PET in LRRK2 mutations: comparison to sporadic Parkinson's disease and evidence for presymptomatic compensation

Dopaminergic neuronal imaging in genetic Parkinson's disease: insights into pathogenesis

OBJECTIVES

To compare the dopaminergic neuronal imaging features of different subtypes of genetic Parkinson's Disease.

METHODS

A retrospective study of genetic Parkinson's diseases cases in which DaTSCAN (123I-FP-CIT) had been performed. Specific non-displaceable binding was calculated for bilateral caudate and putamen for each case. The right:left asymmetry index and striatal asymmetry index was calculated.

RESULTS

Scans were available from 37 cases of monogenetic Parkinson's disease (7 glucocerebrosidase (GBA) mutations, 8 alpha-synuclein, 3 LRRK2, 7 PINK1, 12 Parkin). The asymmetry of radioligand uptake for Parkinson's disease with GBA or LRRK2 mutations was greater than that for Parkinson's disease with alpha synuclein, PINK1 or Parkin mutations.

CONCLUSIONS

The asymmetry of radioligand uptake in Parkinsons disease associated with GBA or LRRK2 mutations suggests that interactions with additional genetic or environmental factors may be associated with dopaminergic neuronal loss.

Nigrostriatal dopamine terminal imaging with dopamine transporter SPECT: an update

This article gives an update on nigrostriatal dopamine terminal imaging, with emphasis on SPECT performed with the presynaptic dopamine transporter (DAT) ligand (123)I-FP-CIT. The paper covers the rational use of this technique in the diagnostic work-up of patients with known or suspected parkinsonian syndromes. In detail, it addresses the impact of the method for the proof or exclusion of neurodegenerative parkinsonism, for its early and preclinical diagnosis, and for the evaluation of disease progression. The importance of normal DAT binding for differentiating symptomatic parkinsonism and relevant tremor syndromes from neurodegeneration is highlighted. Particularly emphasized is the role of DAT SPECT for diagnosing Lewy body dementia and its separation from Alzheimer dementia. Finally, some remarks deal with the economic aspects of the use of these imaging techniques in the clinical setting.

[Advanced Parkinson's disease: clinical characteristics and treatment (part 1)]

INTRODUCTION

A large percentage of patients with Parkinson's disease (PD) develop motor fluctuations, dyskinesias, and severe non-motor symptoms within 3 to 5 years of starting dopaminergic therapy, and these motor complications are refractory to treatment. Several authors refer to this stage of the disease as advanced Parkinson's disease.

OBJECTIVE

To define the clinical manifestations of advanced PD and the risk factors for reaching this stage of the disease.

DEVELOPMENT

This consensus document has been prepared by using an exhaustive literature search and by discussion of the contents by an expert group on movement disorders of the Sociedad Española de Neurología (Spanish Neurology Society), coordinated by two of the authors (JK and MRL).

CONCLUSIONS

Severe motor fluctuations and dyskinesias, axial motor symptoms resistant to levodopa, and cognitive decline are the main signs in the clinical phenotype of advanced PD.

Functional neuroimaging in Parkinson's disease

The use of functional imaging in neurodegenerative diseases has increased in recent years, with applications in research into the underlying pathophysiology, aiding in diagnosis, or evaluating new treatments. In Parkinson's disease (PD), these imaging methods have expanded our understanding of the disease beyond dopaminergic deficits. Moreover, functional imaging methods have described alterations in functional networks relating not only to the motor symptoms, but also to many nonmotor features of PD, such as cognitive dysfunction. From a clinical viewpoint, functional imaging methods can assist in monitoring disease progression, such as in the context of clinical trials, and holds the potential to aid in early diagnosis of PD and differentiation from other parkinsonian disorders.

EFNS/MDS-ES/ENS [corrected] recommendations for the diagnosis of Parkinson's disease

BACKGROUND

A Task Force was convened by the EFNS/MDS-ES Scientist Panel on Parkinson's disease (PD) and other movement disorders to systemically review relevant publications on the diagnosis of PD.

METHODS

Following the EFNS instruction for the preparation of neurological diagnostic guidelines, recommendation levels have been generated for diagnostic criteria and investigations.

RESULTS

For the clinical diagnosis, we recommend the use of the Queen Square Brain Bank criteria (Level B). Genetic testing for specific mutations is recommended on an individual basis (Level B), taking into account specific features (i.e. family history and age of onset). We recommend olfactory testing to differentiate PD from other parkinsonian disorders including recessive forms (Level A). Screening for pre-motor PD with olfactory testing requires additional tests due to limited specificity. Drug challenge tests are not recommended for the diagnosis in de novo parkinsonian patients. There is an insufficient evidence to support their role in the differential diagnosis between PD and other parkinsonian syndromes. We recommend an assessment of cognition and a screening for REM sleep behaviour disorder, psychotic manifestations and severe depression in the initial evaluation of suspected PD cases (Level A). Transcranial sonography is recommended for the differentiation of PD from atypical and secondary parkinsonian disorders (Level A), for the early diagnosis of PD and in the detection of subjects at risk for PD (Level A), although the technique is so far not universally used and requires some expertise. Because specificity of TCS for the development of PD is limited, TCS should be used in conjunction with other screening tests. Conventional magnetic resonance imaging and diffusion-weighted imaging at 1.5 T are recommended as neuroimaging tools that can support a diagnosis of multiple system atrophy (MSA) or progressive supranuclear palsy versus PD on the basis of regional atrophy and signal change as well as diffusivity patterns (Level A). DaTscan SPECT is registered in Europe and the United States for the differential diagnosis between degenerative parkinsonisms and essential tremor (Level A). More specifically, DaTscan is indicated in the presence of significant diagnostic uncertainty such as parkinsonism associated with neuroleptic exposure and atypical tremor manifestations such as isolated unilateral postural tremor. Studies of [(123) I]MIBG/SPECT cardiac uptake may be used to identify patients with PD versus controls and MSA patients (Level A). All other SPECT imaging studies do not fulfil registration standards and cannot be recommended for routine clinical use. At the moment, no conclusion can be drawn as to diagnostic efficacy of autonomic function tests, neurophysiological tests and positron emission tomography imaging in PD.

