Parkinson's disease-like burst firing activity in subthalamic nucleus induced by AAV-α-synuclein is normalized by LRRK2 modulation

Knocking out the LRRK2 gene increases sensitivity to wavelength information in rats

Leucine-rich repeat kinase 2 (LRRK2) is a gene related to familial Parkinson's disease (PD). It has been associated with nonmotor symptoms such as disturbances in the visual system affecting colour discrimination and contrast sensitivity. This study examined how deficiency of LRRK2 impacts visual processing in adult rats. Additionally, we investigated whether these changes can be modelled in wild-type rats by administering the LRRK2 inhibitor PFE360. Visual evoked potentials (VEPs) and steady-state visual evoked potentials (SSVEPs) were recorded in the visual cortex and superior colliculus of female LRRK2-knockout and wild-type rats to study how the innate absence of LRRK2 changes visual processing. Exposing the animals to stimulation at five different wavelengths revealed an interaction between genotype and the response to stimulation at different wavelengths. Differences in VEP amplitudes and latencies were robust and barely impacted by the presence of the LRRK2 inhibitor PFE360, suggesting a developmental effect. Taken together, these results indicate that alterations in visual processing were related to developmental deficiency of LRRK2 and not acute deficiency of LRRK2, indicating a role of LRRK2 in the functional development of the visual system and synaptic transmission.

Translating Pathological Brain Activity Primers in Parkinson's Disease Research

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

The role of LRRK2 in the periphery: link with Parkinson's disease and inflammatory diseases

Parkinson's disease (PD) is currently considered a multisystemic disorder rather than a pure brain disease, in line with the multiple hit hypothesis from Braak. However, despite increasing evidence that the pathology might originate in the periphery, multiple unknown aspects and contradictory data on the pathological processes taking place in the periphery jeopardize the interpretation and therapeutic targeting of PD. Mutations in the leucine-rich-repeat kinase 2 (LRRK2) gene have been widely linked with familial and sporadic PD cases. However, the actual role of LRRK2 in PD pathophysiology is far from understood. There is evidence that LRRK2 may be involved in alpha-synuclein (α-synuclein) pathology and immune cell regulation, but it has also been associated with inflammatory diseases such as inflammatory bowel disease, tuberculosis, leprosy, and several other bacterial infections. In this review, we focus on the different roles of LRRK2 in the periphery. More specifically, we discuss the involvement of LRRK2 in the propagation of α-synuclein pathology and its regulatory role in peripheral inflammation. A deeper understanding of the multidimensional functions of LRRK2 will pave the way for more accurate characterization of PD pathophysiology and its association with other inflammatory diseases.

In vivo susceptibility to energy failure parkinsonism and LRRK2 kinase activity

The G2019S mutation of LRRK2 represents a risk factor for idiopathic Parkinson's disease. Here, we investigate whether LRRK2 kinase activity regulates susceptibility to the environmental toxin 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP). G2019S knock-in mice (bearing enhanced kinase activity) showed greater nigro-striatal degeneration compared to LRRK2 knock-out, LRRK2 kinase-dead and wild-type mice following subacute MPTP treatment. LRRK2 kinase inhibitors PF-06447475 and MLi-2, tested under preventive or therapeutic treatments, protected against nigral dopamine cell loss in G2019S knock-in mice. MLi-2 also rescued striatal dopaminergic terminal degeneration in both G2019S knock-in and wild-type mice. Immunoblot analysis of LRRK2 Serine935 phosphorylation levels confirmed target engagement of LRRK2 inhibitors. However, MLi-2 abolished phosphoSerine935 levels in the striatum and midbrain of both wild-type and G2019S knock-in mice whereas PF-06447475 partly reduced phosphoSerine935 levels in the midbrain of both genotypes. In vivo and ex vivo uptake of the 18-kDa translocator protein (TSPO) ligand [¹⁸F]-VC701 revealed a similar TSPO binding in MPTP-treated wild-type and G2019S knock-in mice which was consistent with an increased GFAP striatal expression as revealed by Real Time PCR. We conclude that LRRK2 G2019S, likely through enhanced kinase activity, confers greater susceptibility to mitochondrial toxin-induced parkinsonism. LRRK2 kinase inhibitors are neuroprotective in this model.

