Yumei Yue, Xiaodan Zhang, Wen Lv, Hsin-Yi Lai, Ting Shen

Abstract Parkinson’s disease is a common neurodegenerative disorder that is associated with abnormal aggregation and accumulation of neurotoxic proteins, including α-synuclein, amyloid-β, and tau,in addition to the impaired elimination of these neurotoxic protein.Atypical parkinsonism, which has the same clinical presentation and neuropathology as Parkinson’s disease, expands the disease landscape within the continuum of Parkinson’s disease and related disorders.The glymphatic system is a waste clearance system in the brain, which is responsible for eliminating the neurotoxic proteins from the interstitial fluid.Impairment of the glymphatic system has been proposed as a significant contributor to the development and progression of neurodegenerative disease, as it exacerbates the aggregation of neurotoxic proteins and deteriorates neuronal damage.Therefore, impairment of the glymphatic system could be considered as the final common pathway to neurodegeneration.Previous evidence has provided initial insights into the potential effect of the impaired glymphatic system on Parkinson’s disease and related disorders; however, many unanswered questions remain.This review aims to provide a comprehensive summary of the growing literature on the glymphatic system in Parkinson’s disease and related disorders.The focus of this review is on identifying the manifestations and mechanisms of interplay between the glymphatic system and neurotoxic proteins, including loss of polarization of aquaporin-4 in astrocytic endfeet, sleep and circadian rhythms, neuroinflammation,astrogliosis, and gliosis.This review further delves into the underlying pathophysiology of the glymphatic system in Parkinson’s disease and related disorders, and the potential implications of targeting the glymphatic system as a novel and promising therapeutic strategy.

Key Words: atypical parkinsonism; glymphatic system; magnetic resonance imaging; neurotoxic proteins; Parkinson’s disease 1Department of Neurology of Sir Run Run Shaw Hospital, Zhejiang University School of Medicine, Zhejiang University, Hangzhou, Zhejiang Province, China; 2Department of Emergency Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China; 3Department of Neurology of the Second Affiliated Hospital and School of Brain Science and Brain Medicine, Interdisciplinary Institute of Neuroscience and Technology, Key Laboratory of Medical Neurobiology of Zhejiang Province, Zhejiang University School of Medicine,Hangzhou, Zhejiang Province, China; 4College of Biomedical Engineering and Instrument Science, Zhejiang University, Hangzhou, Zhejiang Province, China; 5MOE Frontier Science Center for Brain Science and Brain-Machine Integration, State Key Laboratory of Brain-machine Intelligence, School of Brain Science and Brain Medicine, Zhejiang University,

Introduction

Parkinson’s disease (PD) is a common neurodegenerative disorder that mainly affects the elderly (Tysnes and Storstein, 2017).PD is characterized by symptoms such as resting tremor, bradykinesia, and rigidity (Postuma et al., 2015).The typical pathological hallmark of PD is the formation of the Lewy bodies (LBs), which are primarily composed of α-synuclein(Eriksen et al., 2005; Alarcón et al., 2023; Bigi et al., 2023).Additionally, tau aggregation and amyloid-β (Aβ) deposition also contribute to the pathology of PD (Hu et al., 2010).Atypical parkinsonism, which is used to describe a series of neurodegenerative disorders that are distinct pathological entities,including dementia with LBs (DLB), progressive supranuclear palsy (PSP),multiple system atrophy, and corticobasal degeneration, shares some clinical manifestations with PD.There is evidence of neuropathological overlap between atypical parkinsonism and PD, with abnormal protein deposition(Sengupta and Kayed, 2022) and clearance dysfunction being potential common features.

The glymphatic system is a waste clearance system in the brain (Taoka et al., 2017; Xu et al., 2023) that eliminates proteins and other solutes from the interstitial fluid (ISF) (Aukland and Reed, 1993).Since the discovery of the glymphatic system in rodent brains, it has been a hot topic of research (Iliff et al., 2012).Previous studies have revealed that glymphatic clearance dysfunction can occur in aged brains (Kress et al., 2014) and neurodegeneration diseases (Ringstad et al., 2017).Emerging evidence suggests that the impaired glymphatic system may exacerbate the aggregation of neurotoxic proteins (Zou et al., 2019; Ding et al., 2021), highlighting its potential role in neurodegeneration.However, the complex mechanisms underlying the interplay between the glymphatic system and neurotoxic proteins is not yet fully understood.Further research is needed to elucidate the mechanism and exact relationship between the impaired glymphatic system and the accumulation of neurotoxic proteins in PD and its related disorders.

The objective of this review was to provide a comprehensive summary of the growing literature on the glymphatic system in PD and related disorders,identifying the interplay between the glymphatic system and neurotoxic proteins.We aimed to offer a perspective that enhances our understanding of the pathophysiology of the glymphatic system in these diseases and explore its potential as a therapeutic target.

