Water influx into cerebrospinal fluid is primarily controlled by aquaporin-4, not by aquaporin-1: 17O JJVCPE MRI study in knockout mice

Genetic and molecular mechanisms of hydrocephalus

Hydrocephalus is a neurological condition caused by aberrant circulation and/or obstructed cerebrospinal fluid (CSF) flow after cerebral ventricle abnormal dilatation. In the past 50 years, the diagnosis and treatment of hydrocephalus have remained understudied and underreported, and little progress has been made with respect to prevention or treatment. Further research on the pathogenesis of hydrocephalus is essential for developing new diagnostic, preventive, and therapeutic strategies. Various genetic and molecular abnormalities contribute to the mechanisms of hydrocephalus, including gene deletions or mutations, the activation of cellular inflammatory signaling pathways, alterations in water channel proteins, and disruptions in iron metabolism. Several studies have demonstrated that modulating the expression of key proteins, including TGF-β, VEGF, Wnt, AQP, NF-κB, and NKCC, can significantly influence the onset and progression of hydrocephalus. This review summarizes and discusses key mechanisms that may be involved in the pathogenesis of hydrocephalus at both the genetic and molecular levels. While obstructive hydrocephalus can often be addressed by removing the obstruction, most cases require treatment strategies that involve merely slowing disease progression by correcting CSF circulation patterns. There have been few new research breakthroughs in the prevention and treatment of hydrocephalus.

Loss of aquaporin-4 impairs cerebrospinal fluid solute clearance through cerebrospinal fluid drainage pathways

The aquaporin-4 (AQP4) water channel is essential in neurofluid dynamics. AQP4 loss impairs solute exchange between the cerebrospinal fluid (CSF) and interstitial fluid (ISF). However, whether AQP4 expression affects solute clearance from the CSF space to the extracranial space remains unclear. This study aimed to investigate this using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) following the intrathecal administration of gadolinium-based contrast agents (GBCAs) to AQP4 knockout (KO) rats. AQP4 KO rats showed reduced efflux of intrathecal GBCAs to the extracranial spaces through CSF drainage pathways and increased retention of intrathecal GBCAs in the CSF space compared with the controls. These results suggest that AQP4 loss impairs solute clearance from the CSF space to the extracranial spaces via the CSF drainage pathways. This study revealed a close relationship between AQP4 expression and CSF solute clearance, contributing to a better understanding of the function of AQP4 in neurofluid dynamics.

Interstitial fluid flow decreases with age, especially after 50 years

Physiological age-related alterations in the interstitial flow in the brain, which plays an important role in waste product removal, remain unclear. Using [15O]H2O positron emission tomography (PET), water dynamics were evaluated in 63 healthy adult participants aged between 20 and 80 years. Interstitial flow was assessed by influx ratio (IR) and drain rate (DR), using time-activity concentration data. Participants were divided into four age groups with 15-year ranges, to evaluate age-related functional alterations. At least one of the indices declined significantly with age across all groups. A significant linear negative correlation between age and both indicators was found in the scatter plots (IR: R2 = 0.54, DR: R2 = 0.44); both indicators were predominantly lower after age 50 years. These results suggest interstitial flow decreases with age, especially after 50 years. These important findings can contribute to devising therapeutic interventions for neurological diseases characterized by abnormal accumulation of waste products, and suggest the need for taking measures to maintain interstitial flow starting around the age of 50 years.

Clinical magnetic resonance imaging evaluation of glymphatic function

The glymphatic system is a system of specialized perivascular spaces in the brain that facilitates removal of toxic waste solutes from the brain. Evaluation of glymphatic system function by means of magnetic resonance imaging (MRI) has thus far been largely focused on rodents because of the limitations of intrathecal delivery of gadolinium-based contrast agents to humans. This review discusses MRI methods that can be employed clinically for glymphatic-related measurements intended for early diagnosis, prevention, and the treatment of various neurological conditions. Although glymphatic system-based MRI research is in its early stages, recent studies have identified promising noninvasive MRI markers associated with glymphatic system alterations in neurological diseases. However, further optimization in data acquisition, validation, and modeling are needed to investigate the glymphatic system within the clinical setting.

Cerebrospinal fluid-based spatial statistics: towards quantitative analysis of cerebrospinal fluid pseudodiffusivity

BACKGROUND

Cerebrospinal fluid (CSF) circulation is essential in removing metabolic wastes from the brain and is an integral component of the glymphatic system. Abnormal CSF circulation is implicated in neurodegenerative diseases. Low b-value magnetic resonance imaging quantifies the variance of CSF motion, or pseudodiffusivity. However, few studies have investigated the relationship between the spatial patterns of CSF pseudodiffusivity and cognition.

METHODS

We introduced a novel technique, CSF-based spatial statistics (CBSS), to automatically quantify CSF pseudodiffusivity in each sulcus, cistern and ventricle. Using cortical regions as landmarks, we segmented each CSF region. We retrospectively analyzed a cohort of 93 participants with varying degrees of cognitive impairment.

RESULTS

We identified two groups of CSF regions whose pseudodiffusivity profiles were correlated with each other: one group displaying higher pseudodiffusivity and near large arteries and the other group displaying lower pseudodiffusivity and away from the large arteries. The pseudodiffusivity in the third ventricle positively correlated with short-term memory (standardized slope of linear regression = 0.38, adjusted p < 0.001) and long-term memory (slope = 0.37, adjusted p = 0.005). Fine mapping along the ventricles revealed that the pseudodiffusivity in the region closest to the start of the third ventricle demonstrated the highest correlation with cognitive performance.

CONCLUSIONS

CBSS enabled quantitative spatial analysis of CSF pseudodiffusivity and suggested the third ventricle pseudodiffusivity as a potential biomarker of cognitive impairment.

Impact of aquaporin-4 and CD11c + microglia in the development of ependymal cells in the aqueduct: inferences to hydrocephalus

AQP4 is expressed in the endfeet membranes of subpial and perivascular astrocytes and in the ependymal cells that line the ventricular system. The sporadic appearance of obstructive congenital hydrocephalus (OCHC) has been observed in the offspring of AQP4-/- mice (KO) due to stenosis of Silvio's aqueduct. Here, we explore whether the lack of AQP4 expression leads to abnormal development of ependymal cells in the aqueduct of mice. We compared periaqueductal samples from wild-type and KO mice. The microarray-based transcriptome analysis reflected a large number of genes with differential expression (809). Gene sets (GS) associated with ependymal development, ciliary function and the immune system were specially modified qPCR confirmed reduced expression in the KO mice genes: (i) coding for transcription factors for ependymal differentiation (Rfx4 and FoxJ1), (ii) involved in the constitution of the central apparatus of the axoneme (Spag16 and Hydin), (iii) associated with ciliary assembly (Cfap43, Cfap69 and Ccdc170), and (iv) involved in intercellular junction complexes of the ependyma (Cdhr4). By contrast, genes such as Spp1, Gpnmb, Itgax, and Cd68, associated with a Cd11c-positive microglial population, were overexpressed in the KO mice. Electron microscopy and Immunofluorescence of vimentin and γ-tubulin revealed a disorganized ependyma in the KO mice, with changes in the intercellular complex union, unevenly orientated cilia, and variations in the planar cell polarity of the apical membrane. These structural alterations translate into reduced cilia beat frequency, which might alter cerebrospinal fluid movement. The presence of CD11c + microglia cells in the periaqueductal zone of mice during the first postnatal week is a novel finding. In AQP4-/- mice, these cells remain present around the aqueduct for an extended period, showing peak expression at P11. We propose that these cells play an important role in the normal development of the ependyma and that their overexpression in KO mice is crucial to reduce ependyma abnormalities that could otherwise contribute to the development of obstructive hydrocephalus.

