Interaction of elongation factor 1-alpha with leucine-rich repeat kinase 2 impairs kinase activity and microtubule bundling in vitro

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.

The cell biology of Parkinson's disease

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

Identification of Targets from LRRK2 Rescue Phenotypes

Parkinson’s disease (PD) is an age-dependent neurodegenerative condition. Leucine-rich repeat kinase 2 (LRRK2) mutations are the most frequent cause of sporadic and autosomal dominant PD. The exact role of LRRK2 protective variants (R1398H, N551K) together with a pathogenic mutant (G2019S) in aging and neurodegeneration is unknown. We generated the following myc-tagged UAS-LRRK2 transgenic Drosophila: LRRK2 (WT), N551K, R1398H, G2019S single allele, and double-mutants (N551K/G2019S or R1398H/G2019S). The protective variants alone were able to suppress the phenotypic effects caused by the pathogenic LRRK2 mutation. Next, we conducted RNA-sequencing using mRNA isolated from dopaminergic neurons of these different groups of transgenic Drosophila. Using pathway enrichment analysis, we identified the top 10 modules (p < 0.05), with “LRRK2 in neurons in Parkinson’s disease” among the candidates. Further dissection of this pathway identified the most significantly modulated gene nodes such as eEF1A2, ACTB, eEF1A, and actin cytoskeleton reorganization. The induction of the pathway was successfully restored by the R1398H protective variant and R1398H-G2019S or N551K-G2019S rescue experiments. The oxidoreductase family of genes was also active in the pathogenic mutant and restored in protective and rescue variants. In summary, we provide in vivo evidence supporting the neuroprotective effects of LRRK2 variants. RNA sequencing of dopaminergic neurons identified upregulation of specific gene pathways in the Drosophila carrying the pathogenic variant, and this was restored in the rescue phenotypes. Using protective gene variants, our study identifies potential new targets and provides proof of principle of a new therapeutic approach that will further our understanding of aging and neurodegeneration in PD.

Chetomin rescues pathogenic phenotype of LRRK2 mutation in drosophila

Leucine-rich repeat kinase 2 (LRRK2) is a complex protein kinase involved in a diverse set of functions. Mutations in LRRK2 are a common cause of autosomal dominant familial Parkinson's disease. Peroxiredoxin 2 (PRDX2) belongs to a family of anti-oxidants that protect cells from oxidative stress. Importantly, PRDX2 is a cytoplasmic protein, similar to Leucine-rich repeat kinase 2, which localizes predominantly in the cytosol. Here, we demonstrated that Leurice-rich repeat kinase 2 phosphorylates PRDX2 in Drosophila, leading to a loss of dopaminergic neurons, climbing ability and shortened lifespan. These pathogenic phenotypes in the LRRK2 Drosophila were rescued with transgenic expression of PRDX2. Chetomin, a PRDX2 mimic, belongs to a class of epidithio-diketopiperazine fungal secondary metabolites (containing a dithiol group that has hydrogen peroxide-reducing activity). As proof of principle, we demonstrated that Chetomin recapitulated the rescue in these mutant Drosophila. Our findings suggest that Chetomin can be a potential therapeutic compound in LRRK2 linked Parkinson's disease.

Guilt-by-Association - Functional Insights Gained From Studying the LRRK2 Interactome

The Parkinson's disease-associated Leucine-rich repeat kinase 2 (LRRK2) is a complex multi-domain protein belonging to the Roco protein family, a unique group of G-proteins. Variants of this gene are associated with an increased risk of Parkinson's disease. Besides its well-characterized enzymatic activities, conferred by its GTPase and kinase domains, and a central dimerization domain, it contains four predicted repeat domains, which are, based on their structure, commonly involved in protein-protein interactions (PPIs). In the past decades, tremendous progress has been made in determining comprehensive interactome maps for the human proteome. Knowledge of PPIs has been instrumental in assigning functions to proteins involved in human disease and helped to understand the connectivity between different disease pathways and also significantly contributed to the functional understanding of LRRK2. In addition to an increased kinase activity observed for proteins containing PD-associated variants, various studies helped to establish LRRK2 as a large scaffold protein in the interface between cytoskeletal dynamics and the vesicular transport. This review first discusses a number of specific LRRK2-associated PPIs for which a functional consequence can at least be speculated upon, and then considers the representation of LRRK2 protein interactions in public repositories, providing an outlook on open research questions and challenges in this field.

