Rapid lymphatic efflux limits cerebrospinal fluid flow to the brain

Near-Infrared-II Imaging Revealed Hypothermia Regulates Neuroinflammation Following Brain Injury by Increasing the Glymphatic Influx

Advanced in vivo imaging techniques have facilitated the comprehensive visual exploration of animal biological processes, leading to groundbreaking discoveries such as the glymphatic system. However, current limitations of macroscopic imaging techniques impede the precise investigation of physiological parameters regulating this specialized lymphatic transport system. While NIR-II fluorescence imaging has demonstrated advantages in peripheral lymphatic imaging, there are few reports regarding its utilization in the glymphatic system. To address this, a noninvasive transcranial macroscopic NIR-II fluorescence imaging model is developed using a cyanine dye-protein coupled nanoprobe. NIR-II imaging with high temporal and spatial resolution reveals that hypothermia can increase the glymphatic influx by reducing the flow rate of cerebrospinal fluid. In addition, respiratory rate, respiratory amplitude, and heart rate all play a role in regulating the glymphatic influx. Thus, targeting the glymphatic influx may alter the trajectory of immune inflammation following brain injury, providing therapeutic prospects for treating brain injury with mild hypothermia.

Brain clearance is reduced during sleep and anesthesia

It has been suggested that the function of sleep is to actively clear metabolites and toxins from the brain. Enhanced clearance is also said to occur during anesthesia. Here, we measure clearance and movement of fluorescent molecules in the brains of male mice and show that movement is, in fact, independent of sleep and wake or anesthesia. Moreover, we show that brain clearance is markedly reduced, not increased, during sleep and anesthesia.

Lymphatic network drainage resolves cerebral edema and facilitates recovery from experimental cerebral malaria

While brain swelling, associated with fluid accumulation, is a known feature of pediatric cerebral malaria (CM), how fluid and macromolecules are drained from the brain during recovery from CM is unknown. Using the experimental CM (ECM) model, we show that fluid accumulation in the brain during CM is driven by vasogenic edema and not by perivascular cerebrospinal fluid (CSF) influx. We identify that fluid and molecules are removed from the brain extremely quickly in mice with ECM to the deep cervical lymph nodes (dcLNs), predominantly through basal routes and across the cribriform plate and the nasal lymphatics. In agreement, we demonstrate that ligation of the afferent lymphatic vessels draining to the dcLNs significantly impairs fluid drainage from the brain and lowers anti-malarial drug recovery from the ECM syndrome. Collectively, our results provide insight into the pathways that coordinate recovery from CM.

NIR-II nanoprobes for investigating the glymphatic system function under anesthesia and stroke injury

The glymphatic system plays an important role in the transportation of cerebrospinal fluid (CSF) and the clearance of metabolite waste in brain. However, current imaging modalities for studying the glymphatic system are limited. Herein, we apply NIR-II nanoprobes with non-invasive and high-contrast advantages to comprehensively explore the function of glymphatic system in mice under anesthesia and cerebral ischemia-reperfusion injury conditions. Our results show that the supplement drug dexmedetomidine (Dex) enhances CSF influx in the brain, decreases its outflow to mandibular lymph nodes, and leads to significant differences in CSF accumulation pattern in the spine compared to isoflurane (ISO) alone, while both ISO and Dex do not affect the clearance of tracer-filled CSF into blood circulation. Notably, we confirm the compromised glymphatic function after cerebral ischemia-reperfusion injury, leading to impaired glymphatic influx and reduced glymphatic efflux. This technique has great potential to elucidate the underlying mechanisms between the glymphatic system and central nervous system diseases.

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.

Glymphatic-lymphatic coupling: assessment of the evidence from magnetic resonance imaging of humans

The discoveries that cerebrospinal fluid participates in metabolic perivascular exchange with the brain and further drains solutes to meningeal lymphatic vessels have sparked a tremendous interest in translating these seminal findings from animals to humans. A potential two-way coupling between the brain extra-vascular compartment and the peripheral immune system has implications that exceed those concerning neurodegenerative diseases, but also imply that the central nervous system has pushed its immunological borders toward the periphery, where cross-talk mediated by cerebrospinal fluid may play a role in a range of neoplastic and immunological diseases. Due to its non-invasive approach, magnetic resonance imaging has typically been the preferred methodology in attempts to image the glymphatic system and meningeal lymphatics in humans. Even if flourishing, the research field is still in its cradle, and interpretations of imaging findings that topographically associate with reports from animals have yet seemed to downplay the presence of previously described anatomical constituents, particularly in the dura. In this brief review, we illuminate these challenges and assess the evidence for a glymphatic-lymphatic coupling. Finally, we provide a new perspective on how human brain and meningeal clearance function may possibly be measured in future.

Long-term isoflurane anesthesia induces cognitive deficits via AQP4 depolarization mediated blunted glymphatic inflammatory proteins clearance

Perioperative neurocognitive disorders (PND) refer to cognitive deterioration that occurs after surgery or anesthesia. Prolonged isoflurane exposure has potential neurotoxicity and induces PND, but the mechanism is unclear. The glymphatic system clears harmful metabolic waste from the brain. This study sought to unveil the functions of glymphatic system in PND and explore the underlying molecular mechanisms. The PND mice model was established by long term isoflurane anesthesia. The glymphatic function was assessed by multiple in vitro and in vivo methods. An adeno-associated virus was used to overexpress AQP4 and TGN-020 was used to inhibit its function. This research revealed that the glymphatic system was impaired in PND mice and the blunted glymphatic transport was closely associated with the accumulation of inflammatory proteins in the hippocampus. Increasing AQP4 polarization could enhance glymphatic transport and suppresses neuroinflammation, thereby improve cognitive function in the PND model mice. However, a marked impaired glymphatic inflammatory proteins clearance and the more severe cognitive dysfunction were observed when decreasing AQP4 polarization. Therefore, long-term isoflurane anesthesia causes blunted glymphatic system by inducing AQP4 depolarization, enhanced the AQP4 polarization can alleviate the glymphatic system malfunction and reduce the neuroinflammatory response, which may be a potential treatment strategy for PND.

Multisensory gamma stimulation promotes glymphatic clearance of amyloid

The glymphatic movement of fluid through the brain removes metabolic waste1-4. Noninvasive 40 Hz stimulation promotes 40 Hz neural activity in multiple brain regions and attenuates pathology in mouse models of Alzheimer's disease5-8. Here we show that multisensory gamma stimulation promotes the influx of cerebrospinal fluid and the efflux of interstitial fluid in the cortex of the 5XFAD mouse model of Alzheimer's disease. Influx of cerebrospinal fluid was associated with increased aquaporin-4 polarization along astrocytic endfeet and dilated meningeal lymphatic vessels. Inhibiting glymphatic clearance abolished the removal of amyloid by multisensory 40 Hz stimulation. Using chemogenetic manipulation and a genetically encoded sensor for neuropeptide signalling, we found that vasoactive intestinal peptide interneurons facilitate glymphatic clearance by regulating arterial pulsatility. Our findings establish novel mechanisms that recruit the glymphatic system to remove brain amyloid.

