Milestones in PD genetics

Converging roles of ion channels, calcium, metabolic stress, and activity pattern of Substantia nigra dopaminergic neurons in health and Parkinson's disease

Dopamine‐releasing neurons within the Substantia nigra (SN DA) are particularly vulnerable to degeneration compared to other dopaminergic neurons. The age‐dependent, progressive loss of these neurons is a pathological hallmark of Parkinson's disease (PD), as the resulting loss of striatal dopamine causes its major movement‐related symptoms. SN DA neurons release dopamine from their axonal terminals within the dorsal striatum, and also from their cell bodies and dendrites within the midbrain in a calcium‐ and activity‐dependent manner. Their intrinsically generated and metabolically challenging activity is created and modulated by the orchestrated function of different ion channels and dopamine D2‐autoreceptors. Here, we review increasing evidence that the mechanisms that control activity patterns and calcium homeostasis of SN DA neurons are not only crucial for their dopamine release within a physiological range but also modulate their mitochondrial and lysosomal activity, their metabolic stress levels, and their vulnerability to degeneration in PD. Indeed, impaired calcium homeostasis, lysosomal and mitochondrial dysfunction, and metabolic stress in SN DA neurons represent central converging trigger factors for idiopathic and familial PD. We summarize double‐edged roles of ion channels, activity patterns, calcium homeostasis, and related feedback/feed‐forward signaling mechanisms in SN DA neurons for maintaining and modulating their physiological function, but also for contributing to their vulnerability in PD‐paradigms. We focus on the emerging roles of maintained neuronal activity and calcium homeostasis within a physiological bandwidth, and its modulation by PD‐triggers, as well as on bidirectional functions of voltage‐gated L‐type calcium channels and metabolically gated ATP‐sensitive potassium (K‐ATP) channels, and their probable interplay in health and PD.

We propose that SN DA neurons possess several feedback and feed‐forward mechanisms to protect and adapt their activity‐pattern and calcium‐homeostasis within a physiological bandwidth, and that PD‐trigger factors can narrow this bandwidth. We summarize roles of ion channels in this view, and findings documenting that both, reduced as well as elevated activity and associated calcium‐levels can trigger SN DA degeneration.

This article is part of a special issue on Parkinson disease.

Animal models of Parkinson's disease: An updated overview

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

Heterozygote carriers for CNVs in PARK2 are at increased risk of Parkinson's disease

Together with point mutations, homozygous deletions or duplications in PARK2 are responsible for the majority of autosomal recessive juvenile Parkinsonism. It is debated, however, whether heterozygous carriers of these mutations are at increased risk of Parkinson's disease (PD). Our goal was to determine whether heterozygous carriers of copy number variants (CNVs) affecting exons of the PARK2 gene are at risk of PD that is greater than that of non-carriers. We searched for CNVs affecting exons of PARK2 in a sample of 105 749 genotyped Icelanders. In total, 989 carriers, including 24 diagnosed with PD, were identified. The heterozygous carriers were tested for association in a sample of 1415 PD patients and 40 474 controls ≥65 years of age. PD patients were more often heterozygous carriers of PARK2 CNVs than controls [odds ratio (OR) = 1.69, P = 0.03] and compound heterozygous PD patients for a CNV and a missense mutation were not found. Furthermore, we conducted a meta-analysis of studies reporting on case-control samples screened for heterozygous PARK2 CNVs. Ten studies were included in the final analysis, with 4538 cases and 4213 controls. The pooled OR and P-value for the published and Icelandic results showed significant association between PARK2 CNVs and risk of PD (OR = 2.11, P = 2.54 × 10(-6)). Our analysis shows that heterozygous carriers of CNVs affecting exons of PARK2 have greater risk of PD than non-carriers.

Challenges in essential tremor genetics

The field of essential tremor (ET) genetics remains extremely challenging. The relative lack of progress in understanding the genetic etiology of ET, however, does not reflect the lack of a genetic contribution, but rather, the presence of substantial phenotypic and genotypic heterogeneity. A meticulous approach to phenotyping is important for genetic research in ET. The only tool for phenotyping is the clinical history and examination. There is currently no ET-specific serum or imaging biomarker or defining neuropathological feature (e.g., a protein aggregate specific to ET) that can be used for phenotyping, and there is considerable clinical overlap with other disorders such as Parkinson's disease (PD) and dystonia. These issues greatly complicate phenotyping; thus, in some studies, as many as 30-50% of cases labeled as "ET" have later been found to carry other diagnoses (e.g., dystonia, PD) rather than ET. A cursory approach to phenotyping (e.g., merely defining ET as an "action tremor") is likely a major issue in some family studies of ET, and this as well as lack of standardized phenotyping across studies and patient centers is likely to be a major contributor to the relative lack of success of genome wide association studies (GWAS). To dissect the genetic architecture of ET, whole genome sequencing (WGS) in carefully characterized and well-phenotyped discovery and replication datasets of large case-control and familial cohorts will likely be of value. This will allow specific hypotheses about the mode of inheritance and genetic architecture to be tested. There are a number of approaches that still remain unexplored in ET genetics, including the contribution of copy number variants (CNVs), 'uncommon' moderate effect alleles, 'rare' variant large effect alleles (including Mendelian and complex/polygenic modes of inheritance), de novo and gonadal mosaicism, epigenetic changes and non-coding variation. Using these approaches is likely to yield new ET genes.

No dopamine cell loss or changes in cytoskeleton function in transgenic mice expressing physiological levels of wild type or G2019S mutant LRRK2 and in human fibroblasts

Mutations within the LRRK2 gene have been identified in Parkinson’s disease (PD) patients and have been implicated in the dysfunction of several cellular pathways. Here, we explore how pathogenic mutations and the inhibition of LRRK2 kinase activity affect cytoskeleton dynamics in mouse and human cell systems. We generated and characterized a novel transgenic mouse model expressing physiological levels of human wild type and G2019S-mutant LRRK2. No neuronal loss or neurodegeneration was detected in midbrain dopamine neurons at the age of 12 months. Postnatal hippocampal neurons derived from transgenic mice showed no alterations in the seven parameters examined concerning neurite outgrowth sampled automatically on several hundred neurons using high content imaging. Treatment with the kinase inhibitor LRRK2-IN-1 resulted in no significant changes in the neurite outgrowth. In human fibroblasts we analyzed whether pathogenic LRRK2 mutations change cytoskeleton functions such as cell adhesion. To this end we compared the adhesion characteristics of human skin fibroblasts derived from six PD patients carrying one of three different pathogenic LRRK2 mutations and from four age-matched control individuals. The mutant LRRK2 variants as well as the inhibition of LRRK2 kinase activity did not reveal any significant cell adhesion differences in cultured fibroblasts. In summary, our results in both human and mouse cell systems suggest that neither the expression of wild type or mutant LRRK2, nor the inhibition of LRRK2 kinase activity affect neurite complexity and cellular adhesion.