Retrieval Strategy

An online search of the PubMed database was performed to retrieve articles published in English up to September 15, 2023.A combination of the key words (Title/Abstract) was used to optimized the search sensitivity and specificity: “Parkinson’s disease”; “PD”; “atypical parkinsonism”; “α-synuclein”;“amyloid-β”; “tau”; “neurotoxic protein”; “glymphatic system”; “PVSs”;“perivascular spaces”; “diffusion tensor image along the perivascular space”;“DTI-ALPS”; “sleep”; “aquaporin protein-4”; “AQP4”.The results were screened by the title and abstract, and studies exploring the neurotoxic proteins in PD and atypical parkinsonism; the relationship between neurotoxic proteins, sleep, AQP4, and the glymphatic system; and the PVSs and DTI-ALPS change in PD and related disorders were included to investigate the interplay between the glymphatic system and neurotoxic proteins in PD and related disorders.

Overview of the Glymphatic System

The glymphatic system was initially discovered in rodents using two-photon imaging with a small fluorescent tracer (Iliff et al., 2012).The study found that the exchange of cerebrospinal fluid (CSF) with the ISF occurs through the paravascular spaces.The convective ISF bulk flow plays a critical role in the clearance of interstitial solutes.With the rapid development of magnetic resonance imaging (MRI), the glymphatic system has also been identified in the human brain (Taoka et al., 2017; Yokota et al., 2019).The glymphatic system (Figure 1) is composed of various components, including CSF, ISF, perivascular spaces (PVSs), cerebral vessels, glial cells, and astrocyte aquaporin-4 (AQP4) (Liu et al., 2021).CSF is produced by epithelial cells of the choroid plexus and circulates in the subarachnoid space before entering the PVSs, propelled by arterial wall pulsatility.The PVSs are interstitial fluid-filled spaces surrounding the blood vessels and contain the endfeet of astrocytes(Bown et al., 2022).The water channels, AQP4, are widely expressed in astrocytes, which facilitate the movement of CSF into the brain parenchyma(Iliff et al., 2012).The ISF then mixes with the CSF, exchanging waste products and solutes, which is drained into PVSs before eventually reaching the meningeal lymphatic vessels (Liu et al., 2021).

Figure 1 ｜ Schematic representation of the glymphatic system.The glymphatic system is composed of various components, including cerebral spinal fluid (CSF), interstitial fluid (ISF), perivascular spaces, cerebral vessels, glial cells, and astrocyte aquaporin protein-4 (AQP4).Created with BioRender.com.

The waste clearance activity of the glymphatic system can be influenced by several factors, including the sleep-wake cycle, aging process, body posture,sympathetic and parasympathetic innervation, and the activity of glial cells, such as astrocytes (Sun et al., 2018).The clearance of exogenous Aβ was reported to decrease by 40% in older mice when compared to young mice.Additionally, artery pulsatility and perivascular AQP4 polarization are decreased in aged mice, which is accompanied by a reduction in CSF-ISF exchange (Kress et al., 2014).Furthermore, hypertrophy of astrocytic glial fibrillary acidic protein processes occurs with aging, resulting in reactive gliosis that impairs the glymphatic system (Sun et al., 2018), suggesting that the function of the glymphatic system may deteriorate with age.

The sleep-wake cycle is another critical factor affecting the glymphatic system.Studies have found that waste removal by the glymphatic system is dramatically increased during sleep (Benveniste et al., 2019), with a 60%increase in interstitial volume observed during natural sleep (Xie et al.,2013).This leads to a substantial boost in the CSF-ISF exchange, facilitating the removal of waste products.Studies in rodents have shown that the perivascular and parenchymal tracer flow was significantly reduced by 95%in awake mice compared to sleeping mice, and exogenous Aβ clearance was also slower in awake mice (Xie et al., 2013).The CSF flow increased the most during rapid eye movement (REM) sleep (Tuura et al., 2021), suggesting a critical role of REM sleep in glymphatic function.Intriguingly, sleep positions also affect glymphatic system clearance (Lee et al., 2015), with the right recumbent position being optimal for glymphatic system transport.The enhanced function of the glymphatic system during natural sleep may be related to the inhibition of noradrenergic projections from the locus coeruleus(Xie et al., 2013), resulting in dilatation of the interstitial spaces, facilitating solute transport.

Therefore, sleep disturbances, including poor sleep quality and sleep deprivation, have been linked to dysfunction of the glymphatic system and increased accumulation of Aβ and tau (Holth et al., 2019; Semyachkina-Glushkovskaya et al., 2020b).Furthermore, dysfunction of the glymphatic system was reported in obstructive sleep apnea, which may contribute to an increased incidence of Alzheimer’s disease (AD; Roy et al., 2022).Additionally,impaired glymphatic system was found in rapid eye movement sleep behavior disorder (RBD; Si et al., 2022), which is considered as a prodromal symptom of PD (Berg et al., 2015).These findings suggest that aging processes and sleep disturbances may contribute to the development of neurodegenerative diseases through their effects on the glymphatic system.

Impairment of the Glymphatic System in Parkinson’s Disease and Related Disorders

The morphological basis for the impairment of the glymphatic system manifests in multifaceted ways, including astrocytic injury, mislocalization of AQP4 channels, loss of vascular plasticity, enlargement of the PVSs, and neuroinflammation (Wei et al., 2023).The assessment and quantification of these changes can serve as valuable biomarkers for evaluating the glymphatic system.Methods for assessing the glymphatic system represent a current focal point within the research field.