The glymphatic system and cerebral small vessel disease

OBJECTIVES

Cerebral small vessel disease is a group of pathologies in which alterations of the brain's blood vessels contribute to stroke and neurocognitive changes. Recently, a neurotoxic waste clearance system composed of perivascular spaces abutting the brain's blood vessels, termed the glymphatic system, has been identified as a key player in brain homeostasis. Given that small vessel disease and the glymphatic system share anatomical structures, this review aims to reexamine small vessel disease in the context of the glymphatic system and highlight novel aspects of small vessel disease physiology.

MATERIALS AND METHODS

This review was conducted with an emphasis on studies that examined aspects of small vessel disease and on works characterizing the glymphatic system. We searched PubMed for relevant articles using the following keywords: glymphatics, cerebral small vessel disease, arterial pulsatility, hypertension, blood-brain barrier, endothelial dysfunction, stroke, diabetes.

RESULTS

Cerebral small vessel disease and glymphatic dysfunction are anatomically connected and significant risk factors are shared between the two. These include hypertension, type 2 diabetes, advanced age, poor sleep, obesity, and neuroinflammation. There is clear evidence that CSVD hinders the effective functioning of glymphatic system.

CONCLUSION

These shared risk factors, as well as the model of cerebral amyloid angiopathy pathogenesis, hint at the possibility that glymphatic dysfunction could independently contribute to the pathogenesis of cerebral small vessel disease. However, the current evidence supports a model of cascading dysfunction, wherein concurrent small vessel and glymphatic injury hinder glymphatic-mediated recovery and promote the progression of subclinical to clinical disease.

Blood pressure lowering enhances cerebrospinal fluid efflux to the systemic circulation primarily via the lymphatic vasculature

BACKGROUND

Inside the incompressible cranium, the volume of cerebrospinal fluid is directly linked to blood volume: a change in either will induce a compensatory change in the other. Vasodilatory lowering of blood pressure has been shown to result in an increase of intracranial pressure, which, in normal circumstances should return to equilibrium by increased fluid efflux. In this study, we investigated the effect of blood pressure lowering on fluorescent cerebrospinal fluid tracer absorption into the systemic blood circulation.

METHODS

Blood pressure lowering was performed by an i.v. administration of nitric oxide donor (sodium nitroprusside, 5 µg kg-1 min-1) or the Ca2+-channel blocker (nicardipine hydrochloride, 0.5 µg kg-1 min-1) for 10, and 15 to 40 min, respectively. The effect of blood pressure lowering on cerebrospinal fluid clearance was investigated by measuring the efflux of fluorescent tracers (40 kDa FITC-dextran, 45 kDa Texas Red-conjugated ovalbumin) into blood and deep cervical lymph nodes. The effect of nicardipine on cerebral hemodynamics was investigated by near-infrared spectroscopy. The distribution of cerebrospinal fluid tracers (40 kDa horse radish peroxidase,160 kDa nanogold-conjugated IgG) in exit pathways was also analyzed at an ultrastructural level using electron microscopy.

RESULTS

Nicardipine and sodium nitroprusside reduced blood pressure by 32.0 ± 19.6% and 24.0 ± 13.3%, while temporarily elevating intracranial pressure by 14.0 ± 7.0% and 18.2 ± 15.0%, respectively. Blood pressure lowering significantly increased tracer accumulation into dorsal dura, deep cervical lymph nodes and systemic circulation, but reduced perivascular inflow along penetrating arteries in the brain. The enhanced tracer efflux by blood pressure lowering into the systemic circulation was markedly reduced (- 66.7%) by ligation of lymphatic vessels draining into deep cervical lymph nodes.

CONCLUSIONS

This is the first study showing that cerebrospinal fluid clearance can be improved with acute hypotensive treatment and that the effect of the treatment is reduced by ligation of a lymphatic drainage pathway. Enhanced cerebrospinal fluid clearance by blood pressure lowering may have therapeutic potential in diseases with dysregulated cerebrospinal fluid  flow.

Large molecules from the cerebrospinal fluid enter the optic nerve but not the retina of mice

It has been proposed that cerebrospinal fluid (CSF) can enter and leave the retina and optic nerve along perivascular spaces surrounding the central retinal vessels as part of an aquaporin-4 (AQP4) dependent ocular 'glymphatic' system. Here, we injected fluorescent dextrans and antibodies into the CSF of mice at the cisterna magna and measured their distribution in the optic nerve and retina. We found that uptake of dextrans in the perivascular spaces and parenchyma of the optic nerve is highly sensitive to the cisternal injection rate, where high injection rates, in which dextran disperses fully in the sub-arachnoid space, led to uptake along the full length of the optic nerve. Accumulation of dextrans in the optic nerve did not differ significantly in wild-type and AQP4 knockout mice. Dextrans did not enter the retina, even when intracranial pressure was greatly increased over intraocular pressure. However, elevation of intraocular pressure reduced accumulation of fluorescent dextrans in the optic nerve head, and intravitreally injected dextrans left the retina via perivascular spaces surrounding the central retinal vessels. Human IgG distributed throughout the perivascular and parenchymal areas of the optic nerve to a similar extent as dextran following cisternal injection. However, uptake of a cisternally injected AQP4-IgG antibody, derived from a seropositive neuromyelitis optica spectrum disorder subject, was limited by AQP4 binding. We conclude that large molecules injected in the CSF can accumulate along the length of the optic nerve if they are fully dispersed in the optic nerve sub-arachnoid space but that they do not enter the retina.

