Accumulation of the authentic parkin substrate aminoacyl-tRNA synthetase cofactor, p38/JTV-1, leads to catecholaminergic cell death

Experimental Parkinson models and green chemistry approach

Parkinson's is the most common neurodegenerative disease after Alzheimer's. Motor findings in Parkinson's occur as a result of the degeneration of dopaminergic neurons starting in the substantia nigra pars compacta and ending in the putamen and caudate nucleus. Loss of neurons and the formation of inclusions called Lewy bodies in existing neurons are characteristic histopathological findings of Parkinson's. The disease primarily impairs the functional capacity of the person with cardinal findings such as tremor, bradykinesia, etc., as a result of the loss of dopaminergic neurons in the substantia nigra. Experimental animal models of Parkinson's have been used extensively in recent years to investigate the pathology of this disease. These models are generally based on systemic or local(intracerebral) administration of neurotoxins, which can replicate many features of Parkinson's mammals. The development of transgenic models in recent years has allowed us to learn more about the modeling of Parkinson's. Applying animal modeling, which shows the most human-like effects in studies, is extremely important. It has been demonstrated that oxidative stress increases in many neurodegenerative diseases such as Parkinson's and various age-related degenerative diseases in humans and that neurons are sensitive to it. In cases where oxidative stress increases and antioxidant systems are inadequate, natural molecules such as flavonoids and polyphenols can be used as a new antioxidant treatment to reduce neuronal reactive oxygen species and improve the neurodegenerative process. Therefore, in this article, we examined experimental animal modeling in Parkinson's disease and the effect of green chemistry approaches on Parkinson's disease.

Brain-targeted delivery of neuroprotective survival gene minimizing hematopoietic cell contamination: implications for Parkinson's disease treatment

BACKGROUND

Neurodegenerative diseases, including Parkinson's disease, Amyotropic Lateral Sclerosis (ALS) and Alzheimer's disease, present significant challenges for therapeutic development due to drug delivery restrictions and toxicity concerns. Prevailing strategies often employ adeno-associated viral (AAV) vectors to deliver neuroprotective survival genes directly into the central nervous system (CNS). However, these methods have been limited by triggering immunogenic responses and risk of tumorigenicity, resulting from overexpression of survival genes in peripheral blood mononuclear cells (PBMC), thereby increasing the risk of tumorigenicity in specific immune cells. Thus, by coding selectively suppressive microRNA (miRNA) target sequences in AAV genome, we designed CNS-targeted neuroprotective gene expression vector system without leakage to blood cells.

METHODS

To minimize the potential for transgene contamination in the blood, we designed a CNS-specific AAV system. Our system utilized a self-complementary AAV (scAAV), encoding a quadruple repeated target sequence of the hematopoietic cell-specific miR142-3p at the 3' untranslated region (UTR). As a representative therapeutic survival gene for Parkinson's disease treatment, we integrated DX2, an antagonistic splice variant of the apoptotic gene AIMP2, known to be implicated in Parkinson's disease, into the vector.

RESULTS

This configuration ensured that transgene expression was stringently localized to the CNS, even if the vector found its way into the blood cells. A single injection of scAAV-DX2 demonstrated marked improvement in behavior and motor activity in animal models of Parkinson's disease induced by either Rotenone or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Importantly, comprehensive preclinical data adhering to Good Laboratory Practice (GLP) standards revealed no adverse effects in the treated animals.

CONCLUSIONS

Our CNS-specific vector system, which encodes a survival transgene DX2, signifies a promising avenue for safe gene therapy, avoiding unintended expression of survival gene in blood cells, applicable to various neurodegenerative diseases.

Bi-directional regulation of AIMP2 and its splice variant on PARP-1-dependent neuronal cell death; Therapeutic implication for Parkinson's disease

BACKGROUND

Parthanatos represents a critical molecular aspect of Parkinson's disease, wherein AIMP2 aberrantly activates PARP-1 through direct physical interaction. Although AIMP2 ought to be a therapeutic target for the disease, regrettably, it is deemed undruggable due to its non-enzymatic nature and predominant localization within the tRNA synthetase multi-complex. Instead, AIMP2 possesses an antagonistic splice variant, designated DX2, which counteracts AIMP2-induced apoptosis in the p53 or inflammatory pathway. Consequently, we examined whether DX2 competes with AIMP2 for PARP-1 activation and is therapeutically effective in Parkinson's disease.

METHODS

The binding affinity of AIMP2 and DX2 to PARP-1 was contrasted through immunoprecipitation. The efficacy of DX2 in neuronal cell death was assessed under 6-OHDA and H2O2 in vitro conditions. Additionally, endosomal and exosomal activity of synaptic vesicles was gauged in AIMP2 or DX2 overexpressed hippocampal primary neurons utilizing optical live imaging with VAMP-vGlut1 probes. To ascertain the role of DX2 in vivo, rotenone-induced behavioral alterations were compared between wild-type and DX2 transgenic animals. A DX2-encoding self-complementary adeno-associated virus (scAAV) was intracranially injected into 6-OHDA induced in vivo animal models, and their mobility was examined. Subsequently, the isolated brain tissues were analyzed.

RESULTS

DX2 translocates into the nucleus upon ROS stress more rapidly than AIMP2. The binding affinity of DX2 to PARP-1 appeared to be more robust compared to that of AIMP2, resulting in the inhibition of PARP-1 induced neuronal cell death. DX2 transgenic animals exhibited neuroprotective behavior in rotenone-induced neuronal damage conditions. Following a single intracranial injection of AAV-DX2, both behavior and mobility were consistently ameliorated in neurodegenerative animal models induced by 6-OHDA.

CONCLUSION

AIMP2 and DX2 are proposed to engage in bidirectional regulation of parthanatos. They physically interact with PARP-1. Notably, DX2's cell survival properties manifest exclusively in the context of abnormal AIMP2 accumulation, devoid of any tumorigenic effects. This suggests that DX2 could represent a distinctive therapeutic target for addressing Parkinson's disease in patients.

Pan-cancer Analysis Identifies AIMP2 as a Potential Biomarker for Breast Cancer

Introduction

Aminoacyl tRNA synthetase complex interacting with multifunctional protein 2 (AIMP2) is a significant regulator of cell proliferation and apoptosis. Despite its abnormal expression in various tumor types, the specific functions and effects of AIMP2 on tumor immune cell infiltration, proliferation, and migration remain unclear.

Materials and Methods

To assess AIMP2's role in tumor immunity, we conducted a pan-cancer multi-database analysis using the Cancer Genome Atlas (TCGA), Genotype-Tissue Expression (GTEx), and Cancer Cell Lines Encyclopedia (CCLE) datasets, examining expression levels, prognosis, tumor progression, and immune microenvironment. Additionally, we investigated AIMP2's impact on breast cancer (BRCA) proliferation and migration using cell counting kit 8 (CCK-8) assay, transwell assays, and western blot analysis.

