Insights into GBA Parkinson's disease pathology and therapy with induced pluripotent stem cell model systems.

While the link between GBA and Parkinson's disease (PD) was initially unexpected, it is now well established that GBA mutations are the most frequent genetic risk for PD. GBA has also been linked to sporadic PD, dementia with Lewy bodies, and ageing. Thus, GBA represents a promising target to counteract brain disease and the age-related decline of lysosomal function. The exact mechanisms involved in the risk of developing PD in GBA mutation carriers are still unclear and research in this field has faced the major challenge of a lack of proper modeling systems. Induced pluripotent stem cells (iPSCs) as well as advances in disease modeling and genome editing have facilitated studies of human brain disease. With regard to GBA-PD, iPSCs offer several advantages including the possibility of investigating sphingolipid (SPL) biology in relevant cells, the role of dopamine metabolism as well as non-cell autonomous mechanisms that are likely involved in the disease process. This review will summarize findings that emerged from iPSC-based studies in the context of GBA-PD pathology and therapy. We also highlight current advantages and challenges of stem cell models for neurological disease modeling and drug discovery.

Mitochondrial Antigen Presentation: A Vacuolar Path to Autoimmunity in Parkinson's Disease.

Mitochondrial antigens can be presented by MHC molecules and initiate adaptive immune responses but the mechanisms of mitochondrial antigen presentation (MitAP) have remained mostly unknown. A recent study proposes a new model whereby MitAP is mediated by a vesicle transport pathway that is suppressed by the Parkinson's disease (PD) associated proteins PTEN-induced putative kinase 1 (PINK1) and Parkin. This discovery brings a new perspective on the link between mitochondrial dysfunction and autoimmunity in PD.

Refined clinical scoring in comparative EAE studies does not enhance the chance to observe statistically significant differences.

Considering the 3R rules of animal experimentation, we asked if refined scoring of experimental autoimmune encephalomyelitis (EAE) in mice improves documentation of clinical EAE allowing to perform a more powerful statistical analysis. Surprisingly, refined EAE scoring failed to improve statistical outcome comparing the overall disease courses between two groups of mice.

Neuronopathic Gaucher disease models reveal defects in cell growth promoted by Hippo pathway activation.

Gaucher Disease (GD), the most common lysosomal disorder, arises from mutations in the GBA1 gene and is characterized by a wide spectrum of phenotypes, ranging from mild hematological and visceral involvement to severe neurological disease. Neuronopathic patients display dramatic neuronal loss and increased neuroinflammation, whose molecular basis are still unclear. Using a combination of Drosophila dGBA1b loss-of-function models and GD patient-derived iPSCs differentiated towards neuronal precursors and mature neurons we showed that different GD- tissues and neuronal cells display an impairment of growth mechanisms with an increased cell death and reduced proliferation. These phenotypes are coupled with the downregulation of several Hippo transcriptional targets, mainly involved in cells and tissue growth, and YAP exclusion from nuclei. Interestingly, Hippo knock-down in the GBA-KO flies rescues the proliferative defect, suggesting that targeting the Hippo pathway can be a promising therapeutic approach to neuronopathic GD.

Glucocerebrosidase is imported into mitochondria and preserves complex I integrity and energy metabolism.

Mutations in GBA1, the gene encoding the lysosomal enzyme ß-glucocerebrosidase (GCase), which cause Gaucher's disease, are the most frequent genetic risk factor for Parkinson's disease (PD). Here, we employ global proteomic and single-cell genomic approaches in stable cell lines as well as induced pluripotent stem cell (iPSC)-derived neurons and midbrain organoids to dissect the mechanisms underlying GCase-related neurodegeneration. We demonstrate that GCase can be imported from the cytosol into the mitochondria via recognition of internal mitochondrial targeting sequence-like signals. In mitochondria, GCase promotes the maintenance of mitochondrial complex I (CI) integrity and function. Furthermore, GCase interacts with the mitochondrial quality control proteins HSP60 and LONP1. Disease-associated mutations impair CI stability and function and enhance the interaction with the mitochondrial quality control machinery. These findings reveal a mitochondrial role of GCase and suggest that defective CI activity and energy metabolism may drive the pathogenesis of GCase-linked neurodegeneration.

