Alpha-synuclein-induced neurodegeneration is exacerbated in PINK1 knockout mice

Intermittent fasting reduces alpha-synuclein pathology and functional decline in a mouse model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by dopaminergic neuron degeneration and α-synuclein (aSyn) accumulation. Environmental factors play a significant role in PD progression, highlighting the potential of non-pharmacological interventions. This study investigates the therapeutic effects of intermittent fasting (IF) in an rAAV-aSyn mouse model of PD. IF, initiated four weeks post-induction of aSyn pathology, improved motor function and reduced dopaminergic neuron and axon terminal degeneration. Additionally, IF preserved dopamine levels and synaptic integrity in the striatum. Mechanistically, IF enhanced autophagic activity, promoting phosphorylated-aSyn clearance and reducing its accumulation in insoluble brain fractions. Transcriptome analysis revealed IF-induced modulation of inflammation-related genes and microglial activation. Validation in primary cultures confirmed that autophagy activation and inflammatory modulators (CCL17, IL-36RN) mitigate aSyn pathology. These findings suggest that IF exerts neuroprotective effects, supporting further exploration of IF and IF-mimicking therapies as potential PD treatments.

Recent developments in gene therapy for Parkinson's disease

Parkinson's disease (PD) is a progressive, neurodegenerative disorder for which there is currently no cure. Gene therapy has emerged as a novel approach offering renewed hope for the development of treatments that meaningfully alter the course of the disease. In this review, we explore various gene therapy strategies currently being developed targeting key aspects of PD pathogenesis: the restoration of the dopamine system by delivering genes involved in dopamine biosynthesis, reinforcing the inhibitory signaling pathways through glutamic acid decarboxylase (GAD) delivery to increase GABA production, enhancing neuronal survival and development by introducing various neurotrophic factors, delivery of genes to complement recessive familial PD mutations to correct mitochondrial dysfunction, restoring lysosomal function through delivery of GBA1 to increase glucocerebrosidase (GCase) activity, and reducing α-synuclein levels by reducing or silencing SNCA expression. Despite promising early work, challenges remain in developing safe, effective, and long-lasting gene therapies. Key considerations include optimizing viral vectors for targeted delivery, achieving controlled and sustained gene expression using different promoters, minimizing immune responses, and increasing transgene delivery capacity. Future prospects may involve combinatory strategies targeting multiple pathways, such as multi-gene constructs delivered via high-capacity viral systems.

Research models to study lewy body dementia

Lewy body dementia (LBD) encompasses neurodegenerative dementias characterized by cognitive fluctuations, visual hallucinations, and parkinsonism. Clinical differentiation of LBD from Alzheimer's disease (AD) remains complex due to symptom overlap, yet approximately 25% of dementia cases are diagnosed as LBD postmortem, primarily identified by the presence of α-synuclein aggregates, tau tangles, and amyloid plaques. These pathological features position LBD as a comorbid condition of both Parkinson's disease (PD) and AD, with over 50% of LBD cases exhibiting co-pathologies. LBD's mixed pathology complicates the development of comprehensive models that reflect the full spectrum of LBD's etiological, clinical, and pathological features. While existing animal and cellular models have facilitated significant discoveries in PD and AD research, they lack specificity in capturing LBD's unique pathogenic mechanisms, limiting the exploration of therapeutic avenues for LBD specifically. This review assesses widely used PD and AD models in terms of their relevance to LBD, particularly focusing on their ability to replicate human disease pathology and assess treatment efficacy. Furthermore, we discuss potential modifications to these models to advance the understanding of LBD mechanisms and propose innovative research directions aimed at developing models with enhanced etiological, face, predictive, and construct validity.

Rethinking 'rare' PINK1 Parkinson's disease: A meta-analysis of geographical prevalence, phenotypic diversity, and α-synuclein pathology

PTEN-induced kinase 1 (PINK1)-related Parkinson's disease (PD) is traditionally considered a rare autosomal recessive form of early-onset PD (EOPD), lacking classical Lewy body pathology. However, this characterization underestimates and oversimplifies PINK1-PD, largely due to a lack of extensive studies in diverse ethnic populations. This review and meta-analysis explores considerable variations in PINK1 variant rates and the wide heterogeneity influenced by patient- and variant-specific factors, delineating a more precise disease profile. Our findings reveal that PINK1-PD is more common than previously thought, with geographic 'hotspots' where up to 9% of EOPD cases are linked to PINK1 variants, including the pathogenic p.Leu347Pro variant affecting 1 in 1300 West Polynesians. Homozygous PINK1-PD typically manifests around age 35, predominantly affecting the lower limbs, with an excellent response to levodopa. Heterozygous PINK1-PD presents an 'intermediate' phenotype, with a later onset age (around 43 years) than homozygous PINK1-PD but earlier than idiopathic PD (typically after age 65). The severity of the phenotype is influenced by variant zygosity and pathogenicity, interacting with genetic and environmental factors to push some individuals beyond the disease threshold. Notably, females with PINK1-PD have earlier onset age than males, particularly in homozygous cases and when variants occur in the first half of PINK1's kinase domain. Contrary to traditional views, α-synuclein pathology is present in 87.5% of PINK1-PD postmortem cases across ages and variants. We challenge conventional views on PINK1-PD, highlighting distinct phenotypes influenced by zygosity, sex, and a role for α-synuclein pathology, urging for increased recognition and research of this not-so-rare disease.

Experimental models of Parkinson's disease: Challenges and Opportunities

Parkinson's disease (PD) is a widespread neurodegenerative disorder occurs due to the degradation of dopaminergic neurons present in the substantia nigra pars compacta (SNpc). Millions of people are affected by this devastating disorder globally, and the frequency of the condition increases with the increase in the elderly population. A significant amount of progress has been made in acquiring more knowledge about the etiology and the pathogenesis of PD over the past decades. Animal models have been regarded to be a vital tool for the exploration of complex molecular mechanisms involved in PD. Various animals used as models for disease monitoring include vertebrates (zebrafish, rats, mice, guinea pigs, rabbits and monkeys) and invertebrate models (Drosophila, Caenorhabditis elegans). The animal models most relevant for study of PD are neurotoxin induction-based models (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 6-Hydroxydopamine (6-OHDA) and agricultural pesticides (rotenone, paraquat), pharmacological models (reserpine or haloperidol treated rats), genetic models (α-synuclein, Leucine-rich repeat kinase 2 (LRRK2), DJ-1, PINK-1 and Parkin). Several non-mammalian genetic models such as zebrafish, Drosophila and Caenorhabditis elegance have also gained popularity in recent years due to easy genetic manipulation, presence of genes homologous to human PD, and rapid screening of novel therapeutic molecules. In addition, in vitro models (SH-SY5Y, PC12, Lund human mesencephalic (LUHMES) cells, Human induced pluripotent stem cell (iPSC), Neural organoids, organ-on-chip) are also currently in trend providing edge in investigating molecular mechanisms involved in PD as they are derived from PD patients. In this review, we explain the current situation and merits and demerits of the various animal models.

