRKN mRNA levels, thus also reducing protein content and deregulating the E3 ubiquitin ligase action. In fact, following miR-218 overexpression, mitochondria result less ubiquitylated and the autophagy machinery fails to proceed with correct mitochondrial clearance. Since mitophagy defects are associated with various human diseases, these results qualify miR-218 as a promising therapeutic target for human diseases.

A study of residual lesions in horses that recovered from clinical signs of chronic equine dysautonomia

BACKGROUND

Equine dysautonomia (ED) causes degeneration and loss of autonomic neurons. Approximately 50% of chronic cases recover, but it is unclear how they survive neuronal loss.

OBJECTIVES

To assess lesions, autonomic neuron numbers, interstitial cells of Cajal (ICC), and neurodegeneration in recovered cases.

ANIMALS

Thirteen cases (group ED), euthanized 10.3 ± 5.2 (1-16) years from diagnosis and 6 age-matched controls (group C).

METHODS

Prospective, case control; routine post mortem examination, neuron counts in peripheral and enteric ganglia and immunohistochemical assessment of neural networks (Protein gene product [PGP] 9.5), ICC (c-kit), and neurodegeneration (beta-amyloid precursor protein and ubiquitin) in intestine.

RESULTS

Postmortem findings in group ED were small intestinal dilation (4/12, 33%) and muscular hypertrophy (4/12, 33%), and gastric mucosal hypertrophy (3/11, 27%) and ulceration (4/11, 36%). Neuron density was lower in group ED (mean 39% lower for cranial cervical ganglion [P < .001], median 44% lower in celiacomesenteric ganglion [P = .01]). In intestine, neuronal depletion was worst in ileum (median 100% lower in submucosal plexus [P < .001], 91% lower in myenteric plexus [P = .004]). Group ED had less PGP 9.5 staining in ileal myenteric plexus (mean 66% lower [P = .04]) and circular muscle (median 75% lower [P = .006]). In ileum, there was less c-kit staining in myenteric plexus (median 57% lower [P = .02]) but not muscularis externa. Beta-amyloid precursor protein and ubiquitin results were not indicitive of neurodegeneration.

CONCLUSIONS AND CLINICAL IMPORTANCE

Intact ICC in muscularis externa might help maintain motility after neuronal loss. Treatment supporting ICC function warrants investigation.

SQSTM1/p62: A Potential Target for Neurodegenerative Disease

Neurodegenerative diseases, characterized by a progressive loss of brain function, affect the lives of millions of individuals worldwide. The complexity of the brain poses a challenge for scientists trying to map the biochemical and physiological pathways to identify areas of pathological errors. Brain samples of patients with neurodegenerative diseases have been shown to contain large amounts of misfolded and abnormally aggregated proteins, resulting in dysfunction in certain brain centers. Removal of these abnormal molecules is essential in maintaining protein homeostasis and overall neuronal health. Macroautophagy is a major route by which cells achieve this. Administration of certain autophagy-enhancing compounds has been shown to provide therapeutic effects for individuals with neurodegenerative conditions. SQSTM1/p62 is a scaffold protein closely involved in the macroautophagy process. p62 functions to anchor the ubiquitinated proteins to the autophagosome membrane, promoting degradation of unwanted molecules. Modulators targeting p62 to induce autophagy and promote its protective pathways for aggregate protein clearance have high potential in the treatment of these conditions. Additionally, causal relationships have been found between errors in regulation of SQSTM1/p62 and the development of a variety of neurodegenerative disorders, including Alzheimer's, Parkinson's, Huntington's, amyotrophic lateral sclerosis, and frontotemporal lobar degeneration. Furthermore, SQSTM1/p62 also serves as a signaling hub for multiple pathways associated with neurodegeneration, providing a potential therapeutic target in the treatment of neurodegenerative diseases. However, rational design of a p62-oriented autophagy modulator that can balance the negative and positive functions of multiple domains in p62 requires further efforts in the exploration of the protein structure and pathological basis.

