Parkinson's Disease-Linked LRRK2-G2019S Mutation Alters Synaptic Plasticity and Promotes Resilience to Chronic Social Stress in Young Adulthood

Parkinson's LRRK2-G2019S risk gene mutation drives sex-specific behavioral and cellular adaptations to chronic variable stress

Anxiety is a psychiatric non-motor symptom of Parkinson's that can appear in the prodromal period, prior to significant loss of brainstem dopamine neurons and motor symptoms. Parkinson's-related anxiety affects females more than males, despite the greater prevalence of Parkinson's in males. How stress, anxiety and Parkinson's are related and the basis for a sex-specific impact of stress in Parkinson's are not clear. We addressed this using young adult male and female mice carrying a G2019S knockin mutation of leucine-rich repeat kinase 2 ( Lrrk2 G2019S ) and Lrrk2 WT control mice. In humans, LRRK2 G2019S significantly elevates the risk of late-onset Parkinson's. To assess within-sex differences between Lrrk2 G2019S and control mice in stress-induced anxiety-like behaviors in young adulthood, we used a within-subject design whereby Lrrk2 G2019S and Lrrk2 WT control mice underwent tests of anxiety-like behaviors before (baseline) and following a 28 day (d) variable stress paradigm. There were no differences in behavioral measures between genotypes in males or females at baseline, indicating that the mutation alone does not produce anxiety-like responses. Following chronic stress, male Lrrk2 G2019S mice were affected similarly to male wildtypes except for novelty-suppressed feeding, where stress had no impact on Lrrk2 G2019S mice while significantly increasing latency to feed in Lrrk2 WT control mice. Female Lrrk2 G2019S mice were impacted by chronic stress similarly to wildtype females across all behavioral measures. Subsequent post-stress analyses compared cFos immunolabeling-based cellular activity patterns across several stress-relevant brain regions. The density of cFos-activated neurons across brain regions in both male and female Lrrk2 G2019S mice was generally lower compared to stressed Lrrk2 WT mice, except for the nucleus accumbens of male Lrrk2 G2019S mice, where cFos-labeled cell density was significantly higher than all other groups. Together, these data suggest that the Lrrk2 G2019S mutation differentially impacts anxiety-like behavioral responses to chronic stress in males and females that may reflect sex-specific adaptations observed in circuit activation patterns in stress-related brain regions.

LRRK2 mediates haloperidol-induced changes in indirect pathway striatal projection neurons

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 LRRK2 kinase has an important 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.

Swimming short fibrous nasal drops achieving intraventricular administration

Adequate drug delivery across the blood-brain barrier (BBB) is a critical factor in treating central nervous system (CNS) disorders. Inspired by swimming fish and the microstructure of the nasal cavity, this study is the first to develop swimming short fibrous nasal drops that can directly target the nasal mucosa and swim in the nasal cavity, which can effectively deliver drugs to the brain. Briefly, swimming short fibrous nasal drops with charged controlled drug release were fabricated by electrospinning, homogenization, the π-π conjugation between indole group of fibers, the benzene ring of leucine-rich repeat kinase 2 (LRRK2) inhibitor along with charge-dipole interaction between positively charged poly-lysine (PLL) and negatively charged surface of fibers; this enabled these fibers to stick to nasal mucosa, prolonged the residence time on mucosa, and prevented rapid mucociliary clearance. In vitro, swimming short fibrous nasal drops were biocompatible and inhibited microglial activation by releasing an LRRK2 inhibitor. In vivo, luciferase-labelled swimming short fibrous nasal drops delivered an LRRK2 inhibitor to the brain through the nasal mucosa, alleviating cognitive dysfunction caused by sepsis-associated encephalopathy by inhibiting microglial inflammation and improving synaptic plasticity. Thus, swimming short fibrous nasal drops is a promising strategy for the treatment of CNS diseases.

Micro-and mesoscale aspects of neurodegeneration in engineered human neural networks carrying the LRRK2 G2019S mutation

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene have been widely linked to Parkinson's disease, where the G2019S variant has been shown to contribute uniquely to both familial and sporadic forms of the disease. LRRK2-related mutations have been extensively studied, yet the wide variety of cellular and network events related to these mutations remain poorly understood. The advancement and availability of tools for neural engineering now enable modeling of selected pathological aspects of neurodegenerative disease in human neural networks in vitro. Our study revealed distinct pathology associated dynamics in engineered human cortical neural networks carrying the LRRK2 G2019S mutation compared to healthy isogenic control neural networks. The neurons carrying the LRRK2 G2019S mutation self-organized into networks with aberrant morphology and mitochondrial dynamics, affecting emerging structure-function relationships both at the micro-and mesoscale. Taken together, the findings of our study points toward an overall heightened metabolic demand in networks carrying the LRRK2 G2019S mutation, as well as a resilience to change in response to perturbation, compared to healthy isogenic controls.

Leucine-rich repeat kinase 2 limits dopamine D1 receptor signaling in striatum and biases against heavy persistent alcohol drinking

The transition from hedonic alcohol drinking to problematic drinking is a hallmark of alcohol use disorder that occurs only in a subset of drinkers. This transition requires long-lasting changes in the synaptic drive and the activity of striatal neurons expressing dopamine D1 receptor (D1R). The molecular mechanisms that generate vulnerability in some individuals to undergo the transition are less understood. Here, we report that the Parkinson's-related protein leucine-rich repeat kinase 2 (LRRK2) modulates striatal D1R function to affect the behavioral response to alcohol and the likelihood that mice transition to heavy, persistent alcohol drinking. Constitutive deletion of the Lrrk2 gene specifically from D1R-expressing neurons potentiated D1R signaling at the cellular and synaptic level and enhanced alcohol-related behaviors and drinking. Mice with cell-specific deletion of Lrrk2 were more prone to heavy alcohol drinking, and consumption was insensitive to punishment. These findings identify a potential novel role for LRRK2 function in the striatum in promoting resilience against heavy and persistent alcohol drinking.

Multi-method computational evaluation of the inhibitors against leucine-rich repeat kinase 2 G2019S mutant for Parkinson's disease

Leucine-rich repeat kinase 2 G2019S mutant (LRRK2 G2019S) is a potential target for Parkinson's disease therapy. In this work, the computational evaluation of the LRRK2 G2019S inhibitors was conducted via a combined approach which contains a preliminary screening of a large database of compounds via similarity and pharmacophore, a secondary selection via structure-based affinity prediction and molecular docking, and a rescoring treatment for the final selection. MD simulations and MM/GBSA calculations were performed to check the agreement between different prediction methods for these inhibitors. 331 experimental ligands were collected, and 170 were used to build the structure-activity relationship. Eight representative ligand structural models were employed in similarity searching and pharmacophore screening over 14 million compounds. The process for selecting proper molecular descriptors provides a successful sample which can be used as a general strategy in QSAR modelling. The rescoring used in this work presents an alternative useful treatment for ranking and selection.