Changes of cerebrospinal fluid pressure gradient in different body positions under experimental impairment of cerebrospinal fluid pathway: new insight into hydrocephalus development

It is generally accepted that hydrocephalus is a consequence of the disbalance between cerebrospinal fluid (CSF) secretion and absorption which should in turn lead to CSF pressure gradient development and ventricular enlargement. To test CSF pressure gradient role in hydrocephalus development, we experimentally caused CSF system impairment at two sites in cats. In the first group of animals, we caused Sylvian aqueduct obstruction and recorded CSF pressure changes pre and post obstruction at three measuring sites (lateral ventricle -LV, cortical-CSS and lumbar subarachnoid space -LSS) during 15 min periods and in different body positions over 360 degrees. In the second group of experiments, we caused cervical stenosis by epidural plastic semiring implantation and monitored CSF pressure changes pre and post stenosis implantation at two measuring sites (lateral ventricle and lumbar subarachnoid space) during 15 min periods in different body positions over 360 degrees. Both groups of experimental animals had similar CSF pressures before stenosis or obstruction at all measuring points in the horizontal position. During head-up verticalization, CSF pressures inside the cranium gradually became more subatmospheric with no significant difference between LV and CSS, as they are measured at the same hydrostatic level, while CSF pressure inside LSS became more positive, causing the development of a large hydrostatic gradient between the cranial and the spinal space. With cervical stenosis, CSF pressure inside the cranium is positive during head-up verticalization, while in cats with aqueductal obstruction CSF pressure inside the CSS remains negative, as it was during control period. Concomitantly, CSF pressure inside LV becomes less negative, thus creating a small hydrostatic gradient between LV and CSS. Since CSF pressure and gradient changes occur only by shifting body position from the horizontal plane, our results indicate that cervical stenosis in a head-up vertical position reduces blood perfusion of the whole brain, while aqueductal obstruction impairs only the perfusion of the local periventricular brain tissue. It seems that, for evolutionary important bipedal activity, free craniospinal communication and good spinal space compliance represent crucial biophysical parameters for adequate cerebral blood perfusion and prevention of pathophysiological changes leading to the development of hydrocephalus.

Aquaporins: Gatekeepers of Fluid Dynamics in Traumatic Brain Injury

Aquaporins (AQPs), particularly AQP4, play a crucial role in regulating fluid dynamics in the brain, impacting the development and resolution of edema following traumatic brain injury (TBI). This review examines the alterations in AQP expression and localization post-injury, exploring their effects on brain edema and overall injury outcomes. We discuss the underlying molecular mechanisms regulating AQP expression, highlighting potential therapeutic strategies to modulate AQP function. These insights provide a comprehensive understanding of AQPs in TBI and suggest novel approaches for improving clinical outcomes through targeted interventions.

Connecting the Dots: The Cerebral Lymphatic System as a Bridge Between the Central Nervous System and Peripheral System in Health and Disease

As a recently discovered waste removal system in the brain, cerebral lymphatic system is thought to play an important role in regulating the homeostasis of the central nervous system. Currently, more and more attention is being focused on the cerebral lymphatic system. Further understanding of the structural and functional characteristics of cerebral lymphatic system is essential to better understand the pathogenesis of diseases and to explore therapeutic approaches. In this review, we summarize the structural components and functional characteristics of cerebral lymphatic system. More importantly, it is closely associated with peripheral system diseases in the gastrointestinal tract, liver, and kidney. However, there is still a gap in the study of the cerebral lymphatic system. However, we believe that it is a critical mediator of the interactions between the central nervous system and the peripheral system.

Aquaporin 4 Mediates the Effect of Iron Overload on Hydrocephalus After Intraventricular Hemorrhage

BACKGROUND

Iron overload plays an important role in hydrocephalus development following intraventricular hemorrhage (IVH). Aquaporin 4 (AQP4) participates in the balance of cerebrospinal fluid secretion and absorption. The current study investigated the role of AQP4 in the formation of hydrocephalus caused by iron overload after IVH.

METHODS

There were three parts to this study. First, Sprague-Dawley rats received an intraventricular injection of 100 µl autologous blood or saline control. Second, rats had IVH and were treated with deferoxamine (DFX), an iron chelator, or vehicle. Third, rats had IVH and were treated with 2-(nicotinamide)-1,3,4-thiadiazole (TGN-020), a specific AQP4 inhibitor, or vehicle. Rats underwent T2-weighted and T2* gradient-echo magnetic resonance imaging to assess lateral ventricular volume and intraventricular iron deposition at 7, 14, and 28 days after intraventricular injection and were then euthanized. Real-time quantitative polymerase chain reaction, western blot analysis, and immunofluorescence analyses were conducted on the rat brains to evaluate the expression of AQP4 at different time points. Hematoxylin and eosin-stained brain sections were obtained to assess the ventricular wall damage on day 28.

RESULTS

Intraventricular injection of autologous blood caused a significant ventricular dilatation, iron deposition, and ventricular wall damage. There was increased AQP4 mRNA and protein expression in the periventricular tissue in IVH rats through day 7 to day 28. The DFX treatment group had a lower lateral ventricular volume and less intraventricular iron deposition and ventricular wall damage than the vehicle-treated group after IVH. The expression of AQP4 protein in periventricular tissue was also inhibited by DFX on days 14 and 28 after IVH. The use of TGN-020 attenuated hydrocephalus development after IVH and inhibited the expression of AQP4 protein in the periventricular tissue between day 14 and day 28 without a significant effect on intraventricular iron deposition or ventricular wall damage.

CONCLUSIONS

AQP4 located in the periventricular area mediated the effect of iron overload on hydrocephalus after IVH.

Methods for studying mammalian aquaporin biology

Aquaporins (AQPs), transmembrane water-conducting channels, have earned a great deal of scrutiny for their critical physiological roles in healthy and disease cell states, especially in the biomedical field. Numerous methods have been implemented to elucidate the involvement of AQP-mediated water transport and downstream signaling activation in eliciting whole cell, tissue, and organ functional responses. To modulate these responses, other methods have been employed to investigate AQP druggability. This review discusses standard in vitro, in vivo, and in silico methods for studying AQPs, especially for biomedical and mammalian cell biology applications. We also propose some new techniques and approaches for future AQP research to address current gaps in methodology.

Aquaporin-4 Deficiency is Associated with Cognitive Impairment and Alterations in astrocyte-neuron Lactate Shuttle

Cognitive impairment refers to notable declines in cognitive abilities including memory, language, and emotional stability leading to the inability to accomplish essential activities of daily living. Astrocytes play an important role in cognitive function, and homeostasis of the astrocyte-neuron lactate shuttle (ANLS) system is essential for maintaining cognitive functions. Aquaporin-4 (AQP-4) is a water channel expressed in astrocytes and has been shown to be associated with various brain disorders, but the direct relationship between learning, memory, and AQP-4 is unclear. We examined the relationship between AQP-4 and cognitive functions related to learning and memory. Mice with genetic deletion of AQP-4 showed significant behavioral and emotional changes including hyperactivity and instability, and impaired cognitive functions such as spatial learning and memory retention. 18 F-FDG PET imaging showed significant metabolic changes in the brains of AQP-4 knockout mice such as reductions in glucose absorption. Such metabolic changes in the brain seemed to be the direct results of changes in the expression of metabolite transporters, as the mRNA levels of multiple glucose and lactate transporters in astrocytes and neurons were significantly decreased in the cortex and hippocampus of AQP-4 knockout mice. Indeed, AQP-4 knockout mice showed significantly higher accumulation of both glucose and lactate in their brains compared with wild-type mice. Our results show that the deficiency of AQP-4 can cause problems in the metabolic function of astrocytes and lead to cognitive impairment, and that the deficiency of AQP4 in astrocyte endfeet can cause abnormalities in the ANLS system.

From Homeostasis to Pathology: Decoding the Multifaceted Impact of Aquaporins in the Central Nervous System

Aquaporins (AQPs), integral membrane proteins facilitating selective water and solute transport across cell membranes, have been the focus of extensive research over the past few decades. Particularly noteworthy is their role in maintaining cellular homeostasis and fluid balance in neural compartments, as dysregulated AQP expression is implicated in various degenerative and acute brain pathologies. This article provides an exhaustive review on the evolutionary history, molecular classification, and physiological relevance of aquaporins, emphasizing their significance in the central nervous system (CNS). The paper journeys through the early studies of water transport to the groundbreaking discovery of Aquaporin 1, charting the molecular intricacies that make AQPs unique. It delves into AQP distribution in mammalian systems, detailing their selective permeability through permeability assays. The article provides an in-depth exploration of AQP4 and AQP1 in the brain, examining their contribution to fluid homeostasis. Furthermore, it elucidates the interplay between AQPs and the glymphatic system, a critical framework for waste clearance and fluid balance in the brain. The dysregulation of AQP-mediated processes in this system hints at a strong association with neurodegenerative disorders such as Parkinson's Disease, idiopathic normal pressure hydrocephalus, and Alzheimer's Disease. This relationship is further explored in the context of acute cerebral events such as stroke and autoimmune conditions such as neuromyelitis optica (NMO). Moreover, the article scrutinizes AQPs at the intersection of oncology and neurology, exploring their role in tumorigenesis, cell migration, invasiveness, and angiogenesis. Lastly, the article outlines emerging aquaporin-targeted therapies, offering a glimpse into future directions in combatting CNS malignancies and neurodegenerative diseases.

