Parkinson-related LRRK2 mutation R1441C/G/H impairs PKA phosphorylation of LRRK2 and disrupts its interaction with 14-3-3

A Personalized 14-3-3 Disease-Targeting Workflow Yields Repositioning Drug Candidates

Rare diseases typically evade the application of the standard drug discovery and development pipelines due to their understudied molecular etiology and the small market size. Herein, we report a rare disease-directed workflow that rapidly studies the molecular features of the disorder, establishes a high-throughput screening (HTS) platform, and conducts an HTS of thousands of approved drugs to identify and validate repositioning drug candidates. This study examines the pediatric neurological disorder caused by de novo mutations in YWHAG, the gene encoding the scaffolding protein 14-3-3γ, and the workflow discovers nuclear relocalization and a severe drop in 14-3-3γ binding to its phosphorylated protein partners as the key molecular features of the pathogenic hotspot YWHAG mutations. We further established a robust in vitro HTS platform and screened ca. 3000 approved drugs to identify the repositioning drug candidates that restore the deficient 14-3-3γ-phosphotarget interactions. Our workflow can be applied to other 14-3-3-related disorders and upscaled for many other rare diseases.

Molecular dynamics studies reveal the structural impacts of LRRK2 R1441C and LRRK2 D1994A mutations in Parkinson's disease

Parkinson's Disease (PD) is a continuingly deteriorating neurological ailment affecting over 8.5 million patients globally as of 2019, and the numbers are expected to keep rising. To aid in identifying therapeutic targets, molecular dynamics simulations are convenient and cost-effective methods for enriching our knowledge of the molecular pathophysiology of diseases. Many proteins and their corresponding mutations have been identified to contribute to this disease, of which Leucine-rich repeat kinase 2 (LRRK2) is accountable for a significant percentage. Several mutations involving the domains in LRRK2 have been studied, which are known to interfere with various enzymatic processes, ultimately leading to trademark features of PD like aggregation of protein inclusions called Lewy Bodies (LBs), mitochondrial dysfunctions, etc. The precise molecular mechanism of the mutations' pathophysiology is still unclear. This research article looks at the structural effects of mutations, namely the R1441C and D1994A mutations, on the surrounding residues in the protein, offering novel insights into pathophysiological changes at an atomistic level. Our results indicate a gain of electrostatic interactions with a stable αβ motif within the LRR-Roc linker, amongst other changes. This article also highlights the potential involvement and importance of the αβ motif in LRRK2 associated PD.

14-3-3 binding maintains the Parkinson's associated kinase LRRK2 in an inactive state

Leucine-rich repeat kinase 2 (LRRK2) is a central player in cellular signaling and a significant contributor to Parkinson's disease (PD) pathogenesis. 14-3-3 proteins are essential regulators of LRRK2, modulating its activity. Here, we present the cryo- electron microscopy structure of the LRRK2:14-3-3 2 autoinhibitory complex, showing that a 14-3-3 dimer stabilizes an autoinhibited LRRK2 monomer by binding to key phosphorylation sites and the COR-A and COR-B subdomains within the Roc-COR GTPase domain of LRRK2. This interaction locks LRRK2 in an inactive conformation, restricting LRR domain mobility and preventing dimerization and oligomer formation. Our mutagenesis studies reveal that PD-associated mutations at the COR:14-3-3 interface and within the GTPase domain reduce 14-3-3 binding, diminishing its inhibitory effect on LRRK2. These findings provide a structural basis for understanding how LRRK2 likely remains dormant within cells, illuminate aspects of critical PD biomarkers, and suggest therapeutic strategies to enhance LRRK2-14-3-3 interactions to treat PD and related disorders.

14-3-3 phosphorylation inhibits 14-3-3θ's ability to regulate LRRK2 kinase activity and toxicity

LRRK2 mutations are among the most common genetic causes for Parkinson's disease (PD), and toxicity is associated with increased kinase activity. 14-3-3 proteins are key interactors that regulate LRRK2 kinase activity. Phosphorylation of the 14-3-3θ isoform at S232 is dramatically increased in human PD brains. Here we investigate the impact of 14-3-3θ phosphorylation on its ability to regulate LRRK2 kinase activity. Both wildtype and the non-phosphorylatable S232A 14-3-3θ mutant reduced the kinase activity of wildtype and G2019S LRRK2, whereas the phosphomimetic S232D 14-3-3θ mutant had minimal effects on LRRK2 kinase activity, as determined by measuring autophosphorylation at S1292 and T1503 and Rab10 phosphorylation. However, wildtype and both 14-3-3θ mutants similarly reduced the kinase activity of the R1441G LRRK2 mutant. 14-3-3θ phosphorylation did not promote global dissociation with LRRK2, as determined by co-immunoprecipitation and proximal ligation assays. 14-3-3s interact with LRRK2 at several phosphorylated serine/threonine sites, including T2524 in the C-terminal helix, which can fold back to regulate the kinase domain. Interaction between 14-3-3θ and phosphorylated T2524 LRRK2 was important for 14-3-3θ's ability to regulate kinase activity, as wildtype and S232A 14-3-3θ failed to reduce the kinase activity of G2019S/T2524A LRRK2. Finally, we found that the S232D mutation failed to protect against G2019S LRRK2-induced neurite shortening in primary cultures, while the S232A mutation was protective. We conclude that 14-3-3θ phosphorylation destabilizes the interaction of 14-3-3θ with LRRK2 at T2524, which consequently promotes LRRK2 kinase activity and toxicity.

LRRK2 regulates ferroptosis through the system Xc-GSH-GPX4 pathway in the neuroinflammatory mechanism of Parkinson's disease

Parkinson's disease (PD) is the most prevalent neurodegenerative disorder. Neuroinflammation mediated by activated microglia and apoptosis of dopaminergic (DA) neurons in the midbrain are its primary pathological manifestations. Leucine-rich repeat protein kinase 2 (LRRK2) kinase has been observed to increase expression during neuroinflammation, however, the effect of LRRK2 on microglia activation remains poorly understood. In this study, we have established lipopolysaccharide (LPS) treated BV2 cells and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) models for both in vivo and in vitro investigation. Our data in vivo reveal that LRRK2 can promote microglia activation by regulating ferroptosis and activating nuclear factor-κB. Inhibition of LRRK2 expression effectively suppressed the LPS-induced pro-inflammatory cytokines and facilitated the secretion of neuroprotective factors. Importantly, by co-overexpressing LRRK2 and glutathione peroxidase 4 (GPX4), we identified the system Xc-GSH-GPX4 pathway as a crucial component in LRRK2-mediated microglial ferroptosis and inflammatory responses. Using a microglial culture supernatant (MCS) transfer model, we found that inhibiting LRRK2 or downregulating ferroptosis in BV2 cells prevented SH-SY5Y cell apoptosis. Additionally, we observed abundant expression of LRRK2 and P-P65 in the midbrain, which was elevated in the MPTP-induced PD model, along with microglia activation. LRRK2 and P-P65 expression inhibition with PF-06447475 attenuated microglia activation in the nigrostriatal dense part of MPTP-treated mice. Based on our findings, it is evident that LRRK2 plays a critical role in promoting the neuroinflammatory response during the pathogenesis of PD by regulating the system Xc-GSH-GPX4 pathway. Taken together, our data highlights the potential research and therapeutic value of targeting LRRK2 to regulate neuroinflammatory response in PD through ferroptosis.

