The G2385R variant of leucine-rich repeat kinase 2 associated with Parkinson's disease is a partial loss-of-function mutation

14-3-3 phosphorylation inhibits 14-3-3θ's ability to regulate LRRK2 kinase activity and toxicity

LRRK2 mutations are among the most common genetic causes for Parkinson's disease (PD), and toxicity is associated with increased kinase activity. 14-3-3 proteins are key interactors that regulate LRRK2 kinase activity. Phosphorylation of the 14-3-3θ isoform at S232 is dramatically increased in human PD brains. Here we investigate the impact of 14-3-3θ phosphorylation on its ability to regulate LRRK2 kinase activity. Both wildtype and the non-phosphorylatable S232A 14-3-3θ mutant reduced the kinase activity of wildtype and G2019S LRRK2, whereas the phosphomimetic S232D 14-3-3θ mutant had minimal effects on LRRK2 kinase activity, as determined by measuring autophosphorylation at S1292 and T1503 and Rab10 phosphorylation. However, wildtype and both 14-3-3θ mutants similarly reduced the kinase activity of the R1441G LRRK2 mutant. 14-3-3θ phosphorylation did not promote global dissociation with LRRK2, as determined by co-immunoprecipitation and proximal ligation assays. 14-3-3s interact with LRRK2 at several phosphorylated serine/threonine sites, including T2524 in the C-terminal helix, which can fold back to regulate the kinase domain. Interaction between 14-3-3θ and phosphorylated T2524 LRRK2 was important for 14-3-3θ's ability to regulate kinase activity, as wildtype and S232A 14-3-3θ failed to reduce the kinase activity of G2019S/T2524A LRRK2. Finally, we found that the S232D mutation failed to protect against G2019S LRRK2-induced neurite shortening in primary cultures, while the S232A mutation was protective. We conclude that 14-3-3θ phosphorylation destabilizes the interaction of 14-3-3θ with LRRK2 at T2524, which consequently promotes LRRK2 kinase activity and toxicity.

Emerging perspectives on precision therapy for Parkinson's disease: multidimensional evidence leading to a new breakthrough in personalized medicine

PD is a prevalent and progressive neurodegenerative disorder characterized by both motor and non-motor symptoms. Genes play a significant role in the onset and progression of the disease. While the complexity and pleiotropy of gene expression networks have posed challenges for gene-targeted therapies, numerous pathways of gene variant expression show promise as therapeutic targets in preclinical studies, with some already in clinical trials. With the recognition of the numerous genes and complex pathways that can influence PD, it may be possible to take a novel approach to choose a treatment for the condition. This approach would be based on the symptoms, genomics, and underlying mechanisms of the disease. We discuss the utilization of emerging genetic and pathological knowledge of PD patients to categorize the disease into subgroups. Our long-term objective is to generate new insights for the therapeutic approach to the disease, aiming to delay and treat it more effectively, and ultimately reduce the burden on individuals and society.

Cell-autonomous role of leucine-rich repeat kinase in the protection of dopaminergic neuron survival

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease (PD). However, whether LRRK2 mutations cause PD and degeneration of dopaminergic (DA) neurons via a toxic gain-of-function or a loss-of-function mechanism is unresolved and has pivotal implications for LRRK2-based PD therapies. In this study, we investigate whether Lrrk2 and its functional homolog Lrrk1 play a cell-intrinsic role in DA neuron survival through the development of DA neuron-specific Lrrk conditional double knockout (cDKO) mice. Unlike Lrrk germline DKO mice, DA neuron-restricted Lrrk cDKO mice exhibit normal mortality but develop age-dependent loss of DA neurons, as shown by the progressive reduction of DA neurons in the substantia nigra pars compacta (SNpc) at the ages of 20 and 24 months. Moreover, DA neurodegeneration is accompanied with increases in apoptosis and elevated microgliosis in the SNpc as well as decreases in DA terminals in the striatum, and is preceded by impaired motor coordination. Taken together, these findings provide the unequivocal evidence for the cell-intrinsic requirement of LRRK in DA neurons and raise the possibility that LRRK2 mutations may impair its protection of DA neurons, leading to DA neurodegeneration in PD.

Cell autonomous role of leucine-rich repeat kinase in protection of dopaminergic neuron survival

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease (PD), which is the leading neurodegenerative movement disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). However, whether LRRK2 mutations cause PD and degeneration of DA neurons via a toxic gain-of-function or a loss-of-function mechanism is unresolved and has pivotal implications for LRRK2-based PD therapies. In this study, we investigate whether LRRK2 and its functional homologue LRRK1 play an essential, intrinsic role in DA neuron survival through the development of DA neuron-specific LRRK conditional double knockout (cDKO) mice. We first generated and characterized floxed LRRK1 and LRRK2 mice and then confirmed that germline deletions of the floxed LRRK1 and LRRK2 alleles result in null mutations, as evidenced by the absence of LRRK1 and LRRK2 mRNA and protein in the respective homozygous deleted mutant mice. We further examined the specificity of Cre-mediated recombination driven by the dopamine transporter-Cre (DAT-Cre) knockin (KI) allele using a GFP reporter line and confirmed that DAT-Cre-mediated recombination is restricted to DA neurons in the SNpc. Crossing these validated floxed LRRK1 and LRRK2 mice with DAT-Cre KI mice, we then generated DA neuron-restricted LRRK cDKO mice and further showed that levels of LRRK1 and LRRK2 are reduced in dissected ventral midbrains of LRRK cDKO mice. While DA neuron-restricted LRRK cDKO mice of both sexes exhibit normal mortality and body weight, they develop age-dependent loss of DA neurons in the SNpc, as demonstrated by the progressive reduction of DA neurons in the SNpc of LRRK cDKO mice at the ages of 20 and 24 months but the unaffected number of DA neurons at the age of 15 months. Moreover, DA neurodegeneration is accompanied with increases of apoptosis and elevated microgliosis in the SNpc as well as decreases of DA terminals in the striatum, and is preceded by impaired motor coordination. Taken together, these findings provide the unequivocal evidence for the importance of LRRK in DA neurons and raise the possibility that LRRK2 mutations may impair its protection of DA neurons, leading to DA neurodegeneration in PD.

Role of the leucine-rich repeat protein kinase 2 C-terminal tail in domain cross-talk

Leucine-rich repeat protein kinase 2 (LRRK2) is a multi-domain protein encompassing two of biology's most critical molecular switches, a kinase and a GTPase, and mutations in LRRK2 are key players in the pathogenesis of Parkinson's disease (PD). The availability of multiple structures (full-length and truncated) has opened doors to explore intra-domain cross-talk in LRRK2. A helix extending from the WD40 domain and stably docking onto the kinase domain is common in all available structures. This C-terminal (Ct) helix is a hub of phosphorylation and organelle-localization motifs and thus serves as a multi-functional protein : protein interaction module. To examine its intra-domain interactions, we have recombinantly expressed a stable Ct motif (residues 2480-2527) and used peptide arrays to identify specific binding sites. We have identified a potential interaction site between the Ct helix and a loop in the CORB domain (CORB loop) using a combination of Gaussian accelerated molecular dynamics simulations and peptide arrays. This Ct-Motif contains two auto-phosphorylation sites (T2483 and T2524), and T2524 is a 14-3-3 binding site. The Ct helix, CORB loop, and the CORB-kinase linker together form a part of a dynamic 'CAP' that regulates the N-lobe of the kinase domain. We hypothesize that in inactive, full-length LRRK2, the Ct-helix will also mediate interactions with the N-terminal armadillo, ankyrin, and LRR domains (NTDs) and that binding of Rab substrates, PD mutations, or kinase inhibitors will unleash the NTDs.