Bulat-Klarica-Oreskovic Hypothesis: A Comprehensive Review

Classical theories of cerebrospinal fluid (CSF) production and flow are taught throughout medical education. The idea that CSF is produced and/or filtered by the choroid plexus and flows in one direction throughout the ventricular system has been a largely accepted thesis. However, modern studies have called into question the validity of this hypothesis, suggesting that CSF does not move unidirectionally but rather is driven by microvessel contractions in a to-and-fro manner throughout the cerebrospinal system. Moreover, new insights suggest that in addition to CSF production, the exchange of fluids and proteins between the cortical vasculature and the interstitium may function as the brain's version of a lymphatic system. This comprehensive review provides evidence for a different framework of CSF flow. One that includes perivascular pulsations that push CSF back and forth, allowing exchange between the CSF and interstitium, and with CSF production occurring throughout the cerebrospinal system. These findings could be revolutionary in understanding the pathophysiology of CSF flow and in the treatment of pathologies such as intracranial hypertension, hydrocephalus, Alzheimer's disease, and many others.

Multifaceted Roles of Aquaporins in the Pathogenesis of Alzheimer’s Disease

The central nervous system is highly dependent on water, and disturbances in water homeostasis can have a significant impact on its normal functions. The regulation of water balance is, at least in part, carried out via specialized water channels called aquaporins. In the central nervous system, two major aquaporins (AQPs), AQP1 and AQP4, and their potential involvements have been long implicated in the pathophysiology of many brain disorders such as brain edema and Neuromyelitis optica. In addition to these diseases, there is growing attention to the involvement of AQPs in the removal of waste products in Alzheimer’s disease (AD). This indicates that targeting fluid homeostasis is a novel and attractive approach for AD. This review article aims to summarize recent knowledge on the pathological implications of AQPs in AD, discussing unsolved questions and future prospects.

Methylene Blue Delivery Mediated by Focused Ultrasound-Induced Blood-Brain Barrier Disruption Reduces Neural Damage and Amyloid-Beta Plaques by AQP-4 Upregulation

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease worldwide, causing progressive cognitive decline, memory impairment, and neurological deficits. Methylene blue (MB), an antioxidant, has emerged as a potential drug for the treatment of AD owing to its cognitive improvement and neuroprotective functions. Despite the small molecular size of MB, which can cross the BBB, the therapeutic effective dosage using a BBB-permeable delivery system in a specific brain localization remains unclear. In this study, we presented magnetic resonance-guided focused ultrasound (MRgFUS) as a delivery system to enhance BBB permeability for the effective treatment of AD. MRgFUS using two ultrasound intensities (0.25 and 0.32 MPa) was used to intravenously deliver MB to the hippocampal region. Compared with treatment with 0.25 MPa FUS, treatment with 0.32 MPa FUS significantly enhanced MB brain accumulation. Deposition of amyloid-β (Aβ) plaques and neural cell damage was significantly reduced in 0.32 MPa FUS/MB-treated APP/PS1 mice. Furthermore, aquaporin-4 expression increased significantly in the 0.32 MPa FUS and 0.32 MPa FUS/MB groups without glial fibrillary acidic protein activation. The results from this study demonstrate that FUS improved MB delivery to the brain, and FUS/MB combination treatment reduced the number of Aβ plaques. This study revealed the potential of FUS-BBBD as an effective strategy to enhance the efficacy of therapeutic drugs for AD.

Blood Cerebrospinal Fluid Barrier Function Disturbance Can Be Followed by Amyloid-β Accumulation

BACKGROUND

Elucidation of the mechanism of amyloid-β accumulation plays an important role in therapeutic strategies for Alzheimer's disease (AD). The aim of this study is to elucidate the relationship between the function of the blood-cerebrospinal fluid barrier (BCSFB) and the clearance of amyloid-β (Aβ).

METHODS

Twenty-five normal older adult volunteers (60-81 years old) participated in this PET study for clarifying the relationship between interstitial water flow and Aβ accumulation. Water dynamics were analyzed using two indices in [15O]H2O PET, the influx ratio (IR) and drain rate (DR), and Aβ accumulation was assessed qualitatively by [18F]flutemetamol PET.

RESULTS

[15O]H2O PET examinations conducted initially and after 2 years showed no significant changes in both indices over the 2-year period (IR: 1.03 ± 0.21 and 1.02 ± 0.20, DR: 1.74 ± 0.43 and 1.67 ± 0.47, respectively). In [18F]flutemetamol PET, on the other hand, one of the 25 participants showed positive results and two showed positive changes after 2 years. In these three participants, the two indices of water dynamics showed low values at both periods (IR: 0.60 ± 0.15 and 0.60 ± 0.13, DR: 1.24 ± 0.12 and 1.11 ± 0.10).

CONCLUSIONS

Our results indicated that BCSFB function disturbances could be followed by Aβ accumulation, because the reduced interstitial flow preceded amyloid accumulation in the positive-change subjects, and amyloid accumulation was not observed in the older adults with sufficiently high values for the two indices. We believe that further elucidation of interstitial water flow will be the key to developing therapeutic strategies for AD, especially with regard to prevention.

Choroid plexus function in neurological homeostasis and disorders: The awakening of the circadian clocks and orexins

As research regarding the role of circadian rhythms, sleep, and the orexinergic system in neurodegenerative diseases is growing, it is surprising that the choroid plexus (CP) remains underappreciated in this realm. Despite its extensive role in the regulation of circadian rhythms and orexinergic signalling, as well as acting as the primary conduit between cerebrospinal fluid (CSF) and the circulatory system, providing a mechanism by which toxic waste molecules can be removed from the brain, the CP has been largely unexplored in neurodegeneration. In this review, we explore the role of the CP in maintaining brain homeostasis and circadian rhythms, regulating CSF dynamics, and how these functions change across the lifespan, from development to senescence. In addition, we examine the relationship between the CP, orexinergic signalling, and the glymphatic system, highlighting gaps in the literature and areas that require immediate exploration. Finally, we assess current knowledge, including possible therapeutic strategies, regarding the role of the CP in neurological disorders, such as traumatic brain injury, migraine, Alzheimer's disease, and multiple sclerosis.

Tissutal and Fluidic Aspects in Osteopathic Manual Therapy: A Narrative Review

Over the years, several authors have discussed the possibility of considering somatic dysfunction (SD) as a “nosological element” detectable on palpation. There are many aspects to consider regarding the etiology and diagnosis of SD, and the literature on osteopathic issues provides details on physiological signs that characterize it, including tissue texture changes. Recent knowledge suggests that how tissue and, in particular, connective tissue, responds to osteopathic treatment may depend on the modulation of the inflammation degree. Low-grade inflammation (LGI) may act on the extracellular matrix (ECM) and on cellular elements; and these mechanisms may be mediated by biological water. With its molecules organized in structures called exclusion zones (EZ), water could explain the functioning of both healthy and injured tissues, and how they can respond to osteopathic treatment with possible EZ normalization as a result. The relationship between inflammation and DS and the mechanisms involved are described by several authors; however, this review suggests a new model relating to the characteristics of DS and to its clinical implications by linking to LGI. Tissue alterations detectable by osteopathic palpation would be mediated by body fluids and in particular by biological water which has well-defined biophysical characteristics. Research in this area is certainly still to be explored, but our suggestion seems plausible to explain many dynamics related to osteopathic treatment. We believe that this could open up a fascinating scenario of therapeutic possibilities and knowledge in the future.

