Loss of leucine-rich repeat kinase 2 causes impairment of protein degradation pathways, accumulation of alpha-synuclein, and apoptotic cell death in aged mice

α-Synuclein: Multiple pathogenic roles in trafficking and proteostasis pathways in Parkinson's disease

Parkinson's disease (PD) is a common age-related neurodegenerative disorder characterized by the loss of dopaminergic neurons in the midbrain. A hallmark of both familial and sporadic PD is the presence of Lewy body inclusions composed mainly of aggregated α-synuclein (α-syn), a presynaptic protein encoded by the SNCA gene. The mechanisms driving the relationship between α-syn accumulation and neurodegeneration are not completely understood, although recent evidence indicates that multiple branches of the proteostasis pathway are simultaneously perturbed when α-syn aberrantly accumulates within neurons. Studies from patient-derived midbrain cultures that develop α-syn pathology through the endogenous expression of PD-causing mutations show that proteostasis disruption occurs at the level of synthesis/folding in the endoplasmic reticulum (ER), downstream ER-Golgi trafficking, and autophagic-lysosomal clearance. Here, we review the fundamentals of protein transport, highlighting the specific steps where α-syn accumulation may intervene and the downstream effects on proteostasis. Current therapeutic efforts are focused on targeting single pathways or proteins, but the multifaceted pathogenic role of α-syn throughout the proteostasis pathway suggests that manipulating several targets simultaneously will provide more effective disease-modifying therapies for PD and other synucleinopathies.

State of the Art in Sub-Phenotyping Midbrain Dopamine Neurons

Dopaminergic neurons in the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNpc) comprise around 75% of all dopaminergic neurons in the human brain. While both groups of dopaminergic neurons are in close proximity in the midbrain and partially overlap, development, function, and impairments in these two classes of neurons are highly diverse. The molecular and cellular mechanisms underlying these differences are not yet fully understood, but research over the past decade has highlighted the need to differentiate between these two classes of dopaminergic neurons during their development and in the mature brain. This differentiation is crucial not only for understanding fundamental circuitry formation in the brain but also for developing therapies targeted to specific dopaminergic neuron classes without affecting others. In this review, we summarize the state of the art in our understanding of the differences between the dopaminergic neurons of the VTA and the SNpc, such as anatomy, structure, morphology, output and input, electrophysiology, development, and disorders, and discuss the current technologies and methods available for studying these two classes of dopaminergic neurons, highlighting their advantages, limitations, and the necessary improvements required to achieve more-precise therapeutic interventions.

Lysosomal dysfunction in α-synuclein pathology: molecular mechanisms and therapeutic strategies

In orchestrating cell signaling, facilitating plasma membrane repair, supervising protein secretion, managing waste elimination, and regulating energy consumption, lysosomes are indispensable guardians that play a crucial role in preserving intracellular homeostasis. Neurons are terminally differentiated post-mitotic cells. Neuronal function and waste elimination depend on normal lysosomal function. Converging data suggest that lysosomal dysfunction is a critical event in the etiology of Parkinson's disease (PD). Mutations in Glucosylceramidase Beta 1 (GBA1) and leucine-rich repeat kinase 2 (LRRK2) confer an increased risk for the development of parkinsonism. Furthermore, lysosomal dysfunction has been observed in the affected neurons of sporadic PD (sPD) patients. Given that lysosomal hydrolases actively contribute to the breakdown of impaired organelles and misfolded proteins, any compromise in lysosomal integrity could incite abnormal accumulation of proteins, including α-synuclein, the major component of Lewy bodies in PD. Clinical observations have shown that lysosomal protein levels in cerebrospinal fluid may serve as potential biomarkers for PD diagnosis and as signs of lysosomal dysfunction. In this review, we summarize the current evidence regarding lysosomal dysfunction in PD and discuss the intimate relationship between lysosomal dysfunction and pathological α-synuclein. In addition, we discuss therapeutic strategies that target lysosomes to treat PD.

Mechanisms of lysosomal tubulation and sorting driven by LRRK2

Lysosomes are dynamic cellular structures that adaptively remodel their membrane in response to stimuli, including membrane damage. Lysosomal dysfunction plays a central role in the pathobiology of Parkinson's disease (PD). Gain-of-function mutations in Leucine-rich repeat kinase 2 (LRRK2) cause familial PD and genetic variations in its locus increase the risk of developing the sporadic form of the disease. We previously uncovered a process we term LYTL (LYsosomal Tubulation/sorting driven by LRRK2), wherein membrane-damaged lysosomes generate tubules sorted into mobile vesicles. Subsequently, these vesicles interact with healthy lysosomes. LYTL is orchestrated by LRRK2 kinase activity, via the recruitment and phosphorylation of a subset of RAB GTPases. Here, we summarize the current understanding of LYTL and its regulation, as well as the unknown aspects of this process.

Absence of motor impairments or pathological changes in TMEM230 knockout rats

Parkinson's disease (PD), which is the second most common neurodegenerative disorder, is characterized by progressive movement impairment and loss of midbrain dopaminergic neurons in the substantia nigra. Although mutations in TMEM230 are linked to familial PD, the pathogenic mechanism underlying TMEM230-associated PD remains to be elucidated. To explore the effect of TMEM230 depletion in vivo, we created TMEM230 knockout rats using CRISPR-Cas9 technology. TMEM230 knockout rats did not exhibit any core features of PD, including impaired motor function, loss of dopaminergic neurons in the substantia nigra, or altered expression of proteins related to autophagy, the Rab family, or vesicular trafficking. In addition, no glial reactions were observed in TMEM230 knockout rats. These results indicate that depletion of TMEM230 may not lead to dopaminergic neuron degeneration in rats, further supporting that PD-associated TMEM230 mutations lead to dopaminergic neuron death by gain-of-toxic function.

Biphenotypic Cells and α-Synuclein Accumulation in Enteric Neurons of Leucine-Rich Repeat Kinase 2 Knockout Mice

BACKGROUND

Leucine-rich repeat kinase 2 is a molecule that is responsible for familial Parkinson's disease. Our previous findings revealed that leucine-rich repeat kinase 2 is expressed in the enteric nervous system. However, which cells in the enteric nervous system express leucine-rich repeat kinase 2 and whether leucine-rich repeat kinase 2 is associated with the structure of the enteric nervous system remain unclear. The enteric nervous system is remarkable because some patients with Parkinson's disease experience gastrointestinal symptoms before developing motor symptoms.

AIMS

We established a leucine-rich repeat kinase 2 reporter mouse model and performed immunostaining in leucine-rich repeat kinase 2 knockout mice.

METHODS

Longitudinal muscle containing the myenteric plexus prepared from leucine-rich repeat kinase 2 reporter mice was analyzed by immunostaining using anti-green fluorescent protein (GFP) antibody. Immunostaining using several combinations of antibodies characterizing enteric neurons and glial cells was performed on intestinal preparations from leucine-rich repeat kinase 2 knockout mice.