EF1A1/HSC70 Cooperatively Suppress Brain Endothelial Cell Apoptosis via Regulating JNK Activity

AIMS

In our previous study, eEF1A1 was identified to be a new target for protecting brain ischemia injury, but the mechanism remains largely unknown. In this study, we screened the downstream cellular protein molecules interacted with eEF1A1 and found mechanism of eEF1A1 in brain ischemia protection.

METHODS AND RESULTS

Through co-immunoprecipitation and mass spectrometry for searching the interaction of proteins with eEF1A1 in bEnd3 cells, HSC70 was identified to be a binding protein of eEF1A1, which was further validated by Western blot and immunofluorescence. eEF1A1 or HSC70 knockdown, respectively, increased OGD-induced apoptosis of brain vascular endothelial cells, which was detected by Annexin V-FITC/PI staining. HSC70 or eEF1A1 knockdown enhances phosphorylated JNK, phosphorylation of c-JUN (Ser63, Ser73), cleaved caspase-9, and cleaved caspase-3 expression, which could be rescued by JNK inhibitor.

CONCLUSION

In summary, our data suggest that the presence of chaperone forms of interaction between eEF1A1 and HSC70 in brain vascular endothelial cells, eEF1A1 and HSC70 can play a protective role in the process of ischemic stroke by inhibiting the JNK signaling pathway activation.

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.

Increasing microtubule acetylation rescues axonal transport and locomotor deficits caused by LRRK2 Roc-COR domain mutations

Leucine-rich repeat kinase 2 (LRRK2) mutations are the most common genetic cause of Parkinson's disease. LRRK2 is a multifunctional protein affecting many cellular processes and has been described to bind microtubules. Defective microtubule-based axonal transport is hypothesized to contribute to Parkinson's disease, but whether LRRK2 mutations affect this process to mediate pathogenesis is not known. Here we find that LRRK2 containing pathogenic Roc-COR domain mutations (R1441C, Y1699C) preferentially associates with deacetylated microtubules, and inhibits axonal transport in primary neurons and in Drosophila, causing locomotor deficits in vivo. In vitro, increasing microtubule acetylation using deacetylase inhibitors or the tubulin acetylase αTAT1 prevents association of mutant LRRK2 with microtubules, and the deacetylase inhibitor trichostatin A (TSA) restores axonal transport. In vivo knockdown of the deacetylases HDAC6 and Sirt2, or administration of TSA rescues both axonal transport and locomotor behavior. Thus, this study reveals a pathogenic mechanism and a potential intervention for Parkinson's disease.

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.

Changes in actin dynamics and F-actin structure both in synaptoneurosomes of LRRK2(R1441G) mutant mice and in primary human fibroblasts of LRRK2(G2019S) mutation carriers

Converging evidence suggests that the Parkinson's disease-linked leucine-rich repeat kinase 2 (LRRK2) modulates cellular function by regulating actin dynamics. In the present study we investigate the role of LRRK2 in functional synaptic terminals of adult LRRK2-knockout and LRRK2(R1441G)-transgenic mice as well as in primary fibroblasts of LRRK2(G2019S) mutation carriers. We show that lack of LRRK2 decreases and overexpression of mutant LRRK2 age-dependently increases the effect of the actin depolymerizing agent Latrunculin A (LatA) on the synaptic cytoskeleton. Similarly, endogenous mutant LRRK2 increases sensitivity to LatA in primary fibroblasts. Under basal conditions however, these fibroblasts show an increase in F-actin bundles and a decrease in filopodial length which can be rescued by LatA treatment. Our data suggest that LRRK2 alters actin dynamics and F-actin structure both in brain neurons and skin fibroblasts. We hypothesize that increased F-actin bundling represents a compensatory mechanism to protect F-actin from the depolymerizing effect of mutant LRRK2 under basal conditions. Our data further indicate that LRRK2-dependent changes in the cytoskeleton might have functional consequences on postsynaptic NMDA receptor localization.