Mechanisms behind changes of neurodegeneration biomarkers in plasma induced by sleep deprivation

Acute sleep deprivation has been shown to affect cerebrospinal fluid and plasma concentrations of biomarkers associated with neurodegeneration, though the mechanistic underpinnings remain unknown. This study compared individuals who, for one night, were either subject to total sleep deprivation or free sleep, (i) examining plasma concentrations of neurodegeneration biomarkers the morning after sleep deprivation or free sleep and (ii) determining how overnight changes in biomarkers plasma concentrations correlate with indices of meningeal lymphatic and glymphatic clearance functions. Plasma concentrations of amyloid-β 40 and 42, phosphorylated tau peptide 181, glial fibrillary acid protein and neurofilament light were measured longitudinally in subjects who from Day 1 to Day 2 either underwent total sleep deprivation (n = 7) or were allowed free sleep (n = 21). The magnetic resonance imaging contrast agent gadobutrol was injected intrathecally, serving as a cerebrospinal fluid tracer. Population pharmacokinetic model parameters of gadobutrol cerebrospinal fluid-to-blood clearance were utilized as a proxy of meningeal lymphatic clearance capacity and intrathecal contrast-enhanced magnetic resonance imaging as a proxy of glymphatic function. After one night of acute sleep deprivation, the plasma concentrations of amyloid-β 40 and 42 were reduced, but not the ratio, and concentrations of the other biomarkers were unchanged. The overnight change in amyloid-β 40 and 42 plasma concentrations in the sleep group correlated significantly with indices of meningeal lymphatic clearance capacity, while this was not seen for the other neurodegeneration biomarkers. However, overnight change in plasma concentrations of amyloid-β 40 and 42 did not correlate with the glymphatic marker. On the other hand, the overnight change in plasma concentration of phosphorylated tau peptide 181 correlated significantly with the marker of glymphatic function in the sleep deprivation group but not in the sleep group. The present data add to the evidence of the role of sleep and sleep deprivation on plasma neurodegeneration concentrations; however, the various neurodegeneration biomarkers respond differently with different mechanisms behind sleep-induced alterations in amyloid-β and tau plasma concentrations. Clearance capacity of meningeal lymphatics seems more important for sleep-induced changes in amyloid-β 40 and 42 plasma concentrations, while glymphatic function seems most important for change in plasma concentration of phosphorylated tau peptide 181 during sleep deprivation. Altogether, the present data highlight diverse mechanisms behind sleep-induced effects on concentrations of plasma neurodegeneration biomarkers.

In vivo distribution of cerebrospinal fluid tracer in human upper spinal cord and brain stem

BACKGROUNDIntrathecal injection is an attractive route through which drugs can be administered and directed to the spinal cord, restricted by the blood-spinal cord barrier. However, in vivo data on the distribution of cerebrospinal fluid (CSF) substances in the human spinal cord are lacking. We conducted this study to assess the enrichment of a CSF tracer in the upper cervical spinal cord and the brain stem.METHODSAfter lumbar intrathecal injection of a magnetic resonance imaging (MRI) contrast agent, gadobutrol, repeated blood samples and MRI of the upper cervical spinal cord, brain stem, and adjacent subarachnoid spaces (SAS) were obtained through 48 hours. The MRI scans were then analyzed for tracer distribution in the different regions and correlated to age, disease, and amounts of tracer in the blood to determine CSF-to-blood clearance.RESULTSThe study included 26 reference individuals and 35 patients with the dementia subtype idiopathic normal pressure hydrocephalus (iNPH). The tracer enriched all analyzed regions. Moreover, tracer enrichment in parenchyma was associated with tracer enrichment in the adjacent SAS and with CSF-to-blood clearance. Clearance from the CSF was delayed in patients with iNPH compared with younger reference patients.CONCLUSIONA CSF tracer substance administered to the lumbar thecal sac can access the parenchyma of the upper cervical spinal cord and brain stem. Since CSF-to-blood clearance is highly individual and is associated with tracer level in CSF, clearance assessment may be used to tailor intrathecal treatment regimes.FUNDINGSouth-Eastern Norway Regional Health and Østfold Hospital Trust supported the research and publication of this work.

The directional flow generated by peristalsis in perivascular networks-Theoretical and numerical reduced-order descriptions

Directional fluid flow in perivascular spaces surrounding cerebral arteries is hypothesized to play a key role in brain solute transport and clearance. While various drivers for a pulsatile flow, such as cardiac or respiratory pulsations, are well quantified, the question remains as to which mechanisms could induce a directional flow within physiological regimes. To address this question, we develop theoretical and numerical reduced-order models to quantify the directional (net) flow induceable by peristaltic pumping in periarterial networks. Each periarterial element is modeled as a slender annular space bounded internally by a circular tube supporting a periodic traveling (peristaltic) wave. Under reasonable assumptions of a small Reynolds number flow, small radii, and small-amplitude peristaltic waves, we use lubrication theory and regular perturbation methods to derive theoretical expressions for the directional net flow and pressure distribution in the perivascular network. The reduced model is used to derive closed-form analytical expressions for the net flow for simple network configurations of interest, including single elements, two elements in tandem, and a three element bifurcation, with results compared with numerical predictions. In particular, we provide a computable theoretical estimate of the net flow induced by peristaltic motion in perivascular networks as a function of physiological parameters, notably, wave length, frequency, amplitude, and perivascular dimensions. Quantifying the maximal net flow for specific physiological regimes, we find that vasomotion may induce net pial periarterial flow velocities on the order of a few to tens of μ m/s and that sleep-related changes in vasomotion pulsatility may drive a threefold flow increase.

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.

A bibliometric and knowledge-map analysis of the glymphatic system from 2012 to 2022

Objective

To explore the development context, research hotspots and frontiers in the glymphatic system (GS) field from 2012 to 2022 by bibliometric analysis.

Methods

The Web of Science Core Collection (WoSCC) database was searched for articles published between 2012 and 2022. Microsoft Excel was used to manage the data. VOSviewer, CiteSpace, GraphPad Prism, the Web of Science, and an online analysis platform for bibliometrics (http://bibliometric.com/) were used to analyze the countries, institutions, journals, and collaboration networks among authors and the types of articles, developmental directions, references, and top keywords of published articles.

Results

A total of 412 articles were retrieved, including 39 countries/regions, 223 research institutes and 171 academic journals. The subject classifications related to the GS were Neuroscience, Clinical Neuroscience and Radiology/Nuclear Medicine/Medical Imaging. The United States has maintained its dominant and most influential position in GS research. Among research institutions and journals, the Univ Rochester and Journal of Cerebral Blood Flow and Metabolism had the highest number of academic articles, respectively. Nedergaard M had the most published article, and Iliff JJ had the most co-citations. The top two keywords with the highest frequency were "glymphatic system" and "cerebrospinal fluid."

Conclusion

This research provides valuable information for the study of the GS. The bibliometric analysis of this area will encourage potential collaborations among researchers, defining its frontiers and directions for development.

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.