RESULTS

GFP expression in the reporter mice was predominantly in enteric glial cells rather than in enteric neurons. Immunostaining revealed that differences in the structure and proportion of major immunophenotypic cells were not apparent in the knockout mice. Interestingly, the number of biphenotypic cells expressing the neuronal and glial cell markers increased in the leucine-rich repeat kinase 2 knockout mice. Moreover, there was accumulation of α-synuclein in the knockout mice.

CONCLUSIONS

Our present findings suggest that leucine-rich repeat kinase 2 is a newly recognized molecule that potentially regulates the integrity of enteric nervous system and enteric α-synuclein accumulation.

Cell-autonomous role of leucine-rich repeat kinase in the protection of dopaminergic neuron survival

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease (PD). However, whether LRRK2 mutations cause PD and degeneration of dopaminergic (DA) neurons via a toxic gain-of-function or a loss-of-function mechanism is unresolved and has pivotal implications for LRRK2-based PD therapies. In this study, we investigate whether Lrrk2 and its functional homolog Lrrk1 play a cell-intrinsic role in DA neuron survival through the development of DA neuron-specific Lrrk conditional double knockout (cDKO) mice. Unlike Lrrk germline DKO mice, DA neuron-restricted Lrrk cDKO mice exhibit normal mortality but develop age-dependent loss of DA neurons, as shown by the progressive reduction of DA neurons in the substantia nigra pars compacta (SNpc) at the ages of 20 and 24 months. Moreover, DA neurodegeneration is accompanied with increases in apoptosis and elevated microgliosis in the SNpc as well as decreases in DA terminals in the striatum, and is preceded by impaired motor coordination. Taken together, these findings provide the unequivocal evidence for the cell-intrinsic requirement of LRRK in DA neurons and raise the possibility that LRRK2 mutations may impair its protection of DA neurons, leading to DA neurodegeneration in PD.

Deficiency of leucine-rich repeat kinase 2 aggravates thioacetamide-induced acute liver failure and hepatic encephalopathy in mice

BACKGROUND

Hepatic encephalopathy (HE) is closely associated with inflammatory responses. However, as a crucial regulator of the immune and inflammatory responses, the role of leucine-rich repeat kinase 2 (LRRK2) in the pathogenesis of HE remains unraveled. Herein, we investigated this issue in thioacetamide (TAA)-induced HE following acute liver failure (ALF).

METHODS

TAA-induced HE mouse models of LRRK2 wild type (WT), LRRK2 G2019S mutation (Lrrk2G2019S) and LRRK2 knockout (Lrrk2-/-) were established. A battery of neurobehavioral experiments was conducted. The biochemical indexes and pro-inflammatory cytokines were detected. The prefrontal cortex (PFC), striatum (STR), hippocampus (HIP), and liver were examined by pathology and electron microscopy. The changes of autophagy-lysosomal pathway and activity of critical Rab GTPases were analyzed.

RESULTS

The Lrrk2-/--HE model reported a significantly lower survival rate than the other two models (24% vs. 48%, respectively, p < 0.05), with no difference found between the WT-HE and Lrrk2G2019S-HE groups. Compared with the other groups, after the TAA injection, the Lrrk2-/- group displayed a significant increase in ammonium and pro-inflammatory cytokines, aggravated hepatic inflammation/necrosis, decreased autophagy, and abnormal phosphorylation of lysosomal Rab10. All three models reported microglial activation, neuronal loss, disordered vesicle transmission, and damaged myelin structure. The Lrrk2-/--HE mice presented no severer neuronal injury than the other genotypes.

CONCLUSIONS

LRRK2 deficiency may exacerbate TAA-induced ALF and HE in mice, in which inflammatory response is evident in the brain and aggravated in the liver. These novel findings indicate a need of sufficient clinical awareness of the adverse effects of LRRK2 inhibitors on the liver.

Cell autonomous role of leucine-rich repeat kinase in protection of dopaminergic neuron survival

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease (PD), which is the leading neurodegenerative movement disorder characterized by the progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). However, whether LRRK2 mutations cause PD and degeneration of DA neurons via a toxic gain-of-function or a loss-of-function mechanism is unresolved and has pivotal implications for LRRK2-based PD therapies. In this study, we investigate whether LRRK2 and its functional homologue LRRK1 play an essential, intrinsic role in DA neuron survival through the development of DA neuron-specific LRRK conditional double knockout (cDKO) mice. We first generated and characterized floxed LRRK1 and LRRK2 mice and then confirmed that germline deletions of the floxed LRRK1 and LRRK2 alleles result in null mutations, as evidenced by the absence of LRRK1 and LRRK2 mRNA and protein in the respective homozygous deleted mutant mice. We further examined the specificity of Cre-mediated recombination driven by the dopamine transporter-Cre (DAT-Cre) knockin (KI) allele using a GFP reporter line and confirmed that DAT-Cre-mediated recombination is restricted to DA neurons in the SNpc. Crossing these validated floxed LRRK1 and LRRK2 mice with DAT-Cre KI mice, we then generated DA neuron-restricted LRRK cDKO mice and further showed that levels of LRRK1 and LRRK2 are reduced in dissected ventral midbrains of LRRK cDKO mice. While DA neuron-restricted LRRK cDKO mice of both sexes exhibit normal mortality and body weight, they develop age-dependent loss of DA neurons in the SNpc, as demonstrated by the progressive reduction of DA neurons in the SNpc of LRRK cDKO mice at the ages of 20 and 24 months but the unaffected number of DA neurons at the age of 15 months. Moreover, DA neurodegeneration is accompanied with increases of apoptosis and elevated microgliosis in the SNpc as well as decreases of DA terminals in the striatum, and is preceded by impaired motor coordination. Taken together, these findings provide the unequivocal evidence for the importance of LRRK in DA neurons and raise the possibility that LRRK2 mutations may impair its protection of DA neurons, leading to DA neurodegeneration in PD.

The V-ATPase-ATG16L1 axis recruits LRRK2 to facilitate the lysosomal stress response

Leucine-rich repeat kinase 2 (LRRK2), a Rab kinase associated with Parkinson's disease and several inflammatory diseases, has been shown to localize to stressed lysosomes and get activated to regulate lysosomal homeostasis. However, the mechanisms of LRRK2 recruitment and activation have not been well understood. Here, we found that the ATG8 conjugation system regulates the recruitment of LRRK2 as well as LC3 onto single membranes of stressed lysosomes/phagosomes. This recruitment did not require FIP200-containing autophagy initiation complex, nor did it occur on double-membrane autophagosomes, suggesting independence from canonical autophagy. Consistently, LRRK2 recruitment was regulated by the V-ATPase-ATG16L1 axis, which requires the WD40 domain of ATG16L1 and specifically mediates ATG8 lipidation on single membranes. This mechanism was also responsible for the lysosomal stress-induced activation of LRRK2 and the resultant regulation of lysosomal secretion and enlargement. These results indicate that the V-ATPase-ATG16L1 axis serves a novel non-autophagic role in the maintenance of lysosomal homeostasis by recruiting LRRK2.

