Loss of dopaminoreceptive neuron causes L-dopa resistant parkinsonism in tauopathy

Widespread Distribution of α-Synuclein Oligomers in LRRK2-related Parkinson's Disease

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial and sporadic Parkinson's disease (PD). While the clinical features of LRRK2-PD patients resemble those of typical PD, there are significant differences in the pathological findings. The pathological hallmark of definite PD is the presence of α-synuclein (αSYN)-positive Lewy-related pathology; however, approximately half of LRRK2-PD cases do not have Lewy-related pathology. Lewy-related pathology is a late-stage αSYN aggregation that can be visualized with hematoxylin and eosin stains or conventional immunohistochemistry (IHC). Increasing evidence has indicated that αSYN oligomers, which represent the early-stage of αSYN aggregation, may have neurotoxicity. Visualization of αSYN oligomers requires specialized staining techniques, such as αSYN-proximity ligation assay (PLA). The distribution and severity of αSYN oligomers in the human brain of LRRK2-PD patients remain unknown. In this study, we performed phosphorylated αSYN-IHC and αSYN-PLA staining on postmortem brain sections of patients with three pathogenic LRRK2 mutants: p.G2019S (n=5), p.I2020T (n=5), and p.R1441C (n=4). The severity of Lewy-related pathology and αSYN oligomers were assessed semi-quantitatively in the brainstem, limbic lobe, basal ganglia, and cerebral cortex. αSYN oligomers were detected in LRRK2-PD cases even in cases without Lewy-related pathology; a negative correlation was observed between Lewy-related pathology and αSYN oligomers (r=-0.26 [-0.39, -0.12]; P<0.0001). Our findings suggest that αSYN oligomers may represent a common pathological feature of LRRK2-PD. Notably, patients harboring p.G2019S and p.I2020T had significantly higher levels of αSYN oligomers in those without Lewy-related pathology compared to those with Lewy-related pathology. These cases also had a trend toward shorter disease duration. These results imply that in LRRK2-PD, αSYN oligomers may initially accumulate in the brain but do not progress to form Lewy-related pathology. The present study suggests that targeting αSYN oligomers may be a therapeutic strategy for LRRK2-PD even if there is no Lewy-related pathology.

Directionality bias underpins divergent spatiotemporal progression of Alzheimer-related tauopathy in mouse models

Mounting evidence implicates trans-synaptic connectome-based spread as a shared mechanism behind different tauopathic conditions, yet also suggests there is divergent spatiotemporal progression between them. A potential parsimonious explanation for this apparent contradiction could be that different conditions incur differential rates and directional biases in tau transmission along fiber tracts. In this meta-analysis we closely examined this hypothesis and quantitatively tested it using spatiotemporal tau pathology patterns from 11 distinct models across 4 experimental studies. For this purpose, we extended a network-based spread model by incorporating net directionality along the connectome. Our data unambiguously supports the directional transmission hypothesis. First, retrograde bias is an unambiguously better predictor of tau progression than anterograde bias. Second, while spread exhibits retrograde character, our best-fitting biophysical models incorporate the mixed effects of both retrograde- and anterograde-directed spread, with notable tau-strain-specific differences. We also found a nontrivial association between directionality bias and tau strain aggressiveness, with more virulent strains exhibiting less retrograde character. Taken together, our study implicates directional transmission bias in tau transmission along fiber tracts as a general feature of tauopathy spread and a strong candidate explanation for the diversity of spatiotemporal tau progression between conditions. This simple and parsimonious mechanism may potentially fill a critical gap in our knowledge of the spatiotemporal ramification of divergent tauopathies.

miR-425 deficiency promotes necroptosis and dopaminergic neurodegeneration in Parkinson's disease

A major hallmark of Parkinson’s disease (PD) is the degeneration of dopaminergic neurons in the substantia nigra, and the causative mechanism is thought to be the activation of programmed neuronal death. Necroptosis is a regulated process of cell death triggered by RIPK1. Although the pathophysiology of PD has been studied extensively, the cellular mechanism underlying dopaminergic neuron death remains unclear. In this study, we detected a specific miRNA, miR-425, in response to MPTP toxicity and dopaminergic degeneration. In MPTP-treated mice, we observed necroptosis activation and miR-425 deficiency in the substantia nigra, which is correlated with dopaminergic neuron loss. This miRNA targeted RIPK1 transcripts and promoted the phosphorylation of MLKL and necroptosis. Similarly, in the brains of PD patients, miR-425 deficiency and necroptosis activation were also confirmed in dopaminergic neuron. Furthermore, we found that genetic knockdown of miR-425 aggravated MPTP-induced motor deficits and dopaminergic neurodegeneration via early upregulation of necroptotic genes. Intracerebral miR-425 mimics (AgomiR-425) treatment attenuated necroptosis activation and dopaminergic neuron loss, and improved locomotor behaviors. In conclusion, our study suggests that miR-425 deficiency triggers necroptosis of dopaminergic neurons, and targeting miR-425 in MPTP-treated mice restored dysfunctional dopaminergic neurodegeneration and ameliorated behavioral deficits. These findings identify brain delivery of miR-425 as a potential therapeutic approach for the treatment of PD.