Heat Shock Protein 90 in Parkinson's Disease: Profile of a Serial Killer

Parkinson's disease (PD) is the second most common neurodegenerative disease, characterized by abnormal α-synuclein misfolding and aggregation, mitochondrial dysfunction, oxidative stress, as well as progressive death of dopaminergic neurons in the substantia nigra. Molecular chaperones play a role in stabilizing proteins and helping them achieve their proper structure. Previous studies have shown that overexpression of heat shock protein 90 (HSP90) can lead to the death of dopaminergic neurons associated with PD. Inhibiting HSP90 is considered a potential treatment approach for neurodegenerative disorders, as it may reduce protein aggregation and related toxicity, as well as suppress various forms of regulated cell death (RCD). This review provides an overview of HSP90 and its role in PD, focusing on its modulation of proteostasis and quality control of LRRK2. The review also explores the effects of HSP90 on different types of RCD, such as apoptosis, chaperone-mediated autophagy (CMA), necroptosis, and ferroptosis. Additionally, it discusses HSP90 inhibitors that have been tested in PD models. We will highlight the under-investigated neuroprotective effects of HSP90 inhibition, including modulation of oxidative stress, mitochondrial dysfunction, PINK/PARKIN, heat shock factor 1 (HSF1), histone deacetylase 6 (HDAC6), and the PHD2-HSP90 complex-mediated mitochondrial stress pathway. By examining previous literature, this review uncovers overlooked neuroprotective mechanisms and emphasizes the need for further research on HSP90 inhibitors as potential therapeutic strategies for PD. Finally, the review discusses the potential limitations and possibilities of using HSP90 inhibitors in PD therapy.

The Parkinson's disease-associated mutation LRRK2 G2385R alters mitochondrial biogenesis via the PGC-1α-TFAM pathway

Mutations in the Leucine-rich repeat protein kinase 2 (LRRK2) gene are the most frequent cause of familial Parkinson's disease (PD). Although LRRK2 has been extensively studied, the pathogenic mechanism of the LRRK2 G2385R mutation, which is most common in Asian populations, especially in the Chinese Han population, remains unclear. In this study, we demonstrated that the LRRK2 G2385R mutation in HEK293T cells led to a reduction in cellular PGC-1α protein expression and inhibition of mitochondrial biogenesis through the PGC-1α-TFAM pathway. This resulted in a decrease in mitochondrial genome expression, which in turn impaired the normal electron transfer process of the oxidative phosphorylation respiratory chain, leading to mitochondrial dysfunction and onset of apoptosis. The mitochondrial dysfunction and apoptosis caused by the LRRK2 G2385R mutation were significantly alleviated by antioxidant Idebenone, which provides a theoretical basis for the subsequent development of precise treatment specifically for PD patients with LRRK2 G2385R mutation. Further validation of our findings in neurons and animal models are necessary.

Regulation of LRRK2 mRNA stability by ATIC and its substrate AICAR through ARE-mediated mRNA decay in Parkinson's disease

Mutations in LRRK2 are the most common genetic causes of Parkinson's disease (PD). While the enzymatic activity of LRRK2 has been linked to PD, previous work has also provided support for an important role of elevated LRRK2 protein levels, independent of enzymatic activity, in PD pathogenesis. However, the mechanisms underlying the regulation of LRRK2 protein levels remain unclear. Here, we identify a role for the purine biosynthesis pathway enzyme ATIC in the regulation of LRRK2 levels and toxicity. AICAr, the precursor of ATIC substrate, regulates LRRK2 levels in a cell-type-specific manner in vitro and in mouse tissue. AICAr regulates LRRK2 levels through AUF1-mediated mRNA decay. Upon AICAr treatment, the RNA binding protein AUF1 is recruited to the AU-rich elements (ARE) of LRRK2 mRNA leading to the recruitment of the decapping enzyme complex DCP1/2 and decay of LRRK2 mRNA. AICAr suppresses LRRK2 expression and rescues LRRK2-induced dopaminergic neurodegeneration and neuroinflammation in PD Drosophila and mouse models. Together, this study provides insight into a novel regulatory mechanism of LRRK2 protein levels and function via LRRK2 mRNA decay that is distinct from LRRK2 enzymatic functions.

14-3-3 phosphorylation inhibits 14-3-3θ's ability to regulate LRRK2 kinase activity

LRRK2 mutations are among the most common genetic causes for Parkinson's disease (PD), and toxicity is associated with increased kinase activity. 14-3-3 proteins are key interactors that regulate LRRK2 kinase activity. Phosphorylation of the 14-3-3θ isoform at S232 is dramatically increased in human PD brains. Here we investigate the impact of 14-3-3θ phosphorylation on its ability to regulate LRRK2 kinase activity. Both wildtype and the non-phosphorylatable S232A 14-3-3θ mutant reduced the kinase activity of wildtype and G2019S LRRK2, whereas the phosphomimetic S232D 14-3-3θ mutant had minimal effects on LRRK2 kinase activity, as determined by measuring autophosphorylation at S1292 and T1503 and Rab10 phosphorylation. However, wildtype and both 14-3-3θ mutants similarly reduced the kinase activity of the R1441G LRRK2 mutant. 14-3-3θ phosphorylation did not promote global dissociation with LRRK2, as determined by co-immunoprecipitation and proximal ligation assays. 14-3-3s interact with LRRK2 at several phosphorylated serine/threonine sites, including T2524 in the C-terminal helix, which can fold back to regulate the kinase domain. Interaction between 14-3-3θ and phosphorylated T2524 LRRK2 was important for 14-3-3θ's ability to regulate kinase activity, as wildtype and S232A 14-3-3θ failed to reduce the kinase activity of G2019S/T2524A LRRK2. Molecular modeling showed that 14-3-3θ phosphorylation causes a partial rearrangement of its canonical binding pocket, thus affecting the interaction between 14-3-3θ and the C-terminus of LRRK2. We conclude that 14-3-3θ phosphorylation destabilizes the interaction of 14-3-3θ with LRRK2 at T2524, which consequently promotes LRRK2 kinase activity.