Results

Our findings revealed that AIMP2 was overexpressed in 24 tumor tissue types compared to normal tissue and was associated with four tumor stages. Survival analysis indicated that AIMP2 expression was strongly correlated with overall survival (OS) in certain cancer patients, with high AIMP2 expression linked to poorer prognosis in five cancer types.

Conclusion

Finally, siRNA-mediated AIMP2 knockdown inhibited BRCA cell proliferation and migration in vitro. In conclusion, our pan-cancer analysis suggests that AIMP2 may play a crucial role in tumor immunity and could serve as a potential prognostic marker, particularly in BRCA.

Development of 18F-Labeled PET Tracer Candidates for Imaging of the Abelson Non-receptor Tyrosine Kinase in Parkinson's Disease

Activated Abelson non-receptor tyrosine kinase (c-Abl) plays a harmful role in neurodegenerative conditions such as Parkinson's disease (PD). Inhibition of c-Abl is reported to have a neuroprotective effect and be a promising therapeutic strategy for PD. We have previously identified a series of benzo[d]thiazole derivatives as selective c-Abl inhibitors from which one compound showed high therapeutic potential. Herein, we report the development of a complementary positron emission tomography (PET) tracer. In total, three PET tracer candidates were developed and eventually radiolabeled with fluorine-18 for in vivo evaluation studies in mice. Candidate [18F]3 was identified as the most promising compound, since it showed sufficient brain uptake, good washout kinetics, and satisfactory metabolic stability. In conclusion, we believe this tracer provides a good starting point to further validate and explore c-Abl as a target for therapeutic strategies against PD supported by PET.

Structural mechanisms of mitochondrial quality control mediated by PINK1 and parkin

Parkinson's disease (PD) is the second most common neurodegenerative disease and represents a looming public health crisis as the global population ages. While the etiology of the more common, idiopathic form of the disease remains unknown, the last ten years have seen a breakthrough in our understanding of the genetic forms related to two proteins that regulate a quality control system for the removal of damaged or non-functional mitochondria. Here, we review the structure of these proteins, PINK1, a protein kinase, and parkin, a ubiquitin ligase with an emphasis on the molecular mechanisms responsible for their recognition of dysfunctional mitochondria and control of the subsequent ubiquitination cascade. Recent atomic structures have revealed the basis of PINK1 substrate specificity and the conformational changes responsible for activation of PINK1 and parkin catalytic activity. Progress in understanding the molecular basis of mitochondrial quality control promises to open new avenues for therapeutic interventions in PD.

Age-related cataract: GSTP1 ubiquitination and degradation by Parkin inhibits its anti-apoptosis in lens epithelial cells

PURPOSE

Oxidative stress-induced apoptosis of lens epithelial cells (LECs) contributes to the pathogenesis of age-related cataract (ARC). The purpose of this research is to underlie the potential mechanism of E3 ligase Parkin and its oxidative stress-associated substrate in cataractogenesis.

METHODS

The central anterior capsules were obtained from patients with ARC, Emory mice, and corresponding controls. SRA01/04 cells were exposed to H₂O₂ combined with cycloheximide (a translational inhibitor), MG-132 (a proteasome inhibitor), chloroquine (an autophagy inhibitor), Mdivi-1 (a mitochondrial division inhibitor), respectively. Co-immunoprecipitation was employed to detect protein-protein interactions and ubiquitin-tagged protein products. Levels of proteins and mRNA were evaluated by western blotting and quantitative RT-PCR assays.

RESULTS

Glutathione-S-transferase P1 (GSTP1) was identified as a novel Parkin substrate. Compared with corresponding controls, GSTP1 was significantly decreased in the anterior lens capsules obtained from human cataracts and Emory mice. Similarly, GSTP1 was declined in H₂O₂-stimulated SRA01/04 cells. Ectopic expression of GSTP1 mitigated H₂O₂-induced apoptosis, whereas silencing GSTP1 aggregated apoptosis. In addition, H₂O₂ stimulation and Parkin overexpression could promote the degradation of GSTP1 through the ubiquitin-proteasome system, autophagy-lysosome pathway, and mitophagy. After co-transfection with Parkin, the non-ubiquitinatable GSTP1 mutant maintained its anti-apoptotic function, while wildtype GSTP1 failed. Mechanistically, GSTP1 might promote mitochondrial fusion through upregulating Mitofusins 1/2 (MFN1/2).

CONCLUSION

Oxidative stress induces LECs apoptosis via Parkin-regulated degradation of GSTP1, which may provide potential targets for ARC therapy.

System-wide identification of myeloid markers of TB disease and HIV-induced reactivation in the macaque model of Mtb infection and Mtb/SIV co-infection

Mycobacterium tuberculosis (Mtb) has developed specialized mechanisms to parasitize its host cell, the macrophage. These mechanisms allow it to overcome killing by oxidative burst and persist in the wake of an inflammatory response. Mtb infection in the majority of those exposed is controlled in an asymptomatic form referred to as latent tuberculosis infection (LTBI). HIV is a well-known catalyst of reactivation of LTBI to active TB infection (ATB). Through the use of nonhuman primates (NHPs) co-infected with Mtb and Simian Immunodeficiency Virus (Mtb/SIV), we are able to simulate human progression of TB/AIDS comorbidity. The advantage of NHP models is that they recapitulate the breadth of human TB outcomes, including immune control of infection, and loss of this control due to SIV co-infection. Identifying correlates of immune control of infection is important for both vaccine and therapeutics development. Using macaques infected with Mtb or Mtb/SIV and with different clinical outcomes we attempted to identify signatures between those that progress to active infection after SIV challenge (reactivators) and those that control the infection (non-reactivators). We particularly focused on pathways relevant to myeloid origin cells such as macrophages, as these innate immunocytes have an important contribution to the initial control or the lack thereof, following Mtb infection. Using bacterial burden, C-reactive protein (CRP), and other clinical indicators of disease severity as a guide, we were able to establish gene signatures of host disease state and progression. In addition to gene signatures, clustering algorithms were used to differentiate between host disease states and identify relationships between genes. This allowed us to identify clusters of genes which exhibited differential expression profiles between the three groups of macaques: ATB, LTBI and Mtb/SIV. The gene signatures were associated with pathways relevant to apoptosis, ATP production, phagocytosis, cell migration, and Type I interferon (IFN), which are related to macrophage function. Our results suggest novel macrophage functions that may play roles in the control of Mtb infection with and without co-infection with SIV. These results particularly point towards an interplay between Type I IFN signaling and IFN-γ signaling, and the resulting impact on lung macrophages as an important determinant of progression to TB.

