Predictors of Conversion to α-Synucleinopathy Diseases in Idiopathic Rapid Eye Movement Sleep Behavior Disorder

Mechanism and therapeutic targets of the involvement of a novel lysosomal proton channel TMEM175 in Parkinson's disease

Parkinson's disease (PD), recognized as the second most prevalent neurodegenerative disease in the aging population, presents a significant challenge due to the current lack of effective treatment methods to mitigate its progression. Many pathogenesis of PD are related to lysosomal dysfunction. Moreover, extensive genetic studies have shown a significant correlation between the lysosomal membrane protein TMEM175 and the risk of developing PD. Building on this discovery, TMEM175 has been identified as a novel potassium ion channel. Intriguingly, further investigations have found that potassium ion channels gradually close and transform into hydrion "excretion" channels in the microenvironment of lysosomes. This finding was further substantiated by studies on TMEM175 knockout mice, which exhibited pronounced motor dysfunction in pole climbing and suspension tests, alongside a notable reduction in dopamine neurons within the substantia nigra compacta. Despite these advancements, the current research landscape is not without its controversies. In light of this, the present review endeavors to methodically examine and consolidate a vast array of recent literature on TMEM175. This comprehensive analysis spans from the foundational research on the structure and function of TMEM175 to expansive population genetics studies and mechanism research utilizing cellular and animal models.A thorough understanding of the structure and function of TMEM175, coupled with insights into the intricate mechanisms underpinning lysosomal dysfunction in PD dopaminergic neurons, is imperative. Such knowledge is crucial for pinpointing precise intervention targets, thereby paving the way for novel therapeutic strategies that could potentially alter the neurodegenerative trajectory of PD.

The Motor Dysfunction Seen in Isolated REM Sleep Behavior Disorder

BACKGROUND

Isolated Rapid Eye Movement (REM) sleep Behavior Disorder (iRBD) requires quantitative tools to detect incipient Parkinson's disease (PD).

METHODS

A motor battery was designed and compared with the Movement Disorder Society-Unified Parkinson's Disease Rating Scale part III (MDS-UPDRS-III) in people with iRBD and controls. This included two keyboard-based tests (BRadykinesia Akinesia INcoordination tap test and Distal Finger Tapping) and two dual tasking tests (walking and finger tapping).

RESULTS

We included 33 iRBD patients and 29 controls. The iRBD group performed both keyboard-based tapping tests more slowly (P < 0.001, P = 0.020) and less rhythmically (P < 0.001, P = 0.006) than controls. Unlike controls, the iRBD group increased their walking duration (P < 0.001) and had a smaller amplitude (P = 0.001) and slower (P = 0.007) finger tapping with dual task. The combination of the most salient motor markers showed 90.3% sensitivity for 89.3% specificity (area under the ROC curve [AUC], 0.94), which was higher than the MDS-UPDRS-III (minus action tremor) (69.7% sensitivity, 72.4% specificity; AUC, 0.81) for detecting motor dysfunction.

CONCLUSION

Speed, rhythm, and dual task motor deterioration might be accurate indicators of incipient PD in iRBD. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Montreal Cognitive Assessment and the Clock Drawing Test to Identify MCI and Predict Dementia in Isolated REM Sleep Behavior Disorder

BACKGROUND AND OBJECTIVES

Patients with isolated/idiopathic REM sleep behavior disorder (iRBD) are at high risk for developing mild cognitive impairment (MCI) and dementia with Lewy bodies (DLB). However, there is a lack of scientific knowledge regarding the accuracy of cognitive screening tools to identify these conditions in iRBD. This study aimed to determine in iRBD the psychometrics of 2 screening tests to discriminate patients with MCI and those at risk of DLB.