Involvement of Mitochondria in Parkinson's Disease

Mitochondrial dysregulation, such as mitochondrial complex I deficiency, increased oxidative stress, perturbation of mitochondrial dynamics and mitophagy, has long been implicated in the pathogenesis of PD. Initiating from the observation that mitochondrial toxins cause PD-like symptoms and mitochondrial DNA mutations are associated with increased risk of PD, many mutated genes linked to familial forms of PD, including PRKN, PINK1, DJ-1 and SNCA, have also been found to affect the mitochondrial features. Recent research has uncovered a much more complex involvement of mitochondria in PD. Disruption of mitochondrial quality control coupled with abnormal secretion of mitochondrial contents to dispose damaged organelles may play a role in the pathogenesis of PD. Furthermore, due to its bacterial ancestry, circulating mitochondrial DNAs can function as damage-associated molecular patterns eliciting inflammatory response. In this review, we summarize and discuss the connection between mitochondrial dysfunction and PD, highlighting the molecular triggers of the disease process, the intra- and extracellular roles of mitochondria in PD as well as the therapeutic potential of mitochondrial transplantation.

Alpha-synuclein-associated changes in PINK1-PRKN-mediated mitophagy are disease context dependent

Alpha-synuclein (αsyn) aggregates are pathological features of several neurodegenerative conditions including Parkinson disease (PD), dementia with Lewy bodies, and multiple system atrophy (MSA). Accumulating evidence suggests that mitochondrial dysfunction and impairments of the autophagic-lysosomal system can contribute to the deposition of αsyn, which in turn may interfere with health and function of these organelles in a potentially vicious cycle. Here we investigated a potential convergence of αsyn with the PINK1-PRKN-mediated mitochondrial autophagy pathway in cell models, αsyn transgenic mice, and human autopsy brain. PINK1 and PRKN identify and selectively label damaged mitochondria with phosphorylated ubiquitin (pS65-Ub) to mark them for degradation (mitophagy). We found that disease-causing multiplications of αsyn resulted in accumulation of the ubiquitin ligase PRKN in cells. This effect could be normalized by starvation-induced autophagy activation and by CRISPR/Cas9-mediated αsyn knockout. Upon acute mitochondrial damage, the increased levels of PRKN protein contributed to an enhanced pS65-Ub response. We further confirmed increased pS65-Ub-immunopositive signals in mouse brain with αsyn overexpression and in postmortem human disease brain. Of note, increased pS65-Ub was associated with neuronal Lewy body-type αsyn pathology, but not glial cytoplasmic inclusions of αsyn as seen in MSA. While our results add another layer of complexity to the crosstalk between αsyn and the PINK1-PRKN pathway, distinct mechanisms may underlie in cells and brain tissue despite similar outcomes. Notwithstanding, our finding suggests that pS65-Ub may be useful as a biomarker to discriminate different synucleinopathies and may serve as a potential therapeutic target for Lewy body disease.

PINK1-PRKN mediated mitophagy: differences between in vitro and in vivo models

Mitophagy is a key intracellular process that selectively removes damaged mitochondria to prevent their accumulation that can cause neuronal degeneration. During mitophagy, PINK1 (PTEN induced kinase 1), a serine/threonine kinase, works with PRKN/parkin, an E3 ubiquitin ligase, to target damaged mitochondria to the lysosome for degradation. Mutations in the PINK1 and PRKN genes cause early-onset Parkinson disease that is also associated with mitochondrial dysfunction. There are a large number of reports indicating the critical role of PINK1 in mitophagy. However, most of these findings were obtained from in vitro experiments with exogenous PINK1 expression and acute damage of mitochondria by toxins. Recent studies using novel animal models suggest that PINK1-PRKN can also function independent of mitochondria. In this review, we highlight the major differences between in vitro and in vivo models for investigating PINK1 and discuss the potential mechanisms underlying these differences with the aim of understanding how PINK1 functions under different circumstances.Abbreviations: AAV: adeno-associated viruses;AD: Alzheimer disease; CCCP: carbonyl cyanidem-chlorophenyl hydrazone; HD: Huntington disease; MPTP: 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; MTS: mitochondrial targeting sequence; PD: Parkinson diseases; PINK1: PTEN induced kinase 1; PRKN: parkin RBR E3 ubiquitin protein ligase; ROS: reactive oxygen species; UIM, ubiquitin interacting motif.

Neuroprotective approaches to halt Parkinson's disease progression

One of the most significant threats in Parkinson's disease (PD) is neurodegeneration. Neurodegeneration at both nigral as well as non-nigral regions of the brain is considered responsible for disease progression in PD. The key factors that initiate neurodegeneration are oxidative stress, neuroinflammation, mitochondrial complex-1 inhibition, and abnormal α-synuclein (SNCA) protein aggregations. Nigral neurodegeneration results in motor symptoms (tremor, bradykinesia, rigidity, shuffling gait, and postural instability) whereas; non-nigral neurodegeneration is responsible for non-motor symptoms (depression, cognitive dysfunctions, sleep disorders, hallucination, and psychosis). The available therapies for PD aim at increasing dopamine levels. The medications such as Monoamine oxidase B (MAO-B) inhibitors, catechol o-methyltransferase (COMT) inhibitors, Dopamine precursor (Levodopa), dopamine agonists, and dopamine reuptake inhibitors drastically improve the motor symptoms and quality of life only in the early stages of the disease. However, dopa resistant motor symptoms (abnormality in posture, speech impediment, gait, and balance problems), dopa resistant non-motor signs (sleep problems, autonomic dysfunction, mood, and cognitive impairment, pain), and drug-related side effects (motor fluctuations, psychosis, and dyskinesias) are considered responsible for the failure of these therapies. Further, none of the treatments, alone or in combination, are capable of halting the disease progression in the long run. Therefore, there is a need to develop safe and efficient neuroprotective agents, which can slow or stop the disease progression for the better management of PD. In this review, an effort has been made to discuss the various mechanisms responsible for progressive neurodegeneration (disease progression) in PD and also multiple strategies available for halting disease progression.

PTEN-Induced Putative Kinase 1 Dysfunction Accelerates Synucleinopathy

Background:

Mutations in PTEN-induced putative kinase 1 (PINK1) cause autosomal recessive Parkinson’s disease (PD) and contribute to the risk of sporadic PD. However, the relationship between PD-related PINK1 mutations and alpha-synuclein (α-syn) aggregation—a main pathological component of PD—remains unexplored.