Autophagy, apoptosis, and mitochondria: molecular integration and physiological relevance in skeletal muscle

Apoptosis and autophagy are processes resulting from the integration of cellular stress and death signals. Their individual importance is highlighted by the lethality of various mouse models missing apoptosis or autophagy-related genes. In addition to their independent roles, significant overlap exists with respect to the signals that stimulate these processes as well as their effector consequences. While these cellular systems exemplify the programming redundancies that underlie many fundamental biological mechanisms, their intertwined relationship means that dysfunction can promote pathology. Although both autophagic and apoptotic signaling are active in skeletal muscle during various diseases and atrophy, their specific roles here are somewhat unique. Given our growing understanding of how specific changes at the cellular level impact whole-organism physiology, there is an equally growing interest in pharmacological manipulation of apoptosis and/or autophagy for altering human physiology and health.

Parkin-dependent regulation of the MCU complex component MICU1

The mitochondrial Ca2+ uniporter machinery is a multiprotein complex composed by the Ca2+ selective pore-forming subunit, the mitochondrial uniporter (MCU), and accessory proteins, including MICU1, MICU2 and EMRE. Their concerted action is required to fine-tune the uptake of Ca2+ into the mitochondrial matrix which both sustains cell bioenergetics and regulates the apoptotic response. To adequately fulfil such requirements and avoid impairment in mitochondrial Ca2+ handling, the intracellular turnover of all the MCU components must be tightly regulated. Here we show that the MCU complex regulator MICU1, but not MCU and MICU2, is rapidly and selectively degraded by the Ubiquitin Proteasome System (UPS). Moreover, we show that the multifunctional E3 ubiquitin ligase Parkin (PARK2), whose mutations cause autosomal recessive early-onset Parkinson's disease (PD), is a potential candidate involved in this process since its upregulation strongly decreases the basal level of MICU1. Parkin was found to interact with MICU1 and, interestingly, Parkin Ubl-domain, but not its E3-ubquitin ligase activity, is required for the degradation of MICU1, suggesting that in addition to the well documented role in the control of Parkin basal auto-inhibition, the Ubl-domain might exert important regulatory functions by acting as scaffold for the proteasome-mediated degradation of selected substrates under basal conditions, i.e. to guarantee their turnover. We have found that also MICU2 stability was affected upon Parkin overexpression, probably as a consequence of increased MICU1 degradation. Our findings support a model in which the PD-related E3 ubiquitin ligase Parkin directly participates in the selective regulation of the MCU complex regulator MICU1 and, indirectly, also of the MICU2 gatekeeper, thus indicating that Parkin loss of function could contribute to the impairment of the ability of mitochondria to handle Ca2+ and consequently to the pathogenesis of PD.

A Proposed Mechanism for Neurodegeneration in Movement Disorders Characterized by Metal Dyshomeostasis and Oxidative Stress

Shared molecular pathologies between distinct neurodegenerative disorders offer unique opportunities to identify common mechanisms of neuron death, and apply lessons learned from one disease to another. Neurotoxic superoxide dismutase 1 (SOD1) proteinopathy in SOD1-associated familial amyotrophic lateral sclerosis (fALS) is recapitulated in idiopathic Parkinson disease (PD), suggesting that these two phenotypically distinct disorders share an etiological pathway, and tractable therapeutic target(s). Despite 25 years of research, the molecular determinants underlying SOD1 misfolding and toxicity in fALS remain poorly understood. The absence of SOD1 mutations in PD highlights mounting evidence that SOD1 mutations are not the sole cause of SOD1 protein misfolding occasioning oligomerization and toxicity, reinforcing the importance of non-genetic factors, including protein metallation and post-translational modification in determining SOD1 stability and function. We propose that these non-genetic factors underlie the misfolding and dysfunction of SOD1 and other proteins in both PD and fALS, constituting a shared and tractable pathway to neurodegeneration.

mTORC2 (Rictor) in Alzheimer's Disease and Reversal of Amyloid-β Expression-Induced Insulin Resistance and Toxicity in Rat Primary Cortical Neurons

