The Parkinson's disease associated LRRK2 exhibits weaker in vitro phosphorylation of 4E-BP compared to autophosphorylation

Identification of necroptosis-related genes in Parkinson's disease by integrated bioinformatics analysis and experimental validation

Background

Parkinson's disease (PD) is the second most common neurodegeneration disease worldwide. Necroptosis, which is a new form of programmed cell death with high relationship with inflammation, plays a vital role in the progression of PD. However, the key necroptosis related genes in PD are not fully elucidated.

Purpose

Identification of key necroptosis-related genes in PD.

Method

The PD associated datasets and necroptosis related genes were downloaded from the GEO Database and GeneCards platform, respectively. The DEGs associated with necroptosis in PD were obtained by gap analysis, and followed by cluster analysis, enrichment analysis and WGCNA analysis. Moreover, the key necroptosis related genes were generated by PPI network analysis and their relationship by spearman correlation analysis. Immune infiltration analysis was used for explore the immune state of PD brain accompanied with the expression levels of these genes in various types of immune cells. Finally, the gene expression levels of these key necroptosis related genes were validated by an external dataset, blood samples from PD patients and toxin-induced PD cell model using real-time PCR analysis.

Result

Twelve key necroptosis-related genes including ASGR2, CCNA1, FGF10, FGF19, HJURP, NTF3, OIP5, RRM2, SLC22A1, SLC28A3, WNT1 and WNT10B were identified by integrated bioinformatics analysis of PD related dataset GSE7621. According to the correlation analysis of these genes, RRM2 and WNT1 were positively and negatively correlated with SLC22A1 respectively, while WNT10B was positively correlated with both OIF5 and FGF19. As the results from immune infiltration analysis, M2 macrophage was the highest population of immune cell in analyzed PD brain samples. Moreover, we found that 3 genes (CCNA1, OIP5 and WNT10B) and 9 genes (ASGR2, FGF10, FGF19, HJURP, NTF3, RRM2, SLC22A1, SLC28A3 and WNT1) were down- and up- regulated in an external dataset GSE20141, respectively. All the mRNA expression levels of these 12 genes were obviously upregulated in 6-OHDA-induced SH-SY5Y cell PD model while CCNA1 and OIP5 were up- and down- regulated, respectively, in peripheral blood lymphocytes of PD patients.

Conclusion

Necroptosis and its associated inflammation play fundamental roles in the progression of PD and these identified 12 key genes might be served as new diagnostic markers and therapeutic targets for PD.

Parkinson Disease: Translating Insights from Molecular Mechanisms to Neuroprotection

Parkinson disease (PD) used to be considered a nongenetic condition. However, the identification of several autosomal dominant and recessive mutations linked to monogenic PD has changed this view. Clinically manifest PD is then thought to occur through a complex interplay between genetic mutations, many of which have incomplete penetrance, and environmental factors, both neuroprotective and increasing susceptibility, which variably interact to reach a threshold over which PD becomes clinically manifested. Functional studies of PD gene products have identified many cellular and molecular pathways, providing crucial insights into the nature and causes of PD. PD originates from multiple causes and a range of pathogenic processes at play, ultimately culminating in nigral dopaminergic loss and motor dysfunction. An in-depth understanding of these complex and possibly convergent pathways will pave the way for therapeutic approaches to alleviate the disease symptoms and neuroprotective strategies to prevent disease manifestations. This review is aimed at providing a comprehensive understanding of advances made in PD research based on leveraging genetic insights into the pathogenesis of PD. It further discusses novel perspectives to facilitate identification of critical molecular pathways that are central to neurodegeneration that hold the potential to develop neuroprotective and/or neurorestorative therapeutic strategies for PD. SIGNIFICANCE STATEMENT: A comprehensive review of PD pathophysiology is provided on the complex interplay of genetic and environmental factors and biologic processes that contribute to PD pathogenesis. This knowledge identifies new targets that could be leveraged into disease-modifying therapies to prevent or slow neurodegeneration in PD.

Local Translation in Nervous System Pathologies

Dendrites and axons can extend dozens to hundreds of centimeters away from the cell body so that a single neuron can sense and respond to thousands of stimuli. Thus, for an accurate function of dendrites and axons the neuronal proteome needs to be asymmetrically distributed within neurons. Protein asymmetry can be achieved by the transport of the protein itself or the transport of the mRNA that is then translated at target sites in neuronal processes. The latter transport mechanism implies local translation of localized mRNAs. The role of local translation in nervous system (NS) development and maintenance is well established, but recently there is growing evidence that this mechanism and its deregulation are also relevant in NS pathologies, including neurodegenerative diseases. For instance, upon pathological signals disease-related proteins can be locally synthesized in dendrites and axons. Locally synthesized proteins can exert their effects at or close to the site of translation, or they can be delivered to distal compartments like the nucleus and induce transcriptional responses that lead to neurodegeneration, nerve regeneration and other cell-wide responses. Relevant key players in the process of local protein synthesis are RNA binding proteins (RBPs), responsible for mRNA transport to neurites. Several neurological and neurodegenerative disorders, including amyotrophic lateral sclerosis or spinal motor atrophy, are characterized by mutations in genes encoding for RBPs and consequently mRNA localization and local translation are impaired. In other diseases changes in the local mRNA repertoire and altered local protein synthesis have been reported. In this review, we will discuss how deregulation of localized translation at different levels can contribute to the development and progression of nervous system pathologies.

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.

Pathways of protein synthesis and degradation in PD pathogenesis

Since the discovery of protein aggregates in the brains of individuals with Parkinson's disease (PD) in the early 20th century, the scientific community has been interested in the role of dysfunctional protein metabolism in PD etiology. Recent advances in the field have implicated defective protein handling underlying PD through genetic, in vitro, and in vivo studies incorporating many disease models alongside neuropathological evidence. Here, we discuss the existing body of research focused on understanding cellular pathways of protein synthesis and degradation, and how aberrations in either system could engender PD pathology with special attention to α-synuclein-related consequences. We consider transcription, translation, and post-translational modification to constitute protein synthesis, and protein degradation to encompass proteasome-, lysosome- and endoplasmic reticulum-dependent mechanisms. Novel findings connecting each of these steps in protein metabolism to development of PD indicate that deregulation of protein production and turnover remains an exciting area in PD research.

