How genetics research in Parkinson's disease is enhancing understanding of the common idiopathic forms of the disease

Novel insights into the role of circular RNAs in Parkinson disease: An emerging renaissance in the management of neurodegenerative diseases

Parkinson's disease (PD), as a debilitating neurodegenerative disease, particularly affects the elderly population, and is clinically identified by resting tremor, rigidity, and bradykinesia. Pathophysiologically, PD is characterized by an early loss of dopaminergic neurons in the Substantia nigra pars compacta, accompanied by the extensive aggregation of alpha-synuclein (α-Syn) in the form of Lewy bodies. The onset of PD has been reported to be influenced by multiple biological molecules. In this context, circular RNAs (circRNAs), as tissue-specific noncoding RNAs with closed structures, have been recently demonstrated to involve in a set of PD's pathogenic processes. These RNA molecules can either up- or downregulate the expression of α-Syn, as well as moderating its accumulation through different regulatory mechanisms, in which targeting microRNAs (miRNAs) is considered the most common pathway. Since circRNAs have prominent structural and biological characteristics, they could also be considered as promising candidates for PD diagnosis and treatment. Unfortunately, PD has become a global health concern, and a large number of its pathogenic processes are still unclear; thus, it is crucial to elucidate the ambiguous aspects of PD pathophysiology to improve the efficiency of diagnostic and therapeutic strategies. In line with this fact, the current review aims to highlight the interplay between circRNAs and PD pathogenesis, and then discusses the diagnostic and therapeutic potential of circRNAs in PD progression. This study will thus be the first of its kind reviewing the relationship between circRNAs and PD.

The role of amyloids in Alzheimer's and Parkinson's diseases

With varying clinical symptoms, most neurodegenerative diseases are associated with abnormal loss of neurons. They share the same common pathogenic mechanisms involving misfolding and aggregation, and these visible aggregates of proteins are deposited in the central nervous system. Amyloid formation is thought to arise from partial unfolding of misfolded proteins leading to the exposure of hydrophobic surfaces, which interact with other similar structures and give rise to form dimers, oligomers, protofibrils, and eventually mature fibril aggregates. Accumulating evidence indicates that amyloid oligomers, not amyloid fibrils, are the most toxic species that causes Alzheimer's disease (AD) and Parkinson's disease (PD). AD has recently been recognized as the 'twenty-first century plague', with an incident rate of 1% at 60 years of age, which then doubles every fifth year. Currently, 5.3 million people in the US are afflicted with this disease, and the number of cases is expected to rise to 13.5 million by 2050. PD, a disorder of the brain, is the second most common form of dementia, characterized by difficulty in walking and movement. Keeping the above views in mind, in this review we have focused on the roles of amyloid in neurodegenerative diseases including AD and PD, the involvement of amyloid in mitochondrial dysfunction leading to neurodegeneration, are also considered in the review.

A Role for SERCA Pumps in the Neurobiology of Neuropsychiatric and Neurodegenerative Disorders

Calcium (Ca²⁺) is a fundamental regulator of cell fate and intracellular Ca²⁺ homeostasis is crucial for proper function of the nerve cells. Given the complexity of neurons, a constellation of mechanisms finely tunes the intracellular Ca²⁺ signaling. We are focusing on the sarco/endoplasmic reticulum (SR/ER) calcium (Ca²⁺)-ATPase (SERCA) pump, an integral ER protein. SERCA's well established role is to preserve low cytosolic Ca²⁺ levels ([Ca²⁺]_cyt), by pumping free Ca²⁺ ions into the ER lumen, utilizing ATP hydrolysis. The SERCA pumps are encoded by three distinct genes, SERCA1-3, resulting in 12 known protein isoforms, with tissue-dependent expression patterns. Despite the well-established structure and function of the SERCA pumps, their role in the central nervous system is not clear yet. Interestingly, SERCA-mediated Ca²⁺ dyshomeostasis has been associated with neuropathological conditions, such as bipolar disorder, schizophrenia, Parkinson's disease and Alzheimer's disease. We summarize here current evidence suggesting a role for SERCA in the neurobiology of neuropsychiatric and neurodegenerative disorders, thus highlighting the importance of this pump in brain physiology and pathophysiology.

Synapsin III is a key component of α-synuclein fibrils in Lewy bodies of PD brains

Lewy bodies (LB) and Lewy neurites (LN), which are primarily composed of α-synuclein (α-syn), are neuropathological hallmarks of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). We recently found that the neuronal phosphoprotein synapsin III (syn III) controls dopamine release via cooperation with α-syn and modulates α-syn aggregation. Here, we observed that LB and LN, in the substantia nigra of PD patients and hippocampus of one subject with DLB, displayed a marked immunopositivity for syn III. The in situ proximity ligation assay revealed the accumulation of numerous proteinase K-resistant neuropathological inclusions that contained both α-syn and syn III in tight association in the brain of affected subjects. Most strikingly, syn III was identified as a component of α-syn-positive fibrils in LB-enriched protein extracts from PD brains. Finally, a positive correlation between syn III and α-syn levels was detected in the caudate putamen of PD subjects. Collectively, these findings indicate that syn III is a crucial α-syn interactant and a key component of LB fibrils in the brain of patients affected by PD.

Mitochondria and α-Synuclein: Friends or Foes in the Pathogenesis of Parkinson's Disease?

Parkinson’s disease (PD) is a movement disorder characterized by dopaminergic nigrostriatal neuron degeneration and the formation of Lewy bodies (LB), pathological inclusions containing fibrils that are mainly composed of α-synuclein. Dopaminergic neurons, for their intrinsic characteristics, have a high energy demand that relies on the efficiency of the mitochondria respiratory chain. Dysregulations of mitochondria, deriving from alterations of complex I protein or oxidative DNA damage, change the trafficking, size and morphology of these organelles. Of note, these mitochondrial bioenergetics defects have been related to PD. A series of experimental evidence supports that α-synuclein physiological action is relevant for mitochondrial homeostasis, while its pathological aggregation can negatively impinge on mitochondrial function. It thus appears that imbalances in the equilibrium between the reciprocal modulatory action of mitochondria and α-synuclein can contribute to PD onset by inducing neuronal impairment. This review will try to highlight the role of physiological and pathological α-synuclein in the modulation of mitochondrial functions.