Early deficits in an in vitro striatal microcircuit model carrying the Parkinson's GBA-N370S mutation

Understanding medium spiny neuron (MSN) physiology is essential to understand motor impairments in Parkinson's disease (PD) given the architecture of the basal ganglia. Here, we developed a custom three-chambered microfluidic platform and established a cortico-striato-nigral microcircuit partially recapitulating the striatal presynaptic landscape in vitro using induced pluripotent stem cell (iPSC)-derived neurons. We found that, cortical glutamatergic projections facilitated MSN synaptic activity, and dopaminergic transmission enhanced maturation of MSNs in vitro. Replacement of wild-type iPSC-derived dopamine neurons (iPSC-DaNs) in the striatal microcircuit with those carrying the PD-related GBA-N370S mutation led to a depolarisation of resting membrane potential and an increase in rheobase in iPSC-MSNs, as well as a reduction in both voltage-gated sodium and potassium currents. Such deficits were resolved in late microcircuit cultures, and could be reversed in younger cultures with antagonism of protein kinase A activity in iPSC-MSNs. Taken together, our results highlight the unique utility of modelling striatal neurons in a modular physiological circuit to reveal mechanistic insights into GBA1 mutations in PD.

Role of the leucine-rich repeat protein kinase 2 C-terminal tail in domain cross-talk

Leucine-rich repeat protein kinase 2 (LRRK2) is a multi-domain protein encompassing two of biology's most critical molecular switches, a kinase and a GTPase, and mutations in LRRK2 are key players in the pathogenesis of Parkinson's disease (PD). The availability of multiple structures (full-length and truncated) has opened doors to explore intra-domain cross-talk in LRRK2. A helix extending from the WD40 domain and stably docking onto the kinase domain is common in all available structures. This C-terminal (Ct) helix is a hub of phosphorylation and organelle-localization motifs and thus serves as a multi-functional protein : protein interaction module. To examine its intra-domain interactions, we have recombinantly expressed a stable Ct motif (residues 2480-2527) and used peptide arrays to identify specific binding sites. We have identified a potential interaction site between the Ct helix and a loop in the CORB domain (CORB loop) using a combination of Gaussian accelerated molecular dynamics simulations and peptide arrays. This Ct-Motif contains two auto-phosphorylation sites (T2483 and T2524), and T2524 is a 14-3-3 binding site. The Ct helix, CORB loop, and the CORB-kinase linker together form a part of a dynamic 'CAP' that regulates the N-lobe of the kinase domain. We hypothesize that in inactive, full-length LRRK2, the Ct-helix will also mediate interactions with the N-terminal armadillo, ankyrin, and LRR domains (NTDs) and that binding of Rab substrates, PD mutations, or kinase inhibitors will unleash the NTDs.

Molecular mechanism of cognitive impairment associated with Parkinson's disease: A stroke perspective

Parkinson's disease (PD) is a common neurological illness that causes several motor and non-motor symptoms, most characteristically limb tremors and bradykinesia. PD is a slowly worsening disease that arises due to progressive neurodegeneration of specific areas of the brain, especially the substantia nigra of the midbrain. Even though PD has continuously been linked to a higher mortality risk in numerous epidemiologic studies, there have been significant discoveries regarding the connection between PD and stroke. The incidence of strokes such as cerebral infarction and hemorrhage is substantially associated with the development of PD. Moreover, cognitive impairments, primarily dementia, have been associated with stroke and PD. However, the underlying molecular mechanism of this phenomenon is still obscure. This concise review focuses on the relationship between stroke and PD, emphasizing the molecular mechanism of cognition deficit and memory loss evident in PD and stroke. Furthermore, we are also highlighting some potential drug molecules that can target both PD and stroke.

PAK6-mediated phosphorylation of PPP2R2C regulates LRRK2-PP2A complex formation

Mutations in leucine-rich repeat kinase 2 (LRRK2) are a common cause of inherited and sporadic Parkinson's disease (PD) and previous work suggests that dephosphorylation of LRRK2 at a cluster of heterologous phosphosites is associated to disease. We have previously reported subunits of the PP1 and PP2A classes of phosphatases as well as the PAK6 kinase as regulators of LRRK2 dephosphorylation. We therefore hypothesized that PAK6 may have a functional link with LRRK2's phosphatases. To investigate this, we used PhosTag gel electrophoresis with purified proteins and found that PAK6 phosphorylates the PP2A regulatory subunit PPP2R2C at position S381. While S381 phosphorylation did not affect PP2A holoenzyme formation, a S381A phosphodead PPP2R2C showed impaired binding to LRRK2. Also, PAK6 kinase activity changed PPP2R2C subcellular localization in a S381 phosphorylation-dependent manner. Finally, PAK6-mediated dephosphorylation of LRRK2 was unaffected by phosphorylation of PPP2R2C at S381, suggesting that the previously reported mechanism whereby PAK6-mediated phosphorylation of 14-3-3 proteins promotes 14-3-3-LRRK2 complex dissociation and consequent exposure of LRRK2 phosphosites for dephosphorylation is dominant. Taken together, we conclude that PAK6-mediated phosphorylation of PPP2R2C influences the recruitment of PPP2R2C to the LRRK2 complex and PPP2R2C subcellular localization, pointing to an additional mechanism in the fine-tuning of LRRK2 phosphorylation.

Structural and functional complexity of HSP90 in cellular homeostasis and disease

Heat shock protein 90 (HSP90) is a chaperone with vital roles in regulating proteostasis, long recognized for its function in protein folding and maturation. A view is emerging that identifies HSP90 not as one protein that is structurally and functionally homogeneous but, rather, as a protein that is shaped by its environment. In this Review, we discuss evidence of multiple structural forms of HSP90 in health and disease, including homo-oligomers and hetero-oligomers, also termed epichaperomes, and examine the impact of stress, post-translational modifications and co-chaperones on their formation. We describe how these variations influence context-dependent functions of HSP90 as well as its interaction with other chaperones, co-chaperones and proteins, and how this structural complexity of HSP90 impacts and is impacted by its interaction with small molecule modulators. We close by discussing recent developments regarding the use of HSP90 inhibitors in cancer and how our new appreciation of the structural and functional heterogeneity of HSP90 invites a re-evaluation of how we discover and implement HSP90 therapeutics for disease treatment.