Review of Cerebrospinal Fluid Physiology and Dynamics: A Call for Medical Education Reform

BACKGROUND

The flow of cerebrospinal fluid (CSF) has been described as a unidirectional system with the choroid plexus serving as the primary secretor of CSF and the arachnoid granulations as primary reabsorption site. This theory of neurosurgical forefathers has been universally adopted and taught as dogma. Many neuroscientists have found difficulty reconciling this theory with common pathologies, and recent studies have found that this "classic" hypothesis may not represent the full picture.

OBJECTIVE

To review modern CSF dynamic theories and to call for medical education reform.

METHODS

We reviewed the literature from January 1990 to December 2020. We searched the PubMed database using key terms "cerebrospinal fluid circulation," "cerebrospinal fluid dynamics," "cerebrospinal fluid physiology," "glymphatic system," and "glymphatic pathway." We selected articles with a primary aim to discuss either CSF dynamics and/or the glymphatic system.

RESULTS

The Bulat-Klarica-Orešković hypothesis purports that CSF is secreted and reabsorbed throughout the craniospinal axis. CSF demonstrates similar physiology to that of water elsewhere in the body. CSF "circulates" throughout the subarachnoid space in a pulsatile to-and-fro fashion. Osmolarity plays a critical role in CSF dynamics. Aquaporin-4 and the glymphatic system contribute to CSF volume and flow by establishing osmolarity gradients and facilitating CSF movement. Multiple studies demonstrate that the choroid plexus does not play any significant role in CSF circulation.

CONCLUSION

We have highlighted major studies to illustrate modern principles of CSF dynamics. Despite these, the medical education system has been slow to reform curricula and update learning resources.

Glymphatic System Dysfunction in Central Nervous System Diseases and Mood Disorders

The glymphatic system, a recently discovered macroscopic waste removal system in the brain, has many unknown aspects, especially its driving forces and relationship with sleep, and thus further explorations of the relationship between the glymphatic system and a variety of possible related diseases are urgently needed. Here, we focus on the progress in current research on the role of the glymphatic system in several common central nervous system diseases and mood disorders, discuss the structural and functional abnormalities of the glymphatic system which may occur before or during the pathophysiological progress and the possible underlying mechanisms. We emphasize the relationship between sleep and the glymphatic system under pathological conditions and summarize the common imaging techniques for the glymphatic system currently available. The perfection of the glymphatic system hypothesis and the exploration of the effects of aging and endocrine factors on the central and peripheral regulatory pathways through the glymphatic system still require exploration in the future.

Cerebrospinal Fluid Homeostasis and Hydrodynamics: A Review of Facts and Theories

BACKGROUND AND PURPOSE

According to the classical hypothesis, the cerebrospinal fluid (CSF) is actively secreted inside the brain's ventricular system, predominantly by the choroid plexuses, before flowing unidirectionally in a cranio-caudal orientation toward the arachnoid granulations (AGs), where it is reabsorbed into the dural venous sinuses. This concept has been accepted as a doctrine for more than 100 years and was subjected only to minor modifications. Its inability to provide an adequate explanation to questions arising from the everyday clinical practice, in addition to the ever growing pool of experimental data contradicting it, has led to the identification of its limitations. Literature includes an increasing number of studies suggesting a more complex mechanism than that previously described. This review article summarizes the proposed mechanisms of CSF regulation, referring to the key clinical and experimental developments supporting or defying them.

METHODS

A non-systematical literature search of the major databases was performed for studies on the mechanisms of CSF homeostasis. Gray literature was additionally assessed employing a hand-search technique. No restrictions were imposed regarding the time, language, or type of publication.

CONCLUSION

CSF secretion and absorption are expected to take place throughout the entire brain's capillaries network under the regulation of hydrostatic and osmotic gradients. The unidirectional flow is defied, highlighting the possibility of its complete absence. The importance of AGs is brought into question, potentiating the significance of the lymphatic system as the primary site of reabsorption. However, the definition of hydrocephalus and its treatment strategies remain strongly associated with the classical hypothesis.

Cellular Distribution of Brain Aquaporins and Their Contribution to Cerebrospinal Fluid Homeostasis and Hydrocephalus

Brain aquaporins facilitate the movement of water between the four water compartments: blood, cerebrospinal fluid, interstitial fluid, and intracellular fluid. This work analyzes the expression of the four most abundant aquaporins (AQPs) (AQP1, AQP4, AQP9, and AQP11) in the brains of mice and discuss their contribution to hydrocephalus. We analyzed available data from single-cell RNA sequencing of the central nervous system of mice to describe the expression of aquaporins and compare their distribution with that based on qPCR, western blot, and immunohistochemistry assays. Expression of AQP1 in the apical cell membrane of choroid plexus epithelial cells and of AQP4 in ependymal cells, glia limitans, and astrocyte processes in the pericapillary end foot is consistent with the involvement of both proteins in cerebrospinal fluid homeostasis. The expression of both aquaporins compensates for experimentally induced hydrocephalus in the animals. Recent data demonstrate that hypoxia in aged animals alters AQP4 expression in the choroidal plexus and cortex, increasing the ventricle size and intraventricular pressure. Cerebral distensibility is reduced in parallel with a reduction in cerebrospinal fluid drainage and cognitive deterioration. We propose that aged mice chronically exposed to hypoxia represent an excellent experimental model for studying the pathophysiological characteristics of idiopathic normal pressure hydrocephalus and roles for AQPs in such disease.

Metabolic Contribution and Cerebral Blood Flow Regulation by Astrocytes in the Neurovascular Unit

The neurovascular unit (NVU) is a conceptual framework that has been proposed to better explain the relationships between the neural cells and blood vessels in the human brain, focused mainly on the brain gray matter. The major components of the NVU are the neurons, astrocytes (astroglia), microvessels, pericytes, and microglia. In addition, we believe that oligodendrocytes should also be included as an indispensable component of the NVU in the white matter. Of all these components, astrocytes in particular have attracted the interest of researchers because of their unique anatomical location; these cells are interposed between the neurons and the microvessels of the brain. Their location suggests that astrocytes might regulate the cerebral blood flow (CBF) in response to neuronal activity, so as to ensure an adequate supply of glucose and oxygen to meet the metabolic demands of the neurons. In fact, the adult human brain, which accounts for only 2% of the entire body weight, consumes approximately 20–25% of the total amount of glucose and oxygen consumed by the whole body. The brain needs a continuous supply of these essential energy sources through the CBF, because there are practically no stores of glucose or oxygen in the brain; both acute and chronic cessation of CBF can adversely affect brain functions. In addition, another important putative function of the NVU is the elimination of heat and waste materials produced by neuronal activity. Recent evidence suggests that astrocytes play pivotal roles not only in supplying glucose, but also fatty acids and amino acids to neurons. Loss of astrocytic support can be expected to lead to malfunction of the NVU as a whole, which underlies numerous neurological disorders. In this review, we shall focus on historical and recent findings with regard to the metabolic contributions of astrocytes in the NVU.

The glymphatic hypothesis: the theory and the evidence

The glymphatic hypothesis proposes a mechanism for extravascular transport into and out of the brain of hydrophilic solutes unable to cross the blood-brain barrier. It suggests that there is a circulation of fluid carrying solutes inwards via periarterial routes, through the interstitium and outwards via perivenous routes. This review critically analyses the evidence surrounding the mechanisms involved in each of these stages. There is good evidence that both influx and efflux of solutes occur along periarterial routes but no evidence that the principal route of outflow is perivenous. Furthermore, periarterial inflow of fluid is unlikely to be adequate to provide the outflow that would be needed to account for solute efflux. A tenet of the hypothesis is that flow sweeps solutes through the parenchyma. However, the velocity of any possible circulatory flow within the interstitium is too small compared to diffusion to provide effective solute movement. By comparison the earlier classical hypothesis describing extravascular transport proposed fluid entry into the parenchyma across the blood-brain barrier, solute movements within the parenchyma by diffusion, and solute efflux partly by diffusion near brain surfaces and partly carried by flow along "preferred routes" including perivascular spaces, white matter tracts and subependymal spaces. It did not suggest fluid entry via periarterial routes. Evidence is still incomplete concerning the routes and fate of solutes leaving the brain. A large proportion of the solutes eliminated from the parenchyma go to lymph nodes before reaching blood but the proportions delivered directly to lymph or indirectly via CSF which then enters lymph are as yet unclear. In addition, still not understood is why and how the absence of AQP4 which is normally highly expressed on glial endfeet lining periarterial and perivenous routes reduces rates of solute elimination from the parenchyma and of solute delivery to it from remote sites of injection. Neither the glymphatic hypothesis nor the earlier classical hypothesis adequately explain how solutes and fluid move into, through and out of the brain parenchyma. Features of a more complete description are discussed. All aspects of extravascular transport require further study.