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

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

No Detectable Effect on Visual Responses Using Functional MRI in a Rodent Model of α-Synuclein Expression

Parkinson's disease (PD) is a progressive neurodegenerative disease that is typically diagnosed late in its progression. There is a need for biomarkers suitable for monitoring the disease progression at earlier stages to guide the development of novel neuroprotective therapies. One potential biomarker, α-synuclein, has been found in both the familial cases of PD, as well as the sporadic cases and is considered a key feature of PD. α-synuclein is naturally present in the retina, and it has been suggested that early symptoms of the visual system may be used as a biomarker for PD. Here, we use a viral vector to induce a unilateral expression of human wild-type α-synuclein in rats as a mechanistic model of protein aggregation in PD. We employed functional magnetic resonance imaging (fMRI) to investigate whether adeno-associated virus (AAV) mediated expression of human wild-type α-synuclein alter functional activity in the visual system. A total of 16 rats were injected with either AAV-α-synuclein (n = 7) or AAV-null (n = 9) in the substantia nigra pars compacta (SNc) of the left hemisphere. The expression of α-synuclein was validated by a motor assay and postmortem immunohistochemistry. Five months after the introduction of the AAV-vector, fMRI showed robust blood oxygen level-dependent (BOLD) responses to light stimulation in the visual systems of both control and AAV-α-synuclein animals. However, our results demonstrate that the expression of AAV-α-synuclein does not affect functional activation of the visual system. This negative finding suggests that fMRI-based read-outs of visual responses may not be a sensitive biomarker for 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 Ablation Attenuates Αlpha-Synuclein-Induced Neuroinflammation Without Affecting Neurodegeneration or Neuropathology In Vivo

The development of disease-modifying therapies for Parkinson's disease is a major challenge which would be facilitated by a better understanding of the pathogenesis. Leucine-rich repeat kinase 2 (LRRK2) and α-synuclein are key players in Parkinson's disease, but their relationship remains incompletely resolved. Previous studies investigating the effect of LRRK2 on α-synuclein-induced neurotoxicity and neuroinflammation in preclinical Parkinson's disease models have reported conflicting results. Here, we aimed to further explore the functional interaction between α-synuclein and LRRK2 and to evaluate the therapeutic potential of targeting physiological LRRK2 levels. We studied the effects of total LRRK2 protein loss as well as pharmacological LRRK2 kinase inhibition in viral vector-mediated α-synuclein-based Parkinson's disease models developing early- and late-stage neurodegeneration. Surprisingly, total LRRK2 ablation or in-diet treatment with the LRRK2 kinase inhibitor MLi-2 did not significantly modify α-synuclein-induced motor deficits, dopaminergic cell loss, or α-synuclein pathology. Interestingly, we found a significant effect on α-synuclein-induced neuroinflammatory changes in the absence of LRRK2, with a reduced microglial activation and CD4+ and CD8+ T cell infiltration. This observed lack of protection against α-synuclein-induced toxicity should be well considered in light of the ongoing therapeutic development of LRRK2 kinase inhibitors for idiopathic Parkinson's disease. Future studies will be crucial to understand the link between these neuroinflammatory processes and disease progression as well as the role of α-synuclein and LRRK2 in these pathological events.

