Dopamine signaling modulates microglial NLRP3 inflammasome activation: implications for Parkinson's disease.

BACKGROUND: Parkinson's disease (PD) is characterized by the loss of nigral dopaminergic neurons leading to impaired striatal dopamine signaling, α-synuclein- (α-syn-) rich inclusions, and neuroinflammation. Degenerating neurons are surrounded by activated microglia with increased secretion of interleukin-1ß (IL-1ß), driven largely by the NLRP3 inflammasome. A critical role for microglial NLRP3 inflammasome activation in the progression of both dopaminergic neurodegeneration and α-syn pathology has been demonstrated in parkinsonism mouse models. Fibrillar α-syn activates this inflammasome in mouse and human macrophages, and we have shown previously that the same holds true for primary human microglia. Dopamine blocks microglial NLRP3 inflammasome activation in the MPTP model, but its effects in this framework, highly relevant to PD, remain unexplored in primary human microglia and in other in vivo parkinsonism models. METHODS: Biochemical techniques including quantification of IL-1ß secretion and confocal microscopy were employed to gain insight into dopamine signaling-mediated inhibition of the NLRP3 inflammasome mechanism in primary human microglia and the SYN120 transgenic mouse model. Dopamine and related metabolites were applied to human microglia together with various inflammasome activating stimuli. The involvement of the receptors through which these catecholamines were predicted to act were assessed with agonists in both species. RESULTS: We show in primary human microglia that dopamine, L-DOPA, and high extracellular K+, but not norepinephrine and epinephrine, block canonical, non-canonical, and α-syn-mediated NLRP3 inflammasome-driven IL-1ß secretion. This suggests that dopamine acts as an inflammasome inhibitor in human microglia. Accordingly, we provide evidence that dopamine exerts its inhibitory effect through dopamine receptor D1 and D2 (DRD1 and DRD2) signaling. We also show that aged mice transgenic for human C-terminally truncated (1-120) α-syn (SYN120 tg mice) display increased NLRP3 inflammasome activation in comparison to WT mice that is diminished upon DRD1 agonism. CONCLUSIONS: Dopamine inhibits canonical, non-canonical, and α-syn-mediated activation of the NLRP3 inflammasome in primary human microglia, as does high extracellular K+. We suggest that dopamine serves as an endogenous repressor of the K+ efflux-dependent microglial NLRP3 inflammasome activation that contributes to dopaminergic neurodegeneration in PD, and that this reciprocation may account for the specific vulnerability of these neurons to disease pathology.

α-Synuclein evokes NLRP3 inflammasome-mediated IL-1ß secretion from primary human microglia.

Synucleinopathies such as Parkinson's disease (PD) are hallmarked by α-synuclein (α-syn) pathology and neuroinflammation. This neuroinflammation involves activated microglia with increased secretion of interleukin-1ß (IL-1ß). The main driver of IL-1ß secretion from microglia is the NLRP3 inflammasome. A critical link between microglial NLRP3 inflammasome activation and the progression of both α-syn pathology and dopaminergic neurodegeneration has been identified in various PD models in vivo. α-Syn is known to activate the microglial NLRP3 inflammasome in murine models, but its relationship to this inflammasome in human microglia has not been established. In this study, IL-1ß secretion from primary mouse microglia induced by α-syn fibrils was dependent on NLRP3 inflammasome assembly and caspase-1 activity, as previously reported. We show that exposure of primary human microglia to α-syn fibrils also resulted in significant IL-1ß secretion that was dependent on inflammasome assembly and involved the recruitment of caspase-1 protein to inflammasome scaffolds as visualized with superresolution microscopy. While canonical IL-1ß secretion was clearly dependent on caspase-1 enzymatic activity, this activity was less clearly involved for α-syn-induced IL-1ß secretion from human microglia. This work presents similarities between primary human and mouse microglia in the mechanisms of activation of the NLRP3 inflammasome by α-syn, but also highlights evidence to suggest that there may be a difference in the requirement for caspase-1 activity in IL-1ß output. The data represent a novel characterization of PD-related NLRP3 inflammasome activation in primary human microglia and further implicate this mechanism in the pathology underlying PD.

Aß-oligomer uptake and the resulting inflammatory response in adult human astrocytes are precluded by an anti-Aß single chain variable fragment in combination with an apoE mimetic peptide.

