Dopaminergic neuronal release sites have unique characteristics, as only about 25–30% of axonal varicosities contain active zones that support transmitter release. Rim1/2 proteins are important scaffolding molecules that organize these release sites. Our phosphoproteomic analysis showed that several pathways associated with the organization of presynaptic release sites are altered with changes in LRKK2 kinase activity, and we found a decrease in the interaction of phosphorylated Rab3a with various active zone proteins from mouse brain. Therefore, we next examined whether alterations in the number and/or organization of active zones may form the cellular basis for the observed deficits in dopamine release in Lrrk2G2019S mice.

Parkinson’s disease (PD) is a multisystemic disorder that manifests through motor and non-motor symptoms. Motor dysfunction is the most debilitating and it is caused by the degeneration of dopamine-producing neurons in the substantia nigra pars compacta (SNpc). A body of evidence indicates that synapse demise precedes by years neuronal death. Still, early synaptic dysfunctions in PD are poorly deciphered. Here we combined literature metanalysis, proteomics and phosphoproteomics with biochemical, imaging and electrophysiological measurements in neurons, brains and synaptosomes from knockout and knockin mouse models, as well as human iPSC-derived neurons associated with the PD-kinase LRRK2. We show that phosphorylation of LRRK2 at Ser935, which controls LRRK2 subcellular localization, rapidly increases upon brain-derived neurotrophic factor (BDNF) stimulation of differentiated SH-SY5Y cells and primary mouse neurons. Affinity-purification coupled with mass spectrometry (AP-MS/MS) analysis revealed that LRRK2 interactome is significantly reshaped upon BDNF stimulation, with an interconnected network of actin cytoskeleton-associated proteins increasing their binding to LRRK2. Accordingly, LRRK2 knockout neurons exhibit decreased TrkB signaling and fail to induce BDNF-dependent spinogenesis. In vivo , one-month old Lrrk2 knockout mice display defects in spine maturation, a phenotype that disappears with age. In human iPSC-derived cortical neurons, BDNF increases the frequency of miniature excitatory post-synaptic currents (mEPSC) in wild-type but not in the presence of LRRK2 knockout, functionally supporting a distinctive role of LRRK2 in BDNF-synaptic signaling. Finally, Lrrk2 G2019S PD mutant synaptosomes display differentially phosphorylated proteins enriched in categories related to postsynaptic structural organization. Taken together, our study discloses a critical function of LRRK2 in BDNF-dependent synaptic processes and identifies the postsynaptic actin cytoskeleton as a convergent site of LRRK2 pathophysiological activity.

Abstract Parkinson’s disease (PD) is defined pathologically by loss of dopamine-producing neurons in the substantia nigra pars compacta (SNc). Yet synaptic dysfunction emerges much earlier, making it essential to define the mechanisms that drive early nigrostriatal deregulation. In the SNc, molecularly distinct dopamine neuron subtypes show differential susceptibility to PD. Here, we used intersectional genetic mouse models to determine how the PD-linked kinase LRRK2 affects vulnerable dopamine subtypes. Immunofluorescence and proximity-labeling proteomics revealed enriched LRRK2 expression in vulnerable dopamine neuron subclusters. High-resolution imaging showed that pathogenic LRRK2 disrupts presynaptic release-site organization in vulnerable dopamine axons, leading to reduced spontaneous and evoked striatal dopamine release in vivo. Proteomic analyses further showed that mutant LRRK2 increases phosphorylation of RAB3 proteins, impairing their interaction with the active-zone effectors RIM1 and RIM2. Together, these findings highlight a subtype-specific, cell-autonomous mechanism by which pathogenic LRRK2 impairs PD-vulnerable nigrostriatal synapses and provide a framework for therapeutic strategies targeting early synaptic deficits in PD.