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

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

Autophagy and mTOR pathways in mouse embryonic stem cell, lung cancer and somatic fibroblast cell lines

Embryonic developmental stages and regulations have always been one of the most intriguing aspects of science. Since the cancer stem cell discovery, striking for cancer development and recurrence, embryonic stem cells and control mechanisms, as well as cancer cells and cancer stem cell control mechanisms become important research materials. It is necessary to reveal the similarities and differences between somatic and cancer cells which are formed of embryonic stem cells divisions and determinations. For this purpose, mouse embryonic stem cells (mESCs), mouse skin fibroblast cells (MSFs) and mouse lung squamous cancer cells (SqLCCs) were grown in vitro and the differences between these three cell lines signalling regulations of mechanistic target of rapamycin (mTOR) and autophagic pathways were demonstrated by immunofluorescence and real-time polymerase chain reaction. Expressional differences were clearly shown between embryonic, cancer and somatic cells that mESCs displayed higher expressional level of Atg10, Hdac1 and Cln3 which are related with autophagic regulation and Hsp4, Prkca, Rhoa and ribosomal S6 genes related with mTOR activity. LC3 and mTOR protein levels were lower in mESCs than MSFs. Thus, the mechanisms of embryonic stem cell regulation results in the formation of somatic tissues whereas that these cells may be the causative agents of cancer in any deterioration.

The LRRK2 signalling system

The LRRK2 gene is a major contributor to genetic risk for Parkinson's disease and understanding the biology of the leucine-rich repeat kinase 2 (LRRK2, the protein product of this gene) is an important goal in Parkinson's research. LRRK2 is a multi-domain, multi-activity enzyme and has been implicated in a wide range of signalling events within the cell. Because of the complexities of the signal transduction pathways in which LRRK2 is involved, it has been challenging to generate a clear idea as to how mutations and disease associated variants in this gene are altered in disease. Understanding the events in which LRRK2 is involved at a systems level is therefore critical to fully understand the biology and pathobiology of this protein and is the subject of this review.

Genetic Modifiers of Neurodegeneration in a Drosophila Model of Parkinson's Disease

Disease phenotypes can be highly variable among individuals with the same pathogenic mutation. There is increasing evidence that background genetic variation is a strong driver of disease variability in addition to the influence of environment. To understand the genotype-phenotype relationship that determines the expressivity of a pathogenic mutation, a large number of backgrounds must be studied. This can be efficiently achieved using model organism collections such as the Drosophila Genetic Reference Panel (DGRP). Here, we used the DGRP to assess the variability of locomotor dysfunction in a LRRK2 G2019S Drosophila melanogaster model of Parkinson's disease (PD). We find substantial variability in the LRRK2 G2019S locomotor phenotype in different DGRP backgrounds. A genome-wide association study for candidate genetic modifiers reveals 177 genes that drive wide phenotypic variation, including 19 top association genes. Genes involved in the outgrowth and regulation of neuronal projections are enriched in these candidate modifiers. RNAi functional testing of the top association and neuronal projection-related genes reveals that pros, pbl, ct, and CG33506 significantly modify age-related dopamine neuron loss and associated locomotor dysfunction in the Drosophila LRRK2 G2019S model. These results demonstrate how natural genetic variation can be used as a powerful tool to identify genes that modify disease-related phenotypes. We report novel candidate modifier genes for LRRK2 G2019S that may be used to interrogate the link between LRRK2, neurite regulation and neuronal degeneration in PD.

The Drosophila hep pathway mediates Lrrk2-induced neurodegeneration

Although the pathogenesis of Parkinson's disease (PD) remains unclear, mutations in leucine-rich repeat kinase 2 (Lrrk2) are among the major causes of familial PD. Most of these mutations disrupt Lrrk2 kinase and (or) GTPase domain function, resulting in neuronal degeneration. However, the signal pathways underlying Lrrk2-induced neuronal degeneration are not fully understood. There is an expanding body of evidence that suggests a link between Lrrk2 function and MAP kinase (MAPK) cascades. To further investigate this link in vivo, genetic RNAi screens of the MAPK pathways were performed in a Drosophila model to identify genetic modifier(s) that can suppress G2019S-Lrrk2-induced PD-like phenotypes. The results revealed that the knockdown of hemipterous (hep, or JNKK) increased fly survival time, improved locomotor function, and reduced loss of dopaminergic neurons in G2019S-Lrrk2 transgenic flies. Expression of the dominant-negative allele of JNK (JNK-DN), a kinase that is downstream of hep in G2019S-Lrrk2 transgenic flies, elicited a similar effect. Moreover, treatment with the JNK inhibitor SP600125 partially reversed the G2019S-Lrrk2-induced loss of dopaminergic neurons. These results indicate that the hep pathway plays an important role in Lrrk2-linked Parkinsonism in flies. These studies provide new insights into the molecular mechanisms underlying Lrrk2-linked PD pathogenesis and aid in identifying potential therapeutic targets.

Decoding Parkinson's Disease Pathogenesis: The Role of Deregulated mRNA Translation

Mutations in a number of genes cause rare familial forms of Parkinson’s disease and provide profound insight into potential mechanisms governing disease pathogenesis. Recently, a role for translation and metabolism of mRNA has emerged in the development of various neurodegenerative disorders including Parkinson’s disease (PD). In PD, preliminary evidence supports a role for aberrant translation in the disease process stemming from mutations in several genes. Translation control is central to maintaining organism homeostasis under variable environmental conditions and deregulation of this may predispose to certain stressors. Hypothetically, deregulated translation may be detrimental to neuronal viability in PD through the misexpression of a subset of transcripts or through the impact of excessive bulk translation on energy consumption and burden on protein homeostatic mechanisms. While compelling preliminary evidence exists to support a role for translation in PD, much more work is required to identify specific mechanisms linking altered translation to the disease process.

L'RRK de Triomphe: a solution for LRRK2 GTPase activity?

Leucine-rich repeat kinase 2 (LRRK2) is a central protein in the pathogenesis of Parkinson's disease (PD), yet its normal function has proved stubbornly hard to elucidate. Even though it remains unclear how pathogenic mutations affect LRRK2 to cause PD, recent findings provide increasing cause for optimism. We summarise here the developing consensus over the effect of pathogenic mutations in the Ras of complex proteins and C-terminal of Roc domains on LRRK2 GTPase activity. This body of work has been greatly reinforced by our own study of the protective R1398H variant contained within the LRRK2 GTPase domain. Collectively, data point towards the pathogenicity of GTP-bound LRRK2 and strengthen a working model for LRRK2 GTPase function as a GTPase activated by dimerisation. Together with the identification of the protective R1398H variant as a valuable control for pathogenic mutations, we have no doubt that these triumphs for the LRRK2 field will accelerate research towards resolving LRRK2 function and towards new treatments for PD.

