The I2020T Leucine-rich repeat kinase 2 transgenic mouse exhibits impaired locomotive ability accompanied by dopaminergic neuron abnormalities

Modeling Parkinson's disease in LRRK2 rodents

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are associated with familial and sporadic forms of Parkinson's disease (PD). Sporadic PD and LRRK2 PD share main clinical and neuropathological features, namely hypokinesia, degeneration of nigro-striatal dopamine neurons and α-synuclein aggregates in the form of Lewy bodies. Animals harboring the most common LRRK2 mutations, i.e. p.G2019S and p.R1441C/G, have been generated to replicate the parkinsonian phenotype and investigate the underlying pathogenic mechanisms. Disappointingly, however, LRRK2 rodents did not consistently phenocopy hypokinesia and nigro-striatal degeneration, or showed Lewy body-like aggregates. Instead, LRRK2 rodents manifested non-motor signs and dysregulated transmission at dopaminergic and non-dopaminergic synapses that are reminiscent of behavioral and functional network changes observed in the prodromal phase of the disease. LRRK2 rodents also manifested greater susceptibility to different parkinsonian toxins or stressors when subjected to dual-hit or multiple-hit protocols, confirming LRRK2 mutations as genetic risk factors. In conclusion, LRRK2 rodents represent a unique tool to identify the molecular mechanisms through which LRRK2 modulates the course and clinical presentations of PD and to study the interplay between genetic, intrinsic and environmental protective/risk factors in PD pathogenesis.

Mitochondrial function and dynamics in neural stem cells and neurogenesis: Implications for neurodegenerative diseases

Mitochondria have been largely described as the powerhouse of the cell and recent findings demonstrate that this organelle is fundamental for neurogenesis. The mechanisms underlying neural stem cells (NSCs) maintenance and differentiation are highly regulated by both intrinsic and extrinsic factors. Mitochondrial-mediated switch from glycolysis to oxidative phosphorylation, accompanied by mitochondrial remodeling and dynamics are vital to NSCs fate. Deregulation of mitochondrial proteins, mitochondrial DNA, function, fission/fusion and metabolism underly several neurodegenerative diseases; data show that these impairments are already present in early developmental stages and NSC fate decisions. However, little is known about mitochondrial role in neurogenesis. In this Review, we describe the recent evidence covering mitochondrial role in neurogenesis, its impact in selected neurodegenerative diseases, for which aging is the major risk factor, and the recent advances in stem cell-based therapies that may alleviate neurodegenerative disorders-related neuronal deregulation through improvement of mitochondrial function and dynamics.

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.

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.

LRRK2 inhibition potentiates PARP inhibitor cytotoxicity through inhibiting homologous recombination-mediated DNA double strand break repair

PARP inhibitors induce DNA lesions, the repair of which are highly dependent on homologous recombination (HR), and preferentially kill HR- deficient cancers. However, cancer cells have developed several mechanisms to transform HR and confer drug resistance to PARP inhibition. Therefore, there is a great clinical interest in exploring new therapies that induce HR deficiency (HRD), thereby sensitizing cancer cells to PARP inhibitors. Here, we found that GSK2578215A, a high-selective and effective leucine-rich repeat kinase 2 (LRRK2) inhibitor, or LRRK2 depletion suppresses HR preventing the recruitment of RAD51 to DNA damage sites through disruption of the interaction of RAD51 and BRCA2. Moreover, LRRK2 inhibition or depletion increases the susceptibility of ovarian cancer cells to Olaparib in vitro and in vivo. In clinical specimens, LRRK2 high expression is high related with advanced clinical characteristics and poor survival of ovarian cancer patients. All these findings indicate ovarian cancers expressing high levels of LRRK2 are more resistant to treatment potentially through promoting HR. Furthermore, combination treatment with an LRRK2 and PARP inhibitor may be a novel strategy to improve the effectiveness of LRRK2 expression ovarian cancers.

