Ian F. Harrison

Purpose: We developed a dynamic, heavily T2‐weighted (hT2w) FLAIR protocol to resolve acute Mn ²⁺ transport kinetics, as a proxy for Ca ²⁺ , across the choroid plexus (ChP)—cerebrospinal fluid (CSF) interface, in vivo. By quantifying intercompartmental Mn ²⁺ dynamics, and comparing these with Gd‐DOTA, we sought to characterize blood‐CSF‐barrier (BCSFB)‐mediated tracer transport, combining high in‐plane resolution and 4‐min temporal sampling. Methods: hT2w‐FLAIR DCE MRI was conducted in anesthetized C57BL/6 mice, receiving intravenous Mn ²⁺ or Gd‐DOTA. Semi‐quantitative tracer kinetic modeling permitted regional and inter‐tracer comparisons. Results: Mn ²⁺ rapidly crossed from the vasculature into the CSF, across the ChP, with similar arrival times and time‐to‐peak across compartments. Mn ²⁺ delivery into the ChP exhibited a 5.7‐fold steeper uptake rate ( p = 0.031) and a 5.6‐fold higher peak signal amplitude ( p = 0.0028) than in the LV‐CSF. In contrast, Gd‐DOTA exhibited a 2.2‐fold shorter ChP plateau duration ( p = 0.034) and 2.2‐fold faster ChP clearance rate ( p = 0.0047), compared to Mn ²⁺ . Within the CSF, there were significant temporal delays with Gd‐DOTA relative to Mn ²⁺ : a lengthened arrival (eight‐fold, p = 0.045) and time‐to‐peak (six‐fold, p = 0.024). Conclusion: Our hT2w‐FLAIR DCE‐MRI approach captured the early BCSFB transport window for Mn ²⁺ , providing distinct ChP and LV‐CSF kinetics at a higher temporal resolution and with reduced CSF signal contamination compared to prior approaches. These findings highlight that MRI tracer choice strongly shapes measured BCSFB kinetics, and supports the use of Mn ²⁺ as a tool for probing compartment‐specific ChP transport.

Propagation and aggregation of prion proteins, such as tau and α-synuclein (α-syn), are key pathological features of neurodegenerative diseases. Extracellular clearance pathways, such as the glymphatic system, might play a crucial role in the removal of these toxic proteins from the brain. Primarily active during sleep, this system relies on aquaporin-4 (AQP4) water channel expression and polarization to astrocytic endfeet, facilitating interstitial solute clearance. Glymphatic dysfunction has recently been implicated in Parkinson’s disease, but the precise mechanisms underlying the pathogenic effect of this dysfunction remain unclear. This includes how impaired glymphatic function influences α-syn propagation dynamics, in addition to the role of propagating α-syn itself on glymphatic function. In this study, we used a mouse model of α-syn propagation to elucidate the impact of α-syn aggregation on glymphatic function by measuring CSF–interstitial fluid exchange and assessing AQP4 and associated endfoot complex proteins in the brain over time and across different regions. Our results show that direct injection of α-syn preformed fibrils leads to local reduced expression of the AQP4 endfoot complex, but propagation of α-syn pathology induces an enhancement of glymphatic function, suggesting compensatory upregulation in response to increasing α-syn aggregate load. To determine the influence of glymphatic dysfunction on α-syn propagation dynamics, we then took a pharmacological approach to inhibit glymphatic function in this model. Acute glymphatic inhibition significantly reduced brain-to-CSF clearance of misfolded α-syn, and chronic treatment exacerbated α-syn pathology, cerebral atrophy and motor behavioural deficits in mice. Together, our findings show that α-syn clearance and propagation are modulated by glymphatic function. Moreover, they suggest that AQP4 complex dysregulation might contribute to glymphatic impairment associated with Parkinson’s disease, supporting further mechanistic investigation of this protein complex in the disease.

This scientific commentary refers to ‘Loss of glymphatic homeostasis in heart failure’ by Kritsilis et al. (https://doi.org/10.1093/brain/awae411).