Mechanisms of Parkinson's Disease: Lessons from Drosophila

The power of Drosophila genetics has attracted attention in tackling important biomedical challenges such as the understanding and prevention of neurodegenerative diseases. Parkinson's disease (PD) is the most common neurodegenerative movement disorder which results from the relentless degeneration of midbrain dopaminergic neurons. Over the past two decades tremendous advances have been made in identifying genes responsible for inherited forms of PD. The ease of genetic manipulation in Drosophila has spurred the development of numerous models of PD, including expression of human genes carrying pathogenic mutations or the targeted mutation of conserved orthologs. The genetic and cellular analysis of these models is beginning to reveal fundamental insights into the pathogenic mechanisms. Numerous pathways and processes are disrupted in these models but some common themes are emerging. These often implicate aberrant synaptic function, protein aggregation, autophagy, oxidative stress, and mitochondrial dysfunction. Moreover, an impressive list of small molecule compounds have been identified as effective in reversing pathogenic phenotypes, paving the way to explore these for therapeutic interventions.

LRRK2 regulates retrograde synaptic compensation at the Drosophila neuromuscular junction

Parkinson's disease gene leucine-rich repeat kinase 2 (LRRK2) has been implicated in a number of processes including the regulation of mitochondrial function, autophagy and endocytic dynamics; nevertheless, we know little about its potential role in the regulation of synaptic plasticity. Here we demonstrate that postsynaptic knockdown of the fly homologue of LRRK2 thwarts retrograde, homeostatic synaptic compensation at the larval neuromuscular junction. Conversely, postsynaptic overexpression of either the fly or human LRRK2 transgene induces a retrograde enhancement of presynaptic neurotransmitter release by increasing the size of the release ready pool of vesicles. We show that LRRK2 promotes cap-dependent translation and identify Furin 1 as its translational target, which is required for the synaptic function of LRRK2. As the regulation of synaptic homeostasis plays a fundamental role in ensuring normal and stable synaptic function, our findings suggest that aberrant function of LRRK2 may lead to destabilization of neural circuits.

Mutations in the protein LRRK2 have been associated with Parkinson's disease but little is still known about the basic functions of the protein in the brain. Here the authors show that in fruit flies, LRRK2 regulates retrograde homeostatic synaptic compensation at the larval neuromuscular junction.

The function of orthologues of the human Parkinson's disease gene LRRK2 across species: implications for disease modelling in preclinical research

In the period since LRRK2 (leucine-rich repeat kinase 2) was identified as a causal gene for late-onset autosomal dominant parkinsonism, a great deal of work has been aimed at understanding whether the LRRK2 protein might be a druggable target for Parkinson's disease (PD). As part of this effort, animal models have been developed to explore both the normal and the pathophysiological roles of LRRK2. However, LRRK2 is part of a wider family of proteins whose functions in different organisms remain poorly understood. In this review, we compare the information available on biochemical properties of LRRK2 homologues and orthologues from different species from invertebrates (e.g. Caenorhabditis elegans and Drosophila melanogaster) to mammals. We particularly discuss the mammalian LRRK2 homologue, LRRK1, and those species where there is only a single LRRK homologue, discussing examples where each of the LRRK family of proteins has distinct properties as well as those cases where there appear to be functional redundancy. We conclude that uncovering the function of LRRK2 orthologues will help to elucidate the key properties of human LRRK2 as well as to improve understanding of the suitability of different animal models for investigation of LRRK2-related PD.

Phosphoproteomics reveals that Parkinson's disease kinase LRRK2 regulates a subset of Rab GTPases

Mutations in Park8, encoding for the multidomain Leucine-rich repeat kinase 2 (LRRK2) protein, comprise the predominant genetic cause of Parkinson's disease (PD). G2019S, the most common amino acid substitution activates the kinase two- to threefold. This has motivated the development of LRRK2 kinase inhibitors; however, poor consensus on physiological LRRK2 substrates has hampered clinical development of such therapeutics. We employ a combination of phosphoproteomics, genetics, and pharmacology to unambiguously identify a subset of Rab GTPases as key LRRK2 substrates. LRRK2 directly phosphorylates these both in vivo and in vitro on an evolutionary conserved residue in the switch II domain. Pathogenic LRRK2 variants mapping to different functional domains increase phosphorylation of Rabs and this strongly decreases their affinity to regulatory proteins including Rab GDP dissociation inhibitors (GDIs). Our findings uncover a key class of bona-fide LRRK2 substrates and a novel regulatory mechanism of Rabs that connects them to PD.

LRRK2 Kinase Inhibition as a Therapeutic Strategy for Parkinson's Disease, Where Do We Stand?

One of the most promising therapeutic targets for potential disease-modifying treatment of Parkinson's disease (PD) is leucine-rich repeat kinase 2 (LRRK2). Specifically, targeting LRRK2's kinase function has generated a lot of interest from both industry and academia. This work has yielded several published studies showing the feasibility of developing potent, selective and brain permeable LRRK2 kinase inhibitors. The availability of these experimental drugs is contributing to filling in the gaps in our knowledge on the safety and efficacy of LRRK2 kinase inhibition. Recent studies of LRRK2 kinase inhibition in preclinical models point to potential undesired effects in peripheral tissues such as lung and kidney. Also, while strategies are now emerging to measure target engagement of LRRK2 inhibitors, there remains an important need to expand efficacy studies in preclinical models of progressive PD. Future work in the LRRK2 inhibition field must therefore be directed towards developing molecules and treatment regimens which demonstrate efficacy in mammalian models of disease in conditions where safety liabilities are reduced to a minimum.

Abberant protein synthesis in G2019S LRRK2 Drosophila Parkinson disease-related phenotypes

LRRK2 mutations are a frequent cause of familial Parkinson disease (PD) and are also found in a number of sporadic PD cases. PD-linked G2019S and I2020T mutations in the kinase domain of LRRK2 result in elevated kinase activity, which is required for the toxicity of these pathogenic variants in cell and animal models of PD. We recently reported that LRRK2 interacts with and phosphorylates a number of mammalian ribosomal proteins, several of which exhibit increased phosphorylation via both G2019S and I2020T LRRK2. Blocking the phosphorylation of ribosomal protein s15 through expression of phospho-deficient T136A s15 prevents age-associated locomotor deficits and dopamine neuron loss caused by G2019S LRRK2 expression in Drosophila indicating that s15 is a pathogenic LRRK2 substrate. We previously described that G2019S LRRK2 causes an induction of bulk mRNA translation that is blocked by T136A s15 or the protein synthesis inhibitor anisomycin. Here, we report the protective effects of the eIF4E/eIF4G interaction inhibitor 4EGI-1, in preventing neurodegenerative phenotypes in G2019S LRRK2 flies, and discuss how our findings and those of other groups provide a framework to begin investigating the mechanistic impact of LRRK2 on translation.