Pathological Functions of LRRK2 in Parkinson's Disease

Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) are common genetic risk factors for both familial and sporadic Parkinson’s disease (PD). Pathogenic mutations in LRRK2 have been shown to induce changes in its activity, and abnormal increase in LRRK2 kinase activity is thought to contribute to PD pathology. The precise molecular mechanisms underlying LRRK2-associated PD pathology are far from clear, however the identification of LRRK2 substrates and the elucidation of cellular pathways involved suggest a role of LRRK2 in microtubule dynamics, vesicular trafficking, and synaptic transmission. Moreover, LRRK2 is associated with pathologies of α-synuclein, a major component of Lewy bodies (LBs). Evidence from various cellular and animal models supports a role of LRRK2 in the regulation of aggregation and propagation of α-synuclein. Here, we summarize our current understanding of how pathogenic mutations dysregulate LRRK2 and discuss the possible mechanisms leading to neurodegeneration.

Back and to the Future: From Neurotoxin-Induced to Human Parkinson's Disease Models

Parkinson's disease (PD) is an age-related neurodegenerative disorder characterized by motor symptoms such as tremor, slowness of movement, rigidity, and postural instability, as well as non-motor features like sleep disturbances, loss of ability to smell, depression, constipation, and pain. Motor symptoms are caused by depletion of dopamine in the striatum due to the progressive loss of dopamine neurons in the substantia nigra pars compacta. Approximately 10% of PD cases are familial arising from genetic mutations in α-synuclein, LRRK2, DJ-1, PINK1, parkin, and several other proteins. The majority of PD cases are, however, idiopathic, i.e., having no clear etiology. PD is characterized by progressive accumulation of insoluble inclusions, known as Lewy bodies, mostly composed of α-synuclein and membrane components. The cause of PD is currently attributed to cellular proteostasis deregulation and mitochondrial dysfunction, which are likely interdependent. In addition, neuroinflammation is present in brains of PD patients, but whether it is the cause or consequence of neurodegeneration remains to be studied. Rodents do not develop PD or PD-like motor symptoms spontaneously; however, neurotoxins, genetic mutations, viral vector-mediated transgene expression and, recently, injections of misfolded α-synuclein have been successfully utilized to model certain aspects of the disease. Here, we critically review the advantages and drawbacks of rodent PD models and discuss approaches to advance pre-clinical PD research towards successful disease-modifying therapy. © 2020 The Authors.

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.

Progress in LRRK2-Associated Parkinson's Disease Animal Models

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

Human Spinal Motor Neurons Are Particularly Vulnerable to Cerebrospinal Fluid of Amyotrophic Lateral Sclerosis Patients

Amyotrophic lateral sclerosis (ALS) is the most common and devastating motor neuron (MN) disease. Its pathophysiological cascade is still enigmatic. More than 90% of ALS patients suffer from sporadic ALS, which makes it specifically demanding to generate appropriate model systems. One interesting aspect considering the seeding, spreading and further disease development of ALS is the cerebrospinal fluid (CSF). We therefore asked whether CSF from sporadic ALS patients is capable of causing disease typical changes in human patient-derived spinal MN cultures and thus could represent a novel model system for sporadic ALS. By using induced pluripotent stem cell (iPSC)-derived MNs from healthy controls and monogenetic forms of ALS we could demonstrate a harmful effect of ALS-CSF on healthy donor-derived human MNs. Golgi fragmentation—a typical finding in lower organism models and human postmortem tissue—was induced solely by addition of ALS-CSF, but not control-CSF. No other neurodegenerative hallmarks—including pathological protein aggregation—were found, underpinning Golgi fragmentation as early event in the neurodegenerative cascade. Of note, these changes occurred predominantly in MNs, the cell type primarily affected in ALS. We thus present a novel way to model early features of sporadic ALS.

