The glymphatic system and its role in cerebral homeostasis

The brain’s high bioenergetic state is paralleled by high metabolic waste production. Authentic lymphatic vasculature is lacking in brain parenchyma. Cerebrospinal fluid (CSF) flow has long been thought to facilitate central nervous system detoxification in place of lymphatics, but the exact processes involved in toxic waste clearance from the brain remain incompletely understood. Over the past 8 yr, novel data in animals and humans have begun to shed new light on these processes in the form of the “glymphatic system,” a brain-wide perivascular transit passageway dedicated to CSF transport and interstitial fluid exchange that facilitates metabolic waste drainage from the brain. Here we will discuss glymphatic system anatomy and methods to visualize and quantify glymphatic system (GS) transport in the brain and also discuss physiological drivers of its function in normal brain and in neurodegeneration

Waste Clearance in the Brain

Waste clearance (WC) is an essential process for brain homeostasis, which is required for the proper and healthy functioning of all cerebrovascular and parenchymal brain cells. This review features our current understanding of brain WC, both within and external to the brain parenchyma. We describe the interplay of the blood-brain barrier (BBB), interstitial fluid (ISF), and perivascular spaces within the brain parenchyma for brain WC directly into the blood and/or cerebrospinal fluid (CSF). We also discuss the relevant role of the CSF and its exit routes in mediating WC. Recent discoveries of the glymphatic system and meningeal lymphatic vessels, and their relevance to brain WC are highlighted. Controversies related to brain WC research and potential future directions are presented

Developing novel non-invasive MRI techniques to assess cerebrospinal fluid-interstitial fluid (CSF-ISF) exchange

The pathological cascade of events in Alzheimer’s disease (AD) is initiated decades prior to the onset of symptoms. Despite intensive research, the relative time-course/interaction of these events is yet to be determined. Recent evidence suggests that impairments to brain clearance (facilitated by the compartmental exchange of cerebrospinal-fluid (CSF) with interstitial-fluid (ISF)), contributes to the build-up of amyloid and tau (AD hallmarks). Therefore, abnormalities in CSF-ISF exchange dynamics, may represent an early driver of downstream events. Clinical evaluation of this hypothesis is hampered due to the lack of non-invasive CSF-ISF exchange assessment techniques. In this thesis, the primary aim was to develop a physiologically relevant, non-invasive CSF-ISF exchange assessment technique that would circumvent the limitations associated with current procedures (primarily their invasiveness). Towards this goal, animal studies were conducted to investigate the feasibility of a contrast enhanced-magnetic resonance imaging (CE-MRI) approach as a potential non-invasive CSF-ISF exchange imaging technique. Another aim of this thesis was to investigate whether the proposed MRI platform could detect abnormalities in CSF-ISF exchange, a condition hypothesised to occur in the early stages of AD. As such, pharmacological intervention studies were conducted to alter CSF-ISF exchange dynamics. CE-MRI, in conjunction with high-level image post-processing, demonstrated high sensitivity to physiological CSF-ISF exchange. This novel, non-invasive platform, captured dynamic, whole-brain infiltration of contrast agent from the blood to the CSF and into the parenchyma, via a pathway named ‘VEntricular-Cerebral TranspORt (VECTOR)’. Additionally, the platform detected significant abnormalities in CSF-ISF exchange following pharmacological intervention, demonstrating the potential of VECTOR in the study of the parenchymal accumulation of aberrant proteins. Development of this platform is a breakthrough step towards the clinical assessment of CSF-ISF exchange abnormalities to study its role in disease onset/progression, an approach that may inform understanding of the causal sequence of pathological events that occurs in AD development

Fast uncertainty quantification of tracer distribution in the brain interstitial fluid with multilevel and quasi Monte Carlo

Efficient uncertainty quantification algorithms are key to understand the propagation of uncertainty -- from uncertain input parameters to uncertain output quantities -- in high resolution mathematical models of brain physiology. Advanced Monte Carlo methods such as quasi Monte Carlo (QMC) and multilevel Monte Carlo (MLMC) have the potential to dramatically improve upon standard Monte Carlo (MC) methods, but their applicability and performance in biomedical applications is underexplored. In this paper, we design and apply QMC and MLMC methods to quantify uncertainty in a convection-diffusion model of tracer transport within the brain. We show that QMC outperforms standard MC simulations when the number of random inputs is small. MLMC considerably outperforms both QMC and standard MC methods and should therefore be preferred for brain transport models.Comment: Multilevel Monte Carlo, quasi Monte Carlo, brain simulation, brain fluids, finite element method, biomedical computing, random fields, diffusion-convectio

Peristaltic flow in the glymphatic system.