Cellular processes associated with LRRK2 function and dysfunction

Mutations in the leucine-rich repeat kinase 2 (LRRK2)-encoding gene are the most common cause of monogenic Parkinson's disease. The identification of LRRK2 polymorphisms associated with increased risk for sporadic Parkinson's disease, as well as the observation that LRRK2-Parkinson's disease has a pathological phenotype that is almost indistinguishable from the sporadic form of disease, suggested LRRK2 as the culprit to provide understanding for both familial and sporadic Parkinson's disease cases. LRRK2 is a large protein with both GTPase and kinase functions. Mutations segregating with Parkinson's disease reside within the enzymatic core of LRRK2, suggesting that modification of its activity impacts greatly on disease onset and progression. Although progress has been made since its discovery in 2004, there is still much to be understood regarding LRRK2's physiological and neurotoxic properties. Unsurprisingly, given the presence of multiple enzymatic domains, LRRK2 has been associated with a diverse set of cellular functions and signalling pathways including mitochondrial function, vesicle trafficking together with endocytosis, retromer complex modulation and autophagy. This review discusses the state of current knowledge on the role of LRRK2 in health and disease with discussion of potential substrates of phosphorylation and functional partners with particular emphasis on signalling mechanisms. In addition, the use of immune cells in LRRK2 research and the role of oxidative stress as a regulator of LRRK2 activity and cellular function are also discussed.

Chemical genetic approach identifies microtubule affinity-regulating kinase 1 as a leucine-rich repeat kinase 2 substrate

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of autosomal-dominant forms of Parkinson's disease. LRRK2 is a modular, multidomain protein containing 2 enzymatic domains, including a kinase domain, as well as several protein-protein interaction domains, pointing to a role in cellular signaling. Although enormous efforts have been made, the exact pathophysiologic mechanisms of LRRK2 are still not completely known. In this study, we used a chemical genetics approach to identify LRRK2 substrates from mouse brain. This approach allows the identification of substrates of 1 particular kinase in a complex cellular environment. Several of the identified peptides are involved in the regulation of microtubule (MT) dynamics, including microtubule-associating protein (MAP)/microtubule affinity-regulating kinase 1 (MARK1). MARK1 is a serine/threonine kinase known to phosphorylate MT-binding proteins such as Tau, MAP2, and MAP4 at KXGS motifs leading to MT destabilization. In vitro kinase assays and metabolic-labeling experiments in living cells confirmed MARK1 as an LRRK2 substrate. Moreover, we also showed that LRRK2 and MARK1 are interacting in eukaryotic cells. Our findings contribute to the identification of physiologic LRRK2 substrates and point to a potential mechanism explaining the reported effects of LRRK2 on neurite morphology.

An early endosome regulator, Rab5b, is an LRRK2 kinase substrate

Leucine-rich repeat kinase 2 (LRRK2) has been identified as a causative gene for Parkinson's disease (PD). LRRK2 contains a kinase and a GTPase domain, both of which provide critical intracellular signal-transduction functions. We showed previously that Rab5b, a small GTPase protein that regulates the motility and fusion of early endosomes, interacts with LRRK2 and co-regulates synaptic vesicle endocytosis. Using recombinant proteins, we show here that LRRK2 phosphorylates Rab5b at its Thr6 residue in in vitro kinase assays with mass spectrophotometry analysis. Phosphorylation of Rab5b by LRRK2 on the threonine residue was confirmed by western analysis using cells stably expressing LRRK2 G2019S. The phosphomimetic T6D mutant exhibited stronger GTPase activity than that of the wild-type Rab5b. In addition, phosphorylation of Rab5b by LRRK2 also exhibited GTPase activity stronger than that of the unphosphorylated Rab5b protein. Two assays testing Rab5's activity, neurite outgrowth analysis and epidermal growth factor receptor degradation assays, showed that Rab5b T6D exhibited phenotypes that were expected to be observed in the inactive Rab5b, including longer neurite length and less degradation of EGFR. These results suggest that LRRK2 kinase activity functions as a Rab5b GTPase activating protein and thus, negatively regulates Rab5b signalling.

Heterogeneity of leucine-rich repeat kinase 2 mutations: genetics, mechanisms and therapeutic implications

Variation within and around the leucine-rich repeat kinase 2 (LRRK2) gene is associated with familial and sporadic Parkinson's disease (PD). Here, we discuss the prevalence of LRRK2 substitutions in different populations and their association with PD, as well as molecular and cellular mechanisms of pathologically relevant LRRK2 mutations. Kinase activation was proposed as a universal molecular mechanism for all pathogenic LRRK2 mutations, but later reports revealed heterogeneity in the effect of mutations on different activities of LRRK2. One mutation (G2019S) increases kinase activity, whereas mutations in the Ras of complex proteins (ROC)-C-terminus of ROC (COR) bidomain impair the GTPase function of LRRK2. Some risk factor variants, including G2385R in the WD40 domain, actually decrease the kinase activity of LRRK2. We suggest a model where LRRK2 mutations exert different molecular mechanisms but interfere with normal cellular function of LRRK2 at different levels of the same downstream pathway. Finally, we discuss the current state of therapeutic approaches for LRRK2-related PD.

Ribosomal protein s15 phosphorylation mediates LRRK2 neurodegeneration in Parkinson's disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) are a common cause of familial and sporadic Parkinson's disease (PD). Elevated LRRK2 kinase activity and neurodegeneration are linked, but the phosphosubstrate that connects LRRK2 kinase activity to neurodegeneration is not known. Here, we show that ribosomal protein s15 is a key pathogenic LRRK2 substrate in Drosophila and human neuron PD models. Phosphodeficient s15 carrying a threonine 136 to alanine substitution rescues dopamine neuron degeneration and age-related locomotor deficits in G2019S LRRK2 transgenic Drosophila and substantially reduces G2019S LRRK2-mediated neurite loss and cell death in human dopamine and cortical neurons. Remarkably, pathogenic LRRK2 stimulates both cap-dependent and cap-independent mRNA translation and induces a bulk increase in protein synthesis in Drosophila, which can be prevented by phosphodeficient T136A s15. These results reveal a novel mechanism of PD pathogenesis linked to elevated LRRK2 kinase activity and aberrant protein synthesis in vivo.

