Dietary Amino Acids Impact LRRK2-Induced Neurodegeneration in Parkinson's Disease Models.

The G2019S mutation in leucine-rich repeat kinase 2 (LRRK2) is a common cause of Parkinson's disease (PD) and results in age-related dopamine neuron loss and locomotor dysfunction in Drosophila melanogaster through an aberrant increase in bulk neuronal protein synthesis. Under nonpathologic conditions, protein synthesis is tightly controlled by metabolic regulation. Whether nutritional and metabolic influences on protein synthesis can modulate the pathogenic effect of LRRK2 on protein synthesis and thereby impact neuronal loss is a key unresolved question. Here, we show that LRRK2 G2019S-induced neurodegeneration is critically dependent on dietary amino acid content in Drosophila studies with both sexes. Low dietary amino acid concentration prevents aberrant protein synthesis and blocks LRRK2 G2019S-mediated neurodegeneration in Drosophila and rat primary neurons. Unexpectedly, a moderately high-amino acid diet also blocks dopamine neuron loss and motor deficits in Drosophila through a separate mechanism involving stress-responsive activation of 5'-AMP-activated protein kinase (AMPK) and neuroprotective induction of autophagy, implicating the importance of protein homeostasis to neuronal viability. At the highest amino acid diet of the range tested, PD-related neurodegeneration occurs in an age-related manner, but is also observed in control strains, suggesting that it is independent of mutant LRRK2 expression. We propose that dietary influences on protein synthesis and autophagy are critical determinants of LRRK2 neurodegeneration, opening up possibilities for future therapeutic intervention.SIGNIFICANCE STATEMENT Parkinson's disease (PD) prevalence is projected to rise as populations continue to age, yet there are no current therapeutic approaches that delay or stop disease progression. A broad role for leucine-rich repeat kinase 2 (LRRK2) mutations in familial and idiopathic PD has emerged. Here, we show that dietary amino acids are important determinants of neurodegeneration in a Drosophila model of LRRK2 PD. Restricting all amino acids effectively suppresses dopaminergic neuron loss and locomotor deficits and is associated with reduced protein synthesis, while moderately high amino acids similarly attenuate these PD-related phenotypes through a stress-responsive induction of 5'-AMP-activated protein kinase and autophagy. These studies suggest that diet plays an important role in the development of PD-related phenotypes linked to LRRK2.

Genetic and Environmental Factors Influence the Pleomorphy of LRRK2 Parkinsonism.

Missense mutations in the LRRK2 gene were first identified as a pathogenic cause of Parkinson's disease (PD) in 2004. Soon thereafter, a founder mutation in LRRK2, p.G2019S (rs34637584), was described, and it is now estimated that there are approximately 100,000 people worldwide carrying this risk variant. While the clinical presentation of LRRK2 parkinsonism has been largely indistinguishable from sporadic PD, disease penetrance and age at onset can be quite variable. In addition, its neuropathological features span a wide range from nigrostriatal loss with Lewy body pathology, lack thereof, or atypical neuropathology, including a large proportion of cases with concomitant Alzheimer's pathology, hailing LRRK2 parkinsonism as the "Rosetta stone" of parkinsonian disorders, which provides clues to an understanding of the different neuropathological trajectories. These differences may result from interactions between the LRRK2 mutant protein and other proteins or environmental factors that modify LRRK2 function and, thereby, influence pathobiology. This review explores how potential genetic and biochemical modifiers of LRRK2 function may contribute to the onset and clinical presentation of LRRK2 parkinsonism. We review which genetic modifiers of LRRK2 influence clinical symptoms, age at onset, and penetrance, what LRRK2 mutations are associated with pleomorphic LRRK2 neuropathology, and which environmental modifiers can augment LRRK2 mutant pathophysiology. Understanding how LRRK2 function is influenced and modulated by other interactors and environmental factors-either increasing toxicity or providing resilience-will inform targeted therapeutic development in the years to come. This will allow the development of disease-modifying therapies for PD- and LRRK2-related neurodegeneration.

