Elevated LRRK2 autophosphorylation in brain-derived and peripheral exosomes in LRRK2 mutation carriers

The LRRK2-G2019S mutation attenuates repair of brain injury partially by reducing the release of osteopontin-containing monocytic exosome-like vesicles

BACKGROUND

Brain injury has been suggested as a risk factor for neurodegenerative diseases. Accordingly, defects in the brain's intrinsic capacity to repair injury may result in the accumulation of damage and a progressive loss of brain function. The G2019S (GS) mutation in LRRK2 (leucine rich repeat kinase 2) is the most prevalent genetic alteration in Parkinson's disease (PD). Here, we sought to investigate how this LRRK2-GS mutation affects repair of the injured brain.

METHODS

Brain injury was induced by stereotaxic injection of ATP, a damage-associated molecular pattern (DAMP) component, into the striatum of wild-type (WT) and LRRK2-GS mice. Effects of the LRRK2-GS mutation on brain injury and the recovery from injury were examined by analyzing the molecular and cellular behavior of neurons, astrocytes, and monocytes.

RESULTS

Damaged neurons express osteopontin (OPN), a factor associated with brain repair. Following ATP-induced damage, monocytes entered injured brains, phagocytosing damaged neurons and producing exosome-like vesicles (EVs) containing OPN through activation of the inflammasome and subsequent pyroptosis. Following EV production, neurons and astrocytes processes elongated towards injured cores. In LRRK2-GS mice, OPN expression and monocytic pyroptosis were decreased compared with that in WT mice, resulting in diminished release of OPN-containing EVs and attenuated elongation of neuron and astrocyte processes. In addition, exosomes prepared from injured LRRK2-GS brains induced neurite outgrowth less efficiently than those from injured WT brains.

CONCLUSIONS

The LRRK2-GS mutation delays repair of injured brains through reduced expression of OPN and diminished release of OPN-containing EVs from monocytes. These findings suggest that the LRRK2-GS mutation may promote the development of PD by delaying the repair of brain injury.

Extracellular Vesicles as Biomarkers for Parkinson's Disease: How Far from Clinical Translation?

Parkinson's disease (PD) is a neurodegenerative disorder affecting about 10 million people worldwide with a prevalence of about 2% in the over-80 population. The disease brings in also a huge annual economic burden, recently estimated by the Michael J Fox Foundation for Parkinson's Research to be USD 52 billion in the United States alone. Currently, no effective cure exists, but available PD medical treatments are based on symptomatic prescriptions that include drugs, surgical approaches and rehabilitation treatment. Due to the complex biology of a PD brain, the design of clinical trials and the personalization of treatment strategies require the identification of accessible and measurable biomarkers to monitor the events induced by treatment and disease progression and to predict patients' responsiveness. In the present review, we strive to briefly summarize current knowledge about PD biomarkers, focusing on the role of extracellular vesicles as active or involuntary carriers of disease-associated proteins, with particular attention to those research works that envision possible clinical applications.

Post-translational modification and mitochondrial function in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease with currently no cure. Most PD cases are sporadic, and about 5-10% of PD cases present a monogenic inheritance pattern. Mutations in more than 20 genes are associated with genetic forms of PD. Mitochondrial dysfunction is considered a prominent player in PD pathogenesis. Post-translational modifications (PTMs) allow rapid switching of protein functions and therefore impact various cellular functions including those related to mitochondria. Among the PD-associated genes, Parkin, PINK1, and LRRK2 encode enzymes that directly involved in catalyzing PTM modifications of target proteins, while others like α-synuclein, FBXO7, HTRA2, VPS35, CHCHD2, and DJ-1, undergo substantial PTM modification, subsequently altering mitochondrial functions. Here, we summarize recent findings on major PTMs associated with PD-related proteins, as enzymes or substrates, that are shown to regulate important mitochondrial functions and discuss their involvement in PD pathogenesis. We will further highlight the significance of PTM-regulated mitochondrial functions in understanding PD etiology. Furthermore, we emphasize the potential for developing important biomarkers for PD through extensive research into PTMs.

Prospective Role of PAK6 and 14-3-3γ as Biomarkers for Parkinson's Disease

Background

Parkinson's disease is a progressive neurodegenerative disorder mainly distinguished by sporadic etiology, although a genetic component is also well established. Variants in the LRRK2 gene are associated with both familiar and sporadic disease. We have previously shown that PAK6 and 14-3-3γ protein interact with and regulate the activity of LRRK2.

Objective

The aim of this study is to quantify PAK6 and 14-3-3γ in plasma as reliable biomarkers for the diagnosis of both sporadic and LRRK2-linked Parkinson's disease.

Methods

After an initial quantification of PAK6 and 14-3-3γ expression by means of Western blot in post-mortem human brains, we verified the presence of the two proteins in plasma by using quantitative ELISA tests. We analyzed samples obtained from 39 healthy subjects, 40 patients with sporadic Parkinson's disease, 50 LRRK2-G2019S non-manifesting carriers and 31 patients with LRRK2-G2019S Parkinson's disease.

Results

The amount of PAK6 and 14-3-3γ is significantly different in patients with Parkinson's disease compared to healthy subjects. Moreover, the amount of PAK6 also varies with the presence of the G2019S mutation in the LRRK2 gene. Although the generalized linear models show a low association between the presence of Parkinson's disease and PAK6, the kinase could be added in a broader panel of biomarkers for the diagnosis of Parkinson's disease.

Conclusions

Changes of PAK6 and 14-3-3γ amount in plasma represent a shared readout for patients affected by sporadic and LRRK2-linked Parkinson's disease. Overall, they can contribute to the establishment of an extended panel of biomarkers for the diagnosis of Parkinson's disease.

How should we be using biomarkers in trials of disease modification in Parkinson's disease?

The recent validation of the α-synuclein seed amplification assay as a biomarker with high sensitivity and specificity for the diagnosis of Parkinson's disease has formed the backbone for a proposed staging system for incorporation in Parkinson's disease clinical studies and trials. The routine use of this biomarker should greatly aid in the accuracy of diagnosis during recruitment of Parkinson's disease patients into trials (as distinct from patients with non-Parkinson's disease parkinsonism or non-Parkinson's disease tremors). There remain, however, further challenges in the pursuit of biomarkers for clinical trials of disease modifying agents in Parkinson's disease, namely: optimizing the distinction between different α-synucleinopathies; the selection of subgroups most likely to benefit from a candidate disease modifying agent; a sensitive means of confirming target engagement; and the early prediction of longer-term clinical benefit. For example, levels of CSF proteins such as the lysosomal enzyme β-glucocerebrosidase may assist in prognostication or allow enrichment of appropriate patients into disease modifying trials of agents with this enzyme as the target; the presence of coexisting Alzheimer's disease-like pathology (detectable through CSF levels of amyloid-β42 and tau) can predict subsequent cognitive decline; imaging techniques such as free-water or neuromelanin MRI may objectively track decline in Parkinson's disease even in its later stages. The exploitation of additional biomarkers to the α-synuclein seed amplification assay will, therefore, greatly add to our ability to plan trials and assess the disease modifying properties of interventions. The choice of which biomarker(s) to use in the context of disease modifying clinical trials will depend on the intervention, the stage (at risk, premotor, motor, complex) of the population recruited and the aims of the trial. The progress already made lends hope that panels of fluid biomarkers in tandem with structural or functional imaging may provide sensitive and objective methods of confirming that an intervention is modifying a key pathophysiological process of Parkinson's disease. However, correlation with clinical progression does not necessarily equate to causation, and the ongoing validation of quantitative biomarkers will depend on insightful clinical-genetic-pathophysiological comparisons incorporating longitudinal biomarker changes from those at genetic risk with evidence of onset of the pathophysiology and those at each stage of manifest clinical Parkinson's disease.

The role of exosomes in the diagnosis of Parkinson's disease

Parkinson's disease is a common neurodegenerative disease characterized by intracellular aggregation of misfolded α-synuclein as a major pathological hallmark. Exosomes are cell-derived lipid bilayer membrane vesicles with various components, including proteins, RNA, and lipids, that mediate intercellular communication. Currently, exosomes are found to be responsible for transporting misfolded proteins from unhealthy neurons to nearby cells, spreading the disease from cell to cell. Such exosomes can also be found in the cerebrospinal fluid and blood. Thus, exosomes may serve as a potential tool to detect the pathology of Parkinson's disease for clinical diagnosis. In this article, the role and challenges of exosomes in the diagnosis of Parkinson's disease are outlined.