STING signaling in the brain: Molecular threats, signaling activities, and therapeutic challenges

Stimulator of interferon genes (STING) is an innate immune signaling protein critical to infections, autoimmunity, and cancer. STING signaling is also emerging as an exciting and integral part of many neurological diseases. Here, we discuss recent advances in STING signaling in the brain. We summarize how molecular threats activate STING signaling in the diseased brain and how STING signaling activities in glial and neuronal cells cause neuropathology. We also review human studies of STING neurobiology and consider therapeutic challenges in targeting STING to treat neurological diseases.

Lysosomal stress drives the release of pathogenic α-synuclein from macrophage lineage cells via the LRRK2-Rab10 pathway

α-Synuclein and LRRK2 are associated with both familial and sporadic Parkinson's disease (PD), although the mechanistic link between these two proteins has remained elusive. Treating cells with lysosomotropic drugs causes the recruitment of LRRK2 and its substrate Rab10 onto overloaded lysosomes and induces extracellular release of lysosomal contents. Here we show that lysosomal overload elicits the release of insoluble α-synuclein from macrophages and microglia loaded with α-synuclein fibrils. This release occurred specifically in macrophage lineage cells, was dependent on the LRRK2-Rab10 pathway and involved exosomes. Also, the uptake of α-synuclein fibrils enhanced the LRRK2 phosphorylation of Rab10, which was accompanied by an increased recruitment of LRRK2 and Rab10 onto lysosomal surface. Our data collectively suggest that α-synuclein fibrils taken up in lysosomes activate the LRRK2-Rab10 pathway, which in turn upregulates the extracellular release of α-synuclein aggregates, leading to a vicious cycle that could enhance α-synuclein propagation in PD pathology.

The Common Denominators of Parkinson's Disease Pathogenesis and Methamphetamine Abuse

The pervasiveness and mortality associated with methamphetamine abuse have doubled during the past decade, suggesting a possible worldwide substance use crisis. Epitomizing the pathophysiology and toxicology of methamphetamine abuse proclaims severe signs and symptoms of neurotoxic and neurobehavioral manifestations in both humans and animals. Most importantly, chronic use of this drug enhances the probability of developing neurodegenerative diseases manifolds. Parkinson's disease is one such neurological disorder, which significantly and evidently not only shares a number of toxic pathogenic mechanisms induced by methamphetamine exposure but is also interlinked both structurally and genetically. Methamphetamine-induced neurodegeneration involves altered dopamine homeostasis that promotes the aggregation of α-synuclein protofibrils in the dopaminergic neurons and drives these neurons to make them more vulnerable to degeneration, as recognized in Parkinson's disease. Moreover, the pathologic mechanisms such as mitochondrial dysfunction, oxidative stress, neuroinflammation and decreased neurogenesis detected in methamphetamine abusers dramatically resemble to what is observed in Parkinson's disease cases. Therefore, the present review comprehensively cumulates a holistic illustration of various genetic and molecular mechanisms putting across the notion of how methamphetamine administration and intoxication might lead to Parkinson's disease-like pathology and Parkinsonism.

LRRK2 aggravates kidney injury through promoting MFN2 degradation and abnormal mitochondrial integrity

Mitochondrial dysfunction is one of the key features of acute kidney injury (AKI) and associated fibrosis. Leucine-rich repeat kinase 2 (LRRK2) is highly expressed in kidneys and regulates mitochondrial homeostasis. How it functions in AKI is unclear. Herein we reported that LRRK2 was dramatically downregulated in AKI kidneys. Lrrk2-/- mice exhibited less severity of AKI when compared to wild-type counterparts with less mitochondrial fragmentation and decreased reactive oxygen species (ROS) production in proximal renal tubular cells (PTCs) due to mitofusin 2 (MFN2) accumulation. Overexpression of LRRK2 in human PTC cell lines promoted LRRK2-MKK4/JNK-dependent phosphorylation of MFN2Ser27 and subsequently ubiquitination-mediated MFN2 degradation, which in turn exaggerated mitochondrial damage upon ischemia/reperfusion (I/R) mimicry treatment. Lrrk2 deficiency also alleviated AKI-to-chronic kidney disease (CKD) transition with less fibrosis. In vivo pretreatment of LRRK2 inhibitors attenuated the severity of AKI as well as CKD, potentiating LRRK2 as a novel target to alleviate AKI and fibrosis.

The Involvement of Neuroinflammation in the Onset and Progression of Parkinson's Disease

Parkinson's disease is a neurodegenerative disease exhibiting the fastest growth in incidence in recent years. As with most neurodegenerative diseases, the pathophysiology is incompletely elucidated, but compelling evidence implicates inflammation, both in the central nervous system and in the periphery, in the initiation and progression of the disease, although it is not yet clear what triggers this inflammatory response and where it begins. Gut dysbiosis seems to be a likely candidate for the initiation of the systemic inflammation. The therapies in current use provide only symptomatic relief, but do not interfere with the disease progression. Nonetheless, animal models have shown promising results with therapies that target various vicious neuroinflammatory cascades. Translating these therapeutic strategies into clinical trials is still in its infancy, and a series of issues, such as the exact timing, identifying biomarkers able to identify Parkinson's disease in early and pre-symptomatic stages, or the proper indications of genetic testing in the population at large, will need to be settled in future guidelines.

LRRK2 suppresses lysosome degradative activity in macrophages and microglia through MiT-TFE transcription factor inhibition

Cells maintain optimal levels of lysosome degradative activity to protect against pathogens, clear waste, and generate nutrients. Here, we show that LRRK2, a protein that is tightly linked to Parkinson's disease, negatively regulates lysosome degradative activity in macrophages and microglia via a transcriptional mechanism. Depletion of LRRK2 and inhibition of LRRK2 kinase activity enhanced lysosomal proteolytic activity and increased the expression of multiple lysosomal hydrolases. Conversely, the kinase hyperactive LRRK2 G2019S Parkinson's disease mutant suppressed lysosomal degradative activity and gene expression. We identified MiT-TFE transcription factors (TFE3, TFEB, and MITF) as mediators of LRRK2-dependent control of lysosomal gene expression. LRRK2 negatively regulated the abundance and nuclear localization of these transcription factors and their depletion prevented LRRK2-dependent changes in lysosome protein levels. These observations define a role for LRRK2 in controlling lysosome degradative activity and support a model wherein LRRK2 hyperactivity may increase Parkinson's disease risk by suppressing lysosome degradative activity.