At the outset, invasive techniques were employed to assess glymphatic function, includingex vivofluorescence imaging andin vivotwo-photon imaging in rodent models (Iliff et al., 2012; Yang et al., 2013), as well as dynamic positron emission computed tomography (de Leon et al., 2017)and MRI with intrathecal contrast medium (Eide and Ringstad, 2015) in humans.However, the invasiveness and potential side effects associated with these methods have posed challenges when applied to human subjects.There is a growing need for safer and more advanced techniques to explore the glymphatic system in both research and clinical settings.Recently, the quick development of MRI techniques has significantly contributed to the exploration of the glymphatic systemin vivo.The brain glymphatic system has been extensively studied in PD using advanced MRI techniques, such as PVSs number and volume calculation, and diffusion tensor imaging along the perivascular space (DTI-ALPS), as shown in Figure 2 and Table 1.PVS burden, as measured by their number and volume, manually or automatically,has been used to assess structural abnormalities of the glymphatic system.Previous studies have demonstrated a higher PVS burden in PD, which was associated with the severity of symptoms (Donahue et al., 2021; Lin et al.,2022; Tu et al., 2022).Clinical presentations may vary depending on the size of the PVSs; small PVSs (≤ 3 mm) in the basal ganglia (BG) was significantly correlated with the unified Parkinson Disease Rating Scale (UPDRS) I and II,while large PVSs (> 3 mm) in the BG was correlated with UPDRS III and IV(Ramirez et al., 2022).The current research trend further emphasizes the utilization of non-invasive methods to measure the functional status of the glymphatic system.DTI-ALPS, as an approximate measure of glymphatic flow and indirectly represents the transport function of the glymphatic system.The DTI-ALPS index is computed by taking the ratio of diffusion in the perivascular direction along the medullary veins to diffusion perpendicular to the principal fiber track direction (Taoka et al., 2017; Shen et al., 2022).This index serves as a non-invasive biomarker for assessing fluid dynamics in the brain and represents glymphatic function, which offers insight into the fluidopathy in PD (Shen et al., 2022).A lower DTI-ALPS represents functional impairment in the glymphatic system.As expected, PD patients with cognitive impairment have a significantly lower DTI-ALPS index (Chen et al., 2021), which was also observed in PD-related disorders such as RBD (Lee et al., 2022; Si et al., 2022).In addition, the DTI-ALPS index was remarkably lower in PSP patients (Ota et al., 2023), suggesting a disruption in glymphatic activity in PSP.Correlation analyses revealed that the DTI-ALPS index in PD was negatively correlated with clinical profiles, including the age, age at disease onset, UPDRS III score,subscore for rigidity, PD sleep scale score, and a history of diabetes mellitus,while it was positively correlated with the Mini-Mental State Examination and Montreal Cognitive Assessment scores (Bae et al., 2023; Cai et al., 2023; Qin et al., 2023).Longitudinal assessments further showed that PD patients with a low DTI-ALPS deteriorated faster regarding the UPDRS II and III scores, Symbol Digit Modalities Test, and Hopkins Verbal Learning Test (He et al., 2023).Additionally, the volume of lateral ventricle and midbrain tegmentum was significantly correlated with the DTI-ALPS index, confirming both structural abnormalities and functional impairment in PD and related disorders (Ota et al., 2023).

Ultra-high field 7.0T MRI, with a higher spatio-temporal resolution and signal-to-noise ratio, and improves the visualization and assessment of PVSs,especially normal-sized PVSs that are typically invisible with conventional MRI imaging (Bouvy et al., 2014).Moreover, 7.0T MRI provides a clearer visualization of subtle structures in the white matter compared to 3.0T MRI(Vu et al., 2015).The improved resolution and decreased partial volumeeffects of 7.0T MRI also provide a superior tool to evaluate the DTI-ALPS index compared with 1.5T or 3.0T MRI.With 7.0T MRI, our group has extensively explored the PVSs burden and DTI-ALPS index change in PD (Lv et al., 2021;Shen et al., 2021, 2022).Our previous study revealed a more severe PVSs burden of the BG in patients with early-stage PD than healthy controls, which was associated with disease severity and medication dosage (Shen et al.,2021).In another proof-of-concept study, we found that the PVSs burden of the BG was greater in PD freezers compared with PD non-freezers, and was correlated with the progression of the freezing of gait phenotype (Lv et al., 2021).These studies have demonstrated that both global and regional structural impairments of the glymphatic system are already present in the early stage of PD, and are correlated with specific phenotypes, disease severity, and progression.A cross-sectional study was conducted using 7.0T MRI to explore glymphatic dysfunction in patients with PD at different disease stages (Shen et al., 2022); patients with PD were observed to have a lower DTI-ALPS index than healthy controls, and the differences become more pronounced in late-stage PD.Furthermore, the reduced DTI-ALPS index was associated with an increased PVSs burden in the BG, and both measurements were correlated with PD disease severity.These findings align with previous research, which suggests that the glymphatic system becomes more severely impaired as PD disease progresses (Ma et al., 2021).Taken together, these studies suggest that impairment of the glymphatic system plays a critical role in the pathophysiology and underlying mechanism of PD.