α-Synuclein nonhuman primate models of Parkinson's disease

Proper understanding of the mechanism(s) by which α-synuclein misfolds and propagates may hold the key to unraveling the complex pathophysiology of Parkinson's disease. A more complete understanding of the disease itself, as well as establishing animal models that fully recapitulate pathological and functional disease progression, are needed to develop treatments that will delay, halt or reverse the disease course. Traditional neurotoxin-based animal models fail to mimic crucial aspects of Parkinson's and thus are not relevant for the study of neuroprotection and disease-modifying therapies. Therefore, a new era of animal models centered on α-synuclein has emerged with the utility of nonhuman primates in these studies beginning to become important. Indeed, disease modeling in nonhuman primates offers a more similar anatomical and genetic background to humans, and the ability to assess complex behavioral impairments that are difficult to test in rodents. Furthermore, results obtained from monkey studies translate better to applications in humans. In this review, we highlight the importance of α-synuclein in Parkinson's disease and discuss the development of α-synuclein based nonhuman primate models.

Protein Kinases and Parkinson's Disease

Currently, the lack of new drug candidates for the treatment of major neurological disorders such as Parkinson’s disease has intensified the search for drugs that can be repurposed or repositioned for such treatment. Typically, the search focuses on drugs that have been approved and are used clinically for other indications. Kinase inhibitors represent a family of popular molecules for the treatment and prevention of various cancers, and have emerged as strong candidates for such repurposing because numerous serine/threonine and tyrosine kinases have been implicated in the pathobiology of Parkinson’s disease. This review focuses on various kinase-dependent pathways associated with the expression of Parkinson’s disease pathology, and evaluates how inhibitors of these pathways might play a major role as effective therapeutic molecules.

Dynamic Changes in Striatal mGluR1 But Not mGluR5 during Pathological Progression of Parkinson's Disease in Human Alpha-Synuclein A53T Transgenic Rats: A Multi-PET Imaging Study

Parkinson's disease (PD) is a prevalent degenerative disorder affecting the CNS that is primarily characterized by resting tremor and movement deficits. Group I metabotropic glutamate receptor subtypes 1 and 5 (mGluR1 and mGluR5, respectively) are important targets for investigation in several CNS disorders. In the present study, we investigated the in vivo roles of mGluR1 and mGluR5 in chronic PD pathology by performing longitudinal positron emission tomography (PET) imaging in A53T transgenic (A53T-Tg) rats expressing an abnormal human α-synuclein (ASN) gene. A53T-Tg rats showed a dramatic decline in general motor activities with age, along with abnormal ASN aggregation and striatal neuron degeneration. In longitudinal PET imaging, striatal nondisplaceable binding potential (BPND) values for [(11)C]ITDM (N-[4-[6-(isopropylamino) pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[(11)C]methylbenzamide), a selective PET ligand for mGluR1, temporarily increased before PD symptom onset and dramatically decreased afterward with age. However, striatal BPND values for (E)-[(11)C]ABP688 [3-(6-methylpyridin-2-ylethynyl)-cyclohex-2-enone-(E)-O-[(11)C]methyloxime], a specific PET ligand for mGluR5, remained constant during experimental terms. The dynamic changes in striatal mGluR1 BPND values also showed a high correlation in pathological decreases in general motor activities. Furthermore, declines in mGluR1 BPND values were correlated with decreases in BPND values for [(18)F]FE-PE2I [(E)-N-(3-iodoprop-2E-enyl)-2β-carbo-[(18)F]fluoroethoxy-3β-(4-methylphenyl) nortropane], a specific PET ligand for the dopamine transporter, a biomarker for dopaminergic neurons. In conclusion, our results have demonstrated for the first time that dynamic changes occur in mGluR1, but not mGluR5, that accompany pathological progression in a PD animal model.

SIGNIFICANCE STATEMENT

Synaptic signaling by glutamate, the principal excitatory neurotransmitter in the brain, is modulated by group I metabotropic glutamate receptors, including the mGluR1 and mGluR5 subtypes. In the brain, mGluR1 and mGluR5 have distinct functional roles and regional distributions. Their roles in brain pathology, however, are not well characterized. Using longitudinal PET imaging in a chronic rat model of PD, we demonstrated that expression of mGluR1, but not mGluR5, dynamically changed in the striatum accompanying pathological PD progression. These findings imply that monitoring mGluR1 in vivo may provide beneficial information to further understand central nervous system disorders.

Differential interaction between iron and mutant alpha-synuclein causes distinctive Parkinsonian phenotypes in Drosophila

Alpha-synuclein aggregation is the central hallmark of both sporadic and familial Parkinson's disease (PD). Patients with different PD-causing genetic defects of alpha-synuclein usually show distinctive clinical features that are atypical to sporadic PD. Iron accumulation is invariably found in PD. Recent studies showed that mutant and wild-type alpha-synuclein may have differential interaction with iron and mutant alpha-synuclein toxicity could be preferentially exacerbated by iron. We hence hypothesized that iron overload could selectively influence mutant alpha-synuclein toxicity and disease phenotypes. To test the hypothesis, we investigated if Drosophila melanogaster over-expressing A53T, A30P, and wild-type (WT) alpha-synuclein have different responses to iron treatment. We showed that iron treatment induced similar reduction of survival rate in all flies but induced a more severe motor decline in A53T and A30P mutant alpha-synuclein expressing flies, suggesting interaction between mutant alpha-synuclein and iron. Although no significant difference in total head iron content was found among these flies, we demonstrated that iron treatment induced selective DA neuron loss in motor-related PPM3 cluster only in the flies that express A53T and A30P mutant alpha-synuclein. We provided the first in vivo evidence that iron overload could induce distinctive neuropathology and disease phenotypes in mutant but not WT alpha-synuclein expressing flies, providing insights to the cause of clinical features selectively exhibited by mutant alpha-synuclein carriers.