Pathobiology of Cognitive Impairment in Parkinson Disease: Challenges and Outlooks

Cognitive impairment (CI) is a characteristic non-motor feature of Parkinson disease (PD) that poses a severe burden on the patients and caregivers, yet relatively little is known about its pathobiology. Cognitive deficits are evident throughout the course of PD, with around 25% of subtle cognitive decline and mild CI (MCI) at the time of diagnosis and up to 83% of patients developing dementia after 20 years. The heterogeneity of cognitive phenotypes suggests that a common neuropathological process, characterized by progressive degeneration of the dopaminergic striatonigral system and of many other neuronal systems, results not only in structural deficits but also extensive changes of functional neuronal network activities and neurotransmitter dysfunctions. Modern neuroimaging studies revealed multilocular cortical and subcortical atrophies and alterations in intrinsic neuronal connectivities. The decreased functional connectivity (FC) of the default mode network (DMN) in the bilateral prefrontal cortex is affected already before the development of clinical CI and in the absence of structural changes. Longitudinal cognitive decline is associated with frontostriatal and limbic affections, white matter microlesions and changes between multiple functional neuronal networks, including thalamo-insular, frontoparietal and attention networks, the cholinergic forebrain and the noradrenergic system. Superimposed Alzheimer-related (and other concomitant) pathologies due to interactions between α-synuclein, tau-protein and β-amyloid contribute to dementia pathogenesis in both PD and dementia with Lewy bodies (DLB). To further elucidate the interaction of the pathomechanisms responsible for CI in PD, well-designed longitudinal clinico-pathological studies are warranted that are supported by fluid and sophisticated imaging biomarkers as a basis for better early diagnosis and future disease-modifying therapies.

Cellular and subcellular localization of Rab10 and phospho-T73 Rab10 in the mouse and human brain

Autosomal dominant pathogenic mutations in Leucine-rich repeat kinase 2 (LRRK2) cause Parkinson's disease (PD). The most common mutation, G2019S-LRRK2, increases the kinase activity of LRRK2 causing hyper-phosphorylation of its substrates. One of these substrates, Rab10, is phosphorylated at a conserved Thr73 residue (pRab10), and is one of the most abundant LRRK2 Rab GTPases expressed in various tissues. The involvement of Rab10 in neurodegenerative disease, including both PD and Alzheimer's disease makes pinpointing the cellular and subcellular localization of Rab10 and pRab10 in the brain an important step in understanding its functional role, and how post-translational modifications could impact function. To establish the specificity of antibodies to the phosphorylated form of Rab10 (pRab10), Rab10 specific antisense oligonucleotides were intraventricularly injected into the brains of mice. Further, Rab10 knock out induced neurons, differentiated from human induced pluripotent stem cells were used to test the pRab10 antibody specificity. To amplify the weak immunofluorescence signal of pRab10, tyramide signal amplification was utilized. Rab10 and pRab10 were expressed in the cortex, striatum and the substantia nigra pars compacta. Immunofluorescence for pRab10 was increased in G2019S-LRRK2 knockin mice. Neurons, astrocytes, microglia and oligodendrocytes all showed Rab10 and pRab10 expression. While Rab10 colocalized with endoplasmic reticulum, lysosome and trans-Golgi network markers, pRab10 did not localize to these organelles. However, pRab10, did overlap with markers of the presynaptic terminal in both mouse and human cortex, including α-synuclein. Results from this study suggest Rab10 and pRab10 are expressed in all brain areas and cell types tested in this study, but pRab10 is enriched at the presynaptic terminal. As Rab10 is a LRRK2 kinase substrate, increased kinase activity of G2019S-LRRK2 in PD may affect Rab10 mediated membrane trafficking at the presynaptic terminal in neurons in disease.

Caffeine intake interacts with Asian gene variants in Parkinson's disease: a study in 4488 subjects

Background

Caffeine intake reduces risk of Parkinson's disease (PD), but the interaction with genes is unclear. The interaction of caffeine with genetic variants in those at high PD risk has healthcare importance. We investigate interactions of caffeine intake with risk variants found in Asians, and determine PD risk estimates in caffeine-drinkers carrying these variants.

Methods

PD patients and controls without neurological disorders were included. Caffeine intake was assessed using a validated evaluation tool. Leucine rich repeat kinase 2 (LRRK2) risk variants were genotyped. Statistical analysis was conducted with logistic regression models. Gene-caffeine interactions were quantified using attributable proportion (AP) due to interaction (positive interaction defined as AP >0).

Findings

5100 subjects were screened and 4488 subjects (1790 PD, 2698 controls) with genetic data of at least one LRRK2 variant were included. Risk-variant-carriers who were non-caffeine-drinkers had increased PD odds compared to wildtype carriers who were caffeine-drinkers for G2385R [OR 8.6 (2.6-28.1) p < 0.001; AP = 0.71], R1628P [OR 4.6 (1.6-12.8) p = 0.004; AP = 0.50] and S1647T [OR 4.0 (2.0-8.1) p < 0.001; AP = 0.55] variants.

Interpretation

Caffeine intake interacts with LRRK2 risk variants across three different groups of gene carriers. Asymptomatic risk-variant-carriers who are non-caffeine-drinkers have four to eight times greater PD risk compared to wildtype-caffeine-drinkers. Lifestyle modifications to mitigate PD risk in asymptomatic healthy risk variant carriers have potential roles in our Asian cohort.

Funding

This study was supported by the National Medical Research Council (STaR and PD OF LCG 000207 grants) and Duke-NUS Medical School.

Parkinson's-linked LRRK2-G2019S derails AMPAR trafficking, mobility and composition in striatum with cell-type and subunit specificity

Parkinson's (PD) is a multi-factorial disease that affects multiple brain systems and circuits. While defined by motor symptoms caused by degeneration of brainstem dopamine neurons, debilitating non-motor abnormalities in fronto-striatal based cognitive function are common, appear early and are initially independent of dopamine. Young adult mice expressing the PD-associated G2019S missense mutation in Lrrk2 also exhibit deficits in fronto-striatal-based cognitive tasks. In mice and humans, cognitive functions require dynamic adjustments in glutamatergic synapse strength through cell-surface trafficking of AMPA-type glutamate receptors (AMPARs), but it is unknown how LRRK2 mutation impacts dynamic features of AMPAR trafficking in striatal projection neurons (SPNs). Here, we used Lrrk2 G2019S knockin mice to show that surface AMPAR subunit stoichiometry is altered biochemically and functionally in mutant SPNs to favor incorporation of GluA1 over GluA2. GluA1-containing AMPARs were resistant to internalization from the cell surface, leaving an excessive accumulation of GluA1 on the surface within and outside synapses. This negatively impacted trafficking dynamics that normally support synapse strengthening, as GluA1-containing AMPARs failed to increase at synapses in response to a potentiating stimulus and showed significantly reduced surface mobility. Surface GluA2-containing AMPARs were expressed at normal levels in synapses, indicating subunit-selective impairment. Abnormal surface accumulation of GluA1 was independent of PKA activity and was limited to D 1 R SPNs. Since LRRK2 mutation is thought to be part of a common PD pathogenic pathway, our data suggest that sustained, striatal cell-type specific changes in AMPAR composition and trafficking contribute to cognitive or other impairments associated with PD.