A Novel, Selective c-Abl Inhibitor, Compound 5, Prevents Neurodegeneration in Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that affects movement. The nonreceptor tyrosine kinase c-Abl has shown a potential role in the progression of PD. As such, c-Abl inhibition is a promising candidate for neuroprotection in PD and α-synucleinopathies. Compound 5 is a newly synthesized blood-brain barrier penetrant c-Abl inhibitor with higher efficacy than existing inhibitors. The objective of the current study was to demonstrate the neuroprotective effects of compound 5 on the α-synuclein preformed fibril (α-syn PFF) mouse model of PD. Compound 5 significantly reduced neurotoxicity, activation of c-Abl, and Lewy body pathology caused by α-syn PFF in cortical neurons. Additionally, compound 5 markedly ameliorated the loss of dopaminergic neurons, c-Abl activation, Lewy body pathology, neuroinflammatory responses, and behavioral deficits induced by α-syn PFF injection in vivo. Taken together, these results suggest that compound 5 could be a pharmaceutical agent to prevent the progression of PD and α-synucleinopathies.

Parkinson Disease: Translating Insights from Molecular Mechanisms to Neuroprotection

Parkinson disease (PD) used to be considered a nongenetic condition. However, the identification of several autosomal dominant and recessive mutations linked to monogenic PD has changed this view. Clinically manifest PD is then thought to occur through a complex interplay between genetic mutations, many of which have incomplete penetrance, and environmental factors, both neuroprotective and increasing susceptibility, which variably interact to reach a threshold over which PD becomes clinically manifested. Functional studies of PD gene products have identified many cellular and molecular pathways, providing crucial insights into the nature and causes of PD. PD originates from multiple causes and a range of pathogenic processes at play, ultimately culminating in nigral dopaminergic loss and motor dysfunction. An in-depth understanding of these complex and possibly convergent pathways will pave the way for therapeutic approaches to alleviate the disease symptoms and neuroprotective strategies to prevent disease manifestations. This review is aimed at providing a comprehensive understanding of advances made in PD research based on leveraging genetic insights into the pathogenesis of PD. It further discusses novel perspectives to facilitate identification of critical molecular pathways that are central to neurodegeneration that hold the potential to develop neuroprotective and/or neurorestorative therapeutic strategies for PD. SIGNIFICANCE STATEMENT: A comprehensive review of PD pathophysiology is provided on the complex interplay of genetic and environmental factors and biologic processes that contribute to PD pathogenesis. This knowledge identifies new targets that could be leveraged into disease-modifying therapies to prevent or slow neurodegeneration in PD.

TRIP12 ubiquitination of glucocerebrosidase contributes to neurodegeneration in Parkinson's disease

Impairment in glucocerebrosidase (GCase) is strongly associated with the development of Parkinson's disease (PD), yet the regulators responsible for its impairment remain elusive. In this paper, we identify the E3 ligase Thyroid Hormone Receptor Interacting Protein 12 (TRIP12) as a key regulator of GCase. TRIP12 interacts with and ubiquitinates GCase at lysine 293 to control its degradation via ubiquitin proteasomal degradation. Ubiquitinated GCase by TRIP12 leads to its functional impairment through premature degradation and subsequent accumulation of α-synuclein. TRIP12 overexpression causes mitochondrial dysfunction, which is ameliorated by GCase overexpression. Further, conditional TRIP12 knockout in vitro and knockdown in vivo promotes the expression of GCase, which blocks α-synuclein preformed fibrils (α-syn PFFs)-provoked dopaminergic neurodegeneration. Moreover, TRIP12 accumulates in human PD brain and α-synuclein-based mouse models. The identification of TRIP12 as a regulator of GCase provides a new perspective on the molecular mechanisms underlying dysfunctional GCase-driven neurodegeneration in PD.

Understanding the Potential of Genome Editing in Parkinson's Disease

CRISPR is a simple and cost-efficient gene-editing technique that has become increasingly popular over the last decades. Various CRISPR/Cas-based applications have been developed to introduce changes in the genome and alter gene expression in diverse systems and tissues. These novel gene-editing techniques are particularly promising for investigating and treating neurodegenerative diseases, including Parkinson's disease, for which we currently lack efficient disease-modifying treatment options. Gene therapy could thus provide treatment alternatives, revolutionizing our ability to treat this disease. Here, we review our current knowledge on the genetic basis of Parkinson's disease to highlight the main biological pathways that become disrupted in Parkinson's disease and their potential as gene therapy targets. Next, we perform a comprehensive review of novel delivery vehicles available for gene-editing applications, critical for their successful application in both innovative research and potential therapies. Finally, we review the latest developments in CRISPR-based applications and gene therapies to understand and treat Parkinson's disease. We carefully examine their advantages and shortcomings for diverse gene-editing applications in the brain, highlighting promising avenues for future research.

Amyloid-like oligomerization of AIMP2 contributes to α-synuclein interaction and Lewy-like inclusion

Lewy bodies are pathological protein inclusions present in the brain of patients with Parkinson's disease (PD). These inclusions consist mainly of α-synuclein with associated proteins, such as parkin and its substrate aminoacyl transfer RNA synthetase complex-interacting multifunctional protein-2 (AIMP2). Although AIMP2 has been suggested to be toxic to dopamine neurons, its roles in α-synuclein aggregation and PD pathogenesis are largely unknown. Here, we found that AIMP2 exhibits a self-aggregating property. The AIMP2 aggregate serves as a seed to increase α-synuclein aggregation via specific and direct binding to the α-synuclein monomer. The coexpression of AIMP2 and α-synuclein in cell cultures and in vivo resulted in the rapid formation of α-synuclein aggregates with a corresponding increase in toxicity. Moreover, accumulated AIMP2 in mouse brain was largely redistributed to insoluble fractions, correlating with the α-synuclein pathology. Last, we found that α-synuclein preformed fibril (PFF) seeding, adult Parkin deletion, or oxidative stress triggered a redistribution of both AIMP2 and α-synuclein into insoluble fraction in cells and in vivo. Supporting the pathogenic role of AIMP2, AIMP2 knockdown ameliorated the α-synuclein aggregation and dopaminergic cell death in response to PFF or 6-hydroxydopamine treatment. Together, our results suggest that AIMP2 plays a pathological role in the aggregation of α-synuclein in mice. Because AIMP2 insolubility and coaggregation with α-synuclein have been seen in the PD Lewy body, targeting pathologic AIMP2 aggregation might be useful as a therapeutic strategy for neurodegenerative α-synucleinopathies.

Lewy body-associated proteins: victims, instigators, or innocent bystanders? The case of AIMP2 and alpha-synuclein

Lewy bodies (LBs), one of the neuropathological defining hallmarks of Parkinson's disease (PD), are composed of a complex mixture of alpha-synuclein (aSyn) filaments and hundreds of proteins, lipids, and membranous organelles. However, these proteins' role in aSyn aggregation and the biogenesis of LBs remains poorly understood. Previous studies have focused on investigating the role of these proteins as modifiers of aSyn aggregation, inclusion formation, and toxicity; very often, one protein at a time. In a recent study, Ham et al. suggest that one of these proteins, aminoacyl tRNA synthase complex-interacting multifunctional protein 2 (AIMP2), plays a primary role in the initiation of aSyn aggregation and is essential for aSyn inclusion formation and toxicity in cells and several models of synucleinopathies (Ham et al., 2020). Based on in vitro aggregation studies, they proposed a model in which AIMP2 self-associates to form amyloid-like aggregates that interact with monomeric aSyn and catalyze/seed the formation of aSyn fibrils and, eventually, LB-like inclusions. Herein, we present a critical analysis of their results and conclusions, review previous studies on AIMP2 aggregation, and reexamine the role of AIMP2 in regulating aSyn inclusion formation and clearance and aSyn-induced neurodegeneration in Parkinson's disease. We conclude by presenting lesson learned and recommendations on experimental factors and approaches that should be considered in future studies aimed at investigating the potential of targeting LBs-associated proteins, including AIMP2, for developing therapies to treat PD and other synucleinopathies.