From Tethered to Freestanding Stabilizers of 14-3-3 Protein-Protein Interactions through Fragment Linking

Small-molecule stabilization of protein-protein interactions (PPIs) is a promising strategy in chemical biology and drug discovery. However, the systematic discovery of PPI stabilizers remains a largely unmet challenge. Herein we report a fragment-linking approach targeting the interface of 14-3-3 and a peptide derived from the estrogen receptor alpha (ERα) protein. Two classes of fragments-a covalent and a noncovalent fragment-were co-crystallized and subsequently linked, resulting in a noncovalent hybrid molecule in which the original fragment interactions were largely conserved. Supported by 20 crystal structures, this initial hybrid molecule was further optimized, resulting in selective, 25-fold stabilization of the 14-3-3/ERα interaction. The high-resolution structures of both the single fragments, their co-crystal structures and those of the linked fragments document a feasible strategy to develop orthosteric PPI stabilizers by linking to an initial tethered fragment.

LRRK2 phosphorylation status and kinase activity regulate (macro)autophagy in a Rab8a/Rab10-dependent manner

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic cause of Parkinson's disease (PD), with growing importance also for Crohn's disease and cancer. LRRK2 is a large and complex protein possessing both GTPase and kinase activity. Moreover, LRRK2 activity and function can be influenced by its phosphorylation status. In this regard, many LRRK2 PD-associated mutants display decreased phosphorylation of the constitutive phosphorylation cluster S910/S935/S955/S973, but the role of these changes in phosphorylation status with respect to LRRK2 physiological functions remains unknown. Here, we propose that the S910/S935/S955/S973 phosphorylation sites act as key regulators of LRRK2-mediated autophagy under both basal and starvation conditions. We show that quadruple LRRK2 phosphomutant cells (4xSA; S910A/S935A/S955A/S973A) have impaired lysosomal functionality and fail to induce and proceed with autophagy during starvation. In contrast, treatment with the specific LRRK2 kinase inhibitors MLi-2 (100 nM) or PF-06447475 (150 nM), which also led to decreased LRRK2 phosphorylation of S910/S935/S955/S973, did not affect autophagy. In explanation, we demonstrate that the autophagy impairment due to the 4xSA LRRK2 phospho-dead mutant is driven by its enhanced LRRK2 kinase activity. We show mechanistically that this involves increased phosphorylation of LRRK2 downstream targets Rab8a and Rab10, as the autophagy impairment in 4xSA LRRK2 cells is counteracted by expression of phosphorylation-deficient mutants T72A Rab8a and T73A Rab10. Similarly, reduced autophagy and decreased LRRK2 phosphorylation at the constitutive sites were observed in cells expressing the pathological R1441C LRRK2 PD mutant, which also displays increased kinase activity. These data underscore the relation between LRRK2 phosphorylation at its constitutive sites and the importance of increased LRRK2 kinase activity in autophagy regulation and PD pathology.

14-3-3 phosphorylation inhibits 14-3-3θ's ability to regulate LRRK2 kinase activity

LRRK2 mutations are among the most common genetic causes for Parkinson's disease (PD), and toxicity is associated with increased kinase activity. 14-3-3 proteins are key interactors that regulate LRRK2 kinase activity. Phosphorylation of the 14-3-3θ isoform at S232 is dramatically increased in human PD brains. Here we investigate the impact of 14-3-3θ phosphorylation on its ability to regulate LRRK2 kinase activity. Both wildtype and the non-phosphorylatable S232A 14-3-3θ mutant reduced the kinase activity of wildtype and G2019S LRRK2, whereas the phosphomimetic S232D 14-3-3θ mutant had minimal effects on LRRK2 kinase activity, as determined by measuring autophosphorylation at S1292 and T1503 and Rab10 phosphorylation. However, wildtype and both 14-3-3θ mutants similarly reduced the kinase activity of the R1441G LRRK2 mutant. 14-3-3θ phosphorylation did not promote global dissociation with LRRK2, as determined by co-immunoprecipitation and proximal ligation assays. 14-3-3s interact with LRRK2 at several phosphorylated serine/threonine sites, including T2524 in the C-terminal helix, which can fold back to regulate the kinase domain. Interaction between 14-3-3θ and phosphorylated T2524 LRRK2 was important for 14-3-3θ's ability to regulate kinase activity, as wildtype and S232A 14-3-3θ failed to reduce the kinase activity of G2019S/T2524A LRRK2. Molecular modeling showed that 14-3-3θ phosphorylation causes a partial rearrangement of its canonical binding pocket, thus affecting the interaction between 14-3-3θ and the C-terminus of LRRK2. We conclude that 14-3-3θ phosphorylation destabilizes the interaction of 14-3-3θ with LRRK2 at T2524, which consequently promotes LRRK2 kinase activity.

Crystal structure and ligandability of the 14-3-3/pyrin interface

Overactivation of Pyrin is the cause of the inflammatory diseases Mediterranean Fever and Pyrin-associated autoinflammation with neutrophilic dermatosis (PAAND). Binding of 14-3-3 proteins reduces the pro-inflammatory activity of Pyrin, hence small molecules that stabilize the Pyrin/14-3-3 complex could convey an anti-inflammatory effect. We have solved the atomic resolution crystal structures of phosphorylated peptides derived from PyrinpS208 and PyrinpS242 - the two principle 14-3-3 binding sites in Pyrin - in complex with 14-3-3 and analyzed the ligandability of these protein-peptide interfaces by crystal-based fragment soaking. The complex between 14-3-3 and PyrinpS242 appears to be much more amenable for small-molecule binding than that of 14-3-3/PyrinpS208. Consequently, only for the 14-3-3/PyrinpS242 complex could we find an interface-binding fragment, validating protein crystallography and fragment soaking as a method to evaluate the ligandability of protein surfaces.

LRRK2 expression in normal and pathologic human gut and in rodent enteric neural cell lines

Leucine-rich repeat kinase 2 (LRRK2) gene, which is the gene most commonly associated with Parkinson's disease (PD), is also a susceptibility gene for Crohn's disease, thereby suggesting that LRRK2 may sit at the crossroads of gastrointestinal inflammation, Parkinson's, and Crohn's disease. LRRK2 protein has been studied intensely in both CNS neurons and in immune cells, but there are only few studies on LRRK2 in the enteric nervous system (ENS). LRRK2 is present in ENS ganglia and the existing studies on LRRK2 expression in colonic biopsies from PD subjects have yielded conflicting results. Herein, we propose to extend these findings by studying in more details LRRK2 expression in the ENS. LRRK2 expression was evaluated in full thickness segments of colon of 16 Lewy body, 12 non-Lewy body disorders cases, and 3 non-neurodegenerative controls and in various enteric neural cell lines. We showed that, in addition to enteric neurons, LRRK2 is constitutively expressed in enteric glial cells in both fetal and adult tissues. LRRK2 immunofluorescence intensity in the myenteric ganglia was not different between Lewy body and non-Lewy body disorders. Additionally, we identified the cAMP pathway as a key signaling pathway involved in the regulation of LRRK2 expression and phosphorylation in the enteric glial cells. Our study is the first detailed characterization of LRRK2 in the ENS and the first to show that enteric glial cells express LRRK2. Our findings provide a basis to unravel the functions of LRRK2 in the ENS and to further investigate the pathological changes in enteric synucleinopathies.