SIGNIFICANCE STATEMENT

Mutations in LRRK2 are common genetic risks for PD. Lrrk2 G2019S mice fail to exhibit long-term potentiation at corticostriatal synapses and show significant deficits in frontal-striatal based cognitive tasks. While LRRK2 has been implicated generally in protein trafficking, whether G2019S derails AMPAR trafficking at synapses on striatal neurons (SPNs) is unknown. We show that surface GluA1-AMPARs fail to internalize and instead accumulate excessively within and outside synapses. This effect is selective to D 1 R SPNs and negatively impacts synapse strengthening as GluA1-AMPARs fail to increase at the surface in response to potentiation and show limited surface mobility. Thus, LRRK2-G2019S narrows the effective range of plasticity mechanisms, supporting the idea that cognitive symptoms reflect an imbalance in AMPAR trafficking mechanisms within cell-type specific projections.

Modeling Parkinson's disease in LRRK2 rodents

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are associated with familial and sporadic forms of Parkinson's disease (PD). Sporadic PD and LRRK2 PD share main clinical and neuropathological features, namely hypokinesia, degeneration of nigro-striatal dopamine neurons and α-synuclein aggregates in the form of Lewy bodies. Animals harboring the most common LRRK2 mutations, i.e. p.G2019S and p.R1441C/G, have been generated to replicate the parkinsonian phenotype and investigate the underlying pathogenic mechanisms. Disappointingly, however, LRRK2 rodents did not consistently phenocopy hypokinesia and nigro-striatal degeneration, or showed Lewy body-like aggregates. Instead, LRRK2 rodents manifested non-motor signs and dysregulated transmission at dopaminergic and non-dopaminergic synapses that are reminiscent of behavioral and functional network changes observed in the prodromal phase of the disease. LRRK2 rodents also manifested greater susceptibility to different parkinsonian toxins or stressors when subjected to dual-hit or multiple-hit protocols, confirming LRRK2 mutations as genetic risk factors. In conclusion, LRRK2 rodents represent a unique tool to identify the molecular mechanisms through which LRRK2 modulates the course and clinical presentations of PD and to study the interplay between genetic, intrinsic and environmental protective/risk factors in PD pathogenesis.

α-MSH-catabolic enzyme prolylcarboxypeptidase in nucleus accumbens shell ameliorates stress susceptibility in mice through regulating synaptic plasticity

Emerging evidence demonstrates the vital role of synaptic transmission and structural remodeling in major depressive disorder. Activation of melanocortin receptors facilitates stress-induced emotional behavior. Prolylcarboxypeptidase (PRCP) is a serine protease, which splits the C-terminal amino acid of α-MSH and inactivates it. In this study, we asked whether PRCP, the endogenous enzyme of melanocortin system, might play a role in stress susceptibility via regulating synaptic adaptations. Mice were subjected to chronic social defeat stress (CSDS) or subthreshold social defeat stress (SSDS). Depressive-like behavior was assessed in SIT, SPT, TST and FST. Based on to behavioral assessments, mice were divided into the susceptible (SUS) and resilient (RES) groups. After social defeat stress, drug infusion or viral expression and behavioral tests, morphological and electrophysiological analysis were conducted in PFX-fixed and fresh brain slices containing the nucleus accumbens shell (NAcsh). We showed that PRCP was downregulated in NAcsh of susceptible mice. Administration of fluoxetine (20 mg·kg-1·d-1, i.p., for 2 weeks) ameliorated the depressive-like behavior, and restored the expression levels of PRCP in NAcsh of susceptible mice. Pharmacological or genetic inhibition of PRCP in NAcsh by microinjection of N-benzyloxycarbonyl-L-prolyl-L-prolinal (ZPP) or LV-shPRCP enhanced the excitatory synaptic transmission in NAcsh, facilitating stress susceptibility via central melanocortin receptors. On the contrary, overexpression of PRCP in NAcsh by microinjection of AAV-PRCP alleviated the depressive-like behavior and reversed the enhanced excitatory synaptic transmission, abnormal dendritogenesis and spinogenesis in NAcsh induced by chronic stress. Furthermore, chronic stress increased the level of CaMKIIα, a kinase closely related to synaptic plasticity, in NAcsh. The elevated level of CaMKIIα was reversed by overexpression of PRCP in NAcsh. Pharmacological inhibition of CaMKIIα in NAcsh alleviated stress susceptibility induced by PRCP knockdown. This study has revealed the essential role of PRCP in relieving stress susceptibility through melanocortin signaling-mediated synaptic plasticity in NAcsh.

Calcium dysregulation combined with mitochondrial failure and electrophysiological maturity converge in Parkinson's iPSC-dopamine neurons

Parkinson's disease (PD) is characterized by a progressive deterioration of motor and cognitive functions. Although death of dopamine neurons is the hallmark pathology of PD, this is a late-stage disease process preceded by neuronal dysfunction. Here we describe early physiological perturbations in patient-derived induced pluripotent stem cell (iPSC)-dopamine neurons carrying the GBA-N370S mutation, a strong genetic risk factor for PD. GBA-N370S iPSC-dopamine neurons show an early and persistent calcium dysregulation notably at the mitochondria, followed by reduced mitochondrial membrane potential and oxygen consumption rate, indicating mitochondrial failure. With increased neuronal maturity, we observed decreased synaptic function in PD iPSC-dopamine neurons, consistent with the requirement for ATP and calcium to support the increase in electrophysiological activity over time. Our work demonstrates that calcium dyshomeostasis and mitochondrial failure impair the higher electrophysiological activity of mature neurons and may underlie the vulnerability of dopamine neurons in PD.

Genome-wide case-only analysis of gene-gene interactions with known Parkinson's disease risk variants reveals link between LRRK2 and SYT10

The effects of one genetic factor upon Parkinson's disease (PD) risk may be modified by other genetic factors. Such gene-gene interaction (G×G) could explain some of the 'missing heritability' of PD and the reduced penetrance of known PD risk variants. Using the largest single nucleotide polymorphism (SNP) genotype data set currently available for PD (18,688 patients), provided by the International Parkinson's Disease Genomics Consortium, we studied G×G with a case-only (CO) design. To this end, we paired each of 90 SNPs previously reported to be associated with PD with one of 7.8 million quality-controlled SNPs from a genome-wide panel. Support of any putative G×G interactions found was sought by the analysis of independent genotype-phenotype and experimental data. A total of 116 significant pairwise SNP genotype associations were identified in PD cases, pointing towards G×G. The most prominent associations involved a region on chromosome 12q containing SNP rs76904798, which is a non-coding variant of the LRRK2 gene. It yielded the lowest interaction p-value overall with SNP rs1007709 in the promoter region of the SYT10 gene (interaction OR = 1.80, 95% CI: 1.65-1.95, p = 2.7 × 10^-43). SNPs around SYT10 were also associated with the age-at-onset of PD in an independent cohort of carriers of LRRK2 mutation p.G2019S. Moreover, SYT10 gene expression during neuronal development was found to differ between cells from affected and non-affected p.G2019S carriers. G×G interaction on PD risk, involving the LRRK2 and SYT10 gene regions, is biologically plausible owing to the known link between PD and LRRK2, its involvement in neural plasticity, and the contribution of SYT10 to the exocytosis of secretory vesicles in neurons.