Regulation of LRRK2 mRNA stability by ATIC and its substrate AICAR through ARE-mediated mRNA decay in Parkinson's disease

Mutations in LRRK2 are the most common genetic causes of Parkinson's disease (PD). While the enzymatic activity of LRRK2 has been linked to PD, previous work has also provided support for an important role of elevated LRRK2 protein levels, independent of enzymatic activity, in PD pathogenesis. However, the mechanisms underlying the regulation of LRRK2 protein levels remain unclear. Here, we identify a role for the purine biosynthesis pathway enzyme ATIC in the regulation of LRRK2 levels and toxicity. AICAr, the precursor of ATIC substrate, regulates LRRK2 levels in a cell-type-specific manner in vitro and in mouse tissue. AICAr regulates LRRK2 levels through AUF1-mediated mRNA decay. Upon AICAr treatment, the RNA binding protein AUF1 is recruited to the AU-rich elements (ARE) of LRRK2 mRNA leading to the recruitment of the decapping enzyme complex DCP1/2 and decay of LRRK2 mRNA. AICAr suppresses LRRK2 expression and rescues LRRK2-induced dopaminergic neurodegeneration and neuroinflammation in PD Drosophila and mouse models. Together, this study provides insight into a novel regulatory mechanism of LRRK2 protein levels and function via LRRK2 mRNA decay that is distinct from LRRK2 enzymatic functions.

Effects of LRRK2 Inhibitors in Nonhuman Primates

Toxicology studies in nonhuman primates were conducted to evaluate selective, brain penetrant inhibitors of LRRK2. GNE 7915 was limited to 7-day administration in cynomolgus monkeys at 65 mg/kg/day or limited to 14 days in rhesus at 22.5 mg/kg b.i.d. due to physical signs. Compound 25 demonstrated acceptable tolerability at 50 and 225 mg/kg b.i.d. for 7 days in rhesus monkeys. MK-1468 was tolerated during 7-day administration at 100, 200 or 800 mg/kg/day or for 30-day administration at 30, 100, or 500 mg/kg b.i.d. in rhesus monkeys. The lungs revealed hypertrophy of type 2 pneumocytes, with accumulation of intra-alveolar macrophages. Transmission electron microscopy confirmed increased lamellar structures within hypertrophic type 2 pneumocytes. Hypertrophy and hyperplasia of type 2 pneumocytes with accumulation of intra-alveolar macrophages admixed with neutrophils were prominent at peripheral lungs of animals receiving compound 25 or MK-1468. Affected type 2 pneumocytes were immuno-positive for pro-surfactant C, but negative for CD11c, a marker for intra-alveolar macrophages. Accumulation of collagen within alveolar walls, confirmed by histochemical trichrome stain, accompanied changes described for compound 25 and MK-1468. Following a 12-week treatment-free interval, animals previously receiving MK-1468 for 30 days exhibited remodeling of alveolar structure and interstitial components that did not demonstrate reversibility.

Pathogenic LRRK2 compromises the subcellular distribution of lysosomes in a Rab12-RILPL1-dependent manner

Mutations in leucine-rich repeat kinase 2 (LRRK2) cause familial Parkinson's disease (PD). Recent studies have shown that LRRK2 physiologically phosphorylates several Rab family proteins including Rab12 and that this phosphorylation is accelerated by the pathogenic mutations in LRRK2, although the significance in the PD pathogenesis remains unknown. Here we examined the effect of the overexpression of LRRK2 on the distribution of organelles in cultured cells and found that lysosomes become clustered in a perinuclear region upon the overexpression of pathogenic mutant LRRK2 in a manner dependent on its kinase activity. The perinuclear clustering of lysosomes was abolished by knocking out RAB12 as well as its effector protein RILPL1. Re-expression of Rab12 in RAB12 knockout cells suggested that the phosphorylation at Ser106 of Rab12 is required for the perinuclear clustering of lysosomes. Moreover, phosphorylated Rab12 was also accumulated on the clustered lysosomes, and the phosphorylation of Rab12 increased its interaction with RILPL1, leading us to conclude that the increase in the phosphorylation of Rab12 by pathogenic LRRK2 compromised intracellular lysosomal transport via the enhanced interaction of Rab12 with RILPL1. These data suggest the involvement of abnormal regulation of lysosomal transport in the LRRK2-mediated pathogenesis of PD.

Allele-dependent interaction of LRRK2 and NOD2 in leprosy

Leprosy, caused by Mycobacterium leprae, rarely affects children younger than 5 years. Here, we studied a multiplex leprosy family that included monozygotic twins aged 22 months suffering from paucibacillary leprosy. Whole genome sequencing identified three amino acid mutations previously associated with Crohn’s disease and Parkinson’s disease as candidate variants for early onset leprosy: LRRK2 N551K, R1398H and NOD2 R702W. In genome-edited macrophages, we demonstrated that cells expressing the LRRK2 mutations displayed reduced apoptosis activity following mycobacterial challenge independently of NOD2. However, employing co-immunoprecipitation and confocal microscopy we showed that LRRK2 and NOD2 proteins interacted in RAW cells and monocyte-derived macrophages, and that this interaction was substantially reduced for the NOD2 R702W mutation. Moreover, we observed a joint effect of LRRK2 and NOD2 variants on Bacillus Calmette-Guérin (BCG)-induced respiratory burst, NF-κB activation and cytokine/chemokine secretion with a strong impact for the genotypes found in the twins consistent with a role of the identified mutations in the development of early onset leprosy.

The Development and Design Strategy of Leucine-Rich Repeat Kinase 2 Inhibitors: Promising Therapeutic Agents for Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder affecting millions of people worldwide. Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) are the most common genetic risk factor for PD. Elevated LRRK2 kinase activity is found in idiopathic and familial PD cases. LRRK2 mutations are involved in multiple PD pathogeneses, including dysregulation of mitochondrial homeostasis, ciliogenesis, etc. Here, we provide a comprehensive overview of the biological function, structure, and mutations of LRRK2. We also examine recent advances and challenges in developing LRRK2 inhibitors and address prospective protein-based targeting strategies. The binding mechanisms, structure-activity relationships, and pharmacokinetic features of inhibitors are emphasized to provide a comprehensive compendium on the rational design of LRRK2 inhibitors. We hope that this publication can serve as a guide for designing novel LRRK2 inhibitors based on the summarized facts and perspectives.

LRRK2: Genetic mechanisms vs genetic subtypes

In 2004, the identification of pathogenic variants in the LRRK2 gene across several families with autosomal dominant late-onset Parkinson's disease (PD) revolutionized our understanding of the role of genetics in PD. Previous beliefs that genetics in PD was limited to rare early-onset or familial forms of the disease were quickly dispelled. Currently, we recognize LRRK2 p.G2019S as the most common genetic cause of both sporadic and familial PD, with more than 100,000 affected carriers across the globe. The frequency of LRRK2 p.G2019S is also highly variable across populations, with some regions of Asian or Latin America reporting close to 0%, contrasting to Ashkenazi Jews or North African Berbers reporting up to 13% and 40%, respectively. Patients with LRRK2 pathogenic variants are clinically and pathologically heterogeneous, highlighting the age-related variable penetrance that also characterizes LRRK2-related disease. Indeed, the majority of patients with LRRK2-related disease are characterized by a relatively mild Parkinsonism with less motor symptoms with variable presence of α-synuclein and/or tau aggregates, with pathologic pleomorphism widely described. At a functional cellular level, it is likely that pathogenic variants mediate a toxic gain-of-function of the LRRK2 protein resulting in increased kinase activity perhaps in a cell-specific manner; by contrast, some LRRK2 variants appear to be protective reducing PD risk by decreasing the kinase activity. Therefore, employing this information to define appropriate patient populations for clinical trials of targeted kinase LRRK2 inhibition strategies is very promising and demonstrates a potential future application for PD using precision medicine.