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

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

Downregulated LRRK2 gene expression inhibits proliferation and migration while promoting the apoptosis of thyroid cancer cells by inhibiting activation of the JNK signaling pathway

Emerging studies have indicated that leucine-rich repeat kinase 2 (LRRK2) is associated with thyroid cancer (TC). The present study investigated the effect of LRRK2 on the cell cycle and apoptosis in TC, and examined the underlying mechanisms in vitro. To screen TC-associated differentially expressed genes, gene expression microarray analysis was conducted. Retrieval of pathways associated with TC from the Kyoto Encyclopedia of Genes and Genomes database indicated that the c-Jun N-terminal kinase (JNK) signaling pathway serves an essential role in TC. SW579, IHH-4, TFC-133, TPC-1 and Nthy-ori3-1 cell lines were used to screen cell lines with the highest and lowest LRRK2 expression for subsequent experiments. The two selected cell lines were transfected with pcDNA-LRRK2, or small interfering RNA against LRRK2 or SP600125 (a JNK inhibitor). Subsequently, flow cytometry, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling, a 5-ethynyl-2′-deoxyuridine assay and a scratch test was conducted to detect the cell cycle distribution, apoptosis, proliferation and migration, respectively, in each group. The LRRK2 gene was determined to be elevated in TC based on the microarray data of the GSE3678 dataset. The SW579 cell line was identified to exhibit the highest LRRK2 expression, while IHH-4 cells exhibited the lowest LRRK2 expression. LRRK2 silencing, through inhibiting the activation of the JNK signaling pathway, increased the expression levels of genes and proteins associated with cell cycle arrest and apoptosis in TC cells, promoted cell cycle arrest and apoptosis, and inhibited cell migration and proliferation in TC cells, indicating that LRRK2 repression could exert beneficial effects through the JNK signaling pathway on TC cells. These observations demonstrate that LRRK2 silencing promotes TC cell growth inhibition, and facilitates apoptosis and cell cycle arrest. The JNK signaling pathway may serve a crucial role in mediating the anti-carcinogenic activities of downregulated LRRK2 in TC.

The role of LRRK2 in cytoskeletal dynamics

Leucine-rich repeat kinase 2 (LRRK2), a complex kinase/GTPase mutated in Parkinson's disease, has been shown to physically and functionally interact with cytoskeletal-related components in different brain cells. Neurons greatly rely on a functional cytoskeleton for many homeostatic processes such as local and long-distance vesicle transport, synaptic plasticity, and dendrites/axons growth and remodeling. Here, we will review the available data linking LRRK2 and the cytoskeleton, and discuss how this may be functionally relevant for the well-established roles of LRRK2 in intracellular trafficking pathways and outgrowth of neuronal processes in health and disease conditions.

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.

LRRK2 mouse models: dissecting the behavior, striatal neurochemistry and neurophysiology of PD pathogenesis

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of familial Parkinson's disease (PD), resembling the sporadic disorder. Intensive effort has been directed toward LRRK2 mouse modeling and investigation, aimed at reproducing the human disease to inform mechanistic studies of pathogenesis and design of neuroprotective therapies. The physiological function of LRRK2 is still under exploration, but a clear role in striatal neurophysiology and animal behavior has emerged. Alterations in LRRK2 impair dopamine (DA) transmission, regulation and signaling, in addition to corticostriatal synaptic plasticity. Consistently, several subtle abnormalities in motor and nonmotor abilities have been demonstrated in LRRK2 genetic mouse models, generally paralleling preclinical symptoms of early DA dysfunction. However, the variability in model design and phenotypes observed requires a critical approach in interpreting the results, adapting the model used to the specific research question. Etiologically appropriate knockin mice might represent the ultimate animal model in which to study early disease mechanisms and therapies as well as to investigate drug effectiveness and off-target consequences.