Extracellular Vesicles for the Diagnosis of Parkinson's Disease: Systematic Review and Meta-Analysis

Parkinson's disease (PD) biomarkers are needed by both clinicians and researchers (for diagnosis, identifying study populations, and monitoring therapeutic response). Imaging, genetic, and biochemical biomarkers have been widely studied. In recent years, extracellular vesicles (EVs) have become a promising material for biomarker development. Proteins and molecular material from any organ, including the central nervous system, can be packed into EVs and transported to the periphery into easily obtainable biological specimens like blood, urine, and saliva. We performed a systematic review and meta-analysis of articles (published before November 15, 2022) reporting biomarker assessment in EVs in PD patients and healthy controls (HCs). Biomarkers were analyzed using random effects meta-analysis and the calculated standardized mean difference (Std.MD). Several proteins and ribonucleic acids have been identified in EVs in PD patients, but only α-synuclein (aSyn) and leucine-rich repeat kinase 2 (LRRK2) were reported in sufficient studies (n = 24 and 6, respectively) to perform a meta-analysis. EV aSyn was significantly increased in neuronal L1 cell adhesion molecule (L1CAM)-positive blood EVs in PD patients compared to HCs (Std.MD = 1.84, 95% confidence interval = 0.76-2.93, P = 0.0009). Further analysis of the biological sample and EV isolation method indicated that L1CAM-IP (immunoprecipitation) directly from plasma was the best isolation method for assessing aSyn in PD patients. Upcoming neuroprotective clinical trials immediately need peripheral biomarkers for identifying individuals at risk of developing PD. Overall, the improved sensitivity of assays means they can identify biomarkers in blood that reflect changes in the brain. CNS-derived EVs in blood will likely play a major role in biomarker development in the coming years. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Quantitative proteomics and phosphoproteomics of urinary extracellular vesicles define putative diagnostic biosignatures for Parkinson's disease

BACKGROUND

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene have been recognized as genetic risk factors for Parkinson's disease (PD). However, compared to cancer, fewer genetic mutations contribute to the cause of PD, propelling the search for protein biomarkers for early detection of the disease.

METHODS

Utilizing 138 urine samples from four groups, healthy individuals (control), healthy individuals with G2019S mutation in the LRRK2 gene (non-manifesting carrier/NMC), PD individuals without G2019S mutation (idiopathic PD/iPD), and PD individuals with G2019S mutation (LRRK2 PD), we applied a proteomics strategy to determine potential diagnostic biomarkers for PD from urinary extracellular vesicles (EVs).

RESULTS

After efficient isolation of urinary EVs through chemical affinity followed by mass spectrometric analyses of EV peptides and enriched phosphopeptides, we identify and quantify 4476 unique proteins and 2680 unique phosphoproteins. We detect multiple proteins and phosphoproteins elevated in PD EVs that are known to be involved in important PD pathways, in particular the autophagy pathway, as well as neuronal cell death, neuroinflammation, and formation of amyloid fibrils. We establish a panel of proteins and phosphoproteins as novel candidates for disease biomarkers and substantiate the biomarkers using machine learning, ROC, clinical correlation, and in-depth network analysis. Several putative disease biomarkers are further partially validated in patients with PD using parallel reaction monitoring (PRM) and immunoassay for targeted quantitation.

CONCLUSIONS

These findings demonstrate a general strategy of utilizing biofluid EV proteome/phosphoproteome as an outstanding and non-invasive source for a wide range of disease exploration.

LRRK2: Genetic mechanisms vs genetic subtypes

In 2004, the identification of pathogenic variants in the LRRK2 gene across several families with autosomal dominant late-onset Parkinson's disease (PD) revolutionized our understanding of the role of genetics in PD. Previous beliefs that genetics in PD was limited to rare early-onset or familial forms of the disease were quickly dispelled. Currently, we recognize LRRK2 p.G2019S as the most common genetic cause of both sporadic and familial PD, with more than 100,000 affected carriers across the globe. The frequency of LRRK2 p.G2019S is also highly variable across populations, with some regions of Asian or Latin America reporting close to 0%, contrasting to Ashkenazi Jews or North African Berbers reporting up to 13% and 40%, respectively. Patients with LRRK2 pathogenic variants are clinically and pathologically heterogeneous, highlighting the age-related variable penetrance that also characterizes LRRK2-related disease. Indeed, the majority of patients with LRRK2-related disease are characterized by a relatively mild Parkinsonism with less motor symptoms with variable presence of α-synuclein and/or tau aggregates, with pathologic pleomorphism widely described. At a functional cellular level, it is likely that pathogenic variants mediate a toxic gain-of-function of the LRRK2 protein resulting in increased kinase activity perhaps in a cell-specific manner; by contrast, some LRRK2 variants appear to be protective reducing PD risk by decreasing the kinase activity. Therefore, employing this information to define appropriate patient populations for clinical trials of targeted kinase LRRK2 inhibition strategies is very promising and demonstrates a potential future application for PD using precision medicine.

LRRK2 Quantification in Cerebrospinal Fluid of Patients with Parkinson's Disease and Atypical Parkinsonian Syndromes

BACKGROUND

The alteration of leucine-rich repeat kinase 2 (LRRK2) kinase activity is thought to be involved in Parkinson's disease (PD) pathogenesis beyond familiar cases, and LRRK2 inhibitors are currently under investigation. Preliminary data suggest a relationship between LRRK2 alteration and cognitive impairment in PD.

OBJECTIVE

To investigate cerebrospinal fluid (CSF) LRRK2 levels in PD and other parkinsonian disorders, also correlating them with cognitive impairment.

METHODS

In this study, we retrospectively investigated by means of a novel highly sensitive immunoassay the levels of total and phosphorylated (pS1292) LRRK2 in CSF of cognitively unimpaired PD (n = 55), PD with mild cognitive impairment (n = 49), PD with dementia (n = 18), dementia with Lewy bodies (n = 12), atypical parkinsonian syndromes (n = 35), and neurological controls (n = 30).

RESULTS

Total and pS1292 LRRK2 levels were significantly higher in PD with dementia with respect to PD with mild cognitive impairment and PD, and also showed a correlation with cognitive performances.

CONCLUSIONS

The tested immunoassay may represent a reliable method for assessing CSF LRRK2 levels. The results appear to confirm an association of LRRK2 alteration with cognitive impairment in PD. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Characterization of Lipopolysaccharide Effects on LRRK2 Signaling in RAW Macrophages

Dysfunction of the immune system and mitochondrial metabolism has been associated with Parkinson's disease (PD) pathology. Mutations and increased kinase activity of leucine-rich repeat kinase 2 (LRRK2) are linked to both idiopathic and familial PD. However, the function of LRRK2 in the immune cells under inflammatory conditions is contradictory. Our results showed that lipopolysaccharide (LPS) stimulation increased the kinase activity of LRRK2 in parental RAW 264.7 (WT) cells. In addition to this, LRRK2 deletion in LRRK2 KO RAW 264.7 (KO) cells altered cell morphology following LPS stimulation compared to the WT cells, as shown by an increase in the cell impedance as observed by the xCELLigence measurements. LPS stimulation caused an increase in the cellular reactive oxygen species (ROS) levels in both WT and KO cells. However, WT cells displayed a higher ROS level compared to the KO cells. Moreover, LRRK2 deletion led to a reduction in interleukin-6 (IL-6) inflammatory cytokine and cyclooxygenase-2 (COX-2) expression and an increase in lactate production after LPS stimulation compared to the WT cells. These data illustrate that LRRK2 has an effect on inflammatory processes in RAW macrophages upon LPS stimulation.