Antibodies and protein misfolding: From structural research tools to therapeutic strategies

Protein misfolding disorders, including the neurodegenerative conditions Alzheimer's disease (AD) and Parkinson's disease (PD) represent one of the major medical challenges or our time. The underlying molecular mechanisms that govern protein misfolding and its links with disease are very complex processes, involving the formation of transiently populated but highly toxic molecular species within the crowded environment of the cell and tissue. Nevertheless, much progress has been made in understanding these events in recent years through innovative experiments and therapeutic strategies, and in this review we present an overview of the key roles of antibodies and antibody fragments in these endeavors. We discuss in particular how these species are being used in combination with a variety of powerful biochemical and biophysical methodologies, including a range of spectroscopic and microscopic techniques applied not just in vitro but also in situ and in vivo, both to gain a better understanding of the mechanistic nature of protein misfolding and aggregation and also to design novel therapeutic strategies to combat the family of diseases with which they are associated. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.

Involvement of endocytosis and alternative splicing in the formation of the pathological process in the early stages of Parkinson's disease

Parkinson's disease (PD) is the one of most widespread neurodegenerative pathologies. Because of the impossibility of studying the endogenous processes that occur in the brain of patients with PD in the presymptomatic stage, the mechanisms that trigger the disease remain unknown. Thus, the identification of the processes that play an important role in the early stages of the disease in these patients is extremely difficult. In this context, we performed a whole-transcriptome analysis of the peripheral blood of untreated patients with stage 1 PD (Hoehn-Yahr scale). We demonstrated a significant change in the levels of transcripts included in the large groups of processes associated with the functioning of the immune system and cellular transport. Moreover, a significant change in the splicing of genes involved in cellular-transport processes was shown in our study.

Type II kinase inhibitors show an unexpected inhibition mode against Parkinson's disease-linked LRRK2 mutant G2019S

A number of well-known type II inhibitors (ATP-noncompetitive) that bind kinases in their DFG-out conformation were tested against wild-type LRRK2 and the most common Parkinson's disease-linked mutation, G2019S. We found that traditional type II inhibitors exhibit surprising variability in their inhibition mechanism between the wild type (WT) and the G2019S mutant of LRRK2. The type II kinase inhibitors were found to work in an ATP-competitive fashion against the G2019S mutant, whereas they appear to follow the expected noncompetitive mechanism against WT. Because the G2019S mutation lies in the DXG motif (DYG in LRRK2 but DFG in most other kinases) of the activation loop, we explored the structural consequence of the mutation on loop dynamics using an enhanced sampling method called metadynamics. The simulations suggest that the G2019S mutation stabilizes the DYG-in state of LRRK2 through a series of hydrogen bonds, leading to an increase in the conformational barrier between the active and inactive forms of the enzyme and a relative stabilization of the active form. The conformational bias toward the active form of LRRK2 mutants has two primary consequences. (1) The mutant enzyme becomes hyperactive, a known contributor to the Parkinsonian phenotype, as a consequence of being "locked" into the activated state, and (2) the mutation creates an unusual allosteric pocket that can bind type II inhibitors but in an ATP-competitive fashion. Our results suggest that developing type II inhibitors, which are generally considered superior to type I inhibitors because of desirable selectivity profiles, might be especially challenging for the G2019S LRRK2 mutant.

Nanobodies as structural probes of protein misfolding and fibril formation

The deposition of peptides and proteins as amyloid fibrils is a common feature of nearly 50 medical -disorders affecting the brain or a variety of other organs and tissues. These disorders, which include Alzheimer's disease, Parkinson's disease, the prion diseases, and type II diabetes, have an enormous impact on the public health and economy of the modern world. Extensive research is therefore taking place to determine the underlying molecular mechanisms and determinants of the pathological conversion of amyloidogenic proteins from their soluble forms into fibrillar structures. The use of molecular probes and biophysical techniques, such as X-ray crystallography and particularly NMR spectroscopy, are allowing detailed analysis of the mechanism of fibril formation and of the underlying structural and chemical features of the associated pathogenicity. Nanobodies, the antigen-binding domains derived from camelid heavy-chain antibodies, are excellent tools to probe protein aggregation as a result of their exquisite specificity and high affinity and stability, along with their ease of expression and small size; the latter in particular allows them to be used very efficiently in combination with NMR spectroscopy and X-ray crystallography. In this chapter we present an overview of how nanobodies are being used to obtain detailed information on the mechanisms of amyloid formation and on the nature and origin of their links with human diseases.

The parkin mutant phenotype in the fly is largely rescued by metal-responsive transcription factor (MTF-1)

The gene for Parkin, an E3 ubiquitin ligase, is mutated in some familial forms of Parkinson's disease, a severe neurodegenerative disorder. A homozygous mutant of the Drosophila ortholog of human parkin is viable but results in severe motoric impairment including an inability to fly, female and male sterility, and a decreased life span. We show here that a double mutant of the genes for Parkin and the metal-responsive transcription factor 1 (MTF-1) is not viable. MTF-1, which is conserved from insects to mammals, is a key regulator of heavy metal homeostasis and detoxification and plays additional roles in other stress conditions, notably oxidative stress. In contrast to the synthetic lethality of the double mutant, elevated expression of MTF-1 dramatically ameliorates the parkin mutant phenotype, as evidenced by a prolonged life span, motoric improvement including short flight episodes, and female fertility. At the cellular level, muscle and mitochondrial structures are substantially improved. A beneficial effect is also seen with a transgene encoding human MTF-1. We propose that Parkin and MTF-1 provide complementary functions in metal homeostasis, oxidative stress and other cellular stress responses. Our findings also raise the possibility that MTF-1 gene polymorphisms in humans could affect the severity of Parkinson's disease.

PINK1 displays tissue-specific subcellular location and regulates apoptosis and cell growth in breast cancer cells

The PINK1 gene is mutated in the germ line of patients with hereditary early-onset Parkinson disease, and PINK1 prosurvival function at neuronal mitochondria has been related with the etiology of this disease. However, the expression and function of PINK1 protein in nonneuronal tissues has not been determined yet. Here, we have analyzed PINK1 protein expression and subcellular distribution in normal and neoplastic human tissues and investigated the function of PINK1 in breast carcinoma cells. PINK1 protein, as stained by a specific anti-PINK1 monoclonal antibody, was widely expressed in human tissues, displaying high expression in epithelial tissues and in the central nervous system and lower expression in tissues of mesenchymal origin. The subcellular distribution of PINK1 was cytoplasmic granular or cytoplasmic diffuse in most tissues. In breast, PINK1 was also associated with the plasma membrane. Human neoplastic tissues ranged from high PINK1 expression in carcinomas to low expression in sarcomas. In neoplastic tissues, PINK1 displayed a diffuse cytoplasmic localization, with an additional membranous localization in breast carcinoma and squamous carcinoma of lung. In the human breast carcinoma Michigan Cancer Foundation-7 cell line, ectopic expression of cytoplasmic or mitochondrial-targeted PINK1 inhibited apoptosis triggered by hydrogen peroxide and suppressed cell growth in soft agar, whereas PINK1 silencing increased hydrogen peroxide-induced apoptosis. Together, our findings indicate that the physiologic functions of PINK1 go beyond its regulatory role of mitochondria-mediated cell survival in neurons.