METHODS

We retrospectively selected and followed 64 patients with polysomnography-confirmed iRBD seen in sleep clinic between 2006 and 2021, 32 with MCI (mean age 68.44 years, 72% men), 32 without MCI (67.78 years, 66% men), and 32 controls (69.84 years, 47% men). Participants underwent a neurologic evaluation and neuropsychological assessment for MCI diagnosis. They also completed the Montreal Cognitive Assessment (MoCA) and Clock Drawing Test (CDT). Fifty-three patients were followed (mean of 5.10 ± 2.64 years); 6 developed DLB, and 16 developed Parkinson disease. An independent cohort of 10 patients with iRBD who later developed DLB was also recruited and followed. Receiver operating characteristic curves with area under the curve (AUC) were performed assessing the discriminant value of the MoCA and CDT.

RESULTS

The cut-off values that best differentiated patients who developed DLB from controls were on the MoCA total score (≤25/30 with 100% [95% CI 61%-100%] sensitivity and 78% [61%-89%] specificity, AUC = 0.888) and delayed recall (≤3/5 with 83% [44%-97%] sensitivity and 78% [61%-89%] specificity, AUC = 0.875). Both values yielded a sensitivity of 90% (60%-98%) to detect patients at risk of DLB in the independent cohort. Cutoffs that best discriminated patients with MCI from controls were: ≤25/30 (MoCA total score) with 72% [55%-84%] sensitivity, 78% [61%-89%] specificity, AUC = 0.803 and ≤2/5 (MoCA delayed recall) with 63% [45%-77%] sensitivity, 94% [80%-98%] specificity, AUC = 0.843. No acceptable optimal values were found for the CDT.

DISCUSSION

In iRBD, the MoCA demonstrates adequate psychometric properties to identify patients most at risk of developing DLB and to screen for MCI, whereas the CDT does not.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that the MoCA, but not the CDT, is useful in screening patients with iRBD for the risk of developing DLB.

Cerebrospinal-fluid biomarkers for predicting phenoconversion in patients with isolated rapid-eye movement sleep behavior disorder

STUDY OBJECTIVES

Patients with isolated rapid-eye-movement sleep behavior disorder (iRBD) have an increased risk of developing neurodegenerative diseases. This study assessed cerebrospinal-fluid (CSF) biomarkers of neurodegeneration and blood-brain barrier (BBB) alteration in patients with iRBD compared to controls and ascertain whether these biomarkers may predict phenoconversion to alpha-synucleinopathies (Parkinson's Disease (PD), Dementia with Lewy bodies (DLB), Multiple System Atrophy (MSA)).

METHODS

Patients and controls underwent between 2012 and 2016 a neurological assessment, a lumbar puncture for CSF biomarker analysis (β-amyloid42 - Aβ42; total-tau, and phosphorylated tau), and BBB alteration (CSF/serum albumin ratio). All patients with iRBD were followed until 2021 and then classified into patients who converted to alpha-synucleinopathies (iRBD converters, cRBD) or not (iRBD non-converters, ncRBD).

RESULTS

Thirty-four patients with iRBD (mean age 67.12 ± 8.14) and 33 controls (mean age 64.97 ± 8.91) were included. At follow-up (7.63 ± 3.40 years), eight patients were ncRBD and 33 patients were cRBD: eleven converted to PD, 10 to DLB, and two to MSA. Patients with iRBD showed lower CSF Aβ42 levels and higher CSF/serum albumin ratio than controls. Cox regression analysis showed that the phenoconversion rate increases with higher motor impairment (hazard ratio [HR] = 1.23, p = 0.032). CSF Aβ42 levels predicted phenoconversion to DLB (HR = 0.67, p = 0.038) and BBB alteration predicted phenoconversion to PD (HR = 1.20, p = 0.038).

DISCUSSION

This study showed that low CSF Aβ42 levels and high BBB alteration may predict the phenoconversion to DLB and PD in patients with iRBD, respectively. These findings highlight the possibility to discriminate phenoconversion in iRBD patients through CSF biomarkers; however, further studies are needed.