LRRK2 kinase activity regulates GCase level and enzymatic activity differently depending on cell type in Parkinson's disease.

Leucine-rich repeat kinase 2 (LRRK2) is a kinase involved in different cellular functions, including autophagy, endolysosomal pathways, and immune function. Mutations in LRRK2 cause autosomal-dominant forms of Parkinson's disease (PD). Heterozygous mutations in GBA1, the gene encoding the lysosomal enzyme glucocerebrosidase (GCase), are the most common genetic risk factors for PD. Moreover, GCase function is altered in idiopathic PD and in other genetic forms of the disease. Recent work suggests that LRRK2 kinase activity can regulate GCase function. However, both a positive and a negative correlation have been described. To gain insights into the impact of LRRK2 on GCase, we performed a comprehensive analysis of GCase levels and activity in complementary LRRK2 models, including (i) LRRK2 G2019S knock in (GSKI) mice, (ii) peripheral blood mononuclear cell (PBMCs), plasma, and fibroblasts from PD patients carrying LRRK2 G2019S mutation, (iii) patient iPSCs-derived neurons; (iv) endogenous and overexpressed cell models. In some of these models we found a positive correlation between the activities of LRRK2 and GCase, which was further confirmed in cell lines with genetic and pharmacological manipulation of LRRK2 kinase activity. GCase protein level is reduced in GSKI brain tissues and in G2019S iPSCs-derived neurons, but increased in fibroblasts and PBMCs from patients, suggesting cell-type-specific effects. Overall, our study indicates that LRRK2 kinase activity affects both the levels and the catalytic activity of GCase in a cell-type-specific manner, with important implications in the context of therapeutic application of LRRK2 inhibitors in GBA1-linked and idiopathic PD.

Progresses in both basic research and clinical trials of NAD+ in Parkinson's disease.

The decline of nicotinamide adenine dinucleotide (NAD+) levels is a hallmark of aging in multiple organisms and tissues, including the human brain. Hence, agents that increase intracellular NAD+ could have beneficial effects in aging and age-related neurodegenerative diseases. Disturbances in NAD+ metabolism have also been observed in Parkinson's disease (PD), supporting a link between neuronal bioenergetics failure and disease pathogenesis. Here, we review emerging findings revealing key roles for NAD+ and related metabolites in experimental models of dopaminergic neurodegeneration and in PD patients. We discuss how increased NAD+ levels might ameliorate disease phenotypes by restoring neuronal mitochondrial energy metabolism, promoting cellular proteostasis, and modulating the immune system. Finally, we describe ongoing clinical trials targeting NAD+ in PD and highlight the need for further investigations to better delineate the association between NAD+, brain aging and disease, and optimal strategies for efficiently and safely raising NAD+ levels. A more comprehensive understanding of the basic mechanisms linking NAD+, energy metabolism, and PD, and of the impact of life-long NAD+ targeting strategies, are critical to inform future clinical applications.

Claudin-12 is not required for blood-brain barrier tight junction function.

