Silencing of the Pink1 gene expression by conditional RNAi does not induce dopaminergic neuron death in mice

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.

Mitochondrial Bioenergy in Neurodegenerative Disease: Huntington and Parkinson

Strong evidence suggests a correlation between degeneration and mitochondrial deficiency. Typical cases of degeneration can be observed in physiological phenomena (i.e., ageing) as well as in neurological neurodegenerative diseases and cancer. All these pathologies have the dyshomeostasis of mitochondrial bioenergy as a common denominator. Neurodegenerative diseases show bioenergetic imbalances in their pathogenesis or progression. Huntington's chorea and Parkinson's disease are both neurodegenerative diseases, but while Huntington's disease is genetic and progressive with early manifestation and severe penetrance, Parkinson's disease is a pathology with multifactorial aspects. Indeed, there are different types of Parkinson/Parkinsonism. Many forms are early-onset diseases linked to gene mutations, while others could be idiopathic, appear in young adults, or be post-injury senescence conditions. Although Huntington's is defined as a hyperkinetic disorder, Parkinson's is a hypokinetic disorder. However, they both share a lot of similarities, such as neuronal excitability, the loss of striatal function, psychiatric comorbidity, etc. In this review, we will describe the start and development of both diseases in relation to mitochondrial dysfunction. These dysfunctions act on energy metabolism and reduce the vitality of neurons in many different brain areas.

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.

Urolithin A attenuates auditory cell senescence by activating mitophagy

Aging of sensory organs is associated with a decline in mitochondrial function and the accumulation of dysfunctional mitochondria. Impaired mitophagy blocks the turnover of dysfunctional mitochondria and leads to their accumulation. Urolithin A (UA) induces mitophagy in various mammalian cells. This study was aimed at investigating the effect of the mitophagy activator, UA, on premature senescent auditory cells. The levels of cellular senescence-associated p53 and p21 significantly increased in H₂O₂-induced senescent House Ear Institute-Organ of Corti 1 (HEI-OC1) cells and cochlear explants. However, the levels of mitophagy-related molecules significantly decreased. UA significantly decreased the expression of senescence-associated p53 and p21, and increased the expression of mitophagy-related proteins, in H₂O₂-induced senescent cells and cochlear explants. The percentage of β-galactosidase-stained senescent cells also reduced in H₂O₂-treated cells and cochlear explants upon UA pre-treatment. The formation of mitophagosomes and mitophagolysosomes was restored upon UA pre-treatment of H₂O₂-induced senescent cells. The knockdown of mitophagy-related genes (Parkin and Bnip3) resulted in annulment of UA-induced anti-senescent activity. UA significantly increased the ATP content, mitochondrial DNA (mtDNA) integrity, and mitochondrial membrane potential in senescent HEI-OC1 cells. These findings indicate that UA counteracted mitophagy decline and prevented premature senescence in auditory cells. Hence, UA administration might be a promising strategy for preventing mitochondrial dysfunction in patients with age-related hearing loss.

A Critical Review of Zebrafish Models of Parkinson's Disease

A wide variety of human diseases have been modelled in zebrafish, including various types of cancer, cardiovascular diseases and neurodegenerative diseases like Alzheimer’s and Parkinson’s. Recent reviews have summarized the currently available zebrafish models of Parkinson’s Disease, which include gene-based, chemically induced and chemogenetic ablation models. The present review updates the literature, critically evaluates each of the available models of Parkinson’s Disease in zebrafish and compares them with similar models in invertebrates and mammals to determine their advantages and disadvantages. We examine gene-based models, including ones linked to Early-Onset Parkinson’s Disease: PARKIN, PINK1, DJ-1, and SNCA; but we also examine LRRK2, which is linked to Late-Onset Parkinson’s Disease. We evaluate chemically induced models like MPTP, 6-OHDA, rotenone and paraquat, as well as chemogenetic ablation models like metronidazole-nitroreductase. The article also reviews the unique advantages of zebrafish, including the abundance of behavioural assays available to researchers and the efficiency of high-throughput screens. This offers a rare opportunity for assessing the potential therapeutic efficacy of pharmacological interventions. Zebrafish also are very amenable to genetic manipulation using a wide variety of techniques, which can be combined with an array of advanced microscopic imaging methods to enable in vivo visualization of cells and tissue. Taken together, these factors place zebrafish on the forefront of research as a versatile model for investigating disease states. The end goal of this review is to determine the benefits of using zebrafish in comparison to utilising other animals and to consider the limitations of zebrafish for investigating human disease.

New pathogenic insights from large animal models of neurodegenerative diseases

Animal models are essential for investigating the pathogenesis and developing the treatment of human diseases. Identification of genetic mutations responsible for neurodegenerative diseases has enabled the creation of a large number of small animal models that mimic genetic defects found in the affected individuals. Of the current animal models, rodents with genetic modifications are the most commonly used animal models and provided important insights into pathogenesis. However, most of genetically modified rodent models lack overt neurodegeneration, imposing challenges and obstacles in utilizing them to rigorously test the therapeutic effects on neurodegeneration. Recent studies that used CRISPR/Cas9-targeted large animal (pigs and monkeys) have uncovered important pathological events that resemble neurodegeneration in the patient’s brain but could not be produced in small animal models. Here we highlight the unique nature of large animals to model neurodegenerative diseases as well as the limitations and challenges in establishing large animal models of neurodegenerative diseases, with focus on Huntington disease, Amyotrophic lateral sclerosis, and Parkinson diseases. We also discuss how to use the important pathogenic insights from large animal models to make rodent models more capable of recapitulating important pathological features of neurodegenerative diseases.

Autophagy system as a potential therapeutic target for neurodegenerative diseases

Autophagy is an evolutionally conserved process by which cytoplasmic contents including protein aggregates and damaged organelles such as mitochondria and lysosomes, are sequestered by double-membrane structure, autophagosomes, and delivered to the lysosomes for degradation. Recently, considerable efforts have been made to reveal the role of autophagy in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Huntington's disease. Impairment of autophagy aggravates the accumulation of misfolded protein and damaged organelles in neurons, while sufficient autophagic activity reduces such accumulation in nervous system and ameliorates the pathology. Here we summarize recent progress regarding the role of autophagy in several neurodegenerative diseases and the potential autophagy-associated therapies for them.