Altered connectivity in the cognitive control-related prefrontal cortex in Parkinson's disease with rapid eye movement sleep behavior disorder

Rapid eye movement sleep behavior disorder (RBD) frequently occurs in Parkinson's disease (PD), however, the exact pathophysiological mechanism is not clear. The prefrontal cortex (PFC), especially ventrolateral prefrontal cortex (VLPFC), dorsolateral prefrontal cortex (DLPFC), and inferior frontal gyrus (IFG) which may play roles by regulating cognitive control processes. The purpose of this study was to investigate whether there is abnormal functional connectivity (FC) maps and volume changes in PD with RBD(PD-RBD). We recruited 20 PD-RBD, 20 PD without RBD (PD-nRBD), and 20 normal controls (NC). We utilized resting-state functional Magnetic Resonance Imaging (rs-MRI) to explore FC changes based on regions of interest (VLPFC, DLPFC, and IFG), and used voxel-based morphology technology to analyze whole-brain volumes by 3D-T1 structural MRI. Except the REM sleep behavioral disorders questionnaire (RBDSQ), the PD-RBD showed lower visuospatial/executive and attention scores than the NC group. The RBDSQ scores were significantly positively correlated with zFC of right DLPFC to bilateral posterior cingulate cortex (PCC) (P = 0.0362, R = 0.4708, AlphaSim corrected) and also significantly positively correlated with zFC of left VLPFC to right inferior temporal (P = 0.0157, R = 0.5323, AlphaSim corrected) in PD-RBD group. Furthermore, abnormal correlations with zFC values were also found in some cognitive subdomains in PD-RBD group. The study may suggest that in PD-RBD patients, the presence of RBD may be related to the abnormal FC of VLPFC and DLPFC, meanwhile, the abnormal FC of DLPFC and IFG may be related to the mechanisms of cognitive impairment.

TMEM175: A lysosomal ion channel associated with neurological diseases

Lysosomes are acidic intracellular organelles with autophagic functions that are critical for protein degradation and mitochondrial homeostasis, while abnormalities in lysosomal physiological functions are closely associated with neurological disorders. Transmembrane protein 175 (TMEM175), an ion channel in the lysosomal membrane that is essential for maintaining lysosomal acidity, has been proven to coordinate with V-ATPase to modulate the luminal pH of the lysosome to assist the digestion of abnormal proteins and organelles. However, there is considerable controversy about the characteristics of TMEM175. In this review, we introduce the research progress on the structural, modulatory, and functional properties of TMEM175, followed by evidence of its relevance for neurological disorders. Finally, we discuss the potential value of TMEM175 as a therapeutic target in the hope of providing new directions for the treatment of neurodegenerative diseases.

Considering REM Sleep Behavior Disorder in the Management of Parkinson's Disease

Rapid eye movement (REM) sleep behavior disorder (RBD) is the result of the loss of physiological inhibition of muscle tone during REM sleep, characterized by dream-enacting behavior and widely recognized as a prodromal manifestation of alpha-synucleinopathies. Indeed, patients with isolated RBD (iRBD) have an extremely high estimated risk to develop a neurodegenerative disease after a long follow up. Nevertheless, in comparison with PD patients without RBD (PDnoRBD), the occurrence of RBD in the context of PD (PDRBD) seems to identify a unique, more malignant phenotype, characterized by a more severe burden of disease in terms of both motor and non-motor symptoms and increased risk for cognitive decline. However, while some medications (eg, melatonin, clonazepam, etc.) and non-pharmacological options have been found to have some therapeutic benefits on RBD there is no available treatment able to modify the disease course or, at least, slow down the neurodegenerative process underlying phenoconversion. In this scenario, the long prodromal phase may allow an early therapeutic window and, therefore, the identification of multimodal biomarkers of disease onset and progression is becoming increasingly crucial. To date, several clinical (motor, cognitive, olfactory, visual, and autonomic features) neurophysiological, neuroimaging, biological (biofluids or tissue biopsy), and genetic biomarkers have been identified and proposed, also in combination, as possible diagnostic or prognostic markers, along with a potential role for some of them as outcome measures and index of treatment response. In this review, we provide an insight into the present knowledge on both existing and future biomarkers of iRBD and highlight the difference with PDRBD and PDnoRBD, including currently available treatment options.