Animal behavioral assessments in current research of Parkinson's disease

Parkinson's disease (PD), a neurodegenerative disorder, is traditionally classified as a movement disorder. Patients typically suffer from many motor dysfunctions. Presently, clinicians and scientists recognize that many non-motor symptoms are associated with PD. There is an increasing interest in both motor and non-motor symptoms in clinical studies on PD patients and laboratory research on animal models that imitate the pathophysiologic features and symptoms of PD patients. Therefore, appropriate behavioral assessments are extremely crucial for correctly understanding the mechanisms of PD and accurately evaluating the efficacy and safety of novel therapies. This article systematically reviews the behavioral assessments, for both motor and non-motor symptoms, in various animal models involved in current PD research. We addressed the strengths and weaknesses of these behavioral tests and their appropriate applications. Moreover, we discussed potential mechanisms behind these behavioral tests and cautioned readers against potential experimental bias. Since most of the behavioral assessments currently used for non-motor symptoms are not particularly designed for animals with PD, it is of the utmost importance to greatly improve experimental design and evaluation in PD research with animal models. Indeed, it is essential to develop specific assessments for non-motor symptoms in PD animals based on their characteristics. We concluded with a prospective view for behavioral assessments with real-time assessment with mobile internet and wearable device in future PD research.

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.

LRRK2 BAC transgenic rats develop progressive, L-DOPA-responsive motor impairment, and deficits in dopamine circuit function

Mutations in leucine-rich repeat kinase 2 (LRRK2) lead to late-onset, autosomal dominant Parkinson's disease, characterized by the degeneration of dopamine neurons of the substantia nigra pars compacta, a deficit in dopamine neurotransmission and the development of motor and non-motor symptoms. The most prevalent Parkinson's disease LRRK2 mutations are located in the kinase (G2019S) and GTPase (R1441C) encoding domains of LRRK2. To better understand the sequence of events that lead to progressive neurophysiological deficits in vulnerable neurons and circuits in Parkinson's disease, we have generated LRRK2 bacterial artificial chromosome transgenic rats expressing either G2019S or R1441C mutant, or wild-type LRRK2, from the complete human LRRK2 genomic locus, including endogenous promoter and regulatory regions. Aged (18-21 months) G2019S and R1441C mutant transgenic rats exhibit L-DOPA-responsive motor dysfunction, impaired striatal dopamine release as determined by fast-scan cyclic voltammetry, and cognitive deficits. In addition, in vivo recordings of identified substantia nigra pars compacta dopamine neurons in R1441C LRRK2 transgenic rats reveal an age-dependent reduction in burst firing, which likely results in further reductions to striatal dopamine release. These alterations to dopamine circuit function occur in the absence of neurodegeneration or abnormal protein accumulation within the substantia nigra pars compacta, suggesting that nigrostriatal dopamine dysfunction precedes detectable protein aggregation and cell death in the development of Parkinson's disease. In conclusion, our longitudinal deep-phenotyping provides novel insights into how the genetic burden arising from human mutant LRRK2 manifests as early pathophysiological changes to dopamine circuit function and highlights a potential model for testing Parkinson's therapeutics.

Neural stem cells in Parkinson's disease: a role for neurogenesis defects in onset and progression

Parkinson's disease (PD) is the second most common neurodegenerative disorder, leading to a variety of motor and non-motor symptoms. Interestingly, non-motor symptoms often appear a decade or more before the first signs of motor symptoms. Some of these non-motor symptoms are remarkably similar to those observed in cases of impaired neurogenesis and several PD-related genes have been shown to play a role in embryonic or adult neurogenesis. Indeed, animal models deficient in Nurr1, Pitx3, SNCA and PINK1 display deregulated embryonic neurogenesis and LRRK2 and VPS35 have been implicated in neuronal development-related processes such as Wnt/β-catenin signaling and neurite outgrowth. Moreover, adult neurogenesis is affected in both PD patients and PD animal models and is regulated by dopamine and dopaminergic (DA) receptors, by chronic neuroinflammation, such as that observed in PD, and by differential expression of wild-type or mutant forms of PD-related genes. Indeed, an increasing number of in vivo studies demonstrate a role for SNCA and LRRK2 in adult neurogenesis and in the generation and maintenance of DA neurons. Finally, the roles of PD-related genes, SNCA, LRRK2, VPS35, Parkin, PINK1 and DJ-1 have been studied in NSCs, progenitor cells and induced pluripotent stem cells, demonstrating a role for some of these genes in stem/progenitor cell proliferation and maintenance. Together, these studies strongly suggest a link between deregulated neurogenesis and the onset and progression of PD and present strong evidence that, in addition to a neurodegenerative disorder, PD can also be regarded as a developmental disorder.