Role of rodent models in advancing precision medicine for Parkinson's disease

With a current lack of disease-modifying treatments, an initiative toward implementing a precision medicine approach for treating Parkinson's disease (PD) has emerged. However, challenges remain in how to define and apply precision medicine in PD. To accomplish the goal of optimally targeted and timed treatment for each patient, preclinical research in a diverse population of rodent models will continue to be an essential part of the translational path to identify novel biomarkers for patient diagnosis and subgrouping, understand PD disease mechanisms, identify new therapeutic targets, and screen therapeutics prior to clinical testing. This review highlights the most common rodent models of PD and discusses how these models can contribute to defining and implementing precision medicine for the treatment of PD.

Promising biomarkers and therapeutic targets for the management of Parkinson's disease: recent advancements and contemporary research

Parkinson's disease (PD) is one of the progressive neurological diseases which affect around 10 million population worldwide. The clinical manifestation of motor symptoms in PD patients appears later when most dopaminergic neurons have degenerated. Thus, for better management of PD, the development of accurate biomarkers for the early prognosis of PD is imperative. The present work will discuss the potential biomarkers from various attributes covering biochemical, microRNA, and neuroimaging aspects (α-synuclein, DJ-1, UCH-L1, β-glucocerebrosidase, BDNF, etc.) for diagnosis, recent development in PD management, and major limitations with current and conventional anti-Parkinson therapy. This manuscript summarizes potential biomarkers and therapeutic targets, based on available preclinical and clinical evidence, for better management of PD.

Lysosomal Pathogenesis of Parkinson's Disease: Insights From LRRK2 and GBA1 Rodent Models

The discovery of mutations in LRRK2 and GBA1 that are linked to Parkinson's disease provided further evidence that autophagy and lysosome pathways are likely implicated in the pathogenic process. Their protein products are important regulators of lysosome function. LRRK2 has kinase-dependent effects on lysosome activity, autophagic efficacy and lysosomal Ca2+ signaling. Glucocerebrosidase (encoded by GBA1) is a hydrolytic enzyme contained in the lysosomes and contributes to the degradation of alpha-synuclein. PD-related mutations in LRRK2 and GBA1 slow the degradation of alpha-synuclein, thus directly implicating the dysfunction of the process in the neuropathology of Parkinson's disease. The development of genetic rodent models of LRRK2 and GBA1 provided hopes of obtaining reliable preclinical models in which to study pathogenic processes and perform drug validation studies. Here, I will review the extensive characterization of these models, their impact on understanding lysosome alterations in the course of Parkinson's disease and what novel insights have been obtained. In addition, I will discuss how these models fare with respect to the features of a "gold standard" animal models and what could be attempted in future studies to exploit LRRK2 and GBA1 rodent models in the fight against Parkinson's disease.

Early-Onset albuminuria and Associated Renal Pathology in Leucine-Rich Repeat Kinase 2 Knockout Rats

Activating mutations of the leucine-rich repeat kinase 2 (LRRK2) gene are associated with Parkinson disease (PD), prompting development of LRRK2 inhibitors as potential treatment for PD. However, kidney safety concerns have surfaced from LRRK2 knockout (KO) mice and rats and from repeat-dose studies in rodents administered LRRK2 inhibitors. To support drug development of this therapeutic target, we conducted a study of 26 weeks' duration in 2-month-old wild-type and LRRK2 KO Long-Evans Hooded rats to systematically examine the performance of urinary safety biomarkers and to characterize the nature of the morphological changes in the kidneys by light microscopy and by ultrastructural evaluation. Our data reveal the time course of early-onset albuminuria at 3 and 4 months in LRRK2 KO female and male rats, respectively. The increases in urine albumin were not accompanied by concurrent increases in serum creatinine, blood urea nitrogen, or renal safety biomarkers such as kidney injury molecule 1 or clusterin, although morphological alterations in both glomerular and tubular structure were identified by light and transmission electron microscopy at 8 months of age. Diet optimization with controlled food intake attenuated the progression of albuminuria and associated renal changes.

Leucine-rich repeat kinase 2 is protective during acute kidney injury through its activation of autophagy in podocytes

Leucine-rich repeat kinase 2 (LRRK2) is a known regulator of autophagy in a range of cell types. Here, we investigated the role of LRRK2-associated autophagy during acute kidney injury (AKI) and its underlying mechanism(s) of action. Male mice aged 8-weeks were treated with the LRRK2 inhibitor MLi-2 and exposed to lipopolysaccharide (LPS) through intraperitoneal injection or ischemia-reperfusion (IR) surgery. Mice were sacrificed 12 or 24 h post-LPS injection or IR operation and blood was collected for serum creatinine measurements. Kidney cortical tissues were collected for western blot analysis of podocyte-specific markers and autophagy-associated proteins. Renal histopathology was observed through hematoxylin-eosin staining. For cell-based assays, immortalized mouse podocytes were silenced for LRRK2 through siRNA transfection and exposed to LPS or cobalt chloride. Changes in cell viability were investigated using cell counting kit-8, flow cytometry and MTT assays. Expression of podocyte-specific markers and autophagy-associated proteins were analyzed by western blotting. We observed an increase in LRRK2 expression at 12 h post-LPS injection and IR surgery that was accompanied by enhanced autophagy. At 24 h post-treatment, both LRRK2 expression and autophagy declined. Kidney injury was most pronounced in mice treated with MLi-2. Podocytes silenced for LRRK2 showed a loss of cell viability, decreased levels of podocyte-specific protein expression and a suppression of autophagy. Together, these data reveal the protective effects of LRRK2 during AKI through enhanced podocyte autophagy and cell viability.

LRRK2 and Lipid Pathways: Implications for Parkinson's Disease

Genetic alterations in the LRRK2 gene, encoding leucine-rich repeat kinase 2, are a common risk factor for Parkinson's disease. How LRRK2 alterations lead to cell pathology is an area of ongoing investigation, however, multiple lines of evidence suggest a role for LRRK2 in lipid pathways. It is increasingly recognized that in addition to being energy reservoirs and structural entities, some lipids, including neural lipids, participate in signaling cascades. Early investigations revealed that LRRK2 localized to membranous and vesicular structures, suggesting an interaction of LRRK2 and lipids or lipid-associated proteins. LRRK2 substrates from the Rab GTPase family play a critical role in vesicle trafficking, lipid metabolism and lipid storage, all processes which rely on lipid dynamics. In addition, LRRK2 is associated with the phosphorylation and activity of enzymes that catabolize plasma membrane and lysosomal lipids. Furthermore, LRRK2 knockout studies have revealed that blood, brain and urine exhibit lipid level changes, including alterations to sterols, sphingolipids and phospholipids, respectively. In human LRRK2 mutation carriers, changes to sterols, sphingolipids, phospholipids, fatty acyls and glycerolipids are reported in multiple tissues. This review summarizes the evidence regarding associations between LRRK2 and lipids, and the functional consequences of LRRK2-associated lipid changes are discussed.