Table 1 ｜Evaluation of the glymphatic system in Parkinson’s disease (PD) and related disorders

Figure 2 ｜ Non-invasive evaluation of the glymphatic system (unpublished data).(A) Visualization of the perivascular space (PVS) on 7T MRI T2WI images.PVS counting and volume calculation in the basal ganglia to evaluate structural abnormalities of the glymphatic system.(B) Diffusion tensor image along the perivascular space (DTI-ALPS)index for measuring the glymphatic flow.The yellow arrows indicate the PVS in the basal ganglia; the yellow box indicates the medullary vein perpendicular to the lateral ventricle.Unpublished data from the authors’ laboratory.

Neurotoxic Proteins in Parkinson’s disease and Related Disorders

α-Synucleinopathies

The pathological hallmarks of PD include the presence of LBs and Lewy neurites in the brain, which are mainly composed of aggregated α-synuclein protein (Eriksen et al., 2005).These abnormal protein deposits have been detected in multiple brain regions, including the medulla oblongata, pontine tegmentum, substantia nigra, and cortices (Braak et al., 2003), and are believed to play a critical role in the neurodegenerative process underlying PD.Additionally, this pathology has be observed in patients with RBD, DLB,and multiple system atrophy (Spillantini et al., 1998; Liguori et al., 2023).α-synuclein is encoded by the synuclein alpha gene, and a mutation in the synuclein alpha gene results in the expression of insoluble α-synuclein(Guhathakurta et al., 2017), which aggregates intracellularly and forms insoluble deposits that are secreted from cells via an exocytotic pathway(Lee et al., 2005), leading to the accumulation of extracellular α-synuclein.Extracellular α-synuclein can then be internalized by neighboring cells and acts as a catalyzer of intracellular α-synuclein (Lee, 2008), as shown in Figure 3A and D.This prion-like cascade of synucleinopathies has been demonstrated bothin vivoandin vitro(Lee, 2008; Luk et al., 2012).There is mounting evidence linking synucleinopathies to the clearance system; alongside the autophagy lysosomal pathway and ubiquitin proteasome system (Webb et al.,2003), the glymphatic system is also believed to play a role in the clearance of α-synuclein (Nakamura et al., 2016; Hoshi et al., 2017).Therefore, dysfunction of the glymphatic system may result in α-synuclein accumulation (Zou et al.,2019; Ding et al., 2021).

Amyloidopathies

In addition to the hallmark α-synuclein pathology, co-occurring proteinopathies, such as taupathies and amyloidopathies, have been observed in patients with PD (Sengupta and Kayed, 2022).Aβ deposition,which is a hallmark of AD, has also been observed in PD and related disorders,such as DLB and PD dementia (Edison et al., 2008).The Aβ protein is encoded by the amyloid precursor protein gene, which is cleaved by secretase to produce Aβ peptides.The soluble monomeric Aβ fragments aggregate into insoluble oligomers, which form fibrils and accumulate in neurological plaques (Devkota et al., 2021), as shown in Figure 3B.The Aβ fibril surface can subsequently catalyze the formation of Aβ oligomers (Cohen et al., 2013).In AD, Aβ deposition often starts in the medial temporal lobe, including the entorhinal cortex and hippocampus, and spreads to other regions of the brain over time (Sehar et al., 2022).In PD and related disorders, Aβ deposition is less prominent than α-synuclein and tau pathology, but can still be found in various brain regions, including the cortices, hippocampus, striatum, and claustrum (Lim et al., 2019).The accumulation of Aβ plaques has been found to synergistically accelerate α-synuclein aggregation and vice versa (Clinton et al., 2010).Furthermore, evidence has suggested that Aβ accumulation is associated with both cognitive and locomotor functions in PD (Rochester et al., 2017).Although impairment of the glymphatic system has been shown to be responsible for Aβ deposition in AD (Xu et al., 2015), it is yet to be determined whether the system is involved in PD.

Taupathies

Taupathies are characterized by abnormal accumulation of tau proteins,leading to the formation of neurofibrillary tangles in the brain, which is observed in PD and atypical parkinsonism, such as PSP and corticobasal degeneration (Levin et al., 2016).Tau protein is encoded by the microtubuleassociated protein tau gene, which has been identified as a risk gene for PD(Chang et al., 2017).In the normal state, the tau protein is soluble and assists in microtubule stabilization, membrane binding, and axonal transport (Chen et al., 1992).However, in the pathological state, tau becomes insoluble,aggregates within neurons, and can be subsequently released into the extracellular space (Figure 3C).These aggregates can be retaken up by adjacent neurons and glia, contributing to the propagation of taupathies throughout the brain (Kfoury et al., 2012).Tau pathology is often observed in various brain regions, including the entorhinal cortex, medial and basal temporal regions, and primary cortices in AD (Cho et al., 2016) and in the BG,brainstem, and cortical regions in PSP and corticobasal degeneration (Dickson,2012).In PD, tau pathology is primarily observed in the entorhinal areas,limbic system, and lateral temporal areas (Compta et al., 2014).Furthermore,the CSF levels of tau protein were found to be associated with clinical features in PD (Hu et al., 2010).As mentioned earlier, the co-localization of tau and α-synuclein has been reported in the LBs of PD patients (Arima et al., 1999).The interaction between tau and α-synuclein may potentially enhance the aggregation of both proteins, thereby contributing to the pathophysiology of PD (Giasson et al., 2003).A previous study has indicated that tau is eliminated from the brain to the periphery through CSF-ISF exchange (Ishida et al., 2022).The glymphatic system, which plays a critical role in this drainage system(Iliff et al., 2012), is also believed to contribute to tau clearance (Ishida et al.,2022).