Interaction of LRRK2 with kinase and GTPase signaling cascades

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

A Parkinson's disease gene regulatory network identifies the signaling protein RGS2 as a modulator of LRRK2 activity and neuronal toxicity

Mutations in LRRK2 are one of the primary genetic causes of Parkinson's disease (PD). LRRK2 contains a kinase and a GTPase domain, and familial PD mutations affect both enzymatic activities. However, the signaling mechanisms regulating LRRK2 and the pathogenic effects of familial mutations remain unknown. Identifying the signaling proteins that regulate LRRK2 function and toxicity remains a critical goal for the development of effective therapeutic strategies. In this study, we apply systems biology tools to human PD brain and blood transcriptomes to reverse-engineer a LRRK2-centered gene regulatory network. This network identifies several putative master regulators of LRRK2 function. In particular, the signaling gene RGS2, which encodes for a GTPase-activating protein (GAP), is a key regulatory hub connecting the familial PD-associated genes DJ-1 and PINK1 with LRRK2 in the network. RGS2 expression levels are reduced in the striata of LRRK2 and sporadic PD patients. We identify RGS2 as a novel interacting partner of LRRK2 in vivo. RGS2 regulates both the GTPase and kinase activities of LRRK2. We show in mammalian neurons that RGS2 regulates LRRK2 function in the control of neuronal process length. RGS2 is also protective against neuronal toxicity of the most prevalent mutation in LRRK2, G2019S. We find that RGS2 regulates LRRK2 function and neuronal toxicity through its effects on kinase activity and independently of GTPase activity, which reveals a novel mode of action for GAP proteins. This work identifies RGS2 as a promising target for interfering with neurodegeneration due to LRRK2 mutations in PD patients.

RNA metabolism in the pathogenesis of Parkinson׳s disease

Neurodegenerative diseases such as Parkinson׳s disease are progressive disorders of the nervous system that affect the function and maintenance of specific neuronal populations. While most disease cases are sporadic with no known cause, a small percentage of disease cases are caused by inherited genetic mutations. The identification of genes associated with the familial forms of the diseases and subsequent studies of proteins encoded by the disease genes in cellular or animal models have offered much-needed insights into the molecular and cellular mechanisms underlying disease pathogenesis. Recent studies of the familial Parkinson׳s disease genes have emphasized the importance of RNA metabolism, particularly mRNA translation, in the disease process. It is anticipated that continued studies on the role of RNA metabolism in Parkinson׳s disease will offer unifying mechanisms for understanding the cause of neuronal dysfunction and degeneration and facilitate the development of novel and rational strategies for treating this debilitating disease.

In vitro phosphorylation does not influence the aggregation kinetics of WT α-synuclein in contrast to its phosphorylation mutants

The aggregation of alpha-synuclein (α-SYN) into fibrils is characteristic for several neurodegenerative diseases, including Parkinson's disease (PD). Ninety percent of α-SYN deposited in Lewy Bodies, a pathological hallmark of PD, is phosphorylated on serine129. α-SYN can also be phosphorylated on tyrosine125, which is believed to regulate the membrane binding capacity and thus possibly its normal function. A better understanding of the effect of phosphorylation on the aggregation of α-SYN might shed light on its role in the pathogenesis of PD. In this study we compare the aggregation properties of WT α-SYN with the phospho-dead and phospho-mimic mutants S129A, S129D, Y125F and Y125E and in vitro phosphorylated α-SYN using turbidity, thioflavin T and circular dichroism measurements as well as transmission electron microscopy. We show that the mutants S129A and S129D behave similarly compared to wild type (WT) α-SYN, while the mutants Y125F and Y125E fibrillate significantly slower, although all mutants form fibrillar structures similar to the WT protein. In contrast, in vitro phosphorylation of α-SYN on either S129 or Y125 does not significantly affect the fibrillization kinetics. Moreover, FK506 binding proteins (FKBPs), enzymes with peptidyl-prolyl cis-trans isomerase activity, still accelerate the aggregation of phosphorylated α-SYN in vitro, as was shown previously for WT α-SYN. In conclusion, our results illustrate that phosphorylation mutants can display different aggregation properties compared to the more biologically relevant phosphorylated form of α-SYN.

Inhibition of LRRK2 kinase activity stimulates macroautophagy

Leucine Rich Repeat Kinase 2 (LRRK2) is one of the most important genetic contributors to Parkinson's disease. LRRK2 has been implicated in a number of cellular processes, including macroautophagy. To test whether LRRK2 has a role in regulating autophagy, a specific inhibitor of the kinase activity of LRRK2 was applied to human neuroglioma cells and downstream readouts of autophagy examined. The resulting data demonstrate that inhibition of LRRK2 kinase activity stimulates macroautophagy in the absence of any alteration in the translational targets of mTORC1, suggesting that LRRK2 regulates autophagic vesicle formation independent of canonical mTORC1 signaling. This study represents the first pharmacological dissection of the role LRRK2 plays in the autophagy/lysosomal pathway, emphasizing the importance of this pathway as a marker for LRRK2 physiological function. Moreover it highlights the need to dissect autophagy and lysosomal activities in the context of LRRK2 related pathologies with the final aim of understanding their aetiology and identifying specific targets for disease modifying therapies in patients.

Pathogenic Parkinson's disease mutations across the functional domains of LRRK2 alter the autophagic/lysosomal response to starvation

LRRK2 is one of the most important genetic contributors to Parkinson's disease (PD). Point mutations in this gene cause an autosomal dominant form of PD, but to date no cellular phenotype has been consistently linked with mutations in each of the functional domains (ROC, COR and Kinase) of the protein product of this gene. In this study, primary fibroblasts from individuals carrying pathogenic mutations in the three central domains of LRRK2 were assessed for alterations in the autophagy/lysosomal pathway using a combination of biochemical and cellular approaches. Mutations in all three domains resulted in alterations in markers for autophagy/lysosomal function compared to wild type cells. These data highlight the autophagy and lysosomal pathways as read outs for pathogenic LRRK2 function and as a marker for disease, and provide insight into the mechanisms linking LRRK2 function and mutations.