Genetically modified large animal models for investigating neurodegenerative diseases

Neurodegenerative diseases represent a large group of neurological disorders including Alzheimer’s disease, amyotrophic lateral sclerosis, Parkinson’s disease, and Huntington’s disease. Although this group of diseases show heterogeneous clinical and pathological phenotypes, they share important pathological features characterized by the age-dependent and progressive degeneration of nerve cells that is caused by the accumulation of misfolded proteins. The association of genetic mutations with neurodegeneration diseases has enabled the establishment of various types of animal models that mimic genetic defects and have provided important insights into the pathogenesis. However, most of genetically modified rodent models lack the overt and selective neurodegeneration seen in the patient brains, making it difficult to use the small animal models to validate the effective treatment on neurodegeneration. Recent studies of pig and monkey models suggest that large animals can more faithfully recapitulate pathological features of neurodegenerative diseases. In this review, we discuss the important differences in animal models for modeling pathological features of neurodegenerative diseases, aiming to assist the use of animal models to better understand the pathogenesis and to develop effective therapeutic strategies.

Animal Models of Autosomal Recessive Parkinsonism

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder. The neuropathological hallmark of the disease is the loss of dopamine neurons of the substantia nigra pars compacta. The clinical manifestations of PD are bradykinesia, rigidity, resting tremors and postural instability. PD patients often display non-motor symptoms such as depression, anxiety, weakness, sleep disturbances and cognitive disorders. Although, in 90% of cases, PD has a sporadic onset of unknown etiology, highly penetrant rare genetic mutations in many genes have been linked with typical familial PD. Understanding the mechanisms behind the DA neuron death in these Mendelian forms may help to illuminate the pathogenesis of DA neuron degeneration in the more common forms of PD. A key step in the identification of the molecular pathways underlying DA neuron death, and in the development of therapeutic strategies, is the creation and characterization of animal models that faithfully recapitulate the human disease. In this review, we outline the current status of PD modeling using mouse, rat and non-mammalian models, focusing on animal models for autosomal recessive PD.

Shedding light on mitophagy in neurons: what is the evidence for PINK1/Parkin mitophagy in vivo?

Neurons are highly specialised cells with a large bioenergetic demand, and so require a healthy mitochondrial network to function effectively. This network is compromised in many neurological disorders, in which damaged mitochondria accumulate. Dysfunctional mitochondria can be removed via an organelle-specific autophagic pathway, a process known as mitophagy. The canonical mitophagy pathway is dependent on the actions of PINK1 (PTEN-induced putative kinase 1) and Parkin and has been well studied in immortalised cells and cultured neurons. However, evidence for a role of this mitophagy pathway in the brain is still limited, and studies suggest that there may be important differences in how neurons respond to mitochondrial damage in vitro and in vivo. Here, we first describe the evidence for a functional PINK1/Parkin mitophagy pathway in neurons, and review how this pathway is affected in disease models. We then critically evaluate the literature by comparing findings from in vitro models and more recent in vivo studies in flies and mice. The emerging picture implicates that alternative mitophagy pathways operate in neurons in vivo. New mouse models that employ fluorescent biosensors to monitor mitophagy in vivo will be instrumental to understand the relative role of the different clearance pathways in the brain under physiological and pathological conditions.

PINK1 Primes Parkin-Mediated Ubiquitination of PARIS in Dopaminergic Neuronal Survival

Mutations in PTEN-induced putative kinase 1 (PINK1) and parkin cause autosomal-recessive Parkinson's disease through a common pathway involving mitochondrial quality control. Parkin inactivation leads to accumulation of the parkin interacting substrate (PARIS, ZNF746) that plays an important role in dopamine cell loss through repression of proliferator-activated receptor gamma coactivator-1-alpha (PGC-1α) promoter activity. Here, we show that PARIS links PINK1 and parkin in a common pathway that regulates dopaminergic neuron survival. PINK1 interacts with and phosphorylates serines 322 and 613 of PARIS to control its ubiquitination and clearance by parkin. PINK1 phosphorylation of PARIS alleviates PARIS toxicity, as well as repression of PGC-1α promoter activity. Conditional knockdown of PINK1 in adult mouse brains leads to a progressive loss of dopaminergic neurons in the substantia nigra that is dependent on PARIS. Altogether, these results uncover a function of PINK1 to direct parkin-PARIS-regulated PGC-1α expression and dopaminergic neuronal survival.

Parkin and PINK1 functions in oxidative stress and neurodegeneration

Loss-of-function mutations in the genes encoding Parkin and PINK1 are causally linked to autosomal recessive Parkinson's disease (PD). Parkin, an E3 ubiquitin ligase, and PINK1, a mitochondrial-targeted kinase, function together in a common pathway to remove dysfunctional mitochondria by autophagy. Presumably, deficiency for Parkin or PINK1 impairs mitochondrial autophagy and thereby increases oxidative stress due to the accumulation of dysfunctional mitochondria that release reactive oxygen species. Parkin and PINK1 likely have additional functions that may be relevant to the mechanisms by which mutations in these genes cause neurodegeneration, such as regulating inflammation, apoptosis, or dendritic morphogenesis. Here we briefly review what is known about functions of Parkin and PINK1 related to oxidative stress and neurodegeneration.

Mouse Genetic Models of Human Brain Disorders

Over the past three decades, genetic manipulations in mice have been used in neuroscience as a major approach to investigate the in vivo function of genes and their alterations. In particular, gene targeting techniques using embryonic stem cells have revolutionized the field of mammalian genetics and have been at the forefront in the generation of numerous mouse models of human brain disorders. In this review, we will first examine childhood developmental disorders such as autism, intellectual disability, Fragile X syndrome, and Williams-Beuren syndrome. We will then explore psychiatric disorders such as schizophrenia and lastly, neurodegenerative disorders including Alzheimer’s disease and Parkinson’s disease. We will outline the creation of these mouse models that range from single gene deletions, subtle point mutations to multi-gene manipulations, and discuss the key behavioral phenotypes of these mice. Ultimately, the analysis of the models outlined in this review will enhance our understanding of the in vivo role and underlying mechanisms of disease-related genes in both normal brain function and brain disorders, and provide potential therapeutic targets and strategies to prevent and treat these diseases.

Neuroinflammation in Parkinson's disease animal models: a cell stress response or a step in neurodegeneration?