Co-pathology of α-synuclein, Aβ, and tau

Previous studies have identified the presence of mixed neurotoxic protein pathologies in PD and related disorders (Masliah et al., 2001; Lashley et al., 2008; Compta et al., 2011; Irwin et al., 2013).While the α-synuclein pathology plays a critical role, co-occurring Aβ plaques and tau neurofibrillary tangles have been observed in up to 30–40% patients with PD (Irwin et al., 2013).Increasing evidence suggests that there is a synergistic interplay between the α-synuclein, tau, and Aβ pathologies (Compta et al., 2011).An increased cortical Aβ plaque burden was reported to be positively correlated with the severity of cortical α-synuclein density (Lashley et al., 2008).Furthermore,in vitroexperiments have shown that tau and α-synuclein can promote the polymerization and fibril formation of the other (Giasson et al., 2003).Additionally, Aβ peptides may also enhance the aggregation and accumulation of α-synuclein, suggesting that these proteins may interact to accelerate neurodegeneration (Masliah et al., 2001).These studies have suggested that the co-pathology of α-synuclein, tau, and Aβ may contribute to the development and progression of PD.More extensive investigations are required to comprehensively understand the intricate interplay among these proteins and their involvement in the pathology of PD.

Interactions between the Glymphatic System and Neurotoxic Proteins

The impaired glymphatic system and neurotoxic protein accumulation have been extensively reported in neurodegenerative diseases.As we mentioned earlier, the glymphatic system is impaired both structurally and functionally in PD and related disorders, as shown in Figure 4.There has been growing interest in investigating the relationship between the glymphatic system and the accumulation of neurotoxic proteins in these conditions, as summarized in Table 2.It is worth noting that there appears to be a reciprocal interplay between impaired glymphatic function and the accumulation of neurotoxic proteins, where protein deposition can worsen glymphatic dysfunction and vice versa.

Figure 3 ｜ Proteinopathy formation and spread.(A) Formation of Lewy bodies from α-synuclein.The monomeric form of α-synuclein aggregates into oligomers, which further form protofibrils that eventually mature into insoluble fibrils, the main component of Lewy bodies.(B) Formation of amyloid plaques.The amyloid precursor protein is first cleaved by β-secretase and subsequently by γ-secretase to generate amyloid-β (Aβ) peptides, which accumulate and aggregate to form neurotoxic amyloid plaques.(C) Formation of neurofibrillary tangles.The hyperphosphorylated tau proteins assemble and further develop into filamentous neurofibrillary tangles.(D) Cell-to-cell spreading of proteinopathies.Aβ, α-synuclein, and tau seeds can all be transmitted trans-synaptically from one neuron to another, leading to the spread of these protein aggregates throughout the brain.Created with BioRender.com.

Figure 4 ｜ Schematic representation of the glymphatic system in pathological states.The enlargement of the perivascular spaces (PVSs) is indicative of structural abnormalities, while a reduction in the diffusion tensor image along the perivascular space (DTI-ALPS) index reflects functional impairment.Created with BioRender.com.AQP4: Aquaporin-4; CSF: cerebral spinal fluid; ISF: interstitial fluid.