Identification of exosomal biomarkers and its optimal isolation and detection method for the diagnosis of Parkinson's disease: A systematic review and meta-analysis

Recently, there has been growing interest in exosomal biomarkers for their active targeting and specificity for delivering their cargos (proteins, lipids, nucleic acids) from the parent cell to the recipient cell. Currently, the clinical diagnosis of Parkinson's disease (PD) is mainly based on a clinician's neuropsychological examination and motor symptoms (e.g., bradykinesia, rigidity, postural instability, and resting tremor). However, this diagnosis method is not accurate due to overlapping criteria of other neurodegenerative diseases. Exosomes are differentially expressed in PD and a combination of types and contents of exosomes might be used as a biomarker in PD. Here, we systematically reviewed and meta-analyzed exosomal contents, types and sources of exosomes, method of isolation, and protein quantification tools to determine the optimum exosome-related attributes for PD diagnosis. Pubmed, Embase, and ISI Web of Science were searched for relevant studies. 25 studies were included in the meta-analysis. The Ratio of Mean (RoM) with 95% confidence intervals (CI) was calculated to estimate the effect size. Biomarker performances were rated by random-effects meta-analysis with the Restricted Maximum Likelihood (REML) method. The study protocol is available at PROSPERO (CRD42022331885). Exosomal α-synuclein (α-Syn) was significantly altered in PD patients from healthy controls [RoM = 1.67, 95% CI (0.99 to 2.35); p = 0.00] followed by tau [RoM = 1.33, 95% CI (0.79 to 1.87); p = 0.00], PS-129 [RoM = 0.97, 95% CI (0.54 to 1.40); p = 0.00], and DJ-1/PARK7 [RoM = 0.93, 95% CI (0.64 to 1.21); p = 0.00]. Central nervous system derived L1CAM exosome [RoM = 1.24, 95% CI (1.04 to 1.45); p = 0.00] from either plasma [RoM = 1.35, 95% CI (1.09 to 1.61); p = 0.00]; or serum [RoM = 1.47, 95% CI (1.05 to 1.90); p = 0.00] has been found the optimum type of exosome. The exosome isolation by ExoQuick [RoM = 1.16, 95% CI (0.89 to 1.43); p = 0.00] and protein quantification method by ELISA [RoM = 1.28, 95% CI (1.15 to 1.41); p = 0.00] has been found the optimum isolation and quantification method, respectively for PD diagnosis. This meta-analysis suggests that α-Syn in L1CAM exosome derived from blood, isolated by ExoQuick kit, and quantified by ELISA can be used for PD diagnosis.

Extracellular vesicles in the study of Alzheimer's and Parkinson's diseases: Methodologies applied from cells to biofluids

Extracellular vesicles (EVs) are gaining increased importance in fundamental research as key players in disease pathogenic mechanisms, but also in translational and clinical research due to their value in biomarker discovery, either for diagnostics and/or therapeutics. In the first research scenario, the study of EVs isolated from neuronal models mimicking neurodegenerative diseases can open new avenues to better understand the pathological mechanisms underlying these conditions or to identify novel molecular targets for diagnosis and/or therapeutics. In the second research scenario, the easy availability of EVs in body fluids and the specificity of their cargo, which can reflect the cell of origin or disease profiles, turn these into attractive diagnostic tools. EVs with exosome-like characteristics, circulating in the bloodstream and other peripheral biofluids, constitute a non-invasive and rapid alternative to study several conditions, including brain-related disorders. In both cases, several EVs isolation methods are already available, but each neuronal model or biofluid presents its own challenges. Herein, a literature overview on EVs isolation methodologies from distinct neuronal models (cellular culture and brain tissue) and body fluids (serum, plasma, cerebrospinal fluid, urine and saliva) was carried out. Focus was given to approaches employed in the context of Alzheimer's and Parkinson's diseases, and the main research findings discussed. The topics here revised will facilitate the choice of EVs isolation methodologies and potentially prompt new discoveries in EVs research and in the neurodegenerative diseases field.

Molecular Pathways Involved in LRRK2-Linked Parkinson’s Disease: A Systematic Review

Parkinson’s disease is one of the most common neurodegenerative diseases affecting the ageing population, with a prevalence that has doubled over the last 30 years. As the mechanism of the disease is not fully elucidated, the current treatments are unable to effectively prevent neurodegeneration. Studies have found that mutations in Leucine-rich-repeat-kinase 2 (LRRK2) are the most common cause of familial Parkinson’s disease (PD). Moreover, aberrant (higher) LRRK2 kinase activity has an influence in idiopathic PD as well. Hence, the aim of this review is to categorize and synthesize current information related to LRRK2-linked PD and present the factors associated with LRRK2 that can be targeted therapeutically. A systematic review was conducted using the databases PubMed, Medline, SCOPUS, SAGE, and Cochrane (January 2016 to July 2021). Search terms included “Parkinson’s disease”, “mechanism”, “LRRK2”, and synonyms in various combinations. The search yielded a total of 988 abstracts for initial review, 80 of which met the inclusion criteria. Here, we emphasize molecular mechanisms revealed in recent in vivo and in vitro studies. By consolidating the recent updates in the field of LRRK2-linked PD, researchers can further evaluate targets for therapeutic application.

Small Extracellular Vesicles Secreted by Nigrostriatal Astrocytes Rescue Cell Death and Preserve Mitochondrial Function in Parkinson's Disease

Extracellular vesicles (EVs) are emerging as powerful players in cell-to-cell communication both in healthy and diseased brain. In Parkinson's disease (PD)-characterized by selective dopaminergic neuron death in ventral midbrain (VMB) and degeneration of their terminals in striatum (STR)-astrocytes exert dual harmful/protective functions, with mechanisms not fully elucidated. Here, this study shows that astrocytes from the VMB-, STR-, and VMB/STR-depleted brains release a population of small EVs  in a region-specific manner. Interestingly, VMB-astrocytes secreted the highest rate of EVs, which is further exclusively increased in response to CCL3, a chemokine that promotes robust dopaminergic neuroprotection in different PD models. The neuroprotective potential of nigrostriatal astrocyte-EVs is investigated in differentiated versus undifferentiated SH-SY5Y cells exposed to oxidative stress and mitochondrial toxicity. EVs from both VMB- and STR-astrocytes counteract H₂ O₂ -induced caspase-3 activation specifically in differentiated cells, with EVs from CCL3-treated astrocytes showing a higher protective effect. High resolution respirometry further reveals that nigrostriatal astrocyte-EVs rescue neuronal mitochondrial complex I function impaired by the neurotoxin MPP⁺ . Notably, only EVs from VMB-astrocyte fully restore ATP production, again specifically in differentiated SH-SY5Y. These results highlight a regional diversity in the nigrostriatal system for the secretion and activities of astrocyte-EVs, with neuroprotective implications for PD.

Evaluation of Current Methods to Detect Cellular Leucine-Rich Repeat Kinase 2 (LRRK2) Kinase Activity

Background:

Coding variation in the Leucine rich repeat kinase 2 gene linked to Parkinson’s disease (PD) promotes enhanced activity of the encoded LRRK2 kinase, particularly with respect to autophosphorylation at S1292 and/or phosphorylation of the heterologous substrate RAB10.

Objective:

To determine the inter-laboratory reliability of measurements of cellular LRRK2 kinase activity in the context of wildtype or mutant LRRK2 expression using published protocols.

Methods:

Benchmark western blot assessments of phospho-LRRK2 and phospho-RAB10 were performed in parallel with in situ immunological approaches in HEK293T, mouse embryonic fibroblasts, and lymphoblastoid cell lines. Rat brain tissue, with or without adenovirus-mediated LRRK2 expression, and human brain tissues from subjects with or without PD, were also evaluated for LRRK2 kinase activity markers.

Results:

Western blots were able to detect extracted LRRK2 activity in cells and tissue with pS1292-LRRK2 or pT73-RAB10 antibodies. However, while LRRK2 kinase signal could be detected at the cellular level with over-expressed mutant LRRK2 in cell lines, we were unable to demonstrate specific detection of endogenous cellular LRRK2 activity in cell culture models or tissues that we evaluated.

Conclusion:

Further development of reliable methods that can be deployed in multiple laboratories to measure endogenous LRRK2 activities are likely required, especially at cellular resolution.

Elevated Urinary Rab10 Phosphorylation in Idiopathic Parkinson Disease

BACKGROUND

Pathogenic leucine-rich repeat kinase 2 LRRK2 mutations may increase LRRK2 kinase activity and Rab substrate phosphorylation. Genetic association studies link variation in LRRK2 to idiopathic Parkinson disease (iPD) risk.

OBJECTIVES

Through measurements of the LRRK2 kinase substrate pT73-Rab10 in urinary extracellular vesicles, this study seeks to understand how LRRK2 kinase activity might change with iPD progression.