Capturing the domain crosstalk in full length LRRK2 and LRRK2RCKW

LRRK2 is a multi-domain protein with three catalytically inert N-terminal domains (NtDs) and four C-terminal domains, including a kinase and a GTPase domain. LRRK2 mutations are linked to Parkinson's Disease. Recent structures of LRRK2RCKW and a full-length inactive LRRK2 (fl-LRRK2INACT) monomer revealed that the kinase domain drives LRRK2 activation. The LRR domain and also an ordered LRR- COR linker, wrap around the C-lobe of the kinase domain and sterically block the substrate binding surface in fl-LRRK2INACT. Here we focus on the crosstalk between domains. Our biochemical studies of GTPase and kinase activities of fl-LRRK2 and LRRK2RCKW reveal how mutations influence this crosstalk differently depending on the domain borders investigated. Furthermore, we demonstrate that removing the NtDs leads to altered intramolecular regulation. To further investigate the crosstalk, we used Hydrogen-Deuterium exchange Mass Spectrometry (HDX-MS) to characterize the conformation of LRRK2RCKW   and Gaussian Accelerated Molecular Dynamics (GaMD) to create dynamic portraits of fl-LRRK2 and LRRK2RCKW. These models allowed us to investigate the dynamic changes in wild type and mutant LRRK2s. Our data show that the a3ROC helix, the Switch II motif in the ROC domain, and the LRR-ROC linker play crucial roles in mediating local and global conformational changes. We demonstrate how these regions are affected by other domains in fl-LRRK2 and LRRK2RCKW and show how unleashing of the NtDs as well as PD mutations lead to changes in conformation and dynamics of the ROC and kinase domains which ultimately impact kinase and GTPase activities. These allosteric sites are potential therapeutic targets.

The Development and Design Strategy of Leucine-Rich Repeat Kinase 2 Inhibitors: Promising Therapeutic Agents for Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder affecting millions of people worldwide. Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) are the most common genetic risk factor for PD. Elevated LRRK2 kinase activity is found in idiopathic and familial PD cases. LRRK2 mutations are involved in multiple PD pathogeneses, including dysregulation of mitochondrial homeostasis, ciliogenesis, etc. Here, we provide a comprehensive overview of the biological function, structure, and mutations of LRRK2. We also examine recent advances and challenges in developing LRRK2 inhibitors and address prospective protein-based targeting strategies. The binding mechanisms, structure-activity relationships, and pharmacokinetic features of inhibitors are emphasized to provide a comprehensive compendium on the rational design of LRRK2 inhibitors. We hope that this publication can serve as a guide for designing novel LRRK2 inhibitors based on the summarized facts and perspectives.

Structural Insights and Development of LRRK2 Inhibitors for Parkinson's Disease in the Last Decade

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease, characterized by the specific loss of dopaminergic neurons in the midbrain. The pathophysiology of PD is likely caused by a variety of environmental and hereditary factors. Many single-gene mutations have been linked to this disease, but a significant number of studies indicate that mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) are a potential therapeutic target for both sporadic and familial forms of PD. Consequently, the identification of potential LRRK2 inhibitors has been the focus of drug discovery. Various investigations have been conducted in academic and industrial organizations to investigate the mechanism of LRRK2 in PD and further develop its inhibitors. This review summarizes the role of LRRK2 in PD and its structural details, especially the kinase domain. Furthermore, we reviewed in vitro and in vivo findings of selected inhibitors reported to date against wild-type and mutant versions of the LRRK2 kinase domain as well as the current trends researchers are employing in the development of LRRK2 inhibitors.

Pathophysiological evaluation of the LRRK2 G2385R risk variant for Parkinson’s disease

Missense variants in leucine-rich repeat kinase 2 (LRRK2) lead to familial and sporadic Parkinson’s disease (PD). The pathological features of PD patients with LRRK2 variants differ. Here, we report an autopsy case harboring the LRRK2 G2385R, a risk variant for PD occurring mainly in Asian populations. The patient exhibited levodopa-responsive parkinsonism at the early stage and visual hallucinations at the advanced stage. The pathological study revealed diffuse Lewy bodies with neurofibrillary tangles, amyloid plaques, and mild signs of neuroinflammation. Biochemically, detergent-insoluble phospho-α-synuclein was accumulated in the frontal, temporal, entorhinal cortexes, and putamen, consistent with the pathological observations. Elevated phosphorylation of Rab10, a substrate of LRRK2, was also prominent in various brain regions. In conclusion, G2385R appears to increase LRRK2 kinase activity in the human brain, inducing a deleterious brain environment that causes Lewy body pathology.

LRRK2 and Proteostasis in Parkinson's Disease

Parkinson's disease is a neurodegenerative condition initially characterized by the presence of tremor, muscle stiffness and impaired balance, with the deposition of insoluble protein aggregates in Lewy's Bodies the histopathological hallmark of the disease. Although different gene variants are linked to Parkinson disease, mutations in the Leucine-Rich Repeat Kinase 2 (LRRK2) gene are one of the most frequent causes of Parkinson's disease related to genetic mutations. LRRK2 toxicity has been mainly explained by an increase in kinase activity, but alternative mechanisms have emerged as underlying causes for Parkinson's disease, such as the imbalance in LRRK2 homeostasis and the involvement of LRRK2 in aggregation and spreading of α-synuclein toxicity. In this review, we recapitulate the main LRRK2 pathological mutations that contribute to Parkinson's disease and the different cellular and therapeutic strategies devised to correct LRRK2 homeostasis. In this review, we describe the main cellular control mechanisms that regulate LRRK2 folding and aggregation, such as the chaperone network and the protein-clearing pathways such as the ubiquitin-proteasome system and the autophagic-lysosomal pathway. We will also address the more relevant strategies to modulate neurodegeneration in Parkinson's disease through the regulation of LRRK2, using small molecules or LRRK2 silencing.

LRRK2 dynamics analysis identifies allosteric control of the crosstalk between its catalytic domains

The 2 major molecular switches in biology, kinases and GTPases, are both contained in the Parkinson disease-related leucine-rich repeat kinase 2 (LRRK2). Using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and molecular dynamics (MD) simulations, we generated a comprehensive dynamic allosteric portrait of the C-terminal domains of LRRK2 (LRRK2RCKW). We identified 2 helices that shield the kinase domain and regulate LRRK2 conformation and function. One helix in COR-B (COR-B Helix) tethers the COR-B domain to the αC helix of the kinase domain and faces its activation loop, while the C-terminal helix (Ct-Helix) extends from the WD40 domain and interacts with both kinase lobes. The Ct-Helix and the N-terminus of the COR-B Helix create a "cap" that regulates the N-lobe of the kinase domain. Our analyses reveal allosteric sites for pharmacological intervention and confirm the kinase domain as the central hub for conformational control.

Deletion of lrrk2 causes early developmental abnormalities and age-dependent increase of monoamine catabolism in the zebrafish brain

LRRK2 gain-of-function is considered a major cause of Parkinson's disease (PD) in humans. However, pathogenicity of LRRK2 loss-of-function in animal models is controversial. Here we show that deletion of the entire zebrafish lrrk2 locus elicits a pleomorphic transient brain phenotype in maternal-zygotic mutant embryos (mzLrrk2). In contrast to lrrk2, the paralog gene lrrk1 is virtually not expressed in the brain of both wild-type and mzLrrk2 fish at different developmental stages. Notably, we found reduced catecholaminergic neurons, the main target of PD, in specific cell populations in the brains of mzLrrk2 larvae, but not adult fish. Strikingly, age-dependent accumulation of monoamine oxidase (MAO)-dependent catabolic signatures within mzLrrk2 brains revealed a previously undescribed interaction between LRRK2 and MAO biological activities. Our results highlight mzLrrk2 zebrafish as a tractable tool to study LRRK2 loss-of-function in vivo, and suggest a link between LRRK2 and MAO, potentially of relevance in the prodromic stages of PD.

