Mutations in the gene LRRK2 encoding dardarin (PARK8) cause familial Parkinson's disease: clinical, pathological, olfactory and functional imaging and genetic data

Decoding Inhibitor Egression from Wild-Type and G2019S Mutant LRRK2 Kinase: Insights into Unbinding Mechanisms for Precision Drug Design in Parkinson's Disease

Leucine-rich repeat kinase 2 (LRRK2) remains a viable target for drug development since the discovery of the association of its mutations with Parkinson's disease (PD). G2019S (in the kinase domain) is the most common mutation for LRRK2-based PD. Though various types of inhibitors have been developed for the kinase domain to reduce the effect of the mutation, understanding the working of these inhibitors at the molecular level is still ongoing. This study focused on the exploration of the dissociation mechanism (pathways) of inhibitors from (WT and G2019S) LRRK2 kinase (using homology model CHK1 kinase), which is one of the crucial aspects in drug discovery. Here, two ATP-competitive type I inhibitors, PF-06447475 and MLi-2 (Comp1 and Comp2 ), and one non-ATP-competitive type II inhibitor, rebastinib (Comp3), were considered for this investigation. To study the unbinding process, random accelerated molecular dynamics simulations were performed. The binding free energies of the three inhibitors for different egression paths were determined using umbrella sampling. This work found four major egression pathways that were adopted by the inhibitors Comp1 (path1, path2, and path3), Comp2 (path1, path2 and path3), and Comp3 (path3 and path4). Also, the mechanism of unbinding for each path and key residues involved in unbinding were explored. Mutation was not observed to impact the preference of the particular egression pathways for both LRRK2-Comp1 and -Comp2 systems. However, the findings suggested that the size of the inhibitor molecules might have an effect on the preference of the egression pathways. The binding energy and residence time of the inhibitors followed a similar trend to experimental observations. The findings of this work might provide insight into designing more potent inhibitors for the G2019S LRRK2 kinase.

Peripheral cutaneous synucleinopathy characteristics in genetic Parkinson's disease

Background

Cutaneous phosphorylated alpha-synuclein (p-α-syn) deposition is an important biomarker of idiopathic Parkinson's disease (iPD). Recent studies have reported synucleinopathies in patients with common genetic forms of PD.

Objective

This study aimed to detect p-α-syn deposition characteristic in rare genetic PD patients with CHCHD2 or RAB39B mutations. Moreover, this study also aimed to describe peripheral alpha-synuclein prion-like activity in genetic PD patients, and acquire whether the cutaneous synucleinopathy characteristics of genetic PD are consistent with central neuropathologies.

Methods

We performed four skin biopsy samples from the distal leg (DL) and proximal neck (C7) of 161 participants, including four patients with CHCHD2 mutations, two patients with RAB39B mutations, 16 patients with PRKN mutations, 14 patients with LRRK2 mutations, five patients with GBA mutations, 100 iPD patients, and 20 healthy controls. We detected cutaneous synucleinopathies using immunofluorescence staining and a seeding amplification assay (SAA). A systematic literature review was also conducted, involving 64 skin biopsies and 205 autopsies of genetic PD patients with synucleinopathy.

Results

P-α-syn was deposited in the peripheral cutaneous nerves of PD patients with CHCHD2, LRRK2, or GBA mutations but not in those with RAB39B or PRKN mutations. There were no significant differences in the location or rate of α-syn-positive deposits between genetic PD and iPD patients. Peripheral cutaneous synucleinopathy appears to well represent brain synucleinopathy of genetic PD, especially autosomal dominant PD (AD-PD). Cutaneous α-synuclein SAA analysis of iPD and LRRK2 and GBA mutation patients revealed prion-like activity.

Conclusion

P-α-syn deposition in peripheral cutaneous nerves, detected using SAA and immunofluorescence staining, may serve as an accurate biomarker for genetic PD and iPD in the future.

A Next-Generation Sequencing Study in a Cohort of Sicilian Patients with Parkinson's Disease

Parkinson's disease (PD) is a multisystem and multifactorial disorder and, therefore, the application of modern genetic techniques may assist in unraveling its complex pathophysiology. We conducted a clinical-demographic evaluation of 126 patients with PD, all of whom were Caucasian and of Sicilian ancestry. DNA was extracted from the peripheral blood for each patient, followed by sequencing using a Next-Generation Sequencing system. This system was based on a custom gene panel comprising 162 genes. The sample underwent further filtering, taking into account the allele frequencies of genetic variants, their presence in the Human Gene Mutation Database, and their association in the literature with PD or other movement/neurodegenerative disorders. The largest number of variants was identified in the leucine-rich repeat kinase 2 (LRRK2) gene. However, variants in other genes, such as acid beta-glucosidase (GBA), DNA polymerase gamma catalytic subunit (POLG), and parkin RBR E3 ubiquitin protein ligase (PRKN), were also discovered. Interestingly, some of these variants had not been previously associated with PD. Enhancing our understanding of the genetic basis of PD and identifying new variants possibly linked to the disease will contribute to improved diagnostic accuracy, therapeutic developments, and prognostic insights for affected individuals.

Role of the Residue Q1919 in Increasing Kinase Activity of G2019S LRRK2 Kinase: A Computational Study

Mutations in multi-domain leucine-rich repeat kinase 2 (LRRK2) have been an interest to researchers as these mutations are associated with Parkinson's disease. G2019S mutation in LRRK2 kinase domain leads to the formation of additional hydrogen bonds by S2019 which results in stabilization of the active state of the kinase, thereby increasing kinase activity. Two additional hydrogen bonds of S2019 are reported separately. Here, a mechanistic picture of the formation of additional hydrogen bonds of S2019 with Q1919 (also with E1920) is presented using 'active' Roco4 kinase as a homology model and its relationship with the stabilization of the 'active' G2019S LRRK2 kinase. A conformational flipping of residue Q1919 was found which helped to form stable hydrogen bond with S2019 and made 'active' state more stable in G2019S LRRK2. Two different states were found within the 'active' kinase with respect to the conformational change (flipping) in Q1919. Two doubly-mutated systems, G2019S/Q1919A and G2019S/E1920 K, were studied separately to check the effect of Q1919 and E1920. For both cases, the stable S2 state was not formed, leading to a decrease in kinase activity. These results indicate that both the additional hydrogen bonds of S2019 (with Q1919 and E1920) are necessary to stabilize the active G2019S LRRK2.

Translating Pathological Brain Activity Primers in Parkinson's Disease Research

Translational experimental approaches that help us better trace Parkinson's disease (PD) pathophysiological mechanisms leading to new therapeutic targets are urgently needed. In this article, we review recent experimental and clinical studies addressing abnormal neuronal activity and pathological network oscillations, as well as their underlying mechanisms and modulation. Our aim is to enhance our knowledge about the progression of Parkinson's disease pathology and the timing of its symptom's manifestation. Here, we present mechanistic insights relevant for the generation of aberrant oscillatory activity within the cortico-basal ganglia circuits. We summarize recent achievements extrapolated from available PD animal models, discuss their advantages and limitations, debate on their differential applicability, and suggest approaches for transferring knowledge on disease pathology into future research and clinical applications.

LRRK2 and Parkinson's disease: from genetics to targeted therapy

LRRK2 variants are implicated in both familial and sporadic PD. LRRK2-PD has a generally benign clinical presentation and variable pathology, with inconsistent presence of Lewy bodies and marked Alzheimer's disease pathology. The mechanisms underlying LRRK2-PD are still unclear, but inflammation, vesicle trafficking, lysosomal homeostasis, and ciliogenesis have been suggested, among others. As novel therapies targeting LRRK2 are under development, understanding the role and function of LRRK2 in PD is becoming increasingly important. Here, we outline the epidemiological, pathophysiological, and clinical features of LRRK2-PD, and discuss the arising therapeutic approaches targeting LRRK2 and possible future directions for research.