Quantitative expression proteomics and phosphoproteomics profile of brain from PINK1 knockout mice: insights into mechanisms of familial Parkinson's disease

Parkinson's disease (PD) is an age-related, neurodegenerative motor disorder characterized by progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta and presence of α-synuclein-containing protein aggregates. Mutations in the mitochondrial Ser/Thr kinase PTEN-induced kinase 1 (PINK1) are associated with an autosomal recessive familial form of early-onset PD. Recent studies have suggested that PINK1 plays important neuroprotective roles against mitochondrial dysfunction by phosphorylating and recruiting Parkin, a cytosolic E3 ubiquitin ligase, to facilitate elimination of damaged mitochondria via autophagy-lysosomal pathways. Loss of PINK1 in cells and animals leads to various mitochondrial impairments and oxidative stress, culminating in dopaminergic neuronal death in humans. Using a 2-D polyacrylamide gel electrophoresis proteomics approach, the differences in expressed brain proteome and phosphoproteome between 6-month-old PINK1-deficient mice and wild-type mice were identified. The observed changes in the brain proteome and phosphoproteome of mice lacking PINK1 suggest that defects in signaling networks, energy metabolism, cellular proteostasis, and neuronal structure and plasticity are involved in the pathogenesis of familial PD. Mutations in PINK1 are associated with an early-onset form of Parkinson's disease (PD). This study examines changes in the proteome and phosphoproteome of the PINK1 knockout mouse brain. Alterations were noted in several key proteins associated with: increased oxidative stress, aberrant cellular signaling, altered neuronal structure, decreased synaptic plasticity, reduced neurotransmission, diminished proteostasis networks, and altered metabolism. 14-3-3ε, 14-3-3 protein epsilon; 3-PGDH, phosphoglycerate dehydrogenase; ALDOA, aldolase A; APT1, acyl-protein thioesterase 1; CaM, calmodulin; CBR3, carbonyl reductase [NADPH] 3; ENO2, gamma-enolase; HPRT, hypoxanthine-guanine phosphoribosyltransferase; HSP70, heat-shock-related 70 kDa protein 2; IDHc, cytoplasmic isocitrate dehydrogenase [NADP+]; MAPK1, mitogen-activated protein kinase 1; MEK1, MAP kinase kinase 1; MDHc, cytoplasmic malate dehydrogenase; NFM, neurofilament medium polypeptide; NSF, N-ethylmaleimide-sensitive fusion protein; PHB, prohibitin; PINK1, PTEN-induced putative kinase 1; PPIaseA, peptidyl-prolyl cis-trans isomerase A; PSA2, proteasome subunit alpha type-2; TK, transketolase; VDAC-2, voltage-dependent anion-selective channel protein 2.

How to optimize the treatment of early stage Parkinson's disease

The approach to early Parkinson's disease denotes the communication of the diagnosis and important decisions, such as when and how to start treatment. Evidence based medicine and guidelines indicate which drugs have robust evidence of efficacy and tolerability in this specific population. However, de-novo patients may show different characteristics and they may be in a different phase of their disease. In this review, we will give an insight into the appropriate time therapy should be started and the actual knowledge about disease modification therapies. Moreover, the drugs indicated for early treatment will be considered and an indication for the use of these drugs will be given with the support of the actual knowledge.

Is Parkinson's disease truly a prion-like disorder? An appraisal of current evidence

Parkinson's disease (PD) is the world's second most common neurodegenerative disease and most common movement disorder. Characterised by a loss of dopaminergic neurons and the development of intraneuronal inclusions known as Lewy bodies, it has classically been thought of as a cell-autonomous disease. However, in 2008, two groups reported the startling observation of Lewy bodies within embryonic neuronal grafts transplanted into PD patients little more than a decade previously, suggesting that PD pathology can be propagated to neighbouring cells and calling basic assumptions of our understanding of the disease into question. Subsequent research has largely served to confirm this interpretation, pointing towards a prion-like intercellular transfer of misfolded α-synuclein, the main component of Lewy bodies, as central to PD. This shift in thinking offers a revolutionary approach to PD treatment, potentially enabling a transition from purely symptomatic therapy to direct targeting of the pathology that drives disease progression. In this short review, we appraise current experimental support for PD as a prion-like disease, whilst highlighting areas of controversy or inconsistency which must be resolved. We also offer a brief discussion of the therapeutic implications of these discoveries.

α-Synuclein-induced dopaminergic neurodegeneration in a rat model of Parkinson's disease occurs independent of ATP13A2 (PARK9)

Mutations in the ATP13A2 (PARK9) gene cause early-onset, autosomal recessive Parkinson's disease (PD) and Kufor-Rakeb syndrome. ATP13A2 mRNA is spliced into three distinct isoforms encoding a P5-type ATPase involved in regulating heavy metal transport across vesicular membranes. Here, we demonstrate that three ATP13A2 mRNA isoforms are expressed in the normal human brain and are modestly increased in the cingulate cortex of PD cases. ATP13A2 can mediate protection toward a number of stressors in mammalian cells and can protect against α-synuclein-induced toxicity in cellular and invertebrate models of PD. Using a primary cortical neuronal model combined with lentiviral-mediated gene transfer, we demonstrate that human ATP13A2 isoforms 1 and 2 display selective neuroprotective effects toward toxicity induced by manganese and hydrogen peroxide exposure through an ATPase-independent mechanism. The familial PD mutations, F182L and G504R, abolish the neuroprotective effects of ATP13A2 consistent with a loss-of-function mechanism. We further demonstrate that the AAV-mediated overexpression of human ATP13A2 is not sufficient to attenuate dopaminergic neurodegeneration, neuropathology, and striatal dopamine and motoric deficits induced by human α-synuclein expression in a rat model of PD. Intriguingly, the delivery of an ATPase-deficient form of ATP13A2 (D513N) to the substantia nigra is sufficient to induce dopaminergic neuronal degeneration and motor deficits in rats, potentially suggesting a dominant-negative mechanism of action. Collectively, our data demonstrate a distinct lack of ATP13A2-mediated protection against α-synuclein-induced neurotoxicity in the rat nigrostriatal dopaminergic pathway, and limited neuroprotective capacity overall, and raise doubts about the potential of ATP13A2 as a therapeutic target for PD.