Objective:

To investigate whether α-syn pathology is exacerbated in the absence of PINK1 after α-syn preformed fibril (PFF) injection in a PD mouse model and its effects on neurodegeneration.

Methods:

In this study, 10-week-old Pink1 knockout (KO) and wildtype (WT) mice received stereotaxic unilateral striatal injection of recombinant mouse α-syn PFF. Then, α-syn pathology progression, inflammatory responses, and neurodegeneration were analyzed via immunohistochemistry, western blot analysis, and behavioral testing.

Results:

After PFF injection, the total α-syn levels significantly increased, and pathological α-syn was markedly aggregated in Pink1 KO mice compared with Pink1 WT mice. Then, earlier and more severe neuronal loss and motor deficits occurred. Moreover, compared with WT mice, Pink1 KO mice had evident microglial/astrocytic immunoreactivity and prolonged astrocytic activation, and a higher rate of protein phosphatase 2A phosphorylation, which might explain the greater α-syn aggravation and neuronal death.

Conclusion:

The loss of Pink1 function accelerated α-syn aggregation, accumulation and glial activation, thereby leading to early and significant neurodegeneration and behavioral impairment in the PD mouse model. Therefore, our findings support the notion that PINK1 dysfunction increases the risk of synucleinopathy.

PTEN-induced kinase 1 (PINK1) and Parkin: Unlocking a mitochondrial quality control pathway linked to Parkinson's disease

Dissection of the function of two Parkinson's disease-linked genes encoding the protein kinase, PTEN-induced kinase 1 (PINK1) and ubiquitin E3 ligase, Parkin, has illuminated a highly conserved mitochondrial quality control pathway found in nearly every cell type including neurons. Mitochondrial damage-induced activation of PINK1 stimulates phosphorylation-dependent activation of Parkin and ubiquitin-dependent elimination of mitochondria by autophagy (mitophagy). Structural, cell biological and neuronal studies are unravelling the key steps of PINK1/Parkin-dependent mitophagy and uncovering new insights into how the pathway is regulated. The emerging role for aberrant immune activation as a driver of dopaminergic neuron degeneration after loss of PINK1 and Parkin poses new exciting questions on cell-autonomous and noncell-autonomous mechanisms of PINK1/Parkin signalling in vivo.

Genetically Targeted Clinical Trials in Parkinson's Disease: Learning from the Successes Made in Oncology

Clinical trials in neurodegenerative disorders have been associated with high rate of failures, while in oncology, the implementation of precision medicine and focus on genetically defined subtypes of disease and targets for drug development have seen an unprecedented success. With more than 20 genes associated with Parkinson’s disease (PD), most of which are highly penetrant and often cause early onset or atypical signs and symptoms, and an increasing understanding of the associated pathophysiology culminating in dopaminergic neurodegeneration, applying the technologies and designs into the field of neurodegeneration seems a logical step. This review describes some of the methods used in oncology clinical trials and some attempts in Parkinson’s disease and the potential of further implementing genetics, biomarkers and smart clinical trial designs in this disease area.

Aberrant Expression of Circulating MicroRNA Leads to the Dysregulation of Alpha-Synuclein and Other Pathogenic Genes in Parkinson's Disease

A group of circulating microRNAs (miRNAs) have been implicated in the pathogenesis of Parkinson’s disease. However, a comprehensive study of the interactions between pathogenic miRNAs and their downstream Parkinson’s disease (PD)-related target genes has not been performed. Here, we identified the miRNA expression profiles in the plasma and circulating exosomes of Parkinson’s disease patients using next-generation RNA sequencing. Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) analyses showed that the miRNA target genes were enriched in axon guidance, neurotrophin signaling, cellular senescence, and the Transforming growth factor-β (TGF-β), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K)-protein kinase B (AKT) and mechanistic target of rapamycin (mTOR) signaling pathways. Furthermore, a group of aberrantly expressed miRNAs were selected and further validated in individual patient plasma, human neural stem cells (NSCs) and a rat model of PD. More importantly, the full scope of the regulatory network between these miRNAs and their PD-related gene targets in human neural stem cells was examined, and the findings revealed a similar but still varied downstream regulatory cascade involving many known PD-associated genes. Additionally, miR-23b-3p was identified as a novel direct regulator of alpha-synuclein, which is possibly the key component in PD. Our current study, for the first time, provides a glimpse into the regulatory network of pathogenic miRNAs and their PD-related gene targets in PD. Moreover, these PD-associated miRNAs may serve as biomarkers and novel therapeutic targets for PD.

Adeno-Associated Viral Vectors as Versatile Tools for Parkinson's Research, Both for Disease Modeling Purposes and for Therapeutic Uses

It is without any doubt that precision medicine therapeutic strategies targeting neurodegenerative disorders are currently witnessing the spectacular rise of newly designed approaches based on the use of viral vectors as Trojan horses for the controlled release of a given genetic payload. Among the different types of viral vectors, adeno-associated viruses (AAVs) rank as the ones most commonly used for the purposes of either disease modeling or for therapeutic strategies. Here, we reviewed the current literature dealing with the use of AAVs within the field of Parkinson’s disease with the aim to provide neuroscientists with the advice and background required when facing a choice on which AAV might be best suited for addressing a given experimental challenge. Accordingly, here we will be summarizing some insights on different AAV serotypes, and which would be the most appropriate AAV delivery route. Next, the use of AAVs for modeling synucleinopathies is highlighted, providing potential readers with a landscape view of ongoing pre-clinical and clinical initiatives pushing forward AAV-based therapeutic approaches for Parkinson’s disease and related synucleinopathies.

The Nigral Coup in Parkinson's Disease by α-Synuclein and Its Associated Rebels

The risk of Parkinson's disease increases with age. However, the etiology of the illness remains obscure. It appears highly likely that the neurodegenerative processes involve an array of elements that influence each other. In addition, genetic, endogenous, or exogenous toxins need to be considered as viable partners to the cellular degeneration. There is compelling evidence that indicate the key involvement of modified α-synuclein (Lewy bodies) at the very core of the pathogenesis of the disease. The accumulation of misfolded α-synuclein may be a consequence of some genetic defect or/and a failure of the protein clearance system. Importantly, α-synuclein pathology appears to be a common denominator for many cellular deleterious events such as oxidative stress, mitochondrial dysfunction, dopamine synaptic dysregulation, iron dyshomeostasis, and neuroinflammation. These factors probably employ a common apoptotic/or autophagic route in the final stages to execute cell death. The misfolded α-synuclein inclusions skillfully trigger or navigate these processes and thus amplify the dopamine neuron fatalities. Although the process of neuroinflammation may represent a secondary event, nevertheless, it executes a fundamental role in neurodegeneration. Some viral infections produce parkinsonism and exhibit similar characteristic neuropathological changes such as a modest brain dopamine deficit and α-synuclein pathology. Thus, viral infections may heighten the risk of developing PD. Alternatively, α-synuclein pathology may induce a dysfunctional immune system. Thus, sporadic Parkinson's disease is caused by multifactorial trigger factors and metabolic disturbances, which need to be considered for the development of potential drugs in the disorder.

