Glymphatic Cerebrospinal Fluid and Solute Transport Quantified by MRI and PET Imaging

The Glymphatic System and Subarachnoid Lymphatic-Like Membrane: Recent Developments in Cerebrospinal Fluid Research

BACKGROUND

Cerebrospinal fluid (CSF) circulates throughout the ventricles, cranial and spinal subarachnoid spaces, and central spinal cord canal. CSF protects the central nervous system through mechanical cushioning, regulation of intracranial pressure, regulation of metabolic homeostasis, and provision of nutrients. Recently, investigators have characterized the glial-lymphatic (glymphatic) system, the analog of the lymphatic system in the central nervous system, and described a fourth meningeal layer; the subarachnoid lymphatic-like membrane (SLYM)relevant to the CSF.

METHODS

A narrative review was conducted.

RESULTS

In this review, we summarize these advances. We describe the development of the original model, controversies, a revised model, and a new conceptual framework. We characterize the biological functions, influence of sleep-wake cycles, and effect of aging with relevance to the glymphatic system. We highlight the role of the glymphatic system in Alzheimer's disease, idiopathic normal pressure hydrocephalus, ischemic stroke, subarachnoid hemorrhage, and traumatic brain injury. Next, we characterize the structure and role of the SLYM. Finally, we explore the relevance of the glymphatic system and SLYM to neurosurgery.

CONCLUSIONS

This manuscript will inform clinicians and scientists regarding preclinical and translational advances in the understanding of the structure, dynamics, and function of the CSF.

Quantitative MRI of Gd-DOTA Accumulation in the Mouse Brain After Intraperitoneal Administration: Validation by Mass Spectrometry

BACKGROUND

In mice, intraperitoneal (ip) contrast agent (CA) administration is convenient for mapping microvascular parameters over a long-time window. However, continuous quantitative MRI of CA accumulation in brain over hours is still missing.

PURPOSE

To validate a quantitative time-resolved MRI technique for mapping the CA kinetics in brain upon ip administration.

STUDY TYPE

Prospective, animal model.

SPECIMEN

25 C57Bl/6JRj mice underwent MRI.

FIELD STRENGTH/SEQUENCE

7-T, gradient echo sequence.

ASSESSMENT

Gd-DOTA concentration was monitored by MRI (25 s/repetition) over 135 minutes with (N = 15) and without (N = 10) ip mannitol challenge (5 g/kg). After the final repetition, the brains were sampled to quantify gadolinium by mass spectrometry (MS). Upon manual brain segmentation, the average gadolinium concentration was compared with the MS quantification in transcardially perfused (N = 20) and unperfused (N = 5) mice. Precontrast T1-maps were acquired in 8 of 25 mice.

STATISTICAL TESTS

One-tailed Spearman and Pearson correlation between gadolinium quantification by MRI and by MS, D'Agostino-Pearson test for normal distribution, Bland-Altman analysis to evaluate the agreement between MRI and MS. Significance was set at P-value <0.05.

RESULTS

MRI showed that ip administered CA reached the blood compartment (>5 mM) within 10 minutes and accumulated continuously for 2 hours in cerebrospinal fluid (>1 mM) and in brain tissue. The MRI-derived concentration maps showed interindividual differences in CA accumulation (from 0.47 to 0.81 mM at 2 hours) with a consistent distribution resembling the pathways of the glymphatic system. The average in-vivo brain concentration 2 hours post-CA administration correlated significantly (r = 0.8206) with the brain gadolinium quantification by MS for N = 21 paired observations available.

DATA CONCLUSION

The presented experimental and imaging protocol may be convenient for monitoring the spatiotemporal pattern of CA uptake and clearance in the mouse brain over 2 hours. The quantification of the CA from the MRI signal in brain is corroborated by MS.

EVIDENCE LEVEL

N/A TECHNICAL EFFICACY: Stage 1.

How Do Modifiable Risk Factors Affect Alzheimer's Disease Pathology or Mitigate Its Effect on Clinical Symptom Expression?

Epidemiological studies show that modifiable risk factors account for approximately 40% of the population variability in risk of developing dementia, including sporadic Alzheimer's disease (AD). Recent findings suggest that these factors may also modify disease trajectories of people with autosomal-dominant AD. With positron emission tomography imaging, it is now possible to study the disease many years before its clinical onset. Such studies can provide key knowledge regarding pathways for either the prevention of pathology or the postponement of its clinical expression. The former "resistance pathway" suggests that modifiable risk factors could affect amyloid and tau burden decades before the appearance of cognitive impairment. Alternatively, the resilience pathway suggests that modifiable risk factors may mitigate the symptomatic expression of AD pathology on cognition. These pathways are not mutually exclusive and may appear at different disease stages. Here, in a narrative review, we present neuroimaging evidence that supports both pathways in sporadic AD and autosomal-dominant AD. We then propose mechanisms for their protective effect. Among possible mechanisms, we examine neural and vascular mechanisms for the resistance pathway. We also describe brain maintenance and functional compensation as bases for the resilience pathway. Improved mechanistic understanding of both pathways may suggest new interventions.

Is CAA a perivascular brain clearance disease? A discussion of the evidence to date and outlook for future studies

The brain's network of perivascular channels for clearance of excess fluids and waste plays a critical role in the pathogenesis of several neurodegenerative diseases including cerebral amyloid angiopathy (CAA). CAA is the main cause of hemorrhagic stroke in the elderly, the most common vascular comorbidity in Alzheimer's disease and also implicated in adverse events related to anti-amyloid immunotherapy. Remarkably, the mechanisms governing perivascular clearance of soluble amyloid β-a key culprit in CAA-from the brain to draining lymphatics and systemic circulation remains poorly understood. This knowledge gap is critically important to bridge for understanding the pathophysiology of CAA and accelerate development of targeted therapeutics. The authors of this review recently converged their diverse expertise in the field of perivascular physiology to specifically address this problem within the framework of a Leducq Foundation Transatlantic Network of Excellence on Brain Clearance. This review discusses the overarching goal of the consortium and explores the evidence supporting or refuting the role of impaired perivascular clearance in the pathophysiology of CAA with a focus on translating observations from rodents to humans. We also discuss the anatomical features of perivascular channels as well as the biophysical characteristics of fluid and solute transport.

Exploring neurodegenerative disorders using advanced magnetic resonance imaging of the glymphatic system

The glymphatic system, a macroscopic waste clearance system in the brain, is crucial for maintaining neural health. It facilitates the exchange of cerebrospinal and interstitial fluid, aiding the clearance of soluble proteins and metabolites and distributing essential nutrients and signaling molecules. Emerging evidence suggests a link between glymphatic dysfunction and the pathogenesis of neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's disease. These disorders are characterized by the accumulation and propagation of misfolded or mutant proteins, a process in which the glymphatic system is likely involved. Impaired glymphatic clearance could lead to the buildup of these toxic proteins, contributing to neurodegeneration. Understanding the glymphatic system's role in these disorders could provide insights into their pathophysiology and pave the way for new therapeutic strategies. Pharmacological enhancement of glymphatic clearance could reduce the burden of toxic proteins and slow disease progression. Neuroimaging techniques, particularly MRI-based methods, have emerged as promising tools for studying the glymphatic system in vivo. These techniques allow for the visualization of glymphatic flow, providing insights into its function under healthy and pathological conditions. This narrative review highlights current MRI-based methodologies, such as motion-sensitizing pulsed field gradient (PFG) based methods, as well as dynamic gadolinium-based and glucose-enhanced methodologies currently used in the study of neurodegenerative disorders.