Iron in neurodegenerative disorders of protein misfolding: a case of prion disorders and Parkinson's disease

SIGNIFICANCE

Intracellular and extracellular aggregation of a specific protein or protein fragments is the principal pathological event in several neurodegenerative conditions. We describe two such conditions: sporadic Creutzfeldt-Jakob disease (sCJD), a rare but potentially infectious and invariably fatal human prion disorder, and Parkinson's disease (PD), a common neurodegenerative condition second only to Alzheimer's disease in prevalence. In sCJD, a cell surface glycoprotein known as the prion protein (PrP(C)) undergoes a conformational change to PrP-scrapie, a pathogenic and infectious isoform that accumulates in the brain parenchyma as insoluble aggregates. In PD, α-synuclein, a cytosolic protein, forms insoluble aggregates that accumulate in neurons of the substantia nigra and cause neurotoxicity.

RECENT ADVANCES

Although distinct processes are involved in the pathogenesis of sCJD and PD, both share brain iron dyshomeostasis as a common associated feature that is reflected in the cerebrospinal fluid in a disease-specific manner.

CRITICAL ISSUES

Since PrP(C) and α-synuclein play a significant role in maintaining cellular iron homeostasis, it is important to understand whether the aggregation of these proteins and iron dyshomeostasis are causally related. Here, we discuss recent information on the normal function of PrP(C) and α-synuclein in cellular iron metabolism and the cellular and biochemical processes that contribute to iron imbalance in sCJD and PD.

FUTURE DIRECTIONS

Improved understanding of the relationship between brain iron imbalance and protein aggregation is likely to help in the development of therapeutic strategies that can restore brain iron homeostasis and mitigate neurotoxicity.

Interaction of LRRK2 with kinase and GTPase signaling cascades

LRRK2 is a protein that interacts with a plethora of signaling molecules, but the complexity of LRRK2 function presents a challenge for understanding the role of LRRK2 in the pathophysiology of Parkinson's disease (PD). Studies of LRRK2 using over-expression in transgenic mice have been disappointing, however, studies using invertebrate systems have yielded a much clearer picture, with clear effects of LRRK2 expression, knockdown or deletion in Caenorhabditis elegans and Drosophila on modulation of survival of dopaminergic neurons. Recent studies have begun to focus attention on particular signaling cascades that are a target of LRRK2 function. LRRK2 interacts with members of the mitogen activated protein kinase (MAPK) pathway and might regulate the pathway action by acting as a scaffold that directs the location of MAPK pathway activity, without strongly affecting the amount of MAPK pathway activity. Binding to GTPases, GTPase-activating proteins and GTPase exchange factors are another strong theme in LRRK2 biology, with LRRK2 binding to rac1, cdc42, rab5, rab7L1, endoA, RGS2, ArfGAP1, and ArhGEF7. All of these molecules appear to feed into a function output for LRRK2 that modulates cytoskeletal outgrowth and vesicular dynamics, including autophagy. These functions likely impact modulation of α-synuclein aggregation and associated toxicity eliciting the disease processes that we term PD.

Cav1.3 channels control D2-autoreceptor responses via NCS-1 in substantia nigra dopamine neurons

Dopamine midbrain neurons within the substantia nigra are particularly prone to degeneration in Parkinson's disease. Their selective loss causes the major motor symptoms of Parkinson's disease, but the causes for the high vulnerability of SN DA neurons, compared to neighbouring, more resistant ventral tegmental area dopamine neurons, are still unclear. Consequently, there is still no cure available for Parkinson's disease. Current therapies compensate the progressive loss of dopamine by administering its precursor l-DOPA and/or dopamine D2-receptor agonists. D2-autoreceptors and Cav1.3-containing L-type Ca(2+) channels both contribute to Parkinson's disease pathology. L-type Ca(2+) channel blockers protect SN DA neurons from degeneration in Parkinson's disease and its mouse models, and they are in clinical trials for neuroprotective Parkinson's disease therapy. However, their physiological functions in SN DA neurons remain unclear. D2-autoreceptors tune firing rates and dopamine release of SN DA neurons in a negative feedback loop through activation of G-protein coupled potassium channels (GIRK2, or KCNJ6). Mature SN DA neurons display prominent, non-desensitizing somatodendritic D2-autoreceptor responses that show pronounced desensitization in PARK-gene Parkinson's disease mouse models. We analysed surviving human SN DA neurons from patients with Parkinson's disease and from controls, and detected elevated messenger RNA levels of D2-autoreceptors and GIRK2 in Parkinson's disease. By electrophysiological analysis of postnatal juvenile and adult mouse SN DA neurons in in vitro brain-slices, we observed that D2-autoreceptor desensitization is reduced with postnatal maturation. Furthermore, a transient high-dopamine state in vivo, caused by one injection of either l-DOPA or cocaine, induced adult-like, non-desensitizing D2-autoreceptor responses, selectively in juvenile SN DA neurons, but not ventral tegmental area dopamine neurons. With pharmacological and genetic tools, we identified that the expression of this sensitized D2-autoreceptor phenotype required Cav1.3 L-type Ca(2+) channel activity, internal Ca(2+), and the interaction of the neuronal calcium sensor NCS-1 with D2-autoreceptors. Thus, we identified a first physiological function of Cav1.3 L-type Ca(2+) channels in SN DA neurons for homeostatic modulation of their D2-autoreceptor responses. L-type Ca(2+) channel activity however, was not important for pacemaker activity of mouse SN DA neurons. Furthermore, we detected elevated substantia nigra dopamine messenger RNA levels of NCS-1 (but not Cav1.2 or Cav1.3) after cocaine in mice, as well as in remaining human SN DA neurons in Parkinson's disease. Thus, our findings provide a novel homeostatic functional link in SN DA neurons between Cav1.3- L-type-Ca(2+) channels and D2-autoreceptor activity, controlled by NCS-1, and indicate that this adaptive signalling network (Cav1.3/NCS-1/D2/GIRK2) is also active in human SN DA neurons, and contributes to Parkinson's disease pathology. As it is accessible to pharmacological modulation, it provides a novel promising target for tuning substantia nigra dopamine neuron activity, and their vulnerability to degeneration.