The mouse/human cross-species heterodimer of leucine-rich repeat kinase 2: possible significance in the transgenic model mouse of Parkinson's disease

Leucine-rich repeat kinase (LRRK2) is the causal molecule of autosomal dominant Parkinson's disease (PD). We previously reported that intracellular degradation of wild-type (WT) LRRK2 is promoted by formation of heterodimers with the I2020T mutant LRRK2. In the present study, we investigated whether this is also the case for mouse/human cross-species heterodimers, which could be formed in transgenic mice. First, by co-transfection and immunoprecipitation, we identified the cross-species heterodimer of mouse LRRK2 and human LRRK2. Next, we found that the protein level of mouse LRRK2 decreased when co-transfected with human I2020T LRRK2, but not with human WT LRRK2. These results suggested that degradation of mouse LRRK2 was promoted by formation of a cross-species heterodimer with the mutant LRRK2. In I2020T LRRK2-transgenic mice, the lower protein level of brain LRRK2 in comparison with control mice, together with higher expression of the mRNA, suggested that endogenous LRRK2 was degraded by formation of cross-species heterodimers. Our results suggest a new concept of cross-species dimer/oligomer formation in transgenic disease-model mice.

Neuroinflammation in Parkinson's disease animal models: a cell stress response or a step in neurodegeneration?

The motor symptoms of Parkinson's disease are due to the progressive degeneration of dopaminergic neurons in the substantia nigra. Multiple neuroinflammatory processes are exacerbated in Parkinson's disease, including glial-mediated reactions, increased expression of proinflammatory substances, and lymphocytic infiltration, particularly in the substantia nigra. Neuroinflammation is also implicated in the neurodegeneration and consequent behavioral symptoms of many Parkinson's disease animal models, although it is not clear whether these features emulate pathogenic steps in the genuine disorder or if some inflammatory features provide protective stress responses. Here, we compare and summarize findings on neuroinflammatory responses and effects on behavior in a wide range of toxin-based, inflammatory and genetic Parkinson's disease animal models.

Parkinson's disease: animal models and dopaminergic cell vulnerability

Parkinson's disease (PD) is a neurodegenerative disorder that affects about 1.5% of the global population over 65 years of age. A hallmark feature of PD is the degeneration of the dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and the consequent striatal DA deficiency. Yet, the pathogenesis of PD remains unclear. Despite tremendous growth in recent years in our knowledge of the molecular basis of PD and the molecular pathways of cell death, important questions remain, such as: (1) why are SNc cells especially vulnerable; (2) which mechanisms underlie progressive SNc cell loss; and (3) what do Lewy bodies or α-synuclein reveal about disease progression. Understanding the variable vulnerability of the dopaminergic neurons from the midbrain and the mechanisms whereby pathology becomes widespread are some of the primary objectives of research in PD. Animal models are the best tools to study the pathogenesis of PD. The identification of PD-related genes has led to the development of genetic PD models as an alternative to the classical toxin-based ones, but does the dopaminergic neuronal loss in actual animal models adequately recapitulate that of the human disease? The selection of a particular animal model is very important for the specific goals of the different experiments. In this review, we provide a summary of our current knowledge about the different in vivo models of PD that are used in relation to the vulnerability of the dopaminergic neurons in the midbrain in the pathogenesis of PD.

