Advances in elucidating the function of leucine-rich repeat protein kinase-2 in normal cells and Parkinson's disease

Characterizing the role of the dark kinome in neurodegenerative disease - A mini review

BACKGROUND

Drugs that modulate previously unexplored targets could potentially slow or halt the progression of neurodegenerative diseases. Several candidate proteins lie within the dark kinome, those human kinases that have not been well characterized. Much of the kinome (~80%) remains poorly studied, and these targets likely harbor untapped biological potential.

SCOPE OF REVIEW

This review highlights the significance of kinases as mediators of aberrant pathways in neurodegeneration and provides examples of published high-quality small molecules that modulate some of these kinases.

MAJOR CONCLUSIONS

There is a need for continued efforts to develop high-quality chemical tools to illuminate the function of understudied kinases in the brain. Potent and selective small molecules enable accurate pairing of an observed phenotype with a protein target.

GENERAL SIGNIFICANCE

The examples discussed herein support the premise that validation of therapeutic hypotheses surrounding kinase targets can be accomplished via small molecules and they can serve as the basis for disease-focused drug development campaigns.

Focus on the Small GTPase Rab1: A Key Player in the Pathogenesis of Parkinson's Disease

Parkinson’s disease (PD) is the second most frequent neurodegenerative disease. It is characterized by the loss of dopaminergic neurons in the substantia nigra and the formation of large aggregates in the survival neurons called Lewy bodies, which mainly contain α-synuclein (α-syn). The cause of cell death is not known but could be due to mitochondrial dysfunction, protein homeostasis failure, and alterations in the secretory/endolysosomal/autophagic pathways. Survival nigral neurons overexpress the small GTPase Rab1. This protein is considered a housekeeping Rab that is necessary to support the secretory pathway, the maintenance of the Golgi complex structure, and the regulation of macroautophagy from yeast to humans. It is also involved in signaling, carcinogenesis, and infection for some pathogens. It has been shown that it is directly linked to the pathogenesis of PD and other neurodegenerative diseases. It has a protective effect against α–σψν toxicity and has recently been shown to be a substrate of LRRK2, which is the most common cause of familial PD and the risk of sporadic disease. In this review, we analyze the key aspects of Rab1 function in dopamine neurons and its implications in PD neurodegeneration/restauration. The results of the current and former research support the notion that this GTPase is a good candidate for therapeutic strategies.

Impact of Type II LRRK2 inhibitors on signaling and mitophagy

Much effort has been devoted to the development of selective inhibitors of the LRRK2 as a potential treatment for LRRK2 driven Parkinson's disease. In this study, we first compare the properties of Type I (GSK3357679A and MLi-2) and Type II (GZD-824, Rebastinib and Ponatinib) kinase inhibitors that bind to the closed or open conformations of the LRRK2 kinase domain, respectively. We show that Type I and Type II inhibitors suppress phosphorylation of Rab10 and Rab12, key physiological substrates of LRRK2 and also promote mitophagy, a process suppressed by LRRK2. Type II inhibitors also display higher potency towards wild-type LRRK2 compared with pathogenic mutants. Unexpectedly, we find that Type II inhibitors, in contrast with Type I compounds, fail to induce dephosphorylation of a set of well-studied LRRK2 biomarker phosphorylation sites at the N-terminal region of LRRK2, including Ser935. These findings emphasize that the biomarker phosphorylation sites on LRRK2 are likely reporting on the open vs closed conformation of LRRK2 kinase and that only inhibitors which stabilize the closed conformation induce dephosphorylation of these biomarker sites. Finally, we demonstrate that the LRRK2[A2016T] mutant which is resistant to MLi-2 Type 1 inhibitor, also induces resistance to GZD-824 and Rebastinib suggesting this mutation could be exploited to distinguish off target effects of Type II inhibitors. Our observations provide a framework of knowledge to aid with the development of more selective Type II LRRK2 inhibitors.

An Emerging Role for Phosphoinositides in the Pathophysiology of Parkinson’s Disease

Recent data support an involvement of defects in homeostasis of phosphoinositides (PIPs) in the pathophysiology of Parkinson’s disease (PD). Genetic mutations have been identified in genes encoding for PIP-regulating and PIP-interacting proteins, that are associated with familial and sporadic PD. Many of these proteins are implicated in vesicular membrane trafficking, mechanisms that were recently highlighted for their close associations with PD. PIPs are phosphorylated forms of the membrane phospholipid, phosphatidylinositol. Their composition in the vesicle’s membrane of origin, as well as membrane of destination, controls vesicular membrane trafficking. We review the converging evidence that points to the involvement of PIPs in PD. The review describes PD- and PIP-associated proteins implicated in clathrin-mediated endocytosis and autophagy, and highlights the involvement of α-synuclein in these mechanisms.