Glymphatic system dysfunction in neurodegenerative diseases

PURPOSE OF REVIEW

Purpose of this review is to update the ongoing work in the field of glymphatic and neurodegenerative research and to highlight focus areas that are particularly promising.

RECENT FINDINGS

Multiple reports have over the past decade documented that glymphatic fluid transport is broadly suppressed in neurodegenerative diseases. Most studies have focused on Alzheimer's disease using a variety of preclinical disease models, whereas the clinical work is based on various neuroimaging approaches. It has consistently been reported that brain fluid transport is impaired in patients suffering from Alzheimer's disease compared with age-matched control subjects.

SUMMARY

An open question in the field is to define the mechanistic underpinning of why glymphatic function is suppressed. Other questions include the opportunities for using glymphatic imaging for diagnostic purposes and in treatment intended to prevent or slow Alzheimer disease progression.

Clearance of interstitial fluid (ISF) and CSF (CLIC) group-part of Vascular Professional Interest Area (PIA), updates in 2022-2023. Cerebrovascular disease and the failure of elimination of Amyloid-β from the brain and retina with age and Alzheimer's disease: Opportunities for therapy

This editorial summarizes advances from the Clearance of Interstitial Fluid and Cerebrospinal Fluid (CLIC) group, within the Vascular Professional Interest Area (PIA) of the Alzheimer's Association International Society to Advance Alzheimer's Research and Treatment (ISTAART). The overarching objectives of the CLIC group are to: (1) understand the age-related physiology changes that underlie impaired clearance of interstitial fluid (ISF) and cerebrospinal fluid (CSF) (CLIC); (2) understand the cellular and molecular mechanisms underlying intramural periarterial drainage (IPAD) in the brain; (3) establish novel diagnostic tests for Alzheimer's disease (AD), cerebral amyloid angiopathy (CAA), retinal amyloid vasculopathy, amyloid-related imaging abnormalities (ARIA) of spontaneous and iatrogenic CAA-related inflammation (CAA-ri), and vasomotion; and (4) establish novel therapies that facilitate IPAD to eliminate amyloid β (Aβ) from the aging brain and retina, to prevent or reduce AD and CAA pathology and ARIA side events associated with AD immunotherapy.

Deregulation of the Glymphatic System in Alzheimer's Disease: Genetic and Non-Genetic Factors

Alzheimer's disease (AD) is the most prevalent form of dementia and is characterized by progressive degeneration of brain function. AD gradually affects the parts of the brain that control thoughts, language, behavior and mental function, severely impacting a person's ability to carry out daily activities and ultimately leading to death. The accumulation of extracellular amyloid-β peptide (Aβ) and the aggregation of intracellular hyperphosphorylated tau are the two key pathological hallmarks of AD. AD is a complex condition that involves both non-genetic risk factors (35%) and genetic risk factors (58-79%). The glymphatic system plays an essential role in clearing metabolic waste, transporting tissue fluid, and participating in the immune response. Both non-genetic and genetic risk factors affect the glymphatic system to varying degrees. The main purpose of this review is to summarize the underlying mechanisms involved in the deregulation of the glymphatic system during the progression of AD, especially concerning the diverse contributions of non-genetic and genetic risk factors. In the future, new targets and interventions that modulate these interrelated mechanisms will be beneficial for the prevention and treatment of AD.

Using Neuroimaging to Study Cerebral Amyloid Angiopathy and Its Relationship to Alzheimer's Disease

Cerebral amyloid angiopathy (CAA) is characterized by amyloid-β aggregation in the media and adventitia of the leptomeningeal and cortical blood vessels. CAA is one of the strongest vascular contributors to Alzheimer's disease (AD). It frequently co-occurs in AD patients, but the relationship between CAA and AD is incompletely understood. CAA may drive AD risk through damage to the neurovascular unit and accelerate parenchymal amyloid and tau deposition. Conversely, early AD may also drive CAA through cerebrovascular remodeling that impairs blood vessels from clearing amyloid-β. Sole reliance on autopsy examination to study CAA limits researchers' ability to investigate CAA's natural disease course and the effect of CAA on cognitive decline. Neuroimaging allows for in vivo assessment of brain function and structure and can be leveraged to investigate CAA staging and explore its associations with AD. In this review, we will discuss neuroimaging modalities that can be used to investigate markers associated with CAA that may impact AD vulnerability including hemorrhages and microbleeds, blood-brain barrier permeability disruption, reduced cerebral blood flow, amyloid and tau accumulation, white matter tract disruption, reduced cerebrovascular reactivity, and lowered brain glucose metabolism. We present possible areas for research inquiry to advance biomarker discovery and improve diagnostics.

Cerebrospinal fluid solute transport associated with sensorimotor brain activity in rodents

Cerebrospinal fluid (CSF) is crucial for maintaining neuronal homeostasis, providing nutrition, and removing metabolic waste from the brain. However, the relationship between neuronal activity and CSF solute transport remains poorly understood. To investigate the effect of regional neuronal activity on CSF solute transport, Sprague-Dawley rats (all male, n = 30) under anesthesia received an intracisternal injection of a fluorescent tracer (Texas Red ovalbumin) and were subjected to unilateral electrical stimulation of a forelimb. Two groups (n = 10 each) underwent two different types of stimulation protocols for 90 min, one including intermittent 7.5-s resting periods and the other without rest. The control group was not stimulated. Compared to the control, the stimulation without resting periods led to increased transport across most of the cortical areas, including the ventricles. The group that received intermittent stimulation showed an elevated level of solute uptake in limited areas, i.e., near/within the ventricles and on the ventral brain surface. Interhemispheric differences in CSF solute transport were also found in the cortical regions that overlap with the forelimb sensorimotor area. These findings suggest that neuronal activity may trigger local and brain-wide increases in CSF solute transport, contributing to waste clearance.

Visualizing Fluid Flows via Regularized Optimal Mass Transport with Applications to Neuroscience

The regularized optimal mass transport (rOMT) problem adds a diffusion term to the continuity equation in the original dynamic formulation of the optimal mass transport (OMT) problem proposed by Benamou and Brenier. We show that the rOMT model serves as a powerful tool in computational fluid dynamics for visualizing fluid flows in the glymphatic system. In the present work, we describe how to modify the previous numerical method for efficient implementation, resulting in a significant reduction in computational runtime. Numerical results applied to synthetic and real-data are provided.

Glymphatic-System Function Is Associated with Addiction and Relapse in Heroin Dependents Undergoing Methadone Maintenance Treatment

This study investigates the impact of methadone maintenance treatment (MMT) on the brain glymphatic system (GS) in opioid addiction in China. A total of 51 male MMT patients, 48 demographically matched healthy controls (HCs), and 20 heroin dependents (HDs) were recruited for this study. The GS functioning was assessed using diffusion-tensor-imaging analysis along perivascular spaces (DTI-ALPS index) and the bilateral ALPS divergency (DivALPS). Group differences were analyzed utilizing ANOVA and two-sample t-tests. The relationship between DivALPS and relapse rate was explored using regression analysis. The DTI-ALPS index was significantly higher for the left-side brain than the right side in all three groups. There was a significant difference for the right side (p = 0.0098) between the groups. The MMT and HD groups showed significantly higher DTI-ALPS than the HC group (p = 0.018 and 0.016, respectively). The DivALPS varied significantly among the three groups (p = 0.04), with the HD group showing the lowest and the HC group the highest values. Significant negative relationships were found between relapse count, DivALPS (p < 0.0001, Exp(B) = 0.6047), and age (p < 0.0001, Exp(B) = 0.9142). The findings suggest that MMT may contribute to promoting brain GS recovery in heroin addicts, and modulation of the GS may serve as a potential biomarker for relapse risk, providing insights into novel therapeutic strategies.