Human Induced Pluripotent Stem Cell Phenotyping and Preclinical Modeling of Familial Parkinson's Disease

Parkinson's disease (PD) is primarily idiopathic and a highly heterogenous neurodegenerative disease with patients experiencing a wide array of motor and non-motor symptoms. A major challenge for understanding susceptibility to PD is to determine the genetic and environmental factors that influence the mechanisms underlying the variations in disease-associated traits. The pathological hallmark of PD is the degeneration of dopaminergic neurons in the substantia nigra pars compacta region of the brain and post-mortem Lewy pathology, which leads to the loss of projecting axons innervating the striatum and to impaired motor and cognitive functions. While the cause of PD is still largely unknown, genome-wide association studies provide evidence that numerous polymorphic variants in various genes contribute to sporadic PD, and 10 to 15% of all cases are linked to some form of hereditary mutations, either autosomal dominant or recessive. Among the most common mutations observed in PD patients are in the genes LRRK2, SNCA, GBA1, PINK1, PRKN, and PARK7/DJ-1. In this review, we cover these PD-related mutations, the use of induced pluripotent stem cells as a disease in a dish model, and genetic animal models to better understand the diversity in the pathogenesis and long-term outcomes seen in PD patients.

Molecular Pathways Involved in LRRK2-Linked Parkinson’s Disease: A Systematic Review

Parkinson’s disease is one of the most common neurodegenerative diseases affecting the ageing population, with a prevalence that has doubled over the last 30 years. As the mechanism of the disease is not fully elucidated, the current treatments are unable to effectively prevent neurodegeneration. Studies have found that mutations in Leucine-rich-repeat-kinase 2 (LRRK2) are the most common cause of familial Parkinson’s disease (PD). Moreover, aberrant (higher) LRRK2 kinase activity has an influence in idiopathic PD as well. Hence, the aim of this review is to categorize and synthesize current information related to LRRK2-linked PD and present the factors associated with LRRK2 that can be targeted therapeutically. A systematic review was conducted using the databases PubMed, Medline, SCOPUS, SAGE, and Cochrane (January 2016 to July 2021). Search terms included “Parkinson’s disease”, “mechanism”, “LRRK2”, and synonyms in various combinations. The search yielded a total of 988 abstracts for initial review, 80 of which met the inclusion criteria. Here, we emphasize molecular mechanisms revealed in recent in vivo and in vitro studies. By consolidating the recent updates in the field of LRRK2-linked PD, researchers can further evaluate targets for therapeutic application.

Homozygous mutation of the LRRK2 ROC domain as a novel genetic model of parkinsonism

Background

Parkinson’s disease (PD) is one of the most important neurodegenerative disorders in elderly people. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are found in a large proportion of the patients with sporadic and familial PD. Mutations can occur at different locations in the LRRK2. Patients with LRRK2 ROC-COR mutations face an increased risk of typical motor symptoms of PD, along with cognitive decline. An animal model with a monogenic LRRK2 gene mutation is a suitable model for exploring the pathophysiology of PD and identifying potential drug therapies. However, the effect of homozygous (HOM) LRRK2 in PD pathophysiology is unclear.

Methods

We established human LRRK2 (hLRRK2) R1441G HOM transgenic (Tg) mice to explore the phenotype and pathological features that are associated with hLRRK2 R1441G Tg mouse models and discuss the potential clinical relevance. The open field test (OFT) was performed to examine motor and nonmotor behaviors. A CatWalk analysis system was used to study gait function. [¹⁸F]FDOPA PET was used to investigate functional changes in the nigrostriatal pathway in vivo. Transmission electron microscopy was used to examine the morphological changes in mitochondria and lysosomes in the substantia nigra.

Results

The R1441G HOM Tg mice demonstrated gait disturbance and exhibited less anxiety-related behavior and exploratory behavior than mice with hLRRK2 at 12 months old. Additionally, [¹⁸F]FDOPA PET showed a reduction in FDOPA uptake in the striatum of the HOM Tg mice. Notably, there was significant lysosome and autophagosome accumulation in the cytoplasm of dopaminergic neurons in R1441G hemizygous (HEM) and HOM mice. Moreover, it was observed using transmission electron microscopy (TEM) that the mitochondria of R1441G Tg mice were smaller than those of hLRRK2 mice.

Conclusion

This animal provides a novel HOM hLRRK2 R1441G Tg mouse model that reproduces some phenotype of Parkinsonism in terms of both motor and behavioral dysfunction. There is an increased level of mitochondrial fission and no change in the fusion process in the group of HOM hLRRK2 R1441G Tg mouse. This mutant animal model of PD might be used to study the mechanisms of mitochondrial dysfunction and explore potential new drug targets.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00844-9.

Structural Insights and Development of LRRK2 Inhibitors for Parkinson's Disease in the Last Decade

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease, characterized by the specific loss of dopaminergic neurons in the midbrain. The pathophysiology of PD is likely caused by a variety of environmental and hereditary factors. Many single-gene mutations have been linked to this disease, but a significant number of studies indicate that mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) are a potential therapeutic target for both sporadic and familial forms of PD. Consequently, the identification of potential LRRK2 inhibitors has been the focus of drug discovery. Various investigations have been conducted in academic and industrial organizations to investigate the mechanism of LRRK2 in PD and further develop its inhibitors. This review summarizes the role of LRRK2 in PD and its structural details, especially the kinase domain. Furthermore, we reviewed in vitro and in vivo findings of selected inhibitors reported to date against wild-type and mutant versions of the LRRK2 kinase domain as well as the current trends researchers are employing in the development of LRRK2 inhibitors.

Offense and Defense in Granulomatous Inflammation Disease

Granulomatous inflammation (GI) diseases are a group of chronic inflammation disorders characterized by focal collections of multinucleated giant cells, epithelioid cells and macrophages, with or without necrosis. GI diseases are closely related to microbes, especially virulent intracellular bacterial infections are important factors in the progression of these diseases. They employ a range of strategies to survive the stresses imposed upon them and persist in host cells, becoming the initiator of the fighting. Microbe-host communication is essential to maintain functions of a healthy host, so defense capacity of hosts is another influence factor, which is thought to combine to determine the result of the fighting. With the development of gene research technology, many human genetic loci were identified to be involved in GI diseases susceptibility, providing more insights into and knowledge about GI diseases. The current review aims to provide an update on the most recent progress in the identification and characterization of bacteria in GI diseases in a variety of organ systems and clinical conditions, and examine the invasion and escape mechanisms of pathogens that have been demonstrated in previous studies, we also review the existing data on the predictive factors of the host, mainly on genetic findings. These strategies may improve our understanding of the mechanisms underlying GI diseases, and open new avenues for the study of the associated conditions in the future.

