Screening for Novel LRRK2 Inhibitors Using a High-Throughput TR-FRET Cellular Assay for LRRK2 Ser935 Phosphorylation

Background

Mutations in the leucine-rich repeat kinase-2 (LRRK2) have been linked to Parkinson’s disease. Recent studies show that inhibition of LRRK2 kinase activity decreased the level of phosphorylation at its own Ser910 and Ser935, indicating that these sites are prime targets for cellular readouts of LRRK2 inhibition.

Methodology/Principal Findings

Using Time-Resolved Förster Resonance Energy Transfer (TR-FRET) technology, we developed a high-throughput cellular assay for monitoring LRRK2 phosphorylation at Ser935. LRRK2-Green Fluorescence Protein (GFP) fusions were expressed in cells via BacMam. Phosphorylation at Ser935 in these cells is detected using a terbium labeled anti-phospho-Ser935 antibody that generates a TR-FRET signal between terbium and GFP. LRRK2 wild-type and G2019S are constitutively phosphorylated at Ser935 in cells as measured by TR-FRET. The phosphorylation level is reduced for the R1441C mutant and little could be detected for the kinase-dead mutant D1994A. The TR-FRET cellular assay was further validated using reported LRRK2 inhibitors including LRRK2-IN-1 and our results confirmed that inhibition of LRRK2 can reduce the phosphorylation level at Ser935. To demonstrate the utility of this assay for screening, we profiled a small library of 1120 compounds. Three known LRRK2 inhibitors were identified and 16 hits were followed up in the TR-FRET and a cytotoxicity assay. Interestingly, out of the top 16 hits, five are known inhibitors of IκB phosphorylation, two CHK1 and two CDC25 inhibitors. Thirteen hits were further tested in a biochemical LRRK2 kinase activity assay and Western blot analysis for their effects on the phosphorylation of Ser910, Ser935, Ser955 and Ser973.

Conclusions/Significance

We developed a TR-FRET cellular assay for LRRK2 Ser935 phosphorylation that can be applied to the screening for LRRK2 inhibitors. We report for the first time that several compounds such as IKK16, CHK1 inhibitors and {"type":"entrez-nucleotide","attrs":{"text":"GW441756","term_id":"315858226","term_text":"GW441756"}}GW441756 can inhibit LRRK2 Ser935 phosphorylation in cells and LRRK2 kinase activity in vitro.     

Phosphorylation of LRRK2 serines 955 and 973 is disrupted by Parkinson's disease mutations and LRRK2 pharmacological inhibition

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial Parkinson's disease. An amino terminal cluster of constitutively phosphorylated residues, serines 860, 910, 935, 955, and 973, appears to be biologically relevant. Phosphorylation of serines 910 and 935 is regulated in response to LRRK2 kinase activity and is responsible for interaction with 14-3-3 and maintaining LRRK2 in a non-aggregated state. We examined the phosphorylation status of two other constitutive phosphorylation sites, serines 955 and 973. Treatment of LRRK2 expressing cells with the selective LRRK2 inhibitor LRRK2-IN1 revealed that, like Ser910/Ser935, phosphorylation of Ser955 and Ser973 is disrupted by acute inhibition of LRRK2 kinase activity. Additionally, phosphorylation of Ser955 and 973 is disrupted in the context of several Parkinson's disease associated mutations [R1441G/C, Y1699C, and I2020T]. We observed that modification of Ser973 is dependent on the modification of Ser910/Ser935. Ser955Ala and Ser973Ala mutations do not induce relocalization of LRRK2; however, all phosphomutants exhibited similar localization patterns when exposed to LRRK2-IN1. We conclude that the mechanisms of regulation of Ser910/935/955/973 phosphorylation are similar and physiologically relevant. These sites can be utilized as biomarkers for LRRK2 activity as well as starting points for the elucidation of upstream and downstream enzymes that regulate LRRK2.     