Rodent models based on endolysosomal genes involved in Parkinson's disease

Genes associated with endolysosomal function have been recently associated with familial Parkinson's disease and described as risk factors for sporadic cases. This indicates that deficits in this pathway predispose to parkinsonism. To better understand the role of these genes in disease development, rodent models have been created by targeting genes playing a role in endolysosomal function, such as LRRK2, DNAJC6, SYNJ1, VPS35, GBA1, ATP13A2 and TMEM175. Here, we review the latest findings describing parkinsonian features in these animal models secondary to endolysosomal dysfunction. Also, we provide suggestions for further development and application of these animal models to better understand the contribution of endolysosomal dysfunction in Parkinson's disease and provide novel models for testing therapeutic approaches.

Endosomal traffic and glutamate synapse activity are increased in VPS35 D620N mutant knock-in mouse neurons, and resistant to LRRK2 kinase inhibition

Vacuolar protein sorting 35 (VPS35) regulates neurotransmitter receptor recycling from endosomes. A missense mutation (D620N) in VPS35 leads to autosomal-dominant, late-onset Parkinson's disease. Here, we study the basic neurobiology of VPS35 and Parkinson's disease mutation effects in the D620N knock-in mouse and the effect of leucine-rich repeat kinase 2 (LRRK2) inhibition on synaptic phenotypes. The study was conducted using a VPS35 D620N knock-in mouse that expresses VPS35 at endogenous levels. Protein levels, phosphorylation states, and binding ratios in brain lysates from knock-in mice and wild-type littermates were assayed by co-immunoprecipitation and western blot. Dendritic protein co-localization, AMPA receptor surface expression, synapse density, and glutamatergic synapse activity in primary cortical cultures from knock-in and wild-type littermates were assayed using immunocytochemistry and whole-cell patch clamp electrophysiology. In brain tissue, we confirm VPS35 forms complexes with LRRK2 and AMPA-type glutamate receptor GluA1 subunits, in addition to NMDA-type glutamate receptor GluN1 subunits and D2-type dopamine receptors. Receptor and LRRK2 binding was unaltered in D620N knock-in mice, but we confirm the mutation results in reduced binding of VPS35 with WASH complex member FAM21, and increases phosphorylation of the LRRK2 kinase substrate Rab10, which is reversed by LRRK2 kinase inhibition in vivo. In cultured cortical neurons from knock-in mice, pRab10 is also increased, and reversed by LRRK2 inhibition. The mutation also results in increased endosomal recycling protein cluster density (VPS35-FAM21 co-clusters and Rab11 clusters), glutamate transmission, and GluA1 surface expression. LRRK2 kinase inhibition, which reversed Rab10 hyper-phosphorylation, did not rescue elevated glutamate release or surface GluA1 expression in knock-in neurons, but did alter AMPAR traffic in wild-type cells. The results improve our understanding of the cell biology of VPS35, and the consequences of the D620N mutation in developing neuronal networks. Together the data support a chronic synaptopathy model for latent neurodegeneration, providing phenotypes and candidate pathophysiological stresses that may drive eventual transition to late-stage parkinsonism in VPS35 PD. The study demonstrates the VPS35 mutation has effects that are independent of ongoing LRRK2 kinase activity, and that LRRK2 kinase inhibition alters basal physiology of glutamate synapses in vitro.

R1441G but not G2019S mutation enhances LRRK2 mediated Rab10 phosphorylation in human peripheral blood neutrophils

Heterozygous gain-of-kinase function variants in LRRK2 (leucine-rich repeat kinase 2) cause 1-2% of all cases of Parkinson's disease (PD) albeit with incomplete and age-dependent penetrance. All pathogenic LRRK2 mutations reside within the two catalytic domains of LRRK2-either in its kinase domain (e.g. G2019S) with modest effect or its ROC-COR GTPase domain (e.g. R1441G/H) with large effect on LRRK2 kinase activity. We have previously reported assays to interrogate LRRK2 kinase pathway activity in human bio-samples measuring phosphorylation of its endogenous substrate Rab10, that mirrors LRRK2 kinase activation status. Here, we isolated neutrophils from fresh peripheral blood from 101 participants including 42 LRRK2 mutation carriers (21 with the G2019S and 21 with the R1441G mutations), 27 patients with idiopathic PD, and 32 controls. Using a dual approach, LRRK2 dependent Rab10 phosphorylation at Threonine 73 (pRab10Thr73) was measured by quantitative multiplexed immunoblotting for pRab10Thr73/total Rab10 as well as targeted mass-spectrometry for absolute pRab10Thr73 occupancy. We found a significant over fourfold increase in pRab10Thr73 phosphorylation in carriers of the LRRK2 R1441G mutation irrespective of clinical disease status. The effect of the LRRK2 G2019S mutation did not reach statistical significance. Furthermore, we show that LRRK2 phosphorylation at Serine 935 is not a marker for LRRK2 kinase activity in human neutrophils. When analysing pRab10Thr73 phosphorylation in post-mortem brain samples, we observed overall high variability irrespective of clinical and LRRK2 mutation status and attributed this mainly to the adverse effect of the peri- and post-mortem period on the stability of posttranslational modifications such as protein phosphorylation. Overall, in vivo LRRK2 dependent pRab10Thr73 phosphorylation in human peripheral blood neutrophils is a specific, robust and promising biomarker for significant LRRK2 kinase hyperactivation, as with the LRRK2 R1441G mutation. Additional readouts and/or assays may be needed to increase sensitivity to detect modest LRRK2 kinase activation, as with the LRRK2 G2019S mutation. Our assays could be useful for patient stratification and target engagement studies for LRRK2 kinase inhibitors.

