Targeting of Lysosomal Pathway Genes for Parkinson's Disease Modification: Insights From Cellular and Animal Models

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.

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 Potentiality of Natural Products and Herbal Medicine as Novel Medications for Parkinson's Disease: A Promising Therapeutic Approach

As the global population ages, the prevalence of Parkinson's disease (PD) is steadily on the rise. PD demonstrates chronic and progressive characteristics, and many cases can transition into dementia. This increases societal and economic burdens, emphasizing the need to find effective treatments. Among the widely recognized causes of PD is the abnormal accumulation of proteins, and autophagy dysfunction accelerates this accumulation. The resultant Lewy bodies are also commonly found in Alzheimer's disease patients, suggesting an increased potential for the onset of dementia. Additionally, the production of free radicals due to mitochondrial dysfunction contributes to neuronal damage and degeneration. The activation of astrocytes and the M1 phenotype of microglia promote damage to dopamine neurons. The drugs currently used for PD only delay the clinical progression and exacerbation of the disease without targeting its root cause, and come with various side effects. Thus, there is a demand for treatments with fewer side effects, with much potential offered by natural products. In this study, we reviewed a total of 14 articles related to herbal medicines and natural products and investigated their relevance to possible PD treatment. The results showed that the reviewed herbal medicines and natural products are effective against lysosomal disorder, mitochondrial dysfunction, and inflammation, key mechanisms underlying PD. Therefore, natural products and herbal medicines can reduce neurotoxicity and might improve both motor and non-motor symptoms associated with PD. Furthermore, these products, with their multi-target effects, enhance bioavailability, inhibit antibiotic resistance, and might additionally eliminate side effects, making them good alternative therapies for PD treatment.

Genetic Evidence for Endolysosomal Dysfunction in Parkinson’s Disease: A Critical Overview

Parkinson’s disease (PD) is the second most common neurodegenerative disorder in the aging population, and no disease-modifying therapy has been approved to date. The pathogenesis of PD has been related to many dysfunctional cellular mechanisms, however, most of its monogenic forms are caused by pathogenic variants in genes involved in endolysosomal function (LRRK2, VPS35, VPS13C, and ATP13A2) and synaptic vesicle trafficking (SNCA, RAB39B, SYNJ1, and DNAJC6). Moreover, an extensive search for PD risk variants revealed strong risk variants in several lysosomal genes (e.g., GBA1, SMPD1, TMEM175, and SCARB2) highlighting the key role of lysosomal dysfunction in PD pathogenesis. Furthermore, large genetic studies revealed that PD status is associated with the overall “lysosomal genetic burden”, namely the cumulative effect of strong and weak risk variants affecting lysosomal genes. In this context, understanding the complex mechanisms of impaired vesicular trafficking and dysfunctional endolysosomes in dopaminergic neurons of PD patients is a fundamental step to identifying precise therapeutic targets and developing effective drugs to modify the neurodegenerative process in PD.

Genetic variations in GBA1 and LRRK2 genes: Biochemical and clinical consequences in Parkinson disease

Variants in the GBA1 and LRRK2 genes are the most common genetic risk factors associated with Parkinson disease (PD). Both genes are associated with lysosomal and autophagic pathways, with the GBA1 gene encoding for the lysosomal enzyme, glucocerebrosidase (GCase) and the LRRK2 gene encoding for the leucine-rich repeat kinase 2 enzyme. GBA1-associated PD is characterized by earlier age at onset and more severe non-motor symptoms compared to sporadic PD. Mutations in the GBA1 gene can be stratified into severe, mild and risk variants depending on the clinical presentation of disease. Both a loss- and gain- of function hypothesis has been proposed for GBA1 variants and the functional consequences associated with each variant is often linked to mutation severity. On the other hand, LRRK2-associated PD is similar to sporadic PD, but with a more benign disease course. Mutations in the LRRK2 gene occur in several structural domains and affect phosphorylation of GTPases. Biochemical studies suggest a possible convergence of GBA1 and LRRK2 pathways, with double mutant carriers showing a milder phenotype compared to GBA1-associated PD. This review compares GBA1 and LRRK2-associated PD, and highlights possible genotype-phenotype associations for GBA1 and LRRK2 separately, based on biochemical consequences of single variants.

LRRK2, GBA and their interaction in the regulation of autophagy: implications on therapeutics in Parkinson's disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) and glucocerebrosidase (GBA) represent two most common genetic causes of Parkinson’s disease (PD). Both genes are important in the autophagic-lysosomal pathway (ALP), defects of which are associated with α-synuclein (α-syn) accumulation. LRRK2 regulates macroautophagy via activation of the mitogen activated protein kinase/extracellular signal regulated protein kinase (MAPK/ERK) kinase (MEK) and the calcium-dependent adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathways. Phosphorylation of Rab GTPases by LRRK2 regulates lysosomal homeostasis and endosomal trafficking. Mutant LRRK2 impairs chaperone-mediated autophagy, resulting in α-syn binding and oligomerization on lysosomal membranes. Mutations in GBA reduce glucocerebrosidase (GCase) activity, leading to glucosylceramide accumulation, α-syn aggregation and broad autophagic abnormalities. LRRK2 and GBA influence each other: GCase activity is reduced in LRRK2 mutant cells, and LRRK2 kinase inhibition can alter GCase activity in GBA mutant cells. Clinically, LRRK2 G2019S mutation seems to modify the effects of GBA mutation, resulting in milder symptoms than those resulting from GBA mutation alone. However, dual mutation carriers have an increased risk of PD and earlier age of onset compared with single mutation carriers, suggesting an additive deleterious effect on the initiation of PD pathogenic processes. Crosstalk between LRRK2 and GBA in PD exists, but its exact mechanism is unclear. Drugs that inhibit LRRK2 kinase or activate GCase are showing efficacy in pre-clinical models. Since LRRK2 kinase and GCase activities are also altered in idiopathic PD (iPD), it remains to be seen if these drugs will be useful in disease modification of iPD.