Background: The choroid plexus plays an important role in brain homeostasis, including the active secretion of cerebrospinal fluid. Its function and structure have been reported to be affected by normal ageing. However, existing measures of choroid plexus volume may be complicated by partial volume ( in vivo MRI) and tissue fixation artefacts (histology). In this study, we investigate possible changes in choroid plexus volume within the lateral ventricles of aged mice utilising two structural MRI protocols explicitly designed for time-efficient, high-resolution in vivo imaging of the choroid plexus. Methods: Two MRI sequences were utilised to examine in vivo choroid plexus volume in the lateral ventricles of young (∼6 months) and aged (∼24 months) mouse brains: 1) an ultra-long echo-time T2 weighted fast-spin-echo and 2) a multi-TE T2* mapping protocol. A test-retest study was performed on a subset of the data to examine the reproducibility of choroid plexus volume estimation. A two-way ANOVA test was performed to determine possible differences in choroid plexus volume in young and aged mouse groups across the two distinct MRI protocols. Results: Reproducibility tests showed a low test-retest variability of the manual segmentation pipeline for both MRI protocols. A statistically significant reduction of in vivo choroid plexus volume was found in the aged mouse brain. This finding is concordant with previous histology studies that have observed a reduction in epithelial cell height with ageing across a wide range of species. Conclusions: We present an in vivo investigation of changes to lateral ventricle choroid plexus volume in the mouse brain utilising a manual segmentation approach based on two bespoke MRI protocols designed for time-efficient high resolution imaging of the choroid plexus. Furthermore, based on these protocols, we provide evidence for a reduction in choroid plexus volume in the aged brain. This research provides insight for studies utilising MRI measurements of choroid plexus volume as a biomarker of age-related neurologic conditions as it indicates that the ageing process itself does not result in hypertrophy of the choroid plexus, but a decrease in tissue volume.

Propagation and aggregation of prion proteins, such as tau and α-synuclein (αSyn), are key pathological features of neurodegenerative diseases. Extracellular clearance pathways, such as the glymphatic system, may play a crucial role in the removal of these toxic proteins from the brain. Primarily active during sleep, this system relies on aquaporin-4 (AQP4) water channel expression and polarisation to astrocytic endfeet, facilitating interstitial solute clearance. Glymphatic dysfunction has recently been implicated in Parkinson’s disease, however the precise mechanisms underlying the pathogenic effect of this dysfunction remain unclear. This includes how impaired glymphatic function influences αSyn propagation dynamics, and the role of propagating αSyn itself on glymphatic function. In this study, we used a mouse model of αSyn propagation to elucidate the impact of αSyn aggregation on glymphatic function, by measuring CSF-ISF exchange and assessing AQP4 and associated endfoot complex proteins in the brain over time and across different regions. Our results show that direct injection of αSyn pre-formed fibrils leads to reduced expression of the AQP4 endfoot complex, but propagation of endogenous αSyn induces an enhancement of glymphatic function suggesting compensatory upregulation in response to increasing endogenous αSyn load. To determine the influence of glymphatic dysfunction on αSyn propagation dynamics, we then employed a pharmacological approach to inhibit glymphatic function in this model. Acute glymphatic inhibition significantly reduced brain to CSF αSyn clearance, and chronic treatment exacerbated αSyn pathology, neurodegeneration, and motor behavioural deficits in mice. Together our findings show that αSyn clearance and propagation are modulated by glymphatic function and suggest that AQP4 complex dysregulation may contribute to glymphatic impairment associated with Parkinson’s diseases. Summary for the non-scientific community The glymphatic system clears brain waste during sleep. Lopes et al. show that α-synuclein, a protein linked to Parkinson’s, is cleared by this system. Using a mouse model of the disease, they suggest that aquaporin-4 water channels may impair glymphatic function, contributing to α-synuclein buildup in patients’ brains.