Membrane localization of LRRK2 is associated with increased formation of the highly active LRRK2 dimer and changes in its phosphorylation

Autosomal dominant mutations in leucine rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease (PD). Despite the presence of multiple domains, the kinase activity of LRRK2 is thought to represent the primary function of the protein. Alterations in LRRK2 kinase activity are thought to underlie the pathogenesis of its PD-linked mutations; however, many questions regarding basic aspects of LRRK2 function remain unclear, including the cellular mechanisms of LRRK2 regulation and the importance of its unique distribution within the cell. Here, we demonstrate for the first time that the subcellular localization of wild-type LRRK2 is associated with changes in four distinct biochemical properties likely crucial for LRRK2 function. Our data demonstrate for the first time that the wild-type LRRK2 dimer possesses greater kinase activity than its more abundant monomeric counterpart. Importantly, we show that this activated form of LRRK2 is substantially enriched at the membrane of cells expressing endogenous or exogenous LRRK2, and that the membrane-associated fraction of LRRK2 likewise possesses greater kinase activity than cytosolic LRRK2. In addition, membrane-associated LRRK2 binds GTP more efficiently than cytosolic LRRK2 but demonstrates a lower degree of phosphorylation. Our observations suggest that multiple events, including altered protein-protein interactions and post-translational modifications, contribute to the regulation of LRRK2 function, through modulation of membrane association and complex assembly. These findings may have implications for the sites of LRRK2 function within the cell, the identification and localization of bona fide LRRK2 substrates, and efforts to design small molecule inhibitors of LRRK2.

Mechanisms in dominant parkinsonism: The toxic triangle of LRRK2, alpha-synuclein, and tau

Parkinson's disease (PD) is generally sporadic but a number of genetic diseases have parkinsonism as a clinical feature. Two dominant genes, alpha-synuclein (SNCA) and leucine-rich repeat kinase 2 (LRRK2), are important for understanding inherited and sporadic PD. SNCA is a major component of pathologic inclusions termed Lewy bodies found in PD. LRRK2 is found in a significant proportion of PD cases. These two proteins may be linked as most LRRK2 PD cases have SNCA-positive Lewy bodies. Mutations in both proteins are associated with toxic effects in model systems although mechanisms are unclear. LRRK2 is an intracellular signaling protein possessing both GTPase and kinase activities that may contribute to pathogenicity. A third protein, tau, is implicated as a risk factor for PD. We discuss the potential relationship between these genes and suggest a model for PD pathogenesis where LRRK2 is upstream of pathogenic effects through SNCA, tau, or both proteins.

Development of a high-throughput AlphaScreen assay measuring full-length LRRK2(G2019S) kinase activity using moesin protein substrate

Mutations within the LRRK2 (leucine-rich repeat kinase 2) gene predispose humans to develop late-onset Parkinson's disease (PD). The most prevalent of these mutations, G2019S, has been shown to increase LRRK2 kinase activity. Therefore, the discovery of small molecule inhibitors of LRRK2(G2019S) through high-throughput screening (HTS) may provide a novel therapeutic strategy for treating PD. Current biochemical assays monitoring the activity of LRRK2(G2019S) either are radioactive or use short peptidic substrates. Here we describe the development and optimization of a novel HTS AlphaScreen assay for measuring the catalytic activity of full-length LRRK2(G2019S) using its putative physiological protein substrate moesin. The high sensitivity of this optimized 384-well assay allowed the use of enzyme concentrations as low as 0.75nM. The estimated apparent K(m) value for adenosine triphosphate (6 microM) using the glutathione S-transferase-moesin substrate was much lower than the one previously reported using LRRKtide, a synthetic peptide derived from moesin. Testing of nonselective kinase inhibitors (staurosporine, H-1152, and Y-27632) generated potencies consistent with published data. Finally, robotic validation of the assay yielded an average Z' factor of 0.80. Overall, these results indicate that the present HTS AlphaScreen assay might provide a more relevant biochemical approach for the discovery of novel LRRK2(G2019S) inhibitors.

The impact of genetic research on our understanding of Parkinson's disease

Until recently, genetics was thought to play a minor role in the development of Parkinson's disease (PD). Over the last decade, a number of genes that definitively cause PD have been identified, which has led to the generation of disease models based on pathogenic gene variants that recapitulate many features of the disease. These genetic studies have provided novel insight into potential mechanisms underlying the aetiology of PD. This chapter will provide a profile of the genes conclusively linked to PD and will outline the mechanisms of PD pathogenesis implicated by genetic studies. Mitochondrial dysfunction, oxidative stress and impaired ubiquitin-proteasome system function are disease mechanisms that are particularly well supported by genetic studies and are therefore the focus of this chapter.

Genetic basis of Parkinson disease

Over the past few years, considerable progress has been made in understanding the molecular mechanisms of Parkinson disease (PD). Mutations in certain genes are found to cause monogenic forms of the disorder, with autosomal dominant or autosomal recessive inheritance. These genes include alpha-synuclein, parkin, PINK1, DJ-1, LRRK2, and ATP13A2. The monogenic variants are important tools in identifying cellular pathways that shed light on the pathogenesis of this disease. Certain common genetic variants are also likely to modulate the risk of PD. International collaborative studies and meta-analyses have identified common variants as genetic susceptibility risk/protective factors for sporadic PD.