Signaling Mechanisms and Pharmacological Modulators Governing Diverse Aquaporin Functions in Human Health and Disease

The aquaporins (AQPs) are a family of small integral membrane proteins that facilitate the bidirectional transport of water across biological membranes in response to osmotic pressure gradients as well as enable the transmembrane diffusion of small neutral solutes (such as urea, glycerol, and hydrogen peroxide) and ions. AQPs are expressed throughout the human body. Here, we review their key roles in fluid homeostasis, glandular secretions, signal transduction and sensation, barrier function, immunity and inflammation, cell migration, and angiogenesis. Evidence from a wide variety of studies now supports a view of the functions of AQPs being much more complex than simply mediating the passive flow of water across biological membranes. The discovery and development of small-molecule AQP inhibitors for research use and therapeutic development will lead to new insights into the basic biology of and novel treatments for the wide range of AQP-associated disorders.

Aquaporin-4 expression in the human choroid plexus

The choroid plexus (CP) consists of specialized ependymal cells and underlying blood vessels and stroma producing the bulk of the cerebrospinal fluid (CSF). CP epithelial cells are considered the site of the internal blood-cerebrospinal fluid barrier, show epithelial characteristics (basal lamina, tight junctions), and express aquaporin-1 (AQP1) apically. In this study, we analyzed the expression of aquaporins in the human CP using immunofluorescence and qPCR. As previously reported, AQP1 was expressed apically in CP epithelial cells. Surprisingly, and previously unknown, many cells in the CP epithelium were also positive for aquaporin-4 (AQP4), normally restricted to ventricle-lining ependymal cells and astrocytes in the brain. Expression of AQP1 and AQP4 was found in the CP of all eight body donors investigated (3 males, 5 females; age 74–91). These results were confirmed by qPCR, and by electron microscopy detecting orthogonal arrays of particles. To find out whether AQP4 expression correlated with the expression pattern of relevant transport-related proteins we also investigated expression of NKCC1, and Na/K-ATPase. Immunostaining with NKCC1 was similar to AQP1 and revealed no particular pattern related to AQP4. Co-staining of AQP4 and Na/K-ATPase indicated a trend for an inverse correlation of their expression. We hypothesized that AQP4 expression in the CP was caused by age-related changes. To address this, we investigated mouse brains from young (2 months), adult (12 months) and old (30 months) mice. We found a significant increase of AQP4 on the mRNA level in old mice compared to young and adult animals. Taken together, we provide evidence for AQP4 expression in the CP of the aging brain which likely contributes to the water flow through the CP epithelium and CSF production. In two alternative hypotheses, we discuss this as a beneficial compensatory, or a detrimental mechanism influencing the previously observed CSF changes during aging.

Quantitative magnetic resonance imaging for evaluating of the cerebrospinal fluid kinetics with 17O-labeled water tracer: A preliminary report

The aim of this study was to evaluate the feasibility of kinetic analysis of cerebrospinal fluid (CSF) using ¹⁷O-labeled water tracer. Four subjects (two idiopathic normal pressure hydrocephalus (iNPH) and two possible AD dementia patients) were prospectively included. Injectable formulation of ¹⁷O-labeled water containing 10 mol% of H₂¹⁷O (PSO17), was intrathecally administered to the subjects with the lateral decubitus position between the 3rd and 4th lumbar vertebrae. MRI acquisitions were performed in four-time points, before PSO17 administration, 1, 8, and 24 h after PSO17 administration. The 3-dimensional fast spin echo sequence was used. After image registration for all four-time points data, polygonal regions of interest (ROIs) were set in the 14 regions to obtain the signal intensity of CSF. Each signal intensity within the ROI was converted to ¹⁷O concentration [%]. The peak concentration at one hour after administration, the slope of concentration changes after PSO17 administration [%/s], and the root mean square error (RMSE) for evaluating the performance of a fitting were calculated. There was no significant difference in peak concentration between the iNPH and AD group. The slope in the AD group (-2.25 ± 1.62 × 10^-3 [%/h]) was significantly smaller than in the iNPH group (-1.21 ± 2.31 × 10^-3 [%/h]), which suggests the speed of CSF clearance in the iNPH group was slower than AD group. The RMSE indicating the fit to the concentration change in the AD group (4.86 ± 4.74 × 10^-3) was also significantly smaller than in the iNPH group (8.64 ± 7.56 × 10^-3). The kinetic evaluation of CSF using ¹⁷O-labeled water was feasible, and this preliminary study suggests that the differentiation of iNPH and possible AD dementia can be achieved using this method.

Aquaporin-4 Mediates Permanent Brain Alterations in a Mouse Model of Hypoxia-Aged Hydrocephalus

Aquaporin-4 (AQP4) is the principal water channel in the brain being expressed in astrocytes and ependymal cells. AQP4 plays an important role in cerebrospinal fluid (CSF) homeostasis, and alterations in its expression have been associated with hydrocephalus. AQP4 contributes to the development of hydrocephalus by hypoxia in aged mice, reproducing such principal characteristics of the disease. Here, we explore whether these alterations associated with the hydrocephalic state are permanent or can be reverted by reexposure to normoxia. Alterations such as ventriculomegaly, elevated intracranial pressure, and cognitive deficits were reversed, whereas deficits in CSF outflow and ventricular distensibility were not recovered, remaining impaired even one month after reestablishment of normoxia. Interestingly, in AQP4^−/− mice, the impairment in CSF drainage and ventricular distensibility was completely reverted by re-normoxia, indicating that AQP4 has a structural role in the chronification of those alterations. Finally, we show that aged mice subjected to two hypoxic episodes experience permanent ventriculomegaly. These data reveal that repetitive hypoxic events in aged cerebral tissue promote the permanent alterations involved in hydrocephalic pathophysiology, which are dependent on AQP4 expression.

Dynamic oxygen-17 MRI with adaptive temporal resolution using golden-means-based 3D radial sampling

PURPOSE

The aim of this study was to develop a high-resolution 3D oxygen-17 (17 O) MRI method to delineate the kinetics of 17 O-enriched water (H217 O) across the entire mouse brain after a bolus injection via the tail vein.

METHODS

The dynamic 17 O signal was acquired with a golden-means-based 3D radial sampling scheme. To achieve adequate temporal resolution with preserved spatial resolution, a k-space-weighted view sharing strategy was used in image reconstruction with an adaptive window size tailored to the kinetics of the 17 O signal. Simulation studies were performed to determine the adequate image reconstruction parameters. The established method was applied to delineating the kinetics of intravenously injected H217 O in vivo in the post-stroke mouse brain.

RESULTS

The proposed dynamic 17 O-MRI method achieved an isotropic resolution of 1.21 mm (0.77 mm nominal) in mouse brain at 9.4T, with the temporal resolution increased gradually from 3 s at the initial phase of rapid signal increase to 15 s at the steady-state. The high spatial resolution enabled the delineation of the heterogeneous H217 O uptake and washout kinetics in stroke-affected mouse brain.

CONCLUSION

The current study demonstrated a 3D 17 O-MRI method for dynamic monitoring of 17 O signal changes with high spatial and temporal resolution. The method can be utilized to quantify physiological parameters such as cerebral blood flow and blood-brain barrier permeability by tracking injected H217 O. It can also be used to measure oxygen consumption rate in 17 O-oxygen inhalation studies.

Direct imaging of glymphatic transport using H217O MRI

The recently proposed glymphatic pathway for solute transport and waste clearance from the brain has been the focus of intense debate. By exploiting an isotopically enriched MRI tracer, H217O, we directly imaged glymphatic water transport in the rat brain in vivo. Our results reveal glymphatic transport that is dramatically faster and more extensive than previously thought and unlikely to be explained by diffusion alone. Moreover, we confirm the critical role of aquaporin-4 channels in glymphatic transport.