Research progress in the evaluation of glymphatic system function by the DTI-ALPS method

The glymphatic system can remove metabolic wastes from the brain, which plays a significant role in maintaining the homeostasis of the central nervous system. It is an important basis for advanced cognitive functions such as learning and memory. Studies have analyzed the function of glymphatic system by diffusion tensor imaging analysis along the perivascular space (DTI-ALPS) recently. Compared with other invasive examinations that require fluorescent tracer technique or the injection of contrast agents, DTI-ALPS can evaluate the hydromechanics of the glymphatic system via quantifying the diffusion rate of water molecules in different directions, which turns out to be a non-invasive in vivo neuroimaging method. The ALPS-index calculated by the DTI-ALPS method is significantly correlated with the cognitive function in diseases of central nervous system and other system and can reflect the dynamic changes of diseases. In general, ALPS-index is expected to become a novel neuroimaging biomarker for predicting prognosis and clinical effects.

Transport pathways and kinetics of cerebrospinal fluid tracers in mouse brain observed by dynamic contrast-enhanced MRI

Recent studies have suggested the glymphatic system as a key mechanism of waste removal in the brain. Dynamic contrast-enhanced MRI (DCE-MRI) using intracisternally administered contrast agents is a promising tool for assessing glymphatic function in the whole brain. In this study, we evaluated the transport kinetics and distribution of three MRI contrast agents with vastly different molecular sizes in mice. Our results demonstrate that oxygen-17 enriched water (H217O), which has direct access to parenchymal tissues via aquaporin-4 water channels, exhibited significantly faster and more extensive transport compared to the two gadolinium-based contrast agents (Gd-DTPA and GadoSpin). Time-lagged correlation and clustering analyses also revealed different transport pathways for Gd-DTPA and H217O. Furthermore, there were significant differences in transport kinetics of the three contrast agents to the lateral ventricles, reflecting the differences in forces that drive solute transport in the brain. These findings suggest the size-dependent transport pathways and kinetics of intracisternally administered contrast agents and the potential of DCE-MRI for assessing multiple aspects of solute transport in the glymphatic system.

Human brain solute transport quantified by glymphatic MRI-informed biophysics during sleep and sleep deprivation

Whether you are reading, running or sleeping, your brain and its fluid environment continuously interacts to distribute nutrients and clear metabolic waste. Yet, the precise mechanisms for solute transport within the human brain have remained hard to quantify using imaging techniques alone. From multi-modal human brain MRI data sets in sleeping and sleep-deprived subjects, we identify and quantify CSF tracer transport parameters using forward and inverse subject-specific computational modelling. Our findings support the notion that extracellular diffusion alone is not sufficient as a brain-wide tracer transport mechanism. Instead, we show that human MRI observations align well with transport by either by an effective diffusion coefficent 3.5[Formula: see text] that of extracellular diffusion in combination with local clearance rates corresponding to a tracer half-life of up to 5 h, or by extracellular diffusion augmented by advection with brain-wide average flow speeds on the order of 1-9 [Formula: see text]m/min. Reduced advection fully explains reduced tracer clearance after sleep-deprivation, supporting the role of sleep and sleep deprivation on human brain clearance.

A SPECT-based method for dynamic imaging of the glymphatic system in rats

The glymphatic system is a brain-wide waste drainage system that promotes cerebrospinal fluid circulation through the brain to remove waste metabolites. Currently, the most common methods for assessing glymphatic function are ex vivo fluorescence microscopy of brain slices, macroscopic cortical imaging, and MRI. While all these methods have been crucial for expanding our understanding of the glymphatic system, new techniques are required to overcome their specific drawbacks. Here, we evaluate SPECT/CT imaging as a tool to assess glymphatic function in different anesthesia-induced brain states using two radiolabeled tracers, [111In]-DTPA and [99mTc]-NanoScan. Using SPECT, we confirmed the existence of brain state-dependent differences in glymphatic flow and we show brain state-dependent differences of CSF flow kinetics and CSF egress to the lymph nodes. We compare SPECT and MRI for imaging glymphatic flow and find that the two imaging modalities show the same overall pattern of CSF flow, but that SPECT was specific across a greater range of tracer concentrations than MRI. Overall, we find that SPECT imaging is a promising tool for imaging the glymphatic system, and that qualities such as high sensitivity and the variety of available tracers make SPECT imaging a good alternative for glymphatic research.

Continuous positive airway pressure increases CSF flow and glymphatic transport

Respiration can positively influence cerebrospinal fluid (CSF) flow in the brain, yet its effects on central nervous system (CNS) fluid homeostasis, including waste clearance function via glymphatic and meningeal lymphatic systems, remain unclear. Here, we investigated the effect of supporting respiratory function via continuous positive airway pressure (CPAP) on glymphatic-lymphatic function in spontaneously breathing anesthetized rodents. To do this, we used a systems approach combining engineering, MRI, computational fluid dynamics analysis, and physiological testing. We first designed a nasal CPAP device for use in the rat and demonstrated that it functioned similarly to clinical devices, as evidenced by its ability to open the upper airway, augment end-expiratory lung volume, and improve arterial oxygenation. We further showed that CPAP increased CSF flow speed at the skull base and augmented glymphatic transport regionally. The CPAP-induced augmented CSF flow speed was associated with an increase in intracranial pressure (ICP), including the ICP waveform pulse amplitude. We suggest that the augmented pulse amplitude with CPAP underlies the increase in CSF bulk flow and glymphatic transport. Our results provide insights into the functional crosstalk at the pulmonary-CSF interface and suggest that CPAP might have therapeutic benefit for sustaining glymphatic-lymphatic function.

The glymphatic system: a new perspective on brain diseases

The glymphatic system is a brain-wide perivascular pathway driven by aquaporin-4 on the endfeet of astrocytes, which can deliver nutrients and active substances to the brain parenchyma through periarterial cerebrospinal fluid (CSF) influx pathway and remove metabolic wastes through perivenous clearance routes. This paper summarizes the composition, overall fluid flow, solute transport, related diseases, affecting factors, and preclinical research methods of the glymphatic system. In doing so, we aim to provide direction and reference for more relevant researchers in the future.

Disentangling the impact of cerebrospinal fluid formation and neuronal activity on solute clearance from the brain

BACKGROUND

Despite recent attention, pathways and mechanisms of fluid transposition in the brain are still a matter of intense discussion and driving forces underlying waste clearance in the brain remain elusive. Consensus exists that net solute transport is a prerequisite for efficient clearance. The individual impact of neuronal activity and cerebrospinal fluid (CSF) formation, which both vary with brain state and anesthesia, remain unclear.

METHODS

To separate conditions with high and low neuronal activity and high and low CSF formation, different anesthetic regimens in naive rat were established, using Isoflurane (ISO), Medetomidine (MED), acetazolamide or combinations thereof. With dynamic contrast-enhanced MRI, after application of low molecular weight contrast agent (CA) Gadobutrol to cisterna magna, tracer distribution was monitored as surrogate for solute clearance. Simultaneous fiber-based Ca²⁺-recordings informed about the state of neuronal activity under different anesthetic regimen. T2-weighted MRI and diffusion-weighted MRI (DWI) provided size of subarachnoidal space and aqueductal flow as surrogates for CSF formation. Finally, a pathway and mechanism-independent two-compartment model was introduced to provide a measure of efficiency for solute clearance from the brain.