Orchestrated increase of dopamine and PARK mRNAs but not miR-133b in dopamine neurons in Parkinson's disease

Progressive loss of substantia nigra dopamine neurons (SN DA) is a hallmark of aging and of Parkinson's disease (PD). Mutations in PARK genes cause familial PD forms. Increased expression of alpha-synuclein (PARK4) is a disease-triggering event in familial PD and also observed in SN DA neurons in sporadic PD but related transcriptional changes are unknown. With optimized single-cell quantitative real-time polymerase chain reaction analysis, we compared messenger RNA and microRNA levels in SN DA neurons from sporadic PD patients and controls. Non-optimally matched donor ages and RNA integrities are common problems when analyzing human samples. We dissected the influence of distinct ages and RNA integrities of our samples by applying a specifically-optimized, linear-mixed-effects model to quantitative real-time polymerase chain reaction-data. We identified that elevated alpha-synuclein messenger RNA levels in SN DA neurons of human PD brains were positively correlated with corresponding elevated levels of mRNAs for functional compensation of progressive SN DA loss and for enhanced proteasomal (PARK5/UCHL1) and lysosomal (PARK9/ATPase13A2) function, possibly counteracting alpha-synuclein toxicity. In contrast, microRNA miR-133b levels, previously implicated in transcriptional dysregulation in PD, were not altered in SN DA neurons in PD.

Ageing and Parkinson's disease: why is advancing age the biggest risk factor?

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Review of age related processes occurring within substantia nigra neurons.

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Discussion of why these neurons seem to be susceptible to loss with age.

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Review of why SN neurons are particularly sensitive to mitochondrial dysfunction.

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Review of why SN neurons are sensitive to changes in protein degradation pathways.

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Discussion of relevance to Parkinson's disease pathology.

As the second most common age related neurodegenerative disease after Alzheimer's disease, the health, social and economic impact resulting from Parkinson's disease will continue to increase alongside the longevity of the population. Ageing remains the biggest risk factor for developing idiopathic Parkinson's disease. Although research into the mechanisms leading to cell death in Parkinson's disease has shed light on many aspects of the pathogenesis of this disorder, we still cannot answer the fundamental question, what specific age related factors predispose some individuals to develop this common neurodegenerative disease. In this review we focus specifically on the neuronal population associated with the motor symptoms of Parkinson's disease, the dopaminergic neurons of the substantia nigra, and try to understand how ageing puts these neurons at risk to the extent that a slight change in protein metabolism or mitochondrial function can push the cells over the edge leading to catastrophic cell death and many of the symptoms seen in Parkinson's disease. We review the evidence that ageing is important for the development of Parkinson's disease and how age related decline leads to the loss of neurons within this disease, before describing exactly how advancing age may lead to substantia nigra neuronal loss and Parkinson's disease in some individuals.

Significant study of population stratification, sensitivity analysis and trim and fill analyses on GBA mutation and Parkinson's disease

This comprehensive meta-analysis was applied to case-control studies of the association between PD and GBA to assess the joint evidence for the association, the influence of individual studies, and evidence for publication bias. We searched PubMed, Medline, Cochrane Library, reference lists of relevant studies to June 2012, and email contact with authors. For the case-control studies, the authors found 1) support for the association between PD and GBA, both in total group analysis [fixed: OR and 95%CI: 4.825 (3.901-5.968), P < 0.001; random: OR and 95%CI: 4.791 (3.520-6.520), P < 0.001] and in Asia, Europe, Americas, and Israel subgroups analysis [Asia: fixed: OR and 95%CI: 7.495 (4.490-12.511), P < 0.001, random: OR and 95%CI: 7.989 (4.060-15.723), P < 0.001; Americas: fixed: OR and 95%CI: 4.036 (2.460-6.622), P < 0.001, random: OR and 95%CI: 4.065 (2.464-6.707), P < 0.001; Europe: fixed: OR and 95%CI: 3.353 (2.287-4.917), P < 0.001, random: OR and 95%CI: 3.559 (2.148-5.894), P < 0.001; Israel: fixed/random: OR and 95%CI: 6.430 (4.430-9.333), P < 0.001], 2) no evidence that this association was accounted for by any one study, and 3) no evidence for publication bias. In conclusion, GBA mutation status may be significantly associated with PD.

Therapy for Parkinson's disease: what is in the pipeline?

Despite advances in the treatment of Parkinson's disease there are still many unmet needs, including neuroprotection, treatment of motor complications, treatment of dyskinesia, treatment of psychosis, and treatment of nondopaminergic symptoms. In this review, I highlight the obstacles to develop a neuroprotective drug and some of the treatment strategies recently approved or still in clinical trials designed to meet these unmet needs.

The role of inflammation in sporadic and familial Parkinson's disease

The etiology of Parkinson's disease (PD) is complex and most likely involves numerous environmental and heritable risk factors. Interestingly, many genetic variants, which have been linked to familial forms of PD or identified as strong risk factors, also play a critical role in modulating inflammatory responses. There has been considerable debate in the field as to whether inflammation is a driving force in neurodegeneration or simply represents a response to neuronal death. One emerging hypothesis is that inflammation plays a critical role in the early phases of neurodegeneration. In this review, we will discuss emerging aspects of both innate and adaptive immunity in the context of the pathogenesis of PD. We will highlight recent data from genetic and functional studies that strongly support the theory that genetic susceptibility plays an important role in modulating immune pathways and inflammatory reactions, which may precede and initiate neuronal dysfunction and subsequent neurodegeneration. A detailed understanding of such cellular and molecular inflammatory pathways is crucial to uncover pathogenic mechanisms linking sporadic and hereditary PD and devise tailored neuroprotective interventions.

