Diffusion MRI reveals in vivo and non-invasively changes in astrocyte function induced by an aquaporin-4 inhibitor

From Brownian motion to virtual biopsy: a historical perspective from 40 years of diffusion MRI

Diffusion MRI was introduced in 1985, showing how the diffusive motion of molecules, especially water, could be spatially encoded with MRI to produce images revealing the underlying structure of biologic tissues at a microscopic scale. Diffusion is one of several Intravoxel Incoherent Motions (IVIM) accessible to MRI together with blood microcirculation. Diffusion imaging first revolutionized the management of acute cerebral ischemia by allowing diagnosis at an acute stage when therapies can still work, saving the outcomes of many patients. Since then, the field of diffusion imaging has expanded to the whole body, with broad applications in both clinical and research settings, providing insights into tissue integrity, structural and functional abnormalities from the hindered diffusive movement of water molecules in tissues. Diffusion imaging is particularly used to manage many neurologic disorders and in oncology for detecting and classifying cancer lesions, as well as monitoring treatment response at an early stage. The second major impact of diffusion imaging concerns the wiring of the brain (Diffusion Tensor Imaging, DTI), allowing to obtain from the anisotropic movement of water molecules in the brain white-matter images in 3 dimensions of the brain connections making up the Connectome. DTI has opened up new avenues of clinical diagnosis and research to investigate brain diseases, neurogenesis and aging, with a rapidly extending field of application in psychiatry, revealing how mental illnesses could be seen as Connectome spacetime disorders. Adding that water diffusion is closely associated to neuronal activity, as shown from diffusion fMRI, one may consider that diffusion MRI is ideally suited to investigate both brain structure and function. This article retraces the early days and milestones of diffusion MRI which spawned over 40 years, showing how diffusion MRI emerged and expanded in the research and clinical fields, up to become a pillar of modern clinical imaging.

Single-cell analysis of osmoregulation reveals heterogeneity of aquaporin 4 functionality in human astrocytes

The water channel aquaporin 4 (AQP4) contributes to water flow and waste removal across the blood-brain barrier and its levels, organization and localization are perturbed in various neurological diseases, including Alzheimer's Disease. This renders AQP4 a potentially valuable therapeutic target. However, most functional assays aimed at identifying modulators of AQP4 function are performed with primary rodent cells and do not consider inter-cellular variations in AQP4 abundance and presentation. To address this, we have established and applied a robust live cell microscopy assay that captures the contribution of AQP4 in the osmotically driven (de-)quenching of the vital dye Calcein-AM with single-cell resolution. Using human astrocytoma cells, we found that performing measurements on cellular regions instead of whole fields of view yielded a more sensitive readout of the osmotic response, which correlated with AQP4 abundance. Stable co-expression of the two major AQP4 isoforms, but not of the individual isoforms, provoked a faster adaptation to osmotic changes, while siRNA-mediated knockdown of AQP4 had the opposite effect. Post-hoc correlation with the canonical membrane marker CD44 revealed that the speed of the osmotic response scaled with AQP4 membrane enrichment. Coating the substrate with laminin promoted AQP4 membrane enrichment, while cell confinement with fixed-size micropatterns further increased the speed of osmoregulation, underscoring the influence of extracellular factors. The osmotic response of primary fetal astrocytes and human iPSC-derived astrocyte models was comparable to AQP4-deficient astrocytoma cells, in line with their low AQP4 levels and indicative of their immature state. In conclusion, a correlative single-cell approach based on the quantification of Calcein-AM quenching capacity, AQP4 abundance and AQP4 membrane enrichment, allows resolving osmoregulation in a more sensitive manner and reveals heterogeneity between and within human astrocyte (-like) cultures, which could prove crucial for future screens aimed at identifying AQP4 modulators.