Hidden phenotypes of PINK1/Parkin knockout mice

PINK1, a serine/threonine ubiquitin kinase, and Parkin, an E3 ubiquitin ligase, work in coordination to target damaged mitochondria to the lysosome in a process called mitophagy. This review will cover what we have learned from PINK1 and Parkin knockout (KO) mice. Systemic PINK1 and Parkin KO mouse models haven't faithfully recapitulated early onset forms of Parkinson's disease found in humans with recessive mutations in these genes. However, the utilization of these mouse models has given us insight into how PINK1 and Parkin contribute to mitochondrial quality control and function in different tissues beyond the brain such as in heart and adipose tissue. Although PINK1 and Parkin KO mice have been generated over a decade ago, these models are still being used today to creatively elucidate cell-type specific functions. Recently, these mouse models have uncovered that these proteins contribute to innate immunity and cancer phenotypes.

Ubiquitin signalling in neurodegeneration: mechanisms and therapeutic opportunities

Neurodegenerative diseases are characterised by progressive damage to the nervous system including the selective loss of vulnerable populations of neurons leading to motor symptoms and cognitive decline. Despite millions of people being affected worldwide, there are still no drugs that block the neurodegenerative process to stop or slow disease progression. Neuronal death in these diseases is often linked to the misfolded proteins that aggregate within the brain (proteinopathies) as a result of disease-related gene mutations or abnormal protein homoeostasis. There are two major degradation pathways to rid a cell of unwanted or misfolded proteins to prevent their accumulation and to maintain the health of a cell: the ubiquitin–proteasome system and the autophagy–lysosomal pathway. Both of these degradative pathways depend on the modification of targets with ubiquitin. Aging is the primary risk factor of most neurodegenerative diseases including Alzheimer’s disease, Parkinson’s disease and amyotrophic lateral sclerosis. With aging there is a general reduction in proteasomal degradation and autophagy, and a consequent increase of potentially neurotoxic protein aggregates of β-amyloid, tau, α-synuclein, SOD1 and TDP-43. An often over-looked yet major component of these aggregates is ubiquitin, implicating these protein aggregates as either an adaptive response to toxic misfolded proteins or as evidence of dysregulated ubiquitin-mediated degradation driving toxic aggregation. In addition, non-degradative ubiquitin signalling is critical for homoeostatic mechanisms fundamental for neuronal function and survival, including mitochondrial homoeostasis, receptor trafficking and DNA damage responses, whilst also playing a role in inflammatory processes. This review will discuss the current understanding of the role of ubiquitin-dependent processes in the progressive loss of neurons and the emergence of ubiquitin signalling as a target for the development of much needed new drugs to treat neurodegenerative disease.

Classic and evolving animal models in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease with motor and non-motor symptoms. PD is characterized by the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and deficiency of dopamine in the striatal region. The primary objective in PD research is to understand the pathogenesis, targets, and development of therapeutic interventions to control the progress of the disease. The anatomical and physiological resemblances between humans and animals gathered the researcher's attention towards the use of animals in PD research. Due to varying age of onset, symptoms, and progression rate, PD becomes heterogeneous which demands the variety of animal models to study diverse features of the disease. Parkinson is a multifactorial disorder, selection of models become important as not a single model shows all the biochemical features of the disease. Currently, conventional pharmacological, neurotoxin-induced, genetically modified and cellular models are available for PD research, but none of them recapitulate all the biochemical characteristics of the disease. In this review, we included the updated knowledge on the main features of currently available in vivo and in vitro models as well as their strengths and weaknesses.

Intramitochondrial proteostasis is directly coupled to α-synuclein and amyloid β1-42 pathologies

Mitochondrial dysfunction has long been implicated in the neurodegenerative disorder Parkinson's disease (PD); however, it is unclear how mitochondrial impairment and α-synuclein pathology are coupled. Using specific mitochondrial inhibitors, EM analysis, and biochemical assays, we report here that intramitochondrial protein homeostasis plays a major role in α-synuclein aggregation. We found that interference with intramitochondrial proteases, such as HtrA2 and Lon protease, and mitochondrial protein import significantly aggravates α-synuclein seeding. In contrast, direct inhibition of mitochondrial complex I, an increase in intracellular calcium concentration, or formation of reactive oxygen species, all of which have been associated with mitochondrial stress, did not affect α-synuclein pathology. We further demonstrate that similar mechanisms are involved in amyloid-β 1-42 (Aβ42) aggregation. Our results suggest that, in addition to other protein quality control pathways, such as the ubiquitin-proteasome system, mitochondria per se can influence protein homeostasis of cytosolic aggregation-prone proteins. We propose that approaches that seek to maintain mitochondrial fitness, rather than target downstream mitochondrial dysfunction, may aid in the search for therapeutic strategies to manage PD and related neuropathologies.

Enhanced Susceptibility of PINK1 Knockout Rats to α-Synuclein Fibrils

The main neuropathological hallmarks of Parkinson's disease (PD) are loss of dopaminergic neurons in the substantia nigra and intraneuronal protein aggregates immunoreactive for α-synuclein phosphorylated at serine 129 (pS129). Most cases of PD are idiopathic; however, genetic mutations have been identified in several genes linked to familial PD. Mutations in the gene encoding α-synuclein are causally linked to dominantly inherited forms of PD and mutations in the PTEN-induced kinase-1 (PINK1) gene are linked to recessively inherited forms of PD. Because abnormal α-synuclein protein aggregates appear spontaneously in PINK1 knockout (KO) rats, we hypothesize that PINK1-deficiency causes endogenous α-synuclein to be more prone to aggregation. α-Synuclein aggregation does not normally occur in mice or rats, however, it can be induced by intracranial injection of α-synuclein pre-formed fibrils (PFFs), which also induces loss of dopaminergic nigral neurons 3-6 months post-injection. Because PINK1-deficiency is linked to early-onset PD, we further hypothesize that PINK1 KO rats will show earlier PFF-induced neurodegeneration compared to wild-type (WT) rats. Herein, we report that intracranial injection of α-synuclein PFFs into the dorsal striatum induced more abundant pS129 α-synuclein in PINK1 KO rat brains compared to WT littermate controls. Moreover, the synuclein extracted from the brains of PFF-injected PINK1 KO rats was more insoluble compared to PFF-injected WT littermates, suggesting greater progression of α-synuclein pathology in PINK1 KO rats. Four weeks post-injection, PFFs caused significant loss of dopaminergic neurons in the substantia nigra of PINK1 KO rats, but not WT controls. Together, our results indicate that PINK1 deficiency increases vulnerability to α-synuclein aggregation and dopaminergic neurodegeneration in vivo.