Overview of the Impact of Pathogenic LRRK2 Mutations in Parkinson's Disease

Leucine-rich repeat kinase 2 (LRRK2) is a large protein kinase that physiologically phosphorylates and regulates the function of several Rab proteins. LRRK2 is genetically implicated in the pathogenesis of both familial and sporadic Parkinson's disease (PD), although the underlying mechanism is not well understood. Several pathogenic mutations in the LRRK2 gene have been identified, and in most cases the clinical symptoms that PD patients with LRRK2 mutations develop are indistinguishable from those of typical PD. However, it has been shown that the pathological manifestations in the brains of PD patients with LRRK2 mutations are remarkably variable when compared to sporadic PD, ranging from typical PD pathology with Lewy bodies to nigral degeneration with deposition of other amyloidogenic proteins. The pathogenic mutations in LRRK2 are also known to affect the functions and structure of LRRK2, the differences in which may be partly attributable to the variations observed in patient pathology. In this review, in order to help researchers unfamiliar with the field to understand the mechanism of pathogenesis of LRRK2-associated PD, we summarize the clinical and pathological manifestations caused by pathogenic mutations in LRRK2, their impact on the molecular function and structure of LRRK2, and their historical background.

Role of rodent models in advancing precision medicine for Parkinson's disease

With a current lack of disease-modifying treatments, an initiative toward implementing a precision medicine approach for treating Parkinson's disease (PD) has emerged. However, challenges remain in how to define and apply precision medicine in PD. To accomplish the goal of optimally targeted and timed treatment for each patient, preclinical research in a diverse population of rodent models will continue to be an essential part of the translational path to identify novel biomarkers for patient diagnosis and subgrouping, understand PD disease mechanisms, identify new therapeutic targets, and screen therapeutics prior to clinical testing. This review highlights the most common rodent models of PD and discusses how these models can contribute to defining and implementing precision medicine for the treatment of PD.

Brain and Cerebrospinal Fluid α-Synuclein Real-Time Quaking-Induced Conversion Identifies Lewy Body Pathology in LRRK2-PD

BACKGROUND

The neuropathology of Parkinson's disease (PD) associated with leucine-rich repeat kinase 2 (LRRK2) mutations (LRRK2-PD) is heterogeneous and varies with the type of mutation. There are only a few studies evaluating seeding aggregation assays to detect α-synuclein (α-syn) in patients with LRRK2-PD.

OBJECTIVE

We aimed to investigate whether α-syn real-time quaking induced conversion (RT-QuIC) is a sensitive biomarker of synucleinopathy in LRRK2-PD.

METHODS

We studied α-syn RT-QuIC in brain tissue and postmortem ventricular cerebrospinal fluid (CSF) of LRRK2-PD cases with and without Lewy-type pathology.

RESULTS

The accuracy of α-syn RT-QuIC in substantia nigra and CSF samples of patients with LRRK2-PD was 100%. The test also obtained 100% sensitivity to detect misfolded α-syn in substantia nigra of cases with idiopathic PD and was negative in the substantia nigra of all the control brains without Lewy-type pathology.

CONCLUSIONS

Substantia nigra and ventricular CSF RT-QuIC discriminates with high sensitivity and specificity LRRK2 cases with Lewy-type pathology from those without it. RT-QuIC assay could be of particular interest in the selection of cases for clinical trials in this genetic form of PD. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Human Induced Pluripotent Stem Cell Phenotyping and Preclinical Modeling of Familial Parkinson's Disease

Parkinson's disease (PD) is primarily idiopathic and a highly heterogenous neurodegenerative disease with patients experiencing a wide array of motor and non-motor symptoms. A major challenge for understanding susceptibility to PD is to determine the genetic and environmental factors that influence the mechanisms underlying the variations in disease-associated traits. The pathological hallmark of PD is the degeneration of dopaminergic neurons in the substantia nigra pars compacta region of the brain and post-mortem Lewy pathology, which leads to the loss of projecting axons innervating the striatum and to impaired motor and cognitive functions. While the cause of PD is still largely unknown, genome-wide association studies provide evidence that numerous polymorphic variants in various genes contribute to sporadic PD, and 10 to 15% of all cases are linked to some form of hereditary mutations, either autosomal dominant or recessive. Among the most common mutations observed in PD patients are in the genes LRRK2, SNCA, GBA1, PINK1, PRKN, and PARK7/DJ-1. In this review, we cover these PD-related mutations, the use of induced pluripotent stem cells as a disease in a dish model, and genetic animal models to better understand the diversity in the pathogenesis and long-term outcomes seen in PD patients.

A Review on Response to Device-Aided Therapies Used in Monogenic Parkinsonism and GBA Variants Carriers: A Need for Guidelines and Comparative Studies

Parkinson's disease (PD) is in some cases predisposed-or-caused by genetic variants, contributing to the expression of different phenotypes. Regardless of etiology, as the disease progresses, motor fluctuations and/or levodopa-induced dyskinesias limit the benefit of pharmacotherapy. Device-aided therapies are good alternatives in advanced disease, including deep brain stimulation (DBS), levodopa-carbidopa intestinal gel, and continuous subcutaneous infusion of apomorphine. Candidate selection and timing are critical for the success of such therapies. Genetic screening in DBS cohorts has shown a higher proportion of mutation carriers than in general cohorts, suggesting that genetic factors may influence candidacy for advanced therapies. The response of monogenic PD to device therapies is not well established, and the contribution of genetic information to decision-making is still a matter of debate. The limited evidence regarding gene-dependent response to device-aided therapies is reviewed here. An accurate understanding of the adequacy and responses of different mutation carriers to device-aided therapies requires the development of specific studies with long-term monitoring.

Mitochondrial ROS promotes susceptibility to infection via gasdermin D-mediated necroptosis

Although mutations in mitochondrial-associated genes are linked to inflammation and susceptibility to infection, their mechanistic contributions to immune outcomes remain ill-defined. We discovered that the disease-associated gain-of-function allele Lrrk2G2019S (leucine-rich repeat kinase 2) perturbs mitochondrial homeostasis and reprograms cell death pathways in macrophages. When the inflammasome is activated in Lrrk2G2019S macrophages, elevated mitochondrial ROS (mtROS) directs association of the pore-forming protein gasdermin D (GSDMD) to mitochondrial membranes. Mitochondrial GSDMD pore formation then releases mtROS, promoting a switch to RIPK1/RIPK3/MLKL-dependent necroptosis. Consistent with enhanced necroptosis, infection of Lrrk2G2019S mice with Mycobacterium tuberculosis elicits hyperinflammation and severe immunopathology. Our findings suggest a pivotal role for GSDMD as an executer of multiple cell death pathways and demonstrate that mitochondrial dysfunction can direct immune outcomes via cell death modality switching. This work provides insights into how LRRK2 mutations manifest or exacerbate human diseases and identifies GSDMD-dependent necroptosis as a potential target to limit Lrrk2G2019S-mediated immunopathology.

Homozygous mutation of the LRRK2 ROC domain as a novel genetic model of parkinsonism

Background

Parkinson’s disease (PD) is one of the most important neurodegenerative disorders in elderly people. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are found in a large proportion of the patients with sporadic and familial PD. Mutations can occur at different locations in the LRRK2. Patients with LRRK2 ROC-COR mutations face an increased risk of typical motor symptoms of PD, along with cognitive decline. An animal model with a monogenic LRRK2 gene mutation is a suitable model for exploring the pathophysiology of PD and identifying potential drug therapies. However, the effect of homozygous (HOM) LRRK2 in PD pathophysiology is unclear.

Methods

We established human LRRK2 (hLRRK2) R1441G HOM transgenic (Tg) mice to explore the phenotype and pathological features that are associated with hLRRK2 R1441G Tg mouse models and discuss the potential clinical relevance. The open field test (OFT) was performed to examine motor and nonmotor behaviors. A CatWalk analysis system was used to study gait function. [¹⁸F]FDOPA PET was used to investigate functional changes in the nigrostriatal pathway in vivo. Transmission electron microscopy was used to examine the morphological changes in mitochondria and lysosomes in the substantia nigra.

Results

The R1441G HOM Tg mice demonstrated gait disturbance and exhibited less anxiety-related behavior and exploratory behavior than mice with hLRRK2 at 12 months old. Additionally, [¹⁸F]FDOPA PET showed a reduction in FDOPA uptake in the striatum of the HOM Tg mice. Notably, there was significant lysosome and autophagosome accumulation in the cytoplasm of dopaminergic neurons in R1441G hemizygous (HEM) and HOM mice. Moreover, it was observed using transmission electron microscopy (TEM) that the mitochondria of R1441G Tg mice were smaller than those of hLRRK2 mice.