From structure to ætiology: a new window on the biology of leucine-rich repeat kinase 2 and Parkinson's disease

Since the discovery of mutations in leucine-rich repeat kinase 2 (LRRK2) as an underlying genetic cause for the development of Parkinson's disease (PD) in 2004 (Neuron 44, 601-607; Neuron 44, 595-600), and subsequent efforts to develop LRRK2 kinase inhibitors as a therapy for Parkinson's (Expert Opin. Ther. Targets 21, 751-753), elucidating the atomic resolution structure of LRRK2 has been a major goal of research into this protein. At over 250 kDa, the large size and complicated domain organisation of LRRK2 has made this a highly challenging target for structural biologists, however, a number of recent studies using both in vitro and in situ approaches (Nature 588, 344-349; Cell 182, 1508-1518.e1516; Cell 184, 3519-3527.e3510) have provided important new insights into LRRK2 structure and the complexes formed by this protein.

The development of inhibitors of leucine-rich repeat kinase 2 (LRRK2) as a therapeutic strategy for Parkinson's disease: the current state of play

Current therapeutic approaches for Parkinson's disease (PD) are based around treatments that alleviate symptoms but do not slow or prevent disease progression. As such, alternative strategies are needed. A promising approach is the use of molecules that reduce the function of leucine-rich repeat kinase (LRRK2). Gain-of-function mutations in LRRK2 account for a notable proportion of familial Parkinson's disease cases, and significantly, elevated LRRK2 kinase activity is reported in idiopathic Parkinson's disease. Here, we describe progress in finding therapeutically effective LRRK2 inhibitors, summarising studies that range from in vitro experiments to clinical trials. LRRK2 is a complex protein with two enzymatic activities and a myriad of functions. This creates opportunities for a rich variety of strategies and also increases the risk of unintended consequences. We comment on the strength and limitations of the different approaches and conclude that with two molecules under clinical trial and a diversity of alternative options in the pipeline, there is cause for optimism.

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.

Pathogenic LRRK2 requires secondary factors to induce cellular toxicity

Pathogenic mutations in the leucine-rich repeat kinase 2 (LRRK2) gene belong to the most common genetic causes of inherited Parkinson’s disease (PD) and variations in its locus increase the risk to develop sporadic PD. Extensive research efforts aimed at understanding how changes in the LRRK2 function result in molecular alterations that ultimately lead to PD. Cellular LRRK2-based models revealed several potential pathophysiological mechanisms including apoptotic cell death, LRRK2 protein accumulation and deficits in neurite outgrowth. However, highly variable outcomes between different cellular models have been reported. Here, we have investigated the effect of different experimental conditions, such as the use of different tags and gene transfer methods, in various cellular LRRK2 models. Readouts included cell death, sensitivity to oxidative stress, LRRK2 relocalization, α-synuclein aggregation and neurite outgrowth in cell culture, as well as neurite maintenance in vivo. We show that overexpression levels and/or the tag fused to LRRK2 affect the relocalization of LRRK2 to filamentous and skein-like structures. We found that overexpression of LRRK2 per se is not sufficient to induce cellular toxicity or to affect α-synuclein-induced toxicity and aggregate formation. Finally, neurite outgrowth/retraction experiments in cell lines and in vivo revealed that secondary, yet unknown, factors are required for the pathogenic LRRK2 effects on neurite length. Our findings stress the importance of technical and biological factors in LRRK2-induced cellular phenotypes and hence imply that conclusions based on these types of LRRK2-based assays should be interpreted with caution.

The cell biology of Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder resulting from the death of dopamine neurons in the substantia nigra pars compacta. Our understanding of PD biology has been enriched by the identification of genes involved in its rare, inheritable forms, termed PARK genes. These genes encode proteins including α-syn, LRRK2, VPS35, parkin, PINK1, and DJ1, which can cause monogenetic PD when mutated. Investigating the cellular functions of these proteins has been instrumental in identifying signaling pathways that mediate pathology in PD and neuroprotective mechanisms active during homeostatic and pathological conditions. It is now evident that many PD-associated proteins perform multiple functions in PD-associated signaling pathways in neurons. Furthermore, several PARK proteins contribute to non-cell-autonomous mechanisms of neuron death, such as neuroinflammation. A comprehensive understanding of cell-autonomous and non-cell-autonomous pathways involved in PD is essential for developing therapeutics that may slow or halt its progression.

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.

Somatic Mutations in LRRK2 Identify a Subset of Invasive Mammary Carcinomas Associated with High Mutation Burden

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common cause of familial Parkinson disease. Although LRRK2-related Parkinson disease patients have a heightened risk of certain nonskin cancers, including breast cancer, it is unknown whether LRRK2 somatic mutations occur and are associated with breast cancer. The objective of this study was to evaluate the occurrence of LRRK2 somatic mutations in breast cancer and the clinicopathologic features associated with LRRK2-mutated tumors. Using The Cancer Genome Atlas Breast Cancer Project, somatic LRRK2 DNA sequence information was obtained for 93 cases, of which 17 cases (18%) with 18 mutations were identified. LRRK2-mutated mammary carcinomas are enriched with stop-gain, truncating mutations predicted to result in loss of function; missense mutations frequently targeted the GTPase and kinase domains. Tumors displayed predominantly high-grade morphology with abundant granular eosinophilic cytoplasm, resembling mitochondria-rich apocrine-like carcinomas. Exploration of the genomic landscape of LRRK2-mutated carcinomas yielded frequent TP53 deactivation and a remarkably high tumor mutation burden. More important, breast cancers with LRRK2 mutations are associated with reduced patient survival compared with The Cancer Genome Atlas Breast Cancer Project cohort. These findings, for the first time, show that somatic LRRK2 mutations occur frequently in breast cancer, and the high mutation burden seen in this subset of tumors suggests that LRRK2 mutations may herald benefit from immune checkpoint inhibition.

CHIP as a therapeutic target for neurological diseases

Carboxy-terminus of Hsc70-interacting protein (CHIP) functions both as a molecular co-chaperone and ubiquitin E3 ligase playing a critical role in modulating the degradation of numerous chaperone-bound proteins. To date, it has been implicated in the regulation of numerous biological functions, including misfolded-protein refolding, autophagy, immunity, and necroptosis. Moreover, the ubiquitous expression of CHIP in the central nervous system suggests that it may be implicated in a wide range of functions in neurological diseases. Several recent studies of our laboratory and other groups have highlighted the beneficial role of CHIP in the pathogenesis of several neurological diseases. The objective of this review is to discuss the possible molecular mechanisms that contribute to the pathogenesis of neurological diseases in which CHIP has a pivotal role, such as stroke, intracerebral hemorrhage, Alzheimer's disease, Parkinson's disease, and polyglutamine diseases; furthermore, CHIP mutations could also cause neurodegenerative diseases. Based on the available literature, CHIP overexpression could serve as a promising therapeutic target for several neurological diseases.