LRRK2 pathobiology in Parkinson's disease

Mutations in the catalytic Roc-COR and kinase domains of leucine-rich repeat kinase 2 (LRRK2) are a common cause of familial Parkinson's disease (PD). LRRK2 mutations cause PD with age-related penetrance and clinical features identical to late-onset sporadic PD. Biochemical studies support an increase in LRRK2 kinase activity and a decrease in GTPase activity for kinase domain and Roc-COR mutations, respectively. Strong evidence exists that LRRK2 toxicity is kinase dependent leading to extensive efforts to identify selective and brain-permeable LRRK2 kinase inhibitors for clinical development. Cell and animal models of PD indicate that LRRK2 mutations affect vesicular trafficking, autophagy, protein synthesis, and cytoskeletal function. Although some of these biological functions are affected consistently by most disease-linked mutations, others are not and it remains currently unclear how mutations that produce variable effects on LRRK2 biochemistry and function all commonly result in the degeneration and death of dopamine neurons. LRRK2 is typically present in Lewy bodies and its toxicity in mammalian models appears to be dependent on the presence of α-synuclein, which is elevated in human iPS-derived dopamine neurons from patients harboring LRRK2 mutations. Here, we summarize biochemical and functional studies of LRRK2 and its mutations and focus on aberrant vesicular trafficking and protein synthesis as two leading mechanisms underlying LRRK2-linked disease.

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.

Conditional expression of Parkinson's disease-related R1441C LRRK2 in midbrain dopaminergic neurons of mice causes nuclear abnormalities without neurodegeneration

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene cause late-onset, autosomal dominant Parkinson's disease (PD). The clinical and neurochemical features of LRRK2-linked PD are similar to idiopathic disease although neuropathology is somewhat heterogeneous. Dominant mutations in LRRK2 precipitate neurodegeneration through a toxic gain-of-function mechanism which can be modeled in transgenic mice overexpressing human LRRK2 variants. A number of LRRK2 transgenic mouse models have been developed that display abnormalities in dopaminergic neurotransmission and alterations in tau metabolism yet without consistently inducing dopaminergic neurodegeneration. To directly explore the impact of mutant LRRK2 on the nigrostriatal dopaminergic pathway, we developed conditional transgenic mice that selectively express human R1441C LRRK2 in dopaminergic neurons from the endogenous murine ROSA26 promoter. The expression of R1441C LRRK2 does not induce the degeneration of substantia nigra dopaminergic neurons or striatal dopamine deficits in mice up to 2years of age, and fails to precipitate abnormal protein inclusions containing alpha-synuclein, tau, ubiquitin or autophagy markers (LC3 and p62). Furthermore, mice expressing R1441C LRRK2 exhibit normal motor activity and olfactory function with increasing age. Intriguingly, the expression of R1441C LRRK2 induces age-dependent abnormalities of the nuclear envelope in nigral dopaminergic neurons including reduced nuclear circularity and increased invaginations of the nuclear envelope. In addition, R1441C LRRK2 mice display increased neurite complexity of cultured midbrain dopaminergic neurons. Collectively, these novel R1441C LRRK2 conditional transgenic mice reveal altered dopaminergic neuronal morphology with advancing age, and provide a useful tool for exploring the pathogenic mechanisms underlying the R1441C LRRK2 mutation in PD.

LRRK2, a puzzling protein: insights into Parkinson's disease pathogenesis

Leucine-rich repeat kinase 2 (LRRK2) is a large, ubiquitous protein of unknown function. Mutations in the gene encoding LRRK2 have been linked to familial and sporadic Parkinson's disease (PD) cases. The LRRK2 protein is a single polypeptide that displays GTPase and kinase activity. Kinase and GTPase domains are involved in different cellular signaling pathways. Despite several experimental studies associating LRRK2 protein with various intracellular membranes and vesicular structures such as endosomal/lysosomal compartments, the mitochondrial outer membrane, lipid rafts, microtubule-associated vesicles, the golgi complex, and the endoplasmic reticulum its broader physiologic function(s) remain unidentified. Additionally, the cellular distribution of LRRK2 may indicate its role in several different pathways, such as the ubiquitin-proteasome system, the autophagic-lysosomal pathway, intracellular trafficking, and mitochondrial dysfunction. This review discusses potential mechanisms through which LRRK2 may mediate neurodegeneration and cause PD.

