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

The Glymphatic System and Subarachnoid Lymphatic-Like Membrane: Recent Developments in Cerebrospinal Fluid Research

BACKGROUND

Cerebrospinal fluid (CSF) circulates throughout the ventricles, cranial and spinal subarachnoid spaces, and central spinal cord canal. CSF protects the central nervous system through mechanical cushioning, regulation of intracranial pressure, regulation of metabolic homeostasis, and provision of nutrients. Recently, investigators have characterized the glial-lymphatic (glymphatic) system, the analog of the lymphatic system in the central nervous system, and described a fourth meningeal layer; the subarachnoid lymphatic-like membrane (SLYM)relevant to the CSF.

METHODS

A narrative review was conducted.

RESULTS

In this review, we summarize these advances. We describe the development of the original model, controversies, a revised model, and a new conceptual framework. We characterize the biological functions, influence of sleep-wake cycles, and effect of aging with relevance to the glymphatic system. We highlight the role of the glymphatic system in Alzheimer's disease, idiopathic normal pressure hydrocephalus, ischemic stroke, subarachnoid hemorrhage, and traumatic brain injury. Next, we characterize the structure and role of the SLYM. Finally, we explore the relevance of the glymphatic system and SLYM to neurosurgery.

CONCLUSIONS

This manuscript will inform clinicians and scientists regarding preclinical and translational advances in the understanding of the structure, dynamics, and function of the CSF.

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.

Shifting from ependyma to choroid plexus epithelium and the changing expressions of aquaporin-1 and aquaporin-4

The cells of the choroid plexus (CP) epithelium are specialized ependymal cells (ECs) but have distinct properties. The CP cells and ECs form single-cell sheets contiguous to each other at a transitional zone. The CP is underlined by a basal lamina and has barrier properties, whereas the ECs do not. The basal lamina of the CP is continuous with the glia limitans superficialis and, consequently, the CP stroma is continuous with the meninges along entering blood vessels. The CP has previously been reported to express aquaporin-1 (AQP1) mostly apically, and ECs show mostly basolateral aquaporin-4 (AQP4) expression. Recent evidence in various systems has shown that in changing conditions the expression and distribution of AQP4 can be modified, involving phosphorylation and calmodulin-triggered translocation. Studies on the human CP revealed that AQP4 is also expressed in some CP cells, which is likely to be increased during ageing based on mouse data. Moreover, subependymal astrocytic processes in the ependyma-CP transition, forming a glial plate around blood vessels and facing the CP stroma, were strongly positive for AQP4. We propose that the increased AQP4 expression might be a compensatory mechanism for the observed reduction in CSF production in the ageing human brain. The high AQP4 density in the transition zone might facilitate the transport of water into and out of the CP stroma and serve as a drainage and clearing pathway for metabolites in the CNS.

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.

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.

Extracellular matrix protein anosmin-1 overexpression alters dopaminergic phenotype in the CNS and the PNS with no pathogenic consequences in a MPTP model of Parkinson's disease

The development and survival of dopaminergic neurons are influenced by the fibroblast growth factor (FGF) pathway. Anosmin-1 (A1) is an extracellular matrix protein that acts as a major regulator of this signaling pathway, controlling FGF diffusion, and receptor interaction and shuttling. In particular, previous work showed that A1 overexpression results in more dopaminergic neurons in the olfactory bulb. Prompted by those intriguing results, in this study, we investigated the effects of A1 overexpression on different populations of catecholaminergic neurons in the central (CNS) and the peripheral nervous systems (PNS). We found that A1 overexpression increases the number of dopaminergic substantia nigra pars compacta (SNpc) neurons and alters the striosome/matrix organization of the striatum. Interestingly, these numerical and morphological changes in the nigrostriatal pathway of A1-mice did not confer an altered susceptibility to experimental MPTP-parkinsonism with respect to wild-type controls. Moreover, the study of the effects of A1 overexpression was extended to different dopaminergic tissues associated with the PNS, detecting a significant reduction in the number of dopaminergic chemosensitive carotid body glomus cells in A1-mice. Overall, our work shows that A1 regulates the development and survival of dopaminergic neurons in different nuclei of the mammalian nervous system.