Obstacles to the development of a neuroprotective therapy for Parkinson's disease

A neuroprotective or disease-modifying therapy that can slow or stop disease progression and prevent the development of intolerable disability is the major unmet medical need in the treatment of Parkinson's disease (PD). Many putative neuroprotective agents have been identified in the laboratory, but none has been unequivocally demonstrated to provide disease-modifying effects in PD patients, even when clinical trials are positive. Obstacles to defining a neuroprotective therapy in PD include: (1) uncertainty about the cause of PD and precisely what to target, (2) a reliable animal model in which to test putative neuroprotective agents that accurately predicts results in PD patients, (3) insight about which dose to employ in clinical trials and which patient group to study, (4) a clinical trial design that reliably differentiates disease-modifying and symptomatic effects and that is acceptable to regulatory authorities, and (5) the cost and time of the development program. Advances have been made in each of these areas, thereby increasing the prospects of developing a neuroprotective or disease-modifying therapy in the not-too-distant future. These issues are reviewed in the present article.

Protective effect of LRRK2 p.R1398H on risk of Parkinson's disease is independent of MAPT and SNCA variants

The best validated susceptibility variants for Parkinson's disease are located in the α-synuclein (SNCA) and microtubule-associated protein tau (MAPT) genes. Recently, a protective p.N551K-R1398H-K1423K haplotype in the leucine-rich repeat kinase 2 (LRRK2) gene was identified, with p.R1398H appearing to be the most likely functional variant. To date, the consistency of the protective effect of LRRK2 p.R1398H across MAPT and SNCA variant genotypes has not been assessed. To address this, we examined 4 SNCA variants (rs181489, rs356219, rs11931074, and rs2583988), the MAPT H1-haplotype-defining variant rs1052553, and LRRK2 p.R1398H (rs7133914) in Caucasian (n = 10,322) and Asian (n = 2289) series. There was no evidence of an interaction of LRRK2 p.R1398H with MAPT or SNCA variants (all p ≥ 0.10); the protective effect of p.R1398H was observed at similar magnitude across MAPT and SNCA genotypes, and the risk effects of MAPT and SNCA variants were observed consistently for LRRK2 p.R1398H genotypes. Our results indicate that the association of LRRK2 p.R1398H with Parkinson's disease is independent of SNCA and MAPT variants, and vice versa, in Caucasian and Asian populations.

Genetik der Parkinson-Krankheit

Neben 9 eindeutig gesicherten monogenen Parkinson-Formen gibt es zahlreiche bekannte Risiko- oder protektive Genvarianten, die das Risiko für eine Parkinson-Erkrankung modulieren. Unter den monogenen Formen folgen 3 (PARK1/PARK4, PARK8, PARK17) einem autosomal-dominanten Erbgang und 6 einem rezessiven Vererbungsmuster (PARK2, PARK6, PARK7, PARK9, PARK14, PARK15). Ebenfalls 6 Formen gehen mit einem der idiopathischen Parkinson-Krankheit sehr ähnlichen klinischen Bild einher (PARK1/PARK4, PARK2, PARK6, PARK7, PARK8, PARK17), darunter sind PARK8 mit Mutationen im LRRK2-Gen und spätem Krankheitsbeginn bzw. PARK2 mit Mutationen im Parkin-Gen und frühem Erkrankungsalter die weitaus häufigsten. Pathophysiologisch stehen bei den monogenen Formen wie auch bei der idiopathischen Parkinson-Krankheit Mechanismen der oxidativen Modifikation, des gestörten Proteinabbaus sowie der mitochondrialen Dysfunktion im Mittelpunkt, sodass die monogenen Parkinson-Formen als humane Modellerkrankungen für die idiopathische Form dienen können.

Alzheimer's disease and Parkinson's disease genome-wide association study top hits and risk of Parkinson's disease in Korean population

Alzheimer's disease (AD) and Parkinson's disease (PD) have overlapping clinical and pathological features, suggesting a common pathway for these 2 neurodegenerative disorders. Here we investigated the association of both AD and PD GWAS top hits with PD susceptibility. We selected 25 single nucleotide polymorphisms (SNPs) in 9 genes (ABCA7, APOE, BST1, CLU, CR1, LRRK2, PARK16, PICALM, and SNCA) that were genotyped in 1036 PD case patients and 1208 controls. Case patients and controls were all ethnic Koreans. Logistic regression analysis was performed to calculate age- and sex-adjusted odds ratios. None of the AD-susceptibility loci (ABCA7, APOE, CLU, CR1, and PICALM) showed statistically significant association with PD susceptibility. In contrast, we replicated associations of SNCA, LRRK2, BST1, and PARK16 with PD susceptibility in Koreans. Of those, the SNCA SNP rs11931074 showed the most significant association with PD susceptibility (adjusted odds ratio = 1.48; 95% confidence interval = 1.31-1.67; p = 2.20E-10). In a logistic regression analysis with SNPs coded under an additive model, there was no significant genetic interaction between the LRRK2 and the PARK16 locus gene RAB7L1 in PD risk. Our results confirm the associations of SNCA, LRRK2, BST1, and PARK16 with PD susceptibility and fail to show significant associations of AD genome-wide association study (GWAS) top hits with PD susceptibility in a Korean population.