The motor symptoms of Parkinson's disease are due to the progressive degeneration of dopaminergic neurons in the substantia nigra. Multiple neuroinflammatory processes are exacerbated in Parkinson's disease, including glial-mediated reactions, increased expression of proinflammatory substances, and lymphocytic infiltration, particularly in the substantia nigra. Neuroinflammation is also implicated in the neurodegeneration and consequent behavioral symptoms of many Parkinson's disease animal models, although it is not clear whether these features emulate pathogenic steps in the genuine disorder or if some inflammatory features provide protective stress responses. Here, we compare and summarize findings on neuroinflammatory responses and effects on behavior in a wide range of toxin-based, inflammatory and genetic Parkinson's disease animal models.

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

Loss-of-function mutations in the PINK1 gene lead to recessive forms of Parkinson's disease. Animal models with depleted PINK1 expression have failed to reproduce significant nigral dopaminergic neurodegeneration and clear alpha-synuclein pathology, main characteristics of the disease. In this study, we investigated whether alpha-synuclein pathology is altered in the absence of PINK1 in cell culture and in vivo. We observed that downregulation of PINK1 enhanced alpha-synuclein aggregation and apoptosis in a neuronal cell culture model for synucleinopathy. Silencing of PINK1 expression in mouse substantia nigra using recombinant adeno-associated viral vectors did not induce dopaminergic neurodegeneration in a long-term study up to 10 months, nor did it enhance or accelerate dopaminergic neurodegeneration after alpha-synuclein overexpression. However, in PINK1 knockout mice, overexpression of alpha-synuclein in the substantia nigra resulted in enhanced dopaminergic neurodegeneration as well as significantly higher levels of alpha-synuclein phosphorylation at serine 129 at 4 weeks postinjection. In conclusion, our results demonstrate that total loss of PINK1 leads to an increased sensitivity to alpha-synuclein-induced neuropathology and cell death in vivo.

Mutant PINK1 upregulates tyrosine hydroxylase and dopamine levels, leading to vulnerability of dopaminergic neurons

PINK1 mutations cause autosomal recessive forms of Parkinson disease (PD). Previous studies suggest that the neuroprotective function of wild-type (WT) PINK1 is related to mitochondrial homeostasis. PINK1 can also localize to the cytosol; however, the cytosolic function of PINK1 has not been fully elucidated. In this study we demonstrate that the extramitochondrial PINK1 can regulate tyrosine hydroxylase (TH) expression and dopamine (DA) content in dopaminergic neurons in a PINK1 kinase activity-dependent manner. We demonstrate that overexpression of full-length (FL) WT PINK1 can downregulate TH expression and DA content in dopaminergic neurons. In contrast, overexpression of PD-linked G309D, A339T, and E231G PINK1 mutations upregulates TH and DA levels in dopaminergic neurons and increases their vulnerability to oxidative stress. Furthermore transfection of FL WT PINK1 or PINK1 fragments with the PINK1 kinase domain can inhibit TH expression, whereas kinase-dead (KD) FL PINK1 or KD PINK1 fragments upregulate TH level. Our findings highlight a potential novel function of extramitochondrial PINK1 in dopaminergic neurons. Deregulation of these functions of PINK1 may contribute to PINK1 mutation-induced dopaminergic neuron degeneration. However, deleterious effects caused by PINK1 mutations may be alleviated by iron-chelating agents and antioxidant agents with DA quinone-conjugating capacity.

Defining functional gene-circuit interfaces in the mouse nervous system

Complexity in the nervous system is established by developmental genetic programs, maintained by differential genetic profiles and sculpted by experiential and environmental influence over gene expression. Determining how specific genes define neuronal phenotypes, shape circuit connectivity and regulate circuit function is essential for understanding how the brain processes information, directs behavior and adapts to changing environments. Mouse genetics has contributed greatly to current percepts of gene-circuit interfaces in behavior, but considerable work remains. Large-scale initiatives to map gene expression and connectivity in the brain, together with advanced techniques in molecular genetics, now allow detailed exploration of the genetic basis of nervous system function at the level of specific circuit connections. In this review, we highlight several key advances for defining the function of specific genes within a neural network.

Gudu, an Armadillo repeat-containing protein, is required for spermatogenesis in Drosophila

The Drosophila annotated gene CG5155 encodes a protein that contains 10 Armadillo-repeats and has an unknown function. To fill this gap, we performed loss-of-function studies using RNAi. By analysis of four independent Drosophila RNAi lines targeting two non-overlapping regions of the CG5155 transcript, we demonstrate that this gene is required for male fertility. Therefore, we have named this gene Gudu. The transcript of Gudu is highly enriched in adult testes. Knockdown of Gudu by a ubiquitous driver leads to defects in the formation of the individualization complex that is required for spermatid maturation, thereby impairing spermatogenesis. Furthermore, testis-specific knockdown of Gudu by crossing the RNAi lines with the bam-Gal4 driver is sufficient to cause the infertility and defective spermatogenesis. Since Gudu is highly homologous to vertebrate ARMC4, also an Armadillo-repeat-containing protein enriched in testes, our results suggest that Gudu and ARMC4 are a subfamily of Armadillo-repeat containing proteins that may have an evolutionarily conserved function in spermatogenesis.

Pathogenic mutation in VPS35 impairs its protection against MPP(+) cytotoxicity

Parkinson's disease primarily results from progressive degeneration of dopaminergic neurons in the substantia nigra. Both neuronal toxicants and genetic factors are suggested to be involved in the disease pathogenesis. The mitochondrial toxicant 1-methyl-4-phenylpyridinium (MPP(+)) shows a highly selective toxicity to dopaminergic neurons. Recent studies indicate that mutation in the vacuolar protein sorting 35 (vps35) gene segregates with Parkinson's disease in some families, but how mutation in the vps35 gene causes dopaminergic cell death is not known. Here, we report that enhanced VPS35 expression protected dopaminergic cells against MPP(+) toxicity and that this neuroprotection was compromised by pathogenic mutation in the gene. A loss of neuroprotective functions contributes to the pathogenesis of VPS35 mutation in Parkinson's disease.