The In Situ Structure of Parkinson's Disease-Linked LRRK2

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most frequent cause of familial Parkinson's disease. LRRK2 is a multi-domain protein containing a kinase and GTPase. Using correlative light and electron microscopy, in situ cryo-electron tomography, and subtomogram analysis, we reveal a 14-Å structure of LRRK2 bearing a pathogenic mutation that oligomerizes as a right-handed double helix around microtubules, which are left-handed. Using integrative modeling, we determine the architecture of LRRK2, showing that the GTPase and kinase are in close proximity, with the GTPase closer to the microtubule surface, whereas the kinase is exposed to the cytoplasm. We identify two oligomerization interfaces mediated by non-catalytic domains. Mutation of one of these abolishes LRRK2 microtubule-association. Our work demonstrates the power of cryo-electron tomography to generate models of previously unsolved structures in their cellular environment.

From Synaptic Dysfunction to Neuroprotective Strategies in Genetic Parkinson's Disease: Lessons From LRRK2

The pathogenesis of Parkinson’s disease (PD) is thought to rely on a complex interaction between the patient’s genetic background and a variety of largely unknown environmental factors. In this scenario, the investigation of the genetic bases underlying familial PD could unveil key molecular pathways to be targeted by new disease-modifying therapies, still currently unavailable. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are responsible for the majority of inherited familial PD cases and can also be found in sporadic PD, but the pathophysiological functions of LRRK2 have not yet been fully elucidated. Here, we will review the evidence obtained in transgenic LRRK2 experimental models, characterized by altered striatal synaptic transmission, mitochondrial dysfunction, and α-synuclein aggregation. Interestingly, the processes triggered by mutant LRRK2 might represent early pathological phenomena in the pathogenesis of PD, anticipating the typical neurodegenerative features characterizing the late phases of the disease. A comprehensive view of LRRK2 neuronal pathophysiology will support the possible clinical application of pharmacological compounds targeting this protein, with potential therapeutic implications for patients suffering from both familial and sporadic PD.

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.

Physiological and pathological roles of LRRK2 in the nuclear envelope integrity

Mutations in LRRK2 cause autosomal dominant and sporadic Parkinson's disease, but the mechanisms involved in LRRK2 toxicity in PD are yet to be fully understood. We found that LRRK2 translocates to the nucleus by binding to seven in absentia homolog (SIAH-1), and in the nucleus it directly interacts with lamin A/C, independent of its kinase activity. LRRK2 knockdown caused nuclear lamina abnormalities and nuclear disruption. LRRK2 disease mutations mostly abolish the interaction with lamin A/C and, similar to LRRK2 knockdown, cause disorganization of lamin A/C and leakage of nuclear proteins. Dopaminergic neurons of LRRK2 G2019S transgenic and LRRK2 -/- mice display decreased circularity of the nuclear lamina and leakage of the nuclear protein 53BP1 to the cytosol. Dopaminergic nigral and cortical neurons of both LRRK2 G2019S and idiopathic PD patients exhibit abnormalities of the nuclear lamina. Our data indicate that LRRK2 plays an essential role in maintaining nuclear envelope integrity. Disruption of this function by disease mutations suggests a novel phosphorylation-independent loss-of-function mechanism that may synergize with other neurotoxic effects caused by LRRK2 mutations.

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.

Binding of the Human 14-3-3 Isoforms to Distinct Sites in the Leucine-Rich Repeat Kinase 2

Proteins of the 14-3-3 family are well known modulators of the leucine-rich repeat kinase 2 (LRRK2) regulating kinase activity, cellular localization, and ubiquitylation. Although binding between those proteins has been investigated, a comparative study of all human 14-3-3 isoforms interacting with LRRK2 is lacking so far. In a comprehensive approach, we quantitatively analyzed the interaction between the seven human 14-3-3 isoforms and LRRK2-derived peptides covering both, reported and putative 14-3-3 binding sites. We observed that phosphorylation is an absolute prerequisite for 14-3-3 binding and generated binding patterns of 14-3-3 isoforms to interact with peptides derived from the N-terminal phosphorylation cluster (S910 and S935), the Roc domain (S1444) and the C-terminus. The tested 14-3-3 binding sites in LRRK2 preferentially were recognized by the isoforms γ and η, whereas the isoforms ϵ and especially σ showed the weakest or no binding. Interestingly, the possible pathogenic mutation Q930R in LRRK2 drastically increases binding affinity to a peptide encompassing pS935. We then identified the autophosphorylation site T2524 as a so far not described 14-3-3 binding site at the very C-terminus of LRRK2. Binding affinities of all seven 14-3-3 isoforms were quantified for all three binding regions with pS1444 displaying the highest affinity of all measured singly phosphorylated peptides. The strongest binding was detected for the combined phosphosites S910 and S935, suggesting that avidity effects are important for high affinity interaction between 14-3-3 proteins and LRRK2.

The LRRK2 N-terminal domain influences vesicle trafficking: impact of the E193K variant

The LRRK2 protein consists of multiple functional domains, including protein-binding domains at its N and C-terminus. Mutations in the Leucine-rich repeat kinase 2 gene (LRRK2) have been linked to familial and sporadic Parkinson's disease (PD). We have recently described a novel variant falling within the N-terminal armadillo repeats, E193K. Herein, our aim is to investigate the functional impact of LRRK2 N-terminal domain and the E193K variant on vesicle trafficking. By combining Total Internal Reflection Fluorescence (TIRF) microscopy and a synaptopHluorin assay, we found that expression of a construct lacking the N-terminal domain increases the frequency and amplitude of spontaneous synaptic events. Complementary biochemical approaches showed that the E193K variant alters the binding properties of LRRK2, decreases LRRK2 binding to synaptic vesicles, and promotes vesicle fusion. Our results confirm the physiological and pathological relevance of the nature of the LRRK2-associated macro-molecular complex solidifying the idea that different pathological mutations critically alter the scaffolding function of LRRK2 resulting in a perturbation of the vesicular trafficking as a common denominator.