Functional interaction of Parkinson's disease-associated LRRK2 with members of the dynamin GTPase superfamily

Mutations in LRRK2 cause autosomal dominant Parkinson's disease (PD). LRRK2 encodes a multi-domain protein containing GTPase and kinase domains, and putative protein–protein interaction domains. Familial PD mutations alter the GTPase and kinase activity of LRRK2 in vitro. LRRK2 is suggested to regulate a number of cellular pathways although the underlying mechanisms are poorly understood. To explore such mechanisms, it has proved informative to identify LRRK2-interacting proteins, some of which serve as LRRK2 kinase substrates. Here, we identify common interactions of LRRK2 with members of the dynamin GTPase superfamily. LRRK2 interacts with dynamin 1–3 that mediate membrane scission in clathrin-mediated endocytosis and with dynamin-related proteins that mediate mitochondrial fission (Drp1) and fusion (mitofusins and OPA1). LRRK2 partially co-localizes with endosomal dynamin-1 or with mitofusins and OPA1 at mitochondrial membranes. The subcellular distribution and oligomeric complexes of dynamin GTPases are not altered by modulating LRRK2 in mouse brain, whereas mature OPA1 levels are reduced in G2019S PD brains. LRRK2 enhances mitofusin-1 GTP binding, whereas dynamin-1 and OPA1 serve as modest substrates of LRRK2-mediated phosphorylation in vitro. While dynamin GTPase orthologs are not required for LRRK2-induced toxicity in yeast, LRRK2 functionally interacts with dynamin-1 and mitofusin-1 in cultured neurons. LRRK2 attenuates neurite shortening induced by dynamin-1 by reducing its levels, whereas LRRK2 rescues impaired neurite outgrowth induced by mitofusin-1 potentially by reversing excessive mitochondrial fusion. Our study elucidates novel functional interactions of LRRK2 with dynamin-superfamily GTPases that implicate LRRK2 in the regulation of membrane dynamics important for endocytosis and mitochondrial morphology.

ERKed by LRRK2: a cell biological perspective on hereditary and sporadic Parkinson's disease

The leucine rich repeat kinase 2 (LRRK2/dardarin) is implicated in autosomal dominant familial and sporadic Parkinson's disease (PD); mutations in LRRK2 account for up to 40% of PD cases in some populations. LRRK2 is a large protein with a kinase domain, a GTPase domain, and multiple potential protein interaction domains. As such, delineating the functional pathways for LRRK2 and mechanisms by which PD-linked variants contribute to age-related neurodegeneration could result in pharmaceutically tractable therapies. A growing number of recent studies implicate dysregulation of mitogen activated protein kinases 3 and 1 (also known as ERK1/2) as possible downstream mediators of mutant LRRK2 effects. As these master regulators of growth, differentiation, neuronal plasticity and cell survival have also been implicated in other PD models, a set of common cell biological pathways may contribute to neuronal susceptibility in PD. Here, we review the literature on several major cellular pathways impacted by LRRK2 mutations--autophagy, microtubule/cytoskeletal dynamics, and protein synthesis--in context of potential signaling crosstalk involving the ERK1/2 and Wnt signaling pathways. Emerging implications for calcium homeostasis, mitochondrial biology and synaptic dysregulation are discussed in relation to LRRK2 interactions with other PD gene products. It has been shown that substantia nigra neurons in human PD and Lewy body dementia patients exhibit cytoplasmic accumulations of ERK1/2 in mitochondria, autophagosomes and bundles of intracellular fibrils. Both experimental and human tissue data implicate pathogenic changes in ERK1/2 signaling in sporadic, toxin-based and mutant LRRK2 settings, suggesting engagement of common cell biological pathways by divergent PD etiologies.

Development of inducible leucine-rich repeat kinase 2 (LRRK2) cell lines for therapeutics development in Parkinson's disease

The pathogenic mechanism(s) contributing to loss of dopamine neurons in Parkinson's disease (PD) remain obscure. Leucine-rich repeat kinase 2 (LRRK2) mutations are linked, as a causative gene, to PD. LRRK2 mutations are estimated to account for 10% of familial and between 1 % and 3 % of sporadic PD. LRRK2 proximate single nucleotide polymorphisms have also been significantly associated with idiopathic/sporadic PD by genome-wide association studies. LRRK2 is a multidomain-containing protein and belongs to the protein kinase super-family. We constructed two inducible dopaminergic cell lines expressing either human-LRRK2-wild-type or human-LRRK2-mutant (G2019S). Phenotypes of these LRRK2 cell lines were examined with respect to cell viability, morphology, and protein function with or without induction of LRRK2 gene expression. The overexpression of G2019S gene promoted (1) low cellular metabolic activity without affecting cell viability, (2) blunted neurite extension, and (3) increased phosphorylation at S910 and S935. Our observations are consistent with reported general phenotypes in LRRK2 cell lines by other investigators. We used these cell lines to interrogate the biological function of LRRK2, to evaluate their potential as a drug-screening tool, and to investigate screening for small hairpin RNA-mediated LRRK2 G2019S gene knockdown as a potential therapeutic strategy. A proposed LRRK2 kinase inhibitor (i.e., IN-1) decreased LRRK2 S910 and S935 phosphorylation in our MN9DLRRK2 cell lines in a dose-dependent manner. Lentivirus-mediated transfer of LRRK2 G2019S allele-specific small hairpin RNA reversed the blunting of neurite extension caused by LRRK2 G2019S overexpression. Taken together, these inducible LRRK2 cell lines are suitable reagents for LRRK2 functional studies, and the screening of potential LRRK2 therapeutics.

Hexokinase activity is required for recruitment of parkin to depolarized mitochondria

Autosomal recessive parkinsonism genes contribute to maintenance of mitochondrial function. Two of these, PINK1 and parkin, act in a pathway promoting autophagic removal of depolarized mitochondria. Although recruitment of parkin to mitochondria is PINK1-dependent, additional components necessary for signaling are unclear. We performed a screen for endogenous modifiers of parkin recruitment to depolarized mitochondria and identified hexokinase 2 (HK2) as a novel modifier of depolarization-induced parkin recruitment. Hexose kinase activity was required for parkin relocalization, suggesting the effects are shared among hexokinases including the brain-expressed hexokinase 1 (HK1). Knockdown of both HK1 and HK2 led to a stronger block in parkin relocalization than either isoform alone, and expression of HK2 in primary neurons promoted YFP-parkin recruitment to depolarized mitochondria. Mitochondrial parkin recruitment was attenuated with AKT inhibition, which is known to modulate HK2 activity and mitochondrial localization. We, therefore, propose that Akt-dependent recruitment of hexokinases is a required step in the recruitment of parkin prior to mitophagy.

Leucine-rich repeat kinase 2 for beginners: six key questions

There has been intense interest in leucine-rich repeat kinase 2 (LRRK2) since 2004, when mutations in the LRRK2 gene were discovered to cause dominantly inherited Parkinson's disease (PD). This article will address six basic questions about LRRK2 biology as it relates to PD, with particular emphasis on its discovery, current concepts of its physiological and pathological functions, and the strategies being used to discover how LRRK2 dysfunction causes PD.