Modeling Parkinson's Disease With the Alpha-Synuclein Protein

Alpha-synuclein (α-Syn) is a key protein involved in Parkinson's disease (PD) pathology. PD is characterized by the loss of dopaminergic neuronal cells in the substantia nigra pars compacta and the abnormal accumulation and aggregation of α-Syn in the form of Lewy bodies and Lewy neurites. More precisely, the aggregation of α-Syn is associated with the dysfunctionality and degeneration of neurons in PD. Moreover, mutations in the SNCA gene, which encodes α-Syn, cause familial forms of PD and are the basis of sporadic PD risk. Given the role of the α-Syn protein in the pathology of PD, animal models that reflect the dopaminergic neuronal loss and the widespread and progressive formation of α-Syn aggregates in different areas of the brain constitute a valuable tool. Indeed, animal models of PD are important for understanding the molecular mechanisms of the disease and might contribute to the development and validation of new therapies. In the absence of animal models that faithfully reproduce human PD, in recent years, numerous animal models of PD based on α-Syn have been generated. In this review, we summarize the main features of the α-Syn pre-formed fibrils (PFFs) model and recombinant adeno-associated virus vector (rAAV) mediated α-Syn overexpression models, providing a detailed comparative analysis of both models. Here, we discuss how each model has contributed to our understanding of PD pathology and the advantages and weakness of each of them.

The Role of the Antioxidant Response in Mitochondrial Dysfunction in Degenerative Diseases: Cross-Talk between Antioxidant Defense, Autophagy, and Apoptosis

The mitochondrion is an essential organelle important for the generation of ATP for cellular function. This is especially critical for cells with high energy demands, such as neurons for signal transmission and cardiomyocytes for the continuous mechanical work of the heart. However, deleterious reactive oxygen species are generated as a result of mitochondrial electron transport, requiring a rigorous activation of antioxidative defense in order to maintain homeostatic mitochondrial function. Indeed, recent studies have demonstrated that the dysregulation of antioxidant response leads to mitochondrial dysfunction in human degenerative diseases affecting the nervous system and the heart. In this review, we outline and discuss the mitochondrial and oxidative stress factors causing degenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, and Friedreich's ataxia. In particular, the pathological involvement of mitochondrial dysfunction in relation to oxidative stress, energy metabolism, mitochondrial dynamics, and cell death will be explored. Understanding the pathology and the development of these diseases has highlighted novel regulators in the homeostatic maintenance of mitochondria. Importantly, this offers potential therapeutic targets in the development of future treatments for these degenerative diseases.

α-synuclein expression from a single copy transgene increases sensitivity to stress and accelerates neuronal loss in genetic models of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease and is characterized by the formation of α-synuclein-containing protein aggregates called Lewy bodies within the brain. A crucial role for α-synuclein in the pathogenesis of PD is also suggested by the fact that point mutations, increased copy number, or polymorphisms in the α-synuclein gene SNCA all cause or contribute to the development of PD. In addition to SNCA, an increasing number of other genes have been implicated in PD. While mutations in at least some of these genes have been shown to cause the formation of Lewy bodies, the role of α-synuclein in these genetic forms of PD remains poorly defined. Since C. elegans do not have a homolog of α-synuclein, this organism provides the opportunity to identify synergism between α-synuclein and other genes implicated in PD. To do this, we generated a novel C. elegans model in which wild-type α-synuclein is ubiquitously expressed from a single copy transgene, and examined the resulting effect on phenotypic deficits in PD deletion mutants affecting PARK2/pdr-1, PINK1/pink-1, DJ-1/djr-1.1 and ATP13A2/catp-6. While the PD deletion mutants exhibit only mild phenotypic deficits in absence of α-synuclein, expression of wild-type α-synuclein caused increased sensitivity to multiple stresses, induced deficits in dopamine-dependent behavior, and accelerated loss of dopamine neurons. Overall, these results suggest that the recessive loss of function mutations act together with α-synuclein to cause PD, and that α-synuclein lowering strategies may be effective in genetic forms of PD.

Lack of PINK1 alters glia innate immune responses and enhances inflammation-induced, nitric oxide-mediated neuron death

Neuroinflammation is involved in the pathogenesis of Parkinson’s disease (PD) and other neurodegenerative disorders. We show that lack of PINK1- a mitochondrial kinase linked to recessive familial PD – leads to glia type-specific abnormalities of innate immunity. PINK1 loss enhances LPS/IFN-γ stimulated pro-inflammatory phenotypes of mixed astrocytes/microglia (increased iNOS, nitric oxide and COX-2, reduced IL-10) and pure astrocytes (increased iNOS, nitric oxide, TNF-α and IL-1β), while attenuating expression of both pro-inflammatory (TNF-α, IL-1β) and anti-inflammatory (IL-10) cytokines in microglia. These abnormalities are associated with increased inflammation-induced NF-κB signaling in astrocytes, and cause enhanced death of neurons co-cultured with inflamed PINK1^−/− mixed glia and neuroblastoma cells exposed to conditioned medium from LPS/IFN-γ treated PINK1^−/− mixed glia. Neuroblastoma cell death is prevented with an iNOS inhibitor, implicating increased nitric oxide production as the cause for enhanced death. Finally, we show for the first time that lack of a recessive PD gene (PINK1) increases α-Synuclein-induced nitric oxide production in all glia types (mixed glia, astrocytes and microglia). Our results describe a novel pathogenic mechanism in recessive PD, where PINK1 deficiency may increase neuron death via exacerbation of inflammatory stimuli-induced nitric oxide production and abnormal innate immune responses in glia cells.

Are rodent models of Parkinson's disease behaving as they should?

In recent years our understanding of Parkinson's disease has expanded both in terms of pathological hallmarks as well as relevant genetic influences. In parallel with the aetiological discoveries a multitude of PD animal models have been established. The vast majority of these are rodent models based on environmental, genetic and mechanistic insight. A major challenge in many of these models is their ability to only recapitulate some of the complex disease features seen in humans. Although symptom alleviation and clinical signs are of utmost importance in therapeutic research many of these models lack comprehensive behavioural testing. While non-motor symptoms become increasingly important as early diagnostic markers in PD, they are poorly characterized in rodents. In this review we look at well-established and more recent animal models of PD in terms of behavioural characterization and discuss how they can best contribute to progression in Parkinson's research.