Animal models of Parkinson's disease: vertebrate genetics

Parkinson's disease (PD) is a complex genetic disorder that is associated with environmental risk factors and aging. Vertebrate genetic models, especially mice, have aided the study of autosomal-dominant and autosomal-recessive PD. Mice are capable of showing a broad range of phenotypes and, coupled with their conserved genetic and anatomical structures, provide unparalleled molecular and pathological tools to model human disease. These models used in combination with aging and PD-associated toxins have expanded our understanding of PD pathogenesis. Attempts to refine PD animal models using conditional approaches have yielded in vivo nigrostriatal degeneration that is instructive in ordering pathogenic signaling and in developing therapeutic strategies to cure or halt the disease. Here, we provide an overview of the generation and characterization of transgenic and knockout mice used to study PD followed by a review of the molecular insights that have been gleaned from current PD mouse models. Finally, potential approaches to refine and improve current models are discussed.

Regulation of Parkin E3 ubiquitin ligase activity

Parkin is an E3 ubiquitin ligase mutated in autosomal recessive juvenile Parkinson's disease. In addition, it is a putative tumour suppressor, and has roles outside its enzymatic activity. It is critical for mitochondrial clearance through mitophagy, and is an essential protein in most eukaryotes. As such, it is a tightly controlled protein, regulated through an array of external interactions with multiple proteins, posttranslational modifications including phosphorylation and S-nitrosylation, and self-regulation through internal associations. In this review, we highlight some of the recent studies into Parkin regulation and discuss future challenges for gaining a full molecular understanding of the regulation of Parkin E3 ligase activity.

Oxidative stress in genetic mouse models of Parkinson's disease

There is extensive evidence in Parkinson's disease of a link between oxidative stress and some of the monogenically inherited Parkinson's disease-associated genes. This paper focuses on the importance of this link and potential impact on neuronal function. Basic mechanisms of oxidative stress, the cellular antioxidant machinery, and the main sources of cellular oxidative stress are reviewed. Moreover, attention is given to the complex interaction between oxidative stress and other prominent pathogenic pathways in Parkinson's disease, such as mitochondrial dysfunction and neuroinflammation. Furthermore, an overview of the existing genetic mouse models of Parkinson's disease is given and the evidence of oxidative stress in these models highlighted. Taken into consideration the importance of ageing and environmental factors as a risk for developing Parkinson's disease, gene-environment interactions in genetically engineered mouse models of Parkinson's disease are also discussed, highlighting the role of oxidative damage in the interplay between genetic makeup, environmental stress, and ageing in Parkinson's disease.

Mitochondrial dysfunction in genetic animal models of Parkinson's disease

Mitochondria are highly dynamic, multifunctional organelles. Aside from their major role in energy metabolism, they are also crucial for many cellular processes including neurotransmission, synaptic maintenance, calcium homeostasis, cell death, and neuronal survival.

SIGNIFICANCE

Increasing evidence supports a role for abnormal mitochondrial function in the molecular pathophysiology of Parkinson's disease (PD). For three decades we have known that mitochondrial toxins are capable of producing clinical parkinsonism in humans. PD is the most common neurodegenerative movement disorder that is characterized by the progressive loss of substantia nigra dopaminergic neurons leading to a deficiency of striatal dopamine. Although the neuropathology underlying the disease is well defined, it remains unclear why nigral dopaminergic neurons degenerate and die.

RECENT ADVANCES

Most PD cases are idiopathic, but there are rare familial cases. Mutations in five genes are known to unambiguously cause monogenic familial PD: α-synuclein, parkin, DJ-1, PTEN-induced kinase 1 (PINK1), and leucine-rich repeat kinase 2 (LRRK2). These key molecular players are proteins of seemingly diverse function, but with potentially important roles in mitochondrial maintenance and function. Cell and animal-based genetic models have provided indispensable tools for understanding the molecular basis of PD, and have provided additional evidence implicating mitochondrial dysfunction as a primary pathogenic pathway leading to the demise of dopaminergic neurons in PD.

CRITICAL ISSUES

Here, we critically discuss the evidence for mitochondrial dysfunction in genetic animal models of PD, and evaluate whether abnormal mitochondrial function represents a cause or consequence of disease pathogenesis.

FUTURE DIRECTIONS

Mitochondria may represent a potential target for the development of disease-modifying therapies.

Inactivation of Pink1 gene in vivo sensitizes dopamine-producing neurons to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and can be rescued by autosomal recessive Parkinson disease genes, Parkin or DJ-1

Mutations in the mitochondrial PTEN-induced kinase 1 (Pink1) gene have been linked to Parkinson disease (PD). Recent reports including our own indicated that ectopic Pink1 expression is protective against toxic insult in vitro, suggesting a potential role for endogenous Pink1 in mediating survival. However, the role of endogenous Pink1 in survival, particularly in vivo, is unclear. To address this critical question, we examined whether down-regulation of Pink1 affects dopaminergic neuron loss following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the adult mouse. Two model systems were utilized: virally delivered shRNA-mediated knockdown of Pink1 and germ line-deficient mice. In both instances, loss of Pink1 generated significant sensitivity to damage induced by systemic MPTP treatment. This sensitivity was associated with greater loss of dopaminergic neurons in the Substantia Nigra pars compacta and terminal dopamine fiber density in the striatum region. Importantly, we also show that viral mediated expression of two other recessive PD-linked familial genes, DJ-1 and Parkin, can protect dopaminergic neurons even in the absence of Pink1. This evidence not only provides strong evidence for the role of endogenous Pink1 in neuronal survival, but also supports a role of DJ-1 and Parkin acting parallel or downstream of endogenous Pink1 to mediate survival in a mammalian in vivo context.

Parkin, PINK1 and mitochondrial integrity: emerging concepts of mitochondrial dysfunction in Parkinson's disease

Mitochondria are dynamic organelles which are essential for many cellular processes, such as ATP production by oxidative phosphorylation, lipid metabolism, assembly of iron sulfur clusters, regulation of calcium homeostasis, and cell death pathways. The dynamic changes in mitochondrial morphology, connectivity, and subcellular distribution are critically dependent on a highly regulated fusion and fission machinery. Mitochondrial function, dynamics, and quality control are vital for the maintenance of neuronal integrity. Indeed, there is mounting evidence that mitochondrial dysfunction plays a central role in several neurodegenerative diseases. In particular, the identification of genes linked to rare familial variants of Parkinson's disease has fueled research on mitochondrial aspects of the disease etiopathogenesis. Studies on the function of parkin and PINK1, which are associated with autosomal recessive parkinsonism, provided compelling evidence that these proteins can functionally interact to maintain mitochondrial integrity and to promote clearance of damaged and dysfunctional mitochondria. In this review we will summarize current knowledge about the impact of parkin and PINK1 on mitochondria.