LRRK2 and Proteostasis in Parkinson's Disease

Parkinson's disease is a neurodegenerative condition initially characterized by the presence of tremor, muscle stiffness and impaired balance, with the deposition of insoluble protein aggregates in Lewy's Bodies the histopathological hallmark of the disease. Although different gene variants are linked to Parkinson disease, mutations in the Leucine-Rich Repeat Kinase 2 (LRRK2) gene are one of the most frequent causes of Parkinson's disease related to genetic mutations. LRRK2 toxicity has been mainly explained by an increase in kinase activity, but alternative mechanisms have emerged as underlying causes for Parkinson's disease, such as the imbalance in LRRK2 homeostasis and the involvement of LRRK2 in aggregation and spreading of α-synuclein toxicity. In this review, we recapitulate the main LRRK2 pathological mutations that contribute to Parkinson's disease and the different cellular and therapeutic strategies devised to correct LRRK2 homeostasis. In this review, we describe the main cellular control mechanisms that regulate LRRK2 folding and aggregation, such as the chaperone network and the protein-clearing pathways such as the ubiquitin-proteasome system and the autophagic-lysosomal pathway. We will also address the more relevant strategies to modulate neurodegeneration in Parkinson's disease through the regulation of LRRK2, using small molecules or LRRK2 silencing.

Motor Impairments and Dopaminergic Defects Caused by Loss of Leucine-Rich Repeat Kinase Function in Mice

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease (PD), but the pathogenic mechanism underlying LRRK2 mutations remains unresolved. In this study, we investigate the consequence of inactivation of LRRK2 and its functional homolog LRRK1 in male and female mice up to 25 months of age using behavioral, neurochemical, neuropathological, and ultrastructural analyses. We report that LRRK1 and LRRK2 double knock-out (LRRK DKO) mice exhibit impaired motor coordination at 12 months of age before the onset of dopaminergic neuron loss in the substantia nigra (SNpc). Moreover, LRRK DKO mice develop age-dependent, progressive loss of dopaminergic terminals in the striatum. Evoked dopamine (DA) release measured by fast-scan cyclic voltammetry in the dorsal striatum is also reduced in the absence of LRRK. Furthermore, LRRK DKO mice at 20-25 months of age show substantial loss of dopaminergic neurons in the SNpc. The surviving SNpc neurons in LRRK DKO mice at 25 months of age accumulate large numbers of autophagic and autolysosomal vacuoles and are accompanied with microgliosis. Surprisingly, the cerebral cortex is unaffected, as shown by normal cortical volume and neuron number as well as unchanged number of apoptotic cells and microglia in LRRK DKO mice at 25 months. These findings show that loss of LRRK function causes impairments in motor coordination, degeneration of dopaminergic terminals, reduction of evoked DA release, and selective loss of dopaminergic neurons in the SNpc, indicating that LRRK DKO mice are unique models for better understanding dopaminergic neurodegeneration in PD.SIGNIFICANCE STATEMENT Our current study employs a genetic approach to uncover the normal function of the LRRK family in the brain during mouse life span. Our multidisciplinary analysis demonstrates a critical normal physiological role of LRRK in maintaining the integrity and function of dopaminergic terminals and neurons in the aging brain, and show that LRRK DKO mice recapitulate several key features of PD and provide unique mouse models for elucidating molecular mechanisms underlying dopaminergic neurodegeneration in PD.

Altered neural cell junctions and ion-channels leading to disrupted neuron communication in Parkinson's disease

Parkinson's disease (PD) is a neurological disorder that affects the movement of the human body. It is primarily characterized by reduced dopamine levels in the brain. The causative agent of PD is still unclear but it is generally accepted that α-synuclein has a central role to play. It is also known that gap-junctions and associated connexins are complicated structures that play critical roles in nervous system signaling and associated misfunctioning. Thus, our current article emphasizes how, alongside α-synuclein, ion-channels, gap-junctions, and related connexins, all play vital roles in influencing multiple metabolic activities of the brain during PD. It also highlights that ion-channel and gap-junction disruptions, which are primarily mediated by their structural-functional changes and alterations, have a role in PD. Furthermore, we discussed available drugs and advanced therapeutic interventions that target Parkinson's pathogenesis. In conclusion, it warrants creating better treatments for PD patients. Although, dopaminergic replenishment therapy is useful in treating neurological problems, such therapies are, however, unable to control the degeneration that underpins the disease, thereby declining their overall efficacy. This creates an additional challenge and an untapped scope for neurologists to adopt treatments for PD by targeting the ion-channels and gap-junctions, which is well-reviewed in the present article.

Dopamine Transporter, PhosphoSerine129 α-Synuclein and α-Synuclein Levels in Aged LRRK2 G2019S Knock-In and Knock-Out Mice

The G2019S mutation in leucine rich-repeat kinase 2 (LRRK2) is a major cause of familial Parkinson’s disease. We previously reported that G2019S knock-in mice manifest dopamine transporter dysfunction and phosphoSerine129 α-synuclein (pSer129 α-syn) immunoreactivity elevation at 12 months of age, which might represent pathological events leading to neuronal degeneration. Here, the time-dependence of these changes was monitored in the striatum of 6, 9, 12, 18 and 23-month-old G2019S KI mice and wild-type controls using DA uptake assay, Western analysis and immunohistochemistry. Western analysis showed elevation of membrane dopamine transporter (DAT) levels at 9 and 12 months of age, along with a reduction of vesicular monoamine transporter 2 (VMAT2) levels at 12 months. DAT uptake was abnormally elevated from 9 to up to 18 months. DAT and VMAT2 level changes were specific to the G2019S mutation since they were not observed in LRRK2 kinase-dead or knock-out mice. Nonetheless, dysfunctional DAT uptake was not normalized by acute pharmacological inhibition of LRRK2 kinase activity with MLi-2. Immunoblot analysis showed elevation of pSer129 α-syn levels in the striatum of 12-month-old G2019S KI mice, which, however, was not confirmed by immunohistochemical analysis. Instead, total α-syn immunoreactivity was found elevated in the striatum of 23-month-old LRRK2 knock-out mice. These data indicate mild changes in DA transporters and α-syn metabolism in the striatum of 12-month-old G2019S KI mice whose pathological relevance remains to be established.

Significant modulations of linc001128 and linc0938 with miR-24-3p and miR-30c-5p in Parkinson disease

Parkinson disease (PD) is the second most common neurodegenerative disease; the evidence suggests that lncRNAs and miRNAs play an important role in regulating the PD-related genes. The purpose of this research was to introduce two novel lncRNAs as the biomarker of PD diagnosis and treatment. We evaluated the expression profiles of six nodes of two regulatory networks in the PBMCs which had been got from 38 PD patients and 20 healthy individuals by qRT-PCR. Then, we compared the expression of these RNAs in both early and late stages of PD with the controls to determine if their expression could be related to the severity of disease. Further, this study investigated the direct interaction between one of the lncRNAs and target miRNA by using the dual luciferase assay. The results of the expression profiles of six nodes of the two ceRNA networks shown that linc01128, hsa-miR-24-3p and hsa-miR-30c-5p expression were significantly downregulated. While, the Linc00938, LRRK2 and ATP13A2 expression were up-regulated in the PBMC of the PD patients, in comparison to the controls. In addition, this study demonstrated that linc00938 directly sponged hsa-miR-30c-5p. The present study, therefore, for the first time, revealed two candidate lncRNAs as the biomarkers in the PD patients.