Human Cord Blood Derived Unrestricted Somatic Stem Cells Restore Aquaporin Channel Expression, Reduce Inflammation and Inhibit the Development of Hydrocephalus After Experimentally Induced Perinatal Intraventricular Hemorrhage

Intraventricular hemorrhage (IVH) is a severe complication of preterm birth associated with cerebral palsy, intellectual disability, and commonly, accumulation of cerebrospinal fluid (CSF). Histologically, IVH leads to subependymal gliosis, fibrosis, and disruption of the ependymal wall. Importantly, expression of aquaporin channels 1 and 4 (AQP1 and AQP4) regulating respectively, secretion and absorption of cerebrospinal fluids is altered with IVH and are associated with development of post hemorrhagic hydrocephalus. Human cord blood derived unrestricted somatic stem cells (USSCs), which we previously demonstrated to reduce the magnitude of hydrocephalus, as having anti-inflammatory, and beneficial behavioral effects, were injected into the cerebral ventricles of rabbit pups 18 h after glycerol-induced IVH. USSC treated IVH pups showed a reduction in ventricular size when compared to control pups at 7 and 14 days (both, P < 0.05). Histologically, USSC treatment reduced cellular infiltration and ependymal wall disruption. In the region of the choroid plexus, immuno-reactivity for AQP1 and ependymal wall AQP4 expression were suppressed after IVH but were restored following USSC administration. Effects were confirmed by analysis of mRNA from dissected choroid plexus and ependymal tissue. Transforming growth factor beta (TGF-β) isoforms, connective tissue growth factor (CTGF) and matrix metalloprotease-9 (MMP-9) mRNA, as well as protein levels, were significantly increased following IVH and restored towards normal with USSC treatment (P < 0.05). The anti-inflammatory cytokine Interleukin-10 (IL-10) mRNA was reduced in IVH, but significantly recovered after USSC injection (P < 0.05). In conclusion, USSCs exerted anti-inflammatory effects by suppressing both TGF-β specific isoforms, CTGF and MMP-9, recovered IL-10, restored aquaporins expression towards baseline, and reduced hydrocephalus. These results support the possibility of the use of USSCs to reduce IVH consequences in prematurity.

Choroid plexus and CSF: an updated review

In this article, we review the available literature about the functions of the choroid plexus (ChP), including its basic role in cerebrospinal fluid (CSF) secretion, renewal and absorption. We discuss more recently described, lesser-known functions of the ChP, such as its role in circadian rhythm regulation, chemical and immune surveillance and functional implications of ChP disruption, as occurs in neurodegenerative disorders.

Retinal Aquaporin-4 and Regulation of Water Inflow Into the Vitreous Body

Purpose

Details of the posterior eye water dynamics are unclear. Aquaporin-4 (AQP4), a water channel, plays an important role in water dynamics in the central nervous system and is also present in the ocular tissue. The purpose of this study was to reveal the role of AQP4 in the water dynamics of the posterior eye using in vivo JJ vicinal coupling proton exchange (JJVCPE) magnetic resonance imaging (MRI) of AQP4 knockout (KO) mice and their wild-type littermates (controls).

Methods

JJVCPE MRI of the eye was performed on five AQP4 KO mice and seven control mice. We assessed the normalized signal intensities of a region of interest (ROI) set in the vitreous body after H₂¹⁷O administration. The results of the two groups were compared using a two-tailed Mann-Whitney U test.

Results

A statistical analysis revealed that the normalized ROI signal intensities at the steady state were significantly lower (P = 0.010, <0.05) in the AQP4 KO mice (mean ± SD, 84.5% ± 2.7%) than the controls (mean ± SD, 88.8% ± 1.9%).

Conclusions

The present study using JJVCPE MRI of the eye demonstrated that retinal AQP4 has a potential role in the regulation of water inflow into the vitreous body. Absence of AQP4 in the KO mice probably induces lower water outflow from the vitreous body. Our results could help clarify the pathogenesis of various ocular diseases.

Stagnation of glymphatic interstitial fluid flow and delay in waste clearance in the SOD1-G93A mouse model of ALS

Overexpression and mislocalization of aquaporin-4 (AQP4) in the SOD1^G93A mouse model of amyotrophic lateral sclerosis (ALS) have previously been reported. However, how alterations of AQP4 affect interstitial bulk flow in the brain and spinal cord, the so-called glymphatic system, is unclear. Here, we report an enhanced accumulation of disease-associated SOD1 species including SOD1 oligomers in SOD1^G93A;AQP4^-/- mice compared with SOD1^G93A mice during ALS disease progression, as analyzed by sandwich ELISA. By directly injecting SOD1 oligomers into the spinal cord parenchyma, we observed a significantly larger delay in clearance of biotinylated or fluorescent-labeled SOD1 oligomers in AQP4^-/- mice than in wild-type mice. Furthermore, when we injected the fluorescent-labeled tracer protein ovalbumin into the cisterna magna and analyzed the tracer distribution in the cervical spinal cord, approximately 35 % processing ability was found to be reduced in SOD1^G93A mice compared to wild-type mice. These results suggest that the glymphatic system is abnormal and that waste clearance is delayed in SOD1^G93A mice.

Idiopathic Normal Pressure Hydrocephalus and Elderly Acquired Hydrocephalus: Evaluation With Cerebrospinal Fluid Flow and Ventricular Volume Parameters

Purpose

To investigate differences in cerebrospinal fluid (CSF) flow through the aqueduct and to determine whether there is a relationship between CSF flow and ventricular volume parameters in idiopathic normal pressure hydrocephalus (iNPH) patients, elderly acquired hydrocephalus patients and age-matched healthy volunteers by phase-contrast MR (PC-MR).

Methods

A total of 40 iNPH patients and 41 elderly acquired hydrocephalus patients and 26 age-matched healthy volunteers in the normal control (NC) group were included between November 2017 and October 2019 in this retrospective study. The following CSF flow parameters were measured with PC-MR: peak velocity, average velocity (AV), aqueductal stroke volume (ASV), net ASV, and net flow. The following ventricular volume parameters were measured: ventricular volume (VV), brain volume, total intracranial volume, and relative VV. Differences between the iNPH and acquired hydrocephalus groups were compared Mann–Whitney U test and correlations between CSF flow and ventricular volume parameters were assessed using the Spearman correlation coefficient.

Results

Aqueductal stroke volume was significantly higher in the iNPH and acquired hydrocephalus groups than in the NC group, but did not differ significantly between the iNPH group and acquired hydrocephalus group. The AV, net ASV, and net flow in the iNPH and acquired hydrocephalus groups were significantly higher than those in the NC group (P < 0.0001), and those in the acquired hydrocephalus group were significantly higher than those in the iNPH group (P = 0.01, P = 0.007, P = 0.002, respectively). The direction of the AV and net ASV significantly differed among the three groups. There were no associations between the volume parameters and CSF flow according to PC-MR among the three groups.

Conclusion

Compared with iNPH, elderly acquired hydrocephalus demonstrated higher CSF hyperdynamic flow. Although increased CSF flow may contribute to further changes in ventricular morphology, there is no linear relationship between them. These findings might help increase our understanding of flow dynamics in iNPH and elderly acquired hydrocephalus.

The Cranial Bowl in the New Millennium and Sutherland's Legacy for Osteopathic Medicine: Part 1

A theoretical model that does not evolve with new information deriving from scientific research, by changing the assumptions from which it was born, becomes a philosophy; the scientist becomes a scholarch. Cranial manual osteopathic medicine is very controversial, although it is commonly practiced, from the clinician to the nonmedical health worker. The article, divided into two parts, reviews the assumptions with which the cranial model was created, highlighting the scientific innovations and new anatomical-physiological reflections. In the first part we will review the synthesis and movement of cerebrospinal fluid (CSF), the movement of the central and peripheral nervous system; we will highlight the mechanical characteristics of the meninges. The aim of the article is to highlight the need to renew the existing cranial model.