Conclusion

This animal provides a novel HOM hLRRK2 R1441G Tg mouse model that reproduces some phenotype of Parkinsonism in terms of both motor and behavioral dysfunction. There is an increased level of mitochondrial fission and no change in the fusion process in the group of HOM hLRRK2 R1441G Tg mouse. This mutant animal model of PD might be used to study the mechanisms of mitochondrial dysfunction and explore potential new drug targets.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00844-9.

Olfactory dysfunction is associated with motor function only in tremor-dominant Parkinson's disease

BACKGROUND

The prevalence of olfactory impairment in patients with Parkinson's disease (PD) is 50-90%, and therefore, olfactory dysfunction is one of the most prevalent non-motor symptoms (NMSs) in patients with PD. Numerous studies have evaluated the association between motor and non-motor symptoms and olfactory dysfunction in PD.

AIM

In this study, we investigated the relationship between olfactory dysfunction, which is measured using the UPSIT test, with other motor and non-motor symptoms separately in three motor subtypes of PD, including tremor dominant (TD), postural instability and gait difficulty (PIGD), and indeterminate and healthy subjects.

METHODS

We recruited 487 early-stage PD patients (43 PIGD, 406 TD, and 38 indeterminate) and healthy controls (HCs) (n = 197) from the Parkinson Progression Markers Initiative (PPMI). All participants completed motor and non-motor tests at baseline visit and after 4 years of follow-up. Subjects underwent common PD scaling tests.

RESULTS

Olfactory dysfunction was significantly correlated with declined motor functions only in the TD subtype. Also, significant correlations were noticed between olfactory dysfunction and speed-attention processing and executive function in the HCs as well. Finally, no significant or meaningful association was observed in the PIGD and indeterminate subtype. Anosmia and hyposmia subjects in the TD group had the worse motor and non-motor scores compared to normosmia subjects after 4 years.

CONCLUSION

Olfactory dysfunction was significantly correlated with declined motor functions in the TD subtype. This is indicating that olfactory dysfunction may be an early motor and non-motor biomarker only in the TD subtype. However, it is possible that the involvement of olfactory function in other subtypes is not strong enough to make it a useful marker of diseases progression.

Preclinical and clinical evaluation of the LRRK2 inhibitor DNL201 for Parkinson's disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic risk factors for Parkinson's disease (PD). Increased LRRK2 kinase activity is thought to impair lysosomal function and may contribute to the pathogenesis of PD. Thus, inhibition of LRRK2 is a potential disease-modifying therapeutic strategy for PD. DNL201 is an investigational, first-in-class, CNS-penetrant, selective, ATP-competitive, small-molecule LRRK2 kinase inhibitor. In preclinical models, DNL201 inhibited LRRK2 kinase activity as evidenced by reduced phosphorylation of both LRRK2 at serine-935 (pS935) and Rab10 at threonine-73 (pT73), a direct substrate of LRRK2. Inhibition of LRRK2 by DNL201 demonstrated improved lysosomal function in cellular models of disease, including primary mouse astrocytes and fibroblasts from patients with Gaucher disease. Chronic administration of DNL201 to cynomolgus macaques at pharmacologically relevant doses was not associated with adverse findings. In phase 1 and phase 1b clinical trials in 122 healthy volunteers and in 28 patients with PD, respectively, DNL201 at single and multiple doses inhibited LRRK2 and was well tolerated at doses demonstrating LRRK2 pathway engagement and alteration of downstream lysosomal biomarkers. Robust cerebrospinal fluid penetration of DNL201 was observed in both healthy volunteers and patients with PD. These data support the hypothesis that LRRK2 inhibition has the potential to correct lysosomal dysfunction in patients with PD at doses that are generally safe and well tolerated, warranting further clinical development of LRRK2 inhibitors as a therapeutic modality for PD.

Genotype-Phenotype Correlation in Progressive Supranuclear Palsy Syndromes: Clinical and Radiological Similarities and Specificities

The progressive supranuclear palsy (PSP) syndrome encompasses different entities. PSP disease of sporadic origin is the most frequent presentation, but different genetic mutations can lead either to monogenic variants of PSP disease, or to other conditions with a different pathophysiology that eventually may result in PSP phenotype. PSP syndrome of monogenic origin is poorly understood due to the low prevalence and variable expressivity of some mutations. Through this review, we describe how early age of onset, family history of early dementia, parkinsonism, dystonia, or motor neuron disease among other clinical features, as well as some neuroimaging signatures, may be the important clues to suspect PSP syndrome of monogenic origin. In addition, a diagnostic algorithm is proposed that may be useful to guide the genetic diagnosis once there is clinical suspicion of a monogenic PSP syndrome.

Prevalence of ten LRRK2 variants in Parkinson's disease: A comprehensive review

INTRODUCTION

Variants in the leucine-rich repeat kinase 2 gene (LRRK2) are risk factors for Parkinson's disease (PD), but their prevalence varies geographically, reflecting the locations of founder events and dispersion of founders' descendants.

METHODS

A comprehensive literature review was conducted to identify studies providing prevalence estimates for any of ten variants in LRRK2 (G2019S, R1441C, R1441G, R1441H, I2020T, N1437H, Y1699C, S1761R, G2385R, R1628P) among individuals with PD globally. We calculated crude country-specific variant prevalence estimates and, when possible, adjusted estimates for ethno-racial composition. For clinic-based studies, probands were used over other familial cases, whereas for population-based studies, all PD cases were used.

RESULTS

The analysis included 161 articles from 52 countries yielding 581 prevalence estimates across the ten variants. G2019S was the most common variant, exceeding 1.0% in 26 of 51 countries with estimates. The other variants were far less common. G2385R and R1628P were observed almost exclusively in East Asian countries, where they were found in ∼5-10% of cases. All prevalence estimates adjusted for ethno-racial composition were lower than their unadjusted counterparts, although data permitting this adjustment was only available for six countries.

CONCLUSIONS

Except for G2019S, the LRRK2 variants covered in this review were uncommon in most countries studied. However, there were countries with higher prevalence for some variants, reflecting the uneven geographic distribution of LRRK2 variants. The fact that ethno-racial group‒adjusted estimates were lower than crude estimates suggests that estimates derived largely from clinic-based studies may overstate the true prevalence of some LRRK2 variants in PD.

The Role of Olfactory System in the Etiogenesis of Parkinson’s Diseases: An Overview

Parkinson’s disease (PD) mainly affects the dopaminergic neuronal networks of the substantia nigra, which leads to both motor and nonmotor symptoms of the disease. Based on the reports from the previous studies, 95% of the cases are presented along with olfactory dysfunction. The relevant publications from 2002 to 2021 were searched and shortlisted using PubMed and Google Scholar. In this review, we have discussed the correlation between olfactory dysfunction and PD. Olfactory damage presents earlier than the motor symptoms. Because there are no current methodologies for the early detection of PD, olfactory dysfunction can be used as a potential marker for the early detection of PD and hence paving the way for better therapeutic approaches.

LRRK2 mutant knock-in mouse models: therapeutic relevance in Parkinson's disease

Mutations in the leucine-rich repeat kinase 2 gene (LRRK2) are one of the most frequent genetic causes of both familial and sporadic Parkinson's disease (PD). Mounting evidence has demonstrated pathological similarities between LRRK2-associated PD (LRRK2-PD) and sporadic PD, suggesting that LRRK2 is a potential disease modulator and a therapeutic target in PD. LRRK2 mutant knock-in (KI) mouse models display subtle alterations in pathological aspects that mirror early-stage PD, including increased susceptibility of nigrostriatal neurotransmission, development of motor and non-motor symptoms, mitochondrial and autophagy-lysosomal defects and synucleinopathies. This review provides a rationale for the use of LRRK2 KI mice to investigate the LRRK2-mediated pathogenesis of PD and implications from current findings from different LRRK2 KI mouse models, and ultimately discusses the therapeutic potentials against LRRK2-associated pathologies in PD.