PINK1 suppresses alpha-synuclein-induced neuronal injury: a novel mechanism in protein phosphatase 2A activation

Alpha-synuclein (α-Syn) and phosphatase and tensin homolog deleted on chromosome ten (PTEN)-induced putative kinase (PINK) 1 are proteins found in Lewy bodies, which are a pathological hallmark of Parkinson's disease (PD). PINK1 overexpression suppresses α-Syn-induced phenotypes and increases lifespan and health in an animal model of PD. It has been suggested that the two proteins regulate protein phosphatase (PP) 2A activity, but the underlying mechanisms and neuroprotective action of PP2A against PD-associated pathology are unknown. We found that α-Syn overexpression in SK-N-SH neuroblastoma cells and primary cortical neurons caused mitochondrial dysfunction and cell injury via phosphorylation of PP2A at Tyr307 and inhibition of its activity. Concomitant overexpression of PINK1 reversed this effect and restored the activity. The level of phospho-activated Src was increased in cells overexpressing α-Syn, which was reversed by co-expressing PINK1, suggesting that the latter suppressed α-Syn-induced PP2A inactivation by inhibiting Src activity. Calmodulin/Src complex formation was also enhanced in α-Syn-overexpressing cells, which was reversed by co-expression of PINK1 as a result of reduced mitochondrial Ca²⁺ releasing. Interestingly, the protective effects of PINK1 in α-Syn induced models were abolished by treatment with the PP2A inhibitor okadaic acid, indicating that PP2A is a target of PINK1. These findings indicate that PINK1 protects against α-Syn-induced neurodegeneration by promoting the dissociation of the calmodulin/Src complex and inhibiting Src, thereby enhancing PP2A activity. This was supported by the observation that PP2A activity was decreased in PD patients, which was negatively correlated with Hoehn and Yahr scores. Our results provide novel insight into the mechanisms underlying neurodegeneration in PD as well as possible avenues for therapeutic intervention in the treatment of this disease.

Pink1 interacts with α-synuclein and abrogates α-synuclein-induced neurotoxicity by activating autophagy

Parkinson’s disease (PD) is one of the most common neurodegenerative diseases, characterized by degeneration of dopaminergic neurons in the substantia nigra. α-synuclein (α-syn) and PTEN-induced putative kinase (PINK)1 are two critical proteins associated with the pathogenesis of PD. α-syn induces mitochondrial deficits and apoptosis, PINK1 was found to alleviate α-syn-induced toxicity, but the mechanistic details remain obscure. Here, we show that PINK1 interacts with α-syn mainly in the cytoplasm, where it initiates autophagy. This interaction was dependent on the kinase activity of PINK1 and was abolished by deletion of the kinase domain or a G309D point mutation, an inactivating mutation in the kinase domain. Interaction between PINK1 and α-syn stimulated the removal of excess α-syn, which prevented mitochondrial deficits and apoptosis. Our findings provide evidence for a novel mechanism underlying the protective effects of PINK1 against α-syn-induced neurodegeneration and highlight a novel therapeutic target for PD treatment.

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.

PINK1, Parkin, and Mitochondrial Quality Control: What can we Learn about Parkinson's Disease Pathobiology?

The first clinical description of Parkinson's disease (PD) will embrace its two century anniversary in 2017. For the past 30 years, mitochondrial dysfunction has been hypothesized to play a central role in the pathobiology of this devastating neurodegenerative disease. The identifications of mutations in genes encoding PINK1 (PTEN-induced kinase 1) and Parkin (E3 ubiquitin ligase) in familial PD and their functional association with mitochondrial quality control provided further support to this hypothesis. Recent research focused mainly on their key involvement in the clearance of damaged mitochondria, a process known as mitophagy. It has become evident that there are many other aspects of this complex regulated, multifaceted pathway that provides neuroprotection. As such, numerous additional factors that impact PINK1/Parkin have already been identified including genes involved in other forms of PD. A great pathogenic overlap amongst different forms of familial, environmental and even sporadic disease is emerging that potentially converges at the level of mitochondrial quality control. Tremendous efforts now seek to further detail the roles and exploit PINK1 and Parkin, their upstream regulators and downstream signaling pathways for future translation. This review summarizes the latest findings on PINK1/Parkin-directed mitochondrial quality control, its integration and cross-talk with other disease factors and pathways as well as the implications for idiopathic PD. In addition, we highlight novel avenues for the development of biomarkers and disease-modifying therapies that are based on a detailed understanding of the PINK1/Parkin pathway.

Mitochondrial Biology and Neurological Diseases

Mitochondria are extremely active organelles that perform a variety of roles in the cell including energy production, regulation of calcium homeostasis, apoptosis, and population maintenance through fission and fusion. Mitochondrial dysfunction in the form of oxidative stress and mutations can contribute to the pathogenesis of various neurodegenerative diseases such as Parkinson’s (PD), Alzheimer’s (AD), and Huntington’s diseases (HD). Abnormalities of Complex I function in the electron transport chain have been implicated in some neurodegenerative diseases, inhibiting ATP production and generating reactive oxygen species that can cause major damage to mitochondria

Mutations in both nuclear and mitochondrial DNA can contribute to neurodegenerative disease, although the pathogenesis of these conditions tends to focus on nuclear mutations. In PD, nuclear genome mutations in the PINK1 and parkin genes have been implicated in neurodegeneration [1], while mutations in APP, PSEN1 and PSEN2 have been implicated in a variety of clinical symptoms of AD [5]. Mutant htt protein is known to cause HD [2]. Much progress has been made to determine some causes of these neurodegenerative diseases, though permanent treatments have yet to be developed. In this review, we discuss the roles of mitochondrial dysfunction in the pathogenesis of these diseases.

PINK1 in the limelight: multiple functions of an eclectic protein in human health and disease

The gene PINK1 [phosphatase and tensin homologue (PTEN)-induced putative kinase 1] encodes a serine/threonine kinase which was initially linked to the pathogenesis of a familial form of Parkinson's disease. Research on PINK1 has recently unravelled that its multiple functions extend well beyond neuroprotection, implicating this eclectic protein in a growing number of human pathologies, including cancer, diabetes, cardiopulmonary dysfunctions, and inflammation. Extensive studies have identified PINK1 as a crucial player in the mitochondrial quality control pathway, required to label damaged mitochondria and promote their elimination through an autophagic process (mitophagy). Mounting evidence now indicates that PINK1 activities are not restricted solely to mitophagy, and that different subcellular and even sub-mitochondrial pools of PINK1 are involved in distinct signalling cascades to regulate cell metabolism and survival. In this review, we provide a concise overview on the different functions of PINK1 and their potential role in human diseases. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

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.