Research exploring the interplay between the glymphatic system and neurotoxic proteins is predominantly in the preclinical stage.The meningeal lymphatic vessels (mLVs) and deep cervical lymph nodes (Dclns) are components of the peripheral lymphatic system that are connected to the brain glymphatic system, facilitating the removal of waste products from the brain and transporting them into the peripheral lymphatic system (Natale et al., 2021).Since the brain glymphatic system is a complex network of microvessels and lymphatic capillaries, direct ligation of the glymphatic system is quite challenging and may lead to brain tissue damage.Currently, research efforts are primarily focused on manipulating the peripheral lymphatic system to modulate the function of the glymphatic system for study purposes.Most studies utilized invasive interventions, such as ligating mLVs or Dclns outside of the brain (Wang et al., 2019; Zou et al., 2019; Ding et al., 2021).In a transgenic PD mouse model (A53T mice), ligation of Dclns led to blockade of meningeal lymphatic drainage, which worsened α-synuclein deposition in brain (Zou et al., 2019).Additionally, ligation of Dclns exacerbated autophagy dysfunction, impairing the intracellular degradation of α-synuclein, which further contributed to increased α-synuclein deposition in brain.In the α-synuclein preformed fibrils (PFF) mouse model of PD, ligation of mLVs resulted in a notable rise in cortical α-synuclein levels (Ding et al., 2021).This study also provided evidence from another perspective that protein deposition impaired the function of the glymphatic clearance system.They found a reduction in lymphatic drainage after α-synuclein PFF injection, along with an increase in the levels of meningeal macrophages and inflammatory cytokines.In addition, the clearance of total tau and phosphorylated tau were also impaired after ligation of Dclns in A53T mice, suggesting that glymphatic dysfunction could aggravate tau deposition in PD.Investigation on the relationship between Aβ and the glymphatic system have been focused on AD rather than PD.In a transgenic AD mouse model, ligation of Dclns resulted in an increased Aβ load burden and phosphorylated tau accumulation in the brain (Wang et al., 2019).Additionally, ablation of mLVs, which impaired meningeal lymphatics function, has been shown to worsen amyloid pathology in both the meninges and brain (Da Mesquita et al., 2018).These studies implied that impairment of glymphatic clearance function aggravates the accumulation of α-synuclein, Aβ, and tau in neurodegenerative diseases.Theinduced reactive astrogliosis and inflammatory activity further impaired the glymphatic system (Da Mesquita et al., 2018; Wang et al., 2019; Zou et al.,2019; Ding et al., 2021), and consequently, there may be a reciprocal interplay between neurotoxic protein deposition and glymphatic clearance dysfunction.

Table 2 ｜Interactions between the glymphatic system and neurotoxic proteins in Parkinson’s disease (PD) and related disorders

These research findings have also been further substantiated in clinical studies involving patients with PD.Meningeal lymphatic function was found to be impaired in patients with PD, which might parallel the progression of PD (Ding et al., 2021).Vascular deposition of Aβ has been observed in vessels displaying enlarged PVSs (Perosa et al., 2022).In addition, a post-mortem study of patients with multiple system atrophy revealed phosphorylated α-synuclein deposits on periventricular astrocytes, providing evidence that α-synuclein was also cleared through the perivascular glymphatic route(Nakamura et al., 2016).A heavier burden of dilated PVSs in the substantia nigra and BG was found in patients with PD, which was associated with an increased tau protein level in the CSF (Li et al., 2020).The glymphatic activity,assessed by the DTI-ALPS index, was found to be associated with Aβ and tau deposition and neuroinflammation (Ota et al., 2022).In PD, a higher PVSs burden in the BG, which reflected more severe impairment of the glymphatic system, was correlated with a lower Aβ level in the CSF, suggesting impaired clearance or increased deposition of Aβ in the brain (Chen et al., 2022).Thus, impaired glymphatic clearance activity may result in more extensive aggregation of neurotoxic proteins in the brain, subsequently accelerating neurodegenerative processes and cognitive decline.Additionally, in turn, the deposition of these proteins may block the glymphatic pathway, contributing to the enlargement of PVSs and decreased glymphatic activity.

Glymphatic impairment could be considered as a final common pathway in neurodegenerative diseases.The mechanisms underlying the interplay between the glymphatic system and neurotoxic proteins is complex and multifaced.Various risk factors have been identified that can predispose individuals to glymphatic impairment, including aging, hypertension, small vessel disease, sleep dysfunction, and, as the specific focus of our attention,neurotoxic protein deposition (Wei et al., 2023).The underlying mechanisms that govern the interplay between the glymphatic system and neurotoxic proteins, include the loss of polarization of AQP4 in astrocytic endfeet, sleep and circadian rhythms, neuroinflammation, astrogliosis, and gliosis (Xie et al.,2013; Da Mesquita et al., 2018; Benveniste et al., 2019; Wang et al., 2019;Zou et al., 2019; Ding et al., 2021; Tuura et al., 2021).Further investigations at the molecular level will allow for a deeper understanding of the components of the glymphatic system.