Mice with the Rab10 T73V mutation exhibit anxiety-like behavior and alteration of neuronal functions in the striatum

Hyperphosphorylated Rab10 has been implicated in the pathogenesis of neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. However, the neurophysiological function of the evolutionarily conserved Thr73 phosphorylation of Rab10 remains poorly understood. Here, we generated a novel mouse model expressing the non-phosphorylatable T73V mutation of Rab10 and performed a comprehensive series of neurological analyses, including behavioral tests, synaptic evaluations, neuronal and glial staining, assessments of neurite arborization and spine morphogenesis. The Rab10 T73V mutantmice exhibited a characteristic anxiety-like phenotype with other behavioral modules relatively unaffected. Moreover, Rab10 T73V mutant mice displayed striatum-specific synaptic dysfunction, as indicated by aberrantly increased expression levels of synaptic proteins and impaired frequencies of miniature inhibitory postsynaptic currents. The genetic deletion of Rab10 phosphorylation enhanced neurite arborization and accelerated spine maturation in striatal medium spiny neurons. Our findings emphasize the specific role of intrinsic phospho-Rab10 in the regulation of the striatal circuitry and its related behaviors.

Non-Motor Symptoms of Parkinson's Disease: The Neurobiology of Early Psychiatric and Cognitive Dysfunction

Parkinson's disease (PD) is a progressive neurodegenerative disorder that has been recognized for over 200 years by its clinically dominant motor system impairment. There are prominent non-motor symptoms as well, and among these, psychiatric symptoms of depression and anxiety and cognitive impairment are common and can appear earlier than motor symptoms. Although the neurobiology underlying these particular PD-associated non-motor symptoms is not completely understood, the identification of PARK genes that contribute to hereditary and sporadic PD has enabled genetic models in animals that, in turn, have fostered ever deepening analyses of cells, synapses, circuits, and behaviors relevant to non-motor psychiatric and cognitive symptoms of human PD. Moreover, while it has long been recognized that inflammation is a prominent component of PD, recent studies demonstrate that brain-immune signaling crosstalk has significant modulatory effects on brain cell and synaptic function in the context of psychiatric symptoms. This review provides a focused update on such progress in understanding the neurobiology of PD-related non-motor psychiatric and cognitive symptoms.

Iron-induced cytotoxicity mediated by endolysosomal TRPML1 channels is reverted by TFEB

Increased brain iron content has been consistently reported in sporadic Parkinson's disease (PD) patients, and an increase in cytosolic free iron is known to cause oxidative stress and cell death. However, whether iron also accumulates in susceptible brain areas in humans or in mouse models of familial PD remains unknown. In addition, whilst the lysosome functions as a critical intracellular iron storage organelle, little is known about the mechanisms underlying lysosomal iron release and how this process is influenced by lysosome biogenesis and/or lysosomal exocytosis. Here, we report an increase in brain iron content also in PD patients due to the common G2019S-LRRK2 mutation as compared to healthy age-matched controls, whilst differences in iron content are not observed in G2019S-LRRK2 knockin as compared to control mice. Chemically triggering iron overload in cultured cells causes cytotoxicity via the endolysosomal release of iron which is mediated by TRPML1. TFEB expression reverts the iron overload-associated cytotoxicity by causing lysosomal exocytosis, which is dependent on a TRPML1-mediated increase in cytosolic calcium levels. Therefore, approaches aimed at increasing TFEB levels, or pharmacological TRPML1 activation in conjunction with iron chelation may prove beneficial against cell death associated with iron overload conditions such as those associated with PD.

How Well Do Rodent Models of Parkinson's Disease Recapitulate Early Non-Motor Phenotypes? A Systematic Review

The prodromal phase of Parkinson's disease (PD) is characterised by many non-motor symptoms, and these have recently been posited to be predictive of later diagnosis. Genetic rodent models can develop non-motor phenotypes, providing tools to identify mechanisms underlying the early development of PD. However, it is not yet clear how reproducible non-motor phenotypes are amongst genetic PD rodent models, whether phenotypes are age-dependent, and the translatability of these phenotypes has yet to be explored. A systematic literature search was conducted on studies using genetic PD rodent models to investigate non-motor phenotypes; cognition, anxiety/depressive-like behaviour, gastrointestinal (GI) function, olfaction, circadian rhythm, cardiovascular and urinary function. In total, 51 genetic models of PD across 150 studies were identified. We found outcomes of most phenotypes were inconclusive due to inadequate studies, assessment at different ages, or variation in experimental and environmental factors. GI dysfunction was the most reproducible phenotype across all genetic rodent models. The mouse model harbouring mutant A53T, and the wild-type hα-syn overexpression (OE) model recapitulated the majority of phenotypes, albeit did not reliably produce concurrent motor deficits and nigral cell loss. Furthermore, animal models displayed different phenotypic profiles, reflecting the distinct genetic risk factors and heterogeneity of disease mechanisms. Currently, the inconsistent phenotypes within rodent models pose a challenge in the translatability and usefulness for further biomechanistic investigations. This review highlights opportunities to improve phenotype reproducibility with an emphasis on phenotypic assay choice and robust experimental design.

Neuronal Firing and Glutamatergic Synapses in the Substantia Nigra Pars Reticulata of LRRK2-G2019S Mice

Pathogenic mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are frequent causes of familial Parkinson’s Disease (PD), an increasingly prevalent neurodegenerative disease that affects basal ganglia circuitry. The cellular effects of the G2019S mutation in the LRRK2 gene, the most common pathological mutation, have not been thoroughly investigated. In this study we used middle-aged mice carrying the LRRK2-G2019S mutation (G2019S mice) to identify potential alterations in the neurophysiological properties and characteristics of glutamatergic synaptic transmission in basal ganglia output neurons, i.e., substantia nigra pars reticulata (SNr) GABAergic neurons. We found that the intrinsic membrane properties and action potential properties were unaltered in G2019S mice compared to wild-type (WT) mice. The spontaneous firing frequency was similar, but we observed an increased regularity in the firing of SNr neurons recorded from G2019S mice. We examined the short-term plasticity of glutamatergic synaptic transmission, and we found an increased paired-pulse depression in G2019S mice compared to WT mice, indicating an increased probability of glutamate release in SNr neurons from G2019S mice. We measured synaptic transmission mediated by NMDA receptors and we found that the kinetics of synaptic responses mediated by these receptors were unaltered, as well as the contribution of the GluN2B subunit to these responses, in SNr neurons of G2019S mice compared to WT mice. These results demonstrate an overall maintenance of basic neurophysiological and synaptic characteristics, and subtle changes in the firing pattern and in glutamatergic synaptic transmission in basal ganglia output neurons that precede neurodegeneration of dopaminergic neurons in the LRRK2-G2019S mouse model of late-onset PD.