Increased tau phosphorylation follows impeded dopamine clearance in a P301L and novel P301L/COMT-deleted (DM) tau mouse model

In Alzheimer's disease, the phosphorylation of tau is a critical event preceding the formation of neurofibrillary tangles. Previous work exploring the impact of a dopamine blocking antipsychotic on tau phosphorylation in a tau transgenic model suggested that extracellular dopamine may play a regulatory role in the phosphorylation state of tau. In order to test this hypothesis, and in order to develop a mouse model of impaired dopamine metabolism and tauopathy, an extant P301L transgenic tau model of Alzheimer's disease and a novel P301L/catechol-O-methyltransferase deleted model (DM mouse) were treated with the norepinephrine reuptake inhibitor reboxetine, and prefrontal dopamine concentrations and the phosphorylated state of tau was quantified. In two experiments, male and female P301L+/+//COMT+/+ and P301L+/+//COMT-/- (DM) mice were treated with reboxetine 20 mg/kg IP. In one experiment, acutely following reboxetine injection, the prefrontal cortex of mice were microdialyzed for dopamine, and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, utilizing the MetaQuant technique. In another experiment, acutely following reboxetine injections, tau phosphorylation was quantified in the frontal cortex, striatum, and hippocampus of the mice. Reboxetine injections were followed by significant increases from baseline in extracellular dopamine concentrations in P301L and DM mice, with significantly higher peak levels in the DM mice. Treatment was also followed by increases in tau phosphorylation spread throughout brain regions, with a larger impact on female mice. Extracellular dopamine concentrations exert an influence on the phosphorylation state of tau, with surges in dopamine associating with acute increases in tau phosphorylation.

DOPA Decarboxylase Modulates Tau Toxicity

BACKGROUND

The microtubule-associated protein tau accumulates into toxic aggregates in multiple neurodegenerative diseases. We found previously that loss of D₂-family dopamine receptors ameliorated tauopathy in multiple models including a Caenorhabditis elegans model of tauopathy.

METHODS

To better understand how loss of D₂-family dopamine receptors can ameliorate tau toxicity, we screened a collection of C. elegans mutations in dopamine-related genes (n = 45) for changes in tau transgene-induced behavioral defects. These included many genes responsible for dopamine synthesis, metabolism, and signaling downstream of the D₂ receptors.

RESULTS

We identified one dopamine synthesis gene, DOPA decarboxylase (DDC), as a suppressor of tau toxicity in tau transgenic worms. Loss of the C. elegans DDC gene, bas-1, ameliorated the behavioral deficits of tau transgenic worms, reduced phosphorylated and detergent-insoluble tau accumulation, and reduced tau-mediated neuron loss. Loss of function in other genes in the dopamine and serotonin synthesis pathways did not alter tau-induced toxicity; however, their function is required for the suppression of tau toxicity by bas-1. Additional loss of D₂-family dopamine receptors did not synergize with bas-1 suppression of tauopathy phenotypes.

CONCLUSIONS

Loss of the DDC bas-1 reduced tau-induced toxicity in a C. elegans model of tauopathy, while loss of no other dopamine or serotonin synthesis genes tested had this effect. Because loss of activity upstream of DDC could reduce suppression of tau by DDC, this suggests the possibility that loss of DDC suppresses tau via the combined accumulation of dopamine precursor levodopa and serotonin precursor 5-hydroxytryptophan.

Persistent Drug-Induced Parkinsonism in Patients with Normal Dopamine Transporter Imaging

Functional neuroimaging for the dopamine transporter (DAT) is used to distinguish drug-induced parkinsonism (DIP) from subclinical Parkinson’s disease (PD). Although DIP patients who show a normal DAT image are expected to recover completely, some do not. We investigated whether these patients showed changes in striatal DAT activity using semi-quantitative analysis of ¹⁸F-FP-CIT PET data. DIP patients with visually normal DAT images were selected from medical records. The subjects were classified as patients who recovered partially (PR) or completely within 12 months (CR). The ¹⁸F-FP-CIT uptake in each striatal subregion was compared between the CR and the PR groups. In total, 41 and 9 patients of the CR and PR groups were assessed, respectively. The two patient groups were comparable in terms of clinical characteristics including age, sex, and severity of parkinsonism. From semi-quantitative analysis of the PET image, the PR patients showed a relatively lower ligand uptake in the ventral striatum, the anterior putamen and the posterior putamen compared with the CR patients. This result suggests that persistent DIP in patients with visually normal DAT imaging may be associated with subtle decrement of DAT activity.

Dopamine D2 receptor antagonism suppresses tau aggregation and neurotoxicity

BACKGROUND

Tauopathies, including Alzheimer's disease and frontotemporal dementia, are diseases characterized by the formation of pathological tau protein aggregates in the brain and progressive neurodegeneration. Presently no effective disease-modifying treatments exist for tauopathies.

METHODS

To identify drugs targeting tau neurotoxicity, we have used a Caenorhabditis elegans model of tauopathy to screen a drug library containing 1120 compounds approved for human use for the ability to suppress tau-induced behavioral effects.

RESULTS

One compound, the typical antipsychotic azaperone, improved the motility of tau transgenic worms, reduced levels of insoluble tau, and was protective against neurodegeneration. We found that azaperone reduces insoluble tau in a human cell culture model of tau aggregation and that other antipsychotic drugs (flupenthixol, perphenazine, and zotepine) also ameliorate the effects of tau expression in both models.

CONCLUSIONS

Reduction of dopamine signaling through the dopamine D2 receptor with the use of gene knockouts in Caenorhabditis elegans or RNA interference knockdown in human cell culture has similar protective effects against tau toxicity. These results suggest dopamine D2 receptor antagonism holds promise as a potential neuroprotective strategy for targeting tau aggregation and neurotoxicity.