CONCLUSIONS

The diagnosis of PD is still largely based on the correct identification of its clinical features. Selected investigations (genetic, olfactory, and neuroimaging studies) have an ancillary role in confirming the diagnosis, and some of them could be possibly used in the near future to identify subjects in a pre-symptomatic phase of the disease.

Down-regulation of LRRK2 in control and DAT transfected HEK cells increases manganese-induced oxidative stress and cell toxicity

The extra-pyramidal symptoms associated with manganism often overlap with that seen in Parkinsonism suggesting a common link between the two disorders. Since wide deviations are observed in susceptibility and characteristics of the symptoms observed in manganism, these differences may be due to underlying genetic variability. Genes linked to early onset of Parkinsonism which includes ATP13A2 and parkin have already been suggested to promote development of Mn toxicity. Of the other Parkinson-linked genes, mutations in LRRK2, an autosomal dominant gene, represent another likely candidate involved in the development of manganism. In this paper the effect of shRNA LRRK2 knock-down on Mn toxicity was examined in control and DAT transfected HEK293 cells. Results demonstrate that LRRK2 down-regulation potentiates Mn toxicity in both control and DAT-transfected cell as well as potentiates DA toxicity. Combined treatment of Mn and DA further augments cell toxicity, ROS production and JNK phosphorylation in LRRK2 deficient cells compared to controls. Consistent with studies demonstrating that LRRK2 plays a role in the phosphorylation of p38, our results similarly demonstrate a decrease in p38 activation in LRRK2 knock-down cells. Our findings suggest that null mutations in LRRK2 which cause Parkinsonism potentiate Mn toxicity and increase susceptibility to develop manganism.

Manganese homeostasis and transport

The review addresses issues pertinent to Mn accumulation and its mechanisms of transport, its neurotoxicity and mechanisms of neurodegeneration. The role of mitochondria and glia in this process is emphasized. We also discuss gene x environment interactions, focusing on the interplay between genes linked to Parkinson's disease (PD) and sensitivity to Mn.

Dopamine transporter loss in 6-OHDA Parkinson's model is unmet by parallel reduction in dopamine uptake

The dopamine transporter (DAT) regulates synaptic dopamine (DA) in striatum and modulation of DAT can affect locomotor activity. Thus, in Parkinson's disease (PD), DAT loss could affect DA clearance and locomotor activity. The locomotor benefits of L-DOPA may be mediated by transport through monoamine transporters and conversion to DA. However, its impact upon DA reuptake is unknown and may modulate synaptic DA. Using the unilateral 6-OHDA rat PD model, we examined [(3)H]DA uptake dynamics in relation to striatal DAT and tyrosine hydroxylase (TH) protein loss compared with contralateral intact striatum. Despite >70% striatal DAT loss, DA uptake decreased only ∼25% and increased as DAT loss approached 99%. As other monoamine transporters can transport DA, we determined if norepinephrine (NE) and serotonin (5-HT) differentially modulated DA uptake in lesioned striatum. Unlabeled DA, NE, and 5-HT were used, at a concentration that differentially inhibited DA uptake in intact striatum, to compete against [(3)H]DA uptake. In 6-OHDA lesioned striatum, DA was less effective, whereas NE was more effective, at inhibiting [(3)H]DA uptake. Furthermore, norepinephrine transporter (NET) protein levels increased and desipramine was ∼two-fold more effective at inhibiting NE uptake. Serotonin inhibited [(3)H]DA uptake, but without significant difference between lesioned and contralateral striatum. L-DOPA inhibited [(3)H]DA uptake two-fold more in lesioned striatum and inhibited NE uptake ∼five-fold more than DA uptake in naïve striatum. Consequently, DA uptake may be mediated by NET when DAT loss is at PD levels. Increased inhibition of DA uptake by L-DOPA and its preferential inhibition of NE over DA uptake, indicates that NET-mediated DA uptake may be modulated by L-DOPA when DAT loss exceeds 70%. These results indicate a novel mechanism for DA uptake during PD progression and provide new insight into how L-DOPA affects DA uptake, revealing possible mechanisms of its therapeutic and side effect potential.

Expression of human E46K-mutated α-synuclein in BAC-transgenic rats replicates early-stage Parkinson's disease features and enhances vulnerability to mitochondrial impairment

Parkinson's disease (PD), the second most common neurodegenerative disorder, is etiologically heterogeneous, with most cases thought to arise from a combination of environmental factors and genetic predisposition; about 10% of cases are caused by single gene mutations. While neurotoxin models replicate many of the key behavioral and neurological features, they often have limited relevance to human exposures. Genetic models replicate known disease-causing mutations, but are mostly unsuccessful in reproducing major features of PD. In this study, we created a BAC (bacterial artificial chromosome) transgenic rat model of PD expressing the E46K mutation of α-synuclein, which is pathogenic in humans. The mutant protein was expressed at levels ~2-3-fold above endogenous α-synuclein levels. At 12 months of age, there was no overt damage to the nigrostriatal dopamine system; however, (i) alterations in striatal neurotransmitter metabolism, (ii) accumulation and aggregation of α-synuclein in nigral dopamine neurons, and (iii) evidence of oxidative stress suggest this model replicates several preclinical features of PD. Further, when these animals were exposed to rotenone, a mitochondrial toxin linked to PD, they showed heightened sensitivity, indicating that α-synuclein expression modulates the vulnerability to mitochondrial impairment. We conclude that these animals are well-suited to examination of gene-environment interactions that are relevant to PD.