LRRK2 Targeting Strategies as Potential Treatment of Parkinson's Disease

Parkinson's Disease (PD) affects millions of people worldwide with no cure to halt the progress of the disease. Leucine-rich repeat kinase 2 (LRRK2) is the most common genetic cause of PD and, as such, LRRK2 inhibitors are promising therapeutic agents. In the last decade, great progress in the LRRK2 field has been made. This review provides a comprehensive overview of the current state of the art, presenting recent developments and challenges in developing LRRK2 inhibitors, and discussing extensively the potential targeting strategies from the protein perspective. As currently there are three LRRK2-targeting agents in clinical trials, more developments are predicted in the upcoming years.

Structural analysis of the full-length human LRRK2

Mutations in leucine-rich repeat kinase 2 (LRRK2) are commonly implicated in the pathogenesis of both familial and sporadic Parkinson's disease (PD). LRRK2 regulates critical cellular processes at membranous organelles and forms microtubule-based pathogenic filaments, yet the molecular basis underlying these biological roles of LRRK2 remains largely enigmatic. Here, we determined high-resolution structures of full-length human LRRK2, revealing its architecture and key interdomain scaffolding elements for rationalizing disease-causing mutations. The kinase domain of LRRK2 is captured in an inactive state, a conformation also adopted by the most common PD-associated mutation, LRRK2^G2019S. This conformation serves as a framework for structure-guided design of conformational specific inhibitors. We further determined the structure of COR-mediated LRRK2 dimers and found that single-point mutations at the dimer interface abolished pathogenic filamentation in cells. Overall, our study provides mechanistic insights into physiological and pathological roles of LRRK2 and establishes a structural template for future therapeutic intervention in PD.

Optimization of brain-penetrant picolinamide derived leucine-rich repeat kinase 2 (LRRK2) inhibitors

The discovery of potent, kinome selective, brain penetrant LRRK2 inhibitors is the focus of extensive research seeking new, disease-modifying treatments for Parkinson's disease (PD). Herein, we describe the discovery and evolution of a picolinamide-derived lead series. Our initial optimization efforts aimed at improving the potency and CLK2 off-target selectivity of compound 1 by modifying the heteroaryl C-H hinge and linker regions. This resulted in compound 12 which advanced deep into our research operating plan (ROP) before heteroaryl aniline metabolite 14 was characterized as Ames mutagenic, halting its progression. Strategic modifications to our ROP were made to enable early de-risking of putative aniline metabolites or hydrolysis products for mutagenicity in Ames. This led to the discovery of 3,5-diaminopyridine 15 and 4,6-diaminopyrimidine 16 as low risk for mutagenicity (defined by a 3-strain Ames negative result). Analysis of key matched molecular pairs 17 and 18 led to the prioritization of the 3,5-diaminopyridine sub-series for further optimization due to enhanced rodent brain penetration. These efforts culminated in the discovery of ethyl trifluoromethyl pyrazole 23 with excellent LRRK2 potency and expanded selectivity versus off-target CLK2.

LRRK2 G2019S mutation amplifies protein aggregate propagation

This scientific commentary refers to ‘LRRK2 G2019S kinase activity triggers neurotoxic NSF aggregation’, by Pischedda et al. (doi: 10.1093/brain/awab073).