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.

PINK1/PARKIN signalling in neurodegeneration and neuroinflammation

Mutations in the PTEN-induced kinase 1 (PINK1) and Parkin RBR E3 ubiquitin-protein ligase (PARKIN) genes are associated with familial forms of Parkinson’s disease (PD). PINK1, a protein kinase, and PARKIN, an E3 ubiquitin ligase, control the specific elimination of dysfunctional or superfluous mitochondria, thus fine-tuning mitochondrial network and preserving energy metabolism. PINK1 regulates PARKIN translocation in impaired mitochondria and drives their removal via selective autophagy, a process known as mitophagy. As knowledge obtained using different PINK1 and PARKIN transgenic animal models is being gathered, growing evidence supports the contribution of mitophagy impairment to several human pathologies, including PD and Alzheimer’s diseases (AD). Therefore, therapeutic interventions aiming to modulate PINK1/PARKIN signalling might have the potential to treat these diseases. In this review, we will start by discussing how the interplay of PINK1 and PARKIN signalling helps mediate mitochondrial physiology. We will continue by debating the role of mitochondrial dysfunction in disorders such as amyotrophic lateral sclerosis, Alzheimer’s, Huntington’s and Parkinson’s diseases, as well as eye diseases such as age-related macular degeneration and glaucoma, and the causative factors leading to PINK1/PARKIN-mediated neurodegeneration and neuroinflammation. Finally, we will discuss PINK1/PARKIN gene augmentation possibilities with a particular focus on AD, PD and glaucoma.

Structural insight into the interaction between p53 TAD1 and AIMP2-DX2 by NMR

p53 is the most studied tumor suppressor and a key transcriptional factor, with discrete domains that regulate cellular pathways such as apoptosis, angiogenesis, cell-cycle arrest, DNA repair, and senescence. Previous studies have suggested that AIMP2, and ARS-interacting multifunctional protein 2, promote cell death via the protective interaction with p53 upon DNA damage. Also, oncogenic splicing variant of AIMP2 lacking exon2, AIMP2-DX2, compromises the pro-apoptotic activity and anti-proliferative activities of the AIMP2 by competing with AIMP2 for the binding with p53. However, the molecular mechanism for the interaction of p53 and AIMP2 remains elusive. Using NMR spectroscopy, we studied the structural details of the interaction of transactivation domain 1 (TAD1) of p53 with GST domain of AIMP2, which is also common in AIMP2-DX2. The chemical shift perturbation (CSP) experiments demonstrate that amino acid residues from E17 to E28 of p53, known to bind to MDM2 are also involved in binding to AIMP2-DX2. Structure determination of this region based on the transferred-NOE (trNOE) data revealed that TAD1 of the p53 forms a turn structure with hydrophobic interactions by side chains of F19, L22, W23 and L26, distinct from the structure for MDM2 binding. Also, docking results based on NMR CSP data suggest the binding mode of p53 with AIMP2-DX2 GST domain. These data provide the first structural insight into the binding of the p53 TAD1 on AIMP2 and AIMP2-DX2.

Parkin interacting substrate zinc finger protein 746 is a pathological mediator in Parkinson's disease

α-Synuclein misfolding and aggregation plays a major role in the pathogenesis of Parkinson's disease. Although loss of function mutations in the ubiquitin ligase, parkin, cause autosomal recessive Parkinson's disease, there is evidence that parkin is inactivated in sporadic Parkinson's disease. Whether parkin inactivation is a driver of neurodegeneration in sporadic Parkinson's disease or a mere spectator is unknown. Here we show that parkin in inactivated through c-Abelson kinase phosphorylation of parkin in three α-synuclein-induced models of neurodegeneration. This results in the accumulation of parkin interacting substrate protein (zinc finger protein 746) and aminoacyl tRNA synthetase complex interacting multifunctional protein 2 with increased parkin interacting substrate protein levels playing a critical role in α-synuclein-induced neurodegeneration, since knockout of parkin interacting substrate protein attenuates the degenerative process. Thus, accumulation of parkin interacting substrate protein links parkin inactivation and α-synuclein in a common pathogenic neurodegenerative pathway relevant to both sporadic and familial forms Parkinson's disease. Thus, suppression of parkin interacting substrate protein could be a potential therapeutic strategy to halt the progression of Parkinson's disease and related α-synucleinopathies.

More than 20 years of the discovery of Park2

Discovery of Park2 is our finding of a family of young onset parkinsonism, in which this family was thought to be associated with a polymorphism of the manganese superoxide gene. The gene locus of the manganese superoxide dismutase has been known. We were able to pick up a gene for this family and related families in the close approximate position at the long arm of chromosome 6. The gene for this disease has a ubiquitin-like motif in the N-terminus and two RING finger structures. It was shown that this gene had a ubiquitin-protein ligase activity. But it is not elucidated the substrate of this enzyme. Meanwhile, it has become clear that PINK1 and Parkin work together to remove the mitochondria of the lowered membrane potential in the autophagosomes (mitophagy). Now that the molecular mechanisms of mitophagy is under investigation. In addition, many hot topics are going on such as Lewy body in Park2, single heterozygotes, rare clinical manifestations, and so on.

PINK1 and Parkin mitochondrial quality control: a source of regional vulnerability in Parkinson's disease

That certain cell types in the central nervous system are more likely to undergo neurodegeneration in Parkinson's disease is a widely appreciated but poorly understood phenomenon. Many vulnerable subpopulations, including dopamine neurons in the substantia nigra pars compacta, have a shared phenotype of large, widely distributed axonal networks, dense synaptic connections, and high basal levels of neural activity. These features come at substantial bioenergetic cost, suggesting that these neurons experience a high degree of mitochondrial stress. In such a context, mechanisms of mitochondrial quality control play an especially important role in maintaining neuronal survival. In this review, we focus on understanding the unique challenges faced by the mitochondria in neurons vulnerable to neurodegeneration in Parkinson's and summarize evidence that mitochondrial dysfunction contributes to disease pathogenesis and to cell death in these subpopulations. We then review mechanisms of mitochondrial quality control mediated by activation of PINK1 and Parkin, two genes that carry mutations associated with autosomal recessive Parkinson's disease. We conclude by pinpointing critical gaps in our knowledge of PINK1 and Parkin function, and propose that understanding the connection between the mechanisms of sporadic Parkinson's and defects in mitochondrial quality control will lead us to greater insights into the question of selective vulnerability.