Background: The aggregation and spread of misfolded amyloid structured proteins, such as tau and α-synuclein, are key pathological features associated with neurodegenerative disorders, including Alzheimer’s and Parkinson’s disease. These proteins possess a prion-like property, enabling their transmission from cell to cell leading to propagation throughout the central and peripheral nervous systems. While the mechanisms underlying their intracellular spread are still being elucidated, targeting the extracellular space has emerged as a potential therapeutic approach. The glymphatic system, a brain-wide pathway responsible for clearing extracellular metabolic waste from the central nervous system, has gained attention as a promising target for removing these toxic proteins. Methods: In this study, we investigated the impact of long-term modulation of glymphatic function on tau aggregation and spread by chronically treating a mouse model of tau propagation with a pharmacological inhibitor of AQP4, TGN-020. Thy1-hTau.P301S mice were intracerebrally inoculated with tau into the hippocampus and overlying cortex, and subsequently treated with TGN-020 (3 doses/week, 50 mg/kg TGN-020, i.p.) for 10-weeks. During this time, animal memory was studied using cognitive behavioural tasks, and structural MR images were acquired of the brain in vivo prior to brain extraction for immunohistochemical characterisation. Results: Our findings demonstrate increased tau aggregation in the brain and transhemispheric propagation in the hippocampus following the inhibition of glymphatic clearance. Moreover, disruption of the glymphatic system aggravated recognition memory in tau inoculated mice and exacerbated regional changes in brain volume detected in the model. When initiation of drug treatment was delayed for several weeks post-inoculation, the alterations were attenuated. Conclusions: These results indicate that by modulating AQP4 function and, consequently, glymphatic clearance, it is possible to modify the propagation and pathological impact of tau in the brain, particularly during the initial stages of the disease. These findings highlight the critical role of the glymphatic system in preserving healthy brain homeostasis and offer valuable insights into the therapeutic implications of targeting this system for managing neurodegenerative diseases characterized by protein aggregation and spread.

Background: The aggregation and spread of misfolded amyloid structured proteins, such as tau and α-synuclein, are key pathological features associated with neurodegenerative disorders, including Alzheimer’s and Parkinson’s disease. These proteins possess a prion-like property, enabling their transmission from cell to cell leading to propagation throughout the central and peripheral nervous systems. While the mechanisms underlying their intracellular spread are still being elucidated, targeting the extracellular space has emerged as a potential therapeutic approach. The glymphatic system, a brain-wide pathway responsible for clearing extracellular metabolic waste from the central nervous system, has gained attention as a promising target for removing these toxic proteins. Methods: In this study, we investigated the impact of long-term modulation of glymphatic function on tau aggregation and spread by chronically treating a mouse model of tau propagation with a pharmacological inhibitor of AQP4, TGN-020. Thy1-hTau.P301S mice were intracerebrally inoculated with tau into the hippocampus and overlying cortex, and subsequently treated with TGN-020 (3 doses/week, 50mg/kg TGN-020, i.p.) for 10-weeks. During this time, animal memory was studied using cognitive behavioural tasks, and structural MR images were acquired of the brain in vivo prior to brain extraction for immunohistochemical characterisation. Results: Our findings demonstrate increased tau aggregation in the brain and transhemispheric propagation in the hippocampus and cortex following the inhibition of glymphatic clearance. Moreover, disruption of the glymphatic system aggravated recognition memory in tau inoculated mice and exacerbated regional changes in brain volume detected in the model. When initiation of drug treatment was delayed for several weeks post-inoculation, the aforementioned alterations were attenuated. Conclusions: These results indicate that by modulating AQP4 function and, consequently, glymphatic clearance, it is possible to modify the propagation and pathological impact of tau in the brain, particularly during the initial stages of the disease. These findings highlight the critical role of the glymphatic system in preserving healthy brain homeostasis and offer valuable insights into the therapeutic implications of targeting this system for managing neurodegenerative diseases characterized by protein aggregation and spread.

Many neurodegenerative diseases, including Alzheimer’s disease and Parkinson’s disease, are characterised by the accumulation of misfolded protein deposits in the brain, leading to a progressive destabilisation of the neuronal network and neuronal death. Among the proteins that can abnormally accumulate are tau and α-synuclein, which can propagate in a prion-like manner and which upon aggregation, represent the most common intracellular proteinaceous lesions associated with neurodegeneration. For years it was thought that these intracellular proteins and their accumulation had no immediate relationship with extracellular homeostasis pathways such as the glymphatic clearance system; however, mounting evidence has now suggested that this is not the case. The involvement of the glymphatic system in neurodegenerative disease is yet to be fully defined; however, it is becoming increasingly clear that this pathway contributes to parenchymal solute clearance. Importantly, recent data show that proteins prone to intracellular accumulation are subject to glymphatic clearance, suggesting that this system plays a key role in many neurological disorders. In this review, we provide a background on the biology of tau and α-synuclein and discuss the latest findings on the cell-to-cell propagation mechanisms of these proteins. Importantly, we discuss recent data demonstrating that manipulation of the glymphatic system may have the potential to alleviate and reduce pathogenic accumulation of propagation-prone intracellular cytotoxic proteins. Furthermore, we will allude to the latest potential therapeutic opportunities targeting the glymphatic system that might have an impact as disease modifiers in neurodegenerative diseases.