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.

Mutant LRRK2 enhances glutamatergic synapse activity and evokes excitotoxic dendrite degeneration

Mutations in leucine-rich repeat kinase 2 (LRRK2), which are associated with autosomal dominant Parkinson's disease, elicit progressive dendrite degeneration in neurons. We hypothesized that synaptic dysregulation contributes to mutant LRRK2-induced dendritic injury. We performed in vitro whole-cell voltage clamp studies of glutamatergic receptor agonist responses and glutamatergic synaptic activity in cultured rat cortical neurons expressing full-length wild-type and mutant forms of LRRK2. Expression of the pathogenic G2019S or R1441C LRRK2 mutants resulted in larger whole-cell current responses to direct application of AMPA and NMDA receptor agonists. In addition, mutant LRRK2-expressing neurons exhibited an increased frequency of spontaneous miniature excitatory postsynaptic currents (mEPSCs) in conjunction with increased excitatory synapse density as assessed by immunofluorescence for PSD95 and VGLUT1. Mutant LRRK2-expressing neurons showed enhanced vulnerability to acute synaptic glutamate stress. Furthermore, treatment with the NMDA receptor antagonist memantine significantly protected against subsequent losses in dendrite length and branching complexity. These data demonstrate an early association between mutant LRRK2 and increased excitatory synapse activity, implicating an excitotoxic contribution to mutant LRRK2 induced dendrite degeneration.

[Preparation of monospecific antibodies against isoform 2 of translation elongation factor 1A (eEF1A2)]

Amino acid sequences of eukaryotic translation elongation factor isoform 1 (eEF1A1) and 2 (eEF1A2) were compared and two peptide fragments of eEF1A2 were chosen as linear antigenic determinants for generation of monospecific antipeptide antibodies. Selected peptides were synthesized, conjugated to bovine serum albumin (BSA) and used for mice immunizations. Antibodies, produced against the eEF1A2 fragment 330-343 conjugated to BSA, specifically recognized this isoform in the native and partially denatured states but did not interact with the eEF1A1 isoform. It was shown that these monospecific anti-eEF1A2 antibodies could be employed for eEF1A2 detection both by enzyme-linked immunosorbent assay and by immunoblotting.

The regulation and deregulation of Wnt signaling by PARK genes in health and disease

Wingless/Int (Wnt) signaling pathways are signal transduction mechanisms that have been widely studied in the field of embryogenesis. Recent work has established a critical role for these pathways in brain development, especially of midbrain dopaminergic neurones. However, the fundamental importance of Wnt signaling for the normal function of mature neurones in the adult central nervous system has also lately been demonstrated by an increasing number of studies. Parkinson's disease (PD) is the second most prevalent neurodegenerative disease worldwide and is currently incurable. This debilitating disease is characterized by the progressive loss of a subset of midbrain dopaminergic neurones in the substantia nigra leading to typical extrapyramidal motor symptoms. The aetiology of PD is poorly understood but work performed over the last two decades has identified a growing number of genetic defects that underlie this condition. Here we review a growing body of data connecting genes implicated in PD--most notably the PARK genes--with Wnt signaling. These observations provide clues to the normal function of these proteins in healthy neurones and suggest that deregulated Wnt signaling might be a frequent pathomechanism leading to PD. These observations have implications for the pathogenesis and treatment of neurodegenerative diseases in general.