Astrocyte aquaporin mediates a tonic water efflux maintaining brain homeostasis

Brain water homeostasis not only provides a physical protection, but also determines the diffusion of chemical molecules key for information processing and metabolic stability. As a major type of glia in brain parenchyma, astrocytes are the dominant cell type expressing aquaporin water channel. How astrocyte aquaporin contributes to brain water homeostasis in basal physiology remains to be understood. We report that astrocyte aquaporin 4 (AQP4) mediates a tonic water efflux in basal conditions. Acute inhibition of astrocyte AQP4 leads to intracellular water accumulation as optically resolved by fluorescence-translated imaging in acute brain slices, and in vivo by fiber photometry in mobile mice. We then show that aquaporin-mediated constant water efflux maintains astrocyte volume and osmotic equilibrium, astrocyte and neuron Ca2+ signaling, and extracellular space remodeling during optogenetically induced cortical spreading depression. Using diffusion-weighted magnetic resonance imaging (DW-MRI), we observed that in vivo inhibition of AQP4 water efflux heterogeneously disturbs brain water homeostasis in a region-dependent manner. Our data suggest that astrocyte aquaporin, though bidirectional in nature, mediates a tonic water outflow to sustain cellular and environmental equilibrium in brain parenchyma.

Relationship between regional volume changes and water diffusion in fixed marmoset brains: an in vivo and ex vivo comparison

Ex vivo studies of the brain are often employed as experimental systems in neuroscience. In general, brains for ex vivo MRI studies are usually fixed with paraformaldehyde to preserve molecular structure and prevent tissue destruction during long-term storage. As a result, fixing brain tissue causes microstructural changes and a decrease in brain volume. Therefore, the purpose of this study was to investigate the regional effect of brain volume and microstructural changes on the restricted diffusion of water molecules in the common marmoset brain using in vivo and ex vivo brains from the same individual. We used 9.4T magnetic resonance imaging and also compared the T2-weighted images and diffusion-weighted imaging (DWI) data between in vivo and ex vivo brains to investigate changes in brain volume and diffusion of water molecules in 12 common marmosets. We compared fractional anisotropy, mean diffusivity, AD (axial diffusivity), and radial diffusivity values in white matter and gray matter between in vivo and ex vivo brains. We observed that AD showed the strongest correlation with regional volume changes in gray matter. The results showed a strong correlation between AD and changes in brain volume. By comparing the in vivo and ex vivo brains of the same individual, we identified significant correlations between the local effects of perfusion fixation on microstructural and volumetric changes of the brain and alterations in the restricted diffusion of water molecules within the brain. These findings provide valuable insights into the complex relationships between tissue fixation, brain structure, and water diffusion properties in the marmoset brain.

Diffusion magnetic resonance imaging of cerebrospinal fluid dynamics: Current techniques and future advancements

Cerebrospinal fluid (CSF) plays a critical role in metabolic waste clearance from the brain, requiring its circulation throughout various brain pathways, including the ventricular system, subarachnoid spaces, para-arterial spaces, interstitial spaces, and para-venous spaces. The complexity of CSF circulation has posed a challenge in obtaining noninvasive measurements of CSF dynamics. The assessment of CSF dynamics throughout its various circulatory pathways is possible using diffusion magnetic resonance imaging (MRI) with optimized sensitivity to incoherent water movement across the brain. This review presents an overview of both established and emerging diffusion MRI techniques designed to measure CSF dynamics and their potential clinical applications. The discussion offers insights into the optimization of diffusion MRI acquisition parameters to enhance the sensitivity and specificity of diffusion metrics on underlying CSF dynamics. Lastly, we emphasize the importance of cautious interpretations of diffusion-based imaging, especially when differentiating between tissue- and fluid-related changes or elucidating structural versus functional alterations.

The effect of aquaporin-4 inhibition on cerebrospinal fluid-tissue water exchange in mouse brain detected by magnetization transfer indirect spin labeling MRI

The fluid transport of cerebrospinal fluid (CSF) and interstitial fluid in surrounding tissues plays an important role in the drainage pathway that facilitates waste clearance from the brain. This pathway is known as the glymphatic or perivascular system, and its functions are dependent on aquaporin-4 (AQP4). Recently, magnetization transfer indirect spin labeling (MISL) magnetic resonance imaging (MRI) has been proposed as a noninvasive and noncontrast-enhanced method for detecting water exchange between CSF and brain tissue. In this study, we first optimized the MISL sequence at preclinical 3 T MRI, and then studied the correlation of MISL in CSF with magnetization transfer (MT) in brain tissue, as well as the altered water exchange under AQP4 inhibition, using C57BL/6 mice. Results showed a strong correlation of MISL signal with MT signal. With the AQP4 inhibitor, we observed a significant decrease in MISL value (P < 0.05), suggesting that the hampered AQP4 activity led to decreased water exchange between CSF and brain tissue or the impairment of the glymphatic function. Overall, our findings demonstrate the potential application of MISL in assessing brain water exchange at 3 T MRI and its potential clinical translation.