Loss of aquaporin-4 results in glymphatic system dysfunction via brain-wide interstitial fluid stagnation

The glymphatic system is a fluid transport network of cerebrospinal fluid (CSF) entering the brain along arterial perivascular spaces, exchanging with interstitial fluid (ISF), ultimately establishing directional clearance of interstitial solutes. CSF transport is facilitated by the expression of aquaporin-4 (AQP4) water channels on the perivascular endfeet of astrocytes. Mice with genetic deletion of AQP4 (AQP4 KO) exhibit abnormalities in the brain structure and molecular water transport. Yet, no studies have systematically examined how these abnormalities in structure and water transport correlate with glymphatic function. Here, we used high-resolution 3D magnetic resonance (MR) non-contrast cisternography, diffusion-weighted MR imaging (MR-DWI) along with intravoxel-incoherent motion (IVIM) DWI, while evaluating glymphatic function using a standard dynamic contrast-enhanced MR imaging to better understand how water transport and glymphatic function is disrupted after genetic deletion of AQP4. AQP4 KO mice had larger interstitial spaces and total brain volumes resulting in higher water content and reduced CSF space volumes, despite similar CSF production rates and vascular density compared to wildtype mice. The larger interstitial fluid volume likely resulted in increased slow but not fast MR diffusion measures and coincided with reduced glymphatic influx. This markedly altered brain fluid transport in AQP4 KO mice may result from a reduction in glymphatic clearance, leading to enlargement and stagnation of fluid in the interstitial space. Overall, diffusion MR is a useful tool to evaluate glymphatic function and may serve as valuable translational biomarker to study glymphatics in human disease.

Aquaporin-4 Expression Switches from White to Gray Matter Regions during Postnatal Development of the Central Nervous System

Aquaporin-4 (AQP4) is the most abundant water channel in the central nervous system and plays a fundamental role in maintaining water homeostasis there. In adult mice, AQP4 is located mainly in ependymal cells, in the endfeet of perivascular astrocytes, and in the glia limitans. Meanwhile, its expression, location, and function throughout postnatal development remain largely unknown. Here, the expression of AQP4 mRNA was studied by in situ hybridization and RT-qPCR, and the localization and amount of protein was studied by immunofluorescence and western blotting, both in the brain and spinal cord. For this, wild-type mice of the C57BL/6 line, aged 1, 3, 7, 11, 20, and 60 days, and 18 months were used. The results showed a change in both the expression and location of AQP4 in postnatal development compared to those during adult life. In the early stages of postnatal development it appears in highly myelinated areas, such as the corpus callosum or cerebellum, and as the animal grows, it disappears from these areas, passing through the cortical regions of the forebrain and concentrating around the blood vessels. These findings suggest an unprecedented possible role for AQP4 in the early cell differentiation process, during the first days of life in the newborn animal, which will lead to myelination.

Choroid Plexus Aquaporins in CSF Homeostasis and the Glymphatic System: Their Relevance for Alzheimer's Disease

The glymphatic system, a fluid-clearance pathway involved in brain waste clearance, is known to be impaired in neurological disorders, including Alzheimer's disease (AD). For this reason, it is important to understand the specific mechanisms and factors controlling glymphatic function. This pathway enables the flow of cerebrospinal fluid (CSF) into the brain and subsequently the brain interstitium, supported by aquaporins (AQPs). Continuous CSF transport through the brain parenchyma is critical for the effective transport and drainage of waste solutes, such as toxic proteins, through the glymphatic system. However, a balance between CSF production and secretion from the choroid plexus, through AQP regulation, is also needed. Thus, any condition that affects CSF homeostasis will also interfere with effective waste removal through the clearance glymphatic pathway and the subsequent processes of neurodegeneration. In this review, we highlight the role of AQPs in the choroid plexus in the modulation of CSF homeostasis and, consequently, the glymphatic clearance pathway, with a special focus on AD.