Gene therapy targeting mitochondrial pathway in Parkinson's disease

Parkinson's disease (PD) presents a relative selective localization of pathology to substantia nigra and well-defined motor symptoms caused by dopaminergic degeneration that makes it an ideal target for gene therapy. Parallel progress in viral vector systems enables the delivery of therapeutic genes directly into brain with reasonable safety along with sustained transgene expression. To date, gene therapy for PD that has reached clinical trial evaluation is mainly based on symptomatic approach that involves enzyme replacement strategy and restorative approach that depends on the addition of neurotrophic factors. Mitochondrial dysregulation, such as reduced complex I activity, increased mitochondria-derived reactive oxygen species (ROS) production, ROS-mediated mitochondrial DNA damage, bioenergetic failure, and perturbation of mitochondrial dynamics and mitophagy, has long been implicated in the pathogenesis of PD. Many of mutated genes linked to familial forms of PD affect these mitochondrial features. In this review, we discuss the recent progress that has been made in preclinical development of gene therapy targeting the mitochondrial pathway as disease modifying approach for PD. This review focuses on the potential therapeutic efficacy of candidate genes, including Parkin, PINK1, alpha synuclein, PGC-1 alpha, and anti-apoptotic molecules.

Roles of PTEN with DNA Repair in Parkinson's Disease

Oxidative stress is considered to play key roles in aging and pathogenesis of many neurodegenerative diseases such as Parkinson’s disease, which could bring DNA damage by cells. The DNA damage may lead to the cell apoptosis, which could contribute to the degeneration of neuronal tissues. Recent evidence suggests that PTEN (phosphatase and tensin homolog on chromosome 10) may be involved in the pathophysiology of the neurodegenerative disorders. Since PTEN expression appears to be one dominant determinant of the neuronal cell death, PTEN should be a potential molecular target of novel therapeutic strategies against Parkinson’s disease. In addition, defects in DNA damage response and DNA repair are often associated with modulation of hormone signaling pathways. Especially, many observations imply a role for estrogen in a regulation of the DNA repair action. In the present review, we have attempted to summarize the function of DNA repair molecules at a viewpoint of the PTEN signaling pathway and the hormone related functional modulation of cells, providing a broad interpretation on the molecular mechanisms for treatment of Parkinson’s disease. Particular attention will be paid to the mechanisms proposed to explain the health effects of food ingredients against Parkinson’s disease related to reduce oxidative stress for an efficient therapeutic intervention.

Subtle alterations of excitatory transmission are linked to presynaptic changes in the hippocampus of PINK1-deficient mice

Homozygous or heterozygous mutations in the PTEN-induced kinase 1 (PINK1) gene have been linked to early-onset Parkinson's disease (PD). Several neurophysiological studies have demonstrated alterations in striatal synaptic plasticity along with impaired dopamine release in PINK1-deficient mice. Using electrophysiological methods, here we show that PINK1 loss of function causes a progressive increase of spontaneous glutamate-mediated synaptic events in the hippocampus, without influencing long-term potentiation. Moreover, fluorescence analysis reveals increased neurotrasmitter release although our biochemical results failed to detect which presynaptic proteins might be engaged. This study provides a novel role for PINK1 beyond the physiology of nigrostriatal dopaminergic circuit. Specifically, PINK1 might contribute to preserve synaptic function and glutamatergic homeostasis in the hippocampus, a brain region underlying cognition. The subtle changes in excitatory transmission here observed might be a pathogenic precursor to excitotoxic neurodegeneration and cognitive decline often observed in PD. Using electrophysiological and fluorescence techniques, we demonstrate that lack of PINK1 causes increased excitatory transmission and neurotransmitter release in the hippocampus, which might lead to the cognitive decline often observed in Parkinson's disease.

Epigenetic Treatment of Neurodegenerative Disorders: Alzheimer and Parkinson Diseases

Preclinical Research In this review, we discuss epigenetic-driven methods for treating neurodegenerative disorders associated with mitochondrial dysfunction, focusing on carnitinoid antioxidant-histone deacetylase inhibitors that show an ability to reinvigorate synaptic plasticity and protect against neuromotor decline in vivo. Aging remains a major risk factor in patients who progress to dementia, a clinical syndrome typified by decreased mental capacity, including impairments in memory, language skills, and executive function. Energy metabolism and mitochondrial dysfunction are viewed as determinants in the aging process that may afford therapeutic targets for a host of disease conditions, the brain being primary in such thinking. Mitochondrial dysfunction is a core feature in the pathophysiology of both Alzheimer and Parkinson diseases and rare mitochondrial diseases. The potential of new therapies in this area extends to glaucoma and other ophthalmic disorders, migraine, Creutzfeldt-Jakob disease, post-traumatic stress disorder, systemic exertion intolerance disease, and chemotherapy-induced cognitive impairment. An emerging and hopefully more promising approach to addressing these hard-to-treat diseases leverages their sensitivity to activation of master regulators of antioxidant and cytoprotective genes, antioxidant response elements, and mitophagy. Drug Dev Res 77 : 109-123, 2016. © 2016 Wiley Periodicals, Inc.

High-content analysis of α-synuclein aggregation and cell death in a cellular model of Parkinson's disease

BACKGROUND

Alpha-synuclein (α-SYN) aggregates represent a key feature of Parkinson's disease, but the exact relationship between α-SYN aggregation and neurodegeneration remains incompletely understood. Therefore, the availability of a cellular assay that allows medium-throughput analysis of α-SYN-linked pathology will be of great value for studying the aggregation process and for advancing α-SYN-based therapies.

NEW METHOD

Here we describe a high-content neuronal cell assay that simultaneously measures oxidative stress-induced α-SYN aggregation and apoptosis.

RESULTS

We optimized an automated and reproducible assay to quantify both α-SYN aggregation and cell death in human SH-SY5Y neuroblastoma cells.

COMPARISON WITH EXISTING METHODS

Quantification of α-SYN aggregates in cells has typically relied on manual imaging and counting or cell-free assays, which are time consuming and do not allow a concurrent analysis of cell viability. Our high-content analysis method for quantification of α-SYN aggregation allows simultaneous measurements of multiple cell parameters at a single-cell level in a fast, objective and automated manner.

CONCLUSIONS

The presented analysis approach offers a rapid, objective and multiparametric approach for the screening of compounds and genes that might alter α-SYN aggregation and/or toxicity.