METHODS

Using an immunoblotting approach validated in LRRK2 transgenic mice, the ratio of pT73-Rab10 to total Rab10 protein was measured in extracellular vesicles from a cross-section of G2019S LRRK2 mutation carriers (N = 45 participants) as well as 485 urine samples from a novel longitudinal cohort of iPD and controls (N = 85 participants). Generalized estimating equations were used to conduct analyses with commonly used clinical scales.

RESULTS

Although the G2019S LRRK2 mutation did not increase pT73-Rab10 levels, the ratio of pT73-Rab10 to total Rab10 nominally increased over baseline in iPD urine vesicle samples with time, but did not increase in age-matched controls (1.34-fold vs. 1.05-fold, 95% confidence interval [CI], 0.004-0.56; P = 0.046; Welch's t test). Effect estimates adjusting for sex, age, disease duration, diagnosis, and baseline clinical scores identified increasing total Movement Disorder Society-Sponsored Revision of the Unified (MDS-UPDRS) scores (β = 0.77; CI, 0.52-1.01; P = 0.0001) with each fold increase of pT73-Rab10 to total Rab10. Lower Montreal Cognitive Assessment (MoCA) score in iPD is also associated with increased pT73-Rab10.

CONCLUSIONS

These results provide initial insights into peripheral LRRK2-dependent Rab phosphorylation, measured in biobanked urine, where higher levels of pT73-Rab10 are associated with worse disease progression. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Exosome: The “Off-the-Shelf” Cellular Nanocomponent as a Potential Pathogenic Agent, a Disease Biomarker, and Neurotherapeutics

Exosomes are nanosized “off-the-shelf” lipid vesicles released by almost all cell types and play a significant role in cell–cell communication. Exosomes have already been proven to carry cell-specific cargos of proteins, lipids, miRNA, and noncoding RNA (ribonucleic acid). These vesicles can be selectively taken up by the neighboring cell and can regulate cellular functions. Herein, we have discussed three different roles of exosomes in neuroscience. First, we have discussed how exosomes play the role of a pathogenic agent as a part of cell–cell communication and transmit pathogens such as amyloid-beta (Aβ), further helping in the propagation of neurodegenerative and other neurological diseases. In the next section, the review talks about the role of exosomes in biomarker discovery in neurological disorders. Toward the end, we have reviewed how exosomes can be harnessed and engineered for therapeutic purposes in different brain diseases. This review is based on the current knowledge generated in this field and our comprehension of this domain.

Trafficking of the glutamate transporter is impaired in LRRK2-related Parkinson's disease

The Excitatory Amino Acid Transporter 2 (EAAT2) accounts for 80% of brain glutamate clearance and is mainly expressed in astrocytic perisynaptic processes. EAAT2 function is finely regulated by endocytic events, recycling to the plasma membrane and degradation. Noteworthy, deficits in EAAT2 have been associated with neuronal excitotoxicity and neurodegeneration. In this study, we show that EAAT2 trafficking is impaired by the leucine-rich repeat kinase 2 (LRRK2) pathogenic variant G2019S, a common cause of late-onset familial Parkinson’s disease (PD). In LRRK2 G2019S human brains and experimental animal models, EAAT2 protein levels are significantly decreased, which is associated with elevated gliosis. The decreased expression of the transporter correlates with its reduced functionality in mouse LRRK2 G2019S purified astrocytic terminals and in Xenopus laevis oocytes expressing human LRRK2 G2019S. In LRRK2 G2019S knock-in mouse brain, the correct surface localization of the endogenous transporter is impaired, resulting in its interaction with a plethora of endo-vesicular proteins. Mechanistically, we report that pathogenic LRRK2 kinase activity delays the recycling of the transporter to the plasma membrane via Rabs inactivation, causing its intracellular re-localization and degradation. Taken together, our results demonstrate that pathogenic LRRK2 interferes with the physiology of EAAT2, pointing to extracellular glutamate overload as a possible contributor to neurodegeneration in PD.

A new paradigm for diagnosis of neurodegenerative diseases: peripheral exosomes of brain origin

Neurodegenerative diseases are a heterogeneous group of maladies, characterized by progressive loss of neurons. These diseases involve an intricate pattern of cross-talk between different types of cells to maintain specific signaling pathways. A component of such intercellular cross-talk is the exchange of various types of extracellular vesicles (EVs). Exosomes are a subset of EVs, which are increasingly being known for the role they play in the pathogenesis and progression of neurodegenerative diseases, e.g., synucleinopathies and tauopathies. The ability of the central nervous system exosomes to cross the blood–brain barrier into blood has generated enthusiasm in their study as potential biomarkers. However, the lack of standardized, efficient, and ultra-sensitive methods for the isolation and detection of brain-derived exosomes has hampered the development of effective biomarkers. Exosomes mirror heterogeneous biological changes that occur during the progression of these incurable illnesses, potentially offering a more comprehensive outlook of neurodegenerative disease diagnosis, progression and treatment. In this review, we aim to discuss the challenges and opportunities of peripheral biofluid-based brain-exosomes in the diagnosis and biomarker discovery of Alzheimer’s and Parkinson’s diseases. In the later part, we discuss the traditional and emerging methods used for the isolation of exosomes and compare their advantages and disadvantages in clinical settings.

Monogenetic Forms of Parkinson’s Disease – Bridging the Gap Between Genetics and Biomarkers

The therapy of neurodegenerative diseases such as Parkinson’s disease (PD) is still limited to the treatment of symptoms and primarily aimed at compensating for dopaminergic hypofunction. Numerous disease-modifying therapies currently in the pipeline attempt to modify the underlying pathomechanisms. In recent decades, the results of molecular genetics and biomarker research have raised hopes of earlier diagnosis and new neuroprotective therapeutic approaches. As the disease-causing processes in monogenetic forms of PD are better understood than in sporadic PD, these disease subsets are likely to benefit first from disease-modifying therapies. Recent studies have suggested that disease-relevant changes found in genetically linked forms of PD (i.e., PARK-LRRK2, PARK-GBA) can also be reproduced in patients in whom no genetic cause can be found, i.e., those with sporadic PD. It can, therefore, be assumed that as soon as the first causal therapy for genetic forms of PD is approved, more patients with PD will undergo genetic testing and counseling. Regarding future neuroprotective trials in neurodegenerative diseases and objective parameters such as biomarkers with high sensitivity and specificity for the diagnosis and course of the disease are needed. These biomarkers will also serve to monitor treatment success in clinical trials. Promising examples in PD, such as alpha-synuclein species, lysosomal enzymes, markers of amyloid and tau pathology, and neurofilament light chain, are under investigation in blood and CSF. This paper provides an overview of the opportunities and current limitations of monogenetic diagnostic and biomarker research in PD and aims to build a bridge between current knowledge and association with PD genetics and biomarkers.

Therapeutically harnessing extracellular vesicles

The field of extracellular vesicle (EV) research has developed rapidly over the last decade from the study of fundamental biology to a subject of significant clinical relevance. The potential of harnessing EVs in the diagnosis and treatment of diseases - including cancer and neurological and cardiovascular disorders - is now being recognized. Accordingly, the applications of EVs as therapeutic targets, biomarkers, novel drug delivery agents and standalone therapeutics are being actively explored. This Review provides a brief overview of the characteristics and physiological functions of the various classes of EV, focusing on their association with disease and emerging strategies for their therapeutic exploitation.

Circulating extracellular vesicles: friends and foes in neurodegeneration

Extracellular vesicles have been identified as pivotal mediators of intercellular communication with critical roles in physiological and pathological conditions. Via this route, several molecules (e.g., nucleic acids, proteins, metabolites) can be transferred to proximal and distant targets to convey specific information. Extracellular vesicle-associated cargo molecules have been proposed as markers of several disease conditions for their potential of tracking down the generating cell. Indeed, circulating extracellular vesicles may represent biomarkers of dysfunctional cellular quality control systems especially in conditions characterized by the accrual of intracellular misfolded proteins. Furthermore, the identification of extracellular vesicles as tools for the delivery of nucleic acids or other cargo molecules to diseased tissues makes these circulating shuttles possible targets for therapeutic development. The increasing interest in the study of extracellular vesicles as biomarkers resides mainly in the fact that the identification of peripheral levels of extracellular vesicle-associated proteins might reflect molecular events occurring in hardly accessible tissues, such as the brain, thereby serving as a "brain liquid biopsy". The exploitation of extracellular vesicles for diagnostic and therapeutic purposed might offer unprecedented opportunities to develop personalized approaches. Here, we discuss the bright and dark sides of extracellular vesicles in the setting of two main neurodegenerative diseases (i.e., Parkinson's and Alzheimer's diseases). A special focus will be placed on the possibility of using extracellular vesicles as biomarkers for the two conditions to enable disease tracking and treatment monitoring.