Plasma Metabolite Signature Classifies Male LRRK2 Parkinson’s Disease Patients

Parkinson’s disease (PD) is a progressive neurodegenerative disease, causing loss of motor and nonmotor function. Diagnosis is based on clinical symptoms that do not develop until late in the disease progression, at which point the majority of the patients’ dopaminergic neurons are already destroyed. While many PD cases are idiopathic, hereditable genetic risks have been identified, including mutations in the gene for LRRK2, a multidomain kinase with roles in autophagy, mitochondrial function, transcription, molecular structural integrity, the endo-lysosomal system, and the immune response. A definitive PD diagnosis can only be made post-mortem, and no noninvasive or blood-based disease biomarkers are currently available. Alterations in metabolites have been identified in PD patients, suggesting that metabolomics may hold promise for PD diagnostic tools. In this study, we sought to identify metabolic markers of PD in plasma. Using a 1H-13C heteronuclear single quantum coherence spectroscopy (HSQC) NMR spectroscopy metabolomics platform coupled with machine learning (ML), we measured plasma metabolites from approximately age/sex-matched PD patients with G2019S LRRK2 mutations and non-PD controls. Based on the differential level of known and unknown metabolites, we were able to build a ML model and develop a Biomarker of Response (BoR) score, which classified male LRRK2 PD patients with 79.7% accuracy, 81.3% sensitivity, and 78.6% specificity. The high accuracy of the BoR score suggests that the metabolomics/ML workflow described here could be further utilized in the development of a confirmatory diagnostic for PD in larger patient cohorts. A diagnostic assay for PD will aid clinicians and their patients to quickly move toward a definitive diagnosis, and ultimately empower future clinical trials and treatment options.

Recent advances in the pathology of prodromal non-motor symptoms olfactory deficit and depression in Parkinson's disease: clues to early diagnosis and effective treatment

Parkinson’s disease (PD) is a progressive neurodegenerative disease characterized by movement dysfunction due to selective degeneration of dopaminergic neurons in the substantia nigra pars compacta. Non-motor symptoms of PD (e.g., sensory dysfunction, sleep disturbance, constipation, neuropsychiatric symptoms) precede motor symptoms, appear at all stages, and impact the quality of life, but they frequently go unrecognized and remain untreated. Even when identified, traditional dopamine replacement therapies have little effect. We discuss here the pathology of two PD-associated non-motor symptoms: olfactory dysfunction and depression. Olfactory dysfunction is one of the earliest non-motor symptoms in PD and predates the onset of motor symptoms. It is accompanied by early deposition of Lewy pathology and neurotransmitter alterations. Because of the correlation between olfactory dysfunction and an increased risk of progression to PD, olfactory testing can potentially be a specific diagnostic marker of PD in the prodromal stage. Depression is a prevalent PD-associated symptom and is often associated with reduced quality of life. Although the pathophysiology of depression in PD is unclear, studies suggest a causal relationship with abnormal neurotransmission and abnormal adult neurogenesis. Here, we summarize recent progress in the pathology of the non-motor symptoms of PD, aiming to provide better guidance for its effective management.

Mind the Gap: LRRK2 Phenotypes in the Clinic vs. in Patient Cells

Mutations in the Parkinson's disease (PD) protein Leucine Rich Repeat Kinase 2 (LRRK2) have been under study for more than 15 years and our understanding of the cellular phenotypes for the pathogenic mutant forms of LRRK2 has significantly advanced. In parallel to research on LRRK2 mutations in experimental systems, clinical characterization of patients carrying LRRK2 mutations has advanced, as has the analysis of cells that are derived from these patients, including fibroblasts, blood-derived cells, or cells rendered pluripotent. Under the hypothesis that patient clinical phenotypes are a consequence of a cascade of underlying molecular mechanisms gone astray, we currently have a unique opportunity to compare findings from patients and patient-derived cells to ask the question of whether the clinical phenotype of LRRK2 Parkinson's disease and cellular phenotypes of LRRK2 patient-derived cells may be mutually informative. In this review, we aim to summarize the available information on phenotypes of LRRK2 mutations in the clinic, in patient-derived cells, and in experimental models in order to better understand the relationship between the three at the molecular and cellular levels and identify trends and gaps in correlating the data.

Targeted sequencing of Parkinson's disease loci genes highlights SYT11, FGF20 and other associations

Genome-wide association studies (GWAS) have identified numerous loci associated with Parkinson's disease. The specific genes and variants that drive the associations within the vast majority of these loci are unknown. We aimed to perform a comprehensive analysis of selected genes to determine the potential role of rare and common genetic variants within these loci. We fully sequenced 32 genes from 25 loci previously associated with Parkinson's disease in 2657 patients and 3647 controls from three cohorts. Capture was done using molecular inversion probes targeting the exons, exon-intron boundaries and untranslated regions (UTRs) of the genes of interest, followed by sequencing. Quality control was performed to include only high-quality variants. We examined the role of rare variants (minor allele frequency < 0.01) using optimized sequence Kernel association tests. The association of common variants was estimated using regression models adjusted for age, sex and ethnicity as required in each cohort, followed by a meta-analysis. After Bonferroni correction, we identified a burden of rare variants in SYT11, FGF20 and GCH1 associated with Parkinson's disease. Nominal associations were identified in 21 additional genes. Previous reports suggested that the SYT11 GWAS association is driven by variants in the nearby GBA gene. However, the association of SYT11 was mainly driven by a rare 3' UTR variant (rs945006601) and was independent of GBA variants (P = 5.23 × 10-5 after exclusion of all GBA variant carriers). The association of FGF20 was driven by a rare 5' UTR variant (rs1034608171) located in the promoter region. The previously reported association of GCH1 with Parkinson's disease is driven by rare non-synonymous variants, some of which are known to cause dopamine-responsive dystonia. We also identified two LRRK2 variants, p.Arg793Met and p.Gln1353Lys, in 10 and eight controls, respectively, but not in patients. We identified common variants associated with Parkinson's disease in MAPT, TMEM175, BST1, SNCA and GPNMB, which are all in strong linkage disequilibrium with known GWAS hits in their respective loci. A common coding PM20D1 variant, p.Ile149Val, was nominally associated with reduced risk of Parkinson's disease (odds ratio 0.73, 95% confidence interval 0.60-0.89, P = 1.161 × 10-3). This variant is not in linkage disequilibrium with the top GWAS hits within this locus and may represent a novel association. These results further demonstrate the importance of fine mapping of GWAS loci, and suggest that SYT11, FGF20, and potentially PM20D1, BST1 and GPNMB should be considered for future studies as possible Parkinson's disease-related genes.

A More Homogeneous Phenotype in Parkinson's Disease Related to R1441G Mutation in the LRRK2 Gene

Parkinson's disease (PD) is characterized by a great clinical heterogeneity. Nevertheless, the biological drivers of this heterogeneity have not been completely elucidated and are likely to be complex, arising from interactions between genetic, epigenetic, and environmental factors. Despite this heterogeneity, the clinical patterns of monogenic forms of PD have usually maintained a good clinical correlation with each mutation once a sufficient number of patients have been studied. Mutations in LRRK2 are the most commonly known genetic cause of autosomal dominant PD known to date. Furthermore, recent genome-wide association studies have revealed variations in LRRK2 as significant risk factors also for the development of sporadic PD. The LRRK2-R1441G mutation is especially frequent in the population of Basque ascent based on a possible founder effect, being responsible for almost 50% of cases of familial PD in our region, with a high penetrance. Curiously, Lewy bodies, considered the neuropathological hallmark of PD, are absent in a significant subset of LRRK2-PD cases. Indeed, these cases appear to be associated with a less aggressive primarily pure motor phenotype. The aim of our research is to examine the clinical phenotype of R1441G-PD patients, more homogeneous when we compare it with sporadic PD patients or with patients carrying other LRRK2 mutations, and reflect on the value of the observed correlation in the genetic forms of PD. The clinical heterogeneity of PD leads us to think that there may be as many different diseases as the number of people affected. Undoubtedly, genetics constitutes a relevant key player, as it may significantly influence the phenotype, with differences according to the mutation within the same gene, and not only in familial PD but also in sporadic forms. Thus, extending our knowledge regarding genetic forms of PD implies an expansion of knowledge regarding sporadic forms, and this may be relevant due to the future therapeutic implications of all forms of PD.