Summary Coding mutations in the Leucine-rich repeat kinase 2 ( LRRK2 ) gene represent the most common cause of familial Parkinson’s disease (PD), and are frequently observed in idiopathic PD. In addition, variation around the LRRK2 locus has been shown to alter PD risk by genome-wide association studies. Disease-causing mutations cluster within the catalytic core of LRRK2 – composed of GTPase (ROC) and serine-threonine kinase domains – and lead to an increase in kinase activity, resulting in hyperphosphorylation of a subset of RAB GTPases and consequent cellular toxicity. However, the interplay between LRRK2 GTPase and kinase domains, and with the surrounding scaffold regions has remained underexplored, with implications for the prediction of on- and off-target effects associated with kinase inhibition. To address this gap, here we dissected the contributions of kinase, GTPase and scaffold domains to LRRK2 function in murine macrophages and tissues expressing endogenous levels of GTP/GDP-binding deficient Lrrk2 T1348N. Guanosine nucleotide-free Lrrk2 is devoid of both GTPase and kinase activities but maintains the scaffold shell, leading to significant reshaping of Lrrk2 interactome and engagement in novel interactions. This altered functional state leads to impaired autophagy and accumulation of enlarged lysosomes and autophagic cargo in macrophages and kidneys. Since pharmacological inhibition of LRRK2 is under clinical evaluation, our results reveal retained scaffold functions upon loss of catalytic activity that warrant careful consideration.

The end-stage pathology of Parkinson's disease (PD) involves the loss of dopamine-producing neurons in the substantia nigra pars compacta (SNc). However, synaptic deregulation of these neurons begins much earlier. Understanding the mechanisms behind synaptic deficits is crucial for early therapeutic intervention, yet these remain largely unknown. In the SNc, different dopamine neuron subtypes show varying susceptibility patterns to PD, complicating our understanding. This study uses intersectional genetic mouse models to uncover synaptic perturbations in vulnerable dopamine neurons, focusing on the LRRK2 kinase, a protein closely linked to PD. Through a combination of immunofluorescence and advanced proximity labeling methods, we found higher LRRK2 expression in the most vulnerable dopamine neuron subclusters. High-resolution imaging revealed that pathogenic LRRK2 disrupts release sites in vulnerable dopamine axons, leading to decreased in vivo evoked striatal dopamine release in mice with LRRK2 mutations. Proteomic and biochemical analyses indicate that mutant LRRK2 increases the phosphorylation of RAB3 proteins, reducing their interactions with RIM1/2 effector proteins and impacting their synaptic functions. Overall, this research highlights the cell-autonomous dysfunctions caused by mutant LRRK2 in the neurons that are primarily affected by the disease. It also provides a framework for therapeutic strategies for early nigrostriatal synaptic deficits in PD.

Haloperidol is used to manage psychotic symptoms in several neurological disorders through mechanisms that involve antagonism of dopamine D2 receptors that are highly expressed in the striatum. Significant side effects of haloperidol, known as extrapyramidal symptoms, lead to motor deficits similar to those seen in Parkinson’s disease and present a major challenge in clinical settings. The underlying molecular mechanisms responsible for these side effects remain poorly understood. Parkinson’s disease-associated leucine-rich repeat kinase 2 (LRRK2) has an essential role in striatal physiology and a known link to dopamine D2 receptor signaling. Here, we systematically explore convergent signaling of haloperidol and LRRK2 through pharmacological or genetic inhibition of LRRK2 kinase, as well as knock-in mouse models expressing pathogenic mutant LRRK2 with increased kinase activity. Behavioral assays show that LRRK2 kinase inhibition ameliorates haloperidol-induced motor changes in mice. A combination of electrophysiological and anatomical approaches reveals that LRRK2 kinase inhibition interferes with haloperidol-induced changes, specifically in striatal neurons of the indirect pathway. Proteomic studies and targeted intracellular pathway analyses demonstrate that haloperidol induces a similar pattern of intracellular signaling as increased LRRK2 kinase activity. Our study suggests that LRRK2 kinase plays a key role in striatal dopamine D2 receptor signaling underlying the undesirable motor side effects of haloperidol. This work opens up new therapeutic avenues for dopamine-related disorders, such as psychosis, also furthering our understanding of Parkinson’s disease pathophysiology.