LRRK2 G2019S kinase activity triggers neurotoxic NSF aggregation

Parkinson's disease is characterized by the progressive degeneration of dopaminergic neurons within the substantia nigra pars compacta and the presence of protein aggregates in surviving neurons. The LRRK2 G2019S mutation is one of the major determinants of familial Parkinson's disease cases and leads to late-onset Parkinson's disease with pleomorphic pathology, including α-synuclein accumulation and deposition of protein inclusions. We demonstrated that LRRK2 phosphorylates N-ethylmaleimide sensitive factor (NSF). We observed aggregates containing NSF in basal ganglia specimens from patients with Parkinson's disease carrying the G2019S variant, and in cellular and animal models expressing the LRRK2 G2019S variant. We found that LRRK2 G2019S kinase activity induces the accumulation of NSF in toxic aggregates. Of note, the induction of autophagy cleared NSF aggregation and rescued motor and cognitive impairment observed in aged hG2019S bacterial artificial chromosome (BAC) mice. We suggest that LRRK2 G2019S pathological phosphorylation impacts on NSF biochemical properties, thus causing the formation of cytotoxic protein inclusions.

Combined Knockout of Lrrk2 and Rab29 Does Not Result in Behavioral Abnormalities in vivo

BACKGROUND

Coding mutations in the LRRK2 gene, encoding for a large protein kinase, have been shown to cause familial Parkinson's disease (PD). The immediate biological consequence of LRRK2 mutations is to increase kinase activity, suggesting that inhibition of this enzyme might be useful therapeutically to slow disease progression. Genome-wide association studies have identified the chromosomal loci around LRRK2 and one of its proposed substrates, RAB29, as contributors towards the lifetime risk of sporadic PD.

OBJECTIVE

Considering the evidence for interactions between LRRK2 and RAB29 on the genetic and protein levels, we set out to determine whether there are any consequences on brain function with aging after deletion of both genes.

METHODS

We generated a double knockout mouse model and performed a battery of motor and non-motor behavioral tests. We then investigated postmortem assays to determine the presence of PD-like pathology, including nigral dopamine cell count, astrogliosis, microgliosis, and striatal monoamine content.

RESULTS

Behaviorally, we noted only that 18-24-month Rab29-/- and double (Lrrk2-/-/Rab29-/-) knockout mice had diminished locomotor behavior in open field compared to wildtype mice. However, no genotype differences were seen in the outcomes that represented PD-like pathology.

CONCLUSION

These results suggest that depletion of both LRRK2 and RAB29 is tolerated, at least in mice, and support that this pathway might be able to be safely targeted for therapeutics in humans.

Characterization of the Onset, Progression, and Reversibility of Morphological Changes in Mouse Lung after Pharmacological Inhibition of Leucine-Rich Kinase 2 Kinase Activity

Gain-of-function mutations in leucine-rich kinase 2 (LRRK2) are associated with increased incidence of Parkinson disease (PD); thus, pharmacological inhibition of LRRK2 kinase activity is postulated as a disease-modifying treatment of PD. Histomorphological changes in lungs of nonhuman primates (NHPs) treated with small-molecule LRRK2 kinase inhibitors have brought the safety of this treatment approach into question. Although it remains unclear how LRRK2 kinase inhibition affects the lung, continued studies in NHPs prove to be both cost- and resource-prohibitive. To develop a tractable alternative animal model platform, we dosed male mice in-diet with the potent, highly selective LRRK2 kinase inhibitor MLi-2 and induced histomorphological changes in lung within 1 week. Oral bolus dosing of MLi-2 at a frequency modeled to provide steady-state exposure equivalent to that achieved with in-diet dosing induced type II pneumocyte vacuolation, suggesting pulmonary changes require sustained LRRK2 kinase inhibition. Treating mice with MLi-2 in-diet for up to 6 months resulted in type II pneumocyte vacuolation that progressed only modestly over time and was fully reversible after withdrawal of MLi-2. Immunohistochemical analysis of lung revealed a significant increase in prosurfactant protein C staining within type II pneumocytes. In the present study, we demonstrated the kinetics for onset, progression, and rapid reversibility of chronic LRRK2 kinase inhibitor effects on lung histomorphology in rodents and provide further evidence for the derisking of safety and tolerability concerns for chronic LRRK2 kinase inhibition in PD. SIGNIFICANCE STATEMENT: We have defined a mouse model by which the on-target lung effects of leucine-rich kinase 2 (LRRK2) kinase inhibition can be monitored, whereas previous in vivo testing relied solely on nonhuman primates. Data serve to derisk long-term treatment with LRRK2 kinase inhibitors, as all lung changes were mild and readily reversible.

Therapeutic Advances in Diabetes, Autoimmune, and Neurological Diseases

Since 2015, 170 small molecules, 60 antibody-based entities, 12 peptides, and 15 gene- or cell-therapies have been approved by FDA for diverse disease indications. Recent advancement in medicine is facilitated by identification of new targets and mechanisms of actions, advancement in discovery and development platforms, and the emergence of novel technologies. Early disease detection, precision intervention, and personalized treatments have revolutionized patient care in the last decade. In this review, we provide a comprehensive overview of current and emerging therapeutic modalities developed in the recent years. We focus on nine diseases in three major therapeutics areas, diabetes, autoimmune, and neurological disorders. The pathogenesis of each disease at physiological and molecular levels is discussed and recently approved drugs as well as drugs in the clinic are presented.