Mesencephalic complex I deficiency does not correlate with parkinsonism in mitochondrial DNA maintenance disorders

Genetic evidence from recessively inherited Parkinson's disease has indicated a clear causative role for mitochondrial dysfunction in Parkinson's disease. This role has long been discussed based on findings that toxic inhibition of mitochondrial respiratory complex I caused parkinsonism and that tissues of patients with Parkinson's disease show complex I deficiency. Disorders of mitochondrial DNA maintenance are a common cause of inherited neurodegenerative disorders, and lead to mitochondrial DNA deletions or depletion and respiratory chain defect, including complex I deficiency. However, parkinsonism associates typically with defects of catalytic domain of mitochondrial DNA polymerase gamma. Surprisingly, however, not all mutations affecting DNA polymerase gamma manifest as parkinsonism, but, for example, spacer region mutations lead to spinocerebellar ataxia and/or severe epilepsy. Furthermore, defective Twinkle helicase, a close functional companion of DNA polymerase gamma in mitochondrial DNA replication, results in infantile-onset spinocerebellar ataxia, epilepsy or adult-onset mitochondrial myopathy, but not typically parkinsonism. Here we sought for clues for this specificity in the neurological manifestations of mitochondrial DNA maintenance disorders by studying mesencephalic neuropathology of patients with DNA polymerase gamma or Twinkle defects, with or without parkinsonism. We show here that all patients with mitochondrial DNA maintenance disorders had neuronopathy in substantia nigra, most severe in DNA polymerase gamma-associated parkinsonism. The oculomotor nucleus was also affected, but less severely. In substantia nigra, all patients had a considerable decrease of respiratory chain complex I, but other respiratory chain enzymes were not affected. Complex I deficiency did not correlate with parkinsonism, age, affected gene or inheritance. We conclude that the cell number in substantia nigra correlated well with parkinsonism in DNA polymerase gamma and Twinkle defects. However, complex I defect is a general consequence of mitochondrial DNA maintenance defects, and does not explain manifestation of parkinsonism or degree of mesencephalic cell death in patients with mitochondrial DNA maintenance disorders.

Biomarkers in Parkinson's disease (recent update)

Parkinson's disease (PD) is the second most common neurodegenerative disorder mostly affecting the aging population over sixty. Cardinal symptoms including, tremors, muscle rigidity, drooping posture, drooling, walking difficulty, and autonomic symptoms appear when a significant number of nigrostriatal dopaminergic neurons are already destroyed. Hence we need early, sensitive, specific, and economical peripheral and/or central biomarker(s) for the differential diagnosis, prognosis, and treatment of PD. These can be classified as clinical, biochemical, genetic, proteomic, and neuroimaging biomarkers. Novel discoveries of genetic as well as nongenetic biomarkers may be utilized for the personalized treatment of PD during preclinical (premotor) and clinical (motor) stages. Premotor biomarkers including hyper-echogenicity of substantia nigra, olfactory and autonomic dysfunction, depression, hyposmia, deafness, REM sleep disorder, and impulsive behavior may be noticed during preclinical stage. Neuroimaging biomarkers (PET, SPECT, MRI), and neuropsychological deficits can facilitate differential diagnosis. Single-cell profiling of dopaminergic neurons has identified pyridoxal kinase and lysosomal ATPase as biomarker genes for PD prognosis. Promising biomarkers include: fluid biomarkers, neuromelanin antibodies, pathological forms of α-Syn, DJ-1, amyloid β and tau in the CSF, patterns of gene expression, metabolomics, urate, as well as protein profiling in the blood and CSF samples. Reduced brain regional N-acetyl-aspartate is a biomarker for the in vivo assessment of neuronal loss using magnetic resonance spectroscopy and T2 relaxation time with MRI. To confirm PD diagnosis, the PET biomarkers include [(18)F]-DOPA for estimating dopaminergic neurotransmission, [(18)F]dG for mitochondrial bioenergetics, [(18)F]BMS for mitochondrial complex-1, [(11)C](R)-PK11195 for microglial activation, SPECT imaging with (123)Iflupane and βCIT for dopamine transporter, and urinary salsolinol and 8-hydroxy, 2-deoxyguanosine for neuronal loss. This brief review describes the merits and limitations of recently discovered biomarkers and proposes coenzyme Q10, mitochondrial ubiquinone-NADH oxidoreductase, melatonin, α-synculein index, Charnoly body, and metallothioneins as novel biomarkers to confirm PD diagnosis for early and effective treatment of PD.

Nucleotide excision repair in chronic neurodegenerative diseases

Impaired DNA repair involving the nucleotide excision repair (NER)/transcription-coupled repair (TCR) pathway cause human pathologies associated with severe neurological symptoms. These clinical observations suggest that defective NER/TCR might also play a critical role in chronic neurodegenerative disorders (ND), such as Alzheimer's and Parkinson's disease. Involvement of NER/TCR in these disorders is also substantiated by the evidence that aging constitutes the principal risk factor for chronic ND and that this DNA repair mechanism is very relevant for the aging process itself. Our understanding of the exact role of NER/TCR in chronic ND, however, is extremely rudimentary; while there is no doubt that defective NER/TCR can lead to neuronal death, evidence for its participation in the etiopathogenesis of ND is inconclusive thus far. Here we summarize the experimental observations supporting a role for NER/TCR in chronic ND and suggest questions and lines of investigation that might help in addressing this important issue. We also present a preliminary yet unprecedented meta-analysis on human brain microarray data to understand the expression levels of the various NER factors in the anatomical areas relevant for chronic ND pathogenesis. In summary, this review intends to highlight elements supporting a role of NER/TCR in these devastating disorders and to propose potential strategies of investigation.

PLK2 modulates α-synuclein aggregation in yeast and mammalian cells

Phosphorylation of α-synuclein (aSyn) on serine 129 is one of the major post-translation modifications found in Lewy bodies, the typical pathological hallmark of Parkinson's disease. Here, we found that both PLK2 and PLK3 phosphorylate aSyn on serine 129 in yeast. However, only PLK2 increased aSyn cytotoxicity and the percentage of cells presenting cytoplasmic foci. Consistently, in mammalian cells, PLK2 induced aSyn phosphorylation on serine 129 and induced an increase in the size of the inclusions. Our study supports a role for PLK2 in the generation of aSyn inclusions by a mechanism that does not depend directly on serine 129 phosphorylation.

α-Synuclein mutations cluster around a putative protein loop

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We map all five missense SNCA mutations on the proposed α-synuclein protein models.

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4 mutations cluster around the protein loop linking the two legs of the hairpin.

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4 mutations cluster around the point of hairpin convergence for tetramer formation.

With the recent identification of two new pathogenic mutations in α-synuclein, we map the five known pathogenic mutations onto the best available models of the protein structure. We show that four of the five mutations map to a potential fold in the protein with the exception being the A30P mutation in which the substitution would be expected to have a profound effect on protein structure. We discuss this localisation in terms of the proposed mechanisms for mutation pathogenicity.