Neuroprotection by the Immunomodulatory Drug Pomalidomide in the Drosophila LRRK2WD40 Genetic Model of Parkinson's Disease

The search for new disease-modifying drugs for Parkinson's disease (PD) is a slow and highly expensive process, and the repurposing of drugs already approved for different medical indications is becoming a compelling alternative option for researchers. Genetic variables represent a predisposing factor to the disease and mutations in leucine-rich repeat kinase 2 (LRRK2) locus have been correlated to late-onset autosomal-dominant PD. The common fruit fly Drosophila melanogaster carrying the mutation LRRK2 loss-of-function in the WD40 domain (LRRK2^WD40), is a simple in vivo model of PD and is a valid tool to first evaluate novel therapeutic approaches to the disease. Recent studies have suggested a neuroprotective activity of immunomodulatory agents in PD models. Here the immunomodulatory drug Pomalidomide (POM), a Thalidomide derivative, was examined in the Drosophila LRRK2^WD40 genetic model of PD. Mutant and wild type flies received increasing POM doses (1, 0.5, 0.25 mM) through their diet from day 1 post eclosion, until postnatal day (PN) 7 or 14, when POM's actions were evaluated by quantifying changes in climbing behavior as a measure of motor performance, the number of brain dopaminergic neurons and T-bars, mitochondria integrity. LRRK2^WD40 flies displayed a spontaneous age-related impairment of climbing activity, and POM significantly and dose-dependently improved climbing performance both at PN 7 and PN 14. LRRK2^WD40 fly motor disability was underpinned by a progressive loss of dopaminergic neurons in posterior clusters of the protocerebrum, which are involved in the control of locomotion, by a low number of T-bars density in the presynaptic bouton active zones. POM treatment fully rescued the cell loss in all posterior clusters at PN 7 and PN 14 and significantly increased the T-bars density. Moreover, several damaged mitochondria with dilated cristae were observed in LRRK2^WD40 flies treated with vehicle but not following POM. This study demonstrates the neuroprotective activity of the immunomodulatory agent POM in a genetic model of PD. POM is an FDA-approved clinically available and well-tolerated drug used for the treatment of multiple myeloma. If further validated in mammalian models of PD, POM could rapidly be clinically tested in humans.

Recent Developments in LRRK2-Targeted Therapy for Parkinson's Disease

Kinase activating missense mutations in leucine-rich repeat kinase 2 (LRRK2) are pathogenically linked to neurodegenerative Parkinson's disease (PD). Over the past decade, substantial effort has been devoted to the development of potent and selective small molecule inhibitors of LRRK2, as well as their preclinical testing across different Parkinson's disease models. This review outlines the genetic and biochemical evidence that pathogenic missense mutations increase LRRK2 kinase activity, which in turn provides the rationale for the development of small molecule inhibitors as potential PD therapeutics. An overview of progress in the development of LRRK2 inhibitors is provided, which in particular indicates that highly selective and potent compounds capable of clinical utility have been developed. We outline evidence from rodent- and human-induced pluripotent stem cell models that support a pathogenic role for LRRK2 kinase activity, and review the substantial experiments aimed at evaluating the safety of LRRK2 inhibitors. We address challenges still to overcome in the translational therapeutic pipeline, including biomarker development and clinical trial strategies, and finally outline the potential utility of LRRK2 inhibitors for other genetic forms of PD and ultimately sporadic PD. Collective evidence supports the ongoing clinical translation of LRRK2 inhibitors as a therapeutic intervention for PD is greatly needed.

LRRK2 links genetic and sporadic Parkinson's disease

The past two decades in research has revealed the importance of leucine-rich repeat kinase 2 (LRRK2) in both monogenic and sporadic forms of Parkinson's disease (PD). In families, mutations in LRRK2 can cause PD with age-dependent but variable penetrance and genome-wide association studies have found variants of the gene that are risk factors for sporadic PD. Functional studies have suggested that the common mechanism that links all disease-associated variants is that they increase LRRK2 kinase activity, albeit in different ways. Here, we will discuss the roles of LRRK2 in areas of inflammation and vesicular trafficking in the context of monogenic and sporadic PD. We will also provide a hypothetical model that links inflammation and vesicular trafficking together in an effort to outline how these pathways might interact and eventually lead to neuronal cell death. We will also highlight the translational potential of LRRK2-specific kinase inhibitors for the treatment of 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.

Inhibition of LRRK2 or Casein Kinase 1 Results in LRRK2 Protein Destabilization

Mutations and variations in the leucine-rich repeat kinase 2 (LRRK2) gene are strongly associated with an increased risk to develop Parkinson's disease (PD). Most pathogenic LRRK2 mutations display increased kinase activity, which is believed to underlie LRRK2-mediated toxicity. Therefore, major efforts have been invested in the development of potent and selective LRRK2 kinase inhibitors. Several of these compounds have proven beneficial in cells and in vivo, even in a LRRK2 wild-type background. Therefore, LRRK2 kinase inhibition holds great promise as disease-modifying PD therapy, and is currently tested in preclinical and early clinical studies. One of the safety concerns is the development of lung pathology in mice and non-human primates, which is most likely related to the strongly reduced LRRK2 protein levels after LRRK2 kinase inhibition. In this study, we aimed to better understand the molecular consequences of chronic LRRK2 kinase inhibition, which may be pivotal in the further development of a LRRK2 kinase inhibitor-based PD therapy. We found that LRRK2 protein levels are not restored during long-term LRRK2 kinase inhibition, but are recovered upon inhibitor withdrawal. Interestingly, LRRK2 kinase inhibitor-induced destabilization does not occur in all pathogenic LRRK2 variants and the N-terminal part of LRRK2 appears to play a crucial role in this process. In addition, we identified CK1, an upstream kinase of LRRK2, as a regulator of LRRK2 protein stability in cell culture and in vivo. We propose that pharmacological LRRK2 kinase inhibition triggers a cascade that results in reduced CK1-mediated phosphorylation of yet unidentified LRRK2 phosphorylation sites. This process involves the N-terminus of LRRK2 and ultimately leads to LRRK2 protein degradation.

Lrrk promotes tau neurotoxicity through dysregulation of actin and mitochondrial dynamics

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial Parkinson disease. Genetics and neuropathology link Parkinson disease with the microtubule-binding protein tau, but the mechanism of action of LRRK2 mutations and the molecular connection between tau and Parkinson disease are unclear. Here, we investigate the interaction of LRRK and tau in Drosophila and mouse models of tauopathy. We find that either increasing or decreasing the level of fly Lrrk enhances tau neurotoxicity, which is further exacerbated by expressing Lrrk with dominantly acting Parkinson disease-associated mutations. At the cellular level, altering Lrrk expression promotes tau neurotoxicity via excess stabilization of filamentous actin (F-actin) and subsequent mislocalization of the critical mitochondrial fission protein dynamin-1-like protein (Drp1). Biochemically, monomeric LRRK2 exhibits actin-severing activity, which is reduced as increasing concentrations of wild-type LRRK2, or expression of mutant forms of LRRK2 promote oligomerization of the protein. Overall, our findings provide a potential mechanistic basis for a dominant negative mechanism in LRRK2-mediated Parkinson disease, suggest a common molecular pathway with other familial forms of Parkinson disease linked to abnormalities of mitochondrial dynamics and quality control, and raise the possibility of new therapeutic approaches to Parkinson disease and related disorders.

The role of monogenic genes in idiopathic Parkinson's disease

In the past two decades, mutations in multiple genes have been linked to autosomal dominant or recessive forms of monogenic Parkinson's disease (PD). Collectively, these monogenic (often familial) cases account for less than 5% of all PD, the majority being apparently sporadic cases. More recently, large-scale genome-wide association studies have identified over 40 loci that increase risk of PD. Importantly, there is overlap between monogenic and sporadic PD genes, particularly for the loci that contain the genes SNCA and LRRK2, which are mutated in monogenic dominant PD. There have also been reports of idiopathic PD cases with heterozygous variants in autosomal recessive genes suggesting that these mutations may increase risk of PD. These observations suggest that monogenic and idiopathic PD may have shared pathogenic mechanisms. Here, we focus mainly on the role of monogenic PD genes that represent pleomorphic risk loci for idiopathic PD. We also discuss the functional mechanisms that may play a role in increasing risk of disease in both monogenic and idiopathic forms.