Genetic and Environmental Factors Influence the Pleomorphy of LRRK2 Parkinsonism

Missense mutations in the LRRK2 gene were first identified as a pathogenic cause of Parkinson's disease (PD) in 2004. Soon thereafter, a founder mutation in LRRK2, p.G2019S (rs34637584), was described, and it is now estimated that there are approximately 100,000 people worldwide carrying this risk variant. While the clinical presentation of LRRK2 parkinsonism has been largely indistinguishable from sporadic PD, disease penetrance and age at onset can be quite variable. In addition, its neuropathological features span a wide range from nigrostriatal loss with Lewy body pathology, lack thereof, or atypical neuropathology, including a large proportion of cases with concomitant Alzheimer's pathology, hailing LRRK2 parkinsonism as the "Rosetta stone" of parkinsonian disorders, which provides clues to an understanding of the different neuropathological trajectories. These differences may result from interactions between the LRRK2 mutant protein and other proteins or environmental factors that modify LRRK2 function and, thereby, influence pathobiology. This review explores how potential genetic and biochemical modifiers of LRRK2 function may contribute to the onset and clinical presentation of LRRK2 parkinsonism. We review which genetic modifiers of LRRK2 influence clinical symptoms, age at onset, and penetrance, what LRRK2 mutations are associated with pleomorphic LRRK2 neuropathology, and which environmental modifiers can augment LRRK2 mutant pathophysiology. Understanding how LRRK2 function is influenced and modulated by other interactors and environmental factors-either increasing toxicity or providing resilience-will inform targeted therapeutic development in the years to come. This will allow the development of disease-modifying therapies for PD- and LRRK2-related neurodegeneration.

Oral P. gingivalis impairs gut permeability and mediates immune responses associated with neurodegeneration in LRRK2 R1441G mice

Background

The R1441G mutation in the leucine-rich repeat kinase 2 (LRRK2) gene results in late-onset Parkinson’s disease (PD). Peripheral inflammation and gut microbiota are closely associated with the pathogenesis of PD. Chronic periodontitis is a common type of peripheral inflammation, which is associated with PD. Porphyromonas gingivalis (Pg), the most common bacterium causing chronic periodontitis, can cause alteration of gut microbiota. It is not known whether Pg-induced dysbiosis plays a role in the pathophysiology of PD.

Methods

In this study, live Pg were orally administrated to animals, three times a week for 1 month. Pg-derived lipopolysaccharide (LPS) was used to stimulate mononuclear cells in vitro. The effects of oral Pg administration on the gut and brain were evaluated through behaviors, morphology, and cytokine expression.

Results

Dopaminergic neurons in the substantia nigra were reduced, and activated microglial cells were increased in R1441G mice given oral Pg. In addition, an increase in mRNA expression of tumor necrosis factor (TNF-α) and interleukin-1β (IL-1β) as well as protein level of α-synuclein together with a decrease in zonula occludens-1 (Zo-1) was detected in the colon in Pg-treated R1441G mice. Furthermore, serum interleukin-17A (IL-17A) and brain IL-17 receptor A (IL-17RA) were increased in Pg-treated R1441G mice.

Conclusions

These findings suggest that oral Pg-induced inflammation may play an important role in the pathophysiology of LRRK2-associated PD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-020-02027-5.

Clinically Precedented Protein Kinases: Rationale for Their Use in Neurodegenerative Disease

Kinases are an intensively studied drug target class in current pharmacological research as evidenced by the large number of kinase inhibitors being assessed in clinical trials. Kinase-targeted therapies have potential for treatment of a broad array of indications including central nervous system (CNS) disorders. In addition to the many variables which contribute to identification of a successful therapeutic molecule, drug discovery for CNS-related disorders also requires significant consideration of access to the target organ and specifically crossing the blood-brain barrier (BBB). To date, only a small number of kinase inhibitors have been reported that are specifically designed to be BBB permeable, which nonetheless demonstrates the potential for success. This review considers the potential for kinase inhibitors in the context of unmet medical need for neurodegenerative disease. A subset of kinases that have been the focus of clinical investigations over a 10-year period have been identified and discussed individually. For each kinase target, the data underpinning the validity of each in the context of neurodegenerative disease is critically evaluated. Selected molecules for each kinase are identified with information on modality, binding site and CNS penetrance, if known. Current clinical development in neurodegenerative disease are summarized. Collectively, the review indicates that kinase targets with sufficient rationale warrant careful design approaches with an emphasis on improving brain penetrance and selectivity.

Low Levels of LRRK2 Gene Expression are Associated with LRRK2 SNPs and Contribute to Parkinson's Disease Progression

Parkinson's disease (PD) is a chronic neurodegenerative disease that has relatively slow progression with motor symptoms. Leucine-rich repeat kinase 2 (LRRK2) gene mutations and polymorphisms are suggested to be associated with PD. In this study, we aimed to investigate the association between single-nucleotide polymorphisms (SNPs) of the LRRK2 gene, namely, rs11176013, rs10878371, rs11835105, and PD. Genotypes of 132 PD cases and 133 healthy individuals were determined by qRT-PCR. Haplotype analysis was performed. Additionally, LRRK2 mRNA expression levels were determined in 83 PD cases and 55 healthy subjects. The relationship between LRRK2 mRNA levels, the target SNPs, and clinical data was also investigated. Our results indicated that the "GG" genotype and "G" allele of rs11176013 and the "CC" genotype and "C" allele of rs10878371 were more frequent in cases. The "GCG" haplotype was significantly more frequent in cases. LRRK2 mRNA expression levels in patients were significantly lower than those in healthy individuals. The patients with the "CC" genotype for rs10878371 and the "GG" genotype for rs11176013 had decreased LRRK2 mRNA levels. We found that the rs11176013 "GG" genotype and the rs10878371 "CC" genotype were less frequently seen in cases with akinetic rigid or combined akinetic rigid and tremor-dominant initial symptoms. Consequently, our results demonstrate that the rs11176013 and rs10878371 polymorphisms are associated with PD in a Turkish cohort, and moreover, these results suggest that these polymorphisms may affect the expression of the LRRK2 gene and disease progression and thus play a role in the pathogenesis of PD.

Isolated and combined genetic tremor syndromes: a critical appraisal based on the 2018 MDS criteria

The 2018 consensus statement on the classification of tremors proposes a two-axis categorization scheme based on clinical features and etiology. It also defines "isolated" and "combined" tremor syndromes depending on whether tremor is the sole clinical manifestation or is associated with other neurological or systemic signs. This syndromic approach provides a guide to investigate the underlying etiology of tremors, either genetic or acquired. Several genetic defects have been proven to cause tremor disorders, including autosomal dominant and recessive, X-linked, and mitochondrial diseases, as well as chromosomal abnormalities. Furthermore, some tremor syndromes are recognized in individuals with a positive family history, but their genetic confirmation is pending. Although most genetic tremor disorders show a combined clinical picture, there are some distinctive conditions in which tremor may precede the appearance of other neurological signs by years or remain the prominent manifestation throughout the disease course, previously leading to misdiagnosis as essential tremor (ET). Advances in the knowledge of genetically determined tremors may have been hampered by the inclusion of heterogeneous entities in previous studies on ET. The recent classification of tremors therefore aims to provide more consistent clinical data for deconstructing the genetic basis of tremor syndromes in the next-generation and long-read sequencing era. This review outlines the wide spectrum of tremor disorders with defined or presumed genetic etiology, both isolated and combined, unraveling diagnostic clues of these conditions and focusing mainly on ET-like phenotypes. Furthermore, we suggest a phenotype-to-genotype algorithm to support clinicians in identifying tremor syndromes and guiding genetic investigations.

Progress in LRRK2-Associated Parkinson's Disease Animal Models

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most frequent cause of familial Parkinson's disease (PD). Several genetic manipulations of the LRRK2 gene have been developed in animal models such as rodents, Drosophila, Caenorhabditis elegans, and zebrafish. These models can help us further understand the biological function and derive potential pathological mechanisms for LRRK2. Here we discuss common phenotypic themes found in LRRK2-associated PD animal models, highlight several issues that should be addressed in future models, and discuss emerging areas to guide their future development.