Compartmentalized Signaling in Aging and Neurodegeneration

The cyclic AMP (cAMP) signalling cascade is necessary for cell homeostasis and plays important roles in many processes. This is particularly relevant during ageing and age-related diseases, where drastic changes, generally decreases, in cAMP levels have been associated with the progressive decline in overall cell function and, eventually, the loss of cellular integrity. The functional relevance of reduced cAMP is clearly supported by the finding that increases in cAMP levels can reverse some of the effects of ageing. Nevertheless, despite these observations, the molecular mechanisms underlying the dysregulation of cAMP signalling in ageing are not well understood. Compartmentalization is widely accepted as the modality through which cAMP achieves its functional specificity; therefore, it is important to understand whether and how this mechanism is affected during ageing and to define which is its contribution to this process. Several animal models demonstrate the importance of specific cAMP signalling components in ageing, however, how age-related changes in each of these elements affect the compartmentalization of the cAMP pathway is largely unknown. In this review, we explore the connection of single components of the cAMP signalling cascade to ageing and age-related diseases whilst elaborating the literature in the context of cAMP signalling compartmentalization.

Deciphering the LRRK code: LRRK1 and LRRK2 phosphorylate distinct Rab proteins and are regulated by diverse mechanisms

Autosomal dominant mutations in LRRK2 that enhance kinase activity cause Parkinson's disease. LRRK2 phosphorylates a subset of Rab GTPases including Rab8A and Rab10 within its effector binding motif. Here, we explore whether LRRK1, a less studied homolog of LRRK2 that regulates growth factor receptor trafficking and osteoclast biology might also phosphorylate Rab proteins. Using mass spectrometry, we found that in LRRK1 knock-out cells, phosphorylation of Rab7A at Ser72 was most impacted. This residue lies at the equivalent site targeted by LRRK2 on Rab8A and Rab10. Accordingly, recombinant LRRK1 efficiently phosphorylated Rab7A at Ser72, but not Rab8A or Rab10. Employing a novel phospho-specific antibody, we found that phorbol ester stimulation of mouse embryonic fibroblasts markedly enhanced phosphorylation of Rab7A at Ser72 via LRRK1. We identify two LRRK1 mutations (K746G and I1412T), equivalent to the LRRK2 R1441G and I2020T Parkinson's mutations, that enhance LRRK1 mediated phosphorylation of Rab7A. We demonstrate that two regulators of LRRK2 namely Rab29 and VPS35[D620N], do not influence LRRK1. Widely used LRRK2 inhibitors do not inhibit LRRK1, but we identify a promiscuous inhibitor termed GZD-824 that inhibits both LRRK1 and LRRK2. The PPM1H Rab phosphatase when overexpressed dephosphorylates Rab7A. Finally, the interaction of Rab7A with its effector RILP is not affected by LRRK1 phosphorylation and we observe that maximal stimulation of the TBK1 or PINK1 pathway does not elevate Rab7A phosphorylation. Altogether, these findings reinforce the idea that the LRRK enzymes have evolved as major regulators of Rab biology with distinct substrate specificity.

LRRK2 at the pre-synaptic site: A 16-years perspective

Parkinson's disease is a common neurodegenerative disorder and is clinically characterized by bradykinesia, rigidity, and resting tremor. Missense mutations in the leucine-rich repeat protein kinase-2 gene (LRRK2) are a recognized cause of inherited Parkinson's disease. The physiological and pathological impact of LRRK2 is still obscure, but accumulating evidence indicates that LRRK2 orchestrates diverse aspects of membrane trafficking, such as membrane fusion and vesicle formation and transport along actin and tubulin tracks. In the present review, we focus on the special relation between LRRK2 and synaptic vesicles. LRRK2 binds and phosphorylates key actors within the synaptic vesicle cycle. Accordingly, alterations in dopamine and glutamate transmission have been described upon LRRK2 manipulations. However, the different modeling strategies and phenotypes observed require a critical approach to decipher the outcome of LRRK2 at the pre-synaptic site.