Pathogenesis of familial Parkinson's disease: new insights based on monogenic forms of Parkinson's disease

Parkinson's disease (PD) is one of the most common movement disorders caused by the loss of dopaminergic neuronal cells. The molecular mechanisms underlying neuronal degeneration in PD remain unknown; however, it is now clear that genetic factors contribute to the pathogenesis of this disease. Approximately, 5% of patients with clinical features of PD have clear familial etiology, which show a classical recessive or dominant Mendelian mode of inheritance. Over the decade, more than 15 loci and 11 causative genes have been identified so far and many studies shed light on their implication in not only monogenic but also sporadic form of PD. Recent studies revealed that PD-associated genes play important roles in cellular functions, such as mitochondrial functions, ubiquitin-proteasomal system, autophagy-lysosomal pathway and membrane trafficking. Furthermore, the proteins encoded by PD-associated genes can interact with each other and such gene products may share a common pathway that leads to nigral degeneration. However, their precise roles in the disease and their normal functions remain poorly understood. In this study, we review recent progress in knowledge about the genes associated with familial PD.

Leucine-rich repeat kinase 2 mutations and Parkinson's disease: three questions

Mutations in the gene encoding LRRK2 (leucine-rich repeat kinase 2) were first identified in 2004 and have since been shown to be the single most common cause of inherited Parkinson’s disease. The protein is a large GTP-regulated serine/threonine kinase that additionally contains several protein–protein interaction domains. In the present review, we discuss three important, but unresolved, questions concerning LRRK2. We first ask: what is the normal function of LRRK2? Related to this, we discuss the evidence of LRRK2 activity as a GTPase and as a kinase and the available data on protein–protein interactions. Next we raise the question of how mutations affect LRRK2 function, focusing on some slightly controversial results related to the kinase activity of the protein in a variety of in vitro systems. Finally, we discuss what the possible mechanisms are for LRRK2-mediated neurotoxicity, in the context of known activities of the protein.

Parkinson's disease: from monogenic forms to genetic susceptibility factors

Research in Parkinson's disease (PD) genetics has been extremely prolific over the past decade. More than 13 loci and 9 genes have been identified, but their implication in PD is not always certain. Point mutations, duplications and triplications in the alpha-synuclein (SNCA) gene cause a rare dominant form of PD in familial and sporadic cases. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are a more frequent cause of autosomal dominant PD, particularly in certain ethnic groups. Loss-of-function mutations in Parkin, PINK1, DJ-1 and ATP13A2 cause autosomal recessive parkinsonism with early-onset. Identification of other Mendelian forms of PD will be a main challenge for the next decade. In addition, susceptibility variants that contribute to PD have been identified in several populations, such as polymorphisms in the SNCA, LRRK2 genes and heterozygous mutations in the beta-glucocerebrosidase (GBA) gene. Genome-wide associations and re-sequencing projects, together with gene-environment interaction studies, are expected to further define the causal role of genetic determinants in the pathogenesis of PD, and improve prevention and treatment.

The Parkinson disease protein leucine-rich repeat kinase 2 transduces death signals via Fas-associated protein with death domain and caspase-8 in a cellular model of neurodegeneration

Neurodegenerative illnesses such as Parkinson and Alzheimer disease are an increasingly prevalent problem in aging societies, yet no therapies exist that retard or prevent neurodegeneration. Dominant missense mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson disease (PD), but the mechanisms by which mutant forms of LRRK2 disrupt neuronal function and cause cell death remain poorly understood. We report that LRRK2 interacts with the death adaptor Fas-associated protein with death domain (FADD), and that in primary neuronal culture LRRK2-mediated neurodegeneration is prevented by the functional inhibition of FADD or depletion of caspase-8, two key elements of the extrinsic cell death pathway. This pathway is activated by disease-triggering mutations, which enhance the LRRK2-FADD association and the consequent recruitment and activation of caspase-8. These results establish a direct molecular link between a mutant PD gene and the activation of programmed cell death signaling, and suggest that FADD/caspase-8 signaling contributes to LRRK2-induced neuronal death.

Neuroinflammation and oxidation/nitration of alpha-synuclein linked to dopaminergic neurodegeneration

alpha-Synuclein (SYN) is the major component of Lewy bodies, the neuropathological hallmarks of Parkinson's disease (PD). Missense mutations and multiplications of the SYN gene cause autosomal dominant inherited PD. Thus, SYN is implicated in the pathogenesis of PD. However, the mechanism whereby SYN promotes neurodegeneration remains unclear. Familial PD with SYN gene mutations are rare because the majority of PD is sporadic and emerging evidence indicates that sporadic PD may result from genetic and environmental risk factors including neuroinflammation. Hence, we examined the relationship between SYN dysfunction and neuroinflammation in mediating dopaminergic neurodegeneration in mice and dopaminergic neuronal cultures derived from wild-type SYN and mutant A53T SYN transgenic mice in a murine SYN-null (SYNKO) background (M7KO and M83KO, respectively). Stereotaxic injection of an inflammagen, lipopolysaccharide, into substantia nigra of these SYN genetically engineered mice induced similar inflammatory reactions. In M7KO and M83KO, but not in SYNKO mice, the neuroinflammation was associated with dopaminergic neuronal death and the accumulation of insoluble aggregated SYN as cytoplasmic inclusions in nigral neurons. Nitrated/oxidized SYN was detected in these inclusions and abatement of microglia-derived nitric oxide and superoxide provided significant neuroprotection in neuron-glia cultures from M7KO mice. These data suggest that nitric oxide and superoxide released by activated microglia may be mediators that link inflammation and abnormal SYN in mechanisms of PD neurodegeneration. This study advances understanding of the role of neuroinflammation and abnormal SYN in the pathogenesis of PD and opens new avenues for the discovery of more effective therapies for PD.

Structure of the ROC domain from the Parkinson's disease-associated leucine-rich repeat kinase 2 reveals a dimeric GTPase

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of Parkinson's disease (PD). LRRK2 contains a Ras of complex proteins (ROC) domain that may act as a GTPase to regulate its protein kinase activity. The structure of ROC and the mechanism(s) by which it regulates kinase activity are not known. Here, we report the crystal structure of the LRRK2 ROC domain in complex with GDP-Mg(2+) at 2.0-A resolution. The structure displays a dimeric fold generated by extensive domain-swapping, resulting in a pair of active sites constructed with essential functional groups contributed from both monomers. Two PD-associated pathogenic residues, R1441 and I1371, are located at the interface of two monomers and provide exquisite interactions to stabilize the ROC dimer. The structure demonstrates that loss of stabilizing forces in the ROC dimer is likely related to decreased GTPase activity resulting from mutations at these sites. Our data suggest that the ROC domain may regulate LRRK2 kinase activity as a dimer, possibly via the C-terminal of ROC (COR) domain as a molecular hinge. The structure of the LRRK2 ROC domain also represents a signature from a previously undescribed class of GTPases from complex proteins and results may provide a unique molecular target for therapeutics in PD.