The IkappaB kinase family phosphorylates the Parkinson's disease kinase LRRK2 at Ser935 and Ser910 during Toll-like receptor signaling

Mutations in leucine-rich repeat kinase 2 (LRRK2) are strongly associated with late-onset autosomal dominant Parkinson's disease. LRRK2 is highly expressed in immune cells and recent work points towards a link between LRRK2 and innate immunity. Here we demonstrate that stimulation of the Toll-Like Receptor (TLR) pathway by MyD88-dependent agonists in bone marrow-derived macrophages (BMDMs) or RAW264.7 macrophages induces marked phosphorylation of LRRK2 at Ser910 and Ser935, the phosphorylation sites that regulate the binding of 14-3-3 to LRRK2. Phosphorylation of these residues is prevented by knock-out of MyD88 in BMDMs, but not the alternative TLR adaptor protein TRIF. Utilising both pharmacological inhibitors, including a new TAK1 inhibitor, NG25, and genetic models, we provide evidence that both the canonical (IKKα and IKKβ) and IKK-related (IKKε and TBK1) kinases mediate TLR agonist induced phosphorylation of LRRK2 in vivo. Moreover, all four IKK members directly phosphorylate LRRK2 at Ser910 and Ser935 in vitro. Consistent with previous work describing Ser910 and Ser935 as pharmacodynamic biomarkers of LRRK2 activity, we find that the TLR independent basal phosphorylation of LRRK2 at Ser910 and Ser935 is abolished following treatment of macrophages with LRRK2 kinase inhibitors. However, the increased phosphorylation of Ser910 and Ser935 induced by activation of the MyD88 pathway is insensitive to LRRK2 kinase inhibitors. Finally, employing LRRK2-deficient BMDMs, we present data indicating that LRRK2 does not play a major role in regulating the secretion of inflammatory cytokines induced by activation of the MyD88 pathway. Our findings provide the first direct link between LRRK2 and the IKKs that mediate many immune responses. Further work is required to uncover the physiological roles that phosphorylation of LRRK2 by IKKs play in controlling macrophage biology and to determine how phosphorylation of LRRK2 by IKKs impacts upon the use of Ser910 and Ser935 as pharmacodynamic biomarkers.     

No Dopamine Cell Loss or Changes in Cytoskeleton Function in Transgenic Mice Expressing Physiological Levels of Wild Type or G2019S Mutant LRRK2 and in Human Fibroblasts

Mutations within the LRRK2 gene have been identified in Parkinson’s disease (PD) patients and have been implicated in the dysfunction of several cellular pathways. Here, we explore how pathogenic mutations and the inhibition of LRRK2 kinase activity affect cytoskeleton dynamics in mouse and human cell systems. We generated and characterized a novel transgenic mouse model expressing physiological levels of human wild type and G2019S-mutant LRRK2. No neuronal loss or neurodegeneration was detected in midbrain dopamine neurons at the age of 12 months. Postnatal hippocampal neurons derived from transgenic mice showed no alterations in the seven parameters examined concerning neurite outgrowth sampled automatically on several hundred neurons using high content imaging. Treatment with the kinase inhibitor LRRK2-IN-1 resulted in no significant changes in the neurite outgrowth. In human fibroblasts we analyzed whether pathogenic LRRK2 mutations change cytoskeleton functions such as cell adhesion. To this end we compared the adhesion characteristics of human skin fibroblasts derived from six PD patients carrying one of three different pathogenic LRRK2 mutations and from four age-matched control individuals. The mutant LRRK2 variants as well as the inhibition of LRRK2 kinase activity did not reveal any significant cell adhesion differences in cultured fibroblasts. In summary, our results in both human and mouse cell systems suggest that neither the expression of wild type or mutant LRRK2, nor the inhibition of LRRK2 kinase activity affect neurite complexity and cellular adhesion.     

LRRK2 directly phosphorylates Akt1 as a possible physiological substrate: impairment of the kinase activity by Parkinson's disease-associated mutations

LRRK2 is the causal molecule for autosomal-dominant familial Parkinson's disease, although its true function, including its physiological substrates, remains unknown. Here, using in vitro kinase assay with recombinant proteins, we demonstrated for the first time that LRRK2 directly phosphorylates Akt1, a central molecule involved in signal transduction for cell survival and prevention of apoptosis. Ser473, one of two amino acids essential for Akt1 activation, was the target site for LRRK2. A knockdown experiment using intact cells also demonstrated LRRK2-mediated phosphorylation of Akt1 (Ser473), suggesting that Akt1 is a convincing candidate for the physiological substrate of LRRK2. The disease-associated mutations, R1441C, G2019S, and I2020T, exhibited reduced interaction with, and phosphorylation of, Akt1, suggesting one possible mechanism for the neurodegeneration caused by LRRK2 mutations.     