Classification of α-synuclein-induced changes in the AAV α-synuclein rat model of Parkinson's disease using electrophysiological measurements of visual processing

Biomarkers suitable for early diagnosis and monitoring disease progression are the cornerstone of developing disease-modifying treatments for neurodegenerative diseases such as Parkinson's disease (PD). Besides motor complications, PD is also characterized by deficits in visual processing. Here, we investigate how virally-mediated overexpression of α-synuclein in the substantia nigra pars compacta impacts visual processing in a well-established rodent model of PD. After a unilateral injection of vector, human α-synuclein was detected in the striatum and superior colliculus (SC). In parallel, there was a significant delay in the latency of the transient VEPs from the affected side of the SC in late stages of the disease. Inhibition of leucine-rich repeat kinase using PFE360 failed to rescue the VEP delay and instead increased the latency of the VEP waveform. A support vector machine classifier accurately classified rats according to their `disease state' using frequency-domain data from steady-state visual evoked potentials (SSVEP). Overall, these findings indicate that the latency of the rodent VEP is sensitive to changes mediated by the increased expression of α-synuclein and especially when full overexpression is obtained, whereas the SSVEP facilitated detection of α-synuclein across reflects all stages of PD model progression.

LRRK2 and Protein Aggregation in Parkinson's Disease: Insights From Animal Models

Mutations in leucine-rich repeat kinase 2 (LRRK2) instigate an autosomal dominant form of Parkinson’s disease (PD). Despite the neuropathological heterogeneity observed in LRRK2-PD, accumulating evidence suggests that alpha-synuclein and tau pathology are observed in a vast majority of cases. Intriguingly, the presence of protein aggregates spans both LRRK2-PD and idiopathic disease, supportive of a common pathologic mechanism. Thus, it is important to consider how LRRK2 mutations give rise to such pathology, and whether targeting LRRK2 might modify the accumulation, transmission, or toxicity of protein aggregates. Likewise, it is not clear how LRRK2 mutations drive PD pathogenesis, and whether protein aggregates are implicated in LRRK2-dependent neurodegeneration. While animal models have been instrumental in furthering our understanding of a potential interaction between LRRK2 and protein aggregation, the biology is far from clear. We aim to provide a thoughtful overview of the evidence linking LRRK2 to protein aggregation in animal models.

Dopaminergic neurodegeneration induced by Parkinson's disease-linked G2019S LRRK2 is dependent on kinase and GTPase activity

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of late-onset, autosomal-dominant familial Parkinson's disease (PD). LRRK2 functions as both a kinase and GTPase, and PD-linked mutations are known to influence both enzymatic activities. While PD-linked LRRK2 mutations can commonly induce neuronal damage in culture models, the mechanisms underlying these pathogenic effects remain uncertain. Rodent models containing familial LRRK2 mutations often lack robust PD-like neurodegenerative phenotypes. Here, we develop a robust preclinical model of PD in adult rats induced by the brain delivery of recombinant adenoviral vectors with neuronal-specific expression of human LRRK2 harboring the most common G2019S mutation. In this model, G2019S LRRK2 induces the robust degeneration of substantia nigra dopaminergic neurons, a pathological hallmark of PD. Introduction of a stable kinase-inactive mutation or administration of the selective kinase inhibitor, PF-360, attenuates neurodegeneration induced by G2019S LRRK2. Neuroprotection provided by pharmacological kinase inhibition is mediated by an unusual mechanism involving the robust destabilization of human LRRK2 protein in the brain relative to endogenous LRRK2. Our study further demonstrates that G2019S LRRK2-induced dopaminergic neurodegeneration critically requires normal GTPase activity, as hypothesis-testing mutations that increase GTP hydrolysis or impair GTP-binding activity provide neuroprotection although via distinct mechanisms. Taken together, our data demonstrate that G2019S LRRK2 induces neurodegeneration in vivo via a mechanism that is dependent on kinase and GTPase activity. Our study provides a robust rodent preclinical model of LRRK2-linked PD and nominates kinase inhibition and modulation of GTPase activity as promising disease-modifying therapeutic targets.