Cognitive deficits and altered cholinergic innervation in young adult male mice carrying a Parkinson's disease Lrrk2G2019S knockin mutation

Impaired executive function is a common and debilitating non-motor symptom of idiopathic and hereditary Parkinson's disease (PD), but there is little understanding of the underlying pathophysiological mechanisms and circuits. The G2019S mutation in the kinase domain of leucine-rich repeat kinase 2 (LRRK2) greatly increases risk for late-onset PD, and non-manifesting LRRK2^G2019S carriers can also exhibit early and significant cognitive impairment. Here, we subjected young adult male mice carrying a Lrrk2^G2019S knockin mutation to touchscreen-based operant tasks that measure attention, goal-directed learning and cognitive flexibility, all of which rely on frontal-striatal connectivity and are strongly modulated by cholinergic innervation. In a visuospatial attention task, mutant mice exhibited significantly more omissions and longer response latencies than controls that could not be attributed to deficits in motivation, visual sensory perception per se or locomotion, thereby suggesting impairments in divided attention and/or action-selection as well as generally slower information processing speed. Pretreating mice with the acetylcholinesterase inhibitor donepezil normalized both higher omission rates and longer response latencies in the mutants, but did not affect any performance metric in controls. Strikingly, cholinergic fiber density in cortical areas PL/IL and DMS (dorsomedial striatum) was significantly sparser in mutants than in controls, while further behavioral interrogation of the mutants revealed significant impairments in action-outcome associations but preserved cognitive flexibility. These data suggest that the Lrrk2^G2019S mutation negatively impacts cholinergic innervation anatomically and functionally by young adulthood, impairing corticostriatal network function in ways that may contribute to early PD-associated executive function deficits.

Oxidative stress and synaptic dysfunction in rodent models of Parkinson's disease

Parkinson's disease (PD) is a multifactorial disorder involving a complex interplay between a variety of genetic and environmental factors. In this scenario, mitochondrial impairment and oxidative stress are widely accepted as crucial neuropathogenic mechanisms, as also evidenced by the identification of PD-associated genes that are directly involved in mitochondrial function. The concept of mitochondrial dysfunction is closely linked to that of synaptic dysfunction. Indeed, compelling evidence supports the role of mitochondria in synaptic transmission and plasticity, although many aspects have not yet been fully elucidated. Here, we will provide a brief overview of the most relevant evidence obtained in different neurotoxin-based and genetic rodent models of PD, focusing on mitochondrial impairment and synaptopathy, an early central event preceding overt nigrostriatal neurodegeneration. The identification of early deficits occurring in PD pathogenesis is crucial in view of the development of potential disease-modifying therapeutic strategies.

Trafficking of the glutamate transporter is impaired in LRRK2-related Parkinson's disease

The Excitatory Amino Acid Transporter 2 (EAAT2) accounts for 80% of brain glutamate clearance and is mainly expressed in astrocytic perisynaptic processes. EAAT2 function is finely regulated by endocytic events, recycling to the plasma membrane and degradation. Noteworthy, deficits in EAAT2 have been associated with neuronal excitotoxicity and neurodegeneration. In this study, we show that EAAT2 trafficking is impaired by the leucine-rich repeat kinase 2 (LRRK2) pathogenic variant G2019S, a common cause of late-onset familial Parkinson’s disease (PD). In LRRK2 G2019S human brains and experimental animal models, EAAT2 protein levels are significantly decreased, which is associated with elevated gliosis. The decreased expression of the transporter correlates with its reduced functionality in mouse LRRK2 G2019S purified astrocytic terminals and in Xenopus laevis oocytes expressing human LRRK2 G2019S. In LRRK2 G2019S knock-in mouse brain, the correct surface localization of the endogenous transporter is impaired, resulting in its interaction with a plethora of endo-vesicular proteins. Mechanistically, we report that pathogenic LRRK2 kinase activity delays the recycling of the transporter to the plasma membrane via Rabs inactivation, causing its intracellular re-localization and degradation. Taken together, our results demonstrate that pathogenic LRRK2 interferes with the physiology of EAAT2, pointing to extracellular glutamate overload as a possible contributor to neurodegeneration in PD.

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.

Genetic Variants Associated With Resilience in Human and Animal Studies

Resilience is broadly defined as the ability to maintain or regain functioning in the face of adversity and is influenced by both environmental and genetic factors. The identification of specific genetic factors and their biological pathways underpinning resilient functioning can help in the identification of common key factors, but heterogeneities in the operationalisation of resilience have hampered advances. We conducted a systematic review of genetic variants associated with resilience to enable the identification of general resilience mechanisms. We adopted broad inclusion criteria for the definition of resilience to capture both human and animal model studies, which use a wide range of resilience definitions and measure very different outcomes. Analyzing 158 studies, we found 71 candidate genes associated with resilience. OPRM1 (Opioid receptor mu 1), NPY (neuropeptide Y), CACNA1C (calcium voltage-gated channel subunit alpha1 C), DCC (deleted in colorectal carcinoma), and FKBP5 (FKBP prolyl isomerase 5) had both animal and human variants associated with resilience, supporting the idea of shared biological pathways. Further, for OPRM1, OXTR (oxytocin receptor), CRHR1 (corticotropin-releasing hormone receptor 1), COMT (catechol-O-methyltransferase), BDNF (brain-derived neurotrophic factor), APOE (apolipoprotein E), and SLC6A4 (solute carrier family 6 member 4), the same allele was associated with resilience across divergent resilience definitions, which suggests these genes may therefore provide a starting point for further research examining commonality in resilience pathways.

Inhibition of LRRK2 kinase activity promotes anterograde axonal transport and presynaptic targeting of α-synuclein

Pathologic inclusions composed of α-synuclein called Lewy pathology are hallmarks of Parkinson’s Disease (PD). Dominant inherited mutations in leucine rich repeat kinase 2 (LRRK2) are the most common genetic cause of PD. Lewy pathology is found in the majority of individuals with LRRK2-PD, particularly those with the G2019S-LRRK2 mutation. Lewy pathology in LRRK2-PD associates with increased non-motor symptoms such as cognitive deficits, anxiety, and orthostatic hypotension. Thus, understanding the relationship between LRRK2 and α-synuclein could be important for determining the mechanisms of non-motor symptoms. In PD models, expression of mutant LRRK2 reduces membrane localization of α-synuclein, and enhances formation of pathologic α-synuclein, particularly when synaptic activity is increased. α-Synuclein and LRRK2 both localize to the presynaptic terminal. LRRK2 plays a role in membrane traffic, including axonal transport, and therefore may influence α-synuclein synaptic localization. This study shows that LRRK2 kinase activity influences α-synuclein targeting to the presynaptic terminal. We used the selective LRRK2 kinase inhibitors, MLi-2 and PF-06685360 (PF-360) to determine the impact of reduced LRRK2 kinase activity on presynaptic localization of α-synuclein. Expansion microscopy (ExM) in primary hippocampal cultures and the mouse striatum, in vivo, was used to more precisely resolve the presynaptic localization of α-synuclein. Live imaging of axonal transport of α-synuclein-GFP was used to investigate the impact of LRRK2 kinase inhibition on α-synuclein axonal transport towards the presynaptic terminal. Reduced LRRK2 kinase activity increases α-synuclein overlap with presynaptic markers in primary neurons, and increases anterograde axonal transport of α-synuclein-GFP. In vivo, LRRK2 inhibition increases α-synuclein overlap with glutamatergic, cortico-striatal terminals, and dopaminergic nigral-striatal presynaptic terminals. The findings suggest that LRRK2 kinase activity plays a role in axonal transport, and presynaptic targeting of α-synuclein. These data provide potential mechanisms by which LRRK2-mediated perturbations of α-synuclein localization could cause pathology in both LRRK2-PD, and idiopathic PD.

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.