Silencing myostatin gene by RNAi in sheep embryos

Myostatin (MSTN) gene is described as a negative regulator of the skeletal muscle growth. Controlling MSTN gene expression by genetic manipulation could accelerate the muscle growth and meat production of livestock animals. In the present study, several siRNAs targeting sheep MSTN gene were designed and their interfering efficiency was evaluated in vitro. The present study showed that one of the siRNAs, PSL1, could down-regulate the expression of MSTN significantly. PSL1 was ligated into lentivirus vector, GP-Supersilencing, to construct a siRNA expression lentivirus vector. Fibroblast cells were infected by lentivirus particles and positive cells were isolated by flow cytometry. Nucleus of the positive cell was transferred into enucleated oocytes of sheep. The present study showed that 99.4% of the sorted cells displayed green fluorescence. After enucleation of oocytes with microinjection, about 20% of reconstructed embryos can be developed into morulas, and strong green fluorescence could be observed using a fluorescence microscope. This method can be available to produce transgenic cell line for somatic cell nucleus transfer for transgenic animals.

Early exposure to paraquat sensitizes dopaminergic neurons to subsequent silencing of PINK1 gene expression in mice

Environmental exposure, genetic modification, and aging are considered risky for Parkinson's disease (PD). How these risk factors cooperate to induce progressive neurodegeneration in PD remains largely unknown. Paraquat is an herbicide commonly used for weed and grass control. Exposure to paraquat is associated with the increased incidence of PD. In contrast to familial PD, most sporadic PD cases do not have genetic mutation, but may suffer from partial dysfunction of neuron-protective genes as aging. Using conditional transgenic RNAi, we showed that temporal silencing of PINK1 expression in adult mice increased striatal dopamine, the phenotype that could not be induced by constitutive gene silencing. Moreover, early exposure to paraquat sensitized dopaminergic neurons to subsequent silencing of PINK1 gene expression, leading to a significant loss of dopaminergic neurons. Our findings suggest a novel pathogenesis of PD: exposure to environmental toxicants early in the life reduces the threshold of developing PD and partial dysfunction of neuron-protective genes later in the life initiates a process of progressive neurodegeneration to cross the reduced threshold of disease onset.

Temporal expression of mutant LRRK2 in adult rats impairs dopamine reuptake

Parkinson's disease (PD) results from progressive degeneration of dopaminergic neurons. Most PD cases are sporadic, but some have pathogenic mutation in the individual genes. Mutation of the leucine-rich repeat kinase-2 (LRRK2) gene is associated with familial and sporadic PD, as exemplified by G2019S substitution. While constitutive expression of mutant LRRK2 in transgenic mice fails to induce neuron death, transient expression of the disease gene by viral delivery causes a substantial loss of dopaminergic neurons in mice. To further assess LRRK2 pathogenesis, we created inducible transgenic rats expressing human LRRK2 with G2019S substitution. Temporal overexpression of LRRK2(G2019S) in adult rats impaired dopamine reuptake by dopamine transporter (DAT) and thus enhanced locomotor activity, the phenotypes that were not observed in transgenic rats constitutively expressing the gene throughout life time. Reduced DAT binding activity is an early sign of dopaminergic dysfunction in asymptomatic subjects carrying pathogenic mutation in LRRK2. Our transgenic rats recapitulated the initiation process of dopaminergic dysfunction caused by pathogenic mutation in LRRK2. Inducible transgenic approach uncovered phenotypes that may be obscured by developmental compensation in constitutive transgenic rats. Finding in inducible LRRK2 transgenic rats would guide developing effective strategy in transgenic studies: Inducible expression of transgene may induce greater phenotypes than constitutive gene expression, particularly in rodents with short life time.

Depletion of PINK1 affects mitochondrial metabolism, calcium homeostasis and energy maintenance

Loss-of-function mutations in the gene encoding the mitochondrial PTEN-induced putative kinase 1 (PINK1) are a major cause of early-onset familial Parkinson's disease (PD). Recent studies have highlighted an important function for PINK1 in clearing depolarized mitochondria by mitophagy. However, the role of PINK1 in mitochondrial and cellular functioning in physiological conditions is still incompletely understood. Here, we investigate mitochondrial and cellular calcium (Ca2+) homeostasis in PINK1-knockdown and PINK1-knockout mouse cells, both in basal metabolic conditions and after physiological stimulation, using unbiased automated live single-cell imaging in combination with organelle-specific fluorescent probes. Our data reveal that depletion of PINK1 induces moderate fragmentation of the mitochondrial network, mitochondrial membrane depolarization and increased production of reactive oxygen species. This results in reduced uptake of Ca2+ by mitochondria after physiological stimulation. As a consequence, cells with knockdown or knockout of PINK1 display impaired mitochondrial ATP synthesis, which is exacerbated under conditions of increased ATP demand, thereby affecting cytosolic Ca2+ extrusion. The impairment in energy maintenance was confirmed in the brain of PINK1-knockout mice by in vivo bioluminescence imaging. Our findings demonstrate a key role for PINK1 in the regulation of mitochondrial homeostasis and energy metabolism under physiological conditions.

TDP-43 potentiates alpha-synuclein toxicity to dopaminergic neurons in transgenic mice

TDP-43 and α-synuclein are two disease proteins involved in a wide range of neurodegenerative diseases. While TDP-43 proteinopathy is considered a pathologic hallmark of sporadic amyotrophic lateral sclerosis and frontotemporal lobe degeneration, α-synuclein is a major component of Lewy body characteristic of Parkinson's disease. Intriguingly, TDP-43 proteinopathy also coexists with Lewy body and with synucleinopathy in certain disease conditions. Here we reported the effects of TDP-43 on α-synuclein neurotoxicity in transgenic mice. Overexpression of mutant TDP-43 (M337V substitution) in mice caused early death in transgenic founders, but overexpression of normal TDP-43 only induced a moderate loss of cortical neurons in the transgenic mice at advanced ages. Interestingly, concomitant overexpression of normal TDP-43 and mutant α-synuclein caused a more severe loss of dopaminergic neurons in the double transgenic mice as compared to single-gene transgenic mice. TDP-43 potentiated α-synuclein toxicity to dopaminergic neurons in living animals. Our finding provides in vivo evidence suggesting that disease proteins such as TDP-43 and α-synuclein may play a synergistic role in disease induction in neurodegenerative diseases.