Mechanisms of Autoimmune Cell in DA Neuron Apoptosis of Parkinson's Disease: Recent Advancement

Parkinson's disease (PD) is a prevalent neurodegenerative disorder that manifests as motor and nonmotor symptoms due to the selective loss of midbrain DArgic (DA) neurons. More and more studies have shown that pathological reactions initiated by autoimmune cells play an essential role in the progression of PD. Autoimmune cells exist in the brain parenchyma, cerebrospinal fluid, and meninges; they are considered inducers of neuroinflammation and regulate the immune in the human brain in PD. For example, T cells can recognize α-synuclein presented by antigen-presenting cells to promote neuroinflammation. In addition, B cells will accelerate the apoptosis of DA neurons in the case of PD-related gene mutations. Activation of microglia and damage of DA neurons even form the self-degeneration cycle to deteriorate PD. Numerous autoimmune cells have been considered regulators of apoptosis, α-synuclein misfolding and aggregation, mitochondrial dysfunction, autophagy, and neuroinflammation of DA neurons in PD. The evidence is mounting that autoimmune cells promote DA neuron apoptosis. In this review, we discuss the current knowledge regarding the regulation and function of B cell, T cell, and microglia as well as NK cell in PD pathogenesis, focusing on DA neuron apoptosis to understand the disease better and propose potential target identification for the treatment in the early stages of PD. However, there are still some limitations in our work, for example, the specific mechanism of PD progression caused by autoimmune cells in mitochondrial dysfunction, ferroptosis, and autophagy has not been clarified in detail, which needs to be summarized in further work.

R1441C and G2019S LRRK2 knockin mice have distinct striatal molecular, physiological, and behavioral alterations

LRRK2 mutations are closely associated with Parkinson's disease (PD). Convergent evidence suggests that LRRK2 regulates striatal function. Here, by using knock-in mouse lines expressing the two most common LRRK2 pathogenic mutations-G2019S and R1441C-we investigated how LRRK2 mutations altered striatal physiology. While we found that both R1441C and G2019S mice displayed reduced nigrostriatal dopamine release, hypoexcitability in indirect-pathway striatal projection neurons, and alterations associated with an impaired striatal-dependent motor learning were observed only in the R1441C mice. We also showed that increased synaptic PKA activities in the R1441C and not G2019S mice underlie the specific alterations in motor learning deficits in the R1441C mice. In summary, our data argue that LRRK2 mutations' impact on the striatum cannot be simply generalized. Instead, alterations in electrochemical, electrophysiological, molecular, and behavioral levels were distinct between LRRK2 mutations. Our findings offer mechanistic insights for devising and optimizing treatment strategies for PD patients.

Molecular glues: enhanced protein-protein interactions and cell proteome editing

The druggable genome is limited by structural features that can be targeted by small molecules in disease-relevant proteins. While orthosteric and allosteric protein modulators have been well studied, they are limited to antagonistic/agonistic functions. This approach to protein modulation leaves many disease-relevant proteins as undruggable targets. Recently, protein-protein interaction modulation has emerged as a promising therapeutic field for previously undruggable protein targets. Molecular glues and heterobifunctional degraders such as PROTACs can facilitate protein interactions and bring the proteasome into proximity to induce targeted protein degradation. In this review, we discuss the function and rational design of molecular glues, heterobifunctional degraders, and hydrophobic tag degraders. We also review historic and novel molecular glues and targets and discuss the challenges and opportunities in this new therapeutic field.

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 LRRK2911-919, a nine amino acid segment within a flexible interdomain region (LRRK2853-981), 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.

A Phosphosite Mutant Approach on LRRK2 Links Phosphorylation and Dephosphorylation to Protective and Deleterious Markers, Respectively

The Leucine Rich Repeat Kinase 2 (LRRK2) gene is a major genetic determinant of Parkinson’s disease (PD), encoding a homonymous multi-domain protein with two catalytic activities, GTPase and Kinase, involved in intracellular signaling and trafficking. LRRK2 is phosphorylated at multiple sites, including a cluster of autophosphorylation sites in the GTPase domain and a cluster of heterologous phosphorylation sites at residues 860 to 976. Phosphorylation at these latter sites is found to be modified in brains of PD patients, as well as for some disease mutant forms of LRRK2. The main aim of this study is to investigate the functional consequences of LRRK2 phosphorylation or dephosphorylation at LRRK2’s heterologous phosphorylation sites. To this end, we generated LRRK2 phosphorylation site mutants and studied how these affected LRRK2 catalytic activity, neurite outgrowth and lysosomal physiology in cellular models. We show that phosphorylation of RAB8a and RAB10 substrates are reduced with phosphomimicking forms of LRRK2, while RAB29 induced activation of LRRK2 kinase activity is enhanced for phosphodead forms of LRRK2. Considering the hypothesis that PD pathology is associated to increased LRRK2 kinase activity, our results suggest that for its heterologous phosphorylation sites LRRK2 phosphorylation correlates to healthy phenotypes and LRRK2 dephosphorylation correlates to phenotypes associated to the PD pathological processes.

The Roc domain of LRRK2 as a hub for protein-protein interactions: a focus on PAK6 and its impact on RAB phosphorylation

Leucine-rich repeat kinase 2 (LRRK2) has taken center stage in Parkinson's disease (PD) research as mutations cause familial PD and more common variants increase lifetime risk for disease. One unique feature in LRRK2 is the coexistence of GTPase/Roc (Ras of complex) and kinase catalytic functions, bridged by a COR (C-terminal Of Roc) platform for dimerization. Multiple PD mutations are located within the Roc/GTPase domain and concomitantly lead to defective GTPase activity and augmented kinase activity in cells, supporting a crosstalk between GTPase and kinase domains. In addition, biochemical and structural data highlight the importance of Roc as a molecular switch modulating LRRK2 monomer-to-dimer equilibrium and building the interface for interaction with binding partners. Here we review the effects of PD Roc mutations on LRRK2 function and discuss the importance of Roc as a hub for multiple molecular interactions relevant for the regulation of cytoskeletal dynamics and intracellular trafficking pathways. Among the well-characterized Roc interactors, we focused on the cytoskeletal-related kinase p21-activated kinase 6 (PAK6). We report the affinity between LRRK2-Roc and PAK6 measured by microscale thermophoresis (MST). We further show that PAK6 can modulate LRRK2-mediated phosphorylation of RAB substrates in the presence of LRRK2 wild-type (WT) or the PD G2019S kinase mutant but not when the PD Roc mutation R1441G is expressed. These findings support a mechanism whereby mutations in Roc might affect LRRK2 activity through impaired protein-protein interaction in the cell.