Central and peripheral tau retention modulated by an anti-tau antibody

Tau protein blood levels dependent on its distribution to peripheral organs and possible elimination from the body. Thus, the peripheral distribution of CSF-derived tau protein was explored, especially since there is a transition to blood-based biomarkers and the emerging idea that tau pathology may spread beyond brain. Near infrared fluorescence (NIRF) was mainly used to analyze tau (tau-NIRF) distribution after its intracisternal or intravenous injection. There was a striking uptake of blood- or CSF-derived tau-NIRF protein by the skeletal structures, liver, small intestine (duodenum), gall bladder, kidneys, urinary bladder, lymph nodes, heart, and spleen. In aging and in older APP/PS1 mice, tau uptake in regions, such as the brain, liver, and skeleton, was increased. In bone (femur) injected tau protein was associated with integrin-binding sialoprotein (IBSP), a major non-collagenous glycoprotein that is associated with mineralization. Tau-NIRF was cleared slowly from CSF via mainly across the cribriform plate, and cervical lymph nodes. In brain, some of the CSF injected tau protein was associated with NeuN-positive and PDGFRý-positive cells, which may explain its retention. The presence of tau in the bladders suggested excretion routes of tau. CSF anti-tau antibody increased CSF tau clearance, while blood anti-tau antibody decreased tau accumulation in the femur but not in liver, kidney, and spleen. Thus, the data show a body-wide distribution and retention of CSF-derived tau protein, which increased with aging and in older APP/PS1 mice. Further work is needed to elucidate the relevance of tau accumulation in each organ to tauopathy.

Image Analysis Techniques for In Vivo Quantification of Cerebrospinal Fluid Flow

Over the last decade, there has been a tremendously increased interest in understanding the neurophysiology of cerebrospinal fluid (CSF) flow, which plays a crucial role in clearing metabolic waste from the brain. This growing interest was largely initiated by two significant discoveries: the glymphatic system (a pathway for solute exchange between interstitial fluid deep within the brain and the CSF surrounding the brain) and meningeal lymphatic vessels (lymphatic vessels in the layer of tissue surrounding the brain that drain CSF). These two CSF systems work in unison, and their disruption has been implicated in several neurological disorders including Alzheimer's disease, stoke, and traumatic brain injury. Here, we present experimental techniques for in vivo quantification of CSF flow via direct imaging of fluorescent microspheres injected into the CSF. We discuss detailed image processing methods, including registration and masking of stagnant particles, to improve the quality of measurements. We provide guidance for quantifying CSF flow through particle tracking and offer tips for optimizing the process. Additionally, we describe techniques for measuring changes in arterial diameter, which is an hypothesized CSF pumping mechanism. Finally, we outline how these same techniques can be applied to cervical lymphatic vessels, which collect fluid downstream from meningeal lymphatic vessels. We anticipate that these fluid mechanical techniques will prove valuable for future quantitative studies aimed at understanding mechanisms of CSF transport and disruption, as well as for other complex biophysical systems.

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.

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.

Intranasal Pathway for Nanoparticles to Enter the Central Nervous System

Intranasal administration was previously proposed for delivering drugs for central nervous system (CNS) diseases. However, the delivery and elimination pathways, which are very imperative to know for exploring the therapeutic applications of any given CNS drugs, remain far from clear. Because lipophilicity has a high priority in the design of CNS drugs, the as-prepared CNS drugs tend to form aggregates. Therefore, a PEGylated Fe₃O₄ nanoparticle labeled with a fluorescent dye was prepared as a model drug and studied to elucidate the delivery pathways of intranasally administered nanodrugs. Through magnetic resonance imaging, the distribution of the nanoparticles was investigated in vivo. Through ex vivo fluorescence imaging and microscopy studies, more precise distribution of the nanoparticles across the entire brain was disclosed. Moreover, the elimination of the nanoparticles from cerebrospinal fluid was carefully studied. The temporal dose levels of intranasally delivered nanodrugs in different parts of the brain were also investigated.

Plasma neurodegeneration biomarker concentrations associate with glymphatic and meningeal lymphatic measures in neurological disorders

Clearance of neurotoxic brain proteins via cerebrospinal fluid (CSF) to blood has recently emerged to be crucial, and plasma biomarkers of neurodegeneration were newly introduced to predict neurological disease. This study examines in 106 individuals with neurological disorders associations between plasma biomarkers [40 and 42 amino acid-long amyloid-β (Aβ40 and Aβ42), total-tau, glial fibrillary acidic protein (GFAP), and neurofilament light (NfL)] and magnetic resonance imaging measures of CSF-mediated clearance from brain via extra-vascular pathways (proxy of glymphatic function) and CSF-to-blood clearance variables from pharmacokinetic modeling (proxy of meningeal lymphatic egress). We also examine how biomarkers vary during daytime and associate with subjective sleep quality. Plasma concentrations of neurodegeneration markers associate with indices of glymphatic and meningeal lymphatic functions in individual- and disease-specific manners, vary during daytime, but are unaffected by sleep quality. The results suggest that plasma concentrations of neurodegeneration biomarkers associate with measures of glymphatic and meningeal lymphatic function.

Open pathways for cerebrospinal fluid outflow at the cribriform plate along the olfactory nerves

BACKGROUND

Routes along the olfactory nerves crossing the cribriform plate that extend to lymphatic vessels within the nasal cavity have been identified as a critical cerebrospinal fluid (CSF) outflow pathway. However, it is still unclear how the efflux pathways along the nerves connect to lymphatic vessels or if any functional barriers are present at this site. The aim of this study was to anatomically define the connections between the subarachnoid space and the lymphatic system at the cribriform plate in mice.

METHODS

PEGylated fluorescent microbeads were infused into the CSF space in Prox1-GFP reporter mice and decalcification histology was utilized to investigate the anatomical connections between the subarachnoid space and the lymphatic vessels in the nasal submucosa. A fluorescently-labelled antibody marking vascular endothelium was injected into the cisterna magna to demonstrate the functionality of the lymphatic vessels in the olfactory region. Finally, we performed immunostaining to study the distribution of the arachnoid barrier at the cribriform plate region.

FINDINGS

We identified that there are open and direct connections from the subarachnoid space to lymphatic vessels enwrapping the olfactory nerves as they cross the cribriform plate towards the nasal submucosa. Furthermore, lymphatic vessels adjacent to the olfactory bulbs form a continuous network that is functionally connected to lymphatics in the nasal submucosa. Immunostainings revealed a discontinuous distribution of the arachnoid barrier at the olfactory region of the mouse.

INTERPRETATION

Our data supports a direct bulk flow mechanism through the cribriform plate allowing CSF drainage into nasal submucosal lymphatics in mice.

FUNDING

This study was supported by the Swiss National Science Foundation (310030_189226), Dementia Research Switzerland-Synapsis Foundation, the Heidi Seiler Stiftung and the Fondation Dr. Corinne Schuler.