Differences in Stability, Activity and Mutation Effects Between Human and Mouse Leucine-Rich Repeat Kinase 2

Mutations in the Leucine-rich repeat kinase 2 (LRRK2) gene have been implicated in the pathogenesis of Parkinson's disease (PD). Identification of PD-associated LRRK2 mutations has led to the development of novel animal models, primarily in mice. However, the characteristics of human LRRK2 and mouse Lrrk2 protein have not previously been directly compared. Here we show that proteins from different species have different biochemical properties, with the mouse protein being more stable but having significantly lower kinase activity compared to the human orthologue. In examining the effects of PD-associated mutations and risk factors on protein function, we found that conserved substitutions such as G2019S affect human and mouse LRRK2 proteins similarly, but variation around position 2385, which is not fully conserved between humans and mice, induces divergent in vitro behavior. Overall our results indicate that structural differences between human and mouse LRRK2 are likely responsible for the different properties we have observed for these two species of LRRK2 protein. These results have implications for disease modelling of LRRK2 mutations in mice and on the testing of pharmacological therapies in animals.

Mechanisms of Mutant LRRK2 Neurodegeneration

LRRK2 mutations are associated with the loss of neurons, that is to say toxicity, in patients and in experimental model systems. However, the mechanisms by which mutations can be linked to neurodegeneration are not fully defined. Here I will argue that mechanism in this context encompasses a variety of levels of information. Mutations or alterations in gene expression at a genetic level are one set of mechanisms that are reflected at the biochemical level likely in enhanced or persistent function of LRRK2. By impacting cellular pathways, prominently including changes in autophagy but also microtubule function, mitochondria and protein synthesis, in neurons and immune cells, the LRRK2 brain is primed for neurodegeneration in an age-dependent manner. These concepts have implications for not only modeling LRRK2 disease but also perhaps for Parkinson's disease more generally, including the development of therapeutic modalities.

Insights into the Influence of Specific Splicing Events on the Structural Organization of LRRK2

Leucine-rich repeat kinase 2 (LRRK2) is a large protein of unclear function. Rare mutations in the LRRK2 gene cause familial Parkinson's disease (PD) and inflammatory bowel disease. Genome-wide association studies (GWAS) have revealed significant association of the abovementioned diseases at the LRRK2 locus. Cell and systems biology research has led to potential roles that LRRK2 may have in PD pathogenesis, especially the kinase domain (KIN). Previous human expression studies showed evidence of mRNA expression and splicing patterns that may contribute to our understanding of the function of LRRK2. In this work, we investigate and identified significant regional differences in LRRK2 expression at the mRNA level, including a number of splicing events in the Ras of complex protein (Roc) and C-terminal of Roc domain (COR) of LRRK2, in the substantia nigra (SN) and occipital cortex (OCTX). Our findings indicate that the predominant form of LRRK2 mRNA is full length, with shorter isoforms present at a lower copy number. Our molecular modelling study suggests that splicing events in the ROC/COR domains will have major consequences on the enzymatic function and dimer formation of LRRK2. The implications of these are highly relevant to the broader effort to understand the biology and physiological functions of LRRK2, and to better characterize the role(s) of LRRK2 in the underlying mechanism leading to PD.

The genetic architecture of mitochondrial dysfunction in Parkinson's disease

Mitochondrial impairment is a well-established pathological pathway implicated in Parkinson's disease (PD). Defects of the complex I of the mitochondrial respiratory chain have been found in post-mortem brains from sporadic PD patients. Furthermore, several disease-related genes are linked to mitochondrial pathways, such as PRKN, PINK1, DJ-1 and HTRA2 and are associated with mitochondrial impairment. This phenotype can be caused by the dysfunction of mitochondrial quality control machinery at different levels: molecular, organellar or cellular. Mitochondrial unfolded protein response represents the molecular level and implicates various chaperones and proteases. If the molecular level of quality control is not sufficient, the organellar level is required and involves mitophagy and mitochondrial-derived vesicles to sequester whole dysfunctional organelle or parts of it. Only when the impairment is too severe, does it lead to cell death via apoptosis, which defines the cellular level of quality control. Here, we review how currently known PD-linked genetic variants interfere with different levels of mitochondrial quality control. We discuss the graded risk concept of the most recently identified PARK loci (PARK 17-23) and some susceptibility variants in GBA, LRRK2 and SNCA. Finally, the emerging concept of rare genetic variants in candidates genes for PD, such as HSPA9, TRAP1 and RHOT1, complete the picture of the complex genetic architecture of PD that will direct future precision medicine approaches.

Detection of endogenous S1292 LRRK2 autophosphorylation in mouse tissue as a readout for kinase activity

Parkinson’s disease-linked mutations in LRRK2 enhance the kinase activity of the protein, therefore targeting LRRK2 kinase activity is a promising therapeutic approach. Phosphorylation at S935 of LRRK2 and of its Rab GTPase substrates have proven very useful biomarkers to monitor its kinase activity. Complementary to these approaches autophosphorylation of LRRK2 can be used as a direct kinase activity readout but to date detection of autophosphorylation at endogenous levels in vivo has been limited. We developed a fractionation-based enrichment method to successfully detect endogenous S1292 LRRK2 autophosphorylation in mouse tissues and highlight S1292 as a physiological readout candidate for LRRK2 kinase activity in vivo.

P62/SQSTM1 is a novel leucine-rich repeat kinase 2 (LRRK2) substrate that enhances neuronal toxicity

Autosomal-dominant, missense mutations in the leucine-rich repeat protein kinase 2 (LRRK2) gene are the most common genetic predisposition to develop Parkinson's disease (PD). LRRK2 kinase activity is increased in several pathogenic mutations (N1437H, R1441C/G/H, Y1699C, G2019S), implicating hyperphosphorylation of a substrate in the pathogenesis of the disease. Identification of the downstream targets of LRRK2 is a crucial endeavor in the field to understand LRRK2 pathway dysfunction in the disease. We have identified the signaling adapter protein p62/SQSTM1 as a novel endogenous interacting partner and a substrate of LRRK2. Using mass spectrometry and phospho-specific antibodies, we found that LRRK2 phosphorylates p62 on Thr138 in vitro and in cells. We found that the pathogenic LRRK2 PD-associated mutations (N1437H, R1441C/G/H, Y1699C, G2019S) increase phosphorylation of p62 similar to previously reported substrate Rab proteins. Notably, we found that the pathogenic I2020T mutation and the risk factor mutation G2385R displayed decreased phosphorylation of p62. p62 phosphorylation by LRRK2 is blocked by treatment with selective LRRK2 inhibitors in cells. We also found that the amino-terminus of LRRK2 is crucial for optimal phosphorylation of Rab7L1 and p62 in cells. LRRK2 phosphorylation of Thr138 is dependent on a p62 functional ubiquitin-binding domain at its carboxy-terminus. Co-expression of p62 with LRRK2 G2019S increases the neurotoxicity of this mutation in a manner dependent on Thr138. p62 is an additional novel substrate of LRRK2 that regulates its toxic biology, reveals novel signaling nodes and can be used as a pharmacodynamic marker for LRRK2 kinase activity.

