Evy Lobbestael

Phosphorylation of Ezrin/Radixin/Moesin Proteins by LRRK2 Promotes the Rearrangement of Actin Cytoskeleton in Neuronal Morphogenesis

Leucine-rich repeat kinase 2 (LRRK2) functions as a putative protein kinase of ezrin, radixin, and moesin (ERM) family proteins. A Parkinsons disease-related G2019S substitution in the kinase domain of LRRK2 further enhances the phosphorylation of ERM proteins. The phosphorylated ERM (pERM) proteins are restricted to the filopodia of growing neurites in which they tether filamentous actin (F-actin) to the cytoplasmic membrane and regulate the dynamics of filopodia protrusion. Here, we show that, in cultured neurons derived from LRRK2 G2019S transgenic mice, the number of pERM-positive and F-actin-enriched filopodia was significantly increased, and this correlates with the retardation of neurite outgrowth. Conversely, deletion of LRRK2, which lowered the pERM and F-actin contents in filopodia, promoted neurite outgrowth. Furthermore, inhibition of ERM phosphorylation or actin polymerization rescued the G2019S-dependent neuronal growth defects. These data support a model in which the G2019S mutation of LRRK2 causes a gain-of-function effect that perturbs the homeostasis of pERM and F-actin in sprouting neurites critical for neuronal morphogenesis.

Insight into the mode of action of the LRRK2 Y1699C pathogenic mutant

J. Neurochem. (2011) 116, 304–315.

LRRK2 Kinase Activity Is Dependent on LRRK2 GTP Binding Capacity but Independent of LRRK2 GTP Binding

Leucine rich repeat kinase 2 (LRRK2) is a Parkinsons disease (PD) gene that encodes a large multidomain protein including both a GTPase and a kinase domain. GTPases often regulate kinases within signal transduction cascades, where GTPases act as molecular switches cycling between a GTP bound “on” state and a GDP bound “off” state. It has been proposed that LRRK2 kinase activity may be increased upon GTP binding at the LRRK2 Ras of complex proteins (ROC) GTPase domain. Here we extensively test this hypothesis by measuring LRRK2 phosphorylation activity under influence of GDP, GTP or non-hydrolyzable GTP analogues GTPγS or GMPPCP. We show that autophosphorylation and lrrktide phosphorylation activity of recombinant LRRK2 protein is unaltered by guanine nucleotides, when co-incubated with LRRK2 during phosphorylation reactions. Also phosphorylation activity of LRRK2 is unchanged when the LRRK2 guanine nucleotide binding pocket is previously saturated with various nucleotides, in contrast to the greatly reduced activity measured for the guanine nucleotide binding site mutant T1348N. Interestingly, when nucleotides were incubated with cell lysates prior to purification of LRRK2, kinase activity was slightly enhanced by GTPγS or GMPPCP compared to GDP, pointing to an upstream guanine nucleotide binding protein that may activate LRRK2 in a GTP-dependent manner. Using metabolic labeling, we also found that cellular phosphorylation of LRRK2 was not significantly modulated by nucleotides, although labeling is significantly reduced by guanine nucleotide binding site mutants. We conclude that while kinase activity of LRRK2 requires an intact ROC-GTPase domain, it is independent of GDP or GTP binding to ROC.

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 Parkinsons 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.

Identification of protein phosphatase 1 as a regulator of the LRRK2 phosphorylation cycle.