The neurobiology of LRRK2 and its role in the pathogenesis of Parkinson's disease

Leucine-rich repeat kinase 2 (LRRK2) is a large, widely expressed protein of largely unknown function. Mutations in the gene encoding LRRK2 have been linked to multiple diseases, including a prominent association with familial and sporadic Parkinson's disease (PD), as well as inflammatory bowel disorders such as Crohn's disease. The LRRK2 protein possesses both kinase and GTPase signaling domains, as well as multiple protein interaction domains. Experimental studies in both cellular and in vivo models of mutant LRRK2-induced neurodegeneration have given clues to potential function(s) of LRRK2, yet much remains unknown. For example, while it is known that intact kinase and GTPase activity are required for mutant forms of the protein to trigger cell death, the specific targets of these enzymatic activities that mediate the death of neurons are not known. In this review, we discuss the evidence linking LRRK2 to various cellular/neuronal activities such as extrinsic death and inflammatory signaling, lysosomal protein degradation, the cytoskeletal system and neurite outgrowth, vesicle trafficking, mitochondrial dysfunction, as well as multiple points of interaction with several other genes linked to the pathogenesis of PD. In order for more effective therapeutic strategies to be envisioned and implemented, the mechanisms underlying LRRK2-mediated neurodegeneration need to be better characterized. Furthermore, insights into LRRK2-associated PD pathogenesis can potentially advance our understanding of the more common sporadic forms of PD.

Unlocking the secrets of LRRK2 function with selective kinase inhibitors

LRRK2 is currently considered to be a potential therapeutic target for the treatment of Parkinson’s disease. A number of pathological mutations, the majority of which lie in the dual catalytic domains of LRRK2, segregate with Parkinson’s disease in an autosomal-dominant fashion. The most common mutation, G2019S, results in an increase in the kinase activity of LRRK2 and much work has, therefore, gone into the development of potent and specific inhibitors of LRRK2 kinase activity. A number of LRRK2 kinase inhibitors have now been employed in the search for the physiological function of LRRK2 and the targets of LRRK2 kinase activity.

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

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

ATP-competitive LRRK2 inhibitors interfere with monoclonal antibody binding to the kinase domain of LRRK2 under native conditions. A method to directly monitor the active conformation of LRRK2?

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease. LRRK2 kinase activity is required for toxicity in neuronal cell cultures suggesting that selective kinase inhibitors may prevent neurodegeneration in patients. Directly monitoring LRRK2 activity in cells would be advantageous for the development of small molecule LRRK2 inhibitors. Here, we demonstrate that a monoclonal anti-LRRK2 antibody directed against the activation segment binds less efficiently to native LRRK2 protein in the presence of ATP-competitive LRRK2 inhibitors. Since kinase inhibitors prevent autophosphorylation and refolding of the activation segment, we hypothesize that the antibody preferentially binds to the active conformation of LRRK2 under native conditions.

AMP kinase activation mitigates dopaminergic dysfunction and mitochondrial abnormalities in Drosophila models of Parkinson's disease

Mutations in parkin and LRRK2 together account for the majority of familial Parkinson's disease (PD) cases. Interestingly, recent evidence implicates the involvement of parkin and LRRK2 in mitochondrial homeostasis. Supporting this, we show here by means of the Drosophila model system that, like parkin, LRRK2 mutations induce mitochondrial pathology in flies when expressed in their flight muscles, the toxic effects of which can be rescued by parkin coexpression. When expressed specifically in fly dopaminergic neurons, mutant LRRK2 results in the appearance of significantly enlarged mitochondria, a phenotype that can also be rescued by parkin coexpression. Importantly, we also identified in this study that epigallocatechin gallate (EGCG), a green tea-derived catechin, acts as a potent suppressor of dopaminergic and mitochondrial dysfunction in both mutant LRRK2 and parkin-null flies. Notably, the protective effects of EGCG are abolished when AMP-activated protein kinase (AMPK) is genetically inactivated, suggesting that EGCG-mediated neuroprotection requires AMPK. Consistent with this, direct pharmacological or genetic activation of AMPK reproduces EGCG's protective effects. Conversely, loss of AMPK activity exacerbates neuronal loss and associated phenotypes in parkin and LRRK mutant flies. Together, our results suggest the relevance of mitochondrial-associated pathway in LRRK2 and parkin-related pathogenesis, and that AMPK activation may represent a potential therapeutic strategy for these familial forms of PD.

Mutant LRRK2 elicits calcium imbalance and depletion of dendritic mitochondria in neurons

Mutations in the leucine-rich repeat kinase 2 (LRRK2) have been associated with familial and sporadic cases of Parkinson disease. Mutant LRRK2 causes in vitro and in vivo neurite shortening, mediated in part by autophagy, and a parkinsonian phenotype in transgenic mice; however, the underlying mechanisms remain unclear. Because mitochondrial content/function is essential for dendritic morphogenesis and maintenance, we investigated whether mutant LRRK2 affects mitochondrial homeostasis in neurons. Mouse cortical neurons expressing either LRRK2 G2019S or R1441C mutations exhibited autophagic degradation of mitochondria and dendrite shortening. In addition, mutant LRRK2 altered the ability of the neurons to buffer intracellular calcium levels. Either calcium chelators or inhibitors of voltage-gated L-type calcium channels prevented mitochondrial degradation and dendrite shortening. These data suggest that mutant LRRK2 causes a deficit in calcium homeostasis, leading to enhanced mitophagy and dendrite shortening.