Aminoacyl-tRNA synthetases as therapeutic targets

Aminoacyl-tRNA synthetases (ARSs) are essential enzymes for protein synthesis with evolutionarily conserved enzymatic mechanisms. Despite their similarity across organisms, scientists have been able to generate effective anti-infective agents based on the structural differences in the catalytic clefts of ARSs from pathogens and humans. However, recent genomic, proteomic and functionomic advances have unveiled unexpected disease-associated mutations and altered expression, secretion and interactions in human ARSs, revealing hidden biological functions beyond their catalytic roles in protein synthesis. These studies have also brought to light their potential as a rich and unexplored source for new therapeutic targets and agents through multiple avenues, including direct targeting of the catalytic sites, controlling disease-associated protein-protein interactions and developing novel biologics from the secreted ARS proteins or their parts. This Review addresses the emerging biology and therapeutic applications of human ARSs in diseases including autoimmune and rare diseases, and cancer.

Deubiquitinase USP29 Governs MYBBP1A in the Brains of Parkinson's Disease Patients

The inactivation of parkin by mutation or post-translational modification contributes to dopaminergic neuronal death in Parkinson's disease (PD). The substrates of parkin, FBP1 and AIMP2, are accumulated in the postmortem brains of PD patients, and it was recently suggested that these parkin substrates transcriptionally activate deubiquitinase USP29. Herein, we newly identified 160 kDa myb-binding protein (MYBBP1A) as a novel substrate of USP29. Knockdown of parkin increased the level of AIMP2, leading to ultimately USP29 and MYBBP1A accumulation in SH-SY5Y cells. Notably, MYBBP1A was downregulated in the ventral midbrain (VM) of Aimp2 knockdown mice, whereas the upregulation of MYBBP1A was observed in the VM of inducible AIMP2 transgenic mice, as well as in the substantia nigra of sporadic PD patients. These results suggest that AIMP2 upregulates USP29 and MYBBP1A in the absence of parkin activity, contributing to PD pathogenesis.

Quantitative analysis of nasal transcripts reveals potential biomarkers for Parkinson's disease

Patients with Parkinson's disease (PD) oftentimes develop olfactory dysfunction in their early stages, converting the nasal environment into a useful source of potential biomarkers. Here we determined the possible application of nasal fluid cells for PD biomarker identification. Thirty PD patients and 13 age-matched healthy controls were enrolled in this study. Messenger RNA levels of selected PD-related genes were monitored through real-time quantitative PCR. Target gene transcripts can be efficiently amplified from the cDNA library from human nasal fluid cell pellets. And subsequent analysis showed both a marked downregulation of parkin transcripts and an upregulation of AIMP2 in PD patients when compared to controls (cutoff value = 1.753 for with 84.2% sensitivity and 84.6% specificity; 0.359 for parkin with 76.7% sensitivity and 76.9 specificity). Moreover, alteration pattern of parkin and AIMP2 in PD was distinct from another neurodegenerative disease, multiple system atrophy. Analysis in both the early and late stages of PD cases reported that parkin levels inversely correlated with PD stages. Our results validate the practical value of easily accessible nasal fluid cells and the utility of both AIMP2 and parkin as potential biomarkers for PD diagnosis.

USP10 Is a Driver of Ubiquitinated Protein Aggregation and Aggresome Formation to Inhibit Apoptosis

Accumulation of ubiquitinated proteins is cytotoxic, but cells inactivate these cytotoxicities by inducing aggresome formation. We found that ubiquitin-specific protease 10 (USP10) inhibits ubiquitinated protein-induced apoptosis by inducing aggresome formation. USP10 interacted with the ubiquitin receptor p62 and the interaction augmented p62-dependent ubiquitinated protein aggregation and aggresome formation, thereby cooperatively inhibiting apoptosis. We provide evidence that USP10/p62-induced protein aggregates inhibit proteasome activity, which increases the amount of ubiquitinated proteins and promotes aggresome formation. USP10 induced aggresomes containing α-synuclein, a pathogenic protein in Parkinson disease, in cultured cells. In Parkinson disease brains, USP10 was colocalized with α-synuclein in the disease-linked aggresome-like inclusion Lewy bodies, suggesting that USP10 inhibits α-synuclein-induced neurotoxicity by promoting Lewy body formation. Collectively, these findings suggest that USP10 is a critical factor to control protein aggregation, aggresome formation, and cytotoxicity in protein-aggregation-related diseases.

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USP10 induces ubiquitinated protein aggregation and aggresome formation

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USP10 inhibits ubiquitinated protein-induced apoptosis by aggresome formation

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USP10 induces α-synuclein-positive aggresome

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USP10 is colocalized with α-synuclein in Lewy body in Parkinson disease

Cell Death Mechanisms of Neurodegeneration

There are common mechanisms shared by genetically or pathologically distinct neurodegenerative diseases, such as excitotoxicity, mitochondrial deficits and oxidative stress, protein misfolding and translational dysfunction, autophagy and microglia activation. This indicates that although the original cause may differ in individual diseases or even subtypes of certain disorders, these disrupted common cell functions and signaling, together with aging, may lead to final execution of cell death through similar pathways. The variable neurodegenerative disease symptoms are probably caused by the type, location, and connection of the cell populations that suffer from dysfunction and loss. Besides apoptosis, necroptosis, and autophagy, an important form of death termed parthanatos plays a prominent role in stroke and several neurodegenerative diseases, which is due to PARP-1 overactivation, PAR accumulation, nuclear translocation of the mitochondria protein AIF, and large-scale DNA cleavage. Understanding the mechanisms and interactions of cell death signaling will not only help to develop neuroprotective strategies to halt neurodegeneration, but also provide biomarkers for monitoring disease progression and recovery.

PICK1 inhibits the E3 ubiquitin ligase activity of Parkin and reduces its neuronal protective effect

Parkin functions as a multipurpose E3 ubiquitin ligase, and Parkin loss of function is associated with both sporadic and familial Parkinson's disease (PD). We report that the Bin/Amphiphysin/Rvs (BAR) domain of protein interacting with PRKCA1 (PICK1) bound to the really interesting new gene 1 (RING1) domain of Parkin and potently inhibited the E3 ligase activity of Parkin by disrupting its interaction with UbcH7. Parkin translocated to damaged mitochondria and led to their degradation in neurons, whereas PICK1 robustly inhibited this process. PICK1 also impaired the protective function of Parkin against stresses in SH-SY5Y cells and neurons. The protein levels of several Parkin substrates were reduced in young PICK1-knockout mice, and these mice were resistant to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-mediated toxicity. Taken together, the results indicate that PICK1 is a potent inhibitor of Parkin, and the reduction of PICK1 enhances the protective effect of Parkin.