Dopaminergic neuronal loss and dopamine-dependent locomotor defects in Fbxo7-deficient zebrafish

Recessive mutations in the F-box only protein 7 gene (FBXO7) cause PARK15, a Mendelian form of early-onset, levodopa-responsive parkinsonism with severe loss of nigrostriatal dopaminergic neurons. However, the function of the protein encoded by FBXO7, and the pathogenesis of PARK15 remain unknown. No animal models of this disease exist. Here, we report the generation of a vertebrate model of PARK15 in zebrafish. We first show that the zebrafish Fbxo7 homolog protein (zFbxo7) is expressed abundantly in the normal zebrafish brain. Next, we used two zFbxo7-specific morpholinos (targeting protein translation and mRNA splicing, respectively), to knock down the zFbxo7 expression. The injection of either of these zFbxo7-specific morpholinos in the fish embryos induced a marked decrease in the zFbxo7 protein expression, and a range of developmental defects. Furthermore, whole-mount in situ mRNA hybridization showed abnormal patterning and significant decrease in the number of diencephalic tyrosine hydroxylase-expressing neurons, corresponding to the human nigrostriatal or ventral tegmental dopaminergic neurons. Of note, the number of the dopamine transporter-expressing neurons was much more severely depleted, suggesting dopaminergic dysfunctions earlier and larger than those due to neuronal loss. Last, the zFbxo7 morphants displayed severe locomotor disturbances (bradykinesia), which were dramatically improved by the dopaminergic agonist apomorphine. The severity of these morphological and behavioral abnormalities correlated with the severity of zFbxo7 protein deficiency. Moreover, the effects of the co-injection of zFbxo7- and p53-specific morpholinos were similar to those obtained with zFbxo7-specific morpholinos alone, supporting further the contention that the observed phenotypes were specifically due to the knock down of zFbxo7. In conclusion, this novel vertebrate model reproduces pathologic and behavioral hallmarks of human parkinsonism (dopaminergic neuronal loss and dopamine-dependent bradykinesia), representing therefore a valid tool for investigating the mechanisms of selective dopaminergic neuronal death, and screening for modifier genes and therapeutic compounds.

Presynaptic dysfunction in Parkinson's disease: a focus on LRRK2

PD (Parkinson's disease) is a common neurodegenerative disease clinically characterized by bradykinesia, rigidity and resting tremor. Recent studies have proposed that synaptic dysfunction, implicated in numerous studies of animal models of PD, might be a key factor in PD. The molecular defects that lead to PD progression might be hidden at the presynaptic neuron: in fact accumulating evidence has shown that the majority of the genes linked to PD play a critical role at the presynaptic site. In the present paper, we focus on the presynaptic function of LRRK2 (leucine-rich repeat kinase 2), a protein that mutated represents the main genetic cause of familial PD described to date. Neurotransmission relies on proper presynaptic vesicle trafficking; defects in this process, variation in dopamine flow and alteration of presynaptic plasticity have been reported in several animal models of LRRK2 mutations. Furthermore, impaired dopamine turnover has been described in presymptomatic LRRK2 PD patients. Thus, given the pathological events occurring at the synapses of PD patients, the presynaptic site may represent a promising target for early diagnostic therapeutic intervention.

Biomarkers of Parkinson's disease: current status and future perspectives

This review summarizes major advances in biomarker discovery for diagnosis, differential diagnosis and progression of Parkinson's disease (PD), with emphasis on neuroimaging and biochemical markers. Potential strategies to develop biomarkers capable of predicting PD in the prodromal stage before the appearance of motor symptoms or correlating with nonmotor symptoms, an active area of research, are also discussed.

Neuroimaging: current role in detecting pre-motor Parkinson's disease

Convergent evidence suggests a pre-motor period in Parkinson's disease (PD) during which typical motor symptoms have not yet developed although dopaminergic neurons in the substantia nigra have started to degenerate. Advances in different neuroimaging techniques have allowed the detection of functional and structural changes in early PD. This review summarizes the state of the art knowledge concerning structural neuroimaging techniques including magnetic resonance imaging (MRI) and transcranial B-mode-Doppler-sonography (TCS) as well as functional neuroimaging techniques using radiotracer imaging (RTI) with different radioligands in detecting pre-motor PD.

Neuroimaging in the early diagnosis of neurodegenerative disease

Functional imaging may be useful for both the early diagnosis as well as preclinical detection of neurodegenerative disease. Additionally, while structural imaging has traditionally been regarded as a tool to exclude alternate diagnoses, recent advances in magnetic resonance show promise for greater diagnostic specificity. The role of MR and radionuclide imaging in early diagnosis and preclinical detection of dementia and parkinsonism are reviewed here.