Parkinson's disease-associated mutant LRRK2 phosphorylates Rab7L1 and modifies trans-Golgi morphology

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the major genetic cause of autosomal-dominantly inherited Parkinson's disease. LRRK2 is implicated in the regulation of intracellular trafficking, neurite outgrowth and PD risk in connection with Rab7L1, a putative interactor of LRRK2. Recently, a subset of Rab GTPases have been reported as substrates of LRRK2. Here we examine the kinase activity of LRRK2 on Rab7L1 in situ in cells. Phos-tag analyses and metabolic labeling assays revealed that LRRK2 readily phosphorylates Golgi-localized wild-type Rab7L1 but not mutant forms that are distributed in the cytoplasm. In vitro assays demonstrated direct phosphorylation of Rab7L1 by LRRK2. Subsequent screening using Rab7L1 mutants harboring alanine-substitution for every single Ser/Thr residue revealed that Ser72 is a major phosphorylation site, which was confirmed by using a phospho-Ser72-specific antibody. Moreover, LRRK2 pathogenic Parkinson mutants altogether markedly enhanced the phosphorylation at Ser72. The modulation of Ser72 phosphorylation in Rab7L1 resulted in an alteration of the morphology and distribution of the trans-Golgi network. These data collectively support the involvement of Rab7L1 phosphorylation in the LRRK2-mediated cellular and pathogenetic mechanisms.     

Lack of Correlation between the Kinase Activity of LRRK2 Harboring Kinase-Modifying Mutations and Its Phosphorylation at Ser910, 935, and Ser955

Leucine-rich repeat kinase 2 (LRRK2) is extensively phosphorylated in cells within a region amino-terminal to the leucine-rich repeat domain. Since phosphorylation in this region of LRRK2, including Ser910, Ser935, Ser955, and Ser973, is significantly downregulated upon treatment with inhibitors of LRRK2, it has been hypothesized that signaling pathways downstream of the kinase activity of LRRK2 are involved in regulating the phosphorylation of LRRK2, although the precise mechanism has remained unknown. Here we examined the effects of LRRK2 inhibitors on the phosphorylation state at Ser910, Ser935, and Ser955 in a series of kinase-inactive mutants of LRRK2. We found that the responses of LRRK2 to the inhibitors varied among mutants, in a manner not consistent with the above-mentioned hypothesis. Notably, one of the kinase-inactive mutants, T2035A LRRK2, underwent phosphorylation, as well as the inhibitor-induced dephosphorylation, at Ser910, Ser935, and Ser955, to a similar extent to those observed with wild-type LRRK2. These results suggest that the kinase activity of LRRK2 is not involved in the common mechanism of inhibitor-induced dephosphorylation of LRRK2.     

Rab29 activation of the Parkinson's disease‐associated LRRK2 kinase

Parkinson's disease predisposing LRRK2 kinase phosphorylates a group of Rab GTPase proteins including Rab29, within the effector‐binding switch II motif. Previous work indicated that Rab29, located within the PARK16 locus mutated in Parkinson's patients, operates in a common pathway with LRRK2. Here, we show that Rab29 recruits LRRK2 to the trans‐Golgi network and greatly stimulates its kinase activity. Pathogenic LRRK2 R1441G/C and Y1699C mutants that promote GTP binding are more readily recruited to the Golgi and activated by Rab29 than wild‐type LRRK2. We identify conserved residues within the LRRK2 ankyrin domain that are required for Rab29‐mediated Golgi recruitment and kinase activation. Consistent with these findings, knockout of Rab29 in A549 cells reduces endogenous LRRK2‐mediated phosphorylation of Rab10. We show that mutations that prevent LRRK2 from interacting with either Rab29 or GTP strikingly inhibit phosphorylation of a cluster of highly studied biomarker phosphorylation sites (Ser910, Ser935, Ser955 and Ser973). Our data reveal that Rab29 is a master regulator of LRRK2, controlling its activation, localization, and potentially biomarker phosphorylation.     