Age-related LRRK2 G2019S Mutation Impacts Microglial Dopaminergic Fiber Refinement and Synaptic Pruning Involved in Abnormal Behaviors

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most frequent cause of autosomal dominant Parkinson's disease (PD), producing psychiatric and motor symptoms. We conducted this study to explore whether microglial dopaminergic (DAergic) fiber refinement and synaptic pruning are involved in the abnormal behavioral phenotypes of carriers of the LRRK2 G2019S mutation, by employing young and middle-aged PD model mice. The results revealed a characteristic late-onset hyperactivity and a progressive decline in the motor coordination of the LRRK2 G2019S mutation mice. LRRK2 G2019S mutation-induced aberrant microglial morphogenesis, with more branches and junctions per cell, resulted in excessive microglial refinement of dopaminergic (DAergic) fibers. Moreover, aberrant synaptic pruning distinctly impacted the prefrontal cortex (PFC) and dorsal striatum (DS), with significantly higher spine density in the PFC but the opposite effects in the DS region. Furthermore, LRRK2 G2019S mutation remodeled the inflammatory transcription landscape of microglia, rendering certain cerebral areas highly susceptible to microglial immune response. These findings indicate that LRRK2 G2019S mutation induces the production of inflammatory cytokines and mediates abnormal microglial morphogenesis and activity, resulting in abnormal phagocytosis, synaptic pruning and loss of DAergic fibers during aging, and, eventually, PD-related behavioral abnormalities.

Chronic and Acute Manipulation of Cortical Glutamate Transmission Induces Structural and Synaptic Changes in Co-cultured Striatal Neurons

In contrast to the prenatal topographic development of sensory cortices, striatal circuit organization is slow and requires the functional maturation of cortical and thalamic excitatory inputs throughout the first postnatal month. While mechanisms regulating synapse development and plasticity are quite well described at excitatory synapses of glutamatergic neurons in the neocortex, comparatively little is known of how this translates to glutamate synapses onto GABAergic neurons in the striatum. Here we investigate excitatory striatal synapse plasticity in an in vitro system, where glutamate can be studied in isolation from dopamine and other neuromodulators. We examined pre-and post-synaptic structural and functional plasticity in GABAergic striatal spiny projection neurons (SPNs), co-cultured with glutamatergic cortical neurons. After synapse formation, medium-term (24 h) TTX silencing increased the density of filopodia, and modestly decreased dendritic spine density, when assayed at 21 days in vitro (DIV). Spine reductions appeared to require residual spontaneous activation of ionotropic glutamate receptors. Conversely, chronic (14 days) TTX silencing markedly reduced spine density without any observed increase in filopodia density. Time-dependent, biphasic changes to the presynaptic marker Synapsin-1 were also observed, independent of residual spontaneous activity. Acute silencing (3 h) did not affect presynaptic markers or postsynaptic structures. To induce rapid, activity-dependent plasticity in striatal neurons, a chemical NMDA receptor-dependent “long-term potentiation (LTP)” paradigm was employed. Within 30 min, this increased spine and GluA1 cluster densities, and the percentage of spines containing GluA1 clusters, without altering the presynaptic signal. The results demonstrate that the growth and pruning of dendritic protrusions is an active process, requiring glutamate receptor activity in striatal projection neurons. Furthermore, NMDA receptor activation is sufficient to drive glutamatergic structural plasticity in SPNs, in the absence of dopamine or other neuromodulators.

Increased Excitability and Heightened Magnitude of Long-Term Potentiation at Hippocampal CA3-CA1 Synapses in a Mouse Model of Neonatal Hyperoxia Exposure

Preterm infants exposed to supraphysiological oxygen (hyperoxia) during the neonatal period have hippocampal atrophy and cognitive dysfunction later in childhood and as adolescents. Previously, we reported that 14-week-old adult mice exposed to hyperoxia as newborns had spatial memory deficits and hippocampal shrinkage, findings that mirror those of human adolescents who were born preterm. The area CA1 region of the hippocampus that is crucial for spatial learning and memory is highly vulnerable to oxidative stress. In this study, we investigated the long-term impact of neonatal hyperoxia exposure on hippocampal CA3-CA1 synaptic function. Male and female C57BL/6J mouse pups were continuously exposed to either 85% normobaric oxygen or air between postnatal days 2-14. Hippocampal slice electrophysiology at CA3-CA1 synapses was then performed at 14 weeks of age. We observed that hyperoxia exposed mice have heightened strength of basal synaptic transmission measured in input-output curves, increased fiber volley amplitude indicating increased axonal excitability, and heightened LTP magnitude at CA3-CA1 synapses, likely a consequence of increased postsynaptic depolarization during tetanus. These data demonstrate that supraphysiological oxygen exposure during the critical neonatal developmental period leads to pathologically heightened CA3-CA1 synaptic function during early adulthood which may contribute to hippocampal shrinkage and learning and memory deficits we previously reported. Furthermore, these results will help shed light on the consequences of hyperoxia exposure on the development of hippocampal synaptic circuit abnormalities that could be contributing to cognitive deficits in children born preterm.

Pathway-specific dysregulation of striatal excitatory synapses by LRRK2 mutations

LRRK2 is a kinase expressed in striatal spiny projection neurons (SPNs), cells which lose dopaminergic input in Parkinson’s disease (PD). R1441C and G2019S are the most common pathogenic mutations of LRRK2. How these mutations alter the structure and function of individual synapses on direct and indirect pathway SPNs is unknown and may reveal pre-clinical changes in dopamine-recipient neurons that predispose toward disease. Here, R1441C and G2019S knock-in mice enabled thorough evaluation of dendritic spines and synapses on pathway-identified SPNs. Biochemical synaptic preparations and super-resolution imaging revealed increased levels and altered organization of glutamatergic AMPA receptors in LRRK2 mutants. Relatedly, decreased frequency of miniature excitatory post-synaptic currents accompanied changes in dendritic spine nano-architecture, and single-synapse currents, evaluated using two-photon glutamate uncaging. Overall, LRRK2 mutations reshaped synaptic structure and function, an effect exaggerated in R1441C dSPNs. These data open the possibility of new neuroprotective therapies aimed at SPN synapse function, prior to disease onset.

Parkinson’s disease is caused by progressive damage to regions of the brain that regulate movement. This leads to a loss in nerve cells that produce a signaling molecule called dopamine, and causes patients to experience shakiness, slow movement and stiffness. When dopamine is released, it travels to a part of the brain known as the striatum, where it is received by cells called spiny projection neurons (SPNs), which are rich in a protein called LRRK2. Mutations in this protein have been shown to cause the motor impairments associated with Parkinson’s disease.

SPNs send signals to other regions of the brain either via a ‘direct’ route, which promotes movement, or an ‘indirect’ route, which suppresses movement. Previous studies suggest that mutations in the gene for LRRK2 influence the activity of these pathways even before dopamine signaling has been lost. Yet, it remained unclear how different mutations independently affected each pathway. To investigate this further, Chen et al. studied two of the mutations most commonly found in the human gene for LRRK2, known as G2019S and R1441C. This involved introducing one of these mutations in to the genetic code of mice, and using fluorescent proteins to mark single SPNs in either the direct or indirect pathway.

The experiments showed that both mutations disrupted the connections between SPNs in the direct and indirect pathway, which altered the activity of nerve cells in the striatum. Chen et al. found that individual connections were more strongly affected by the R1441C mutation. Further experiments showed that this was caused by the re-organization of a receptor protein in the nerve cells of the direct pathway, which increased how SPNs responded to inputs from other nerve cells.