Abstract Parkinson’s disease (PD) is a multisystemic disorder that manifests through motor and non-motor symptoms. Motor dysfunction is the most debilitating and it is caused by the degeneration of dopamine-producing neurons in the substantia nigra pars compacta (SNpc). Increasing evidence suggests that synapse dysfunction precedes neuronal loss by years. Still, early synaptic alterations in PD remain poorly understood. Here we integrate literature meta-analysis, proteomics and phosphoproteomics with biochemical, imaging and electrophysiological measurements in neurons and brains from knockout and knockin Lrrk2 mouse models, as well as human iPSC-derived neurons lacking LRRK2. We demonstrate that phosphorylation of LRRK2 at Ser935 and of RAB proteins is induced by brain-derived neurotrophic factor (BDNF) stimulation in differentiated SH-SY5Y cells and primary mouse neurons. Affinity-purification coupled with mass spectrometry (AP-MS/MS) revealed a significant remodelling of the LRRK2 interactome following BDNF treatment, with enhanced association of LRRK2 to a network of actin cytoskeleton-related proteins. Gene-ontology analyses of both literature-curated LRRK2 interactors and phospho-proteome from striatal tissues with elevated LRRK2 activity (G2019S knockin mice) highlight synapse-actin remodelling as major affected pathways. We further observed that loss of LRRK2 impairs BDNF signaling and alters postsynaptic density architecture. One month-old Lrrk2 knockout mice display structural alterations in dendritic protrusions, a phenotype that normalizes with age. In human iPSC-derived neurons, BDNF enhances the frequency of miniature excitatory post-synaptic currents (mEPSC) in wild-type but not in LRRK2 knockout neurons, which appear to bypass this regulation through developmental compensation. Taken together, our study discloses a critical role of LRRK2 in BDNF-dependent synaptic modulation and identifies the synaptic actin cytoskeleton as a convergent site of LRRK2’s pathophysiological activity.

Parkinson's disease (PD) is a multisystem disorder presenting motor and non-motor symptoms. Motor dysfunction is the most debilitating, caused by the degeneration of dopamine-producing neurons. Increasing evidence indicates that synapse demise occurs years before neuronal death. Yet, the early synaptic dysfunctions in PD remain poorly understood. Leucine-Rich Repeat Kinase 2 (LRRK2), a serine/threonine kinase and GTPase relevant for both familial and sporadic forms of PD, has been increasingly associated with synaptic processes. These include the phosphorylation of key synaptic proteins and interactions with cytoskeletal components. Brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF) are fundamental for synapse maturation, maintenance, and plasticity. Recent findings indicate that neurotrophic signaling is impaired in PD. In this review, we critically discuss the significance of identifying and clarifying the early molecular events leading to synaptic dysfunction in PD. We examine how mutant LRRK2 affects these processes and the relationship between LRRK2 and BDNF signaling from both mechanistic and therapeutic perspectives.