An imbalance between production and clearance of soluble amyloid-ß (Aß) initiates the pathological process in sporadic Alzheimer's disease (AD). Aß-specific antibodies seemed promising as therapeutic option in AD mouse models. In patients, however, vascular side-effects and Aß-antibody complex-induced microglial and/or perivascular macrophage inflammatory responses were encountered. To prevent inflammatory reactions, we designed a single chain variable fragment (scFv-h3D6), based on monoclonal antibody bapineuzumab (mAb-h3D6), but lacking the Fc region. ScFv-h3D6 reduced Aß-oligomer burden and prevented AD-associated behavioral and cellular changes in 3xTg-AD mice. As scFv-h3D6 lacks the Fc-tail, it cannot enhance Fc-receptor mediated Aß clearance by microglia and probably exerts its beneficial effects in 3xTg-AD mice through other mechanisms. ScFv-h3D6 restored the increased apoE and apoJ levels in 3xTg-AD brains back to normal. ApoE and apoJ influence cholesterol transport, Aß aggregation and clearance, and their genetic variants are risk factors for sporadic AD. Astrocytes are constitutive scavengers of soluble Aß from the CNS. We previously found apoE and apoJ to inhibit Aß uptake by adult human astrocytes, in vitro, and thus to potentially protect astrocytes from Aß cytotoxicity. In the present study, scFv-h3D6 and mAb-h3D6 inhibited Aß-oligomer uptake by adult human astrocytes. ApoE- and apoJ- mimetic peptides (MP) affected Aß uptake as well as Aß-induced cytokine release similar to intact apoE and apoJ, without interfering with the strong inhibitory effects of scFv-h3D6 on Aß-oligomer uptake. These results suggest that combining Aß-specific scFv and apoE-MP, that inhibits Aß oligomer-induced cytokine release by astrocytes, could offer advantages over currently used therapeutics.

Quantification of clusterin in paired cerebrospinal fluid and plasma samples.

BACKGROUND: Clusterin (ApoJ) is an amyloid-associated protein and plays an important role in Alzheimer's disease (AD) pathology. Recent genome-wide association studies have indicated that certain genetic variants increase the risk of developing AD. To determine if the expression of clusterin is different in AD patients, both systemically and locally in the brain, differs between (subgroups of) AD patients and non-AD cases, an assay available that detects clusterin in both plasma and cerebrospinal fluid (CSF) with equal sensitivity would be helpful. METHODS: We compared four different commercially available antibodies in their ability to detect recombinant clusterin and immune-purified human clusterin. Specificity was tested on western blot and in ELISA systems, and selection was based on the ability to detect clusterin in CSF and plasma. A sandwich ELISA was developed and validated with monoclonal antibody G7 as capture, and rabbit polyclonal (Alexis) antibodies for detection. RESULTS: Our ELISA measured clusterin concentrations in plasma and CSF with dynamic ranges of 2-70 mg/L and 0.5-40 mg/L, respectively. The assays showed 99.8% recovery in CSF and 97% recovery in plasma. Intra-assay coefficient of variation was 1.4% and inter-assay 8.8%. The assay shows no cross-reactivity with related apolipoproteins. Clusterin quantification is dependent on the type of storage for plasma samples. A single freeze/thaw cycle caused fluctuations of clusterin concentrations in plasma, while clusterin in CSF is stable for up to five cycles. CONCLUSIONS: We have successfully developed a clusterin ELISA that reliably measures CSF and plasma clusterin concentrations. In a pilot study, all samples gave results that were well within the dynamic range of the assay, with low variations. Freshly stored plasma samples are crucial for accurate clusterin quantification.

Genetic screening of a Dutch population with isolated GH deficiency (IGHD).

OBJECTIVE: Five per cent to 30% of cases of idiopathic isolated GH deficiency (IGHD) have first-degree relatives with short stature, which is suggestive of a genetic aetiology. The HYPOPIT study aimed to obtain an overall picture of gene encoding pituitary GH (GH1) and gene encoding GH releasing hormone-receptor (GHRHR) defects in a Dutch IGHD cohort and to relate them with clinical parameters. DESIGN, PATIENTS AND MEASUREMENTS: Genetic analysis was performed of exons and exon-intron boundaries of GH1 and GHRHR in 89 Caucasian IGHD patients from 81 families, using denaturing high-performance liquid chromatography (dHPLC), DNA sequencing and multiplex ligation-dependent probe amplification. In addition, we performed functional studies on novel identified GH1 exonic variants. RESULTS: Five different heterozygous GH1 mutations were present in 5 out of 81 participating families (6.1%), whereas no mutations in GHRHR were found. Patients with IGF-I SDS < -4.0 and peak GH levels < 5.7 mU/l had a mutation frequency of 40%, in contrast to 6.8% in patients with only one criterion, and 0.0% in patients with none of these criteria (P = 0.00007). Five new GH1 and two GHRHR variants were also identified; two of them (GH1 F92L and D153H) caused a marked reduction of GH secretion in vitro. CONCLUSION: GH1 and GHRHR mutations are rare in Caucasian Dutch IGHD patients, which suggests the involvement of other genetic determinants in the aetiology of IGHD. IGF-I < -4.0 and peak GH levels < 5.7 mU/l are strong predictors of GH1 mutations in the studied population.