(G2019S) LRRK2 activates MKK4-JNK pathway and causes degeneration of SN dopaminergic neurons in a transgenic mouse model of PD

(G2019S) mutation of leucine-rich repeat kinase 2 (LRRK2) is the most common genetic cause of both familial and sporadic Parkinson's disease (PD) cases. Twelve- to sixteen-month-old (G2019S) LRRK2 transgenic mice prepared by us displayed progressive degeneration of substantia nigra pars compacta (SNpc) dopaminergic neurons and parkinsonism phenotypes of motor dysfunction. LRRK2 is a member of mixed lineage kinase subfamily of mitogen-activated protein kinase kinase kinases (MAPKKKs). We hypothesized that (G2019S) mutation augmented LRRK2 kinase activity, leading to overphosphorylation of downstream MAPK kinase (MKK) and resulting in activation of neuronal death signal pathway. Consistent with our hypothesis, (G2019S) LRRK2 expressed in HEK 293 cells exhibited an augmented kinase activity of phosphorylating MAPK kinase 4 (MKK4) at Ser(257), and protein expression of active phospho-MKK4(Ser257) was upregulated in the SN of (G2019S) LRRK2 transgenic mice. Protein level of active phospho-JNK(Thr183/Tyr185) and phospho-c-Jun(Ser63), downstream targets of phospho-MKK4(Ser257), was increased in the SN of (G2019S) LRRK2 mice. Upregulated mRNA expression of pro-apoptotic Bim and FasL, target genes of phospho-c-Jun(Ser63), and formation of active caspase-9, caspase-8 and caspase-3 were also observed in the SN of (G2019S) LRRK2 transgenic mice. Our results suggest that mutant (G2019S) LRRK2 activates MKK4-JNK-c-Jun pathway in the SN and causes the resulting degeneration of SNpc dopaminergic neurons in PD transgenic mice.     

The E3 ligase TRIM1 ubiquitinates LRRK2 and controls its localization,degradation, and toxicity

Missense mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial Parkinson’s disease (PD); however, pathways regulating LRRK2 subcellular localization, function, and turnover are not fully defined. We performed quantitative mass spectrometry–based interactome studies to identify 48 novel LRRK2 interactors, including the microtubule-associated E3 ubiquitin ligase TRIM1 (tripartite motif family 1). TRIM1 recruits LRRK2 to the microtubule cytoskeleton for ubiquitination and proteasomal degradation by binding LRRK2_911–919, a nine amino acid segment within a flexible interdomain region (LRRK2_853–981), which we designate the “regulatory loop” (RL). Phosphorylation of LRRK2 Ser910/Ser935 within LRRK2 RL influences LRRK2’s association with cytoplasmic 14-3-3 versus microtubule-bound TRIM1. Association with TRIM1 modulates LRRK2’s interaction with Rab29 and prevents upregulation of LRRK2 kinase activity by Rab29 in an E3-ligase–dependent manner. Finally, TRIM1 rescues neurite outgrowth deficits caused by PD-driving mutant LRRK2 G2019S. Our data suggest that TRIM1 is a critical regulator of LRRK2, controlling its degradation, localization, binding partners, kinase activity, and cytotoxicity.     

Characterization of TAE684 as a potent LRRK2 kinase inhibitor

Leucine-rich repeat kinase 2 (LRRK2) is linked to Parkinson's disease and may represent an attractive therapeutic target. Here we report a 2,4-dianilino-5-chloro-pyrimidine, TAE684, a previously reported inhibitor of anaplastic lymphoma kinase (ALK), is also a potent inhibitor of LRRK2 kinase activity (IC(50) of 7.8nM against wild-type LRRK2, 6.1nM against the G2019S mutant). TAE684 substantially inhibits Ser910 and Ser935 phosphorylation of both wild-type LRRK2 and G2019S mutant at a concentration of 0.1-0.3μM in cells and in mouse spleen and kidney, but not in brain, following oral doses of 10mg/kg.     

GSK2578215A; A potent and highly selective 2-arylmethyloxy-5-substitutent-N-arylbenzamide LRRK2 kinase inhibitor

Leucine-rich repeat kinase 2 (LRRK2) is a promising therapeutic target for some forms of Parkinson's disease. Here we report the discovery and characterization of 2-arylmethyloxy-5-subtitutent-N-arylbenzamides with potent LRRK2 activities exemplified by GSK2578215A which exhibits biochemical IC(50)s of around 10nM against both wild-type LRRK2 and the G2019S mutant. GSK2578215A exhibits exceptionally high selectivity for LRRK2 across the kinome, substantially inhibits Ser910 and Ser935 phosphorylation of both wild-type LRRK2 and G2019S mutant at a concentration of 0.3-1.0μM in cells and in mouse spleen and kidney, but not in brain, following intraperitoneal injection of 100mg/kg.     