Mammalian target of rapamycin complex 1 (mTORC1), a nutrient sensor and central controller of cell growth and proliferation, is altered in various models of Alzheimer's disease (AD). Even less studied or understood in AD is mammalian target of rapamycin complex 2 (mTORC2) that influences cellular metabolism, in part through the regulations of Akt/PKB and SGK. Dysregulation of insulin/PI3K/Akt signaling is another important feature of AD pathogenesis. We found that both total mTORC1 and C2 protein levels and individual C1 and C2 enzymatic activities were decreased in human AD brain samples. In two rodent AD models, mTORC1 and C2 activities were also decreased. In a neuronal culture model of AD characterized by accumulation of cellular amyloid-β (Aβ)42, mTORC1 activity was reduced. Autophagic vesicles and markers were correspondingly increased and new protein synthesis was inhibited, consistent with mTORC1 hypofunction. Interestingly, mTORC2 activity in neural culture seemed resistant to the effects of intracellular amyloid. In various cell lines, Aβ expression provoked insulin resistance, characterized by inhibition of stimulated Akt phosphorylation, and an increase in negative mTORC1 regular, p-AMPK, itself a nutrient sensor. Rapamycin decreased phospho-mTOR and to lesser degree p-Rictor. This further suppression of mTORC1 activity protected cells from Aβ-induced toxicity and insulin resistance. More striking, Rictor over-expression fully reversed the Aβ-effects on primary neuronal cultures. Finally, using in vitro assay, Rictor protein addition completely overcame oligomeric Aβ-induced inhibition of the PDK-Akt activation step. We conclude that striking a new balance by restoring mTORC2 abundance and/or inhibition of mTORC1 has therapeutic potential in AD.

Activating Autophagy as a Therapeutic Strategy for Parkinson's Disease

Parkinson's disease is a progressive neurodegenerative disease characterized by Lewy body pathology of which the primary constituent is aggregated misfolded alpha-synuclein protein. Currently, there are no clinical therapies for treatment of the underlying alpha-synuclein dysfunction and accumulation, and the standard of care for patients with Parkinson's disease focuses only on symptom management, creating an immense therapeutic gap that needs to be filled. Defects in autophagy have been strongly implicated in Parkinson's disease. Here, we review evidence from human, mouse, and cell culture studies to briefly explain these defects in autophagy in Parkinson's disease and the necessity for autophagy to be carefully and precisely tuned to maintain neuron survival. We summarize recent experimental agents for treating alpha-synuclein accumulation in α-synuclein Parkinson's disease and related synucleinopathies. Most of the efforts for developing experimental agents have focused on immunotherapeutic strategies, but we discuss why those efforts are misplaced. Finally, we emphasize why increasing autophagy flux for alpha-synuclein clearance is the most promising therapeutic strategy. Activating autophagy has been successful in preclinical models of Parkinson's disease and yields promising results in clinical trials.

Molecular Biology Digest of Cell Mitophagy

The homeostasis of eukaryotic cells relies on efficient mitochondrial function. The control of mitochondrial quality is framed by the combination of distinct but interdependent mechanisms spanning biogenesis, regulation of dynamic network, and finely tuned degradation either through ubiquitin-proteasome system or autophagy (mitophagy). There is continuous evolution on the pathways orchestrating the mitochondrial response to stress signals and the organelle adaptation to quality control during acute and subtle dysfunctions. Notably, it remains indeed ill-defined whether active mitophagy leads to cell survival or death by defective mitochondrial degradation. Above all, uncharted is whether and how pharmacologically tackle these mechanisms may lead to conceive novel therapeutic strategies for treating conditions associated with the defective mitochondria. Here, we attempt to provide a chronological and comprehensive overview of the determining discoveries, which have led to the current knowledge of mitophagy.

Dysregulation of Ubiquitin-Proteasome System in Neurodegenerative Diseases

The ubiquitin-proteasome system (UPS) is one of the major protein degradation pathways, where abnormal UPS function has been observed in cancer and neurological diseases. Many neurodegenerative diseases share a common pathological feature, namely intracellular ubiquitin-positive inclusions formed by aggregate-prone neurotoxic proteins. This suggests that dysfunction of the UPS in neurodegenerative diseases contributes to the accumulation of neurotoxic proteins and to instigate neurodegeneration. Here, we review recent findings describing various aspects of UPS dysregulation in neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease.