Update on CSF Biomarkers in Parkinson's Disease

Progress in developing disease-modifying therapies in Parkinson’s disease (PD) can only be achieved through reliable objective markers that help to identify subjects at risk. This includes an early and accurate diagnosis as well as continuous monitoring of disease progression and therapy response. Although PD diagnosis still relies mainly on clinical features, encouragingly, advances in biomarker discovery have been made. The cerebrospinal fluid (CSF) is a biofluid of particular interest to study biomarkers since it is closest to the brain structures and therefore could serve as an ideal source to reflect ongoing pathologic processes. According to the key pathophysiological mechanisms, the CSF status of α-synuclein species, markers of amyloid and tau pathology, neurofilament light chain, lysosomal enzymes and markers of neuroinflammation provide promising preliminary results as candidate biomarkers. Untargeted approaches in the field of metabolomics provide insights into novel and interconnected biological pathways. Markers based on genetic forms of PD can contribute to identifying subgroups suitable for gene-targeted treatment strategies that might also be transferable to sporadic PD. Further validation analyses in large PD cohort studies will identify the CSF biomarker or biomarker combinations with the best value for clinical and research purposes.

Opposite Roles of Co-enzyme Q10 and Formaldehyde in Neurodegenerative Diseases

Most of neurodegenerative diseases (NDD) have no cure. The common etiology of neurodegenerations is unclear. Air pollutant-gaseous formaldehyde is notoriously known to induce demyelination and cognitive impairments. Unexpectedly, an amount of formaldehyde has been detected in the brains. Multiple factors can induce the generation and accumulation of endogenous formaldehyde. Excessive formaldehyde can induce oxidative stress to generate H2O2; in turn, H2O2 promote formaldehyde production. Clinical investigations have shown that an abnormal high level of formaldehyde but low level of coenzyme Q10 (coQ10) was observed in patients with NDD. Further studies have proven that excessive formaldehyde directly inactivates coQ10, reduces the ATP generation, enhances oxidative stress, initiates inflammation storm, induces demyelination; subsequently, it results in neurodegeneration. Although the low water solubility of coQ10 limits its clinical application, nanomicellar water-soluble coQ10 exhibits positive therapeutical effects. Hence, nanopackage of coQ10 may be a promising strategy for treating NDD.

The LRRK2 G2019S mutation alters astrocyte-to-neuron communication via extracellular vesicles and induces neuron atrophy in a human iPSC-derived model of Parkinson's disease

Astrocytes are essential cells of the central nervous system, characterized by dynamic relationships with neurons that range from functional metabolic interactions and regulation of neuronal firing activities, to the release of neurotrophic and neuroprotective factors. In Parkinson's disease (PD), dopaminergic neurons are progressively lost during the course of the disease, but the effects of PD on astrocytes and astrocyte-to-neuron communication remain largely unknown. This study focuses on the effects of the PD-related mutation LRRK2 G2019S in astrocytes generated from patient-derived induced pluripotent stem cells. We report the alteration of extracellular vesicle (EV) biogenesis in astrocytes and identify the abnormal accumulation of key PD-related proteins within multivesicular bodies (MVBs). We found that dopaminergic neurons internalize astrocyte-secreted EVs and that LRRK2 G2019S EVs are abnormally enriched in neurites and fail to provide full neurotrophic support to dopaminergic neurons. Thus, dysfunctional astrocyte-to-neuron communication via altered EV biological properties may participate in the progression of PD.

Pathways to Parkinson's disease: a spotlight on 14-3-3 proteins

14-3-3s represent a family of highly conserved 30 kDa acidic proteins. 14-3-3s recognize and bind specific phospho-sequences on client partners and operate as molecular hubs to regulate their activity, localization, folding, degradation, and protein-protein interactions. 14-3-3s are also associated with the pathogenesis of several diseases, among which Parkinson's disease (PD). 14-3-3s are found within Lewy bodies (LBs) in PD patients, and their neuroprotective effects have been demonstrated in several animal models of PD. Notably, 14-3-3s interact with some of the major proteins known to be involved in the pathogenesis of PD. Here we first provide a detailed overview of the molecular composition and structural features of 14-3-3s, laying significant emphasis on their peculiar target-binding mechanisms. We then briefly describe the implication of 14-3-3s in the central nervous system and focus on their interaction with LRRK2, α-Synuclein, and Parkin, three of the major players in PD onset and progression. We finally discuss how different types of small molecules may interfere with 14-3-3s interactome, thus representing a valid strategy in the future of drug discovery.

Extracellular Vesicles as Novel Diagnostic and Prognostic Biomarkers for Parkinson's Disease

The elderly population will significantly increase in the next decade and, with it, the proportion of people affected by age-related diseases. Among them, one of the most invalidating is Parkinson's disease (PD), characterized by motor- and non-motor dysfunctions which strongly impair the quality of life of affected individuals. PD is characterized by the progressive degeneration of dopaminergic neurons, with consequent dopamine depletion, and the accumulation of misfolded α-synuclein aggregates. Although 150 years have passed since PD first description, no effective therapies are currently available, but only palliative treatments. Importantly, PD is often diagnosed when the neuronal loss is elevated, making difficult any therapeutic intervention. In this context, two key challenges remain unanswered: (i) the early diagnosis to avoid the insurgence of irreversible symptoms; and (ii) the reliable monitoring of therapy efficacy. Research strives to identify novel biomarkers for PD diagnosis, prognosis, and therapeutic follow-up. One of the most promising sources of biomarkers is represented by extracellular vesicles (EVs), a heterogeneous population of nanoparticles, released by all cells in the microenvironment. Brain-derived EVs are able to cross the blood-brain barrier, protecting their payload from enzymatic degradation, and are easily recovered from biofluids. Interestingly, EV content is strongly influenced by the specific pathophysiological status of the donor cell. In this manuscript, the role of EVs as source of novel PD biomarkers is discussed, providing all recent findings concerning relevant proteins and miRNAs carried by PD patient-derived EVs, from several biological specimens. Moreover, the contribution of mitochondria-derived EVs will be dissected. Finally, the promising possibility to use EVs as source of markers to monitor PD therapy efficacy will be also examined. In the future, larger cohort studies will help to validate these EV-associated candidates, that might be effectively used as non-invasive and robust source of biomarkers for PD.

The role of extracellular vesicles in the physiological and pathological regulation of the blood-brain barrier

Extracellular vesicles (EVs) are a subclass of biological nanoparticles secreted by most cell types. Once secreted, EVs can travel long distances to deliver their content to target cells thereby playing a key role in cell-to-cell communication and supporting both physiological and pathological processes. In recent years, the functional versatility of EVs has come to be more widely appreciated. Their heterogeneous structure encloses solubilized bioactive cargoes including proteins and nucleic acids. EVs mirror the secreting cell in composition therefore representing a novel source of diagnostic and prognostic biomarkers. Moreover, due to their unique structure, EVs constitute a promising class of biocompatible nanovehicles for drug delivery as well. Importantly, and of burgeoning interest, is the fact that EVs have the intrinsic ability to breach biological barriers including the complex blood-brain barrier (BBB), whose restrictive nature represents a significant therapeutic challenge. EVs have been shown to contribute to the progression of a variety of brain diseases including metastatic brain cancer, neurodegenerative diseases, and acute pathologies including infections and ischemia. In this review, the role of EVs in the maintenance and regulation of the BBB under normal physiological and pathologic conditions are discussed. Applications of EVs as therapeutic and diagnostic tools in the treatment of diseases that affect the central nervous system are presented as are limitations hindering their broad translation and potential solutions to resolve them.

Seeking progress in disease modification in Parkinson disease

OBJECTIVE

Disease modification in Parkinson disease (PD) has remained an elusive goal, in spite of large investments over several decades. Following a large meeting of experts, this review article discusses the state of the science, possible reasons for past PD trials' failures to demonstrate disease-modifying benefit, and potential solutions.