A direct interaction between leucine-rich repeat kinase 2 and specific β-tubulin isoforms regulates tubulin acetylation

Mutations in LRRK2, encoding the multifunctional protein leucine-rich repeat kinase 2 (LRRK2), are a common cause of Parkinson disease. LRRK2 has been suggested to influence the cytoskeleton as LRRK2 mutants reduce neurite outgrowth and cause an accumulation of hyperphosphorylated Tau. This might cause alterations in the dynamic instability of microtubules suggested to contribute to the pathogenesis of Parkinson disease. Here, we describe a direct interaction between LRRK2 and β-tubulin. This interaction is conferred by the LRRK2 Roc domain and is disrupted by the familial R1441G mutation and artificial Roc domain mutations that mimic autophosphorylation. LRRK2 selectively interacts with three β-tubulin isoforms: TUBB, TUBB4, and TUBB6, one of which (TUBB4) is mutated in the movement disorder dystonia type 4 (DYT4). Binding specificity is determined by lysine 362 and alanine 364 of β-tubulin. Molecular modeling was used to map the interaction surface to the luminal face of microtubule protofibrils in close proximity to the lysine 40 acetylation site in α-tubulin. This location is predicted to be poorly accessible within mature stabilized microtubules, but exposed in dynamic microtubule populations. Consistent with this finding, endogenous LRRK2 displays a preferential localization to dynamic microtubules within growth cones, rather than adjacent axonal microtubule bundles. This interaction is functionally relevant to microtubule dynamics, as mouse embryonic fibroblasts derived from LRRK2 knock-out mice display increased microtubule acetylation. Taken together, our data shed light on the nature of the LRRK2-tubulin interaction, and indicate that alterations in microtubule stability caused by changes in LRRK2 might contribute to the pathogenesis of Parkinson disease.

RISC in PD: the impact of microRNAs in Parkinson's disease cellular and molecular pathogenesis

Parkinson's disease (PD) is a debilitating neurodegenerative disease characterized primarily by the selective death of dopaminergic (DA) neurons in the substantia nigra pars compacta of the midbrain. Although several genetic forms of PD have been identified, the precise molecular mechanisms underlying DA neuron loss in PD remain elusive. In recent years, microRNAs (miRNAs) have been recognized as potent post-transcriptional regulators of gene expression with fundamental roles in numerous biological processes. Although their role in PD pathogenesis is still a very active area of investigation, several seminal studies have contributed significantly to our understanding of the roles these small non-coding RNAs play in the disease process. Among these are studies which have demonstrated specific miRNAs that target and down-regulate the expression of PD-related genes as well as those demonstrating a reciprocal relationship in which PD-related genes act to regulate miRNA processing machinery. Concurrently, a wealth of knowledge has become available regarding the molecular mechanisms that unify the underlying etiology of genetic and sporadic PD pathogenesis, including dysregulated protein quality control by the ubiquitin-proteasome system and autophagy pathway, activation of programmed cell death, mitochondrial damage and aberrant DA neurodevelopment and maintenance. Following a discussion of the interactions between PD-related genes and miRNAs, this review highlights those studies which have elucidated the roles of these pathways in PD pathogenesis. We highlight the potential of miRNAs to serve a critical regulatory role in the implicated disease pathways, given their capacity to modulate the expression of entire families of related genes. Although few studies have directly linked miRNA regulation of these pathways to PD, a strong foundation for investigation has been laid and this area holds promise to reveal novel therapeutic targets for PD.

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.