Delayed clearance of cerebrospinal fluid tracer from choroid plexus in idiopathic normal pressure hydrocephalus

Impaired clearance of amyloid-β from choroid plexus is one proposed mechanism behind amyloid deposition in Alzheimer's disease. The present study examined whether clearance from choroid plexus of a cerebrospinal fluid tracer, serving as a surrogate marker of a metabolic waste product, is altered in idiopathic normal pressure hydrocephalus (iNPH), one sub-type of dementia. In a prospective observational study of close to healthy individuals (reference cohort; REF) and individuals with iNPH, we performed standardized T1-weighted magnetic resonance imaging scans before and through 24 h after intrathecal administration of a cerebrospinal fluid tracer (the magnetic resonance imaging contrast agent gadobutrol). Changes in normalized T1 signal within the choroid plexus and cerebrospinal fluid of lateral ventricles were quantified using FreeSurfer. The normalized T1 signal increased to maximum within choroid plexus and cerebrospinal fluid of lateral ventricles 6-9 h after intrathecal gadobutrol in both the REF and iNPH cohorts (enrichment phase). Peak difference in normalized T1 signals between REF and iNPH individuals occurred after 24 h (clearance phase). The results gave evidence for gadobutrol resorption from cerebrospinal fluid by choroid plexus, but with delay in iNPH patients. Whether choroid plexus has a role in iNPH pathogenesis in terms of delayed clearance of amyloid-β remains to be shown.

Measuring water exchange across the blood-brain barrier using MRI

The blood-brain barrier (BBB) regulates the transfer of solutes and essential nutrients into the brain. Growing evidence supports BBB dysfunction in a range of acute and chronic brain diseases, justifying the need for novel research and clinical tools that can non-invasively detect, characterize, and quantify BBB dysfunction in-vivo. Many approaches already exist for measuring BBB dysfunction in man using positron emission tomography and magnetic resonance imaging (e.g. dynamic contrast-enhanced MRI measurements of gadolinium leakage). This review paper focusses on MRI measurements of water exchange across the BBB, which occurs through a wide range of pathways, and is likely to be a highly sensitive marker of BBB dysfunction. Key mathematical models and acquisition methods are discussed for the two main approaches: those that utilize contrast agents to enhance relaxation rate differences between the intravascular and extravascular compartments and so enhance the sensitivity of MRI signals to BBB water exchange, and those that utilize the dynamic properties of arterial spin labelling to first isolate signal from intravascular spins and then estimate the impact of water exchange on the evolving signal. Data from studies in healthy and pathological brain tissue are discussed, in addition to validation studies in rodents.

Altered Waste Disposal System in Aging and Alzheimer's Disease: Focus on Astrocytic Aquaporin-4

Among the diverse cell types included in the general population named glia, astrocytes emerge as being the focus of a growing body of research aimed at characterizing their heterogeneous and complex functions. Alterations of both their morphology and activities have been linked to a variety of neurological diseases. One crucial physiological need satisfied by astrocytes is the cleansing of the cerebral tissue from waste molecules. Several data demonstrate that aquaporin-4 (AQP-4), a protein expressed by astrocytes, is crucially important for facilitating the removal of waste products from the brain. Aquaporins are water channels found in all district of the human organism and the most abundant isoform in the brain is AQP-4. This protein is involved in a myriad of astrocytic activities, including calcium signal transduction, potassium buffering, synaptic plasticity, astrocyte migration, glial scar formation and neuroinflammation. The highest density of AQP-4 is found at the astrocytic domains closest to blood vessels, the endfeet that envelop brain vessels, with low to zero expression in other astrocytic membrane regions. Increased AQP-4 expression and loss of polarization have recently been documented in altered physiological conditions. Here we review the latest findings related to aging and Alzheimer’s disease (AD) on this topic, as well as the available knowledge on pharmacological tools to target AQP-4.

Glymphatic imaging using MRI

In recent years, the existence of a mass transport system in the brain via cerebrospinal fluid (CSF) or interstitial fluid (ISF) has been suggested by many studies. The glymphatic system is hypothesized to be a waste clearance system of the CSF through the perivascular and interstitial spaces in the brain. Tracer studies have primarily been used to visualize or evaluate the waste clearance system in the brain, and evidence for this system has accumulated. The initial study that identified the glymphatic system was an in vivo tracer study in mice. In that study, fluorescent tracers were injected into the cisterna magna and visualized by two-photon microscopy. MRI has also been used to evaluate glymphatic function primarily with gadolinium-based contrast agents (GBCAs) as tracers. A number of GBCA studies evaluating glymphatic function have been conducted using either intrathecal or intravenous injections. Stable isotopes, such as ¹⁷ O-labeled water, may also be used as tracers since they can be detected by MRI. In addition to tracer studies, several other approaches have been used to evaluate ISF dynamics within the brain, including diffusion imaging. Phase contrast evaluation is a powerful method for visualizing flow within the CSF space. In order to evaluate the movement of water within tissue, diffusion-weighted MRI represents another promising technique, and several studies have utilized diffusion techniques for the evaluation of the glymphatic system. This review will discuss the findings of these diffusion studies. Level of Evidence: 5 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2019. J. Magn. Reson. Imaging 2020;51:11-24.

Increased cerebral vascularization and decreased water exchange across the blood-brain barrier in aquaporin-4 knockout mice

Aquaporin-4 (AQP4) plays an important role in regulating water exchange across the blood-brain barrier (BBB) and brain-cerebrospinal fluid interface. Studies on AQP-4 knockout mice (AQP4-KO) have reported considerable protection from brain edema induced by acute water intoxication and ischemic stroke, identifying AQP4 as a potential target for therapeutic interventions. However, the long-term effects of chronic AQP4 suppression are yet to be elucidated. In the current study, we evaluated the physiological and structural changes in adult AQP4-KO mice using magnetic resonance imaging (MRI) and immunohistochemical analysis. Water exchange across BBB was assessed by tracking an intravenous bolus injection of oxygen-17 (¹⁷O) water (H₂¹⁷O) using ¹⁷O-MRI. Cerebral blood flow (CBF) was quantified using arterial spin-labeling (ASL) MRI. Capillary density was determined by immunohistochemical staining for glucose transporter-1 (GLUT1). Compared to wildtype control mice, AQP4-KO mice showed a significant reduction in peak and steady-state H₂¹⁷O uptake despite unaltered CBF. Interestingly, a 22% increase in cortical capillary density was observed in AQP4-KO mice. These results suggest that increased cerebral vascularization may be an adaptive response to chronic reduction in water exchange across BBB in AQP4-KO mice.

Physiological and pathological impact of AQP1 knockout in mice

Aquaporin 1 (AQP1) is a glycoprotein responsible for water passive transport quickly across biological membrane. Here, we reviewed the structural and functional impacts of AQP1 knockout (AQP1-KO) in animal or cell culture models. AQP1 gene deletion can cause a large number of abnormalities including the disturbance in epithelial fluid secretion, polyhydramnios, deficiency of urinary concentrating function, and impairment of pain perception. AQP1-KO mice also displayed aberrations of cardiovascular, gastrointestinal and hepatobiliary, and kidney functions as well as placenta and embryo development. Moreover, AQP1-KO perturbed tumor angiogenesis and led to reduced brain injury upon trauma. On the cellular level, AQP1-KO caused neuroinflammation, aberrant cell proliferation and migration, and macrophages infiltration. Mechanistic studies confirmed that AQP1 gene products regulate the secretory function and participated in balancing the osmotic water flux across the peritoneal membrane. The available data indicated that AQP1 might serve as a potential target for developing novel therapeutic approaches against diverse human diseases.

Aquaporin-4 facilitator TGN-073 promotes interstitial fluid circulation within the blood-brain barrier: [17O]H2O JJVCPE MRI study

The blood–brain barrier (BBB), which imposes significant water permeability restriction, effectively isolates the brain from the systemic circulation. Seemingly paradoxical, the abundance of aquaporin-4 (AQP-4) on the inside of the BBB strongly indicates the presence of unique water dynamics essential for brain function. On the basis of the highly specific localization of AQP-4, namely, astrocyte end feet at the glia limitans externa and pericapillary Virchow–Robin space, we hypothesized that the AQP-4 system serves as an interstitial fluid circulator, moving interstitial fluid from the glia limitans externa to pericapillary Virchow–Robin space to ensure proper glymphatic flow draining into the cerebrospinal fluid. The hypothesis was tested directly using the AQP-4 facilitator TGN-073 developed in our laboratory, and [¹⁷O]H₂O JJ vicinal coupling proton exchange MRI, a method capable of tracing water molecules delivered into the blood circulation. The results unambiguously showed that facilitation of AQP-4 by TGN-073 increased turnover of interstitial fluid through the system, resulting in a significant reduction in [¹⁷O]H₂O contents of cortex with normal flux into the cerebrospinal fluid. The study further suggested that in addition to providing the necessary water for proper glymphatic flow, the AQP-4 system produces a water gradient within the interstitial space promoting circulation of interstitial fluid within the BBB.