METHODS

The National Institute of Neurological Disorders and Stroke (NINDS) convened a meeting including leaders in the field and representatives of key stakeholder groups to discuss drug therapy with the goal of disease modification in PD.

RESULTS

Important lessons can be learned from previous attempts, as well as from other fields. The selection process for therapeutic targets and agents differs among various organizations committed to therapeutic development. The areas identified as critical to target in future research include the development of relevant biomarkers, refinements of the targeted patient populations, considerations of novel trial designs, and improving collaborations between all stakeholders.

CONCLUSIONS

We identify potential barriers to progress in disease modification for Parkinson's and propose a set of research priorities that may improve the likelihood of success.

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.

Secreted Extracellular Vesicle Molecular Cargo as a Novel Liquid Biopsy Diagnostics of Central Nervous System Diseases

Secreted extracellular vesicles (EVs) are heterogeneous cell-derived membranous granules which carry a large diversity of molecules and participate in intercellular communication by transferring these molecules to target cells by endocytosis. In the last decade, EVs’ role in several pathological conditions, from etiology to disease progression or therapy evasion, has been consolidated, including in central nervous system (CNS)-related disorders. For this review, we performed a systematic search of original works published, reporting the presence of molecular components expressed in the CNS via EVs, which have been purified from plasma, serum or cerebrospinal fluid. Our aim is to provide a list of molecular EV components that have been identified from both nonpathological conditions and the most common CNS-related disorders. We discuss the methods used to isolate and enrich EVs from specific CNS-cells and the relevance of its components in each disease context.

Serum Uric Acid in LRRK2 Related Parkinson's Disease: Longitudinal Data from the PPMI Study

BACKGROUND

Previous studies have highlighted serum uric acid as a putative idiopathic Parkinson's disease (iPD) biomarker. Only one study, so far, showed higher levels of serum uric acid in leucine-rich repeat kinase 2 (LRRK + 2) carriers compared to those who developed PD, however a longitudinal comparison between LRRK2 + PD and healthy controls (HC) has not been performed.

OBJECTIVE

The aim of this study was to determine whether there are longitudinal differences in serum uric acid between iPD, LRRK2 + PD and HC and their association with motor and non-motor features.

METHODS

Longitudinal data of uric acid of 282 de novo iPD, 144 LRRK2 + PD patients, and 195 age-matched HC were obtained from the Parkinson's Progression Markers Initiative (PPMI) database. We also used longitudinal Montreal Cognitive Assessment (MoCA), Movement Disorder Society-Unified Parkinson's Disease Rating Scale part III (MDS-UPDRS-III), Geriatric Depression Scale (GDS) scores, and DaTSCAN striatal binding ratios (SBRs).

RESULTS

Longitudinal uric acid measurements were significantly lower in LRRK2 + PD patients compared to HC up to 5 years follow-up. There was no significant impact or correlation of adjusted or unadjusted uric acid levels with MoCA, MDS-UPDRS III, or GDS scores, the presence of RBD or DAT-SCAN SBRs.

CONCLUSION

LRRK2 + PD group had significantly lower uric acid concentrations compared to HC after adjusting for age, sex and baseline BMI up to 5 years follow-up. There were no significant associations between uric acid levels and indices of disease severity. These findings identify serum uric acid as a marker linked to LRRK2 + PD.

Characterization of the Onset, Progression, and Reversibility of Morphological Changes in Mouse Lung after Pharmacological Inhibition of Leucine-Rich Kinase 2 Kinase Activity

Gain-of-function mutations in leucine-rich kinase 2 (LRRK2) are associated with increased incidence of Parkinson disease (PD); thus, pharmacological inhibition of LRRK2 kinase activity is postulated as a disease-modifying treatment of PD. Histomorphological changes in lungs of nonhuman primates (NHPs) treated with small-molecule LRRK2 kinase inhibitors have brought the safety of this treatment approach into question. Although it remains unclear how LRRK2 kinase inhibition affects the lung, continued studies in NHPs prove to be both cost- and resource-prohibitive. To develop a tractable alternative animal model platform, we dosed male mice in-diet with the potent, highly selective LRRK2 kinase inhibitor MLi-2 and induced histomorphological changes in lung within 1 week. Oral bolus dosing of MLi-2 at a frequency modeled to provide steady-state exposure equivalent to that achieved with in-diet dosing induced type II pneumocyte vacuolation, suggesting pulmonary changes require sustained LRRK2 kinase inhibition. Treating mice with MLi-2 in-diet for up to 6 months resulted in type II pneumocyte vacuolation that progressed only modestly over time and was fully reversible after withdrawal of MLi-2. Immunohistochemical analysis of lung revealed a significant increase in prosurfactant protein C staining within type II pneumocytes. In the present study, we demonstrated the kinetics for onset, progression, and rapid reversibility of chronic LRRK2 kinase inhibitor effects on lung histomorphology in rodents and provide further evidence for the derisking of safety and tolerability concerns for chronic LRRK2 kinase inhibition in PD. SIGNIFICANCE STATEMENT: We have defined a mouse model by which the on-target lung effects of leucine-rich kinase 2 (LRRK2) kinase inhibition can be monitored, whereas previous in vivo testing relied solely on nonhuman primates. Data serve to derisk long-term treatment with LRRK2 kinase inhibitors, as all lung changes were mild and readily reversible.

Inflammatory Diseases Among Norwegian LRRK2 Mutation Carriers. A 15-Years Follow-Up of a Cohort

The first families with LRRK2 related Parkinson’s disease (PD) were presented around 15 years ago and numerous papers have described the characteristics of the LRRK2 phenotype. The prevalence of autosomal dominant PD varies around the world mainly depending on local founder effects. The highest prevalence of LRRK2 G2019S PD in Norway is located to the central part of the country and most families could be traced back to common ancestors. The typical Norwegian LRRK2 phenotype is not different from classical PD and similar to that seen in most other LRRK2 families. The discovery of LRRK2 PD has allowed us to follow-up multi-incident families and to study their phenotype longitudinally. In the Norwegian LRRK2 families there has been a significantly higher incidence of inflammatory diseases like multiple sclerosis and rheumatoid arthritis that seen in other PD populations. Recent studies in LRRK2 mechanisms have indicated that this protein may be crucial in initiating disease processes. In this short survey of 100 Norwegian mutation carriers followed through more than 15 years are presented. The prevalence of inflammatory diseases among these cases is highlighted. The role of LRRK2 in the conversion process from carrier status to PD phenotype is still unknown and disease generating mechanisms important for initiating LRRK2 PD are still to be identified.

Potential of extracellular vesicles in the Parkinson's disease - Pathological mediators and biomarkers

Parkinson's disease (PD) is an age-related neurodegenerative disorder characterized by the progressive deterioration of motor function. Histopathologically, it is widely accepted that the progressive death of selected dopaminergic neuronal populations and the accumulation of hallmark Lewy bodies (LBs) composed of α-synuclein (α-syn) might be the two vital pathogenesis. Extracellular vesicles (EVs) are cell-derived membranous vesicles that are liberated from virtually all cell types including neurons, and harbor a variety of proteins, DNA, mRNA, and lipids. The roles of these vesicles include cell-cell signaling, removal of unwanted proteins, and transfer of pathogens (including misfolded proteins) between cells. In PD, EVs not only enhance the spread of α-syn at distant sites and reduce their clearance but also mediate other PD pathogenesis such as the activation of microglia and the dysfunction of autophagy and lysosomal degradation systems. Recently, clinical evidence for the diagnostic performance of EV-associated biomarkers, particularly exosome biomarkers, has merged. In this regard, we reviewed the recent understanding of the biological roles of EVs as important tools for biomarker discovery and pathological regulators of PD, and discuss the main concerns and challenges for the application of EV biomarkers in the clinical setting.