Neurotrophic factors and Parkinson's disease: the emergence of a new player?

The administration of neurotrophic factors is a potential approach to the therapy of neurodegenerative disorders such as Parkinson's disease. However, issues relating to compound delivery and potential side effects have limited the clinical application of this treatment strategy. The identification of CDNF and MANF, which constitute a new class of neurotrophic factors active against dopaminergic neurons, may provide new hope for therapeutic approaches to neurodegenerative disorders based on neurotrophic factors or downstream components of their signaling pathways.

Assessment of the progression of Parkinson's disease: a metabolic network approach

BACKGROUND

Clinical research into Parkinson's disease has focused increasingly on the development of interventions that slow the neurodegeneration underlying this disorder. These investigations have stimulated interest in finding objective biomarkers that show changes in the rate of disease progression with treatment. Through radiotracer-based imaging of nigrostriatal dopaminergic function, a specific class of biomarkers to monitor the progression of Parkinson's disease has been identified, and these biomarkers were used in the clinical trials of drugs with the potential to modify the course of the disease. However, in some of these studies there was discordance between the imaging outcome measures and blinded clinical ratings of disease severity. Research is underway to identify and validate alternative ways to image brain metabolism, through which the efficacy of new therapies for Parkinson's disease and related disorders can be assessed.

RECENT DEVELOPMENTS

During recent years, spatial covariance analysis has been used with (18)F-fluorodeoxyglucose PET to detect abnormal patterns of brain metabolism in patients with neurodegenerative disorders. Rapid, automated, voxel-based algorithms have been used with metabolic imaging to quantify the activity of disease-specific networks. This approach has helped to characterise the unique metabolic patterns associated with the motor and cognitive features of Parkinson's disease. The results of several studies have shown correction of abnormal motor, but not cognitive, network activity by treatment with dopaminergic therapy and deep brain stimulation. The authors of a longitudinal imaging study of early-stage Parkinson's disease reported substantial differences in the development of these metabolic networks over a follow-up of 4 years. WHERE NEXT?: Developments in network imaging have provided the basis for several new applications of metabolic imaging in the study of Parkinson's disease. A washout study is currently underway to determine the long-duration effects of dopaminergic therapy on the network activity related to Parkinson's disease, which will be useful to plan future trials of disease-modifying drugs. Network approaches are also being applied to the study of atypical parkinsonian syndromes. The characterisation of specific patterns associated with atypical parkinsonian syndromes and classic Parkinson's disease will be the basis for a fully automated imaging-based procedure for early differential diagnosis. Efforts are underway to quantify the networks related to Parkinson's disease with less invasive imaging methods. Assessments of network activity with perfusion-weighted MRI show excellent concordance with measurements done with established radiotracer techniques. This approach will ultimately enable the assessment of abnormal network activity in people who are genetically at risk of Parkinson's disease.

rAAV-mediated nigral human parkin over-expression partially ameliorates motor deficits via enhanced dopamine neurotransmission in a rat model of Parkinson's disease

We hypothesized that over-expressing the E3 ligase, parkin, whose functional loss leads to Parkinson's disease, in the nigrostriatal tract might be protective in the unilateral 6-hydroxydopamine (6-OHDA) rat lesion model. Recombinant adeno-associated virus (rAAV) encoding human parkin or green fluorescent protein (GFP) was injected into the rat substantia nigra 6 weeks prior to a four-site striatal 6-OHDA lesion. Vector-mediated parkin over-expression significantly ameliorated motor deficits as measured by amphetamine-induced rotational behavior and spontaneous behavior in the cylinder test but forelimb akinesia as assessed by the stepping test was unaffected. rAAV-mediated human parkin was expressed in the nigrostriatal tract, the substantia pars reticulata, and the subthalamic nucleus. However, in lesioned animals, there was no difference between nigral parkin and GFP-transduction on lesion-induced striatal tyrosine hydroxylase (TH) innervation or nigral TH positive surviving neurons. A second lesion experiment was performed to determine if striatal dopamine (DA) neurotransmission was enhanced as measured biochemically. In this second group of parkin and GFP treated rats, behavioral improvement was again observed. In addition, striatal TH and DA levels were slightly increased in the parkin-transduced group. In a third experiment, we evaluated parkin and GFP transduced rats 6 weeks after vector injection without DA depletion. When challenged with amphetamine, parkin treated rats tended to display asymmetries biased away from the treated hemisphere. Nigral parkin over-expression induced increases in both striatal TH and DA levels. Therefore, while parkin over-expression exerted no protective effect on the nigrostriatal DA system, parkin appeared to enhance the efficiency of nigrostriatal DA transmission in intact nigral DA neurons likely due to the observed increases in TH.

Prolonged toxicokinetics and toxicodynamics of paraquat in mouse brain

BACKGROUND

Paraquat (PQ) has been implicated as a risk factor for the Parkinson disease phenotype (PDP) in humans and mice using epidemiologic or experimental approaches. The toxicokinetics (TK) and toxicodynamics (TD) of PQ in the brain are not well understood.

OBJECTIVES

The TK and TD of PQ in brain were measured after single or repeated doses.

METHODS

Brain regions were analyzed for PQ levels, amount of lipid peroxidation, and functional activity of the 20S proteasome.

RESULTS

Paraquat (10 mg/kg, ip) was found to be persistent in mouse ventral midbrain (VM) with an apparent half-life of approximately 28 days and was cumulative with a linear pattern between one and five doses. PQ was also absorbed orally with a concentration in brain rising linearly after single doses between 10 and 50 mg/kg. The level of tissue lipid peroxides (LPO) was differentially elevated in three regions, being highest in VM, lower in striatum (STR), and least in frontal cortex (FCtx), with the earliest significant elevation detected at 1 day. An elevated level of LPO was still present in VM after 28 days. Despite the cumulative tissue levels of PQ after one, three, and five doses, the level of LPO was not further increased. The activity of the 20S proteasome in the striatum was altered after a single dose and reduced after five doses.