LRRK2 and idiopathic Parkinson's disease

The etiology of idiopathic Parkinson's disease (iPD) is multifactorial, and both genetics and environmental exposures are risk factors. While mutations in leucine-rich repeat kinase-2 (LRRK2) that are associated with increased kinase activity are the most common cause of autosomal dominant PD, the role of LRRK2 in iPD, independent of mutations, remains uncertain. In this review, we discuss how the architecture of LRRK2 influences kinase activation and how enhanced LRRK2 substrate phosphorylation might contribute to pathogenesis. We describe how oxidative stress and endolysosomal dysfunction, both of which occur in iPD, can activate non-mutated LRRK2 to a similar degree as pathogenic mutations. Similarly, environmental toxicants that are linked epidemiologically to iPD risk can also activate LRRK2. In aggregate, current evidence suggests an important role for LRRK2 in iPD.

LRRK2 mutant knock-in mouse models: therapeutic relevance in Parkinson's disease

Mutations in the leucine-rich repeat kinase 2 gene (LRRK2) are one of the most frequent genetic causes of both familial and sporadic Parkinson's disease (PD). Mounting evidence has demonstrated pathological similarities between LRRK2-associated PD (LRRK2-PD) and sporadic PD, suggesting that LRRK2 is a potential disease modulator and a therapeutic target in PD. LRRK2 mutant knock-in (KI) mouse models display subtle alterations in pathological aspects that mirror early-stage PD, including increased susceptibility of nigrostriatal neurotransmission, development of motor and non-motor symptoms, mitochondrial and autophagy-lysosomal defects and synucleinopathies. This review provides a rationale for the use of LRRK2 KI mice to investigate the LRRK2-mediated pathogenesis of PD and implications from current findings from different LRRK2 KI mouse models, and ultimately discusses the therapeutic potentials against LRRK2-associated pathologies in PD.

Implications of disease-related mutations at protein-protein interfaces

Protein-protein interfaces have been attracting great attention owing to their critical roles in protein-protein interactions and the fact that human disease-related mutations are generally enriched in them. Recently, substantial research progress has been made in this field, which has significantly promoted the understanding and treatment of various human diseases. For example, many studies have discovered the properties of disease-related mutations. Besides, as more large-scale experimental data become available, various computational approaches have been proposed to advance our understanding of disease mutations from the data. Here, we overview recent advances in characteristics of disease-related mutations at protein-protein interfaces, mutation effects on protein interactions, and investigation of mutations on specific diseases.

LRRK2 at Striatal Synapses: Cell-Type Specificity and Mechanistic Insights

Mutations in leucine-rich repeat kinase 2 (LRRK2) cause Parkinson’s disease with a similar clinical presentation and progression to idiopathic Parkinson’s disease, and common variation is linked to disease risk. Recapitulation of the genotype in rodent models causes abnormal dopamine release and increases the susceptibility of dopaminergic neurons to insults, making LRRK2 a valuable model for understanding the pathobiology of Parkinson’s disease. It is also a promising druggable target with targeted therapies currently in development. LRRK2 mRNA and protein expression in the brain is highly variable across regions and cellular identities. A growing body of work has demonstrated that pathogenic LRRK2 mutations disrupt striatal synapses before the onset of overt neurodegeneration. Several substrates and interactors of LRRK2 have been identified to potentially mediate these pre-neurodegenerative changes in a cell-type-specific manner. This review discusses the effects of pathogenic LRRK2 mutations in striatal neurons, including cell-type-specific and pathway-specific alterations. It also highlights several LRRK2 effectors that could mediate the alterations to striatal function, including Rabs and protein kinase A. The lessons learned from improving our understanding of the pathogenic effects of LRRK2 mutations in striatal neurons will be applicable to both dissecting the cell-type specificity of LRRK2 function in the transcriptionally diverse subtypes of dopaminergic neurons and also increasing our understanding of basal ganglia development and biology. Finally, it will inform the development of therapeutics for Parkinson’s disease.

Gαs-Protein Kinase A (PKA) Pathway Signalopathies: The Emerging Genetic Landscape and Therapeutic Potential of Human Diseases Driven by Aberrant Gαs-PKA Signaling

Many of the fundamental concepts of signal transduction and kinase activity are attributed to the discovery and crystallization of cAMP-dependent protein kinase, or protein kinase A. PKA is one of the best-studied kinases in human biology, with emphasis in biochemistry and biophysics, all the way to metabolism, hormone action, and gene expression regulation. It is surprising, however, that our understanding of PKA's role in disease is largely underappreciated. Although genetic mutations in the PKA holoenzyme are known to cause diseases such as Carney complex, Cushing syndrome, and acrodysostosis, the story largely stops there. With the recent explosion of genomic medicine, we can finally appreciate the broader role of the Gαs-PKA pathway in disease, with contributions from aberrant functioning G proteins and G protein-coupled receptors, as well as multiple alterations in other pathway components and negative regulators. Together, these represent a broad family of diseases we term the Gαs-PKA pathway signalopathies. The Gαs-PKA pathway signalopathies encompass diseases caused by germline, postzygotic, and somatic mutations in the Gαs-PKA pathway, with largely endocrine and neoplastic phenotypes. Here, we present a signaling-centric review of Gαs-PKA-driven pathophysiology and integrate computational and structural analysis to identify mutational themes commonly exploited by the Gαs-PKA pathway signalopathies. Major mutational themes include hotspot activating mutations in Gαs, encoded by GNAS, and mutations that destabilize the PKA holoenzyme. With this review, we hope to incite further study and ultimately the development of new therapeutic strategies in the treatment of a wide range of human diseases. SIGNIFICANCE STATEMENT: Little recognition is given to the causative role of Gαs-PKA pathway dysregulation in disease, with effects ranging from infectious disease, endocrine syndromes, and many cancers, yet these disparate diseases can all be understood by common genetic themes and biochemical signaling connections. By highlighting these common pathogenic mechanisms and bridging multiple disciplines, important progress can be made toward therapeutic advances in treating Gαs-PKA pathway-driven disease.