Cerebrospinal fluid dynamics and intracranial pressure elevation in neurological diseases

The fine balance between the secretion, composition, volume and turnover of cerebrospinal fluid (CSF) is strictly regulated. However, during certain neurological diseases, this balance can be disrupted. A significant disruption to the normal CSF circulation can be life threatening, leading to increased intracranial pressure (ICP), and is implicated in hydrocephalus, idiopathic intracranial hypertension, brain trauma, brain tumours and stroke. Yet, the exact cellular, molecular and physiological mechanisms that contribute to altered hydrodynamic pathways in these diseases are poorly defined or hotly debated. The traditional views and concepts of CSF secretion, flow and drainage have been challenged, also due to recent findings suggesting more complex mechanisms of brain fluid dynamics than previously proposed. This review evaluates and summarises current hypotheses of CSF dynamics and presents evidence for the role of impaired CSF dynamics in elevated ICP, alongside discussion of the proteins that are potentially involved in altered CSF physiology during neurological disease. Undoubtedly CSF secretion, absorption and drainage are important aspects of brain fluid homeostasis in maintaining a stable ICP. Traditionally, pharmacological interventions or CSF drainage have been used to reduce ICP elevation due to over production of CSF. However, these drugs are used only as a temporary solution due to their undesirable side effects. Emerging evidence suggests that pharmacological targeting of aquaporins, transient receptor potential vanilloid type 4 (TRPV4), and the Na+-K+-2Cl- cotransporter (NKCC1) merit further investigation as potential targets in neurological diseases involving impaired brain fluid dynamics and elevated ICP.

AQP1 and AQP4 Contribution to Cerebrospinal Fluid Homeostasis

Aquaporin 1 (AQP1), expressed in epithelial cells of the choroid plexus, and aquaporin 4 (AQP4) present in ependymal cells and glia limitants have been proposed to play a significant role in cerebrospinal fluid (CSF) production and homeostasis. However, the specific contribution of each water channel to these functions remains unknown, being a subject of debate during the last years. Here, we analyzed in detail how AQP1 and AQP4 participate in different aspects of the CSF homeostasis such as the load and drainage of ventricles, and further explored if these proteins play a role in the ventricular compliance. To do that, we carried out records of intraventricular pressure and CSF outflow, and evaluated ventricular volume by magnetic resonance imaging in AQP1^−/−, AQP4^−/−, double AQP1^−/−-AQP4^−/− knock out and wild type mice controls. The analysis performed clearly showed that both AQPs have a significant participation in the CSF production, and additionally revealed that the double AQP1-AQP4 mutation alters the CSF drainage and the ventricular compliance. The data reported here indicate a significant extra-choroidal CSF formation mediated by AQP4, supporting the idea of an important and constant CSF production/absorption process, sustained by efflux/influx of water between brain capillaries and interstitial fluid. Moreover, our results suggest the participation of AQPs in structural functions also related with CSF homeostasis such as the distensibility capacity of the ventricular system.

The Effect of Aquaporin-4 Knockout on Interstitial Fluid Flow and the Structure of the Extracellular Space in the Deep Brain

It has been reported that aquaporin-4 (AQP4) deficiency impairs transportation between the cerebrospinal fluid and interstitial fluid (ISF) as well as the clearance of interstitial solutes in the superficial brain. However, the effect of AQP4 on ISF flow in the deep brain remains unclear. This study compared the brain ISF flow in the caudate nucleus and thalamus of normal rats (NO) and AQP4 knockout rats (KO) using tracer-based magnetic resonance imaging. The rate of brain ISF flow slowed to different degrees in the two regions of KO rats’ brains. Compared with NO rats, the half-life of ISF in the thalamus of KO rats was significantly prolonged, with a corresponding decrease in the clearance coefficient. The tortuosity of the brain extracellular space (ECS) was unchanged in the thalamus of KO rats. In the caudate nucleus of KO rats, the volume fraction of the ECS and the diffusion coefficient were increased, with significantly decreased tortuosity; no significant changes in brain ISF flow were demonstrated. Combined with a change in the expression of glial fibrillary acidic protein and AQP4 in two brain regions, we found that the effect of AQP4 knockout on ISF flow and ECS structure in these two regions differed. This difference may be related to the distribution of astrocytes and the extent of AQP4 decline. This study provides evidence for the involvement of AQP4 in ISF transportation in the deep brain and provides a basis for the establishment of a pharmacokinetic model of the brain’s interstitial pathway.

Cerebrospinal fluid volumetric net flow rate and direction in idiopathic normal pressure hydrocephalus

The aim of the present study was to examine cerebrospinal fluid (CSF) volumetric net flow rate and direction at the cranio-cervical junction (CCJ) and cerebral aqueduct in individuals with idiopathic normal pressure hydrocephalus (iNPH) using cardiac-gated phase-contrast magnetic resonance imaging (PC-MRI). An in-depth, pixel-by-pixel analysis of regions of interest from the CCJ and cerebral aqueduct, respectively, was done in 26 iNPH individuals, and in 4 healthy subjects for validation purposes. Results from patients were compared with over-night measurements of static and pulsatile intracranial pressure (ICP). In iNPH, CSF net flow at CCJ was cranially directed in 17/22 as well as in 4/4 healthy subjects. Estimated daily CSF volumetric net flow rate at CCJ was 6.9 ± 9.9 L/24 h in iNPH patients and 4.5 ± 5.0 L/24 h in healthy individuals. Within the cerebral aqueduct, the CSF net flow was antegrade in 7/21 iNPH patients and in 4/4 healthy subjects, while it was retrograde (i.e. towards ventricles) in 14/21 iNPH patients. Estimated daily CSF volumetric net flow rate in cerebral aqueduct was 1.1 ± 2.2 L/24 h in iNPH while 295 ± 53 mL/24 h in healthy individuals. Magnitude of cranially directed CSF net flow in cerebral aqueduct was highest in iNPH individuals with signs of impaired intracranial compliance. The study results indicate CSF flow volumes and direction that are profoundly different from previously assumed. We hypothesize that spinal CSF formation may serve to buffer increased demand for CSF flow through the glymphatic system during sleep and during deep inspiration to compensate for venous outflow.

Combined effects of aquaporin-4 and hypoxia produce age-related hydrocephalus

Aquaporin-4, present in ependymal cells, in glia limiting and abundantly in pericapillary astrocyte foot processes, and aquaporin-1, expressed in choroid plexus epithelial cells, play an important role in cerebrospinal fluid production and may be involved in the pathophysiology of age-dependent hydrocephalus. The finding that brain aquaporins expression is regulated by low oxygen tension led us to investigate how hypoxia and elevated levels of cerebral aquaporins may result in an increase in cerebrospinal fluid production that could be associated with a hydrocephalic condition. Here we have explored, in young and aged mice exposed to hypoxia, whether aquaporin-4 and aquaporin-1 participate in the development of age-related hydrocephalus. Choroid plexus, striatum, cortex and ependymal tissue were analyzed separately both for mRNA and protein levels of aquaporins. Furthermore, parameters such as total ventricular volume, intraventricular pressure, cerebrospinal fluid outflow rate, ventricular compliance and cognitive function were studied in wild type, aquaporin-1 and aquaporin-4 knock-out animals subjected to hypoxia or normoxia. Our data demonstrate that hypoxia is involved in the development of age-related hydrocephalus by a process that depends on aquaporin-4 channels as a main route for cerebrospinal fluid movement. Significant increases in aquaporin-4 expression that occur over the course of animal aging, together with a reduced cerebrospinal fluid outflow rate and ventricular compliance, contribute to produce more severe hydrocephalus related to hypoxic events in aged mice, with a notable impairment in cognitive function. These results indicate that physiological events and/or pathological conditions presenting with cerebral hypoxia/ischemia contribute to the development of chronic adult hydrocephalus.