CONCLUSIONS

These data have implications for PQ as a risk factor in humans and in rodent models of the PDP.

Alpha-synuclein protects cerebellar granule neurons against 6-hydroxydopamine-induced death

The physiological role of alpha-synuclein, a protein found enriched in intraneuronal deposits characterizing Parkinson's disease, is debated. While its aggregation is usually considered linked to neuropathology, its normal function may be related to fundamental processes of synaptic transmission and plasticity. By using antisense oligonucleotide strategy, we report in this study that alpha-synuclein silencing in cultured cerebellar granule cells results in widespread death of these neurons, thus demonstrating an essential pro-survival role of the protein towards primary neurons. To study alpha-synuclein expression and processing in a Parkinson's disease model of neurotoxicity, we exposed differentiated cultures of cerebellar granule neurons to toxic concentrations of 6-hydroxydopamine (6-OHDA). This resulted in neuronal death accompanied by a decrease of the monomeric form of alpha-synuclein, which was due to both decreased synthesis of the protein and its increased mono-ubiquitination accompanied by nuclear translocation. The essential neuroprotective role of alpha-synuclein was confirmed by the fact that subchronic valproate treatment, which increases alpha-synuclein expression and prevents its nuclear translocation in cerebellar granule cells exposed to 6-OHDA, significantly protected these neurons from 6-OHDA insult. In agreement with the pro-survival role of alpha-synuclein in this model, subtoxic concentrations of alpha-synuclein antisense oligonucleotides, aggravated 6-OHDA toxicity towards granule neurons. Our results demonstrate that normal alpha-synuclein expression is essential for the viability of primary neurons and that its pro-survival role is abolished in 6-OHDA neurotoxic challenge. These results are relevant to more precisely define the role of alpha-synuclein in neuronal cells and to better understand its putative involvement in neurodegeneration.

PINK1 mutants associated with recessive Parkinson's disease are defective in inhibiting mitochondrial release of cytochrome c

Mutations in PTEN-induced kinase 1 (PINK1) gene cause recessive familial type 6 of Parkinson's disease (PARK6). We investigated molecular mechanisms underlying PINK1 neuroprotective function and PARK6 mutation-induced loss of PINK1 function. Overexpression of wild-type PINK1 blocked mitochondrial release of apoptogenic cytochrome c, caspase-3 activation and apoptotic cell death induced by proteasome inhibitor MG132. N-terminal truncated PINK1 (NDelta35), which lacks mitochondrial localization sequence, did not block MG132-induced cytochrome c release and cytotoxicity. Despite mitochondrial expression, PARK6 mutant (E240K), (H271Q), (G309D), (L347P), (E417G) and C-terminal truncated (CDelta145) PINK1 failed to inhibit MG132-induced cytochrome c release and caspase-3 activation. Overexpression of wild-type PINK1 blocked cytochrome c release and cell death caused by atractyloside, which opens mitochondrial permeability transition pore (mPTP). PARK6 PINK1 mutants failed to inhibit atractyloside-induced cytochrome c release. These results suggest that PINK1 exerts anti-apoptotic effect by inhibiting the opening of mPTP and that PARK6 mutant PINK1 loses its ability to prevent mPTP opening and cytochrome c release.

Parkinson's disease and genetics

BACKGROUND

Idiopathic Parkinson disease (IPD) is a condition of unknown cause. Several factors are believed to contribute to its onset, and many studies have been conducted in search of the possible etiology of Parkinson disease.

REVIEW SUMMARY

Genetic factors have become relevant when trying to explain the onset of Parkinson disease. The studies are divided into 2 categories: epidemiological and studies that analyze twins from families with members suffering from Parkinson disease, thus looking for the responsible genetic mutations. In this article we address this controversial topic, reviewing some of the most significant studies trying to provide evidence which relates genetics to Parkinson disease.

CONCLUSION

We present current epidemiological studies and the most important genetic factors related to Parkinson disease, including the latest information currently available on each issue.

A common genetic factor for Parkinson disease in ethnic Chinese population in Taiwan

BACKGROUND

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, characterized clinically by resting tremor, bradykinesia, postural instability and rigidity. The prevalence of PD is approximately 2% of the population over 65 years of age and 1.7 million PD patients (age > or = 55 years) live in China. Recently, a common LRRK2 variant Gly2385Arg was reported in ethnic Chinese PD population in Taiwan. We analyzed the frequency of this variant in our independent PD case-control population of Han Chinese from Taiwan.

METHODS

305 patients and 176 genetically unrelated healthy controls were examined by neurologists and the diagnosis of PD was based on the published criteria. The region of interest was amplified with standard polymerase chain reaction (PCR). PCR fragments then were directly sequenced in both forward and reverse directions. Differences in genotype frequencies between groups were assessed by the X2 test, while X2 analysis was used to test for the Hardy-Weinberg equilibrium.

RESULTS

Of the 305 patients screened we identified 27 (9%) with heterozygous G2385R variant. This mutation was only found in 1 (0.5%) in our healthy control samples (odds ratio = 16.99, 95% CI: 2.29 to 126.21, p = 0.0002). Sequencing of the entire open reading frame of LRRK2 in G2385R carriers revealed no other variants.

CONCLUSION

These data suggest that the G2385R variant contributes significantly to the etiology of PD in ethnic Han Chinese individuals. With consideration of the enormous and expanding aging Chinese population in mainland China and in Taiwan, this variant is probably the most common known genetic factor for PD worldwide.

Neurodegeneration and neuroprotection in multiple sclerosis and other neurodegenerative diseases

Multiple sclerosis is considered a disease of myelin destruction; Parkinson's disease (PD), one of dopaminergic neuron depletion; ALS, a disease of motor neuron death; and Alzheimer's, a disease of plaques and tangles. Although these disorders differ in important ways, they also have common pathogenic features, including inflammation, genetic mutations, inappropriate protein aggregates (e.g., Lewy bodies, amyloid plaques), and biochemical defects leading to apoptosis, such as oxidative stress and mitochondrial dysfunction. In most disorders, it remains uncertain whether inflammation and protein aggregation are neurotoxic or neuroprotective. Elucidating the mechanisms that orchestrate neuronal diseases should facilitate development of neuroprotective and neurorestorative strategies.