Evolution patterns of probable REM sleep behavior disorder predicts Parkinson's disease progression

The course of REM sleep behavior disorder (RBD) variates in the early stage of Parkinson's disease. We aimed to delineate the association between the evolution pattern of probable RBD (pRBD) and the progression of Parkinson's disease (PD). 281 de novo PD patients from the Parkinson's Progression Markers Initiative database were included. Patients were followed up for a mean of 6.8 years and were classified into different groups according to the evolution patterns of pRBD. Disease progression was compared among groups using survival analysis, where the endpoint was defined as progression to Hoehn-Yahr stage 3 or higher for motor progression and progression to mild cognitive impairment for cognitive decline. At the 4th year of follow-up, four types of pRBD evolution patterns were identified: (1) non-RBD-stable (55.5%): patients persistently free of pRBD; (2) late-RBD (12.1%): patients developed pRBD during follow-up; (3) RBD-stable (24.9%): patients showed persistent pRBD, and (4) RBD-reversion (7.5%): patients showed pRBD at baseline which disappeared during follow-up. The RBD-reversion type showed the fastest motor progression while the RBD-stable type showed the fastest cognitive decline. At baseline, the RBD-reversion type showed the most severe gray matter atrophy in the middle frontal gyrus, while the RBD-stable type showed gray matter atrophy mainly in the para-hippocampal gyrus. Four types of early pRBD evolution patterns featured different brain lesions and predicted different courses of motor and cognitive decline in PD.

Possible predictors of phenoconversion in isolated REM sleep behaviour disorder: a systematic review and meta-analysis

BACKGROUND

A number of promising biomarkers for predicting imminent α-synucleinopathies have been suggested in isolated rapid eye movement sleep behaviour disorder (iRBD). However, existing evidence is conflicting without quantitative evaluation.

METHODS

PubMed, Web of Science and ClinicalTrials.gov were searched through June 2021 to identify possible predictors of phenoconversion from iRBD to Parkinson's disease (PD). The pooled HRs and standardised mean differences (SMDs) with 95% CIs were calculated using fixed-effects or random-effects model.

RESULTS

A total of 123 studies were included in the meta-analysis. Significant motor dysfunction (HR 1.83, 95% CI 1.33 to 2.51, I²=86.8%, p<0.001), constipation (HR 1.52, 95% CI 1.26 to 1.84, I²=8.3%, p=0.365), orthostatic hypotension (HR 1.93, 95% CI 1.05 to 3.53, I²=54.9%, p=0.084), hyposmia (HR 2.78, 95% CI 1.83 to 4.23, I²=23.9%, p=0.255), mild cognitive impairment (HR 2.27, 95% CI 1.58 to 3.27, I²=0%, p=0.681) and abnormal colour vision (SMD -0.34, 95% CI -0.63 to -0.05, I²=45.6%, p=0.087) correlated with susceptibility to PD. The process can also be traced by putaminal dopamine transporter imaging (HR 2.60, 95% CI 1.94 to 3.48, I²=0%, p=0.781) and tonic electromyographic activity (HR 1.50, 95% CI 1.04 to 2.15, I²=70%, p=0.018).

CONCLUSIONS

The predictive value of each biomarker was initially highlighted with comprehensive evaluation. Combining specific predictors with high sensitivity is promising for detecting phenoconversion in the prodromal stage. Large-scale and multicentre studies are pivotal to extend our findings.

Resting-State Functional Network Topology Alterations of the Occipital Lobe Associated With Attention Impairment in Isolated Rapid Eye Movement Behavior Disorder

Purpose

This study investigates the topological properties of brain functional networks in patients with isolated rapid eye movement sleep behavior disorder (iRBD).

Participants and Methods

A total of 21 patients with iRBD (iRBD group) and 22 healthy controls (HCs) were evaluated using resting-state functional MRI (rs-fMRI) and neuropsychological measures in cognitive and motor function. Data from rs-fMRI were analyzed using graph theory, which included small-world properties, network efficiency, network local efficiency, nodal shortest path, node efficiency, and network connectivity, as well as the relationship between behavioral characteristics and altered brain topological features.