Parkin Protects against Oxygen-Glucose Deprivation/Reperfusion Insult by Promoting Drp1 Degradation

Ischemic stroke results in severe brain damage and remains one of the leading causes of death and disability worldwide. Effective neuroprotective therapies are needed to reduce brain damage resulting from ischemic stroke. Mitochondria are crucial for cellular energy production and homeostasis. Modulation of mitochondrial function mediates neuroprotection against ischemic brain damage. Dynamin-related protein 1 (Drp1) and parkin play a key role in regulating mitochondrial dynamics. They are potential therapeutic targets for neuroprotection in ischemic stroke. Protective effects of parkin-Drp1 pathway on mitochondria were assessed in a cellular ischemia-reperfusion injury model. Mouse neuroblastoma Neuro2a (N2a) cells were subjected to oxygen-glucose deprivation/reperfusion (OGDR) insult. OGDR induces mitochondrial fragmentation. The expression of Drp1 protein is increased after OGDR insult, while the parkin protein level is decreased. The altered protein level of Drp1 after OGDR injury is mediated by parkin through ubiquitin proteasome system (UPS). Drp1 depletion protects against OGDR induced mitochondrial damage and apoptosis. Meanwhile, parkin overexpression protects against OGDR induced apoptosis and mitochondrial dysfunction, which is attenuated by increased expression of Drp1. Our data demonstrate that parkin protects against OGDR insult through promoting degradation of Drp1. This neuroprotective potential of parkin-Drp1 pathway against OGDR insult will pave the way for developing novel neuroprotective agents for cerebral ischemia-reperfusion related disorders.

Parkin Regulation and Neurodegenerative Disorders

Parkin is a unique, multifunctional ubiquitin ligase whose various roles in the cell, particularly in neurons, are widely thought to be protective. The pivotal role that Parkin plays in maintaining neuronal survival is underscored by our current recognition that Parkin dysfunction represents not only a predominant cause of familial parkinsonism but also a formal risk factor for the more common, sporadic form of Parkinson’s disease (PD). Accordingly, keen research on Parkin over the past decade has led to an explosion of knowledge regarding its physiological roles and its relevance to PD. However, our understanding of Parkin is far from being complete. Indeed, surprises emerge from time to time that compel us to constantly update the paradigm of Parkin function. For example, we now know that Parkin’s function is not confined to mere housekeeping protein quality control (QC) roles but also includes mitochondrial homeostasis and stress-related signaling. Furthermore, emerging evidence also suggest a role for Parkin in several other major neurodegenerative diseases including Alzheimer’s disease (AD) and Amyotrophic Lateral Sclerosis (ALS). Yet, it remains truly amazing to note that a single enzyme could serve such multitude of functions and cellular roles. Clearly, its activity has to be tightly regulated. In this review, we shall discuss this and how dysregulated Parkin function may precipitate neuronal demise in various neurodegenerative disorders.

It Is All about (U)biquitin: Role of Altered Ubiquitin-Proteasome System and UCHL1 in Alzheimer Disease

Free radical-mediated damage to macromolecules and the resulting oxidative modification of different cellular components are a common feature of aging, and this process becomes much more pronounced in age-associated pathologies, including Alzheimer disease (AD). In particular, proteins are particularly sensitive to oxidative stress-induced damage and these irreversible modifications lead to the alteration of protein structure and function. In order to maintain cell homeostasis, these oxidized/damaged proteins have to be removed in order to prevent their toxic accumulation. It is generally accepted that the age-related accumulation of “aberrant” proteins results from both the increased occurrence of damage and the decreased efficiency of degradative systems. One of the most important cellular proteolytic systems responsible for the removal of oxidized proteins in the cytosol and in the nucleus is the proteasomal system. Several studies have demonstrated the impairment of the proteasome in AD thus suggesting a direct link between accumulation of oxidized/misfolded proteins and reduction of this clearance system. In this review we discuss the impairment of the proteasome system as a consequence of oxidative stress and how this contributes to AD neuropathology. Further, we focus the attention on the oxidative modifications of a key component of the ubiquitin-proteasome pathway, UCHL1, which lead to the impairment of its activity.