Parkinson's disease therapeutics: new developments and challenges since the introduction of levodopa

The demonstration that dopamine loss is the key pathological feature of Parkinson's disease (PD), and the subsequent introduction of levodopa have revolutionalized the field of PD therapeutics. This review will discuss the significant progress that has been made in the development of new pharmacological and surgical tools to treat PD motor symptoms since this major breakthrough in the 1960s. However, we will also highlight some of the challenges the field of PD therapeutics has been struggling with during the past decades. The lack of neuroprotective therapies and the limited treatment strategies for the nonmotor symptoms of the disease (ie, cognitive impairments, autonomic dysfunctions, psychiatric disorders, etc.) are among the most pressing issues to be addressed in the years to come. It appears that the combination of early PD nonmotor symptoms with imaging of the nigrostriatal dopaminergic system offers a promising path toward the identification of PD biomarkers, which, once characterized, will set the stage for efficient use of neuroprotective agents that could slow down and alter the course of the disease.

Centrosomal aggregates and Golgi fragmentation disrupt vesicular trafficking of DAT

Lewy bodies containing the centrosomal protein γ-tubulin and fragmentation of Golgi apparatus (GA) have been described in nigral neurons of Parkinson's disease (PD) patients. However, the relevance of these features in PD pathophysiology remains unknown. We analyzed the impact of proteasome inhibition in the formation of γ-tubulin-containing aggregates as well as on GA structure. SH-SY5Y cells were treated with the proteasome inhibitor Z-Leu-Leu-Leu-al (MG132) to induce centrosomal-protein aggregates. Then, microtubules (MTs) and Golgi dynamics, as well as the vesicular transport of dopamine transporter (DAT) were evaluated both in vitro and in living cells. MG132 treatment induced γ-tubulin aggregates which altered microtubule nucleation. MG132-treated cells containing γ-tubulin aggregates showed fragmentation of GA and perturbation of the trans-Golgi network. Under these conditions, the DAT accumulated at the centrosomal-Golgi region indicating that the vesicular transport of DAT was disrupted. Thus, centrosomal aggregates and fragmentation of GA are 2 closely related processes that could result in the disruption of the vesicular transport of DAT toward the plasma membrane in a model of dopaminergic neuronal degeneration.

Cerebrospinal fluid amyloid β and tau in LRRK2 mutation carriers

OBJECTIVE

The goal of the current investigation was to examine a cohort of symptomatic and asymptomatic LRRK2 mutation carriers, in order to address whether the reported alterations in amyloid β (Aβ) and tau species in the CSF of patients with sporadic Parkinson disease (PD) are a part of PD pathogenesis, the aging process, or a comorbid disease in patients with PD, and to explore the possibility of Aβ and tau as markers of early or presymptomatic PD.

METHODS

CSF Aβ42, total tau, and phosphorylated tau were measured with Luminex assays in 26 LRRK2 mutation carriers, who were either asymptomatic (n = 18) or had a phenotype resembling sporadic PD (n = 8). All patients also underwent PET scans with 18F-6-fluoro-l-dopa (FD), 11C-(±)-α-dihydrotetrabenazine (DTBZ), and 11C-d-threo-methylphenidate (MP) to measure dopaminergic function in the striatum. The levels of CSF markers were then compared to each PET measurement.

RESULTS

Reduced CSF Aβ42 and tau levels correlated with lower striatal dopaminergic function as determined by all 3 PET tracers, with a significant association between Aβ42 and FD uptake. When cases were restricted to carriers of the G2019S mutation, the most common LRRK2 variant in our cohort, significant correlations were also observed for tau.

CONCLUSIONS

The disposition of Aβ and tau is likely important in both LRRK2-related and sporadic PD, even during early phases of the disease. A better understanding of their production, aggregation, and degradation, including changes in their CSF levels, may provide insights into the pathogenesis of PD and the potential utility of these proteins as biomarkers.

DJ-1 and αSYN in LRRK2 CSF do not correlate with striatal dopaminergic function

Previous studies demonstrated decreased levels of DJ-1 and α-synuclein (αSYN) in human cerebrospinal fluid (CSF) in patients with Parkinson's disease (PD), but neither marker correlated with PD severity, raising the possibility that they may be excellent progression markers during early or preclinical phases of PD. Individuals carrying the leucine-rich repeat kinase 2 (LRRK2) gene mutation are at increased risk for PD, and the phenotype of LRRK2 patients is almost identical to sporadic PD. To determine whether dopaminergic dysfunction in the basal ganglia, as determined by positron emission tomography (PET) scans, correlates with CSF levels of DJ-1 and αSYN during preclinical stages, Luminex assays were used to analyze CSF samples from asymptomatic LRRK2 mutation carriers, along with carriers who presented with a clinical diagnosis of PD. The data revealed no statistically significant relationship between PET scan evidence of loss of striatal dopaminergic function and the CSF biomarkers DJ-1 and αSYN, except for a weak correlation between DJ-1 and methylphenidate binding, suggesting that the use of these potential biomarkers on their own to screen LRRK2 gene mutation carriers for PD is not appropriate.