AIMP2-DX2 Promotes the Proliferation, Migration, and Invasion of Nasopharyngeal Carcinoma Cells

Nasopharyngeal carcinoma (NPC) is a head and neck tumor with high degree of malignancy and with high incidence especially in southern China. AIMP2-DX2, one isoform of the aminoacyl-tRNA synthetase interacting multifunctional proteins (AIMPs), is shown to be a potential target in many cancers. However, the detailed mechanisms of AIMP2-DX2 in NPC development remain to be elucidated. Here, we found that the mRNA expression level of AIMP2-DX2 was significantly increased in NPC specimens, compared with normal nasopharyngeal tissues. Microarray immunohistochemical analysis of NPC specimens and Kaplan–Meier analysis showed that patients with high AIMP2-DX2 protein expression had shorter overall survival than those with low AIMP2-DX2 level. Furthermore, mRNA and protein expression levels of AIMP2-DX2 were both increased in cultured NPC cell lines (5-8F, CNE-2Z, and CNE-1), by being compared with normal nasopharyngeal cell line NP69. Overexpression of AIMP2-DX2 remarkably promoted the cell viability, cell migration, and invasion of cultured NPC cells. Genetic knockdown of AIMP2-DX2 by shRNA lentiviruses significantly suppressed the proliferation, migration, and invasion and induced apoptosis of NPC cells. Inhibition of AIMP2-DX2 decreased the highly expressed level of matrix metalloproteinase- (MMP-) 2 and MMP-9, further suppressed proliferation, migration, and invasion in cultured NPC cells in vitro, and inhibited tumor growth in a xenograft mouse model in vivo. Taken together, these results suggest that AIMP2-DX2 plays an important role in the regulation of NPC and could be a potential therapeutic target and prognostic indicator for the treatment of NPC.

The M458L missense mutation disrupts the catalytic properties of Parkin

Parkin encodes an E3 ubiquitin ligase, and mutations affecting its catalytic potential are implicated in autosomal recessive Parkinson's disease (PD). The M458L mutation of parkin and its enzymatic effects require characterization. Therefore, we examined the enzymatic activity of Parkin with M458L mutation. We show that the M458L mutant retains its autoubiquitination potential in vitro but not in cells. Fas-associated factor 1 and p38 (substrates of Parkin) are able to bind to the M458L mutant in cells; however, these Parkin substrates are not ubiquitinated and degraded in M458L mutant-transfected cells. Moreover, M458L mutant fails to protect the mitochondria against hydrogen peroxide_, leading to cell death. Considering the role of mitochondrial dysfunction in PD pathogenesis, our results imply a causative role for the M458L mutation in neurodegeneration.

Activation mechanisms of the E3 ubiquitin ligase parkin

Monogenetic, familial forms of Parkinson's disease (PD) only account for 5-10% of the total number of PD cases, but analysis of the genes involved therein is invaluable to understanding PD-associated neurodegenerative signaling. One such gene, parkin, encodes a 465 amino acid E3 ubiquitin ligase. Of late, there has been considerable interest in the role of parkin signaling in PD and in identifying its putative substrates, as well as the elucidation of the mechanisms through which parkin itself is activated. Its dysfunction underlies both inherited and idiopathic PD-associated neurodegeneration. Here, we review recent literature that provides a model of activation of parkin in the setting of mitochondrial damage that involves PINK1 (PTEN-induced kinase-1) and phosphoubiquitin. We note that neuronal parkin is primarily a cytosolic protein (with various non-mitochondrial functions), and discuss potential cytosolic parkin activation mechanisms.

Twenty years since the discovery of the parkin gene

Nearly 20 years have passed since we identified the causative gene for a familial Parkinson's disease, parkin (now known as PARK2), in 1998. PARK2 is the most common gene responsible for young-onset Parkinson's disease. It codes for the protein Parkin RBR E3 ubiquitin-protein ligase (PARK2), which directly links to the ubiquitin-proteasome as a ubiquitin ligase. PARK2 is involved in mitophagy, which is a type of autophagy, in collaboration with PTEN-induced putative kinase 1 (PINK1). The PINK1 gene (previously known as PARK6) is also a causative gene for young-onset Parkinson's disease. Both gene products may be involved in regulating quality control within the mitochondria. The discovery of PARK2 as a cause of young-onset Parkinson's disease has had a major impact on other neurodegenerative diseases. The involvement of protein degradation systems has been implicated as a common mechanism for neurodegenerative diseases in which inclusion body formation is observed. The discovery of the involvement of PARK2 in Parkinson's disease focused attention on the involvement of protein degradation systems in neurodegenerative diseases. In this review, we focus on the history of the discovery of PARK2, the clinical phenotypes of patients with PARK2 mutations, and its functional roles.

Mechanisms of Parkinson's disease-related proteins in mediating secondary brain damage after cerebral ischemia

Both Parkinson's disease (PD) and stroke are debilitating conditions that result in neuronal death and loss of neurological functions. These two conditions predominantly affect aging populations with the deterioration of the quality of life for the patients themselves and a tremendous burden to families. While the neurodegeneration and symptomology of PD develop chronically over the years, post-stroke neuronal death and dysfunction develop rapidly in days. Despite the discrepancy in the pathophysiological time frame and severity, both conditions share common molecular mechanisms that include oxidative stress, mitochondrial dysfunction, inflammation, endoplasmic reticulum stress, and activation of various cell death pathways (apoptosis/necrosis/autophagy) that synergistically modulate the neuronal death. Emerging evidence indicates that several proteins associated with early-onset familial PD play critical roles in mediating the neuronal death. Importantly, mutations in the genes encoding Parkin, PTEN-induced putative kinase 1 and DJ-1 mediate autosomal recessive forms of PD, whereas mutations in the genes encoding leucine-rich repeat kinase 2 and α-synuclein are responsible for autosomal dominant PD. This review discusses the significance of these proteins with the emphasis on the role of α-synuclein in mediating post-ischemic brain damage.

Ubiquitin and Parkinson's disease through the looking glass of genetics

Biochemical alterations found in the brains of Parkinson's disease (PD) patients indicate that cellular stress is a major driver of dopaminergic neuronal loss. Oxidative stress, mitochondrial dysfunction, and ER stress lead to impairment of the homeostatic regulation of protein quality control pathways with a consequent increase in protein misfolding and aggregation and failure of the protein degradation machinery. Ubiquitin signalling plays a central role in protein quality control; however, prior to genetic advances, the detailed mechanisms of how impairment in the ubiquitin system was linked to PD remained mysterious. The discovery of mutations in the α-synuclein gene, which encodes the main protein misfolded in PD aggregates, together with mutations in genes encoding ubiquitin regulatory molecules, including PTEN-induced kinase 1 (PINK1), Parkin, and FBX07, has provided an opportunity to dissect out the molecular basis of ubiquitin signalling disruption in PD, and this knowledge will be critical for developing novel therapeutic strategies in PD that target the ubiquitin system.