Integrative analyses identify HIF-1α as a potential protective role with immune cell infiltration in adamantinomatous craniopharyngioma

Craniopharyngiomas (CPs) are histologically benign tumors located in the sellar–suprasellar region. Although the transcriptome development in recent years have deepened our knowledge to the tumorigenesis process of adamantinomatous craniopharyngioma (ACP), the peritumoral immune infiltration of tumor is still not well understood. In this study, weighted gene coexpression network analysis (WGCNA) was applied to identify different gene modules based on clinical characteristics and gene expression, and then, the protein–protein interaction (PPI) network with the Cytohubba plug-in were performed to screen pivotal genes. In addition, immune cell infiltration (ICI) analysis was used to evaluate the immune microenvironment of ACP patients. In total, 8,568 differential expression genes were identified based on our datasets and two microarray profiles from the public database. The functional enrichment analysis revealed that upregulated genes were mainly enriched in immune-related pathways while downregulated genes were shown in the hormone and transduction of signaling pathways. The WGCNA investigated the most relevant modules, and 1,858 hub genes was detected, from which the PPI network identified 14 pivotal genes, and the Hypoxia-inducible factor 1-alpha (HIF-1α) pathway including four critical genes may be involved in the development of ACP. Moreover, naïve CD4+ and CD8+ T cells were decreased while specific subtypes of T cells were significantly increased in ACP patients according to ICI analysis. Validation by immunofluorescence staining revealed a higher expression of HIF-1α in ACP (ACP vs. control) and adult-subtype (adult vs. children), suggesting a possible state of immune system activation. Notably, children with low HIF-1α scores were related to the hypothalamus involvement and hydrocephalus symptoms. In this study, we successfully identified HIF-1α as a key role in the tumorigenesis and development of ACP through comprehensive integrated analyses and systematically investigated the potential relationship with immune cells in ACP. The results may provide valuable resources for understanding the underlying mechanisms of ACP and strengthen HIF-1α as a potential immunotherapeutic target in clinical application.

Glioblastoma disrupts the ependymal wall and extracellular matrix structures of the subventricular zone

Background

Glioblastoma (GBM) is the most aggressive and common type of primary brain tumor in adults. Tumor location plays a role in patient prognosis, with tumors proximal to the lateral ventricles (LVs) presenting with worse overall survival, increased expression of stem cell genes, and increased incidence of distal tumor recurrence. This may be due in part to interaction of GBM with factors of the subventricular zone (SVZ), including those contained within the cerebrospinal fluid (CSF). However, direct interaction of GBM tumors with CSF has not been proved and would be hindered in the presence of an intact ependymal cell layer.

Methods

Here, we investigate the ependymal cell barrier and its derived extracellular matrix (ECM) fractones in the vicinity of a GBM tumor. Patient-derived GBM cells were orthotopically implanted into immunosuppressed athymic mice in locations distal and proximal to the LV. A PBS vehicle injection in the proximal location was included as a control. At four weeks post-xenograft, brain tissue was examined for alterations in ependymal cell health via immunohistochemistry, scanning electron microscopy, and transmission electron microscopy.

Results

We identified local invading GBM cells within the LV wall and increased influx of CSF into the LV-proximal GBM tumor bulk compared to controls. In addition to the physical disruption of the ependymal cell barrier, we also identified increased signs of compromised ependymal cell health in LV-proximal tumor-bearing mice. These signs include increased accumulation of lipid droplets, decreased cilia length and number, and decreased expression of cell channel proteins. We additionally identified elevated numbers of small fractones in the SVZ within this group, suggesting increased indirect CSF-contained molecule signaling to tumor cells.

Conclusions

Our data is the first to show that LV-proximal GBMs physically disrupt the ependymal cell barrier in animal models, resulting in disruptions in ependymal cell biology and increased CSF interaction with the tumor bulk. These findings point to ependymal cell health and CSF-contained molecules as potential axes for therapeutic targeting in the treatment of GBM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00354-8.

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.

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.