BACKGROUND: The blood-brain barrier (BBB) ensures central nervous system (CNS) homeostasis by strictly controlling the passage of molecules and solutes from the bloodstream into the CNS. Complex and continuous tight junctions (TJs) between brain endothelial cells block uncontrolled paracellular diffusion of molecules across the BBB, with claudin-5 being its dominant TJs protein. However, claudin-5 deficient mice still display ultrastructurally normal TJs, suggesting the contribution of other claudins or tight-junction associated proteins in establishing BBB junctional complexes. Expression of claudin-12 at the BBB has been reported, however the exact function and subcellular localization of this atypical claudin remains unknown. METHODS: We created claudin-12-lacZ-knock-in C57BL/6J mice to explore expression of claudin-12 and its role in establishing BBB TJs function during health and neuroinflammation. We furthermore performed a broad standardized phenotypic check-up of the mouse mutant. RESULTS: Making use of the lacZ reporter allele, we found claudin-12 to be broadly expressed in numerous organs. In the CNS, expression of claudin-12 was detected in many cell types with very low expression in brain endothelium. Claudin-12lacZ/lacZ C57BL/6J mice lacking claudin-12 expression displayed an intact BBB and did not show any signs of BBB dysfunction or aggravated neuroinflammation in an animal model for multiple sclerosis. Determining the precise localization of claudin-12 at the BBB was prohibited by the fact that available anti-claudin-12 antibodies showed comparable detection and staining patterns in tissues from wild-type and claudin-12lacZ/lacZ C57BL/6J mice. CONCLUSIONS: Our present study thus shows that claudin-12 is not essential in establishing or maintaining BBB TJs integrity. Claudin-12 is rather expressed in cells that typically lack TJs suggesting that claudin-12 plays a role other than forming classical TJs. At the same time, in depth phenotypic screening of clinically relevant organ functions of claudin-12lacZ/lacZ C57BL/6J mice suggested the involvement of claudin-12 in some neurological but, more prominently, in cardiovascular functions.

Claudin-3-deficient C57BL/6J mice display intact brain barriers.

The tight junction protein claudin-3 has been identified as a transcriptional target of the Wnt/ß-catenin signaling pathway regulating blood-brain barrier (BBB) maturation. In neurological disorders loss of claudin-3 immunostaining is observed at the compromised BBB and blood-cerebrospinal fluid barrier (BCSFB). Although these observations support a central role of claudin-3 in regulating brain barriers' tight junction integrity, expression of claudin-3 at the brain barriers has remained a matter of debate. This prompted us to establish claudin-3-/- C57BL/6J mice to study the role of claudin-3 in brain barrier integrity in health and neuroinflammation. Bulk and single cell RNA sequencing and direct comparative qRT-PCR analysis of brain microvascular samples from WT and claudin-3-/- mice show beyond doubt that brain endothelial cells do not express claudin-3 mRNA. Detection of claudin-3 protein at the BBB in vivo and in vitro is rather due to junctional reactivity of anti-claudin-3 antibodies to an unknown antigen still detected in claudin-3-/- brain endothelium. We confirm expression and junctional localization of claudin-3 at the BCSFB of the choroid plexus. Our study clarifies that claudin-3 is not expressed at the BBB and shows that absence of claudin-3 does not impair brain barrier function during health and neuroinflammation in C57BL/6J mice.

Publisher Correction: Claudin-3-deficient C57BL/6J mice display intact brain barriers.

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The NAD+ Precursor Nicotinamide Riboside Rescues Mitochondrial Defects and Neuronal Loss in iPSC and Fly Models of Parkinson's Disease.

While mitochondrial dysfunction is emerging as key in Parkinson's disease (PD), a central question remains whether mitochondria are actual disease drivers and whether boosting mitochondrial biogenesis and function ameliorates pathology. We address these questions using patient-derived induced pluripotent stem cells and Drosophila models of GBA-related PD (GBA-PD), the most common PD genetic risk. Patient neurons display stress responses, mitochondrial demise, and changes in NAD+ metabolism. NAD+ precursors have been proposed to ameliorate age-related metabolic decline and disease. We report that increasing NAD+ via the NAD+ precursor nicotinamide riboside (NR) significantly ameliorates mitochondrial function in patient neurons. Human neurons require nicotinamide phosphoribosyltransferase (NAMPT) to maintain the NAD+ pool and utilize NRK1 to synthesize NAD+ from NAD+ precursors. Remarkably, NR prevents the age-related dopaminergic neuronal loss and motor decline in fly models of GBA-PD. Our findings suggest NR as a viable clinical avenue for neuroprotection in PD and other neurodegenerative diseases.