VPS35 regulates parkin substrate AIMP2 toxicity by facilitating lysosomal clearance of AIMP2

Vacuolar protein sorting-associated protein 35 (VPS35) is involved in retrograde transport of proteins from endosomes to trans-Golgi network. Gene mutations in VPS35 are linked to autosomal dominant late-onset Parkinson's disease (PD). Although the identification of VPS35 mutations has provided novel insight about its interactions with several PD-associated genes including leucine-rich repeat kinase 2 (LRRK2) and α-synuclein, little information is available about the molecular mechanisms of cell death downstream of VPS35 dysfunction. In this study, we showed that VPS35 has a role in the lysosomal degradation of parkin substrate aminoacyl tRNA synthetase complex-interacting multifunctional protein 2 (AIMP2), of which accumulation leads to poly(ADP-ribose) polymerase-1 (PARP1)-dependent cell death. VPS35 was co-immunoprecipitated with AIMP2, as well as lysosome-associated membrane protein-2a (Lamp2a). Interestingly, this association was disrupted by PD-associated VPS35 mutant D620N. VPS35 overexpression prevented AIMP2-potentiated cell death and PARP1 activation in SH-SY5Y cells. More importantly, knockdown of VPS35 led to PARP1 activation and cell death, which was AIMP2 dependent. These findings provide new mechanistic insights into the role of VPS35 in the regulation of AIMP2 levels and cell death. As AIMP2 accumulation was reported in PD patient's brains and involved in dopaminergic cell death, identification of VPS35 as a novel regulator of AIMP2 clearance via lysosomal pathway provides alternative venue to control dopaminergic cell death in PD.

Parkin promotes proteasomal degradation of synaptotagmin IV by accelerating polyubiquitination

Parkin is an E3 ubiquitin ligase whose mutations cause autosomal recessive juvenile Parkinson's disease (PD). Unlike the human phenotype, parkin knockout (KO) mice show no apparent dopamine neuron degeneration, although they demonstrate reduced expression and activity of striatal mitochondrial proteins believed to be necessary for neuronal survival. Instead, parkin-KO mice show reduced striatal evoked dopamine release, abnormal synaptic plasticity, and non-motor symptoms, all of which appear to mimic the preclinical features of Parkinson's disease. Extensive studies have screened candidate synaptic proteins responsible for reduced evoked dopamine release, and synaptotagmin XI (Syt XI), an isoform of Syt family regulating membrane trafficking, has been identified as a substrate of parkin in humans. However, its expression level is unaltered in the striatum of parkin-KO mice. Thus, the target(s) of parkin and the molecular mechanisms underlying the impaired dopamine release in parkin-KO mice remain unknown. In this study, we focused on Syt IV because of its highly homology to Syt XI, and because they share an evolutionarily conserved lack of Ca²⁺-binding capacity; thus, Syt IV plays an inhibitory role in Ca²⁺-dependent neurotransmitter release in PC12 cells and neurons in various brain regions. We found that a proteasome inhibitor increased Syt IV protein, but not Syt XI protein, in neuron-like, differentiated PC12 cells, and that parkin interacted with and polyubiquitinated Syt IV, thereby accelerating its protein turnover. Parkin overexpression selectively degraded Syt IV protein, but not Syt I protein (indispensable for Ca²⁺-dependent exocytosis), thus enhancing depolarization-dependent exocytosis. Furthermore, in parkin-KO mice, the level of striatal Syt IV protein was increased. Our data indicate a crucial role for parkin in the proteasomal degradation of Syt IV, and provide a potential mechanism of parkin-regulated, evoked neurotransmitter release.

Proteomic studies associated with Parkinson's disease

INTRODUCTION

Parkinson's disease (PD) is an insidious disorder affecting more than 1-2% of the population over the age of 65. Understanding the etiology of PD may create opportunities for developing new treatments. Genomic and transcriptomic studies are useful, but do not provide evidence for the actual status of the disease. Conversely, proteomic studies deal with proteins, which are real time players, and can hence provide information on the dynamic nature of the affected cells. The number of publications relating to the proteomics of PD is vast. Therefore, there is a need to evaluate the current proteomics literature and establish the connections between the past and the present to foresee the future. Areas covered: PubMed and Web of Science were used to retrieve the literature associated with PD proteomics. Studies using human samples, model organisms and cell lines were selected and reviewed to highlight their contributions to PD. Expert commentary: The proteomic studies associated with PD achieved only limited success in facilitating disease diagnosis, monitoring and progression. A global system biology approach using new models is needed. Future research should integrate the findings of proteomics with other omics data to facilitate both early diagnosis and the treatment of PD.

c-Abl and Parkinson's Disease: Mechanisms and Therapeutic Potential

Although the etiology of Parkinson's disease (PD) is poorly understood, oxidative stress has long been implicated in the pathogenesis of the disease. However, multifaceted and divergent signaling cascades downstream of oxidative stress have posed challenges for researchers to identify a central component of the oxidative stress-induced pathways causing neurodegeneration in PD. Since 2010, c-Abl-a non-receptor tyrosine kinase and an indicator of oxidative stress-has shown remarkable potential as a future promising drug target in PD therapeutics. Although, the constitutively active form of c-Abl, Bcr-Abl, has a long history in chronic myeloid leukemia and acute lymphocytic leukemia, the role of c-Abl in PD and relevant neurodegenerative diseases was completely unknown. Recently, others and we have identified and validated c-Abl as an important pathogenic mediator of the disease, where activated c-Abl emerges as a common link to various PD-related inducers of oxidative stress relevant to both sporadic and familial forms of PD and α-synucleinopathies. This review discusses the role of c-Abl in PD and the latest advancement on c-Abl as a drug target and as a prospective biomarker.

Viral Vector-Based Modeling of Neurodegenerative Disorders: Parkinson's Disease

Gene therapy methods are increasingly used to model Parkinson's disease (PD) in animals in an effort to test experimental therapeutics within a more relevant context to disease pathophysiology and neuropathology. We have detailed several criteria that are critical or advantageous to accurately modeling PD in a murine model or in a nonhuman primate. Using these criteria, we then evaluate approaches made to model PD using viral vectors to date, including both adeno-associated viruses and lentiviruses. Lastly, we comment on the consideration of aging as a critical factor for modeling PD.

Parkin and mitophagy in cancer

Mitophagy, the selective engulfment and clearance of mitochondria, is essential for the homeostasis of a healthy network of functioning mitochondria and prevents excessive production of cytotoxic reactive oxygen species from damaged mitochondria. The mitochondrially targeted PTEN-induced kinase-1 (PINK1) and the E3 ubiquitin ligase Parkin are well-established synergistic mediators of the mitophagy of dysfunctional mitochondria. This pathway relies on the ubiquitination of a number of mitochondrial outer membrane substrates and subsequent docking of autophagy receptor proteins to selectively clear mitochondria. There are also alternate Parkin-independent mitophagy pathways mediated by BCL2/adenovirus E1B 19 kDa protein-interacting protein 3 and Nip-3 like protein X as well as other effectors. There is increasing evidence that ablation of mitophagy accelerates a number of pathologies. Familial Parkinsonism is associated with loss-of-function mutations in PINK1 and Parkin. A growing number of studies have observed a correlation between impaired Parkin activity and enhanced cancer development, leading to the emerging concept that Parkin activity, or mitophagy in general, is a tumour suppression mechanism. This review examines the molecular mechanisms of mitophagy and highlights the potential links between Parkin and the hallmarks of cancer that may influence tumour development and progression.