Expression of PINK1 in the brain, eye and ear of mouse during embryonic development

PINK1 is a 581 amino acid protein with a serine/threonine kinase domain and an N-terminal mitochondrial targeting motif. The enzyme is expressed in the brain as well as in several tissues such as heart, skeletal muscle, liver, kidney, pancreas and testis. In the present study, we have investigated by Western blot analysis and immunohistochemistry the presence and distribution of PINK1 in the brain, eye and inner ear of mouse during embryonic development. In the brain we detected two PINK1 molecular isoforms of 55 kDa and 66 kDa. Immunoreactive perikarya first appeared at stage E15 in the diencephalon within the thalamus, the hypothalamus, the periventricular layers of the third ventricle and in the rhombencephalon at level of the pons. Subsequently, new PINK1-positive neurons were found in the midbrain within the floor and the periventricular layers of the ventral wall of the mesencephalic vesicle (stage E17) as well as in the neopallial cortex, the tegmentum of the midbrain and the periventricular region of the caudal part of the rhombencephalon (stage E19). At P0, PINK1-immunoreactive cells appeared in the striatum, the mantle layer and caudal part of the medulla oblongata and the cerebellum. The spatio-temporal expression of PINK1 and its heterogeneous distribution suggest that the enzyme might be involved in neuroregulatory processes during embryogenesis. In the eye, PINK1-immunoreactivity was found in the lens and in the cornea, whereas in the inner ear the enzyme was expressed in the ependymal and subependymal cells of the saccule and in the semicircular canals indicating that PINK1 plays a role in the development of these sensory organs.

Transgenic animal models of neurodegeneration based on human genetic studies

The identification of genes linked to neurodegenerative diseases such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD) and Parkinson's disease (PD) has led to the development of animal models for studying mechanism and evaluating potential therapies. None of the transgenic models developed based on disease-associated genes have been able to fully recapitulate the behavioral and pathological features of the corresponding disease. However, there has been enormous progress made in identifying potential therapeutic targets and understanding some of the common mechanisms of neurodegeneration. In this review, we will discuss transgenic animal models for AD, ALS, HD and PD that are based on human genetic studies. All of the diseases discussed have active or complete clinical trials for experimental treatments that benefited from transgenic models of the disease.

Dopaminergic cell damage and vulnerability to MPTP in Pink1 knockdown zebrafish

The functions of PTEN (phosphatase/tensin homolog)-induced putative kinase (PINK1), which is mutated in early-onset Parkinson's disease, are poorly understood. We characterized a PINK1 antibody and found colocalization of PINK1-like immunoreactivity with aminergic markers. We inactivated translation of Pink1 using morpholino-oligonucleotides (MOs) in larval zebrafish. Dopaminergic neurons consisted of two sets of neuron populations, marked by complementary expression of two tyrosine hydroxylase genes th1 and th2. Translation inhibition of pink1 resulted in reduction of both th mRNA forms until day 5 or 7, respectively. The affected dopaminergic neurons were in one group expressing th1 and three groups expressing th2. Lack of Pink1 sensitized the fish to subeffective doses of MPTP, which caused a locomotor deficit and facilitated loss of th1 in one diencephalic dopaminergic cell group. Control experiments with pink1 mRNA and control MO suggested that effects with the splice site targeting MO were specific. Distinct groups of dopaminergic neurons are thus sensitive to loss of Pink1. Sensitization of the pink1 morphant fish to MPTP toxicity suggests that genetic factors play a role in toxin-induced Parkinson's disease.

Deletion of the WD40 domain of LRRK2 in Zebrafish causes Parkinsonism-like loss of neurons and locomotive defect

LRRK2 plays an important role in Parkinson's disease (PD), but its biological functions are largely unknown. Here, we cloned the homolog of human LRRK2, characterized its expression, and investigated its biological functions in zebrafish. The blockage of zebrafish LRRK2 (zLRRK2) protein by morpholinos caused embryonic lethality and severe developmental defects such as growth retardation and loss of neurons. In contrast, the deletion of the WD40 domain of zLRRK2 by morpholinos targeting splicing did not induce severe embryonic developmental defects; rather it caused Parkinsonism-like phenotypes, including loss of dopaminergic neurons in diencephalon and locomotion defects. These neurodegenerative and locomotion defects could be rescued by over-expressing zLRRK2 or hLRRK2 mRNA. The administration of L-dopa could also rescue the locomotion defects, but not the neurodegeneration. Taken together, our results demonstrate that zLRRK2 is an ortholog of hLRRK2 and that the deletion of WD40 domain of zLRRK2 provides a disease model for PD.

Parkinson's disease (PD) is a degenerative disease of the brain (central nervous system) that often impairs motor skills, speech, and other functions. PD was long thought to be caused by environmental factors, but the discovery of several gene mutations in the patients (mostly with familial form of PD) clearly demonstrated the involvement of genetic factors in the development of PD. Among the identified genes, LRRK2 was discovered to be one of the most important genetic causes of PD. The biological function of LRRK2 was, however, largely unknown. In this study, we studied the function of LRRK2 in zebrafish by blocking the normal function of LRRK2. The zebrafish showed features of neurodegeneration and locomotion defects, similar to those of PD patients. The defects of the fish could be rescued by expressing the normal protein of LRRK2, and the locomotion defect could also be rescued by the administration of L-dopa that is commonly used for treating PD patients. We have therefore developed a zebrafish model for PD that can be used for understanding the mechanism underlying the development of PD and will be helpful for future screening of new drugs to treat PD.