Clinical and brain imaging characteristics in leucine-rich repeat kinase 2-associated PD and asymptomatic mutation carriers

The objective of this research was to evaluate a possible endophenotype in leucine-rich repeat kinase 2 (LRRK2)-associated Parkinson's disease (PD). Ten symptomatic LRRK2 patients, 24 sporadic Parkinson's disease patients as well as 10 asymptomatic LRRK2 mutation carriers and 29 matched healthy controls underwent comprehensive clinical assessments with respect to motor and non-motor symptoms. Transcranial sonography and magnetic resonance imaging (voxel-based morphometry [VBM]) were assessed to evaluate morphological imaging characteristics. LRRK2 patients had an earlier onset of motor symptoms and a more benign phenotype of motor and non-motor characteristics compared to sporadic Parkinson's disease patients. However, depression scores were higher in LRRK2 patients. No clinical differences were found regarding motor and non-motor symptoms in asymptomatic LRRK2 mutation carriers in comparison to controls. Transcranial sonography showed hyperechogenicity of the substantia nigra in both patients' cohorts as well as in asymptomatic LRRK2 mutation carriers. Voxel-based morphometry revealed increased gray matter volume of the cerebellum and precentral gyrus in LRRK2 patients and of the cuneus in asymptomatic LRRK2 mutation carriers. In contrast, we found decreased basal ganglia gray matter volume in LRRK2 patients compared to controls. Increased gray matter volume of different anatomical structures associated with motor loops in LRRK2 patients and asymptomatic LRRK2 mutation carriers compared to age-matched sporadic Parkinson's disease patients and controls might indicate compensatory mechanism in LRRK2 mutation carriers due to motor network plasticity not only in the symptomatic stage of the disease but even in the premotor phase. Substantia nigra hyperechogenicity in yet unaffected LRRK2 mutation carriers indicates morphologic alterations in an asymptomatic stage of disease.

Milestones in neuroimaging

In the last 25 years there have been enormous advances in brain imaging. In addition to utility in diagnosis, these have led to novel insights into the pathogenesis of basal ganglia disease and the role of dopamine and the basal ganglia in normal health. The authors review highlights of this work, with a focus on advances in Parkinson's disease, the dystonias, Huntington's disease, and the role of dopamine in cognition and reward signaling. Emerging areas for future development include studies of functional connectivity, the analysis of default mode networks, studies of novel neurochemical pathways, methods to study disease pathogenesis, and the application of imaging techniques to investigate animal models of disease.

In vivo quantification of dopamine transporters in mice with unilateral 6-OHDA lesions using [11C]methylphenidate and PET

Quantification of the binding of [11C]methylphenidate to the dopamine transporter (DAT) using positron emission tomography (PET) is often used to evaluate the integrity of dopaminergic neurons in the striatal regions of the brain. Over the past decade, many genetically engineered mouse models of human disease have been developed and have become particularly useful for the study of disease onset and progression over time. Quantitative imaging of small structures such as the mouse brain is especially challenging. Thus, the aims of this study were (1) to evaluate the accuracy of quantifying DAT binding using in vivo PET and (2) to examine the impact of different methodologies.

METHODS

Eight mice were scanned with [11C]methylphenidate under true or transient equilibrium conditions using a bolus and constant infusion protocol or a bolus injection protocol to evaluate the accuracy of the Logan graphical approach for [11C]methylphenidate imaging in mice. Displacement with unlabeled methylphenidate (0.1, 3 and 10 mg/kg) was used to verify specific binding. In a second experiment, 30 mice were lesioned by injection of 6-hydroxydopamine (6-OHDA) at doses of 0, 2 or 4 μg (n=10) into the right striatum to assess the dose-dependent correlation between the PET signal and dopaminergic degeneration. In addition, we performed test-retest experiments and used ex vivo autoradiography (AR) to validate the effect of partial volume on the accuracy of the [11C]methylphenidate PET quantification in the mouse striatum.

RESULTS

The binding potentials (BPND) calculated from the Logan graphical analysis under transient equilibrium conditions (1.03±0.1) were in excellent agreement with those calculated at true equilibrium (1.07±0.1). Displacement of specific binding with 0.1, 3 and 10mg/kg methylphenidate resulted in 38%, 77% and 81% transporter occupancy in the striatum. Intra-striatal injections of 6-OHDA caused a dose-dependent decrease in the specific binding of [11C]methylphenidate to the DAT in the striatum. The BPND was reduced by 49% and 61% after injection with 2 and 4 μg of 6-OHDA, respectively. The test-retest reproducibility was 6% in the healthy striatum and 27% in the lesioned striatum. In addition, only a small (15%) difference was found between the [11C]methylphenidate DVR-1 values determined by PET and AR on the healthy side, and no differences were observed on the lesioned side.

CONCLUSION

The present work demonstrates for the first time that [11C]methylphenidate PET is useful for the quantification of striatal dopamine transporters at the dopaminergic nerve terminals in the mouse striatum; therefore, this marker may be used as a biomarker in genetically engineered mouse models of neurodegenerative disorders. However, only changes resulting in greater than 10% differences in BPND values can reliably be detected in vivo.

The Parkinson's disease protein LRRK2 impairs proteasome substrate clearance without affecting proteasome catalytic activity

Leucine-rich repeat kinase 2 (LRRK2) mutations are the most common known cause of Parkinson's disease (PD). The clinical features of LRRK2 PD are indistinguishable from idiopathic PD, with accumulation of α-synuclein and/or tau and/or ubiquitin in intraneuronal aggregates. This suggests that LRRK2 is a key to understanding the aetiology of the disorder. Although loss-of-function does not appear to be the mechanism causing PD in LRRK2 patients, it is not clear how this protein mediates toxicity. In this study, we report that LRRK2 overexpression in cells and in vivo impairs the activity of the ubiquitin-proteasome pathway, and that this accounts for the accumulation of diverse substrates with LRRK2 overexpression. We show that this is not mediated by large LRRK2 aggregates or sequestration of ubiquitin to the aggregates. Importantly, such abnormalities are not seen with overexpression of the related protein LRRK1. Our data suggest that LRRK2 inhibits the clearance of proteasome substrates upstream of proteasome catalytic activity, favouring the accumulation of proteins and aggregate formation. Thus, we provide a molecular link between LRRK2, the most common known cause of PD, and its previously described phenotype of protein accumulation.