Autophagy is a highly conserved homeostatic process essential for the bulk degradation of cytoplasmic components and aggregated proteins. Multiple evidence indicates that impairment of (macro)autophagy leads to neurodegeneration, such as Parkinson disease (PD). Our previous work showed that p21 activated kinase 6 (PAK6) interacts with the PD-associated leucine-rich repeat kinase (LRRK2) to promote neurite outgrowth in the mouse striatum; still the function of PAK6 in the brain is largely unknown. Here, we found that downregulation of neuronal but not glial mbt , the D. melanogaster homolog of PAK6, impairs autophagy-lysosomal function. PAK6 overexpression in cells and in C. elegans increases transcription factor EB (TFEB) nuclear translocation in a kinase activity-dependent manner. Mechanistically, PAK6 forms a complex with TFEB to regulate its nuclear localization in a manner dependent on phosphorylation of and binding to 14-3-3 proteins and phosphorylation of TFEB at S467. In line with its ability to promote neuronal autophagy, mbt downregulation exacerbates alpha-synuclein toxicity in Drosophila dopaminergic neurons. Moreover, PAK6 overexpression in the substantia nigra of mutant LRRK2 mice reduces the burden of phosphorylated alpha-synuclein in dopaminergic neurons. Altogether, our study uncovers a novel role of PAK6 as a positive regulator of autophagy via TFEB and suggests that modulating its activity may represent a way to selectively turn on autophagy in neurons, with implications for the treatment of neurodegenerative disorders. ### Competing Interest Statement The authors have declared no competing interest. * AD : Alzheimer Disease ALP : autophagic lysosomal pathway α-syn : alpha-synuclein AMPK : AMP-activated protein kinase CLEAR : Coordinated Lysosomal Expression and Regulation DA : dopamine FCS : fluorescence correlation spectroscopy GCase : Glucocerebrosidase KD : knockdown KO : knockout LRRK2 : leucine-rich repeat kinase 2 mbt : mushroom bodies tiny mTORC1 : mechanistic target of rapamycin complex 1 OE : overexpressing PAK4 : p21 activated kinase 4 PAK5 : p21 activated kinase 5 PAK6 : p21 activated kinase 6 PD : Parkinson Disease WT : wild type

This protocol describes how to isolate striatal synaptosomes form brain tissue and process them for imaging purposes.

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.

ABSTRACTBackgroundThe role of the gut-brain axis has been recently highlighted as a major contributor to Parkinson’s Disease (PD) physiopathology, with numerous studies investigating bidirectional transmission of pathological protein aggregates, such as α-Synuclein (αSyn), in the context of neurodegenerative disease. However, little and often conflictual evidence is available from human studies due to patient heterogeneity, sampling variability, and the employment of antibodies with affinity for different αSyn epitopes.ObjectivesWe aimed to investigate the enteric nervous system (ENS) in PD by characterizing αSyn alterations and glial responses in duodenum biopsies of PD patients by employing topography-specific sampling and conformation-specific αSyn antibodies.Methods18 Patients with symptomatic PD who underwent Duodopa Percutaneous Endoscopic Gastrostomy and Jejunal Tube (PEG-J) procedure, as well as 18 age- and -sex-matched Healthy Control subjects undergoing routine diagnostic endoscopy, were included in the study. A mean of 4 duodenal wall biopsies were sampled from each patient. Immunohistochemistry was performed for anti-aggregated αSyn (5G4) and GFAP antibodies. Morphometrical-semi-quantitative analysis was performed to characterize αSyn-5G4+ and GFAP+ density and size.ResultsElevated immunoreactivity for aggregated α-Syn was identified in all biopsies of PD patients compared to controls. αSyn-5G4+ colocalized with neuronal marker β-III-tubulin. Evaluation of enteric glia cells revealed an increased size and density when compared with controls, suggesting reactive gliosis.ConclusionsDuodenal biopsy may represent a feasible and reliable tool for characterizing PD pathology in the GI tract and discerning patients from controls. Future studies are required to confirm these findings in a prodromal or early PD phase.