RESULTS

Anatomical imaging, DWI and Ca²⁺-recordings confirmed that conditions with distinct levels of neuronal activity and CSF formation were achieved. A sleep-resembling condition, with reduced neuronal activity and enhanced CSF formation was achieved using ISO+MED and an awake-like condition with high neuronal activity using MED alone. CA distribution in the brain correlated with the rate of CSF formation. The cortical brain state had major influence on tracer diffusion. Under conditions with low neuronal activity, higher diffusivity suggested enlargement of extracellular space, facilitating a deeper permeation of solutes into brain parenchyma. Under conditions with high neuronal activity, diffusion of solutes into parenchyma was hindered and clearance along paravascular pathways facilitated. Exclusively based on the measured time signal curves, the two-compartment model provided net exchange ratios, which were significantly larger for the sleep-resembling condition than for the awake-like condition.

CONCLUSIONS

Efficiency of solute clearance in brain changes with alterations in both state of neuronal activity and CSF formation. Our clearance pathway and mechanism agnostic kinetic model informs about net solute transport, solely based on the measured time signal curves. This rather simplifying approach largely accords with preclinical and clinical findings.

Imaging of the meningeal lymphatic network in healthy adults: A 7T MRI study

BACKGROUND AND PURPOSE

Meningeal lymphatic vessels (MLVs) along the dural venous sinuses are suspected to be important in connecting the glymphatic and peripheral lymphatic system. Understanding the topography of MLVs may clarify the role of the glymphatic system in neurological diseases. The aim of this analysis was to use high resolution pre- and post-contrast FLAIR 7T MRI to identify and characterize the morphology of MLV in a cohort of healthy volunteers.

MATERIALS AND METHODS

MRI examinations of seventeen healthy volunteers enrolled as controls in a larger 7T MRI study were reviewed. Pre- and post-contrast 3-D FLAIR subtractions and MP2RAGE sequences were spatially normalized and reviewed for signal intensity and enhancement patterns within putative MLVs along pre-determined dural and venous structures. Frequency of occurrence of MLVs at the above-described locations and patterns of their enhancement were analyzed.

RESULTS

Putative MLVs are commonly located along the superior sagittal sinus (SSS) and cortical veins. A "fixed enhancement" signal pattern was more frequent at these locations (p<.05). The morphology of MLVs along the SSS qualitatively changes in an antero-posterior direction. Lack of signal was more frequent along the straight and transverse sinuses (p<.05).

CONCLUSION

Putative MLVs in healthy individuals are concentrated along the SSS and cortical veins. FLAIR signal and enhancement characteristics suggest these structures may transport proteinaceous fluid. Pathways connecting MLVs to cervical lymph nodes however remain unclear.

The Neurovasculome: Key Roles in Brain Health and Cognitive Impairment: A Scientific Statement From the American Heart Association/American Stroke Association

BACKGROUND

Preservation of brain health has emerged as a leading public health priority for the aging world population. Advances in neurovascular biology have revealed an intricate relationship among brain cells, meninges, and the hematic and lymphatic vasculature (the neurovasculome) that is highly relevant to the maintenance of cognitive function. In this scientific statement, a multidisciplinary team of experts examines these advances, assesses their relevance to brain health and disease, identifies knowledge gaps, and provides future directions.

METHODS

Authors with relevant expertise were selected in accordance with the American Heart Association conflict-of-interest management policy. They were assigned topics pertaining to their areas of expertise, reviewed the literature, and summarized the available data.

RESULTS

The neurovasculome, composed of extracranial, intracranial, and meningeal vessels, as well as lymphatics and associated cells, subserves critical homeostatic functions vital for brain health. These include delivering O2 and nutrients through blood flow and regulating immune trafficking, as well as clearing pathogenic proteins through perivascular spaces and dural lymphatics. Single-cell omics technologies have unveiled an unprecedented molecular heterogeneity in the cellular components of the neurovasculome and have identified novel reciprocal interactions with brain cells. The evidence suggests a previously unappreciated diversity of the pathogenic mechanisms by which disruption of the neurovasculome contributes to cognitive dysfunction in neurovascular and neurodegenerative diseases, providing new opportunities for the prevention, recognition, and treatment of these conditions.

CONCLUSIONS

These advances shed new light on the symbiotic relationship between the brain and its vessels and promise to provide new diagnostic and therapeutic approaches for brain disorders associated with cognitive dysfunction.

Prospective Safety Study of Intrathecal Gadobutrol in Different Doses

BACKGROUND AND PURPOSE

In our clinical practice, we increasingly use intrathecal contrast-enhanced glymphatic MR imaging to assess CSF disturbances. However, because intrathecal MR imaging contrast agents such as gadobutrol (Gadovist; 1.0 mmol/mL) are used off-label, a thorough understanding of the safety profile is required.

MATERIALS AND METHODS

We performed a prospective safety study from August 2020 to June 2022 of intrathecal gadobutrol, including consecutive patients who received either 0.50, 0.25, or 0.10 mmol. Serious and nonserious adverse events were recorded systematically at 1-3 days, 4 weeks, and >6 months after the intrathecal administration.

RESULTS

The study included 196 patients who received intrathecal gadobutrol, including patients assessed for idiopathic normal pressure hydrocephalus (iNPH, n = 144) or patients examined for other CSF disorders (non-iNPH cohort; n = 52). The intrathecal gadobutrol doses were either 0.50 mmol (n = 56), 0.25 mmol (n = 111), or 0.10 mmol (n = 29). No serious adverse events were observed. Nonserious adverse events on days 1-3 after intrathecal gadobutrol were, to some degree, dose-dependent but mild-to-moderate, including severe headache, nausea, and/or dizziness in 6/196 (6.3%) patients, and they were more common in the non-iNPH than in the iNPH cohort. At 4 weeks, none reported severe nonserious adverse events, and 9/179 (5.0%) patients had mild-to-moderate symptoms. After >6 months, 2 patients reported mild headache.

CONCLUSIONS

The present study adds to the accumulating evidence that intrathecal gadobutrol in doses up to 0.50 is safe.

Quantification approaches for magnetic resonance imaging following intravenous gadolinium injection: A window into brain-wide glymphatic function

The glymphatic system is a brain-wide network of perivascular pathways along which cerebrospinal fluid and interstitial fluid rapidly exchange, facilitating solute and waste clearance from the brain parenchyma. The characterization of this exchange process in humans has relied primarily upon serial magnetic resonance imaging following intrathecal gadolinium-based contrast agent injection. However, less invasive approaches are needed. Here, we administered a gadolinium-based contrast agent intravenously in eight healthy participants and acquired magnetic resonance imaging scans prior to and 30, 90, 180, and 360 min post contrast injection. Using a region-of-interest approach, we observed that peripheral tissues and blood vessels exhibited high enhancement at 30 min after contrast administration, likely reflecting vascular and peripheral interstitial distribution of the gadolinium-based contrast agent. Ventricular, grey matter and white matter enhancement peaked at 90 min, declining thereafter. Using k-means clustering, we identify distinct distribution volumes reflecting the leptomeningeal perivascular network, superficial grey matter and deep grey/white matter that exhibit a sequential enhancement pattern consistent with parenchymal contrast enhancement via the subarachnoid cerebrospinal fluid compartment. We also outline the importance of correcting for (otherwise automatic) autoscaling of signal intensities, which could potentially lead to misinterpretation of gadolinium-based contrast agent distribution kinetics. In summary, we visualize and quantify delayed tissue enhancement following intravenous administration of gadolinium-based contrast agent in healthy human participants.