Non-motor and motor features in LRRK2 transgenic mice

Background

Non-motor symptoms are increasingly recognized as important features of Parkinson’s disease (PD). LRRK2 mutations are common causes of familial and sporadic PD. Non-motor features have not been yet comprehensively evaluated in LRRK2 transgenic mouse models.

Objective

Using a transgenic mouse model overexpressing the R1441G mutation of the human LRRK2 gene, we have investigated the longitudinal correlation between motor and non-motor symptoms and determined if specific non-motor phenotypes precede motor symptoms.

Methodology

We investigated the onset of motor and non-motor phenotypes on the LRRK2^R1441G BAC transgenic mice and their littermate controls from 4 to 21 month-old using a battery of behavioral tests. The transgenic mutant mice displayed mild hypokinesia in the open field from 16 months old, with gastrointestinal dysfunctions beginning at 6 months old. Non-motor features such as depression and anxiety-like behaviors, sensorial functions (pain sensitivity and olfaction), and learning and memory abilities in the passive avoidance test were similar in the transgenic animals compared to littermate controls.

Conclusions

LRRK2^R1441G BAC transgenic mice displayed gastrointestinal dysfunction at an early stage but did not have abnormalities in fine behaviors, olfaction, pain sensitivity, mood disorders and learning and memory compared to non-transgenic littermate controls. The observations on olfaction and gastrointestinal dysfunction in this model validate findings in human carriers. These mice did recapitulate mild Parkinsonian motor features at late stages but compensatory mechanisms modulating the progression of PD in these models should be further evaluated.

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.

LRRK2: an éminence grise of Wnt-mediated neurogenesis?

The importance of leucine-rich repeat kinase 2 (LRRK2) to mature neurons is well-established, since mutations in PARK8, the gene encoding LRRK2, are the most common known cause of Parkinson’s disease. Nonetheless, despite the LRRK2 knockout mouse having no overt neurodevelopmental defect, numerous lines of in vitro data point toward a central role for this protein in neurogenesis. Roles for LRRK2 have been described in many key processes, including neurite outgrowth and the regulation of microtubule dynamics. Moreover, LRRK2 has been implicated in cell cycle control, suggesting additional roles in neurogenesis that precede terminal differentiation. However, we contend that the suggested function of LRRK2 as a scaffolding protein at the heart of numerous Wnt signaling cascades provides the most tantalizing link to neurogenesis in the developing brain. Numerous lines of evidence show a critical requirement for multiple Wnt pathways in the development of certain brain regions, not least the dopaminergic neurons of the ventral mid-brain. In conclusion, these observations indicate a function of LRRK2 as a subtle yet critical mediator of the action of Wnt ligands on developing neurons. We suggest that LRRK2 loss- or gain-of-function are likely modifiers of developmental phenotypes seen in animal models of Wnt signaling deregulation, a hypothesis that can be tested by cross-breeding relevant genetically modified experimental strains.