Development of a multiplexed targeted mass spectrometry assay for LRRK2-phosphorylated Rabs and Ser910/Ser935 biomarker sites

Mutations that increase the protein kinase activity of LRRK2 are one of the most common causes of familial Parkinson's disease. LRRK2 phosphorylates a subset of Rab GTPases within their Switch-II motif, impacting interaction with effectors. We describe and validate a new, multiplexed targeted mass spectrometry assay to quantify endogenous levels of LRRK2-phosphorylated Rab substrates (Rab1, Rab3, Rab8, Rab10, Rab35 and Rab43) as well as total levels of Rabs, LRRK2 and LRRK2-phosphorylated at the Ser910 and Ser935 biomarker sites. Exploiting this assay, we quantify for the first time the relative levels of each of the pRab proteins in different cells (mouse embryonic fibroblasts, human neutrophils) and mouse tissues (brain, kidney, lung and spleen). We define how these components are impacted by Parkinson's pathogenic mutations (LRRK2[R1441C] and VPS35[D620N]) and LRRK2 inhibitors. We find that the VPS35[D620N], but not LRRK2[R1441C] mutation, enhances Rab1 phosphorylation in a manner blocked by administration of an LRRK2 inhibitor, providing the first evidence that endogenous Rab1 is a physiological substrate for LRRK2. We exploit this assay to demonstrate that in Parkinson's patients with VPS35[D620N] mutations, phosphorylation of multiple Rab proteins (Rab1, Rab3, Rab8, Rab10 and Rab43) is elevated. We highlight the benefits of this assay over immunoblotting approaches currently deployed to assess LRRK2 Rab signalling pathway.

Syd/JIP3 controls tissue size by regulating Diap1 protein turnover downstream of Yorkie/YAP

How organisms control organ size is not fully understood. We found that Syd/JIP3 is required for proper wing size in Drosophila. JIP3 mutations are associated with organ size defects in mammals. The underlying mechanisms are not well understood. We discovered that Syd/JIP3 inhibition results in a downregulation of the inhibitor of apoptosis protein 1 (Diap1) in the Drosophila wing. Correspondingly, Syd/JIP3 deficient tissues exhibit ectopic cell death and yield smaller wings. Syd/JIP3 inhibition generated similar effects in mammalian cells, indicating a conserved mechanism. We found that Yorkie/YAP stimulates Syd/JIP3 in Drosophila and mammalian cells. Notably, Syd/JIP3 is required for the full effect of Yorkie-mediated tissue growth. Thus Syd/JIP3 regulation of Diap1 functions downstream of Yorkie/YAP to control growth. This study provides mechanistic insights into the recent and perplexing link between JIP3 mutations and organ size defects in mammals, including in humans where de novo JIP3 variants are associated with microcephaly.

Endogenous Rab29 does not impact basal or stimulated LRRK2 pathway activity

Mutations that enhance LRRK2 protein kinase activity cause inherited Parkinson's disease. LRRK2 phosphorylates a group of Rab GTPase proteins, including Rab10 and Rab12, within the effector-binding switch-II motif. Previous work has indicated that the PARK16 locus, which harbors the gene encoding for Rab29, is involved in Parkinson's, and that Rab29 operates in a common pathway with LRRK2. Co-expression of Rab29 and LRRK2 stimulates LRRK2 activity by recruiting LRRK2 to the surface of the trans Golgi network. Here, we report that knock-out of Rab29 does not influence endogenous LRRK2 activity, based on the assessment of Rab10 and Rab12 phosphorylation, in wild-type LRRK2, LRRK2[R1441C] or VPS35[D620N] knock-in mouse tissues and primary cell lines, including brain extracts and embryonic fibroblasts. We find that in brain extracts, Rab12 phosphorylation is more robustly impacted by LRRK2 inhibitors and pathogenic mutations than Rab10 phosphorylation. Transgenic overexpression of Rab29 in a mouse model was also insufficient to stimulate basal LRRK2 activity. We observed that stimulation of Rab10 and Rab12 phosphorylation induced by agents that stress the endolysosomal system (nigericin, monensin, chloroquine and LLOMe) is suppressed by LRRK2 inhibitors but not blocked in Rab29 deficient cells. From the agents tested, nigericin induced the greatest increase in Rab10 and Rab12 phosphorylation (5 to 9-fold). Our findings indicate that basal, pathogenic, as well as nigericin and monensin stimulated LRRK2 pathway activity is not controlled by Rab29. Further work is required to establish how LRRK2 activity is regulated, and whether other Rab proteins can control LRRK2 by targeting it to diverse membranes.

The 14-3-3 Proteins as Important Allosteric Regulators of Protein Kinases

Phosphorylation by kinases governs many key cellular and extracellular processes, such as transcription, cell cycle progression, differentiation, secretion and apoptosis. Unsurprisingly, tight and precise kinase regulation is a prerequisite for normal cell functioning, whereas kinase dysregulation often leads to disease. Moreover, the functions of many kinases are regulated through protein–protein interactions, which in turn are mediated by phosphorylated motifs and often involve associations with the scaffolding and chaperon protein 14-3-3. Therefore, the aim of this review article is to provide an overview of the state of the art on 14-3-3-mediated kinase regulation, focusing on the most recent mechanistic insights into these important protein–protein interactions and discussing in detail both their structural aspects and functional consequences.