The Evolving Landscape of Exosomes in Neurodegenerative Diseases: Exosomes Characteristics and a Promising Role in Early Diagnosis

Neurodegenerative diseases (ND) remains to be one of the biggest burdens on healthcare systems and serves as a leading cause of disability and death. Alzheimer's disease (AD) is among the most common of such disorders, followed by Parkinson's disease (PD). The basic molecular details of disease initiation and pathology are still under research. Only recently, the role of exosomes has been linked to the initiation and progression of these neurodegenerative diseases. Exosomes are small bilipid layer enclosed extracellular vesicles, which were once considered as a cellular waste and functionless. These nano-vesicles of 30-150 nm in diameter carry specific proteins, lipids, functional mRNAs, and high amounts of non-coding RNAs (miRNAs, lncRNAs, and circRNAs). As the exosomes content is known to vary as per their originating and recipient cells, these vesicles can be utilized as a diagnostic biomarker for early disease detection. Here we review exosomes, their biogenesis, composition, and role in neurodegenerative diseases. We have also provided details for their characterization through an array of available techniques. Their updated role in neurodegenerative disease pathology is also discussed. Finally, we have shed light on a novel field of salivary exosomes as a potential candidate for early diagnosis in neurodegenerative diseases and compared the biomarkers of salivary exosomes with other blood/cerebrospinal fluid (CSF) based exosomes within these neurological ailments.

Extracellular vesicles - propagators of neuropathology and sources of potential biomarkers and therapeutics for neurodegenerative diseases

Neurodegenerative diseases are characterised by the irreversible degeneration of neurons in the central or peripheral nervous systems. These include amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD) and prion diseases. Small extracellular vesicles (sEVs), a type of EV involved in cellular communication, have been well documented as propagating neurodegenerative diseases. These sEVs carry cargo, such as proteins and RNA, to recipient cells but are also capable of promoting protein misfolding, thus actively contributing to the progression of these diseases. sEV secretion is also a compensatory process for lysosomal dysfunction in the affected cells, despite inadvertently propagating disease to recipient cells. Despite this, sEV miRNAs have biomarker potential for the early diagnosis of these diseases, while stem cell-derived sEVs and those generated through exogenous assistance demonstrate the greatest therapeutic potential. This Review will highlight novel advancements in the involvement of sEVs as propagators of neuropathology, biomarkers and potential therapeutics in neurodegenerative diseases.

Insights Into the Proteomic Profiling of Extracellular Vesicles for the Identification of Early Biomarkers of Neurodegeneration

Extracellular vesicles (EVs) are involved in the development and progression of neurodegenerative diseases, including Alzheimer's and Parkinson's disease. Moreover, EVs have the capacity to modify the physiology of neuronal circuits by transferring proteins, RNA, lipids, and metabolites. The proteomic characterization of EVs (exosomes and microvesicles) from preclinical models and patient samples has the potential to reveal new proteins and molecular networks that affect the normal physiology prior to the appearance of traditional biomarkers of neurodegeneration. Noteworthy, many of the genetic risks associated to the development of Alzheimer's and Parkinson's disease affect the crosstalk between mitochondria, endosomes, and lysosomes. Recent research has focused on determining the role of endolysosomal trafficking in the onset of neurodegenerative diseases. Proteomic studies indicate an alteration of biogenesis and molecular content of EVs as a result of endolysosomal and autophagic dysfunction. In this review, we discuss the status of EV proteomic characterization and their usefulness in discovering new biomarkers for the differential diagnosis of neurodegenerative diseases. Despite the challenges related to the failure to follow a standard isolation protocol and their implementation for a clinical setting, the analysis of EV proteomes has revealed the presence of key proteins with post-translational modifications that can be measured in peripheral fluids.

Exosome markers of LRRK2 kinase inhibition

Hyper-activated LRRK2 is linked to Parkinson’s disease susceptibility and progression. Quantitative measures of LRRK2 inhibition, especially in the brain, maybe critical in the development of successful LRRK2-targeting therapeutics. In this study, two different brain-penetrant and selective LRRK2 small-molecule kinase inhibitors (PFE-360 and MLi2) were orally administered to groups of cynomolgus macaques. Proposed pharmacodynamic markers in exosomes from urine and cerebrospinal fluid (CSF) were compared to established markers in peripheral blood mononuclear cells (PBMCs). LRRK2 kinase inhibition led to reductions in exosome-LRRK2 protein and the LRRK2-substrate pT73-Rab10 in urine, as well as reduced exosome-LRRK2 and autophosphorylated pS1292-LRRK2 protein in CSF. We propose orthogonal markers for LRRK2 inhibition in urine and CSF can be used in combination with blood markers to non-invasively monitor the potency of LRRK2-targeting therapeutics.

Urinary Biomarkers for Neurodegenerative Diseases

Global incidence of neurodegenerative diseases (NDDs) such as Alzheimer's disease (AD) and Parkinson's disease (PD) is rapidly increasing, but the diagnosis of these diseases at their early stage is challenging. Therefore, the availability of reproducible and reliable biomarkers to diagnose such diseases is more critical than ever. In addition, biomarkers could be used not only to diagnose diseases but also to monitor the development of disease therapeutics. Urine is an excellent biofluid that can be utilized as a source of biomarker to diagnose not only several renal diseases but also other diseases because of its abundance in invasive sampling. However, urine was conventionally regarded as inappropriate as a source of biomarker for neurodegenerative diseases because it is anatomically distant from the central nervous system (CNS), a major pathologic site of NDD, in comparison to other biofluids such as cerebrospinal fluid (CSF) and plasma. However, recent studies have suggested that urine could be utilized as a source of NDD biomarker if an appropriate marker is predetermined by metabolomic and proteomic approaches in urine and other samples. In this review, we summarize such studies related to NDD.

The Current State-of-the Art of LRRK2-Based Biomarker Assay Development in Parkinson's Disease

Evidence is mounting that LRRK2 function, particularly its kinase activity, is elevated in multiple forms of Parkinson's disease, both idiopathic as well as familial forms linked to mutations in the LRRK2 gene. However, sensitive quantitative markers of LRRK2 activation in clinical samples remain at the early stages of development. There are several measures of LRRK2 activity that could potentially be used in longitudinal studies of disease progression, as inclusion/exclusion criteria for clinical trials, to predict response to therapy, or as markers of target engagement. Among these are levels of LRRK2, phosphorylation of LRRK2 itself, either by other kinases or via auto-phosphorylation, its in vitro kinase activity, or phosphorylation of downstream substrates. This is advantageous on many levels, in that multiple indices of elevated kinase activity clearly strengthen the rationale for targeting this kinase with novel therapeutic candidates, and provide alternate markers of activation in certain tissues or biofluids for which specific measures are not detectable. However, this can also complicate interpretation of findings from different studies using disparate measures. In this review we discuss the current state of LRRK2-focused biomarkers, the advantages and disadvantages of the current pallet of outcome measures, the gaps that need to be addressed, and the priorities that the field has defined.

Pharmacodynamic Biomarkers for Emerging LRRK2 Therapeutics

Genetic studies have identified variants in the LRRK2 gene as important components of Parkinson's disease (PD) pathobiology. Biochemical and emergent biomarker studies have coalesced around LRRK2 hyperactivation in disease. Therapeutics that diminish LRRK2 activity, either with small molecule kinase inhibitors or anti-sense oligonucleotides, have recently advanced to the clinic. Historically, there have been few successes in the development of therapies that might slow or halt the progression of neurodegenerative diseases. Over the past few decades of biomedical research, retrospective analyses suggest the broad integration of informative biomarkers early in development tends to distinguish successful pipelines from those that fail early. Herein, we discuss the biomarker regulatory process, emerging LRRK2 biomarker candidates, assays, underlying biomarker biology, and clinical integration.

Elevated In Vitro Kinase Activity in Peripheral Blood Mononuclear Cells of Leucine-Rich Repeat Kinase 2 G2019S Carriers: A Novel Enzyme-Linked Immunosorbent Assay-Based Method

Background

Leucine‐rich repeat kinase 2 kinase inhibitors are being vigorously pursued as potential therapeutic options; however, there is a critical need for sensitive and quantitative assays of leucine‐rich repeat kinase 2 function and target engagement.

Objectives

Our objective was to compare collection and storage protocols for peripheral blood mononuclear cells, and to determine the optimal conditions for downstream analyses of leucine‐rich repeat kinase 2 in PD cohorts.

Methods

Here, we describe enzyme‐linked immunosorbent assay–based assays capable of detecting multiple aspects of leucine‐rich repeat kinase 2 function at endogenous levels in human tissues.

Results

In peripheral blood mononuclear cells from both healthy and affected carriers of the G2019S mutation in leucine‐rich repeat kinase 2, we report, for the first time, significantly elevated in vitro kinase activity, while detecting a significant increase in pS935/leucine‐rich repeat kinase 2 in idiopathic PD patients.