Vascular risk factors and astrocytic marker for the glymphatic system activity

OBJECTIVES

Glymphatic system maintains brain fluid circulation via active transportation of astrocytic aquaporin-4 in perivascular space. The diffusion tensor imaging analysis along the perivascular space (DTI-ALPS) is an established method measuring perivascular glymphatic activity, but comprehensive investigations into its influential factors are lacking.

METHODS

Community-dwelling older adults underwent brain MRI scans, neuropsychiatric, and multi-domain assessments. Blood biomarker tests included glial fibrillary acidic protein (GFAP) for astrocyte injury.

RESULTS

In 71 enrolled participants, the DTI-ALPS index was associated with modifiable factors, including lipid profile (high-density lipoprotein, r = 0.396; very-low-density lipoprotein, r =  - 0.342), glucose intolerance (diabetes mellitus, standardized mean difference (SMD) = 0.7662; glycated hemoglobin, r =  - 0.324), obesity (body mass index, r =  - 0.295; waist, r =  - 0.455), metabolic syndrome (SMD =   - 0.6068), cigarette-smoking (SMD =  - 0.6292), and renal clearance (creatinine, r =  - 0.387; blood urea nitrogen, r =  - 0.303). Unmodifiable associative factors of DTI-ALPS were age (r =  - 0.434) and sex (SMD = 1.0769) (all p < 0.05). A correlation of DTI-ALPS and blood GFAP was noticed (r =  - 0.201, one-tailed t-test for the assumption that astrocytic injury impaired glymphatic activity, p = 0.046). Their cognitive correlations diverged, domain-specific for DTI-ALPS (Facial Memory Test, r = 0.272, p = 0.022) but global cognition-related for blood GFAP (MoCA, r =  - 0.264, p = 0.026; ADAS-cog, r = 0.304, p = 0.010).

CONCLUSION

This correlation analysis revealed multiple modifiable and unmodifiable association factors to the glymphatic image marker. The DTI-ALPS index correlated with various metabolic factors that are known to increase the risk of vascular diseases such as atherosclerosis. Furthermore, the DTI-ALPS index was associated with renal indices, and this connection might be a link of water regulation between the two systems. In addition, the astrocytic biomarker, plasma GFAP, might be a potential marker of the glymphatic system; however, more research is needed to confirm its effectiveness.

Measuring water exchange on a preclinical MRI system using filter exchange and diffusion time dependent kurtosis imaging

PURPOSE

Filter exchange imaging (FEXI) and diffusion time (t)-dependent diffusion kurtosis imaging (DKI(t)) are both sensitive to water exchange between tissue compartments. The restrictive effects of tissue microstructure, however, introduce bias to the exchange rate obtained by these two methods, as their interpretation conventionally rely on the Kärger model of barrier limited exchange between Gaussian compartments. Here, we investigated whether FEXI and DKI(t) can provide comparable exchange rates in ex vivo mouse brains.

THEORY AND METHODS

FEXI and DKI(t) data were acquired from ex vivo mouse brains on a preclinical MRI system. Phase cycling and negative slice prewinder gradients were used to minimize the interferences from imaging gradients.

RESULTS

In the corpus callosum, apparent exchange rate (AXR) from FEXI correlated with the exchange rate (the inverse of exchange time, 1/τex ) from DKI(t) along the radial direction. In comparison, discrepancies between FEXI and DKI(t) were found in the cortex due to low filter efficiency and confounding effects from tissue microstructure.

CONCLUSION

The results suggest that FEXI and DKI(t) are sensitive to the same exchange processes in white matter when separated from restrictive effects of microstructure. The complex microstructure in gray matter, with potential exchange among multiple compartments and confounding effects of microstructure, still pose a challenge for FEXI and DKI(t).