As an important component of the glymphatic system, the molecular mechanism involving astrocytic AQP4 channels has been intensively studied.A postmortem study revealed that α-synuclein accumulation was negatively correlated with astrocytic AQP4 expression in brain tissues of patients with PD (Hoshi et al., 2017).In rodent models, Aqp4deletion led to a reduction in glymphatic activity, which impaired the clearance of exogenous α-synuclein from the brain (Zhang et al., 2023).Overexpression of α-synuclein suppressedAqp4expression and polarization, accelerating glymphatic dysfunction.Consistently, reduced expression ofAqp4worsened the deposition of α-synuclein, resulting in the loss of dopaminergic neurons and behavioral deficits (Cui et al., 2021).Tau clearance has also been identified as an AQP4-dependent component of the glymphatic pathway.Following the deletion ofAqp4, levels of tau in the CSF elevated, leading to pathological tau deposition and increased neurodegeneration in the brain (Ishida et al., 2022).Thus, astrocytic AQP4 channels play a pivotal role in glymphatic clearance,representing one of the molecular mechanisms that exacerbate protein deposition in PD.The links between sleep, glymphatic clearance, and protein aggregation have provided compelling evidence that sleep and circadian rhythms are crucial mechanisms underlying neurodegeneration.Studies have demonstrated that the functional state of the glymphatic system changes with the sleepwake cycle (Xie et al., 2013; Benveniste et al., 2019; Tuura et al., 2021).Waste removal by the glymphatic system is dramatically increased during sleep, especially in the REM stage (Xie et al., 2013; Benveniste et al., 2019;Tuura et al., 2021).During sleep, the volume of CSF in the brain interstitial space increases, promoting the exchange of waste products between the ISF and CSF (Boespflug and Iliff, 2018).In addition, glymphatic fluid flow and waste clearance has been proven to be under circadian control (Hablitz et al., 2020).Recent studies have shown that sleep plays an important role in the regulation of Aβ dynamics; specifically, the clearance rates of Aβ were increased by 2.5 times during sleep compared to wakefulness (Xie et al., 2013), and sleep deprivation has been shown to increase Aβ levels and aggravate plaque formation (Kang et al., 2009).In the elderly, reduced slow wave sleep, insomnia, and sleep apnea have been associated with higher levels of soluble Aβ42in the brain tissues (Lucey and Bateman, 2014),suggesting that sleep is a critical for regulating Aβ dynamics and plays an important role in the pathogenesis of AD.Sleep disturbance is also a common prodromal factor in PD patients, with an incidence rate of 52.6%, second only to the gastrointestinal symptoms (Durcan et al., 2019).Previous studies have found that reduced sleep duration, poor sleep quality, and non-apnea sleep disorders increase the risk of PD (Hsiao et al., 2017; Lysen et al., 2019).Most individuals with RBD eventually developing PD and LBD, and RBD has been extensively studied as a potential biomarker of PD (Galbiatiet al., 2019).Previous studies reported a decrease in the DTI-ALPS index in patients with RBD (Si et al., 2022), indicating abnormalities in glymphatic function.It is well acknowledged that RBD shares similar pathological hallmarks with PD, such as α-synuclein deposition (Liguori et al., 2023).Sleep disturbance may impair the glymphatic system, leading to ineffective elimination of neurotoxic proteins,contributing to the development of PD (Si et al., 2022).These observations underscore the significant role of sleep and circadian rhythm in the pathway towards neurodegeneration in PD.

Neuroinflammation impairs glymphatic function and further exacerbates the inflammatory response, while glymphatic impairment aggravates inflammation by suppressing cytokine clearance (Mogensen et al., 2021).Specifically, loss of AQP4 polarization has been noted in neuroinflammation.Aqp4deletion increased the levels of neuroinflammatory factors, and rendered the dopaminergic neurons more vulnerable to α-synuclein toxicity(Zhang et al., 2023).Furthermore, theAqp4deficiency promoted activation of the microglia and modulated astrocyte-to-microglia communication in neuroinflammation (Sun et al., 2016).As a hallmark of neuroinflammation,activated microglia can release neuroinflammatory mediators and proinflammatory cytokines that may impact the function of astrocytes and astrocytic AQP4 channels (Tjalkens et al., 2017), subsequently influencing glymphatic clearance.It is evident that these molecular mechanisms mutually influence each other, establishing a feedback loop that contributes to neurodegeneration in PD and related disorders.

In general, the impaired glymphatic system and neurotoxic protein accumulation may result in a variety of consequences, including neuroinflammation, oxidative stress, impaired neuronal function, synaptic transmission deficits, and neurodegeneration, which ultimately lead to development and progression of PD and related disorders (Wang et al.,2019; Zou et al., 2019; Ding et al., 2021).However, it is worth noting that current research findings may represent just the tip of the iceberg, as the intricate mechanisms governing the interplay between the glymphatic system and neurotoxic proteins remain to be fully understood.Further studies are needed to identify the precise mechanisms underlying these relationships.Understanding these pathological processes is crucial for developing targeted therapeutic approaches aimed at restoring the glymphatic system,reducing proteinopathies, and potentially slowing down the progression of neurodegenerative diseases.

Therapeutic Potential of the Glymphatic System in Parkinson’s Disease

In general, mounting evidence has demonstrated that promoting glymphatic system function may be an efficient way to alleviate neurodegeneration.The glymphatic system can be modulated through various interventions, including pharmacological agents, body position (Lee et al., 2015), and sleep (Xie et al.,2013).These interventions may have therapeutic potential in the treatment of PD and related disorders by promoting the clearance of toxic waste products from the brain.