These findings suggest that LRRK2 mutations disrupt neural activity in the striatum before dopamine levels become depleted. This discovery could help researchers identify new therapies for treating the early stages of Parkinson’s disease before the symptoms of dopamine loss arise.

Association of caffeine and related analytes with resistance to Parkinson disease among LRRK2 mutation carriers: A metabolomic study

OBJECTIVE

To identify markers of resistance to developing Parkinson disease (PD) among LRRK2 mutation carriers (LRRK2+), we carried out metabolomic profiling in individuals with PD and unaffected controls (UC), with and without the LRRK2 mutation.

METHODS

Plasma from 368 patients with PD and UC in the LRRK2 Cohort Consortium (LCC), comprising 118 LRRK2+/PD+, 115 LRRK2+/UC, 70 LRRK2-/PD+, and 65 LRRK2-/UC, and CSF available from 68 of them, were analyzed by liquid chromatography with mass spectrometry. For 282 analytes quantified in plasma and CSF, we assessed differences among the 4 groups and interactions between LRRK2 and PD status, using analysis of covariance models adjusted by age, study site cohort, and sex, with p value corrections for multiple comparisons.

RESULTS

Plasma caffeine concentration was lower in patients with PD vs UC (p < 0.001), more so among LRRK2+ carriers (by 76%) than among LRRK2- participants (by 31%), with significant interaction between LRRK2 and PD status (p = 0.005). Similar results were found for caffeine metabolites (paraxanthine, theophylline, 1-methylxanthine) and a nonxanthine marker of coffee consumption (trigonelline) in plasma, and in the subset of corresponding CSF samples. Dietary caffeine was also lower in LRRK2+/PD+ compared to LRRK2+/UC with significant interaction effect with the LRRK2+ mutation (p < 0.001).

CONCLUSIONS

Metabolomic analyses of the LCC samples identified caffeine, its demethylation metabolites, and trigonelline as prominent markers of resistance to PD linked to pathogenic LRRK2 mutations, more so than to idiopathic PD. Because these analytes are known both as correlates of coffee consumption and as neuroprotectants in animal PD models, the findings may reflect their avoidance by those predisposed to develop PD or their protective effects among LRRK2 mutation carriers.

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.

Maternal Programming of Social Dominance via Milk Cytokines

Regular physical activity improves physical and mental health. Here we found that the effect of physical activity extends to the next generation. Voluntary wheel running of dams, from postpartum day 2 to weaning, increased the social dominance and reproductive success, but not the physical/metabolic health, of their otherwise sedentary offspring. The individual's own physical activity did not improve dominance status. Maternal exercise did not disrupt maternal care or the maternal and offspring microbiota. Rather, the development of dominance behavior in the offspring of running mothers could be explained by the reduction of LIF, CXCL1, and CXCL2 cytokines in breast milk. These data reveal a cytokine-mediated lactocrine pathway that responds to the mother's postpartum physical activity and programs offspring social dominance. As dominance behaviors are highly relevant to the individual's survival and reproduction, lactocrine programming could be an evolutionary mechanism by which a mother promotes the social rank of her offspring.

From Synaptic Dysfunction to Neuroprotective Strategies in Genetic Parkinson's Disease: Lessons From LRRK2

The pathogenesis of Parkinson’s disease (PD) is thought to rely on a complex interaction between the patient’s genetic background and a variety of largely unknown environmental factors. In this scenario, the investigation of the genetic bases underlying familial PD could unveil key molecular pathways to be targeted by new disease-modifying therapies, still currently unavailable. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are responsible for the majority of inherited familial PD cases and can also be found in sporadic PD, but the pathophysiological functions of LRRK2 have not yet been fully elucidated. Here, we will review the evidence obtained in transgenic LRRK2 experimental models, characterized by altered striatal synaptic transmission, mitochondrial dysfunction, and α-synuclein aggregation. Interestingly, the processes triggered by mutant LRRK2 might represent early pathological phenomena in the pathogenesis of PD, anticipating the typical neurodegenerative features characterizing the late phases of the disease. A comprehensive view of LRRK2 neuronal pathophysiology will support the possible clinical application of pharmacological compounds targeting this protein, with potential therapeutic implications for patients suffering from both familial and sporadic PD.

Progress in LRRK2-Associated Parkinson's Disease Animal Models

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most frequent cause of familial Parkinson's disease (PD). Several genetic manipulations of the LRRK2 gene have been developed in animal models such as rodents, Drosophila, Caenorhabditis elegans, and zebrafish. These models can help us further understand the biological function and derive potential pathological mechanisms for LRRK2. Here we discuss common phenotypic themes found in LRRK2-associated PD animal models, highlight several issues that should be addressed in future models, and discuss emerging areas to guide their future development.

Sleep Spindles and Fragmented Sleep as Prodromal Markers in a Preclinical Model of LRRK2-G2019S Parkinson's Disease

Sleep disturbances co-occur with and precede the onset of motor symptoms in Parkinson's disease (PD). We evaluated sleep fragmentation and thalamocortical sleep spindles in mice expressing the p.G2019S mutation of the leucine-rich repeat kinase 2 (LRRK2) gene, one of the most common genetic forms of PD. Thalamocortical sleep spindles are oscillatory events that occur during slow-wave sleep that are involved in memory consolidation. We acquired data from electrocorticography, sleep behavioral measures, and a rotarod-based motor enrichment task in 28 LRRK2-G2019S knock-in mice and 27 wild-type controls (8–10 month-old males). Sleep was more fragmented in LRRK2-G2019S mice; sleep bouts were shorter and more numerous, even though total sleep time was similar to controls. LRRK2-G2019S animals expressed more sleep spindles, and individual spindles were longer in duration than in controls. We then chronically administered the LRRK2-inhibitor MLi-2 in-diet to n = 12 LRRK2-G2019S and n = 15 wild-type mice for a within-subject analysis of the effects of kinase inhibition on sleep behavior and physiology. Treatment with MLi-2 did not impact these measures. The data indicate that the LRRK2-G2019S mutation could lead to reduced sleep quality and altered sleep spindle physiology. This suggests that sleep spindles in LRRK2-G2019S animals could serve as biomarkers for underlying alterations in sleep networks resulting from the LRRK2-G2019S mutation, and further evaluation in human LRRK2-G2019S carriers is therefore warranted.

LRRK2 mutation alters behavioral, synaptic, and nonsynaptic adaptations to acute social stress

Parkinson's disease (PD) risk is increased by stress and certain gene mutations, including the most prevalent PD-linked mutation LRRK2-G2019S. Both PD and stress increase risk for psychiatric symptoms, yet it is unclear how PD-risk genes alter neural circuitry in response to stress that may promote psychopathology. Here we show significant differences between adult G2019S knockin and wild-type (wt) mice in stress-induced behaviors, with an unexpected uncoupling of depression-like and hedonia-like responses in G2019S mice. Moreover, mutant spiny projection neurons in nucleus accumbens (NAc) lack an adaptive, stress-induced change in excitability displayed by wt neurons, and instead show stress-induced changes in synaptic properties that wt neurons lack. Some synaptic alterations in NAc are already evident early in postnatal life. Thus G2019S alters the magnitude and direction of behavioral responses to stress that may reflect unique modifications of adaptive plasticity in cells and circuits implicated in psychopathology in humans.NEW & NOTEWORTHY Depression is associated with Parkinson's disease (PD), and environmental stress is a risk factor for both. We investigated how LRRK2-G2019S PD mutation affects depression-like behaviors, synaptic function, and intrinsic neuronal excitability following stress. In response to stress, the mutation drives abnormal synaptic changes, prevents adaptive changes in intrinsic excitability, and leads to aberrant behaviors, thus defining new ways in which PD mutations derail adaptive plasticity in response to stress that may contribute to disease onset.