The effect of a novel AQP4 facilitator, TGN-073, on glymphatic transport captured by diffusion MRI and DCE-MRI

The glymphatic system is a low resistance pathway, by which cerebrospinal fluid enters the brain parenchyma along perivascular spaces via AQP4 channels. It is hypothesised that the resulting convective flow of the interstitial fluid provides an efficient mechanism for the removal of waste toxins from the brain. Therefore, enhancing AQP4 function might protect against neurodegenerative diseases such as Alzheimer's disease (AD), in which the accumulation of harmful proteins and solutes is a hallmark feature. Here, we test the effect of a putative AQP4 facilitator, TGN-073, on glymphatic transport in a normal rat brain by employing different MRI techniques. Surgical procedures were undertaken to catheterise the cisterna magna, thereby enabling infusion of the MRI tracer. Followed by the intraperitoneal injection of either TGN-073, or the vehicle. Using a paramagnetic contrast agent (Gd-DTPA) as the MRI tracer, dynamic 3D T1 weighted imaging of the glymphatic system was undertaken over two hours. Further, the apparent diffusion coefficient was measured in different brain regions using diffusion-weighted imaging (DWI). While physiological parameters and arterial blood gas analysis were monitored continuously. We found that rats treated with TGN-073 showed the distribution of Gd-DTPA was more extensive and parenchymal uptake was higher compared with the vehicle group. Water diffusivity was increased in the brain of TGN-073 treated group, which indicates greater water flux. Also, MRI showed the glymphatic transport and distribution in the brain is naturally heterogeneous, which is consistent with previous studies. Our results indicate that compounds such as TGN-073 can improve glymphatic function in the brain. Since glymphatic impairment due to AQP4 dysfunction is potentially associated with several neurological disorders such as AD, dementia and traumatic brain injury, enhancing AQP4 functionality might be a promising therapeutic target.

Glymphatic system dysfunction in patients with occipital lobe epilepsy

BACKGROUND AND PURPOSE

We aimed to investigate the glymphatic system function in patients with occipital lobe epilepsy (OLE) and healthy controls using diffusion tensor image analysis along the perivascular space (DTI-ALPS) index.

METHODS

We retrospectively included 23 patients with OLE and 30 healthy controls. The participants underwent brain MRI, which was normal, and diffusion tensor imaging. We used the DSI Studio for data preprocessing, obtained the fiber orientation and diffusivities, and calculated the DTI-ALPS index from the diffusivity values associated with the projection and association fibers in the left hemisphere.

RESULTS

There were no differences in mean age (31.6 years [range: 13-58] vs. 31.3 years [range: 20-57], p = .912) and male sex ratio (10/23 [43.5%] vs. 15/30 [50.0%]) between the groups. Compared to healthy controls, the diffusivities in patients with OLE were higher along the Y-axis in the projection fiber and along the Z-axis in the association fiber and lower along the Y-axis in the association fiber. The DTI-ALPS index in patients with OLE was lower than that in the healthy controls (1.421 ± 0.171 vs. 1.667 ± 0.271, p < .001, 95% confidence interval of difference = 0.117-0.376, Test statistic t = 3.823). We found no association between the DTI-ALPS index and clinical characteristics in OLE.

CONCLUSION

The DTI-ALPS index in patients with OLE was significantly lower than that in healthy controls, suggesting glymphatic system dysfunction in OLE. The DTI-ALPS index could help assess the glymphatic system function in patients with epilepsy.

Novel developments in non-contrast enhanced MRI of the perivascular clearance system: What are the possibilities for Alzheimer's disease research?

The cerebral waste clearance system (i.e, glymphatic or intramural periarterial drainage) works through a network of perivascular spaces (PVS). Dysfunction of this system likely contributes to aggregation of Amyloid-β and subsequent toxic plaques in Alzheimer's disease (AD). A promising, non-invasive technique to study this system is MRI, though applications in dementia are still scarce. This review focusses on recent non-contrast enhanced (non-CE) MRI techniques which determine and visualise physiological aspects of the clearance system at multiple levels, i.e., cerebrospinal fluid flow, PVS-flow and interstitial fluid movement. Furthermore, various MRI studies focussing on aspects of the clearance system which are relevant to AD are discussed, such as studies on ageing, sleep alterations, and cognitive decline. Additionally, the complementary function of non-CE to CE methods is elaborated upon. We conclude that non-CE studies have great potential to determine which parts of the waste clearance system are affected by AD and in which stages of cognitive impairment dysfunction of this system occurs, which could allow future clinical trials to target these specific mechanisms.