In Vitro Modeling of Leucine-Rich Repeat Kinase 2 G2019S-Mediated Parkinson's Disease Pathology

Leucine-rich repeat kinase 2 (LRRK2) G2019S (glycine to serine) is the most common mutation associated with sporadic and familial Parkinson's disease (PD) with 80% penetrance by age 70. This mutation is found worldwide, with up to 40% of individuals in the North African Arab population carrying the mutation. Induced pluripotent stem cells derived from fibroblasts of patients carrying the LRRK2 G2019S mutation have been a critical source of cells for generating dopaminergic neurons and studying G2019S-related pathology. These studies have elucidated LRRK2-related mechanisms of mitochondrial dysregulation, increased reactive oxygen species, truncated and simplified neurites, and cell death. These phenotypes are thought to result from the G2019S mutation increasing substrate access and therefore increasing the catalytic rate of the serine/threonine kinase. In this article, we critically review the contributions of in vitro modeling to the current knowledge on LRRK2 G2019S. We also analyze the role of patient-derived cell lines for the identification and validation of therapeutic targets, emphasizing their importance as part of a 3R approach to translational research and personalized medicine.

LRRK2 G2019S-induced mitochondrial DNA damage is LRRK2 kinase dependent and inhibition restores mtDNA integrity in Parkinson's disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with increased risk for developing Parkinson's disease (PD). Previously, we found that LRRK2 G2019S mutation carriers have increased mitochondrial DNA (mtDNA) damage and after zinc finger nuclease-mediated gene mutation correction, mtDNA damage was no longer detectable. While the mtDNA damage phenotype can be unambiguously attributed to the LRRK2 G2019S mutation, the underlying mechanism(s) is unknown. Here, we examine the role of LRRK2 kinase function in LRRK2 G2019S-mediated mtDNA damage, using both genetic and pharmacological approaches in cultured neurons and PD patient-derived cells. Expression of LRRK2 G2019S induced mtDNA damage in primary rat midbrain neurons, but not in cortical neuronal cultures. In contrast, the expression of LRRK2 wild type or LRRK2 D1994A mutant (kinase dead) had no effect on mtDNA damage in either midbrain or cortical neuronal cultures. In addition, human LRRK2 G2019S patient-derived lymphoblastoid cell lines (LCL) demonstrated increased mtDNA damage relative to age-matched controls. Importantly, treatment of LRRK2 G2019S expressing midbrain neurons or patient-derived LRRK2 G2019S LCLs with the LRRK2 kinase inhibitor GNE-7915, either prevented or restored mtDNA damage to control levels. These findings support the hypothesis that LRRK2 G2019S-induced mtDNA damage is LRRK2 kinase activity dependent, uncovering a novel pathological role for this kinase. Blocking or reversing mtDNA damage via LRRK2 kinase inhibition or other therapeutic approaches may be useful to slow PD-associated pathology.

The LRRK2 Variant E193K Prevents Mitochondrial Fission Upon MPP+ Treatment by Altering LRRK2 Binding to DRP1

Mutations in leucine-rich repeat kinase 2 gene (LRRK2) are associated with familial and sporadic Parkinson's disease (PD). LRRK2 is a complex protein that consists of multiple domains, including 13 putative armadillo-type repeats at the N-terminus. In this study, we analyzed the functional and molecular consequences of a novel variant, E193K, identified in an Italian family. E193K substitution does not influence LRRK2 kinase activity. Instead it affects LRRK2 biochemical properties, such as phosphorylation at Ser935 and affinity for 14-3-3ε. Primary fibroblasts obtained from an E193K carrier demonstrated increased cellular toxicity and abnormal mitochondrial fission upon 1-methyl-4-phenylpyridinium treatment. We found that E193K alters LRRK2 binding to DRP1, a crucial mediator of mitochondrial fission. Our data support a role for LRRK2 as a scaffolding protein influencing mitochondrial fission.

LRRK2 Phosphorylation: Behind the Scenes

Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) are known today as the most common genetic cause of Parkinson's disease (PD). LRRK2 is a large protein that is hypothesized to regulate other proteins as a scaffold in downstream signaling pathways. This is supported by the multiple domain composition of LRRK2 with several protein-protein interaction domains combined with kinase and GTPase activity. LRRK2 is highly phosphorylated at sites that are strictly controlled by upstream regulators, including its own kinase domain. In cultured cells, most pathogenic mutants display increased autophosphorylation at S1292, but decreased phosphorylation at sites controlled by other kinases. We only begin to understand how LRRK2 phosphorylation is regulated and how this impacts its physiological and pathological function. Intriguingly, LRRK2 kinase inhibition, currently one of the most prevailing disease-modifying therapeutic strategies for PD, induces LRRK2 dephosphorylation at sites that are also dephosphorylated in pathogenic variants. In addition, LRRK2 kinase inhibition can induce LRRK2 protein degradation, which might be related to the observed inhibitor-induced adverse effects on the lung in rodents and non-human primates, as it resembles the lung pathology in LRRK2 knock-out animals. In this review, we will provide an overview of how LRRK2 phosphorylation is regulated and how this complex regulation relates to several molecular and cellular features of LRRK2.

Fatigue correlates with LRRK2 G2385R variant in Chinese Parkinson's disease patients

INTRODUCTION

Fatigue is common in patients with Parkinson's disease (PD). The leucine-rich repeat kinase 2 (LRRK2) G2385R variant predisposes individuals to develop PD in China. The aim of this study was to evaluate whether the LRRK2 G2385R variant is associated with fatigue in patients with PD.

METHODS

Fatigue was evaluated by the Parkinson Fatigue Scale (PFS) in 329 PD patients and 180 controls, a cut-off score of ≥3.3 was used to define the presence of fatigue. All the enrolled PD patients were assessed by a comprehensive battery of motor and non-motor questionnaires. PD patients were genotyped for the G2385R variant. Associations of fatigue with the clinical assessments and with the G2385R variant in PD patients were analyzed by logistic regression.

RESULTS

Fatigue frequency was 55.62%. A logistic regression model found that the female sex (OR = 10.477; 95%CI: 2.806-39.120; p < 0.001), motor function (OR = 1.060; 95%CI: 1.012-1.110; p = 0.013), sleep disturbance (OR = 0.943; 95%CI: 0.910-0.976; p = 0.001) and depression severity (OR = 0.843; 95%CI: 0.736-0.965; p = 0.013) collectively predict the presence of fatigue in PD patients. After adjustment for demographics and associated clinical factors, the G2385R variant was associated with an increased risk for the presence of fatigue (OR = 10.699; 95% CI = 2.387-47.958; p = 0.002) in the PD population in this study.

CONCLUSION

We confirm that fatigue in PD patients is common, and we have strengthened the associations between fatigue and female sex, motor severity and non-motor symptoms, particularly depression and sleep disturbances. Overall, we found that carriers of the G2385R variant were more prone to fatigue than non-carriers in PD patients.