Autophagy system as a potential therapeutic target for neurodegenerative diseases

Autophagy is an evolutionally conserved process by which cytoplasmic contents including protein aggregates and damaged organelles such as mitochondria and lysosomes, are sequestered by double-membrane structure, autophagosomes, and delivered to the lysosomes for degradation. Recently, considerable efforts have been made to reveal the role of autophagy in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Huntington's disease. Impairment of autophagy aggravates the accumulation of misfolded protein and damaged organelles in neurons, while sufficient autophagic activity reduces such accumulation in nervous system and ameliorates the pathology. Here we summarize recent progress regarding the role of autophagy in several neurodegenerative diseases and the potential autophagy-associated therapies for them.

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

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

Emerging Therapeutic Strategies for Parkinson’s Disease and Future Prospects: A 2021 Update

Parkinson’s disease (PD) is a neurodegenerative disorder pathologically distinguished by degeneration of dopaminergic neurons in the substantia nigra pars compacta. Muscle rigidity, tremor, and bradykinesia are all clinical motor hallmarks of PD. Several pathways have been implicated in PD etiology, including mitochondrial dysfunction, impaired protein clearance, and neuroinflammation, but how these factors interact remains incompletely understood. Although many breakthroughs in PD therapy have been accomplished, there is currently no cure for PD, only trials to alleviate the related motor symptoms. To reduce or stop the clinical progression and mobility impairment, a disease-modifying approach that can directly target the etiology rather than offering symptomatic alleviation remains a major unmet clinical need in the management of PD. In this review, we briefly introduce current treatments and pathophysiology of PD. In addition, we address the novel innovative therapeutic targets for PD therapy, including α-synuclein, autophagy, neurodegeneration, neuroinflammation, and others. Several immunomodulatory approaches and stem cell research currently in clinical trials with PD patients are also discussed. Moreover, preclinical studies and clinical trials evaluating the efficacy of novel and repurposed therapeutic agents and their pragmatic applications with encouraging outcomes are summarized. Finally, molecular biomarkers under active investigation are presented as potentially valuable tools for early PD diagnosis.

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.

Regulation of neuronal autophagy and the implications in neurodegenerative diseases

Neurons are highly polarized and post-mitotic cells with the specific requirements of neurotransmission accompanied by high metabolic demands that create a unique challenge for the maintenance of cellular homeostasis. Thus, neurons rely heavily on autophagy that constitutes a key quality control system by which dysfunctional cytoplasmic components, protein aggregates, and damaged organelles are sequestered within autophagosomes and then delivered to the lysosome for degradation. While mature lysosomes are predominantly located in the soma of neurons, the robust, constitutive biogenesis of autophagosomes occurs in the synaptic terminal via a conserved pathway that is required to maintain synaptic integrity and function. Following formation, autophagosomes fuse with late endosomes and then are rapidly and efficiently transported by the microtubule-based cytoplasmic dynein motor along the axon toward the soma for lysosomal clearance. In this review, we highlight the recent knowledge of the roles of autophagy in neuronal health and disease. We summarize the available evidence about the normal functions of autophagy as a protective factor against neurodegeneration and discuss the mechanism underlying neuronal autophagy regulation. Finally, we describe how autophagy function is affected in major neurodegenerative diseases with a special focus on Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis.

LRRK2 signaling in neurodegeneration: two decades of progress

Leucine-rich repeat kinase 2 (LRRK2) is a complex GTPase/kinase orchestrating cytoskeletal dynamics and multiple steps of the endolysosomal pathway through interaction with a host of partners and phosphorylation of a subset of Rab GTPases. Mutations in LRRK2 cause late-onset Parkinson's disease (PD) and common variants in the locus containing LRRK2 have been associated with sporadic PD, progressive supranuclear palsy as well as a number of inflammatory diseases. This review encompasses the major discoveries in the field of LRRK2 pathobiology, from the initial gene cloning to the latest progress in LRRK2 inhibition as a promising therapeutic approach to fight neurodegeneration.

Drosophila as a model to study autophagy in neurodegenerative diseases and digestive tract

Autophagy regulates cellular homeostasis by degrading and recycling cytosolic components and damaged organelles. Disruption of autophagic flux has been shown to induce or facilitate neurodegeneration and accumulation of autophagic vesicles is overt in neurodegenerative diseases. The fruit fly Drosophila has been used as a model system to identify new factors that regulate physiology and disease. Here we provide a historical perspective of how the fly models have offered mechanistic evidence to understand the role of autophagy in neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Charcot-Marie-Tooth neuropathy, and polyglutamine disorders. Autophagy also plays a pivotal role in maintaining tissue homeostasis and protecting organism health. The gastrointestinal tract regulates organism health by modulating food intake, energy balance, and immunity. Growing evidence is strengthening the link between autophagy and digestive tract health in recent years. Here, we also discuss how the fly models have advanced the understanding of digestive physiology regulated by autophagy.

Alpha-Synuclein and Cognitive Decline in Parkinson Disease

Parkinson disease (PD) is the second most common neurodegenerative disorder in elderly people. It is characterized by the aggregation of misfolded alpha-synuclein throughout the nervous system. Aside from cardinal motor symptoms, cognitive impairment is one of the most disabling non-motor symptoms that occurs during the progression of the disease. The accumulation and spreading of alpha-synuclein pathology from the brainstem to limbic and neocortical structures is correlated with emerging cognitive decline in PD. This review summarizes the genetic and pathophysiologic relationship between alpha-synuclein and cognitive impairment in PD, together with potential areas of biomarker advancement.

LRRK2 along the Golgi and lysosome connection: a jamming situation

Parkinson's disease (PD) is an age-related neurodegenerative disorder, clinically characterized by bradykinesia, rigidity, and resting tremor. Leucine-Rich Repeat Kinase 2 (LRRK2) is a large, multidomain protein containing two enzymatic domains. Missense mutations in its coding sequence are amongst the most common causes of familial PD. The physiological and pathological impact of LRRK2 is still obscure, but accumulating evidence supports a role for LRRK2 in membrane and vesicle trafficking, mainly functioning in the endosome-recycling system, (synaptic) vesicle trafficking, autophagy, and lysosome biology. LRRK2 binds and phosphorylates key regulators of the endomembrane systems and is dynamically localized at the Golgi. The impact of LRRK2 on the Golgi may reverberate throughout the entire endomembrane system and occur in multiple intersecting pathways, including endocytosis, autophagy, and lysosomal function. This would lead to overall dysregulation of cellular homeostasis and protein catabolism, leading to neuronal dysfunction and accumulation of toxic protein species, thus underlying the possible neurotoxic effect of LRRK2 mutations causing PD.