LRRK2 and α-Synuclein: Distinct or Synergistic Players in Parkinson's Disease?

Parkinson's disease (PD) is the most common neurodegenerative movement disorder, characterized by prominent degeneration of dopaminergic neurons in the substantia nigra and aggregation of the protein α-synuclein within intraneuronal inclusions known as Lewy bodies. Ninety percent of PD cases are idiopathic while the remaining 10% are associated with gene mutations that affect cellular functions ranging from kinase activity to mitochondrial quality control, hinting at a multifactorial disease process. Mutations in LRRK2 and SNCA (the gene coding for α-synuclein) cause monogenic forms of autosomal dominant PD, and polymorphisms in either gene are also associated with increased risk of idiopathic PD. Although Lewy bodies are a defining neuropathological feature of PD, an appreciable subset of patients with LRRK2 mutations present with a clinical phenotype indistinguishable from idiopathic PD but lack Lewy pathology at autopsy, suggesting that LRRK2-mediated PD may occur independently of α-synuclein aggregation. Here, we examine whether LRRK2 and α-synuclein, as mediators of neurodegeneration in PD, exist in common or distinct pathways. Specifically, we review evidence from preclinical models and human neuropathological studies examining interactions between the two proteins. Elucidating the degree of interplay between LRRK2 and α-synuclein will be necessary for treatment stratification once effective targeted disease-modifying therapies are developed.

Olfactory Dysfunction in Familial and Sporadic Parkinson's Disease

This minireview discusses our current understanding of the olfactory dysfunction that is frequently observed in sporadic and familial forms of Parkinson's disease and parkinsonian syndromes. We review the salient characteristics of olfactory dysfunction in these conditions, discussing its prevalence and characteristics, how neuronal processes and circuits are altered in Parkinson's disease, and what is assessed by clinically used measures of olfactory function. We highlight how studies of monogenic Parkinson's disease and investigations in ethnically diverse populations have contributed to understanding the mechanisms underlying olfactory dysfunction. Furthermore, we discuss how imaging and system-level approaches have been used to understand the pathogenesis of olfactory dysfunction. We discuss the challenging, remaining gaps in understanding the basis of olfactory dysfunction in neurodegeneration. We propose that insights could be obtained by following longitudinal cohorts with familial forms of Parkinson's disease using a combination of approaches: a multifaceted longitudinal assessment of olfactory function during disease progression is essential to identify not only how dysfunction arises, but also to address its relationship to motor and non-motor Parkinson's disease symptoms. An assessment of cohorts having monogenic forms of Parkinson's disease, available within the Genetic Epidemiology of Parkinson's Disease (GEoPD), as well as other international consortia, will have heuristic value in addressing the complexity of olfactory dysfunction in the context of the neurodegenerative process. This will inform our understanding of Parkinson's disease as a multisystem disorder and facilitate the more effective use of olfactory dysfunction assessment in identifying prodromal Parkinson's disease and understanding disease progression.

Genetic predispositions of Parkinson's disease revealed in patient-derived brain cells

Parkinson's disease (PD) is the second most prevalent neurological disorder and has been the focus of intense investigations to understand its etiology and progression, but it still lacks a cure. Modeling diseases of the central nervous system in vitro with human induced pluripotent stem cells (hiPSC) is still in its infancy but has the potential to expedite the discovery and validation of new treatments. Here, we discuss the interplay between genetic predispositions and midbrain neuronal impairments in people living with PD. We first summarize the prevalence of causal Parkinson's genes and risk factors reported in 74 epidemiological and genomic studies. We then present a meta-analysis of 385 hiPSC-derived neuronal lines from 67 recent independent original research articles, which point towards specific impairments in neurons from Parkinson's patients, within the context of genetic predispositions. Despite the heterogeneous nature of the disease, current iPSC models reveal converging molecular pathways underlying neurodegeneration in a range of familial and sporadic forms of Parkinson's disease. Altogether, consolidating our understanding of robust cellular phenotypes across genetic cohorts of Parkinson's patients may guide future personalized drug screens in preclinical research.

The Emerging Functions of LRRK2 and Rab GTPases in the Endolysosomal System

The leucine-rich repeat kinase 2 (LRRK2), the most common causative gene for autosomal-dominant familial Parkinson’s disease, encodes a large protein kinase harboring multiple characteristic domains. LRRK2 phosphorylates a set of Rab GTPases in cells, which is enhanced by the Parkinson-associated LRRK2 mutations. Accumulating evidence suggests that LRRK2 regulates intracellular vesicle trafficking and organelle maintenance including Golgi, endosomes and lysosomes. Furthermore, genetic knockout or inhibition of LRRK2 cause lysosomal abnormalities in rodents and primates, and cells from Parkinson’s patients with LRRK2 mutations also exhibit altered lysosome morphology. Cell biological studies on LRRK2 in a diverse cellular context further strengthen the potential connection between LRRK2 and regulation of the endolysosomal system, part of which is mediated by Rab phosphorylation by LRRK2. We will focus on the latest advances on the role of LRRK2 and Rab in relation to the endolysosomal system, and discuss the possible link to the pathomechanism of Parkinson’s disease.

Update on the association between alpha-synuclein and tau with mitochondrial dysfunction: Implications for Parkinson's disease

The critical role of mitochondrial dysfunction in the pathological mechanisms of neurodegenerative disorders, particularly Parkinson's disease (PD), is well established. Compelling evidence indicates that Parkinson's proteins (e.g., α-synuclein, Parkin, PINK1, DJ-1, and LRRK2) are associated with mitochondrial dysfunction and oxidative stress in PD. Significantly, there is a possible central role of alpha-synuclein (α-Syn) in the occurrence of mitochondrial dysfunction and oxidative stress by the mediation of different signaling pathways. Also, tau, traditionally considered as the main component of neurofibrillary tangles, aggregates and amplifies the neurotoxic effects on mitochondria by interacting with α-Syn. Moreover, oxidative stress caused by mitochondrial dysfunction favors assembly of both α-Syn and tau and also plays a key role in the formation of protein aggregates. In this review, we provide an overview of the relationship between these two pathological proteins and mitochondrial dysfunction in PD, and also summarize the underlying mechanisms in the interplay of α-Syn aggregation and phosphorylated tau targeting the mitochondria, to find new strategies to prevent PD processing.

LRRK2 Biology from structure to dysfunction: research progresses, but the themes remain the same

Since the discovery of leucine-rich repeat kinase 2 (LRRK2) as a protein that is likely central to the aetiology of Parkinson’s disease, a considerable amount of work has gone into uncovering its basic cellular function. This effort has led to the implication of LRRK2 in a bewildering range of cell biological processes and pathways, and probable roles in a number of seemingly unrelated medical conditions. In this review we summarise current knowledge of the basic biochemistry and cellular function of LRRK2. Topics covered include the identification of phosphorylation substrates of LRRK2 kinase activity, in particular Rab proteins, and advances in understanding the activation of LRRK2 kinase activity via dimerisation and association with membranes, especially via interaction with Rab29. We also discuss biochemical studies that shed light on the complex LRRK2 GTPase activity, evidence of roles for LRRK2 in a range of cell signalling pathways that are likely cell type specific, and studies linking LRRK2 to the cell biology of organelles. The latter includes the involvement of LRRK2 in autophagy, endocytosis, and processes at the trans-Golgi network, the endoplasmic reticulum and also key microtubule-based cellular structures. We further propose a mechanism linking LRRK2 dimerisation, GTPase function and membrane recruitment with LRRK2 kinase activation by Rab29. Together these data paint a picture of a research field that in many ways is moving forward with great momentum, but in other ways has not changed fundamentally. Many key advances have been made, but very often they seem to lead back to the same places.