Investigating the Impact of Delivery Routes for Exon Skipping Therapies in the CNS of DMD Mouse Models

Nucleic acid-based therapies have demonstrated great potential for the treatment of monogenetic diseases, including neurologic disorders. To date, regulatory approval has been received for a dozen antisense oligonucleotides (ASOs); however, these chemistries cannot readily cross the blood–brain barrier when administered systemically. Therefore, an investigation of their potential effects within the central nervous system (CNS) requires local delivery. Here, we studied the brain distribution and exon-skipping efficacy of two ASO chemistries, PMO and tcDNA, when delivered to the cerebrospinal fluid (CSF) of mice carrying a deletion in exon 52 of the dystrophin gene, a model of Duchenne muscular dystrophy (DMD). Following intracerebroventricular (ICV) delivery (unilateral, bilateral, bolus vs. slow rate, repeated via cannula or very slow via osmotic pumps), ASO levels were quantified across brain regions and exon 51 skipping was evaluated, revealing that tcDNA treatment invariably generates comparable or more skipping relative to that with PMO, even when the PMO was administered at higher doses. We also performed intra-cisterna magna (ICM) delivery as an alternative route for CSF delivery and found a biased distribution of the ASOs towards posterior brain regions, including the cerebellum, hindbrain, and the cervical part of the spinal cord. Finally, we combined both ICV and ICM injection methods to assess the potential of an additive effect of this methodology in inducing efficient exon skipping across different brain regions. Our results provide useful insights into the local delivery and associated efficacy of ASOs in the CNS in mouse models of DMD. These findings pave the way for further ASO-based therapy application to the CNS for neurological disease.

Human parasagittal dura is a potential neuroimmune interface

Parasagittal dura (PSD) is located on both sides of the superior sagittal sinus and harbours arachnoid granulations and lymphatic vessels. Efflux of cerebrospinal fluid (CSF) to human PSD has recently been shown in vivo. Here we obtain PSD volumes from magnetic resonance images in 76 patients under evaluation for CSF disorders and correlate them to age, sex, intracranial volumes, disease category, sleep quality, and intracranial pressure. In two subgroups, we also analyze tracer dynamics and time to peak tracer level in PSD and blood. PSD volume is not explained by any single assessed variable, but tracer level in PSD is strongly associated with tracer in CSF and brain. Furthermore, peak tracer in PSD occurs far later than peak tracer in blood, implying that PSD is no major efflux route for CSF. These observations may indicate that PSD is more relevant as a neuroimmune interface than as a CSF efflux route.

Glymphatic-assisted perivascular brain delivery of intrathecal small gold nanoparticles

Nanoparticles are ultrafine particulate matter having considerable potential for treatment of central nervous system (CNS) disorders. Despite their tiny size, the blood-brain barrier (BBB) restricts their access to the CNS. Their direct cerebrospinal fluid (CSF) administration bypasses the BBB endothelium, but still fails to give adequate brain uptake. We present a novel approach for efficient CNS delivery of ¹¹¹In-radiolabelled gold nanoparticles (AuNPs; 10-15 nm) via intra-cisterna magna administration, with tracking by SPECT imaging. To accelerate CSF brain influx, we administered AuNPs intracisternally in conjunction with systemic hypertonic saline, which dramatically increased the parenchymal AuNP uptake, especially in deep brain regions. AuNPs entered the CNS along periarterial spaces as visualized by MRI of gadolinium-labelled AuNPs and were cleared from brain within 24 h and excreted through the kidneys. Thus, the glymphatic-assisted perivascular network augment by systemic hypertonic saline is a pathway for highly efficient brain-wide distribution of small AuNPs.

Whole CNS 3D Cryo-Fluorescence Tomography Shows CSF Clearance along Nasal Lymphatics, Spinal Nerves, and Lumbar/Sacral Lymph Nodes

Unwanted proteins and metabolic waste in cerebral spinal fluid are cleared from the brain by meningeal and nasal lymphatics and the perineural sheath of cranial nerves; however, the distribution and clearance of cerebral spinal fluid (CSF) along the subarachnoid space of the entire spinal cord is not fully understood. Cryo-fluorescence tomography (CFT) was used to follow the movement of tracers from the ventricular system of the brain down through the meningeal lining of the spinal cord and out to the spinal lymphatic nodes. Isoflurane-anesthetized mice were infused into the lateral cerebroventricle with 5.0 µL of quantum dots [QdotR 605 ITKTM amino (PEG)] over two mins. Mice were allowed to recover (ca 2-3 min) and remained awake and ambulatory for 5, 15, 30, 60, and 120 min after which they were euthanized, and the entire intact body was frozen at -80^°. The entire mouse was sectioned, and white light and fluorescent images were captured after each slice to produce high resolution three-dimensional volumes. Tracer appeared throughout the ventricular system and central canal of the spinal cord and the entire subarachnoid space of the CNS. A signal could be visualized in the nasal cavity, deep cervical lymph nodes, thoracic lymph nodes, and more superficial submandibular lymph nodes as early as 15 min post infusion. A fluorescent signal could be visualized along the dorsal root ganglia and down the proximal extension of the spinal nerves of the thoracic and lumbar segments at 30 min. There was a significant accumulation of tracer in the lumbar and sacral lymph nodes between 15-60 min. The dense fluorescent signal in the thoracic vertebrae noted at 5- and 15-min post infusion was significantly reduced by 30 min. Indeed, all signals in the spinal cord were ostensibly absent by 120 min, except for trace amounts in the coccyx. The brain still had some residual signal at 120 min. These data show that Qdots with a hydrodynamic diameter of 16-20 nm rapidly clear from the brain of awake mice. These data also clearly demonstrate the rapid distribution and efflux of traces along a major length of the vertebral column and the potential contribution of the spinal cord in the clearance of brain waste.

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.

Emerging Roles of Meningeal Lymphatic Vessels in Alzheimer's Disease

Meningeal lymphatic vessels (mLVs), the functional lymphatic system present in the meninges, are the key drainage route responsible for the clearance of molecules, immune cells, and cellular debris from the cerebrospinal fluid and interstitial fluid into deep cervical lymph nodes. Aging and ApoE4, the two most important risk factors for Alzheimer's disease (AD), induce mLV dysfunction, decrease cerebrospinal fluid influx and outflux, and exacerbate amyloid pathology and cognitive dysfunction. Dysfunction of mLVs results in the deposition of metabolic products, accelerates neuroinflammation, and promotes the release of pro-inflammatory cytokines in the brain. Thus, mLVs represent a novel therapeutic target for treating neurodegenerative and neuroinflammatory diseases. This review aims to summarize the structure and function of mLVs and to discuss the potential effect of aging and ApoE4 on mLV dysfunction, as well as their roles in the pathogenesis of AD.