Clinically Precedented Protein Kinases: Rationale for Their Use in Neurodegenerative Disease

Kinases are an intensively studied drug target class in current pharmacological research as evidenced by the large number of kinase inhibitors being assessed in clinical trials. Kinase-targeted therapies have potential for treatment of a broad array of indications including central nervous system (CNS) disorders. In addition to the many variables which contribute to identification of a successful therapeutic molecule, drug discovery for CNS-related disorders also requires significant consideration of access to the target organ and specifically crossing the blood-brain barrier (BBB). To date, only a small number of kinase inhibitors have been reported that are specifically designed to be BBB permeable, which nonetheless demonstrates the potential for success. This review considers the potential for kinase inhibitors in the context of unmet medical need for neurodegenerative disease. A subset of kinases that have been the focus of clinical investigations over a 10-year period have been identified and discussed individually. For each kinase target, the data underpinning the validity of each in the context of neurodegenerative disease is critically evaluated. Selected molecules for each kinase are identified with information on modality, binding site and CNS penetrance, if known. Current clinical development in neurodegenerative disease are summarized. Collectively, the review indicates that kinase targets with sufficient rationale warrant careful design approaches with an emphasis on improving brain penetrance and selectivity.

LRRK2 and the Endolysosomal System in Parkinson's Disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) cause autosomal dominant familial Parkinson’s disease (PD), with pathogenic mutations enhancing LRRK2 kinase activity. There is a growing body of evidence indicating that LRRK2 contributes to neuronal damage and pathology both in familial and sporadic PD, making it of particular interest for understanding the molecular pathways that underlie PD. Although LRRK2 has been extensively studied to date, our understanding of the seemingly diverse functions of LRRK2 throughout the cell remains incomplete. In this review, we discuss the functions of LRRK2 within the endolysosomal pathway. Endocytosis, vesicle trafficking pathways, and lysosomal degradation are commonly disrupted in many neurodegenerative diseases, including PD. Additionally, many PD-linked gene products function in these intersecting pathways, suggesting an important role for the endolysosomal system in maintaining protein homeostasis and neuronal health in PD. LRRK2 activity can regulate synaptic vesicle endocytosis, lysosomal function, Golgi network maintenance and sorting, vesicular trafficking and autophagy, with alterations in LRRK2 kinase activity serving to disrupt or regulate these pathways depending on the distinct cell type or model system. LRRK2 is critically regulated by at least two proteins in the endolysosomal pathway, Rab29 and VPS35, which may serve as master regulators of LRRK2 kinase activity. Investigating the function and regulation of LRRK2 in the endolysosomal pathway in diverse PD models, especially in vivo models, will provide critical insight into the cellular and molecular pathophysiological mechanisms driving PD and whether LRRK2 represents a viable drug target for disease-modification in familial and sporadic PD.

Precision medicine in Parkinson's disease patients with LRRK2 and GBA risk variants - Let's get even more personal

Parkinson's disease (PD) is characterized by motor deficits and a wide variety of non-motor symptoms. The age of onset, rate of disease progression and the precise profile of motor and non-motor symptoms display considerable individual variation. Neuropathologically, the loss of substantia nigra dopaminergic neurons is a key feature of PD. The vast majority of PD patients exhibit alpha-synuclein aggregates in several brain regions, but there is also great variability in the neuropathology between individuals. While the dopamine replacement therapies can reduce motor symptoms, current therapies do not modify the disease progression. Numerous clinical trials using a wide variety of approaches have failed to achieve disease modification. It has been suggested that the heterogeneity of PD is a major contributing factor to the failure of disease modification trials, and that it is unlikely that a single treatment will be effective in all patients. Precision medicine, using drugs designed to target the pathophysiology in a manner that is specific to each individual with PD, has been suggested as a way forward. PD patients can be stratified according to whether they carry one of the risk variants associated with elevated PD risk. In this review we assess current clinical trials targeting two enzymes, leucine-rich repeat kinase 2 (LRRK2) and glucocerebrosidase (GBA), which are encoded by two most common PD risk genes. Because the details of the pathogenic processes coupled to the different LRRK2 and GBA risk variants are not fully understood, we ask if these precision medicine-based intervention strategies will prove "precise" or "personalized" enough to modify the disease process in PD patients. We also consider at what phases of the disease that such strategies might be effective, in light of the genes being primarily associated with the risk of developing disease in the first place, and less clearly linked to the rate of disease progression. Finally, we critically evaluate the notion that therapies targeting LRRK2 and GBA might be relevant to a wider segment of PD patients, beyond those that actually carry risk variants of these genes.