Monitoring therapeutic effects in Parkinson's disease by serial imaging of the nigrostriatal dopaminergic pathway

PET and SPECT are very sensitive techniques to detect in-vivo nigrostriatal degeneration in Parkinson's disease, even in the pre-motor phase of the disease. Furthermore, these techniques are able to measure disease progression. However, caution must be used in the interpretation of studies in which therapeutic effects in Parkinson's disease were also monitored by serial imaging of nigrostriatal neurons, as disparity between imaging and clinical outcomes has been reported in several clinical studies.

Biomarkers for Parkinson's disease

Biomarkers for detecting the early stages of Parkinson's disease (PD) could accelerate development of new treatments. Such biomarkers could be used to identify individuals at risk for developing PD, to improve early diagnosis, to track disease progression with precision, and to test the efficacy of new treatments. Although some progress has been made, there are many challenges associated with developing biomarkers for detecting PD in its earliest stages.

The role of dopaminergic imaging in patients with symptoms of dopaminergic system neurodegeneration

Diagnosis of neurological and psychiatric conditions associated with disturbances of dopaminergic functioning can be challenging, especially in the early stages, and may be assisted with biomarkers such as dopamine system imaging. Distinguishing between Alzheimer's disease and dementia with Lewy bodies is a major diagnostic challenge. Clinical diagnosis of Parkinson's disease is straightforward with classic presentation, but accurate distinction among Parkinsonian variants may be difficult; non-Parkinson's disease conditions are commonly misdiagnosed as Parkinson's disease, and ~20% of patients with Parkinson's disease are not clinically diagnosed despite coming to medical attention. Early and accurate diagnosis is desirable to improve management. Imaging of the dopamine transporter using single-photon emission computed tomography may be of particular utility in this regard. Abnormal imaging indicates underlying nigrostriatal neurodegeneration, supportive of a diagnosis of Parkinson's disease, atypical parkinsonism or dementia with Lewy bodies, and identifies patient groups in whom dopaminergic therapy may be beneficial. Normal imaging supports diagnosis of a condition not involving nigrostriatal neurodegeneration such as Alzheimer's disease, essential tremor or drug-induced parkinsonism and hence a different therapeutic approach. In patients in whom there was diagnostic uncertainty between degenerative parkinsonism and non-degenerative tremor disorders, baseline imaging with the dopamine transporter ligand [(123)I]ioflupane (DaTscan™) has shown 78% sensitivity and 97% specificity with reference to clinical diagnosis at 3 years, versus 93% and 46%, respectively, for baseline clinical diagnosis. In a Phase III trial of [(123)I]ioflupane in patients with initial clinical diagnosis of probable or possible dementia with Lewy bodies or non-Lewy body dementia, mean specificity for excluding non-Lewy body dementia (predominantly Alzheimer's disease) was 90.4%. Using clinical diagnosis as a reference against which to assess sensitivity and specificity of dopamine transporter imaging is a limitation, but definitive diagnosis via pathological confirmation is generally not feasible. In a series of patients with post-mortem brain examination, imaging using [(123)I]ioflupane has demonstrated higher sensitivity (88%) and specificity (100%) for differentiating dementia with Lewy bodies from non-Lewy body dementia than clinical diagnosis (75% and 42%, respectively). Dopaminergic system imaging may be particularly valuable in patients with clinically inconclusive parkinsonism or a clinical diagnosis of possible dementia with Lewy bodies; it is not helpful in differentiating between Parkinson's disease and atypical parkinsonism, although postsynaptic dopaminergic imaging may be of utility. Other potential uses of dopamine transporter imaging include identification of patients with premotor Parkinson's disease, monitoring disease progression in testing novel therapeutics, and as an inclusion criterion for entry into clinical trials.

Aberrant striatal synaptic plasticity in monogenic parkinsonisms

In the recent past, the pathogenesis of Parkinson's disease (PD) has evolved from a neurodegenerative disorder considered entirely sporadic to a disease with an unequivocal genetic component. Indeed, different inherited forms of PD have been discovered and characterized, although the functional roles of the gene products identified are still under intense investigation. To gain a better understanding of the cellular and molecular pathogenic mechanisms of hereditary forms of PD, different animal models have been generated. Although most of the rodent models display neither obvious behavioral impairment nor evidence for neurodegeneration, remarkable abnormalities of dopamine-mediated neurotransmission and corticostriatal synaptic plasticity have been described, indicative of a fundamental distortion of network function within the basal ganglia. The picture emerging from a critical review of recent data on monogenic parkinsonisms suggests that mutations in PD genes might cause developmental rearrangements in the corticobasal ganglia circuitry, compensating the dopaminergic dysfunction observed both in mice and humans, in order to maintain proper motor function.

Subclinical signs in LRRK2 mutation carriers

BACKGROUND

Several non-motor features have been reported to precede the motor signs of Parkinson's disease (PD) by several years. However, the time of onset of non-motor and motor symptoms is still debated. Healthy individuals carrying a PD-related mutation are candidates for studying the earliest disease signs.

OBJECTIVES

To describe clinically healthy family members of PD patients carrying a LRRK2 mutation (LRRK2-PD).

METHODS

A total of 47 family members of LRRK2-PD patients were included in the present study and were screened for the p.G2019S and p.N1437H substitutions in the LRRK2 gene. A standardized case report form was filled out in each case, including general medical evaluation, neurological examination with UPDRS, an olfaction test, mood, sleep and cognitive questionnaires.