Parkin Protects Against Misfolded SOD1 Toxicity by Promoting Its Aggresome Formation and Autophagic Clearance

Mutations in Cu/Zn superoxide dismutase (SOD1) cause autosomal dominant amyotrophic lateral sclerosis (ALS), a devastating neurodegenerative disease with no effective treatment. Despite ample evidence indicating involvement of mutation-induced SOD1 protein misfolding and aggregation in ALS pathogenesis, the molecular mechanisms that control cellular management of misfolded, aggregation-prone SOD1 mutant proteins remain unclear. Here, we report that parkin, an E3 ubiquitin-protein ligase which is linked to Parkinson's disease, is a novel regulator of cellular defense against toxicity induced by ALS-associated SOD1 mutant proteins. We find that parkin mediates K63-linked polyubiquitination of SOD1 mutants in cooperation with the UbcH13/Uev1a E2 enzyme and promotes degradation of these misfolded SOD1 proteins by the autophagy-lysosome system. In response to strong proteotoxic stress associated with proteasome impairment, parkin promotes sequestration of misfolded and aggregated SOD1 proteins to form perinuclear aggresomes, regulates positioning of lysosomes around misfolded SOD1 aggresomes, and facilitates aggresome clearance by autophagy. Our findings reveal parkin-mediated cytoprotective mechanisms against misfolded SOD1 toxicity and suggest that enhancing parkin-mediated cytoprotection may provide a novel therapeutic strategy for treating ALS.

Proteomic Profiling of Cranial (Superior) Cervical Ganglia Reveals Beta-Amyloid and Ubiquitin Proteasome System Perturbations in an Equine Multiple System Neuropathy

Equine grass sickness (EGS) is an acute, predominantly fatal, multiple system neuropathy of grazing horses with reported incidence rates of ∼2%. An apparently identical disease occurs in multiple species, including but not limited to cats, dogs, and rabbits. Although the precise etiology remains unclear, ultrastructural findings have suggested that the primary lesion lies in the glycoprotein biosynthetic pathway of specific neuronal populations. The goal of this study was therefore to identify the molecular processes underpinning neurodegeneration in EGS. Here, we use a bottom-up approach beginning with the application of modern proteomic tools to the analysis of cranial (superior) cervical ganglion (CCG, a consistently affected tissue) from EGS-affected patients and appropriate control cases postmortem. In what appears to be the proteomic application of modern proteomic tools to equine neuronal tissues and/or to an inherent neurodegenerative disease of large animals (not a model of human disease), we identified 2,311 proteins in CCG extracts, with 320 proteins increased and 186 decreased by greater than 20% relative to controls. Further examination of selected proteomic candidates by quantitative fluorescent Western blotting (QFWB) and subcellular expression profiling by immunohistochemistry highlighted a previously unreported dysregulation in proteins commonly associated with protein misfolding/aggregation responses seen in a myriad of human neurodegenerative conditions, including but not limited to amyloid precursor protein (APP), microtubule associated protein (Tau), and multiple components of the ubiquitin proteasome system (UPS). Differentially expressed proteins eligible for in silico pathway analysis clustered predominantly into the following biofunctions: (1) diseases and disorders, including; neurological disease and skeletal and muscular disorders and (2) molecular and cellular functions, including cellular assembly and organization, cell-to-cell signaling and interaction (including epinephrine, dopamine, and adrenergic signaling and receptor function), and small molecule biochemistry. Interestingly, while the biofunctions identified in this study may represent pathways underpinning EGS-induced neurodegeneration, this is also the first demonstration of potential molecular conservation (including previously unreported dysregulation of the UPS and APP) spanning the degenerative cascades from an apparently unrelated condition of large animals, to small animal models with altered neuronal vulnerability, and human neurological conditions. Importantly, this study highlights the feasibility and benefits of applying modern proteomic techniques to veterinary investigations of neurodegenerative processes in diseases of large animals.

The ubiquitin-proteasome system in neurodegeneration

SIGNIFICANCE

Impairment of the ubiquitin-proteasome system (UPS) has been implicated in the pathogenesis of a wide variety of neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's diseases. The most significant risk factor for the development of these disorders is aging, which is associated with a progressive decline in UPS activity and the accumulation of oxidatively modified proteins. To date, no therapies have been developed that can specifically up-regulate this system.

RECENT ADVANCES

In the neurodegenerative brain, dysfunction of the UPS has been associated with the deposition of ubiquitinated protein aggregates and widespread disruption of the proteostasis network. Recent research has identified further evidence of impairment in substrate ubiquitination and proteasomal degradation, which could contribute to the loss of cellular proteostasis in neurodegenerative disease. Novel strategies for activation of the UPS by genetic manipulation and treatment with synthetic compounds have also recently been identified.