Animal models of Parkinson's disease: An updated overview

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose etiology, besides a minority of genetic cases, is still largely unknown. Animal models have contributed to elucidate PD etiology and pathogenesis, as well as its cellular and molecular mechanisms, leading to the general hypothesis that this neurological disorder is due to complex interactions between environmental and genetic factors. However, the full understanding of PD is still very far from being achieved, and new potential treatments need to be tested to further improve patients' quality of life and, possibly, slow down the neurodegenerative process. In this context, animal models of PD are required to address all these issues. "Classic" models are based on neurotoxins that selectively target catecholaminergic neurons (such as 6-hydroxydopamine, 1-methyl-1,2,3,6-tetrahydropiridine, agricultural pesticides, etc.), while more recent models employ genetic manipulations that either introduce mutations similar to those find in familial cases of PD (α-synuclein, DJ-1, PINK1, Parkin, etc.) or selectively disrupt nigrostriatal neurons (MitoPark, Pitx3, Nurr1, etc.). Each one of these models has its own advantages and limitations, thus some are better suited for studying PD pathogenesis, while others are more pertinent to test therapeutic treatments. Here, we provide a critical and updated review of the most used PD models.

Intracellular Dynamics of Synucleins: "Here, There and Everywhere"

Synucleins are small, soluble proteins expressed primarily in neural tissue and in certain tumors. The synuclein family consists of three members: α-, β-, and γ-synucleins present only in vertebrates. Members of the synuclein family have high sequence identity, especially in the N-terminal regions. The synuclein gene family came into the spotlight, when one of its members, α-synuclein, was found to be associated with Parkinson's disease and other neurodegenerative disorders, whereas γ-synuclein was linked to several forms of cancer. There are a lot of controversy and exciting debates concerning members of the synuclein family, including their normal functions, toxicity, role in pathology, transmission between cells and intracellular localization. Important findings which remain undisputable for many years are synuclein localization in synapses and their role in the regulation of synaptic vesicle trafficking, whereas their presence and function in mitochondria and nucleus is a debated topic. In this review, we present the data on the localization of synucleins in two intracellular organelles: the nucleus and mitochondria.

Mitophagy and the mitochondrial unfolded protein response in neurodegeneration and bacterial infection

Mitochondria are highly dynamic and structurally complex organelles that provide multiple essential metabolic functions. Mitochondrial dysfunction is associated with neurodegenerative conditions such as Parkinson’s disease, as well as bacterial infection. Here, we explore the roles of mitochondrial autophagy (mitophagy) and the mitochondrial unfolded protein response (UPR^mt) in the response to mitochondrial dysfunction, focusing in particular on recent evidence on the role of mitochondrial import efficiency in the regulation of these stress pathways and how they may interact to protect the mitochondrial pool while initiating an innate immune response to protect against bacterial pathogens.

The roles of PINK1, parkin, and mitochondrial fidelity in Parkinson's disease

Understanding the function of genes mutated in hereditary forms of Parkinson's disease yields insight into disease etiology and reveals new pathways in cell biology. Although mutations or variants in many genes increase the susceptibility to Parkinson's disease, only a handful of monogenic causes of parkinsonism have been identified. Biochemical and genetic studies reveal that the products of two genes that are mutated in autosomal recessive parkinsonism, PINK1 and Parkin, normally work together in the same pathway to govern mitochondrial quality control, bolstering previous evidence that mitochondrial damage is involved in Parkinson's disease. PINK1 accumulates on the outer membrane of damaged mitochondria, activates Parkin's E3 ubiquitin ligase activity, and recruits Parkin to the dysfunctional mitochondrion. Then, Parkin ubiquitinates outer mitochondrial membrane proteins to trigger selective autophagy. This review covers the normal functions that PINK1 and Parkin play within cells, their molecular mechanisms of action, and the pathophysiological consequences of their loss.

Parkinson's disease: animal models and dopaminergic cell vulnerability

Parkinson's disease (PD) is a neurodegenerative disorder that affects about 1.5% of the global population over 65 years of age. A hallmark feature of PD is the degeneration of the dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and the consequent striatal DA deficiency. Yet, the pathogenesis of PD remains unclear. Despite tremendous growth in recent years in our knowledge of the molecular basis of PD and the molecular pathways of cell death, important questions remain, such as: (1) why are SNc cells especially vulnerable; (2) which mechanisms underlie progressive SNc cell loss; and (3) what do Lewy bodies or α-synuclein reveal about disease progression. Understanding the variable vulnerability of the dopaminergic neurons from the midbrain and the mechanisms whereby pathology becomes widespread are some of the primary objectives of research in PD. Animal models are the best tools to study the pathogenesis of PD. The identification of PD-related genes has led to the development of genetic PD models as an alternative to the classical toxin-based ones, but does the dopaminergic neuronal loss in actual animal models adequately recapitulate that of the human disease? The selection of a particular animal model is very important for the specific goals of the different experiments. In this review, we provide a summary of our current knowledge about the different in vivo models of PD that are used in relation to the vulnerability of the dopaminergic neurons in the midbrain in the pathogenesis of PD.

Potentiation of neurotoxicity in double-mutant mice with Pink1 ablation and A53T-SNCA overexpression

The common age-related neurodegeneration of Parkinson's disease can result from dominant causes like increased dosage of vesicle-associated alpha-synuclein (SNCA) or recessive causes like deficiency of mitophagy factor PINK1. Interactions between these triggers and their convergence onto shared pathways are crucial, but currently conflicting evidence exists. Here, we crossed previously characterized mice with A53T-SNCA overexpression and with Pink1 deletion to generate double mutants (DMs). We studied their lifespan and behavior, histological and molecular anomalies at late and early ages. DM animals showed potentiated phenotypes in comparison with both single mutants (SMs), with reduced survival and strongly reduced spontaneous movements from the age of 3 months onwards. In contrast to SMs, a quarter of DM animals manifested progressive paralysis at ages >1 year and exhibited protein aggregates immunopositive for pSer129-SNCA, p62 and ubiquitin in spinal cord and basal brain. Brain proteome quantifications of ubiquitination sites documented altered degradation of SNCA and the DNA-damage marker H2AX at the age of 18 months. Global brain transcriptome profiles and qPCR validation experiments identified many consistent transcriptional dysregulations already at the age of 6 weeks, which were absent from SMs. The observed downregulations for Dapk1, Dcaf17, Rab42 and the novel SNCA-marker Lect1 as well as the upregulations for Dctn5, Mrpl9, Tmem181a, Xaf1 and H2afx reflect changes in ubiquitination, mitochondrial/synaptic/microtubular/cell adhesion dynamics and DNA damage. Thus, our study confirmed that SNCA-triggered neurotoxicity is exacerbated by the absence of PINK1 and identified a novel molecular signature that is detectable early in the course of this double pathology.