Modeling sporadic Parkinson's disease by silencing the ubiquitin E3 ligase component, SKP1A

Large-scale transcriptomics analysis of gene expression profile of sporadic Parkinson's disease (PD) substantia nigra (SN) has identified a number of differentially expressed genes participating in the neurotoxic cascade of DA neurons death, in particular those related to handling of proteins, dopaminergic transmission and iron metabolism. One of them, SKP1A (p19, S-phase kinase-associated protein 1A), an essential component of the ubiquitin-E3 ligase Skp1, Cullin 1, F-box protein (SCF) complex, has been found to be significantly decreased in the SN pars compacta of post-mortem parkinsonian brains. Recently, a new genetic cell model of sporadic PD was developed by knocking-down SKP1A in SN-derived cell-line infected with short hairpin RNA lentiviruses. SKP1A deficiency resulted in increased susceptibility to cell death and a decline in the expression of dopaminergic phenotypic markers. SKP1A-silenced cells were unable to arrest at G(0)/G(1,) when induced to differentiate, entering into an aberrant cell cycle and progressive death. During this process the cells developed rounded aggregates with characteristics of LB-like inclusions (aggresomes) including immunoreactivity for gamma-tubulin, alpha-synuclein, ubiquitin, tyrosine hydroxylase, Hsc-70 and proteasome subunit. In conclusion, future studies should focus on a careful consideration of crucial dopaminergic interacting genes, as emerged from human sporadic PD, which will serve as a basis for the development of a slowly progressive genetic animal model of sporadic PD, with the potential of evaluating drugs with "disease modifying activity".

Behavioral phenotyping of mouse models of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative movement disorder afflicting millions of people in the United States. The advent of transgenic technologies has contributed to the development of several new mouse models, many of which recapitulate some aspects of the disease; however, no model has been demonstrated to faithfully reproduce the full constellation of symptoms seen in human PD. This may be due in part to the narrow focus on the dopamine-mediated motor deficits. As current research continues to unmask PD as a multi-system disorder, animal models should similarly evolve to include the non-motor features of the disease. This requires that typically cited behavioral test batteries be expanded. The major non-motor symptoms observed in PD patients include hyposmia, sleep disturbances, gastrointestinal dysfunction, autonomic dysfunction, anxiety, depression, and cognitive decline. Mouse behavioral tests exist for all of these symptoms and while some models have begun to be reassessed for the prevalence of this broader behavioral phenotype, the majority has not. Moreover, all behavioral paradigms should be tested for their responsiveness to L-DOPA so these data can be compared to patient response and help elucidate which symptoms are likely not dopamine-mediated. Here, we suggest an extensive, yet feasible, battery of behavioral tests for mouse models of PD aimed to better assess both non-motor and motor deficits associated with the disease.

Recent advances in the manipulation of murine gene expression and its utility for the study of human neurological disease

Transgenic mouse models have vastly contributed to our knowledge of the genetic and molecular pathways underlying the pathogenesis of neurological disorders that affect millions of people worldwide. Not only have they allowed the generation of disease models mimicking the human pathological state but they have also permitted the exploration of the pathological role of specific genes through the generation of knock-out and knock-in models. Classical constitutive transgenic mice have several limitations however, due to behavioral adaptation process occurring and conditional mouse models are time-consuming and often lack extensive spatial or temporal control of gene manipulation. These limitations could be overcome by means of innovative methods that are now available such as RNAi, viral vectors and large cloning DNA vectors. These tools have been extensively used for the generation of mouse models and are characterized by the superior control of transgene expression that has been proven invaluable in the assessment of novel treatments for neurological diseases and to further investigate the molecular processes underlying the etiopathology of neurological disorders. Furthermore, in association with classical transgenic mouse models, they have allowed the validation of innovative therapeutic strategies for the treatment of human neurological disorders. This review describes how these tools have overcome the limitations of classical transgenic mouse models and how they have been of value for the study of human neurological diseases.

The multicopy gene Sly represses the sex chromosomes in the male mouse germline after meiosis

Small-interfering RNAs have been used to disrupt the function of the more than 100 copies of the Sly gene on the mouse Y chromosome, leading to defective sex chromosome repression during spermatid differentiation and, as a consequence, sperm malformations and near-sterility.

Studies of mice with Y chromosome long arm deficiencies suggest that the male-specific region (MSYq) encodes information required for sperm differentiation and postmeiotic sex chromatin repression (PSCR). Several genes have been identified on MSYq, but because they are present in more than 40 copies each, their functions cannot be investigated using traditional gene targeting. Here, we generate transgenic mice producing small interfering RNAs that specifically target the transcripts of the MSYq-encoded multicopy gene Sly (Sycp3-like Y-linked). Microarray analyses performed on these Sly-deficient males and on MSYq-deficient males show a remarkable up-regulation of sex chromosome genes in spermatids. SLY protein colocalizes with the X and Y chromatin in spermatids of normal males, and Sly deficiency leads to defective repressive marks on the sex chromatin, such as reduced levels of the heterochromatin protein CBX1 and of histone H3 methylated at lysine 9. Sly-deficient mice, just like MSYq-deficient mice, have severe impairment of sperm differentiation and are near sterile. We propose that their spermiogenesis phenotype is a consequence of the change in spermatid gene expression following Sly deficiency. To our knowledge, this is the first successful targeted disruption of the function of a multicopy gene (or of any Y gene). It shows that SLY has a predominant role in PSCR, either via direct interaction with the spermatid sex chromatin or via interaction with sex chromatin protein partners. Sly deficiency is the major underlying cause of the spectrum of anomalies identified 17 y ago in MSYq-deficient males. Our results also suggest that the expansion of sex-linked spermatid-expressed genes in mouse is a consequence of the enhancement of PSCR that accompanies Sly amplification.

During meiosis in the male mouse, the X and Y chromosomes are transcriptionally silenced, and retain a significant degree of repression after meiosis. Postmeiotically, X and Y chromosome–encoded genes are consequently expressed at a low level, with the exception of genes present in many copies, which can achieve a higher level of expression. Gene amplification is a notable feature of the X and Y chromosomes, and it has been proposed that this serves to compensate for the postmeiotic repression. The long arm of the mouse Y chromosome (MSYq) has multicopy genes organized in clusters over several megabases. On the basis of analysis of mice carrying MSYq deletions, we proposed that MSYq encodes genetic information that is crucial for postmeiotic repression of the sex chromosomes and for sperm differentiation. The gene(s) responsible for these functions were, however, unknown. In this study, using transgenically delivered small interfering RNA, we disrupted the function of Sly, a gene that is present in more than 100 copies on MSYq. Sly-deficient males have major sperm differentiation problems together with a remarkable postmeiotic derepression of genes encoded on the X and Y chromosomes. Furthermore, the epigenetic modifications normally associated with sex chromosome repression are altered. Our data thus show that the SLY protein is required to mediate postmeiotic repression of the X and Y chromosomes. It is likely that the sperm differentiation problems in Sly-deficient males are largely a consequence of the derepression of the sex chromosomes in spermatids. We propose that the postmeiotic repressive effect of Sly on genes encoded on the X and Y chromosomes drove their massive amplification in the mouse.