LRRK2 phosphorylates Snapin and inhibits interaction of Snapin with SNAP-25

Leucine-rich repeat kinase 2 (LRRK2) is a gene that, upon mutation, causes autosomal-dominant familial Parkinson's disease (PD). Yeast two-hybrid screening revealed that Snapin, a SNAP-25 (synaptosomal-associated protein-25) interacting protein, interacts with LRRK2. An in vitro kinase assay exhibited that Snapin is phosphorylated by LRRK2. A glutathione-S-transferase (GST) pull-down assay showed that LRRK2 may interact with Snapin via its Ras-of-complex (ROC) and N-terminal domains, with no significant difference on interaction of Snapin with LRRK2 wild type (WT) or its pathogenic mutants. Further analysis by mutation study revealed that Threonine 117 of Snapin is one of the sites phosphorylated by LRRK2. Furthermore, a Snapin T117D phosphomimetic mutant decreased its interaction with SNAP-25 in the GST pull-down assay. SNAP-25 is a component of the SNARE (Soluble NSF Attachment protein REceptor) complex and is critical for the exocytosis of synaptic vesicles. Incubation of rat brain lysate with recombinant Snapin T117D, but not WT, protein caused decreased interaction of synaptotagmin with the SNARE complex based on a co-immunoprecipitation assay. We further found that LRRK2-dependent phosphorylation of Snapin in the hippocampal neurons resulted in a decrease in the number of readily releasable vesicles and the extent of exocytotic release. Combined, these data suggest that LRRK2 may regulate neurotransmitter release via control of Snapin function by inhibitory phosphorylation.

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.

Cellular effects of LRRK2 mutations

Mutations in LRRK2 (leucine-rich repeat kinase 2) are a relatively common cause of inherited PD (Parkinson's disease), but the mechanism(s) by which mutations lead to disease are poorly understood. In the present paper, I discuss what is known about LRRK2 in cellular models, focusing specifically on assays that have been used to tease apart the effects of LRRK2 mutations on cellular phenotypes. LRRK2 expression has been suggested to cause loss of neuronal viability, although because it also has a strong effect on the length of neurites on these cells, whether this is true toxicity or not is unclear. Also, LRRK2 mutants can promote the redistribution of LRRK2 from diffuse cytosolic staining to more discrete structures, at least at high expression levels achieved in transfection experiments. The relevance of these phenotypes for PD is not yet clear, and a great deal of work is needed to understand them in more depth.

Human leucine-rich repeat kinase 1 and 2: intersecting or unrelated functions?

Mutations in LRRK2 (leucine-rich repeat kinase 2) are associated with both familial and sporadic PD (Parkinson's disease). LRRK1 (leucine-rich repeat kinase 1) shares a similar domain structure with LRRK2, but it is not linked to PD. LRRK proteins belong to a gene family known as ROCO, which codes for large proteins with several domains. All ROCO proteins have a ROC (Ras of complex proteins) GTPase domain followed by a domain of unknown function [COR (C-terminal of ROC)]. LRRK2, LRRK1 and other ROCO proteins also possess a kinase domain. To date, the function of LRRK1 and both the physiological and the pathological roles of LRRK2 are only beginning to unfold. The comparative analysis of these two proteins is a strategy to single out the specific properties of LRRKs to understand their cellular physiology. This comparison is the starting point to unravel the pathways that may lead to PD and eventually to develop therapeutic strategies for its treatment. In the present review, we discuss recently published results on LRRK2 and its paralogue LRRK1 concerning their evolutionary significance, biochemical properties and potential functional roles.

The LRRK2 R1628P variant plays a protective role in Han Chinese population with Alzheimer's disease

AIMS

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most prevalent neurodegenerative disorders that may share some overlapping etiologies. Mutations within leucine-rich repeat kinase 2 (LRRK2) have been reported to be responsible for PD, and the location of LRRK2 is within a linkage peak for sporadic AD (SAD). The aim of this study was to investigate two Asian-specific LRRK2 variants, R1628P and G2385R, with the association of Han Chinese SAD.

METHODS

Genotyping of R1628P and G2385R was performed by PCR-restriction fragment length polymorphism (RFLP) analysis in 390 patients with SAD and 545 unrelated age- and sex-matched healthy controls.

RESULTS

The frequency of the C allele within R1628P was more than three times higher in control group (1.7%) than in patients with SAD (0.5%) (OR 0.264; 95% CI, 0.088-0.792, P = 0.018). After stratification by the presence of one or two apolipoprotein E ε4 alleles, the protective effect becomes stronger (ε44: OR 0.028; 95% CI, 0.003-0.303, P = 0.003; ε4: OR 0.104; 95% CI, 0.013-0.818, P = 0.031). However, no difference was found in G2385R variant.