A clinical primer for the glymphatic system

The complex and dynamic system of fluid flow through the perivascular and interstitial spaces of the central nervous system has new-found implications for neurological diseases. Cerebrospinal fluid movement throughout the CNS parenchyma is more dynamic than could be explained via passive diffusion mechanisms alone. Indeed, a semi-structured glial-lymphatic (glymphatic) system of astrocyte-supported extracellular perivascular channels serves to directionally channel extracellular fluid, clearing metabolites and peptides to optimize neurologic function. Clinical studies of the glymphatic network has to date proven challenging, with most data gleaned from rodent models and post-mortem investigations. However, increasing evidence suggests that disordered glymphatic function contributes to the pathophysiology of CNS aging, neurodegenerative disease, and CNS injuries, as well as normal pressure hydrocephalus. Unlocking such pathophysiology could provide important avenues toward novel therapeutics. We here provide a multidisciplinary overview of glymphatics and critically review accumulating evidence regarding its structure, function, and hypothesized relevance to neurological disease. We highlight emerging technologies of relevance to the longitudinal evaluation of glymphatic function in health and disease. Finally, we discuss the translational opportunities and challenges of studying glymphatic science.

Evaluation of aquaporins in the cerebrospinal fluid in patients with idiopathic normal pressure hydrocephalus

Brain aquaporin 1 (AQP1) and AQP4 are involved in cerebrospinal fluid (CSF) homeostasis and might participate in the origin of hydrocephalus. Studies have shown alterations of perivascular AQP4 expression in idiopathic normal pressure hydrocephalus (iNPH) and Alzheimer's disease (AD). Due to the overlapping of clinical signs between iNPH and certain neurological conditions, mainly AD, specific biomarkers might improve the diagnostic accuracy for iNPH. The goal of the present study was to analyze and quantify the presence of AQP1 and AQP4 in the CSF of patients with iNPH and AD to determine whether these proteins can be used as biomarkers of iNPH. We examined AQP1 and AQP4 protein levels in the CSF of 179 participants (88 women) classified into 5 groups: possible iNPH (81 participants), hydrocephalus associated with other neurological disorders (13 participants), AD (41 participants), non-AD dementia (32 participants) and healthy controls (12 participants). We recorded each participant's demographic and clinical variables and indicated, when available in the clinical history, the record of cardiovascular and respiratory complications. An ELISA showed virtually no AQP content in the CSF. Information on the vascular risk factors (available for 61 patients) confirmed some type of vascular risk factor in 86% of the patients with possible iNPH and 58% of the patients with AD. In conclusion, the ELISA analysis showed insufficient sensitivity to detect the presence of AQP1 and AQP4 in CSF, ruling out the possible use of these proteins as biomarkers for diagnosing iNPH.

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.

Hydrocephalus Revisited: New Insights into Dynamics of Neurofluids on Macro- and Microscales

New experimental and clinical findings question the historic view of hydrocephalus and its 100-year-old classification. In particular, real-time magnetic resonance imaging (MRI) evaluation of cerebrospinal fluid (CSF) flow and detailed insights into brain water regulation on the molecular scale indicate the existence of at least three main mechanisms that determine the dynamics of neurofluids: (1) inspiration is a major driving force; (2) adequate filling of brain ventricles by balanced CSF upsurge is sensed by cilia; and (3) the perivascular glial network connects the ependymal surface to the pericapillary Virchow-Robin spaces. Hitherto, these aspects have not been considered a common physiologic framework, improving knowledge and therapy for severe disorders of normal-pressure and posthemorrhagic hydrocephalus, spontaneous intracranial hypotension, and spaceflight disease.

The impact of hypoxia on blood-brain, blood-CSF, and CSF-brain barriers

The blood-brain barrier (BBB), blood-cerebrospinal fluid (CSF) barrier (BCSFB), and CSF-brain barriers (CSFBB) are highly regulated barriers in the central nervous system comprising complex multicellular structures that separate nerves and glia from blood and CSF, respectively. Barrier damage has been implicated in the pathophysiology of diverse hypoxia-related neurological conditions, including stroke, multiple sclerosis, hydrocephalus, and high-altitude cerebral edema. Much is known about the damage to the BBB in response to hypoxia, but much less is known about the BCSFB and CSFBB. Yet, it is known that these other barriers are implicated in damage after hypoxia or inflammation. In the 1950s, it was shown that the rate of radionucleated human serum albumin passage from plasma to CSF was five times higher during hypoxic than normoxic conditions in dogs, due to BCSFB disruption. Severe hypoxia due to administration of the bacterial toxin lipopolysaccharide is associated with disruption of the CSFBB. This review discusses the anatomy of the BBB, BCSFB, and CSFBB and the impact of hypoxia and associated inflammation on the regulation of those barriers.