Expression analysis of Lrrk1, Lrrk2 and Lrrk2 splice variants in mice

Missense mutations in the leucine-rich repeat kinase 2 gene (LRRK2) are linked to autosomal dominant forms of Parkinson's disease (PD). In order to get insights into the physiological role of Lrrk2, we examined the distribution of Lrrk2 mRNA and different splice variants in the developing murine embryo and the adult brain of Mus musculus. To analyse if the Lrrk2-paralog, Lrrk1, may have redundant functions in PD-development, we also compared Lrrk1 and Lrrk2 expression in the same tissues. Using radioactive in situ hybridization, we found ubiquitous expression of both genes at low level from embryonic stage E9.5 onward, which progressively increased up until birth. The developing central nervous system (CNS) displayed no prominent Lrrk2 mRNA signals at these time-points. However, in the entire postnatal brain Lrrk2 became detectable, showing strongest level in the striatum and the cortex of adult mice; Lrrk1 was only detectable in the mitral cell layer of the olfactory bulb. Thus, due to the non-overlapping expression patterns, a redundant function of Lrrk2 and Lrrk1 in the pathogenesis of PD seems to be unlikely. Quantification of Lrrk2 mRNA and protein level in several brain regions by real-time PCR and Western blot verified the striatum and cortex as hotspots of postnatal Lrrk2 expression. Strong expression of Lrrk2 is mainly found in neurons, specifically in the dopamine receptor 1 (DRD1a) and 2 (DRD2)-positive subpopulations of the striatal medium spiny neurons. Finally, we identified 2 new splice-variants of Lrrk2 in RNA-samples from various adult brain regions and organs: a variant with a skipped exon 5 and a truncated variant terminating in an alternative exon 42a. In order to identify the origin of these two splice variants, we also analysed primary neural cultures independently and found cell-specific expression patterns for these variants in microglia and astrocytes.

Type II kinase inhibitors show an unexpected inhibition mode against Parkinson's disease-linked LRRK2 mutant G2019S

A number of well-known type II inhibitors (ATP-noncompetitive) that bind kinases in their DFG-out conformation were tested against wild-type LRRK2 and the most common Parkinson's disease-linked mutation, G2019S. We found that traditional type II inhibitors exhibit surprising variability in their inhibition mechanism between the wild type (WT) and the G2019S mutant of LRRK2. The type II kinase inhibitors were found to work in an ATP-competitive fashion against the G2019S mutant, whereas they appear to follow the expected noncompetitive mechanism against WT. Because the G2019S mutation lies in the DXG motif (DYG in LRRK2 but DFG in most other kinases) of the activation loop, we explored the structural consequence of the mutation on loop dynamics using an enhanced sampling method called metadynamics. The simulations suggest that the G2019S mutation stabilizes the DYG-in state of LRRK2 through a series of hydrogen bonds, leading to an increase in the conformational barrier between the active and inactive forms of the enzyme and a relative stabilization of the active form. The conformational bias toward the active form of LRRK2 mutants has two primary consequences. (1) The mutant enzyme becomes hyperactive, a known contributor to the Parkinsonian phenotype, as a consequence of being "locked" into the activated state, and (2) the mutation creates an unusual allosteric pocket that can bind type II inhibitors but in an ATP-competitive fashion. Our results suggest that developing type II inhibitors, which are generally considered superior to type I inhibitors because of desirable selectivity profiles, might be especially challenging for the G2019S LRRK2 mutant.

Insights into LRRK2 function and dysfunction from transgenic and knockout rodent models

Mutations in the LRRK2 (leucine-rich repeat kinase 2) gene on chromosome 12 cause autosomal dominant PD (Parkinson's disease), which is indistinguishable from sporadic forms of the disease. Numerous attempts have therefore been made to model PD in rodents via the transgenic expression of LRRK2 and its mutant variants and to elucidate the function of LRRK2 by knocking out rodent Lrrk2. Although these models often only partially recapitulate PD pathology, they have helped to elucidate both the normal and pathological function of LRRK2. In particular, LRRK2 has been suggested to play roles in cytoskeletal dynamics, synaptic machinery, dopamine homoeostasis and autophagic processes. Our understanding of how these pathways are affected, their contribution towards PD development and their interaction with one another is still incomplete, however. The present review summarizes the findings from LRRK2 rodent models and draws potential connections between the apparently disparate cellular processes altered, in order to better understand the underlying mechanisms of LRRK2 dysfunction and illuminate future therapeutic interventions.