RESULTS

Thirty-two study participants were positive, and 15 were negative for a LRRK2 mutation. Higher UPDRS motor scores, more frequent reports of urinary problems, and fewer hours of sleep were found in mutation carriers compared to non-carriers. The mutation carriers with UPDRS ≥8 were all aged over 50 years, had shorter overall sleeping hours, more frequent urinary and constipation problems, higher mood scores and body mass index. Deterioration of olfaction was not detected in either group.

CONCLUSION

Healthy LRRK2 mutation carriers presented subclinical parkinsonian motor and non-motor signs in the apparent absence of olfactory loss. Longitudinal studies will determine whether these changes precede alterations detectable by neuroimaging.

Biopterin levels in the cerebrospinal fluid of patients with PARK8 (I2020T)

PARK8 is the most common form of familial Parkinson's disease (PD). We measured biopterin and monoamine metabolite levels in the cerebrospinal fluids of 7 PARK8 patients (I2020T mutation in leucine-rich repeat kinase 2), 2 asymptomatic mutation carriers, and 21 sporadic PD patients. The biopterin levels in PARK8 patients were significantly higher than those in sporadic PD patients, although the symptoms were comparable in both groups, suggesting that PARK8 patients exhibit parkinsonian symptoms with higher biopterin levels than sporadic PD patients.

Mood and cognition in leucine-rich repeat kinase 2 G2019S Parkinson's disease

The behavioral and cognitive features of the leucine-rich repeat kinase G2019S mutation in Parkinson's disease in the Ashkenazi Jewish population are not well described; therefore, we sought to more systematically characterize these features using a semistructured psychiatric interview and neuropsychological testing. Twenty-one Ashkenazi Jewish patients having the leucine-rich repeat kinase G2019S mutation were compared with age- and sex-matched Ashkenazi Jewish patients with Parkinson's disease without mutations. Although overall rates of affective disorders were not greater in mutation carriers, the carriers exhibited a 6-fold increased risk of premorbid affective disorders (odds ratio, 6.0; P = .10), as determined by the Structured Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders-IV. Of interest, we identified 2 leucine-rich repeat kinase carriers with bipolar disorder; no mutation-negative subjects had this diagnosis. Performance on the Hopkins Verbal Learning Test-Revised, Judgment of Line Orientation, and Frontal Assessment Battery was consistent with previous reports and did not differ between groups. Study findings suggest a possible association between premorbid mood disorders and leucine-rich repeat kinase Parkinson's disease, warranting further evaluation.

Genetic LRRK2 models of Parkinson's disease: Dissecting the pathogenic pathway and exploring clinical applications

Dominantly inherited mutations in leucine-rich repeat kinase 2 are the most common cause of familial Parkinson's disease. Understanding leucine-rich repeat kinase 2 biology and pathophysiology is central to the elucidation of Parkinson's disease etiology and development of disease intervention. Recently, a number of genetic mouse models of leucine-rich repeat kinase 2 have been reported utilizing different genetic approaches. Some similarities in Parkinson's disease-related pathology emerge in these genetic models despite lack of substantial neuropathology and clinical syndromes of Parkinson's disease. The systematic characterization of these models has begun to shed light on leucine-rich repeat kinase 2 biology and pathophysiology and is expected to offer the identification and validation of drug targets. In this review, we summarize the progress of genetic leucine-rich repeat kinase 2 mouse models and discuss their utility in understanding much needed knowledge regarding early-stage (presymptomatic) disease progression, identifying drug targets, and exploring the potential to aid compound screening focused on inhibitors of kinase activity of leucine-rich repeat kinase 2.

Dopamine turnover increases in asymptomatic LRRK2 mutations carriers

Increase in dopamine (DA) turnover was found to occur early in symptomatic Parkinson's disease (PD) and to be functionally related to the dopamine transporter (DAT). The objectives of this study were to examine changes in DA turnover in the asymptomatic PD phase; to compare them with changes in other dopaminergic markers, and to investigate a possible relationship between DAT and DA turnover. Eight subjects from families at increased risk of PD due to LRRK2 mutation were investigated. Positron emission tomography imaging was performed with: ¹⁸F-fluorodopa to determine the effective DA distribution volume (EDV), the inverse of DA turnover, and the DA uptake rate K(occ), a marker of DA synthesis and storage; ¹¹C-methylphenidate (MP, a DAT marker) and ¹¹C-dihydrotetrabenazine (DTBZ, a VMAT2 marker) to estimate the binding potentials BP(ND_MP) and BP(ND_DTBZ). On average, EDV showed the largest reduction from age-matched control values (42%) followed by BP(ND_MP) (23%) and BP(ND_DTBZ) (17%), whereas K(occ) remained in the normal range for all subjects. No correlation was found between EDV and any other marker. DA turnover was found to be elevated in asymptomatic mutation carriers at increased risk of PD. Such change was determined to be larger than and statistically independent from changes observed with the other markers. These results support a compensatory role of increased DA turnover in presymptomatic disease and indicate that at this stage, in contrast to the symptomatic PD phase, increased turnover is not related to DAT.

[Nuclear medicine imaging in patients with Parkinson's syndrome: an update]

Nuclear medicine imaging using positron emission tomography (PET) or single photon emission computed tomography (SPECT) has enabled to study not only the metabolism and blood flow in specific brain areas but also the quantification of the function of distinct molecules. With respect to Parkinson's disease PET and later SPECT allowed the number of dopaminergic neurons to be assessed in vivo. These quantifications are relevant to establishing a clinical diagnosis, assessing the progression of the disease or the survival of transplanted dopaminergic neurons. In addition both techniques have markedly contributed to our understanding of the pathophysiology of this disorder. More recently, molecular imaging has been directed towards understanding the pathophysiology of non-motor symptoms in this disorder.