Identification of molecular glues of the SLP76/14-3-3 protein-protein interaction

The stabilisation of protein-protein interactions (PPIs) through molecular glues is a novel and promising approach in drug discovery. In stark contrast to research in protein-protein inhibition the field of stabilisation remains underdeveloped with comparatively few examples of small-molecule stabilisers of PPIs reported to date. At the same time identifying molecular glues has received recent sustained interest, especially in the fields of targeted protein degradation and 14-3-3 PPIs. The hub-protein 14-3-3 has a broad interactome with more than 500 known protein partners which presents a great opportunity for therapeutic intervention. In this study we have developed an HTRF assay suitable for HTS of the 14-3-3/SLP76 PPI and have completed a proof of concept screen against a chemically diverse library of 20 K molecules. The adaptor protein SLP76 has been reported to interact with 14-3-3 proteins downstream of the TCR playing an important role in mediating its own proteasomal degradation. We believe that stabilisation of this PPI could be exploited to potentiate degradation of SLP76 and therefore inhibit TCR signalling. This would represent an interesting alternative to other approaches in the field of targeted protein degradation. Here we disclose 16 novel stabilisers of the 14-3-3/SLP76 PPI across multiple different chemotypes. Based on the early results presented here we would recommend this approach to find molecular glues with broad applicability in the field of 14-3-3 PPIs.

Pathways to Parkinson's disease: a spotlight on 14-3-3 proteins

14-3-3s represent a family of highly conserved 30 kDa acidic proteins. 14-3-3s recognize and bind specific phospho-sequences on client partners and operate as molecular hubs to regulate their activity, localization, folding, degradation, and protein-protein interactions. 14-3-3s are also associated with the pathogenesis of several diseases, among which Parkinson's disease (PD). 14-3-3s are found within Lewy bodies (LBs) in PD patients, and their neuroprotective effects have been demonstrated in several animal models of PD. Notably, 14-3-3s interact with some of the major proteins known to be involved in the pathogenesis of PD. Here we first provide a detailed overview of the molecular composition and structural features of 14-3-3s, laying significant emphasis on their peculiar target-binding mechanisms. We then briefly describe the implication of 14-3-3s in the central nervous system and focus on their interaction with LRRK2, α-Synuclein, and Parkin, three of the major players in PD onset and progression. We finally discuss how different types of small molecules may interfere with 14-3-3s interactome, thus representing a valid strategy in the future of drug discovery.

Markov State Models and Molecular Dynamics Simulations Provide Understanding of the Nucleotide-Dependent Dimerization-Based Activation of LRRK2 ROC Domain

Mutations in leucine-rich repeat kinase 2 (LRRK2) are recognized as the most frequent cause of Parkinson's disease (PD). As a multidomain ROCO protein, LRRK2 is characterized by the presence of both a Ras-of-complex (ROC) GTPase domain and a kinase domain connected through the C-terminal of an ROC domain (COR). The bienzymatic ROC-COR-kinase catalytic triad indicated the potential role of GTPase domain in regulating kinase activity. However, as a functional GTPase, the detailed intrinsic regulation of the ROC activation cycle remains poorly understood. Here, combining extensive molecular dynamics simulations and Markov state models, we disclosed the dynamic structural rearrangement of ROC's homodimer during nucleotide turnover. Our study revealed the coupling between dimerization extent and nucleotide-binding state, indicating a nucleotide-dependent dimerization-based activation scheme adopted by ROC GTPase. Furthermore, inspired by the well-known R1441C/G/H PD-relevant mutations within the ROC domain, we illuminated the potential allosteric molecular mechanism for its pathogenetic effects through enabling faster interconversion between inactive and active states, thus trapping ROC in a prolonged activated state, while the implicated allostery could provide further guidance for identification of regulatory allosteric pockets on the ROC complex. Our investigations illuminated the thermodynamics and kinetics of ROC homodimer during nucleotide-dependent activation for the first time and provided guidance for further exploiting ROC as therapeutic targets for controlling LRRK2 functionality in PD treatment.

Genetic Imaging of Neuroinflammation in Parkinson's Disease: Recent Advancements

Parkinson's disease (PD) is one of the most prevalent neurodegenerative aging disorders characterized by motor and non-motor symptoms due to the selective loss of midbrain dopaminergic (DA) neurons. The decreased viability of DA neurons slowly results in the appearance of motor symptoms such as rigidity, bradykinesia, resting tremor, and postural instability. These symptoms largely depend on DA nigrostriatal denervation. Pharmacological and surgical interventions are the main treatment for improving clinical symptoms, but it has not been possible to cure PD. Furthermore, the cause of neurodegeneration remains unclear. One of the possible neurodegeneration mechanisms is a chronic inflammation of the central nervous system, which is mediated by microglial cells. Impaired or dead DA neurons can directly lead to microglia activation, producing a large number of reactive oxygen species and pro-inflammatory cytokines. These cytotoxic factors contribute to the apoptosis and death of DA neurons, and the pathological process of neuroinflammation aggravates the primary morbid process and exacerbates ongoing neurodegeneration. Therefore, anti-inflammatory treatment exerts a robust neuroprotective effect in a mouse model of PD. Since discovering the first mutation in the α-synuclein gene (SNCA), which can cause disease-causing, PD has involved many genes and loci such as LRRK2, Parkin, SNCA, and PINK1. In this article, we summarize the critical descriptions of the genetic factors involved in PD's occurrence and development (such as LRRK2, SNCA, Parkin, PINK1, and inflammasome), and these factors play a crucial role in neuroinflammation. Regulation of these signaling pathways and molecular factors related to these genetic factors can vastly improve the neuroinflammation of PD.

The development of inhibitors of leucine-rich repeat kinase 2 (LRRK2) as a therapeutic strategy for Parkinson's disease: the current state of play

Current therapeutic approaches for Parkinson's disease (PD) are based around treatments that alleviate symptoms but do not slow or prevent disease progression. As such, alternative strategies are needed. A promising approach is the use of molecules that reduce the function of leucine-rich repeat kinase (LRRK2). Gain-of-function mutations in LRRK2 account for a notable proportion of familial Parkinson's disease cases, and significantly, elevated LRRK2 kinase activity is reported in idiopathic Parkinson's disease. Here, we describe progress in finding therapeutically effective LRRK2 inhibitors, summarising studies that range from in vitro experiments to clinical trials. LRRK2 is a complex protein with two enzymatic activities and a myriad of functions. This creates opportunities for a rich variety of strategies and also increases the risk of unintended consequences. We comment on the strength and limitations of the different approaches and conclude that with two molecules under clinical trial and a diversity of alternative options in the pipeline, there is cause for optimism.