Phosphorylation of 4E-BP1 in the mammalian brain is not altered by LRRK2 expression or pathogenic mutations

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are a common cause of autosomal dominant familial Parkinson's disease (PD). LRRK2 encodes a multi-domain protein containing GTPase and kinase enzymatic domains. Disease-associated mutations in LRRK2 variably influence enzymatic activity with the common G2019S variant leading to enhanced kinase activity. Mutant LRRK2 induces neuronal toxicity through a kinase-dependent mechanism suggesting that kinase activity is important for mediating the pathogenic effects of LRRK2 mutations. A number of LRRK2 kinase substrates have been identified in vitro but whether they represent authentic physiological substrates in mammalian cells or tissues is not yet clear. The eukaryotic initiation factor 4E (eIF4E)-binding protein, 4E-BP1, was recently identified as a potential substrate of LRRK2 kinase activity in vitro and in Drosophila with phosphorylation occurring at Thr37 and Thr46. Here, we explore a potential interaction of LRRK2 and 4E-BP1 in mammalian cells and brain. We find that LRRK2 can weakly phosphorylate 4E-BP1 in vitro but LRRK2 overexpression is not able to alter endogenous 4E-BP1 phosphorylation in mammalian cells. In mammalian neurons LRRK2 and 4E-BP1 display minimal co-localization, whereas the subcellular distribution, protein complex formation and covalent post-translational modification of endogenous 4E-BP1 are not altered in the brains of LRRK2 knockout or mutant LRRK2 transgenic mice. In the brain, the phosphorylation of 4E-BP1 at Thr37 and Thr46 does not change in LRRK2 knockout or mutant LRRK2 transgenic mice, nor is 4E-BP1 phosphorylation altered in idiopathic or G2019S mutant PD brains. Collectively, our results suggest that 4E-BP1 is neither a major nor robust physiological substrate of LRRK2 in mammalian cells or brain.

The synaptic function of LRRK2

Mutations in LRRK2 (leucine-rich repeat kinase 2) are the most frequent genetic lesions so far found in familial as well as sporadic forms of PD (Parkinson's disease), a neurodegenerative disease characterized by the dysfunction and degeneration of dopaminergic and other neuronal types. The molecular and cellular mechanisms underlying LRRK2 action remain poorly defined. Synaptic dysfunction has been increasingly recognized as an early event in the pathogenesis of major neurological disorders. Using Drosophila as a model system, we have shown that LRRK2 controls synaptic morphogenesis. Loss of dLRRK (Drosophila LRRK2) results in synaptic overgrowth at the Drosophila neuromuscular junction synapse, whereas overexpression of wild-type dLRRK, hLRRK2 (human LRRK2) or the pathogenic hLRRK2-G2019S mutant has the opposite effect. Alteration of LRRK2 activity also affects synaptic transmission in a complex manner. LRRK2 exerts its effects on synaptic morphology by interacting with distinct downstream effectors at the pre- and post-synaptic compartments. At the postsynapse, LRRK2 functionally interacts with 4E-BP (eukaryotic initiation factor 4E-binding protein) and the microRNA machinery, both of which negatively regulate protein synthesis. At the presynapse, LRRK2 phosphorylates and negatively regulates the microtubule-binding protein Futsch and functionally interacts with the mitochondrial transport machinery. These results implicate compartment-specific synaptic dysfunction caused by altered protein synthesis, cytoskeletal dynamics and mitochondrial transport in LRRK2 pathogenesis and offer a new paradigm for understanding and ultimately treating LRRK2-related PD.

Mechanisms of LRRK2-mediated neurodegeneration

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene represent the most common cause of familial Parkinson's disease (PD), whereas common variation at the LRRK2 locus is associated with an increased risk of idiopathic PD. Considerable progress has been made toward understanding the biological functions of LRRK2 and the molecular mechanisms underlying the pathogenic effects of disease-associated mutations. The development of neuronal culture models and transgenic or viral-based rodent models have proved useful for identifying a number of emerging pathways implicated in LRRK2-dependent neuronal damage, including the microtubule network, actin cytoskeleton, autophagy, mitochondria, vesicular trafficking, and protein quality control. However, many important questions remain to be posed and answered. Elucidating the molecular mechanisms and pathways underlying LRRK2-mediated neurodegeneration is critical for the identification of new molecular targets for therapeutic intervention in PD. In this review we discuss recent advances and unanswered questions in understanding the pathophysiology of LRRK2.

Microtubule destabilization is shared by genetic and idiopathic Parkinson's disease patient fibroblasts

Data from both toxin-based and gene-based models suggest that dysfunction of the microtubule system contributes to the pathogenesis of Parkinson's disease, even if, at present, no evidence of alterations of microtubules in vivo or in patients is available. Here we analyze cytoskeleton organization in primary fibroblasts deriving from patients with idiopathic or genetic Parkinson's disease, focusing on mutations in parkin and leucine-rich repeat kinase 2. Our analyses reveal that genetic and likely idiopathic pathology affects cytoskeletal organization and stability, without any activation of autophagy or apoptosis. All parkinsonian fibroblasts have a reduced microtubule mass, represented by a higher fraction of unpolymerized tubulin in respect to control cells, and display significant changes in microtubule stability-related signaling pathways. Furthermore, we show that the reduction of microtubule mass is so closely related to the alteration of cell morphology and behavior that both pharmacological treatment with microtubule-targeted drugs, and genetic approaches, by transfecting the wild type parkin or leucine-rich repeat kinase 2, restore the proper microtubule stability and are able to rescue cell architecture. Taken together, our results suggest that microtubule destabilization is a point of convergence of genetic and idiopathic forms of parkinsonism and highlight, for the first time, that microtubule dysfunction occurs in patients and not only in experimental models of Parkinson's disease. Therefore, these data contribute to the knowledge on molecular and cellular events underlying Parkinson's disease and, revealing that correction of microtubule defects restores control phenotype, may offer a new therapeutic target for the management of the disease.

High LRRK2 levels fail to induce or exacerbate neuronal alpha-synucleinopathy in mouse brain

The G2019S mutation in the multidomain protein leucine-rich repeat kinase 2 (LRRK2) is one of the most frequently identified genetic causes of Parkinson’s disease (PD). Clinically, LRRK2(G2019S) carriers with PD and idiopathic PD patients have a very similar disease with brainstem and cortical Lewy pathology (α-synucleinopathy) as histopathological hallmarks. Some patients have Tau pathology. Enhanced kinase function of the LRRK2(G2019S) mutant protein is a prime suspect mechanism for carriers to develop PD but observations in LRRK2 knock-out, G2019S knock-in and kinase-dead mutant mice suggest that LRRK2 steady-state abundance of the protein also plays a determining role. One critical question concerning the molecular pathogenesis in LRRK2(G2019S) PD patients is whether α-synuclein (aSN) has a contributory role. To this end we generated mice with high expression of either wildtype or G2019S mutant LRRK2 in brainstem and cortical neurons. High levels of these LRRK2 variants left endogenous aSN and Tau levels unaltered and did not exacerbate or otherwise modify α-synucleinopathy in mice that co-expressed high levels of LRRK2 and aSN in brain neurons. On the contrary, in some lines high LRRK2 levels improved motor skills in the presence and absence of aSN-transgene-induced disease. Therefore, in many neurons high LRRK2 levels are well tolerated and not sufficient to drive or exacerbate neuronal α-synucleinopathy.