Discovery of 4-alkylamino-7-aryl-3-cyanoquinoline LRRK2 kinase inhibitors

Mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with familial Parkinson’s disease (PD). The kinase activity of this complex protein is increased by pathogenic mutations. Inhibition of LRRK2 kinase activity has therefore emerged as a promising approach for the treatment of PD. Herein we report our findings on a series of 4-alkylamino-7-aryl-3-cyanoquinolines that exhibit kinase inhibitory activity against both wild type and G2019S mutant LRRK2. Activity was determined in both biochemical and cellular assays. Compound 14 was further evaluated in an in vivo pharmacodynamic study and found to significantly inhibit Ser935 phosphorylation after oral dosing.     

<Emphasis Type="Italic">LRRK2</Emphasis> Low-penetrance Mutations (Gly2019Ser) and Risk Alleles (Gly2385Arg)—Linking Familial and Sporadic Parkinson’s Disease

The identification of mutations in the leucine-rich repeat kinase 2 (LRRK2) gene as a cause of autosomal dominant Parkinson’s disease (PD) was a major step forward in the genetic dissection of this disorder. However, what makes LRRK2 unique among the known PD-causing genes is that a low-penetrance mutation, Gly2019Ser, is a frequent determinant not only of familial, but also of sporadic PD in several populations from South Europe, North Africa and Middle East. Moreover, a different polymorphic variant, Gly2385Arg, is a frequent risk factor for PD among Chinese and Japanese populations. Currently, the Gly2019Ser and Gly2385Arg variants represent the most relevant PD-causing mutation and risk allele, respectively, linking the etiology of the familial and the sporadic forms of this disease. Understanding how the dysfunction of LRRK2 protein leads to neurodegeneration might provide crucial insights for unraveling the molecular mechanisms of PD and for developing disease-modifying therapies. Special issue dedicated to Dr. Moussa Youdim.     

Rac1 protein rescues neurite retraction caused by G2019S leucine-rich repeat kinase 2 (LRRK2)

Mutations in leucine-rich repeat kinase 2 (LRRK2) are currently the most common genetic cause of familial late-onset Parkinson disease, which is clinically indistinguishable from idiopathic disease. The most common pathological mutation in LRRK2, G2019S LRRK2, is known to cause neurite retraction. However, molecular mechanisms underlying regulation of neurite length by LRRK2 are unknown. Here, we demonstrate a novel interaction between LRRK2 and the Rho GTPase, Rac1, which plays a critical role in actin cytoskeleton remodeling necessary for the maintenance of neurite morphology. LRRK2 binds strongly to endogenous or expressed Rac1, while showing weak binding to Cdc42 and no binding to RhoA. Co-expression with LRRK2 increases Rac1 activity, as shown by increased binding to the p21-activated kinase, which modulates actin cytoskeletal dynamics. LRRK2 constructs carrying mutations that inactivate the kinase or GTPase activities do not activate Rac1. Interestingly, LRRK2 does not increase levels of membrane-bound Rac1 but dramatically changes the cellular localization of Rac1, causing polarization, which is augmented further when LRRK2 is co-expressed with constitutively active Rac1. Four different disease-related mutations in LRRK2 altered binding to Rac1, with the G2019S and R1441C LRRK2 mutations attenuating Rac1 binding and the Y1699C and I2020T LRRK2 mutations increasing binding. Co-expressing Rac1 in SH-SY5Y cells rescues the G2019S mutant phenotype of neurite retraction. We hypothesize that pathological mutations in LRRK2 attenuates activation of Rac1, causing disassembly of actin filaments, leading to neurite retraction. The interactions between LRRK2 and Rho GTPases provide a novel pathway through which LRRK2 might modulate cellular dynamics and contribute to the pathophysiology of Parkinson disease.     

Inhibition of LRRK2 kinase activity stimulates macroautophagy

Leucine Rich Repeat Kinase 2 (LRRK2) is one of the most important genetic contributors to Parkinson's disease. LRRK2 has been implicated in a number of cellular processes, including macroautophagy. To test whether LRRK2 has a role in regulating autophagy, a specific inhibitor of the kinase activity of LRRK2 was applied to human neuroglioma cells and downstream readouts of autophagy examined. The resulting data demonstrate that inhibition of LRRK2 kinase activity stimulates macroautophagy in the absence of any alteration in the translational targets of mTORC1, suggesting that LRRK2 regulates autophagic vesicle formation independent of canonical mTORC1 signaling. This study represents the first pharmacological dissection of the role LRRK2 plays in the autophagy/lysosomal pathway, emphasizing the importance of this pathway as a marker for LRRK2 physiological function. Moreover it highlights the need to dissect autophagy and lysosomal activities in the context of LRRK2 related pathologies with the final aim of understanding their aetiology and identifying specific targets for disease modifying therapies in patients.     