CRITICAL ISSUES

Here, we discuss the specific roles of the UPS in the healthy central nervous system and establish how dysfunctional components can contribute to neurotoxicity in the context of disease.

FUTURE DIRECTIONS

Knowledge of the UPS components that are specifically or preferentially involved in neurodegenerative disease will be critical in the development of targeted therapies which aim at limiting the accumulation of misfolded proteins without gross disturbance of this major proteolytic pathway.

Intact protein analysis of ubiquitin in cerebrospinal fluid by multiple reaction monitoring reveals differences in Alzheimer's disease and frontotemporal lobar degeneration

The impairment of the ubiquitin-proteasome system (UPS) is thought to be an early event in neurodegeneration, and monitoring UPS alterations might serve as a disease biomarker. Our aim was to establish an alternate method to antibody-based assays for the selective measurement of free monoubiquitin in cerebrospinal fluid (CSF). Free monoubiquitin was measured with liquid chromatography-multiple reaction monitoring mass spectrometry (LC-MS/MS) in CSF of patients with Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), behavioral variant of frontotemporal dementia (bvFTD), Creutzfeldt-Jakob disease (CJD), Parkinson's disease (PD), primary progressive aphasia (PPA), and progressive supranuclear palsy (PSP). The LC-MS/MS method showed excellent intra- and interassay precision (4.4-7.4% and 4.9-10.3%) and accuracy (100-107% and 100-106%). CSF ubiquitin concentration was increased compared with that of controls (33.0 ± 9.7 ng/mL) in AD (47.5 ± 13.1 ng/mL, p < 0.05) and CJD patients (171.5 ± 103.5 ng/mL, p < 0.001) but not in other neurodegenerative diseases. Receiver operating characteristic curve (ROC) analysis of AD vs control patients revealed an area under the curve (AUC) of 0.832, and the specificity and sensitivity were 75 and 75%, respectively. ROC analysis of AD and FTLD patients yielded an AUC of 0.776, and the specificity and sensitivity were 53 and 100%, respectively. In conclusion, our LC-MS/MS method may facilitate ubiquitin determination to a broader community and might help to discriminate AD, CJD, and FTLD patients.

Neddylation dysfunction in Alzheimer's disease

Ubiquitin-dependent proteolysis is a major mechanism that downregulates misfolded proteins or those that have finished a programmed task. In the last two decades, neddylation has emerged as a major regulatory pathway for ubiquitination. Central to the neddylation pathway is the amyloid precursor protein (APP)-binding protein APP-BP1, which together with Uba3, plays an analogous role to the ubiquitin-activating enzyme E1 in nedd8 activation. Activated nedd8 covalently modifies and activates a major class of ubiquitin ligases called Cullin-RING ligases (CRLs). New evidence suggests that neddylation also modifies Type-1 transmembrane receptors such as APP. Here we review the functions of neddylation and summarize evidence suggesting that dysfunction of neddylation is involved in Alzheimer's disease.

Ubiquitination increases parkin activity to promote autophagic α-synuclein clearance

Parkinson’s disease (PD) is a movement disorder associated with genetic and age related causes. Although autosomal recessive early onset PD linked to parkin mutations does not exhibit α-Synuclein accumulation, while autosomal dominant and sporadic PD manifest with α-Synuclein inclusions, loss of dopaminergic substantia nigra neurons is a common denominator in PD. Here we show that decreased parkin ubiquitination and loss of parkin stability impair interaction with Beclin-1 and alter α-Synuclein degradation, leading to death of dopaminergic neurons. Tyrosine kinase inhibition increases parkin ubiquitination and interaction with Beclin-1, promoting autophagic α-Synuclein clearance and nigral neuron survival. However, loss of parkin via deletion increases α-Synuclein in the blood compared to the brain, suggesting that functional parkin prevents α-Synuclein release into the blood. These studies demonstrate that parkin ubiquitination affects its protein stability and E3 ligase activity, possibly leading to α-Synuclein sequestration and subsequent clearance.