Modulation of microglial pro-inflammatory and neurotoxic activity for the treatment of Parkinson's disease

Parkinson's disease (PD) is a debilitating movement disorder resulting from a progressive degeneration of the nigrostriatal dopaminergic pathway and depletion of neurotransmitter dopamine in the striatum. Molecular cloning studies have identified nearly a dozen genes or loci that are associated with small clusters of mostly early onset and genetic forms of PD. The etiology of the vast majority of PD cases remains unknown, and the precise molecular and biochemical processes governing the selective and progressive degeneration of the nigrostriatal dopaminergic pathway are poorly understood. Current drug therapies for PD are symptomatic and appear to bear little effect on the progressive neurodegenerative process. Studies of postmortem PD brains and various cellular and animal models of PD in the last 2 decades strongly suggest that the generation of pro-inflammatory and neurotoxic factors by the resident brain immune cells, microglia, plays a prominent role in mediating the progressive neurodegenerative process. This review discusses literature supporting the possibility of modulating the activity of microglia as a neuroprotective strategy for the treatment of PD.

The genetics of neurodegenerative diseases

In the last 50 years, an enormous amount of progress has been made in dissecting the etiology of hereditary neurodegenerative diseases, including the dementias, the parkinsonisms, the ataxias and the motor-neuron diseases. In addition, these genetic findings are beginning to provide insights into the pathogeneses of the sporadic forms of the diseases. Through animal and cellular modeling studies we are beginning to gain insights into the pathogenic pathways to disease. This mechanistic understanding is now leading to therapeutic strategies based on this new understanding. As yet, however, no mechanistic therapies are in use in the clinic.

Glucocerebrosidase mutations are not found in association with LRRK2 G2019S in subjects with parkinsonism

Alteration G2019S in the leucine-rich repeat kinase 2 gene (LRRK2) has been identified in several populations of patients with parkinsonism, including Ashkenazi Jewish subjects with Parkinson disease. Mutations in glucocerebrosidase (GBA), the enzyme deficient in Gaucher disease, are also identified at an increased frequency among Parkinson probands, including those of Ashkenazi Jewish ancestry. A Taqman Assay-by-Design SNP genotyping strategy was utilized to establish whether G2019S was found in association with GBA mutations. Among 37 subjects with parkinsonism who were heterozygous for a GBA mutation, none carried G2019S. Furthermore, G2019S was not found in 18 patients with Gaucher disease who developed parkinsonian manifestations and 11 other Gaucher probands with parkinsonism in a first degree relative. Among 45 patients with Gaucher disease without a history of parkinsonism, one G2019S carrier was found. These findings suggest that GBA and LRRK2 mutations are discrete risk factors for parkinsonism in both Ashkenazi Jewish and non-Jewish subjects.

Kinase activity is required for the toxic effects of mutant LRRK2/dardarin

Mutations in the LRRK2 gene, coding for dardarin, cause dominantly inherited Parkinson's disease (PD). Dardarin is a large protein, and mutations are found throughout the gene including the kinase domain. However, it is not clear if kinase activity is important for the damaging effects of pathogenic mutations. In this study, we noted two cellular phenotypes associated with mutant dardarin. First, pathogenic mutations increase the tendency of dardarin to form inclusion bodies. Secondly, neurons and neuronal cell lines undergo cell death after expression of mutant protein. Manipulating activity by replacing the kinase domain with a 'kinase-dead' version blocks inclusion body formation and strongly delays cell death. This predicts that kinase inhibitors will be useful therapeutic agents in patients with LRRK2 mutations and, perhaps, in sporadic PD. We also show that dardarin protein is expressed within human midbrain neurons and that C-terminal epitopes are also found in some Lewy bodies.

PET imaging of the basal ganglia

The present chapter reviews PET imaging in basal ganglia disorders; Parkinson's disease is used as a model of these disorders because the neurochemical pathobiology of this disease is well known and great advances in the imaging area have been achieved. Other basal ganglia disorders including Tourette's syndrome, dystonia, Huntington's chorea and Wilson's disease are also dealt with. With PET and SPECT techniques, the whole integrative dopaminergic network of neurons can be studied, which plays an important role in differential diagnostics. Furthermore, pharmacological effects of medication can be visualized and the role of stereotaxic neurosurgery can be evaluated. Finally, functional imaging gives clues about the prognosis and rehabilitation aspects of the basal ganglia disorders.

Leucine-rich repeat kinase 2: Relevance to Parkinson's disease

Human leucine-rich repeat kinase 2 (LRRK2) is a novel kinase belonging to the ROCO protein superfamily (Ras of complex proteins (Roc) with a C-terminal of Roc domain). This large complex protein of 280kDa contains several functional domains including leucine-rich repeats, Ras-related GTPase, mitogen-activated protein kinase kinase kinase (MAPKKK), and WD40 repeats. While definitive functions of LRRK2 have yet to be described, the domain structure of LRRK2 suggests that it plays an important role in the regulation of signal transduction cascades through its dual enzymatic activities of GTPase and MAPKKK. Moreover, mutations in LRRK2 have been found to be thus far the most frequent cause of late-onset familial and idiopathic Parkinson's disease. Further investigations should allow for the elucidation of how pathogenic mutations trigger changes in the structure and function of LRRK2 that lead to aberrant signal transduction and neurodegeneration in Parkinson's disease.

RecurrentLRRK2 (Park8) mutations in early-onset Parkinson's disease

Mutations in LRRK2 (leucine-rich repeat kinase 2) have been associated with autosomal dominant Parkinson's disease (PD) and cluster in several 3' exons of the gene. The majority of mutations have been detected in late-onset cases (age at onset >50 years). We screened 5 of the 51 exons of LRRK2 that previously have been reported to harbor mutations in 98 early-onset and 42 late-onset PD patients. We identified two mutations (c.4321C>T, c.6055G>A) in three early-onset patients. Screening of an additional 220 early-onset PD patients for these mutations revealed another mutation carrier. In conclusion, LRRK2 mutations need to be considered also in early-onset PD.