The prion hypothesis in Parkinson's disease: Braak to the future

Parkinson’s disease (PD) is a progressive neurodegenerative disorder typified by the presence of intraneuronal inclusions containing aggregated alpha synuclein (αsyn). The progression of parkinsonian pathology and clinical phenotype has been broadly demonstrated to follow a specific pattern, most notably described by Braak and colleagues. In more recent times it has been hypothesized that αsyn itself may be a critical factor in mediating transmission of disease pathology from one brain area to another. Here we investigate the growing body of evidence demonstrating the ability of αsyn to spread transcellularly and induce pathological aggregation affecting neurons by permissive templating and provide a critical analysis of some irregularities in the hypothesis that the progression of PD pathology may be mediated by such a prion-like process. Finally we discuss some key questions that remain unanswered which are vital to determining the potential contribution of a prion-like process to the pathogenesis of PD.

hnRNP A3 binds to GGGGCC repeats and is a constituent of p62-positive/TDP43-negative inclusions in the hippocampus of patients with C9orf72 mutations

Genetic analysis revealed the hexanucleotide repeat expansion GGGGCC within the regulatory region of the gene C9orf72 as the most common cause of familial amyotrophic lateral sclerosis and the second most common cause of frontotemporal lobar degeneration. Since repeat expansions might cause RNA toxicity via sequestration of RNA-binding proteins, we searched for proteins capable of binding to GGGGCC repeats. In vitro-transcribed biotinylated RNA containing hexanucleotide GGGGCC or, as control, AAAACC repeats were incubated with nuclear protein extracts. Using stringent filtering protocols 20 RNA-binding proteins with a variety of different functions in RNA metabolism, translation and transport were identified. A subset of these proteins was further investigated by immunohistochemistry in human autopsy brains. This revealed that hnRNP A3 formed neuronal cytoplasmic and intranuclear inclusions in the hippocampus of patients with C9orf72 repeat extensions. Confocal microcopy showed that these inclusions belong to the group of the so far enigmatic p62-positive/TDP-43 negative inclusions characteristically seen in autopsy cases of diseased C9orf72 repeat expansion carriers. Thus, we have identified one protein component of these pathognomonic inclusions.

Role of LRRK2 kinase activity in the pathogenesis of Parkinson's disease

Interest in studying the biology of LRRK2 (leucine-rich repeat kinase 2) started in 2004 when missense mutations in the LRRK2 gene were linked to an inherited form of Parkinson's disease with clinical and pathological presentation resembling the sporadic syndrome. LRRK2 is a complex molecule containing domains implicated in protein interactions, as well as kinase and GTPase activities. The observation that the common G2019S mutation increases kinase activity in vitro suggests that altered phosphorylation of LRRK2 targets may have pathological outcomes. Given that protein kinases are ideal targets for drug therapies, much effort has been directed at understanding the role of LRRK2 kinase activity on disease onset. However, no clear physiological substrates have been identified to date, indicating that much research is still needed to fully understand the signalling pathways orchestrated by LRRK2 and deregulated under pathological conditions.

Evolution of neurodegeneration

A number of neurodegenerative diseases principally affect humans as they age and are characterized by the loss of specific groups of neurons in different brain regions. Although these disorders are generally sporadic, it is now clear that many of them have a substantial genetic component. As genes are the raw material with which evolution works, we might benefit from understanding these genes in an evolutionary framework. Here, I will discuss how we can understand whether evolution has shaped genes involved in neurodegeneration and the implications for practical issues, such as our choice of model systems for studying these diseases, and more theoretical concerns, such as the level of selection against these phenotypes.

New horizons in the pathogenesis, assessment and management of movement disorders

In this review, we shall outline recent advances in our understanding of the movement disorders which geriatricians encounter in their clinical practice. Many of these diseases are no longer simply considered disorders of movement: carefully conducted longitudinal studies have shown that concomitant cognitive dysfunction, neuropsychiatric disturbance and behavioural issues are frequent and exert a heavy burden on the individual and their carers. Great progress has been made in understanding the molecular and cellular processes that drive the pathological changes in these conditions, as have advances in neuroimaging and preclinical drug discovery programmes. Unfortunately, this is yet to translate into disease-modifying therapies for these progressive disorders. Advances have been also made in non-pharmacological interventions such as tailored physiotherapy and speech therapy programmes. The important contribution of palliative care has been recognised and increasingly incorporated into the multidisciplinary approach. The UK is at the forefront of research into these conditions and geriatricians are well placed to contribute to research through recruiting patients to observational studies or therapeutic trials, particularly with the support of agencies such as the National Institute for Health Research-Dementias & Neurodegenerative Diseases Research Network (NIHR-DeNDRoN).

The central theme of Parkinson's disease: α-synuclein

Parkinson's disease (PD) is the second most common neurodegenerative disorder, defined by the presence of resting tremor, muscular rigidity, bradykinesia, and postural instability. PD is characterized by the progressive loss of dopaminergic neurons within the substantia nigra pars compacta of the midbrain. The neuropathological hallmark of the disease is the presence of intracytoplasmic inclusions, called Lewy bodies (LBs) and Lewy neurites (LNs), containing α-synuclein, a small protein which is widely expressed in the brain. The α-synuclein gene, SNCA, is located on chromosome 4q22.1; SNCA-linked PD shows an autosomal dominant inheritance pattern with a relatively early onset age, and it usually progresses rapidly. Three missense mutations, A53T, A30P, and E46K, in addition to gene multiplications of the SNCA have been described so far. Although it is clear that LBs and LNs contain mainly the α-synuclein protein, the mechanism(s) which leads α-synuclein to accumulate needs to be elucidated. The primary question in the molecular pathology of PD is how wild-type α-synuclein aggregates in PD, and which interacting partner(s) plays role(s) in the aggregation process. It is known that dopamine synthesis is a stressfull event, and α-synuclein expression somehow affects the dopamine synthesis. The aberrant interactions of α-synuclein with the proteins in the dopamine synthesis pathway may cause disturbances in cellular mechanisms. The normal physiological folding state of α-synuclein is also important for the understanding of pathological aggregates. Recent studies on the α-synuclein protein and genome-wide association studies of the α-synuclein gene show that PD has a strong genetic component, and both familial and idiopathic PD have a common denominator, α-synuclein, at the molecular level. It is clear that the disease process in Parkinson's disease, as in other neurodegenerative disorders, is very complicated; there can be several different molecular pathways which are responsible for diverse and possibly also unrelated functions inside the neuron, playing roles in PD pathogenesis.