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

BACKGROUND: Coding variation in the Leucine rich repeat kinase 2 gene linked to Parkinson's disease (PD) promotes enhanced activity of the encoded LRRK2 kinase, particularly with respect to autophosphorylation at S1292 and/or phosphorylation of the heterologous substrate RAB10. OBJECTIVE: To determine the inter-laboratory reliability of measurements of cellular LRRK2 kinase activity in the context of wildtype or mutant LRRK2 expression using published protocols. METHODS: Benchmark western blot assessments of phospho-LRRK2 and phospho-RAB10 were performed in parallel with in situ immunological approaches in HEK293T, mouse embryonic fibroblasts, and lymphoblastoid cell lines. Rat brain tissue, with or without adenovirus-mediated LRRK2 expression, and human brain tissues from subjects with or without PD, were also evaluated for LRRK2 kinase activity markers. RESULTS: Western blots were able to detect extracted LRRK2 activity in cells and tissue with pS1292-LRRK2 or pT73-RAB10 antibodies. However, while LRRK2 kinase signal could be detected at the cellular level with over-expressed mutant LRRK2 in cell lines, we were unable to demonstrate specific detection of endogenous cellular LRRK2 activity in cell culture models or tissues that we evaluated. CONCLUSION: Further development of reliable methods that can be deployed in multiple laboratories to measure endogenous LRRK2 activities are likely required, especially at cellular resolution.

HSP90 Inhibitor, NVP-AUY922, Improves Myelination in Vitro and Supports the Maintenance of Myelinated Axons in Neuropathic Mice.

Hereditary demyelinating neuropathies linked to peripheral myelin protein 22 (PMP22) involve the disruption of normal protein trafficking and are therefore relevant targets for chaperone therapy. Using a small molecule HSP90 inhibitor, EC137, in cell culture models, we previously validated the chaperone pathway as a viable target for therapy development. Here, we tested five commercially available inhibitors of HSP90 and identified BIIB021 and AUY922 to support Schwann cell viability and enhance chaperone expression. AUY922 showed higher efficacy, compared to BIIB021, in enhancing myelin synthesis in dorsal root ganglion explant cultures from neuropathic mice. For in vivo testing, we randomly assigned 2-3 month old C22 and 6 week old Trembler J (TrJ) mice to receive two weekly injections of either vehicle or AUY922 (2 mg/kg). By the intraperitoneal (i.p.) route, the drug was well-tolerated by all mice over the 5 month long study, without influence on body weight or general grooming behavior. AUY922 improved the maintenance of myelinated nerves of both neuropathic models and attenuated the decline in rotarod performance and peak muscle force production in C22 mice. These studies highlight the significance of proteostasis in neuromuscular function and further validate the HSP90 pathway as a therapeutic target for hereditary neuropathies.

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.

Inducible HSP70 is critical in preventing the aggregation and enhancing the processing of PMP22.

Chaperones, also called heat shock proteins (HSPs), transiently interact with proteins to aid their folding, trafficking, and degradation, thereby directly influencing the transport of newly synthesized molecules. Induction of chaperones provides a potential therapeutic approach for protein misfolding disorders, such as peripheral myelin protein 22 (PMP22)-associated peripheral neuropathies. Cytosolic aggregates of PMP22, linked with a demyelinating Schwann cell phenotype, result in suppression of proteasome activity and activation of proteostatic mechanisms, including the heat shock pathway. Although the beneficial effects of chaperones in preventing the aggregation and improving the trafficking of PMP22 have been repeatedly observed, the requirement for HSP70 in events remains elusive. In this study, we show that activation of the chaperone pathway in fibroblasts from PMP22 duplication-associated Charcot-Marie-Tooth disease type 1A patient with an FDA-approved small molecule increases HSP70 expression and attenuates proteasome dysfunction. Using cells from an HSP70.1/3(-/-) (inducible HSP70) mouse model, we demonstrate that under proteotoxic stress, this chaperone is critical in preventing the aggregation of PMP22, and this effect is aided by macroautophagy. When examined at steady-state, HSP70 appears to play a minor role in the trafficking of wild-type-PMP22, while it is crucial for preventing the buildup of the aggregation-prone Trembler-J-PMP22. HSP70 aids the processing of Trembler-J-PMP22 through the Golgi and its delivery to lysosomes via Rab7-positive vesicles. Together, these results demonstrate a key role for inducible HSP70 in aiding the processing and hindering the accumulation of misfolded PMP22, which in turn alleviates proteotoxicity within the cells.