AIMP2 Controls Intestinal Stem Cell Compartments and Tumorigenesis by Modulating Wnt/β-Catenin Signaling

Wnt/β-catenin (CTNNB1) signaling is crucial for the proliferation and maintenance of intestinal stem cells (ISC), but excessive activation leads to ISC expansion and eventually colorectal cancer. Thus, negative regulators are required to maintain optimal levels of Wnt/β-catenin signaling. Aminoacyl-tRNA synthetase-interacting multifunctional proteins (AIMP) function in protein synthesis, but have also been implicated in signaling cascades affecting angiogenesis, immunity, and apoptosis. In this study, we investigated the relationship between AIMP2 and Wnt/β-catenin signaling in a murine model of intestinal homeostasis and tumorigenesis. Hemizygous deletion of Aimp2 resulted in enhanced Wnt/β-catenin signaling, increased proliferation of cryptic epithelial cells, and expansion of ISC compartments. In an Apc(Min/+) background, Aimp2 hemizygosity increased adenoma formation. Mechanistically, AIMP2 disrupted the interaction between AXIN and Dishevelled-1 (DVL1) to inhibit Wnt/β-catenin signaling by competing with AXIN. Furthermore, AIMP2 inhibited intestinal organoid formation and growth by suppressing Wnt/β-catenin signaling in an Aimp2 gene dosage-dependent manner. Collectively, our results showed that AIMP2 acts as a haploinsufficient tumor suppressor that fine-tunes Wnt/β-catenin signaling in the intestine, illuminating the regulation of ISC abundance and activity. Cancer Res; 76(15); 4559-68. ©2016 AACR.

It's all about talking: two-way communication between proteasomal and lysosomal degradation pathways via ubiquitin

Selective degradation of proteins requires a fine-tuned coordination of the two major proteolytic pathways, the ubiquitin-proteasome system (UPS) and autophagy. Substrate selection and proteolytic activity are defined by a plethora of regulatory cofactors influencing each other. Both proteolytic pathways are initiated by ubiquitylation to mark substrate proteins for degradation, although the size and/or topology of the modification are different. In this context E3 ubiquitin ligases, ensuring the covalent attachment of activated ubiquitin to the substrate, are of special importance. The regulation of E3 ligase activity, competition between different E3 ligases for binding E2 conjugation enzymes and substrates, as well as their interplay with deubiquitylating enzymes (DUBs) represent key events in the cross talk between the UPS and autophagy. The coordination between both degradation routes is further influenced by heat shock factors and ubiquitin-binding proteins (UBPs) such as p97, p62, or optineurin. Mutations in enzymes and ubiquitin-binding proteins or a general decline of both proteolytic systems during aging result in accumulation of damaged and aggregated proteins. Thus further mechanistic understanding of how UPS and autophagy communicate might allow therapeutic intervention especially against age-related diseases.

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.

Lysosomal Enzyme Glucocerebrosidase Protects against Aβ1-42 Oligomer-Induced Neurotoxicity

Glucocerebrosidase (GCase) functions as a lysosomal enzyme and its mutations are known to be related to many neurodegenerative diseases, including Gaucher’s disease (GD), Parkinson’s disease (PD), and Dementia with Lewy Bodies (DLB). However, there is little information about the role of GCase in the pathogenesis of Alzheimer’s disease (AD). Here we demonstrate that GCase protein levels and enzyme activity are significantly decreased in sporadic AD. Moreover, Aβ_1–42 oligomer treatment results in neuronal cell death that is concomitant with decreased GCase protein levels and enzyme activity, as well as impairment in lysosomal biogenesis and acidification. Importantly, overexpression of GCase promotes the lysosomal degradation of Aβ_1–42 oligomers, restores the lysosomal impairment, and protects against the toxicity in neurons treated with Aβ_1–42 oligomers. Our findings indicate that a deficiency of GCase could be involved in progression of AD pathology and suggest that augmentation of GCase activity may be a potential therapeutic option for the treatment of AD.

Involvement of inhibitory PAS domain protein in neuronal cell death in Parkinson's disease

Inhibitory PAS domain protein (IPAS), a repressor of hypoxia-inducible factor-dependent transcription under hypoxia, was found to exert pro-apoptotic activity in oxidative stress-induced cell death. However, physiological and pathological processes associated with this activity are not known. Here we show that IPAS is a key molecule involved in neuronal cell death in Parkinson’s disease (PD). IPAS was ubiquitinated by Parkin for proteasomal degradation following carbonyl cyanide m-chlorophenyl hydrazone treatment. Phosphorylation of IPAS at Thr12 by PTEN-induced putative kinase 1 (PINK1) was required for ubiquitination to occur. Activation of the PINK1–Parkin pathway attenuated IPAS-dependent apoptosis. IPAS was markedly induced in the midbrain following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration, and IPAS-deficient mice showed resistance to MPTP-induced degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc). A significant increase in IPAS expression was found in SNpc neurons in patients with sporadic PD. These results indicate a mechanism of neurodegeneration in PD.

Nigral overexpression of alpha-synuclein in the absence of parkin enhances alpha-synuclein phosphorylation but does not modulate dopaminergic neurodegeneration

BACKGROUND

Alpha-synuclein is a key protein in the pathogenesis of Parkinson's disease. Mutations in the parkin gene are the most common cause of early-onset autosomal recessive Parkinson's disease, probably through a loss-of-function mechanism. However, the molecular mechanism by which loss of parkin function leads to the development of the disease and the role of alpha-synuclein in parkin-associated Parkinson's disease is still not elucidated. Conflicting results were reported about the effect of the absence of parkin on alpha-synuclein-mediated neurotoxicity using a transgenic approach. In this study, we investigated the effect of loss of parkin on alpha-synuclein neuropathology and toxicity in adult rodent brain using viral vectors. Therefore, we overexpressed human wild type alpha-synuclein in the substantia nigra of parkin knockout and wild type mice using two different doses of recombinant adeno-associated viral vectors.

RESULTS

No difference was observed in nigral dopaminergic cell loss between the parkin knockout mice and wild type mice up to 16 weeks after viral vector injection. However, the level of alpha-synuclein phosphorylated at serine residue 129 in the substantia nigra was significantly increased in the parkin knockout mice compared to the wild type mice while the total expression level of alpha-synuclein was similar in both groups. The increased alpha-synuclein phosphorylation was confirmed in a parkin knockdown cell line.

CONCLUSIONS

These findings support a functional relationship between parkin and alpha-synuclein phosphorylation in rodent brain.