Conclusions

Quantitative assays such as these described here could potentially uncover specific markers of leucine‐rich repeat kinase 2 function that are predictive of disease progression, aid in patient stratification, and be a critical component of upcoming clinical trials. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

The Future of Targeted Gene-Based Treatment Strategies and Biomarkers in Parkinson's Disease

Biomarkers and disease-modifying therapies are both urgent unmet medical needs in the treatment of Parkinson's disease (PD) and must be developed concurrently because of their interdependent relationship: biomarkers for the early detection of disease (i.e., prior to overt neurodegeneration) are necessary in order for patients to receive maximal therapeutic benefit and vice versa; disease-modifying therapies must become available for patients whose potential for disease diagnosis and prognosis can be predicted with biomarkers. This review provides an overview of the milestones achieved to date in the therapeutic strategy development of disease-modifying therapies and biomarkers for PD, with a focus on the most common and advanced genetically linked targets alpha-synuclein (SNCA), leucine-rich repeat kinase-2 (LRRK2) and glucocerebrosidase (GBA1). Furthermore, we discuss the convergence of the different pathways and the importance of patient stratification and how these advances may apply more broadly to idiopathic PD. The heterogeneity of PD poses a challenge for therapeutic and biomarker development, however, the one gene- one target approach has brought us closer than ever before to an unprecedented number of clinical trials and biomarker advancements.

LRRK2 Phosphorylation, More Than an Epiphenomenon

Mutations in the Leucine Rich Repeat Kinase 2 (LRRK2) gene are linked to autosomal dominant Parkinson's disease (PD), and genetic variations at the LRRK2 locus are associated with an increased risk for sporadic PD. This gene encodes a kinase that is physiologically multiphosphorylated, including clusters of both heterologous phosphorylation and autophosphorylation sites. Several pieces of evidence indicate that LRRK2's phosphorylation is important for its pathological and physiological functioning. These include a reduced LRRK2 heterologous phosphorylation in PD brains or after pharmacological inhibition of LRRK2 kinase activity as well as the appearance of subcellular LRRK2 accumulations when this protein is dephosphorylated at heterologous phosphosites. Nevertheless, the regulatory mechanisms governing LRRK2 phosphorylation levels and the cellular consequences of changes in LRRK2 phosphorylation remain incompletely understood. In this review, we present current knowledge on LRRK2 phosphorylation, LRRK2 phosphoregulation, and how LRRK2 phosphorylation changes affect cellular processes that may ultimately be linked to PD mechanisms.

What Have We Learned from Cerebrospinal Fluid Studies about Biomarkers for Detecting LRRK2 Parkinson's Disease Patients and Healthy Subjects with Parkinson's-Associated LRRK2 Mutations?

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common known cause of autosomal dominant Parkinson’s disease (PD) and sporadic PD (sPD). The clinical presentation of LRRK2 PD is similar to sPD, and except for genetic testing, no biochemical or imaging markers can differentiate LRRK2 PD from sPD. Discovery of such biomarkers could indicate neuropathological mechanisms that are unique to or increased in LRRK2 PD. This review discusses findings in 17 LRRK2 - related CSF studies found on PubMed. Most of these studies compared analyte concentrations between four diagnostic groups: LRRK2 PD patients, sPD patients, asymptomatic control subjects carrying PD-associated LRRK2 mutations (LRRK2 CTL), and healthy control subjects lacking LRRK2 mutations (CTL). Analytes examined in these studies included Aβ1-42, tau, α-synuclein, oxidative stress markers, autophagy-related proteins, pteridines, neurotransmitter metabolites, exosomal LRRK2 protein, RNA species, inflammatory cytokines, mitochondrial DNA (mtDNA), and intermediary metabolites. FINDINGS: Pteridines, α-synuclein, mtDNA, 5-hydroxyindolacetic acid, β-D-glucose, lamp2, interleukin-8, and vascular endothelial growth factor were suggested to differentiate LRRK2 PD from sPD patients; 8-hydroxy-2’-deoxyguanosine (8-OHdG), 8-isoprostane (8-ISO), 2-hydroxybutyrate, mtDNA, lamp2, and neopterin may differentiate between LRRK2 CTL and LRRK2 PD subjects; and soluble oligomeric α-synuclein, 8-OHdG, and 8-ISO might differentiate LRRK2 CTL from CTL subjects. CONCLUSIONS: The low numbers of investigations of each analyte, small sample sizes, and methodological differences limit conclusions that can be drawn from these studies. Further investigations are indicated to determine the validity of the analytes identified in these studies as possible biomarkers for LRRK2 PD patients and/or LRRK2 CTL subjects.

Long-Term Outcomes of Genetic Parkinson's Disease

Parkinson’s disease (PD) is a progressive neurodegenerative disorder that affects 1–2% of people by the age of 70 years. Age is the most important risk factor, and most cases are sporadic without any known environmental or genetic causes. Since the late 1990s, mutations in the genes SNCA, PRKN, LRRK2, PINK1, DJ-1, VPS35, and GBA have been shown to be important risk factors for PD. In addition, common variants with small effect sizes are now recognized to modulate the risk for PD. Most studies in genetic PD have focused on finding new genes, but few have studied the long-term outcome of patients with the specific genetic PD forms. Patients with known genetic PD have now been followed for more than 20 years, and we see that they may have distinct and different prognoses. New therapeutic possibilities are emerging based on the genetic cause underlying the disease. Future medication may be based on the pathophysiology individualized to the patient’s genetic background. The challenge is to find the biological consequences of different genetic variants. In this review, the clinical patterns and long-term prognoses of the most common genetic PD variants are presented.

Quantitative Measurements of LRRK2 in Human Cerebrospinal Fluid Demonstrates Increased Levels in G2019S Patients

Leucine-rich repeat kinase 2 (LRRK2) mutations are among the most significant genetic risk factors for developing late onset Parkinson’s disease (PD). To understand whether a therapeutic can modulate LRRK2 levels as a potential disease modifying strategy, it is important to have methods in place to measure the protein with high sensitivity and specificity. To date, LRRK2 measurements in cerebrospinal fluid (CSF) have used extracellular vesicle enrichment via differential ultracentrifugation and western blot detection. Our goal was to develop a methodology which could be deployed in a clinical trial, therefore throughput, robustness and sensitivity were critical. To this end, we developed a Stable Isotope Standard Capture by Anti-peptide Antibody (SISCAPA) assay which is capable of detecting LRRK2 from 1 ml of human CSF. The assay uses a commercially available LRRK2 monoclonal antibody (N241A/34) and does not require extracellular vesicle enrichment steps. The assay includes stable isotope peptide addition which allows for absolute quantitation of LRRK2 protein. We determined that the assay performed adequately for CSF measurements and that blood contamination from traumatic lumbar puncture does not pose a serious analytical challenge. We then applied this technique to 106 CSF samples from the MJFF LRRK2 Cohort Consortium which includes healthy controls, sporadic PD patients and LRRK2 mutation carriers with and without PD. Of the 105 samples that had detectable LRRK2 signal, we found that the PD group with the G2019S LRRK2 mutation had significantly higher CSF LRRK2 levels compared to all other groups. We also found that CSF LRRK2 increased with the age of the participant. Taken together, this work represents a step forward in our ability to measure LRRK2 in a challenging matrix like CSF which has implications for current and future LRRK2 therapeutic clinical trials.

Potential Roles of Exosomes in Parkinson's Disease: From Pathogenesis, Diagnosis, and Treatment to Prognosis

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease in the world, after Alzheimer's disease (AD), affecting approximately 1% of people over 65 years of age. Exosomes were once considered to be cellular waste and functionless. However, our understanding about exosome function has increased, and exosomes have been found to carry specific proteins, lipids, functional messenger RNAs (mRNAs), high amounts of non-coding RNAs (including microRNAs, lncRNAs, and circRNAs) and other bioactive substances. Exosomes have been shown to be involved in many physiological processes in vivo, including intercellular communication, cell migration, angiogenesis, and anti-tumor immunity. Moreover, exosomes may be pivotal in the occurrence and progression of various diseases. Therefore, exosomes have several diverse potential applications due to their unique structure and function. For instance, exosomes may be used as biological markers for the diagnosis and prognosis of various diseases, or as a natural carrier of drugs for clinical treatment. Here, we review the potential roles of exosomes in the pathogenesis, diagnosis, treatment, and prognosis of PD.