Brain washing and neural health: role of age, sleep, and the cerebrospinal fluid melatonin rhythm

The brain lacks a classic lymphatic drainage system. How it is cleansed of damaged proteins, cellular debris, and molecular by-products has remained a mystery for decades. Recent discoveries have identified a hybrid system that includes cerebrospinal fluid (CSF)-filled perivascular spaces and classic lymph vessels in the dural covering of the brain and spinal cord that functionally cooperate to remove toxic and non-functional trash from the brain. These two components functioning together are referred to as the glymphatic system. We propose that the high levels of melatonin secreted by the pineal gland directly into the CSF play a role in flushing pathological molecules such as amyloid-β peptide (Aβ) from the brain via this network. Melatonin is a sleep-promoting agent, with waste clearance from the CNS being highest especially during slow wave sleep. Melatonin is also a potent and versatile antioxidant that prevents neural accumulation of oxidatively-damaged molecules which contribute to neurological decline. Due to its feedback actions on the suprachiasmatic nucleus, CSF melatonin rhythm functions to maintain optimal circadian rhythmicity, which is also critical for preserving neurocognitive health. Melatonin levels drop dramatically in the frail aged, potentially contributing to neurological failure and dementia. Melatonin supplementation in animal models of Alzheimer's disease (AD) defers Aβ accumulation, enhances its clearance from the CNS, and prolongs animal survival. In AD patients, preliminary data show that melatonin use reduces neurobehavioral signs such as sundowning. Finally, melatonin controls the mitotic activity of neural stem cells in the subventricular zone, suggesting its involvement in neuronal renewal.

Transport Pathways and Kinetics of Cerebrospinal Fluid Tracers in Mouse Brain Observed by Dynamic Contrast-Enhanced MRI

Background: Recent studies have suggested the glymphatic system as a solute transport pathway and waste removal mechanism in the brain. Imaging intracisternally administered tracers provides the opportunity of assessing various aspects of the glymphatic function. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) allows the evaluation of both the kinetics and spatial distribution of tracer transport in the whole brain. However, assessing mouse glymphatic function by DCE-MRI has been challenged by the small size of a mouse brain and the limited volume of fluids that can be delivered intracisternally without significantly altering the intracranial pressure. Further, previous studies in rats suggest that assessment of glymphatic function by DCE-MRI is dependent on the molecular size of the contrast agents. Methods: We established and validated an intracisternal infusion protocol in mice that allowed the measurements of the entire time course of contrast agent transport for 2 hours. The transport kinetics and distribution of three MRI contrast agents with drastically different molecular weights (MWs): Gd-DTPA (MW=661.8 Da, n=7), GadoSpin-P (MW=200 kDa, n=6), and oxygen-17 enriched water (H 2 17 O, MW=19 Da, n=7), were investigated. Results: The transport of H 2 17 O was significantly faster and more extensive than the two gadolinium-based contrast agents. Time-lagged correlation analysis and clustering analysis comparing the kinetics of Gd-DTPA and H 2 17 O transport also showed different cluster patterns and lag time between different regions of the brain, suggesting different transport pathways for H 2 17 O because of its direct access to parenchymal tissues via the aquaporin-4 water channels. Further, there were also significant differences in the transport kinetics of the three tracers to the lateral ventricles, which reflects the differences in forces that drive tracer transport in the brain. Conclusions: Comparison of the transport kinetics and distribution of three MRI contrast agents with different molecular sizes showed drastically different transport profiles and clustering patterns, suggesting that the transport pathways and kinetics in the glymphatic system are size-dependent.

Hypothermia impairs glymphatic drainage in traumatic brain injury as assessed by dynamic contrast-enhanced MRI with intrathecal contrast

Introduction

The impact of hypothermia on the impaired drainage function of the glymphatic system in traumatic brain injury (TBI) is not understood.

Methods

Male Sprague-Dawley rats undergoing controlled cortical impact injury (CCI) were subjected to hypothermia or normothermia treatment. The rats undergoing sham surgery without CCI were used as the control. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with intrathecal administration of low- and high-molecular-weight contrast agents (Gd-DTPA and hyaluronic acid conjugated Gd-DTPA) was performed after TBI and head temperature management. The semiquantitative kinetic parameters characterizing the contrast infusion and cleanout in the brain, including influx rate, efflux rate, and clearance duration, were calculated from the average time-intensity curves.

Results and discussion

The qualitative and semiquantitative results of DCE-MRI obtained from all examined perivascular spaces and most brain tissue regions showed a significantly increased influx rate and efflux rate and decreased clearance duration among all TBI animals, demonstrating a significant impairment of glymphatic drainage function. This glymphatic drainage dysfunction was exacerbated when additional hypothermia was applied. The early glymphatic drainage reduction induced by TBI and aggravated by hypothermia was linearly related to the late increased deposition of p-tau and beta-amyloid revealed by histopathologic and biochemical analysis and cognitive impairment assessed by the Barnes maze and novel object recognition test. The glymphatic system dysfunction induced by hypothermia may be an indirect alternative pathophysiological factor indicating injury to the brain after TBI. Longitudinal studies and targeted glymphatic dysfunction management are recommended to explore the potential effect of hypothermia in TBI.

Early distribution of18 F-labeled AAV9 vectors in the cerebrospinal fluid after intracerebroventricular or intracisternal magna infusion in non-human primates

BACKGROUND

The delivery of adeno-associated virus (AAV) vectors via the cerebrospinal fluid (CSF) has emerged as a valuable method for widespread transduction in the central nervous system. Although infusion into the cerebral ventricles is a common protocol in preclinical studies of small animals, the cisterna magna has been recognized as an alternative target for clinical studies because it can be reached in a less invasive manner using an intrathecal catheter via the subarachnoid space from a lumbar puncture.

METHODS

We evaluated the early distribution of fluorine-18-labeled AAV9 vectors infused into the lateral ventricle or cisterna magna of four non-human primates using positron emission tomography. The expression of the green fluorescent protein was immunohistochemically determined.

RESULTS

In both approaches, the labeled vectors diffused into the broad arachnoid space around the brain stem and cervical spinal cord within 30 min. Both infusion routes efficiently transduced neurons in the cervical spinal cord.

CONCLUSIONS

For gene therapy that primarily targets the cervical spinal cord and brainstem, such as amyotrophic lateral sclerosis, cisterna magna infusion would be a feasible and effective administration method.

Physiology of Glymphatic Solute Transport and Waste Clearance from the Brain

This review focuses on the physiology of glymphatic solute transport and waste clearance, using evidence from experimental animal models as well as from human studies. Specific topics addressed include the biophysical characteristics of fluid and solute transport in the central nervous system, glymphatic-lymphatic coupling, as well as the role of cerebrospinal fluid movement for brain waste clearance. We also discuss the current understanding of mechanisms underlying increased waste clearance during sleep.

The Role of Glymphatic System in Alzheimer’s and Parkinson’s Disease Pathogenesis

Alzheimer’s disease (AD) is the most common cause of neurodegenerative dementia, whilst Parkinson’s disease (PD) is a neurodegenerative movement disorder. These two neurodegenerative disorders share the accumulation of toxic proteins as a pathological hallmark. The lack of definitive disease-modifying treatments for these neurogenerative diseases has led to the hypothesis of new pathogenic mechanisms to target and design new potential therapeutic approaches. The recent observation that the glymphatic system is supposed to be responsible for the movement of cerebrospinal fluid into the brain and clearance of metabolic waste has led to study its involvement in the pathogenesis of these classic proteinopathies. Aquaporin-4 (AQP4), a water channel located in the endfeet of astrocyte membrane, is considered a primary driver of the glymphatic clearance system, and defective AQP4-mediated glymphatic drainage has been linked to proteinopathies. The objective of the present review is to present the recent body of knowledge that links the glymphatic system to the pathogenesis of AD and PD disease and other lifestyle factors such as sleep deprivation and exercise that may influence glymphatic system function. We will also focus on the potential neuroimaging approaches that could identify a neuroimaging marker to detect glymphatic system changes.