The Cell Biology of LRRK2 in Parkinson's Disease

Point mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial Parkinson's disease (PD) and are implicated in a significant proportion of apparently sporadic PD cases. Clinically, LRRK2-driven PD is indistinguishable from sporadic PD, making it an attractive genetic model for the much more common sporadic PD. In this review, we highlight recent advances in understanding LRRK2's subcellular functions using LRRK2-driven PD models, while also considering some of the limitations of these model systems. Recent developments of particular importance include new evidence of key LRRK2 functions in the endolysosomal system and LRRK2's regulation of and by Rab GTPases. Additionally, LRRK2's interaction with the cytoskeleton allowed elucidation of the LRRK2 structure and appears relevant to LRRK2 protein degradation and LRRK2 inhibitor therapies. We further discuss how LRRK2's interactions with other PD-driving genes, such as the VPS35, GBA1, and SNCA genes, may highlight cellular pathways more broadly disrupted in PD.

14-3-3 Proteins: Novel Pharmacological Targets in Neurodegenerative Diseases

14-3-3 proteins are a family of proteins expressed throughout the body and implicated in many diseases, from cancer to neurodegenerative disorders. While these proteins do not have direct enzymatic activity, they form a hub for many signaling pathways via protein-protein interactions (PPIs). 14-3-3 interactions have proven difficult to target with traditional pharmacological methods due to the unique nature of their binding. However, recent advances in compound development utilizing a range of tools, from thermodynamic binding site analysis to computational molecular modeling techniques, have opened the door to targeting these interactions. Compounds are already being developed targeting 14-3-3 interactions with potential therapeutic implication for neurodegenerative disorders, but challenges still remain in optimizing specificity and target engagement to avoid unintended negative consequences arising from targeting 14-3-3 signaling networks.

LRRK2; a dynamic regulator of cellular trafficking

Parkinson's disease (PD) represents the second most common neurodegenerative disorder, characterized clinically by bradykinesia, resting tremor, rigidity and postural instability, and a variety of non-motor features. The etiology of PD is unknown, however genetic, environmental and inflammatory factors may influence disease onset and progression. Genetic variability in leucine-rich repeat kinase 2 confers significant genotypic and population-attributable risk for LRRK2-parkinsonism that is clinically indistinguishable from idiopathic PD. Nevertheless, the age-associated midbrain pathology observed post-mortem in LRRK2-parkinsonism may involve the abnormal accumulation of either α-synuclein or tau, or just the loss of dopaminergic neurons and gliosis. While diverse biological functions have been described for this multi-domain protein in many cell types, evidence suggests LRRK2 may sense endosomal trafficking to orchestrate dynamic changes in vesicular flux and cytoskeletal architecture. This review posits the long-held belief that synaptic-axonal dysfunction and terminal degeneration may precede dopaminergic cell loss, and provocatively questions how facets of LRRK2 biology may influence this molecular pathogenesis.

Alterations in cyclic nucleotide signaling are implicated in healthy aging and age-related pathologies of the brain

It is not only important to consider how hormones may change with age, but also how downstream signaling pathways that couple to hormone receptors may change. Among these hormone-coupled signaling pathways are the 3',5'-cyclic guanosine monophosphate (cGMP) and 3',5'-cyclic adenosine monophosphate (cAMP) intracellular second messenger cascades. Here, we test the hypothesis that dysfunction of cAMP and/or cGMP synthesis, execution, and/or degradation occurs in the brain during healthy and pathological diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. Although most studies report lower cyclic nucleotide signaling in the aged brain, with further reductions noted in the context of age-related diseases, there are select examples where cAMP signaling may be elevated in select tissues. Thus, therapeutics would need to target cAMP/cGMP in a tissue-specific manner if efficacy for select symptoms is to be achieved without worsening others.

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.

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.

Reading the phosphorylation code: binding of the 14-3-3 protein to multivalent client phosphoproteins

Many major protein-protein interaction networks are maintained by 'hub' proteins with multiple binding partners, where interactions are often facilitated by intrinsically disordered protein regions that undergo post-translational modifications, such as phosphorylation. Phosphorylation can directly affect protein function and control recognition by proteins that 'read' the phosphorylation code, re-wiring the interactome. The eukaryotic 14-3-3 proteins recognizing multiple phosphoproteins nicely exemplify these concepts. Although recent studies established the biochemical and structural basis for the interaction of the 14-3-3 dimers with several phosphorylated clients, understanding their assembly with partners phosphorylated at multiple sites represents a challenge. Suboptimal sequence context around the phosphorylated residue may reduce binding affinity, resulting in quantitative differences for distinct phosphorylation sites, making hierarchy and priority in their binding rather uncertain. Recently, Stevers et al. [Biochemical Journal (2017) 474: 1273-1287] undertook a remarkable attempt to untangle the mechanism of 14-3-3 dimer binding to leucine-rich repeat kinase 2 (LRRK2) that contains multiple candidate 14-3-3-binding sites and is mutated in Parkinson's disease. By using the protein-peptide binding approach, the authors systematically analyzed affinities for a set of LRRK2 phosphopeptides, alone or in combination, to a 14-3-3 protein and determined crystal structures for 14-3-3 complexes with selected phosphopeptides. This study addresses a long-standing question in the 14-3-3 biology, unearthing a range of important details that are relevant for understanding binding mechanisms of other polyvalent proteins.

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.

The Current State-of-the Art of LRRK2-Based Biomarker Assay Development in Parkinson's Disease

Evidence is mounting that LRRK2 function, particularly its kinase activity, is elevated in multiple forms of Parkinson's disease, both idiopathic as well as familial forms linked to mutations in the LRRK2 gene. However, sensitive quantitative markers of LRRK2 activation in clinical samples remain at the early stages of development. There are several measures of LRRK2 activity that could potentially be used in longitudinal studies of disease progression, as inclusion/exclusion criteria for clinical trials, to predict response to therapy, or as markers of target engagement. Among these are levels of LRRK2, phosphorylation of LRRK2 itself, either by other kinases or via auto-phosphorylation, its in vitro kinase activity, or phosphorylation of downstream substrates. This is advantageous on many levels, in that multiple indices of elevated kinase activity clearly strengthen the rationale for targeting this kinase with novel therapeutic candidates, and provide alternate markers of activation in certain tissues or biofluids for which specific measures are not detectable. However, this can also complicate interpretation of findings from different studies using disparate measures. In this review we discuss the current state of LRRK2-focused biomarkers, the advantages and disadvantages of the current pallet of outcome measures, the gaps that need to be addressed, and the priorities that the field has defined.

Pharmacodynamic Biomarkers for Emerging LRRK2 Therapeutics

Genetic studies have identified variants in the LRRK2 gene as important components of Parkinson's disease (PD) pathobiology. Biochemical and emergent biomarker studies have coalesced around LRRK2 hyperactivation in disease. Therapeutics that diminish LRRK2 activity, either with small molecule kinase inhibitors or anti-sense oligonucleotides, have recently advanced to the clinic. Historically, there have been few successes in the development of therapies that might slow or halt the progression of neurodegenerative diseases. Over the past few decades of biomedical research, retrospective analyses suggest the broad integration of informative biomarkers early in development tends to distinguish successful pipelines from those that fail early. Herein, we discuss the biomarker regulatory process, emerging LRRK2 biomarker candidates, assays, underlying biomarker biology, and clinical integration.