A cluster of phosphorylation sites in LRRK2 (leucine-rich repeat kinase 2), including Ser910, Ser935, Ser955 and Ser973, is important for PD (Parkinsons disease) pathogenesis as several PD-linked LRRK2 mutants are dephosphorylated at these sites. LRRK2 is also dephosphorylated in cells after pharmacological inhibition of its kinase activity, which is currently proposed as a strategy for disease-modifying PD therapy. Despite this importance of LRRK2 dephosphorylation in mutant LRRK2 pathological mechanism(s) and in LRRK2s response to inhibition, the mechanism by which this occurs is unknown. Therefore we aimed to identify the phosphatase for LRRK2. Using a panel of recombinant phosphatases, we found that PP1 (protein phosphatase 1) efficiently dephosphorylates LRRK2 in vitro. PP1 activity on LRRK2 dephosphorylation was confirmed in cells using PP1 inhibition to reverse LRRK2 dephosphorylation induced by the potent LRRK2 kinase inhibitor LRRK2-IN1 as well as in R1441G mutant LRRK2. We also found that PP1 and LRRK2 can form a complex in cells. Furthermore, we observed that PP1 inhibition modulates LRRK2s cellular phenotype by reducing skein-like LRRK2-positive structures associated with dephosphorylation. In conclusion, the present study reveals PP1 as the physiological LRRK2 phosphatase, responsible for LRRK2 dephosphorylation observed in PD mutant LRRK2 and after LRRK2 kinase inhibition.

Phosphorylation of LRRK2: from kinase to substrate

The PD (Parkinsons disease) protein LRRK2 (leucine-rich repeat kinase 2) occurs in cells as a highly phosphorylated protein, with the majority of phosphosites clustering in the region between the ankyrin repeat and leucine-rich repeat domains. The observation that several pathogenic variants of LRRK2 display strongly reduced cellular phosphorylation suggests that phosphorylation of LRRK2 is involved in the PD pathological process. Furthermore, treatment of cells with inhibitors of LRRK2 kinase activity, which are currently considered as potential disease-modifying therapeutics for PD, leads to a rapid decrease in the phosphorylation levels of LRRK2. For these reasons, understanding the cellular role and regulation of LRRK2 as a kinase and as a substrate has become the focus of intense investigation. In the present review, we discuss what is currently known about the cellular phosphorylation of LRRK2 and how this relates to its function and dysfunction.

Differential protein-protein interactions of LRRK1 and LRRK2 indicate roles in distinct cellular signaling pathways

Genetic studies show that LRRK2, and not its closest paralogue LRRK1, is linked to Parkinsons disease. To gain insight into the molecular and cellular basis of this discrepancy, we searched for LRRK1‐ and LRRK2‐specific cellular processes by identifying their distinct interacting proteins. A protein microarray‐based interaction screen was performed with recombinant 3xFlag‐LRRK1 and 3xFlag‐LRRK2 and, in parallel, co‐immunoprecipitation followed by mass spectrometry was performed from SH‐SY5Y neuroblastoma cell lines stably expressing 3xFlag‐LRRK1 or 3xFlag‐LRRK2. We identified a set of LRRK1‐ and LRRK2‐specific as well as common interactors. One of our most prominent findings was that both screens pointed to epidermal growth factor receptor (EGF‐R) as a LRRK1‐specific interactor, while 14‐3‐3 proteins were LRRK2‐specific. This is consistent with phosphosite mapping of LRRK1, revealing phosphosites outside of 14‐3‐3 consensus binding motifs. To assess the functional relevance of these interactions, SH‐SY5Y‐LRRK1 and ‐LRRK2 cell lines were treated with LRRK2 kinase inhibitors that disrupt 14‐3‐3 binding, or with EGF, an EGF‐R agonist. Redistribution of LRRK2, not LRRK1, from diffuse cytoplasmic to filamentous aggregates was observed after inhibitor treatment. Similarly, EGF induced translocation of LRRK1, but not of LRRK2, to endosomes. Our study confirms that LRRK1 and LRRK2 can carry out distinct functions by interacting with different cellular proteins.

Immunohistochemical detection of transgene expression in the brain using small epitope tags.