The flow inside the perivascular space (PVS) is modeled using a first-principles approach in order to investigate how the cerebrospinal fluid (CSF) enters the brain through a permeable layer of glial cells. Lubrication theory is employed to deal with the flow in the thin annular gap of the perivascular space between an impermeable artery and the brain tissue. The artery has an imposed peristaltic deformation and the deformable brain tissue is modeled by means of an elastic Hooke\u27s law. The perivascular flow model is solved numerically, discovering that the peristaltic wave induces a steady streaming to/from the brain which strongly depends on the rigidity and the permeability of the brain tissue. A detailed quantification of the through flow across the glial boundary is obtained for a large parameter space of physiologically relevant conditions. The parameters include the elasticity and permeability of the brain, the curvature of the artery, its length and the amplitude of the peristaltic wave. A steady streaming component of the through flow due to the peristaltic wave is characterized by an in-depth physical analysis and the velocity across the glial layer is found to flow from and to the PVS, depending on the elasticity and permeability of the brain. The through CSF flow velocity is quantified to be of the order of micrometers per seconds

Impact of aquaporin (AQP1 and AQP4) genetic variation on the relationship between sleep quality and Alzheimer’s disease pathological hallmarks

Alzheimer’s disease (AD) is widely recognised as a growing global health issue with far ranging social and economic implications. The accumulation of Amyloid-β (Aβ) in the brain is a pathological hallmark of AD. A recently discovered lymphatic–like system in the central nervous system (termed the glymphatic system) has been postulated to be both implicit in the clearance of Aβ from the brain, and most effective during sleep—making sleep an important consideration in the investigation of AD. Central nervous system expressed water channel proteins, namely Aquaporin 1 and 4, have been suggested to play a pivotal role in glymphatic function and thus, clearance of Aβ from the brain. However, to-date this has only been investigated in AD rodent models and one human study of aquaporin/Aβ protein co-localisation in post mortem brain tissue. To partially address this gap in knowledge, the current study sought to investigate whether genetic variations (single nucleotide polymorphisms, SNPs) within the genes encoding aquaporin 1 (AQP1) and aquaporin 4 (AQP4), were associated with AD risk, brain Aβ burden and self-reported sleep parameters. Further, this study aimed to determine whether genetic variation moderated the relationship between sleep parameters and brain Aβ burden. This study was observational and cross-sectional in design, and utilised Genome-Wide Association Study, Pittsburgh Sleep Quality Index (PSQI), and Aβ positron emission tomography data from the larger Australian Imaging, Biomarkers and Lifestyle (AIBL) study. Genetic variation in AQP1 and AQP4 SNPs was not associated with either an increased AD risk or differences in brain Aβ burden. However, genetic variation in AQP4, specifically rs12968026, was associated with altered, self-reported, “overall” sleep quality (PSQI total score). Further, this study reports that several SNPs in AQP1 and AQP4 moderate the conditional effect that three PSQI-determined sleep parameters, namely, sleep latency (time taken to fall asleep, in minutes), sleep duration (length of sleep, in hours) and daytime dysfunction (disruption of daytime activities due to sleepiness), had on brain Aβ burden. Taken together, the results of this study add weight to the argument that the glymphatic system, is a major biological mechanism underpinning Aβ clearance from the brain. The findings also engender a greater understanding of what factors may moderate a sleep-AD phenotype relationship, and suggest that interventions targeted at improving suboptimal sleep parameters may be most effective at delaying AD onset when tailored to the genetics of the individual

Importance of CSF circulation following ischaemic stroke: A novel MRI investigation of CSF parenchymal flow