Novel therapeutics for hydrocephalus: Insights from animal models

Hydrocephalus is a cerebrospinal fluid physiological disorder that causes ventricular dilation with normal or high intracranial pressure. The current regular treatment for hydrocephalus is cerebrospinal fluid shunting, which is frequently related to failure and complications. Meanwhile, considering that the current nonsurgical treatments of hydrocephalus can only relieve the symptoms but cannot eliminate this complication caused by primary brain injuries, the exploration of more effective therapies has become the focus for many researchers. In this article, the current research status and progress of nonsurgical treatment in animal models of hydrocephalus are reviewed to provide new orientations for animal research and clinical practice.

Regulation of aquaporin-4 expression in the central nervous system investigated using M23-AQP4 null mouse

In astrocytes, unknown mechanisms regulate the expression of M1 and M23 isoforms of water channel aquaporin-4 (M1-AQP4 and M23-AQP4). The ratio between these two isoforms controls the AQP4 assembly state in the plasma membrane known as orthogonal arrays of particles (OAPs). To give new insights into these mechanisms, here, we explore the regulation of AQP4 expression in the spinal cord of a CRISPR/Cas9 M23-null mouse model (M23-null). In the M23-null spinal cord OAP assembly, the perivascular localization of AQP4 and M1-AQP4 protein were drastically reduced. In heterozygous, M1-AQP4 was proportionally reduced with M23-AQP4, maintaining the isoform ratio unaffected. We hypothesize a role of the M23-AQP4 in the regulation of M1-AQP4 expression. M1-AQP4 transcription, splicing and M1-AQP4 protein degradation were found to be unaffected in M23-null spinal cord and in M23-null astrocyte primary culture. The translational control was investigated by mRNA-protein pull down and quantitative mass spectrometry, to isolate and quantify AQP4 mRNA binding proteins (AQP4-RBPs). Compared to WT, in M23-null spinal cord, the interaction between AQP4 mRNA and polypyrimidine tract binding protein 1, a positive regulator of AQP4 translation, was higher, while interaction with the RNA helicase DDX17 was lower. In astrocyte primary cultures, DDX17 knockdown upregulated AQP4 protein expression and increased cell swelling, leaving AQP4 mRNA levels unchanged. Here, we identify AQP4-RBPs and provide evidence that in mouse spinal cord M23-AQP4 deletion changes the interaction between AQP4 mRNA and some RBPs involved in AQP4 translation. We describe for the first time the RNA helicase DDX17 as a regulator of AQP4 expression in astrocytes.

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.

Role of aquaporins in hydrocephalus: what do we know and where do we stand? A systematic review

INTRODUCTION

Glymphatic fluid circulation may be considered the lymphatic system of the brain and the main role of such system seems to be played by aquaporins (AQPs), a family of proteins which regulates water exchange, in particular AQP4 and 1. Alterations of glymphatic fluid circulation through AQPs variations are now emerging as central elements in the pathophysiology of different brain conditions, like hydrocephalus. This systematic review provides an insight about the role of AQPs in hydrocephalus establishment and compensation, investigating their possible role as diagnostic tools or therapeutic targets.

METHODS

PubMed database was screened searching for the relevant existing literature in English language published until February 29th 2020, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement.

RESULTS

A total of 40 articles met the inclusion criteria for our systematic analysis. AQP4 resulted the most studied water channel, followed by AQP1. The changes in cerebrospinal fluid (CSF), brain parenchyma and choroid plexus (CP) in different hydrocephalus type were analyzed. Moreover, important pharmacological interactions regarding AQP and molecules or conditions were discussed. A very interesting result is the general consensus on increase of AQP4 in hydrocephalic patients, unless in patients suffering from idiopathic normal pressure hydrocephalus, where AQP4 shows a tendency in reduction.

CONCLUSION

AQP seem to play a central role in the pathophysiology of hydrocephalus and in its compensation mechanisms. Further studies are required to definitively establish their precise roles and their quantitative changes to allow their utilization as diagnostic tools or therapeutic targets.

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.