Neuroimaging in Parkinson's disease

Parkinson's disease (PD) is a common disorder in which the primary features can be related to dopamine deficiency. Changes on structural imaging are limited, but a wealth of abnormalities can be detected using positron emission tomography, single photon emission computed tomography, or functional magnetic resonance imaging to detect changes in neurochemical pathology or functional connectivity. The changes detected on these studies may reflect the disease process itself and/or compensatory responses to the disease, or they may arise in association with disease- and/or treatment-related complications. This review will focus mainly on neurochemical and metabolic studies and reviews various approaches to the assessment of dopaminergic function as well as the function of other neurotransmitters that may be affected in PD. A number of clinical applications are highlighted, including diagnostic utility, identification of preclinical disease, changes associated with motor and nonmotor complications of PD, and the effects of various therapeutic interventions.

Behavioral, neurochemical, and electrophysiological changes in an early spontaneous mouse model of nigrostriatal degeneration

In idiopathic Parkinson's disease, clinical symptoms do not emerge until consistent neurodegeneration has occurred. The late appearance of symptoms implies the existence of a relatively long preclinical period during which several disease-induced neurochemical changes take place to mask the existence of the disease and delay its clinical manifestations. The aim of this study was to examine the neurochemical, neurophysiological, and behavioral changes induced by the loss of nigrostriatal innervation in the En1+/-;En2-/- mouse, in the 10 months following degeneration, compared to En2 null mutant mice. Behavioral analysis (Pole-test, Beam-walking test, and Inverted grid test) and field potential recordings in the striatum indicated that loss of ~70% of nigrostriatal neurons produced no significant functional effects until 8 months of age, when En1+/-;En2-/- animals started to show frank motor deficits and electrophysiological alterations in corticostriatal plasticity. Similarly, alterations in dopamine homeostasis, dopamine turnover, and dopamine innervation were observed in aged animals compared to young En1+/-;En2-/- mice. These data suggests that in En1+/-;En2-/- mice nigrostriatal degeneration in the substantia nigra is functionally compensated.

Imaging dopamine transporters in Parkinson’s disease

The dopamine transporter (DAT) is responsible for clearance of dopamine from the synaptic cleft after its release. Imaging DAT availability provides a measure of dopamine terminal function and a method for detecting striatal dopamine deficiency states present in idiopathic Parkinson’s disease and atypical neurodegenerative Parkinsonian disorders such as multiple system atrophy and progressive supranuclear palsy. DAT imaging with PET or single photon emission computed tomography can be used to support a diagnosis of dopamine-deficient parkinsonism in cases where this is suspected and rationalize the use of dopaminergic agents as therapy. It can also detect subclinical dopaminergic dysfunction when present in subjects at risk of Parkinson’s disease, such as relatives of patients, susceptibility gene mutation carriers, and subjects with late-onset hyposmia or sleep disorders. Finally, the presence of normal DAT availability on imaging can help exclude nondopamine-deficient syndromes, such as dystonic and severe essential tremors, drug-induced and psychogenic parkinsonism that, on occasion, mimic Parkinson’s disease.

(18)F-AV-133: A Selective VMAT2-binding Radiopharmaceutical for PET Imaging of Dopaminergic Neurons

The early detection and monitoring of neurodegenerative diseases, including Parkinson disease, Alzheimer disease, dementia with Lewy bodies and other dementias, and movement disorders, represent a significant unmet medical need. Tools for accurate and early differential diagnosis are necessary to determine the appropriate treatment for patients and to minimize inappropriate use of potentially harmful treatments. Such diagnostic imaging tools are expected to permit monitoring of disease progression and will thus accelerate testing and development of disease-modifying drugs. The new imaging tests may be useful as prognostic tools by identifying humans with neurodegenerative diseases before the clinical manifestations become evident.

Enhanced striatal dopamine transmission and motor performance with LRRK2 overexpression in mice is eliminated by familial Parkinson's disease mutation G2019S

PARK8/LRRK2 (leucine-rich repeat kinase 2) was recently identified as a causative gene for autosomal dominant Parkinson's disease (PD), with LRRK2 mutation G2019S linked to the most frequent familial form of PD. Emerging in vitro evidence indicates that aberrant enzymatic activity of LRRK2 protein carrying this mutation can cause neurotoxicity. However, the physiological and pathophysiological functions of LRRK2 in vivo remain elusive. Here we characterize two bacterial artificial chromosome (BAC) transgenic mouse strains overexpressing LRRK2 wild-type (Wt) or mutant G2019S. Transgenic LRRK2-Wt mice had elevated striatal dopamine (DA) release with unaltered DA uptake or tissue content. Consistent with this result, LRRK2-Wt mice were hyperactive and showed enhanced performance in motor function tests. These results suggest a role for LRRK2 in striatal DA transmission and the consequent motor function. In contrast, LRRK2-G2019S mice showed an age-dependent decrease in striatal DA content, as well as decreased striatal DA release and uptake. Despite increased brain kinase activity, LRRK2-G2019S overexpression was not associated with loss of DAergic neurons in substantia nigra or degeneration of nigrostriatal terminals at 12 months. Our results thus reveal a pivotal role for LRRK2 in regulating striatal DA transmission and consequent control of motor function. The PD-associated mutation G2019S may exert pathogenic effects by impairing these functions of LRRK2. Our LRRK2 BAC transgenic mice, therefore, could provide a useful model for understanding early PD pathological events.