Imaging perivascular space structure and function using brain MRI

In this article, we provide an overview of current neuroimaging methods for studying perivascular spaces (PVS) in humans using brain MRI. In recent years, an increasing number of studies highlighted the role of PVS in cerebrospinal/interstial fluid circulation and clearance of cerebral waste products and their association with neurological diseases. Novel strategies and techniques have been introduced to improve the quantification of PVS and to investigate their function and morphological features in physiological and pathological conditions. After a brief introduction on the anatomy and physiology of PVS, we examine the latest technological developments to quantitatively analyze the structure and function of PVS in humans with MRI. We describe the applications, advantages, and limitations of these methods, providing guidance and suggestions on the acquisition protocols and analysis techniques that can be applied to study PVS in vivo. Finally, we review the human neuroimaging studies on PVS across the normative lifespan and in the context of neurological disorders.

Glymphatic system dysfunction in patients with cluster headache

INTRODUCTION

The aim of this study was to investigate alterations of the glymphatic system function in patients with cluster headache.

METHODS

We enrolled patients with cluster headache and healthy controls, and they underwent brain magnetic resonance imaging (MRI), including diffusion tensor imaging (DTI). We used the MRIcron and DSI studio programs for DTI preprocessing and DTI analysis with perivascular space (DTI-ALPS) index calculation.

RESULTS

Fourteen patients with cluster headache and 23 healthy controls were enrolled. The DTI-ALPS indexes of the groups were significantly different. The DTI-ALPS index for the patients with cluster headache was lower than that for the healthy controls (1.586 vs. 1.786, p = 0.044). There was a significant negative correlation between the DTI-ALPS index and age in the patients with cluster headache (r = -0.549, p = 0.042). However, the DTI-ALPS index was not associated with other clinical characteristics, including disease duration and headache intensity (r = -0.405, p = 0.150; r = -0.048, p = 0.869, respectively).

CONCLUSION

Patients with cluster headache had a lower DTI-ALPS index than the healthy controls; this might indicate glymphatic system dysfunction in the patients with cluster headache. Further research is required to determine whether glymphatic system dysfunction is related to the pathophysiology of cluster headache.

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.

Clearance Systems in the Brain, From Structure to Function

As the most metabolically active organ in the body, there is a recognized need for pathways that remove waste proteins and neurotoxins from the brain. Previous research has indicated potential associations between the clearance system in the brain and the pathological conditions of the central nervous system (CNS), due to its importance, which has attracted considerable attention recently. In the last decade, studies of the clearance system have been restricted to the glymphatic system. However, removal of toxic and catabolic waste by-products cannot be completed independently by the glymphatic system, while no known research or article has focused on a comprehensive overview of the structure and function of the clearance system. This thesis addresses a neglected aspect of linkage between the structural composition and main components as well as the role of neural cells throughout the clearance system, which found evidence that the components of CNS including the glymphatic system and the meningeal lymphatic system interact with a neural cell, such as astrocytes and microglia, to carry out vital clearance functions. As a result of this evidence that can contribute to a better understanding of the clearance system, suggestions were identified for further clinical intervention development of severe conditions caused by the accumulation of metabolic waste products and neurotoxins in the brain, such as Alzheimer’s disease (AD) and Parkinson’s disease (PD).

Neuroimaging measures of iron and gliosis explain memory performance in aging

Evidence from animal and histological studies has indicated that accumulation of iron in the brain results in reactive gliosis that contributes to cognitive deficits. The current study extends these findings to human cognitive aging and suggests that magnetic resonance imaging (MRI) techniques like quantitative relaxometry can be used to study iron and its effects in vivo. The effects of iron on microstructure and memory performance were examined using a combination of quantitative relaxometry and multicompartment diffusion imaging in 35 young (21.06 ± 2.18 years) and 28 older (72.58 ± 6.47 years) adults, who also completed a memory task. Replicating past work, results revealed age‐related increases in iron content (R2*) and diffusion, and decreases in memory performance. Independent of age group, iron content was significantly related to restricted (intracellular) diffusion in regions with low‐moderate iron (hippocampus, caudate) and to all diffusion metrics in regions with moderate‐high iron (putamen, globus pallidus). This pattern is consistent with different stages of iron‐related gliosis, ranging from astrogliosis that may influence intracellular diffusion to microglial proliferation and increased vascular permeability that may influence all sources of diffusion. Further, hippocampal restricted diffusion was significantly related to memory performance, with a third of this effect related to iron content; consistent with the hypothesis that higher iron‐related astrogliosis in the hippocampus is associated with poorer memory performance. These results demonstrate the sensitivity of MRI to iron‐related gliosis and extend our understanding of its impact on cognition by showing that this relationship also explains individual differences in memory performance.