CONCLUSION

Our study suggested that R1628P variant within LRRK2 plays a protective role in Han Chinese population with SAD and such effect has an interaction with the APOE genotype.

Leucine-rich repeat kinase 2 functionally interacts with microtubules and kinase-dependently modulates cell migration

Recent studies indicate that the Parkinson's disease-linked leucine-rich repeat kinase 2 (LRRK2) modulates cytoskeletal functions by regulating actin and tubulin dynamics, thereby affecting neurite outgrowth. By interactome analysis we demonstrate that the binding of LRRK2 to tubulins is significantly enhanced by pharmacological LRRK2 inhibition in cells. Co-incubation of LRRK2 with microtubules increased the LRRK2 GTPase activity in a cell-free assay. Destabilization of microtubules causes a rapid decrease in cellular LRRK2(S935) phosphorylation indicating a decreased LRRK2 kinase activity. Moreover, both human LRRK2(G2019S) fibroblasts and mouse LRRK2(R1441G) fibroblasts exhibit alterations in cell migration in culture. Treatment of mouse fibroblasts with the selective LRRK2 inhibitor LRRK2-IN1 reduces cell motility. These findings suggest that LRRK2 and microtubules mutually interact both in non-neuronal cells and in neurons, which might contribute to our understanding of its pathogenic effects in Parkinson's disease.

Biochemical characterization of highly purified leucine-rich repeat kinases 1 and 2 demonstrates formation of homodimers

Leucine-rich repeat kinase 1 and 2 (LRRK1 and LRRK2) are large multidomain proteins containing kinase, GTPase and multiple protein-protein interaction domains, but only mutations in LRRK2 are linked to familial Parkinson's disease (PD). Independent studies suggest that LRRK2 exists in the cell as a complex compatible with the size of a dimer. However, whether this complex is truly a homodimer or a heterologous complex formed by monomeric LRRK2 with other proteins has not been definitively proven due to the limitations in obtaining highly pure proteins suitable for structural characterization. Here, we used stable expression of LRRK1 and LRRK2 in HEK293T cell lines to produce recombinant LRRK1 and LRRK2 proteins of greater than 90% purity. Both purified LRRKs are folded, with a predominantly alpha-helical secondary structure and are capable of binding GTP with similar affinity. Furthermore, recombinant LRRK2 exhibits robust autophosphorylation activity, phosphorylation of model peptides in vitro and ATP binding. In contrast, LRRK1 does not display significant autophosphorylation activity and fails to phosphorylate LRRK2 model substrates, although it does bind ATP. Using these biochemically validated proteins, we show that LRRK1 and LRRK2 are capable of forming homodimers as shown by single-particle transmission electron microscopy and immunogold labeling. These LRRK dimers display an elongated conformation with a mean particle size of 145 Å and 175 Å respectively, which is disrupted by addition of 6M guanidinium chloride. Immunogold staining revealed double-labeled particles also in the pathological LRRK2 mutant G2019S and artificial mutants disrupting GTPase and kinase activities, suggesting that point mutations do not hinder the dimeric conformation. Overall, our findings indicate for the first time that purified and active LRRK1 and LRRK2 can form dimers in their full-length conformation.

From global phosphoproteomics to individual proteins: the case of translation elongation factor eEF1A

Phosphoproteomics is often aimed at deciphering the modified components of signaling pathways in certain organisms, tissues and pathologies. Phosphorylation of housekeeping proteins, albeit important, usually attracts less attention. Here, we provide targeted analysis of eukaryotic translation elongation factor 1A (eEF1A), which is the main element of peptide elongation machinery. There are 97% homologous A1 and A2 isoforms of eEF1A; their expression in mammalian tissues is mutually exclusive and differentially regulated in development. The A2 isoform reveals proto-oncogenic properties and specifically interacts with some cellular proteins. Several tyrosine residues shown experimentally to be phosphorylated in eEF1A1 are hardly solution accessible, so their phosphorylation could be linked with structural rearrangement of the protein molecule. The possible role of tyrosine phosphorylation in providing the background for structural differences between the 'extended' A1 isoform and the compact oncogenic A2 isoform is discussed. The 'road map' for targeted analysis of any protein of interest using phosphoproteomics data is presented.