Effect of LRRK2 G2385R Variant on Subthalamic Deep Brain Stimulation Efficacy in Parkinson's Disease in a Han Chinese Population

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective treatment for advanced Parkinson's disease (PD). The G2385R variant of LRRK2 is a risk factor for PD in Han Chinese individuals. We retrospectively compared the clinical outcomes of STN-DBS surgery between PD Han Chinese G2385R variant carriers and non-carriers. Fifty-seven PD patients with bilateral STN-DBS were enrolled, including 8 G2385R+ variant carriers (G2385R+ group) and 49 non-carriers (G2385R- group). Clinical data included Unified Parkinson's Disease Rating Scale (UPDRS) parts I to IV, levodopa equivalent daily dose (LEDD), Mini-Mental State Examination Scale (MMSE) score, and Hamilton Depression Rating Scale (HAMD) score measured prior to DBS and 12 months post-DBS. DBS settings were also recorded. All PD patients benefited from STN-DBS surgery. There were no statistical differences between the two groups in terms of motor function, daily living activities, and LEDD reductions at 12 months post-DBS. The rigidity of the post-surgical G2385R+ group was significantly improved compared with that of the G2385R- group (P = 0.045). Post-surgical voltage in the G2385R+ group was significantly higher than that in the G2385R- group (P = 0.033). STN-DBS outcomes were not influenced by the LRRK2 G2385R variant in Han Chinese patients.

The LRRK2-RAB axis in regulation of vesicle trafficking and α-synuclein propagation

LRRK2 and SNCA, the gene for α-synuclein, are the two of the most important genetic factors of Parkinson's disease (PD). A-synuclein is aggregated and accumulated in neurons and glia in PD and considered the pathogenic culprit of the disease. A-synuclein aggregates spread from a few discrete regions of the brain to larger areas as the disease progresses through cell-to-cell propagation mechanism. LRRK2 is involved in the regulation of vesicle trafficking, in particular in the endolysosomal and autophagic pathways. Studies also suggest that LRRK2 might regulate the pathogenic actions of α-synuclein. However, the relationship between these two proteins in the pathogenesis of PD remains elusive. Here, we review the current literature on the pathophysiological function of LRRK2 with an emphasis on its role in the endolysosomal and autophagic pathways. We also propose a potential mechanism by which LRRK2 is involved in the regulation of aggregation and the propagation of α-synuclein.

Astrocytic Oxidative/Nitrosative Stress Contributes to Parkinson's Disease Pathogenesis: The Dual Role of Reactive Astrocytes

Parkinson’s disease (PD) is the second most common neurodegenerative disease worldwide; it is characterized by dopaminergic neurodegeneration in the substantia nigra pars compacta, but its etiology is not fully understood. Astrocytes, a class of glial cells in the central nervous system (CNS), provide critical structural and metabolic support to neurons, but growing evidence reveals that astrocytic oxidative and nitrosative stress contributes to PD pathogenesis. As astrocytes play a critical role in the production of antioxidants and the detoxification of reactive oxygen and nitrogen species (ROS/RNS), astrocytic oxidative/nitrosative stress has emerged as a critical mediator of the etiology of PD. Cellular stress and inflammation induce reactive astrogliosis, which initiates the production of astrocytic ROS/RNS and may lead to oxidative/nitrosative stress and PD pathogenesis. Although the cause of aberrant reactive astrogliosis is unknown, gene mutations and environmental toxicants may also contribute to astrocytic oxidative/nitrosative stress. In this review, we briefly discuss the physiological functions of astrocytes and the role of astrocytic oxidative/nitrosative stress in PD pathogenesis. Additionally, we examine the impact of PD-related genes such as α-synuclein, protein deglycase DJ-1( DJ-1), Parkin, and PTEN-induced kinase 1 (PINK1) on astrocytic function, and highlight the impact of environmental toxicants, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), rotenone, manganese, and paraquat, on astrocytic oxidative/nitrosative stress in experimental models.

Mutant LRRK2 mediates peripheral and central immune responses leading to neurodegeneration in vivo

Missense mutations in the leucine rich repeat kinase 2 (LRRK2) gene result in late-onset Parkinson's disease. The incomplete penetrance of LRRK2 mutations in humans and LRRK2 murine models of Parkinson's disease suggests that the disease may result from a complex interplay of genetic predispositions and persistent exogenous insults. Since neuroinflammation is commonly associated with the pathogenesis of Parkinson's disease, we examine a potential role of mutant LRRK2 in regulation of the immune response and inflammatory signalling in vivo. Here, we show that mice overexpressing human pathogenic LRRK2 mutations, but not wild-type mice or mice overexpressing human wild-type LRRK2 exhibit long-term lipopolysaccharide-induced nigral neuronal loss. This neurodegeneration is accompanied by an exacerbated neuroinflammation in the brain. The increased immune response in the brain of mutant mice subsequently has an effect on neurons by inducing intraneuronal LRRK2 upregulation. However, the enhanced neuroinflammation is unlikely to be triggered by dysfunctional microglia or infiltrated T cells and/or monocytes, but by peripheral circulating inflammatory molecules. Analysis of cytokine kinetics and inflammatory pathways in the peripheral immune cells demonstrates that LRRK2 mutation alters type II interferon immune response, suggesting that this increased neuroinflammatory response may arise outside the central nervous system. Overall, this study suggests that peripheral immune signalling plays an unexpected-but important-role in the regulation of neurodegeneration in LRRK2-associated Parkinson's disease, and provides new targets for interfering with the onset and progression of the disease.

Transmission of α-synuclein seeds in neurodegenerative disease: recent developments

Cell-to-cell transmission of proteopathic alpha-synuclein (α-syn) seeds is increasingly thought to underlie the progression of neurodegenerative diseases including Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, and related synucleinopathies. As such, it is important to understand the chemical and biological relationships between cells and pathological aggregates of α-syn. This brief review updates our understanding of the templated spread of α-syn pathology in neurodegenerative disease from the perspective of proteopathic α-syn seeds, including how these seeds are processed by cells as well as their effects on cellular function. Recent advances in understanding the conformations of α-syn seeds are highlighted, and the possible structural basis for the observed heterogeneity of synucleinopathies is discussed. Finally, we propose the possibility that some known risk factors for synucleinopathies may in fact potentiate the cell-to-cell transmission of α-syn pathology via imbalances in interrelated cell biological processes.

LRRK2, alpha-synuclein, and tau: partners in crime or unfortunate bystanders?

The identification of genetic forms of Parkinson's disease (PD) has tremendously expanded our understanding of the players and mechanisms involved. Mutations in the genes encoding for alpha-synuclein (aSyn), LRRK2, and tau have been associated with familial and sporadic forms of the disease. aSyn is the major component of Lewy bodies and Lewy neurites, which are pathognomonic protein inclusions in PD. Hyperphosphorylated tau protein accumulates in neurofibrillary tangles in the brains of Alzheimer's disease patients but is also seen in the brains of PD patients. LRRK2 is a complex multi-domain protein with kinase and GTPase enzymatic activity. Since aSyn and tau are phosphoproteins, we review the possible interplay between the three proteins. Understanding the interplay between LRRK2, aSyn and tau is extremely important, as this may enable the identification of novel targets and pathways for therapeutic intervention.

Integrated Analysis of Whole Exome Sequencing and Copy Number Evaluation in Parkinson's Disease

Genetic studies of the familial forms of Parkinson’s disease (PD) have identified a number of causative genes with an established role in its pathogenesis. These genes only explain a fraction of the diagnosed cases. The emergence of Next Generation Sequencing (NGS) expanded the scope of rare variants identification in novel PD related genes. In this study we describe whole exome sequencing (WES) genetic findings of 60 PD patients with 125 variants validated in 51 of these cases. We used strict criteria for variant categorization that generated a list of variants in 20 genes. These variants included loss of function and missense changes in 18 genes that were never previously linked to PD (NOTCH4, BCOR, ITM2B, HRH4, CELSR1, SNAP91, FAM174A, BSN, SPG7, MAGI2, HEPHL1, EPRS, PUM1, CLSTN1, PLCB3, CLSTN3, DNAJB9 and NEFH) and 2 genes that were previously associated with PD (EIF4G1 and ATP13A2). These genes either play a critical role in neuronal function and/or have mouse models with disease related phenotypes. We highlight NOTCH4 as an interesting candidate in which we identified a deleterious truncating and a splice variant in 2 patients. Our combined molecular approach provides a comprehensive strategy applicable for complex genetic disorders.