Loss of PINK1 in medaka fish (Oryzias latipes) causes late-onset decrease in spontaneous movement

Parkinson's disease is a neurodegenerative disease associated with the degeneration of dopaminergic neurons in the substantia nigra. The PTEN-induced kinase 1 gene (PINK1) is responsible for recessive inherited familial Parkinson's disease (PARK6). Neither the function of PINK1 nor its role in the prevention of Parkinson's disease is fully understood. Gene disruption of PINK1 causes remarkably different phenotypes in animal models such as Drosophila melanogaster, zebrafish, and mouse, none of which recapitulate Parkinson's-disease-like symptoms. We established PINK1-gene-disrupted medaka fish. These mutant fish grew normally at first, then developed significant decrease in the frequency of spontaneous swimming movements in the late-adult stage. Although the mutants did not show any dopaminergic cell loss, the amount of 3,4-dihydroxyphenylacetic acid, a major metabolite of dopamine, decreased. Thus, PINK1 contributes to the maintenance of dopamine metabolism, even before the selective death of dopaminergic neurons. Our animal model is therefore a valuable tool to detect pathogenesis in Parkinson's patients in the early stages.

Inflammation in transgenic mouse models of neurodegenerative disorders

Much evidence is available that inflammation contributes to the development of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease. Our review investigates how well current mouse models reflect this aspect of the pathogenesis. Transgenic models of AD have been available for several years and are the most extensively studied. Modulation of cytokine levels, activation of microglia and, to a lesser extent, activation of the complement system have been reported. Mouse models of PD and HD so far show less evidence for the involvement of inflammation. An increasing number of transgenic mouse strains is being created to model human neurodegenerative diseases. A perfect model should reflect all aspects of a disease. It is important to evaluate continuously the models for their match with the human disease and reevaluate them in light of new findings in human patients. Although none of the transgenic mouse models recapitulates all aspects of the human disorder they represent, all models have provided valuable information on basic molecular pathways. In particular, the mouse models of Alzheimer disease have also led to the development of new therapeutic strategies such as vaccination and modulation of microglial activity.

A sporadic Parkinson disease model via silencing of the ubiquitin-proteasome/E3 ligase component SKP1A

The aim of this study was to develop a new model of sporadic Parkinson disease (PD) based on silencing of the SKP1A gene, a component of the ubiquitin-proteasome/E3 ligase complex, Skp1, Cullin 1, F-box protein, which was found to be highly decreased in the substantia nigra of sporadic PD patients. Initially, an embryonic mouse substantia nigra-derived cell line (SN4741 cells) was infected with short hairpin RNA lentiviruses encoding the murine transcript of the SKP1A gene or with scrambled vector. SKP1A silencing resulted in increased susceptibility to neuronal damages induced by the parkinsonism-inducing neurotoxin 1-methyl-4-phenylpyridinium ion and serum starvation, in parallel with a decline in the expression of the dopaminergic markers, dopamine transporter and vesicular monoamine transporter-2. SKP1A-deficient cells presented a delay in completion of the cell cycle and the inability to arrest at the G(0)/G(1) phase when induced to differentiate. Instead, the cells progressed through S phase, developing rounded aggregates with characteristics of aggresomes including immunoreactivity for gamma-tubulin, alpha-synuclein, ubiquitin, tyrosine hydroxylase, Hsc-70 (70-kDa heat shock cognate protein), and proteasome subunit, and culminating in a lethal phenotype. Conversely, stably enforced expression of wild type SKP1A duplicated the survival index of naïve SN4741 cells under proteasomal inhibition injury, suggesting a new structural role of SKP1 in dopaminergic neuronal function, besides its E3 ligase activity. These results link, for the first time, SKP1 to dopamine neuronal function and survival, suggesting an essential role in sporadic PD. In summary, this new model has reproduced to a significant extent the molecular alterations described in sporadic PD at the cellular level, implicating Skp1 as a potential modifier in sporadic PD neurodegeneration.

PINK1: one protein, multiple neuroprotective functions

Mutations in the PINK1 gene are the second most frequent cause of autosomal recessive parkinsonism after Parkin, and can represent a risk factor towards sporadic Parkinson’s disease. The PINK1 protein product has been implicated in several functions, mostly aimed at protecting neuronal cells against different types of stress. Growing evidence indicates that PINK1 interplays with other proteins mutated in familial forms of Parkinson’s disease, such as Parkin, DJ-1 and α-synuclein. These interactions contribute to the definition of a complex scenario in which the mainteinance of mitochondrial homeostasis and the clearance of misfolded proteins and damaged organelles represent key neuroprotective processes. In this review, we summarize the current knowledge on PINK1 characteristics and functions, and analyze the network of functional relationships that link PINK1 to other Parkinson’s disease-related proteins.

Cell death pathways in Parkinson's disease: role of mitochondria

Parkinson's disease (PD) is a progressive neurodegenerative disease and is characterized pathologically by selective loss of nigrostriatal dopaminergic neurons and the formation of Lewy bodies. Although in the majority of cases the cause of PD is unknown, mitochondrial dysfunction, environmental toxins, oxidative stress, and abnormal protein accumulation may all be involved in disease pathogenesis. The discovery of genes causing rare familial forms of PD (including alpha-synuclein, parkin, DJ-1, PINK1, and LRRK2) has shed light on our understanding of the molecular mechanisms of the development of the disease. Further studies from transgenic or toxin-induced experimental models have also provided insights into the etiology of human disease. Recently, accumulating evidence has suggested that mitochondrial dysfunction is one of the key players in molecular cell death pathways of PD. In this review, we provide an overview of the role of mitochondria in the pathogenesis of both sporadic and familial forms of PD. We also discuss the links between different pathways and highlight novel therapeutic opportunities which target mitochondria.