Intranasal Delivery of Liposomes to Glioblastoma by Photostimulation of the Lymphatic System

The blood-brain barrier (BBB) limits the delivery of majority of cancer drugs and thereby complicates brain tumor treatment. The nasal-brain-lymphatic system is discussed as a pathway for brain drug delivery overcoming the BBB. However, in most cases, this method is not sufficient to achieve a therapeutic effect due to brain drug delivery in a short distance. Therefore, it is necessary to develop technologies to overcome the obstacles facing nose-to-brain delivery of promising pharmaceuticals. In this study, we clearly demonstrate intranasal delivery of liposomes to the mouse brain reaching glioblastoma (GBM). In the experiments with ablation of the meningeal lymphatic network, we report an important role of meningeal pathway for intranasal delivery of liposomes to the brain. Our data revealed that GBM is characterized by a dramatic reduction of intranasal delivery of liposomes to the brain that was significantly improved by near-infrared (1267 nm) photostimulation of the lymphatic vessels in the area of the cribriform plate and the meninges. These results open new perspectives for non-invasive improvement of efficiency of intranasal delivery of cancer drugs to the brain tissues using nanocarriers and near-infrared laser-based therapeutic devices, which are commercially available and widely used in clinical practice.

Brain borders at the central stage of neuroimmunology

The concept of immune privilege suggests that the central nervous system is isolated from the immune system. However, recent studies have highlighted the borders of the central nervous system as central sites of neuro-immune interactions. Although the nervous and immune systems both function to maintain homeostasis, under rare circumstances, they can develop pathological interactions that lead to neurological or psychiatric diseases. Here we discuss recent findings that dissect the key anatomical, cellular and molecular mechanisms that enable neuro-immune responses at the borders of the brain and spinal cord and the implications of these interactions for diseases of the central nervous system.

Effects of increased intracranial pressure on cerebrospinal fluid influx, cerebral vascular hemodynamic indexes, and cerebrospinal fluid lymphatic efflux

The glymphatic-lymphatic fluid transport system (GLFTS) consists of glymphatic pathway and cerebrospinal fluid (CSF) lymphatic outflow routes, allowing biological liquids from the brain parenchyma to access the CSF along with perivascular space and to be cleaned out of the skull through lymphatic vessels. It is known that increased local pressure due to physical compression of tissue improves lymphatic transport in peripheral organs, but little is known about the exact relationship between increased intracranial pressure (IICP) and GLFTS. In this study, we verify our hypothesis that IICP significantly impacts GLFTS, and this effect depends on severity of the IICP. Using a previously developed inflating balloon model to induce IICP and inject fluorescent tracers into the cisterna magna, we found significant impairment of the glymphatic circulation after IICP. We further found that cerebrovascular occlusion occurred, and cerebrovascular pulsation decreased after IICP. IICP also interrupted the drainage of deep cervical lymph nodes and dorsal meningeal lymphatic function, enhancing spinal lymphatic outflow to the sacral lymph nodes. Notably, these effects were associated with the severity of IICP. Thus, our findings proved that the intensity of IICP significantly impacts GLFTS. This may have translational applications for preventing and treating related neurological disorders.

Impaired peri-olfactory cerebrospinal fluid clearance is associated with ageing, cognitive decline and dyssomnia

BACKGROUND

Animal experiments have demonstrated the dependency of cerebrospinal fluid clearance function on age and sleep, which partially underlay the cognitive decline in the elderly. However, human evidence is lacking, which could be mainly attributed to the limited methods of cerebrospinal fluid clearance function assessment.

METHOD

Serial T1-weighted and T2-fluid attenuated inversion recovery imaging were performed in 92 patients before and at multiple time points including 4.5 h, 15 h and 39 h after intrathecal injection of contrast agent to visualize the putative meningeal lymphatic pathway, peri-olfactory nerve pathway, and peri-optic nerve pathway. We defined the clearance function as the percentage change in signal unit ratio of critical locations in these pathways from baseline to 39 h after intrathecal injection, and further analysed their relationships with age, sleep, and cognitive function.

FINDINGS

Cerebrospinal fluid clearance through the putative meningeal lymphatic and perineural pathways were clearly visualized. The clearance function of putative meningeal lymphatic and perineural pathways were impaired with ageing (all P < 0.05). The clearance function through peri-olfactory nerve pathway in inferior turbinate was positively correlated with sleep quality and cognitive function (both P < 0.05), and mediated the association of sleep quality with cognitive function (percent change in β [bootstrap 95% CI]: 33% [-0.220, -0.007]).

INTERPRETATION

The impaired clearance through putative peri-olfactory nerve pathway may explain the cognitive decline in patients with sleep disturbance. The study shows a promising method to assess cerebrospinal fluid clearance function of putative peri-neural pathways via dynamic magnetic resonance imaging with intrathecal injection of contrast agent.

FUNDING

This work was supported by the National Natural Science Foundation of China (81971101, 82171276 and 82101365).

Theory of sleep/wake cycles affecting brain elastography

As elastography of the brain finds increasing clinical applications, fundamental questions remain about baseline viscoelastic properties of the brainin vivo. Furthermore, the underlying mechanisms of how and why elastographic measures can change over time are still not well understood. To study these issues, reverberant shear wave elastography using an optical coherence tomography scanner is implemented on a mouse model, both under awake conditions and in a sleep state where there are known changes in the glymphatic fluid flow system in the brain. We find that shear wave speed, a measure of stiffness, changes by approximately 12% between the two states, sleep versus awake, in the entire cortical brain imaging volume. Our microchannel flow model of biphasic (fluid plus solid) tissue provides a plausible rheological model based on the fractal branching vascular and perivascular system, plus a second parallel system representing the finer scale glymphatic fluid microchannels. By adjusting the glymphatic system fluid volume proportional to the known sleep/wake changes, we are able to approximately predict the measured shear wave speeds and their change with the state of the glymphatic system. The advantages of this model are that its main parameters are derived from anatomical measures and are linked to other major derivations of branching fluid structures including Murray's Law. The implications for clinical studies are that elastography of the brain is strongly influenced by the regulation or dysregulation of the vascular, perivascular, and glymphatic systems.

Perivascular spaces and their role in neuroinflammation

It is uncontested that perivascular spaces play critical roles in maintaining homeostasis and priming neuroinflammation. However, despite more than a century of intense research on perivascular spaces, many open questions remain about the anatomical compartment surrounding blood vessels within the CNS. The goal of this comprehensive review is to summarize the literature on perivascular spaces in human neuroinflammation and associated animal disease models. We describe the cell types taking part in the morphological and functional aspects of perivascular spaces and how those spaces can be visualized. Based on this, we propose a model of the cascade of events occurring during neuroinflammatory pathology. We also discuss current knowledge gaps and limitations of the available evidence. An improved understanding of perivascular spaces could advance our comprehension of the pathophysiology of neuroinflammation and open a new therapeutic window for neuroinflammatory diseases such as multiple sclerosis.