Genetic causes of PD: A pathway to disease modification

The underline neuropathology of Parkinson disease is pleiomorphic and its genetic background diverse. Possibly because of this heterogeneity, no effective disease modifying therapy is available. In this paper we give an overview of the genetics of Parkinson disease and explain how this is relevant for the development of new therapies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Long-Term Exposure to PFE-360 in the AAV-α-Synuclein Rat Model: Findings and Implications

Parkinson's disease (PD) is a progressive neurodegenerative disorder associated with impaired motor function and several non-motor symptoms, with no available disease modifying treatment. Intracellular accumulation of pathological α-synuclein inclusions is a hallmark of idiopathic PD, whereas, dominant mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with familial PD that is clinically indistinguishable from idiopathic PD. Recent evidence supports the hypothesis that an increase in LRRK2 kinase activity is associated with the development of not only familial LRRK2 PD, but also idiopathic PD. Previous reports have shown preclinical effects of LRRK2 modulation on α-synuclein-induced neuropathology. Increased subthalamic nucleus (STN) burst firing in preclinical neurotoxin models and PD patients is hypothesized to be causally involved in the development of the motor deficit in PD. To study a potential pathophysiological relationship between α-synuclein pathology and LRRK2 kinase activity in PD, we investigated the effect of chronic LRRK2 inhibition in an AAV-α-synuclein overexpression rat model. In this study, we report that chronic LRRK2 inhibition using PFE-360 only induced a marginal effect on motor function. In addition, the aberrant STN burst firing and associated neurodegenerative processes induced by α-synuclein overexpression model remained unaffected by chronic LRRK2 inhibition. Our findings do not strongly support LRRK2 inhibition for the treatment of PD. Therefore, the reported beneficial effects of LRRK2 inhibition in similar α-synuclein overexpression rodent models must be considered with prudence and additional studies are warranted in alternative α-synuclein-based models.

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

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

Modeling α-Synuclein Propagation with Preformed Fibril Injections

The aggregation of α-synuclein (α-syn) has been implicated in the pathogenesis of many neurodegenerative disorders, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). Postmortem analyses of α-syn pathology, especially that of PD, have suggested that aggregates progressively spread from a few discrete locations to wider brain regions. The neuron-to-neuron propagation of α-syn has been suggested to be the underlying mechanism by which aggregates spread throughout the brain. Many cellular and animal models has been created to study cell-to-cell propagation. Recently, it has been shown that a single injection of preformed fibrils (PFFs) made of recombinant α-syn proteins into various tissues and organs of many different animal species results in widespread α-syn pathology in the central nervous system (CNS). These PFF models have been extensively used to study the mechanism by which aggregates spread throughout the brain. Here, we review what we have learned from PFF models, describe the nature of PFFs and the neuropathological features, neurophysiological characteristics, and behavioral outcomes of the models.

Downregulation of tyrosine hydroxylase phenotype after AAV injection above substantia nigra: Caution in experimental models of Parkinson's disease

Adeno-associated virus (AAV) vector-mediated delivery of human α-synuclein (α-syn) gene in rat substantia nigra (SN) results in increased expression of α-syn protein in the SN and striatum which can progressively degenerate dopaminergic neurons. Therefore, this model is thought to recapitulate the neurodegeneration in Parkinson's disease. Here, using AAV to deliver α-syn above the SN in male and female rats resulted in clear expression of human α-syn in the SN and striatum. The protein was associated with moderate behavioral deficits and some loss of tyrosine hydroxylase (TH) in the nigrostriatal areas. However, the immunohistochemistry results were highly variable and showed little to no correlation with behavior and the amount of α-syn present. Expression of green fluorescent protein (GFP) was used as a control to monitor gene delivery and expression efficacy. AAV-GFP resulted in a similar or greater TH loss compared to AAV-α-syn and therefore an additional vector that does not express a protein was tested. Vectors with double-floxed inverse open reading frame (DIO ORF) encoding fluorescent proteins that generate RNA that is not translated also resulted in TH downregulation in the SN but showed no significant behavioral deficits. These results demonstrate that although expression of wild-type human α-syn can cause neurodegeneration, the variability and lack of correlation with outcome measures are drawbacks with the model. Furthermore, design and control selection should be considered carefully because of conflicting conclusions due to AAV downregulation of TH, and we recommend caution with having highly regulated TH as the only marker for the dopamine system.