Purification, crystallization and preliminary X-ray crystallographic analysis of mammalian translation elongation factor eEF1A2

Translation elongation factor eEF1A2 was purified to homogeneity from rabbit muscle by two consecutive ion-exchange column-chromatography steps and this mammalian eEF1A2 was successfully crystallized for the first time. Protein crystals obtained using ammonium sulfate as precipitant diffracted to 2.5 Å resolution and belonged to space group P6(1)22 or P6(3)22 (unit-cell parameters a = b = 135.4, c = 304.6 Å). A complete native data set was collected to 2.7 Å resolution.

Models for LRRK2-Linked Parkinsonism

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder characterized by the selective loss of dopaminergic neurons and the presence of Lewy bodies. The pathogenesis of PD is not fully understood, but it appears to involve both genetic susceptibility and environmental factors. Treatment for PD that prevents neuronal death progression in the dopaminergic system and abnormal protein deposition in the brain is not yet available. Recently, mutations in the leucine-rich repeat kinase 2 (LRRK2) gene have been identified to cause autosomal-dominant late-onset PD and contribute to sporadic PD. Here, we review the recent models for LRRK2-linked Parkinsonism and their utility in studying LRRK2 neurobiology, pathogenesis, and potential therapeutics.

PINK1-Interacting Proteins: Proteomic Analysis of Overexpressed PINK1

Recent publications suggest that the Parkinson's disease- (PD-) related PINK1/Parkin pathway promotes elimination of dysfunctional mitochondria by autophagy. We used tandem affinity purification (TAP), SDS-PAGE, and mass spectrometry as a first step towards identification of possible substrates for PINK1. The cellular abundance of selected identified interactors was investigated by Western blotting. Furthermore, one candidate gene was sequenced in 46 patients with atypical PD. In addition to two known binding partners (HSP90, CDC37), 12 proteins were identified using the TAP assay; four of which are mitochondrially localized (GRP75, HSP60, LRPPRC, and TUFM). Western blot analysis showed no differences in cellular abundance of these proteins comparing PINK1 mutant and control fibroblasts. When sequencing LRPPRC, four exonic synonymous changes and 20 polymorphisms in noncoding regions were detected. Our study provides a list of putative PINK1 binding partners, confirming previously described interactions, but also introducing novel mitochondrial proteins as potential components of the PINK1/Parkin mitophagy pathway.

LRRK2 kinase regulates synaptic morphology through distinct substrates at the presynaptic and postsynaptic compartments of the Drosophila neuromuscular junction

Mutations in leucine-rich repeat kinase 2 (LRRK2) are linked to familial as well as sporadic forms of Parkinson's disease (PD), a neurodegenerative disease characterized by dysfunction and degeneration of dopaminergic and other types of neurons. The molecular and cellular mechanisms underlying LRRK2 action remain poorly defined. Here, we show that LRRK2 controls synaptic morphogenesis at the Drosophila neuromuscular junction. Loss of Drosophila LRRK2 results in synaptic overgrowth, whereas overexpression of Drosophila LRRK or human LRRK2 has opposite effects. Alteration of LRRK2 activity also affects neurotransmission. LRRK2 exerts its effects on synaptic morphology by interacting with distinct downstream effectors at the presynaptic and postsynaptic compartments. At the postsynapse, LRRK2 interacts with the previously characterized substrate 4E-BP, an inhibitor of protein synthesis. At the presynapse, LRRK2 phosphorylates and negatively regulates the microtubule (MT)-binding protein Futsch. These results implicate synaptic dysfunction caused by deregulated protein synthesis and aberrant MT dynamics in LRRK2 pathogenesis and offer a new paradigm for understanding and ultimately treating PD.