Factors Associated With Dysfunction of Glymphatic System in Patients With Glioma

Objectives

Rodent experiments have provided some insights into the changes of glymphatic function associated with glioma growth. The diffusion tensor image analysis along the perivascular space (DTI-ALPS) method offers an opportunity for the noninvasive investigation of the glymphatic system in patients with glioma. We aimed to investigate the factors associated with glymphatic function changes in patients with glioma.

Materials and Methods

A total of 201 glioma patients (mean age = 47.4 years, 116 men; 86 grade II, 52 grade III, and 63 grade IV) who had preoperative diffusion tensor imaging for calculation of the ALPS index were retrospectively included. Information collected from each patient included sex, age, tumor grade, isocitrate dehydrogenase 1 (IDH1) mutation status, peritumoral brain edema volume, tumor volume, and ALPS index. Group differences in the ALPS index according to sex, tumor grade, and IDH1 mutation status were assessed using analysis of covariance with age adjustment. Linear regression analyses were performed to identify the factors associated with the ALPS index.

Results

Group comparisons revealed that the ALPS index of grade II/III gliomas was significantly higher than that of grade IV gliomas (p < 0.001). The ALPS index of IDH1 mutant gliomas was significantly higher than that of IDH1 wild-type gliomas (p < 0.001). On multivariable linear regression analysis, IDH1 mutation (β = 0.308, p < 0.001) and peritumoral brain edema volume (β = −0.353, p < 0.001) were the two independent factors associated with the ALPS index.

Conclusion

IDH1 wild-type gliomas and gliomas with larger peritumoral brain edema volumes were associated with a lower ALPS index, which may reflect impaired glymphatic function.

Exploring cell membrane water exchange in aquaporin-4-deficient ischemic mouse brain using diffusion-weighted MRI

BACKGROUND

Aquaporin-4 is a membrane channel protein that is highly expressed in brain astrocytes and facilitates the transport of water molecules. It has been suggested that suppression of aquaporin-4 function may be an effective treatment for reducing cellular edema after cerebral infarction. It is therefore important to develop clinically applicable measurement systems to evaluate and better understand the effects of aquaporin-4 suppression on the living body.

METHODS

Animal models of focal cerebral ischemia were created by surgically occluding the middle cerebral artery of wild-type and aquaporin-4 knockout mice, after which multi-b-value multi-diffusion-time diffusion-weighted imaging measurements were performed. Data were analyzed with both the apparent diffusion coefficient (ADC) model and a compartmental water-exchange model.

RESULTS

ADCs were estimated for five different b value ranges. The ADC of aquaporin-4 knockout mice in the contralateral region was significantly higher than that of wild-type mice for each range. In contrast, aquaporin-4 knockout mice had significantly lower ADC than wild-type mice in ischemic tissue for each b-value range. Genotype-dependent differences in the ADC were particularly significant for the lowest ranges in normal tissue and for the highest ranges in ischemic tissue. The ADCs measured at different diffusion times were significantly different for both genotypes. Fitting of the water-exchange model to the ischemic region data found that the water-exchange time in aquaporin-4 knockout mice was approximately 2.5 times longer than that in wild-type mice.

CONCLUSIONS

Multi-b-value multi-diffusion-time diffusion-weighted imaging may be useful for in vivo research and clinical diagnosis of aquaporin-4-related diseases.

Glymphatic imaging and modulation of the optic nerve

Optic nerve health is essential for proper function of the visual system. However, the pathophysiology of certain neurodegenerative disease processes affecting the optic nerve, such as glaucoma, is not fully understood. Recently, it was hypothesized that a lack of proper clearance of neurotoxins contributes to neurodegenerative diseases. The ability to clear metabolic waste is essential for tissue homeostasis in mammals, including humans. While the brain lacks the traditional lymphatic drainage system identified in other anatomical regions, there is growing evidence of a glymphatic system in the central nervous system, which structurally includes the optic nerve. Named to acknowledge the supportive role of astroglial cells, this perivascular fluid drainage system is essential to remove toxic metabolites from the central nervous system. Herein, we review existing literature describing the physiology and dysfunction of the glymphatic system specifically as it relates to the optic nerve. We summarize key imaging studies demonstrating the existence of a glymphatic system in the optic nerves of wild-type rodents, aquaporin 4-null rodents, and humans; glymphatic imaging studies in diseases where the optic nerve is impaired; and current evidence regarding pharmacological and lifestyle interventions that may help promote glymphatic function to improve optic nerve health. We conclude by highlighting future research directions that could be applied to improve imaging detection and guide therapeutic interventions for diseases affecting the optic nerve.