G2019s LRRK2 promotes mitochondrial fission and increases TNFα-mediated neuroinflammation responses

Leucine rich-repeat kinase 2 (LRRK2) is involved in the pathogenesis of Parkinson's disease (PD). LRRK2 has kinase and GTPase activities, and mediates several cell functions, including vesicle trafficking, apoptosis, autophagy, mitochondrial dynamics, and neuroinflammation. G2019S (GS) is the most prevalent mutation of LRRK2. The mutation increases kinase activity, suggesting that this activity is crucial for PD pathogenesis. The activation and inhibition of LRRK2 kinase increases and reduces the levels of proinflammatory cytokines, respectively suggesting that the role of LRRK2 in neuroinflammation is critical for the pathology of PD. Previously, we demonstrated that microglial activation by lipopolysaccharide (LPS) increases mitochondrial fission via the activation of LRRK2 kinase, while LRRK2 kinase inhibition diminishes the fission morphology and release of tumor necrosis factor-alpha (TNFα) in BV2 or rat primary microglia and the brains of GS transgenic mice. In this study, the ectopic expression of GS LRRK2 in BV2 cells significantly elevated the expression of Drp1 along the fragmented mitochondria and decreased mitochondria size compared with controls. GS LRRK2-transfected BV2 cells displayed significantly increased TNFα release and neuronal death. Inhibition of LRRK2 kinase alleviated these features. TNFα levels in brains of GS mice were significantly increased compared to those in their littermates. These data further support our previous findings concerning LPS-induced neuroinflammation and mitochondrial fission in microglia via LRRK2 kinase activation.

The unlikely partnership between LRRK2 and α-synuclein in Parkinson's disease

Our understanding of the mechanisms underlying Parkinson's disease, the once archetypical nongenetic neurogenerative disorder, has dramatically increased with the identification of α-synuclein and LRRK2 pathogenic mutations. While α-synuclein protein composes the aggregates that can spread through much of the brain in disease, LRRK2 encodes a multidomain dual-enzyme distinct from any other protein linked to neurodegeneration. In this review, we discuss emergent datasets from multiple model systems that suggest these unlikely partners do interact in important ways in disease, both within cells that express both LRRK2 and α-synuclein as well as through more indirect pathways that might involve neuroinflammation. Although the link between LRRK2 and disease can be understood in part through LRRK2 kinase activity (phosphotransferase activity), α-synuclein toxicity is multilayered and plausibly interacts with LRRK2 kinase activity in several ways. We discuss common protein interactors like 14-3-3s that may regulate α-synuclein and LRRK2 in disease. Finally, we examine cellular pathways and outcomes common to both mutant α-synuclein expression and LRRK2 activity and points of intersection. Understanding the interplay between these two unlikely partners in disease may provide new therapeutic avenues for PD.

Crystal structure of the WD40 domain dimer of LRRK2

Leucine-rich repeat kinase 2 (LRRK2) is a large multidomain protein with both a Ras of complex (ROC) domain and a kinase domain (KD) and, therefore, exhibits both GTPase and kinase activities. Human genetics studies have linked LRRK2 as a major genetic contributor to familial and sporadic Parkinson's disease (PD), a neurodegenerative movement disorder that inflicts millions worldwide. The C-terminal region of LRRK2 is a Trp-Asp-40 (WD40) domain with poorly defined biological functions but has been implicated in microtubule interaction. Here, we present the crystal structure of the WD40 domain of human LRRK2 at 2.6-Å resolution, which reveals a seven-bladed WD40 fold. The structure displays a dimeric assembly in the crystal, which we further confirm by measurements in solution. We find that structure-based and PD-associated disease mutations in the WD40 domain including the common G2385R polymorphism mainly compromise dimer formation. Assessment of full-length LRRK2 kinase activity by measuring phosphorylation of Rab10, a member of the family of Rab GTPases known to be important kinase substrates of LRRK2, shows enhancement of kinase activity by several dimerization-defective mutants including G2385R, although dimerization impairment does not always result in kinase activation. Furthermore, mapping of phylogenetically conserved residues onto the WD40 domain structure reveals surface patches that may be important for additional functions of LRRK2. Collectively, our analyses provide insights for understanding the structures and functions of LRRK2 and suggest the potential utility of LRRK2 kinase inhibitors in treating PD patients with WD40 domain mutations.

Chemosensory dysfunction in neurodegenerative diseases

A number of neurodegenerative diseases are accompanied by disordered smell function. The degree of dysfunction can vary among different diseases, such that olfactory testing can aid in differentiating, for example, Alzheimer's disease (AD) from major affective disorder and Parkinson's disease (PD) from progressive supranuclear palsy. Unfortunately, altered smell function often goes unrecognized by patients and physicians alike until formal testing is undertaken. Such testing uniquely probes brain regions not commonly examined in physical examinations and can identify, in some cases, patients who are already in the "preclinical" stage of disease. Awareness of this fact is one reason why the Quality Standards Committee of the American Academy of Neurology has designated smell dysfunction as one of the key diagnostic criteria for PD. The same recommendation has been made by the Movement Disorder Society for both the diagnosis of PD and identification of prodromal PD. Similar suggestions are proposed to include olfactory dysfunction as an additional research criterion for the diagnosis of AD. Although taste impairment, i.e., altered sweet, sour, bitter, salty, and umami perception, has also been demonstrated in some disorders, taste has received much less scientific attention than smell. In this review, we assess what is known about the smell and taste disorders of a wide range of neurodegenerative diseases and describe studies seeking to understand their pathologic underpinnings.

Analysis of macroautophagy related proteins in G2019S LRRK2 Parkinson's disease brains with Lewy body pathology

LRRK2, the gene encoding the multidomain kinase Leucine-Rich Repeat Kinase 2 (LRRK2), has been linked to familial and sporadic forms of Parkinson's disease (PD), as well as cancer, leprosy and Crohn's disease, establishing it as a target for discovery therapeutics. LRRK2 has been associated with a range of cellular processes, however its physiological and pathological functions remain unclear. The most prevalent LRRK2 mutations in PD have been shown to affect macroautophagy in various cellular models while a role in autophagy signalling has been recapitulated in vivo. Dysregulation of autophagy has been implicated in PD pathology, and this raises the possibility that differential autophagic activity is relevant to disease progression in PD patients carrying LRRK2 mutations. To examine the relevance of LRRK2 to the regulation of macroautophagy in a disease setting we examined the levels of autophagic markers in the basal ganglia of G2019S LRRK2 PD post-mortem tissue, in comparison to pathology-matched idiopathic PD (iPD), using immunoblotting (IB). Significantly lower levels of p62 and LAMP1 were observed in G2019S LRRK2 PD compared to iPD cases. Similarly, an increase in ULK1 was observed in iPD but was not reflected in G2019S LRRK2 PD cases. Furthermore, examination of p62 by immunohistochemistry (IH) recapitulated a distinct signature for G2019S PD. IH of LAMP1, LC3 and ULK1 broadly correlated with the IB results. Our data from a small but pathologically well-characterized cases highlights a divergence of G2019S PD carriers in terms of autophagic response in alpha-synuclein pathology affected brain regions compared to iPD.

Physiological and pathological functions of LRRK2: implications from substrate proteins

Leucine-rich repeat kinase 2 (LRRK2) encodes a 2527-amino acid (aa) protein composed of multiple functional domains, including a Ras of complex proteins (ROC)-type GTP-binding domain, a carboxyl terminal of ROC (COR) domain, a serine/threonine protein kinase domain, and several repeat domains. LRRK2 is genetically involved in the pathogenesis of both sporadic and familial Parkinson's disease (FPD). Parkinson's disease (PD) is the second most common neurodegenerative disorder, manifesting progressive motor dysfunction. PD is pathologically characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta, and the presence of intracellular inclusion bodies called Lewy bodies (LB) in the remaining neurons. As the most frequent PD-causing mutation in LRRK2, G2019S, increases the kinase activity of LRRK2, an abnormal increase in LRRK2 kinase activity is believed to contribute to PD pathology; however, the precise biological functions of LRRK2 involved in PD pathogenesis remain unknown. Although biochemical studies have discovered several substrate proteins of LRRK2 including Rab GTPases and tau, little is known about whether excess phosphorylation of these substrates is the cause of the neurodegeneration in PD. In this review, we summarize latest findings regarding the physiological and pathological functions of LRRK2, and discuss the possible molecular mechanisms of neurodegeneration caused by LRRK2 and its substrates.