Leucine-rich repeat kinase 2 and alpha-synuclein: intersecting pathways in the pathogenesis of Parkinson's disease?

Although Parkinson's disease (PD) is generally a sporadic neurological disorder, the discovery of monogenic, hereditable forms of the disease has been crucial in delineating the molecular pathways that lead to this pathology. Genes responsible for familial PD can be ascribed to two categories based both on their mode of inheritance and their suggested biological function. Mutations in parkin, PINK1 and DJ-1 cause of recessive Parkinsonism, with a variable pathology often lacking the characteristic Lewy bodies (LBs) in the surviving neurons. Intriguingly, recent findings highlight a converging role of all these genes in mitochondria function, suggesting a common molecular pathway for recessive Parkinsonism. Mutations in a second group of genes, encoding alpha-synuclein (α-syn) and LRRK2, are transmitted in a dominant fashion and generally lead to LB pathology, with α-syn being the major component of these proteinaceous aggregates. In experimental systems, overexpression of mutant proteins is toxic, as predicted for dominant mutations, but the normal function of both proteins is still elusive. The fact that α-syn is heavily phosphorylated in LBs and that LRRK2 is a protein kinase, suggests that a link, not necessarily direct, exists between the two. What are the experimental data supporting a common molecular pathway for dominant PD genes? Do α-syn and LRRK2 target common molecules? Does LRRK2 act upstream of α-syn? In this review we will try to address these of questions based on the recent findings available in the literature.

ARHGEF7 (Beta-PIX) acts as guanine nucleotide exchange factor for leucine-rich repeat kinase 2

Background

Mutations within the leucine-rich repeat kinase 2 (LRRK2) gene are a common cause of familial and sporadic Parkinson's disease. The multidomain protein LRRK2 exhibits overall low GTPase and kinase activity in vitro.

Methodology/Principal Findings

Here, we show that the rho guanine nucleotide exchange factor ARHGEF7 and the small GTPase CDC42 are interacting with LRRK2 in vitro and in vivo. GTPase activity of full-length LRRK2 increases in the presence of recombinant ARHGEF7. Interestingly, LRRK2 phosphorylates ARHGEF7 in vitro at previously unknown phosphorylation sites. We provide evidence that ARHGEF7 might act as a guanine nucleotide exchange factor for LRRK2 and that R1441C mutant LRRK2 with reduced GTP hydrolysis activity also shows reduced binding to ARHGEF7.

Conclusions/Significance

Downstream effects of phosphorylation of ARHGEF7 through LRRK2 could be (i) a feedback control mechanism for LRRK2 activity as well as (ii) an impact of LRRK2 on actin cytoskeleton regulation. A newly identified familial mutation N1437S, localized within the GTPase domain of LRRK2, further underlines the importance of the GTPase domain of LRRK2 in Parkinson's disease pathogenesis.

Heterodimerization of Lrrk1-Lrrk2: Implications for LRRK2-associated Parkinson disease

LRRK2 mutations are recognized as the most frequent genetic cause of both familial and sporadic parkinsonism identified to date. A remarkable feature of this form of parkinsonism is the variable penetrance of symptom manifestation resulting in a wide range of age-at-onset in patients. Herein we use a functional approach to identify the Lrrk1 protein as a potential disease modifier demonstrating an interaction and heterodimer formation with Lrrk2. In addition, evaluation of LRRK1 variants in our large Lrrk2 p.G2019S-parkinsonism series from a Tunisian (n=145) identified a missense mutation (p.L416M) resulting in an average 6.2 years younger age at disease onset. In conclusion we show that the interaction of Lrrk1-Lrrk2 can form protein dimers and this interaction may influence the age of symptomatic manifestation in Lrrk2-parkinsonism patients.