The glymphatic system: implications for drugs for central nervous system diseases

In the past decade, evidence for a fluid clearance pathway in the central nervous system known as the glymphatic system has grown. According to the glymphatic system concept, cerebrospinal fluid flows directionally through the brain and non-selectively clears the interstitium of metabolic waste. Importantly, the glymphatic system may be modulated by particular drugs such as anaesthetics, as well as by non-pharmacological factors such as sleep, and its dysfunction has been implicated in central nervous system disorders such as Alzheimer disease. Although the glymphatic system is best described in rodents, reports using multiple neuroimaging modalities indicate that a similar transport system exists in the human brain. Here, we overview the evidence for the glymphatic system and its role in disease and discuss opportunities to harness the glymphatic system therapeutically; for example, by improving the effectiveness of intrathecally delivered drugs.

Quantifying cerebrospinal fluid dynamics: A review of human neuroimaging contributions to CSF physiology and neurodegenerative disease

Cerebrospinal fluid (CSF), predominantly produced in the ventricles and circulating throughout the brain and spinal cord, is a key protective mechanism of the central nervous system (CNS). Physical cushioning, nutrient delivery, metabolic waste, including protein clearance, are key functions of the CSF in humans. CSF volume and flow dynamics regulate intracranial pressure and are fundamental to diagnosing disorders including normal pressure hydrocephalus, intracranial hypotension, CSF leaks, and possibly Alzheimer's disease (AD). The ability of CSF to clear normal and pathological proteins, such as amyloid-beta (Aβ), tau, alpha synuclein and others, implicates it production, circulation, and composition, in many neuropathologies. Several neuroimaging modalities have been developed to probe CSF fluid dynamics and better relate CSF volume and flow to anatomy and clinical conditions. Approaches include 2-photon microscopic techniques, MRI (tracer-based, gadolinium contrast, endogenous phase-contrast), and dynamic positron emission tomography (PET) using existing approved radiotracers. Here, we discuss CSF flow neuroimaging, from animal models to recent clinical-research advances, summarizing current endeavors to quantify and map CSF flow with implications towards pathophysiology, new biomarkers, and treatments of neurological diseases.

Assessing pulsatile waveforms of paravascular cerebrospinal fluid dynamics using dynamic diffusion-weighted imaging (dDWI)

Cerebrospinal fluid (CSF) in the paravascular spaces of the surface arteries (sPVS) is a vital pathway in brain waste clearance. Arterial pulsations may be the driving force of the paravascular flow, but its pulsatile pattern remains poorly characterized, and no clinically practical method for measuring its dynamics in the human brain is available. In this work, we introduce an imaging and quantification framework for in-vivo non-invasive assessment of pulsatile fluid dynamics in the sPVS. It used dynamic Diffusion-Weighted Imaging (dDWI) at a lower b-values of 150s/mm² and retrospective gating to detect the slow flow of CSF while suppressing the fast flow of adjacent arterial blood. The waveform of CSF flow over a cardiac cycle was revealed by synchronizing the measurements with the heartbeat. A data-driven approach was developed to identify sPVS and allow automatic quantification of the whole-brain fluid waveforms. We applied dDWI to twenty-five participants aged 18–82 y/o. Results demonstrated that the fluid waveforms across the brain showed an explicit cardiac-cycle dependency, in good agreement with the vascular pumping hypothesis. Furthermore, the shape of the CSF waveforms closely resembled the pressure waveforms of the artery wall, suggesting that CSF dynamics is tightly related to artery wall mechanics. Finally, the CSF waveforms in aging participants revealed a strong age effect, with a significantly wider systolic peak observed in the older relative to younger participants. The peak widening may be associated with compromised vascular compliance and vessel wall stiffening in the older brain. Overall, the results demonstrate the feasibility, reproducibility, and sensitivity of dDWI for detecting sPVS fluid dynamics of the human brain. Our preliminary data suggest age-related alterations of the paravascular pumping. With an acquisition time of under six minutes, dDWI can be readily applied to study fluid dynamics in normal physiological conditions and cerebrovascular/neurodegenerative diseases.

Concise Review: Recent advances in neuroimaging techniques to assist clinical trials on cell-based therapies in neurodegenerative diseases

Neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), are progressive disorders for which a curative therapy is still lacking. Cell-based therapy aims at replacing dysfunctional cellular populations by repairing damaged tissue and by enriching the microenvironment of selective brain areas, and thus constitutes a promising disease-modifying treatment of neurodegenerative diseases. Scientific research has engineered a wide range of human-derived cellular populations to help overcome some of the logistical, safety, and ethical issues associated with this approach. Open-label studies and clinical trials in human participants have employed neuroimaging techniques, such as positron emission tomography (PET) and magnetic resonance imaging (MRI), to assess the success of the transplantation, to evaluate the functional integration of the implanted tissue into the host environment and to understand the pathophysiological changes associated with the therapy. Neuroimaging has constituted an outcome measure of large, randomized clinical trials, and has given answers to clarify the pathophysiology underlying some of the complications linked with this therapy. Novel PET radiotracers and MRI sequences for the staging of neurodegenerative diseases and to study alterations at molecular level significantly expands the translational potential of neuroimaging to assist pre-clinical and clinical research on cell-based therapy in these disorders. This concise review summarizes the current use of neuroimaging in human studies of cell-based replacement therapy and focuses on future application of PET and MRI techniques to evaluate the pathophysiology and treatment efficacy, as well as to aid patient selection and as an outcome measure to improve treatment success.

Decreased CSF clearance and increased brain amyloid in Alzheimer's disease

BACKGROUND

In sporadic Alzheimer's disease (AD), brain amyloid-beta (Aβ) deposition is believed to be a consequence of impaired Aβ clearance, but this relationship is not well established in living humans. CSF clearance, a major feature of brain glymphatic clearance (BGC), has been shown to be abnormal in AD murine models. MRI phase contrast and intrathecally delivered contrast studies have reported reduced CSF flow in AD. Using PET and tau tracer 18F-THK5117, we previously reported that the ventricular CSF clearance of the PET tracer was reduced in AD and associated with elevated brain Aβ levels.

METHODS

In the present study, we use two PET tracers, 18F-THK5351 and 11C-PiB to estimate CSF clearance calculated from early dynamic PET frames in 9 normal controls and 15 AD participants.

RESULTS

we observed that the ventricular CSF clearance measures were correlated (r = 0.66, p < 0.01), with reductions in AD of 18 and 27%, respectively. We also replicated a significant relationship between ventricular CSF clearance (18F-THK5351) and brain Aβ load (r =  - 0.64, n = 24, p < 0.01). With a larger sample size, we extended our observations to show that reduced CSF clearance is associated with reductions in cortical thickness and cognitive performance.

CONCLUSIONS

Overall, the findings support the hypothesis that failed CSF clearance is a feature of AD that is related to Aβ deposition and to the pathology of AD. Longitudinal studies are needed to determine whether failed CSF clearance is a predictor of progressive amyloidosis or its consequence.