Localized Modification of Water Molecule Transport After Focused Ultrasound-Induced Blood-Brain Barrier Disruption in Rat Brain

Interstitial solutes can be removed by various overlapping clearance systems, including blood-brain barrier (BBB) transport and glymphatic clearance. Recently, focused ultrasound (FUS)-induced BBB disruption (BBBD) has been applied to visualize glymphatic transport. Despite evidence that FUS-BBBD might facilitate glymphatic transport, the nature of fluid movement within the sonication region is yet to be determined. In this study, we sought to determine whether FUS-BBBD may facilitate the local movement of water molecules. Two different FUS conditions (0.60-0.65 MPa and 0.75-0.80 MPa) were used to induce BBBD in the caudate-putamen and thalamus regions of healthy Sprague-Dawley rats. The water diffusion caused by FUS-BBBD was analyzed using the apparent diffusion coefficient (ADC), axial diffusivity, radial diffusivity (RD), and fractional anisotropy, obtained at 5 min, 24 and 48 h, as well as the water channel expression of aquaporin-4 (AQP-4) immunostaining at 48 h after FUS-induced BBBD. In addition, hematoxylin and eosin histopathology and Fluoro-Jade C (FJC) immunostaining were performed to analyze brain damage. The signal changes in ADC and RD in the sonication groups showed significant and transient reduction at 5 min, with subsequent increases at 24 and 48 h after FUS-induced BBBD. When we applied higher sonication conditions, the ADC and RD showed enhancement until 48 h, and became comparable to contralateral values at 72 h. AQP-4 expression was upregulated after FUS-induced BBBD in both sonication conditions at 48 h. The results of this study provide preliminary evidence on how mechanical forces from FUS alter water dynamics through diffusion tensor imaging (DTI) measures and AQP4 expression.

Neurofluid as Assessed by Diffusion-Weighted Imaging

The glymphatic system hypothesis is associated with the circulation of cerebrospinal fluid (CSF) in the skull and interstitial fluid (ISF) in the brain. There are several imaging techniques to visualize the dynamics of CSF and ISF. Magnetic resonance imaging (MRI) is one of the promising modalities for glymphatic imaging and diffusion MRI is expected imaging tool. Several disorders are associated with glymphatic dysfunction or impairment in the dynamics of CSF or ISF. The Central Nervous System interstitial fluidopathy concept has been proposed to encompass diseases with pathologies that are predominantly associated with abnormal ISF/CSF dynamics.

Peritumoral Brain Edema in Meningiomas May Be Related to Glymphatic Dysfunction

The pathogenesis of peritumoral brain edema (PTBE) in meningiomas remains unclear. The glymphatic system is recently recognized as a pathway for waste clearance and maintaining fluid balance in the brain parenchymal interstitium. We aimed to investigate if the PTBE volume of meningiomas correlates with their glymphatic function. A total of 80 meningioma patients (mean age, 58.8 years; 37 men) and 44 normal subjects (mean age 53.3 years; 23 men) who had preoperative diffusion-tensor imaging for calculation of the analysis along the perivascular space (ALPS) index were retrospectively included. Information collected from each patient included sex, age, tumor grade, Ki-67 index, tumor location, tumor volume, PTBE volume and ALPS index. Comparisons of ALPS index among meningiomas without PTBE, meningiomas with PTBE, and normal subjects were performed using analysis of covariance with Bonferroni correction and adjustments for age and sex. Pearson correlation coefficient and multivariable linear regression analyses were performed to identify factors associated with PTBE volume. Group comparisons revealed that the ALPS index was significantly higher (P < 0.05) in meningiomas without PTBE vs. meningiomas with PTBE and normal subjects. On the other hand, ALPS index was not different between meningiomas with PTBE and normal subjects. On Pearson correlation and multivariable linear regression analyses, the ALPS index was the only factor significantly (P < 0.05) associated with PTBE volume. In conclusion, PTBE volume inversely correlated with ALPS index in meningiomas. PTBE formation in meningiomas may be related to glymphatic dysfunction.