It has been proposed that intracranial pressure (ICP) elevation and collateral failure are responsible for unexplained early neurological deterioration (END) in stroke. Our aim was to investigate whether cerebrospinal fluid (CSF) dynamics, rather than oedema, are responsible for elevation of ICP after ischaemic stroke. Permanent middle cerebral artery occlusion (pMCAO) was induced with an intraluminal filament. At 24 hours after stroke, baseline ICP was measured, and CSF dynamics were probed via a steady-state infusion method. For the first time, we found a significant correlation between the baseline ICP at 24 hours post-stroke and the value of CSF outflow resistance. Results show that CSF outflow resistance, rather than oedema, was the mechanism responsible for ICP elevation following ischaemic stroke. This challenges current concepts and suggests the possibility that intracranial hypertension may be occurring undetected in a much wider range of stroke patients than is currently considered to be the case. Over the last decade, there has been significant renewed interest in the anatomical pathways and physiological mechanisms for the circulation of CSF. The glymphatic system is one such pathway that has been recently characterised. This network drives CSF into the brain along periarterial spaces and interstitial fluid (ISF) out along perivenous spaces. Aquaporin-4 (AQP4) water channels, densely expressed at the vascular endfeet of astrocytes, facilitate glymphatic transport. Glymphatic failure has been linked to a broad range of neurodegenerative diseases including ischaemic stroke. Accordingly, if the glymphatic circulation is a major outflow route for CSF, glymphatic dysfunction following ischaemic stroke could alter CSF dynamics and, therefore, ICP. Nevertheless, the glymphatic hypothesis is still controversial. All in vivo and biomechanical modelling studies that have investigated the glymphatic system have been based on utilizing a solute tracer to track the movement of CSF within the intracranial space. Since 99% of CSF is water, it is questionable whether nonwater tracer molecules can ever show the real dynamic flow of CSF. Hence, we sought the develop of a new MRI method to directly image CSF dynamics in-vivo, by exploiting an isotopically enriched MRI tracer, namely, H217O. Our results reveal glymphatic flow that is dramatically faster and more extensive than previously thought. Moreover, we confirm the critical role of aquaporin-4 (AQP4) channels in glymphatic flow by imaging CSF water dynamics in the brain using H217O alongside a potent blocker of AQP4. We hope in future that this new method can be used to investigate the responsible mechanism for the increased CSF resistance and ICP elevation following ischaemic stroke

Development and characterisation of a zebrafish larval model to investigate mechanisms for pathophysiology of intracranial hypertension in cryptococcal meningitis

Cryptococcosis is a fungal infection caused by members of the genus Cryptococcus. Worldwide, the most prevalent pathogen of this genus is the encapsulated saprophyte species Cryptococcus neoformans. Cryptococcosis most commonly occurs as an opportunistic airborne lung infection which can disseminate to most organ systems. The central nervous system appears particularly susceptible to developing a pathology from the infection, with more than half of cryptococcosis patients diagnosed with cryptococcal meningitis despite infection in multiple organs. Cryptococcal meningitis (CM) is a meningoencephalitis (infection of the brain parenchyma and meninges) which globally accounts for 19% (13-24) of AIDS-related mortality (Rajansingham et al., 2022). In 2020, reports show annual incidence of 152 000 cases of cryptococcal meningitis, resulting in 112 000 cryptococcal-related deaths, almost half of which are in eastern and southern Africa (Rajansingham et al., 2022). 50-70% of CM cases present with a pathologically elevated intracranial pressure (intracranial hypertension) (Graybill et al., 2000; Jarvis et al., 2014; Kagimu et al., 2022;). This thesis aims to improve our understanding of intracranial hypertension in CM to help identify potential targets for treatment, by developing and testing new models of intracranial hypertension in CM. Three different approaches were used – theoretical, in vitro rheology and in vivo in zebrafish. Zebrafish was chosen as the core experimental system in which to develop new models due to its physiology, tractability for live imaging and susceptibility to cryptococcosis. In zebrafish larvae, the dynamic nature of cranial vasculature compartments and the CSF during infection was examined using wide field and light sheet microscopy techniques. The physical properties of tissues and fluids when interacting with cryptococcal yeast cells was modelled with theoretical and in vitro rheological measurements. In vitro it was found that viscosity of fluids may increase in the presence of heat killed C. neoformans, but whether this change is pathologically significant requires further investigation. Using a model of cryptococcal infection in zebrafish larvae, a “pulsation” phenomenon was identified, consisting of vasodilation and constriction in the cranial vasculature with an impact on vessel wall permeability. The findings in this work, are reflective of the CM pathology as seen in human patients and suggest impaired CSF and blood flow homeostasis may contribute to intracranial hypertension in CM