BackgroundIn vivo overexpression of proteins is a powerful approach to study their biological function, generate disease models or evaluate gene therapy approaches. In order to investigate an exogenously expressed protein, specific and sensitive detection is essential. Unfortunately, antibodies that allow histological detection of the protein of interest are not always readily available. The use of an epitope tag fused to the protein can circumvent this problem as well as provide the possibility to discriminate endogenous from overexpressed proteins. In order to minimize impact on the bioactivity and biodistribution of the overexpressed protein, preference is given to small tags.ResultsIn the present study, we evaluated several small epitope tags together with corresponding anti-tag antibodies for the detection of overexpressed proteins in rat brain, using eGFP as a reference. We generated several lentiviral vectors encoding eGFP with different N-terminally fused small epitope tags (AU1, flag, 3flag, HA, myc and V5). After confirmation of their functionality in cell culture, we injected these lentiviral vectors stereotactically into the striatum of rats and prepared paraformaldehyde fixed floating sections for immunohistochemical analysis. Using multiple antibodies and antibody dilutions per epitope tag, we extensively assessed the efficiency of several anti-tag antibodies for chromogenic immunohistochemical detection of the epitope tagged eGFPs by determining the proportion of immunoreactivity detected by anti-tag antibodies compared to anti-GFP antibody. Using fluorescence immunohistochemistry and confocal microscopy, we also quantified the proportion of eGFP-positive cells detected by anti-tag antibodies. Our results show that all the examined small epitope tags could be detected by anti-tag antibodies both in cell extracts as well as in vivo, although to varying degrees depending on the tag and antibody used. Using the presented protocol, V5/anti-V5 and HA/HA11 tag/antibody combinations provided the most sensitive detection in brain tissue. We confirmed the applicability of these optimized in vivo tag detection conditions for a difficult to detect protein, firefly luciferase (fLuc), using lentiviral vector constructs expressing V5 tagged and 3flag tagged fLuc protein.ConclusionsWe show here that several small epitope tags are useful for immunohistochemical detection of exogenous proteins in vivo. Our study also provides a generic methodology which is broadly applicable for the detection of overexpressed transgenes in mammalian brain tissue.

Kinases as Targets for Parkinson's Disease: From Genetics to Therapy

Intense research efforts are currently directed at elucidating the etiology of Parkinsons disease (PD). One approach that has begun to shed light on the PD pathogenic pathways is the identification of disease genes through genetic linkage or association studies. These studies have revealed that several kinases may be involved in PD, as some PD genes encode kinases themselves while other PD genes are found in the same cellular pathways as kinases. Two of these kinases stand out as potential drug targets for novel PD therapy, namely leucine rich repeat kinase 2 (LRRK2) and the alpha-synuclein (α-syn) phosphorylating polo-like kinase 2 (PLK2). Indeed, both α- syn and LRRK2 show genetic linkage as well as genetic association with PD, indicating their relevance to a large number of PD cases. Also, due to the dominant mode of α-syn and LRRK2 inheritance and based on current knowledge of LRRK2 and α-syn phosphorylation by PLK2, inhibition of LRRK2 and PLK2 may constitute a potential therapy for PD. Here we discuss the function of these kinases as well as progress in their validation as drug targets for the treatment of PD.

Pharmacological LRRK2 kinase inhibition induces LRRK2 protein destabilization and proteasomal degradation

Leucine-rich repeat kinase 2 (LRRK2) kinase activity is increased in several pathogenic mutations, including the most common mutation, G2019S, and is known to play a role in Parkinson’s disease (PD) pathobiology. This has stimulated the development of potent, selective LRRK2 kinase inhibitors as one of the most prevailing disease-modifying therapeutic PD strategies. Although several lines of evidence support beneficial effects of LRRK2 kinase inhibitors, many questions need to be answered before clinical applications can be envisaged. Using six different LRRK2 kinase inhibitors, we show that LRRK2 kinase inhibition induces LRRK2 dephosphorylation and can reduce LRRK2 protein levels of overexpressed wild type and G2019S, but not A2016T or K1906M, LRRK2 as well as endogenous LRRK2 in mouse brain, lung and kidney. The inhibitor-induced reduction in LRRK2 levels could be reversed by proteasomal inhibition, but not by lysosomal inhibition, while mRNA levels remained unaffected. In addition, using LRRK2 S910A and S935A phosphorylation mutants, we show that dephosphorylation of these sites is not required for LRRK2 degradation. Increasing our insight in the molecular and cellular consequences of LRRK2 kinase inhibition will be crucial in the further development of LRRK2-based PD therapies.