Fractional anisotropy from diffusion tensor imaging correlates with acute astrocyte and myelin swelling in neonatal swine models of excitotoxic and hypoxic-ischemic brain injury

The specific cytopathology that causes abnormal fractional anisotropy (FA) and mean diffusivity (MD) from diffusion tensor imaging (DTI) after neonatal hypoxia-ischemia (HI) is not completely understood. The panoply of cell types in the brain might contribute differentially to changes in DTI metrics. Because glia are the predominant cell type in brain, we hypothesized that changes in FA and MD would signify perturbations in glial microstructure. Using a 3-Tesla clinical scanner, we conducted in vivo DTI MRI in nine neonatal piglets at 20-96 h after excitotoxic brain injury from striatal quinolinic acid injection or global HI. FA and MD from putamen, caudate, and internal capsule in toto were correlated with astrocyte swelling, neuronal excitotoxicity, and white matter injury. Low FA correlated with more swollen astrocytes immunophenotyped by aquaporin-4 (AQP4), glial fibrillary acidic protein (GFAP), and glutamate transporter-1 (GLT-1). Low FA was also related to the loss of neurons with perineuronal GLT-1+ astrocyte decorations, large myelin swellings, lower myelin density, and oligodendrocyte cell death identified by 2',3'-cyclic nucleotide 3'-phosphodiesterase, bridging integrator-1, and nuclear morphology. MD correlated with degenerating oligodendrocytes and depletion of normal GFAP+ astrocytes but not with astrocyte or myelin swelling. We conclude that FA is associated with cytotoxic edema in astrocytes and oligodendrocyte processes as well as myelin injury at the cellular level. MD can detect glial cell death and loss, but it may not discern subtle pathology in swollen astrocytes, oligodendrocytes, or myelin. This study provides a cytopathologic basis for interpreting DTI in the neonatal brain after HI.

Imaging for central nervous system (CNS) interstitial fluidopathy: disorders with impaired interstitial fluid dynamics

After the introduction of the glymphatic system hypothesis, an increasing number of studies on cerebrospinal fluid and interstitial fluid dynamics within the brain have been investigated and reported. A series of diseases are known which develop due to abnormality of the glymphatic system including Alzheimer's disease, traumatic brain injury, stroke, or other disorders. These diseases or disorders share the characteristics of the glymphatic system dysfunction or other mechanisms related to the interstitial fluid dynamics. In this review article, we propose "Central Nervous System (CNS) Interstitial Fluidopathy" as a new concept encompassing diseases whose pathologies are majorly associated with abnormal interstitial fluid dynamics. Categorizing these diseases or disorders as "CNS interstitial fluidopathies," will promote the understanding of their mechanisms and the development of potential imaging methods for the evaluation of the disease as well as clinical methods for disease treatment or prevention. In other words, having a viewpoint of the dynamics of interstitial fluid appears relevant for understanding CNS diseases or disorders, and it would be possible to develop novel common treatment methods or medications for "CNS interstitial fluidopathies."

Correction: Diffusion MRI reveals in vivo and non-invasively changes in astrocyte function induced by an aquaporin-4 inhibitor

[This corrects the article DOI: 10.1371/journal.pone.0229702.].

The Glymphatic System: A Review of the Challenges in Visualizing its Structure and Function with MR Imaging

The central nervous system (CNS) was previously thought to be the only organ system lacking lymphatic vessels to remove waste products from the interstitial space. Recently, based on the results from animal experiments, the glymphatic system was hypothesized. In this hypothesis, cerebrospinal fluid (CSF) enters the periarterial spaces, enters the interstitial space of the brain parenchyma via aquaporin-4 (AQP4) channels in the astrocyte end feet, and then exits through the perivenous space, thereby clearing waste products. From the perivenous space, the interstitial fluid drains into the subarachnoid space and meningeal lymphatics of the parasagittal dura. It has been reported that the glymphatic system is particularly active during sleep. Impairment of glymphatic system function might be a cause of various neurodegenerative diseases such as Alzheimer’s disease, normal pressure hydrocephalus, glaucoma, and others. Meningeal lymphatics regulate immunity in the CNS. Many researchers have attempted to visualize the function and structure of the glymphatic system and meningeal lymphatics in vivo using MR imaging. In this review, we aim to summarize these in vivo MR imaging studies and discuss the significance, current limitations, and future directions. We also discuss the significance of the perivenous cyst formation along the superior sagittal sinus, which is recently discovered in the downstream of the glymphatic system.