Friedreich ataxia (FRDA) is a rare, inherited neurodegenerative disease caused by an expanded GAA repeat in the first intron of the FXN gene, leading to transcriptional silencing and reduced expression of frataxin. Frataxin participates in the mitochondrial assembly of FeS clusters, redox cofactors of the respiratory complexes I, II and III. To date it is still unclear how frataxin deficiency culminates in the decrease of bioenergetics efficiency in FRDA patients’ cells. We previously demonstrated that in healthy cells frataxin is closely attached to the mitochondrial cristae, which contain both the FeS cluster assembly machinery and the respiratory chain complexes, whereas in FRDA patients’ cells with impaired respiration the residual frataxin is largely displaced in the matrix. To gain novel insights into the function of frataxin in the mitochondrial pathophysiology, and in the upstream metabolic defects leading to FRDA disease onset and progression, here we explored the potential interaction of frataxin with the FeS cluster-containing respiratory complexes I, II and III. Using healthy cells and different FRDA cellular models we found that frataxin interacts with these three respiratory complexes. Furthermore, by EPR spectroscopy, we observed that in mitochondria from FRDA patients’ cells the decreased level of frataxin specifically affects the FeS cluster content of complex I. Remarkably, we also found that the frataxin-like protein Nqo15 from T. thermophilus complex I ameliorates the mitochondrial respiratory phenotype when expressed in FRDA patient’s cells. Our data point to a structural and functional interaction of frataxin with complex I and open a perspective to explore therapeutic rationales for FRDA targeted to this respiratory complex.

In Parkinson's Disease (PD), recent evidence points towards the involvement of gut-brain axis as one of the primary physio-pathological mechanisms underlying α-Syn aggregation and propagation to CNS. Furthermore, gastrointestinal dysfunctions represent one of the main non-motor symptoms in PD, often preceding the development of proper motor symptoms. Our aim was to investigate the enteric nervous system (ENS) in PD by characterizing α-Syn alterations and glial responses in stomach-duodenum biopsies of PD patients.

Mutations in LRRK2 cause familial Parkinson’s disease and common variants increase disease risk. LRRK2 kinase activity and cellular localization are tightly regulated by phosphorylation of key residues, primarily Ser1292 and Ser935, which impacts downstream phosphorylation of its substrates, among which Rab10. A comprehensive characterization of LRRK2 activity and phosphorylation in brain as a function of age and mutations is missing. Here, we monitored Ser935 and Ser1292 phosphorylation in midbrain, striatum, and cortex of 1, 6, and 12 months-old mice carrying G2019S and R1441C mutations or murine bacterial artificial chromosome (BAC)-Lrrk2-G2019S. We observed that G2019S and, at a greater extent, R1441C brains display decreased phospho-Ser935, while Ser1292 autophosphorylation increased in G2019S but not in R1441C brain, lung, and kidney compared to wild-type. Further, Rab10 phosphorylation, is elevated in R1441C carrying mice, indicating that the effect of LRRK2 mutations on substrate phosphorylation is not generalizable. In BAC-Lrrk2-G2019S striatum and midbrain, Rab10 phosphorylation, but not Ser1292 autophosphorylation, decreases at 12-months, pointing to autophosphorylation and substrate phosphorylation as uncoupled events. Taken together, our study provides novel evidence that LRRK2 phosphorylation in mouse brain is differentially impacted by mutations, brain area, and age, with important implications as diagnostic markers of disease progression and stratification.

The activation of the G-protein coupled receptor CXCR4 by its ligand CXCL12α is involved in a large variety of physiological and pathological processes, including the growth of B cells precursors and of motor axons, autoimmune diseases, stem cell migration, inflammation, and several neurodegenerative conditions. Recently, we demonstrated that CXCL12α potently stimulates the functional recovery of damaged neuromuscular junctions via interaction with CXCR4. This result prompted us to test the neuroregeneration activity of small molecules acting as CXCR4 agonists, endowed with better pharmacokinetics with respect to the natural ligand. We focused on NUCC-390, recently shown to activate CXCR4 in a cellular system. We designed a novel and convenient chemical synthesis of NUCC-390, which is reported here. NUCC-390 was tested for its capability to induce the regeneration of motor axon terminals completely degenerated by the presynaptic neurotoxin α-Latrotoxin. NUCC-390 was found to strongly promote the functional recovery of the neuromuscular junction, as assayed by electrophysiology and imaging. This action is CXCR4 dependent, as it is completely prevented by AMD3100, a well-characterized CXCR4 antagonist. These data make NUCC-390 a strong candidate to be tested in human therapy to promote nerve recovery of function after different forms of neurodegeneration.