LRRK2 Phosphorylation, More Than an Epiphenomenon

Mutations in the Leucine Rich Repeat Kinase 2 (LRRK2) gene are linked to autosomal dominant Parkinson's disease (PD), and genetic variations at the LRRK2 locus are associated with an increased risk for sporadic PD. This gene encodes a kinase that is physiologically multiphosphorylated, including clusters of both heterologous phosphorylation and autophosphorylation sites. Several pieces of evidence indicate that LRRK2's phosphorylation is important for its pathological and physiological functioning. These include a reduced LRRK2 heterologous phosphorylation in PD brains or after pharmacological inhibition of LRRK2 kinase activity as well as the appearance of subcellular LRRK2 accumulations when this protein is dephosphorylated at heterologous phosphosites. Nevertheless, the regulatory mechanisms governing LRRK2 phosphorylation levels and the cellular consequences of changes in LRRK2 phosphorylation remain incompletely understood. In this review, we present current knowledge on LRRK2 phosphorylation, LRRK2 phosphoregulation, and how LRRK2 phosphorylation changes affect cellular processes that may ultimately be linked to PD mechanisms.

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.

Mitochondrial Homeostasis and Signaling in Parkinson's Disease

The loss of dopaminergic (DA) neurons in the substantia nigra leads to a progressive, long-term decline of movement and other non-motor deficits. The symptoms of Parkinson's disease (PD) often appear later in the course of the disease, when most of the functional dopaminergic neurons have been lost. The late onset of the disease, the severity of the illness, and its impact on the global health system demand earlier diagnosis and better targeted therapy. PD etiology and pathogenesis are largely unknown. There are mutations in genes that have been linked to PD and, from these complex phenotypes, mitochondrial dysfunction emerged as central in the pathogenesis and evolution of PD. In fact, several PD-associated genes negatively impact on mitochondria physiology, supporting the notion that dysregulation of mitochondrial signaling and homeostasis is pathogenically relevant. Derangement of mitochondrial homeostatic controls can lead to oxidative stress and neuronal cell death. Restoring deranged signaling cascades to and from mitochondria in PD neurons may then represent a viable opportunity to reset energy metabolism and delay the death of dopaminergic neurons. Here, we will highlight the relevance of dysfunctional mitochondrial homeostasis and signaling in PD, the molecular mechanisms involved, and potential therapeutic approaches to restore mitochondrial activities in damaged neurons.

Binding of the Human 14-3-3 Isoforms to Distinct Sites in the Leucine-Rich Repeat Kinase 2

Proteins of the 14-3-3 family are well known modulators of the leucine-rich repeat kinase 2 (LRRK2) regulating kinase activity, cellular localization, and ubiquitylation. Although binding between those proteins has been investigated, a comparative study of all human 14-3-3 isoforms interacting with LRRK2 is lacking so far. In a comprehensive approach, we quantitatively analyzed the interaction between the seven human 14-3-3 isoforms and LRRK2-derived peptides covering both, reported and putative 14-3-3 binding sites. We observed that phosphorylation is an absolute prerequisite for 14-3-3 binding and generated binding patterns of 14-3-3 isoforms to interact with peptides derived from the N-terminal phosphorylation cluster (S910 and S935), the Roc domain (S1444) and the C-terminus. The tested 14-3-3 binding sites in LRRK2 preferentially were recognized by the isoforms γ and η, whereas the isoforms ϵ and especially σ showed the weakest or no binding. Interestingly, the possible pathogenic mutation Q930R in LRRK2 drastically increases binding affinity to a peptide encompassing pS935. We then identified the autophosphorylation site T2524 as a so far not described 14-3-3 binding site at the very C-terminus of LRRK2. Binding affinities of all seven 14-3-3 isoforms were quantified for all three binding regions with pS1444 displaying the highest affinity of all measured singly phosphorylated peptides. The strongest binding was detected for the combined phosphosites S910 and S935, suggesting that avidity effects are important for high affinity interaction between 14-3-3 proteins and LRRK2.

LRRK2 Biology from structure to dysfunction: research progresses, but the themes remain the same

Since the discovery of leucine-rich repeat kinase 2 (LRRK2) as a protein that is likely central to the aetiology of Parkinson’s disease, a considerable amount of work has gone into uncovering its basic cellular function. This effort has led to the implication of LRRK2 in a bewildering range of cell biological processes and pathways, and probable roles in a number of seemingly unrelated medical conditions. In this review we summarise current knowledge of the basic biochemistry and cellular function of LRRK2. Topics covered include the identification of phosphorylation substrates of LRRK2 kinase activity, in particular Rab proteins, and advances in understanding the activation of LRRK2 kinase activity via dimerisation and association with membranes, especially via interaction with Rab29. We also discuss biochemical studies that shed light on the complex LRRK2 GTPase activity, evidence of roles for LRRK2 in a range of cell signalling pathways that are likely cell type specific, and studies linking LRRK2 to the cell biology of organelles. The latter includes the involvement of LRRK2 in autophagy, endocytosis, and processes at the trans-Golgi network, the endoplasmic reticulum and also key microtubule-based cellular structures. We further propose a mechanism linking LRRK2 dimerisation, GTPase function and membrane recruitment with LRRK2 kinase activation by Rab29. Together these data paint a picture of a research field that in many ways is moving forward with great momentum, but in other ways has not changed fundamentally. Many key advances have been made, but very often they seem to lead back to the same places.

Recent Developments in LRRK2-Targeted Therapy for Parkinson's Disease

Kinase activating missense mutations in leucine-rich repeat kinase 2 (LRRK2) are pathogenically linked to neurodegenerative Parkinson's disease (PD). Over the past decade, substantial effort has been devoted to the development of potent and selective small molecule inhibitors of LRRK2, as well as their preclinical testing across different Parkinson's disease models. This review outlines the genetic and biochemical evidence that pathogenic missense mutations increase LRRK2 kinase activity, which in turn provides the rationale for the development of small molecule inhibitors as potential PD therapeutics. An overview of progress in the development of LRRK2 inhibitors is provided, which in particular indicates that highly selective and potent compounds capable of clinical utility have been developed. We outline evidence from rodent- and human-induced pluripotent stem cell models that support a pathogenic role for LRRK2 kinase activity, and review the substantial experiments aimed at evaluating the safety of LRRK2 inhibitors. We address challenges still to overcome in the translational therapeutic pipeline, including biomarker development and clinical trial strategies, and finally outline the potential utility of LRRK2 inhibitors for other genetic forms of PD and ultimately sporadic PD. Collective evidence supports the ongoing clinical translation of LRRK2 inhibitors as a therapeutic intervention for PD is greatly needed.