Mutations in LRRK2/dardarin associated with Parkinson disease are more toxic than equivalent mutations in the homologous kinase LRRK1

Several mutations have been found in the leucine-rich repeat kinase 2 gene (LRRK2), encoding the protein dardarin, which are associated with autosomal dominant Parkinson disease. We have previously shown that mutant LRRK2/dardarin is toxic to neurons and neuron-like cell lines in culture and that some mutations are also associated with an inclusion-body phenotype. There is a homologous kinase, LRRK1, which has a similar domain structure but is not known to carry mutations causing Parkinson disease. In the current study, we introduced mutations at equivalent residues in both LRRK2 and LRRK1 to determine their effects in cells. We show that mutations in dardarin are more prone to form inclusion bodies in transfected cells and are more toxic than equivalent mutations in LRRK1. This work suggests that dardarin/LRRK2 is inherently more damaging than LRRK1.     

Ancient Origin of the Parkinson Disease Gene LRRK2

Dominant mutations in the LRRK2 gene, a member of the Roco family, cause both familial and sporadic Parkinson disease. LRRK genes had so far been detected only in bilaterian animals. In deuterostomes, including humans, two LRRK genes (LRRK1 and LRRK2) exist, while in protostomes a single LRRK gene has been found. In this study, I combine structural and phylogenetic analyses to show that the cnidarian Nematostella vectensis has four LRRK genes. One of them is a bona fide orthologue of the human LRRK2 gene, demonstrating that this gene has an ancient origin. Two others are, respectively, orthologues of the deuterostome LRRK1 and the protostome LRRK genes. The fourth gene is probably cnidarian-specific. This precise characterization of the early evolution of LRRK genes in animals has important implications, because it indicates that the Drosophila and Caenorhabditis LRRK genes, which are studied to gain an understanding of LRRK2 function, are not true orthologues of the human Parkinson disease gene. Novel functional insights are also gained by comparison of the structures of LRRK2 genes in distantly related species.     

Discovery of 4-ethoxy-7H-pyrrolo[2,3-d]pyrimidin-2-amines as potent,selective and orally bioavailable LRRK2 inhibitors

Inhibition of LRRK2 kinase activity with small molecules has emerged as a potential novel therapeutic treatment for Parkinson’s disease. Herein we disclose the discovery of a 4-ethoxy-7H-pyrrolo[2,3-d]pyrimidin-2-amine series as potent LRRK2 inhibitors identified through a kinase-focused set screening. Optimization of the physicochemical properties and kinase selectivity led to the discovery of compound 7, which exhibited potent in vitro inhibition of LRRK2 kinase activity, good physicochemical properties and kinase selectivity across the kinome. Moreover, compound 7 was able to penetrate into the CNS, and in vivo pharmacology studies revealed significant inhibition of Ser935 phosphorylation in the brain of both rats (30 and 100?mg/kg) and mice (45?mg/kg) following oral administration.     

LRRK1 protein kinase activity is stimulated upon binding of GTP to its Roc domain

Human leucine-rich repeat kinase 1 (LRRK1) is a multi-domain protein of unknown function belonging to the ROCO family of complex proteins. Here, we report the molecular characterization of human LRRK1 and show, for the first time, that LRRK1 is both a functional protein kinase and a GDP/GTP-binding protein. Binding of GTP to LRRK1 is specific, requires the GTPase-like Roc domain, and leads to a stimulation of LRRK1 kinase activity. LRRK1 is the first example of a GTP-regulated protein kinase harboring both the kinase effector domain and the GTP-binding regulatory domain. Hence, we propose a model in which LRRK1 cycles between a GTP-bound active and a GDP-bound inactive state. Moreover, we mutated LRRK1 to mimic mutations previously identified in LRRK2/dardarin, the only human paralogue of LRRK1, that have been linked to autosomal-dominant parkinsonism. We demonstrate that three of four mutations analyzed significantly downregulate LRRK1 kinase activity. Ultimately, the results presented for LRRK1 may contribute to the elucidation of LRRK2's role in the pathogenesis of Parkinson's disease.