Origins of Parkinson's Disease in Brain Development: Insights From Early and Persistent Effects of LRRK2-G2019S on Striatal Circuits

Late-onset Parkinson's disease (PD) is dominated clinically and experimentally by a focus on dopamine neuron degeneration and ensuing motor system abnormalities. There are, additionally, a number of non-motor symptoms - including cognitive and psychiatric - that can appear much earlier in the course of the disease and also significantly impair quality of life. The neurobiology of such cognitive and psychiatric non-motor symptoms is poorly understood. The recognition of genetic forms of late-onset PD, which are clinically similar to idiopathic forms in both motor and non-motor symptoms, raises the perspective that brain cells and circuits - and the behaviors they support - differ in significant ways from normal by virtue of the fact that these mutations are carried throughout life, including especially early developmental critical periods where circuit structure and function is particularly susceptible to the influence of experience-dependent activity. In this focused review, we support this central thesis by highlighting studies of LRRK2-G2019S mouse models. We describe work that shows that in G2019S mutants, corticostriatal activity and plasticity are abnormal by P21, the end of a period of excitatory synaptogenesis in striatum. Moreover, by young adulthood, impaired striatal synaptic and non-synaptic forms of plasticity likely underlie altered and variable performance by mutant mice in validated tasks that test for depression-like and anhedonia-like behaviors. Mechanistically, deficits in cellular, synaptic and behavioral plasticity may be unified by mutation-linked defects in trafficking of AMPAR subunits and other membrane channels, which in turn may reflect impairment in the function of the Rab family of GTPases, a major target of LRRK2 phosphorylation. These findings underscore the need to better understand how PD-related mutant proteins influence brain structure and function during an extended period of brain development, and offer new clues for future therapeutic strategies to target non-motor cognitive or psychiatric symptoms of PD.

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.

The molecular and cellular mechanisms of depression: a focus on reward circuitry

Depression is a complex disorder that takes an enormous toll on individual health. As affected individuals display a wide variation in their clinical symptoms, the precise neural mechanisms underlying the development of depression remain elusive. Although it is impossible to phenocopy every symptom of human depression in rodents, the preclinical field has had great success in modeling some of the core affective and neurovegetative depressive symptoms, including social withdrawal, anhedonia, and weight loss. Adaptations in select cell populations may underlie these individual depressive symptoms and new tools have expanded our ability to monitor and manipulate specific cell types. This review outlines some of the most recent preclinical discoveries on the molecular and neurophysiological mechanisms in reward circuitry that underlie the expression of behavioral constructs relevant to depressive symptoms.

Are we listening to everything the PARK genes are telling us?

The cardinal motor symptoms that define Parkinson's disease (PD) clinically have been recognized for over 200 years. That these symptoms arise following the loss of dopamine neurons in the substantia nigra has been known for the last 50. These long-established facts have fueled a broadly held expectation that degenerating dopaminergic neurons alone hold the key to understanding and curing PD. This prevalent expectation is at odds with the observation that many nonmotor symptoms, including depression and cognitive inflexibility among others, can appear years earlier than the overt dopaminergic neuron degeneration that drives motor abnormalities and are not improved by levodopa treatment. Thus, preserving or rescuing dopamine neuron health and function is of paramount importance, but this alone fails to capture the underlying neurobiology of earlier-appearing nonmotor symptoms. Insight into the complete landscape of disease-related abnormalities and the context in which they arise can be gleaned from a more comprehensive consideration of the PARK genes that are known to cause PD. Here, we make the case that a full incorporation of research showing when and where PARK genes are expressed as well as the impact of gene mutation on function throughout life, in tandem with research studying how dopaminergic neuron degeneration begins, is essential for a full understanding of the multi-dimensional etiology of PD. A broad view may also reveal something about long-term adjustments cells and systems make in response to gene mutation and help to identify mechanisms conferring the resilience or susceptibility of some cells and systems over others.

Induced pluripotent stem cell-based modeling of mutant LRRK2-associated Parkinson's disease

Recent advances in cell reprogramming have enabled assessment of disease-related cellular traits in patient-derived somatic cells, thus providing a versatile platform for disease modeling and drug development. Given the limited access to vital human brain cells, this technology is especially relevant for neurodegenerative disorders such as Parkinson's disease (PD) as a tool to decipher underlying pathomechanisms. Importantly, recent progress in genome-editing technologies has provided an ability to analyze isogenic induced pluripotent stem cell (iPSC) pairs that differ only in a single genetic change, thus allowing a thorough assessment of the molecular and cellular phenotypes that result from monogenetic risk factors. In this review, we summarize the current state of iPSC-based modeling of PD with a focus on leucine-rich repeat kinase 2 (LRRK2), one of the most prominent monogenetic risk factors for PD linked to both familial and idiopathic forms. The LRRK2 protein is a primarily cytosolic multi-domain protein contributing to regulation of several pathways including autophagy, mitochondrial function, vesicle transport, nuclear architecture and cell morphology. We summarize iPSC-based studies that contributed to improving our understanding of the function of LRRK2 and its variants in the context of PD etiopathology. These data, along with results obtained in our own studies, underscore the multifaceted role of LRRK2 in regulating cellular homeostasis on several levels, including proteostasis, mitochondrial dynamics and regulation of the cytoskeleton. Finally, we expound advantages and limitations of reprogramming technologies for disease modeling and drug development and provide an outlook on future challenges and expectations offered by this exciting technology.

Functional and behavioral consequences of Parkinson's disease-associated LRRK2-G2019S mutation

LRRK2 mutation is the most common inherited, autosomal dominant cause of Parkinson's disease (PD) and has also been observed in sporadic cases. Most mutations result in increased LRRK2 kinase activity. LRRK2 is highly expressed in brain regions that receive dense, convergent innervation by dopaminergic and glutamatergic axons, and its levels rise developmentally coincident with glutamatergic synapse formation. The onset and timing of expression suggests strongly that LRRK2 regulates the development, maturation and function of synapses. Several lines of data in mice show that LRRK2-G2019S, the most common LRRK2 mutation, produces an abnormal gain of pathological function that affects synaptic activity, spine morphology, persistent forms of synapse plasticity and behavioral responses to social stress. Effects of the mutation can be detected as early as the second week of postnatal development and can last or have consequences that extend into adulthood and occur in the absence of dopamine loss. These data suggest that the generation of neural circuits that support complex behaviors is modified by LRRK2-G2019S. Whether such alterations impart vulnerability to neurons directly or indirectly, they bring to the forefront the idea that neural circuits within which dopamine neurons eventually degenerate are assembled and utilized in ways that are distinct from circuits that lack this mutation and may contribute to non-motor symptoms observed in humans with PD.