Results

Rey-Osterrieth complex figure test (ROCFT-copy), symbol digital modalities test (SDMT), auditory verbal learning test (AVLT)-N1, AVLT-N2, AVLT-N3, and AVLT-N1-3 scores were significantly lower in patients with iRBD than in HC (P &lt; 0.05), while trail making test A (TMT-A), TMT-B, and Unified Parkinson’s Disease Rating Scale Part-III (UPDRS-III) scores were higher in patients with iRBD (P &lt; 0.05). Compared with the HCs, patients with iRBD had no difference in the small-world attributes (P &gt; 0.05). However, there was a significant decrease in network global efficiency (P = 0.0052) and network local efficiency (P = 0.0146), while an increase in characteristic path length (P = 0.0071). There was lower nodal efficiency in occipital gyrus and nodal shortest path in frontal, parietal, temporal lobe, and cingulate gyrus. Functional connectivities were decreased between the nodes of occipital with the regions where they had declined nodal shortest path. There was a positive correlation between TMT-A scores and the nodal efficiency of the right middle occipital gyrus (R = 0.602, P = 0.014).

Conclusion

These results suggest that abnormal behaviors may be associated with disrupted brain network topology and functional connectivity in patients with iRBD and also provide novel insights to understand pathophysiological mechanisms in iRBD.

Parkinson's disease and multiple system atrophy patient iPSC-derived oligodendrocytes exhibit alpha-synuclein-induced changes in maturation and immune reactive properties

Our results demonstrate the existence of early cellular pathways and network alterations in oligodendrocytes in the alpha-synucleinopathies Parkinson’s disease and multiple system atrophy. They further reveal the involvement of an immune component triggered by alpha-synuclein protein, as well as a connection between (epi)genetic changes and immune reactivity in multiple system atrophy. The knowledge generated in this study could be used to devise novel therapeutic approaches to treat synucleinopathies.

Limited evidence has shed light on how aSYN proteins affect the oligodendrocyte phenotype and pathogenesis in synucleinopathies that include Parkinson’s disease (PD) and multiple system atrophy (MSA). Here, we investigated early transcriptomic changes within PD and MSA O4⁺ oligodendrocyte lineage cells (OLCs) generated from patient-induced pluripotent stem cells (iPSCs). We found impaired maturation of PD and MSA O4⁺ OLCs compared to controls. This phenotype was associated with changes in the human leukocyte antigen (HLA) genes, the immunoproteasome subunit PSMB9, and the complement component C4b for aSYN p.A53T and MSA O4⁺ OLCs, but not in SNCA^trip O4⁺ OLCs despite high levels of aSYN assembly formation. Moreover, SNCA overexpression resulted in the development of O4⁺ OLCs, whereas exogenous treatment with aSYN species led to significant toxicity. Notably, transcriptome profiling of genes encoding proteins forming Lewy bodies and glial cytoplasmic inclusions revealed clustering of PD aSYN p.A53T O4⁺ OLCs with MSA O4⁺ OLCs. Our work identifies early phenotypic and pathogenic changes within human PD and MSA O4⁺ OLCs.

Predictors of RBD progression and conversion to synucleinopathies

Purpose of review

Rapid eye movement (REM) sleep behaviour disorder (RBD) is considered the expression of the initial neurodegenerative process underlying synucleinopathies and constitutes the most important marker of their prodromal phase. This article reviews recent research from longitudinal research studies in isolated RBD (iRBD) aiming to describe the most promising progression biomarkers of iRBD and to delineate the current knowledge on the level of prediction of future outcome in iRBD patients at diagnosis.

Recent findings

Longitudinal studies revealed the potential value of a variety of biomarkers, including clinical markers of motor, autonomic, cognitive, and olfactory symptoms, neurophysiological markers such as REM sleep without atonia and electroencephalography, genetic and epigenetic markers, cerebrospinal fluid and serum markers, and neuroimaging markers to track the progression and predict phenoconversion. To-date the most promising neuroimaging biomarker in iRBD to aid the prediction of phenoconversion is striatal presynaptic striatal dopaminergic dysfunction.