Transport in the Brain Extracellular Space: Diffusion, but Which Kind?

The mechanisms of transport of substances in the brain parenchyma have been a hot topic in scientific discussion in the past decade. This discussion was triggered by the proposed glymphatic hypothesis, which assumes a directed flow of cerebral fluid within the parenchyma, in contrast to the previous notion that diffusion is the main mechanism. However, when discussing the issue of “diffusion or non-diffusion”, much less attention was given to the question that diffusion itself can have a different character. In our opinion, some of the recently published results do not fit into the traditional understanding of diffusion. In this regard, we outline the relevant new theoretical approaches on transport processes in complex random media such as concepts of diffusive diffusivity and time-dependent homogenization, which expands the understanding of the forms of transport of substances based on diffusion.

A real-time in vivo clearance assay for quantification of glymphatic efflux

Glymphatic fluid transport eliminates metabolic waste from the brain including amyloid-β, yet the methodology for studying efflux remains rudimentary. Here, we develop a method to evaluate glymphatic real-time clearance. Efflux of Direct Blue 53 (DB53, also T-1824 or Evans Blue) injected into the striatum is quantified by imaging the DB53 signal in the vascular compartment, where it is retained due to its high affinity to albumin. The DB53 signal is detectable as early as 15 min after injection and the efflux kinetics are sharply reduced in mice lacking the water channel aquaporin 4 (AQP4). Pharmacokinetic modeling reveal that DB53 efflux is consistent with the existence of two efflux paths, one with fast kinetics (T_1/2 = 50 min) and another with slow kinetics (T_1/2 = 240 min), in wild-type mice. This in vivo methodology will aid in defining the physiological variables that drive efflux, as well as the impact of brain states or disorders on clearance kinetics.

CSF circulation and dispersion yield rapid clearance from intracranial compartments

In this paper, we used a computational model to estimate the clearance of a tracer driven by the circulation of cerebrospinal fluid (CSF) produced in the choroid plexus (CP) located within the lateral ventricles. CSF was assumed to exit the subarachnoid space (SAS) via different outflow routes such as the parasagittal dura, cribriform plate, and/or meningeal lymphatics. We also modelled a reverse case where fluid was produced within the spinal canal and absorbed in the choroid plexus in line with observations on certain iNPH patients. No directional interstitial fluid flow was assumed within the brain parenchyma. Tracers were injected into the foramen magnum. The models demonstrate that convection in the subarachnoid space yields rapid clearance from both the SAS and the brain interstitial fluid and can speed up intracranial clearance from years, as would be the case for purely diffusive transport, to days.

The meningeal lymphatic vessels and the glymphatic system: Potential therapeutic targets in neurological disorders

The recent discovery of the meningeal lymphatic vessels (mLVs) and glymphatic pathways has challenged the long-lasting dogma that the central nervous system (CNS) lacks a lymphatic system and therefore does not interact with peripheral immunity. This discovery has reshaped our understanding of mechanisms underlying CNS drainage. Under normal conditions, a close connection between mLVs and the glymphatic system enables metabolic waste removal, immune cell trafficking, and CNS immune surveillance. Dysfunction of the glymphatic-mLV system can lead to toxic protein accumulation in the brain, and it contributes to the development of a series of neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. The identification of precise cerebral transport routes is based mainly on indirect, invasive imaging of animals, and the results cannot always be applied to humans. Here we review the functions of the glymphatic-mLV system and evidence for its involvement in some CNS diseases. We focus on emerging noninvasive imaging techniques to evaluate the human glymphatic-mLV system and their potential for preclinical diagnosis and prevention of neurodegenerative diseases. Potential strategies that target the glymphatic-mLV system in order to treat and prevent neurological disorders are also discussed.

Significance of developmental meningeal lymphatic dysfunction in experimental post-traumatic injury

Understanding the pathological mechanisms unfolding after chronic traumatic brain injury (TBI) could reveal new therapeutic entry points. During the post-TBI sequel, the involvement of cerebrospinal fluid drainage through the meningeal lymphatic vessels was proposed. Here, we used K14-VEGFR3-Ig transgenic mice to analyze whether a developmental dysfunction of meningeal lymphatic vessels modifies post-TBI pathology. To this end, a moderate TBI was delivered by controlled cortical injury over the temporal lobe in male transgenic mice or their littermate controls. We performed MRI and a battery of behavioral tests over time to define the post-TBI trajectories. In vivo analyses were integrated by ex-vivo quantitative and morphometric examinations of the cortical lesion and glial cells.

In post-TBI K14-VEGFR3-Ig mice, the recovery from motor deficits was protracted compared to littermates. This outcome is coherent with the observed slower hematoma clearance in transgenic mice during the first two weeks post-TBI. No other genotype-related behavioral differences were observed, and the volume of cortical lesions imaged by MRI in vivo, and confirmed by histology ex-vivo, were comparable in both groups. However, at the cellular level, post-TBI K14-VEGFR3-Ig mice exhibited an increased percentage of activated Iba1 microglia in the hippocampus and auditory cortex, areas that are proximal to the lesion.

Although not impacting or modifying the structural brain damage and post-TBI behavior, a pre-existing dysfunction of meningeal lymphatic vessels is associated with morphological microglial activation over time, possibly representing a sub-clinical pathological imprint or a vulnerability factor. Our findings suggest that pre-existing mLV deficits could represent a possible risk factor for the overall outcome of TBI pathology.

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Developmental deficit in the meningeal lymphatic vessels contributes to sustain the chronic neuroinflammation and represent a susceptibility factor in TBI, despite the lack of a functional phenotype.

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Development and progression of TBI-related cortical lesion is not exacerbated by developmental deficit in meningeal lymphatics.

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Meningeal lymphatic developmental deficits result in increased neuroinflammation, suggesting a sub-clinical pathological imprint or a vulnerability factor.

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Congenital mLV deficit affects the interstitial fluid dynamics and the post-TBI hematoma resolution.