Evaluation of the Glymphatic System With Diffusion Tensor Imaging-Along the Perivascular Space in Cancer Pain

Cancer pain (CP) is one of the most common symptoms affecting life quality, and there is considerable variation in pain experience among patients with malignant tumors. Previously, it has been found that the fluid drainage function in the brain can be regulated by peripheral pain stimulation. However, the relationship between cancer pain and functional changes of the glymphatic system (an important pathway for fluid drainage in the brain) remains unclear. In this study, 97 participants were enrolled, which included 40 participants in the cancer pain (CP) group, 27 participants in the painless cancer (PLC) group and 30 participants in the control (NC) group. Differences in glymphatic system function among the three groups and between before and after pain pharmacological intervention were analyzed by measuring diffusivity and the index along the perivascular space (ALPS index) using diffusion tensor imaging. We found that diffusivity and the ALPS index were significantly lower in the CP group than in the PLC and NC group and increased following intervention with pain relief. Moreover, the ALPS index was negatively correlated with the degree of pain in the CP group. The present study verified that alterations in glymphatic function are closely related to cancer pain, and the quantification of functional changes reflects pain severity. Our findings support the use of neuroimaging biomarkers for cancer pain assessment and indicate that pain can be alleviated by regulating brain function status.

Cerebral Microcirculation, Perivascular Unit, and Glymphatic System: Role of Aquaporin-4 as the Gatekeeper for Water Homeostasis

In the past, water homeostasis of the brain was understood as a certain quantitative equilibrium of water content between intravascular, interstitial, and intracellular spaces governed mostly by hydrostatic effects i.e., strictly by physical laws. The recent achievements in molecular bioscience have led to substantial changes in this regard. Some new concepts elaborate the idea that all compartments involved in cerebral fluid homeostasis create a functional continuum with an active and precise regulation of fluid exchange between them rather than only serving as separate fluid receptacles with mere passive diffusion mechanisms, based on hydrostatic pressure. According to these concepts, aquaporin-4 (AQP4) plays the central role in cerebral fluid homeostasis, acting as a water channel protein. The AQP4 not only enables water permeability through the blood-brain barrier but also regulates water exchange between perivascular spaces and the rest of the glymphatic system, described as pan-cerebral fluid pathway interlacing macroscopic cerebrospinal fluid (CSF) spaces with the interstitial fluid of brain tissue. With regards to this, AQP4 makes water shift strongly dependent on active processes including changes in cerebral microcirculation and autoregulation of brain vessels capacity. In this paper, the role of the AQP4 as the gatekeeper, regulating the water exchange between intracellular space, glymphatic system (including the so-called neurovascular units), and intravascular compartment is reviewed. In addition, the new concepts of brain edema as a misbalance in water homeostasis are critically appraised based on the newly described role of AQP4 for fluid permeation. Finally, the relevance of these hypotheses for clinical conditions (including brain trauma and stroke) and for both new and old therapy concepts are analyzed.

Quantitative analysis of macroscopic solute transport in the murine brain

Background

Understanding molecular transport in the brain is critical to care and prevention of neurological disease and injury. A key question is whether transport occurs primarily by diffusion, or also by convection or dispersion. Dynamic contrast-enhanced (DCE-MRI) experiments have long reported solute transport in the brain that appears to be faster than diffusion alone, but this transport rate has not been quantified to a physically relevant value that can be compared to known diffusive rates of tracers.

Methods

In this work, DCE-MRI experimental data is analyzed using subject-specific finite-element models to quantify transport in different anatomical regions across the whole mouse brain. The set of regional effective diffusivities (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$D_{eff}$$\end{document}Deff), a transport parameter combining all mechanisms of transport, that best represent the experimental data are determined and compared to apparent diffusivity (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$D_{app}$$\end{document}Dapp), the known rate of diffusion through brain tissue, to draw conclusions about dominant transport mechanisms in each region.

Results

In the perivascular regions of major arteries, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$D_{eff}$$\end{document}Deff for gadoteridol (550 Da) was over 10,000 times greater than \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$D_{app}$$\end{document}Dapp. In the brain tissue, constituting interstitial space and the perivascular space of smaller blood vessels, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$D_{eff}$$\end{document}Deff was 10–25 times greater than \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$D_{app}$$\end{document}Dapp.

Conclusions

The analysis concludes that convection is present throughout the brain. Convection is dominant in the perivascular space of major surface and branching arteries (Pe > 1000) and significant to large molecules (> 1 kDa) in the combined interstitial space and perivascular space of smaller vessels (not resolved by DCE-MRI). Importantly, this work supports perivascular convection along penetrating blood vessels.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-021-00290-z.

FLUID study: study protocol for an open-label, single-centre pilot study to investigate the efFect of Lemborexant on sleep management in Japanese sUbjects aged 50 years and older with Insomnia Disorder

INTRODUCTION

Bidirectional associations have been reported between sleep disturbance and both cognitive impairment, including Alzheimer's disease and amyloid beta-peptide (Aβ) accumulation. These relationships can be explained by the glymphatic system, which acts as a garbage drainage system in the brain. As interstitial fluid dynamics are suggested to increase during sleep, clearance of Aβ can be influenced by sleep disturbance or deprivation. We hypothesised that using lemborexant, an orexin receptor antagonist, to improve sleep quality would also improve the function of the glymphatic system. We plan to examine the effect of lemborexant on sleep quality and the glymphatic system among patients with insomnia disorder.

METHODS AND ANALYSIS

This pilot study is designed as an open-label, single-arm, single-centre trial. Thirty patients aged 50 years and over with insomnia will be recruited. The participants will take lemborexant (5 mg) at bedtime for 12 weeks and undergo a home-based sleep study at baseline and weeks 4 and 12, as well as MRI examinations to evaluate the glymphatic system at baseline and week 12. The primary outcome will be changes in objective sleep parameters as evaluated using a sleep monitoring system. The secondary outcomes will be changes in subjective sleep parameters. The relationships between changes in sleep parameters and the glymphatic system will be evaluated using diffusion tensor image analysis along the perivascular space, which is called the ALPS-index. Sleep parameters and the ALPS-index will be analysed using a paired t-test or Pearson's correlation coefficient.

ETHICS AND DISSEMINATION

The study protocol was approved by Nagoya University Certified Review Board. The findings from this research will be published in peer-reviewed journals and be presented at local, national and international conferences.

TRIAL REGISTRATION NUMBER

jRCTs041210024.

Neurofluids-Deep inspiration, cilia and preloading of the astrocytic network

With the advent of real-time MRI, the motion and passage of cerebrospinal fluid can be visualized without gating and exclusion of low-frequency waves. This imaging modality gives insights into low-volume, rapidly oscillating cardiac-driven movement as well as sustained, high-volume, slowly oscillating inspiration-driven movement. Inspiration means a spontaneous or artificial increase in the intrathoracic dimensions independent of body position. Alterations in thoracic diameter enable the thoracic and spinal epidural venous compartments to be emptied and filled, producing an upward surge of cerebrospinal fluid inside the spine during inspiration; this surge counterbalances the downward pooling of venous blood toward the heart. Real-time MRI, as a macroscale in vivo observation method, could expand our knowledge of neurofluid dynamics, including how astrocytic fluid preloading is adjusted and how brain buoyancy and turgor are maintained in different postures and zero gravity. Along with these macroscale findings, new microscale insights into aquaporin-mediated fluid transfer, its sensing by cilia, and its tuning by nitric oxide will be reviewed. By incorporating clinical knowledge spanning several disciplines, certain disorders-congenital hydrocephalus with Chiari malformation, idiopathic intracranial hypertension, and adult idiopathic hydrocephalus-are interpreted and reviewed according to current concepts, from the basics of the interrelated systems to their pathology.