Previous studies have revealed the important role of sleep in PD (Xie et al., 2013; Lee et al., 2015; Benveniste et al., 2019; Tuura et al., 2021).The regulation of sleep and the circadian rhythm has emerged as an extensively explored therapeutic target in recent years.Melatonin is a hormone that plays a crucial role in regulating the sleep-wake cycle, circadian rhythm,and body temperature rhythm; nocturnal melatonin secretion was found to be reduced in patients with sleep disorders (Micic et al., 2015).In vitrostudies have demonstrated that melatonin has the potential to inhibit the formation of α-synuclein fibrils and destabilize fibrils that have formed, which may facilitate to alleviate α-synuclein toxicity and neuronal damage (Ono et al., 2012).The effects of melatonin on PD have been studied, which have revealed notable improvements in sleep symptoms in PD patients following melatonin administration, accompanied by a trend toward reducing the UPDRS III score (Liguori et al., 2022).Therefore, melatonin supplementation in PD may ameliorate symptoms and potentially slow down disease progression.Adenosine is another neuromodulator for sleep-wake regulation and increases after sleep deprivation (Leenaars et al., 2018).A prior clinical trial has found that an adenosine A2A receptor antagonist, istradefylline, could reduce the daily OFF time of patients with PD and was tolerated well (Mizuno et al., 2013).Slow wave activity (SWA) during sleep was also associated with glymphatic system function, with an inverse relationship between SWA and tau and Aβ accumulation in AD (Semyachkina-Glushkovskaya et al., 2020a).SWA augmentation, such as with olanzapine, tiagabine, and baclofen (Semyachkina-Glushkovskaya et al., 2020a) has the protential to enhance neurotoxic protein clearance and alleviate neurodegenerative disease severity.

The role of light can also be observed in the regulation of the sleep-wake cycle and circadian rhythm.Bright light therapy has been utilized in PD patients to consolidate the sleep rhythm and improve the sleep-wake cycle (Videnovic et al., 2017), potentially alleviating the disease burden.Bright light therapy has been shown to significantly improve symptoms in PD, including depression,tremor, and motor symptoms (Paus et al., 2007).Photobiomodulation therapy refers to the application of visible and near infrared light, which has shown promise in enhancing the clearance of Aβ in an AD mouse model, indicating its potential neuroprotective role by improving lymphatic drainage activity(Hamblin, 2016).A randomized controlled trial has revealed significant improvements in mobility, dynamic balance, fine motor skill, and cognitive function in patients with PD following photobiomodulation therapy, with no side effects observed (Liebert et al., 2021).

As mentioned, AQP4 is the critical component of the glymphatic system,and its deletion can aggravate glymphatic dysfunction.Therefore, the modulation of AQP4 represents a potential therapeutic target that may hold promising treatment effects.In pre-clinical research in a mouse model,the facilitator of AQP4 (TGN-073 (N-(3-benzyloxypyridin-2-yl)-benzenesulfonamide)) promoted fluid turnover and interstitial circulation in the PVSs(Huber et al., 2018), suggesting AQP4 activity facilitators as a promising avenue for the potential treatment of PD.Another potential pharmacological target is the adrenergic system.A previous study reported reduced levels of norepinephrine during sleep, and the local delivery of norepinephrine receptor antagonists in animals resulted in an increased clearance rate of the glymphatic system (Zhao et al., 2020), which implies that norepinephrine modulation can improve glymphatic system function.While evidence on the therapeutic effects of promoting glymphatic system function in PD is not conclusive, it remains a promising target for the treatment of PD.Therefore,further investigations and large sample clinical trials are warranted.

Limitations

This study has some limitations that should be noted.Firstly, most of the studies we retrieved on the impairment of the glymphatic system were focused on PD.Currently, there is relatively limited research exploring atypical parkinsonism, which may restrict our understanding of the glymphatic system from a disease-specific perspective.Secondly, there is a lack of longitudinal studies investigating the glymphatic system in PD and related disorders,which could potentially compromise the reliability of the results and leaves us uncertain about whether impairment of the glymphatic system progresses with the advancement of the disease.Thirdly, large-scale clinical trials to investigate the efficiency and safety of treatments targeting the glymphatic system in patients with PD and related disorders are warranted.Longitudinal follow-up assessments of the glymphatic system before and after treatment are also needed.

Conclusion

The investigation of glymphatic dysfunction and neurotoxic protein clearance in PD has provided valuable insight into the underlying mechanisms of the disease pathogenesis and progression, and has paved the way for the identification of novel therapeutic targets.Studies have demonstrated that impairment of the glymphatic system occurs in the prodromal stage or early stage of PD and worsens as the disease progresses.This impairment hampers the efficient clearance of neurotoxic proteins from the brain, leading to the accumulation of α-synuclein, tau, and Aβ, which triggers reactive astrogliosis,inflammatory activity, oxidative stress, impaired neuronal function, synaptic transmission deficits, and neurodegeneration.This further impairs the glymphatic system, creating a vicious cycle that perpetuates disease progression.The accumulating evidence suggests the glymphatic system to be a promising therapeutic target, with the aim to restore the glymphatic system, reduce proteinopathies, and subsequently slow down the progression of PD and related disorders.Promoting the critical components of glymphatic clearance by light or medicine interference may improve PD symptoms.Future studies to identify the efficiency and to explore new factors related to glymphatic system function are needed.

Author contributions:YY conducted the literature search and wrote the first draft of the manuscript; XZ and WL contributed to literature search; HYL revised the manuscript; TS contributed to conception of the article, creation of the illustrations, and also revised the manuscript.All authors approved the final manuscript.

Conflicts of interest:The authors declare that there are no competing interests.

Data availability statement:Not applicable.

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