Summary

There is a variety of potential biomarkers for monitoring disease progression and predicting iRBD conversion into synucleinopathies. A combined multimodal biomarker model could offer a more sensitive and specific tool. Further longitudinal studies are warranted to iRBD as a high-risk population for early neuroprotective interventions and disease-modifying therapies.

Translocation of TMEM175 Lysosomal Potassium Channel to the Plasma Membrane by Dynasore Compounds

TMEM175 (transmembrane protein 175) coding sequence variants are associated with increased risk of Parkinson’s disease. TMEM175 is the ubiquitous lysosomal K⁺ channel regulated by growth factor receptor signaling and direct interaction with protein kinase B (PKB/Akt). In the present study, we show that the expression of mouse TMEM175 results in very small K⁺ currents through the plasma membrane in Xenopus laevis oocytes, in good accordance with the previously reported intracellular localization of the channel. However, the application of the dynamin inhibitor compounds, dynasore or dyngo-4a, substantially increased TMEM175 currents measured by the two-electrode voltage clamp method. TMEM175 was more permeable to cesium than potassium ions, voltage-dependently blocked by 4-aminopyridine (4-AP), and slightly inhibited by extracellular acidification. Immunocytochemistry experiments indicated that dyngo-4a increased the amount of epitope-tagged TMEM175 channel on the cell surface. The coexpression of dominant-negative dynamin, and the inhibition of clathrin- or caveolin-dependent endocytosis increased TMEM175 current much less than dynasore. Therefore, dynamin-independent pharmacological effects of dynasore may also contribute to the action on the channel. TMEM175 current rapidly decays after the withdrawal of dynasore, raising the possibility that an efficient internalization mechanism removes the channel from the plasma membrane. Dyngo-4a induced about 20-fold larger TMEM175 currents than the PKB activator SC79, or the coexpression of a constitutively active mutant PKB with the channel. In contrast, the allosteric PKB inhibitor MK2206 diminished the TMEM175 current in the presence of dyngo-4a. These data suggest that, in addition to the lysosomes, PKB-dependent regulation also influences TMEM175 current in the plasma membrane.

Development and Validation of a Prognostic Model for Cognitive Impairment in Parkinson's Disease With REM Sleep Behavior Disorder

The presentation and progression of Parkinson’s disease (PD) are not uniform, but the presence of rapid eye movement sleep behavior disorder (RBD) in PD patients may indicate a worse prognosis than isolated PD. Increasing evidence suggests that patients with comorbid PD and RBD (PD-RBD) are more likely to develop cognitive impairment (CI) than those with isolated PD; however, the predictors of CI in PD-RBD patients are not well understood. This study aimed to develop a prognostic model for predicting mild cognitive impairment (MCI) in PD-RBD patients. The data of PD-RBD patients were extracted from the Parkinson’s Progression Markers Initiative study (PPMI), and the sample was randomly divided into a training set (n = 96) and a validation set (n = 24). PD-MCI as defined by the level II Movement Disorder Society (MDS) diagnostic criteria was the outcome of interest. The demographic features, clinical assessments, dopamine transporter (DAT) imaging data, cerebrospinal fluid (CSF) analyses and genetic data of PD patients were considered candidate predictors. We found that performance on the University of Pennsylvania Smell Identification Test (UPSIT), the mean signal and asymmetry index of the putamen on DAT imaging, p-tau/α-syn and p-tau in CSF, and rs55785911 genotype were predictors of PD-MCI in PD-RBD patients. A C-index of 0.81 was obtained with this model, and a C-index of 0.73 was obtained in the validation set. Favorable results of calibrations and decision curve analysis demonstrated the efficacy and feasibility of this model. In conclusion, we developed a prognostic model for predicting MCI in PD-RBD patients; the model displayed good discrimination and calibration and may be a convenient tool for clinical application. Larger samples and external validation sets are needed to validate this model.