A Critical LRRK at the Synapse? The Neurobiological Function and Pathophysiological Dysfunction of LRRK2

Since the discovery of LRRK2 mutations causal to Parkinson's disease (PD) in the early 2000s, the LRRK2 protein has been implicated in a plethora of cellular processes in which pathogenesis could occur, yet its physiological function remains elusive. The development of genetic models of LRRK2 PD has helped identify the etiological and pathophysiological underpinnings of the disease, and may identify early points of intervention. An important role for LRRK2 in synaptic function has emerged in recent years, which links LRRK2 to other genetic forms of PD, most notably those caused by mutations in the synaptic protein α-synuclein. This point of convergence may provide useful clues as to what drives dysfunction in the basal ganglia circuitry and eventual death of substantia nigra (SN) neurons. Here, we discuss the evolution and current state of the literature placing LRRK2 at the synapse, through the lens of knock-out, overexpression, and knock-in animal models. We hope that a deeper understanding of LRRK2 neurobiology, at the synapse and beyond, will aid the eventual development of neuroprotective interventions for PD, and the advancement of useful treatments in the interim.

Parkinson's: A Disease of Aberrant Vesicle Trafficking

Parkinson's disease (PD) is a leading cause of neurodegeneration that is defined by the selective loss of dopaminergic neurons and the accumulation of protein aggregates called Lewy bodies (LBs). The unequivocal identification of Mendelian inherited mutations in 13 genes in PD has provided transforming insights into the pathogenesis of this disease. The mechanistic analysis of several PD genes, including α-synuclein (α-syn), leucine-rich repeat kinase 2 (LRRK2), PTEN-induced kinase 1 (PINK1), and Parkin, has revealed central roles for protein aggregation, mitochondrial damage, and defects in endolysosomal trafficking in PD neurodegeneration. In this review, we outline recent advances in our understanding of these gene pathways with a focus on the emergent role of Rab (Ras analog in brain) GTPases and vesicular trafficking as a common mechanism that underpins how mutations in PD genes lead to neuronal loss. These advances have led to previously distinct genes such as vacuolar protein-sorting-associated protein 35 (VPS35) and LRRK2 being implicated in a common signaling pathway. A greater understanding of these common nodes of vesicular trafficking will be crucial for linking other PD genes and improving patient stratification in clinical trials underway against α-syn and LRRK2 targets.

RIPK protein kinase family: Atypical lives of typical kinases

Receptor Interacting Protein Kinases (RIPKs) are a family of Ser/Thr/Tyr kinases whose functions, regulation and pathophysiologic roles have remained an enigma for a long time. In recent years, these proteins garnered significant interest due to their roles in regulating a variety of host defense functions including control of inflammatory gene expression, different forms of cell death, and cutaneous and intestinal barrier functions. In addition, there is accumulating evidence that while these kinases seemingly follow typical kinase blueprints, their functioning in cells can take forms that are atypical for protein kinases. Lastly, while these kinases generally belong to distinct areas of innate immune regulation, there are emerging overarching themes that may unify the functions of this kinase family. Our review seeks to discuss the biology of RIPKs, and how typical and atypical features of this family informs the activity of a rapidly growing repertoire of RIPK inhibitors.

LRRK2-Related Parkinson's Disease Due to Altered Endolysosomal Biology With Variable Lewy Body Pathology: A Hypothesis

Mutations in the gene encoding for leucine-rich repeat kinase 2 (LRRK2) are associated with both familial and sporadic Parkinson's disease (PD). LRRK2 encodes a large protein comprised of a GTPase and a kinase domain. All pathogenic variants converge on enhancing LRRK2 kinase substrate phosphorylation, and distinct LRRK2 kinase inhibitors are currently in various stages of clinical trials. Although the precise pathophysiological functions of LRRK2 remain largely unknown, PD-associated mutants have been shown to alter various intracellular vesicular trafficking pathways, especially those related to endolysosomal protein degradation events. In addition, biochemical studies have identified a subset of Rab proteins, small GTPases required for all vesicular trafficking steps, as substrate proteins for the LRRK2 kinase activity in vitro and in vivo. Therefore, it is crucial to evaluate the impact of such phosphorylation on neurodegenerative mechanisms underlying LRRK2-related PD, especially with respect to deregulated Rab-mediated endolysosomal membrane trafficking and protein degradation events. Surprisingly, a significant proportion of PD patients due to LRRK2 mutations display neuronal cell loss in the substantia nigra pars compacta in the absence of any apparent α-synuclein-containing Lewy body neuropathology. These findings suggest that endolysosomal alterations mediated by pathogenic LRRK2 per se are not sufficient to cause α-synuclein aggregation. Here, we will review current knowledge about the link between pathogenic LRRK2, Rab protein phosphorylation and endolysosomal trafficking alterations, and we will propose a testable working model whereby LRRK2-related PD may present with variable LB pathology.