Central nervous system zoning: How brain barriers establish subdivisions for CNS immune privilege and immune surveillance

The central nervous system (CNS) coordinates all our body functions. Neurons in the CNS parenchyma achieve this computational task by high speed communication via electrical and chemical signals and thus rely on a strictly regulated homeostatic environment, which does not tolerate uncontrolled entry of blood components including immune cells. The CNS thus has a unique relationship with the immune system known as CNS immune privilege. Previously ascribed to the presence of blood-brain barriers and the lack of lymphatic vessels in the CNS parenchyma prohibiting, respectively, efferent and afferent connections with the peripheral immune system, it is now appreciated that CNS immune surveillance is ensured by cellular and acellular brain barriers that limit immune cell and mediator accessibility to specific compartments at the borders of the CNS. CNS immune privilege is established by a brain barriers anatomy resembling the architecture of a medieval castle surrounded by two walls bordering a castle moat. Built for protection and defense this two-walled rampart at the outer perimeter of the CNS parenchyma allows for accommodation of different immune cell subsets and efficient monitoring of potential danger signals derived from inside or outside of the CNS parenchyma. It enables effective mounting of immune responses within the subarachnoid or perivascular spaces, while leaving the CNS parenchyma relatively undisturbed. In this study, we propose that CNS immune privilege rests on the proper function of the brain barriers, which allow for CNS immune surveillance but prohibit activation of immune responses from the CNS parenchyma unless it is directly injured.

Sensitivity analysis on a network model of glymphatic flow

Intracranial cerebrospinal and interstitial fluid (ISF) flow and solute transport have important clinical implications, but limited in vivo access to the brain interior leaves gaping holes in human understanding of the nature of these neurophysiological phenomena. Models can address some gaps, but only insofar as model inputs are accurate. We perform a sensitivity analysis using a Monte Carlo approach on a lumped-parameter network model of cerebrospinal and ISF in perivascular and extracellular spaces in the murine brain. We place bounds on model predictions given the uncertainty in input parameters. Péclet numbers for transport in penetrating perivascular spaces (PVSs) and within the parenchyma are separated by at least two orders of magnitude. Low permeability in penetrating PVSs requires unrealistically large driving pressure and/or results in poor perfusion and are deemed unlikely. The model is most sensitive to the permeability of penetrating PVSs, a parameter whose value is largely unknown, highlighting an important direction for future experiments. Until the value of the permeability of penetrating PVSs is more accurately measured, the uncertainty of any model that includes flow in penetrating PVSs is so large that absolute numbers have little meaning and practical application is limited.

The Underlying Role of the Glymphatic System and Meningeal Lymphatic Vessels in Cerebral Small Vessel Disease

There is a growing prevalence of vascular cognitive impairment (VCI) worldwide, and most research has suggested that cerebral small vessel disease (CSVD) is the main contributor to VCI. Several potential physiopathologic mechanisms have been proven to be involved in the process of CSVD, such as blood-brain barrier damage, small vessels stiffening, venous collagenosis, cerebral blood flow reduction, white matter rarefaction, chronic ischaemia, neuroinflammation, myelin damage, and subsequent neurodegeneration. However, there still is a limited overall understanding of the sequence and the relative importance of these mechanisms. The glymphatic system (GS) and meningeal lymphatic vessels (mLVs) are the analogs of the lymphatic system in the central nervous system (CNS). As such, these systems play critical roles in regulating cerebrospinal fluid (CSF) and interstitial fluid (ISF) transport, waste clearance, and, potentially, neuroinflammation. Accumulating evidence has suggested that the glymphatic and meningeal lymphatic vessels played vital roles in animal models of CSVD and patients with CSVD. Given the complexity of CSVD, it was significant to understand the underlying interaction between glymphatic and meningeal lymphatic transport with CSVD. Here, we provide a novel framework based on new advances in main four aspects, including vascular risk factors, potential mechanisms, clinical subtypes, and cognition, which aims to explain how the glymphatic system and meningeal lymphatic vessels contribute to the progression of CSVD and proposes a comprehensive insight into the novel therapeutic strategy of CSVD.

A network model of glymphatic flow under different experimentally-motivated parametric scenarios

Flow of cerebrospinal fluid (CSF) through perivascular spaces (PVSs) in the brain delivers nutrients, clears metabolic waste, and causes edema formation. Brain-wide imaging cannot resolve PVSs, and high-resolution methods cannot access deep tissue. However, theoretical models provide valuable insight. We model the CSF pathway as a network of hydraulic resistances, using published parameter values. A few parameters (permeability of PVSs and the parenchyma, and dimensions of PVSs and astrocyte endfoot gaps) have wide uncertainties, so we focus on the limits of their ranges by analyzing different parametric scenarios. We identify low-resistance PVSs and high-resistance parenchyma as the only scenario that satisfies three essential criteria: that the flow be driven by a small pressure drop, exhibit good CSF perfusion throughout the cortex, and exhibit a substantial increase in flow during sleep. Our results point to the most important parameters, such as astrocyte endfoot gap dimensions, to be measured in future experiments.

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We model the CSF pathway as a network of hydraulic resistances

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Predictions are bracketed by analyzing parametric scenarios for unknown parameters

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Low-resistance PVSs and high-resistance parenchyma produce realistic flows

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Astrocyte endfoot gap size is among the important parameters to be measured

Applications in Awake Animal Magnetic Resonance Imaging

There are numerous publications on methods and applications for awake functional MRI across different species, e.g., voles, rabbits, cats, dogs, and rhesus macaques. Each of these species, most obviously rhesus monkey, have general or unique attributes that provide a better understanding of the human condition. However, much of the work today is done on rodents. The growing number of small bore (≤30 cm) high field systems 7T- 11.7T favor the use of small animals. To that point, this review is primarily focused on rodents and their many applications in awake function MRI. Applications include, pharmacological MRI, drugs of abuse, sensory evoked stimuli, brain disorders, pain, social behavior, and fear.

Fluid transport in the brain

The brain harbors a unique ability to, figuratively speaking, shift its gears. During wakefulness, the brain is geared fully toward processing information and behaving, while homeostatic functions predominate during sleep. The blood-brain barrier establishes a stable environment that is optimal for neuronal function, yet the barrier imposes a physiological problem; transcapillary filtration that forms extracellular fluid in other organs is reduced to a minimum in brain. Consequently, the brain depends on a special fluid [the cerebrospinal fluid (CSF)] that is flushed into brain along the unique perivascular spaces created by astrocytic vascular endfeet. We describe this pathway, coined the term glymphatic system, based on its dependency on astrocytic vascular endfeet and their adluminal expression of aquaporin-4 water channels facing toward CSF-filled perivascular spaces. Glymphatic clearance of potentially harmful metabolic or protein waste products, such as amyloid-β, is primarily active during sleep, when its physiological drivers, the cardiac cycle, respiration, and slow vasomotion, together efficiently propel CSF inflow along periarterial spaces. The brain's extracellular space contains an abundance of proteoglycans and hyaluronan, which provide a low-resistance hydraulic conduit that rapidly can expand and shrink during the sleep-wake cycle. We describe this unique fluid system of the brain, which meets the brain's requisites to maintain homeostasis similar to peripheral organs, considering the blood-brain-barrier and the paths for formation and egress of the CSF.