Neuroinflammation-Driven Lymphangiogenesis in CNS Diseases

The central nervous system (CNS) undergoes immunosurveillance despite the lack of conventional antigen presenting cells and lymphatic vessels in the CNS parenchyma. Additionally, the CNS is bathed in a cerebrospinal fluid (CSF). CSF is continuously produced, and consequently must continuously clear to maintain fluid homeostasis despite the lack of conventional lymphatics. During neuroinflammation, there is often an accumulation of fluid, antigens, and immune cells to affected areas of the brain parenchyma. Failure to effectively drain these factors may result in edema, prolonged immune response, and adverse clinical outcome as observed in conditions including traumatic brain injury, ischemic and hypoxic brain injury, CNS infection, multiple sclerosis (MS), and brain cancer. Consequently, there has been renewed interest surrounding the expansion of lymphatic vessels adjacent to the CNS which are now thought to be central in regulating the drainage of fluid, cells, and waste out of the CNS. These lymphatic vessels, found at the cribriform plate, dorsal dural meninges, base of the brain, and around the spinal cord have each been implicated to have important roles in various CNS diseases. In this review, we discuss the contribution of meningeal lymphatics to these processes during both steady-state conditions and neuroinflammation, as well as discuss some of the many still unknown aspects regarding the role of meningeal lymphatics in neuroinflammation. Specifically, we focus on the observed phenomenon of lymphangiogenesis by a subset of meningeal lymphatics near the cribriform plate during neuroinflammation, and discuss their potential roles in immunosurveillance, fluid clearance, and access to the CSF and CNS compartments. We propose that manipulating CNS lymphatics may be a new therapeutic way to treat CNS infections, stroke, and autoimmunity.

Repetitive transcranial magnetic stimulation increases the brain's drainage efficiency in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease with high prevalence rate among the elderly population. A large number of clinical studies have suggested repetitive transcranial magnetic stimulation (rTMS) as a promising non-invasive treatment for patients with mild to moderate AD. However, the underlying cellular and molecular mechanisms remain largely uninvestigated. In the current study, we examined the effect of high frequency rTMS treatment on the cognitive functions and pathological changes in the brains of 4- to 5-month old 5xFAD mice, an early pathological stage with pronounced amyloidopathy and cognitive deficit. Our results showed that rTMS treatment effectively prevented the decline of long-term memories of the 5xFAD mice for novel objects and locations. Importantly, rTMS treatment significantly increased the drainage efficiency of brain clearance pathways, including the glymphatic system in brain parenchyma and the meningeal lymphatics, in the 5xFAD mouse model. Significant reduction of Aβ deposits, suppression of microglia and astrocyte activation, and prevention of decline of neuronal activity as indicated by the elevated c-FOS expression, were observed in the prefrontal cortex and hippocampus of the rTMS-treated 5xFAD mice. Collectively, these findings provide a novel mechanistic insight of rTMS in regulating brain drainage system and β-amyloid clearance in the 5xFAD mouse model, and suggest the potential use of the clearance rate of contrast tracer in cerebrospinal fluid as a prognostic biomarker for the effectiveness of rTMS treatment in AD patients.

Brain Barriers and brain fluids research in 2020 and the fluids and barriers of the CNS thematic series on advances in in vitro modeling of the blood-brain barrier and neurovascular unit

This editorial discusses advances in brain barrier and brain fluid research in 2020. Topics include: the cerebral endothelium and the neurovascular unit; the choroid plexus; the meninges; cerebrospinal fluid and the glymphatic system; disease states impacting the brain barriers and brain fluids; drug delivery to the brain. This editorial also highlights the recently completed Fluids Barriers CNS thematic series entitled, Advances in in vitro modeling of the bloodbrain barrier and neurovascular unit. Such in vitro modeling is progressing rapidly.

Not Just a Bystander: The Emerging Role of Astrocytes and Research Tools in Studying Cognitive Dysfunctions in Schizophrenia

Cognitive dysfunction is one of the core symptoms in schizophrenia, and it is predictive of functional outcomes and therefore useful for treatment targets. Rather than improving cognitive deficits, currently available antipsychotics mainly focus on positive symptoms, targeting dopaminergic/serotoninergic neurons and receptors in the brain. Apart from investigating the neural mechanisms underlying schizophrenia, emerging evidence indicates the importance of glial cells in brain structure development and their involvement in cognitive functions. Although the etiopathology of astrocytes in schizophrenia remains unclear, accumulated evidence reveals that alterations in gene expression and astrocyte products have been reported in schizophrenic patients. To further investigate the role of astrocytes in schizophrenia, we highlighted recent progress in the investigation of the effect of astrocytes on abnormalities in glutamate transmission and impairments in the blood–brain barrier. Recent advances in animal models and behavioral methods were introduced to examine schizophrenia-related cognitive deficits and negative symptoms. We also highlighted several experimental tools that further elucidate the role of astrocytes. Instead of focusing on schizophrenia as a neuron-specific disorder, an additional astrocytic perspective provides novel and promising insight into its causal mechanisms and treatment. The involvement of astrocytes in the pathogenesis of schizophrenia and other brain disorders is worth further investigation.

High-altitude cerebral edema: its own entity or end-stage acute mountain sickness?

High-altitude cerebral edema (HACE) and acute mountain sickness (AMS) are neuropathologies associated with rapid exposure to hypoxia. However, speculation remains regarding the exact etiology of both HACE and AMS and whether they share a common mechanistic pathology. This review outlines the basic principles of HACE development, highlighting how edema could develop from 1) a progression from cytotoxic swelling to ionic edema or 2) permeation of the blood brain barrier (BBB) with or without ionic edema. Thereafter, discussion turns to the available neuroimaging literature in the context of cytotoxic, ionic, or vasogenic edema in both HACE and AMS. Although HACE is clearly caused by an increase in brain water of ionic and/or vasogenic origin, there is very little evidence that this type of edema is present when AMS develops. However, cerebral vasodilation, increased intracranial blood volume, and concomitant intracranial fluid shifts from the extracellular to the intracellular space, as interpreted from changes in diffusion indices within white matter, are observed consistently in persons acutely exposed to hypoxia and with AMS. Therefore, herein we explore the idea that intracellular swelling occurs alongside AMS, and is a critical precursor to extracellular ionic edema formation. We propose that this process produces a subtle modulation of the BBB, which either together with or independent of vasogenic edema provides a transvascular segue from the end-stage of AMS to HACE. Ultimately, this review seeks to shed light on the possible processes underlying HACE pathophysiology, and thus highlights potential avenues for future prevention and treatment.

Does Impaired Glymphatic Drainage Cause Glymphedema? A Review Tailored to Neurocritical Care and Neurosurgery

Research into the glymphatic system reached an inflection point with steep trajectory in 2012 when it was formally recognized and named, but the historical roots for it are solid and deep, dating back to pioneers such as Cushing, Weed, and Dandy. We provide an overview of key discoveries of the glymphatic system, which promotes bulk flow of fluid and solutes throughout the brain parenchyma. We also discuss the lymphatic drainage of the central nervous system. Evidence is building that failure of the glymphatic system causes glymphedema in patients commonly managed by neurocritical care and neurosurgery specialists. We review research supporting this for decompressive craniectomy, subarachnoid hemorrhage, and normal-pressure hydrocephalus. We argue that it is time for a paradigm shift from the traditional model of cerebrospinal fluid circulation to a revised model that incorporates the glymphatic pathway and lymphatic clearance. These recent breakthroughs will inspire new therapeutic approaches to recognize, reverse, and restore glymphatic dysfunction and to leverage this pathway to deliver brain-wide therapeutics.