Supportive evidence for 11 loci from genome-wide association studies in Parkinson's disease

Genome-wide association studies have identified a number of susceptibility loci in sporadic Parkinson's disease (PD). Recent larger studies and meta-analyses have greatly expanded the list of proposed association signals. We performed a case-control replication study in a Scandinavian population, analyzing samples from 1345 unrelated PD patients and 1225 control subjects collected by collaborating centers in Norway and Sweden. Single-nucleotide polymorphisms representing 18 loci previously reported at genome-wide significance levels were genotyped, as well as 4 near-significant, suggestive, loci. We replicated 11 association signals at p < 0.05 (SNCA, STK39, MAPT, GPNMB, CCDC62/HIP1R, SYT11, GAK, STX1B, MCCC1/LAMP3, ACMSD, and FGF20). The more recently nominated susceptibility loci were well represented among our positive findings, including 3 which have not previously been validated in independent studies. Conversely, some of the more well-established loci failed to replicate. While future meta-analyses should corroborate disease associations further on the level of common markers, efforts to pinpoint functional variants and understand the biological implications of each risk locus in PD are also warranted.

Transcriptome analysis using next-generation sequencing

Up to date research in biology, biotechnology, and medicine requires fast genome and transcriptome analysis technologies for the investigation of cellular state, physiology, and activity. Here, microarray technology and next generation sequencing of transcripts (RNA-Seq) are state of the art. Since microarray technology is limited towards the amount of RNA, the quantification of transcript levels and the sequence information, RNA-Seq provides nearly unlimited possibilities in modern bioanalysis. This chapter presents a detailed description of next-generation sequencing (NGS), describes the impact of this technology on transcriptome analysis and explains its possibilities to explore the modern RNA world.

Pathogenesis of Parkinson's disease

Parkinson's disease is a common adult-onset neurodegenerative disorder whose pathogenesis remains essentially unknown. Currently, it is believed that the neurodegenerative process in Parkinson's disease is a combination of both cell-autonomous and non-cell-autonomous mechanisms. Proposed cell-autonomous mechanisms include alterations in mitochondrial bioenergetics, dysregulation of calcium homeostasis, and impaired turnover of mitochondria. As for the proposed non-cell-autonomous mechanisms, they involve prion-like behavior of misfolded proteins and neuroinflammation. This suggests that cell death in Parkinson's disease is caused by a multifactorial cascade of pathogenic events and argues that effective neuroprotective therapy for Parkinson's disease may have to rely on multiple drug interventions.

Stem cells and regenerative therapies for Parkinson's disease

Currently the mainstay of Parkinson's disease (PD) therapy is the pharmacological replacement of the loss of the dopaminergic nigrostriatal pathway using drugs such as dopamine agonists and levodopa. Whilst these drugs effectively ameliorate some of the motor features of PD, they do not improve many of the nonmotor features that arise secondary to pathology outside of this system, nor do they slow the progressive neurodegeneration that is a characteristic of the disease. Regenerative therapies for PD seek to fill this therapeutic gap, with cell transplantation being the most explored approach to date. A number of different cell sources have been used in this therapeutic approach, but to date, the most successful has been the use of fetal ventral mesencephalic (VM) tissue that contains within it the developing nigral dopaminergic cells. Cell transplantation for PD was pioneered in the 1980-1990s, with several successful open-label trials of fetal VM transplantation in patients with relatively advanced PD. Whilst these findings were not replicated in two subsequent double-blind sham-surgery controlled trials, there were reasons to explain this outside of the one drawn at the time that these therapies are ineffective. Indeed all these studies have provided evidence that following the transplantation of fetal VM tissue, dopaminergic cells can survive long term, produce dopamine, and bring about clinical improvements in younger patients over many years. The use of fetal tissue, irrespective of its true efficacy, will never become a widely available therapy for PD for a host of practical and ethical reasons, and thus much work has been put in recently to exploring the utility of stem cells as a source of nigral dopaminergic neurons. In this respect, the advent of embryonic stem cell and induced pluripotent cells has heralded a new era in cell therapy for PD, and several groups have now demonstrated that these cells can form dopaminergic neurons which improve functional deficits in animal models of PD. Whilst encouraging, problems with respect to the immunogenicity and tumorigenicity of these cells means that they will need to be used in the clinic cautiously. Other regenerative therapies in PD have been tried over the years and include the use of trophic factors. This has primarily involved glial cell line-derived neurotrophic factor (GDNF) and again has produced mixed clinical effects, and in order to try and resolve this, a new trial of intraputamenal GDNF is now being planned. In addition, a new trial for platelet derived growth factor as a treatment for PD has just completed recruitment, and PYM50028 (Cogane) an oral agent shown in animal models to reduce the effects of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) lesioning by the induction of growth factors is currently under investigation in a multicentre Phase II trial. Overall, there are a number of promising new regenerative therapies being developed and tested in PD, although the true long-term efficacy of any of these in large numbers of patients is still not known.

The management of gastrointestinal symptoms in Parkinson's disease

The involvement of the autonomic nervous system in Parkinson's disease causes many non-motor symptoms, among which gastrointestinal complaints are prominent. Drooling, dyspepsia, constipation, abdominal pain and fecal incontinence are frequently a source of patient distress. Dysphagia is recognized as causing both discomfort and increased risk of serious complications. Although a diagnosis can often be established based on the reports of patients and/or caregivers, and additional testing is seldom required, these diagnoses are clearly under recognized in clinical practice. These symptoms respond to the same treatment measures used in the general population, although certain drugs with a potential to increase parkinsonian symptoms should be avoided. Increased and early identification of these symptoms can result in a significant improvement in the quality of life of Parkinson's disease patients.