Isolated RING2 domain of parkin is sufficient for E2-dependent E3 ligase activity

The E3 ubiquitin ligase activity of the parkin protein is implicated in playing a protective role against neurodegenerative disorders including Parkinson's, Huntington's, and Alzheimer's diseases. Parkin has four zinc-containing domains: RING0, RING1, IBR (in-between ring), and RING2. Mutational analysis of full-length parkin suggests that the C-terminal RING2 domain contains the catalytic core. Here, a catalytically competent recombinant RING2 containing an N-terminal GB1 solubility peptide is described. In cell-free in vitro ubiqitination reactions, the RING2 construct catalyzes the transfer of ubiquitin from the E2 enzyme UbcH7 to the attached GB1 tag. This intramolecular autoubiquitination reaction indicates that (a) ubiquitination by RING2 can occur in the absence of other parkin domains and (b) UbcH7 can interact directly with RING2 to transfer its bound ubiquitin. Mass spectrometry identified sites of mono- and diubiquitin attachment to two surface-exposed lysine residues (Lys24 and Lys39) on the GB1 peptide. The sites of diubiquitination involved Lys11 and Lys48 linkages, which have been identified as general signals for proteasome degradation. Cleaving the linker between the GB1 tag and RING2 resulted in loss of ubiquitination activity, indicating that the substrate must be tethered to RING2 for proper presentation to the active site. Atomic absorption spectrometry and selective mutation of zinc ligands indicated that only one of the two zinc binding sites on RING2, the N-terminal site, needs to be occupied by zinc for expression of ubiquitination activity. This is consistent with the hypothesis that the second, C-terminal, zinc binding site on RING2 has a regulatory rather than a catalytic function.

Nilotinib-induced autophagic changes increase endogenous parkin level and ubiquitination, leading to amyloid clearance

Alzheimer's disease (AD) is a neurodegenerative disorder associated with amyloid accumulation and autophagic changes. Parkin is an E3 ubiquitin ligase involved in proteasomal and autophagic clearance. We previously demonstrated decreased parkin solubility and interaction with the key autophagy enzyme beclin-1 in AD, but tyrosine kinase inhibition restored parkin-beclin-1 interaction. In the current studies, we determined the mechanisms of nilotinib-induced parkin-beclin-1 interaction, which leads to amyloid clearance. Nilotinib increased endogenous parkin levels and ubiquitination, which may enhance parkin recycling via the proteasome, leading to increased activity and interaction with beclin-1. Parkin solubility was decreased and autophagy was altered in amyloid expressing mice, suggesting that amyloid stress affects parkin stability, leading to failure of protein clearance via the lysosome. Isolation of autophagic vacuoles revealed amyloid and parkin accumulation in autophagic compartments but nilotinib decreased insoluble parkin levels and facilitated amyloid deposition into lysosomes in wild type, but not parkin(-/-) mice, further underscoring an essential role for endogenous parkin in amyloid clearance. These results suggest that nilotinib boosts the autophagic machinery, leading to increased level of endogenous parkin that undergoes ubiquitination and interacts with beclin-1 to facilitate amyloid clearance. These data suggest that nilotinib-mediated autophagic changes may trigger parkin response via increased protein levels, providing a therapeutic strategy to reduce Aβ and Tau in AD.

KEY MESSAGE

Parkin solubility (stability) is decreased in AD and APP transgenic mice. Nilotinib-induced autophagic changes increase endogenous parkin level. Increased parkin level leads to ubiquitination and proteasomal recycling. Re-cycling decreases insoluble parkin and increases parkin-beclin-1 interaction. Beclin-1-parkin interaction enhances amyloid clearance.

Parkin overexpression ameliorates hippocampal long-term potentiation and β-amyloid load in an Alzheimer's disease mouse model

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by a severe decline of memory performance. A widely studied AD mouse model is the APPswe/PSEN1ΔE9 (APP/PS1) strain, as mice exhibit amyloid plaques as well as impaired memory capacities. To test whether restoring synaptic plasticity and decreasing β-amyloid load by Parkin could represent a potential therapeutic target for AD, we crossed APP/PS1 transgenic mice with transgenic mice overexpressing the ubiquitin ligase Parkin and analyzed offspring properties. Overexpression of Parkin in APP/PS1 transgenic mice restored activity-dependent synaptic plasticity and rescued behavioral abnormalities. Moreover, overexpression of Parkin was associated with down-regulation of APP protein expression, decreased β-amyloid load and reduced inflammation. Our data suggest that Parkin could be a promising target for AD therapy.